Method for manufacturing resistance spot welded joints, and method for manufacturing automotive parts

Abstract

To provide a manufacturing method of a resistance spot welded joint and a manufacturing method of an automobile component, with which bonding failure can be avoided without increasing a scattering generating frequency, in a plate set in which a plate thickness ratio is 4.5 or more and the thinnest metal plate is arranged on a surface.SOLUTION: In a manufacturing method of a resistance spot welded joint, a first pressure application time with an applied pressure of P1, a second pressure application time with an applied pressure of P2, and a third pressure application time with an applied pressure of P3 are provided in sequence, wherein P1, P2, and P3 satisfy P2≤0.95*P1, P2<P3, 0.80*P1≤P3. Further, in the manufacturing method of a resistance spot welded joint, a time integral value S of electric current from a start time point D1 to an end time point D4 of electric conduction, a time integral value S1 of electric current from the D1 to a start time point D2 of the second pressure application time, and a time integral value S2 of electric current from the D1 to an end time point D3 of the second pressure application time, satisfy 0.20*S≤S1 and S2≤0.80*S, and a length of the second pressure application time is 20-200 msec.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a resistance spot welding joint and a method for manufacturing automotive parts.

Background Art

[0002] In automotive skeletal parts, for example, in a center pillar or the like, high-strength steel plates with a relatively thick plate thickness may be employed. This is to achieve collision safety of the vehicle body and weight reduction through component integration. On the other hand, on the outermost side of the automobile, parts such as a highly designed side member formed using a thin steel plate of, for example, 0.6 to 0.8 mm are arranged. The thin steel plate on the outside of the automobile is often made of soft steel in order to ensure workability.

[0003] For the above reasons, in the assembly of automotive parts, a plate assembly composed of a thick steel plate and a thin steel plate may be spot welded. Also, the plate assembly to be spot welded may have a three-layer configuration such as a thin steel plate - thick steel plate - thick steel plate. The plate thickness ratio of such a plate assembly may be, for example, 4.5 or more. Here, the plate thickness ratio is a value defined as tsum / tmin, where the plate thickness of the thinnest steel plate, that is, the thinnest steel plate, is tmin (mm), and the total plate thickness of the steel plates included in the plate assembly is tsum (mm).

[0004] However, when spot welding a plate assembly with a plate thickness ratio of 4.5 or more and the thinnest steel plate arranged on the surface, poor jointing is likely to occur. The nugget formed inside the plate assembly (originally referring to the melted and solidified part, but in this specification, both the melted part and the melted and solidified part are referred to as the nugget) does not grow up to the thinnest steel plate arranged on the outermost surface of the plate assembly, thereby causing poor jointing between the thinnest steel plate and the adjacent steel plate in contact with it.

[0005] In the plate assembly described above, one reason why the nugget does not grow down to the thinnest steel plate is that the thinnest steel plate is in contact with the spot welding electrode. The spot welding electrode has a structure in which a coolant flows inside, cooling the steel plate. Therefore, the temperature of the thinnest steel plate in contact with the electrode does not rise as easily as the temperature of the steel plates inside the plate assembly.

[0006] Another reason why the nugget doesn't grow down to the thinnest steel sheet is that the temperature rise starts from the center of the sheet assembly. The further away from the center of the sheet assembly, the slower the temperature rises.

[0007] Furthermore, if the thinnest steel sheet is mild steel, the growth of the nugget down to the thinnest steel sheet is further hindered. This is because thin mild steel is easily deformed. When thin mild steel is placed on the surface of a plate assembly, the mild steel easily deforms under pressure and current, 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 dissipation from the mild steel to the electrode.

[0008] For the reasons stated above, the thinnest steel plate on the surface of the plate assembly does not easily reach a higher temperature, and therefore melting and solidification are unlikely to occur in the thinnest steel plate. If the nugget does not grow down to the thinnest steel plate, the joint strength cannot be ensured. This hinders the use of thick, high-strength steel sheets in automotive frame components and narrows the range of choices for the thickness of steel sheets used in plate assemblies. Therefore, spot welding technology capable of solving these problems is eagerly awaited.

[0009] Patent Document 1 discloses a method for manufacturing a resistance spot welded joint by welding together a plate assembly made by overlapping multiple metal plates using resistance spot welding, characterized in that the resistance spot welding is performed in two stages, a first stage and a second stage, and the second stage welding is performed with higher pressure, lower current, or the same current and longer energizing time, or the same energizing time, compared to the first stage welding.

[0010] Patent Document 2 discloses a spot welding method having the steps of pre-energization, first energization, second energization, and third energization. In this method, pre-energization improves the contact surface between steel plates, a constant welding current is applied in the first energization to generate a nugget due to heat generated by the electrical contact resistance between the steel plates, the current is lower than that of the first energization in the second energization to suppress scattering and grow the nugget radially, and the current is higher than that of the second energization in the third energization to grow the nugget not only radially but mainly in the thickness direction, thereby reducing the pressure applied by the spot welding electrode.

[0011] Patent Document 3 discloses a resistance spot welding method comprising a plate assembly with a plate thickness ratio of more than 3, in which a thin plate is superimposed on at least one of two or more superimposed thick plates, which is then sandwiched between a pair of electrodes and joined by applying current while applying pressure. In this welding method, the current application and pressure pattern is divided into two or more multi-stage steps to perform welding, and in this case, the pressure applied in the first step: F1 and the pressure applied in the second step: F2 satisfy the relationship F1 > F2. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2005-262259 [Patent Document 2] Japanese Patent Publication No. 2018-30178 [Patent Document 3] International Publication No. 2016 / 088319 [Overview of the project] [Problems that the invention aims to solve]

[0013] When spot welding, it is also necessary to suppress spatter. Spatter refers to the phenomenon of localized heating of the base material, causing melting and scattering, or the metal itself. Spatter impairs the surface properties of resistance spot welded joints. Furthermore, if spatter occurs significantly, the scattering of molten metal reduces the nugget diameter, impairing the joint strength. One method to suppress spatter is to reduce the welding current value. However, reducing the welding current value inhibits nugget growth. Therefore, a method that maintains nugget diameter while suppressing spatter is highly desired.

[0014] According to the technology described in Patent Document 1, resistance spot welded joints with nuggets of the required size can be easily manufactured even in large plate assemblies with a plate thickness ratio exceeding 5, without adding extra steps or causing spatter. However, in the technology described in Patent Document 1, the first stage of welding is performed with low pressure and high current. It is thought that spatter is likely to occur in this first stage of welding.

[0015] In the technology described in Patent Document 2, spot welding is divided into four steps, the current value is optimized in each step, and the pressure is reduced towards the end of the welding process. However, spatter is likely to occur towards the end of this welding process.

[0016] In the technology described in Patent Document 3, spot welding is divided into multiple steps, and the pressure applied in the second step is reduced. However, spatter is likely to occur in this second step.

[0017] In view of the above circumstances, the object of the present invention is to provide a method for manufacturing resistance spot welding joints and a method for manufacturing automotive parts that can avoid joint defects without increasing the frequency of spatter in a plate assembly where the plate thickness ratio is 4.5 or more and the thinnest metal plate is placed on the surface. [Means for solving the problem]

[0018] The gist of this invention is as follows:

[0019] (1) A method for manufacturing a resistance spot welded joint according to one aspect of the present invention comprises a step of forming a nugget that joins a plate assembly, which is constructed by stacking two or more metal plates, by sandwiching the plate assembly between a pair of electrodes and applying current, wherein the thinnest metal plate is placed on at least one surface of the plate assembly, and the plate thickness ratio of the plate assembly, tsum / tmin, calculated based on the plate thickness tmin (mm) of the thinnest metal plate and the total plate thickness tsum (mm) of the metal plates included in the plate assembly, is 4.5 or more, and in the step of forming the nugget, a first pressurizing time with a pressurizing force of P1 (kN) between the pair of electrodes, a second pressurizing time with a pressurizing force of P2 (kN) between the pair of electrodes, and a third pressurizing time with a pressurizing force of P3 (kN) between the pair of electrodes are provided in order, and P1, P2, and P3 satisfy equations (1), (2), and (3), P2 ≤ 0.95 × P1 ···(1) P2 <P3···(2) 0.80 × P1 ≤ P3 ···(3) The time integral value S of the current during the period from the start time D1 to the end time D4 of the nugget forming process, the time integral value S1 of the current during the period from time D1 to the start time D2 of the second pressurization time, and the time integral value S2 of the current during the period from time D1 to the end time D3 of the second pressurization time satisfy equations (4) and (5), 0.20 × S ≤ S1 ···(4) S² ≤ 0.80 × S···(5) The duration of the second pressurization time is 20 to 200 msec. (2) The method for manufacturing a resistance spot welded joint as described in (1) above preferably further comprises a step of modifying the nugget by applying current to the plate assembly after the step of forming the nugget, while maintaining the pressure applied to the pair of electrodes. (3) The manufacturing method of the resistance spot welding joint described in (2) above preferably further includes a step of cooling the plate assembly by stopping the energization of the plate assembly while maintaining the pressing force of the pair of electrodes between the step of forming the nugget and the step of modifying the nugget. (4) In the manufacturing method of the resistance spot welding joint according to any one of (1) to (3) above, preferably, P2 ≧ 0.50 × P1. (5) In the manufacturing method of the resistance spot welding joint according to any one of (1) to (4) above, preferably, P1 is 2.5 kN or more and 8.0 kN or less. (6) In the manufacturing method of the resistance spot welding joint according to any one of (1) to (5) above, preferably, the welding current in the step of forming the nugget is 4 kA or more and 12 kA or less.

[0020] (7) The manufacturing method of an automotive part according to another aspect of the present invention includes the manufacturing method of the resistance spot welding joint according to any one of (1) to (6) above.

Advantages of the Invention

[0021] According to the present invention, there can be provided a manufacturing method of a resistance spot welding joint capable of avoiding joint defects without increasing the occurrence frequency of scattering in a plate assembly having a plate thickness ratio of 4.5 or more and in which the thinnest metal plate is arranged on the surface, and a manufacturing method of an automotive part.

Brief Description of the Drawings

[0022] [Figure 1] It is a schematic diagram of the change over time of the pressing force and the current value in the manufacturing method of the resistance spot welding joint according to the present embodiment. [Figure 2] It is a schematic diagram showing the relationship between the pressing force and the size of the nugget in the manufacturing method of the resistance spot welding joint according to the present embodiment. [Figure 3] It is a schematic diagram showing the relationship between the pressing force and the size of the nugget in the conventional manufacturing method of the resistance spot welding joint. [Figure 4A]This is a cross-sectional view of a nugget obtained by the method for manufacturing a resistance spot welded joint according to this embodiment. [Figure 4B] This is a cross-sectional view of a nugget obtained by a conventional resistance spot welding joint manufacturing method. [Figure 5] This is a schematic diagram illustrating the changes over time in the applied pressure and current values ​​in a method for manufacturing resistance spot welded joints, which includes a step for modifying the nugget. [Figure 6] This is a schematic diagram of the changes over time in the applied pressure and current values ​​in a method for manufacturing a resistance spot welded joint, which includes steps for cooling the nugget and modifying the nugget. [Modes for carrying out the invention]

[0023] The manufacturing method for the resistance spot welded joint according to this embodiment includes a step of forming a nugget 12 that joins a plate assembly 11, which is made by stacking two or more metal plates 111, by sandwiching the plate assembly 11 between a pair of electrodes E and applying current, wherein the thinnest metal plate 111S is placed on at least one surface of the plate assembly 11, and the plate thickness ratio tsum / tmin of the plate assembly 11, calculated based on the plate thickness tmin (mm) of the thinnest metal plate and the total plate thickness tsum (mm) of the metal plates included in the plate assembly, is 4.5 or more, and in the step of forming the nugget 12, a first pressurizing time is set with the pressurizing force of the pair of electrodes E to P1 (kN), and the pressurizing force of the pair of electrodes E A second pressurization time with a pressure of P2 (kN) and a third pressurization time with a pressurizing force of a pair of electrodes E of P3 (kN) are provided in sequence, such that P1, P2, and P3 satisfy equations (1), (2), and (3), and the time integral value S of the current during the period from the start of energization in the nugget formation process D1 to the end of energization D4, the time integral value S1 of the current during the period from time D1 to the start of the second pressurization time D2, and the time integral value S2 of the current during the period from time D1 to the end of the second pressurization time D3 satisfy equations (4) and (5), and the length of the second pressurization time is 20 to 200 msec. P2 ≤ 0.95 × P1 ···(1) P2 <P3···(2) 0.80 × P1 ≤ P3 ···(3) 0.20 × S ≤ S1 ···(4) S² ≤ 0.80 × S···(5)

[0024] (Nugget formation process) The manufacturing method of the resistance spot-welded joint 1 according to this embodiment includes a step of spot-welding a plate assembly 11 made by stacking two or more metal plates 111. Spot welding is performed by sandwiching the plate assembly 11 between a pair of spot-welding electrodes E and applying current. When current is applied, resistance heating occurs in the portion sandwiched by the electrodes E, the metal plates 111 melt, and a nugget 12 that joins the metal plates 111 is formed. At this time, in order to stabilize the current and suppress spatter, the pair of electrodes E pressurize the plate assembly 11.

[0025] (Electrode E) The pair of electrodes E are spot welding electrodes. Spot welding electrodes E are rod-shaped electrodes that directly contact the plate assembly and transmit welding current and pressure. In addition, spot welding electrodes E are cooled by a coolant such as water during spot welding. Therefore, heat is transferred from the plate assembly 11 to electrodes E while spot welding electrodes E are in contact with the plate assembly 11. When the current is stopped while the pressure is maintained, the plate assembly 11 is cooled by electrodes E.

[0026] (Arrangement of the thinnest metal plate 111 in the plate assembly 11) The thinnest metal plate 111S is placed on at least one surface of the plate assembly 11. Typical examples of plate assembly 11 on which the thinnest metal plate 111S is placed on the surface include (1) a plate assembly 11 in which the thinnest metal plate 111S and a thick metal plate 111 are stacked on top of each other, (2) a plate assembly 11 in which the thinnest metal plate 111S is further stacked on top of a plurality of stacked thick metal plates 111, and (3) a plate assembly 11 in which a thick metal plate 111 is sandwiched between two thinnest metal plates 111 of the same thickness.

[0027] When the thinnest metal sheet 111S is placed on the surface of the sheet assembly 11, bonding defects are likely to occur in the thinnest metal sheet 111S. However, the thinnest metal sheet 111S is often used as an exterior component of machine parts. For example, in some automobile parts, the thinnest metal sheet 111S is used as an exterior component, and the thicker metal sheet 111 is used as a structural component. In such parts, it is necessary for the thinnest metal sheet 111S to be placed on the surface of the sheet assembly 11.

[0028] (Plate thickness ratio of board assembly 11) The plate thickness ratio of the plate assembly 11 is set to 4.5 or higher. The plate thickness ratio is the value tsum / tmin obtained by dividing the total plate thickness tsum (mm) of the metal plates included in the plate assembly by the plate thickness tmin (mm) of the thinnest metal plate. The plate thickness ratio of the plate assembly 11 may be 4.8 or higher, 5.0 or higher, or 5.2 or higher. There is no particular upper limit to the plate thickness ratio of the plate assembly 11, but for example, the plate thickness ratio may be 10 or less, 9.5 or less, or 9.0 or less.

[0029] The larger the plate thickness ratio, the more likely it is that joining defects will occur in the thinnest metal plate 111S placed on the surface of the plate assembly 11. However, increasing the plate thickness ratio can lead to improved performance of machine parts. For example, some automobile parts are constructed by joining thick steel plates used as structural members and thin steel plates used as exterior members. In such parts, it is desirable to increase the plate thickness of the structural members to ensure strength, or to decrease the plate thickness of the exterior members to reduce the weight of the parts. Therefore, there is a strong demand for technology that can join plate assemblies 11 with high plate thickness ratios.

[0030] (First, second, and third pressurization times) The process of forming the nugget 12 includes, as shown in Figure 1, a first pressurization time with a pressurization force of P1 (kN) on the pair of electrodes E, a second pressurization time with a pressurization force of P2 (kN) on the pair of electrodes E, and a third pressurization time with a pressurization force of P3 (kN) on the pair of electrodes E. The pressurization forces P1, P2, and P3 satisfy equations (1), (2), and (3). P2 ≤ 0.95 × P1 ···(1) P2 <P3···(2) 0.80 × P1 ≤ P3 ···(3) In other words, in the manufacturing method according to this embodiment, a period of time is provided in the middle of the process of forming the nugget 12 during which the pressing force is reduced. Furthermore, in the manufacturing method according to this embodiment, the pressing force at the end of the process of forming the nugget 12 is higher than the pressing force in the middle. Note that the coefficient of P1 in equation (1) may be replaced with 0.90, 0.85, or 0.80. The coefficient of P1 in equation (3) may be replaced with 0.85, 0.90, or 0.95.

[0031] In the manufacturing method according to this embodiment, the start, end, and length of the second pressurization period are further defined.

[0032] (Start and end times of the second pressurization period) The second pressurization period begins after the heating and melting of the plate assembly 11 using electrode E has progressed. On the other hand, the second pressurization period ends before the formation of the nugget 12 is completed. In the manufacturing method according to this embodiment, the start and end times of the second pressurization period are determined using the time integral value of the current being supplied. The time integral value of the current being supplied serves as an indicator of the progress of the nugget formation process.

[0033] Specifically, in the manufacturing method according to this embodiment, equations (4) and (5) are satisfied. 0.20 × S ≤ S1 ···(4) S² ≤ 0.80 × S···(5) S is the time integral of the current during the period from the start time D1 to the end time D4 of the process of energizing the nugget formation. S is an index value of the total amount of heat introduced into the plate assembly 11 for the formation of the nugget 12. S1 is the time integral of the current during the period from time D1 to time D2, the start of the second pressurization period. S1 is an index value of the amount of heat supplied to the plate assembly 11 up to time D2, the start of the second pressurization period. S2 is the time integral of the current during the period from time D1 to time D3, the end of the second pressurization period. S2 is an index value of the amount of heat supplied to the plate assembly 11 up to time D3, the end of the second pressurization period. Note that the subsequent energization described later is not included in the "energization process for forming the nugget 12" and is therefore not considered when calculating S, S1, and S2. On the other hand, the preheating energization described later is included in the "energization process for forming the nugget 12" and is therefore considered when calculating S, S1, and S2.

[0034] (4) If spot welding is performed under conditions that satisfy equation (4), the start of the second pressurization time is set to be after the formation of the nugget 12 has progressed by approximately 20%. (5) If spot welding is performed under conditions that satisfy equation (5), the end of the second pressurization time is set to be before the formation of the nugget 12 has progressed by approximately 80%. The coefficient of S in equation (4), "0.20", may be replaced with 0.22, 0.25, or 0.30. The coefficient of S in equation (5), "0.80", may be replaced with 0.75, 0.70, or 0.60.

[0035] (Length of the second pressurization period) The length of the second pressurization time shall be within the range of 20 to 200 msec. The length of the second pressurization time may be 30 msec or more, 50 msec or more, or 80 msec or more. The length of the second pressurization time may be 180 msec or less, 150 msec or less, or 120 msec or less.

[0036] (Effects and Benefits) In the manufacturing method according to this embodiment, a multi-stage pressurization period is provided when forming the nugget 12. The pressurization force at the start of current application is set to P1, the pressurization force is reduced to P2 during current application, and the pressurization force is increased to P3 after a predetermined time has elapsed. The inventors have found that by applying such a multi-stage pressurization period to spot welding, the interface between the thinnest metal plate 111S placed on the surface of the plate assembly 11 and the metal plate 111 in contact with it can be stably melted. The reason for this is presumed to be as follows.

[0037] When applying current to electrode E, it is necessary to pressurize the plate assembly 11 using electrode E. If the pressurizing force is insufficient, the welding will become unstable, resulting in spatter or welding defects. Spatter refers to the phenomenon of localized heating and melting of the base material, or the resulting metal splatter.

[0038] However, as schematically shown in Figure 3, increasing the pressure promotes heat transfer from the plate assembly 11 to the electrode E. This is because increasing the pressure increases the contact area between the plate assembly 11 and the electrode E. Consequently, during spot welding, the temperature rises and the base material melts in the center of the plate assembly 11 in the thickness direction, while the temperature does not rise sufficiently in the surface layer of the plate assembly 11. This prevents the melting of the interface between the thinnest metal plate 111S on the surface of the plate assembly 11 and the metal plate 111 in contact with it. As a result, the nugget 12 does not grow down to the thinnest metal plate 111S, resulting in a defective joint.

[0039] Therefore, in the manufacturing method according to this embodiment, as schematically shown in Figure 2, the pressure is reduced in the middle of the spot welding. This suppresses heat transfer from the plate assembly 11 to the electrode E. At the same time, heat transfer continues from the central part of the plate assembly 11, which is a higher temperature region, to the surface part of the plate assembly 11. As a result, the surface part of the plate assembly 11 is heated. This phenomenon is called reheating. As a result, the melting of the base material in the surface part of the plate assembly 11 is promoted. As a result, the nugget 12 grows along the plate thickness direction and reaches the thinnest metal plate 111S. This suppresses bonding defects.

[0040] Furthermore, in the manufacturing method according to this embodiment, as shown in Figure 1, the pressure P1 during the first pressurizing time and the pressure P3 during the third pressurizing time are kept high. As a result, scattering can be suppressed in the manufacturing method according to this embodiment.

[0041] However, in order to obtain the above-mentioned effects, the applied pressures P1 to P3 must satisfy equations (1) to (3) above.

[0042] The applied pressure P1 is not particularly limited, and conditions used in ordinary spot welding can be appropriately adopted.

[0043] The applied pressure P2 must satisfy equation (1). That is, P2 must be 95% or less of P1. If P2 is greater than 95% of P1, the effect of promoting the melting of the plate assembly 11 will not be obtained.

[0044] The applied pressure P3 must satisfy equations (2) and (3). That is, P3 must be greater than P2 and at least 80% of P1. If P3 does not satisfy equations (2) and (3), the rate of spatter will increase towards the end of spot welding, which may lead to a higher rate of welding defects.

[0045] In addition, the start, end, and length of the second pressurization period must be within the range described above.

[0046] If the second pressurization period is started too early, i.e., equation (4) is not satisfied, the spatter rate may increase, potentially leading to a higher welding defect rate. Also, if the second pressurization period is started too late, i.e., equation (5) is not satisfied, the reheating effect will be small, making it difficult to melt the base material on the surface of the plate assembly 11. Furthermore, the spatter rate may increase, potentially leading to a higher welding defect rate. This is because the current is not stable at the beginning and end of spot welding, making spatter more likely.

[0047] To avoid spatter, the start of the second pressurization period may be delayed, or the end of the second pressurization period may be brought forward. However, the length of the second pressurization period must be 20 msec or longer. If the second pressurization period is too short, the melting of the base material in the surface layer of the plate assembly 11 will be hindered, resulting in a poor joint. Therefore, the start and end times of the second pressurization period should be selected within a range that allows the second pressurization period to be 20 msec or longer. Note that if the second pressurization period is too long, the welding stability may be impaired. For this reason, the length of the second pressurization period should be 200 msec or less.

[0048] The experimental results demonstrating the effects of the manufacturing method according to this embodiment are shown in Figures 4A and 4B. Figure 4A is a cross-sectional view of the nugget 12 obtained by the manufacturing method in which a second pressurization time was provided to satisfy the above-described conditions. Figure 4B is a cross-sectional view of the nugget 12 obtained by the manufacturing method in which a second pressurization time was not provided, but the pressurization force, welding current value, and energization time were the same as in Figure 4A. The nugget 12 in Figure 4A, in which a second pressurization time was provided, melted deeper into the thinnest metal plate 111S on the surface of the plate assembly 11 than the nugget 12 in Figure 4B, in which a second pressurization time was not provided.

[0049] (Modification of Nugget 12) The manufacturing method according to this embodiment may further include a step of modifying the nugget 12 by applying a post-current to the plate assembly 11 while maintaining the applied pressure of the pair of electrodes E, as shown in Figure 5, after the step of forming the nugget 12. Post-current application means applying a post-thermal current between the pair of electrodes E. A post-thermal current is a current applied in resistance welding of materials that harden by welding, after welding, for the purpose of tempering or annealing the hardened weld. By modifying the nugget 12, the joint strength of the resistance spot welded joint 1 can be further increased. If the metal plate 111 is a zinc-plated steel plate, post-current application can also suppress LME that occurs inside or at the edges of the corona bond.

[0050] The current value and energizing time during post-welding are not particularly limited, and appropriate values ​​can be adopted depending on the material of the plate assembly. For example, even if the current value is at the same level as the welding current, shortening the energizing time can prevent the nugget from melting. The current value and energizing time can be freely combined.

[0051] Furthermore, when calculating the time integral value S of the current during the period from the start time D1 to the end time D4 of the nugget formation process, subsequent energization is not considered. S is a value used to determine the start and end times of the second pressurization period. On the other hand, since subsequent energization is performed to modify the nugget 12, it does not melt the plate assembly 11. Subsequent energization does not affect the size of the nugget 12 and does not cause scattering. Therefore, when determining the start and end times of the second pressurization period, it is not necessary to consider the existence of subsequent energization. As shown in Figure 5, when the energization for forming the nugget 12 (sometimes called the main energization) and the energization for modifying the nugget 12 are performed continuously, the point in time when the current value begins to decrease is considered to be the end time D4 of the energization for the nugget formation process.

[0052] (Cooling of nugget 12 before modification) In the manufacturing method according to this embodiment, as shown in Figure 5, the steps of forming the nugget 12 and modifying the nugget 12 may be carried out in succession. On the other hand, as shown in Figure 6, a step of cooling the plate assembly 11 by stopping the supply of current to the plate assembly 11 while maintaining the applied pressure of the pair of electrodes E may be further included between the steps of forming the nugget 12 and the steps of modifying the nugget 12. The cooling step can be appropriately provided depending on the material of the metal plates 111 that constitute the plate assembly 11. The applied pressure during cooling is not particularly limited. For example, cooling may be performed by ending the supply of current while maintaining the applied pressure P3 during the third pressurizing time. On the other hand, the applied pressure may be changed from P3 after the end of the supply of current.

[0053] (Upper and lower limits of P1) The pressure P1 applied during the first pressurization time can be appropriately selected within a range that can suppress scattering. Furthermore, the pressure P1 can be optimized based on the material of the plate assembly 11 and the total thickness tsum of the metal plates included in the plate assembly. For example, P1 may be in the range of 2.5kN or more and 8.0kN or less. P1 may be 3.0kN or more, 3.5kN or more, or 4.0kN or more. P1 may be 6.5kN or less, 6.0kN or less, or 5.0kN or less.

[0054] (Lower limit of P2) The lower limit of the applied pressure P2 during the second pressurization time is not particularly limited, but it may be, for example, 50% or more of P1. That is, P2 ≥ 0.50 × P1. This can further suppress the occurrence of scattering during the second pressurization time. P2 may be 60% or more, 65% or more, or 70% or more of P1.

[0055] (Lower limit of P3) The lower limit of P3 can be any value within the range defined by equations (2) and (3). Alternatively, P3 may be limited to 1.1 times, 1.2 times, or 1.3 times or more P2. This makes it possible to more reliably suppress scattering during the third pressurization time.

[0056] (Current value) The value of the welding current in the process of forming the nugget 12 is not particularly limited. Here, the welding current is the current that flows to melt the base material and form the weld, and is also called the main current. The application of the main current is sometimes referred to as the main current. In contrast, the current that preheats the base material without melting it is called the preheating current. The term "current application" in the process of forming the nugget 12 is a concept that includes both the application of the welding current (main current application) and the application of the preheating current (preheating current application).

[0057] The higher the welding current, the more likely spatter is to occur. On the other hand, the higher the welding current, the larger the size of the nugget 12 becomes, and the more joint defects are suppressed. The welding current can be optimized based on the material of the plate assembly 11 and the total thickness tsum of the metal plates included in the plate assembly. For example, the welding current may be 4kA or more and 12kA or less. The welding current may be 5kA or more, 6kA or more, or 7kA or more. The welding current may be 10kA or less, 9kA or less, or 8kA or less. In the process of forming the nugget 12, the current value may be kept constant as shown in Figure 1, etc., or the current value may be changed as appropriate according to the degree of growth of the nugget 12.

[0058] (Manufacturing methods for automotive parts) Another embodiment of the present invention provides a method for manufacturing an automotive part, comprising the method for manufacturing a resistance spot welded joint 1 according to this embodiment. This makes it possible to manufacture an automotive part with a high plate thickness ratio and suppressed joint defects without generating spatter. An automotive part is, for example, a center pillar. This makes it possible to easily manufacture an automotive part with a plate thickness ratio of 4.5 or more, and with the thinnest metal plate 111S placed on the surface, while suppressing joint defects. In the manufacture of automotive parts, thin plates used as exterior materials and thick plates used as structural materials are often joined. Therefore, the method for manufacturing an automotive part according to this embodiment is extremely suitable for manufacturing an automotive part having a resistance spot welded joint 1 with a large plate thickness ratio.

[0059] However, the application of the method for manufacturing resistance spot welded joints according to this embodiment is not particularly limited. For example, the method for manufacturing resistance spot welded joints according to this embodiment may be applied to the manufacture of home appliances.

[0060] Although embodiments of the present invention have been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical spirit of the invention.

[0061] (Preheating and power supply) In the process of forming the nugget 12, a preheating current may be applied before the welding current is applied. A preheating current is a current applied to preheat the weld area before welding. The application of the preheating current is sometimes referred to as preheating current application. Preheating current application can further improve welding stability in the initial stages of main current application.

[0062] Furthermore, preheating is taken into consideration when calculating the time integral S of the welding current during the period from the start time D1 to the end time D4 of the nugget formation process. This is because the amount of heat supplied during preheating affects the formation of the nugget 12 during the main energizing. When preheating is performed, the start time D1 of the nugget formation process is the start time of the preliminary energizing.

[0063] (Electrode E) The type of electrode E for resistance spot welding is not particularly limited. For example, various spot welding electrodes E specified in JIS C 9304:1999 can be used in the manufacturing method of the resistance spot welded joint 1 according to this embodiment. Furthermore, the tip diameter of the electrode that pressurizes the thinner steel plate may be made smaller than the tip diameter of the electrode that pressurizes the opposite steel plate. This increases the current density in the thinner steel plate and reduces the cooling effect of the electrode. This promotes heating of the thinner steel plate, and the reheating effect of the pressurization process described above is superimposed, ensuring that the base material in the surface layer melts properly.

[0064] (Composition of board assembly 11) If tsum / tmin is 4.5 or greater, the number of metal plates 111 included in the plate assembly 11 can be any value of two or more. The effects of the manufacturing method according to this embodiment are achieved regardless of the number of metal plates 111. Normally, when there are three or more metal plates 111, a large heat input is required to join the thinnest metal plate 111S placed on the surface of the plate assembly 11 to the other metal plates 111. This makes spattering more likely to occur during welding. However, according to the manufacturing method according to this embodiment, both spattering and joining defects can be suppressed in a plate assembly 11 composed of three or more metal plates 111.

[0065] The number of metal plates 111 with the minimum thickness may be two or more. Therefore, the metal plates 111 with the minimum thickness may be placed on both surfaces of the plate assembly 11. On the other hand, if the number of metal plates 111 with the minimum thickness is two or more, one metal plate 111 with the minimum thickness may be placed on the surface of the plate assembly 11, and the other metal plates 111 with the minimum thickness may be placed inside the plate assembly 11.

[0066] If the number of metal plates 111 constituting the plate assembly 11 is 2 to 4, then naturally, the thicknesses tmin of these metal plates must differ from one another. This is because the thickness ratios when 2, 3, or 4 metal plates 111 of the same thickness are stacked are 2, 3, or 4, respectively, which are below the lower limit of the thickness ratio "4.5" for which the manufacturing method of the resistance spot welding joint 1 according to this embodiment targets welding. On the other hand, if the number of metal plates 111 included in the plate assembly 11 is 5 or more, the thicknesses of these metal plates may be the same. In this case, all metal plates 111 are considered to be the thinnest metal plate 111S. The thickness ratio of a plate assembly 11 composed of 5 metal plates 111 with the same thickness value t is 5, which satisfies the requirements of the plate assembly 11 targeted by the manufacturing method according to this embodiment. It is difficult to join all the metal plates 111 included in such a plate assembly 11 by ordinary resistance spot welding. However, according to the manufacturing method of this embodiment, the growth of the nugget 12 can be promoted, and all the metal plates 111 can be joined together.

[0067] (Types of metal plates 111) The type of metal plate 111 is not particularly limited. The effects of the manufacturing method according to this embodiment are achieved regardless of the type of metal plate 111. Examples of metal plates 111 include steel plates, aluminum plates, stainless steel plates, and titanium plates.

[0068] If the metal plate 111 is a steel plate, it is preferable that the tensile strength of the steel plate be, for example, 980 MPa or more. By making one or more of the multiple steel plates high-strength steel plates with a tensile strength of 980 MPa or more, the rigidity of the resistance spot welded joint 1 can be increased. On the other hand, the steel plates may be mild steel with a tensile strength of less than 980 MPa. For example, if the resistance spot welded joint 1 is an automobile part, the thinnest metal plate 111S placed on the surface of the plate assembly 11 may be made of mild steel, and the other steel plates may be made of high-strength steel plates.

[0069] One or more of the multiple metal plates 111 may have plating. If the metal plate 111 is a steel plate, examples of plating applied to the surface of the steel plate include hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, and aluminum plating.

[0070] The total thickness tsum of the metal plates included in the plate assembly is not particularly limited, but is preferably in the range of 2.0 mm to 6.0 mm. This ensures the rigidity of the resistance spot weld joint 1 while more effectively suppressing spatter during spot welding. Furthermore, the thickness tmin of the thinnest metal plate placed on the surface of the plate assembly 11 is preferably in the range of 0.3 mm to 1.5 mm.

[0071] (Welding conditions) The end of the first pressurization period (i.e., the start of the second pressurization period) is defined by equation (4). On the other hand, the start and length of the first pressurization period are not particularly limited. Considering the stability of the spot weld, it is preferable to start pressurization before starting to energize for the formation of the nugget 12.

[0072] The start of the third pressurization period (i.e., the end of the second pressurization period) is defined by equation (5). On the other hand, the end of the third pressurization period and its length are not particularly limited. Considering the stability of the spot weld, it is preferable to release the electrode E and terminate the pressurization after the energization for the formation of the nugget 12 has finished.

[0073] The conditions for the steps of cooling the nugget 12 and modifying the nugget 12 are not particularly limited, and appropriate pressurizing and energizing conditions can be adopted according to the plate assembly 11. Cooling and post-energizing do not melt the metal plate 111 and the nugget 12. Cooling and post-energizing do not cause spatter and do not affect the size of the nugget 12. Therefore, these steps are not considered to promote or suppress joint defects. On the other hand, these steps are beneficial for improving the mechanical properties of the nugget 12, such as toughness, and for increasing the joint strength of the resistance spot welded joint 1. [Examples]

[0074] The effects of one aspect of the present invention will be further explained in detail by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention.

[0075] Three types of plate assemblies, each consisting of three or four steel plates, were prepared.

[0076] [Table 1]

[0077] Various spot welds were performed on these plate assemblies under the following conditions. In some cases, cooling and post-current were performed after spot welding. • Welding machine: Servo-pressure stationary welding machine, single-phase AC (frequency 50kHz). • Electrode: Dome radius (DR) Cr-Cu. • Electrode tip shape: φ6mm R40mm. • Welding conditions: As described in Tables 2 and 3.

[0078] The definitions of the terms listed in Tables 2 and 3 will be explained using the symbols D1 to D7 shown in Figure 6. D1 to D7 are defined as follows: D1: The point at which the process of forming the nuggets begins to be energized. • D2: The start of the second pressurization period. • D3: The end of the second pressurization period. D4: The point at which the power supply for the nugget formation process ends. D5: The point at which power is applied to the process of modifying the nuggets, i.e., the start of post-power application. D6: The point at which power is turned off. • D7: The point at which the electrode opening is initiated.

[0079] The terms listed in Table 2 are defined as follows: ●Nugget formation process (main power application): Period D1 to D4. • Current value I1: Current value in D1~D4. • Power supply time TI1: Length from D1 to D4. ●First pressurization period: Period D1-D2. • Pressure P1: Pressure applied between D1 and D2. • Pressurization time TP1: Length from D1 to D2. ●Second pressurization period: Period D2-D3. • Pressure P2: Pressure applied in D2~D3. • Pressurization time TP2: Length from D2 to D3. ●Third pressurization period: Period D3-D4. • Pressure P3: Pressure applied in D3~D4. • Pressurization time TP3: Length from D3 to D4. ●Time integral of current • S1 / S: Ratio of integral value S1 to integral value S • S² / S: Ratio of the integral value S² to the integral value S ●Cooling process (fourth pressurization time): Period D4 to D5. • Pressure CP1: Pressure at D4~D5. • Pressurization time TCP1: Length from D4 to D5. ●Nugget modification process (post-energization, fifth pressurization time): Period D5 to D6. • Current value AI1: Current value in D5~D6. • The length of the energizing time TAI1:D5~D6. • Pressure AP1: Pressure at D5~D6. • Pressurization time TAP1: Length from D5 to D6. ●Holding process (sixth pressurization time): Period D6 to D7. • Pressure CP1: Pressure applied in D6~D7. • Pressurization time TCP1: Length from D6 to D7. Furthermore, the applied pressure and current values ​​at various stages or time points were kept constant at the values ​​listed in the table throughout the entire period in which they were applied.

[0080] In some cases, some of the steps and times listed above were omitted. For example, in test condition 1, which did not include the cooling and nugget modification steps, the sixth pressurization time began immediately after the third pressurization time ended. In test condition 4, which did not include the third pressurization time, the cooling step, and the nugget modification step, the sixth pressurization time began immediately after the second pressurization time ended. The omitted steps or time conditions are indicated by the symbol "-".

[0081] The definitions of terms used in Tables 2 and 3 are merely for convenience to more clearly express the manufacturing conditions. For example, in the manufacturing method according to this embodiment, the pressurization during the first pressurization time may start before D1, but the notation of pressurization before D1 is omitted in Tables 2 and 3. Also, in the manufacturing method according to this embodiment, the pressurization during the third pressurization time may continue from D4 onwards. Therefore, the fourth pressurization time and the sixth pressurization time listed in the table can be considered as part of the third pressurization time. However, in order to further clarify the manufacturing conditions applied to the example, the pressurization time is divided and shown before and after D4 in Tables 2 and 3. Therefore, in Table 2, the energization time TI1 of the nugget formation process is equal to the sum of the first pressurization time TP1, the second pressurization time TP2, and the third pressurization time TP3.

[0082] [Table 2]

[0083] [Table 3]

[0084] The nugget diameter of the resistance spot welded joint manufactured using the procedure described above was measured using the following method. First, the nugget was cut along a plane passing through its center and perpendicular to the steel plate surface. The cross-section was prepared and observed with an optical microscope. The nugget diameter was then confirmed along the joint surface between the thinnest steel plate on the surface of the plate assembly and the steel plate adjacent to it. If the nugget did not extend to the thinnest steel plate on the surface of the plate assembly, the nugget diameter was considered to be 0 mm. The nugget diameter was then evaluated according to the following criteria and recorded in Table 4. However, welding was performed five times for each condition, and the nugget diameter of the resistance spot welded joint with the smallest nugget diameter was evaluated according to the following criteria. Nugget diameter of 4√t (=3.1mm) or more: ◎ Nugget diameter is 3√t (=2.3mm) or more and less than 4√t: ○ Nugget diameter less than 3√t (=2.3mm): × Note that 't' represents the thickness of the thinnest steel plate placed on the surface of the plate assembly. In all examples, 't' was 0.6 mm. Resistance spot welded joints with a nugget diameter of less than 3√t were judged to have a faulty joint.

[0085] Furthermore, the area near the electrode tip was observed during manufacturing, and the frequency of spatter occurrence was visually measured. Welding was performed five times for each condition. In test conditions where spatter occurred two or more times, it was determined that the spatter suppression was insufficient.

[0086] [Table 4]

[0087] Under test condition 2, the start of the second pressurization period was too early, resulting in insufficient S1 / S. Consequently, the frequency of scattering was high under test condition 2.

[0088] Under test condition 3, the end of the second pressurization period was too late, resulting in an excessive S2 / S ratio. Consequently, the frequency of scattering was high under test condition 3.

[0089] Under test condition 4, a third pressurization time was not provided, resulting in an S2 / S ratio of 1.0. Consequently, the frequency of scattering was high under test condition 4. In addition, due to the excessive amount of scattering, the nugget diameter was also insufficient under test condition 4.

[0090] Under test condition 5, the applied pressure P2 during the second pressurization time was too high. As a result, nugget growth was not promoted under test condition 5, and the nugget diameter was insufficient.

[0091] Under test condition 6, the duration of the second pressurization was too short. As a result, nugget growth was not promoted under test condition 6, and the nugget diameter was insufficient.

[0092] Under test condition 7, the duration of the second pressurization was too long. As a result, the frequency of scattering was high under test condition 7. In addition, the excessive amount of scattering resulted in an insufficient nugget diameter under test condition 7.

[0093] On the other hand, in test conditions 1 and 8-12, the applied pressure P2 during the second pressurization period, as well as the start, end, and duration of the second pressurization period, were all appropriate. Therefore, under these test conditions, both the frequency of scattering and bonding defects were suppressed. [Explanation of Symbols]

[0094] 1. Resistance spot welding joint 11 Board set 111 Metal plate 111S Thinnest metal plate 12 nuggets E-electrode tmin: Thickness of the thinnest metal sheet tsum Total thickness of metal plates included in the plate assembly P1 Pressure applied during the first pressurization time P2 Pressure applied during the second pressurization time P3 Pressure applied during the third pressurization time D1 The point at which power is applied to the nugget formation process. D2 The start of the second pressurization period D3 End of the second pressurization period D4 The point at which the power supply for the nugget formation process ends. D5 The point at which power is applied to the process of modifying the nuggets. D6 The point at which the power supply for the nugget modification process ends. D7 The point at which the electrode opening was initiated. Time integral of current during the period from S D1 to D4 S1 Time integral of the current during the period from D1 to D2 S2 Time integral of current during the period from D1 to D3

Claims

1. A method for manufacturing resistance spot welded joints, The process includes a step of forming a nugget that joins two or more metal plates together by sandwiching the plate assembly between a pair of electrodes and applying an electric current, The thinnest metal plate is placed on at least one surface of the aforementioned plate assembly. The thickness ratio of the plate assembly, tsum / tmin, calculated based on the thickness tmin (mm) of the thinnest metal plate and the total thickness tsum (mm) of the metal plates included in the plate assembly, is 4.5 or greater. In the process of forming the nugget, A first pressurizing time where the pressurizing force of the pair of electrodes is P1 (kN), A second pressurizing time where the pressurizing force of the pair of electrodes is P2 (kN), A third pressurizing time where the pressurizing force of the pair of electrodes is P3 (kN), They are set up in order, P1, P2, and P3 satisfy equations (1), (2), and (3), P2 ≤ 0.95 × P1 ... (1) P2<P3...(2) 0.80 × P1 ≤ P3 ... (3) The time integral value S of the current during the period from the start time D1 to the end time D4 of the nugget forming process, the time integral value S1 of the current during the period from time D1 to the start time D2 of the second pressurization time, and the time integral value S2 of the current during the period from time D1 to the end time D3 of the second pressurization time satisfy equations (4) and (5), 0.20 × S ≤ S1 ... (4) S² ≤ 0.80 × S ... (5) The length of the second pressurization time is 20 to 200 msec. A method for manufacturing resistance spot welded joints.

2. The method for manufacturing a resistance spot welded joint according to claim 1, further comprising the step of modifying the nugget by applying current to the plate assembly after the step of forming the nugget, while maintaining the applied pressure of the pair of electrodes.

3. The method for manufacturing a resistance spot welded joint according to claim 2, further comprising a step of cooling the plate assembly by stopping the supply of current to the plate assembly while maintaining the applied pressure of the pair of electrodes between the step of forming the nugget and the step of modifying the nugget.

4. A method for manufacturing a resistance spot welded joint according to claim 1, characterized in that P2 ≥ 0.50 × P1.

5. A method for manufacturing a resistance spot welded joint according to claim 1, characterized in that P1 is 2.5 kN or more and 8.0 kN or less.

6. The method for manufacturing a resistance spot welded joint according to claim 1, characterized in that the welding current in the step of forming the nugget is 4 kA or more and 12 kA or less.

7. 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 6.

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