Steel members and steel plates

Abstract

A steel member according to the present invention has a specific chemical composition, and if a position corresponding to 1 / 4 the thickness from the surface in the thickness direction is taken as the 1 / 4 depth position, the metal structure at the 1 / 4 depth position contains, in volume fractions, 90% or more of martensite. With respect to the metal structure at the 1 / 4 depth position, among the crystal grains each having a body-centered structure, the ratio (L49-56°) / (L4-12°) of the length L49-56° of a grain boundary having a rotation angle of 49° to 56° to the length L4-12° of a grain boundary having a rotation angle of 4° to 12° is 1.10 or more as measured using the <011> direction as the rotation axis; and the tensile strength is more than 1,500 MPa.

Description

[Technical Field]

[0001] The present invention relates to a steel member and a steel plate. This application claims priority based on Japanese Patent Application No. 2021-193323, filed on November 29, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] In the field of automotive steel sheets, the application of steel sheets with high tensile strength (high-strength steel sheets) is expanding in order to improve both fuel economy and collision safety against the backdrop of recent stricter environmental regulations and collision safety standards. 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 steel strength increases, its ductility decreases, resulting in the problem of fracture at high-pressure locations when processed into complex shapes. Furthermore, as steel strength 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, especially those with tensile strengths 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 technology has been adopted as a technology 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 technology is a hot forming technology in which the material to be formed is heated and then formed.

[0005] With 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. Furthermore, with hot stamping technology, the steel is quenched at the same time as it is formed using a press die, so the steel (steel component) has sufficient strength after forming.

[0006] For example, Patent Document 1 discloses that hot stamping technology 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 set higher CO2 reduction targets, and automobile manufacturers 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 the rapidly advancing trend of electric vehicles, in order to protect not only passengers but also batteries from collisions and to offset the resulting weight increase. For example, steel members used in automobiles and other vehicles require higher strength (greater than 1.5 GPa) than those described in the aforementioned Patent Document 1 and those currently commonly used in steel members formed by hot stamping.

[0008] However, with increasing strength, many metallic materials experience a deterioration in their properties, particularly their increased susceptibility to hydrogen embrittlement. It is known that hydrogen embrittlement susceptibility increases for steel components when the tensile strength exceeds 1.2 GPa, and there have been cases of hydrogen embrittlement cracking in bolt steel, which has been strengthened ahead of the automotive industry. Therefore, there are concerns that hot-stamped components with tensile strengths exceeding 1.5 GPa will be even more susceptible to hydrogen embrittlement.

[0009] Steel components used in automobiles are at risk of hydrogen embrittlement cracking due to hydrogen generated during automobile manufacturing and use. During manufacturing, hydrogen is generated during the material heating process and electrodeposition coating process, and some of it is absorbed by the steel components. During use, hydrogen is also generated due to corrosion of the steel components. As mentioned above, the susceptibility of steel to hydrogen embrittlement is extremely high, especially in the strength range above 1.5 GPa, so it is thought that even trace amounts of hydrogen can cause hydrogen embrittlement. Therefore, in order to apply hot-stamped components with tensile strengths above 1.5 GPa to automobile bodies to further reduce the weight of automobiles, it is necessary to sufficiently reduce the risk of hydrogen embrittlement cracking.

[0010] Regarding high-strength steel materials having a tensile strength exceeding 1.5 GPa, for example, Patent Document 2 discloses a hot-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. However, from the viewpoint of hydrogen embrittlement resistance in the manufacture and use of automobiles, there is room for further improvement in these technologies. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2002-102980 [Patent Document 2] Japanese Patent Application Publication No. 2012-180594 [Patent Document 3] Japanese Patent Publication No. 2012-1802 [Patent Document 4] International Publication No. 2015 / 182596 [Patent Document 5] International Publication No. 2015 / 182591 Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention has been made to solve the above problems, and an object of the present invention is to provide a steel member having high tensile strength and excellent resistance to hydrogen embrittlement, and a steel plate suitable as a material for the steel member. [Means for solving the problem]

[0013] The present inventors investigated the influence of the metallographic structure and the steel sheet used as a raw material on the properties of a steel member having high tensile strength and excellent hydrogen embrittlement resistance, and obtained the following findings.

[0014] (a) Most 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 mass% C, which ensures strength after heat treatment. To further reduce the weight of vehicle bodies, the inventors conducted detailed studies to obtain steel members with high strength exceeding 1.5 GPa after heat treatment by increasing the C content. As a result, they found that an ultra-high tensile strength exceeding 1.5 GPa can be obtained after heat treatment including quenching, such as hot stamping, by setting the C content to 0.26 mass% or more.

[0015] On the other hand, as the tensile strength increased to over 1.5 GPa, the susceptibility to hydrogen embrittlement increased, raising concerns about the risk of hydrogen embrittlement cracking due to hydrogen generated in the corrosive environment during use in automobiles.

[0016] (b) The present inventors have investigated methods for improving the hydrogen embrittlement resistance of high-strength steel members with a tensile strength of over 1.5 GPa. As a result, they have found that hydrogen embrittlement resistance can be improved by controlling the length of grain boundaries with specific rotation angles.

[0017] Based on the above findings, the present inventors have developed a steel member and a steel plate that is the raw material for the steel member, which have significantly improved hydrogen embrittlement resistance and a tensile strength of more than 1.5 GPa. The present invention is summarized as follows. [1] A steel member according to an embodiment of the present invention contains, by mass%, C: 0.26 to 0.65%, Si: 0 to 2.00%, Mn: 0.05 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.100%, Cu: 0.10 to 2.50%, Ni: 0 to 0.30%, Cr: 0 to 0.50%, B: 0 to 0.0100%, Mo: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Al: 0 to 1.00%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: A metallurgical structure having a chemical composition consisting of 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, wherein when a position at 1 / 4 of the thickness from the surface in the thickness direction is defined as a 1 / 4 depth position, the metallurgical structure at the 1 / 4 depth position contains 90% or more martensite by volume fraction, and in the metallurgical structure at the 1 / 4 depth position, among the grain boundaries of crystal grains having a body-centered structure, <011> The ratio L49-56° / L4-12°, which is the ratio of the length L49-56° of the grain boundary where the rotation angle is 49 to 56° and the length L4-12° of the grain boundary where the rotation angle is 4 to 12°, is 1.10 or more, and the tensile strength is greater than 1500 MPa. [2] The steel member according to [1] further comprises the following: [Ni] / [Cu]<0.50, where [Ni] is the Ni content, [Cu] is the Cu content, [Nb] is the Nb content, [Ti] is the Ti content, and [N] is the N content, in mass%, in the chemical composition; [Ti]=3×[N]+0.010 to 3×[N]+0.020 when [Nb]=0.01 or more and less than 0.03; [Ti]=5×[N]+0.010 to 5×[N]+0.020 when [Nb]=0.03 or more and less than 0.05; and [Ti]=7×[N]+0.010 to 7×[N] when [Nb]=0.05 to 0.10. + 0.020, and the metal structure at the 1 / 4 depth position may contain NbTi(C,N), and the size of the NbTi(C,N) may be 20 μm or less. [3] The steel member according to [1] or [2] may have a coating on the surface. [4] In the steel member described in [3], the coating may be mainly composed of an Fe—Al alloy. [5] In the steel member described in [3], the coating may be mainly composed of an Fe—Zn alloy. [6] A steel sheet according to another embodiment of the present invention comprises, in mass%, C: 0.26 to 0.65%, Si: 0 to 2.00%, Mn: 0.05 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.100%, Cu: 0.1 0~2.50%, Ni:0~0.30%, Cr:0~0.50%, B:0~0.0100%, Mo:0~1.00%, V:0~1.00%, Ca:0~0.0 10%, Mg:0~0.010%, Al:0~1.00%, Sn:0~1.00%, W:0~1.00%, Sb:0~1.00%, Zr:0~1.00%, S The steel sheet has a chemical composition consisting of E: 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 in a metallographic structure at a 1 / 4 depth position, which is a position at 1 / 4 of the sheet thickness from the surface in the sheet thickness direction, the metallographic structure is surrounded by a boundary where the crystal orientation misorientation is 5° or more, and the volume fraction of a region within the boundary where the average crystal orientation misorientation is 0.4° or more and 3.0° or less is 85% or less, and the volume fraction of retained austenite in the metallographic structure at the 1 / 4 depth position is 5% or less. [7] In the steel sheet described in [6], the metal structure may be surrounded by a boundary having a crystal orientation mismatch of 5° or more, and the volume fraction of the region within the boundary having an average crystal orientation mismatch of 0.4° or more and 3.0° or less may be 10% or less. [8] The steel sheet according to [6] or [7] may have a coating on the surface. [9] In the steel sheet according to [8], the coating may be an Al-based coating.

[10] In the steel sheet according to [8], the coating may be a Zn-based coating. [Effects of the Invention]

[0018] According to the above aspects of the present invention, it is possible to provide a steel member having high tensile strength and excellent hydrogen embrittlement resistance, and a steel sheet suitable as a material for the steel member. Such a steel member has high strength but a low risk of hydrogen embrittlement, and can therefore be used safely in vehicle bodies. DETAILED DESCRIPTION OF THE INVENTION

[0019] (A) Steel parts A steel member according to one embodiment of the present invention (steel member according to the present embodiment) has a predetermined chemical composition, and when a position at 1 / 4 of the thickness from the surface in the thickness direction is defined as a 1 / 4 depth position, the metal structure at the 1 / 4 depth position contains 90% or more martensite by volume fraction, and in the metal structure at the 1 / 4 depth position, among the grain boundaries of crystal grains having a body-centered structure, <011> The ratio L49-56° / L4-12°, which is the ratio of the length of the grain boundary where the rotation angle is 49 to 56° and the length of the grain boundary where the rotation angle is 4 to 12°, is 1.10 or more, and the tensile strength is greater than 1500 MPa (1.5 GPa). The steel member may have a coating on its surface. Each requirement is explained in detail below.

[0020] (A1) Chemical composition The steel member according to this embodiment has a predetermined chemical composition, specifically, the chemical composition of the steel member is, in mass %, C: 0.26 to 0.65%, Si: 0 to 2.00%, Mn: 0.05 to 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.100%, Cu: 0.10 to 2.50%, Ni: 0 to 0.30%, Cr: 0 to 0.50%, B: 0 to 0.0100%, Mo: 0 to 1.00%, V: 0 to 1.00%, and Ca. : 0-0.010%, Mg: 0-0.010%, Al: 0-1.00%, Sn: 0-1.00%, W: 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. Here, the chemical composition of a steel member refers to the chemical composition of a portion excluding the surface (for example, the position at 1 / 4 of the thickness from the surface of the steel member in the thickness direction: 1 / 4 depth position). Hereinafter, % relating to content is mass % unless otherwise specified. 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 sheet 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 (steel member obtained by 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 to 0.32% or more. On the other hand, if the C content exceeds 0.65%, the strength of the steel member after quenching becomes excessively high, and hydrogen embrittlement resistance decreases significantly. Therefore, the C content is set to 0.65% or less. The C content is preferably set to 0.60% or less.

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

[0023] Mn: 0.05 to 3.00% Mn is an extremely effective element for improving the hardenability of steel and ensuring stable strength after quenching. Mn also lowers the Ac3 point, promoting lower quenching temperatures. However, if the Mn content is less than 0.05%, this effect is insufficient. Therefore, the Mn content is set to 0.05% or more. Preferably, the Mn content is set to 0.15% 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 set to 2.50% or less, and more preferably set to 1.50% or less.

[0024] P:0.050% or less P is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, if the P content exceeds 0.050%, the reduction in hydrogen embrittlement resistance becomes significant. Therefore, the P content is limited to 0.050% or less. It is preferable to limit the P content to 0.020% or less. Since a small P content is preferable, it may be 0%, but from the viewpoint of cost, it may be 0.001% or more.

[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. Since a lower S content is preferable, 0% is acceptable, but from the viewpoint of cost, it may be 0.0001% or more.

[0026] N: 0.010% or less N is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, if the N content exceeds 0.010%, coarse nitrides are formed in the steel, significantly reducing hydrogen embrittlement resistance. Therefore, the N content is set to 0.010% or less. There is no need to particularly set a lower limit for the N content, and it may be 0%, but setting the N content to less than 0.0002% increases steelmaking costs and is 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 oxides are formed in the steel, significantly reducing hydrogen embrittlement resistance. Therefore, the O content is set to 0.010% or less. There is no need to particularly set a lower limit for 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] Nb: 0.01 to 0.10% Nb is an element that forms fine carbonitrides in steel, and the grain refinement effect of these carbonitrides suppresses Cu hot embrittlement cracking during the hot rolling process and improves the hydrogen embrittlement resistance of steel members. To achieve these effects, the Nb content is set to 0.01% or more. The Nb content is preferably set to 0.02% or more. On the other hand, if the Nb content exceeds 0.10%, the carbonitrides become coarse, promoting bending cracks during the continuous casting process. Furthermore, the solute Nb inhibits the development of grain boundaries with specific rotation angles in the steel member, as described below, 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 set to 0.08% or less.

[0029] Ti: 0.005 to 0.100% Ti is an element that forms fine carbonitrides together with Nb in steel, and the grain refinement effect of these carbonitrides suppresses Cu hot embrittlement cracking during the hot rolling process and improves the hydrogen embrittlement resistance of steel members. Ti also preferentially bonds with N in steel to form nitrides, suppresses the consumption of solute B due to the precipitation of BN, and promotes the effect of B in improving hardenability, which will be described later. To achieve the above effects, the Ti content is set to 0.005% or more. The Ti content is preferably set to 0.010% or more, and more preferably 0.015% or more. On the other hand, if the Ti content exceeds 0.100%, the carbonitrides become coarse, promoting bending straightening cracks during the continuous casting process. Furthermore, solute Ti inhibits the development of grain boundaries with specific rotation angles 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, resulting in a decrease in the strength of the steel member after quenching. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably set to 0.080% or less.

[0030] Cu: 0.10 to 2.50% Cu is an element that develops grain boundaries with a specific rotation angle and improves the hydrogen embrittlement resistance of steel members. Cu is also an element that is effective in improving the hardenability of steel and ensuring stable strength of steel members after quenching. Cu is also an element that improves corrosion resistance in corrosive environments. To obtain these effects, the Cu content is set to 0.10% or more. The Cu content is preferably set to 0.20% or more. On the other hand, if the Cu content exceeds 2.50%, the above effects saturate and the cost increases. Therefore, the Cu content is set to 2.50% or less. The Cu content is preferably set to 1.50% or less, and more preferably set to 1.00% or less.

[0031] The chemical composition of the steel member according to this embodiment may contain elements other than those described above, i.e., the balance may be Fe and impurities. Alternatively, to improve various properties (hardenability, strength, hydrogen embrittlement resistance, deoxidation resistance, corrosion resistance, etc.) of the steel member and parts including this steel member, one or more elements selected from the group consisting of Ni, Cr, B, Mo, V, Ca, Mg, Al, Sn, W, Sb, Zr, Se, Bi, As, Ta, Re, Os, Ir, Tc, Co, and REM may be further contained within the ranges shown below. These elements are optional elements and do not necessarily need to be contained, so the lower limit is 0%. These elements may be contained as impurities as long as their content is equal to or less than the upper limit described below.

[0032] Ni: 0 to 0.30% Ni is an element effective in 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 during the production of steel sheets. Therefore, Ni may be added. To obtain the above effects, the Ni content is preferably 0.06% or more, more preferably 0.10% or more, and even more preferably 0.20% or more. The Ni content may be set according to the Cu content. On the other hand, if the Ni content exceeds 0.30%, the development of specific grain boundaries, as described below, is inhibited, and the limiting hydrogen content of the steel member decreases. Furthermore, Ni generates fumes that are harmful to humans when laser cutting or welding is performed during the steel member manufacturing process. Therefore, if Ni is contained, the Ni content should be 0.30% or less. The Ni content is preferably 0.20% or less, more preferably 0.15% or less, and even more preferably 0.10% or less.

[0033] Cr: 0 to 0.50% Cr is an element that is effective in improving the hardenability of steel and ensuring stable strength of the steel member after quenching. Therefore, Cr may be added. 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 0.50%, the above effects saturate and costs increase. Furthermore, since Cr has the effect of stabilizing iron carbides, if the Cr content exceeds 0.50%, coarse iron carbides may remain undissolved during heat treatment of the steel sheet, which may reduce the hydrogen embrittlement resistance of the steel member. Furthermore, Cr generates fumes that are harmful to humans when laser cutting or welding is performed in the steel member manufacturing process. Therefore, the Cr content is set to 0.50% or less. The Cr content is preferably set to 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% or less.

[0034] B: 0 to 0.0100% B is an element that has the effect of improving the hardenability of steel even in small amounts. Furthermore, B segregates at grain boundaries, strengthening the grain boundaries and improving hydrogen embrittlement resistance, and it is an element that suppresses the grain growth of austenite when the steel sheet is heated. Therefore, B may be added. To obtain the above effects, the B content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0100%, a large amount of coarse compounds precipitates, reducing the hydrogen embrittlement resistance of the steel member. Therefore, if B is contained, the B content should be 0.0100% or less. The B content is preferably 0.0080% or less.

[0035] Mo: 0 to 1.00% Mo is an extremely effective element for improving the hardenability of steel and for ensuring stable strength of steel members after quenching. In particular, when Mo is added in combination with the above-mentioned B, a synergistic effect of improving hardenability can be obtained. Therefore, Mo may be added. To obtain the above-mentioned effect, the Mo content is preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, Mo is an element that stabilizes iron carbides. If the Mo 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. Furthermore, the cost increases significantly. Therefore, if Mo is contained, the Mo content is set to 1.00% or less. Preferably, the Mo content is set to 0.80% or less.

[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 refinement effect and hydrogen trapping effect of the carbides. Therefore, V may be added. To obtain the above effects, 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 effects saturate and the economic efficiency decreases. Therefore, if V is contained, the V content is set to 1.00% or less.

[0037] Ca: 0 to 0.010% Ca is an element that has the effect of refining inclusions in steel and improving the hydrogen embrittlement resistance of the steel member after quenching. Therefore, Ca may be added. To obtain the above effects, 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 0.010%, the effect saturates and the cost increases. Therefore, if Ca is contained, the Ca content is set to 0.010% or less. The Ca content is preferably set to 0.005% or less, and more preferably set to 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 be added. To obtain the above effects, 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 the cost increases. Therefore, if Mg is added, the Mg content is set to 0.010% or less. The Mg content is preferably 0.005% or less, and more preferably 0.004% or less.

[0039] Al: 0 to 1.00% Al is an element that is generally used as a deoxidizer for steel, and therefore may 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 the economic efficiency decreases. Therefore, if Al is contained, the Al content is set to 1.00% or less.

[0040] Sn: 0 to 1.00% Sn is an element that improves corrosion resistance in a corrosive environment. Therefore, it is preferable to add Sn. If the Sn content is less than 0.01%, these effects are insufficient. Therefore, when Sn is added, 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 set to 1.00% or less.

[0041] W: 0 to 1.00% W is an element that is effective in improving the hardenability of steel and in stably ensuring the strength of steel members after quenching. Therefore, it may be contained. W is also an element that improves corrosion resistance in a corrosive environment. To obtain the above effects, the W content is preferably 0.01% or more. The W content is more preferably 0.05% or more, and even more preferably 0.10% or more. On the other hand, if the W content exceeds 1.00%, the above effects saturate and the economic efficiency decreases. Therefore, if W is added, the W content is set to 1.00% or less.

[0042] Sb: 0 to 1.00% Sb is an element that improves corrosion resistance in a corrosive environment, and therefore may 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 set to 1.00% or less.

[0043] Zr: 0 to 1.00% Zr is an element that improves corrosion resistance in a corrosive environment, and therefore may 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.

[0044] Se: 0 to 1.00% Se is an element that improves hydrogen embrittlement resistance, and therefore may 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, so if Se is contained, the Se content is set to 1.00% or less.

[0045] Bi: 0 to 1.00% Bi is an element that improves hydrogen embrittlement resistance, and therefore may 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, so if Bi is added, the Bi content is set to 1.00% or less.

[0046] As: 0 to 1.00% As is an element that improves hydrogen embrittlement resistance, and therefore 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, so if As is contained, the As content is set to 1.00% or less.

[0047] Ta: 0 to 1.00% Ta is an element that improves hydrogen embrittlement resistance, and therefore may be added. 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, so if Ta is added, the Ta content is set to 1.00% or less.

[0048] Re:0~1.00% Re is an element that improves hydrogen embrittlement resistance, and therefore may 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, so if Re is added, the Re content is set to 1.00% or less.

[0049] Os: 0 to 1.00% Os is an element that improves hydrogen embrittlement resistance, and therefore may 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 added, the Os content is set to 1.00% or less.

[0050] Ir: 0 to 1.00% Ir is an element that improves hydrogen embrittlement resistance, and therefore may be added. 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, so if Ir is added, the Ir content is set to 1.00% or less.

[0051] Tc: 0 to 1.00% Tc is an element that improves hydrogen embrittlement resistance, and therefore may 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, so if Tc is added, the Tc content is set to 1.00% or less.

[0052] Co: 0 to 1.00% Co is an element that improves corrosion resistance in a corrosive environment, and therefore may 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 the economic efficiency decreases. Therefore, if Co is added, the Co content is set to 1.00% or less.

[0053] REM: 0 to 0.30% Like Ca, REM is an element that refines inclusions in steel and improves the hydrogen embrittlement resistance of the steel member after quenching. Therefore, REM may be added. To obtain the above effects, 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 the cost increases. Therefore, if REM is added, the REM content is set to 0.30% or less. The REM content is preferably set to 0.20% or less.

[0054] 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, and this alloy contains, for example, Sc, Y, La, Ce, Pr, and Nd.

[0055] In the chemical composition of the steel member of this embodiment, the elements other than those described above, that is, the remainder, are Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial production of steel sheets due to raw materials such as ores and scraps, and various factors in the production process, and are acceptable within a range that does not adversely affect the properties of the steel member according to the present embodiment. The industrial production method includes the blast furnace steelmaking method and the electric furnace steelmaking method, and includes the level (impurity level) of components mixed in during production by either method.

[0056] The chemical composition of the steel member can be determined by the following method. This can be obtained by performing elemental analysis using a common method such as ICP-AES from a depth of 1 / 4 of the steel part (a depth of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable). C and S can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method. Here, the surface that serves as the reference for the 1 / 4 depth position is the surface of the steel member, but if the surface has a coating (if the steel member has a steel substrate and a coating formed on the surface of the steel substrate), the surface is the surface of the steel substrate excluding the coating. Therefore, if the steel member has a steel substrate and a coating formed on the surface of the steel substrate, the chemical composition of the steel member refers to the chemical composition of the steel substrate. This also applies to the 1 / 4 depth position that defines the metal structure.

[0057] In the steel member according to this embodiment, in order to obtain a grain boundary having a specific rotation angle, as described later, the contents of each element satisfy the above ranges, and it is preferable that, when the Ni content is [Ni] and the Cu content is [Cu], in mass%, the relationship [Ni] / [Cu]<0.50 is satisfied. The ratio [Ni] / [Cu] is more preferably 0.30 or less, even more preferably 0.20 or less, and even more preferably 0.15 or less. On the other hand, from the viewpoint of suppressing hot brittle cracking due to Cu, [Ni] / [Cu] is preferably 0.01 or more, more preferably 0.03 or more, and even more preferably 0.05 or more.

[0058] In the steel member according to this embodiment, when the Nb content is [Nb], the Ti content is [Ti], and the N content is [N], in mass%, it is preferable that the Ti content and the N content satisfy a predetermined relationship according to the Nb content as follows: [Nb]=0.01 or more and less than 0.03 [Ti] = 3 × [N] + 0.010 ~ 3 × [N] + 0.020 (meaning 3 × [N] + 0.010 ≦ [Ti] ≦ 3 × [N] + 0.020.) [Nb]=0.03 or more and less than 0.05 [Ti] = 5 × [N] + 0.010 ~ 5 × [N] + 0.020 (meaning 5 × [N] + 0.010 ≦ [Ti] ≦ 5 × [N] + 0.020.) At [Nb] = 0.05 to 0.10, [Ti] = 7 × [N] + 0.010 ~ 7 × [N] + 0.020 (meaning 7 × [N] + 0.010 ≦ [Ti] ≦ 7 × [N] + 0.020.) By satisfying the above relationship, the Nb and Ti contained in the steel material are precipitated finely, with just the right amount of excess and deficiency, and free Nb and free Ti are reduced. The refinement of carbonitrides can suppress bending straightening cracks during the continuous casting process. Furthermore, the finely precipitated carbonitrides have a grain refinement effect, suppressing Cu embrittlement cracks during the hot rolling process described below. Furthermore, the reduction in free Nb and free Ti makes it easier for grain boundaries with specific rotation angles, described below, to be formed.

[0059] (A2) Metal structure of steel components The metal structure at the 1 / 4 depth position of the steel member according to this embodiment is a structure containing martensite at a volume fraction of 90% or more. If the steel member has the above chemical composition and the volume fraction of martensite is 90% or more, a tensile strength of more than 1.5 GPa can be obtained. Preferably, the volume fraction is 95% or more of martensite. More preferably, it is 97% or more. The volume fraction of martensite may be 100%. If the volume fraction of martensite is small, it becomes difficult to obtain a tensile strength of more than 1.5 GPa.

[0060] The metal structure at the 1 / 4 depth position of the steel member may contain retained austenite, bainite, ferrite, and / or pearlite as the remainder other than martensite. Martensite includes not only fresh martensite but also tempered martensite and auto-tempered martensite. Auto-tempered martensite is tempered martensite that is formed during cooling during quenching without undergoing heat treatment for tempering. The martensite that is formed is tempered in situ due to the self-heating that accompanies martensitic transformation.

[0061] The fraction of the microstructure in the metal structure of the steel member can be measured by the following method. The area fraction of martensite (including tempered martensite and auto-tempered martensite) is measured using a transmission electron microscope (TEM) and an electron beam diffraction device attached to the TEM. Specifically, a measurement sample is cut out from the widthwise end of the steel member in the cross section perpendicular to the rolling direction, including a position at 1 / 4 of the width of the steel member (1 / 4 width position) and a position at 1 / 4 depth of the steel member, and used as a thin film sample for TEM observation. 2 The above range is observed by TEM. The electron diffraction patterns of thin film samples are used to distinguish between martensite and bainite, which have body-centered cubic lattices, and retained austenite, which has a face-centered cubic lattice. Iron carbide (Fe3C) in martensite and bainite is then identified through the diffraction patterns, and the precipitation morphology is observed to measure the microstructural fractions of martensite and bainite. Specifically, if the precipitation morphology is three-directional precipitation, it is determined to be martensite (tempered martensite), and if it is unidirectional precipitation, it is determined to be bainite. If no iron carbide precipitation is observed, it is also determined to be martensite (fresh martensite). The structure fractions of martensite and bainite measured by TEM are measured as area fractions (area %), but since the steel member according to this embodiment has an isotropic metal structure, the area fraction values ​​can be directly converted to volume fractions. Carbides are observed to distinguish between martensite and bainite, but in this embodiment, carbides are not included in the volume fraction of the structure. When ferrite or pearlite is present as a residual structure, it can be easily confirmed using an optical microscope or a scanning electron microscope. Specifically, a measurement sample is cut out from a cross section perpendicular to the rolling direction, including a position at 1 / 4 the width of the steel member and a position at 1 / 4 the depth of the steel member, to be used 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 the ferrite and pearlite, and a 40,000 μm area is observed using a scanning electron microscope. 2 By observing the above range, the presence of ferrite or pearlite can be confirmed. Pearlite is a structure in which ferrite and cementite are arranged in alternating layers, while bainite is distinguished as granular precipitation of cementite.

[0062] In the steel member according to this embodiment, in the metal structure at the 1 / 4 depth position, among the grain boundaries of crystal grains having a body-centered structure, <011> The ratio L49-56° / L4-12°, which is the ratio of the length L49-56° of the grain boundary where the rotation angle is 49 to 56° with respect to the direction as the rotation axis, to the length L4-12° of the grain boundary where the rotation angle is 4 to 12°, is 1.10 or more. L49-56° / L4-12° is preferably 1.20 or more, and more preferably 1.30 or more. There is no upper limit to L49-56° / L4-12°, but since it rarely exceeds 3.00, the upper limit may be set to 3.00. In a metal structure mainly composed of martensite, the grain boundaries of crystal grains having a body-centered structure mainly consist of one or more of the following types: 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°. Among these, when considering the hydrogen-mediated ductile fracture behavior, which is the fracture behavior of hydrogen embrittlement cracking, the grain boundaries with a rotation angle of 49 to 56° are the most effective in suppressing the initiation and propagation of ductile cracks, while the grain boundaries with a rotation angle of 4 to 12° are the least effective. Therefore, in the steel member according to this embodiment, in the metal structure at the 1 / 4 depth position, <011> The length L49-56° of the grain boundary where the rotation angle is 49 to 56° is made longer than the length L4-12° of the grain boundary where the rotation angle is 4 to 12°, with the direction as the rotation axis, thereby improving hydrogen embrittlement resistance. If L49-56° / L4-12° is less than 1.10, the effect of improving hydrogen embrittlement resistance is insufficient. <011> Grain boundaries with a rotation angle of 57 to 63° and grain boundaries with a rotation angle of 64 to 72° with respect to the direction as the rotation axis have high crack propagation resistance, but hydrogen is likely to accumulate and cracks are likely to occur, so they do not contribute much to improving hydrogen embrittlement resistance. Therefore, in this embodiment, there are no limitations on their length.

[0063] <011> The grain boundary with a rotation angle of A to B degrees is the boundary between adjacent crystal grains. <011> This refers to grain boundaries that overlap when rotated by A to B degrees around the direction as the rotation axis.

[0064] The grains with a body-centered structure in the metal structure (internal structure) at 1 / 4 depth of the steel member <011> The ratio of the length L49-56° of the grain boundary where the rotation angle is 49 to 56°, to the length L4-12° of the grain boundary where the rotation angle is 4 to 12°, L49-56° / L4-12°, can be measured by the following method. A sample is cut from a position at least 50 mm away from the end of the steel component so that a cross section perpendicular to the surface (thickness cross section) can be observed. The sample length, depending on the measurement device, should be approximately 10 mm long enough to allow observation in the rolling direction. Crystal orientation information is obtained by EBSD analysis of a 50 μm x 50 μm measurement area at a measurement interval of 0.1 μm, centered at a position 1 / 4 of the thickness from the surface of the steel component (1 / 4 depth position). EBSD analysis is performed using a device consisting of, for example, a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) at an analysis speed of 200–300 points per second. Scanning electron microscopes and EBSD detectors with equivalent or better performance than those mentioned above may also be used, but JEOL and TSL products are preferred. Next, for the obtained crystal orientation information, among the grain boundaries of crystal grains with a body-centered structure, <011> The ratio of the length of the grain boundary with a rotation angle of 49 to 56 degrees to the length of the grain boundary with a rotation angle of 4 to 12 degrees was calculated, and the ratio of the length of the grain boundary with a body-centered structure was calculated. <011> The ratio of the length of the grain boundary where the rotation angle is 49 to 56° with respect to the direction as the rotation axis to the length of the grain boundary where the rotation angle is 4 to 12°, L49-56° / L4-12°, is obtained. The length of the above-mentioned grain boundary can be easily calculated, for example, by using the "Inverse Pole Figure Map" and "Axis Angle" functions installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. 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 with an arbitrary direction as the rotation axis. The above analysis is performed for all crystal grains included in the measurement area, <011> The length of the two types of grain boundaries described above can be calculated using the direction as the rotation axis. Measurements are performed for five fields, and the average value of L49-56° / L4-12° for each field is taken as L49-56° / L4-12° in this embodiment. As for grain boundaries other than those described above, it is also possible to calculate the length of grain boundaries with rotation angles of 57 to 63° and those with rotation angles of 64 to 72°.

[0065] Furthermore, in the steel member according to this embodiment, the metal structure at the 1 / 4 depth position preferably contains NbTi(C,N) (carbonitride of Nb and Ti), and the size of NbTi(C,N) is preferably 20 μm or less. The inclusion of fine carbonitrides suppresses cracks due to bending straightening during the continuous casting process. Furthermore, the finely precipitated carbonitrides have a grain refinement effect, which suppresses Cu embrittlement cracks during the hot rolling process described below. Furthermore, the finely precipitated carbonitrides improve hydrogen embrittlement resistance. The size of NbTi(C,N) is preferably 10 μm or less, and more preferably 8 μm or less.

[0066] NbTi(C,N) can be measured by the following method. The size of NbTi(C,N) is measured using a scanning electron microscope. Specifically, a measurement sample is cut out from the cross section of the steel member in a direction perpendicular to the rolling direction, at a position 1 / 4 of the width from the end in the width direction (1 / 4 width position) and including a position 1 / 4 of the depth of the steel member, to serve as the sample for observation. The cut sample is mechanically polished and then mirror-finished. Next, a scanning electron microscope is used to observe NbTi(C,N) that appears with bright contrast in the backscattered electron image. The size of NbTi(C,N) is measured by the maximum Feret diameter. Ten visual fields are measured, and the average value of the Feret diameter of the NbTi(C,N) with the largest Feret diameter in each visual field is taken as the size of NbTi(C,N).

[0067] (A3) Surface of steel member The steel member according to this embodiment may have a coating on a part or all of its surface. The coating may be a coating mainly made of an Fe-Al alloy or a coating mainly made of an Fe-Zn alloy. The coating is also called a film, an alloyed plating layer, or an intermetallic compound layer. A coating primarily made of an Fe-Al alloy is a coating containing 70% by mass or more of Fe and Al, and a coating primarily made of an Fe-Zn alloy is a coating containing 70% by mass or more of Fe and Zn. A coating primarily made of an Fe-Al alloy may contain, in addition to Fe and Al, 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 balance being impurities. A coating primarily made of an Fe-Zn alloy may contain, in addition to Fe and 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 balance being impurities. The coating provides corrosion resistance, which has the effect of improving hydrogen embrittlement resistance when used in automobiles. The thickness of the coating is preferably 10 to 100 μm.

[0068] When a coating is formed on the surface of the steel member, the surface used as the reference for the above-mentioned 1 / 4 depth is the surface excluding the coating.

[0069] The 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 in the longitudinal direction) and a width quarter portion (a position one-quarter the width from the width end in the width direction) of the steel member, and observed. The observation range under the microscope is, for example, 400 times magnification, and the area is 40,000 μm 2 The thickness of the coating is measured in 10 random fields of view and the average value is taken as the coating thickness. When observing using a BSE image (or COMPO image), a clear difference in contrast can be seen between the coating and the base steel (steel 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 taken at 20 equally spaced locations within the observation photograph, with the distance between measurement locations being 6.5 μm. When measuring, five fields of view are observed in the same manner as above, and the average value is used to determine the coating thickness. The chemical composition of the coating can be determined by spot elemental analysis (beam diameter 1 μm or less) using an electron probe microanalyzer (EPMA) on the same observation area as above to determine the Fe, Al, and Zn contents in the coating. A total of 10 points are analyzed on the coating in any 10 fields of view, and the average value is taken as the Fe, Al, and Zn contents in the coating. The same method can be used to determine the Fe, Al, and Zn contents in the coating even if elements other than Fe, Al, and Zn are present.

[0070] (A4) Shape of steel members 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" refers to both formed bodies and flat plates. The steel member may also be a tailored property material with varying strength depending on the location. In this case, it is preferable that at least a portion of the steel member has a tensile strength of more than 1.5 GPa. The tailored property material may be formed by joining steel plates with different chemical compositions, strengths, and thicknesses, or may be a steel plate that has been partially heat-treated. The steel member may also have a decarburized layer or a soft layer in part of its surface.

[0071] (A5) Characteristics of steel members By controlling the metal structure at the quarter depth position as described above, the steel member according to this embodiment has a high strength of more than 1500 MPa (1.5 GPa) in tensile strength and excellent resistance to hydrogen embrittlement, reducing the risk of hydrogen embrittlement when applied to automobiles. The tensile strength is preferably 1800 MPa or more. On the other hand, in terms of hydrogen embrittlement resistance, the tensile strength is preferably 3000 MPa or less, and more preferably 2900 MPa or less.

[0072] In this embodiment, hydrogen embrittlement resistance is evaluated by a hydrogen charging delayed fracture test or corrosion test intended for application to automobile body parts. For example, a steel member is hydrogen charged and subjected to four-point bending to apply stress, and the test is observed indoors to see if it cracks or not, or the steel member is placed in a corrosion acceleration tester and evaluated to see if it cracks or not over a certain number of cycles.

[0073] (B) Steel plate Next, a steel plate (hereinafter sometimes referred to as the steel plate according to the present embodiment) that is the raw material for the steel member according to the present embodiment will be described. The steel plate described below can be subjected to a heat treatment described below to obtain the steel member.

[0074] (B1) Chemical composition of steel plate The range of the chemical composition of the steel plate according to this embodiment is the same as the chemical composition of the steel member according to this embodiment described above, and the reasons for the limitations are also the same. The chemical composition of a steel plate can be obtained, for example, by performing elemental analysis at a representative position (1 / 4 depth position) from the surface of the steel plate in the plate thickness direction using a general method such as ICP. Here, the surface that serves as the reference for the 1 / 4 depth position is the surface of the steel sheet, but if the surface has a coating (if the steel sheet has a base steel sheet and a coating formed on the surface of the base steel sheet), the surface is the surface of the base steel sheet excluding the coating. This also applies to the 1 / 4 depth position that defines the metal structure.

[0075] (B2) Metal structure of steel plate In the steel plate according to this embodiment, in the metal structure at the 1 / 4 depth position, the volume fraction of a region (also called granular bainite) surrounded by a boundary with an average crystal orientation misorientation of 5° or more, and within that boundary, where the average crystal orientation misorientation is 0.4° or more and 3.0° or less, is 85% or less. Granular bainite is a matrix structure that provides the internal structure of the steel member described above in the heat treatment described below. If the volume fraction of granular bainite exceeds 85%, the L49-56° / L4-12° of the steel member described above may be less than 1.10. The volume fraction of granular bainite is preferably 80% or less, more preferably 10% or less, and even more preferably 5% or less. The volume fraction of granular bainite may be 2% or more.

[0076] Furthermore, the steel sheet according to this embodiment has a volume fraction of retained austenite of 5% or less. If the volume fraction of retained austenite exceeds 5%, the shape of the steel sheet may be deteriorated. The remaining structure may contain ferrite or pearlite, and, depending on the manufacturing conditions described below, bainite or martensite. The martensite mentioned above also includes tempered and auto-tempered martensite. Auto-tempered martensite is tempered martensite that is formed during cooling during quenching without undergoing heat treatment for tempering. The martensite that is formed is tempered in situ by the heat generated by martensitic transformation.

[0077] Next, we will explain how to measure the volume fraction of granular bainite, which is a crystal grain with an average crystal orientation misorientation of 0.4° or more and 3.0° or less, inside crystal grains surrounded by crystal grain boundaries with an average crystal orientation misorientation of 5° or more. A sample is cut from a position at least 50 mm away from the edge of the steel plate so that a cross section perpendicular to the surface (thickness cross section) can be observed. The sample should be large enough to allow observation of approximately 10 mm in the rolling direction, depending on the measurement device. Crystal orientation information is obtained by EBSD analysis of the cut sample over a 100 μm x 100 μm measurement area, centered at 1 / 4 of the plate thickness from the surface, at a measurement interval of 0.2 μm. EBSD analysis is performed using a device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) at an analysis speed of 200–300 points per second. Scanning electron microscopes and EBSD detectors with equivalent or better performance than those mentioned above may also be used, but JEOL and TSL products are preferred.

[0078] The area fraction of granular bainite can be easily calculated from the obtained crystal orientation information, for example, by using the "Grain Average Misorientation" function included in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. This function calculates the misorientation between adjacent measurement points for a body-centered crystal grain, and then calculates the average value for all measurement points within the crystal grain. The area fraction of granular bainite can be obtained by defining regions with a misorientation of 5° or more as crystal grains using the obtained crystal orientation information and calculating the area fraction of regions within the crystal grain where the average crystal orientation misorientation is 0.4° or more and 3.0° or less using the "Grain Average Misorientation" function. Measurements are performed over five fields of view, and the average value of the area fraction of granular bainite for each field of view is defined as the area fraction of granular bainite in this embodiment. In the steel sheet according to this embodiment, the area fraction is considered to be equal to the volume fraction, and this is defined as the volume fraction of granular bainite in this embodiment.

[0079] The volume fractions of the other structures of the steel plate according to this embodiment can be determined in the same manner as in the steel member described above.

[0080] (B3) Surface of steel plate The steel sheet according to this embodiment may have a coating on a portion of its surface. The coating may be a coating mainly composed of Al (Al-based coating) or a coating mainly composed of Zn (Zn-based coating). The coating is also called a film or a plating layer. A coating mainly composed of Al is a coating containing 70% by mass or more of Al, and a coating mainly composed of Zn is a coating containing 70% by mass or more of Zn. The coating mainly composed of Al may contain, in addition to Al, 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 further 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.

[0081] (B4) Shape of steel plate 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 a tailored property material in which steel plates with different strengths or thicknesses are joined together.

[0082] Next, a method for manufacturing a steel sheet according to this embodiment will be described.

[0083] (C) Steel plate manufacturing method 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, is not limited, but it can be manufactured, for example, by using a manufacturing method including the steps shown below. (i) a slab preparation step of melting steel having the above-mentioned chemical composition and casting it to produce slabs; (ii) a hot rolling step in which the obtained slab is hot rolled to form a hot rolled steel sheet; (iii) a winding process for winding the hot-rolled steel sheet; (iv) a hot-rolled sheet annealing step in which the hot-rolled steel sheet after the coiling step is annealed as necessary; (v) a cold rolling step of descaling the hot-rolled steel sheet after the coiling step or the hot-rolled sheet annealing step, if necessary, and cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; (vi) an annealing step of annealing the hot-rolled steel sheet or the cold-rolled steel sheet as necessary to obtain an annealed steel sheet; (vii) A coating step, where necessary, of applying a coating to the hot-rolled steel sheet, the cold-rolled steel sheet or the annealed steel sheet to obtain a coated steel sheet.

[0084] Preferred conditions for each step are explained below. For conditions not explained, known conditions can be applied.

[0085] <Slab preparation process> In the slab preparation process, steel having the above-mentioned chemical composition is melted and cast to produce a slab to be subjected to hot rolling. For example, a slab can be produced by melting molten steel having the above-mentioned chemical composition using a converter or an electric furnace, and then by continuous casting. Instead of continuous casting, an ingot casting method, thin slab casting method, or the like can also be used. When continuous casting is performed, it is preferable that the average casting speed is 0.3 to 2.0 m / min and the bending straightening temperature is 500°C or higher and 1000°C or lower. An average casting speed exceeding 2.0 m / min is undesirable because it reduces the homogeneity of the slab. Generally, a slower casting speed results in uniform solidification, resulting in a slab with good homogeneity and alloying element dispersion. Therefore, the homogeneity of slabs produced at an average casting speed of 2.0 m / min or less during continuous casting is sufficiently high. Steel sheets produced using slabs produced under these conditions are likely to have a large grain boundary length ratio L49-56° / L4-12° in the steel member after heat treatment, as described below. Therefore, steel sheets with excellent hydrogen embrittlement resistance can be achieved. On the other hand, an average casting speed of 0.3 m / min or more is preferable because productivity is low. A bending correction temperature of less than 500°C is not preferable because deformation ability decreases, making bending correction difficult, and a bending correction temperature of more than 1000°C is not preferable because temperature fluctuations cause deterioration of the shape. The average casting speed is more preferably 0.6 to 0.9 m / min, and the straightening temperature is more preferably 600 to 800°C. In order to suppress cracking during bending straightening, as described above, it is preferable that [Ti]=3×[N]+0.010 to 3×[N]+0.020 (3×[N]+0.010≦[Ti]≦3×[N]+0.020) when [Nb]=0.01 or more and less than 0.03, [Ti]=5×[N]+0.010 to 5×[N]+0.020 (5×[N]+0.010≦[Ti]≦5×[N]+0.020) when [Nb]=0.03 or more and less than 0.05, and [Ti]=7×[N]+0.010 to 7×[N]+0.020 (7×[N]+0.010≦[Ti]≦7×[N]+0.020) when [Nb]=0.05 to 0.10. In addition, in slabs, it is preferable that [Ni] / [Cu]≧0.5 in order to suppress Cu hot embrittlement in the subsequent hot rolling process. On the other hand, in steel members, it is preferable that [Ni] / [Cu]<0.5 in order to increase the L49-56° / L4-12°. In this case, Cu hot embrittlement can be suppressed by controlling the Nb and Ti contents as described above.

[0086] <Hot rolling process> In the hot rolling process, the slab is heated, subjected to rough rolling, descaling if necessary, and finally subjected to finish rolling. In the hot rolling process, the hot rolling conditions are not limited. Preferably, the finish rolling temperature is 900°C or higher and 1060°C or lower.

[0087] <Winding process> In the coiling process, for example, the hot-rolled steel sheet after the hot rolling process 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 volume fraction of granular bainite in the steel sheet, it is preferable to coil the steel sheet at 450°C or higher, and more preferably at 550°C or higher.

[0088] <Hot-rolled sheet annealing process> In the annealing process 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. Annealing the hot-rolled steel sheet is preferable because it softens the hot-rolled steel sheet and reduces the load in the subsequent cold-rolling process.

[0089] <Cold rolling process> In the cold rolling process, the hot-rolled steel sheet after the hot-rolled sheet annealing process (or the hot-rolled steel sheet after the coiling process if the hot-rolled sheet annealing process is not performed) is descaled and cold-rolled to obtain a cold-rolled steel sheet. Descaling and cold rolling are not necessarily required, but if cold rolling is performed, it is preferable that the cumulative reduction in cold rolling be 30% or more from the viewpoint of ensuring good flatness. On the other hand, in order to avoid excessive rolling load, it is preferable that the cumulative reduction in cold rolling be 80% or less. The descaling method is not particularly limited, but pickling is preferred. When pickling is performed, it is preferable to remove only iron scale by pickling with hydrochloric acid or sulfuric acid.

[0090] <Annealing process> When annealing is performed before coating, the hot-rolled steel sheet or cold-rolled steel sheet is annealed in a temperature range of 700 to 950°C to obtain an annealed steel sheet. The annealing step is preferable because it can soften the cold-rolled steel sheet and make it easier to thread the sheet in the subsequent plating step.

[0091] <Coating process> When a coating is formed on the surface, a coating is formed on the surface of the steel sheet (hot-rolled steel sheet after the coiling process, hot-rolled steel sheet after the hot-rolled sheet annealing process, cold-rolled steel sheet after the cold-rolling process, or annealed steel sheet after the annealing process) in the coating process to produce a coated 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 the coating include an Al-based coating containing Al and a Zn-based coating containing Zn.

[0092] When an Al-based coating is formed by hot dip plating, the plating bath often contains Fe as an impurity in addition to Al. In addition to the elements mentioned above, the plating bath may also 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 misch metals, as long as the Al content is 70 mass% or more. When hot-dip plating is performed, the annealed steel sheet after the annealing process may be cooled to room temperature and then heated again to perform plating, or 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 cooling to room temperature.

[0093] There are no particular limitations on the pre-treatment and post-treatment for coating, and possible treatments include pre-coating, solvent application, alloying, temper rolling, etc. As an alloying treatment, for example, annealing at 450 to 800°C is possible. 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.

[0094] (D) Manufacturing method of steel components The method for manufacturing the steel member according to this embodiment is not limited, but the steel member can be manufactured by using the above-described steel plate with a manufacturing method including the steps described below, for example.

[0095] <Heat treatment process> 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 carried out under conditions in which the steel sheet obtained by the method described below is heated to a temperature of Ac3 point to (Ac3 point + 300)°C at an average heating rate of 1.0 to 1000°C / s, and then cooled to a temperature of Ms point or lower at an average cooling rate equal to or higher than the upper critical cooling rate. A temperature rise rate of less than 1.0°C / sec is undesirable because it reduces the productivity of the heat treatment, while a temperature rise rate of more than 1000°C / sec is undesirable because it results in a duplex structure and reduces the limiting hydrogen content. Furthermore, if the heat treatment temperature is lower than the Ac3 point (°C), ferrite will remain after cooling, resulting in insufficient strength, which is undesirable.On the other hand, if the heat treatment temperature is higher than the Ac3 point + 300°C, the structure will become coarse-grained, which is undesirable, resulting in a lower limit of 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. If the material is cooled below the upper critical cooling rate, ferrite and pearlite are formed, resulting in insufficient strength. During heating, the temperature may be held within ±10° C. of the heating temperature for 1 to 300 seconds. After cooling to a temperature equal to or lower than the Ms point, the steel member may be tempered at a temperature in the range of about 100 to 600° C. in order to adjust the strength of the steel member.

[0096] The Ac3 point, Ms point and upper critical cooling rate are measured by the following method. A rectangular test piece measuring 30 mm wide and 200 mm long was cut out from the steel plate according to this embodiment. The test piece was heated to 1000°C in a nitrogen atmosphere at a heating rate of 10°C / sec, held at that temperature for 5 minutes, and then cooled to room temperature at various cooling rates. The cooling rate was set from 1°C / sec to 100°C / sec in 10°C / sec increments (however, after 1°C / sec, the rate was changed to 10°C / sec). The Ac3 point and Ms point were measured by measuring the change in thermal expansion 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 precipitation did not occur was defined as the upper critical cooling rate. The Ms point obtained from the change in thermal expansion when cooled at or above the upper critical cooling rate was defined as the Ms point of the steel sheet. The Ac3 point and Ms point are measured five times in the above manner and the average is calculated.

[0097] Here, during the above series of heat treatments, hot forming such as hot stamping may be performed while the steel sheet is being heated to a temperature range of Ac3 point to (Ac3 point + 300)°C and then cooled to the Ms point, i.e., while cooling at or above the upper critical cooling rate. Examples of hot forming include bending, drawing, stretch forming, hole expansion, and flanging. Furthermore, the present invention may be applied to forming methods other than press forming, such as roll forming, as long as a means for cooling the steel sheet is provided simultaneously with or immediately after forming. Repeated hot forming may be performed if the above thermal history is followed. Furthermore, the above series of heat treatments may be repeated multiple times.

[0098] As described above, the steel member according to this embodiment includes both a formed body obtained by hot forming a steel plate and a flat plate obtained by only heat treatment.

[0099] Furthermore, as the steel member according to this embodiment, a steel member having regions with different strengths may be obtained by subjecting a part of the steel plate serving as the raw material to hot forming or heat treatment.

[0100] 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. [Example]

[0101] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0102] Example 1 Steels having the chemical compositions shown in Tables 1-1 and 1-2 (continuation of Table 1-1) were melted and continuously cast under the conditions shown in Table 2-1 to obtain slabs for hot rolling. The resulting slabs were hot-rolled and coiled at a temperature of 850°C or less to produce 3.2 mm-thick hot-rolled steel sheets. The hot-rolled steel sheets were pickled and then cold-rolled to produce 1.6 mm-thick cold-rolled steel sheets. After cold rolling, some of the steel sheets were annealed by heating to 760°C and holding for 10 seconds. They were then immersed in an aluminum-plating bath at 680°C containing 10% Si, 2% Fe, and the balance being impurities to produce aluminum-plated steel sheets (coating in Table 2-2: Al). Some of the steel sheets were immersed in a zinc-plating bath containing Zn and impurities to produce zinc-plated steel sheets (coating in Table 2-2: Zn). The coating thickness was adjusted to 25 μm. The chemical composition of these steel plates was measured at a position 1 / 4 of the way through the plate thickness from the surface, and was found to be the same as that of the slab. The [Ni] / [Cu] ratio was as shown in Table 2-1.

[0103] [Table 1-1]

[0104] [Table 1-2]

[0105] The volume fraction of the region (granular bainite) surrounded by a boundary with a crystal orientation misorientation of 5° or more and having an average crystal orientation misorientation of 0.4° or more and 3.0° or less within the boundary, and the volume fraction of retained austenite were evaluated using the methods described above. The evaluation results are shown in Table 2-2. The remainder of the metallography, not shown in the table, was ferrite and pearlite.

[0106] [Table 2-1]

[0107] [Table 2-2]

[0108] As shown in Tables 2-1 and 2-2, steel sheets having the prescribed chemical composition and metal structure were obtained from Examples B1 to B38, which satisfied the range of the present invention. On the other hand, Comparative Examples b1 to b15, which did not satisfy the range of the present invention, did not satisfy the chemical composition and metal structure. Comparative Examples b11 and b12 suffered from cracks during continuous casting, and Comparative Example b15 suffered from cracks during hot rolling, so they could not proceed to the next process.

[0109] <Example 2> The steel plate produced in Example 1 was subjected to heat treatment in which it was heated to the heating temperature shown in Table 3-1 at the heating rate shown in Table 3-1, held at a temperature within ±10°C of this heating temperature for 60 seconds, and cooled to a temperature below the Ms point at the average cooling rate shown in Table 3-1, thereby obtaining steel members C1 to C38 and c1 to c12. The chemical composition of these steel members at a position 1 / 4 of the plate thickness from the surface was the same as that of the slab.

[0110] In the obtained steel member, the volume fraction of martensite and the grain boundaries of the crystal grains having a body-centered structure are determined by the above-mentioned method. <011> The direction was used as the rotation axis, and the ratio of the grain boundary length L49-56° where the rotation angle is 49 to 56° to the grain boundary length L4-12° where the rotation angle is 4 to 12°, L49-56° / L4-12°, and the size of NbTi(C,N) in the metal structure were measured. In addition, the tensile strength and the limiting hydrogen content Hc, which is an index of hydrogen embrittlement resistance, were evaluated in the following manner. The evaluation results are shown in Table 3-2.

[0111] <Tensile strength> The tensile test was performed in accordance with the provisions of ASTM standard E8. After grinding the steel member to a thickness of 1.2 mm, except for the edges, a half-size ASTM standard E8 plate test piece (parallel length: 32 mm, parallel plate width: 6.25 mm) was taken so that the test direction was parallel to the rolling direction. A strain gauge (gauge length: 5 mm) was attached to the center of the width and length of the parallel portion of the test piece, and a room temperature tensile test was performed at a strain rate of 3 mm / min to measure the tensile strength (maximum strength). In this example, a tensile strength of more than 1500 MPa was evaluated as having high strength.

[0112] <Limit hydrogen amount Hc> Hydrogen embrittlement resistance was evaluated by measuring the limit of hydrogen content (Hc) without cracking during four-point bending of hydrogen-loaded specimens. Specifically, strip specimens 8 mm wide and 68 mm long were cut from the steel components, avoiding the edges. Strain gauges (5 mm long) similar to those used in tensile tests were attached to the surface of the specimens at the center of the width and length directions. The specimens were then bent using a four-point support jig to a strain equivalent to half the tensile strength. The four-point bend specimens with various hydrogen loadings were observed for cracking, and the limit of hydrogen content (Hc) without cracking was determined. For Al-based coatings, the amount of hydrogen storage was varied by changing the furnace dew point during heat treatment. For uncoated and Zn-based coatings, hydrogen was stored by immersing the specimens in ammonium thiocyanate solutions of various concentrations after four-point bending. The amount of hydrogen stored in the steel components was measured by temperature-programmed hydrogen analysis, where the steel components were heated at a rate of 100°C / hr and the amount of diffusible hydrogen released up to 250°C was defined as the amount of hydrogen contained in the steel components. In this example, the hydrogen embrittlement resistance was evaluated as excellent 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.

[0113] [Table 3-1]

[0114] [Table 3-2]

[0115] As shown in Tables 3-1 and 3-2, invention examples C1 to C38, which satisfy the range of the present invention, exhibited good results in both metal structure and properties, while comparison examples c1 to c12, which do not satisfy the range of the present invention, were inferior in at least one of strength and hydrogen embrittlement resistance. [Industrial Applicability]

[0116] According to the present invention, it is possible to obtain high-strength steel members and steel plates with excellent hydrogen embrittlement resistance. The coated 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 used in automotive components, they contribute to improving fuel economy and collision safety.

Claims

1. In mass%, C: 0.26-0.65%, Si: 0-2.00%, Mn: 0.05-3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0.01 to 0.10%, Ti: 0.005-0.100%, Cu: 0.10-2.50%, Ni: 0 to 0.30%, Cr: 0 to 0.50%, B: 0 to 0.0100%, Mo: 0-1.00%, V: 0-1.00%, Ca: 0-0.010%, Mg: 0 to 0.010%, Al: 0-1.00%, Sn: 0-1.00%, W: 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. and a chemical composition consisting of When the position of 1 / 4 of the thickness from the surface in the thickness direction is defined as the 1 / 4 depth position, The metal structure at the 1 / 4 depth position contains martensite in a volume fraction of 90% or more, In the metallographic structure at the 1 / 4 depth position, among grain boundaries of crystal grains having a body-centered structure, the ratio L49-56°, which is the length of a grain boundary having a rotation angle of 49 to 56° with the <011> direction as the rotation axis, to the length L4-12°, which is the length of a grain boundary having a rotation angle of 4 to 12°, is 1.10 or more; The tensile strength is greater than 1500 MPa. A steel member characterized by:

2. Furthermore, in the chemical composition, when the Ni content, Cu content, Nb content, Ti content, and N content are defined as [Ni], [Cu], [Cu], [Nb], [Ti], and [N], respectively, by mass%, [Ni] / [Cu]<0.50; When [Nb] is 0.01 or more and less than 0.03, [Ti] is 3×[N]+0.010 to 3×[N]+0.020, When [Nb] is 0.03 or more and less than 0.05, [Ti] is 5 × [N] + 0.010 to 5 × [N] + 0.020, When [Nb] is 0.05 to 0.10, [Ti] is 7 × [N] + 0.010 to 7 × [N] + 0.020, The metal structure at the 1 / 4 depth position contains NbTi(C,N), and the size of the NbTi(C,N) is 20 μm or less. The steel member according to claim 1 .

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. The steel member according to claim 3 .

5. The coating is mainly composed of an Fe-Zn alloy. The steel member according to claim 3 .

6. In mass%, C: 0.26-0.65%, Si: 0-2.00%, Mn: 0.05-3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0.01 to 0.10%, Ti: 0.005-0.100%, Cu: 0.10-2.50%, Ni: 0 to 0.30%, Cr: 0 to 0.50%, B: 0 to 0.0100%, Mo: 0-1.00%, V: 0-1.00%, Ca: 0-0.010%, Mg: 0 to 0.010%, Al: 0-1.00%, Sn: 0-1.00%, W: 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 Remainder: Fe and impurities and a chemical composition consisting of In a metal structure at a 1 / 4 depth position, which is a position of 1 / 4 of the plate thickness from the surface in the plate thickness direction, the volume fraction of a region surrounded by a boundary where the crystal orientation misorientation is 5° or more, and within the boundary, the average crystal orientation misorientation is 0.4° or more and 3.0° or less, is 85% or less; In the metal structure at the 1 / 4 depth position, the volume fraction of retained austenite is 5% or less. A steel plate characterized by:

7. In the metal structure, the volume fraction of the region surrounded by a boundary having a crystal orientation misorientation of 5° or more, and having an average crystal orientation misorientation of 0.4° or more and 3.0° or less within the boundary, is 10% or less. The steel sheet according to claim 6 .

8. having a coating on the surface; The steel sheet according to claim 6 or 7.

9. The coating is an Al-based coating. The steel sheet according to claim 8 .

10. The coating is a Zn-based coating. The steel sheet according to claim 8 .

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