Method for manufacturing a spot welded joint, and spot welded joint

Abstract

The present invention provides a method for manufacturing a spot-welded joint and a spot-welded joint that can avoid joint defects without increasing the frequency of spatter in plate assemblies with a high plate thickness ratio and a large total thickness. [Solution] A method for manufacturing a spot-welded joint according to one aspect of the present disclosure is provided in which the total thickness tsum of the steel plates is 4.0 mm or more, the plate thickness ratio is 4.0 or more and 8.0 or less, and the following conditions are met: 5.0 ≤ I1, 0.4 ≤ I1 / A ≤ 1.5, 20 ≤ T1 ≤ 140, 200 ≤ I1 × T1 ≤ 1500, 8.0 ≤ I2, I1 ≤ I2, 0.4 ≤ I2 / A ≤ 1.5, 20 ≤ T2 ≤ 140, T1 + T2 ≤ 240, I3 ≤ I2 × 0.9, and 60 × (tsum / 2) ≤ T3 ≤ 200 × (tsum / 2), where I1 to I3 are the current values ​​from the first energization to the third energization, T1 to T3 are the energization times from the first energization to the third energization, and A is the energized area of ​​the thin-thick interface immediately before the start of the first energization.

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a spot weld joint and a spot weld joint.

Background Art

[0002] In automobile skeletal members, for example, in a center pillar or the like, high-strength steel plates with a relatively large 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, components 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 to ensure workability.

[0003] Due to the above reasons, in the assembly of automobile 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, for example, thin steel plate - thick steel plate - thick steel plate.

[0004] Methods for manufacturing a weld joint by spot welding a plate assembly composed of a thick steel plate and a thin steel plate are disclosed in, for example, Patent Documents 1 to 4.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the manufacturing of automotive parts, the plate thickness ratio of a plate assembly may be, for example, 5.0 or higher. The plate thickness ratio is defined as tsum / tmin, where tmin (mm) is the thickness of the thinnest steel plate and tsum (mm) is the total thickness of the steel plates included in the assembly.

[0007] However, when spot welding a plate assembly with a plate thickness ratio of 5.0 or more, and where the thinnest steel plate is placed on the surface, joint defects are likely to occur. The nugget formed inside the plate assembly does not grow to the thinnest steel plate placed on the outermost surface of the assembly, resulting in a joint defect between the thinnest steel plate and the adjacent steel plate in contact with it. Generally, the term "nugget" refers to the molten and solidified portion that occurs in the weld in lap resistance welding. However, in this disclosure, the molten portion formed during lap resistance welding is also referred to as a nugget.

[0008] 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. During spot welding, heat is transferred from the steel plate to the spot welding electrode. Therefore, the temperature of the thinnest steel plate in contact with the electrode does not rise as easily as that of the steel plates inside the plate assembly.

[0009] 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.

[0010] 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.

[0011] For the reasons stated above, the thinnest steel sheet 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 sheet. If the nugget does not grow down to the thinnest steel sheet, 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 the steel sheets that make up the plate assembly. If the use of high-strength steel sheets or thick plates is hindered, the number of parts in the automobile body may increase, potentially leading to an increase in weight.

[0012] Furthermore, spot welding requires the suppression of spatter. Spatter refers to the phenomenon of localized heating and melting of the base material, or the resulting metal scattering. Spatter impairs the surface properties of the spot-welded joint. Also, if spatter occurs significantly, the nugget diameter decreases due to the scattering of molten metal, impairing the joint strength. One means of suppressing spatter is to reduce the welding current value. However, reducing the welding current value inhibits nugget growth. Therefore, it is extremely difficult to maintain a sufficient nugget diameter while suppressing spatter. The technologies disclosed in Patent Documents 1 to 4 do not ensure a sufficient nugget diameter in plate assemblies with a plate thickness ratio of 5.0 or more while suppressing spatter.

[0013] In view of the above circumstances, the object of this disclosure is to provide a method for manufacturing a spot-welded joint and a spot-welded joint that can avoid joint defects without increasing the frequency of spatter in plate assemblies with a high plate thickness ratio and a large total thickness. [Means for solving the problem]

[0014] The gist of this disclosure is as follows:

[0015] (1) A method for manufacturing a spot-welded joint according to one aspect of the present disclosure is a method for manufacturing a spot-welded joint comprising the step of applying current to a plate assembly of three or more stacked steel plates, wherein the thinnest steel plate is placed on the surface of the plate assembly, the total thickness tsum in units of mm of all the steel plates included in the plate assembly is 4.0 mm or more, the thickness tmin in units of mm of the thinnest steel plate and the plate thickness ratio tsum / tmin of the plate assembly calculated from tsum are 4.0 or more and 8.0 or less, the current application comprises at least a first current application, a second current application and a third current application, and the current application satisfies the following formula: 5.0≦I1……………………………………………………Equation 1 0.4≦I1 / A≦1.5…………………………………………Equation 2 20≦T1≦140………………………………………………Equation 3 200≦I1×T1≦1500……………………………………Equation 4 8.0≦I2……………………………………………………Equation 5 I1≦I2…………………………………………………………Equation 6 0.4≦I2 / A≦1.5…………………………………………Equation 7 20≦T2≦140………………………………………………Equation 8 T1+T2≦240………………………………………………Equation 9 I3≦I2×0.9………………………………………………Equation 10 60×(tsum / 2)≦T3≦200×(tsum / 2)…Equation 11 I1 is the current value in units kA for the first energization, T1 is the energization time in units ms for the first energization, I2 is the current value in units kA for the second energization, T2 is the energization time in units ms for the second energization, I3 is the current value in units kA for the third energization, T3 is the energization time in units ms for the third energization, and A is the distance in units mm between the thinnest steel plate and the adjacent steel plate immediately before the start of the first energization. 2is the energized area in, and when the I1 and the I2 are the same, the main energization has an energization pause time between the first energization and the second energization. (2) Preferably, in the method for manufacturing a spot weld joint described in (1) above, the main energization satisfies the following formula. I1 < I2………………………… Formula 6a (3) Preferably, in the method for manufacturing a spot weld joint described in (1) or (2) above, the main energization further has an i-th energization (4 ≦ i), and the main energization satisfies the following formula, I3 < Ii…………………………………… Formula 12 Ii ≦ I(i + 1)………………………… Formula 13 Ii is the current value of the i-th energization, I(i + 1) is the current value of the (i + 1)-th energization, and when the Ii and the I(i + 1) are the same, the main energization has an energization pause time between the i-th energization and the (i + 1)-th energization. (4) Preferably, in the method for manufacturing a spot weld joint described in any one of (1) to (3) above, the main energization satisfies the following formula, P2 < Pj…………………………………… Formula 14 P2 is the pressing force of the second energization, and Pj is the pressing force of the j-th energization (3 ≦ j). (5) Preferably, in the method for manufacturing a spot weld joint described in any one of (1) to (4) above, after the main energization ends, the method further includes a step of post-energizing the plate assembly. (6) Preferably, in the method for manufacturing a spot weld joint described in (5) above, after the main energization ends and before the post-energization starts, the method further includes a step of cooling the plate assembly.

[0016] (7) A spot welded joint according to another aspect of the present disclosure is a spot welded joint comprising three or more stacked steel plates and a nugget for joining the steel plates, wherein the thinnest steel plate is placed on the surface of the spot welded joint, the total thickness tsum in units of mm of all the steel plates included in the spot welded joint is 4.0 mm or more, the plate thickness ratio tsum / tmin of the spot welded joint calculated from the thickness tmin in units of mm of the thinnest steel plate and tsum is 4.0 or more and 8.0 or less, and the following formula is satisfied at the mating surfaces of all the steel plates: 4.0√t≦d………………………………Equation 15 d is the diameter of the nugget measured at the mating surface, and t is the thickness of the thinner of the two steel plates that make up the mating surface. [Effects of the Invention]

[0017] According to this disclosure, a method for manufacturing a spot-welded joint and a spot-welded joint can be provided that can avoid joint defects in plate assemblies with a high plate thickness ratio without increasing the frequency of spatter. [Brief explanation of the drawing]

[0018] [Figure 1] This is a schematic cross-sectional view of an example of a plate assembly during the main energization of spot welding. [Figure 2] This is a current value-elapsed time graph of an example of a method for manufacturing a spot-welded joint without any power interruption time between the first and second energization. [Figure 3] This is a current value-elapsed time graph of an example of a method for manufacturing a spot-welded joint that has a power-off period between the first and second energization. [Figure 4] This is a current value-elapsed time graph of an example of a method for manufacturing a spot-welded joint with a fourth current supply. [Figure 5] This is a current value-elapsed time graph of an example of a method for manufacturing a spot-welded joint that includes initial energization, cooling, and post-energization. [Figure 6] This is a schematic cross-sectional view of an example of a spot-welded joint. [Modes for carrying out the invention]

[0019] (1. Method for manufacturing spot welded joints) A method for manufacturing a spot-welded joint according to one aspect of the present disclosure comprises a step of applying current to a plate assembly 10 made of three or more stacked steel plates 11, the thinnest steel plate 11min being placed on the surface of the plate assembly 10, the total thickness tsum of all steel plates 11 included in the plate assembly 10 in units of mm being 4.0 mm or more, the thickness tmin of the thinnest steel plate 11min in units of mm being tmin, and the plate thickness ratio tsum / tmin of the plate assembly 10 calculated from tsum being 4.0 or more and 8.0 or less, the current application comprising at least a first current application, a second current application, and a third current application, and the current application satisfying the following formula, 5.0≦I1……………………………………………………Equation 1 0.4≦I1 / A≦1.5…………………………………………Equation 2 20≦T1≦140………………………………………………Equation 3 200≦I1×T1≦1500……………………………………Equation 4 8.0≦I2……………………………………………………Equation 5 I1≦I2…………………………………………………………Equation 6 0.4≦I2 / A≦1.5…………………………………………Equation 7 20≦T2≦140………………………………………………Equation 8 T1+T2≦240………………………………………………Equation 9 I3≦I2×0.9………………………………………………Equation 10 60×(tsum / 2)≦T3≦200×(tsum / 2)…Equation 11 I1 is the current value in units kA for the first energization, T1 is the energization time in units ms for the first energization, I2 is the current value in units kA for the second energization, T2 is the energization time in units ms for the second energization, I3 is the current value in units kA for the third energization, T3 is the energization time in units ms for the third energization, and A is the distance in units mm between the thinnest steel plate 11 min and the adjacent steel plate 11 immediately before the start of the first energization. 2 This is the energized area, and if I1 and I2 are the same, this energization has an energization pause time between the first energization and the second energization.

[0020] (Hontsutsu S1) The method for manufacturing a spot-welded joint according to this embodiment includes the step of applying current S1 to a plate assembly 10 made by stacking two or more steel plates 11, as illustrated in Figure 1. This current application S1 is performed by sandwiching the plate assembly 10 between a pair of electrodes 2 and passing a welding current through it. This current application S1 causes resistance heating in the portion sandwiched by the electrodes 2. As a result, the steel plates 11 melt and a nugget 12 that joins the steel plates 11 is formed. At this time, the pair of electrodes 2 pressurize the plate assembly 10 in order to stabilize the current and suppress spatter.

[0021] The steel plate 11 can have any shape suitable for spot welding. In other words, the term "steel plate 11" is a concept that encompasses not only flat steel plates but also flat regions of steel members with three-dimensional shapes. For example, the flange portion of a hat-shaped member can also be considered a steel plate 11.

[0022] The pair of electrodes 2 are spot welding electrodes 2. The spot welding electrodes 2 are rod-shaped electrodes 2 that directly contact the plate assembly 10 to conduct the welding current and transmit pressure to the plate assembly 10. In addition, the tips of the spot welding electrodes 2 are cooled by a coolant such as water during spot welding. Therefore, heat is transferred from the plate assembly 10 to the electrodes 2 while the spot welding electrodes 2 are in contact with the plate assembly 10. When the current is stopped while maintaining the applied pressure, the plate assembly 10 is cooled by the electrodes 2.

[0023] (Arrangement of the thinnest steel plate 11 min in the plate assembly 10) The thinnest steel plate 11min is placed on the surface of the plate assembly 10. The thinnest steel plate 11min is the thinnest of the three or more steel plates 11 included in the plate thickness. Note that there may be two or more thinnest steel plates 11min. In this case, it is sufficient that at least one thinnest steel plate 11min is placed on at least one surface.

[0024] For example, in each of the plate assembly examples shown in Table 1, the thinnest steel plate 11, which is the thinnest steel plate 11min, is placed on the surface of the plate assembly 10. Any of the plate assembly 10s in Table 1 can be used in the method for manufacturing spot-welded joints according to this embodiment. In plate assembly examples A and B, the first and third steel plates 11 are the steel plates 11 placed on the surface of the plate assembly 10, and in example C, the first and fourth steel plates 11 are the steel plates 11 placed on the surface of the plate assembly 10. The plate assembly 10 illustrated in Figure 1 corresponds to plate assembly example A in Table 1.

[0025] [Table 1]

[0026] When the thinnest steel sheet 11min is placed on the surface of the plate assembly 10, bonding defects are likely to occur at the interface between the thinnest steel sheet 11min and the adjacent steel sheet 11. Hereinafter, the interface between the thinnest steel sheet 11min and the steel sheet 11 in contact with it may be referred to as the "thin-thick interface C". The thinnest steel sheet 11min is often used as an exterior component of machine parts. For example, in some automobile parts, the thinnest steel sheet 11min is used as an exterior component, and the thicker steel sheet 11 is used as a structural component. In such parts, the thinnest steel sheet 11min needs to be placed on the surface of the plate assembly 10. And, a sufficient nugget diameter needs to be secured at the thin-thick interface C.

[0027] (Ratio of board thickness in board assembly 10) The plate thickness ratio of the plate assembly 10 is set to be between 4.0 and 8.0. The plate thickness ratio is the value tsum / tmin obtained by dividing the total thickness tsum (mm) of all steel plates 11 included in the plate assembly 10 by the thickness tmin (mm) of the thinnest steel plate 11min. The plate thickness ratio of the plate assembly 10 may be 5.0 or more, 5.2 or more, 5.5 or more, or 5.8 or more. The plate thickness ratio of the plate assembly 10 may be 7.8 or less, 7.5 or less, or 7.0 or less. The thickness tmin of the thinnest steel plate 11min is not particularly limited, but it is preferably in the range of 0.3 mm to 1.5 mm.

[0028] (Total thickness of board assembly 10: tsum) From the viewpoint of increasing the strength of the mechanical structural component to which the spot welded joint 1 according to this embodiment is applied, the tsum is set to 4.0 mm or more. Preferably, the tsum is set to 4.5 mm or more, or 5.0 mm or more.

[0029] (First power supply, second power supply, and third power supply) This energization S1 comprises at least a first energization, a second energization performed after the first energization, and a third energization performed after the second energization. These energization times are distinguished based on differences in current values, as shown in Figure 1. Alternatively, these energization times are separated by a period of pause in energization, as shown in Figure 2. The i-th energization (4≦i), described later, is also distinguished based on differences in current values ​​or a period of pause in energization. It is also permissible to include a period of pause in energization between two energization times with different current values, as shown in Figure 2.

[0030] In the energization pattern illustrated in Figure 1, the current values ​​for the first, second, and third energizations are different from each other. These energizations are separated at the point where the current value changes.

[0031] In the energizing pattern illustrated in Figure 2, the current values ​​for the first and second energizing are the same. However, the energizing pattern illustrated in Figure 2 includes an energizing pause time. The first and second energizing are separated during the energizing pause time. As is clear from Equation 6 described later, the current value I1 for the first energizing and the current value I2 for the second energizing may be the same. In this case, the energizing has an energizing pause time between the first and second energizing. During the energizing pause time, heat is transferred from the plate assembly 10 to the electrode 2, while no resistive heating occurs. Therefore, the temperature of the plate assembly 10 decreases slightly during the energizing pause time.

[0032] Below, we define the current value in the nth (n≦1) energization as In, and the energization time in the nth energization as Tn. That is, I1 is the current value in the first energization, T1 is the energization time in the first energization, I2 is the current value in the second energization, T2 is the energization time in the second energization, I3 is the current value in the third energization, and T3 is the energization time in the third energization. The current value In in the nth energization is the value of the welding current flowed through the plate assembly 10 in the nth energization. The energization time Tn in the nth energization is the length of time for which the welding current of current value In is applied. The unit of all current values ​​is kA. The unit of all energization times is ms (milliseconds).

[0033] Furthermore, the distance between the thinnest steel plate 11 min and the adjacent steel plate 11 immediately before the start of the first energization, in units of mm 2 The energized area is defined as A. The energized area A is a value measured before the start of the energization in the board assembly 10 to be energized, or in a board assembly 10 having the same configuration as the board assembly 10 to be energized.

[0034] The procedure for measuring the current-conducting area A is as follows: First, pressure-sensitive paper is inserted between the thinnest steel plate 11min and the adjacent steel plate 11. Next, the pressure P1 applied during the current application is applied to the plate assembly 10 in which the pressure-sensitive paper is inserted. No current is applied when the plate assembly 10 is pressurized. Then, the electrodes are released and the pressure-sensitive paper is removed from the plate assembly 10 with the pressurized force at zero. Discoloration occurs on the pressure-sensitive paper due to the pressurization. The area of ​​this discolored area is considered to be the current-conducting area A between the thinnest steel plate 11min and the adjacent steel plate 11.

[0035] It is not necessary to measure the current-carrying area A each time a spot weld is performed. When spot welding is performed on a plate assembly 10 of the same configuration under the same conditions, the current-carrying area A will be substantially constant. In the method for manufacturing spot-welded joints according to this embodiment, the current-carrying area A identified by the procedure described above is considered to be the current-carrying area A for all spot welds performed on a plate assembly 10 of the same configuration under the same conditions.

[0036] This power supply, which has a first power supply, a second power supply, and a third power supply, satisfies the following equation. 5.0≦I1……………………………………………………Equation 1 0.4≦I1 / A≦1.5…………………………………………Equation 2 20≦T1≦140………………………………………………Equation 3 200≦I1×T1≦1500……………………………………Equation 4 8.0≦I2……………………………………………………Equation 5 I1≦I2…………………………………………………………Equation 6 0.4≦I2 / A≦1.5…………………………………………Equation 7 20≦T2≦140………………………………………………Equation 8 T1+T2≦240………………………………………………Equation 9 I3≦I2×0.9………………………………………………Equation 10 60×(tsum / 2)≦T3≦200×(tsum / 2)…Equation 11 The technical significance and effects of these formulas will be described later.

[0037] (Effects and Benefits) In the method for manufacturing spot-welded joints according to this embodiment, a plate assembly 10 having a total thickness tsum of 4.0 mm or more and a plate thickness ratio tsum / tmin of 4.0 or more and 8.0 or less is welded.

[0038] 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 10 with a high plate thickness ratio.

[0039] Furthermore, by increasing the total thickness tsum of the plate assembly 10, the strength of the machine part to which the spot-welded joint 1 obtained by the manufacturing method according to this embodiment is applied can be dramatically increased.

[0040] However, spot welding such a plate assembly 10 while suppressing both spatter and joint defects is difficult. The larger the plate thickness ratio tsum / tmin, the more likely joint defects are to occur at the thin-thick interface C. Also, the larger the total thickness tsum, the more difficult it becomes to flatten the gaps between the steel plates 11 by pressurizing before current is applied. If the gaps between the steel plates 11 are not sufficiently flattened and the contact area between the steel plates 11 is insufficient when the welding current is applied, spatter will occur when high current is applied in the initial stage of current application. However, if the applied pressure is increased to suppress spatter, the contact area between the steel plates 11 becomes too large, resulting in insufficient current density at the thin-thick interface C, making melting of the thin-thick interface C difficult.

[0041] Here, according to the inventors' research results, it has been found that by optimizing the applied pressure and contact area before the start of current application, and by applying a short, high current in the initial stages of current application, the thin-to-thick interface C can be sufficiently melted.

[0042] However, it has been newly discovered that even if the thin-thick interface C can be sufficiently melted by the above-described means, it is difficult to melt the other interfaces of the steel plates 11. Increasing the pressure increases the contact area of ​​the steel plates 11 at interfaces other than the thin-thick interface C, and the current density decreases. Also, when the total thickness tsum of the plate assembly 10 is large, it takes time for the molten region generated at the thin-thick interface C to grow to the other interfaces.

[0043] Therefore, the inventors decided to carry out the energization in at least three stages. In the first energization, the following equation is satisfied. 5.0≦I1……………………………………………………Equation 1 0.4≦I1 / A≦1.5…………………………………………Equation 2 20≦T1≦140………………………………………………Equation 3 200≦I1×T1≦1500……………………………………Equation 4 The first energization is performed with a high current for a short duration. By increasing the current value and heat input during the first energization, the melting of the thin-thick interface C can be promoted. In Equation 4, I1 × T1 is an index value of the heat input during the first energization. In addition, by making the first energization short, scattering can be suppressed.

[0044] Furthermore, the first energization that satisfies the above conditions promotes contact between the thinnest steel plate 11min and the steel plate 11 in contact with it. Even if there is a large gap in the thin-to-thickness interface C before the start of this energization, the thinnest steel plate 11min and the steel plate 11 in contact with it are in close contact at the end of the first energization. This suppresses scattering during the second energization.

[0045] Furthermore, in order to sufficiently melt the thin-thick interface C, it is necessary to increase I1 / A. I1 / A is an index value of the current density at the thin-thick interface C. If I1 / A is lower than 0.4, it becomes difficult to melt the thin-thick interface C. On the other hand, if the index value of current density I1 / A exceeds 1.5, scattering will occur with a high probability, even if the energizing time T1 of the first energization is shortened. Therefore, the above effect can be obtained by performing the first energization in a manner that satisfies Equation 2. The energized area A can be kept within an appropriate range by adjusting the applied pressure according to the size of the gap between the plate assembly 10 and the steel plate 11.

[0046] In the first energization, the current value I1 may be 5.2kA or more, 5.5kA or more, or 6.0kA or more. The upper limit of the current value I1 is not particularly limited, but for example it may be 8.0kA or less, 7.5kA or less, or 7.0kA or less. I1 / A is set to 0.4 or more and 1.5 or less. I1 / A may be 0.5 or more, 0.6 or more, or 0.8 or more. I1 / A may be 1.4 or less, 1.3 or less, or 1.0 or less. The energization time T1 may be 250ms or more, 300ms or more, or 400ms or more. The energization time T1 may be 1400ms or less, 1200ms or less, or 1000ms or less. I1×T1 may be 300 or more, 400 or more, or 500 or more. I1×T1 may be 1400 or less, 1300 or less, or 1200 or less.

[0047] During the second energization, the following equation is satisfied. 8.0≦I2……………………………………………………Equation 5 I1≦I2…………………………………………………………Equation 6 0.4≦I2 / A≦1.5…………………………………………Equation 7 20≦T2≦140………………………………………………Equation 8 T1+T2≦240………………………………………………Equation 9 In the second energization, as shown in Formula 5 and Formula 6, a current equal to or greater than that in the first energization flows. Further, in the second energization, as shown in Formula 8, a large current flows only for a short time. By making the second energization time short, scattering in the second energization is suppressed. Also, by making the current value of the second energization equal to or greater than that of the first energization, melting of the thin-thick interface C is further promoted. In addition, by making the current value of the second energization equal to or greater than that of the first energization, heat generation at the welded portion is further promoted, and the energization time after the third energization can be shortened. The second energization that satisfies the above formula can shorten the total welding time.

[0048] In the case of disturbances, contact between the steel plates 11 is promoted during the first energization, and the current value I2 of the second energization is increased more than the current value I1 of the first energization. In this case, the thin-thick interface C is melted by the second energization.

[0049] As described above, the current values of the first energization and the second energization may be the same, and an energization pause time may be provided between the first energization and the second energization. During the energization pause time, the temperature of the welded portion decreases, and the occurrence of scattering is suppressed. Also, in this case, the melting region of the thin-thick interface C formed by the first energization is expanded by the second energization.

[0050] In the second energization, the current value I2 may be 8.2 kA or more, 8.5 kA or more, or 9.0 kA or more. The upper limit of the current value I2 is not particularly limited, but may be, for example, 10.0 kA or less, 9.5 kA or less, or 9.2 kA or less. Also, I1 < I2 may be satisfied. I2 may be 1.1 times or more, 1.2 times or more, or 1.3 times or more of I1. I2 / A may be 0.5 or more, 0.6 or more, or 0.8 or more. I2 / A may be 1.4 or less, 1.3 or less, or 1.2 or less. T2 may be 40 ms or more, 60 ms or more, or 80 ms or more. T2 may be 120 ms or less, 100 ms or less, or 90 ms or less. T1 + T2 may be 220 ms or less, 210 ms or less, or 200 ms or less.

[0051] In the third energization, the following formula is satisfied. I3 ≤ I2 × 0.9………………………………………………… Equation 10 60 × (tsum / 2) ≤ T3 ≤ 200 × (tsum / 2)… Equation 11 In the third energization that is performed after the first energization and the second energization which are short-time and high-current, the current value I3 is made lower than that of the second energization. Specifically, as shown in Equation 10, the current value I3 of the third energization is made 90% or less of the current value I2 of the second energization. Also, in the third energization, the energization time is determined according to the total thickness of the plate assembly 10. According to the third energization, the nugget diameter at interfaces other than the thin-thick interface C can be sufficiently enlarged.

[0052] In the third energization, the current value I3 may be 0.85 times or less, 0.8 times or less, or 0.75 times or less of I2. The energization time T3 may be 70 × (tsum / 2) or more, 80 × (tsum / 2) or more, or 100 × (tsum / 2) or more. The energization time T3 may be 180 × (tsum / 2) or less, 150 × (tsum / 2) or less, or 120 × (tsum / 2) or less.

[0053] Above, the most basic aspect of the method for manufacturing the spot welding joint according to the present embodiment has been described. Hereinafter, a more preferable aspect will be described.

[0054] (Relationship between the current value I1 of the first energization and the current value I2 of the second energization) When the gap between the steel plates in the plate assembly before welding, that is, the plate gap, is large, as shown in FIG. 2, it is preferable to make the current value I1 of the first energization smaller than the current value I2 of the second energization. That is, instead of the above Equation 6, it is preferable that this energization satisfies the following Equation 6a. I1 < I2………………………… Equation 6a

[0055] When the plate gap is large in the plate assembly before welding, scattering is likely to occur during welding. However, by performing this energization so as to satisfy Equation 6a, the occurrence of scattering in the first energization can be suppressed. Also, the plate gap is crushed in the first energization. Thereby, scattering due to the plate gap can be suppressed after the second energization.

[0056] On the other hand, when the plate gap is small in the plate assembly before welding, as shown in FIG. 3, it is allowed to set I1 and I2 to the same value. For example, when the plate gap is 0.5 mm or less on all mating surfaces of the plate assembly, it is preferable to set I1 to the same value as I2 to further promote the growth of the nugget.

[0057] (The i-th energization (4 ≤ i)) As illustrated in FIG. 4, this energization may further have an energization time after the third energization. Hereinafter, the i-th (4 ≤ i) energization is referred to as the i-th energization. When this energization has the i-th energization, it is preferable that this energization satisfies the following formula. I3 < Ii…………………………………… Formula 12 Ii ≤ I(i + 1)………………………… Formula 13 Note that when Ii and I(i + 1) are the same, this energization has an energization pause time between the i-th energization and the (i + 1)-th energization.

[0058] Ii is the current value of the i-th energization, and I(i + 1) is the current value of the (i + 1)-th energization. As shown in Formula 12, it is preferable to increase the current value in the energization after the fourth energization compared to the third energization. Also, as shown in Formula 13, in the energization after the fourth energization, it is preferable to make the current value constant or increase it step by step. Thereby, both the nugget diameter at the thin-thick interface C and the nugget diameter at the interfaces other than the thin-thick interface C can be further enlarged.

[0059] (Pressing force) In the method for manufacturing the spot weld joint according to the present embodiment, the pressing force can be appropriately selected. Preferably, this energization satisfies the following formula. P2 < Pj…………………………………… Formula 14 P2 is the pressing force of the second energization, and Pj is the pressing force of the j-th energization (3 ≤ j). As shown in Formula 14, it is preferable to increase the pressing force in the energization after the third energization compared to the second energization.

[0060] Increasing the pressure reduces the amount of spatter. From the viewpoint of suppressing spatter, it is preferable to increase the pressure. On the other hand, increasing the pressure increases the contact area between the electrode and the steel plate, which increases the amount of heat transferred from the plate assembly 10 to the electrode 2, making it more difficult for the temperature of the plate assembly 10 to rise. From the viewpoint of promoting the growth of the nugget 12, it is preferable to decrease the applied pressure. In the method for manufacturing spot welded joints according to this embodiment, in the second energization, it is preferable to set the pressure low within a range where spatter does not occur in order to promote the growth of the nugget 12. On the other hand, in the energization from the third energization onward, it is preferable to increase the applied pressure from the viewpoint of suppressing the occurrence of spatter.

[0061] (After energization S3) As shown in Figure 5, the method for manufacturing a spot-welded joint according to this embodiment may further include a step of applying a post-energization S3 to the plate assembly 10 after the initial energization S1. Post-energization S3 is the application of a post-thermal current between the pair of electrodes 2. A post-thermal current is a current applied in resistance welding of materials that harden by welding, after welding, for the purpose of tempering, annealing, or mitigating solidification segregation within the nugget. By modifying the weld, the joint strength of the spot-welded joint 1 can be further increased. If the steel plate 11 is a zinc-plated steel plate, post-energization S3 can also suppress LME that occurs inside or at the edges of the corona bond.

[0062] The current value and energizing time in post-energization S3 are not particularly limited, and appropriate values ​​can be adopted depending on the material of the plate assembly 10. For example, even if the current value is at the same level as the welding current, shortening the energizing time can prevent the nugget 12 from melting. Also, since the current in post-energization is set lower than the third and fourth energizations, spatter and joint defects will not occur. Therefore, in post-energization, the current value and energizing time can be freely combined.

[0063] (Cooling S2 before powering on S3) As shown in Figure 5, the method for manufacturing a spot-welded joint according to this embodiment may further include a step of stopping the energization while maintaining the applied pressure after the completion of the main energization S1 and before the start of the post-energization S3. The electrode 2 for spot welding is configured to have a coolant flowing through its interior. By stopping the energization while maintaining the applied pressure, heat can be transferred from the plate assembly 10 to the electrode 2, thereby cooling the plate assembly 10 S2. This causes quenching hardening of the nugget 12, thereby increasing the shear tensile strength of the spot-welded joint 1. The applied pressure in cooling S2 is not particularly limited. Cooling S2 does not cause spatter or joining defects. Therefore, the applied pressure in cooling S2 can be freely set. For example, cooling S2 may be performed by ending the energization while maintaining the applied pressure during the last energization time of the main energization. Alternatively, the applied pressure may be changed after the completion of the main energization.

[0064] (2. Spot welded joint 1) A spot welded joint 1 according to another aspect of the present disclosure comprises three or more stacked steel plates 11 and a nugget 12 for joining the steel plates 11, wherein the thinnest steel plate 11min is placed on the surface of the spot welded joint 1, the total thickness tsum in units of mm of all steel plates included in the spot welded joint is 4.0 mm or more, the plate thickness ratio tsum / tmin of the spot welded joint 1 calculated from the thickness tmin in units of mm of the thinnest steel plate 11min and tsum is 4.0 or more and 8.0 or less, and the following formula is satisfied at the mating surfaces of all steel plates 11: 4.0√t≦d………………………………Equation 15 d is the diameter of the nugget 12 measured at the mating surface, and t is the thickness of the thinner of the two steel plates 11 that make up the mating surface.

[0065] The details of the spot-welded joint 1 according to this embodiment will be described below. The preferred embodiment of the method for manufacturing the spot-welded joint described above is also applicable to the spot-welded joint 1 according to this embodiment.

[0066] (Steel plate 11) The spot welding joint 1 has three or more stacked steel plates 11. On the surface of the spot welding joint 1, the thinnest steel plate 11min, which is the thinnest among all the steel plates 11, is arranged. Also, the total thickness tsum in mm of all the steel plates 11 included in the plate stack 10 is 4.0 mm or more, and the plate thickness ratio tsum / tmin of the spot welding joint 1, calculated from the thickness tmin in mm of the thinnest steel plate 11min and tsum, is 4.0 or more and 8.0 or less. The position of the thinnest steel plate 11min in the steel plates 11 of the spot welding joint 1 and the plate thickness ratio of the spot welding joint 1 are the same as those of the plate stack 10 before spot welding described above. Various aspects of the steel plates 11 constituting the plate stack 10 before spot welding are also applicable to the steel plates 11 of the spot welding joint 1.

[0067] (Nugget 12) The spot welding joint 1 has a nugget 12 that joins a plurality of steel plates 11. The nugget 12 satisfies the following formula on the mating surface of all the steel plates 11. 4.0√t≦d…………………………………Formula 15 d is the diameter of the nugget 12 measured on the mating surface, and t is the thickness of the thinner one of the two steel plates 11 constituting the mating surface. Note that on all mating surfaces, the nugget diameter d may be 4.2√d or more, 4.5√d or more, or 5.0√d or more.

[0068] For example, the spot welding joint 1 illustrated in FIG. 6 has three steel plates 11. The topmost steel plate 11 is the thinnest steel plate 11min with a thickness of tmin. The thickness of the second steel plate 11 from the top is tx, and the thickness of the third steel plate 11 from the top is ty. In the spot welding joint 1 of FIG. 6, there are two mating surfaces of the steel plates 11. The upper mating surface is the thin-thick interface C. The diameter of the nugget 12 measured at the thin-thick interface C is da. The diameter of the nugget 12 measured at the interface other than the thin-thick interface C, that is, the lower mating surface, is db. In the spot welding joint 1 illustrated in FIG. 6, if tx < ty, the following formulas are all satisfied. 4×(tmin) 1 / 2 ≦da 4×(tx) 1 / 2 ≦db On the other hand, if tx > ty, then all of the following equations are satisfied. 4 × (tmin) 1 / 2 ≦da 4×(ty) 1 / 2 ≤db Furthermore, if tx = ty, then either tx or ty can be used in the formula defining db.

[0069] The cross-sectional test for measuring the nugget diameter d is performed on a cross section perpendicular to the surface of the steel plate 11. The cross-sectional test specimen is prepared by cutting a vertical plane passing through the center of the weld point. The nugget diameter d is measured by the cross-sectional macro test specified in JIS Z 3139:2009 "Cross-sectional testing method for spot, projection and seam welds".

[0070] (Effects and Benefits) The spot-welded joint 1 according to this embodiment has a large total thickness tsum and a plate thickness ratio tsum / tmin of steel plates 11. For example, by applying the spot-welded joint 1 according to this embodiment to an automobile part, it is possible to increase the plate thickness of the frame member to ensure strength while decreasing the plate thickness of the exterior member to reduce the weight of the part. The spot-welded joint 1 according to this embodiment contributes to increasing the strength and reducing the weight of various mechanical structural parts.

[0071] Furthermore, in the spot-welded joint 1 according to this embodiment, a sufficiently large nugget diameter d is ensured at all mating surfaces. Therefore, the spot-welded joint 1 according to this embodiment has high joint strength.

[0072] While embodiments of the present disclosure have been described above, the disclosure is not limited thereto and can be modified as appropriate without departing from the technical idea. Below, a more preferred example of a method for manufacturing a spot-welded joint and a spot-welded joint 1 according to the present embodiment will be described. The preferred example illustrated below is applicable to both the method for manufacturing a spot-welded joint and the spot-welded joint 1.

[0073] (Steel plate 11) The tensile strength of the steel plate 11 is preferably 980 MPa or higher. By making one or more of the multiple steel plates 11 high-strength steel plates with a tensile strength of 980 MPa or higher, the strength of the automobile parts manufactured using the steel plates 11 can be increased. On the other hand, the steel plates 11 may be mild steel with a tensile strength of less than 980 MPa. For example, when a spot-welded joint 1 is used as an automobile part, the thinnest steel plate 11min placed on the surface of the plate assembly 10 may be mild steel, and the other steel plates 11 may be high-strength steel plates.

[0074] One or more of the multiple steel plates 11 may have a plating. Examples of plating applied to the surface of the steel plates 11 include hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, and aluminum plating. [Examples]

[0075] The effects of one aspect of this disclosure will be further illustrated by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of this disclosure. This disclosure is not limited to these examples of conditions. This disclosure may adopt various conditions as long as they do not depart from its gist and achieve its objectives.

[0076] Three steel plates were stacked to form various plate assemblies as shown in Table 2. Steel plate 1 was placed on one surface of the plate assemblies, steel plate 3 was placed on the other surface of the plate assemblies, and steel plate 2 was placed between steel plate 1 and steel plate 2. In all plate assemblies, steel plate 1 was a TS270MPa class steel plate, steel plate 2 was a TS1180MPa class steel plate, and steel plate 3 was a TS980MPa class steel plate. In all plate assemblies, steel plate 1 was the thinnest steel plate. In all plate assemblies, the gap between steel plate 1 and steel plate 2 was 0.7 mm, and the gap between steel plate 2 and steel plate 3 was also 0.7 mm.

[0077] [Table 2]

[0078] These plate assemblies were subjected to main energization, consisting of a first energization, a second energization, and a third energization. The energization conditions are shown below. In the spot welding of Example No. 9, the current values ​​for the second energization and the third energization were the same. In the spot welding of Example No. 9, there was no energization pause between the first and second energization, but there was an energization pause between the second and third energization. In the other spot welding examples, there was no energization pause during the main energization.

[0079] [Table 3]

[0080] [Table 4]

[0081] [Table 5]

[0082] Cross-sectional tests were conducted on various spot-welded joints manufactured by applying current to plate assemblies, and the nugget diameter was measured. The presence or absence of spatter during spot welding was also evaluated. The presence or absence of spatter was determined visually. For samples where spatter leakage from between the steel plates was visually confirmed, "Present" was noted in the "Spatter Presence / Absence" column. The cross-sectional test results and spatter evaluation results are shown below.

[0083] [Table 6]

[0084] In the manufacturing of the spot-welded joint in Example No. 1, the following equation was not satisfied. 0.4≦I2 / A≦1.5…………………………………………Equation 7 In the manufacturing of the spot-welded joint in Example No. 1, the current density at the thin-to-thick interface was too high, resulting in spatter.

[0085] In the manufacturing of the spot-welded joint in Example No. 4, the following equation was not satisfied. 20≦T1≦140………………………………………………Equation 3 200≦I1×T1≦1500……………………………………Equation 4 In the manufacturing of the spot-welded joint in Example No. 4, the initial energization was too long, and the heat input during the initial energization was too large, resulting in spatter. Furthermore, the nugget diameter could not be secured in the spot-welded joint in Example No. 4. This is presumed to be because the molten metal spattered and scattered, resulting in a smaller nugget.

[0086] In the manufacturing of the spot-welded joint in Example No. 5, the following equation was not satisfied. 200≦I1×T1≦1500……………………………………Equation 4 In the manufacturing of the spot-welded joint in Example No. 5, the heat input during the first energization was too high, resulting in spatter. Furthermore, the nugget diameter could not be secured in the spot-welded joint in Example No. 5. This is presumed to be because the molten metal spattered and scattered, resulting in a smaller nugget.

[0087] In the manufacturing of the spot-welded joint in Example No. 8, the following equation was not satisfied. 60×(tsum / 2)≦T3≦200×(tsum / 2)…Equation 11 In the manufacturing of the spot-welded joint in Example No. 8, the third current application was too short. As a result, in the spot-welded joint in Example No. 8, the nugget could not be grown sufficiently during the third current application, and the nugget diameter could not be secured at interfaces other than the thin-thickness interface.

[0088] In the manufacturing of the spot-welded joint in Example No. 9, the following equation was not satisfied. I1≦I2…………………………………………………………Equation 6 I3≦I2×0.9………………………………………………Equation 10 In the manufacturing of the spot-welded joint in Example No. 9, the current value for the second current application was insufficient, while the current value for the third current application was excessive. As a result, the nugget diameter could not be secured at the thin-to-thick interface in the spot-welded joint in Example No. 9.

[0089] On the other hand, in spot-welded joints manufactured under conditions that satisfied all of Equations 1 to 11, a sufficient nugget diameter was ensured at all overlapping surfaces. Furthermore, no spatter occurred during the actual energization performed under conditions that satisfied all of Equations 1 to 11. [Explanation of Symbols]

[0090] 1. Spot welded joint 10 board set 11 Steel plate 11min Thinnest steel plate 12 nuggets 2 electrodes C thin-thick interface

Claims

1. A method for manufacturing a spot-welded joint, comprising a step of applying current to a plate assembly made of three or more stacked steel plates, The thinnest steel plate, which is the thinnest of the aforementioned steel plates, is placed on the surface of the aforementioned plate assembly. The total thickness tsum of all the steel plates included in the aforementioned plate assembly, in units of mm, shall be 4.0 mm or more. The thickness tmin of the thinnest steel plate in units of mm, and the thickness ratio tsum / tmin of the plate assembly, calculated from tsum, are set to be between 4.0 and 8.

0. The aforementioned energization comprises at least a first energization, a second energization, and a third energization, The aforementioned energization satisfies the following equation, 5.0≦I1……………………………………………………Equation 1 0.4≦I1 / A≦1.5…………………………………………Formula 2 20≦T1≦140………………………………………………Equation 3 200≦I1×T1≦1500……………………………………Formula 4 8.0≦I2……………………………………………………Equation 5 I1≦I2…………………………………………………………Equation 6 0.4≦I2 / A≦1.5…………………………………………Equation 7 20≦T2≦140………………………………………………Equation 8 T1+T2≦240………………………………………………Equation 9 I3≦I2×0.9………………………………………………Equation 10 60×(tsum / 2)≦T3≦200×(tsum / 2)…Formula 11 I1 is the current value in units of kA for the first energization, T1 is the energizing time of the first energizing in ms, I2 is the current value in units of kA for the second energization, T2 is the energizing time of the second energizing in ms. I3 is the current value in units of kA for the third energization, T3 is the energizing time of the third energizing operation in ms. A is the distance, in units of mm, between the thinnest steel plate and the adjacent steel plate immediately before the start of the first energization. ２ This is the area under current flow. If I1 and I2 are the same, the main energization has an energization pause time between the first energization and the second energization. A method for manufacturing spot-welded joints.

2. The method for manufacturing a spot welded joint according to claim 1, characterized in that the current application satisfies the following formula. I1<I2…………………………Equation 6a

3. The aforementioned current supply further has an i-th current supply (4 ≤ i), The aforementioned energization satisfies the following equation, I3<Ii……………………………………Equation 12 Ii ≤ I(i+1)…………………………Equation 13 Ii is the current value of the i-th energization, and I(i+1) is the current value of the (i+1)th energization. If Ii and I(i+1) are the same, the main energization has an energization pause between the i-energy and the (i+1)-energy. A method for manufacturing a spot welded joint according to claim 1, characterized by its features.

4. The aforementioned energization satisfies the following equation, P2<Pj……………………………………Equation 14 P2 is the applied pressure of the second energization, Pj is the applied pressure at the jth current stage (3 ≤ j). A method for manufacturing a spot welded joint according to claim 1, characterized by its features.

5. A method for manufacturing a spot-welded joint according to any one of claims 1 to 4, further comprising the step of applying a second current to the plate assembly after the completion of the first current application.

6. The method for manufacturing a spot-welded joint according to claim 5, further comprising the step of cooling the plate assembly after the completion of the main energization and before the start of the post-energization.

7. Three or more stacked steel plates, A nugget for joining the aforementioned steel plates, A spot welded joint comprising, The thinnest steel plate, which is the thinnest steel plate, is placed on the surface of the spot welded joint. The total thickness tsum of all the steel plates included in the spot welded joint is 4.0 mm or more. The plate thickness ratio tsum / tmin of the spot welded joint, calculated from the thickness tmin and tsum in units of mm of the thinnest steel plate, is 4.0 or more and 8.0 or less. At all of the aforementioned steel plate joints, the following equation is satisfied: 4.0√t≦d………………………………Equation 15 d is the diameter of the nugget measured at the mating surface, t is the thickness of the thinner of the two steel plates that make up the mating surface. Spot welded joint.

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