Steel member and steel sheet

Abstract

A purpose of the present invention is to provide a steel member which has high strength and excellent collision resistance characteristics, and another purpose of the present invention is to provide a steel sheet from which the steel member can be produced. A steel sheet according to the present invention is characterized by having a specific chemical composition, and is also characterized in that, at the position of 1 / 4 depth, the number density of groups having an aspect ratio of 3.0 or more among groups consisting of two or more Ti- and Nb-based precipitates having an equivalent circle diameter of 1 µm or more and a shortest distance of 10 µm or less is 4.0 / mm2 or less. In addition, a steel member according to the present invention is produced from the steel sheet.

Description

Steel members and steel plates

[0001] This disclosure relates to steel members and steel plates. This application claims priority under Japanese Patent Application No. 2025-016824, filed in Japan on February 4, 2025, the contents of which are incorporated herein by reference.

[0002] In the field of automotive steel sheets, the application of high-strength steel sheets with high tensile strength is expanding in order to improve both fuel efficiency and collision safety, against the backdrop of increasingly stringent environmental regulations and collision safety standards. However, as strength increases, the press formability of the steel sheets decreases, making it difficult to manufacture products with complex shapes.

[0003] Specifically, as the strength of the steel sheet increases, its ductility decreases, leading to the problem of fracture at highly processed areas when processed into complex shapes. Furthermore, the increased strength of the steel sheet also causes springback and wall warping due to residual stress after processing, resulting in a deterioration of dimensional accuracy. Therefore, press forming of high-strength steel sheets, especially those with a tensile strength of 780 MPa or more, into products with complex shapes is not easy. While roll forming makes it easier to process high-strength steel sheets, its application is limited to parts with a uniform cross-section along the longitudinal direction.

[0004] Therefore, in recent years, hot stamping technology has been adopted as a press forming technique for materials that are difficult to form, such as high-strength steel sheets. Hot stamping technology is a hot forming technique in which the material to be formed is heated before forming.

[0005] In this technology, the material is heated before shaping. Therefore, the steel is soft during shaping and has good formability. This allows even high-strength steel plates to be precisely shaped into complex forms. Furthermore, in hot stamping technology, quenching is performed simultaneously with shaping using a press die, so the resulting steel component has sufficient strength.

[0006] However, in general, as the strength of steel members increases, their anti-collision properties decrease. That is, the higher the strength of the steel member, the easier it is for cracks to occur in the steel plate at an early stage when the member is deformed during a collision. From the perspectives of weight reduction, high strength, and improved collision safety of automotive members in recent years, steel members with high strength and excellent anti-collision properties are required.

[0007] For example, Patent Document 1 discloses a steel plate having a steel structure in which the total area ratio of martensite and bainite to the entire structure is 95% or more and 100% or less, the balance consisting of one or two of ferrite and retained austenite, the average grain size of prior austenite grains exceeding 5 μm, and the length of the long axis being 20 to 80 μm, with 5 particles / mm ２ or less present, and having a tensile strength of 1320 MPa or more. Patent Document 1 discloses that a high-strength steel plate excellent in the anti-delayed fracture property of the sheared end face can be obtained by the technique described in Patent Document 1.

[0008] Patent Document 2 discloses a steel plate having a structure in which the total area ratio of martensite and bainite is 95% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite, the density of inclusion particles having a long axis length of 20 μm or more and 80 μm or less and a shortest distance between inclusion particles longer than 10 μm, and the density of inclusion particle groups having a long axis length of 20 μm or more and a shortest distance between inclusion particles of 10 μm or less and consisting of two or more inclusion particles having a long axis length of 0.3 μm or more, with a total of 5 particles / mm ２ or less, the local P concentration being 0.060 mass% or less from the surface of the steel plate to the 3 / 4 position in the plate thickness direction at the 1 / 4 position, the Mn segregation degree in the above position range being 1.50 or less, and the tensile strength being 1320 MPa or more. Patent Document 2 discloses that a high-strength steel plate excellent in the anti-delayed fracture property not only of the steel plate base material but also of the cut end face itself can be obtained by the technique described in Patent Document 2.

[0009] International Publication No. 2018 / 062381 International Publication No. 2020 / 129402

[0010] However, in Patent Documents 1 and 2, the impact resistance characteristics of the members are not considered.

[0011] The present disclosure has been made in view of the above circumstances. An object of the present disclosure is to provide a steel member having high strength and excellent impact resistance characteristics, and a steel sheet capable of manufacturing this steel member.

[0012] The gist of the present disclosure is as follows. [1] The chemical composition is, in mass%, C: more than 0.40%, 0.70% or less, Si: 2.00% or less, Mn: 0.01 to 3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%, Co: 0 to 3.00%, Ni: 0 to 3.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.2000%, Mg: 0 to 0.2000%, REM: 0 to 0.3000%, Sb: 0 to 1.00%, Sn: 0 to 1.00%, Zr: 0 to 1.00%, As: 0 to 1.00%, Se: 0 to 1.0000%, Bi: 0 to 1.0000%, Ta: 0 to 1.0000%, Re: 0 to 1.0000%, Os: 0 to 1.0000%, Ir: 0 to 1.0000%, Tc: 0 to 1.0000%, and the balance: Fe and impurities, when the range from the position of 1 / 8 of the plate thickness to the position of 3 / 8 of the plate thickness centered on the position of 1 / 4 of the plate thickness in the plate thickness direction from the surface is defined as the 1 / 4 depth position, the metal structure at the 1 / 4 depth position is, in volume%, martensite, bainite and tempered martensite: 90% or more in total, and ferrite, pearlite, cementite and retained austenite: 0 to 10% in total, and at the 1 / 4 depth position, the number density of the group having an aspect ratio of 3.0 or more among the group consisting of two or more Ti, Nb-based precipitates having a circle equivalent diameter of 1 μm or more and a shortest distance of 10 μm or less is 4.0 pieces / mm ２A steel member characterized by the following: [2] The chemical composition is, in mass%, Cr: 0.01 to 1.00%, B: 0.0001 to 0.0100%, Mo: 0.01 to 1.00%, W: 0.01 to 2.00%, Co: 0.01 to 3.00%, Ni: 0.01 to 3.00%, Cu: 0.01 to 1.00%, V: 0.01 to 1.00%, Ca: 0.0001 to 0.2000%, Mg: 0.0001 to 0.2000%, REM: 0.0001 to 0.3000%, Sb: 0.01 to 1.00%, Sn: 0.01 to 1.00%, Zr: 0.01 to 1.00%, The steel member according to [1] above, characterized in that it contains one or more of the following: As: 0.01 to 1.00%, Se: 0.0001 to 1.0000%, Bi: 0.0001 to 1.0000%, Ta: 0.0001 to 1.0000%, Re: 0.0001 to 1.0000%, Os: 0.0001 to 1.0000%, Ir: 0.0001 to 1.0000%, and Tc: 0.0001 to 1.0000%. [3] The steel member according to [1] or [2] above, characterized in that when the position 50 μm deep from the surface in the plate thickness direction is defined as the 50 μm depth position, the Vickers hardness at the 50 μm depth position is 50 HV or less than the Vickers hardness at a position 1 / 4 of the plate thickness in the plate thickness direction from the surface. [4] The steel member according to any one of the above items [1] to [3], characterized in that the surface is coated. [5] The steel member according to the above item [4], characterized in that the coating is an Al-based coating or a Zn-based coating.[6] The chemical composition, in mass%, is: C: greater than 0.40% and 0.70% or less, Si: 2.00% or less, Mn: 0.01 to 3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%, Co: 0 to 3.00%, Ni: 0 to 3.00%, Cu: 0 to 1.00%, V Composed of: 0-1.00%, Ca: 0-0.2000%, Mg: 0-0.2000%, REM: 0-0.3000%, Sb: 0-1.00%, Sn: 0-1.00%, Zr: 0-1.00%, As: 0-1.00%, Se: 0-1.0000%, Bi: 0-1.0000%, Ta: 0-1.0000%, Re: 0-1.0000%, Os: 0-1.0000%, Ir: 0-1.0000%, Tc: 0-1.0000%, and the remainder: Fe and impurities. When the 1 / 4 depth position is defined as the range from the position 1 / 8 of the plate thickness to the position 3 / 8 of the plate thickness from the surface, with the center position being 1 / 4 of the plate thickness in the thickness direction from the surface, the number density of the group consisting of 2 or more Ti,Nb precipitates with an equivalent circle diameter of 1 μm or more and a shortest distance of 10 μm or less, where the aspect ratio is 3.0 or more, is 4.0 particles / mm. ２A steel sheet characterized by the following: [7] The chemical composition is, in mass%, Cr: 0.01 to 1.00%, B: 0.0001 to 0.0100%, Mo: 0.01 to 1.00%, W: 0.01 to 2.00%, Co: 0.01 to 3.00%, Ni: 0.01 to 3.00%, Cu: 0.01 to 1.00%, V: 0.01 to 1.00%, Ca: 0.0001 to 0.2000%, Mg: 0.0001 to 0.2000%, REM: 0.0001 to 0.3000%, Sb: 0.01 to 1.00%, Sn: 0.01 to 1.00%, Zr: 0.01 to 1.00%, The steel sheet according to [6] above, characterized in that it contains one or more of the following: As: 0.01 to 1.00%, Se: 0.0001 to 1.0000%, Bi: 0.0001 to 1.0000%, Ta: 0.0001 to 1.0000%, Re: 0.0001 to 1.0000%, Os: 0.0001 to 1.0000%, Ir: 0.0001 to 1.0000%, and Tc: 0.0001 to 1.0000%. [8] D, which is a decarburization index. Ｃ The steel sheet according to [6] or [7] above, characterized in that the ratio is 0.10 or higher. [9] The steel sheet according to any one of [6] to [8] above, characterized in that the surface is coated.

[10] The steel sheet according to [9] above, characterized in that the coating is an Al-based coating or a Zn-based coating.

[0013] According to the above embodiments of this disclosure, it is possible to provide a steel member having high strength and excellent impact resistance, as well as a steel plate on which this steel member can be manufactured.

[0014] This is a diagram illustrating how to determine the central position of Ti,Nb precipitates. The decarburization index is D. Ｃ This is a diagram to explain the calculation method.

[0015] In steel members manufactured by hot stamping onto steel plates, having a tensile strength of 2.2 GPa or more, it is difficult to ensure excellent impact resistance. The inventors of this invention have investigated how to achieve high strength and excellent impact resistance in steel members and have obtained the following findings.

[0016] (A) When Ti,Nb precipitates with large particle sizes are present in close proximity in a row, the impact resistance of the steel member decreases. This is presumed to be due to (a) the separation of the interface between the matrix phase and the Ti,Nb precipitates when the steel member deforms, and the formation of cracks, and (b) the promotion of crack propagation due to the close proximity and row arrangement of the Ti,Nb precipitates. (B) Groups consisting of two or more Ti,Nb precipitates having the above-described morphology (Ti,Nb precipitate group) are already formed at the steel sheet stage, and their persistence after hot stamping reduces the impact resistance of the steel member. (C) In the steel sheet manufacturing method, by controlling the heating conditions of the slab before hot rolling and the rough rolling conditions, the number density of Ti,Nb precipitate groups having the above-described morphology (Ti,Nb precipitate group with a high aspect ratio) can be reduced, and as a result, the impact resistance of the steel member is improved.

[0017] The steel members according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the steel members according to this embodiment will be explained.

[0018] The steel member according to this embodiment has the following chemical composition. Note that the numerical limit ranges indicated by "~" below include both the lower and upper limits. Values ​​indicated as "less than" or "greater than" do not include the numerical range. All percentages for chemical composition represent mass percentages.

[0019] The steel member according to this embodiment has a chemical composition in mass percent of: C: greater than 0.40% and 0.70% or less, Si: 2.00% or less, Mn: 0.01 to 3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, and the remainder being Fe and impurities. Each element will be described in detail below.

[0020] C: greater than 0.40%, 0.70% or less. Carbon (C) is an element that greatly affects the strength of steel members. If the C content is 0.40% or less, the desired strength cannot be obtained in the steel member. Therefore, the C content should be greater than 0.40%. Preferably, the C content is 0.42% or more, 0.45% or more, 0.47% or more, or 0.50% or more. On the other hand, if the C content is greater than 0.70%, the strength of the steel member becomes too high, and the impact resistance of the steel member decreases. Therefore, the C content should be 0.70% or less. Preferably, the C content is 0.68% or less, 0.65% or less, or 0.60% or less.

[0021] Si: 2.00% or less. Si has tempering softening resistance and has the effect of suppressing the decrease in strength due to auto-tempering during hot stamping quenching. The Si content may be 0%. However, it may be actively included to obtain the above effect. From the viewpoint of obtaining higher strength, it is preferable that the Si content be 0.005% or more. The Si content is more preferably 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, 0.15% or more, or 0.20% or more. On the other hand, if the Si content exceeds 2.00%, surface scale problems occur. That is, after pickling the scale generated during hot rolling, patterns caused by surface irregularities occur, resulting in an inferior surface appearance. Also, if plating treatment is performed on the surface of the steel sheet before it is hot stamped and formed into a steel member, the plating performance deteriorates. Furthermore, if the Si content exceeds 2.00%, the impact resistance characteristics of the steel member decrease. For this reason, the Si content should be 2.00% or less. The Si content is preferably 1.50% or less, 1.00% or less, less than 0.80%, 0.70% or less, 0.60% or less, 0.50% or less, 0.45% or less, or 0.40% or less.

[0022] Mn: 0.01 to 3.00% Mn is an element that improves the strength and hardenability of steel members. To obtain this effect and high strength, the Mn content should be 0.01% or more. Preferably, the Mn content is 0.05% or more, 0.10% or more, 0.30% or more, 0.50% or more, 0.60% or more, 1.00% or more, greater than 1.00%, 1.05% or more, or 1.10% or more. On the other hand, if the Mn content exceeds 3.00%, the impact resistance of the steel member decreases. Therefore, the Mn content should be 3.00% or less. Preferably, the Mn content is 2.80% or less, 2.50% or less, 2.00% or less, or 1.50% or less.

[0023] P: 0.100% or less. P is an element that segregates at grain boundaries and reduces the strength of the grain boundaries. If the P content exceeds 0.100%, the strength of the grain boundaries decreases significantly, and the impact resistance properties of the steel member deteriorate. For this reason, the P content should be 0.100% or less. Preferably, the P content is 0.050% or less, 0.040% or less, or 0.030% or less. The P content may be 0%. However, since excessively reducing the P content increases refining costs, the P content may be 0.001% or more, 0.003% or more, or 0.005% or more.

[0024] S: 0.0100% or less. S is an element that affects nonmetallic inclusions in steel and reduces the impact resistance of steel members. If the S content exceeds 0.0100%, the impact resistance of steel members decreases significantly. Therefore, the S content should be 0.0100% or less. Preferably, the S content is 0.0080% or less or 0.0050% or less. The S content may be 0%. However, if the S content is excessively reduced, the manufacturing cost of the desulfurization process will increase, so the S content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or more than 0.0020%.

[0025] Al: 0.001 to 1.000% Al is an element used as a deoxidizing agent for molten steel. If deoxidation is insufficient, the impact resistance of the steel member will decrease due to the excess oxides produced. In order to sufficiently deoxidize the molten steel, the Al content should be 0.001% or more. Preferably, the Al content is 0.010% or more or 0.030% or more. On the other hand, if the Al content exceeds 1.000%, many nonmetallic inclusions will be formed, and the impact resistance of the steel member will decrease. For this reason, the Al content should be 1.000% or less. Preferably, the Al content is 0.700% or less, 0.500% or less, 0.300% or less, 0.200% or less, 0.100% or less or 0.080% or less.

[0026] N: 0.0200% or less. If the N content exceeds 0.0200%, the number density of Ti,Nb precipitates increases, reducing the impact resistance of the steel member. Therefore, the N content should be 0.0200% or less. Preferably, the N content is 0.0150% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less. The N content may be 0%. However, reducing the N content to less than 0.0005% significantly increases the cost of removing N, which is economically undesirable. Therefore, it is preferable that the N content be 0.0005% or more. It is more preferable that the N content be 0.0010% or more.

[0027] O: 0.0100% or less. When oxygen is present in large amounts in steel, it forms coarse oxides that act as fracture initiation points. As a result, the impact resistance of the steel member decreases. If the oxygen content exceeds 0.0100%, the impact resistance of the steel member decreases significantly. Therefore, the oxygen content should be 0.0100% or less. Preferably, the oxygen content is 0.0080% or less or 0.0050% or less. The oxygen content may also be 0%. However, if the oxygen content is reduced to less than 0.0005%, the cost of oxygen removal increases significantly, which is economically undesirable. Therefore, it is preferable that the oxygen content be 0.0005% or more or 0.0010% or more.

[0028] Nb: 0.001 to 0.100% Nb, as a solid solution element, improves the impact resistance of steel members by refining the metal structure. If the Nb content is less than 0.001%, the impact resistance of the steel members decreases. Therefore, the Nb content should be 0.001% or more. Preferably, the Nb content is 0.005% or more or 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, the number density of Ti,Nb precipitates increases, which reduces the impact resistance of the steel members. Therefore, the Nb content should be 0.100% or less. Preferably, the Nb content is 0.080% or less, 0.070% or less, or 0.060% or less.

[0029] Ti: 0.001 to 0.200% Ti forms carbonitrides in the steel, improving the strength of the steel member through precipitation strengthening. It also refines the metal structure, improving the impact resistance of the steel member. If the Ti content is less than 0.001%, the impact resistance of the steel member decreases. Therefore, the Ti content should be 0.001% or more. Preferably, the Ti content is 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if the Ti content exceeds 0.200%, the number density of Ti,Nb precipitates increases, which reduces the impact resistance of the steel member. Therefore, the Ti content should be 0.200% or less. Preferably, the Ti content is 0.150% or less, 0.100% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0030] The remainder of the chemical composition of the steel member according to this embodiment may be Fe and impurities. Examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and / or during the steelmaking process, and are permissible within a range that does not impair the properties of the steel member according to this embodiment.

[0031] The steel member according to this embodiment may contain one or more of the following elements as optional elements in place of a portion of Fe. If the following optional elements are not included, the content is 0%.

[0032] Cr: 0-1.00% Cr is an element that improves the hardenability of steel and has the effect of improving the strength of steel members. Cr also has the effect of improving the corrosion resistance of steel members. Therefore, Cr may be included as needed. To ensure the above effects are reliably achieved, it is preferable that the Cr content be 0.01% or more. More preferably, the Cr content is 0.03% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Cr content exceeds 1.00%, carbides present after hot rolling, cold rolling, or annealing (including after plating) may stabilize, and carbides may remain after heat treatment (hot stamping), potentially reducing the impact resistance of the steel member. Therefore, the Cr content should be 1.00% or less. Preferably, the Cr content is 0.95% or less, or 0.60% or less.

[0033] B: 0 to 0.0100% B has the effect of improving hardenability in heat treatment (hot stamping) and thereby improving the strength of steel members. Therefore, B may be included as needed. In order to reliably exhibit the above effect, it is preferable that the B content be 0.0001% or more. The B content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the B content exceeds 0.0100%, the above effect will saturate, and cracks may occur during hot rolling, and the impact resistance of the steel member may decrease due to borides. Therefore, the B content should be 0.0100% or less. The B content is preferably 0.0090% or less, 0.0060% or less, or 0.0040% or less.

[0034] Mo: 0-1.00% Mo is an element that improves the hardenability of steel and has the effect of improving the strength of steel members. In addition, Mo has the effect of improving the corrosion resistance of steel members. For this reason, Mo may be included as needed. In order to reliably exhibit the above effects, it is preferable that the Mo content be 0.01% or more. The Mo content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the Mo content is greater than 1.00%, carbides present after hot rolling, cold rolling, or annealing (including after plating) may stabilize, and carbides may remain after heat treatment (hot stamping), which may reduce the impact resistance of the steel member. For this reason, the Mo content should be 1.00% or less. The Mo content is preferably 0.80% or less or 0.60% or less.

[0035] W: 0-2.00% W is an element that improves the hardenability of steel and contributes to improving the strength of steel members. W also has the effect of improving the corrosion resistance of steel members. Therefore, W may be included as needed. In order to reliably exhibit the above effects, it is preferable that the W content be 0.01% or more. More preferably, the W content is 0.05% or more or 0.10% or more. On the other hand, if the W content exceeds 2.00%, the hot workability may deteriorate and productivity may decrease. Therefore, the W content should be 2.00% or less. Preferably, the W content is 1.80% or less, 0.80% or less or 0.60% or less.

[0036] Co: 0-3.00% Co is an element that has the effect of raising the Ms point and improving the impact resistance of steel members. For this reason, Co may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Co content be 0.01% or more. More preferably, the Co content is 0.05% or more or 0.10% or more. On the other hand, if the Co content exceeds 3.00%, the Ms point becomes excessively high, which reduces the strength of the steel member. For this reason, the Co content should be 3.00% or less. Preferably, the Co content is 1.00% or less, 0.80% or less, less than 0.60%, or 0.30% or less.

[0037] Ni: 0-3.00% Ni is an element that improves the hardenability of steel and has the effect of improving the strength of steel members. Ni also has the effect of improving the corrosion resistance of steel members. Therefore, Ni may be included as needed. In order to reliably exhibit the above effects, it is preferable that the Ni content be 0.01% or more. The Ni content is more preferably 0.04% or more, 0.06% or more, or 0.10% or more. On the other hand, if the Ni content exceeds 3.00%, the amount of retained austenite produced increases, and the strength of the steel member decreases. Therefore, the Ni content should be 3.00% or less. The Ni content is preferably 1.80% or less, 1.00% or less, 0.90% or less, 0.60% or less, or less than 0.30%.

[0038] Cu: 0-1.00% Cu is an element that improves the hardenability of steel and has the effect of improving the strength of steel members. Cu also has the effect of improving the corrosion resistance of steel members. Therefore, Cu may be included as needed. In order to reliably exhibit the above effects, it is preferable that the Cu content be 0.01% or more. The Cu content is more preferably 0.05% or more, 0.07% or more, 0.10% or more, or 0.20% or more. On the other hand, if the Cu content exceeds 1.00%, the above effects saturate and the alloy cost increases. Therefore, the Cu content is 1.00% or less. The Cu content is preferably 0.80% or less or 0.60% or less.

[0039] V: 0-1.00% V has the effect of forming carbonitrides in the steel and improving the strength of the steel member through precipitation strengthening. Furthermore, as a solid solution element, it has the effect of improving the strength and impact resistance of the steel member by refining the metal structure. For this reason, V may be included as needed. In order to reliably exhibit the above effects, it is preferable that the V content be 0.01% or more. The V content is more preferably 0.04% or more, 0.10% or more, or 0.20% or more. On the other hand, if the V content exceeds 1.00%, excessive carbonitrides are generated and the impact resistance of the steel member decreases. For this reason, the V content should be 1.00% or less. The V content is preferably 0.80% or less or 0.60% or less.

[0040] Ca: 0 to 0.2000% Ca has the effect of refining inclusions in the steel and preventing cracking during hot stamping and impact caused by inclusions. Therefore, Ca may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Ca content be 0.0001% or more. More preferably, the Ca content is 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, if the Ca content exceeds 0.2000%, the effect of refining inclusions in the steel becomes saturated, and the alloy cost increases. Therefore, the Ca content is 0.2000% or less. Preferably, the Ca content is 0.1800% or less, 0.1000% or less, or 0.0180% or less.

[0041] Mg: 0 to 0.2000% Mg has the effect of refining inclusions in the steel and preventing cracking during hot stamping and impact caused by inclusions. Therefore, Mg may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Mg content be 0.0001% or more. The Mg content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the Mg content exceeds 0.2000%, the effect of refining inclusions in the steel becomes saturated, and the alloy cost increases. Therefore, the Mg content is 0.2000% or less. The Mg content is preferably 0.1800% or less, 0.1000% or less, or 0.0400% or less.

[0042] REM: 0 to 0.3000% REM has the effect of refining inclusions in steel and preventing cracking during hot stamping and impact caused by inclusions. Therefore, REM may be included as needed. In order to reliably exhibit the above effect, it is preferable that the REM content be 0.0001% or more. More preferably, the REM content is 0.0005% or more or 0.0010% or more. On the other hand, if the REM content exceeds 0.3000%, the effect of refining inclusions in steel saturates, and the alloy cost increases. Therefore, the REM content is 0.3000% or less. Preferably, the REM content is 0.2000% or less, 0.1000% or less, or 0.0600% or less. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements.

[0043] Sb: 0-1.00% Sb is an element that improves the corrosion resistance of steel components in corrosive environments. Therefore, Sb may be included as needed. To ensure the above effect is reliably achieved, the Sb content is preferably 0.01% or more. More preferably, the Sb content is 0.02% or more or 0.05% or more. On the other hand, even if the Sb content exceeds 1.00%, the above effect becomes saturated, so the Sb content should be 1.00% or less. Preferably, the Sb content is 0.80% or less, 0.60% or less, or 0.16% or less.

[0044] Sn: 0-1.00% Sn has the effect of improving the corrosion resistance of steel members. Therefore, Sn may be included as needed. In order to reliably exert this effect, it is preferable that the Sn content be 0.01% or more. More preferably, the Sn content is 0.02% or more or 0.05% or more. On the other hand, even if the Sn content exceeds 1.00%, the above effect will saturate, so the Sn content should be 1.00% or less. Preferably, the Sn content is 0.80% or less, 0.60% or less or 0.20% or less.

[0045] Zr: 0-1.00% Zr has the effect of improving the strength of steel members by forming carbonitrides in the steel and strengthening through precipitation. Furthermore, it has the effect of suppressing the formation of BN by fixing N as nitride and enhancing the hardenability improvement effect of B. For this reason, Zr may be included as needed. In order to reliably exhibit the above effects, it is preferable that the Zr content be 0.01% or more. More preferably, the Zr content is 0.02% or more or 0.05% or more. On the other hand, if the Zr content exceeds 1.00%, excessive carbonitrides are formed and the impact resistance characteristics of the steel member decrease. For this reason, the Zr content should be 1.00% or less. Preferably, the Zr content is 0.80% or less, 0.60% or less or 0.10% or less.

[0046] As: 0-1.00% As has the effect of improving hydrogen embrittlement resistance. Therefore, As may be included as needed. In order to reliably exhibit the above effect, it is preferable that the As content be 0.01% or more. The As content is more preferably 0.02% or more or 0.05% or more. On the other hand, if the As content exceeds 1.00%, the effect saturates and the cost increases. Therefore, the As content is 1.00% or less. The As content is preferably 0.80% or less, 0.60% or less or 0.12% or less.

[0047] Se: 0 to 1.0000% Se has the effect of improving hydrogen embrittlement resistance. Therefore, Se may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Se content be 0.0001% or more. The Se content is more preferably 0.0002% or more or 0.0005% or more. On the other hand, if the Se content exceeds 1.0000%, the effect saturates and the cost increases. Therefore, the Se content is 1.0000% or less. The Se content is preferably 0.0800% or less, 0.0600% or less or 0.0200% or less.

[0048] Bi: 0 to 1.0000% Bi has the effect of improving hydrogen embrittlement resistance. Therefore, Bi may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Bi content be 0.0001% or more. More preferably, the Bi content is 0.0005% or more or 0.0010% or more. On the other hand, if the Bi content exceeds 1.0000%, the effect saturates and the cost increases. Therefore, the Bi content is 1.0000% or less. Preferably, the Bi content is 0.0800% or less or 0.0600% or less.

[0049] Ta: 0 to 1.0000% Ta has the effect of improving hydrogen embrittlement resistance. Therefore, Ta may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Ta content be 0.0001% or more. More preferably, the Ta content is 0.0020% or more or 0.0050% or more. On the other hand, if the Ta content exceeds 1.0000%, the effect saturates and the cost increases. Therefore, the Ta content is 1.0000% or less. Preferably, the Ta content is 0.0800% or less, 0.0600% or less or 0.0400% or less.

[0050] Re: 0 to 1.0000% Re has the effect of improving hydrogen embrittlement resistance. Therefore, Re may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Re content be 0.0001% or more. The Re content is more preferably 0.0020% or more or 0.0050% or more. On the other hand, if the Re content exceeds 1.0000%, the effect saturates and the cost increases. Therefore, the Re content is 1.0000% or less. The Re content is preferably 0.0800% or less or 0.0600% or less.

[0051] Os: 0 to 1.0000% Os has the effect of improving hydrogen embrittlement resistance. Therefore, Os may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Os content be 0.0001% or more. More preferably, the Os content is 0.0020% or more or 0.0050% or more. On the other hand, if the Os content exceeds 1.0000%, the effect saturates and the cost increases. Therefore, the Os content is 1.0000% or less. Preferably, the Os content is 0.0800% or less or 0.0600% or less.

[0052] Ir: 0 to 1.0000% Ir has the effect of improving hydrogen embrittlement resistance. Therefore, Ir may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Ir content be 0.0001% or more. More preferably, the Ir content is 0.0010% or more or 0.0050% or more. On the other hand, if the Ir content exceeds 1.0000%, the effect saturates and the cost increases. Therefore, the Ir content is 1.0000% or less. Preferably, the Ir content is 0.0800% or less, 0.0600% or less or 0.0400% or less.

[0053] Tc: 0 to 1.0000% Tc has the effect of improving hydrogen embrittlement resistance. Therefore, Tc may be included as needed. In order to reliably exhibit the above effect, it is preferable that the Tc content be 0.0001% or more. The Tc content is more preferably 0.0010% or more or 0.0050% or more. On the other hand, if the Tc content exceeds 1.0000%, the effect saturates and the cost increases. Therefore, the Tc content is 1.0000% or less. The Tc content is preferably 0.0800% or less, 0.0600% or less or 0.0400% or less.

[0054] The chemical composition of the steel member described above can be determined by the following method. A test specimen is taken from the surface of the steel member in the thickness direction, within a range of 1 / 8 to 3 / 8 of the plate thickness, and this specimen can be measured using a general method such as ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-non-dispersive infrared absorption method. If the steel member has a coating on its surface, the coating should be removed by mechanical grinding, and then the chemical composition should be analyzed in the same manner.

[0055] Next, the metallographic structure of the steel member according to this embodiment will be described. In the steel member according to this embodiment, when the 1 / 4 depth position is defined as the range from the position of 1 / 8 of the plate thickness to the position of 3 / 8 of the plate thickness from the surface, centered at the position of 1 / 4 of the plate thickness in the thickness direction from the surface, the metallographic structure at the 1 / 4 depth position is, by volume %, martensite, bainite, and tempered martensite: 90% or more in total, and ferrite, pearlite, cementite, and retained austenite: 0 to 10% in total. At the 1 / 4 depth position, the number density of the group consisting of two or more Ti,Nb precipitates with an equivalent circle diameter of 1 μm or more and a shortest distance of 10 μm or less, and with an aspect ratio of 3.0 or more, is 4.0 particles / mm ２ The following applies:

[0056] In this embodiment, the microstructure at a depth of 1 / 4 is defined. This is because the microstructure at this position represents a typical microstructure of the steel member. The 1 / 4 depth position is the range from a position 1 / 8 of the plate thickness from the surface to a position 3 / 8 of the plate thickness, and can be rephrased as "the range starting from a position 1 / 8 of the plate thickness from the surface and ending at a position 3 / 8 of the plate thickness from the surface." Here, the surface of the steel member refers to the surface of the steel plate if the steel member consists only of a steel plate, and to the interface between the coating and the steel plate if the steel member has a coating on its surface. The interface between the coating and the steel plate is determined by the BSE COMPO image described later.

[0057] Martensite, bainite, and tempered martensite: Total of 90% or more. Martensite, bainite, and tempered martensite are high-strength structures. If the total volume percentage of martensite, bainite, and tempered martensite is less than 90%, the strength of the steel member will decrease. Therefore, the total volume percentage of martensite, bainite, and tempered martensite should be 90% or more. It is not necessary for all of martensite, bainite, and tempered martensite to be present; if only one type is present, its volume percentage must be 90% or more. If any two types are present, the total volume percentage of those two types must be 90% or more. Preferably, the total volume percentage of martensite, bainite, and tempered martensite is 93% or more, 95% or more, or 97% or more. A higher total volume percentage of martensite, bainite, and tempered martensite is preferable, so it may be 100%.

[0058] Ferrite, pearlite, cementite, and retained austenite: 0-10% in total. If the volume fraction of ferrite, pearlite, cementite, and retained austenite is too high, the desired amount of martensite, bainite, and tempered martensite cannot be secured, and the strength of the steel member will decrease. Therefore, in relation to the total volume fraction of martensite, bainite, and tempered martensite, the total volume fraction of ferrite, pearlite, cementite, and retained austenite should be 10% or less. It is not necessary for all of ferrite, pearlite, cementite, and retained austenite to be present; if any one of them is present, it is sufficient if its volume fraction (or the sum of their volume fractions) is 10% or less. Preferably, the total volume fraction of ferrite, pearlite, cementite, and retained austenite is 7% or less, 5% or less, or 3% or less. Since a lower total volume fraction of ferrite, pearlite, cementite, and retained austenite is preferable, it may be 0%. Precipitates (excluding cementite) and inclusions may be present, but their volume fractions are included in the volume fractions of the structures in which they exist (martensite, bainite, tempered martensite, ferrite, pearlite, and retained austenite).

[0059] The volume fraction of each microstructure is measured by the following method: A test specimen is taken from any position at least 10 mm away from the end face of the steel member (if it is not possible to take a test specimen from this position, take one from a position that avoids the end) in a cross section parallel to the thickness direction (thickness cross section) so that the metallographic structure at a depth of 1 / 4 is observed. The cross section of the above test specimen is polished in stages using silicon carbide sandpaper from #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, the specimen is polished at room temperature using colloidal silica that does not contain alkaline solutions to remove the strain introduced into the surface layer of the test specimen.

[0060] Crystal orientation information is obtained by electron backscatter diffraction measurement at an arbitrary position in the longitudinal direction of the cross-section of the polished specimen, with the measurement interval being 0.1 μm, in a region extending from 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness, centered at a position 200 μm in the longitudinal direction and 1 / 4 of the plate thickness from the surface in the thickness direction. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (AMETEK Velocity detector) is used. At this time, the vacuum level inside the apparatus is 9.6 × 10⁻⁶. －５ The Pa value should be below 25kV, the acceleration voltage 25kV, and the irradiation current level 16. During measurement, the Phase should be set to "iron-α" or "iron-γ".

[0061] Furthermore, the same area as the EBSD measurement area is observed at a magnification of 1000x or more using an FE-SEM (thermal field emission scanning electron microscope, JEOL JSM-7200F). To identify the observation position when observing the same area as the EBSD measurement area, Vickers indentations are stamped at three of the four corners of the EBSD measurement area, within a range of 100 μm from each corner. After that, the surface contaminants are polished off, leaving the metallic structure of the observation surface, and then Nital etching is performed. By using the Vickers indentations as markers, the same area as the EBSD observation surface can be observed.

[0062] For contaminant removal, methods such as buff polishing using alumina particles with a particle size of 0.1 μm or less, polishing using colloidal silica that does not contain alkaline solutions at room temperature, or Ar ion sputtering can be used.

[0063] In tissue observation using a thermal field emission scanning electron microscope, the volume fractions of cementite and pearlite are determined by the following method.

[0064] In microstructure observation, regions with high brightness in the secondary electron image are identified as cementite. Regions with low brightness are identified as ferrite. A structure in which ferrite and cementite are arranged alternately in layers (lamellar structure) is identified as pearlite. Note that if the structure has been processed by rolling or molding, the cementite in the lamellar structure may be curved or partially fragmented. However, even these deformed cementites are identified as pearlite if they are arranged alternately in layers with ferrite. The area ratio of pearlite is obtained by calculating the area ratio of the regions identified as pearlite. In addition, in regions other than those identified as pearlite, the area ratio of cementite is obtained by calculating the area ratio of granular regions with an equivalent circle diameter of 2 μm or more within the regions identified as cementite. The area ratios of cementite and pearlite are considered as the volume ratios of cementite and pearlite, respectively.

[0065] Note that the ferrite and cementite that make up pearlite are not included in the volume percentage of ferrite and cementite in the metal structure. Similarly, the cementite that makes up bainite and tempered martensite is not included in the volume percentage of cementite in the metal structure.

[0066] Next, for regions other than those identified as cementite and pearlite by microstructural observation using FE-SEM, regions with an fcc crystal structure and regions with a bcc crystal structure are determined from the crystal orientation information obtained by EBSD measurement. Specifically, first, the EBSD measurement results and the microstructural image obtained by FE-SEM observation are superimposed using Vickers indentations as markers. From the microstructural image obtained by FE-SEM observation, regions other than those identified as cementite and pearlite are determined from regions with an fcc crystal structure and regions with a bcc crystal structure using the crystal orientation information obtained by EBSD measurement and the "PhaseMap" function installed in the "OIMAnalysis®" software attached to the EBSD analyzer. At this time, the locations of cementite and pearlite may be identified by comparing the grain boundary map using 15° grain boundaries, as described later, with the location of Vickers indentations. The area fraction of retained austenite is obtained by calculating the area fraction of the region with the crystal structure fcc. The area fraction of retained austenite is considered to be the volume fraction of retained austenite.

[0067] Also, for the region with a bcc crystal structure, using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, it is determined whether it is ferrite, bainite, martensite, or tempered martensite, and the area ratio is measured. Specifically, under the condition that a boundary with a crystal orientation difference of 15° or more is regarded as a grain boundary (15° grain boundary), a region with a Grain Average Misorientation value (GAM value) of 1.0° or less is discriminated as ferrite. Also, a region with a GAM value exceeding 1.0° is discriminated as "martensite, bainite, and tempered martensite". By calculating the area ratios of the regions discriminated as ferrite and "martensite, bainite, and tempered martensite" respectively, the area ratios of ferrite and "martensite, bainite, and tempered martensite" are obtained. Also, the area ratios of ferrite and "martensite, bainite, and tempered martensite" are regarded as the volume ratios of ferrite and "martensite, bainite, and tempered martensite" respectively.

[0068] At the 1 / 4 depth position, the number density of the group with an aspect ratio of 3.0 or more among the groups composed of two or more Ti, Nb-based precipitates with an equivalent circle diameter of 1 μm or more and a shortest distance of 10 μm or less: 4.0 pieces / mm ２ Hereinafter, the Ti, Nb-based precipitate is a structure harder than steel. When a Ti, Nb-based precipitate with an equivalent circle diameter (equivalent circular diameter) of 1 μm or more exists in the steel, cracks are generated due to the peeling of the interface between the steel and the Ti, Nb-based precipitate or the cracking of the Ti, Nb-based precipitate itself during deformation. Also, when these Ti, Nb-based precipitates exist side by side at a narrow interval, the connection between cracks becomes easy, the propagation of cracks is promoted, and the collision resistance characteristics of the steel member deteriorate. When the number density of the group with an aspect ratio of 3.0 or more among the groups composed of two or more Ti, Nb-based precipitates with an equivalent circle diameter of 1 μm or more and a shortest distance of 10 μm or less is ２ more than 4.0 pieces / mm, the collision resistance characteristics of the steel member are significantly deteriorated. Therefore, the number density of the above precipitate group is ２ ２The following applies: The number density of the above precipitate group is preferably 3.4 particles / mm³. ２ Below, 3.0 pieces / mm ２ Below, 2.5 pieces / mm ２ Below, 2.0 pieces / mm ２ The following or 1.5 pieces / mm ２ The following is preferable: A lower number density for the above precipitate group is preferable, so 0.0 particles / mm ２ or 0.5 pieces / mm ２ That's fine too.

[0069] The number density of groups consisting of two or more Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a shortest distance of 10 μm or less, and with an aspect ratio of 3.0 or more, is measured by the following method. Ti,Nb precipitates are identified by energy-dispersive X-ray spectroscopy (EDS). The measurement uses an analytical apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-6380A) and an EDS detector (Bruker AXS XFlash 5030T). The vacuum level inside the apparatus is 2.0 × 10⁻⁶. －４ The pressure (Pa) is set to 20 kV, the acceleration voltage to 20 kV, the irradiation current to 35 μA, and the irradiation time to 32 μs.

[0070] A test specimen is cut from any position at least 10 mm away from the end face of the steel member (if it is not possible to take a test specimen from this position, a position avoiding the end face is used). The surface of the cut test specimen is mechanically ground, and then polished in stages using silicon carbide sandpaper from #600 to #1500 up to a depth of 1 / 4 of the steel member. Finally, a mirror finish is achieved using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water, and this specimen is prepared for SEM observation. The surface of this specimen is measured over 4 mm. ２ SEM observation and EDS measurements will be performed in the above areas.

[0071] Observation is performed at a measurement magnification of 500x. Steel and Ti,Nb precipitates are distinguished by the brightness of the backscattered electron image and EDS measurement. Regions with low brightness are considered precipitates. Next, the composition of the above precipitates is measured by EDS. The elements to be analyzed are C, Si, Mn, P, S, Al, N, O, Nb, and Ti. Cr, B, Mo, W, Co, Ni, Cu, V, Ca, Mg, REM, Sb, Sn, Zr, As, Se, Bi, Ta, Re, Os, Ir, and Tc may be added as needed. Among the precipitates with a COUNT number of 1000 or more, precipitates in which the sum of the concentrations of Ti and Nb is 10% or more by weight are identified as Ti,Nb precipitates. The equivalent circular diameter of Ti,Nb precipitates is calculated, and Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more are classified as precipitate groups by performing the following procedure.

[0072] For a given Ti,Nb precipitate A to belong to the same group as any other Ti,Nb precipitate B, it means that the length of the line segment (shortest distance) connecting the center positions of precipitate A and precipitate B is less than or equal to 10 μm. This is calculated for all combinations of Ti,Nb precipitates to identify those belonging to the same group. Furthermore, the distance between the centers of the two Ti,Nb precipitates that are furthest apart among those belonging to the same group is considered the major axis of the Ti,Nb precipitate group. Also, the equivalent circular diameter of the largest Ti,Nb precipitate among those belonging to the same group is considered the minor axis of the Ti,Nb precipitate group. By identifying Ti,Nb precipitate groups with an aspect ratio (major axis / minor axis) of 3.0 or greater, and calculating the number of these groups divided by the measurement area, the number density of groups consisting of two or more Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a shortest distance of 10 μm or less, and with an aspect ratio of 3.0 or greater, is obtained. The major axis of the Ti,Nb precipitate groups with an aspect ratio of 3.0 or greater is at least 3 μm or greater.

[0073] The central position of the Ti,Nb precipitate is determined by the following method. As shown in Figure 1, the lower left of the backscattered electron image is taken as the origin, with the horizontal direction as the x-axis and the vertical direction as the y-axis. Central position of the precipitate (coordinates: x ｃ , y ｃ x coordinate of )ｃ The x-coordinate of the precipitate is the average of the x-coordinate of the pixel with the maximum x-coordinate and the x-coordinate of the pixel with the minimum x-coordinate. The y-coordinate of the center position of the precipitate is y ｃ We will calculate the same for this as well.

[0074] In the steel member according to this embodiment, when the 50 μm depth position is defined as a position 50 μm deep from the surface in the thickness direction (a range of 45 to 55 μm deep from the surface in the thickness direction is acceptable), it is preferable that the Vickers hardness at the 50 μm depth position is 50 HV or more smaller than the Vickers hardness at a position 1 / 4 of the thickness from the surface in the thickness direction (a range of -20 μm to +20 μm in the thickness direction centered on the 1 / 4 position from the surface is acceptable). That is, it is preferable that the condition "(Vickers hardness at the 1 / 4 thickness position from the surface) - (Vickers hardness at the 50 μm depth position) ≥ 50 HV" is satisfied. By making the Vickers hardness at the 50 μm depth position 50 HV or more smaller than the Vickers hardness at the 1 / 4 thickness position from the surface in the thickness direction, better bendability can be obtained. As a result, better impact resistance can be obtained in the steel member. To further improve flexibility, it is preferable that the Vickers hardness at a depth of 50 μm is 60 HV or more, or 100 HV or more, less than the Vickers hardness at a position 1 / 4 of the plate thickness in the direction from the surface to the plate thickness.

[0075] Vickers hardness is measured by the following method. In accordance with JIS Z 2244-1:2024, Vickers hardness tests are performed at a depth of 50 μm and at a position 1 / 4 of the plate thickness from the surface in the thickness direction. In the Vickers hardness test, five measurements are taken at each position on the cross-section (thickness cross-section) of the polished specimen, with a load of 0.098 N at the 50 μm depth position and a load of 9.8 N at the position 1 / 4 of the plate thickness from the surface in the thickness direction, with a load holding time of 10 seconds. The Vickers hardness at each position is obtained by calculating the average of three points excluding the maximum and minimum values.

[0076] The steel member according to this embodiment may have a coating on part or all of its surface. The coating may be an Al-based coating (a coating mainly composed of an Fe-Al alloy) or a Zn-based coating (a coating mainly composed of an Fe-Zn alloy). The coating is also called a film, an alloyed plating layer, or an intermetallic compound layer. Having a coating can improve corrosion resistance. The thickness of the coating is preferably 5 to 100 μm.

[0077] Al-based coatings (coatings mainly composed of Fe-Al alloys) are coatings containing a total of 70% by mass or more of Fe and Al, and Zn-based coatings (coatings mainly composed of Fe-Zn alloys) are coatings containing a total of 70% by mass or more of Fe and Zn.

[0078] Al-based coatings (coatings mainly composed of Fe-Al alloys) may contain, in addition to Fe and Al, one or more of the following: 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.

[0079] Zn-based coatings (coatings mainly composed of Fe-Zn alloys) may contain, in addition to Fe and Zn, one or more of the following: 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.

[0080] The thickness of the coating can be determined by cross-sectional observation using a scanning electron microscope. A test specimen is cut from any position at least 10 mm away from the end face. The cross-section of the cut test specimen is mechanically polished and then mirror-finished. The observation range with the scanning electron microscope is, for example, 400x magnification, covering an area of ​​40,000 μm. ２ The above range applies.

[0081] Cross-sectional observation using BSE COMPO imaging reveals a clear contrast difference between the coating and the underlying steel plate. Therefore, the thickness of the coating can be measured by measuring the thickness from the outermost surface to the point where the contrast changes. Measurements are taken at 20 equally spaced points within the observation photograph, with a distance of 6.5 μm between each measurement point. In addition, five fields of view are observed using the above procedure, and the average value is used to determine the thickness of the coating.

[0082] The chemical composition of the coating can be determined by performing elemental analysis of spots (beam diameter 1 μm or less) using an electron probe microanalyzer (EPMA) within the same observation range as described above, thereby determining the concentrations of Fe, Al, and Zn contained in the coating. Ten points are analyzed in any 10 fields of view of the coating, and the average value is taken as the concentration of Fe, Al, and Zn contained in the coating. The same method is used to determine the concentrations of elements other than Fe, Al, and Zn that are present.

[0083] The plate thickness of the steel member according to this embodiment is not particularly limited, but may be 0.4 to 5.0 mm. From the viewpoint of increasing the energy absorbed during impact, the plate thickness may preferably be 0.8 to 5.0 mm or 1.0 to 5.0 mm.

[0084] Strength: Vickers hardness of 630 HV or more. In this embodiment, Vickers hardness is used as an alternative indicator for tensile strength. The steel member according to this embodiment may have a Vickers hardness of 630 HV or more at a position 1 / 4 of the plate thickness from the surface in the thickness direction. If the Vickers hardness at a position 1 / 4 of the plate thickness from the surface in the thickness direction is 630 HV or more, in this embodiment it can be determined that it has sufficient strength (tensile strength: 2.2 GPa or more). The Vickers hardness at a position 1 / 4 of the plate thickness from the surface in the thickness direction is preferably 660 HV or more, 680 HV or more, or 690 HV or more. The Vickers hardness at a position 1 / 4 of the plate thickness from the surface in the thickness direction is also 850 HV or less. The Vickers hardness at a position 1 / 4 of the plate thickness from the surface in the thickness direction is measured by the method described above.

[0085] VDA bending angle (α): 40° or more The VDA bending angle is an indicator of the collision resistance characteristics of a steel member. The higher the VDA bending angle of the steel plate constituting the steel member, the better the collision resistance characteristics of the steel member. In this embodiment, it is preferable that the steel member has a VDA bending angle of 40° or more (converted to a plate thickness of 2.0 mm), which can be obtained by performing a bending test in accordance with the following VDA standard. If the VDA bending angle of the steel plate constituting the steel member is 40° or more, in this embodiment it can be determined that the steel member has excellent collision resistance characteristics, and the collision safety of the automobile member can be improved.

[0086] The bending test shall be performed by the following method: A test specimen shall be taken from any position at least 10 mm away from the end face of the steel member, and the bending test shall be performed in accordance with the German Association of the Automotive Industry standard VDA 238-100:2017-04 under the following conditions: Determine the bending angle (VDA bending angle) when the bending load has decreased by 60 N from the maximum point. If cracking occurs before the bending load decreases by 60 N from the maximum point, determine the bending angle at the time of cracking and use that as the VDA bending angle. Note that the VDA bending angle differs depending on the relationship between the bending ridge direction and the rolling direction of the steel plate, but when taking a test specimen from the member, a specimen taken from any direction may be used.

[0087] If the plate thickness is not 2.0 mm, the VDA bending angle obtained from the bending test at that plate thickness is corrected to a value equivalent to a 2.0 mm plate thickness using the following formula: VDA bending angle = α ｔ -23.05 × (1 - t / 2.0) In the above formula, α ｔ The VDA bending angle obtained from the bending test is shown, and t represents the plate thickness (mm).

[0088] The conditions for the bending test are as follows: Specimen dimensions: 60 mm x 30 mm Bending edge: Parallel to the 30 mm side Test method: Roll support, punch pressing Roll diameter: φ30 mm Punch shape: Tip radius = 0.4 mm Distance between rolls: (2.0 x plate thickness + 0.50) to (2.0 x plate thickness + 0.55) mm Pressing speed: 20 mm / min Testing machine: SHIMADZU AUTOGRAPH 20kN

[0089] Next, a steel sheet according to this embodiment, which can be used to manufacture the steel members described above, will be described. Since the steel sheet according to this embodiment has the same chemical composition as the steel members described above, the reasons for limitation and the measurement method will be omitted from the explanation.

[0090] In this embodiment, when the steel plate is defined as having a 1 / 4 depth position centered at a position 1 / 4 of the plate thickness from the surface, and extending from a position 1 / 8 of the plate thickness to a position 3 / 8 of the plate thickness from the surface, the number density of the group consisting of two or more Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a shortest distance of 10 μm or less, and having an aspect ratio of 3.0 or more, is 4.0 particles / mm². ２ The following applies. Note that the surface used as the reference for depth position is the surface of the steel plate; however, if the steel plate has a coating on its surface, it refers to the interface between the coating and the base steel plate. In the case of steel plates, the surface is determined by the GD-OES analysis described later.

[0091] The number density of the group consisting of two or more Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a shortest distance of 10 μm or less at a 1 / 4 depth position, and with an aspect ratio of 3.0 or more, is 4.0 particles / mm². ２ The Ti,Nb precipitates described below have a structure that is harder than steel. When Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more are present in steel, cracks are generated during deformation due to delamination at the interface between the steel and the Ti,Nb precipitates or cracking of the Ti,Nb precipitates themselves. Furthermore, if these Ti,Nb precipitates are arranged at close intervals, cracks are easily linked together, crack propagation is accelerated, and the impact resistance of the steel member is reduced. Among the group consisting of two or more Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a shortest distance of 10 μm or less, the number density of the group with an aspect ratio of 3.0 or more is 4.0 particles / mm². ２ If the number density is excessive, the state of the Ti,Nb precipitate group in the steel member obtained by heat treatment cannot be favorably controlled, and the impact resistance properties of the steel member are significantly reduced. Therefore, the number density of the precipitate group should be 4.0 particles / mm³. ２ The following applies: The number density of the above precipitate group is preferably 3.4 particles / mm³. ２ Below, 3.0 pieces / mm２ Below, 2.5 pieces / mm ２ Below, 2.0 pieces / mm ２ The following or 1.5 pieces / mm ２ The following is preferable: A lower number density for the above precipitate group is preferable, so 0.0 particles / mm ２ or 0.5 pieces / mm ２ That's fine too.

[0092] The method for measuring the number density of Ti,Nb precipitates is the same as for steel members, so the explanation is omitted.

[0093] If the rolling direction of the steel plate is not known in advance, the rolling direction of the steel plate may be determined by the following method. A test piece is taken from any position at least 50 mm away from the end face of the steel plate so that the thickness cross section can be observed. After finishing the thickness cross section of the taken test piece with mirror polishing, it is observed using an FE-SEM at magnifications of 100x, 200x, 500x, and 1000x using BSE COMPO images. Depending on the size of the inclusion, the observation result at an appropriate magnification that allows the inclusion dimensions to be measured is selected. The observation range is 500 μm or wider and covers the entire thickness of the plate, and areas with low brightness are determined to be inclusions. Multiple fields of view may be used for observation. Next, using the thickness cross section initially observed using the above method as a reference, a test piece is taken so that the thickness cross section can be observed rotated in 5° increments within the range of 0° to 180° around the thickness axis, and the cross section is observed using the same method as above. The average length of the major axis of multiple inclusions in each cross-section is calculated for each cross-section. The cross-section with the highest average length of the major axis of the inclusions is identified. The direction parallel to the major axis direction of the inclusions in that cross-section is determined to be the rolling direction. In the case of steel members, the rolling direction of the steel plates constituting the steel member can be determined in a similar manner.

[0094] D, the decarburization index Ｃ : 0.10 or higher The steel sheet according to this embodiment has a decarburization index (an index representing the decarburization state of the surface layer) D Ｃ It is preferable that the decarburization index of steel sheets is D ＣBy setting the decarburization index to 0.10 or higher, the Vickers hardness of the surface layer (at a depth of 50 μm) of the steel member after heat treatment (hot stamping) can be reduced. As a result, the bendability of the steel member can be further improved, and the impact resistance of the steel member can be further improved. Ｃ The decarburization index D Ｃ It may be 0.30 or less.

[0095] The decarburization index of steel sheets is obtained by the following method. A test specimen is taken from any position at least 50 mm away from the edge of the steel sheet, and the elemental concentration distribution is measured from the surface of the steel sheet toward the thickness direction using a glow discharge optical emission spectrometer (GD-OES: Horiba Marcus-type high-frequency glow discharge optical emission spectrometer, GD-PROFILER-HR). The measurement range is set to a depth of at least 120 μm from the measurement starting surface. The measurement conditions are set to an analysis diameter of 4 mmφ, a sputtering rate of 4 μm / min, an argon pressure of 600 Pa, an RF output of 35 W, and a measurement interval of 0.02 μm or less. The elements to be analyzed are C and Fe. If the contained components have been identified in advance, they should be specified as the elements to be analyzed.

[0096] In this embodiment, the region where the Fe concentration is 90% by mass or more in the GD-OES analysis is determined to be a steel plate, and the depth position of the measurement point where the Fe concentration first reaches 90% by mass or more from the start of measurement is defined as the surface of the steel plate. If the steel plate has a coating on its surface, part or all of the coating is removed by mechanical polishing or chemical polishing so that measurement is possible up to a depth of 120 μm from the surface of the steel plate (the interface between the steel plate and the coating) before being subjected to GD-OES analysis. The position where the Fe concentration first reaches 90% by mass in the GD-OES analysis is considered to be the interface between the steel plate and the coating.

[0097] The average value of the measured carbon concentration in the region from a depth of 100 μm to 120 μm from the surface of the steel plate is calculated, and the resulting average value is considered to be the carbon concentration of the steel plate.

[0098] By performing GD-OES analysis to a depth of 120 μm from the surface of the steel plate, a curve graph like the one shown in Figure 2 is obtained. For each unit depth (measurement interval), the difference between the C concentration of the steel plate and the measured C concentration at that measurement location is calculated to obtain the C reduction amount. If the C concentration at the measurement point is higher than the C concentration of the steel plate, the C reduction amount is set to 0. The area of ​​the C-deficient region is obtained by calculating the integral of the product of the unit depth and the C reduction amount (area of ​​region A in Figure 2). The product of the C concentration of the steel plate and the depth of 120 μm is used as the reference area (area of ​​region B in Figure 2). The value obtained by dividing the area of ​​the C-deficient region by the reference area (area of ​​region A / area of ​​region B) is multiplied by 0.6 to obtain the decarburization index, D Ｃ To obtain.

[0099] In this embodiment, when the steel sheet is defined as having a 1 / 4 depth position centered at a position 1 / 4 of the sheet thickness from the surface, and extending from 1 / 8 of the sheet thickness to 3 / 8 of the sheet thickness from the surface, the metal structure at the 1 / 4 depth position may consist of, by volume %, pearlite: 40% or more, and ferrite, bainite, martensite, tempered martensite, cementite, and retained austenite: a total of 60% or less. Furthermore, it is preferable that the volume fraction of retained austenite among the structures other than pearlite be less than 10%. If the volume fraction of retained austenite in the metal structure of the steel sheet is 10% or more, the distribution of elements such as Mn becomes heterogeneous in the steel member after hot stamping heating, and the desired impact resistance cannot be obtained. The method for measuring the volume fraction of each structure is the same as for the steel member, so the explanation is omitted.

[0100] In the steel sheet according to this embodiment, the tensile strength is preferably 1300 MPa or less. If the tensile strength of the steel sheet is 1300 MPa or less, shear workability can be ensured, and blank materials for heat treatment can be easily manufactured. The tensile strength of the steel sheet is measured by taking a JIS No. 5 tensile test specimen from a position 1 / 4 of the sheet width from the end face of the steel sheet, along a direction perpendicular to the rolling direction, and performing a tensile test at room temperature in accordance with JIS Z 2241:2022. The tensile test is performed twice at a tensile speed of 10 mm / min, and the average value is used as the representative value.

[0101] The steel plate according to this embodiment may have the above-described coating on part or all of its surface. The type of coating and measurement method are the same as for steel members, so a description will be omitted.

[0102] The thickness of the steel plate according to this embodiment is not particularly limited, but may be 0.4 to 5.0 mm. From the viewpoint of increasing the energy absorbed during impact, the plate thickness may preferably be 0.8 to 5.0 mm or 1.0 to 5.0 mm.

[0103] Next, a preferred method for manufacturing the steel plate according to this embodiment will be described. In this embodiment, the slab temperature and steel plate temperature refer to the surface temperature of the slab and the surface temperature of the steel plate, respectively.

[0104] In the preferred method for manufacturing steel sheets according to this embodiment, a slab having the above-described chemical composition is obtained by continuous casting, the slab is inserted into a heating furnace before it cools to a temperature range of 350°C or lower, and the slab is heated such that the following equations (A) and (B) are satisfied, when the residence time in the temperature range of 1050°C or higher is ts (minutes) and the holding time in the temperature range of 1200°C or higher is th (minutes): ts ≤ 120 …(A) 0.65 × ts ≤ th …(B) In rough rolling, the reduction ratio of the final stage and the stage before the final stage is set to 45% or less, and the rolling completion temperature is set to 1050°C or higher. The following describes each step.

[0105] A slab having the chemical composition described above is obtained by continuous casting. Then, before it cools to a temperature range of 350°C or below, the slab is inserted into a heating furnace and heating is started. Ti,Nb precipitates precipitate during the cooling process to room temperature after continuous casting, and then coarseen and stabilize. By starting heating before the slab cools to a temperature range of 350°C or below, the precipitation, coarsening, and stabilization of Ti,Nb precipitates can be suppressed.

[0106] The slab inserted into the heating furnace is heated so as to satisfy the above formulas (A) and (B), where ts (minutes) is the residence time in the temperature range of 1050°C or higher and th (minutes) is the holding time in the temperature range of 1200°C or higher. By satisfying formula (A), the coarsening of TiN, which does not dissolve during slab heating, among the Ti,Nb precipitates, can be suppressed. Furthermore, by satisfying formula (B), the residence time in the temperature range of 1050°C or higher but less than 1200°C is shortened, suppressing the coarsening of Ti,Nb precipitates, and promoting the dissolution of Ti,Nb precipitates other than TiN in the temperature range of 1200°C or higher. As a result, the number density of the Ti,Nb precipitate group can be reduced. To refine the metal structure of the steel sheet, it is preferable that th be less than 100 minutes.

[0107] After the above heating, in rough rolling, the reduction ratio of the final stage and the stage before the final stage should be set to 45% or less, and the rolling completion temperature (the exit temperature of the final stage of rough rolling) should be 1050°C or higher. By performing rough rolling under these conditions, it is possible to suppress the cracking and dispersion of Ti,Nb precipitates in the rolling direction, and the increase in the aspect ratio of the Ti,Nb precipitate group. Note that the reduction ratio here refers to the thickness of the sheet before rolling, t ０ The thickness of the sheet after rolling is t １ When this is the case, (1-t １ / t ０ It can be expressed as ) × 100 (%).

[0108] After rough rolling, finish rolling and coiling are performed. Furthermore, the hot-rolled steel sheet may be cold-rolled or annealed as needed. Also, a coating may be formed after cold rolling or annealing as needed. The conditions for these processes are not particularly limited, but in order to refine the metal structure of the steel sheet, the total reduction ratio of cold rolling should be 30% or more, and the heating temperature for annealing should be (Ac ３ Point -150)℃ or higher, (Ac ３It is preferable to keep the temperature below (Act + 150°C). Furthermore, in order to further improve the bendability of the steel member and obtain better impact resistance, it is preferable to decarburize the surface layer of the steel sheet during annealing (also called decarburization annealing). Decarburization annealing may be performed after the hot rolling process or after the cold rolling process. Also, a coating may be formed after decarburization annealing. The decarburization conditions are a humid atmosphere containing nitrogen, hydrogen, or oxygen with a dew point of 1°C or higher, and (Act ３ Point -150)℃ or higher, (Ac ３ It is preferable that the residence time in the temperature range below (point + 150°C) is 5 to 1200 seconds. Here, residence time refers to the time it takes for the steel plate temperature to rise before reaching Ac ３ After raising the temperature to -150°C and holding it for a certain period of time, Ac ３ This is the total time it takes to cool down to -150°C. ３ The point can be determined by the formula described later.

[0109] The coating method is not particularly limited and can include hot-dip galvanizing, electroplating, vacuum deposition, cladding, and thermal spraying. Hot-dip galvanizing is the most widely used method industrially. Examples of coatings include Al-based coatings containing Al and Zn-based coatings containing Zn.

[0110] When forming an Al-based coating by hot-dip plating, the plating bath often contains Fe as an impurity in addition to Al. Furthermore, as long as the total amount of Fe and Al is 70% by mass or more, the plating bath may also contain one or more of the following in addition to Al and Fe: Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM.

[0111] When forming a Zn-based coating by hot-dip plating, the plating bath often contains Fe as an impurity in addition to Zn. Furthermore, as long as the total amount of Fe and Zn is 70% by mass or more, the plating bath may also contain one or more of the following in addition to Zn and Fe: Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM.

[0112] When performing hot-dip plating, the steel sheet may be cooled to room temperature after the annealing or decarburization annealing process and then reheated for plating. Alternatively, after annealing or decarburization annealing, the sheet may be cooled to a temperature near the plating bath temperature (for example, 650 to 750°C for Al-based coatings, or 420 to 500°C for Zn-based coatings), and hot-dip plating may be performed without first cooling to room temperature.

[0113] There are no particular limitations on the pre-treatment and post-treatment of the coating; pre-coating, solvent application, alloying treatment, temper rolling, etc., are possible. For alloying treatment, the coated steel sheet can be heated to, for example, 450 to 800°C. As a post-treatment, skin pass rolling with a reduction of 0.1 to 0.5% may be performed to adjust the shape of the steel sheet.

[0114] The steel member according to this embodiment can be stably manufactured using a manufacturing method that includes, for example, the following steps, applied to the steel plate according to this embodiment obtained by the method described above.

[0115] A steel member is obtained by heat-treating the steel plate according to this embodiment. The heat treatment is performed, for example, by heating the steel plate obtained by the above method at an average heating rate of 1.0 to 1000°C / second. ３ Point~(Ac ３ It is preferable to heat the material to a temperature range of (point + 300°C) and then cool it to a temperature range below the Ms point at an average cooling rate of 30.0°C / second or higher.

[0116] An average heating rate of less than 1.0°C / second is undesirable because it reduces the productivity of the heat treatment. On the other hand, an average heating rate exceeding 1000°C / second is also undesirable because it results in a mixed grain structure and reduces the critical hydrogen content.

[0117] Furthermore, the heat treatment temperature (maximum heating temperature) is Ac ３ If the temperature is below the Ms point (°C), the average cooling rate is less than 30.0°C / second, or the cooling stop temperature is above the Ms point (°C), it is undesirable because the volume fraction of martensite, bainite, and tempered martensite will be insufficient after cooling, which may result in insufficient strength of the steel member. On the other hand, if the heat treatment temperature is below the Ms point (°C), ３ Temperatures above 300°C are undesirable because they can cause the metal structure to coarseen, leading to a decrease in the critical hydrogen content and a reduction in impact resistance.

[0118] During heating, Ac ３ Point~(Ac ３ The temperature may be maintained at a temperature of (Ms point + 300°C) for 1 to 300 seconds. Alternatively, after cooling to a temperature below the Ms point, a tempering treatment may be performed by maintaining the steel member at a temperature of 100 to 600°C for 10 to 60 minutes to adjust its strength.

[0119] Ac ３ The point and Ms point are calculated using the content (mass%) of each element in the chemical composition of the steel sheet (heat-treated steel sheet) from the following formulas (1) and (2). Substitute 0 for elements that are not present. Ac ３ (℃)=854-179×C+44×Si-14×Mn-18×Ni-2×Cr+4×Co…(1) Ms(℃)=521-353×C-22×Si-24×Mn-17×Ni-8×Cu-16×Mo+10×Co…(2)

[0120] Here, during the above series of heat treatments, Ac ３ Point~(Ac ３ It is preferable to perform hot forming, such as hot stamping, while the material is being heated to a temperature range of (point + 300°C) and then cooled to the Ms point. Examples of hot forming include bending, drawing, stretching, and hole widening (stretch flange forming). In addition, other forming methods besides press forming, such as roll forming, may be applied, provided that a means is provided to cool the steel sheet simultaneously with or immediately after forming. Repeated hot forming may be performed according to the above-described thermal history. Furthermore, the above series of heat treatments may be repeated multiple times.

[0121] When performing hot forming by hot stamping (including when using a flat die), it is preferable to set the forming start temperature to 700°C or higher, cool the die, and set the release temperature (the surface temperature of the steel member when removing it from the die) to be below the Ms point and below 250°C. This improves the dimensional accuracy of the steel member.

[0122] Furthermore, the above heat treatment may be performed on a portion of the steel sheet that will be used as the material for hot forming or heat treatment. In this case, a steel member with regions of different strengths can be obtained.

[0123] The above series of heat treatments can be carried out by any method; for example, heating may be performed by high-frequency heating, electric heating, infrared heating, or furnace heating. Cooling may also be performed by water cooling, mold cooling, etc. In addition, the atmosphere inside the heating furnace may be air, combustion gas, or nitrogen gas. Furthermore, the dew point inside the heating furnace may be controlled to suppress hydrogen generation during the heat treatment.

[0124] Next, embodiments of the present invention will be described. The conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0125] Steel having the chemical compositions shown in Tables 1A to 2D was melted and continuously cast. Subsequently, the slabs were heated under the conditions shown in Tables 3A to 3C, followed by hot rolling, cold rolling, and annealing (cold rolling or annealing was omitted under some conditions) to obtain steel plates with a thickness of 1.6 to 3.5 mm.

[0126] Furthermore, after slab heating and rough rolling under the conditions shown in the table, the finish rolling completion temperature is Ac ３ Finish rolling was performed to achieve a score of 1.5°C or higher, and the sheet was wound to a winding temperature in the range of 750°C to 500°C. Furthermore, in some embodiments, the hot-rolled steel sheet was cold-rolled with a total reduction ratio of 30% or more. In addition, in some embodiments, after cold rolling, the sheet was treated in an atmosphere with a dew point of less than 1°C (Ac ３ Point -150)℃ or higher, (Ac ３ Annealing was performed by holding the material at a temperature of (1°C + 150°C) or lower for 1000 seconds or less. Examples of this annealing process are indicated as "normal" in the table. In addition, for some examples, in order to form a decarburized layer on the surface, after cold rolling, the material was annealed in a humid atmosphere containing nitrogen, hydrogen, or oxygen with a dew point of 1°C or higher (Ac ３ Point -150)℃ or higher, (Ac ３Decarburization annealing was performed under conditions where the residence time was 5 to 1200 seconds in a temperature range of 150°C or lower. Examples of this annealing process are indicated as "decarburization" in the table. Furthermore, in some examples, an Al-based coating or a Zn-based coating was applied to the steel sheet after cold rolling, annealing, or decarburization annealing under the conditions described above.

[0127] The following terms are used for each item in the table: Insertion temperature: The temperature at which the slab obtained by continuous casting is inserted into the heating furnace. Residence time ts: Residence time in the temperature range of 1050°C or higher. Heating temperature: The maximum heating temperature of the slab. Holding time th: Holding time in the temperature range of 1200°C or higher.

[0128] The obtained steel sheet was subjected to the above-described method to determine the Ti,Nb precipitate group and the decarburization index D Ｃ The following was evaluated. The results obtained are shown in Tables 4A to 4C. In the tables, the number density of precipitate groups represents the number density of groups consisting of two or more Ti,Nb precipitates with an equivalent circle diameter of 1 μm or more and a shortest distance of 10 μm or less at a 1 / 4 depth position, and whose aspect ratio is 3.0 or more. The metal structure was also measured in the same manner as the steel members described above. In the metal structure of the steel sheet according to the present invention example, the volume fraction of pearlite was 40% or more, and the structures other than pearlite consisted of one or more of ferrite, bainite, martensite, tempered martensite, cementite, and retained austenite, with a total volume fraction of 0 to 60%. The volume fraction of retained austenite was less than 10%. Furthermore, a JIS No. 5 tensile test specimen was taken from a position 1 / 4 of the sheet width from the end face of the steel sheet, along a direction perpendicular to the rolling direction, and a tensile test was performed at room temperature in accordance with JIS Z 2241:2022 to measure the tensile strength. In the steel plate according to the present invention, the tensile strength was less than 1300 MPa.

[0129] From the obtained steel plates, a hot-stamping base plate measuring 240 mm in width and 200 mm in length was taken, and a plate-shaped steel member was obtained by hot stamping under the conditions shown in Tables 4A to 4C. The starting temperature for hot stamping was 700°C or higher (except for production No. 84), and the average cooling rate was 30.0°C / second or higher. In some examples, a second hot stamping was performed under the same conditions after a series of hot stampings. In addition, in some examples, a tempering treatment was performed after hot stamping, involving holding at a temperature range of 100 to 600°C for 10 to 60 minutes. Note that the holding time in the table is A ３ Point~(Ac ３ The holding time is in the temperature range of (point + 300°C), and the cooling stop temperature is the release temperature from the plate-shaped mold.

[0130] The obtained steel members were evaluated for their microstructure, Ti, Nb precipitate group, Vickers hardness, and VDA bending angle (α) using the method described above. However, when measuring the VDA bending angle, test pieces were taken so that the bending ridge direction (parallel to the 30 mm side) was parallel to the rolling direction of the steel sheet. The obtained results are shown in Tables 5A to 5C. Underlined text in the tables indicates that the results are outside the scope of this disclosure, that the manufacturing conditions are undesirable, or that the characteristic values ​​are undesirable.

[0131] If the Vickers hardness at a point 1 / 4 of the plate thickness from the surface was 630 HV or higher, it was judged to have high strength and was deemed acceptable. On the other hand, if the Vickers hardness at a point 1 / 4 of the plate thickness from the surface was less than 630 HV, it was judged to lack high strength and was deemed unacceptable.

[0132] If the VDA bending angle was 40° or more and less than 50° (calculated for a 2.0 mm thick plate), it was judged to have excellent collision resistance and was marked as "Good" in the table. On the other hand, if the VDA bending angle was less than 40° (calculated for a 2.0 mm thick plate), it was judged to lack excellent collision resistance and was marked as "Poor" in the table. Furthermore, if the VDA bending angle was 50° or more (calculated for a 2.0 mm thick plate), it was judged to have even better collision resistance and was marked as "Excellent" in the table.

[0133]

[0134]

[0135]

[0136]

[0137]

[0138]

[0139]

[0140]

[0141]

[0142]

[0143]

[0144]

[0145]

[0146]

[0147]

[0148]

[0149]

[0150] As can be seen from the above, the steel members relating to this disclosure have high strength and excellent impact resistance. Furthermore, it can be seen that steel members having the above characteristics can be manufactured from the steel plates relating to this disclosure.

[0151] According to the above embodiments of this disclosure, it is possible to provide a steel member having high strength and excellent impact resistance, as well as a steel plate on which this steel member can be manufactured.

Claims

1. The chemical composition, in mass percent, is as follows: C: greater than 0.40%, 0.70% or less, Si: 2.00% or less, Mn: 0.01 to 3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%, Co: 0 to 3.00%, Ni: 0 to 3.00%, Cu: 0 to 1.00%, V Composed of: 0-1.00%, Ca: 0-0.2000%, Mg: 0-0.2000%, REM: 0-0.3000%, Sb: 0-1.00%, Sn: 0-1.00%, Zr: 0-1.00%, As: 0-1.00%, Se: 0-1.0000%, Bi: 0-1.0000%, Ta: 0-1.0000%, Re: 0-1.0000%, Os: 0-1.0000%, Ir: 0-1.0000%, Tc: 0-1.0000%, and the remainder: Fe and impurities. When the 1 / 4 depth position is defined as the range from the position 1 / 8 of the plate thickness to the position 3 / 8 of the plate thickness from the surface, centered at a position 1 / 4 of the plate thickness in the thickness direction from the surface, the metallic structure at the 1 / 4 depth position is, by volume %, 90% or more of martensite, bainite, and tempered martensite, and 0-10% of ferrite, pearlite, cementite, and retained austenite. Among the group consisting of two or more Ti,Nb precipitates with an equivalent circle diameter of 1 μm or more and a shortest distance of 10 μm or less at the 1 / 4 depth position, the number density of the group with an aspect ratio of 3.0 or more is 4.0 particles / mm². ２ A steel member characterized by the following:

2. The chemical composition is as follows, in mass%, Cr: 0.01-1.00%, B: 0.0001-0.0100%, Mo: 0.01-1.00%, W: 0.01-2.00%, Co: 0.01-3.00%, Ni: 0.01-3.00%, Cu: 0.01-1.00%, V: 0.01-1.00%, Ca: 0.0001-0.2000%, Mg: 0.0001-0.2000%, REM: 0.0001-0.3000%, Sb: 0.01-1.00%, Sn: 0.01-1.00%, Zr: 0.01-1.00% The steel member according to claim 1, characterized by containing one or more of the following: As: 0.01 to 1.00%, Se: 0.0001 to 1.0000%, Bi: 0.0001 to 1.0000%, Ta: 0.0001 to 1.0000%, Re: 0.0001 to 1.0000%, Os: 0.0001 to 1.0000%, Ir: 0.0001 to 1.0000%, and Tc: 0.0001 to 1.0000%.

3. The steel member according to claim 1, characterized in that when the position 50 μm deep from the surface in the thickness direction is defined as the 50 μm depth position, the Vickers hardness at the 50 μm depth position is 50 HV or less than the Vickers hardness at a position 1 / 4 of the thickness from the surface in the thickness direction.

4. The steel member according to claim 2, characterized in that when the position 50 μm deep from the surface in the thickness direction is defined as the 50 μm depth position, the Vickers hardness at the 50 μm depth position is 50 HV or less than the Vickers hardness at a position 1 / 4 of the thickness from the surface in the thickness direction.

5. The steel member according to any one of claims 1 to 4, characterized in that it has a coating on the surface.

6. The steel member according to claim 5, characterized in that the coating is an Al-based coating or a Zn-based coating.

7. The chemical composition, in mass percent, is as follows: C: greater than 0.40%, 0.70% or less, Si: 2.00% or less, Mn: 0.01 to 3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%, Co: 0 to 3.00%, Ni: 0 to 3.00%, Cu: 0 to 1.00%, V Composed of: 0-1.00%, Ca: 0-0.2000%, Mg: 0-0.2000%, REM: 0-0.3000%, Sb: 0-1.00%, Sn: 0-1.00%, Zr: 0-1.00%, As: 0-1.00%, Se: 0-1.0000%, Bi: 0-1.0000%, Ta: 0-1.0000%, Re: 0-1.0000%, Os: 0-1.0000%, Ir: 0-1.0000%, Tc: 0-1.0000%, and the remainder: Fe and impurities. When the 1 / 4 depth position is defined as the range from the position 1 / 8 of the plate thickness to the position 3 / 8 of the plate thickness from the surface, centered at a position 1 / 4 of the plate thickness in the thickness direction from the surface, the number density of the group consisting of two or more Ti,Nb precipitates with an equivalent circle diameter of 1 μm or more and a shortest distance of 10 μm or less, and with an aspect ratio of 3.0 or more, is 4.0 particles / mm². ２ A steel plate characterized by the following:

8. The chemical composition is as follows, in mass%, Cr: 0.01-1.00%, B: 0.0001-0.0100%, Mo: 0.01-1.00%, W: 0.01-2.00%, Co: 0.01-3.00%, Ni: 0.01-3.00%, Cu: 0.01-1.00%, V: 0.01-1.00%, Ca: 0.0001-0.2000%, Mg: 0.0001-0.2000%, REM: 0.0001-0.3000%, Sb: 0.01-1.00%, Sn: 0.01-1.00%, Zr: 0.01-1.00% The steel sheet according to claim 7, characterized by containing one or more of the following: As: 0.01 to 1.00%, Se: 0.0001 to 1.0000%, Bi: 0.0001 to 1.0000%, Ta: 0.0001 to 1.0000%, Re: 0.0001 to 1.0000%, Os: 0.0001 to 1.0000%, Ir: 0.0001 to 1.0000%, and Tc: 0.0001 to 1.0000%.

9. D, the decarburization index Ｃ The steel plate according to claim 7, characterized in that the ratio is 0.10 or greater.

10. D, the decarburization index Ｃ The steel plate according to claim 8, characterized in that the ratio is 0.10 or greater.

11. The steel plate according to any one of claims 7 to 10, characterized in that it has a coating on the surface.

12. The steel sheet according to claim 11, characterized in that the coating is an Al-based coating or a Zn-based coating.

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