Manufacturing method for spot welded joints

Abstract

To provide a new improved manufacturing method for a spot-welded joint, the method enabling LME cracking and hydrogen embrittlement cracking to be more surely suppressed.SOLUTION: A manufacturing method for a spot-welded joint is provided in which a plate combination constituted of a plurality of overlapped steel plates is spot-welded, wherein one or more of the plurality of steel plates are high-strength steel plates having a tensile strength of 980 MPa or higher and at least either of the high-strength steel plates and the steel plates arranged adjacently to the high-strength steel plates have galvanized layers. The manufacturing method for a spot-welded joint includes: a main energizing step of forming a nugget; a first cooling step; and a second cooling step in which a cooling speed is faster than in the first cooling step, wherein retention time which is time elapsing after energization is finished until an electrode releases pressurization is 0.5 (sec) or more.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] For example, spot welding is mainly used to join steel sheets together when assembling structural components for automobiles. In spot welding, a set of steel sheets is sandwiched between a pair of electrodes and current is applied while pressure is applied, causing the joining interface between the steel sheets to melt and solidify. This forms a nugget at the joining interface between the steel sheets, joining the steel sheets together. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-103377 [Patent Document 2] Patent No. 6108018 [Patent Document 3] Patent No. 6777173 [Patent Document 4] Patent No. 5987982 [Patent Document 5] Patent No. 5151615 [Patent Document 6] Patent No. 5429327 [Patent Document 7] Japanese Patent Application Publication No. 2020-157358 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, when the plurality of steel sheets includes a high-strength steel sheet, and at least one of the high-strength steel sheet and a steel sheet adjacent to the high-strength steel sheet has a zinc-based coating, it is known that spot welding can cause LME (Liquid Metal Embrittlement) cracking or hydrogen embrittlement cracking.

[0005] The mechanism by which LME cracking occurs is generally as follows: The zinc-based coating layer on the surface of the steel sheet melts due to Joule heat during welding, turning into molten zinc. Meanwhile, after the current and pressure are applied, the steel sheet undergoes thermal contraction, and tensile stress due to this thermal contraction is applied to the area around the nugget. As a result, the molten zinc penetrates the grain boundaries, reducing their strength. This causes LME cracking. Meanwhile, hydrogen embrittlement cracking is cracking caused by hydrogen in the steel sheet embrittling the steel sheet. The hydrogen that causes hydrogen embrittlement cracking can be present in the steel sheet beforehand, but can also penetrate into the steel sheet from, for example, processing oil applied to the surface of the steel sheet.

[0006] Patent Documents 1 to 7 disclose techniques for suppressing LME cracking or hydrogen embrittlement cracking, and these techniques provide certain effects. However, the properties required for automotive structural components are becoming stricter every year, and further improvements in suppressing LME cracking and hydrogen embrittlement cracking are needed.

[0007] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a new and improved method for manufacturing a spot-welded joint that can more reliably suppress LME cracking and hydrogen embrittlement cracking. [Means for solving the problem]

[0008] In order to solve the above problems, according to one aspect of the present invention, there is provided a method for manufacturing a spot welding joint for spot welding a plate assembly composed of a plurality of overlapped steel plates, wherein one or more of the plurality of steel plates are high-strength steel plates with a tensile strength of 980 MPa or more, and at least one of the high-strength steel plate and the steel plate adjacent to the high-strength steel plate has a zinc-based plating. The method for manufacturing the spot welding joint includes a main energization step of forming a nugget, a first cooling step, and a second cooling step with a faster cooling rate than the first cooling step. The holding time, which is the time from the end of energization to the release of the pressure applied by the electrode to the steel plate, is 0.5 (sec) or more. After the second cooling step, the spot welding joint has a two-layer structure in a cross-sectional view passing through the center of the nugget and perpendicular to the surface of the plate assembly, where the nugget has an inner structure and an outer structure. The ratio of the diameter of the inner structure to the diameter of the nugget in the direction parallel to the surface of the plate assembly is 40 to 100%, the ratio of the diameter of the inner structure to the diameter of the nugget in the direction perpendicular to the surface of the plate assembly is 0 to 80%, and the aspect ratio of the diameter of the inner structure is 25% or less. There is provided a method for manufacturing a spot welding joint characterized by the above.

[0009] Here, in the first cooling step, 0 < t1 ≦ 2 (sec), 0.6 × I0 < I1 ≦ I0 (kA), the cooling rate is 500 °C / sec or less. In the second cooling step, 0.5 (sec) < t2, 0 ≦ I < I1 (kA), the cooling rate is 2000 °C / sec or more. I0 is the current value during the main energization step, t1 and I1 may be the time and I1 may be the current value during the first cooling step, and t2 and I2 may be the time and current value during the second cooling step.

[0010] Also, in the main energization step, the nugget diameter may be set to 3√t to 6√t, where t is the minimum plate thickness among the plate thicknesses of the plurality of steel plates.

[0011] Before the main energization step, a preliminary energization step of 0 < tp < <0.1 (sec), I0 < Ip < 20 (kA) may be performed. tp is the time of the preliminary energization step, I0 is the current value during the main energization step, and Ip may be the current value during the preliminary energization step.

[0012] Immediately before the start of main energization, one or more of the following conditions may be satisfied: the impact angle is 1° or more; the total gap is 0.5 mm or more; and the clearance is 0.1 mm or more. [Effects of the Invention]

[0013] According to the above-described aspects of the present invention, LME cracking and hydrogen embrittlement cracking can be more reliably suppressed. [Brief explanation of the drawings]

[0014] [Figure 1] 6 is a graph showing an outline of each current application pattern in the examples. [Figure 2] 5A and 5B are cross-sectional photographs of nuggets obtained by each current application pattern. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, the present embodiment will be described with reference to the drawings. Numerical ranges indicated with "to" include the upper and lower limits. Numerical ranges indicated with "greater than" or "less than" do not include the upper or lower limits.

[0016] <1. Overview of this embodiment> As described above, when a sheet assembly includes a high-strength steel sheet and at least one of the high-strength steel sheet and a steel sheet adjacent to the high-strength steel sheet has a zinc-based coating, there is a concern that LME cracking or hydrogen embrittlement cracking may occur due to spot welding.

[0017] One possible technique for suppressing LME cracking is to increase the hold time. Here, the hold time is the time from the end of current application to the release of the electrode pressure on the steel sheet. However, with this method, the hardness of the nugget (weld) increases due to rapid cooling by the electrode (which cools after current application is stopped by water cooling), raising the concern of hydrogen embrittlement cracking at the nugget edge where toughness is reduced.

[0018] Therefore, in this embodiment, the cooling process after the current application process is divided into two stages. In the first cooling process, which has a slow cooling rate, rapid cooling by the electrode is avoided and an increase in nugget hardness is suppressed. That is, in the first cooling process, current is passed through the electrode to slowly cool the molten portion (while heat is input). The outer structure of the nugget is formed in this first cooling process. Then, a second cooling process, which has a faster cooling rate than the first cooling process, is performed to form the inner structure of the nugget and obtain a double nugget structure. In the second cooling process, current may or may not be passed through the electrode. Because the outer structure is softer than the inner structure, a decrease in toughness at the nugget edge is suppressed. Therefore, hydrogen embrittlement cracking is suppressed. Furthermore, by extending the holding time, LME cracking that occurs directly below the electrode, in the shoulder portion, and immediately outside the nugget can also be suppressed.

[0019] Therefore, according to this embodiment, the double nugget structure makes it possible to prevent both LME cracking and hydrogen embrittlement cracking. Furthermore, when preliminary current is also applied, the effect of suppressing spattering improves the effect of suppressing LME cracking.

[0020] If the nugget structure has a single-layer structure consisting of only the outer microstructure, the first cooling step described above must be performed for a long time, which increases the cycle time. On the other hand, if the nugget structure has a single-layer structure consisting of only the inner microstructure (i.e., if the nugget is formed by conventional spot welding), the toughness of the nugget edge decreases, making it more susceptible to hydrogen embrittlement cracking.

[0021] <2. Mechanism of this embodiment> As described above, the method for manufacturing a spot-welded joint according to this embodiment not only involves holding a high-strength steel sheet (e.g., a galvanized high-strength steel sheet) in contact with a zinc-based coating for a long period of time, but also gradually cooling the molten portion (first cooling step) while current is passed through the electrode (while heat is being input) after the main current application step. This first cooling step forms the outer structure of the nugget. Then, a second cooling step, which has a faster cooling rate than the first cooling step, is performed to form the inner structure of the nugget, resulting in a double nugget structure. This makes it possible to form a double nugget with inner and outer structures that have different cooling steps (cooling stages), thereby simultaneously solving LME cracking and hydrogen embrittlement cracking.

[0022] Generally, hydrogen embrittlement cracking is caused by the presence of hydrogen in the steel sheet composition or the intrusion of hydrogen from processing oil or the atmosphere, as well as the generation of tensile stress. For this reason, methods have been proposed to suppress hydrogen in the steel by specifying the steel sheet composition. However, this technology is not very effective against hydrogen intrusion from processing oil or the atmosphere. Therefore, when the cooling rate increases due to heat removal from the electrode after the current application process, the toughness of the nugget edge decreases, causing hydrogen embrittlement cracking due to the generated tensile stress and hydrogen absorbed from the processing oil that adhered during previous processes (such as press working).

[0023] To solve this problem, the inventors focused on a method for forming a double nugget with inner and outer structures that undergo different cooling processes. Specifically, as described above, by continuing to hold the electrode while current is applied (while heat is being input) after the main current application process, the cooling rate is slowed without generating stress. This improves the toughness of the nugget edge, making it possible to reduce hydrogen embrittlement cracking and improve CTS (peel strength). In other words, when an attempt is made to peel a welded joint, stress concentrates on the outside of the nugget, but the improved toughness of the outer structure of the nugget improves peel strength.

[0024] Furthermore, LME cracking is caused by the temperature around the nugget being above the coating melting point and the generation of tensile stress. Therefore, a method has been proposed in which the temperature of the molten part is lowered without generating stress by continuing to apply pressure with the electrodes until the temperature drops below the coating melting point. In other words, the temperature of the molten part is lowered by extending the holding time. However, simply extending the holding time increases the risk of hydrogen embrittlement cracking due to rapid cooling by the electrodes. Therefore, in this embodiment, in the first cooling step, pressure (holding) with the electrodes is continued while current is passed through the electrodes. Then, the current is stopped and pressure (holding) is performed for a long period of time. This makes it possible to simultaneously suppress hydrogen embrittlement cracking and LME cracking.

[0025] <3. Manufacturing method of spot welded joint according to this embodiment> Next, a method for manufacturing a spot-welded joint according to this embodiment will be described. The method for manufacturing a spot-welded joint according to this embodiment is a method for manufacturing a spot-welded joint by spot-welding a sheet assembly made of a plurality of overlapping steel sheets. Here, at least one of the plurality of steel sheets is a high-strength steel sheet having a tensile strength of 980 MPa or more, and at least one of the high-strength steel sheet and a steel sheet adjacent to the high-strength steel sheet has a zinc-based coating.

[0026] The type of zinc-based coating is not particularly limited, and may be, for example, hot-dip galvanizing (GI), galvannealed hot-dip galvanizing (GA), or electrogalvanizing (EG). The composition of the zinc-based coating is also not particularly limited, and various zinc-based coatings, such as Zn-Mg-Al coating, can be used. The number of steel sheets is also not particularly limited, and may be about 2 to 3 sheets, or 4 or more sheets.

[0027] The method for manufacturing a spot welded joint according to this embodiment includes a main current application step, a first cooling step, and a second cooling step, and the holding time is set to 0.5 (sec) or more.

[0028] This current application process is a process for forming a nugget. More specifically, a sheet set consisting of a plurality of steel sheets is sandwiched between a pair of electrodes and pressurized while current is applied, thereby melting and solidifying the joining interface between the steel sheets. This forms a nugget at the joining interface between the steel sheets and expands the nugget.

[0029] The first cooling step is a cooling step using heat generated by current application immediately after the nugget expansion is completed (i.e., the main current application step is completed), in which heat is generated by current application at the joining interface between the steel sheets but the nugget does not grow. The first cooling step may be performed while maintaining the current value of the main current application step, and does not have to be clearly distinguishable from the main current application step in the current application pattern exemplified in FIG.

[0030] The second cooling step is a cooling step that occurs immediately after the first cooling step, and has a faster cooling rate than the first cooling step. In the second cooling step, the joining interface between the steel sheets may or may not be electrically heated. The second cooling step is preferably a step in which the steel sheets are cooled by extracting heat from the electrodes without electrically heating the joining interface between the steel sheets.

[0031] The holding time is the time from the end of current application to the release of pressure from the electrodes, and in this embodiment, the holding time is set to 0.5 seconds or more. This makes it possible to prevent LME cracking. If the holding time is less than 0.5 seconds, there is a risk of LME cracking occurring. If no current application heat generation is performed in the second cooling step, the time t2 of the second cooling step and the holding time become equal.

[0032] Thus, in this embodiment, while performing the two-stage cooling process, the holding time is set to 0.5 (sec) or more. As a result, a double-structured nugget is formed. Also, since the outer structure of the nugget is softer than the inner structure, a decrease in the toughness at the end of the nugget is suppressed. Here, for example, the hardness (Ho) of the outer structure satisfies Hb < Ho < 1.15Hb with respect to the base material hardness (Hb), and the hardness (Hi) of the inner structure satisfies Ho < Hi. The hardness can be measured, for example, using a Vickers hardness tester. Hydrogen embrittlement cracks are suppressed by the double-structured nugget. Furthermore, by extending the holding time to 0.5 (sec) or more, LME cracks generated directly under the electrode, at the shoulder, and directly outside the nugget can also be suppressed simultaneously.

[0033] Here, the double structure of the nugget will be described in detail. In a cross-sectional view (cross-sectional view in the thickness direction) passing through the center of the nugget and perpendicular to the surface of the plate assembly, the nugget has a two-layer structure having an inner structure and an outer structure. Furthermore, the ratio of the diameter of the inner structure to the diameter of the nugget (outer structure) in the direction parallel to the surface of the plate assembly is 40 to 100%, and the ratio of the diameter of the inner structure to the diameter of the nugget (outer structure) in the direction perpendicular to the surface of the plate assembly is 0 to 80%. Furthermore, the aspect ratio of the diameter of the inner structure is 25% or less. When the nugget structure satisfies the above requirements, hydrogen embrittlement cracks and LME cracks are suppressed.

[0034] The ratio of the diameter of the inner structure to the diameter of the nugget (outer structure) in the direction parallel to the surface of the plate assembly is preferably 60 to 90%. Furthermore, the ratio of the diameter of the inner structure to the diameter of the nugget (outer structure) in the direction perpendicular to the surface of the plate assembly is preferably 50 to 65%.

[0035] The inner structure and the outer structure of the nugget can be discriminated by cross-sectional observation by corrosion using picral solution. The diameters of the outer structure and the inner structure are the maximum diameters of the outer structure and the inner structure in the cross-section obtained by cutting the center of the nugget.

[0036] When the ratio of the diameter of the inner structure to the diameter of the slug (outer structure) in the direction parallel to the surface of the plate assembly is 40 to 100% (40% or more), the effect of alleviating the segregation at the slug end can be obtained. When the ratio is less than 40%, the segregation at the slug end is not alleviated, so the strength in the peeling direction of the joint may decrease and hydrogen embrittlement cracking may occur.

[0037] When the ratio of the diameter of the inner structure to the diameter of the slug (outer structure) in the direction perpendicular to the surface of the plate assembly is 0 to 80%, the effect of suppressing the increase in hardness at the slug end can be obtained. When the ratio exceeds 80%, the slow cooling effect is not sufficient and the hardness at the slug end increases, so there is a risk of hydrogen embrittlement cracking. In addition, in order to make the ratio of the diameter of the inner structure to the diameter of the slug (outer structure) in the direction perpendicular to the surface of the plate assembly less than 20%, for example, it is necessary to perform downslope energization or the like, and the tact time increases significantly. Therefore, the ratio of the diameter of the inner structure to the diameter of the slug (outer structure) in the direction perpendicular to the surface of the plate assembly is preferably 20 to 80%.

[0038] The aspect ratio of the slug diameter of the inner structure is {(the thickness of the inner structure in the plate thickness direction) / (the diameter of the inner structure in the plate width direction)} × 100 (%). This aspect ratio needs to be 25% or less in order to sufficiently slow-cool the slug end (first cooling step). When the ratio exceeds 25%, the slow cooling effect is not sufficient, the toughness at the slug end becomes low, and there is a risk of hydrogen embrittlement cracking.

[0039] In order to obtain the double structure described above, for example, the first cooling step and the second cooling step may be set as follows. That is, in the first cooling step, 0 < t1 ≦ 2 (sec), 0.6 × I0 < I1 ≦ I0 (kA), and the cooling rate is 500 °C / sec or less. In the second cooling step, 0.5 (sec) < t2, 0 ≦ I2 < I1 (kA), and the cooling rate is 2000 °C / sec or more. Here, I0 is the current value during the main energization step, t1 and I1 are the time and current value of the first cooling step, and t2 and I2 are the time and current value of the second cooling step.

[0040] As mentioned above, I1 may be equal to I0. In this case, the same magnitude of current is passed for a long time. Even in this case, it is possible to distinguish between the main current passing process and the first cooling process. That is, in the first half of the current passing, the resistance value of the joint interface between the steel sheets is high, and the amount of heat generated in response to the current passing is large. The current passing at this time is the main current passing process. In the second half of the current passing, that is, after the nugget has fully grown, the current path widens and heat generation ceases, so the amount of heat generation decreases. That is, the joint interface between the steel sheets is cooled. The current passing at this time is the first cooling process.

[0041] It is preferable not to provide a cooling time (a time during which the current value is set to zero) between the first cooling step and the main current application step. In this case, a nugget having a double structure and satisfying the above aspect ratio can be easily formed.

[0042] As mentioned above, I2 may be 0. In this case, the time for the second cooling step is the retention time.

[0043] The current value I1 in the first cooling step and the current value I2 in the second cooling step may or may not be constant. An example of the latter is a downslope. When each current value is a downslope, it is necessary to provide a difference in cooling rate between them. It is preferable that the current value I1 in the first cooling step and the current value I2 in the second cooling step are constant. This is because in order to make the current value I1 in the first cooling step and the current value I2 in the second cooling step a downslope, the current values ​​need to be controlled in detail and the construction time becomes long. When the current value I1 in the first cooling step and the current value I2 in the second cooling step are not constant, each current value is a time average value.

[0044] The cooling rate is the cooling rate at the end of the nugget, which can be obtained by resistance welding simulation software. The end of the nugget is a 100-μm square area set at the position closest to the position corresponding to the maximum diameter of the outer structure within the nugget in the cross-section (a cross-section perpendicular to the plate surface) obtained by cutting the center of the nugget. The temperature of the end of the nugget is the average temperature of the 100-μm square area. And the cooling rate of the first cooling process is the value obtained by dividing the difference between the temperature of the end of the nugget at the start of the first cooling process and the temperature of the end of the nugget at the end of the first cooling process by the time of the first cooling process. The cooling rate of the second cooling process is the value obtained by dividing the difference between the temperature of the end of the nugget at the start of the second cooling process and the temperature of the end of the nugget at the end of the second cooling process by the time of the second cooling process. The pressing force during the first cooling process and the second cooling process is not particularly limited. For example, it may be about 0.5 to 1.5 times the pressing force P0 during the main energization.

[0045] By setting the first cooling process and the second cooling process as described above, the above-mentioned double-structured nugget can be formed.

[0046] In the main energization process, the nugget diameter may be set to 3√t to 6√t. t is the minimum plate thickness among the plate thicknesses of a plurality of steel plates. The nugget diameter here is substantially the diameter (maximum diameter) of the nugget (outer structure) in the direction perpendicular to the surface of the plate stack.

[0047] Before the main energization process, a preliminary energization process with 0 < tp < 0.1 (sec) and I0 < Ip < 20 (kA) may be performed. Here, tp is the time of the preliminary energization process, I0 is the current value during the main energization process, and Ip is the current value during the preliminary energization process. By performing the preliminary energization process before the main energization process, the generation of spatter can be suppressed, and the LME cracking of the electrode shoulder caused by spatter generation can be suppressed.

[0048] In addition, one or more of the following conditions may be satisfied: the impact angle must be 1° or greater, the total gap must be 0.5 mm or greater, and the clearance must be 0.1 mm or greater. The impact angle refers to the inclination of the mating surfaces of the steel sheets when the plane perpendicular to the line connecting the pair of electrodes is set to 0°. The gap refers to the size of the gap between the steel sheets in the welded area. When there are three or more steel sheets and two or more gaps, the gap refers to the sum of the sizes of the multiple gaps. Clearance refers to the error when clamping the workpiece (steel sheet) using the electrodes. Specifically, in spot welding, where both electrodes start from a distance from the steel sheet and are then brought closer, the clearance is the gap between the other electrode and the workpiece (steel sheet) when the other electrode contacts the steel sheet and pressure is applied. The impact angle, gap, and clearance are collectively referred to as welding disturbance. In this embodiment, even when these welding disturbances are met, it is possible to manufacture a spot welded joint in which hydrogen embrittlement cracking and LME cracking are suppressed.

[0049] As described above, according to this embodiment, it is possible to manufacture a spot-welded joint in which hydrogen embrittlement cracking and LME cracking are simultaneously suppressed. [Example]

[0050] Next, an example of this embodiment will be described. In this example, two galvannealed steel sheets with a strength of 1180 MPa and a thickness of 1.6 mm were prepared as the base steel sheets, and these were overlapped and spot welded. Specifically, spot welding was performed with Pleton oil applied to the mating surfaces of the steel sheets under the conditions of an impact angle of 3°, a gap of 2 mm, and a clearance of 0.3 mm. The welded product was stored for more than two days, embedded in resin, and the cross section was observed. The overall conditions for spot welding are as shown in Table 1 below.

[0051] [Table 1]

[0052] In Table 1, the pressure is the force with which the electrode presses the steel sheet assembly, and in this example, pressure was applied at this pressure from the main current application process to the second cooling process. The main current application time and the main current value are conditions for the main current application process. The cooling time is the cooling time when cooling is performed between the main current application process and the first cooling process. When this cooling is performed, no current is applied to the electrode. The post-current application time and the post-current value are conditions for the first cooling process. The holding time is the time for the second cooling process. In other words, in this example, no current was applied in the second cooling process. Therefore, the time for the second cooling process is the same as the holding time. Note that when post-current application is not performed, the holding time is the time for the first cooling process. In Test No. 5, the total time for the main current application and the first cooling process was 960 (msec). The results are shown in Table 2 and Figure 2.

[0053] [Table 2]

[0054] The classifications in Table 2 are a combination of the test number and holding time in Table 1. Ip, Pp, and tp indicate the current value, pressure, and time, respectively, of the preliminary current application process. I0, P0, and t0 indicate the current value, pressure, and time, respectively, of the main current application process. I1, P1, and t1 indicate the current value, pressure, and time, respectively, of the first cooling process. I2, P2, and t2 indicate the current value, pressure, and time, respectively, of the second cooling process.

[0055] The thickness direction inner / outer ratio in Table 2 refers to the ratio of the inner microstructure diameter to the nugget diameter in the direction perpendicular to the sheet assembly surface. The width direction inner / outer ratio refers to the ratio of the inner microstructure diameter to the nugget diameter in the direction parallel to the sheet assembly surface. The inner microstructure aspect / horizontal ratio refers to the aspect ratio of the inner microstructure diameter. Each value is in percent. If the nugget formed a double structure, the double structure item was marked with a ◯; if the nugget did not form a double structure, the double structure item was marked with an ×. Hydrogen embrittlement cracking and LME cracking were evaluated as × if the respective cracks were found, and as ◯ if they were not found. The column items in Figure 2 indicate the holding time, and the row items indicate the test number. In Figure 2, the part indicated by reference numeral 100 is the outer microstructure, and the part indicated by reference numeral 200 is the inner microstructure.

[0056] Classifications 1-20 to 1-2000 reproduce the conventional spot welding method. As shown in Figure 2, the nugget did not have a double structure, and either LME cracking or hydrogen embrittlement cracking was observed. Note that LME cracking and hydrogen embrittlement cracking can be distinguished by analyzing the components of each crack in a cross section perpendicular to the sheet assembly surface. If Zn is detected in the cracked area, it can be determined to be LME cracking, and if Zn is not detected in the cracked area, it can be determined to be hydrogen embrittlement cracking.

[0057] In sections 2-20 to 2-2000, a cooling period was provided between the main current application process and the first cooling process. In all examples, the inner structure aspect / width ratio was outside the range of this embodiment. For this reason, LME cracking was found in section 2-20 (see Figure 2). Sections 2-20 to 2-2000 are conditions that are generally considered to be post-current application, but it is known that post-current application is difficult to stably exert its effects. Although no LME cracking was found in this example, there is still room for improvement before it can be applied to actual manufacturing.

[0058] Sections 3-20 to 3-2000 are examples that partially correspond to this embodiment. However, in section 3-20, the holding time was 20 msec, which was shorter than the holding time (0.5 (sec)) of this embodiment. Therefore, the nugget had a double structure and each ratio was within the range of this embodiment, but LME cracking was found (see Figure 2). Sections 3-600 and 3-2000 satisfied all the requirements of this embodiment, so the nugget had a double structure and each ratio was within the range of this embodiment. Furthermore, neither hydrogen embrittlement cracking nor LME cracking was found.

[0059] Classes 4-20 to 4-2000 are examples in which the first cooling process was performed with a downslope. In none of the cases did the nugget form a double structure, but neither hydrogen embrittlement cracking nor LME cracking was found. However, in order to perform the first cooling process with a downslope, the current value had to be precisely controlled, which was time-consuming. Furthermore, there is a concern that the CTS improvement effect may be reduced in Classes 4-20 to 4-2000.

[0060] Sections 5-20 to 5-2000 are examples that partially correspond to this embodiment. However, in section 5-20, the holding time was 20 msec, which was shorter than the holding time (0.5 (sec)) of this embodiment. Therefore, the nugget had a double structure and each ratio was within the range of this embodiment, but LME cracking was found (see Figure 2). Sections 5-600 and 5-2000 satisfied all the requirements of this embodiment, so the nugget had a double structure and each ratio was within the range of this embodiment. Furthermore, neither hydrogen embrittlement cracking nor LME cracking was found.

[0061] Next, the test conditions were changed in various ways based on the 3-600 category, and the double structure of the nugget was varied. The results are shown in Table 3.

[0062] [Table 3]

[0063] As shown in Table 3, when one or more of the thickness direction inner / outer ratio, width direction inner / outer ratio, and inner structure length / width ratio did not satisfy the requirements of this embodiment, either hydrogen embrittlement cracking or LME cracking was found. Note that in Class 3-600-5, neither hydrogen embrittlement cracking nor LME cracking was found, but the takt time was longer than in the other invention examples.

[0064] As explained above, it has been revealed that the welded joint manufactured by the manufacturing method of the spot welded joint according to this embodiment has a nugget with a predetermined double structure, and is free from both hydrogen embrittlement cracking and LME cracking.

[0065] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Claims

1. A method for manufacturing a spot welded joint by spot welding a plate assembly made of a plurality of overlapping steel plates, At least one of the plurality of steel plates is a high-strength steel plate having a tensile strength of 980 MPa or more, At least one of the high-strength steel plate and the steel plate adjacent to the high-strength steel plate has a zinc-based coating, The method for manufacturing the spot welded joint includes a main current application process for forming a nugget, a first cooling step and a second cooling step having a cooling rate faster than that of the first cooling step, The holding time, which is the time from the end of current application to the release of the pressure applied by the electrodes to the steel sheet, is 0.5 (sec) or more, and The spot welded joint after the second cooling step is The nugget has a two-layer structure having an inner structure and an outer structure in a cross section passing through the center of the nugget and perpendicular to the sheet assembly surface, The ratio of the diameter of the inner structure to the diameter of the nugget in a direction parallel to the plate assembly surface is 40 to 100%, The ratio of the diameter of the inner structure to the diameter of the nugget in a direction perpendicular to the plate assembly surface is 0 to 80%, A method for manufacturing a spot welded joint, characterized in that the aspect ratio of the diameter of the inner structure is 25% or less.

2. In the first cooling step, 0<t1≦2 (sec), 0.6×I0<I1≦I0 (kA), and a cooling rate of 500° C. / sec or less; In the second cooling step, 0.5 (sec) < t2, 0 ≦ I2 < I1 (kA), and the cooling rate is 2000° C. / sec or more, I0 is the current value during the main current application step, t1 and I1 are the time and current value of the first cooling step, 2. The method for manufacturing a spot welded joint according to claim 1, wherein t2 and I2 are the time and current value of the second cooling step.

3. In the main current application process, the nugget diameter is set to 3√t to 6√t, 3. The method for manufacturing a spot welded joint according to claim 1, wherein t is the smallest thickness among the thicknesses of the plurality of steel plates.

4. Before the main current application process, a preliminary current application process of 0<tp<0.1 (sec) and I0<Ip<20 (kA) is performed, tp is the time of the pre-energization step, I0 is the current value during the main current application step, The method for manufacturing a spot welded joint according to any one of claims 1 to 3, wherein Ip is a current value during the pre-energization step.

5. 5. The method for manufacturing a spot welded joint according to claim 1, wherein immediately before the start of main current flow, one or more of the following conditions are satisfied: an impact angle of 1° or more; a total plate gap of 0.5 mm or more; and a clearance of 0.1 mm or more.

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