Resistance welding system and manufacturing method of resistance welding joint

Abstract

Provided are a resistance welding system and a method for manufacturing a resistance welded joint, capable of suppressing the occurrence of scattering during resistance welding that is adapted and controlled, and manufacturing a resistance welded joint having an optimal nugget diameter. A resistance welding system according to a first embodiment of the present invention is provided with a pair of electrodes, a power supply device, and a resistance welding control device having a pre-energizing unit and a main welding energizing unit. The main welding energization unit calculates the cumulative amount of heat per unit volume and unit time on the basis of the energization time of the main welding on the basis of the cumulative amount of heat per unit volume, which is obtained in advance and which enables good welding in the plate group. A pre-energizing unit that performs pre-energizing so as to satisfy I1gt by adjusting the inter-electrode resistance, the inter-electrode voltage, or the welding current, which generates the calculated cumulative heat generation per unit volume and unit time; i2, 750 * t < = (I1) 2 * T1 < = 1550 * t, and 10 < = T1 < = 50.

Description

Technical Field

[0001] The invention relates to a resistance welding system and a method for manufacturing a resistance welding joint. Background Art

[0002] Resistance welding is widely used for overlapping welding of steel plates. Resistance welding is a technique that uses electricity to weld the welded parts and uses the resistance heat generated by the electricity to weld the welded parts. Resistance spot welding refers to welding that is performed by clamping the overlapped welded parts with the tip of an appropriately polished electrode, concentrating the pressure on the tip of the electrode to pass current, thereby locally heating and welding, and then pressurizing and holding the electrode to perform welding.

[0003] The welding system used to implement this welding includes a pair of electrodes sandwiching the workpiece, a power supply device that allows the welding current to flow between the pair of electrodes, and a resistance welding control device that controls the power supply device. In order to achieve good welding, the Joule heating value must be adjusted to an appropriate value. If the Joule heating value is too high, it will scatter from the workpiece, and good welding cannot be performed in the workpiece. If it is too low, it will not be heated sufficiently, so poor welding will occur. The optimal Joule heating value is maintained by adjusting the welding current and the power-on time.

[0004] Various indicators have been proposed to evaluate whether the Joule heating value is too large or too small, and the welding current is usually used as an indicator. If the detected current is too low, the welding current is increased, and if it is too high, the welding current is reduced. In recent years, the cumulative heating value per unit volume that can be welded well in the welded workpiece, which is previously calculated, has been used as a control indicator for the Joule heating value. By adaptively controlling the Joule heating value based on the cumulative heating value per unit volume, stable and good weldability with less deviation can always be ensured regardless of the type of welded workpiece or the wear condition of the electrode tip.

[0005] For example, Patent Document 1 discloses a resistance spot welding method having a main welding process and a test welding process before the main welding process. In the test welding process, the change in the instantaneous calorific value per unit volume and the cumulative calorific value per unit volume are stored as target values. In the first power-on step of the main welding process, a current value that does not cause scattering is selected, and welding is performed by constant current control. After the second power-on step of the main welding process, welding is performed based on the change curve of the instantaneous calorific value per unit volume stored as the target value in the test welding process. In a certain power-on step, when the change in the instantaneous calorific value deviates from the change curve used as the reference, adaptive control is performed to control the welding current in such a way that the cumulative calorific value in the power-on step is consistent with the cumulative calorific value in the power-on step previously calculated in the test welding process.

[0006] In Patent Document 2, a resistance spot welding method for performing main welding and test welding is disclosed. In the test welding, it is set to a state where there is a gap of 0.2 to 2.0 mm in the overlapping surface of metal plates. On this basis, pre - energization and main welding are performed by constant - current control. In addition, the change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume calculated based on the inter - electrode voltage or inter - electrode resistance when the optimal nugget is formed respectively in this pre - energization and this main welding are stored. And, in the main welding, for the pre - energization and the main welding respectively, welding is performed based on the change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount stored in the pre - energization and the main welding of the test welding. In this pre - energization or this main welding, when the change amount of the instantaneous heat generation amount per unit volume deviates from the reference change curve, the welding current is controlled so that the cumulative heat generation amount per unit volume in this pre - energization or this main welding is respectively consistent with the cumulative heat generation amount per unit volume previously obtained in the pre - energization or the main welding of the test welding. When the welding current of the pre - energization of the test welding is set as I1 and the welding current of the main welding of the test welding is set as I2, the relationship of I1 < I2 is satisfied.

[0007] In Patent Document 3, a resistance spot welding method for performing main welding and test welding before the main welding is disclosed. The test welding is set to be performed under two or more welding conditions. In the test welding, for each welding condition, pre - energization by constant - current control is performed in the same energization mode, and the inter - electrode voltage or inter - electrode resistance at the time of this pre - energization is stored. In the main welding, energization is performed by constant - current control, and the change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume calculated based on the inter - electrode voltage or inter - electrode resistance when the optimal nugget is formed are stored. Further, in the main welding, pre - energization by constant - current control is performed in the same energization mode as the test welding, the inter - electrode voltage or inter - electrode resistance in this pre - energization is compared with the inter - electrode voltage or inter - electrode resistance stored in the pre - energization of the test welding for each welding condition, and the change curve of the instantaneous heat generation amount per unit volume and the cumulative heat generation amount per unit volume in the main welding of the test welding stored under the welding condition with the smallest difference are set as the target values of the main welding in the main welding. Then, as the main welding, adaptive control for controlling the welding current is performed according to this target value.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Publication No. 5999293

[0011] Patent Document 2: Japanese Patent Publication No. 6913062

[0012] Patent Document 3: Japanese Patent No. 6471841 Summary of the invention

[0013] Problems to be solved by the invention

[0014] When the Joule heating value is adaptively controlled based on the cumulative heating value per unit volume, the occurrence of scattering becomes a problem. Scattering is a phenomenon in which the welded parts are locally overheated and melted and scattered during overlap resistance welding, or the molten metal thereof. If scattering occurs, the melting depth of the weld is reduced, and the resistance of the weld drops sharply. As a result, the instantaneous heating value per unit volume of the weld is sharply reduced.

[0015] In the case where the resistance of the welded part decreases sharply due to scattering, if the adaptive control of the Joule heating value based on the cumulative heating value per unit volume is continued, the resistance welding control device increases the Joule heating value sharply to compensate for the decrease in instantaneous heating value. However, such excessive heat input may cause excessive indentations on the plate surface, weld nubs, blowholes, adhesion and other welding defects.

[0016] In addition, scattering also has an adverse effect on post-energization. In post-energization, current is passed through the hardened weld to perform heat treatment such as tempering. As a result, various mechanical strength characteristics of the weld can be improved. However, when scattering occurs during welding, the post-energization does not function properly due to the rapid temperature drop caused by the scattering, and sometimes the improvement in joint strength brought about by the post-energization cannot be expected.

[0017] Patent Documents 1 to 3 do not investigate these phenomena.

[0018] As one of the methods for avoiding this phenomenon, sometimes, when the occurrence of scattering is detected, the adaptive control using the appropriate instantaneous heat value per unit volume and the cumulative heat value is temporarily suspended and switched to constant current control in which a certain welding current that enables good welding is passed. If such a control logic is set, even if scattering occurs, poor welding can be avoided and a good nugget can be obtained. However, in order to maximize the effect of the adaptive control based on the Joule heat value, it is preferable to reduce the chance of switching the adaptive control based on the Joule heat value to the constant current control as much as possible.

[0019] In view of the above situation, an object of the present invention is to provide a resistance welding system and a method for manufacturing a resistance welding joint that suppress the occurrence of scattering in adaptively controlled resistance welding and can manufacture a resistance welding joint with an optimal weld nugget diameter and improved joint strength.

[0020] Means used to solve problems

[0021] The gist of the present invention is as follows.

[0022] (1) A resistance welding system according to a first embodiment of the present invention is a resistance welding system for performing resistance spot welding on a plate group of two or more superimposed steel plates, the resistance welding system comprising: a pair of electrodes sandwiching the plate group; a power supply device for causing current to flow between the pair of electrodes; and a resistance welding control device having a pre-energizing unit for pre-energizing the pair of electrodes, and a main welding energizing unit for performing main welding at the pair of electrodes after the pre-energizing, the resistance welding control device controlling the power supply device with respect to welding current and energizing time; the main welding energizing unit calculates instantaneous calorific value per unit volume and per unit time based on the energizing time of the main welding according to the cumulative calorific value per unit volume that enables good welding of the plate group obtained in advance, and adjusts the inter-electrode resistance, inter-electrode voltage, or welding current that generates the instantaneous calorific value per unit volume and per unit time calculated above, thereby adaptively controlling the main welding; the pre-energizing unit performs the pre-energizing in a manner that satisfies Formulas 1, 2, and 3.

[0023] I1>I2…(Formula 1)

[0024] 750×t≤(I1) 2 ×T1≤1550×t…(Formula 2)

[0025] 10≤T1≤50…(Formula 3)

[0026] Here, in the above-mentioned Formula 1, Formula 2 and Formula 3, I1 is the preheating current (kA) flowing into the above-mentioned plate group during the above-mentioned pre-energization; I2 is the above-mentioned constant current controlled welding current (kA) when the above-mentioned cumulative heat generation per unit volume is obtained by flowing the constant current controlled welding current into the above-mentioned plate group with the same energization time as the above-mentioned main welding; T1 is the time when the above-mentioned preheating current flows, that is, the preheating energization time (msec); t is the total plate thickness (mm) of the above-mentioned steel plates contained in the above-mentioned plate group.

[0027] (2) In the resistance welding system described in (1) above, preferably, the steel plates included in the plate group are three overlapping plates; one of the steel plates is a thin plate with a thickness of less than 0.8 mm; two of the steel plates are thick plates with a thickness of more than 1.0 mm; the thin plates are arranged on the surface of the plate group; and a total thickness ratio calculated by dividing the total thickness (mm) of the steel plates included in the plate group by the minimum value of the thickness of the steel plates arranged on the surface of the plate group is 3.5 or more.

[0028] (3) The resistance welding system described in (1) or (2) above preferably further satisfies Expression 4.

[0029] I2+0.3×t<I max <I2+0.8×t…(Formula 4)

[0030] Here, in the above formula 4, I max It is the maximum value (kA) of the welding current in the main welding.

[0031] (4) The resistance welding system described in any one of (1) to (3) above preferably further satisfies Expression 5 in consideration of interference such as a plate gap and a flow splitting condition.

[0032] 30 / h<K<100 / h…(Formula 5)

[0033] Here, in the above formula 5, K is the maximum value (kA / sec) of the rate of change of the above welding current in the above main welding; h is the total plate thickness ratio calculated by dividing the above total plate thickness (mm) of the above steel plates included in the above plate group by the minimum value of the plate thickness of the above steel plates arranged on the surface of the above plate group.

[0034] (5) In the resistance welding system described in any one of (1) to (4) above, preferably, at least one of the steel plates included in the plate group is a high-strength steel plate having a tensile strength of 980 MPa or more; the resistance welding control device further includes a post-energization unit for performing post-energization on the pair of electrodes after the main welding; the post-energization unit performs the post-energization in a manner that satisfies Equations 6 and 7.

[0035] I2×0.5≤I3≤I2×1.2…(Formula 6)

[0036] 200≤T3≤2000…(Formula 7)

[0037] Here, in the above equations 6 and 7, I3 is the post-thermal current (kA) flowing into the plate group during the post-energization; T3 is the time during which the post-thermal current flows, that is, the post-thermal energization time (msec).

[0038] (6) A method for manufacturing a resistance welded joint according to a second embodiment of the present invention comprises the following steps: pre-energizing a pair of electrodes sandwiching a plate group of two or more stacked steel plates; and, following the pre-energizing, performing main welding at the pair of electrodes; calculating the instantaneous heat generation per unit volume and per unit time based on the energizing time of the main welding according to the cumulative heat generation per unit volume that enables good welding in the plate group obtained in advance, and adjusting the inter-electrode resistance, inter-electrode voltage or welding current that generates the instantaneous heat generation per unit volume and per unit time calculated above, thereby adaptively controlling the main welding; and performing the pre-energizing in a manner that satisfies Formulas 1, 2 and 3.

[0039] I1>I2…(Formula 1)

[0040] 750×t≤(I1) 2 ×T1≤1550×t…(Formula 2)

[0041] 10≤T1≤50…(Formula 3)

[0042] Here, in the above-mentioned Formula 1, Formula 2 and Formula 3, I1 is the preheating current (kA) flowing into the above-mentioned plate group during the above-mentioned pre-energization; I2 is the above-mentioned constant current controlled welding current (kA) when constant current controlled welding is performed in the above-mentioned plate group with the same energization time as the above-mentioned main welding to obtain the above-mentioned cumulative heat generation per unit volume; T1 is the time when the above-mentioned preheating current flows, that is, the preheating energization time (msec); t is the total plate thickness (mm) of the above-mentioned steel plates contained in the above-mentioned plate group.

[0043] (7) In the method for manufacturing a resistance welded joint described in (6) above, it is preferred that the steel plates included in the plate group are three overlapping plates; one of the steel plates is a thin plate having a thickness of less than 0.8 mm; two of the steel plates are thick plates having a thickness of more than 1.0 mm; the thin plates are arranged on the surface of the plate group; and a total thickness ratio calculated by dividing the total thickness (mm) of the steel plates included in the plate group by the minimum value of the thickness of the steel plates arranged on the surface of the plate group is greater than 3.5.

[0044] (8) The method for manufacturing a resistance welded joint described in (6) or (7) above preferably further satisfies Expression 4.

[0045] I2+0.3×t<I max <I2+0.8×t…(Formula 4)

[0046] Here, in the above formula 4, I max It is the maximum value (kA) of the welding current in the main welding.

[0047] (9) The method for manufacturing a resistance welded joint according to any one of (6) to (8) above preferably further satisfies Expression 5 in consideration of interference such as a plate gap and a flow splitting condition.

[0048] 30 / h<K<100 / h…(Formula 5)

[0049] Here, in the above formula 5, K is the maximum value (kA / sec) of the rate of change of the above welding current in the above main welding; h is the total plate thickness ratio calculated by dividing the above total plate thickness (mm) of the above steel plates included in the above plate group by the minimum value of the plate thickness of the above steel plates arranged on the surface of the above plate group.

[0050] (10) In the method for manufacturing a resistance welded joint described in any one of (6) to (9) above, it is preferred that at least one of the steel plates included in the plate group is a high-strength steel plate having a tensile strength of 980 MPa or more; and the method also includes a step of performing post-energization on a pair of the electrodes after the main welding; and the post-energization is performed in a manner that satisfies Equations 6 and 7.

[0051] I2×0.5≤I3≤I2×1.2…(Formula 6)

[0052] 200≤T3≤2000…(Formula 7)

[0053] Here, in the above equations 6 and 7, I3 is the post-thermal current (kA) flowing to the plate group during the post-energization; T3 is the time during which the post-thermal current flows, that is, the post-thermal energization time (msec).

[0054] Effects of the Invention

[0055] According to the present invention, it is possible to provide a resistance welding system and a method for manufacturing a resistance welded joint that suppress the occurrence of spattering during adaptively controlled resistance welding and that can manufacture a resistance welded joint having an optimal nugget diameter. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is a schematic diagram of an example of the resistance welding system according to the first embodiment.

[0057] Figure 2 This is an example of a diagram (weldability diagram, weld lobe) showing the relationship between current and nugget diameter.

[0058] Figure 3 This is a conceptual diagram used to explain the mechanism of scattering.

[0059] Figure 4 It is a schematic diagram of resistance welding of a plate group with a plate gap.

[0060] Figure 5 This is a conceptual diagram of impurity removal by pre-energization. DETAILED DESCRIPTION

[0061] (1. Resistance welding system)

[0062] like Figure 1As illustrated, the resistance welding system of the first embodiment of the present invention is a resistance welding system for performing resistance spot welding on a plate group 60 of two or more superimposed steel plates, and comprises: a pair of electrodes 51, 52, sandwiching the plate group 60; a power supply device 10, allowing current to flow between the pair of electrodes 51, 52; and a resistance welding control device 20, having a pre-energizing section for performing pre-energization S1 on the pair of electrodes 51, 52 and a main welding energizing section for performing main welding S2 in the pair of electrodes 51, 52 after the pre-energization S1, and the resistance welding control device 20 is used for welding. The power supply device 10 is controlled by the current and the power-on time. For the main welding S2, the main welding power-on unit calculates the instantaneous heat value q per unit volume and per unit time based on the cumulative heat value per unit volume that can perform good welding in the plate group 60 that is calculated in advance, and the inter-electrode resistance, inter-electrode voltage or welding current that generates the calculated instantaneous heat value q per unit volume and per unit time are adjusted to perform adaptive control. The pre-power-on unit performs pre-power-on S1 to satisfy Formulas 1, 2 and 3.

[0063] I1>I2…(Formula 1)

[0064] 750×t≤(I1) 2 ×T1≤1550×t…(Formula 2)

[0065] 10≤T1≤50…(Formula 3)

[0066] Here, in Formula 1, Formula 2 and Formula 3, I1 is the preheating current (kA) flowing into the plate group 60 in the pre-power-on S1, I2 is the constant current controlled welding current (kA) when the constant current controlled welding current flows into the plate group 60 for the same power-on time as the main welding S2 to obtain the cumulative heat generation Q per unit volume, T1 is the time for the preheating current to flow, that is, the preheating power-on time (msec), and t is the total plate thickness (mm) of the steel plates contained in the plate group 60.

[0067] (Plate set 60)

[0068] The resistance welding system according to the first embodiment is a system for performing resistance spot welding on a plate group 60 in which two or more steel plates are stacked. The steel plates included in the plate group 60 may be stacked in three or more sheets. In addition, there may be a slight gap between the steel plates included in the plate group 60. The gap between the steel plates is called a plate gap.

[0069] (Electrodes 51, 52, Power Supply Device 10 and Resistance Welding Control Device 20)

[0070] The resistance welding system of the first embodiment includes: a pair of electrodes 51, 52 sandwiching a plate group 60; a power supply device 10 that allows current to flow between the pair of electrodes 51, 52; and a resistance welding control device 20 that controls the power supplied from the power supply device 10 according to the welding current and the power-on time. The electrodes 51, 52 are so-called resistance spot welding electrodes, that is, rod-shaped electrodes that directly contact the plate group 60 during resistance spot welding to pass the welding current and transmit the pressure. The power supply device 10 can be assembled into the resistance welding system or can be a commercial power supply connected to the resistance welding device for use.

[0071] (Pre-energization part and main welding energization part)

[0072] The resistance welding control device 20 includes: a pre-energizing unit that performs pre-energizing S1 on a pair of electrodes 51 and 52; and a main welding energizing unit that performs main welding S2 in an adaptively controlled manner on the pair of electrodes 51 and 52 after the pre-energizing S1. For the convenience of explanation, the main welding S2 performed by the main welding energizing unit will be explained first, and then the pre-energizing S1 performed by the pre-energizing unit will be explained.

[0073] (Main welding S2)

[0074] In the main welding S2, a high current is passed through the plate group 60 while applying pressure, and the plate group 60 is welded by resistance heat. Through the main welding S2 and the subsequent cooling, a nugget is formed in the plate group 60. The nugget joins the plurality of steel plates included in the plate group 60. In the main welding S2, the current passed through the plate group 60 to form the nugget is called the welding current, and the time during which the welding current flows is called the energization time.

[0075] Adaptive control is performed on the main welding S2. Adaptive control is a control that changes the characteristics of the control system in a manner that satisfies the required conditions according to the characteristics of the control object / environmental disturbances. In the main welding S2 of the resistance welding system related to the first embodiment, the cumulative calorific value Q per unit volume is used as an indicator of adaptive control. The Joule heating value (for example, the inter-electrode resistance and inter-electrode voltage, or the welding current, the power-on time) in the main welding S2 is adjusted so that the cumulative calorific value Q per unit volume becomes a specified value.

[0076] The cumulative calorific value Q per unit volume in main welding S2 is a value obtained by integrating the value q calculated by the following formula A during the period from the start to the end of main welding S2.

[0077] q=(V×I) / (S×t)…Formula A

[0078] In formula A, V is the voltage between electrodes, I is the welding current, S is the contact area between electrodes 51, 52 and plate group 60, and t is the total plate thickness (mm) of the steel plates included in plate group 60. The value q obtained by formula A is the instantaneous heat generation q per unit volume and unit time. This is because the welding current generates resistance heating when it passes through the columnar portion of the total plate thickness t with the contact area S.

[0079] In addition, if formula B is used to rewrite formula A, it becomes formula C.

[0080] R=(r×t) / S…Formula B

[0081] q=(V×I×R) / (r×t 2 )

[0082] =(V 2 ) / (r×t 2 )…Formula C

[0083] In Formula B, r is the resistivity of the welded material. R obtained from Formula B is the resistance of the columnar portion of the total plate thickness t when the contact area is S.

[0084] According to Formula C, the instantaneous heat generation q per unit volume and per unit time can be calculated based on the inter-electrode voltage V, the total plate thickness t of the welded parts, and the resistivity r of the welded parts, and is not affected by the contact area S between the electrodes 51, 52 and the plate group 60. If resistance spot welding is repeated, the electrode tips of the electrodes 51, 52 may wear and S may change, but the instantaneous heat generation q per unit volume and per unit time is not affected by the wear of the electrode tips of the electrodes 51, 52. In addition, Formula C calculates the instantaneous heat generation q based on the inter-electrode voltage V, but the instantaneous heat generation q can also be calculated based on the welding current I. In this case, it is also unnecessary to use the contact area S between the electrodes 51, 52 and the plate group 60. On the other hand, since the resistivity of the welded parts, i.e., the steel plates, changes according to the temperature of the steel plates, q is not constant even if I and V are fixed values ​​throughout the power-on time of the main welding S2. In addition, if scattering E occurs, the total plate thickness t of the steel plates at the locations clamped by the electrodes 51, 52 is greatly reduced.

[0085] When the instantaneous heat generation q per unit volume / unit time is integrated during the energization time of the main welding S2 , the integrated heat generation per unit volume applied to welding is obtained as Q. The integrated heat generation per unit volume Q can be calculated without using the contact area S between the electrodes 51 , 52 and the plate assembly 60 .

[0086] In the resistance welding system related to the first embodiment, the cumulative calorific value Q per unit volume is used as an indicator of adaptive control. Specifically, the cumulative calorific value Q that enables good welding in the plate group 60 is first calculated. Next, based on the cumulative calorific value Q and the power-on time of the main welding S2, the instantaneous calorific value q per unit volume and per unit time that can reproduce the cumulative calorific value Q is calculated. Then, the Joule calorific value is adjusted in the main welding S2 to obtain the instantaneous calorific value q per unit volume and per unit time. The predetermined instantaneous calorific value q per unit volume and per unit time can also be achieved by adjusting the welding current. In addition, the predetermined instantaneous calorific value q per unit volume and per unit time can also be achieved by adjusting the inter-electrode resistance or the inter-electrode voltage.

[0087] The method for determining the cumulative calorific value Q that enables good welding in the plate group 60 is not limited. The cumulative calorific value Q can be determined by simulation. In addition, the cumulative calorific value Q can also be determined by test welding. Test welding is welding performed before welding (main welding) to obtain the resistance welded joint to be manufactured to explore welding conditions. An example of test welding is described below.

[0088] In the test welding, first, a plate group identical to the plate group 60 to be welded is prepared. In the plate group for test welding, interference such as plate gaps and shunts (tributaries) is suppressed as much as possible. Next, the plate group for test welding is pre-energized and welded. For pre-energization and welding, for example, constant current control is performed. The preheating energization time in the pre-energization of the test welding is set to a value in the range of approximately 15 to 33 msec, for example. The preheating current in the pre-energization of the test welding is made as large as possible within a range where scattering does not occur. The energization time in the test welding is set according to the total plate thickness t (mm) of the steel plates included in the plate group. For example, the energization time in the test welding can be set to approximately (t÷2)×10×20 msec. However, these matters are merely benchmarks for setting the preheating energization time, preheating current, and energization time. The preheating energization time, preheating current, and energization time in the test welding can be appropriately set according to the properties of the plate group and the desired resistance welded joint.

[0089] The preheating power-on time, preheating current, main welding current, and power-on time are set to be constant, and the welding current during welding is changed in various ways while performing multiple test welding. Then, the diameter of the nugget obtained by the resistance spot welding is measured. Thus, a graph showing the relationship between the welding current and the nugget diameter (weld lobe weldability graph) is prepared. An example of a weldability graph is shown in Figure 2 middle. Figure 2 This is a weldability diagram of a plate set consisting of one thin plate and two thick plates. In addition, the thin plate is configured as the outer plate of the plate set. The detailed plate set and detailed welding conditions of the thin plate and thick plate are shown in Table 1 below.

[0090] [Table 1]

[0091] Figure 2 Preparation conditions of solderability diagram

[0092] sheet 270Mpa grade alloyed hot-dip galvanized steel sheet thickness 0.6mm thick plate 980Mpa grade alloyed hot-dip galvanized steel sheet thickness 1.6mm × 2 pieces overlap Plate Gap none Add pressure 3.8kN Warm-up power-on time 1 cycle (power frequency 60Hz. Approximately 17msec) Preheating current 13kA Preheating current control method Constant current control Power-on time 22 cycles (power frequency 60Hz. Approximately 367msec) Welding current range 5~8kA Current control method Constant current control

[0093] Figure 2 The dashed curve in the middle shows the relationship between the core diameter and welding current measured at the overlapped surface of the thin plate and the thick plate. Figure 2 The solid line curve represents the relationship between the diameter of the weld nugget measured at the overlapping surface of two thick plates and the welding current. Figure 2 The dotted line marked as "4√t (1.6mm)" in the figure represents the qualified reference value of the diameter of the weld nugget at the overlapping surface of the two thick plates. Figure 2 The dotted line "4√t (0.6mm)" in the figure indicates the acceptable reference value of the nugget diameter at the overlapped surface of the thin plate and the thick plate. 4√t is an indicator of the acceptable reference value of the nugget diameter in general, where t is the thickness of the thinner steel plate of the two steel plates constituting the overlapped surface where the nugget diameter is measured. "4√t (1.6mm)" is approximately 5.1mm, and "4√t (0.6mm)" is approximately 3.1mm. Figure 2 The hollow data points correspond to the weld nuggets, which are scattered during welding.

[0094] according to Figure 2 It can be assumed from the weldability diagram that by setting the welding current to 6.5 kA or more, the nugget diameter at the overlapping surface of two thick plates and the nugget diameter at the overlapping surface of a thin plate and a thick plate can both be within the acceptable range. Figure 2 It can be assumed from the weldability diagram that by setting the welding current to 7.3 kA or less, the occurrence of scattering can be suppressed. Figure 2 For the plate group corresponding to the weldability diagram, it can be judged that the welding current in the appropriate welding is in the range of 6.5 to 7.3 kA, and the optimal welding current is 7.0 kA.

[0095] Next, the cumulative current Q per unit volume achieved by controlling the welding current in an appropriate welding current range (i.e., 6.5 to 7.3 kA. The optimal welding current is 7.0 kA) is calculated. For example, the inter-electrode voltage or inter-electrode resistance is measured at the welding current of the test welding, and by substituting them into the above-mentioned formula C, the instantaneous heat generation q per unit volume and per unit time and the cumulative heat generation Q per unit volume can be calculated. The cumulative current Q per unit volume thus obtained can be used in the main welding as the "preliminarily calculated cumulative heat generation Q per unit volume that enables good welding in the plate group 60".

[0096] (Pre-power on S1)

[0097] In the pre-energization S1 performed before the main welding S2, current is passed through the pair of electrodes 51 and 52 sandwiching the plate group 60, and the plate group 60 is preheated by resistance heat. The purpose of the pre-energization S1 is to preheat the plate group 60, and it is not necessary to melt the plate group 60. However, it is allowed that the plate group 60 is slightly melted. The current flowing into the plate group 60 in the pre-energization S1 is called the preheating current, and the time when the preheating current flows is called the preheating energization time.

[0098] Unlike the adaptively controlled main welding S2, the pre-energization S1 may not be adaptively controlled. For example, the pre-energization S1 may also be controlled by constant current. Here, "constant current control" includes:

[0099] (1) The power source is a DC power source and the current is controlled to be constant; and

[0100] (2) Make the power source an AC power source and control the effective current to a certain

[0101] The concept of both.

[0102] On the other hand, in the case of a DC type, in addition to constant current control, the pre-energization S1 also includes the concepts of always keeping the current constant and controlling the current waveform to keep the effective current constant. For example, the current waveform of the pre-energization S1 can be a sine waveform, a square waveform, or a non-sinusoidal waveform (triangular waveform, oblique waveform, Sinc waveform).

[0103] Furthermore, pre-current S1 is performed so as to satisfy Expressions 1, 2, and 3.

[0104] I1>I2…(Formula 1)

[0105] 750×t≤(I1) 2 ×T1≤1550×t…(Formula 2)

[0106] 10≤T1≤50…(Formula 3)

[0107] Here, the definitions of symbols included in Formula 1, Formula 2, and Formula 3 are as follows.

[0108] I1: preheating current flowing to the plate group 60 in the pre-energization S1 (kA)

[0109] I2: welding current (kA) of the main welding S2 controlled by constant current when the plate group 60 is subjected to the main welding S2 controlled by constant current for the same energization time as the main welding S2 to obtain the cumulative calorific value Q per unit volume

[0110] T1: The time during which the preheating current flows, i.e. the preheating power-on time (msec)

[0111] t: Total thickness of the steel plates included in the plate group 60 (mm)

[0112] In addition, when the power supply used in the pre-energization S1 is an AC current, I1 in Formula 1 is the peak current value during a period in which the effective current is constant, and I1 in Formula 2 is the execution current. In addition, when the power supply used in welding (test welding or welding simulation) for obtaining I2 is an AC power supply, I2 in Formula 1 is the peak current value during a period in which the effective current is constant, and I2 in Formula 2 is the effective current.

[0113] When the welding power source used in the pre-energization S1 is an AC type, it is difficult to control the current every 1 msec like a DC type, and the welding power source controls the current every 0.5 cyc. Therefore, I1 in Formula 2 is not a peak current value but an effective current value.

[0114] When the welding power source used in the welding (test welding or welding simulation) for obtaining I2 is an AC type, I2 is an effective current value including a peak current value.

[0115] In many cases, the pre-energization S1 is a short-time high current control of about 1 cyc. When the welding power source is a DC type, since the current can be controlled every 1 msec, the peak current value can be specified. However, when the welding power source is an AC type, the current can sometimes only be controlled every 0.5 cyc. Therefore, it is preferable to specify the effective current value instead of the peak current value.

[0116] When the current value or execution current in the pre-energization S1 is not constant, I1 in Formula 1 is the maximum current value (peak current) in the pre-energization, and I1 in Formula 2 is the average current value (effective current) in the pre-energization.

[0117] I2 will be described in detail. I2 is a value corresponding to the reference value of the adaptive control used in the above-mentioned main welding S2, "the cumulative heating value Q per unit volume that enables good welding in the plate group 60 that is previously determined". Under the conditions that the welding current is set to I2 and the power-on time of the main welding is set to the same as the main welding S2 based on the adaptive control, the cumulative heating value Q per unit volume obtained when the main welding S2 based on the constant current control is performed on the plate group 60 is substantially consistent with the cumulative heating value Q per unit volume that enables good welding in the plate group 60 that is previously determined.

[0118] When the cumulative calorific value Q per unit volume that enables good welding in the plate group 60 is determined in advance through test welding, the appropriate welding current obtained through the test welding can be used as I2. Figure 2 In the main welding in which the power-on time of the test welding described above is approximately 367 msec and the welding current is determined to be appropriate within the range of 6.5 to 7.3 kA, and 7 kA is particularly preferred. Based on the appropriate welding current obtained in the test welding, the cumulative calorific value Q per unit volume that enables good welding in the plate group 60, which is determined in advance, is determined. The appropriate welding current obtained in the test welding can be used as I2.

[0119] The resistance welding system according to the first embodiment is a system that performs the main welding S2 by adaptive control based on the cumulative calorific value Q per unit volume. Therefore, the welding current in the main welding S2 is usually not constant. On the other hand, I2 is the welding current when it is assumed that the main welding is performed by constant current control to obtain the same calorific value as the main welding S2. That is, I2 is a value that serves as an index of the welding current in the main welding S2.

[0120] According to Formula 1, the preheating current I1 in the preheating S1 is larger than I2. The resistance welding system according to the first embodiment is a system that performs the preheating S1 with a current substantially larger than that of the main welding S2. According to Formula 2, the value (I1) as an index of the amount of heat generated in the preheating S1 is 2 ×T1 is limited to a range corresponding to the thickness of the plate group 60. In addition, according to Formula 3, the preheating energization time T1 in the pre-energization S1 is set to a very short value. If Formulas 1 to 3 are considered comprehensively, it can be said that the resistance welding system according to the first embodiment can perform the pre-energization S1 at a high current and in a short time.

[0121] exist Figure 1 A specific example of the resistance welding system described above is shown in FIG. Figure 1 The overall structure of the resistance welding system is shown. Here, the resistance welding system is connected to a power supply device 10 for use. Figure 1 Reference numeral 20 in the drawing denotes a resistance welding control device. Figure 1 The resistance welding control device 20 illustrated in FIG. 2 includes a power element 26 , an interface 25 , a CPU 23 , an A / D converter 24 , a ROM 21 , and a RAM 22 .

[0122] The power element 26 adjusts the welding current by adjusting the trigger angle (conduction angle) or the duty cycle of the primary voltage. The welding current obtained after the current is adjusted by the power element 26 is stepped down to a low voltage and high current through the welding transformer 30 (including a rectifier), and the current is passed between the pair of electrodes 51 and 52. A plate group 60 as a workpiece to be welded is sandwiched between the pair of electrodes 51 and 52. The plate group 60 is obtained by stacking two or more steel plates.

[0123] The clamping pressure between the pair of electrodes 51 and 52 is adjusted by a clamping pressure control mechanism (not shown). The clamping pressure control mechanism is, for example, a cylinder or a servo motor. A high current is passed through the plate group 60 clamped between the pair of electrodes 51 and 52, and the welded parts themselves undergo resistance heating. The plate group 60 is welded by the resistance heating.

[0124] The inter-electrode voltage between the pair of electrodes 51 and 52 is measured by the inter-electrode voltage detection circuit 40, and the measured value is converted by the A / D converter 24 and input to the CPU 23. The CPU 23 operates according to the control program stored in the ROM 21, controls the trigger angle of the power element 26 or the duty ratio of the primary voltage, and adjusts the heat generation per unit time (in this case, per cycle or per msec) and per unit volume. Figure 1 Reference numeral 25 indicates an interface 25 for turning on a power element 26 by a gate signal output from the CPU 23. The RAM 22 is used for temporarily storing various data. The ROM 21 stores a control program for executing the pre-energization S1 and the main welding S2.

[0125] (Effect)

[0126] In the resistance welding system of the first embodiment, in the main welding S2, the Joule heating value is adjusted with the cumulative heating value Q per unit volume (the cumulative heating value during the conduction of the welding current) as an index. As a result, stable welding can always be performed regardless of the interference state of the plate group 60, which is the workpiece to be welded, and the wear condition of the electrode tips of the electrodes 51 and 52. In other words, the cumulative heating value Q per unit volume that enables good welding with less welding deviation is not affected by the interference state of the workpiece to be welded and the wear condition of the tip of the electrodes 51 and 52. Generally, the contact area of ​​the electrodes 51 and 52 with the workpiece to be welded changes according to the wear condition of the electrodes 51 and 52. Therefore, it is conceivable that it is difficult to calculate the instantaneous heating value per unit volume. If the calculation is actually performed, the phenomenon that the conduction current changes due to the change in the contact area of ​​the electrodes 51 and 52 with the workpiece to be welded and the phenomenon that the Joule heating value also changes accordingly cancel each other out, and the instantaneous heating value per unit volume can be calculated even without considering the contact area of ​​the electrodes 51 and 52 with the workpiece to be welded. According to the resistance welding system related to the first embodiment, regardless of the interference state of the workpiece to be welded or the wear condition of the tip of the electrodes 51 and 52, the accumulated calorific value Q per unit volume that enables good welding is always used as an indicator, and the Joule calorific value is adjusted based on it, so that good welding can be performed.

[0127] However, if scattering E occurs during the main welding S2, it becomes difficult to continue the adaptive control of the main welding S2. If scattering E occurs, the melting depth of the weld portion decreases, and the resistance value of the weld portion drops sharply. As a result, the instantaneous heating value per unit volume of the weld portion decreases sharply. In the case where the resistance value of the weld portion decreases sharply due to scattering E, if the adaptive control of the Joule heating value based on the cumulative heating value Q per unit volume is continued, the resistance welding control device 20 of the resistance welding system increases the Joule heating value sharply in order to compensate for the reduction in instantaneous heating value. However, such excessive heat input may cause excessive indentation on the plate surface, weld nodules, pores, adhesion, etc., and excessive heat input may cause poor welding.

[0128] Therefore, the resistance welding system according to the first embodiment has a pre-energizing unit for pre-energizing S1 before main welding S2. By pre-energizing S1, impurity layers such as oxide films are removed, the crimping portion P is enlarged, and the occurrence of scattering E in main welding S2 is suppressed.

[0129] exist Figure 3 A conceptual diagram for explaining the mechanism of occurrence of scattered E is shown in FIG. Figure 3 The growth of the molten zone M during the main welding S2 is shown. Figure 3The elliptical region surrounded by a solid line provided on the superimposed surface of the two steel plates is a molten portion M. The elliptical region surrounded by a dotted line is a crimping portion P. The crimping portion P is a region pressed by the pair of electrodes 51 , 52 .

[0130] The further the main welding S2 progresses downward, the more the temperature of the weld increases upward. Therefore, the molten portion M grows as the main welding S2 progresses. In addition, the crimping portion P also grows as the main welding S2 progresses. This is because the steel plate softens and deforms due to the temperature rise of the weld and its surroundings. As long as the molten portion M is inside the crimping portion P, the molten metal is sealed in the steel plate and scattering E does not occur. However, if the molten portion M grows to the outside of the crimping portion P, the molten metal scatters and scattering E occurs. In addition, in the case where the plate group 60 has a plate gap, scattering E is more likely to occur. This is because, as Figure 4 As shown, if there is a plate gap, the crimping portion P becomes smaller.

[0131] Therefore, the resistance welding system according to the first embodiment performs pre-energization S1 before main welding S2. The pre-energization S1 softens and deforms the steel sheet. As a result, the crimping portion P is enlarged, and the molten portion M is prevented from growing outside the crimping portion P.

[0132] Furthermore, the short-time high-current pre-energization S1 also has the effect of removing the impurity layer such as the oxide film layer on the surface of the steel plate. The impurity layer is an oxide film with a thickness of about several nanometers. If the impurity layer is removed, the new surface of the steel plate is exposed. When the new surfaces are in contact with each other, the melting temperature is reduced to 1 / 3, the influence of interference is reduced, and an environment that is easy to weld can be created.

[0133] exist Figure 5 2 shows a conceptual diagram of impurity removal by pre-energization S1. By pre-energization S1 with a short time and high current, the impurity layer 602 on the surface of the steel plate 601 is destroyed to form a new surface 603. Thus, a through-circuit is formed. By forming a through-circuit, the HAZ area is widened and scattering is suppressed.

[0134] However, if the plate gap is removed by the usual pre-energization S1, the nugget diameter may not be ensured. If the pre-energization S1 is performed, the contact resistance between the steel plates and the contact resistance between the electrodes 51 and 52 and the steel plates are reduced. If the contact resistance is reduced, the heat generation is reduced, and the growth of the nugget is suppressed.

[0135] This problem is particularly significant when the total plate thickness ratio (a value calculated by dividing the total plate thickness (mm) of the steel plates included in the plate group 60 by the minimum plate thickness of the steel plates arranged on the surface of the plate group 60) is large. In the plate group 60 with a large total plate thickness ratio, thin steel plates are arranged on the surface of the plate group 60. If such a plate group 60 is preheated, the contact resistance between the thin steel plates arranged on the surface of the plate group 60 and the steel plates in contact with it decreases. As a result, the heat generation at the superimposed surface of the thin plate and the steel plate in contact with it decreases, and there is a possibility that the thin plate will be poorly joined.

[0136] Therefore, in the resistance welding system according to the first embodiment, the pre-energization S1 is performed so as to satisfy the formula (1).

[0137] I1>I2…(Formula 1)

[0138] Generally speaking, the preheating current is smaller than the welding current. For example, in JIS Z3001-6:2013 "Welding Terminology - Part 6: Resistance Welding" Figure 2 In the example of the resistance spot welding sequence illustrated in , the preheating current is set to a value smaller than the welding current. On the other hand, the preheating current I1 in the pre-energization S1 satisfying Formula 1 is a value substantially larger than the welding current in the main welding S2. By setting the preheating current to such a large value, the preheating energization time can be shortened. By passing the preheating current under the conditions of a short time and a high current, it is possible to suppress the occurrence of scattering while suppressing the decrease in contact resistance and ensure the nugget diameter. As a result, the effect of improving the joint strength (especially CTS) can also be obtained.

[0139] However, if the pre-energization S1 with a high current is continued for a long time, there is a possibility that scattering E will occur during the pre-energization S1. If scattering E occurs during the pre-energization S1, the plate thickness of the portion sandwiched by the electrodes 51 and 52 will be significantly reduced, and the appropriate cumulative calorific value per unit volume Q will change for this portion. Therefore, it becomes difficult to appropriately perform the main welding S2.

[0140] Therefore, the resistance welding system according to the first embodiment performs the pre-energization S1 so as to satisfy Expressions 2 and 3.

[0141] 750×t≤(I1) 2 ×T1≤1550×t…(Formula 2)

[0142] 10≤T1≤50…(Formula 3)

[0143] By performing the pre-energization S1 in a manner satisfying Formula 2 and Formula 3, the occurrence of scattering E in the pre-energization S1 is suppressed. In addition, according to Formula 3, by setting T1 to less than 50 msec, the decrease in contact resistance can be suppressed as described above. However, if the preheating energization time is too short or the heat input in the pre-energization S1 is too small, the scattering E suppression effect in the main welding S2 cannot be obtained. Therefore, in Formula 2 and Formula 3, (I1) 2 ×T1 is set to 750×t or more, and T1 is set to 10 msec or more.

[0144] The most basic form of the resistance welding system according to the first embodiment has been described above. Hereinafter, a preferred form will be described.

[0145] (Plate group 60 and total plate thickness ratio)

[0146] The plate group 60 welded by the resistance welding system is not particularly limited, and for example, a three-plate overlapping plate group consisting of two thick plates and one thin plate with a total plate thickness ratio of 3.5 or more is preferred. That is, the steel plates included in the plate group 60 are three overlapping plates, one of the steel plates is a thin plate with a plate thickness of 0.8 mm or less, and two of the steel plates are thick plates with a plate thickness of 1.0 mm or more, and the thin plates are arranged on the surface of the plate group 60. The total plate thickness ratio h calculated by dividing the total plate thickness (mm) of the steel plates included in the plate group 60 by the minimum value of the plate thickness of the steel plates arranged on the surface of the plate group 60 is preferably 3.5 or more and 7.0 or less.

[0147] The total thickness ratio h is a value obtained by dividing the total thickness of the steel plates included in the plate group 60 by the thickness of the thinner steel plate of the two steel plates arranged on the surface of the plate group 60. When the two steel plates arranged on the surface of the plate group 60 have the same thickness, the thickness of either steel plate may be used for calculation of the total thickness ratio.

[0148] A typical example of the plate group 60 having such a structure is an automobile part. Automobile parts sometimes have a structure obtained by welding a frame member that ensures the strength of the part and an exterior member that ensures the beauty of the part. The material of the frame member is a high-strength thick plate, and the material of the exterior member is a low-strength thin plate. In addition, the exterior member is arranged on the surface of the automobile part. Therefore, the joint of the automobile part having the above structure is set as a welded joint of two thick plates and one thin plate, and the thin plate is arranged on the surface of the welded joint.

[0149] The total plate thickness ratio is an indicator of the difficulty of resistance welding of the plate group 60. The smaller the thickness of the thin plate arranged on the surface of the plate group 60, the more difficult it is to grow the nugget to the thin plate and join the thin plate with other plates. Therefore, the greater the total plate thickness ratio, which is the value obtained by dividing the total plate thickness of the plate group 60 by the plate thickness of the thin plate on the surface of the plate group 60, the more difficult it is to join the plate group 60 by resistance welding. On the other hand, if the design and use of the parts are taken into consideration, there are cases where the total plate thickness ratio is preferably larger. For example, in automobile parts, the larger the total plate thickness ratio, the thinner the wall of the exterior parts can be, thereby reducing the weight of the parts and improving the fuel consumption of the automobile.

[0150] The resistance welding system of the first embodiment can optimally join a set of three overlapping plates with a high plate thickness ratio. The resistance welding system of the first embodiment fuses the overlapping surfaces of two thick plates by pre-energizing S1. As a result, the occurrence of scattering E in the adaptively controlled main welding S2 can be suppressed and the nugget diameter at the overlapping surface of the two thick plates can be enlarged.

[0151] In addition, the usual pre-energization S1 may make it difficult to join thin plates. If pre-energization S1 is performed, the contact resistance at the overlapping surface of the thin plate and the thick plate decreases, and the heat generated at the overlapping surface during the main welding S2 decreases. As a result, the nugget can be prevented from growing to the overlapping surface of the thin plate and the thick plate. However, in the resistance welding system related to the first embodiment, the pre-energization S1 time is limited to a short time by equations 2 and 3. The pre-energization S1 performed by the resistance welding system related to the first embodiment can fuse only the overlapping surfaces of the two thick plates without damaging the contact resistance at the overlapping surfaces of the thin plate and the thick plate. Therefore, the resistance welding system related to the first embodiment can also expand the diameter of the nugget at the overlapping surface of the thin plate and the thick plate.

[0152] (The maximum value of welding current I max )

[0153] The resistance welding system according to the first embodiment is preferably configured to perform the main welding S2 in a manner satisfying Formula 4. In Formula 4, I max is the maximum value of the welding current in the main welding S2 (kA). t is the total plate thickness (mm) of the steel plates included in the plate group 60. I2 is the welding current (kA) of the main welding S2 controlled by constant current when the plate group 60 is subjected to the main welding S2 controlled by constant current for the same energization time as the main welding S2 to obtain the cumulative calorific value Q per unit volume.

[0154] I2+0.3×t<I max <I2+0.8×t…(Formula 4)

[0155] During the main welding S2, the welding current varies by adaptive control. Equation 4 sets the lower limit value and upper limit value of the welding current based on the above-mentioned I2 and the thickness of the plate group 60. According to Equation 4, by performing the main welding S2 to satisfy the relationship of I max < the relationship of I2 + 0.8×t, the occurrence of spatter E during the main welding S2 is further suppressed. In addition, by performing the main welding S2 to satisfy the relationship of I2 + 0.3×t < I max the relationship, the nugget diameter can be enlarged to further improve the joint strength.

[0156] (The maximum value K of the change rate of the welding current)

[0157] Regarding the resistance welding system of the first embodiment, it is preferable to be configured to consider disturbances such as the plate gap and the shunt condition, and perform the main welding S2 to satisfy Equation 5. In Equation 5, K is the maximum value of the change rate of the welding current (kA / sec) during the main welding S2, and h is the total plate thickness ratio calculated by dividing the total plate thickness (mm) of the steel plates included in the plate group 60 by the minimum value of the plate thickness of the steel plate arranged on the surface of the plate group 60.

[0158] 30 / h < K < 100 / h…(Equation 5)

[0159] The maximum value K of the change rate of the welding current is the maximum value of the change amount per unit time of the welding current during the period from the start to the end of the main welding S2 (kA / sec). The unit time is 1 / 120 second (approximately 8.3 msec) or 1 / 100 second (approximately 10 msec).

[0160] The larger the change rate of the welding current, the easier it is for spatter E to occur during the main welding S2. According to Equation 5, by performing adaptive control on the welding current to satisfy K < 100 / h, the occurrence of spatter E during the main welding S2 is further suppressed. On the other hand, from the viewpoint of improving the response speed of the adaptive control, it is preferable that the change rate of the welding current is larger. According to Equation 5, by performing adaptive control on the welding current to satisfy 30 / h < K, the response speed of the adaptive control is further improved.

[0161] (Post - energization S3)

[0162] In the plate group 60 that is resistance spot - welded by the resistance welding system of the first embodiment, it is preferable that one or more of the above - mentioned steel plates included in the plate group 60 are high - strength steel plates with a tensile strength of 980 MPa or more. In this case, the resistance welding control device 20 of the resistance welding system of the first embodiment preferably further has a post - energization part that performs post - energization S3 to a pair of electrodes 51, 52 after the main welding S2.

[0163] Post-energization S3 is energization for passing post-heat current to the weld. Post-heat current is a current that flows for the purpose of tempering or annealing the hardened weld after welding in resistance welding of steel materials that have been hardened by welding. Post-heat current is sometimes called tempering current. The period during which post-heat current flows is called post-heat energization time.

[0164] The energization conditions in the post-energization S3 satisfy Equations 6 and 7.

[0165] I2×0.5≤I3≤I2×1.2…(Formula 6)

[0166] 200≤T3≤2000…(Formula 7)

[0167] Here, in equations 6 and 7, I3 is the post-heat current (kA) flowing to the plate group during post-energization, and T3 is the time during which the post-heat current flows, i.e., the post-heat energization time (msec). As described above, I2 is the constant-current controlled welding current (kA) when the cumulative calorific value per unit volume is obtained by welding the plate group under constant-current control for the same energization time as the main welding. In addition, post-energization is performed, for example, under constant-current control, but various waveforms can be applied as in the pre-energization.

[0168] Generally, the greater the tensile strength TS of the steel plate, the greater the cross tensile strength CTS of the weld formed by resistance welding. However, when the TS of the steel plate is approximately 980 MPa or more, the greater the TS of the steel plate, the smaller the CTS of the weld. The reason for the decrease in CTS may be the embrittlement of the weld.

[0169] According to the post-energization S3 in accordance with the above-mentioned energization conditions, the embrittled weld structure can be optimally modified. Therefore, even when the plate group 60 has more than one high-strength steel plate, a weld with a higher CTS can be manufactured. According to the research results of the inventors of the present application, if the post-energization is performed under the above-mentioned conditions after the pre-energization and main welding by the resistance welding system related to the first embodiment, the strength can be obtained by about 1.5 times compared with the CTS of the resistance welded joint obtained by the usual resistance spot welding under the condition without interference.

[0170] In addition, when the tensile strength of all the steel plates included in the plate group is 1500 MPa or less, for example, the post-energization conditions may be defined using Equations 6A and 7A instead of Equations 6 and 7.

[0171] I2×0.8≤I3≤I2×1.2…(Formula 6A)

[0172] 200≤T3≤400…(Formula 7A)

[0173] Furthermore, when the tensile strength of one or more of the steel plates included in the plate group is 2000 MPa or more, for example, the post-energization condition may be defined using Equation 6B and Equation 7B instead of Equation 6 and Equation 7.

[0174] I2×0.5≤I3≤I2×0.7…(Formula 6B)

[0175] 1000≤T3≤2000…(Formula 7B)

[0176] (2. Method for manufacturing resistance welded joint)

[0177] Next, a method for manufacturing a resistance welded joint according to a second embodiment of the present invention is described. The method for manufacturing a resistance welded joint according to the second embodiment comprises: a process of pre-energizing a pair of electrodes 51 and 52 of a plate group 60 sandwiching two or more superimposed steel plates S1; and a process of performing main welding S2 in a pair of electrodes 51 and 52 after pre-energizing S1; for the main welding S2, an instantaneous heat generation q per unit volume and per unit time is calculated based on the energizing time of the main welding S2 according to the cumulative heat generation per unit volume that enables good welding in the plate group 60 obtained in advance, and adaptive control is performed by adjusting the inter-electrode voltage or welding current to generate the calculated instantaneous heat generation q per unit volume and per unit time, so as to perform pre-energizing S1 in a manner that satisfies equations 1, 2, and 3.

[0178] I1>I2…(Formula 1)

[0179] 750×t≤(I1) 2 ×T1≤1550×t…(Formula 2)

[0180] 10≤T1≤50…(Formula 3)

[0181] Here, in Formula 1, Formula 2 and Formula 3, I1 is the preheating current (kA) flowing into the plate group 60 in the pre-energization S1, I2 is the welding current (kA) of the main welding S2 controlled by constant current when the plate group 60 is subjected to the main welding S2 controlled by constant current for the same energization time as the main welding S2 to obtain the cumulative heat generation Q per unit volume, T1 is the time for which the preheating current flows, that is, the preheating energization time (msec), and t is the total plate thickness (mm) of the steel plates contained in the plate group 60.

[0182] The method for manufacturing a resistance welded joint according to the second embodiment is similar to the resistance welding system according to the first embodiment, and is a system for resistance welding a plate group 60 in which two or more steel plates are stacked. The steel plates included in the plate group 60 may be stacked in three or more sheets. In addition, there may be a slight gap between the steel plates included in the plate group 60.

[0183] The method for manufacturing a resistance welded joint according to the second embodiment includes: a step of pre-energizing a pair of electrodes 51 and 52 sandwiching a plate group 60; and a step of performing main welding S2 on the pair of electrodes 51 and 52 after the pre-energizing S1. Hereinafter, the main welding S2 will be described first, and then the pre-energizing S1 will be described.

[0184] (Main welding S2)

[0185] In the main welding S2, a high current is passed while applying pressure to the plate group 60, and the plate group 60 is welded by resistance heating. The phenomenon occurring in the plate group 60 in the main welding S2 is as described in the resistance welding system according to the first embodiment.

[0186] Adaptive control is performed for the main welding S2. In the main welding S2 of the method for manufacturing a resistance welded joint according to the second embodiment, the cumulative calorific value Q per unit volume is set as an index of adaptive control, similarly to the resistance welding system according to the first embodiment. The Joule calorific value (for example, inter-electrode resistance, inter-electrode voltage, or welding current) in the main welding S2 is adjusted so that the cumulative calorific value Q per unit volume becomes a predetermined value.

[0187] In the main welding S2 of the method for manufacturing a resistance welded joint according to the second embodiment, the cumulative calorific value Q per unit volume is used as an indicator of adaptive control, similarly to the resistance welding system according to the first embodiment. Specifically, the cumulative calorific value Q that enables good welding in the plate group 60 is first obtained in advance. Next, based on the cumulative calorific value Q and the power-on time of the main welding S2, the instantaneous calorific value q per unit volume and per unit time that can reproduce the cumulative calorific value Q is obtained. In addition, the Joule calorific value is adjusted in the main welding S2 to obtain the instantaneous calorific value q per unit volume and per unit time. The instantaneous calorific value q per unit volume and per unit time may also be achieved by adjusting the welding current. In addition, the instantaneous calorific value q per unit volume and per unit time may also be achieved by adjusting the inter-electrode resistance or the inter-electrode voltage. The method for determining the cumulative calorific value Q that enables good welding in the plate group 60 is not limited. For example, the cumulative calorific value Q used in the method for manufacturing a resistance welded joint according to the second embodiment may be determined by the method exemplified in the resistance welding system according to the first embodiment.

[0188] (Pre-power on S1)

[0189] In the pre-energization S1 performed before the main welding S2, as in the resistance welding system related to the first embodiment, current is passed to the pair of electrodes 51 and 52 sandwiching the plate group 60, and the plate group 60 is preheated by resistance heat. The purpose of the pre-energization S1 is to preheat the plate group 60, and it is not necessary to melt the plate group 60. However, it is acceptable for the plate group 60 to melt slightly.

[0190] In the main welding S2 of the method for manufacturing a resistance welded joint according to the second embodiment, pre-current application S1 is performed so as to satisfy Expressions 1, 2, and 3, similarly to the resistance welding system according to the first embodiment.

[0191] I1>I2…(Formula 1)

[0192] 750×t≤(I1) 2 ×T1≤1550×t…(Formula 2)

[0193] 10≤T1≤50…(Formula 3)

[0194] (Effect)

[0195] In the method for manufacturing a resistance welded joint according to the second embodiment, in the main welding S2, the Joule heating value is adjusted using the instantaneous heating value per unit volume (the cumulative heating value during the welding current flow) as an index. As a result, welding can always be performed stably regardless of the interference state of the welded workpiece, i.e., the plate group 60, and the wear condition of the tip of the electrodes 51 and 52. In other words, the cumulative heating value Q per unit volume that enables stable and good welding is not affected by the interference state of the welded workpiece and the wear condition of the tip of the electrodes 51 and 52.

[0196] However, if scattering E occurs during the main welding S2, it becomes difficult to continue the adaptive control of the main welding S2. Therefore, in the method for manufacturing a resistance welded joint according to the second embodiment, pre-energization S1 is performed before the main welding S2. By pre-energization S1, impurity layers such as oxide films are removed, the crimping portion P is enlarged, and the occurrence of scattering E in the main welding S2 is suppressed.

[0197] However, when the plate gap is removed by the usual pre-energization S1, the nugget diameter may not be ensured. If the pre-energization S1 is performed, the contact resistance between the steel plates and the contact resistance between the electrodes 51 and 52 and the steel plates are reduced. If the contact resistance is reduced, the heat generation is reduced and the growth of the nugget is suppressed. Therefore, in the method for manufacturing a resistance welded joint according to the second embodiment, the pre-energization S1 is performed in a manner that satisfies Formula 1.

[0198] I1>I2…(Formula 1)

[0199] Generally, the preheating current is smaller than the welding current. On the other hand, the preheating current I1 in the preheating current S1 satisfying Formula 1 is substantially larger than the welding current in the main welding S2. By setting the preheating current to such a large value, the preheating current-on time can be shortened. By shortening the preheating current-on time, the decrease in contact resistance can be suppressed, and the nugget diameter can be ensured.

[0200] However, if the pre-energization S1 with a high current is continued for a long time, there is a possibility that scattering E will occur during the pre-energization S1. Therefore, in the method for manufacturing a resistance welded joint according to the second embodiment, the pre-energization S1 is performed so that Expressions 2 and 3 are satisfied.

[0201] 750×t≤(I1) 2 ×T1≤1550×t…(Formula 2)

[0202] 10≤T1≤50…(Formula 3)

[0203] By performing the pre-energization S1 in a manner satisfying Formula 2 and Formula 3, the occurrence of scattering E in the pre-energization S1 is suppressed. In addition, according to Formula 3, by making T1 less than 50 msec, the decrease in contact resistance can be suppressed as described above. However, if the preheating energization time is too short or the heat input in the pre-energization S1 is too small, the scattering E suppression effect in the main welding S2 cannot be obtained. Therefore, in Formula 2 and Formula 3, (I1) 2 ×T1 is set to be 750×t or more, and T1 is set to be 10 msec or more.

[0204] The most basic form of the method for manufacturing a resistance welded joint according to the second embodiment has been described above. Hereinafter, a preferred form will be described.

[0205] (Plate group 60 and total plate thickness ratio)

[0206] The welded member, i.e., the plate group 60 in the method for manufacturing a resistance welded joint is not particularly limited. For example, as in the resistance welding system related to the first embodiment, a plate group consisting of two thick plates and one thin plate with a total plate thickness ratio of 3.5 or more is preferably used. That is, the steel plates included in the plate group 60 are three overlapping plates, one of the steel plates is a thin plate with a plate thickness of 0.8 mm or less, and two of the steel plates are thick plates with a plate thickness of 1.0 mm or more. The thin plates are arranged on the surface of the plate group 60, and the total plate thickness ratio h calculated by dividing the total plate thickness (mm) of the steel plates included in the plate group 60 by the minimum value of the plate thickness of the steel plates arranged on the surface of the plate group 60 is preferably 3.5 or more and 7.0 or less.

[0207] The method for manufacturing a resistance welded joint according to the second embodiment is similar to the resistance welding system according to the first embodiment, and can perform optimal welding in a set of three overlapping plates with a high plate thickness ratio. The method for manufacturing a resistance welded joint according to the second embodiment fuses the overlapping surfaces of two thick plates by pre-energizing S1. As a result, the occurrence of scattering E in the adaptively controlled main welding S2 can be suppressed, and the diameter of the nugget at the overlapping surface of the two thick plates can be expanded.

[0208] In addition, the usual pre - energization S1 is sometimes difficult to perform the joining of thin plates. However, in the manufacturing method of the resistance - welded joint according to the second embodiment, the pre - energization time is limited to a short time by Formula 2 and Formula 3. The pre - energization S1 performed by the manufacturing method of the resistance - welded joint according to the second embodiment can fuse only the superposed surfaces of two thick plates without impairing the contact resistance at the superposed surfaces of the thin plate and the thick plate. Therefore, the manufacturing method of the resistance - welded joint according to the second embodiment can also enlarge the nugget diameter at the superposed surfaces of the thin plate and the thick plate.

[0209] (The maximum value I of the welding current max )

[0210] Similar to the resistance - welding system according to the first embodiment, the manufacturing method of the resistance - welded joint according to the second embodiment preferably performs the main welding S2 in a manner that satisfies Formula 4.

[0211] I2 + 0.3×t<I max <I2 + 0.8×t…(Formula 4)

[0212] According to Formula 4, by performing the main welding S2 in a manner that satisfies the relationship of I max <I2 + 0.8×t, the occurrence of spatter E in the main welding S2 is further suppressed. In addition, by performing the main welding S2 in a manner that satisfies the relationship of I2 + 0.3×t < I max , the nugget diameter can be enlarged to further improve the joining strength.

[0213] (The maximum value K of the change rate of the welding current)

[0214] Similar to the resistance - welding system according to the first embodiment, the manufacturing method of the resistance - welded joint according to the second embodiment preferably takes into account disturbances such as the plate gap and the shunt condition, and is configured to perform the main welding S2 in a manner that satisfies Formula 5.

[0215] 30 / h<K<100 / h…(Formula 5)

[0216] The larger the change rate of the welding current, the easier it is to generate spatter E during the main welding S2. According to Formula 5, by adaptively controlling the welding current to satisfy K < 100 / h, the occurrence of spatter E in the main welding S2 is further suppressed. On the other hand, from the viewpoint of improving the response speed of the adaptive control, it is preferable that the change rate of the welding current is large. According to Formula 5, by adaptively controlling the welding current to satisfy 30 / h < K, the response speed of the adaptive control is further improved.

[0217] (Post - energization S3)

[0218] In the plate group 60 joined by the method for manufacturing a resistance welded joint according to the second embodiment, it is preferred that, similar to the resistance welding system according to the first embodiment, one or more of the steel plates included in the plate group 60 is a high-strength steel plate having a tensile strength of 980 MPa or more. In this case, the method for manufacturing a resistance welded joint according to the second embodiment preferably further includes a step of performing post-energization S3 on the pair of electrodes 51 and 52 after the main welding S2. The energization conditions in the post-energization S3 satisfy equations 6 and 7.

[0219] I2×0.5≤I3≤I2×1.2…(Formula 6)

[0220] 200≤T3≤2000…(Formula 7)

[0221] Here, in equations 6 and 7, I3 is the post-heat current (kA) flowing to the plate group in the post-energization, and T3 is the time for the post-heat current to flow, that is, the post-heat energization time (msec). As described above, I2 is the constant-current controlled welding current (kA) when the cumulative calorific value per unit volume is obtained by energizing the plate group with the constant-current controlled welding current for the same energization time as the main welding. In addition, the post-energization is performed, for example, by constant current control, but various waveforms can be used in the post-energization as in the pre-energization.

[0222] The welded portion is well tempered by the post-energization S3 in accordance with the above-mentioned energization conditions. Therefore, even when the plate group 60 includes one or more high-strength steel plates, a welded portion having a high CTS can be manufactured.

[0223] The optimal post-energization condition may be further selected according to the strength of the steel plate. For example, when the tensile strength of all the steel plates included in the plate group is 1500 MPa or less, it is preferable to use equations 6A and 7A instead of equations 6 and 7 to define the post-energization condition.

[0224] I2×0.8≤I3≤I2×1.2…(Formula 6A)

[0225] 200≤T3≤400…(Formula 7A)

[0226] On the other hand, when the tensile strength of one or more steel plates included in the plate group is 2000 MPa or more, it is preferable to define the post-energization conditions using Formula 6B and Formula 7B instead of Formula 6 and Formula 7.

[0227] I2×0.5≤I3≤I2×0.7…(Formula 6B)

[0228] 1000≤T3≤2000…(Formula 7B)

[0229] The embodiments of the present invention are described above, but the present invention is not limited thereto, and can be appropriately changed within the scope of the technical concept of the present invention. The following describes the best examples of the resistance welding system of the first embodiment of the present invention and the method for manufacturing the resistance welding joint of the second embodiment. In addition, the structure illustrated below can be applied to any of the first embodiment and the second embodiment unless otherwise specifically denied.

[0230] (Tensile strength of steel plate)

[0231] One or more of the steel plates included in the plate group 60 is preferably a high-strength steel plate. The tensile strength TS of the high-strength steel plate may be, for example, 980 MPa or more, 1000 MPa or more, 1100 MPa or more, or 1200 MPa or more. In the case where the plate group 60 includes both thin plates and thick plates, the thick plates are generally made of high-strength steel plates. High-strength steel plates are very effective as a means of improving the strength of resistance welded joints. The lower limit of the TS of the high-strength steel plate is not particularly limited, and for example, the TS of the high-strength steel plate may be less than 2000 MPa, less than 1800 MPa, or less than 1600 MPa.

[0232] The plate group 60 may also include a mild steel plate having a lower strength than a high-strength steel plate. The tensile strength of the mild steel plate is, for example, 600 MPa or less, 500 MPa or less, or 400 MPa or less. When the plate group 60 includes both a thin plate and a thick plate, the thin plate is generally made of a mild steel plate in many cases. Since the mild steel plate has high processability, it is suitable as a material for exterior parts of automobile parts that require a creative appearance. The lower limit value of the TS of the mild steel plate is not particularly limited, and for example, the TS of the mild steel plate may be 200 MPa or more, 250 MPa or more, or 300 MPa or more.

[0233] (Surface treatment of steel plates)

[0234] One or more of the steel plates included in the plate group 60 may be plated steel plates having a coating layer on the surface. The type of coating layer is not limited, but is preferably hot-dip galvanizing, alloy hot-dip galvanizing, electrogalvanizing, zinc-nickel plating, or aluminum plating.

[0235] [Example]

[0236] The effects of one form of the present invention are described in more detail by way of examples. However, the conditions in the examples are only one conditional example adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one conditional example. The present invention may adopt various conditions as long as it does not depart from the gist of the present invention and achieves the purpose of the present invention.

[0237] Various plate groups were prepared by sequentially stacking the steel plates 1, 2, and 3 listed in Table 2. In some of the plate groups, Figure 4 The spacer 70 shown in the figure provides a gap (plate gap) between the steel plate 2 and the steel plate 3. The interval between the spacers is set to 40 mm. The size of the plate gap is recorded in Table 2. In addition, in some plate groups, the main welding is performed after pre-setting the nugget on both sides of the welding point. The nugget generates a shunt in the main welding. The interval between the center of the nugget and the center of the welding point is set to 10 mm.

[0238] Pre-energization and welding were performed on these plate groups. However, pre-energization was omitted for some of the plate groups. In addition, post-energization was performed on some of the plate groups after welding. The pressure between energizations was set to a constant value. The conditions of pre-energization, main welding, and post-energization are recorded in Tables 3 and 4. The omitted energization conditions are recorded as "-" in Tables 3 and 4. In addition, in Table 3, inappropriate values ​​are underlined.

[0239] It was visually confirmed whether or not scattering occurred during resistance welding under the conditions described in Tables 3 and 4. The presence or absence of scattering is described in Table 5.

[0240] The nugget diameter is measured along the superposition surface of the steel plates in a cross section passing through the center of the nugget and perpendicular to the surface of the plate group. The nugget diameter measured along the superposition surface of Steel Plate 1 and Steel Plate 2 is recorded in the "Steel Plate 1 / Steel Plate 2" column of Table 5, and the nugget diameter measured along the superposition surface of Steel Plate 2 and Steel Plate 3 is recorded in the "Steel Plate 2 / Steel Plate 3" column of Table 5. In the case where the nugget does not grow to the superposition surface of the steel plates, the nugget diameter is determined to be 0 mm.

[0241] When CTS can be measured, the joint strength is evaluated. The evaluation index of the joint strength is the cross tensile strength CTS of steel plate 1 and steel plate 2. CTS is measured in accordance with JIS Z3137:1999 "Test piece dimensions and test methods for cross tensile tests of resistance spot welded and convex welded joints". In addition, the measurement objects of CTS are steel plate 1 and steel plate 2. When CTS is measured, its value is recorded in Table 5.

[0242] In addition, for the samples with shunts set as interference, the CTS measurement was omitted. This is because the shunt is formed by setting other nuggets on both sides of the weld spot, but the nuggets brought by the weld spot become an obstacle to the CTS measurement. For the joints where the CTS measurement was omitted, CTS is recorded as "-".

[0243] The joints with a nugget diameter of 3.1 mm or more at the overlapped surface of steel plate 1 and steel plate 2 and a nugget diameter of 6.3 mm or more at the overlapped surface of steel plate 2 and steel plate 3 were judged to be joints with the optimal nugget diameters. Inappropriate nugget diameters are underlined in Table 5. The joints with CTS of steel plates 2 and 3 of 7.15 MPa or more were judged to be joints with significantly excellent bonding strength.

[0244] [Table 2]

[0245] Table 2

[0246]

[0247] [Table 3]

[0248] Table 3

[0249]

[0250] [Table 4]

[0251] Table 4

[0252]

[0253] [Table 5]

[0254] Table 5

[0255]

[0256] No.0 is an example in which welding is performed under constant current control without interference and pre-energization is omitted. The welding of No.0 is equivalent to the so-called ordinary resistance spot welding. In addition, post-energization is not performed in No.0. Since there is no interference, no scattering occurs during the welding of No.0. The CTS of No.0 is used as the evaluation standard of the CTS of the embodiment. The joint having a CTS 1.5 times that of No.0 is judged to be a joint with excellent joint strength.

[0257] In the welding of No. 1, No. 19 and No. 23, pre-energization was omitted, and the main welding was performed by constant current control. The welding of No. 1, No. 19 and No. 23 is equivalent to the so-called ordinary resistance spot welding. In the joints of No. 1, No. 19 and No. 23, no nugget was formed at the overlapping surface of steel plate 2 and steel plate 3, so that poor joining occurred. In addition, in the joints of No. 1 and No. 19, the nugget diameter at the overlapping surface of steel plate 1 and steel plate 2 was insufficient.

[0258] In the welding of No. 2, No. 13, No. 20 and No. 24, pre-energization was omitted. As a result, in the welding of No. 2, No. 13, No. 20 and No. 24, scattering occurred at the stage of main welding. In addition, in the joint of No. 13, the nugget diameter at the overlapping surface of steel plates 1 and 2 and the nugget diameter at the overlapping surface of steel plates 2 and 3 were insufficient. In the joint of No. 20, the nugget diameter at the overlapping surface of steel plates 1 and 2 was insufficient.

[0259] In the welding of No. 3 and No. 14, I1 is smaller than I2, and does not satisfy Formula 1. As a result, in the joints of No. 3 and No. 14, the nugget diameter at the overlapping surface of the steel plates 2 and 3 is insufficient.

[0260] In welding No.6, (I1) 2 ×T1 is insufficient and does not satisfy Formula 2. As a result, in the welding of No. 6, spalling occurred at the stage of main welding. In addition, in the joint of No. 6, the nugget diameter at the overlapping surface of the steel plates 2 and 3 is insufficient.

[0261] In welding No.7, No.10 and No.17, (I1) 2 ×T1 is excessive and does not satisfy Formula 2. As a result, in the welding of No. 7, No. 10, and No. 17, scattering occurred at the stage of pre-energization. In addition, in the joint of No. 7, the diameter of the nugget at the overlapping surface of steel plates 2 and 3 is insufficient, and in the joint of No. 17, the diameter of the nugget at the overlapping surface of steel plates 1 and 2 is insufficient.

[0262] In the welding of No. 12 and No. 21, the main welding was performed under constant current control. In the joint of No. 12, the nugget diameter at the overlapping surface of steel plates 2 and 3 was insufficient. In the joint of No. 21, no nugget was formed at the overlapping surface of steel plates 2 and 3, resulting in poor joining. In addition, in the joint of No. 21, the nugget diameter at the overlapping surface of steel plates 1 and 2 was also insufficient.

[0263] On the other hand, according to welding conditions that satisfy all of Formulas 1 to 3, the occurrence of spattering can be suppressed, and a joint having an optimum nugget diameter can be manufactured.

[0264] Description of symbols

[0265] 10 power supply device; 20 resistance welding control device; 21 ROM; 22 RAM; 23 CPU; 24 A / D converter; 25 interface; 26 power element; 30 welding transformer; 40 inter-electrode voltage detection circuit; 51 electrode; 52 electrode; 60 plate group; 601 steel plate; 602 impurity layer; 603 new surface; 70 spacer; P crimping part; M molten part; E scattering; S1 pre-energization; S2 main welding; S3 post-energization; Q cumulative heat generation per unit volume; q instantaneous heat generation per unit volume and unit time.

Claims

1. A resistance welding system for performing resistance spot welding on a plate assembly of two or more superimposed steel plates, characterized in that: The above resistance welding system has: a pair of electrodes sandwiching the plate assembly; a power supply device for causing current to flow between the pair of electrodes; and The resistance welding control device comprises: a pre-energizing unit for pre-energizing the pair of electrodes; and a main welding energizing unit for performing main welding at the pair of electrodes after the pre-energizing, wherein the resistance welding control device controls the power supply device with respect to welding current and energizing time. The main welding current supply unit calculates the instantaneous heating value per unit volume and per unit time based on the current supply time of the main welding according to the cumulative heating value per unit volume that can perform good welding in the plate group obtained in advance, and adjusts the inter-electrode resistance, inter-electrode voltage or welding current that generates the calculated instantaneous heating value per unit volume and per unit time, thereby adaptively controlling the main welding. The pre-energization unit performs the pre-energization in a manner that satisfies equations 1, 2, and 3. I1>I2…Formula 1 750×t≤(I1) 2 ×T1≤1550×t…Formula 2 10≤T1≤50…Formula 3 Here, in the above formula 1, the above formula 2 and the above formula 3, I1 is the preheating current flowing to the above plate group during the above pre-energization, and the unit is kA; I2 is the constant current controlled welding current when the constant current controlled welding current is passed to the plate group for the same power-on time as the main welding so as to obtain the cumulative calorific value per unit volume, and the unit is kA; T1 is the time for the above-mentioned preheating current to flow, i.e., the preheating power-on time, in msec; t is the total plate thickness of the above-mentioned steel plates included in the above-mentioned plate group, and the unit is mm.

2. The resistance welding system according to claim 1, characterized in that The steel plates included in the plate group are three overlapping plates. One of the steel plates is a thin plate having a thickness of 0.8 mm or less. The two steel plates in the above steel plates are thick plates with a thickness of 1.0 mm or more. The thin plate is arranged on the surface of the plate group. A total thickness ratio calculated by dividing the total thickness of the steel plates included in the plate group in units of mm by the minimum value of the thickness of the steel plates arranged on the surface of the plate group is 3.5 or more.

3. The resistance welding system according to claim 1 or 2, characterized in that: It also satisfies Equation 4, I2+0.3×t<I max <I2+0.8×t…Formula 4 Here, in the above formula 4, I max It is the maximum value of the welding current in the main welding, and the unit is kA.

4. The resistance welding system according to claim 1 or 2, characterized in that: Considering the interference of plate gap, diversion conditions, etc., equation 5 is also satisfied. 30 / h<K<100 / h…Formula 5 Here, in the above formula 5, K is the maximum value of the rate of change of the welding current in the main welding, and the unit is kA / sec; h is a total plate thickness ratio calculated by dividing the total plate thickness of the steel plates included in the plate group in units of mm by the minimum value of the plate thickness of the steel plates arranged on the surfaces of the plate group.

5. The resistance welding system according to claim 1 or 2, characterized in that: One or more of the steel plates included in the plate group is a high-strength steel plate having a tensile strength of 980 MPa or more. The resistance welding control device further comprises a post-energizing unit for performing post-energization to the pair of electrodes after the main welding. The post-energization unit performs the post-energization in a manner satisfying equations 6 and 7. I2×0.5≤I3≤I2×1.2…Formula 6 200≤T3≤2000…Formula 7 Here, in the above formula 6 and the above formula 7, I3 is the post-heat current flowing to the above-mentioned plate group during the above-mentioned post-energization, and the unit is kA; T3 is the time during which the post-heat current flows, that is, the post-heat conduction time, and the unit is msec.

6. A method for manufacturing a resistance welding joint, characterized in that: The following processes are available: Pre-energizing a pair of electrodes sandwiching a plate group of two or more stacked steel plates; and After the above-mentioned pre-energization, the main welding is carried out at the pair of the above-mentioned electrodes. The instantaneous heating value per unit volume and per unit time is calculated based on the energization time of the main welding according to the cumulative heating value per unit volume that can perform good welding on the plate group obtained in advance, and the inter-electrode resistance, inter-electrode voltage or welding current that generates the calculated instantaneous heating value per unit volume and per unit time are adjusted to adaptively control the main welding. The above pre-energization is performed in a manner that satisfies equations 1, 2, and 3. I1>I2…Formula 1 750×t≤(I1) 2 ×T1≤1550×t…Formula 2 10≤T1≤50…Formula 3 Here, in the above formula 1, the above formula 2 and the above formula 3, I1 is the preheating current flowing to the above plate group during the above pre-energization, and the unit is kA; I2 is the constant current controlled welding current when the plate group is subjected to constant current controlled welding for the same power-on time as the main welding to obtain the cumulative calorific value per unit volume, and the unit is kA; T1 is the time for the above-mentioned preheating current to flow, i.e., the preheating power-on time, in msec; t is the total plate thickness of the above-mentioned steel plates included in the above-mentioned plate group, and the unit is mm.

7. The method for manufacturing a resistance welded joint according to claim 6, characterized in that: The steel plates included in the plate group are three overlapping plates. One of the steel plates is a thin plate having a thickness of 0.8 mm or less. The two steel plates in the above steel plates are thick plates with a thickness of 1.0 mm or more. The thin plate is arranged on the surface of the plate group. A total thickness ratio calculated by dividing the total thickness of the steel plates included in the plate group in units of mm by the minimum value of the thickness of the steel plates arranged on the surface of the plate group is 3.5 or more.

8. The method for manufacturing a resistance welded joint according to claim 6 or 7, characterized in that: It also satisfies Equation 4, I2+0.3×t<I max <I2+0.8×t…Formula 4 Here, in the above formula 4, I max It is the maximum value of the welding current in the main welding, and the unit is kA.

9. The method for manufacturing a resistance welded joint according to claim 6 or 7, characterized in that: Considering the interference of plate gap, diversion conditions, etc., equation 5 is also satisfied. 30 / h<K<100 / h…Formula 5 Here, in the above formula 5, K is the maximum value of the rate of change of the welding current in the main welding, and the unit is kA / sec; h is a total plate thickness ratio calculated by dividing the total plate thickness of the steel plates included in the plate group in units of mm by the minimum value of the plate thickness of the steel plates arranged on the surfaces of the plate group.

10. The method for manufacturing a resistance welded joint according to claim 6 or 7, characterized in that: One or more of the steel plates included in the plate group is a high-strength steel plate having a tensile strength of 980 MPa or more, The method for manufacturing the resistance welded joint further comprises a step of performing post-energization on the pair of electrodes after the main welding. The above-mentioned post-power-on is performed in a manner that satisfies equations 6 and 7. I2×0.5≤I3≤I2×1.2…Formula 6 200≤T3≤2000…Formula 7 Here, in the above formula 6 and the above formula 7, I3 is the post-heat current flowing to the above-mentioned plate group during the above-mentioned post-energization, and the unit is kA; T3 is the time during which the post-heat current flows, that is, the post-heat conduction time, and the unit is msec.

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