Steel member and steel sheet

Abstract

In the present invention, a steel member and a steel sheet have a prescribed chemical composition. At the 1 / 4 depth position, carbide groups in the steel member have a number density of 10.0×10-4 per μm2 or less and carbide groups in the steel sheet have a number density of 10.0×10-3 per μm2 or less, said carbide groups composed of three carbides with an equivalent circle diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less.

Description

Steel members and steel plates

[0001] This application claims priority to Japanese Patent Application No. 2024-100116, filed on June 21, 2024, the contents of which are incorporated herein by reference.

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

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

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

[0005] In this technology, the material is heated before being formed. Therefore, the steel is soft during forming and has good formability. This allows even high-strength steel sheets to be formed into complex shapes with high precision. In addition, hot stamping technology involves quenching the material while forming it using a press die. By including the necessary amount of alloy elements that improve hardenability, the steel part after forming has sufficient strength.

[0006] However, in general, the toughness of steel members decreases as the strength increases. In recent years, from the viewpoint of reducing the weight, increasing the strength, and improving collision safety of automobile members, steel members having high strength and excellent toughness are required.

[0007] For example, Patent Document 1 discloses a steel sheet in which the area ratio of martensite and bainite to the entire structure is 95% or more and 100% or less in total, the remainder being one or both of ferrite and retained austenite, the average grain size of prior austenite grains is more than 5 μm, and the number of inclusions having a major axis length of 20 to 80 μm is 5 / mm ２ Patent Document 1 discloses a steel sheet having a steel structure present as follows and having a tensile strength of 1,320 MPa or more. Patent Document 1 discloses that the technology described in Patent Document 1 makes it possible to obtain a high-strength steel sheet having excellent delayed fracture resistance at a sheared edge.

[0008] Patent Document 2 describes a steel sheet having a total area ratio of martensite and bainite of 95% to 100%, with the remainder being one or more selected from ferrite and retained austenite, in which the density of inclusion particles having a major axis length of 20 μm to 80 μm and in which the shortest distance between the inclusion particles is longer than 10 μm, and the density of inclusion particle groups having a major axis length of 20 μm to 80 μm, which are inclusion particles having a major axis length of 0.3 μm or more and consisting of two or more inclusions in which the shortest distance between the inclusion particles is 10 μm or less, is 5 particles / mm ２ and a steel sheet having a structure in which the local P concentration is 0.060 mass% or less from the 1 / 4 position to the 3 / 4 position in the sheet thickness direction from the steel sheet surface, the Mn segregation degree in the above-mentioned positional range is 1.50 or less, and the tensile strength is 1320 MPa or more. Patent Document 2 discloses that the technology described in Patent Document 2 makes it possible to obtain a high-strength steel sheet that is excellent in delayed fracture resistance not only in the delayed fracture that occurs in the steel sheet base material but also in the delayed fracture resistance of the cut edge itself.

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

[0010] However, Patent Documents 1 and 2 do not take into consideration the toughness of the members.

[0011] The present disclosure has been made in view of the above-mentioned circumstances, and aims to provide a steel member having high strength and excellent toughness, and a steel plate from which the steel member can be manufactured.

[0012] The gist of the present invention is as follows: [1] A steel sheet having a chemical composition, in mass %, of C: more than 0.40% and not more than 1.00%, Si: not more than 2.00%, Mn: more than 1.00% and not more than 3.00%, P: not more than 0.100%, S: not more than 0.0100%, Al: 0.001 to 1.000%, N: not more than 0.0200%, O: not more than 0.0100%, Nb: 0 to 0.100%, Ti: 0 to 0.200%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%, Co: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, V : 0 to 1.00%, Ca: 0 to 0.2000%, Mg: 0 to 0.2000%, REM: 0 to 0.3000%, Sb: 0 to 1.00%, Sn: 0 to 1.00%, Zr: 0 to 1.00%, As: 0 to 1.00%, Se: 0 to 1.0000%, Bi: 0 to 1.0000%, Ta: 0 to 1.0000%, Re: 0 to 1.0000%, Os: 0 to 1.0000%, Ir: 0 to 1.0000%, Tc: 0 to 1.0000%, and the balance: Fe and impurities, When a range from a position of 1 / 8 of the plate thickness from the surface to a position of 3 / 8 of the plate thickness, with a position of 1 / 4 of the plate thickness from the surface as the center, is defined as a 1 / 4 depth position, the metal structure at the 1 / 4 depth position is, in volume %, martensite, bainite, and tempered martensite: a total of 90% or more, and at the 1 / 4 depth position, the number density of a carbide group consisting of three carbides having a circle equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less is 10.0 × 10 －４ pieces / μm ２a standard deviation of the circle-equivalent diameter of prior austenite grains is 6.0 μm or less. [2] The chemical composition is, in mass%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Cr: 0.01 to 1.00%, B: 0.0001 to 0.0100%, Mo: 0.01 to 1.00%, W: 0.01 to 2.00%, Co: 0.01 to 1.00%, Ni: 0.01 to 1.00%, 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%, The steel member according to the above [1], characterized in that it contains one or more selected from the group consisting of Sn: 0.01 to 1.00%, Zr: 0.01 to 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 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 a position 50 μm deep from the surface in the plate thickness direction is defined as a 50 μm depth position, the Vickers hardness at the 50 μm depth position is lower by 50 HV or more than the Vickers hardness at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction. [4] The steel member according to any one of [1] to [3] above, characterized in that the surface has a coating. [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: more than 0.40% and not more than 1.00%, Si: not more than 2.00%, Mn: more than 1.00% and not more than 3.00%, P: not more than 0.100%, S: not more than 0.0100%, Al: 0.001 to 1.000%, N: not more than 0.0200%, O: not more than 0.0100%, Nb: 0 to 0.100%, Ti: 0 to 0.200%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%, Co: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, V : 0 to 1.00%, Ca: 0 to 0.2000%, Mg: 0 to 0.2000%, REM: 0 to 0.3000%, Sb: 0 to 1.00%, Sn: 0 to 1.00%, Zr: 0 to 1.00%, As: 0 to 1.00%, Se: 0 to 1.0000%, Bi: 0 to 1.0000%, Ta: 0 to 1.0000%, Re: 0 to 1.0000%, Os: 0 to 1.0000%, Ir: 0 to 1.0000%, Tc: 0 to 1.0000%, and the balance: Fe and impurities, When a range from a position 1 / 8 of the plate thickness from the surface to a position 3 / 8 of the plate thickness, with the position 1 / 4 of the plate thickness from the surface as the center, is defined as a 1 / 4 depth position, the metal structure at the 1 / 4 depth position is composed of, by volume, pearlite: 40% or more, and ferrite, bainite, martensite, tempered martensite, carbides, and retained austenite: a total of 60% or less, and at the 1 / 4 depth position, the number density of a carbide group consisting of three carbides having an equivalent circle diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less is 10.0 x 10 －３ pieces / μm ２The steel sheet is characterized in that the dispersion index of the pearlite is 0.50 to 1.00 or less. [7] The chemical composition, in mass%, is: Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Cr: 0.01 to 1.00%, B: 0.0001 to 0.0100%, Mo: 0.01 to 1.00%, W: 0.01 to 2.00%, Co: 0.01 to 1.00%, Ni: 0.01 to 1.00%, 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%, [8] The steel sheet according to the above [6], characterized in that it contains one or more elements selected from the group consisting of Sn: 0.01 to 1.00%, Zr: 0.01 to 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 to 1.0000%, Ir: 0.0001 to 1.0000%, and Tc: 0.0001 to 1.0000%. Ｃ [9] The steel sheet according to any one of [6] to [8] above, characterized in that the surface has a coating.

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

[0013] According to the above aspects of the present disclosure, it is possible to provide a steel member having high strength and excellent toughness, and a steel plate from which this steel member can be manufactured.

[0014] FIG. 1 is a diagram showing an image in which carbides having a particle size of 0.5 μm or more in equivalent circle diameter are displayed. FIG. 2 is a diagram for explaining how to count carbide groups. Ｃ FIG. 10 is a diagram for explaining a calculation method of

[0015] It is difficult to ensure excellent toughness in a steel member having a tensile strength of 2.2 GPa or more that is manufactured by hot stamping a steel plate. As a result of investigations into how to achieve high strength and excellent toughness in a steel member, the inventors have obtained the following findings.

[0016] In steel members having a strength of 2.2 GPa or more, carbide groups present within a certain distance in the steel form cracks and promote the propagation of the cracks. This makes it difficult to ensure excellent toughness in the steel member. Therefore, it is difficult to obtain excellent toughness in a steel member by simply reducing the number density of individual carbides, as in conventional technology. Even if the average number density of individual carbides is reduced, if there are areas where carbides are densely packed, the number density of the carbide groups will increase, degrading the toughness of the steel member. Therefore, by reducing the number density of carbide groups consisting of three carbides present within a certain distance, it is possible to achieve both a tensile strength of 2.2 GPa or more and excellent toughness.

[0017] Furthermore, in steel members with a high Mn content, the variation in the grain size of prior austenite grains after hot stamping increases, which tends to cause microscale residual stress and deteriorate the toughness of the steel member. In order to reduce the variation in the grain size of prior austenite grains after hot stamping, it is important to favorably control the arrangement of ferrite and pearlite in the base steel sheet to be subjected to hot stamping. In order to obtain such a metal structure in the steel sheet, it is effective to perform a large reduction in the final stage of finish rolling in hot rolling and then immediately quench the steel sheet.

[0018] The steel member according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the steel member according to this embodiment will be described.

[0019] The steel member according to this embodiment has the following chemical composition. Note that the numerical ranges described below, separated by "to", include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" do not include the numerical range. All percentages regarding the chemical composition represent mass %.

[0020] The steel member according to this embodiment has a chemical composition, in mass%, of C: more than 0.40% and 1.00% or less, Si: 2.00% or less, Mn: more than 1.00% and 3.00% or less, 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, and the balance: Fe and impurities. Each element will be described in detail below.

[0021] C: More than 0.40% and 1.00% or less C is an element that significantly 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 is set to more than 0.40%. The C content is preferably 0.45% or more or 0.50% or more. On the other hand, if the C content is more than 1.00%, the strength of the steel member becomes too high, and the toughness of the steel member decreases. Therefore, the C content is set to 1.00% or less. The C content is preferably 0.90% or less, 0.80% or less, or 0.75% or less.

[0022] Si: 2.00% or less Si has temper softening resistance and is effective in suppressing strength reduction due to auto-tempering during hot stamp quenching. The Si content may be 0%. However, Si may be actively added to achieve the above effect. From the viewpoint of obtaining higher strength, the Si content is preferably 0.005% or more. The Si content is more preferably 0.01% or more, 0.02% or more, 0.03% or more, 0.15% or more, or 0.20% or more. On the other hand, if the Si content exceeds 2.00%, problems with surface scale occur. That is, after pickling to remove scale formed during hot rolling, patterns due to surface irregularities occur, resulting in poor surface appearance. Furthermore, when a plating treatment is performed on the surface of a steel sheet before it is hot stamped and formed into a steel member, platability deteriorates. Furthermore, if the Si content exceeds 2.00%, the toughness of the steel member deteriorates. Therefore, the Si content is set to 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: More than 1.00% and 3.00% or less Mn is an element that improves the strength of a steel member and the hardenability of the steel. If the Mn content is 1.00% or less, when the cooling rate after hot stamping is low, the volume fractions of martensite, bainite, and tempered martensite in the metal structure of the steel member may decrease, resulting in insufficient strength of the steel member. Therefore, the Mn content is set to more than 1.00%. The Mn content is preferably 1.20% or more, 1.40% or more, or 1.60% or more. On the other hand, if the Mn content is more than 3.00%, even if the manufacturing method described below is applied, it is not possible to suppress the variation in the grain size of prior austenite grains in the metal structure of the steel member, and the toughness of the steel member may decrease. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.60% or less, or 2.40% 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 is significantly reduced, resulting in a decrease in the toughness of the steel member. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.050% or less, 0.040% or less, or 0.030% or less. The P content may be 0%. However, since excessive reduction of the P content increases refining costs, the P content may be set to 0.001% or more.

[0025] S: 0.0100% or less S is an element that affects non-metallic inclusions in steel and deteriorates the toughness of steel members. If the S content exceeds 0.0100%, the toughness of the steel members deteriorates significantly. Therefore, the S content is set to 0.0100% or less. The S content is preferably set to 0.0080% or less or 0.0050% or less. The S content may be 0%. However, since excessive reduction of the S content increases the manufacturing cost of the desulfurization process, the S content may be set to 0.0001% or more.

[0026] Al: 0.001 to 1.000% Al is an element used as a deoxidizer for molten steel. If deoxidation is insufficient, the toughness of the steel member will deteriorate due to the excess oxides produced. In order to sufficiently deoxidize the molten steel, the Al content is set to 0.001% or more. The Al content is preferably 0.010% or more or 0.030% or more. On the other hand, if the Al content exceeds 1.000%, many non-metallic inclusions will be formed, deteriorating the toughness of the steel member. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 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%, a large amount of coarse nitrides is formed in the steel, significantly reducing the toughness of the steel member. Therefore, the N content is set to 0.0200% or less. The N content is preferably 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 denitrification cost, which is economically undesirable. Therefore, the N content is preferably set to 0.0005% or more. The N content is more preferably set to 0.0010% or more.

[0028] O: 0.0100% or less When a large amount of O is contained in steel, it forms coarse oxides that become the starting point of fracture. As a result, the toughness of the steel member decreases. If the O content exceeds 0.0100%, the toughness of the steel member significantly deteriorates. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less or 0.0050% or less. The O content may be 0%. However, if the O content is reduced to less than 0.0005%, the cost of deoxidization increases significantly, which is economically undesirable. Therefore, the O content is preferably set to 0.0005% or more or 0.0010% or more.

[0029] The balance of the chemical composition of the steel member according to the present 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 to the extent that they do not impair the properties of the steel member according to the present embodiment.

[0030] 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. When the following optional elements are not contained, the content is 0%.

[0031] Nb: 0.001 to 0.100% Nb, as a solid solution element, has the effect of refining the metal structure and improving the toughness of the steel member. Therefore, Nb may be contained as necessary. To ensure the above effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides is formed, deteriorating the toughness of the steel member. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, 0.070% or less, or 0.060% or less.

[0032] Ti: 0.001 to 0.200% Ti forms carbonitrides in steel and has the effect of improving the strength of steel members through precipitation strengthening. It also refines the metal structure, thereby improving the toughness of steel members. Therefore, Ti may be contained as necessary. To ensure the above effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of carbonitrides is formed, deteriorating the toughness of the steel members. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.150% or less, 0.100% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0033] Cr: 0.01 to 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 contained as necessary. To ensure the above effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.03% or more or 0.05% 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 be stabilized, and carbides may remain after hot stamping, resulting in a decrease in the toughness of the steel member. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.95% or less or 0.60% or less.

[0034] B: 0.0001 to 0.0100% B has the effect of improving the hardenability during hot stamping or during cooling after hot stamping, thereby improving the strength of the steel member. Therefore, B may be added as necessary. To ensure the above effect, the B content is preferably 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 becomes saturated, and cracks may occur during hot rolling and borides may reduce the toughness of the steel member. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0090% or less or 0.0060% or less.

[0035] Mo: 0.01 to 1.00% Mo is an element that improves the hardenability of steel and has the effect of improving the strength of steel members. Mo also has the effect of improving the corrosion resistance of steel members. Therefore, Mo may be contained as necessary. To ensure the above effects, the Mo content is preferably 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 exceeds 1.00%, carbides present after hot rolling, cold rolling, or annealing (including after plating) are stabilized, and carbides may remain after hot stamping, resulting in a decrease in the toughness of the steel member. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.80% or less or 0.60% or less.

[0036] W: 0.01 to 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 contained as necessary. To ensure the above effects, the W content is preferably 0.01% or more. The W content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the W content exceeds 2.00%, hot workability may deteriorate, resulting in reduced productivity. Therefore, the W content is set to 2.00% or less. The W content is preferably 1.80% or less or 0.60% or less.

[0037] Co: 0.01 to 1.00% Co improves the corrosion resistance of steel members. Therefore, Co may be contained as necessary. To ensure the above effects, the Co content is preferably 0.01% or more. The Co content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the Co content exceeds 1.00%, the hardenability of the steel decreases, thereby reducing the strength of the steel member. Therefore, the Co content is set to 1.00% or less. The Co content is preferably 0.80% or less or 0.60% or less.

[0038] Ni: 0.01 to 1.00% Ni is an element that improves the hardenability of steel and has the effect of improving the strength of steel members. Ni also has the effect of improving the corrosion resistance of steel members. Therefore, Ni may be contained as necessary. To ensure the above effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.04% or more or 0.06% or more. On the other hand, even if the Ni content exceeds 1.00%, the above effects saturate and the alloy cost increases. Therefore, the Ni content is set to 1.00% or less. The Ni content is preferably 0.90% or less or 0.60% or less.

[0039] Cu: 0.01 to 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 contained as necessary. To ensure the above effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more or 0.07% or more. On the other hand, even if the Cu content exceeds 1.00%, the above effects saturate and the alloy cost increases. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less or 0.60% or less.

[0040] V: 0.01 to 1.00% V forms carbonitrides in steel and has the effect of improving the strength of steel members through precipitation strengthening. Furthermore, as a solid solution element, V also has the effect of refining the metal structure, thereby improving the strength and toughness of steel members. Therefore, V may be contained as necessary. To ensure the above effect, the V content is preferably 0.01% or more. The V content is more preferably 0.04% or more or 0.10% or more. On the other hand, if the V content exceeds 1.00%, a large amount of carbonitrides is formed, deteriorating the toughness of the steel members. Therefore, the V content is set to 1.00% or less. The V content is preferably 0.80% or less or 0.60% or less.

[0041] Ca: 0.0001 to 0.2000% Ca has the effect of refining inclusions in steel and preventing cracks caused by the inclusions during hot stamping and collision. Therefore, Ca may be added as necessary. To ensure the above effect, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the Ca content exceeds 0.2000%, the effect of refining inclusions in steel saturates and alloy costs increase. Therefore, the Ca content is set to 0.2000% or less. The Ca content is preferably 0.1800% or less or 0.1000% or less.

[0042] Mg: 0.0001 to 0.2000% Mg has the effect of refining inclusions in steel and preventing cracks caused by the inclusions during hot stamping and collision. Therefore, Mg may be added as necessary. To ensure the above effect, the Mg content is preferably 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 steel saturates and the alloy cost increases. Therefore, the Mg content is set to 0.2000% or less. The Mg content is preferably 0.1800% or less or 0.1000% or less.

[0043] REM: 0.0001 to 0.3000% REM has the effect of refining inclusions in steel and preventing cracks caused by the inclusions during hot stamping and collision. Therefore, REM may be added as necessary. To ensure the above effect, the REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the REM content exceeds 0.3000%, the effect of refining inclusions in steel saturates and alloy costs increase. Therefore, the REM content is set to 0.3000% or less. The REM content is preferably 0.2000% or less or 0.1000% or less. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total content of these elements.

[0044] Sb: 0.01 to 1.00% Sb is an element that improves the corrosion resistance of steel members in a corrosive environment. Therefore, Sb may be contained as necessary. In order to reliably exert the above-mentioned effects, the Sb content is preferably 0.01% or more. The Sb content is more preferably 0.02% or more or 0.05% or more. On the other hand, even if the Sb content exceeds 1.00%, the above-mentioned effects saturate, so the Sb content is set to 1.00% or less. The Sb content is preferably 0.80% or less or 0.60% or less.

[0045] Sn: 0.01 to 1.00% Sn has the effect of improving the corrosion resistance of steel members. Therefore, Sn may be contained as necessary. To ensure this effect, the Sn content is preferably 0.01% or more. The Sn content is more preferably 0.02% or more or 0.05% or more. On the other hand, even if the Sn content exceeds 1.00%, the above effect saturates, so the Sn content is set to 1.00% or less. The Sn content is preferably 0.80% or less or 0.60% or less.

[0046] Zr: 0.01 to 1.00% Zr has the effect of forming carbonitrides in steel and improving the strength of steel members through precipitation strengthening. Furthermore, Zr fixes N as nitrides, suppressing the formation of BN and enhancing the hardenability-improving effect of B. Therefore, Zr may be contained as needed. To ensure the above effects, the Zr content is preferably 0.01% or more. The Zr content is more preferably 0.02% or more or 0.05% or more. On the other hand, if the Zr content exceeds 1.00%, a large amount of carbonitrides is formed, reducing the toughness of the steel member. Therefore, the Zr content is set to 1.00% or less. The Zr content is preferably 0.80% or less or 0.60% or less.

[0047] As: 0.01 to 1.00% As has the effect of improving hydrogen embrittlement resistance. Therefore, As may be contained as necessary. In order to reliably exert the above effect, the As content is preferably 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 costs increase. Therefore, the As content is set to 1.00% or less. The As content is preferably 0.80% or less or 0.60% or less.

[0048] Se: 0.0001 to 1.0000% Se has the effect of improving hydrogen embrittlement resistance. Therefore, Se may be contained as necessary. In order to reliably exert the above effect, the Se content is preferably 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 set to 1.0000% or less. The Se content is preferably 0.0800% or less or 0.0600% or less.

[0049] Bi: 0.0001 to 1.0000% Bi has the effect of improving hydrogen embrittlement resistance. Therefore, Bi may be contained as necessary. In order to reliably exert the above effect, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 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 set to 1.0000% or less. The Bi content is preferably 0.0800% or less or 0.0600% or less.

[0050] Ta: 0.0001 to 1.0000% Ta has the effect of improving hydrogen embrittlement resistance. Therefore, Ta may be contained as necessary. In order to reliably exert the above effect, the Ta content is preferably 0.0001% or more. The Ta content is more preferably 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 set to 1.0000% or less. The Ta content is preferably 0.0800% or less or 0.0600% or less.

[0051] Re: 0.0001 to 1.0000% Re has the effect of improving hydrogen embrittlement resistance. Therefore, Re may be contained as necessary. In order to reliably exert the above effect, the Re content is preferably 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 set to 1.0000% or less. The Re content is preferably 0.0800% or less or 0.0600% or less.

[0052] Os: 0.0001 to 1.0000% Os has the effect of improving hydrogen embrittlement resistance. Therefore, Os may be contained as necessary. To ensure the above effect, the Os content is preferably 0.0001% or more. The Os content is more preferably 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 set to 1.0000% or less. The Os content is preferably 0.0800% or less or 0.0600% or less.

[0053] Ir: 0.0001 to 1.0000% Ir has the effect of improving hydrogen embrittlement resistance. Therefore, Ir may be contained as necessary. To ensure the above effect, the Ir content is preferably 0.0001% or more. The Ir content is more preferably 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 set to 1.0000% or less. The Ir content is preferably 0.0800% or less or 0.0600% or less.

[0054] Tc: 0.0001 to 1.0000% Tc has the effect of improving hydrogen embrittlement resistance. Therefore, Tc may be contained as necessary. In order to reliably exert the above effect, the Tc content is preferably 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 set to 1.0000% or less. The Tc content is preferably 0.0800% or less or 0.0600% or less.

[0055] The chemical composition of the above-mentioned steel member can be determined by the following method. A test piece is taken from the surface of the steel member in the range of 1 / 8 to 3 / 8 of the plate thickness in the plate thickness direction, and this test piece can be measured by 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 surface coating is removed by mechanical grinding, and then the chemical composition is analyzed in the same manner.

[0056] Next, the metallographic structure of the steel member according to this embodiment will be described. In the steel member according to this embodiment, when a range from a position 1 / 8 of the plate thickness from the surface to a position 3 / 8 of the plate thickness, with the position 1 / 4 of the plate thickness from the surface as the center, is defined as a 1 / 4 depth position, the metallographic structure at the 1 / 4 depth position is, in volume %, martensite, bainite, and tempered martensite: a total of 90% or more, and the number density of a carbide group at the 1 / 4 depth position, which is made up of three carbides having a circle equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less, is 10.0 × 10 －４ pieces / μm ２ The standard deviation of the circle-equivalent diameter of the prior austenite grains is 6.0 μm or less.

[0057] In this embodiment, the metallographic structure at the 1 / 4 depth position is defined. The reason for this is that the metallographic structure at this position represents a representative metallographic structure of a steel member. The 1 / 4 depth position refers to a range from a position 1 / 8 of the plate thickness from the surface to a position 3 / 8 of the plate thickness, which can be rephrased as "a 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." Note that the surface of the steel member referred to here refers to the surface of the steel plate when the steel member is made of only a steel plate, or refers to the interface between the coating and the steel plate when the steel member has a coating on its surface. The interface between the coating and the steel plate is identified using a BSE image (or a COMPO image) as described below.

[0058] Martensite, bainite, and tempered martensite: 90% or more in total Martensite, bainite, and tempered martensite are high-strength structures. If the total volume fraction of martensite, bainite, and tempered martensite is less than 90%, the strength of the steel member decreases. Therefore, the total volume fraction of martensite, bainite, and tempered martensite is set to 90% or more. The total volume fraction of martensite, bainite, and tempered martensite is preferably 93% or more, 95% or more, or 97% or more. The larger the total volume fraction of martensite, bainite, and tempered martensite, the better, so it may be 100%.

[0059] Ferrite, pearlite, carbide, and retained austenite: 0 to 10% in total. If the volume fractions of ferrite, pearlite, carbide, and retained austenite are too high, the desired amount of martensite, bainite, and tempered martensite cannot be secured, resulting in reduced strength in the steel member. Therefore, in relation to the total volume fractions of martensite, bainite, and tempered martensite, the total volume fraction of ferrite, pearlite, carbide, and retained austenite is preferably 10% or less. The total volume fraction of ferrite, pearlite, carbide, and retained austenite is more preferably 7% or less, 5% or less, or 3% or less. The smaller the total volume fraction of ferrite, pearlite, carbide, and retained austenite, the better, so it may be 0%. Precipitates and inclusions other than carbides may be present, but their volume fractions are included in the volume fraction of the structure in which they exist (martensite, bainite, tempered martensite, ferrite, pearlite, and retained austenite).

[0060] The volume fraction of each structure is measured using 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 a test specimen cannot be taken from this position, a position avoiding the end) so that the metal structure can be observed at a quarter depth in a cross section parallel to the plate thickness direction (plate thickness cross section). The cross section of the test specimen is polished using silicon carbide paper ranging from #600 to #1500 in grades, and then mirror-finished using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. Next, the specimen is polished at room temperature using colloidal silica without an alkaline solution to remove strain introduced into the surface layer of the sample.

[0061] At any position in the longitudinal direction of the cross section of the polished test piece, a region extending from a position 1 / 8 of the plate thickness from the surface to a position 3 / 8 of the plate thickness from the surface, centered at a position 200 μm in the longitudinal direction and a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, is measured at measurement intervals of 0.1 μm by electron backscatter diffraction to obtain crystal orientation information. For the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (Velocity detector manufactured by AMETEK) is used. At this time, the degree of vacuum in the device is 9.6 × 10 －５ The pressure is set to 0.2 Pa or less, the acceleration voltage is 25 kV, and the probe current level is 16. Furthermore, during measurement, "iron-α" and "iron-γ" are set as the phases and measurement is performed.

[0062] Furthermore, the same region as the EBSD measurement region is observed at 1000x magnification using a FE-SEM: thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL). When observing the same region as the EBSD measurement region, Vickers indentations are stamped at three of the four corners of the EBSD measurement region within 100 μm of each of the four corners so that the observation position can be identified. Thereafter, surface contamination is polished and removed, leaving the metal structure of the observation surface, and the surface is then etched with nital. Using the Vickers indentations as a guide, the same region as the EBSD observation surface can be observed.

[0063] Contamination can be removed by buffing 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.

[0064] In observing the structure using a thermal field emission scanning electron microscope, the volume fractions of carbide and pearlite are determined by the following method.

[0065] In structural observation, regions with high brightness in the secondary electron image are identified as carbides. Regions with low brightness are identified as ferrite. A structure in which ferrite and carbides are arranged in alternating layers (lamellar structure) is identified as pearlite. Note that when a structure is processed by rolling or molding, the carbides in the lamellar structure may be curved or partially cut. However, even these deformed carbides are identified as pearlite if they are arranged in alternating layers with ferrite. The pearlite area ratio is obtained by calculating the area ratio of the region identified as pearlite. Furthermore, the carbide area ratio is obtained by calculating the area ratio of granular regions with a circle equivalent diameter of 0.5 μm or more among the regions identified as carbides in regions other than the region identified as pearlite. The area ratios of carbide and pearlite are considered to be the volume fractions of carbide and pearlite, respectively. Note that the ferrite and carbides that constitute pearlite are not included in the volume fractions of ferrite and carbide in the metal structure. Carbides that constitute bainite and tempered martensite are not included in the volume fraction of carbides in the metal structure.

[0066] Next, for regions other than those determined to be carbides and pearlite by FE-SEM structural observation, regions with an fcc crystal structure and regions with a bcc crystal structure are distinguished from each other based on the crystal orientation information obtained by EBSD measurement. Specifically, first, the EBSD measurement results and the structural image obtained by FE-SEM observation are superimposed using the Vickers indentation as a marker. For regions other than those determined to be carbides and pearlite from the structural image obtained by FE-SEM observation, regions with an fcc crystal structure and regions with a bcc crystal structure are distinguished from each other based on the crystal orientation information obtained by EBSD measurement and the "PhaseMap" function installed in the software "OIMAnalysis (registered trademark)" attached to the EBSD analyzer. In this case, the positions of carbides and pearlite may be identified by comparing the grain boundary map using the 15° grain boundary described below with the position of the Vickers indentation. The area fraction of the region having the fcc crystal structure is calculated to obtain the area fraction of the retained austenite. The area fraction of the retained austenite is regarded as the volume fraction of the retained austenite.

[0067] Furthermore, for regions with a bcc crystal structure, the "Grain Average Misorientation" function installed in the software "OIMAnalysis (registered trademark)" attached to the EBSD analyzer is used to determine whether the region is ferrite, bainite, martensite, or tempered martensite, and the area ratio is measured. Specifically, under the condition that a boundary with a crystal orientation misorientation of 15° or more is considered to be a grain boundary (15° grain boundary), a region with a Grain Average Misorientation value (GAM value) of 1.0° or less is determined to be ferrite. Furthermore, a region with a GAM value of more than 1.0° is determined to be "martensite, bainite, and tempered martensite." The area ratios of ferrite and the regions determined to be "bainite, martensite, and tempered martensite" are calculated to obtain the area ratios of ferrite and "bainite, martensite, and tempered martensite." The area ratios of ferrite and "bainite, martensite, and tempered martensite" are considered to be the volume ratios of ferrite and "bainite, martensite, and tempered martensite," respectively.

[0068] Number density of a group of three carbides having a circle-equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less: 10.0 × 10 －４ pieces / μm ２ A group of three carbides having a circle-equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less forms a crack and promotes the propagation of the crack. This makes it difficult to ensure excellent toughness in the steel member. In this embodiment, in order to obtain the desired toughness in the steel member, the number density of the group of three carbides having a circle-equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less is set to 10.0×10 －４ pieces / μm ２ The number density of the carbide group is preferably 5.0 × 10 －４ pieces / μm ２ or less or 3.0 x 10 －４ pieces / μm ２ The smaller the number density of the carbide group, the better. －４ pieces / μm２ It may also be possible to use the following.

[0069] The number density of the carbide groups is measured using the image analysis software "Fiji (ImageJ)." First, a structural image obtained by observation using the thermal field emission scanning electron microscope described above is loaded as the analysis target, and a binarization process is performed to separate the image into high-brightness and low-brightness regions using the "Auto" function in the "Threshold" tab. However, since the structural image is intended to observe carbide groups over a wide area, the region observed using the thermal field emission scanning electron microscope described above at a magnification of 1000x is a region ranging from the surface to 1 / 4 of the plate thickness in the plate thickness direction, 120 μm in the direction perpendicular to the plate thickness direction, and 80 μm in the plate thickness direction. This is because, when the magnification is high and the photographed area is narrow, or when the field of view is discrete, the average number density of carbides can be evaluated, but the number density of the carbide groups cannot be appropriately evaluated. If the binarization process is not performed appropriately using the "Auto" function, adjustments may be made to the extent that the regions identified as ferrite based on brightness as described above are not classified as high-brightness regions after binarization. However, adjustments are made so that the number of high-brightness regions does not decrease drastically compared to the original tissue image used for analysis.

[0070] Next, the "Analyze Particles" function is used to extract carbides having a particle size of 0.5 μm or more in equivalent circle diameter. Specifically, the area (size) of the above-mentioned high brightness area is extracted. ２ The above areas are designated as the analysis target. This allows the image shown in Figure 1 to be obtained. The black areas in Figure 1 are carbides with a particle size of 0.5 μm or more in equivalent circle diameter. Next, using the "Analyze" function, select "Centroid" in the "Set Measurements" tab and output the center coordinates (X and Y coordinates of the center) of the extracted carbides. The distance between the centers of each carbide is calculated from the center coordinates of each carbide.

[0071] As a result, when a certain carbide A is used as a reference and other carbides B and C exist that are 10 μm or less apart from carbide A, the three carbides A, B, and C are identified as one carbide group. In this case, every time there are two other carbides that are 10 μm or less apart from a certain carbide, one carbide group is identified. For example, as shown in FIG. 2 , when three other carbides (carbides B, C, and D) exist around carbide A, the carbide groups are identified as three (carbide group a consisting of carbides A, B, and C; carbide group b consisting of carbides A, B, and D; and carbide group c consisting of carbides A, C, and D). This makes it possible to identify that the more densely packed the carbides are, the greater the number of carbide groups. Similarly, the number of carbide groups is calculated using all carbides as a reference, and the sum of the numbers of carbide groups (total number of carbide groups) is calculated excluding overlaps. The number density of the carbide groups is obtained by dividing the total number of carbide groups by the area of ​​the measurement range. "Excluding overlaps" means that when the combination of carbides that make up a carbide group is the same, it is treated as one precipitate group.

[0072] Standard deviation of circle-equivalent diameter of prior austenite grains: 6.0 μm or less If the standard deviation of the circle-equivalent diameter of prior austenite grains exceeds 6.0 μm, the grain size variation of the prior austenite grains is too large, and in the case of a steel member containing a large amount of Mn, as in the chemical composition according to this embodiment, excellent toughness cannot be obtained. Therefore, the standard deviation of the circle-equivalent diameter of prior austenite grains is set to 6.0 μm or less. The standard deviation of the circle-equivalent diameter of prior austenite grains is preferably 5.0 μm or less, 4.5 μm or less, 4.0 μm or less, or 3.5 μm or less. The standard deviation of the circle-equivalent diameter of prior austenite grains may be 1.0 μm or more, or 1.5 μm or more.

[0073] The standard deviation of the circle-equivalent diameter of prior austenite grains is obtained using the following method. A sample is cut from any position at least 10 mm away from the end face of the steel member (if a sample cannot be taken from this position, a position avoiding the end) so that the cross section of the plate thickness can be observed. The size of the sample depends on the measuring device, but it should be large enough to observe approximately 10 mm in the longitudinal direction. The cross section of the sample is polished using silicon carbide paper ranging from #600 to #1500 in stages, and then finished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. The observation surface is then finished by electrolytic polishing.

[0074] Using the polished test piece, a measurement range of 100 μm in the longitudinal direction and 100 μm in the thickness direction was measured at a quarter depth position, with a measurement interval of 0.2 μm, by electron backscatter diffraction to obtain crystal orientation information. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used, for example, an EBSD analyzer consisting of a JEOL JSM-7200F and an AMETEK Velocity detector. At this time, the degree of vacuum in the EBSD analyzer was 9.6×10 －５ The pressure is set to 0.2 Pa or less, the acceleration voltage is 25 kV, and the probe current level is 16. Furthermore, during measurement, "iron-α" and "iron-γ" are set as the phases and measurement is performed.

[0075] Using the obtained crystal orientation information and the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, regions with a bcc crystal structure are extracted. For these regions, a crystal orientation map of the prior austenite grains is created by the method described in Acta Materialia, 58 (2010), 6393-6403. For this crystal orientation map, the "Grain Properties" function is used to determine the value calculated by the Area method under the condition that interfaces with a crystal orientation difference of 15° or more are considered to be grain boundaries, thereby obtaining the standard deviation of the circle-equivalent diameter of the prior austenite grains.

[0076] 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), the Vickers hardness at the 50 μm depth position is preferably 50 HV or more lower than the Vickers hardness at a position ¼ of the thickness from the surface in the thickness direction (a range of −20 μm to +20 μm in the thickness direction from the ¼ position in the thickness direction is acceptable). In other words, it is preferable to satisfy the following relationship: "(Vickers hardness at a position ¼ of the thickness from the surface in the thickness direction) - (Vickers hardness at a position 50 μm deep) ≥ 50 HV." By making the Vickers hardness at the 50 μm depth position 50 HV or more lower than the Vickers hardness at a position ¼ of the thickness from the surface in the thickness direction, it is possible to obtain superior bendability while ensuring high strength and excellent toughness. To further improve the bendability, it is more preferable that the Vickers hardness at a depth of 50 μm is 100 HV or more lower than the Vickers hardness at a position ¼ of the plate thickness from the surface in the plate thickness direction.

[0077] Vickers hardness is measured by the following method. In accordance with JIS Z 2244-1:2020, a Vickers hardness test is performed at a depth of 50 μm and at a position 1 / 4 of the thickness from the surface in the thickness direction. In the Vickers hardness test, a load of 0.098 N is applied to the polished cross section (thickness cross section) of the test piece at a depth of 50 μm, and a load of 9.8 N is applied to the position 1 / 4 of the thickness from the surface in the thickness direction, with a load holding time of 10 seconds. Five points are measured at each position, and the average value of the three points excluding the maximum and minimum values ​​is calculated to obtain the Vickers hardness at each position.

[0078] The steel member according to this embodiment may have a coating on a part or all of its surface. The coating may be an Al-based coating (a coating mainly made of an Fe-Al-based alloy) or a Zn-based coating (a coating mainly made of an Fe-Zn-based alloy). The coating is also called a film, an alloyed plating layer, or an intermetallic compound layer. The presence of the coating can improve corrosion resistance. The thickness of the coating is preferably 5 to 100 μm.

[0079] An Al-based coating (a coating mainly made of an Fe-Al-based alloy) is a coating containing 70 mass% or more of Fe and Al in total, and a Zn-based coating (a coating mainly made of an Fe-Zn-based alloy) is a coating containing 70 mass% or more of Fe and Zn in total.

[0080] The Al-based coating (a coating mainly composed of an Fe—Al-based alloy) may contain, in addition to Fe and Al, one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities.

[0081] The Zn-based coating (a coating mainly composed of an Fe—Zn-based alloy) may contain, in addition to Fe and Zn, one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities.

[0082] The thickness of the coating can be determined by cross-sectional observation using a scanning electron microscope. A test piece is cut out from an arbitrary position at least 10 mm away from the end face. The cross-section of the cut test piece is mechanically polished and then mirror-finished. The observation range using a scanning electron microscope is, for example, 400 times magnification and 40,000 μm in area. ２ The above range applies.

[0083] When a cross section is observed using a BSE image (or COMPO image), a clear difference in contrast is observed between the coating and the base steel (steel sheet). 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 locations within the observation photograph, with the distance between measurement locations being 6.5 μm. Furthermore, when measuring, five visual fields are observed in the same manner as above, and the average value is used to determine the coating thickness.

[0084] The chemical composition of the coating can be determined by performing spot elemental analysis (beam diameter 1 μm or less) on the same observation range as above using an electron probe microanalyzer (EPMA) to determine the concentrations of Fe, Al, and Zn contained in the coating. A total of 10 points are analyzed on the coating in any 10 fields of view, and the average values ​​are used as the concentrations of Fe, Al, and Zn contained in the coating. The same method can be used even when elements other than Fe, Al, and Zn are contained in the coating.

[0085] The 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 absorption energy during a collision, the thickness may be preferably 0.8 to 5.0 mm or 1.0 to 5.0 mm.

[0086] Strength: Vickers hardness of 630 HV or more In this embodiment, Vickers hardness is used as an alternative index of tensile strength. The steel member according to this embodiment may have a Vickers hardness of 630 HV or more at a position of 1 / 4 of the plate thickness from the surface in the plate thickness direction. If the Vickers hardness at a position of 1 / 4 of the plate thickness from the surface in the plate thickness direction is 630 HV or more, it can be determined that the steel member has sufficient strength (tensile strength: 2.2 GPa or more) in this embodiment. The Vickers hardness at a position of 1 / 4 of the plate thickness from the surface in the plate thickness direction is preferably 650 HV or more, 680 HV or more, or 690 HV or more. The Vickers hardness at a position of 1 / 4 of the plate thickness from the surface in the plate thickness direction may be 850 HV or less. The Vickers hardness at a position of 1 / 4 of the plate thickness from the surface in the plate thickness direction is measured by the method described above.

[0087] Charpy impact value at -100°C (K －１００ ): 20 J / cm ２ The steel member according to this embodiment has a Charpy impact value of 20 J / cm at −100° C., obtained by performing a Charpy impact test under the following conditions: ２ The Charpy impact value at -100°C is preferably 20 J / cm or more. ２ If the above is true, it can be determined that the toughness is excellent in this embodiment.

[0088] The Charpy impact value at -100°C is the Charpy impact absorption energy at -100°C divided by the cross-sectional area of ​​the test specimen. The Charpy impact absorption energy at -100°C is determined by tests conforming to JIS Z 2242:2018 and JIS B 7722:2018. Test specimens are taken from flat portions of the steel member at least 10 mm away from the end face (for example, the top plate or other flat portions in the case of a hat-shaped member). The coating film on the steel member is peeled off, and test specimens measuring 10 mm wide and 55 mm long are taken with the original material thickness intact (including the notch shape described below, with the shape conforming to JIS Z 2242:2018 except for the thickness). Even if the steel member has a coating, the test specimen is taken without removing the coating. A 2 mm deep V-notch (notch angle: 45°, notch bottom radius: 0.25 mm, notch bottom width: 8 mm, notch position (center): 27.5 mm from the longitudinal end of the test piece) is made in the sampled test piece. Three test pieces are stacked and fixed with screws, and a Charpy impact test is performed. However, if the plate thickness is less than 2.0 mm, the test is performed with three test pieces stacked, but if the plate thickness is 2.0 mm or more, the test is performed with a single test piece without stacking. Note that the Charpy impact value differs depending on the relationship between the notch depth direction and the rolling direction of the steel plate, but test pieces may be taken from either direction when taking test pieces from the member.

[0089] VDA bending angle (α): 40° or more The steel member according to this embodiment preferably has a VDA bending angle of 40° or more (equivalent to a plate thickness of 2.0 mm) 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, the steel member can be determined to have excellent bendability, and the collision safety of the automobile member can be further improved.

[0090] The bending test is performed using the following method. A test piece is taken from an arbitrary position at least 10 mm away from the end face of the steel member, and a bending test is performed under the following conditions in accordance with the German Association of the Automotive Industry standard VDA 238-100:2017. The bending angle (VDA bending angle) is determined when the bending load drops by 60 N from the maximum point. If a crack occurs before the bending load reaches the maximum point, the bending angle at the time when the crack occurs is determined and used as the VDA bending angle. Note that the value of the VDA bending angle varies depending on the relationship between the bending ridge direction and the rolling direction of the steel plate, but test pieces taken from either direction may be used when taking test pieces from the member.

[0091] If the plate thickness is not 2.0 mm, the VDA bending angle obtained by performing a bending test at that plate thickness is corrected to a value equivalent to a plate thickness of 2.0 mm using the following formula: VDA bending angle = α ｔ -23.05 × (1 - t / 2.0) In the above formula, α ｔ indicates the VDA bending angle obtained by the bending test, and t indicates the plate thickness (mm).

[0092] The conditions for the bending test are as follows: Test piece dimensions: 60 mm x 30 mm Bending ridge: Parallel to the 30 mm side Test method: Roll support, punch pressing Roll diameter: φ30 mm Punch shape: Tip R = 0.4 mm Roll distance: (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 20 kN

[0093] Next, a description will be given of a steel sheet according to this embodiment from which the above-mentioned steel members can be manufactured. Since the steel sheet according to this embodiment has the same chemical composition as the above-mentioned steel members, the reasons for limitations and measurement methods will not be described.

[0094] In the steel plate according to this embodiment, when a range from a position of 1 / 8 of the plate thickness from the surface to a position of 3 / 8 of the plate thickness, with the position of 1 / 4 of the plate thickness from the surface as the center, is defined as a 1 / 4 depth position, the metal structure at the 1 / 4 depth position is composed of, by volume %, pearlite: 40% or more, ferrite, bainite, martensite, tempered martensite, carbides, and retained austenite: 60% or less in total, and the number density of a carbide group consisting of three carbides having a circle equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less at the 1 / 4 depth position is 10.0 × 10 －３ pieces / μm ２ or less, and the dispersion index of the pearlite is 0.50 to 1.00. The surface that serves as the reference for the depth position is the surface of the steel sheet, but in cases where the steel sheet has a coating on the surface, it refers to the interface between the coating and the base steel sheet. In the case of a steel sheet, the surface is determined by GD-OES analysis, which will be described later.

[0095] Pearlite: 40% or more The formation of pearlite suppresses the generation of coarse spheroidal carbides in high-strength materials. Furthermore, the carbides constituting pearlite are crushed into fine carbides in the subsequent cold rolling process, allowing them to be sufficiently dissolved in the subsequent annealing process and the heating process before hot stamping. If the volume fraction of pearlite is less than 40%, the number of coarse spheroidal carbides increases, making it easier for carbide clusters to form. Therefore, the volume fraction of pearlite is set to 40% or more. The volume fraction of pearlite is preferably 50% or more, 60% or more, 70% or more, or 80% or more. From the viewpoint of productivity, the volume fraction of pearlite is preferably 95% or less or 90% or less.

[0096] Ferrite, bainite, martensite, tempered martensite, carbides, and retained austenite: 60% or less in total If the volume fraction of ferrite, bainite, martensite, tempered martensite, carbides, and retained austenite exceeds 60% in total, it is impossible to obtain a desired amount of pearlite. Therefore, the volume fraction of ferrite, bainite, martensite, tempered martensite, carbides, and retained austenite is set to 60% or less in total. The volume fractions of ferrite, bainite, martensite, tempered martensite, carbides, and retained austenite are preferably 50% or less, 40% or less, 30% or less, or 20% or less in total. Furthermore, in relation to the volume fraction of pearlite, the volume fraction of ferrite, bainite, martensite, tempered martensite, carbides, and retained austenite is preferably 5% or more or 10% or more in total. Precipitates and inclusions other than carbides may be present, but their volume fraction is included in the volume fraction of the structure in which they exist (ferrite, pearlite, bainite, martensite, tempered martensite, and retained austenite).

[0097] The method for measuring the volume fraction of each structure is the same as that for the steel member, and therefore the explanation will be omitted.

[0098] Number density of a group of three carbides having a circle-equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less: 10.0 × 10 －３ pieces / μm ２ Even when a steel plate is subjected to hot stamping to produce a steel member, the number of carbides decreases due to the heat treatment performed by hot stamping, but the positions of the carbides remain unchanged. In order to obtain the desired toughness by suitably controlling the number density of the carbide groups in the steel member, it is important to control the positions of the carbides in the steel plate. When the number density of the carbide groups consisting of three carbides each having a circle equivalent diameter (circle equivalent diameter) of 0.5 μm or more and a center-to-center distance of 10 μm or less is 10.0 × 10 －３ pieces / μm ２If the number density of the carbide group in the steel member exceeds 10.0×10, it is not possible to preferably control the number density of the carbide group in the steel member, and the toughness of the steel member may deteriorate. Therefore, the number density of the carbide group consisting of three carbides having a circle equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less should be 10.0×10 －３ pieces / μm ２ The number density of the carbide group is preferably 5.0 × 10 －３ pieces / μm ２ or less or 3.0 x 10 －３ pieces / μm ２ The smaller the number density of the carbide group, the better. －３ pieces / μm ２ It may also be possible to use the following.

[0099] The number density of the carbide group is measured by the same method as for the steel member, except for the method for extracting the carbides. After performing binarization processing on the structure image by the same method as for the steel member, carbides with a circle equivalent diameter of 0.5 μm or more are extracted using the "Analyze Particles" function. At this time, carbides with an area of ​​0.20 μm or more are extracted by targeting areas with high brightness. ２ The region where the "Circularity" value is 0.3 to 1.0 is specified as the analysis target. By specifying the "Circularity" value as the analysis target, carbides that make up pearlite, which is a lamellar structure, can be excluded from the analysis. The rest is the same as for steel members.

[0100] Pearlite dispersion index: 0.50 to 1.00 If the pearlite dispersion index is less than 0.50, it is not possible to reduce the standard deviation of the circle-equivalent diameter of prior austenite grains in the steel member. Therefore, the pearlite dispersion index is set to 0.50 or more. The pearlite dispersion index is preferably 0.60 or more, 0.70 or more, or 0.80 or more. Since the pearlite dispersion index does not exceed 1.00, the upper limit is 1.00.

[0101] The pearlite dispersion index is obtained by the following method. First, a scanning electron microscope is used to obtain a secondary electron image of a thickness cross section parallel to the rolling direction, centered at a position 1 / 4 of the thickness from the surface in the thickness direction, covering a range of 30 μm parallel to the rolling direction and 30 μm perpendicular to the rolling direction. Next, ten 30 μm long straight lines parallel to the rolling direction are drawn at 2 μm intervals in the obtained secondary electron image. Next, the grain boundaries or phase boundaries intersecting with these lines are classified into A / A boundaries (boundaries between ferrite and ferrite, i.e., ferrite grain boundaries) and A / B boundaries (boundaries between ferrite and pearlite), and the number of intersections between each boundary and the lines is calculated. Next, the number of intersections with the A / B boundaries is divided by the total number of intersections, i.e., the sum of the number of intersections with the A / A boundaries and the number of intersections with the A / B boundaries, to obtain the proportion of A / B boundaries in that field of view. The same procedure is performed on three different fields of view within the same sample, and the average proportion of A / B boundaries in the three fields of view is calculated. The obtained average value is taken as the dispersion index of pearlite.

[0102] If the rolling direction of the steel sheet is not known in advance, the rolling direction of the steel sheet may be determined by the following method. A test piece is taken from an arbitrary position at least 50 mm away from the end face of the steel sheet so that the thickness cross section can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. Depending on the size of the inclusion, an appropriate magnification at which the inclusion dimensions can be measured is selected. The observation range is 500 μm or more in width and across the entire thickness of the sheet, and areas with low brightness are determined to be inclusions. Observation may be performed from multiple fields of view. Next, using the thickness cross section initially observed by the above method as a reference, a test piece is taken so that the thickness cross section can be observed by rotating it in 5° increments in the range of 0° to 180° around the thickness direction, and the cross section is observed using the same method as above. The average length of the major axis of the multiple inclusions in each cross section is calculated for each cross section. The cross section where the average value of the major axis length of the obtained inclusions is greatest 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 plate that constitutes the steel member can also be determined in a similar manner.

[0103] Decarbonization index DＣ 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 D of the steel sheet is 0.10 or more. Ｃ By making the Vickers hardness at a surface layer (at a depth of 50 μm) of the steel member after hot stamping, the Vickers hardness at a surface layer (at a depth of 50 μm) can be reduced, thereby improving the bendability of the steel member.

[0104] The decarburization index of a steel sheet is obtained by the following method. A glow discharge optical emission spectrometer (GD-OES: a Marcus-type high-frequency glow discharge optical emission spectrometer, GD-PROFILER-HR, manufactured by HORIBA, Ltd.) is used to measure the element concentration distribution from the surface of the steel sheet toward the sheet thickness direction. Here, the measurement range is set to a position at a depth of 120 μm or more from the measurement start surface. The measurement conditions are 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 analyzed are C and Fe. Furthermore, if the contained components are determined in advance, they are specified as the analyzed elements.

[0105] In this embodiment, the region where the Fe concentration is 90% by mass or more in the GD-OES analysis is determined to be the steel sheet, 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 determined to be the surface of the steel sheet. If the steel sheet has a coating on its surface, the coating may be partially or completely removed by mechanical polishing or chemical polishing before GD-OES analysis so that measurements can be made up to a depth of 120 μm from the surface of the steel sheet (the interface between the steel sheet and the coating). 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 sheet and the coating.

[0106] The average of the measured values ​​of the C concentration in the region from a position 100 μm deep from the surface of the steel sheet to a position 120 μm deep from the surface is calculated, and the obtained average value is regarded as the C concentration of the steel sheet.

[0107] A curve graph such as that shown in Figure 3 is obtained by performing GD-OES analysis from the surface of the steel plate to a depth of 120 µm. The amount of carbon reduction is obtained by calculating the difference between the carbon concentration of the steel plate and the measured carbon concentration at each unit depth (measurement interval). If the carbon concentration at the measurement point is higher than the carbon concentration of the steel plate, the amount of carbon reduction is set to 0. The area of ​​the carbon-depleted region is obtained by integrating the product of the unit depth and the amount of carbon reduction (area of ​​region A in Figure 3). The product of the carbon concentration of the steel plate and a depth of 120 µm is set to the reference area (area of ​​region B in Figure 3). The value obtained by dividing the area of ​​the carbon-depleted 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 get.

[0108] In the steel sheet according to this embodiment, the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface in the sheet thickness direction, which is an alternative index of tensile strength, is preferably 380 HV or less. If the Vickers hardness at a position 1 / 4 of the sheet thickness from the surface in the sheet thickness direction is 380 HV or less, it can be determined that the tensile strength is 1300 MPa or less, which makes it easier to shear the steel sheet before heat treatment. The Vickers hardness of the steel sheet is measured by the same method as for the steel member.

[0109] The steel sheet according to this embodiment may have the above-described coating on a part or all of the surface. The type and measurement method thereof are the same as those for the steel member, and therefore description thereof will be omitted.

[0110] 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 absorption energy during a collision, the thickness may preferably be 0.8 to 5.0 mm or 1.0 to 5.0 mm.

[0111] Next, a preferred method for manufacturing the steel sheet according to this embodiment will be described. Note that the temperature of the steel sheet in this embodiment refers to the surface temperature of the steel sheet.

[0112] In a preferred method for producing a steel sheet according to this embodiment, finish rolling is performed so that the finish rolling completion temperature is in the temperature range of 850 to 1000°C and the reduction in the final stage of finish rolling is 40% or more, water cooling is started within 4.0 seconds after the completion of finish rolling, and cooling is performed so that the average cooling rate in the temperature range from the finish rolling completion temperature to 650°C is 50°C / second or more, intermediate air cooling is performed in a temperature range of 580 to 650°C, and the temperature is held for 4.0 seconds or more, coiling is performed so that the coiling temperature is in the temperature range of 450 to 580°C, cold rolling is performed so that the total reduction is 30% or more, annealing is performed in a temperature range of 730°C or higher, and cooling is performed so that the average cooling rate in the temperature range from the annealing temperature to 650°C is 5°C / second or more, and holding in a temperature range of 500 to 650°C for 200 seconds or more. Each step will be described below.

[0113] Finish rolling completion temperature: 850 to 1000°C By setting the finish rolling completion temperature (the temperature at the delivery side of the final stage of finish rolling) to 850°C or higher, the rolling load is reduced. This makes it possible to prevent the steel sheet from breaking. Furthermore, by setting the finish rolling completion temperature to 1000°C or lower, it is possible to prevent the crystal grains from becoming coarse, and to prevent breakage during cold rolling. Therefore, it is preferable that the finish rolling completion temperature be in the temperature range of 850 to 1000°C.

[0114] Reduction ratio in the final stage of finish rolling: 40% or more By setting the reduction ratio in the final stage of finish rolling to 40% or more, it is possible to promote the generation of granular austenite. By combining with the manufacturing conditions described later, it is possible to favorably control the dispersion index of pearlite. Note that the reduction ratio in the final stage of finish rolling is the ratio of the entry plate thickness in the final stage of finish rolling to t ０ The thickness of the final stage of finish rolling is t １ When this is done, (1-t １ / t ０ ) × 100 (%).

[0115] The slab to be subjected to the hot rolling consisting of rough rolling and finish rolling is not particularly limited except that it has the above-mentioned chemical composition. For example, a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or electric furnace, etc. and then by continuous casting can be used. Instead of continuous casting, an ingot casting method, thin slab casting method, etc. may also be used. The rough rolling conditions and finish rolling conditions are not particularly limited as long as the finish rolling completion temperature and the reduction rate in the final stage of finish rolling are within the above-mentioned ranges.

[0116] Time from completion of finish rolling to start of cooling: within 4.0 seconds By starting water cooling within 4.0 seconds after completion of finish rolling, it is possible to suppress the growth of granular austenite generated from the austenite interface. Under the manufacturing conditions described below, ferrite is generated from the austenite interface and the austenite transforms into pearlite, which ultimately makes it possible to favorably control the dispersion index of pearlite.

[0117] Average cooling rate in the temperature range from the finish rolling completion temperature to 650°C: 50°C / second or more By setting the average cooling rate in the temperature range from the finish rolling completion temperature to 650°C to 50°C / second or more, it is possible to suppress the precipitation of coarse spherical carbides. As a result, it is possible to reduce the number density of carbide groups in the steel sheet. Therefore, it is preferable to set the average cooling rate in the temperature range from the finish rolling completion temperature to 650°C to 50°C / second or more.

[0118] In this embodiment, the average cooling rate is the temperature difference between the start point and the end point of the set range divided by the elapsed time from the start point to the end point.

[0119] Intermediate air cooling: Holding in the temperature range of 580 to 650°C for 4.0 seconds or more By performing intermediate air cooling in the temperature range of 580 to 650°C for 4.0 seconds or more, it is possible to suppress the temperature rise due to reheating after coiling and to suppress the stabilization of carbides. Furthermore, in order to suppress the formation of carbide groups, it is effective to sufficiently promote pearlite transformation by holding in this temperature range. Simply lowering the coiling temperature increases the coil temperature due to transformation heat after coiling, and carbides are stabilized due to the concentration of alloy elements, making it easier for carbide groups to form. However, by holding in the temperature range of 580 to 650°C for 4.0 seconds or more and promoting the transformation in advance, it is possible to suppress the formation of carbide groups after coiling. Therefore, it is preferable to hold in the temperature range of 580 to 650°C for 4.0 seconds or more during intermediate air cooling. Note that when holding in this temperature range, the temperature may be constant or may vary within the temperature range of 580 to 650°C.

[0120] Coiling temperature: 450 to 580°C By setting the coiling temperature to 450°C or higher, an increase in hardness of the steel sheet can be suppressed, and cold rolling can be easily performed. By setting the coiling temperature to 580°C or lower, stabilization of carbides can be suppressed. Therefore, the coiling temperature is preferably set in the temperature range of 450 to 580°C.

[0121] Total reduction rate of cold rolling: 30% or more By setting the total reduction rate of cold rolling to 30% or more, carbides in the steel can be crushed, and dissolution of the carbides can be promoted by heating in a subsequent process. As a result, the number density of carbide groups in the steel sheet can be reduced. Therefore, it is preferable that the total reduction rate of cold rolling is 30% or more. The total reduction rate of cold rolling is the thickness of the sheet before the first cold rolling to t ２ The thickness of the plate after the final cold rolling is t ３ When this is done, (1-t ３ / t ２ ) × 100 (%).

[0122] Annealing temperature: 730°C or higher, annealing time: 30 seconds or higher By setting the annealing temperature to 730°C or higher and the annealing time, which is the holding time in the temperature range of 730°C or higher, to 30 seconds or higher, it is possible to dissolve the spheroidal carbides present in the structure. Therefore, it is preferable that the annealing temperature is 730°C or higher and the annealing time is 30 seconds or higher. On the other hand, if the annealing time is held for more than 3000 seconds, the above effect saturates and the cost increases, so the upper limit of the annealing time is set to 3000 seconds.

[0123] Average cooling rate in the temperature range from the annealing temperature to 650°C: 5°C / second or more By setting the average cooling rate in the temperature range from the annealing temperature to 650°C to 5°C / second or more, it is possible to suppress the formation of coarse spheroidal carbides in the high temperature range. When pearlite transformation occurs in the high temperature range, spheroidal carbides precipitate in the ferrite. If the amount of these spheroidal carbides is large, carbide clusters are likely to form, which is not desirable. Therefore, the average cooling rate in the temperature range from the annealing temperature to 650°C is set to 5°C / second or more.

[0124] Holding time in the temperature range of 500 to 650°C: 200 seconds or more Holding for 200 seconds or more in the temperature range of 500 to 650°C provides a high driving force in the low temperature range to cause pearlite transformation, thereby forming pearlite that separates the ferrite. This makes it possible to suppress the formation of carbide groups.

[0125] The cooling method after holding in the temperature range of 500 to 650°C is not particularly limited. A coating may be formed during or after the annealing process. In order to improve the bendability of the steel member, it is preferable to decarburize the surface layer of the steel sheet during annealing (also called decarburization annealing). Alternatively, a coating may be formed after decarburization annealing. The decarburization conditions are a humid atmosphere containing nitrogen, hydrogen, or oxygen and having 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 of 5 to 1200 seconds is the time required for the steel sheet temperature to rise to Ac ３ After heating to -150°C and holding for a certain period of time, ３ The total time required to cool the sample to -150°C. ３The point can be determined using the formula described below.

[0126] The coating method is not particularly limited, and may be hot-dip plating, electroplating, vacuum deposition, cladding, thermal spraying, etc. Hot-dip plating is the most widely used method industrially. Examples of coatings include Al-based coatings containing Al and Zn-based coatings containing Zn.

[0127] When an Al-based coating is formed by hot dip plating, the plating bath often contains Fe as an impurity in addition to Al. In addition to the elements mentioned above, the plating bath may contain one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, as long as the total content of Fe and Al is 70 mass % or more.

[0128] When a Zn-based coating is formed by hot dip plating, the plating bath often contains Fe as an impurity in addition to Zn. In addition to Zn and Fe, the plating bath may also contain one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, as long as the total content of Fe and Zn is 70 mass % or more.

[0129] When hot-dip plating is performed, the steel sheet after the annealing step or the decarburization annealing step may be cooled to room temperature and then heated again to perform plating, or the steel sheet may be cooled or heated to a temperature close to the plating bath temperature (for example, 650 to 750°C for an Al-based coating, or 420 to 500°C for a Zn-based coating) after annealing or decarburization annealing, and then hot-dip plating may be performed without once cooling to room temperature.

[0130] There are no particular limitations on the pre-treatment and post-treatment for coating, and pre-coating, solvent application, alloying treatment, temper rolling, etc. are possible. As an alloying treatment, the coated steel sheet can be heated to, for example, 450 to 800°C. Furthermore, as a post-treatment, skin-pass rolling with a reduction of 0.1 to 0.5% may be performed to adjust the shape of the steel sheet.

[0131] The steel member according to this embodiment can be stably produced by using the steel plate according to this embodiment obtained by the method described above, for example, by a production method including the steps described below.

[0132] The steel plate according to this embodiment is subjected to a heat treatment to obtain a steel member. For example, the heat treatment is performed by heating the steel plate obtained by the above-described method at an average temperature rising rate of 1.0 to 1000°C / sec. ３ Point~(Ac ３ It is preferable to heat the material to a temperature range of the Ms point +300°C and then cool it to a temperature range of the Ms point or lower at an average cooling rate of 15.0°C / sec or higher.

[0133] An average heating rate of less than 1.0°C / sec is undesirable because it reduces the productivity of the heat treatment, whereas an average heating rate of more than 1000°C / sec is undesirable because it results in a duplex structure in the metal structure and reduces the limiting hydrogen content.

[0134] In addition, the heat treatment temperature (maximum heating temperature) is Ac ３ If the cooling rate is less than the Ms point (°C), the spherical carbides present in the metal structure cannot be sufficiently dissolved, making it impossible to reduce the number density of the carbides after cooling, making it difficult to ensure excellent toughness in the steel member, and the volume fractions of martensite, bainite, and tempered martensite are insufficient, which may result in insufficient strength of the steel member, which is undesirable. If the average cooling rate is less than 15.0°C / sec or the cooling stop temperature exceeds the Ms point (°C), the volume fractions of martensite, bainite, and tempered martensite are insufficient after cooling, which may result in insufficient strength of the steel member, which is undesirable. On the other hand, if the heat treatment temperature is (Ac ３ If the temperature exceeds the temperature point +300°C, the metal structure may become coarse grained, the limit hydrogen content may decrease, and the toughness may deteriorate, which is not preferable.

[0135] When heating, Ac ３ Point~(Ac ３ After cooling to a temperature equal to or lower than the Ms point, the steel member may be subjected to a tempering treatment in which the steel member is held in a temperature range of 100 to 600°C for 10 to 60 minutes in order to adjust the strength of the steel member.

[0136] A.C. ３The Ac point and Ms point are calculated from the following formulas (1) and (2) using the content (mass%) of each element in the chemical composition of the steel sheet (steel sheet for heat treatment). 0 is substituted for elements that do not contain the corresponding element. ３ (℃)=854-179×C+44×Si-14×Mn-18×Ni-2×Cr…(1) Ms(℃)=521-353×C-22×Si-24×Mn-17×Ni-8×Cu-16×Mo…(2)

[0137] Here, during the series of heat treatments, Ac ３ Point~(Ac ３ After heating to a temperature range of (Ms point +300°C), it is preferable to perform hot forming such as hot stamping while cooling to the Ms point. Examples of hot forming include bending, drawing, stretch forming, and hole expansion (stretch flange forming). Furthermore, forming methods other than press forming, such as roll forming, may be applied as long as a means for cooling the steel sheet simultaneously with or immediately after forming is provided. If the above-mentioned thermal history is followed, repeated hot forming may be performed. Furthermore, the above series of heat treatments may be repeated multiple times.

[0138] When hot forming by hot stamping (including when a flat die is used) is performed, it is preferable that the forming start temperature is 700° C. or higher, cooling is performed using the die, and the demolding temperature (the surface temperature of the steel member when it is removed from the die) is equal to or lower than the Ms point and less than 250° C. This improves the dimensional accuracy of the steel member.

[0139] The heat treatment may be performed on a portion of the steel sheet that is the raw material for hot forming or heat treatment, in which case a steel member having regions with different strengths can be obtained.

[0140] The above series of heat treatments can be carried out by any method. For example, heating may be carried out by high-frequency heating, electrical heating, infrared heating, or furnace heating. Cooling may also be carried out by water cooling, mold cooling, or the like. The atmosphere in the heating furnace may be air, combustion gas, or nitrogen gas. The dew point in the heating furnace may be controlled to suppress hydrogen generation during the heat treatment.

[0141] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0142] Slabs having the chemical compositions shown in Tables 1A to 2D were heated to 1100°C or higher, and then hot-rolled, cold-rolled, and annealed under the conditions shown in Tables 3A to 4E to obtain steel plates having a thickness of 1.6 to 3.5 mm.

[0143] In some examples, in order to form a decarburized layer in the surface layer portion, after cold rolling, the steel sheet is heated in a humid atmosphere containing nitrogen, hydrogen or oxygen and having a dew point of 1°C or higher (Ac ３ Point -150)℃ or higher, (Ac ３ The decarburization annealing was performed under the condition that the residence time in the temperature range of not more than the temperature point +150°C was 5 to 1200 seconds. In some examples, the steel sheet after the annealing or decarburization annealing was coated with an Al-based coating or a Zn-based coating under the above-mentioned conditions.

[0144] In Tables 3A to 3C, "average cooling rate after rolling" indicates the average cooling rate in the temperature range from the finish rolling completion temperature to 650°C. In Tables 3A to 3C, "intermediate air-cooling time" indicates the holding time in the temperature range from 580 to 650°C. In Tables 4A to 4E, "average cooling rate after annealing" indicates the average cooling rate in the temperature range from the annealing temperature to 650°C. In Tables 4A to 4E, "holding time after cooling" indicates the holding time in the temperature range from 500 to 650°C.

[0145] The obtained steel sheet was subjected to the above-mentioned method to determine the metal structure, carbide group, pearlite dispersion index, decarburization index D ＣThe results obtained are shown in Tables 4A to 4E. Note that underlines in the tables indicate that the results are outside the scope of the present disclosure, that the manufacturing conditions are not within the preferred range, or that the characteristic values ​​are unpreferable. Furthermore, in the metal structure of the steel sheet, the structure other than pearlite was one or more of ferrite, bainite, martensite, tempered martensite, carbide, and retained austenite. Furthermore, the Vickers hardness of the steel sheet according to the present invention at a position 1 / 4 of the thickness from the surface in the thickness direction was 380 HV or less.

[0146] Furthermore, the "number density of carbide groups" in Tables 4A to 4E and Tables 5A to 5E indicates the number density of carbide groups consisting of three carbides having a circle equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less at the 1 / 4 depth position.

[0147] From the obtained steel plate, a hot stamping blank having a width of 240 mm and a length of 200 mm was taken, and hot stamping was carried out under the conditions shown in Tables 5A to 5E to obtain a plate-shaped steel member. The hot stamping start temperature was 700°C or higher, and the demolding temperature (cooling stop temperature) was the Ms point or lower and less than 250°C. Some examples were subjected to a tempering treatment under the conditions shown in the table after hot stamping. The obtained steel member was measured by the above-mentioned methods for the metallographic structure, carbide group, standard deviation of the circle equivalent diameter of prior austenite grains, Vickers hardness, and Charpy impact value at -100°C (K －１００ ), and VDA bend angle (α). When measuring the Charpy impact value, the test specimens were taken so that the notch depth direction was perpendicular to the rolling direction of the steel sheet. When measuring the VDA bend angle, the test specimens were taken so that the bend ridge direction (parallel to the 30 mm side) was parallel to the rolling direction of the steel sheet.

[0148] The results are shown in Tables 5A to 5E. In Tables 5A to 5E, the structures other than the hard structures (martensite, bainite, and tempered martensite) were one or more of ferrite, pearlite, carbide, and retained austenite.

[0149] When the Vickers hardness at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction was 630 HV or more, the plate was judged to have high strength and pass, whereas when the Vickers hardness at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction was less than 630 HV, the plate was judged to have low strength and fail.

[0150] Charpy impact value at -100°C is 20 J / cm ２ If the Charpy impact value at -100°C was 20 J / cm or more, the specimen was judged to have excellent toughness and passed the test, and the result was recorded as "Good" in the table. ２ If the toughness was less than 1 / 2, the specimen was judged to have no excellent toughness and was rejected, and the result was recorded as "Bad" in the table.

[0151] When the VDA bending angle was 40° or more in terms of a plate thickness of 2.0 mm, it was determined that the sheet had particularly excellent bendability.

[0152]

[0153]

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[0172]

[0173] As can be seen from the above, the steel member according to the present disclosure has high strength and excellent toughness. Furthermore, it can be seen that the steel member having the above properties can be manufactured from the steel plate according to the present disclosure.

[0174] According to the above aspects of the present disclosure, it is possible to provide a steel member having high strength and excellent toughness, and a steel plate from which this steel member can be manufactured.

Claims

1. The chemical composition, in mass%, is: C: over 0.40% and not more than 1.00%, Si: not more than 2.00%, Mn: over 1.00% and not more than 3.00%, P: not more than 0.100%, S: not more than 0.0100%, Al: 0.001 to 1.000%, N: not more than 0.0200%, O: not more than 0.0100%, Nb: 0 to 0.100%, Ti: 0 to 0.200%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%, Co: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, V : 0 to 1.00%, Ca: 0 to 0.2000%, Mg: 0 to 0.2000%, REM: 0 to 0.3000%, Sb: 0 to 1.00%, Sn: 0 to 1.00%, Zr: 0 to 1.00%, As: 0 to 1.00%, Se: 0 to 1.0000%, Bi: 0 to 1.0000%, Ta: 0 to 1.0000%, Re: 0 to 1.0000%, Os: 0 to 1.0000%, Ir: 0 to 1.0000%, Tc: 0 to 1.0000%, and the balance: Fe and impurities, When a range from a position of 1 / 8 of the plate thickness from the surface to a position of 3 / 8 of the plate thickness, with a position of 1 / 4 of the plate thickness from the surface as the center, is defined as a 1 / 4 depth position, the metal structure at the 1 / 4 depth position is, in volume %, martensite, bainite, and tempered martensite: a total of 90% or more, and at the 1 / 4 depth position, the number density of a carbide group consisting of three carbides having a circle equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less is 10.0 × 10 －４ pieces / μm ２ a standard deviation of the circle-equivalent diameter of prior austenite grains is 6.0 μm or less.

2. The chemical composition is, in mass%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Cr: 0.01 to 1.00%, B: 0.0001 to 0.0100%, Mo: 0.01 to 1.00%, W: 0.01 to 2.00%, Co: 0.01 to 1.00%, Ni: 0.01 to 1.00%, 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%, 2. The steel member according to claim 1, further comprising at least one selected from the group consisting of Sn: 0.01 to 1.00%, Zr: 0.01 to 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 to 1.0000%, Ir: 0.0001 to 1.0000%, and Tc: 0.0001 to 1.0000%.

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

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

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

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

7. Chemical composition, in mass%, is: C: over 0.40% and not more than 1.00%, Si: not more than 2.00%, Mn: over 1.00% and not more than 3.00%, P: not more than 0.100%, S: not more than 0.0100%, Al: 0.001 to 1.000%, N: not more than 0.0200%, O: not more than 0.0100%, Nb: 0 to 0.100%, Ti: 0 to 0.200%, Cr: 0 to 1.00%, B: 0 to 0.0100%, Mo: 0 to 1.00%, W: 0 to 2.00%, Co: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, V : 0 to 1.00%, Ca: 0 to 0.2000%, Mg: 0 to 0.2000%, REM: 0 to 0.3000%, Sb: 0 to 1.00%, Sn: 0 to 1.00%, Zr: 0 to 1.00%, As: 0 to 1.00%, Se: 0 to 1.0000%, Bi: 0 to 1.0000%, Ta: 0 to 1.0000%, Re: 0 to 1.0000%, Os: 0 to 1.0000%, Ir: 0 to 1.0000%, Tc: 0 to 1.0000%, and the balance: Fe and impurities, When a range from a position of 1 / 8 of the plate thickness from the surface to a position of 3 / 8 of the plate thickness, with a position of 1 / 4 of the plate thickness from the surface as the center, is defined as a 1 / 4 depth position, the metal structure at the 1 / 4 depth position is composed of, in volume %, pearlite: 40% or more, and ferrite, bainite, martensite, tempered martensite, carbides, and retained austenite: 60% or less in total, and at the 1 / 4 depth position, the number density of a carbide group consisting of three carbides having a circle equivalent diameter of 0.5 μm or more and a center-to-center distance of 10 μm or less is 10.0 × 10 －３ pieces / μm ２ The steel sheet is characterized in that the dispersion index of the pearlite is 0.50 to 1.00 or less.

8. The chemical composition is, in mass%, Nb: 0.001 to 0.100%, Ti: 0.001 to 0.200%, Cr: 0.01 to 1.00%, B: 0.0001 to 0.0100%, Mo: 0.01 to 1.00%, W: 0.01 to 2.00%, Co: 0.01 to 1.00%, Ni: 0.01 to 1.00%, 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%, 8. The steel sheet according to claim 7, further comprising one or more elements selected from the group consisting of Sn: 0.01 to 1.00%, Zr: 0.01 to 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 to 1.0000%, Ir: 0.0001 to 1.0000%, and Tc: 0.0001 to 1.0000%.

9. Decarbonization indicator D Ｃ The steel sheet according to claim 7, wherein the value of σ is 0.10 or more.

10. Decarbonization indicator D Ｃ The steel sheet according to claim 8, wherein the value of σ is 0.10 or more.

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

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