Resistance spot welding method and resistance spot welding device

Abstract

To provide a technique that can achieve high-quality welding, in resistance spot welding to a work-piece.SOLUTION: A work-piece having a plurality of metal plates overlapped with each other is welded using a resistance spot welding device. First energization control is performed to a pair of electrodes so that first current flow between the pair of electrodes holding the work-piece. Subsequently, second energization control is performed to the pair of electrodes so that the current flowing between the pair of electrodes reduces from the first current to second current smaller than the first current. Subsequently, third energization control is performed to the pair of electrodes so that the current flowing between the pair of electrodes increases from the second current to third current larger than the second current. Subsequently, fourth energization control is performed to the pair of electrodes so that fourth current flows between the pair of electrodes.SELECTED DRAWING: Figure 3

Description

Technical Field

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

Background Art

[0002] Regarding resistance spot welding, in order to efficiently form a weld nugget while suppressing the generation of spatter, a technique of flowing a constant current before this energization is already known. Patent Document 1 discloses performing energization control so as to flow a constant current for a certain period after initial energization in which the welding current is gradually increased, and then flowing an even larger constant current.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Regarding resistance spot welding, when welding a workpiece in which a plurality of metal plates are stacked, the welding quality may deteriorate due to variations in heat generation between the metal plates. Variations in heat generation can be caused by differences in resistance between the metal plates. The resistance difference can be caused by, for example, differences in tensile strength or plate thickness between the metal plates.

[0005] When there are variations in heat generation between the metal plates, non-uniform weld nuggets are formed in the workpiece, and insufficient penetration may occur for some of the metal plates.

[0006] Therefore, according to one aspect of the present disclosure, it is desirable to provide a technique capable of achieving high-quality welding with respect to resistance spot welding of a workpiece in which a plurality of metal plates are stacked.

Means for Solving the Problems

[0007] According to one aspect of the present disclosure, there is provided a resistance spot welding method for welding a work in which a plurality of metal plates are stacked, using a resistance spot welding apparatus. The resistance spot welding apparatus includes a pair of electrodes that sandwich the work on both sides in the stacking direction of the plurality of metal plates.

[0008] The resistance spot welding method includes performing first energization control on the pair of electrodes so that a first current flows between the pair of electrodes sandwiching the work. The resistance spot welding method may further include performing second energization control on the pair of electrodes so that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current. The second energization control may be performed following the first energization control.

[0009] The resistance spot welding method may further include performing third energization control on the pair of electrodes so that the current flowing through the pair of electrodes increases from the second current to a third current greater than the second current. The third energization control may be performed following the second energization control.

[0010] The resistance spot welding method may further include performing fourth energization control on the pair of electrodes so that a fourth current flows between the pair of electrodes. The fourth energization control may be performed following the third energization control. The fourth current may be a constant current. The first current may be a constant current smaller than the fourth current. The third current may be a current greater than the fourth current.

[0011] According to this resistance spot welding method, by the first and second energization controls, it is possible to promote the warping of the metal plates so that the current conduction path concentrates in a region where the path is narrow. Further, by the third energization control, by temporarily flowing a large current, it is possible to promote the shunt to the metal plate surface and realize melting over a wide range in the stacking direction. Also, by suppressing the variation in the material properties in the stacking direction due to this melting, it is possible to suppress the variation in melting between the plurality of metal plates when finishing the welding of the work in the fourth energization control.

[0012] Therefore, according to this resistance spot welding method, high-quality welding of a workpiece in which a plurality of metal plates are stacked can be achieved. Furthermore, since the third energization control is performed, in the fourth energization control, the fourth current can be suppressed to a relatively small current to achieve good welding. Therefore, according to this resistance spot welding method, high-quality welding can be achieved while suppressing spatter.

[0013] According to one aspect of the present disclosure, the plurality of metal plates may include at least two metal plates having different resistances, tensile strengths, or plate thicknesses from each other. When the plurality of metal plates include metal plates with different resistances, heat generation and melting may vary among the plurality of metal plates due to the difference in resistance.

[0014] Metal plates with different strengths also have different resistances from each other. Metal plates with different plate thicknesses also have different resistances from each other. Therefore, even when the plurality of metal plates include metal plates with different tensile strengths or plate thicknesses, similarly, melting variations may occur among the plurality of metal plates.

[0015] According to one aspect of the present disclosure, by the above-described first, second, third, and fourth energization controls, variations in melting among a plurality of metal plates due to differences in resistance, tensile strength, or plate thickness can be suppressed, and the welding quality is improved.

[0016] According to one aspect of the present disclosure, the workpiece may include a workpiece in which two metal plates located at both ends in the stacking direction among the plurality of metal plates have different resistances, tensile strengths, or plate thicknesses from each other. The above-described resistance spot welding method is effective in improving the welding quality for such a workpiece.

[0017] According to one aspect of the present disclosure, the work may include a work in which a plurality of metal plates are stacked such that the resistance, tensile strength, or plate thickness increases or decreases in the stacking direction. According to one aspect of the present disclosure, among the plurality of metal plates, the tensile strength of the first metal plate located at the first end in the stacking direction is different from the tensile strength of the second metal plate located at the center in the stacking direction, and the tensile strength of the third metal plate located at the second end, which is the end opposite to the first end in the stacking direction, is smaller than the sum of the tensile strengths of the first metal plate and the second metal plate, and the tensile strength of the second metal plate located at the center in the stacking direction is the tensile strength of one metal plate sandwiched between the first metal plate and the third metal plate located at both ends in the stacking direction among the plurality of metal plates, or the sum of the tensile strengths of one or more metal plates sandwiched between the first metal plate and the third metal plate. The above-described resistance spot welding method can improve the welding quality for a work that satisfies such conditions.

[0018] According to one aspect of the present disclosure, the ratio H1 / H2 of the total plate thickness H1, which is the sum of the thicknesses of the plurality of metal plates in the stacking direction of the work, to the thickness H2 of the metal plate with the smaller thickness among the two metal plates located at both ends in the stacking direction of the plurality of metal plates may be 3.5 or more. The above-described resistance spot welding method can improve the welding quality for a work that satisfies such conditions.

[0019] According to one aspect of the present disclosure, the work may include a work having a strength difference between both sides of 445 MPa or more. According to one aspect of the present disclosure, the work may include a work having a strength difference between both sides of 255 MPa or more and a strength ratio of 4.29 or more. Here, the strength difference between both sides is the difference in tensile strength between the two metal plates located at both ends in the stacking direction in the work, and the strength ratio is a value obtained by dividing the sum of the tensile strengths of the plurality of metal plates in the work by the tensile strength of the metal plate with the smaller tensile strength among the two metal plates located at both ends in the stacking direction. The above-described resistance spot welding method can improve the welding quality for a work that satisfies such conditions.

[0020] According to one aspect of the present disclosure, the plurality of metal plates may include high-tensile steel plates. According to the resistance spot welding method of the present disclosure, the welding quality for a workpiece including high-tensile steel plates can be improved.

[0021] According to one aspect of the present disclosure, a resistance spot welding apparatus for welding a workpiece in which a plurality of metal plates are stacked may be provided. The resistance spot welding apparatus may include a pair of electrodes and a control unit. The pair of electrodes may be arranged so as to sandwich the workpiece on both sides in the stacking direction of the plurality of metal plates.

[0022] The control unit may be configured to control energization between the pair of electrodes. The control unit may perform first energization control for the pair of electrodes so that a first current flows between the pair of electrodes sandwiching the workpiece.

[0023] The control unit may perform second energization control for the pair of electrodes so that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current. The second energization control may be performed following the first energization control.

[0024] The control unit may perform third energization control for the pair of electrodes so that the current flowing through the pair of electrodes increases from the second current to a third current larger than the second current. The third energization control may be performed following the second energization control.

[0025] The control unit may perform fourth energization control for the pair of electrodes so that a fourth current flows between the pair of electrodes. The fourth energization control may be performed following the third energization control. The fourth current may be a constant current. The first current may be a constant current smaller than the fourth current. The third current may be a current larger than the fourth current.

[0026] According to this resistance spot welding apparatus, the welding quality can be improved in the same manner as the above-described resistance spot welding method.

Brief Description of the Drawings

[0027]

Figure 1

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Figure 3

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Figure 9

DETAILED DESCRIPTION OF THE INVENTION

[0028] Exemplary embodiments of the present disclosure will be described below with reference to the drawings. The resistance spot welding apparatus 1 shown in FIG. 1 is configured to weld a workpiece W in which a plurality of metal plates are stacked by resistance spot welding.

[0029] The workpiece W can include a plurality of steel plates as the plurality of metal plates. The workpiece W can include at least two steel plates having different resistances. The workpiece W can include at least two steel plates having different tensile strengths. The workpiece W can include at least two steel plates having different plate thicknesses. The higher the tensile strength of the steel plate, the greater the resistance. The greater the plate thickness of the steel plate, the greater the resistance. The resistance referred to in this specification is electrical resistance.

[0030] The exemplary work W shown in FIG. 1 is a work in which three steel plates are stacked, and includes a first steel plate P1, a second steel plate P2, and a third steel plate P3.

[0031] Although not limited, the first steel plate P1 can be, for example, a hot-dip galvanized steel plate. For example, the first steel plate P1 can be a high-tensile steel plate with a tensile strength of 440 MPa (megapascals) or more. For example, the first steel plate P1 can be a steel plate of SCGA440 standard with a thickness of 1.4 mm.

[0032] Although not limited, the second steel plate P2 can be, for example, a hot-dip galvanized steel plate. For example, the second steel plate P2 can be a high-tensile steel plate with a tensile strength of 1180 MPa or more. For example, the second steel plate P2 can be a steel plate of SCGA1180 standard with a thickness of 1.4 mm. Such a high-tensile steel plate with extremely high tensile strength is also called a super high-tensile material.

[0033] Although not limited, the third steel plate P3 can be, for example, a cold-rolled steel plate. For example, the third steel plate P3 can be a high-tensile steel plate with a tensile strength of 1470 MPa or more. For example, the third steel plate P3 can be a steel plate of SPC1470 standard with a thickness of 2 mm.

[0034] In the exemplary work W shown in FIG. 1, the first steel plate P1, the second steel plate P2, and the third steel plate P3 are stacked in order. Hereinafter, the direction in which a plurality of metal plates constituting the work W are arranged is expressed as the stacking direction. The stacking direction corresponds to the normal directions of the first steel plate P1, the second steel plate P2, and the third steel plate P3, and the normal direction of the work surface. The stacking direction corresponds to the thickness directions of the first steel plate P1, the second steel plate P2, and the third steel plate P3, and the thickness direction of the work.

[0035] The resistance spot welding apparatus 1 has a resistance welder 20. The resistance welder 20 welds a plurality of metal plates arranged as the work W in the stacking direction by resistance spot welding.

[0036] The resistance welder 20 includes 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 above the workpiece W so as to sandwich the workpiece W in the stacking direction together with the first electrode 21. The first electrode 21 is relatively movable in the vertical direction with respect to the second electrode 22.

[0037] The first electrode 21 and the second electrode 22 each come into contact with the workpiece W during welding. The first electrode 21 comes into contact with the third steel plate P3 which is a metal plate located at the lowermost layer of the workpiece W. The second electrode 22 comes into contact with the first steel plate P1 which is a metal plate located at the uppermost layer of the workpiece W. The first electrode 21 and the second electrode 22 sandwich the workpiece W so as to apply pressure to both sides in the stacking direction. In this state, a welding current is supplied between the first electrode 21 and the second electrode 22 and flows through the workpiece W. Due to the resistance heat generated by the welding current, the workpiece W is welded.

[0038] As an electrical system for the resistance welder 20, the resistance spot welding apparatus 1 includes a welding power source 30, a current sensor 40, and a control unit 50 as shown in FIG. 2. The welding power source 30 is configured to supply a welding current between the first electrode 21 and the second electrode 22. The current sensor 40 is provided in a line between the welding power source 30 and the first electrode 21 or the second electrode 22, detects the current I supplied between the first electrode 21 and the second electrode 22, and is configured to input the detection signal to the control unit 50. The current I is the above-described welding current.

[0039] The control unit 50 controls the energization between the first electrode 21 and the second electrode 22 by controlling the welding power source 30. Specifically, as energization control, the control unit 50 is configured to control the current I flowing between the first electrode 21 and the second electrode 22 so that the current I flowing between the first electrode 21 and the second electrode 22 changes according to the current profile shown in FIG. 3.

[0040] Specifically, the control unit 50 is configured to perform feedback control on the current I flowing between the first electrode 21 and the second electrode 22 based on the current I detected by the current sensor 40. Hereinafter, the first electrode 21 and the second electrode 22 are collectively referred to as a pair of electrodes 21, 22.

[0041] When a welding start instruction for the work W is input through an operation unit (not shown), the control unit 50 starts the control process shown in FIG. 4, and controls the current I between the pair of electrodes 21, 22 so that the melting current changes according to the current profile shown in FIG. 3. By this control, good resistance spot welding for the work W is realized.

[0042] Specifically, the control unit 50 performs first energization control on the pair of electrodes 21, 22 so that a first current I1 flows between the pair of electrodes 21, 22 that sandwich the work W during a period C1 from the welding start time point T0 of the work W to a first time point T1 when a predetermined time has elapsed (S110). The first current I1 is a constant current.

[0043] Subsequent to the first energization control (S110), the control unit 50 performs second energization control on the pair of electrodes 21, 22 so that the current I flowing between the pair of electrodes 21, 22 decreases from the first current I1 to a second current I2 that is smaller than the first current I1 during a period C2 from the first time point T1 to a second time point T2 (S120). The second current I2 is greater than zero.

[0044] Subsequent to the second energization control (S120), the control unit 50 performs third energization control on the pair of electrodes 21, 22 so that the current I flowing through the pair of electrodes 21, 22 increases from the second current I2 to a third current I3 that is greater than the second current during a period C3 from the second time point T2 to a third time point T3 (S130).

[0045] Furthermore, the control unit 50 performs third energization control on the pair of electrodes 21, 22 so that the current I flowing through the pair of electrodes 21, 22 maintains the third current I3 during a period C4 from the third time point T3 to a fourth time point T4 (S130).

[0046] Following the third energization control (S130), the control unit 50 performs fourth energization control on the pair of electrodes 21 and 22 so that a fourth current I4 smaller than the third current I3 flows between the pair of electrodes 21 and 22 during a period C5 from a fourth time point T4 to a fifth time point T5 (S140).

[0047] During the period C5 from the fourth time point T4 to the fifth time point T5, energization control is performed so that a constant current flows between the pair of electrodes 21 and 22 as the fourth current I4. Also, during the period C1 from the welding start time point T0 to the first time point T1, energization control is performed so that a constant current flows between the pair of electrodes 21 and 22 as the first current I1. However, the first current I1 is smaller than the fourth current I4 for reasons described later.

[0048] During the periods C3 and C4 from the second time point T2 to the fourth time point T4, for reasons described later, the current between the pair of electrodes 21 and 22 is controlled so that a current larger than the fourth current I4 flows between the pair of electrodes 21 and 22 as the third current I3.

[0049] The control unit 50 continuously performs such first, second, third, and fourth energization controls as a series of energization controls from the time point T0, and stops the energization between the pair of electrodes 21 and 22 at the fifth time point T5. Then, the control process ends. At the fifth time point T5, the resistance spot welding of the workpiece W is completed.

[0050] The purpose of the energization control according to the above-described current profile will be described. The energization from the welding start time point T0 to the first time point T1 in the present embodiment corresponds to the first stage of the initial energization, and is executed to heat the workpiece W to such an extent that the workpiece W does not melt due to resistance heating within the workpiece W. The first current I1 and the time length of the period C1 from the time point T0 to the time point T1 are determined within a range where the workpiece W does not melt.

[0051] FIG. 5 shows a specific example of a current profile that can be adopted when the workpiece W includes, as the first steel plate P1, a steel plate of SCGA440 standard with a thickness of 1.4 mm, as the second steel plate P2, a steel plate of SCGA1180 standard with a thickness of 1.4 mm, and as the third steel plate P3, a steel plate of SPC1470 standard with a thickness of 2 mm (hereinafter referred to as the target workpiece). The pressing force between the pair of electrodes 21, 21 is 5.51 kN (kilo-newton).

[0052] The horizontal axis shown in FIG. 5 represents time, and the vertical axis represents current I. The graph shown in FIG. 5 details the ratio of the time lengths between time points T0, T1, T2, T3, T4, T5 and the ratio of currents I1, I2, I3, I4 with the time and current I at the origin being zero.

[0053] FIG. 6A conceptually shows that the inside of the workpiece W is heated in the first stage of initial energization. The dashed line in FIG. 6A schematically shows the heated part in the workpiece W in the case where the resistance of the second steel plate P2 is higher than that of the first steel plate P1 and the resistance of the third steel plate P3 is higher than that of the second steel plate P2. The part indicated by the dashed line corresponds to the resistance center of the workpiece W. This example of the combination of the first steel plate P1, the second steel plate P2, and the third steel plate P3 includes the combination of the first steel plate P1, the second steel plate P2, and the third steel plate P3 in the above-mentioned target workpiece.

[0054] The first stage of initial energization is performed not only for the purpose of simply heating the resistance center but also for the purpose of causing warping in the plurality of metal plates constituting the workpiece W, thereby limiting the current-carrying path in the workpiece W to a narrow region.

[0055] The warping of the metal plate referred to here is such that the adjacent metal plates are separated in the stacking direction as they are farther from the center of the current-carrying path connecting the first electrode 21 and the second electrode 22 in the stacking direction.

[0056] FIG. 6A shows that due to the warp of the first steel plate P1, the first steel plate P1 and the second steel plate P2 are separated from each other in a region away from the center of the energization path. Similarly, due to the warp of the third steel plate P3, the second steel plate P2 and the third steel plate P3 are separated from each other in a region away from the center of the energization path.

[0057] Due to such separation, the energization path is limited, and current flows intensively inside the work W. Hereinafter, the intensive flow of current between a pair of electrodes 21 and 22 through a limited region within the work W at a desired current density is expressed as current concentration.

[0058] The energization from the first time point T1 to the second time point T2 corresponds to the second stage of the initial energization, and is performed for the purpose of heating the region within the work W sandwiched between the first electrode 21 and the second electrode 22 by gently heating the work W.

[0059] The broken line in FIG. 6B conceptually shows that in the second stage of the initial energization, heat spreads from the resistance center, and the region within the work W sandwiched between the first electrode 21 and the second electrode 22 is heated as a whole.

[0060] This heating plays a role in stably forming the warp of the metal plate. That is, by providing a process of gradually decreasing the current I as in the current control from the first time point T1 to the second time point T2, the warp of the metal plate is stably generated, and current concentration is stably realized.

[0061] In the present embodiment, the control unit 50 controls the current flowing between the pair of electrodes 21 and 22 based on the detection signal from the current sensor 40. However, when the warp of the metal plate cannot be stably realized, the current density when current flows in the stacking direction of the work W changes, and the mode of heat generation changes. Therefore, in order to realize stable current concentration, it is important to perform down-slope current control (S120) between the first time point T1 and the second time point T2.

[0062] The energization from the second time point T2 to the fourth time point T4 is energization for promoting shunt flow to the surface of the metal plate with relatively low resistance by passing a large current, and realizing heat generation and melting from the upper surface to the lower surface of the work W between the pair of electrodes 21 and 22. Hereinafter, the periods C3 and C4 from the second time point T2 to the fourth time point T4 are also expressed as the large current section L.

[0063] When a steel plate with relatively low resistance, such as the first steel plate P1 of the target work, exists on the surface of the work W, if a large current is not passed, sufficient Joule heat does not generate in the low-resistance steel plate, and there is a possibility that welding to the first steel plate P1 becomes insufficient. By passing a large current, it is possible to suppress deterioration of welding quality due to insufficient heat generation in the relatively low-resistance steel plate.

[0064] FIG. 7A shows that by passing a large current in the third energization control, extensive melting occurs between the pair of electrodes 21 and 22, and thereby material penetration occurs between adjacent steel plates in the first steel plate P1, the second steel plate P2, and the third steel plate P3. Due to the penetration, the resistance difference between the metal plates is alleviated.

[0065] The energization from the fourth time point T4 to the fifth time point T5 corresponds to this energization and is performed to finish welding the work W. The fourth current I4 and the time length of the period C5 from the time point T4 to the time point T5 (that is, this energization time) are determined so that an appropriate welding nugget G is formed in the work W.

[0066] According to this energization, melting in the work W spreads from the resistance center to the periphery, and an appropriate welding nugget G is formed in the work W. Due to the alleviation of the resistance difference caused by the penetration generated by the previous large current, the melting in the work W spreads more uniformly than in the case where there is no third energization control. As a result, an appropriate welding nugget G is formed in the work W.

[0067] Figures 7B and 7C show that the welding nugget G gradually grows due to this energization. As shown in Figure 7C, when the welding nugget G spreads over the entire plurality of metal plates (first steel plate P1, second steel plate P2, third steel plate P3) in the work W, good welding is achieved.

[0068] Furthermore, in this embodiment, since current concentration is achieved, efficient heat generation and melting of the metal plates can be realized, and it is possible to form an appropriate welding nugget G with a relatively small current.

[0069] Furthermore, in this embodiment, in the process up to the second stage of the initial energization, current concentration can be stably achieved, so the current margin in this energization can be increased. That is, a relatively wide range of currents can be allowed as the fourth current I4 for achieving appropriate welding. The thick arrow attached to the fourth current I4 in Figure 5 means that the margin of the fourth current I4 is wide.

[0070] Furthermore, in this embodiment, since penetration in a wide range in the stacking direction of the work W can be realized by the third energization control, in this energization, a small current can be used to suppress spatter and achieve good welding.

[0071] As described above, the resistance spot welding apparatus 1 and the resistance spot welding method of this embodiment have been described. This welding method functions significantly when welding a work W in which a plurality of metal plates with different resistances are stacked. Examples of a plurality of metal plates with different resistances include a plurality of metal plates with different tensile strengths and a plurality of metal plates with different plate thicknesses.

[0072] When there is a resistance difference between metal plates in the work W, the resistance heat generation is smaller at the low-resistance part compared to other parts. In this embodiment, a large current can promote the shunt to the relatively low-resistance metal plate. Furthermore, it can cause the melting of the material between adjacent metal plates and relax the resistance difference. Therefore, it is possible to weld a plurality of metal plates with different resistances well.

[0073] The resistance spot welding method according to the current profile including the above-described large current section L functions particularly effectively when a workpiece W including three metal plates, such as the combination of the first steel plate P1, the second steel plate P2, and the third steel plate P3, satisfies at least any one of the first condition, the second condition, the third condition, and the fourth condition represented by the following inequalities. · First condition: X1 < X3 · Second condition: X1 > X2 and (X1 + X2) > X3 · Third condition: X1 < X2 and (X1 + X2) > X3 · Fourth condition: X1 ≧ X2 > X3

[0074] Here, it is assumed that the first metal plate in the workpiece W is a metal plate with a tensile strength of X1, the second metal plate is a metal plate with a tensile strength of X2, and the third metal plate is a metal plate with a tensile strength of X3.

[0075] The first metal plate and the third metal plate are metal plates located at both ends in the stacking direction of the workpiece W, and the second metal plate is a metal plate sandwiched between the first metal plate and the second metal plate. The first metal plate corresponds to the first steel plate P1 illustrated in FIG. 1. The second metal plate corresponds to the second steel plate P2. The third metal plate corresponds to the third steel plate P3. However, the first metal plate, the second metal plate, and the third metal plate may be respectively associated with the third steel plate P3, the second steel plate P2, and the first steel plate P1 in this order. That is, in understanding the first condition, the second condition, the third condition, and the fourth condition, the first metal plate, the second metal plate, and the third metal plate may be understood as being stacked from top to bottom, or may be understood as being stacked from bottom to top. In other words, the first metal plate and the tensile strength X1 may be read as the third metal plate and the tensile strength X3, and the third metal plate and the tensile strength X3 may be read as the first metal plate and the tensile strength X1.

[0076] The first condition is that the tensile strength X1 of the first metal plate located at one end of the workpiece W is smaller than the tensile strength X3 of the third metal plate located at the opposite end of the workpiece W. That is, it is a condition that the two metal plates located at both ends of the workpiece W have different tensile strengths X1 and X3.

[0077] The second condition is that the tensile strength X1 of the first metal plate located at the first end in the stacking direction of the workpiece W is greater than the tensile strength X2 of the second metal plate located at the center of the workpiece W, and the tensile strength X3 of the third metal plate located at the second end in the stacking direction of the workpiece W is smaller than the sum of the tensile strength X1 of the first metal plate and the tensile strength X2 of the second metal plate. The second end in the stacking direction of the workpiece W is the end opposite to the first end in the stacking direction of the workpiece W.

[0078] The third condition is that the tensile strength X1 of the first metal plate located at the first end of the workpiece W is smaller than the tensile strength X2 of the second metal plate located at the center of the workpiece W, and the tensile strength X3 of the third metal plate located at the second end of the workpiece W is smaller than the sum of the tensile strength X1 of the first metal plate and the tensile strength X2 of the second metal plate.

[0079] The second condition and the third condition can be summarized and expressed as follows. X1≒X2 and (X1 + X2)>X3 That is, the second condition and the third condition are that the tensile strength X1 of the first metal plate located at the first end of the workpiece W is different from the tensile strength X2 of the second metal plate located at the center of the workpiece W, and the tensile strength X3 of the third metal plate located at the second end of the workpiece W is smaller than the sum of the tensile strength X1 of the first metal plate and the tensile strength X2 of the second metal plate.

[0080] The fourth condition is that the plurality of metal plates are stacked so that the tensile strength increases or decreases in the stacking direction. The stacking direction here includes the direction from the first metal plate to the third metal plate and the direction from the third metal plate to the first metal plate.

[0081] The tensile strengths X1, X2, X3 of the first condition, the second condition, the third condition, and the fourth condition may be read as resistances X1, X2, X3. The tensile strengths X1, X2, X3 may be read as plate thicknesses X1, X2, X3. The greater the tensile strength, the greater the resistance. The greater the plate thickness, the greater the tensile strength.

[0082] When the first condition is satisfied, the resistance center of the work W is shifted toward the third metal plate in the stacking direction of the work W due to the influence of the third metal plate with a large resistance. When the fourth condition is satisfied, the resistance center of the work W is shifted toward the first metal plate in the stacking direction of the work W due to the influence of the first metal plate with a large resistance.

[0083] Therefore, when applying a conventional welding method to a work W that satisfies any of these conditions and simply growing the welding nugget G from the resistance center, insufficient welding may occur on the metal plate located at the end of the work W.

[0084] According to the present embodiment, as shown in FIG. 7A, it is also possible to melt the first metal plate with relatively low resistance by a large current, and appropriate welding of the work W can be realized. Therefore, sputtering can also be suppressed.

[0085] The first condition, second condition, third condition, and fourth condition described above can also be applied when the workpiece W is a stack of four or more metal plates. When the "second metal plate" is replaced with a "stack of a plurality of metal plates" and the tensile strength X2 of the second metal plate is replaced with the sum of the tensile strengths of the metal plates constituting the "stack of a plurality of metal plates", and the above conditions are satisfied, the resistance spot welding method of the present embodiment functions effectively for good welding of the workpiece W. When the workpiece W is a stack of four or more metal plates, the "second metal plate located at the center of the workpiece W" in the second condition and the third condition is one or more metal plates sandwiched between two metal plates located at both ends of the workpiece W. In this case, the "tensile strength X2 of the second metal plate located at the center of the workpiece W" is the sum of the tensile strengths of one or more metal plates sandwiched between two metal plates located at both ends of the workpiece W.

[0086] According to the welding tests in various environments, it has been found that the resistance spot welding method of the present embodiment also contributes to proper welding of the workpiece W when the workpiece W satisfies either one of the following fifth condition and sixth condition. · Fifth condition: The difference in strength ΔX between both sides is 445 MPa or more. · Sixth condition: The difference in strength ΔX between both sides is 255 MPa or more, and the strength ratio R is 4.29 or more.

[0087] The difference in strength ΔX between both sides here is the difference in the tensile strengths of the metal plates located at both ends in the stacking direction of the workpiece W. That is, the difference in strength ΔX between both sides is the difference in the tensile strengths of the two metal plates in contact with the pair of electrodes 21, 22 in the workpiece W. Using the above tensile strengths X1, X2, X3, the difference in strength ΔX between both sides can be expressed by the formula ΔX = |X1 - X3|.

[0088] The strength ratio R corresponds to the ratio R = R1 / R2, where R1 is the sum of the tensile strengths in the stacking direction of the plurality of metal plates constituting the work W, and R2 is the tensile strength of the metal plate with the lower tensile strength among the two metal plates located at both ends in the stacking direction of the plurality of metal plates. Using the above-mentioned tensile strengths X1, X2, and X3, R1 can be expressed by the formula R1 = (X1 + X2 + X3), and R2 can be expressed by the formula R2 = min{X1, X3} using the MIN function. In this case, the strength ratio R = (X1 + X2 + X3) / min{X1, X3}.

[0089] The greater the difference ΔX in strength on both sides, the less the melting of the metal plate with relatively low tensile strength progresses due to low resistance. The greater the strength ratio R, the more the deviation of the welding nugget G occurs. Therefore, the resistance spot welding method of the present embodiment has high utility value for the work W that satisfies the above-mentioned fifth condition or sixth condition.

[0090] The resistance spot welding method of the present embodiment also functions approximately effectively when welding a work W whose plate thickness satisfies the following seventh condition. · Seventh condition: The plate thickness ratio H is 3.5 or more.

[0091] The plate thickness ratio H here is the ratio H1 / H2, where H1 is the total plate thickness, which is the sum of the thicknesses in the stacking direction of the plurality of metal plates constituting the work W, and H2 is the thickness of the metal plate with the smaller thickness among the two metal plates located at both ends in the stacking direction of the plurality of metal plates.

[0092] [Modification Example] The current profile is not limited to the example shown in FIG. 3, and may be changed to any of the current profiles in FIGS. 8A, 8B, 8C, 9A, 9B, and 9C. As can be understood from the comparison between FIG. 3 and FIGS. 8A, 8B, 8C, 9A, 9B, and 9C, the initial energization including the downslope and the large current before the main energization particularly contribute to good welding.

[0093] In the current profile shown in FIG. 8A, the current waveform in the high-current section L1 is not trapezoidal as shown in FIG. 3, but triangular with an upslope and a sharp peak. In the current profile shown in FIG. 8B, the current waveform in the high-current section L2 is rectangular. In the current profile shown in FIG. 8C, the current waveform in the high-current section L3 is triangular with a downslope starting from the peak.

[0094] In the current profile shown in FIG. 9A, the current waveform in the high-current section L4 is trapezoidal with an upslope and a downslope. In the current profile shown in FIG. 9B, the current waveform in the high-current section L5 is triangular with an upslope and a downslope and a sharp peak. In the current profile shown in FIG. 9C, the current waveform in the high-current section L6 is trapezoidal starting from the peak.

[0095] [Other Embodiments] The present disclosure is not limited to the above embodiments and can take various forms. For example, the above-described resistance spot welding apparatus 1 and resistance spot welding method may be used for welding a work W in which two metal plates are stacked as a plurality of metal plates.

[0096] The functions of one component in the above embodiments may be distributed and provided in a plurality of components. The functions of a plurality of components may be integrated into one component. A part of the configuration of the above embodiments may be omitted. At least a part of the configuration of the above embodiments may be added to or replaced with the configuration of other above embodiments. All aspects included in the technical idea specified from the language described in the claims are embodiments of the present disclosure.

[0097] [Technical Ideas Disclosed in this Specification] It can be understood that the following technical ideas are disclosed in this specification. [Item 1] A resistance spot welding method for welding a work in which a plurality of metal plates are stacked, using a resistance spot welding apparatus including a pair of electrodes that sandwich the work on both sides in the stacking direction of the plurality of metal plates, comprising: Performing first energization control on the pair of electrodes so that a first current flows between the pair of electrodes sandwiching the work; Subsequent to the first energization control, performing second energization control on the pair of electrodes so that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current; Subsequent to the second energization control, performing third energization control on the pair of electrodes so that the current flowing through the pair of electrodes increases from the second current to a third current greater than the second current; Subsequent to the third energization control, performing fourth energization control on the pair of electrodes so that a fourth current flows between the pair of electrodes; Including: The fourth current is a constant current; The first current is a constant current smaller than the fourth current; The third current is greater than the fourth current; A resistance spot welding method. [Item 2] The resistance spot welding method according to item 1, wherein the plurality of metal plates include at least two metal plates having different resistances, tensile strengths, or plate thicknesses from each other. [Item 3] The resistance spot welding method according to item 1 or item 2, wherein the work includes a work in which two metal plates located at both ends in the stacking direction among the plurality of metal plates have different resistances, tensile strengths, or plate thicknesses from each other. [Item 4] The work is: A work in which the plurality of metal plates are stacked so that the resistance, tensile strength, or plate thickness increases or decreases in the stacking direction, and Among the plurality of metal plates, the tensile strength of the first metal plate located at the first end in the stacking direction is different from the tensile strength of the second metal plate located at the center in the stacking direction, and the tensile strength of the third metal plate located at the second end, which is the end opposite to the first end in the stacking direction, is smaller than the sum of the tensile strength of the first metal plate and the tensile strength of the second metal plate. The tensile strength of the second metal plate located at the center in the stacking direction is the tensile strength of one metal plate sandwiched between the first metal plate and the third metal plate located at both ends in the stacking direction among the plurality of metal plates, or the sum of the tensile strengths of one or more metal plates sandwiched between the first metal plate and the third metal plate. Work The resistance spot welding method according to any one of Items 1 to 3, including at least one of them. [Item 5] The ratio H1 / H2 of the total plate thickness H1, which is the sum of the thicknesses of the plurality of metal plates in the stacking direction in the work, to the thickness H2 of the metal plate with the smaller thickness among the two metal plates located at both ends in the stacking direction of the plurality of metal plates is 3.5 or more. The resistance spot welding method according to any one of Items 1 to 4. [Item 6] The work is A work with a strength difference between both sides of 445 MPa or more, and A work in which the strength difference between both sides is 255 MPa or more and the strength ratio is 4.29 or more including at least one of them, The strength difference between both sides is the difference in tensile strength between the two metal plates located at both ends in the stacking direction in the work, and the strength ratio is the value obtained by dividing the sum of the tensile strengths of the plurality of metal plates in the work by the tensile strength of the metal plate with the smaller tensile strength among the two metal plates located at both ends in the stacking direction. The resistance spot welding method according to any one of Items 1 to 5. [Item 7] The plurality of metal plates include high-tensile steel plates. The resistance spot welding method according to any one of Items 1 to 6. [Item 8] A resistance spot welding apparatus for welding a work in which a plurality of metal plates are overlapped A pair of electrodes arranged to sandwich the workpiece on both sides in the stacking direction of the plurality of metal plates, A control unit configured to control energization between the pair of electrodes, and, the control unit, executes first energization control for the pair of electrodes so that a first current flows between the pair of electrodes sandwiching the workpiece, subsequent to the first energization control, second energization control for the pair of electrodes is executed so that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current, subsequent to the second energization control, third energization control for the pair of electrodes is executed so that the current flowing through the pair of electrodes increases from the second current to a third current larger than the second current, subsequent to the third energization control, fourth energization control for the pair of electrodes is executed so that a fourth current flows between the pair of electrodes, the fourth current is a constant current, the first current is a constant current smaller than the fourth current, the third current is larger than the fourth current, Resistance spot welding apparatus.

Description of symbols

[0098] 1... Resistance spot welding apparatus, 20... Resistance welder, 21... First electrode, 22... Second electrode, 30... Welding power source, 40... Current sensor, 50... Control unit.

Claims

1. A resistance spot welding method for welding a work in which a plurality of metal plates are stacked, using a resistance spot welding apparatus including a pair of electrodes that sandwich the work on both sides in the stacking direction of the plurality of metal plates, the method comprising: performing first energization control on the pair of electrodes so that a first current flows between the pair of electrodes sandwiching the work; subsequent to the first energization control, performing second energization control on the pair of electrodes so that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current; subsequent to the second energization control, performing third energization control on the pair of electrodes so that the current flowing through the pair of electrodes increases from the second current to a third current greater than the second current; subsequent to the third energization control, performing fourth energization control on the pair of electrodes so that a fourth current flows between the pair of electrodes; including the fourth current being a constant current, the first current being a constant current smaller than the fourth current, the third current being greater than the fourth current, a resistance spot welding method.

2. The resistance spot welding method according to claim 1, wherein the plurality of metal plates include at least two metal plates having different resistances, tensile strengths, or plate thicknesses from each other.

3. The resistance spot welding method according to claim 1, wherein the work includes a work in which two metal plates located at both ends in the stacking direction among the plurality of metal plates have different resistances, tensile strengths, or plate thicknesses from each other.

4. The work is a work in which the plurality of metal plates are stacked such that the resistance, tensile strength, or plate thickness increases or decreases in the stacking direction, and Among the plurality of metal plates, the tensile strength of the first metal plate located at the first end in the stacking direction is different from the tensile strength of the second metal plate located at the center in the stacking direction, and the tensile strength of the third metal plate located at the second end, which is the end opposite to the first end in the stacking direction, is smaller than the sum of the tensile strength of the first metal plate and the tensile strength of the second metal plate, and the tensile strength of the second metal plate located at the center in the stacking direction is the tensile strength of one metal plate sandwiched between the first metal plate and the third metal plate located at both ends in the stacking direction among the plurality of metal plates, or the sum of the tensile strengths of one or more metal plates sandwiched between the first metal plate and the third metal plate. Work The resistance spot welding method according to claim 1, including at least one of the above.

5. The ratio H1 / H2 of the total plate thickness H1, which is the sum of the thicknesses of the plurality of metal plates in the stacking direction in the work, to the thickness H2 of the metal plate with the smaller thickness among the two metal plates located at both ends in the stacking direction of the plurality of metal plates is 3.5 or more. The resistance spot welding method according to claim 1.

6. The work is A work with a strength difference between both sides of 445 MPa or more, and A work in which the strength difference between both sides is 255 MPa or more and the strength ratio is 4.29 or more including at least one of the above, The strength difference between both sides is the difference in tensile strength between the two metal plates located at both ends in the stacking direction in the work, and the strength ratio is the value obtained by dividing the sum of the tensile strengths of the plurality of metal plates in the work by the tensile strength of the metal plate with the smaller tensile strength among the two metal plates located at both ends in the stacking direction. The resistance spot welding method according to claim 1.

7. The plurality of metal plates include high-tensile steel plates. The resistance spot welding method according to any one of claims 1 to 6.

8. A resistance spot welding apparatus for welding a work in which a plurality of metal plates are stacked, A pair of electrodes arranged to sandwich the workpiece on both sides in the stacking direction of the plurality of metal plates; A control unit configured to control energization between the pair of electrodes; Comprising: The control unit: Executes first energization control for the pair of electrodes so that a first current flows between the pair of electrodes sandwiching the workpiece; Subsequent to the first energization control, second energization control for the pair of electrodes is executed so that the current flowing between the pair of electrodes decreases from the first current to a second current smaller than the first current; Subsequent to the second energization control, third energization control for the pair of electrodes is executed so that the current flowing through the pair of electrodes increases from the second current to a third current larger than the second current; Subsequent to the third energization control, fourth energization control for the pair of electrodes is executed so that a fourth current flows between the pair of electrodes; The fourth current is a constant current; The first current is a constant current smaller than the fourth current; The third current is larger than the fourth current; A resistance spot welding device.

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