Spot welding method and method for manufacturing welded joint

Abstract

To provide a spot welding method that can form a weld joint with high joint strength, from high-strength steel plate of 1180 MPa grade or higher.SOLUTION: The spot welding method includes: a step of pinching two overlapped steel plates in a plate thickness direction with a pair of electrodes; a first current-carrying step of carrying current values Ia(kA) between the pair of electrodes; a first non-current-carrying step of stopping the current-carrying for more than 300 ms after the first current-carrying step; a second current-carrying step of carrying currents values Ib(kA) between the pair of electrodes for a current-carrying time of 100 ms or more and 1000 ms or less, after the first non-current-carrying step; a second non-current-carrying step of stopping the current-carrying for 200 ms or more, after the second current-carrying step; and a third current-carrying step of carrying current values Ic(kA) between the pair of electrodes, during time of 100 ms or more and 1000 ms or less, after the second non-current-carrying step, where the current values Ia, Ib and Ic satisfy the following formulas (1) and (2): 0.70<Ib / Ia<0.99 (1) and 0.65<Ic / Ia<0.80 (2).SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] There has been a demand for improved occupant safety in vehicles, and for this purpose, the strength of vehicle bodies has been improved. On the other hand, against the backdrop of worsening issues such as global warming, efforts to improve the fuel efficiency of automobiles are accelerating. It is known that reducing the weight of vehicle bodies is an effective way to improve fuel efficiency.

[0003] To achieve both weight reduction and crashworthiness in automobiles, there is a need to increase the strength and ductility of the steel sheets used in automobile bodies. One way to achieve this is to increase the C and Mn content of the steel sheets. However, spot welding is used to join steel sheets, but increasing the C and Mn content in the steel sheets makes the hardenability too high, and joints formed by spot welding become significantly embrittled with hardening, resulting in a deterioration in the strength of the welded joint (JIS Z 3140).

[0004] As a spot welding method capable of increasing joint strength, three-stage current welding has been proposed (for example, Patent Document 1). Patent Document 1 describes a first current application step in which a sheet assembly including two or more steel sheets, including at least one steel sheet having a C content of more than 0.30 mass% and not more than 0.70 mass%, is sandwiched between a pair of electrodes in the sheet thickness direction and current is applied at a current value I1 (kA) while applying pressure; After the first energization step, a time t of 16 ms or more and 200 ms or less c1 a first de-energizing step of de-energizing the After the first de-energizing step, a second energizing step of energizing at a current value I2 (kA) for a time t2 (ms) (where 0.6≦I2 / I1≦1.1 and 50≦t2≦1000 are satisfied); After the second energization step, time t c2 The second de-energization step (where 3.5 × 10 -3 ×Ms 2 -3.3×Ms+1100 <tc2 ≦9000. Here, Ms (℃) = 561 - 474 × [C] - 33 × [Mn] - 17 × [Ni] - 17 × [Cr] - 21 × [Mo]. After the second de-energizing step, a third energizing step of energizing the coil at a current value I3 (kA) for a time t3 (ms) (where 0.4≦I3 / I1≦1.0 and 200≦t3 are satisfied); A spot welding method is disclosed in which the above steps are performed continuously.

[0005] The spot welding method disclosed in Patent Document 1 aims to alleviate segregation and regulate grain size in the second current application step, and also applies tempering in the third current application step after the second no-current application step. This is said to ensure high joint strength even in sheet assemblies that include high-strength steel sheets of 780 MPa class or higher. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-154390 Summary of the Invention [Problem to be solved by the invention]

[0007] To further reduce the weight of automobiles and improve their crashworthiness, the use of high-strength, high-ductility steel sheets of 1180 MPa class or higher has been considered. High-strength, high-ductility steel sheets of 1180 MPa class or higher not only contain a large amount of C, but also contain large amounts of other alloying elements (e.g., Si, Mn), making spot welding difficult. Even with the spot welding method disclosed in Patent Document 1, it was difficult to ensure joint strength. Therefore, a spot welding method for steel sheets of 1180 MPa class or higher was needed.

[0008] An embodiment of the present invention provides a spot welding method capable of forming a welded joint with high joint strength even in high-strength steel plates, such as those of 1180 MPa class or higher, and a method for manufacturing a welded joint using the spot welding method. [Means for solving the problem]

[0009] Aspect 1 of the present invention is a step of overlapping a first steel sheet and a second steel sheet and sandwiching them between a pair of electrodes in the sheet thickness direction; a first current application step of applying a current value Ia (kA) between the pair of electrodes; a first de-energizing step of stopping the energization for 300 ms or more after the first energizing step; a second current-carrying step of applying a current between the pair of electrodes at a current value Ib (kA) for a current-carrying time of 100 ms or more and 1000 ms or less after the first current-carrying step; a second de-energizing step of stopping the energization for 200 ms or more after the second energizing step; a third current-carrying step of applying a current between the pair of electrodes at a current value Ic (kA) for a current-carrying time of 100 ms or more and 1000 ms or less after the second current-deactivating step, This is a spot welding method in which the current values ​​Ia, Ib, and Ic satisfy the following formulas (1) and (2). 0.70 <Ib / Ia<0.99 (1) 0.65 <Ic / Ia<0.80 (2)

[0010] Aspect 2 of the present invention is The first steel plate is C: 0.25% by mass or more and 0.50% by mass or less, Tensile strength is 1180 MPa or more, and The spot welding method according to aspect 1 satisfies the following formula (3): (TS1) × (EL1) 0.5 >5200 (3) Here, TS1 is the tensile strength (MPa) of the first steel plate, EL1 is the elongation (%) of the first steel plate.

[0011] Aspect 3 of the present invention is The first steel plate is Si: 1.00% by mass or more, and Mn: 1.00% by mass or more The spot welding method according to aspect 1 or 2, further comprising one or two of the following:

[0012] A fourth aspect of the present invention is In the spot welding method according to any one of aspects 1 to 3, the second steel plate has a tensile strength of 500 MPa or less.

[0013] A fifth aspect of the present invention is In the spot welding method according to any one of aspects 1 to 4, the first steel plate has a thickness t1 of 1.0 mm or more, and the second steel plate has a thickness t2 of less than 1.0 mm.

[0014] A sixth aspect of the present invention is A method for manufacturing a welded joint by spot welding two or more steel plates, The spot welding is performed by the spot welding method according to any one of the first to fifth aspects of the present invention, which is a method for producing a welded joint. [Effects of the Invention]

[0015] According to one embodiment of the present invention, it is possible to provide a spot welding method that can form a welded joint with high joint strength, even in high-strength steel plates, such as those of 1180 MPa class or higher, and a method for manufacturing a welded joint using the spot welding method. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic cross-sectional view illustrating a method for spot welding two steel plates. [Figure 2] FIG. 2(a) is a graph illustrating a current pattern during spot welding in the spot welding method according to embodiment 1, and FIG. 2(b) is a graph showing the temperature of the welded portion in the current pattern shown in FIG. 2(a). [Figure 3]FIG. 3(a) is a graph illustrating the current pattern during spot welding performed using the spot welding method described in Patent Document 1, and FIG. 3(b) is a graph showing the temperature of the welded portion in the current pattern shown in FIG. 3(a). [Figure 4] FIG. 4 is a cross-sectional photograph for explaining the measurement positions of the hardness of the welded joint. [Figure 5] 5(a) to 5(c) are hardness distribution diagrams showing the results of hardness measurements of welded joints. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present inventor discovered that when spot welding high-strength steel plates of 1180 MPa class or higher is performed using the spot welding method disclosed in Patent Document 1, the joint strength of the welded joint may not be sufficient in some cases, and conducted extensive research.

[0018] Steel sheets containing a large amount of carbon and having excellent strength and ductility tend to have a welded nugget that is very hard and embrittled during welding. Therefore, a process called tempering current is required to temper the weld and increase its ductility. However, because the additive elements such as silicon and manganese, which are added to ensure the ductility of steel sheets, exhibit resistance to tempering softening, sufficient heating is required for tempering.

[0019] When tempering by resistance heating caused by current flow, it is difficult to temper the entire weld evenly because the center of the nugget, where the current density is highest, becomes hotter than other parts. The inventors have therefore discovered that by appropriately controlling the current value in the three current application processes (first to third current application processes) and the time for which current application is stopped in the non-current application process between the current application processes, the entire weld can be uniformly tempered when the third current application process is completed, and as a result, the strength of the joint can be improved.

[0020] <Embodiment 1: Spot welding method> A method for spot welding steel sheets according to a first embodiment of the present invention will be described in detail below.

[0021] The spot welding method according to the first embodiment includes: A process of overlapping two steel plates (a first steel plate and a second steel plate) and sandwiching them between a pair of electrodes in the thickness direction; and spot welding the stacked steel plates together. The spot welding process is a first current application step of applying a current value Ia (kA) between the pair of electrodes; a first de-energizing step of stopping the energization for 300 ms or more after the first energizing step; a second current-carrying step of applying a current between the pair of electrodes at a current value Ib (kA) for a current-carrying time of 100 ms or more and 1000 ms or less after the first current-carrying step; a second de-energizing step of stopping the energization for 200 ms or more after the second energizing step; a third current-carrying step of applying a current between the pair of electrodes at a current value Ic (kA) for a current-carrying time of 100 ms or more and 1000 ms or less after the second current-deactivating step, The current values ​​Ia, Ib, and Ic satisfy the following formulas (1) and (2). 0.70 <Ib / Ia<0.99 (1) 0.65 <Ic / Ia<0.80 (2)

[0022] Each step will be described in detail below.

[0023] [Process of sandwiching stacked steel sheets between a pair of electrodes] As shown in Fig. 1, two steel plates (a first steel plate P1 and a second steel plate P2) are overlapped, and the overlapped portion (overlapped portion) is sandwiched between a pair of electrodes (a first electrode 11 and a second electrode 12) in the plate thickness direction. When overlapping the two steel plates, only at least a portion to be welded may overlap, or the entire plates may overlap. The overlapping portion of two steel sheets is pressed from above and below with a first electrode 11 and a second electrode 12 while an electric current is applied, causing the interface between the steel sheets to melt due to Joule heat generated between the sheets. In preparation for this, before the current is applied, the overlapping portion of the steel sheets is clamped and fixed between the electrodes.

[0024] (First steel plate P1, second steel plate P2) The spot welding method of the first embodiment is particularly suitable for welding sheet assemblies that include high-strength steel sheets with a high carbon content. For example, it is preferable that the first steel plate P1 contains C: 0.25 mass % or more and 0.50 mass % or less, and satisfies the following formula (3). (TS1) × (EL1) 0.5 >5200 (3) where TS1 is the tensile strength (MPa) of the first steel plate, EL1 is the elongation (%) of the first steel sheet.

[0025] The first steel sheet P1 satisfying formula (3) is a high-strength, high-ductility steel sheet excellent in both tensile strength and elongation. The tensile strength of the first steel sheet P1 is preferably 1180 MPa or more, and more preferably 1470 MPa or more. The upper limit of the tensile strength TS1 of the first steel sheet P1 is not particularly limited, but is, for example, 1900 MPa or less. Such steel sheets are used, for example, as frame members for automobile bodies. The spot welding method of the first embodiment is suitable for spot welding such high-strength, high-ductility steel plates.

[0026] The alloy elements that may be contained in the first steel plate P1 will be described. The first steel plate P1 preferably contains C: 0.25 mass % or more and 0.50 mass % or less. The first steel plate P1 may further contain one or more of the alloying elements (1) to (4) described below. (1) One or both of Si: 1.00 mass% or more and Mn: 1.00 mass% or more (2) One or more of P: more than 0% by mass and not more than 0.050% by mass, S: more than 0% by mass and not more than 0.010% by mass, Al: 0.01% by mass or more and not more than 0.10% by mass, and N: more than 0% by mass and not more than 0.010% by mass (3) Cr, Mo, Cu, Ni: more than 0 mass% and 0.300 mass% or less in total (4) V, Nb, Ti: more than 0 mass% and 0.100 mass% or less in total

[0027] Each element will be described in detail below.

[0028] (C: 0.25 mass% or more and 0.50 mass% or less) C may be added to ensure the strength and ductility of the steel sheet, but it also hardens the welded joint during welding, which can reduce the strength of the joint. Therefore, the C content is preferably set to a range of 0.25% by mass to 0.50% by mass.

[0029] (One or both of Si: 1.00 mass% or more and Mn: 1.00 mass% or more) These elements have the effect of improving the tensile strength of steel sheets. On the other hand, these elements also increase the temper softening resistance of steel sheets. In spot welding, the toughness of the welded joint is increased by tempering the hard phase (martensite) that occurs in the weld, thereby improving the strength of the welded joint. If the temper softening resistance is high, softening due to tempering is suppressed, which can hinder the improvement of the strength of the welded joint. In the spot welding method of embodiment 1, the welded portion can be sufficiently tempered by controlling the conditions of the three-stage current application, and therefore a high-strength welded joint can be formed in a steel plate that contains one or both of Si and Mn and has high temper softening resistance.

[0030] The Si content is preferably 1.00 mass % or more, more preferably 1.20 mass % or more, and is preferably 3.00 mass % or less, more preferably 2.70 mass % or less. The Mn content is preferably 1.00% by mass or more, more preferably 1.20% by mass or more, and is preferably 3.00% by mass or less, more preferably 2.70% by mass or less.

[0031] (P: more than 0% by mass and 0.050% by mass or less) P is inevitably present as an impurity element. P reduces the elongation (EL) of the steel sheet. The P content is preferably 0.050% by mass or less, and more preferably 0.030% by mass or less. The lower the P content, the better, and 0% by mass is most preferable. However, due to constraints in the manufacturing process, etc., there are cases where P remains at more than 0% by mass, for example, about 0.001% by mass.

[0032] (S: more than 0% by mass and 0.010% by mass or less) S is inevitably present as an impurity element. S forms sulfide-based inclusions such as MnS, which become the starting points for cracks in the steel sheet. The S content is preferably 0.010% by mass or less, more preferably 0.005% by mass or less. The lower the S content, the better, and 0% by mass is most preferable. However, due to constraints in the manufacturing process, etc., there are cases where S remains at more than 0% by mass, for example, about 0.001% by mass.

[0033] (Al: 0.01 mass% or more and 0.10 mass% or less) Al functions as a deoxidizing element, reducing the amount of oxygen in molten steel, thereby reducing the number density of inclusions and improving the basic quality of the steel. To effectively exert this effect, the Al content is preferably 0.01 mass% or more, more preferably 0.015 mass% or more, and particularly preferably 0.02 mass% or more. On the other hand, an excessive Al content promotes the formation of ferrite, making it difficult to obtain the desired metal structure (tempered martensite). The Al content is preferably 0.10 mass% or less, more preferably 0.08 mass% or less, and particularly preferably 0.06 mass% or less.

[0034] (N: more than 0% by mass and less than 0.010% by mass) N is inevitably present as an impurity element. It is most preferable that the N content be 0% by mass, but N inevitably remains in the steel during the manufacturing process. From the above viewpoints, it is preferable to reduce the N content, and the N content is preferably 0.010% by mass or less, more preferably 0.008% by mass or less, and particularly preferably 0.006% by mass or less.

[0035] (Cr, Mo, Cu, Ni: total of more than 0 mass% and 0.300 mass% or less) Cr, Mo, Cu, and Ni are elements that can improve the hardenability, increase the strength of the steel, and contribute to improving the balance between strength and ductility, and can be added selectively. On the other hand, if added in excess, the tempering softening resistance becomes too high, making it difficult to ensure joint strength even with three-stage current application. The total amount of Cr, Mo, Cu, and Ni is preferably 0.300% by mass or less, more preferably 0.150% by mass or less, and particularly preferably 0.120% by mass or less, and is preferably more than 0% by mass, more preferably 0.020% by mass or more.

[0036] (V, Nb, Ti: more than 0 mass% and 0.100 mass% or less in total) V, Nb, and Ti are elements that react with C to form fine carbides in steel sheets, contributing to microstructural refinement, and can be added selectively. However, if added in excess, coarse carbides may be formed during steel sheet manufacturing or welding, potentially deteriorating the ductility and joint strength of the steel. The total content of V, Nb, and Ti is preferably 0.100% by mass or less, more preferably 0.050% by mass or less, and particularly preferably 0.020% by mass or less, and is preferably more than 0% by mass, more preferably 0.005% by mass or more.

[0037] (balance: Fe and unavoidable impurities) In a preferred embodiment, the balance of the first steel plate P1 is iron and unavoidable impurities. The unavoidable impurities include trace elements (e.g., As, Sb, Sn, etc.) that are introduced due to the conditions of raw materials, materials, manufacturing facilities, etc.

[0038] The second steel sheet P2 may be, for example, a mild steel sheet having a tensile strength of 500 MPa or less. The spot welding method according to the first embodiment is also suitable for spot welding between a high-strength steel sheet and a mild steel sheet. Mild steel sheets are used, for example, for the outer panels of automobile bodies. The second steel plate P2 may be a high-strength steel plate (having a tensile strength of, for example, 1180 MPa or more).

[0039] The thickness of the first steel plate P1 and the second steel plate P2 is not particularly limited, but may be a thickness suitable for steel plates used in automobile bodies. The frame of the vehicle body is formed from thick steel plates (for example, 1.0 mm or more) to make it less likely to deform during a collision, and the outer plates covering the outside of the vehicle body are sometimes formed from thin steel plates (for example, less than 1.0 mm). Therefore, the plate thickness t1 of the first steel plate P1 may be, for example, 1.0 mm or more, and the plate thickness t2 of the second steel plate P2 may be, for example, less than 1.0 mm.

[0040] [Spot welding process for stacked steel plates] The spot welding method of the first embodiment is performed using three-stage current application. The nugget and HAZ are entirely transformed into martensite in the first current application step and the first non-current application step, and then tempered in two separate steps, the second current application step and the third current application step. Therefore, even in steel sheets with high temper softening resistance, the nugget and HAZ can be entirely tempered sufficiently.

[0041] FIG. 2(a) is a graph illustrating a current pattern during spot welding, and FIG. 2(b) is a graph showing the temperature of the welded portion in the current pattern shown in FIG. 2(a). The process of spot welding stacked steel plates is as shown in Figure 2(a) and Figure 2(b). The method includes (i) a first energization step, (ii) a first non-energization step, (iii) a second energization step, (iv) a second non-energization step, and (v) a third energization step.

[0042] The spot welding method according to embodiment 1 is similar to the conventional spot welding method disclosed in Patent Document 1 in that it uses three-stage current application, but the detailed conditions are different, resulting in a completely different metal structure after welding. FIG. 3(a) shows a graph illustrating the current pattern during spot welding disclosed in Patent Document 1, and FIG. 3(b) is a graph showing the temperature of the welded portion in the current pattern shown in FIG. 3(a). By comparing the graphs in FIGS. 2(a) and 2(b) relating to the spot welding method of embodiment 1 with the graphs in FIGS. 3(a) and 3(b) relating to the spot welding method of Patent Document 1, the characteristics of the spot welding method of embodiment 1 will be understood in more detail.

[0043] The first to third current-carrying steps and the first and second current-deactivating steps in the spot welding method of the first embodiment will be described below.

[0044] (i) First energization process In the first current application process, a current of Ia (kA) is applied between a pair of electrodes that hold two overlapping steel sheets together. Heat generated during current application creates a molten region, forming a weld (nugget) on the steel sheets. The current Ia and current application time are set so that no expulsion occurs and a nugget of appropriate dimensions is formed. The appropriate ranges for the current Ia and current application time vary depending on the composition, strength, and thickness of the steel sheet to be welded, the tip diameter of the welding tip used during spot welding, etc., but for example, the current Ia can be set within the range of 4 kA to 9 kA, and the current application time can be set within the range of 100 ms to 1,000 ms.

[0045] It is preferable to conduct a preliminary experiment to determine the optimal current value Ia. Two steel sheets to be welded are overlapped and clamped between a pair of electrodes, and current is passed through the steel sheets while varying the current value in increments of 0.5 kA or 1.0 kA. The maximum current value at which welding can be performed without generating any expulsion is preferably used as the "current value Ia."

[0046] (ii) First non-current process In the first de-energization step, the current flow between the pair of electrodes is stopped for 300 ms or more, preferably 300 ms to 2000 ms. The first de-energization step is carried out for a time sufficient to cool the molten pool formed in the first de-energization step to form a nugget, and further to cool the nugget and the surrounding HAZ to transform them into martensite. Upon completion of the first de-energization step, the metal structures of the nugget and HAZ become martensite (as-quenched martensite). If the de-energization time is too short, the nugget portion and HAZ cannot be transformed into martensite, whereas if it is too long, the productivity decreases, which is undesirable.

[0047] In the spot welding method disclosed in Patent Document 1, the first non-energization step is short, lasting from 16 ms to 200 ms. As can be seen from Figure 3(b), because the first non-energization step is short, the nugget and HAZ remain at a temperature higher than point A3 when the next second energization step begins. Therefore, the nugget and HAZ have not yet transformed into martensite at the completion of the first non-energization step.

[0048] (iii) Second energization process In the second current application step of the spot welding method according to the first embodiment, current is applied between the pair of electrodes at a current value Ib (kA) for a current application time of 100 ms to 1000 ms. The current value Ib is set so that its relationship with the current value Ia in the first current application step satisfies the following formula (1): 0.70 <Ib / Ia<0.99 (1)

[0049] The purpose of the second current application process is to fully temper the martensite formed in the nugget center (the central part of the nugget where the current density is high when current is applied), the nugget outer edge (the part surrounding the nugget center), and the HAZ. By controlling the current application time and current value in the second current application process as described above, the nugget outer edge and HAZ can be heated to a temperature suitable for tempering and softening the martensite. Note that the temperature of the nugget outer edge and HAZ must be below the A3 point when current is applied. If they are heated above the A3 point, they will be quenched, and martensite (as-quenched martensite) will form after cooling. On the other hand, the center of the nugget, where the current density is high when current is passed through, becomes hotter than the outer edge of the nugget and the HAZ, and is therefore heated to a temperature above the A3 point.

[0050] The martensite at the outer edge of the nugget and in the HAZ can be tempered by the second current application process, but the martensite in the center of the nugget is heated above the A3 point in the second current application process, and is therefore converted back into martensite in the next second non-current application process.

[0051] The current value Ib (set so as to satisfy the formula (1)) and the current application time must be set appropriately for the following reasons. If the current value Ib is too low and Ib / Ia is 0.70 or less, the hardness of the outer edge of the nugget and the HAZ cannot be reduced, and the joint strength of the final welded joint may be reduced. A similar problem can occur if the power-on time is too short.

[0052] If the current value Ib is too high and Ib / Ia is 0.99 or more, the outer edge of the nugget and the HAZ are heated to the austenite region and hardened by re-quenching, which may reduce the joint strength of the final welded joint. A similar problem can occur if the power is turned on for too long.

[0053] In the spot welding method disclosed in Patent Document 1, the purpose of the second current application step is to regulate the grain size near the fusion boundary within the nugget, melt the center of the nugget without crossing the fusion boundary created in the first current application step, and apply appropriate heat to the area near the edge of the nugget. In other words, as shown in Figure 3(b), the center of the nugget is heated to a temperature above the melting point, while the outer edge of the nugget and the HAZ are heated to a temperature above the A3 point. As a result, the entire nugget and HAZ are heated to a temperature above the A3 point, and the entire nugget and HAZ are transformed into martensite by the subsequent second non-current application step.

[0054] (iv) Second non-current process In the second non-energization step of the spot welding method according to the first embodiment, the passage of current between the pair of electrodes is stopped for 200 ms or more, preferably for 200 ms to 1500 ms. In the second non-energization step, the center of the nugget, which was heated to a temperature equal to or higher than the A3 point in the second energization step, is transformed into martensite. Meanwhile, the outer edge of the nugget and the HAZ remain tempered martensite because they were tempered in the second energization step. When the second non-energizing process is completed, the central part of the nugget will have a metal structure consisting of as-quenched martensite and its surroundings (the outer edge of the nugget and the HAZ) consisting of tempered martensite. If the time for stopping current application is too short, the central portion of the nugget cannot be transformed into martensite, whereas if it is too long, the productivity decreases, which is undesirable.

[0055] In the spot welding method disclosed in Patent Document 1, the nugget (including the center and outer edge of the nugget) and the entire HAZ are converted to martensite by the second non-current-passing step, as shown in Fig. 3(b). When the second non-current-passing step is completed, the entire nugget and the HAZ become as-quenched martensite.

[0056] (v) Third energization process In the third current application step of the spot welding method according to the first embodiment, a current Ic (kA) is applied between the pair of electrodes for a current application time of 100 ms to 1000 ms. The current Ic is set so that its relationship with the current Ia in the first current application step satisfies the following formula (2): 0.65 <Ic / Ia<0.80 (2)

[0057] The purpose of the third current application step is to temper the as-quenched martensite in the center of the nugget while maintaining the outer edge and HAZ of the nugget as tempered martensite. By controlling the current duration and current value of the third current application step as described above, the center of the nugget can be heated to a temperature suitable for tempering and softening the martensite. Note that during current application, the temperatures of the center, outer edge, and HAZ must all be below the A3 point.

[0058] The current value Ic (set so as to satisfy the formula (2)) and the current application time must be set appropriately for the following reasons. If the current value Ic is too low, with Ic / Ia being 0.65 or less, the hardness of the central portion of the nugget cannot be reduced, and the joint strength of the finally obtained welded joint may decrease. A similar problem can occur if the power-on time is too short.

[0059] If the current value Ic is too high and Ic / Ia is 0.80 or more, the nugget and part or all of the HAZ will be heated to the austenite region and hardened by re-quenching, which may reduce the joint strength of the final welded joint. A similar problem can occur if the power is turned on for too long.

[0060] In the third current-flow process, the temperature of the nugget outer edge and HAZ is lower than the temperature of the nugget center, where the current density is higher. Therefore, even if the nugget center is heated to a temperature that can sufficiently temper the martensite in the third current-flow process, the temperature of the nugget outer edge and HAZ may be too low to temper the martensite. However, since the martensite in the nugget outer edge and HAZ has already been tempered in the second current-flow process, there is no problem even if they are not heated to a temperature that can temper them in the third current-flow process.

[0061] In the spot welding method disclosed in Patent Document 1, the third current-flow process is capable of tempering the entire nugget and HAZ. However, with a high-strength steel sheet that has a high temper-softening resistance (i.e., a high content of additive elements), it is difficult to fully soften the entire nugget and HAZ by the third current-flow process alone. As a result, with the spot welding method disclosed in Patent Document 1, only the center of the nugget may be tempered, and the outer edge of the nugget and the HAZ may not be tempered (or may be insufficiently tempered).

[0062] According to the spot welding method of the first embodiment, the martensite in the outer edge portion of the nugget and the HAZ is tempered in the second current-flow process, and the martensite in the center portion of the nugget is tempered in the third current-flow process, so that the entire weld can be sufficiently softened even in steel sheets with high temper softening resistance. As a result, brittle fracture of the weld joint can be suppressed, and the strength of the weld joint can be improved.

[0063] [Electrodes 11 and 12] Next, the electrodes 11 and 12 used in spot welding will be described in detail with reference to FIG.

[0064] As shown in FIG. 1, the first electrode 11 has an electrode tip 11x that contacts the steel sheet, and the second electrode 12 has an electrode tip 12x that contacts the steel sheet. The first electrode 11 shown in Fig. 1 is a cylindrical smooth-tip electrode. The shape of the first electrode 11 can be any electrode shape commonly used in spot welding, such as a smooth-tip type (Fig. 1) or a DR type. The shape and dimensions of the second electrode 12 are also similar to those of the first electrode 11.

[0065] The overlapping steel plates are pressed between electrode tips 11x, 12x of a pair of upper and lower electrodes (first electrode 11 and second electrode 12). When the load applied to the overlapping portion by the first electrode 11 and the second electrode 12 (referred to as the "electrode load") is high, it is effective in suppressing deformation of the steel sheets due to heating and melting during welding, and in suppressing the occurrence of flash. However, if the electrode load is too high, the welded portion may be crushed and deformed during welding, which may reduce the remaining thickness of the plate. If the electrode load is too low, the amount of heat generated in the welded portion during welding that is dissipated via the first electrode 11 and the second electrode 12 is reduced, making surface flash more likely to occur.

[0066] The lower limit of the electrode load is preferably 1.5 kN, more preferably 2.0 kN. The electrode load need not be excessively high, and the upper limit is preferably 7.0 kN, more preferably 6.0 kN.

[0067] As the material for the first electrode 11 and the second electrode 12, electrode materials generally used in spot welding, such as pure copper, chromium copper, and alumina-dispersed copper, can be used.

[0068] <Embodiment 2: Method for manufacturing welded joint> The second embodiment is a method for manufacturing a welded joint by the spot welding method according to the first embodiment. The welded joint produced by the production method of embodiment 2 has high joint strength (for example, high tensile shear strength (TSS), high cross tensile strength (CTS), etc.).

[0069] The tensile shear test is performed in accordance with JIS Z 3136:1999. The cross tensile test is performed in accordance with JIS Z 3137:1999. The tensile shear strength (TSS) and cross tensile strength (CTS) are each measured multiple times (2 to 3 times), and the average value is used for evaluation.

[0070] The criteria for determining whether tensile shear strength (TSS) and cross tensile strength (CTS) are acceptable or not shall vary depending on the plate thickness, as specified in JIS Z 3140. In terms of tensile shear strength (TSS), it is preferable that it is equal to or greater than the "average value" of A-class and AF-class joints in Table 7 of JIS Z 3140:2017 "Inspection methods and criteria for spot welds," and more preferably 1.10 times or more of the "average value." In the case of cross tensile strength (CTS), it is preferably 1.14 times or more the "average value" of A class and AF class joints in Table 9 of JIS Z 3140:2017, and more preferably 1.20 times or more the "average value."

[0071] In the case of a steel plate having a thickness of 1.4 mm and a tensile strength of 1463 MPa, the tensile shear strength (TSS) of the welded joint is preferably 12.5 kN or more, more preferably 13.8 kN or more, and the cross tensile strength (CTS) of the welded joint is preferably 5.05 kN or more, more preferably 5.32 kN or more. [Example]

[0072] Welding test piece Two steel plates with the chemical composition and physical properties (tensile strength TS, elongation EL) shown in Table 1 were prepared and stacked to prepare a welded test specimen (a laminate of two steel plates). 0.5 >5200 is met.

[0073] [Table 1]

[0074] The welded test specimens were prepared in the specified test piece shape (for cross tension test: width 50 mm x length 150 mm, for cross-sectional observation: 40 mm x 40 mm).

[0075] Welding method The welding machine used was a DC inverter spot welder with an air cylinder type pressure mechanism. The first electrode 11 and the second electrode 12 used electrode tips made of chromium copper and having a DR shape (electrode diameter 13.0 mm, tip diameter 6.0 mm).

[0076] The prepared welded test piece was clamped and pressed between the first electrode 11 and the second electrode 12. The electrode load of the first electrode 11 and the second electrode 12 was set to 4.0 kN.

[0077] A preliminary welding experiment was conducted using direct current. When the current value was between 4 kA and 5 kA, welding was performed without any flashing, but when the current value was between 6 kA and 8 kA, flashing occurred. Therefore, the current value Ia in the first current application step was determined to be 5 kA.

[0078] A welding test was carried out under the conditions shown in Table 2, including (i) the first energization step, (ii) the first non-energization step, (iii) the second energization step, (iv) the second non-energization step, and (v) the third energization step. The welding current was a direct current. In the table, "-" means that the step was not performed, and underlined values ​​indicate that the value is outside the scope of the embodiments of the present invention. Note that "-" is not underlined even if it is outside the scope of the present invention.

[0079] In each test, three specimens were welded, and the joint strength and the hardness distribution of the cross section of the weld were measured.

[0080] (Measurement of joint strength) A cross tensile test was conducted to evaluate the joint strength of the welded joint. The cross tensile test was conducted in accordance with JIS Z 3137:1999. The cross tensile strength (CTS) was measured twice and the average value was calculated. The average CTS value is shown in Table 3.

[0081] The cross tensile strength (CTS) of a welded joint is rated as "poor" if it is less than 5.05 kN, "good" if it is 5.05 kN or more, and "excellent" if it is 5.32 kN or more. The values ​​for which the CTS measurement results were judged to be "poor" are underlined.

[0082] [Table 2]

[0083] [Table 3]

[0084] Test Nos. 7, 11, 13 and 15 were "Examples" that satisfied all of the welding conditions specified in embodiment 1. Therefore, the cross tensile strength (CTS) of the welded joints was high. On the other hand, Test Nos. 1 to 6, 8 to 10, 12, 14, and 16 were "comparative examples" that did not satisfy the welding conditions specified in embodiment 1. Therefore, the CTS was low. The points in each comparative example where the welding conditions were not satisfied are summarized below.

[0085] In Test No. 1, only the first current application step was performed, and the other steps were not performed. In Test Nos. 2 to 5, the first and second current-carrying steps were performed, but the third current-carrying step was not performed. In Test No. 2, the ratio of the current value Ib in the second current-carrying step to the current value Ia in the first current-carrying step (Ib / Ia) did not satisfy formula (1).

[0086] In Test Nos. 6, 8 to 10, and 11, the ratio (Ic / Ia) of the current value Ic in the third current application step to the current value Ia in the first current application step did not satisfy formula (2). In Test No. 14, the time period of the first de-energization step was shorter than the time period specified in the first embodiment. In Test No. 16, the time period of the second de-energization step was shorter than the time period specified in the first embodiment.

[0087] (Hardness distribution in the cross section of the weld) For Tests Nos. 1, 4, and 11, the welds were cut in the thickness direction, and the nugget and HAZ were exposed on the cross section. A micro Vickers hardness tester was used to measure the Vickers hardness of the center of the nugget, the outer edge of the nugget, and the HAZ. Figure 4 is an optical microscope photograph of the cross section of the weld. The area enclosed by the dashed rectangle is the hardness measurement range, and each point within the rectangle is the hardness measurement position. Figures 5(a) to (c) show the hardness measurement results (hardness distribution with hardness indicated at the measurement positions).

[0088] Test No. 1 (Fig. 5(a)) shows the hardness of the nugget and HAZ after the first energization step (and first de-energization step), Test No. 4 (Fig. 5(b)) shows the hardness of the nugget and HAZ after the second energization step (and second de-energization step), and Test No. 11 (Fig. 5(c)) shows the hardness of the nugget and HAZ after the third energization step. In other words, Tests No. 1 and 4 show the state of Test No. 11 at an intermediate stage.

[0089] In Test No. 1 shown in Figure 5(a), the Vickers hardness of the entire nugget and HAZ was approximately 650 Hv or higher (approximately 650 Hv or higher and approximately 730 Hv or lower). This is thought to be because the nugget formed in the first current application process and its surrounding HAZ were cooled in the first current removal process, causing the entire nugget and HAZ to transform into martensite. There were also some areas within the nugget with low hardness (646 Hv, 641 Hv), which is presumably due to defects formed within the nugget.

[0090] In Test No. 4 shown in Figure 5(b), the Vickers hardness of the nugget center was approximately 650 Hv or more (approximately 650 Hv to approximately 720 Hv), while the Vickers hardness of the nugget outer edge and HAZ decreased to less than approximately 650 Hv (approximately 400 Hv to approximately 640 Hv). This is thought to be because the martensite formed in the nugget outer edge and HAZ was tempered and softened by the second current application process, and the nugget center became martensite again, resulting in an increase in hardness.

[0091] In Test No. 11 shown in Figure 5(c), the Vickers hardness of the entire nugget (center and outer edge of the nugget) and the HAZ was reduced to less than approximately 650 Hv (approximately 370 Hv to 570 Hv). This is thought to be because the martensite in the center of the nugget was also tempered by the third current application process, softening the nugget and HAZ.

[0092] In this way, it was confirmed that by performing three-stage current application under specific conditions, as in the spot welding method according to the first embodiment, it is possible to soften the nugget and HAZ as a whole. [Explanation of symbols]

[0093] 11 First electrode 12 Second electrode P1 First steel plate P2 Second steel plate

Claims

1. a step of overlapping the first steel plate and the second steel plate and sandwiching them between a pair of electrodes in the plate thickness direction; a first current application step of applying a current value Ia (kA) between the pair of electrodes to form a weld; a first non-energizing step in which, after the first energizing step, energization is stopped for 300 ms or more to transform the welded portion into martensite; a second current-carrying step of applying current between the pair of electrodes at a current value Ib (kA) for a current-carrying time of 100 ms or more and 1000 ms or less after the first current-deactivating step; a second de-energization step of stopping the energization for 200 ms or more after the second energization step; a third current-carrying step of applying a current between the pair of electrodes at a current value Ic (kA) for a current-carrying time of 100 ms or more and 1000 ms or less after the second current-deactivating step, A spot welding method, wherein current values ​​Ia, Ib, and Ic satisfy the following formulas (1) and (2). 0.70<Ib / Ia<0.99 (1) 0.65<Ic / Ia<0.80 (2)

2. The first steel plate is C: 0.25% by mass or more and 0.50% by mass or less, Tensile strength is 1180 MPa or more, and The spot welding method according to claim 1, wherein the following formula (3) is satisfied: (TTS1)×(EL1) ０．５ >5200 (3) Here, TS1 is the tensile strength (MPa) of the first steel plate, EL1 is the elongation (%) of the first steel plate.

3. The first steel plate is Si: 1.00% by mass or more, and Mn: 1.00% by mass or more The spot welding method according to claim 1 , further comprising one or two of the following:

4. The spot welding method according to claim 2 , wherein the second steel plate has a tensile strength of 500 MPa or less.

5. The spot welding method according to claim 1, wherein the first steel plate has a thickness t1 of 1.0 mm or more, and the second steel plate has a thickness t2 of less than 1.0 mm.

6. A method for manufacturing a welded joint by spot welding two or more steel plates, comprising: A method for manufacturing a welded joint, wherein the spot welding is performed by the spot welding method according to any one of claims 1 to 5.

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