Steel member and steel sheet

Abstract

This steel member has a chemical composition containing, in terms of mass%, 0.26%-0.40% of C, 0%-2.00% of Si, 0.001%-3.000% of Mn, 0.100% or less of P, 0.0100% or less of S, 0.001%-1.000% of Al, 0.020% or less of N, 0.010% or less of O, and 0.410%-2.000% of V, wherein when the position at 1 / 4 of the thickness from a surface in the thickness direction is referred to as the 1 / 4-depth position, the steel member has a V-based precipitate in a metal structure at the 1 / 4-depth position; and among the V-based precipitate, the V content contained in a V-based precipitate having a maximum diameter of less than 500 nm is 0.005 mass% or more, and the V content contained in a V-based precipitate having a maximum diameter of 500 nm or more is 0.010 mass% or more.

Description

Steel members and steel plates

[0001] This application claims priority to Japanese Patent Application No. 2024-087913, filed on May 30, 2024, the contents of which are incorporated herein by reference.

[0002] In the field of automotive steel sheets, in response to recent tightening of environmental regulations and collision safety standards, the application of steel sheets having high tensile strength (high-strength steel sheets) is expanding in order to improve both fuel economy and collision safety. However, as the strength of steel sheets increases, the press formability of the steel sheets decreases, making it difficult to manufacture products with complex shapes.

[0003] Specifically, as the strength of steel sheets increases, their ductility decreases, resulting in the problem of fracture at highly processed locations when they are processed into complex shapes. Furthermore, as the strength of steel sheets increases, residual stress after processing causes springback and wall warping, resulting in poor dimensional accuracy. Therefore, it is not easy to press-form high-strength steel sheets, particularly those with a tensile strength of 780 MPa or more, into products with complex shapes. While roll forming, rather than press forming, makes it easier to process high-strength steel sheets, its application is limited to parts with a uniform cross section in the longitudinal direction.

[0004] Therefore, in recent years, hot stamping has been adopted as a technique for manufacturing extremely strong automotive parts with high dimensional accuracy, as disclosed in, for example, Patent Documents 1 to 3. Hot stamping is a hot forming technique in which a material to be formed is heated and then formed.

[0005] In hot stamping, the material is heated before being formed. Therefore, the steel material is soft during forming and has good formability. This allows for accurate forming into complex shapes without forming defects such as cracks. In addition, in hot stamping, the steel material (steel member) is quenched simultaneously with forming using a press die, so the steel material (steel member) after forming has sufficient strength.

[0006] For example, Patent Document 1 discloses that hot stamping can impart a tensile strength of 1400 MPa or more to a steel member obtained by forming a steel plate.

[0007] In recent years, countries around the world have been experiencing higher CO ２ Automobile companies have set reduction targets and are working to reduce fuel consumption while taking collision safety into consideration. Higher strength materials are required for vehicle bodies, not only for gasoline-powered vehicles but also for electric vehicles, which are rapidly being developed, in order to protect not only passengers but also batteries from collisions and to offset the resulting weight increase. For example, steel members used in automobiles and the like require higher strength (tensile strength exceeding 1500 MPa) than those described in the aforementioned Patent Document 1 or those generally used for steel members currently formed by hot stamping.

[0008] Regarding high-strength steel materials having a tensile strength exceeding 1.5 GPa (1500 MPa), for example, Patent Document 2 discloses a hot-press-formed press-formed product having excellent toughness and a tensile strength of 1.8 GPa or more. Patent Document 3 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and also having good toughness and ductility. Patent Document 4 discloses a steel material having a high tensile strength of 1.8 GPa or more and also having good toughness and weldability. Patent Document 5 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and also having good toughness and weldability.

[0009] However, with increasing strength, many metallic materials experience deterioration in various properties, particularly increased susceptibility to hydrogen embrittlement. It is known that steel members experience increased susceptibility to hydrogen embrittlement when their tensile strength is 1.2 GPa or higher, and there are concerns that hot-stamped members with tensile strengths exceeding 1.5 GPa will experience even greater susceptibility to hydrogen embrittlement. In order to further reduce the weight of automobile bodies by applying hot-stamped members with tensile strengths exceeding 1.5 GPa to automobile bodies, it is desirable to further improve hydrogen embrittlement resistance. However, hydrogen embrittlement resistance has not been adequately studied in conventional technology.

[0010] Japanese Patent Publication No. 2002-102980 Japanese Patent Publication No. 2012-180594 Japanese Patent Publication No. 2012-1802 International Publication No. 2015 / 182596 International Publication No. 2015 / 182591

[0011] In view of the above background, an object of the present invention is to provide a steel member having high strength and excellent resistance to hydrogen embrittlement, and a steel plate suitable as a material for the steel member.

[0012] In order to obtain a steel member having high tensile strength and excellent resistance to hydrogen embrittlement, the present inventors investigated the influence of the microstructure and the steel sheet used as the base material on these properties, and as a result, obtained the following findings.

[0013] (a) Many commonly used steel sheets that exhibit a tensile strength of about 1.5 GPa (1500 MPa) after heat treatment, including quenching, such as hot stamping, contain about 0.20% by mass of C, and this C ensures strength after heat treatment. In order to further reduce the weight of vehicle bodies, the inventors conducted detailed studies to obtain steel members that have a high strength of more than 1.5 GPa after heat treatment by increasing the C content. As a result, they found that by setting the C content to 0.26% by mass or more, ultra-high tensile strength of more than 1.5 GPa can be stably obtained after heat treatment, including quenching, such as hot stamping.

[0014] On the other hand, as the tensile strength increases to over 1.5 GPa, the susceptibility to hydrogen embrittlement increases, and there is concern that hydrogen embrittlement cracking may occur due to hydrogen generated in the corrosive environment during the manufacturing of steel members and during use (in automobiles).

[0015] (b) The present inventors have investigated methods for improving the hydrogen embrittlement resistance of high-strength steel members having a tensile strength of more than 1.5 GPa. As a result, they have found that higher hydrogen embrittlement resistance can be obtained by precipitating V-based precipitates that act as hydrogen trapping sites in the steel member and controlling the V content of each V-based precipitate by size.

[0016] The present inventors have made the invention in light of the above findings. The gist of the present invention is as follows. [1] A steel member according to one aspect of the present invention has a chemical composition, in mass %, of C: 0.26 to 0.40%, Si: 0 to 2.00%, Mn: 0.001 to 3.000%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.020% or less, O: 0.010% or less, V: 0.410 to 2.000%, N: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, V: 0.410 to 2.000%, and N: 0.100% or less. b: 0-0.10%, Ti: 0-0.200%, Cu: 0-2.00%, Ni: 0-2.00%, Cr: 0-1.00%, B: 0-0.0200%, Mo: 0-2 .00%, Ca: 0 to 0.020%, Mg: 0 to 0.020%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.000%, Zr: 0 to 1.000%. , Se: 0 to 1.000%, Bi: 0 to 1.000%, As: 0 to 1.000%, Ta: 0 to 1.000%, Re: 0 to 1.000%, Os: 0 to 1.000%, Ir: 0 to 1.000%, Tc: 0 to 1.000%, Co: 0 to 1.000%, REM: 0 to 0.300%, and the balance: Fe and impurities. When a position at 1 / 4 of the thickness from the surface in the thickness direction is defined as the 1 / 4 depth position, the metal structure at the 1 / 4 depth position has V-based precipitates, and among the V-based precipitates, the V content contained in V-based precipitates having a maximum diameter of less than 500 nm is 0.005 mass % or more, and the V content contained in V-based precipitates having a maximum diameter of 500 nm or more is 0.010 mass % or more. [2] In the steel member according to [1], the chemical composition may include, in mass%, Nb: 0.01 to 0.10%.[3] The steel member according to [1] or [2], wherein the chemical composition is, in mass%, Ti: 0.005 to 0.200%, Cu: 0.10 to 2.00%, Ni: 0.10 to 2.00%, Cr: 0.03 to 1.00%, B: 0.0005 to 0.0200%, Mo: 0.10 to 2.00%, Ca: 0.001 to 0.020%, Mg: 0.001 to 0.020%, Sn: 0.01 to 1.00%, W: 0.10 to 1.00%, Sb: 0.010 to 1.000%, Zr: [4] The steel member according to any one of [1] to [3] may have a coating on the surface. [5] The steel member according to [4] may have a coating mainly composed of an Fe—Al-based alloy. [6] In the steel member according to [4], the coating may be mainly made of an Fe—Zn alloy.[7] A steel sheet according to another embodiment of the present invention has a chemical composition, in mass%, of C: 0.26 to 0.40%, Si: 0 to 2.00%, Mn: 0.001 to 3.000%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.020% or less, O: 0.010% or less, V: 0.410 to 2.000%, N b: 0-0.10%, Ti: 0-0.200%, Cu: 0-2.00%, Ni: 0-2.00%, Cr: 0-1.00%, B: 0-0.0200%, Mo: 0-2 .00%, Ca: 0 to 0.020%, Mg: 0 to 0.020%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.000%, Zr: 0 to 1.000%. , Se: 0 to 1.000%, Bi: 0 to 1.000%, As: 0 to 1.000%, Ta: 0 to 1.000%, Re: 0 to 1.000%, Os: 0 to 1.000%, Ir: 0 to 1.000%, Tc: 0 to 1.000%, Co: 0 to 1.000%, REM: 0 to 0.300%, and the balance: Fe and impurities. When a position at 1 / 4 of the sheet thickness from the surface in the sheet thickness direction is defined as a 1 / 4 depth position, the metal structure at the 1 / 4 depth position has V-based precipitates, and among the V-based precipitates, the V content contained in V-based precipitates having a maximum diameter of less than 500 nm is 0.010 mass % or more, and the V content contained in V-based precipitates having a maximum diameter of 500 nm or more is 0.015 mass % or more. [8] In the steel plate according to [7], the chemical composition may include, in mass%, Nb: 0.01 to 0.10%.[9] The steel sheet according to [7] or [8], wherein the chemical composition is, in mass%, Ti: 0.005 to 0.200%, Cu: 0.10 to 2.00%, Ni: 0.10 to 2.00%, Cr: 0.03 to 1.00%, B: 0.0005 to 0.0200%, Mo: 0.10 to 2.00%, Ca: 0.001 to 0.020%, Mg: 0.001 to 0.020%, Sn: 0.01 to 1.00%, W: 0.10 to 1.00%, Sb: 0.010 to 1.000%, Zr:

[10] The steel sheet according to any one of [7] to [9] may have a coating on the surface.

[11] The steel sheet according to

[10] may have one or more elements selected from the group consisting of 0.010 to 1.000%, Se: 0.010 to 1.000%, Bi: 0.010 to 1.000%, As: 0.010 to 1.000%, Ta: 0.010 to 1.000%, Re: 0.010 to 1.000%, Os: 0.010 to 1.000%, Ir: 0.010 to 1.000%, Tc: 0.010 to 1.000%, Co: 0.010 to 1.000%, and REM: 0.010 to 0.300%.

[12] In the steel sheet according to

[10] , the coating may be a Zn-based coating.

[0017] According to the above-described aspects of the present invention, it is possible to provide a steel member having high strength and excellent resistance to hydrogen embrittlement, and a steel plate suitable as a material for the steel member.

[0018] A steel member according to one embodiment of the present invention (steel member according to this embodiment), a steel plate according to one embodiment of the present invention suitable as a material thereof (steel plate according to this embodiment), and a manufacturing method thereof will be described. In this embodiment, in the case of a steel member, a position at 1 / 4 of the thickness from the surface in the thickness direction (in the case of a steel plate, the thickness direction of the steel plate (plate thickness direction)) will be described as the 1 / 4 depth position. In addition, in the case of a steel plate, a position at 1 / 4 of the thickness from the surface in the plate thickness direction will be described as the 1 / 4 depth position. Here, the surface serving as the reference for the 1 / 4 depth position is the surface of the steel member. However, in the case of a steel member having a coating, i.e., a steel member having a base steel material and a coating formed on the surface of the base steel material, the surface refers to the surface of the base steel material excluding the coating (the interface between the coating and the base steel material). Similarly, in the case of a steel plate, the surface serving as the reference for the 1 / 4 depth position is the surface of the steel plate, and in the case of a steel plate having a coating, the surface refers to the surface of the base steel plate excluding the coating (the interface between the coating and the base steel plate).

[0019] <Steel Member> The steel member according to this embodiment has a predetermined chemical composition, and has V-based precipitates in a metallographic structure at a quarter depth position, wherein the V-based precipitates having a maximum diameter of less than 500 nm contain a V content of 0.005 mass% or more relative to the mass of the entire steel, and the V-based precipitates having a maximum diameter of 500 nm or more contain a V content of 0.010 mass% or more relative to the mass of the entire steel. Each of these will be described below.

[0020] [Chemical Composition] The chemical composition of the steel member according to this embodiment is, in mass %, C: 0.26 to 0.40%, Si: 0 to 2.00%, Mn: 0.001 to 3.000%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.020% or less, O: 0.010% or less, V: 0.410 to 2.000%, Nb: 0 to 0.10%, Ti: 0 to 0.200%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Cr: 0 to 1.00%, B: 0 to 0.0200%, and Mo: 0 to 2.00%, Ca: 0-0.020%, Mg: 0-0.020%, Sn: 0-1.00%, W: 0-1.00%, Sb: 0-1.000%, Zr: 0-1.000%, Se: 0-1.000%, Bi: 0-1.000%, As: 0-1.000%, Ta: 0-1.000%, Re: 0-1.000%, Os: 0-1.000%, Ir: 0-1.000%, Tc: 0-1.000%, Co: 0-1.000%, REM: 0-0.300%, and the balance: Fe and impurities. The reasons for limiting the content of each element are as follows. Numerical ranges separated by "-" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range. Furthermore, unless otherwise specified, percentages regarding the content of elements are mass percent.

[0021] (C: 0.26 to 0.40%) C is an element that improves the hardenability of steel and improves the strength of the steel member obtained after the steel plate is subjected to heat treatment (quenching) such as hot stamping. If the C content is less than 0.26%, it becomes difficult to stably ensure the target strength (over 1.5 GPa (1500 MPa)) of the steel member after quenching (obtained after quenching). Therefore, the C content is set to 0.26% or more. The C content is preferably set to 0.28% or more, more preferably set to 0.30% or more, and even more preferably set to 0.31% or more. On the other hand, if the C content exceeds 0.40%, the strength of the steel member after quenching increases and the toughness tends to decrease. Therefore, the C content is set to 0.40% or less. The C content is preferably set to 0.38% or less, and more preferably set to 0.36% or less.

[0022] (Si: 0 to 2.00%) Si does not necessarily need to be contained (it may be 0%), but it is an effective element for improving the hardenability of steel and ensuring stable strength of the steel member after quenching. Therefore, it may be contained. To obtain the above effects, the Si content is preferably 0.10% or more, and more preferably 0.35% or more. On the other hand, if the Si content in steel exceeds 2.00%, the heating temperature required for austenite transformation during heat treatment (quenching) becomes significantly high. This may increase the cost required for heat treatment or may result in residual ferrite during heating, reducing the strength of the steel member. Therefore, the Si content is set to 2.00% or less. The Si content is preferably set to 1.50% or less.

[0023] (Mn: 0.001 to 3.000%) Mn is an extremely effective element for improving the hardenability of steel and stably ensuring strength after quenching. Mn also lowers the Ac3 point and promotes lowering the quenching temperature. To achieve these effects, the Mn content is set to 0.001% or more. The Mn content is preferably set to 0.005% or more, more preferably set to 0.010% or more or 0.015% or more, and even more preferably set to 0.100% or more. On the other hand, if the Mn content exceeds 3.000%, the hydrogen embrittlement resistance of the steel member after quenching deteriorates. Therefore, the Mn content is set to 3.000% or less. The Mn content is preferably set to 2.500% or less, 1.900% or less, 1.500% or less, or 1.000% or less.

[0024] (P: 0.100% or less) P is an element that reduces the hydrogen embrittlement resistance of a steel member after quenching. In particular, if the P content exceeds 0.100%, the reduction in hydrogen embrittlement resistance becomes significant. Therefore, the P content is limited to 0.100% or less. The P content is preferably limited to 0.060% or less or 0.020% or less. Since a lower P content is preferable, the P content may be 0%, but from the viewpoint of steelmaking costs, it may be 0.001% or more.

[0025] (S: 0.0100% or less) S is an element that reduces the toughness of a steel member after quenching. In particular, if the S content exceeds 0.0100%, the reduction in toughness becomes significant. Therefore, the S content is limited to 0.0100% or less. The S content is preferably limited to 0.0050% or less or 0.0030% or less. Since a small S content is preferable, the S content may be 0%, but from the viewpoint of steelmaking costs, it may be 0.0001% or more.

[0026] (Al: 0.001 to 1.000%) Al is an element generally used as a deoxidizer for steel. To obtain the above effects, the Al content is set to 0.001% or more. The Al content is preferably set to 0.005% or more, and more preferably set to 0.010% or more. On the other hand, if the Al content exceeds 1.000%, the above effects saturate and economic efficiency decreases. Therefore, when Al is contained, the Al content is set to 1.000% or less. The Al content may also be set to 0.500% or less or 0.100% or less.

[0027] (N: 0.020% or less) N is an element that reduces the toughness of a steel member after quenching. In particular, if the N content exceeds 0.020%, coarse nitrides are formed in the steel, significantly reducing toughness. Therefore, the N content is set to 0.020% or less. The N content is preferably set to 0.015% or less, and more preferably set to 0.010% or less. There is no need to particularly limit the lower limit of the N content, and it may be 0%, but setting the N content to less than 0.001% increases steelmaking costs and is therefore economically undesirable. Therefore, the N content may be set to 0.001% or more, or 0.002% or more, or 0.003% or more.

[0028] (O: 0.010% or less) O ​​is an element that reduces the toughness of a steel member after quenching. In particular, if the O content exceeds 0.010%, coarse oxides are formed in the steel, significantly reducing toughness. Therefore, the O content is set to 0.010% or less. There is no need to particularly limit the lower limit of the O content, and it may be 0%, but setting the O content to less than 0.001% increases steelmaking costs and is therefore economically undesirable. Therefore, the O content may be set to 0.001% or more, or 0.002% or more, or 0.003% or more.

[0029] (V: 0.410 to 2.000%) V is an element that forms carbides in steel and improves the hydrogen embrittlement resistance of steel members through the hydrogen trapping and grain refinement effects of these carbides. Therefore, the V content is set to 0.410% or more. When the C content is 0.26 to 0.40%, if the V content is less than 0.410%, V does not precipitate sufficiently, and sufficient hydrogen embrittlement resistance is not obtained. The V content is preferably 0.450% or more, more preferably 0.500% or more, and even more preferably 0.600% or more. On the other hand, if the V content exceeds 2.000%, the above effects saturate and economic efficiency decreases. Therefore, if V is contained, the V content is set to 2.000% or less. The V content is preferably 1.500% or less, and more preferably 0.800% or less.

[0030] The steel member according to this embodiment may have a chemical composition containing the above elements (basic elements), with the balance being Fe and impurities. On the other hand, for the purpose of improving various properties, the steel member may further contain one or more of the following elements (optional elements). Since the optional elements do not necessarily need to be contained, the lower limit of their content is 0%.

[0031] (Nb: 0 to 0.10%) Nb is an element that precipitates together with V in steel, thereby promoting the precipitation of V-based precipitates. Therefore, Nb may be contained. To obtain the above effects, the Nb content is preferably 0.01% or more. The Nb content is more preferably 0.02% or more. On the other hand, if the Nb content exceeds 0.10%, the above effects saturate and economic efficiency decreases. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably 0.08% or less, more preferably 0.07% or less, and even more preferably 0.06% or less or 0.05% or less.

[0032] (Ti: 0 to 0.200%) Ti is an element that forms fine carbides, carbonitrides, etc., and improves the hydrogen embrittlement resistance of steel members through the grain refinement effect. Ti also preferentially bonds with N in steel to form nitrides, suppresses the consumption of solute B due to the precipitation of BN, and promotes the hardenability improvement effect of B, described below. Therefore, Ti may be contained. To achieve the above effects, the Ti content is preferably 0.005% or more. The Ti content is more preferably 0.010% or more, and even more preferably 0.015% or more. On the other hand, if the Ti content exceeds 0.200%, the amount of TiC precipitation increases and C is consumed, resulting in a decrease in the strength of the steel member after quenching. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.080% or less, and more preferably 0.060% or less.

[0033] (Cu: 0 to 2.00%) Cu is an element effective for improving the hardenability of steel and stably ensuring the strength of steel members after quenching. Cu is also an element that improves corrosion resistance in corrosive environments. Therefore, Cu may be contained. To obtain the above effects, the Cu content is preferably 0.10% or more. The Cu content is more preferably 0.20% or more. On the other hand, if the Cu content exceeds 2.00%, the above effects saturate and costs increase. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less, and more preferably 1.00% or less or 0.60% or less.

[0034] (Ni: 0 to 2.00%) Ni is an element effective in improving the hardenability of steel and stably ensuring the strength of steel members after quenching. Therefore, it may be contained. To obtain the above effects, the Ni content is preferably 0.10% or more, and more preferably 0.15% or more. On the other hand, if the Ni content exceeds 2.00%, the above effects saturate and costs increase. Therefore, the Ni content is set to 2.00% or less. The Ni content is preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.20% or less.

[0035] (Cr: 0 to 1.00%) Cr is an effective element for improving the hardenability of steel and stably ensuring the strength of steel members after quenching. Therefore, Cr may be contained. To obtain the above effects, the Cr content is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Cr content exceeds 1.00%, the above effects saturate and costs increase. In addition, since Cr has the effect of stabilizing iron carbides, if the Cr content exceeds 1.00%, coarse iron carbides may remain undissolved during heat treatment of the steel plate, reducing the toughness of the steel member. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.50% or less, more preferably 0.20% or less, and even more preferably 0.15% or less.

[0036] (B: 0 to 0.0200%) B is an element that has the effect of improving the hardenability of steel even in trace amounts. Furthermore, B segregates at grain boundaries, strengthening the grain boundaries and improving hydrogen embrittlement resistance, and it is an element that suppresses austenite grain growth when the steel sheet is heated. Therefore, B may be contained. To obtain the above effects, the B content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0200%, a large amount of coarse compounds precipitates, reducing the hydrogen embrittlement resistance of the steel member. Therefore, if B is contained, the B content is set to 0.0200% or less. The B content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0037] (Mo: 0 to 2.00%) Mo is a highly effective element for improving the hardenability of steel and ensuring stable strength of steel members after quenching. In particular, when Mo is added in combination with the above-mentioned B, a synergistic effect of improving hardenability is obtained. Therefore, Mo may be added. To obtain the above effect, the Mo content is preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, Mo is an element that has the effect of stabilizing iron carbides. If the Mo content exceeds 2.00%, coarse iron carbides may remain undissolved during heat treatment of the steel plate, reducing the toughness of the steel member after quenching. In addition, costs may increase significantly. Therefore, if Mo is added, the Mo content is set to 2.00% or less. The Mo content is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.40% or less.

[0038] (Ca: 0 to 0.020%) Ca is an element that has the effect of refining inclusions in steel and increasing the toughness of the steel member after quenching. Therefore, it may be contained. To obtain the above effect, the Ca content is preferably 0.001% or more, and more preferably 0.002% or more. On the other hand, if the Ca content exceeds 0.020%, the effect saturates and the cost increases. Therefore, if Ca is contained, the Ca content is set to 0.020% or less. The Ca content is preferably 0.010% or less, more preferably 0.006% or less, and even more preferably 0.004% or less.

[0039] (Mg: 0 to 0.020%) Mg is an element that has the effect of refining inclusions in steel and increasing the toughness of the steel member after quenching. Therefore, it may be contained. To obtain the above effect, the Mg content is preferably 0.001% or more. The Mg content is more preferably 0.002% or more. On the other hand, if the Mg content exceeds 0.020%, the effect saturates and the cost increases. Therefore, if Mg is contained, the Mg content is 0.020% or less. The Mg content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.004% or less.

[0040] (Sn: 0 to 1.00%) Sn is an element that improves corrosion resistance in a corrosive environment. Therefore, Sn may be contained. To obtain the above effects, the Sn content is preferably 0.01% or more. The Sn content is more preferably 0.03% or more, and even more preferably 0.05% or more. On the other hand, if the Sn content exceeds 1.00%, the grain boundary strength decreases, and the hydrogen embrittlement resistance of the steel member after quenching decreases. Therefore, if Sn is contained, the Sn content is 1.00% or less. The Sn content may also be 0.50% or less or 0.10% or less.

[0041] (W: 0 to 1.00%) W is an extremely effective element for improving the hardenability of steel and stably ensuring the strength of steel members after quenching. Therefore, it may be contained. To obtain the above effects, the W content is preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, W is an element that has the effect of stabilizing iron carbides. If the W content exceeds 1.00%, coarse iron carbides may remain undissolved during heat treatment of the steel plate, reducing the toughness of the steel member after quenching. In addition, costs may increase significantly. Therefore, if W is contained, the W content is set to 1.00% or less. The W content is preferably 0.80% or less or 0.60% or less.

[0042] (Sb: 0 to 1.000%) Sb is an element that improves corrosion resistance in a corrosive environment. Therefore, it may be contained. To obtain the above effects, the Sb content is preferably 0.010% or more, and more preferably 0.040% or more. On the other hand, if the Sb content exceeds 1.000%, the grain boundary strength decreases, and the hydrogen embrittlement resistance of the steel member after quenching decreases. Therefore, if Sb is contained, the Sb content is set to 1.000% or less. The Sb content may also be 0.800% or less, 0.500% or less, or 0.100% or less.

[0043] (Zr: 0 to 1.000%) Zr is an element that improves corrosion resistance in a corrosive environment. Therefore, it may be contained. To obtain the above effects, the Zr content is preferably 0.010% or more, and more preferably 0.100% or more. On the other hand, if the Zr content exceeds 1.000%, the grain boundary strength decreases, and the hydrogen embrittlement resistance of the steel member after quenching decreases. Therefore, if Zr is contained, the Zr content is set to 1.000% or less. The Zr content may also be set to 0.500% or less or 0.300% or less.

[0044] (Se: 0 to 1.000%) Se is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effects, the Se content is preferably 0.010% or more, and more preferably 0.100% or more. On the other hand, if the Se content exceeds 1.000%, the effect saturates and the cost increases. Therefore, if Se is contained, the Se content is 1.000% or less. The Se content may also be 0.500% or less or 0.300% or less.

[0045] (Bi: 0 to 1.000%) Bi is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effect, the Bi content is preferably 0.010% or more, and more preferably 0.050% or more. On the other hand, if the Bi content exceeds 1.000%, the effect saturates and the cost increases. Therefore, if Bi is contained, the Bi content is 1.000% or less. The Bi content may also be 0.500% or less or 0.200% or less.

[0046] (As: 0 to 1.000%) As is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effects, the As content is preferably 0.010% or more, more preferably 0.100% or more, and even more preferably 0.200% or more. On the other hand, if the As content exceeds 1.000%, the effect saturates and the cost increases. Therefore, if As is contained, the As content is 1.000% or less. The As content may be 0.500% or less.

[0047] (Ta: 0 to 1.000%) Ta is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effects, the Ta content is preferably 0.010% or more, and more preferably 0.100% or more. On the other hand, if the Ta content exceeds 1.000%, the effect saturates and the cost increases. Therefore, if Ta is contained, the Ta content is 1.000% or less. The Ta content may also be 0.500% or less or 0.300% or less.

[0048] (Re: 0 to 1.000%) Re is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effect, the Re content is preferably 0.010% or more, and more preferably 0.100% or more. On the other hand, if the Re content exceeds 1.000%, the effect saturates and the cost increases. Therefore, if Re is contained, the Re content is 1.000% or less. The Re content may also be 0.500% or less or 0.300% or less.

[0049] (Os: 0 to 1.000%) Os is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effects, the Os content is preferably 0.010% or more, and more preferably 0.100% or more. On the other hand, if the Os content exceeds 1.000%, the effect saturates and the cost increases. Therefore, if Os is contained, the Os content is set to 1.000% or less. The Os content may also be 0.500% or less or 0.200% or less.

[0050] (Ir: 0 to 1.000%) Ir is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effects, the Ir content is preferably 0.010% or more, more preferably 0.100% or more, and even more preferably 0.200% or more. On the other hand, if the Ir content exceeds 1.000%, the effect saturates and the cost increases. Therefore, if Ir is contained, the Ir content is 1.000% or less. The Ir content may also be 0.500% or less or 0.300% or less.

[0051] (Tc: 0 to 1.000%) Tc is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effects, it is preferable that the Tc content be 0.010% or more. On the other hand, if the Tc content exceeds 1.000%, the effect saturates and the cost increases. Therefore, if Tc is contained, the Tc content is set to 1.000% or less.

[0052] (Co: 0 to 1.000%) Co is an element that improves corrosion resistance in a corrosive environment. Therefore, it may be contained. To obtain the above effects, the Co content is preferably 0.010% or more. On the other hand, if the Co content exceeds 1.000%, the above effects saturate and economic efficiency decreases. Therefore, if Co is contained, the Co content is 1.000% or less. The Co content may also be 0.500% or less, 0.400% or less, or 0.300% or less.

[0053] (REM: 0 to 0.300%) REM, like Ca, is an element that refines inclusions in steel and improves the toughness of steel members after quenching. Therefore, REM may be added. To obtain the above effect, the REM content is preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the REM content exceeds 0.300%, the effect saturates and costs increase. Therefore, if REM is added, the REM content is set to 0.300% or less. The REM content is preferably 0.200% or less, 0.100% or less, or 0.050% or less. Here, REM refers to a total of 17 elements, including Sc, Y, and lanthanides such as La, Ce, and Nd, and the REM content refers to the total content of these elements. REM is added to molten steel using, for example, an Fe—Si-REM alloy, which contains, for example, Sc, Y, La, Ce, Pr, and Nd.

[0054] (Balance: Fe and Impurities) In the chemical composition of the steel member according to this embodiment, the remainder is Fe and impurities other than the elements described above. Here, "impurities" refer to components that are mixed in during the industrial production of steel plate due to various factors in the manufacturing process, such as raw materials such as ore and scrap, and are acceptable to the extent that they do not have a clear adverse effect on the properties of the steel member according to this embodiment. Industrial production methods include blast furnace steelmaking and electric furnace steelmaking, and include the levels (impurity levels) of elements mixed in during production by either method. Impurities may also include Pb. The chemical composition of the steel member can be determined by the following method. A test piece is taken from a 1 / 4 depth position of the steel member (a range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable), and the test piece is subjected to elemental analysis using a common method such as ICP-AES. C and S, which are difficult to measure by ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method.

[0055] As described above, the surface serving as the reference for the 1 / 4 depth position is the surface of the steel member. In other words, if the steel member has a coating on its surface, the chemical composition of the steel member is the chemical composition of the base steel material excluding the coating.

[0056] [Metal Structure (Microstructure)] (V-Based Precipitates) The steel member according to this embodiment has V-based precipitates in the metal structure at the 1 / 4 depth position. Furthermore, among the V-based precipitates present at the 1 / 4 depth position, the V content contained in the V-based precipitates with a maximum diameter of less than 500 nm (which can also be said to be the V content present as V-based precipitates with a maximum diameter of less than 500 nm, among the V content in the chemical composition of the steel member) is 0.005 mass% or more, and the V content contained in the V-based precipitates with a maximum diameter of 500 nm or more (which can also be said to be the V content present as V-based precipitates with a maximum diameter of 500 nm, among the V content in the chemical composition of the steel member) is 0.010 mass% or more. V-based precipitates serve as hydrogen trapping sites, and thus the presence of V-based precipitates improves hydrogen embrittlement resistance. In particular, V-based precipitates with a maximum diameter of less than 500 nm primarily have the effect of trapping hydrogen generated in the corrosive environment in which the steel member is used. If the V content of these V-based precipitates (the V content contained in the V-based precipitates) is less than 0.005 mass%, the desired effect cannot be obtained. The V content of V-based precipitates having a maximum diameter of less than 500 nm is preferably 0.007 mass% or more, more preferably 0.010 mass% or more, and even more preferably 0.020 mass% or more. On the other hand, V-based precipitates having a maximum diameter of 500 nm or more mainly have the effect of trapping hydrogen generated during the heat treatment performed on the steel sheet to obtain the steel member described below. If the V content of these V-based precipitates is less than 0.010 mass%, the desired effect cannot be obtained. The V content of V-based precipitates having a maximum diameter of 500 nm or more is preferably 0.050 mass% or more, more preferably 0.100 mass% or more, and even more preferably 0.150 mass% or more. By simultaneously controlling V-based precipitates having a maximum diameter of less than 500 nm and V-based precipitates having a maximum diameter of 500 nm or more, it is possible to dramatically improve the hydrogen embrittlement resistance of the steel member compared to the case where either one of the precipitates is present. Although there is no upper limit on the V content of the V-based precipitates having a maximum diameter of less than 500 nm, since the V content of the steel member is limited from an economic standpoint, the V content of the V-based precipitates having a maximum diameter of less than 500 nm may be 2.000 mass% or less.Although there is no upper limit to the V content of V-based precipitates having a maximum diameter of 500 nm or more, the V content of the V-based precipitates having a maximum diameter of 500 nm or more may be 2.000 mass% or less because the V content of the steel member is limited from an economical standpoint. Furthermore, although there is no upper limit to the maximum diameter of the V-based precipitates having a maximum diameter of 500 nm or more, it may be 10,000 nm or less from an economical standpoint.

[0057] Whether or not V-based precipitates are present in the metallographic structure at the 1 / 4 depth position (the presence of V-based precipitates), the V content of V-based precipitates with a maximum diameter of less than 500 nm, and the V content of V-based precipitates with a maximum diameter of 500 nm or more are determined by the FFF and extraction residue measurements described below, respectively. The V content of V-based precipitates with a maximum diameter of less than 500 nm is determined according to the procedure described in Japanese Patent No. 4,572,001. Specifically, for test specimens collected from a position 30 mm or more away from the end of a steel member, precipitates contained at a depth of 1 / 4 of the test specimen (a depth in the thickness direction from the surface is acceptable if it is within 1 / 8 to 3 / 8 of the thickness) were extracted and separated by constant current electrolysis at 500 mA using a 10% by mass acetylacetone-1% by mass tetramethylammonium chloride-methanol solution. The V-based precipitates were then dispersed and recovered using a solvent suitable for dispersing the V-based precipitates determined using the Hansen Solubility Parameter. The V-based precipitate dispersion was then introduced into a field-flow fractionation (FFF) device to separate precipitate particles of less than 500 nm in size. The separated precipitate particles were then directly introduced into an ICP mass spectrometer, where the particles were decomposed to the atomic level using the thermal energy of the plasma, and then ionized, allowing mass analysis of the elements contained in the precipitates by size. The signal intensity chromatogram of the obtained V mass spectrum was integrated, and the V content was quantified using a calibration curve prepared by flow injection. The V content of V-based precipitates with a maximum diameter of 500 nm or more is determined by subtracting the V content of V-based precipitates less than 500 nm from the V content of all sizes of V-based precipitates (the sum of the V content of V-based precipitates with a maximum diameter of less than 500 nm and the V content of V-based precipitates with a maximum diameter of 500 nm or more). The V content of V-based precipitates of all sizes can be determined by extracting and separating precipitates contained at a depth of 1 / 4 in the sample (a depth in the range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable) under the same electrolysis conditions as above, filtering and recovering them using a Nuclepore filter with a pore size of 0.2 μm, and then subjecting them to pretreatment (such as acid decomposition) as described in JIS G 1258-0 to 3:2017, followed by ICP optical emission spectrometry. Furthermore, if the V content of all sizes of V-based precipitates is 0.001% or more, it is determined that V-based precipitates are present in the metallographic structure.

[0058] In order to obtain a tensile strength of more than 1.5 GPa, the metal structure of the steel member according to this embodiment preferably contains martensite at a volume fraction of 75% or more as the metal structure (microstructure) at the 1 / 4 depth position. The volume fraction of martensite is more preferably 90% or more. The volume fraction of martensite may be 100%. Martensite includes not only so-called fresh martensite, but also tempered martensite and auto-tempered martensite. Auto-tempered martensite is tempered martensite that is formed during cooling during quenching without undergoing heat treatment for tempering, and is formed by in-situ tempering of the formed martensite due to self-heating associated with martensitic transformation.

[0059] The metal structure at the 1 / 4 depth position of the steel member may contain retained austenite, bainite, ferrite, and / or pearlite in addition to martensite.

[0060] The volume fraction of each structure in the metal structure of a steel member can be measured by the following method. The volume fraction of martensite (including fresh martensite, tempered martensite, and auto-tempered martensite) is measured using a transmission electron microscope (TEM) and an electron beam diffraction device attached to the TEM. Specifically, a test piece including a 1 / 4 depth position of the steel member is cut out from a position 30 mm or more away from the end of the steel member, and used as a thin film sample for TEM observation. A 400 μm section of this thin film sample at a 1 / 4 depth position of the steel member (a range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable) is measured. ２ The above range is observed with a TEM. The electron diffraction pattern of the thin film sample is used to distinguish between martensite and bainite, which are body-centered cubic lattices, and retained austenite, which is a face-centered cubic lattice. Iron carbides (Fe ３C) is found from the diffraction pattern, and the precipitation morphology is observed to measure the structure fractions of martensite and bainite. Specifically, if the precipitation morphology is tridirectional precipitation, it is determined to be martensite (tempered martensite or auto-tempered martensite), and if it is unidirectional limited precipitation, it is determined to be bainite. Martensite (fresh martensite) is also determined when no iron carbide precipitation is observed. The structure fractions of martensite and bainite measured by TEM are measured as area percentages, but since the steel member according to this embodiment has an isotropic metal structure, the area fraction value can be directly converted to volume fraction. Iron carbides are observed to distinguish between martensite and bainite, and in this embodiment, the iron carbides are included in the volume fraction of martensite or bainite. The volume fraction of retained austenite is measured using X-ray diffraction. Specifically, a test piece is cut out from a position 30 mm or more away from the end of the steel member to serve as a sample for X-ray diffraction. The cut-out test specimen is chemically polished using hydrofluoric acid and hydrogen peroxide solution to a depth of 1 / 4 from the surface (a depth of 1 / 8 to 3 / 8 of the thickness from the surface is acceptable). If chemical polishing takes a long time, for example, it may be polished using waterproof paper to about 1 / 8 of the thickness from the surface, and then chemically polished further to a depth of 1 / 4. The measurement conditions are a Co tube with a 2θ range of 45° to 105°. The diffracted X-ray intensity of the face-centered cubic lattice (retained austenite) contained in the steel member is measured, and the volume fraction of retained austenite is calculated from the area ratio of the diffraction curve. The presence of ferrite or pearlite as the residual structure can be easily confirmed using an optical microscope or scanning electron microscope. Specifically, a test specimen is cut out from a position 30 mm or more away from the end to serve as a sample for observation. The cross section of the cut-out test specimen in the thickness direction (a cross section parallel to the thickness direction) is mechanically polished and then mirror-finished. Next, the sample is etched with a nital etching solution to reveal ferrite and pearlite, and a scanning electron microscope is used to measure the area of ​​40,000 μm at a depth of 1 / 4 of the steel member (a range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable). ２By observing the above range, the presence of ferrite or pearlite is confirmed. A structure in which ferrite and cementite are arranged in alternating layers is determined to be pearlite. In this embodiment, the above-mentioned V-based precipitates are not included in the metal structure. Therefore, their presence is not taken into consideration when measuring the above-mentioned volume fraction. In other words, the volume fraction of V-based precipitates is included in the volume fraction of the structure in which V-based precipitates are present (martensite, bainite, retained austenite, ferrite, or pearlite).

[0061] [Tensile Strength] The steel member according to this embodiment is targeted to have a tensile strength of more than 1500 MPa (1.500 GPa) in order to achieve both fuel economy and crashworthiness when applied to automotive components. The tensile strength is preferably 1600 MPa or more, and more preferably 1700 MPa or more. On the other hand, in order to ensure toughness, the tensile strength is preferably 2500 MPa or less, and more preferably less than 2400 MPa, less than 2300 MPa, less than 2200 MPa, less than 2100 MPa, or less than 2000 MPa. The tensile strength is obtained by performing a tensile test at room temperature in accordance with the provisions of ASTM Standard E8. The tensile speed is 3 mm / min. However, in cases where a test specimen cannot be obtained due to the shape of the steel member, the tensile strength may be obtained by measuring the Vickers hardness and converting it to tensile strength. The Vickers hardness is obtained by performing a Vickers hardness test at a ¼ depth position, with the cross section in the thickness direction as the measurement surface. The Vickers hardness test is performed in accordance with JIS Z2244-1:2020, with a load of 98 N and a load holding time of 10 seconds, with measurements taken at five points and the average value calculated. The average Vickers hardness is then multiplied by 3.30 to convert it into tensile strength.

[0062] The shape of the steel member according to this embodiment is not particularly limited. That is, the steel member may be a formed body obtained by forming a steel plate into a predetermined shape. In this embodiment, the formed body is also referred to as a "steel member." The steel member may also be a tailored property material having different strengths and thicknesses depending on the location. In this case, it is preferable that at least a portion of the steel member has a tensile strength of greater than 1500 MPa. For example, 10% or more by volume of the steel member may have a tensile strength of greater than 1500 MPa, or 30% or more by volume of the steel member may have a tensile strength of greater than 1500 MPa. The tailored property material may be formed by joining together steel plates according to this embodiment, which will be described later, and then hot forming them, or by joining a steel plate according to this embodiment and an arbitrary steel plate and then hot forming them. Alternatively, a portion of the steel member may be heat-treated to adjust the strength of that portion. When the steel member has a portion having a tensile strength of greater than 1500 MPa and a portion having a tensile strength of 1500 MPa or less, it is sufficient that at least the portion having a tensile strength of greater than 1500 MPa has the above-described chemical composition and metallographic structure.

[0063] [Coating] The steel member according to this embodiment may have a coating on a portion or all of its surface. The coating may be a coating primarily made of an Fe—Al alloy, or a coating primarily made of an Fe—Zn alloy. The coating is also referred to as a film, an alloyed plating layer, or an intermetallic compound layer. A coating primarily made of an Fe—Al alloy is a coating containing 70 mass% or more of Fe and Al in total, and a coating primarily made of an Fe—Zn alloy is a coating containing 70 mass% or more of Fe and Zn in total. A coating primarily made of an Fe—Al alloy may further contain, in addition to Fe and Al, one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. The coating primarily made of an Fe-Zn alloy may contain, in addition to Fe and Zn, one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. The presence of the coating provides corrosion resistance, which has the effect of further improving hydrogen embrittlement resistance during use of the steel member (in automobiles). The thickness of the coating is preferably 10 to 100 μm.

[0064] The thickness of the coating can be determined by observing the cross section in the thickness direction with a scanning electron microscope. Specifically, a test piece is cut out from a position 30 mm or more away from the end of the steel member and observed. The observation range with the microscope is, for example, 400 times magnification, and an area of ​​40,000 μm ２The thickness of the cut specimen is within the above range. The cross section in the thickness direction is mechanically polished and then mirror-finished. Next, the specimen is observed using a BSE image (or a COMPO image), and the coating thickness is measured at 20 locations with a distance of 6.5 μm between measurement locations, and the average of the measured values ​​is calculated. Observation using a BSE image (or a COMPO image) reveals a clear contrast difference between the coating and the base steel (steel plate substrate). Therefore, the coating thickness can be determined by measuring the thickness from the outermost surface to the point where the contrast changes. Ten fields of view are observed in the same manner as above, and the average of the measured values ​​from the 10 fields of view is used to determine the coating thickness. Furthermore, the chemical composition of the coating can be determined by performing spot elemental analysis (beam diameter: 1.0 μm or less) using an electron probe microanalyzer (EPMA) on the same observation area as above to determine the Fe, Al, and Zn contents contained in the coating. A total of 10 points are analyzed in any 10 fields of view, and the average values ​​are used to determine the Fe, Al, and Zn contents in the coating. Even when elements other than Fe, Al, and Zn are contained, the same method is used to determine the content.

[0065] <Steel Plate> Next, the steel plate according to the present embodiment will be described. The steel plate according to the present embodiment can be used as a material for the steel member according to the present embodiment by performing heat treatment on the steel plate according to the present embodiment.

[0066] [Chemical Composition] The chemical composition of the steel plate according to this embodiment needs to be set so as to obtain desirable properties for the steel member after heat treatment. However, since the chemical composition does not substantially change due to heat treatment, the chemical composition of the steel plate according to this embodiment may be the same as that of the steel member according to this embodiment. The chemical composition of the steel plate may be measured in the same manner as for the steel member by taking a test piece from the 1 / 4 depth position of the steel plate (a range of 1 / 8 to 3 / 8 of the plate thickness from the surface in the plate thickness direction is acceptable). Here, the surface serving as the reference for the 1 / 4 depth position is the surface of the steel plate. However, if the steel plate has a coating, i.e., if the steel plate has a base steel plate and a coating formed on the surface of the base steel plate, the surface means the surface of the base steel plate excluding the coating.

[0067] [Metal structure] (V-based precipitates) The steel sheet according to this embodiment has V-based precipitates in the metal structure at the ¼ depth position, and the V content contained in the V-based precipitates having a maximum diameter of less than 500 nm (which can also be said to be the content of V present as V-based precipitates having a maximum diameter of less than 500 nm out of the V content in the chemical composition of the steel sheet) is 0.010 mass% or more, and the V content contained in the V-based precipitates having a maximum diameter of 500 nm or more (which can also be said to be the content of V present as V-based precipitates having a maximum diameter of 500 nm or more out of the V content in the chemical composition of the steel sheet) is 0.015 mass% or more. Although the V content of V-based precipitates may vary depending on the heat treatment, the steel sheet according to this embodiment has the above-described V-based precipitates. Therefore, in a steel member obtained by subjecting this steel sheet to a heat treatment such as hot stamping, among the V-based precipitates present at the 1 / 4 depth, the V content of V-based precipitates with a maximum diameter of less than 500 nm can be 0.005% by mass or more, and the V content of V-based precipitates with a maximum diameter of 500 nm or more can be 0.010% by mass or more. In other words, the hydrogen embrittlement resistance of a steel member obtained using the steel sheet according to this embodiment as a raw material is improved. Therefore, the V-based precipitates are controlled as described above. The V content of V-based precipitates with a maximum diameter of less than 500 nm is preferably 0.020% by mass or more, more preferably 0.025% by mass or more, and even more preferably 0.030% by mass or more. The V content contained in V-based precipitates having a maximum diameter of 500 nm or more is preferably 0.100 mass% or more, more preferably 0.150 mass% or more, and even more preferably 0.200 mass% or more, or 0.300 mass% or more. The V content contained in V-based precipitates having a maximum diameter of less than 500 nm and the V content contained in V-based precipitates having a maximum diameter of 500 nm or more may both be 2.000 mass% or less. The upper limit of the maximum diameter of V-based precipitates having a maximum diameter of 500 nm or more is not limited, but may be 10,000 nm or less.

[0068] The size and V content of the V-based precipitates in the metal structure at the 1 / 4 depth position can be measured in the same manner as for the steel member.

[0069] The metal structure at the 1 / 4 depth position of the steel plate according to this embodiment is not limited, but is often ferrite or pearlite. Bainite, martensite, and retained austenite may be contained within the conditions of the manufacturing method described below. The martensite mentioned above includes not only so-called fresh martensite but also tempered martensite and auto-tempered martensite. It is desirable that the steel plate before heat treatment be a soft steel plate. Therefore, the volume fraction of martensite in the steel plate is preferably less than 50%, and more preferably less than 10%. The metal structure at the 1 / 4 depth position of the steel plate can be measured in the same manner as for the steel member.

[0070] The shape of the steel plate according to this embodiment is not particularly limited. That is, the steel plate may be a flat plate (single plate) or a tailored property material (blank) in which steel plates with different strengths or thicknesses are joined together.

[0071] [Coating] The steel sheet according to this embodiment may have a coating on a portion of its surface. The coating may be an Al-based coating or a Zn-based coating. The coating is also referred to as a film or a plating layer. An Al-based coating is a coating containing 70% by mass or more of Al, and a Zn-based coating is a coating containing 70% by mass or more of Zn. The Al-based coating may contain one or more of Fe, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. The Zn-based coating may contain, in addition to Zn, one or more of Fe, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. By providing the above-described coating on the steel sheet according to this embodiment, a steel member obtained by subjecting the steel sheet to a heat treatment such as hot stamping will have a coating mainly composed of the Fe—Al alloy or a coating mainly composed of the Fe—Zn alloy. In other words, the hydrogen embrittlement resistance of a steel member obtained using the steel sheet according to this embodiment as a raw material is improved. The thickness of the coating is preferably 10 to 100 μm. The chemical composition and thickness of the coating can be measured in the same manner as for the steel member.

[0072] <Method for manufacturing steel sheet> There are no limitations on the method for manufacturing the steel sheet according to this embodiment, which is suitable as a material for the steel member according to this embodiment, and the steel sheet can be manufactured, for example, by using a manufacturing method including the steps shown below: (i) a casting step of melting steel having the above-described chemical composition and casting it to manufacture a slab, (ii) a hot rolling step of hot rolling the obtained slab to obtain a hot-rolled steel sheet, (iii) a holding step of holding the hot-rolled steel sheet after the hot-rolling step in a temperature range of 500 to 800°C for 10 seconds or more, (iv) a coiling step of coiling the hot-rolled steel sheet after the holding step, (v) a cold rolling step of descaling the hot-rolled steel sheet after the coiling step and cold-rolling it to obtain a cold-rolled steel sheet, (vi) an annealing step of annealing the hot-rolled steel sheet or cold-rolled steel sheet to obtain an annealed steel sheet, and (vii) a coating step of forming a coating on the surface of the hot-rolled steel sheet, cold-rolled steel sheet, or annealed steel sheet, as necessary.

[0073] Preferred conditions for each step are explained below. For conditions not explained, known conditions can be applied.

[0074] <Casting Step> In the casting step, steel having the above-described chemical composition is melted and cast to produce a slab to be subjected to hot rolling. For example, molten steel having the above-described chemical composition is melted using a converter or an electric furnace, and a slab produced by continuous casting can be used. Instead of continuous casting, an ingot casting method, a thin slab casting method, or the like may be employed. Known casting conditions may be used.

[0075] <Hot Rolling Process> In the hot rolling process, the obtained slab is heated, and the heated slab is subjected to hot rolling including rough rolling and finish rolling to obtain a hot-rolled steel sheet. At this time, the slab heating temperature is 1150°C or higher, the rough rolling completion temperature is 1050°C or higher, the time (residence time) from the start of the final reduction in rough rolling to the start of finish rolling is 60 seconds or more, and in the finish rolling, the cumulative reduction rate at 1050°C or lower is 60% or more. By the above rolling, precipitation nuclei of V-based precipitates, particularly V-based precipitates with a maximum diameter of 500 nm or more, are formed. If the slab heating temperature is less than 1150°C, the rough rolling completion temperature will be below 1050°C. If the rough rolling completion temperature is less than 1050°C, it is difficult to ensure the time from the start of the final reduction in rough rolling to the start of finish rolling. If the time from the start of the final reduction in rough rolling to the start of finish rolling is less than 60 seconds, the precipitation of V-based precipitates will be insufficient. If the cumulative reduction rate at 1050°C or less is less than 60%, the precipitation of V-based precipitates will be insufficient. The finish rolling temperature is not limited, but may be 800°C or higher, which is the upper limit of the holding step described below. The upper limit of the slab heating temperature is not limited, but from the perspective of heating economy, the slab heating temperature may be 1400°C or lower. Furthermore, the upper limit of the time from the start of the final reduction in rough rolling to the start of finish rolling is not limited, but from the perspective of productivity and ensuring the finish rolling temperature, 240 seconds or less is preferred.

[0076] <Holding Step> In the holding step, the steel sheet after the hot rolling step (after the completion of finish rolling) is held in a temperature range of 500 to 800°C for 10 seconds or more. This holding step causes V-based precipitates to precipitate. In particular, V precipitates in the precipitation nuclei formed in the hot rolling step, forming V-based precipitates with a maximum diameter of 500 nm or more, and V-based precipitates with a maximum diameter of less than 500 nm precipitate at the phase interface in the steel sheet. If the holding time is short, the desired V-based precipitates will not precipitate sufficiently. The upper limit of the holding time is not limited, but may be 120 seconds or less from the perspective of productivity. The upper limit of the holding time is preferably 60 seconds or less, and more preferably 30 seconds or less. The holding does not have to be at a constant temperature, and the temperature may vary. For example, it is acceptable for the steel sheet to be slowly cooled for 10 seconds or more, with the steel sheet temperature being in the range of 500 to 800°C. Since V-based precipitates are unlikely to precipitate in the temperature range from the finish rolling completion temperature to 800°C and in the temperature range below 500°C, the cooling conditions in these temperature ranges are not limited.

[0077] When the chemical composition of the slab contains Nb, Nb precipitates are formed during the hot rolling process and act as nuclei for V precipitation, promoting the formation of composite precipitates with V. As a result, V-based precipitates of 500 nm or more are particularly likely to be formed. The composite precipitates of Nb and V are included in V-based precipitates.

[0078] <Coiling step> In the coiling step, the hot-rolled steel sheet after the holding step is coiled into a hot-rolled coil. The coiling conditions are not particularly limited, but it is preferable to set the coiling temperature to 400°C or higher in order to reduce the load in cold rolling.

[0079] <Cold Rolling Step> In the cold rolling step, the hot-rolled steel sheet after the coiling step is descaled and cold-rolled to obtain a cold-rolled steel sheet. Descaling and cold rolling are not necessarily required, but if cold rolling is performed, the cumulative reduction in cold rolling is preferably 30% or more from the viewpoint of ensuring good flatness. On the other hand, in order to avoid excessive rolling load, the cumulative reduction in cold rolling is preferably 80% or less. The descaling method is not particularly limited, but pickling is preferred. Furthermore, if pickling is performed, it is preferred to remove only iron scale by pickling with hydrochloric acid or sulfuric acid.

[0080] <Annealing step> In the annealing step, the steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) is annealed. The annealing conditions are not limited, but when annealing is performed, the hot-rolled steel sheet or cold-rolled steel sheet is annealed in a temperature range of 700 to 950°C to obtain an annealed steel sheet. By undergoing the annealing step, the cold-rolled steel sheet can be softened, which makes it easier to pass the sheet in the subsequent coating step. The annealing step may be performed as needed, or may not be performed.

[0081] <Coating step> When a coating is formed on the surface, a coating is formed on the surface of a steel sheet (a hot-rolled steel sheet after a coiling step, a cold-rolled steel sheet after a cold-rolling step, or an annealed steel sheet after an annealing step) to produce a coated steel sheet (if the coating is a plating layer, it is a plated steel sheet). The coating method is not particularly limited, and possible methods include hot-dip galvanization, electroplating, vacuum deposition, cladding, and thermal spraying. Hot-dip galvanization is the most widely used method industrially. Examples of coatings include Al-based coatings containing Al and Zn-based coatings containing Zn.

[0082] When an Al-based coating is formed by hot-dip plating, the coating bath often contains Fe as an impurity in addition to Al. Furthermore, as long as the coating bath contains 70 mass% or more of Al, the coating bath may contain one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM in addition to the elements described above. When a Zn-based coating is formed by hot-dip plating, the coating bath often contains Fe as an impurity in addition to Zn. Furthermore, as long as the Zn content is 70% by mass or more, the plating bath may further contain one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM in addition to the above elements. When hot-dip plating is performed, the steel sheet after annealing may be cooled to room temperature and then heated again to perform plating, or after annealing, the steel sheet may be cooled to a temperature close to the plating bath temperature (for example, 650 to 750 ° C. for an Al-based coating, 420 to 510 ° C. for a Zn-based coating), and then hot-dip plating may be performed without once cooling to room temperature.

[0083] There are no particular limitations on the pre-treatment and post-treatment for coating, and pre-coating, solvent application, alloying treatment, temper rolling, etc. are possible. As an alloying treatment, the coated steel sheet can be heated to, for example, 450 to 800°C. Furthermore, as a post-treatment, temper rolling is useful for shape adjustment, etc., and allows for a reduction of, for example, 0.1 to 0.5%.

[0084] <Method for manufacturing steel member> The method for manufacturing a steel member according to this embodiment is not limited, but the steel member can be manufactured by, for example, using a manufacturing method including the steps described below on the steel plate according to this embodiment obtained by the method described above.

[0085] <Heat Treatment Step> In the heat treatment step, the steel sheet according to this embodiment having a predetermined chemical composition is heat-treated to produce a steel member. The heat treatment is performed, for example, by heating the steel sheet obtained by the above-described method to a temperature between the Ac3 point and (Ac3 point + 300)°C at an average heating rate of 1.0 to 1000°C / s, and then cooling to the Ms point or below at an average cooling rate equal to or greater than the upper critical cooling rate. An average heating rate of less than 1.0°C / s to the heat treatment temperature is undesirable because it reduces the productivity of the heat treatment. On the other hand, an average heating rate of more than 1000°C / s is undesirable because it results in a duplex grain structure and reduced toughness. Furthermore, a heat treatment temperature (maximum heating temperature) below the Ac3 point (°C) is undesirable because ferrite remains after cooling, reducing the volume fraction of martensite in the metal structure of the steel member and resulting in insufficient strength. On the other hand, a heat treatment temperature above (Ac3 point + 300)°C is undesirable because it results in coarse grained structure and reduced toughness. The upper critical cooling rate is the minimum cooling rate at which austenite is supercooled to form martensite without precipitating ferrite or pearlite in the metal structure. Cooling at an average cooling rate below the upper critical cooling rate may result in the formation of ferrite or pearlite, resulting in a decrease in the volume fraction of martensite in the metal structure of the steel member, resulting in insufficient strength. During heating, the steel member may be held at a temperature range of Ac3 point to (Ac3 point + 300) °C for 1 to 300 seconds. After cooling to a temperature below the Ms point, the steel member may be tempered at a temperature range of about 100 to 600 °C to adjust its strength.

[0086] The Ac3 point, Ms point, and upper critical cooling rate are measured by the following method. A strip-shaped test piece measuring 30 mm wide and 200 mm long is cut from the steel plate according to this embodiment. The test piece is heated to 1000°C in a nitrogen atmosphere at a heating rate of 10°C / s, held at that temperature for 5 minutes, and then cooled to room temperature at various cooling rates. The cooling rate is set from 1°C / s to 100°C / s in 10°C / s intervals (however, after 1°C / s, it is changed to 10°C / s). The Ac3 point and Ms point are measured by measuring the change in thermal expansion of the test piece during heating and cooling. The average value of the Ac3 points obtained in all tests is taken as the Ac3 point of the steel plate. Furthermore, the metal structure of each test piece cooled at the above cooling rates is observed, and the minimum cooling rate at which ferrite precipitation does not occur is taken as the upper critical cooling rate. The average value of the Ms points obtained from the change in thermal expansion when cooled at or above the upper critical cooling rate is taken as the Ms point of the steel sheet.

[0087] Here, during the above series of heat treatments, after heating to a temperature range of Ac3 point to (Ac3 point + 300) ° C., while cooling to the Ms point, i.e., a step of cooling at or above the upper critical cooling rate, hot forming such as hot stamping may be performed at the same time. Examples of hot forming include bending, drawing, stretch forming, and hole expansion (stretch flange forming). Furthermore, forming methods other than press forming, such as roll forming, may be applied as long as a means for cooling the steel sheet simultaneously with or immediately after forming is provided. Repeated hot forming may be performed according to the above thermal history. Furthermore, the above series of heat treatments may be repeated multiple times. When hot forming by hot stamping (including when a flat mold is used), it is preferable to set the forming start temperature to 700 ° C. or higher, perform cooling using the mold, and set the demolding temperature (the surface temperature of the steel member when removed from the mold) to less than 150 ° C. This improves the dimensional accuracy of the steel member.

[0088] The heat treatment may be performed on a portion of the steel sheet that is the raw material for hot forming or heat treatment, in which case a steel member having regions with different strengths can be obtained.

[0089] The above series of heat treatments can be carried out by any method. For example, heating may be carried out by high-frequency heating, electrical heating, infrared heating, or furnace heating. Cooling may also be carried out by water cooling, mold cooling, or the like. The atmosphere in the heating furnace may be air, combustion gas, or nitrogen gas. The dew point in the heating furnace may be controlled to suppress hydrogen generation during the heat treatment.

[0090] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0091] Example 1 Steels having the chemical compositions shown in Tables 1-1 and 1-2 were melted to obtain slabs. The obtained slabs were heated to 1,150°C or higher and hot-rolled under the conditions shown in Table 2-1 to obtain steel sheets (hot-rolled steel sheets) having a thickness of 2.7 mm. The finish rolling temperature was 800°C or higher. The obtained hot-rolled steel sheets were held at 500 to 800°C for the holding times shown in Table 2-1 and coiled at a coiling temperature of 400°C or higher. They were then pickled and cold-rolled to obtain cold-rolled steel sheets having a thickness of 1.6 mm. Some of the cold-rolled steel sheets were annealed under the conditions shown in Table 2-1 to obtain annealed steel sheets. Some of the annealed steel sheets (annealed steel sheets) were immersed in an Al-plating bath at 680°C containing 10 mass% Si, 2 mass% Fe, and the balance being impurities, to obtain Al-plated steel sheets having an Al-based coating on the surface (coating: Al in Table 2-2). Furthermore, some of the steel sheets after annealing (annealed steel sheets) were immersed in a Zn plating bath containing Zn and impurities at 450°C to obtain Zn-plated steel sheets having a Zn-based coating on the surface (coating in Table 2-2: Zn). The thickness of the coating was adjusted to about 25 μm in all cases. (In Table 2-2, "-" for coating indicates that no coating was formed.)

[0092] Test specimens were cut from the obtained steel sheets, and analysis using an FFF-ICP device and measurement of the extracted residue were performed as described above to confirm the presence or absence of V-based precipitates and to determine the V content contained in the V-based precipitates. Test specimens were also cut from the obtained steel sheets, and the metal structure was observed at a 1 / 4 depth position. As a result, although not shown in the table, in all of the inventive examples, the volume fraction of martensite was less than 50%.

[0093]

[0094]

[0095]

[0096]

[0097] As shown in Tables 1-1 to 2-2, steel sheets having predetermined chemical compositions and V-based precipitates were obtained in Examples B1 to B16, which satisfied the ranges of the present invention. On the other hand, in Comparative Examples b1 to b8, the chemical compositions or the V content of the V-based precipitates did not satisfy the ranges of the present invention.

[0098] Example 2 A hot stamping blank having a width of 200 mm and a length of 300 mm was taken from the steel sheet obtained in Example 1, and the hot stamping blank was heated under the conditions shown in Table 3-1. The holding time in the temperature range of Ac3 point to (Ac3 point + 300) °C was 120 seconds. The heating furnace was an air atmosphere, with a dew point of -5 °C. The hot stamping blank was removed from the heating furnace and allowed to cool to 750 °C, after which it was sandwiched between dies equipped with a cooling device to obtain a plate-shaped steel member. The demolding temperature was less than 150 °C. The average cooling rate to the Ms point or below was as shown in Table 3-1. In the steel members in which the plated steel sheet was used as a base sheet for hot stamping, an Fe—Al-based alloy layer containing Fe and Al in total of 70 mass% or more (in Table 3-2, the column for coating: Fe—Al) or an Fe—Zn-based alloy layer containing Fe and Zn in total of 70 mass% or more (in Table 3-2, the column for coating: Fe—Zn) was formed on the surface to a thickness of 10 to 100 μm.

[0099] Test specimens were cut out from the obtained steel members, and analysis using an FFF-ICP apparatus and measurement of the extracted residue were carried out in the same manner as described above to confirm the presence or absence of V-based precipitates and to determine the V content contained in the V-based precipitates.

[0100] Furthermore, the metal structure of the obtained steel members was observed at a 1 / 4 depth position. As a result, although not shown in the table, in all of the inventive examples, the volume fraction of martensite was 75% or more.

[0101] Further, the obtained steel members were subjected to a tensile test and a hydrogen embrittlement test by the following methods to evaluate the tensile strength and hydrogen embrittlement resistance.

[0102] <Tensile strength> The tensile test was carried out in accordance with the provisions of ASTM standard E8. Half-size plate-shaped test pieces (parallel portion length: 32 mm, parallel portion plate width: 6.25 mm) according to ASTM standard E8 were taken from the soaked portion of the steel member so that the test direction was parallel to the rolling direction of the steel plate constituting the steel member. A strain gauge (gauge length: 5 mm, FLAB-5 manufactured by Tokyo Measuring Instruments Research Institute Co., Ltd.) was then attached to the center of the width and length of the parallel portion of the test piece, and a room temperature tensile test was carried out at a tension rate of 3 mm / min to measure the tensile strength (maximum strength). In this example, a tensile strength of more than 1500 MPa was evaluated as high strength.

[0103] <Hydrogen Embrittlement Resistance> Hydrogen embrittlement resistance was evaluated by a four-point bending test on test specimens, based on the amount of hydrogen (Hc) that could be absorbed in the test specimens up to the limit at which cracks did not occur. Specifically, test specimens were cut out as strips 6 mm wide and 68 mm long, avoiding the edges of the steel member to be evaluated, so that the longitudinal direction was parallel to the longitudinal direction of the steel plate constituting the steel member. Strain gauges (gauge length: 5 mm, FLAB-5, manufactured by Tokyo Measuring Instruments Laboratory Co., Ltd.) similar to those used in the tensile test were attached to the center of the surface of the test specimen in the width and length directions. Then, based on the stress-strain curve measured by the above-mentioned tensile test on the steel member to be evaluated, the test specimens were bent along their longitudinal direction using a four-point support jig until a strain equivalent to 3 / 5 of the tensile strength was generated on the surface of the test specimen. The jig used had a 10 mm spacing between the inner pins (two inner load application points) and a 60 mm spacing between the outer pins (two outer support points). Test specimens with different amounts of absorbed hydrogen (hydrogen absorption capacity) were observed for the occurrence of cracks, and the maximum (limit) hydrogen content Hc (mass ppm) at which cracks did not occur was determined. For steel members with Fe-Al coatings, the dew point in the heating furnace during the heat treatment process was changed to vary only the amount of hydrogen absorption, and the occurrence of cracks was observed within 72 hours after four-point bending. For steel members without coatings or steel members with Fe-Zn coatings, multiple test specimens were immersed in aqueous solutions of ammonium thiocyanate with different concentrations after four-point bending to absorb hydrogen, and the occurrence of cracks was observed within 72 hours after immersion. The amount of hydrogen absorbed in the steel members was determined by temperature-programmed hydrogen analysis using test specimens prepared under the same conditions as the test specimens subjected to four-point bending. Specifically, the amount of diffusible hydrogen released by heating at a rate of 100°C / hr up to 250°C was defined as the amount of hydrogen contained in the steel members. In this example, a steel member having a tensile strength of 2000 MPa or less was evaluated as having excellent hydrogen embrittlement resistance when the hydrogen content Hc was 0.7 mass ppm or more, and when the hydrogen content Hc was 0.5 mass ppm or more, when the steel member had a tensile strength of more than 2000 MPa.

[0104]

[0105]

[0106] As shown in Tables 3-1 and 3-2, Invention Examples C1 to C16, which satisfied the range of the present invention, had high strength and excellent hydrogen embrittlement resistance. On the other hand, Comparative Examples c1 to c8 had chemical compositions and / or V-based precipitates outside the range of the present invention, and were inferior in tensile strength or hydrogen embrittlement resistance.

[0107] According to the present invention, it is possible to provide a steel member having high strength and excellent hydrogen embrittlement resistance, and a steel plate suitable as a material for such a steel member. Therefore, the present invention has high industrial applicability.

Claims

1. Chemical composition, in mass%, is: C: 0.26 to 0.40%, Si: 0 to 2.00%, Mn: 0.001 to 3.000%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.020% or less, O: 0.010% or less, V: 0.410 to 2.000%, Nb: 0 to 0.10%, Ti: 0 to 0.200%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Cr: 0 to 1.00%, B: 0 to 0.0200%, Mo: 0 to 2.00%, Ca: 0 to 0.020%, Mg: 0 to 0.020%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Se: 0 to 1.000%, Bi: 0 to 1.000%, As: 0 to 1.000%, Ta: 0 to 1.000%, Re: 0 to 1.000%, Os: 0 to 1.000%, Ir: 0 to 1.000%, Tc: 0 to 1.000%, Co: 0 to 1.000%, REM: 0 to 0.300%, and the balance: Fe and impurities, wherein when a position at 1 / 4 of the thickness from the surface in the thickness direction is defined as a 1 / 4 depth position, the metal structure at the 1 / 4 depth position has V-based precipitates, wherein, among the V-based precipitates, the V-based precipitates having a maximum diameter of less than 500 nm have a V content of 0.005% by mass or more, and the V-based precipitates having a maximum diameter of 500 nm or more have a V content of 0.010% by mass or more.

2. The steel member according to claim 1, characterized in that the chemical composition contains, in mass %, Nb: 0.01 to 0.10%.

3. The chemical composition is, in mass%, Ti: 0.005 to 0.200%, Cu: 0.10 to 2.00%, Ni: 0.10 to 2.00%, Cr: 0.03 to 1.00%, B: 0.0005 to 0.0200%, Mo: 0.10 to 2.00%, Ca: 0.001 to 0.020%, Mg: 0.001 to 0.020%, Sn: 0.01 to 1.00%, W: 0.10 to 1.00%, Sb: 0.010 to 1.000%, Zr: 0.010 to 1.000%, Se: 0.010 to 1.000%, Bi: 0.010 to 1.000%, 3. The steel member according to claim 1, comprising one or more elements selected from the group consisting of As: 0.010 to 1.000%, Ta: 0.010 to 1.000%, Re: 0.010 to 1.000%, Os: 0.010 to 1.000%, Ir: 0.010 to 1.000%, Tc: 0.010 to 1.000%, Co: 0.010 to 1.000%, and REM: 0.010 to 0.300%.

4. The steel member according to claim 1 or 2, characterized in that the surface has a coating.

5. The steel member according to claim 3, characterized in that the surface has a coating.

6. The steel member according to claim 4, characterized in that the coating is mainly composed of an Fe-Al alloy.

7. The steel member according to claim 5, wherein the coating is mainly composed of an Fe-Al alloy.

8. The steel member according to claim 4, wherein the coating is mainly composed of an Fe-Zn alloy.

9. The steel member according to claim 5, wherein the coating is mainly composed of an Fe-Zn alloy.

10. The chemical composition, in mass%, is: C: 0.26 to 0.40%, Si: 0 to 2.00%, Mn: 0.001 to 3.000%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.020% or less, O: 0.010% or less, V: 0.410 to 2.000%, Nb: 0 to 0.10%, Ti: 0 to 0.200%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Cr: 0 to 1.00%, B: 0 to 0.0200%, Mo: 0 to 2.00%, Ca: 0 to 0.020%, Mg: 0 to 0.020%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Se: 0 to 1.000%, Bi: 0 to 1.000%, As: 0 to 1.000%, Ta: 0 to 1.000%, Re: 0 to 1.000%, Os: 0 to 1.000%, Ir: 0 to 1.000%, Tc: 0 to 1.000%, Co: 0 to 1.000%, REM: 0 to 0.300%, and the balance: Fe and impurities, and when a position at 1 / 4 of the sheet thickness from the surface in the sheet thickness direction is defined as a 1 / 4 depth position, the metal structure at the 1 / 4 depth position has V-based precipitates, wherein, among the V-based precipitates, the V-based precipitates having a maximum diameter of less than 500 nm have a V content of 0.010 mass% or more, and the V-based precipitates having a maximum diameter of 500 nm or more have a V content of 0.015 mass% or more.

11. The steel sheet according to claim 10, characterized in that the chemical composition contains, in mass %, Nb: 0.01 to 0.10%.

12. The chemical composition is, in mass%, Ti: 0.005 to 0.200%, Cu: 0.10 to 2.00%, Ni: 0.10 to 2.00%, Cr: 0.03 to 1.00%, B: 0.0005 to 0.0200%, Mo: 0.10 to 2.00%, Ca: 0.001 to 0.020%, Mg: 0.001 to 0.020%, Sn: 0.01 to 1.00%, W: 0.10 to 1.00%, Sb: 0.010 to 1.000%, Zr: 0.010 to 1.000%, Se: 0.010 to 1.000%, Bi: 0.010 to 1.000%, 12. The steel sheet according to claim 10, comprising one or more elements selected from the group consisting of As: 0.010 to 1.000%, Ta: 0.010 to 1.000%, Re: 0.010 to 1.000%, Os: 0.010 to 1.000%, Ir: 0.010 to 1.000%, Tc: 0.010 to 1.000%, Co: 0.010 to 1.000%, and REM: 0.010 to 0.300%.

13. Steel sheet according to claim 10 or 11, characterized in that the surface has a coating.

14. Steel sheet according to claim 12, characterized in that the surface has a coating.

15. The steel sheet according to claim 13, wherein the coating is an Al-based coating.

16. The steel sheet according to claim 14, wherein the coating is an Al-based coating.

17. The steel sheet according to claim 13, characterized in that the coating is a Zn-based coating.

18. The steel sheet according to claim 14, characterized in that the coating is a Zn-based coating.

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