Method for manufacturing resistance spot-weld joint

Abstract

To provider a spot-welding method capable of suppressing LME splitting.SOLUTION: In a method for manufacturing a resistance spot-weld joint 1, an applied pressure lowering time T2 indicating a length of a period from a time point b when a pair of electrodes X are started to be moved in a direction of separating from each other to a time point c when a pressure applied to a material to be welded by the pair of electrodes X becomes 10% of the applied pressure at the time of terminating energizing a welding current is set to 18×t2-65×t+109 or higher (t: total plate thickness of material to be welded by unit mm) by a unit msec, or an average electrode releasing speed during a period from the time point b when the pair of electrodes X are started to be moved in the direction of separating from each other to the time point c when the pressure applied to the material to be welded by the pair of electrodes X becomes 10% of the applied pressure at the time of terminating energizing the welding current is set to 30 mm / sec or lower.SELECTED DRAWING: Figure 1-2

Description

[Technical field]

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

[0002] In recent years, in the automotive industry, there has been a demand for lighter car bodies to improve fuel efficiency and reduce CO2 emissions, and for stronger car body components to improve collision safety. To meet these demands, it is effective to use high-strength steel plates for car body components and various parts. Resistance spot welding is mainly used for assembling car bodies and installing parts.

[0003] Resistance spot welding is a type of resistance welding in which overlapping workpieces are clamped between the tips of electrodes with appropriately shaped tips, and the welding current and pressure are concentrated in a relatively small area to locally heat the area while simultaneously applying pressure with the electrode. In resistance spot welding, the molten and solidified part of the weld is called a nugget.

[0004] However, when resistance spot welding is performed on high-strength plated steel sheets, there is a problem that LME (Liquid Metal Embrittlement) cracks occur directly under the electrode and directly outside the corona bond. Corona bond refers to the solid-phase welded ring-shaped part that occurs around the nugget mentioned above.

[0005] LME cracking occurs when the heat generated during resistance spot welding melts the plating components of the plating layer, causing the alloy components to penetrate into the grain boundaries of the steel sheet structure at the weld, and tensile stress acts on the welded parts. LME cracking is more pronounced under conditions where welding disturbances such as gaps, strike angles, and clearances occur. In automobile bodies, LME cracking at welded parts reduces the strength of the welded parts, and there is a known technology to suppress LME cracking at welded parts by controlling the speed of the resistance spot welding sequence.

[0006] For example, Patent Document 1 provides a spot welding method for welded members composed of multiple steel plates with at least the weld portion overlapped, characterized in that at least the overlapping surface of the weld portion of at least one of the multiple steel plates is coated with a zinc-based plating, the multiple steel plates have a total plate thickness t (mm) of 1.35 mm or more, and a post-weld holding time Ht (seconds) from the end of welding current flow between the welding electrodes to the time when the welding electrodes and the welded members are no longer in contact with each other is within the range of the following formula (A). 0.015t 2 +0.020≦Ht≦0.16t 2 -0.40t+0.70···(A)

[0007] Patent Document 2 also provides a drive control method for a welding gun arm, which is a method for controlling the operation of a motor that drives a gun arm for welding a workpiece, and which is characterized in that in a series of operations in which the gun arm pressurizes the workpiece and then releases this pressure to displace the gun arm in a direction away from the workpiece, the acceleration and maximum speed of the motor in the release stage in which the gun arm releases the pressure on the workpiece are set to smaller values ​​than in the displacement stage in which the gun arm displaces in a direction away from the workpiece. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2017-47475 A [Patent Document 2] Japanese Patent Application Publication No. 11-267852 Summary of the Invention [Problem to be solved by the invention]

[0009] However, in the technology of Patent Document 1, the pressure holding time from the end of welding current to when the electrode and the welded member start to separate is long, and the electrode release time from the pressure holding time to when the welding electrode and the welded member are not in contact with each other is short. In Patent Document 1, the holding time is said to be the period from the end of welding current to when the welding electrode and the welded member are not in contact with each other, but in a normal spot welding device, the welding electrode is released quickly. In addition, Patent Document 1 does not consider the release speed of the welding electrode at all. Therefore, it is considered that the holding time described in Patent Document 1 is almost the pressure holding time from the end of welding current to when the electrode and the welded member start to separate. If the pressure holding time is long, when producing parts by spot welding, the welding time per point becomes long, and productivity decreases. In addition, in the technology of Patent Document 2, when spot welding plated steel sheets, if the electrode release speed is fast, the residual stress of the welded part increases, and LME cracking occurs.

[0010] The present invention has been made in consideration of the above-mentioned circumstances, and has an object to provide a manufacturing method for a resistance spot welded joint that can suppress LME cracking. [Means for solving the problem]

[0011] The gist of the present invention is as follows.

[0012] (1) A method for welding a workpiece consisting of a plurality of overlapping steel sheets and having a total sheet thickness t in mm, comprising the steps of: sandwiching the workpiece between a pair of electrodes and applying a pressure; passing a welding current to the pair of electrodes to form a nugget; and moving the pair of electrodes in a direction away from each other to release the pair of electrodes, wherein at least one of the workpieces is a plated steel sheet, and immediately before the pressure is applied to the workpieces, any one or more of the following conditions is satisfied: a clearance of 0.1 mm or more, an impact angle of 0.5° or more, and a sheet gap of 0.1 mm or more; and a pressure force drop time, which is the length of the period from when the pair of electrodes start to move in the direction away from each other to when the pressure applied to the workpieces by the pair of electrodes becomes 10% of the pressure force at the time when the passage of the welding current is completed, is expressed in msec as 18×t 2 A manufacturing method for resistance spot welded joints with a joint strength of -65×t+109 or more. (2) The method for producing a resistance spot welded joint according to (1), characterized in that the pressure drop time, in msec, is 18×t2-65×t+214 or more. (3) A method for manufacturing a resistance spot welded joint, comprising the steps of: sandwiching a workpiece consisting of a plurality of overlapping steel sheets and having a total sheet thickness t in mm between a pair of electrodes and applying a pressure; passing a welding current to the pair of electrodes to form a nugget; and releasing the pair of electrodes by moving the pair of electrodes in a direction away from each other, wherein at least one of the workpieces is a plated steel sheet, and immediately before the pressure is applied to the workpieces, one or more of the following conditions are satisfied: a clearance of 0.1 mm or more, an impact angle of 0.5° or more, and a sheet gap of 0.1 mm or more, and the average electrode release speed during the period from when the pair of electrodes start to move in the direction away from each other to when the pressure applied by the pair of electrodes to the workpieces becomes 10% of the pressure at the end of the passage of the welding current is 30 mm / sec or less. (4) The method for producing a resistance spot welded joint according to (3), wherein the average electrode release speed is set to 10 mm / sec or less. (5) A method for manufacturing a resistance spot welded joint according to any one of (1) to (4), characterized in that immediately before the application of the pressure to the workpiece, one or more of the following is satisfied: the clearance is 0.2 mm or more, the impact angle is 1° or more, and the plate gap is 0.3 mm or more. (6) A method for manufacturing a resistance spot welded joint according to any one of (1) to (5), characterized in that a holding time, which is the length of the period from the time when the flow of the welding current is terminated to the time when the pair of electrodes start to move in the direction away from each other, is 100 msec or less. Effect of the Invention

[0013] According to the present invention, it is possible to provide a manufacturing method for a resistance spot welded joint that is capable of suppressing LME cracking. [Brief description of the drawings]

[0014] [Figure 1-1] FIG. 1 is a schematic diagram of a method for manufacturing a resistance spot welded joint. [Figure 1-2] FIG. 2 is a diagram showing a resistance spot welding sequence. [Diagram 2] FIG. 4 is a graph showing changes in applied pressure over time in resistance spot welding performed at various average electrode opening speeds in Example 1. [Diagram 3] 1 shows the results of cross-sectional observation in which the steel plate after resistance spot welding in Example 2 was cut in the plate thickness direction and the cross section was observed. [Figure 4] 1 shows the results of cross-sectional observation in which the steel plate after resistance spot welding in Example 3 was cut in the plate thickness direction and the cross section was observed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] When resistance spot welding is performed on workpieces including plated steel sheets, if the electrode is released rapidly as in the past, residual stress in the weld increases. This causes LME cracking due to components penetrating into the grain boundaries. Therefore, in this invention, we focused on reducing residual stress in order to improve LME cracking even with a short holding time.

[0016] The inventors have found that by slowly releasing the electrodes after the passage of the welding current is terminated, the impact on the weld can be mitigated, the residual stress generated can be reduced, the plating components of the plating layer can be prevented from penetrating into the grain boundaries, and LME cracking can also be prevented directly below the electrodes and directly outside the corona bond. The inventors have also found that, as specific means for slowly releasing the electrodes, for example, setting the pressure drop time to a predetermined value or more and setting the average electrode release speed to a predetermined value or less are effective.

[0017] The manufacturing method of the resistance spot welded joint according to the first embodiment and the second embodiment will be described in detail below. The manufacturing method of the resistance spot welded joint according to the first embodiment is characterized in that the pressure drop time is set to a predetermined value or more, and the manufacturing method of the resistance spot welded joint according to the second embodiment is characterized in that the average electrode release speed is set to a predetermined value or less. Below, first, the common points between the first embodiment and the second embodiment will be described, and then the first embodiment and the second embodiment will be described separately.

[0018] A method for manufacturing a resistance spot welded joint will be described with reference to the schematic diagram of FIG. 1-1 and the resistance spot welding sequence of FIG. 1-2.

[0019] The manufacturing method of a resistance spot welded joint includes a step S1 of clamping a workpiece consisting of multiple overlapping steel plates 11 between a pair of electrodes X and applying a pressure, and a subsequent step S2 of passing a welding current through the pair of electrodes X to form a nugget 12.

[0020] At least one of the steel sheets 11 constituting the workpiece is a plated steel sheet. Plated steel sheets have high corrosion resistance. Therefore, by using plated steel sheets for at least one of the workpieces of the resistance spot welded joint 1, the corrosion resistance of the resistance spot welded joint 1 can be improved. The type of plating on the plated steel sheet is not particularly limited, and may be, for example, hot-dip galvanizing or alloyed hot-dip galvanizing. The plating component is not limited to zinc-based, and the manufacturing method of the resistance spot welded joint according to the first embodiment and the second embodiment can be effectively used even when a plating layer (such as aluminum) that causes LME cracking is formed on the steel sheet. Examples of components that cause LME cracking in the steel sheet include Zn, Sn, Cu, Cd, In, Hg, Bi, Na, Cu-Pb, Cu-Sn, Zn-Sn, Cu-Pd, Cd-Zn, and Al-Sn-Cu. Incidentally, the plating causes LME cracking during spot welding, but in the manufacturing method of resistance spot welded joints, this problem is solved by slowly releasing electrode X.

[0021] The type of steel plate 11 is not particularly limited, but is preferably a high-strength steel plate having a tensile strength of, for example, 780 MPa or more. Also, for example, it is preferable that the carbon equivalent Ceq represented by the following formula is 0.15% or more. Ceq(%)=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 The thickness of the steel plates 11 is not particularly limited, and can be appropriately selected depending on the application of the resistance spot welded joint 1. The number of steel plates 11 is also not particularly limited. Although the number of steel plates 11 is two in Fig. 1-1, it may be three or more.

[0022] The manufacturing method for resistance spot welded joints includes a step S3 in which, after forming a nugget 12 and joining a plurality of steel sheets 11, the pair of electrodes X are moved in a direction away from each other to release the pair of electrodes X. Here, "releasing the electrodes X" refers to a series of operations from when the movement of the electrodes X starts until the electrodes X and the steel sheet 11 are separated and the applied pressure becomes zero. The average electrode release speed described later refers to the rate of change over time of the distance between the tips of the pair of electrodes X when the electrodes are released. Since the steel sheet 11 is plastically deformed by the application of pressure, when the electrodes X are slowly released, it takes some time from when the pair of electrodes X start to be moved in a direction away from each other until the applied pressure becomes zero.

[0023] In addition, the spot welding according to the first and second embodiments is performed under conditions that satisfy at least one of the following conditions immediately before starting the step S1 of applying pressure to the welded material: a clearance of 0.1 mm or more, an impact angle of 0.5° or more, and a plate gap of 0.1 mm or more. The clearance refers to an error when the workpiece is sandwiched using the electrodes X. Specifically, in spot welding that starts with both electrodes X separated from the steel plates 11 and moves the electrodes X closer to each other, the clearance is the gap between the other electrode X and the workpiece when one electrode X contacts the steel plate 11 and pressure is applied. The impact angle refers to the inclination of the mating surface of the steel plates 11 when a plane perpendicular to a straight line connecting the pair of electrodes X is set to 0°. The plate gap refers to the size of the gap between the steel plates 11 in the welded portion. When there are three or more steel plates 11 and two or more gaps, the plate gap refers to the total value of the size of the multiple gaps. Plate gaps, strike angles, clearances, etc. are collectively called welding disturbances.

[0024] When such welding disturbances exist, the residual stress of the welded portion is likely to increase. Therefore, when welding disturbances exist, LME cracking is highly likely to occur. Therefore, satisfying the above-mentioned conditions at the start of spot welding is disadvantageous from the viewpoint of suppressing LME cracking. On the other hand, in spot welding at manufacturing sites, welding disturbances may be unavoidable from the viewpoint of improving work efficiency. Therefore, the manufacturing method of the resistance spot welded joint is effective in the actual working environment at such manufacturing sites. Moreover, in the manufacturing method of the resistance spot welded joint according to the first and second embodiments, the electrode X is slowly released to solve the problem of LME cracking caused by welding disturbances.

[0025] (First embodiment) Above, the common components between the first embodiment and the second embodiment have been described. Next, the first embodiment will be described with reference to the resistance spot welding sequence shown in FIG.

[0026] In the manufacturing method for a resistance spot welded joint according to the first embodiment, the pressure drop time T2 is controlled to gradually release the electrodes X. The pressure drop time T2 is the length of the period from time b when the pair of electrodes X start to move away from each other to time c when the pressure applied by the pair of electrodes X to the workpiece becomes 10% of the pressure P at time a when the passage of the welding current ends. In FIG. 1-2, the symbol a is assigned to the time when the current value becomes 0, the symbol b is assigned to the time when the pressure starts to decrease, and the symbol c is assigned to the time when the pressure becomes 0.10×P. This is because the decrease in the pressure starts from the moment when the pair of electrodes X start to move away from each other.

[0027] In the first embodiment, the pressure drop time T2 is determined according to the plate thickness of the workpieces to be welded, because the stress caused by deformation of the workpieces when pressure is applied and the cooling rate of the workpieces to be welded change according to the plate thickness. The pressure drop time T2 is 18×t in msec. 2It is necessary to set the pressure drop time T2 to 18×t or more. Note that t is the total plate thickness of the workpiece in mm. This reduces the residual stress in the weld, and reduces LME cracking even under welding disturbance conditions. 2 If it is less than -65 × t + 109 (msec), the effect of reducing LME cracking cannot be sufficiently obtained, and LME cracking may occur even under small welding disturbance conditions.

[0028] The pressure drop time T2 is expressed in msec as 18 × t 2 It may be -65×t+214 or more. In this case, the effect of reducing LME cracking can be increased under welding disturbance conditions, and the effect of reducing LME cracking can be achieved even under large welding disturbance conditions. The longer the pressure drop time T2, the greater the effect of reducing LME cracking, but extending the pressure drop time T2 can reduce productivity. Therefore, it is preferable to set an appropriate upper limit value from the viewpoint of productivity.

[0029] The length of the period from time a when the welding current is stopped to time b when the pair of electrodes X start to move away from each other is referred to as the holding time T1. Conventionally, those skilled in the art have recognized that the longer the holding time T1, the more the residual stress in the weld can be reduced. However, in the manufacturing method of the resistance spot welded joint according to the first embodiment, the holding time T1 is not particularly essential and may be 0 msec. This is because LME cracking can be suppressed by setting the pressure drop time T2 to a predetermined value or more. Among LME cracks in spot welding, cracks immediately outside the pressure welded portion can be suppressed by increasing the hold time T1 using, for example, the method described in Patent Document 1. The manufacturing method for a resistance spot welded joint according to the first embodiment (and the manufacturing method for a resistance spot welded joint according to the second embodiment described later) can be effective in suppressing LME cracks immediately outside the corona bond in addition to LME cracks immediately outside the pressure welded portion.

[0030] Second embodiment Next, a second embodiment will be described with reference to the resistance spot welding sequence shown in FIG.

[0031] In the manufacturing method for a resistance spot welded joint according to the second embodiment, the average electrode opening speed is controlled to slowly open the electrodes X. The average electrode opening speed is the average value of the electrode opening speed during the period from time b when the pair of electrodes X start to move away from each other to time c when the pressure applied by the pair of electrodes X to the workpiece becomes 10% of the pressure P at time a when the passage of the welding current is terminated. The average electrode opening speed can be calculated by dividing the change in the distance between the tips of the electrodes X from time b to time c by the time from time b to time c. The time from time b to time c refers to the above-mentioned pressure force drop time T2.

[0032] In the manufacturing method of the resistance spot welded joint according to the second embodiment, the average electrode release speed is set to 30 mm / sec or less. If the average electrode release speed is 30 mm / sec or less, the residual stress in the welded portion can be reduced, and LME cracking can be reduced even under welding disturbance conditions.

[0033] The average electrode release speed may be set to 10 mm / sec or less, which can further increase the effect of reducing LME cracking.

[0034] In the manufacturing method of the resistance spot welded joint according to the second embodiment, the hold time T1 is not particularly essential and may be 0 msec. This is because LME can be suppressed by setting the average electrode opening speed to a predetermined value or more.

[0035] Hereinafter, more preferred aspects of the manufacturing methods for the resistance spot welded joint according to the first and second embodiments will be described. Unless otherwise specified, the aspects described below can be applied to both the manufacturing methods for the resistance spot welded joint according to the first and second embodiments.

[0036] The manufacturing method of the resistance spot welded joint of the first and second embodiments may be performed under conditions that satisfy at least one of the following conditions immediately before applying a pressure to the workpiece: a clearance of 0.2 mm or more, an impact angle of 1° or more, and a plate gap of 0.3 mm or more. Even under such severe welding disturbance conditions, the manufacturing method of the resistance spot welded joint of the first and second embodiments can exhibit the effect of reducing LME cracking. This allows for a margin in the forming accuracy of the workpiece, and reduces the defect rate. In addition, the welding equipment can be further simplified, the welding preparation time can be shortened, and the burden on the worker can be reduced. Although there are no particular limitations on the upper limits of disturbance, it is preferable that the clearance be 2.0 mm or less, the strike angle be 5° or less, and the plate gap be 2.0 mm or less, for example.

[0037] The holding time T1 may be 100 msec or less. This can shorten the tact time and further improve manufacturing efficiency. When the holding time is 100 msec or less, LME cracking is particularly likely to occur in normal resistance spot welding. Therefore, in the prior art, the holding time T1 is set to more than 100 msec, and the plating is solidified before the electrodes are released, thereby reducing the risk of LME cracking. However, the manufacturing methods for the resistance spot welded joints of the first and second embodiments do not require the holding time T1, and even if the holding time T1 is 100 msec or less, there is little risk of LME cracking. Therefore, in situations where the tact time should be shortened, the manufacturing methods for the resistance spot welded joints of the first and second embodiments are particularly effective. The value of the welding current in step S2 for passing a welding current to form nugget 12 can be appropriately set so as to prevent the generation of spatter and obtain a desired nugget diameter. The value of the welding current is not particularly limited, but is, for example, 6.0 to 8.0 kA. EXAMPLES

[0038] The effects of one embodiment of the present invention will be explained more specifically with reference to an example. However, the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0039] Example 1 A resistance spot welding test was performed on multiple overlapping steel plates using a spot welding device with a pair of electrodes. At this time, the behavior of the welding device was confirmed using a current pressure sensor. For welding, a stationary servo pressure was used as the welding device, and two sheets of GA980TRIP (sheet thickness: 1.4, 1.6, 2.0 mm) of the same thickness were used as the steel plates. In the test, the average electrode release speed was changed to 1, 10, 20, 30, 50, and 100 (mm / sec), which changed the pressure force drop speed. An example of the relationship between each average electrode release speed after current application and the pressure force is shown in Figure 2. The cycle in Figure 2 means 1 / 50 sec (20 msec). The welding conditions, disturbance conditions, and the presence or absence of LME cracks just outside the corona bond after welding are shown in Table 1 below. The lower limit value of formula (1) and the lower limit value of formula (2) in Table 1 are values ​​calculated by substituting the total sheet thickness t into the following formulas (1) and (2). 18×t 2 -65×t+109: (1) formula 18×t 2 -65×t+214: (2) formula

[0040] [Table 1]

[0041] Whether or not LME cracking occurred immediately outside the corona bond was judged by cutting the steel plate in the plate thickness direction so as to include the nugget and observing the cross section. As shown in Table 1, there were no LME cracks in Nos. 2, 6, 7, 9, 10, 11, 12, 13, 17, 18, and 19, which had an average electrode release speed of 30 mm / sec or less. 2-65×t+109 (more than msec). In particular, in No. 10, 11, 17, 18, and 19, the pressure drop time was 18×t 2 When the welding was performed at -65×t+214 or higher, no LME cracking occurred, despite the severe welding disturbance conditions.

[0042] Example 2 In Example 2, a resistance spot welding test was performed. Just before applying a pressure to the workpiece, the impact angle was 3° and the clearance was 0.3 mm. Two sheets of GA980TRIP (sheet thickness: 1.6 mm) of the same thickness were used. A servo pressure single-phase AC welding machine was used. The welding conditions were: pressure: 400 kgf, squeeze time: 600 msec (30 cyc), welding time: 400 msec (20 cyc), hold time T1: 80 msec (4 cyc, set to 1 cyc). The squeeze time is the time from when the electrode pressure command signal is issued to when the welding current starts to flow.

[0043] In the test, two types of welding currents, 7.0kA and 7.5kA, were applied to each plate pair. At this time, two types of tests were conducted for each welding current: an example of the invention with a pressure drop time of 191msec and an average electrode release speed of 10mm / sec, and a comparative example with a pressure drop time of 58msec and an average electrode release speed of 100mm / sec. The cross-sectional observation results are shown in Figure 3. As can be seen from Figure 3, no LME cracks were found just outside the corona bond in the steel sheets tested as inventive examples. On the other hand, LME cracks were observed just outside the corona bond in the steel sheets tested as comparative examples. The LME cracks refer to the areas surrounded by dotted lines in Figure 3.

[0044] Example 3 In Example 3, a resistance spot welding test was performed by changing the welding current to 6.0 kA, 6.5 kA, 7.0 kA, 7.5 kA, and 8.0 kA. Just before applying pressure to the workpiece, the impact angle was 3° and the clearance was 0.3 mm. Two sheets of GA980TRIP (thickness: 1.6 mm) with the same thickness were used. In addition, a servo pressure single-phase AC welding machine was used. The welding conditions were as follows: pressure: 400 kgf, squeeze time: 600 msec (30 cyc), welding time: 400 msec (20 cyc), holding time T1: 80 msec (4 cyc, set to 1 cyc).

[0045] At this time, for each welding current, tests were conducted on an example of the invention with a pressure drop time of 191 msec and an average electrode release speed of 10 mm / sec, and on a comparative example with a pressure drop time of 58 msec and an average electrode release speed of 100 mm / sec. The cross-sectional observation results are shown in Figure 4. As can be seen from Figure 4, no LME cracks were found just outside the corona bond in the steel sheets tested as inventive examples. On the other hand, LME cracks were observed just outside the corona bond in the steel sheets tested as comparative examples. The LME cracks refer to the area surrounded by the dotted line in Figure 4. [Industrial Applicability]

[0046] According to the present invention, it is possible to provide a manufacturing method for resistance spot welded joints that can suppress LME cracking by slowing down the average electrode release speed even if the pressure holding time is short. In addition, since the pressure holding time is short, the tact time is also short, and it is expected to improve productivity in the automotive industry. Therefore, the present invention has high industrial applicability. [Explanation of symbols]

[0047] 1 Resistance spot welded joints 11 Steel plate 12. Nugget X electrode a) The point at which the welding current is stopped b) The point at which the pair of electrodes start to move away from each other c The point at which the pressure applied by the pair of electrodes to the workpiece becomes 10% of the pressure at the end of the welding current. T1 retention time T2 Pressure drop time P is the welding force at the end of the welding current.

Claims

1. A process of applying a pressure to a workpiece having a total plate thickness t (unit: mm) composed of a plurality of overlapping steel plates, by sandwiching the workpiece between a pair of electrodes; applying a welding current to the pair of electrodes to form a nugget; releasing the pair of electrodes by moving the pair of electrodes away from each other; Equipped with At least one of the workpieces is a plated steel sheet, Immediately before applying the pressure to the workpiece, one or more of the following conditions is satisfied: a clearance of 0.1 mm or more, a strike angle of 0.5° or more, and a plate gap of 0.1 mm or more; The average electrode release speed during the period from when the pair of electrodes start to move in the direction away from each other to when the pressure applied by the pair of electrodes to the workpiece becomes 10% of the pressure applied at the end of the passage of the welding current is set to 30 mm / sec or less. A method for manufacturing resistance spot welded joints.

2. The method for manufacturing a resistance spot welded joint according to claim 1, wherein the average electrode release speed is set to 10 mm / sec or less.

3. 3. The method for manufacturing a resistance spot welded joint according to claim 1, wherein immediately before the application of the pressure to the workpiece, one or more of the following is satisfied: the clearance is 0.2 mm or more, the impact angle is 1° or more, and the plate gap is 0.3 mm or more.

4. 4. The method for producing a resistance spot welded joint according to claim 1, wherein a holding time, which is a length of a period from the time when the flow of the welding current is terminated to the time when the pair of electrodes start to move in the direction away from each other, is 100 msec or less.

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