Steel member and steel sheet

Abstract

This steel member has a specific chemical composition, and if a range that is centered on the position corresponding to 1 / 4 the thickness from the surface in the thickness direction, and extends from the position corresponding to 1 / 8 the thickness to the position corresponding to 3 / 8 the thickness from the surface in the thickness direction is taken as the 1 / 4 depth position, and among the grain boundaries of crystal grains each having a body-centered structure at the 1 / 4 depth position, if L49-56° is the length of a grain boundary having a rotation angle of 49° to 56°, L64-72° is the length of a grain boundary having a rotation angle of 64° to 72°, L57-63° is the length of a grain boundary having a rotation angle of 57° to 63° and L4-12° is the length of a grain boundary having a rotation angle of 4° to 12° as measured using the <011> direction as the rotation axis, the ratio of the sum of L49-56° and L64-72° to the sum of L57-63° and L4-12°, that is (L49-56° + L64-72°) / (L57-63° + L4-12°), is 1.30 or more. This steel member has a tensile strength of more than 1,500 MPa.

Description

Steel members and steel plates

[0001] The present invention relates to a steel member and a steel plate. This application claims priority to Japanese Patent Application No. 2023-038695, filed on March 13, 2023, 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 press-forming difficult-to-form materials such as high-strength steel sheets, 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 this technology, the material is heated before being formed. Therefore, the steel is soft during forming and has good formability. This allows even high-strength steel sheets to be formed into complex shapes with high precision. In addition, in hot stamping, the steel is quenched simultaneously with forming using a press die, so the steel (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 not only for gasoline-powered vehicles but also for electric vehicles, which are rapidly being developed, 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 (a strength exceeding 1.5 GPa (1500 MPa)) that exceeds the strengths described in the aforementioned Patent Document 1 and those currently commonly used in steel members formed by hot stamping.

[0008] Regarding high-strength steel materials having a tensile strength exceeding 1.5 GPa, 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. Patent Document 5 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and also having good toughness.

[0009] 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

[0010] Many metallic materials experience deterioration in various properties as their strength increases, particularly in their susceptibility to hydrogen embrittlement. It is known that steel members exhibit increased susceptibility to hydrogen embrittlement when their tensile strength exceeds 1.2 GPa, and there are concerns that hot-stamped members with tensile strengths exceeding 1.5 GPa will exhibit even greater susceptibility to hydrogen embrittlement. To further reduce the weight of automobile bodies by using hot-stamped members with tensile strengths exceeding 1.5 GPa, it is desirable to further improve their resistance to hydrogen embrittlement.

[0011] 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] The present inventors investigated the influence of the metallographic structure and the steel sheet used as the base material on the properties of a steel member having high tensile strength and excellent hydrogen embrittlement resistance, and 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, an ultra-high tensile strength of more than 1.5 GPa can be obtained after heat treatment, including quenching, such as hot stamping.

[0014] On the other hand, as the tensile strength is increased 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 use in an automobile.

[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, and have found that the hydrogen embrittlement resistance can be improved by controlling the length ratio between grain boundaries having specific rotation angles.

[0016] (c) Furthermore, the present inventors have discovered that the length of a grain boundary having a specific rotation angle varies greatly depending on the state of existence of Mo (molybdenum) in the steel, and that the state of existence of Mo can be controlled by the manufacturing conditions.

[0017] 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 0.65%, Si: 0 to 2.00%, Mn: 0 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, Mo: 0.10 to 2.00%, 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%, and B: 0 to 0.02%. 00%, W: 0-1.00%, V: 0-1.00%, Ca: 0-0.020%, Mg: 0-0.010%, Al: 0-1.00%, Sn: 0-1.00%, Sb: 0-1.00%, Zr: 0-1.00%, Se : 0-1.00%, Bi: 0-1.00%, As: 0-1.00%, Ta: 0-1.00%, Re: 0-1.00%, Os: 0-1.00%, Ir: 0-1.00%, Tc: 0-1.00%, Co: 0-1.0 0%, REM: 0 to 0.30%, and the balance: Fe and impurities. When a range from a position of 1 / 4 of the thickness from the surface in the thickness direction, centered at a position of 1 / 4 of the thickness, to a position of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is defined as a 1 / 4 depth position, among grain boundaries of crystal grains having a body-centered structure at the 1 / 4 depth position, the length of the grain boundary having a rotation angle of 49 to 56° with the <011> direction as the rotation axis is defined as L49-56°, and the rotation angle is defined as L50-50°. [2] The steel member according to [1] may have a chemical composition, in mass%, of Nb: 0.01 to 0.10%, Nb-based precipitates are present at the 1 / 4 depth position, and the Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel member. [3] The steel member according to [1] or [2] may have a coating on the surface. [4] The steel member according to [3] may have the coating mainly composed of an Fe—Al alloy or an Fe—Zn alloy.[5] In the steel member according to any one of [1] to [4], the chemical composition may be, in mass%, W: more than 0% and less than 0.10%, or Mo: more than 1.00% and less than 2.00%. [6] In the steel member according to any one of [1] to [5], the (L49-56°+L64-72°) / (L57-63°+L4-12°)) may be less than 1.40. [7] In another aspect of the present invention, a steel sheet has a chemical composition, in mass%, of C: 0.26 to 0.65%, Si: 0 to 2.00%, Mn: 0 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, Mo: 0.10 to 2.00%, Nb: 0 to 0.10%, Ti: 0 to 0.200%, Cu: 0-2.00%, Ni: 0-2.00%, Cr: 0-1.00%, B: 0-0.0200%, W: 0-1.00%, V: 0-1.0 0%, Ca: 0-0.020%, Mg: 0-0.010%, Al: 0-1.00%, Sn: 0-1.00%, Sb: 0-1.00%, Zr: 0-1.00% , Se: 0-1.00%, Bi: 0-1.00%, As: 0-1.00%, Ta: 0-1.00%, Re: 0-1.00%, Os: 0-1.00%, Ir: 0-1.00%, Tc: 0-1.00%, Co: 0-1.00%, REM: 0-0.30%, and the balance: Fe and impurities, and when a range from a position 1 / 8 to a position 3 / 8 of the thickness in the thickness direction from the surface, centered at a position 1 / 4 of the thickness, is defined as a 1 / 4 depth position, at the 1 / 4 depth position, the range is surrounded by a boundary where the crystal orientation misorientation is 5° or more, and the area fraction of a region within the boundary where the average crystal orientation misorientation is 0.4 to 3.0° is 80% or less. [8] The steel sheet according to [7] may have the chemical composition, in mass%, of Nb: 0.01 to 0.10%, Nb-based precipitates are present at the 1 / 4 depth position, and the Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel sheet. [9] The steel sheet according to [7] or [8] may have a coating on the surface.

[10] The steel sheet according to [9] may have the coating be an Al-based coating or a Zn-based coating.

[11] The steel sheet according to any of [7] to

[10] may have the chemical composition, in mass%, of W: more than 0% and less than 0.10%, or Mo: more than 1.00% and less than 2.00%.

[0018] 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.

[0019] <Steel Member> A steel member according to one embodiment of the present invention (steel member according to this embodiment) will be described. Hereinafter, the range from 1 / 8 to 3 / 8 of the thickness in the thickness direction from the surface, with the center at the 1 / 4 position of the thickness from the surface, will be described as the 1 / 4 depth position. The steel member according to this embodiment has a predetermined chemical composition, and at the 1 / 4 depth position, (L49-56° + L64-72°) / (L57-63° + L4-12°) is 1.30 or more, and the tensile strength is greater than 1500 MPa. Each of these will be described.

[0020] [Chemical Composition] The chemical composition of the steel member according to this embodiment is, in mass %, C: 0.26 to 0.65%, Si: 0 to 2.00%, Mn: 0 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, Mo: 0.10 to 2.00%, 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%, W: 0 to 1.00%, V: 0 to 1.00%, and 00%, Ca: 0-0.020%, Mg: 0-0.010%, Al: 0-1.00%, Sn: 0-1.00%, Sb: 0-1.00%, Zr: 0-1.00%, Se: 0-1.00%, Bi: 0-1.00%, As: 0-1.00%, Ta: 0-1.00%, Re: 0-1.00%, Os: 0-1.00%, Ir: 0-1.00%, Tc: 0-1.00%, Co: 0-1.00%, REM: 0-0.30%, and the balance: Fe and impurities. The reasons for limiting the content of each element are as follows.

[0021] (C: 0.26 to 0.65%) 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 quenching such as hot stamping. If the C content is less than 0.26%, it becomes difficult to ensure sufficient strength (more than 1.5 GPa (1500 MPa)) in 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, and more preferably set to 0.31% or more or 0.32% or more. Furthermore, to obtain even higher tensile strength, for example, 2300 MPa or more, the C content is preferably set to 0.45% or more. On the other hand, if the C content exceeds 0.65%, the strength of the steel member after quenching becomes excessively high, resulting in a significant decrease in hydrogen embrittlement resistance. Therefore, the C content is set to 0.65% or less. The C content is preferably 0.60% or less, and more preferably 0.55% 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, more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, if the Si content in the 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 1.80% or less, more preferably 1.50% or less, and even more preferably 1.10% or less.

[0023] (Mn: 0 to 3.00%) Mn does not necessarily need to be contained (it may be 0%), but it is a very effective element for improving the hardenability of steel and ensuring stable strength after quenching. Mn also lowers the Ac3 point and promotes lowering the quenching temperature. Therefore, Mn may be contained. To obtain the above effects, the Mn content is preferably 0.05% or more, more preferably 0.15% or more, and even more preferably 0.25% or more or 0.30% or more. On the other hand, if the Mn content exceeds 3.00%, the hydrogen embrittlement resistance of the steel member after quenching deteriorates. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, more preferably 1.80% or less, and even more preferably 1.50% or less.

[0024] (P: 0.100% or less) P is an element that reduces the hydrogen embrittlement resistance of steel members 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 0.055% or less, and more preferably to 0.020% or less. Since a lower P content is preferable, the P content may be 0%, but may be 0.001% or more from the viewpoint of cost.

[0025] (S: 0.0100% or less) S is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, if the S content exceeds 0.0100%, the hydrogen embrittlement resistance is significantly reduced. Therefore, the S content is limited to 0.0100% or less. The S content is preferably limited to 0.0050% or less, and more preferably to 0.0030% or less. Since a low S content is preferable, the S content may be 0%, but from the viewpoint of cost, it may be 0.0001% or more.

[0026] (N: 0.020% or less) N is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, if the N content exceeds 0.020%, coarse nitrides are formed in the steel, significantly reducing hydrogen embrittlement resistance. Therefore, the N content is set to 0.020% or less. The N content is preferably 0.015% or less, 0.010% or less, or 0.006% 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 0.0002% increases steelmaking costs and is therefore economically undesirable. Therefore, the N content may be set to 0.0002% or more, 0.0008% or more, or 0.001% or more.

[0027] (O: 0.010% or less) O ​​is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, if the O content exceeds 0.010%, coarse nitrides are formed in the steel, significantly reducing hydrogen embrittlement resistance. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.007% or less, 0.005% or less, or 0.003% 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 0.0002% increases steelmaking costs and is therefore economically undesirable. Therefore, the O content may be set to 0.0002% or more, 0.0008% or more, or 0.001% or more.

[0028] (Mo: 0.10 to 2.00%) Mo is an important element in the steel member according to this embodiment. Mo is an element that segregates at grain boundaries and is effective for promoting the development of grain boundaries having the above-mentioned specific rotation angle. Mo is also an element that is effective for improving the hardenability of steel and stably ensuring the strength of the steel member after quenching. Mo is also an element that improves corrosion resistance in a corrosive environment. If the Mo content is less than 0.10%, sufficient effects cannot be obtained. Therefore, the Mo content is set to 0.10% or more. The Mo content is preferably set to 0.20% or more, more preferably set to 0.40% or more, and may be set to more than 1.00%. On the other hand, if the Mo content exceeds 2.00%, the above effects saturate and economic efficiency decreases. Therefore, the Mo content is set to 2.00% or less. The Mo content is preferably set to 1.50% or less, and more preferably set to 1.00% or less.

[0029] (Nb: 0 to 0.10%) Nb is an element that forms fine carbides, nitrides, or carbonitrides in steel, and suppresses Cu hot embrittlement cracking during the hot rolling process due to the grain refinement effect of these precipitates. Furthermore, in the steel sheet according to this embodiment, enriching Mo in the Nb-based precipitates (higher than the Mo concentration (Mo content) of the base steel) improves the hydrogen embrittlement resistance of the steel member. Therefore, the Nb content may be 0%, but Nb may also be contained. To obtain the above effect, 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 carbonitrides become coarse, promoting bending straightening cracks during the continuous casting process. Furthermore, solute Nb inhibits the development of grain boundaries with a specific rotation angle in the steel member, as described below, thereby reducing the hydrogen embrittlement resistance of the steel member. Therefore, the Nb content is set to 0.10% or less. The Nb content is preferably 0.08% or less.

[0030] (Ti: 0 to 0.200%) Ti is an element that forms fine carbides, carbonitrides, etc. with Nb in steel, and due to the grain refinement effect, suppresses Cu hot embrittlement cracking during the hot rolling process and improves the hydrogen embrittlement resistance of the steel member. Ti also preferentially bonds with N in the steel to form nitrides, suppresses the consumption of solute B due to the precipitation of BN, and promotes the effect of B improving hardenability, which will be described later. Therefore, Ti may not be contained, that is, the Ti content may be 0%, but Ti may also be contained. To obtain 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 carbonitrides, etc., become coarse, promoting bending straightening cracks during the continuous casting process. Furthermore, solute Ti inhibits the development of grain boundaries having a specific rotation angle in the steel member, as described below, reducing the hydrogen embrittlement resistance of the steel member. Furthermore, in addition to carbonitrides with Nb and TiN, the amount of TiC precipitated increases, consuming C, reducing the strength of the steel member after quenching. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably set to 0.080% or less, and more preferably to 0.050% or less.

[0031] (Cu: 0 to 2.00%) Cu is an element effective in 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 not be contained, that is, the Cu content may be 0%, but Cu may also 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.

[0032] (Ni: 0 to 2.00%) Ni is an element effective for improving the hardenability of steel and stably ensuring the strength of steel members after quenching. Ni is also an element that has the effect of suppressing Cu hot embrittlement cracking in the production of steel sheets. Therefore, Ni may not be contained, that is, the Ni content may be 0%, but Ni may also be contained. To obtain the above effects, the Ni content is preferably 0.10% or more, and more preferably 0.20% 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 or 0.10% or less.

[0033] (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 absent, i.e., the Cr content may be 0%, but Cr may also 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. Furthermore, 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, which may reduce the hydrogen embrittlement resistance 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 0.30% or less, and even more preferably 0.15% or less.

[0034] (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 during heating of the steel sheet. Therefore, B may not be contained, i.e., the B content may be 0%, but B may also be contained. To obtain the above effects, the B content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more. On the other hand, if the B content exceeds 0.0200%, many coarse compounds precipitate, 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, and more preferably 0.0050% or less.

[0035] (W: 0 to 1.00%) W is a highly effective element for improving the hardenability of steel and stably ensuring the strength of steel members after quenching. Therefore, W may be absent, i.e., the W content may be 0%, but W may also be contained. To obtain the above effects, the W content is preferably 0.01% or more, and more preferably 0.10% or more or 0.20% or more. On the other hand, W is an element that stabilizes iron carbides. If the W content exceeds 1.00%, coarse iron carbides may remain undissolved when the steel plate is heated, which may reduce the hydrogen embrittlement resistance 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, and may be less than 0.10%.

[0036] (V: 0 to 1.00%) V is an element that forms fine carbides in steel and improves the hydrogen embrittlement resistance of steel members due to the grain refining effect and hydrogen trapping effect of the carbides. Therefore, V may not be contained, that is, the V content may be 0%, but V may also be contained. To obtain the above effect, the V content is preferably 0.01% or more, and more preferably 0.10% or more. On the other hand, if the V content exceeds 1.00%, the above effect saturates and economic efficiency decreases. Therefore, if V is contained, the V content is set to 1.00% or less. The V content is preferably 0.50% or less or 0.20% or less.

[0037] (Ca: 0 to 0.020%) Ca is an element that has the effect of refining inclusions in steel and increasing the hydrogen embrittlement resistance of steel members after quenching. Therefore, Ca may not be contained, that is, the Ca content may be 0%, but Ca may also 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 costs increase. Therefore, if Ca is contained, the Ca content is set to 0.020% or less. The Ca content is preferably 0.006% or less, and more preferably 0.004% or less.

[0038] (Mg: 0 to 0.010%) Mg is an element that has the effect of refining inclusions in steel and improving hydrogen embrittlement resistance after heat treatment. Therefore, Mg may not be contained, that is, the Mg content may be 0%, but Mg may also 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 0.010%, the effect saturates and costs increase. Therefore, if Mg is contained, the Mg content is set to 0.010% or less. The Mg content is preferably 0.005% or less, more preferably 0.004% or less.

[0039] (Al: 0 to 1.00%) Al is an element commonly used as a deoxidizer for steel. Therefore, Al may not be contained, that is, the Al content may be 0%, but Al may also be contained. In order to obtain the above effects, the Al content is preferably 0.01% or more. On the other hand, if the Al content exceeds 1.00%, the above effects saturate and economic efficiency decreases. Therefore, if Al is contained, the Al content is 1.00% or less. The Al content is preferably 0.20% or less, and may be 0.05% or less.

[0040] (Sn: 0 to 1.00%) Sn is an element that improves corrosion resistance in a corrosive environment. Therefore, Sn may not be contained, that is, the Sn content may be 0%, but Sn may also 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 is preferably 0.30% or less, and may be 0.10% or less.

[0041] (Sb: 0 to 1.00%) Sb is an element that improves corrosion resistance in a corrosive environment. Therefore, Sb may not be contained, that is, the Sb content may be 0%, but Sb may also be contained. To obtain the above effects, the Sb content is preferably 0.01% or more. On the other hand, if the Sb content exceeds 1.00%, 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 1.00% or less. The Sb content is preferably 0.30% or less, and may be 0.20% or less.

[0042] (Zr: 0 to 1.00%) Zr is an element that improves corrosion resistance in a corrosive environment. Therefore, Zr may not be contained, that is, the Zr content may be 0%, but Zr may also be contained. To obtain the above effects, the Zr content is preferably 0.01% or more. On the other hand, if the Zr content exceeds 1.00%, 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 1.00% or less. The Zr content is preferably 0.35% or less.

[0043] (Se: 0 to 1.00%) Se is an element that improves hydrogen embrittlement resistance. Therefore, Se may not be contained, that is, the Se content may be 0%, but Se may also be contained. To obtain the above effects, the Se content is preferably 0.01% or more. On the other hand, if the Se content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if Se is contained, the Se content is 1.00% or less. The Se content is preferably 0.40% or less.

[0044] (Bi: 0 to 1.00%) Bi is an element that improves hydrogen embrittlement resistance. Therefore, Bi may not be contained, that is, the Bi content may be 0%, but Bi may also be contained. To obtain the above effects, the Bi content is preferably 0.01% or more. On the other hand, if the Bi content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if Bi is contained, the Bi content is 1.00% or less. The Bi content is preferably 0.30% or less.

[0045] (As: 0 to 1.00%) As is an element that improves hydrogen embrittlement resistance. Therefore, As may not be contained, that is, the As content may be 0%, but As may be contained. To obtain the above effects, the As content is preferably 0.01% or more. On the other hand, if the As content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if As is contained, the As content is 1.00% or less. The As content is preferably 0.40% or less.

[0046] (Ta: 0 to 1.00%) Ta is an element that improves hydrogen embrittlement resistance. Therefore, Ta may not be contained, that is, the Ta content may be 0%, but Ta may also be contained. To obtain the above effects, the Ta content is preferably 0.01% or more. On the other hand, if the Ta content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if Ta is contained, the Ta content is 1.00% or less. The Ta content is preferably 0.50% or less.

[0047] (Re: 0 to 1.00%) Re is an element that improves hydrogen embrittlement resistance. Therefore, Re may not be contained, that is, the Re content may be 0%, but Re may also be contained. To obtain the above effects, the Re content is preferably 0.01% or more. On the other hand, if the Re content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if Re is contained, the Re content is 1.00% or less. The Re content is preferably 0.40% or less.

[0048] (Os: 0 to 1.00%) Os is an element that improves hydrogen embrittlement resistance. Therefore, Os may not be contained, that is, the Os content may be 0%, but Os may also be contained. To obtain the above effects, the Os content is preferably 0.01% or more. On the other hand, if the Os content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if Os is contained, the Os content is 1.00% or less. The Os content is preferably 0.20% or less.

[0049] (Ir: 0 to 1.00%) Ir is an element that improves hydrogen embrittlement resistance. Therefore, Ir may not be contained, that is, the Ir content may be 0%, but Ir may also be contained. To obtain the above effects, the Ir content is preferably 0.01% or more. On the other hand, if the Ir content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if Ir is contained, the Ir content is 1.00% or less. The Ir content is preferably 0.30% or less.

[0050] (Tc: 0 to 1.00%) Tc is an element that improves hydrogen embrittlement resistance. Therefore, Tc may not be contained, that is, the Tc content may be 0%, but Tc may also be contained. To obtain the above effects, the Tc content is preferably 0.01% or more. On the other hand, if the Tc content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if Tc is contained, the Tc content is set to 1.00% or less. The Tc content is preferably 0.40% or less, and more preferably 0.15% or less.

[0051] (Co: 0 to 1.00%) Co is an element that improves corrosion resistance in a corrosive environment. Therefore, Co may not be contained, that is, the Co content may be 0%, but Co may also be contained. To obtain the above effects, the Co content is preferably 0.01% or more. On the other hand, if the Co content exceeds 1.00%, the above effects saturate and economic efficiency decreases. Therefore, if Co is contained, the Co content is set to 1.00% or less. The Co content is preferably 0.40% or less, and more preferably 0.10% or less.

[0052] (REM: 0 to 0.30%) Like Ca, REM is an element that refines inclusions in steel and improves the hydrogen embrittlement resistance of steel members after quenching. Therefore, REM may be absent, i.e., the REM content may be 0%, but REM may also be contained. To obtain the above effect, the REM content is preferably 0.01% or more, and more preferably 0.02% or more. On the other hand, if the REM content exceeds 0.30%, the effect saturates and costs increase. Therefore, if REM is contained, the REM content is set to 0.30% or less. The REM content is preferably 0.20% 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.

[0053] (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 within a range that does not adversely affect 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. It is obtained by performing elemental analysis using a common method such as ICP-AES 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). C and S, which are difficult to measure using ICP-AES, can be measured using combustion-infrared absorption, N using inert gas fusion-thermal conductivity, and O using inert gas fusion-non-dispersive infrared absorption. However, since the chemical composition at the 1 / 4 depth position does not substantially change during the manufacturing process, when the analytical value of the chemical composition of the molten steel or the steel plate is known, the analytical value of the chemical composition of the molten steel or the steel plate may be used as the chemical composition of the steel member.

[0054] Here, the surface that serves as the reference for the 1 / 4 depth position is the surface of the steel member, but if the steel member has a coating, i.e., if the steel member has a coating on its surface, the surface means the surface of the base steel material excluding the coating.

[0055] [Metal Structure] In the steel member according to this embodiment, the metal structure (microstructure) at the 1 / 4 depth position is specified as described below.

[0056] (L49-56°+L64-72°) / (L57-63°+L4-12°): 1.30 or more) Grain boundaries of crystal grains having a body-centered structure mainly consist of one or more of the following: grain boundaries with a rotation angle of 4 to 12°, grain boundaries with a rotation angle of 49 to 56°, grain boundaries with a rotation angle of 57 to 63°, and grain boundaries with a rotation angle of 64 to 72°. In particular, martensite often consists mainly of the above four types of grain boundaries. Of these, grain boundaries with a rotation angle of 49 to 56° and grain boundaries with a rotation angle of 64 to 72° are effective in suppressing the initiation and propagation of ductile cracks with respect to hydrogen-mediated ductile fracture behavior, which is the fracture behavior of hydrogen embrittlement cracking. On the other hand, grain boundaries with a rotation angle of 57 to 63° and grain boundaries with a rotation angle of 4 to 12° are not effective. Therefore, in the steel member according to this embodiment, at the 1 / 4 depth position, the sum (L49-56° + L64-72°) of the length of the grain boundary where the rotation angle is 49 to 56° (L49-56°) and the length of the grain boundary where the rotation angle is 64 to 72° (L64-72°) with respect to the sum (L57-63° + L4-12°) of the length of the grain boundary where the rotation angle is 57 to 63° (L57-63°) and the length of the grain boundary where the rotation angle is 4 to 12° (L4-12°) is increased to improve hydrogen embrittlement resistance. If the ratio (L49-56° + L64-72°) / (L57-63° + L4-12°) is less than 1.30, sufficient improvement in hydrogen embrittlement resistance cannot be obtained. The ratio (L49-56° + L64-72°) / (L57-63° + L4-12°) is preferably 1.50 or more, more preferably 1.60 or more. There is no upper limit, but it is substantially 2.80 or less. To improve hydrogen embrittlement resistance, a higher ratio (L49-56° + L64-72°) / (L57-63° + L4-12°) is preferable. However, if a very high hydrogen embrittlement resistance is not required and only the minimum necessary hydrogen embrittlement resistance is sufficient, the ratio (L49-56° + L64-72°) / (L57-63° + L4-12°) may be limited to a lower range. For example, it may be less than 1.40, if necessary. Here, the grain boundary having a rotation angle of A to B° with the <011> direction as the rotation axis means a grain boundary in which adjacent crystal grains on either side of the grain boundary overlap when rotated by A to B° around the <011> direction as the rotation axis.

[0057] (L49-56° + L64-72°) / (L57-63° + L4-12°) can be measured using the following method. A sample is cut from a position at least 50 mm away from the end of the steel member so that a cross section perpendicular to the surface (thickness cross section) can be observed. The sample should be long enough to observe a cross section of approximately 10 mm in the thickness direction, depending on the measurement device. The cut sample's cross section is then polished. For example, the cross section of the cut sample is polished using waterproof paper from #320 to #1200 or larger, and then polished to a mirror finish using a diamond suspension with a particle size of 3 to 1 μm. Next, electrolytic polishing is performed to remove strain introduced into the surface layer of the cross section. Crystal orientation information is obtained by performing EBSD analysis on a 50 μm x 50 μm measurement area at a measurement interval of 0.1 μm at a 1 / 4 depth position, which is between 1 / 8 and 3 / 8 of the way from the surface to the thickness direction, with the center at 1 / 4 of the way from the surface of the steel member in the thickness direction. Here, the EBSD analysis is performed using, for example, a device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) at an analysis speed of 200 to 300 points per second. A scanning electron microscope and an EBSD detector with performance equal to or better than those described above may also be used, but JEOL and TSL detectors are preferred. Next, from the obtained crystal orientation information, the lengths of the grain boundaries of crystal grains having a body-centered structure, where the rotation angle is 49 to 56° with respect to the <011> direction as the rotation axis, the lengths of the grain boundaries where the rotation angle is 64 to 72°, the lengths of the grain boundaries where the rotation angle is 57 to 63°, and the lengths of the grain boundaries where the rotation angle is 4 to 12°, are determined, and the respective results are used to calculate (L49-56°+L64-72°) / (L57-63°+L4-12°). The length of the grain boundary can be easily calculated, for example, by using the "Inverse Pole Figure Map" and "Axis Angle" functions included in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. With these functions, the total length of the grain boundary of a crystal grain having a body-centered structure can be calculated by specifying a specific rotation angle around an arbitrary direction as the rotation axis.The above analysis is performed on all crystal grains included in the measurement region, and the lengths of the four types of grain boundaries mentioned above are calculated using the <011> direction as the rotation axis. Five fields of view are measured, and the average value of (L49-56° + L64-72°) / (L57-63° + L4-12°) for each field of view is taken as (L49-56° + L64-72°) / (L57-63° + L4-12°) in this embodiment. The formation of grain boundaries with rotation angles of 57 to 63° and grain boundaries with rotation angles of 4 to 12°, with the <011> direction as the rotation axis, is suppressed by segregating Mo to the grain boundaries.

[0058] (Preferably, Nb-based precipitates are present, and the Mo concentration (content) of the Nb-based precipitates is 4.5 times or more the Mo content of the steel member.) In order to further enhance the above effect, it is preferable that Nb-based precipitates that dissolve Mo are present at the 1 / 4 depth position. However, even if Nb-based precipitates are present at the 1 / 4 depth position, if the Mo concentration is less than 4.5 times the Mo content of the steel member, the effect cannot be sufficiently obtained. Therefore, it is preferable that Nb-based precipitates are present whose Mo concentration is 4.5 times or more the Mo content of the steel member. The Mo concentration of the Nb-based precipitates is preferably 8.0 times or more, more preferably 10.0 times or more the Mo content of the steel member. Furthermore, the size of the Nb-based precipitates is preferably 15 μm or less. The Nb-based precipitates of interest in this embodiment are precipitates containing 50 mass % or more of Nb, such as Nb carbide, Nb carbonitride, Nb nitride, NbTi carbide, and NbTi carbonitride.

[0059] The presence or absence of Nb-based precipitates and the Mo concentration (content) of the Nb-based precipitates are determined by the following method. A sample is taken from a position 1 / 4 of the plate width (short side) from the width direction end of the steel member so that the cross section of the steel member in the thickness direction can be observed. A COMPO image is obtained for this sample using a scanning electron microscope to confirm the presence of Nb-based precipitates. Nb-based precipitates contain a large amount of Nb, which is a heavier element than Fe, and therefore appear brighter than the iron substrate. The Mo content contained in the Nb-based precipitates can be determined by performing spot elemental analysis (beam diameter: 0.5 μm) on these bright Nb-based precipitates using an electron probe microanalyzer (EPMA). During the measurement, observation is performed at a magnification that allows observation of the Nb-based precipitates, and the field of view is changed until 10 Nb-based precipitates are found, or until the total area of ​​the observation field reaches 1,250,000 μm. ２ Observe until it reaches 1,250,000 μm. ２ If no Nb-based precipitates are observed in the observation field, it is determined that no Nb-based precipitates are present at the 1 / 4 depth position. Each observed Nb-based precipitate is analyzed, and the average value is taken as the Mo content of the Nb-based precipitate. That is, if 10 Nb-based precipitates are observed, the average of the Mo contents of the 10 Nb-based precipitates is taken as the Mo content of the Nb-based precipitates. If there are fewer than 10 Nb-based precipitates, the average of the Mo contents of the Nb-based precipitates is taken as the Mo content of the Nb-based precipitates. Nb-based precipitates may also contain C, N, Ti, Cr, and B. In the measurement, Nb-based precipitates are those with a size of 0.5 μm or more and an Nb concentration of 50 mass% or more. Most of the Nb-based precipitates in the steel member of this embodiment have a size of 1.0 to 12.0 μm. The size of the Nb-based precipitates is defined as the average value of the distance between parallel lines in the horizontal direction sandwiching the second region (horizontal Feret diameter) and the distance between parallel lines in the vertical direction sandwiching the second region (vertical Feret diameter). The horizontal direction is the longitudinal direction of the steel member, and the vertical direction is the thickness direction perpendicular to the longitudinal direction.

[0060] In order to obtain a tensile strength of more than 1.5 GPa, the steel member according to this embodiment preferably contains martensite at an area fraction (area %) of 75% or more. Furthermore, since grain boundaries having a specific rotation angle, which will be described later, are easier to control in martensite than in a body-centered structure, a high area fraction of martensite is preferable. The martensite fraction is more preferably 90% or more in area fraction. The area fraction of martensite may be 100%. Martensite also includes tempered martensite and auto-tempered martensite. Auto-tempered martensite is tempered martensite formed during cooling during quenching without heat treatment for tempering, and is formed by in-situ tempering of the martensite formed by self-heating associated with martensitic transformation.

[0061] The steel member may contain retained austenite and / or bainite in addition to martensite at the 1 / 4 depth position. Ferrite and pearlite are not contained, and the area fractions of ferrite and pearlite are 0%. The total area fraction of martensite, retained austenite, and bainite is preferably 98% or more or 99% or more, and more preferably 100%. The area fraction of martensite is preferably 95% or more or 97% or more, and more preferably 98% or more. The upper limit of the area fraction of martensite is 100%.

[0062] The area fraction of the metal structure of a steel member can be measured by the following method. The area 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. Because the TEM also observes the structure in the thickness direction of a thin film sample, the structure fraction is essentially a volume fraction. However, the steel member of this embodiment has a structure mainly composed of martensite, and the size of the structure identified as martensite is much larger than the thickness of the thin film. Therefore, in the TEM photograph (field of view), there is almost no boundary between martensite and a structure other than martensite (substantially bainite) in the depth direction of the photograph (field of view). If a boundary exists, it is excluded from the observation area. Therefore, in this embodiment, the area fraction of each structure is calculated based on the area ratio of each structure in the TEM photograph (field of view). Specifically, a measurement sample is cut out from the widthwise end of the steel member at a position 1 / 4 of the width of the steel member (1 / 4 width position) and including a position 1 / 4 depth of the steel member, to serve as a thin film sample for TEM observation. The thin film sample was placed at 400 μm depth from the steel member at a depth of 1 / 4 of the thickness. 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), and if it is unidirectional precipitation, it is determined to be bainite. Martensite (fresh martensite) is also determined when no iron carbide precipitation is observed. Carbides are observed to distinguish between martensite and bainite, but in this embodiment, the carbides themselves are not included in the area fraction of the structure. The area fraction of retained austenite is measured using X-ray diffraction. Specifically, a measurement sample is cut out from a position 1 / 4 of the width of the steel member from the widthwise end (1 / 4 width position) to serve as the sample for X-ray diffraction. The cut sample is chemically polished from the surface to a depth of 1 / 4 of the thickness using hydrofluoric acid and hydrogen peroxide. If chemical polishing takes too long, for example, the surface may be polished using waterproof paper down to about 1 / 8 of the thickness, and then chemical polishing may be continued down to 1 / 4 of the thickness. Measurement conditions are as follows: a Co tube is used, and 2θ is in the 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 volume fraction obtained by X-ray diffraction of retained austenite is regarded as the area fraction. Measuring the area fractions of martensite, retained austenite, and bainite is complicated. Instead of measuring these area fractions, the area fractions of ferrite and pearlite may be measured using the following method, and the value obtained by subtracting the sum from 100% may be regarded as the total area fraction of martensite, retained austenite, and bainite. The presence of ferrite or pearlite can be easily confirmed using an optical microscope or scanning electron microscope. Specifically, a measurement sample including a position of 1 / 4 width and a position of 1 / 4 depth of the steel member is cut out to serve as a sample for observation. The cut sample is mechanically polished and then mirror-finished. Next, the sample is etched with a nital etching solution to reveal ferrite and pearlite, and a 40,000 μm area at a position of 1 / 4 depth of the steel member is measured using a scanning electron microscope. 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 as pearlite, and cementite is determined as granular precipitation of bainite. The structure fraction measured by a scanning electron microscope is an area fraction (area %). Here, in this embodiment, the above-mentioned Nb-based precipitates themselves are not considered as the area fraction of the Nb-based precipitates, but rather as the area fraction of the metal structure surrounding the Nb-based inclusions. Therefore, their presence is not taken into consideration when measuring the above area fraction.

[0063] [Tensile Strength] The steel member according to this embodiment has a tensile strength of more than 1500 MPa (1.5 GPa) in order to contribute to improving both fuel economy and crashworthiness when applied to automobile components. The tensile strength is preferably 1800 MPa or more, and more preferably 2300 MPa or more. On the other hand, in terms of hydrogen embrittlement resistance, the tensile strength is preferably 3150 MPa or less, and more preferably 2850 MPa or less. The tensile strength can be measured in accordance with ASTM E8M-22. Specifically, a tensile test piece of subsize test piece (parallel portion width: 6.0±0.1 mm, gauge length: 25.0±0.1 mm) in Table 1 of ASTM E8M-22 can be taken and subjected to a tensile test to measure the tensile strength (TS). When the steel member is small, for example, and the tensile test piece cannot be obtained, the average hardness in the plate thickness direction is measured by a Vickers hardness test (HV1 with a test force of 9.807 N) in accordance with JIS Z2244-1:2020, and the obtained average hardness in the plate thickness direction is converted into tensile strength using a known hardness conversion table (for example, SAE J417-1983). This value may be regarded as the tensile strength of the steel member according to this embodiment.

[0064] The shape of the steel member according to this embodiment is not particularly limited. That is, the steel member may be a flat plate, or a formed body formed from a steel plate into a predetermined shape. Hot-formed steel members are often formed bodies, but in this embodiment, the term "steel member" encompasses both formed bodies and flat plates. The steel member may also be part of a tailored property material whose strength varies depending on the location. In this case, the tailored property material is a steel member that combines the steel member according to this embodiment with a steel member other than this embodiment, and the entire tailored property material does not need to satisfy the above-described chemical composition, metallographic structure (here, L49-56° + L64-72°) / (L57-63° + L4-12°), and tensile strength. It is sufficient that at least a portion of the tailored property material satisfies the above-described chemical composition, metallographic structure (here, L49-56° + L64-72°) / (L57-63° + L4-12°), and tensile strength, and there is no need to specify the proportions thereof. Tailored property materials may be made by joining steel plates with different chemical compositions, strengths, or thicknesses, or may be made by partially heat-treating steel plates. Furthermore, the steel members may have a decarburized layer or a soft layer on part of their surface.

[0065] [Thickness] The thickness of the steel member (which may also be referred to as the thickness of the steel plate constituting the steel member when the steel member is a member obtained by processing a steel plate) is not limited. In the case of an automotive steel member manufactured by hot stamping, the thickness may be 0.6 mm or more or 0.8 mm or more, based on the main thickness range in which the steel member is used. For the same reason, the thickness may be 4.0 mm or less or 2.5 mm or less.

[0066] [Coating] The steel member according to this embodiment may have a coating on part 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. It is not necessary to exclude coatings other than coatings primarily made of an Fe—Al alloy and coatings primarily made of an Fe—Zn alloy; other coatings, such as Sn-based coatings or non-metallic resin coatings, or multiple layers thereof, may also be used. A coating primarily made of an Fe—Al alloy is a coating containing a total of 70% by mass or more of Fe and Al, and a coating primarily made of an Fe—Zn alloy is a coating containing a total of 70% by mass or more of Fe and Zn. The coating mainly made of an Fe-Al alloy may further contain 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 Fe and Al, with the balance being impurities. The coating mainly made of an Fe-Zn alloy may further contain 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 Fe and Zn, with the balance being impurities. The total content of impurities may be 1% or less. The coating provides corrosion resistance, thereby improving hydrogen embrittlement resistance in automotive use. The thickness of the coating is preferably 10 to 100 μm.

[0067] The chemical composition and thickness of the coating can be determined by observing the cross section with a scanning electron microscope. Specifically, a measurement sample is cut out from a longitudinal half portion (a position half the length from the longitudinal end) and a width quarter portion (a position one-quarter the width from the width 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 coating is within the above range. The cut sample is mechanically polished and then mirror-finished. Next, the thickness of the coating is measured in 10 random fields of view, and the average value is used as the coating thickness. When observed using a BSE image (or COMPO image), a clear difference in contrast is confirmed between the coating and the base steel (steel plate substrate). Therefore, the thickness of the coating can be determined by measuring the thickness from the outermost surface to the point where the contrast changes. Measurements are made at 20 equally spaced locations within the observation photograph, with the distance between measurement locations being 6.5 μm. Furthermore, five fields of view are observed as described above, and the average value is used to determine the coating thickness. Furthermore, the chemical composition of the coating can be determined by performing spot elemental analysis (beam diameter: 0.5 μm) on the same observation area as above using an electron probe microanalyzer (EPMA) to determine the Fe, Al, and Zn contents contained in the coating. A total of 10 points are analyzed in the coating in 10 random fields of view, and the average value is used as 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.

[0068] <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.

[0069] [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 equivalent to the chemical composition of the steel member according to this embodiment. The chemical composition of the steel plate can be measured from the 1 / 4 depth position (a range of 1 / 8 to 3 / 8 of the thickness from the surface in the plate thickness direction) using the same method as for the steel member. As described above, since the chemical composition at the 1 / 4 depth position does not substantially change during the manufacturing process, if the analytical value of the chemical composition of the molten steel is known, the analytical value of the chemical composition of the molten steel may be used as the chemical composition of the steel plate. 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 refers to the surface of the base steel plate excluding the coating.

[0070] [Metal structure] The metal structure at a 1 / 4 depth position is defined, which is in the range from 1 / 8 to 3 / 8 of the thickness in the thickness direction from the surface, with the center being a 1 / 4 position of the thickness (plate thickness) in the thickness direction from the surface.

[0071] (Area fraction of regions surrounded by boundaries with a crystal orientation misorientation of 5° or more, where the average crystal orientation misorientation within the boundaries is 0.4 to 3.0°: 80% or less) In the steel sheet according to this embodiment, at the 1 / 4 depth position, the area fraction of regions (crystal grains) surrounded by boundaries with a crystal orientation misorientation of 5° or more, where the average crystal orientation misorientation within the boundaries is 0.4 to 3.0°, is 80% or less. This region is the base structure from which the metal structure of the steel member described above is obtained in the heat treatment described below. If the area fraction of this region exceeds 80%, the steel member may have a ratio (L49-56° + L64-72°) / (L57-63° + L4-12°) of less than 1.30. The area fraction of this region is preferably 50% or less, more preferably 35% or less, and even more preferably 30% or less. The area fraction of this region may be 1% or more, or may be 10% or more. As long as the area fraction of the above-mentioned region is 80% or less, there are no restrictions on the metal structure of the other portions.

[0072] This section describes a method for measuring the area fraction of crystal grains with an average crystal orientation misorientation of 0.4 to 3.0° within crystal grains surrounded by crystal boundaries with a crystal orientation misorientation of 5° or more (hereinafter referred to as the "area fraction of a specific structure"). A sample is cut from a position 50 mm or more away from the edge of the steel sheet so that a cross section perpendicular to the surface (thickness cross section) can be observed. The size of the sample is such that a cross section of approximately 10 mm in the thickness direction can be observed, depending on the measurement device. Crystal orientation information is obtained by performing EBSD analysis on a 100 μm × 100 μm measurement area, centered at a position ¼ of the thickness from the surface, at a measurement interval of 0.2 μm for the cut sample. Here, the EBSD analysis is performed at an analysis speed of 200 to 300 points per second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL). A scanning electron microscope and EBSD detector having performance equal to or better than those described above may be used, but those manufactured by JEOL and TSL are preferred.

[0073] The area fraction of the specific structure can be easily calculated from the obtained crystal orientation information by using, for example, the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. This function calculates the misorientation between adjacent measurement points for a crystal grain having a body-centered structure, and then calculates the average value for all measurement points within the crystal grain. With respect to the obtained crystal orientation information, regions with a misorientation of 5° or more are defined as crystal grains, and the "Grain Average Misorientation" function calculates the area fraction of regions within the crystal grains where the average crystal orientation misorientation is 0.4 to 3.0° relative to the observation field, thereby obtaining the area fraction of the specific structure. Measurements are performed in five fields, and the average value of the area fractions of the specific structure in each field is defined as the area fraction of the specific structure in this embodiment.

[0074] The area fraction of the metal structure of the steel plate according to this embodiment can be determined in the same manner as in the steel member described above.

[0075] (Preferably, Nb-based precipitates are present, and the Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel sheet.) The Nb-based precipitates containing dissolved Mo have the effect of dragging austenite (γ) grain boundaries during heat treatment at a temperature equal to or higher than the Ac3 point. When the Nb-based precipitates partially dissolve while the γ grain boundaries are being dragged by the Nb-based precipitates containing dissolved Mo, the dissolved Mo segregates at the γ grain boundaries. The grain boundaries where Mo is segregated promote the formation of lath martensite with a specific crystal orientation during the γ→α' (martensite) transformation. As laths of lath martensite with a specific crystal orientation collide and coalesce as they grow, resulting in the formation of grain boundaries with the specific rotation angle described above. The presence of Nb-based precipitates containing concentrated Mo can increase the amount of Mo segregating at grain boundaries during heat treatment. To achieve the above-mentioned effect, it is preferable that Nb-based precipitates that dissolve Mo are present at the 1 / 4 depth position. However, even if Nb-based precipitates are present at the 1 / 4 depth position, if their Mo concentration is less than 4.5 times the Mo content of the steel sheet, the effect cannot be sufficiently obtained. Therefore, it is preferable that the Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel sheet. The Mo concentration of the Nb-based precipitates is more preferably 8.0 times or more the Mo content of the steel sheet, and even more preferably 10.0 times or more. Furthermore, it is preferable that the size of the Nb-based precipitates is 15 μm or less. In this embodiment, the Nb-based precipitates are precipitates containing 50 mass% or more of Nb, such as Nb carbides, Nb carbonitrides, Nb nitrides, NbTi carbides, and NbTi carbonitrides.

[0076] The presence or absence of Nb-based precipitates and the Mo concentration of the Nb-based precipitates can be determined by the same method as that described for the steel member.

[0077] The shape of the steel plate according to this embodiment is not particularly limited. That is, the steel plate may be a flat plate, or may be part of an original plate of a tailored property material in which steel plates with different strengths or thicknesses are joined together.

[0078] [Thickness] The thickness of the steel sheet according to this embodiment is not limited. In the case of a hot stamping steel sheet for automotive parts, the thickness may be 0.6 mm or more or 0.8 mm or more, based on the main thickness range of the steel sheet. For the same reason, the thickness may be 4.0 mm or less or 2.5 mm or less.

[0079] [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 (Al-based coating) or a Zn-based coating (Zn-based coating). The coating is also referred to as a film or a plating layer. It is not necessary to exclude coatings other than Al-based coatings and Zn-based coatings. The steel sheet may be coated with other coatings, such as Sn-based coatings or non-metallic resin coatings, or multiple coating layers thereof. 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 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, 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 total content of impurities may be 1% or less. The thickness of the coating is preferably 10 to 100 μm. The chemical composition and thickness of the coating on the steel sheet can be determined using the same methods as those for measuring the chemical composition and thickness of the coating on the steel member described above.

[0080] <Method for manufacturing steel plate> There are no particular limitations on the method for manufacturing the steel plate according to this embodiment, which is suitable as a material for the steel member according to this embodiment. For example, the steel plate can be manufactured using a manufacturing method including the steps described below. (i) a casting process in which steel having the above-mentioned chemical composition is melted and cast to produce a slab; (ii) a hot rolling process in which the obtained slab is heated and then hot-rolled to form a hot-rolled steel sheet (hot-rolled steel sheet); (iii) a winding process in which the hot-rolled steel sheet is coiled; (iv) a hot-rolled sheet annealing process in which the hot-rolled steel sheet after the winding process is annealed as necessary; (v) a cold rolling process in which the hot-rolled steel sheet after the coiling process or the hot-rolled sheet annealing process is descaled as necessary and cold-rolled to form a cold-rolled steel sheet (cold-rolled steel sheet); (vi) an annealing process in which the hot-rolled steel sheet or the cold-rolled steel sheet is annealed as necessary to form an annealed steel sheet; (vii) a coating process in which the hot-rolled steel sheet, the cold-rolled steel sheet, or the annealed steel sheet is coated as necessary to form a coated steel sheet.

[0081] Preferred conditions for each step are explained below. For conditions not explained, known conditions can be applied.

[0082] <Casting Process> In the casting process, steel having the above-described chemical composition is melted and cast to produce a slab for hot rolling. For example, molten steel having the above-described chemical composition is melted using a converter or electric furnace, and a slab produced by continuous casting can be used. Instead of continuous casting, ingot casting, thin slab casting, or other methods may be used. During casting, Mo must be uniformly (macroscopically uniformly) dissolved. However, Mo is difficult to dissolve because it has a high melting point and is a heavy element. Therefore, if the pouring temperature is lower than the liquidus temperature + 10°C, Mo will not dissolve uniformly. Therefore, the pouring temperature is set to the liquidus temperature + 10°C or higher. While the upper limit of the pouring temperature is not limited, a temperature of 1650°C or lower is preferred. The liquidus temperature is determined by the chemical composition of the molten steel and can be calculated by thermodynamic calculation. While the thermodynamic calculation method is not particularly limited, it is recommended to use integrated thermodynamic calculation software: Thermo-Calc. Furthermore, to ensure uniform solidification, the casting speed (Vc) is slowed. Specifically, the casting speed is set to 0.9 m / min or less. Generally, a slow casting speed increases the possibility of cracks occurring during bending straightening, but the steel sheet according to this embodiment contains a predetermined amount of Mo or more. This Mo has the effect of suppressing the precipitation of grain boundary ferrite, which segregates at grain boundaries and causes cracks, so cracks do not occur even at this casting speed. The casting speed is preferably set to 0.5 m / min or less.

[0083] <Hot Rolling Step> In the hot rolling step, the slab is heated and subjected to rough rolling, followed by descaling as necessary, and finally finish rolling. When obtaining Nb-based precipitates in which Mo is concentrated (for example, the Mo concentration is 4.5 times or more the Mo content of the mother steel sheet), it is preferable to perform hot rolling under the following conditions. That is, in the heating performed prior to hot rolling, the heating temperature is set to the solution temperature of the Nb-based precipitates + 5°C or higher. The Nb-based precipitates precipitated in the casting step are coarse, and there is no Mo concentration. In order to obtain Nb-based precipitates in which Mo is concentrated, it is necessary to melt the slab once and then finely precipitate them again by hot rolling. If the heating temperature is less than the solution temperature of the Nb-based precipitates + 5°C, the coarse Nb-based precipitates precipitated in the casting step cannot be dissolved. Furthermore, in this hot rolling process, when obtaining predetermined Nb-based precipitates, it is preferable to set the time from the end of rough rolling to the start of finish rolling to 10 seconds or less. In the steel sheet according to this embodiment, Nb-based precipitates are precipitated in fine ferrite after finish rolling. If the Nb-based precipitates are finely dispersed, Mo dissolves in the solid solution, increasing the Mo concentration. At the end of rough rolling, the steel is usually in the γ region, and if the time from the end of rough rolling to the start of finish rolling exceeds 10 seconds, Nb-based precipitates precipitate in the γ state, and fine Nb-based precipitates with concentrated Mo cannot be obtained. It is more preferable to set the time from the end of rough rolling to the start of finish rolling to 7 seconds or less.

[0084] <Coiling step> In the coiling step, for example, the hot-rolled steel sheet after the hot rolling step is coiled at a temperature range of 850°C or lower. If the coiling temperature exceeds 850°C, the steel sheet is coiled before transformation has progressed much, and the progress of transformation in the coil may result in a defective coil shape, which is undesirable. On the other hand, in order to reduce the area fraction of the specific structure in the steel sheet, coiling is preferably performed at 450°C or higher, and more preferably at 550°C or higher.

[0085] <Hot-rolled sheet annealing step> In the annealing step of the hot-rolled steel sheet, annealing may be performed, for example, in an atmosphere containing 80% by volume or more of nitrogen or in air at 450 to 950°C for 5 hours or more, as necessary. Hot-rolled sheet annealing is preferable because it can soften the hot-rolled steel sheet and reduce the load in the subsequent cold-rolling step.

[0086] <Cold Rolling Step> In the cold rolling step, the hot-rolled steel sheet after the hot-rolled sheet annealing step (the hot-rolled steel sheet after the coiling step if the hot-rolled sheet annealing step is not performed) is descaled, and then 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.

[0087] <Annealing step> When annealing is performed before the coating step, the hot-rolled steel sheet or the cold-rolled steel sheet is annealed in a temperature range of 700 to 950°C to obtain an annealed steel sheet. The annealing step is preferable because it can soften the cold-rolled steel sheet and facilitates sheet threading in the subsequent plating step.

[0088] <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 hot-rolled steel sheet after a hot-rolled sheet annealing 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, thermal spraying, etc. 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.

[0089] When an Al-based coating is formed by hot-dip plating, the plating bath often contains Fe as an impurity in addition to Al. Furthermore, as long as the plating bath contains 70 mass% or more of Al, the plating bath may contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, or misch metal in addition to the elements described above. When hot-dip plating is performed, the annealed steel sheet after the annealing step may be cooled to room temperature and then heated again for plating, or the steel sheet may be cooled to 650 to 750°C, which is close to the plating bath temperature, after annealing, and then hot-dip plating may be performed without first cooling to room temperature.

[0090] 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, for example, annealing at 450 to 800°C is possible. Furthermore, as a post-treatment, temper rolling is useful for shape adjustment, etc., and for example, a reduction of 0.1 to 0.5% is possible.

[0091] <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.

[0092] <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 method described below at an average heating rate of 1.0 to 1000°C / s to a temperature between the Ac3 point and (Ac3 point + 300)°C, and then cooling it to the Ms point or below at an average cooling rate equal to or greater than the upper critical cooling rate. A heating rate of less than 1.0°C / s is undesirable because it reduces the productivity of the heat treatment. On the other hand, a heating rate of more than 1000°C / s is undesirable because it results in a duplex structure and a reduced critical hydrogen content. Furthermore, a heat treatment temperature less than the Ac3 point (°C) is undesirable because ferrite remains after cooling, resulting in insufficient strength. On the other hand, a heat treatment temperature greater than the Ac3 point + 300°C is undesirable because it results in a coarse-grained structure and a reduced critical hydrogen content. 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 structure. Cooling at a rate below the upper critical cooling rate results in the formation of ferrite or pearlite, resulting in insufficient strength. During heating, the steel may be held at a temperature within ±10°C of the heating temperature for 1 to 300 seconds. After cooling to a temperature below the Ms point, tempering may be performed at a temperature in the range of about 100 to 600°C to adjust the strength of the steel member.

[0093] 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 thermal expansion change of the test piece during heating and cooling. Furthermore, among the test pieces cooled at the above cooling rates, the minimum cooling rate at which ferrite and pearlite did not precipitate is defined as the upper critical cooling rate. The Ms point obtained from the thermal expansion change when cooled at a rate equal to or higher than the upper critical cooling rate is defined as the Ms point of the steel member.

[0094] Here, during the above series of heat treatments, after heating to a temperature range of Ac3 point to (Ac3 point + 300) °C, hot forming such as hot stamping may be performed while cooling to the Ms point, i.e., while performing a cooling process at or above the upper critical cooling rate. Examples of hot forming include bending, drawing, stretch forming, hole expansion, and 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. If the above thermal history is followed, repeated hot forming may be performed. Furthermore, the above series of heat treatments may be repeated multiple times.

[0095] The heat treatment may be performed on a portion of the steel sheet material by hot forming or heat treatment, in which case a steel member having regions with different strengths can be obtained.

[0096] 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, city gas, or nitrogen gas. The dew point in the heating furnace may be controlled to suppress hydrogen generation during the heat treatment.

[0097] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0098] Example 1 Steels having the chemical compositions shown in Tables 1-1 and 1-2 (the balance being Fe and impurities) were melted and continuously cast under the conditions shown in Table 2-1 to obtain slabs for hot rolling. The obtained slabs were heated to 1260°C, which is at least 5°C above the solid solution temperature of Nb-based precipitates, hot-rolled, and coiled at a temperature of 850°C or lower and 450°C or higher, to obtain steel sheets (hot-rolled steel sheets) with a thickness of 2.7 mm. The hot-rolled steel sheets were pickled and then cold-rolled to obtain steel sheets (cold-rolled steel sheets) with a thickness of 1.6 mm. After cold rolling, some of the steel sheets were annealed by heating to 760°C and holding for 10 seconds, and then immersed in an Al-plating bath at 680°C containing 10% Si and 2% Fe, with the balance being impurities, to obtain Al-plated steel sheets (coating: Al in Table 2-2). Some of the steel sheets were immersed in a Zn plating bath containing Zn and impurities to form Zn-plated steel sheets (coating in Table 2-2: Zn). The thickness of the coating was adjusted to 30 μm.

[0099]

[0100]

[0101] In the obtained steel sheets, the area fraction of regions (i.e., specific structures) surrounded by boundaries with a crystal orientation misorientation of 5° or more and having an average crystal orientation misorientation of 0.4 to 3.0° within the boundaries was evaluated using the method described above. The evaluation results are shown in Table 2-2. Furthermore, when Nb-based precipitates were present in the obtained steel sheets, the ratio of the Mo concentration of the Nb-based precipitates to the Mo content of the steel sheets was measured. The results are shown in Table 2-2.

[0102]

[0103]

[0104] As shown in Tables 2-1 and 2-2, steel sheets having the prescribed chemical composition and metallographic structure were obtained for Examples B1 to B14, which satisfied the range of the present invention. On the other hand, Comparative Examples b1 to b9, which did not satisfy the range of the present invention, did not satisfy the chemical composition and metallographic structure.

[0105] Example 2 The steel plates (B1 to B14 and b1 to b9) produced in Example 1 were subjected to a heat treatment in which they were heated to the heating temperatures shown in Table 3-1 at the heating rates shown in Table 3-1, held at a temperature within ±10°C of the heating temperature for 90 seconds, and cooled to a temperature equal to or lower than the Ms point at the average cooling rates shown in Table 3-1, thereby obtaining steel members C1 to C14 and c1 to c9.

[0106] The martensite area fraction and (L49-56° + L64-72°) / (L57-63° + L4-12°) ratio of the obtained steel members were measured using the methods described above to confirm the presence or absence of Nb-based inclusions. If Nb-based precipitates were present, the ratio of their Mo concentration to the Mo content in the chemical composition of the steel member (or the chemical composition of the base steel plate if coated) was measured using the methods described above. The results are shown in Table 3-2. Although not shown in the table, the metal structure of the inventive examples consisted of retained austenite and / or bainite, in addition to martensite. Furthermore, the size of the Nb-based precipitates present was 0.5 to 15 μm. Furthermore, the tensile strength and the critical hydrogen content Hc, which is an index of hydrogen embrittlement resistance, were evaluated as follows. The evaluation results are shown in Table 3-2. Furthermore, the steel members obtained from the steel plate having the Al-based coating had an Fe—Al-based coating on the surface, and the steel members obtained from the steel plate having the Zn-based coating had an Fe—Zn-based coating on the surface. The Fe—Al-based coating was a coating containing approximately 10 mass% Si, with the remainder consisting of Fe and Al and 1% or less of impurities. The Fe—Zn-based coating was a coating consisting of an Fe—Zn alloy and 1% or less of impurities.

[0107] <Tensile Strength> The tensile test was carried out in accordance with the provisions of ASTM Standard E8M-22. That is, after the steel member was ground evenly on both sides to a thickness of 1.2 mm, avoiding the ends, a sub-size tensile test specimen (gauge length: 25.0±0.1 mm (parallel portion length: 32.0 mm), parallel portion plate width: 6.0±0.1 mm) according to Table 1 of ASTM Standard E8M-22 was taken. A strain gauge (gauge length: 5 mm) was then attached to the center of the width and length of the parallel portion of the test specimen, and a room temperature tensile test was carried out at a strain rate of 3 mm / min to measure the tensile strength (TS). In this example, a tensile strength of more than 1500 MPa was evaluated as having high strength.

[0108] <Limited Hydrogen Content Hc> Hydrogen embrittlement resistance was evaluated by four-point bending of hydrogen-absorbed test specimens and measuring the limit of hydrogen content Hc without cracking. Specifically, strip-shaped test specimens measuring 8 mm wide and 68 mm long were cut out, avoiding the edges of the steel members. Strain gauges (gauge length: 5 mm) similar to those used in the tensile test were attached to the center of the width and length of the surface of the test specimen. The test specimens were then bent using a four-point support jig so that a strain equivalent to half the stress of the tensile strength obtained in the tensile test was generated on the surface of the test specimen (four-point bending test specimens). The four-point bending test specimens with various hydrogen contents were observed for cracking, and the limit of hydrogen content Hc without cracking was determined. In the case of Al-based coatings, the hydrogen absorption amount was varied by changing the furnace dew point during heat treatment. In the case of uncoated and Zn-based coatings, the specimens were immersed in ammonium thiocyanate solutions of various concentrations for 72 hours after four-point bending to absorb hydrogen. The hydrogen absorbed in the steel member was determined by temperature-rising hydrogen analysis at a rate of 100°C / hr, and the amount of diffusible hydrogen released up to 250°C was taken as the amount of hydrogen contained in the steel member. In this example, the steel member was evaluated as having excellent hydrogen embrittlement resistance when Hc was 0.6 mass ppm or more for a tensile strength of 1500 to less than 2000 MPa, 0.4 mass ppm or more for a tensile strength of 2000 to less than 2500 MPa, and 0.2 mass ppm or more for a tensile strength of 2500 MPa or more.

[0109]

[0110]

[0111] As shown in Tables 3-1 and 3-2, Invention Examples C1 to C14 had chemical compositions and metal structures that satisfied the ranges of the present invention, and exhibited good results in both tensile strength and hydrogen embrittlement resistance. On the other hand, Comparative Examples c1 to c9 did not have chemical compositions or metal structures that satisfied the ranges of the present invention, and were inferior in at least one of strength and hydrogen embrittlement resistance.

[0112] According to the present invention, it is possible to obtain high-strength steel members and steel plates with excellent hydrogen embrittlement resistance. The steel members according to the present invention are particularly suitable for use as automotive frame components. Because the steel members according to the present invention have high strength and excellent hydrogen embrittlement resistance, when applied to automotive components, they contribute to improving fuel economy and collision safety.

Claims

1. The chemical composition, in mass%, is C: 0.26-0.65%, Si: 0-2.00%, Mn: 0 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, Mo: 0.10-2.00%, Nb: 0 to 0.10%, Ti: 0-0.200%, Cu: 0-2.00%, Ni: 0-2.00%, Cr: 0-1.00%, B: 0 to 0.0200%, W: 0-1.00%, V: 0-1.00%, Ca: 0-0.020%, Mg: 0 to 0.010%, Al: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 1.00%, Zr: 0 to 1.00%, Se: 0-1.00%, Bi: 0-1.00%, As: 0 to 1.00%, Ta: 0 to 1.00%, Re: 0 to 1.00%, Os: 0-1.00%, Ir: 0-1.00%, Tc: 0-1.00%, Co: 0-1.00%, REM: 0 to 0.30%, and The balance is Fe and impurities. When a range from a position of 1 / 8 to a position of 3 / 8 of the thickness in the thickness direction from the surface is defined as a 1 / 4 depth position, At the 1 / 4 depth position, among the grain boundaries of crystal grains having a body-centered structure, when the length of the grain boundary where the rotation angle is 49 to 56° with the <011> direction as the rotation axis is defined as L49-56°, the length of the grain boundary where the rotation angle is 64 to 72° is defined as L64-72°, the length of the grain boundary where the rotation angle is 57 to 63° is defined as L57-63°, and the length of the grain boundary where the rotation angle is 4 to 12° is defined as L4-12°, the ratio of the sum of L49-56° and L64-72° to the sum of L57-63° and L4-12°, i.e., (L49-56° + L64-72°) / (L57-63° + L4-12°), is 1.30 or more, The tensile strength is greater than 1500 MPa. A steel member characterized by:

2. The chemical composition is, in mass%, Nb: 0.01 to 0.10%, Nb-based precipitates are present at the 1 / 4 depth position, The Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel member. The steel member according to claim 1 , characterized in that it is

3. The surface has a coating. The steel member according to claim 1 or 2.

4. The coating is mainly composed of an Fe—Al alloy or an Fe—Zn alloy, The steel member according to claim 3 .

5. The chemical composition is, in mass%, W: more than 0% and less than 0.10%, or Mo: more than 1.00% and less than 2.00%. The steel member according to claim 1 or 2.

6. The (L49-56°+L64-72°) / (L57-63°+L4-12°) is less than 1.40; The steel member according to claim 1 or 2.

7. The chemical composition, in mass%, is C: 0.26-0.65%, Si: 0-2.00%, Mn: 0 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, Mo: 0.10-2.00%, Nb: 0 to 0.10%, Ti: 0-0.200%, Cu: 0-2.00%, Ni: 0-2.00%, Cr: 0-1.00%, B: 0 to 0.0200%, W: 0-1.00%, V: 0-1.00%, Ca: 0-0.020%, Mg: 0 to 0.010%, Al: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 1.00%, Zr: 0 to 1.00%, Se: 0-1.00%, Bi: 0-1.00%, As: 0 to 1.00%, Ta: 0 to 1.00%, Re: 0 to 1.00%, Os: 0-1.00%, Ir: 0-1.00%, Tc: 0-1.00%, Co: 0-1.00%, REM: 0 to 0.30%, and The balance is Fe and impurities. When a range from a position of 1 / 8 to a position of 3 / 8 of the thickness in the thickness direction from the surface to the surface, centered at a position of 1 / 4 of the thickness, is defined as a 1 / 4 depth position, At the 1 / 4 depth position, The surface area of ​​the region surrounded by a boundary having a crystal misorientation of 5° or more, and the average crystal misorientation within the boundary being 0.4 to 3.0°, is 80% or less. A steel plate characterized by:

8. The chemical composition is, in mass%, Nb: 0.01 to 0.10%, Nb-based precipitates are present at the 1 / 4 depth position, The Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel plate. The steel sheet according to claim 7 , characterized in that

9. having a coating on the surface; The steel sheet according to claim 7 or 8.

10. The coating is an Al-based coating or a Zn-based coating. The steel sheet according to claim 9 .

11. The chemical composition is, in mass%, W: more than 0% and less than 0.10%, or Mo: more than 1.00% and less than 2.00%. The steel sheet according to claim 7 or 8.