Patent Application
20230055479
The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is formed into various shapes by press working or the like to be used, and particularly relates to a hot-rolled steel sheet that has high strength and has excellent ductility and shearing workability.
Priority is claimed on Japanese Patent Application No. 2020-010944, filed on Jan. 27, 2020, the content of which is incorporated herein by reference.
In recent years, from the viewpoint of protecting the global environment, efforts have been made to reduce the amount of carbon dioxide gas emitted in many fields. Vehicle manufacturers are also actively developing techniques for reducing the weight of vehicle bodies for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of vehicle bodies since the emphasis is placed on improvement in collision resistance to secure the safety of the occupants.
In order to achieve both vehicle body weight reduction and collision resistance, an investigation has been conducted to make a member thin by using a high strength steel sheet. Therefore, a steel sheet having both high strength and excellent formability is strongly desired. Several techniques have been proposed from the related art to meet these demands.
Since there are various working methods for vehicle members, the required formability differs depending on members to which the working methods are applied, but among these, ductility is placed as important indices for formability.
In addition, vehicle members are formed by press forming, and the press-formed blank sheet is often manufactured by highly productive shearing working. The blank sheet manufactured by shearing working needs to have excellent end surface accuracy after the shearing working.
In the technique for improving ductility, for example, Patent Document 1 discloses a high strength steel sheet for a vehicle having excellent collision resistant safety and formability, in which residual austenite having an average grain size of 5 μm or less is dispersed in ferrite having an average grain size of 10 μm or less. In the steel sheet containing residual austenite in the metallographic structure, while the austenite is transformed into martensite during working and large elongation is exhibited due to transformation-induced plasticity, the formation of full hard martensite impairs hole expansibility. Patent Document 1 discloses that not only ductility but also hole expansibility are improved by refining the ferrite and the residual austenite.
Patent Document 2 discloses a high strength steel sheet having excellent ductility and stretch flangeability and having a tensile strength of 980 MPa or more, in which a second phase including residual austenite and/or martensite is finely dispersed in crystal grains.
In the technique for improving shearing workability, for example, Patent Document 3 discloses a technique for controlling burr height after punching by controlling a ratio ds/db of the ferrite grain size ds of the surface layer to ferrite crystal grain db of an inside to 0.95 or less.
Patent Document 4 discloses a technique for improving separations or burrs on an end surface of a plate by reducing a P content.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 1111-61326
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2005-179703
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. H10-168544
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2005-298924
The techniques disclosed in Patent Documents 1 to 4 are all techniques of improving either ductility or an end surface property after shearing working. However, Patent Documents 1 to 3 do not refer to a technique for achieving both of the properties. Patent Document 4 refers to both shearing workability and press formability. However, since the strength of a steel sheet disclosed in Patent Document 4 is less than 850 MPa, it may be difficult to apply the steel sheet to a member having a high strength of 980 MPa or more.
In addition, in particular, a high strength steel sheet of 980 MPa or more has objects that a proportion of a sheared section to an end surface after shearing working is not stable and an accuracy of a cut end surface varies.
The present invention has been made in view of the above objects of the related art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent ductility and shearing workability.
In view of the above objects, the present inventors have conducted intensive studies on a chemical composition of a hot-rolled steel sheet and a relationship between a metallographic structure and mechanical properties. As a result, the following findings (a) to (h) were obtained, and the present invention was completed.
The expression of having excellent shearing workability refers to that a proportion of a sheared section to an end surface after shearing working (hereinafter, may be referred to as a sheared section proportion) is stable (the amount of change in the sheared section proportion is small).
In addition, the expression of having excellent strength or having high strength refers to that tensile strength is 980 MPa or more.
(a) In order to obtain the excellent tensile (maximum) strength, it is preferable to use a full hard structure. That is, it is preferable to include martensite, tempered martensite, and/or bainite in a structure.
(b) However, since the full hard structure is a structure having poor ductility, excellent ductility cannot be secured simply with the metallographic structure mainly having the full hard structures.
(c) In order to allow a hot-rolled steel sheet having high strength to also have excellent ductility, it is effective to contain an appropriate amount of ferrite having high ductility.
(d) Since the ferrite is generally soft, it is necessary to use Ti, Nb, V, and the like as precipitation hardening elements in order to obtain desired strength. Therefore, it is effective to perform intermediate air cooling in the hot rolling process to obtain an appropriate amount of precipitation hardened ferrite.
(e) A full hard structure is generally formed in a phase transformation at 600° C. or lower, but in this temperature range, a large number of a grain boundary having a crystal misorientation of 60° and a grain boundary having a crystal misorientation of 7° about the <110> direction are formed.
(f) When forming the grain boundary having a crystal misorientation of 600 about the <110> direction, dislocations are less likely to accumulate in a full hard structure. In a hard phase, in this metallographic structure in which the density of grain boundaries is high and grain boundaries are uniformly dispersed (that is, a total length of the grain boundaries having a crystal misorientation of 60° about the <110> direction is large), dislocations are less likely to accumulate in the full hard structure during shearing working, cracks are less likely to occur from inside the full hard structure. As a result, even in a case where a hard phase occasionally exists near a cutting edge of a shearing tool, cracks are less likely to occur, and a proportion of the sheared section is kept constant, that is, the proportion of the sheared section is stabilized.
(g) In order to uniformly disperse the grain boundary having a crystal misorientation of 60° about the <110> direction in the hard phase, a standard deviation of a Mn concentration is required to be equal to or less than a certain value. In order to set the standard deviation of the Mn concentration to be equal to or less than a certain value, when a slab is heated, it is effective to retain the slab in a temperature range of 700° C. to 850° C. for 900 seconds or longer, and then further heat the slab, retain in a temperature range of 1100° C. or higher for 6000 seconds or longer, and perform hot rolling so that a total sheet thickness is reduced by 90% or more in the temperature range of 850° C. to 1100° C.
(h) In order to increase the length of the grain boundary having a crystal misorientation of 600 about the <110> direction and decrease the length of the grain boundary having a crystal misorientation of 7° about the <110> direction, it is effective to perform coiling at 400° C. to 600° C.
The gist of the present invention made based on the above 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 %:
C: 0.050% to 0.250%;
Si: 0.05% to 3.00%;
Mn: 1.00% to 4.00%;
one or two or more of Ti, Nb, or V: 0.060% to 0,500% in total;
sol. Al: 0.001% to 2.000%;
P: 0.100% or less;
S: 0.0300% or less;
N: 0.1000% or less;
O: 0.0100% or less;
Cu: 0% to 2.00%;
Cr: 0% to 2.00%;
Mo: 0% to 1.00%;
Ni: 0% to 2.00%;
B: 0% to 0.0100%;
Ca: 0% to 0.0200%;
Mg: 0% to 0.0200%;
REM: 0% to 0.1000%0;
Bi: 0% to 0.020%;
one or two or more of Zr, Co, Zn, or W: 0% to 1.00% in total;
Sn: 0% to 0.050%; and
a remainder consisting of Fe and impurities,
in which a metallographic structure
has a tensile strength of 980 MPa or more.
(2) In the hot-rolled steel sheet according to (1), an average grain size of a surface layer may be less than 3.0 μm.
(3) The hot-rolled steel sheet according to (1) or (2) may include, as the chemical composition, by mass %, one or two or more selected from the group consisting of
Cu: 0.01% to 2.00%,
Cr: 0.01% to 2.00%,
Mo: 0.01% to 1.00%,
Ni: 0.02% to 2.00%,
B: 0.0001% to 0.0100%,
Ca: 0.0005% to 0.0200%,
Mg: 0.0005% to 0.0200%,
REM: 0.0005% to 0.1000%, and
Bi: 0.0005% to 0.020%.
According to the above aspect of the present invention, it is possible to obtain a hot-rolled steel sheet having excellent strength, ductility, and shearing workability. Further, according to a preferred embodiment according to the present invention, it is possible to obtain a hot-rolled steel sheet having the above-mentioned various properties and further suppressing the occurrence of cracking inside a bend, that is, having excellent resistance to cracking inside a bend.
The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for vehicle members, mechanical structural members, and building members.
The chemical composition and metallographic structure of a hot-rolled steel sheet (hereinafter, sometimes simply referred to as a steel sheet) according to the present embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the scope of the present invention.
The numerical limit range described with “to” in between includes the lower limit and the upper limit. Regarding the numerical value indicated by “less than” or “more than”, the value does not fall within the numerical range.
In the following description, % regarding the chemical composition of the hot-rolled steel sheet is mass % unless otherwise specified.
1. Chemical Composition
The hot-rolled steel sheet according to the present embodiment includes, by mass %, C: 0.050% to 0.250%, Si: 0.05% to 3.00%, Mn: 1.00% to 4.00%, one or two or more of Ti, Nb, or V: 0.060% to 0.500% in total, sol. Al: 0.001% to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and a remainder consisting of Fe and impurities. Each element will be described in detail below.
(1-1) C: 0.050% to 0.250%
C increases an area fraction of the hard phase and increases the strength of the ferrite by combining with precipitation hardening elements such as Ti, Nb, and V. When the C content is less than 0.050%, it is difficult to obtain a desired strength. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more and more preferably 0.070% or more.
On the other hand, when the C content is more than 0.250%, the area fraction of the ferrite decreases, so that the ductility of the hot-rolled steel sheet decreases. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.150% or less, less than 0.150%, or 0.130% or less.
(1-2) Si: 0.05% to 3.00%
Si has an action of promoting the formation of ferrite to improve the ductility of the hot-rolled steel sheet and an action of solid solution strengthening the ferrite to increase the strength of the hot-rolled steel sheet. In addition, Si has an action of making the steel sound by deoxidation (suppressing the occurrence of defects such as blow holes in the steel). When the Si content is less than 0.05%, an effect by the action cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.30% or more, 0.50% or more, or 0.80% or more.
However, when the Si content is more than 3.00%, the surface properties, the chemical convertibility, the ductility and the weldability of the hot-rolled steel sheet are significantly deteriorated, and the A3 transformation point is significantly increased. This makes it difficult to perform hot rolling in a stable manner. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less and more preferably 2.50% or less.
(1-3) Mn: 1.00% to 4.00%
Mn has actions of suppressing ferritic transformation and high-strengthening the hot-rolled steel sheet. When the Mn content is less than 1.00%, the tensile strength of 980 MPa or more cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.50% or more and more preferably 1.80% or more.
On the other hand, when the Mn content is more than 4.00%, an angular difference of the crystal grain in the hard phase becomes non-uniform due to the segregation of Mn, and the sheared section proportion becomes unstable. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less or 3.50% or less.
(1-4) One or Two or More of Ti, Nb, or V: 0.060 to 0.500% in Total
Ti, Nb, and V are elements that are finely precipitated in steel as carbides and nitrides and improve the strength of steel by precipitation hardening. In addition, Ti, Nb, and V are elements that fix C by forming the carbides and suppress the formation of cementite, which is harmful to shearing workability. In order to obtain these effects, the total amount of Ti, Nb, and V is set to 0.060% or more. It is not necessary that all of Ti, Nb, and V are contained, and any one of these elements may be contained. One of Ti, Nb, and V may be contained and the amount thereof may be 0.060% or more, and two or more of Ti, Nb, and V are contained and the total amount thereof may be 0.060% or more. The total amount of Ti, Nb, and V is preferably 0.080% or more.
On the other hand, when the total amount of Ti, Nb, and V is more than 0.500%, the workability is deteriorated. Therefore, the total amount of Ti, Nb, and V is set to 0.500% or less. The total amount of Ti, Nb, and V is preferably 0.300% or less, and more preferably 0.250% or less.
(1-5) Sol. Al: 0.001% to 2.000%
Similar to Si, Al has an action of making the steel sheet sound by deoxidizing, and also has an action of promoting the formation of ferrite and increasing the ductility of the hot-rolled steel sheet. When a sol. Al content is less than 0.001%, an effect by the action cannot be obtained. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more or 0.030% or more.
On the other hand, when the sol. Al content is more than 2.000%, the above effects are saturated and this case is not economically preferable. Thus, the sol. Al content is set to 2.000% or less. The sol. Al content is preferably 1.500% or less, 1.000% or less, 0.500% or less, or 0.100% or less.
The sol. Al in the present embodiment means acid-soluble Al, and refers to solid solution Al present in steel in a solid solution state.
(1-6) P: 0.100% or Less
P is an element that is generally contained as an impurity and is also an element having an action of enhancing the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, although P may be positively contained, P is an element that is easily segregated, and when the P content is more than 0.100%, the ductility is significantly decreased due to the boundary segregation. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less.
The lower limit of the P content does not need to be particularly specified, but is preferably 0.001% from the viewpoint of refining cost.
(1-7) S: 0.0300% or Less
S is an element that is contained as an impurity and forms sulfide-based inclusions in the steel to decrease the ductility of the hot-rolled steel sheet. When the S content is more than 0.0300%, the ductility of the hot-rolled steel sheet is significantly decreased. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less.
The lower limit of the S content does not need to be particularly specified, but is preferably 0.0001% from the viewpoint of refining cost.
(1-8) N: 0.1000% or Less
N is an element contained in steel as an impurity and has an action of decreasing the ductility of the hot-rolled steel sheet. When the N content is more than 0.1000%, the ductility of the hot-rolled steel sheet is significantly decreased. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less and more preferably 0.0700% or less.
Although the lower limit of the N content does not need to be particularly specified, in a case where one or two or more of Ti, Nb, or V are contained to further refine the metallographic structure, the N content is preferably 0.0010% or more and more preferably 0.0020% or more to promote the precipitation of carbonitride.
(1-9) O: 0.0100% or Less
When a large amount of O is contained in the steel, O forms a coarse oxide that becomes the origin of fracture, and causes brittle fracture and hydrogen-induced cracks. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less and 0.0050% or less.
The O content may be 0.0005% or more or 0.0010% or more to disperse a large number of fine oxides when the molten steel is deoxidized.
The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, and the like, and are allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment.
The hot-rolled steel sheet according to the present embodiment may contain Ti, Nb, V, Cu, Cr, Mo, Ni, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W, and Sn as optional elements, instead of a part of Fe. In a case where the above optional elements are not contained, the lower limit of the content thereof is 0%. Hereinafter, the above optional elements will be described in detail.
(1-10) Cu: 0.01% to 2.00%, Cr: 0.01% to 2.00%, Mo: 0.01% to 1.00%, Ni: 0.02% to 2.00%, and B: 0.0001% to 0.0100%
Cu, Cr, Mo, Ni and B all have an action of enhancing the hardenability of the hot-rolled steel sheet and increasing the tensile strength. In addition, Cu and Mo have an action of being precipitated as carbides in the steel to increase the strength of the hot-rolled steel sheet. Further, in a case where Cu is contained, Ni has an action of effectively suppressing the grain boundary crack of the slab caused by Cu. Therefore, one or two or more of these elements may be contained.
As described above, Cu has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of being precipitated as carbide in the steel at a low temperature to enhance the strength of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Cu content is preferably 0.01% or more and more preferably 0.05% or more.
However, when the Cu content is more than 2.00%, grain boundary cracks may occur in the slab in some cases. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less and 1.00% or less.
As described above, Cr has an action of enhancing the hardenability of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Cr content is preferably 0.01% or more or 0.05% or more.
However, when the Cr content is more than 2.00%, the chemical convertibility of the hot-rolled steel sheet is significantly decreased. Accordingly, the Cr content is set to 2.00% or less.
As described above, Mo has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of being precipitated as carbides in the steel to enhance the strength of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Mo content is preferably 0.01% or more or 0.02% or more.
However, even when the Mo content is set to be more than 1.00%, the effect by the action is saturated, and this case is not economically preferable. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less and 0.20% or less.
As described above, Ni has an action of enhancing the hardenability of the hot-rolled steel sheet. In addition, when Cu is contained, Ni has an action of effectively suppressing the grain boundary crack of the slab caused by Cu. In order to more reliably obtain the effect by the action, the Ni content is preferably 0.02% or more.
Since Ni is an expensive element, it is not economically preferable to contain a large amount of Ni, Therefore, the Ni content is set to 2.00% or less.
As described above, B has an action of enhancing the hardenability of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the B content is preferably 0.0001% or more or 0.0002% or more.
However, when the B content is more than 0.0100%, the ductility of the hot-rolled steel sheet is significantly decreased, and thus the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.
(1-11) Ca: 0.0005% to 0.0200%, Mg: 0.0005% to 0.0200% c, REM: 0.0005% to 0.1000%, and Bi: 0.0005% to 0.020%
All of Ca, Mg, and REM have an action of enhancing the formability of the hot-rolled steel sheet by adjusting the shape of inclusions in the steel to a preferable shape. In addition, Bi has an action of enhancing the formability of the hot-rolled steel sheet by refining the solidification structure. Therefore, one or two or more of these elements may be contained. In order to more reliably obtain the effect by the action, it is preferable that any one or more of Ca, Mg, REM, and Bi is 0.0005% or more.
However, when the Ca content or Mg content is more than 0.0200%, or when the REM content is more than 0.1000%, the inclusions are excessively formed in the steel, and thus the ductility of the hot-rolled steel sheet may be decreased in some cases. In addition, even when the Bi content is more than 0.020%, the above effect by the action is saturated, and this case is not economically preferable. Therefore, the Ca content and Mg content are set to 0.0200% or less, the REM content is set to 0.1000% or less, and the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less.
Here, REM refers to a total of 17 elements including Sc, Y, and lanthanoid, and the REM content refers to the total amount of these elements. In a case of the lanthanoid, lanthanoid is industrially added in the form of misch metal.
(1-12) One or Two or More of Zr, Co, Zn, or W: 0% to 1.00% in Total and Sn: 0% to 0.050%
Regarding Zr, Co, Zn, and W, the present inventors have confirmed that even when the total amount of these elements is 1.00% or less, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. Therefore, one or two or more of Zr, Co, Zn, or W may be contained in a total of 1.00% or less.
Further, the present inventors have confirmed that the effect of the hot-rolled steel sheet according to the present embodiment is not impaired even if a small amount of Sn is contained. However, when a large amount of Sn is contained, a defect may occur during hot rolling, and thus, the Sn content is set to 0.050% or less.
The above-described chemical composition of the hot-rolled steel sheet may be measured by a general analytical method. For example, inductively coupled plasma-atomic emission spectrometry (ICP-AES) may be used for measurement. In addition, sol. Al may be measured by the ICP-AES using a filtrate after heat-decomposing a sample with an acid. C and S may be measured by using a combustion-infrared absorption method, and N may be measured by using the inert gas melting-thermal conductivity method.
2. Metallographic Structure of Hot-Rolled Steel Sheet
Next, the metallographic structure 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 metallographic structure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite, has a ratio L60/L7 of a length L60 of a grain boundary having a crystal misorientation of 60° to a length L7 of a grain boundary having a crystal misorientation of 7° about a <110> direction of 0.60 or more, and has a standard deviation of a Mn concentration of 0.60 mass % or less. Therefore, the hot-rolled steel sheet according to the present embodiment can obtain excellent strength, ductility, and shearing workability.
In the present embodiment, a microstructural fraction, L60/L7, and a standard deviation of the Mn concentration in the metallographic structure at a depth of ¼ of the sheet thickness from a surface and a center position in a sheet width direction in a cross section parallel to a rolling direction are defined. The reason for defining the metallographic structure at the depth of ¼ of the sheet thickness from the surface and the center position in the sheet width direction in the cross section parallel to the rolling direction is that the metallographic structure at this position is a typical metallographic structure of the steel sheet.
The position at the depth of ¼ of the sheet thickness from the surface is a region between a depth of ⅛ of the sheet thickness from the surface and a depth of ⅜ of the sheet thickness from the surface.
(2-1) Area Fraction of Residual Austenite: Less than 3.0%
The residual austenite is a structure that is present as a face-centered cubic lattice even at room temperature. The residual austenite increases the ductility of the hot-rolled steel sheet due to transformation-induced plasticity (TRIP). On the other hand, the residual austenite has an action of being transformed into high-carbon martensite during shearing working to inhibit stable crack initiation, which causes the sheared section proportion to become unstable. When the area fraction of the residual austenite is 3.0% or more, the action is manifested, shearing workability of the hot-rolled steel sheet is deteriorated. Therefore, the area fraction of the residual austenite is set to less than 3.0%. The area fraction of the residual austenite is preferably less than 1.0%. Since less residual austenite is preferable, the area fraction of the residual austenite may also be 0%.
As the measurement method of the area fraction of the residual austenite, methods by X-ray diffraction, electron back scatter diffraction image (EBSP, electron back scattering diffraction pattern) analysis, and magnetic measurement and the like may be used and the measured values may differ depending on the measurement method. In the present embodiment, the area fraction of the residual austenite is measured by X-ray diffraction.
In the measurement of the area fraction of the residual austenite by X-ray diffraction in the present embodiment, first, the integrated intensities of a total of 6 peaks of α(110), α(200), α(211), γ(111), γ(200), and γ(220) are obtained in the cross section parallel to the rolling direction at a depth of ¼ of the sheet thickness of the hot-rolled steel sheet (region between a depth of ⅛ of the sheet thickness from the surface and a depth of ⅜ of the sheet thickness from the surface) and the center position in the sheet width direction, using Co-Kα rays, and the area fraction of the residual austenite is obtained by calculation using the strength averaging method.
(2-2) Area Fraction of Ferrite: 15.0% or More and Less than 60.0%
Ferrite is a structure formed when fec transforms into bcc at a relatively high temperature. The ferrite has a high work hardening rate, and thus has an action of enhancing the strength-ductility balance of the hot-rolled steel sheet. In order to obtain the action, the area fraction of the ferrite is set to 15.0% or more. The area fraction of the ferrite is preferably 20.0% or more. On the other hand, since the ferrite has low strength, it is not possible to obtain a desired tensile strength when the area fraction is excessive. Therefore, the ferrite area fraction is set to less than 60.0%. Preferably, the ferrite area fraction is 50.0% or less, 45.0% or less, or 40.0% or less.
(2-3) Area Fraction of Pearlite: Less than 5.0%
Pearlite is a lamellar metallographic structure in which cementite is precipitated in layers between ferrite, and is a soft metallographic structure as compared with bainite and martensite. When the area fraction of the pearlite is 5.0% or more, carbon is consumed by the cementite contained in the pearlite, the strength of martensite or bainite, which is the remainder in microstructure, is lowered, and the tensile strength of 980 MPa or more cannot be obtained. Therefore, the area fraction of the pearlite is set to less than 5.0%. The area fraction of the pearlite is preferably 3.0% or less, 2.0% or less, or 1.0% or less. In order to improve the ductility of the hot-rolled steel sheet, it is preferable to reduce the area fraction of the pearlite as possible, a lower limit thereof is set to 0%.
(2-4) Bainite, Martensite, and Tempered Martensite: More than 32.0% and 85.0% or Less in Total
The hot-rolled steel sheet according to the present embodiment may contain a full hard structure including one or two or more of bainite, martensite, and tempered martensite, with a total area fraction of more than 32.0% and 85.0% or less, as the remainder in microstructure other than the residual austenite, ferrite, and pearlite. By setting the total area fraction of bainite, martensite, and tempered martensite to more than 32.0%, the strength of the hot-rolled steel sheet can be enhanced. Therefore, the total area fraction of bainite, martensite and, tempered martensite is preferably more than 32.0%. More preferably, the total area fraction is 35.0% or more, 40.0% or more, more than 43.0%, or 50.0% or more.
In addition, by setting the total area fraction of bainite, martensite, and tempered martensite to 85.0% or less, the ductility of the hot-rolled steel sheet can be increased. Therefore, the total area fraction of bainite, martensite and, tempered martensite is preferably 85.0% or less. More preferably, the total area fraction is 80.0% or less, 75.0% or less, or 70.0% or less.
One of bainite, martensite, and tempered martensite may be contained, and the area fraction thereof may be more than 32.0% and 85.0% or less. Two or more kinds of bainite, martensite, and tempered martensite may be contained, and the total area fraction thereof may be more than 32.0% and 85.0% or less.
Measurement of the area fraction of the ferrite and the pearlite is conducted in the following manner.
The cross section perpendicular to the rolling direction is mirror-finished and polished at a room temperature with colloidal silica without containing an alkaline solution for 8 minutes to remove the strain introduced into the surface layer of a sample A region with a length of 50 μm and between a depth of ⅛ of the sheet thickness from the surface and a depth of ⅜ of the sheet thickness from the surface is measured by electron backscatter diffraction at a measurement interval of 0.1 μm, such that a measurement can be performed at a depth of ¼ of the sheet thickness from a surface and a center position in a sheet width direction in a cross section parallel to a rolling direction, in a random position of the sample cross section in a longitudinal direction, to obtain crystal orientation information.
For the measurement, an EBSD analyzer configured of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. In this case, the EBSD analyzer is set such that the degree of vacuum inside is 9.6×10−5 Pa or less, an acceleration voltage is 15 kV, an irradiation current level is 13, and an electron beam irradiation level is 62. Further, a reflected electron image is captured in the same visual field. First, crystal grains in which ferrite and cementite are precipitated in layers are identified from a reflected electron image, and the area fraction of the crystal grains is calculated to obtain the area fraction of pearlite. Then, for crystal grains excluding the crystal grains determined as the pearlite, a region where a Grain Average Misorientation value is 1.0° or less is determined to be ferrite, using the obtained crystal orientation information in a “Grain Average Misorientation” installed in the software “OIM Analysis (registered trademark)” (manufactured by AMETEK, Inc.) attached to the EBSD analyzer. When determining the area fraction of the region determined as the ferrite, the area fraction of the ferrite is obtained.
The area fraction of remainder in microstructure (a full hard structure including one or two or more of bainite, martensite, and tempered martensite) is obtained by subtracting the area fraction of the residual austenite, the area fraction of ferrite, and the area fraction of pearlite, from 100%.
(2-5) Ratio L60/L7 of a Length L60 of Grain Boundary having Crystal misorientation of 600 to a Length L7 of Grain Boundary having Crystal misorientation of 7° about <110> Direction: 0.60 or More
In order to obtain a high strength of 980 MPa or more, the primary phase is required to have a full hard structure. The full hard structure is generally formed in a phase transformation at 600° C. or lower, but in this temperature range, a large number of a grain boundary having a crystal misorientation of 60° and a grain boundary having a crystal misorientation of 7° about the <110> direction are formed. When forming the grain boundary having a crystal misorientation of 60° about the <110> direction, dislocations are less likely to accumulate in a full hard structure. Therefore, in the hard phase, in this metallographic structure in which the density of grain boundaries is high and grain boundaries are uniformly dispersed (that is, a total length of the grain boundaries having a crystal misorientation of 60° about the <110> direction is large), since the hard phase is less likely to be deformed, strain is less likely to be concentrated inside the full hard structure and cracks are stably initiated regardless of the presence or absence of the hard phase near a cutting edge of a shearing tool. As a result, the sheared section proportion becomes stable.
On the other hand, in the grain boundary having a crystal misorientation of 7° about the <110> direction, dislocations are likely to accumulate in a hard phase. Therefore, in the hard phase, in the metallographic structure in which the density of the grain boundaries having a crystal misorientation of 7° about the <110> direction is high, since the hard phase is easily deformed, dislocation is introduced into the hard phase during shearing working and the initiation of cracks from the inside of the hard phase is promoted, so that the sheared section proportion changes depending on the presence or absence of the hard phase near the cutting edge of the shearing tool. As a result, the sheared section proportion becomes unstable.
Therefore, when the length of a grain boundary having a crystal misorientation of 60° is set to L60 and the length of the grain boundary having a crystal misorientation of 7° about a <110> direction is set to L7, stability of the sheared section proportion is dominated by L60/L7. In a case where Loo/L7 is less than 0.60, the sheared section proportion becomes unstable due to the above action. Therefore, in order to improve the shearing workability of the hot-rolled steel sheet, it is necessary to set L6/L7 to 0.60 or more. L60/L7 is preferably 0.63 or more, 0.65 or more, or 0.70 or more. The upper limit of L6/L7 does not need to be specified, but may be 1.50 or less and 1.00 or less.
The grain boundary having a crystal misorientation of X° about the <110> direction refers to a grain boundary having a crystallographic relationship in which the crystal orientations of the crystal grain A and the crystal grain B are the same by rotating one crystal grain B by X° along the <110> axis, when two adjacent crystal grain A and crystal grain B are specified at a certain grain boundary. However, considering the measurement accuracy of the crystal orientation, an orientation difference of ±4° is allowed from the matching orientation relationship.
In the present embodiment, the length L60 of a grain boundary having a crystal misorientation of 60° and the length L7 of a grain boundary having a crystal misorientation of 7° about the <110> direction are measured by using the electron back scatter diffraction pattern-orientation image microscopy (EBSP-OIM) method.
In the EBSP-OIM method, a crystal orientation of an irradiation point can be measured for a short time period in such manner that a highly inclined sample in a scanning electron microscope (SEM) is irradiated with electron beams, a Kikuchi pattern formed by back scattering is photographed by a high sensitive camera, and the photographed image is processed by a computer.
The EBSP-OIM method is carried out using an EBSD analyzer configured of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector, and OIM Analysis (registered trademark) manufactured by AMETEK, Inc. In the EBSP-OIM method, since the fine structure of the sample surface and the crystal orientation can be analyzed, the length of the grain boundary having a specific crystal misorientation can be quantitatively determined. The analyzable area of the EBSP-OIM method is a region that can be observed by the SEM. The EBSP-OIM method makes it possible to analyze a region with a minimum resolution of 20 nm, which varies depending on the resolution of the SEM.
In measuring the length of specific grain boundaries of the metallographic structure at the depth of ¼ of the sheet thickness from the steel sheet surface (region between a depth of ⅛ of the sheet thickness from the surface and a depth of ⅜ of the sheet thickness from the surface) and the center position in the sheet width direction in the cross section parallel to the rolling direction, the analysis is performed with 1200 fold magnification, in a region of 40 μm×30 μm, for at least 5 visual fields. Moreover, an average value of the lengths of the grain boundary having a crystal misorientation of 600 about the <110> direction is calculated to obtain L60. Similarly, an average value of the lengths of the grain boundary having a crystal misorientation of 7° about the <110> direction is calculated to obtain L7. As described above, the orientation difference of ±4° is allowed.
Ferrite and pearlite are soft phases and have a small effect on a dislocation accumulation effect inside the hard phase. In addition, residual austenite is not a structure formed by a phase transformation at 600° C. or lower and has no effect of dislocation accumulation. Therefore, in the present measurement method, ferrite, pearlite, and residual austenite are not included as a target in the analysis. That is, in the present embodiment, the length L60 of a grain boundary having a crystal misorientation of 60° and the length L7 of a grain boundary having a crystal misorientation of 7° about the <110> direction are the lengths of the full hard structure (one or two or more of bainite, martensite, and tempered martensite). The pearlite is identified in the same manner as the measurement method of the area fraction of the pearlite and the ferrite is identified in the same manner as the measurement method of the area fraction of the ferrite, so that the pearlite and the ferrite can be excluded from the analysis target. In addition, the EBSP-OIM method, the residual austenite having a crystal structure of fcc can be excluded from the analysis target.
(2-6) Standard Deviation of Mn Concentration: 0.60 Mass % or Less
The standard deviation of the Mn concentration at a depth of ¼ of the sheet thickness from a surface of the hot-rolled steel sheet according to the present embodiment (region between a depth of ⅛ of the sheet thickness from the surface and a depth of ⅜ of the sheet thickness from the surface) and the center position in the sheet width direction is 0.60 mass % or less. Accordingly, the grain boundary having a crystal misorientation of 60° about the <110> direction can be uniformly dispersed. As a result, the sheared section proportion can be stabilized. The standard deviation of the Mn concentration is preferably 0.55 mass % or less, 0.50 mass % or less, or 0.45 mass % or less.
A lower limit of the standard deviation of the Mn concentration is preferably as small as the value from the viewpoint of stabilizing the sheared section proportion, but a practical lower limit is 0.10 mass % due to the restrictions of the manufacturing process.
The standard deviation of the Mn concentration is measured by the following method.
After the L cross section of the hot-rolled steel sheet is mirror polished, the center position in the sheet width direction at the depth of ¼ of the sheet thickness from the surface (region between a depth of ⅛ of the sheet thickness from the surface and a depth of ⅜ of the sheet thickness from the surface) is measured using an electron probe microanalyzer (EPMA) to measure the standard deviation of the Mn concentration. The measurement condition is set such that an acceleration voltage is 15 kV and the magnification is 5000 times, and a distribution image in the range of 20 μm in the sample rolling direction and 20 μm in the sample sheet thickness direction is measured. More specifically, the measurement interval is set to 0.1 μm, and the Mn concentration at 40000 or more points is measured. Then, a standard deviation based on the Mn concentration obtained from all the measurement point is calculated to obtain the standard deviation of the Mn concentration.
(2-7) Average Grain Size of Surface Layer: Less than 3.0 m
As the strength of the steel sheet becomes higher, cracks are likely to initiate from an inside of a bend during bending (hereinafter referred to as cracking inside a bend). When making the grain size of the surface layer finer, it is possible to suppress cracking inside a bend of the hot-rolled steel sheet.
The mechanism of the cracking inside a bend is presumed as follows. During bending, compressive stress is generated inside the bend. At first, bending proceeds while uniformly deforming the entire inside of the bend, but when the bending amount increases, the deformation cannot be carried out only by uniform deformation, and the deformation proceeds due to the concentration of strain locally (generation of shear deformation band). As this shear deformation band further propagates, cracks along the shearing band are initiated from the inner surface of the bend and propagate. The reason why the cracking inside a bend is more likely to be initiated along with the high-strengthening is presumed that when uniform deformation is less likely to proceed due to the decrease in work hardening ability along with the strength increasing and a deformation bias is likely to occur, a shear deformation band is formed at an early stage of working (or in a mild working condition).
According to the research by the present inventors, it was found that the cracking inside a bend becomes remarkable in the steel sheet having the tensile strength of 980 MPa or more. Furthermore, the present inventors have found that as the grain size of the surface layer of the hot-rolled steel sheet is finer, the local strain concentration is further suppressed and the cracking inside a bend becomes difficult to occur. In order to obtain the action, it is preferable that the average grain size of the surface layer of the hot-rolled steel sheet is less than 3.0 μm. It is more preferable that the average grain size is 2.5 μm or less. The lower limit is not particularly limited, and may be 1.0 m or more, 1.5 μm or more, or 2.0 μm or more.
In the present embodiment, the surface layer is a region from the surface of the hot-rolled steel sheet to a position at a depth of 50 μm from the surface.
The grain size of the surface layer is measured by using the EBSP-OIM method. In the cross section parallel to the rolling direction, a region from the surface of the hot-rolled steel sheet to a position at a depth of 50 μm from the surface and the center position in the sheet width direction is analyzed with 1200 fold magnification, in a region of 40 μm×30 μm, for at least 5 visual field, a place where the angular difference between adjacent measurement points is 5° or more is defined as a grain boundary, and an area average grain size is calculated. The obtained area average grain size is defined as the average grain size of the surface layer.
Since the residual austenite is not a structure formed by phase transformation at 600° C. or lower and has no effect of dislocation accumulation, the residual austenite is not included as a target in the analysis in the present measurement method. That is, in the present embodiment, the average grain size of the surface layer is a size of ferrite, pearlite, and full hard structure (one or two or more of bainite, martensite, and tempered martensite). The EBSP-OIM method, the residual austenite having a crystal structure of fcc can be excluded from the analysis target.
3. Tensile Strength Properties
Among the mechanical properties of the hot-rolled steel sheet, the tensile strength properties (tensile strength and total elongation) were evaluated in accordance with JIS Z 2241: 2011. A test piece is a No. 5 test piece of JIS Z 2241: 2011. The sampling position of the tensile test piece may be ¼ portion from the end portion in the sheet width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction.
The hot-rolled steel sheet according to the present embodiment has a tensile (maximum) strength of 980 MPa or more. When the tensile strength is less than 980 MPa, an applicable component is limited, and the contribution of weight reduction of the vehicle body is small. An upper limit does not need to be particularly be limited, and may be 1400 MPa or 1350 MPa from the viewpoint of suppressing wearing of a die.
Further, the product (TS×El) of the tensile strength and the total elongation which are indices of ductility is preferably 15000 MPa-% or more. By setting the product of the tensile strength and the total elongation to 15000 MPa-% or more, it is possible to obtain a hot-rolled steel sheet that greatly contributes to weight reduction of a vehicle body without limiting applicable components.
4. Sheet Thickness
The sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited and may be 0.5 to 8.0 mm. By setting the sheet thickness of the hot-rolled steel sheet to 0.5 mm or more, it becomes easy to secure the rolling completion temperature, and it is also possible to reduce rolling force, and to easily perform hot rolling. Therefore, the sheet thickness of the hot-rolled steel sheet according to the present embodiment may be 0.5 mm or more. The sheet thickness is preferably 1.2 mm or more and 1.4 mm or more. In addition, when the sheet thickness is set to 8.0 mm or less, the metallographic structure can be easily refined, and the above-described metallographic structure can be easily secured. Therefore, the sheet thickness may be 8.0 mm or less. The sheet thickness is preferably 6.0 mm or less.
5. Others
(5-1) Plating Layer
The hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and metallographic structure may be a surface-treated steel sheet provided with a plating layer on the surface for the purpose of improving corrosion resistance and the like. The plating layer may be an electro plating layer or a hot-dip plating layer. Examples of the electro plating layer include electrogalvanizing and electro Zn—Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, and hot-dip Zn—Al—Mg—Si alloy plating.
The plating adhesion amount is not particularly limited and may be the same as before. Further, it is also possible to further enhance the corrosion resistance by applying an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.
6. Manufacturing Conditions
A suitable method for manufacturing the hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metallographic structure is as follows.
In order to obtain the hot-rolled steel sheet according to the present embodiment, it is effective that after performing heating the slab under predetermined conditions, hot rolling is performed and accelerated cooling is performed to a predetermined temperature range, thereafter, slow cooling is performed, and the cooling history is controlled until coiling.
In the suitable method for manufacturing the hot-rolled steel sheet according to the present embodiment, the following steps (1) to (7) are sequentially performed. The temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
(1) The slab is retained in a temperature range of 700° C. to 850° C. for 900 seconds or longer, then further heated, and retained in a temperature range of 1100° C. or higher for 6000 seconds or longer.
(2) Hot rolling is performed in a temperature range of 850° C. to 1100° C. so that the total sheet thickness is reduced by 90% or more.
(3) The Hot rolling is completed so that a hot rolling completion temperature Tf becomes equal to or higher than a temperature T1 (° C.) represented by Formula <1>.
(4) Within one second after the completion of the hot rolling, cooling is performed to a temperature range of hot rolling completion temperature Tf−50° C. or lower. Then, accelerated cooling is performed to a temperature range of 600° C. to 730° C. at an average cooling rate of 50° C./s or higher.
Here, cooling to a temperature range of hot rolling completion temperature Tf-50° C. or lower within one second after the completion of the hot rolling is a more preferable cooling condition.
(5) In the temperature range of 600° C. to 730° C., slow cooling at an average cooling rate of lower than 5° C./s is performed for 2.0 seconds or longer.
(6) Cooling is performed to a temperature range of 600° C. or lower at an average cooling rate of 50° C./s or higher.
(7) Coiling is performed in a temperature range of 400° C. to 600° C.
T1(° C.)=868−396×[C]−68.1×[Mn]+24.6×[Si]−36.1×[Ni]−24.8×[Cr]−20.7×[Cu]+250×[sol. Al] <1>
However, the [element symbol] in Formula <1> indicates the content (mass %) of each element in the steel. When the element is not contained, substitution is performed with 0.
(6-1) Slab, Slab Temperature When Subjected to Hot Rolling, and Retention Time
As a slab to be subjected to hot rolling, a slab obtained by continuous casting, a slab obtained by casting and blooming, and the like can be used, and slabs obtained by performing hot working or cold working on these slabs as necessary can be used.
The slab to be subjected to hot rolling is preferably retained in a temperature range of 700° C. to 850° C. during heating for 900 seconds or longer, then further heated and retained in a temperature range of 1100° C. or higher for 6000 seconds or longer. During retaining in the temperature range of 700° C. to 850° C., the steel sheet temperature may be fluctuated or be constant in the temperature range. Furthermore, during retaining in the temperature range of 1100° C. or higher, the steel sheet temperature may be fluctuated or be constant in the temperature range of 1100° C. or higher.
In the austenite transformation in the temperature range of 700° C. to 850° C., when Mn is distributed between the ferrite and the austenite and the transformation time becomes longer, Mn can be diffused in the ferrite region. Accordingly, the Mn microsegregation unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. By reducing the standard deviation of the Mn concentration, it is possible to uniformly disperse the grain boundaries having a crystal misorientation of 60° about the <110> direction in a final metallographic structure, and stabilize the sheared section proportion.
Further, in order to make the austenite grains uniform during slab heating, it is preferable to heat the slab in the temperature range of 1100° C. or higher for 6000 seconds or longer.
In hot rolling, it is preferable to use a reverse mill or a tandem mill for multi-pass rolling. Particularly, from the viewpoint of industrial productivity, it is more preferable that at least the final several stages are hot-rolled using a tandem mill.
(6-2) Rolling Reduction of Hot Rolling: Total Sheet Thickness Reduction of 90% or More in Temperature Range of 850° C. to 1100° C.
By performing hot rolling so that the sheet thickness is reduced by 90% or more in total in the temperature range of 850° C. to 1100° C., recrystallized austenite grains are mainly refined, and the accumulation of strain energy is promoted to unrecrystallized austenite grains. In addition, the recrystallization of austenite is promoted and the atomic diffusion of Mn is promoted, so that it is possible to reduce the standard deviation of the Mn concentration.
By reducing the standard deviation of the Mn concentration, it is possible to uniformly disperse the grain boundaries having a crystal misorientation of 60° about the <110> direction in the final metallographic structure, and stabilize the sheared section proportion. Therefore, it is preferable to perform the hot rolling in a temperature range of 850° C. to 1100° C. so that the total sheet thickness is reduced by 90% or more.
The sheet thickness reduction in a temperature range of 850° C. to 1100° C. can be expressed as (t0−t1)/t0×100(%) when an inlet sheet thickness before the first pass in the rolling in this temperature range is to and an outlet sheet thickness after the final pass in the rolling in this temperature range is t1.
(6-3) Hot Rolling Completion Temperature Tf: T1 (° C.) or Higher
The hot rolling completion temperature Tf is preferably set to T1 (° C.) or higher. By setting the hot rolling completion temperature Tf to T1 (° C.) or higher, an excessive increase in the number of ferrite nucleation sites in the austenite can be suppressed, and the formation of the ferrite in the final structure (the metallographic structure of the hot-rolled steel sheet after manufacturing) can be suppressed, and it is possible to obtain the hot-rolled steel sheet having high strength.
(6-4) Within One Second after Completion of Hot Rolling, Cooling to Temperature Range of Hot Rolling Completion Temperature Tf−50° C. or Lower, and then, Accelerated Cooling to Temperature Range of 600° C. to 730° C. at Average Cooling Rate of 50° C./s or Higher
It is preferable that, within one second after the completion of the hot rolling, cooling is performed to a temperature range of hot rolling completion temperature Tf-50° C. or lower, and then, accelerated cooling is performed to a temperature range of 600° C. to 730° C. at an average cooling rate of 50° C./s or higher. Here, cooling to a temperature range of hot rolling completion temperature Tf−50° C. or lower within one second after the completion of the hot rolling is a more preferable cooling condition.
In order to suppress the growth of austenite crystal grains that have been refined by hot rolling, it is more preferable that cooling is performed by 50° C. or more within 1 second after the completion of the hot rolling, that is, cooling is performed to reach a temperature range of hot rolling completion temperature Tf −50° C. or lower within 1 second after the completion of the hot rolling. In order to perform cooling to a temperature range of hot rolling completion temperature Tf−50° C. or lower within one second after the completion of the hot rolling, cooling at a large average cooling rate is performed immediately after the completion of the hot rolling, for example, cooling water may be sprayed on the surface of the steel sheet. When cooling is performed to a temperature range of Tf−50° C. or lower within one second after the completion of the hot rolling, the grain size of the surface layer can be refined and resistance to cracking inside a bend of the hot-rolled steel sheet can be improved.
In addition, when performing accelerated cooling to a temperature range of 730° C. or lower at an average cooling rate of 50° C./s or higher after the completion of the hot rolling or after the above cooling, it is possible to suppress the formation of ferrite and pearlite having a small amount of precipitation hardening. Accordingly, the strength of the hot-rolled steel sheet is enhanced.
The average cooling rate referred herein is a value obtained by dividing the temperature drop width of the steel sheet from the start of accelerated cooling (when introducing a steel sheet to cooling equipment) to the completion of accelerated cooling (when deriving a steel sheet from cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling.
In cooling after the completion of the hot rolling, when the average cooling rate during accelerated cooling to the temperature range of 600° C. to 730° C. is 50° C./s or higher, ferritic transformation and/or pearlitic transformation in which the amount of precipitation hardening inside the steel sheet is small is suppressed and a tensile strength of 980 MPa or more can be obtained. Therefore, after the completion of the hot rolling, the engine is accelerated cooling is performed to a temperature range of 600° C. to 730° C. at an average cooling rate of 50° C./s or higher.
The upper limit of the average cooling rate is not particularly specified, but when the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, the average cooling rate is preferably 300° C./s or lower.
(6-5) In Temperature Range of 600° C. to 730° C., Slow Cooling at Average Cooling Rate of Lower than 5° C./s is Performed for 2.0 Seconds or Longer.
The precipitation hardened ferrite can be sufficiently precipitated by performing slow cooling at an average cooling rate of lower than 5° C./s for 2.0 seconds or longer in a temperature range of 600° C. to 730° C. As a result, both the strength and the ductility of the hot-rolled steel sheet can be obtained.
The average cooling rate referred here refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the accelerated cooling to a start temperature of the slow cooling by the time required from the stop of accelerated cooling to the start of the slow cooling.
When the time for slow cooling in the temperature range of 600° C. to 730° C. is 2.0 seconds or longer, the area fraction of the precipitation hardened ferrite reaches a desired amount, and it is possible to obtain the action. Accordingly, in the temperature range of 600° C. to 730° C., slow cooling at an average cooling rate of lower than 5° C./s is performed for 2.0 seconds or longer. The time for the slow cooling is preferably 3.0 seconds or longer and more preferably 4.0 seconds or longer.
The upper limit of the time for the slow cooling is determined by the equipment layout, and may be shorter than 10.0 seconds. In addition, although the lower limit of the average cooling rate for slow cooling is not particularly set, raising the temperature without cooling may require a large investment in equipment. Therefore, the lower limit may be set to 0° C./s or higher.
(6-6) Average Cooling Rate to Temperature Range of 600° C. or Lower: 50° C./s or Higher
In order to suppress the area fraction of the pearlite to obtain the tensile strength of 980 MPa or more, the average cooling rate from the cooling stop temperature of the slow cooling to 600° C. is set to 50° C./s or higher. Accordingly, the primary phase structure can be full hard.
The average cooling rate referred here refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the slow cooling at the average cooling rate of lower than 5° C./s to the coiling temperature by the time required from the stop of the slow cooling at the average cooling rate of lower than 5° C./s to 600° C.
When the average cooling rate is 50° C./s or higher, the area fraction of pearlite is reduced, and the strength and ductility of the hot-rolled steel sheet are improved. Therefore, the average cooling rate from the cooling stop temperature of the slow cooling at the average cooling rate of lower than 5° C./s to the temperature range of 600° C. or lower is set to 50° C./s or higher.
(6-7) Coiling Temperature: 400° C. to 600° C.
The coiling temperature is in the temperature range of 400° C. to 600° C. When setting the coiling temperature to 400° C. or higher, it is possible to decrease the transformation driving force from austenite to bcc and it is also possible to decrease the deformation strength of austenite. Therefore, when transformation from austenite into bainite and martensite is performed, the length L7 of the grain boundary having a crystal misorientation of 7° about the <110> direction decreases, and the length L60 of the grain boundary having a crystal misorientation of 60° about the <110> direction increases. Accordingly, L60/L7 can be set to 0.60 or more. As a result, the sheared section proportion can be stabilized.
When setting the coiling temperature to 600° C. or lower, the area fraction of ferrite can be set to less than 60%, and a desired tensile strength can be obtained. Therefore, the coiling temperature is preferably set to the temperature range of 400° C. to 600° C. The coiling temperature is more preferably 450° C. or higher. In addition, the coiling temperature is more preferably 550° C. or lower.
Next, the effects of one aspect of the present invention will be described more specifically by way of examples, but the conditions in the examples are condition examples adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to these condition examples. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
Steels having chemical compositions shown in Steel Nos. A to T in Tables 1 and 2 were melted and continuously cast to manufacture slabs having a thickness of 240 to 300 mm. The obtained slabs were used to obtain hot-rolled steel sheets shown in Table 4 under the manufacturing conditions shown in Table 3.
The slab was allowed to retain in the temperature range of 700° C. to 850° C. for the retention time shown in Table 3, and then further heated to the heating temperature shown in Table 3 and retained. In addition, the average cooling rate of the slow cooling was set to lower than 5° C./s.
For the obtained hot-rolled steel sheet, the area fraction of the metallographic structure, L60/L7, the standard deviation of the Mn concentration, and the average grain size of the surface layer were determined by the above-described method. The obtained measurement results are shown in Table 4.
Evaluation Method of Properties of Hot-Rolled Steel Sheet
(1) Tensile Strength Properties
Among the mechanical properties of the obtained hot-rolled steel sheet, the tensile strength properties (tensile strength TS and total elongation EL) and the total elongation were evaluated according to JIS Z 2241: 2011. A test piece was a No. 5 test piece of JIS Z 2241: 2011. The sampling position of the tensile test piece may be ¼ portion from the end portion in the sheet width direction, and the direction perpendicular to the rolling direction was the longitudinal direction.
In a case where the tensile strength TS ≥980 MPa and the tensile strength TS×total elongation El ≥15000 (MPa·%) were satisfied, the hot-rolled steel sheet was determined to be as acceptable as a hot-rolled steel sheet having excellent strength and ductility. On the other hand, in a case where any one of tensile strength TS ≥980 MPa and tensile strength TS×total elongation El ≥15000 (MPa·%) was not satisfied, it was determined that the hot-rolled steel sheet does not have excellent strength and ductility, which is fail.
(2) Shearing Workability
The shearing workability of the hot-rolled steel sheet was evaluated by determining the amount of change in the sheared section proportion by a punching test. Five punched holes were prepared at the center position of sheet width, with a hole diameter of 10 mm, a clearance of 15%, and a punching speed of 3 m/s. Next, with respect to the five punched holes, a state of the end surfaces parallel to the rolling direction at ten places (two end surfaces per one punched hole) was photographed with an optical microscope view.
In the obtained observation photograph, the end surface as shown in (a) of
The shear droop is an R-shaped smooth surface. The sheared section is a punched end surface separated by shearing deformation. The fractured section is a punched end surface separated by cracks initiated from the vicinity of the cutting edge after the completion of the shearing deformation. The burr is a surface having projections projecting from a lower surface of a hot-rolled steel sheet.
In the observation photographs of 10 end surfaces obtained from 5 end surfaces, a proportion of the sheared section to the end surface was measured, and the difference between the maximum value and the minimum value of the obtained sheared section proportion (%) is defined as the amount of change in a sheared section proportion (%). As shown in (a) of
When the amount of change in the sheared section proportion is 20% or less, it is determined to be a hot-rolled steel sheet having excellent shearing workability which is acceptable. On the other hand, when the amount of change in the sheared section proportion is more than 20%, it is determined to be a hot-rolled steel sheet having poor shearing workability which is fail.
(3) Resistance to Cracking Inside Bend
As a bending test piece, a strip-shaped test piece having a size of 100 mm×30 min was cut out from a ½ position in the width direction of the hot-rolled steel sheet, and the resistance to cracking inside a bend was evaluated by the following bending test.
Regarding both bending (L-axis bending) in which a bending ridge is parallel to the rolling direction (L direction) and bending (C-axis bending) in which a bending ridge is parallel to the direction perpendicular to the rolling direction (C direction), the resistance to cracking inside a bend is studied in accordance with JIS Z 2248: 2014 (V block 90° bending test), the minimum bending radius at which cracks are not initiated is determined, and a value obtained by dividing an average value R of the minimum bending radii of the L axis and the C axis by the sheet thickness t is defined as a limit bending R/t, which is an index value of bendability. When the R/t≤2.5, it was determined that the hot-rolled steel sheet was excellent in resistance to cracking inside a bend.
However, regarding the presence or absence of cracks, a crack was observed with an optical microscope, after mirror polishing the cross section obtained by cutting the test piece after the V block 90° bending test on a plane parallel to the bending direction and perpendicular to the sheet surface, and when the crack length observed inside the bend of the test piece is more than 30 μm, it is determined that there is a crack.
0.036
0.259
3.20
0.89
0.050
87
5537
P
Q
R
S
T
0.9
24
748
40
638
110
0.62
0.63
0.66
6.3
10
11.2
11
68.5
12
0.32
26
27
28
80.4
29
64.8
8.2
30
21.6
22
28
29
14567
14976
14181
10
13534
11
12
35
26
27
14671
28
13495
29
14660
30
As can be seen from Table 4, the production Nos. 1, 2, 6, and 13 to 25 according to Invention Example, hot-rolled steel sheets having excellent strength, ductility and shearing workability were obtained. Furthermore, in Production Nos. 1, 2, 14 to 21, and 23 to 25 in which the average grain size of the surface layer is less than 3.0 μm, a hot-rolled steel sheet having excellent resistance to cracking inside a bend, in addition to having the above-mentioned various properties was obtained.
On the other hand, the production Nos. 3 to 5, 7 to 12 and 26 to 30 which are comparative examples were poor in any one or more of the strength, the ductility, and shearing workability.
According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent strength, ductility, and shearing workability. Further, according to a preferred embodiment according to the present invention, it is possible to obtain a hot-rolled steel sheet having the above-mentioned various properties and further suppressing the occurrence of cracking inside a bend, that is, having excellent resistance to cracking inside a bend.
The hot-rolled steel sheet according to the present invention is suitable as an industrial material used for vehicle members, mechanical structural members, and building members.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-010944 | Jan 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/046384 | 12/11/2020 | WO |
