Spot welded joint, method for manufacturing spot welded joint, and automotive parts

Abstract

To provide a spot welded joint which includes a high-strength steel sheet having tensile strength of 1700 MPa or more and can suppress melt boundary fracture, and to provide a method for manufacturing the spot welded joint, and an automobile component.SOLUTION: A spot welded joint according to one aspect of the present invention includes: plate sets including a first steel sheet having a C content of 0.28 mass% or more and tensile strength of 1700 MPa or more, and a second steel sheet having a C content of less than 0.28 mass%; and a nugget for joining the plate sets. At least one of the first steel sheet is overlapped with the second steel sheet, and a melt boundary hardness gradient in an entire hardness evaluation region of the first steel sheet is -0.5 to 0 Hv / μm or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a spot weld joint, a method for manufacturing a spot weld joint, and automotive parts.

Background Art

[0002] Resistance spot welding can join steel plates cheaply and quickly. Therefore, resistance spot welding is used in various applications such as joining high-strength steel plates, which are materials for automotive body members.

[0003] On the other hand, in the automotive field, the strength and thickness of steel plates are increasing. By increasing the strength and reducing the thickness of the steel plate, it is possible to reduce the weight while maintaining the strength of the automotive body member, and to obtain effects such as improved fuel efficiency.

[0004] Here, for steel plates with a tensile strength of 1700 MPa or more, a decrease in the joining strength of spot weld joints has become a problem. For example, in a spot weld joint with a base metal steel plate having a tensile strength of 1780 MPa, there is a problem that the cross-tension strength (CTS) is significantly reduced. The cross-tension strength means the strength of the joint with respect to the stress in the direction of peeling the steel plates included in the plate assembly.

[0005] One of the factors for the decrease in CTS is the decrease in toughness near the end of the nugget. In high-strength steel plates, toughness is ensured by applying heat treatments under various conditions to optimize the metal structure. However, when high-strength steel plates are spot welded, the metal structure changes in the nugget and its surrounding heat-affected zone, and as a result, the spot weld joint is likely to break. Due to the above circumstances, a joining method for suppressing the embrittlement of the end of the nugget is required.

[0006] Patent Document 1 discloses a joint structure comprising: a first steel plate member; a second steel plate member superimposed on the first steel plate member in the thickness direction; a third steel plate member superimposed on the second steel plate member in the thickness direction on the side of the second steel plate member opposite to the side on which the first steel plate member is superimposed; and a welded portion formed in the superimposed portion of the first steel plate member, the second steel plate member, and the third steel plate member to join them together, wherein the third steel plate member is folded back at the end of the second steel plate member and has a folded portion disposed between the first steel plate member and the second steel plate member, and the folded portion is joined to the first steel plate member and the second steel plate member by the welded portion.

[0007] Patent Document 2 discloses a resistance spot welding method in which, before the start of current application for the main welding, the workpiece to be welded is pressurized until it reaches an initial setting pressure, and then the pressure applied during the main welding is set using a parameter that serves as an indicator of the pressure obtained from the start of pressurization before the start of current application for the main welding until the initial setting pressure is reached.

[0008] Patent Document 3 discloses a resistance spot welding method in which a main weld and a test weld preceding the main weld are performed, and the test weld is performed under two or more welding conditions. In the test weld, for each welding condition, the pressurization parameter from the start of pressurization of the workpiece before energization begins until the set pressure is reached, the time change curve of the instantaneous heat generation, and the cumulative heat generation are stored. Furthermore, in the main weld, the workpiece is pressed under the same conditions as the test weld before energization begins, and the pressurization parameter at this time is compared with the parameter stored in the test weld for each welding condition to set target values ​​for the time change curve of the instantaneous heat generation and the cumulative heat generation of the main weld. An adaptive control welding method is then performed in which the amount of current is controlled according to these target values.

[0009] Patent Document 4 discloses a spot-welded member formed by spot-welding two or more steel plates having a carbon content of 0.05 mass% or more and 0.5 mass% or less, a tensile strength TS of 780 MPa or more, and a plate thickness t of 1.2 mm or more, wherein the shape, size, and hardness of the nugget region, heat-affected hardening region, and heat-affected softening region of the spot-welded area, as well as the end position of the sheet separation, satisfy predetermined conditions. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2020-157348 [Patent Document 2] Japanese Patent Publication No. 2021-112773 [Patent Document 3] International Publication No. 2019 / 160141 [Patent Document 4] Japanese Patent Publication No. 2016-209919 [Overview of the project] [Problems that the invention aims to solve]

[0011] The inventors have discovered that the embrittlement of a nugget can be suppressed by layering a high-strength steel plate with a high carbon content and a low-strength steel plate with a low carbon content. When a plate assembly composed of a high-strength steel plate and a low-strength steel plate is spot-welded, the molten metal of the high-strength steel plate and the molten metal of the low-strength steel plate mix, and the carbon is diluted. As a result, the carbon content of the nugget becomes lower than that of the high-strength steel plate. The hardness of steel is proportional to its carbon content. Therefore, by diluting the carbon in the nugget, the embrittlement of the nugget can be suppressed.

[0012] However, even in spot-welded joints where nugget embrittlement was suppressed, sufficient CTS (Critical Tissue Saturation) was often not ensured. The inventors of this invention conducted a detailed investigation into the fracture morphology of welds in spot-welded joints combining high-strength steel plates and low-strength steel plates. As a result, it was discovered that in spot-welded joints combining high-strength steel plates and low-strength steel plates, cracks propagate along the molten boundary of the nugget. The molten boundary is the outer edge of the nugget; in other words, it is the boundary between the nugget, where melting and solidification occur during spot welding, and the heat-affected zone, where melting and solidification do not occur during spot welding.

[0013] None of the patent documents 1 to 4 focus on this type of fracture, nor do they provide any means to solve it.

[0014] Specifically, Patent Document 1 is a technology for steel plates with a tensile strength of 1500 MPa or less, and does not consider spot welding of steel plates with a tensile strength of 1700 MPa or more. Furthermore, Patent Document 1 does not consider fractures that occur along the molten boundary, nor does it disclose a nugget configuration that can suppress such fractures.

[0015] Patent documents 2 and 3 address the issue of suppressing scattering and do not consider fractures that occur along the molten boundary. Furthermore, neither patent documents 2 nor 3 disclose a nugget configuration that can suppress such fractures.

[0016] Patent Document 4 discloses that spot-welded joints may undergo either plug fracture or delamination fracture during a cross-tensile test, but it does not specifically examine whether cracks propagate along the molten boundary in these fractures. Furthermore, Patent Document 4 does not disclose a nugget configuration that can suppress this fracture.

[0017] The present invention aims to provide a spot-welded joint containing a high-strength steel plate with a tensile strength of 1700 MPa or more, capable of suppressing fusion boundary fracture, a method for manufacturing the same, and an automobile part. [Means for solving the problem]

[0018] The gist of this invention is as follows:

[0019] (1) A spot welded joint according to one aspect of the present invention comprises a plate assembly including a first steel plate having a carbon content of 0.28% by mass or more and a tensile strength of 1700 MPa or more, and a second steel plate having a carbon content of less than 0.28% by mass, and a nugget for joining the plate assembly, wherein at least one of the first steel plates is overlapped with the second steel plate, and the hardness evaluation region of the first steel plate is defined in a cross section of the spot welded joint that passes through the center of the nugget and is perpendicular to the surface of the plate assembly, with respect to a first imaginary line along the overlapping surface of the first steel plate and the second steel plate, and parallel to the first imaginary line, and When the region between the second imaginary line, which is 400 μm away from the first steel sheet and located inside the first steel sheet, is defined as the region between the imaginary line and the first imaginary line, and the melting boundary hardness gradient is defined as the slope of the linear approximation obtained by the least squares method of the hardness distribution of a region with a width of 280 μm centered on the melting boundary, obtained by continuously performing micro-Vickers hardness measurements at 70 μm intervals with a load of 50 gf along a third imaginary line perpendicular to the melting boundary which is the outer edge of the nugget, from the outside to the inside of the nugget, within the hardness evaluation region, the measured melting boundary hardness gradient in all of the hardness evaluation regions of the first steel sheet is -0.5 to 0 Hv / μm. (2) The spot welded joint described in (1) above preferably has a P segregation evaluation region defined as a 200 μm square rectangular region located inside the hardness evaluation region and inside the nugget, where one or more of its vertices coincide with the molten boundary and the extension direction of one of its four sides coincides with the thickness direction of the plate assembly; the average P concentration defined as the average value of the P concentration measured at each P concentration measurement point arranged at 0.5 μm intervals along the vertical and horizontal sides of the P segregation evaluation region; a P segregation point defined as a P concentration measurement point in the P segregation evaluation region having a P concentration of 10 times or more the average P concentration; and the degree of P segregation defined as the ratio of the number of P segregation points to the total number of P concentration measurement points, wherein the degree of P segregation is 0.3% or more. (3) The spot welded joint described in (1) above preferably has a difference in carbon content between the first steel plate and the second steel plate that are stacked on top of each other of 0.07% or less. (4) The spot welded joint described in (2) above preferably has a difference in carbon content between the first steel plate and the second steel plate that are stacked on top of each other of 0.07% or less.

[0020] (5) A method for manufacturing a spot-welded joint according to another aspect of the present invention is a method for manufacturing a spot-welded joint according to any one of (1) to (4) above, comprising the steps of: applying pressure using a spot welding electrode to a plate assembly including a first steel plate having a carbon content of 0.28% by mass or more and a tensile strength of 1700 MPa or more, and a second steel plate having a carbon content of less than 0.28% by mass, and then applying a main current to the plate assembly to locally melt the plate assembly; and after the main current is applied, holding the pressure applied to the plate assembly with the current substantially stopped, and cooling the plate assembly, wherein at least one of the first steel plates is stacked with the second steel plate, and the pressure FE (N), welding current IW (kA), and average plate thickness h (mm) of the steel plates included in the plate assembly satisfy the following formulas A and B, and during the holding, the pressure FE satisfies formula A, and the holding time tH (msec) satisfies formula C. 1960×h≦FE≦3920×h (Formula A) 0.5 ≤ IW ≤ 9 ··· (Equation B) 0 ≤ tH ≤ 200 ··· (Equation C) (6) The method for manufacturing the spot welding joint according to (5) above preferably continuously executes the step of performing the main energization and the step of holding, and after the step of holding, releases the pressing force. (7) The method for manufacturing the spot welding joint according to (5) above preferably holds the pressing force on the plate assembly in a state where the energization is substantially stopped between the step of performing the main energization and the step of performing the holding, thereby cooling the plate assembly, and reheats the nugget by performing energization after segregation relaxation on the nugget formed in the plate assembly. The method further includes steps of cooling and reheating. The pressing force FE (N) in the cooling and the energization after segregation relaxation satisfies the formula A, the cooling time tS (msec) in the cooling satisfies the following formula D, the welding current IW (kA) in the main energization and the reheating current IP (kA) in the energization after segregation relaxation satisfy the following formula E, and the energization time tP (msec) in the energization after segregation relaxation satisfies the following formula F. 1 ≤ tS ≤ 400 ··· (Equation D) 0.6 × IW ≤ IP ≤ 0.9 × IW ··· (Equation E) 50 ≤ tP ≤ 1000 ··· (Equation F)

[0021] (8) An automotive part according to another aspect of the present invention includes the spot welding joint according to any one of (1) to (4) above.

Advantages of the Invention

[0022] According to the present invention, it is possible to provide a spot welding joint including a high-strength steel plate with a tensile strength of 1700 MPa or more and capable of suppressing melting boundary fracture, a method for manufacturing the same, and an automotive part.

Brief Description of the Drawings

[0023] [Figure 1] It is a schematic cross-sectional view of a spot welding joint according to the present embodiment. [Figure 2] It is a schematic diagram of a method for measuring the melting boundary hardness gradient. [Figure 3A] This is a schematic diagram of the P-segregation evaluation region. [Figure 3B] This is a schematic diagram of the P-segregation evaluation region. [Figure 3C] This is a schematic diagram of the P-segregation evaluation region. [Figure 4] This is a flowchart of the method for manufacturing a spot-welded joint according to this embodiment. [Figure 5] This is a schematic cross-sectional view of an example of a spot-welded joint where the plate assembly consists of three steel plates. [Figure 6] This is a schematic cross-sectional view of an example of a spot-welded joint where the plate assembly consists of three steel plates. [Figure 7] This is a schematic cross-sectional view of an example of a spot-welded joint where the plate assembly consists of three steel plates. [Modes for carrying out the invention]

[0024] (1. Spot welded joints) A spot-welded joint 1 according to one aspect of the present invention, as illustrated in Figures 1 and 2, comprises a plate assembly 11 including a first steel plate 111 having a carbon content of 0.28% by mass or more and a tensile strength of 1700 MPa or more, and a second steel plate 112 having a carbon content of less than 0.28% by mass, and a nugget 12 for joining the plate assembly 11, wherein at least one of the first steel plates 111 is overlapped with the second steel plate 112, and the hardness evaluation region 111H of the first steel plate 111 is defined in a cross section of the spot-welded joint 1 that passes through the center of the nugget 12 and is perpendicular to the surface of the plate assembly 11, along a first virtual line VL1 that is parallel to the first virtual line VL1, and a first virtual line that is parallel to the first virtual line VL1. The hardness evaluation region 111H is defined as the region between the imaginary line VL1, which is 400 μm away from the second imaginary line VL2, which is located inside the first steel plate 111, and the molten boundary hardness gradient is defined as the slope of the linear approximation obtained by the least squares method of the hardness distribution of a region with a width of 280 μm centered on the molten boundary 12B, obtained by continuously performing micro-Vickers hardness measurements at 70 μm intervals with a load of 50 gf from the outside to the inside of the nugget 12 along a third imaginary line VL3 which is perpendicular to the molten boundary 12B, which is the outer edge of the nugget 12, within the hardness evaluation region 111H, and the molten boundary hardness gradient in all hardness evaluation regions 111H of the first steel plate 111 is -0.5 to 0 Hv / μm. However, if the thickness of the first steel plate 111 superimposed on the second steel plate 112 is less than 400 μm, the entire interior of the first steel plate 111 may be considered as the hardness evaluation region 111H.

[0025] (A plate assembly 11 consisting of a first steel plate 111 and a second steel plate 112) The spot-welded joint 1 according to this embodiment has a plate assembly 11 including a first steel plate 111 and a second steel plate 112. The plate assembly 11 is a welding base material formed by overlapping steel plates.

[0026] The first steel sheet 111 is defined as a steel sheet with a carbon content of 0.28% by mass or more and a tensile strength of 1700 MPa or more. The second steel sheet 112 is defined as a steel sheet with a carbon content of less than 0.28% by mass. The sheet assembly 11 includes one or more first steel sheets 111 and one or more second steel sheets 112. Note that a steel sheet with a carbon content of 0.28% by mass or more and a tensile strength of less than 1700 MPa does not fall under either the first steel sheet 111 or the second steel sheet 112. The sheet assembly 11 may include such a steel sheet.

[0027] (Nuggets 12) The spot-welded joint 1 according to this embodiment has a nugget 12. The nugget 12 is a part of the weld formed by resistance welding, and is the metal that melted and solidified during welding. The nugget 12 joins the plate assembly 11. In other words, the nugget 12 joins multiple overlapping steel plates included in the plate assembly 11. The outer edge of the nugget 12 is called the molten boundary 12B.

[0028] (Positional relationship between the first steel plate 111 and the second steel plate 112) As shown in Figure 1, at least one of the first steel plates 111 included in the plate assembly 11 is overlapped with the second steel plate 112. Preferably, all of the first steel plates 111 included in the plate assembly 11 are overlapped with the second steel plate 112. This causes carbon dilution around the overlapping surface 11A of the first steel plate 111 and the second steel plate 112. The nugget 12 is obtained when the molten first steel plate 111 and the second steel plate 112 solidify. Therefore, around the overlapping surface 11A of the first steel plate 111 and the second steel plate 112, the carbon concentration of the nugget 12 is lower than that of the first steel plate 111 and higher than that of the second steel plate 112.

[0029] (Melting boundary hardness gradient) In the spot-welded joint 1 according to this embodiment, the molten boundary hardness gradient in all hardness evaluation regions 111H of the first steel plate 111, as determined by hardness measurements performed continuously from the outside to the inside of the nugget, is -0.5 to 0 HV / μm. The method for measuring the molten boundary hardness gradient and the definitions of terms used to specify the measurement method will be described below with reference to Figure 2.

[0030] The fusion boundary hardness gradient is a value evaluated in a cross section of the spot welded joint 1 that passes through the center of the nugget 12 and is perpendicular to the surface of the plate assembly 11. Figures 1 and 2 are cross-sectional views of the spot welded joint 1 according to this embodiment in the said cross section. Hereinafter, the said cross section will simply be referred to as the "cross section".

[0031] As shown in Figure 2, the imaginary line along the overlapping surface 11A of the first steel plate 111 and the second steel plate 112 in the cross-section is referred to as the first imaginary line VL1. The imaginary line parallel to the first imaginary line VL1, separated from the first imaginary line VL1 by 400 μm, and located inside the first steel plate 111 is referred to as the second imaginary line VL2. Furthermore, any straight line perpendicular to the molten boundary 12B, which is the outer edge of the nugget 12, is referred to as the third imaginary line VL3.

[0032] The region between the first virtual line VL1 and the second virtual line VL2 is defined as the hardness evaluation region 111H of the first steel plate 111. In the spot-welded joint 1 illustrated in Figure 1, there is only one overlapping surface 11A between the first steel plate 111 and the second steel plate 112, and therefore there is only one hardness evaluation region 111H. However, there are cases where the number of overlapping surfaces 11A between the first steel plate 111 and the second steel plate 112 is two or more. For example, as shown in Figure 6, in a plate assembly 11 in which the first steel plate 111 is sandwiched between two second steel plates 112, there are two overlapping surfaces 11A between the first steel plate 111 and the second steel plate 112. In such a plate assembly, there are also two hardness evaluation regions 111H. If the thickness of the first steel sheet 111 is less than 800 μm, the two hardness evaluation regions overlap. In this case, the melt boundary hardness gradient may be measured for each of the overlapping regions, or, if measurement is possible in the overlapping region, the melt boundary hardness may be measured only for the overlapping region. The same applies to the measurement of P segregation degree, which will be described later.

[0033] The melting boundary hardness gradient is a value obtained by continuously performing micro-Vickers hardness measurements across the melting boundary 12B within the hardness evaluation region 111H. Specifically, within the hardness evaluation region 111H, micro-Vickers hardness measurements with a load of 50gf are continuously performed at 70μm intervals along the third virtual line VL3, from the outside to the inside of the nugget 12. The slope of the linear approximation line obtained by the least squares method of the hardness distribution in a region with a width of 280μm centered on the melting boundary 12B is defined as the melting boundary hardness gradient. Note that the hardness measurement is performed continuously from the outside to the inside of the nugget. Therefore, if the hardness decreases from the outside to the inside of the nugget, the melting boundary hardness gradient will be a negative value. The melting boundary hardness gradient only needs to be measured once for each hardness evaluation region 111H.

[0034] In the spot welded joint 1 according to this embodiment, the molten boundary hardness gradient is set to -0.5 to 0 HV / μm in all of the one or more hardness evaluation regions 111H. The molten boundary hardness gradient may be -0.4 HV / μm or higher, -0.3 HV / μm or higher, or -0.2 HV / μm or higher in all of the hardness evaluation regions 111H. The molten boundary hardness gradient may also be -0.1 HV / μm or lower.

[0035] (Effects and effects of the spot welded joint 1 according to this embodiment) In the spot-welded joint 1 according to this embodiment, at least one of the first steel plates 111 is overlapped with the second steel plate 112. As a result, the carbon content of the nugget 12 is lower than that of the first steel plate 111. Consequently, in the spot-welded joint 1 according to this embodiment, the embrittlement of the nugget 12, which is characteristic of spot-welded joints 1 made of high-strength steel plates, is suppressed.

[0036] However, in the spot-welded joint 1 where the first steel plate 111 and the second steel plate 112 are overlapped, there is a tendency for fracture to propagate along the fusion boundary 12B. This fusion boundary fracture reduces the joint strength of the spot-welded joint 1, especially the CTS.

[0037] The inventors considered that the difference in hardness between the nugget 12 and the first steel plate 111 was the cause of molten boundary fracture. The inventors then found that molten boundary fracture could be suppressed by suppressing the difference in hardness between the nugget 12 and the first steel plate 111 and setting the molten boundary hardness gradient to -0.5 to 0 Hv / μm.

[0038] Furthermore, the areas where fusion boundary fracture is a concern are the regions in the nugget 12 where the degree of carbon dilution is high and the boundary with the first steel plate 111. The regions where the degree of carbon dilution is high are the peripheral areas of the joint surface between the first steel plate 111 and the second steel plate 112. Therefore, in the spot welded joint 1 according to this embodiment, the fusion boundary hardness gradient is evaluated in the hardness evaluation region 111H, which is within 400 μm from the joint surface. If the number of hardness evaluation regions 111H is two or more, the fusion boundary hardness gradient is within a predetermined range in all hardness evaluation regions 111H. On the other hand, at the joint surfaces of the two first steel plates 111 or the joint surfaces of the two second steel plates 112, the difference in carbon concentration between the nugget 12 and the base steel plate is small. Also, even in regions away from the joint surface between the first steel plate 111 and the second steel plate 112, the difference in carbon concentration between the nugget 12 and the base steel plate is small. In regions where a hardness difference between the nugget 12 and the base steel plate is unlikely to occur, the molten boundary hardness gradient is not particularly limited.

[0039] One method for setting the molten boundary hardness gradient within a predetermined range is to apply current to the nugget 12 after segregation relaxation under the conditions described later. Current application after segregation relaxation is a post-welding process that, immediately after solidification following resistance welding, promotes the diffusion of segregated elements by maintaining the nugget 12 at a high temperature, thereby improving the toughness of the weld metal. Current application after segregation relaxation differs from spot welding (main welding) in that it does not melt the nugget 12 or the base metal.

[0040] By applying current after segregation relaxation, the nugget 12 and its surroundings are kept at a high temperature, thereby homogenizing the hardness around the molten boundary 12B. Normally, P segregation occurs inside the nugget 12 of a spot-welded joint 1 containing high-strength steel plate. However, in the nugget 12 after segregation relaxation and application of current, P segregation is also eliminated.

[0041] On the other hand, it is also possible to set the molten boundary hardness gradient within a predetermined range without applying current after segregation relaxation. P segregation remains inside the nugget 12 obtained without applying current after segregation relaxation. Specifically, the degree of P segregation of the nugget 12, as evaluated by the measurement method described below, is 0.3% or more. By evaluating the degree of P segregation, it is possible to estimate whether or not current was applied after segregation relaxation. By omitting current application after segregation relaxation, the manufacturing efficiency of the spot welded joint 1 can be increased.

[0042] First, we will explain the definition of terms used to describe the measurement site for P segregation. As shown in Figures 3A to 3C, the P segregation evaluation area 12P is defined as a 200 μm square rectangular area located within the hardness evaluation area 111H and within the nugget 12, in a cross section of the spot welded joint 1 that passes through the center of the nugget 12 and is perpendicular to the surface of the plate assembly 11, where one or more of its vertices coincide with the molten boundary, and the extension direction of one of its four sides coincides with the thickness direction of the plate assembly 11.

[0043] Figure 3A is a schematic diagram of a cross-section of a spot-welded joint 1 having two steel plates, where the plate assembly 11 passes through the center of the nugget 12 and is perpendicular to the surface of the plate assembly 11. In the spot-welded joint 1 illustrated in Figure 3A, the cross-section of the nugget 12 is elliptical. In such a spot-welded joint 1, the P segregation evaluation region 12P is set inside the hardness evaluation region 111H and inside the nugget 12. The P segregation evaluation region 12P also has a rectangular shape of 200 μm on each side. Furthermore, one or more of the four vertices of the P segregation evaluation region 12P are in contact with the outer edge of the nugget 12, i.e., the fusion boundary 12B. In addition, the extension direction of one of the four sides of the P segregation evaluation region 12P is set to coincide with the thickness direction of the plate assembly 11. Note that trace cavities are not considered to be inside the nugget 12. If the aforementioned location contains trace cavities or other defects and is unsuitable for measuring the degree of P segregation, it is permissible to move the measurement location slightly away from the aforementioned location. For example, the degree of P segregation measured at the position closest to the edge of nugget 12 is expected to be almost identical to the degree of P segregation measured at a position approximately 100 μm away from this position.

[0044] Figure 3B is a schematic diagram of a spot welded joint 1 in which the plate assembly 11 has one first steel plate 111 and two second steel plates 112, and the first steel plate 111 is placed on the surface of the plate assembly 11, with the cross section passing through the center of the nugget 12 and perpendicular to the surface of the plate assembly 11. In such a spot welded joint 1, the P segregation evaluation region 12P can be set in the same way as the spot welded joint 1 illustrated in Figure 3A.

[0045] Figure 3C is a schematic diagram of a spot-welded joint 1 in which the plate assembly 11 has one first steel plate 111 and two second steel plates 112, and the first steel plate 111 is located inside the plate assembly 11, with the cross section passing through the center of the nugget 12 and perpendicular to the surface of the plate assembly 11. In such a spot-welded joint 1, there are two hardness evaluation regions 111H, and therefore there are also two P segregation evaluation regions 12P.

[0046] In this P segregation evaluation region 12P, the P concentration is measured. The P concentration is measured at P concentration measurement points arranged at 0.5 μm intervals along the vertical and horizontal sides of the P segregation evaluation region 12P. Although it is not necessary to distinguish between the vertical and horizontal sides of the P segregation evaluation region 12P, for convenience in this embodiment, the sides extending along the thickness direction of the plate assembly 11 are referred to as vertical sides, and the sides perpendicular to the vertical sides are referred to as horizontal sides. Since the lengths of the vertical and horizontal sides of the P segregation evaluation region 12P are 200 μm, the number of P concentration measurement points is (200 ÷ 0.5). 2 This amounts to 160,000 points. The average value of the P concentrations measured at these measurement points is defined as the average P concentration. Furthermore, among these measurement points, those with a P concentration 10 times or more than the average P concentration are defined as P segregation points. The ratio of the number of P segregation points to the total number of P concentration measurement points is considered to be the degree of P segregation of spot welded joint 1.

[0047] One example of a method for setting the molten boundary hardness gradient within a predetermined range without applying current after segregation relaxation is to optimize the combination of the first steel plate 111 and the second steel plate 112, and to optimize the current application conditions as described below. In the spot welded joint 1 obtained in this way, the difference in carbon content between the overlapping first steel plate 111 and the second steel plate 112 is 0.07% or less. To dilute the carbon in the nugget 12, it is preferable that the carbon content of the second steel plate 112 be small. On the other hand, by setting the difference in carbon content between the overlapping first steel plate 111 and the second steel plate 112 to 0.07% or less, the hardness difference between the nugget 12 and the first steel plate 111 can be reduced. The difference in carbon content between the overlapping first steel plate 111 and the second steel plate 112 may be 0.06% or less, 0.05% or less, 0.04% or less, or 0.03% or less. Naturally, the current may be applied after segregation relaxation, and the difference in carbon content between the first steel plate 111 and the second steel plate 112 may be kept within the above-mentioned range.

[0048] (2. Method for manufacturing spot welded joint 1) Next, a method for manufacturing a spot-welded joint 1 according to another aspect of the present invention will be described. The method for manufacturing a spot-welded joint 1 according to another aspect of the present invention is shown in the flowchart of Figure 4, (Main energizing process S1) A process in which the main energizing process is applied while applying pressure to the plate assembly 11, which includes a first steel plate 111 having a carbon content of 0.28 mass% or more and a tensile strength of 1700 MPa or more, and a second steel plate 112 having a carbon content of less than 0.28 mass%, to locally melt the plate assembly 11. (Holding process S2) After the initial energization, the process of holding the pressure applied to the plate assembly 11 while the energization is substantially stopped. The assembly is equipped such that at least one of the first steel plates 111 is stacked with the second steel plate 112, and the applied pressure FE (N), welding current IW (kA), and average plate thickness h (mm) of the steel plates included in the plate assembly 11 satisfy the following equations A and B, and during holding, the applied pressure FE satisfies equation A, and the holding time tH (msec) satisfies the following equation C. 1960×h≦FE≦3920×h (Formula A) 0.5≦IW≦9 (Formula B) 0≦tH≦200 (Formula C)

[0049] (Main energization process S1) First, the plate assembly 11 is energized while applying pressure using a spot welding electrode. This energization is the process of heating the base material, which is the material to be welded in resistance welding, in order to melt it. This energization causes the plate assembly 11 to melt locally. For example, by using a standard spot welding electrode as specified in JIS C 9304:1999, the plate assembly 11 can be heated by applying pressure while energizing it.

[0050] The plate assembly 11 subject to this energization includes a first steel plate 111 having a carbon content of 0.28% by mass or more and a tensile strength of 1700 MPa or more, and a second steel plate 112 having a carbon content of less than 0.28% by mass. At least one of the first steel plates 111 is stacked with the second steel plate 112.

[0051] (Holding process S2) Following the initial energizing process S1, the pressure applied to the plate assembly 11 is maintained with the energization substantially stopped. Note that typical spot welding electrodes have internal channels for refrigerant flow. During spot welding, the tip of the electrode is cooled by a refrigerant such as water. Therefore, by maintaining pressure applied to the plate assembly 11 with no or near-total energization, heat transfer occurs from the weld to the electrode, cooling the weld. When the initial energizing process S1 and the holding process S2 are performed consecutively, the molten metal solidifies during the holding process S2, forming a nugget 12 that joins the plate assembly 11. On the other hand, when the cooling process S11 and the post-segregation relaxation energizing process S12, described later, are also performed, the reheated nugget 12 is cooled during the holding process S2.

[0052] In the energization process S1 and the holding process S2, the following conditions must be met. 1960×h≦FE≦3920×h (Formula A) 0.5≦IW≦9 (Formula B) 0≦tH≦200 (Formula C) The definitions of the symbols included in the above formulas A, B, and C are as follows: FE: Pressure (N) applied during the energizing process S1 and the holding process S2. h: Average plate thickness (mm) of the steel plates included in plate assembly 11 IW: Welding current (kA) in the main energizing process S1 tH: Holding time (msec) of the applied pressure in holding process S2.

[0053] The applied pressure FE is the force with which the electrodes clamp the plate assembly 11. The applied pressure FE may be a constant value, or it may be varied as appropriate during the manufacturing of the spot welded joint 1. For example, the applied pressure FE applied to the main energizing process S1 and the holding process S2, as well as the cooling process S11 and the post-segregation relaxation energizing process S12 described later, may be different. Furthermore, the applied pressure FE may fluctuate from the start to the end of the main energizing process. The same applies to cooling, post-segregation relaxation energizing, and holding.

[0054] However, in the current application process S1, the applied pressure FE must always satisfy equation A. If the applied pressure FE is too small relative to the average plate thickness h in the current application process S1, gaps may form between the steel plates during current application, potentially leading to welding defects. Conversely, if the applied pressure FE is too large relative to the average plate thickness h, the electrodes may indent into the interior of the plate assembly 11 during current application, potentially causing unevenness on the surface of the spot welded joint 1.

[0055] The welding current value IW in the current application process S1 must satisfy equation B. If the welding current value IW is insufficient, a nugget 12 of sufficient size for joining the plate assembly 11 may not be formed. On the other hand, if the welding current value IW is too high, the spot welding becomes unstable, resulting in various welding defects such as spatter. The melting current value IW may be a constant value, or it may be varied as appropriate during the manufacturing of the spot welded joint 1. However, in the current application process S1, the melting current value IW must always satisfy equation B.

[0056] In the holding process S2, the applied pressure FE must satisfy equation A. If the applied pressure FE is insufficient, heat transfer from the weld to the electrode will be hindered. On the other hand, if the applied pressure FE is excessive, the electrode may sink inward towards the plate assembly 11, potentially causing irregularities on the surface of the spot welded joint 1.

[0057] The holding time tH for the applied pressure in the holding process S2 is the time during which the applied pressure FE satisfies equation A. Providing a holding time is not necessarily required; for example, the holding time tH may be set to 0 msec. That is, the electrodes may be released simultaneously with the end of the main energizing process S1, or simultaneously with the end of the post-segregation relaxation energizing process S12 described later, thereby reducing the applied pressure to less than 1960 × h. However, considering the stability of the main energizing process S1 or the post-segregation relaxation energizing process S12, the holding time tH may be set to more than 0 msec. On the other hand, if the holding time tH exceeds 200 msec, the manufacturing efficiency of the spot welded joint 1 will be impaired.

[0058] In the manufacturing method of the spot welded joint 1 according to this embodiment, current application after segregation relaxation is not essential. For example, the current application step and the holding step may be performed continuously, and the pressure may be released after the holding step. Generally, current application after segregation relaxation is not performed after the pressure is released. By omitting current application after segregation relaxation, the manufacturing efficiency of the spot welded joint 1 is improved.

[0059] (Cooling process S11 and energizing process S12 after segregation relaxation) On the other hand, the manufacturing method of the spot welded joint 1 according to this embodiment is as shown in the flowchart of Figure 4, between the energizing step S1 and the holding step S2, (Cooling process S11) A process to cool the plate assembly 11 by maintaining the pressure applied to the plate assembly 11 while the power supply is substantially stopped, (Segregation relaxation and current application process S12) A process to reheat the nuggets 12 formed on the plate assembly 11 by applying current after segregation relaxation, The following may be further provided. In this case, it is preferable that the applied pressure FE(N) during cooling satisfies equation A, the cooling time tS(msec) during cooling satisfies equation D below, the welding current IW(kA) during the main energization and the reheating current IP(kA) during the energization after segregation relaxation satisfies equation E below, and the energization time tP(msec) after segregation relaxation satisfies equation F below. 1≦tS≦400 (Formula D) 0.6×IW≦IP≦0.9×IW (Formula E) 50≦tP≦1000 (Formula F)

[0060] (Cooling process S11) In the cooling step S11, after the completion of the energizing step S1, the plate assembly 11 is cooled by maintaining the pressure applied to the plate assembly 11 while the energizing is substantially stopped. This causes the molten metal to solidify and form the nugget 12. The cooling step S11 plays the same role as the holding step S2 in the manufacturing method of the spot welded joint 1, which does not have the segregation relaxation post-energizing step S12.

[0061] (Electrification step S12 after segregation relaxation) In the post-segregation relaxation energizing process S12, the nuggets 12 formed on the plate assembly 11 are reheated by energizing them after segregation relaxation. This heat-treats the nuggets 12 and their surroundings, bringing the molten boundary hardness gradient within a predetermined range.

[0062] In the cooling step S11 and the post-segregation relaxation energizing step S12, it is preferable that the applied pressure FE(N) in the cooling step S11 and the post-segregation relaxation energizing step S12 satisfies the above formula A, and furthermore, it is preferable that the following conditions are met. 1≦tS≦400 (Formula D) 0.6×IW≦IP≦0.9×IW (Formula E) 50≦tP≦1000 (Formula F) The definitions of the symbols included in the above formulas D, E, and F are as follows: tS: Cooling time (msec) in cooling process S11 IP: Reheating current (kA) in the post-segregation relaxation energizing process S12 tP: Time (msec) of energizing after segregation relaxation in the energizing process S12

[0063] In the cooling process S11, the applied pressure FE must also satisfy equation A. If the applied pressure FE is insufficient, heat transfer from the weld to the electrode will be hindered. On the other hand, if the applied pressure FE is excessive, the electrode may sink inward towards the plate assembly 11, potentially causing irregularities on the surface of the spot welded joint 1.

[0064] The cooling time tS is the time during the cooling process S11 when the applied pressure FE satisfies equation A. If the cooling time tS is insufficient, reheating will begin before the temperature of the weld has sufficiently decreased, which may result in improper heat treatment. On the other hand, if the cooling time tS is too long, the manufacturing efficiency of the spot welded joint 1 will be impaired.

[0065] Equation E defines the relationship between the current value IW in the main energizing process S1 and the current value IP in the post-segregation relaxation energizing process S12. If the current value IP in the post-segregation relaxation energizing process S12 is too high compared to the current value IW in the main energizing process S1, the nugget 12 may remelt. On the other hand, if the current value IP in the post-segregation relaxation energizing process S12 is too low compared to the current value IW in the main energizing process S1, the heat treatment may not be performed properly.

[0066] Equation F defines the energizing time tP after segregation relaxation. The energizing time after segregation relaxation is the time during which the applied pressure FE satisfies equation A and the current value IP satisfies equation E. If the energizing time tP after segregation relaxation is too short, the heat treatment may not be performed properly. On the other hand, if the energizing time tP after segregation relaxation is too long, the manufacturing efficiency of the spot welded joint 1 will be impaired.

[0067] (3. Automotive parts) Another embodiment of the present invention provides an automotive part comprising a spot-welded joint 1 according to the above-described embodiment. The automotive part according to this embodiment has a first steel plate 111 with a tensile strength of 1700 MPa or more, and therefore has high strength. Furthermore, in the automotive part according to this embodiment, the carbon content of the nugget 12 is suppressed, so the brittleness of the nugget 12 is suppressed. In addition, in the automotive part according to this embodiment, the molten boundary hardness gradient is within a predetermined range, so fracture along the molten boundary 12B, which is the boundary between the nugget 12 and the base material, is suppressed. Therefore, the automotive part according to this embodiment has high joint strength. The type of automotive part according to this embodiment is not particularly limited, but examples include bumpers and B-pillars, which are important components for ensuring collision safety. Also, A-pillars, side sills, floor members, front side members, kick parts, rear side members, front suspension towers, tunnel reinforcements, torque boxes, seat frames, battery case frames and the joints between these pillars (joints between B-pillars and side sills, joints between B-pillars and roof rails, joints between roof cross members and roof rails) may also be automotive parts according to this embodiment. However, the applications of the spot-welded joint 1 according to this embodiment are not limited to automotive parts.

[0068] Although embodiments of the present invention have been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical spirit of the invention. Examples of other embodiments of the spot-welded joint and its manufacturing method according to this embodiment are described below. The embodiments described below are applicable to both the spot-welded joint and its manufacturing method.

[0069] (plate thickness ratio) In the spot-welded joint 1 according to this embodiment, the plate thickness ratio may be 2 or more and 5 or less. Here, the plate thickness ratio is defined as the value obtained by dividing the total plate thickness of the steel plates included in the plate assembly 11 by the plate thickness of the thinnest steel plate placed on the surface of the plate assembly 11. According to the above definition, in the spot-welded joint 1 according to this embodiment, the plate thickness ratio is necessarily 2 or more. On the other hand, the plate thickness ratio may be 2.1 or more, 2.5 or more, or 3.0 or more. This increases the design freedom of the machine parts composed of the spot-welded joint 1 according to this embodiment. Furthermore, in the spot-welded joint 1 according to this embodiment, it is preferable to set the plate thickness ratio to 5 or less. This further reduces the occurrence rate of joint defects. The plate thickness ratio may be 4.8 or less, 4.5 or less, or 4.0 or less.

[0070] (Thickness and tensile strength of the steel plate, etc.) The thickness of the first steel plate 111 and the second steel plate 112 is not particularly limited. A particularly suitable thickness for the first steel plate 111 and the second steel plate 112 is 0.8 mm or more and 4.0 mm or less. The total thickness of the plate assembly 11 is also not particularly limited.

[0071] As stated above, the tensile strength of the first steel plate 111 is 1700 MPa or more, but it may be 1800 MPa or more, 1900 MPa or more, 2000 MPa or more, or 2100 MPa or more. The upper limit of the tensile strength of the first steel plate 111 is not particularly limited, but for example it may be 2800 MPa or less, 2500 MPa or less, or 2300 MPa or less.

[0072] The tensile strength of the second steel sheet 112 is not particularly limited. For example, it is preferable that the tensile strength of the second steel sheet 112 be less than 1800 MPa, 1700 MPa or less, 1500 MPa or less, or 1200 MPa or less. The tensile strength of the second steel sheet 112 may be 470 MPa or more, 500 MPa or more, or 600 MPa or more.

[0073] One or both of the first steel plate 111 and the second steel plate 112 may have a plating layer on their surface. Examples of plating layers include hot-dip galvanizing, hot-dip alloy galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, electro-Al plating, etc. For example, the first steel plate 111 and the second steel plate 112 may be steel plates for hot stamping, and the spot-welded joint 1 may be a hot-stamping member.

[0074] (Construction of the board structure) The total number of steel plates included in the plate assembly 11 can be any value of two or more. Similarly, the number of first steel plates 111 and second steel plates 112 can also be any value of one or more. The order in which the steel plates are stacked is also arbitrary, as long as at least one of the first steel plates 111 is stacked with a second steel plate 112.

[0075] For example, the plate assembly 11 may have a configuration in which one second steel plate 112 is sandwiched between two first steel plates 111, as shown in Figure 5. In the plate assembly 11 of Figure 5, there are two overlapping surfaces 11A between the first steel plate 111 and the second steel plate 112. Therefore, the molten boundary hardness gradient must be within a predetermined range in the hardness evaluation region 111H of both first steel plates 111.

[0076] The plate assembly 11 may have a configuration in which one first steel plate 111 is sandwiched between two second steel plates 112, as shown in Figure 6. In the plate assembly 11 of Figure 6, there are two overlapping surfaces 11A between the first steel plate 111 and the second steel plates 112. Therefore, the molten boundary hardness gradient must be within a predetermined range in both the hardness evaluation region 111H on one surface side and the hardness evaluation region 111H on the other surface side of the first steel plate 111.

[0077] The plate assembly 11 may have a configuration in which two first steel plates 111 are overlapped and one second steel plate 112 is placed on top of them, as shown in Figure 7. In the plate assembly 11 of Figure 7, there is one overlapping surface 11A between the first steel plate 111 and the second steel plate 112. It is sufficient that the molten boundary hardness gradient is within a predetermined range only in the hardness evaluation region 111H of the first steel plate 111 that is overlapped with the second steel plate 112. Since the risk of molten boundary 12B fracture is low at the overlapping surface of the two first steel plates 111, the molten boundary hardness gradient is not particularly limited.

[0078] The plate assembly 11 may include four or more steel plates. Furthermore, the plate assembly 11 may include steel plates that do not fall under either the first steel plate 111 or the second steel plate 112, as long as at least one of the first steel plates 111 is overlapped with the second steel plate 112. [Examples]

[0079] The effects of one aspect of the present invention will be further explained in detail by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention.

[0080] Various plate assemblies were created by combining steel plates A to F. For plate assemblies other than those in Invention Example 2, a method for manufacturing spot welded joints including the main energizing process and the holding process was applied to produce various spot welded joints. For the plate assemblies in Invention Example 2, a method for manufacturing spot welded joints including the main energizing process, the cooling process, the energizing process after segregation relaxation, and the holding process was applied. The molten boundary hardness gradient and P segregation degree of the spot welded joints were measured using the procedure described above. In addition, a cross tensile test was performed on the spot welded joints to cause peeling fracture of the weld, and the fracture mode was observed. Spot welded joints that showed a molten boundary fracture mode were judged as unacceptable (×), and spot welded joints that showed a plug fracture mode were judged as acceptable (〇).

[0081] The welding conditions were as follows: • Average board thickness in the board assembly: 1.6mm • Pressure applied during this energization process: 3.92N • Welding current value during this energization process: 7.0kA • Pressure applied during the holding process: 3.92N • Holding time in the holding process: 200 msec (If there is an energization process after segregation relaxation) • Pressure applied during the cooling process: 3.92N • Cooling time in the cooling process: 80 msec • Pressure applied during the energization process after segregation relaxation: 3.92N • Reheating current value in the energization process after segregation relaxation: 6.3kA • Post-segregation relaxation energizing process: Energy application time after segregation relaxation: 600 msec

[0082] Table 1 shows the carbon content of steel plates A to F. Table 2 describes the types of steel plates constituting the plate assembly, the fusion boundary hardness gradient, the degree of P segregation, and the fracture mode.

[0083] [Table 1]

[0084] [Table 2]

[0085] In the spot-welded joints of Examples 2, 4, and 6-10 of the present invention, the fusion boundary hardness gradient was within a predetermined range. When these spot-welded joints were subjected to cross-tensile tests, no fusion boundary fracture occurred.

[0086] On the other hand, in Comparative Examples 1, 3, and 5, the molten boundary hardness gradient was too small. That is, in these comparative examples, the hardness difference between the nugget and the first steel plate was not suppressed. It is presumed that this was because the difference in carbon content between the overlapping first and second steel plates was more than 0.07%, and moreover, no current was applied after segregation relaxation. When cross tensile tests were performed on these spot-welded joints, molten boundary fracture occurred.

[0087] In Comparative Example 11, the molten boundary hardness gradient was within a predetermined range, but the second steel plate was not included in the plate assembly. In Comparative Example 11, the nugget was significantly embrittlement, resulting in molten boundary fracture. [Explanation of Symbols]

[0088] 1. Spot welded joint 11 Board set 111 Daiichi Steel Plate 111H Hardness evaluation range 112 Second steel plate 11A Overlap surface between the first steel plate and the second steel plate 12 nuggets 12B Melting boundary 12P P segregation evaluation area i Indentation VL1 First Virtual Line VL2 Second Virtual Line VL3 Third Virtual Line S1 Main energization process S11 Cooling process S12 Energization process after segregation relaxation S2 holding process

Claims

1. A plate assembly comprising a first steel plate having a carbon content of 0.28% by mass or more and a tensile strength of 1700 MPa or more, and a second steel plate having a carbon content of less than 0.28% by mass, A nugget for joining the aforementioned plate assembly, A spot welded joint comprising, At least one of the first steel plates is superimposed on the second steel plate. The hardness evaluation region of the first steel plate is defined as the region between a first virtual line along the overlapping surface of the first and second steel plates, and a second virtual line parallel to the first virtual line, spaced 400 μm apart from the first virtual line, and located inside the first steel plate, in a cross section passing through the center of the nugget of the spot welded joint and perpendicular to the surface of the plate assembly. When the melting boundary hardness gradient is defined as the slope of a linear approximation obtained by the least squares method of the hardness distribution in a region with a width of 280 μm centered on the melting boundary, obtained by continuously performing micro-Vickers hardness measurements at a load of 50 gf at 70 μm intervals from the outside to the inside of the nugget along a third imaginary line perpendicular to the melting boundary which is the outer edge of the nugget, within the hardness evaluation region, then The measured molten boundary hardness gradient in all hardness evaluation regions of the first steel sheet is -0.5 to 0 Hv / μm. The P segregation evaluation region is defined as a 200 μm square rectangular region located within the hardness evaluation region and within the nugget, where one or more of its vertices coincide with the melting boundary, and the extension direction of one of its four sides coincides with the thickness direction of the plate assembly. The average P concentration is defined as the average value of the P concentrations measured at each P concentration measurement point arranged at 0.5 μm intervals along the vertical and horizontal sides of the P segregation evaluation region. A P segregation point is defined as a P concentration measurement point in the P segregation evaluation region having a P concentration of 10 times or more the average P concentration. When the degree of P segregation is defined as the ratio of the number of P segregation points to the total number of P concentration measurement points, The degree of P segregation is 0.3% or more. Spot welded joint.

2. The spot-welded joint according to claim 1, characterized in that the difference in carbon content between the first steel plate and the second steel plate, which are stacked on top of each other, is 0.07% or less.

3. A method for manufacturing a spot welded joint, A process of locally melting a plate assembly, which includes a first steel plate having a carbon content of 0.28% by mass or more and a tensile strength of 1700 MPa or more, and a second steel plate having a carbon content of less than 0.28% by mass, by applying pressure using a spot welding electrode while applying current to the plate assembly. After the aforementioned energization, the process involves maintaining the applied pressure to the plate assembly while substantially stopping the energization, and cooling the plate assembly. Equipped with, At least one of the first steel plates is superimposed on the second steel plate. The applied pressure FE (N), welding current IW (kA), and average plate thickness h (mm) of the steel plates included in the plate assembly satisfy the following equations A and B. In the holding described above, the applied pressure FE satisfies formula A, and the holding time tH (msec) satisfies the following formula C. The spot-welded joint comprises a plate assembly including a first steel plate having a carbon content of 0.28% by mass or more and a tensile strength of 1700 MPa or more, and a second steel plate having a carbon content of less than 0.28% by mass, and a nugget for joining the plate assembly, wherein at least one of the first steel plates is overlapped with the second steel plate, and the hardness evaluation region of the first steel plate is defined in a cross section of the spot-welded joint that passes through the center of the nugget and is perpendicular to the surface of the plate assembly, along a first imaginary line parallel to the overlapping surface of the first steel plate and the second steel plate, and a line parallel to the first imaginary line and spaced 400 μm apart from the first imaginary line. Furthermore, if we define the region between the second imaginary line located inside the first steel plate and the molten boundary hardness gradient as the slope of the linear approximation obtained by the least squares method of the hardness distribution in a region with a width of 280 μm centered on the molten boundary, obtained by continuously performing micro-Vickers hardness measurements at a load of 50 gf at 70 μm intervals from the outside to the inside of the nugget along a third imaginary line perpendicular to the molten boundary which is the outer edge of the nugget within the hardness evaluation region, then the measured molten boundary hardness gradient in all of the hardness evaluation regions of the first steel plate is -0.5 to 0 Hv / μm. A method for manufacturing spot-welded joints. 1960×h≦FE≦3920×h...(Formula A) 0.5≦IW≦9...(Formula B) 0≦tH≦200...(Formula C)

4. The process of applying the current and the process of holding are performed in succession. After the holding step, the pressing force is released. The method for manufacturing a spot welded joint according to claim 3.

5. Between the process of applying the current and the process of holding, The process involves cooling the plate assembly by maintaining the applied pressure on the plate assembly while substantially stopping the power supply, The process involves reheating the nuggets formed on the plate assembly by applying an electric current to them after segregation relaxation, Furthermore, The applied pressure FE(N) during the cooling and the energization after segregation relaxation satisfies the above formula A. The cooling time tS (msec) in the above cooling satisfies the following equation D, The welding current IW (kA) in the current application and the reheating current IP (kA) in the current application after segregation relaxation satisfy the following formula E: The current application time tP (msec) after segregation relaxation in the aforementioned current application after segregation relaxation satisfies the following equation F. The method for manufacturing a spot welded joint according to claim 3. 1≦tS≦400 (Formula D) 0.6×IW≦IP≦0.9×IW...(Formula E) 50≦tP≦1000...(Formula F)

6. An automotive part comprising a spot-welded joint according to claim 1 or 2.

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