Hot-rolled steel sheets and parts

Abstract

The hot-rolled steel sheet according to the present invention has a desired chemical composition, and in the internal region, the average aspect ratio of prior-austenite grains is 4.00-6.00, the area ratio of the martensite is 90% or more, and the value obtained by dividing the average aspect ratio of prior-austenite grains in the surface layer region by the average aspect ratio of prior-austenite grains in the internal region is less than 0.950.

Description

[Technical Field]

[0001] The present disclosure relates to a hot-rolled steel sheet and a part, and more particularly to a hot-rolled steel sheet having high strength, excellent bendability, and resistance to crack propagation in the thickness direction, and a part manufactured using the same. This application claims priority based on Japanese Patent Application No. 2023-178084, filed on October 16, 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, efforts have been made to reduce the weight of automobiles and machine parts. By ensuring rigidity through optimal part design, it is possible to reduce the weight of automobiles and machine parts. Furthermore, for blank-formed parts such as press-formed parts, weight can be reduced by reducing the thickness of the part material.

[0003] However, when trying to ensure the strength properties of parts, such as static fracture strength and yield strength, while reducing the plate thickness, it is necessary to use high-strength materials. In particular, the application of higher-strength steel sheets to automobile suspension parts, such as lower arms, trail links, and knuckles, has begun to be considered. These automobile suspension parts are manufactured by subjecting steel sheets to burring, stretch flange forming, bending, and the like. Therefore, steel sheets used for these automobile suspension parts are required to have excellent formability, particularly excellent bendability.

[0004] Furthermore, in the above-mentioned automobile suspension parts, if the steel sheet is made high-strength, cracks are likely to occur during forming or in the early stage of deformation during a collision. Therefore, the steel sheet used in the above-mentioned automobile suspension parts is also required to have excellent collision resistance properties.

[0005] For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet having a structure in which a martensite phase accounts for 95% or more in area ratio at a quarter-thickness position of the steel sheet and the average aspect ratio of prior austenite grains is 3.0 or more, and in a stress relaxation test, the 5-minute relaxation stress value when 400 MPa is applied is 20 MPa or less and the tensile strength is 1180 MPa or more. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent No. 7010418 Summary of the Invention [Problem to be solved by the invention]

[0007] However, no consideration is given to crashworthiness in Patent Document 1. The present inventors thought that improvement in crashworthiness could be expected by improving resistance to crack propagation in the plate thickness direction.

[0008] The present disclosure has been made in view of the above-mentioned problems, and aims to provide a hot-rolled steel sheet having high strength, excellent bendability, and resistance to crack propagation in the plate thickness direction, and a part manufactured using the same. [Means for solving the problem]

[0009] The present inventors have found that high strength and excellent bendability can be obtained in a hot-rolled steel sheet by controlling the average aspect ratio of prior austenite grains in the inner region. Furthermore, the inventors have found that by making the average aspect ratio of the prior austenite grains in the surface region smaller than the average aspect ratio of the prior austenite grains in the inner region, it is possible to obtain excellent bendability and resistance to crack propagation in the plate thickness direction in a high-strength hot-rolled steel plate.

[0010] The gist of the present disclosure, which was made based on the above findings, is as follows. (1) Chemical composition, in mass%, C: 0.050~0.120%, Si: 0 to 3.00% Mn: 1.20~3.00%, Al: 0.010~0.400%, P: 0~0.080%, S: 0 to 0.0100%, N: 0 to 0.0050%, O: 0 to 0.0100%, Ti: 0 to 0.180% Nb: 0 to 0.100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0~2.000%, Mo: 0 to 3.000%, Ni: 0 to 0.500% B: 0~0.0100%, Ca: 0 to 0.0500%, Mg: 0 to 0.050% REM: 0 to 0.1000%, 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% the balance being Fe and impurities, In the internal region which is a region from the surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness from the surface, The average aspect ratio of prior austenite grains is 4.00 to 6.00, The area ratio of martensite is 90% or more, a value obtained by dividing an average aspect ratio of prior austenite grains in a surface region that is a region from the surface to a depth of 1 / 15 of the plate thickness from the surface by the average aspect ratio of the prior austenite grains in the internal region is less than 0.950. (2) The chemical composition is in mass%: Ti: 0.001 to 0.180%, 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: 0.001 to 0.050%, REM: 0.0001 to 0.1000%, 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 The hot-rolled steel sheet according to (1) above, characterized in that it contains one or more selected from the group consisting of Sn: 0.001 to 0.050%. (3) The hot-rolled steel sheet according to (1) or (2), characterized in that the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface layer region by the average aspect ratio of the prior austenite grains in the internal region is 0.900 or less. (4) The hot-rolled steel sheet according to any one of (1) to (3) above, wherein the average aspect ratio of the prior austenite grains in the inner region is 4.00 to 5.50. (5) A part comprising the hot-rolled steel sheet according to any one of (1) to (4) above. [Effects of the Invention]

[0011] According to the above aspects of the present disclosure, it is possible to provide a hot-rolled steel sheet having high strength, excellent bendability, and resistance to crack propagation in the plate thickness direction, and a part manufactured using the same. DETAILED DESCRIPTION OF THE INVENTION

[0012] A hot-rolled steel sheet and a part (hereinafter, sometimes referred to as a hot-rolled steel sheet and a part according to the present embodiment) according to an embodiment of the present disclosure will be described. However, the present disclosure is not limited to the configuration disclosed in the present embodiment, and various modifications are possible within the scope of the present disclosure.

[0013] The individual constituent elements of the present disclosure will be described in detail below. First, the reasons for limiting the chemical composition of the hot-rolled steel sheet according to the present embodiment will be described. Below, numerical ranges indicated with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical compositions are mass % unless otherwise specified.

[0014] The chemical composition of the hot-rolled steel sheet according to this embodiment contains, in mass%, C: 0.050-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, Al: 0.010-0.400%, P: 0-0.080%, S: 0-0.0100%, N: 0-0.0050%, O: 0-0.0100%, and the balance: Fe and impurities. Each element will be described in detail below.

[0015] C: 0.050 to 0.120% C is an important element for improving the strength of a hot-rolled steel sheet. To obtain the desired strength, the C content is set to 0.050% or more. The C content is preferably 0.055% or more, 0.060% or more, or 0.070% or more. On the other hand, if the C content exceeds 0.120%, the bendability of the hot-rolled steel sheet deteriorates. Therefore, the C content is set to 0.120% or less. The C content is preferably 0.100% or less, and more preferably 0.090% or less.

[0016] Si: 0 to 3.00% Si is an element that has the effect of suppressing the formation of carbides during ferrite transformation and improving the toughness of the hot-rolled steel sheet. Since Si is not necessarily contained, the Si content may be 0%. To reliably obtain the above effect, the Si content is preferably 0.10% or more. The Si content is more preferably 0.50% or more, or 0.70% or more. On the other hand, if the Si content exceeds 3.00%, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.00% or less, 1.50% or less, or 1.20% or less.

[0017] Mn: 1.20~3.00% Mn is an element that is effective in improving the hardenability and the strength of the hot-rolled steel sheet by solid solution strengthening. To obtain the desired strength, the Mn content is set to 1.20% or more. The Mn content is preferably 1.30% or more, more preferably 1.50% or more. On the other hand, if the Mn content exceeds 3.00%, MnS, which adversely affects the bendability of the hot-rolled steel sheet, is likely to be formed. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, 2.50% or less, or 2.20% or less.

[0018] Al: 0.010 to 0.400% Al has the effect of improving the quality of steel by deoxidizing it and also has the effect of controlling ferrite transformation. Furthermore, if the Al content is less than 0.010%, the bendability of the hot-rolled steel sheet deteriorates. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.015% or more, 0.020% or more, or 0.030% or more. On the other hand, if the Al content exceeds 0.400%, cluster-like precipitates of alumina are generated, which increases the cracking susceptibility of the slab and makes the slab difficult to handle. Therefore, the Al content is set to 0.400% or less. The Al content is preferably 0.300% or less, 0.250% or less, or 0.200% or less.

[0019] P: 0 to 0.080% P is an element that affects the weldability of hot-rolled steel sheets. In particular, if the P content exceeds 0.080%, the weldability of the hot-rolled steel sheets deteriorates significantly. Furthermore, the cracking sensitivity of the slabs increases, making the slabs difficult to handle. Therefore, the P content is set to 0.080% or less. The P content is preferably 0.040% or less, 0.020% or less, or 0.010% or less. The P content may be 0%. From the viewpoint of refining costs, the P content may be 0.001% or more.

[0020] S: 0 to 0.0100% S is an element that affects the bendability of hot-rolled steel sheets. In particular, if the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which are harmful to the bendability of the hot-rolled steel sheet, are generated. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, or 0.0060% or less. The S content may be 0%. From the viewpoint of refining costs, the S content may be 0.0001% or more.

[0021] N: 0 to 0.0050% N is an element that combines with Ti to form Ti nitrides. In particular, if the N content exceeds 0.0050%, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the N content is set to 0.0050% or less. The N content is preferably 0.0040% or less, or 0.0030% or less. The N content may be 0%. From the viewpoint of refining costs, the N content may be 0.0001% or more.

[0022] O: 0 to 0.0100% O is an element that, when contained in steel in large amounts, forms coarse oxides that become fracture initiation sites, causing brittle fracture and hydrogen-induced cracking. If the O content exceeds 0.0100%, brittle fracture and hydrogen-induced cracking are more likely to occur. In addition, the bendability of the hot-rolled steel sheet deteriorates. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0030% or less. Since O may not be contained, the O content may be 0%. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more, or 0.0010% or more.

[0023] The hot-rolled steel sheet according to this embodiment may contain the above chemical components, with the balance being Fe and impurities. In this embodiment, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, and / or substances that are allowed to exist within a range that does not adversely affect the properties of the hot-rolled steel sheet according to this embodiment.

[0024] Although not essential for providing the desired properties, the following optional elements may be contained to reduce manufacturing variations and further improve the strength of the hot-rolled steel sheet. However, since the inclusion of these elements is not essential, the lower limit of the content of these elements is 0%.

[0025] Ti: 0.001 to 0.180% Ti precipitates in steel as carbides or nitrides, and has the effect of refining the metal structure through a pinning effect and increasing the strength and yield ratio of the hot-rolled steel sheet through precipitation strengthening. To ensure these effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.180%, excessive precipitation of TiC will occur, deteriorating the bendability of the hot-rolled steel sheet. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0.160% or less, or 0.150% or less.

[0026] Nb: 0.001 to 0.100% Nb has the effect of increasing the strength of the hot-rolled steel sheet by refining the crystal grain size of the hot-rolled steel sheet and by precipitation strengthening of NbC. To reliably obtain this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, the above effects saturate. Also, the bendability of the hot-rolled steel sheet deteriorates. Therefore, even when Nb is contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, or 0.060% or less.

[0027] V: 0.001 to 1.000% V has the effect of increasing the strength of hot-rolled steel sheets by strengthening through precipitates, strengthening through grain refinement by inhibiting the growth of ferrite crystal grains, and strengthening through dislocation strengthening by inhibiting recrystallization. To reliably obtain these effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the V content is excessive, a large amount of carbonitrides will precipitate, deteriorating the bendability of the hot-rolled steel sheet. Therefore, the V content is set to 1.000% or less. The V content is preferably 0.800% or less, or 0.600% or less.

[0028] Cu: 0.001 to 1.000% Cu exists in the form of fine particles in steel and has the effect of increasing the strength of the hot-rolled steel sheet. To reliably obtain this effect, the Cu content is preferably 0.001% or more. The Cu content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Cu content is excessive, the weldability of the hot-rolled steel sheet deteriorates. Therefore, the Cu content is set to 1.000% or less. The Cu content is preferably 0.800% or less, and more preferably 0.600% or less.

[0029] Cr: 0.001 to 2.000% Cr is an element effective in improving the strength of hot-rolled steel sheets. To reliably obtain this effect, the Cr content is preferably 0.001% or more. The Cr content is more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if the Cr content is excessive, the bendability of the hot-rolled steel sheet deteriorates. Therefore, the Cr content is set to 2.000% or less. The Cr content is preferably 1.500% or less, 1.200% or less, or 1.000% or less.

[0030] Mo: 0.001 to 3.000% Mo is an element effective in strengthening ferrite precipitation. To ensure this effect, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if the Mo content is excessive, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the Mo content is set to 3.000% or less. The Mo content is preferably 2.500% or less, 2.000% or less, or 1.500% or less.

[0031] Ni: 0.001 to 0.500% Ni has the effect of suppressing phase transformation at high temperatures and increasing the strength of the hot-rolled steel sheet. To reliably obtain this effect, the Ni content is preferably 0.001% or more. The Ni content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Ni content is excessive, the weldability of the hot-rolled steel sheet deteriorates. Therefore, the Ni content is set to 0.500% or less. The Ni content is preferably 0.300% or less, and more preferably 0.150% or less.

[0032] B: 0.0001 to 0.0100% B has the effect of suppressing phase transformation at high temperatures and increasing the strength of the hot-rolled steel sheet. To reliably obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the B content is excessive, B precipitates are formed, reducing the strength of the hot-rolled steel sheet. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, and more preferably 0.0050% or less.

[0033] Ca: 0.0001 to 0.0500% Ca disperses many fine oxides during deoxidation of molten steel, thereby refining the structure of the hot-rolled steel sheet. Ca also fixes S in the steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and improves the bendability of the hot-rolled steel sheet. To ensure these effects, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more, or 0.0010% or more. On the other hand, even if the Ca content exceeds 0.0500%, the above effect saturates. Therefore, the Ca content is set to 0.0500% or less. The Ca content is preferably 0.0300% or less, and more preferably 0.0200% or less.

[0034] Mg: 0.001 to 0.050% Mg has the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of the hot-rolled steel sheet. To ensure this effect, the Mg content is preferably 0.001% or more. The Mg content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Mg content exceeds 0.050%, excessive inclusions are formed in the steel, resulting in a decrease in the yield ratio of the hot-rolled steel sheet. Therefore, the Mg content is set to 0.050% or less. The Mg content is preferably 0.040% or less, or 0.030% or less.

[0035] REM: 0.0001 to 0.1000% REM has the effect of increasing the yield ratio of hot-rolled steel sheets by adjusting the shape of inclusions in steel to a preferred shape. To reliably obtain this effect, the REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the REM content exceeds 0.1000%, excessive inclusions are generated in the steel, resulting in a decrease in the yield ratio of the hot-rolled steel sheet. Therefore, the REM content is set to 0.1000% or less. The REM content is preferably 0.0800% or less, or 0.0600% or less. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metal.

[0036] Bi: 0.001 to 0.100% Bi has the effect of increasing the yield ratio of a hot-rolled steel sheet by refining the solidification structure. To reliably obtain this effect, the Bi content is preferably 0.001% or more. The Bi content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Bi content exceeds 0.100%, the above effects become saturated, which is economically undesirable. Therefore, the Bi content is set to 0.100% or less. The Bi content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.

[0037] Ta: 0.001 to 0.100% Ta, like V, has the effect of increasing the strength of the hot-rolled steel sheet by forming fine carbides in the steel. To ensure this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Ta content exceeds 0.100%, the bendability 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.

[0038] Zr: 0.001 to 0.500% Zr has the effect of increasing the strength of the hot-rolled steel sheet through solid solution strengthening. To reliably obtain this effect, the Zr content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if the Zr content exceeds 0.500%, the bendability of the hot-rolled steel sheet deteriorates. Therefore, the Zr content is set to 0.500% or less. The Zr content is preferably 0.300% or less, and more preferably 0.100% or less.

[0039] Co: 0.001 to 3.000% Co has the effect of increasing the strength of the hot-rolled steel sheet through solid solution strengthening. To reliably obtain this effect, the Co content is preferably 0.001% or more. The Co content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Co content exceeds 3.000%, the bendability of the hot-rolled steel sheet deteriorates. 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.

[0040] Zn: 0.001 to 0.200% Zn has the effect of increasing the strength of the hot-rolled steel sheet through solid solution strengthening. To reliably obtain this effect, the Zn content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if the Zn content exceeds 0.200%, the bendability of the hot-rolled steel sheet deteriorates. 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.

[0041] W: 0.001 to 0.200% W has the effect of increasing the strength of the hot-rolled steel sheet through solid solution strengthening. To reliably obtain this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005%, 0.010%, or more. On the other hand, if the W content exceeds 0.200%, the bendability of the hot-rolled steel sheet deteriorates. Therefore, the W content is set to 0.200% or less. The W content is preferably 0.150% or less, more preferably 0.100% or less.

[0042] Sb: 0.001 to 0.500% Sb has the effect of suppressing the generation of oxides that serve as fracture initiation sites, thereby improving the bendability of the hot-rolled steel sheet. To ensure this effect, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, even if Sb is contained in a large amount, the above effect saturates, so the Sb content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.100% or less.

[0043] As: 0.001 to 0.050% As has the effect of reducing the austenite single-phase temperature, thereby refining prior austenite grains and improving the bendability of the hot-rolled steel sheet. To reliably obtain this effect, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more, more preferably 0.010% or more. On the other hand, since the above effects saturate even when As is contained in a large amount, the As content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

[0044] Sn: 0.001 to 0.050% Sn has the effect of improving the bendability of the hot-rolled steel sheet by suppressing the generation of oxides that serve as the starting point for fracture. To reliably obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, even if Sn is contained in a large amount, the above effect saturates, so the Sn content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

[0045] The chemical composition of the above-mentioned hot-rolled steel sheet can be analyzed using a spark discharge optical emission spectrometer or the like. Values ​​of C and S are determined by burning the steel sheet in an oxygen stream using a gas composition analyzer or the like and measuring the values ​​by infrared absorption. Values ​​of O and N are determined by melting a test piece taken from the steel sheet in a helium stream and measuring the values ​​by thermal conductivity. If the hot-rolled steel sheet has a plating layer or a coating film on the surface, the plating layer or the coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

[0046] Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described. In the hot-rolled steel sheet according to this embodiment, in an internal region from the surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness from the surface, the average aspect ratio of prior austenite grains is 4.00 to 6.00, the area fraction of martensite is 90% or more, and the value obtained by dividing the average aspect ratio of prior austenite grains in the surface layer region from the surface to a depth of 1 / 15 of the plate thickness from the surface by the average aspect ratio of the prior austenite grains in the internal region is less than 0.950.

[0047] As described above, in this embodiment, the internal region refers to the region extending from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface. In other words, it refers to the region extending from 1 / 8 of the plate thickness depth from the surface as the starting point to 3 / 8 of the plate thickness depth from the surface as the ending point. The surface region refers to the region from the surface to a depth of 1 / 15 of the plate thickness from the surface. In other words, it refers to the region that starts at the surface and ends at a depth of 1 / 15 of the plate thickness from the surface. When the hot-rolled steel sheet has a plating layer, a coating film, or the like on the surface, the surface here refers to the interface between the steel sheet and the plating layer, the coating film, or the like.

[0048] internal area Average aspect ratio of prior austenite grains: 4.00 to 6.00 If the average aspect ratio of the prior austenite grains in the inner region is less than 4.00, the strength of the hot-rolled steel sheet decreases and the resistance to crack propagation in the sheet thickness direction deteriorates. Therefore, the average aspect ratio of the prior austenite grains in the inner region is set to 4.00 or more. The average aspect ratio of the prior austenite grains in the inner region is preferably 4.20 or more, 4.50 or more, or 5.00 or more. On the other hand, if the average aspect ratio of the prior austenite grains in the inner region exceeds 6.00, the bendability of the hot-rolled steel sheet deteriorates. Therefore, the average aspect ratio of the prior austenite grains in the inner region is set to 6.00 or less. The average aspect ratio of the prior austenite grains in the inner region is preferably 5.80 or less, 5.50 or less, or 5.30 or less. From the viewpoint of obtaining better bendability in the hot-rolled steel sheet, the average aspect ratio of the prior austenite grains in the inner region is preferably 4.00 to 5.50.

[0049] The aspect ratio of a prior austenite grain is the value obtained by dividing the major axis of the prior austenite grain by the minor axis, and is a value of 1.00 or more.

[0050] The average aspect ratio of prior austenite grains is measured by the following method. A sample is taken from a hot-rolled steel sheet at a position 1 / 4 from the end face in the sheet width direction so that the metal structure of the cross section (thickness direction × rolling direction cross section) normal to the sheet width direction can be observed. The size of the sample depends on the measuring device, but it may be, for example, a rectangular parallelepiped measuring the entire thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the observation surface is mirror-polished and then etched using an etching solution (a saturated aqueous solution of picric acid, an aqueous solution containing a surfactant and oxalic acid) by the Bechet-Beaujard method in accordance with JIS G 0551:2020. Grains that turn black due to the corrosion are identified as prior austenite grains. The observation surface revealing the prior austenite grains is observed using an optical microscope, and eight fields of view, each 200 μm in the thickness direction × 600 μm in the rolling direction, are photographed at a magnification of 1000x or more for the internal region (the region from 1 / 8 of the sheet thickness depth from the surface to 3 / 8 of the sheet thickness depth from the surface). The ratio of the major axis to the minor axis measured for each prior austenite grain is calculated from the photographed structure, and the ratio is weighted by the area of ​​each prior austenite grain to calculate the average, thereby obtaining the average aspect ratio of the prior austenite grains. For example, if the major axis / minor axis of a prior austenite grain G1 is r1 and its area is A1, and the major axis / minor axis of another prior austenite grain G2 is r2 and its area is A2, the average aspect ratio of the two prior austenite grains is calculated as (A1 × r1 + A2 × r2) / (A1 + A2). If the prior austenite grains cannot be sufficiently revealed by the above-mentioned method, the prior austenite grains are identified by the reconstruction method described in "Studies on High-Precision Reconstruction Methods for Austenite Structures in Steel" (Hata Kengo, Wakita Masayuki, Fujiwara Tomoya, Kono Kaori, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30), and the average aspect ratio of the prior austenite grains is determined. The EBSD measurement data used in the reconstruction method is obtained as follows. After colloidal polishing or electrolytic polishing is performed on the above-mentioned observation field (field of view of 200 μm in the sheet thickness direction × 600 μm in the rolling direction), crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.1 μm. For the measurements, an EBSD analysis system consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high speed EBSD detector) was used. The vacuum level inside the system was 9.6 × 10 -5 The acceleration voltage is 25 kV and the probe current level is 16. To display the EBSD map, version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX / TSL Solution is used for the obtained crystal orientation information.

[0051] When prior austenite grains having an equivalent circle diameter of less than 2 μm are included, they are excluded from the above measurement because prior austenite grains having an equivalent circle diameter of less than 2 μm do not adversely affect the properties of the hot-rolled steel sheet according to this embodiment. The rolling direction of the hot-rolled steel sheet is determined by the following method. Test specimens are taken so that the thickness cross section of the hot-rolled steel plate can be observed. The direction perpendicular to the plate surface is the Z direction, and a total of 12 test specimens are taken by rotating the plate 30° around this Z direction. The thickness cross section of the taken test specimens is polished, and the prior austenite grain boundaries are revealed using the above-mentioned etching solution. The average aspect ratio of the prior austenite grains is calculated using the intercept method. The test specimen with the largest average aspect ratio of the prior austenite grains is identified, and the direction from which the test specimen was taken is determined to be the rolling direction of the hot-rolled steel plate. In other words, the direction parallel to the thickness cross section of the test specimen and perpendicular to the thickness direction is determined to be the rolling direction of the hot-rolled steel plate.

[0052] Martensite area ratio: 90% or more Martensite is a structure that increases the strength of a hot-rolled steel sheet. If the area fraction of martensite is less than 90%, the desired strength cannot be obtained. Therefore, the area fraction of martensite is set to 90% or more. The area fraction of martensite is preferably 92% or more, 94% or more, or 96% or more. The area fraction of martensite may be 100%.

[0053] Residual tissue: 10% or less The metal structure of the internal region of the hot-rolled steel sheet according to this embodiment may contain, in addition to martensite, bainite, ferrite, pearlite, and retained austenite as a residual structure. The area ratio of the residual structure may be 10% or less in relation to the area ratio of martensite. Furthermore, the area ratio of the residual structure may be 8% or less, 6% or less, or 4% or less in relation to the area ratio of martensite. Since the residual structure does not need to be included, the area ratio of the residual structure may be 0%.

[0054] The area ratio of martensite and the remaining structure is measured by the following method. A test piece is taken from the hot-rolled steel sheet so that the metal structure can be observed at the 1 / 4 position in the thickness direction (from the surface to the 1 / 8 position in the thickness direction to the 3 / 8 position in the thickness direction). The cross section of the test piece is mirror-polished and etched with LePera. Then, a 200 μm (thickness direction) × 600 μm (perpendicular to the thickness direction) area at the 1 / 4 position in the thickness direction is observed using a thermal field emission scanning electron microscope (FE-SEM) (JEOL JSM-7001F), and image analysis is performed.

[0055] In Repelle corrosion, martensite and retained austenite are not corroded, so by calculating the area ratio of the uncorroded area, the total area ratio of martensite and retained austenite is obtained.

[0056] The area fraction of retained austenite is obtained by X-ray diffraction. A test piece taken from a hot-rolled steel sheet is ground from the sheet surface to the 1 / 4 position in the sheet thickness direction (from the surface to the 1 / 8 position in the sheet thickness direction to the 3 / 8 position in the sheet thickness direction), and the exposed surface is used as the observation surface. This observation surface is mirror-polished and then finished by electrolytic polishing. For the observation surface, the integrated intensity of a total of five peaks, α(200), α(211), γ(200), γ(220), and γ(311), is determined using a Rigaku RINT-2500 Mo-Kα, and the volume fraction of retained austenite is calculated using the intensity averaging method. This volume fraction of retained austenite is considered to be the area fraction of retained austenite.

[0057] The total area fraction of martensite is obtained by subtracting the area fraction of retained austenite obtained by X-ray diffraction from the sum of the area fractions of martensite and retained austenite obtained by observation using the Fe-SEM. If the total area fraction of martensite is calculated to be a negative value, the total area fraction of martensite is considered to be 0%.

[0058] The area ratio of pearlite is obtained by the following method. For the same area (200 μm × 600 μm) as that used to determine the area ratio of martensite and retained austenite, only the corroded layer is removed by polishing and the specimen is mirror-finished, then etched using nital solution, and observed using an FE-SEM, followed by image analysis. The area where cementite and ferrite are arranged in a lamellar shape is determined as pearlite, and the area ratio of this area is calculated to obtain the area ratio of pearlite.

[0059] The area ratio of ferrite is obtained by the following method. The following operation is performed on regions other than the region determined to be pearlite by the above method. The same area (200 μm × 600 μm) used to determine the area fraction of martensite and retained austenite was subjected to colloidal polishing or electrolytic polishing, and then crystal orientation information was obtained by electron backscatter diffraction at a measurement interval of 0.2 μm. For the measurements, an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity® ultra-high-speed EBSD detector) was used. The vacuum level in the analyzer was 9.6 × 10 Pa or less, the acceleration voltage was 25 kV, and the probe current level was 16.

[0060] The following analysis is performed on the obtained crystal orientation information using version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX / TSL Solutions. Measurement points with a crystal orientation misorientation of 15° or more are considered to be crystal grain boundaries, and the area surrounded by these crystal grain boundaries is considered to be crystal grains. Next, the difference in crystal orientation between all measurement points within a crystal grain is calculated, and the average value of these differences is calculated to obtain the GAM value (Grain Average Misorientation) of the crystal grain. Crystal grains with a GAM value of 0.5° or less are considered to be ferrite, and their area fraction is calculated to obtain the ferrite area fraction.

[0061] The area fraction of bainite is obtained by subtracting the area fractions of martensite, retained austenite, pearlite, and ferrite obtained above from 100%. If the area fraction of bainite is calculated as a negative value, the area fraction of bainite is set to 0%. In this embodiment, the area ratio of the metallographic structure is calculated by image analysis using FE-SEM, X-ray diffraction, and EBSD analysis, so the total of each structure may not be 100%. In such cases, the area ratio of each structure is corrected so that the total is 100%. For example, if the total of the area ratios of each structure is 103%, the area ratio of each structure is corrected by multiplying it by "100 / 103".

[0062] The observation conditions for FE-SEM are as follows: Electron gun type: Thermal emission type Current exposure number: 9 WD (Working Distance): 10mm Accelerating voltage: 20 kV Objective aperture number: 4 Number of pixels: 5120 x 3840

[0063] The average aspect ratio of the prior austenite grains in the surface region divided by the average aspect ratio of the prior austenite grains in the inner region: less than 0.950 If the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface region by the average aspect ratio of the prior austenite grains in the inner region is 0.950 or more, the desired bendability cannot be obtained in the hot-rolled steel sheet. Therefore, the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface region by the average aspect ratio of the prior austenite grains in the inner region is set to be less than 0.950. The value obtained by dividing the average aspect ratio of the prior austenite grains in the surface region by the average aspect ratio of the prior austenite grains in the inner region is preferably 0.930 or less, or 0.900 or less. The lower limit of the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface region by the average aspect ratio of the prior austenite grains in the inner region is not particularly limited, but may be 0.800 or more, or 0.850 or more.

[0064] The average aspect ratio of the prior austenite grains in the surface region is obtained by measuring the surface region (the region from the surface to a depth of 1 / 15 of the plate thickness from the surface) using the same method as that used to measure the average aspect ratio of the prior austenite grains in the internal region.

[0065] Tensile strength (TS): {(C-0.06) / 0.01} x 45 + 1180 MPa or more The hot-rolled steel sheet according to this embodiment may have a tensile strength of {(C-0.06) / 0.01}×45+1180 MPa or more, where C is the C content in mass% in the chemical composition. By achieving a tensile strength of this value or more, the applicable parts are not limited, and the contribution to vehicle weight reduction can be increased. There is no particular need to set an upper limit to the tensile strength, but from the viewpoint of suppressing die wear, it may be set to 1500 MPa or less or 1300 MPa or less.

[0066] Tensile strength is evaluated by conducting a tensile test in accordance with JIS Z 2241:2022. The test piece used is a No. 5 test piece of JIS Z 2241:2022. The tensile test piece is taken from a quarter section from the end in the plate width direction, with the direction perpendicular to the rolling direction as the longitudinal direction. If a No. 5 test piece cannot be obtained because the hot-rolled steel sheet or part is small or has a complex shape, a small rectangular piece with a parallel section of any width may be obtained and a tensile test may be performed using this small piece to determine the tensile strength. The direction of the rectangular piece is perpendicular to the rolling direction.

[0067] Maximum bending angle: 30° or more In the hot-rolled steel sheet according to this embodiment, the maximum bending angle obtained in a bending test based on the VDA standard described below may be 30° or more, and the maximum bending angle is preferably 40° or more.

[0068] The test specimens used for the bending test are 60 mm (rolling direction) x 30 mm (width direction) specimens taken from hot-rolled steel sheets. Using these specimens, bending tests are performed under the following conditions based on the VDA standard (VDA238-100:2017-04) specified by the German Association of the Automotive Industry. If the thickness of the test piece exceeds 1.6 mm, the surface on the punch side is ground to make the thickness 1.6 mm before the bending test. If the thickness of the test piece is 1.6 mm or less, the maximum bending angle obtained by the following formula is used. However, in the following formula, α t denotes the maximum bending angle obtained in the bending test, t denotes the plate thickness, and uEL denotes the uniform elongation (total elongation at the maximum test force). The uniform elongation is the value obtained by conducting a tensile test using the method described above. Maximum bending angle when plate thickness is 1.6 mm or less = α t -13.852×(1-t / 1.6)×(uEL+0.22) 0.292

[0069] Test piece dimensions: 60 mm (rolling direction) x 30 mm (width direction) Bending ridge: parallel to the width direction Test method: Roll support, punch press Roll diameter: φ30mm Punch shape: Tip R = 0.4 mm Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm Push-in speed: 20mm / min Testing machine: SHIMADZU AUTOGRAPH 20kN

[0070] Collision resistance properties: W2 / W1>0.150 In this embodiment, the impact resistance of a hot-rolled steel sheet is evaluated by the crack propagation resistance in the sheet thickness direction. The crack propagation resistance in the sheet thickness direction is obtained by calculating the ratio of energy W2 to energy W1 (W2 / W1) from the displacement-load curve when the hot-rolled steel sheet is punched. The hot-rolled steel sheet according to this embodiment may have W2 / W1 greater than 0.150. The hot-rolled steel sheet is punched using a φ10 cylindrical punch to form a circular hole with a diameter of 10 mm, with the burr facing the die side, under conditions of a clearance of 12.5%. Here, W2 is the integral of the load after the maximum load with respect to the displacement (∫Fds), and W1 is the integral of the load before the maximum load with respect to the displacement (∫Fds). Note that F is the load and s is the displacement.

[0071] The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 1.2 mm or more. It is preferably 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to obtain the above-mentioned metal structure after hot rolling. Therefore, the plate thickness may be 8.0 mm or less, and is preferably 6.0 mm or less.

[0072] The hot-rolled steel sheet according to this embodiment, having the above-described chemical composition and metallographic structure, may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized coating and electrolytic Zn-Ni alloy coating. Examples of hot-dip plated layers include hot-dip galvanized coating, alloyed hot-dip galvanized coating, hot-dip aluminum coating, hot-dip Zn-Al alloy coating, hot-dip Zn-Al-Mg alloy coating, and hot-dip Zn-Al-Mg-Si alloy coating. The coating weight is not particularly limited and may be the same as conventional coatings. Furthermore, it is possible to further enhance corrosion resistance by carrying out an appropriate chemical conversion treatment after plating (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0073] The hot-rolled steel sheet according to the present embodiment has high strength, excellent bendability, and resistance to crack propagation in the thickness direction, and therefore can be suitably used for parts, particularly automobile parts. Among automobile parts, the hot-rolled steel sheet according to the present embodiment can be suitably used for automobile suspension parts such as lower arms, trail links, and knuckles. These automobile parts may be made solely of the hot-rolled steel sheet according to the present embodiment, or may be formed by joining the hot-rolled steel sheet according to the present embodiment with another steel sheet. Parts manufactured using the hot-rolled steel sheet according to this embodiment have the same chemical composition as the above-described steel sheet. Furthermore, the parts may contain both processed and unprocessed parts. The unprocessed parts have the same metallurgical structure as the above-described steel sheet. The processed parts generally have the same metallurgical structure as the above-described steel sheet, but in cases where the parts have been heavily processed or at the edges of the parts, the metallurgical structure may not be present, or it may be difficult to determine whether the metallurgical structure is present. Therefore, when measuring the metallurgical structure of a part, the edges are avoided and the measurement is performed on the unprocessed parts. If there are no unprocessed parts, the measurement is performed on the parts that have not been heavily processed. An unprocessed or unprocessed part refers to, for example, a flat part of the part, a part where the thickness change due to processing is small, and a part that has not been subjected to punching, hole expansion, bending, or the like. As an example, in the case of the above-described part, a test piece is taken from the flat and largest area part near the center of gravity and examined.

[0074] Next, a preferred method for manufacturing the hot-rolled steel sheet according to this embodiment will be described. According to the manufacturing method described below, the hot-rolled steel sheet according to this embodiment can be stably manufactured. Note that the temperature of the slab and the temperature of the steel sheet in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.

[0075] A preferred method for producing a hot-rolled steel sheet according to this embodiment is as follows: (1) before rough rolling, a step of applying strain to a slab having the above-mentioned chemical composition one or more times so that the width direction strain is 3 to 15% in total; (2) A process of performing finish rolling on the strained slab so that the total reduction is 46 to 60% in a temperature range below 1040°C and the reduction is 20 to 30% in a temperature range of 1020 to 1040°C; (3) After the completion of finish rolling, accelerated cooling is performed at an average cooling rate of 30°C / s or more until the temperature reaches 200°C. Each step will be described below.

[0076] (1) Strain application before rough rolling Before rough rolling, it is preferable to impart strain to a slab having the above-described chemical composition one or more times so that the total width direction strain is 3 to 15%. This makes it possible to preferably control the ratio between the average aspect ratio of the prior austenite grains in the inner region and the average aspect ratio of the prior austenite grains in the surface region. The imparting of strain may be carried out after the slab has been heated for rough rolling.

[0077] The total strain applied to the slab in the width direction can be expressed as (1-w1 / w0) x 100 (%), where w0 is the width direction length of the slab before the first strain is applied and w1 is the width direction length of the slab after the final strain is applied. As a method for imparting strain in the width direction of the slab, for example, a method of imparting strain using a roll installed so that the rotation axis is perpendicular to the plate surface of the slab can be mentioned.

[0078] The slab to be strained is not particularly limited except for the chemical composition described above. For example, a slab can be produced by melting molten steel having the above chemical composition using a converter or electric furnace, and then by continuous casting. Instead of continuous casting, an ingot casting method, thin slab casting method, or the like may be used. The heating temperature for the slab before rough rolling may be in the range of 1100 to 1300°C.

[0079] (2) Finish rolling In the finish rolling, the strained slab is preferably rolled at a total reduction rate of 46 to 60% in a temperature range below 1040° C. and at a reduction rate of 20 to 30% in a temperature range of 1020 to 1040° C. By setting the total reduction rate to 46 to 60% in a temperature range below 1040° C., in combination with the above-mentioned strain application, the average aspect ratio of the prior austenite grains in the inner region can be preferably controlled.

[0080] The total reduction rate in the temperature range below 1040°C can be expressed as (1-t1 / t0) x 100 (%), where t0 is the entry thickness of the first rolling in the temperature range below 1040°C and t1 is the exit thickness of the last rolling in the temperature range below 1040°C.

[0081] Furthermore, by performing finish rolling at a rolling reduction rate of 20 to 30% in a temperature range of 1020 to 1040°C, the ratio between the average aspect ratio of the prior austenite grains in the inner region and the average aspect ratio of the prior austenite grains in the surface region can be preferably controlled. The reduction ratio can be expressed as (1-t3 / t2) x 100 (%), where t2 is the entry thickness and t3 is the delivery thickness.

[0082] (3) Accelerated cooling down to 200°C After the finish rolling is completed, it is preferable to accelerate cooling at an average cooling rate of 30° C. / s or more until the temperature reaches 200° C. By carrying out accelerated cooling under these conditions, it is possible to obtain the desired amount of martensite.

[0083] The average cooling rate in this embodiment is the temperature difference between the start point and the end point of the set range divided by the elapsed time from the start point to the end point. [Example]

[0084] Next, the effects of one embodiment of the present disclosure will be explained in more detail using examples, but the conditions in the examples are examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the object of the present disclosure is achieved.

[0085] Steels having the chemical compositions shown in Tables 1 and 2 were melted and continuously cast into slabs with thicknesses of 240 to 300 mm. The resulting slabs were used to obtain hot-rolled steel sheets shown in Table 4 under the manufacturing conditions shown in Table 3. The thicknesses of the obtained hot-rolled steel sheets were 1.2 to 8.0 mm.

[0086] For the obtained hot-rolled steel sheets, the average aspect ratio and metal structure of the prior austenite grains in the inner region, the average aspect ratio of the prior austenite grains in the surface region, tensile strength, maximum bending angle, and impact resistance index (W2 / W1) were determined using the methods described above. Regarding the maximum bending angle, when the thickness of the hot-rolled steel sheet was 1.6 mm or less, the value corrected by the above formula was used. In some cases, the metal structure of the inner region of the hot-rolled steel sheet contains, in addition to martensite, bainite, ferrite, pearlite, and retained austenite as residual structures. The measurement results are shown in Table 4.

[0087] Evaluation method for the properties of hot-rolled steel sheets If the tensile strength was {(C-0.06) / 0.01}×45+1180 MPa or more, the hot-rolled steel sheet was judged to have high strength and passed the test. On the other hand, if the tensile strength was less than {(C-0.06) / 0.01}×45+1180 MPa, the hot-rolled steel sheet was judged to have low strength and failed the test.

[0088] When the maximum bending angle was 30% or more, the hot-rolled steel sheet was judged to have excellent bendability and to have passed the test. On the other hand, when the maximum bending angle was less than 30%, the hot-rolled steel sheet was judged to have poor bendability and to have passed the test.

[0089] When W2 / W1 was more than 0.150, the hot-rolled steel sheet was judged to have excellent crack propagation resistance in the thickness direction and excellent crash resistance properties, and was therefore judged to have passed the test.On the other hand, when W2 / W1 was 0.150 or less, the hot-rolled steel sheet was judged to have poor crash resistance properties, and was therefore judged to have failed the test.

[0090] [Table 1]

[0091] [Table 2]

[0092] [Table 3]

[0093] [Table 4]

[0094] It can be seen from Table 4 that the hot-rolled steel sheets according to the examples of the present invention have high strength, as well as excellent bendability and resistance to crack propagation in the sheet thickness direction. On the other hand, it is clear that the hot-rolled steel sheets according to the comparative examples are inferior in one or more of the above properties. Furthermore, for all examples, lower arms (components) were manufactured by press working. The flat portion of the lower arm was evaluated in the same manner as described above. The measurement results and evaluation results were the same as those shown in Table 4. [Industrial Applicability]

[0095] According to the above aspects of the present disclosure, it is possible to provide a hot-rolled steel sheet having high strength, excellent bendability, and resistance to crack propagation in the plate thickness direction, and a part manufactured using the same.

Claims

1. The chemical composition, in mass%, is C: 0.050-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, Al: 0.010-0.400%, P: 0 to 0.080%, S: 0 to 0.0100%, N: 0 to 0.0050%, O: 0 to 0.0100%, Ti: 0 to 0.180%, Nb: 0 to 0.100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0 to 0.500%, B: 0 to 0.0100%, Ca: 0-0.0500%, Mg: 0 to 0.050%, REM: 0-0.1000%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0 to 0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0 to 0.500%, As: 0 to 0.050%, and Sn: 0 to 0.050%; the balance being Fe and impurities; In an internal region which is a region from 1 / 8 depth of the plate thickness from the surface to 3 / 8 depth of the plate thickness from the surface, The average aspect ratio of prior austenite grains is 4.00 to 6.00, The area ratio of martensite is 90% or more, a value obtained by dividing an average aspect ratio of prior austenite grains in a surface layer region that is a region from the surface to a depth of 1 / 15 of the plate thickness from the surface by the average aspect ratio of the prior austenite grains in the internal region is less than 0.

950.

2. The chemical composition is, in mass %, Ti: 0.001 to 0.180%, Nb: 0.001-0.100%, V: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Cr: 0.001-2.000%, Mo: 0.001-3.000%, Ni: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Ca: 0.0001-0.0500%, Mg: 0.001-0.050%, REM: 0.0001-0.1000%, Bi: 0.001-0.100%, Ta: 0.001 to 0.100%, Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001-0.200%, W: 0.001-0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, and The hot-rolled steel sheet according to claim 1, further comprising at least one element selected from the group consisting of Sn: 0.001 to 0.050%.

3. 2. The hot-rolled steel sheet according to claim 1, wherein the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface layer region by the average aspect ratio of the prior austenite grains in the inner region is 0.900 or less.

4. A hot-rolled steel sheet as described in claim 2, characterized in that the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface region by the average aspect ratio of the prior austenite grains in the internal region is 0.900 or less.

5. The hot-rolled steel sheet according to any one of claims 1 to 4, characterized in that the average aspect ratio of the prior austenite grains in the inner region is 4.00 to 5.

50.

6. A part comprising the hot-rolled steel sheet according to any one of claims 1 to 4.

7. A part comprising the hot-rolled steel sheet described in claim 5.

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