Patent Application
20240401163
The present invention relates to a hot-rolled steel sheet.
Priority is claimed on Japanese Patent Application No. 2021-168623, filed Oct. 14, 2021, the content of which is incorporated herein by reference.
In recent years, a weight of a vehicle body has been reduced by applying a high strength steel sheet for the purpose of improving fuel efficiency and collision safety of a vehicle. However, high-strengthening of a steel sheet generally causes deterioration of toughness. Therefore, in the development of the high strength steel sheet, it is an important issue to achieve high-strengthening without deteriorating the toughness.
In general, in order to improve toughness, a method of improving toughness by performing rolling at a low temperature and applying a high cumulative strain in a state of unrecrystallized austenite is known. However, when a rolling reduction in the state of unrecrystallized austenite is increased, there is a problem that an aspect ratio of prior austenite grains increases and anisotropy in toughness increases.
For example, Patent Document 1 discloses a hot-rolled steel sheet which has a texture in which, in a thickness middle portion which is a steel sheet portion partitioned between a ⅜ thickness position and a ⅝ thickness position of a sheet thickness from a surface of a steel sheet, an average value of X-ray random intensity ratios of {100}<011> to {223}<110> orientation groups on a sheet surface is 6.5 or less and an X-ray random intensity ratio of a {332}<113> crystal orientation is 5.0 or less, and has a microstructure in which a total area ratio of tempered martensite, martensite, and lower bainite is more than 85% and an average grain size is 12.0 μm or less.
[Patent Document 1] Japanese Patent No. 5621942
However, in the technique disclosed in Patent Document 1, from the viewpoint of improving the fuel efficiency of a vehicle and improving collision safety, there is room for further improvement in a reduction in anisotropy in toughness in a high strength steel sheet.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent toughness, and reduced anisotropy in toughness.
The present inventors investigated a relationship between a texture and mechanical properties of a hot-rolled steel sheet, and as a result, found that anisotropy in toughness can be further reduced even in a hot-rolled steel sheet having a tensile strength of 1,180 MPa or more. The present inventors found that in a rolled steel sheet, different textures develop on a surface and an inside. In addition, the present inventors found that, in order to reduce the anisotropy in toughness, it is more effective to control a texture in an austenite region than a texture of martensite after rapid cooling. Furthermore, the present inventors found that it is effective to preferably control hot rolling conditions in order to obtain a texture having a desired crystal orientation.
The gist of the present invention made on the basis of the above-mentioned findings is as follows.
(1) A hot-rolled steel sheet according to an aspect of the present invention includes, as a chemical composition, by mass %:
According to the above-described aspect according to the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent toughness, and reduced anisotropy in toughness.
Hereinafter, a hot-rolled steel sheet according to the present embodiment will be described in detail.
First, reasons for limiting a chemical composition of the hot-rolled steel sheet according to the present embodiment will be described. In addition, numerical limiting ranges described below using “to” include the lower limit and the upper limit in the ranges. Numerical values indicated as “less than” or “more than” do not fall within the numerical range. In addition, all % regarding the chemical composition means mass %.
The hot-rolled steel sheet according to the present embodiment includes, as a chemical composition, by mass %: C: 0.100% to 0.500%; Si: 0.100% to 3.000%; Mn: 0.50% to 3.00%; P: 0.100% or less; S: 0.0100% or less; Al: 1.000% or less; N: 0.0100% or less; and a remainder: Fe and impurities. Hereinafter, each element will be described in detail.
C is an important element for improving strength of the hot-rolled steel sheet. When a C content is less than 0.100%, the strength of the hot-rolled steel sheet decreases. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.150% or more, 0.170% or more, 0.200% or more, or 0.220% or more.
On the other hand, when the C content is more than 0.500%, toughness of the hot-rolled steel sheet deteriorates. Therefore, the C content is set to 0.500% or less. The C content is preferably 0.450% or less, 0.400% or less, or 0.370% or less.
Si is an element having an effect of improving the strength of the hot-rolled steel sheet. When a Si content is less than 0.100%, the strength of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 0.100% or more. The Si content is preferably 0.200% or more, 0.300% or more, 0.400% or more, or 0.500% or more. The Si content is more preferably more than 1.000%, and still more preferably 1.100% or more.
However, when the Si content is more than 3.000%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 3.000% or less. The Si content is preferably 2.700% or less, 2.500% or less, or 2.300% or less.
Mn is an element effective for improving the strength of the hot-rolled steel sheet by improving hardenability and solid solution strengthening. When a Mn content is less than 0.50%, the strength of the hot-rolled steel sheet decreases. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably 1.00% or more, 1.20% or more, or 1.50% or more.
On the other hand, when the Mn content is more than 3.00%, MnS that increases anisotropy in the toughness of the hot-rolled steel sheet is generated. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, 2.30% or less, or 2.00% or less.
P is an impurity element, and the lower a P content is, the more preferable it is. When the P content is more than 0.100%, deterioration of workability and weldability of the hot-rolled steel sheet becomes significant, and fatigue properties also deteriorate. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.070% or less, 0.050% or less, or 0.030% or less.
A lower limit of the P content is not particularly specified. However, since an excessive reduction in the P content causes an increase in manufacturing cost, the P content may be set to 0.001% or more or 0.005% or more.
S is an impurity element, and the lower a S content is, the more preferable it is. When the S content is more than 0.0100%, a large amount of inclusions such as MnS that increase the anisotropy in the toughness of the hot-rolled steel sheet are generated. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.
A lower limit of the S content is not particularly specified. However, since an excessive reduction in the S content causes an increase in the manufacturing cost, the S content may be set to 0.0005% or more, or 0.0010 or more.
Al is an element that acts as a deoxidizing agent in a steelmaking stage and is effective for improving cleanliness of steel. However, when an Al content is more than 1.000%, alumina precipitated in the form of clusters is generated, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Al content is set to be 1.000% or less. The Al content is preferably 0.700% or less, 0.500% or less, or 0.400% or less.
A lower limit of the Al content is not particularly specified. However, since an excessive reduction in the Al content causes an increase in the manufacturing cost, the Al content may be set to 0.001% or more, or 0.005% or more.
N is an impurity element. When a N content is more than 0.0100%, a coarse Ti nitride is formed at a high temperature, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.
A lower limit of the N content is not particularly specified. However, since an excessive reduction in the N content causes an increase in the manufacturing cost, the N content may be set to 0.0010% or more.
The hot-rolled steel sheet according to the present embodiment may contain the above elements, and the remainder of Fe and impurities. Examples of the impurities include those that are unavoidably incorporated from steel raw materials or scrap and/or in a steelmaking process or elements that are allowed in a range in which the properties of the hot-rolled steel sheet according to the present embodiment are not inhibited.
In order to improve various properties, the hot-rolled steel sheet according to the present embodiment may contain optional elements shown below instead of a portion of Fe. In order to reduce an alloy cost, it is not necessary to intentionally include these optional elements in steel. Therefore, lower limits of the amounts of these optional elements are all 0%.
Ti is an element effective for suppressing austenite recrystallization and grain growth between stands of hot rolling. By suppressing the recrystallization of austenite between the stands, strain can be further accumulated. As a result, the texture of the hot-rolled steel sheet can be preferably controlled. In a case of reliably obtaining the effect, a Ti content is preferably set to 0.02% or more.
On the other hand, when the Ti content is more than 0.20%, inclusions attributed to TiN are generated, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.20% or less.
Nb is an element effective for suppressing austenite recrystallization and grain growth between the stands of hot rolling. By suppressing the recrystallization of austenite between the stands, strain can be further accumulated. As a result, the texture of the hot-rolled steel sheet can be preferably controlled. In a case of reliably obtaining the effect, a Nb content is preferably set to 0.010% or more.
On the other hand, when the Nb content is more than 0.100%, the effect is saturated. Therefore, the Nb content is set to 0.100% or less.
Ca is an element having an effect of refining the structure of the hot-rolled steel sheet by dispersing a number of fine oxides during deoxidation of molten steel. In addition, Ca is also an element that fixes S in steel as spherical CaS, suppresses the generation of elongated inclusions such as MnS, and reduces the anisotropy in the toughness of the hot-rolled steel sheet. In a case of reliably obtaining these effects, a Ca content is preferably set to 0.0001% or more.
On the other hand, when the Ca content is more than 0.0060%, the above effects are saturated. Therefore, the Ca content is set to 0.0060% or less.
Mo is an element effective for precipitation hardening of ferrite. In a case of reliably obtaining this effect, a Mo content is preferably set to 0.01% or more. On the other hand, when the Mo content is more than 0.50%, cracking susceptibility of a slab increases, and it becomes difficult to handle the slab. Therefore, the Mo content is set to 0.50% or less.
Cr is an effective element for improving the strength of the hot-rolled steel sheet. In a case of reliably obtaining this effect, a Cr content is preferably set to 0.01% or more.
However, when the Cr content is more than 1.00%, ductility of the hot-rolled steel sheet deteriorates. Therefore, the Cr content is set to 1.00% or less.
V improves the strength of the hot-rolled steel sheet through strengthening with precipitates and refinement of ferrite grains. In a case of reliably obtaining this effect, a V content is preferably set to 0.01% or more.
On the other hand, when the V content is more than 0.50%, a large amount of carbonitrides is precipitated, and formability of the hot-rolled steel sheet deteriorates. Therefore, the V content is set to be 0.50% or less.
Cu is an element that is solid-solubilized in steel and contributes to an improvement in the strength of steel. Cu is also an element that improves hardenability. In a case of reliably obtaining these effects, a Cu content is preferably set to 0.01% or more.
On the other hand, when the Cu content is more than 0.50%, surface properties of the hot-rolled steel sheet deteriorate, and there are cases where chemical convertibility and corrosion resistance deteriorate. Therefore, the Cu content is set to 0.50% or less.
Ni is an element that is solid-solubilized in steel and contributes to an increase in the strength of the steel. Ni is also an element that improves hardenability. In a case of reliably obtaining these effects, a Ni content is preferably set to 0.01% or more.
On the other hand, since an alloy cost of Ni is high, including a large amount of Ni causes an increase in the cost. In addition, when the Ni content is more than 0.50%, there are cases where the weldability of the hot-rolled steel sheet deteriorates. Therefore, the Ni content is set to 0.50% or less.
Sn has an effect of suppressing internal oxidation and an effect of improving the strength. In a case of reliably obtaining the effects, a Sn content is preferably set to 0.001% or more.
On the other hand, when a large amount of Sn is contained, there are cases where defects occur during hot rolling. Therefore, the Sn content is set to 0.050% or less.
The chemical composition described above may be measured by a general analysis method. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.
In a case where the hot-rolled steel sheet includes a plating layer on a surface, the chemical composition may be analyzed after removing the plating layer from the surface by mechanical grinding.
Next, a microstructure of the hot-rolled steel sheet according to the present embodiment will be described.
In the hot-rolled steel sheet according to the present embodiment, a microstructure in a region from a depth of ⅛ of a sheet thickness from a surface to a depth of ⅜ of the sheet thickness from the surface includes, by area %, 90% to 100% of martensite, and 0% to 10% of a remainder in microstructure, in a texture of a region from the surface to the depth of ⅛ of the sheet thickness from the surface, pole densities of {001}<110>, {111}<110>, and {112}<110> orientation groups are 2.0 or more, and in a texture of a region from the depth of ⅛ of the sheet thickness from the surface to a depth of ½ of the sheet thickness from the surface, a pole density of a {110}<112> orientation is 5.0 or less.
In the present embodiment, area % of martensite and the remainder in microstructure in the region from the depth of ⅛ of the sheet thickness from the surface to the depth of ⅜ of sheet thickness from the surface is specified because the microstructure at this position represents a typical microstructure of the hot-rolled steel sheet. Hereinafter, each specifying will be described in detail.
When an area ratio of martensite is less than 90%, the strength of the hot-rolled steel sheet deteriorates, and a desired strength cannot be obtained. Therefore, the area ratio of martensite is set to 90% or more. The area ratio of martensite is preferably 92% or more, 95% or more, or 97% or more, and more preferably 100%.
In the present embodiment, martensite refers to fresh martensite and tempered martensite. In the present embodiment, it is not necessary to distinguish between fresh martensite and tempered martensite, and thus both will be collectively referred to as martensite.
Tempered martensite is obtained by tempering fresh martensite and has a lower dislocation density than fresh martensite. A preferred manufacturing method of the hot-rolled steel sheet according to the present embodiment, which will be described later, does not include a heat treatment for the purpose of tempering after rapid cooling. However, there are cases where tempered martensite is generated during cooling after hot rolling or by reheating after coiling.
The microstructure of the hot-rolled steel sheet according to the present embodiment may contain bainite as the remainder in microstructure. When an area ratio of the remainder in microstructure is more than 10%, the strength of the hot-rolled steel sheet decreases, and a desired strength cannot be obtained. Therefore, the area ratio of the remainder in microstructure is set to 10% or less. The area ratio of the remainder in microstructure is preferably 8% or less, 5% or less, or 3% or less, and more preferably 0%.
The area ratio of each structure is obtained by the following method.
A test piece for structure observation is collected from a ¼ thickness position of the hot-rolled steel sheet (a region from the depth of ⅛ of the sheet thickness from the surface to the depth of ⅜ of the sheet thickness from the surface) and from a sheet width center position so that a sheet thickness cross section parallel to a rolling direction serves an observed section. The observed section is mirror-polished and then corroded with a 3 volume % Nital solution. Three visual fields of the observed section after the corrosion were photographed at a magnification of 2,000-fold using an optical microscope and a scanning electron microscope (SEM). Each photographed visual field is set to 500 μm×500 μm. Image analysis is performed on the photographs, and the area ratio of each structure is calculated. The area ratio of each structure is obtained by calculating an average value of the area ratios obtained for the three visual fields.
Since martensite is a structure having a substructure called a block or a packet in grains, it is possible to distinguish martensite from other microstructures in an electron channeling contrast image for which the scanning electron microscope is used.
Among structures that are each an aggregate of lath-shaped crystal grains and do not contain any Fe-based carbides having a major axis of 20 nm or more in the structure, a structure that is not martensite or a structure in which Fe-based carbides having a major axis of 20 nm or more are contained in the structure and the Fe-based carbides have a single variant, that is, the Fe-based carbides extend in the same direction, is regarded as the bainite. Here, the Fe-based carbides extending in the same direction refer to Fe-based carbides for which a difference in extending direction between the Fe-based carbides is 5° or less.
Average Grain Size of Prior Austenite Grains: More than 5.0 μm and 30.0 μm or Less
In the hot-rolled steel sheet according to the present embodiment, at the ¼ thickness position (the region from the depth of ⅛ of the sheet thickness from the surface to the depth of ⅜ of the sheet thickness from the surface), an average grain size of prior austenite grains may be more than 5.0 μm and 30.0 μm or less. By setting the average grain size of the prior austenite grains to more than 5.0 μm, the predetermined texture required in the present embodiment can be stably obtained, and the anisotropy in the toughness of the hot-rolled steel sheet can be further reduced. The average grain size of the prior austenite grains is preferably 6.0 μm or more, 7.0 μm or more, 8.0 μm or more, or 9.0 μm or more.
On the other hand, when the average grain size of the prior austenite grains is more than 30.0 μm, there are cases where a desired strength cannot be obtained. Therefore, the average grain size of the prior austenite grains is preferably set to 30.0 μm or less.
The average grain size of the prior austenite grains is obtained by the following method.
A test piece for structure observation is collected from the ¼ thickness position of the hot-rolled steel sheet (the region from the depth of ⅛ of the sheet thickness from the surface to the depth of ⅜ of the sheet thickness from the surface) and from the sheet width center position so that the sheet thickness cross section parallel to the rolling direction serves an observed section. The observed section is mirror-polished and then corroded with a 3 volume % Nital solution, and the microstructure is observed with a scanning electron microscope (SEM). A range in which approximately 10,000 crystal grains are observed in one visual field is photographed in three visual fields by SEM observation. The obtained photograph is subjected to image analysis using image analysis software (WinROOF) to calculate the average grain size of the prior austenite grains. For one of the prior austenite grains included in the observed visual field, an average value of a shortest diameter and a longest diameter is calculated, and the average value is used as the grain size of the prior austenite grains. The above operation is performed on all the prior austenite grains except for the prior austenite grains which are not entirely included in the photographed visual fields, such as crystal grains in an end portion of the photographed visual field, and the grain sizes of all the prior austenite grains in the photographed visual fields are obtained. The average grain size of the prior austenite grains in the photographed visual fields is obtained by calculating a value obtained by dividing the sum of the obtained grain sizes of the prior austenite grains by the total number of prior austenite grains of which grain sizes are measured. This operation is performed for each of all the photographed visual fields, and the average grain size of the prior austenite grains of all the photographed visual fields is calculated, thereby obtaining the average grain size of the prior austenite grains.
Pole Densities of {001}<110>, {111}<110>, and {112}<110> Orientation Group in Texture of Region from Surface to Depth of ⅛ of Sheet Thickness from Surface: 2.0 or More
When the pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture of the region (hereinafter, sometimes referred to as a surface layer region) from the surface to the depth of ⅛ of the sheet thickness from the surface are less than 2.0, the occurrence of fine cracks in the surface layer region cannot be suppressed. As a result, the anisotropy in the toughness of the hot-rolled steel sheet increases. Therefore, the pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture of the surface layer region are set to 2.0 or more. The pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture of the surface layer region are preferably 2.2 or more, 2.5 or more, or 2.7 or more.
An upper limit of the pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture of the surface layer region is not particularly specified, but may be set to 9.0 or less, 8.0 or less, 7.0 or less, or 5.0 or less from the viewpoint of suppressing the deterioration of ductility.
Pole Density of {110}<112> Orientation in Texture of Region from Depth of ⅛ of Sheet Thickness from Surface to Depth of ½ of Sheet Thickness from Surface: 5.0 or Less
The pole density of the {110}<112> orientation in the texture of the region (hereinafter, sometimes referred to as an internal region) from the depth of ⅛ of the sheet thickness from the surface to the depth of ½ of the sheet thickness from the surface is more than 5.0, the anisotropy in the toughness of the hot-rolled steel sheet increases. Therefore, the pole density of the {110}<112> orientation in the texture of the internal region is set to 5.0 or less. The pole density of the {110}<112> orientation in the texture of the internal region is preferably 4.6 or less, 4.2 or less, or 4.0 or less.
A lower limit of the pole density of the {110}<112> orientation in the texture of the internal region is not particularly specified, but may be set to 2.0 or more or 2.5 or more from the viewpoint of suppressing the deterioration of the strength.
For the pole densities, a device in which a scanning electron microscope and an EBSD analyzer are combined and OIM analysis (registered trademark) manufactured by AMETEK Inc. are used. From an orientation distribution function (ODF) that is calculated by using orientation data measured by an electron backscattering diffraction (EBSD) method and a spherical harmonic function and displays a three-dimensional texture, the pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture of the surface layer region and the pole density of the {110}<112> in the texture of the internal region are obtained.
A measurement range is set to, for the surface layer region, the region from the surface to the depth of ⅛ of the sheet thickness from the surface and for the internal region, the region from the depth of ⅛ of the sheet thickness from the surface to the depth of ½ of the sheet thickness from the surface. Measurement pitches are set to 5 μm/step.
It should be noted that {hkl} indicates a crystal plane parallel to a rolled surface and <uvw> indicates a crystal direction parallel to the rolling direction. That is, {hkl}<uvw> indicates a crystal in which {hkl} is oriented in a sheet surface normal direction and <uvw> is oriented in the rolling direction.
The rolling direction of the hot-rolled steel sheet can be determined by the following method.
First, a test piece is collected so that a sheet thickness cross section of the hot-rolled steel sheet can be observed. The sheet thickness cross section of the collected test piece is finished by mirror polishing and then observed using an optical microscope. An observation range is set to an overall thickness of the sheet thickness, and a region with dark brightness is determined to be an inclusion. Among inclusions, in inclusions having a major axis length of 40 μm or more, a direction parallel to a direction in which the inclusion extends is determined to be the rolling direction.
A tensile strength of the hot-rolled steel sheet according to the present embodiment is set to 1,180 MPa or more from the viewpoint of improving collision safety of a vehicle or the like or reducing a weight of a vehicle body. The tensile strength is preferably 1,250 MPa or more, 1,300 MPa or more, 1,350 MPa or more, or 1,400 MPa or more.
An upper limit of the tensile strength is not particularly specified, but is preferably 2,000 MPa or less, 1,600 MPa or less, 1,500 MPa or less, or 1,400 MPa or less.
The tensile strength is measured according to JIS Z 2241:2011. A No. 5 test piece of JIS Z 2241:2011 is used as a test piece, and a test direction is set to a direction perpendicular to the rolling direction.
The sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited and may be set to 1.2 to 8.0 mm. When the sheet thickness of the hot-rolled steel sheet is less than 1.2 mm, it is difficult to secure a rolling completion temperature, a rolling force becomes excessive, and there are cases where it is difficult to perform hot rolling.
When the sheet thickness is more than 8.0 mm, it becomes difficult to control the texture, and there are cases where it is difficult to obtain the above-described texture. Therefore, the sheet thickness may be set to 8.0 mm or less.
The hot-rolled steel sheet according to the present embodiment may have a plating layer on the surface. As the plating layer, an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized layer, a hot-dip galvannealed layer, or the like is an exemplary example.
Next, a preferred manufacturing method of the hot-rolled steel sheet according to the present embodiment will be described. The preferred manufacturing method of the hot-rolled steel sheet according to the present embodiment includes the following steps (a) to (d). Unless otherwise specified, a temperature in the following description refers to a surface temperature of the steel sheet.
(a) A heating step of heating a slab having the above-described chemical composition to a temperature range of 1,100° C. or higher and lower than 1,350° C.
(b) A finish rolling step of performing finish rolling on the heated slab using a rolling mill having a plurality of stands, in which the following conditions (I) to (V) are satisfied.
(I) A finish rolling start temperature is set to 800° C. or higher.
(II) In each of the last four stands among the plurality of stands, rolling is performed so that σ represented by Expression (1) becomes 40 to 80.
Here, T is a temperature (° C.) immediately before entering each stand, & is an equivalent plastic strain, and ε′ is a strain rate.
(III) Interpass times between the last four stands are set to 0.2 to 10.0 seconds.
(IV) A cumulative rolling reduction of the last four stands is set to 60% or larger.
(V) A finishing temperature is set to 800° C. to 950° C.
(c) A cooling step of starting cooling within 1.0 second after completion of the finish rolling, and performing cooling to a temperature range of 300° C. or lower so that an average cooling rate in a temperature range of the finishing temperature to 300° C. is 100° C./s or faster.
(d) A coiling step of performing coiling after the cooling.
Hereinafter, each step will be described.
In the heating step, it is preferable to heat the slab having the above-mentioned chemical composition to a temperature range of 1,100° C. or higher and lower than 1,350° C. A method of manufacturing the slab does not need to be particularly limited, and a commonly used method can be applied in which molten steel having the above-described chemical composition is melted in a converter or the like and is cast into a slab by a casting method such as continuous casting. In addition, an ingot-making and blooming method may be used.
In the slab, most of carbonitride-forming elements such as Ti are present in the slab as coarse carbonitrides in a non-uniform distribution. The coarse precipitates (carbonitrides) present in a non-uniform distribution deteriorate various properties (for example, tensile strength, toughness, and hole expansibility) of the hot-rolled steel sheet. Therefore, the slab before hot rolling is heated to solid-solubilize the coarse precipitates. In order to sufficiently solid-solubilize these coarse precipitates before hot rolling, a heating temperature of the slab is preferably set to 1,100° C. or higher. However, an excessively high heating temperature for the slab causes the generation of surface defects and a decrease in yield due to scale removal. Therefore, the heating temperature of the steel material is preferably set to lower than 1,350° C.
The slab is heated to the temperature range of 1,100° C. or higher and lower than 1,350° C. and held for a predetermined time. However, when a holding time is longer than 4,800 seconds, the amount of scale generated increases. As a result, a rolled-in scale or the like is likely to occur in the subsequent finish rolling step, and there are cases where surface quality of the hot-rolled steel sheet deteriorates. Therefore, the holding time in the temperature range of 1,100° C. or higher and lower than 1,350° C. is preferably set to 4,800 seconds or shorter.
Rough rolling may be performed on the slab between the heating step and the finish rolling step. Conditions of the rough rolling are not particularly limited as long as desired sheet bar dimensions can be obtained.
In the finish rolling step, the heated slab is subjected to finish rolling using the rolling mill having the plurality of stands. Here, it is preferable to satisfy the conditions (I) to (V) to be described below.
It is preferable to perform descaling before the finish rolling or during rolling between the rolling stands during the finish rolling.
The finish rolling start temperature (an entry-side temperature of a first pass of the finish rolling) is preferably set to 800° C. or higher. When the finish rolling start temperature is lower than 800° C., rolling in some of the plurality of rolling stands (particularly the stands in the first half) is performed at a temperature in a ferrite/austenite dual phase region. As a result, a worked structure remains after the finish rolling, and there are cases where the strength and toughness of the hot-rolled steel sheet deteriorate. Therefore, the finish rolling start temperature is preferably set to 800° C. or higher.
The finish rolling start temperature is preferably set to 1,100° C. or lower in order to suppress coarsening of austenite and to preferably control the texture in the surface layer region and in the internal region.
Here, T is the temperature (° C.) immediately before entering each stand (that is, the entry-side temperature), ε is the equivalent plastic strain, and ε′ is the strain rate.
The fact that ε in each of the last four stands is 40 to 80 can be rephrased as follows: ε of the fourth stand from the last stand, ε of the third stand from the last stand, ε of the second stand from the last stand, and ε of the last stand are all 40 to 80.
When there is even one stand in which σ is less than 40, there are cases where strain necessary for the development of the texture in the surface layer region is not suitably applied in each of the last four stands. As a result, there are cases where in the texture of the region from the surface to the depth of ⅛ of the sheet thickness from the surface, the pole densities of the {001}<110>, {111}<110>, and {112}<110> orientation groups cannot be preferably controlled. Therefore, σ in each of the last four stands is preferably set to 40 or more.
In addition, when there is even one stand in which σ is more than 80, the texture of the internal region cannot be preferably controlled, and there are cases where the anisotropy in the toughness of the hot-rolled steel sheet increases. Therefore, σ in each of the last four stands is preferably set to 80 or less.
In addition, ¿, which is the equivalent plastic strain, can be obtained by ε=(2/√3)×(h/H) when an entry-side sheet thickness is represented by h and an exit-side sheet thickness is represented by H. In addition, the strain rate &′ can be obtained by ε′=ε/t when a rolling time is t(s). In addition, the rolling time t refers to a time during which strain is applied to the steel sheet when the steel sheet and the rolling roll come into contact with each other.
(III) Interpass Times between Last Four Stands: 0.2 to 10.0 Seconds
When there is even one pass in which the interpass time is longer than 10.0 seconds between the last four stands, recovery and recrystallization between the passes progresses. As a result, it becomes difficult to accumulate strain, and there are cases where a desired structure cannot be obtained in the hot-rolled steel sheet. Therefore, the interpass times between the last four stands are preferably set to 10.0 seconds or shorter.
The interpass times between the last four stands are preferably short, but a reduction in the interpass times are limited in terms of an installation space of each stand and a rolling rate. In addition, when the interpass times between the last four stands become shorter than 0.2 seconds, the number of unrecrystallized grains significantly increases, and there are cases where a desired texture cannot be obtained. Therefore, the interpass times between the last four stands are preferably set to 0.2 seconds or longer.
The interpass times between the last four stands are 0.2 to 10.0 seconds can be rephrased as follows: interpass time between the fourth stand from the last stand and the third stand from the last stand, the interpass time between the third stand from the last stand and the second stand from the last stand, and the interpass time between the second stand from the last stand and the last stand are all 0.2 to 10.0 seconds.
When the cumulative rolling reduction of the last four stands is smaller than 60%, there are cases where a dislocation density introduced into unrecrystallized austenite decreases. When the dislocation density introduced into unrecrystallized austenite decreases, it becomes difficult to obtain a desired structure, and there are cases where the strength and toughness of the hot-rolled steel sheet deteriorate. Therefore, the cumulative rolling reduction of the last four stands is preferably set to 60% or larger.
When the cumulative rolling reduction of the last four stands is larger than 97%, there are cases where a shape of the hot-rolled steel sheet deteriorates. Therefore, the cumulative rolling reduction of the last four stands may be set to 97% or smaller.
The cumulative rolling reduction of the last four stands can be represented by {1−(t1/t0)}×100(%) when an inlet sheet thickness of the fourth stand from the last stand is represented by t0 and an outlet sheet thickness of the last stand is represented by t1.
When the finish rolling finishing temperature (exit-side temperature of the last stand) is lower than 800° C., the rolling is performed at a temperature in the ferrite/austenite dual phase region. Therefore, there are cases where the worked structure remains after the rolling and the strength and toughness of the hot-rolled steel sheet decrease. Therefore, the finishing temperature is preferably set to 800° C. or higher.
In addition, in the slab having the chemical composition according to the present embodiment, an unrecrystallized austenite region is a temperature range of approximately 950° C. or lower. Therefore, when the finishing temperature is higher than 950° C., austenite grains grow, and a grain length of martensite in the hot-rolled steel sheet obtained after the cooling increases. As a result, it becomes difficult to obtain a desired texture, and there are cases where the strength and toughness of the hot-rolled steel sheet decrease. Therefore, the finishing temperature is preferably set to 950° C. or lower.
In the cooling step, it is preferable that cooling is started within 1.0 second after the completion of the finish rolling, and cooling to a temperature range of 300° C. or lower is performed so that an average cooling rate in a temperature range of the finishing temperature to 300° C. is 100° C./s or faster.
In the present embodiment, it is preferable that cooling equipment is installed at a rear stage of finish rolling equipment, and the cooling is performed while the steel sheet after the finish rolling passes through the cooling equipment. The cooling equipment is preferably equipment that can cool the steel sheet at an average cooling rate of 100° C./s or faster. Examples of the cooling equipment include water cooling equipment using water as a cooling medium.
The average cooling rate in the cooling step is a value obtained by dividing a temperature drop width of the steel sheet from when the cooling is started to when the cooling is ended by a time required from when the cooling is started to when the cooling is ended. When the cooling is started refers to a time when the steel sheet is introduced into the cooling equipment, and when the cooling is ended refers to a time when the steel sheet is taken out of the cooling equipment.
Examples of the cooling equipment include equipment having no intermediate air cooling section and equipment having at least one intermediate air cooling section. In the present embodiment, any cooling equipment may be used. Even in a case where cooling equipment having an air cooling section is used, the average cooling rate from the start of cooling to the end of cooling may be 100° C./s or faster.
Hereinafter, the reasons for limiting cooling conditions will be described. A cooling stop temperature is 300° C. or lower, and this condition will be described in the coiling step.
Cooling Start Time: Within 1.0 Second after Completion of Finish Rolling
It is preferable to start cooling immediately after the completion of the finish rolling. When the cooling start time is longer than 1.0 second, recrystallization proceeds, cooling is performed in a state where the strain is released, and there are cases where a desired texture cannot be obtained in the hot-rolled steel sheet.
Therefore, it is preferable to start the cooling within 1.0 second after the completion of the finish rolling.
When the average cooling rate in the temperature range of the finishing temperature to 300° C. is slower than 100° C./s, bainite or ferrite is likely to be formed, and there are cases where a desired amount of martensite cannot be obtained. Therefore, the average cooling rate in the temperature range of the finishing temperature to 300° C. is preferably set to 100° C./s or faster.
In the coiling step, the steel sheet cooled to a temperature range of 300° C. or lower is preferably coiled. Since the steel sheet is coiled immediately after the cooling, a coiling temperature is almost equal to the cooling stop temperature. When the coiling temperature is higher than 300° C., polygonal ferrite or bainite is generated, and there are cases where the strength of the hot-rolled steel sheet decreases. Therefore, the coiling temperature is preferably set to a temperature range of 300° C. or lower.
After the coiling, the hot-rolled steel sheet may be subjected to temper rolling according to a conventional method, or subjected to pickling to remove the scale formed on the surface. Alternatively, a plating treatment such as aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electrogalvanizing, and hot-dip galvannealing, or a chemical conversion treatment may be performed.
The hot-rolled steel sheet according to the present embodiment can be stably manufactured by the preferred manufacturing method described above.
Next, examples of the present invention will be described. Conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. The present invention may adopt various conditions to achieve the object of the present invention without departing from the scope of the present invention.
Molten steels having the chemical compositions shown in Table 1 were melted in a converter and slabs were obtained by a continuous casting method. Next, these slabs were heated under the conditions shown in Tables 2A and 2B, subjected to rough rolling, and then subjected to finish rolling under the conditions shown in Tables 2A and 2B. After the finish rolling was completed, the slabs were cooled and coiled under the conditions shown in Tables 3A and 3B to obtain hot-rolled steel sheets having the sheet thicknesses shown in Tables 3A and 3B.
In the heating step, holding times at the heating temperatures shown in Tables 2A and 2B were set to 4,800 seconds or shorter.
In addition, as cooling after the finish rolling, water cooling was performed in which the steel sheet was passed through water cooling equipment having no intermediate air cooling section. An average cooling rate in Tables 3A and 3B is a value obtained by dividing a temperature drop width of the steel sheet from when the steel sheet was introduced into the water cooling equipment to when the steel sheet was taken out of the water cooling equipment by a time required for the steel sheet to be passed through the water cooling equipment.
A test piece was collected from the obtained hot-rolled steel sheet, an area ratio of each structure and pole densities of textures were measured and a tensile test was conducted by the above-described methods.
The obtained results are shown in Tables 4A and 4B.
In a case where an obtained tensile strength was 1,180 MPa or more, the hot-rolled steel sheet was determined to be acceptable as having high strength. On the other hand, in a case where the obtained tensile strength was less than 1,180 MPa, the hot-rolled steel sheet was determined to be unacceptable as not having high strength.
A Charpy impact test was conducted to evaluate the toughness of the hot-rolled steel sheets, and a ductile-brittle transition temperature was measured. For the measurement of the ductile-brittle transition temperature, a C-direction notch Charpy impact test was conducted using a V-notch test piece having a subsize of 2.5 mm according to JIS Z 2242:2018. A temperature at which a brittle fracture surface ratio became 50% was defined as the ductile-brittle transition temperature. In addition, for hot-rolled steel sheets having a final sheet thickness of less than 2.5 mm, an overall thickness was measured.
In a case where the obtained ductile-brittle transition temperature was −50° C. or lower, the hot-rolled steel sheet was determined to be acceptable as having excellent toughness. On the other hand, in a case where the obtained ductile-brittle transition temperature was higher than −50° C., the hot-rolled steel sheet was determined to be unacceptable as being inferior in toughness.
Furthermore, the anisotropy in toughness was evaluated by the following method. According to JIS Z 2242:2018, an absorbed energy of a C-direction notch and an absorbed energy of an L-direction notch were measured by a Charpy impact test using a V-notch test piece having a subsize of 2.5 mm. The Charpy impact test was conducted at −60° C. A difference between the absorbed energy of the L-direction notch and the absorbed energy of the C-direction notch was calculated, and in a case where the difference was ±15 J or less, the hot-rolled steel sheet was determined to be acceptable as having reduced anisotropy in toughness. On the other hand, in a case where the difference between the absorbed energy of the L-direction notch and the absorbed energy of the C-direction notch was more than ±15 J, the hot-rolled steel sheet was determined to be unacceptable as not having reduced anisotropy in toughness.
3.100
0.080
0.600
0.050
0.30
3.10
38
11.0
82
1,220
33
H
I
57
780
98
960
0.1
U
V
W
X
1,120
790
1,150
32
82
35
86
32
86
11.0
11.0
H
400
I
2.0
U
V
W
X
1.2
350
6.0
1.7
14
1.6
17
6.4
20
6.2
23
1.8
24
H
81
19
25
I
1.9
21
−16
14
−17
17
22
20
18
23
−42
−22
24
1,150
−19
25
1,020
18
U
7.8
V
W
X
1.9
88
12
7.3
1.9
1.8
1.9
8.0
5.2
7.6
5.2
7.3
5.9
7.3
7.9
69
31
1.8
67
33
29
−16
1,170
18
21
1,150
18
16
1,090
16
18
16
23
−19
22
−19
19
−18
17
−19
1,088
18
1,100
−16
From Tables 4A and 4B, it can be seen that the hot-rolled steel sheets according to the present invention examples had high strength, excellent toughness, and reduced anisotropy in toughness. On the other hand, it can be seen that in the hot-rolled steel sheets according to the comparative examples, any of the properties deteriorated.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-168623 | Oct 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/038035 | 10/12/2022 | WO |
