EP4660342A1 - Hot-rolled steel sheet - Google Patents

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, and has GAM_S/GAM_I of 0.70 to 1.05, and in a microstructure at a position of 1/4 depth from the surface in the sheet thickness direction, the area ratio of a region having a GAM value of more than 0.6° is 50% or more, and the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is less than 15%, and a standard deviation of the area average value of the GAM values of grains in a region from the surface to a depth of 200 µm in the sheet thickness direction is 0.25 to 0.65°.

Description

TECHNICAL FIELD
• The present invention relates to a hot-rolled steel sheet.
• Priority is claimed on Japanese Patent Application No. 2023-013129, filed January 31, 2023 , the content of which is incorporated herein by reference.
BACKGROUND ART
• In recent years, weight reduction of vehicle components has been promoted. Designing an optimum shape as the component shape ensures stiffness and thereby makes it possible to reduce the weights of vehicle components. Furthermore, in blank-formed components such as a press-formed component, the weights can be reduced by reducing the sheet thicknesses of component materials.
• In the case of attempting to ensure the strength properties of components such as static fracture strength and yield strength while reducing the sheet thicknesses, it becomes necessary to use high-strength materials. In particular, for vehicle suspension parts such as lower control arms, trailing arms, and knuckles, studies have begun about the application of higher than 780 MPa-grade steel sheets. These vehicle suspension parts are manufactured by performing burring, stretch flanging, bending forming, or the like on steel sheets. Therefore, steel sheets that are applied to these vehicle suspension parts are required to have excellent formability, particularly, ductility and hole expandability.
• For example, Patent Document 1 discloses a high strength steel sheet in which a microstructure is substantially a two-phase microstructure of ferrite and bainite, and a carbide containing Ti and Mo is dispersed and precipitated in a ferrite phase.
Citation List Patent Document
• Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2003-321725
SUMMARY OF INVENTION Technical Problem
• However, Patent Document 1 does not consider ductility and hole expandability.
• The vehicle suspension part as described above is manufactured by forming a hot-rolled steel sheet in a plurality of steps. Therefore, a hot-rolled steel sheet applied to a vehicle suspension part is required to have excellent bendability even after being subjected to a certain degree of prestrain in a pre-step. When composite deformation including compression is generated as prestrain in forming in a plurality of steps, development of irregularities on a surface layer of the hot-rolled steel sheet increases, which increases the risk of fracture in subsequent bending forming. The present inventors have found that such a problem does not occur in a cold-rolled steel sheet having a small surface roughness, and is a problem unique to a hot-rolled steel sheet.
• The present invention has been made in view of the above circumstances. An object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bendability after prestrain application.
Solution to Problem
• The gist of the present invention is as follows.
1. (1) A hot-rolled steel sheet according to an aspect of the present invention has a chemical composition including, in mass%,
   • C: 0.045 to 0.120%,
   • Si: 0 to 3.00%,
   • Mn: 1.20 to 2.60%,
   • Ti: 0.020 to 0.180%,
   • Al: 0.010 to 0.400%,
   • P: 0.080% or less,
   • S: 0.0100% or less,
   • N: 0.0050% or less,
   • O: 0.010% or less,
   • Nb: 0 to 0.100%,
   • V: 0 to 1.000%,
   • Cu: 0 to 1.000%,
   • Cr: 0 to 2.000%,
   • Mo: 0 to 3.000%,
   • Ni: 0 to 0.500%,
   • B: 0 to 0.0100%,
   • Ca: 0 to 0.0500%,
   • Mg: 0 to 0.0500%,
   • REM: 0 to 0.100%,
   • Bi: 0 to 0.100%,
   • Ta: 0 to 0.100%,
   • Zr: 0 to 0.500%,
   • Co: 0 to 3.000%,
   • Zn: 0 to 0.200%,
   • W: 0 to 0.200%,
   • Sb: 0 to 0.500%,
   • As: 0 to 0.050%,
   • Sn: 0 to 0.050%, and
   • a remainder including Fe and impurities,
   • in which GAM_S/GAM_I, which is a ratio of GAM_I, which is an area average value of GAM values of grains at a position of 1/4 depth from a surface in a sheet thickness direction, to GAM_S, which is an area average value of GAM values of grains in a region from the surface to a depth of 200 µm in the sheet thickness direction, is 0.70 to 1.05, and
   • in a microstructure at the position of 1/4 depth from the surface in the sheet thickness direction,
   • the area ratio of a region having a GAM value of more than 0.6° is 50% or more, and the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is less than 15%, and
   • a standard deviation of the area average value of the GAM values of grains in a region from the surface to a depth of 200 µm in the sheet thickness direction is 0.25 to 0.65°.
2. (2) In the hot-rolled steel sheet according to (1), the chemical composition may include, in mass%,
   one or more selected from the group consisting of:
   • Nb: 0.001 to 0.100%,
   • V: 0.001 to 1.000%,
   • Cu: 0.001 to 1.000%,
   • Cr: 0.001 to 2.000%,
   • Mo: 0.001 to 3.000%,
   • Ni: 0.001 to 0.500%,
   • B: 0.0001 to 0.0100%,
   • Ca: 0.0001 to 0.0500%,
   • Mg: 0.0001 to 0.0500%,
   • REM: 0.001 to 0.100%,
   • Bi: 0.001 to 0.100%,
   • Ta: 0.001 to 0.100%,
   • Zr: 0.001 to 0.500%,
   • Co: 0.001 to 3.000%,
   • Zn: 0.001 to 0.200%,
   • W: 0.001 to 0.200%,
   • Sb: 0.001 to 0.500%,
   • As: 0.001 to 0.050%, and
   • Sn: 0.001 to 0.050%.
3. (3) The hot-rolled steel sheet according to (1) or (2), in which
   • in the microstructure at the position of 1/4 depth from the surface in the sheet thickness direction,
   • the area ratio of a region having a GAM value of more than 0.6° and less than 2.0° is 50% or more.
4. (4) The hot-rolled steel sheet according to (1) or (2), in which
   • in the microstructure at the position of 1/4 depth from the surface in the sheet thickness direction,
   • the area ratio of a region having a GAM value of 2.0° or more is 50% or more. Advantageous Effects of Invention
• According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet that provides a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bendability after prestrain application.
BRIEF DESCRIPTION OF DRAWINGS
• [FIG. 1] FIG. 1 is a view for explaining a method of forming a hat component.
DESCRIPTION OF EMBODIMENTS
• Hereinafter, a hot-rolled steel sheet according to the present embodiment will be described in detail. However, the present invention is not limited only to a configuration disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present invention.
• Numerical limiting ranges expressed below using "to" include the lower limit and the upper limit in the ranges. Numerical values expressed with "less than" and "more than" are not included in numerical ranges. "%" regarding chemical compositions all indicates "mass%".
• The chemical composition of the hot-rolled steel sheet according to the present embodiment includes, in mass%, C: 0.045 to 0.120%, Si: 0 to 3.00%, Mn: 1.20 to 2.60%, Ti: 0.020 to 0.180%, Al: 0.010 to 0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, and a remainder: Fe and impurities.
• Hereinafter, each element will be described in detail.
C: 0.045 to 0.120%
• C is an element necessary for obtaining a desired tensile strength of the hot-rolled steel sheet. When the C content is less than 0.045%, the desired tensile strength cannot be obtained in the hot-rolled steel sheet. Therefore, the C content is set to 0.045% or more. The C content is preferably 0.050% or more, more preferably 0.060% or more, and still more preferably 0.080% or more.
• On the other hand, when the C content is more than 0.120%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the C content is set to 0.120% or less. The C content is preferably 0.110% or less and more preferably 0.100% or less.
Si: 0 to 3.00%
• Si is an element that improves the tensile strength of the hot-rolled steel sheet by solid solution strengthening. However, the hot-rolled steel sheet according to the present embodiment ensures sufficient tensile strength even without containing Si. Therefore, the Si content may be 0%. The Si content is preferably 0.01% or more and more preferably 0.03% or more.
• On the other hand, when the content of Si is too large, hot rolling may be difficult due to insufficient ductility or the like. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less and more preferably 1.50% or less. In the hot-rolled steel sheet according to the present embodiment, the strength, elongation and hole expandability of the hot-rolled steel sheet can be realized in a high balance by setting the Si content to 0 to 3.00%.
Mn: 1.20 to 2.60%
• Mn is an element necessary for improving the strength of the hot-rolled steel sheet. When the Mn content is less than 1.20%, the desired tensile strength cannot be obtained in the hot-rolled steel sheet. Therefore, the Mn content is set to 1.20% or more. The Mn content is preferably 1.40% or more and more preferably 1.60% or more.
• On the other hand, when the Mn content is more than 2.60%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Mn content is set to 2.60% or less. The Mn content is preferably 2.30% or less and more preferably 2.20% or less.
Ti: 0.020 to 0.180%
• Ti is an element that increases the strength of the hot-rolled steel sheet by forming a fine nitride in steel. When the Ti content is less than 0.020%, the desired tensile strength cannot be obtained in the hot-rolled steel sheet. Therefore, the Ti content is set to 0.020% or more. The Ti content is preferably 0.050% or more and more preferably 0.080% or more.
• On the other hand, when the Ti content is more than 0.180%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0.160% or less and more preferably 0.150% or less.
Al: 0.010 to 0.400%
• Al is an element that acts as a deoxidizer and improves the cleanliness of the steel. When the Al content is less than 0.010%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) are formed in the steel. Such inclusions degrade the workability, particularly the hole expandability, of the hot-rolled steel sheet. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.020% or more and more preferably 0.030% or more.
• On the other hand, when the Al content is more than 0.400%, casting becomes difficult. Therefore, the Al content is set to 0.400% or less. The Al content is preferably 0.300% or less, more preferably 0.200% or less, and still more preferably 0.100% or less.
P: 0.080% or less
• P is an element that segregates at grain boundaries in steel and promotes embrittlement of the grain boundaries. When the P content is too large, elongation and hole expandability of the hot-rolled steel sheet are likely to be reduced, and furthermore, slab cracking and the like due to embrittlement may occur, and hot rolling may be difficult. Therefore, the P content is set to 0.080% or less. The P content is preferably 0.020% or less and more preferably 0.010% or less.
• The P content is preferably as low as possible, and is preferably 0%. However, when the P content is excessively reduced, P removal cost significantly increases, and thus the P content may be 0.001% or more.
S: 0.0100% or less
• S is an element that embrittles slabs by being present as a sulfide. In addition, S is also an element that degrades the workability of the hot-rolled steel sheet. When the S content is more than 0.0100%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less and more preferably 0.0050% or less.
• The S content is preferably as low as possible, and is preferably 0%. However, when the S content is excessively reduced, S removal cost significantly increases, and thus the S content may be 0.0005% or more.
N: 0.0050% or less
• N is an element that forms a coarse nitride in steel and deteriorates the hole expandability of the hot-rolled steel sheet. When the N content is too large, elongation and hole expandability of the hot-rolled steel sheet are likely to be reduced due to excessive generation of nitrides and the like, and furthermore, slab cracking and the like due to embrittlement may occur, and hot rolling may be difficult. Therefore, the N content is set to 0.0050% or less. The N content is preferably 0.0040% or less and more preferably 0.0035% or less.
• The N content is preferably as low as possible, and is preferably 0%. However, when the N content is excessively reduced, N removal cost significantly increases, and thus the N content may be 0.0005% or more.
O: 0.010% or less
• O is an element that forms an oxide and lowers the workability of the hot-rolled steel sheet. When the O content is more than 0.010%, an oxide is excessively generated, and the like, so that the hole expandability of the hot-rolled steel sheet is likely to be reduced. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.008% or less and more preferably 0.006% or less.
• The O content is preferably as low as possible, and is preferably 0%. However, when the O content is excessively reduced, O removal cost significantly increases, and thus the O content may be 0.001% or more.
• 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 substances that are mixed from ore as a raw material, a scrap, a manufacturing environment, or the like, or substances acceptable within a range not adversely affecting the hot-rolled steel sheet according to the present embodiment.
• The chemical composition of the hot-rolled steel sheet according to the present embodiment may contain the following optional elements instead of a part of Fe. The lower limit of the content when optional elements are not contained is 0%.
• Hereinafter, each optional element will be described.
Nb: 0.001 to 0.100%
• Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb is also an element that increases the strength of the hot-rolled steel sheet by forming a fine carbide. In order to reliably obtain these effects, the Nb content is preferably set to 0.001% or more. The Nb content is more preferably 0.010% or more and still more preferably 0.030% or more.
• On the other hand, when the Nb content is more than 0.100%, the toughness of a cast slab is deteriorated, and it may be difficult to perform hot rolling. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less and more preferably 0.060% or less.
V: 0.001 to 1.000%
• V is an element that increases the strength of the hot-rolled steel sheet by forming a fine carbide in steel. In order to reliably obtain this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.050% or more and still more preferably 0.100% or more.
• On the other hand, when the V content is more than 1.000%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the V content is set to 1.000% or less. The V content is preferably 0.500% or less and more preferably 0.300% or less.
Cu: 0.001 to 1.000%
• Cu has an action of enhancing the hardenability of the hot-rolled steel sheet, and an action of increasing the strength of the hot-rolled steel sheet by being precipitated as a carbide in steel at a low temperature. In order to more reliably obtain an effect of these actions, the Cu content is preferably set to 0.001% or more. The Cu content is more preferably 0.050% or more and still more preferably 0.100% or more.
• On the other hand, when the Cu content is more than 1.000%, intergranular cracking of the slab may occur. Therefore, the Cu content is set to 1.000% or less. The Cu content is preferably 0.500% or less and more preferably 0.300% or less.
Cr: 0.001 to 2.000%
• Cr is an element exhibiting an effect similar to that of Mn. In order to reliably obtain an effect of increasing the strength of the hot-rolled steel sheet by containing Cr, the Cr content is preferably set to 0.001% or more. The Cr content is more preferably 0.050% or more and still more preferably 0.100% or more.
• On the other hand, even when Cr is contained in an amount exceeding 2.000%, the above effect is saturated. Therefore, the Cr content is set to 2.000% or less. From the viewpoint of reducing the alloy cost, the Cr content is preferably 1.000% or less and more preferably 0.500% or less.
Mo: 0.001 to 3.000%
• Mo is an element that increases the strength of the hot-rolled steel sheet by forming a fine carbide in steel. In order to reliably obtain this effect, the Mo content is preferably set to 0.001% or more. The Mo content is more preferably 0.050% or more and still more preferably 0.100% or more.
• On the other hand, when the Mo content is more than 3.000%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Mo content is set to 3.000% or less. The Mo content is preferably 2.000% or less and more preferably 1.000% or less.
Ni: 0.001 to 0.500%
• Ni is an element that enhances hardenability of the hot-rolled steel sheet. In addition, when Cu is contained, Ni has an action of effectively suppressing intergranular cracking of the slab caused by Cu. In order to reliably obtain an effect of the above action, the Ni content is preferably set to 0.001% or more. The Ni content is more preferably 0.050% or more and still more preferably 0.100% or more.
• On the other hand, since Ni is an expensive element, it is not economically preferable to contain Ni in a large amount. Therefore, the Ni content is set to 0.500% or less. From the viewpoint of reducing the alloy cost, the Ni content is preferably 0.300% or less and more preferably 0.200% or less.
B: 0.0001 to 0.0100%
• B is an element that increases the strength of the hot-rolled steel sheet. In order to reliably obtain this effect, the B content is preferably set to 0.0001% or more. The B content is more preferably 0.0005% or more and still more preferably 0.0010% or more.
• On the other hand, even when B is contained in an amount exceeding 0.0100%, the above effect is saturated. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0070% or less and more preferably 0.0050% or less.
Ca: 0.0001 to 0.0500%
• Ca is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferable shape. In order to reliably obtain this effect, the Ca content is preferably set to 0.0001% or more. The Ca content is preferably 0.0010% or more and more preferably 0.0050% or more.
• On the other hand, when the Ca content is more than 0.0500%, inclusions are excessively generated in steel, and conversely, the ductility and hole expandability of the hot-rolled steel sheet may be deteriorated. Therefore, the Ca content is set to 0.0500% or less. The Ca content is preferably 0.0300% or less and more preferably 0.0100% or less.
Mg: 0.0001 to 0.0500%
• Mg is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferable shape. In order to reliably obtain this effect, the Mg content is preferably set to 0.0001% or more. The Mg content is preferably 0.0010% or more and more preferably 0.0020% or more.
• On the other hand, when the Mg content is more than 0.0500%, inclusions are excessively generated in steel, and conversely, the ductility and hole expandability of the hot-rolled steel sheet may be deteriorated. Therefore, the Mg content is set to 0.0500% or less. The Mg content is preferably 0.0300% or less and more preferably 0.0100% or less.
REM: 0.001 to 0.100%
• REM is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferable shape. In order to reliably obtain this effect, the REM content is preferably set to 0.001% or more. The REM content is preferably 0.003% or more and more preferably 0.005% or more.
• On the other hand, when the REM content is more than 0.100%, inclusions are excessively generated in steel, and conversely, the ductility and hole expandability of the hot-rolled steel sheet may be deteriorated. Therefore, the REM content is set to 0.100% or less. The REM content is preferably 0.050% or less and more preferably 0.030% or less.
• Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoid, and the content of REM refers to the total content of these elements. Lanthanoid is industrially added in a form of misch metal.
Bi: 0.001 to 0.100%
• Bi is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by refining the solidified structure. In order to reliably obtain this effect, the Bi content is preferably set to 0.001% or more. The Bi content is preferably 0.002% or more and more preferably 0.003% or more.
• On the other hand, even when the Bi content is more than 0.100%, the above effect is saturated, which is not economically preferable. Therefore, the Bi content is set to 0.100% or less. From the viewpoint of reducing the alloy cost, the Bi content is preferably 0.050% or less and more preferably 0.030% or less.
Ta: 0.001 to 0.100%
• Similarly to V, Ta is an element that increases the strength of the hot-rolled steel sheet by forming a fine carbide in steel. In order to reliably obtain this effect, the Ta content is preferably set to 0.001% or more. The Ta content is preferably 0.005% or more and still more preferably 0.010% or more.
• On the other hand, when the Ta content is more than 0.100%, the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.080% or less and more preferably 0.050% or less.
Zr: 0.001 to 0.500%
• Zr is an element that increases the strength of the hot-rolled steel sheet by solid solution strengthening. In order to reliably obtain this effect, the Zr content is preferably set to 0.001% or more. The Zr content is more preferably 0.005% or more and still more preferably 0.010% or more.
• On the other hand, when the Zr content is more than 0.500%, the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, the Zr content is set to set to 0.500% or less. The Zr content is preferably 0.300% or less and more preferably 0.100% or less.
Co: 0.001 to 3.000%
• Co is an element that increases the strength of the hot-rolled steel sheet by solid solution strengthening. In order to reliably obtain this effect, the Co content is preferably set to 0.001% or more. The Co content is more preferably 0.005% or more and still more preferably 0.010% or more.
• On the other hand, when the Co content is more than 3.000%, the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, the Co content is set to 3.000% or less. The Co content is preferably 1.000% or less and more preferably 0.500% or less.
Zn: 0.001 to 0.200%
• Zn is an element that increases the strength of the hot-rolled steel sheet by solid solution strengthening. In order to reliably obtain this effect, the Zn content is preferably set to 0.001% or more. The Zn content is preferably 0.005% or more and still more preferably 0.010% or more.
• On the other hand, when the Zn content is more than 0.200%, the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, the Zn content is set to 0.200% or less. The Zn content is preferably 0.150% or less and more preferably 0.100% or less.
W: 0.001 to 0.200%
• W is an element that increases the strength of the hot-rolled steel sheet by solid solution strengthening. In order to reliably obtain this effect, the W content is preferably set to 0.001% or more. The W content is more preferably 0.005% or more and still more preferably 0.010% or more.
• On the other hand, when the W content is more than 0.200%, the ductility and hole expandability of the hot-rolled steel sheet deteriorate. Therefore, the W content is set to 0.200% or less. The W content is preferably 0.150% or less and more preferably 0.100% or less.
Sb: 0.001 to 0.500%
• Sb is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by suppressing generation of an oxide serving as a starting point of fracture. In order to reliably obtain this effect, the Sb content is preferably set to 0.001% or more. The Sb content is more preferably 0.005% or more and still more preferably 0.10% or more.
• On the other hand, since the above effect is saturated even when a large amount of Sb is contained, the Sb content is set to 0.500% or less. The Sb content is preferably 0.300% or less and more preferably 0.100% or less.
As: 0.001 to 0.050%
• As is an element that enhances the hole expandability of the hot-rolled steel sheet by lowering the austenite single phase-forming temperature to make the prior austenite grains finer. When this effect is reliably obtained, the As content is preferably set to 0.001% or more. The As content is preferably 0.005% or more and still more preferably 0.010% or more.
• On the other hand, since the above effect is saturated even when a large amount of As is contained, the As content is set to 0.050% or less. The As content is preferably 0.040% or less and more preferably 0.030% or less.
Sn: 0.001 to 0.050%
• Sn is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by suppressing generation of an oxide serving as a starting point of fracture. When this effect is reliably obtained, the Sn content is preferably set to 0.001% or more. The Sn content is preferably 0.005% or more and still more preferably 0.010% or more.
• On the other hand, since the above effect is saturated even when a large amount of Sn is contained, the Sn content is set to 0.050% or less. The Sn content is preferably 0.040% or less and more preferably 0.030% or less.
• The chemical composition of the hot-rolled steel sheet described above may be analyzed using a spark discharge optical emission spectrometer or the like. C and S adopt values identified by burning in an oxygen stream using a gas component analyzer or the like and measuring by an infrared absorption method. In addition, N adopts a value identified by melting a test piece collected from a steel sheet in a helium gas flow and measuring the melted test piece by a thermal conductivity method.
• When the hot-rolled steel sheet includes a plating layer on the surface, the chemical composition may be analyzed after the plating layer is removed by mechanical grinding or the like as necessary.
• 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, GAM_S/GAM_I, which is a ratio of GAM_I, which is an area average value of GAM values of grains at a position of 1/4 depth from a surface in a sheet thickness direction, to GAM_S, which is an area average value of GAM values of grains in a region from the surface to a depth of 200 µm in the sheet thickness direction, is 0.70 to 1.05, and in a microstructure at the position of 1/4 depth from the surface in the sheet thickness direction, the area ratio of a region having a GAM value of more than 0.6° is 50% or more, and the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is less than 15%, and a standard deviation of the area average value of the GAM values of grains in a region from the surface to a depth of 200 µm in the sheet thickness direction is 0.25 to 0.65°.
• In the present embodiment, a microstructure at 1/4 position from an end surface in a width direction is defined. Here, the 1/4 position from the end surface in the width direction is a w/4 position from the end surface in the width direction when the length in the width direction is w.
• That is, the "x/y position (here, x and y are natural numbers satisfying x<y.) from an end surface" means a position moved from the end surface in the width direction of the steel sheet toward the central part of the steel sheet by a distance of x/y of the sheet width in the width direction. For example, when the sheet width of the steel sheet is 1 m, the "1/4 position from the end surface" means a position at a distance of 0.25 m from the end surface in the width direction of the steel sheet.
• The "sheet thickness x/y position (here, x and y are natural numbers satisfying x<y.)" means a position moved from the surface (sheet surface) in the sheet thickness direction of the steel sheet toward the central part of the steel sheet by a distance (depth) of x/y of a sheet thickness t in the sheet thickness direction. For example, when the sheet thickness t of the steel sheet is 2 mm, the "sheet thickness 1/8 position" means a position at a depth of 0.25 mm from the surface in the sheet thickness direction of the steel sheet.
• When the steel sheet has a coating such as a plating layer on the surface, the "surface of the steel sheet" means an interface between the steel sheet and the coating, and the "sheet thickness t" means the sheet thickness of the steel sheet (base metal) excluding the coating.
• Hereinafter, each definition will be described.
GAM_S/GAM_I: 0.70 to 1.05
• The "GAM (Grain Average Misorientation) value" of grain is measured by an electron backscatter pattern (EBSP) method. A grain having a small GAM value improves ductility of the hot-rolled steel sheet, but reduces strength. In addition, the strength depends on the average properties of the hot-rolled steel sheet in the sheet thickness direction, whereas bendability depends on the properties of the sheet surface. Therefore, the present inventors have found that the strength and bendability of the hot-rolled steel sheet after prestrain application can be improved by arranging grains having a small GAM value and excellent ductility in a surface layer region of the hot-rolled steel sheet.
• When GAM_S/GAM_I, which is a ratio of GAM_I, which is an area average value of GAM values of grains at a position of 1/4 depth from a surface in a sheet thickness direction (hereinafter, it may be referred to as an internal region), to GAMs, which is an area average value of GAM values of grains in a region from the surface to a depth of 200 µm in the sheet thickness direction (hereinafter, it may be referred to as a surface layer region), is less than 0.70, the GAM value of grain in the surface layer region is small, and a desired strength cannot be obtained in the hot-rolled steel sheet. Therefore, GAM_S/GAM_I is set to 0.70 or more. GAM_S/GAM_I is preferably set to 0.80 or more.
• On the other hand, when GAM_S/GAM_I is more than 1.05, the GAM value of grain in the surface layer region becomes too large, and desired bendability cannot be obtained after prestrain application. Therefore, GAM_S/GAM_I is set to 1.05 or less. GAM_S/GAM_I is preferably set to 0.95 or less.
Standard deviation of area average value of GAM values of grains in surface layer region: 0.25 to 0.65°
• The fracture in the hot-rolled steel sheet occurs mainly in grains having a large GAM value. However, when moderate nonuniformity is imparted in the metallographic structure, the grains having a small GAM value are preferentially deformed at the time of prestrain, and deformation of the grains having a large GAM value is suppressed, so that it is possible to suppress the fracture of the grains having a large GAM value at the time of bending forming after prestrain application. When the standard deviation of the area average value of the GAM values in the region from the surface to a depth of 200 µm in the sheet thickness direction (the surface layer region) is less than 0.25°, the deformation of the grains having a large GAM value cannot be suppressed, and desired bendability cannot be obtained after prestrain application. Therefore, the standard deviation of the area average value of the GAM values of grains in the surface layer region is set to 0.25° or more. The standard deviation of the area average value of the GAM values of grains in the surface layer region is preferably set to 0.35° or more.
• On the other hand, when the nonuniformity is too large, excessive strain concentration occurs, which causes deterioration of bendability after prestrain application. Therefore, the standard deviation of the area average value of the GAM values of grains in the surface layer region is set to 0.65° or less. The standard deviation of the area average value of the GAM values of grains in the surface layer region is preferably set to 0.55° or less.
• The GAM values of grains in the internal region and the surface layer region are measured by the following method.
• First, a sample is collected so that a microstructure of a cross section with the width direction as a normal direction (the sheet thickness direction × a cross section in the rolling direction) can be observed at the 1/4 position from the end surface in the width direction of the hot-rolled steel sheet. The size of the sample may be, for example, a rectangular parallelepiped having a total thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction, depending on the measuring device. Next, the observed section of the sample is mirror-polished, and then polished using colloidal silica containing no alkaline solution at room temperature for 8 minutes to remove strain introduced into the surface of the sample. A region of 200 µm in the sheet thickness direction around a 1/4 depth position from the surface in the sheet thickness direction and 400 µm or more at an arbitrary position in the rolling direction (a rectangular region having a center at a 1/4 depth position in the sheet thickness direction, the rectangular region having a length of 200 µm (short side) in the sheet thickness direction and a length of 400 µm or more (long side) in the rolling direction) of the polished sample is measured at a measurement interval of 0.2 µm to obtain crystal orientation information.
• For the measurement by the EBSP method, an EBSD analyzer including a thermal field emission scanning electron microscope (JSM-7001F, manufactured by JEOL Ltd.) and an EBSD detector (HIKARI detector, manufactured by TSL Solutions Ltd.) is used. At this time, the degree of vacuum in the EBSD analyzer is 9.6 × 10^-5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the irradiation level of the electron beam is 62. From the obtained crystal orientation information, using a "Grain Average Misorientation" function installed on the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, a region surrounded by grain boundaries having an orientation difference of 15° or more is regarded as one grain in the EBSD analysis image, and the average value of orientation differences between adjacent pixels in the grain is calculated to obtain the GAM value of the grain. It is to be noted that the defined grains having an equivalent circle diameter of 0.6 µm or less may have a large measurement error and thus are excluded from the measurement. GAM_I, which is an area average value of GAM values of grains in the internal region, is obtained by performing calculation for all grains using the obtained GAM value, the area of grains measured in the EBSD analysis image, and the following formula (1).
• In the following formula (1), GAM_i represents the GAM value of the i-th grain, A_i represents the area of the i-th grain, and n represents the number of grains included in the measurement range.
  [Formula 1] $$\left({\displaystyle {\sum }_{i=1}^n{A}_i\times {\mathit{GAM}}_i}\right)/\left({\displaystyle {\sum }_{i=1}^n{A}_i}\right)$$
• In addition, GAM_S, which is an area average value of GAM values of grains in the surface layer region, is obtained by performing measurement in the same manner for a region of 200 µm in the sheet thickness direction around a depth position of 100 µm from the surface in the sheet thickness direction and 400 µm or more at an arbitrary position in the rolling direction of the polished sample.
• Further, by using the area average value GAM_S of the GAM values obtained for grains in the surface layer region and the following formula (2), the standard deviation of the area average value of the GAM values of grains in the surface layer region is obtained.
• In the following formula (2), GAM_i represents the GAM value of the i-th grain, A_i represents the area of the i-th grain, and n represents the number of grains included in the measurement range.
  [Formula 2] $$\sqrt{\left({\displaystyle {\sum }_{i=1}^n{A}_i\times {\left({\mathit{GAM}}_i-{\mathit{GAM}}_{\mathrm{S}}\right)}^2}\right)/\left({\displaystyle {\sum }_{i=1}^n{A}_i}\right)}$$
• The rolling direction of the hot-rolled steel sheet is determined by the following method.
• A test piece is collected so that a cross section parallel to the sheet surface of the hot-rolled steel sheet can be observed. In the collected test piece, a cross section at which the distance from the surface is 1/4 position of the sheet thickness is finished by mirror polishing, and then observed using an optical microscope. The observation range is set to 500 µm × 500 µm or more, and a direction parallel to the elongation direction of the grains is determined as the rolling direction. In the observed cross section, a direction orthogonal to the determined rolling direction is determined as the width direction of the hot-rolled steel sheet.
Area ratio of region having GAM value of more than 0.6°: 50% or more
• In the microstructure in the internal region, when the area ratio of a region having a GAM value of more than 0.6° is less than 50%, a desired strength cannot be obtained in the hot-rolled steel sheet. Therefore, the area ratio of a region having a GAM value of more than 0.6° is set to 50% or more. The area ratio of a region having a GAM value of more than 0.6° is preferably 80% or more, more preferably 90% or more, and still more preferably 95% or more.
• The area ratio of a region having a GAM value of more than 0.6° may be 100%.
Sum of area ratio of region having GAM value of more than 3.0° and area ratio of residual austenite: less than 15%
• When the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is 15% or more, a desired strength may not be obtained or a desired hole expandability may not be obtained in the hot-rolled steel sheet. Therefore, the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is less than 15%. The sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is preferably 10% or less and more preferably 5% or less.
• The sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite may be 0% or 1% or more.
• Here, desired strength, ductility, and degree of bendability after prestrain application vary depending on the applied vehicle component. The hot-rolled steel sheet according to the present embodiment may have the above-described chemical composition and metallographic structure, and then may have either microstructure of a first or second aspect described below depending on the desired strength, ductility, and degree of bendability after prestrain application.
(First aspect) Area ratio of region having GAM value of more than 0.6° and less than 2.0°: 50% or more
• The first aspect is a microstructure relatively suitable in a case where it is required to achieve both strength and ductility at higher levels. In the present embodiment, by setting the area ratio of a region having a GAM value of more than 0.6° and less than 2.0° to 50% or more, it is possible to achieve both strength and ductility at higher levels in the hot-rolled steel sheet. In the first aspect, the area ratio of a region having a GAM value of more than 0.6° and less than 2.0° is preferably 70% or more and more preferably 85% or more.
• The area ratio of a region having a GAM value of more than 0.6° and less than 2.0° may be 100%.
• In the first aspect, the region having a GAM value of 2.0° or more and the region having a GAM value of 0.6° or less at a total area ratio of 0 to 50% may be included as the remainder in microstructure other than the region having a GAM value of more than 0.6° and less than 2.0°.
(Second aspect) Area ratio of region having GAM value of 2.0° or more: 50% or more
• The second aspect is a microstructure relatively suitable in a case where higher strength is required. In the present embodiment, by setting the area ratio of a region having a GAM value of 2.0° or more to 50% or more, higher strength can be obtained in the hot-rolled steel sheet. The area ratio of a region having a GAM value of 2.0° or more is preferably 70% or more and more preferably 85% or more.
• The area ratio of a region having a GAM value of 2.0° or more may be 100%.
• In the second aspect, the region having a GAM value of less than 2.0° at an area ratio of 0 to 50% may be included as the remainder in microstructure other than the region having a GAM value of 2.0° or more.
• The area ratio of a region having a GAM value of more than 0.6°, the area ratio of a region having a GAM value of more than 0.6° and less than 2.0°, the area ratio of a region having a GAM value of 2.0° or more, and the area ratio of a region having a GAM value of more than 3.0° are measured by the following methods.
• The GAM values of grains in the internal region are calculated in the same manner as in measuring the GAM values of grains in the internal region described above. The area ratio of grains having an obtained GAM value of more than 0.6°, the area ratio of grains having an obtained GAM value of more than 0.6° and less than 2.0°, the area ratio of grains having an obtained GAM value of 2.0° or more, and the area ratio of a region having an obtained GAM value of more than 3.0° are calculated to obtain the area ratio of each region.
• The area ratio of residual austenite is measured by the following method.
• In the measurement of the area ratio of residual austenite by X-ray diffraction in the present embodiment, first, a sample is collected so that a microstructure in a region of 1 mm or more at an arbitrary position in the rolling direction and 1 mm or more around a 1/4 position from the end surface in the width direction can be observed in the cross section at a 1/4 position from the surface in the sheet thickness direction of the hot-rolled steel sheet. The integrated intensity of a total of six peaks α(110), α(200), α(211), γ(111), γ(200), and y(220) of the sample is determined using a Co-Kα ray. Next, the volume percentage of residual austenite is calculated from the integrated intensity using an intensity average method. The obtained volume percentage of residual austenite is regarded as the area ratio of residual austenite.
Mechanical properties Tensile strength (TS): 940 MPa or more
• The tensile strength may be 940 MPa or more. By setting the tensile strength to 940 MPa or more, contribution to weight reduction of a vehicle body can be increased, and the hot-rolled steel sheet can be suitably applied to a vehicle component. The upper limit of the tensile strength is not particularly limited, but may be 1400 MPa or less from the viewpoint of suppressing die wear.
Uniform elongation (uEl): 3.0% or more
• The uniform elongation may be 3.0% or more. By setting the uniform elongation to 3.0% or more, the hot-rolled steel sheet can be suitably applied to a vehicle component. The upper limit of the uniform elongation is not particularly limited, but may be 10.0% or less.
• The tensile strength and the uniform elongation are measured by performing a tensile test in accordance with JIS Z 2241:2022 using a No. 5 test piece of JIS Z 2241:2022. The tensile test piece is collected at the center position in the width direction, and the direction perpendicular to the rolling direction and the sheet thickness direction (width direction) is taken as the longitudinal direction.
• When the No. 5 test piece cannot be collected from the hot-rolled steel sheet to be measured, a minute test piece having the width direction as the longitudinal direction can be substituted as a test piece for measuring the tensile strength.
Hole expansion ratio: 40% or more
• The hole expansion ratio may be 40% or more. By setting the hole expansion ratio to 40% or more, the hot-rolled steel sheet can be suitably applied to a vehicle component. The upper limit of the hole expansion ratio is not particularly limited, but may be 80% or less.
• The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.
Bendability after prestrain application
• Bendability after prestrain application is evaluated by performing a bending test on the hot-rolled steel sheet after draw bending process. The draw bending process is performed, for example, by forming a hat component 10 with a forming height of 60 mm under the conditions shown in FIG. 1. The test piece is collected from the hot-rolled steel sheet such that the longitudinal direction of the test piece is the width direction of the hot-rolled steel sheet and the size is 240 mm × 60 mm. Conditions for the draw bending process are as follows: the width of a punch 1 is 75 mm, the corner R of the punch 1 is "sheet thickness × 5 (mm)", the corner R of a die 2 is "sheet thickness × 3.125 mm", the clearance between the punch 1 and the die 2 is "sheet thickness + 0.9 mm", and the blank holder force (BHF) is "sheet thickness × 6.25 (ton)". The conditions shown in FIG. 1 are conditions when the sheet thickness is 1.6 mm.
• In the forming of the hat component 10 as shown in FIG. 1, when a vertical wall 11 is formed, the steel sheet comes into contact with the punch 1 while being subjected to bending and unbending deformation, so that it is possible to reproduce a recessed part formed in a flat-R portion in the vicinity of the vertical wall portion of the vehicle suspension part. In the bending test to be described later, a test piece is collected from the vertical wall portion 11 of the hat component 10 such that the die 2 side is outside the bending and the stroke direction D of the punch 1 is in the bending axis direction.
• In the bending test, using the test piece described above, a bending test is performed under the following conditions based on the VDA standard (VDA238-100) defined by the Verband der Automobilindustrie.
• In the case where the tensile strength of the hot-rolled steel sheet is less than 1040 MPa, if the maximum bending angle obtained by a bending test after prestrain application is 70° or more, it can be determined that the hot-rolled steel sheet has excellent bendability after prestrain application.
• Also, in the case where the tensile strength of the hot-rolled steel sheet is 1040 MPa or more, if the maximum bending angle obtained by a bending test after prestrain application is 50° or more, it can be determined that the hot-rolled steel sheet has excellent bendability after prestrain application.
• When the sheet thickness of the test piece after prestrain application exceeds 1.6 mm, the surface on the punch side is ground to set the sheet thickness to 1.6 mm, and then the bending test is performed.
• In addition, when the sheet thickness of the test piece after prestrain application is 1.6 mm or less, the maximum bending angle obtained by the following formula is adopted. In the following formula, α₀ represents the maximum bending angle obtained by the bending test, t represents the sheet thickness, and uEL represents uniform elongation. Maximum bending angle in the case of 1.6 mm or less = α₀ - 13.852 × (1 - t/1.6) × (uEL + 0.22)^0.292
• Test piece dimensions: 60 mm (rolling direction) × 30 mm (width direction)
• Sheet thickness of test piece: 1.6 mm
• Bending ridge line: direction parallel to width direction
• Test method: roll support, punch pushing
• Roll diameter: φ30 mm
• Punch shape: tip R = 0.4 mm
• Roll-to-roll distance: 2.0 × sheet thickness (mm) + 0.5 mm
• Pushing speed: 20 mm/min
• Tester: SHIMADZU AUTOGRAPH 20 kN
• Further, in the first and second aspects described above, since the desired strength, ductility, and degree of bendability after prestrain application are different, each aspect may have the following strength, ductility, and bendability after prestrain application. Since desired hole expandability is equivalent in either aspect, description thereof is omitted.
(First aspect) Tensile strength: 940 MPa or more, uniform elongation: 4.0% or more
• In the first aspect, the tensile strength may be 940 MPa or more, and the uniform elongation may be 4.0% or more. In the first aspect, the tensile strength may be 980 MPa or more. Also, in the first aspect, the uniform elongation may be 5.0% or more.
(First aspect) Bendability after prestrain application: maximum bending angle after prestrain application of 70° or more
• In the first aspect, after prestrain application, the maximum bending angle obtained by the bending test described above may be 70° or more.
(Second aspect) Tensile strength: 1040 MPa or more, uniform elongation: 3.0% or more
• In the second aspect, the tensile strength may be 1040 MPa or more, and the uniform elongation may be 3.0% or more. In the second aspect, the tensile strength may be 1140 MPa or more. Also, in the second aspect, the uniform elongation may be 4.0% or more.
(Second aspect) Bendability after prestrain application: maximum bending angle after prestrain application of 50° or more
• In the second aspect, after prestrain application, the maximum bending angle obtained by the bending test described above may be 50° or more.
• The hot-rolled steel sheet according to the present embodiment may be a surface-treated steel sheet by providing a plating layer on the surface for the purpose of improving corrosion resistance and the like. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electro-galvanizing and electric Zn-Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, alloying hot-dip galvanizing, 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 in the conventional plating layer. In addition, it is also possible to further enhance corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical treatment solution) after plating.
• Next, a preferred manufacturing method for the hot-rolled steel sheet according to the present embodiment will be described. According to the manufacturing method described below, the hot-rolled steel sheet according to the present embodiment can be stably manufactured. 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.
• Steps (1) to (3) described below are steps common in the first and second aspects. For the subsequent steps, steps (4) and (5) correspond to the first aspect and step (6) corresponds to the second aspect.
• A preferred method for manufacturing a hot-rolled steel sheet according to the present embodiment includes the steps of:
1. (1) applying strain two or more times in the width direction to a slab having the above-described chemical composition before rough rolling, setting a difference between a temperature at the time of the first strain application and a temperature at the time of the final strain application to 20°C to 40°C, and performing descaling every time strain is applied in the width direction;
2. (2) performing rough rolling on the slab to which the strain has been applied; and
3. (3) performing finish rolling such that a difference between a finish rolling start temperature and a finishing temperature is 60°C or higher and lower than 120°C, and the finishing temperature is 950°C or lower,
   and further includes one or more steps of the following (4) to (6).
4. (4) After completion of finish rolling, accelerated cooling to a temperature range of 580°C to 680°C at an average cooling rate of 30°C/s or faster, and slow cooling (air cooling) in this temperature range for 2.0 seconds or longer.
5. (5) After completion of slow cooling, accelerated cooling to 300°C at an average cooling rate of 30°C/s or faster.
6. (6) After completion of finish rolling, accelerated cooling to 300°C at an average cooling rate of 30°C/s or faster.
• Hereinafter, each step will be described.
(1) Strain application before rough rolling: common in first and second aspects
• It is preferable to apply strain two or more times in the width direction to a slab having the above-described chemical composition before rough rolling, setting a difference between a temperature at the time of the first strain application in the width direction and a temperature at the time of the final strain application in the width direction to 20°C to 40°C. This makes it possible to control the nonuniformity in the surface layer region to an appropriate state. As a result, the standard deviation of the area average value of the GAM values of grains in the surface layer region of the hot-rolled steel sheet can be controlled within a preferable range. Here, the "width direction of the slab" is a direction orthogonal to the conveyance direction of the slab and the sheet thickness direction, and the conveyance direction of the slab corresponds to the rolling direction in a later step.
• Even if the number of times of applying strain in the width direction is only one, the standard deviation of the area average value of the GAM values of grains in the surface layer region of the hot-rolled steel sheet cannot be controlled within a preferable range. Therefore, as described above, the number of times of applying strain in the width direction is two or more.
• In addition, descaling is preferably performed every time strain is applied in the width direction of the slab, that is, descaling is preferably performed every time before multiple times of strain application. Descaling is preferably performed by injecting water. By performing descaling every time strain is applied in the width direction of the slab, it is possible to suppress a rapid rise of the surface layer temperature due to heat recuperation. As a result, the difference between a temperature at the time of the first strain application and a temperature at the time of the final strain application can be preferably controlled to 20°C to 40°C. When strain is applied in the width direction of the slab, it is usual to perform descaling once only before the first strain application. In this case, by applying strain in the width direction multiple times, the surface layer temperature rises due to heat recuperation, and it is difficult to control the temperature rise to 20°C to 40°C.
• The strain may be applied after slab heating for rough rolling is performed.
• Examples of a method of applying strain in the width direction of the slab include a method of applying strain in the width direction (pressing down in the width direction) to the slab by passing the slab between rolls installed so that a rotary shaft is perpendicular to a sheet surface of the slab and the conveyance direction.
• The slab to which strain is applied is not particularly limited except for having the above-described chemical composition. For example, a slab manufactured by melting molten steel having the above chemical composition using a converter, an electric furnace, or the like and a continuous casting method can be used. Instead of the continuous casting method, an ingot-making method, a thin slab casting method, or the like may be adopted. In the slab heating before rough rolling, the heating temperature may be set to a temperature range of 1100°C to 1300°C.
(2) Rough rolling: common in first and second aspects
• The conditions for rough rolling are not particularly limited, and the rough rolling can be, for example, a step of performing rolling a plurality of times at a temperature of 1100°C or higher to set the sheet thickness to 30 to 60 mm.
(3) Finish rolling: common in first and second aspects
• In the finish rolling step, it is preferable to perform finish rolling such that a difference between a finish rolling start temperature (inlet side temperature) and a finishing temperature (outlet side temperature) is 60°C or higher and lower than 120°C, and the finishing temperature is 950°C or lower. This makes it possible to control GAM_S/GAM_I, which is a ratio of GAM_I, which is an area average value of GAM values of grains in the internal region, to GAM_S, which is an area average value of GAM values of grains in the surface layer region, within a preferable range. The lower limit of the finishing temperature is not particularly limited, and may be appropriately determined according to the rolling load limitation of the equipment. In order to suppress a rapid increase in load, the finishing temperature can be, for example, 850°C or higher.
(4) Slow cooling (air cooling) in temperature range of 580°C to 680°C: corresponding to first aspect
• After completion of the finish rolling, the steel sheet is accelerated cooled to a temperature range of 580°C to 680°C at an average cooling rate of 30°C/s or faster, and slowly cooled (air-cooled) in this temperature range for 2.0 seconds or longer. The area ratio of a region having a GAM value of more than 0.6° and less than 2.0° can be increased by slow cooling (air cooling) in a temperature range of 580°C to 680°C for 2.0 seconds or longer.
• The slow cooling (air cooling) in the present embodiment refers to cooling with an average cooling rate of 20°C/s or slower.
(5) Accelerated cooling after slow cooling (air cooling): corresponding to first aspect
• After slow cooling (air cooling) in the temperature range of 580°C to 680°C (first aspect), accelerated cooling is performed to 300°C at an average cooling rate of 30°C/s or faster. After slow cooling (air cooling), accelerated cooling is performed to 300°C at an average cooling rate of 30°C/s or faster, whereby a desired microstructure can be obtained.
• After being accelerated cooled to 300°C, the steel sheet may be air cooled to room temperature, or may be coiled into a coil shape and then water-cooled.
(6) Accelerated cooling to 300°C: corresponding to second aspect
• After completion of the finish rolling, accelerated cooling is performed to 300°C at an average cooling rate of 30°C/s or faster. By performing accelerated cooling to 300°C at an average cooling rate of 30°C/s or faster without performing slow cooling (air cooling) in the middle of the accelerated cooling, the area ratio of a region having a GAM value of more than 0.6° (including a region having a GAM value of 2.0° or more) can be increased.
• After being accelerated cooled to 300°C, the steel sheet may be air cooled to room temperature, or may be coiled into a coil shape and then water-cooled.
• The average cooling rate in the present embodiment is a value obtained by dividing a temperature difference between a start point and an end point of a set range by an elapsed time from the start point to the end point.
Examples
• Next, Examples of the present invention will be described, but conditions in Examples are examples of conditions adopted to confirm feasibility and an effect of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
• Slabs having the chemical compositions shown in Tables 1A to 2B were manufactured by continuous casting. Using the obtained slabs, hot-rolled steel sheets having a sheet thickness of 3.0 mm were manufactured under the conditions shown in Tables 3A to 3C.
• Blanks in Tables 1A to 2B indicate that the element is not intentionally contained.
• After slab heating, strain was applied two or more times in the width direction. The difference between a temperature at the time of the first strain application in the width direction and a temperature at the time of the final strain application in the width direction is described in the column of "Temperature difference at time of strain application in width direction" in the table.
• Regarding manufacture Nos. 1 to 18 and 25 to 45, after completion of the finish rolling, accelerated cooling was performed at an average cooling rate of 30°C/s or faster to the "start temperature of slow cooling" in the table. The slow cooling was performed by air cooling, and the average cooling rate in the slow cooling was 20°C/s or slower.
• Also, after slow cooling, accelerated cooling was performed by "Average cooling rate until reaching 300°C after completion of slow cooling" in the table. After the accelerated cooling was stopped, coiling was immediately performed.
• For manufacture Nos. 19 to 24 and 46 to 63, after the completion of the finish rolling, accelerated cooling was performed according to "Average cooling rate until reaching 300°C after completion of finish rolling" in the table without performing slow cooling. After the accelerated cooling was stopped, coiling was immediately performed.
• When the difference between a finish rolling start temperature and a finishing temperature was 60°C or higher and lower than 120°C, "OK" was written in the column of "Difference between finish rolling start temperature and finishing temperature of 60°C or higher and lower than 120°C" in Tables 3A to 3C. On the other hand, when this condition was not satisfied, "NG" was written in the column.
• For the obtained hot-rolled steel sheet, the metallographic structure, the tensile strength (TS), the uniform elongation (uEl), the hole expansion ratio (λ), and the bendability after prestrain application were evaluated by the above-described methods.
• The obtained results are shown in Tables 4A to 5C.
• A case where the tensile strength (TS) was 940 MPa or more was determined to be acceptable as having high strength. On the other hand, a case where the tensile strength (TS) was less than 940 MPa was determined to be unacceptable as not having high strength.
• A case where the uniform elongation (uEl) was 3.0% or more was determined to be acceptable as having excellent ductility. On the other hand, a case where the uniform elongation (uEl) was less than 3.0% was determined to be unacceptable as not having excellent ductility.
• A case where the hole expansion ratio (λ) was 40% or more was determined to be acceptable as having excellent hole expandability. On the other hand, a case where the hole expansion ratio (λ) was less than 40% was determined to be unacceptable as not having excellent hole expandability.
• The bendability (maximum bending angle) after prestrain application was evaluated according to the following criteria depending on the tensile strength.
· In case where tensile strength is less than 1040 MPa
• A case where the bendability (maximum bending angle) after prestrain application was 70° or more was determined to be acceptable as having excellent bendability after prestrain application. On the other hand, a case where the bendability (maximum bending angle) after prestrain application was less than 70° was determined to be unacceptable as not having excellent bendability after prestrain application.
· In case where tensile strength is 1040 MPa or more
• A case where the bendability (maximum bending angle) after prestrain application was 50° or more was determined to be acceptable as having excellent bendability after prestrain application. On the other hand, a case where the bendability (maximum bending angle) after prestrain application was less than 50° was determined to be unacceptable as not having excellent bendability after prestrain application. [Table 1A]

  Steel	Chemical composition (mass%), remainder being Fe and impurities
  	C	Si	Mn	Ti	Al	P	S	N	O	Nb	V	Cu	Cr	Mo
  A	0.025	0.93	2.00	0.114	0.182	0.008	0.0037	0.0023	0.002	0.007
  B	0.126	0.90	1.29	0.127	0.107	0.008	0.0037	0.0035	0.001	0.012
  C	0.071	3.17	1.65	0.098	0.081	0.009	0.0046	0.0035	0.003	0.013
  D	0.058	0.57	2.73	0.110	0.098	0.008	0.0043	0.0029	0.003	0.011
  E	0.086	0.98	1.05	0.093	0.070	0.009	0.0038	0.0024	0.002	0.014			0.104
  F	0.082	0.90	1.64	0.207	0.114	0.008	0.0042	0.0017	0.001	0.012
  G	0.066	0.89	2.00	0.002	0.109	0.007	0.0039	0.0023	0.003	0.011
  H	0.060	0.79	2.19	0.101	0.410	0.007	0.0044	0.0024	0.002	0.007
  I	0.075	1.01	2.03	0.095	0.005	0.008	0.0042	0.0031	0.002	0.011
  J	0.072	0.99	2.10	0.139	0.156	0.091	0.0039	0.0025	0.002	0.007
  K	0.066	0.81	1.86	0.124	0.058	0.009	0.0114	0.0016	0.002	0.012		0.190
  L	0.073	1.03	2.16	0.094	0.132	0.007	0.0044	0.0098	0.001	0.015			0.195
  M	0.070	0.87	1.99	0.112	0.208	0.008	0.0042	0.0025	0.012					0.114
  N	0.073	0.87	2.08	0.092	0.171	0.008	0.0041	0.0018	0.001
  O	0.072	0.77	2.14	0.121	0.071	0.007	0.0040	0.0029	0.003	0.009	0.013		0.100
  P	0.069	0.94	1.92	0.085	0.171	0.007	0.0038	0.0035	0.002	0.007	0.009
  Q	0.114	0.92	1.28	0.123	0.189	0.009	0.0037	0.0025	0.001	0.008
  R	0.048	0.62	2.48	0.091	0.054	0.008	0.0039	0.0027	0.002	0.010			0.188
  S	0.072	1.56	1.85	0.103	0.072	0.009	0.0044	0.0022	0.002	0.013		0.125
  T	0.094	0.01	2.24	0.092	0.162	0.007	0.0037	0.0033	0.002	0.005
  U	0.067	0.96	2.05	0.172	0.055	0.009	0.0047	0.0018	0.003	0.011			0.100
  V	0.066	0.99	2.18	0.102	0.136	0.008	0.0045	0.0024	0.001	0.033				0.125
  W	0.073	1.08	2.02	0.124	0.194	0.008	0.0042	0.0029	0.002	0.008	0.290		0.100
  X	0.068	0.89	2.16	0.131	0.340	0.007	0.0039	0.0018	0.003	0.010
  Y	0.076	0.82	1.88	0.110	0.021	0.009	0.0043	0.0032	0.002	0.010			0.100
  Z	0.076	0.86	1.62	0.114	0.110	0.008	0.0041	0.0019	0.003	0.010

  The underline represents that it is outside of the range of the present invention.

  [Table 1B]

  Steel	Chemical composition (mass%), remainder being Fe and impurities
  	C	Si	Mn	Ti	Al	P	S	N	O	Nb	v	Cu	Cr	Mo
  AA	0.071	0.76	2.21	0.046	0.045	0.006	0.0022	0.0021	0.002	0.004	0.014
  AB	0.073	0.89	1.98	0.093	0.146	0.009	0.0031	0.0031	0.001
  AC	0.076	0.74	2.19	0.121	0.071	0.007	0.0043	0.0030	0.003	0.009	0.107
  AD	0.070	0.97	2.08	0.092	0.173	0.007	0.0038	0.0036	0.002	0.007	0.008
  AE	0.115	0.88	1.26	0.114	0.191	0.009	0.0037	0.0024	0.001	0.009
  AF	0.046	0.57	2.47	0.088	0.054	0.008	0.0038	0.0027	0.002	0.010			0.207
  AG	0.074	1.59	1.88	0.105	0.074	0.009	0.0044	0.0021	0.002	0.012		0.120
  AH	0.092	0.01	2.17	0.091	0.157	0.007	0.0038	0.0033	0.002	0.005
  AI	0.065	0.94	2.05	0.158	0.055	0.009	0.0047	0.0019	0.003	0.012
  AJ	0.066	1.04	2.07	0.098	0.148	0.008	0.0043	0.0026	0.001	0.032				0.125
  AK	0.076	1.05	2.08	0.123	0.203	0.008	0.0041	0.0028	0.002	0.008	0.306
  AL	0.072	0.90	2.10	0.124	0.329	0.007	0.0035	0.0017	0.003	0.010
  AM	0.081	0.83	1.93	0.105	0.020	0.009	0.0043	0.0033	0.002	0.010
  AN	0.079	0.87	1.59	0.118	0.104	0.008	0.0042	0.0019	0.003	0.010

  [Table 2A]

  Steel	Chemical composition (mass%), remainder being Fe and impurities
  	Ni	B	Ca	Mg	REM	Bi	Ta	Zr	Co	Zn	W	Sb	As	Sn
  A
  B
  C			0.0030	0.0021
  D						0.002
  E							0.011
  F										0.154
  G		0.0038
  H					0.003
  I											0.165
  J								0.356
  K									0.193
  L		0.0013
  M														0.020
  N
  O							0.023
  P												0.060
  Q								0.304
  R														0.040
  S
  T	0.052												0.005
  U			0.0031							0.113
  V
  W						0.003					0.157
  X				0.0021					0.175
  Y					0.096
  Z		0.0010

  The underline represents that it is outside of the range of the present invention.

  [Table 2B]

  Steel	Chemical composition (mass%), remainder being Fe and impurities
  	Ni	B	Ca	Mg	REM	Bi	Ta	Zr	Co	Zn	W	Sb	As	Sn
  AA							0.014
  AB		0.0019
  AC		0.0022					0.018
  AD		0.0023										0.062
  AE		0.0017						0.288
  AF		0.0018												0.045
  AG		0.0019
  AH	0.051	0.0021											0.004
  AI		0.0022	0.0027							0.126
  AJ		0.0021
  AK		0.0022				0.004					0.157
  AL		0.0024		0.0024					0.176
  AM		0.0020			0.089
  AN		0.0018

  [Table 5A]

  Manufacture No.	Steel	Tensile strength TS	Uniform elongation uEL	Hole expansion ratio λ	Maximum bending angle after prestrain application	Note
  		MPa	%	%	°
  1	A	780	10.1	104	111	Comparative Example
  2	B	1230	3.8	37	69	Comparative Example
  3	C	Slab cracking				Comparative Example
  4	D	1127	4.1	37	121	Comparative Example
  5	E	845	8.9	71	115	Comparative Example
  6	F	966	6.9	34	115	Comparative Example
  7	G	922	7.7	41	120	Comparative Example
  8	H	Slab cracking				Comparative Example
  9	I	1013	5.6	36	107	Comparative Example
  10	J	Slab cracking				Comparative Example
  11	K	948	6.3	35	110	Comparative Example
  12	L	Slab cracking				Comparative Example
  13	M	977	5.8	35	115	Comparative Example
  14	N	1024	5.5	70	69	Comparative Example
  15	AA	1035	5.9	42	68	Comparative Example
  16	AA	1005	6.0	78	68	Comparative Example
  17	AA	1005	5.6	78	69	Comparative Example
  18	AA	938	6.1	75	81	Comparative Example
  19	N	1116	4.4	70	49	Comparative Example
  20	AA	1131	4.8	41	48	Comparative Example
  21	AA	1094	4.8	76	48	Comparative Example
  22	AA	1130	4.7	76	46	Comparative Example
  23	AA	930	5.1	40	81	Comparative Example

  The underline represents that it is outside of the range of the present invention or the property is not preferable.

  [Table 5B]

  Manufacture No.	Steel	Tensile strength TS	Uniform elongation uEL	Hole expansion ratio λ	Maximum bending angle after prestrain application	Note
  		MPa	%	%	°
  24	P	1039	4.2	59	72	Present Invention Example
  25	N	1006	6.0	58	73	Present Invention Example
  26	N	1013	6.1	40	72	Present Invention Example
  27	N	989	5.5	76	74	Present Invention Example
  28	N	980	6.0	58	101	Present Invention Example
  29	N	1005	5.1	53	75	Present Invention Example
  30	N	981	6.7	41	107	Present Invention Example
  31	N	960	7.1	70	105	Present Invention Example
  32	N	1029	5.9	61	111	Present Invention Example
  33	O	1010	5.9	53	92	Present Invention Example
  34	P	985	5.5	76	91	Present Invention Example
  35	Q	1039	5.9	54	88	Present Invention Example
  36	R	942	6.7	70	98	Present Invention Example
  37	S	1065	5.1	54	94	Present Invention Example
  38	T	1050	5.4	60	100	Present Invention Example
  39	U	983	6.3	68	112	Present Invention Example
  40	V	1035	5.8	41	109	Present Invention Example
  41	W	1036	5.5	51	79	Present Invention Example
  42	X	961	6.0	73	114	Present Invention Example
  43	Y	996	6.5	66	106	Present Invention Example
  44	Z	961	6.2	63	110	Present Invention Example
  45	AA	981	6.0	48	93	Present Invention Example
  46	AB	1173	4.1	42	51	Present Invention Example
  47	AB	1151	4.2	40	52	Present Invention Example
  48	AB	1175	4.4	45	54	Present Invention Example

  [Table 5C]

  Manufacture No.	Steel	Tensile strength TS	Uniform elongation uEL	Hole expansion ratio λ	Maximum bending angle after prestrain application	Note
  		MPa	%	%	°
  49	AB	1065	5.4	54	70	Present Invention Example
  50	AB	1055	5.0	70	61	Present Invention Example
  51	AB	1172	4.4	65	70	Present Invention Example
  52	AC	1194	3.7	55	51	Present Invention Example
  53	AD	1168	4.0	54	68	Present Invention Example
  54	AE	1290	3.1	65	52	Present Invention Example
  55	AF	1043	5.3	66	95	Present Invention Example
  56	AG	1211	3.7	52	52	Present Invention Example
  57	AH	1085	3.9	66	58	Present Invention Example
  58	AI	1115	4.3	65	74	Present Invention Example
  59	AJ	1125	4.7	41	73	Present Invention Example
  60	AK	1192	3.9	51	63	Present Invention Example
  61	AL	1070	4.0	46	69	Present Invention Example
  62	AM	1166	4.4	48	56	Present Invention Example
  63	AN	1065	4.8	52	85	Present Invention Example

• As can be seen from Tables 4A to 5C, the hot-rolled steel sheets according to the present invention examples have high strength and excellent ductility and hole expandability, and have excellent bendability after prestrain application.
• On the other hand, it can be seen that the steel sheets according to comparative examples are inferior in any one or more of the properties.
INDUSTRIAL APPLICABILITY
• According to the above aspect of the present invention, a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bendability after prestrain application is provided.

Claims (4)

1. A hot-rolled steel sheet having a chemical composition comprising, in mass%,

   C: 0.045 to 0.120%,

   Si: 0 to 3.00%,

   Mn: 1.20 to 2.60%,

   Ti: 0.020 to 0.180%,

   Al: 0.010 to 0.400%,

   P: 0.080% or less,

   S: 0.0100% or less,

   N: 0.0050% or less,

   O: 0.010% or less,

   Nb: 0 to 0.100%,

   V: 0 to 1.000%,

   Cu: 0 to 1.000%,

   Cr: 0 to 2.000%,

   Mo: 0 to 3.000%,

   Ni: 0 to 0.500%,

   B: 0 to 0.0100%,

   Ca: 0 to 0.0500%,

   Mg: 0 to 0.0500%,

   REM: 0 to 0.100%,

   Bi: 0 to 0.100%,

   Ta: 0 to 0.100%,

   Zr: 0 to 0.500%,

   Co: 0 to 3.000%,

   Zn: 0 to 0.200%,

   W: 0 to 0.200%,

   Sb: 0 to 0.500%,

   As: 0 to 0.050%,

   Sn: 0 to 0.050%, and

   a remainder comprising Fe and impurities,

   wherein GAM_S/GAM_I, which is a ratio of GAM_I, which is an area average value of GAM values of grains at a position of 1/4 depth from a surface in a sheet thickness direction, to GAM_S, which is an area average value of GAM values of grains in a region from the surface to a depth of 200 µm in the sheet thickness direction, is 0.70 to 1.05, and

   in a microstructure at the position of 1/4 depth from the surface in the sheet thickness direction,

   the area ratio of a region having a GAM value of more than 0.6° is 50% or more, and the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is less than 15%, and

   a standard deviation of the area average value of the GAM values of grains in a region from the surface to a depth of 200 µm in the sheet thickness direction is 0.25 to 0.65°.
2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition comprises, in mass%,
   one or more selected from the group consisting of:

   Nb: 0.001 to 0.100%,

   V: 0.001 to 1.000%,

   Cu: 0.001 to 1.000%,

   Cr: 0.001 to 2.000%,

   Mo: 0.001 to 3.000%,

   Ni: 0.001 to 0.500%,

   B: 0.0001 to 0.0100%,

   Ca: 0.0001 to 0.0500%,

   Mg: 0.0001 to 0.0500%,

   REM: 0.001 to 0.100%,

   Bi: 0.001 to 0.100%,

   Ta: 0.001 to 0.100%,

   Zr: 0.001 to 0.500%,

   Co: 0.001 to 3.000%,

   Zn: 0.001 to 0.200%,

   W: 0.001 to 0.200%,

   Sb: 0.001 to 0.500%,

   As: 0.001 to 0.050%, and

   Sn: 0.001 to 0.050%.
3. The hot-rolled steel sheet according to claim 1 or 2, wherein in the microstructure at the position of 1/4 depth from the surface in the sheet thickness direction,
   the area ratio of a region having a GAM value of more than 0.6° and less than 2.0° is 50% or more.
4. The hot-rolled steel sheet according to claim 1 or 2, wherein in the microstructure at the position of 1/4 depth from the surface in the sheet thickness direction,
   the area ratio of a region having a GAM value of 2.0° or more is 50% or more.