Method for manufacturing resistance spot welding joint

Abstract

Provided is a method for manufacturing a resistance spot welded joint in which nugget diameter stability is excellent. In test welding, preliminary energization by constant current control in two or more patterns and main energization are performed, preliminary energization is performed under the same energization conditions and a pressing force of less than 2.5 kN, and the electric characteristics between the electrodes during the preliminary energization and a curve of the change over time of the instantaneous heat generation amount per unit volume during the main energization are stored. In main welding, the preliminary energization by the constant current control is performed under the same energization conditions as the preliminary energization of the test welding, the target value of the main energization in the main welding is set by comparing the electric characteristics between the electrodes of the preliminary energization, and then adaptive control welding for controlling the energization amount of the main energization is performed.

Description

Method for manufacturing resistance spot welded joints

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

[0002] Resistance spot welding, a type of lap resistance welding, is commonly used to join overlapping metal sheets, such as overlapping steel sheets. This welding method involves sandwiching two or more overlapping metal sheets (to be welded) and applying pressure from above and below with a pair of electrodes while passing a high welding current between the upper and lower electrodes for a short period of time to join them. This welding method utilizes resistance heating generated by passing a high welding current to obtain a spot-like weld. This spot-like weld is called a nugget. A nugget is a portion of the overlapping metal sheets that melts and solidifies at the contact point when a current is passed through them. This nugget joins the metal sheets together in a spot-like manner.

[0003] To obtain good weld quality, it is important to form a nugget with a diameter within an appropriate range. Hereinafter, the diameter of the nugget will also be referred to as the nugget diameter. The nugget diameter is determined by the welding conditions, such as the welding pressure, welding current, and welding time. Therefore, to obtain a nugget diameter within the appropriate range, it is necessary to appropriately set the above-mentioned welding conditions according to the conditions of the workpieces, such as the material, plate thickness, and number of overlapping sheets of the workpieces.

[0004] For example, during the manufacture of automobiles, thousands of resistance spot welds are performed on each vehicle. Furthermore, it is necessary to weld the workpieces (workpieces) that flow in one after another. In this case, if the conditions of the workpieces at each welding point, such as the material, plate thickness, and number of overlapping sheets, are the same, the same nugget diameter can be obtained under the same welding conditions, such as the welding pressure, welding current, and welding time.

[0005] However, if there is a disturbance during welding, such as a previously welded point (previously welded point) near the welding point, or if the surface of the workpiece is uneven and a contact point of the workpiece is near the welding point, the current will be shunted to the previously welded point or the contact point. In such a situation, even if welding is performed under specified conditions, the current density at the desired welding position directly below the electrode will be low. As a result, the required nugget diameter will not be obtained. To compensate for this lack of heat generation and obtain the required nugget diameter, a high welding current must be set in advance.

[0006] Furthermore, if the area around the welding point is tightly constrained due to the surface irregularities or shape of the workpiece, or if there is a foreign object between the metal sheets around the welding point, the gap between the metal sheets will become larger, which will narrow the contact diameter between the metal sheets and make expulsion more likely to occur.

[0007] International Publication No. 2019 / 035367

[0008] As a solution to the above problem, the inventors previously proposed a resistance spot welding method in which a plurality of overlapping metal plates are sandwiched between a pair of electrodes and joined by applying current while applying pressure, wherein a main welding and a test welding prior to the main welding are performed, and the test welding is performed under two or more welding conditions, wherein in the test welding, for each of the welding conditions, a pre-current is applied using constant current control with the same current pattern, and the electrical characteristics between the electrodes during the pre-current are stored, and then in the main current application, a time change curve of the instantaneous heat generation per unit volume and a cumulative heat generation per unit volume are calculated from the electrical characteristics between the electrodes when a proper nugget is formed by applying current using constant current control, and further, in the main welding, The present inventors have developed a resistance spot welding method, which discloses in Patent Document 1, a method for performing a pre-energization using constant current control with the same energization pattern as the test welding, comparing the electrical characteristics between the electrodes in the pre-energization with the electrical characteristics between the electrodes stored in the pre-energization of the test welding for each of the welding conditions, setting the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume in the main energization of the test welding stored under the welding conditions with the smallest difference as target values ​​for the main energization in the main welding, and then performing adaptive control welding in which the amount of energization is controlled in accordance with the target values ​​for the main energization.

[0009] The technology described in Patent Document 1 makes it possible to obtain a nugget diameter within an appropriate range without expulsion, even when disturbances are present. However, with the technology described in Patent Document 1, a difference between the nugget diameter of the resistance spot welded joint actually obtained by the actual welding and the target diameter (hereinafter also referred to as nugget diameter deviation) may occur depending on the type and degree of disturbance. Improvements in this respect are currently desired from the perspective of further improving product precision.

[0010] The present invention was developed in consideration of the above-mentioned current situation, and aims to provide a manufacturing method of a resistance spot welded joint that reduces deviation in nugget diameter and enables stable production of a nugget of a target diameter regardless of the type or degree of disturbance (hereinafter also referred to as having excellent nugget diameter stability). Note that in this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits, respectively.

[0011] The inventors conducted extensive research to achieve the above-mentioned objective. First, the inventors performed test welding under a variety of patterns (welding conditions) according to the technique described in Patent Document 1. They then selected the optimal patterns (welding conditions) from the test welding, i.e., the time change curves of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume (hereinafter also referred to as time change curves) obtained under conditions that most closely resemble the disturbance conditions in the actual welding, as the target values ​​for the actual welding current. The inventors then performed adaptive control welding, conceiving a possible reduction in the deviation in nugget diameter. For example, assuming a gap between metal sheets as the disturbance, test welding was performed to simulate gaps with different disturbance strengths, such as gaps of 1.0 mm, 1.5 mm, and 2.0 mm. Next, the inventors compared the difference between the inter-electrode electrical characteristics of the pre-current in the actual welding and the inter-electrode electrical characteristics of the pre-current in each test welding, and selected the test welding pattern that minimized the difference (hereinafter also referred to as a comparison of the inter-electrode electrical characteristics of the pre-current). Next, the time change curve, etc., of the main current flow stored in the selected test welding is set as a target value, and the main current flow for the main welding is performed by adaptive control welding. The inventors thought that this would reduce the deviation of the nugget diameter.

[0012] The inventors conducted various experiments and studies based on the above idea. However, even when test welding was performed with many patterns simulating disturbances of different strengths, there were cases in which the deviation in nugget diameter could not be sufficiently reduced.

[0013] Therefore, the inventors conducted further experiments and studies, and as a result, found that the deviation of the nugget diameter can be stably reduced by reducing the pressure in the pre-current application of the test welding, particularly by setting it to less than 2.5 kN.

[0014] The inventors consider the reason for the above as follows. Specifically, when the disturbance conditions are similar, for example, when the same type of disturbance exists but the intensity is different, the inter-electrode electrical characteristics obtained during pre-current conduction are similar. This tendency is particularly pronounced when the pre-current conduction force is large. Therefore, when comparing the inter-electrode electrical characteristics of the pre-current conduction, the time change curve, etc., of a test welding performed under conditions similar to the level of the disturbance in the actual welding is not necessarily selected as the target value for the actual welding current, resulting in a deviation in the nugget diameter. In this regard, by reducing the pre-current conduction force, particularly to a value less than 2.5 kN, the intensity of the disturbance is reflected in the inter-electrode electrical characteristics of the pre-current conduction, preferably the integrated value of the inter-electrode resistance. This improves the accuracy of selecting the optimal time change curve, etc., of the test welding as the target value for the actual welding current, i.e., the time change curve, etc., obtained under conditions similar to the disturbance in the actual welding (hereinafter also referred to as target value selection accuracy for the actual welding). As a result, deviations in nugget diameter are stably reduced, making it possible to stably obtain nuggets of the target diameter.

[0015] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows.

[0016] 1. A method for manufacturing a resistance spot welded joint, in which a workpiece made of overlapping metal plates is sandwiched between a pair of electrodes and joined by applying pressure and current to produce a resistance spot welded joint, wherein main welding and test welding prior to the main welding are performed, and the test welding is performed in two or more patterns, wherein for each pattern, pre-current is applied using constant current control under the same current conditions and a pressure of less than 2.5 kN, and the electrical characteristics between the electrodes during the pre-current are stored, and then main current is applied using constant current control, and a time-varying curve of the instantaneous heat generation per unit volume at which a nugget of a target diameter is formed and a cumulative heat generation per unit volume are stored, and further, during the main welding, a pre-energization using constant current control under the same energization conditions as the pre-energization of the test welding, comparing the electrical characteristics between the electrodes in the pre-energization with the electrical characteristics between the electrodes stored in the pre-energization of the test welding for each pattern, setting the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume in the main energization of the test welding, which are stored for the pattern with the smallest difference, as target values ​​for the main energization of the test welding, and then performing adaptive control welding in which the amount of energization is controlled in accordance with the target values ​​for the main energization of the test welding.

[0017] 2. The method for manufacturing a resistance spot welded joint as set forth in claim 1, wherein in the adaptive control welding, welding is performed based on the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation amount set as the target values, and if the time change curve of the instantaneous heat generation per unit volume deviates from the reference time change curve, the amount of current flow is controlled to compensate for the deviation within the remaining current flow time so that the cumulative heat generation per unit volume in the main current flow for the main welding matches the cumulative heat generation per unit volume set as the target value.

[0018] 3. The method for manufacturing a resistance spot welded joint according to 1 or 2 above, wherein the electrical property between the electrodes is an integral value of interelectrode resistance.

[0019] 4. The method for manufacturing a resistance spot welded joint according to any one of 1 to 3 above, wherein the test welding is performed by simulating the same type of disturbance in two or more patterns.

[0020] 5. The method for manufacturing a resistance spot welded joint according to 4 above, wherein the same type of disturbance is a plate gap.

[0021] According to the present invention, it is possible to reduce deviations in nugget diameter and stably obtain nuggets of a target diameter regardless of the type or degree of disturbance. In particular, according to the present invention, when welding workpieces that flow one after another in actual work such as automobile manufacturing (the state of disturbance varies for each welding position and workpiece), it is possible to obtain nuggets of a target diameter by effectively responding to variations in the state of disturbance, which is extremely advantageous in terms of improving product precision.

[0022] The present invention will be described based on the following embodiments. A welding device that can be used in a manufacturing method for a resistance spot-welded joint according to one embodiment of the present invention may be equipped with a pair of upper and lower electrodes and capable of freely controlling the welding pressure and welding current during welding. There are no particular limitations on the pressure mechanism (e.g., air cylinder or servo motor), type (e.g., stationary type, robot gun), or electrode shape. The device may also have a storage device that stores time-varying curves, etc.

[0023] First, test welding and actual welding in a method for manufacturing a resistance spot welded joint according to one embodiment of the present invention will be described.

[0024] Test welding: Test welding is performed using two or more, preferably three or more, patterns. In particular, test welding is performed by simulating the same type of disturbance, preferably two or more, more preferably three or more, patterns. In this case, it goes without saying that the intensity of the disturbance (e.g., the magnitude of the disturbance if the disturbance is a gap) differs for each pattern. Examples of disturbance types include gaps, clearances, strike angles, and misalignment. The method for manufacturing a resistance spot-welded joint according to one embodiment of the present invention is particularly effective for gaps. There is no particular upper limit to the number of test welding patterns. From the standpoint of efficiency, the number of test welding patterns is preferably five or less.

[0025] Here, the gap refers to the gap (the distance between the mating surfaces) between the metal plates to be welded (before pressure is applied by the electrodes). In this case, it is preferable to perform test welding in multiple patterns in which the gap is varied between 0 mm (no gap) and 3.0 mm, for example, in the following states (disturbance strength): no gap, 1.0 mm, 2.0 mm, and 3.0 mm. Note that the gap variation width for each test welding pattern may or may not be constant. The same applies below. Furthermore, when the welded material is a stack of three or more metal plates, the test welding may simulate gaps between the metal plates where gaps are expected to occur during actual welding (actual welding). In this case, for example, a gap may be simulated between the outermost plate (the metal plate arranged outermost in the welded material) and the metal plate adjacent to the outermost plate.

[0026] The clearance is the gap between the electrode and the metal plate (the distance between the metal plate and one electrode at the time when the opposite electrode first comes into contact with the metal plate during welding). In this case, it is preferable to perform test welding in a plurality of patterns in which the clearance is changed within a range of 0 mm (no clearance) to 3.0 mm, for example, in the states (intensity of disturbance) of no clearance, 1.0 mm, 2.0 mm, and 3.0 mm.

[0027] The impact angle is the inclination of the electrode relative to the direction perpendicular to the surface of the metal plate. In this case, it is preferable to perform test welding in a plurality of patterns in which the impact angle is changed in the range of 0 degrees (no impact angle) to 10 degrees, for example, in the states of no impact angle, impact angle: 1 degree, impact angle: 5 degrees, and impact angle: 10 degrees (disturbance strength).

[0028] The misalignment is the distance between contact point 1 and contact point 2 on the same plane (a plane parallel to the surface of the metal plate) when contact point 1 and contact point 2 are projected onto the same plane when viewed from a direction perpendicular to the surface of the metal plate. Here, contact point 1 is the point of contact between one electrode and the surface of the metal plate. Contact point 2 is the point of contact between the other electrode and the surface of the metal plate. In this case, it is preferable to perform test welding in a plurality of patterns in which the misalignment is changed in a range from 0 mm (no misalignment) to 2.0 mm, for example, in states (intensity of disturbance) of no misalignment, 1.0 mm, 1.5 mm, and 2.0 mm.

[0029] In the test welding, a pattern in which welding is performed under a simulated disturbance condition may be combined with a pattern in which welding is performed under a disturbance-free condition. Furthermore, patterns simulating different types of disturbances, such as a plate gap and a misalignment, may be combined.

[0030] In test welding, preliminary current and main current are applied for each pattern as follows.

[0031] (Pre-current for test welding) The pre-current for test welding is controlled by constant current control. The electrical characteristics between the electrodes during the pre-current are stored. In a method for manufacturing a resistance spot welded joint according to one embodiment of the present invention, it is extremely important to control the applied pressure (set applied pressure) during the pre-current for test welding as follows.

[0032] Pressing force: less than 2.5 kN As described above, in order to improve the accuracy of target value selection in actual welding and obtain excellent stability of nugget diameter, it is effective to reduce the pressing force in the pre-current of test welding. Therefore, the pressing force in the pre-current of test welding is less than 2.5 kN, preferably 2.0 kN or less, and more preferably 1.5 kN or less. There is no particular lower limit for the pressing force in the pre-current of test welding. For example, the pressing force in the pre-current of test welding is preferably 1.0 kN or more.

[0033] The current conditions for the test welding pre-current other than the pressure are not particularly limited as long as they are constant current control. For example, the current value I1 for the test welding pre-current is preferably in the range of 1 to 2 kA. The current application time T1 for the test welding pre-current is preferably in the range of 20 to 100 ms.

[0034] The target value for the actual welding current is determined by comparing the electrical characteristics between the electrodes of the pre-current stored for each test welding pattern. Therefore, the current conditions (pressure, welding current (hereinafter simply referred to as current), and current duration) for the pre-current in test welding are the same for all patterns. In other words, "pre-current is performed by constant current control under the same current conditions for each test welding pattern" means that pre-current is performed by constant current control with the same pressure, current, and current duration for all test welding patterns. However, an error of about 5% is acceptable for each of the pressure, current, and current duration. Even if an error occurs, the pressure must be less than 2.5 kN.

[0035] The electrical characteristic between the electrodes means an integral value of the interelectrode resistance or an integral value of the interelectrode voltage. In particular, from the viewpoint of improving the accuracy of selecting the target value in the actual welding, the electrical characteristic between the electrodes is preferably an integral value of the interelectrode resistance (for example, an integral value of the interelectrode resistance at the end of the preliminary current).

[0036] (Main current application for test welding) In test welding, a pre-current is applied for each pattern, followed by a main current application. In the main current application for test welding, current is applied under constant current control to form a nugget of the target diameter, and the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume are stored.

[0037] The energization conditions for the main energization of test welding are not particularly limited as long as they are constant current control and a nugget of the target diameter is obtained. For example, the current value I2 for the main energization of test welding is preferably in the range of 3 to 9 kA. It is also preferable to satisfy the relationship I1 < I2. The energization time T2 for the main energization of test welding is preferably in the range of 20 to 300 ms. The pressing force for the main energization of test welding is preferably in the range of 3 to 7 kN. Furthermore, a rest period may be provided between the preliminary energization and the main energization of test welding. Note that the energization conditions (pressure, current value, and energization time) for the main energization of test welding do not need to be the same for all patterns.

[0038] The target diameter of the nugget may be set based on the required specifications, and is preferably selected from the range of 3.0√t to 6.0√t, and more preferably from the range of 4.0√t to 5.0√t, where t is the thickness (mm) of the thinnest metal plate among the metal plates to be welded.

[0039] Test welding patterns other than those described above are not particularly limited, and can be set, for example, by conducting a preliminary welding test in advance (specifically, using workpieces of the same steel type and thickness as those actually used, and conducting welding under various conditions with constant current control in a state simulating the above-mentioned disturbance or in a state without any disturbance).

[0040] After the test welding, the actual welding is carried out. In the actual welding, the preliminary current and the actual current are applied as follows.

[0041] (Pre-energization of Actual Welding) In actual welding, first, pre-energization is performed using constant current control under the same energization conditions as in test welding. Then, the electrical characteristics between the electrodes in the pre-energization are compared with the electrical characteristics between the electrodes stored in the pre-energization of the test welding for each pattern, and the time change curve, etc., for the actual current in the test welding stored in the pattern with the smallest difference is set as the target value for the actual current in the actual welding.

[0042] Note that "pre-current is conducted under constant current control under the same current conditions as pre-current for test welding" means that the pressure, current, and current duration are set to the same values ​​as those used in pre-current for test welding, and pre-current for actual welding is conducted under constant current control. However, an error of about 5% is acceptable for each of the pressure, current, and current duration.

[0043] (Main Welding Current) Next, for the main current, adaptive control welding is performed, controlling the amount of current flow according to the target values ​​set after the pre-current. In adaptive control welding for the main current, for example, welding is performed based on the time-varying curve of the instantaneous heat generation per unit volume and the cumulative heat generation amount set as target values ​​after the pre-current. If the time-varying curve of the instantaneous heat generation per unit volume follows the reference time-varying curve, welding is continued and the welding is terminated. However, if the time-varying curve of the instantaneous heat generation per unit volume deviates from the reference time-varying curve, the amount of current flow is controlled to compensate for the deviation within the remaining current flow time so that the cumulative heat generation per unit volume during the main current flow matches the target value. Adaptive control welding can be performed, for example, using a control device that directly measures or calculates changes in the current value, interelectrode voltage, interelectrode resistance, and heat generation amount during welding as electrical signals and controls input parameters such as the current value and interelectrode voltage based on these values.

[0044] The calorific value may be calculated, for example, from the electrical characteristics between the electrodes. There are no particular limitations on the method for calculating the calorific value, and the method disclosed in Japanese Patent Laid-Open No. 11-33743, for example, may be adopted. The calculation procedure for the calorific value q per unit volume and unit time and the cumulative calorific value Q per unit volume using this method is as follows:

[0045] Let t' be the total thickness of the materials to be welded, r be the electrical resistivity of the materials to be welded, V be the voltage between the electrodes, I be the current value, and S be the area of ​​contact between the electrodes and the materials to be welded. In this case, the welding current passes through a columnar portion with a cross-sectional area of ​​S and a thickness of t', generating resistance heat. The amount of heat q generated per unit volume and unit time in this columnar portion can be calculated using the following equation (1): q = (V * I) / (S * t') (1) Furthermore, the electrical resistance R of this columnar portion can be calculated using the following equation (2): R = (r * t') / S (2) By solving equation (2) for S and substituting it into equation (1), the amount of heat generated q can be calculated using the following equation (3): q = (V * I * R) / (r * t') ２ ) = (V ２ ) / (r・t' ２ ) ... (3).

[0046] As is clear from equation (3), the calorific value q per unit volume and unit time can be calculated from the interelectrode voltage V, the total thickness t' of the workpieces to be welded, and the electrical resistivity r of the workpieces, and is not affected by the contact area S between the electrode and the workpieces. While equation (3) calculates the calorific value from the interelectrode voltage V, the calorific value q can also be calculated from the current value I, and in this case, it is not necessary to use the contact area S between the electrode and the workpieces. Furthermore, by accumulating the calorific value q per unit volume and unit time over the current flow time, the cumulative calorific value Q per unit volume can be obtained. As is clear from equation (3), this cumulative calorific value Q per unit volume can also be calculated without using the contact area S between the electrode and the workpieces to be welded. Needless to say, other calculation formulas may also be used.

[0047] Furthermore, there are no particular limitations on the materials to be welded, and the method can be applied to welding steel sheets of various strengths, from mild steel to ultra-high tensile steel sheets, as well as plated steel sheets and light metal sheets such as aluminum alloys, and can also be applied to sheet assemblies in which three or more steel sheets are stacked together. Note that a steel sheet having a plated layer on the surface that comes into contact with the electrode may be placed.

[0048] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0049] For the two-sheet stacking assembly shown in Table 1 or the three-sheet stacking assembly (where metal sheet 1, metal sheet 2, and metal sheet 3 were stacked in this order), test welding was performed under the conditions shown in Table 2, and actual welding was performed under the conditions shown in Table 3 to produce resistance spot welded joints. In the plating column in Table 1, no description indicates an unplated steel sheet, and GA indicates an alloyed hot-dip galvanized steel sheet. The cycle in Table 2, which is the unit of current flow time, refers to the cycle at 50 Hz. At 50 Hz, one cycle is 20 ms, and for example, 16 cycles is 320 ms.

[0050] In the test welding, the same current flow conditions were used for each pattern, and a pre-current was applied under constant current control, and the integral value of the inter-electrode resistance during the pre-current was stored. In addition, in the actual welding, current was applied under constant current control, and the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume were stored, which resulted in the formation of nuggets of the target diameter. The target diameter of the nuggets was 4.0√t in all cases.

[0051] For the actual welding, a pre-current was applied under constant current control under the same current conditions as the test welding. The integrated value of the interelectrode resistance during the pre-current for the actual welding was compared with the integrated value of the interelectrode resistance stored during the pre-current for the test welding for each pattern. The time-varying curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume for the actual current stored for the pattern with the smallest difference were set as the target values ​​for the actual welding. Then, for the actual welding, adaptive control welding was performed, controlling the amount of current according to the target values.

[0052] Then, the stability of the nugget diameter was evaluated in the following manner.

[0053] [Evaluation of Nugget Diameter Stability] For each manufactured joint, the welded portion was cut and the cross section was etched. Next, the cross section of each joint was observed with an optical microscope, and the nugget diameter formed between the metal sheets was measured. The stability of the nugget diameter was evaluated according to the following criteria. The evaluation results are also shown in Table 3. Pass A (particularly excellent): The deviation in nugget diameter was within 0.1√t in all cases, regardless of the intensity of the disturbance during the actual welding. Pass B (even better): The deviation in nugget diameter was within 0.3√t in all cases, regardless of the intensity of the disturbance during the actual welding (except Pass A). Pass C (excellent): The deviation in nugget diameter was within 0.5√t in all cases, regardless of the intensity of the disturbance during the actual welding (except Pass A and Pass B). Fail (poor): The deviation in nugget diameter exceeded 0.5√t for at least one intensity of the disturbance during the actual welding. The deviation in nugget diameter can also be considered as the absolute value of the difference between the nugget diameter (mm) of the joint obtained in the actual welding and the target diameter (mm).

[0054]

[0055]

[0056]

[0057] In all of the inventive examples, excellent stability of the nugget diameter was obtained, whereas in the comparative examples, the stability of the nugget diameter was not sufficient.

Claims

1. A method for manufacturing a resistance spot welded joint, in which a workpiece consisting of multiple overlapping metal plates is sandwiched between a pair of electrodes and joined by applying pressure and current to produce a resistance spot welded joint, wherein the method comprises: performing main welding and test welding prior to the main welding; and performing the test welding in two or more patterns; in the test welding, for each pattern, a pre-current is applied by constant current control under the same current conditions and with a pressure of less than 2.5 kN, and storing the electrical characteristics between the electrodes during the pre-current; then, in the main welding, a current is applied by constant current control, and the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume at which a nugget of the target diameter is formed are stored; and further, in the main welding, a pre-energization using constant current control under the same energization conditions as the pre-energization of the test welding, comparing the electrical characteristics between the electrodes in the pre-energization with the electrical characteristics between the electrodes stored in the pre-energization of the test welding for each pattern, setting the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation per unit volume in the main energization of the test welding, which are stored for the pattern with the smallest difference, as target values ​​for the main energization of the test welding, and then performing adaptive control welding in which the amount of energization is controlled in accordance with the target values ​​for the main energization of the test welding.

2. A method for manufacturing a resistance spot welded joint as set forth in claim 1, wherein in the adaptive control welding, welding is performed based on the time change curve of the instantaneous heat generation per unit volume and the cumulative heat generation set as the target values, and if the time change curve of the instantaneous heat generation per unit volume deviates from the reference time change curve, the amount of current flow is controlled to compensate for the deviation within the remaining current flow time so that the cumulative heat generation per unit volume in the main current flow for the main welding matches the cumulative heat generation per unit volume set as the target value.

3. The method for manufacturing a resistance spot welded joint according to claim 1, wherein the electrical characteristic between the electrodes is an integral value of the interelectrode resistance.

4. The method for manufacturing a resistance spot welded joint according to claim 2, wherein the electrical characteristic between the electrodes is an integral value of the interelectrode resistance.

5. A method for manufacturing a resistance spot welded joint according to any one of claims 1 to 4, wherein the test welding is performed by simulating the same type of disturbance in two or more patterns.

6. The method for manufacturing a resistance spot welded joint according to claim 5, wherein the same type of disturbance is a gap.

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