Spot welding member and method for manufacturing spot welding member

Abstract

A spot welding member according to one embodiment of the present disclosure comprises: a first metal plate and a second metal plate disposed on the surfaces of a spot welding member; a first internal metal plate group configured from one or more internal metal plates disposed between the first metal plate and the second metal plate; a first spot welding part for joining the first metal plate and the first internal metal plate group; and a second spot welding part for joining the second metal plate and the first internal metal plate group. An indentation of the first spot welding part is provided to the first metal plate and the one or more internal metal plates, and an indentation of the second spot welding part is provided to the first metal plate and the second metal plate. The indentation of the first spot welding part is set apart from the indentation of the second spot welding part. A first nugget is set apart from the second metal plate, and a second nugget is set apart from the first metal plate.

Description

Spot-welded member and method for manufacturing spot-welded member

[0001] This application claims priority to Japanese Patent Application No. 2023-137103, filed on August 25, 2023, the contents of which are incorporated herein by reference.

[0002] Spot welding is a resistance welding technique in which overlapping base metals are clamped between the tips of electrodes, and a current is passed through them while applying pressure, causing the metals to melt and join with the Joule heat generated. In lap resistance welding such as spot welding, the molten and solidified part that forms at the weld is called a nugget.

[0003] Spot welding is highly efficient and is therefore used to manufacture a variety of mechanical structural components. For example, thousands of spot welds are formed per automobile during production. However, spot welding presents a problem in that welding defects are likely to occur in sheet assemblies with a large sheet thickness ratio. A sheet assembly is a group of multiple stacked metal sheets that are the target of spot welding. The sheet thickness ratio of a sheet assembly is the total thickness of the metal sheets in the sheet assembly divided by the thickness of the thinner metal sheet located on the surface of the sheet assembly.

[0004] The nugget that joins the metal plates is formed in the center of the sheet assembly or in its vicinity. If the diameter of the nugget is small or if the nugget is formed at a position far from the thin plates on the surface of the sheet assembly, the nugget and the thin plates on the surface of the sheet assembly will not overlap, resulting in poor joining of the thin plates. In particular, in sheet assemblies with a sheet thickness ratio of more than 6.0, poor joining of the thin plates on the surface of the sheet assembly is likely to occur.

[0005] Poor joints in spot welding are particularly problematic in the manufacture of automobile parts. Automobile parts are typically made up of thin mild steel sheets that form the exterior and high-strength steel sheets that form the frame. When spot welding such parts, the thin sheet that forms the exterior is inevitably placed on the outermost surface of the assembly.

[0006] Furthermore, in spot welding between high-strength steel sheets and mild steel, nuggets tend to be formed disproportionately on the high-strength steel sheets. This is because high-strength steel sheets contain more alloying elements than mild steel, and these alloying elements increase the electrical resistance of the high-strength steel sheets. In spot welding between high-strength steel sheets and mild steel, resistance heating occurs preferentially in the high-strength steel sheets, which have higher electrical resistance. Therefore, in spot welding between high-strength steel sheets and mild steel, the nugget does not grow to the mild steel sheets, which may result in poor joining of the mild steel.

[0007] Thus, poor joints are likely to occur in resistance spot welding of sheet assemblies in which thin mild steel is arranged on the outermost surface. It is generally believed that poor joints are particularly likely to occur in sheet assemblies with a sheet thickness ratio of more than 6.0. Poor joints are likely to occur in spot welding of three or more overlapping steel sheets. Poor joints are particularly likely to occur when the number of overlapping steel sheets is four or more.

[0008] Reducing the sheet thickness ratio to avoid poor joints reduces the design freedom of mechanical structural components. To avoid poor joints, notches can be formed in the steel sheets at the joint to reduce the number of steel sheets in the joint. However, this method reduces the strength of the component. Therefore, a technology to prevent poor joints in spot welding of sheet assemblies with a high sheet thickness ratio and three or more metal sheets is desired.

[0009] Patent Document 1 discloses a resistance spot welding method in which a plate assembly in which a thinner steel plate is overlapped on at least one of two or more overlapping thick steel plates is sandwiched between a pair of electrodes and resistance spot welding is performed while applying a pressure, the method comprising a preliminary step and a main step, in which a high electrode pressure is applied in the preliminary step to eliminate gaps in the plate assembly, and then in the main step, welding is performed at a low pressure, for a short time, and with a high current in the early stage of welding, and welding is performed at a high pressure in the later stage of welding.

[0010] Patent Document 2 discloses a resistance spot welding method in which a plate assembly, in which a thin plate is overlapped on at least one of two or more overlapping thick plates, is sandwiched between a pair of electrodes and a pressure is applied, and the method comprises: once the pressure reaches a set value P, starting a preliminary current flow at a low current; once all of the overlapping plates are in contact with each other, fixing the position of the electrodes; and then starting main current flow; fixing the electrode position suppresses thermal expansion of the plate assembly after the start of main current flow, thereby increasing the pressure; and setting the set pressure P and maximum pressure Pm to satisfy Pm≧1.5×P and P≦4 kN.

[0011] Patent Document 3 discloses a resistance spot welding method for high-strength thin steel sheets, which comprises: sandwiching two or more overlapping high-strength thin steel sheets between a pair of electrodes and applying a pressure; after welding a first point, moving the position of the electrodes; and, after the welded portion of the first point has cooled to a temperature equal to or lower than the Mf point, welding a second point so as to overlap the welded portion of the first point.

[0012] Patent Document 4 discloses a spot welding method for spot welding a stack formed by stacking three or more workpieces, the spot welding method comprising the steps of: clamping the stack between a first welding tip and a second welding tip; and electrically connecting an inner workpiece, of two workpieces in the stack that has the greatest contact resistance and faces the inside of the stack, to an earth electrode or an electrode of the same polarity as the second welding tip that is close to an outer workpiece that is adjacent to the inner workpiece and faces the outside of the stack; causing a current to flow from the first electrode tip to the second welding tip, thereby generating a current from the first electrode tip to the electrode via the inner workpiece, and pressing the stack with a first pressing force by the first welding tip and the second welding tip to join the inner workpiece to another workpiece; and pressing the stack with a second pressing force greater than the first pressing force to join the inner workpiece to the outer workpiece.

[0013] JP 2009-241112 A JP 2008-290098 A JP 2010-172945 A JP 2012-71333 A

[0014] In the techniques of Patent Documents 1 to 4, it is necessary to increase the welding current value in order to promote nugget growth and avoid poor joints. However, increasing the welding current value makes expulsion more likely to occur. Explosion refers to the phenomenon in which the base material is locally overheated and melts and splashes during lap resistance welding such as spot welding, or the metal itself. A large amount of expulsion deteriorates welding workability. Furthermore, if expulsion occurs, deep dents may form in the welded portion, reducing joint strength. With the techniques of Patent Documents 1 to 4, it is difficult to avoid both poor joints and expulsion.

[0015] In view of the above circumstances, an object of the present invention is to provide a spot-welded component with high strength reliability that can avoid both poor joining and the occurrence of expulsion when spot welding a plate assembly having three or more metal plates, and a method for manufacturing a spot-welded component.

[0016] The gist of the present invention is as follows.

[0017] (1) A spot-welded component according to one aspect of the present disclosure comprises: one first metal plate and one second metal plate arranged on a surface of the spot-welded component; a first internal metal plate group consisting of one or more internal metal plates arranged between the first metal plate and the second metal plate; a first spot weld having a first nugget joining the first metal plate and the first internal metal plate group; and a second spot weld having a second nugget joining the second metal plate and the first internal metal plate group, wherein indentations of the first spot weld are provided on the first metal plate and the internal metal plate, indentations of the second spot weld are provided on the first metal plate and the second metal plate, the indentations of the first spot weld are separated from the indentations of the second spot weld, the first nugget is separated from the second metal plate, and the second nugget is separated from the first metal plate. (2) Preferably, in the spot-welded component described in (1) above, the number of the internal metal plates constituting the first internal metal plate group to which the first nugget is joined is two or more. (3) Preferably, the spot-welded component described in (1) or (2) above has a second internal metal plate group composed of one or more internal metal plates arranged between the first internal metal plate group and the second metal plate, the first nugget is spaced from the second internal metal plate group, and the second nugget joins the second metal plate, the first internal metal plate group, and the second internal metal plate group. (4) Preferably, in the spot-welded component described in any one of (1) to (3) above, the distance between the center of the indentation in the first spot weld and the center of the indentation in the second spot weld is 10 mm or more and 50 mm or less in plan view. (5) Preferably, in the spot-welded component according to any one of (1) to (4) above, the second metal plate is thicker than the first metal plate.(6) Preferably, in the spot-welded component described in (5) above, the value obtained by dividing the total thickness of the first metal plate and the first internal metal plate group joined by the first nugget by the thickness of the first metal plate is less than 6.0, and the value obtained by dividing the total thickness of the first metal plate, the second metal plate, and the internal metal plate of the spot-welded component by the thickness of the first metal plate is greater than 6.5. (7) Preferably, in the spot-welded component described in any one of (1) to (6) above, the first metal plate, the second metal plate, and the internal metal plate are steel plates, the first metal plate has a Vickers hardness of 150 HV or less, and at least one of the second metal plate and the internal metal plate has a hardness of 300 HV or more. (8) Preferably, in the spot-welded component described in any one of (1) to (7) above, an adhesive is provided at one or more joint interfaces. (9) Preferably, in the spot-welded component described in any one of (1) to (8) above, the number of metal plates to which the second nugget is joined is three or more, and the value obtained by dividing the total thickness of the metal plates to which the second nugget is joined by the thickness of the second metal plate is 1.8 to 4.2.

[0018] (10) A method for manufacturing a spot-welded member according to another aspect of the present disclosure includes the steps of: performing first spot welding on a first internal metal plate group consisting of a first metal plate and one or more internal metal plates stacked together to form a joined body having a first nugget joining the first metal plate and the first internal metal plate group; and performing second spot welding on the internal metal plate arranged on the surface of the joined body to form a second nugget joining the first internal metal plate group and the second metal plate of the joined body, wherein the welding point of the second spot welding is spaced from the welding point of the first spot welding, and the second spot welding is terminated before the second nugget grows to the first metal plate. (11) Preferably, in the method for manufacturing a spot-welded member described above in (10), the number of internal metal plates constituting the first internal metal plate group to be subjected to the first spot welding is two or more. (12) Preferably, in the method for manufacturing a spot-welded member described in (10) or (11) above, a second internal metal plate group consisting of one or more internal metal plates and the second metal plate are overlapped in order on the internal metal plate arranged on the surface of the joined body, and the second spot welding is performed. (13) Preferably, in the method for manufacturing a spot-welded member described in any one of (10) to (12) above, the distance between the center of the weld point of the first spot weld and the center of the weld point of the second spot weld is 10 mm to 50 mm in plan view. (14) Preferably, in the method for manufacturing a spot-welded member described in any one of (10) to (13) above, the second metal plate is thicker than the first metal plate. (15) Preferably, in the manufacturing method of spot-welded components described in (14) above, the value obtained by dividing the total thickness of the first metal plate and the first internal metal plate group of the joined body by the thickness of the first metal plate is less than 6.0, and the value obtained by dividing the total thickness of the first metal plate, the second metal plate, and the internal metal plate of the spot-welded component by the thickness of the first metal plate is more than 6.5.(16) Preferably, in the method for manufacturing a spot-welded member according to any one of (10) to (15) above, the first metal plate, the second metal plate, and the internal metal plate are steel plates, the first metal plate has a Vickers hardness of 150 HV or less, and at least one of the second metal plate and the internal metal plate has a hardness of 300 HV or more. (17) Preferably, the method for manufacturing a spot-welded member according to any one of (10) to (16) above further includes, before the first spot welding, applying an adhesive to one or more of a surface of the first metal plate and a surface of the internal metal plate, and the surface to which the adhesive is applied constitutes a joining interface of the joined body. (18) Preferably, in the method for manufacturing a spot-welded member according to any one of (10) to (17) above, the method further comprises, before the second spot welding, applying an adhesive to one or more of the surfaces of the joined body and the surfaces of the metal plates to be placed thereon, and the surfaces to which the adhesive is applied form the joining interface of the spot-welded member. (19) Preferably, in the method for manufacturing a spot-welded member according to any one of (10) to (18) above, the number of the metal plates to be joined by the second nugget is three or more, and the value obtained by dividing the total thickness of the metal plates to be joined by the second nugget by the thickness of the second metal plate is 1.8 to 4.2.

[0019] According to the present invention, it is possible to provide a spot-welded component with high strength reliability that can avoid both poor joining and the occurrence of expulsion when spot welding a plate assembly having three or more metal plates, and a method for manufacturing a spot-welded component.

[0020] FIG. 1 is a cross-sectional schematic diagram of a first spot weld to two metal plates and a joined body. FIG. 2 is a cross-sectional schematic diagram of the joined body of FIG. 1 and a second spot weld to one metal plate, and a spot-welded member. FIG. 3 is a cross-sectional schematic diagram of the joined body of FIG. 3 and a second spot weld to one metal plate, and a spot-welded member. FIG. 1 is a cross-sectional schematic diagram of the joined body of FIG. 1 and a second spot weld to two metal plates, and a spot-welded member. FIG. 1 is a cross-sectional schematic diagram of the joined body of FIG. 1 and a second spot weld to three metal plates, and a spot-welded member. FIG. 2 is a cross-sectional schematic diagram of the spot-welded member in which surface expulsion has occurred. FIG. 3 is a cross-sectional photograph of a spot-welded member of Example 3. FIG. 4 is a cross-sectional photograph of a spot-welded member of Example 4. FIG. 4 is a cross-sectional photograph of an example of a spot-welded member having five metal plates.

[0021] (1. Manufacturing Method of Spot-Welded Member 1) A manufacturing method of a spot-welded member 1 according to one aspect of the present invention includes the steps of: performing a first spot weld S1 on a first internal metal plate group 1131 composed of one stacked first metal plate 111 and one or more internal metal plates 113, to form a joined body 13 having a first nugget 121 that joins the first metal plate 111 and the first internal metal plate group 1131; and performing a second spot weld S2 on the internal metal plate 113 arranged on the surface of the joined body 13, to form a second nugget 122 that joins the first internal metal plate group 1131 and the second metal plate 112 of the joined body 13; and separating a welding point WP of the second spot weld S2 from a welding point WP of the first spot weld S1, and terminating the second spot weld S2 before the second nugget 122 grows to the first metal plate 111.

[0022] (First Spot Welding S1) First, a plate assembly is prepared by stacking one or more internal metal plates 113 on a first internal metal plate group 1131, which is formed by stacking one or more internal metal plates 113, and then stacking a first metal plate 111 on the first internal metal plate group 1131. A first spot welding S1 is performed on this plate assembly. In spot welding, the stacked metal plates are clamped between the tips of a pair of spot welding electrodes E, and a current is passed through the metal plates while applying a pressure. Indirect spot welding, which is resistance welding without clamping the metal plates between the tips of the pair of electrodes E, is not considered spot welding in the manufacturing method of the spot-welded member 1 according to this embodiment. The metal plates are melted by Joule heat generated by the current. The molten zone grows from near the center of the plate assembly toward the surface of the plate assembly. The plate assembly refers to multiple stacked metal plates to be spot-welded. When the current is terminated, heat transfer occurs from the metal plates to the electrodes E, causing the molten metal to solidify. The nugget formed by the solidification of the molten metal joins the metal plates.

[0023] The first metal plate 111 is part of the multiple metal plates that the spot-welded component 1 has. The internal metal plate 113 that constitutes the first internal metal plate group 1131 is also part of the multiple metal plates that the spot-welded component 1 has. The first metal plate 111 is disposed on the surface of the spot-welded component 1. The internal metal plate 113 is disposed inside the spot-welded component 1. However, when the first spot welding S1 is completed, the internal metal plate 113 is disposed on the surface of the joined body 13.

[0024] Note that the spot-welded component 1 may have a second internal metal plate group 1132 composed of one or more internal metal plates 113 (see FIGS. 5A and 5B ). The first internal metal plate group 1131 and the second internal metal plate group 1132 are distinguished based on whether or not they are the target of the first spot welding S1. The first internal metal plate group 1131 is the target of the first spot welding S1. Therefore, a first nugget 121 is formed in the first internal metal plate group 1131. On the other hand, the second internal metal plate group 1132 is not the target of the first spot welding S1. Therefore, the first nugget 121 is not formed in the second internal metal plate group 1132. In other words, the second internal metal plate group 1132 is separated from the first nugget 121. Details of the second internal metal plate group 1132 will be described later.

[0025] The first spot welding S1 forms a joined body 13 as shown in FIG. 1 and other drawings. The joined body 13 includes a first metal plate 111 and an internal metal plate 113 stacked together, and a first nugget 121. The first metal plate 111 is disposed on one surface of the joined body 13. At the end of the first spot welding S1, the internal metal plate 113 is disposed on the surface of the joined body 13. The first nugget 121 joins the first metal plate 111 and a first internal metal plate group 1131.

[0026] A first indentation 1211 is formed on the surface of the joined body 13 by the first spot welding S1. That is, the first indentation 1211 is formed on the first metal plate 111 and the internal metal plate 113 disposed on the surface of the joined body 13. The indentation is a depression on the surface of the base material caused by the electrode tip as a result of spot welding.

[0027] (Second Spot Welding S2) Next, one second metal plate 112 is overlapped on the internal metal plate 113 arranged on the surface of the joined body 13 (see FIG. 2, etc.). Second spot welding S2 is performed on the overlapped joined body 13 and second metal plate 112. A second nugget 122 is formed by the second spot welding S2, and the spot-welded member 1 shown in FIG. 2, etc. is obtained.

[0028] The second metal plate 112 is one of the multiple metal plates included in the spot-welded member 1. The second metal plate 112 is disposed on the surface of the spot-welded member 1 opposite to the first metal plate 111.

[0029] The spot-welded component 1 has a first metal plate 111, a second metal plate 112, an inner metal plate 113, a first nugget 121, and a second nugget 122. Because the first spot weld S1 is not applied to the second metal plate 112, the second metal plate 112 is spaced apart from the first nugget 121.

[0030] The second spot weld S2 forms second indentations 1221 on both surfaces of the spot-welded member 1. That is, the first metal plate 111 and the second metal plate 112 are provided with second indentations 1221.

[0031] The second nugget 122 joins the first internal metal plate group 1131 of the joined body 13 and the second metal plate 112. However, the second spot welding S2 is terminated before the second nugget 122 grows to the first metal plate 111. Therefore, the second nugget 122 does not join all of the metal plates of the spot-welded member 1. The second nugget 122 is separated from the first metal plate 111.

[0032] In the second spot weld S2, the weld point WP of the second spot weld S2 is spaced apart from the weld point WP of the first spot weld S1. The weld point WP is a location on the metal plate where the spot welding electrode E comes into contact. By spaced apart the weld point WP of the first spot weld S1 and the weld point WP of the second spot weld S2, the second nugget 122 is spaced apart from the first nugget 121. By observing the cross section of the spot-welded member 1, it can be determined whether the first nugget 121 is spaced apart from the second nugget 122.

[0033] (Effects) In the first spot welding S1, only a portion of the metal plates included in the spot-welded member 1 are welded. As a result, in the first spot welding S1, the plate thickness ratio of the plate assembly is reduced. The plate thickness ratio of the plate assembly is the value obtained by dividing the total thickness of the metal plates in the plate assembly by the thickness of the thinner of the metal plates located on the surface of the plate assembly. By producing the plate assembly from only a portion of the metal plates, the total thickness of the plate assembly can be reduced, and the plate thickness ratio can be reduced.

[0034] Reducing the sheet thickness ratio in the first spot weld S1 reduces the welding current value required to grow the first nugget 121 to the first metal sheet 111. Reducing the sheet thickness ratio makes it possible to avoid poor welding of the first metal sheet 111. Furthermore, reducing the sheet thickness ratio also makes it possible to reduce the welding current value and avoid expulsion in the first spot weld S1.

[0035] In the second spot welding S2, all of the metal plates included in the spot-welded member 1 appear to be welded. However, in the second spot welding S2, the second nugget 122 is not grown to the first metal plate 111. Because the first metal plate 111 is joined to the first internal metal plate group 1131 in the first spot welding S1, there is no need in the second spot welding S2 to grow the second nugget 122 to the first metal plate 111. Therefore, in the second spot welding S2, a lower welding current value can be selected than in spot welding of a typical plate assembly with a high plate thickness ratio.

[0036] Increasing the welding current value to grow the nugget to the first metal sheet 111 increases the risk of surface expulsion occurring on the surface of the second metal sheet 112. Surface expulsion is a phenomenon in which molten metal from the nugget erupts from the surface of the metal sheet (see FIG. 6 ). Surface expulsion protrudes from the surface of the weld or adheres to the surface of the metal sheet, damaging the aesthetic appearance of the spot-welded component 1. Surface expulsion also impairs welding workability. However, the manufacturing method for the spot-welded component 1 according to this embodiment can extremely efficiently suppress expulsion during the second spot weld S2.

[0037] Furthermore, in the second spot weld S2, the weld point WP of the second spot weld S2 is separated from the weld point WP of the first spot weld S1. This makes it possible to suppress current shunting in the second spot weld S2. Current shunting refers to current flowing in a parallel circuit formed by existing weld points in spot welding. In the second spot weld S2, the first nugget 121 becomes an existing weld point that causes current shunting. If current shunting to an existing weld point occurs, the current flowing through the second metal sheet increases, further increasing the risk of surface expulsion occurring on the surface of the second metal sheet 112.

[0038] In the manufacturing method of the spot-welded member 1 according to this embodiment, the welding point WP of the second spot weld S2 is separated from the welding point WP of the first spot weld S1 to suppress current shunting, which further reliably suppresses expulsion at the second spot weld S2.

[0039] The most basic aspect of the method for manufacturing the spot-welded member 1 according to this embodiment has been described above. A more preferred aspect will now be described.

[0040] (Spot welding of four or more metal plates) As shown in Fig. 1 , the number of internal metal plates 113 constituting the first internal metal plate group 1131 subjected to the first spot welding S1 may be one. On the other hand, as shown in Fig. 3 and Fig. 4 , the number of internal metal plates 113 constituting the first internal metal plate group 1131 subjected to the first spot welding S1 may be two or more. In this case, one internal metal plate 113 is disposed on the surface of the joined body 13, and the remaining internal metal plates 113 are disposed inside the joined body 13.

[0041] As shown in FIG. 2 , the second spot welding S2 may be performed after only the second metal plate 112 is placed on the internal metal plate 113 arranged on the surface of the joined body 13. In this case, the second metal plate 112 is directly placed on the internal metal plate 113 arranged on the surface of the joined body 13. On the other hand, as shown in FIGS. 5A and 5B , the second spot welding S2 may be performed after a second internal metal plate group 1132 consisting of one or more internal metal plates 113 and the second metal plate 112 are placed on the internal metal plate 113 arranged on the surface of the joined body 13. That is, a second internal metal plate group 1132 consisting of another internal metal plate 113 may be placed between the second metal plate 112 and the first internal metal plate group 1131 of the joined body 13. In FIG. 5A , the second internal metal plate group 1132 includes one internal metal plate 113. 5B, the number of internal metal plates 113 included in the second internal metal plate group 1132 is two. The number of internal metal plates 113 included in the second internal metal plate group 1132 may be three or more.

[0042] 5A and 5B , the second metal plate 112 is overlapped with the first internal metal plate group 1131 included in the joined body 13 via the additionally provided second internal metal plate group 1132. Then, a second spot weld S2 is applied to the joined body 13, the second internal metal plate group 1132, and the second metal plate 112.

[0043] 3, 4, 5A, and 5B, the number of metal plates included in the spot-welded component 1 can be set to four or more. This makes it possible to manufacture a complex spot-welded component 1 having a large number of metal plates.

[0044] 5A and 5B, the number of metal plates included in the joined body 13 is two. On the other hand, as illustrated in Fig. 3, the number of metal plates included in the joined body 13 may be three or more, and two or more metal plates may be spot-welded to the joined body 13 as illustrated in Fig. 5A and 5B.

[0045] (Distance between the centers of the welding points WP) The distance between the welding points WP in the first spot weld S1 and the welding points WP in the second spot weld S2 is not particularly limited. From the viewpoint of further reducing shunt current and more reliably suppressing expulsion, the distance D between the centers of the welding points WP in the first spot weld S1 and the welding points WP in the second spot weld S2 is preferably 10 mm or more, 15 mm or more, 20 mm or more, or 30 mm or more in a plan view. On the other hand, from the viewpoint of further improving the load transmission efficiency of the spot-welded member 1, the distance D between the centers of the welding points WP in the first spot weld S1 and the welding points WP in the second spot weld S2 is preferably 50 mm or less, 45 mm or less, 40 mm or less, or 35 mm or less in a plan view.

[0046] The center of the welding point WP is the location where the central axis of the electrode E was located during spot welding. The center of the first indentation 1211 formed on the surface of the joined body 13 is considered to be the center of the welding point WP in the first spot weld S1. The center of the second indentation 1221 formed on the surface of the spot-welded member 1 is considered to be the center of the welding point WP in the second spot weld S2.

[0047] In manufacturing the spot-welded member 1, the first spot weld S1 and the second spot weld S2 may each be performed two or more times. For example, if the metal plate is a flange portion of a hat-shaped member, the first spot weld S1 and the second spot weld S2 are performed on the flange portion multiple times along the extension direction of the flange portion. In this case, the distance D between the center of the weld point WP of the first spot weld S1 and the center of the weld point WP of the second spot weld S2 means the distance D between the center of the weld point WP of the first spot weld S1 and the center of the weld point WP of the second spot weld S2 that is closest to it.

[0048] (Thickness of Second Metal Plate 112) The thickness of the second metal plate 112 is not particularly limited. From the viewpoint of reliably joining the second metal plate 112 to at least the adjacent internal metal plate 113, it is preferable that the second metal plate 112 be thicker than the first metal plate 111. This makes it easier to grow the second nugget 122 into the second metal plate 112. The thickness of the second metal plate 112 may be 1.1 times or more, 1.2 times or more, 1.5 times or more, or 2.0 times or more the thickness of the first metal plate 111.

[0049] (Thickness ratio) The thickness ratio of the joined body 13, i.e., the value obtained by dividing the total thickness of the first metal plate 111 and the first internal metal plate group 1131 of the joined body 13 by the thickness of the first metal plate 111, is preferably less than 6.0. This makes it easier to grow the first nugget 121 up to the first metal plate 111. The thickness ratio of the joined body 13 may be 5.8 or less, 5.5 or less, or 5.0 or less.

[0050] The thickness ratio of the spot-welded component 1 based on the thickness of the first metal plate 111, i.e., the value obtained by dividing the total thickness of the first metal plate 111, the second metal plate 112, and the inner metal plate 113 of the spot-welded component 1 by the thickness of the first metal plate 111, is preferably greater than 6.5. This further improves the strength and rigidity of the spot-welded component 1. The thickness ratio of the spot-welded component 1 based on the thickness of the first metal plate 111 may be 6.8 or more, 7.0 or more, 7.2 or more, or 7.5 or more. When the number of metal plates joined by the second nugget 122 is three or more, the thickness ratio of the second nugget 122 is preferably 1.8 to 4.2. The number of metal plates joined by the second nugget 122 is the sum of the number of the second metal plate 112 and the inner metal plate 113 joined by the second nugget 122. When determining the number of metal plates joined by the second nugget 122, it is not taken into consideration whether the internal metal plate 113 belongs to the first internal metal plate group 1131 or the second internal metal plate group 1132. The plate thickness ratio for the second nugget 122 is the total thickness of the metal plates joined by the second nugget 122 divided by the thickness of the second metal plate 112. To ensure that the number of metal plates joined by the second nugget 122 is three or more, the number of internal metal plates 113 constituting the first internal metal plate group 1131 may be two or more, as shown in FIG. 4 . In the spot-welded component 1 illustrated in FIG. 4 , the number of internal metal plates 113 constituting the first internal metal plate group 1131 is two, and the number of internal metal plates 113 constituting the second internal metal plate group 1132 is zero. The second nugget 122 in Fig. 4 joins two internal metal plates 113 included in the first internal metal plate group 1131 and the second metal plate 112. Therefore, the number of metal plates joined by the second nugget 122 in Fig. 4 is three. On the other hand, in order to join three or more metal plates to the second nugget 122, the number of internal metal plates 113 constituting the second internal metal plate group 1132 may be one or more, as shown in Fig. 5A. In the spot-welded member 1 illustrated in Fig. 5A, the number of internal metal plates 113 constituting the first internal metal plate group 1131 is one, and the number of internal metal plates 113 constituting the second internal metal plate group 1132 is one.The second nugget 122 in FIG. 5A joins one internal metal plate 113 in the first internal metal plate group 1131, one internal metal plate 113 in the second internal metal plate group 1132, and the second metal plate 112. Therefore, the number of metal plates joined by the second nugget 122 in FIG. 5A is also three. In either case, the thickness ratio of the second nugget 122 is preferably 1.8 to 4.2. By setting the thickness ratio of the second nugget 122 to 1.8 or more, the occurrence of surface expulsion from the second metal plate 112 can be further suppressed. Furthermore, by setting the thickness ratio of the second nugget 122 to 4.2 or less, the bond between the second nugget 122 and the adjacent internal metal plate can be further strengthened. Increasing the thickness of the second metal plate increases the cross-sectional area through which current flows and reduces electrical resistance. Therefore, current shunting toward the first nugget increases. For this reason, if the current value is constant, the formation of the second nugget will be insufficient. Normally, the current is increased to compensate for this, which makes surface expulsion more likely to occur. The thickness ratio for the second nugget 122 may be set to 2.0 or more, 2.5 or more, or 3.0 or more. The thickness ratio for the second nugget 122 may be set to 4.0 or less, 3.5 or less, or 3.2 or less.

[0051] (Type of Metal Plate) The type of metal plate is not particularly limited. In the manufacturing method of the spot-welded component 1 according to this embodiment, any material that can be spot-welded can be used as the welding base material. A suitable material can be used depending on the application of the spot-welded component 1. Examples of metal plates include steel plates, aluminum plates, and stainless steel plates.

[0052] A suitable example of the metal plate is a steel plate. In particular, it is preferable that one or more of the metal plates be high-strength steel plates. For example, when the spot-welded component 1 is an automobile part, by using one or more of the metal plates as high-strength steel plates, the thickness of the metal plates can be reduced while ensuring the strength of the spot-welded component 1. By reducing the thickness of the metal plates, the weight of the spot-welded component 1 can be reduced.

[0053] When the metal plates are steel plates, it is preferable that the first metal plate 111 be mild steel and that one or more of the second metal plate 112 and the inner metal plate 113 be high-strength steel plates. For example, it is preferable that the Vickers hardness of the first metal plate 111 be 150 HV or less, 140 HV or less, 130 HV or less, or 120 HV or less, and that the hardness of one or more of the second metal plate 112 and the inner metal plate 113 be 300 HV or more, 320 HV or more, 380 HV or more, or 440 HV or more. The first metal plate 111 is disposed on the outermost surface of the spot-welded member 1, and is therefore suitable, for example, for use as an exterior component of an automobile. By using thin mild steel for the first metal plate 111, the formability of the first metal plate 111 can be improved, thereby improving the aesthetic appearance of the automobile. Furthermore, from the viewpoint of improving the corrosion resistance of automobiles, zinc-based plated steel sheets (electrogalvanized steel sheets, hot-dip galvanized steel sheets, galvannealed hot-dip galvanized steel sheets, zinc-nickel plated steel sheets) are desirable. The second metal sheet 112 and the internal metal sheet 113 are suitable for, for example, automobile frame components. By using a high-strength steel sheet for one or more of the second metal sheet 112 and the internal metal sheet 113, the collision safety of the automobile can be improved. Examples of high-strength steel sheets include press-formed cold-rolled steel sheets, hot-rolled steel sheets, and hot-stamped steel sheets. In the case of automobile body parts requiring corrosion resistance, zinc-based plated steel sheets (hot-dip galvanized steel sheets, galvannealed hot-dip galvanized steel sheets, zinc-based plated hot-stamped steel sheets) and aluminum-based plated hot-stamped steel sheets may be included.

[0054] (Application of Adhesive) Weld bonding may be applied to the manufacturing method of the spot-welded member 1 according to this embodiment. Weld bonding is a method in which an adhesive is applied to the joint interface 14 in advance and then spot welding is performed. Therefore, the manufacturing method of the spot-welded member 1 according to this embodiment may include, in addition to the first spot welding S1 and the second spot welding S2, a step of bonding the metal plates using an adhesive. By using both bonding and spot welding, the rigidity and joint strength of the spot-welded member 1 can be further increased. Weld bonding may be applied to either the first spot welding S1 or the second spot welding S2, or to both.

[0055] For example, the manufacturing method of the spot-welded component 1 may further include a step of applying an adhesive to one or more of the surface of the first metal plate 111 and the surface of the internal metal plate 113 before the first spot welding S1. In this case, the surface to which the adhesive is applied is defined as the joint interface 14 of the joined body 13. The joint interface 14 of the joined body 13 refers to the mating surfaces of the overlapping metal plates in the joined body 13.

[0056] For example, the manufacturing method of the spot-welded component 1 may further include a step of applying an adhesive to one or more of the surfaces of the joined body 13 and the surfaces of the metal plates overlapping the joined body 13 before the second spot welding S2. In this case, the surface to which the adhesive is applied is defined as the joint interface 14 of the spot-welded component 1. The joint interface 14 of the spot-welded component 1 refers to the mating surfaces of the overlapping metal plates in the spot-welded component 1.

[0057] The spot-welded component 1 has two or more joint interfaces 14. From the viewpoint of improving rigidity and joint strength, it is preferable to provide an adhesive at at least one of the joint interfaces 14. For example, it is preferable to provide an adhesive at the joint interface 14 between the first metal plate 111 and the second metal plate 112, and it is even more preferable to provide an adhesive at all of the joint interfaces 14. The type of adhesive is not particularly limited. Examples of adhesives include a thermosetting epoxy adhesive, urethane adhesive, or rubber adhesive that hardens during heat treatment (140°C to 190°C) used in electrodeposition coating of automobiles.

[0058] (2. Spot-welded component 1) A spot-welded component 1 according to another aspect of the present invention includes one first metal plate 111 and one second metal plate 112 arranged on the surface of the spot-welded component 1, a first internal metal plate group 1131 consisting of one or more internal metal plates 113 arranged between the first metal plate 111 and the second metal plate 112, a first spot weld having a first nugget 121 joining the first metal plate 111 and the first internal metal plate group 1131, and a second nugget 122 joining the second metal plate 112 and the first internal metal plate group 1131. and a second spot weld formed on the inner metal plate 113, wherein a first indentation 1211 of the first spot weld is formed on the first metal plate 111 and the inner metal plate 113, and a second indentation 1221 of the second spot weld is formed on the first metal plate 111 and the second metal plate 112, the indentation of the first spot weld being spaced apart from the indentation of the second spot weld, the first nugget 121 being spaced apart from the second metal plate 112, and the second nugget 122 being spaced apart from the first metal plate 111. The spot-welded component 1 according to this embodiment will be described in detail below. The preferred aspects of the method for manufacturing the spot-welded component 1 described above can also be applied to the spot-welded component 1 according to this embodiment.

[0059] 2 and other figures, the spot-welded component 1 has a first metal plate 111, a second metal plate 112, and an internal metal plate 113. The first metal plate 111 and the second metal plate 112 are disposed on the surface of the spot-welded component 1. The internal metal plate 113 is disposed between the first metal plate 111 and the second metal plate 112.

[0060] 2 and other figures, the spot-welded component 1 further includes a first spot-welded portion having a first nugget 121 and a second spot-welded portion having a second nugget 122. The spot-welded portion is a general term that includes the nugget and the heat-affected zone (HAZ). The spot-welded portion also has an indentation (i.e., a depression on the surface of the base material caused by the electrode tip).

[0061] The first nugget 121 joins the first metal plate 111 to the first inner metal plate group 1131. The second nugget 122 joins the second metal plate 112 to the first inner metal plate group 1131.

[0062] The first nugget 121 and the second nugget 122 are distinguished based on their corresponding indentations. One of the first indentations 1211, i.e., the indentation of the first spot weld, is provided on the surface of the spot-welded component 1, and the other is provided inside the spot-welded component 1. The second indentations 1221, i.e., the indentations of the second spot weld, are both provided on the surface of the spot-welded component 1. By observing the appearance of the spot-welded component 1, it is possible to identify locations where indentations are provided on both surfaces and locations where indentations are provided on only one surface. A nugget located at a location where indentations are provided on only one surface of the spot-welded component 1 is a first nugget 121. A nugget located at a location where indentations are provided on both surfaces of the spot-welded component 1 is a second nugget 122.

[0063] The first metal plate 111 and the second metal plate 112 are distinguished based on the nuggets provided thereon. The metal plate on which the first nugget 121 is provided is the first metal plate 111. The metal plate on which the second nugget 122 is provided is the second metal plate 112. In other words, the first indentation 1211 is provided on the first metal plate 111 and the internal metal plate 113 constituting the first internal metal plate group 1131, and the second indentation 1221 is provided on the first metal plate 111 and the second metal plate 112. Note that, when observing the appearance of the spot-welded component 1, it can be seen that the number of indentations differs between the first surface and the second surface of the spot-welded component 1. It can be determined that the metal plate with the most indentations is the first metal plate 111.

[0064] The first indentation 1211 (i.e., the indentation of the first spot weld) is spaced apart from the second indentation 1221 (i.e., the indentation of the second spot weld). By observing the surface of the first metal plate 111, the outer edges of the first indentation 1211 and the second indentation 1221 can be easily identified. Therefore, whether the first indentation 1211 and the second indentation 1221 are spaced apart from each other can be easily determined based on an external observation of the spot-welded component 1. Alternatively, whether the first indentation 1211 and the second indentation 1221 are spaced apart from each other may be determined by observing a cross section of the nugget. A method for observing the cross section will be described later.

[0065] 2 and other drawings, the first nugget 121 is spaced apart from the second metal plate 112. The second nugget 122 is spaced apart from the first metal plate 111. That is, both the first nugget 121 and the second nugget 122 join only some of the multiple metal plates included in the spot-welded member 1.

[0066] (Effects) The first nugget 121 is spaced apart from the second metal plate 112. Similarly, the second nugget 122 is spaced apart from the first metal plate 111. That is, the first nugget 121 and the second nugget 122 join only some of the metal plates included in the spot-welded component 1. Therefore, a high welding current is not required to manufacture the first nugget 121 and the second nugget 122, and expulsion is suppressed. The first nugget 121 and the second nugget 122 can be easily manufactured.

[0067] Furthermore, poor welding can be avoided when manufacturing the first nugget 121 and the second nugget 122. Generally, when joining a sheet assembly with a high sheet thickness ratio, it is difficult to grow the nugget up to the thinnest metal sheet arranged on the surface of the sheet assembly. Therefore, poor welding is likely to occur in the thinnest metal sheet arranged on the surface of the sheet assembly. However, when manufacturing the first nugget 121, the second metal sheet 112 is not the welding target, and the sheet thickness ratio is reduced. When manufacturing the second nugget 122, there is no need to grow the nugget up to the first metal sheet 111, and therefore the sheet thickness ratio is substantially reduced. Therefore, the first nugget 121 can reliably join the first metal sheet 111 and its adjacent metal sheet. Furthermore, the second nugget 122 can reliably join the second metal sheet 112 and its adjacent metal sheet.

[0068] In addition, because the first indentation 1211 and the second indentation 1221 are spaced apart, shunting is suppressed during the second spot welding S2. By suppressing shunting, expulsion during the second spot welding S2 is more reliably suppressed.

[0069] The most basic aspect of the spot-welded member 1 according to this embodiment has been described above. A more preferred aspect will now be described.

[0070] (Number of Metal Plates Joined by the First Nugget 121 and the Second Nugget 122) The first nugget 121 joins the first metal plate 111 and the first internal metal plate group 1131. In the spot-welded member 1 illustrated in FIGS. 2 , 5A, and 5B , the first nugget 121 joins the first metal plate 111 and one internal metal plate 113. Therefore, the first internal metal plate group 1131 includes one internal metal plate 113, and the number of metal plates joined by the first nugget 121 is two. On the other hand, as illustrated in FIG. 4 , the first nugget 121 may join the first metal plate 111 and two or more internal metal plates 113. That is, the number of internal metal plates 113 that make up the first internal metal plate group 1131 may be two or more, and the number of metal plates may be three or more.

[0071] The second nugget 122 joins the second metal plate 112 and the first internal metal plate group 1131. In the spot-welded member 1 illustrated in FIG. 2 , the second nugget 122 joins the second metal plate 112 and one internal metal plate 113. Therefore, the first internal metal plate group 1131 includes one internal metal plate 113, and the number of metal plates joined by the second nugget 122 is two. On the other hand, as illustrated in FIG. 4 , the second nugget 122 may join the second metal plate 112 and two or more internal metal plates 113. That is, the number of internal metal plates 113 that make up the first internal metal plate group 1131 may be two or more, and the number of metal plates may be three or more.

[0072] 5A and 5B , a second internal metal plate group 1132 composed of one or more internal metal plates 113 that are not joined to the first nugget 121 but are joined to the second nugget 122 may be disposed between the first internal metal plate group 1131 and the second metal plate 112. As shown in FIG. 5A , the number of internal metal plates 113 included in the second internal metal plate group 1132 may be one. On the other hand, as shown in FIG. 5B , similar to the first internal metal plate group 1131, the number of internal metal plates 113 that constitute the second internal metal plate group 1132 may also be two or more. In this case, the first indentation 1211 provided in the internal metal plate 113 is provided in the internal metal plate 113 of the first internal metal plate group 1131 that is adjacent to the second internal metal plate group 1132. Therefore, the first internal metal plate group 1131 and the second internal metal plate group 1132 can be distinguished from the positions of the first nugget 121 and the second nugget 122 and the position of the first indentation 1211 made on the internal metal plate 113.

[0073] By making the number of metal plates joined by the first nugget 121 and / or the second nugget 122 three or more, and making the number of metal plates included in the spot-welded component 1 four or more, it becomes possible to manufacture a complex spot-welded component 1 having a large number of metal plates.

[0074] (Distance Between Indentation Centers) In the spot-welded member 1 according to this embodiment, the distance D between the center of the first indentation 1211 (i.e., the indentation of the first spot welded portion) and the center of the second indentation 1221 (i.e., the indentation of the second spot welded portion) is not particularly limited. From the viewpoint of further reducing shunting and more reliably suppressing expulsion, it is preferable that the distance D between the center of the first indentation 1211 and the center of the second indentation 1221 be 10 mm or more, 15 mm or more, 20 mm or more, or 30 mm or more in a plan view. On the other hand, from the viewpoint of further improving the load transmission efficiency of the spot-welded member 1, it is preferable that the distance D between the center of the first indentation 1211 and the center of the second indentation 1221 be 50 mm or less, 45 mm or less, 40 mm or less, or 35 mm or less in a plan view.

[0075] (Thickness of Second Metal Plate 112) The thickness of the second metal plate 112 is not particularly limited. From the viewpoint of reliably joining the second metal plate 112 to the internal metal plate 113, it is preferable that the second metal plate 112 be thicker than the first metal plate 111. The thickness of the second metal plate 112 may be 1.1 times or more, 1.2 times or more, 1.5 times or more, or 2.0 times or more the thickness of the first metal plate 111.

[0076] (Thickness ratio) The value obtained by dividing the total thickness of the first metal plate 111 and the first internal metal plate group 1131 to which the first nugget 121 is joined by the thickness of the first metal plate 111 is preferably less than 6.0, 5.8 or less, 5.5 or less, or 5.0 or less. This makes it easier to grow the first nugget 121 up to the first metal plate 111.

[0077] The sheet thickness ratio of the spot-welded component 1 based on the thickness of the first metal plate 111, i.e., the value obtained by dividing the total thickness of the first metal plate 111, the second metal plate 112, and the inner metal plate 113 of the spot-welded component 1 by the thickness of the first metal plate 111, is preferably greater than 6.5. This further improves the strength and rigidity of the spot-welded component 1. The sheet thickness ratio of the spot-welded component 1 based on the thickness of the first metal plate 111 may also be 6.8 or more, 7.0 or more, 7.2 or more, or 7.5 or more. As illustrated in FIGS. 4, 5A, and 5B, when the number of metal plates joined by the second nugget 122 is three or more, the value obtained by dividing the total thickness of the metal plates joined by the second nugget 122 by the thickness of the second metal plate 112 is preferably 1.8 to 4.2. The value obtained by dividing the total thickness of the metal plates to which the second nugget 122 is joined by the thickness of the second metal plate 112 is referred to as the thickness ratio of the second nugget 122. By setting the thickness ratio of the second nugget 122 to 1.8 or more, the occurrence of surface expulsion from the second metal plate 112 can be further suppressed. Furthermore, by setting the thickness ratio of the second nugget 122 to 4.2 or less, the joining between the second nugget 122 and the adjacent internal metal plate can be further strengthened. The thickness ratio of the second nugget 122 may be 2.0 or more, 2.5 or more, or 3.0 or more. The thickness ratio of the second nugget 122 may be 4.0 or less, 3.5 or less, or 3.2 or less.

[0078] (Type of Metal Plate) As described with respect to the manufacturing method of the spot-welded member 1, the type of metal plate is not particularly limited. A suitable example of a metal plate is a steel plate. When the first metal plate 111, the second metal plate 112, and the internal metal plate 113 are steel plates, the Vickers hardness of the first metal plate 111 is preferably 150 HV or less, 140 HV or less, 130 HV or less, or 120 HV or less, and the hardness of one or more of the second metal plate 112 and the internal metal plate 113 is 300 HV or more, 320 HV or more, 350 HV or more, or 380 HV. There is no particular upper limit for the hardness of the first metal plate 111, the second metal plate 112, and the internal metal plate 113, but the hardness of these metal plates may be, for example, 730 HV or less or 700 HV or less.

[0079] (Adhesive) An adhesive may be provided at one or more joint interfaces 14 of the spot-welded component 1. The joint interfaces 14 of the spot-welded component 1 refer to the mating surfaces of the overlapping metal plates in the spot-welded component 1. This further increases the rigidity and joint strength of the spot-welded component 1. From the viewpoint of improving the rigidity and joint strength, it is preferable to provide an adhesive at at least one joint interface 14. For example, it is preferable to provide an adhesive at the joint interface 14 between the first metal plate 111 and the second metal plate 112, and it is even more preferable to provide an adhesive at all joint interfaces 14.

[0080] Although the embodiment of the present invention has been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical concept of the invention. A more preferred example of the spot-welded component 1 according to this embodiment and its manufacturing method will be described below. The preferred aspects described below are applicable to both the spot-welded component 1 and its manufacturing method.

[0081] (Tensile Strength of Metal Plates) Spot welding is widely used in the manufacture of automotive parts. Therefore, the manufacturing method of the spot-welded member 1 according to this embodiment may be applied to automotive parts. In this case, for example, the first metal plate 111 is an automotive exterior part that requires high workability. The other metal plates are automotive frame parts that require high strength. Therefore, the tensile strength of the first metal plate 111 may be 440 MPa or less, 420 MPa or less, or 400 MPa or less, and the tensile strength of one or more metal plates other than the first metal plate 111 may be 980 MPa or more, 1180 MPa or more, or 1470 MPa or more. This facilitates the processing of automotive exterior parts and ensures the strength of automotive frame parts.

[0082] (Surface Treatment of Metal Sheet) One or more metal sheets may have a surface treatment layer. The surface treatment layer may be, for example, a plating layer. The composition of the plating layer may be suitable for the composition of the underlying metal sheet. For example, when the metal sheet is a steel sheet, the plating layer may be, for example, a zinc-based plating layer or an aluminum-based plating layer. The zinc-based plating layer is a plating layer having an average Zn content of 30% by mass or more, or 50% by mass or more. Examples include electrogalvanizing, hot-dip galvanizing, galvannealed hot-dip galvanizing, zinc-nickel plating, hot-stamped galvannealed hot-dip galvanizing, zinc-nickel plating, and galvannealed hot-dip galvanizing. The aluminum-based plating layer is a plating layer having an average Al content of 20% by mass or more, or 30% by mass or more. For example, hot-stamped aluminum plating. A coating film may be provided on the surface of the spot-welded member 1. The coating film may be, for example, an electrodeposition coating film provided on the surface of the spot-welded member 1 after spot welding.

[0083] (Thickness of Metal Plates) There are no particular limitations on the thickness of the metal plates of the spot-welded member 1. For example, the thickness of the first metal plate 111 is preferably 0.50 to 1.1 mm. The thickness of the second metal plate 112 is preferably 0.9 to 2.6 mm. The thickness of the inner metal plate 113 is preferably 0.9 to 2.6 mm.

[0084] (Nugget Diameter) The sizes of the first nugget 121 and the second nugget 122 are not particularly specified. As long as the above requirements are met, an appropriate nugget diameter can be adopted depending on the material, thickness, number, etc. of the metal plates. For example, if the thickness of the first metal plate 111 is 0.5 mm to 1.1 mm and the total thickness of the metal plates in the spot-welded component 1 is 3.5 mm to 9.0 mm, it is preferable that the diameter of the first nugget 121 be 3√(t1) to 9√(t1) and the diameter of the second nugget 122 be 3√(t2) to 9√(t2). Note that t1 is the thickness of the first metal plate 111, and t2 is the thickness of the second metal plate 112.

[0085] Here, the diameter of the first nugget 121 refers to the width of the nugget measured on a virtual line along the joint interface 14 between the first metal plate 111 and the adjacent internal metal plate 113. The diameter of the second nugget 122 refers to the width of the nugget measured on a virtual line along the joint interface 14 between the second metal plate 112 and the adjacent internal metal plate 113. The width of the molten solidified portion is measured on a plane that includes the center of the first dent and the center of the second dent and is perpendicular to the first metal plate 111. The nugget diameter can be controlled by appropriately changing the current flow conditions depending on the material, thickness, number, etc. of the metal plates.

[0086] (Pressure-welded portion) In the welded portion where the second nugget 122 is provided, the first metal plate 111 and the first inner metal plate group 1131 are not joined by the second nugget 122. However, in the welded portion where the second nugget 122 is provided, it is preferable that a pressure-welded portion is provided between the first metal plate 111 and the first inner metal plate group 1131. The pressure-welded portion is a portion where the base material is solid-state welded, but not melted or solidified. By pressure-welding the first metal plate 111 in the second spot welding S2, the joining strength of the spot-welded member 1 is further increased.

[0087] When a pressure-welded portion is formed between the first metal plate 111 and the first inner metal plate group 1131, the presence of the pressure-welded portion can be confirmed by a chisel test. In the chisel test, a chisel is driven into the interface between the first metal plate 111 and the first inner metal plate group 1131 at the spot-welded portion to fracture the joint. When a plug fracture occurs on the first metal plate 111 side, the joint is determined to fall into one of the following situations: (1) A first nugget 121 joining the first metal plate 111 and the first inner metal plate group 1131 was formed in the joint subjected to the chisel test. (2) A second nugget 122 and a pressure-welded portion joining the first metal plate 111 and the first inner metal plate group 1131 were formed in the joint subjected to the chisel test. Then, the nugget formed in the fractured portion can be observed by cutting the fractured portion and etching the cross section. If the nugget is separated from the first metal plate 111, it is determined that the nugget is the second nugget 122 and that a pressure-welded portion was provided between the first metal plate 111 and the first internal metal plate group 1131.

[0088] (Welding Conditions) The conditions for the first spot welding S1 and the second spot welding S2 are not particularly specified. As long as the above-mentioned requirements are met, appropriate welding conditions can be adopted depending on the material, thickness, number, etc. of the metal plates. Examples of suitable welding conditions are listed below.

[0089] In each of the first spot weld S1 and the second spot weld S2, the current may be applied once or twice or more. When the current is applied multiple times, the current values ​​for the first and second currents may be the same or different. When the plate thickness exceeds 5 mm, pulsation welding may be performed, in which current is applied multiple times at the same current value. Furthermore, an upslope may be applied at the start of current application, and a downslope may be applied at the end of current application. The current waveforms for these upslope and downslope may be continuous or pulsed. The upslope reduces the risk of expulsion and hydrogen embrittlement cracking when welding ultra-high strength steel sheets when there is a large gap between the metal sheets or when welding galvanized hot-stamped steel sheets with high surface resistance. The downslope reduces the risk of hydrogen embrittlement cracking and LME cracking due to galvanization when welding ultra-high strength steel sheets.

[0090] The current value may also be changed during current application. For example, a high current may be applied in the early stages of current application and then reduced in the later stages. This promotes nugget formation. Alternatively, a low current may be applied in the early stages of current application and then increased in the later stages. This reduces the occurrence of expulsion, especially when the gap between the metal sheets is large.

[0091] The conditions of the number of times of current application, slope control, and current value during welding may be freely combined depending on the plate combination. If necessary, the pressure during welding may be changed.

[0092] The welding power source may be a DC inverter, an AC inverter, or a single-phase AC. The electrode shape may be DR, R, CR, or CF type. The electrode material may be chromium copper, chromium-zirconium copper, or alumina-dispersed copper. When welding galvanized steel sheets, if LME cracks occur at the indentation, alumina-dispersed copper is preferable.

[0093] In a more preferred example of welding conditions, a DR-type electrode having an electrode tip of 5 mm to 8 mm is used, the pressure is 250 kgf to 700 kgf (2.451 kN to 6.864 kN), the current time is 12 cycles to 60 cycles (240 msec to 1200 msec), the current value is 6 kA to 13 kA, and the holding time is 1 cycle to 60 cycles (20 msec to 1200 msec).

[0094] (Applications) The application of the spot-welded member 1 and the manufacturing method thereof according to this embodiment is not particularly limited. For example, it is preferable to incorporate the manufacturing method of the spot-welded member 1 into a manufacturing process of an automobile part. For example, the first spot weld S1 can be applied to a side frame assembly process, and the second spot weld S2 can be applied to a body assembly process.

[0095] In this case, a side frame is manufactured by first spot welding S1 to an outer plate (side panel) and a B-pillar outer (reinforcement material). This side frame has a configuration similar to the joined body 13 having two metal plates shown in FIG. 1. The outer side panel corresponds to the first metal plate 111 of the joined body 13, and the B-pillar outer corresponds to the first internal metal plate group 1131 of the joined body 13. Next, a body is manufactured by second spot welding S2 to the side frame (joint body 13), the side sill outer, and the side sill inner. The body has a configuration similar to the spot-welded member 1 shown in FIG. 5A. The side sill outer corresponds to the second internal metal plate group 1132, and the side sill inner corresponds to the second metal plate 112.

[0096] Alternatively, a side frame may be manufactured by performing first spot welding S1 on the outer plate, the B-pillar outer, and the side sill outer. This side frame has a configuration similar to the joined body 13 having three metal plates shown in FIG. 3. The outer plate corresponds to the first metal plate 111 of the joined body 13, and the B-pillar outer and the side sill outer correspond to a first internal metal plate group 1131 of the joined body 13. The internal metal plate 113 adjacent to the first metal plate 111 is the B-pillar outer. Next, a body is manufactured by performing second spot welding S2 on the side frame (joint body 13) and the side sill inner. The body has a configuration similar to the spot-welded member 1 shown in FIG. 4. The side sill inner corresponds to the second metal plate 112.

[0097] In either example, if the outer plate is joined to the B-pillar outer by the first spot welding S1, there is no need to form a nugget at the interface between the outer plate and the B-pillar outer by the second spot welding S2. The second spot welding S2 is performed for the purpose of joining the reinforcements together.

[0098] (Evaluation Method) In the spot-welded component 1, whether the first indentation 1211 is separated from the second indentation 1221 can usually be easily determined by visually observing the surface of the first metal plate 111. If the outer edges of the first indentation 1211 and the second indentation 1221 are unclear, the cross sections of the first nugget 121 and the second nugget 122 are observed. By cutting the spot-welded component 1 along a plane that passes through the center of the first indentation 1211 and the center of the second indentation 1221 and is perpendicular to the first metal plate 111, and appropriately adjusting the cut surface, the cross sections of the first nugget 121 and the second nugget 122 become visible. As shown in FIG. 2 and other figures, if the first nugget 121 is separated from the second nugget 122 in the cross section, the first indentation 1211 is considered to be separated from the second indentation 1221. In this case, it is also considered that the welding point WP of the second spot weld S2 was separated from the welding point WP of the first spot weld S1 during the production of the spot-welded component 1.

[0099] In the spot-welded component 1, whether the first nugget 121 is separated from the second metal plate 112 is determined by observing a cross section of the first nugget 121. The spot-welded component 1 is cut along a plane that passes through the center of the first indentation 1211 and is perpendicular to the first metal plate 111, and the cut surface is appropriately adjusted, so that the cross section of the first nugget 121 can be visually observed. If the second metal plate 112 is not melted and not incorporated into the first nugget 121, it is determined that the first nugget 121 is separated from the second metal plate 112.

[0100] In the spot-welded component 1, whether the second nugget 122 is separated from the first metal sheet 111 is determined by observing a cross section of the second nugget 122. The cross section of the second nugget 122 can be visually observed by cutting the spot-welded component 1 along a plane that passes through the center of the second indentation 1221 and is perpendicular to the first metal sheet 111, and then appropriately adjusting the cut surface. If the first metal sheet 111 is not melted and not incorporated into the second nugget 122, it is determined that the second nugget 122 is separated from the first metal sheet 111. In this case, it is determined that the second spot welding S2 was completed in the manufacture of the spot-welded component 1 before the second nugget 122 grew to the first metal sheet 111.

[0101] The distance D between the center of the first indentation 1211 and the center of the second indentation 1221 can be easily measured by visually observing and identifying the centers of the first metal plate 111. Furthermore, the distance D between the center of the first indentation 1211 and the center of the second indentation 1221 is considered to be the same as the distance D between the center of the weld point WP in the first spot weld S1 and the center of the weld point WP in the second spot weld S2.

[0102] The thicknesses of the first metal plate 111, the second metal plate 112, and the internal metal plate 113 are measured at locations sufficiently distant from the first nugget 121 and the second nugget 122. Specifically, the locations where no deformation due to the first spot weld S1 and the second spot weld S2 occurs are used as the locations where the thicknesses of the metal plates are measured.

[0103] The Vickers hardness of the first metal plate 111, the second metal plate 112, and the inner metal plate 113 is measured in accordance with JIS Z 2244:2009 "Vickers hardness test - Test method." The hardness measurement is performed at a location where no deformation occurs due to the first spot weld S1 and the second spot weld S2. The hardness measurement is also performed on a cross section perpendicular to the surface of the metal plate. The indentation is formed at the t / 4 position. The t / 4 position refers to a position where the depth from the surface of the metal plate is t / 4, where t refers to the thickness of the metal plate. The test force is 0.5 kgf. Three tests are performed. The measurement points are spaced 0.5 mm apart. The arithmetic mean value of the values ​​obtained in each test is considered to be the hardness of the metal plate.

[0104] The tensile strength of the first metal plate 111, the second metal plate 112, and the inner metal plate 113 is measured in accordance with JIS Z 2241:2011 "Method of tensile testing for metallic materials." Alternatively, the tensile strength may be calculated from a Vickers hardness test. In this case, SAE J 417 is used for conversion.

[0105] The effects of one embodiment of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0106] (Example 1) Spot-welded components were produced by spot welding various sheet combinations made of materials 1 to 4 shown in Table 1. The columns "Material 1" to "Material 4" in Table 1 list the plating type, the tensile strength (unit: MPa) of the steel sheet, and the thickness t (unit: mm), in that order.・GA270: GA plated steel sheet, tensile strength 270 MPa class (110 HV) ・GA980: GA plated steel sheet, tensile strength 980 MPa class (300 HV) ・GA1180: GA plated steel sheet, tensile strength 1180 MPa class (390 HV) ・GA1470: GA plated steel sheet, tensile strength 1470 MPa class (460 HV) ・CR1470: Unplated steel sheet, tensile strength 1470 MPa class (460 HV) ・Al1800: Al plated hot stamped steel sheet, tensile strength 1800 MPa class (550 HV) Note that GA plating refers to alloyed hot-dip galvanizing.

[0107] The plate assembly was stacked in the order of material 1, material 2, material 3, and material 4. Material 1 corresponds to the first metal plate, material 4 corresponds to the second metal plate, and materials 2 and 3 correspond to the internal metal plates. The "total thickness" listed in Table 1 is the total value (unit: mm) of the thicknesses of materials 1 to 4. The "plate thickness ratio" listed in Table 1 is the value obtained by dividing the total thickness of the plate assembly by the thickness of material 1.

[0108]

[0109] The plate assemblies shown in Table 1 were spot welded under the conditions shown in Table 2. For examples in which four steel plates were joined by two spot welding passes, the numbers of the metal plates used in the first spot welding, the plate thickness ratio of the joined body, the welding conditions for the first spot welding, the welding conditions for the second spot welding, and the weld point center-to-center spacing are listed. The "plate thickness ratio of the joined body" is the value obtained by dividing the total thickness of the metal plates used in the first spot welding by the thickness of Material 1. The "weld point center-to-center spacing" is the distance between the centers of the weld points of the first spot welding and the second spot welding. For examples in which four steel plates were joined by a single spot welding pass, only the welding conditions for that spot welding are listed. The welding condition column lists the applied pressure, welding current, and welding time. One cycle was 1 / 50 seconds.

[0110]

[0111] The spot-welded components obtained by spot welding were subjected to a chisel test as specified in JIS Z 3144:2013, "Field Test Methods for Spot and Projection Welds." Specifically, a chisel was pressed between Material 1 and Material 2 near the first nugget, applying a tensile force in the thickness direction of the weld. This test determined whether the bond strength between Material 1 and Material 2 was satisfactory. The first nugget was fractured between Material 1 and Material 2 of each test specimen by driving the chisel. The nugget diameter was then evaluated using a three-level scale in the "Chisel Test Results of the Bonding Interface Between Material 1 and Material 2" column in Table 3, in accordance with the method described in JIS Z 3139 6.1 (Cross-Section Macro Test). The evaluation criteria were as follows: Examples rated "C" were determined to have poor bonding in Material 1. The following criteria were met: Nugget diameter of 4.25√t or more (t is the thickness of material 1): A Nugget diameter of 3√t or more but less than 4.25√t: B Nugget diameter of less than 3√t: C The same chisel test was also performed on the second nugget as on the first nugget. Then, the plug diameters of the first and second nuggets were measured in accordance with JIS Z 3144:2013, "Field Test Method for Spot and Projection Welds." The plug diameter refers to the average diameter of the plug-shaped fracture portion measured after a fracture test. A plug fracture refers to a button-shaped fracture in the nugget, the heat-affected zone, or the base metal outside the nugget. As a result, it was confirmed that the plug diameter of the first nugget was 4.25√t or more (t is the thickness of material 1) in all of the present invention examples. Furthermore, it was confirmed that the plug diameter of the second nugget was also 4.25√t or more (t is the thickness of material 4) in all of the present invention examples.

[0112] Thereafter, whether or not the second nugget of the spot-welded component had grown to material 1 was confirmed by cross-sectional observation. The observation was performed on a cut surface that passed through the center of the second indentation and was perpendicular to material 1. The observation results were recorded in the "Material 1 interface nugget formation in second spot welding" column. If a second nugget had been formed at the interface between material 1 and material 2, "Yes" was recorded in the "Material 1 interface nugget formation in second spot welding" column, and otherwise "No" was recorded.

[0113] Furthermore, the amount of surface expulsion in Material 4 was evaluated and listed in Table 3. The occurrence of surface expulsion in the second spot welding was evaluated by cross-sectional observation. When traces of the weld metal of the nugget protruding toward the metal surface from near the indentation of the second spot weld were confirmed, "NG" was entered in the "Surface Explosion" column. When minute molten spatter was confirmed only on the surface, "NORMAL" was entered in the "Surface Explosion" column. When the result was not judged to be either NG or NORMAL, "GOOD" was entered in the "Surface Explosion" column.

[0114]

[0115] In Examples 1 and 8, Materials 1 to 4 were joined by a single spot welding. In these examples, a joining failure occurred in Material 1.

[0116] In Examples 2 and 12, the welding point of the second spot weld was not spaced apart from the welding point of the first spot weld. As a result, the second spot welds in Examples 2 and 12 suffered from significant surface expulsion.

[0117] In Example 7, in the second spot welding, the second nugget was grown to Material 1. As a result, in the second spot welding of Example 7, significant surface expulsion occurred.

[0118] In other examples, materials 1 to 4 were joined by first spot welding and second spot welding, the welding point of the second spot welding was separated from the welding point of the first spot welding, and the second spot welding was terminated before the second nugget grew to the first metal sheet. In these examples, there was no joint failure in material 1, and surface expulsion was sufficiently suppressed, resulting in spot-welded components with high strength reliability.

[0119] FIG. 7A is a cross-sectional photograph of a spot-welded component of Example 3. The metal plates in FIG. 7A are, from top to bottom, GA270-0.65t (Material 1), GA980-1.6t (Material 2), CR1470-1.6t (Material 3), and CR1470-1.6t (Material 4). The nugget on the right side of FIG. 7A is the first nugget, and the nugget on the left side of FIG. 7A is the second nugget. The center-to-center spacing of the weld points in the spot-welded component of FIG. 7A is 12 mm. In addition, in the production of the spot-welded component of FIG. 7A, the first spot welds were made on Materials 1 and 2.

[0120] 7A shows that the thin metal plate arranged at the top of the page is securely joined to the adjacent metal plate, and also shows that no surface expulsion occurred during the second spot welding.

[0121] FIG. 7B is a cross-sectional photograph of the spot-welded component of Example 4. The metal plates in FIG. 7B are, from top to bottom, GA270-0.65t (Material 1), GA980-1.6t (Material 2), CR1470-1.6t (Material 3), and CR1470-1.6t (Material 4). The nugget on the right side of FIG. 7B is the first nugget, and the nugget on the left side of FIG. 7B is the second nugget. The center-to-center spacing of the weld points in the spot-welded component of FIG. 7B was 12 mm. In addition, in the production of the spot-welded component of FIG. 7B, the first spot welds were made on Materials 1, 2, and 3.

[0122] 7B shows that the thin metal plate arranged at the top of the page is securely joined to the adjacent metal plate, and also shows that no surface expulsion occurred during the second spot welding.

[0123] (Example 2: Sheet Thickness Ratio for Second Nugget) Spot-welded components were produced by spot welding various sheet combinations made of Materials 1 to 4 shown in Table 4. The columns of "Material 1" to "Material 4" in Table 1 list the plating type and the tensile strength (unit: MPa) of the steel sheet, as well as the thickness t (unit: mm), in that order.

[0124] The sheet assembly was stacked in the order of material 1, material 2, material 3, and material 4. Material 1 corresponds to the first metal sheet, material 4 corresponds to the second metal sheet, and materials 2 and 3 correspond to the inner metal sheets. Table 4 lists the materials targeted by the first spot welding. In addition, materials 2, 3, and 4 were joined in the second spot welding. The "sheet thickness ratio for the second nugget" in Table 4 is the value obtained by dividing the total thickness of materials 2 to 4 joined by the second nugget by the thickness of the second metal sheet (i.e., material 4). For ease of explanation, the joint shape is listed in Table 4.

[0125]

[0126]

[0127] A chisel test was performed on the spot-welded components obtained by spot welding. Specifically, a chisel was pressed into (1) between material 1 and material 2 and (2) between material 3 and material 4 near the first nugget to apply a tensile force in the thickness direction of the weld. This test determined whether the bond strength between material 1 and material 2 and the bond strength between material 3 and material 4 were satisfactory. A first nugget was fractured between material 1 and material 2 of each test specimen by driving the chisel. Furthermore, a second nugget was fractured between material 3 and material 4 of each test specimen by driving the chisel. The nugget diameters were evaluated on a three-point scale in the "Chisel test results of the bond interface between material 1 and material 2" column in Table 6, based on the method described in JIS Z 3139 6.1 (cross-sectional macroscopic test). The evaluation criteria were as follows. Examples rated "C" were determined to have poor bonding in material 1.・Nugget diameter is 4.25√t (t is the thickness of material 4) or more: A ・Nugget diameter is 3√t or more and less than 4.25√t: B ・Nugget diameter is less than 3√t: C

[0128] Furthermore, the amount of surface expulsion in Material 4 was evaluated and listed in Table 6. The occurrence of surface expulsion in the second spot welding was evaluated by cross-sectional observation. When traces of the weld metal of the nugget protruding toward the metal surface from near the indentation of the second spot weld were confirmed, "NG" was entered in the "Surface Explosion" column. When minute molten spatter was confirmed only on the surface, "NORMAL" was entered in the "Surface Explosion" column. When the result was not judged to be either NG or NORMAL, "GOOD" was entered in the "Surface Explosion" column.

[0129]

[0130] In the spot-welded components of Examples A1 to A11, the number of metal plates joined by the second nugget was three or more, and the value obtained by dividing the total thickness of the metal plates joined by the second nugget by the thickness of the second metal plate was 1.8 to 4.2. In these examples, both the first nugget joint strength and the second nugget joint strength were excellent, and the occurrence of expulsion was suppressed. In the spot-welded component of Example A12, the number of metal plates joined by the second nugget was three or more, and the value obtained by dividing the total thickness of the metal plates joined by the second nugget by the thickness of the second metal plate was less than 1.8. In Example A12, surface expulsion, in which the surface was slightly melted, occurred. However, because surface expulsion affecting strength did not occur in Example A12, Example A12 is also considered an inventive example.

[0131] (Example 3: Spot-welded component having five metal plates) The following five metal plates were spot-welded. Material 1 was used as the first metal plate, and Material 5 was used as the second metal plate. Material 1: GA270-0.65t Material 2: CR980-1.2t Material 3: CR980-1.2t Material 4: Al1800-1.6t Material 5: Al1800-1.6t First, a first spot welding was performed on Materials 1, 2, and 3 to form a joint having a first nugget. Next, Materials 4 and 5 were overlapped on Material 3 of the joint, and a second spot welding was performed to form a second nugget joining Materials 2, 3, 4, and 5. The weld point of the second spot welding was spaced from the weld point of the first spot welding. The second spot welding was terminated before the second nugget grew to the first metal plate.

[0132] A cross-sectional photograph of the spot-welded component obtained by the above-described procedure is shown in Figure 8. In the spot-welded component of Figure 8, poor joining was avoided. In addition, in the spot-welded component of Figure 8, the occurrence of expulsion was also avoided.

[0133] S1 First spot weld S2 Second spot weld 1 Spot welded member 111 First metal plate 112 Second metal plate 113 Internal metal plate 1131 First internal metal plate group 1132 Second internal metal plate group 121 First nugget 1211 First indentation 122 Second nugget 1221 Second indentation 13 Joint 14 Joint interface D Distance between center of first nugget and center of second nugget E Electrode WP Welding point

Claims

1. A spot-welded component comprising: a first metal plate and a second metal plate arranged on a surface of the spot-welded component; a first internal metal plate group consisting of one or more internal metal plates arranged between the first metal plate and the second metal plate; a first spot weld having a first nugget joining the first metal plate and the first internal metal plate group; and a second spot weld having a second nugget joining the second metal plate and the first internal metal plate group, wherein an indentation of the first spot weld is provided on the first metal plate and the internal metal plate, an indentation of the second spot weld is provided on the first metal plate and the second metal plate, the indentation of the first spot weld is separated from the indentation of the second spot weld, the first nugget is separated from the second metal plate, and the second nugget is separated from the first metal plate.

2. A spot-welded component as described in claim 1, wherein the number of the inner metal plates constituting the first inner metal plate group to which the first nugget is joined is two or more.

3. A spot welded component as described in claim 1 or 2, further comprising a second internal metal plate group consisting of one or more internal metal plates arranged between the first internal metal plate group and the second metal plate, the first nugget being spaced apart from the second internal metal plate group, and the second nugget joining the second metal plate, the first internal metal plate group, and the second internal metal plate group.

4. A spot-welded component as described in claim 1 or 2, wherein the distance between the center of the indentation of the first spot welded portion and the center of the indentation of the second spot welded portion is 10 mm or more and 50 mm or less in a plan view.

5. A spot-welded component according to claim 1 or 2, wherein the second metal plate is thicker than the first metal plate.

6. The spot-welded component according to claim 5, wherein a value obtained by dividing the total thickness of the first metal plate and the first internal metal plate group joined by the first nugget by the thickness of the first metal plate is less than 6.0, and a value obtained by dividing the total thickness of the first metal plate, the second metal plate, and the internal metal plate of the spot-welded component by the thickness of the first metal plate is more than 6.

5.

7. The spot-welded component according to claim 1 or 2, wherein the first metal plate, the second metal plate, and the internal metal plate are steel plates, the first metal plate has a Vickers hardness of 150 HV or less, and at least one of the second metal plate and the internal metal plate has a hardness of 300 HV or more.

8. A spot-welded component according to claim 1 or 2, wherein an adhesive is provided at one or more of the joining interfaces.

9. The spot welded component according to claim 1, wherein the number of the metal plates to which the second nugget is joined is three or more, and a value obtained by dividing the total thickness of the metal plates to which the second nugget is joined by the thickness of the second metal plates is 1.8 to 4.

2.

10. A method for manufacturing a spot-welded member, comprising: a step of performing a first spot welding on a first internal metal plate group consisting of a first metal plate and one or more internal metal plates stacked together to form a joint having a first nugget joining the first metal plate and the first internal metal plate group; a step of superimposing a second metal plate on the internal metal plate arranged on the surface of the joint, and performing a second spot welding to form a second nugget joining the first internal metal plate group and the second metal plate of the joint; wherein a welding point of the second spot welding is separated from a welding point of the first spot welding; and the second spot welding is terminated before the second nugget grows to the first metal plate.

11. A method for manufacturing a spot-welded component as set forth in claim 10, wherein the number of the internal metal plates constituting the first internal metal plate group subjected to the first spot welding is two or more.

12. A method for manufacturing a spot-welded member as described in claim 10 or 11, in which a second internal metal plate group consisting of one or more internal metal plates and the second metal plate are stacked in order on the internal metal plate arranged on the surface of the joint body, and the second spot welding is performed.

13. A method for manufacturing a spot-welded component as described in claim 10 or 11, wherein the distance between the center of the weld point in the first spot weld and the center of the weld point in the second spot weld is 10 mm or more and 50 mm or less in a plan view.

14. The method for manufacturing a spot-welded component according to claim 10 or 11, wherein the second metal plate is thicker than the first metal plate.

15. A method for manufacturing a spot-welded component as described in claim 14, wherein a value obtained by dividing the total thickness of the first metal plate and the first group of internal metal plates of the joined body by the thickness of the first metal plate is less than 6.0, and a value obtained by dividing the total thickness of the first metal plate, the second metal plate, and the internal metal plate of the spot-welded component by the thickness of the first metal plate is more than 6.

5.

16. A method for manufacturing a spot-welded component as described in claim 10 or 11, wherein the first metal plate, the second metal plate, and the internal metal plate are steel plates, the Vickers hardness of the first metal plate is 150 HV or less, and the hardness of at least one of the second metal plate and the internal metal plate is 300 HV or more.

17. A method for manufacturing a spot-welded component as described in claim 10 or 11, further comprising a step of applying an adhesive to one or more of a surface of the first metal plate and a surface of the internal metal plate prior to the first spot welding, and the surface to which the adhesive is applied is defined as a joining interface of the joined body.

18. A method for manufacturing a spot-welded component as described in claim 10 or 11, further comprising a step of applying an adhesive to one or more of the surface of the joint body and the surface of the metal plate to be overlaid thereon before the second spot welding, and the surface to which the adhesive is applied is made to be the joint interface of the spot-welded component.

19. A method for manufacturing a spot-welded component as set forth in claim 10 or 11, wherein the number of the metal plates to which the second nugget is joined is 3 or more, and a value obtained by dividing the total thickness of the metal plates to which the second nugget is joined by the thickness of the second metal plates is 1.8 to 4.2.

Images