Resistance Spot Welding Method

Abstract

To provide a resistance spot welding method enabling suppression of expulsion and surface flash even when a lot of elements excluding Al and Zn are included in a plating layer.SOLUTION: In a resistance spot welding method, a plate set in which a plurality of steel plates are overlapped are sandwiched with a pair of electrodes, the steel plates are joined through energization while pressurizing the steel plates, at least one of the plurality of steel plates is a surface-treated steel plate having a plating layer having Al or Zn as the main component; the energization is divided into two or more stages of at least initial energization and main energization; an energization stop time is provided in the initial energization and the main energization; and when an initial energization time is t1[ms], and the energization stop time is tc[ms], a relation between t1 and tc is predetermined one among a total plate thickness T[mm], pressurization pressure P[kN], a plated thickness D[μm], and a ratio X of a content (mass%) of elements excluding Al and Zn, included in the plating layer.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a resistance spot welding method. [Background technology]

[0002] In general, resistance spot welding, which is a type of lap resistance welding, is used to join overlapping steel sheets. In order to obtain good weld quality in resistance spot welding, it is necessary to suppress the scattering of molten metal, or so-called splashing.

[0003] As a measure against the occurrence of expulsion, Patent Document 1 proposes a method in which a current pattern is a multi-stage current pattern of three or more stages, welding conditions such as current flow time and welding current are adjusted so that the appropriate current range (ΔI: a current range in which a nugget having a desired nugget diameter or more and a molten residual thickness of 0.05 mm or more can be stably formed) is 1.0 kA or more, preferably 2.0 kA or more, and a cooling time is provided between each stage. Patent Document 2 proposes a method in which a current pattern is a two-stage current pattern of initial current flow and main current flow, welding conditions such as current flow time and welding current are adjusted to increase the welding current value at the limit of expulsion occurrence, and a cooling time is provided between each stage. Patent Document 3 proposes a method in which a current pattern is a three-stage current pattern of initial current flow, constant current flow, and main current flow, and welding conditions such as current flow time and welding current are adjusted to suppress heat generation during the constant current period and to make the temperature distribution of the plate assembly uniform by heat conduction in order to increase the welding current value at the limit of expulsion occurrence. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3849539 [Patent Document 2] Patent Publication No. 2021-79416 [Patent Document 3] Patent Publication No. 2021-79410 [Non-patent literature]

[0005] [Non-Patent Document 1] Ji et al., Journal of Mechanical Science and Technology 28 (11) (2014) 4761~4769 Summary of the Invention [Problem to be solved by the invention]

[0006] When welding a plating layer containing a large amount of elements other than Al and Zn, expulsion may not be prevented even at a constant current. However, Patent Documents 1 to 3 do not include any idea of ​​adjusting the welding conditions according to the component composition of the plating layer.

[0007] The present disclosure has been made in consideration of the above-mentioned circumstances, and aims to provide a resistance spot welding method that can suppress expulsion even when the plating layer contains large amounts of elements other than Al and Zn. [Means for solving the problem]

[0008] The inventors have conducted extensive research to achieve the above object, and as a result, have found that expulsion can be suppressed and nugget formation can be stabilized by appropriately repeating cooling and heating in accordance with the component composition and plating amount of the plating layer.

[0009] Although there are some unclear points regarding the stabilization mechanism of the nugget formation as described above, the inventors currently consider it as follows. It is believed that expulsion that occurs during resistance spot welding is significantly affected by the heat generation form at the beginning of current flow. This heat generation form is correlated with the wet spread of the plating layer between the steel sheets. Furthermore, the wet spread of the plating layer is correlated with the electrical path (current flow diameter or resistance). Since the compositional components and plating amount of the plating layer affect the wet spread of the plating layer, it is believed that by appropriately repeating heating and cooling according to the compositional components and plating amount of the plating layer, the wet spread of the plating layer is stabilized, resulting in stable welding and suppression of expulsion.

[0010] The present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows.

[0011] [1] A resistance spot welding method in which a sheet assembly made of multiple overlapping steel sheets is sandwiched between a pair of electrodes and joined by passing electricity through the electrodes while applying pressure. At least one of the plurality of steel sheets is a surface-treated steel sheet having a plating layer mainly composed of Al or Zn, The energization is divided into at least two stages, an initial energization and a main energization, and a current stop time is provided between the initial energization and the main energization, a resistance spot welding method in which, when an initial current flow time is t1 [ms] and the current flow pause time is tc [ms], the relationship between t1 and tc satisfies any one of the following formulas (1) to (3) depending on a ratio X of a content (mass%) of elements other than Al and Zn in the plating layer in relation to a total sheet thickness T [mm], a press force P [kN], a plating thickness D [μm], and the content (mass%) of elements other than Al and Zn in the plating layer: Note When 0.01≦X<0.20, (0.05+0.1×X)×(1+(D×T) / P) / 50 ≦ tc / (t1+tc) ≦ (1.2+0.5×X)×(1+(D×T) / P) / 50 …(1) When 0.20≦X<0.40, (0.03 + 0.2×X)×(1 + (D×T) / P) / 50 ≤ tc / (t1 + tc) ≤ (1.1 + X)×(1 + (D×T) / P) / 50 …(2) When 0.40 ≤ X ≤ 0.50, (-0.09 + 0.5×X)×(1 + (D×T) / P) / 50 ≤ tc / (t1 + tc) ≤ (0.9 + 1.5×X)×(1 + (D×T) / P) / 50 …(3)

[0012] [2] The resistance spot welding method according to [1], wherein the surface-treated steel sheet is subjected to hot stamping at 850 degrees or higher and then spot welded.

[0013] [3] The resistance spot welding method according to [1] or [2], wherein t1 is 5 ms or more and 100 ms or less.

[0014] [4] The resistance spot welding method according to any one of [1] to [3], wherein tc is 20 ms or less.

[0015] [5] The resistance spot welding method according to any one of [1] to [4], wherein t1 [ms] and the main energization time tm [ms] satisfy t1 < tm. [Advantages of the Invention]

[0016] According to the present disclosure, even when the plating layer contains a large amount of elements other than Al and Zn, a resistance spot welding method capable of suppressing spattering can be provided. [Brief Description of the Drawings]

[0017] [Figure 1] It is a diagram for explaining the outline of the resistance spot welding method. [Diagram 2] It is a schematic diagram of the energization pattern in the resistance spot welding method using an inverter DC resistance spot welder. [Diagram 3] It is a conceptual diagram of the energization pattern in the resistance spot welding method using an AC resistance spot welder. [Figure 4] FIG. 13 is a diagram showing the relationship between the upper and lower limits of tc / (t1+tc) and the ratio X when the total sheet thickness T is 3.0 mm, the plating thickness is 20 μm, and the applied pressure P is 3.5 kN. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, an embodiment of the present invention will be described. Note that the present invention is not limited to the following embodiment. In addition, in this specification, a numerical range expressed using "to" means a range including the numerical values ​​before and after "to" as the lower and upper limits.

[0019] As shown in Fig. 1, the resistance spot welding method according to the present disclosure is a method in which a sheet set 3 consisting of multiple overlapping steel sheets 1, 2 is sandwiched between a pair of upper and lower electrodes 4, 5, and while applying pressure, a high welding current is passed between the upper and lower electrodes for a short period of time to form and join a joint 6. A point-like weld 6 is obtained by utilizing resistance heat generated by passing a high-current welding current. This point-like weld 6 is called a nugget, and is a portion where the steel sheets 1, 2 melt and solidify at the contact point of the steel sheets when a current is passed through the overlapping steel sheets, thereby joining the steel sheets together in a point-like manner.

[0020] At least one of the multiple steel sheets is a surface-treated steel sheet having a plating layer mainly composed of Al or Zn. Two or more or three or more of the multiple steel sheets may be surface-treated steel sheets having a plating layer mainly composed of Al or Zn. All of the multiple steel sheets may be surface-treated steel sheets having a plating layer mainly composed of Al or Zn.

[0021] The plating layer containing Al or Zn as a main component means that the ratio (mass %) of the total content of Al and Zn contained in the plating layer is 0.50 or more.

[0022] The plating layer further contains elements other than Al and Zn. The content (mass%) of the elements other than Al and Zn contained in the plating layer is set to be 0.01 or more and 0.50 or less. Here, the ... The content (mass%) of the elements other than Al and Zn contained in the plating layer is defined by the following formula. X = (100-[Al]-[Zn]) / 100 [Al]: The amount of Al contained in the plating layer (mass%) [Zn]: The amount of Zn contained in the plating layer (mass%) The element contained in the plating layer other than Al and Zn may be, for example, at least one selected from the group consisting of Fe, Si, Mg, Sn, and Ni.

[0023] In particular, the plating layer of a steel sheet for hot stamping contains Zn, Mg, Sn, etc., which have low vapor pressure. Therefore, in resistance spot welding in which the steel sheet for hot stamping is used as the joined material, there is a problem that expulsion is likely to occur even at a low current. However, according to the present resistance spot welding method, expulsion can be suppressed even in resistance spot welding in which the steel sheet for hot stamping is used as the joined material.

[0024] In the present resistance spot welding method, the current is divided into at least two stages, an initial current and a main current, and a current rest time is provided between the initial current and the main current. As described above, it is considered that the expulsion occurring during resistance spot welding is greatly affected by the heat generation form at the initial stage of current. This heat generation form is correlated with the wet spreading of the plating layer between the steel sheets. It is considered that the occurrence of expulsion is suppressed when the wet spreading of the plating layer extends over a wide range. Therefore, the present inventors have found that the wet spreading of the plating layer is stabilized by adjusting the initial current time t1 [ms] and the subsequent current rest time tc [ms] according to the component composition and plating amount of the plating layer, and have arrived at the present resistance spot welding method.

[0025] In the present resistance spot welding method, when an initial current flow time is t1 [ms] and the current rest time is tc [ms], the relationship between t1 and tc in relation to a total sheet thickness T [mm], a press force P [kN], a plating thickness D [μm], and a proportion X of the content (mass%) of elements other than Al and Zn contained in the plating layer satisfies any one of the following formulas (1) to (3) depending on the proportion X. When 0.01≦X<0.20, (0.05+0.1×X)×(1+(D×T) / P) / 50 ≦ tc / (t1+tc) ≦ (1.2+0.5×X)×(1+(D×T) / P) / 50 …(1) When 0.20≦X<0.40, (0.03+0.2×X)×(1+(D×T) / P) / 50 ≦ tc / (t1+tc) ≦(1.1+X)×(1+(D×T) / P) / 50 …(2) When 0.40≦X≦0.50, (-0.09+0.5×X)×(1+(D×T) / P) / 50 ≦ tc / (t1+tc) ≦ (0.9+1.5×X)×(1+(D×T) / P) / 50 …(3)

[0026] The current interruption time tc is the time without current flow after the end of the initial current flow time t1, and is the time from the end of the initial current flow to the start of main current flow. By setting the current interruption time tc so as to satisfy the above formulas (1) to (3), the temperature of the coating layer can be lowered before the coating layer that has spread between the steel sheets by wetting is discharged outside the sheet assembly, even if the coating layer has a low melting point or boiling point. This ensures a sufficient current flow diameter between the steel sheets and prevents the occurrence of splashing. The welding current value may or may not be constant during the initial current flow and main current flow.

[0027] As the ratio X of elements other than Al and Zn contained in the coating layer increases, a residence time is required to sufficiently wet and spread the coating layer between the steel sheets. Therefore, the ratio of tc in t1 and tc needs to be increased as the ratio X increases. For example, FIG. 4 is a diagram showing the relationship between the upper and lower limits of tc / (t1+tc) and the ratio X when the total sheet thickness T is 3.0 mm, the coating thickness is 20 μm, and the pressure P is 3.5 kN. The upper and lower limits of tc / (t1+tc) were calculated based on the above formulas (1) to (3). As shown in FIG. 4, as the ratio X of elements other than Al and Zn increases, both the upper and lower limits of tc / (t1+tc) increase.

[0028] 2 and 3 show schematic diagrams of the current patterns in the above-mentioned resistance spot welding method. FIG. 2 shows the current patterns in an inverter DC resistance spot welding machine, and FIG. 3 shows the current patterns in an AC resistance spot welding machine. The dotted lines in FIG. 3 indicate, for example, root mean square values ​​(hereinafter referred to as "RMS"), and the period during which the current value is equal to or greater than the RMS is the current flow time, and the period during which the current value is less than the RMS is the rest time. In FIG. 2(a) and FIG. 3(a), the current flow pattern is shown in outline when the current flow consists of two stages, an initial current flow and a main current flow, but the current flow pattern is not limited to two stages. In one example, as shown in FIG. 2(b) and FIG. 3(b), a multi-stage current flow of three or more stages including two or more initial current flows may be performed before the main current flow. When a multi-stage current flow is performed, it is sufficient that the initial current flow time t1 of the first initial current flow and the current flow rest time tc immediately thereafter satisfy the above formulas (1) to (3). However, as shown in FIG. 2(b) and FIG. 3(b), current application with the same initial current application time t1 and current application pause time tc as the first stage may be repeated in multiple stages.

[0029] t1 [ms] may be controlled to satisfy formulas (1) to (3), but is preferably 5 ms or more and 100 ms or less. If t1 is 5 ms or more, the coating layer melts and the molten coating can wet and spread between the steel sheets. If t1 is 100 ms or less, the molten coating can be prevented from being swept out of the system.

[0030] tc [ms] should be controlled to satisfy the formulas (1) to (3), and it is preferably 20 ms or less. If tc is 20 ms or less, the plating spreading between the steel plates is stable and the molten state of the nugget can be maintained. Also, although the lower limit of tc is not particularly limited, it is preferably 5 ms or more in order to stabilize the plating spreading between the steel plates.

[0031] tm [ms] is not particularly limited, but in order to obtain an appropriate nugget diameter, it is preferable that t1 < tm, and more preferably 100 ms or more. Also, from the viewpoint of construction time constraints, tm is preferably 1000 ms or less.

[0032] In the above, the relationship between the initial energization time t1 and the pause time tc with respect to the occurrence of splash during welding has been described. Hereinafter, other factors that affect the occurrence of splash during resistance spot welding will also be described.

[0033] The total plate thickness T [mm] is the total plate thickness of a plurality of steel plates that are overlapped to form a plate stack. When the total plate thickness T becomes small, the nugget easily reaches the steel plate surface during excessive heat generation. For this reason, it is considered that splash is likely to occur. The total plate thickness T [mm] should be controlled to satisfy the formulas (1) to (3), and for example, it is preferably 0.8 mm or more and preferably 10 mm or less.

[0034] The pressure P [kN] is the pressure applied to the plate assembly by the pair of electrodes from the start of the initial current flow to the end of the main current flow. The pressure P may be constant throughout the welding, but it does not have to be constant. It is considered that if the pressure P increases, the discharge of the molten metal plating around the welded part is promoted, the current flow diameter becomes insufficient, and splashing is likely to occur. Therefore, the pressure P [kN] is controlled to satisfy the formulas (1) to (3). The pressure P [kN] may be controlled to satisfy the formulas (1) to (3), and is preferably set to 2 kN or more and 10 kN or less, for example. The pressure P is measured as follows. The pressure P can be measured using a pressure gauge such as a load cell.

[0035] The plating thickness D [μm] is the plating layer thickness per one side of the surface-treated steel sheet. When multiple steel sheets each have a plating layer, the plating thickness D is the average value of the plating layer thicknesses of the multiple sheets. When the plating thickness D of the surface-treated steel sheet to be welded is small, the current flow diameter is insufficient. For this reason, it is considered that the heat generated at the beginning of the current flow becomes excessive, and splashing is likely to occur. As a result, the amount of the plating layer in contact with the base material in the vicinity of the welded part decreases, and the current flow diameter decreases. For this reason, it is considered that splashing is likely to occur. Therefore, the plating thickness D is adjusted to satisfy the formulas (1) to (3). The plating thickness D may be adjusted to satisfy the formulas (1) to (3), but for example, it is preferable to set it to 10 μm or more from the viewpoint of corrosion resistance, and it is preferable to set it to 40 μm or less from the viewpoint of manufacturing costs. The plating thickness is measured as follows. That is, ten locations are randomly selected from the steel plate, the steel plate is cut in the thickness direction at those locations, the cross section is mirror-polished, and the plating thickness is measured using an optical microscope or a secondary electron image or a backscattered electron image (magnification 500x), and the plating thickness is taken as the average value of the ten locations.

[0036] The proportion X of the content (mass%) of elements other than Al and Zn is a value indicating the total content (mass%) of elements other than Al and Zn contained in the coating layer as a percentage. When multiple steel sheets each have a coating layer, the proportion Xc of the content (mass%) of elements other than Al and Zn is calculated by formula (4). In formula (4), steel sheets 1 and 2 are one and the other of two opposing steel sheets. Xc=(T1×X1+T2×X2) / (T1+T2) …(4) however, T1: plating thickness of steel sheet 1, T2: plating thickness of steel sheet 2 X1: Content (mass%) of elements other than Al and Zn in steel plate 1 X2: Content (mass%) of elements other than Al and Zn in steel plate 2

[0037] For the reasons described above, the above formulas (1) to (3) were found. The coefficients in the formulas were determined by experiment to be optimal for X.

[0038] The steel type of the steel plates constituting the plate assembly is not particularly limited. The manufacturing method of the steel plate may be any method such as cold rolling or hot rolling, and the structure of the steel plate may be any method. In addition, there is no problem in using hot-pressed steel plates as the steel plates of the plate assembly. In addition, the thickness of the steel plate is not particularly limited, and in one example, it may be a thickness that can be used for a general automobile body. The thickness of the steel plate may be, for example, 0.5 mm or more and 4.0 mm or less.

[0039] The plate combination may be a combination of multiple sheets of the same type of steel plate or multiple sheets of different types of steel plate. In addition, there is no problem even if the thicknesses of the steel plates are different, and a combination of a steel plate having a plating layer with a lower melting point than the steel plate and a steel plate having no plating layer may be used.

[0040] The sheet assembly may be a sheet assembly in which three or more steel sheets are overlapped. When a sheet assembly in which three or more steel sheets are overlapped is used as the joined materials, the welding conditions are adjusted so that the above formulas (1) to (3) are satisfied for the sheet assembly between two opposing steel sheets in which the product of the plating thickness D and the sheet thickness T is the smallest.

[0041] Furthermore, the current value and the time during which the current is applied may be constant throughout the welding, but they do not have to be constant. Also, there is no problem in using a control method in which parameters such as the resistance value and the voltage value during the welding are monitored and the current value and the time during which the current is applied are changed in response to the fluctuations.

[0042] Furthermore, resistance spot welding may be performed with a gap between the steel sheets or with the electrode inclined relative to the steel sheets.

[0043] Using the above-mentioned resistance spot welding method, a welded member can be obtained in which a plurality of steel sheets, including at least one surface-treated steel sheet having a metal plating layer, are welded together. When the above-mentioned resistance spot welding method is used for welding, the occurrence of expulsion at the welded portion can be suppressed, so that a welded member can be produced from surface-treated steel sheets having a low-melting point plating layer.

[0044] The welding device that can be used in this resistance spot welding method is equipped with a pair of upper and lower electrodes, and can arbitrarily control the pressure and welding current during welding. The pressure mechanism (air cylinder, servo motor, etc.), type (fixed type, robot gun, etc.), and electrode shape of the welding device are not particularly limited. Examples of the shape of the electrode tip include DR type (dome radius type), R diameter (radius type), and D type (dome type) described in JISC 9304:1999.

[0045] Conditions other than those mentioned above can be determined in the usual manner. EXAMPLES

[0046] Examples of the present disclosure are shown below. Note that the present disclosure is not limited to the following examples. For each of the two-ply and three-ply plate pairs in which steel plate 1 and steel plate 2 shown in Table 1 were overlapped, resistance spot welding was performed under the conditions shown in Table 2 to prepare joints (welded members). Note that the melting points of the base metals of the test materials in this example are in the range of 1400 to 1570°C, and the melting points of the platings are in the range of 400 to 700°C. The tensile strengths shown in Table 1 are tensile strengths obtained by preparing JIS No. 5 tensile test pieces from each steel plate in a direction parallel to the rolling direction and conducting tensile tests in accordance with the provisions of JIS Z 2241:2011.

[0047] The welding equipment used was an inverter DC resistance spot welder or an AC resistance spot welder, and the electrodes were DR-type chromium copper electrodes. The two electrodes were the same. The current was passed in two stages, an initial current and a main current, and when an inverter DC resistance spot welder was used, the current value (welding current) during current was kept constant. The main current time tm was 420 ms. The pressure was the pressure at the end of current, and was kept constant during current and until the end of current. Resistance spot welding was performed at room temperature, with the electrodes always water-cooled.

[0048] For each of the obtained joints, the welded portion was cut in the thickness direction of the steel plate, the cross section was mirror-polished, and the nugget diameter was measured using an optical microscope (magnification: 10x). In the case of three-ply steel plates, the same test was performed on the plate combination with the smallest product of the plating thickness D and the plate thickness T.

[0049] The occurrence of expulsion during welding was also observed. Ten joints were fabricated and observed under the same conditions, and the results were judged according to the following criteria: (A) to (C) were deemed acceptable, and (D) was deemed unacceptable. Here, ΔW is the range between the welding current value at which expulsion occurs and the welding current value at which 4√t occurs, determined by experimentally determining the weld lobe (relationship between current value and nugget diameter) at 0.5kA intervals for the welding current, and determining the welding current value at which 4√t occurs from the approximation curve of the data thus obtained. Note that t is the thickness of the thinnest steel plate used. Incidentally, in spot welding, the required nugget diameter is determined for each steel plate thickness t, and the formula for this calculation is 4√t. (A): ΔW is 1.0kA or more for all 10 joints (B): 1 out of 10 has a ΔW of less than 1.0 kA (C): 2 out of 10 have ΔW less than 1.0 kA (D): ΔW is less than 1.0 kA for 3 or more of 10 bodies.

[0050] As shown in Table 2, the evaluation results for ΔW above are shown, and all of the joints (examples of the present invention) in which the pause time tc was set so as to satisfy the requirements of this disclosure were rated (passed) as either (A) to (C).

[0051] The occurrence of expulsion during welding was also observed. Ten joints were fabricated and observed under the same conditions, and the results were judged according to the following criteria. The results are shown in Table 2. (E) to (G) were judged as pass, and (H) was judged as fail. (E): No expulsion occurred in all 10 joints. (F): 1 out of 10 bodies have been shattered (G): 2 out of 10 have scattering (H): 3 or more of 10 bodies have been shattered

[0052] As shown in Table 2, all of the joints (invention examples) in which the downtime tc was set so as to satisfy the requirements of the present disclosure were rated as either E to G (passed).

[0053] [Table 1]

[0054] [Table 2] [Explanation of symbols]

[0055] 1, 2 steel plate 3 board set 4, 5 electrodes 6 Welding section (nugget)

Claims

1. In a resistance spot welding method in which a sheet set of multiple overlapping steel sheets is sandwiched between a pair of electrodes and joined by passing current through the electrodes while applying pressure, At least one of the plurality of steel sheets is a surface-treated steel sheet having a plating layer mainly composed of Al or Zn, The energization is divided into at least two stages, an initial energization and a main energization, and a current stop time is provided between the initial energization and the main energization, When an initial current application time is t1 [ms] and the current application rest time is tc [ms], the relationship between t1 and tc satisfies any one of the following formulas (1) to (3) in relation to a total sheet thickness T [mm], a pressure P [kN], a plating thickness D [μm], and a ratio X of a content (mass%) of elements other than Al and Zn contained in the plating layer, according to the ratio X: a resistance spot welding method, wherein the t1 [ms] and the main current flow time tm [ms] satisfy t1 < tm. Note When 0.01≦X<0.20, (0.05+0.1×X)×(1+(D×T) / P) / 50≦tc / (t1+tc)≦(1.2+0.5×X)×(1+(D×T) / P) / 50…(1) When 0.20≦X<0.40, (0.03+0.2×X)×(1+(D×T) / P) / 50≦tc / (t1+tc)≦(1.1+X)×(1+(D×T) / P) / 50…(2) When 0.40≦X≦0.50, (-0.09+0.5×X)×(1+(D×T) / P) / 50≦tc / (t1+tc)≦(0.9+1.5×X)×(1+(D×T) / P) / 50…(3)

2. 2. The resistance spot welding method according to claim 1, wherein the surface-treated steel sheet is subjected to a hot stamping treatment in which the surface-treated steel sheet is heated to 850° C. or more, and then spot welding is performed.

3. The resistance spot welding method according to claim 1 or 2, wherein t1 is 5 ms or more and 100 ms or less.

4. 3. The resistance spot welding method according to claim 1, wherein tc is 20 ms or less.

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