Resistance spot welding method and manufacturing method for welded components

Abstract

To provide a resistance spot welding method that can stably form a nugget with a desired nugget diameter, even if there is disturbance.SOLUTION: In a resistance spot welding method, assembled plates 3 constituted of a plurality of overlapped steel plates 1 and 2 are pinched by a pair of electrodes 4 and 5, so that the assembled plates are jointed to each other by being pressurized and energized. The energization has energization in two or more steps including energization in the first step and energization in a second step by which a nugget with a predetermined nugget diameter is formed after the energization in the first step. When a target amount of heat generation in the energization in the first step is defined as E1 and a target amount of heat generation in the energization in the second step is defined as E2, E1 / E2 is in a predetermined range corresponding to a plate thickness ratio R of the assembled plates 3. Electric currents between the pair of electrodes 4 and 5 are controlled while an electric signal is monitored during welding.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a resistance spot welding method and a method for manufacturing a welded component.

[0002] Generally, resistance spot welding, a type of lap resistance welding, is used to join overlapping steel sheets. This welding method is a method in which two or more overlapping steel sheets are sandwiched and pressed from above and below with a pair of electrodes while a high welding current is passed between the upper and lower electrodes for a short period of time to join them. A spot-like weld is obtained by utilizing the resistance heat generated by passing a high welding current. This spot-like weld is called a "nugget" and is the part where the overlapping steel sheets melt and solidify at the contact point when a current is passed through the steel sheets. The steel sheets are joined at a spot by this nugget.

[0003] In order to obtain good weld quality, it is important to adjust the nugget diameter to an appropriate range. The nugget diameter is determined by welding conditions such as welding current, current application time, electrode shape, and pressure. Therefore, in order to obtain an appropriate nugget diameter, it is necessary to set the above welding conditions appropriately according to the conditions of the workpieces, such as the material, plate thickness, and number of overlapping sheets of the workpieces.

[0004] For example, when manufacturing an automobile, several thousand spot welds are applied to each vehicle, and it is necessary to weld the workpieces (workpieces) that flow in one after another. In this case, if the condition of the workpieces at each welding point, such as the material, plate thickness, and number of overlapping sheets, is the same, the same nugget diameter can be obtained under the same welding conditions, such as the welding current, current flow time, and pressure.

[0005] However, in continuous welding, the contact surface of the electrode with the workpiece gradually wears away, and the contact area gradually becomes larger than the initial state. If the same welding current as in the initial state is applied to the electrode with the expanded contact area, the current density in the workpiece decreases, and the temperature rise in the welded portion decreases, resulting in a smaller nugget diameter. For this reason, the electrode is polished or replaced every several hundred to several thousand welding points to prevent the tip diameter of the electrode from expanding too much.

[0006] In addition, resistance welding devices equipped with a function (stepper function) that increases the welding current value after a predetermined number of welding operations to compensate for the decrease in current density due to electrode wear have been used in the past. To use this stepper function, it is necessary to properly set the above-mentioned welding current change pattern in advance.

[0007] However, it takes a lot of time and money to derive welding current change patterns corresponding to many welding conditions and conditions of materials to be welded through tests, etc. In addition, in actual welding, the progress of electrode wear varies, so it cannot be said that a predetermined welding current change pattern is always appropriate.

[0008] Furthermore, if there is a disturbance during welding, for example, if there is a previously welded point (previously welded point) near the point to be welded, or if the surface unevenness of the material to be welded is large and there is a contact point of the material to be welded near the point to be welded, the current will be shunted to the previously welded point and the contact point during welding. In such a state, even if welding is performed under specified conditions, the current density at the position to be welded directly below the electrode will decrease, and it will not be possible to obtain a nugget of the required diameter. To compensate for this lack of heat generation and obtain a nugget of the required diameter, it is necessary to set a high welding current in advance.

[0009] Furthermore, if the area around the welding point is tightly constrained due to surface irregularities or the shape of the components, or if a foreign object is trapped between the steel sheets around the welding point, the gap between the steel sheets (hereinafter also referred to as the "sheet gap") will become larger, narrowing the contact diameter between the steel sheets and making splashing more likely to occur.

[0010] In order to solve such welding instability, so-called adaptive control welding has been proposed. In adaptive control welding, changes in the welding phenomenon due to the above-mentioned electrode wear and disturbances are directly measured or calculated as electrical signals in the form of changes in current, voltage, resistance and heat generation during welding, and input parameters such as the welding current and voltage are controlled based on the values.

[0011] Patent Document 1 describes a method for controlling welding conditions of a resistance welding machine that aims to perform good welding by detecting the welding current and the voltage between the tips, simulating the welded part by heat conduction calculations, and estimating the nugget formation state of the welded part during welding.

[0012] Patent Document 2 describes a resistance welding system that uses a welding system that calculates the cumulative heat generation amount per unit volume that will allow the workpieces to be well welded based on the plate thickness and current flow time of the workpieces, and adjusts the welding current or voltage to generate the calculated amount of heat generation per unit volume and unit time, thereby achieving good welding regardless of the type of workpieces to be welded or the wear state of the electrodes.

[0013] Patent Document 3 describes a resistance spot welding method in which a welding pattern is divided into two steps: a step for securing a current path directly under the electrode, followed by a step for forming a nugget of a specified diameter; the change in instantaneous heat generation per unit volume over time and the cumulative heat generation per unit volume, which are calculated from the electrical characteristics between the electrodes when a current is passed under constant current control in test welding to form an appropriate nugget, are each stored as target values; and the current flow is controlled so that the cumulative heat generation in this welding matches the cumulative heat generation previously determined in test welding, thereby obtaining a nugget diameter of a certain size or more.

[0014] Patent Document 4 describes a resistance spot welding method in which, when storing the cumulative heat generation amount in test welding, a disturbance state is simulated and stored, and then the amount of current is controlled so that the cumulative heat generation amount in the actual welding matches the cumulative heat generation amount previously determined in the test welding, thereby obtaining a nugget diameter of a certain size or more.

[0015] Patent Document 5 describes a resistance spot welding method in which pressure is applied until an initial set pressure is reached before current begins to flow for the actual welding, the pressure is similarly measured during the actual welding, and the pressure during current flow is set using a pressure index parameter obtained from the start of current flow until the initial set pressure is reached, thereby enabling a stable production of a desired nugget diameter. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] Japanese Patent Application Publication No. 10-94883 [Patent Document 2] Japanese Patent Application Publication No. 11-33743 [Patent Document 3] International Publication No. 2015 / 049998 [Patent Document 4] JP 2019-34341 A [Patent Document 5] International Publication No. 2020 / 095847 Summary of the Invention [Problem to be solved by the invention]

[0017] However, the technology described in Patent Document 1 requires complex calculations to estimate the nugget temperature based on a heat conduction model (heat conduction simulation), etc., which not only complicates the configuration of the welding control device but also makes the welding control device itself expensive.

[0018] In addition, the technology described in Patent Document 2 is believed to be able to perform good welding even if the electrodes are worn to a certain extent by controlling the cumulative heat generation amount to a target value. However, when the set conditions of the workpieces and the actual conditions of the workpieces are significantly different, for example, when there is a large gap between the metal plates to be welded, even if the final cumulative heat generation amount can be matched to the target value, the form of heat generation, that is, the change over time in the temperature distribution of the welded part, deviates from the heat pattern that results in the desired good welded part, and the required nugget diameter cannot be obtained or expulsion occurs.

[0019] Furthermore, in the techniques described in Patent Documents 3 to 5, the accumulated heat amount is stored regardless of the presence or absence of disturbance, but the means for setting the conditions is not specified, and various experiments are required for each plate combination having a different plating, so there is a problem that it may not be an optimal solution for fluctuations due to disturbance.

[0020] The present invention has been made in consideration of the above problems, and has an object to provide a resistance spot welding method that can stably form a nugget with a desired nugget diameter even when external disturbances are present. [Means for solving the problem]

[0021] The present inventors conducted intensive experimental studies on a method for setting conditions that can solve the above problems and stably form a nugget with a desired nugget diameter even when disturbances exist. It has been known that the sheet thickness ratio (i.e., the total value of the thicknesses of the steel sheets constituting the sheet combination / the minimum value of the thicknesses of the steel sheets constituting the sheet combination) has a large effect on nugget formation. Therefore, the present inventors conducted a summary focusing on the definition of the relationship between the sheet thickness ratio and the amount of heat generation.

[0022] As a result, when spot welding is performed by sandwiching a stack of multiple steel plates between a pair of electrodes and energizing while applying pressure, the energization is divided into two or more stages including a first-stage energization and a second-stage energization for forming a nugget, and the target heat generation amount E1 in the first-stage energization and the target heat generation amount E2 required to form a nugget with a desired nugget diameter in the second-stage energization are set such that E1 / E2 satisfies requirements according to the plate thickness ratio R of the steel plates constituting the stack. Then, by controlling the current between the pair of electrodes while monitoring the electrical signal during welding, it has been found that a nugget with a desired nugget diameter can be stably formed even when there are disturbances, and thus the present invention has been completed.

[0023] That is, the present invention for solving the above problems is as follows. [1] In a resistance spot welding method in which a stack of multiple steel plates is sandwiched between a pair of electrodes and joined by energizing while applying pressure, the energization has two or more stages including a first-stage energization and a second-stage energization for forming a nugget with a predetermined nugget diameter after the first-stage energization, as the target heat generation amount E1 in the first-stage energization, the target heat generation amount E2 required to form a nugget with the predetermined nugget diameter in the second-stage energization, the pressing force P [N], the difference ΔTS [MPa] in the maximum tensile strength of the steel plates constituting the stack, and the plate thickness ratio R of the steel plates constituting the stack, E1 / E2 satisfies any one of the following formulas (1) to (3), and the current between the pair of electrodes is controlled while monitoring the electrical signal during welding. A resistance spot welding method characterized by this. When 2.0 ≦ R ≦ 2.5, (0.08 + 0.004×R)×(1 + ΔTS / P) ≦ E1 / E2 ≦ (0.4 + 0.02×R)×(1 + ΔTS / P) (1) When 2.5 < R ≦ 5, (0.07 + 0.008×R)×(1 + ΔTS / P) ≦ E1 / E2 ≦ (0.35 + 0.04×R)×(1 + ΔTS / P) (2) When 5 < R ≦ 10, (0.03 + 0.016×R)×(1 + ΔTS / P) ≦ E1 / E2 ≦ (0.25 + 0.06×R)×(1 + ΔTS / P) (3)

[0024] [2] Test welding is performed prior to the actual welding using the two or more stages of current application, In the test welding, a time change curve of an instantaneous heat generation amount per unit volume and an accumulated heat generation amount per unit volume, which are calculated from an electrical characteristic between the pair of electrodes when a current is passed by constant current control to form a nugget having the predetermined nugget diameter, are stored; The resistance spot welding method according to [1], wherein, in the current supply for the main welding, a time change curve of the instantaneous heat generation per unit volume and a cumulative heat generation per unit volume in the test welding are set to target values, and the current supply is controlled in accordance with the target values.

[0025] [3] The resistance spot welding method according to [1] or [2], wherein E1 / E2 is less than 0.9.

[0026] [4] The resistance spot welding method according to any one of [1] to [3] above, wherein ΔTS is 440 MPa or more.

[0027] [5] A method for producing a welded component, comprising joining two or more overlapping steel plates by the resistance spot welding method according to any one of [1] to [4] above to obtain a welded component. Effect of the Invention

[0028] According to the present invention, even when disturbances exist, a nugget having a desired diameter can be stably formed. [Brief description of the drawings]

[0029] [Figure 1] FIG. 1 is a diagram for explaining an outline of a resistance spot welding method. [Diagram 2] FIG. 11 is a diagram showing an appropriate range of the ratio E1 / E2 of the target heat generation amount E1 in the first stage of current application to the target heat generation amount E2 in the second stage of current application, for stably forming a nugget having a desired diameter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. As shown in Fig. 1, the resistance spot welding method according to the present invention is a resistance spot welding method in which a sheet set 3 consisting of a plurality of overlapping steel sheets 1 and 2 is sandwiched between a pair of electrodes 4 and 5 and joined by passing current while applying pressure. A spot-like weld 6 is formed by resistance heat generated by the above-mentioned current passing. This spot-like weld 6 is called a "nugget" and is a portion where the steel sheets 1 and 2 melt and solidify at the contact point between the overlapping steel sheets 1 and 2 when a welding current is passed through the overlapping steel sheets 1 and 2, thereby joining the steel sheets 1 and 2 to each other in a spot-like manner.

[0031] The steel sheets 1 and 2 to be joined may be steel sheets having a plating layer. Two or more of the multiple steel sheets 1 and 2 may be steel sheets having a plating layer. Furthermore, all of the multiple steel sheets 1 and 2 may be steel sheets having a plating layer. The plating layer may be provided on only one side (one surface) of the steel sheets 1 and 2, or on both sides.

[0032] An example of the steel sheet having the above-mentioned plating layer is a zinc-based plated steel sheet. The zinc-based plated steel sheet is a steel sheet in which a zinc-based plating is coated on the steel sheet by various methods such as hot-dip plating, electroplating, vapor deposition plating, and thermal spraying. Examples of the zinc-based plating include hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, electrogalvanizing, and electrogalvanizing-nickel alloy plating, but are not limited thereto, and all known zinc-based platings containing zinc can be applied. Examples of such zinc-based plated steel sheets include hot-dip galvanized steel sheets (GI) that have not been subjected to alloying treatment, and alloyed hot-dip galvanized steel sheets (GA) that have been subjected to alloying treatment.

[0033] It is preferable that the difference ΔTS in maximum tensile strength between the steel sheets 1 and 2 constituting the sheet pair 3 is 440 MPa or more. This makes the change in heat generation form due to the difference in tensile strength clear, so that stable welding can be performed by controlling the target heat generation amount in the present invention.

[0034] The present invention is a resistance spot welding method for controlling current during welding while monitoring electrical signals during welding. Here, the energization has two or more stages of energization including a first-stage energization and a second-stage energization for forming a nugget with a predetermined nugget diameter after the first-stage energization. The first-stage energization is an energization performed before energization for forming a predetermined nugget diameter and is an initial energization for securing an energization path. On the other hand, the second-stage energization is the main energization for forming a nugget with a desired nugget diameter.

[0035] In the present invention, the target heat generation amount E1 for securing the energization path in the first-stage energization and the target heat generation amount E2 required for forming a nugget with a predetermined nugget diameter in the second-stage energization are determined based on the ratio E1 / E2 and the plate thickness ratio R of the steel plates 1 and 2 constituting the plate assembly 3, and energization is performed.

[0036] Specifically, the resistance spot welding method according to the present invention has two or more stages of energization including a first-stage energization and a second-stage energization for forming a nugget with a predetermined nugget diameter after the first-stage energization. Then, as the target heat generation amount E1 in the first-stage energization, the target heat generation amount E2 required for forming a nugget with a predetermined nugget diameter in the second-stage energization, the pressing force P [N], the difference ΔTS [MPa] in the maximum tensile strength of the steel plates constituting the plate assembly, and the plate thickness ratio R of the steel plates constituting the plate assembly, E1 / E2 satisfies any one of the following formulas (1) to (3), and the current between a pair of electrodes is controlled while monitoring the electrical signal during welding. When 2.0 ≤ R ≤ 2.5, (0.08 + 0.004 × R) × (1 + ΔTS / P) ≤ E1 / E2 ≤ (0.4 + 0.02 × R) × (1 + ΔTS / P) (1) When 2.5 < R ≤ 5, (0.07 + 0.008 × R) × (1 + ΔTS / P) ≤ E1 / E2 ≤ (0.35 + 0.04 × R) × (1 + ΔTS / P) (2) When 5 < R ≤ 10, (0.03 + 0.016 × R) × (1 + ΔTS / P) ≤ E1 / E2 ≤ (0.25 + 0.06 × R) × (1 + ΔTS / P) (3)

[0037] In the above formulas (1) to (3), a large value of E1 / E2 indicates that it is necessary to increase the amount of heat generated in the initial stage of current flow (first stage current flow) and stabilize the contact between steel sheets 1 and 2. In addition, when ΔTS is large, a higher target heat generation amount E1 is required to form a nugget, but a correlation was observed in which the effect of the difference due to ΔTS becomes smaller when the pressure P is increased.

[0038] In addition, when the sheet thickness ratio R is small, the formation of the nugget is easy to stabilize, and the value of E1 / E2 may be relatively small, but when the sheet thickness ratio R exceeds a certain value, it becomes difficult to stabilize the contact area, and the required target heat generation amount E1 also becomes large. It was confirmed that when the sheet thickness ratio R is very large and exceeds 5, the target heat generation amount E1 for the sheet thickness ratio R increases rapidly. By sorting out these tendencies, the inventors have found that by setting the target heat generation amounts E1 and E2 so that E1 / E2 satisfies any one of the above formulas (1) to (3) and controlling the current between the electrodes 4 and 5 during welding, a nugget with a desired nugget diameter can be stably formed even when disturbances exist.

[0039] Figure 2 shows the optimum range of E1 / E2 for stably forming a nugget with the desired diameter, calculated for a ΔTS of 710 MPa and a press force of 3,500 N. In Figure 2, the solid line indicates the lower limit of E1 / E2 for stably forming a nugget with the desired diameter, and the dotted line indicates the upper limit. As shown in Figure 2, as the sheet thickness ratio R increases, both the lower and upper limits that E1 / E2 must satisfy increase, and a larger amount of heat is required.

[0040] It is also preferable that E1 / E2 is less than 0.9, which makes it possible to suppress initial rapid heat generation when there is a difference in tensile strength, and to perform stable welding.

[0041] The method of calculating the target heat generation amounts E1 and E2 is not particularly limited, but an example is disclosed in Patent Document 3, and the above method can be adopted in the present invention as well. The calculation procedure for the heat generation amount q per unit volume / unit time and the cumulative heat generation amount Q per unit volume according to the method disclosed in Patent Document 3 is as follows.

[0042] That is, let t be the total thickness of the materials to be welded, such as steel plates, r be the electrical resistivity of the materials to be welded, V be the voltage between the electrodes, I be the welding current, and S be the area of ​​contact between the electrodes and the materials to be welded. In this case, the welding current I passes through a columnar portion with a cross-sectional area of ​​S and thickness t, generating resistance heat. The amount of heat q per unit volume and unit time in this columnar portion can be calculated using the following equation (4). q=(V I) / (S t) (4)

[0043] Moreover, the electrical resistance R of the columnar portion is calculated by the following formula (5). R = (r t) / S (5)

[0044] By solving equation (5) for S and substituting this into equation (4), the amount of heat generated, q, is given by the following equation (6). q=(V I R) / (r t 2 )=(V 2 ) / (r t 2 ) (6)

[0045] As is clear from the above formula (6), the amount of heat generated per unit volume and unit time q can be calculated from the interelectrode voltage V, the total thickness t of the workpieces, and the electrical resistivity r of the workpieces, and is not affected by the contact area S between the electrode and the workpieces. Although formula (6) calculates the amount of heat generated q from the interelectrode voltage V, it can also be calculated from the interelectrode current I, and in this case, it is not necessary to use the contact area S between the electrode and the workpieces. Then, by accumulating the amount of heat generated q per unit volume and unit time over the current application period, the accumulated amount of heat generated per unit volume Q during welding can be obtained. As is clear from formula (6), this accumulated amount of heat generated per unit volume Q can also be calculated without using the contact area S between the electrode and the workpieces.

[0046] The pressure P set in the present invention may be appropriately set depending on the material and thickness of the steel sheets 1 and 2 constituting the sheet pair 3, which is the material to be welded. For example, when a sheet pair of two 270-2000 MPa class steel sheets, each having a thickness of 1.4 mm and a material that is either not plated or has a Zn-containing plated layer on its surface, is used as the material to be welded, the set pressure is preferably 1.0-7.0 kN. When a sheet pair of three 270-2000 MPa class steel sheets, each having a thickness of 1.4 mm and a material that is either not plated or has a Zn-containing plated layer on its surface, is used as the material to be welded, the set pressure is preferably 2.0 kN-10.0 kN.

[0047] The present invention performs adaptive control welding, in which welding is performed using the above-mentioned target value for the amount of heat generated as a reference, and if the time change in the instantaneous amount of heat generated per unit volume is along the reference time change curve, the welding is continued as is and the welding is terminated. However, if the time change in the instantaneous amount of heat generated per unit volume deviates from the reference time change curve, the amount of current is controlled so that the accumulated amount of heat generated per unit volume in the actual welding matches the accumulated amount of heat generated per unit volume set as the target value in order to compensate for the difference in the amount of heat generated within the remaining current flow time.

[0048] The desired nugget diameter of the nugget formed by the second-stage current flow need only be greater than or equal to what is required for production, but it is desirable to aim for a nugget diameter between the sheets that is greater than or equal to 4√t (mm), where t (mm) is the thickness of the thinner sheet of the two sheets that make up the gap.

[0049] In the present invention, it is preferable to perform test welding prior to actual welding by two or more stages of current application, and in the test welding, store a time-varying curve of the instantaneous heat generation amount per unit volume and a cumulative heat generation amount per unit volume calculated from the electrical characteristics between a pair of electrodes when a nugget of a predetermined diameter is formed by current application under constant current control, and in current application for actual welding, set the time-varying curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume in the test welding to target values, and control the current application amount according to the target values. This makes it possible to form a nugget of the desired diameter without causing expulsion.

[0050] (Method of manufacturing welded components) A method for producing a welded component according to the present invention is characterized in that a plurality of overlapping steel plates are joined by the above-described resistance spot welding method according to the present invention.

[0051] As described above, in the resistance spot welding method according to the present invention, the current is passed through two or more stages including a first stage current and a second stage current for forming a nugget, and the target heat generation amount E1 in the first stage current and the target heat generation amount E2 required for forming a nugget of a desired diameter in the second stage current are set so that E1 / E2 satisfies a requirement according to the sheet thickness ratio R of the steel sheets constituting the sheet assembly, and the current between the pair of electrodes is controlled while monitoring the electrical signal during welding. This makes it possible to form a nugget of a desired diameter even when disturbances exist. Therefore, by joining multiple overlapping steel sheets using the resistance spot welding method according to the present invention, a welded member having a nugget of a desired diameter and high weld strength can be manufactured. EXAMPLES

[0052] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0053] For the plate assemblies composed of the steel plates shown in Table 1, when there was no plate gap or when there was a plate gap, the main welding was performed under the conditions shown in Table 2 to produce welded joints. For each of the obtained welded joints, the welded part was cut, the cross section was etched, and then observed with an optical microscope to measure the nugget diameter. For the plate gap, for laboratory experiments, a spacer was inserted between the steel plates so that the gap between the steel plates at a point 25 mm away from the welding target position was 2 mm on both sides. The measured nugget diameter and the variation of the measured nugget diameter from the target nugget diameter were evaluated as follows. At that time, the target nugget diameter was compared with the variation when there was a plate gap, based on the case when there was no plate gap. The obtained evaluation results are shown in Table 2.

[0054] [Table 1]

[0055] [Table 2]

[0056] Diameter Judgment: ○ (Pass): The nugget diameter is larger than the target nugget diameter. × (Fail): The nugget diameter is smaller than the target nugget diameter.

[0057] Diameter Variation: ○ (Pass): The nugget diameter varies from the target nugget diameter by less than 0.3 × (plate thickness (mm)) square root (mm). × (Fail): The nugget diameter varies from the target nugget diameter by more than the square root (mm) of 0.3 × (plate thickness (mm)). Note that (plate thickness) here refers to the thickness (mm) of the thinnest steel plate in the plate assembly.

[0058] As is clear from Table 2, in all of the inventive examples, not only were the desired nugget diameters obtained, but the variation was small, regardless of whether or not there was a gap. On the other hand, in the comparative examples, a sufficient nugget diameter was not obtained, and the variation was large. [Industrial Applicability]

[0059] According to the present invention, even when disturbances exist, a nugget having a desired diameter can be stably formed. [Explanation of symbols]

[0060] 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, The energization has two or more stages including a first stage energization and a second stage energization for forming a nugget having a predetermined nugget diameter after the first stage energization, a target heat generation amount E1 in the first stage of current application, a target heat generation amount E2 necessary to form a nugget of the specified nugget diameter in the second stage of current application, a pressing force P [N], a maximum tensile strength difference ΔTS [MPa] of the steel sheets constituting the sheet combination, and a sheet thickness ratio R of the steel sheets constituting the sheet combination, E1 / E2 satisfying any of the following formulas (1) to (3), and controlling the current between the pair of electrodes while monitoring an electrical signal during welding. In the case of 2.0≦R≦2.5, (0.08+0.004×R)×(1+ΔTS / P)≦E1 / E2≦(0.4+0.02×R)×(1+ΔTS / P) (1) In the case where 2.5<R≦5, (0.07+0.008×R)×(1+ΔTS / P)≦E1 / E2≦(0.35+0.04×R)×(1+ΔTS / P) (2) In the case of 5<R≦10, (0.03+0.016×R)×(1+ΔTS / P)≦E1 / E2≦(0.25+0.06×R)×(1+ΔTS / P) (3)

2. A test welding is performed prior to the main welding by the two or more stages of current application, In the test welding, a time change curve of an instantaneous heat generation amount per unit volume and an accumulated heat generation amount per unit volume, which are calculated from an electrical characteristic between the pair of electrodes when a current is passed by constant current control to form a nugget having the predetermined nugget diameter, are stored; 2. The resistance spot welding method according to claim 1, wherein, in the current supply for the main welding, a time change curve of the instantaneous heat generation amount per unit volume and the accumulated heat generation amount per unit volume in the test welding are set to target values, and the current supply amount is controlled in accordance with the target values.

3. 3. The resistance spot welding method according to claim 1, wherein E1 / E2 is less than 0.

9.

4. 3. The resistance spot welding method according to claim 1, wherein ΔTS is 440 MPa or more.

5. A method for producing a welded component, comprising joining two or more overlapping steel plates by the resistance spot welding method according to claim 1 or 2 to obtain a welded component.

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