Resistance spot welding method and resistance spot welding device

Abstract

The present disclosure provides a resistance spot welding method and a resistance spot welding device capable of suppressing cracks in a high tensile strength steel sheet without being limited by the material. The resistance spot welding method according to one aspect of the present disclosure includes a welding process of welding a workpiece made of a plurality of steel sheets by a resistance spot welding device. The resistance spot welding device includes a first electrode in contact with a first steel sheet, the first steel sheet being a high tensile strength steel sheet among the plurality of steel sheets, and a second electrode in contact with a second steel sheet and sandwiching the workpiece together with the first electrode, the second steel sheet being a steel sheet having a tensile strength lower than that of the first steel sheet among the plurality of steel sheets. In the welding process, the first steel sheet is compressed in a direction intersecting a thickness direction of the first steel sheet at least from a start of welding of the workpiece to an end of the welding of the workpiece.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a resistance spot welding method and a resistance spot welding device. BACKGROUND

[0002] In resistance spot welding of a high tensile strength steel sheet (i.e., a high-strength material) having a large tensile strength, a tensile stress is generated in a welded portion when pressing by an electrode is released. This tensile stress is caused by the restoring force of the material and the solidification of the molten portion.

[0003] The above-described tensile stress can cause a crack in the welded portion after welding. Therefore, a method of suppressing a crack by specifying the zinc component amount of a steel sheet has been proposed (see Japanese Patent Application Publication No. 2020-179413). SUMMARY

[0004] In the above-described resistance spot welding method, since the zinc component amount needs to be adjusted, the material of the steel sheet is limited. Moreover, a test needs to be repeatedly performed so that the zinc component amount is within the specified range.

[0005] One aspect of the present disclosure preferably provides a resistance spot welding method that is not limited by the material and can suppress a crack of a high tensile strength steel sheet.

[0006] One aspect of the present disclosure relates to a resistance spot welding method including a welding process of a workpiece made of a plurality of steel sheets by a resistance spot welding device. The resistance spot welding device includes a first electrode configured to contact a first steel sheet, the first steel sheet being a high tensile strength steel sheet among the plurality of steel sheets, and a second electrode configured to contact a second steel sheet and sandwich the workpiece with the first electrode, the second steel sheet being a steel sheet having a smaller tensile strength than the first steel sheet among the plurality of steel sheets. In the welding process, the first steel sheet is compressed in a direction intersecting a thickness direction of the first steel sheet at least from a start of welding of the workpiece to an end of welding of the workpiece.

[0007] According to the above-described configuration, the workpiece is welded in a state where the first steel sheet as a high tensile strength steel sheet has generated a compressive stress. As a result, a tensile stress generated after welding is canceled by the compressive stress, thereby suppressing a crack of the first steel sheet.

[0008] In one aspect of the present disclosure, in the welding process, the first steel sheet can be compressed in a direction intersecting a thickness direction of the first steel sheet until cooling after welding of the workpiece ends. According to the above-described configuration, the first steel sheet continuously generates a compressive stress until cooling ends without generating a tensile stress, and thus, a suppression effect on a crack can be promoted.

[0009] In one aspect of the present disclosure, in the process of welding, the first steel sheet can be compressed in a direction intersecting the thickness direction of the first steel sheet before starting the welding of the workpiece. According to the above configuration, the welding of the workpiece can be started while the first steel sheet is in a state of dynamic stability. Thus, the welding quality can be improved.

[0010] In one aspect of the present disclosure, in the process of welding, the first steel sheet can be compressed in a direction intersecting the thickness direction of the first steel sheet before starting the welding of the workpiece. According to the above configuration, the welding of the workpiece can be started while the first steel sheet is in a state of dynamic stability. Thus, the welding quality can be improved.

[0011] Another aspect of the present disclosure relates to a resistance spot welding device configured to perform welding on a workpiece, in which the workpiece is composed of a plurality of steel sheets. The resistance spot welding device includes a first electrode configured to contact a first steel sheet, in which the first steel sheet is a high tensile strength steel sheet among the plurality of steel sheets; a second electrode configured to contact a second steel sheet and to sandwich the workpiece together with the first electrode, in which the second steel sheet is a steel sheet having a tensile strength smaller than the first steel sheet among the plurality of steel sheets; and a compression mechanism configured to compress the first steel sheet in a direction intersecting the thickness direction of the first steel sheet at least from the process of welding of the workpiece to the end of the welding of the workpiece.

[0012] According to the above configuration, the tensile stress generated after the welding is offset by the compression stress, thereby suppressing the cracking of the first steel sheet.

[0013] In one aspect of the present disclosure, the compression mechanism can be configured to compress the first steel sheet in a direction intersecting the thickness direction of the first steel sheet until the end of the cooling after the welding of the workpiece. According to the above configuration, the compression stress is continuously applied to the first steel sheet until the end of the cooling at which the tensile stress is no longer generated, and thus the suppression effect on the cracking can be promoted.

[0014] In one aspect of the present disclosure, in the process of welding, the first steel sheet can be compressed in a direction intersecting the thickness direction of the first steel sheet before starting the welding of the workpiece. According to the above configuration, the welding of the workpiece can be started while the first steel sheet is in a state of dynamic stability. Thus, the welding quality can be improved.

[0015] In one aspect of the present disclosure, the compression mechanism can have a first pressing portion and a second pressing portion facing each other. The first pressing portion and the second pressing portion can be configured to compress the welding portion of the first steel sheet in a direction intersecting the thickness direction of the first steel sheet by sandwiching the first steel sheet. According to the above configuration, the compression stress can be surely applied to the first steel sheet. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a schematic view of a resistance spot welding device according to an embodiment.

[0017] Figure 2A is a schematic view showing one process of welding a workpiece by the resistance spot welding device.

[0018] Figure 2B is a schematic view showing a next process of Figure 2A .

[0019] Figure 2C is a schematic view showing a next process of Figure 2B .

[0020] Figure 3A is a schematic view showing a next process of Figure 2C .

[0021] Figure 3B is a schematic view showing a next process of Figure 3A .

[0022] Figure 4 is a flowchart of a resistance spot welding method according to an embodiment. DETAILED DESCRIPTION

[0023] An exemplary embodiment of the present disclosure will be described below with reference to the accompanying drawings.

[0024] [1. First Embodiment]

[0025] [1-1. Configuration]

[0026] Figure 1 The resistance spot welding device 1 shown in FIG. 1 is configured to perform welding on a workpiece W that is formed by stacking a first steel sheet P1 and a second steel sheet P2. The resistance spot welding device 1 includes a resistance welding machine 2 and a pressing mechanism 3.

[0027] The first steel sheet P1 is a so-called high tensile strength steel sheet having a tensile strength of 980 MPa or more and 3000 MPa or less. The tensile strength of the first steel sheet P1 is preferably 1180 MPa or more. The second steel sheet P2 is a non-high tensile strength steel sheet having a tensile strength of 270 MPa or more and less than 980 MPa, or a high tensile strength steel sheet having a tensile strength less than that of the first steel sheet P1. In the present embodiment, the second steel sheet P2 is stacked on the first steel sheet P1.

[0028] The first steel sheet P1 is in contact with the second steel sheet P2 at a welding portion X. In addition, the first steel sheet P1 also has a region separated from the second steel sheet P2 in the thickness direction. That is, the workpiece W has a gap S provided between the first steel sheet P1 and the second steel sheet P2.

[0029] The workpiece W has a protruding portion W1 that protrudes downward, and the welding portion X is disposed in the protruding portion W1. The second steel sheet P2 is disposed on the inner side of the bent portion of the first steel sheet P1 that constitutes the protruding portion W1.

[0030] <Resistance welding machine>

[0031] The resistance welding machine 2 performs resistance spot welding on the first steel sheet P1 and the second steel sheet P2 disposed as the workpiece W along the thickness direction.

[0032] The resistance welding machine 2 has a first electrode 21 and a second electrode 22. The first electrode 21 is disposed below the workpiece W. The second electrode 22 is disposed so as to sandwich the workpiece W together with the first electrode 21 in the thickness direction above the workpiece W. The first electrode 21 is capable of relative movement along the vertical direction with respect to the second electrode 22.

[0033] The first electrode 21 and the second electrode 22 each come into contact with the workpiece W at the time of welding. Specifically, the first electrode 21 is configured to come into contact with the first steel sheet P1. The second electrode 22 is configured to come into contact with the second steel sheet P2. A welding current flows between the first electrode 21 and the second electrode 22 via the workpiece W.

[0034] <Compression mechanism>

[0035] The compression mechanism 3 is configured to compress the first steel sheet P1 in a direction intersecting the thickness direction of the first steel sheet P1 at least from the start of welding of the workpiece W until the end of welding of the workpiece W.

[0036] Specifically, the compression mechanism 3 continuously compresses the first steel sheet P1 from before the start of welding of the workpiece W until the end of cooling after welding of the workpiece W. The compression mechanism 3 has a base 31, a first pressing portion 32, a second pressing portion 33, and a drive portion 34.

[0037] The base 31 is a portion that holds the workpiece W. The base 31 has the peripheral portion of the protruding portion W1 of the workpiece W placed thereon. The base 31 is disposed so as not to overlap the welding portion X in the vertical direction.

[0038] The first pressing portion 32 and the second pressing portion 33 are disposed so as to face each other in the horizontal direction. The first pressing portion 32 and the second pressing portion 33 sandwich at least the portion of the first steel sheet P1 that contains the welding portion X in a direction intersecting the thickness direction (specifically, a direction orthogonal to the thickness direction), thereby compressing that portion.

[0039] The drive portion 34 moves the second pressing portion 33 in the horizontal direction. The drive portion 34 is, for example, a cylinder that performs an action by oil pressure, air pressure, electric power, a spring, or the like. The second pressing portion 33 is moved by the drive portion 34 in a direction closer to the first pressing portion 32 and in a direction farther from the first pressing portion 32.

[0040] As shown in FIG. 1, the first pressing portion 32 is in contact with the convex portion Wl of the workpiece W before the welding is started (i.e., a state in which the first electrode 21 and the second electrode 22 are away from the workpiece W). Specifically, the first pressing portion 32 is in contact with the convex portion Wl of the workpiece W from the outside. Figure 2A

[0041] From the state shown in FIG. 1, the second pressing portion 33 is moved toward the first pressing portion 32, and thereby, as shown in FIG. 2, the convex portion Wl of the workpiece W is sandwiched between the first pressing portion 32 and the second pressing portion 33. Thus, the portion of the first steel sheet Pl constituting the bottom wall of the convex portion Wl is compressed in the direction intersecting the thickness direction (specifically, the direction orthogonal to the thickness direction). Figure 2A Figure 2B

[0042] Further, in the present embodiment, the second steel sheet P2 is disposed inside the first steel sheet Pl constituting the convex portion Wl, and the first pressing portion 32 and the second pressing portion 33 do not directly compress the second steel sheet P2 in the direction intersecting the thickness direction. However, it is also possible to compress the second steel sheet P2 in the direction intersecting the thickness direction with a smaller compression force than the compression force with which the first steel sheet Pl is compressed.

[0043] In the state in which the first steel sheet Pl is compressed, as shown in FIG. 3, the resistance spot welding device 1 moves the first electrode 21 and the second electrode 22, and thereby, the workpiece W is pressed in the thickness direction by the first electrode 21 and the second electrode 22. Figure 2C

[0044] Further, it is also possible to compress the first steel sheet Pl by the first pressing portion 32 and the second pressing portion 33 after the workpiece W is pressed by the first electrode 21 and the second electrode 22. Further, in the present embodiment, the direction in which the first electrode 21 and the second electrode 22 sandwich the workpiece W is inclined with respect to the thickness direction of the first steel sheet Pl at the welding portion. However, the direction in which the first electrode 21 and the second electrode 22 sandwich the workpiece W can also be parallel to the thickness direction of the first steel sheet Pl at the welding portion.

[0045] The resistance spot welding device 1 performs the welding of the workpiece W by supplying a current between the first electrode 21 and the second electrode 22 sandwiching the workpiece W. As the welding of the workpiece W proceeds, a nugget N is formed at the welding portion of the workpiece W.

[0046] That is, the resistance spot welding device 1 performs the welding of the workpiece W while compressing the welding portion of the first steel sheet Pl in the direction intersecting the thickness direction. By the welding performed by the resistance spot welding device 1, the first steel sheet Pl and the second steel sheet P2 are joined to each other in the thickness direction at the welding portion of the workpiece W by means of the nugget N. ​​​​

[0047] After the welding is completed (i.e., after the power supply is stopped), as shown in FIG. 6, the resistance welding machine 2 moves the second electrode 22 away from the workpiece W while maintaining the state in which the first steel sheet PI is compressed by the first pressing portion 32 and the second pressing portion 33. During this period, the welded portion of the workpiece W is cooled, for example, by the first electrode 21. Figure 3A

[0048] After the cooling is completed, as shown in FIG. 7, the resistance spot welding device 1 moves the second pressing portion 33 to be away from the workpiece W and the first pressing portion 32, thereby releasing the compression of the first steel sheet PI. Figure 3B

[0049] [1-2. Manufacturing method]

[0050] Figure 4 The resistance spot welding method according to the present embodiment includes a configuration process S10 and a welding process S20. The resistance spot welding method according to the present embodiment is implemented, for example, using the resistance spot welding device 1 shown in FIG. 1. Figure 1

[0051] <Configuration process>

[0052] In this process, the workpiece W, which is obtained by laminating the first steel sheet PI and the second steel sheet P2 in the thickness direction, is placed on the base 31 of the compression mechanism 3 so that the first steel sheet PI is positioned below.

[0053] Further, the shape of a die for press forming the first steel sheet PI can be adjusted so that the compression stress is generated in the first steel sheet PI. Thus, the compression stress can be applied to the first steel sheet PI in advance.

[0054] <Welding process>

[0055] In this process, the laminated first steel sheet PI and the second steel sheet P2 are subjected to welding by the resistance spot welding device 1. The welding process S20 includes a compression process S21, a power supply process S22, a cooling process S23, and a release process S24.

[0056] (Compression process)

[0057] In this process, the first steel sheet PI is compressed in a direction intersecting the thickness direction by the first pressing portion 32 and the second pressing portion 33 of the compression mechanism 3 before the power supply process S22 (i.e., before the welding is started).

[0058] (Power supply process)

[0059] ​​​In this process, the workpiece W is sandwiched by the first electrode 21 and the second electrode 22 in a state where the first steel sheet P1 is compressed by the compression mechanism 3. Then, the first electrode 21 and the second electrode 22 are energized, and welding of the workpiece W is performed while the first steel sheet P1 is compressed.

[0060] (cooling process)

[0061] In this process, after the energization between the first electrode 21 and the second electrode 22 is stopped, the welded portion of the workpiece W is cooled in a state where the first steel sheet P1 is compressed by the compression mechanism 3.

[0062] (releasing process)

[0063] In this process, when the cooling of the welded portion of the workpiece W is completed, the compression applied to the first steel sheet P1 by the compression mechanism 3 is released.

[0064] [1-3. Effects]

[0065] According to the above-described embodiments, the following effects can be obtained.

[0066] (1a) The workpiece W is welded in a state where the first steel sheet P1, which is a high tensile strength steel sheet, has a compressive stress. As a result, a tensile stress generated after welding is canceled by the compressive stress, thereby suppressing cracking of the first steel sheet P1.

[0067] (1b) The first steel sheet P1 is compressed until the cooling of the workpiece W is completed, whereby the first steel sheet P1 continuously has a compressive stress until the cooling, in which a tensile stress is not generated, is completed. Therefore, the suppression effect on cracking can be promoted.

[0068] (1c) The first steel sheet P1 is compressed before the welding of the workpiece W is started, whereby the welding of the workpiece W can be started in a state where the first steel sheet P1 is dynamically stable. Thus, the welding quality can be improved.

[0069] (1d) The first steel sheet P1 is sandwiched by the first pressing portion 32 and the second pressing portion 33 that face each other, whereby the first steel sheet P1 can be stably compressed.

[0070] [2. Other embodiments]

[0071] The embodiments of the present disclosure have been described above, but the present disclosure is not limited to the above-described embodiments and can be implemented in various ways.

[0072] (2a) In the resistance spot welding device and the resistance spot welding method of the above-described embodiments, the workpiece can not have a protrusion and a gap. For example, the workpiece can be a workpiece in which a plurality of flat steel sheets are seamlessly stacked.

[0073] (2b) In the resistance spot welding device and the resistance spot welding method of the above embodiment, the compression mechanism can start compressing the first steel sheet at the same time as the start of welding, or can start compressing the first steel sheet after the start of welding. In addition, the compression mechanism can release the compression of the first steel sheet at the same time as the end of welding (i.e., the stop of the application of current), or can release the compression of the first steel sheet during the cooling process.

[0074] (2c) In the resistance spot welding device and the resistance spot welding method of the above embodiment, the workpiece can have three or more steel sheets. That is, one or more steel sheets can be arranged between the first steel sheet and the second steel sheet.

[0075] (2d) In the resistance spot welding device and the resistance spot welding method of the above embodiment, the first steel sheet can be stacked on the second steel sheet. In addition, the direction in which the first electrode and the second electrode face each other (i.e., the direction in which the workpiece is sandwiched) is not limited to the vertical direction. For example, the first electrode and the second electrode can be configured to sandwich the workpiece in the horizontal direction.

[0076] (2e) The functions of one constituent element in the above embodiment can be shared by a plurality of constituent elements, or the functions of a plurality of constituent elements can be integrated into one constituent element. Also, a part of the configuration of the above embodiment can be omitted. In addition, at least a part of the configuration of the above embodiment can be added to the configuration of the other embodiment, or at least a part of the configuration of the above embodiment can be replaced with the configuration of the other embodiment, and the like. In addition, all modes included in the technical idea determined by the statements recited in the claims are embodiments of the present disclosure.

Claims

1. A resistance spot welding method including a welding process of performing welding on a workpiece made of a plurality of steel sheets by a resistance spot welding device, characterized by, the resistance spot welding device having: a first electrode configured to be in contact with the first steel sheet, wherein the first steel sheet being a high tensile strength steel sheet among the plurality of steel sheets; and a second electrode configured to contact a second steel sheet and sandwich the workpiece with the first electrode, wherein the second steel sheet is a steel sheet having a smaller tensile strength than the first steel sheet among the plurality of steel sheets, and in the welding process, the first steel sheet is compressed in a direction intersecting a thickness direction of the first steel sheet at least from a welding process of the workpiece to an end of the welding of the workpiece, in the welding process, the first steel sheet is compressed in the direction intersecting the thickness direction of the first steel sheet until an end of cooling after the welding of the workpiece, in the welding process, the first steel sheet is compressed in the direction intersecting the thickness direction of the first steel sheet by a first pressing portion and a second pressing portion opposed to each other sandwiching the first steel sheet.

2. The resistance spot welding method according to claim 1, characterized by, in the welding process, the first steel sheet is compressed in the direction intersecting the thickness direction of the first steel sheet before starting the welding on the workpiece.

3. A resistance spot welding device configured to perform welding on a workpiece, wherein, the workpiece is made of a plurality of steel sheets, the resistance spot welding device characterized by having: a first electrode configured to contact a first steel sheet, wherein the first steel sheet is a high tensile strength steel sheet among the plurality of steel sheets; a second electrode configured to contact a second steel sheet and sandwich the workpiece with the first electrode, wherein the second steel sheet is a steel sheet having a smaller tensile strength than the first steel sheet among the plurality of steel sheets; and a compression mechanism configured to compress the first steel sheet in a direction intersecting a thickness direction of the first steel sheet at least from a welding process of the workpiece to an end of the welding of the workpiece, the compression mechanism configured to compress the first steel sheet in the direction intersecting the thickness direction of the first steel sheet until an end of cooling after the welding of the workpiece, the compression mechanism having a first pressing portion and a second pressing portion opposed to each other, the first pressing portion and the second pressing portion configured to compress a welding portion of the first steel sheet in the direction intersecting the thickness direction of the first steel sheet by sandwiching the first steel sheet.

4. The resistance spot welding device according to claim 3, characterized by, the compression mechanism configured to compress the first steel sheet in the direction intersecting the thickness direction of the first steel sheet before starting the welding on the workpiece.

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