Hot-rolled steel sheet

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, in a crystal orientation distribution function of a texture in a surface layer region, a maximum value A within a range of Φ = 0 to 60° and φ1 = 50 to 90° in a cross section of φ2 = 45° is 6.0 or less, a maximum value B within a range of Φ = 120 to 180° and φ1 = 50 to 90° in the cross section of φ2 = 45° is 6.0 or less, |ΦA - 35°| is 10° or less, and |ΦB - 145°| is 10° or less, 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%.

Description

TECHNICAL FIELD

[0001] The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bending properties under tension in a rolling direction.

[0002] Priority is claimed on Japanese Patent Application No. 2023-013128, filed January 31, 2023, the content of which is incorporated herein by reference.BACKGROUND ART

[0003] 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 blankformed components such as a press-formed component, the weights can be reduced by reducing the sheet thicknesses of component materials.

[0004] 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 having high strength. Since vehicle components are manufactured by subjecting steel sheets to various processes, steel sheets that are applied to vehicle components are required to have excellent formability, particularly, ductility and hole expandability.

[0005] During the manufacture of vehicle components, the steel sheet may be bent under tension. Bending under tension is often performed along the rolling direction of the steel sheet. Therefore, the steel sheets that are applied to vehicle components are also required to have excellent bending properties under tension particularly in the rolling direction.

[0006] For example, Patent Document 1 discloses a hot-rolled steel sheet including a microstructure including 70% or more of ferrite in terms of area ratio and pearlite, having a sheet thickness T0 of 6 to 25 mm and an average grain size of ferrite grains GC inside the sheet thickness of 5 to 15 µm, the hot-rolled steel sheet including a fine grain layer formed from a surface in a sheet thickness direction, the average grain size of ferrite grains being less than 1.0 times the average grain size GC, the fine grain layer including a specific fine grain layer in which the average grain size of ferrite grains is 0.1 to 0.4 times the average grain size GC, a predetermined formula being satisfied when the thickness of the specific fine grain layer is TF0 and the thickness of an ultrafine grain layer in which the average grain size of ferrite grains is less than 0.1 times the average grain size GC is TF1 among the fine grain layer, and the average grain size of ferrite grains in the specific fine grain layer and the ultrafine grain layer being 0.1 to 0.4 times the average grain size GC.

[0007] Patent Document 2 discloses a high strength hot rolled steel sheet having excellent hole expansibility and weld fatigue properties, in which a random intensity ratio of a { 110}<111> to { 110}<001> orientation group of a sheet thickness cross section in a region from an outermost layer to a sheet thickness of 1 / 6 is 3.5 or less.Citation ListPatent Document

[0008] Patent Document 1: Japanese Patent No. 6477020 Patent Document 2: Japanese Patent No. 6701954 SUMMARY OF INVENTIONTechnical Problem

[0009] However, Patent Documents 1 and 2 do not consider bending properties under tension in the rolling direction.

[0010] 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 bending properties under tension in the rolling direction. Solution to Problem

[0011] The present inventors have found that bendability of a hot-rolled steel sheet can be further improved and bending properties under tension can be improved by controlling a maximum value A within a range of Φ = 0 to 60° and φ 1 = 50 to 90° in a cross section of φ 2 = 45° and a maximum value B within a range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° in a crystal orientation distribution function of a texture in a surface layer region (a region from a surface to a depth of 500 µm in a sheet thickness direction) of the hot-rolled steel sheet and setting peak positions of Φ at which the maximum values are located within a desired range.

[0012] In addition, the present inventors have found that it is effective to control rough rolling conditions and finish rolling conditions of hot rolling in order to preferably control the texture in the surface layer region of the hot-rolled steel sheet.

[0013] 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 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 in a crystal orientation distribution function of a texture in a region at 1 / 4 position from an end surface in a width direction and from a surface to a depth of 500 µm in a sheet thickness direction, a maximum value A within a range of Φ = 0 to 60° and φ 1 = 50 to 90° in a cross section of φ 2 = 45° is 6.0 or less, a maximum value B within a range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is 6.0 or less, when a peak position of Φ where the maximum value A is located is Φ A and a peak position of Φ where the maximum value B is located is Φ B , |Φ A - 35°| is 10° or less, and |Φ B - 145°| is 10° or less, 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%. (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) The hot-rolled steel sheet according to (1) or (2), in which in the crystal orientation distribution function of the texture in a region from the surface to a depth of 500 µm in the sheet thickness direction, an absolute value of a difference between the maximum value A and the maximum value B is 3.0 or less at each of 1 / 4 position from the end surface in the width direction, 1 / 4 - 15 mm position from the end surface in the width direction, and 1 / 4 + 15 mm position from the end surface in the width direction. (4) The hot-rolled steel sheet according to any one of (1) to (3), in which in the 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° and less than 2.0° is 50% or more. (5) The hot-rolled steel sheet according to any one of (1) to (3), in which in the 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 2.0° or more is 50% or more. Advantageous Effects of Invention

[0014] According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bending properties under tension in the rolling direction.

[0015] Also, according to a preferred aspect of the present invention, it is possible to provide a hot-rolled steel sheet having the above various properties and further having excellent bending properties under tension also in the width direction.BRIEF DESCRIPTION OF DRAWINGS

[0016] [FIG. 1] A view for explaining a test method of a tension bending test.DESCRIPTION OF EMBODIMENTS

[0017] 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.

[0018] 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%".

[0019] 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.

[0020] Hereinafter, each element will be described in detail.C: 0.045 to 0.120%

[0021] 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.

[0022] 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%

[0023] 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.

[0024] 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%

[0025] 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.

[0026] 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%

[0027] 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.

[0028] 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%

[0029] 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.

[0030] 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

[0031] 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.

[0032] 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

[0033] 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.

[0034] 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

[0035] 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.

[0036] 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

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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%.

[0041] Hereinafter, each optional element will be described.Nb: 0.001 to 0.100%

[0042] 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.

[0043] 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%

[0044] 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.

[0045] 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%

[0046] 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.

[0047] 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%

[0048] 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.

[0049] 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%

[0050] 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.

[0051] 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%

[0052] 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.

[0053] 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%

[0054] 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.

[0055] 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%

[0056] 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.

[0057] 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%

[0058] 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.

[0059] 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%

[0060] 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.

[0061] 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.

[0062] 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%

[0063] 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.

[0064] 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%

[0065] 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.

[0066] 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%

[0067] 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.

[0068] 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%

[0069] 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.

[0070] 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%

[0071] 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.

[0072] 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%

[0073] 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.

[0074] 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%

[0075] 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.

[0076] 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%

[0077] 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.

[0078] 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%

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] Next, a texture of the hot-rolled steel sheet according to the present embodiment will be described.

[0084] In the hot-rolled steel sheet according to the present embodiment, in a crystal orientation distribution function of a texture in a region from a surface to a depth of 500 µm in a sheet thickness direction, a maximum value A within a range of Φ = 0 to 60° and φ 1 = 50 to 90° in a cross section of φ 2 = 45° is 6.0 or less, a maximum value B within a range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is 6.0 or less, and when a peak position of Φ where the maximum value A is located is Φ A and a peak position of Φ where the maximum value B is located is Φ B , |Φ A - 35°| is 10° or less, and |Φ B - 145°| is 10° or less.

[0085] In the present embodiment, a texture in a region at 1 / 4 position from an end surface in a width direction and from a surface to a depth of 500 µm in a sheet thickness 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] Hereinafter, each definition will be described.Maximum value A

[0090] In the crystal orientation distribution function of the texture in the region from the surface to a depth of 500 µm in the sheet thickness direction (hereinafter, it may be referred to as the surface layer region), when the maximum value A of the pole density within a range of Φ = 0 to 60° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° exceeds 6.0, excellent bending properties under tension cannot be obtained in the rolling direction. Therefore, the maximum value A in the range of Φ = 0 to 60° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is set to 6.0 or less. The maximum value A within the range of Φ = 0 to 60° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is preferably 5.0 or less and more preferably 4.0 or less. Here, φ 2 , Φ, and φ 1 in the crystal orientation distribution function are, for example, rotation angles in each of Bunge-Euler notation described in FIG. 4 of Light Metals 60 (2010), 12 p 666-675.

[0091] When the maximum value A within the range of Φ = 0 to 60° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is less than 1.0, the pole density in other orientations increases, and the bending properties under tension in the rolling direction may deteriorate. Therefore, the maximum value A within the range of Φ = 0 to 60° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° may be 1.0 or more.

[0092] When the peak position of Φ at which the maximum value A within the range of Φ = 0 to 60° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is located is Φ A , when |Φ A - 35°| exceeds 10°, excellent bending properties under tension cannot be obtained in the rolling direction. Therefore, |Φ A - 35°| is set to 10° or less. |Φ A - 35°| is preferably 5° or less.Maximum value B

[0093] In the crystal orientation distribution function of the texture in the surface layer region, when the maximum value B within the range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° exceeds 6.0, excellent bending properties under tension cannot be obtained in the rolling direction. Therefore, the maximum value B within the range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is 6.0 or less. The maximum value B within the range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is preferably 5.0 or less and more preferably 4.0 or less.

[0094] When the maximum value B within the range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is less than 1.0, the pole density in other orientations increases, and the bending properties under tension in the rolling direction may deteriorate. Therefore, the maximum value B within the range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° may be 1.0 or more.

[0095] When the peak position of Φ at which the maximum value B within the range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45° is located is Φ B , when |Φ B - 145°| exceeds 10°, excellent bending properties under tension cannot be obtained in the rolling direction. Therefore, |Φ B - 145°| is set to 10° or less. |Φ B - 145°| is preferably 5° or less.

[0096] The maximum value A and the maximum value B and the peak positions of Φ at which these maximum values are located are measured by the following method.

[0097] 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 from the surface to a depth of 500 µm in the sheet thickness direction and a region of 2000 µm or more at an arbitrary position in the rolling direction of the polished sample are measured at a measurement interval of 5.0 µm.

[0098] For the measurement, an apparatus combining a scanning electron microscope and an EBSD analyzer and OIM Analysis (registered trademark) manufactured by TSL Solutions are used. The sample is analyzed by an electron back scattering diffraction (EBSD) method. A crystal orientation distribution function (ODF) is calculated from the obtained orientation data.

[0099] From the obtained crystal orientation distribution function, the maximum value A within the range of Φ = 0 to 60° and φ 1 = 50 to 90° in the cross section of φ 2 = 45°, the maximum value B within the range of Φ = 120 to 180° and φ 1 = 50 to 90° in the cross section of φ 2 = 45°, and the peak positions of Φ at which these maximum values are located are obtained.

[0100] The rolling direction of the hot-rolled steel sheet is determined by the following method.

[0101] 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.Difference between maximum value A and maximum value B

[0102] In the crystal orientation distribution function of the texture in the surface layer region, by setting an absolute value of a difference between the maximum value A and the maximum value B to 3.0 or less at each of the 1 / 4 position from the end surface in the width direction, 1 / 4 - 15 mm position from the end surface in the width direction, and 1 / 4 + 15 mm position from the end surface in the width direction, excellent bending properties under tension can be obtained not only in the rolling direction but also in the width direction. Therefore, it is preferable to set all of the absolute value of the difference between the maximum value A and the maximum value B at the 1 / 4 position from the end surface in the width direction, the absolute value of the difference between the maximum value A and the maximum value B at the 1 / 4 - 15 mm position from the end surface in the width direction, and the absolute value of the difference between the maximum value A and the maximum value B at the 1 / 4 + 15 mm position from the end surface in the width direction to 3.0 or less.

[0103] Here, the 1 / 4 + 15 mm position from the end surface in the width direction is a position advanced by 15 mm in a direction opposite to the end surface from the "w / 4 position from the end surface in the width direction" when the length in the width direction is w. Also, the 1 / 4 - 15 mm position from the end surface in the width direction is a position advanced by 15 mm in a direction of the end surface from the "w / 4 position from the end surface in the width direction" when the length in the width direction is w.

[0104] The absolute value of the difference between the maximum value A and the maximum value B at each position is obtained by performing EBSD analysis by the above-described method and calculating a crystal orientation distribution function at each of the 1 / 4 position from the end surface in the width direction, the 1 / 4 - 15 mm position from the end surface in the width direction, and the 1 / 4 + 15 mm position from the end surface in the width direction.

[0105] Next, a microstructure of the hot-rolled steel sheet according to the present embodiment will be described.

[0106] In the hot-rolled steel sheet according to the present embodiment, 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%.

[0107] In the present embodiment, a microstructure at the 1 / 4 position from the end surface in the width direction and at the position of 1 / 4 depth from the surface in the sheet thickness direction is defined.Area ratio of region having GAM value of more than 0.6°: 50% or more

[0108] 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.

[0109] 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%

[0110] 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 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.

[0111] 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.

[0112] Here, desired strength, ductility, and degree of bending properties under tension vary depending on the applied vehicle component. The hot-rolled steel sheet according to the present embodiment may have the above-described chemical composition, texture, and microstructure, and then may have either microstructure of a first aspect or a second aspect described below depending on the desired strength, ductility, and degree of bending properties under tension.(First aspect) Area ratio of region having GAM value of more than 0.6° and less than 2.0°: 50% or more

[0113] 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 60% or more and more preferably 70% or more.

[0114] The area ratio of a region having a GAM value of more than 0.6° and less than 2.0° may be 100%.

[0115] 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

[0116] 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 60% or more and more preferably 70% or more.

[0117] The area ratio of a region having a GAM value of 2.0° or more may be 100%.

[0118] 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.

[0119] 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.

[0120] The "GAM value" of each microstructure of the hot-rolled steel sheet is measured by an electron backscatter pattern (EBSP) method.

[0121] 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 around a 1 / 4 depth position from the surface in the sheet thickness direction of the polished sample 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) 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.

[0122] A region where the crystal structure is fcc and a region where the crystal structure is bcc are specified from the obtained crystal orientation information, using a "Phase Map" function installed on software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. In the region where the crystal structure is bcc, a region surrounded by grain boundaries having an orientation difference of 15° or more is regarded as one grain, and the average value of orientation differences between adjacent pixels in the grain is calculated to calculate the GAM value of the grain. 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 grains having an obtained GAM value of more than 3.0° are calculated to obtain the area ratio of each region.

[0123] 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.

[0124] The area ratio of residual austenite is measured by the following method.

[0125] 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 γ(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 propertiesTensile strength (TS): 940 MPa or more

[0126] 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

[0127] 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.

[0128] 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.

[0129] 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 (λ): 40% or more

[0130] 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.

[0131] The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.Bending properties under tension

[0132] The bending properties under tension can be evaluated by performing a tension bending test by the method shown in FIG. 1. In the present embodiment, when the length of the steel sheet before the test is L 0 and the length of the steel sheet at fracture is L MAX , L MAX / L 0 when the tension bending test has been performed in the rolling direction or the width direction is used as an index of the bending properties under tension.

[0133] In the tension bending test in the rolling direction, in FIG. 1, the rolling direction is arranged in the direction of L 0 , a punch is pushed down, and depression amount h when the steel sheet fractures is measured. Here, the depression amount h is a stroke amount (mm) of the punch from a position at which the punch comes into contact with the steel sheet until the steel sheet fractures. L MAX is obtained as twice the length L in FIG. 1 (L MAX = 2L). The length L is obtained by an equation "L = h / cosθ", and θ, which is an angle formed between the depressing direction of the punch and the sheet surface of the steel sheet at fracture, is obtained by an equation "θ = arctan(h / (L 0 / 2))". As to whether or not the steel sheet has fractured, it is determined that the steel sheet has fractured when the pressing load of the punch decreases by 20% or more within 1 second. The tension bending test in the width direction is similar to the tension bending test in the rolling direction except that the width direction is arranged in the L 0 direction.

[0134] In the case where the tensile strength is less than 1040 MPa, L MAX / L 0 when the tension bending test has been performed in the rolling direction may be 1.028 or more. In the case where the tensile strength is less than 1040 MPa, if L MAX / L 0 when the tension bending test has been performed in the rolling direction is 1.028 or more, it can be determined that the steel sheet has excellent bending properties under tension in the rolling direction.

[0135] Also, in the case where the tensile strength is less than 1040 MPa, if L MAX / L 0 when a tension bending test has been performed in the width direction is 1.028 or more, it can be determined that the steel sheet has excellent bending properties under tension also in the width direction.

[0136] In the case where the tensile strength is 1040 MPa or more, L MAX / L 0 when the tension bending test has been performed in the rolling direction may be 1.018 or more. In the case where the tensile strength is 1040 MPa or more, if L MAX / L 0 when the tension bending test has been performed in the rolling direction is 1.018 or more, it can be determined that the steel sheet has excellent bending properties under tension in the rolling direction.

[0137] Also, in the case where the tensile strength is 1040 MPa or more, if L MAX / L 0 when the tension bending test has been performed in the width direction is 1.018 or more, it can be determined that the steel sheet has excellent bending properties under tension also in the width direction.

[0138] The tension bending test shown in FIG. 1 is performed under the following conditions. The pressurizing force by a blank holder may be set to such a degree that the hot-rolled steel sheet does not move. Initial sheet thickness t 0 of the hot-rolled steel sheet before the test is set to 1.5 mm. When the sheet thickness of the hot-rolled steel sheet is larger than 1.5 mm, mechanical grinding is performed from one surface of the hot-rolled steel sheet to set the sheet thickness to 1.5 mm, and then the test is performed with the mechanically ground surface on the punch side.

[0139] In addition, when the sheet thickness of the hot-rolled steel sheet is less than 1.5 mm, the test is performed without performing mechanical grinding. However, when the sheet thickness of the hot-rolled steel sheet is less than 1.5 mm, the bending properties under tension are evaluated by an index of L MAX / L 0 - (1.5 - t 0 ) × 0.0242 instead of L MAX / L 0 .

[0140] L 0 : Steel sheet length before test Punch speed P: 3 mm / min Inter-die span Dd: 100 mm Punch tip radius Rp: 20 mm Die tip radius Rd: 20 mm

[0141] Further, in the first and second aspects described above, since the desired strength, ductility, and bending properties under tension are different, the steel sheet may have the following strength, ductility, and bending properties under tension in each aspect. 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

[0142] 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 less. Also, in the first aspect, the uniform elongation may be 5.0% or less.(First aspect) Bending properties under tension

[0143] In the first aspect, L MAX / L 0 when the tension bending test has been performed in the rolling direction may be 1.028 or more.

[0144] Also, in the first aspect, L MAX / L 0 when the tension bending test has been performed in the width direction may be 1.028 or more.(Second aspect) Tensile strength: 1040 MPa or more, uniform elongation: 3.0% or more

[0145] 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 1080 MPa or less. In the second aspect, the uniform elongation may be 4.0% or less.(Second aspect) Bending properties under tension

[0146] In the second aspect, L MAX / L 0 when the tension bending test has been performed in the rolling direction may be 1.018 or more.

[0147] Also, in the second aspect, L MAX / L 0 when the tension bending test has been performed in the width direction may be 1.018 or more.

[0148] 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.

[0149] 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.

[0150] 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.

[0151] A preferred method for manufacturing a hot-rolled steel sheet according to the present embodiment includes the steps of: (1) applying strain once or multiple times to a slab having the above-described chemical composition before rough rolling such that the strain in the width direction is 3 to 15% in total; (2) performing rough rolling on the slab to which the strain has been applied; and (3) performing finish rolling such that a difference in inlet side temperature between a path immediately preceding a final path and the final path is in a temperature range of 30°C or higher and a finishing temperature is in a temperature range of 920°C or higher, and further includes one or more steps of the following (4) to (6). (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) After completion of slow cooling, accelerated cooling to 300°C at an average cooling rate of 30°C / s or faster. (6) After completion of finish rolling, accelerated cooling to 300°C at an average cooling rate of 30°C / s or faster.

[0152] Hereinafter, each step will be described.(1) Strain application before rough rolling: common in first and second aspects

[0153] Before rough rolling, strain is applied once or multiple times to the slab having the above-described chemical composition such that the strain in the width direction is 3 to 15% in total. This makes it possible to improve uniformity of the texture while reducing unevenness of the surface layer of the slab. The strain may be applied after slab heating for rough rolling is performed.

[0154] When the strain applied in the width direction of the slab is less than 3% or more than 15% in total, the peak positions of the maximum value A and the maximum value B in the texture of the hot-rolled steel sheet may not 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.

[0155] In addition, by applying strain not only once but also multiple times in the width direction of the slab, it is possible to suppress an increase in the change in the texture in the width direction of the hot-rolled steel sheet. As a result, it is possible to reduce the absolute value of the difference between the maximum value A and the maximum value B at each of the 1 / 4 position from the end surface in the width direction, the 1 / 4 - 15 mm position from the end surface in the width direction, and the 1 / 4 + 15 mm position from the end surface in the width direction of the hot-rolled steel sheet.

[0156] The total strain applied in the width direction of the slab can be expressed by (1 - w 1 / w 0 ) × 100 (%), where w 0 is the length in the width direction of the slab before the first strain application, and w 1 is the length in the width direction of the slab after the last strain application.

[0157] 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.

[0158] 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

[0159] 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

[0160] In the finish rolling step, finish rolling is performed so that the difference in inlet side temperature between the path immediately preceding the final path and the final path is in a temperature range of 30°C or higher and the finishing temperature is in a temperature range of 920°C or higher. When the difference in inlet side temperature between the path immediately preceding the final path and the final path is lower than 30°C, the peak positions of the maximum value A and the maximum value B in the texture of the hot-rolled steel sheet may not be controlled within a preferable range. Also, when the finishing temperature is lower than 920°C, the maximum value A and the maximum value B cannot be controlled to preferable values.

[0161] Examples of a method of setting the difference in inlet side temperature between the path immediately preceding the final path and the final path to 30°C or higher include control by controlling the injection amount of a coolant such as water from a cooling device such as a cooling spray immediately after rolling, controlling the conveyance speed of the steel sheet during rolling, and the like.

[0162] The path immediately preceding the final path is a path one stage before the final path. For example, in a case where the finish rolling is performed in paths of F1, F2,..., F6, and F7, the path immediately preceding the final path refers to the path of F6.

[0163] Also, the finishing temperature is an outlet side temperature of the final path of the finish rolling.(4) Slow cooling (air cooling) in temperature range of 580°C to 680°C: corresponding to first aspect

[0164] 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.

[0165] 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 completion of slow cooling (air cooling): corresponding to first aspect

[0166] After completion of 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 completion of 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.

[0167] 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

[0168] 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 2.0° or more can be increased.

[0169] 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.

[0170] 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

[0171] 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.

[0172] 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.

[0173] Blanks in Tables 1A to 2B indicate that the element is not intentionally contained.

[0174] In Tables 3A to 3C, when "Difference in inlet side temperature between path immediately preceding final path and final path of 30°C or higher", "OK" was written in the column of the condition, and when the condition was not satisfied, "NG" was written.

[0175] For manufacture Nos. 1 to 20 and 29 to 49, 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.

[0176] For manufacture Nos. 21 to 28 and 50 to 67, after 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.

[0177] For the obtained hot-rolled steel sheet, the texture, the microstructure, the tensile strength (TS), the uniform elongation (uEl), the hole expansion ratio (λ), and the bending properties under tension (L MAX / L 0 in the rolling direction (L direction) and L MAX / L 0 in the width direction (C direction)) were evaluated by the above-described methods.

[0178] The obtained results are shown in Tables 4A to 5C. However, for an example in which the sheet thickness of the hot-rolled steel sheet was less than 1.5 mm, the value of L MAX / L 0 - (1.5 - t 0 ) × 0.0242 was written instead of L MAX / L 0 .

[0179] Also, when "in the crystal orientation distribution function of the texture in the region from the surface to a depth of 500 µm in the sheet thickness direction, the absolute value of the difference between the maximum value A and the maximum value B at the 1 / 4 position from the end surface in the width direction, the 1 / 4 - 15 mm position from the end surface in the width direction, and the 1 / 4 + 15 mm position from the end surface in the width direction is 3.0 or less", "OK" was written in the column of "Absolute value of difference between maximum value A and maximum value B at each of three positions in width direction of 3.0 or less" in the table. On the other hand, when the absolute value was more than 3.0, "NG" was written in the column.

[0180] 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.

[0181] 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.

[0182] 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.

[0183] The bending properties under tension were evaluated according to the following criteria depending on the tensile strength.· In case where tensile strength is less than 1040 MPa

[0184] Bending properties under tension (L MAX / L 0 in rolling direction (L direction)): acceptable if 1.028 or more, unacceptable if less than 1.028.

[0185] In addition, a case where L MAX / L 0 in the width direction (C direction) was 1.028 or more was determined as having excellent bending properties under tension also in the width direction. · In case where tensile strength is 1040 MPa or more bending properties under tension (L MAX / L 0 in rolling direction (L direction)): acceptable if 1.018 or more, unacceptable if less than 1.018.

[0186] A case where L MAX / L 0 in the width direction (C direction) was 1.018 or more was determined as having excellent bending properties under tension also in the width direction. [Table 1A]SteelChemical (mass%), Fe and impuritiesCSiMnTiAlPSNONbVCuCrMoA0.0250.912.000.1120.1910.0080.00370.00240.0020.007B0.1220.881.310.1270.1070.0080.00360.00340.0010.012C0.0713.091.710.0950.0820.0090.00450.00350.0030.013D0.0590.592.800.1050.0980.0080.00430.00300.0030.011E0.0860.981.080.0960.0690.0090.00390.00240.0020.0140.099F0.0790.861.690.2050.1190.0080.00400.00170.0010.011G0.0660.911.980.0020.1070.0070.00380.00240.0030.011H0.0620.822.180.1020.4130.0070.00430.00230.0020.007I0.0721.022.130.0930.0050.0080.00430.00300.0020.011J0.0691.022.040.1340.1560.0890.00380.00260.0020.007K0.0680.831.940.1240.0570.0090.01190.00160.0020.0120.182L0.0770.992.080.0980.1300.0070.00460.00980.0010.0150.195M0.0710.832.050.1080.2040.0080.00420.00250.0120.113N0.0750.852.000.0880.1660.0080.00410.00190.001O0.0720.772.220.1160.0690.0070.00420.00290.0030.0090.0130.100P0.0720.962.000.0870.1660.0070.00370.00360.0020.0070.009Q0.1110.891.280.1170.1820.0090.00380.00250.0010.008R0.0480.592.580.0890.0550.0080.00390.00280.0020.0100.192S0.0701.571.860.1070.0740.0090.00440.00210.0020.0120.130T0.0900.012.230.0910.1630.0070.00380.00320.0020.005U0.0640.922.110.1650.0550.0090.00460.00190.0030.0110.100V0.0641.022.090.1010.1420.0080.00450.00250.0010.0320.123W0.0751.032.080.1220.1990.0080.00400.00280.0020.0080.2940.100X0.0690.882.120.1270.3450.0070.00370.00180.0030.010Y0.0780.821.940.1090.0200.0090.00410.00320.0020.0100.100Z0.0770.831.610.1130.1090.0080.00410.00180.0030.010The underline represents that it is outside of the range of the present invention. [Table 1B] SteelChemical composition (mass%), remainder being Fe and impuritiesCSiMnTiAlPSNONbVCuCrMoAA0.0680.742.140.0440.0460.0060.00220.00210.0020.0040.015AB0.0740.902.010.0900.1500.0090.00300.00300.001AC0.0720.772.220.1160.0690.0070.00420.00290.0030.0090.106AD0.0720.962.000.0870.1660.0070.00370.00360.0020.0070.008AE0.1110.891.280.1170.1820.0090.00380.00250.0010.008AF0.0480.592.580.0890.0550.0080.00390.00280.0020.0100.210AG0.0701.571.860.1070.0740.0090.00440.00210.0020.0120.120AH0.0900.012.230.0910.1630.0070.00380.00320.0020.005AI0.0640.922.110.1650.0550.0090.00460.00190.0030.011AJ0.0641.022.090.1010.1420.0080.00450.00250.0010.0320.123AK0.0751.032.080.1220.1990.0080.00400.00280.0020.0080.294AL0.0690.882.120.1270.3450.0070.00370.00180.0030.010AM0.0780.821.940.1090.0200.0090.00410.00320.0020.010AN0.0770.831.610.1130.1090.0080.00410.00180.0030.010 [Table 2A] SteelChemical composition (mass%), remainder being Fe and impuritiesNiBCaMgREMBiTaZrCoZnWSbAsSnABC0.00310.0020D0.002E0.011F0.162G0.0037H0.003I0.170J0.344K0.192L0.0013M0.020NO0.023P0.059Q0.296R0.041ST0.0510.005U0.00290.118VW0.0030.152X0.00210.182Y0.092Z0.0010 The underline represents that it is outside of the range of the present invention. [Table 2B] SteelChemical composition (mass%), remainder being Fe and impuritiesNiBCaMgREMBiTaZrCoZnWSbAsSnAA0.014AB0.0020AC0.00210.018AD0.00230.061AE0.00180.293AF0.00190.044AG0.0019AH0.0530.00210.004AI0.00220.00280.123AJ0.0022AK0.00210.0040.155AL0.00230.00250.185AM0.00190.088AN0.0018 [Table 5A] Manufacture No.SteelTensile strength TSUniform elongation uELHole expansion ratio λL MAX / L 0 (L direction bending)L MAX / L 0 (C direction bending)NoteMPa%%--1A74810.81041.0731.066Comparative Example2B12384.3371.0191.018Comparative Example3CSlab crackingComparative Example4D11237.8371.0461.042Comparative Example5E8458.0711.0481.046Comparative Example6F9686.4341.0341.028Comparative Example7G9227.0411.0391.028Comparative Example8HSlab crackingComparative Example9I10336.0361.0311.028Comparative Example10JSlab crackingComparative Example11K9816.6351.0361.032Comparative Example12LSlab crackingComparative Example13M9826.8351.0371.030Comparative Example14N9866.1701.0261.018Comparative Example15AA9736.1421.0261.020Comparative Example16AA9636.1781.0311.018Present Invention Example17AA9815.6781.0261.018Comparative Example18P9925.9411.0261.018Comparative Example19N9975.9401.0261.024Comparative Example20AA9814.8401.0291.028Present Invention Example21N10615.4701.0151.007Comparative Example22AA10825.3411.0141.007Comparative Example23AA10495.1811.0211.015Present Invention Example24AA10975.2761.0161.015Comparative Example25P10984.7451.0140.999Comparative 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.SteelTensile strength TSUniform elongation uELHole expansion ratio λL MAX / L 0 (L direction bending)L MAX / L 0 (C direction bending)NoteMPa%%--26N10964.7401.0161.017Comparative Example27AA10675.3401.0191.018Present Invention Example28P10555.7591.0261.023Present Invention Example29N10066.2581.0291.030Present Invention Example30N10015.5401.0301.033Present Invention Example31N10026.0761.0291.030Present Invention Example32N9806.2581.0331.033Present Invention Example33N9985.1531.0291.028Present Invention Example34N9816.0411.0341.034Present Invention Example35N9606.9701.0381.038Present Invention Example36N9936.3611.0301.030Present Invention Example37O10065.6531.0291.029Present Invention Example38P10025.9761.0281.028Present Invention Example39Q9815.7541.0291.029Present Invention Example40R9826.2701.0301.030Present Invention Example41S10226.0541.0301.031Present Invention Example42T9945.8601.0351.035Present Invention Example43U9676.7681.0341.034Present Invention Example44V9786.8411.0301.030Present Invention Example45W10335.2511.0301.029Present Invention Example46X9617.0731.0321.032Present Invention Example47Y10036.3661.0291.029Present Invention Example48Z9616.2631.0351.035Present Invention Example49AA9816.0481.0341.034Present Invention Example50AB11734.2421.0191.019Present Invention Example The underline represents that it is outside of the range of the present invention or the property is not preferable. [Table 5C] Manufacture No.SteelTensile strength TSUniform elongation uELHole expansion ratio λL MAX / L 0 (L direction bending)L MAX / L 0 (C direction bending)NoteMPa%%--51AB11514.3401.0191.020Present Invention Example52AB11754.0451.0191.019Present Invention Example53AB11324.4541.0211.020Present Invention Example54AB11793.8701.0221.018Present Invention Example55AB11724.4651.0281.021Present Invention Example56AC11943.8551.0241.018Present Invention Example57AD11684.2541.0231.020Present Invention Example58AE12903.5651.0221.020Present Invention Example59AF10435.5661.0271.026Present Invention Example60AG12113.5521.0211.018Present Invention Example61AH12003.8661.0211.019Present Invention Example62AI11154.3651.0261.024Present Invention Example63AJ11254.9411.0251.020Present Invention Example64AK11924.2511.0191.018Present Invention Example65AL11524.4461.0201.018Present Invention Example66AM11663.9481.0221.018Present Invention Example67AN11125.1521.0271.022Present Invention Example

[0187] 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 bending properties under tension in the rolling direction.

[0188] 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

[0189] According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bending properties under tension in the rolling direction.

[0190] Also, according to a preferred aspect of the present invention, it is possible to provide a hot-rolled steel sheet having the above various properties and further having excellent bending properties under tension also in the width direction.

Claims

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 in a crystal orientation distribution function of a texture in a region at 1 / 4 position from an end surface in a width direction and from a surface to a depth of 500 µm in a sheet thickness direction, a maximum value A within a range of Φ = 0 to 60° and φ1 = 50 to 90° in a cross section of φ2 = 45° is 6.0 or less, a maximum value B within a range of Φ = 120 to 180° and φ1 = 50 to 90° in the cross section of φ2 = 45° is 6.0 or less, when a peak position of Φ where the maximum value A is located is ΦA and a peak position of Φ where the maximum value B is located is ΦB, |ΦA - 35°| is 10° or less, and |ΦB - 145°| is 10° or less, 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%.

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, wherein in the crystal orientation distribution function of the texture in a region from the surface to a depth of 500 µm in the sheet thickness direction, an absolute value of a difference between the maximum value A and the maximum value B is 3.0 or less at each of 1 / 4 position from the end surface in the width direction, 1 / 4 - 15 mm position from the end surface in the width direction, and 1 / 4 + 15 mm position from the end surface in the width direction.

4. The hot-rolled steel sheet according to claim 2, wherein in the crystal orientation distribution function of the texture in a region from the surface to a depth of 500 µm in the sheet thickness direction, an absolute value of a difference between the maximum value A and the maximum value B is 3.0 or less at each of 1 / 4 position from the end surface in the width direction, 1 / 4 - 15 mm position from the end surface in the width direction, and 1 / 4 + 15 mm position from the end surface in the width direction.

5. The hot-rolled steel sheet according to any one of claims 1 to 4, wherein in the 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° and less than 2.0° is 50% or more.

6. The hot-rolled steel sheet according to any one of claims 1 to 4, wherein in the 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 2.0° or more is 50% or more.

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