Method for manufacturing resistance spot welded joints and method for manufacturing automobile parts

Abstract

To provide a resistance spot weld joint and a method for manufacturing an automobile component which can simply avoid joint failures in a plate set where a plate thickness ratio is 4.5 or more and a thinnest steel plate is arranged thereon.SOLUTION: There is provided a method for manufacturing a resistance spot weld joint, wherein a pressurization force P1 of a first pressurization step, a pressurization force Pr of a pressurization release step and a pressurization force P2 of a second pressurization step satisfy P1≤P2, 2<P1 and Pr≤2, a current value I1 of a first energization step and a current value I2 of a second energization step satisfy I1≤I2 and 4≤I1, an energization time T1 of the first energization step satisfies 50×tsum / 2<T1<200×tsum / 2, an energization time T2 of the second energization step satisfies 50×tsum / 2<T2, and when the first pressurization step includes a holding step, a holding time Th1 of the holding step satisfies Th1<300, and a time Tr of the pressurization release step satisfies 20<Tr.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Relatively thick, high-strength steel sheets are often used for automotive frame components, such as center pillars. This is done to ensure the collision safety of the vehicle body and to achieve weight reduction through component integration. On the other hand, highly decorative side members, for example, made from thin steel sheets of 0.6 to 0.8 mm, are placed on the outermost parts of the vehicle. Mild steel is often used for the thin steel sheets on the exterior of the vehicle to ensure workability.

[0003] For the reasons mentioned above, in the assembly of automotive parts, sheet assemblies consisting of thick and thin steel sheets may be spot welded. Furthermore, the sheet assemblies to be spot welded may also have a three-layer structure, such as a thin steel sheet-thick sheet-thick sheet. The sheet thickness ratio of such sheet assemblies may be, for example, 4.5 or more. Here, the sheet thickness ratio is a value defined as tsum / tmin, where tmin (mm) is the thickness of the thinnest steel sheet, and tsum (mm) is the total thickness of the steel sheets included in the sheet assembly.

[0004] However, when spot welding a sheet assembly in which the sheet thickness ratio is 4.5 or more and the thinnest steel sheet is arranged on the surface, poor joints are likely to occur. A nugget (which originally refers to a melted and solidified portion, but in this specification, both the molten portion and the melted and solidified portion are referred to as a nugget) formed inside the sheet assembly does not grow to the thinnest steel sheet arranged on the outermost surface of the sheet assembly, which causes poor joints between the thinnest steel sheet and the adjacent steel sheet in contact with it.

[0005] In the above-described sheet assembly, one of the reasons why the nugget does not grow to the thinnest steel sheet is that the thinnest steel sheet is in contact with the spot welding electrode. The spot welding electrode has a configuration in which a coolant flows inside to cool the steel sheet. Therefore, the temperature of the thinnest steel sheet in contact with the electrode is less likely to rise than the temperature of the steel sheets inside the sheet assembly.

[0006] Another reason why the nugget does not grow to the thinnest steel sheet is that the temperature rise starts from the center of the sheet pair. The further away from the center of the sheet pair, the more difficult it is for the temperature to rise.

[0007] Furthermore, if the thinnest steel sheet is mild steel, the nugget is further prevented from growing to the thinnest steel sheet because thin mild steel is easily deformed. When thin mild steel is placed on the surface of a sheet assembly, the mild steel is easily deformed by pressure and current flow, and the contact area between the mild steel and the adjacent steel sheet tends to increase. The larger the contact area, the larger the cross-sectional area of ​​the current path and the lower the current density. In addition, the larger the contact area between the mild steel and the electrode, the more significant the heat transfer from the mild steel to the electrode.

[0008] For these reasons, the temperature of the thinnest steel sheet at the surface of the sheet assembly is difficult to increase, and therefore melting and solidification are difficult to occur in the thinnest steel sheet. If the nugget does not grow to the thinnest steel sheet, the joint strength cannot be ensured. This prevents the use of thick (1.6 mm or more) high-strength steel sheets in automotive frame parts and limits the options for thickness of the steel sheets that make up the sheet assembly. Therefore, a spot welding technology that can solve these problems is desired.

[0009] Patent Document 1 discloses a method for producing a resistance spot welded joint by resistance spot welding a plate assembly made up of multiple overlapping metal plates, wherein the resistance spot welding is performed in two stages, a first stage and a second stage, and the second stage welding uses a higher welding 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.

[0010] In Patent Document 2, when performing resistance spot welding while sandwiching a plate assembly in which a thin steel plate is superimposed on at least one of two or more thick plates that are superimposed between a pair of electrodes and applying a pressing force, when the pressing force reaches a set value P, the positions of the pair of electrodes are fixed, and by suppressing the thermal expansion of the plate assembly after the start of energization by fixing the electrode positions, the pressing force is increased, and the initial set pressing force P and the maximum value Pm of the pressing force are Pm≧1.5×P P≦4kN There is disclosed a resistance spot welding method characterized in that it is set as such.

[0011] In Patent Document 3, in a resistance spot welding method in which a plate assembly in which a thin steel plate is superimposed on at least one of two or more superimposed steel plates and at least one of the thin steel plate and the steel plate overlapping it is a plated steel plate is sandwiched between a pair of electrodes and resistance spot welding is performed while applying a pressing force, the welding construction process is divided into a first stage and a second stage, and when starting the first stage, even if overshoot occurs in the pressing force when the electrode contacts the thin steel plate, it is controlled not to exceed 2.5 kN. In the first stage, resistance spot welding is performed with a low pressing force and a short time, in the second stage, resistance spot welding is performed with a high pressing force and a long time, and further, resistance spot welding is performed within a range where the radius d (mm) of the contact portion between the thin steel plate and the electrode before the start of energization in the first stage and the thickness t (mm) of the thin steel plate satisfy d < 7t. There is disclosed a resistance spot welding method characterized in that.

[0012] In Patent Document 4, there is disclosed a spot welding method for joining a plurality of steel plates by overlapping them, which includes a preliminary energization process of gradually loading current, a first energization process of performing constant energization at a current value I1, then a second energization process of performing energization at a current value I2, and further a third energization process of performing energization at a current value I3, and is characterized in that the relationship is I1 > I2 and I2 < I3.

Prior Art Documents

Patent Documents

[0013] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-262259 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-203319 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-268604 [Patent Document 4] International Publication No. WO2015 / 170687 Summary of the Invention [Problem to be solved by the invention]

[0014] However, the techniques of Patent Documents 1 to 4 require significant limitations on the pressure and current value during the current application process and complex control, making it impossible to easily avoid poor joints in spot welding of sheet assemblies in which the sheet thickness ratio is 4.5 or more and the thinnest steel sheet is arranged on the surface.

[0015] In view of the above circumstances, an object of the present invention is to provide a method for manufacturing a resistance spot welded joint and a method for manufacturing an automotive part that can easily avoid poor joining in a sheet assembly having a sheet thickness ratio of 4.5 or more and in which the thinnest steel sheet is arranged on the surface. [Means for solving the problem]

[0016] The gist of the present invention is as follows.

[0017] [1] A manufacturing method of a resistance spot welded joint according to one aspect of the present invention is a manufacturing method of a resistance spot welded joint in which a sheet assembly formed by stacking two or more steel sheets is spot welded, wherein, when the sheet thickness of the thinnest steel sheet among the steel sheets included in the sheet assembly is tmin (mm) and the total sheet thickness of the steel sheets included in the sheet assembly is tsum (mm), tsum / tmin is 4.5 or more, the thinnest steel sheet is arranged on at least one surface of the sheet assembly, the manufacturing method includes, in order, a first pressurizing step in which the sheet assembly is clamped between tips of a pair of electrodes and pressed, a pressure release step in which the pressure applied by the pair of electrodes is reduced, and a second pressurizing step in which the sheet assembly is clamped between the tips of the pair of electrodes and pressed, wherein the applied pressure P1 (kN) in the first pressurizing step, the applied pressure Pr (kN) in the pressure release step, and the applied pressure P2 (kN) in the second pressurizing step are The formulas (1) to (3) are satisfied, the first pressurizing step includes a first current-carrying step of passing current between the pair of electrodes, and the current value in the pressure-releasing step is 0, the second pressurizing step includes a second current-carrying step of passing current between the pair of electrodes, the current value I1 (kA) in the first current-carrying step and the current value I2 (kA) in the second current-carrying step satisfy formulas (4) and (5), the current-carrying time T1 (msec) in the first current-carrying step satisfies formula (6), and the current-carrying time T2 (msec) in the second current-carrying step satisfies formula (7). In the case where the first pressurizing step includes a first holding step after the first current-carrying step in which a pressure that satisfies formula (2) is maintained with the current value set to 0, the holding time Th1 (msec) in the first holding step satisfies formula (8), and the time Tr (msec) in the pressure-releasing step satisfies formula (9). P1≦P2 (1) 2 <P1···(2) Pr≦2 (3) I1≦I2 (4) 4≦I1 (5) 50×tsum / 2 <T1<200×tsum / 2···(6) 50×tsum / 2 <T2···(7) Th1<300 (8) 20 <Tr···(9) [2] In the method for manufacturing a resistance spot welded joint described above in [1], the pressing force P1 (kN) in the first pressing step may further satisfy formula (10). P1<2.0×tsum / 2 (10) [3] In the method for manufacturing a resistance spot welded joint described in [1] above, the pressurizing force P1 (kN) in the first pressurizing step may further satisfy formula (11). P1≦5 (11) [4] In the method for manufacturing a resistance spot welded joint according to any one of [1] to [3] above, the pressurizing force P2 (kN) in the second pressurizing step may further satisfy formula (12). 1.2 × P1 ≦ P2 (12) [5] In the method for manufacturing a resistance spot welded joint described in any one of [1] to [4] above, after the second pressurizing step, a third pressurizing step may be further provided, which includes a third current application step for the purpose of modifying the nugget. [6] A manufacturing method of an automobile part according to another aspect of the present invention includes a manufacturing method of a resistance spot welded joint according to any one of [1] to [3] above. [7] Another aspect of the present invention provides a method for manufacturing an automobile part, comprising the method for manufacturing a resistance spot welded joint described in [4] above. [8] Another aspect of the present invention provides a method for manufacturing an automobile part, comprising the method for manufacturing a resistance spot welded joint described in [5] above. [Effects of the Invention]

[0018] The present invention provides a method for manufacturing a resistance spot welded joint and a method for manufacturing an automotive part that can easily avoid poor joining in a sheet assembly having a sheet thickness ratio of 4.5 or more and in which the thinnest steel sheet is arranged on the surface. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a graph showing an example of changes in pressing force and current value over time in a method for manufacturing a resistance spot welded joint according to one aspect of the present invention. [Figure 2]1 is a schematic diagram of a method for manufacturing a resistance spot welded joint according to one aspect of the present invention. [Figure 3] FIG. 10 is a conceptual diagram illustrating the influence of current application time in the first current application step. [Figure 4] 10 is a cross-sectional photograph showing the experimental results of the influence of current application time in the first current application step. DETAILED DESCRIPTION OF THE INVENTION

[0020] A manufacturing method of a resistance spot welded joint 1 according to one embodiment of the present invention is a manufacturing method of a resistance spot welded joint 1, for example, as shown in Figures 1 and 2, in which a plate assembly 11 formed by stacking two or more steel plates 111 is spot welded, and when the thickness of the thinnest steel plate 111min among the steel plates 111 included in the plate assembly 11 is tmin (mm) and the total thickness of the steel plates included in the plate assembly is tsum (mm), tsum / tmin is 4.5 or more, and the thinnest steel plate 111min is arranged on at least one surface of the plate assembly 11. Here, the manufacturing method of the resistance spot welded joint 1 includes a first pressurizing step S1 in which the sheet set 11 is sandwiched between the tips of a pair of electrodes E and pressurized, a pressure release step S2 in which the pressure applied by the pair of electrodes E is reduced, and a second pressurizing step S3 in which the sheet set 11 is sandwiched between the tips of the pair of electrodes E and pressurized, in that order, the pressurized force P1 (kN) in the first pressurizing step, the pressurized force Pr (kN) in the pressurizing release step, and the pressurized force P2 (kN) in the second pressurizing step satisfy equations (1) to (3), the first pressurizing step S1 includes a first current application step in which current is applied between the pair of electrodes, the current value in the pressure release step S2 is 0, and the second pressurizing step S3 includes a second current application step in which current is applied between the pair of electrodes E and the pair of electrodes E. The first current flow step S1 includes a second current flow step in which current is passed between the first and second current flow steps, and the current value I1 (kA) of the first current flow step and the current value I2 (kA) of the second current flow step satisfy the formulas (4) and (5), the current flow time T1 (msec) of the first current flow step satisfies the formula (6), and the current flow time T2 (msec) of the second current flow step satisfies the formula (7). If the first pressure application step S1 includes a first holding step in which the current value is set to 0 and a pressure that satisfies the formula (2) is maintained, the holding time Th1 (msec) of the first holding step satisfies the formula (8), and the time Tr (msec) of the pressure release step satisfies the formula (9). P1≦P2 (1) 2 <P1···(2) Pr≦2 (3) I1≦I2 (4) 4≦I1 (5) 50×tsum / 2 <T1<200×tsum / 2···(6) 50×tsum / 2 <T2···(7) Th1<300 (8) 20 <Tr···(9) Details will be explained below. Note that the "thinnest steel plate 111 min" may be referred to as the "thinnest steel plate 111 min" hereinafter.

[0021] First, the technical concept of the method for manufacturing a resistance spot welded joint according to this embodiment will be described. The inventors conducted extensive research into a method for manufacturing a resistance spot welded joint that can avoid poor welding in sheet assemblies in which the sheet thickness ratio is 4.5 or more and the thinnest steel sheet is arranged on the surface.

[0022] The inventors first tried to increase the heat input by lengthening the welding time, but the results of the experiment revealed that the effect of extending the welding time on increasing the nugget diameter was saturated at a certain level.

[0023] Figure 4 shows cross-sectional photographs of resistance spot-welded joints fabricated under various welding conditions. In all spot welds, the welding pressure was 3.5 kN, the current was 8 kA, the sheet thickness ratio of the sheet assembly was 7, and the thinnest steel sheet was positioned on the surface of the sheet assembly. The welding time alone was varied between 40 and 400 msec. Within the welding time range of 40 to 200 msec, the nugget size increased with increasing welding time, and the nugget diameter at the interface between the thinnest steel sheet on the surface of the sheet assembly and the adjacent steel sheet also tended to increase. However, this tendency disappeared when the welding time exceeded 200 msec. The nugget diameter at the interface between the thinnest steel sheet on the surface of the sheet assembly and the adjacent steel sheet was smaller in resistance spot-welded joints fabricated with a welding time of 400 msec than in resistance spot-welded joints fabricated with a welding time of 200 msec.

[0024] The reason for this is believed to be that the surface of the sheet assembly is cooled by the spot welding electrode. The longer the current flow time, the greater the heat input, which is the product of the current value and the current flow time. However, as shown in Figure 4, the longer the current flow time, the greater the depression in the spot weld, i.e., the deeper the indentation. This increases the contact area between the steel sheet and the electrode. This is thought to promote heat transfer from the steel sheet to the electrode and reduce the current density, further hindering the melting of the thinnest steel sheet on the surface of the sheet assembly.

[0025] Another method for increasing the heat input is to increase the current value. However, as the current value increases, expulsion from the steel sheet interface of the steel sheet 111 becomes more likely when the number of steel sheets is three or more, making it difficult to obtain a sound weld. Increasing the welding pressure can suppress expulsion, but on the other hand, the cross-sectional area of ​​the current path increases, reducing the current density and making it difficult to increase the temperature of the weld. Furthermore, it is estimated that increasing the welding pressure increases indentation and increases the contact area between the steel sheet and the electrode. For these reasons, it is believed that increasing the current value does not achieve an increase in the nugget diameter at the interface between the thinnest steel sheet and the adjacent steel sheet.

[0026] Taking the above circumstances into consideration, the present inventors have conducted further research and have found that poor joining can be easily avoided by spot welding that satisfies the following conditions. (1) After the first pressurizing step S1 including the first current-carrying step, a pressure-releasing step S2 is provided in which the pressure applied by the pair of electrodes E is reduced to 2 kN or less. (2) After the pressure release step S2, a second pressure step S3 including a second current application step is carried out.

[0027] In the pressure release step S2, the pressure applied by the pair of electrodes E is reduced to 2 kN or less. This pressure is significantly smaller than the pressure applied in conventional spot welding. This is to suppress heat transfer from the thinnest steel sheet 111 to the electrodes E. As described above, the electrodes E used in conventional spot welding have the function of cooling the steel sheet 111. However, by reducing the pressure applied by the electrodes E to 2 kN or less, the amount of heat transferred from the steel sheet 111 to the electrodes E is significantly reduced. Furthermore, since the central portion of the sheet set 11 is heated during the first pressure step S1, which includes the first current application step, heat transfer occurs from the central portion of the sheet set 11 to the thinnest steel sheet 111 in the pressure release step S2, causing the thinnest steel sheet 111 to heat up. This phenomenon is called reheating. Reheating melts the thinnest steel sheet 111, increasing the nugget diameter between the thinnest steel sheet 111 and the adjacent steel sheet 111.

[0028] Furthermore, after the pressure release step S2, a second pressure step S3 including a second current application step is carried out. The width of the fusion zone of the thinnest steel sheet 111 min created by the pressure release step S2 is further enlarged by the second pressure step S3. This makes it possible to reliably avoid poor joining.

[0029] Next, a specific configuration of the method for manufacturing a resistance spot welded joint according to this embodiment will be described.

[0030] (board group 11) In the method for manufacturing a resistance spot-welded joint according to this embodiment, a sheet assembly 11 formed by stacking two or more steel sheets 111 is spot-welded. The sheet assembly 11 has a sheet thickness ratio of 4.5 or more. The sheet thickness ratio is a value defined as tsum / tmin, where tmin (mm) is the sheet thickness of the thinnest steel sheet (thinnest steel sheet 111min) and tsum (mm) is the total sheet thickness of the steel sheets 111 included in the sheet assembly. In the method for manufacturing a resistance spot-welded joint according to this embodiment, the thinnest steel sheet 111min is disposed on one or both surfaces of the sheet assembly 11.

[0031] When the thinnest steel sheet 111min is disposed on the surface of the sheet assembly 11, if the sheet thickness ratio is 4.5 or more, the nugget 12 is unlikely to reach the interface between the thinnest steel sheet 111min and the adjacent steel sheet 111. Therefore, a sheet assembly 11 that satisfies the above requirements is disadvantageous in terms of ensuring joining strength. On the other hand, sheet assemblies 11 that satisfy the above requirements are in high demand, for example, in automobile parts. For example, in automobile parts manufactured by spot welding a frame part made of thick steel sheets and an exterior part made of thin steel sheets, it is preferable that the sheet thickness ratio be 4.5 or more. From the viewpoint of reducing the weight of the automobile body, a larger sheet thickness ratio is preferable. For example, the sheet thickness ratio may be 5.0 or more, 5.5 or more, or 6.0 or more.

[0032] The number of steel sheets 111 constituting the sheet assembly 11 is not particularly limited as long as it is two or more. For example, the number of steel sheets 111 may be three or more, or four or more. In typical spot welding, when the number of steel sheets 111 constituting the sheet assembly 11 is three or more and the number of tsums increases, the nugget diameter is secured by increasing the pressing force and current value. However, increasing the current value makes expulsion more likely to occur between the steel sheets 111. Therefore, it is difficult to suppress poor welding when the number of steel sheets 111 is three or more. On the other hand, according to the manufacturing method of this embodiment, the pressure release process S2 and the second pressure process S3 are used to expand the molten region at the interface between the thinnest steel sheet 111min and the adjacent steel sheet 111. By employing these processes, the pressure-welded portion at the steel sheet interface in the steel sheet 111 expands, and the nugget grows gradually. Therefore, expulsion and poor welding can be suppressed even when the number of steel sheets 111 is three or more.

[0033] When the sheet set 11 includes three or more steel sheets 111, the number of the thinnest steel sheets 111min may be two or more. In this case, one or two of the plurality of thinnest steel sheets 111min may be disposed on one or both surfaces of the sheet set 11. Because the nugget 12 is easily formed inside the sheet set 11, the thinnest steel sheets 111min included in the sheet set 11 do not cause poor joining. Furthermore, the nugget diameter enlargement phenomenon using the pressure release step S2 and the second current application step S3 described above occurs in both of the pair of electrodes E. Therefore, when the thinnest steel sheets 111min are disposed on both surfaces of the sheet set 11, the nugget diameter enlargement effect is obtained for both of the thinnest steel sheets 111min.

[0034] When the number of steel plates 111 constituting the plate set 11 is two to four, it is naturally necessary for the plate thicknesses of these steel plates 111 to be different from one another. This is because when two, three, or four steel plates 111 of the same plate thickness are stacked, the plate thickness ratio is 2, 3, or 4, which is below the lower limit of the plate thickness ratio of "4.5" that is to be welded by the manufacturing method of the resistance spot welded joint 1 according to the present embodiment. On the other hand, when the number of steel plates 111 constituting the plate set 11 is five or more, the plate thicknesses of the steel plates 111 constituting the plate set 11 may all be the same. This is because the plate thickness ratio of a plate set 11 in which five steel plates 111 of the same plate thickness are stacked is 5. Even in such a plate set 11, the effect of increasing the nugget diameter using the pressure release process S2 and the second pressure process S3 described above can be enjoyed.

[0035] The configuration of the steel plate 111 other than the plate thickness ratio is not particularly limited, but a suitable example will be shown below.

[0036] The total thickness tsum of the steel plates 111 included in the plate set 11 is preferably within a range of, for example, 2 mm to 6 mm. This makes it possible to more effectively suppress the occurrence of expulsion during spot welding while ensuring the rigidity of the resistance spot welded joint. In addition, the thickness tmin of the thinnest steel plate 111min arranged on the surface of the plate set 11 is preferably within a range of, for example, 0.3 mm to 1.5 mm.

[0037] The tensile strength of the steel plate 111 is preferably, for example, 980 MPa or more. By using a high-strength steel plate with a tensile strength of 980 MPa or more for one or more of the multiple steel plates 111, the rigidity of the resistance spot-welded joint 1 can be increased. On the other hand, the steel plate 111 may be mild steel with a tensile strength of less than 980 MPa. For example, when the resistance spot-welded joint 1 is used as an automobile part, the thinnest steel plate 111min arranged on the surface of the plate assembly 11 may be mild steel, and the other steel plates 111 may be high-strength steel plates.

[0038] One or more of the steel sheets 111 may be plated steel sheets. Examples of plating include hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, and aluminum plating.

[0039] (First pressurization step S1, pressure release step S2, and second pressurization step S3) As shown in Figures 1 and 2, the above-described sheet set 11 is joined through a first pressure application step S1 including a first current application step, a pressure release step S2, and a second pressure application step S3 including a second current application step to form a resistance spot welded joint 1. If necessary, a third pressure application step S4 including a third current application step may be further performed. Note that the phrase "the first pressure application step includes the first current application step" means that the period from the start to the end of the first pressure application step includes the period from the start to the end of the first current application step. This also applies to the second pressure application step and second current application step, and the third pressure application step and third current application step.

[0040] The first pressurizing step S1 and the second pressurizing step S3 are performed by clamping and applying pressure to the sheet pair 11 between the tips of a pair of electrodes E for spot welding, similar to normal spot welding. The first current-carrying step included in the first pressurizing step S1 and the second current-carrying step included in the second pressurizing step S3 are performed by passing current between the pair of electrodes E, similar to normal spot welding. In the pressure-release step S2 performed between the first pressurizing step S1 and the second pressurizing step S3, the pressure applied by the pair of electrodes E is reduced to 2 kN or less. The conditions for performing these steps must be determined taking into consideration their interrelationships. The conditions for performing these steps are described below. The pressure, current, holding time, and pressure-release step time described below are all set values ​​input into the spot welding machine.

[0041] (definition) Generally, spot welding conditions are expressed by a time-pressure graph and a time-current graph as shown in Figure 1. In this graph, the pressure release step S2 refers to a period during spot welding in which the pressure is set to 2 kN or less and the current value is set to 0 kA. In other words, the pressure release step is a period in which the following formula (3) is satisfied and no current application step is included. The condition values ​​in this pressure release step S2 are defined as follows: · Pressure release process time Tr: The length of time during which the pressure is kept below 2kN. Pressure Pr during pressure release process: If the pressure is set to a constant value, this refers to the pressure. If the pressure is set to fluctuate, this refers to the minimum pressure during the period when the pressure is 2kN or less. In addition, there are cases where the pressure is set to more than 0 kN and 2 kN or less immediately after the start or end of spot welding, but such a low pressure period immediately after the start or immediately before the end is not included in the pressure release process S2, which is a process carried out between the first pressure process S1 and the second pressure process S3.

[0042] The first pressurizing step S1 is a pressurizing period provided before the pressurizing release step S2 for forming the nugget 12. As defined by the following formula (2), the pressurizing force in the first pressurizing step S1 exceeds 2 kN, and in this respect the first pressurizing step S1 and the pressurizing release step S2 are distinguished. The first pressurizing step S1 includes a first current application step, and the condition values ​​in the first pressurizing step S1 and the first current application step are defined as follows: Current application time T1 of the first current application step: The length of the period during which the current value is 4 kA or more during the first current application step. In other words, T1 is the period during which the following formula (5) is satisfied. For example, if the first current application step is an upslope current application, the period during which the current value is less than 4 kA is not included in the current application time T1. Current value I1 of the first current flow step: If the current value is set to be constant, this refers to that current value. If the current value is set to fluctuate, this refers to the maximum current value during the period in which the following formula (5) is satisfied and the current value is 4 kA or more in the case of DC, or the maximum effective value during the period in which the effective value of the current is 4 kA or more in the case of AC. Pressure P1 in the first pressure step: If the pressure is set to be constant, this refers to that pressure. If the pressure is set to fluctuate, this refers to the maximum pressure during the period when a pressure of more than 2 kN is applied, which satisfies the following formula (2).

[0043] Furthermore, in the first pressurizing step S1, if there is a first holding step after the first current application step in which the pressure satisfying equation (2) is maintained with the current value set to 0, the holding time of the first holding step is defined as follows. Holding time Th1 of the first holding step: This refers to the length of the period during which, in the first pressurizing step S1, a pressure of more than 2 kN is maintained, satisfying the following formula (2), with the current value set to 0 after the first current application step. Note that, although pressure application usually begins before the start of current application in the first current application step, this period of pressure application before current application is not included in Th1.

[0044] The second pressurizing step S3 is a pressurizing period for enlarging the nugget diameter, which is provided after the pressurizing release step S2, and includes the second current application step. Specifically, the second pressurizing step S3 refers to the period from when the pressurizing force is increased to P1 or more after the pressurizing release step S2 until the pressurizing force becomes less than P1 after the second current application step. For definitional reasons and technical reasons described below, the pressurizing force P2 of the second pressurizing step S3 must be P1 or more at least after the pressurizing release step S2 is completed. This second pressurizing step S3 includes the second current application step, and the condition values ​​in the second pressurizing step S3 and the second current application step are defined as follows: Current application time T2 of the second current application step: the length of the period during which the current value is I1 or more in the second current application step. In other words, T2 is the period during which the following formula (4) is satisfied: Current value I2 of the second current application step: If the current value is set to be constant, this refers to that current value. If the current value is set to fluctuate, this refers to the maximum current value during the period in which the following formula (4) is satisfied and the current value is equal to or greater than I1 for DC, or the maximum effective value during the period in which the effective value of the current is equal to or greater than I1 for AC. Pressure P2 in the second pressure step: If the pressure is set to be constant, this refers to that pressure. If the pressure is set to fluctuate, this refers to the maximum pressure during the period when a pressure equal to or greater than P1 that satisfies the following formula (1) is applied. The second pressurizing step S3 is a pressurizing period for enlarging the nugget diameter, and as long as the second current application step is included, the second holding step after the second current application step may or may not be included.

[0045] The third pressurizing step S4 refers to a pressurizing period that includes a third current-applying step, which is a current-applying period for modifying the nugget 12, that is provided after the second pressurizing step S3. The second pressurizing step S3 and the third pressurizing step S4 may be provided consecutively or discontinuously. When the second pressurizing step S3 and the third pressurizing step S4 are provided discontinuously, the second pressurizing step S3 refers to the period from when the pressurizing force is increased to or greater than P1 after the pressure-releasing step S2 until the pressurizing force becomes less than P1, and the third pressurizing step S4 refers to the pressurized period that includes the subsequent third current-applying step. On the other hand, when the second pressurizing step S3 and the third pressurizing step S4 are performed consecutively, the second pressurizing step S3 refers to a short period of time from when the pressurizing force is increased to or greater than P1 after the pressurizing release step S2 until the pressurizing force becomes less than P1 or the current value becomes less than I1, and the third pressurizing step S4 refers to a period that follows the second pressurizing step S3 and continues until the pressurizing force becomes 0. Note that when the second pressurizing step S3 and the third pressurizing step S4 are performed consecutively, the second current application step and the third current application step may also be performed consecutively. The third pressurizing step S4 is a pressurizing period for modifying the nugget, and as long as the third current application step is included, the third holding step after the third current application step may or may not be included.

[0046] (Pressure) The pressure P1 (kN) in the first pressurizing step S1, the pressure Pr (kN) in the pressure-releasing step S2, and the pressure P2 (kN) in the second pressurizing step S3 satisfy the formulas (1) to (3). P1≦P2 (1) 2 <P1···(2) Pr≦2 (3)

[0047] Formula (2) defines the lower limit of P1. That is, the pressure P1 in the first pressure application step S1 must be greater than 2 kN. If P1 is insufficient, the contact state between the electrode E and the steel sheet 111 becomes unstable, making it difficult to perform spot welding normally. P1 may be set to 2.2 kN or more, 2.5 kN or more, 3.0 kN or more, or 3.5 kN or more.

[0048] There is no particular upper limit for P1, but P1 may also satisfy the formula (10), for example. P1<2.0×tsum / 2 (10) As described above, tsum is the total thickness (in mm) of the steel sheets 111 included in the sheet set 11. Equation (10) is an equation that determines the upper limit of P1 according to the total sheet thickness of the sheet set 11. When P1 satisfies equation (10), the contact area between the electrode E and the steel sheet 111 and the contact area between the steel sheets 111 themselves can be reduced in the first pressurizing step S1, thereby achieving a further improvement in current density and a further suppression of heat dissipation.

[0049] Instead of determining the upper limit of P1 based on the total plate thickness of the plate assembly 11, it may be determined as in the following formula (11). P1≦5 (11) Even when formula (11) is satisfied, the contact area between the electrode E and the steel sheet 111 or the contact area between the steel sheets 111 can be reduced in the first pressurizing step S1 to improve the current density and further increase the diameter of the nugget 12. P1 may be set to 4.8 kN or less, 4.5 kN or less, or 4.2 kN or less.

[0050] Equation (1) defines the lower limit of P2. That is, the pressure P2 in the second pressurizing step S3 must be equal to or greater than the above-mentioned P1. This is to prevent splashing. In this embodiment, the current range that does not cause splashing is referred to as the "appropriate current range." The higher the pressure, the wider the appropriate current range. As will be described later, the current value I2 must be increased in the second pressurizing step S3. Therefore, in order to widen the appropriate current range, P2 is set to be equal to or greater than P1.

[0051] Preferably, P2 is 1.2 times or more as large as P1. That is, P2 may satisfy the formula (11). 1.2 × P1 ≦ P2 (12) P2 may be 1.3 times or more, 1.5 times or more, or 1.8 times or more as large as P1.

[0052] Formula (3) defines the upper limit of Pr. That is, the pressure Pr in the pressure release step S2 needs to be 2 kN or less. This suppresses heat transfer from the steel sheet 111 to the electrode E and promotes nugget growth through heat recovery. The smaller Pr is, the better, and it may be 0 kN. That is, the electrode E and the steel sheet 111 may be separated from each other in the pressure release step S2. Pr may also be 1.8 kN or less, 1.5 kN or less, or 1.0 kN or less.

[0053] (current value) The current value I1 (kA) in the first current application step and the current value I2 (kA) in the second current application step satisfy the formulas (4) and (5). I1≦I2 (4) 4≦I1 (5)

[0054] Equation (5) defines the lower limit of I1. That is, the current value I1 in the first current application step must be 4 kA or more. This increases the current density, thereby enlarging the diameter of the nugget 12. I1 may be set to 4.2 kA or more, 4.5 kA or more, or 5.0 kA or more. Furthermore, sufficient heat generation in the first pressure application step S1 allows reheating to occur in the subsequent pressure release step S2. Note that the upper limit of I1 is not particularly limited. If I1 is excessive, welding may become unstable due to expulsion, etc. However, the expulsion limit current can be determined, for example, by conducting a preliminary test. I1 may be set to a value equal to or less than this expulsion limit current. I1 may also be set to 14.0 kA or less, 10.0 kA or less, or 6.0 kA or less.

[0055] Formula (4) defines the lower limit of I2. That is, the current value I2 in the second current application step must be equal to or greater than the current value I1 in the first current application step. Increasing I2 can further increase the diameter of the nugget 12 formed through the first pressure application step S1 and pressure release step S2. I2 may be set to 1.1 times or more, 1.2 times or more, or 1.3 times or more of I1.

[0056] In the second current application step, the nugget 12, which serves as the current path, is formed in advance, so the current application is naturally stable. Therefore, even if a current value greater than I1 is applied to the second current application step, there is little possibility that problems such as expulsion will occur. On the other hand, to further stabilize the welding, I2 may be set to 14 kA or less, 12 kA or less, or 10 kA or less.

[0057] (Power-on time T1 of the first power-on process) The current application time T1 (msec) in the first current application step satisfies the formula (6). 50×tsum / 2 <T1<200×tsum / 2···(6) Here, “tsum” in the formula (6) is the total plate thickness (mm) of the steel plates 111 included in the plate set 11.

[0058] Equation (6) defines the upper and lower limits of the energization time T1 based on tsum. The range of energization time T1 defined by equation (6) is shorter than that of normal spot welding. Experiments by the inventors have shown that if T1 is too long, the nugget 12 grows along the plate surface direction rather than along the plate thickness direction, and as a result, the effect of avoiding poor welding by the pressure release step S2 cannot be obtained.

[0059] FIG. 3 shows a conceptual diagram of this phenomenon. When the energization time T1 in the first energization step is set to less than 200×tsum / 2 and the pressure release step S2 is performed, the nugget 12 grows in the sheet thickness direction due to reheating. As a result, the nugget diameter at the interface between the thinnest steel sheet 111min and the steel sheet 111 in contact therewith can be enlarged. On the other hand, when the energization time T1 in the first energization step is set to 200×tsum / 2 or more, the nugget 12 does not grow sufficiently in the sheet thickness direction even if reheating is achieved in the pressure release step S2. This is because, as shown in FIG. 4, the longer T1 is, the greater the influence of heat transfer from the steel sheet 111 to the electrode E, which promotes solidification of the nugget 12 near the surface of the sheet assembly 11. As a result, the nugget diameter at the interface between the thinnest steel sheet 111min and the steel sheet 111 in contact therewith cannot be enlarged. For these reasons, T1 is set to less than 200×tsum / 2. T1 may be 180×tsum / 2 or less, 150×tsum / 2 or less, or 135×tsum / 2 or less.

[0060] On the other hand, if T1 is too short, the amount of heat input in the first current application process is insufficient, making it impossible to obtain a nugget 12 with a sufficient diameter. Therefore, T1 is set to be greater than 50×tsum / 2. T1 may be 75×tsum / 2 or more, 90×tsum / 2 or more, or 120×tsum / 2 or more.

[0061] (Power supply time T2 of the second power supply process) The energization time T2 (msec) of the second energization step satisfies the formula (7). 50×tsum / 2 <T2···(7) Here, "tsum" in formula (7) is the total plate thickness (mm) of the steel plates included in the plate assembly.

[0062] Equation (7) defines the lower limit of the current flow time T2 based on tsum. If T2 is insufficient, the effect of enlarging the nugget diameter by the second current flow process will not be sufficient. Therefore, T2 is set to be greater than 50×tsum / 2. T2 may also be set to be 60×tsum / 2 or more, 70×tsum / 2 or more, or 80×tsum / 2 or more. Note that there is no particular upper limit to T2, but considering productivity, T2 may be set to be 150×tsum / 2 or less, 130×tsum / 2 or less, or 100×tsum / 2 or less, for example.

[0063] (Retention time Th1 of the first retention step) When the first pressure application process S1 includes a first holding process after the first current application process in which the pressure satisfying equation (2) is maintained with the current value set to 0, the holding time Th1 (msec) of the first holding process satisfies equation (8). Th1<300 (8) Note that "the first pressurizing step includes the first holding step" means that the period from the start to the end of the first pressurizing step includes the period from the start to the end of the first holding step.

[0064] Formula (8) defines the upper limit of Th1, and specifically, Th1 is set to less than 300 msec. Generally, the holding time Th1 is set to stabilize the spot welding and to harden the nugget 12. However, in the manufacturing method according to this embodiment, the shorter the holding time Th1, the better. If Th1 is too long, heat removal may proceed before the pressure release step S2 begins, causing the nugget 12 to solidify, which may hinder the growth of the nugget 12 in the pressure release step S2. Th1 may be 200 msec or less, 150 msec or less, or 100 msec or less. The lower limit of Th1 is not particularly limited, and may be 0 msec, i.e., the first holding step may not be set. However, considering the stability of the spot welding, Th1 may be set to 20 msec or more, 40 msec or more, or 60 msec or more.

[0065] (Time Tr of pressure release step S2) The time Tr (msec) of the pressure release step S2 satisfies the formula (9). 20 <Tr···(9)

[0066] Equation (9) defines the lower limit of Tr, and specifically, Tr is set to be greater than 20 msec. If Tr is too short, the effect of increasing the diameter of the nugget 12 due to reheating cannot be fully achieved. From the viewpoint of suppressing poor joining, a longer Tr is preferable, so Tr may be set to, for example, 50 msec or more, 80 msec or more, or 100 msec or more. Even if solidification of the nugget 12 is completed in the pressure release step S2, the nugget diameter can be increased by applying current in the second current application step included in the subsequent second pressure application step S3. However, considering productivity, Tr may be set to, for example, 400 msec or less, 300 msec or less, or 250 msec or less.

[0067] (others) The first pressurizing step S1 and the pressurizing release step S2 must be performed consecutively. This is because the pressurizing release step S2 is a step for growing the nugget 12 using heat generated in the first current-carrying step included in the first pressurizing step S1. Therefore, the pressurizing release step S2 must be performed before the heat diffuses into the base material of the resistance spot welded joint 1 or is dissipated to the electrode E. Specifically, the first pressurizing step S1 and the pressurizing release step S2 are performed consecutively so that the above-described holding time Th1 satisfies formula (8). On the other hand, the pressurizing release step S2 and the second pressurizing step S3 do not need to be performed consecutively. This is because the nugget diameter can be enlarged by the second current-carrying step included in the subsequent second pressurizing step S3. Another pressurizing step including another current-carrying step may be provided between the pressurizing release step S2 and the second current-carrying step S3.

[0068] Furthermore, a third pressurizing step S4 may be performed after the second pressurizing step S3, including a third current application step for modifying the nugget 12. The third current application step is a so-called post-current application step for the purpose of heat treating the nugget to modify its properties. The third current application step may be, for example, current application for reducing residual stress or current application for tempering. Various conditions may be applied to the third current application step depending on the components of the nugget 12, the configuration of the sheet assembly 11, the application of the resistance spot welded joint 1, and the like.

[0069] Other welding conditions are not particularly limited, and any conditions can be applied to the manufacturing method according to this embodiment. For example, unlike the first pressurizing step S1, the holding time of the second holding step after the second current application step in the second pressurizing step S3 is not particularly limited. The holding time of the second holding step may affect the cooling rate and degree of quench hardening of the nugget 12, but does not affect the diameter of the nugget 12 or the frequency of joint defects. Therefore, the holding time of the second holding step in the second pressurizing step S3 can be appropriately set depending on the components of the nugget 12, the configuration of the sheet assembly 11, the application of the resistance spot welded joint 1, and other factors. Other welding conditions can also be appropriately selected from known conditions. Furthermore, the tip shape of the electrode E is not particularly limited. Electrodes of various shapes, such as DR, CR, CF, and R, can be used as the electrode E in the manufacturing method according to this embodiment.

[0070] Next, a method for manufacturing an automotive part according to another aspect of the present invention will be described. The method for manufacturing an automotive part according to this embodiment includes the method for manufacturing a resistance spot-welded joint according to this embodiment described above. This makes it possible to easily manufacture an automotive part that has a sheet thickness ratio of 4.5 or more and in which the thinnest steel sheet is located on the surface, yet in which poor joining is suppressed. When manufacturing automotive parts, thin sheets used as exterior materials are often joined to thick sheets used as structural materials. Therefore, the method for manufacturing an automotive part according to this embodiment is extremely suitable for manufacturing automotive parts having resistance spot-welded joints with large sheet thickness ratios.

[0071] However, the applications of the method for manufacturing a resistance spot welded joint according to the present embodiment described above are not particularly limited. For example, the method for manufacturing a resistance spot welded joint according to the present embodiment may be applied to the manufacture of home appliances. [Example]

[0072] The effects of one embodiment of the present invention will be explained in more detail using examples. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0073] Example 1 Three types of plate assemblies were prepared, each having the tensile strength (TS) and plate thickness shown in Table 1. In each plate assembly, steel plates 1, 2, 3, and 4 were stacked in this order. In each plate assembly, steel plate 1 was the thinnest steel plate arranged on the surface of the plate assembly.

[0074] [Table 1]

[0075] These plate assemblies were spot welded using the spot welding machine and electrodes shown below. Welding machine: Servo pressure stationary welding machine, DC (frequency 50Hz) Electrode: Dome radius (DR) Cr-Cu Electrode tip shape: φ6mm R40mm

[0076] The welding conditions were as shown in Table 2. The current and pressure were constant as shown in Table 2. Values ​​outside the range of the invention are underlined. A supplementary explanation of the values ​​shown in Table 2 is provided below. Since the applied pressure in the pressure release step of Comparative Example 1 exceeds 2 kN, according to the above definition, the time Tr of the pressure release step S2 is 0. However, for convenience, the length of the period during which the applied pressure is at the value shown in Table 2 is recorded as Tr. In the first current application step of Comparative Example 4, there is no period during which the current value I1 satisfies equation (5), and therefore, by definition, the current application time T1 is 0. However, for convenience, the length of the period during which the current value is the value listed in Table 2 is listed in Table 2 as T1. Although the pressure in the first current application step in Comparative Example 7 is 2 kN or less, for convenience, the first stage in Comparative Example 7 is treated as the first current application step. The pressure is set to the value shown in Table 2, and the time during which current application is stopped is shown as holding time Th1. Although the pressure applied in the second current application step in Comparative Example 9 is less than P1, for convenience, the second stage in Comparative Example 9 is treated as the second current application step. Although the current value in the second current application step in Comparative Example 10 is less than I1, for convenience, the second stage in Comparative Example 10 is treated as the second current application step.

[0077] Furthermore, the obtained resistance spot welded joint was cut on a plane passing through the center of the nugget and perpendicular to the steel sheet surface to prepare a cross section, and the nugget diameter between steel sheet 1 and steel sheet 2 was confirmed using an optical microscope. The nugget diameter is also shown in Table 3. The meanings of the symbols in Table 3 are as follows: Nugget diameter between steel plate 1 and steel plate 2 is 4√t=3.1(mm) or more: ◎ Nugget diameter between steel plate 1 and steel plate 2 is 3√t=2.3 (mm) or more: Yes Nugget diameter between steel plate 1 and steel plate 2 is less than 3√t=2.3 (mm): × Even if the nugget diameter between steel plate 1 and steel plate 2 is 3√t=2.3 (mm) or more, expulsion occurs: △ Here, t refers to the thickness of steel plate 1, which is the thinnest steel plate arranged on the surface of the sheet assembly.

[0078] [Table 2]

[0079] In Comparative Example 1, formula (3) was not satisfied. That is, the applied pressure Pr in the pressure release step was excessive in Comparative Example 1. Therefore, in Comparative Example 1, the nugget solidified early, and the nugget diameter was not ensured.

[0080] In Comparative Example 2, formula (6) was not satisfied. Specifically, in Comparative Example 2, the current application time T1 of the first current application step exceeded the upper limit value of formula (6). In Comparative Example 2, the heat input increased, but the amount of heat removed from the steel sheet 1 to the electrode also increased, and the current density decreased. As a result, the nugget solidified early on the surface of the sheet combination, and the nugget diameter was not ensured.

[0081] In Comparative Example 3, formula (6) was not satisfied. Specifically, in Comparative Example 3, the current application time T1 in the first pressurizing step did not reach the lower limit of formula (6). Therefore, in Comparative Example 3, the steel sheet was not sufficiently melted during the first current application step, and the nugget diameter was not ensured.

[0082] In Comparative Example 4, formula (5) was not satisfied. That is, the current value I1 in the first pressurizing step was insufficient in Comparative Example 4. Therefore, in Comparative Example 3, the nugget did not grow to the steel sheet 1 during the first current application step, and the nugget diameter was not ensured.

[0083] In Comparative Example 5, formula (9) was not satisfied. That is, the time Tr in the pressure release step was insufficient in Comparative Example 5. Therefore, in Comparative Example 5, the heat recovery in the pressure release step was insufficient, and the nugget diameter was not ensured.

[0084] In Comparative Example 6, formula (8) was not satisfied. That is, the holding time Th1 in the first pressurizing step was excessive in Comparative Example 6. Therefore, in Comparative Example 6, the nugget solidified early, and the nugget diameter was not ensured.

[0085] In Comparative Example 7, formula (2) was not satisfied. That is, the pressure P1 in the first pressurizing step was insufficient in Comparative Example 7. Therefore, in Comparative Example 7, expulsion occurred during the first pressurizing step, and the nugget diameter was not ensured.

[0086] In Comparative Example 8, formula (7) was not satisfied. That is, the current application time T2 in the second pressurizing step was insufficient in Comparative Example 8. Therefore, in Comparative Example 8, the growth of the nugget along the plate surface direction was insufficient, and the nugget diameter was not ensured.

[0087] In Comparative Example 9, formula (1) was not satisfied. That is, in Comparative Example 9, the pressure P2 in the second pressurizing step was lower than P1. Therefore, in Comparative Example 9, although the nugget diameter was ensured, expulsion occurred in the second pressurizing step.

[0088] In Comparative Example 10, formula (4) was not satisfied. That is, the current I2 in the second current application step was insufficient in Comparative Example 10. Therefore, in Comparative Example 10, the growth of the nugget along the sheet surface direction was insufficient, and the nugget diameter was not ensured.

[0089] On the other hand, in the resistance spot-welded joints of Examples 1 to 9 obtained by spot welding in which all of formulas (1) to (9) were satisfied, a nugget diameter of sufficient size was ensured and poor joining was suppressed. [Explanation of symbols]

[0090] 1 Resistance spot welded joints 11 Board set 111 Steel plate 111min Thinnest steel plate 12 Nuggets tmin: The thickness of the thinnest steel plate tsum Total thickness of steel plates included in the plate assembly E-electrode S1 First pressurization step S2 Pressure release process S3 Second pressurization step P1 Pressure in the first pressure step Pr Pressure during pressure release process P2 Pressure in the second pressurizing step I1 Current value of the first current application process I2 Current value of the second current application process T1: Current application time of the first current application process Th1: Holding time of the first holding step T2: Second energization time Tr: Pressure release process time

Claims

1. A method for manufacturing a resistance spot welded joint by spot welding a plate assembly formed by stacking two or more steel plates, When the plate thickness of the thinnest steel plate among the steel plates included in the plate combination is tmin (mm) and the total plate thickness of the steel plates included in the plate combination is tsum (mm), tsum / tmin is 4.5 or more, The thinnest steel plate is disposed on at least one surface of the plate assembly, The manufacturing method comprises: a first pressing step of sandwiching the plate assembly between tips of a pair of electrodes and applying pressure; a pressure release step of reducing the pressure applied to the pair of electrodes; a second pressing step of sandwiching the plate assembly between the tips of the pair of electrodes and applying pressure; in order, The pressure P1 (kN) in the first pressurizing step, the pressure Pr (kN) in the pressurizing and releasing step, and the pressure P2 (kN) in the second pressurizing step satisfy the following formulas (1) to (3): the first pressurizing step includes a first current-carrying step of applying current between the pair of electrodes, The current value in the pressure release step is 0, the second pressurizing step includes a second current-carrying step of applying current between the pair of electrodes, a current value I1 (kA) in the first current-flowing step and a current value I2 (kA) in the second current-flowing step satisfy equations (4) and (5), The energization time T1 (msec) of the first energization step satisfies the formula (6), The energization time T2 (msec) of the second energization step satisfies the formula (7), In the first pressurizing step, when a first holding step is included in which a pressing force that satisfies the formula (2) is maintained with the current value set to 0 after the first current application step, a holding time Th1 (msec) of the first holding step satisfies the formula (8), The time Tr (msec) of the pressure release step satisfies the formula (9). Method for manufacturing resistance spot welded joints. P1≦P2 (1) 2<P1...(2) Pr≦2 (3) I1≦I2 (4) 4≦I1 (5) 50×tsum / 2<T1<200×tsum / 2...(6) 50×tsum / 2<T2...(7) Th1<300...(8) 20<Tr...(9)

2. The method for manufacturing a resistance spot welded joint according to claim 1, wherein the applied pressure P1 (kN) in the first pressurizing step further satisfies formula (10). P1<2.0×tsum / 2...(10)

3. The method for manufacturing a resistance spot welded joint according to claim 1, wherein the applied pressure P1 (kN) in the first pressurizing step further satisfies formula (11). P1≦5 (11)

4. The method for manufacturing a resistance spot welded joint according to any one of claims 1 to 3, wherein the pressing force P2 (kN) in the second pressing step further satisfies formula (12). 1.2 × P1 ≦ P2 (12)

5. The method for manufacturing a resistance spot welded joint according to any one of claims 1 to 3, further comprising a third pressurizing step including a third current application step for the purpose of modifying the nugget after the second pressurizing step.

6. A method for manufacturing an automotive part, comprising the method for manufacturing a resistance spot welded joint according to any one of claims 1 to 3.

7. A method for manufacturing an automotive part, comprising the method for manufacturing a resistance spot welded joint according to claim 4.

8. A method for manufacturing an automotive part, comprising the method for manufacturing a resistance spot welded joint according to claim 5.

Images