Hot rolled steel sheet and method of producing same

TECHNICAL FIELD

The present invention relates to a hot rolled steel sheet which has composite structure and which shows high strength, excellent formability, and excellent fracture properties, and a method of producing the same.

Priority is claimed on Japanese Patent Application No. 2011-060909, filed in Japan on Mar. 18, 2011, and Japanese Patent Application No. 2011-064633, filed in Japan on Mar. 23, 2011, the contents of which are incorporated herein by reference.

BACKGROUND ART

In recent years, in order to reduce the weight of automobiles, attempts to increase the strength of steel sheets have been performed. In general, increasing the strength of the steel sheet leads to a deterioration of the formability such as a hole expansibility, and thinning the sheet thickness for weight reduction leads to a decrease in fatigue life. Accordingly, in order to develop a steel sheet which shows the high strength and which enables the weight reduction of automobiles, it is important to achieve improvements in the formability such as the hole expansibility and in the fatigue properties in addition to the increase in the strength of the steel sheet.

Conventionally, it is known that an excellent fatigue life can be obtained by producing steel which has composite structure consisting of ferrite and martensite. As a steel sheet which shows the high strength and in which the hole expansibility is intended to be improved by producing the steel which has the composite structure, Patent Document 1 discloses a high strength hot rolled steel sheet where a fraction of the microstructure of the steel which consists of the mixed structure of ferrite, martensite, and residual austenite is appropriately controlled. The characteristic values of the steel sheet which is obtained by the technique are tensile strength of 590 MPa or more and hole expanding ratio of approximately 50%.

Patent Document 2 discloses a high strength hot rolled steel sheet which consists of a mixed structure of ferrite and martensite, which is precipitation-strengthened by carbides of Ti or Nb. The characteristic values of the steel sheet which is obtained by the disclosed technique are tensile strength of 780 MPa or more and hole expanding ratio of approximately 50%.

However, for example, for steel sheets which are used as suspension members or the like of the automobile, a steel sheet which shows excellent coexistence of the tensile strength with the hole expansibility, such as tensile strength of 590 MPa or more and hole expanding ratio of 60% or more as the characteristic values thereof, is anticipated. In particular, a steel sheet which has hole expanding ratio of 90% or more when the tensile strength is 590 MPa to less than 780 MPa and which has hole expanding ratio of 60% or more when the tensile strength is 780 MPa to 980 MPa is anticipated.

In addition, since the variation of each measurement of the hole expanding ratio is comparatively large, it is necessary to reduce a standard deviation σ of the hole expanding ratio which is an index representing the variation, in addition to an average λave of the hole expanding ratio in order to improve the hole expansibility. As described above, in the steel sheets which are used as the suspension members of the automobiles, a steel sheet which has preferably standard deviation σ of the hole expanding ratio of 15% or less and which has more preferably standard deviation σ of the hole expanding ratio is 10% or less is anticipated.

In addition, for example, in a case where the automobile drives over a curb and a strong impact load is applied to the suspension parts, fracture may occur from a punching surface of the suspension parts as a starting point. In particular, since the notch sensitivity increases with an increase in the strength of the steel sheet, the fracture from the punching end face are strongly concerned. For this reason, for the steel sheets which are used as structural materials of the suspension parts or the like, it is necessary to improve the fracture properties. As indices representing the fracture properties, resistance of crack initiation Jc (unit: J/m²) and resistance of crack propagation T. M. (tearing modulus) (unit: J/m³) which are the characteristic values which are obtained by a three point bending test with notch, and fracture appearance transition temperature vTrs (unit: ° C.) and Charpy absorbed energy E (unit: J) which are obtained by a Charpy impact test may be exemplified. The resistance of crack initiation Jc represents the resistance to the initiation of cracks (the start of fracture) from the steel sheet which composes the structural material when the impact load is applied. On the other hand, the resistance of crack propagation T. M. represents the resistance to large-scale fracture (the propagation of fracture) of the steel sheet which composes the structural material. In order not to decrease the safety of the structural material when the impact load is applied, it is important to improve both of the resistances.

Conventionally, techniques, in which the characteristic values, in particular, the resistance of crack initiation Jc and the resistance of crack propagation T. M. which are characteristic values obtained by the three point bending test with notch intend to be improved, have not been disclosed.

In addition, repeated stress is applied to the suspension parts for the automobile. Therefore, since occurrence of the fatigue fracture is concerned, excellent fatigue properties are also required for the steel sheets which are used as structural materials such as suspension parts.

RELATED ART DOCUMENTS
Patent Documents

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H6-145792

[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H9-125194

SUMMARY OF INVENTION
Technical Problem

The present invention was achieved in consideration of the problems described above. An object of the present invention is to provide a hot rolled steel sheet, which has an excellent balance between tensile properties and formability and furthermore which has excellent fracture properties and fatigue properties, and a method of producing the same.

Specifically, the present invention is to provide the hot rolled steel sheet which has composite structure and which shows high strength, wherein the hot rolled steel sheet has the properties such that: the tensile strength TS is 590 MPa or more and the n value (work hardening coefficient) is 0.13 or more as the tensile properties; the average λave of the hole expanding ratio is 60% or more and the standard deviation σ of the hole expanding ratio is 15% or less as the formability; the resistance of crack initiation Jc is 0.5 MJ/m²or more, the resistance of crack propagation T. M. is 600 MJ/m³or more, the fracture appearance transition temperature vTrs is −13° C. or lower, and the Charpy absorbed energy E is 16 J or more as the fracture properties; and the fatigue life in plane bending is 400000 times or more as the fatigue properties.

In particular, the present invention is to provide the hot rolled steel sheet in which, when the tensile strength TS is 590 MPa to less than 780 MPa, in the above-described properties, the average λave of the hole expanding ratio is 90% or more, the resistance of crack initiation Jc is 0.9 MJ/m²or more, and the Charpy absorbed energy E is 35 J or more.

Solution to Problem

An aspect of the present invention employs the following.

(1) A hot rolled steel sheet according to an aspect of the invention includes, as a chemical composition, by mass %, 0.03% to 0.1% of C, 0.5% to 3.0% of Mn, at least one of Si and Al so as to satisfy a condition of 0.5%≦Si+Al≦4.0%, limited to 0.1% or less of P, limited to 0.01% or less of S, limited to 0.02% or less of N, at least one selected from 0.001% to 0.3% of Ti, 0.0001% to 0.02% of Rare Earth Metal, and 0.0001% to 0.01% of Ca, and a balance consisting of Fe and unavoidable impurities, and as a metallographic structure, a ferrite as a primary phase, at least one of a martensite and a residual austenite as a secondary phase, and plural inclusions, wherein: amounts expressed in mass % of each element in the chemical composition satisfy a following Expression 1; an average grain size of the ferrite which is the primary phase is 2 μm to 10 μm; an area fraction of the ferrite which is the primary phase is 90% to 99%; an area fraction of the martensite and the residual austenite which are the secondary phase is 1% to 10% in total; when a cross section whose normal direction corresponds to a transverse direction of the steel sheet is observed at 30 of visual fields by 0.0025 mm², an average of a maximum of a ratio of a major axis to a minor axis of each of the inclusions in each of the visual fields is 1.0 to 8.0; when a group of inclusions in which a major axis of each of the inclusions is 3 μm or more and an interval in a rolling direction between the inclusions is 50 μm or less are defined as inclusion-cluster, and when an inclusion in which the interval is more than 50 μm are defined as an independent-inclusion, a total length in the rolling direction of both the inclusion-cluster whose length in the rolling direction is 30 μm or more and the independent-inclusion whose length in the rolling direction is 30 μm or more is 0 mm to 0.25 mm per 1 mm²of the cross section; a texture satisfies that an X-ray random intensity ratio of a {211} plane which is parallel to a rolling surface is 1.0 to 2.4; and a tensile strength is 590 MPa to 980 MPa.

12.0≦(Ti/48)/(S/32)+{(Ca/40)/(S/32)+(Rare Earth Metal/140)/(S/32)}×15≦150 (Expression 1)

(2) The hot rolled steel sheet according to (1) may further includes, as the chemical composition, by mass %, at least one of 0.001% to 0.1% of Nb, 0.0001% to 0.0040% of B, 0.001% to 1.0% of Cu, 0.001% to 1.0% of Cr, 0.001% to 1.0% of Mo, 0.001% to 1.0% of Ni, and 0.001% to 0.2% of V.

(3) In the hot rolled steel sheet according to (1) or (2), when the hot rolled steel sheet includes, as the chemical composition, by mass %, at least one of 0.0001% to 0.02% of Rare Earth Metal and 0.0001% to 0.01% of Ca, the Ti content may be 0.001% to less than 0.08%.

(4) In the hot rolled steel sheet according to any one of (1) to (3), amounts expressed in mass % of each element in the chemical composition may satisfy a following Expression 2; and the average of the maximum in the ratio of the major axis to the minor axis of each of the inclusions in each of the visual fields may be 1.0 to 3.0.

0.3≦(Rare Earth Metal/140)/(Ca/40) (Expression 2)

(5) In the hot rolled steel sheet according to any one of (1) to (4), an area fraction of a bainite and a pearlite in the metallographic structure may be 0% to less than 5.0% in total.

(6) In the hot rolled steel sheet according to any one of (1) to (5), a total number of MnS precipitates and CaS precipitates having a major axis of 3 μm or more may be 0% to less than 70% as compared with a total number of the inclusions having the major axis of 3 μm or more.

(7) In the hot rolled steel sheet according to any one of (1) to (6), an average grain size of the secondary phase may be 0.5 μm to 8.0 μm.

(8) A method of producing the hot rolled steel sheet according to any one of (1) to (7) includes: a heating process of heating a steel piece which composed of the chemical composition according to any one of (1) to (4) to a range of 1200° C. to 1400° C.; a first rough rolling process of rough rolling the steel piece in a temperature range of higher than 1150° C. to 1400° C. so that a cumulative reduction is 10% to 70% after the heating process; a second rough rolling process of rough rolling in a temperature range of higher than 1070° C. to 1150° C. so that a cumulative reduction is 10% to 25% after the first rough rolling process; a finish rolling process of finish rolling so that a start temperature is 1000° C. to 1070° C. and a finish temperature is Ar3+60° C. to Ar3+200° C. to obtain a hot rolled steel sheet after the second rough rolling process; a first cooling process of cooling the hot rolled steel from the finish temperature so that a cooling rate is 20° C./second to 150° C./second after the finish rolling process; a second cooling process of cooling in a temperature range of 650° C. to 750° C. so that the cooling rate is 1° C./second to 15° C./second and a cooling time is 1 second to 10 seconds after the first cooling process; a third cooling process of cooling to a temperature range of 0° C. to 200° C. so that the cooling rate is 20° C./second to 150° C./second after the second cooling process; and a coiling process of coiling the hot rolled steel sheet after the third cooling process.

(9) In the method of producing the hot rolled steel sheet according to (8), in the first rough rolling process, the rough rolling may be conducted so that the cumulative reduction is 10% to 65%.

Advantageous Effects of Invention

According to the above aspects of the present invention, it is possible to obtain a steel sheet which has an excellent balance between tensile properties and formability and furthermore which has excellent fracture properties and fatigue properties.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view showing test piece size for evaluation of fatigue properties.

FIG. 2A is an explanatory view for the three point bending test with notch.

FIG. 2B shows a notched test piece before the three point bending test with notch and is a cross sectional view which includes the notch whose a normal direction corresponds to a transverse direction of a steel sheet.

FIG. 2C shows a notched test piece which is forcibly fractured after the three point bending test with notch and shows a fracture surface which includes the notch.

FIG. 3A is a load displacement curve which is obtained by the three point bending test with notch.

FIG. 3B is a graph showing a relationship between an amount of crack propagation Δa and processing energy J per 1 m².

FIG. 4A is a schema of an inclusion-cluster which is a group of inclusions.

FIG. 4B is a schema of an independent-inclusion which exists independently.

FIG. 4C is a schema of an inclusion-cluster which includes an inclusion whose length in a rolling direction is 30 μm or more.

FIG. 5 is a diagram which shows a relationship between a total length M in the rolling direction of the inclusions, an average of a maximum of a ratio of a major axis to a minor axis of the inclusions, and an average λave of the hole expanding ratio.

FIG. 6 is a diagram which shows a relationship between the total length M in the rolling direction of the inclusions, the average of the maximum of the ratio of the major axis to the minor axis of the inclusions, and a standard deviation σ of the hole expanding ratio.

FIG. 7 is a diagram which shows a relationship between the total length M in the rolling direction of the inclusions and resistance of crack propagation T. M.

FIG. 8 is a diagram which shows a relationship between S content, Ti content, REM content, and Ca content and the total length M in the rolling direction of the inclusions.

FIG. 9A is a diagram which shows a relationship between cumulative reduction in a first rough rolling process and the total length M in the rolling direction of the inclusions.

FIG. 9B is a diagram which shows a relationship between the cumulative reduction in the first rough rolling process and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions.

FIG. 9C is a diagram which shows a relationship between cumulative reduction in second rough rolling process and an X-ray random intensity ratio of {211} plane.

FIG. 9D is a diagram which shows a relationship between the cumulative reduction in the second rough rolling process and an average grain size of ferrite.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a preferable embodiment of the present invention will be described in detail. However, the present invention is not limited only to the configuration which is disclosed in the embodiment, and various modifications are possible without departing from the aspect of the present invention.

First, description will be given of the basic research results which have led to the completion of the present invention. To start with, description will be given of a measurement method of characteristic values which are required in the hot rolled steel sheet according to the embodiment.

The tensile properties were determined from a tensile test with the following conditions. From a portion of ½ in the sheet width of a test steel sheet, test pieces were prepared so that a tensile direction was parallel to a transverse direction of the test steel sheet. The tensile test was conducted using the test pieces. Then, tensile strength (TS: Tensile Strength) and yield point (YP: Yield Point) were determined. Here, in a case where a clear yield point is not observed, 0.2% proof stress was regarded as the yield point. In addition, n value (work hardening coefficient) is determined as an approximate value of an n-th power law hardening rule based on true stress and true strain which were calculated from the tensile test. Here, a range of the strain when the n value is determined is to be 3% to 12%.

The hole expansibility was evaluated from a hole expansion test with the following conditions. From the portion of ½ in the sheet width of the test steel sheet, 20 test pieces where the length in the rolling direction was 150 mm and the length in the transverse direction was 150 mm were prepared for each test steel sheet. Using the test pieces, the hole expansion test was conducted with the following conditions. The evaluation of the hole expansibility was conducted with the average λave of the hole expanding ratio (unit: %) which was determined by arithmetically averaging 20 test results and with the standard deviation σ (unit: %) which was determined from the following Expression 1. Here, λi in the following Expression 1 represents the i-th hole expanding ratio in the total of 20 tests.

$\begin{matrix} σ^{2} = \frac{1}{20} \sum_{i = 1}^{20} {(λ i - λ ave)}^{2} & (Expression 1) \end{matrix}$

The conditions of the hole expansion test were as follows. In the test piece, a punching hole of 10 mm as an was provided by using a punching punch with a diameter of 10 mm under condition where a punching clearance which was obtained by dividing the intervals between the punching punch and the die hole by the sheet thickness of the test piece was to be 12.5%. Next, in the punching hole in the test piece, a conical punch with an angle of 60° was inserted from the same direction as the punching punch and the inner hole diameter Df was measured at a point of time where crack which was initiated in the punching end surface penetrated in the sheet thickness direction of the test piece. Then, the hole expanding ratio λi (unit: %) was determined from the following Expression 2. Here, the penetration of the crack in the sheet thickness was visually observed.

λi={(Df−D0)/D0}×100 (Expression 2)

The fatigue properties were evaluated from a fatigue test with the following conditions. Test pieces with the size shown in FIG. 1 were prepared from the test steel sheets which were as-hot-rolled. In FIG. 1, the test piece for the fatigue test is shown as 11, the rolling direction is shown as RD (Rolling Direction), and the transverse direction is shown as TD (Transverse Direction). Repeated stress by plane bending was applied to a neck section of the center of the test pieces and the fatigue life in plane bending, which was the number of repetitions until the test pieces was fatigue-fractured, was measured. The condition of the repeated stress which was applied to the test pieces in the fatigue test was completely reversed. Specifically, in a case where the stress amplitude=σ₀, the conditions of the fatigue test were controlled so that the stress change over time was a sine wave where the maximum stress=σ₀, the minimum stress=−σ₀, and the average of the stress=0. The stress amplitude σ₀was to be within a range of 45%±10 MPa as compared with the tensile strength TS of the test steel sheet. In addition, the fatigue test was conducted at least three times under conditions with the same stress amplitude σ₀, and the average of the fatigue life in plane bending by arithmetically averaging each test result was determined. The fatigue properties were evaluated by the average of the fatigue life in plane bending.

The fracture properties were evaluated by the resistance of crack initiation Jc (unit: J/m²) and the resistance of crack propagation T. M. (unit: J/m³) which were obtained by the three point bending test with notch to be described later, and the fracture appearance transition temperature vTrs (unit: ° C.) and the Charpy absorbed energy E (unit: J) which were obtained by the Charpy impact test.

The conditions of the three point bending test with notch were as follows. Five or more of the notched test pieces shown in FIG. 2A and FIG. 2B were prepared from one test steel sheet so that the longitudinal direction of the test piece was parallel to the transverse direction of the test steel sheet and the displacement direction of the three point bending test with notch corresponded to the rolling direction of the test steel sheet. FIG. 2A is an explanatory view for the three point bending test with notch. In FIG. 2A, a test piece for the three point bending test with notch is shown as 21, a notch is shown as 21, a load point is shown as 22, support points are shown as 23, and the displacement direction is shown as 24. FIG. 2B is a cross sectional view of the notched test piece 21 before the three point bending test with notch which includes the notch 21a whose the normal direction corresponds to the transverse direction TD of the test steel sheet. In FIG. 2B, the sheet thickness direction is shown as ND (Normal Direction). As shown in the figures, the longitudinal direction of the test piece 21 was 20.8 mm, the thickness in the displacement direction 24 of the test piece 21 was 5.2 mm, the depth of the displacement direction 24 of the notch 21a was 2.6 mm, the thickness C (value where the depth of the displacement direction 24 of the notch 21a was subtracted from the thickness of the displacement direction 24 of the test piece 21) of the displacement direction 24 of the ligament was 2.6 mm, and the sheet thickness B of the test steel sheet was 2.9 mm.

As shown in FIG. 2A, using the test piece 21, both end sections in the longitudinal direction of the test piece 21 were set as the support points 23 and the central portion thereof was set as the load point 22, and the amount of displacement (stroke) in the displacement direction 24 of the load point were variously changed, thereby conducting the three point bending test with notch. The test piece 21 after the three point bending test with notch was subjected to a heat treatment where the test piece was held for 30 minutes at 250° C. in the atmosphere and then was air-cooled. By the heat treatment, the fracture surface which was derived from the three point bending test with notch was oxidized and colored. The test piece 21 after the heat treatment was cooled using liquid nitrogen to the temperature of the liquid nitrogen, and then the test piece 21 was forcibly fractured at the temperature so that the crack propagated along the displacement direction 24 from the notch 21a of the test piece 21. FIG. 2C exemplifies a fracture surface which includes the notch in the notched test piece 21 which was forcibly fractured after the three point bending test with notch. In the fracture surface, as a result of the oxidizing and coloring, it was possible to clearly distinguish the fracture surface derived from the three point bending test with notch from the fracture surface derived from the forced fracture. In FIG. 2C, the fracture surface derived from the three point bending test with notch is shown as 21b, the fracture surface derived from the forced fracture is shown as 21c, the depth of the fracture surface 21b at a position of ¼ in the sheet thickness of the test steel sheet is shown as L1, the depth of the fracture surface 21b at a position of ½ in the sheet thickness of the test steel sheet is shown as L2s, and the depth of the fracture surface 21b at a position of ¾ in the sheet thickness of the test steel sheet is shown as L3. The fracture surface 21b was observed, L1, L2, and L3 were measured, and then the amount of crack propagation Δa (unit: m) was determined from the following Expression 3.

Δa=(L1+L2+L3)/3 (Expression 3)

FIG. 3A exemplifies a load displacement curve obtained by the three point bending test with notch. As shown in FIG. 3A, by integrating the load displacement curve, processing energy A (unit: J) corresponding to the energy which was applied to the test piece 21 by the test was determined. Then, using the processing energy A, the sheet thickness B of the test steel sheet before the three point bending test with notch, and the thickness C of the displacement direction 24 of the ligament, processing energy J (unit: J/m²) per 1 m²was determined from the following Expression 4.

J=(2×A)/(B×C) (Expression 4)

FIG. 3B is a graph showing the relationship between the amount of crack propagation Δa and the processing energy J per 1 m²when the stroke conditions are variously changed in the three point bending test with notch. As shown in FIG. 3B, an intersection between a linear regression line with respect to Δa and J and a straight line which passed through an origin and whose inclination was 3×(YP+TS)/2 was determined. The value of the processing energy J per 1 m²in the intersection was regarded as the resistance of crack initiation Jc (unit: J/m²) which was a value which represented the resistance to the initiation of crack of the test steel sheet. In addition, an inclination of the linear regression line was regarded as the resistance of crack propagation T. M. (unit: J/m³) which represented the resistance to the propagation of crack of the test steel sheet. The resistance of crack initiation Jc is an index value which represents the degree of the processing energy which is necessary for initiating the crack. Specifically, the resistance of crack initiation Jc represents the resistance to the initiation of the crack (the start of the fracture) from the steel sheet which composes the structural material when the impact load is applied. The resistance of crack propagation T. M. is an index value which represents the degree of the processing energy which is necessary for propagating the crack. Specifically, the resistance of crack propagation T. M. represents the resistance to large-scale fracture (the propagation of the fracture) of the steel sheet which composes the structural material. The fracture properties of the steel sheet were evaluated by the resistance of crack initiation Jc and the resistance of crack propagation T. M.

The conditions of the Charpy impact test were as follows. V notched test pieces were prepared so that the longitudinal direction of the test piece was parallel to the transverse direction of the test steel sheet. Regarding the test piece size, the length of the test piece in the longitudinal direction was 55 mm, the thickness in the direction where the impact was applied to the test piece was 10 mm, the thickness in a direction which intersected with the longitudinal direction and the impact direction of the test piece was 2.5 mm, and a depth of the V notch was 2 mm and an angle thereof was 45°. By conducting the Charpy impact test using the test pieces, the fracture appearance transition temperature vTrs (unit: ° C.) and Charpy absorbed energy E (unit: J) were determined. Here, the fracture appearance transition temperature vTrs was to be a temperature where a fraction of the ductile fracture was 50%, and the Charpy absorbed energy E was to be a value which was obtained when the test temperature was room temperature (23° C.±5° C.). The fracture properties of the steel sheet were evaluated by the fracture appearance transition temperature vTrs and the Charpy absorbed energy E.

As the above-described characteristic values, the hot rolled steel sheet according to the embodiment satisfies that the tensile strength TS is 590 MPa or more, the average λave of the hole expanding ratio is 60% or more, the standard deviation σ of the hole expanding ratio is 15% or less, the fatigue life in plane bending is 400000 times or more, the resistance of crack initiation Jc is 0.5 MJ/m²or more, the resistance of crack propagation T. M. is 600 MJ/m³or more, the fracture appearance transition temperature vTrs is −13° C. or lower, and the Charpy absorbed energy E is 16 J or more.

Next, description will be given of the measurement method of the chemical composition, the observation method of the metallographic structure, and the like of the hot rolled steel sheet according to the embodiment.

The chemical composition of the steel sheet was quantitatively analyzed using EPMA (Electron Probe Micro-Analyzer: electron probe X-ray micro-analysis), AAS (Atomic Absorption Spectrometer: atomic absorption spectrometry), ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometer: inductively coupled plasma emission spectroscopy spectrometry), or ICP-MS (Inductively Coupled Plasma-Mass Spectrometer: inductively coupled plasma mass analysis spectrometry).

The observation of the metallographic structure of the steel sheet was conducted using the following methods. Test pieces for metallographic structure observation were cut out from a portion of ¼ in the sheet width of the steel sheet, so that a cross section (hereinafter, L cross section) whose normal direction corresponded to the transverse direction was an observed section. Then, the test pieces were mirror-polished. Using the test pieces after mirror polishing, inclusions which were included in the metallographic structure were observed at a magnification of 400-fold by an optical microscope so that the observed area was at the vicinity of the central portion of the sheet thickness in the above-described L cross section. In addition, Nital etching or Le Pera etching were conducted on the test pieces after mirror polishing, and the observation was conducted of the metallic phases such as ferrite, martensite, residual austenite, bainite, pearlite, and the like.

The average grain size of ferrite was determined as follows. The crystal orientation distribution was measured by 1 μm steps using an EBSD (Electron Back-Scattered diffraction Pattern) method, so that the observed area was at the central portion of the sheet thickness in the L cross section and was an area of 500 μm in the normal direction and 500 μm in the rolling direction. Then, points where the misorientation was 15° or more were connected, which was regarded as high-angle grain boundaries. The arithmetic average of equivalent circle diameters of each crystal grain which was surrounded by the high-angle grain boundaries were determined and were regarded as the average grain size of the ferrite. At this time, among each of the measurement points which were measured by the EBSD method, crystal grains where the IQ (Image Quality) value was 100 or more were regarded as the ferrite, and the crystal grains where the IQ value was 100 or less were regarded as metallic phases with the exception of the ferrite.

Area fractions such as ferrite, martensite, residual austenite, bainite, pearlite, and the like were determined by image analysis of metallographic micrograph.

In addition, for the investigation of the inclusions, the total length M (unit: mm/mm²) in the rolling direction of the inclusions which were defined as described below was measured.

The existence of the inclusions causes a deterioration of the hole expansibility, because the inclusions form voids in the steel during the deformation of steel sheet and promote the ductile fracture. Moreover, as the shape of the inclusions is elongated in the rolling direction of the steel sheet, the stress concentration in the vicinity of the inclusions during plastic deformation of steel sheet increases. Specifically, in addition to the existence of the inclusions, the hole expansibility is drastically influenced by the shape of the inclusions. Conventionally, it is known that the hole expansibility drastically deteriorates with an increase in the length in the rolling direction of individual inclusions.

The present inventors discover that, when plural inclusions such as elongated inclusions, spherical inclusions, or the like are formed into a group by being distributed with predetermined intervals in the rolling direction of the steel sheet which is the direction of crack propagation, the hole expansibility deteriorates in common with the inclusions which are elongated individually. This seems to be caused by inducing large stress concentrations in the vicinity of the groups, which is derived from the synergistic effect of the strains which are induced in the vicinity of each inclusion which composes the groups during the deformation of the steel sheet. Quantitatively, it was discovered that the hole expansibility deteriorates by the existence of the group of inclusions, in which a major axis of each of the inclusions is 3 μM or more and the inclusions are lined up so that an interval to other adjacent inclusions on a line in the rolling direction of the steel sheet is 50 μm or less, in common with the inclusion which exists independently and is elongated. Hereinafter, the group of the inclusions in which the respective major axes are 3 μm or more and the intervals in the rolling direction between the inclusions are 50 μm or less is referred to as an inclusion-cluster. In addition, in contrast with the inclusion-cluster, the inclusion which exists independently and in which the interval in the rolling direction between the inclusions is more than 50 μm is referred to as an independent-inclusion. The above-described major axis represents the longest diameter in the cross-sectional shape of the observed inclusion and usually corresponds to the diameter in the rolling direction.

As described above, in order to improve the hole expansibility of the steel sheet, it is important to control the shape and distribution of the inclusions as described below.

FIG. 4A is a schema of the inclusion-cluster which is the group of inclusions. In FIG. 4A, the inclusions in which the respective major axes are 3 μm or more are shown as 41a to 41e, the intervals between inclusions in the rolling direction are shown as F, the inclusion-cluster is shown as G, and the length of the inclusion-cluster in the rolling direction is shown as GL. As shown in FIG. 4A, the group of inclusions in which the interval F is 50 μm or less along the rolling direction RD of the steel sheet, specifically, one group which includes the inclusion 41b, the inclusion 41c, and the inclusion 41d, is regarded as the inclusion-cluster G. The length GL in the rolling direction of the inclusion-cluster G is measured. The inclusion-cluster G where the length GL is 30 μm or more has an influence on the hole expansibility of the steel sheet. The inclusion-cluster G where the length GL in the rolling direction is less than 30 μm has a small influence on the hole expansibility. In addition, inclusions in which the major axis is less than 3 μm are not included in the constituent of the inclusion-cluster G since the influence on the hole expansibility is small even if the interval F is 50 μm or less. In addition, in FIG. 4A, the inclusion 41a and the inclusion 41e are respectively regarded as the independent-inclusions.

FIG. 4B is a schema of the independent-inclusions. In FIG. 4B, inclusions in which the respective major axes are 3 μm or more are shown as 41f to 41h, the independent-inclusions are shown as H, and the length of the independent-inclusion in the rolling direction is shown as HL. As shown in FIG. 4B, the inclusions in which the interval F is more than 50 μm along the rolling direction RD of the steel sheet, specifically, the inclusion 41f, the inclusion 41g, and the inclusion 41h, are respectively regarded as the independent-inclusions H. The length HL in the rolling direction of the independent-inclusion H is measured. The independent-inclusion H where the length HL is 30 μm or more has an influence on the hole expansibility of the steel sheet. The independent-inclusion H where the length HL in the rolling direction is less than 30 μm has a small influence on the hole expansibility.

FIG. 4C is a schema of the inclusion-cluster G which includes the inclusion where the length in the rolling direction is 30 μm or more. In FIG. 4C, inclusions in which the respective major axes are 3 μm or more are shown as 41i to 41l. In addition, in FIG. 4C, the inclusion 41j has a length (major axis) in the rolling direction of 30 μm or more. In FIG. 4C, one group which includes the inclusion 41j and the inclusion 41k and in which the interval F is 50 μm or less along the rolling direction RD of the steel sheet is regarded as the inclusion-cluster G, and the inclusions 41i and the inclusions 41l are respectively regarded as the independent-inclusions H. As described above, since the inclusion 41k where the interval F to the inclusion 41j is 50 μm or less exists even when the major axis of the inclusion 41j is 30 μm or more, the inclusion 41j is regarded as a part of the inclusion-cluster G. In addition, hereafter, the independent-inclusion H which is not included in the inclusion-cluster G and whose length HL in the rolling direction is 30 μm or more is referred to as elongated inclusion.

The length GL in the rolling direction of the inclusion-cluster G and the length HL in the rolling direction of the elongated inclusion (independent-inclusion H where the length HL in the rolling direction was 30 μm or more) were entirely measured in an observed visual field, and the total length I (unit: mm) of GL and HL was determined by conducting the measurements for plural visual fields. A total length M (unit: mm/mm²) which was a converted value per 1 mm²of area was determined from the total length I based on the following Expression 5. The total length M has an influence on the hole expansibility of the steel sheet. Here, S is the total area (unit: mm²) of the observed visual field.

M=I/S (Expression 5)

The reason why the total length M which is the converted value per 1 mm²of area from the total length I should be determined, instead of the average of the total length I which is the length in the rolling direction of the above-described inclusions, is as follows.

When the number of the inclusion-clusters G and the elongated inclusions (the independent-inclusions H where the length HL in the rolling direction is 30 μm or more) in the metallographic structure of the steel sheet is small, the cracks propagate while voids which are formed at the periphery of the inclusions are interrupted during the deformation of the steel sheet. On the other hand, when the number of the above-described inclusions is large, voids at the periphery of the inclusions are formed into long continuous void by being connected without being interrupted, which may promote the ductile fracture. The influence of the number of the inclusions is not represented by the average of the total length I but may be represented by the total length M. Accordingly, from this point, the total length M per 1 mm²of area in the length GL in the rolling direction of the inclusion-cluster G and in the length HL in the rolling direction of the elongated inclusions was determined. As described above, the total length M has an influence on the hole expansibility of the steel sheet.

The total length M has an influence on the fracture properties of the steel sheet in addition to the hole expansibility of the steel sheet. During the deformation of the steel sheet, the stress is concentrated on the inclusion-clusters G and elongated inclusions (independent-inclusions H where the length HL in the rolling direction is 30 μm or more) and the initiation and propagation of cracks occur from the inclusions as a starting point. Therefore, in a case where the value of the total length M is large, the resistance of crack initiation Jc and the resistance of crack propagation T. M. decrease. In addition, the Charpy absorbed energy E, which is the energy required to fracture a test piece in a temperature range where ductile fracture occurs, is an index influenced by both of the resistance of crack initiation Jc and the resistance of crack propagation T. M. In a case where the value of the total length M is large, the Charpy absorbed energy E is also decreased similarly.

Furthermore, the total length M also has an influence on the fatigue properties of the steel sheet. It was found that the fatigue life tended to decrease with an increase in the value of the total length M. The reason for the above seems that the number of the inclusion-clusters G or the elongated inclusions, which act as the starting point of the fatigue fracture, increases with an increase in the value of the total length M, so that the fatigue life decreases as the result.

From the above point of view, the total length M in the rolling direction of the inclusions was measured, and therewith, the average λave of the hole expanding ratio, the resistance of crack initiation Jc, the resistance of crack propagation T. M., the Charpy absorbed energy E, the fatigue life, and the like were evaluated.

In addition to the total length M, as the investigation of the inclusions, measurement was conducted for the ratio of the major axis to the minor axis of the inclusion, which was represented by dividing the major axis of the inclusion by the minor axis of the inclusion. The respective ratios of the major axis to the minor axis were entirely measured for the inclusions in an observed visual field, and a maximum therein was determined. 30 times of the measurements were conducted with different visual fields. Then, an average of the respective maxima of the ratios of the major axis to the minor axis which were determined at each visual field was determined. Specifically, after the cross section (L cross section) where was at a portion of ¼ in the sheet width of the steel sheet and whose normal direction corresponded to the transverse direction was mirror-polished, the inclusions were observed using an electron microscope at 30 of arbitrary visual fields in the vicinity of the central portion of the sheet thickness in the L cross section so that one visual field was to be 0.0025 mm²(50 μm×50 μm), the maximum of the ratio of the major axis to the minor axis of the inclusions in each visual field was determined, and the average of the 30 visual fields was determined.

In a case where the shape of each of the inclusions is round and the average of the maximum of the ratio of the major axis to the minor axis is small even when the total length M in the rolling direction of the inclusions is the same values, the stress concentration in the vicinity of the inclusions during the deformation of the steel sheet decreases, and the average λave of the hole expanding ratio, the resistance of crack initiation Jc, and the Charpy absorbed energy E are preferably improved. Therefore, the ratio of the major axis to the minor axis of the inclusions is determined. In addition, since it was found from experiments that the average of the maximum of the ratio of the major axis to the minor axis of the inclusions and the standard deviation σ of the hole expanding ratio had a correlation, the average in regard to the ratio of the major axis to the minor axis was measured from the point of view of evaluating the standard deviation σ of the hole expanding ratio.

In addition to the chemical composition and metallographic structure of the steel sheet, the texture of the steel sheet was measured. The measurement of the texture was conducted using X-ray diffraction measurement. The X-ray diffraction measurement was conducted by a diffractometer method or the like using an appropriate X-ray tube. As a test piece for X-ray diffraction measurement, test pieces in which the length in the transverse direction was 20 mm and the length in the rolling direction were 20 mm was cut out from a portion of ½ in the sheet width of the steel sheet. After mechanically polishing the test pieces so that a position of ½ in the sheet thickness of the steel sheet was the measurement surface, strain was removed by electrolytic polishing or the like. The test piece for X-ray diffraction measurement and a reference standard which did not have the texture in a specific orientation were measured using the X-ray diffraction method or the like under the same conditions, a value where the X-ray intensity of the steel sheet was divided by the X-ray intensity of the reference standard was regarded as the X-ray random intensity ratio. Here, the X-ray random intensity ratio is synonymous with the pole density. In addition, instead of the X-ray diffraction measurement, the texture may be measured using the EBSD method or an ECP (Electron Channeling Pattern) method. In addition, as the texture of the steel sheet, the X-ray random intensity ratio of the {211} plane (which was synonymous with the pole density of the {211} plane or with the {211} plane intensity) was measured.

Next, description will be given of the limitation range and reasons for the limitation relating to the total length M and the average of the ratio of the major axis to the minor axis in order that the properties of the hot rolled steel sheet according to the embodiment satisfy that the average λave of the hole expanding ratio is 60% or more, the standard deviation σ of the hole expanding ratio is 15% or less, and the resistance of crack propagation T. M. is 600 MJ/m³or more.

FIG. 5 is a diagram which shows a relationship between the total length M in the rolling direction of the inclusions, the average of the maximum of the ratio of the major axis to the minor axis of the inclusions, and the average λave of the hole expanding ratio. FIG. 6 is a diagram which shows a relationship between the total length M in the rolling direction of the inclusions, the average of the maximum of the ratio of the major axis to the minor axis of the inclusions, and the standard deviation σ of the hole expanding ratio.

As shown in FIG. 5, the average λave of the hole expanding ratio of the steel sheet is improved with a decrease in the value of the total length M in the rolling direction of the inclusions and with a decrease in the average of the maximum of the ratio of the major axis to the minor axis. In addition, as shown in FIG. 6, the standard deviation a of the hole expanding ratio is improved with the decrease in the average of the maximum of the ratio of the major axis to the minor axis of the inclusions. Here, it is shown that each data which is plotted in FIG. 5 and FIG. 6 satisfies the configuration of the hot rolled steel sheet according to the embodiment with the exception of a configuration relating to the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis.

From FIG. 5 and FIG. 6, it is understood that the average λave of the hole expanding ratio can be controlled to 60% or more and the standard deviation σ can be controlled to 15% or less by controlling the total length M in the rolling direction of the inclusions to 0 mm/mm²to 0.25 mm/mm²and by controlling the average of the maximum of the ratio of the major axis to the minor axis to 1.0 to 8.0. The reason for the above seems that the stress concentration is relieved in the vicinity of the inclusions during the plastic deformation of the steel sheet by decreasing the value of the total length M and the average of the ratio of the major axis to the minor axis as described above. It is preferable that the total length M in the rolling direction of the inclusions is 0 mm/mm²to 0.20 mm/mm², and it is more preferable that the total length M in the rolling direction of the inclusions is 0 mm/mm²to 0.15 mm/mm². In addition, it is understood that the average λave of the hole expanding ratio can be controlled to 65% or more and the standard deviation σ can be controlled to 10% or less by preferably controlling the average of the maximum of the ratio of the major axis to the minor axis to 1.0 to 3.0. It is more preferable that the average of the maximum of the ratio of the major axis to the minor axis is 1.0 to 2.0.

FIG. 7 is a diagram which shows a relationship between the total length M in the rolling direction of the inclusions and the resistance of crack propagation T. M. From the diagram, it is understood that, in a case where the total length M in the rolling direction of the inclusions is 0 mm/mm²to 0.25 mm/mm², in addition to the average λave and the standard deviation σ of the hole expanding ratio, the resistance of crack propagation T. M. of 600 MJ/m³or more is also satisfied. In general, in order to prevent the fracture of the steel sheet which composes the structural material, it is important to improve the resistance of crack propagation T. M. As mentioned above, the resistance of crack propagation T. M. tends to depend on the total length M in the rolling direction of the inclusions, and it is found that controlling the total length M to the range is important.

As described above, by controlling the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions, it is possible to satisfy the properties such as the average λave of the hole expanding ratio, the standard deviation σ of the hole expanding ratio, and the resistance of crack propagation T. M. In addition, as mentioned above, the total length M also improves the fatigue properties. Next, description will be given of a method which controls the total length M and the average of the ratio of the major axis to the minor axis to the ranges.

The present inventors found that the inclusion-cluster G and the elongated inclusion (independent-inclusion H where the length HL in the rolling direction was 30 μm or more), which caused the increase in the total length M in the rolling direction of the inclusions or the average of the maximum of the ratio of the major axis to the minor axis of the inclusions, were MnS precipitates which were elongated by the rolling or residues of desulfurizing agent which was added for desulfurization at steel making. In addition, it was found that, although the influence was not large as compared with the MnS precipitates or the residues of desulfurizing agent, CaS which precipitated without oxides and sulfides of REM (Rare Earth Metal) as a nucleus and precipitates of calcium aluminate or the like which was a mixture of CaO and alumina may also increase the total length M or the average of the ratio of the major axis to the minor axis. Since CaS and the precipitates of calcium aluminate or the like may become a shape which is elongated in the rolling direction by rolling, the hole expansibility of the steel sheet, the fracture properties, or the like may deteriorate. As a result of the investigation of the method which suppressed the inclusions in order to improve the properties such as the average λave of the hole expanding ratio, the standard deviation σ of the hole expanding ratio, and the resistance of crack propagation T. M., it was found that the following was important.

First, it is important to reduce the S content which bonds to Mn in order to suppress the MnS precipitates. From the point of view, in the hot rolled steel sheet according to the embodiment, in order to totally reduce the entire S content in the steel, the upper limit thereof is to be 0.01 mass %.

In addition, since TiS precipitates are formed at a higher temperature than the MnS formation temperature range when Ti is added, it is possible to reduce the amount of MnS precipitates. Similarly, since sulfides of REM or Ca are formed when REM or Ca are added, it is possible to reduce the amount of MnS precipitates. Therefore, the hot rolled steel sheet according to the embodiment contains at least one selected from the group consisting of, by mass %, Ti: 0.001% to 0.3%, REM: 0.0001% to 0.02%, and Ca: 0.0001% to 0.01%. Although it is possible to reduce the amount of MnS precipitates by selecting Ca, in order to suppress the precipitation of CaS, calcium aluminate, or the like, the upper limit of the Ca content is to be 0.01 mass %. The limitation range and reasons for the limitation of the chemical composition of the hot rolled steel sheet will be described later in detail.

Furthermore, in order to suppress the MnS precipitates, it is necessary to stoichiometrically include the larger amount of Ti, REM, or Ca than that of S. Therefore, the relationship between the S content, the Ti content, the REM content, and the Ca content and the total length M in the rolling direction of the inclusions was investigated. FIG. 8 is a diagram which shows a relationship between the S content, the Ti content, the REM content, and the Ca content and the total length M in the rolling direction of the inclusions. It was found that, when the value of (Ti/48)/(S/32)+{(Ca/40)/(S/32)+(REM/140)/(S/32)}×15 was 12.0 to 150, the total length M was 0 mm/mm²to 0.25 mm/mm². Specifically, in the hot rolled steel sheet according to the embodiment, it is necessary that the amounts expressed in mass % of each element in the chemical composition satisfy the following Expression 6. By satisfying the Expression 6, it is considered that the formation of elongated MnS precipitates is suppressed. In addition, although not shown in the diagram, it was found that, in a case where the following Expression 6 was satisfied, the average of the maximum of the ratio of the major axis to the minor axis of the inclusions was 1.0 to 8.0. Furthermore, it was found that, even in a case where all of Ti, REM, and Ca were simultaneously included in the steel, or in a case where at least one selected from Ti, REM, and Ca was included in the steel, the total length M was 0 mm/mm²to 0.25 mm/mm²and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions was 1.0 to 8.0, when the following Expression 6 was satisfied.

12.0≦(Ti/48)/(S/32)+{(Ca/40)/(S/32)+(REM/140)/(S/32)}×15≦150 (Expression 6)

In order to control the total length M to 0 mm/mm²to 0.25 mm/mm²and to control the average of the ratio of the major axis to the minor axis to 1.0 to 8.0, in addition to satisfying the Expression 6, the cumulative reduction is to be 10% to 70% in a temperature range of higher than 1150° C. to 1400° C. in the first rough rolling process as described later. The method of producing the hot rolled steel sheet according to the embodiment will be described later in detail.

According to the above-described configuration, it is possible to control the total length M and the average of the ratio of the major axis to the minor axis. However, in order to further improve the properties of the steel sheet, it is preferable to reduce CaS which precipitates without oxides and sulfides of REM as the nucleus and to reduce the precipitates of calcium aluminate or the like. In order to reduce the precipitates, the amounts expressed in mass % of each element in the chemical composition may satisfy the following Expression 7. It was found that, when the following Expression 7 was satisfied, the average of the maximum of the ratio of the major axis to the minor axis of the inclusions was preferably 1.0 to 3.0. Moreover, in a case where Ti or REM is added to steel, since the Ca content may be as small as possible, it is not necessary to determine an upper limit of the following Expression 7.

0.3≦(REM/140)/(Ca/40) (Expression 7)

In a case where REM is sufficiently added as compared with Ca so as to satisfy the Expression 7, CaS or the like crystallizes or precipitates while spherical REM oxides or REM sulfides act as the nuclei. On the other hand, since the REM oxides or the REM sulfides which act as the nuclei are reduced when the ratio of REM to Ca is reduced and the Expression 7 is not satisfied, CaS or the like in which the REM oxides or the REM sulfides do not act as the nuclei precipitates excessively. The inclusions may have a shape which is elongated in the rolling direction due to the rolling. As described above, when the Expression 7 is satisfied, the ratio of the major axis to the minor axis of the inclusions is preferably controlled.

In order to control the average of the maximum of the ratio of the major axis to the minor axis of the inclusions to 1.0 to 3.0, in addition to satisfying the Expression 7, it is preferable that the cumulative reduction is 10% to 65% in a temperature range of higher than 1150° C. to 1400° C. in the first rough rolling process as described later. The method of producing the hot rolled steel sheet according to the embodiment will be described later in detail.

Subsequently, description will be given of the base elements of the hot rolled steel sheet according to the embodiment and of the limitation range and reasons for the limitation. Hereinafter, the % in the description represents mass %.

C: 0.03% to 0.1%

C (carbon) is an element which contributes to an improvement in the tensile strength TS. When the C content is insufficient, the fracture appearance transition temperature vTrs may increase due to the coarsening of the metallographic structure. In addition, when the C content is insufficient, it may be difficult to obtain the intended area fraction of martensite and residual austenite. On the other hand, when the C content is excessive, the average λave of the hole expanding ratio, the resistance of crack initiation Jc, and the Charpy absorbed energy E may decrease. For this reason, the C content is to be 0.03% to 0.1%. Preferably, the C content may be 0.04% to 0.08%. More preferably, the C content may be 0.04% to 0.07%.

Mn: 0.5% to 3.0%

Mn (manganese) is an element contributing to an improvement in the tensile strength TS of the steel sheet as an element of solid solution strengthening. In order to obtain the intended tensile strength TS, the Mn content is to be 0.5% or more. However, when the Mn content is more than 3.0%, cracking during the hot rolling occurs readily. For this reason, the Mn content is to be 0.5% to 3.0%. In addition, when the Mn content is more than 3.0%, ferrite transformation is suppressed and the area fraction of the martensite and the residual austenite may increase. To preferably control the area fraction of the ferrite which is the primary phase and the martensite and the residual austenite which are the secondary phase, the Mn content may be 0.8% to 2.0%. More preferably, the Mn content may be 1.0% to 1.5%.

0.5%≦Si+Al≦4.0%

In order to obtain the intended tensile strength TS and the intended area fraction of the ferrite, at least one selected from the group consisting of Si (silicon) and Al (aluminum) is contained. In order to obtain the effect, at least one of Si and Al is contained and the amount of Si+Al is to be 0.5% or more. However, when at least one of Si and Al is contained and the amount of Si+Al is more than 4.0%, the average λave of the hole expanding ratio may decrease. Preferably, the content may be 1.5% to 3.0%. Even more preferably, the content may be 1.8% to 2.6%.

Si: 0.5% to 2.0%

Si (silicon) is an element that contributes to the improvement of the tensile strength TS of the steel and to the promotion of the ferrite transformation. In order to obtain the intended tensile strength and the intended area fraction of the ferrite, it is preferable that the Si content is 0.5% or more. However, when the Si content is more than 2.0%, the strength may excessively increase and the average λave of the hole expanding ratio may decrease. For this reason, preferably, the Si content may be 0.5% to 2.0%.

Al: 0.005% to 2.0%

Al (aluminum) is an element which deoxidizes molten steel, and an element which contributes to an improvement in the tensile strength TS. In order to sufficiently obtain the effect, it is preferable that the Al content is 0.005% or more. However, when the Al content is more than 2.0%, the strength may excessively increase and the average λave of the hole expanding ratio may decrease. For this reason, preferably, the Al content may be 0.005% to 2.0%.

The hot rolled steel sheet according to the embodiment further contains at least one selected from the group consisting of Ti, REM, and Ca in the following content.

Ti: 0.001% to 0.3%

Ti (titanium) is an element contributing to an improvement of the tensile strength TS of the steel sheet by finely precipitating as TiC. In addition, Ti is an element which suppresses the precipitation of MnS which is elongated during rolling by precipitating as TiS. Therefore, the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions may decrease. In order to obtain the effect, the Ti content is to be 0.001% or more. However, when the Ti content is more than 0.3%, the strength may excessively increase, and the average λave of the hole expanding ratio, the resistance of crack initiation Jc, and the Charpy absorbed energy E may decrease. For this reason, the Ti content is to be 0.001% to 0.3%. Preferably, the Ti content may be 0.01% to 0.3%. More preferably, the Ti content may be 0.05% to 0.18%. Most preferably, the Ti content may be 0.08% to 0.15%.

REM: 0.0001% to 0.02%

REM (Rare Earth Metal) is element which suppresses the formation of MnS by bonding to S in the steel. In addition, REM is element which decreases the average of the maximum of the ratio of the major axis to the minor axis of the inclusions and the total length M in the rolling direction by spheroidizing the shape of the sulfides such as MnS. When the REM content is less than 0.0001%, the effect of suppressing the formation of MnS and the effect of spheroidizing the shape of the sulfides such as MnS may not be sufficiently obtained. In addition, when the REM content is more than 0.02%, the inclusions which include the REM oxides may excessively form, and the average λave of the hole expanding ratio, the resistance of crack initiation Jc, and the Charpy absorbed energy E may decrease. For this reason, the REM content is to be 0.0001% to 0.02%. Preferably, the REM content may be 0.0005% to 0.005%. More preferably, the REM content may be 0.001% to 0.004%.

Here, REM represents a generic name for a total of 17 elements, specifically 15 elements from lanthanum with atomic number 57 to lutetium with atomic number 71, scandium with atomic number 21, and yttrium with atomic number 39. In general, REM is supplied in the state of misch metal which is a mixture of the elements, and is added to the steel.

Ca: 0.0001% to 0.01%

Ca (calcium) is an element which suppresses the formation of MnS by bonding to S in the steel. In addition, Ca is an element which decreases the average of the maximum of the ratio of the major axis to the minor axis of the inclusions and the total length M in the rolling direction by spheroidizing the shape of the sulfides such as MnS. When the Ca content is less than 0.0001%, the effect of suppressing the formation of MnS and the effect of spheroidizing the shape of the sulfides such as MnS may not be sufficiently obtained. In addition, when the Ca content is more than 0.01%, CaS and the calcium aluminate which tend to be inclusions with an elongated shape may excessively form, and the total length M and the average of the ratio of the major axis to the minor axis may increase. For this reason, the Ca content is to be 0.0001% to 0.01%. Preferably, the Ca content may be 0.0001% to 0.005%. More preferably, the Ca content may be 0.001% to 0.003%. Furthermore preferably, the Ca content may be 0.0015% to 0.0025%.

In the hot rolled steel sheet according to the embodiment, at least one of Ti, REM, and Ca is included as described above, and simultaneously, the amounts expressed in mass % of each element in the chemical composition satisfy the following Expression 8. Here, detailed description will be given of the impurity S. By satisfying the following Expression 8, the amount of MnS precipitates in the steel decreases, and it is possible to obtain an effect of decreasing the average of the maximum of the ratio of the major axis to the minor axis of the inclusions and the total length M in the rolling direction of the inclusions. Thereby, the total length M in the rolling direction of the inclusions is controlled to 0 mm/mm²to 0.25 mm/mm²and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions is controlled to 1.0 to 8.0. As a result, it is possible to obtain an effect of improving the average λave of the hole expanding ratio of the steel sheets, the standard deviation σ, the resistance of crack initiation Jc, the resistance of crack propagation T. M., the Charpy absorbed energy E, and the fatigue life. When the value of the following Expression 8 is less than 12.0, the above effects may not be obtained. Preferably, the above value may be 30.0 or more. In addition, since it is preferable that the amount of S which is the impurity decreases, it is not necessary to determine an upper limit of the following Expression 8. However, in a case where the following Expression 8 is 150 or less, the above effect may preferably obtained.

12.0≦(Ti/48)/(S/32)+{(Ca/40)/(S/32)+(REM/140)/(S/32)}×15≦150 (Expression 8)

When the large amount of Ti is included within the above range, the tensile strength TS of the steel sheet is improved. For example, when the Ti content is 0.08% to 0.3%, it is possible to control the tensile strength TS of the steel sheet to 780 MPa to 980 MPa, and simultaneously, to control the fatigue life in plane bending to 500000 times or more. The reason for the above is derived from the precipitation strengthening of TiC. On the other hand, when Ti is not added, or when the small amount of Ti is included within the above range, the formability and the fracture properties of the steel sheet are improved. For example, when Ti is not added, or when the Ti content is 0.001% to less than 0.08%, although the tensile strength TS of the steel sheet is 590 MPa to less than 780 MPa, it is possible to control the average λave of the hole expanding ratio to 90% or more, the resistance of crack initiation Jc to 0.9 MJ/m²or more, and the Charpy absorbed energy E to 35 J or more. The reason for the above is derived from the decrease in the amount of TiC precipitates. As described above, depending on the purpose of the steel sheet, it is preferable to control the Ti content. When Ti is not added, in order to control the total length M and the average of the ratio of the major axis to the minor axis, it is preferable that at least one of REM and Ca is contained. In addition, when the small amount of Ti is included within the above range, in order to control the total length M and t average of the ratio of the major axis to the minor axis, it is preferable that at least one of REM and Ca is contained. Specifically, when at least one of 0.0001% to 0.02% of REM and 0.0001% to 0.01% of Ca is contained, it is preferable that the Ti content is 0.001% to less than 0.08%. When at least one of 0.0001% to 0.02% of REM and 0.0001% to 0.005% of Ca is contained, it is more preferable that the Ti content is 0.01% to less than 0.08%.

In addition, from the point of view of suppressing the average of the maximum of the ratio of the major axis to the minor axis of the inclusions, it is preferable that the amount of Ca and REM satisfies the following Expression 9. When the following Expression 9 is satisfied, the average of the maximum of the ratio of the major axis to the minor axis of the inclusions is preferably controlled to 1.0 to 3.0. Specifically, it is preferable that the amounts expressed in mass % of each element in the chemical composition satisfy the following Expression 9 and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions is 1.0 to 3.0. More preferably, the above value may be 1.0 to 2.0. As a result, it is possible to obtain further excellent effects for the average λave of the hole expanding ratio, the standard deviation σ of the hole expanding ratio, the resistance of crack initiation Jc, the Charpy absorbed energy E, and the like. The reason for the above is derived from the fact that, in a case where REM is sufficiently added as compared with Ca so as to satisfy the following Expression 9, CaS or the like crystallizes or precipitates while spherical REM oxides or REM sulfides act as the nuclei.

0.3≦(REM/140)/(Ca/40) (Expression 9)

The hot rolled steel sheet according to the embodiment contains unavoidable impurities in addition to the base elements described above. Herein, the unavoidable impurities indicate elements such as P, S, N, O, Pb, Cd, Zn, As, Sb, and the like which contaminate unavoidably from auxiliary materials such as scrap and the like and from producing processes. In the elements, P, S, and N are limited to the following in order to obtain satisfactory the effects. In addition, it is preferable that the unavoidable impurities with the exception of P, S, and N are respectively limited to 0.02% or less. Even when 0.02% or less of each impurity is included, the above effects are not affected. Although the limitation range of the impurities includes 0%, it is industrially difficult to be stably 0%. Hereinafter, the % in the description represents mass %.

P: 0.1% or Less

P (phosphorus) is an impurity which is unavoidably contaminated. When the P content is more than 0.1%, the amount of P segregation at the grain boundaries increases, which leads to a deterioration in the average λave of the hole expanding ratio, the resistance of crack initiation Jc, and the Charpy absorbed energy E. For this reason, the P content is limited to 0.1% or less. Since it is preferable that the P content is as small as possible, the limitation range includes 0%. However, it is not technically easy to control the P content to 0%, and also the production cost of the steel increases in order to be stably less than 0.0001%. Therefore, preferably, the limitation range of the P content may be 0.0001% to 0.1%. More preferably, the limitation range may be 0.001% to 0.03%.

S: 0.01% or Less

S (sulfur) is an impurity which is unavoidably contaminated. When the S content is more than 0.01%, the large amount of MnS is formed in the steel during the heating of the steel piece and MnS is elongated by hot rolling. Therefore, the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions may increase, and it is not possible to obtain the intended properties such as the average λave of the hole expanding ratio, the standard deviation σ, the resistance of crack initiation Jc, the resistance of crack propagation T. M., the Charpy absorbed energy E, and the fatigue life. For this reason, the S content is limited to 0.01% or less. Since it is preferable that the S content is as small as possible, the limitation range includes 0%. However, it is not technically easy to control the S content to 0%, and also the production cost of the steel increases in order to be stably less than 0.0001%. Therefore, preferably, the limitation range of the S content may be 0.0001% to 0.01%. In addition, in a case where desulfurization using a desulfurizing agent is not conducted during the secondary refining, it may be difficult to control the S content to less than 0.003%. In this case, preferably, the S content may be 0.003% to 0.01%.

N: 0.02% Or Less

N (nitrogen) is an impurity which is unavoidably contaminated. When the N content is more than 0.02%, N forms precipitates with Ti and Nb, and the amount of TiC precipitates is reduced. As a result, the tensile strength TS of the steel sheet decreases. For this reason, the N content is limited to 0.02% or less. Since it is preferable that the N content is as small as possible, the limitation range includes 0%. However, it is not technically easy to control the N content to 0%, and also the production cost of the steel increases in order to be stably less than 0.0001%. Therefore, preferably, the limitation range of the N content may be 0.0001% to 0.02%. In addition, in order to more effectively suppress a decrease in the tensile strength TS, it is preferable that the N content is 0.005% or less.

The hot rolled steel sheet according to the embodiment may further contain at least one selected from the group consisting of Nb, B, Cu, Cr, Mo, Ni, and V as optional elements, in addition to the above mentioned base elements and impurities. Hereinafter, limitation range and reasons for the limitation of the optional elements will be described. In addition, the % in the description represents mass %.

Nb: 0.001% to 0.1%

Nb (niobium) is an element contributing to the improvement of the tensile strength TS of the steel by refining the grains. In order to obtain the effect, it is preferable that the Nb content is 0.001% or more. However, when the Nb content is more than 0.1%, the temperature range where dynamic recrystallization occurs during hot rolling may be narrowed. Therefore, a rolling texture which is in non-recrystallized state and which leads to increase the X-ray random intensity ratio of the {211} plane remains excessively after the hot rolling. Detailed description will be given of the texture. When the X-ray random intensity ratio of the {211} plane is excessively increased as the texture, the average λave of the hole expanding ratio, the resistance of crack initiation Jc, and the Charpy absorbed energy E may deteriorate. For this reason, preferably, the Nb content may be 0.001% to 0.1%. More preferably, the Nb content may be 0.002% to 0.07%. Most preferably, the Nb content may be 0.002% to less than 0.02%. In addition, as long as the Nb content is 0% to 0.1%, each of the characteristic values of the hot rolled steel sheet is not negatively influenced.

B: 0.0001% to 0.0040%

B (boron) is an element contributing to the improvement of the tensile strength TS of the steel by refining the grains. In order to obtain the effect, it is preferable that the B content is 0.0001% or more. However, when the B content is more than 0.0040%, the temperature range where dynamic recrystallization occurs during hot rolling may be narrowed. Therefore, a rolling texture which is in non-recrystallized state and which leads to increase the X-ray random intensity ratio of the {211} plane remains excessively after the hot rolling. When the X-ray random intensity ratio of the {211} plane is excessively increased as the texture, the average λave of the hole expanding ratio, the resistance of crack initiation Jc, and the Charpy absorbed energy E may deteriorate. For this reason, preferably, the B content may be 0.0001% to 0.0040%. More preferably, the B content may be 0.0001% to 0.0020%. Most preferably, the B content may be 0.0005% to 0.0015%. In addition, as long as the B content is 0% to 0.0040%, each of the characteristic values of the hot rolled steel sheet is not negatively influenced.

Cu: 0.001% to 1.0%

Cu is an element which has an effect of improving the tensile strength TS of the hot rolled steel sheet by precipitation strengthening or solid solution strengthening. However, when the Cu content is less than 0.001%, the effect is not obtained. On the other hand, when the Cu content is more than 1.0%, the strength may excessively increase, and the average λave of the hole expanding ratio may decrease. For this reason, preferably, the Cu content may be 0.001% to 1.0%. More preferably, the Cu content may be 0.2% to 0.5%. In addition, as long as the Cu content is 0% to 1.0%, each of the characteristic values of the hot rolled steel sheet is not negatively influenced.

Cr: 0.001% to 1.0%

Similarly, Cr is an element which has an effect of improving the tensile strength TS of the hot rolled steel sheet by precipitation strengthening or solid solution strengthening. However, when the Cr content is less than 0.001%, the effect is not obtained. On the other hand, when the Cr content is more than 1.0%, the strength may excessively increase, and the average λave of the hole expanding ratio may decrease. For this reason, preferably, the Cr content may be 0.001% to 1.0%. More preferably, the Cr content may be 0.2% to 0.5%. In addition, as long as the Cr content is 0% to 1.0%, each of the characteristic values of the hot rolled steel sheet is not negatively influenced.

Mo: 0.001% to 1.0%

Similarly, Mo is an element which has an effect of improving the tensile strength TS of the hot rolled steel sheet by precipitation strengthening or solid solution strengthening. However, when the Mo content is less than 0.001%, the effect is not obtained. On the other hand, when the Mo content is more than 1.0%, the strength may excessively increase, and the average λave of the hole expanding ratio may decrease. For this reason, preferably, the Mo content may be 0.001% to 1.0%. More preferably, the Mo content may be 0.001% to 0.03%. Furthermore preferably, the Mo content may be 0.02% to 0.2%. In addition, as long as the Mo content is 0% to 1.0%, each of the characteristic values of the hot rolled steel sheet is not negatively influenced.

Ni: 0.001% to 1.0%

Similarly, Ni is an element which has an effect of improving the tensile strength TS of the hot rolled steel sheet by precipitation strengthening or solid solution strengthening. However, when the Ni content is less than 0.001%, the effect is not obtained. On the other hand, when the Ni content is more than 1.0%, the strength may excessively increase, and the average λave of the hole expanding ratio may decrease. For this reason, preferably, the Ni content may be 0.001% to 1.0%. More preferably, the Ni content may be 0.05% to 0.2%. In addition, as long as the Ni content is 0% to 1.0%, each of the characteristic values of the hot rolled steel sheet is not negatively influenced.

V: 0.001% to 0.2%

Similarly, V is an element which has an effect of improving the tensile strength TS of the hot rolled steel sheet by precipitation strengthening or solid solution strengthening. However, when the V content is less than 0.001%, the effect is not obtained. On the other hand, when the V content is more than 0.2%, the strength may excessively increase, and the average λave of the hole expanding ratio may decrease. For this reason, preferably, the V content may be 0.001% to 0.2%. More preferably, the V content may be 0.005% to 0.2%. Furthermore preferably, the V content may be 0.01% to 0.2%. Most preferably, the V content may be 0.01% to 0.15%. In addition, as long as the V content is 0% to 0.2%, each of the characteristic values of the hot rolled steel sheet is not negatively influenced.

In addition, the hot rolled steel sheet according to the embodiment may contain 0% to 1% in total of Zr, Sn, Co, W, and Mg as necessary.

Next, description will be given of the metallographic structure and the texture of the hot rolled steel sheet according to the embodiment.

The metallographic structure of the hot rolled steel sheet according to the embodiment includes a ferrite as a primary phase, at least one of a martensite and a residual austenite as a secondary phase, and plural inclusions. By forming the mixed structure, it is possible to achieve both the high tensile strength TS and elongation (n value). The reason for the above seems that the ductility is ensured by the ferrite which is the primary phase and comparatively soft and that the tensile strength TS is ensured by the secondary phase which is hard. In addition, by forming the mixed structure, the preferable fatigue properties are obtained. The reason for the above seems that the propagation of the fatigue cracks is suppressed by the martensite and the residual austenite which are the secondary phase and are comparatively hard. In order to obtain the effect, in the metallographic structure of the hot rolled steel sheet according to the embodiment, the area fraction of the primary phase is to be 90% to 99%, and the area fraction of the martensite and the residual austenite which are the secondary phase is to be 1% to 10% in total. When the area fraction of the primary phase is less than 90%, since the metallographic structure is not controlled to the intended mixed structure, it is not possible to obtain the above effect. On the other hand, it is technically difficult to control the area fraction of the primary phase to more than 99%. In addition, when the area fraction of the secondary phase is more than 10% in total, the ductile fracture is promoted, and the average λave of the hole expansion value, the resistance of crack initiation Jc, and the Charpy absorbed energy E deteriorate. On the other hand, when the area fraction of the secondary phase is less than 1% in total, since the metallographic structure is not controlled to the intended mixed structure, it is not possible to obtain the above effect. Preferably, the area fraction of the primary phase may be 95% to 99%, and the area fraction of the martensite and the residual austenite which are the secondary phase may be 1% to 5% in total.

In addition, in the metallographic structure, in addition to the ferrite which is the primary phase, the martensite and the residual austenite which are the secondary phase, and the plural inclusions, a small amount of bainite, pearlite, cementite, or the like may be included. In the metallographic structure, preferably, the area fraction of the bainite and the pearlite may be 0% to less than 5.0% in total. As a result, it is preferable that the metallographic structure is controlled to the intended mixed structure and the above effect is obtained.

The average grain size of the ferrite which is the primary phase is to be 2 μm to 10 μm. When the average grain size of the ferrite which is the primary phase is 10 μm or less, it is possible to obtain the intended fracture appearance transition temperature vTrs. In addition, in order to control the average grain size of the ferrite which is the primary phase to less than 2 μm, it is necessary to select strict producing conditions, and the load on the producing facility is large. For this reason, the average grain size of the ferrite which is the primary phase is to be 2 μm to 10 μm. Preferably, the average grain size may be 2 μm to 7 μm. Furthermore preferably, the average grain size may be 2 μm to 6 μm.

It is preferable that the average grain size of the martensite and the residual austenite which are the secondary phase is 0.5 μm to 8.0 μm. When the average grain size of the secondary phase is more than 8.0 μm, the stress concentration which is induced in the vicinity of the secondary phase may increase, and the properties such as the average λave of the hole expanding ratio may decrease. In addition, in order to control the average grain size of the secondary phase to less than 0.5 μm, it is necessary to select strict producing conditions, and the load on the producing facility is large. For this reason, the average grain size of the secondary phase may be 0.5 μm to 8.0 μm.

In regard to the inclusions which are included in the metallographic structure, when the L cross section whose normal direction corresponds to the transverse direction of the steel sheet is observed at 30 of visual fields by 0.0025 mm², the average of the maximum of the ratio of the major axis to the minor axis of the inclusions in each of the visual fields is to be 1.0 to 8.0. When the above average of the ratio of the major axis to the minor axis is more than 8.0, the stress concentration in the vicinity of the inclusions during the deformation of the steel sheet increases, and it is not possible to obtain the intended properties of the average λave of the hole expanding ratio, the standard deviation σ, the resistance of crack initiation Jc, and the Charpy absorbed energy E. On the other hand, although the lower limit of the above average of the ratio of the major axis to the minor axis is not particularly limited, it is technically difficult to control the above value to less than 1.0. For this reason, the above average of the ratio of the major axis to the minor axis is to be 1.0 to 8.0. In addition, preferably, the above average of the ratio of the major axis to the minor axis may be 1.0 to 3.0. When the above average of the ratio of the major axis to the minor axis is 1.0 to 3.0, it is possible to obtain the preferable effect for the average λave of the hole expanding ratio, the standard deviation a of the hole expanding ratio, the resistance of crack initiation Jc, and the Charpy absorbed energy E.

In addition, in regard to the inclusions which are included in the metallographic structure, when a group of the inclusions in which a major axis of each of the inclusions is 3 μm or more and the interval F in the rolling direction between the inclusions is 50 μm or less are defined as the inclusion-cluster G, and when an inclusion in which the interval F is more than 50 μm are defined as the independent-inclusion H, the total length M in the rolling direction of both the inclusion-cluster G whose length in the rolling direction GL is 30 μm or more and the independent-inclusion H whose length in the rolling direction HL is 30 μm or more is to be 0 mm to 0.25 mm per 1 mm²of the L cross section whose normal direction corresponds to the transverse direction of the steel sheet. When the inclusions satisfy the above condition, it is possible to obtain the preferable effect for the average λave of the hole expanding ratio, the standard deviation σ of the hole expanding ratio, the resistance of crack initiation Jc, the resistance of crack propagation T. M., the Charpy absorbed energy E, and the fatigue properties. In addition, the total length M may be zero. Preferably, the total length M may be 0 mm to 0.15 mm per 1 mm²of the L cross section whose normal direction corresponds to the transverse direction of the steel sheet.

In addition, in regard to the inclusions which are included in the metallographic structure, it is preferable that a total number of MnS precipitates and CaS precipitates having the major axis of 3 μm or more is 0% to less than 70% as compared with the total number of the inclusions having the major axis of 3 μm or more. When the total number of MnS precipitates and CaS precipitates which are included in the inclusions is 0% to less than 70%, it is possible to preferably control the total length M and the average of the ratio of the major axis to the minor axis. In addition, since the inclusions having the major axis is less than 3 μm have a small influence on the properties such as the average λave of the hole expanding ratio and the like, it is not necessary to take account of the inclusions.

In addition, the inclusions as described above mainly indicate the sulfides such as MnS and CaS, the oxides such as CaO—Al₂O₃compound (calcium aluminate), the residues of the desulfurizing agent such as CaF₂, and or the like in the steel.

In regard to the texture of the hot rolled steel sheet according to the embodiment, the X-ray random intensity ratio of the {211} plane ({211} plane intensity) is to be 1.0 to 2.4. When the {211} plane intensity is more than 2.4, the anisotropy of the steel sheet is excessive. Thus, at hole expanding, the reduction of sheet thickness increases at the end surface in the rolling direction which is subjected to tensile strain in the transverse direction, high stress is induced in the end surface, and the cracks tend to initiate and propagate. As a result, the average λave of the hole expanding ratio deteriorates. In addition, when the {211} plane intensity is more than 2.4, the resistance of crack initiation Jc and the Charpy absorbed energy E also deteriorate. On the other hand, it is technically difficult to control the {211} plane intensity to less than 1.0. For this reason, the {211} plane intensity is to be 1.0 to 2.4. Preferably, the {211} plane intensity may be 1.0 to 2.0. In addition, the X-ray random intensity ratio of the {211} plane, the {211} plane intensity, and the pole density of the {211} plane are synonymous. In addition, although the X-ray random intensity ratio of the {211} plane is basically measured by the X-ray diffraction method, since differences in the measurement results are not observed even when the measurement is conducted by the EBSD method or the ECP method, the measurement may be conducted by the EBSD method or the ECP method.

In addition, the measurement method of the chemical composition, the metallographic structure, and the texture, and the definitions such as the X-ray random intensity ratio, the total length M in the rolling direction of the inclusions, and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions are as described above.

In the hot rolled steel sheet according to the embodiment, the chemical composition, the metallographic structure, and the texture are satisfied, so that the tensile strength TS is 590 MPa to 980 MPa. In addition, in the hot rolled steel sheet according to the embodiment, the chemical composition, the metallographic structure, and the texture are satisfied, so that the average λave of the hole expanding ratio is 60% or more, the standard deviation σ of the hole expanding ratio is 15% or less, the fatigue life in plane bending is 400000 times or more, the resistance of crack initiation Jc is 0.5 MJ/m²or more, the resistance of crack propagation T. M. is 600 MJ/m³or more, the fracture appearance transition temperature vTrs is 13° C. or lower, and the Charpy absorbed energy E is 16 J or more.

In the hot rolled steel sheet according to the embodiment, as described above, it is preferable to control the tensile strength TS by controlling the Ti content in accordance with the intended use of the steel sheet. For example, although the tensile strength TS of the steel sheet is 590 MPa to less than 780 MPa when the Ti content is 0.001 to less than 0.08%, it is possible to control the average λave of the hole expanding ratio to 90% or more, the resistance of crack initiation Jc to 0.9 MJ/m², and the Charpy absorbed energy E to 35 J or more in the above properties. For example, when the Ti content is 0.08% to 0.3%, it is possible to control the tensile strength TS of the steel sheet to 780 MPa to 980 MPa, and it is possible to control the fatigue life in plane bending to 500000 times or more in the above properties. As described above, in a case where the Ti content is changed in accordance with the intended use of the steel sheet, in order to control the total length M and the average of the ratio of the major axis to the minor axis to the intended limitation range, the amount of REM and Ca may be controlled as necessary as described above.

Next, description will be given of the method of producing the hot rolled steel sheet according to the embodiment.

A method of producing the hot rolled steel sheet according to the embodiment includes: a heating process of heating a steel piece which consists of the above-described chemical composition to a range of 1200° C. to 1400° C.; a first rough rolling process of rough rolling the steel piece in a temperature range of higher than 1150° C. to 1400° C. so that a cumulative reduction is 10% to 70% after the heating process; a second rough rolling process of rough rolling in a temperature range of higher than 1070° C. to 1150° C. so that a cumulative reduction is 10% to 25% after the first rough rolling process; a finish rolling process of finish rolling so that a start temperature is 1000° C. to 1070° C. and a finish temperature is Ar3+60° C. to Ar3+200° C. to obtain a hot rolled steel sheet after the second rough rolling process; a first cooling process of cooling the hot rolled steel from the finish temperature so that a cooling rate is 20° C./second to 150° C./second after the finish rolling process; a second cooling process of cooling in a temperature range of 650° C. to 750° C. so that the cooling rate is 1° C./second to 15° C./second and a cooling time is 1 second to 10 seconds after the first cooling process; a third cooling process of cooling to a temperature range of 0° C. to 200° C. so that the cooling rate is 20° C./second to 150° C./second after the second cooling process; and a coiling process of coiling the hot rolled steel sheet after the third cooling process. In addition, Ar3 represents a temperature where the ferrite transformation starts during cooling.

In the heating process, a steel piece which consists of the above-described chemical composition and which is obtained by continuous casting or the like is heated in a heating furnace. In order to obtain the intended tensile strength TS, the heating temperature in the process is to be 1200° C. to 1400° C. When the temperature is less than 1200° C., the precipitates which include Ti and Nb are not sufficiently dissolved and coarsen in the steel piece, so that the precipitation strengthening by the precipitates of Ti and Nb may not be obtained. Therefore, the intended tensile strength TS may not be obtained. In addition, when the temperature is less than 1200° C., MnS is not sufficiently dissolved in the steel piece, so that it may not be possible to make S precipitate as the sulfides with Ti, REM, and Ca. Therefore, the intended properties for the average λave of the hole expansion value, the resistance of crack initiation Jc, and the Charpy absorbed energy E may not be obtained. On the other hand, when the steel piece is heated to more than 1400° C., the above effects are saturated and the heating cost also increases.

In the first rough rolling process, rough rolling is conducted to the steel piece which was taken from the heating furnace. In the first rough rolling, rough rolling is conducted so that a cumulative reduction is 10% to 70% in a temperature range of higher than 1150° C. to 1400° C. When the cumulative reduction in the temperature range is more than 70%, both the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions may increase. Therefore, the properties such as the average λave of the hole expanding ratio, the standard deviation σ, the resistance of crack initiation Jc, the resistance of crack propagation T. M., the Charpy absorbed energy E, and the fatigue life may deteriorate. On the other hand, although the lower limit of the cumulative reduction in the first rough rolling process is not particularly limited, the above value is to be 10% or more in consideration of production efficiency and the like in the subsequent processes. In addition, preferably, the cumulative reduction in the first rough rolling process may be 10% to 65%. Thereby, under the condition where the composition of the steel piece satisfies 0.3≦(REM/140)/(Ca/40), it is possible to control the average of the ratio of the major axis to the minor axis to 1.0 to 3.0. In addition, by controlling the temperature range to higher than 1150° C. to 1400° C., it is possible to obtain the above effects.

In the second rough rolling process, rough rolling is conducted so that a cumulative reduction is 10% to 25% in a temperature range of higher than 1070° C. to 1150° C. When the cumulative reduction is less than 10%, the average grain size of the metallographic structure may coarsen, and the intended average grain size of the ferrite which is 2 μm to 10 μm may not be obtained. As a result, the intended fracture appearance transition temperature vTrs may not be obtained. On the other hand, when the cumulative reduction is more than 25%, the {211} plane intensity as the texture may increase. As a result, the intended properties such as the average λave of the hole expanding ratio, the resistance of crack initiation Jc, and the Charpy absorbed energy E may not be obtained. In addition, by controlling the temperature range to higher than 1070° C. to 1150° C., it is possible to obtain the above effect.

Here, description will be given of the basic research results relating to the first rough rolling process and the second rough rolling process. By using the test steels which consisted of the steel composition a as shown in the following Table 1, steel sheets were produced by variously changing the cumulative reduction in the first rough rolling and the second rough rolling, and the properties of the steel sheets were investigated. In addition, the producing conditions with the exception of the cumulative reduction in the first rough rolling and the second rough rolling of the hot rolled steel sheet according to the embodiment were satisfied.

[Table 1]

FIG. 9A is a diagram which shows a relationship between the cumulative reduction in the first rough rolling process and the total length M in the rolling direction of the inclusions. FIG. 9B is a diagram which shows a relationship between the cumulative reduction in the first rough rolling process and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions. FIG. 9C is a diagram which shows a relationship between the cumulative reduction in the second rough rolling process and the {211} plane intensity. FIG. 9D is a diagram which shows a relationship between the cumulative reduction in the second rough rolling process and the average grain size of the ferrite. In addition, the cumulative reduction represents a ratio of reduction of the steel piece in the first rough rolling process and the second rough rolling process on the basis of the thickness of the steel piece after the heating process. Specifically, the cumulative reduction of the rough rolling in the first rough rolling process is defined as {(thickness of the steel piece before first reduction in a temperature range of higher than 1150° C. to 1400° C.−thickness of the steel piece after final reduction in a temperature range of higher than 1150° C. to 1400° C.)/thickness of the steel piece after the heating process×100%}. The cumulative reduction of the rough rolling in the second rough rolling process is defined as {(thickness of the steel piece before first reduction in a temperature range of higher than 1070° C. to 1150° C.−thickness of the steel piece after final reduction in a temperature range of higher than 1070° C. to 1150° C.)/thickness of the steel piece after the heating process×100%}.

From FIG. 9A, it is understood that, when the cumulative reduction is more than 70% in a temperature range of higher than 1150° C. to 1400° C., the total length M in the rolling direction of the inclusions is excessive, and the total length M of 0 mm/mm²to 0.25 mm/mm²which is the intended range is not obtained. In addition, from FIG. 9B, it is understood that, when the cumulative reduction is more than 70% in a temperature range of higher than 1150° C. to 1400° C., the average of the maximum of the ratio of the major axis to the minor axis of the inclusions is excessive, and the average of the ratio of the major axis to the minor axis of 1.0 to 8.0 which is the intended range is not obtained. The reason for the above seems that, as the cumulative reduction of the rough rolling which is conducted in a higher temperature range of higher than 1150° C. to 1400° C. increases, the inclusions tend to be elongated by rolling. In addition, from FIG. 9B, it is understood that, when the cumulative reduction is 65% or less, the average of the ratio of the major axis to the minor axis of 1.0 to 3.0 is obtained.

From FIG. 9C, it is understood that, when the cumulative reduction in a temperature range of higher than 1070° C. to 1150° C. is more than 25%, the {211} plane intensity is excessive, and the intended {211} plane intensity of 1.0 to 2.4 is not obtained. The reason for the above seems that, when the cumulative reduction of the rough rolling which is conducted in a temperature range which is a comparatively low temperature such as higher than 1070° C. to 1150° C. is excessively large, the recrystallization does not proceed uniformly after the rough rolling, and a non-recrystallized structure which leads to increase the {211} plane intensity remains even after the finish rolling, so that the {211} plane intensity increases.

From FIG. 9D, it is understood that, when the cumulative reduction in a temperature range of higher than 1070° C. to 1150° C. is less than 10%, the average grain size of the ferrite is excessive, and the intended average grain size of 2 μm to 10 μm is not obtained. The reason for the above seems that, as the cumulative reduction of the rough rolling which is conducted in a temperature range which is a low temperature such as higher than 1070° C. to 1150° C. decreases, the grain size of the austenite after recrystallization increases, and the average grain size of the ferrite of the steel sheet also increases.

After the second rough rolling process, as the finish rolling process, finish rolling is conducted to the steel piece in order to obtain the hot rolled steel sheet. In the finish rolling process, the start temperature is to be 1000° C. to 1070° C. When the start temperature of the finish rolling is 1000° C. to 1070° C., dynamic recrystallization is promoted in the finish rolling. As a result, the rolling texture which is the non-recrystallized state is relieved, and it is possible to obtain the intended {211} plane intensity of 1.0 to 2.4.

In addition, in the finish rolling process, the finish temperature is to be Ar3+60° C. to Ar3+200° C. In order to obtain the intended {211} plane intensity of 1.0 to 2.4 by preventing the rolling texture which is the no-recrystallized state and which leads to increase the {211} plane intensity from remaining, the finish temperature is controlled to Ar3+60° C. or more. Preferably, the temperature may be Ar3+100° C. or more. In addition, in order to obtain the intended average grain size of the ferrite by preventing the grain size from excessively coarsening, the finish temperature is controlled to Ar3+200° C. or less.

In addition, Ar3 is determined from the following Expression 10. In the following Expression 10, the calculation is conducted using the amounts expressed in mass % of each element in the chemical composition.

Ar3=868−396×C+25×Si−68×Mn−36×Ni−21×Cu−25×Cr+30×Mo (Expression 10)

Subsequently, the hot rolled steel sheet which is obtained by the finish rolling process is cooled in a run out table or the like. The cooling of the hot rolled steel sheet is conducted by the first cooling process to the third cooling process to be described below. In the first cooling process, the hot rolled steel sheet which is at the finish temperature of the finish rolling is cooled to a temperature of 650° C. to 750° C. so that a cooling rate is 20° C./second to 150° C./second. Subsequently, in the second cooling process, the cooling rate is changed to 1° C./second to 15° C./second, and cooling is conducted in a temperature range of 650° C. to 750° C. for a cooling time of 1 second to 10 seconds. Subsequently, in the third cooling process, the cooling rate is again returned to 20° C./second to 150° C./second, and cooling is conducted to a temperature range of 0° C. to 200° C. As described above, in the second cooling process, by conducting the cooling of the hot rolled steel sheet under the cooling rate which is slower than those of the first cooling process and the third cooling process, it is possible to promote the ferrite transformation. As a result, it is possible to obtain the hot rolled steel sheet which has the intended mixed structure.

When the cooling rate of the first cooling process is less than 20° C./second, the grain size of the ferrite may increase, and the fracture appearance transition temperature vTrs may deteriorate. In addition, due to the restriction of the producing facility, it is difficult to control the cooling rate in the first cooling process to more than 150° C./second. For this reason, the cooling rate in the first cooling process is to be 20° C./second to 150° C./second.

In order to promote the ferrite transformation and to control the area fraction of the martensite and the residual austenite which are the secondary phase to the intended range, the cooling rate in the second cooling process is to be 15° C./second or less. In addition, even when the cooling rate in the second cooling process is less than 1° C./second, the effect is saturated. For this reason, the cooling rate in the second cooling process is to be 1° C./second to 15° C./second.

In addition, in order to promote the ferrite transformation and to control the area fraction of the martensite and the residual austenite to the intended range, the temperature range where the second cooling process is conducted is to be 750° C. or less where the ferrite transformation is promoted. In addition, when the temperature range where the second cooling process is conducted is less than 650° C., the formation of the pearlite or the bainite is promoted, and therefore, the fraction of the martensite and the residual austenite may be excessively small. For this reason, the temperature range where the second cooling process is conducted is to be 650° to 750° C.

In addition, when the cooling time in the second cooling process is more than 10 seconds, the formation of the pearlite which causes the deterioration in the tensile strength TS and the fatigue life is promoted, and therefore, the fraction of the martensite and the residual austenite may be excessively small. In addition, in order to promote the ferrite transformation, the cooling time in the second cooling process is to be 1 second or more. For this reason, the cooling time in the second cooling process is to be 1 second to 10 seconds.

When the cooling rate in the third cooling process is less than 20° C./second, the formation of the pearlite and the bainite is promoted, and therefore, the fraction of the martensite and the residual austenite may be excessively small. In addition, due to the restriction of the producing facility, it is difficult to control the cooling rate in the third cooling process to more than 150° C./second. For this reason, the cooling rate in the third cooling process is to be 20° C./second to 150° C./second.

In addition, when the finish temperature of the cooling in the third cooling process is higher than 200° C., the formation of the bainite is promoted during the coiling process which is the subsequent process, and therefore, the fraction of the martensite and the residual austenite may be excessively small. In addition, due to the restriction of the producing facility, it is difficult to control the finish temperature of the cooling in the third cooling process to less than 0° C. For this reason, the finish temperature of the cooling in the third cooling process is to be 0° C. to 200° C.

In addition, for example, the cooling rate of 20° C./second or more is obtained by the cooling such as water-cooling or mist-cooling. In addition, for example, the cooling rate of 15° C./second or less is obtained by the cooling such as air-cooling.

Subsequently, as the coiling process, the hot rolled steel sheet is coiled.

The above are the producing conditions of the hot rolling method according to the embodiment. However, as necessary, in order to improve the ductility by the introduction of moving dislocations and to correct the shape of the steel sheet, the skin pass rolling may be conducted. In addition, as necessary, in order to remove scale which adheres to the surface of the hot rolled steel sheet, the pickling may be conducted. In addition, as necessary, by using the obtained hot rolled steel sheet, the skin pass rolling which is in-line or off-line or the cold rolling may be conducted.

In addition, as necessary, in order to improve the corrosion resistance of the steel sheet, the coating such as a hot dip coating may be conducted. In addition to the hot dip coating, the alloying may be conducted.

EXAMPLE

Hereinafter, the effects of an aspect of the present invention will be described in detail with reference to the following examples. However, the condition in the examples is an example condition employed to confirm the operability and the effects of the present invention, so that the present invention is not limited to the example condition. The present invention can employ various types of conditions as long as the conditions do not depart from the scope of the present invention and can achieve the object of the present invention.

Molten steels having the steel compositions A to MMMM as shown in Tables 2 to 4 were obtained. Each of the molten steels was made by conducting converter smelting and secondary refining. The secondary refining was conducted in a RH (Ruhrstahl-Hausen) vacuum degasser, and desulfurization was conducted by appropriately adding CaO—CaF2-MgO based desulfurizing agent. In some of the steel compositions, in order to suppress the remaining of the desulfurizing agent which tends to be the elongated inclusion, steels having S content which corresponds to that after the primary refining in the converter were produced without conducting desulfurization. Steel pieces were obtained by continuous casting using the molten steels, the hot rolling was conducted under the producing conditions as shown in Tables 5 to 7, and the obtained steel sheets were coiled. The sheet thickness of the obtained hot rolled steel sheets was to be 2.9 mm.

The characteristic values of the obtained hot rolled steel sheets, such as the metallographic structures, the texture, and the inclusions are shown in Tables 8 to 10. The mechanical properties of the obtained hot rolled steel sheets are shown in Tables 11 to 13. The measurement methods of the metallographic structure, the texture, and the inclusions, and the measurement methods of the mechanical properties are described above. As the tensile properties, when the tensile strength TS was 590 MPa or more and the n value was 0.13 or more, it was judged to be acceptable. As the formability, when the average λave of the hole expanding ratio was 60% or more and the standard deviation σ of the hole expanding ratio was 15% or less, it was judged to be acceptable. As the fracture properties, when the resistance of crack initiation Jc was 0.5 MJ/m²or more, the resistance of crack propagation T. M. was 600 MJ/m³or more, the fracture appearance transition temperature vTrs was 13° C. or lower, and the Charpy absorbed energy E was 16 J or more, it was judged to be acceptable. As the fatigue properties, when the bending plane fatigue life was 400000 times or more, it was judged to be acceptable. In addition, the underlined value in the tables indicates out of the range of the present invention. In addition, in the tables, by using the amounts expressed in mass % of each element in the chemical composition, a value of (Ti/48)/(S/32)+{(Ca/40)/(S/32)+(REM/140)/(S/32)}×15 is represented as “*1”, and a value of (REM/140)/(Ca/40) is represented as “*2”.

In Tables 2 to 13, the producing results and the evaluation results are shown. All of the Examples satisfied the ranges of the present invention and are excellent in, as the hot rolled steel sheet, the tensile properties, the formability, the fracture properties, and the fatigue properties. On the other hand, the Comparative Examples did not satisfy the ranges of the present invention as the hot rolled steel sheet.

In Comparative Example 11, since the C content was insufficient, the average grain size of the primary phase coarsened. Therefore, the fracture properties of the steel sheet deteriorated.

In Comparative Example 12, since the C content was insufficient, the average grain size of the primary phase coarsened and the area fraction of the secondary phase decreased. Therefore, the tensile properties and the fracture properties of the steel sheet deteriorated.

In Comparative Example 26, since the S content was excessive, the total length M in the rolling direction of the inclusions increased. Therefore, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 27, since the value of “*1” was insufficient, the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 28, since the Mn content was excessive, the area fraction of the secondary phase increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 30, since the reduction in the first rough rolling process was excessive, the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions increased. Therefore, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 32, since the reduction in the second rough rolling process was excessive, the {211} plane intensity increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 35, since the reduction in the second rough rolling process was insufficient, the average grain size of the primary phase coarsened. Therefore, the fracture properties of the steel sheet deteriorated.

In Comparative Example 36, since the start temperature in the finish rolling process was low, the {211} plane intensity increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 37, since the finish temperature in the finish rolling process was low, the {211} plane intensity increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 38, since the finish temperature in the finish rolling process was high, the average grain size of the primary phase coarsened. Therefore, the fracture properties of the steel sheet deteriorated.

In Comparative Example 39, since the cooling rate in the first cooling process was slow, the average grain size of the primary phase coarsened. Therefore, the fracture properties of the steel sheet deteriorated.

In Comparative Example 40, since the finish temperature of the cooling in the third cooling process was high, the area fraction of the secondary phase decreased. Therefore, the tensile properties and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 41, since the cooling rate in the third cooling process was slow, the area fraction of the secondary phase decreased. Therefore, the tensile properties and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 51, since the C content was insufficient, the average grain size of the primary phase coarsened and the area fraction of the secondary phase decreased. Therefore, the tensile properties, the fracture properties, and the fatigue properties of the steel sheet decreased.

In Comparative Example 67, since the value of “*1” was insufficient, the total length M in the rolling direction of the inclusions increased. Therefore, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 68, since the value of “*1” was insufficient, the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions increased. Therefore, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 69, since the Mn content was excessive, the area fraction of the secondary phase increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 70, since the heating temperature in the heating process was low, the tensile strength was insufficient.

In Comparative Example 71, since the reduction in the first rough rolling process was excessive, the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions increased. Therefore, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 73, since the reduction in the second rough rolling process was excessive, the {211} plane intensity increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 76, since the reduction in the second rough rolling process was insufficient, the average grain size of the primary phase coarsened. Therefore, the fracture properties of the steel sheet deteriorated.

In Comparative Example 77, since the start temperature in the finish rolling process was low, the {211} plane intensity increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 78, since the finish temperature in the finish rolling process was low, the {211} plane intensity increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 79, since the finish temperature in the finish rolling process was high, the average grain size of the primary phase coarsened. Therefore, the fracture properties of the steel sheet deteriorated.

In Comparative Example 80, since the cooling rate in the third cooling process was slow, the average grain size of the primary phase coarsened and the area fraction of the secondary phase decreased. Therefore, the tensile properties, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 81, since the finish temperature of the cooling in the third cooling process was high, the area fraction of the secondary phase decreased. Therefore, the tensile properties and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 84, since all of Ti, REM, or Ca were not contained, the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions increased. Therefore, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 85, since the cooling rate in the second cooling process was fast, the area fraction of the secondary phase increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 86, since the value of “*1” was insufficient, the total length M in the rolling direction of the inclusions increased. Therefore, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 91, since the cooling temperature in the second cooling process was high, the area fraction of the secondary phase increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 92, since the cooling time in the second cooling process was long, the area fraction of the primary phase decreased and the area fraction of the pearlite increased. Therefore, the tensile properties and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 93, since the cooling time in the second cooling process was short, the area fraction of the secondary phase increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 94, since the C content was excessive, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 95, since the Mn content was insufficient, the tensile properties of the steel sheet deteriorated.

In Comparative Examples 96 and 97, since the amount of Si+Al was excessive, the formability of the steel sheet deteriorated.

In Comparative Examples 98 and 99, since the amount of Si+Al content was insufficient, the tensile properties and the fracture properties of the steel sheet deteriorated.

In Comparative Example 100, since the P content was excessive, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 101, since the N content was excessive, the tensile properties of the steel sheet deteriorated.

In Comparative Example 102, since the Ti content was excessive, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 103, since the REM content was excessive, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 104, since the Ca content was excessive, the total length M in the rolling direction of the inclusions and the average of the maximum of the ratio of the major axis to the minor axis of the inclusions increased. Therefore, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 105, since the Ti content was insufficient, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 106, since the REM content was insufficient, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 107, since the Ca content was insufficient, the formability, the fracture properties, and the fatigue properties of the steel sheet deteriorated.

In Comparative Example 108, since the Nb content was excessive, the {211} plane intensity increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 109, since the B content was excessive, the {211} plane intensity increased. Therefore, the formability and the fracture properties of the steel sheet deteriorated.

In Comparative Example 110, since the Cu content was excessive, the formability of the steel sheet deteriorated.

In Comparative Example 111, since the Cr content was excessive, the formability of the steel sheet deteriorated.

In Comparative Example 112, since the Mo content was excessive, the formability of the steel sheet deteriorated.

In Comparative Example 113, since the Ni content was excessive, the formability of the steel sheet deteriorated.

In Comparative Example 114, since the V content was excessive, the formability of the steel sheet deteriorated.

[Table 2]

[Table 3]

[Table 4]

[Table 5]

[Table 6]

[Table 7]

[Table 8]

[Table 9]

[Table 10]

[Table 11]

[Table 12]

[Table 13]

INDUSTRIAL APPLICABILITY

According to the aspect of the present invention, it is possible to obtain a steel sheet which has an excellent balance between tensile properties and formability and furthermore which has excellent fracture properties and fatigue properties. Accordingly, the present invention has significant industrial applicability.

REFERENCE SIGNS LIST

- 41
  a to 41l INCLUSIONS IN WHICH MAJOR AXIS OF EACH OF INCLUSIONS IS 3 μm OR MORE
- F INTERVAL BETWEEN INCLUSIONS IN ROLLING DIRECTION
- G INCLUSION-CLUSTER
- GL LENGTH OF INCLUSION-CLUSTER IN ROLLING DIRECTION
- H INDEPENDENT-INCLUSION
- HL LENGTH OF INDEPENDENT-INCLUSION IN ROLLING DIRECTION

TABLE 1

STEEL
CHEMICAL COMPOSITION (unit: mass %)
Ar3

COMPOSITION
C
Si
Mn
P
S
Al
N
Ti
REM
Ca
(° C.)

a
0.040
1.25
1.25
0.007
0.001
0.025
0.0035
0.07
0.0025
0.002
798

TABLE 2

STEEL
CHEMICAL COMPOSITION (unit: mass %)

COMPOSITION
C
Si
Mn
P
S
Al
N
Ti

EXAMPLE
1
A
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

EXAMPLE
2
B
0.055
1.35
1.85
0.008
0.0010
0.020
0.0025
0.13

EXAMPLE
3
C
0.062
1.05
2.50
0.011
0.0040
0.029
0.0029
0.28

EXAMPLE
4
D
0.057
1.95
1.35
0.009
0.0010
0.026
0.0021
0.12

EXAMPLE
5
E
0.065
1.35
1.70
0.010
0.0040
0.028
0.0020
0.25

EXAMPLE
6
F
0.080
1.15
1.90
0.011
0.0010
0.025
0.0029
0.18

EXAMPLE
7
G
0.061
0.50
1.85
0.012
0.0030
0.025
0.0027
0.13

EXAMPLE
8
H
0.060
0.55
1.87
0.008
0.0035
0.028
0.0029
0.13

EXAMPLE
9
I
0.058
1.36
2.00
0.011
0.0045
0.027
0.0028
0.14

EXAMPLE
10
J
0.059
1.17
1.86
0.012
0.0035
0.021
0.0026
0.08

EXAMPLE
11
K

0.028

1.00
1.90
0.012
0.0040
0.023
0.0024
0.12

EXAMPLE
12
L

0.015

1.30
1.90
0.011
0.0040
0.021
0.0020
0.12

EXAMPLE
13
M
0.065
1.09
1.91
0.006
0.0040
0.028
0.0029
0.13

EXAMPLE
14
N
0.068
1.13
1.80
0.005
0.0040
0.022
0.0025
0.14

EXAMPLE
15
O
0.060
1.27
1.70
0.011
0.0040
0.025
0.0022
0.13

EXAMPLE
16
P
0.061
1.35
1.90
0.012
0.0040
0.027
0.0025
0.13

EXAMPLE
17
Q
0.062
1.25
1.80
0.009
0.0040
0.021
0.0024
0.12

EXAMPLE
18
R
0.055
1.23
1.90
0.011
0.0040
0.029
0.0023
0.11

EXAMPLE
19
S
0.059
1.20
1.89
0.012
0.0040
0.027
0.0027
0.13

EXAMPLE
20
T
0.060
1.30
1.83
0.014
0.0040
0.020
0.0026
0.14

EXAMPLE
21
U
0.057
1.05
1.86
0.008
0.0038
0.022
0.0020
0.12

EXAMPLE
22
V
0.059
1.04
1.87
0.009
0.0040
0.024
0.0029
0.13

EXAMPLE
23
W
0.062
1.10
1.83
0.011
0.0040
0.023
0.0024
0.11

EXAMPLE
24
X
0.061
1.17
1.85
0.012
0.0035
0.024
0.0023
0.13

EXAMPLE
25
Y
0.060
1.15
1.86
0.014
0.0043
0.026
0.0021
0.12

COMPARATIVE EXAMPLE
26
Z
0.061
1.18
1.87
0.009

0.0110

0.024
0.0022
0.13

COMPARATIVE EXAMPLE
27
AA
0.055
1.35
1.75
0.008
0.0100
0.025
0.0021
0.13

COMPARATIVE EXAMPLE
28
BB
0.048
0.51

3.05

0.011
0.0040
0.030
0.0024
0.13

CHEMICAL COMPOSITION (unit: mass %)

REM
Ca
※1
※2
Si + Al
OTHER ELEMENTS

EXAMPLE
1
0.0040
0.0038
48.66
0.30
1.27
V = 0.015%

EXAMPLE
2
0.0025
0.0020
119.24
0.36
1.37
—

EXAMPLE
3

0.0000

0.0000

46.67
∞
1.08
V = 0.03%

EXAMPLE
4

0.0000

0.0000

80.00
∞
1.98
—

EXAMPLE
5

0.0000

0.0003
42.57
0.00
1.38
—

EXAMPLE
6

0.0000

0.0004
124.80
0.00
1.17
—

EXAMPLE
7
0.0050

0.0000

34.60
∞
0.53
V = 0.08%

EXAMPLE
8
0.0050
0.0003
30.69
4.76
0.58
V = 0.08%

EXAMPLE
9
0.0040
0.0034
32.86
0.34
1.39
Nb = 0.019%

EXAMPLE
10
0.0055
0.0050
37.39
0.31
1.19
—

EXAMPLE
11
0.0040
0.0037
34.53
0.31
1.02
—

EXAMPLE
12

0.0400

0.0036
65.09
3.17
1.32
—

EXAMPLE
13
0.0040
0.0037
36.20
0.31
1.12
B = 0.0010%

EXAMPLE
14
0.0180

0.0000

38.76
∞
1.15
Cr = 0.1%, Mo = 0.03%

EXAMPLE
15

0.0000

0.0050
36.67
0.00
1.30
—

EXAMPLE
16

0.0000

0.0040
33.67
0.00
1.38
—

EXAMPLE
17
0.0010
0.0031
30.16
0.09
1.27
—

EXAMPLE
18
0.0020
0.0042
32.65
0.14
1.26
—

EXAMPLE
19
0.0032
0.0044
37.61
0.21
1.23
—

EXAMPLE
20
0.0034
0.0040
38.25
0.24
1.32
—

EXAMPLE
21
0.0027
0.0025
31.38
0.31
1.07
Cu = 0.2%, Ni = 0.1%

EXAMPLE
22
0.0031
0.0024
31.52
0.37
1.06
V = 0.02%

EXAMPLE
23
0.0055
0.0040
35.05
0.39
1.12
—

EXAMPLE
24
0.0038
0.0035
40.48
0.31
1.19
—

EXAMPLE
25
0.0044
0.0029
30.21
0.43
1.18
—

COMPARATIVE EXAMPLE
26
0.0034
0.0041
13.41
0.24
1.20
—

COMPARATIVE EXAMPLE
27
0.0015
0.0023

11.94

0.19
1.38
—

COMPARATIVE EXAMPLE
28
0.0032
0.0022
31.01
0.42
0.54
—

The underlined value in the table indicates out of the range of the present invention.

The ※1 in the table indicates (Ti/48)/(S/32) + {(Ca/40)/(S/32) + (REM/140)/(S/32)} × 15.

The ※2 in the table indicates (REM/140)/(Ca/40).

TABLE 3

STEEL
CHEMICAL COMPOSITION (unit: mass %)

COMPO-

Si +
OTHER

SITION
C
Si
Mn
P
S
Al
N
Ti
REM
Ca
※1
※2
Al
ELEMENTS

EXAMPLE
42
CC
0.040
1.25
1.25
0.007
0.0030
0.023
0.0021
0.05
0.0040
0.0038
30.88
0.30
1.27
—

EXAMPLE
43
DD
0.055
1.35
1.20
0.008
0.0010
0.020
0.0025
0.05
0.0025
0.0020
65.90
0.36
1.37
—

EXAMPLE
44
EE
0.062
1.05
1.48
0.011
0.0040
0.029
0.0029
0.08

0.0000

0.0000

13.37
∞
1.08
V = 0.02%

EXAMPLE
45
FF
0.057
1.95
0.70
0.009
0.0010
0.026
0.0021
0.04

0.0000

0.0000

26.67
∞
1.98
—

EXAMPLE
46
GG
0.065
1.35
1.05
0.010
0.0040
0.028
0.0020
0.08

0.0000

0.0003
13.40
0.00
1.38
—

EXAMPLE
47
HH
0.090
1.15
1.25
0.011
0.0010
0.025
0.0029
0.08

0.0000

0.0004
56.80
0.00
1.17
—

EXAMPLE
48
II
0.061
0.50
1.85
0.012
0.0030
0.025
0.0027
0.05
0.0050

0.0000

16.83
∞
0.53
V = 0.01%

EXAMPLE
49
JJ
0.060
0.55
1.87
0.008
0.0035
0.028
0.0029
0.05
0.0050
0.0002
15.11
7.14
0.58
V = 0.02%

EXAMPLE
50
KK
0.040
1.50
1.51
0.007
0.0015
0.025
0.0025

0.00

0.0034
0.0028
30.17
0.35
1.53
—

COM-
51
LL

0.020

1.30
1.35
0.006
0.0040
0.021
0.0021
0.05
0.0045
0.0040
24.19
0.32
1.32
—

PARATIVE

EXAMPLE

EXAMPLE
52
MM
0.058
1.36
1.35
0.011
0.0045
0.027
0.0028
0.06
0.0040
0.0034
21.00
0.34
1.39
Nb = 0.012%

EXAMPLE
53
NN
0.031
1.00
1.25
0.012
0.0040
0.023
0.0024
0.04
0.0040
0.0037
21.20
0.31
1.02
—

EXAMPLE
54
OO
0.065
1.09
1.26
0.006
0.0040
0.028
0.0029
0.05
0.0040
0.0037
22.86
0.31
1.12
B = 0.0009%

EXAMPLE
55
PP
0.068
1.13
1.15
0.005
0.0040
0.022
0.0025
0.06
0.0100

0.0000

18.57
∞
1.15
Cr = 0.2%,

Mo = 0.05%

EXAMPLE
56
QQ
0.060
1.27
0.83
0.011
0.0040
0.025
0.0022
0.05

0.0000

0.0050
23.33
0.00
1.30
—

EXAMPLE
57
RR
0.061
1.35
1.25
0.012
0.0040
0.027
0.0025
0.05

0.0000

0.0040
20.33
0.00
1.38
—

EXAMPLE
58
SS
0.062
1.25
1.15
0.009
0.0040
0.021
0.0024
0.04
0.0010
0.0031
16.82
0.09
1.27
—

EXAMPLE
59
TT
0.055
1.23
1.25
0.011
0.0040
0.029
0.0023
0.03
0.0020
0.0042
19.31
0.14
1.26
—

EXAMPLE
60
UU
0.059
1.20
1.24
0.012
0.0040
0.027
0.0027
0.05
0.0032
0.0044
24.28
0.21
1.23
—

EXAMPLE
61
VV
0.060
1.30
1.18
0.014
0.0040
0.020
0.0026
0.06
0.0034
0.0040
24.91
0.24
1.32
—

EXAMPLE
62
WW
0.057
1.05
1.21
0.008
0.0038
0.022
0.0020
0.04
0.0027
0.0025
17.35
0.31
1.07
Cu = 0.2%,

Ni = 0.2%

EXAMPLE
63
XX
0.059
1.04
1.22
0.009
0.0040
0.024
0.0029
0.05
0.0031
0.0024
18.19
0.37
1.06
V = 0.01%

EXAMPLE
64
YY
0.062
1.10
1.18
0.011
0.0040
0.023
0.0024
0.03
0.0055
0.0040
21.17
0.39
1.12
—

EXAMPLE
65
ZZ
0.061
1.17
1.20
0.012
0.0035
0.024
0.0023
0.05
0.0036
0.0035
25.25
0.31
1.19
—

EXAMPLE
66
AAA
0.060
1.15
1.21
0.014
0.0043
0.026
0.0021
0.04
0.0035
0.0031
17.64
0.32
1.18
—

COM-
67
BBB
0.061
1.18
1.22
0.009
0.0080
0.024
0.0022
0.05
0.0034
0.0041

11.77

0.24
1.20
—

PARATIVE

EXAMPLE

COM-
68
CCC
0.055
1.35
1.10
0.008
0.0100
0.025
0.0021
0.05
0.0015
0.0023
6.61
0.19
1.38
—

PARATIVE

EXAMPLE

COM-
69
DDD
0.048
0.51

3.05

0.011
0.0040
0.030
0.0024
0.05
0.0032
0.0022
17.68
0.42
0.54
—

PARATIVE

EXAMPLE

The underlined value in the table indicates out of the range of the present invention.

The ※1 in the table indicates (Ti/48)/(S/32) + {(Ca/40)/(S/32) + (REM/140)/(S/32)} × 15.

The ※2 in the table indicates (REM/140)/(Ca/40).

TABLE 4

STEEL
CHEMICAL COMPOSITION (unit: mass %)

COMPOSITION
C
Si
Mn
P
S
Al
N
Ti

EXAMPLE
82
EEE
0.060
1.10
1.80
0.010
0.0010
0.020
0.0020

0.00

EXAMPLE
83
FFF
0.060
1.31
1.75
0.008
0.0030
0.025
0.0025

0.00

COMPARATIVE EXAMPLE
84
GGG
0.065
1.60
0.50
0.010
0.0030
0.028
0.0025

0.00

COMPARATIVE EXAMPLE
85
HHH
0.078
1.50
1.20
0.010
0.0025
0.025
0.0021
0.13

COMPARATIVE EXAMPLE
86
JJJ
0.064
1.50
1.80
0.010
0.0015
0.025
0.0031
0.02

EXAMPLE
87
A
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

EXAMPLE
89
A
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

EXAMPLE
90
KKK
0.060
1.25
1.95
0.010
0.0049
0.025
0.0040
0.13

COMPARATIVE EXAMPLE
91
A
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
92
A
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
93
A
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
94
LLL

0.110

1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
95
MMM
0.040
1.25

0.48

0.007
0.0030
0.023
0.0021
0.05

COMPARATIVE EXAMPLE
96
NNN
0.060

2.55

1.90
0.007
0.0030
1.580
0.0021
0.13

COMPARATIVE EXAMPLE
97
OOO
0.060
1.60
1.90
0.007
0.0030

2.430

0.0021
0.13

COMPARATIVE EXAMPLE
98
PPP
0.031

0.47

1.25
0.012
0.0040
0.007
0.0024
0.04

COMPARATIVE EXAMPLE
99
QQQ
0.031

0.45

1.25
0.012
0.0040

0.004

0.0024
0.04

COMPARATIVE EXAMPLE
100
RRR
0.059
1.17
1.86

0.110

0.0035
0.021
0.0026
0.08

COMPARATIVE EXAMPLE
101
SSS
0.055
1.35
1.20
0.008
0.0010
0.020

0.0250

0.05

COMPARATIVE EXAMPLE
102
TTT
0.062
1.05
2.00
0.011
0.0040
0.029
0.0029

0.31

COMPARATIVE EXAMPLE
103
UUU
0.060
1.10
1.80
0.010
0.0010
0.020
0.0020

0.00

COMPARATIVE EXAMPLE
104
VVV
0.060
1.31
1.75
0.008
0.0030
0.025
0.0025

0.00

COMPARATIVE EXAMPLE
105
WWW
0.062
1.05
1.35
0.011
0.0040
0.029
0.0029
0.0008

COMPARATIVE EXAMPLE
106
XXX
0.060
1.10
1.80
0.010
0.0010
0.020
0.0020

0.00

COMPARATIVE EXAMPLE
107
YYY
0.060
1.31
1.75
0.008
0.0030
0.025
0.0025

0.00

COMPARATIVE EXAMPLE
108
ZZZ
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
109
AAAA
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
110
BBBB
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
111
CCCC
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
112
DDDD
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
113
EEEE
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

COMPARATIVE EXAMPLE
114
FFFF
0.060
1.25
1.90
0.007
0.0030
0.023
0.0021
0.13

EXAMPLE
115
GGGG
0.058
1.36
2.00
0.011
0.0045
0.027
0.0028
0.14

EXAMPLE
116
HHHH
0.065
1.09
1.91
0.006
0.0040
0.028
0.0029
0.13

EXAMPLE
117
IIII
0.057
1.05
1.86
0.008
0.0038
0.022
0.0020
0.12

EXAMPLE
118
JJJJ
0.068
1.13
1.80
0.005
0.0040
0.022
0.0025
0.14

EXAMPLE
119
KKKK
0.068
1.13
1.80
0.005
0.0040
0.022
0.0025
0.14

EXAMPLE
120
LLLL
0.057
1.05
1.86
0.008
0.0038
0.022
0.0020
0.12

EXAMPLE
121
MMMM
0.059
1.04
1.87
0.009
0.0040
0.024
0.0029
0.13

STEEL
CHEMICAL COMPOSITION (unit: mass %)

COMPOSITION
REM
Ca
※1
※2
Si + Al
OTHER ELEMENTS

EXAMPLE
82
EEE
0.0090

0.0000

30.86
∞
1.12
V = 0.12%

EXAMPLE
83
FFF

0.0000

0.0060
24.00
0.00
1.34
V = 0.13%

COMPARATIVE EXAMPLE
84
GGG

0.0000

0.0000

0.00
∞
1.63
V = 0.12%

COMPARATIVE EXAMPLE
85
HHH
0.0039
0.0038
58.26
0.29
1.53
—

COMPARATIVE EXAMPLE
86
JJJ
0.0001
0.0001
9.92
0.29
1.53
V = 0.1%

EXAMPLE
87
A
0.0040
0.0038
48.66
0.30
1.27
V = 0.015%

EXAMPLE
89
A
0.0040
0.0038
48.66
0.30
1.27
V = 0.015%

EXAMPLE
90
KKK
0.0040
0.0035
29.19
0.33
1.28
—

COMPARATIVE EXAMPLE
91
A
0.0040
0.0038
48.66
0.30
1.27
V = 0.015%

COMPARATIVE EXAMPLE
92
A
0.0040
0.0038
48.66
0.30
1.27
V = 0.015%

COMPARATIVE EXAMPLE
93
A
0.0040
0.0038
48.66
0.30
1.27
V = 0.015%

COMPARATIVE EXAMPLE
94
LLL
0.0040
0.0038
48.66
0.30
1.27
—

COMPARATIVE EXAMPLE
95
MMM
0.0040
0.0038
30.88
0.30
1.27
—

COMPARATIVE EXAMPLE
96
NNN
0.0040
0.0038
48.66
0.30

4.13

—

COMPARATIVE EXAMPLE
97
OOO
0.0040
0.0038
48.66
0.30

4.03

—

COMPARATIVE EXAMPLE
98
PPP
0.0040
0.0037
21.20
0.31

0.48

—

COMPARATIVE EXAMPLE
99
QQQ
0.0040
0.0037
21.20
0.31

0.45

—

COMPARATIVE EXAMPLE
100
RRR
0.0055
0.0050
37.39
0.31
1.19
—

COMPARATIVE EXAMPLE
101
SSS
0.0025
0.0020
65.90
0.36
1.37
—

COMPARATIVE EXAMPLE
102
TTT

0.0000

0.0000

51.67
∞
1.08
—

COMPARATIVE EXAMPLE
103
UUU

0.0250

0.0000

85.71
∞
1.12
—

COMPARATIVE EXAMPLE
104
VVV

0.0000

0.0130

52.00
0.00
1.34
—

COMPARATIVE EXAMPLE
105
WWW

0.0000

0.0000

0.13
∞
1.08
—

COMPARATIVE EXAMPLE
106
XXX
0.00008

0.0000

0.27
∞
1.12
—

COMPARATIVE EXAMPLE
107
YYY

0.0000

0.00009
0.36
0.00
1.34
—

COMPARATIVE EXAMPLE
108
ZZZ
0.0040
0.0038
48.66
0.30
1.27

Nb = 0.11%

COMPARATIVE EXAMPLE
109
AAAA
0.0040
0.0038
48.66
0.30
1.27

B = 0.0042%

COMPARATIVE EXAMPLE
110
BBBB
0.0040
0.0038
48.66
0.30
1.27

Cu = 1.1%

COMPARATIVE EXAMPLE
111
CCCC
0.0040
0.0038
48.66
0.30
1.27

Cr = 1.1%

COMPARATIVE EXAMPLE
112
DDDD
0.0040
0.0038
48.66
0.30
1.27

Mo = 1.1%

COMPARATIVE EXAMPLE
113
EEEE
0.0040
0.0038
48.66
0.30
1.27

Ni = 1.1%

COMPARATIVE EXAMPLE
114
FFFF
0.0040
0.0038
48.66
0.30
1.27

V = 0.22%

EXAMPLE
115
GGGG
0.0040
0.0034
32.86
0.34
1.39

Nb = 0.0008%

EXAMPLE
116
HHHH
0.0040
0.0037
36.20
0.31
1.12

B = 0.00009%

EXAMPLE
117
IIII
0.0027
0.0025
31.38
0.31
1.07

Cu = 0.0007%, Ni = 0.1%

EXAMPLE
118
JJJJ
0.01800

0.0000

38.76
∞
1.15

Cr = 0.0008%, Mo = 0.03%

EXAMPLE
119
KKKK
0.0180

0.0000

38.76
∞
1.15
Cr = 0.1%, Mo = 0.0008%

EXAMPLE
120
LLLL
0.0027
0.0025
31.38
0.31
1.07
Cu = 0.2%, N i= 0.0009%

EXAMPLE
121
MMMM
0.0031
0.0024
31.52
0.37
1.06

V = 0.0008%

The underlined value in the table indicates out of the range of the present invention.

The ※1 in the table indicates (Ti/48)/(S/32) + {(Ca/40)/(S/32) + (REM/140)/(S/32)} × 15.

The ※2 in the table indicates (REM/140)/(Ca/40).

TABLE 5

PRODUCTION CONDITIONS

FIRST ROUGH
SECOND ROUGH

Ar3
USAGE OF
HEATING
ROLLING PROCESS
ROLLING PROCESS

TRANSFOR-
REFINING
PROCESS
START

START
FINISH

STEEL
MATION
DESULFURIZING
HEATING
TEM-
FINISH

TEM-
TEM-

COM-
TEMPER-
AGENT IN
TEMPER-
PERA-
TEMPER-
REDUC-
PERA-
PERA-
REDUC-

POSI-
ATURE
SECONDARY
ATURE
TURE
ATURE
TION
TURE
TURE
TION

TION
(° C.)
REFINING
(° C.)
(° C.)
(° C.)
(%)
(° C.)
(° C.)
(%)

EXAMPLE
1
A
746
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE
2
B
754
use
1250
1250
1151
65
1150
1074
21

EXAMPLE
3
C
700
nonuse
1250
1250
1151
65
1150
1071
21

EXAMPLE
4
D
802
use
1250
1250
1151
65
1150
1077
21

EXAMPLE
5
E
760
nonuse
1250
1250
1151
65
1150
1075
21

EXAMPLE
6
F
736
use
1250
1250
1151
65
1150
1071
21

EXAMPLE
7
G
731
nonuse
1250
1250
1151
65
1150
1072
21

EXAMPLE
8
H
731
nonuse
1250
1250
1151
65
1150
1074
21

EXAMPLE
9
I
743
nonuse
1250
1250
1151
65
1150
1078
21

EXAMPLE
10
J
747
nonuse
1250
1250
1151
65
1150
1072
21

COMPARATIVE
11
K
753
nonuse
1250
1250
1151
65
1150
1074
21

EXAMPLE

COMPARATIVE
12
L
765
nonuse
1250
1250
1151
65
1150
1074
21

EXAMPLE

EXAMPLE
13
M
740
nonuse
1250
1250
1151
65
1150
1080
21

EXAMPLE
14
N
745
nonuse
1250
1250
1151
65
1150
1073
21

EXAMPLE
15
O
760
nonuse
1250
1250
1151
65
1150
1072
21

EXAMPLE
16
P
748
nonuse
1250
1250
1151
65
1150
1071
21

EXAMPLE
17
Q
752
nonuse
1250
1250
1151
65
1150
1078
21

EXAMPLE
18
R
748
nonuse
1250
1250
1151
65
1150
1073
21

EXAMPLE
19
S
746
nonuse
1250
1250
1151
65
1150
1079
21

EXAMPLE
20
T
752
nonuse
1250
1250
1151
65
1150
1078
21

PRODUCTION CONDITIONS

FINISH

SECOND
THIRD

ROLLING

COOLING PROCESS
COOLING PROCESS

PROCESS
FIRST

COOLING
COOLING

COOLING

START
FINISH
COOLING

START
FINISH

FINISH

STEEL
TEM-
TEM-
PROCESS

TEM-
TEM-

COOL-
TEM-

COM-
PERA-
PERA-
COOLING
COOLING
PERA-
PERA-
COOLING
ING
PERA-

POSI-
TURE
TURE
RATE
RATE
TURE
TURE
TIME
RATE
TURE

TION
(° C.)
(° C.)
(° C./sec.)
(° C./sec.)
(° C.)
(° C.)
(sec.)
(° C./sec.)
(° C.)

EXAMPLE
1
A
1012
887
29
10
750
670
8
29
25

EXAMPLE
2
B
1014
889
30
10
730
650
8
30
25

EXAMPLE
3
C
1011
895
33
10
720
650
7
33
25

EXAMPLE
4
D
1017
907
27
10
730
650
8
27
25

EXAMPLE
5
E
1015
888
32
10
700
650
5
32
25

EXAMPLE
6
F
1011
893
35
10
750
700
5
35
25

EXAMPLE
7
G
1012
891
31
10
750
690
6
31
25

EXAMPLE
8
H
1014
891
31
10
750
650
10
31
25

EXAMPLE
9
I
1018
892
27
10
750
670
8
27
25

EXAMPLE
10
J
1012
887
30
10
750
670
8
30
25

COMPARATIVE
11
K
1014
892
26
10
750
670
8
26
25

EXAMPLE

COMPARATIVE
12
L
1014
892
25
10
750
670
8
25
25

EXAMPLE

EXAMPLE
13
M
1020
892
30
10
750
670
8
30
25

EXAMPLE
14
N
1013
892
31
5
700
670
6
31
25

EXAMPLE
15
O
1012
892
28
10
750
670
8
28
25

EXAMPLE
16
P
1011
891
31
10
750
670
8
31
25

EXAMPLE
17
Q
1018
891
34
10
750
670
8
34
25

EXAMPLE
18
R
1013
890
30
10
750
670
8
30
25

EXAMPLE
19
S
1019
891
33
10
750
670
8
33
25

EXAMPLE
20
T
1018
893
29
10
750
670
8
29
25

PRODUCTION CONDITIONS

FIRST ROUGH
SECOND ROUGH

Ar3
USAGE OF
HEATING
ROLLING PROCESS
ROLLING PROCESS

TRANSFOR-
REFINING
PROCESS
START

START
FINISH

STEEL
MATION
DESULFURIZING
HEATING
TEM-
FINISH

TEM-
TEM-

COM-
TEMPER-
AGENT IN
TEMPER-
PERA-
TEMPER-
REDUC-
PERA-
PERA-
REDUC-

POSI-
ATURE
SECONDARY
ATURE
TURE
ATURE
TION
TURE
TURE
TION

TION
(° C.)
REFINING
(° C.)
(° C.)
(° C.)
(%)
(° C.)
(° C.)
(%)

EXAMPLE
21
U
737
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE
22
V
743
nonuse
1250
1250
1151
65
1150
1077
21

EXAMPLE
23
W
747
nonuse
1250
1250
1151
65
1150
1072
21

EXAMPLE
24
X
747
nonuse
1250
1260
1151
65
1150
1079
21

EXAMPLE
25
Y
747
nonuse
1250
1250
1151
65
1150
1072
21

COMPARATIVE
26
Z
746
nonuse
1250
1250
1151
65
1150
1073
21

EXAMPLE

COMPARATIVE
27
AA
761
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE

COMPARATIVE
28
BB
654
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE

EXAMPLE
29
A
746
nonuse
1170
1170
1151
65
1120
1078
21

COMPARATIVE
30
A
746
nonuse
1250
1250
1151
75
1150
1079
11

EXAMPLE

EXAMPLE
31
A
746
nonuse
1250
1250
1151
70
1150
1072
16

COMPARATIVE
32
A
746
nonuse
1250
1250
1151
58
1150
1080
28

EXAMPLE

EXAMPLE
33
A
746
nonuse
1250
1250
1151
61
1150
1072
25

EXAMPLE
34
A
746
nonuse
1248
1248
1151
67
1150
1076
10

COMPARATIVE
35
A
746
nonuse
1249
1249
1151
70
1150
1072
5

EXAMPLE

COMPARATIVE
36
A
746
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE

COMPARATIVE
37
A
746
nonuse
1250
1250
1151
65
1150
1074
21

EXAMPLE

COMPARATIVE
38
A
746
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE

COMPARATIVE
39
A
746
nonuse
1250
1250
1151
65
1150
1075
21

EXAMPLE

COMPARATIVE
40
A
746
nonuse
1250
1250
1151
65
1150
1075
21

EXAMPLE

COMPARATIVE
41
A
746
nonuse
1250
1250
1151
65
1150
1075
21

EXAMPLE

PRODUCTION CONDITIONS

FINISH

SECOND
THIRD

ROLLING

COOLING PROCESS
COOLING PROCESS

PROCESS
FIRST

COOLING
COOLING

COOLING

START
FINISH
COOLING

START
FINISH

FINISH

STEEL
TEM-
TEM-
PROCESS

TEM-
TEM-

COOL-
TEM-

COM-
PERA-
PERA-
COOLING
COOLING
PERA-
PERA-
COOLING
ING
PERA-

POSI-
TURE
TURE
RATE
RATE
TURE
TURE
TIME
RATE
TURE

TION
(° C.)
(° C.)
(° C./sec.)
(° C./sec.)
(° C.)
(° C.)
(sec.)
(° C./sec.)
(° C.)

EXAMPLE
21
U
1010
894
30
10
750
670
8
30
25

EXAMPLE
22
V
1017
892
32
10
750
670
8
32
25

EXAMPLE
23
W
1012
887
27
10
750
670
8
27
25

EXAMPLE
24
X
1019
889
28
10
750
670
8
28
25

EXAMPLE
25
Y
1012
893
33
10
750
670
8
33
25

COMPARATIVE
26
Z
1013
886
32
10
750
670
8
32
25

EXAMPLE

COMPARATIVE
27
AA
1010
887
25
10
750
670
8
25
25

EXAMPLE

COMPARATIVE
28
BB
1010
845
28
10
750
670
8
28
25

EXAMPLE

EXAMPLE
29
A
1018
889
26
10
750
670
8
26
25

COMPARATIVE
30
A
1019
891
27
10
750
670
8
27
25

EXAMPLE

EXAMPLE
31
A
1012
885
35
10
750
670
8
35
25

COMPARATIVE
32
A
1020
888
34
10
750
670
8
34
25

EXAMPLE

EXAMPLE
33
A
1012
892
26
10
750
670
8
26
25

EXAMPLE
34
A
1016
886
27
10
750
670
8
27
25

COMPARATIVE
35
A
1012
889
27
10
750
670
8
27
25

EXAMPLE

COMPARATIVE
36
A
960
880
30
10
750
670
8
30
25

EXAMPLE

COMPARATIVE
37
A
1014
800
34
10
750
670
8
34
25

EXAMPLE

COMPARATIVE
38
A
1010
970
26
10
750
670
8
26
25

EXAMPLE

COMPARATIVE
39
A
1015
880
30
10
750
670
8
30
25

EXAMPLE

COMPARATIVE
40
A
1015
880
30
10
750
670
8
30
400

EXAMPLE

COMPARATIVE
41
A
1015
880
30
10
750
670
8
15
25

EXAMPLE

TABLE 6

PRODUCTION CONDITIONS

USAGE OF

REFINING
HEATING

Ar3
DESULFURIZING
PROCESS
FIRST ROUGH ROLLING PROCESS
SECOND ROUGH ROLLING PROCESS

TRANSFORMATION
AGENT
HEATING
START
FINISH

START
FINISH

STEEL
TEMPERATURE
IN SECONDARY
TEMPERATURE
TEMPERATURE
TEMPERATURE
REDUCTION
TEMPERATURE
TEMPERATURE
REDUCTION

COMPOSITION
(° C.)
REFINING
(° C.)
(° C.)
(° C.)
(%)
(° C.)
(° C.)
(%)

EXAMPLE
42
CC
798
nonuse
1250
1250
1151
65
1150
1072
21

EXAMPLE
43
DD
798
use
1250
1250
1151
65
1150
1074
21

EXAMPLE
44
EE
769
nonuse
1250
1250
1151
65
1150
1071
21

EXAMPLE
45
FF
847
use
1250
1250
1151
65
1150
1077
21

EXAMPLE
46
GG
805
nonuse
1250
1250
1151
65
1150
1075
21

EXAMPLE
47
HH
776
use
1250
1250
1151
65
1150
1071
21

EXAMPLE
48
II
731
nonuse
1250
1250
1151
65
1150
1072
21

EXAMPLE
49
JJ
731
nonuse
1250
1250
1151
65
1150
1074
21

EXAMPLE
50
KK
787
use
1250
1250
1151
65
1150
1072
21

COMPARATIVE
51
LL
801
nonuse
1250
1250
1151
65
1150
1072
21

EXAMPLE

EXAMPLE
52
MM
787
nonuse
1250
1250
1151
65
1150
1078
21

EXAMPLE
53
NN
796
nonuse
1250
1250
1151
65
1150
1074
21

EXAMPLE
54
OO
784
nonuse
1250
1250
1151
65
1150
1080
21

EXAMPLE
55
PP
788
nonuse
1250
1250
1151
65
1150
1073
21

EXAMPLE
56
QQ
820
nonuse
1250
1250
1151
65
1150
1072
21

EXAMPLE
57
RR
793
nonuse
1250
1250
1151
65
1150
1071
21

EXAMPLE
58
SS
796
nonuse
1250
1250
1151
65
1150
1078
21

EXAMPLE
59
TT
792
nonuse
1250
1250
1151
65
1150
1073
21

EXAMPLE
60
UU
790
nonuse
1250
1250
1151
65
1150
1079
21

EXAMPLE
61
VV
797
nonuse
1250
1250
1151
65
1150
1078
21

PRODUCTION CONDITIONS

FINISH
FIRST

ROLLING
COOLING
SECOND COOLING PROCESS
THIRD COOLING PROCESS

PROCESS
PROCESS

COOLING
COOLING

COOLING

START
FINISH
COOLING
COOLING
START
FINISH
COOLING
COOLING
FINISH

STEEL
TEMPERATURE
TEMPERATURE
RATE
RATE
TEMPERATURE
TEMPERATURE
TIME
RATE
TEMPERATURE

COMPOSITION
(° C.)
(° C.)
(° C./sec.)
(° C./sec.)
(° C.)
(° C.)
(sec.)
(° C./sec.)
(° C.)

EXAMPLE
42
CC
1012
887
29
10
750
670
8
29
25

EXAMPLE
43
DD
1014
889
30
10
750
670
8
30
25

EXAMPLE
44
EE
1011
895
33
10
750
670
8
33
25

EXAMPLE
45
FF
1017
907
27
10
750
670
8
27
25

EXAMPLE
46
GG
1015
888
32
10
750
670
8
32
25

EXAMPLE
47
HH
1011
893
35
10
750
670
8
35
25

EXAMPLE
48
II
1012
891
31
10
750
670
8
31
25

EXAMPLE
49
JJ
1014
891
31
10
750
670
8
31
25

EXAMPLE
50
KK
1012
890
30
10
750
670
8
30
25

COMPARATIVE EXAMPLE
51
LL
1012
890
30
10
750
670
8
30
25

EXAMPLE
52
MM
1018
892
27
10
750
670
8
27
25

EXAMPLE
53
NN
1014
892
26
10
750
670
8
26
25

EXAMPLE
54
OO
1020
892
30
10
750
670
8
30
25

EXAMPLE
55
PP
1013
892
31
10
750
670
8
31
25

EXAMPLE
56
QQ
1012
892
28
10
750
670
8
28
25

EXAMPLE
57
RR
1011
891
31
10
750
670
8
31
25

EXAMPLE
58
SS
1018
891
34
10
750
670
8
34
25

EXAMPLE
59
TT
1013
890
30
10
750
670
8
30
25

EXAMPLE
60
UU
1019
891
33
10
750
670
8
33
25

EXAMPLE
61
VV
1018
893
29
10
750
670
8
29
25

PRODUCTION CONDITIONS

USAGE OF

REFINING
HEATING

Ar3
DESULFURIZING
PROCESS
FIRST ROUGH ROLLING PROCESS
SECOND ROUGH ROLLING PROCESS

TRANSFORMATION
AGENT
HEATING
START
FINISH

START
FINISH

STEEL
TEMPERATURE
IN SECONDARY
TEMPERATURE
TEMPERATURE
TEMPERATURE
REDUCTION
TEMPERATURE
TEMPERATURE
REDUCTION

COMPOSITION
(° C.)
REFINING
(° C.)
(° C.)
(° C.)
(%)
(° C.)
(° C.)
(%)

EXAMPLE
62
WW
778
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE
63
XX
788
nonuse
1250
1250
1151
65
1150
1077
21

EXAMPLE
64
YY
791
nonuse
1250
1250
1151
65
1150
1072
21

EXAMPLE
65
ZZ
791
nonuse
1250
1250
1151
65
1150
1079
21

EXAMPLE
66
AAA
791
nonuse
1250
1250
1151
65
1150
1072
21

COMPARATIVE
67
BBB
790
nonuse
1250
1250
1151
65
1150
1073
21

EXAMPLE

COMPARATIVE
68
CCC
805
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE

COMPARATIVE
69
DDD
654
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE

COMPARATIVE
70
CC
798
nonuse
1170
1170
1151
65
1120
1078
21

EXAMPLE

COMPARATIVE
71
CC
798
nonuse
1250
1250
1151
75
1150
1079
11

EXAMPLE

EXAMPLE
72
CC
798
nonuse
1250
1250
1151
70
1150
1072
16

COMPARATIVE
73
CC
798
nonuse
1250
1250
1151
58
1150
1080
28

EXAMPLE

EXAMPLE
74
CC
798
nonuse
1250
1250
1151
61
1150
1072
25

EXAMPLE
75
CC
798
nonuse
1248
1248
1151
67
1150
1076
10

COMPARATIVE
76
CC
798
nonuse
1249
1249
1151
70
1150
1072
8

EXAMPLE

COMPARATIVE
77
CC
798
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE

COMPARATIVE
78
CC
798
nonuse
1250
1250
1151
65
1150
1074
21

EXAMPLE

COMPARATIVE
79
CC
798
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE

COMPARATIVE
80
CC
798
nonuse
1250
1250
1151
65
1150
1075
21

EXAMPLE

COMPARATIVE
81
CC
798
nonuse
1250
1250
1151
65
1150
1075
21

EXAMPLE

PRODUCTION CONDITIONS

FINISH
FIRST

ROLLING
COOLING
SECOND COOLING PROCESS
THIRD COOLING PROCESS

PROCESS
PROCESS

COOLING
COOLING

COOLING

START
FINISH
COOLING
COOLING
START
FINISH
COOLING
COOLING
FINISH

STEEL
TEMPERATURE
TEMPERATURE
RATE
RATE
TEMPERATURE
TEMPERATURE
TIME
RATE
TEMPERATURE

COMPOSITION
(° C.)
(° C.)
(° C./sec.)
(° C./sec.)
(° C.)
(° C.)
(sec.)
(° C./sec.)
(° C.)

EXAMPLE
62
WW
1010
894
30
10
750
670
8
30
25

EXAMPLE
63
XX
1017
892
32
10
750
670
8
32
25

EXAMPLE
64
YY
1012
887
27
10
750
670
8
27
25

EXAMPLE
65
ZZ
1019
889
28
10
750
670
8
28
25

EXAMPLE
66
AAA
1012
893
33
10
750
670
8
33
25

COMPARATIVE EXAMPLE
67
BBB
1013
886
32
10
750
670
8
32
25

COMPARATIVE EXAMPLE
68
CCC
1010
887
25
10
750
670
8
25
25

COMPARATIVE EXAMPLE
69
DDD
1010
850
28
10
750
670
8
28
25

COMPARATIVE EXAMPLE
70
CC
1018
889
26
10
750
670
8
26
25

COMPARATIVE EXAMPLE
71
CC
1019
891
27
10
750
670
8
27
25

EXAMPLE
72
CC
1012
885
35
10
750
670
8
35
25

COMPARATIVE EXAMPLE
73
CC
1020
888
34
10
750
670
8
34
25

EXAMPLE
74
CC
1012
892
26
10
750
670
8
26
25

EXAMPLE
75
CC
1016
886
27
10
750
670
8
27
25

COMPARATIVE EXAMPLE
76
CC
1012
889
27
10
750
670
8
27
25

COMPARATIVE EXAMPLE
77
CC
960
880
30
10
750
670
8
30
25

COMPARATIVE EXAMPLE
78
CC
1014
820
34
10
750
670
8
34
25

COMPARATIVE EXAMPLE
79
CC
1010
1015
26
10
750
670
8
26
25

COMPARATIVE EXAMPLE
80
CC
1015
880
25
10
750
670
8
17
25

COMPARATIVE EXAMPLE
81
CC
1015
880
30
10
750
670
8
30
400

TABLE 7

PRODUCTION CONDITIONS

USAGE OF

REFINING
HEATING

Ar3
DESULFURIZING
PROCESS
FIRST ROUGH ROLLING PROCESS
SECOND ROUGH ROLLING PROCESS

TRANSFORMATION
AGENT IN
HEATING
START
FINISH

START
FINISH
RE-

STEEL
TEMPERATURE
SECONDARY
TEMPERATURE
TEMPERATURE
TEMPERATURE
REDUCTION
TEMPERATURE
TEMPERATURE
DUCTION

COMPOSITION
(° C.)
REFINING
(° C.)
(° C.)
(° C.)
(%)
(° C.)
(° C.)
(%)

EXAMPLE
82
EEE
749
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE
83
FFF
758
nonuse
1200
1200
1151
65
1150
1072
21

COMPARATIVE
84
GGG
848
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
85
HHH
793
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
86
JJJ
758
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

EXAMPLE
87
A
746
nonuse
1200
1200
1151
65
1150
1072
13

EXAMPLE
89
A
746
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE
90
KKK
743
nonuse
1200
1200
1151
65
1150
1072
21

COMPARATIVE
91
A
746
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
92
A
746
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
93
A
746
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
94
LLL
726
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
95
MMM
851
nonuse
1250
1250
1151
70
1150
1072
16

EXAMPLE

COMPARATIVE
96
NNN
779
nonuse
1248
1248
1151
67
1150
1076
10

EXAMPLE

COMPARATIVE
97
OOO
755
nonuse
1248
1248
1151
67
1150
1076
10

EXAMPLE

COMPARATIVE
98
PPP
782
nonuse
1250
1250
1151
65
1150
1074
21

EXAMPLE

COMPARATIVE
99
QQQ
782
nonuse
1250
1250
1151
65
1150
1074
21

EXAMPLE

COMPARATIVE
100
RRR
747
nonuse
1250
1250
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
101
SSS
798
use
1250
1250
1151
65
1150
1074
21

EXAMPLE

PRODUCTION CONDITIONS

FINISH
FIRST

ROLLING
COOLING
SECOND COOLING PROCESS
THIRD COOLING PROCESS

PROCESS
PROCESS

COOLING
COOLING

COOLING

START
FINISH
COOLING
COOLING
START
FINISH
COOLING
COOLING
FINISH

STEEL
TEMPERATURE
TEMPERATURE
RATE
RATE
TEMPERATURE
TEMPERATURE
TIME
RATE
TEMPERATURE

COMPOSITION
(° C.)
(° C.)
(° C./sec.)
(° C./sec.)
(° C.)
(° C.)
(sec.)
(° C./sec.)
(° C.)

EXAMPLE
82
EEE
1012
887
29
10
750
670
8
29
25

EXAMPLE
83
FFF
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
84
GGG
1012
910
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
85
HHH
1012
887
29
25
750
670
3.2
29
25

COMPARATIVE EXAMPLE
86
JJJ
1012
887
29
10
750
670
8
29
25

EXAMPLE
87
A
1012
911
25
10
750
670
8
29
25

EXAMPLE
89
A
1012
887
29
8
730
650
10
21
100

EXAMPLE
90
KKK
1012
927
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
91
A
1012
887
29
10
830
670
8
29
25

COMPARATIVE EXAMPLE
92
A
1012
887
29
6
750
751
15
29
25

COMPARATIVE EXAMPLE
93
A
1012
887
29
14
740
660
0.7
29
25

COMPARATIVE EXAMPLE
94
LLL
1012
887
29
10
750
730
8
29
25

COMPARATIVE EXAMPLE
95
MMM
1012
915
35
10
750
670
8
35
25

COMPARATIVE EXAMPLE
96
NNN
1016
886
27
10
750
670
8
27
25

COMPARATIVE EXAMPLE
97
OOO
1016
886
27
10
750
670
8
27
25

COMPARATIVE EXAMPLE
98
PPP
1014
892
26
10
750
670
8
26
25

COMPARATIVE EXAMPLE
99
QQQ
1014
892
26
10
750
670
8
26
25

COMPARATIVE EXAMPLE
100
RRR
1012
887
30
10
750
670
8
30
25

COMPARATIVE EXAMPLE
101
SSS
1014
889
30
10
750
670
8
30
25

PRODUCTION CONDITIONS

USAGE OF

REFINING
HEATING

Ar3
DESULFURIZING
PROCESS
FIRST ROUGH ROLLING PROCESS
SECOND ROUGH ROLLING PROCESS

TRANSFORMATION
AGENT IN
HEATING
START
FINISH

START
FINISH
RE-

STEEL
TEMPERATURE
SECONDARY
TEMPERATURE
TEMPERATURE
TEMPERATURE
REDUCTION
TEMPERATURE
TEMPERATURE
DUCTION

COMPOSITION
(° C.)
REFINING
(° C.)
(° C.)
(° C.)
(%)
(° C.)
(° C.)
(%)

COMPARATIVE
102
TTT
734
nonuse
1250
1250
1151
65
1150
1071
21

EXAMPLE

COMPARATIVE
103
UUU
749
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
104
VVV
758
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
105
WWW
778
nonuse
1250
1250
1151
65
1150
1071
21

EXAMPLE

COMPARATIVE
106
XXX
749
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
107
YYY
758
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
108
ZZZ
746
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
109
AAAA
746
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
110
BBBB
723
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
111
CCCC
719
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
112
DDDD
779
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
113
EEEE
707
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

COMPARATIVE
114
FFFF
746
nonuse
1200
1200
1151
65
1150
1072
21

EXAMPLE

EXAMPLE
115
GGGG
743
nonuse
1250
1250
1151
65
1150
1078
21

EXAMPLE
116
HHHH
740
nonuse
1250
1250
1151
65
1150
1080
21

EXAMPLE
117
IIII
742
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE
118
JJJJ
748
nonuse
1250
1250
1151
65
1150
1073
21

EXAMPLE
119
KKKK
744
nonuse
1250
1250
1151
65
1150
1073
21

EXAMPLE
120
LLLL
741
nonuse
1250
1250
1151
65
1150
1070
21

EXAMPLE
121
MMMM
743
nonuse
1250
1250
1151
65
1150
1077
21

PRODUCTION CONDITIONS

FINISH
FIRST

ROLLING
COOLING
SECOND COOLING PROCESS
THIRD COOLING PROCESS

PROCESS
PROCESS

COOLING
COOLING

COOLING

START
FINISH
COOLING
COOLING
START
FINISH
COOLING
COOLING
FINISH

STEEL
TEMPERATURE
TEMPERATURE
RATE
RATE
TEMPERATURE
TEMPERATURE
TIME
RATE
TEMPERATURE

COMPOSITION
(° C.)
(° C.)
(° C./sec.)
(° C./sec.)
(° C.)
(° C.)
(sec.)
(° C./sec.)
(° C.)

COMPARATIVE EXAMPLE
102
TTT
1011
895
33
10
720
650
7
33
25

COMPARATIVE EXAMPLE
103
UUU
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
104
VVV
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
105
WWW
1011
895
33
10
750
670
8
33
25

COMPARATIVE EXAMPLE
106
XXX
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
107
YYY
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
108
ZZZ
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
109
AAAA
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
110
BBBB
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
111
CCCC
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
112
DDDD
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
113
EEEE
1012
887
29
10
750
670
8
29
25

COMPARATIVE EXAMPLE
114
FFFF
1012
887
29
10
750
670
8
29
25

EXAMPLE
115
GGGG
1018
892
27
10
750
670
8
27
25

EXAMPLE
116
HHHH
1020
892
30
10
750
670
8
30
25

EXAMPLE
117
IIII
1010
894
30
10
750
670
8
30
25

EXAMPLE
118
JJJJ
1013
892
31
5
700
670
6
31
25

EXAMPLE
119
KKKK
1013
892
31
5
700
670
6
31
25

EXAMPLE
120
LLLL
1010
894
30
10
750
670
8
30
25

EXAMPLE
121
MMMM
1017
892
32
10
750
670
8
32
25

TABLE 8

METALLOGRAPHIC STRUCTURE

SECONDARY PHASE

PRIMARY PHASE
MARTENSITE (M) AND

FERRITE (F)
RESIDUAL AUSTENITE (γ)

AVERAGE

AVERAGE
AREA
AREA

GRAIN

GRAIN
FRACTION
FRACTION

STEEL

AREA
SIZE
AREA FRACTION
SIZE
OF BAINITE
OF PEARLITE

COMPOSITION
CONSTITUENT METALLIC PHASE
FRACTION F
(μm)
M (%)
γ (%)
M + γ (%)
(μm)
(%)
(%)

EXAMPLE
1
A
ferrite, martensite, residual austenite
93.1
4.22
5.9
1.0
6.9
3.4
0.00
0.00

EXAMPLE
2
B
ferrite, martensite
93.8
4.25
6.2
0.0
6.2
3.4
0.00
0.00

EXAMPLE
3
C
ferrite, martensite, residual austenite
92.8
4.22
5.7
1.5
7.2
3.4
0.00
0.00

EXAMPLE
4
D
ferrite, martensite, residual austenite
93.5
4.16
4.7
1.8
6.5
3.3
0.00
0.00

EXAMPLE
5
E
ferrite, martensite, residual austenite
92.4
4.19
6.1
1.5
7.6
3.4
0.00
0.00

EXAMPLE
6
F
ferrite, martensite, residual austenite
90.5
4.20
7.4
2.1
9.5
3.4
0.00
0.00

EXAMPLE
7
G
ferrite, martensite, residual austenite
93.0
4.19
5.5
1.5
7.0
3.4
0.00
0.00

EXAMPLE
8
H
ferrite, martensite, residual austenite
93.1
4.20
6.1
0.8
6.9
3.4
0.00
0.00

EXAMPLE
9
I
ferrite, martensite, residual austenite
93.4
3.60
5.4
1.2
6.6
2.9
0.00
0.00

EXAMPLE
10
J
ferrite, martensite, residual austenite
93.5
4.21
5.8
1.0
6.8
3.4
0.00
0.00

COMPARATIVE EXAMPLE
11
K
ferrite, martensite, residual austenite
97.3

10.04
1.9
0.8
2.7
7.1
0.00
0.00

COMPARATIVE EXAMPLE
12
L
ferrite, martensite

99.1

10.21
0.9
0.0

0.9

7.7
0.00
0.00

EXAMPLE
13
M
ferrite, martensite, residual austenite
92.4
3.90
7.1
0.5
7.6
3.1
0.00
0.00

EXAMPLE
14
N
ferrite, martensite, residual austenite
92.1
4.21
6.4
1.5
7.9
3.4
0.00
0.00

EXAMPLE
15
O
ferrite, martensite, residual austenite
93.1
4.17
5.5
1.4
6.9
3.3
0.00
0.00

EXAMPLE
16
P
ferrite, martensite, residual austenite
93.0
4.21
5.8
1.2
7.0
3.4
0.00
0.00

EXAMPLE
17
Q
ferrite, martensite, residual austenite
92.8
4.18
5.9
1.3
7.2
3.3
0.00
0.00

EXAMPLE
18
R
ferrite, martensite, residual austenite
93.8
4.20
4.8
1.4
6.2
3.4
0.00
0.00

EXAMPLE
19
S
ferrite, martensite, residual austenite
93.2
4.17
5.3
1.5
6.8
3.3
0.00
0.00

EXAMPLE
20
T
ferrite, martensite, residual austenite
93.1
4.25
6.0
0.9
6.9
3.4
0.00
0.00

INCLUSIONS

TEXTURE
AVERAGE OF

X-RAY
MAXIMUM OF
TOTAL
NUMBER

RANDOM
RATIO OF
LENGTH M
PERCENTAGE

INTENSITY
MAJOR
IN ROLLING
OF MnS

STEEL
RATIO OF
AXIS TO
DIRECTION
AND CaS

COMPOSITION
{211} PLANE
MINOR AXIS
(mm/mm²)
(%)
ELONGATED INCLUSIONS OBSERVED MAINLY

EXAMPLE
1
A
2.31
3.0
0.03
5.00
calcium aluminate

EXAMPLE
2
B
2.30
1.5
0.04
5.00
calcium aluminate, residual desulfurizing agent

EXAMPLE
3
C
2.25
1.0
0.00
—
none

EXAMPLE
4
D
2.32
1.5
0.02
10.00
residual desulfurizing agent

EXAMPLE
5
E
2.31
4.5
0.00
—
none

EXAMPLE
6
F
2.27
4.5
0.02
10.00
residual desulfurizing agent

EXAMPLE
7
G
2.00
1.0
0.00
—
none

EXAMPLE
8
H
2.05
1.0
0.00
—
none

EXAMPLE
9
I
2.27
2.8
0.14
5.00
calcium aluminate

EXAMPLE
10
J
2.32
2.9
0.18
5.00
calcium aluminate

COMPARATIVE EXAMPLE
11
K
2.27
3.0
0.12
4.00
calcium aluminate

COMPARATIVE EXAMPLE
12
L
2.10
3.0
0.11
4.00
calcium aluminate

EXAMPLE
13
M
2.27
3.0
0.12
7.00
calcium aluminate

EXAMPLE
14
N
2.27
1.0
0.00
—
none

EXAMPLE
15
O
2.28
8.0
0.13
25.00
calcium aluminate, CaS

EXAMPLE
16
P
2.29
8.0
0.19
25.00
calcium aluminate, CaS

EXAMPLE
17
Q
2.28
7.0
0.23
25.00
calcium aluminate, CaS

EXAMPLE
18
R
2.29
5.8
0.14
25.00
calcium aluminate, CaS

EXAMPLE
19
S
2.28
4.8
0.12
25.00
calcium aluminate, CaS

EXAMPLE
20
T
2.26
4.0
0.11
25.00
calcium aluminate, CaS

METALLOGRAPHIC STRUCTURE

SECONDARY PHASE

PRIMARY PHASE
MARTENSITE (M) AND

FERRITE (F)
RESIDUAL AUSTENITE (γ)

AVERAGE

AVERAGE
AREA
AREA

GRAIN

GRAIN
FRACTION
FRACTION

STEEL

AREA
SIZE
AREA FRACTION
SIZE
OF BAINITE
OF PEARLITE

COMPOSITION
CONSTITUENT METALLIC PHASE
FRACTION F
(μm)
M (%)
γ (%)
M + γ (%)
(μm)
(%)
(%)

EXAMPLE
21
U
ferrite, martensite, residual austenite
93.5
4.19
5.3
1.2
6.5
3.3
0.00
0.00

EXAMPLE
22
V
ferrite, martensite, residual austenite
93.2
4.22
5.9
0.9
6.8
3.4
0.00
0.00

EXAMPLE
23
W
ferrite, martensite, residual austenite
92.8
4.20
6.0
1.2
7.2
3.4
0.00
0.00

EXAMPLE
24
X
ferrite, martensite, residual austenite
93.0
4.20
6.4
0.6
7.0
3.4
0.00
0.00

EXAMPLE
25
Y
ferrite, martensite, residual austenite
93.1
4.20
6.2
0.7
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
26
Z
ferrite, martensite, residual austenite
93.0
4.20
5.8
1.2
7.0
3.4
0.00
0.00

COMPARATIVE EXAMPLE
27
AA
ferrite, martensite, residual austenite
93.8
4.15
5.6
0.6
6.2
3.3
0.00
0.00

COMPARATIVE EXAMPLE
28
BB
ferrite, martensite, residual austenite

83.7

4.15
12.8
3.5

16.3
3.3
0.00
0.00

EXAMPLE
29
A
ferrite, martensite, residual austenite
93.1
4.24
5.7
1.2
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
30
A
ferrite, martensite, residual austenite
93.1
4.20
6.1
0.8
6.9
3.4
0.00
0.00

EXAMPLE
31
A
ferrite, martensite, residual austenite
93.1
4.20
6.5
0.4
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
32
A
ferrite, martensite, residual austenite
93.1
3.90
5.1
1.8
6.9
3.1
0.00
0.00

EXAMPLE
33
A
ferrite, martensite, residual austenite
93.1
4.20
5.5
1.4
6.9
3.4
0.00
0.00

EXAMPLE
34
A
ferrite, martensite, residual austenite
93.1
6.00
6.1
0.8
6.9
4.8
0.00
0.00

COMPARATIVE EXAMPLE
35
A
ferrite, martensite, residual austenite
93.1

10.20
5.5
1.4
6.9
7.8
0.00
0.00

COMPARATIVE EXAMPLE
36
A
ferrite, martensite, residual austenite
93.1
3.70
5.7
1.2
6.9
3.0
0.00
0.00

COMPARATIVE EXAMPLE
37
A
ferrite, martensite, residual austenite
93.1
3.70
5.9
1.0
6.9
3.0
0.00
0.00

COMPARATIVE EXAMPLE
38
A
ferrite, martensite, residual austenite
93.1

10.05
5.8
1.1
6.9
7.7
0.00
0.00

COMPARATIVE EXAMPLE
39
A
ferrite, martensite, residual austenite
93.0

10.10
6.0
1.0
7.0
7.5
0.00
0.00

COMPARATIVE EXAMPLE
40
A
ferrite, bainite
95.5
4.90
0.0
0.0

0.0

3.9
4.50
0.00

COMPARATIVE EXAMPLE
41
A
ferrite, pearlite, bainite
94.5
5.50
0.0
0.0

0.0

4.4
3.50
2.00

INCLUSIONS

TEXTURE
AVERAGE OF

X-RAY
MAXIMUM OF
TOTAL
NUMBER

RANDOM
RATIO OF
LENGTH M
PERCENTAGE

INTENSITY
MAJOR
IN ROLLING
OF MnS

STEEL
RATIO OF
AXIS TO
DIRECTION
AND CaS

COMPOSITION
{211} PLANE
MINOR AXIS
(mm/mm²)
(%)
ELONGATED INCLUSIONS OBSERVED MAINLY

EXAMPLE
21
U
2.26
2.8
0.21
20.00
calcium aluminate

EXAMPLE
22
V
2.27
2.0
0.20
20.00
calcium aluminate

EXAMPLE
23
W
2.31
1.0
0.10
7.00
calcium aluminate

EXAMPLE
24
X
2.30
1.0
0.00
5.00
calcium aluminate

EXAMPLE
25
Y
2.26
3.0
0.25
20.00
calcium aluminate

COMPARATIVE EXAMPLE
26
Z
2.32
4.0

0.40

50.00
calcium aluminate, MnS

COMPARATIVE EXAMPLE
27
AA
2.25

9.0

0.30

75.00
MnS

COMPARATIVE EXAMPLE
28
BB
2.32
1.3
0.24
10.00
calcium aluminate

EXAMPLE
29
A
2.30
3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
30
A
2.30

9.0

0.48

5.00
calcium aluminate

EXAMPLE
31
A
2.30
8.0
0.25
5.00
calcium aluminate

COMPARATIVE EXAMPLE
32
A

2.50

3.0
0.25
5.00
calcium aluminate

EXAMPLE
33
A
2.40
2.9
0.24
5.00
calcium aluminate

EXAMPLE
34
A
2.30
5.0
0.15
5.00
calcium aluminate

COMPARATIVE EXAMPLE
35
A
2.25
7.0
0.20
5.00
calcium aluminate

COMPARATIVE EXAMPLE
36
A

2.60

3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
37
A

3.46

3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
38
A
1.84
3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
39
A
2.38
3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
40
A
2.38
3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
41
A
2.38
3.0
0.06
5.00
calcium aluminate

The underlined value in the table indicates out of the range of the present invention.

TABLE 9

METALLOGRAPHIC STRUCTURE

SECONDARY PHASE

PRIMARY PHASE
MARTENSITE (M) AND

FERRITE (F)
RESIDUAL AUSTENITE (γ)

AVERAGE

AVERAGE
AREA
AREA

GRAIN

GRAIN
FRACTION
FRACTION

STEEL

AREA
SIZE
AREA FRACTION
SIZE
OF BAINITE
OF PEARLITE

COMPOSITION
CONSTITUENT METALLIC PHASE
FRACTION F
(μm)
M (%)
γ (%)
M + γ (%)
(μm)
(%)
(%)

EXAMPLE
42
CC
ferrite, martensite, residual austenite
95.7
5.22
3.2
1.1
4.3
4.2
0.00
0.00

EXAMPLE
43
DD
ferrite, martensite, residual austenite
93.8
4.25
4.7
1.6
6.2
3.4
0.00
0.00

EXAMPLE
44
EE
ferrite, martensite, residual austenite
92.8
4.22
5.4
1.8
7.2
3.4
0.00
0.00

EXAMPLE
45
FF
ferrite, martensite, residual austenite
93.5
4.16
4.9
1.6
6.5
3.3
0.00
0.00

EXAMPLE
46
GG
ferrite, martensite, residual austenite
92.4
4.19
5.7
1.9
7.6
3.4
0.00
0.00

EXAMPLE
47
HH
ferrite, martensite, residual austenite
90.5
4.20
7.1
2.4
9.5
3.4
0.00
0.00

EXAMPLE
48
II
ferrite, martensite, residual austenite
93.0
4.19
5.3
1.8
7.0
3.4
0.00
0.00

EXAMPLE
49
JJ
ferrite, martensite, residual austenite
93.1
4.20
5.2
1.7
6.9
3.4
0.00
0.00

EXAMPLE
50
KK
ferrite, martensite, residual austenite
93.5
5.40
4.9
1.6
6.5
4.3
0.00
0.00

COMPARATIVE EXAMPLE
51
LL
ferrite, martensite, residual austenite

99.1

10.09
0.7
0.2

0.9

7.7
0.00
0.00

EXAMPLE
52
MM
ferrite, martensite, residual austenite
93.4
4.30
5.0
1.7
6.6
3.4
0.00
0.00

EXAMPLE
53
NN
ferrite, martensite, residual austenite
96.9
5.90
2.3
0.8
3.1
4.7
0.00
0.00

EXAMPLE
54
OO
ferrite, martensite, residual austenite
92.4
4.22
5.7
1.9
7.6
3.4
0.00
0.00

EXAMPLE
55
PP
ferrite, martensite, residual austenite
92.1
4.21
6.0
2.0
7.9
3.4
0.00
0.00

EXAMPLE
56
QQ
ferrite, martensite, residual austenite
93.1
4.17
5.2
1.7
6.9
3.3
0.00
0.00

EXAMPLE
57
RR
ferrite, martensite, residual austenite
93.0
4.21
5.3
1.8
7.0
3.4
0.00
0.00

EXAMPLE
58
SS
ferrite, martensite, residual austenite
92.8
4.18
5.4
1.8
7.2
3.3
0.00
0.00

EXAMPLE
59
TT
ferrite, martensite, residual austenite
93.8
4.20
4.7
1.6
6.2
3.4
0.00
0.00

EXAMPLE
60
UU
ferrite, martensite, residual austenite
93.2
4.17
5.1
1.7
6.8
3.3
0.00
0.00

EXAMPLE
61
VV
ferrite, martensite, residual austenite
93.1
4.25
5.2
1.7
6.9
3.4
0.00
0.00

INCLUSIONS

TEXTURE
AVERAGE OF

X-RAY
MAXIMUM OF
TOTAL
NUMBER

RANDOM
RATIO OF
LENGTH M
PERCENTAGE

INTENSITY
MAJOR
IN ROLLING
OF MnS

STEEL
RATIO OF
AXIS TO
DIRECTION
AND CaS

COMPOSITION
{211} PLANE
MINOR AXIS
(mm/mm²)
(%)
ELONGATED INCLUSIONS OBSERVED MAINLY

EXAMPLE
42
CC
2.31
3.0
0.03
5.00
calcium aluminate

EXAMPLE
43
DD
2.30
1.5
0.04
5.00
calcium aluminate, residual desulfurizing agent

EXAMPLE
44
EE
2.25
1.0
0.22
—
none

EXAMPLE
45
FF
2.32
1.5
0.02
5.00
residual desulfurizing agent

EXAMPLE
46
GG
2.31
4.5
0.24
—
none

EXAMPLE
47
HH
2.27
4.5
0.02
5.00
residual desulfurizing agent

EXAMPLE
48
II
2.00
1.0
0.17
—
none

EXAMPLE
49
JJ
2.05
1.0
0.18
—
none

EXAMPLE
50
KK
2.30
2.0
0.05
17.50
residual desulfurizing agent, CaS

COMPARATIVE EXAMPLE
51
LL
2.30
2.0
0.10
20.00
calcium aluminate, CaS

EXAMPLE
52
MM
2.27
2.8
0.14
22.50
calcium aluminate, REM oxide, CaS

EXAMPLE
53
NN
2.27
3.0
0.12
20.00
calcium aluminate, REM oxide, CaS

EXAMPLE
54
OO
2.27
3.0
0.12
20.00
calcium aluminate, REM oxide, CaS

EXAMPLE
55
PP
2.27
1.0
0.18
—
none

EXAMPLE
56
QQ
2.28
8.0
0.13
20.00
calcium aluminate, CaS

EXAMPLE
57
RR
2.29
8.0
0.19
20.00
calcium aluminate, CaS

EXAMPLE
58
SS
2.28
7.0
0.23
20.00
calcium aluminate, CaS

EXAMPLE
59
TT
2.29
5.8
0.14
20.00
calcium aluminate, CaS

EXAMPLE
60
UU
2.28
4.8
0.12
20.00
calcium aluminate, CaS

EXAMPLE
61
VV
2.26
4.0
0.11
20.00
calcium aluminate, CaS

METALLOGRAPHIC STRUCTURE

SECONDARY PHASE

PRIMARY PHASE
MARTENSITE (M) AND

FERRITE (F)
RESIDUAL AUSTENITE (γ)

AVERAGE

AVERAGE
AREA
AREA

GRAIN

GRAIN
FRACTION
FRACTION

STEEL

AREA
SIZE
AREA FRACTION
SIZE
OF BAINITE
OF PEARLITE

COMPOSITION
CONSTITUENT METALLIC PHASE
FRACTION F
(μm)
M (%)
γ (%)
M + γ (%)
(μm)
(%)
(%)

EXAMPLE
62
WW
ferrite, martensite, residual austenite
93.5
4.19
4.9
1.6
6.5
3.3
0.00
0.00

EXAMPLE
63
XX
ferrite, martensite, residual austenite
93.2
4.22
5.1
1.7
6.8
3.4
0.00
0.00

EXAMPLE
64
YY
ferrite, martensite, residual austenite
92.8
4.20
5.4
1.8
7.2
3.4
0.00
0.00

EXAMPLE
65
ZZ
ferrite, martensite, residual austenite
93.0
4.20
5.3
1.8
7.0
3.4
0.00
0.00

EXAMPLE
66
AAA
ferrite, martensite, residual austenite
93.1
4.20
5.2
1.7
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
67
BBB
ferrite, martensite, residual austenite
93.0
4.20
5.3
1.8
7.0
3.4
0.00
0.00

COMPARATIVE EXAMPLE
68
CCC
ferrite, martensite, residual austenite
93.8
4.15
4.7
1.6
6.2
3.3
0.00
0.00

COMPARATIVE EXAMPLE
69
DDD
ferrite, martensite, residual austenite

83.7

4.15
12.2
4.1

16.3
3.3
0.00
0.00

COMPARATIVE EXAMPLE
70
CC
ferrite, martensite, residual austenite
95.7
4.24
3.2
1.1
4.3
3.4
0.00
0.00

COMPARATIVE EXAMPLE
71
CC
ferrite, martensite, residual austenite
93.1
4.20
5.2
1.7
6.9
3.4
0.00
0.00

EXAMPLE
72
CC
ferrite, martensite, residual austenite
93.1
4.20
5.2
1.7
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
73
CC
ferrite, martensite, residual austenite
93.1
3.90
5.2
1.7
6.9
3.1
0.00
0.00

EXAMPLE
74
CC
ferrite, martensite, residual austenite
93.1
4.20
5.2
1.7
6.9
3.4
0.00
0.00

EXAMPLE
75
CC
ferrite, martensite, residual austenite
93.1
6.00
5.2
1.7
6.9
4.8
0.00
0.00

COMPARATIVE EXAMPLE
76
CC
ferrite, martensite, residual austenite
93.1

10.10
5.2
1.7
6.9
7.8
0.00
0.00

COMPARATIVE EXAMPLE
77
CC
ferrite, martensite, residual austenite
93.1
3.70
5.2
1.7
6.9
3.0
0.00
0.00

COMPARATIVE EXAMPLE
78
CC
ferrite, martensite, residual austenite
93.1
3.70
5.2
1.7
6.9
3.0
0.00
0.00

COMPARATIVE EXAMPLE
79
CC
ferrite, martensite, residual austenite
93.1

10.08
5.2
1.7
6.9
7.6
0.00
0.00

COMPARATIVE EXAMPLE
80
CC
ferrite, pearlite, bainite
93.9

10.10
0.0
0.0

0.0

7.7
5.10
1.00

COMPARATIVE EXAMPLE
81
CC
ferrite, bainite
95.2
4.90
0.0
0.0

0.0

3.9
4.80
0.00

INCLUSIONS

TEXTURE
AVERAGE OF

X-RAY
MAXIMUM OF
TOTAL
NUMBER

RANDOM
RATIO OF
LENGTH M
PERCENTAGE

INTENSITY
MAJOR
IN ROLLING
OF MnS

STEEL
RATIO OF
AXIS TO
DIRECTION
AND CaS

COMPOSITION
{211} PLANE
MINOR AXIS
(mm/mm²)
(%)
ELONGATED INCLUSIONS OBSERVED MAINLY

EXAMPLE
62
WW
2.26
2.8
0.21
19.00
calcium aluminate, REM oxide, CaS

EXAMPLE
63
XX
2.27
2.0
0.20
10.00
calcium aluminate

EXAMPLE
64
YY
2.31
1.0
0.10
10.00
calcium aluminate

EXAMPLE
65
ZZ
2.30
1.0
0.00
17.50
calcium aluminate, REM oxide, CaS

EXAMPLE
66
AAA
2.26
3.0
0.25
21.50
calcium aluminate, REM oxide, CaS

COMPARATIVE EXAMPLE
67
BBB
2.32
4.0

0.40

40.00
calcium aluminate, MnS

COMPARATIVE EXAMPLE
68
CCC
2.25

9.0

0.45

75.00
MnS

COMPARATIVE EXAMPLE
69
DDD
2.32
1.3
0.24
10.00
calcium aluminate

COMPARATIVE EXAMPLE
70
CC
2.30
3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
71
CC
2.30

9.0

0.48

5.00
calcium aluminate

EXAMPLE
72
CC
2.30
8.0
0.25
5.00
calcium aluminate

COMPARATIVE EXAMPLE
73
CC

2.50

3.0
0.25
5.00
calcium aluminate

EXAMPLE
74
CC
2.40
2.9
0.24
5.00
calcium aluminate

EXAMPLE
75
CC
2.30
5.0
0.15
5.00
calcium aluminate

COMPARATIVE EXAMPLE
76
CC
2.25
7.0
0.20
5.00
calcium aluminate

COMPARATIVE EXAMPLE
77
CC

2.60

3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
78
CC

3.46

3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
79
CC
1.84
3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
80
CC
2.38
3.0
0.06
5.00
calcium aluminate

COMPARATIVE EXAMPLE
81
CC
2.38
3.0
0.06
5.00
calcium aluminate

The underlined value in the table indicates out of the range of the present invention.

TABLE 10

METALLOGRAPHIC STRUCTURE

SECONDARY PHASE

PRIMARY PHASE
MARTENSITE (M) AND

FERRITE (F)
RESIDUAL AUSTENITE (γ)

AREA

AVERAGE

AVERAGE
AREA
FRACTION

GRAIN
AREA FRACTION
GRAIN
FRACTION
OF

STEEL

AREA
SIZE
M
γ
M + γ
SIZE
OF BAINITE
PEARLITE

COMPOSITION
CONSTITUENT METALLIC PHASE
FRACTION F
(μm)
(%)
(%)
(%)
(μm)
(%)
(%)

EXAMPLE
82
EEE
ferrite, martensite, residual austenite
93.1
4.60
5.8
1.1
6.9
3.7
0.00
0.00

EXAMPLE
83
FFF
ferrite, martensite, residual austenite
93.9
5.10
5.6
0.5
6.1
4.1
0.00
0.00

COMPARATIVE EXAMPLE
84
GGG
ferrite, martensite
93.9
5.20
6.1
0.0
6.1
4.2
0.00
0.00

COMPARATIVE EXAMPLE
85
HHH
ferrite, martensite, residual austenite

88.5

4.20
9.0
2.5

11.5
3.4
0.00
0.00

COMPARATIVE EXAMPLE
86
JJJ
ferrite, martensite, residual austenite
94.1
4.50
5.4
0.5
5.9
3.6
0.00
0.00

EXAMPLE
87
A
ferrite, martensite, residual austenite
92.6
9.80
6.4
1.0
7.4
7.8
0.00
0.00

EXAMPLE
89
A
ferrite, martensite, pearlite, bainite
91.0
4.50
4.5
0.0
4.5
3.6
2.00
2.50

EXAMPLE
90
KKK
ferrite, martensite, residual austenite
95.5
9.40
4.0
0.5
4.5
7.8
0.00
0.00

COMPARATIVE EXAMPLE
91
A
ferrite, martensite, residual austenite

89.0

4.90
9.0
2.0

11.0
3.9
0.00
0.00

COMPARATIVE EXAMPLE
92
A
ferrite, martensite, residual austenite, pearlite, bainite

89.0

5.20
2.0
1.0
3.0
4.2
2.00
6.00

COMPARATIVE EXAMPLE
93
A
ferrite, martensite, residual austenite

89.0

4.00
8.0
3.0

11.0
3.2
0.00
0.00

COMPARATIVE EXAMPLE
94
LLL
ferrite, martensite, residual austenite
93.1
4.22
5.9
1.0
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
95
MMM
ferrite, martensite, residual austenite
93.1
4.20
5.2
1.7
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
96
NNN
ferrite, martensite, residual austenite
93.1
6.00
6.1
0.8
6.9
4.8
0.00
0.00

COMPARATIVE EXAMPLE
97
OOO
ferrite, martensite, residual austenite
93.1
6.00
6.1
0.8
6.9
4.8
0.00
0.00

COMPARATIVE EXAMPLE
98
PPP
ferrite, martensite, residual austenite

88.7

5.90
8.9
2.4

11.3
4.7
0.00
0.00

COMPARATIVE EXAMPLE
99
QQQ
ferrite, martensite, residual austenite

87.6

5.90
9.5
2.9

12.4
4.7
0.00
0.00

COMPARATIVE EXAMPLE
100
RRR
ferrite, martensite, residual austenite
93.2
4.21
5.8
1.0
6.8
3.4
0.00
0.00

COMPARATIVE EXAMPLE
101
SSS
ferrite, martensite, residual austenite
93.8
4.25
4.7
1.6
6.2
3.4
0.00
0.00

INCLUSIONS

TEXTURE
AVERAGE OF

X-RAY
MAXIMUM OF
TOTAL
NUMBER

RANDOM
RATIO OF
LENGTH M
PERCENTAGE

INTENSITY
MAJOR
IN ROLLING
OF MnS

STEEL
RATIO OF
AXIS TO
DIRECTION
AND CaS

COMPOSITION
{211} PLANE
MINOR AXIS
(mm/mm²)
(%)
ELONGATED INCLUSIONS OBSERVED MAINLY

EXAMPLE
82
EEE
2.15
1.0
0.21
5.00
MnS

EXAMPLE
83
FFF
2.00
8.0
0.20
5.00
calcium aluminate

COMPARATIVE EXAMPLE
84
GGG
2.20

12.0

0.60

80.00
MnS

COMPARATIVE EXAMPLE
85
HHH
2.30
2.9
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
86
JJJ
2.20
6.0

0.45

65.00
MnS

EXAMPLE
87
A
2.30
3.0
0.03
5.00
calcium aluminate

EXAMPLE
89
A
2.30
3.0
0.03
5.00
calcium aluminate

EXAMPLE
90
KKK
2.00
4.0
0.25
50.00
CaS, MnS

COMPARATIVE EXAMPLE
91
A
2.30
3.0
0.25
5.00
calcium aluminate

COMPARATIVE EXAMPLE
92
A
2.30
3.0
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
93
A
2.30
3.0
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
94
LLL
2.31
3.0
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
95
MMM
2.30
8.0
0.25
5.00
calcium aluminate

COMPARATIVE EXAMPLE
96
NNN
2.30
5.0
0.15
5.00
calcium aluminate

COMPARATIVE EXAMPLE
97
OOO
2.30
5.0
0.15
5.00
calcium aluminate

COMPARATIVE EXAMPLE
98
PPP
2.27
3.0
0.12
20.00
calcium aluminate, REM oxide, CaS

COMPARATIVE EXAMPLE
99
QQQ
2.27
3.0
0.12
20.00
calcium aluminate, REM oxide, CaS

COMPARATIVE EXAMPLE
100
RRR
2.32
2.9
0.18
5.00
calcium aluminate

COMPARATIVE EXAMPLE
101
SSS
2.30
1.5
0.04
5.00
calcium aluminate, residual desulfurizing agent

METALLOGRAPHIC STRUCTURE

SECONDARY PHASE

PRIMARY PHASE
MARTENSITE (M) AND

FERRITE (F)
RESIDUAL AUSTENITE (γ)

AREA

AVERAGE

AVERAGE
AREA
FRACTION

GRAIN
AREA FRACTION
GRAIN
FRACTION
OF

STEEL

AREA
SIZE
M
γ
M + γ
SIZE
OF BAINITE
PEARLITE

COMPOSITION
CONSTITUENT METALLIC PHASE
FRACTION F
(μm)
(%)
(%)
(%)
(μm)
(%)
(%)

COMPARATIVE EXAMPLE
102
TTT
ferrite, martensite, residual austenite
92.8
4.22
5.7
1.5
7.2
3.4
0.00
0.00

COMPARATIVE EXAMPLE
103
UUU
ferrite, martensite, residual austenite
93.1
4.60
5.8
1.1
6.9
3.7
0.00
0.00

COMPARATIVE EXAMPLE
104
VVV
ferrite, martensite, residual austenite
93.9
5.10
5.6
0.5
6.1
4.1
0.00
0.00

COMPARATIVE EXAMPLE
105
WWW
ferrite, martensite, residual austenite
92.8
4.22
5.4
1.8
7.2
3.4
0.00
0.00

COMPARATIVE EXAMPLE
106
XXX
ferrite, martensite, residual austenite
93.1
4.60
5.8
1.1
6.9
3.7
0.00
0.00

COMPARATIVE EXAMPLE
107
YYY
ferrite, martensite, residual austenite
93.9
5.10
5.6
0.5
6.1
4.1
0.00
0.00

COMPARATIVE EXAMPLE
108
ZZZ
ferrite, martensite, residual austenite
93.1
4.22
5.9
1.0
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
109
AAAA
ferrite, martensite, residual austenite
93.1
4.22
5.9
1.0
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
110
BBBB
ferrite, martensite, residual austenite
93.1
4.22
5.9
1.0
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
111
CCCC
ferrite, martensite, residual austenite
93.1
4.22
5.9
1.0
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
112
DDDD
ferrite, martensite, residual austenite
93.1
4.22
5.9
1.0
6.9
3.4
0.00
0.00

COMPARATIVE EXAMPLE
113
EEEE
ferrite, martensite, residual austenite
93.1
4.22
5.9
1.0
6.0
3.4
0.00
0.00

COMPARATIVE EXAMPLE
114
FFFF
ferrite, martensite, residual austenite
93.1
4.22
5.9
1.0
6.9
3.4
0.00
0.00

EXAMPLE
115
GGGG
ferrite, martensite, residual austenite
93.4
3.91
5.4
1.2
6.6
3.1
0.00
0.00

EXAMPLE
116
HHHH
ferrite, martensite, residual austenite
92.4
4.23
7.1
0.5
7.6
3.4
0.00
0.00

EXAMPLE
117
IIII
ferrite, martensite, residual austenite
93.5
4.19
5.3
1.2
6.5
3.3
0.00
0.00

EXAMPLE
118
JJJJ
ferrite, martensite, residual austenite
92.1
4.21
6.4
1.5
7.9
3.4
0.00
0.00

EXAMPLE
119
KKKK
ferrite, martensite, residual austenite
92.1
4.21
6.4
1.5
7.9
3.4
0.00
0.00

EXAMPLE
120
LLLL
ferrite, martensite, residual austenite
93.5
4.19
5.3
1.2
6.5
3.3
0.00
0.00

EXAMPLE
121
MMMM
ferrite, martensite, residual austenite
93.2
4.22
5.9
0.9
6.8
3.4
0.00
0.00

INCLUSIONS

TEXTURE
AVERAGE OF

X-RAY
MAXIMUM OF
TOTAL
NUMBER

RANDOM
RATIO OF
LENGTH M
PERCENTAGE

INTENSITY
MAJOR
IN ROLLING
OF MnS

STEEL
RATIO OF
AXIS TO
DIRECTION
AND CaS

COMPOSITION
{211} PLANE
MINOR AXIS
(mm/mm²)
(%)
ELONGATED INCLUSIONS OBSERVED MAINLY

COMPARATIVE EXAMPLE
102
TTT
2.25
1.0
0.00
—
none

COMPARATIVE EXAMPLE
103
UUU
2.15
1.0
0.21
5.00
MnS

COMPARATIVE EXAMPLE
104
VVV
2.00

11.0

0.51

5.00
calcium aluminate

COMPARATIVE EXAMPLE
105
WWW
2.25

10.1

0.48

5.00
MnS

COMPARATIVE EXAMPLE
106
XXX
2.15

10.5

0.53

5.00
MnS

COMPARATIVE EXAMPLE
107
YYY
2.00

11.2

0.49

5.00
calcium aluminate

COMPARATIVE EXAMPLE
108
ZZZ

2.58

3.0
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
109
AAAA

2.61

3.0
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
110
BBBB
2.31
3.0
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
111
CCCC
2.31
3.0
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
112
DDDD
2.31
3.0
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
113
EEEE
2.31
3.0
0.03
5.00
calcium aluminate

COMPARATIVE EXAMPLE
114
FFFF
2.31
3.0
0.03
5.00
calcium aluminate

EXAMPLE
115
GGGG
2.27
2.8
0.14
5.00
calcium aluminate

EXAMPLE
116
HHHH
2.27
3.0
0.12
7.00
calcium aluminate

EXAMPLE
117
IIII
2.26
2.8
0.21
20.00
calcium aluminate

EXAMPLE
118
JJJJ
2.27
1.0
0.00
—
none

EXAMPLE
119
KKKK
2.27
1.0
0.00
—
none

EXAMPLE
120
LLLL
2.26
2.8
0.21
20.00
calcium aluminate

EXAMPLE
121
MMMM
2.27
2.0
0.20
20.00
calcium aluminate

The underlined value in the table indicates out of the range of the present invention.

TABLE 11

MECHANICAL PROPERTIES

FRACTURE PROPERTIES

THREE POINT
CHARPY TEST

BENDING TEST
FRACTURE

TENSILE
FORMABILITY
RESISTANCE
RESISTANCE
APPEARANCE

FATIGUE

PROPERTIES
HOLE EXPANSION TEST
OF CRACK
OF CRACK
TRANSITION

PROPERTIES

TENSILE

AVERAGE
STANDARD
INITIATION
PROPAGATION
TEMPERATURE
ABSORBED
FATIGUE

STEEL
STRENGTH TS

λ ave
DEVIATION σ
Jc
T.M.
vTrs
ENERGY
LIFE

COMPOSITION
(MPa)
n VALUE
(%)
(λ)
(MJ/m³)
(MJ/m³)
(° C.)
E(J)
(times)

EXAMPLE
1
A
820
0.13
68
10
0.60
893
−62
23.4
676000

EXAMPLE
2
B
800
0.13
75
9
0.69
880
−61
27.4
668000

EXAMPLE
3
C
815
0.13
75
7
0.69
933
−62
27.4
700000

EXAMPLE
4
D
790
0.13
75
8
0.69
906
−64
27.4
684000

EXAMPLE
5
E
790
0.13
64
13
0.55
933
−63
21.2
700000

EXAMPLE
6
F
790
0.13
64
11
0.55
906
−63
21.2
684000

EXAMPLE
7
G
824
0.13
90
7
0.88
933
−63
36.0
700000

EXAMPLE
8
H
825
0.13
90
7
0.88
933
−63
36.0
700000

EXAMPLE
9
I
824
0.14
65
9
0.56
746
−79
21.7
588000

EXAMPLE
10
J
704
0.15
75
10
0.69
693
−63
27.4
556000

COMPARATIVE EXAMPLE
11
K
772
0.14
61
10
0.51
773
0
19.5
604000

COMPARATIVE EXAMPLE
12
L
765
0.12
62
9
0.52
786
6
20.0
480000

EXAMPLE
13
M
815
0.13
65
10
0.56
773
−71
21.7
604000

EXAMPLE
14
N
790
0.13
83
8
0.79
933
−63
32.0
700000

EXAMPLE
15
O
790
0.13
63
15
0.54
760
−64
20.6
596000

EXAMPLE
16
P
790
0.13
62
15
0.52
680
−63
20.0
548000

EXAMPLE
17
Q
790
0.13
61
15
0.51
626
−63
19.5
516000

EXAMPLE
18
R
790
0.13
60
13
0.50
746
−63
18.9
588000

EXAMPLE
19
S
790
0.13
61
10
0.51
773
−64
19.5
604000

EXAMPLE
20
T
790
0.13
62
11
0.52
786
−62
20.0
612000

EXAMPLE
21
U
790
0.13
65
9
0.56
653
−63
21.7
532000

EXAMPLE
22
V
790
0.13
68
8
0.60
666
−62
23.4
540000

EXAMPLE
23
W
790
0.13
80
7
0.75
800
−63
30.3
620000

EXAMPLE
24
X
790
0.13
67
8
0.59
933
−63
22.9
700000

EXAMPLE
25
Y
790
0.13
65
10
0.56
602
−63
21.7
501600

COMPARATIVE EXAMPLE
26
Z
790
0.13
50
18
0.37
400
−63
13.2
380000

COMPARATIVE EXAMPLE
27
AA
794
0.13
40
20
0.25
533
−64
7.5
460000

COMPARATIVE EXAMPLE
28
BB
820
0.13
45
8
0.31
613
−64
10.3
508000

EXAMPLE
29
A
774
0.14
66
10
0.57
853
−62
22.3
652000

COMPARATIVE EXAMPLE
30
A
785
0.14
40
18
0.25
293
−63
7.5
316000

EXAMPLE
31
A
790
0.13
60
10
0.50
600
−63
18.9
500000

COMPARATIVE EXAMPLE
32
A
790
0.13
52
10
0.40
600
−71
14.3
500000

EXAMPLE
33
A
790
0.13
65
9
0.56
613
−63
21.7
508000

EXAMPLE
34
A
790
0.13
65
9
0.56
733
−14
21.7
580000

COMPARATIVE EXAMPLE
35
A
790
0.13
62
10
0.52
666
−8
20.0
540000

COMPARATIVE EXAMPLE
36
A
802
0.13
53
10
0.41
853
−77
14.9
652000

COMPARATIVE EXAMPLE
37
A
810
0.13
45
10
0.31
853
−77
10.3
652000

COMPARATIVE EXAMPLE
38
A
785
0.13
60
10
0.50
853
−11
18.9
652000

COMPARATIVE EXAMPLE
39
A
790
0.13
60
10
0.50
853
−11
18.9
652000

COMPARATIVE EXAMPLE
40
A
775
0.11
60
9
0.50
853
−44
18.9
360000

COMPARATIVE EXAMPLE
41
A
774
0.11
60
10
0.50
853
−27
18.9
350000

The underlined value in the table indicates out of the range of the present invention.

TABLE 12

MECHANICAL PROPERTIES

FRACTURE PROPERTIES

FORMABILITY
THREE POINT

HOLE
BENDING TEST

EXPANSION
RESIS-
RESIS-

TEST
TANCE
TANCE
CHARPY TEST

TENSILE

STAN-
OF
OF
FRACTURE

PROPERTIES

DARD
CRACK
CRACK
APPEARANCE

FATIGUE

STEEL
TENSILE

AVER-
DEVI-
INITI-
PROPA-
TRANSITION

PROPERTIES

COM-
STRENGTH

AGE
ATION
ATION
GATION
TEMPERATURE
ABSORBED
FATIGUE

POSI-
TS
n
λ ave
σ
Jc
T.M.
vTrs
ENERGY
LIFE

TION
(MPa)
VALUE
(%)
(λ)
(MJ/m³)
(MJ/m³)
(° C.)
E(J)
(times)

EXAMPLE
42
CC
600
0.15
98
10
1.00
893
−35
41.4
576000

EXAMPLE
43
DD
610
0.15
105
9
1.09
880
−61
45.4
568000

EXAMPLE
44
EE
815
0.16
105
7
1.09
640
−82
45.4
424000

EXAMPLE
45
FF
600
0.15
105
8
1.09
906
−64
45.4
584000

EXAMPLE
46
GG
600
0.15
94
13
0.95
613
−63
39.2
408000

EXAMPLE
47
HH
600
0.15
94
11
0.95
906
−63
39.2
584000

EXAMPLE
48
II
610
0.15
120
7
1.27
706
−63
54.0
464000

EXAMPLE
49
JJ
621
0.15
120
7
1.27
693
−63
54.0
456000

EXAMPLE
50
KK
600
0.15
100
8
1.02
866
−30
42.6
560000

COMPARATIVE
51
LL

575

0.12
100
8
1.02
800
3
42.6
310000

EXAMPLE

EXAMPLE
52
MM
609
0.16
95
9
0.96
746
−60
39.7
488000

EXAMPLE
53
NN
595
0.16
95
10
0.96
773
−16
39.7
504000

EXAMPLE
54
OO
600
0.15
95
10
0.96
773
−62
39.7
504000

EXAMPLE
55
PP
608
0.15
113
8
1.19
693
−63
50.0
456000

EXAMPLE
56
QQ
600
0.15
93
15
0.93
760
−64
38.6
496000

EXAMPLE
57
RR
600
0.15
92
15
0.92
680
−63
38.0
448000

EXAMPLE
58
SS
600
0.15
91
15
0.91
626
−63
37.5
416000

EXAMPLE
59
TT
600
0.15
90
13
0.90
746
−63
36.9
488000

EXAMPLE
60
UU
600
0.15
91
10
0.91
773
−64
37.5
504000

EXAMPLE
61
VV
600
0.15
92
11
0.92
786
−62
38.0
512000

EXAMPLE
62
WW
610
0.15
95
9
0.96
653
−63
39.7
432000

EXAMPLE
63
XX
608
0.15
98
8
1.00
666
−62
41.4
440000

EXAMPLE
64
YY
600
0.15
110
7
1.15
800
−63
48.3
520000

EXAMPLE
65
ZZ
600
0.15
97
8
0.98
933
−63
40.9
600000

EXAMPLE
66
AAA
600
0.15
95
10
0.69
602
−63
39.7
401600

COMPARATIVE
67
BBB
600
0.15
80
18
0.77
400
−63
31.2
280000

EXAMPLE

COMPARATIVE
68
CCC
604
0.15
70
20
0.64
333
−64
25.5
240000

EXAMPLE

COMPARATIVE
69
DDD
630
0.15
58
8
0.48
613
−64
15.6
408000

EXAMPLE

COMPARATIVE
70
CC

584

0.16
96
10
0.97
853
−62
40.3
552000

EXAMPLE

COMPARATIVE
71
CC
595
0.16
70
18
0.64
293
−63
25.5
216000

EXAMPLE

EXAMPLE
72
CC
600
0.15
90
10
0.90
600
−63
36.9
400000

COMPARATIVE
73
CC
600
0.15
57
10
0.49
600
−71
15.8
400000

EXAMPLE

EXAMPLE
74
CC
600
0.15
95
9
0.96
613
−63
39.7
408000

EXAMPLE
75
CC
600
0.15
95
9
0.96
733
−14
39.7
480000

COMPARATIVE
76
CC
600
0.15
92
10
0.92
666
−11
38.0
440000

EXAMPLE

COMPARATIVE
77
CC
612
0.15
56
10
0.47
853
−77
15.7
552000

EXAMPLE

COMPARATIVE
78
CC
620
0.14
58
10
0.49
853
−77
15.9
552000

EXAMPLE

COMPARATIVE
79
CC
595
0.15
90
10
0.90
853
−11
36.9
552000

EXAMPLE

COMPARATIVE
80
CC

585

0.12
91
8
0.91
853
−11
37.5
340000

EXAMPLE

COMPARATIVE
81
CC

585

0.11
90
8
0.90
853
−44
36.9
330000

EXAMPLE

The underlined value in the table indicates out of the range of the present invention.

TABLE 13

MECHANICAL PROPERTIES

FRACTURE PROPERTIES

FORMABILITY
THREE POINT

HOLE
BENDING TEST

EXPANSION
RESIS-
RESIS-

TEST
TANCE
TANCE
CHARPY TEST

TENSILE

STAN-
OF
OF
FRACTURE

FATIGUE

PROPERTIES

DARD
CRACK
CRACK
APPEARANCE

PRO-

STEEL
TENSILE

AVER-
DEVI-
INITI-
PROPA-
TRANSITION

PERTIES

COM-
STRENGTH

AGE
ATION
ATION
GATION
TEMPERATURE
ABSORBED
FATIGUE

POSI-
TS
n
λ ave
σ
Jc
T.M.
vTrs
ENERGY
LIFE

TION
(MPa)
VALUE
(%)
(λ)
(MJ/m³)
(MJ/m³)
(° C.)
E(J)
(times)

EXAMPLE
82
EEE
590
0.13
69
8
0.61
653
−77
24.0
532000

EXAMPLE
83
FFF
600
0.13
63
15
0.54
666
−66
20.6
540000

COMPARATIVE
84
GGG
595
0.13
45
22
0.31
133
−64
10.3
220000

COMPARATIVE
85
HHH
850
0.13
55
13
0.44
893
−86
15.4
676000

COMPARATIVE
86
JJJ
600
0.13
50
18
0.37
333
−79
13.2
340000

EXAMPLE
87
A
810
0.13
64
12
0.55
893
−14
21.2
676000

EXAMPLE
89
A
815
0.13
65
10
0.56
893
−79
17.2
400000

EXAMPLE
90
KKK
820
0.13
63
12
0.54
600
−14
20.6
500000

COMPARATIVE
91
A
855
0.13
56
13
0.45
600
−70
15.7
500000

EXAMPLE

COMPARATIVE
92
A

585

0.12
65
13
0.56
893
−64
21.7
376000

EXAMPLE

COMPARATIVE
93
A
830
0.13
58
13
0.47
893
−90
15.8
676000

EXAMPLE

COMPARATIVE
94
LLL
820
0.13
58
10
0.46
893
−62
15.6
676000

EXAMPLE

COMPARATIVE
95
MMM

572

0.15
90
10
0.90
600
−63
36.9
400000

EXAMPLE

COMPARATIVE
96
NNN

981

0.13
57
15
0.56
733
−14
21.7
580000

EXAMPLE

COMPARATIVE
97
OOO

983

0.13
55
15
0.56
733
−14
21.7
580000

EXAMPLE

COMPARATIVE
98
PPP

584

0.12
95
10
0.96
773
−16
39.7
384000

EXAMPLE

COMPARATIVE
99
QQQ

572

0.12
95
10
0.96
773
−16
39.7
391000

EXAMPLE

COMPARATIVE
100
RRR
704
0.15
56
10
0.44
693
−63
15.3
556000

EXAMPLE

COMPARATIVE
101
SSS

578

0.15
105
9
1.09
880
−61
45.4
568000

EXAMPLE

COMPARATIVE
102
TTT

982

0.13
59
7
0.48
933
−62
15.7
700000

EXAMPLE

COMPARATIVE
103
UUU
595
0.13
56
8
0.46
653
−77
15.5
532000

EXAMPLE

COMPARATIVE
104
VVV
600
0.13
57
19
0.44
297
−66
14.8
213000

EXAMPLE

COMPARATIVE
105
WWW
600
0.15
65
18
0.51
302
−62
16.3
230000

EXAMPLE

COMPARATIVE
106
XXX
590
0.13
55
20
0.49
288
−77
15.8
222000

EXAMPLE

COMPARATIVE
107
YYY
595
0.13
57
19
0.48
300
−66
15.2
232000

EXAMPLE

COMPARATIVE
108
ZZZ
820
0.13
56
10
0.44
893
−62
15.1
676000

EXAMPLE

COMPARATIVE
109
AAAA
820
0.13
58
10
0.42
893
−62
14.9
676000

EXAMPLE

COMPARATIVE
110
BBBB

981

0.13
54
10
0.60
893
−62
23.4
676000

EXAMPLE

COMPARATIVE
111
CCCC

983

0.13
53
10
0.60
893
−62
23.4
676000

EXAMPLE

COMPARATIVE
112
DDDD

982

0.13
54
10
0.60
893
−62
23.4
676000

EXAMPLE

COMPARATIVE
113
EEEE

981

0.13
52
10
0.60
893
−62
23.4
676000

EXAMPLE

COMPARATIVE
114
FFFF

982

0.13
55
10
0.60
893
−62
23.4
676000

EXAMPLE

EXAMPLE
115
GGGG
791
0.13
65
9
0.56
746
−79
21.7
588000

EXAMPLE
116
HHHH
785
0.13
65
10
0.56
773
−71
21.7
604000

EXAMPLE
117
IIII
781
0.13
65
9
0.56
653
−63
21.7
532000

EXAMPLE
118
JJJJ
782
0.13
83
8
0.79
933
−63
32.0
700000

EXAMPLE
119
KKKK
780
0.13
83
8
0.79
933
−63
32.0
700000

EXAMPLE
120
LLLL
782
0.13
65
9
0.56
653
−63
21.7
532000

EXAMPLE
121
MMMM
781
0.13
68
8
0.60
666
−62
23.4
540000

The underlined value in the table indicates out of the range of the present invention.

Number	Date	Country	Kind
2011-060909	Mar 2011	JP	national
2011-064633	Mar 2011	JP	national

Number	Name	Date	Kind
7347902	Mega	Mar 2008	B2
7780797	Okamoto	Aug 2010	B2
20100000633	Yokoi	Jan 2010	A1
20100112377	Kaneko	May 2010	A1

Number	Date	Country
101696483	Apr 2010	CN
6-145792	May 1994	JP
9-125194	May 1997	JP
2008-138231	Jun 2008	JP
2009-24227	Apr 2010	JP
2010-90476	Apr 2010	JP
2010090476	Apr 2010	JP
10-2009-0115885	Nov 2009	KR
WO 2011111758	Sep 2011	WO

Hot rolled steel sheet and method of producing same

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

Field of Search

CPC

International Classifications

Term Extension

Abstract

Description

Claims

Priority Claims (2)

PCT Information

US Referenced Citations (4)

Foreign Referenced Citations (9)

Non-Patent Literature Citations (4)

Related Publications (1)

Entry
Korean Notice of Allowance and English translation thereof, dated Aug. 18, 2015, for counterpart Korean Application No. 10-2013-7023879.
Chinese Office Action and Search Report, dated Oct. 30, 2014, for Chinese Application No. 201280013216.0, with English translation of Search Report.
Taiwanese Office Action for Application No. 101109250 dated Dec. 24, 2013, with English translation.
International Search Report issued in PCT/JP2012/056927 mailed Jun. 19, 2012.