Method for manufacturing resistance spot welded joint, and method for manufacturing automobile part

Abstract

To provide a method for manufacturing a resistance spot welded joint which can avoid joint failure without increasing an expulsion generation frequency, in a plate set having a high plate thickness ratio, and a method for manufacturing an automobile part.SOLUTION: A method for manufacturing a resistance spot welded joint has a plate thickness ratio of a plate set of 4.5 or more and 8.0 or less, wherein main energization includes first energization, second energization, third energization, and fourth energization, a current value I1 in the first energization is set at 6.0 kA or more, energization time t1 in the first energization is set at 100 msec or less, I1 and t1 satisfy 200≤I1×t1≤1,500, a current value I2 in the second energization satisfies 5.5≤I2≤I1×0.90, a current value I3 in the third energization satisfies I2+0.5≤I3≤I2+2.0, a current value I4 in the fourth energization satisfies I3+0.5≤I4≤I3+2.0, and energization time ti in the second energization to the fourth energization satisfies 30×tsum≤ti≤150×tsum.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 plates are often used for the frame components of automobiles, such as center pillars. This is to ensure the collision safety of the vehicle body and to achieve weight reduction through component integration. On the other hand, at the outermost part of the automobile, parts such as side members with high design quality, which are formed using thin steel plates of, for example, 0.6 to 0.8 mm, are placed. The thin steel plates on the outer side of the automobile are often made of mild steel to ensure workability.

[0003] For the reasons mentioned above, in the assembly of automobile parts, a sheet assembly consisting of a thick steel plate and a thin steel plate may be spot welded. In addition, the sheet assembly to be spot welded may have a three-ply structure, for example, a thin steel plate-thick steel plate-thick steel plate. The sheet thickness ratio of such a sheet assembly 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 sheet thickness of the thinnest steel plate, i.e., the thinnest steel plate, and tsum (mm) is the total sheet thickness of the steel plates included in the sheet assembly.

[0004] However, when spot welding a sheet assembly having a sheet thickness ratio of 4.5 or more and the thinnest steel sheet arranged on the surface, poor joints are likely to occur. A nugget (originally referring to a melted and solidified part, but in this specification, both the molten part and the melted and solidified part are called nuggets) 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-mentioned 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 electrode for spot welding. The electrode for spot welding has a configuration in which a coolant flows inside and cools 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 the nugget does not grow to the thinnest steel plate is that the temperature rise starts at the center of the plate. The further away from the center of the plate, the slower the temperature rise.

[0007] Furthermore, when the thinnest steel sheet is mild steel, the nugget is further prevented from growing to the thinnest steel sheet. This is because thin mild steel is easily deformed. When thin mild steel is arranged 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 is likely to become large. 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 the above reasons, the temperature of the thinnest steel sheet on 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, high-strength steel plates in automobile frame members and limits the options for thickness of steel plates that compose plate assemblies. Therefore, a spot welding technique that can solve these problems is desired.

[0009] Patent Document 1 discloses that when a plate assembly made up of multiple overlapping metal plates is welded and joined by resistance spot welding, the spot welding is performed in two stages, a first stage and a second stage, and the second stage welding is performed with a higher pressing force, a lower current, and a longer current flow time than the first stage welding. I , welding current I I , energizing time T I In relation to the thickness tm of the thinnest metal plate among the plurality of metal plates, 0.8tm≦PI ≦5tm, 3tm+5≦I I , 2≦T I ≦6 (where tm is the thickness (mm) of the thinnest metal plate among the multiple metal plates), and the second stage pressure P II , welding current I II , energizing time T II , 1.1P I ≦P II ≦10P I , 0.5 II ≦I II < II , T I ≦T II ≦10T I It is disclosed that it is preferable to perform welding that satisfies the above.

[0010] Patent Document 2 discloses a spot welding method for joining a plate assembly consisting of a thin plate and multiple thick plates overlapping one side of the thin plate by applying pressure and electricity with a pair of electrodes, characterized in that the amount of electricity passing between the pair of electrodes is increased stepwise without being reduced midway until it reaches a maximum value.

[0011] Patent Document 3 discloses a spot welding method in which a plate assembly consisting of a thin plate and several thick plates overlapping one side of it is joined by clamping and applying pressure between a pair of electrodes and passing a current between the pair of electrodes, the spot welding method having the steps of passing a first current value while applying a first pressure force to the welded portion of the plate assembly, and passing a second current value greater than the first current value while applying a second pressure force greater than the first pressure force to the welded portion, the spot welding method increasing the first pressure force to the second current value in synchronization with the timing of continuously increasing the current value from the first current value to the second current value.

[0012] Patent Document 4 discloses a spot welding method for joining a plate assembly consisting of three or more metal plates with a plate thickness ratio of 5 or more by spot welding, in which a current for growing a nugget is continued to flow during a vertical growth period set within a period of 0.1 seconds from the start of current flow, thereby causing the nugget to grow in the plate thickness direction and melt into all of the metal plates. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] JP 2005-262259 A [Patent Document 2] JP 2019-98345 A [Patent Document 3] JP 2019-147187 A [Patent Document 4] JP 2020-82168 A Summary of the Invention [Problem to be solved by the invention]

[0014] In spot welding, it is also necessary to suppress expulsion. Explosion is a phenomenon in which the base material is locally heated and melts and splashes, or the metal. Explosion impairs the surface properties of resistance spot welded joints. In addition, if expulsion occurs significantly, the nugget diameter becomes smaller due to the splashing of molten metal, and the joint strength is impaired. One method for suppressing expulsion is to reduce the welding current value. However, reducing the welding current value inhibits the growth of the nugget. Therefore, a method for ensuring the nugget diameter while suppressing the occurrence of expulsion is desired.

[0015] According to the technology of Patent Document 1, it is said that it is possible to easily produce a resistance spot welded joint having a nugget of a required size without adding extra steps and without causing expulsion, even in a large plate assembly with a plate thickness ratio exceeding 5. However, in the technology of Patent Document 1, the first stage welding is performed with a low pressure and a high current. With regard to this first stage welding, there is room for improvement from the viewpoint of suppressing the amount of expulsion.

[0016] In the technology of Patent Document 2, it is said that the vertical growth of the nugget is promoted by so-called up-slope current flow, so that the nugget can be melted into the thin plate. However, from the viewpoint of melting the interface between the thin plate and the thick plate more stably, there is room for improvement in the welding conditions in the initial stage of this current flow.

[0017] In the techniques of Patent Documents 3 and 4, it is said that it is possible to reliably join plates having a large plate thickness ratio by increasing the pressure from the first to the second pressure in synchronization with the timing of increasing the current value from the first to the second current value. However, from the viewpoint of facilitating control of the welding device, it is preferable that the welding conditions are simplified.

[0018] In view of the above circumstances, an object of the present invention is to provide a method for manufacturing a resistance spot welded joint that can avoid poor joining without increasing the frequency of expulsion in sheet assemblies with a high sheet thickness ratio, and a method for manufacturing an automotive component. [Means for solving the problem]

[0019] The gist of the present invention is as follows.

[0020] (1) The method for manufacturing a resistance spot welding joint according to one aspect of the present invention includes a step of directly energizing a stack of two or more steel plates, wherein the thinnest steel plate is disposed on at least one surface of the stack of plates, and the plate thickness ratio tsum / tmin of the stack of plates, calculated from the plate thickness tmin per unit mm of the thinnest steel plate and the total plate thickness tsum per unit mm of the steel plates included in the stack of plates, is 4.5 or more and 8.0 or less. The direct energization includes first energization, second energization, third energization, and fourth energization. In the first energization, the current value I1 per unit kA is 6.0 kA or more, and the energization time t1 per unit msec (millisecond) in the first energization is 100 msec or less. I1 and t1 satisfy Equation 1, 200 ≦ I1 × t1 ≦ 1500 ··· (Equation 1) In the second energization, the current value I2 per unit kA satisfies Equation 2, 5.5 ≦ I2 ≦ I1 × 0.90 ··· (Equation 2) In the third energization, the current value I3 per unit kA satisfies Equation 3, I2 + 0.5 ≦ I3 ≦ I2 + 2.0 ··· (Equation 3) In the fourth energization, the current value I4 per unit kA satisfies Equation 4, I3 + 0.5 ≦ I4 ≦ I3 + 2.0 ··· (Equation 4) In the i-th energization (i = 2 to 4), the energization time ti per unit msec (millisecond) satisfies Equation 5. 30 × tsum ≦ ti ≦ 150 × tsum ··· (Equation 5) (2) Preferably, in the method for manufacturing a resistance spot welding joint according to (1) above, the pressing force P1 per unit kN in the first energization and the pressing force Pi per unit kN in the i-th energization (i = 2 to 4) satisfy Equation 6. Pi < P1 ····················· (Equation 6) (3) Preferably, the method for manufacturing a resistance spot welding joint according to (1) or (2) above further includes a step of post-energizing the stack of plates after the direct energization. (4) Preferably, the method for manufacturing a resistance spot welding joint according to (3) above further includes a step of stopping energization while maintaining the pressing force between the direct energization and the post-energization. (5) Preferably, in the method for producing a resistance spot welded joint according to any one of (1) to (4) above, the main current flow has a fifth current flow after the fourth current flow, and a current value I5 in the fifth current flow in kA satisfies Equation 7, I4+0.5≦I5≦I4+2.0 (Formula 7) The energization time ti in the unit of msec (milliseconds) in the i-th energization (i=2 to 5) satisfies Equation 8. 30×tsum≦ti≦120×tsum (Formula 8) (6) Preferably, in the method for producing a resistance spot welded joint described in (5) above, the main current flow includes a sixth current flow after the fifth current flow, and a current value I6 in kA in the sixth current flow satisfies Equation 9, I5+0.5≦I6≦I5+2.0 (Formula 9) The energization time ti in the unit of msec (milliseconds) in the i-th energization (i=2 to 6) satisfies Equation 10. 30×tsum≦ti≦100×tsum (Formula 10)

[0021] (7) Preferably, in the method for manufacturing a resistance spot welded joint described in any one of (1) to (6) above, the tip diameter of the electrode in contact with the thinnest steel sheet among the electrodes used in the main current supply is 6.5 mm or more and 10 mm or less, and the current value I2 is greater than 6.0 kA. (8) Preferably, in the method for producing a resistance spot welded joint according to any one of (1) to (7) above, the pressure Pi in kN at the i-th current application (i=1 to 6) and the total sheet thickness tsum in mm satisfy Formula 11, Pi>tsum (Eq. 11) The current value I2 is greater than 6.0 kA.

[0022] (9) A method for producing an automobile component according to another aspect of the present invention includes the method for producing a resistance spot welded joint according to any one of (1) to (8) above. Effect of the Invention

[0023] According to the present invention, it is possible to provide a method for manufacturing a resistance spot welded joint that can avoid poor joining without increasing the frequency of expulsion in sheet assemblies with a high sheet thickness ratio, and a method for manufacturing an automobile part. [Brief description of the drawings]

[0024] [Figure 1] FIG. 4 is a schematic diagram showing an example of a change in current value over time during main current application. [Diagram 2] FIG. 2 is a schematic cross-sectional view showing an example of a plate assembly and an electrode. [Diagram 3] FIG. 4 is a schematic diagram showing an example of changes in current value and welding force over time in a method for producing a resistance spot welded joint including main current application, cooling, and post-current application. [Figure 4] 1 is a graph showing a current flow pattern of an embodiment. [Diagram 5] 1 is a graph showing the relationship between the diameter of a nugget obtained by spot welding using a single-stage current supply and the welding current value. [Figure 6] 5 is a cross-sectional photograph of a nugget obtained by spot welding according to the current pattern of FIG. 4. [Figure 7] 5 is a graph showing the relationship between the diameter of a nugget obtained by spot welding in which the current value of a fourth current in the current pattern of FIG. 4 is variously changed, and the final current. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] (1. Manufacturing method of resistance spot welded joints) A method for manufacturing a resistance spot welded joint according to one embodiment of the present invention includes a step of applying a main current S1 to a sheet assembly 1 in which two or more steel sheets 11 are stacked, the thinnest steel sheet 11min is arranged on at least one surface of the sheet assembly 1, and a sheet thickness ratio tsum / tmin of the sheet assembly 1, which is calculated from a sheet thickness tmin in mm of the thinnest steel sheet 11min and a total sheet thickness tsum in mm of the steel sheets 11 included in the sheet assembly 1, is set to 4.5 to 8.0, and the main current S1 includes a first current, a second current, a third current, and a fourth current, and the first current In the first current, the current value I1 in kA is 6.0 kA or more, the current flow time t1 in msec (millisecond) in the first current is 100 msec or less, I1 and t1 satisfy formula 1, the current value I2 in kA in the second current satisfies formula 2, the current value I3 in kA in the third current satisfies formula 3, the current value I4 in kA in the fourth current satisfies formula 4, and the current flow time ti in msec (millisecond) in the i-th current (i=2 to 4) satisfies formula 5. 200≦I1×t1≦1500 (Formula 1) 5.5≦I2≦I1×0.90 (Formula 2) I2+0.5≦I3≦I2+2.0 (Formula 3) I3+0.5≦I4≦I3+2.0 (Formula 4) 30×tsum≦ti≦150×tsum (Formula 5) Hereinafter, the method for manufacturing a resistance spot welded joint according to this embodiment will be described in detail with reference to FIG.

[0026] The method for manufacturing a resistance spot welded joint according to this embodiment includes a step of passing a main current S1 through a sheet assembly 1 formed by stacking two or more steel sheets 11. The main current S1 is performed by sandwiching the sheet assembly 1 between a pair of electrodes 2 and passing a welding current through the sheet assembly 1. The main current S1 generates resistance heating in the portion sandwiched between the electrodes 2. This melts the steel sheets 11, and a nugget is formed to join the steel sheets 11. At this time, the pair of electrodes 2 apply pressure to the sheet assembly 1 to stabilize the current and suppress the occurrence of expulsion.

[0027] The steel plate 11 can have any shape suitable for spot welding. In other words, the term "steel plate" is a concept that includes not only flat steel but also a flat region of a steel member having a three-dimensional shape. For example, the flange portion of a hat-shaped member is also considered to be the steel plate 11.

[0028] The pair of electrodes 2 are electrodes for spot welding. The electrodes for spot welding are rod-shaped electrodes that directly contact the sheet group 1 to pass the welding current and transmit the pressing force. The tip of the electrode for spot welding is cooled by a coolant such as water during spot welding. Therefore, while the electrodes for spot welding are in contact with the sheet group 1, heat is transferred from the sheet group 1 to the electrodes 2. When the current is stopped while maintaining the pressing force, the sheet group 1 is cooled by the electrodes 2.

[0029] (Arrangement of the thinnest steel plate 11min in plate group 1) The thinnest steel plate 11min is disposed on at least one surface of the plate set 1. Typical examples of the plate set 1 in which the thinnest steel plate 11min is disposed on the surface include (1) a plate set 1 in which the thinnest steel plate 11min and a thick steel plate 11 are stacked, (2) a plate set 1 in which the thinnest steel plate 11min is further stacked on a plurality of stacked thick steel plates 11, and (3) a plate set 1 in which a thick steel plate 11 is sandwiched between two thinnest steel plates 11min of the same thickness.

[0030] If the thinnest steel plate 11min is arranged on the surface of the plate group 1, poor joining of the thinnest steel plate 11min is likely to occur. However, the thinnest steel plate 11min is often used as an exterior member of a machine part. For example, in some automobile parts, the thinnest steel plate 11min is used as an exterior member, and the thick steel plate 11 is used as a frame member. In such parts, the thinnest steel plate 11min needs to be arranged on the surface of the plate group 1.

[0031] Hereinafter, the thinnest steel sheet 11min may be simply referred to as a "thin sheet." Also, the interface between the thin sheet and the steel sheet 11 in contact with it may be referred to as a "thin-thick interface."

[0032] (Thickness ratio of plate group 1) The thickness ratio of the plate group 1 is 4.5 or more and 8.0 or less. The thickness ratio is the value tsum / tmin obtained by dividing the total thickness tsum (mm) of the steel plates 11 included in the plate group 1 by the thickness tmin (mm) of the thinnest steel plate 11min. The thickness ratio of the plate group 1 may be 4.8 or more, 5.0 or more, or 5.2 or more. The thickness ratio of the plate group 1 may be 7.8 or less, 7.5 or less, or 7.0 or less.

[0033] The larger the plate thickness ratio, the more likely it is that poor joining of the thinnest steel plate 11min arranged on the surface of the plate assembly 1 will occur. However, an increase in the plate thickness ratio can improve the performance of mechanical parts. For example, some automobile parts are formed by joining thick steel plates used as frame members and thin steel plates used as exterior members. In such parts, it is desirable to increase the plate thickness of the frame member to ensure strength, or to reduce the plate thickness of the exterior member to reduce the weight of the part. Therefore, a technology capable of joining plate assembly 1 with a high plate thickness ratio is desired.

[0034] 1, the main current S1 includes a first current, a second current, a third current, and a fourth current. The current values ​​I1, I2, I3, and I4 in the unit of kA of the first current to the fourth current, and the current application times t1, t2, t3, and t4 in the unit of msec (milliseconds) of the first current to the fourth current are within the ranges shown in Table 1.

[0035] [Table 1]

[0036] The current value Ii in the ith current (i=1 to 4) is the value of the welding current passed through the sheet pair in the ith current. The current duration ti in the ith current is the length of time during which the welding current of the current value Ii is passed.

[0037] (Current value I1 and current flow time t1 in the first current flow) The current value I1 in the unit of kA in the first current flow is set to be 6.0 kA or more. The current value I1 may be set to be 6.2 kA or more, 6.5 kA or more, or 6.8 kA or more.

[0038] The energization time t1 in the unit of msec (milliseconds) in the first energization is set to 100 msec or less. The energization time t1 may be 90 msec or less, 85 msec or less, or 80 msec or less.

[0039] Furthermore, in the first current application, the current value I1 (kA) and the current application time t1 (msec) satisfy the following formula 1. 200≦I1×t1≦1500 (Formula 1) I1×t1 is an index value of the heat input in the first current flow. I1×t1 may be 220kA·msec or more, 250kA·msec or more, or 300kA·msec or more. I1×t1 may be 1480kA·msec or less, 1450kA·msec or less, or 1400kA·msec or less.

[0040] The upper limit of the current value I1 in the first current flow is not particularly limited. As long as I1×t1 is within the above-mentioned range, the current value I1 in the first current flow can be any value equal to or greater than 6.0 kA. However, in order to more reliably suppress the occurrence of splashing, the current value I1 in the first current flow may be equal to or less than 15.0 kA, equal to or less than 14.0 kA, or equal to or less than 13.0 kA.

[0041] The lower limit of the energization time t1 of the first energization is not particularly limited. As long as I1×t1 is within the above-mentioned range, the energization time t1 of the first energization can be any value of 100 msec or less. However, from the viewpoint of more reliably suppressing the occurrence of splashing, the energization time t1 of the first energization may be 10 msec or more, 20 msec or more, or 30 msec or more.

[0042] (Current value I2 and current flow time t2 in the second current flow) A current value I2 in kA in the second current supply is set to a value corresponding to the current value I1 in the first current supply. Specifically, the current value I2 in the second current supply satisfies the following formula 2. 5.5≦I2≦I1×0.90 (Formula 2) The current value I2 in the second current flow may be I1 x 0.85 or less, I1 x 0.80 or less, or I1 x 0.70 or less. The current value I2 in the second current flow may be 8.0 kA or less, 7.0 kA or less, or 6.0 kA or less.

[0043] The lower limit of the current value I2 in the second current passing is not particularly limited, but may be, for example, 5 kA or more, 5.2 kA or more, or 5.5 kA or more. In the formula 2, the lower limit of the current value I2 is set to 5.5 kA. This further promotes melting of the thin-thick interface. The current value I2 in the second current passing may be more than 6.0 kA, 6.2 kA or more, or 6.5 kA or more.

[0044] The current flow time t2 in the unit of msec (milliseconds) in the second current flow is set to a value corresponding to the total sheet thickness tsum in the unit of mm of the steel sheets 11 included in the sheet assembly 1. Specifically, the current flow time t2 in the second current flow satisfies the following formula 5. 30×tsum≦ti≦150×tsum (Formula 5) Here, the symbol "ti" in formula 5 refers to the current flow time in the i-th current flow. When formula 5 is applied to the second current flow, i=2, and formula 5 is rewritten as follows: 30×tsum≦t2≦150×tsum As will be described later, formula 5 also applies to the third and fourth energizations.

[0045] The left side of the formula 5 may be 35×tsum, 40×tsum, or 45×tsum. The right side of the formula 5 may be 95×tsum, 90×tsum, or 80×tsum. The preferred modified example of the formula 5 is also applicable not only to the second current passing, but also to the third current passing and the fourth current passing described later.

[0046] (Current value I3 and current flow time t3 in the third current flow) A current value I3 in kA in the third current supply is set to a value corresponding to the current value I2 in the second current supply. Specifically, the current value I3 in the third current supply satisfies the following formula 3. I2+0.5≦I3≦I2+2.0 (Formula 3) As shown in formula 3, the current value I3 in the third energization is higher than the current value I2 in the second energization. The left side of formula 3 may be I2+0.6, I2+0.7, or I2+0.8. The right side of formula 3 may be I2+1.9, I2+1.8, or I2+1.7.

[0047] The current application time t3 in the third current application, measured in msec (milliseconds), satisfies the following formula 5, like the current application time t2 in the second current application. 30×tsum≦ti≦150×tsum (Formula 5) Here, the symbol "ti" in formula 5 refers to the current application time for the i-th current application. When formula 5 is applied to the third current application, i=3, and formula 5 can be rewritten as follows: 30×tsum≦t3≦150×tsum

[0048] (Current value I4 and current flow time t4 in the fourth current flow) A current value I4 in kA in the fourth energization corresponds to the current value I3 in the third energization. Specifically, the current value I4 in the fourth energization satisfies the following formula 4. I3+0.5≦I4≦I3+2.0 (Formula 4) As shown in formula 4, the current value I4 in the fourth energization is higher than the current value I3 in the third energization. The left side of formula 4 may be I3+0.6, I3+0.7, or I3+0.8. The right side of formula 4 may be I3+1.9, I3+1.8, or I3+1.7.

[0049] The energization time t4 in the fourth energization in the unit of msec (milliseconds) satisfies the following formula 5, like the energization time t3 in the third energization. 30×tsum≦ti≦150×tsum (Formula 5) Here, the symbol "ti" in formula 5 refers to the current application time for the i-th current application. When formula 5 is applied to the fourth current application, i=4, and formula 5 is rewritten as follows: 30×tsum≦t4≦150×tsum

[0050] (Action and effect) In the first current, a high current of 6.0 kA or more is passed through the sheet group 1. In addition, in the first current, I1×t1 is set to 200 or more. This promotes heat generation at the joint interface (thin-thick interface) between the thinnest steel sheet 11min (thin sheet) arranged on the surface of the sheet group 1 and the adjacent steel sheet 11. As a result, the thin-thick interface is rapidly heated. Then, melting of the thin-thick interface is promoted at the time of the first current or at the time of the second current or later. However, high currents are prone to cause splashing. Therefore, the length t1 of the first current is set to 100 msec or less, and I1×t1 is set to 1500 or less. This suppresses the amount of splashing to within an acceptable range.

[0051] In the second current flow, the current value I2 is reduced below the current value I1 in the first current flow. This prevents the second current flow from causing splashing. Also, in the second current flow, the current flow time t2 is set to 30×tsum or more. This allows the second current flow to grow the molten part in a direction perpendicular to the plate thickness. When the molten part is grown in a direction perpendicular to the plate thickness, the cross-sectional area of ​​the current path increases and the current density decreases. By reducing the current density in the second current flow, it is possible to prevent splashing in the subsequent third and fourth current flows.

[0052] In the third current, the current value I3 is increased to be higher than the current value I2 in the second current. In addition, in the third current, the current flow time t3 is set to 30×tsum or more. This allows the third current to further grow the molten zone in the direction perpendicular to the plate thickness. The third current can also grow the molten zone in the plate thickness direction. The possibility of splashing occurring in the third current is reduced. This is because the cross-sectional area of ​​the current path is expanded in the second current and the current density is reduced. However, if the current value I3 is extremely excessive, splashing may occur in the third current. Therefore, in the third current, the upper limit value of the current value I3 is set to I2+2.0 to further suppress the occurrence of splashing.

[0053] In the fourth current, the current value I4 is increased to be higher than the current value I3 in the third current. In addition, in the fourth current, the current flow time t4 is set to 30×tsum or more. In the first current to the third current prior to the fourth current, the molten part is grown in the direction perpendicular to the plate thickness and the current density is reduced, so the possibility of splashing occurring in the fourth current is greatly reduced. Therefore, in the fourth current, the current value I4 and the heat input can be set to even larger values. As a result, the fourth current can grow the molten part in the direction perpendicular to the plate thickness and in the plate thickness direction. However, if the current value I4 is extremely excessive, splashing may occur in the fourth current. Therefore, in the third current, the upper limit value of the current value I4 is set to I3+2.0, thereby further suppressing the occurrence of splashing.

[0054] In the method for manufacturing a resistance spot welded joint according to this embodiment, the first to fourth energizations described above are performed in order, thereby making it possible to stably melt the thin-thick interface while suppressing the occurrence of expulsion, thereby producing a resistance spot welded joint having excellent joint strength.

[0055] The most basic aspect of the method for manufacturing a resistance spot welded joint according to this embodiment has been described above. A more preferred aspect will now be described.

[0056] (Pressure forces P1 to P4 in the first to fourth energizations) As shown in Fig. 3, it is preferable that the pressing force P1 in kN per unit in the first energization and the pressing force Pi in kN per unit in the i-th energization (i = 2 to 4) satisfy Equation 6. Pi < P1 ··· (Equation 6) That is, it is preferable that the pressing force P1 in the first energization is greater than the pressing forces P2, P3, and P4 in the second, third, and fourth energizations.

[0057] As the pressing force increases, the amount of warping generated can be decreased. From the viewpoint of suppressing warping, it is preferable to increase the pressing force. On the other hand, as the pressing force increases, the amount of heat transfer from the plate assembly 1 to the electrode 2 increases, and it becomes difficult for the temperature of the plate assembly 1 to rise. From the viewpoint of promoting the growth of the nugget, it is preferable to decrease the pressing force.

[0058] In the method for manufacturing a spot weld joint according to the present embodiment, the first energization is most likely to generate warping. This is because, in the first energization, while the current value I1 is large, the cross-sectional area of the current path is small. Therefore, in the first energization, it is preferable to increase the pressing force. On the other hand, in the second energization and subsequent energizations where warping is less likely to occur, it is preferable to decrease the pressing force below P1.

[0059] (Post-energization S3) As shown in Fig. 3, the method for manufacturing a resistance spot weld joint according to the present embodiment may further include a step of performing a post-energization S3 on the plate assembly 1 after the main energization S1. The post-energization S3 is to pass a post-heating current between the pair of electrodes 2. The post-heating current is a current that is passed for the purpose of annealing or tempering the hardened welded portion after welding in resistance welding of a material that hardens by welding. By modifying the welded portion, the joining strength of the resistance spot weld joint can be further enhanced. When the steel sheet 11 is a zinc-based plated steel sheet, LME generated inside or at the end of the corona bond can also be suppressed by the post-energization S3.

[0060] The current value and current duration in the post-current S3 are not particularly limited, and can be appropriately selected according to the material of the sheet assembly 1. For example, even if the current value is the same as the welding current, the nugget can be prevented from melting by shortening the current duration. Furthermore, the post-current does not cause expulsion or poor joining. Therefore, the current value and current duration can be freely combined in the post-current.

[0061] (Cooling S2 before post-energization S3) As shown in FIG. 3, the manufacturing method of the resistance spot welded joint according to the present embodiment may further include a step of stopping the current application while maintaining the pressing force between the main current application S1 and the post-current application S3. The electrode 2 for spot welding is configured so that a coolant flows inside it. By stopping the current application while maintaining the pressing force, heat can be transferred from the plate group 1 to the electrode 2, and the plate group 1 can be cooled S2. This causes quench hardening in the nugget, thereby increasing the shear tensile strength of the resistance spot welded joint. The pressing force in the cooling S2 is not particularly limited. The cooling S2 does not cause expulsion or joint failure. Therefore, the pressing force can be freely set in the cooling S2. For example, the cooling S2 may be performed by ending the current application while maintaining the pressing force P4 in the fourth current application. On the other hand, the pressing force may be changed from P4 after the end of the fourth current application.

[0062] (Fifth and Sixth Energization) The main energization S1 may further include a fifth energization after the fourth energization. For example, in the fifth energization, the current value I5 in kA satisfies the formula 7. That is, the current value I5 of the fifth energization is greater than the current value I4 of the fourth energization. I4+0.5≦I5≦I4+2.0 (Formula 7) Furthermore, when the main current S1 has a fifth current, it is preferable that the current application times ti (i=2 to 5) in the second current, the third current, the fourth current, and the fifth current in msec (milliseconds) satisfy formula 8. 30×tsum≦ti≦120×tsum (Formula 8)

[0063] The main energization S1 may further include a sixth energization after the fifth energization. For example, in the sixth energization, the current value I6 in kA satisfies formula 9. That is, the current value I6 of the sixth energization is greater than the current value I5 of the fifth energization. I5+0.5≦I6≦I5+2.0 (Formula 9) Furthermore, when the main current S1 has a sixth current, it is preferable that the current application time ti (i=2 to 6) in the unit of msec (milliseconds) for the second current, the third current, the fourth current, the fifth current, and the sixth current satisfy formula 10. 30×tsum≦ti≦100×tsum (Formula 10)

[0064] By passing a larger current after the fourth current is finished, the nugget can be grown even further. When the fifth or sixth current is passed after the fourth current, it is preferable to shorten the current passing times of the second to fourth currents. This makes it possible to further suppress the frequency of expulsion during the fifth or sixth current. One or more current passing stages may be provided after the sixth current. That is, the main current S1 may have seven or more current passing stages. In this case, the occurrence of expulsion can be suppressed by appropriately selecting the current passing time and heat input during the entire main current.

[0065] (2. Manufacturing methods for automobile parts) A manufacturing method of an automobile part according to another aspect of the present invention includes the manufacturing method of a resistance spot welded joint according to the present embodiment. This makes it possible to manufacture an automobile part with a high plate thickness ratio and suppressed joint failure without generating splashing. The automobile part is, for example, a center pillar. This makes it possible to easily manufacture an automobile part with a plate thickness ratio of 4.5 or more and with the thinnest steel plate 11min arranged on the surface, while suppressing joint failure. In manufacturing automobile parts, a thin plate used as an exterior member and a thick plate used as a structural material are often joined together. Therefore, the manufacturing method of an automobile part according to the present embodiment is extremely suitable for manufacturing an automobile part having a resistance spot welded joint with a large plate thickness ratio.

[0066] However, the application of the method for manufacturing a resistance spot welded joint according to the present embodiment described above is 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.

[0067] Although the embodiment of the present invention has been described above, the present invention is not limited to this, and can be modified as appropriate without departing from the technical concept of the invention.

[0068] (Back-up power supply) The manufacturing method for a resistance spot welded joint according to this embodiment may include a step of pre-energizing before the main energization S1. Pre-energizing refers to passing a preheating current through the sheet assembly 1. The pre-heating current is a current passed to preheat the welded portion before welding. By passing pre-energizing, the welding stability in the first energization can be further improved. When passing pre-energizing, the point at which the current value reaches 6.0 kA is regarded as the end of pre-energizing and the start of the first energization.

[0069] (electrode 2) There is no particular limitation on the type of the resistance spot welding electrode 2. For example, various spot welding electrodes specified in JIS C 9304:1999 can be used in the manufacturing method of the resistance spot welded joint according to this embodiment.

[0070] (Tip diameter of electrode 2) In the manufacturing method of the resistance spot welded joint according to this embodiment, the tip diameter φ of the electrode 2 in contact with the thinnest steel sheet 11min of the pair of electrodes 2 used in the main current flow S1 may be set to 6.5 mm or more and 10 mm or less, and the current value I2 may be set to more than 6.0 kA.

[0071] In the manufacture of resistance spot welded joints, the tip diameter of the electrode used in the main current flow is generally set to φ6.0 mm. In the manufacturing method of a resistance spot welded joint according to this embodiment, the tip diameter of the electrode 2 used in the main current flow S1 may also be set to φ6.0 mm. However, if the tip diameter of the electrode 2 in contact with the thinnest steel sheet 11 min is set to φ6.5 mm or more, the growth of the molten zone can be made preferably slow.

[0072] In addition, simply increasing the tip diameter of the electrode 2 in contact with the thinnest steel sheet 11min increases the contact area between the electrode tip and the steel sheet and the area of ​​the thin-thick interface, reducing the current density, and also increasing the cooling ability of the electrode 2, which tends to result in a small nugget diameter at the thin-thick interface. Therefore, it is preferable to set the current value I2 to more than 6.0 kA. As described above, the current value I2 is premised on being I1 x 0.90 or less, so increasing I2 to more than 6.0 kA essentially means increasing the current value I1 to more than 6.67 kA.

[0073] As described above, if the tip diameter of the electrode 2 in contact with the thinnest steel sheet 11 min is set to φ6.5 mm or more and the current value I2 is set to more than 6.0 kA, the contact area and the current density can be favorably balanced, and as a result, the growth of the molten zone can be favorably slowed. If the growth of the molten zone is favorably slowed, not only can the time for the nugget to grow be secured, but also the time for the corona bond to grow can be secured. In addition, since the contact area between the electrode tip and the steel sheet is large, the occurrence of expulsion is also suppressed. In other words, while suppressing the occurrence of expulsion, the corona bond can be grown in the direction perpendicular to the sheet thickness, and the nugget can be favorably grown in the direction perpendicular to the sheet thickness and in the sheet thickness direction.

[0074] On the other hand, if the tip diameter of the electrode 2 that contacts the thinnest steel sheet 11min exceeds φ10 mm, the current density tends to decrease excessively and the cooling ability tends to increase excessively. As a result, nugget formation may become unstable. In addition, if the tip diameter of the electrode 2 exceeds φ10 mm, the current value must be increased more than necessary, which is disadvantageous in terms of energy consumption, load on the equipment, and cost.

[0075] The tip diameter of the electrode 2 in contact with the thinnest steel sheet 11min is preferably 7.0 mm or more, more preferably 7.5 mm or more, while the tip diameter of the electrode 2 in contact with the thinnest steel sheet 11min is preferably 9.5 mm or less, more preferably 9.0 mm or less.

[0076] In the manufacturing method of the resistance spot welded joint according to the present embodiment, the tip diameter φ of the electrode 2 other than the electrode 2 in contact with the thinnest steel sheet 11min among the pair of electrodes 2 used in the main current flow S1 is not particularly limited. For example, the tip diameter of this electrode 2 may be 6.5 mm or more and 9.5 mm or less. Also, preferably, the tip diameter of the electrode 2 other than the electrode in contact with the thinnest steel sheet 11min may be 0.8 times or more and 1.8 times or less the tip diameter of the electrode 2 in contact with the thinnest steel sheet 11min.

[0077] In the method for manufacturing a resistance spot welded joint according to this embodiment, the radius of curvature R of the tips of the pair of electrodes 2 used in the main current flow S1 is not particularly limited. For example, the radius of curvature R may be 20 mm or more and 200 mm or less.

[0078] (Relationship between pressure Pi and total plate thickness tsum) In the manufacturing method of the resistance spot welded joint according to this embodiment, the pressing force Pi in the unit of kN and the total sheet thickness tsum in the unit of mm in the i-th current application (i = 1 to 4) satisfy Equation 11, Pi>tsum (Eq. 11) The current value I2 may be greater than 6.0 kA.

[0079] In the manufacturing method of the resistance spot welded joint according to this embodiment, the pressure Pi applied to the sheet assembly 1 during the main current flow S1 is set to a value corresponding to the total sheet thickness tsum (in mm) of the steel sheets 11 included in the sheet assembly 1, specifically, by making Pi>tsum, the growth of the molten part can be made preferably slow.

[0080] However, if the pressure Pi is simply set to Pi>tsum, the contact area between the electrode tip and the steel sheet and the area of ​​the thin-thick interface increase, the current density decreases, and the cooling ability of the electrode 2 also increases, which tends to result in a small nugget diameter at the thin-thick interface. Therefore, it is preferable to set the current value I2 to more than 6.0 kA. As described above, the current value I2 is premised on being I1×0.90 or less, so increasing I2 to more than 6.0 kA essentially means increasing the current value I1 to more than 6.67 kA.

[0081] As described above, if the pressure Pi is set to Pi>tsum and the current value I2 is set to more than 6.0 kA, the contact area and the current density can be favorably balanced, and as a result, the growth of the molten zone can be favorably slowed. If the growth of the molten zone is favorably slowed, not only can the time for the nugget to grow be secured, but also the time for the corona bond to grow can be secured. In addition, since the pressure Pi is increased, the occurrence of expulsion is also suppressed. In other words, while suppressing the occurrence of expulsion, the corona bond can be grown in the direction perpendicular to the plate thickness, and the nugget can be favorably grown in the direction perpendicular to the plate thickness and in the plate thickness direction.

[0082] On the other hand, the upper limit of the pressure Pi is not particularly limited, but in order to avoid an excessive decrease in current density or an excessive increase in cooling capacity, the upper limit of the pressure Pi may be set to 1.3×tsum or 1.2×tsum.

[0083] (Composition of board assembly 1) If tsum / tmin is 4.5 or more and 8.0 or less, the number of steel plates 11 included in the plate group 1 can be any value of 2 or more. The effect of the manufacturing method of the resistance spot welded joint according to this embodiment is exerted regardless of the number of steel plates 11. Usually, when the number of steel plates 11 is three or more, a large heat input is required to join the thinnest steel plate 11min arranged on the surface of the plate group 1 to the other steel plates 11. This makes it easy for expulsion to occur during welding. However, according to the manufacturing method of the resistance spot welded joint according to this embodiment, it is possible to suppress both expulsion and poor joining in the plate group 1 composed of three or more steel plates 11.

[0084] The number of the thinnest steel plates 11min may be two or more. Therefore, the thinnest steel plates 11min may be disposed on both surfaces of the plate group 1. On the other hand, when the number of the thinnest steel plates 11min is two or more, one may be disposed on the surface of the plate group 1 and the other may be disposed inside the plate group 1.

[0085] When the number of steel plates 11 constituting the plate group 1 is 2 to 4, it is necessary that the plate thicknesses of these steel plates 11 are different from each other. This is because when two, three, or four steel plates 11 having the same plate thickness are stacked, the plate thickness ratio is 2, 3, or 4, respectively, which is below the lower limit value "4.5" of the plate thickness ratio described above. On the other hand, when the number of steel plates 11 included in the plate group 1 is 5 or more, the plate thicknesses of these steel plates 11 may be the same. In this case, all steel plates 11 are regarded as the thinnest steel plate 11min. The plate group 1 composed of five steel plates 11 having the same plate thickness has a plate thickness ratio of 5. This plate group 1 satisfies the requirements of the plate group 1 targeted by the manufacturing method of the resistance spot welded joint according to this embodiment. It is difficult to join all the steel plates 11 included in such a plate group 1 by conventional resistance spot welding. However, according to the manufacturing method of the resistance spot welded joint according to this embodiment, it is possible to promote the growth of nuggets and join all the steel plates 11.

[0086] (Structure of Steel Plate 11) The specific configuration of the steel plate 11 is not particularly limited, but a suitable example will be described below.

[0087] The tensile strength of the steel plate 11 is preferably, for example, 980 MPa or more. By using one or more of the multiple steel plates 11 as a high-strength steel plate 11 having a tensile strength of 980 MPa or more, for example, the strength of an automobile part manufactured using the steel plate can be increased. On the other hand, the steel plate 11 may be a mild steel having a tensile strength of less than 980 MPa. For example, when a resistance spot welded joint is used as an automobile part, the thinnest steel plate 11min arranged on the surface of the plate assembly 1 may be a mild steel, and the other steel plates 11 may be a high-strength steel plate 11.

[0088] One or more of the multiple steel sheets 11 may be plated. Examples of the plating provided on the surface of the steel sheet 11 include hot-dip galvanizing, alloyed hot-dip galvanizing, electrolytic galvanizing, and aluminum plating.

[0089] The total thickness tsum of the steel plates 11 included in the plate set 1 is not particularly limited, but is preferably within a range of, for example, 2.0 mm to 6.0 mm. This makes it possible to more effectively suppress the occurrence of expulsion during spot welding while realizing high strength for the manufactured automobile parts. In addition, the thickness tmin of the thinnest steel plate 11min arranged on the surface of the plate set 1 is preferably within a range of, for example, 0.3 mm to 1.5 mm. EXAMPLES

[0090] The effects of one embodiment of the present invention will be explained more specifically with reference to an example. However, the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0091] (Experiment 1: Comparison of 1-step current flow and 4-step current flow, and confirmation of nugget growth in 4-step current flow) First, the following steel plates 1 to 3 were stacked in order to prepare a plate assembly. Steel plate 1: Cold-rolled steel plate (thickness 0.6 mm) with a tensile strength of 270 MPa Steel plate 2: Cold-rolled steel plate (thickness 2.0 mm) with a tensile strength of 1470 MPa Steel plate 3: Cold-rolled steel plate (thickness 1.4 mm) with a tensile strength of 1180 MPa In the plate assembly, tmin was 0.6 mm, tsum was 4.0 mm, and the plate thickness ratio tsum / tmin was 6.7.

[0092] Next, the plate assembly was spot-welded under the following conditions. -Gap: 0mm or 1mm ·Power supply: DC power supply Tip shape of pair of electrodes: both φ6mm-R40mm ·Holding time: 200msec Pressure: 4kN (constant) from start to finish of energization Comparison example (single-stage current flow) current flow pattern: current value and current flow time were changed in various ways. Current flow pattern in the embodiment (see FIG. 4): The first current I1 is about 8kA, and t1 is about 40msec. The second current I2 is about 6kA, and t2 is about 200msec. The third current I3 is about 7kA, and t3 is about 100msec. The fourth current I4 is about 8kA, and t4 is about 300msec.

[0093] First, the results of the comparative example (single-stage current flow) are shown in FIG. 5. In the comparative example, multiple spot welding was performed while changing the current value and current flow time in various ways. The nugget diameters of various resistance spot welded joints obtained by this were measured and plotted in FIG. 5. The horizontal axis was the welding current value, and the vertical axis was the nugget diameter. The graph of the nugget diameter measured at the interface (thin-thick interface) between steel plate 1 and steel plate 2 (represented by large ◯ and small ◯ in FIG. 5) was labeled "thin-thick interface". The graph of the nugget diameter measured at the interface (thick-thick interface) between steel plate 2 and steel plate 3 (represented by ◇ and □ in FIG. 5) was labeled "thick-thick interface". In addition, no splashing occurred when manufacturing the resistance spot welded joints corresponding to the black data points in FIG. 5. Splashing occurred when manufacturing the resistance spot welded joints corresponding to the white data points in FIG. 5. The current at which this splashing occurred was set as the upper limit.

[0094] As shown in Figure 5, in the case of single-stage current flow, no good current conditions could be found. When the sheet gap was 0 mm, in order to achieve a nugget diameter of 4 mm at the thin-thick interface, the welding current value had to be 7 kA, as shown by the large circle in Figure 5. However, in this case, expulsion occurred. When the sheet gap was 1 mm, it was not possible to achieve a nugget diameter of 4 mm at the thin-thick interface, as shown by the small circle in Figure 5. The possible reasons for the smaller nugget diameter when the sheet gap was 1 mm are thought to be the increased contact area between the electrode and steel sheet, which reduces the current density, and the increased cooling ability of the electrode.

[0095] On the other hand, a good nugget diameter was ensured according to the current pattern shown in Fig. 4. Also, according to the current pattern shown in Fig. 4, the amount of expulsion was small.

[0096] Fig. 6 shows a cross-sectional photograph of a welded joint obtained by the current pattern of Fig. 4. For reference, Fig. 6 also shows a cross-sectional photograph when the current pattern of Fig. 4 is stopped midway. That is, Fig. 6 also shows a cross-sectional photograph of a welded part at an intermediate stage of spot welding. (1) The photograph shown in the row in which "Time (ms)" is written as "40" is a cross-sectional view of a weld obtained by spot welding in which the current flow was stopped 40 msec after the current flow started (i.e., only the first current flow was performed). (2) The photograph shown in the row in which "Time (ms)" is written as "240" is a cross-sectional view of a weld obtained by spot welding in which the current flow was stopped 240 msec after the current flow started (i.e., only the first current flow and the second current flow were performed). (3) The photograph shown in the row in which "Time (ms)" is written as "340" is a cross-sectional view of a weld obtained by spot welding in which current flow was stopped 340 msec after the start of current flow (i.e., only the first current flow, the second current flow, and the third current flow were performed). (4) The photograph shown in the row in which "Time (ms)" is written as "640" is a cross-sectional view of a weld obtained by spot welding in which the current flow was stopped 640 msec after the current flow started (i.e., all of the first to fourth current flows were performed).

[0097] In all of the cross-sectional photographs of the welds obtained by spot welding in which all of (4) the first to fourth currents were applied, as shown in Fig. 6, it was confirmed that the nugget had grown to the thin-thick interface. In other words, the current pattern shown in Fig. 4 was able to avoid poor joining without increasing the frequency of expulsion in sheet assemblies with a high sheet thickness ratio.

[0098] A detailed examination of the nugget growth process revealed the following. During the period 240 to 340 msec from the start of current flow, under disturbance conditions with a sheet gap of 1 mm, nugget growth toward the thin sheet side was delayed compared to disturbance conditions with a sheet gap of 0 mm. It is presumed that the reason that the nugget diameter at the thin-thick interface is smaller when the sheet gap is 1 mm than when the sheet gap is 0 mm is because the contact area between the thin sheet (steel sheet 1) and the electrode increases, reducing the current density and strengthening the electrode cooling ability.

[0099] Furthermore, the inventors further optimized the current flow pattern by variously changing the final current (i.e., the current value I4 of the fourth current flow) in the current flow pattern shown in Figure 4. The results of the optimization are shown in Figure 7.

[0100] Under the disturbance condition of 0 mm gap (GAP0), the nugget diameter at the thin-thick interface exceeded the target value (3.1 mm) when the final current was in the range of 7.5 to 8.5 kA, as shown by △ in Fig. 7. In addition, expulsion occurred when the final current was over 8.5 kA.

[0101] Under the disturbance condition of 1 mm gap (GAP1), the nugget diameter at the thin-thick interface exceeded the target value when the final current was in the range of more than 7.5 to 9 kA, as shown by the square in Figure 7. In addition, expulsion occurred when the final current was more than 9 kA.

[0102] The nugget diameter at the thick-thick interface exceeded the target value (4.7 mm) at all current values, as shown by ◯ and × in Figure 7. The presence or absence of a gap did not affect the nugget diameter at the thick-thick interface.

[0103] (Experiment 2: Comparison of multi-stage current application under various conditions) The following three types of plate assemblies were spot welded under the following conditions.

[0104] ·Plate set 1 (plate thickness ratio: 6.7, total plate thickness tsum: 4.0mm) First steel plate: tensile strength 270MPa, thickness 0.6mm Second steel plate: tensile strength 1180MPa, thickness 2.0mm The third steel plate: tensile strength 980MPa, thickness 1.4mm

[0105] ·Plate set 2 (plate thickness ratio: 5.3, total plate thickness tsum: 3.2mm) First steel plate: tensile strength 270MPa, thickness 0.6mm Second steel plate: Tensile strength 1180MPa, thickness 1.6mm The third steel plate: tensile strength 980MPa, thickness 1.0mm

[0106] ·Plate set 3 (plate thickness ratio: 6.7, total plate thickness tsum: 4.0mm) First steel plate: tensile strength 270MPa, thickness 0.6mm Second steel plate: Tensile strength 1180MPa, thickness 1.2mm The third steel plate: tensile strength 1180MPa, thickness 1.2mm Fourth steel plate: tensile strength 980MPa, thickness 1.0mm

[0107] Welding machine: Servo pressure stationary welding machine DC inverter (frequency 50Hz) Electrode: Dome Radius (DR) Cr-Cu Tip shape of a pair of electrodes: Both are φ6mm-R40mm (except for examples 22 and 24) (In Examples 22 and 24, the tip diameter of the electrode in contact with the thinnest steel plate was 8 mm, and the tip diameter of the other electrodes was 9 mm.) ·Holding time: 200msec Other conditions: See Tables 2A and 2B Among the welding conditions listed in Tables 2A and 2B, those that are inappropriate are underlined.

[0108] The obtained resistance spot welded joint was cut in a plane passing through the center of the nugget and perpendicular to the steel plate surface to prepare a cross section. Then, using an optical microscope, the nugget diameter between the first steel plate (the thinnest steel plate arranged on the outermost surface of the plate assembly) and the second steel plate (the steel plate adjacent to the first steel plate) was confirmed. The evaluation criteria for the nugget diameter were as follows: 4.5√t(t=0.6)=3.5(mm) or more: Very Good 4√t(t=0.6) =3.1(mm) or more: Good 3√t(t=0.6) =2.3(mm) or more: Fair 3√t(t=0.6) = Less than 2.3(mm): Bad

[0109] The number of tests per condition was 5. The smallest nugget diameter obtained from the five tests was evaluated according to the above criteria and listed in Table 3. The frequency of expulsion was also evaluated and listed in Table 3. Conditions in which no expulsion occurred were judged to be good.

[0110] [Table 2A]

[0111] [Table 2B]

[0112] [Table 3]

[0113] Examples 1, 2, 8 to 10, and 16 to 25 are examples in which the first to fourth energizations were appropriately performed. According to these examples, in a sheet assembly with a high sheet thickness ratio, poor joining could be avoided without increasing the frequency of expulsion.

[0114] In Example 3, the current value I2 in the second energization was excessive. In Example 3, the frequency of splashing was high.

[0115] In Example 4, the current value I3 in the third energization was excessive, and the current value I4 in the fourth energization was insufficient. In Example 4, the frequency of splashing was high.

[0116] In Example 5, the current value I4 in the fourth energization was excessive. And in Example 5, the frequency of splashing was high.

[0117] In Example 6, the current value I1 in the first current flow was insufficient, and the current value I2 in the second current flow was excessive relative to I1. Thus, the frequency of expulsion was high in Example 6. Although I2 in Example 6 was the same as I2 in Example 1, the reason why the frequency of expulsion in Example 6 was much higher than that in Example 1 is presumably due to the insufficient current value in the first current flow. It is presumed that the thin-thick interface was not sufficiently heated in the first current flow, and the cross-sectional area of ​​the current path was not expanded, resulting in an excessive current density in the second current flow, causing expulsion.

[0118] In Example 7, I1×t1 was excessive. And in Example 7, the frequency of splashing was high.

[0119] In Example 11, the first current was not passed. And, in Example 11, the frequency of expulsion was high. Although I2 in Example 11 was the same as I2 in Example 1, the reason why the frequency of expulsion in Example 11 was much higher than that in Example 1 is presumably due to the absence of the first current. It is presumed that the current density became excessive in the second current because the thin-thick interface was not heated before the start of the second current and the cross-sectional area of ​​the current path was not expanded, causing expulsion.

[0120] In Example 12, the current passing time t2 in the second current passing was insufficient. In Example 12, the frequency of expulsion was high. This is presumably because the growth of the nugget in the second current passing was insufficient. As a result, the cross-sectional area of ​​the current path was insufficient at the time of the third current passing or the fourth current passing, resulting in an excessive current density.

[0121] In Example 13, the current flow time t3 in the third current flow was insufficient. In Example 13, the frequency of expulsion was high. This is presumably because the growth of the nugget in the third current flow was insufficient. As a result, the cross-sectional area of ​​the current path was insufficient at the time of the fourth current flow, and the current density became excessive.

[0122] In Example 14, the energization time t4 in the fourth energization was insufficient, and the nugget diameter was insufficient in Example 14. This is presumably because the growth of the nugget in the fourth energization was insufficient.

[0123] In Example 15, the current value I2 in the second current passing was excessive. And, in Example 15, the frequency of expulsion was high. This is presumably because the nugget grew excessively in the second current passing.

[0124] Although not shown in the table, in Example 1, after the main current application, cooling was performed by stopping the current application while maintaining the pressure, and then post-current application was performed. Also, although not shown in the table, the automobile parts manufactured under the same manufacturing conditions for the resistance spot welded joints as in Examples 1, 2, 8 to 10, and 16 to 25 had reduced expulsion and reduced joint failure. [Explanation of symbols]

[0125] S1 Main energized S2 cooling After S3 energization I1 Current value at first current I2 Current value during second current flow I3 Current value during third current flow I4 Current value in the fourth current flow t1 Current flow time for the first current t2 Second current application time t3 Current flow time in the third current flow t4 Current flow time in the fourth current flow 1 board set 11 Steel plate 11min The thinnest steel plate 2 electrodes tsum Total plate thickness in mm tmin: thickness of the thinnest steel plate in mm

Claims

1. A manufacturing method of a resistance spot welded joint comprising a step of applying a main current to a plate assembly in which two or more steel plates are stacked, The thinnest steel plate is placed on at least one surface of the plate assembly; The plate thickness ratio tsum / tmin of the plate assembly calculated from the plate thickness tmin in mm of the thinnest steel plate and the total plate thickness tsum in mm of the steel plates included in the plate assembly is set to 4.5 to 8.0, The main energization includes a first energization, a second energization, a third energization, and a fourth energization, In the first current supply, a current value I1 in kA is set to 6.0 kA or more; The first current application time t1 in msec is set to 100 msec or less, I1 and t1 satisfy Formula 1, 200≦I1×t1≦1500...(Formula 1) In the second current supply, a current value I2 in kA satisfies Equation 2, 5.5≦I2≦I1×0.90...(Formula 2) A current value I3 in the unit of kA in the third current supply satisfies Equation 3, I2+0.5≦I3≦I2+2.0...(Formula 3) A current value I4 in the unit of kA in the fourth energization satisfies Equation 4, I3+0.5≦I4≦I3+2.0...(Formula 4) The current application time ti in msec for the i-th current application (i=2 to 4) satisfies Equation 5. 30×tsum≦ti≦150×tsum...(Formula 5) A method for manufacturing resistance spot welded joints.

2. The pressure P1 in the unit of kN in the first current application and the pressure Pi in the unit of kN in the i-th current application (i = 2 to 4) satisfy Equation 6. Pi<P1・・・・・・・・・・・・・・・・・・(Formula 6) The method for manufacturing a resistance spot welded joint according to claim 1 .

3. The method further includes a step of post-energizing the plate assembly after the main energization. The method for manufacturing a resistance spot welded joint according to claim 1 .

4. The method further includes a step of stopping the energization while maintaining the pressure between the main energization and the post-energization. The method for manufacturing a resistance spot welded joint according to claim 3 .

5. The main energization includes a fifth energization after the fourth energization, The current value I5 in the unit of kA in the fifth energization satisfies Equation 7, I4+0.5≦I5≦I4+2.0... (Formula 7) The current application time ti in msec for the i-th current application (i=2 to 5) satisfies Equation 8. 30×tsum≦ti≦120×tsum...(Formula 8) The method for manufacturing a resistance spot welded joint according to claim 1 .

6. The main energization includes a sixth energization after the fifth energization, A current value I6 in the unit of kA in the sixth energization satisfies Equation 9, I5+0.5≦I6≦I5+2.0... (Formula 9) The current application time ti in msec for the i-th current application (i=2 to 6) satisfies Equation 10. 30×tsum≦ti≦100×tsum...(Formula 10) The method for manufacturing a resistance spot welded joint according to claim 5 .

7. Among the electrodes used for the main current application, the tip diameter of the thinnest electrode in contact with the steel plate is set to 6.5 mm or more and 10 mm or less, and the current value I2 is set to more than 6.0 kA. The method for manufacturing a resistance spot welded joint according to claim 1 .

8. The pressing force Pi in the unit of kN in the i-th current application (i = 1 to 4) and the total sheet thickness tsum in the unit of mm satisfy Formula 11, Pi>tsum (Equation 11) The current value I2 is set to more than 6.0 kA. The method for manufacturing a resistance spot welded joint according to claim 1 .

9. A method for manufacturing an automobile part, comprising the method for manufacturing a resistance spot welded joint according to any one of claims 1 to 8.

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