Spot welded joint, and method for manufacturing a spot welded joint.

Abstract

A spot-weld joint according to one embodiment of the present disclosure has four or more overlapped steel sheets and a nugget joining the four or more steel sheets, wherein at least one of the four or more steel sheets is a medium-high carbon steel sheet having a carbon amount of 0.26 mass% or more, at least two of the four or more steel sheets are low carbon steel sheets having a carbon amount of 0.05 mass%-0.24 mass% inclusive, and the low carbon steel sheets are disposed on both surfaces of the spot-weld joint.

Description

Technical Field

[0005] , , , , ,

[0004] ,

[0003] , ,

[0001] The present disclosure relates to a spot weld joint and a method for manufacturing a spot weld joint. This application claims priority based on Japanese Patent Application No. 2023-217964 filed in Japan on December 25, 2023, and incorporates its content herein.

Background Art

[0002] In recent years, in the automotive industry, for the purpose of improving fuel efficiency by reducing the weight of the vehicle body, the application of medium and high carbon steel sheets to the vehicle body has been promoted. The medium and high carbon steel sheet (middle or high carbon steel sheet) is a general term for medium carbon steel sheets and high carbon steel sheets. By increasing the strength of the steel sheets constituting the vehicle body, the required strength can be ensured with a smaller amount of steel sheets, and the weight reduction of the vehicle body can be achieved. ​​​​​​​​​​Patent Document 1 discloses a resistance welding method in which a thin sheet material having a smaller thickness than each of the stacked sheet materials is placed on top of two or more stacked sheet materials and resistance welded with a pair of electrodes, wherein the thin sheet material is hemmed to have a folded portion, one electrode is brought into contact with the folded portion of the thin sheet material, and the other electrode is brought into contact with the lower surface of the lowest sheet material of the two or more sheet materials, and the two or more sheet materials and the thin sheet material are resistance welded together with the one and the other electrodes.

[0006] Patent Document 2 discloses a pillar structure for a vehicle comprising: a pillar member having an outer wall extending in the vertical direction of the vehicle and located on the outer side in the vehicle width direction, a front wall extending inward in the vehicle width direction from the front end of the outer wall in the vehicle longitudinal direction, and a rear wall extending inward in the vehicle width direction from the rear end of the outer wall in the vehicle longitudinal direction; a reinforcing wall facing the outer wall of the pillar member, a front wall extending inward in the vehicle width direction from the front end of the reinforcing wall in the vehicle longitudinal direction, a rear wall extending inward in the vehicle width direction from the rear end of the reinforcing wall in the vehicle longitudinal direction, and a pair of front and rear folded wall portions formed by folding back from the inner ends in the vehicle width direction of the front and rear walls, wherein the overlapping portion of the front wall and the folded wall portion located on the front side in the vehicle longitudinal direction is joined to the front wall of the pillar member by spot welding, and the overlapping portion of the rear wall and the folded wall portion located on the rear side in the vehicle longitudinal direction is joined to the rear wall of the pillar member by spot welding.

[0007] Patent Document 3 discloses an impact energy absorbing structure for the upper part of an automobile body, comprising a structural member extending in the longitudinal direction, formed by joining the flange portions of an outer panel and an inner panel spaced apart from the outer panel toward the interior of the vehicle, to create a closed structure, the structure comprising two metal energy absorbing panels having a thickness smaller than the thickness of the outer panel and the thickness of the inner panel, the energy absorbing panels having substantially the same cross-sectional shape and substantially the same thickness, the inner panel having a shell portion spaced apart toward the interior of the vehicle, and a flange portion connected to the shell portion, the two energy absorbing panels being arranged in the longitudinal direction of the structural member such that adjacent terminal portions overlap each other, and being attached to the structural member by joining the flange portions to the joint of the structural member without joining the shell portions.

[0008] Patent Document 4 discloses a spot welding method for joining multiple overlapping high-tensile steel plates by applying current while clamping and pressing them with a pair of electrodes, characterized in that a sacrificial plate is interposed between the high-tensile steel plate located furthest out in the thickness direction among the multiple high-tensile steel plates and the electrode.

[0009] Patent Document 5 discloses a spot welded joint comprising a plate assembly having a plurality of stacked steel plates, each containing one or more steel plates with a tensile strength of 1800 MPa or more, and a nugget for joining the plurality of steel plates of the plate assembly, satisfying one or both of the requirements of a P segregation degree of 0.250% or less and a Mn segregation degree of 1.500% or less, and having a nugget carbon content index Cnug of less than 0.3400 mass%. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2009-291824 [Patent Document 2] Japanese Patent Publication No. 2010-254190 [Patent Document 3] Japanese Patent Application Publication No. 9-39834 [Patent Document 4] Japanese Patent Publication No. 2019-147188 [Patent Document 5] Japanese Patent Publication No. 2023-37221 [Overview of the project] [Problems that the invention aims to solve]

[0011] In recent years, there has been an increasing need to join four or more steel plates. Furthermore, there is a demand to further improve the cross-tensile strength of spot-welded joints of four or more steel plates, including medium- and high-carbon steel plates, compared to conventional technologies.

[0012] The object of this disclosure is to provide a spot-welded joint comprising four or more steel plates, one or more of which are medium-to-high carbon steel plates, having high cross-tensile strength, and a method for manufacturing the same. [Means for solving the problem]

[0013] The gist of this disclosure is as follows:

[0014] (1) A spot welded joint according to one aspect of the present disclosure is a spot welded joint having four or more stacked steel plates and a nugget for joining the four or more steel plates, wherein at least one of the four or more steel plates is a medium-high carbon steel plate with a carbon content of 0.26 mass% or more, and at least two of the four or more steel plates are low carbon steel plates with a carbon content of 0.05 mass% or more and 0.24 mass% or less, and the low carbon steel plates are arranged on both surfaces of the spot welded joint. (2) Preferably, in the spot welded joint described in (1) above, one or both of the low-carbon steel plates arranged on the surface of the spot welded joint have a Vickers hardness of 150 Hv or more. (3) Preferably, in the spot welded joint described in (1) or (2) above, the Vickers hardness of the medium-to-high carbon steel sheet is 530 Hv or higher. (4) Preferably, in the spot welded joint described in any one of the above items (1) to (3), the thickness of one or both of the low-carbon steel plates arranged on the surface of the spot welded joint is 0.9 mm or more. (5) Preferably, in the spot welded joint described in any one of the above items (1) to (4), when the spot welded joint is viewed in plan along the thickness direction of the steel plate, the minimum distance between the end face of the low-carbon steel plate and the center of the nugget is in the range of 6 to 50 mm in one or both of the low-carbon steel plates arranged on the surface of the spot welded joint. (6) Preferably, in the spot welded joint described in (5) above, when the spot welded joint is viewed in plan along the thickness direction of the steel plate, the distance between the end face of the low-carbon steel plate and the center of the nugget is within the range of 6 to 50 mm over the entire end face of the low-carbon steel plate. (7) Preferably, in the spot welded joint described in any one of the above items (1) to (6), when the spot welded joint is viewed in plan along the thickness direction of the steel plate, one or both ends of the low carbon steel plate arranged on the surface of the spot welded joint have a convex portion and a concave portion, and when the spot welded joint is viewed in plan along the thickness direction of the steel plate, the nugget is located inside the convex portion. (8) Preferably, in the spot welded joint described in (7) above, the spot welded joint has a sub-nugget, and when the spot welded joint is viewed in plan along the thickness direction of the steel plate, the sub-nugget is located inside the recess, and the sub-nugget joins the steel plates, excluding the low-carbon steel plate having the recess in which the sub-nugget is located. (9) Preferably, the spot welded joint described in any one of the above items (1) to (8) satisfies formulas 1 and 2, 0.8×H1≦H2≦1.2×H1: Formula 1 810×(C+Mn / 100+Si / 90)+220≦H2: Formula 2 The symbol H1 represents the average Vickers hardness of the end of the nugget, the symbol H2 represents the average Vickers hardness of the center of the nugget, and the symbols C, Mn, and Si represent the carbon, Mn, and Si content of the nugget, respectively. (10) Preferably, in the spot welded joint described in any one of the above items (1) to (9), the low carbon steel plates arranged on both surfaces of the spot welded joint are separated.

[0015] (11) A method for manufacturing a spot-welded joint according to another aspect of the present disclosure comprises the steps of: stacking four or more steel plates to form a plate assembly; and spot-welding the plate assembly, wherein at least one of the four or more steel plates is a medium-high carbon steel plate with a carbon content of 0.26 mass% or more; at least two of the four or more steel plates are low carbon steel plates with a carbon content of 0.05 mass% or more and 0.24 mass% or less; and the low carbon steel plates are placed on both surfaces of the plate assembly. (12) Preferably, in the method for manufacturing a spot welded joint described in (11) above, one or both of the low-carbon steel plates arranged on the surface of the spot welded joint have a Vickers hardness of 150 Hv or more. (13) Preferably, in the method for manufacturing a spot welded joint described in (11) or (12) above, the Vickers hardness of the medium-high carbon steel sheet is 530 Hv or higher. (14) Preferably, in the method for manufacturing a spot welded joint described in any one of the above items (11) to (13), the thickness of one or both of the low-carbon steel plates arranged on the surface of the spot welded joint is 0.9 mm or more. (15) Preferably, in the method for manufacturing a spot-welded joint described in any one of the above items (11) to (14), in the spot welding, the minimum distance between the end face of the low-carbon steel plate and the welding point is set to be within the range of 6 to 50 mm for one or both of the low-carbon steel plates arranged on the surface of the spot-welded joint. (16) Preferably, in the method for manufacturing the spot welding joint described in (15) above, in the spot welding, the distance between the end face of the low-carbon steel plate and the welding point is within the range of 6 to 50 mm over the entire end face of the low-carbon steel plate. (17) Preferably, in the method for manufacturing the spot welding joint according to any one of (11) to (16) above, one or both ends of the low-carbon steel plates arranged on the surface of the plate assembly have convex portions and concave portions, and the spot welding is performed at the convex portions. (18) Preferably, in the method for manufacturing the spot welding joint described in (17) above, the spot welding is further performed at the concave portions. (19) Preferably, in the method for manufacturing the spot welding joint according to any one of (11) to (18) above, post-current application is not performed after the spot welding. (20) Preferably, in the method for manufacturing the spot welding joint according to any one of (11) to (19) above, the low-carbon steel plates arranged on both surfaces of the plate assembly are separated.

Advantages of the Invention

[0016] According to the present disclosure, it is possible to provide a spot welding joint including four or more steel plates, one or more of which is a medium-high carbon steel plate, and having high cross-tensile strength, and a method for manufacturing the same.

Brief Description of the Drawings

[0017] [Figure 1] It is a cross-sectional view of a spot welding joint in which low-carbon steel plates are arranged on both surfaces. [Figure 2] It is a cross-sectional view of a spot welding joint in which low-carbon steel plates are not arranged on both surfaces. [Figure 3] It is a cross-sectional view of a spot welding joint in which low-carbon steel plates are arranged on only one surface. [Figure 4A] It is a plan view of a spot welding joint in which the size of the low-carbon steel plate arranged on the surface is small. [Figure 4B] It is a plan view of a spot welding joint in which the size of the low-carbon steel plate arranged on the surface is small. [Figure 4C] This is a plan view of a spot-welded joint with small-sized low-carbon steel plates arranged on the surface. [Figure 5A] This is a plan view of a spot-welded joint in which a low-carbon steel plate is placed on the surface and has convex and concave portions on its end faces. [Figure 5B] This is a plan view of a spot-welded joint having a secondary nugget in the recess. [Figure 6] These are the results of the cross-tensile test in Experiment 1. [Figure 7A] This is a cross-sectional view of example A1 after a cross-tensile test. [Figure 7B] This is a cross-sectional view of example A2 after a cross-tensile test. [Figure 7C] This is a cross-sectional view of example A3 after a cross-tensile test. [Figure 7D] This is a cross-sectional view of example A4 after a cross-tensile test. [Figure 7E] This is a cross-sectional view of example A5 after a cross-tensile test. [Figure 8] These are the results of the cross-tensile test in Experiment 2. [Figure 9A] This is a cross-sectional view of example B1 after a cross-tensile test. [Figure 9B] This is a cross-sectional view of example B2 after a cross-tensile test. [Figure 9C] This is a cross-sectional view of example B3 after a cross-tensile test. [Figure 9D] This is a cross-sectional view of example B4 after a cross-tensile test. [Figure 9E] This is a cross-sectional view of example B5 after a cross-tensile test. [Figure 9F] This is a cross-sectional view of example B6 after a cross-tensile test. [Figure 10A] This diagram illustrates a method for manufacturing a B-pillar having a spot-welded joint with low-carbon steel plates on both surfaces. [Figure 10B] This diagram illustrates a method for manufacturing a B-pillar having a spot-welded joint with low-carbon steel plates on both surfaces. [Figure 10C] This diagram illustrates a method for manufacturing a B-pillar having a spot-welded joint with low-carbon steel plates on both surfaces. [Figure 10D] This diagram illustrates a method for manufacturing a B-pillar having a spot-welded joint with low-carbon steel plates on both surfaces. [Figure 11] This is a schematic diagram illustrating the method for measuring the chemical composition of chicken nuggets. [Figure 12] This is a schematic diagram illustrating the method for measuring the average Vickers hardness of the nugget's edge and the average Vickers hardness of the nugget's center. [Modes for carrying out the invention]

[0018] (1. Spot welded joint 1) A spot welded joint 1 according to one aspect of the present disclosure, illustrated in Figure 1, is a spot welded joint 1 having four or more stacked steel plates 11 and a nugget 12 that joins the four or more steel plates 11, wherein at least one of the four or more steel plates 11 is a medium-high carbon steel plate 111 with a carbon content of 0.26 mass% or more, and at least two of the four or more steel plates 11 are low carbon steel plates 112 with a carbon content of 0.05 mass% or more and 0.24 mass% or less, and the low carbon steel plates 112 are arranged on both surfaces of the spot welded joint 1. The details of the spot welded joint 1 will be described below.

[0019] The spot-welded joint 1 according to this embodiment comprises four or more stacked steel plates 11 and a nugget 12 that joins these steel plates 11. The multiple steel plates 11 may be formed into various mechanical structural members by press working or the like. For example, the steel plates 11 may be hat-shaped members. In this case, the nugget 12 is positioned on the flange portion of the hat-shaped member.

[0020] (Medium-high carbon steel sheet 111) One or more of the multiple steel plates 11 are medium-high carbon steel plates 111. A medium-high carbon steel plate 111 is a steel plate 11 with a carbon content of 0.26 mass% or more. The concept of medium-high carbon steel plate 111 encompasses both medium-carbon steel plates and high-carbon steel plates. The carbon content of the medium-high carbon steel plate 111 may be 0.27 mass% or more, 0.29 mass% or more, or 0.31 mass% or more. There is no particular upper limit to the carbon content of the medium-high carbon steel plate 111. For example, the carbon content of the medium-high carbon steel plate 111 may be 0.58 mass% or less, 0.54 mass% or less, or 0.50 mass% or less.

[0021] A suitable example of the medium-high carbon steel sheet 111 is a hot-stamped steel sheet, that is, a steel sheet that has been hot-stamped to form a predetermined shape. Hot-stamped steel sheets are typically press-formed and used as structural members for automobile bodies. Furthermore, because hot-stamped steel sheets are hardened during the hot-stamping process, they possess high hardness.

[0022] (Low-carbon steel sheet 112) Furthermore, two or more of the multiple steel plates 11 are low-carbon steel plates 112. Low-carbon steel plates 112 are steel plates 11 with a carbon content of 0.05 mass% or more and 0.24 mass% or less. The carbon content of the low-carbon steel plates 112 may be 0.07 mass% or more, 0.09 mass% or more, or 0.11 mass% or more. The carbon content of the low-carbon steel plates 112 may be 0.23 mass% or less, 0.20 mass% or less, or 0.18 mass% or less.

[0023] (Arrangement of medium-high carbon steel sheet 111 and low carbon steel sheet 112) Both steel plates 11 placed on both surfaces of the spot-welded joint 1 are low-carbon steel plates 112. That is, the low-carbon steel plates 112 are placed on the surface of the spot-welded joint 1. Since the steel plates 11 on both surfaces are low-carbon steel plates 112, the medium-high carbon steel plates 111 are necessarily placed inside the spot-welded joint 1. If the number of low-carbon steel plates 112 is three or more, one or more low-carbon steel plates 112 may be placed inside the spot-welded joint 1.

[0024] The number of steel plates 11 placed inside the spot welded joint 1 is two or more. The steel plates 11 placed inside the spot welded joint 1 may all be medium-high carbon steel plates 111, as illustrated in Figure 1. On the other hand, both low-carbon steel plates 112 and medium-high carbon steel plates 111 may be placed inside the spot welded joint 1. Furthermore, steel plates 11 that do not fall under either low-carbon steel plates 112 or medium-high carbon steel plates 111 may be placed inside the spot welded joint 1. For example, steel plates 11 with a carbon content of less than 0.05 mass% or steel plates 11 with a carbon content of more than 0.24 mass% but less than 0.26 mass% may be placed inside the spot welded joint 1.

[0025] (Effects and Benefits) In the spot-welded joint 1 according to this embodiment, the medium-high carbon steel plate 111 is placed only inside the spot-welded joint 1. The low-carbon steel plate 112 is placed on the surface of the spot-welded joint 1. As a result, the cross-tensile strength (CTS), shear tensile strength (TSS), and L-shaped tensile strength (LTS) are dramatically improved. The reason for this effect is not clear, but the phenomena confirmed in the inventor's experiments to date, and the considerations based on these, are explained below.

[0026] In prior art, attempts have been made to prevent the embrittlement of the nugget 12 by diluting the carbon in the nugget 12 by combining a medium-high carbon steel sheet 111 and a low carbon steel sheet 112. For example, Patent Document 5 discloses a technique to improve CTS by diluting the carbon in the nugget 12 by overlapping a steel sheet 11 with high tensile strength and a steel sheet 11 with low tensile strength and spot welding them together. However, in the spot-welded joint 1 according to this embodiment, it is presumed that the improvement in CTS is not due to the dilution of the carbon in the nugget 12. This is because the arrangement of the low carbon steel sheet 112 in the spot-welded joint 1 according to this embodiment is not the optimal configuration for diluting the carbon in the nugget 12.

[0027] For example, if a spot-welded joint 1 has two medium-high carbon steel plates 111 and two low-carbon steel plates 112, the optimal configuration for diluting the carbon in the nugget 12 is to place one or two low-carbon steel plates 112 between the two medium-high carbon steel plates 111, as illustrated in Figure 2. Alternatively, as illustrated in Figure 3, arranging the medium-high carbon steel plates 111 and low-carbon steel plates 112 alternately is also considered to facilitate dilution of the carbon in the nugget 12. On the other hand, if two medium-high carbon steel plates 111 are placed adjacent to each other, as in the spot-welded joint 1 illustrated in Figure 1, it is expected that the dilution of the carbon in the nugget 12 will not be sufficient in the region along the joint surface 111S of the medium-high carbon steel plates. The low-carbon steel sheet 112 sandwiching the two medium-high carbon steel sheets 111 dilutes the carbon on the upper and lower sides of the nugget 12 in Figure 1, but it is presumed that this effect does not extend to the region along the joint surface 111S of the two medium-high carbon steel sheets.

[0028] However, when the inventors conducted cross-tensile tests on two medium-high carbon steel plates 111 of the spot-welded joints 1 shown in Figures 1, 2, and 3, it became clear that the spot-welded joint 1 shown in Figure 1 had the highest CTS. In light of this fact, it is presumed that the main reason for the improvement in CTS in the spot-welded joint 1 according to this embodiment is not the dilution of carbon in the nugget 12.

[0029] The inventors performed cross-tensile tests on various spot-welded joints 1 and fractured the welds. They then observed the cross-sections of the fractured areas. It became clear that in the spot-welded joints 1 according to this embodiment, fracture along the molten boundary did not occur. The molten boundary is the boundary between the molten portion (nugget 12) and the unmolten base material. In the spot-welded joints 1 according to this embodiment, fracture occurred inside the nugget 12 or in the unmolten base material. On the other hand, in the spot-welded joints 1 illustrated in Figures 2 and 3, fracture occurred along the molten boundary. It is presumed that one of the reasons why the configuration of the spot-welded joints 1 according to this embodiment improves CTS is that it suppresses fracture along the molten boundary.

[0030] However, our experiments have confirmed that simply suppressing fracture along the molten boundary is not sufficient to improve CTS. It is presumed that there are other reasons besides protection of the molten boundary that the spot welded joint 1 according to this embodiment improves CTS.

[0031] The reason why the spot-welded joint 1 according to this embodiment produced the above-described phenomenon is not clear at present. The inventors presume that the low-carbon steel plates 112 placed on both surfaces of the spot-welded joint 1 increase the total plate thickness of the spot-welded joint 1 and suppress the deformation of the steel plate 11 placed inside the spot-welded joint 1, thereby preventing the concentration of strain at the molten boundary.

[0032] The most basic embodiment of the spot-welded joint 1 according to this embodiment has been described above. A more preferred embodiment will be described below.

[0033] (Hardness of the low-carbon steel plate 112 placed on the surface of the spot-welded joint 1) Preferably, the Vickers hardness of one or both of the low-carbon steel plates 112 placed on the surface of the spot-welded joint 1 is 150 Hv or higher. There is a good correlation between the Vickers hardness and tensile strength of the steel plate 11, and 150 Hv is approximately equivalent to 450 MPa. By increasing the hardness of the low-carbon steel plate 112 to 150 Hv or higher, fracture along the molten boundary can be further suppressed by reducing strain concentration at the molten boundary, and the CTS of the spot-welded joint 1 can be further improved. The hardness of the low-carbon steel plate 112 placed on the surface of the spot-welded joint 1 may be 160 Hv or higher, 180 Hv or higher, or 190 Hv or higher.

[0034] (Hardness of medium-high carbon steel sheet 111) The Vickers hardness of the medium-high carbon steel sheet 111 is preferably 530 Hv or higher. The Vickers hardness of the medium-high carbon steel sheet 111 may be 540 Hv or higher, 560 Hv or higher, or 600 Hv or higher.

[0035] (Thickness of low-carbon steel sheet 112) Preferably, the thickness of one or both of the low-carbon steel plates 112 placed on the surface of the spot-welded joint 1 is 0.9 mm or more. By making the thickness of the low-carbon steel plates 112 0.9 mm or more, fracture along the fusion boundary can be further suppressed and the CTS of the spot-welded joint 1 can be further improved. The thickness of the low-carbon steel plates 112 placed on the surface of the spot-welded joint 1 may be 1.0 mm or more, 1.1 mm or more, or 1.2 mm or more. There is no particular upper limit to the thickness of the low-carbon steel plates 112 placed on the surface of the spot-welded joint 1. For example, the thickness of the low-carbon steel plates 112 placed on the surface of the spot-welded joint 1 may be 2.0 mm or less, 2.3 mm or less, or 2.6 mm or less.

[0036] (Size and shape of low-carbon steel sheet 112) Generally, the steel plate 11 included in a spot-welded joint 1 constitutes a skeletal member or exterior member of a mechanical structural component, and is therefore large in size. On the other hand, the size of the low-carbon steel plate 112 placed on the surface of the spot-welded joint 1 may be small. For example, when the spot-welded joint 1 is viewed in plan along the thickness direction, the minimum value of the distance D between the end face of the low-carbon steel plate 112 and the center of the nugget 12 may be within the range of 6 to 50 mm for one or both of the low-carbon steel plates 112 placed on the surface of the spot-welded joint 1. Preferably, when the spot-welded joint 1 is viewed in plan along the thickness direction, the distance D between the end face of the low-carbon steel plate 112 and the center of the nugget 12 may be within the range of 6 to 50 mm over the entire length of the end face.

[0037] Examples of smaller-sized low-carbon steel sheets 112 are shown in Figures 4A to 4C. Figures 4A to 4C are plan views of the end of a spot-welded joint 1 having multiple nuggets 12, along the thickness direction of the steel sheet 11. The embodiments described below may be applied to only one surface of the spot-welded joint 1 or to both surfaces.

[0038] In the spot-welded joint 1 shown in Figure 4A, multiple rectangular low-carbon steel plates 112 are arranged to cover each of the multiple nuggets 12. The multiple low-carbon steel plates 112 are very small compared to the medium-high carbon steel plates 111 that are superimposed on them. The distance between the center of the nugget 12 and the end face of the low-carbon steel plate 112 covering it is in the range of 6 to 50 mm across the entire end face of the low-carbon steel plate 112.

[0039] In the spot-welded joint 1 shown in Figure 4B, multiple circular low-carbon steel plates 112 are arranged to cover each of the multiple nuggets 12. The multiple low-carbon steel plates 112 are very small compared to the medium-high carbon steel plates 111 that are superimposed on them. The distance between the center of the nugget 12 and the end face of the low-carbon steel plate 112 covering it is in the range of 6 to 50 mm across the entire end face of the low-carbon steel plate 112.

[0040] In the spot-welded joint 1 shown in Figure 4C, a single low-carbon steel plate 112 is positioned along the end of the spot-welded joint 1, covering each of the multiple nuggets 12. The low-carbon steel plate 112 in Figure 4C is larger than the low-carbon steel plates 112 illustrated in Figures 4A and 4B. However, the low-carbon steel plate 112 in Figure 4C is still very small compared to the medium-to-high-carbon steel plate 111 superimposed on it. The minimum distance between the center of the nugget 12 and the end face of the low-carbon steel plate 112 covering it is in the range of 6 to 50 mm.

[0041] By setting the distance between the center of the nugget 12 and the end face of the low-carbon steel plate 112 provided on the surface of the spot-welded joint 1 to 6 mm or more, the CTS improvement effect of the low-carbon steel plate 112 can be further enhanced. On the other hand, by setting the distance D (or the minimum value of distance D) between the center of the nugget 12 and the end face of the low-carbon steel plate 112 provided on the surface of the spot-welded joint 1 to 50 mm or less, and reducing the size of the low-carbon steel plate 112, the weight of the spot-welded joint 1 can be reduced. The configurations illustrated in Figures 4A and 4B are most preferable for reducing the weight of the spot-welded joint 1. On the other hand, the configuration illustrated in Figure 4C has fewer parts compared to Figures 4A and 4B, so it is possible to ensure manufacturing efficiency while reducing the weight of the spot-welded joint 1.

[0042] (The protrusions 1121 and recesses 1122 of the low-carbon steel sheet 112) As illustrated in Figures 5A and 5B, when the spot-welded joint 1 is viewed in plan along the thickness direction of the steel plate 11, one or both ends of the low-carbon steel plate 112 placed on the surface of the spot-welded joint 1 have a protrusion 1121 and a recess 1122, and the nugget 12 may be located inside the protrusion 1121. This makes it possible to reduce the weight of the low-carbon steel plate 112.

[0043] As illustrated in Figure 5B, a secondary nugget 13 may be provided on the spot-welded joint 1. The secondary nugget 13 does not join all of the steel plates 11 included in the spot-welded joint 1. When the spot-welded joint 1 is viewed in plan along the thickness direction of the steel plate 11, the secondary nugget 13 is positioned inside the recess 1122 of the low-carbon steel plate 112. As a result, the secondary nugget 13 does not have the function of joining the low-carbon steel plate 112 having the recess 1122 to other steel plates 11. The secondary nugget 13 is designed to join multiple steel plates 11, excluding the low-carbon steel plate 112 having the recess 1122. This provides the effect of improving member rigidity by increasing the number of spot welds while reducing mass and improving CTS.

[0044] (Vickers hardness of nugget 12) Preferably, the spot welded joint 1 satisfies formulas 1 and 2. 0.8×H1≦H2≦1.2×H1: Formula 1 810×(C+Mn / 100+Si / 90)+220≦H2: Formula 2 In Equations 1 and 2, the symbol H1 represents the average Vickers hardness of the end of the nugget 12, the symbol H2 represents the average Vickers hardness of the center of the nugget 12, and the symbols C, Mn, and Si represent the carbon, Mn, and Si content of the nugget 12, respectively. The average Vickers hardness of the end of the nugget 12 is the average value of the Vickers hardness at measurement point A shown in Figure 12. The average Vickers hardness of the center of the nugget 12 is the average value of the Vickers hardness at measurement point B shown in Figure 12. The measurement methods for the average Vickers hardness of the end and center of the nugget 12 will be described later.

[0045] In nuggets 12 that satisfy Equation 1, the variation in internal hardness is small. Furthermore, in nuggets 12 that satisfy Equation 2, no tempering softening occurs in the center. Therefore, nuggets 12 that satisfy both Equation 1 and Equation 2 have not been energized afterwards.

[0046] Post-welding is a process of applying a post-thermal current to the nugget 12 formed by spot welding. Post-thermal current is an electric current applied to the hardened weld after spot welding of steel materials that harden during welding, for the purpose of tempering or annealing the welded area. Post-welding improves the mechanical properties of the welded area, but increases the welding cycle time. Spot welded joints that satisfy Equations 1 and 2 can be manufactured without post-welding, thus contributing to a reduction in welding cycle time and manufacturing costs.

[0047] (Hemming structure and separation structure) A hemming structure can be applied to the spot welded joint 1. A hemming structure is a structure in which the steel plate placed on the surface of the spot welded joint is made of a single bent steel plate. A spot welded joint to which a hemming structure is applied is obtained by (1) bending a low-carbon steel plate placed on one surface of the plate assembly along its edge to form a bent portion with a steel plate placed inside the plate assembly, (2) placing the bent portion on the other surface of the plate assembly, and (3) spot welding the bent portion. In a spot welded joint to which a hemming structure is applied, the low-carbon steel plates placed on both surfaces of the spot welded joint are integrated.

[0048] On the other hand, naturally, a hemming structure is not required for the spot welded joint 1. That is, the low-carbon steel plates on both surfaces of the spot welded joint may be separated. A preferred example of a spot welded joint configuration in which two low-carbon steel plates are separated is the weld of the B-pillar shown in Figure 10D. Details of the B-pillar in Figure 10D will be described later.

[0049] Hemmed structures can reduce the number of parts in mechanical structural components. However, hemmed structures can only be applied to the ends of mechanical structural components. A structure in which two low-carbon steel plates are separated is preferable because it can be applied to various welded joints of mechanical structural components. Furthermore, achieving a hemmed structure requires incorporating a hemming process into the manufacturing method of spot welded joints. Therefore, hemmed structures increase manufacturing costs. A structure in which two low-carbon steel plates are separated contributes to reducing manufacturing costs.

[0050] (2. Method for manufacturing spot welded joint 1) A method for manufacturing a spot-welded joint 1 according to another aspect of this disclosure comprises the steps of: stacking four or more steel plates 11 to form a plate assembly; and spot-welding the plate assembly, wherein at least one of the four or more steel plates 11 is a medium-high carbon steel plate 111 with a carbon content of 0.26 mass% or more, and at least two of the four or more steel plates 11 are low-carbon steel plates 112 with a carbon content of 0.05 mass% or more and 0.24 mass% or less, and the low-carbon steel plates 112 are placed on both surfaces of the plate assembly. Details of the method for manufacturing the spot-welded joint 1 will be described below. Note that the preferred embodiment of the spot-welded joint 1 described above can be applied to the method for manufacturing the spot-welded joint 1.

[0051] In the manufacturing method of the spot-welded joint 1, first, four or more steel plates 11 are stacked to form a plate assembly. At least one of the four or more steel plates 11 is a medium-high carbon steel plate 111. A medium-high carbon steel plate 111 is a steel plate 11 with a carbon content of 0.26 mass% or more. In addition, at least two of the four or more steel plates 11 are low-carbon steel plates 112. A low-carbon steel plate 112 is a steel plate 11 with a carbon content of 0.05 mass% or more and 0.24 mass% or less.

[0052] The low-carbon steel sheets 112 are placed on both surfaces of the plate assembly. If there are three or more low-carbon steel sheets 112, two of them are placed on the surface of the plate assembly, and the remaining low-carbon steel sheets 112 are placed inside the plate assembly. The medium- and high-carbon steel sheets 111 are necessarily placed inside the plate assembly.

[0053] Next, the plate assembly is spot-welded. This forms a nugget 12 that joins multiple steel plates 11. The conditions for spot welding are not particularly limited. A person skilled in the art can appropriately select values ​​according to the thickness, composition, and strength of the multiple steel plates 11.

[0054] A suitable example of spot welding involves using a DR-type electrode with a tip of 5mm to 9mm, applying a pressure of 250kgf to 700kgf (2.451kN to 6.864kN), energizing for 12 to 60 cycles (240msec to 1200msec), using a current of 6kA to 13kA, and holding for 1 to 60 cycles (20msec to 1200msec). The number of energizing passes can be one or more. If the plate assembly includes hot-stamped steel plates that are prone to surface scattering, an upslope may be used at the start of energizing. If low-temperature cracking or LME cracking is likely to occur at the nugget ends, slow cooling may be performed using a downslope at the end of energizing. The downslope in this case may be pulsed. Furthermore, if the plate thickness ratio of the plate assembly is large, it is acceptable to apply a high current value of 10.5kA to 14.0kA within the first 7 cycles of energization, and then reduce the current value to 10kA or less thereafter.

[0055] The nugget diameter formed by this welding is preferably 3√t or greater, and more preferably 4√t or greater. The nugget diameter is measured along the joint surface of the steel plates, i.e., the joining interface. "t" is the thickness of the thinner of the two steel plates that make up the joining interface where the nugget diameter is measured. There is no specific upper limit for the nugget diameter, but for example, it can be 8√t or less. In addition, structural adhesives to improve rigidity and NVH characteristics, and sealants for waterproofing may be present between the steel plates.

[0056] (Effects and Benefits) According to the manufacturing method of the spot welded joint 1 of this embodiment, a low-carbon steel plate 112 is arranged on the surface, and the spot welded joint 1 having a high CTS can be easily manufactured.

[0057] The most basic embodiment of the method for manufacturing the spot-welded joint 1 according to this embodiment has been described above. A more preferred embodiment will be described below.

[0058] (Hardness of the low-carbon steel sheet 112 placed on the surface of the plate assembly) Preferably, the Vickers hardness of one or both of the low-carbon steel plates 112 arranged on the surface of the plate assembly is 150 Hv or higher. By increasing the hardness of the low-carbon steel plates 112 arranged on the surface of the plate assembly to 150 Hv or higher, the CTS of the spot welded joint 1 can be further improved. The hardness of the low-carbon steel plates 112 arranged on the surface of the plate assembly may be 160 Hv or higher, 180 Hv or higher, or 190 Hv or higher.

[0059] (Hardness of medium-high carbon steel sheet 111) The Vickers hardness of the medium-high carbon steel sheet 111 is preferably 530 Hv or higher. This further improves the rigidity of the spot-welded joint 1. The Vickers hardness of the medium-high carbon steel sheet 111 may be 540 Hv or higher, 560 Hv or higher, or 600 Hv or higher.

[0060] (Thickness of low-carbon steel sheet 112) The thickness of one or both of the low-carbon steel plates 112 arranged on the surface of the plate assembly is preferably 0.9 mm or more. By making the thickness of the low-carbon steel plates 112 arranged on the surface of the plate assembly 0.9 mm or more, the CTS of the spot welded joint 1 can be further improved. The thickness of the low-carbon steel plates 112 arranged on the surface of the plate assembly may be 1.0 mm or more, 1.1 mm or more, or 1.2 mm or more.

[0061] (Size and shape of low-carbon steel plate 112, and location of welding points) The size of the low-carbon steel sheets 112 arranged on the surface of the plate assembly may be small. For example, one or both of the low-carbon steel sheets 112 arranged on the surface of the plate assembly may be small in size as illustrated in Figures 4A to 4C, and the minimum distance between the end face of the low-carbon steel sheet 112 and the welding point may be within the range of 6 to 50 mm. Preferably, the distance between the end face of the low-carbon steel sheet 112 and the welding point may be within the range of 6 to 50 mm over the entire end face. The welding point refers to the point where the center of the electrode used in spot welding and the low-carbon steel sheet 112 come into contact.

[0062] By setting the distance between the welding point and the end face of the low-carbon steel plate 112 placed on the surface of the plate assembly to 6 mm or more, the CTS of the spot-welded joint 1 can be further improved. On the other hand, by setting the distance (or the minimum value of said distance) between the welding point and the end face of the low-carbon steel plate 112 provided on the surface of the plate assembly to 50 mm or less, and reducing the size of the low-carbon steel plate 112, the weight of the spot-welded joint 1 can be reduced.

[0063] (Positions of the protrusions 1121 and recesses 1122 of the low-carbon steel sheet 112, and the welding points) As illustrated in Figures 5A and 5B, one or both ends of the low-carbon steel sheet 112 arranged on the surface of the plate assembly may have a protrusion 1121 and a recess 1122. In this case, spot welding may be performed on the protrusion 1121. This forms a nugget 12 inside the protrusion 1121 that joins the protrusion 1121 of the low-carbon steel sheet 112 to the other steel sheet 11. This makes it possible to reduce the weight of the low-carbon steel sheet 112.

[0064] As illustrated in Figure 5B, spot welding may also be performed on the recess 1122. This makes it possible to form a sub-nugget 13 that joins multiple steel plates 11, excluding the low-carbon steel plate 112 having the recess 1122.

[0065] (Powered on afterwards) The manufacturing method for the spot-welded joint 1 may include a step of post-energizing the nugget 12. Post-energizing is a step of passing a post-thermal current through the nugget 12 formed by spot welding. Post-thermal current is an electric current that is passed through the hardened weld after welding in spot welding of steel materials that harden by welding, for the purpose of tempering or annealing the hardened weld. Post-energizing improves the mechanical properties of the weld.

[0066] On the other hand, in the manufacturing method of the spot-welded joint 1, post-welding is not required after spot welding. Post-welding increases the welding cycle time. Omitting post-welding contributes to shortening the welding cycle time and reducing manufacturing costs. In the spot-welded joint 1 according to this embodiment, the joint strength is ensured by the low-carbon steel plates placed on both surfaces. Therefore, it is preferable to omit post-welding in the manufacturing of the spot-welded joint 1.

[0067] (Hemming structure and separation structure) In the method for manufacturing the spot-welded joint 1, the low-carbon steel sheet may be hemmed. Specifically, in the step of forming a plate assembly by stacking four or more steel sheets, (1) a low-carbon steel sheet placed on one surface of the plate assembly may be bent along its edge to form a bent portion with respect to the steel sheet placed inside the plate assembly, and (2) the bent portion may be placed on the other surface of the plate assembly. By spot welding the bent portion, a spot-welded joint 1 with a hemmed structure is obtained. In the spot-welded joint 1 with a hemmed structure, the low-carbon steel sheets placed on both surfaces of the spot-welded joint are integrated.

[0068] On the other hand, naturally, the spot-welded joint 1 does not necessarily have to have a hemming structure applied to it. That is, the low-carbon steel plates placed on both surfaces of the plate assembly may be separated.

[0069] While embodiments of the present disclosure have been described above, the disclosure is not limited thereto and can be modified as appropriate without departing from the technical spirit of the disclosure. Further preferred examples of the spot-welded joint 1 and its manufacturing method according to the present embodiment are described below. The embodiments illustrated below can be applied to both the spot-welded joint 1 and its manufacturing method according to the present embodiment.

[0070] (Various forms of the steel plate 11) The thickness of the medium-high carbon steel plate 111 placed inside the spot-welded joint 1 or the plate assembly is not particularly limited. For example, it is preferable that the thickness of the medium-high carbon steel plate 111 be 0.7 mm or more, 0.8 mm or more, or 1.0 mm or more. It is also preferable that the thickness of the medium-high carbon steel plate 111 be 2.6 mm or less, 2.3 mm or less, or 2.1 mm or less. The thickness of the low-carbon steel plate 112 placed inside the spot-welded joint 1 or the plate assembly, and the steel plate 11 that does not fall under either the medium-high carbon steel plate 111 or the low-carbon steel plate 112 (hereinafter referred to as the "third steel plate") are also not particularly limited. The plate thickness of the medium-high carbon steel plate 111 described above can be applied to the low-carbon steel plate 112 or the third steel plate placed inside the spot-welded joint 1 or the plate assembly.

[0071] Preferably, the steel plate placed inside the spot welded joint 1 is thicker than the low-carbon steel plate 112 placed on the surface of the spot welded joint 1. For example, it is preferable that the thickness of the steel plate placed inside the spot welded joint 1 is more than 1.0 times, 1.1 times or more, 1.2 times or more, or 1.4 times or more than the thickness of the low-carbon steel plate 112 placed on the surface of the spot welded joint 1.

[0072] The tensile strength of the medium-high carbon steel sheet 111 is preferably, for example, 1700 MPa or more, 1780 MPa or more, or 1900 MPa or more. The tensile strength of the medium-high carbon steel sheet 111 is preferably, for example, 2700 MPa or less, 2600 MPa or less, or 2500 MPa or less.

[0073] The tensile strength of the low-carbon steel sheet 112 is preferably, for example, 340 MPa or more, 450 MPa or more, or 590 MPa or more. The tensile strength of the low-carbon steel sheet 112 is preferably, for example, 1580 MPa or less, 1550 MPa or less, or 1470 MPa or less.

[0074] The tensile strength of the third steel plate is preferably, for example, 250 MPa or more, 270 MPa or more, or 340 MPa or more. The tensile strength of the third steel plate is preferably, for example, 1580 MPa or less, 1550 MPa or less, or 1470 MPa or less.

[0075] The steel sheet 11 may have a surface treatment layer. The surface treatment layer is, for example, a chemical conversion treatment layer and a plating layer. The composition of the chemical conversion treatment layer and the plating layer can be suitable for the composition of the underlying steel sheet 11. The plating layer is preferably, for example, a zinc-based plating layer and an aluminum-based plating layer. A zinc-based plating layer is a plating layer having a Zn content of 50% by mass or more. Examples of zinc-based plating layers include hot-dip galvanized layers, alloyed hot-dip galvanized layers, electro-galvanized layers, zinc-aluminum plating layers, zinc-aluminum-magnesium plating layers, and hot-stamped hot-dip galvanized layers.

[0076] In the case of hot-stamped galvanized steel sheets, the Zn content is 20% by mass or more. It may also contain a few percent of any of the elements Al, Mg, or Ni. A zinc oxide layer may be provided on the surface of the galvanized sheet after hot stamping. An aluminum-based plating layer is a plating layer with an Al content of 20% by mass or more. An example of a steel sheet 11 having an aluminum-based plating layer is a hot-stamped aluminum-silicon plated steel sheet. A layer of Al oxide, Ti oxide, or zinc oxide may be provided on the surface of the aluminum plating after hot stamping.

[0077] (Application) The application of the spot welded joint 1 according to this embodiment is not particularly limited. A suitable application is in the skeletal parts of an automobile body. Specific examples of applications include, for example, A-pillars, B-pillars, roof rails, side sills, floor members, the joint between the A-pillar and B-pillar, the joint between the A-pillar and the roof rail, the joint between the upper and lower parts of the A-pillar, the joint between the B-pillar and the side sill, the joint between the A-pillar and the side sill, the joint between the floor member and the floor cross member, the joint between the front side member and the floor, the joint between the front side member and the side sill, and the skeletal joint parts of the battery case.

[0078] An example of application to the B-pillar is shown in Figures 10A to 10D. The B-pillar consists of an outer reinforcement 32 made of a medium-high carbon steel sheet 111, an inner reinforcement 34 made of a low carbon steel sheet 112, a third steel sheet, the side panel outer 31, and a hinge reinforcement 33. All parts except the hinge reinforcement 33 are joined at the flange portion of the B-pillar. A pre-formed reinforcing low carbon steel sheet 30 can be added to this B-pillar.

[0079] The reinforcing low-carbon steel plate 30 is joined to the flange portion at the tip of the side panel outer 31 by spot welding (see Figure 10B). The flange portion of the outer reinforcement 32 and the flange portion of the inner reinforcement 34 are overlapped on the flange portion of the side panel outer 31 having the reinforcing low-carbon steel plate 30 (see Figure 10C). As a result, the flange portion at the tip of the B-pillar becomes a four-layer plate assembly with the reinforcing low-carbon steel plate 30 and the inner reinforcement 34, made of low-carbon steel, positioned on the surface, and the third steel plate, the side panel outer and the outer reinforcement 32, made of medium-high carbon steel, positioned on the inside. By performing spot welding on the four-layer overlapping portion of the flange (see Figure 10D), the cross tensile strength of the upper part of the B-pillar is further improved. The spot welds shown in black in Figure 10D correspond to the nuggets 12 of the spot welded joint 1 according to this embodiment, and the spot welds shown in white correspond to the sub-nuggets 13 of the spot welded joint 1 according to this embodiment. Although not shown in the diagram, spot welds also exist in other parts of the B-pillar flange.

[0080] In the B-pillar illustrated in Figures 10A to 10D, the reinforcing low-carbon steel plate 30 is placed further outside the side panel outer 31. However, if the inner reinforcement 34 is a medium-high carbon steel plate 111, the low-carbon steel plate 30 is placed further inside (towards the interior) of the inner reinforcement 34. A similar method can be applied to other automotive structural parts such as the A-pillar, roof rails, and side sills.

[0081] (Measurement method) The carbon content of steel sheets is measured according to the gas component analysis specified in JIS G 1211-2:2011 "Iron and steel - Carbon determination method - Part 2: Combustion - Gas volume method," which is referenced in JIS G 0321:2017 "Product analysis method for steel materials and permissible variation thereof." The hardness of steel sheets is measured according to "JIS Z 2244:2009 Vickers hardness test - Test method." The hardness measurement position is outside the nugget and the heat-affected zone (HAZ) formed around it. If the steel sheet has hardened areas such as bent sections, the hardness is measured in a state where work hardening has not occurred. The measuring force is 500g. The tensile strength of steel sheets is measured according to JIS Z 2241:2011 "Tensile test method for metallic materials." The shape of the test piece for the tensile test can be appropriately selected according to the shape of the steel sheet.

[0082] The carbon, manganese, and silicon content of the nugget is calculated based on the chemical composition of the multiple steel plates constituting the spot weld joint, and the area each steel plate occupies within the nugget.

[0083] The carbon content of the steel sheet is measured by gas component analysis as specified in JIS G 1211-2:2011 "Iron and steel - Method for determining carbon - Part 2: Combustion - Gas volume method". The Mn and Si content of the steel sheet is measured by ICP analysis as specified in the JIS G 1258 standard series. Furthermore, by observing a cross-section parallel to the thickness direction and including approximately the center of the nugget indentation, the proportion of the area occupied by each of the multiple steel sheets in the nugget can be determined. The weighted average of the carbon, Mn, and Si content of the multiple steel sheets, weighted by this area, can be considered as the carbon, Mn, and Si content of the nugget.

[0084] Figure 11 shows an example of a method for measuring the chemical composition of a nugget. Figure 11 is a schematic diagram of a cross-section of a nugget. The area enclosed by the imaginary line VL along the interface of the overlapping steel plates and the molten boundary 12F is the area that each of the multiple steel plates occupies in the nugget. The molten boundary 12F is the boundary between the nugget 12 and the unmolten base material. In Figure 11A, the areas that steel plates 11A, 11B, 11C, and 11D occupy in the nugget are the areas of the regions labeled with symbols 12A, 12B, 12C, and 12D, respectively. The average value of the chemical composition of steel plates 11A, 11B, 11C, and 11D, weighted by the areas of these regions, can be considered as the chemical composition of the nugget.

[0085] The average Vickers hardness of the ends of nugget 12 and the average Vickers hardness of the center of nugget 12 are measured by the following procedure. First, the spot welded joint is cut. The cut surface includes approximately the center of the nugget indentation and is parallel to the plate thickness direction. A schematic diagram of the cut surface is shown in Figure 12. Next, the Vickers hardness of nugget 12 is measured according to "JIS Z 2244:2009 Vickers hardness test - Test method". The test force is 0.5 kgf. The measurement locations are as follows. (Measurement location A) An area on the midline X between the two surfaces 1S of the spot welded joint 1, and at a distance of 0.2 mm or more and 1.2 mm or less from the molten boundary 12F. (Measurement location B) An area on the midline X between the two surfaces 1S of the spot welded joint 1, and at a distance of 1.0 mm or less from the center 12C of the nugget 12.

[0086] The center 12C of the nugget 12 in the cross-section is the midpoint of the two intersection points Y of the median line X and the molten boundary 12F.

[0087] Five Vickers hardness measurements will be performed at measurement point A. The interval between hardness measurement positions will be 0.20 mm. The interval between hardness measurement positions refers to the distance between the centers of the indentations I formed by the Vickers hardness measurements. The average Vickers hardness at measurement point A will be considered as the average Vickers hardness of the end of the nugget 12.

[0088] At measurement point B, five Vickers hardness measurements will be performed. The spacing between hardness measurement positions at measurement point B is not limited. It is desirable that the hardness measurement positions at measurement point B be distributed roughly evenly. The average Vickers hardness value at measurement point B will be considered as the average Vickers hardness of the center of the nugget 12.

[0089] For example, Vickers hardness measurements may be performed continuously along the intermediate line X, with the two intersection points Y of the intermediate line X and the molten boundary 12F as the start and end points. In this case, the interval between hardness measurement positions should be 0.20 mm, at least near the molten boundary 12F. Then, Vickers hardness measurement results are extracted from the hardness measurement positions that satisfy the above requirements. Based on the extracted measurement results, the average Vickers hardness of the ends of the nugget 12 and the average Vickers hardness of the center of the nugget 12 can be calculated.

[0090] However, if there is a weld defect such as a shrinkage cavity (a cavity formed by solidification shrinkage) in the center of the nugget, the hardness measurement position should be outside the weld defect at the midline X. In this case, the hardness measurement positions at measurement point B do not need to be evenly spaced. An example of a shrinkage cavity is shown in Figure 9B, etc.

[0091] If there are many incomplete welds on the intermediate line X, making it difficult to measure the hardness properly, the hardness can be measured at measurement point B on the 1 / 4 thickness line XX instead of the intermediate line X. The 1 / 4 thickness line XX is the intermediate line between the intermediate line X and the surface 1S. Measurement point B on the 1 / 4 thickness line XX is an area on the 1 / 4 thickness line XX where the distance from the center 12C of the nugget 12 in a direction perpendicular to the thickness direction is 1.0 mm or less.

[0092] There are two quarter-thickness wires XX. Therefore, the hardness is measured at measurement point B for both quarter-thickness wires XX. The average hardness at measurement point B, which is identified for the quarter-thickness wire XX with the higher average hardness at measurement point B, can be considered as the average Vickers hardness of the center of the nugget 12. [Examples]

[0093] The effects of one aspect of this disclosure will be further illustrated by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of this disclosure. This disclosure is not limited to these examples of conditions. This disclosure may adopt various conditions as long as they do not depart from its gist and achieve its objectives.

[0094] (Experiment 1) Various plate sets A1 to A5 were prepared using steel grades "2.5G-1.6t" and "980-1.2t" from the steel plates listed in Table 1. The units for the alloying element content listed in Table 1 are mass%. Table 1 only lists the content of C, Si, and Mn, which are representative alloying elements of steel plates. Steel grade "2.5G-1.6t" corresponds to medium-high carbon steel plate 111, while the other steel grades correspond to low carbon steel plate 112. None of the steel grades had surface treatment layers such as plating layers.

[0095] [Table 1]

[0096] The configuration of plate assemblies A1 to A5 was as shown in Table 2. Spot welding was then performed on plate assemblies A1 to A5 as shown in Table 2 to manufacture spot-welded joints. The spot welding conditions for all plate assemblies were as follows. • Welding machine: Single-phase AC stationary spot welding machine • Electrode: DR type, tip diameter φ6mm, 40R (chromium copper) • Pressing force: 400 kgf • Power-on time: 0.5 seconds (30 cycles / 60Hz) • Current: Adjust to around 7kA • Hold time: 0.33 seconds (20 cycles / 60Hz)

[0097] Next, a cross-tensile test was performed on spot welded joints A1 to A5. The cross-tensile test was conducted in accordance with JIS Z 3137:1999 "Specimen dimensions and test method for cross-tensile testing of resistance spot and projection welded joints". The results of the cross-tensile test are shown in Figure 6 and Table 2. Cross-sectional photographs of the spot welded joints after the cross-tensile test are shown in Figures 7A to 7E. The cross-tensile test was performed on steel plates of steel grade "2.5G-1.6t" (steel plates marked with an asterisk in Table 2). Specifically, a test force was applied in the vertical direction to the steel plate marked with reference numeral 111 in Figures 7A to 7E to separate the two.

[0098] [Table 2]

[0099] Example A1 was obtained by spot welding two medium-high carbon steel plates. Compared to Example A1, Examples A3 to A5 showed almost no improvement in CTS. In Examples A4 and A5, a low-carbon steel plate was sandwiched between the medium-high carbon steel plates, so the carbon in the nugget may have been diluted. However, in Examples A4 and A5, no improvement in CTS due to carbon dilution was obtained. On the other hand, the CTS of Example A2, in which low-carbon steel plates were placed on both surfaces of the spot-welded joint, was dramatically improved compared to Example A1.

[0100] Figures 7A to 7E show cross-sectional photographs of Examples A1 to A5 after the cross tensile test. As shown in Figures 7C to 7E, in Examples A3 to A5, the medium-high carbon steel plate fractured at the fusion boundary. On the other hand, as shown in Figure 7B, in Example A2, where low-carbon steel plates were placed on both surfaces of the spot-welded joint, fracture of the medium-high carbon steel plate at the fusion boundary was suppressed. In Example A2, fracture occurred at the nugget.

[0101] (Experiment 2) Various plate assemblies B1 to B6 were created using the three types of steel plates listed in Table 1. The configurations of plate assemblies B1 to B6 were as shown in Table 3. Spot welds were then applied to plate assemblies B1 to B6 as shown in Table 3 to produce spot-welded joints. Cross-tensile tests were then performed on the spot-welded joints B1 to B6. The spot welding conditions and cross-tensile test method were the same as in Experiment 1.

[0102] The results of the cross-tensile test are shown in Figure 8 and Table 3. Cross-sectional photographs of the spot-welded joints after the cross-tensile test are shown in Figures 9A to 9F. The cross-tensile test was performed on steel plates of steel grades "2.5G-1.6t" and "590-1.6t". Specifically, the steel plates marked with an asterisk in Table 3 and Figures 9A to 9F were subjected to vertical force to separate them.

[0103] [Table 3]

[0104] Example B1 was obtained by spot welding a medium-high carbon steel sheet to a low carbon steel sheet with a tensile strength of 590 MPa. Compared to Example B1, there was almost no improvement in CTS in Examples B5 and B6. The CTS of Examples B2 to B4 were improved compared to Example B1. And the CTS of Example B2, in which a low carbon steel sheet with a tensile strength of 980 MPa was placed on both surfaces of the spot-welded joint, was the highest value among all the plate assemblies.

[0105] Figures 9A to 9F show cross-sectional photographs of Examples B1 to B6 after cross-tensile testing. As shown in Figures 9E and 9F, in Examples B5 and B6, the medium-high carbon steel plate fractured at the fusion boundary. The CTS in these examples showed little improvement compared to Example B1. On the other hand, as shown in Figures 9B to 9D, in Examples B2 to B4, fracture of the medium-high carbon steel plate at the fusion boundary was suppressed, and an improvement in CTS was confirmed. However, the CTS of Example B2, in which low-carbon steel plates with a tensile strength of 980 MPa were placed on both surfaces of the spot-welded joint, was even better than that of Examples B3 and B4.

[0106] (Experiment 3) Various plate assemblies C1 to C16 were prepared using the steel plates listed in Table 4. The units for the alloying element content listed in Table 4 are mass%. Table 4 only lists the content of C, Si, and Mn, which are representative alloying elements of steel plates. Steel grades "Al1.8G-1.6t" and "2.5G-1.6t" correspond to medium-high carbon steel plates 111, "GA270-0.65t" corresponds to the third type of steel plate, and all other steel grades correspond to low carbon steel plates 112.

[0107] [Table 4]

[0108] The configuration of plate assemblies C1 to C16 was as shown in Table 5. Spot welds were then applied to plate assemblies C1 to C16 as shown in Table 5 to produce spot-welded joints. The spot welding conditions for all plate assemblies were as follows. • Welding machine: Single-phase AC stationary spot welding machine • Electrode: DR type, tip diameter φ6mm, 40R (chromium copper) • Pressing force: 400 kgf • Power-on time: 0.5 seconds (30 cycles / 60Hz) • Current: Adjust to around 7.5kA • Hold time: 0.33 seconds (20 cycles / 60Hz)

[0109] Next, cross-tensile tests were performed on spot-welded joints C1 to C16. The cross-tensile tests were conducted in accordance with JIS Z 3137:1999, "Specimen dimensions and test method for cross-tensile tests of resistance spot and projection welded joints." The results of the cross-tensile tests are shown in Table 5. The cross-tensile tests were performed on the steel plates marked with an asterisk in Table 5. The dimensions of the steel plates other than those marked with an asterisk were 50 mm in length and 50 mm in width.

[0110] [Table 5]

[0111] Example C1 was obtained by spot welding a medium-high carbon steel sheet to a low carbon steel sheet with a tensile strength of 590 MPa. Compared to Example C1, Examples C2, C3, and C6 showed little improvement in CTS. On the other hand, Examples C4, C5, and C7-C16, in which the low carbon steel sheet was placed on both surfaces of the plate assembly, showed improved CTS compared to Example C1.

[0112] (Experiment 4) Spot welding was performed to investigate the effect of post-energization on nugget hardness. The welding target was the plate assembly indicated by symbol A2 in Table 2. The welding conditions were as follows:

[0113] Condition D1 (No power supply afterwards 1) • Welding machine: Inverter DC stationary spot welding machine • Electrode: DR type, tip diameter φ7mm, 40R (chromium copper) • Pressing force: 450 kgf • Power-on time: 0.5 seconds (30 cycles / 60Hz) Current: 7.6kA • Hold time: 0.33 seconds (20 cycles / 60Hz)

[0114] Condition D2 (No post-energization 2) • Welding machine: Inverter DC stationary spot welding machine • Electrode: DR type, tip diameter φ7mm, 40R (chromium copper) • Pressing force: 450 kgf • Power-on time: 0.5 seconds (30 cycles / 60Hz) ·Current: 8.2kA • Hold time: 0.33 seconds (20 cycles / 60Hz)

[0115] Condition D3 (with post-energization 1) • Welding machine: Inverter DC stationary spot welding machine • Electrode: DR type, tip diameter φ7mm, 40R (chromium copper) • Pressing force: 450 kgf • Power-on time: 0.5 seconds (30 cycles / 60Hz) Current: 7.6kA • Pause time: 1.65 seconds (99 cycles / 60Hz) • Post-power-on time: 0.5 seconds (30 cycles / 60Hz) • Current applied after power supply: 4.8kA • Hold time: 0.33 seconds (20 cycles / 60Hz)

[0116] Condition D4 (with post-energization 2) • Welding machine: Inverter DC stationary spot welding machine • Electrode: DR type, tip diameter φ7mm, 40R (chromium copper) • Pressing force: 450 kgf • Power-on time: 0.5 seconds (30 cycles / 60Hz) Current: 7.6kA • Pause time: 1.65 seconds (99 cycles / 60Hz) • Post-power-on time: 0.5 seconds (30 cycles / 60Hz) • Current flow rate: 5.5kA • Hold time: 0.33 seconds (20 cycles / 60Hz)

[0117] Under conditions D1 and D2, the current values ​​differ, but other welding conditions are the same. Post-welding was not performed under both conditions D1 and D2. Conditions D3 and D4 are almost the same as condition D1, but post-welding was performed. Under conditions D3 and D4, the current during post-welding differs. H1, H2, C, Si, and Mn of the spot welded joints obtained under these conditions were measured and recorded in Table 6. Symbol H1 is the average Vickers hardness of the nugget end, symbol H2 is the average Vickers hardness of the nugget center, and symbols C, Mn, and Si are the carbon, Mn, and Si content of the nugget, respectively. The measurement method for these values ​​was as described above. In addition, the values ​​obtained by the following formula are recorded in the "H2 Lower Limit" column of Table 6. Lower limit of H2 = 810 × (C + Mn / 100 + Si / 90) + 220

[0118] [Table 6]

[0119] The nuggets obtained under condition D1 or condition D2 are not tempered and satisfy equations 1 and 2. 0.8×H1≦H2≦1.2×H1: Formula 1 810×(C+Mn / 100+Si / 90)+220≦H2: Formula 2

[0120] The nuggets obtained under condition D3 were broadly softened by subsequent energization. As a result, the nuggets obtained under condition D3 do not satisfy equation 2.

[0121] In the nuggets obtained under condition D4, the edges are softened, but the center of the nugget is re-hardened. The nuggets obtained under condition D4 do not satisfy Equation 1. [Explanation of Symbols]

[0122] 1. Spot welded joint 11 Steel plate 111 Medium- and high-carbon steel sheet 111S Medium-High Carbon Steel Sheet Joint Surface 112 Low-carbon steel sheet 1121 Convex part 1122 recess 12 nuggets 13 Side Nuggets D: Distance between the end face of the low-carbon steel plate and the center of the nugget 30: Low-carbon steel sheet 31: Side panel outer (third steel plate) 32: Outer reinforcement (medium-high carbon steel) 33: Hinge Reinforce 34: Inner reinforcement (low carbon steel)

Claims

1. Four or more stacked steel plates, A nugget for joining four or more of the aforementioned steel plates A spot welded joint having, Of the four or more steel plates, at least one is a medium-to-high carbon steel plate with a carbon content of 0.31 mass% or more. Of the four or more steel plates, at least two are low-carbon steel plates with a carbon content of 0.05 mass% or more and 0.24 mass% or less. The low-carbon steel plate is placed on both surfaces of the spot-welded joint. The low-carbon steel plates arranged on both surfaces of the spot-welded joint are separated. Spot welded joint.

2. The spot welded joint according to claim 1, wherein one or both of the low-carbon steel plates disposed on the surface of the spot welded joint have a Vickers hardness of 150 Hv or more.

3. The spot welded joint according to claim 1 or 2, wherein the Vickers hardness of the medium-high carbon steel plate is 530 Hv or more.

4. The spot welded joint according to claim 1 or 2, wherein the thickness of one or both of the low-carbon steel plates arranged on the surface of the spot welded joint is 0.9 mm or more.

5. The spot welded joint according to claim 1 or 2, wherein when the spot welded joint is viewed in plan along the thickness direction of the steel plate, the minimum distance between the end face of the low-carbon steel plate and the center of the nugget on one or both of the low-carbon steel plates arranged on the surface of the spot welded joint is within the range of 6 to 50 mm.

6. The spot welded joint according to claim 5, wherein, when the spot welded joint is viewed in plan along the thickness direction of the steel plate, the distance between the end face of the low-carbon steel plate and the center of the nugget is within the range of 6 to 50 mm over the entire end face of the steel plate.

7. When the spot welded joint is viewed in plan along the thickness direction of the steel plate, one or both ends of the low-carbon steel plate arranged on the surface of the spot welded joint have a convex portion and a concave portion, When the spot welded joint is viewed in plan along the thickness direction of the steel plate, the nugget is located inside the protrusion. A spot welded joint according to claim 1 or 2.

8. The spot welded joint has a secondary nugget, When the spot welded joint is viewed in plan along the thickness direction of the steel plate, the sub-nugget is located inside the recess. The sub-nugget joins the steel plates, excluding the low-carbon steel plate having the recess on which the sub-nugget is arranged. The spot welded joint according to claim 7.

9. Four or more stacked steel plates, A nugget for joining four or more of the aforementioned steel plates A spot welded joint having, Of the four or more steel plates, at least one is a medium-to-high carbon steel plate with a carbon content of 0.26 mass% or more. Of the four or more steel plates, at least two are low-carbon steel plates with a carbon content of 0.05 mass% or more and 0.24 mass% or less. The low-carbon steel plate is placed on both surfaces of the spot-welded joint. Satisfying equations 1 and 2, 0.8×H1≦H2≦1.2×H1: Formula 1 810×(C+Mn / 100+Si / 90)+220≦H2: Formula 2 The symbol H1 represents the average Vickers hardness of the end of the nugget, The symbol H2 represents the average Vickers hardness of the center of the nugget. The symbols C, Mn, and Si represent the carbon, manganese, and silicon content of the nugget, respectively. Spot welded joint.

10. The process of stacking four or more steel plates to form a plate assembly, The process of spot welding the aforementioned plate assembly, Equipped with, Of the four or more steel plates mentioned above, at least one is a medium-to-high carbon steel plate with a carbon content of 0.31 mass% or more. Of the four or more steel plates mentioned above, at least two are low-carbon steel plates with a carbon content of 0.05 mass% or more and 0.24 mass% or less. The low-carbon steel plates are placed on both surfaces of the aforementioned plate assembly. The low-carbon steel plates arranged on both surfaces of the aforementioned plate assembly are separated. A method for manufacturing spot-welded joints.

11. The method for manufacturing a spot welded joint according to claim 10, wherein one or both of the low-carbon steel plates disposed on the surface of the spot welded joint have a Vickers hardness of 150 Hv or more.

12. The method for manufacturing a spot welded joint according to claim 10 or 11, wherein the Vickers hardness of the medium-high carbon steel sheet is 530 Hv or more.

13. The method for manufacturing a spot welded joint according to claim 10 or 11, wherein the thickness of one or both of the low-carbon steel plates arranged on the surface of the spot welded joint is 0.9 mm or more.

14. The method for manufacturing a spot-welded joint according to claim 10 or 11, wherein in the spot welding, the minimum distance between the end face of the low-carbon steel plate and the welding point is within the range of 6 to 50 mm in one or both of the low-carbon steel plates arranged on the surface of the spot-welded joint.

15. The method for manufacturing a spot-welded joint according to claim 14, wherein, in the spot welding, the distance between the end face of the low-carbon steel plate and the welding point is within the range of 6 to 50 mm over the entire end face of the low-carbon steel plate.

16. One or both ends of the low-carbon steel plates arranged on the surface of the plate assembly have a convex portion and a concave portion, The spot welding is performed on the aforementioned protrusion. A method for manufacturing a spot welded joint according to claim 10 or 11.

17. Furthermore, the method for manufacturing a spot-welded joint according to claim 16, wherein spot welding is performed in the recess.

18. The method for manufacturing a spot-welded joint according to claim 10 or 11, characterized in that no post-energization is performed after the spot welding.

Images