Method for manufacturing spot welded joint, and spot welded joint

Abstract

To provide a manufacturing method of a spot welded joint which enables a wide proper current range ΔI to be secured when welding a plate set which has a high plate thickness ratio and the thinnest metal plate arranged on the outermost surface, and also to provide a spot welded joint which has the high cross tensile strength and is obtained by welding a plate set which has a high plate thickness ratio and the thinnest metal plate arranged on the outermost surface.SOLUTION: A spot welded joint according to an aspect of the present invention comprises: a plate set obtained by overlapping a plurality of metal plates each other; and a nugget joining the plate set. The plate thickness ratio of the plate set is equal to or greater than 4.0. The first metal plate is arranged on the outermost surface of one of the plate set. The outer edge of the nugget observed in a cross section for observation reaches the inside of the first metal plate, and has a shape that is recessed from the first metal plate to the center in the plate thickness direction of the nugget in a region facing the first metal plate.SELECTED DRAWING: Figure 3A

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a spot welded joint, and to a spot welded joint. [Background technology]

[0002] In recent years, particularly in the automotive field, spot welding has been mainly used when welding steel sheets together in processes such as assembling car bodies and installing parts, and various procedures and conditions have been proposed. Spot welding is a type of resistance welding in which overlapping base materials (multiple metal sheets) are clamped between the tip of an electrode (electrode tip) with an appropriately shaped tip, and current and pressure are concentrated in a relatively small area to locally heat it while simultaneously applying pressure with the electrode (JIS Z 3001-6).

[0003] When performing spot welding, it is necessary to consider the welding conditions according to the plate thickness ratio of a plate assembly formed by combining multiple metal plates. The plate thickness ratio is a value calculated by dividing the total plate thickness T of the plate assembly by the plate thickness t of the thinnest metal plate in the plate assembly (see Figure 1).

[0004] When the sheet thickness ratio of a sheet pair is high and the thinnest metal sheet is located on the surface of the sheet pair, poor welding is likely to occur. This is because the nugget (the molten and solidified part that occurs in the weld in lap resistance welding such as spot welding) that joins the sheet pair is generated starting from the point farthest from the water-cooled electrode, that is, the center of the sheet pair. If the current density during welding is too low, the melting that starts from the center of the sheet pair may not reach the thinnest metal sheet. In this case, a nugget that reaches the thinnest metal sheet is not formed, and the thinnest metal sheet is not joined to the other metal sheets (see Figure 2).

[0005] Increasing the welding current value is one way to avoid such poor welding. However, if the welding current value is too high, expulsion (a phenomenon in which the base material, such as a metal plate, is locally overheated and melts and splashes during lap resistance welding such as spot welding, or the metal itself) occurs, which leads to poor welding.

[0006] The difference between the welding current value that can ensure a nugget diameter that can join all of the multiple metal sheets (i.e., the lower limit of the welding current value) and the welding current value that can suppress the occurrence of expulsion (i.e., the upper limit of the welding current value) is hereinafter referred to as the appropriate current range ΔI. The larger the sheet thickness ratio in the sheet assembly, the smaller the appropriate current range ΔI. In other words, the larger the sheet thickness ratio, the less robust the welding conditions are.

[0007] On the other hand, in order to increase the degree of freedom in designing mechanical structural parts, spot welding that allows a high plate thickness ratio is essential. For example, in automobile parts that use thick high-strength steel plates as frame parts and thin low-strength steel plates as exterior parts, the higher the plate thickness ratio, the thinner the low-strength steel plates are, and the lighter the parts are, resulting in improved fuel efficiency.

[0008] As a prior art related to spot welding, for example, Patent Document 1 discloses a spot welding method in which, when spot welding is performed on a stack formed by stacking multiple workpieces, the objective is to sufficiently grow a nugget on the contact surface between the workpieces, the method comprising the steps of clamping the stack between a first welding tip and a second welding tip, abutting a pressure member against the outermost workpiece located at the outermost part of the stack and against which the first welding tip is abutting, and applying pressure to the stack from the outermost workpiece side with the pressure member, and maintaining the pressure by the pressure member and passing current between the first welding tip and the second welding tip.

[0009] Patent Document 2 discloses a resistance spot welding method having an object of suppressing the influence of disturbances such as plate gaps and stably welding three or more metal plates, the method including a machining condition setting step of setting machining conditions including a pressing force and a current value for forming a molten portion at multiple interfaces of three or more metal plates, for each of the multiple interfaces; a composite condition setting step of setting composite conditions composed of multiple stages of machining conditions by combining the machining conditions set in the machining condition setting step in order from a machining condition with a high pressing force by a pair of electrodes or a machining condition with a low current value; and a welding step of welding three or more metal plates based on the composite condition set in the composite condition setting step.

[0010] Patent Document 3 addresses the issue of obtaining an appropriate welded joint in spot welding of multiple stacked steel plates, where there is a gap of up to about 2 mm between the steel plates and the plate thickness ratio (sum of the thicknesses of the stacked steel plates / thickness of the thinnest steel plate) is large. It has a preliminary energization process of gradually applying current, a first energization process of performing constant energization at current value I1, then a second energization process of performing energization at current value I2, and further a third energization process of performing energization at current value I3. The relationship is I1 > I2 and I2 < I3. During the third energization process, it discloses a spot welding method of reducing the pressing force by the spot welding electrode.

[0011] Patent Document 4 aims to provide a resistance spot welding method capable of forming nuggets of the required size without scatter in a plate assembly with a large plate thickness ratio. In resistance spot welding of a plate assembly with two or more thick metal plates stacked and a thin metal plate stacked on one of them, sandwiched between a pair of electrode tips, when performing resistance spot welding in two stages of a first stage and a second stage, one of the pair of electrode tips, the electrode tip in contact with the thin plate, has a curved surface with a tip having a radius of curvature R1 of preferably 60 mm or more, and the other electrode tip in contact with the thick plate has a curved surface with a tip having a radius of curvature larger than the radius of curvature R1 of the tip of the electrode tip in contact with the thin plate or the plane. It discloses a resistance spot welding method in which the welding in the second stage is a welding with a higher pressing force than the welding in the first stage.

[0012] However, the technologies disclosed in Patent Documents 1 to 4 are not preferable in that the welding conditions are complex or special electrode tips and welding apparatuses are required.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

[0014] The present invention provides a method for manufacturing a spot welded joint that can ensure a wide appropriate current range ΔI when welding a sheet pair having a high sheet thickness ratio and the thinnest metal sheet arranged on the outermost surface. Furthermore, the present invention provides a spot welded joint having high cross tensile strength obtained by welding a sheet pair having a high sheet thickness ratio and the thinnest metal sheet arranged on the outermost surface. [Means for solving the problem]

[0015] The gist of the present invention is as follows. (1) A spot welded joint according to one embodiment of the present invention comprises a plate assembly in which a plurality of metal plates are overlapped, and a nugget that joins the plate assembly. When the thinnest metal plate in the plate assembly is defined as a first metal plate, the first metal plate is arranged on one outermost surface of the plate assembly, and a plate thickness ratio of the plate assembly, calculated by dividing the total plate thickness of the plate assembly by the plate thickness of the first metal plate, is 4.0 or more. An outer edge of the nugget, as observed in an observation cross section that is perpendicular to the surface of the plate assembly and passes through the central axis of the nugget in a planar view, reaches the inside of the first metal plate. The outer edge of the nugget, as observed in the observation cross section, has a shape that is recessed from the first metal plate toward the center of the nugget in the plate thickness direction in a region facing the first metal plate. (2) In the spot welded joint described in (1) above, the number of the metal plates may be three or more, and the nugget diameter at the contact surface between the first metal plate and an adjacent metal plate, measured at the observation cross-section, may be larger than the nugget diameter at the contact surfaces between the other metal plates. (3) In the spot welding joint described in the above (1) or (2), the number of the metal plates is three or more. The metal plate disposed on the outermost surface of the other side of the plate stack is defined as the second metal plate, and the metal plate adjacent to the second metal plate is defined as the third metal plate. When the thickness of the thinner one of the second metal plate and the third metal plate is defined as t´, the nugget diameter at the contact surface between the second metal plate and the third metal plate measured in the observation cross section may be 3√t´ or more. (4) The method for manufacturing a spot welding joint according to another aspect of the present invention includes a step of performing spot welding by sandwiching a stack of a plurality of metal plates between a first electrode tip and a second electrode tip facing each other. The plate thickness ratio of the plate stack, which is calculated by dividing the total plate thickness of the plate stack by the plate thickness of the thinnest metal plate in the plate stack, is set to 4.0 or more. The thinnest metal plate is disposed on the outermost surface of the plate stack, and the second electrode tip is disposed on the side where the thinnest metal plate is disposed. When applying a pressing force by pressing the first electrode tip and the second electrode tip against the plate stack, the ratio S2 / S1 of the contact area S2 between the thinnest metal plate and the second electrode tip to the contact area S1 between the first electrode tip and the metal plate is set to 0.10 ≦ S2 / S1 ≦ 0.90. (5) In the method for manufacturing a spot welding joint described in the above (4), when applying a pressing force by pressing the first electrode tip and the second electrode tip against the plate stack, S2 / S1 may be set to 0.10 ≦ S2 / S1 ≦ 0.90 by bending the plate stack toward the second electrode tip. (6) In the method for manufacturing a spot welding joint described in the above (5), the number of the metal plates is three or more, the plate thickness ratio of the plate stack is set to 5.0 or more, and when applying a pressing force by pressing the first electrode tip and the second electrode tip against the plate stack, the contact area S4 between the thinnest metal plate and the metal plate in contact therewith and the contact area S3 between the metal plate in contact with the first electrode tip and the metal plate in contact therewith may satisfy the relationship S4 < S3. (7) In the method for manufacturing a spot welded joint described in (5) or (6) above, before the plate group formed by stacking the multiple metal plates is clamped using the opposing first electrode tip and second electrode tip, the method may further include the steps of contacting the first electrode tip with one surface of the plate group, positioning the second electrode tip away from the other surface of the plate group, and pressing the first electrode tip against the plate group to apply a pressure, thereby bringing the second electrode tip into contact with the other surface of the plate group. (8) In the method for manufacturing a spot welded joint described above in (7), the electrode on which the second electrode tip is provided may be a fixed electrode, and the electrode on which the first electrode tip is provided may be a movable electrode. (9) In the method for manufacturing a spot welded joint described in (7) or (8) above, a plate assembly fixing means may be used to fix the periphery of the weld point of the plate assembly. (10) In the manufacturing method of a spot welded joint described in (9) above, the plate assembly fixing means may be two plate assembly fixing means, and the welding point may be arranged on a straight line formed by two points fixed by the two plate assembly fixing means. (11) In the manufacturing method of a spot welded joint described in (9) above, the plate assembly fixing means may be three or more plate assembly fixing means, and the three or more plate assembly fixing means may be arranged so that the points at which the plate assembly is fixed using the plate assembly fixing means form a regular polygon, and the welding point may be arranged inside the regular polygon. (12) In the manufacturing method of a spot welded joint described in (9) above, the plate assembly fixing means may be arranged so that the location where the plate assembly is fixed using the plate assembly fixing means forms a circle or an ellipse, and the welding point may be arranged inside the circle or ellipse. (13) In the method for manufacturing a spot welded joint according to any one of (5) to (11) above, the first electrode tip and the second electrode tip may have substantially the same tip shape. (14) In the method for manufacturing a spot welded joint described in any one of (7) to (13) above, a clearance, which is the size of a gap between the tip of the second electrode tip and the plate assembly at the stage of contacting the first electrode tip with the plate assembly and starting pressure application, may be 0.5 to 2.5 mm. (15) In the method for manufacturing a spot welded joint according to any one of (5) to (13) above, the plate assembly may be bent toward the second electrode tip by an amount of 0.5 to 2.5 mm. Effect of the Invention

[0016] According to the present invention, it is possible to provide a manufacturing method for a spot welded joint that can ensure a wide appropriate current range ΔI when welding a sheet pair having a high sheet thickness ratio and the thinnest metal sheet arranged on the outermost surface. Furthermore, according to the present invention, it is possible to provide a spot welded joint having high cross tensile strength obtained by welding a sheet pair having a high sheet thickness ratio and the thinnest metal sheet arranged on the outermost surface. [Brief description of the drawings]

[0017] [Figure 1] FIG. 4 is a schematic cross-sectional view of a plate assembly for explaining a plate thickness ratio. [Diagram 2] FIG. 2 is a schematic cross-sectional view of a spot welded joint in which a joining defect has occurred. [Figure 3A] 4 is a cross-sectional photograph of an example of a spot-welded joint obtained by the manufacturing method for a spot-welded joint according to the present embodiment. [Figure 3B] 1 is a schematic cross-sectional view of an example of a spot-welded joint obtained by a method for manufacturing a spot-welded joint according to an embodiment of the present invention. FIG. [Figure 4] 1 is a schematic cross-sectional view illustrating an example of a state in which a plate assembly is sandwiched between a first electrode tip and a second electrode tip that face each other. FIG. [Diagram 5] 13 is a schematic cross-sectional view illustrating an example of a state in which the plate assembly is bent toward the second electrode chip. FIG. [Figure 6] FIG. 11 is a schematic cross-sectional view of a plate assembly, illustrating an example of the locations where S1 to S4 are measured. [Figure 7]FIG. 11 is a schematic cross-sectional view of a plate assembly, illustrating an example of the locations where S1 to S4 are measured. [Figure 8] 5A to 5C are schematic cross-sectional views of the plate assembly in each step of bending the plate assembly toward the second electrode chip. [Figure 9] FIG. 4 is a perspective view illustrating an example of the shape of a plate assembly fixing means. [Figure 10A] FIG. 13 is a cross-sectional schematic diagram of a nugget of a spot-welded joint of Example 3. [Figure 10B] FIG. 11 is a cross-sectional schematic diagram of a nugget of a spot-welded joint of Example 4. [Figure 10C] FIG. 11 is a cross-sectional schematic diagram of a nugget of a spot-welded joint of Example 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The inventors have repeatedly studied means for expanding the appropriate current range ΔI in the welding of a sheet assembly having a high sheet thickness ratio and the thinnest metal sheet arranged on the outermost surface, and have found that the shape of the nugget becomes asymmetric when the contact area between the electrode tip and the sheet assembly is made different between the first surface and the second surface of the sheet assembly.

[0019] The smaller the contact area between the electrode tip and the sheet assembly, the higher the current density and the greater the temperature rise of the metal sheets. Therefore, it was presumed that when spot welding is performed by sandwiching a sheet assembly using opposing first and second electrode tips, if the contact area of ​​the first electrode tip is made larger than the contact area of ​​the second electrode tip, the nugget will be formed biased toward the second electrode tip.

[0020] The inventors applied the method of forming a nugget biased toward one of the sheet surfaces, which was obtained based on this finding, to spot welding of sheet assemblies with a large sheet thickness ratio. By reducing the contact area between the thinnest metal sheet and the electrode tip and forming a nugget biased toward the thinnest metal sheet, it became possible to join the thinnest metal sheet to another metal sheet even with a low current value.

[0021] Furthermore, the inventors have found that the contact area of ​​the electrode tip can be suitably controlled by applying pressure to the plate assembly with a clearance between one electrode tip and the plate assembly (i.e., with one electrode tip separated from the plate assembly and the other electrode tip in contact with the plate assembly). This is presumably because applying pressure with a clearance causes the plate assembly to bend, which changes the contact area of ​​the electrode tip.

[0022] In normal spot welding, a pair of electrode tips are arranged above and below, the plate assembly is placed on the lower electrode tip, and then the upper electrode tip is moved downward to apply pressure to the plate assembly. Therefore, in normal spot welding, there is no clearance at the time of pressure application, and the plate assembly is not bent. In addition, there have been concerns that if spot welding is performed while the plate assembly is bent, distortion of the joint or LME (liquid metal embrittlement) may occur. On the other hand, no advantage has been recognized for spot welding while the plate assembly is bent. For the above reasons, those skilled in the art have considered the clearance between the electrode tip and the plate assembly to be a disturbance element that should be eliminated. However, according to the knowledge of the present inventors, the appropriate current range ΔI can be expanded by bending the plate assembly. However, it has also been found that excessive bending of the plate assembly makes it easier for splashing to occur, narrowing the appropriate current range ΔI.

[0023] A method for manufacturing a spot welded joint according to one aspect of the present invention, which is obtained based on the above findings, includes a step of sandwiching a sheet assembly in which a plurality of overlapping metal sheets are stacked with a first electrode tip and a second electrode tip facing each other and spot welding the sheet assembly, in which a sheet thickness ratio of the sheet assembly, calculated by dividing the total sheet thickness of the sheet assembly by the sheet thickness of the thinnest metal sheet in the sheet assembly, is set to 4.0 or more, the thinnest metal sheet is arranged on the outermost surface of the sheet assembly, the second electrode tip is arranged on the side where the thinnest metal sheet is arranged, and when the first electrode tip and the second electrode tip are pressed against the sheet assembly to apply a pressure, a ratio S2 / S1 of a contact area S2 between the thinnest metal sheet and the second electrode tip and a contact area S1 between the first electrode tip and the metal sheet is set to 0.10≦S2 / S1≦0.90. Hereinafter, the manufacturing method for a spot welded joint according to this embodiment will be described in detail.

[0024] The manufacturing method of the spot welded joint 1 according to the present embodiment includes a step of sandwiching a plate set 11 in which a plurality of metal plates 111 are overlapped with a first electrode tip A and a second electrode tip B facing each other to perform spot welding. Here, the material of the plurality of metal plates 111 is not particularly limited. For example, the metal plate 111 may be a steel plate. If at least one or more steel plates are high-strength steel plates having a tensile strength of 780 MPa or more or 980 MPa or more, it is preferable because the strength of the spot welded joint 1 can be dramatically increased. On the other hand, the metal plate 111 may be an aluminum plate or the like. The metal plate 111 may have a plating layer on its surface. The tensile strengths of the plurality of metal plates 111 may be different. The number of the plurality of metal plates 111 may be any value of 2 or more.

[0025] The thicknesses of the metal plates 111 are appropriately selected so that the plate thickness ratio of the plate set 11 obtained by stacking the metal plates 111 is 4.0 or more. The plate thickness ratio is defined as the ratio of the total plate thickness T of the plate set 11 (i.e., the sum of the plate thicknesses of the metal plates 111) to the thinnest metal plate 111 in the plate set 11. min This value is calculated by dividing the thickness of the thinnest metal plate 111 by the thickness t. min is disposed on the outermost surface of the plate assembly 11.

[0026] In spot welding such a sheet assembly 11, if the welding current value is too low, the melting starts from the center of the sheet assembly 11 and reaches the thinnest metal sheet 111. min In this case, as shown in FIG. min The nugget 12 extending to the thinnest metal plate 111 is not formed. minand the other metal sheets 111 are not joined. On the other hand, if the welding current value is too high, expulsion occurs, which leads to poor joining. The difference between the welding current value that can ensure a nugget diameter that can join all of the multiple metal sheets 111 (i.e., the lower limit of the welding current value) and the welding current value that can suppress the occurrence of expulsion (i.e., the upper limit of the welding current value) is hereinafter referred to as the appropriate current range ΔI. The larger the sheet thickness ratio in the sheet assembly, the smaller the appropriate current range ΔI. In other words, the larger the sheet thickness ratio, the less robust the welding conditions are.

[0027] On the other hand, in order to increase the degree of freedom in designing mechanical structural parts, spot welding that allows a high plate thickness ratio is essential. For example, in an automobile part in which a thick high-strength steel plate is used as a frame part and a thin low-strength steel plate is used as an exterior member, the higher the plate thickness ratio is, the more the thickness of the low-strength steel plate is reduced, and the weight of the part is reduced, resulting in effects such as improved fuel efficiency. For the above reasons, in the manufacturing method of the spot welded joint according to this embodiment, the plate thickness ratio of the plate assembly 11 is set to 4.0 or more. The plate thickness ratio may be 4.5 or more, 5.0 or more, 5.5 or more, or 6.0 or more. There is no particular upper limit to the plate thickness ratio, but the plate thickness ratio may be, for example, 8.0 or less, 7.5 or less, or 7.0 or less.

[0028] In order to reliably join a plate assembly having a high plate thickness ratio, in the method for manufacturing a spot welded joint according to this embodiment, the second electrode tip B is placed on the thinnest metal plate 111. min When the first electrode tip A and the second electrode tip B are pressed against the plate assembly 11 to apply pressure, the thinnest metal plate 111 min Various experiments have revealed that the ratio S2 / S1 of the contact area S2 between the first electrode tip A and the second electrode tip B to the contact area S1 between the first electrode tip A and the metal plate 111 should be set to 0.10≦S2 / S1≦0.90. Here, the "contact area" is a value measured using pressure-sensitive paper, which will be described later. The measurement points for S1 and S2 are the areas surrounded by dashed lines in FIG. 6.

[0029] During spot welding, current flows from one electrode tip to the other electrode tip via the sheet group 11. The narrower the path through which this current flows (current path), the higher the current density becomes, and the higher the maximum temperature of the metal sheet 111 during welding. Here, in normal spot welding, the contact area between one electrode and the sheet group surface is substantially equal to the contact area between the other electrode and the sheet group surface. Therefore, the sheet group 11 is heated almost uniformly.

[0030] On the other hand, in the manufacturing method of the spot welded joint according to the present embodiment, the thinnest metal plate 111 min The contact area S2 between the first electrode tip A and the second electrode tip B is set to 90% or less of the contact area S1 between the first electrode tip A and the metal plate 111. min The contact surface between the first electrode tip A and the second electrode tip B is heated more strongly than the contact surface between the first electrode tip A and the metal plate 111. As a result, the nugget 12 obtained by the manufacturing method for a spot welded joint according to this embodiment has a thickness of 1 mm, as shown in Figs. 3A and 3B. min By forming the nugget 12 under such conditions, the thinnest metal sheet 111 can be spot-welded with a relatively low current value. min It becomes possible to reliably join the metal plate 111 to the other metal plate 111. In other words, by satisfying the relational expression of S2 / S1≦0.90, it is possible to significantly expand the appropriate current range ΔI in spot welding of the sheet assembly 11 having a high sheet thickness ratio. It may be S2 / S1≦0.8, S2 / S1≦0.7, or S2 / S1≦0.6.

[0031] However, if S2 / S1 is too small, the thinnest metal plate 111 min and the second electrode tip B, scattering is likely to occur at the contact surface between the first electrode tip B and the second electrode tip B, and the appropriate current range ΔI becomes narrow. Therefore, S2 / S1 is set to 0.10≦S2 / S1. It may be 0.20≦S2 / S1, 0.30≦S2 / S1, or 0.40≦S2 / S1.

[0032] The means for controlling S2 / S1 within the above range is not particularly limited. For example, as shown in FIG. minBy making the size of the second electrode tip B that contacts the first electrode tip A smaller than the size of the first electrode tip A, it is possible to make S2 / S1 satisfy 0.10≦S2 / S1≦0.90. According to this measure, the method for manufacturing a spot welded joint according to this embodiment can be carried out without making major changes to existing spot welding equipment.

[0033] 5, when the first electrode tip A and the second electrode tip B are pressed against the plate set 11 to apply pressure, the plate set 11 may be bent toward the second electrode tip B (so as to be convex toward the second electrode tip B side). min and the second electrode tip B, and the contact area S2 between the first electrode tip A and the metal plate 111 can be decreased, and the contact area S1 between the first electrode tip A and the metal plate 111 can be increased. In this case, the radius of curvature of the plate set 11 is not particularly limited. A radius of curvature of the plate set 11 that satisfies 0.10≦S2 / S1≦0.90 may be appropriately selected according to the electrode tip shape, etc. By this means, the method for manufacturing a spot welded joint according to this embodiment can be performed while the tip shapes of the first electrode tip A and the second electrode tip B are substantially the same. This can simplify the welding work environment.

[0034] The amount by which the plate group 11 is bent toward the second electrode tip B (in other words, the movement distance of the first electrode tip A when the first electrode tip A is pressed against the plate group 11) is not particularly limited. However, for example, it is preferable to set the amount by which the plate group 11 is bent toward the second electrode tip B to 0.5 to 2.5 mm. By setting the amount by which the plate group 11 is bent toward the second electrode tip B to 0.5 mm or more, S2 / S1 can be set within a more preferable range. In addition, by setting the amount by which the plate group 11 is bent toward the second electrode tip B to 2.5 mm or less, the joining strength of the spot welded joint can be further increased. As will be described later, in order to further increase the joining strength of the spot welded joint, the thinnest metal plate 111 arranged on one outermost layer of the plate group 11 is preferably set to 0.5 mm to 2.5 mm. minIn addition to joining it to another metal plate 111, it is preferable to also increase the joining strength between the metal plate arranged on the outermost surface of the other side of the plate assembly and the other metal plate 111. By setting the amount of bending of the plate assembly 11 toward the second electrode chip B to 2.5 mm or less, the diameter of the nub of the metal plate arranged on the other outermost surface of the plate assembly can be increased, and the joining strength between the metal plate arranged on the other outermost surface of the plate assembly and the other metal plate 111 can be further increased.

[0035] In the case of controlling the contact area between the electrode chip and the surface of the plate assembly by bending the plate assembly 11, and when the number of metal plates 111 is 3 or more, it is preferable to further control the contact area between the metal plates 111 inside the plate assembly 11. For example, when pressing the first electrode chip A and the second electrode chip B against the plate assembly 11 to apply a pressing force, the thinnest metal plate 111 min and the contact area S4 with the metal plate 111 in contact therewith, and the metal plate 111 in contact with the first electrode chip A A and the contact area S3 between this and the metal plate 111 in contact therewith may satisfy the relationship S4 < S3. In this case, the plate thickness ratio of the plate assembly 11 may be 5.0 or more.

[0036] When the number of metal plates constituting the plate assembly 11 is 3, as shown in FIG. 6, the metal plate 111 in contact with the thinnest metal plate 111 min and the metal plate 111 in contact with the first electrode chip A A are the same. On the other hand, when the number of metal plates constituting the plate assembly 11 is 4 or more, as shown in FIG. 7, the metal plate 111 in contact with the thinnest metal plate 111 min and the metal plate 111 in contact with the first electrode chip A A are different. Although the number of metal plates is 4 in FIG. 7, of course, the number of metal plates can be 5 or more. For reference, the contact surfaces corresponding to S1 to S4 are shown in FIGS. 6 and 7.

[0037] When the plate assembly 11 is bent so as to satisfy both the relationship of 0.10 ≦ S2 / S1 ≦ 0.90 and the relationship of S4 < S3, the formation location of the nugget can be made to deviate more significantly toward the thinnest metal plate 111. min Therefore, the appropriate current range ΔI in spot welding of a plate assembly with a high plate thickness ratio can be further expanded. In the plate assembly 11, it is also allowed that there are two or more of the thinnest metal plates 111. min When a plurality of the thinnest metal plates 111 are included in the plate assembly, those that are not on the outermost surface of the plate assembly (i.e., those inside the plate assembly) do not cause poor bonding, so they can be treated in the same way as other metal plates. min When a plurality of the thinnest metal plates 111 are included in the plate assembly and the thinnest metal plates 111 are arranged on both outermost surfaces of the plate assembly 11, spot welding in the above-described procedure is performed on one of the two thinnest metal plates 111, and then spot welding in the above-described procedure is performed again on the other thinnest metal plate 111. Thereby, the bonding strength can be ensured. Even if spot welding is performed twice at the same location, the bonding strength is not impaired. Also, depending on the structure and use of the component, it may not be necessary to ensure the bonding strength of all the metal plates. In this case, spot welding may be performed only on the thinnest metal plate 111 arranged on the outermost surface on the side where the bonding strength is required. In other words, a method for manufacturing a spot weld joint that performs the above-described spot welding on one or more of the two outermost surfaces of the plate assembly can be regarded as the manufacturing method according to the present embodiment. The thinnest metal plate 111 min When a plurality of the thinnest metal plates 111 are included in the plate assembly and the thinnest metal plates 111 are arranged on both outermost surfaces of the plate assembly 11, min If the thinnest metal plates 111 are arranged, min For one of the two thinnest metal plates 111, spot welding in the above-described procedure is performed, and then for the other thinnest metal plate 111, min Spot welding in the above-described procedure may be performed again. Thereby, the bonding strength can be ensured. Even if spot welding is performed twice at the same location, the bonding strength is not impaired. Also, depending on the structure and use of the component, it may not be necessary to ensure the bonding strength of all the metal plates. In this case, spot welding may be performed only on the thinnest metal plate 111 arranged on the outermost surface on the side where the bonding strength is required. In other words, a method for manufacturing a spot weld joint that performs the above-described spot welding on one or more of the two outermost surfaces of the plate assembly can be regarded as the manufacturing method according to the present embodiment. min Only for the thinnest metal plate 111 arranged on the outermost surface on the side where the bonding strength is required, the above-described spot welding may be performed. In other words, a method for manufacturing a spot weld joint that performs the above-described spot welding on one or more of the two outermost surfaces of the plate assembly can be regarded as the manufacturing method according to the present embodiment.

[0038] The contact area is measured using pressure-sensitive paper. For example, when measuring the contact area between the electrode tip and the metal plate, the electrode tip is pressed against the metal plate with a predetermined pressure while the pressure-sensitive paper is sandwiched between the electrode tip and the metal plate. The electrode tip is then separated from the metal plate, the pressure-sensitive paper is collected, and the area of ​​the discolored area is measured, so that the contact area between the electrode tip and the metal plate can be estimated. When measuring the contact area between metal plates, the electrode tip is pressed against the metal plate with a predetermined pressure while the pressure-sensitive paper is sandwiched between the metal plates, and then the pressure-sensitive paper is collected and the area of ​​the discolored area is measured. The contact area during spot welding can be estimated by investigating the relationship between the pressure and the contact area before spot welding is performed.

[0039] The means for bending the plate set 11 toward the second electrode tip B is not particularly limited. For example, as shown in Fig. 8, before the plate set 11, which is a stack of a plurality of metal plates 111, is sandwiched between the opposing first electrode tip A and second electrode tip B, the plate set 11 may be bent toward the second electrode tip B by further providing the following steps to the manufacturing method of the spot welded joint: step s1 of contacting the first electrode tip A with one surface of the plate set 11; step s2 of arranging the second electrode tip B in a state separated from the other surface of the plate set 11; and step s3 of pressing the first electrode tip A against the plate set 11 to apply a pressure, thereby bringing the second electrode tip B into contact with the other surface of the plate set 11. In this case, it is preferable that the electrode on which the second electrode tip B is arranged is a fixed electrode, and the electrode on which the first electrode tip A is arranged is a movable electrode. Note that step s1 of bringing the first electrode tip A into contact with one surface of the plate set 11 may be performed after step s2 of arranging the second electrode tip B away from the other surface of the plate set 11.

[0040] When bending the plate set 11 in the procedure shown in Fig. 8, it is preferable to fix the periphery of the welding point of the plate set 11 using plate set fixing means C. The welding point is a location where the first electrode tip and the second electrode tip contact each other in the plate set 11. The configuration of the plate set fixing means C is not particularly limited, but a preferred embodiment is exemplified below.

[0041] The plate assembly fixing means C may be configured to include, for example, a clamp capable of fixing the plate assembly 11, and a mounting surface on which the plate assembly 11 can be placed. In Fig. 8, the plate assembly fixing means C is disposed below the plate assembly 11, but it may be disposed above it. In Fig. 8, the plate assembly fixing means C is disposed so as to face the first electrode tip A, but it may be disposed so as to face the second electrode tip B.

[0042] In Fig. 8, two plate assembly fixing means C are provided, which support the plate assembly 11 at two points. In this case, it is preferable to place the welding point on a straight line formed by the two points fixed by the two plate assembly fixing means C. This allows stable pressure application to the plate assembly. More preferably, the welding point is placed at the midpoint of the two points fixed by the two plate assembly fixing means C.

[0043] The number of plate assembly fixing means C may be three or more. In this case, it is preferable to arrange three or more plate assembly fixing means C so that the places where the plate assembly 11 is fixed using the plate assembly fixing means C form a regular polygon, and to arrange the welding points inside the regular polygon. More preferably, the welding points are arranged at the center of gravity of the regular polygon. Also, when the number of plate assembly fixing means C is four, the figure formed by the places where the plate assembly 11 is fixed using the plate assembly fixing means C may be a rhombus instead of a square.

[0044] On the other hand, the plate assembly fixing means C may have a mounting surface having a circular, elliptical, or polygonal hole capable of accommodating an electrode tip. According to the plate assembly fixing means C having such a mounting surface, when the plate assembly 11 is pressed, the periphery of the welding point is fixed at the end of the hole C1 of the mounting surface. In other words, the portion where the plate assembly 11 is fixed using the plate assembly fixing means C forms a circle, an ellipse, or a polygon, and the welding point is disposed inside the circle, the ellipse, or the polygon. Preferably, the welding point is disposed at the center of the circle or the ellipse, or the center of gravity of the polygon. According to such a configuration, the pressing of the plate assembly 11 using the electrode tip becomes more stable. FIG. 9 shows an example of the plate assembly fixing means C in which the hole C1 is circular.

[0045] The clearance, i.e., the size of the gap between the tip of the second electrode tip and the sheet assembly at the stage when the first electrode tip is brought into contact with the sheet assembly and pressure application is started, is not particularly limited. However, for example, the clearance is preferably 0.5 to 2.5 mm. By making the clearance 0.5 mm or more, S2 / S1 can be set within a more preferable range. Also, by making the clearance 2.5 mm or less, the joining strength of the spot welded joint can be further increased. As will be described later, in order to further increase the joining strength of the spot welded joint, it is necessary to make the thinnest metal plate 111 arranged on the outermost layer on one side of the sheet assembly. min In addition to joining the metal plate 111 to the other metal plate 111, it is preferable to also increase the joining strength between the metal plate arranged on the outermost layer of the other plate set and the other metal plate 111. By setting the clearance to 2.5 mm or less, the nugget diameter of the metal plate arranged on the outermost layer of the other plate set can be increased, and the joining strength between the metal plate arranged on the outermost layer of the other plate set and the other metal plate 111 can be further increased. The clearance is approximately the same as the amount by which the plate set 11 is bent toward the second electrode tip B as described above.

[0046] Next, a spot welded joint 1 according to another aspect of the present invention will be described. The spot welded joint 1 according to another aspect of the present invention is an example of a spot welded joint obtained by the manufacturing method of a spot welded joint according to the present embodiment described above. However, it should be noted that a spot welded joint 1 that satisfies the requirements described below has high cross tensile strength regardless of its manufacturing method, and is therefore considered to be a spot welded joint according to the present embodiment.

[0047] As illustrated in FIGS. 3A and 3B , the spot welded joint 1 according to this embodiment includes a sheet assembly 11 in which a plurality of metal sheets 111 are overlapped, and a nugget 12 that joins the sheet assembly 11. The thinnest metal sheet 111 in the sheet assembly 11 is min The first metal plate 111 min When the first metal plate 111 is defined as min is disposed on one outermost surface of the plate assembly 11, and the total plate thickness of the plate assembly 11 is determined by the first metal plate 111. minThe sheet thickness ratio of the sheet assembly 11 is 4.0 or more, which is calculated by dividing the sheet thickness of the sheet assembly 11 by the sheet thickness of the first metal plate 111. The outer edge of the nugget 12 observed in the observation cross section that is perpendicular to the surface of the sheet assembly 11 and passes through the center axis of the nugget 12 in a plan view is min The outer edge of the nugget 12 observed in the observation cross section reaches the inside of the first metal plate 111. min In the area facing the first metal plate 111 min The nugget 12 has a shape that is recessed from the center toward the center in the plate thickness direction.

[0048] The material and thickness of the multiple metal plates 111 are not particularly limited. The items exemplified in the manufacturing method of the spot welded joint according to the embodiment described above can be appropriately applied. min The number of metal plates arranged inside the plate group 11 may be two or more. Even if the thickness of the metal plate arranged inside the plate group 11 is small, it does not hinder the joining of the plate group 11. In addition, the thinnest metal plate 111 min However, for the sheet pair 11 arranged on both outermost surfaces, this embodiment may be applied to one of the outermost surfaces where higher cross tensile strength is required, or this embodiment may be applied to both outermost surfaces. As described above, by carrying out the manufacturing method for a spot welded joint according to this embodiment twice at the same location, this embodiment can be applied to both outermost surfaces, thereby increasing the strength of all metal sheets. On the other hand, depending on the structure and application of the part, there are cases where it is not necessary to ensure the joining strength of all metal sheets. In this case, the thinnest metal sheet 111 arranged on the outermost surface on the side where joining strength is required is used. min In other words, the configuration of this embodiment may be applied only to the thinnest metal plate 111 on one or more of the two outermost surfaces of the plate assembly. min A spot welded joint that satisfies the above requirements can be considered to be a spot welded joint according to this embodiment.

[0049] The reason why the plate thickness ratio of the plate set 11 is set to 4.0 or more, and the reason why the first metal plate 111 which is the thinnest metal plate minThe reason why is disposed on one outermost surface of the sheet assembly 11 is as explained in the manufacturing method for a spot welded joint according to the present embodiment described above. Moreover, the preferred sheet thickness ratios exemplified in the manufacturing method for a spot welded joint according to the present embodiment can be appropriately applied to the spot welded joint 1 according to the present embodiment.

[0050] In the spot welded joint 1 according to this embodiment, the outer edge of the nugget 12 observed in a cross section perpendicular to the surface of the metal plate 111 and passing through the central axis of the nugget 12 is the first metal plate 111. min As a result, the spot welded joint 1 according to this embodiment is formed by welding the first metal plate 111 disposed on one of the outermost surfaces of the plate assembly 11. min can be reliably joined to another metal plate 111.

[0051] In the spot welded joint 1 according to this embodiment, the outer edge of the nugget 12 observed in the observation cross section is in contact with the first metal plate 111. min In the area facing the first metal plate 111 min The nugget 12 has a shape recessed from the surface toward the center in the sheet thickness direction of the nugget 12. Here, the observation cross section means a cross section that is perpendicular to the surface of the sheet assembly 11 and passes through the central axis of the nugget 12 in a plan view. Since the cross section of a normal nugget 12 has an elliptical shape, the nugget 12 of the spot welded joint 1 according to this embodiment is significantly different from a normal one in terms of the cross-sectional shape. This recess is formed, for example, during the process of the manufacturing method of the spot welded joint according to this embodiment described above.

[0052] The electrode tip used in the spot welding device has a configuration in which a coolant flows inside the electrode tip. Therefore, the electrode tip cools the metal sheet 111 during spot welding. However, when the spot welded joint 1 is manufactured with S2 / S1 in the range of 0.10 to 0.90, the thinnest metal sheet (i.e., the first metal sheet 111) minThe periphery of the contact surface between the first metal plate 111 and the second electrode tip B is less likely to be cooled by the second electrode tip B. Therefore, when the spot welded joint 1 is manufactured with S2 / S1 in the range of 0.10 to 0.90, min Melting of the metal plate 111 is more likely to progress around the contact surface between the first metal plate 111 and the second electrode tip B than directly under the contact surface. For the reasons described above, the outer edge of the nugget 12 of the spot welded joint 1 according to this embodiment can have a shape that is recessed toward its center. In other words, the outer edge of the nugget 12 of the spot welded joint 1 according to this embodiment can be recessed toward its center by the contact surface between the first metal plate 111 and the second electrode tip B. min In the region facing the first metal plate 111, the first metal plate 111 may have two protrusions extending toward the outside of the sheet set 11. As a result, the spot welded joint 1 according to this embodiment has a first metal plate 111 disposed on one of the outermost surfaces of the sheet set 11. min can be reliably joined to another metal plate 111.

[0053] When the number of metal plates 111 is three or more, the first metal plate 111 measured on the observation cross section min The nugget diameter D1 at the contact surface P with the adjacent metal plate 111 is min It is preferable that the nugget diameter D1 at the contact surface P is larger than the nugget diameter at the contact surface p between the metal plates 111 other than the metal plates 111. The nugget diameter D1 at the contact surface P is the distance between two intersections of the contact surface P and the outer edge of the nugget 12, measured on the cross section for observation. Similarly, the nugget diameter at the contact surface p is the distance between two intersections of the contact surface p and the outer edge of the nugget 12, measured on the cross section for observation. When the number of metal plates 111 is four or more, the number of contact surfaces p is two or more. In this case, the above requirement is satisfied as long as the nugget diameter at the contact surface P is larger than the nugget diameters at all of the contact surfaces p.

[0054] As described above, when a spot welded joint is manufactured with S2 / S1 in the range of 0.10 to 0.90, the first metal plate 111 minThe melting of the metal plate 111 is more likely to progress around the contact surface between the first metal plate 111 and the second electrode tip B than directly under the contact surface. min As a result, the spot welded joint 1 according to this embodiment has a shape that spreads toward the first metal plate 111 disposed on one outermost surface of the plate assembly 11. min This allows the metal plate 111 to be joined to the other metal plate 111 more reliably.

[0055] In addition, when the number of metal plates 111 is three or more, the other outermost surface of the plate set 11 (i.e., the first metal plate 111 min The metal plate to be placed on the outermost surface opposite to the outermost surface on which the second metal plate 111 is placed is A and the second metal plate 111 A The metal plate adjacent to the third metal plate 111 B and the second metal plate 111 A and the third metal plate 111 B When the thinner plate thickness of the two plates is defined as t', the thickness of the second metal plate 111 measured at the cross section for observation is A and the third metal plate 111 B The nugget diameter D2 at the contact surface with the first electrode tip A is preferably 3√t' or more, and more preferably 3.5√t' or more or 4√t' or more. A The second metal plate 111 is the same as the first metal plate 111. A and the third metal plate 111 B When the plate thicknesses of the first metal plate 111 and the second metal plate 112 are the same, A and the third metal plate 111 B The plate thickness is defined as t'. In a cross tensile test of the spot welded joint 1, the first metal plate 111 is most likely to peel off. min The interface between the first metal plate 111 and the adjacent metal plate 111 is the second most likely to peel off. A and a third metal plate 111 adjacent thereto. BTherefore, in addition to the nugget diameter D1 at the interface where peeling is most likely to occur, the nugget diameter D2 at the interface where peeling is second most likely to occur is increased to 3√t' or more, thereby further increasing the cross tensile strength of the spot welded joint 1. For example, in manufacturing the spot welded joint 1, the amount by which the sheet assembly 11 is bent toward the second electrode tip B (or the clearance) is set to 2.5 mm or less, thereby A and a third metal plate 111 adjacent thereto. B By increasing the nugget diameter D2 at the interface with the first and second bonding members, the bonding strength between the first and second bonding members can be further increased. In the spot welded joint and the manufacturing method of the spot welded joint according to this embodiment, a configuration in which a metal plate inside the sheet group 11 is thinner than the metal plate at the outermost surface of the sheet group 11 is also permitted. This is because a metal plate arranged inside the sheet group 11 does not hinder the joining of the sheet group 11 even if it has a small thickness. In this case, the first metal plate is defined as "the thinner of the metal plates arranged at the outermost surface of the sheet group" rather than "the thinnest metal plate" (a case in which the two metal plates arranged at the outermost surface of the sheet group have the same thickness is not taken into consideration in this embodiment). Therefore, in this case, the spot welded joint of this embodiment has a plate assembly in which multiple metal plates are overlapped, and a nugget that joins the plate assembly, and when the thinner of the metal plates arranged on the outermost surface of the plate assembly is defined as the first metal plate, the plate thickness ratio of the plate assembly, calculated by dividing the total plate thickness of the plate assembly by the plate thickness of the first metal plate, is 4.0 or more, the outer edge of the nugget observed in an observation cross section that is perpendicular to the surface of the plate assembly and passes through the central axis of the nugget in a planar view, reaches the inside of the first metal plate, and the outer edge of the nugget observed in the observation cross section has a shape that is recessed from the first metal plate toward the center of the nugget in the plate thickness direction in the area facing the first metal plate, and the thinnest metal plate may be arranged inside the plate assembly. EXAMPLES

[0056] Example 1 Various spot welded joints were produced under the following conditions: -Welding machine: Fixed AC spot welding machine -Plate assembly: (1) Thin plate (plate thickness 0.7mm) SCGA270C (2) Thick plate (1.6mm thickness) SCGA780DP (3) Thick plate (2.3mm thickness) SCGA980DP (Overlapped in the order of 1 to 3 above) -Electrode: DR6φ-40R (both the first and second electrode tips) -Pressure: 400kgf (3.92kN) -Squeeze: 50 cycles -Electrification: 28cyc -Holding: 18cyc -Clearance: Within the range of 0~2.8mm - Distance between board assembly fixing means: 60mm

[0057] The contact area was measured by placing pressure-sensitive paper (a commercially available pressure / surface pressure measurement film) between the plates and applying pressure without passing any current. The plate thickness ratio of the plate assembly calculated from the plate thickness was 6.6. In addition, in the plate assembly, the thinnest metal plate, thin plate (1), was arranged on the outermost surface of the plate assembly. The clearance means the size of the gap between the tip of the second electrode tip and the plate assembly at the stage when the first electrode tip is brought into contact with the plate assembly and pressure application is started. When the first electrode tip is pressed against the plate assembly under the condition that there is a clearance to apply pressure, and the second electrode tip is brought into contact with the other surface of the plate assembly, the plate assembly is bent toward the second electrode tip.

[0058] The contact area during welding and the appropriate current range ΔI are shown in Table 1. 薄 / 厚 indicates the appropriate current range between the thin plate and the thick plate (i.e., the appropriate current range between the above steel plates 1 and 2), and ΔI 厚 / 厚 indicates the appropriate current range between thick plates (i.e., the appropriate current range between steel plates 2 and 3 above).

[0059] [Table 1]

[0060] Example 4 is an example where spot welding was performed without providing a clearance. This corresponds to spot welding by the conventional technology. In this case, S1 and S2 are substantially equal, and S2 / S1 is 1.00. Therefore, for a plate assembly with a plate thickness ratio of 6.6, ΔI 薄 / 厚 In the conventional spot welding, therefore, it is usual to restrict the sheet thickness ratio to a low value. On the other hand, in Examples 1, 2, 5, and 6 in which clearance was provided, a wide appropriate current range was ensured even for a plate assembly with a plate thickness ratio of 6.6. In Examples 1, 2, 5, and 6, S2 / S1 was controlled within the range of 0.10 to 0.90. Moreover, in Examples 2 and 6 which further satisfied the condition S3>S4, an even wider appropriate current range was ensured compared to Examples 1 and 5. In Example 3, by providing a clearance, the appropriate current range between the thin plate and the thick plate could be further increased. However, in Example 3, the appropriate current range between the thick plates was narrowed. This is because expulsion was more likely to occur due to S2 / S1 being less than 0.10. The above results show that by controlling S2 / S1 within the range of 0.10 to 0.90, a wide appropriate current range ΔI can be ensured when welding a sheet assembly having a high sheet thickness ratio and in which the thinnest metal sheet is arranged on the outermost surface.

[0061] Example 2 Various spot welded joints were produced under the conditions shown in Table 2. The welding conditions other than the electrode type and clearance shown in Table 2 were the same as those in Example 1.

[0062] [Table 2]

[0063] A cross tensile test was conducted on the spot welded joints manufactured under the conditions in Table 2 to measure their cross tensile strength, and the results are shown in Table 3. The cross tensile test was conducted in accordance with JIS Z 3137:1999 "Specimen dimensions and test method for cross tensile test of resistance spot and projection welded joints". During the test, tensile stress was applied between the thin plate (1) and the thick plate (2). Therefore, the cross tensile strength obtained by this test is an evaluation index of the joining strength of the thin plate (1) and the thick plate (2). Spot welded joints with a cross tensile strength of 3.4 kN or more were judged to be spot welded joints with excellent cross tensile strength.

[0064] In addition, the spot welded joints manufactured under the conditions in Table 2 were cut in a plane perpendicular to the plate surface and passing through the central axis of the nugget in a plan view. The nugget shape was evaluated in the observation cross section obtained in this way, and the results are shown in Table 3. Specifically, the following elements were measured. - Nugget diameter at the contact surface between the thinnest metal sheet (first metal sheet) and the adjacent metal sheet (referred to as "thin nugget" in Table 3) -The nugget diameter at the contact surface between the metal plate (second metal plate) arranged on the outermost surface opposite the outermost surface on which the first metal plate is arranged and the adjacent metal plate (referred to as "thick nugget" in Table 3) -The reciprocal of the radius of curvature R of the outer edge of the nugget that faces the first metal plate (represented as "1 / R" in Table 3) When the outer edge of the nugget is convex in the direction away from the nugget center, 1 / R is expressed as a positive value, and when the outer edge of the nugget is convex in the direction toward the nugget center, 1 / R is expressed as a negative value. When the radius of curvature is 0, 1 / R cannot be calculated, so it is expressed as "-" in Table 3. When 1 / R is a negative value, the outer edge of the nugget has a shape that is concave toward the center of the nugget in the area facing the first metal plate. In addition, when the outer edge of the nugget did not reach the inside of the first metal plate, the value of the thin nugget D1 was recorded as "0". We also determined whether the nugget diameter D2 of the thick nugget was equal to or greater than the above-mentioned 3√t' (mm). The thicknesses of the two metal plates (2, 3) that form the thick nugget are 1.6mm and 2.3mm, respectively, so the thinner plate thickness t' is 1.6mm, and 3√t' is 3.8mm. If D2 was 3.8mm or greater, we entered "O" in the "D2>3√t'" column, and otherwise entered "X" in the same column. Furthermore, the size relationship between the thin nugget D1 and the thick nugget D2 was also evaluated. If the thin nugget D1 was larger than the thick nugget D2, a "◯" was entered in the "D1>D2" column, and otherwise a "×" was entered in the same column.

[0065] [Table 3]

[0066] In the spot-welded joints of Examples 1 and 2, the outer edge of the nugget reached the inside of the first metal plate, and furthermore, the area facing the first metal plate had a shape that was recessed toward the center of the nugget. Therefore, the spot-welded joints of Examples 1 and 2 had excellent cross tensile strength. In addition, the spot-welded joint of Example 1 had even better cross tensile strength because the thin nugget D1 was larger than the nugget diameter (thick nugget D2) at the contact surface between the other metal plates. In the spot-welded joints of Examples 3 and 4, the outer edge of the nugget did not reach the inside of the first metal plate, and furthermore, the area facing the first metal plate was not recessed toward the center of the nugget, so the cross tensile strength of the spot-welded joints of Examples 3 and 4 was insufficient. In the spot welded joint of Example 5, the outer edge of the nugget reached the inside of the first metal plate, and the cross tensile strength also met the judgment criteria, but the area facing the first metal plate was not recessed toward the center of the nugget, and the nugget shape according to this embodiment was not achieved. In addition, the electrode with W (tungsten) embedded in the tip used in Example 5 is expensive and has a disadvantage in terms of cost. By using a tungsten electrode, the spot welded joint of Example 5 was able to increase the cross tensile strength slightly more than Examples 3 and 4. However, the cross tensile strength of the spot welded joint of Example 5 was lower than that of Invention Examples 1 and 2, which used chromium copper electrodes. For reference, cross-sectional schematic diagrams of spot-welded joints of Examples 3, 4, and 5 are shown in Figures 10A to 10C. In Example 3 (Figure 10A) and Example 4 (Figure 10B), the outer edge of the nugget did not reach the inside of the first metal plate. In Example 5 (Figure 10C), the outer edge of the nugget reached the inside of the first metal plate, but the nugget did not have a dent. [Industrial Applicability]

[0067] According to the present invention, it is possible to provide a manufacturing method for a spot welded joint that can ensure a wide appropriate current range ΔI when welding a sheet pair having a high sheet thickness ratio and the thinnest metal sheet arranged on the outermost surface. Furthermore, according to the present invention, it is possible to provide a spot welded joint having high cross tensile strength obtained by welding a sheet pair having a high sheet thickness ratio and the thinnest metal sheet arranged on the outermost surface. Therefore, the present invention has high industrial applicability. [Explanation of symbols]

[0068] 1 Spot welded joints 11 Board set 111 Metal plate 111 min The thinnest metal plate (first metal plate) 111 A Metal plate in contact with the first electrode tip A (second metal plate) 111 B Third Metal Plate 12. Nugget A First electrode tip B. Second electrode tip C Plate assembly fixing means C1 Hole D1: Nugget diameter at the contact surface between the first metal plate and the adjacent metal plate D2: Nugget diameter at the contact surface between the second metal plate and the adjacent metal plate

Claims

1. A plate assembly made by stacking multiple metal plates, A nugget that joins the plate assembly; Equipped with When the thinnest metal plate in the plate assembly is defined as the first metal plate, The first metal plate is disposed on one outermost surface of the plate set, The plate thickness ratio of the plate assembly, calculated by dividing the total plate thickness of the plate assembly by the plate thickness of the first metal plate, is 4.0 or more; An outer edge of the nugget, which is observed in an observation cross section that is perpendicular to a surface of the plate assembly and passes through a central axis of the nugget in a plan view, reaches the inside of the first metal plate, The outer edge of the nugget observed in the observation cross section has a shape recessed from the first metal plate toward the center of the nugget in the sheet thickness direction in a region facing the first metal plate, The number of the metal plates is three or more, The nugget diameter at the contact surface between the first metal plate and the adjacent metal plate, which is measured at the observation cross section, is larger than the nugget diameter at the contact surface between the other metal plates. Spot welded joints.

2. The number of the metal plates is three or more, The metal plate arranged on the other outermost surface of the plate assembly is defined as a second metal plate, the metal plate adjacent to the second metal plate is defined as a third metal plate, and the thinner plate thickness of the second metal plate and the third metal plate is defined as t'. The nugget diameter at the contact surface between the second metal plate and the third metal plate measured at the observation cross section is 3√t' or more.

2. The spot welded joint according to claim 1.

3. The method includes a step of sandwiching a sheet set including a plurality of overlapping metal sheets between a first electrode tip and a second electrode tip facing each other and spot welding the sheet set, The plate thickness ratio of the plate assembly, calculated by dividing the total plate thickness of the plate assembly by the plate thickness of the thinnest metal plate in the plate assembly, is 4.0 or more; The thinnest metal plate is disposed on the outermost surface of the plate assembly; The second electrode tip is disposed on the side where the thinnest metal plate is disposed; When the first electrode tip and the second electrode tip are pressed against the plate assembly to apply a pressure, a ratio S2 / S1 of a contact area S2 between the thinnest metal plate and the second electrode tip to a contact area S1 between the first electrode tip and the metal plate is set to 0.10≦S2 / S1≦0.

90. A method for manufacturing spot welded joints.

4. When the first electrode tip and the second electrode tip are pressed against the plate set to apply a pressure, the plate set is bent toward the second electrode tip, so that S2 / S1 satisfies the relationship 0.10≦S2 / S1≦0.

90. The method for manufacturing a spot welded joint according to claim 3 .

5. The number of the metal plates is three or more, The plate thickness ratio of the plate assembly is 5.0 or more, When the first electrode tip and the second electrode tip are pressed against the plate assembly to apply a pressure, A contact area S4 between the thinnest metal plate and the metal plate in contact therewith, and The contact area S3 between the metal plate in contact with the first electrode tip and a metal plate in contact therewith is set so as to satisfy the relationship S4<S3. The method for manufacturing a spot welded joint according to claim 4 .

6. Before the plate set in which the plurality of metal plates are stacked is sandwiched between the first electrode tip and the second electrode tip facing each other, contacting the first electrode tip with one surface of the plate assembly; positioning the second electrode tip away from the other surface of the plate assembly; pressing the first electrode tip against the plate set and applying a pressure thereto, thereby bringing the second electrode tip into contact with the other surface of the plate set; The method for manufacturing a spot welded joint according to claim 4 or 5, further comprising:

7. The method for manufacturing a spot welded joint according to claim 6, characterized in that the electrode on which the second electrode tip is provided is a fixed electrode, and the electrode on which the first electrode tip is provided is a movable electrode.

8. 8. The method for manufacturing a spot welded joint according to claim 6, further comprising the step of fixing the periphery of the weld point of the plate assembly using a plate assembly fixing means.

9. The plate assembly fixing means is two plate assembly fixing means, The welding point is arranged on a straight line formed by two points fixed by the two plate assembly fixing means. The method for manufacturing a spot welded joint according to claim 8 .

10. The plate assembly fixing means is three or more plate assembly fixing means, Three or more of the plate assembly fixing means are arranged so that the portions where the plate assembly is fixed using the plate assembly fixing means form a regular polygon; The welding points are arranged inside the regular polygon. The method for manufacturing a spot welded joint according to claim 8 .

11. The plate assembly fixing means is arranged so that a portion where the plate assembly is fixed using the plate assembly fixing means forms a circle or an ellipse; The welding points are positioned inside the circle or ellipse. The method for manufacturing a spot welded joint according to claim 8 .

12. The method for manufacturing a spot welded joint according to any one of claims 4 to 10, characterized in that the first electrode tip and the second electrode tip have substantially the same tip shape.

13. The manufacturing method of the spot welded joint according to any one of claims 6 to 12, characterized in that a clearance, which is a size of a gap between the tip of the second electrode tip and the plate assembly at a stage of contacting the first electrode tip with the plate assembly and starting pressure application, is 0.5 to 2.5 mm.

14. The method for manufacturing a spot welded joint according to any one of claims 4 to 12, characterized in that the amount by which the plate assembly is bent toward the second electrode tip is 0.5 to 2.5 mm.

Images