Steel member and steel sheet

Abstract

The purpose of the present invention is to provide a steel member having high strength and excellent impact resistance properties, and to provide a steel sheet from which the steel member can be manufactured. A steel sheet according to the present invention has a prescribed chemical composition containing 0.60-3.00% Co and 0.30-3.00% Ni, and at a 1 / 4 depth position, has an equivalent circle diameter of 1 μm or greater and a number density of Ti-Nb precipitates of 70 particles / mm2 or less, the ratio Ti / Nb of the Ti concentration to the Nb concentration thereof being 1.5 or greater. Furthermore, a steel member according to the present invention is manufactured from said 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. 2025-016813, 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, generally, steel members have a reduced impact resistance property with an increase in strength. 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 viewpoints of weight reduction, strength increase, and improvement of collision safety of automotive members in recent years, steel members having high strength and excellent impact resistance properties are required.

[0007] For example, Patent Document 1 discloses a hot stamping formed body having a microstructure in which the area ratio of martensite is 90 to 100%, the area of regions having an average GAIQ value of 60,000 or more within a unit crystal grain is 30 area% or more, and the number density of carbides having a circle-equivalent diameter of 0.20 μm or more is 50 pieces / mm ２ or less, and having a tensile strength of 1500 MPa or more. Patent Document 1 discloses that by promoting auto-tempering during hot stamping, a hot stamping formed body having high strength and excellent bendability can be obtained.

[0008] Patent Document 2 discloses a hot press member having a microstructure in which the volume fraction of martensite is 95% or more, the average crystal grain size of prior austenite is 7 μm or less, the number of cementites having a particle size of 0.10 μm or more is 2 pieces / μm ２ or more, and the number of Ti-based carbides having a particle size of 0.10 μm or less is 0.20 pieces / μm ２ or more, and having a tensile strength of 2100 MPa or more. Patent Document 2 discloses that by the technique described in Patent Document 2, a hot press member having a tensile strength of 2100 MPa or more and excellent stress corrosion cracking resistance properties can be obtained.

[0009] International Publication No. 2020 / 189761 International Publication No. 2022 / 044510

[0010] In Patent Document 1, in order to promote auto-tempering, it is necessary to set the cooling stop temperature after hot stamping to a temperature range of 180 to 220°C and to minimize the average cooling rate in the temperature range below the cooling stop temperature. From the viewpoints of improving productivity and stably ensuring strength, there is room for improvement.

[0011] In Patent Document 2, the stress corrosion cracking resistance is improved by dispersing coarse cementite and fine Ti-based carbides in a member, but sufficient consideration is not given to the impact resistance of the member.

[0012] 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, and a steel plate capable of manufacturing this steel member.

[0013] 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%, Co: 0.60 to 3.00%, Ni: 0.30 to 3.00%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%, 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, and 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 metallographic 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 Ti, Nb-based precipitates having a circle equivalent diameter of 1 μm or more and a Ti / Nb which is the ratio of the Ti concentration to the Nb concentration of 1.5 or more is 70 pieces / mm ２A steel member characterized by the following: [2] The chemical composition is as follows, 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%, 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%, As: 0.01 to 1.00%, Se: 0.0001 to 1.0000%, The steel member according to [1] above, characterized in that it contains one or more of the following: 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 more 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 [1] to [3] above, characterized in that the surface is coated. [5] The steel member according to [4] above, 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%, Co: 0.60 to 3.00%, Ni: 0.30 to 3.00%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%. Consists of: Cu: 0-1.00%, V: 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 Ti,Nb precipitates at the 1 / 4 depth position is 70 particles / mm², where the equivalent circle diameter is 1 μm or more and the ratio of Ti concentration to Nb concentration, Ti / Nb, is 1.5 or more. ２A steel plate characterized by the following: [7] The chemical composition is as follows, 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%, 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%, As: 0.01 to 1.00%, Se: 0.0001 to 1.0000%, The steel sheet according to [6] above, characterized in that it contains one or more of the following: 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.

[0014] 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.

[0015] D, the decarburization index Ｃ This is a diagram to explain the calculation method.

[0016] 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.

[0017] (A) When the particle size is large and the number density of Ti,Nb precipitates is high due to a large Ti / Nb ratio (Ti concentration to Nb concentration), the impact resistance of the steel member decreases. This is presumed to be because (a) Ti,Nb precipitates with large particle size and large Ti / Nb ratio are prone to cracking, and (b) cracking of Ti,Nb precipitates promotes crack formation and linkage. (B) The number density and particle size of Ti,Nb precipitates decrease by including Co and Ni. This is presumed to be because Co and Ni lower the free energy of austenite, suppressing the formation of Ti,Nb precipitates. (C) Ti,Nb precipitates are already formed at the steel sheet stage, and their persistence after hot stamping reduces the impact resistance of the steel member. (D) In ​​the method for manufacturing steel sheets, by controlling the heating conditions of the slab before hot rolling, the number density of coarse Ti,Nb precipitates with a high Ti / Nb ratio can be reduced, and as a result, the impact resistance properties of the steel member are improved.

[0018] 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.

[0019] 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.

[0020] 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%, Co: 0.60 to 3.00%, Ni: 0.30 to 3.00%, and the remainder being Fe and impurities. Each element will be described in detail below.

[0021] C: Over 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, 0.46% 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.

[0022] 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, 0.70% or less, 0.50% or less, 0.45% or less, or 0.40% or less.

[0023] 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, or 1.00% 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, or 2.00% or less.

[0024] 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 also be 0%. However, since excessively reducing the P content increases refining costs, the P content may be 0.001% or more, or 0.004% or more.

[0025] 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 or 0.0010% or more.

[0026] 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 or 0.100% or less.

[0027] 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, or 0.0050% 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.

[0028] 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.

[0029] 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.

[0030] 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, reducing 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.

[0031] Co: 0.60-3.00% Co reduces the number density of Ti,Nb precipitates by stabilizing austenite. If the Co content is less than 0.60%, the number density of Ti,Nb precipitates cannot be sufficiently reduced. Therefore, the Co content should be 0.60% or more. Preferably, the Co content is 0.70% or more, 0.80% or more, 1.00% or more, 1.30% or more, or 1.50% or more. On the other hand, if the Co content exceeds 3.00%, the Ms point becomes excessively high, reducing the strength of the steel member. Therefore, the Co content should be 3.00% or less. Preferably, the Co content is 2.80% or less, 2.60% or less, 2.40% or less, or 2.20% or less.

[0032] Ni: 0.30-3.00% Ni, like Co, reduces the number density of Ti,Nb precipitates by stabilizing austenite. If the Ni content is less than 0.30%, the number density of Ti,Nb precipitates cannot be sufficiently reduced. Therefore, the Ni content should be 0.30% or more. Preferably, the Ni content is greater than 0.50%, 0.70% or more, 1.00% or more, or 1.20% or more. On the other hand, if the Ni content is greater than 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. Preferably, the Ni content is 2.80% or less, 2.60% or less, 2.40% or less, or 2.20% or less.

[0033] 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.

[0034] 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%.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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. The REM content is more preferably 0.0005% or more, 0.0010% or more, 0.0050% or more, 0.0100% or more, or 0.0150% or more. On the other hand, if the REM content exceeds 0.3000%, the effect of refining inclusions in steel becomes saturated, and the alloy cost increases. Therefore, the REM content is 0.3000% or less. The REM content is preferably 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.

[0044] 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 0.20% or less.

[0045] 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.

[0046] 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 fixing N as nitride, suppressing the formation of BN, 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. On the other hand, if the Zr content exceeds 1.00%, excessive carbonitrides are formed, and the impact resistance properties 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, 0.10% or less, or 0.06% or less.

[0047] 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 0.10% or less.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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 0.0200% or less.

[0055] 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.

[0056] Next, the microstructure 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 1 / 8 of the plate thickness to the position 3 / 8 of the plate thickness from the surface, centered at the position 1 / 4 of the plate thickness in the thickness direction from the surface, the microstructure 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, and at the 1 / 4 depth position, the number density of Ti,Nb precipitates is 70 particles / mm², with an equivalent circle diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or more. ２ The following applies:

[0057] 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.

[0058] 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%.

[0059] 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).

[0060] 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.

[0061] At an arbitrary position in the longitudinal direction of the cross-section of the test piece after polishing, a region ranging from a position 1 / 8 of the plate thickness from the surface to a position 3 / 8 of the plate thickness from the surface, with a length of 200 μm in the longitudinal direction and centered at a position 1 / 4 of the plate thickness in the plate thickness direction from the surface, is measured at a measurement interval of 0.1 μm by the electron backscatter diffraction method to obtain crystal orientation information. For the measurement, an EBSD analysis device composed of a thermal field emission type scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (Velocity detector manufactured by AMETEK) is used. At this time, the vacuum degree in the device is 9.6×10 －５ Pa or less, the acceleration voltage is 25 kV, and the irradiation current level is 16. Also, at the time of measurement, "iron-α" and "iron-γ" are set as the Phase and the measurement is performed.

[0062] Also, the same region as the EBSD measurement region is observed at a magnification of 1000 times or more using a FE-SEM: thermal field emission type scanning electron microscope (JSM-7200F manufactured by JEOL). When observing the same region as the EBSD measurement region, Vickers indentations are engraved within a range of 100 μm from the four corners of the EBSD measurement region for three of the four corners of the EBSD measurement region so that the observation position can be specified. Then, leaving the metal structure of the observation surface, the surface contamination is polished and removed, and nital etching is performed. By using the Vickers indentation as a mark, the same region as the EBSD observation surface can be observed.

[0063] For removing the contamination, 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 an alkaline solution at room temperature, or Ar ion sputtering may be used.

[0064] In the observation of the structure using a thermal field emission type scanning electron microscope, the volume ratios of cementite and pearlite are determined by the following method.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] Furthermore, for regions with a bcc crystal structure, the "Grain Average Misorientation" function included in the "OIMAnalysis®" software attached to the EBSD analyzer is used to determine whether it is ferrite, bainite, martensite, or tempered martensite, and the area fraction is measured. Specifically, under conditions where boundaries with a crystal orientation difference of 15° or more are considered grain boundaries (15° grain boundaries), regions with a Grain Average Misorientation value (GAM value) of 1.0° or less are identified as ferrite. Regions with a GAM value greater than 1.0° are identified as "martensite, bainite, and tempered martensite." The area ratios of ferrite and the regions identified as "martensite, bainite, and tempered martensite" are calculated to obtain the respective area ratios of ferrite and "martensite, bainite, and tempered martensite." Furthermore, the area ratios of ferrite and "martensite, bainite, and tempered martensite" are considered to be the volume ratios of ferrite and "martensite, bainite, and tempered martensite," respectively.

[0069] At a depth of 1 / 4, the number density of Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or more is 70 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, Ti,Nb precipitates with a Ti / Nb ratio of 1.5 or more are prone to cracking of the precipitates themselves, promoting crack formation and coupling, thereby reducing the impact resistance of the steel member. The number density of Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or more is 70 particles / mm³. ２ If the number density is too high, the impact resistance properties of the steel member will decrease. Therefore, the number density of the Ti,Nb precipitates is 70 particles / mm³. ２The following applies: The number density of the Ti,Nb precipitates is preferably 65 particles / mm³. ２ Below, 60 pieces / mm ２ Below, 58 pieces / mm ２ Below, 55 pieces / mm ２ Below, 50 pieces / mm ２ The following or 48 pieces / mm ２ The following applies: While a lower number density of the Ti,Nb precipitates is preferable, excessively low numbers may cause the metal structure to coarseen, potentially reducing the impact resistance of the steel component. Therefore, the lower limit of the number density of the Ti,Nb precipitates is 5 particles / mm². ２ Or 10 pieces / mm ２ This may also be the case. Furthermore, the lower limit of the number density of the above Ti,Nb precipitates is 15 particles / mm ２ , 20 pieces / mm ２ , 25 pieces / mm ２ or 30 pieces / mm ２ This may also be done. Examples of Ti,Nb precipitates include Ti and Nb carbides, Ti and Nb nitrides, Ti and Nb carbonitrides, and Ti and Nb sulfides.

[0070] The number density of Ti,Nb precipitates with an equivalent circle diameter of 1 μm or more and a Ti / Nb ratio of 1.5 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.

[0071] 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.

[0072] 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 containing Ti and Nb, where 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 for Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more, the ratio of Ti concentration to Nb concentration is calculated. This identifies Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or more. By dividing the number of identified Ti,Nb precipitates by the measurement area, the number density of Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or more is obtained.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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. The plate thickness may also be 4.0 mm or less, less than 4.0 mm, or 3.5 mm or less.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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).

[0087] 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

[0088] 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 not be explained.

[0089] 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 Ti,Nb precipitates at the 1 / 4 depth position is 70 particles / mm², where the equivalent circle diameter is 1 μm or more and the ratio of Ti concentration to Nb concentration, Ti / Nb, is 1.5 or more. 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.

[0090] At a depth of 1 / 4, Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or more have a number density of 70 particles / mm². ２ The Ti,Nb precipitates described below have a structure that is harder than steel. When Ti,Nb precipitates with an equivalent circle 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, Ti,Nb precipitates with a Ti / Nb ratio of 1.5 or higher are prone to cracking of the precipitates themselves, promoting crack formation and linkage. The above Ti,Nb precipitates remain even after hot stamping, reducing the impact resistance of the steel member. The number density of Ti,Nb precipitates with an equivalent circle diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or higher is 70 particles / mm². ２ If the number density is too high, the impact resistance of the steel member obtained by heat treatment will decrease. Therefore, the number density of the Ti,Nb precipitates is 70 particles / mm³. ２ The following applies: The number density of the Ti,Nb precipitates is preferably 65 particles / mm³. ２ Below, 60 pieces / mm ２ Below, 58 pieces / mm ２ Below, 55 pieces / mm ２ Below, 50 pieces / mm ２ The following or 48 pieces / mm ２The following applies: While a lower number density of the Ti,Nb precipitates is preferable, excessively low numbers may cause the metal structure to coarseen, potentially reducing the impact resistance of the steel component. Therefore, the lower limit of the number density of the Ti,Nb precipitates is 5 particles / mm². ２ Or 10 pieces / mm ２ This may also be the case. Furthermore, the lower limit of the number density of the above Ti,Nb precipitates is 15 particles / mm ２ , 20 pieces / mm ２ , 25 pieces / mm ２ or 30 pieces / mm ２ That is also acceptable.

[0091] The method for measuring the number density of Ti,Nb precipitates is the same as for steel components, so the explanation is omitted.

[0092] 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.

[0093] 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 DＣ By 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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 1 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 1). The product of the C concentration of the steel plate and the depth of 120 μm is taken as the reference area (area of ​​region A / area of ​​region B in Figure 1). 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.

[0098] 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 6%. By setting the volume fraction of retained austenite to less than 6% in the metal structure of the steel sheet, the concentrations of Co and Ni are homogenized, and the formation of localized Ti,Nb precipitates is suppressed. The method for measuring the volume fraction of each structure is the same as for steel members, so the explanation is omitted.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] In the preferred method for manufacturing the steel sheet 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 450°C or lower, and the slab is heated to 1100°C or higher such that the residence time in the temperature range of 800 to 950°C is ts (minutes) and the holding time in the temperature range of 1100°C or higher is th (minutes), and ts and th satisfy the following formulas (A) and (B): 0.80 × ts < th < 1.20 × ts (A) ts < 75 (B) The following describes each step.

[0104] A slab having the chemical composition described above is obtained by continuous casting. Then, before it cools to a temperature range of 450°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 450°C or below, the precipitation, coarsening, and stabilization of Ti,Nb precipitates can be suppressed.

[0105] The slab inserted into the heating furnace is heated to 1100°C or higher, such that ts (minutes) is the residence time in the temperature range of 800-950°C and th (minutes) is the holding time in the temperature range of 1100°C or higher, and ts and th satisfy the above equations (A) and (B). In the temperature range of 800-950°C, Ti,Nb precipitates become coarse and stable. If ts is too long, the Ti,Nb precipitates will not dissolve sufficiently even with subsequent heating for solution treatment (high temperature holding) and will remain in the final structure. Coarse Ti,Nb precipitates with a Ti / Nb ratio of 1.5 or higher generate cracks during deformation due to delamination at the interface with the steel and cracking of the Ti,Nb precipitates themselves, reducing the impact resistance of the steel member. Therefore, it is undesirable for a large amount of such Ti,Nb precipitates to remain.

[0106] Furthermore, if the th is too short or the slab holding temperature is too low, the dissolution of Ti,Nb precipitates will be insufficient, and the amount of coarse Ti,Nb precipitates with a Ti / Nb ratio of 1.5 or higher will increase.

[0107] Furthermore, if the holding time (th) in the temperature range above 1100°C is too long, too much Nb dissolves in the Ti,Nb precipitate, and the amount of Ti,Nb precipitates with a Ti / Nb ratio of 1.5 or higher increases.

[0108] Therefore, in the preferred manufacturing method of the steel sheet according to this embodiment, when the residence time in the temperature range of 800 to 950°C is ts (minutes) and the holding time in the temperature range of 1100°C or higher is th (minutes), the slab is heated to 1100°C or higher such that ts and th satisfy the above formulas (A) and (B). Depending on the state of the Ti,Nb precipitates after passing through the temperature range of 800 to 950°C, the Nb in the Ti,Nb precipitates is not excessively dissolved in the temperature range of 1100°C or higher, and the number density of the Ti,Nb precipitates is not excessively high, thereby reducing the number density of Ti,Nb precipitates that have an equivalent circle diameter of 1 μm or more and a Ti / Nb ratio of Ti concentration to Nb concentration of 1.5 or higher.

[0109] After heating the slab, rough rolling, finish rolling, and coiling are performed. Furthermore, the hot-rolled steel sheet may be cold-rolled or annealed as needed. Additionally, 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 ３ It is preferable to keep the temperature below (point + 150°C).

[0110] Furthermore, in order to further improve the bendability of the steel member and obtain superior 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, (Ac 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] A steel member is obtained by heat-treating the steel plate according to this embodiment. The heat treatment is performed, for example, on the steel plate obtained by the above method, at an average heating rate of 1.0 to 1000°C / second, Ac 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 of Ms point or lower and below 150°C at an average cooling rate of 30.0°C / second or more.

[0118] 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.

[0119] 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), the volume fraction of martensite, bainite, and tempered martensite will be insufficient after cooling, which may result in insufficient strength of the steel member, and this is undesirable. Also, if the cooling stop temperature is 150°C or higher, the microstructure will soften, which may result in insufficient strength of the steel member, and this is also undesirable. On the other hand, if the heat treatment temperature is (Ac 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.

[0120] During heating, Ac ３ Point~(Ac ３ The material may be held at a temperature of (Ms point + 300°C) for 1 to 300 seconds. Alternatively, after cooling to a temperature below the Ms point and below 150°C, a tempering treatment may be performed by holding the material at a temperature of 100 to 140°C for 10 to 30 minutes to adjust the strength of the steel component.

[0121] 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)

[0122] Here, during the above series of heat treatments, Ac ３ Point~(Ac ３It 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.

[0123] 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 150°C. This improves the dimensional accuracy of the steel member and ensures stable strength of the steel member.

[0124] 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.

[0125] 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.

[0126] 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.

[0127] 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 3D, and then hot-rolled, cold-rolled, and annealed (cold-rolling or annealing was omitted under some conditions) to obtain steel plates with a thickness of 1.6 to 3.5 mm.

[0128] After heating the slab under the conditions shown in the table, rough rolling is performed so that the rough rolling completion temperature is in the temperature range of 1040 to 1000°C, and 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 ３ Decarburization 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.

[0129] The following are the terms used for each item in Tables 3A to 3D: Insertion temperature: The temperature at which the slab obtained by continuous casting was inserted into the heating furnace. Residence time ts: Residence time in the temperature range of 800 to 950°C. Heating temperature: The maximum heating temperature of the slab. Holding time th: Holding time in the temperature range of 1100°C or higher.

[0130] The obtained steel sheet was analyzed using the method described above to determine its microstructure, Ti, Nb precipitates, and decarburization index D. Ｃ The following was evaluated. The results obtained are shown in Tables 3A to 3D. The items in Tables 3A to 3D (and Tables 5A to 5D) are as follows: Number density of Ti,Nb precipitates: The number density of Ti,Nb precipitates at a 1 / 4 depth position that have an equivalent circle diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or more.

[0131] Furthermore, in the metallographic structure of the steel sheet according to the present invention, 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 6%. In addition, 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 sheet according to the present invention, the tensile strength was less than 1300 MPa.

[0132] 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 4D. The starting temperature for hot stamping was 700°C or higher (except for production No. 88), 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 some examples, a tempering treatment was performed after hot stamping, holding the plate at a temperature range of 100 to 140°C for 10 to 30 minutes. In Tables 4A to 4D, the cooling stop temperature is the release temperature from the flat mold. The metal structure, Ti, Nb precipitates, Vickers hardness, and VDA bending angle (α) of the obtained steel members were evaluated 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 plate. The results obtained are shown in Tables 5A to 5D.

[0133] Underlined text in the table indicates that the information is outside the scope of this disclosure, that the manufacturing conditions are undesirable, or that the characteristic values ​​are undesirable.

[0134] 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.

[0135] 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.

[0136]

[0137]

[0138]

[0139]

[0140]

[0141]

[0142]

[0143]

[0144]

[0145]

[0146]

[0147]

[0148]

[0149]

[0150]

[0151]

[0152]

[0153]

[0154]

[0155]

[0156] 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.

[0157] 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

The chemical composition is expressed in mass percent. C: more than 0.40%, less than 0.70%, Si: 2.00% or less, Mn: 0.01 to 3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001-1.000%, N: 0.0200% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Co: 0.60-3.00%, Ni: 0.30-3.00%, Cr: 0-1.00%, B: 0 to 0.0100%, Mo: 0-1.00%, W: 0-2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ca: 0-0.2000%, Mg: 0-0.2000%, REM: 0-0.3000%, Sb: 0 to 1.00%, Sn: 0-1.00%, 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: 0-1.0000%, Ir: 0-1.0000%, Tc: 0-1.0000%, and The remainder consists of 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, with the center point being 1 / 4 of the plate thickness in the thickness direction from the surface, The metallic structure at the aforementioned 1 / 4 depth position is, by volume %, Martensite, bainite, and tempered martensite: more than 90% in total, Ferrite, pearlite, cementite, and retained austenite: totaling 0-10%, At the aforementioned 1 / 4 depth position, the number density of Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or more is 70 particles / mm². ２ A steel member characterized by the following: 　 The aforementioned chemical composition is, in mass%, Cr: 0.01-1.00%, B: 0.0001 to 0.0100%, Mo: 0.01-1.00%, W: 0.01-2.00%, Cu: 0.01 to 1.00%, V: 0.01 to 1.00%, Ca: 0.0001-0.2000%, Mg: 0.0001-0.2000%, REM: 0.0001-0.3000%, Sb: 0.01 to 1.00%, Sn: 0.01-1.00%, Zr: 0.01-1.00%, 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: 0.0001-1.0000%, Ir: 0.0001 to 1.0000%, and The steel member according to claim 1, characterized in that it contains one or more Tc in an amount of 0.0001 to 1.0000%. 　 When the position 50 μm deep from the surface in the thickness direction is defined as the 50 μm depth position, The steel member according to claim 1, characterized in that the Vickers hardness at a depth of 50 μm is 50 HV or less than the Vickers hardness at a position 1 / 4 of the plate thickness in the direction from the surface to the plate thickness. 　 When the position 50 μm deep from the surface in the thickness direction is defined as the 50 μm depth position, The steel member according to claim 2, characterized in that the Vickers hardness at a depth of 50 μm is 50 HV or less than the Vickers hardness at a position 1 / 4 of the plate thickness in the direction from the surface to the plate thickness. 　 The steel member according to any one of claims 1 to 4, characterized in that it has a coating on the surface. 　 The steel member according to claim 5, characterized in that the coating is an Al-based coating or a Zn-based coating. 　 The chemical composition is expressed in mass percent. C: more than 0.40%, less than 0.70%, Si: 2.00% or less, Mn: 0.01 to 3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001-1.000%, N: 0.0200% or less, O: 0.0100% or less, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Co: 0.60-3.00%, Ni: 0.30-3.00%, Cr: 0-1.00%, B: 0 to 0.0100%, Mo: 0-1.00%, W: 0-2.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, Ca: 0-0.2000%, Mg: 0-0.2000%, REM: 0-0.3000%, Sb: 0 to 1.00%, Sn: 0-1.00%, 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: 0-1.0000%, Ir: 0-1.0000%, Tc: 0-1.0000%, and The remainder consists of 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, with the center point being 1 / 4 of the plate thickness in the thickness direction from the surface, At the aforementioned 1 / 4 depth position, the number density of Ti,Nb precipitates with an equivalent circular diameter of 1 μm or more and a Ti / Nb ratio of 1.5 or more is 70 particles / mm². ２ A steel plate characterized by the following: 　 The aforementioned chemical composition is, in mass%, Cr: 0.01-1.00%, B: 0.0001 to 0.0100%, Mo: 0.01-1.00%, W: 0.01-2.00%, Cu: 0.01 to 1.00%, V: 0.01 to 1.00%, Ca: 0.0001-0.2000%, Mg: 0.0001-0.2000%, REM: 0.0001-0.3000%, Sb: 0.01 to 1.00%, Sn: 0.01-1.00%, Zr: 0.01-1.00%, 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: 0.0001-1.0000%, Ir: 0.0001 to 1.0000%, and The steel sheet according to claim 7, characterized in that it contains one or more Tc in an amount of 0.0001 to 1.0000%. 　 D, the decarburization index Ｃ The steel plate according to claim 7, characterized in that the ratio is 0.10 or greater. 　 D, the decarburization index Ｃ The steel plate according to claim 8, characterized in that the ratio is 0.10 or greater. 　 The steel plate according to any one of claims 7 to 10, characterized in that it has a coating on the aforementioned surface. 　 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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