Steel member

Abstract

This steel member has: a first steel material; one or more second steel materials; and a resistance spot welding part that joins the first steel material and one or more of the second steel materials, the resistance spot welding part having a nugget and an HAZ. The base material part of the first steel material has a prescribed chemical composition. At the 1 / 4-depth position, the number density of transition carbides having an equivalent circle diameter of at least 5 nm is at least 20 per μm2. The tensile strength is 2100 MPa or greater. At a prescribed position in the HAZ, the number density of transition carbides having an equivalent circle diameter of at least 5 nm is at least 20 per μm2. In the resistance spot welding part, the difference ΔHV between the Vickers hardness at a position 50 μm toward the nugget side from the boundary between the nugget and the HAZ and the Vickers hardness at a position 500 μm toward the nugget side from said boundary is less than 70 HV.

Description

steel parts

[0001] This application claims priority from Japanese Patent Application No. 2024-100128, 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 has been adopted as a technique for manufacturing automobile parts with very high strength and high dimensional accuracy, as disclosed in, for example, Patent Document 1. Hot stamping is a hot forming technique in which a steel sheet to be formed is heated and then formed.

[0005] In this technique, the steel sheet is heated before being formed. Therefore, the steel sheet is soft during forming and has good formability. This allows even high-strength steel sheets to be formed into complex shapes with high precision. Furthermore, in hot stamping, quenching is performed simultaneously with forming using a press die, resulting in a metal structure mainly composed of martensite, and the steel material after forming (hot-stamped product) has sufficient strength.

[0006] For example, Patent Document 1 discloses that hot stamping can impart a tensile strength of 1400 MPa or more to a steel member (hot-stamped body) obtained by forming a steel plate.

[0007] In recent years, countries around the world have been experiencing higher CO ２ Automobile companies have set reduction targets and are working to reduce fuel consumption while taking collision safety into consideration. Higher strength materials are required for vehicle bodies, not only for gasoline-powered vehicles but also for electric vehicles, which are being rapidly developed, in order to protect not only passengers but also batteries from collisions and to offset the resulting weight increase. For example, hot-stamped steel sheets used in automobiles and the like require higher-strength steel sheets (tensile strength exceeding 1500 MPa) than those described in the aforementioned Patent Document 1 and those generally used for hot-stamped steel sheets currently formed by hot stamping.

[0008] However, as the strength of a hot stamped steel sheet increases, its toughness, ductility, and bendability tend to deteriorate, and its impact absorption capacity during a collision tends to decrease.

[0009] In response to these problems, Patent Document 2 discloses a hot-stamped steel sheet having a tensile strength of 2000 MPa or more. Patent Document 2 discloses that a hot-stamped steel sheet having excellent strength and toughness can be obtained by performing a double heat treatment to set the average grain size of prior austenite grains to 5.0 μm or less and the average Mn concentration at the grain boundaries of the prior austenite grains to 1.0 mass % or less.

[0010] Furthermore, when a hot-stamped steel sheet is applied to an automobile component, it is joined to other steel materials by resistance spot welding or the like. The joint strength of a resistance spot weld tends to decrease as the strength of the steel material used increases.

[0011] To address this issue, for example, Patent Document 3 discloses a technique for improving cross tensile strength in a spot-welded joint formed from multiple steel plates, including at least one steel plate with a tensile strength of 750 to 2500 MPa.

[0012] However, the technology of Patent Document 3 requires the addition of current to heat-treat the welded portion, which extends the time required to complete the joining, impairing productivity and potentially limiting the welding locations to which the technology can be applied.

[0013] Japanese Patent Publication No. 2002-102980 Japanese Patent Publication No. 6966023 International Publication No. 2014 / 196499

[0014] A steel member in which at least one steel material constituting the steel member has both high strength and excellent impact absorption, and in which the joint strength at the resistance spot welds is also excellent, has not been proposed until now. Therefore, an object of the present invention is to provide a steel member in which at least one steel material constituting the steel member has both high strength and excellent impact absorption, and in which the joint strength at the resistance spot welds is also excellent.

[0015] The present inventors have investigated methods for improving the impact absorption of high-strength hot-stamped steel sheets. As a result, they have found that precipitating transition carbides in the hot-stamped steel sheet improves the bendability of the steel sheet constituting the sheet and enhances its impact absorption. They have also found that in steel members having resistance spot welds obtained using the high-strength hot-stamped steel sheet, precipitating transition carbides at predetermined positions in the HAZ and suppressing softening near the fusion boundary of the nugget increases the joint strength.

[0016] The present invention has been made in view of the above findings. The gist of the present invention is as follows. [1] A steel member according to one aspect of the present invention is a steel member having a first steel material that is a hot-stamped product, one or more second steel materials, and a resistance spot welded portion that joins the first steel material and the one or more second steel materials and includes a nugget and a HAZ, wherein a base metal portion of the first steel material that is a portion other than the resistance spot welded portion contains, in mass %, C: 0.40 to 1.00%, Si: 0.01 to 2.00%, Mn: 1.00% or more but less than 5.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0150% or less, O: 0.0100% or less, B: 0 to 0.0100%, Cr: 0 to 0.50%, and Mo: 0 to 0.50%. , W: 0-3.00%, Ti: 0-0.100%, Nb: 0-0.100%, Co: 0-1.000%, Ni: 0-1.00%, Cu: 0-1.00%, V: 0- 0.500%, Ca: 0-1.0000%, Mg: 0-1.0000%, REM: 0-0.0050%, Sn: 0-0.100%, Sb: 0-0.020%, Zr : 0 to 0.100%, As: 0 to 0.100%, and the balance: Fe and impurities, and when a position at 1 / 4 of the thickness from the surface of the first steel material in the thickness direction is defined as a 1 / 4 depth position, the number density of transition carbides having a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position of the base material of the first steel material is 20 pieces / μm ２ or more, the tensile strength of the base metal portion of the first steel material is 2100 MPa or more, and when a position 0.2 mm from an end of a contact surface between the first steel material and the second steel material in the HAZ of the resistance spot weld is set as a reference point toward the first steel material in the thickness direction of the first steel material, and a virtual line is drawn passing through the reference point in a direction perpendicular to the thickness direction of the first steel material, at a position 0.2 mm from the reference point along the virtual line toward the nugget, the number density of transition carbides having a circle equivalent diameter of 5 nm or more is 20 / μm ２In the resistance spot weld, a difference ΔHV between the Vickers hardness at a position 50 μm toward the nugget from the boundary between the nugget and the HAZ on the imaginary line and the Vickers hardness at a position 500 μm toward the nugget from the boundary is less than 70 HV. [2] In the steel member described in [1], when a position 20 μm from the surface of the first steel material in the thickness direction is defined as a 20 μm depth position, the hardness of the base material portion of the first steel material at the 20 μm depth position may be lower than the hardness of the base material portion of the first steel material at the 1 / 4 depth position. [3] In the steel member described in [2], the hardness of the base material portion of the first steel material at the 20 μm depth position may be lower by 100 HV or more in Vickers hardness than the hardness of the base material portion of the first steel material at the 1 / 4 depth position. [4] The steel member according to any one of [1] to [3], wherein the chemical composition of the base material is, in mass%, B: 0.0002 to 0.0100%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, W: 0.01 to 3.00%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, Co: 0.010 to 1.000%, Ni: 0.01 to 1.00%, Cu: 0. 0.01 to 1.00%, V: 0.010 to 0.500%, Ca: 0.0001 to 1.0000%, Mg: 0.0001 to 1.0000%, REM: 0.0001 to 0.0050%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.020%, Zr: 0.001 to 0.100%, and As: 0.001 to 0.100%.[5] The steel member according to any one of [1] to [4], wherein a base metal portion of the second steel material other than the resistance spot welds contains, in mass%, C: 0.40 to 1.00%, Si: 0.01 to 2.00%, Mn: 1.00% or more and less than 5.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0150% or less, O: 0.0100% or less, B: 0 to 0.0100%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, W: 0 to 3.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Co: 0 to 1.0 and the balance: Fe and impurities, and when a position at ¼ of the thickness from the surface of the second steel material in the thickness direction is defined as a ¼ depth position, the number density of transition carbides having a circle equivalent diameter of 5 nm or more at the ¼ depth position of the base material portion of the second steel material is 20 pieces / μm. ２ or more, the tensile strength of the base metal portion of the second steel material is 2100 MPa or more, and when a position 0.2 mm from an end of a contact surface between the first steel material and the second steel material in the HAZ of the resistance spot weld is set as a reference point toward the second steel material in the thickness direction of the second steel material, and a virtual line is drawn passing through the reference point in a direction perpendicular to the thickness direction of the second steel material, at a position 0.2 mm from the reference point along the virtual line toward the nugget, the number density of transition carbides having a circle equivalent diameter of 5 nm or more is 20 / μm ２ It may be more than that.

[0017] According to the above aspect of the present invention, a steel member can be obtained in which at least one steel material constituting the steel member has both high strength and excellent impact absorption, and the joint strength at the resistance spot welds is also excellent.

[0018] FIG. 1 is an example of a schematic diagram of a cross section at a position including a resistance spot weld of a steel member according to the present embodiment. FIG. 2 is an example of a schematic diagram of a cross section at a position including a resistance spot weld of a steel member according to the present embodiment, and is an example where steel members corresponding to a first steel material and a second steel material are both the first steel material in the present embodiment. FIG. 3 is an example of a schematic diagram of a cross section at a position including a resistance spot weld of a steel member according to the present embodiment, and is an example where the first steel material is sandwiched between two second steel materials. FIG. 4 is an example of a shape of the first steel material of the steel member according to the present embodiment. FIG. 5 is a diagram explaining the sampling position when test pieces for joint strength measurement are sampled from the steel member according to the present embodiment. FIG. 6 is a diagram explaining the sampling position when test pieces for joint strength measurement are sampled from the steel member according to the present embodiment.

[0019] A steel member according to one embodiment of the present invention (steel member according to this embodiment) will be described. As shown in Fig. 1 , a steel member 100 according to this embodiment has a first steel material 11, a second steel material 12, and a resistance spot weld 13 that joins the first steel material 11 and the second steel material 12. The resistance spot weld 13 has a HAZ 2 and a nugget 3. As shown in Fig. 3 , the first steel material 11 may be sandwiched between two second steel materials 12. Each of these will be described below.

[0020] <First steel material> In this embodiment, the position at 1 / 4 of the thickness from the surface of the first steel material in the thickness direction is referred to as the 1 / 4 depth position, and the position at 20 μm from the surface in the thickness direction is referred to as the 20 μm depth position. The first steel material 11 is a hot stamped product obtained by hot stamping a steel plate. This first steel material 11 has a predetermined chemical composition described later, and at the 1 / 4 depth position, the number density of transition carbides having a circle equivalent diameter of 5 nm or more is 20 / μm ２ and the tensile strength of the base material portion of the first steel material is 2100 MPa or more. In the base material portion of the first steel material, it is preferable that the hardness at a depth of 20 μm is smaller than the hardness at the ¼ depth position. The chemical composition, number density of transition carbides, hardness, and tensile strength are all specified for the base material portion of the first steel material, i.e., the portion other than the resistance spot welds of the first steel material.

[0021] In the steel member according to this embodiment, the base material portion and the resistance spot welded portion of the first steel material are distinguished by hardness and microstructural observation. Specifically, the welded portion is cut so that a plate thickness cross section (a cross section parallel to the thickness direction) passing through the center of the welded portion can be observed. The cross section is then mirror-polished and etched with a picric acid solution to reveal the microstructure. Within the revealed microstructure, a portion exhibiting a solidified microstructure is determined to be a portion that was once melted and solidified by welding, and this region is designated as the nugget. Additionally, the Vickers hardness distribution is measured around the nugget in this cross section. The Vickers hardness at a depth of 1 / 4 or 3 / 4, 10 mm or more away from the edge of the nugget in a direction perpendicular to the thickness direction, is designated as the base material hardness. The region excluding the nugget that is softer than the base material hardness is designated as the HAZ-softened portion, and the region closer to the nugget than the HAZ-softened portion that exhibits a hardness equal to or greater than the base material hardness is designated as the HAZ-hardened portion. The boundary between the HAZ-hardened portion and the nugget is designated as the fusion boundary. The HAZ-softened and HAZ-hardened areas are collectively referred to as the HAZ. The HAZ-softened and HAZ-hardened areas are areas affected by the heat of welding and are included in the resistance spot weld along with the nugget. The conditions for Vickers hardness measurement are not limited as long as they allow accurate measurement of hardness; for example, the measurement interval can be 0.2 mm and the measurement load can be 1.96 N. The area other than the resistance spot weld is considered to be the base material.

[0022] (Chemical composition) The base material portion of the first steel material has a chemical composition containing the following elements. Numerical ranges defined by "to" include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" do not include the values ​​in the numerical range. Hereinafter, unless otherwise specified, percentages regarding the content of elements are mass%.

[0023] C: 0.40 to 1.00% C is an element that improves the hardenability of steel and the strength of the hot-stamped body obtained after hot stamping the steel sheet. If the C content is less than 0.40%, it becomes difficult to ensure sufficient strength in the hot-stamped body. Therefore, the C content is set to 0.40% or more. The C content is preferably set to 0.42% or more, 0.44% or more, or 0.45% or more. On the other hand, if the C content exceeds 1.00%, the strength of the hot-stamped body becomes too high, which may result in reduced bendability and ductility. Therefore, the C content is set to 1.00% or less. The C content is preferably set to 0.75% or less, 0.65% or less, 0.60% or less, or 0.55% or less.

[0024] Si: 0.01 to 2.00% Si is an element effective in improving the hardenability of steel and ensuring stable strength of hot-stamped products. To achieve the above effects, the Si content is set to 0.01% or more. The Si content is preferably set to 0.05% or more, 0.10% or more, or 0.20% or more. On the other hand, if the Si content in steel exceeds 2.00%, the heating temperature required for austenite transformation during heat treatment (quenching) becomes significantly higher. This may increase the cost of heat treatment or reduce the strength of the hot-stamped product due to residual ferrite during heating. Furthermore, if the Si content exceeds 2.00%, the behavior of scale formation during steel sheet production may change, impairing the appearance of the product surface. Therefore, the Si content is set to 2.00% or less. The Si content is preferably set to 1.50% or less, 1.00% or less, or 0.70% or less.

[0025] Mn: 1.00% or more, less than 5.00% Mn contributes to improving the strength of hot-stamped steel through solid-solution strengthening. Furthermore, Mn is a highly effective element for improving the hardenability of steel and ensuring stable strength of hot-stamped steel. If the Mn content is less than 1.00%, the volume fraction of martensite in the metal structure of the hot-stamped steel may decrease when the cooling rate after hot stamping is low, potentially resulting in insufficient strength of the hot-stamped steel. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably set to 1.20% or more, 1.30% or more, or 1.50% or more. On the other hand, if the Mn content is 5.00% or more, coarse inclusions such as MnS are likely to form in the steel, potentially resulting in reduced bendability and ductility. Furthermore, even if the steel member manufacturing method described below is applied, it may be difficult to favorably control the precipitation state of transition carbides and the hardness distribution near the fusion boundary, making it difficult to ensure impact absorption and joint strength of resistance spot welds. Therefore, the Mn content is set to less than 5.00%, and is preferably set to 4.50% or less, 4.00% or less, 3.00% or less, or 2.50% or less.

[0026] 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 hot-stamped steel. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less, 0.035% or less, or 0.010% or less. There is no need to particularly limit the lower limit of the P content, and the lower limit is 0%. However, reducing the P content to less than 0.001% significantly increases the dephosphorization cost, which is economically undesirable. In practical operation, the P content may be set to 0.001% or more.

[0027] S: 0.0100% or less S is an element that forms inclusions in steel. If the S content exceeds 0.0100%, a large amount of inclusions will be generated in the steel, reducing the toughness of the hot-stamped steel. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0040% or less, 0.0020% or less, or 0.0010% or less. There is no need to particularly limit the lower limit of the S content, and the lower limit is 0%. However, reducing the S content to less than 0.0001% significantly increases the desulfurization cost, which is economically undesirable. In practical operation, the S content may be set to 0.0001% or more, or 0.0002% or more.

[0028] Al: 0.001 to 1.000% Al is an element that deoxidizes molten steel to improve its soundness (suppressing the occurrence of defects such as blowholes in the steel). If the Al content is less than 0.001%, deoxidation is insufficient. Therefore, the Al content is set to 0.001% or more. The Al content is preferably 0.010% or more or 0.020% or more. On the other hand, if the Al content exceeds 1.000%, coarse oxides and nitrides are formed in the steel, reducing the toughness of the hot-stamped steel. Furthermore, the heating temperature required for austenite transformation during heat treatment (quenching) becomes significantly higher. This may increase the cost of heat treatment or result in residual ferrite during heating, reducing the strength of the hot-stamped steel. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.500% or less, 0.300% or less, or 0.100% or less.

[0029] N: 0.0150% or less N is an element that forms nitrides in steel. Because these nitrides become the starting point of fracture, the N content is set to 0.0150% or less. The N content is preferably set to 0.0100% or less or 0.0050% or less. There is no need to specify a lower limit for the N content, and the lower limit is 0%. However, reducing the N content to less than 0.0001% significantly increases the cost of denitrification, which is economically undesirable. Therefore, the N content may be set to 0.0001% or more, 0.0004% or more, or 0.0010% or more.

[0030] O: 0.0100% or less O is an element that, when contained in large amounts in steel, forms coarse oxides that become fracture initiation sites and deteriorate the toughness of the hot-stamped steel. Therefore, the O content is set to 0.0100% or less. The O content is preferably set to 0.0050% or less, 0.0030% or less, or 0.0020% or less. There is no need to specify a lower limit for the O content, and the lower limit is 0%. However, reducing the O content to less than 0.0001% significantly increases the cost of deoxidation, which is economically undesirable. Therefore, the O content may be set to 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0031] The hot-stamped steel according to this embodiment may have a chemical composition containing the above elements (basic elements), with the balance being Fe and impurities. On the other hand, for the purpose of improving various properties, one or more of the following elements (optional elements) may also be contained. Since the optional elements do not necessarily need to be contained, the lower limit of the content is 0%.

[0032] B: 0 to 0.0100% B is an element that has the effect of improving the hardenability of steel even in small amounts. Furthermore, B segregates at grain boundaries, strengthening the grain boundaries. Therefore, B may be added. To obtain the above effects, the B content is preferably 0.0002% or more. The B content is more preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, if the B content exceeds 0.0100%, a large amount of coarse compounds precipitates, reducing the toughness of the hot-stamped steel. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0033] Cr: 0 to 0.50% Cr is an effective element for improving the hardenability of steel and stably ensuring the strength of hot-stamped products. Therefore, Cr may be added. To obtain the above effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, if the Cr content exceeds 0.50%, the amount of coarse carbide precipitation increases in the hot-stamping steel sheet. Furthermore, alloy costs increase. Therefore, the Cr content is set to 0.50% or less. The Cr content is preferably set to 0.45% or less, 0.40% or less, or 0.30% or less.

[0034] Mo: 0 to 0.50% Mo is a highly effective element for improving the hardenability of steel and ensuring stable strength of hot-stamped steel. In particular, when added in combination with B, a synergistic effect of improving hardenability is achieved. Therefore, Mo may be added. To achieve the above effect, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more, 0.07% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Mo content exceeds 0.50%, the amount of coarse carbide precipitation increases in the hot stamping steel sheet. Furthermore, alloying costs increase. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.45% or less, 0.40% or less, or 0.35% or less.

[0035] W: 0 to 3.00% W is an element effective in improving the hardenability of steel and stably ensuring the strength of hot-stamped steel. To achieve the above effects, the W content is preferably 0.01% or more. The W content is more preferably 0.10% or more, 0.15% or more, 0.20% or more, or 0.25% or more. On the other hand, if the W content exceeds 3.00%, the amount of coarse carbide precipitates increases in the steel sheet for hot stamping. In addition, the alloy cost increases. Therefore, the W content is set to 3.00% or less. The W content is preferably 2.00% or less, 1.00% or less, or 0.50% or less.

[0036] Ti: 0 to 0.100% Ti forms fine carbides, carbonitrides, etc. with Nb in steel, and their grain refinement suppresses Cu hot embrittlement cracking during the hot rolling process and improves the hydrogen embrittlement resistance of the hot-stamped steel. Ti also preferentially bonds with N in the steel to form nitrides, suppressing the consumption of solute B due to the precipitation of BN and promoting the hardenability-improving effect of B. Therefore, Ti may be added. To achieve the above effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, if the Ti content exceeds 0.100%, excessive coarse TiN is formed, deteriorating the toughness of the hot-stamped steel. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.

[0037] Nb: 0 to 0.100% Nb is an element that improves the strength of the hot-stamped body through solid solution strengthening and contributes to the refinement of prior austenite grains by forming carbonitrides. Therefore, Nb may be added as necessary. When Nb is added, the Nb content is preferably 0.001% or more to ensure the above effects. The Nb content is more preferably 0.010% or more, 0.020% or more, 0.025% or more, or 0.035% or more. On the other hand, if the Nb content exceeds 0.100%, excessive coarse carbides may be formed, which may suppress the formation of transition carbides that contribute to improving the impact absorption and joint strength of the hot-stamped body. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.090% or less, 0.070% or less, or 0.050% or less.

[0038] Co: 0 to 1.000% Co is an element that has the effect of raising the martensitic transformation start temperature (Ms point) and improving the toughness of the hot-stamped body. Therefore, Co may be contained. To obtain the above effect, the Co content is preferably 0.010% or more. The Co content is more preferably 0.050% or more, 0.150% or more, or 0.250% or more. On the other hand, Co is an expensive element, and adding a large amount increases the alloy cost. Therefore, the Co content is set to 1.000% or less. The Co content is preferably 0.800% or less, 0.600% or less, or 0.400% or less.

[0039] Ni: 0 to 1.00% Ni is an element effective for improving the hardenability of steel and stably ensuring the strength of hot-stamped steel sheets. Ni also has the effect of suppressing Cu hot embrittlement cracking during the production of steel sheets. Therefore, Ni may be added. To obtain the above effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, if the Ni content exceeds 1.00%, the above effects saturate and alloy costs increase. Therefore, the Ni content is set to 1.00% or less. The Ni content is preferably 0.90% or less, 0.80% or less, 0.50% or less, or 0.20% or less.

[0040] Cu: 0 to 1.00% Cu is an element effective for improving the hardenability of steel and stably ensuring the strength of hot-stamped bodies. Cu also improves corrosion resistance in corrosive environments. Therefore, it may be added. To obtain the above effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.10% or more, 0.15% or more, 0.20% or more, or 0.30% or more. On the other hand, if the Cu content exceeds 1.00%, the above effects saturate and alloy costs increase. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less, 0.60% or less, or 0.40% or less.

[0041] V: 0 to 0.500% V is an element that improves the strength of the hot-stamped steel sheet through solid solution strengthening. Therefore, V may be added. To reliably obtain the above effect, the V content is preferably 0.010% or more. The V content is more preferably 0.050% or more, 0.100% or more, or 0.150% or more. On the other hand, if the V content exceeds 0.500%, excessive carbonitrides are formed, deteriorating the toughness of the hot-stamped steel sheet. Therefore, the V content is set to 0.500% or less. The V content is preferably 0.450% or less, 0.400% or less, or 0.350% or less.

[0042] Ca: 0 to 1.0000% Ca is an element that improves the deformability of the hot-stamped steel by adjusting the shape of inclusions. Therefore, it may be added. To obtain the above effect, the Ca content is preferably 0.0001% or more, and more preferably 0.0005% or more, 0.0010% or more, or 0.0015% or more. However, even if a large amount is added, the above effect saturates and excessive costs are incurred, so the Ca content is set to 1.0000% or less. The Ca content is preferably 0.0400% or less, 0.0200% or less, 0.0150% or less, 0.0100% or less, or 0.0050% or less.

[0043] Mg: 0 to 1.0000% Mg is an element that has the effect of deoxidizing molten steel and improving the quality of the steel. Therefore, Mg may be added. To obtain the above effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the Mg content exceeds 1.0000%, oxides in the steel increase, adversely affecting the toughness of the hot-stamped steel. Therefore, the Mg content is set to 1.0000% or less. The Mg content is preferably 0.0400% or less, 0.0200% or less, 0.0150% or less, 0.0100% or less, or 0.0050% or less.

[0044] REM: 0 to 0.0050% REM is an element that improves the deformability of hot-stamped steel by adjusting the shape of inclusions. Therefore, it may be added. To obtain the above-mentioned effect, the REM content is preferably 0.0001% or more, and more preferably 0.0010% or more. However, even if a large amount is added, the above-mentioned effect saturates and excessive costs are incurred, so the REM content is set to 0.0050% or less. The REM content is preferably 0.0040% or less, or 0.0020% or less. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements.

[0045] Sn: 0 to 0.100% Sn is an element that improves hydrogen embrittlement resistance. Therefore, it may be contained. To obtain the above effects, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.010% or more or 0.030% or more. On the other hand, if the Sn content exceeds 0.100%, the effect saturates and the alloy cost increases. Therefore, if Sn is contained, the Sn content is 0.100% or less. The Sn content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.

[0046] Sb: 0 to 0.020% Sb is an element that has the effect of improving the corrosion resistance of the hot-stamped steel. Therefore, Sb may be added. To obtain the above effect, the Sb content is preferably 0.001% or more, and more preferably 0.005% or more. However, even if a large amount is added, the above effect saturates, so the Sb content is set to 0.020% or less. The Sb content is preferably 0.015% or less.

[0047] Zr: 0 to 0.100% Zr is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and improves the toughness of hot-stamped steel sheets. Therefore, it may be added. To obtain the above effects, the Zr content is preferably 0.001% or more, and more preferably 0.010% or more or 0.020% or more. On the other hand, if Zr is added in large amounts, deterioration of surface properties may become apparent. Therefore, the Zr content is set to 0.100% or less. The Zr content is preferably 0.080% or less or 0.050% or less.

[0048] As: 0 to 0.100% As is an element that inhibits grain boundary migration and suppresses grain growth, thereby reducing the crystal grain size after annealing and the prior austenite grain size after hot stamping, thereby improving the deformability of the hot-stamped steel. Therefore, As may be added. To obtain the above effect, the As content is preferably 0.001% or more, more preferably 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, excessive As content reduces hot ductility and may cause cracking during casting and hot rolling, so the As content is set to 0.100% or less. The As content is preferably 0.060% or less.

[0049] Balance: Fe and impurities In the chemical composition of the base material of the first steel material according to this embodiment, the elements other than those described above, i.e., the balance, are Fe and impurities. That is, the base material of the first steel material according to this embodiment may have a chemical composition containing the basic elements with the balance being Fe and impurities, or may have a chemical composition containing the basic elements and one or more optional elements with the balance being Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of steel sheets due to various factors in the raw materials, such as ores and scrap, and the manufacturing process, and are acceptable within a range that does not adversely affect the properties of the steel member according to this embodiment. The industrial production method includes the blast furnace steelmaking method and the electric furnace steelmaking method, and includes the level (impurity level) of components mixed in during production by either method.

[0050] The chemical composition of the base metal portion of the first steel product can be determined by the following method. A test piece is taken from the base metal portion of the first steel product in the range of 1 / 4 depth position to 3 / 4 depth position, and the test piece is subjected to elemental analysis by a general method such as ICP-AES. C and S, which are difficult to measure with ICP-AES, 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.

[0051] (Number Density of Transition Carbides) In the steel member according to this embodiment, the number density of transition carbides having a circle equivalent diameter of 5 nm or more at a ¼ depth position of the base material portion of the first steel material is 20 pieces / μm ２ That's all. A high-strength hot-stamped steel sheet needs to contain a large amount of C and other alloy elements to obtain high tensile strength. However, as strength increases, the toughness of the hot-stamped steel sheet generally decreases, resulting in a decrease in impact absorption. In response to this, the inventors have conducted studies and found that impact absorption can be improved by allowing transition carbides to exist in the hot-stamped steel sheet and controlling their size and number density. Specifically, transition carbides with a circle equivalent diameter of 5 nm or more to be present in a density of 20 particles / μm ２ Therefore, it has been found that the impact absorption property is improved when the transition carbides having a circle equivalent diameter of 5 nm or more are present at a number density of 20 / μm or more at a ¼ depth position of the base material part of the first steel material constituting the steel member according to the present embodiment, which is a hot stamped body. ２ or more. Here, transition carbide is a general term for iron carbides other than cementite that are generated when martensite is tempered, and includes ε carbide, η carbide, and χ carbide. The reason why transition carbides with a circle equivalent diameter of 5 nm or more are targeted is because transition carbides smaller than this do not sufficiently improve the impact absorption properties. Furthermore, even if the circle equivalent diameter of a transition carbide is 5 nm or more, if the number density is 20 particles / μm ２ If the density is less than 23 particles / μm, the effect is not sufficient. ２Although there is no upper limit to the circle-equivalent diameter of the transition carbides, if the size is too large, it becomes difficult to obtain a sufficient number density, which is not preferable.

[0052] The circle-equivalent diameter of transition carbides and the number density of transition carbides with circle-equivalent diameters of 5 nm or more can be determined by observing a thin-film TEM observation sample using a field emission transmission electron microscope (FE-TEM) (JEM-2100F manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray spectroscopy (EDX). Specifically, a test piece approximately 10 mm square is taken from the base material of the first steel material, and both sides are mechanically or chemically polished to prepare a thin-film sample (approximately 60 μm thick, φ3 mm) at a 1 / 4 depth position (a range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable). Then, double-sided jet electrolytic polishing is performed, leaving a hole in the center, to obtain a thin-film TEM observation sample. Thin-film TEM observation is performed at an accelerating voltage of 200 kV. To reduce variation between locations, at least five fields of view of approximately 100 to 300 nm square are observed for the sample. The type of observed precipitates is identified using the diffraction pattern and the results of EDX analysis. To reduce the variation in number density depending on the field of view, the number density in each field of view is counted and the average value is used as the representative value of the number density.

[0053] (Hardness) In the base material portion of the first steel material, the hardness at a depth of 20 μm is preferably smaller than that at a ¼ depth. Lowering (softening) the hardness near the surface further improves impact absorption. Although an effect can be obtained if the hardness at a depth of 20 μm is smaller than that at a ¼ depth, to obtain a more pronounced effect, it is more preferable that the hardness at the depth of 20 μm is 100 HV or more lower in Vickers hardness than that at the ¼ depth. The hardness at the depth of 20 μm can be reduced, for example, by decarburizing the surface in an annealing process during the production of the steel sheet, as described below. The reason for targeting the depth of 20 μm is that, due to the measurement principle, the hardness near the outermost surface varies greatly.

[0054] The Vickers hardness at the 1 / 4 depth position and the 20 μm depth position is measured by the following method. A test piece is taken from the base material portion of the first steel material, and the plate thickness cross section of the steel plate is buffed. Then, a Vickers hardness test is performed in accordance with JIS Z2244-1:2020. The Vickers hardness test is performed using a micro Vickers hardness tester, with a load of 0.098 N and a load holding time of 10 seconds. For each depth position, five points are measured with the measurement points spaced at least three times the size of the indentation, and the average of the three points excluding the maximum and minimum values ​​is calculated to determine the Vickers hardness at each depth position.

[0055] In the first steel member according to this embodiment, the constituent phases of the metal structure of the steel plate constituting the steel member are not limited, but it is preferable that the steel member have the metal structure shown below. In the first steel member according to this embodiment, the base material portion preferably has a metal structure containing the amount of martensite shown below. In the following description of the metal structure, "%" means "volume %." The metal structure at the 1 / 4 depth position of the base material portion preferably contains more than 90.0% martensite. Since martensite is an effective structure for increasing the tensile strength of the steel plate after hot stamping, the volume fraction of martensite is preferably more than 90.0%. If the volume fraction of martensite is 90.0% or less, the tensile strength of the base material portion of the first steel member may be less than 2100 MPa, resulting in insufficient strength. The volume fraction of martensite is more preferably more than 91.0%, more than 93.0%, or more than 95.0%. There is no need to particularly specify an upper limit for the volume fraction of martensite. However, to significantly increase the volume fraction of martensite, it is necessary to excessively increase the heating temperature or cooling rate of the hot stamping steel sheet during the hot stamping process, which significantly impairs the productivity of the hot-stamped steel sheet (first steel material). Therefore, the volume fraction of martensite is preferably 99.0% or less or 98.0% or less. In this embodiment, martensite includes not only untempered fresh martensite but also tempered martensite containing iron carbides. The remainder of the metal structure may contain ferrite, pearlite, bainite, or retained austenite, and may further contain precipitates such as cementite or oxides present alone. Since ferrite, pearlite, bainite, retained austenite, and precipitates are not required to be present, the lower limits of the volume fractions of ferrite, pearlite, bainite, retained austenite, and precipitates are all 0%. The volume fraction of transition carbides is included in the volume fraction of the structure containing transition carbides (martensite, ferrite, pearlite, bainite, and retained austenite). The retained austenite has the effect of improving the ductility of the steel sheet after hot stamping.To achieve this effect, it is preferable that the volume fraction of retained austenite is 0.5% or more, 1.0% or more, or 2.0% or more. On the other hand, excessively increasing the volume fraction of retained austenite requires austempering at high temperature after hot stamping, which significantly reduces the productivity of the hot-stamped body (first steel material). Furthermore, if retained austenite is contained in excess, the deformability of the base metal portion of the first steel material may deteriorate. Therefore, it is preferable that the volume fraction of retained austenite is less than 9.0%, less than 7.0%, less than 5.0%, or less than 4.0%.

[0056] In this embodiment, the volume fractions of each structure are measured using the following method. A test specimen is taken from the base material of the first steel material, and the cross-section of the steel plate constituting the steel material is buffed. Then, the structure is observed at a 1 / 4 depth position (a depth of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable). Specifically, the polished surface is subjected to nital corrosion or electrolytic polishing, and then a structural photograph is taken using an optical microscope and a scanning electron microscope (SEM). Image analysis of the obtained structural photograph is performed based on brightness differences or differences in the morphology of iron carbides present within the phases to obtain the area fractions of ferrite, pearlite, bainite, tempered martensite, and precipitates. Subsequently, similar observation positions are subjected to Lepera corrosion, and a structural photograph is taken using an optical microscope or a scanning electron microscope (SEM). Image analysis of the obtained structural photograph is performed to calculate the total area fraction of "retained austenite and fresh martensite." Furthermore, after electrolytic polishing of the thickness cross section at the same observation position, the area fraction of retained austenite was measured using an SEM equipped with an electron backscatter pattern analyzer (EBSP). The area fraction of retained austenite was obtained by calculating the area fraction of regions with an fcc crystal structure from the crystal orientation information obtained by EBSP analysis. The area fraction of fresh martensite was obtained by subtracting the area fraction of retained austenite from the sum of the area fractions of the above-mentioned "retained austenite and fresh martensite." Based on these results, the area fractions of ferrite, pearlite, bainite, martensite (tempered martensite and fresh martensite), retained austenite, and precipitates were obtained. The area fractions were then considered to be equivalent to the volume fractions of each structure. In structural observation, tempered martensite can be distinguished from fresh martensite by the presence of iron carbides within it. Tempered martensite can also be distinguished from bainite by the fact that the iron carbides present within it are elongated in multiple directions rather than in a single direction. Stretching in a single direction means that the difference between the stretching directions is within 5°.

[0057] [Coating] The first steel material of the steel member according to this embodiment may have a coating on a portion or all of its surface. The coating may be a coating primarily made of an Fe—Al alloy, or a coating primarily made of an Fe—Zn alloy. The coating is also referred to as a film, an alloyed plating layer, or an intermetallic compound layer. A coating primarily made of an Fe—Al alloy is a coating containing 70% by mass or more of Fe and Al in total, and a coating primarily made of an Fe—Zn alloy is a coating containing 70% by mass or more of Fe and Zn in total. The coating primarily made of an Fe—Al alloy may further 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. The coating primarily made of an Fe-Zn 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. The coating provides corrosion resistance, which has the effect of improving hydrogen embrittlement resistance when used in automobiles. The thickness of the coating is preferably 5 to 100 μm.

[0058] The chemical composition and thickness of the coating can be determined by observing the cross section using a scanning electron microscope and an electron probe microanalyzer (EPMA). The observation range is, for example, 400 times magnification and 40,000 μm in area. ２The thickness of the test piece cut from the base material of the first steel material is mechanically polished and then mirror-finished. The thickness of the coating is then measured at 20 locations using a BSE image (or COMPO image) at a measurement interval of 6.5 μm, and the average of the measured values ​​is calculated. Observation using a BSE image (or COMPO image) reveals a clear contrast difference between the coating and the base steel (steel sheet substrate). Therefore, the thickness of the coating can be measured by measuring the thickness from the outermost surface to the point where the contrast changes. Ten fields of view are observed in the same manner as above, and the average of the 10 fields of view is used to determine the coating thickness. The chemical composition of the coating can be determined by performing spot elemental analysis (beam diameter: 1 μm or less) using an electron probe microanalyzer (EPMA) on the same observation area as above to determine the Fe, Al, and Zn contents in the coating. A total of 10 points are analyzed in any 10 fields of view, and the average values ​​are used to determine the Fe, Al, and Zn contents in the coating. Even when elements other than Fe, Al, and Zn are contained, the same method is used to determine the content.

[0059] The reference surface for the above-mentioned 1 / 4 depth position and 20 μm depth position is the surface of the first steel material, but if the first steel material has a coating, the surface means the surface of the base steel material excluding the coating.

[0060] (Tensile Strength) The tensile strength of the base material portion of the first steel material according to this embodiment is 2100 MPa or more, taking into consideration its contribution to improving fuel economy and collision safety when applied to automotive parts. The tensile strength of the base material portion of the first steel material is preferably 2200 MPa or more, 2300 MPa or more, or 2500 MPa or more. There is no upper limit to the tensile strength, but if the tensile strength is too high, there is a concern that the impact absorption performance will decrease, so the tensile strength may be less than 2900 MPa or less than 2700 MPa. The tensile strength is determined by taking a No. 5 test piece or a No. 13B test piece described in JIS Z2241:2022 from the base material portion of the first steel material and performing a tensile test on this test piece at room temperature according to the test method described in JIS Z2241:2022. If a tensile test specimen cannot be obtained due to the shape of the member, the tensile strength may be calculated by converting the Vickers hardness (load 0.098 N) at the 1 / 4 depth position of the base material portion of the first steel material. Specifically, if the difference between the Vickers hardness at the 20 μm depth position and the Vickers hardness at the 1 / 4 depth position is less than 100 HV (including cases where the Vickers hardness at the 20 μm depth position is greater than the Vickers hardness at the 1 / 4 depth position), the tensile strength is calculated by multiplying the Vickers hardness at the 1 / 4 depth position by 3.35. If the difference between the Vickers hardness at the 20 μm depth position and the Vickers hardness at the 1 / 4 depth position is 100 HV or more, the tensile strength is calculated by multiplying the Vickers hardness at the 1 / 4 depth position by 3.30.

[0061] (Thickness) The thickness of the first steel material according to this embodiment is not limited, but considering its use as an automobile part, the thickness of the flat portion of the base material of the first steel material is preferably 0.4 to 5.0 mm, and more preferably 1.0 to 3.5 mm.

[0062] <Second Steel Material> The second steel material to be joined to the first steel material is not limited. The number of second steel materials may be one or more. For example, as shown in FIGS. 1 and 2, one second steel material may be used for one first steel material. On the other hand, as shown in FIG. 3, two second steel materials may be used for one first steel material. The second steel material may be, for example, a known steel material having a tensile strength of less than 2100 MPa in the base material. Depending on the performance required of the steel member, a steel material having a tensile strength different from that of the first steel material can be used as the second steel material. When multiple second steel materials are used, the multiple second steel materials may each be different steel materials having different performance. The second steel material may be a hot-stamped body. On the other hand, when a high-strength member is to be formed as a whole, the second steel material may be a steel material having a tensile strength of 2100 MPa or more in the base material. When the second steel material has a tensile strength of 2100 MPa or more, it preferably has the same chemical composition and number density of transition carbides as the first steel material described above. The same steel material as the first steel material may be used as the second steel material. When the same steel material as the first steel material is used as the second steel material, the second steel material has the same chemical composition, tensile strength, and number density of transition carbides as the first steel material. When the second steel material has a tensile strength of 2100 MPa or more, or when the same steel material as the first steel material is used as the second steel material, it is preferable to reduce the hardness (soften) near the surface, as with the first steel material.

[0063] <Resistance Spot Weld> A resistance spot weld is a weld formed by resistance spot welding and includes a nugget and a HAZ. The nugget is a portion formed when the first steel material and the second steel material are melted by the heat of welding and then solidified again, and the HAZ is a portion that is not melted by the heat of welding but has undergone a change in structure. In the case of high-strength steel, as shown in FIG. 1 , the HAZ 2 generally includes a high-hardness portion (HAZ-hardened portion 21) and a low-hardness portion (HAZ-softened portion 22) depending on the maximum temperature reached. The resistance spot weld joins the first steel material and the second steel material.

[0064] (Number Density of Transition Carbides in HAZ) In conventional steel members, increasing the strength of the steel used improves the tensile strength of the base material, but there is a problem in that the joint strength of the resistance spot weld is likely to decrease (for example, the tensile shear strength TSS of the joint is likely to decrease). The present inventors conducted research into improving joint strength and found that the HAZ, particularly the HAZ hardened portion, is the fracture path. As a result of further research, the present inventors found that joint strength can be increased by precipitating transition carbides of a predetermined size in the HAZ (particularly the HAZ hardened portion). The steel member according to this embodiment is controlled based on the above findings, and transition carbides are precipitated in the HAZ. Specifically, in the case of the resistance spot weld 13 as shown in FIG. 1 , a reference point MA is set to a position 0.2 mm toward the first steel material 11 in the thickness direction of the first steel material 11 from an end EP of a contact surface CF between the first steel material 11 and the second steel material 12 in the HAZ 2, and when an imaginary line IL is drawn passing through this reference point MA in a direction perpendicular to the thickness direction of the first steel material 11, a measurement point MRP is set to a position 0.2 mm from the reference point MA toward the nugget 3 along the imaginary line IL, and at the measurement point MRP (a range of 0.2 μm in the thickness direction of the first steel material 11 and 0.4 μm in the direction perpendicular to the thickness direction from the measurement point MRP is acceptable), the number density of transition carbides having a circle equivalent diameter of 5 nm or more is 20 / μm ２ The reason why transition carbides having a circle equivalent diameter of 5 nm or more are targeted is that transition carbides smaller than this do not sufficiently improve the joint strength. Furthermore, even if the circle equivalent diameter of transition carbides is 5 nm or more, the number density is 20 particles / μm ２ If the density is less than 23 particles / μm, the effect is not sufficient. ２That's all. There is no upper limit to the circle-equivalent diameter of the transition carbides, but if the size becomes too large, it becomes difficult to obtain a sufficient number density, which is undesirable. Normally, when resistance spot welding is performed, the nugget and its surroundings are in contact with the steel materials (a contact surface CF is formed), but outside of that (in the direction away from the nugget from the end EP), the steel materials are not in contact with each other (are separated). When checking the contact surface CF by cross-sectional observation, the contact surface CF may be separated due to the effects of cutting or polishing. However, since the boundary between the contact surface CF and the location where the steel sheets do not contact each other is angled with respect to the surface of the steel materials as shown in FIG. 1, the end EP can be determined from its shape characteristics.

[0065] In the steel member according to this embodiment, when the same steel as the first steel is used as the second steel, or when the second steel has a tensile strength of 2100 MPa or more, the number density of transition carbides having a circle equivalent diameter of 5 nm or more is 20 / μm in the HAZ, even at positions symmetrical with respect to the contact surface CF. ２ That is, as shown in Fig. 2, when a reference point MA2 is set at a position 0.2 mm from an end EP of a contact surface CF between the first steel material 11 and the second steel material 12 toward the second steel material 12 in the thickness direction of the second steel material 12, and an imaginary line IL2 is drawn passing through the reference point MA2 in a direction perpendicular to the thickness direction of the second steel material 12, a measurement point MRP2 is set at a position 0.2 mm from the reference point MA2 toward the nugget 3 along the imaginary line IL2, and the number density of transition carbides having a circle equivalent diameter of 5 nm or more is 20 / µm or more at the measurement point MRP2 (a range of 0.2 µm in the thickness direction of the second steel material 12 and 0.4 µm in the direction perpendicular to the thickness direction from the measurement point MRP2 is acceptable). ２ When the steel member 100 according to this embodiment is formed by joining two second steel materials together so as to sandwich a first steel material as shown in Fig. 3, the number density of transition carbides at a predetermined position in the HAZ, with each contact surface CF as a reference, satisfies the above.

[0066] The circle-equivalent diameter of transition carbides in the HAZ and the number density of transition carbides with circle-equivalent diameters of 5 nm or more can be determined by observing a thin-film TEM observation sample in the same manner as the base material of the first steel using a field emission transmission electron microscope (FE-TEM) (JEM-2100F manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray spectroscopy (EDX). However, the sample is prepared as follows. After peeling the weld at low temperature, a test piece is taken from a portion of the first steel including the resistance spot weld, and both sides are mechanically or chemically polished to prepare a thin-film sample (approximately 60 μm thick, with dimensions of φ3 mm or less) at the measurement point MRP described above. Then, double-sided jet electrolytic polishing is performed, and a hole is formed near the measurement point MRP by electrolytic polishing to obtain a thin-film TEM observation sample. Thin-film TEM observation is performed at the measurement point MRP (a range of 0.2 μm in the thickness direction and 0.4 μm in the direction perpendicular to the thickness direction, centered on the MRP, is acceptable). Thin-film TEM observation is performed at an accelerating voltage of 200 kV. To reduce variation between locations, at least five visual fields of approximately 100 to 300 nm square are observed on the sample. The type of observed precipitates is identified using the diffraction pattern and EDX analysis results. To reduce variation between visual fields, the number density is counted in each visual field, and the average value is used as the representative number density value. When the same steel as the first steel is used as the second steel, or when the second steel has a tensile strength of 2100 MPa or more, a test piece is also taken from a portion of the second steel including a resistance spot weld, and thin-film TEM observation is performed in the same manner at measurement point MRP2 to determine the number density.

[0067] (Difference ΔHV between the hardness near the fusion boundary of the nugget and the hardness inside the nugget) As a result of studies by the present inventors, it was found that in steel members obtained by spot welding using, as at least one material, a hot-stamped steel sheet having a Mn content of 1.00% or more, a softened region with a lower Mn concentration than the surrounding area is formed near the fusion boundary of the nugget, and this softened region may become a fracture path, thereby reducing joint strength. Therefore, in the resistance spot welds of steel members according to this embodiment, the formation of a softened region near the fusion boundary of the nugget is suppressed, thereby preventing a reduction in joint strength. Specifically, as shown in Figure 1, the Vickers hardness at position N1 on the imaginary line IL, which is 50 µm toward the nugget from the boundary (fusion boundary MB) between the nugget 3 and the HAZ 2, is defined as the hardness of the nugget's fusion boundary vicinity, and the Vickers hardness at position N2, which is 500 µm toward the nugget from the fusion boundary MB, is defined as the hardness of the nugget's interior. ΔHV, which is the difference between the hardness of the nugget's fusion boundary vicinity and the hardness of the nugget's interior, is set to less than 70 HV. If ΔHV is 70 HV or more, the joint strength decreases. ΔHV is preferably set to less than 60 or less than 50. When the second steel material is the same as the first steel material, or when the second steel material has a tensile strength of 2100 MPa or more, as shown in Figure 2, for any one imaginary line IL, the Vickers hardness at position N1 on IL is the hardness of the nugget fusion boundary vicinity, and the Vickers hardness at position N2 on IL is the hardness of the nugget interior, and the difference ΔHV between the hardness of the nugget fusion boundary vicinity and the hardness of the nugget interior is less than 70 HV. When the steel member is formed by joining two second steel materials so as to sandwich the first steel material, as shown in Figure 3, for any one imaginary line IL, the Vickers hardness at position N1 on IL is the hardness of the nugget fusion boundary vicinity, and the Vickers hardness at position N2 on IL is the hardness of the nugget interior, and the difference ΔHV between the hardness of the nugget fusion boundary vicinity and the hardness of the nugget interior is less than 70 HV.

[0068] ΔHV is determined by the following method. A test piece is taken from a portion including a resistance spot weld, and the cross section of the steel plate is buffed. Then, a Vickers hardness test is performed in accordance with JIS Z2244-1:2020. The Vickers hardness test is performed using a micro Vickers hardness tester, with a load of 0.098 N and a load holding time of 10 seconds. The test is performed at one point near the fusion boundary of the nugget and one point inside the nugget. The measured value near the fusion boundary of the nugget is designated HV1, the measured value inside the nugget is designated HV2, and the difference between HV2 and HV1 (HV2 - HV1) is designated ΔHV.

[0069] <Manufacturing method> The manufacturing method of the steel member according to this embodiment is not limited, and the effect can be obtained as long as it has the above-mentioned configuration, but a manufacturing method including the following steps is preferable because it allows stable manufacturing: (I) a welding step in which a first steel material and a second steel material are joined by resistance spot welding to form a steel member, and (II) a heat treatment step in which the steel member after the welding step is heated to 80 to 300°C and held at that temperature for 6 to 1800 seconds. Preferred conditions for each step will be described.

[0070] [Welding Process] In the welding process, a first steel material and a second steel material are joined by resistance spot welding to form a steel member. The first steel material and the second steel material used for welding have the above-described configuration. During welding, the first steel material and the second steel material to be welded are overlapped with each other so that they at least partially overlap. An electrode is pressed against the overlapping portion with a pressure F, and current is applied to form a nugget. The welding conditions are the current application time and pressure, as follows: When the thicknesses (mm) of the flat portions of the overlapping steel materials (the first steel material and the second steel material) are t1 and t2, respectively, the current application time Wt (seconds) is set to a range satisfying Wt≦Wtmax, where Wtmax=(t1+t2)×0.097. The pressure F (kN) is set to a range satisfying Fmin≦F. Here, Fmin = (t1 + t2) × 0.71 + 2.05. To satisfy the above formula, shortening the current application time prevents the nugget from solidifying during current application, and increasing the pressure to promote cooling by the electrode can prevent local softening of the fusion boundary. Because too short a current application time increases the likelihood of flashing, it is preferable to set the current application time Wt (seconds) within a range that satisfies Wtmin ≦ Wt. Here, Wtmin = (t1 + t2) × 0.062. Furthermore, because too large a pressure can result in significant reduction in plate thickness when flashing occurs and potentially reduce the strength of the weld, it is preferable to set the pressure F (kN) within a range that satisfies F ≦ Fmax. Here, Fmax = (t1 + t2) × 1.24 + 2.14. Other known conditions may be applied. When a first steel material and two or more (n) second steel materials are joined by resistance spot welding to form a steel member, the Wtmax and Wtmin are Wtmax = (t1 + t2 + ... + t(n+1)) × 0.097, Wtmin = (t1 + t2 + ... + t(n+1)) × 0.062, and the Fmin and Fmax are Fmin = (t1 + t2 + ... + t(n+1)) × 0.71 + 2.05, Fmax = (t1 + t2 + ... + t(n+1)) × 1.24 + 2.14.As long as the above conditions are satisfied, welding may be performed by passing current multiple times, with no current-carrying cooling periods in between, until a desired nugget diameter is obtained (for example, if the minimum thickness of the overlapped steel materials is tmin, a nugget diameter of 4 × √tmin (mm) or more is preferred). When current is passed multiple times, current is passed so that each pass satisfies the above range.

[0071] [Heat Treatment Step] In the heat treatment step, the steel members after the welding step are heated to 80 to 300°C and held at that temperature for 6 to 1800 seconds. In this step, a predetermined amount of transition carbides is precipitated. If the heating temperature is less than 80°C, the number density of the transition carbides may be insufficient in the base metal portion of the first steel material and the resistance spot welds of the first steel material. On the other hand, if the heating temperature is more than 300°C, the transition carbides may coarsen, and the number density of the transition carbides may be insufficient in the base metal portion of the first steel material and the resistance spot welds of the first steel material. The heating temperature is preferably 100 to 220°C. If the holding time at 80 to 300°C is less than 6 seconds, the number density of the transition carbides may be insufficient in the base metal portion of the first steel material and the resistance spot welds of the first steel material. On the other hand, if the holding time exceeds 1800 seconds, the transition carbides may coarsen, and the number density of the transition carbides may decrease in the base material portion of the first steel material and the resistance spot welds of the first steel material. The holding time is preferably 40 to 1600 seconds. The above heat treatment needs to be performed after welding, but it is not preferable to perform the same heat treatment on the hot stamped product before welding (i.e., perform heat treatment twice, before and after welding). This is because the transition carbides may coarsen, and the number density of the transition carbides may be insufficient in the base material portion or the HAZ of the spot welds. Furthermore, the HAZ-softened portion of the spot welds may become excessively soft, which may reduce the joint strength or the tensile strength of the base material. Furthermore, this may result in reduced productivity, increased manufacturing costs, and CO2 emissions. ２ This also leads to an increase in emissions.

[0072] (Method for manufacturing first steel material) The first steel material, which is a hot-stamped product, can be manufactured by a manufacturing method having the following steps: (i) A hot stamping step in which a steel sheet having a predetermined chemical composition is heated to a temperature range of the higher of the Ac3 point and 800°C to 950°C, and held in this temperature range for 60 to 720 seconds, after which forming is initiated in a temperature range above 750°C, and cooling to less than 80°C so that the average cooling rate from the forming start temperature to 300°C is 15 to 500°C / second. Preferred conditions are described below.

[0073] [Hot Stamping Process] In the hot stamping process, a steel sheet having a predetermined chemical composition and carbide number density is heated to a temperature range between the higher of the Ac3 point and 800°C and 950°C or less, and held at this temperature range for 60 to 720 seconds. The heated steel sheet is then removed from the heating furnace and forming is initiated in a temperature range above 750°C. The steel sheet is then cooled to less than 80°C so that the average cooling rate from the forming start temperature to 300°C is 15 to 500°C / s. This process sufficiently dissolves the carbides present in the steel sheet and increases its strength. If the heating temperature is below the Ac3 point or below 800°C, the carbides may not be sufficiently dissolved, resulting in an insufficient number density of transition carbides in the base metal portion of the first steel material and the resistance spot welds of the first steel material. If the holding time in the above temperature range is less than 60 seconds, the carbides may not dissolve sufficiently, resulting in an insufficient number density of transition carbides in the base material portion of the first steel material and the resistance spot welds of the first steel material. Furthermore, if the heating temperature is less than Ac3 or less than 800°C, or if the holding time in the above temperature range is less than 60 seconds, the austenite transformation may be insufficient, and sufficient strength may not be obtained after the hot stamping process. On the other hand, if the heating temperature is too high or the holding time is too long, the crystal grains become coarse, resulting in insufficient toughness and bendability of the formed body after the hot stamping process. Therefore, the heating temperature should be 950°C or less, and the holding time should be 720 seconds or less. If the hot stamping starting temperature is 750°C or less, the volume fraction of martensite in the metal structure of the hot stamped body may be insufficient, and sufficient strength may not be obtained. Furthermore, if the average cooling rate from the starting temperature to 300°C during cooling to less than 80°C is less than 15°C / second, sufficient hardening may not occur, and the hot stamped body may not obtain sufficient tensile strength. On the other hand, if the average cooling rate to 300°C exceeds 500°C / s, the cooling rate varies greatly from part to part, causing distortion in the shape of the molded body. The Ac3 point can be determined from the change point in the thermal expansion coefficient when the heating rate is set to 5°C / s, using a plate Formaster test or the like.

[0074] In addition, the steel sheet to be subjected to the hot stamping process has a number density of one or more carbides of Nb, Ti, Fe, Mo, W and Cr having a circle equivalent diameter of 0.2 μm or more at a position of 1 / 4 of the sheet thickness from the surface of the steel sheet, of 0.5 pieces / μm ２ It is preferable to use a steel plate having a particle size of less than 0.5 / μm. In the first steel material of the steel member according to this embodiment, transition carbides are precipitated by hot stamping, welding, and heat treatment under predetermined conditions, thereby obtaining transition carbides of a predetermined size and number density. However, if carbides are present in the steel plate to be used as a base material and the carbides do not dissolve sufficiently during heating in hot stamping, the predetermined transition carbides cannot be obtained even after hot stamping, welding, and heat treatment. For example, coarse carbides formed in the steel plate tend not to dissolve during heating in hot stamping and remain undissolved. Therefore, in the steel plate to be used as a base material, coarse carbides are reduced, and the number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 μm or more at a position from the surface to 1 / 4 of the plate thickness is reduced to 0.5 / μm. ２ It is preferable that the number of coarse carbides having an equivalent circle diameter of 0.2 μm or more is less than 0.5 / μm. ２ If this is the case, a large amount of carbide will remain undissolved during heating for hot stamping, which may make it difficult to preferably precipitate transition carbides in the base metal portion and resistance spot welds of the first steel material.

[0075] The number density of one or more carbides of Nb, Ti, Fe, Mo, W, and Cr having an equivalent circle diameter of 0.2 μm or more can be determined by the following method. A cross section of a steel sheet (a cross section parallel to the rolling direction and thickness direction) is mirror-polished and then etched with nital. The sample is observed using a scanning electron microscope (SEM) at a location 1 / 4 of the thickness from the surface (a range of 1 / 8 to 3 / 8 of the thickness from the surface in the thickness direction is acceptable). Observed precipitates in a 50 μm square region at the same depth are subjected to composition analysis using EPMA. If the precipitate contains one or more of Nb, Ti, Fe, Mo, W, and Cr, and C, it is determined to be the target carbide. To reduce variation in number density due to visual field, at least five or more visual fields of a 50 μm square region are observed, and the number density of the carbides having an equivalent circle diameter of 0.2 μm or more is counted, and the average value is used as the representative number density value.

[0076] The chemical composition of the steel sheet to be subjected to the hot stamping process (the steel sheet that will be used to produce the first steel material, also referred to as the steel sheet for hot stamping) may be the same as the chemical composition of the base material of the first steel material. This is because the chemical composition of the base material does not change in principle even after hot stamping, welding, and heat treatment. The chemical composition of the steel sheet can be determined in the same manner as the chemical composition of the base material of the first steel material.

[0077] When the hardness at the 20 μm depth position is made smaller than the hardness at the ¼ depth position in the first steel material of the steel member according to this embodiment, it is preferable to decarburize the surface layer portion at the steel plate stage before hot stamping, so that the hardness at the 20 μm depth position in the steel plate is made smaller than the hardness at the ¼ depth position. In this case, the hardness at the 20 μm depth position is also smaller than the hardness at the ¼ depth position in the first steel material that has become the steel member according to this embodiment.

[0078] (Method for manufacturing steel plate) A steel plate that is the raw material for the first steel material can be manufactured by a manufacturing method having the following steps: (A) a heating step of heating a slab having the same chemical composition as the base material of the first steel material to 1150 to 1350°C, (B) a hot rolling step of hot rolling the slab after the heating step to a finish rolling temperature of 800 to 950°C to obtain a steel plate, (C) a cooling step, which is started within 5.0 seconds from the completion of the hot rolling step, of cooling the steel plate after the hot rolling step to 750°C or less at an average cooling rate of 10 to 100°C / sec, (D) a coiling step of coiling the steel plate after the cooling step at 500 to 750°C, and (E) a cold rolling step of cold rolling the steel plate after the coiling step at a thickness reduction rate of 10 to 60%. (F) an annealing step in which the steel sheet after the cold rolling step is heated to 730°C or higher, held for 40 seconds or more, and then cooled to a temperature range of 500 to 650°C at an average cooling rate of 5°C / second or more, and held in that temperature range for 200 seconds or more. Furthermore, if necessary, one or more of the following steps may be further included: (G) a coating step in which a coating is formed on the surface of the steel sheet after the annealing step; and (H) a skin-pass rolling step in which the steel sheet is subjected to skin-pass rolling at a reduction ratio of 0.05 to 2.0% after the annealing step or the coating step. Preferred conditions for each step are described below.

[0079] [Heating Step] In the heating step, the slab is heated prior to hot rolling. The heating temperature is set to 1150 to 1350°C. If the heating temperature is less than 1150°C, the carbides formed during casting do not dissolve, and coarse carbides remain even after the hot rolling step. On the other hand, from the viewpoint of suppressing scale loss and saving energy, the slab heating temperature is set to 1350°C or less. The chemical composition of the slab subjected to the heating step may be the same as the chemical composition of the steel sheet to be obtained.

[0080] [Hot Rolling Process] In the hot rolling process, the slab after the heating process is hot rolled at a finish rolling temperature of 800 to 950°C to obtain a steel sheet. If the finish rolling temperature (surface temperature at the final pass exit side) is less than 800°C, many unrecrystallized regions flattened in the rolling direction remain, which may result in anisotropy in the properties of the steel sheet. On the other hand, if the finish rolling temperature exceeds 950°C, the crystal grains of the steel sheet for hot stamping become coarse. If the crystal grains of the steel sheet for hot stamping are coarse, the crystal grains after hot stamping also become coarse, resulting in a decrease in the toughness and bendability of the formed body. Furthermore, if the finish rolling temperature exceeds 950°C, coarse carbides are formed, which may result in a large amount of undissolved carbides during heating for hot stamping.

[0081] [Cooling Process] After the completion of finish rolling, if the steel sheet is allowed to remain at a temperature above 750°C for a long period of time, coarse carbides are generated. Therefore, in the cooling process, the steel sheet after the hot rolling process is cooled so that the average cooling rate from the start of cooling to a cooling stop temperature of 750°C or less is 10 to 100°C / s. Furthermore, this cooling is initiated by water cooling within 5.0 seconds from the completion of the hot rolling process. If the average cooling rate to a cooling stop temperature (water cooling stop temperature) of 750°C or less is less than 10°C / s, if the time from the completion of the hot rolling process to the start of the cooling process exceeds 5.0 seconds, or if the cooling stop temperature is above 750°C, a large amount of coarse carbides are generated. On the other hand, if the average cooling rate to a cooling stop temperature of 750°C or less exceeds 100°C / s, it is difficult to uniformly cool the steel sheet, and defects in the sheet shape may occur. The higher the Mn content in the steel sheet (hot-rolled steel sheet) obtained in the hot rolling process, the more stabilized the carbides become, and the number density of coarse carbides increases in the steel sheet for hot stamping. Therefore, when the Mn content is 1.25% or more, it is preferable to set the average cooling rate to 40 to 100°C / sec.

[0082] [Coiling Process] In the coiling process, the steel sheet after the cooling process is coiled at 500 to 750°C. If the coiling temperature is less than 500°C, hard phases such as bainite and martensite are formed, making cold rolling impossible or increasing the cold rolling load in some cases. On the other hand, if the coiling temperature is more than 750°C, Cr and Mn are excessively concentrated in the ferrite grain boundaries and cementite present in pearlite, increasing the amount of precipitation of coarse carbides in the steel sheet for hot stamping. In addition, dissolution of carbides during hot stamping may be inhibited, resulting in a large amount of undissolved carbides.

[0083] [Cold Rolling Step] In the cold rolling step, the steel sheet after the coiling step is cold rolled at a thickness reduction rate (rolling reduction) of 10 to 60% to adjust to a predetermined thickness. Before cold rolling, the steel sheet after the coiling step may be subjected to skin pass rolling and / or pickling according to a conventional method. Furthermore, after cold rolling, treatment such as degreasing may be performed according to a conventional method.

[0084] [Annealing Process] In the annealing process, the steel sheet after the cold rolling process is heated to an annealing temperature (soaking temperature) of 730°C or higher and held at that temperature for 40 seconds or more. The steel sheet is then cooled to a temperature range of 500 to 650°C at an average cooling rate of 5°C / second or higher and held at this temperature range for 200 seconds or more. This annealing refines the carbides in the steel sheet for hot stamping. If the annealing temperature is lower than 730°C or the holding time at the annealing temperature is shorter than 40 seconds, an excessive amount of coarse carbides will be generated. There are no upper limits for the annealing temperature and the holding time at the annealing temperature. However, if the annealing temperature is too high or the holding time at the annealing temperature is too long, the metal structure of the steel sheet will coarsen due to grain growth. Therefore, it is preferable that the annealing temperature be 920°C or lower and the holding time at the annealing temperature be 500 seconds or shorter. Furthermore, if the average cooling rate to the temperature range of 500 to 650°C is less than 5°C / second, or if the holding time in the temperature range of 500 to 650°C is less than 200 seconds, the amount of coarse carbides produced will be excessive. There is no upper limit to the holding time in the temperature range of 500 to 650°C, but if the holding time is too long, productivity will decrease and production costs will increase, so it may be set to 1000 seconds or less. There are no limitations on the cooling after holding in the temperature range of 500 to 650°C.

[0085] Furthermore, in the annealing process, when the hardness at a position 20 μm from the surface of the steel sheet is to be smaller than the hardness at a position ¼ of the sheet thickness from the surface, it is preferable to set the annealing temperature to 730 to 920°C, set the oxygen potential of the atmosphere at -1.50 to -0.50, and set the holding time to 120 to 500 seconds. By performing heat treatment under the above conditions, the surface layer of the steel sheet can be decarburized and the surface layer of the steel sheet for hot stamping can be softened, which is preferable. If the annealing temperature is less than 730°C, the oxygen potential is less than -1.50, or the holding time is less than 120 seconds, decarburization is insufficient and the surface layer of the steel sheet for hot stamping cannot be softened. In order to further decarburize the surface layer of the steel sheet and sufficiently soften the surface layer of the steel sheet, it is preferable to set the annealing temperature to 750°C or higher, the oxygen potential to -0.95 or higher, and the holding time to 180 seconds or higher. On the other hand, if the annealing temperature exceeds 920°C or the holding time at the annealing temperature exceeds 500 seconds, the metal structure of the steel sheet for hot stamping becomes coarse due to grain growth. If the oxygen potential exceeds -0.50, Fe and other alloy elements are oxidized in the surface layer, resulting in insufficient decarburization and insufficient softening of the surface layer of the steel sheet for hot stamping. In addition, a scale pattern is formed on the surface, impairing the surface properties. Here, the oxygen potential is expressed as log 10 (P Ｈ２Ｏ / P Ｈ２ ) is a value defined by the formula P in the oxygen potential formula. Ｈ２Ｏ is the H atmosphere in the annealing furnace ２ The partial pressure of O is P Ｈ２ is the H atmosphere in the annealing furnace ２ indicates the partial pressure of

[0086] [Coating step] When forming a coating on the surface of the first steel material, the coating may be formed on the surface at the stage of steel sheet. The coating method is not particularly limited, and possible methods include hot-dip galvanization, electroplating, vacuum deposition, cladding, thermal spraying, etc. Hot-dip galvanization is the most widely used method industrially. Examples of the coating include Al-based coatings containing Al and Zn-based coatings containing Zn.

[0087] When an Al-based coating is formed by hot-dip plating, the coating bath often contains Fe as an impurity in addition to Al. Furthermore, as long as the coating bath contains 70 mass% or more of Al, the coating 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 in addition to the elements described above. When a Zn-based coating is formed by hot-dip plating, the coating bath often contains Fe as an impurity in addition to Zn. Furthermore, as long as the Zn content is 70% by mass or more, the plating bath may further 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 in addition to the above elements. When hot-dip plating is performed, the steel sheet after the annealing step may be cooled to room temperature and then heated again to perform plating, or the steel sheet after the annealing step 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), and then hot-dip plating may be performed without once cooling to room temperature.

[0088] There are no particular limitations on the pre-treatment and post-treatment of the coating, and possible treatments include pre-coating, solvent application, alloying treatment, etc. As an alloying treatment, the coated steel sheet can be heated to, for example, 450 to 800°C to alloy the coating.

[0089] [Skin-pass rolling process] The production of a steel sheet for hot stamping may further include a skin-pass rolling process in which the steel sheet is subjected to skin-pass rolling after the annealing process or the coating process. By performing skin-pass rolling, the diffusion rate of elements in the material increases, making it easier for carbides to dissolve during hot stamping. In this case, the number density of transition carbides can be increased. To achieve this effect, the reduction ratio of the skin-pass rolling is preferably 0.05% or more, more preferably 0.1% or more. On the other hand, if the reduction ratio of the skin-pass rolling exceeds 2.0%, the load on the skin-pass process becomes large, which causes an increase in material costs. Therefore, when performing skin-pass rolling, the reduction ratio is preferably 2.0% or less.

[0090] (Method for manufacturing second steel material) The method for manufacturing the second steel material is not limited. When the same steel material as the first steel material is used as the second steel material, or when the second steel material is a steel material having a tensile strength of 2100 MPa or more, the second steel material may be manufactured by the same method as the first steel material.

[0091] 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.

[0092] Example 1 Molten steel was cast using a vacuum melting furnace to obtain steel materials having the chemical compositions shown in Tables 1-1 and 1-2. The obtained steel materials were heated to 1250°C and held there for 60 minutes, and then hot-rolled at the finish rolling temperatures shown in Tables 2-1 to 2-3. After hot rolling was completed, the steel sheets were water-cooled under the conditions shown in Tables 2-1 to 2-3. The water-cooling stop temperature was 700°C (800°C for Test No. 30). After water-cooling was stopped, the steel sheets were allowed to cool naturally, and the cooling end temperature was set to the coiling temperature shown in Tables 2-1 to 2-3. The hot-rolled steel sheets were then charged into an electric heating furnace held at this coiling temperature and held there for 60 minutes. The hot-rolled steel sheets were then furnace-cooled to room temperature at an average cooling rate of 20°C / hour to simulate slow cooling after coiling. The furnace-cooled hot-rolled steel sheets were pickled and then cold-rolled to obtain cold-rolled steel sheets having a thickness of 1.4 mm. The thickness reduction rate during cold rolling was set to 46%. Blank cells in Tables 1-1 and 1-2 indicate that the content of the element in question was below the lower limit of measurement.

[0093] The obtained cold-rolled steel sheets were annealed at the annealing temperatures (soaking temperatures) shown in Tables 2-1 to 2-3 using a continuous annealing simulator or a hot-dip galvanizing simulator. During annealing, they were heated to the annealing temperatures listed in Tables 2-1 to 2-3 at an average heating rate of 8°C / s. The atmosphere in the annealing furnace was a nitrogen-hydrogen-water vapor atmosphere containing 3% by volume of hydrogen, with the oxygen potential set as listed in Tables 2-1 to 2-3. After holding at the annealing temperature, they were cooled to 400 to 700°C under the conditions listed in Tables 2-1 to 2-3 and held at that temperature (holding temperature) for 50 to 300 seconds. Subsequently, they were cooled to room temperature at a cooling rate of 10°C / s to obtain annealed steel sheets. In some examples, the annealing step was omitted. Some cold-rolled steel sheets were held at 400 to 700°C, and then, while the steel sheets were being cooled to room temperature, they were immersed in a hot-dip galvanizing bath or a hot-dip aluminum plating bath to be hot-dip galvanized or hot-dip aluminum plated, thereby obtaining hot-dip galvanized steel sheets (GI) or hot-dip aluminum plated steel sheets (AL). Some of the hot-dip galvanized examples were alloyed by heating at 500°C for 20 seconds to obtain galvannealed hot-dip galvannealed steel sheets (GA). The obtained annealed steel sheets or plated steel sheets (GI, GA, or AL) were subjected to skin-pass rolling under the conditions shown in Tables 2-1 to 2-3 to obtain steel sheets for hot stamping.

[0094] A plate Formaster test piece 10 mm wide and 79 mm long was taken from the hot stamping steel sheet, and the test piece was heated at a heating rate of 5°C / sec to determine the Ac3 point from the change in thermal expansion coefficient. Also, a test piece for SEM observation was taken from the hot stamping steel sheet, and the number density of one or more carbides of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 µm or more was measured by the method described above.

[0095] Furthermore, a hot stamping blank measuring 240 mm in width and 400 mm in length was extracted from the hot stamping steel plate, and hot stamping was performed to obtain a hat member (first steel material) with the shape shown in FIG. 4 . In the hot stamping process, the hot stamping blank was heated under the conditions shown in Tables 3-1 and 3-2. The hot stamping blank was then removed from the heating furnace and, with a heating temperature of −80°C as the forming start temperature, sandwiched between a mold equipped with a cooling device to perform hat forming. Subsequently, the blank was cooled in the mold to a cooling stop temperature of 40°C at an average cooling rate of 5 to 70°C / sec from the forming start temperature to 300°C. Next, as shown in FIG. 5 , a closing plate (second steel material) measuring 1.4 mm in thickness, 130 mm in width, and 140 mm in length was spot welded to the flange portion of the hat member to obtain a steel member. A cold-rolled steel sheet with a tensile strength of 590 MPa was used as the second steel material. Spot welding was performed under the following conditions. The pressure and current duration were as shown in Tables 3-1 and 3-2. Test pieces measuring 30 mm in width and 50 mm in length were spot-welded from the first and second steel materials, and the current value at which a nugget diameter of 5.9 mm was measured by cross-sectional observation of the center of the weld was previously investigated. This current value was used. The holding time was 0.20 seconds. The resulting steel members were placed in an electric heating furnace and heat-treated under the conditions shown in Tables 3-1 and 3-2. Heat treatment was omitted for some steel members. In Test No. 51, the hot-stamped body after the hot stamping process was subjected to heat treatment in an electric heating furnace at a heating temperature of 170°C and a holding time of 1200 seconds, followed by spot welding in the same manner to obtain a steel member. Heat treatment was omitted for this steel member. In Test No. 52, the hot-stamped body after the hot stamping process was subjected to heat treatment in an electric heating furnace at a heating temperature of 170°C and a holding time of 1200 seconds, followed by spot welding in the same manner to obtain a steel member. The obtained steel members were placed in an electric heating furnace and heat treated again under the conditions shown in Tables 3-1 and 3-2.

[0096] A test piece for component analysis was taken from the punch bottom of the hat member of the steel member (base material of the first steel material), and the chemical composition was measured by the method described above. Also, a test piece for structure observation was taken from the punch bottom of the hat member of the steel member, and the metal structure at a 1 / 4 depth position was observed by the method described above.

[0097] In addition, test pieces for thin-film TEM observation were taken from the punch bottom of the hat member of the steel member, and the number density of transition carbides having a circle-equivalent diameter of 5 nm or more in the base material was measured using the method described above. Test pieces for hardness measurement were taken from the punch bottom of the hat member of the steel member, and the Vickers hardness was measured at a 1 / 4 depth position and a 20 μm depth position using the method described above. Test pieces for thin-film TEM observation were taken from a flange portion including a spot weld of the steel member, and the number density of transition carbides having a circle-equivalent diameter of 5 nm or more in the HAZ of the resistance spot weld was measured using the method described above. In addition, test pieces for hardness measurement were taken from a flange portion including a spot weld of the steel member, and the hardness of the vicinity of the fusion boundary of the nugget and the inside of the nugget was measured using the method described above, and the difference between these hardnesses was determined.

[0098] Further, a tensile test piece of No. 13B was taken from the vertical wall of the hat member (the base material portion of the first steel material) of the steel member along the longitudinal direction of the hat member in accordance with JIS Z2241:2022, and a tensile test was performed at room temperature at a tension rate of 10 mm / min to determine the tensile strength. If the obtained tensile strength was 2100 MPa or more, it was judged to have high strength and pass. On the other hand, if the obtained tensile strength was less than 2100 MPa, it was judged to have low strength and fail.

[0099] In addition, a 60 mm square test piece for bending tests was taken from the punch bottom of the hat member of the steel member, and a bending test was performed in accordance with the German Association of the Automotive Industry standard VDA 238-100. The test piece was bent so that the bend ridge direction was perpendicular to the rolling direction of the hot stamping steel sheet, and the bend angle (VDA bend angle) was determined at the time when the bending load decreased by 60 N from the highest point. If a crack occurred before the bending load reached its highest point (maximum value), the bend angle at the time when the crack occurred was determined and used as the VDA bend angle. The VDA bend angle is an indicator of the impact absorption of a steel member, and the higher the VDA bend angle of the steel plate constituting the steel member, the better the impact absorption of the steel member. In this example, a VDA bend angle of 40 ° or more was deemed to have excellent impact absorption and was judged to have passed. On the other hand, a VDA bend angle of less than 40 ° was deemed to have no excellent impact absorption and was judged to have failed.

[0100] Furthermore, test pieces for measuring joint strength, 20 mm wide and 200 mm long as shown by the dotted line in Figure 5, were taken from the flange portion including the spot weld of the steel member, and a tensile test was performed on these test pieces at room temperature under conditions of a chuck distance of 100 mm and a tensile speed of 10 mm / min to determine the maximum load (TSS). When the product of the tensile strength (TSS) and TSS was 36,000 MPa·kN or more, the test piece was judged to have high joint strength and pass.

[0101] Tables 4-1 to 4-3 show the results of measuring the number density, hardness, tensile strength, and VDA bending angle of transition carbides, as well as the results of evaluating the mechanical properties of the steel members. "Early fracture" in Table 4-2 indicates that the tensile test specimen broke early in the tensile test, making it impossible to measure the tensile strength. Although not shown in the tables, the chemical composition of the base material of the first steel was the same as the chemical composition of the steel shown in Tables 1-1 and 1-2. Furthermore, the number density of carbides of one or more of Nb, Ti, Fe, Mo, W, and Cr having a circle equivalent diameter of 0.2 μm or more in the base steel plate (steel plate for hot stamping) of the first steel was 0.4 / μm for the steel members according to the present invention. ２ Among the steel members according to the comparative examples, the steel members of test numbers 27 to 30 and 39 to 46 had a crack density of 0.5 pieces / μm or less. ２ Furthermore, in the metal structure of the base material portion of the first steel material of the steel member according to the present invention, the volume fraction of martensite at the 1 / 4 depth position was 92.0% or more, and the total volume fraction of structures other than martensite was 8.0% or less.

[0102]

[0103]

[0104]

[0105]

[0106]

[0107]

[0108]

[0109]

[0110]

[0111]

[0112] As shown in Tables 1-1 to 4-3, the steel members according to the present invention have the chemical composition of the base metal portion of the first steel material, the number density of transition carbides in the base metal portion and HAZ of the first steel material, and the difference in hardness between the nugget fusion boundary vicinity and the nugget interior within the specified ranges, and therefore have high base metal strength, excellent impact absorption (high VDA bend angle), and high joint strength. It can be seen that when the Vickers hardness at a depth of 20 μm in the base metal portion of the first steel material is lower than the Vickers hardness at a depth of 1 / 4, the VDA bend angle is 43° or more, resulting in particularly excellent impact absorption. It can be seen that when the Vickers hardness at a depth of 20 μm is 100 HV or more lower than the Vickers hardness at a depth of 1 / 4, the VDA bend angle is 46° or more, resulting in even better impact absorption. On the other hand, the steel members according to the comparative examples have one or more of the chemical composition of the base material portion of the first steel material, the number density of transition carbides in the base material portion and HAZ of the first steel material, and the difference in hardness between the portion near the fusion boundary of the nugget and the inside of the nugget outside the specified range, and it is found that they have inferior properties in one or more of the base material strength, impact absorption, and joint strength.

[0113] Example 2 Steels A, B, C, F, and P having the chemical compositions shown in Tables 1-1 and 1-2 in Example 1 were obtained by casting molten steel using a vacuum melting furnace. These steels were used to produce hot stamping steel sheets using the same steel materials and under the same conditions as those for Test Nos. 4, 5, 8, 22, 28, and 38 in Example 1. Hot stamping was performed using the obtained hot stamping steel sheets under the same conditions as those for Test Nos. 4, 5, 8, 22, 28, and 38 in Example 1, to obtain hat members having the shapes shown in FIG. 4. In this Example 2, the symbols for the hat members produced under the same conditions as those for Test Nos. 4, 5, 8, 22, 28, and 38 in Example 1 are referred to as B1, C1, F1, P1, C2, and A1, respectively.

[0114] Next, one first steel material and one second steel material were selected from the obtained hat members (including cases where the first steel material and the second steel material were the same type). As shown in FIG. 6 , the hat members were placed face-to-face and spot-welded at the flanges to obtain steel members. Spot welding was performed under the conditions shown in Table 5. Test pieces measuring 30 mm wide and 50 mm long were taken from the first steel material and the second steel material, spot-welded, and the current value at which a nugget diameter of 5.9 mm was measured by cross-sectional observation of the center of the weld was previously investigated. This current value was used. The holding time was 0.20 seconds. The obtained steel members were placed in an electric heating furnace and heat-treated under the conditions shown in Table 5. Heat treatment was omitted for some steel members. Test specimens were collected from the first and second steel materials constituting the steel members, and in the same manner as in Example 1, the chemical composition, metal structure, number density of transition carbides having a circle equivalent diameter of 5 nm or more in the base material and HAZ, hardness of the base material, hardness near the fusion boundary of the nugget and inside the nugget, tensile properties, bendability, and joint strength were investigated. Tables 6-1 to 6-3 show the results of measuring the number density, hardness, tensile strength, and VDA bend angle of the transition carbides, as well as the results of evaluating the mechanical properties of the steel members. When calculating the value of TS x TSS, the higher of the TS values ​​of the first and second steel materials was used as the TS value. Although not shown in the tables, the chemical compositions of the base materials of the first and second steel materials were identical to the chemical compositions of the steel materials shown in Tables 1-1 and 1-2. In addition, in the metal structure of the base material portions of the first steel material and the second steel material of the steel member according to the present invention, the volume fraction of martensite at the 1 / 4 depth position was 92.0% or more, and the total volume fraction of structures other than martensite was 8.0% or less.

[0115]

[0116]

[0117]

[0118]

[0119] As shown in Tables 5 and 6-1 to 6-3, the steel members according to the present invention (Test Nos. 53, 54, 56, and 57) have chemical compositions of the base metal portions of the first and second steel members, number densities of transition carbides in the base metal portions and HAZ of the first and second steel members, and differences in hardness between the nugget fusion boundary vicinity and the nugget interior within specified ranges, and therefore have high base metal strength, excellent impact absorption (high VDA bend angle), and high joint strength. The steel member according to the present invention (Test No. 61) has chemical compositions of the base metal portion of the first steel member, number densities of transition carbides in the base metal portion and HAZ of the first steel member, and differences in hardness between the nugget fusion boundary vicinity and the nugget interior within specified ranges, and therefore have high base metal strength, excellent impact absorption (high VDA bend angle), and high joint strength. On the other hand, in the steel members according to the comparative examples, at least the number density of transition carbides in the base material and HAZ of the first steel material, and the difference in hardness between the area near the fusion boundary of the nugget and the inside of the nugget, are outside the specified range, and it is found that one or more of the properties of impact absorption and joint strength are inferior.

[0120] According to the present invention, a steel member can be obtained in which at least one steel material constituting the steel member has both high strength and excellent impact absorption, and the joint strength at the resistance spot welds is also excellent. Therefore, the present invention has high industrial applicability.

[0121] REFERENCE SIGNS LIST 100 Steel member 11 First steel material 12 Second steel material 13 Resistance spot welded portion 2 HAZ 21 HAZ hardened portion 22 HAZ softened portion 3 Nugget CF Contact surface EP End of contact surface MA, MA2 Reference points MRP, MRP2 Measurement points IL, IL2 Virtual line MB Fusion boundary

Claims

1. A steel member having a first steel material which is a hot stamp formed body, one or more second steel materials, and a resistance spot welded portion which joins the first steel material and the one or more second steel materials and includes a nugget and a HAZ, wherein a base metal portion of the first steel material which is a portion other than the resistance spot welded portion contains, in mass %, C: 0.40 to 1.00%, Si: 0.01 to 2.00%, Mn: 1.00% or more but less than 5.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0150% or less, O: 0.0100% or less, B: 0 to 0.0100%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, W. : 0 to 3.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Co: 0 to 1.000%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, V: 0 to 0.500%, Ca: 0 to 1.0000%, Mg: 0 to 1.0000%, REM: 0 to 0.0050%, Sn: 0 to 0.100%, Sb: 0 to 0.020%, Zr: 0 to 0.100%, As: 0 to 0.100%, and the balance: Fe and impurities, and when a position at 1 / 4 of the thickness from the surface of the first steel material in the thickness direction is defined as a 1 / 4 depth position, At the 1 / 4 depth position of the base material portion of the first steel material, the number density of transition carbides having a circle equivalent diameter of 5 nm or more is 20 pieces / μm ２ or more, the tensile strength of the base metal portion of the first steel material is 2100 MPa or more, and when a position 0.2 mm from an end of a contact surface between the first steel material and the second steel material in the HAZ of the resistance spot weld toward the first steel material in the thickness direction of the first steel material is set as a reference point, and an imaginary line is drawn passing through the reference point in a direction perpendicular to the thickness direction of the first steel material, the number density of transition carbides having a circle equivalent diameter of 5 nm or more is 20 / μm at a position 0.2 mm from the reference point along the imaginary line in the direction of the nugget ２ and in the resistance spot weld, a difference ΔHV between a Vickers hardness at a position on the imaginary line 50 μm toward the nugget from the boundary between the nugget and the HAZ and a Vickers hardness at a position 500 μm toward the nugget from the boundary is less than 70 HV.

2. A steel member according to claim 1, characterized in that, when a position 20 μm from the surface of the first steel material in the thickness direction is defined as a 20 μm depth position, the hardness of the base material portion of the first steel material at the 20 μm depth position is smaller than the hardness of the base material portion of the first steel material at the 1 / 4 depth position.

3. A steel member according to claim 2, characterized in that the hardness of the base material portion of the first steel material at the 20 μm depth position is 100 HV or more lower in Vickers hardness than the hardness of the base material portion of the first steel material at the 1 / 4 depth position.

4. The chemical composition of the base material is, in mass%, B: 0.0002 to 0.0100%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, W: 0.01 to 3.00%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, Co: 0.010 to 1.000%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, V: 0.010 to 0.500%, Ca: 0.0001 to 1.0000%, Mg: 0.0001 to 1.0000%, REM: 0.0001 to 0.0050%, The steel member according to any one of claims 1 to 3, characterized in that it contains one or more elements selected from the group consisting of Sn: 0.001 to 0.100%, Sb: 0.001 to 0.020%, Zr: 0.001 to 0.100%, and As: 0.001 to 0.100%.

5. The base metal portion of the second steel material, which is the portion other than the resistance spot welded portion, contains, in mass %, C: 0.40 to 1.00%, Si: 0.01 to 2.00%, Mn: 1.00% or more but less than 5.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0150% or less, O: 0.0100% or less, B: 0 to 0.0100%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, W: 0 to 3.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Co: 0 to 1.000%, Ni: 0 to 1.00%, The second steel material has a chemical composition consisting of Cu: 0 to 1.00%, V: 0 to 0.500%, Ca: 0 to 1.0000%, Mg: 0 to 1.0000%, REM: 0 to 0.0050%, Sn: 0 to 0.100%, Sb: 0 to 0.020%, Zr: 0 to 0.100%, As: 0 to 0.100%, and the balance: Fe and impurities, and when a position at 1 / 4 of the thickness from the surface of the second steel material in the thickness direction is defined as a 1 / 4 depth position, the number density of transition carbides having a circle equivalent diameter of 5 nm or more at the 1 / 4 depth position of the base material portion of the second steel material is 20 pieces / μm ２ or more, the tensile strength of the base metal portion of the second steel material is 2100 MPa or more, and when a position 0.2 mm from an end of a contact surface between the first steel material and the second steel material in the HAZ of the resistance spot weld toward the second steel material in the thickness direction of the second steel material is set as a reference point, and an imaginary line is drawn passing through the reference point in a direction perpendicular to the thickness direction of the second steel material, the number density of transition carbides having a circle equivalent diameter of 5 nm or more is 20 / μm at a position 0.2 mm from the reference point along the imaginary line in the direction of the nugget ２ The steel member according to any one of claims 1 to 3, wherein:

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