Hot-rolled steel sheet

Abstract

Disclosed is a hot-rolled steel sheet having an excellent balance of strength, elongation, and hole expandability, and also having excellent impact characteristics. This hot-rolled steel sheet has a prescribed chemical composition, the prior austenite particle size being 25 [mu] m or less, the area ratio of a region having a GAM value of more than 0.6 DEG but less than 2.0 DEG being 50% or more but less than 100%, the area ratio of a region having a GAM value of 0.6 DEG or less being 0% or more but less than 50%, the area ratio of a region having a GAM value of 2.0 DEG or more being more than 0% but less than 50%, and satisfying the relationships 1.7 < = LGr / LGt and 1.20 < = (LGr / LGt) / (LMr / LMt). Here, LGr is the average area of the projected length of the original austenite crystal grains in the rolling direction, LGt is the average area of the projected length of the original austenite crystal grains in the plate thickness direction, LMr is the average area of the projected length of a region having a GAM value of 2.0 DEG or more in the rolling direction, and LMt is the average area of the projected length of a region having a GAM value of 2.0 DEG or more in the plate thickness direction.

Description

Technical Field

[0001] The invention discloses a hot-rolled steel plate. Background Art

[0002] Hot-rolled steel sheets are used as raw materials for automobile chassis, structural parts, frames, and frame parts. For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet having a predetermined composition and a predetermined steel structure and excellent punchability.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2012-062562 Summary of the Invention

[0006] Problems to be solved by the invention

[0007] Conventional hot-rolled steel sheets have room for improvement in terms of the balance between strength, elongation, and hole expandability, as well as in terms of collision characteristics.

[0008] Means for solving problems

[0009] The present inventors have conducted intensive research and have found that by (1) using a slab having an appropriate chemical composition, (2) designing the finishing conditions during hot rolling to control the morphology of prior austenite grains, and (3) controlling the cooling rate during cooling to control the transformation behavior (for example, by controlling the cooling conditions in the run-out table), it is possible to produce a hot-rolled steel sheet having an excellent balance of strength, ductility, and hole expandability, as well as excellent collision characteristics. Furthermore, the present inventors have found that the hot-rolled steel sheet produced in this manner has a predetermined chemical composition and a characteristic steel structure, thereby having an excellent balance of strength, ductility, and hole expandability, as well as excellent collision characteristics.

[0010] Based on the above findings, the present application discloses the following various aspects as means for solving the above-mentioned problems.

[0011] <Method 1>

[0012] A hot-rolled steel plate, wherein the hot-rolled steel plate comprises, by mass%,

[0013] C: 0.045% to 0.120%,

[0014] Si: 0% to 3.00%,

[0015] Mn: 1.20% to 2.60%,

[0016] Ti: 0.020% to 0.180%,

[0017] Al: 0.010% to 0.400%,

[0018] P: 0%~0.080%,

[0019] S: 0% to 0.0100%,

[0020] N: 0% to 0.0050%,

[0021] O: 0% to 0.010%,

[0022] Nb: 0% to 0.100%,

[0023] V: 0%~1.000%,

[0024] Cu: 0% to 1.000%,

[0025] Cr: 0% to 2.000%,

[0026] Mo: 0% to 3.000%,

[0027] Ni: 0% to 0.500%,

[0028] B: 0% to 0.0100%,

[0029] Ca: 0% to 0.0500%,

[0030] Mg: 0% to 0.050%,

[0031] REM: 0%~0.100%,

[0032] Bi: 0% to 0.100%,

[0033] Ta: 0%~0.100%,

[0034] Zr: 0% to 0.500%,

[0035] Co: 0% to 3.000%,

[0036] Zn: 0% to 0.200%,

[0037] W: 0% to 0.200%,

[0038] Sb: 0% to 0.500%,

[0039] As: 0% to 0.050%, and

[0040] Sn: 0%~0.050%,

[0041] The remainder consists of Fe and impurities;

[0042] The original austenite grain size is less than 25μm,

[0043] The area ratio of the region where the GAM value exceeds 0.6° and is less than 2.0° is 50% or more and less than 100%.

[0044] The area ratio of the region with a GAM value of 0.6° or less is 0% or more and less than 50%,

[0045] The area ratio of regions with a GAM value of 2.0° or more exceeds 0% and is 50% or less.

[0046] The following relations (1) and (2) are satisfied:

[0047] 1.7≤LGr / LGt (1)

[0048] 1.20≤(LGr / LGt) / (LMr / LMt) (2)

[0049] LGr: Area average of the projected length of the original austenite grains in the rolling direction

[0050] LGt: Area average of the projected length of the prior austenite grains in the plate thickness direction

[0051] LMr: Area average of the projected length in the rolling direction of the region having a GAM value of 2.0° or more LMt: Area average of the projected length in the plate thickness direction of the region having a GAM value of 2.0° or more.

[0052] <Method 2>

[0053] According to the hot-rolled steel sheet of the above-mentioned embodiment 1, the area ratio of the region having a GAM value of 0.6° or less is 0% to 45%.

[0054] <Method 3>

[0055] The hot-rolled steel sheet according to the above-mentioned aspect 1 or 2, wherein the area ratio of the region having a GAM value of 2.0° or greater is more than 0% and 20% or less.

[0056] <Method 4>

[0057] The hot-rolled steel sheet according to any one of aspects 1 to 3 above, wherein the following relationship (1-1) is satisfied:

[0058] 1.7≤LGr / LGt≤10.0(1-1).

[0059] <Method 5>

[0060] The hot-rolled steel sheet according to any one of aspects 1 to 4, wherein the following relationship (2-1) is satisfied:

[0061] 1.20≤(LGr / LGt) / (LMr / LMt)≤5.00(2-1).

[0062] Effects of the Invention

[0063] The hot-rolled steel sheet disclosed herein has an excellent balance among strength, elongation, and hole expandability, and also has excellent collision characteristics. DETAILED DESCRIPTION

[0064] 1. Hot rolled steel plate

[0065] An embodiment of a hot-rolled steel sheet will be described below, but the hot-rolled steel sheet disclosed herein is not limited to the following embodiment.

[0066] The present disclosure relates to a hot-rolled steel plate comprising, by mass%,

[0067] C: 0.045% to 0.120%,

[0068] Si: 0% to 3.00%,

[0069] Mn: 1.20% to 2.60%,

[0070] Ti: 0.020% to 0.180%,

[0071] Al: 0.010% to 0.400%,

[0072] P: 0%~0.080%,

[0073] S: 0% to 0.0100%,

[0074] N: 0% to 0.0050%,

[0075] O: 0% to 0.010%,

[0076] Nb: 0% to 0.100%,

[0077] V: 0%~1.000%,

[0078] Cu: 0% to 1.000%,

[0079] Cr: 0% to 2.000%,

[0080] Mo: 0% to 3.000%,

[0081] Ni: 0% to 0.500%,

[0082] B: 0% to 0.0100%,

[0083] Ca: 0% to 0.0500%,

[0084] Mg: 0% to 0.050%,

[0085] REM: 0%~0.100%,

[0086] Bi: 0% to 0.100%,

[0087] Ta: 0%~0.100%,

[0088] Zr: 0% to 0.500%,

[0089] Co: 0% to 3.000%,

[0090] Zn: 0% to 0.200%,

[0091] W: 0% to 0.200%,

[0092] Sb: 0% to 0.500%,

[0093] As: 0% to 0.050%, and

[0094] Sn: 0%~0.050%,

[0095] The remainder consists of Fe and impurities.

[0096] In the hot-rolled steel sheet of the present disclosure, the prior austenite grain size is 25 μm or less.

[0097] In the hot-rolled steel sheet disclosed herein, the area ratio of the region with a GAM value exceeding 0.6° and less than 2.0° is 50% or more and less than 100%, the area ratio of the region with a GAM value of 0.6° or less is 0% or more and less than 50%, and the area ratio of the region with a GAM value of 2.0° or more is greater than 0% and less than 50%.

[0098] In the hot-rolled steel sheet of the present disclosure, the following relationships (1) and (2) are satisfied:

[0099] 1.7≤LGr / LGt (1)

[0100] 1.20≤(LGr / LGt) / (LMr / LMt) (2)

[0101] LGr: Area average of the projected length of the original austenite grains in the rolling direction

[0102] LGt: Area average of the projected length of the prior austenite grains in the plate thickness direction

[0103] LMr: Area average of the projected length in the rolling direction of the region having a GAM value of 2.0° or more LMt: Area average of the projected length in the plate thickness direction of the region having a GAM value of 2.0° or more.

[0104] 1.1 Chemical composition

[0105] The reasons for limiting the chemical composition of the hot-rolled steel sheet will be described. In the following description, "%" for each component means mass %.

[0106] (C: 0.045% to 0.120%)

[0107] C is an element that increases the strength of hot-rolled steel sheets. If the C content is too low, the region with a GAM value of 0.6° or less becomes excessive, and the strength of the hot-rolled steel sheet is likely to decrease. On the other hand, if the C content is too high, the region with a GAM value of 2.0° or more becomes excessive, and the elongation and hole expandability of the hot-rolled steel sheet are likely to decrease. In the hot-rolled steel sheet disclosed herein, the balance between the strength, elongation and hole expandability of the hot-rolled steel sheet is improved by setting the C content to 0.045% to 0.120%. The C content may also be 0.050% or more, 0.055% or more, or 0.060% or less, or 0.115% or less, 0.110% or less, 0.105% or less, or 0.100% or less.

[0108] (Si: 0% to 3.00%)

[0109] Si acts as a deoxidizer and is an element that affects the morphology of carbides, etc., and is an element that can increase the tensile strength of hot-rolled steel sheets. However, the hot-rolled steel sheets disclosed herein can ensure sufficient tensile strength even if they do not contain Si. On the other hand, if the Si content is too high, hot rolling may become difficult due to insufficient ductility. In the hot-rolled steel sheets disclosed herein, the balance between the strength, elongation, and hole expandability of the hot-rolled steel sheets is improved by setting the Si content to 0% to 3.00%. The Si content may also be greater than 0%, 0.001% or more, 0.005% or more, 0.010% or more, 0.030% or more, 0.050% or more, 0.100% or more, 0.200% or more, 0.300% or more, 0.400% or more, or 0.500% or less, or 2.50% or less, 2.00% or less, 1.80% or less, or 1.50% or less.

[0110] (Mn: 1.20% to 2.60%)

[0111] Mn is an element that can increase the tensile strength of hot-rolled steel sheets. If the Mn content is too low, the region with a GAM value of 0.6° or less becomes excessive, and the strength of the hot-rolled steel sheet is likely to decrease. On the other hand, if the Mn content is too high, the region with a GAM value of 2.0° or more becomes excessive, and the elongation of the hot-rolled steel sheet is likely to decrease. In the hot-rolled steel sheet disclosed herein, the balance between the strength, elongation and hole expandability of the hot-rolled steel sheet is improved by setting the Mn content to 1.20% to 2.60%. The Mn content may also be 1.25% or more, 1.30% or more, 1.35% or more, or 1.40% or less, or 2.50% or less, 2.40% or less, 2.30% or less, or 2.20% or less.

[0112] (Ti: 0.020% to 0.180%)

[0113] Ti is a strengthening element that can contribute to increasing the strength of hot-rolled steel sheets through precipitation strengthening, grain refinement strengthening and / or dislocation strengthening. In addition, Ti is an element that can serve as a phase deformation nucleus. Specifically, in the hot-rolled steel sheet disclosed herein, by causing TiC to precipitate at a high density, it can act as a nucleus for phase transformation. If the Ti content is too little, such a function will not be exerted, and the balance between the strength, elongation and hole expandability of the hot-rolled steel sheet will be easily deteriorated due to coarsening of the original austenite grains. On the other hand, if the Ti content is too much, excessive precipitates will be generated, and the hole expandability of the hot-rolled steel sheet will be easily reduced. In the hot-rolled steel sheet disclosed herein, by setting the Ti content to 0.020% to 0.180%, the balance between the strength, elongation and hole expandability of the hot-rolled steel sheet is improved. The Ti content may be 0.040% or more, 0.060% or more, 0.080% or more, or 0.100% or more, or 0.175% or less, 0.170% or less, 0.165% or less, or 0.160% or less.

[0114] (Al: 0.010% to 0.400%)

[0115] Al is an element that acts as a deoxidizer. If the Al content is too low, deoxidation is likely to become insufficient, inclusions are likely to be excessively generated, and the hole expandability of the hot-rolled steel plate is likely to decrease. On the other hand, if the Al content is too high, cracking of the slab may occur, making hot rolling difficult. In the hot-rolled steel plate disclosed herein, by setting the Al content to 0.010% to 0.400%, cracking of the slab can be suppressed, and the balance between the strength, elongation and hole expandability of the hot-rolled steel plate can be improved. The Al content may also be 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or less, 0.350% or less, 0.300% or less, 0.250% or less, or 0.200% or less.

[0116] (P: 0% to 0.080%)

[0117] P is an element that segregates at the grain boundaries in steel and promotes grain boundary embrittlement. If the P content is too high, the elongation and hole expansion of the hot-rolled steel sheet are likely to decrease, and cracking of the slab due to embrittlement may occur, making hot rolling difficult. In the hot-rolled steel sheet disclosed herein, by setting the P content to 0% to 0.080%, cracking of the slab can be suppressed, and the balance of the strength, elongation and hole expansion of the hot-rolled steel sheet can be improved. The P content may also be 0.001% or more, 0.002% or more, 0.003% or more, or 0.004% or less, or 0.050% or less, 0.030% or less, 0.015% or less, or 0.010% or less.

[0118] (S: 0% to 0.0100%)

[0119] S is an element that generates inclusions such as MnS in steel and reduces the ductility of hot-rolled steel sheets. If the S content is too high, inclusions are excessively generated, and the hole expandability of the hot-rolled steel sheet is easily reduced. In the hot-rolled steel sheet disclosed herein, by setting the S content to 0% to 0.0100%, the balance between the strength, elongation, and hole expandability of the hot-rolled steel sheet is improved. The S content may also be greater than 0.0001%, greater than 0.0010%, greater than 0.0015%, or greater than 0.0020%, or less than 0.0090%, less than 0.0075%, less than 0.0060%, or less than 0.0050%.

[0120] (N: 0% to 0.0050%)

[0121] Nitrogen forms coarse nitrides in steel and is an element that reduces the workability of hot-rolled steel sheets. If the N content is too high, excessive nitride formation can easily reduce the elongation and hole expandability of the hot-rolled steel sheet, and further, cracking of the slab due to embrittlement may occur, making hot rolling difficult. In the hot-rolled steel sheet disclosed herein, by setting the N content to 0% to 0.0050%, cracking of the slab can be suppressed, and the balance between the strength, elongation, and hole expandability of the hot-rolled steel sheet can be improved. The N content can also be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0015% or less, or 0.0048% or less, 0.0045% or less, 0.0042% or less, or 0.0040% or less.

[0122] (O: 0% to 0.010%)

[0123] O forms oxides, which reduce the workability of hot-rolled steel sheets. Excessive O content can lead to excessive oxide formation, which can reduce the hole expandability of the hot-rolled steel sheet. In the hot-rolled steel sheet disclosed herein, the O content is adjusted to 0% to 0.010%, which improves the balance between strength, elongation, and hole expandability. The O content can be 0.001% or higher, or 0.008%, 0.006%, 0.005%, or 0.004% or lower.

[0124] The basic chemical composition of the hot-rolled steel sheet disclosed herein is as described above. Furthermore, the hot-rolled steel sheet disclosed herein may also contain at least one of the following elements as needed. Since these elements may not be present, the lower limit of their content is 0%.

[0125] (Nb: 0% to 0.100%)

[0126] Like Ti, Nb is an element effective for controlling the morphology of carbides and can be added arbitrarily. On the other hand, if the Nb content is too high, not only will the effect be saturated, but precipitates may also be formed. In the hot-rolled steel sheet disclosed herein, the Nb content is 0% to 0.100%. The Nb content may also be 0.001% or more, 0.003% or more, 0.005% or more, or 0.007% or less, or 0.090% or less, 0.070% or less, 0.050% or less, 0.045% or less, 0.040% or less, 0.035% or less, or 0.030% or less.

[0127] (V: 0% to 1.000%)

[0128] V is an element that can contribute to the increase in strength of hot-rolled steel sheets through precipitation strengthening, grain refinement strengthening and / or dislocation strengthening, and can be added arbitrarily. On the other hand, if the V content is too much, in addition to saturation of the effect, precipitates may also be generated. In the hot-rolled steel sheet disclosed herein, the V content is 0% to 1.000%. The V content may also be greater than 0.001%, greater than 0.003%, greater than 0.005%, or greater than 0.007%, or less than 0.900%, less than 0.700%, less than 0.500%, less than 0.300%, less than 0.250%, less than 0.200%, less than 0.150%, or less than 0.100%.

[0129] (Cu: 0% to 1.000%)

[0130] Cu is an element that can contribute to improving at least one of strength and corrosion resistance, and can be added arbitrarily. On the other hand, excessive Cu content may lead to a decrease in toughness, etc. In the hot-rolled steel sheet disclosed herein, the Cu content is 0% to 1.000%. The Cu content may also be 0.001% or more, 0.005% or more, or 0.010% or less, or 0.800% or less, 0.600% or less, 0.400% or less, 0.350% or less, 0.250% or less, or 0.150% or less.

[0131] (Cr: 0% to 2.000%)

[0132] Cr is an element that can improve the hardenability of steel and contribute to at least one of improving strength and corrosion resistance, and can be added arbitrarily. On the other hand, if Cr is excessively contained, in addition to increasing the alloy cost, it may also lead to a decrease in toughness. In the hot-rolled steel sheet disclosed herein, the Cr content is 0% to 2.000%. The Cr content may also be greater than 0.001%, greater than 0.005%, or greater than 0.010%, or less than 1.500%, less than 1.000%, less than 0.800%, less than 0.700%, less than 0.600%, or less than 0.500%.

[0133] (Mo: 0% to 3.000%)

[0134] Mo is an element that can improve the hardenability of steel and contribute to at least one of improving strength and corrosion resistance, and can be added arbitrarily. On the other hand, if Mo is contained excessively, the deformation resistance during processing may increase. In the hot-rolled steel sheet disclosed herein, the Mo content is 0% to 3.000%. The Mo content can also be greater than 0.001%, greater than 0.005%, or greater than 0.010%, or less than 2.500%, less than 2.000%, less than 1.500%, less than 1.000%, less than 0.600%, less than 0.500%, less than 0.400%, or less than 0.300%.

[0135] (Ni: 0% to 0.500%)

[0136] Ni is an element that can improve the hardenability of steel and contribute to at least one of improving strength and heat resistance, and can be added arbitrarily. On the other hand, if Ni is excessively contained, the effect is saturated and there is a possibility of increasing manufacturing costs. In the hot-rolled steel sheet disclosed herein, the Ni content is 0% to 0.500%. The Ni content can also be 0.001% or more, 0.005% or more, or 0.010% or less, or 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, 0.250% or less, 0.200% or less, or 0.150% or less.

[0137] (B: 0% to 0.0100%)

[0138] B is an element that contributes to increasing the strength of steel and can be added arbitrarily. In the hot-rolled steel sheet disclosed herein, the B content is 0% to 0.0100%. The B content can also be 0.0001% or more, 0.0003% or more, or 0.0005% or less, or 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, or 0.0025% or less.

[0139] (Ca: 0% to 0.0500%)

[0140] Ca is an element that can control the morphology of sulfides and can be added arbitrarily. On the other hand, if Ca is contained excessively, the effect is saturated and there is a possibility of increasing manufacturing costs. In the hot-rolled steel sheet disclosed herein, the Ca content is 0% to 0.0500%. The Ca content can also be 0.0001% or more, 0.0003% or more, or 0.0005% or less, or 0.0300% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, or 0.0025% or less.

[0141] (Mg: 0% to 0.050%)

[0142] Mg is an element that helps control the morphology of sulfides and can be added arbitrarily. However, excessive Mg content can reduce toughness. In the hot-rolled steel sheet disclosed herein, the Mg content is 0% to 0.050%. The Mg content can also be 0.001% or higher, or 0.040% or lower, 0.030% or lower, 0.020% or lower, 0.015% or lower, 0.010% or lower, or 0.005% or lower.

[0143] (REM: 0% to 0.100%)

[0144] REM, like Ca, is an element that can be added in trace amounts to control the form of sulfides, and can also be added arbitrarily. On the other hand, if REM is excessively contained, coarse inclusions may be generated. In the hot-rolled steel sheet disclosed herein, the REM content is 0% to 0.100%. The REM content may also be greater than 0.001%, greater than 0.003%, or greater than 0.005%, or less than 0.080%, less than 0.060%, or less than 0.040%. In addition, "REM" is a general term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanoid elements, and "REM content" is the total content of these elements.

[0145] (Bi: 0% to 0.100%)

[0146] Bi is an element that can contribute to improving corrosion resistance, etc., and can be added arbitrarily. On the other hand, if Bi is contained excessively, the effect is saturated and there is a possibility of increasing manufacturing costs. In the hot-rolled steel sheet disclosed herein, the Bi content is 0% to 0.100%. The Bi content can also be 0.001% or more or 0.002%, or less than 0.070%, less than 0.050%, less than 0.030%, less than 0.010%, less than 0.008%, less than 0.006%, or less than 0.004%.

[0147] (Ta: 0% to 0.100%)

[0148] Ta is an element that can help control the morphology of carbides and increase strength, and can be added arbitrarily. On the other hand, excessive Ta content can reduce toughness due to precipitation of Ta carbides, etc. In the hot-rolled steel sheet disclosed herein, the Ta content is 0% to 0.100%. The Ta content can also be 0.001% or more, 0.005% or more, or 0.010% or less, or 0.080% or less, 0.060% or less, or 0.040% or less.

[0149] (Zr: 0% to 0.500%)

[0150] Zr is an element that helps control the morphology of sulfides and can be added arbitrarily. However, excessive Zr content can saturate the effect and potentially increase manufacturing costs. In the hot-rolled steel sheet disclosed herein, the Zr content is 0% to 0.500%. The Zr content can also be 0.001% or higher, 0.005% or higher, or 0.010% or lower, or 0.400% or lower, 0.300% or lower, or 0.200% or lower.

[0151] (Co: 0% to 3.000%)

[0152] Co is an element that can help improve at least one of hardenability and heat resistance, and can be added arbitrarily. On the other hand, if Co is excessively contained, workability may be reduced, which also leads to an increase in raw material costs. In the hot-rolled steel sheet disclosed herein, the Co content is 0% to 3.000%. The Co content can also be more than 0.001%, more than 0.010%, more than 0.030% or more than 0.050%, or less than 2.000%, less than 1.000%, less than 0.800%, less than 0.600%, less than 0.400%, less than 0.200%, less than 0.180%, less than 0.160% or less than 0.140%.

[0153] (Zn: 0% to 0.200%)

[0154] Zn is an element that can control the morphology of inclusions and can be added arbitrarily. On the other hand, excessive Zn content can lead to the formation of large amounts of precipitates and inclusions. In the hot-rolled steel sheet disclosed herein, the Zn content is 0% to 0.200%. The Zn content can also be 0.001% or higher, 0.010% or higher, 0.030% or higher, or 0.050% or lower, and can also be 0.180% or lower, 0.160% or lower, or 0.140% or lower.

[0155] (W: 0% to 0.200%)

[0156] W is an element that improves the hardenability of steel and contributes to increased strength, and can be added arbitrarily. On the other hand, excessive W content can lead to the formation of coarse inclusions. In the hot-rolled steel sheet disclosed herein, the W content is 0% to 0.200%. The W content can also be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or less, or 0.180% or less, 0.160% or less, or 0.140% or less.

[0157] (Sb: 0% to 0.500%)

[0158] Sb is an element that can contribute to improved corrosion resistance and can be added arbitrarily. On the other hand, excessive Sb content can lead to reduced toughness. In the hot-rolled steel sheet disclosed herein, the Sb content is 0% to 0.500%. The Sb content can also be 0.001% or more, 0.010% or more, 0.030% or more, or 0.050% or less, or 0.400% or less, 0.300% or less, or 0.200% or less.

[0159] (As: 0% to 0.050%)

[0160] As is an element that can help improve the machinability of steel and can be added arbitrarily. On the other hand, excessive As content can reduce workability. In the hot-rolled steel sheet disclosed herein, the As content is 0% to 0.050%. The As content can also be 0.001% or more, or 0.005% or less, or 0.030% or less, 0.010% or less, 0.009% or less, 0.008% or less, or 0.007% or less.

[0161] (Sn: 0% to 0.050%)

[0162] Sb is an element that can contribute to improved corrosion resistance and can be added arbitrarily. On the other hand, excessive Sn content can lead to reduced toughness. In the hot-rolled steel sheet disclosed herein, the Sn content is 0% to 0.050%. The Sn content can also be 0.001% or higher, 0.005% or higher, or 0.010% or lower, and can also be 0.047% or lower, 0.045% or lower, or 0.043% or lower.

[0163] (Remainder: Fe and impurities)

[0164] Regarding the chemical composition of the hot-rolled steel sheet disclosed herein, the remainder other than the above-mentioned components is Fe and impurities. Impurities refer to components that are mixed into raw materials such as ores and scrap during the industrial production of hot-rolled steel sheets due to various reasons during the manufacturing process.

[0165] The chemical composition of the hot-rolled steel sheet can be analyzed using a spark discharge emission spectrometer or the like. C and S are determined by burning in an oxygen stream using a gas composition analyzer or the like and measuring by infrared absorption. N is determined by melting a test piece taken from the hot-rolled steel sheet in a helium stream and measuring by thermal conductivity.

[0166] 1.2 Original austenite grain size

[0167] The original austenite grain size in the hot-rolled steel sheet disclosed herein is 25 μm or less. In this way, by refining the original austenite grains, the strain concentration in the microstructure is alleviated, the mechanical properties of the hot-rolled steel sheet are improved, and the hole expandability and collision characteristics of the hot-rolled steel sheet are improved. In particular, when the original austenite grain size is 20 μm or less, 18 μm or less, 15 μm or less, 12 μm or less, 10 μm or less, or 8 μm or less, the balance of the strength, elongation, hole expandability, and collision characteristics of the hot-rolled steel sheet tends to become good. The lower limit of the original austenite grain size is not particularly limited, and may also be greater than 0 μm, greater than 1 μm, greater than 3 μm, greater than 5 μm, or greater than 7 μm.

[0168] In addition, the "original austenite grain size" of the hot-rolled steel plate refers to the average grain size of the original austenite grains. The average grain size of the original austenite grains is measured as follows. First, a sample is collected at a position 1 / 4 of the end face of the hot-rolled steel plate in the plate width direction so that the metal structure of the section (plate thickness direction × rolling direction section) with the plate width direction set as the normal direction can be observed. Although the size of the sample also depends on the measuring device, for example, it can be set as a rectangular parallelepiped with the full thickness in the plate thickness direction, 15 mm in the rolling direction, and 10 mm in the plate width direction. Then, after the observation surface is mirror-polished, it is corroded by the Bechet-Beaujard method using a saturated aqueous solution of picric acid. The grains that appear black due to corrosion are regarded as original austenite grains. The observation surface showing original austenite grains is observed using an optical microscope, and the area of ​​0.05 mm 2 More than 8 fields of view (total 0.40mm 2 The above). Then, based on the steel structure photograph taken by the optical microscope, the equivalent circle diameter of each original austenite grain is calculated. Except for the original austenite grains that are not included in the shooting field of view as a whole, such as the ends of the shooting field of view, the equivalent circle diameters of all the original austenite grains contained in each shooting field of view are calculated as described above. The average grain size of the original austenite grains is obtained by calculating the area average (average weighted by area) of the equivalent circle diameters of the original austenite grains obtained in each shooting field of view. Here, the area average is when the equivalent circle diameter of a certain original austenite grain G1 is D1 and the area is A1, and the equivalent circle diameter of a different original austenite grain G2 is D2 and the area is A2, the area average D of the equivalent circle diameters of the two original austenite grains can be calculated as D = (A1×D1+A2×D2) / (A1+A2).

[0169] In this disclosure, the term "a position x / y from the end surface (where x and y are natural numbers satisfying x < y)" refers to a position that is shifted from the end surface of the steel plate in the plate width direction toward the center of the steel plate by a distance x / y of the plate width. For example, if the steel plate has a width of 1 meter, the term "a position 1 / 4 from the end surface" refers to a position that is 0.25 meters from the end surface of the steel plate in the plate width direction.

[0170] In this disclosure, the term "plate thickness x / y position (where x and y are natural numbers satisfying x < y)" refers to a position that is shifted from the surface (plate surface) of the steel plate in the plate thickness direction toward the center of the steel plate by a distance x / y (depth) equal to the plate thickness t. For example, if the plate thickness t of the steel plate is 2 mm, the "plate thickness 1 / 8 position" refers to a position at a depth of 0.25 mm from the surface of the steel plate in the plate thickness direction. Furthermore, when the steel plate has a coating such as a plating layer on its surface, the "surface of the steel plate" refers to the interface between the steel plate and the coating, and the "plate thickness t" refers to the thickness of the steel plate (base material) excluding the coating.

[0171] In the present disclosure, the plate width direction is a direction perpendicular to the rolling direction and the plate thickness direction.

[0172] When the rolling direction of the steel plate is unclear, the following method can be used as a method for determining the rolling direction of the steel plate. After the plate thickness section of the steel plate is finished by mirror polishing, the S concentration is measured using an electron probe microanalyzer (EPMA). The measurement conditions are to set the acceleration voltage to 15kV, the measurement interval to 1μm, and measure the distribution image of a 500μm square area in the center of the plate thickness. At this time, the extended area with a high S concentration is determined to be an inclusion such as MnS. Observation can also be performed in multiple fields of view. Next, using the plate thickness section initially observed using the above method as a reference, rotate it every 5° within the range of 0° to 180° with the plate thickness direction as the axis, and observe the section parallel to the plane obtained by such rotation using the above method. The average value of the length of the major axis of the multiple inclusions in each section is calculated for each section, and the section with the maximum average value of the length of the major axis of the inclusions is determined. The direction parallel to the major axis direction of the inclusions in the cross section is determined as the rolling direction.

[0173] 1.3 Area ratio

[0174] In the hot-rolled steel sheet disclosed herein, the area ratios of regions with a GAM (Grain Average Misorientation) value exceeding 0.6° and less than 2.0°, regions with a GAM value of 0.6° or less, and regions with a GAM value of 2.0° or greater are determined as follows. Furthermore, in the hot-rolled steel sheet, "regions with a GAM value of 0.6° or less" are often relatively soft. "Regions with a GAM value of 2.0° or greater" are often relatively hard. "Regions with a GAM value exceeding 0.6° and less than 2.0°" often have an intermediate degree of hardness.

[0175] (Area ratio of regions where the GAM value exceeds 0.6° and is less than 2.0°)

[0176] In the hot-rolled steel sheet disclosed herein, the area ratio of the region having a GAM value exceeding 0.6° and less than 2.0° is 50% or more and less than 100%. When the area ratio of the region having a GAM value exceeding 0.6° and less than 2.0° is 50% or more, the hot-rolled steel sheet tends to have an excellent balance among strength, elongation, and hole expandability. The area ratio of the region having a GAM value exceeding 0.6° and less than 2.0° may also be 55% or more, 60% or more, 70% or more, 75% or more, 80% or more, or 99% or less, 95% or less, 90% or less, 85% or less, 80% or less, 75% or less, 70% or less, 65% or less, 60% or less, or 55% or less. In particular, when the area ratio of the region having a GAM value exceeding 0.6° and less than 2.0° is 55% to 95%, the hot-rolled steel sheet tends to have an excellent balance among strength, elongation, and hole expandability.

[0177] (Area ratio of regions with GAM values ​​of 0.6° or less)

[0178] In the hot-rolled steel sheet disclosed herein, the area ratio of the region with a GAM value of 0.6° or less is 0% or more and less than 50%. When the area ratio of the region with a GAM value of 0.6° or less is less than 50%, the balance between strength, elongation, and hole expandability of the hot-rolled steel sheet is likely to be excellent. The hot-rolled steel sheet disclosed herein only needs to have the region with a GAM value of more than 0.6° and less than 2.0° and the region with a GAM value of 2.0° or more described later. The area ratio of the region with a GAM value of 0.6° or less may be 0%. The area ratio of the region with a GAM value of 0.6° or less may be 45% or less, 40% or less, 35% or less, 30% or less, 25% or less, 20% or less, 15% or less, 10% or less, or 5% or less, or may be greater than 0%, 5% or greater, 10% or greater, 15% or greater, 20% or greater, 25% or greater, 30% or greater, 35% or greater, 40% or greater, or 45% or greater. In particular, when the area ratio of the region having a GAM value of 0.6° or less is 0% to 45%, the balance among strength, elongation, and hole expandability of the hot-rolled steel sheet tends to be further improved.

[0179] (Area ratio of regions with a GAM value of 2.0° or more)

[0180] In the hot-rolled steel sheet disclosed herein, the area ratio of the region having a GAM value of 2.0° or more exceeds 0% and is less than 50%. When the hot-rolled steel sheet disclosed herein has a region having a GAM value of 2.0° or more in addition to the region having a GAM value of more than 0.6° and less than 2.0°, the balance among strength, elongation, and hole expandability is likely to be excellent. The area ratio of the region having a GAM value of 2.0° or more may be less than 45%, less than 40%, less than 35%, less than 30%, less than 25%, less than 20%, less than 15%, less than 10%, or less than 5%, or may be more than 1%, more than 5%, more than 10%, more than 15%, more than 20%, more than 25%, more than 30%, more than 35%, more than 40%, or more than 45%. In particular, when the area ratio of the region with a GAM value of 2.0° or higher exceeds 0% and is less than 20%, particularly 1% to 20%, and particularly 1% to 10%, the balance between strength, elongation, and hole expandability of the hot-rolled steel sheet tends to be further improved.

[0181] The "GAM value" of each region of a hot-rolled steel sheet is measured using the EBSP (Electron Back Scatter Pattern) method. Within each measurement region (e.g., within a single grain (defined as a region surrounded by grain boundaries with an orientation difference of 15° or greater)), the average of the orientation differences between adjacent pixels (measurement points) is used as the GAM value for that measurement region (grain). The area ratio of regions with a GAM value exceeding 0.6° and less than 2.0°, the area ratio of regions with a GAM value of 0.6° or less, and the area ratio of regions with a GAM value of 2.0° or greater are measured using the following method. First, a sample is collected from the hot-rolled steel sheet at a position 1 / 4 of the way from the end face in the sheet width direction, so that the metallographic structure of the cross section (sheet thickness direction x rolling direction cross section) with the sheet width direction as the normal direction can be observed. The sample size depends on the measuring instrument, but for example, a rectangular parallelepiped with the full thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction can be used. Next, the observation surface of the sample was mirror-polished and then polished for 8 minutes at room temperature using colloidal silica without an alkaline solution to remove the strain introduced into the sample surface. Centered on a depth position of 1 / 4 of the surface in the thickness direction of the sample, an area of ​​200 μm in the thickness direction and 400 μm or more at any position in the rolling direction was measured by the EBSP method at a measurement interval of 0.2 μm (a rectangular area centered on a depth position of 1 / 4 in the thickness direction, having a length of 200 μm in the thickness direction (short side) and a length of 400 μm or more in the rolling direction (long side)). In the above measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL) was used. At this time, the vacuum degree in the EBSD analysis device was set to 9.6×10 ―5 Pa or less, with the accelerating voltage set to 15 kV, the irradiation current level set to 13, and the electron beam irradiation level set to 62. The GAM value can be calculated using the "OIM Analysis (registered trademark)" software included with the EBSD analyzer. Furthermore, grains with an equivalent circle diameter of 0.6 μm or less were excluded due to the potential for significant measurement error.

[0182] 1.4 Relationship (1): 1.7≤LGr / LGt

[0183] The hot-rolled steel sheet disclosed in the present invention satisfies the relationship (1) of 1.7≤LGr / LGt. Here, LGr is the area average of the projected lengths of the original austenite grains in the rolling direction, and LGt is the area average of the projected lengths of the original austenite grains in the plate thickness direction. In addition, "area average" refers to an average weighted by area. Here, "area" refers to the area of ​​a cross section (plate thickness direction × rolling direction cross section) with the plate width direction as the normal direction. For example, when a prior austenite grain G1 has a rolling direction projection length of LGr1, a plate thickness direction projection length of LGt1, and an area of ​​A1, and a different prior austenite grain G2 has a rolling direction projection length of LGr2, a plate thickness direction projection length of LGt2, and an area of ​​A2, the area average LGr of the rolling direction projection lengths of these two prior austenite grains is LGr = (A1×LGr1+A2×LGr2) / (A1+A2), and the area average LGt of the plate thickness direction projection lengths is LGt = (A1×LGt1+A2×LGt2) / (A1+A2). In the hot-rolled steel sheet of the present disclosure, satisfying the above-mentioned relationship (1) means, in other words, that the prior austenite grains are elongated in the rolling direction. In addition, as described above, in the hot-rolled steel sheet of the present disclosure, the area ratio of the region where the prior austenite grain size is small and the GAM value exceeds 0.6° and is less than 2.0° is 50% or more and less than 100%. That is, in the hot-rolled steel sheet disclosed in the present invention, it can be said that fine grains with an excellent balance between strength and ductility are elongated along the rolling direction. In the past, it was generally believed that when the LGr / LGt in the hot-rolled steel sheet was large, the elongation and hole expansion of the hot-rolled steel sheet would decrease. Therefore, in the previous process, the LGr / LGt was controlled in a manner that was small, that is, the grains were equiaxed. In contrast, in the hot-rolled steel sheet disclosed in the present invention, by satisfying the relationship (2) described later together with the relationship (1), the balance between strength, elongation and hole expansion is improved. In addition, the hot-rolled steel sheet disclosed in the present invention makes the original austenite grains elongate along the rolling direction, so that cracks are difficult to expand in the plate thickness direction, and the collision characteristics become excellent.

[0184] In the above relationship (1), the upper limit of LGl / LGt is not particularly limited. In the hot-rolled steel sheet of the present disclosure, the following relationship (1-1) may also be satisfied. LGr / LGt may be 10.0 or less, 9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, or 5.0 or less. LGr / LGt may also be 2.0 or more, 2.5 or more, 3.0 or more, or 4.0 or more.

[0185] 1.7≤LGr / LGt≤10.0 (1-1)

[0186] In addition, the "LGr / LGt" in the hot-rolled steel plate is measured as follows. First, a sample is collected at a position 1 / 4 of the end face of the hot-rolled steel plate in the plate width direction so that the metal structure of the section (plate thickness direction × rolling direction section) with the plate width direction set as the normal direction can be observed. Although the size of the sample also depends on the measuring device, it can be set as a rectangular parallelepiped with the full thickness in the plate thickness direction, 15 mm in the rolling direction, and 10 mm in the plate width direction. Then, after the observation surface is mirror-polished, it is corroded by the Bechet-Beaujard method using a saturated aqueous solution of picric acid. The grains that appear black due to corrosion are regarded as original austenite grains. The observation surface showing the original austenite grains is observed using an optical microscope, and the area of ​​0.05 mm 2 More than 8 fields of view (total 0.40mm 2 Then, based on the steel structure photograph taken with an optical microscope, the area of ​​each prior austenite grain is calculated, and the projection length in the rolling direction and the projection length in the plate thickness direction are measured respectively, and the ratio of the values ​​averaged by the respective areas is taken as LGr / LGt. In the case where the prior austenite grains cannot be fully presented by the above method, the prior austenite grains are determined by the reconstruction method described in "Study on Reconstruction Methods for Austenite Structure of Steel Towards High Precision" (Hata Kengo, Wakita Masayuki, Fujiwara Tomoya, Kono Kaori, Shin-Nippon Steel & Sumitomo Metal Technology Report No. 404 (2016), pp. 24-30), and the LGr / LGt of the prior austenite grains is calculated.

[0187] 1.5 Relationship (2): 1.20 ≤ (LGr / LGt) / (LMr / LMt)

[0188] The hot-rolled steel sheet disclosed in the present invention satisfies the relationship (2) of 1.20≤(LGr / LGt) / (LMr / LMt). Here, LGr and LGt are as described above, LMr is the area average of the projected length in the rolling direction of the region with a GAM value of 2.0° or more, and LMt is the area average of the projected length in the plate thickness direction of the region with a GAM value of 2.0° or more. That is, in the hot-rolled steel sheet disclosed in the present invention, the LGr / LGt of the original austenite grains is 1.20 times or more of the LMr / LMt of the hard phase. In this way, by making the LGr / LGt of the original austenite grains larger than the LMr / LMt of the hard phase by a certain amount (the LMr / LMt of the hard phase is smaller than the LGr / LGt of the original austenite grains by a certain amount), the hard phase is dispersed, and the deviation in the strength and ductility of the steel sheet as a whole is reduced, thereby achieving an excellent balance among the strength, elongation, and hole expandability of the hot-rolled steel sheet.

[0189] In the above relationship (2), the upper limit of (LGr / LGt) / (LMr / LMt) is not particularly limited. In the hot-rolled steel sheet disclosed herein, the following relationship (2-1) may also be satisfied. (LGr / LGt) / (LMr / LMt) may also be 5.00 or less, 4.80 or less, 4.50 or less, 4.30 or less, 4.00 or less, 3.80 or less, 3.60 or less, or 3.40 or less. In addition, (LGr / LGt) / (LMr / LMt) may also be 1.40 or more, 1.60 or more, 1.80 or more, 2.00 or more, 2.20 or more, or 2.40 or more.

[0190] 1.20≤(LGr / LGt) / (LMr / LMt)≤5.00 (2-1)

[0191] In addition, "LMr / LMt" in the hot-rolled steel sheet is measured as follows. First, a sample is collected at a position 1 / 4 of the end face of the hot-rolled steel sheet in the sheet width direction so that the metal structure of the section (sheet thickness direction × rolling direction section) with the sheet width direction set as the normal direction can be observed. Although the size of the sample also depends on the measuring device, for example, it can be set to a rectangular parallelepiped with the full thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the observation surface of the sample is mirror-polished and then polished for 8 minutes at room temperature using colloidal silica that does not contain an alkaline solution to remove the strain introduced into the sample surface. The above-mentioned sample was measured at a depth position of 1 / 4 from the surface in the thickness direction by the EBSP method at a measurement interval of 0.2 μm, with a range of 200 μm in the thickness direction and 400 μm or more at any position in the rolling direction (a rectangular region centered at a depth position of 1 / 4 in the thickness direction, having a length of 200 μm in the thickness direction (short side) and a length of 400 μm or more in the rolling direction (long side)). In the above-mentioned measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) was used. At this time, the vacuum degree in the EBSD analysis device was set to 9.6×10 -5Pa or less, the acceleration voltage is set to 15kV, the irradiation current level is set to 13, and the irradiation level of the electron beam is set to 62. Next, the GAM value is calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. In addition, for grains whose equivalent circle diameters of the defined grains are less than 0.6μm, they are excluded because there is a possibility of large measurement errors. Based on the information of the calculated GAM value, the area with a GAM value of 2.0° or more is determined. The area of ​​each area with a GAM value of 2.0° or more is calculated, and the projection length in the rolling direction and the projection length in the plate thickness direction are measured respectively according to their shapes, and the ratio of the values ​​after taking the average of the areas in each is taken as LMr / LMt.

[0192] 1.6 Mechanical properties, etc.

[0193] The hot-rolled steel sheet disclosed herein has the above-described chemical composition and steel structure, and thus has an excellent balance among strength, elongation, and hole expandability, and also has excellent collision characteristics.

[0194] (Tensile Strength TS)

[0195] The hot-rolled steel sheet disclosed herein has excellent strength. For example, the hot-rolled steel sheet disclosed herein may also have a tensile strength TS of 960 MPa or more. The tensile strength TS may also be 970 MPa or more or 980 MPa or more. The upper limit of the tensile strength TS is not particularly limited, and may be, for example, 1200 MPa or less, 1150 MPa or less or 1100 MPa or less. In addition, the tensile test for measuring the tensile strength TS of the hot-rolled steel sheet is carried out in accordance with JIS Z 2241, by collecting a No. 5 test piece from a direction parallel to the rolling right-angle direction (plate width direction) of the steel sheet in the length direction of the test piece. In the case where the above-mentioned No. 5 test piece cannot be collected from the hot-rolled steel sheet to be measured, a small test piece with the plate width direction as the length direction may be used as a substitute for the test piece for measuring the tensile strength TS.

[0196] (Uniform elongation uEL)

[0197] The hot-rolled steel sheet disclosed in the present invention has excellent ductility. For example, the hot-rolled steel sheet disclosed in the present invention may also have a uniform elongation uEL of 4.0% to 12.0%. The uniform elongation uEL may also be 5.0% or more, 6.0% or more, 6.5% or more, 7.0% or more, 7.5% or more, or 8.0% or less, or 11.5% or less, 11.0% or less, 10.5% or less, 10.0% or less, 9.5% or less, or 9.0% or less. In addition, the tensile test for determining the uniform elongation uEL of the hot-rolled steel sheet is carried out in accordance with JIS Z 2241, and a No. 5 test piece is collected in a direction in which the length direction of the test piece is parallel to the rolling right-angle direction of the steel sheet (sheet width direction).

[0198] (Hole expansion)

[0199] The hot-rolled steel sheet disclosed in the present invention has excellent hole expansion properties. For example, the hot-rolled steel sheet disclosed in the present invention may also have a hole expansion ratio λ of 40% to 110%. The hole expansion ratio λ may also be 45% or more, or 50% or more, or less than 100%, less than 90%, less than 80%, or less than 70%. In addition, with respect to the hole expansion properties of the hot-rolled steel sheet, a circular hole with a diameter of 10 mm was punched out under the condition of a clearance of 12.5%, with the burr on the die side, and formed using a 60° conical punch, and evaluated using the hole expansion ratio (%). Five hole expansion tests were carried out, and the average value was taken as the hole expansion ratio λ.

[0200] (Collision characteristics)

[0201] The hot-rolled steel sheet disclosed in the present invention has excellent collision characteristics. The collision characteristics of the hot-rolled steel sheet can be evaluated, for example, by the crack propagation resistance in the thickness direction of the plate. The crack propagation resistance in the thickness direction of the plate is determined by the displacement-load curve when punching the hot-rolled steel sheet. For example, it is determined by the ratio W2 / W1 of energy W2 to energy W1. Here, W2=∫Fds (after the maximum load), W1=∫Fds (before the maximum load). The hot-rolled steel sheet disclosed in the present invention may also satisfy 0.15≤W2 / W1. The value of the ratio W2 / W1 may also be greater than 0.17, greater than 0.18, greater than 0.19, or greater than 0.20.

[0202] (plate thickness)

[0203] The thickness of the hot-rolled steel sheet is not particularly limited, and may be, for example, 0.5 mm to 10.0 mm. The upper limit of the thickness may be 8.0 mm, 6.0 mm, or 4.0 mm.

[0204] (use)

[0205] As described above, the hot-rolled steel sheets disclosed herein have an excellent balance of strength, ductility, and hole expandability, as well as excellent crash resistance. Such hot-rolled steel sheets can be used, for example, as raw materials for automotive chassis, structural components, frames, and frame components. They are particularly suitable as raw materials for automotive chassis. Specific examples of automotive chassis include lower control arms, upper control arms, and trailing arms.

[0206] 2. Manufacturing method of hot-rolled steel sheet

[0207] An example of a method for manufacturing a hot-rolled steel sheet disclosed herein is described below, but the method for manufacturing a hot-rolled steel sheet is not limited to the method described below. One embodiment relates to a method for manufacturing a hot-rolled steel sheet, characterized in that the method comprises:

[0208] The heating process of heating the slab,

[0209] The heated slab is subjected to a hot rolling process.

[0210] A cooling step of cooling a hot-rolled steel sheet obtained by hot rolling, and a coiling step of coiling the cooled hot-rolled steel sheet, wherein the slab contains, by mass%,

[0211] C: 0.045% to 0.120%,

[0212] Si: 0% to 3.00%,

[0213] Mn: 1.20% to 2.60%,

[0214] Ti: 0.020% to 0.180%,

[0215] Al: 0.010% to 0.400%,

[0216] P: 0%~0.080%,

[0217] S: 0% to 0.0100%,

[0218] N: 0% to 0.0050%,

[0219] O: 0% to 0.010%,

[0220] Nb: 0% to 0.100%,

[0221] V: 0%~1.000%,

[0222] Cu: 0% to 1.000%,

[0223] Cr: 0% to 2.000%,

[0224] Mo: 0% to 3.000%,

[0225] Ni: 0% to 0.500%,

[0226] B: 0% to 0.0100%,

[0227] Ca: 0% to 0.0500%,

[0228] Mg: 0% to 0.050%,

[0229] REM: 0%~0.100%,

[0230] Bi: 0% to 0.100%,

[0231] Ta: 0%~0.100%,

[0232] Zr: 0% to 0.500%,

[0233] Co: 0% to 3.000%,

[0234] Zn: 0% to 0.200%,

[0235] W: 0% to 0.200%,

[0236] Sb: 0% to 0.500%,

[0237] As: 0% to 0.050%, and

[0238] Sn: 0%~0.050%,

[0239] The remainder consists of Fe and impurities;

[0240] The hot rolling process includes rough rolling and finish rolling.

[0241] The starting temperature ST of the finish rolling is 1000℃~1150℃,

[0242] The finishing rolling includes more than 2 high temperature difference rollings.

[0243] In the high temperature differential rolling, the temperature difference ΔT between the rolling temperature in the rolling stand performing the high temperature differential rolling and the rolling temperature in the immediately preceding rolling stand is 30° C. or more.

[0244] In the finish rolling, the total reduction ratio after the second high temperature difference rolling is 50% or more,

[0245] The finishing temperature FT of the finishing rolling is below 940°C.

[0246] The time from the end of the finish rolling to the start of the cooling is within 2.0 seconds,

[0247] In the cooling process, accelerated cooling is performed after the cooling starts, and the cooling stop temperature of the accelerated cooling is 520° C. to 720° C.

[0248] In the cooling step, the slow cooling time in the temperature range of 720° C. to 470° C. is 2.0 seconds or longer.

[0249] 2.1 Heating process

[0250] In the heating process, the slab having the above-mentioned chemical composition is heated. When the heating temperature is too low, the dissolution of carbides and nitrides becomes insufficient. On the other hand, when the heating temperature is too high, the amount of scale generated increases, thereby reducing the yield. In this regard, the heating temperature of the slab in the heating process can be, for example, 1100°C to 1300°C. The heating temperature can also be above 1150°C or above 1200°C, or below 1260°C. The heating time of the slab in the heating process is sufficient as long as the time can allow the entire slab to reach the target temperature. The heating time can also be, for example, above 6000 seconds (100 minutes) or above 9000 seconds (150 minutes). In particular, by maintaining the temperature at a temperature of 1150°C or above for 6000 seconds (100 minutes), a higher effect can be easily obtained.

[0251] 2.2 Hot rolling process

[0252] In the hot rolling process, the slab heated in the heating process is subjected to hot rolling. The hot rolling process includes rough rolling and finish rolling.

[0253] (Rough rolling)

[0254] The conditions for rough rolling are not particularly limited; the slab may be rolled at a predetermined temperature and a predetermined reduction ratio. The temperature during rough rolling may be, for example, below the heating temperature in the heating step and above the start temperature ST of the finish rolling described below. The reduction ratio during rough rolling may be, for example, a reduction ratio at 800-1150°C that results in a 90% or greater reduction in thickness.

[0255] (Finishing rolling)

[0256] In the finishing rolling, the slab (rough section) that has been rough-rolled is rolled multiple times using multiple stands. The starting temperature ST of the finishing rolling is 1000°C to 1150°C. If the starting temperature ST is too low, the hot-rolled steel sheet finally manufactured does not meet the above-mentioned requirements of the original austenite grain size, and the hole expandability and the like are easily reduced. On the other hand, if the starting temperature ST is too high, the structure of the steel cannot be properly controlled, and the hot-rolled steel sheet finally manufactured does not meet the above-mentioned requirements of the original austenite grain size, relationships (1) and (2), and the collision characteristics are easily reduced. These problems can be eliminated by setting the starting temperature ST to 1000°C to 1150°C. The starting temperature ST can also be 1050°C to 1150°C.

[0257] Finishing rolling includes two or more high-temperature differential rollings. In the high-temperature differential rolling, the temperature difference ΔT between the rolling temperature in the rolling stand where the high-temperature differential rolling is performed and the rolling temperature in the immediately preceding rolling stand is 30°C or more. Here, the rolling temperature is the temperature on the inlet side of the rolling stand, that is, the surface temperature of the steel plate measured immediately before the steel plate is rolled in the rolling stand. Through the first high-temperature differential rolling, TiC, which serves as the nucleus for phase transformation, precipitates at a high density in the steel structure, and through the second high-temperature differential rolling, a driving force for producing a high-quality steel structure can be obtained. If the high-temperature differential rolling is performed less than once, the steel structure cannot be produced with high quality, and the hot-rolled steel plate finally produced does not satisfy the above-mentioned relationship (2), and thus the hole expandability is easily reduced. If the high-temperature differential rolling is performed more than two times, the steel structure can be produced with high quality, and the balance of strength, elongation and hole expandability of the hot-rolled steel plate finally produced is excellent. The number of high-temperature differential rollings can also be changed according to the number of rolling stands for finishing rolling. The number of high temperature differential rolling times may be, for example, 2 to 10 times, or more than 3 times or more than 4 times, or less than 7 times or less than 6 times. The temperature difference ΔT of the high temperature differential rolling may be 30°C or more, or 35°C or more, 40°C or more, 45°C or more, or 50°C or more, or 150°C or less, 100°C or less, 80°C or less, 60°C or less, or 50°C or less. The temperature difference ΔT of the high temperature differential rolling may be controlled, for example, by controlling the amount of cooling material such as water sprayed from a cooling device such as a cooling sprayer immediately after rolling, or by controlling the conveying speed of the steel plate during rolling.

[0258] In the finishing rolling, the total reduction ratio after the second high temperature differential rolling is 50% or more. If the total reduction ratio after the second high temperature differential rolling is too low, the structure of the steel cannot be properly controlled, and the hot-rolled steel sheet finally manufactured does not satisfy the above-mentioned relationship (2), so that the hole expandability is easily reduced. If the total reduction ratio after the second high temperature differential rolling is 50% or more, such problems can be eliminated. The total reduction ratio after the second high temperature differential rolling can also be 55% or more, 60% or more, or 65% or more. If the total reduction ratio after the second high temperature differential rolling is too high, the anisotropy of the structure increases and the hole expandability is easily reduced. As long as the total reduction ratio after the second high temperature differential rolling is 80% or less, such problems can be eliminated, and therefore the total reduction ratio after the second high temperature differential rolling is preferably 80% or less. The upper limit of the total reduction ratio after the second high temperature differential rolling can also be 75% or less or 70% or less. Even when three or more hot-difference rolling passes are performed, the total reduction ratio described above refers to the total reduction ratio after the second hot-difference rolling pass. Furthermore, the total reduction ratio after the second hot-difference rolling pass refers to the thickness reduction ratio after the second hot-difference rolling pass (which may include hot-difference rolling) relative to the thickness after the second hot-difference rolling pass. Of course, the second hot-difference rolling pass does not constitute the final stage (final stand) of finish rolling.

[0259] The second high-temperature differential rolling is preferably performed at a predetermined temperature because the densely precipitated TiC easily suppresses the reduction of dislocations in the steel structure. This allows the value of (LGr / LGt) / (LMr / LMt) to be optimally controlled, improving collision characteristics. For example, in the case of steel with a Ti content of 0.1-0.13%, a Nb content of 0.008-0.02%, a V content of less than 0.01%, a Mo content of less than 0.01%, and a B content of less than 0.0001%, the rolling temperature for the second high-temperature differential rolling is preferably set to 980-1000°C.

[0260] The finishing temperature FT of the finishing rolling is below 940°C. If the finishing temperature FT is too high, the structure of the steel cannot be properly controlled, and the hot-rolled steel sheet finally manufactured does not meet the above-mentioned requirements of the original austenite grain size, relationships (1) and (2), so that the collision characteristics are easily reduced. If the finishing temperature FT is below 940°C, such problems can be eliminated. The finishing temperature FT can also be below 920°C or below 900°C. The lower limit of the finishing temperature FT is not particularly limited as long as the requirements of the cooling process described later can be achieved. For example, the finishing temperature FT can also be above 750°C, above 770°C, above 800°C, above 830°C or above 850°C.

[0261] 2.3 Cooling process

[0262] In the cooling process, the hot-rolled steel sheet obtained by hot rolling is cooled. The time from the completion of the finish rolling to the start of cooling is within 2.0 seconds. If this time is too long, the prior austenite grain size exceeds 25 μm due to grain coarsening, and the balance between strength, elongation, and hole expandability of the resulting hot-rolled steel sheet is likely to deteriorate. By limiting this time to within 2.0 seconds, this problem can be eliminated. Alternatively, this time may be within 1.8 seconds, 1.6 seconds, 1.4 seconds, or 1.2 seconds.

[0263] In the cooling process, accelerated cooling is performed after the cooling starts. "Accelerated cooling" refers to cooling under cooling conditions with a cooling rate of 20°C / s to 200°C / s. What is important is that the cooling stop temperature of the accelerated cooling is 520°C to 720°C. The phase change in the region with a GAM value of 2.0° mainly occurs in the slow cooling after the cooling of the accelerated cooling is stopped. By setting the stop temperature of the accelerated cooling to 520°C to 720°C, the amount of the region with a GAM value below 2.0° becomes appropriate. Outside this temperature, the proportion of the region with a GAM value of 2.0° or more sometimes increases excessively, thereby reducing the uniform elongation.

[0264] In the cooling process, the slow cooling time in the temperature range of 720°C to 470°C is 2.0 seconds or more. "Slow cooling" refers to cooling under cooling conditions with a cooling rate of less than 20°C / s. By setting the slow cooling time in the temperature range of 720°C to 470°C to 2.0 seconds or more, the area ratio of the region with a GAM value exceeding 0.6° and less than 2.0° is 50% or more. For example, by slowly cooling the hot-rolled steel sheet in the run-out roller table (ROT), the slow cooling time in the temperature range of 720°C to 470°C can be 2.0 seconds or more. The slow cooling time can also be 2.2 seconds or more, 2.4 seconds or more, 2.6 seconds or more, 2.8 seconds or more, or 3.0 seconds or more. In particular, by setting the slow cooling time in the temperature range of 680°C to 580°C to 3.0 seconds or more, the amount of regions with a GAM value of 2.0° or more can be more appropriately controlled. The upper limit of the slow cooling time is not particularly limited, and the optimal slow cooling time can be determined taking into account productivity and the like. The slow cooling time may be, for example, 5.0 seconds or less, 4.5 seconds or less, 4.0 seconds or less, or 3.5 seconds or less. If the slow cooling time is too short, the region with a GAM value of 2.0° or more tends to be excessively generated, and the above-mentioned relationship (2) is not satisfied, resulting in a poor balance among strength, elongation, and hole expandability of the hot-rolled steel sheet finally produced.

[0265] In the cooling process, the average cooling rate from the slow cooling to 300°C is preferably 30°C / s or more. If the average cooling rate is low, softening due to tempering occurs, and the strength of the hot-rolled steel sheet finally manufactured is easily reduced. By setting the average cooling rate to 30°C / s or more, such problems can be more effectively eliminated. The average cooling rate may also be 35°C / s or more, 40°C / s or more, 45°C / s or more, or 50°C / s or more. The upper limit of the average cooling rate is not particularly limited. For example, the average cooling rate may also be 120°C / s or less, 110°C / s or less, 100°C / s or less, 90°C / s or less, or 80°C / s or less. In addition, when the coiling temperature is lower than 300°C, the average cooling rate from 300°C to the coiling temperature is not particularly limited.

[0266] 2.4 Coiling process

[0267] In the coiling process, the hot-rolled steel sheet cooled in the cooling process is coiled. The coiling conditions are not particularly limited. The coiling temperature in the coiling process is, for example, 300°C or lower. This coiling temperature may also be 200°C or lower, 100°C or lower, or 50°C or lower, or 0°C or higher, or 20°C or higher.

[0268] As described above, the hot-rolled steel sheet of the present disclosure can be produced by (1) using a slab having an appropriate chemical composition, (2) designing the finishing rolling conditions during hot rolling to control the morphology of the prior austenite grains, and (3) controlling the cooling rate during cooling to control the phase transformation behavior (for example, controlling the cooling conditions in the run-out table (ROT)). Furthermore, in addition to the above-mentioned processes, other processes may be performed during the production of the hot-rolled steel sheet. For example, a tempering process may be optionally performed after the coiling process.

[0269] Example

[0270] 1. Production of hot-rolled steel plates

[0271] Slabs having the chemical compositions shown in Tables 1 and 2 below were subjected to a heating process and a hot rolling process under the conditions shown in Table 3 below. After the hot rolling process, the hot rolling process was followed by a cooling process and a coiling process under the conditions shown in Table 3, thereby producing a hot-rolled steel sheet (steel strip) with a thickness of 3.0 mm. The chemical compositions of the hot-rolled steel sheet (steel strip) were confirmed to be substantially the same as those of the slab, and were those shown in Tables 1 and 2. In Table 3, "rolling with a ΔT ≥ 30°C was performed two or more times" means "high-temperature differential rolling was performed two or more times." "High-temperature differential rolling" means that the temperature difference ΔT between the rolling temperature in the rolling stand performing the high-temperature differential rolling and the rolling temperature in the immediately preceding rolling stand was 30°C or greater. In Table 3, cases in which high-temperature differential rolling was performed two or more times are indicated by "○," and cases in which high-temperature differential rolling was performed once or less are indicated by "X." In Table 3, "total reduction ratio after the second ΔT temperature difference" means "total reduction ratio after the second high-temperature differential rolling." In Table 3, "accelerated cooling" refers to cooling after the start of cooling, and the cooling rate is 20° C. / s to 200° C. / s.

[0272]

[0273]

[0274] Table 3

[0275]

[0276] The underlined part indicates that the steel composition is outside the prescribed range or the manufacturing conditions are not preferred.

[0277] 2. Determination of prior austenite grain size

[0278] The prior austenite grain size was measured for each hot-rolled steel sheet. The prior austenite grain size was measured using the method described above. The results are shown in Table 4 below.

[0279] 3. Determination of GAM value and area ratio

[0280] For each hot-rolled steel sheet, the area ratio of the region with a GAM value exceeding 0.6° and less than 2.0°, the area ratio of the region with a GAM value of 0.6° or less, and the area ratio of the region with a GAM value of 2.0° or greater were measured. The GAM value and area ratio were measured as described above. The results are shown in Table 4 below.

[0281] 4. Determination of LGr / LGt and (LGr / LGt) / (LMr / LMt)

[0282] For each hot-rolled steel plate, LGr (the area average of the projection length of the original austenite grains in the rolling direction), LGt (the area average of the projection length of the original austenite grains in the plate thickness direction), LMr (the area average of the projection length of the region with a GAM value of 2.0° or more in the rolling direction), and LMt (the area average of the projection length of the region with a GAM value of 2.0° or more in the plate thickness direction) were measured, and "LGr / LGt" and "(LGr / LGt) / (LMr / LMt)" were calculated. The measurement / calculation methods of LGr / LGt and LMr / LMt are as described above. The results are shown in Table 4 below.

[0283] 5. Evaluation of mechanical properties

[0284] 5.1 Tensile strength TS, uniform elongation uEL and hole expansion ratio λ

[0285] For each hot-rolled steel sheet, tensile strength TS, uniform elongation uEL, and hole expansion λ were measured. The measurement methods were as described above. The results are shown in Table 4 below. Furthermore, Table 4 indicates that the steel sheets meeting the mechanical properties of "tensile strength TS: 960 MPa or higher," "uniform elongation uEL: 4.0% to 12.0%," and "hole expansion λ: 40% to 110%"—particularly preferred for applications in automotive chassis—are considered to have an excellent balance of strength, elongation, and hole expansion.

[0286] 5.2 Collision characteristics

[0287] Based on the crack propagation resistance in the thickness direction of each hot-rolled steel plate, the collision characteristics of each hot-rolled steel plate are evaluated. The crack propagation resistance in the thickness direction is determined by the displacement-load curve when punching the hot-rolled steel plate. Specifically, it is determined by the ratio W2 / W1 of the following energy W2 to the energy W1. Here, W2 = ∫Fds (after the maximum load), W1 = ∫Fds (before the maximum load), and F is the punching load (N), and S is the punching stroke (mm). In this embodiment, the case of satisfying 0.15≤W2 / W1 is evaluated as having excellent collision characteristics, and the case of satisfying 0.2<W2 / W1 is evaluated as having particularly excellent collision characteristics. The results are shown in Table 4 below.

[0288] Table 4

[0289]

[0290] The underlined part indicates that the steel composition and structure are outside the specified range or the characteristics are not preferred.

[0291] 6. Results and Investigation

[0292] The results shown in Tables 1 to 4 show the following:

[0293] In No. 1, the C content of the hot-rolled steel sheet was too low, so the region of the GAM value of 0.6° or less became excessive, and the strength of the hot-rolled steel sheet decreased.

[0294] In No. 2, the C content of the hot-rolled steel sheet was too high, so the region of GAM value 2.0° or higher became excessive, and the elongation and hole expandability of the hot-rolled steel sheet decreased.

[0295] In No. 3, the Si content of the hot-rolled steel sheet was too high, so the ductility was insufficient, and hot rolling became difficult.

[0296] In No. 4, the Mn content of the hot-rolled steel sheet was too high, so the region of the GAM value of 2.0° or more became excessive, and the elongation of the hot-rolled steel sheet decreased.

[0297] In No. 5, the Mn content of the hot-rolled steel sheet was too low, so the region of the GAM value of 0.6° or less became excessive, and the strength of the hot-rolled steel sheet decreased.

[0298] In No. 6, the Ti content of the hot-rolled steel sheet was too high, and thus, precipitates were excessively generated, and thus the hole expandability of the hot-rolled steel sheet was reduced.

[0299] In No. 7, the Ti content of the hot-rolled steel sheet is too low, so the strength-enhancing effect due to precipitation strengthening, grain refinement strengthening, and / or dislocation strengthening cannot be obtained. In addition, the nuclei for phase transformation cannot be sufficiently generated, and the prior austenite grains coarsen, thereby reducing the strength and hole expandability of the hot-rolled steel sheet.

[0300] In No. 8, the Al content of the hot-rolled steel sheet was too high, so cracking of the slab occurred, making hot rolling difficult.

[0301] In No. 9, the Al content of the hot-rolled steel sheet was too low, so deoxidation was insufficient, and inclusions were excessively generated, thereby reducing the hole expandability of the hot-rolled steel sheet.

[0302] In No. 10, the P content of the hot-rolled steel sheet was too high, so cracking of the slab due to embrittlement occurred, making hot rolling difficult.

[0303] In No. 11, the S content of the hot-rolled steel sheet was too high, and thus, inclusions were excessively generated, thereby reducing the hole expandability of the hot-rolled steel sheet.

[0304] In No. 12, the N content of the hot-rolled steel sheet was too high, so cracking of the slab due to embrittlement occurred, making hot rolling difficult.

[0305] In No. 13, the O content of the hot-rolled steel sheet was too high, and thus oxides were excessively generated, thereby reducing the hole expandability of the hot-rolled steel sheet.

[0306] In No. 15, the finish rolling start temperature ST was too low, so the prior austenite grains were coarsened and the hole expandability of the hot-rolled steel sheet was reduced.

[0307] For No. 16, the finishing rolling start temperature ST is too high, and the finishing rolling end temperature FT is also too high, so the steel structure cannot be properly controlled, the original austenite grains of the hot-rolled steel sheet are coarsened, and the prescribed relationships (1) and (2) are not satisfied, thereby reducing the collision characteristics of the hot-rolled steel sheet.

[0308] In No. 17, high temperature differential rolling with a rolling temperature difference ΔT of 30° C. or more was performed less than once, and thus high-quality steel structure could not be produced. The hot-rolled steel sheet did not satisfy the prescribed relationship (2), and the hole expandability of the hot-rolled steel sheet was reduced.

[0309] In No. 18, the total reduction ratio after the second high temperature differential rolling was too low, so the steel structure could not be properly controlled, and the hot-rolled steel sheet did not satisfy the prescribed relationship (2), resulting in reduced hole expandability of the hot-rolled steel sheet.

[0310] In No. 19, the time from the end of finish rolling to the start of cooling was too long, so the prior austenite grain size exceeded 25 μm due to coarsening of the crystal grains, and the hole expandability of the hot-rolled steel sheet was reduced.

[0311] In No. 20, the cooling stop temperature of the accelerated cooling was too high, so the region with a GAM value of 2.0° or more increased excessively, and the elongation of the hot-rolled steel sheet decreased.

[0312] In No. 21, the cooling stop temperature of the accelerated cooling was too low, so the region with a GAM value of 2.0° or more increased excessively, and the elongation of the hot-rolled steel sheet decreased.

[0313] In No. 38, the finishing rolling start temperature ST is too high, so the steel structure cannot be properly controlled, the prior austenite grains of the hot-rolled steel sheet coarsen, and the hot-rolled steel sheet does not satisfy the prescribed relationship (2), thereby reducing the collision characteristics of the hot-rolled steel sheet.

[0314] For No. 39, although the finishing rolling start temperature ST was properly controlled, the finishing rolling end temperature FT was too high, so the steel structure could not be properly controlled, the original austenite grains of the hot-rolled steel sheet coarsened, and the hot-rolled steel sheet did not satisfy the prescribed relationship (2), thereby reducing the collision characteristics of the hot-rolled steel sheet.

[0315] For No. 40, the slow cooling time in the temperature range of 720°C to 470°C was too short, so the region with a GAM value exceeding 0.6° and less than 2.0° was not sufficiently generated in the hot-rolled steel sheet. The hot-rolled steel sheet did not satisfy the prescribed relationship (2), and the elongation of the hot-rolled steel sheet was reduced.

[0316] Regarding No. 44, similarly to No. 7, the Ti content of the hot-rolled steel sheet is too low, so the strength-enhancing effect due to precipitation strengthening, grain refinement strengthening, and / or dislocation strengthening cannot be obtained. In addition, the nuclei for phase transformation cannot be sufficiently generated, the prior austenite grains coarsen, and the strength and hole expandability of the hot-rolled steel sheet are reduced.

[0317] In contrast, hot-rolled steel sheets Nos. 14, 22-37, and 41-43 exhibited an excellent balance among strength, elongation, and hole expandability, as well as superior crash characteristics. In No. 42, the second hot-difference rolling was performed at 990°C, and then at a specified temperature between 980°C and 1000°C. This resulted in significantly superior crash characteristics compared to No. 34, which performed the second hot-difference rolling at 965°C. The results for Nos. 14, 22-37, and 41-43 indicate that hot-rolled steel sheets meeting the following requirements (A) to (D) exhibit an excellent balance among strength, elongation, and hole expandability, as well as superior crash characteristics.

[0318] (A) The hot-rolled steel sheet contains, 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% to 0.080%, S: 0% to 0.0100%, N: 0% to 0.0050%, O: 0% to 0.010%, Nb: 0% to 0.100%, V: 0% to 1.000%, Cu: 0% to 1.000%, Cr: 0% to 2.000%, and Mo: 0% to 3.0 00%, Ni: 0% to 0.500%, B: 0% to 0.0100%, Ca: 0% to 0.0500%, Mg: 0% to 0.050%, 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% and Sn: 0% to 0.050%, and the remainder is composed of Fe and impurities.

[0319] (B) The prior austenite grain size of the hot-rolled steel sheet is 25 μm or less.

[0320] (C) In the hot-rolled steel sheet, the area ratio of regions having a GAM value exceeding 0.6° and less than 2.0° is 50% or more and less than 100%, the area ratio of regions having a GAM value of 0.6° or less is 0% or more and less than 50%, and the area ratio of regions having a GAM value of 2.0° or more is greater than 0% and less than 50%.

[0321] (D) The hot-rolled steel sheet satisfies the following relationships (1) and (2):

[0322] 1.7≤LGr / LGt (1)

[0323] 1.20≤(LGr / LGt) / (LMr / LMt) (2)

[0324] LGr: Area average of the projected length of the original austenite grains in the rolling direction

[0325] LGt: Area average of the projected length of the prior austenite grains in the plate thickness direction

[0326] LMr: Area average of the projected length in the rolling direction of the region having a GAM value of 2.0° or more LMt: Area average of the projected length in the plate thickness direction of the region having a GAM value of 2.0° or more.

Claims

1. A hot-rolled steel sheet, comprising, by mass%, 0.045% to 0.120% of C, 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%~0.080%, S: 0% to 0.0100%, N: 0% to 0.0050%, O: 0% to 0.010%, Nb: 0% to 0.100%, V: 0%~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 0.050%, REM: 0%~0.100%, Bi: 0% to 0.100%, Ta: 0%~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%, and Sn: 0%~0.050%, The remainder consists of Fe and impurities; and The original austenite grain size is less than 25μm, The area ratio of the region where the GAM value exceeds 0.6° and is less than 2.0° is 50% or more and less than 100%. The area ratio of the region with a GAM value of 0.6° or less is 0% or more and less than 50%, The area ratio of regions with a GAM value of 2.0° or more exceeds 0% and is 50% or less. The following relations (1) and (2) are satisfied: 1.7≤LGr / LGt (1) 1.20≤(LGr / LGt) / (LMr / LMt) (2) LGr: Area average of the projected length of the original austenite grains in the rolling direction LGt: Area average of the projected length of the prior austenite grains in the plate thickness direction LMr: Area average of the rolling direction projection length of the area with a GAM value of 2.0° or more LMt: Area average of the projected length in the plate thickness direction of the region with a GAM value of 2.0° or more.

2. The hot-rolled steel sheet according to claim 1, wherein The area ratio of the region with a GAM value of 0.6° or less is 0% to 45%.

3. The hot-rolled steel sheet according to claim 1 or 2, wherein: The area ratio of the region having a GAM value of 2.0° or greater is more than 0% and 20% or less.

4. The hot-rolled steel sheet according to any one of claims 1 to 3, wherein The following relationship (1-1) is satisfied: 1.7≤LGr / LGt≤10.0(1-1).

5. The hot-rolled steel sheet according to any one of claims 1 to 4, wherein The following relationship (2-1) is satisfied: 1.20≤(LGr / LGt) / (LMr / LMt)≤5.00(2-1).

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