Method for manufacturing spot welded joint

Abstract

To provide a method for manufacturing a spot welded joint which can weld a plate set having three or more metal plates and a plate thickness ratio of 5.0 or more, and sets expulsion generation at a low level.SOLUTION: A method for manufacturing a spot welded joint has a plate thickness ratio of a plate set of 5.0 or more, and satisfies a following expression 1 to expression 5, wherein R1 is a tip curvature radius of a first electrode which is an electrode arranged on a side of a first metal plate, R2 is a tip curvature radius of a second electrode which is an electrode arranged on the side of a second metal plate, T1 is a length of first energization time, I1 is a welding current value in the first energization time, I2 is a welding current value in second energization time, and I0 is an expulsion generation current value.SELECTED DRAWING: Figure 1A

Description

[Technical field]

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

[0002] Spot welding can join metal sheets cheaply and quickly, and is therefore used in a variety of applications, such as joining high-strength metal sheets that are used in automobile body components.

[0003] However, it is difficult to join sheet combinations with a large sheet thickness ratio by spot welding. The sheet thickness ratio is the thickness of the sheet combination divided by the thickness of the thinner of the metal sheets arranged on the surface of the sheet combination. The nugget formed by spot welding grows from the center of the sheet combination toward the surface. The larger the sheet thickness ratio, the thinner the metal sheets on the surface of the sheet combination become, and it becomes difficult for the nugget to grow to the metal sheets. As a result, for example, as shown in FIG. 5, a phenomenon occurs in which the thin sheet arranged on the surface of the sheet combination is not joined to the other metal sheets, i.e., a poor joint occurs.

[0004] In addition, when the metal plates constituting the plate assembly are steel plates and part of them is a high-strength steel plate, the possibility of poor joints is further increased. In general, in plate assemblies with a large plate thickness ratio, the thick plate that provides the strength of the spot-welded joint is a high-strength steel plate, and the thin plate that provides the exterior of the spot-welded joint is a mild steel plate. The alloy element content of mild steel is smaller than that of high-strength steel. Therefore, the electrical resistance value of mild steel is also smaller than that of high-strength steel. When a plate assembly combining a mild steel plate and a high-strength steel plate is spot welded, the high-strength steel plate is more likely to generate heat, and the nugget tends to grow biased toward the high-strength steel plate. This further promotes poor joints of the mild steel plates arranged on the surface of the plate assembly.

[0005] A plate assembly having a plate thickness ratio exceeding 5.0 is sometimes called a "difficult-to-weld plate assembly." Various techniques have been proposed to suppress joint defects in difficult-to-weld plate assemblies.

[0006] Patent Document 1 discloses a method for manufacturing a resistance spot welded joint, in which a plate assembly made of multiple overlapping metal plates is welded and joined by resistance spot welding to produce a resistance spot welded joint, the plate assembly being a plate assembly having a plate thickness ratio of 5 or more, in which a thin plate is overlapped on at least one of two or more overlapping thick plates, the resistance spot welding being a two-stage welding including a first stage and a second stage, and the second stage welding having a higher pressure, a lower current or the same current, and a longer current flow time or the same current flow time as the first stage welding.

[0007] Patent Document 2 discloses a resistance spot welding method in which a plate assembly in which a thin metal plate is overlapped on one of two or more overlapping thick metal plates is sandwiched between a pair of electrode tips and welded by resistance spot welding, the electrode tip of one of the pair of electrode tips that contacts the thin metal plate is an electrode tip having a curved tip with a predetermined radius of curvature R1, and the electrode tip of the other electrode tip that contacts the thick metal plate is an electrode tip having a flat tip or a curved tip having a radius of curvature larger than the radius of curvature R1 of the tip of the electrode tip that contacts the thin metal plate, the resistance spot welding is performed in two stages, a first stage and a second stage, the second stage welding is performed with a higher welding pressure than the first stage welding, and the welding current value of the second stage is set to be equal to or lower than the welding current value of the first stage welding. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 3922263 [Patent Document 2] Patent No. 4543823 Summary of the Invention [Problem to be solved by the invention]

[0009] One method to prevent poor joining of sheet assemblies with a large sheet thickness ratio is to increase the heat input and promote the growth of the nugget. However, the greater the heat input, the more likely expulsion occurs. Explosion is the phenomenon in which the base metal is locally overheated during lap resistance welding and melts and splashes, or the metal itself. In general, expulsion is classified into surface expulsion, which occurs on the outer surface of the electrode tip and the base metal in contact, internal expulsion, which occurs between the base metals, and surface expulsion, where metal inside the nugget splashes out to the surface.

[0010] Both of the welding methods disclosed in Patent Document 1 and Patent Document 2 are two-stage welding, and the first stage welding is performed with a low pressure. In such a welding method, it is considered that the first stage low pressure welding is likely to cause expulsion. In particular, when the welding methods disclosed in Patent Document 1 and Patent Document 2 are applied to a plate assembly having a plate thickness ratio of 5.0 or more and three or more metal plates, it is estimated that the first stage low pressure welding is very likely to cause center expulsion at the thick plate interface. The thick plate interface refers to the interface between metal plates other than the thin plate arranged on the surface of the plate assembly, and in the plate assembly illustrated in FIG. 5, it refers to the interface between the first and second metal plates from the bottom.

[0011] When expulsion occurs, the volume of the molten metal that constitutes the nugget decreases, and the nugget diameter decreases. Therefore, the occurrence of expulsion increases the risk of poor welding. In spot welding, it is also necessary to keep the amount of expulsion to a low level. However, it is not easy to achieve both an increase in the nugget diameter and a reduction in the amount of expulsion.

[0012] In view of the above circumstances, an object of the present invention is to provide a manufacturing method for spot welded joints that can weld a plate assembly having three or more metal plates and a plate thickness ratio of 5.0 or more without poor joining, and that can reduce the amount of expulsion. [Means for solving the problem]

[0013] The gist of the present invention is as follows.

[0014] (1) A method for manufacturing a spot welded joint according to one embodiment of the present invention includes a step of spot welding a sheet assembly including three or more metal sheets using a pair of electrodes, wherein a thickness of a first metal sheet arranged on one surface of the sheet assembly is smaller than a thickness of a second metal sheet arranged on the other surface of the sheet assembly, and a sheet thickness ratio, which is a value obtained by dividing the thickness of the sheet assembly by the thickness of the first metal sheet, is 5.0 or more, the spot welding has a first current flow time and a second current flow time after the first current flow time, and satisfies the following formulas 1, 2, 3, 4, and 5:

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[0015] According to the present invention, it is possible to provide a manufacturing method for a spot welded joint that is capable of welding a plate assembly having three or more metal plates and a plate thickness ratio of 5.0 or more without joint failure and that can reduce the amount of expulsion. [Brief description of the drawings]

[0016] [Figure 1A] FIG. 4 is a schematic diagram showing an example of a change in welding current value over time in the manufacturing method of a spot welded joint according to the present embodiment. [Figure 1B] 10 is a schematic diagram of another example of a change in welding current value over time in the manufacturing method of a spot welded joint according to the present embodiment. FIG. [Diagram 2] 4 is a schematic cross-sectional view of a plate assembly and an electrode in a manufacturing method of a spot welded joint according to the present embodiment. FIG. [Diagram 3] 11A and 11B are diagrams for explaining a method of estimating the contact radius when the electrode tip region pressed into the interior of the metal plate is approximated as a circular arc-shaped solid of revolution; [Figure 4] 1 is a cross-sectional schematic view of a spot welded joint obtained by a manufacturing method according to an embodiment of the present invention. FIG. [Diagram 5] 4 is a schematic cross-sectional view of a spot welded joint in which poor bonding of the first metal plate has occurred. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The manufacturing method of the spot welded joint according to the present embodiment includes a step of spot welding a sheet set 11 including three or more metal sheets using a pair of electrodes, the thickness of a first metal sheet 111 arranged on one surface of the sheet set 11 being smaller than the thickness of a second metal sheet 112 arranged on the other surface of the sheet set 11, a sheet thickness ratio, which is a value obtained by dividing the thickness of the sheet set 11 by the thickness of the first metal sheet 111, is 5.0 or more, the spot welding has a first current flow time and a second current flow time after the first current flow time, and satisfies the following formulas 1, 2, 3, 4, and 5,

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[0018] The manufacturing method of the spot welded joint according to this embodiment includes a step of spot welding the sheet set 11 using a pair of electrodes. Spot welding is a type of resistance welding in which overlapping base materials are sandwiched between the tips of electrodes with appropriately shaped tips, and current and pressure are concentrated in a relatively small area to locally heat the material, while simultaneously applying pressure with the electrodes. In this embodiment, the base materials refer to the sheet set 11.

[0019] (Composition of board group 11) 2, the plate set 11 is composed of three or more stacked metal plates. In this embodiment, for convenience, the thinner of the metal plates arranged on the surface of the plate set 11 is referred to as the first metal plate 111, and the thicker one is referred to as the second metal plate 112. In other words, the thickness t1 of the first metal plate 111 arranged on one surface of the plate set 11 is smaller than the thickness t2 of the second metal plate 112 arranged on the other surface of the plate set 11.

[0020] In the manufacturing method for spot welding according to this embodiment, the sheet thickness ratio of the sheet group 11 is set to be 5.0 or more. The sheet thickness ratio is a value obtained by dividing the thickness t of the sheet group 11 by the thickness t1 of the first metal plate 111. The thickness t of the sheet group 11 is the total value of the sheet thicknesses of all the metal plates included in the sheet group 11.

[0021] (electrode tip curvature radius) A pair of electrodes used in spot welding are so-called spot welding electrodes. Spot welding electrodes are rod-shaped electrodes that directly contact the base material, i.e., the sheet pair 11, during spot welding to pass the welding current and transmit the pressure. The tip of the electrode on the side that contacts the sheet pair 11 is rounded. The shape of the electrode tip is standardized in JIS C 9304:1999. In this standard, the radius of curvature of the electrode tip on the side that contacts the sheet pair 11 is called the tip radius of curvature.

[0022] In the manufacturing method of a spot welded joint according to this embodiment, for convenience, the electrode arranged on the side of the thin first metal plate 111 is referred to as the first electrode E1, and the electrode arranged on the side of the thicker second metal plate 112 is referred to as the second electrode E2. The tip curvature radius R1, measured in mm, of the first electrode E1 and the tip curvature radius R2, measured in mm, of the second electrode E2 satisfy the following formula 1.

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[0023] According to formula 1, the tip of the first electrode E1 is sharper than the tip of the second electrode E2. Specifically, the tip curvature radius R2 of the second electrode E2 is about 3.84 to 19.45 times the tip curvature radius R1 of the first electrode E1.

[0024] Formula 1 is an index value of the ratio between the contact radius of the first electrode E1 and the first metal plate 111 and the contact radius of the second electrode E2 and the second metal plate 112. The smaller the contact radius between the electrode and the metal plate, the smaller the current path and the higher the current density. Therefore, the smaller the contact radius between the electrode and the metal plate, the greater the amount of heat generated by electrical current in the metal plate. In the manufacturing method of the spot welded joint according to this embodiment, it is estimated that the contact radius of the second electrode E2 and the second metal plate 112 is within a range of about 1.4 to about 2.1 times the contact radius of the first electrode E1 and the first metal plate 111.

[0025] The process of deriving Equation 1 is explained below. Generally, if the amount of electrode pressing is the same, the larger the radius of curvature of the electrode tip, the larger the contact radius between the electrode and the metal plate. However, if the pressure is the same, the larger the radius of curvature of the electrode tip, the smaller the amount of electrode pressing. Experimental results showed a tendency for the contact radius to increase as the radius of curvature of the tip increases, but the two did not have a linear correlation.

[0026] Therefore, the inventors estimated the relationship between the tip curvature radius and the contact radius based on the following assumptions. The shape of the electrode tip area pressed into the metal plate is a solid of revolution of an arch shape consisting of circular arcs and chords, as shown in Figure 3. If the applied pressure is constant, the volume of the electrode tip area pressed into the metal plate is constant regardless of the tip curvature radius.

[0027] The volume of a solid of revolution of an arc is calculated using the following formula: V = (1 / 6) × π × h × (3 × a 2 +h 2 ) R = (a 2 +h 2 ) / (2×h) In the above formula, h is the height of the rotating body, R is the radius of the arc, and a is the value obtained by multiplying the chord length of the arc by 1 / 2. The height h of the rotating body corresponds to the indentation depth of the electrode, the value a obtained by multiplying the chord length of the arc by 1 / 2 corresponds to the contact radius, and the radius R of the arc corresponds to the radius of curvature of the tip of the electrode.

[0028] If we rearrange the above equation assuming that V is constant, the contact radius a is roughly proportional to the 0.25th power of the curvature radius R. This study result closely approximated the relationship between the actual contact radius and the tip curvature radius. Based on the above, the inventors have determined that (R2 / R1) 0.25 was used as an index value of the ratio between the contact radius of the second electrode E2 and the contact radius of the first electrode E1.

[0029] (First energization time and second energization time) As shown in Figures 1A and 1B, spot welding has a first current flow time and a second current flow time. The first current flow time is a period during which the welding current value is high, which is provided at the beginning of spot welding. The second current flow time is a period during which the welding current value is low, which is provided after the first current flow time. The start of the first current flow time is the time when current flow starts. The end of the first current flow time is the time when the current value starts to decrease.

[0030] In the manufacturing method of the spot welded joint according to the present embodiment, the welding current value I1 in the first current flow time and the welding current value I2 in the second current flow time are determined based on the expulsion limit current value and the tip curvature radius of the electrode. Specifically, the manufacturing method of the spot welded joint according to the present embodiment satisfies the following formulas 2, 3, 4, and 5.

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[0031] α is a coefficient for determining the upper and lower limits of I1. α is a value within the numerical range defined by the above formula 5. The value obtained by substituting the minimum value of α into formula 2 is the lower limit of I1. The value obtained by substituting the maximum value of α into formula 2 is the upper limit of I1.

[0032] The welding current value I1 in the first current supply time may be set constant throughout the entire first current supply time, as illustrated in FIG. 1A. On the other hand, the welding current value I1 in the first current supply time may be set to vary slightly during the period from the start to the end of the first current supply time, as illustrated in FIG. 1B. In addition, when I1 varies slightly during the period from the start to the end of the first current supply time, it is not necessary for the formula to be satisfied during the entire period from the start to the end of the first current supply time. In this case, if the length of time during which a current value of 0.9 times or more of the value obtained by substituting α=0.210 into formula 2 (i.e., the lower limit value of I1) flows is 0.5×T1 or more, formula 2 is considered to be satisfied.

[0033] (Limiting current value at which scattering occurs) The expulsion limit current value is the maximum current value at which expulsion does not occur in the sheet group 11 when a pair of identical electrodes is used to pass current through the sheet group 11 to be welded for T1 (msec). The expulsion limit current value can be determined by the procedure described below.

[0034] First, the plate group 11 to be welded in the manufacturing method of the spot welded joint according to the present embodiment is spot welded at various welding current values. Hereinafter, the spot welding for identifying the limit current value at which expulsion occurs is referred to as test welding, and the welding for manufacturing the welded joint is referred to as main welding. In the test welding, the welding time is made to match the value T1 scheduled as the length of the first welding time. In other words, in the manufacturing method of the spot welded joint according to the present embodiment, the plate group 11 and the length of the first welding time need to match the plate group 11 and the welding time in the test welding. In addition, in the test welding, both of the pair of electrodes are the same first electrode E1. In addition, it is preferable to select the constraint condition of the test piece in the test welding assuming the disturbance in the actual production. That is, it is preferable to perform the test welding in the environment in which the manufacturing method of the spot welded joint according to the present embodiment is actually performed, or in an environment simulating the environment. In particular, when a plate gap occurs, it becomes difficult to form a nugget on the thin plate side, and expulsion is likely to occur from the thick plate side interface, making welding difficult. Therefore, it is preferable to perform test welding under conditions where a gap is set at the thick plate interface.

[0035] Next, the relationship between the welding current value during test welding and the occurrence of expulsion is summarized, and the maximum welding current value during test welding in which no expulsion occurred is adopted as the expulsion occurrence limit current value.

[0036] The conditions of the test welding are substantially the same as the conditions of the first current flow time in the main welding, except for the welding current and the tip shape of the pair of electrodes. The limiting current value for expulsion obtained by the test welding is the limiting current value for the welding current value that can be passed without causing expulsion when performing normal spot welding using a pair of electrodes with the same tip shape. The limiting current value for expulsion can also be understood as the upper limit of the welding current value that can be adopted when performing spot welding using a pair of electrodes with the same shape, with respect to the sheet assembly 11 in the main welding and the length T1 of the first welding time.

[0037] On the other hand, in the manufacturing method of a spot welded joint according to the present embodiment, a welding current value equal to or greater than the lower limit value defined by Equations 2 and 5 is applied to the sheet set 11 during the first current application time.

[0038] (Action and effect) Next, the functions and effects of various components in the method for manufacturing a spot welded joint according to this embodiment will be described.

[0039] In the manufacturing method of the spot welded joint according to this embodiment, a sheet pair 11 having a sheet thickness ratio of 5.0 or more is used as a weld base material. In such a sheet pair 11, it is difficult to stably join a thin metal plate (first metal plate 111) arranged on the surface of the sheet pair 11. This is because, as shown in Fig. 5, there are cases where a nugget 12 does not grow to the first metal plate 111.

[0040] However, if the welding current value is increased to promote the growth of the nugget 12, expulsion is likely to occur. Moreover, in the manufacturing method of a spot welded joint according to this embodiment, the number of metal plates included in the sheet assembly 11 is three or more. In spot welding of such a sheet assembly 11, if the welding current value is increased, expulsion is likely to occur. In particular, expulsion is likely to occur between the second metal plate 112 and a metal plate in contact with it.

[0041] To solve this problem, in the manufacturing method of a spot welded joint according to this embodiment, the tip curvature radius R1 of the first electrode and the tip curvature radius R2 of the second electrode in contact with the first metal sheet 111 are set to satisfy formula 1. When formula 1 is satisfied, as shown in Fig. 2, the current density is high in the thin first metal sheet 111 and the current density is low in the thick second metal sheet 112. This facilitates heating of the first metal sheet 111 and reduces the amount of splashing in the second metal sheet 112.

[0042] With an electrode combination that satisfies Formula 1, the amount of expulsion in the second metal sheet 112 is kept at a low level, which creates room for increasing the welding current value. In other words, the upper limit of the welding current value that can suppress the amount of expulsion within an acceptable range is increased with an electrode combination that satisfies Formula 1.

[0043] As shown in the examples described later, when the tip curvature radii of the first electrode E1 and the second electrode E2 are the same, there is no range of welding current values ​​at which no expulsion occurs and good penetration into the thin sheet side occurs during the first current flow time. In addition, as the tip curvature radius increases, the current value at which no expulsion occurs increases, but it is found that a nugget is not formed on the thin sheet side. In other words, it is found that as the tip curvature radius of the first electrode E1 increases, a higher welding current is required to form a nugget on the thin sheet side.

[0044] On the other hand, when the tip curvature radius R2 of the second electrode E2 is made larger than the tip curvature radius R1 of the first electrode E1, it is found that the amount of expulsion is suppressed to a low level while a nugget is formed on the thin sheet side. In other words, by selecting an appropriate balance between the tip curvature radius and the current flow conditions, a range of welding current values ​​is created in which no expulsion occurs and good penetration into the thin sheet side is achieved.

[0045] In the manufacturing method for a spot welded joint according to this embodiment, during the first current application time, which is the initial stage of spot welding, a large welding current value I1 defined by formula 2 is applied to the sheet pair 11. This causes extremely large resistance heating to occur near the tip of the first electrode E1, where the current density is low, i.e., in the first metal sheet, and melts the interface between the first metal sheet and the metal sheet in contact with it. In addition, because the range of welding current values ​​at which expulsion does not occur is expanded by the combination of electrodes that satisfy formula 1, even if such a welding current value is applied to the sheet pair 11, the amount of expulsion is suppressed to a low level.

[0046] However, if the length T1 of the first current flow time is too short, it is not possible to obtain a sufficient penetration depth for joining the first metal sheets even if the welding current value and the current density are increased, so according to formula 4, the length T1 of the first current flow time is limited to 15 msec or more.

[0047] In this embodiment, the upper limit of the welding current value that reduces the amount of expulsion is increased as shown in the examples. However, if the welding current value I1 during the first current flow time is excessive, expulsion cannot be avoided even with an electrode combination that satisfies formula 1. Therefore, the upper limit of the welding current value I1 during the first current flow time is specified by formula 2.

[0048] In addition, it is not preferable to extend the length T1 of the first current flow time unnecessarily. Even if the electrode shape satisfies formula 1, if the length T1 of the first current flow time exceeds 100 msec, the welded part will overheat and the occurrence of expulsion will be unavoidable. Therefore, in accordance with formula 4, the length T1 of the first current flow time is limited to 100 msec or less.

[0049] The length T1 of the first current flow time is limited to 100 msec or less. Therefore, although it is possible to melt the interface between the first metal sheet 111 and the adjacent metal sheet by using the first current flow time, it is difficult to grow the molten part to the second metal sheet 112. In other words, it is difficult to form the nugget 12 that joins all the metal sheets by using only the first current flow time.

[0050] Therefore, in the manufacturing method of the spot welded joint according to the present embodiment, a second current flow time is provided following the first current flow time. During the second current flow time, the molten part formed at the interface between the first metal sheet 111 and the adjacent metal sheet during the first current flow time is expanded toward the second metal sheet 112. This forms a nugget 12 that joins all the metal sheets.

[0051] In the initial stage of spot welding, there is a gap between the metal plates. This gap increases the electrical resistance value of the current path formed between the first electrode E1 and the second electrode E2. That is, resistance heating is likely to occur in the initial stage of spot welding. On the other hand, as spot welding progresses, the metal plates soften, the gap between the metal plates is filled, and the current density of the plate assembly 11 decreases. This phenomenon may be referred to as the fitting of the plate assembly 11.

[0052] Considering the above circumstances, it is preferable to provide the time for increasing the welding current value at the initial stage of spot welding. Therefore, in the method for manufacturing a spot welding joint according to the present embodiment, a first energization time with a high welding current value is provided at the initial stage of spot welding, and a second energization time is provided after the first energization time.

[0053] However, as described above, when the state where the welding current value is excessive exceeds 100 msec, the amount of splash generation exceeds the allowable range. Therefore, the welding current value I2 in the second energization time is defined to be lower than I1 according to Equation 3. Thereby, the amount of splash generation in the second metal plate is reduced. However, in the method for manufacturing a spot welding joint according to the present embodiment, I2 can be set flexibly. As long as I2 < I1 is satisfied, I2 is not particularly limited.

[0054] The method for manufacturing a spot welding joint according to the present embodiment is particularly advantageous for suppressing intermediate splash generated at the interface between the second metal plate and the metal plate in contact therewith. This is because the contact area between the second metal plate and the second electrode is wide and the current density in the second metal plate is low, so the second metal plate is less likely to generate heat.

[0055] Due to the effects achieved by the method for manufacturing a spot welding joint according to the present embodiment, a spot welding joint composed of a plate assembly having three or more metal plates and a plate thickness ratio of 5.0 or more, as illustrated in FIG. 4, can be manufactured without joint failure and significant splash generation.

[0056] The most basic aspect of the method for manufacturing a spot welded joint according to this embodiment has been described above. A more preferred aspect will now be described.

[0057] (Preferable range of the length T1 of the first current application time and the coefficient α) It is preferable that the length T1 of the first current supply time satisfies the formula (6) and the coefficient α satisfies the formula (7).

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[0058] (Cooling time) The spot welding may further include a cool time between the first current application time and the second current application time. The cool time refers to the time during which the current value is substantially set to zero while maintaining the pressure between the electrodes. The spot welding electrode is configured so that a coolant flows inside it to prevent the electrode tip from wearing out. Therefore, while the plate group 11 is being pressed by the electrode, heat is constantly being dissipated from the plate group 11 to the electrode. While the current is flowing, the amount of resistance heat generation is greater than the amount of heat dissipation, so the temperature of the plate group 11 rises. However, during the non-current application time, the plate group 11 is cooled by heat dissipation.

[0059] By providing the cooling time, the overheated state of the plate set 11 that occurs during the first current supply time is alleviated, and therefore, the occurrence of splashing during the second current supply time is more suitably suppressed.

[0060] (Pressure) In the manufacturing method of the spot welded joint according to the present embodiment, the pressure between the electrodes is not particularly limited. In general, the pressure between the electrodes is appropriately set according to the welding current value and the sheet assembly 11. In the manufacturing method of the spot welded joint according to the present embodiment, the pressure can be appropriately set according to the welding current value.

[0061] The pressure may be constant, or may be changed depending on the degree of progress of the spot weld. In the manufacturing method of a spot welded joint according to the present embodiment, it is preferable to keep the pressure constant throughout the first current flow time and the second current flow time. By keeping the pressure constant, the effect of suppressing the occurrence of expulsion in the first current flow step can be improved.

[0062] (Preheat time) In spot welding, a preheating time is sometimes set. The preheating time is the time during which a preheating current flows before the welding current flows. The preheating current is a current that flows to preheat the weld before welding.

[0063] In the manufacturing method of the spot welded joint according to the present embodiment, the current value may be increased in a slope manner at the beginning of spot welding, thereby providing a preheating time, as shown in Fig. 1B. The length of the preheating time is defined as the length from the start of current flow to the point at which the current value has increased to 0.9 times the value obtained by substituting α = 0.210 into Equation 2 (i.e., the lower limit value of I1).

[0064] The preheating time has the effect of making the plate group 11 conform to each other and stabilizing the spot welding. In a normal method for manufacturing a spot welded joint, a preheating time is provided to obtain this effect. However, in the method for manufacturing a spot welded joint according to the present embodiment, it is preferable to make the preheating time as short as possible. For example, it is preferable to set the length of the preheating time from the start of spot welding to 50 msec or less. More preferably, the length of the preheating time is 25 msec or less, or 10 msec or less. As described above, when the plate group 11 is conformed to each other, the resistance value of the plate group 11 decreases, and the amount of resistance heat generated in the first metal plate 111 decreases. In order to melt the interface between the first metal plate 111 and the metal plate adjacent thereto quickly and sufficiently in the early stage of spot welding, it is better not to conform the plate group 11 to each other.

[0065] (Metal plate material) As long as the metal plates can be joined by spot welding, there is no particular limitation on the material of the metal plates. The effect of the method for manufacturing a spot welded joint according to the present embodiment can be achieved regardless of the material of the metal plates. Therefore, various materials can be used as the metal plates depending on the application of the spot welded joint 1.

[0066] A suitable example of the metal plate is a steel plate. In particular, it is preferable that the first metal plate 111 is a mild steel plate having a tensile strength of 440 MPa or less, and at least one of the other metal plates is a high-strength steel plate having a tensile strength of 980 MPa or more. For example, when the spot welded joint 1 is an automobile part, the thin first steel plate is used as an exterior member of the automobile, and the other steel plate (e.g., the second steel plate) is used as a skeletal member of the automobile. The exterior member is subjected to various press processes to improve the aesthetic appearance of the automobile. Therefore, it is preferable that the exterior member is made of mild steel. On the other hand, the skeletal member is responsible for the strength and collision safety of the automobile. Therefore, it is preferable that the skeletal member is made of a high-strength steel plate.

[0067] The metal plate may be made of a material other than steel, such as an aluminum plate, a stainless steel plate, or a titanium plate. A plating layer may be provided on the surface of the metal plate. The plating layer may be, for example, a zinc-based plating layer, an aluminum-based plating layer, or a nickel-based plating layer.

[0068] The thickness of the metal plate is not particularly limited. For example, from the viewpoint of further suppressing poor joining, the thickness of the second metal plate 112 is preferably 15% or more, 17% or more, or 20% or more of the total thickness of the metal plates included in the plate set 11. Alternatively, the thickness of the second metal plate 112 is preferably 1.2 times or more, 1.3 times or more, or 1.4 times or more of the thickness of the first metal plate 111.

[0069] For example, when the spot welded joint 1 is used as an automobile part, the first metal plate 111 may have a thickness of 0.5 to 2.3 mm, and the second metal plate 112 may have a thickness of 0.8 to 2.6 mm. The thicknesses of the first metal plate 111 and the second metal plate 112 refer to the thicknesses at the spot welds. When metal plates having an inconsistent thickness, such as tailored blanks, are used as the metal plates included in the sheet set 11, the thickness measured at the welds is regarded as the thickness of the metal plates.

[0070] Although the embodiment of the present invention has been described above, the present invention is not limited thereto, and can be appropriately modified without departing from the technical spirit of the invention. A more preferable example of the method for manufacturing a spot welded joint according to the present embodiment will be described below.

[0071] (electrode tip diameter) In the manufacturing method of the spot welded joint according to the present embodiment, the tip curvature radii of the pair of electrodes are determined to satisfy formula 1. As long as the requirements of formula 1 are satisfied, the tip shape of the electrodes is not particularly limited, and a value according to the tip curvature radius can be appropriately adopted. For example, the tip diameter of the first electrode E1 arranged on the side of the thin first metal plate 111 may be smaller than that of the second electrode E2 arranged on the side of the thick second metal plate 112. For example, when the tip diameter of the first electrode E1 is 6 mm, the tip diameter of the second electrode E2 may be 6 mm, 7 mm, or 8 mm.

[0072] (electrode tip curvature radius) In the manufacturing method of a spot welded joint according to this embodiment, the ratio of the tip curvature radii of a pair of electrodes is specified by Formula 1. As long as this regulation is satisfied, the tip curvature radii of the electrodes can be freely selected according to the thickness, material, etc. of the sheet assembly 11.

[0073] For example, the tip curvature radius R1 of the first electrode may be 10 mm or more, 20 mm or more, or 30 mm or more. By increasing R1, the indentation depth can be reduced to further improve the appearance quality and nugget growth at the thick plate side interface can be promoted. Therefore, R1 is preferably 10 mm or more. On the other hand, as described above, it is preferable that the first electrode E1 has a shape that is more pointed than the second electrode E2. By reducing R1, nugget formation on the thin plate side is promoted, so R1 is preferably 40 mm or less.

[0074] Also, for example, the tip curvature radius R2 of the second electrode may be 60 mm, 100 mm, or 300 mm. By increasing R2, the effect of suppressing splashing can be further enhanced. Therefore, R2 is preferably 60 mm or more. On the other hand, by decreasing R2, the contact state between the electrode and the metal plate can be stabilized when a disturbance occurs. Therefore, R2 is preferably 300 mm or less.

[0075] (Welding current value I2 and welding time T2 during the second weld time) The second current flow time is set to allow the molten portion generated at the interface between the first metal plate 111 and the metal plate in contact therewith during the first current flow time to grow to the second metal plate 112. It is sufficient that the heat input amount sufficient to achieve this purpose is achieved during the second current flow time.

[0076] The welding current value I2 and the welding time T2 in the second current conduction time can be freely set within the above-mentioned range. The welding current value I2 can also be appropriately changed in the second current conduction time. For example, a current conduction pattern in which the welding current value is not constant, such as a slope current conduction in which the welding current value is gradually increased or decreased, or a stepwise current conduction in which the welding current value is increased or decreased in stages, may be applied to the second current conduction time. Of course, the welding current value may be constant in the second current conduction time. A post-current conduction time may be provided after the second current conduction time. The post-current conduction time is a time during which a post-heating current is passed through the nugget 12 to perform a heat treatment such as tempering.

[0077] In spot welding of a sheet group 11 having a high sheet thickness ratio, the most difficult thing is to join a thin metal plate arranged on the surface of the sheet group 11 to another metal plate. In the manufacturing method of a spot welded joint according to the present embodiment, this most difficult item is achieved in the first current flow time. Therefore, in the second current flow time performed following the first current flow time, the current flow pattern can be designed very flexibly. From the viewpoint of further suppressing the occurrence of expulsion, it is preferable to set the welding current value I2 in the second current flow time to a low value and the length T2 of the second current flow time to a large value. On the other hand, from the viewpoint of improving the efficiency of the welding work, it is preferable to set the welding current value I2 to a high value and the length T2 to a small value within a range not exceeding the upper limit value defined by the formula 3. For example, the current flow time T2 may be 80t to 200t (msec) with respect to the total sheet thickness t (mm) of the sheet group 11. EXAMPLES

[0078] 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.

[0079] A plate assembly with a plate thickness ratio of 5.62 consisting of three overlapping sheets of 270 MPa-class steel plate A with a plate thickness of 0.65 mm, 980 MPa-class steel plate B with a plate thickness of 1.8 mm, and non-plated 1.5 GPa-class hot stamp steel plate C with a plate thickness of 1.2 mm was used as the welding base material. Steel plate A was the first metal plate, and steel plate C was the second metal plate. A plate gap of 1 mm was inserted between steel plates B and C, and the occurrence of expulsion and the state of nugget formation on the thinner plate side were evaluated.

[0080] In all examples, the welding pressure was constant at 3.92 kN from the start of the first welding time to the end of the second welding time. The welding time T2 in the second welding time was 360 msec, and the welding current value I2 was 7.0 kA.

[0081] The evaluation criteria for "splattering" were as follows: If no scattering occurred at all or only slight scattering like that of a sparkler occurred, it was judged as "none." If small splashes occurred that did not affect the working environment or the quality of the welds, the result was judged as "passable." If splashing occurred to an extent that would affect the working environment or the final quality of the weld (nugget diameter after the second current), it was judged as "Yes." The specific evaluation method of the weld quality was as follows. For each electrode combination, the nugget diameter on the thick plate side formed after the second current was compared with the nugget diameter on the thick plate side formed with a constant current of 360 msec for the current flow time and 7.0 kA for the welding current. If the expulsion occurring during the first current flow is large, the nugget diameter on the thick plate side formed after the second current flow varies. Therefore, the nugget formation ratio on the thick plate side (= nugget diameter on the thick plate side after the second current flow in the present invention / nugget diameter on the thick plate side at constant current flow) was used as an index, and if the ratio was 0.90 or less, it was determined that expulsion occurred "present." Note that in comparative examples in which R1 and R2 were the same value and did not satisfy formula 1, the measurement of the nugget formation ratio on the thick plate side was omitted.

[0082] The criteria for judging "thin plate side nugget formation" were as follows. If a nugget was not formed on steel plate A, it was judged as "unacceptable." If a nugget was formed on steel plate A, but the penetration rate of the nugget into steel plate A was less than 10% of the plate thickness of steel plate A, it was judged as "passable." If the penetration rate of the nugget into steel plate A was 10% or more of the plate thickness of steel plate A, it was judged as "good." The nugget penetration rate is the value obtained by dividing the penetration depth of the nugget in steel plate A by the plate thickness of steel plate A. The penetration rate is measured on a cross section passing through the center of the nugget impression and perpendicular to steel plate A.

[0083] "α" in the table is a value calculated by substituting the expulsion limit current value I0, the welding current value I1 in the first current flow time, the length T1 of the first current flow time, the tip curvature radius R1 of the first electrode, and the tip curvature radius R2 of the second electrode into formula 2. If "α" in the table does not satisfy formula 5, it is determined that the welding conditions in the first current flow time were inappropriate. The expulsion limit current value I0 can be back-calculated from the values ​​R1, R2, T1, I1, and α in the table. Note that in comparative examples in which R1 and R2 are the same value and do not satisfy formula 1, the calculation of α was omitted.

[0084] [Table 1]

[0085] [Table 2]

[0086] [Table 3]

[0087] [Table 4]

[0088] Values ​​outside the range of the invention are underlined in Tables 1 to 4. It can be seen that according to the welding conditions that satisfy the formulas 1 to 7, no expulsion occurs and sufficient nugget penetration depth is obtained in the thin steel sheet A.

[0089] In Comparative Examples 1 to 8 and Comparative Examples 56 to 61, the tip radius of curvature R1 of the first electrode and the tip radius of curvature R2 of the second electrode were the same, and formula 1 was not satisfied. In most of these Comparative Examples, a nugget was not formed on the steel sheet A. In Comparative Examples 8 and 61, a nugget was formed on the steel sheet A, but the occurrence of expulsion could not be suppressed.

[0090] In Comparative Examples 13, 17, 21, 29, 34, 39, 45, 50, and 55, the coefficient α exceeded the upper limit defined in Equation 5. That is, in these Comparative Examples, the welding current value I1 in the first current flow time was excessive. In all of the Comparative Examples in which the welding current value I1 in the first current flow time was excessive, the occurrence of expulsion could not be suppressed. [Explanation of symbols]

[0091] 1 Spot welded joints 11 Board set 111 Daiichi Metal Plate 112 Second metal plate 12. Nugget E1 first electrode R1 Radius of curvature of the tip of the first electrode E2 second electrode R2 Radius of curvature of the tip of the second electrode T1 Length of first energization time I1 Welding current value during the first weld time T2 Length of the second energization time I2 Welding current value during the second welding time

Claims

1. The method includes a step of spot welding a plate assembly including three or more metal plates using a pair of electrodes, a thickness of a first metal plate arranged on one surface of the plate set is smaller than a thickness of a second metal plate arranged on the other surface of the plate set; A plate thickness ratio, which is a value obtained by dividing the thickness of the plate assembly by the thickness of the first metal plate, is 5.0 or more, the spot welding has a first current flow time and a second current flow time subsequent to the first current flow time, The following formulas 1, 2, 3, 4, and 5 are satisfied, [0010] R1 is the tip curvature radius of the first electrode, which is the electrode disposed on the first metal plate side, in mm; R2 is the tip curvature radius of the second electrode, which is the electrode disposed on the second metal plate side, in mm; T1 is the length of the first current application time in msec, I1 is the welding current value in A during the first weld time; I2 is the welding current value in A during the second weld time; I0 is the limiting current value at which expulsion occurs in A; The expulsion limit current value is a maximum current value that does not cause expulsion in the plate assembly when current is applied to the plate assembly by a pair of the first electrodes for the period T1, α is a value that satisfies Equation 5 A method for manufacturing spot welded joints.

2. The T1 satisfies formula 6, The α satisfies the formula 7 [0025] The method for manufacturing a spot welded joint according to claim 1 .

3. The spot welding further has a cool time between the first energization time and the second energization time.

3. The method for manufacturing a spot welded joint according to claim 1 or 2.

4. The pressure applied during the first current application time and the second current application time is kept constant.

3. The method for manufacturing a spot welded joint according to claim 1 or 2.

5. The preheating time, which is the length from the start of current flow to the point at which the current value has increased to 0.9 times the value obtained by substituting α = 0.210 into Equation 2, is set to 50 msec or less.

3. The method for manufacturing a spot welded joint according to claim 1 or 2.

6. The metal plate is a steel plate.

3. The method for manufacturing a spot welded joint according to claim 1 or 2.

7. The first metal plate is a mild steel plate having a tensile strength of 440 MPa or less, At least one of the metal plates excluding the first metal plate is a high-strength steel plate having a tensile strength of 980 MPa or more. The method for manufacturing a spot welded joint according to claim 6 .

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