JP6468767B2 - Spot welding method - Google Patents

Description

  The present invention relates to a spot welding method for welding by energizing a pair of electrodes in contact with a plurality of metal plates.

  In the automobile manufacturing process, a plurality of superposed metal plates (mainly steel plates) are joined by spot welding. In spot welding, a pair of electrodes are brought into contact with a plurality of metal plates and energized, and a part of the plurality of metal plates is melted by resistance heating at this time to form a nugget. The plates are joined.

After spot welding, an inspection method using a chisel is performed as a method for inspecting the quality of the nugget. In this inspection method, a chisel is driven between metal plates, and at this time, whether or not the nugget is good is determined based on whether or not the nugget is peeled off (see, for example, Patent Document 1 below).

Japanese Patent Laid-Open No. 5-99601 JP 2013-78782 A

  In recent years, for the purpose of reducing the weight of automobiles, high-strength steel sheets such as high-tensile steel sheets (high-tensile steel) and ultra-high-tensile steel sheets (super-high tensile steel) are sometimes used for automobile parts. When such high-strength steel plates are joined together by spot welding, the nugget and its surroundings have high hardness, so if the nugget is inspected using the chisel as described above, the nugget is likely to crack. The problem arises.

  For example, in the above-mentioned Patent Document 2, a first step of energizing a welding current to form a nugget, a second step of energizing at a current value equal to or lower than the welding current, a third step of cooling the welded portion, A spot welding method is shown in which a fourth step of sequentially energizing at a current value larger than the above welding current and heating the welded portion to the recrystallization temperature range is performed. In the second step, a wide softening region is formed outside the nugget by passing a current equal to or lower than the welding current through the nugget. In the third step, the entire nugget is heated in the next fourth step by sufficiently cooling the vicinity of the nugget. Further, in the fourth step, the nugget is heated to the recrystallization temperature range, thereby forming an equiaxed structure and increasing the strength of the nugget (particularly the cross tensile strength). In this way, by forming a softened zone outside the nugget and generating a recrystallized structure in the nugget, the toughness of the nugget and its surroundings (heat affected zone) is improved, and damage to the nugget due to the chisel test is improved. Can be prevented.

  However, in order to generate a recrystallized structure in the nugget, it is necessary to pass a very high current in the fourth step (18.0 to 24.0 kA). For this reason, a large-capacity welding machine is required, and the equipment cost increases.

  In view of the above circumstances, the problem to be solved by the present invention is to prevent damage to the nugget by the chisel test as much as possible by a low-cost method.

  In order to solve the above problems, the present invention provides a main energization for forming a nugget by energizing a pair of electrodes in contact with a plurality of metal plates, and an average current value of the main energization after the main energization. A low current post-energization that is energized at a current value that is smaller than the nugget, and a high current post-energization that is energized at a current value greater than the average current value of the main energization after the low current post-energization. Provided is a spot welding method for continuously performing.

  As described above, by applying the low current after energization at a current value that does not expand the nugget after the main energization, the periphery of the nugget (mainly the heat affected zone) can be softened. Then, by applying current after high current, a recrystallized structure can be generated inside the nugget, and the toughness of the joint can be increased. In addition, according to the verification by the present inventors, without energization stop period between main energization and low current after energization, and between low current after energization and high current after energization, It has been clarified that the recrystallized structure can be generated inside the nugget while suppressing the current value of the high current post-energization by continuously conducting the current and the high current post-energization. Thereby, since a recrystallized structure | tissue can be produced | generated inside a nugget and a nugget can be softened using the welding machine of a comparatively small capacity | capacitance, installation cost is suppressed. As described above, according to the spot welding method of the present invention, it is possible to increase the toughness of the nugget and its surroundings by a low cost method and prevent damage due to the chisel test as much as possible.

It is sectional drawing which shows a mode that the spot welding method which concerns on one Embodiment of this invention is performed. It is a graph which shows the electricity supply pattern in the said spot welding method. It is sectional drawing of the nugget vicinity of the metal plate welded by said spot method. It is a graph which shows the other example of an electricity supply pattern. It is a graph which shows the other example of an electricity supply pattern. It is a graph which shows the other example of an electricity supply pattern.

  FIG. 1 shows how a spot welding method according to an embodiment of the present invention is performed. The spot welding method of this embodiment is what is called direct spot welding. Specifically, a plurality of stacked metal plates W1 and W2 (two in the illustrated example) are sandwiched and pressed between the pair of electrodes 10 and 20 and energized in this state. The contact portion is melted by resistance heating to form the nugget N. In the present embodiment, the metal plates W1 and W2 are steel plates. In particular, one or both of the metal plates W1 and W2 are high-strength steel plates, specifically, high-tensile steel plates having a tensile strength exceeding 490 MPa, or tensile strength of 980 MPa. Consists of super high strength steel plates exceeding In addition, the material of metal plate W1, W2 is not limited, For example, the normal steel plate whose intensity | strength is 490 Mpa or less can also be used for one or both of these.

  As described above, current is passed between the electrodes 10 and 20 in a state where the metal plates W1 and W2 are sandwiched and pressed by the pair of electrodes 10 and 20. The energization pattern at this time is shown in FIG. In the figure, the vertical axis represents the current value, and the horizontal axis represents the elapsed time (time). Specifically, first, the current value flowing between the electrodes 10 and 20 is gradually increased from 0 to the current value I1 (rise period P0). Then, by energizing for a predetermined time with current value I1, a part of metal plates W1 and W2 is melted to form a nugget (main energization P1). The energization time of the main energization P1 is set so that a nugget of a desired size is formed, and is set to about 5 to 30 cycles, for example. In the illustrated example, the current value of the main energization P1 is constant, but the main energization P1 may be performed while changing the current value. In this case, the average current value during the main energization P1 is I1.

  After the main energization P1, the current value is decreased to a current value I2 smaller than the average current value I1 of the main energization P1, and the current value I2 is energized for a predetermined time (low-current post-energization P2). By this low current post-energization P2, the periphery of the nugget (heat-affected zone) is cooled relatively slowly, and this portion is softened. The main energization P1 and the low current post-energization P2 are performed continuously, and no energization stop period is provided between them. The current value I2 of the low current post-energization P2 is set to such a size that the nugget formed by the main energization P1 is not enlarged, that is, a size that does not further melt the metal plates W1 and W2. Specifically, the current values I2 and I4 in the low current post-energization P2 are set to 80% or less, preferably 60% or less of the average current value I1 of the main energization P1, for example. The current values I2 and I4 are preferably not less than a predetermined value in order to stably soften the periphery of the nugget. For example, the current values I2 and I4 are set to 25% or more, preferably 40% or more of the average current value I1 of the main energization P1. Is done.

  After the low current post-energization P2, the current value is increased to a current value I3 larger than the average current value I1 of the main energization P1, and the current value I3 is energized for a predetermined time (high current post-energization P3). As a result, the dendrite (dendritic) structure disappears near the outer peripheral edge of the nugget, and a recrystallized structure appears. Specifically, as shown in FIG. 3, a doughnut-shaped recrystallized structure M is generated in the vicinity of the inner peripheral portion of the nugget N, and thereby the nugget (particularly the outer peripheral end) is softened. The low-current post-energization P2 and the high-current post-energization P3 are performed continuously, and no energization stop period is provided between them. The energization time of the high current post-energization P3 is set to be sufficiently shorter than the energization time of the main energization P1 and the low current post-energization P2, for example, about 3 to 5 cycles. Thus, spot welding is completed.

  Thus, the recrystallized structure M is generated in the nugget N, and the dendrite structure is eliminated, so that the nugget N (particularly the outer peripheral edge) is softened and the toughness of the nugget N can be increased. In particular, in the present embodiment, no energization stop period is provided between the main energization P1 and the low-current post-energization P2, and between the low-current post-energization P2 and the high-current post-energization P3. That is, after the main energization P1 is completed (that is, after the nugget having a predetermined diameter is formed), the energization is continuously performed without setting the current value to 0 until the post-energization is completed. Thereby, even when the current value I3 of the energization P3 after the high current is made relatively small, the recrystallized structure M can be generated inside the nugget N and the outer peripheral edge of the nugget can be softened, so the capacity is relatively small. A welding machine can be used, and equipment costs can be reduced. The current value I3 can be, for example, not more than twice the average current value I1 of the main energization P1.

  The present invention is not limited to the above embodiment. For example, in the embodiment shown in FIG. 4, after the high current post-energization P3 in the energization pattern of FIG. Specifically, after the main energization P1, the first low current post-energization P2, the high current post-energization P3, and the second low current post-energization P4 are continuously performed.

  In the embodiment shown in FIG. 5, after the second low current energization P4 of the energization pattern of FIG. Specifically, after the main energization P1, the first low-current post-energization P2, the first high-current post-energization P3, the second low-current post-energization P4, and the second high-current post-energization P5 are continuously performed. Is going.

  In the embodiment shown in FIG. 6, the low current post-energization P6 is further applied after the second high current post-energization P5 of the energization pattern of FIG. Specifically, after the main energization P1, the first low current post-energization P2, the first high current post-energization P3, the second low current post-energization P4, the second high current post-energization P5, and the third The low-current post-energization P6 is continuously performed.

  In the embodiment shown in FIGS. 4 to 6, the current values of the low current post-energizations P2, P4, and P6 are the same value, but these current values may be different. In these embodiments, the current values of the high current post-energizations P3 and P5 are the same, but these current values may be different.

  Moreover, although the case where this invention was applied to direct spot welding was shown in said embodiment, this invention can also be applied to other spot welding, for example, indirect spot welding and series spot welding.

  In order to investigate preferable conditions in the spot welding method of the present invention, the following tests were conducted.

  First, as shown in Table 1 below, direct spot welding was performed on a plurality of test pieces with different energization patterns. Specifically, the two high-tensile steel plates (both SPC980DU1.2t) were energized with the energization pattern shown in FIG. Y1 to Y4 vary the current values I2 and I4 of the currents P2 and P4 after low current to 2.0 kA and the current values I3 and I5 of the currents P3 and P5 after high current in the range of 8.6 to 10.8 kA. It was. Y5 to Y8 vary the current values I2 and I4 of the currents P2 and P4 after the low current to 3.0 kA, and the current values I3 and I5 of the currents P3 and P5 after the high current in the range of 8.6 to 10.8 kA. It was.

  When the average hardness of the heat-affected zone was measured for each test piece after welding, all the test pieces were approximately 420 HV or less, but the current values I2 and I4 of the currents P2 and P4 after low current were 3.0 kA. Specimens Y5 to Y8 indicated a hardness of 400 HV or less more stably. From this, it can be said that the current values I2 and I4 of the low current post-energization P2 and P4 are preferably 25% or more, preferably 40% or more of the average current value I1 of the main energization P1.

  Next, as shown in Table 2 below, the current values I2 and I4 of the currents P2 and P4 after low current are fixed at 3.0 kA, and the current values I3 and I5 of the currents P3 and P5 after high current are 8.6. Differentiated in the range of ~ 10.8 kA. Other conditions are the same as above. Y9, Y11, Y13, and Y15 in Table 2 are the same conditions as Y5, Y6, Y7, and Y8 in Table 1, respectively.

  When the average hardness of the heat affected zone was measured for each test piece after welding, no significant difference was found. On the other hand, when the hardness of the outer periphery edge of the nugget was measured, it became clear that the hardness of the outer periphery edge of the nugget decreased as the current values I3 and I5 of the energization P3 and P5 after high current increased. Further, when the cross-section of the nugget of Y9 and Y15 was observed, the formation of a doughnut-shaped recrystallized structure was confirmed in the vicinity of the outer peripheral portion inside the nugget of Y9, as shown in FIG. The recrystallized structure as described above was not confirmed inside.

  Furthermore, the test piece Y9 to Y15 was subjected to the same load as that of the chisel test, and the presence or absence of breakage (cracking) of the nugget at this time was confirmed. Specifically, a 1.2-t shim was inserted into the gap between the metal plates of each test piece, and pushed into the position where the distance from the nugget was 3 mm with a hammer. And each test piece was cut | disconnected in the position which crosses a nugget, and the presence or absence of the crack in the cross section was confirmed.

  As a result, cracking of the nugget may be confirmed in Y11 to Y15 in which the current value I3 of the post-high current energization P3 (and the current value I5 of the post-high current energization P5, and so on) is 10.1 kA or less. . Specifically, in Y11, 12, and 13, minute cracks of 2 mm or less occurred at the outer peripheral edge of the nugget (indicated by Δ in Table 2). In Y15, a relatively large crack exceeding 2 mm occurred at the outer peripheral edge of the nugget (indicated by x in Table 2). On the other hand, cracking of the nugget was not confirmed in Y9 and Y10 in which the current values I3 and I5 of the energization P3 and P5 after the high current were larger than 10.1 kA (indicated by ◯ in Table 2).

  From the results shown in Table 2, even when Y9, where the current value I3 of the high current post-energization P3 is less than twice the average current value I1 of the main energization P1, the nugget is sufficiently softened by generating a recrystallized structure. In other words, it can be said that cracking of the nugget by the tage test can be prevented. On the other hand, in order to sufficiently soften the nugget, the current value I3 of the high current post-energization P3 is made larger than 1.4 times the average current value I1 of the main energization P1, thereby sufficiently softening the outer peripheral edge of the nugget. It can be said that it is preferable.

10, 20 Electrode M Recrystallized structure N Nugget W1, W2 Metal plate P0 Ascending period P1 Main energization P2, P4, P6 Low current energization P3, P5 High current after energization

Claims (2)

A main energization that forms a nugget by energizing a pair of electrodes in contact with a plurality of metal plates, and a current that is smaller than the average current value of the main energization and does not expand the nugget after the main energization By continuously performing a low current post-energization with a current value and a high current post-energization with a current value larger than the average current value of the main energization after the low current post-energization , the inside of the nugget A spot welding method for generating a recrystallized structure .   The spot welding method according to claim 1, wherein a current value of the high current post-energization is equal to or less than twice an average current value of the main energization.

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