RESISTANCE SPOT WELDING METHOD AND WELD MEMBER PRODUCTION METHOD

Abstract

A resistance spot welding method comprises: performing test welding; and performing actual welding after the test welding, wherein the test welding is performed under each of two or more welding conditions. In the test welding, for each of the welding conditions, an electrode force parameter from when electrode force application to parts to be welded starts to when a set electrode force is reached before start of current passage and a time variation curve of an instantaneous amount of heat generated and a cumulative amount of heat generated are stored. In the actual welding: electrode force application to the parts to be welded is performed under each of the same conditions as in the test welding before start of current passage, and a corresponding electrode force parameter and the parameter stored in the test welding are compared for each of the welding conditions to set a target of a time variation curve of an instantaneous amount of heat generated and a cumulative amount of heat generated in the actual welding; and adaptive control welding is performed to control a current passage amount according to the target.

Claims

1. A resistance spot welding method of squeezing, by a pair of electrodes, parts to be welded which are a plurality of overlapping metal sheets, and passing a current while applying an electrode force to join the parts to be welded, the resistance spot welding method comprising: performing test welding; and performing actual welding after the test welding, wherein the test welding is performed under each of two or more welding conditions, in the test welding, for each of the welding conditions, an electrode force parameter from when electrode force application to the parts to be welded starts to when a set electrode force is reached is stored before start of current passage, and a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume that are calculated from an electrical property between the electrodes in forming an appropriate nugget by performing current passage by constant current control are stored, and in the actual welding: electrode force application to the parts to be welded is performed under each of the same conditions as in the test welding before start of current passage, an electrode force parameter from when the electrode force application starts to when the set electrode force is reached and the electrode force parameter stored in the test welding are compared to determine a difference therebetween for each of the welding conditions of the test welding, and a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume in the test welding that are stored for a welding condition corresponding to a smallest difference are set as a target in the actual welding; and adaptive control welding is performed to control a current passage amount according to the target.

2. The resistance spot welding method according to claim 1, wherein in the adaptive control welding, welding is performed with the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated being set as the target, and in the case where an amount of time variation of an instantaneous amount of heat generated per unit volume differs from the time variation curve, the current passage amount is controlled in order to compensate for the difference from the time variation curve within a remaining welding time so that a cumulative amount of heat generated per unit volume in the actual welding matches the cumulative amount of heat generated per unit volume set as the target.

3. The resistance spot welding method according to claim 1, wherein in the test welding, at least one welding condition is that welding is performed in a simulated state of a disturbance, and an other welding condition is that welding is performed in a state of no disturbance.

4. The resistance spot welding method according to claim 1, wherein the test welding is performed under each of three or more welding conditions.

5. The resistance spot welding method according to claim 1, wherein in the test welding, a current pattern is divided into two or more steps in at least one welding condition, and in the actual welding, in the case where the target in the adaptive control welding is set based on data stored in the test welding for the welding condition divided into the two or more steps, a current pattern in the actual welding is divided into two or more steps as with the current pattern in the test welding and the adaptive control welding is performed for each step in the actual welding.

6. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 1.

7. The resistance spot welding method according to claim 2, wherein in the test welding, at least one welding condition is that welding is performed in a simulated state of a disturbance, and an other welding condition is that welding is performed in a state of no disturbance.

8. The resistance spot welding method according to claim 2, wherein the test welding is performed under each of three or more welding conditions.

9. The resistance spot welding method according to claim 3, wherein the test welding is performed under each of three or more welding conditions.

10. The resistance spot welding method according to claim 7, wherein the test welding is performed under each of three or more welding conditions.

11. The resistance spot welding method according to claim 2, wherein in the test welding, a current pattern is divided into two or more steps in at least one welding condition, and in the actual welding, in the case where the target in the adaptive control welding is set based on data stored in the test welding for the welding condition divided into the two or more steps, a current pattern in the actual welding is divided into two or more steps as with the current pattern in the test welding and the adaptive control welding is performed for each step in the actual welding.

12. The resistance spot welding method according to claim 3, wherein in the test welding, a current pattern is divided into two or more steps in at least one welding condition, and in the actual welding, in the case where the target in the adaptive control welding is set based on data stored in the test welding for the welding condition divided into the two or more steps, a current pattern in the actual welding is divided into two or more steps as with the current pattern in the test welding and the adaptive control welding is performed for each step in the actual welding.

13. The resistance spot welding method according to claim 4, wherein in the test welding, a current pattern is divided into two or more steps in at least one welding condition, and in the actual welding, in the case where the target in the adaptive control welding is set based on data stored in the test welding for the welding condition divided into the two or more steps, a current pattern in the actual welding is divided into two or more steps as with the current pattern in the test welding and the adaptive control welding is performed for each step in the actual welding.

14. The resistance spot welding method according to claim 7, wherein in the test welding, a current pattern is divided into two or more steps in at least one welding condition, and in the actual welding, in the case where the target in the adaptive control welding is set based on data stored in the test welding for the welding condition divided into the two or more steps, a current pattern in the actual welding is divided into two or more steps as with the current pattern in the test welding and the adaptive control welding is performed for each step in the actual welding.

15. The resistance spot welding method according to claim 8, wherein in the test welding, a current pattern is divided into two or more steps in at least one welding condition, and in the actual welding, in the case where the target in the adaptive control welding is set based on data stored in the test welding for the welding condition divided into the two or more steps, a current pattern in the actual welding is divided into two or more steps as with the current pattern in the test welding and the adaptive control welding is performed for each step in the actual welding.

16. The resistance spot welding method according to claim 9, wherein in the test welding, a current pattern is divided into two or more steps in at least one welding condition, and in the actual welding, in the case where the target in the adaptive control welding is set based on data stored in the test welding for the welding condition divided into the two or more steps, a current pattern in the actual welding is divided into two or more steps as with the current pattern in the test welding and the adaptive control welding is performed for each step in the actual welding.

17. The resistance spot welding method according to claim 10, wherein in the test welding, a current pattern is divided into two or more steps in at least one welding condition, and in the actual welding, in the case where the target in the adaptive control welding is set based on data stored in the test welding for the welding condition divided into the two or more steps, a current pattern in the actual welding is divided into two or more steps as with the current pattern in the test welding and the adaptive control welding is performed for each step in the actual welding.

18. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 2.

19. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 3.

20. A weld member production method comprising joining a plurality of overlapping metal sheets by the resistance spot welding method according to claim 4.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In the accompanying drawings:

[0056] FIG. 1 is a diagram schematically illustrating an example of the case of performing welding on a sheet combination of two overlapping sheets in a state of no disturbance;

[0057] FIG. 2 is a diagram schematically illustrating an example of the case of performing welding on a sheet combination of three overlapping sheets in a state of no disturbance;

[0058] FIG. 3 is a diagram schematically illustrating an example of the case of performing welding on a sheet combination of two overlapping sheets having a sheet gap;

[0059] FIG. 4 is a diagram schematically illustrating an example of the case of performing welding on a sheet combination of three overlapping sheets having a sheet gap;

[0060] FIG. 5 is a diagram schematically illustrating an example of the case of performing welding on a sheet combination of two overlapping sheets having an existing weld;

[0061] FIG. 6 is a diagram schematically illustrating an example of the case of performing welding on a sheet combination of three overlapping sheets having an existing weld;

[0062] FIG. 7 is a diagram schematically illustrating an example of a current pattern of one step in test welding; and

[0063] FIG. 8 is a diagram schematically illustrating an example of a current pattern of two steps in test welding.

DETAILED DESCRIPTION

[0064] The presently disclosed techniques will be described below by way of embodiments.

[0065] One of the disclosed embodiments relates to a resistance spot welding method of squeezing, by a pair of electrodes, parts to be welded which are a plurality of overlapping metal sheets, and passing a current while applying an electrode force to join the parts to be welded, the resistance spot welding method comprising: performing test welding; and performing actual welding after the test welding, wherein the test welding is performed under each of two or more welding conditions, in the test welding, for each of the welding conditions, an electrode force parameter from when electrode force application to the parts to be welded starts to when a set electrode force is reached is stored before start of current passage, and a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume that are calculated from an electrical property between the electrodes in forming an appropriate nugget by performing current passage by constant current control are stored, and in the actual welding: electrode force application to the parts to be welded is performed under each of the same conditions as in the test welding before start of current passage, an electrode force parameter from when the electrode force application starts to when the set electrode force is reached and the electrode force parameter stored in the test welding are compared to determine a difference therebetween for each of the welding conditions of the test welding, and a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume in the test welding that are stored for a welding condition corresponding to a smallest difference are set as a target in the actual welding; and adaptive control welding is performed to control a current passage amount according to the target.

[0066] Any welding device that includes a pair of upper and lower electrodes and is capable of freely controlling each of the electrode force and the welding current during welding may be used in the resistance spot welding method according to one of the disclosed embodiments. The force mechanism (air cylinder, servomotor, etc.), the type (stationary, robot gun, etc.), the electrode shape, and the like are not limited. Herein, the electrical property between the electrodes means the interelectrode resistance or the voltage between electrodes.

[0067] The test welding and the actual welding in the resistance spot welding method according to one of the disclosed embodiments will be described below.

[0068] Test Welding

[0069] In the test welding, first, electrode force application to the parts to be welded is performed to a set electrode force, and, after the set electrode force is reached, current passage is performed by constant current control. Such test welding is performed under each of two or more welding conditions (i.e. disturbance states) and preferably under each of three or more welding conditions. No upper limit is placed on the number of welding conditions, but the number of welding conditions is preferably ten from the perspective of efficiency. In the test welding, an electrode force parameter from when the electrode force application to the parts to be welded starts to when the set electrode force is reached is stored before the start of current passage for each welding condition.

[0070] Examples of the electrode force parameter include the time T.sub.F from when the electrode force application starts to when the set electrode force F (the set electrode force at the start of current passage) is reached and the electrode force change rate (increase rate) F (=F/T.sub.F) from when the electrode force application starts to when the set electrode force is reached, as illustrated in FIGS. 4 and 5.

[0071] The electrode force parameter may be set based on the torque or rotational speed of the servomotor, the strain of the electrodes or the gun, the displacement of the electrodes, etc. (hereafter also referred to as servomotor torque, etc.). For example, the set electrode force at the start of current passage may be set based on the servomotor torque, etc., and the time from when the electrode force application starts to when the set value is reached by the servomotor torque, etc., the torque change rate or rotational speed of the servomotor from when the electrode force application starts to when the set value is reached, and the displacement rate of the electrodes from when the electrode force application starts to when the set value is reached may each be set as the electrode force parameter.

[0072] For example, the torque of the servomotor torque begins to increase rapidly when the electrodes come into contact with the metal sheets as the parts to be welded, and then reaches a stable value and is saturated when a sufficient electrode force is applied to the steel sheets. Accordingly, the time from when the torque begins to increase to when the torque is saturated may be taken to be the time from when the electrode force application starts to when the set electrode force is reached.

[0073] The rotational speed of the servomotor becomes unstable when the electrodes come into contact with the metal sheets as the parts to be welded, and then decreases gradually. When the set electrode force is reached, the electrodes stop moving, so that the rotational speed reaches 0. Accordingly, the time from when the rotational speed becomes unstable and begins to decrease to when the rotational speed reaches 0 may be taken to be the time from when the electrode force application starts to when the set electrode force is reached.

[0074] Further, for example, the time at which the torque begins to increase may be taken to be the time at which the electrode force application starts, and the time at which the rotational speed reaches 0 may be taken to be the time at which the set electrode force is reached.

[0075] Moreover, the following may each be used as the electrode force parameter: [0076] the time from when the electrode force application starts (from the start of electrode force application) to the start of current passage, [0077] the strain of the welding gun from when the electrode force application starts to when the set electrode force is reached, and [0078] the displacement of the electrodes from when the electrode force application starts to when the set electrode force is reached.

[0079] For example, the time from when the electrode force application starts to the start of current passage can be expressed as [the time from when the electrode force application starts to when the set electrode force is reached]+[the time from when the set electrode force is reached to the start of current passage]. Hence, by comparing the parameter in the actual welding and the parameter stored in the test welding in consideration of [the time from when the set electrode force is reached to the start of current passage], the optimal one of the plurality of time variation curves of the instantaneous amounts of heat generated, etc. can be selected as the target based on the effect of a disturbance.

[0080] Thereafter, current passage is performed by constant current control, and a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume that are calculated from the electrical property between the electrodes in forming an appropriate nugget are stored.

[0081] The unit of the instantaneous amount of heat generated is not limited, and may be in conformity with the setting (e.g. J/cyc or J/ms) of the resistance spot welding device used.

[0082] The combination of the welding conditions in the test welding where data are stored is preferably made up of at least one welding condition of performing welding in a simulated state of a disturbance expected in the actual welding and a welding condition of performing welding in a state of no disturbance.

[0083] Examples of the disturbance expected in the actual welding include the above-mentioned disturbances such as current shunting and a sheet gap, e.g. an existing weld within 40 mm from the welding position (electrode center position) and a gap of 0.2 mm or more between the mating surfaces of the steel sheets as the parts to be welded.

[0084] For example, in the case where it is expected that there is a gap of 0.2 mm or more between the mating surfaces of the metal sheets as the parts to be welded and the gap varies for each welding position, preferably one welding condition in the test welding is that welding is performed in a state in which there is a gap of 0.2 mm to 3.0 mm (preferably 0.5 mm to 3.0 mm) between the mating surfaces of the metal sheets as the parts to be welded, and another welding condition is that welding is performed in a state of no disturbance. The upper limit of the gap between the mating surfaces of the metal sheets as the parts to be welded as expected is practically about 3.0 mm.

[0085] Herein, the gap between the mating surfaces of the metal sheets is the gap between the mating surfaces of the metal sheets (the distance between the mating surfaces) at the welding position before the electrode force is applied.

[0086] Particularly in the case where large variations in disturbance in the actual welding are expected, it is preferable to perform the test welding under each of three or more welding conditions each corresponding to a different disturbance state simulated.

[0087] In this case, for example, a welding condition that welding is performed in a state of no disturbance, a welding condition that there is a gap of 0.5 mm or more and less than 1.5 mm between the mating surfaces of the metal sheets as the parts to be welded, and a welding condition that there is a gap of 1.5 mm or more and less than 2.5 mm between the mating surfaces of the metal sheets as the parts to be welded may be combined.

[0088] The current pattern in the test welding may be one step. However, particularly in the case where the distance between the welding position and an existing weld is short and the effect of current shunting to the existing weld is significant, the current pattern is preferably divided into two or more steps. No upper limit is placed on the number of steps, but the number of steps is typically about five.

[0089] No limit is placed on the welding condition in each step, but the following relationship is preferably satisfied:

[0090] 0.3IxIx

[0091] where I1 is the welding current in the first step in the test welding, and Ix (x: an integer of 2 to n, n: the total number of steps) is the welding current in the second step onward.

[0092] Moreover, a cooling time may be provided between the steps.

[0093] After the set electrode force is reached, preliminary current passage may be performed before the start of current passage (before the first step in the case where the current pattern is divided into two or more steps). The current pattern in the preliminary current passage is not limited, and may be a current pattern by constant current control or a current pattern of an upslope.

[0094] In the case where the preliminary current passage is performed in the test welding for a welding condition and the target of the adaptive control welding in the actual welding is set based on data stored in the test welding for the welding condition, in the actual welding, preliminary current passage is performed under the same condition as the test welding for the welding condition, or preliminary current passage is performed by adaptive control welding with the time variation curve of the instantaneous amount of heat generated and the cumulative amount of heat generated which are stored in the preliminary current passage of the test welding for the welding condition.

[0095] Test welding conditions other than the above are not limited. For example, the test welding conditions other than the above may be determined and set by performing a preliminary welding test for the same steel type and thickness as the parts to be welded, under various conditions by constant current control in each of a state of no disturbance and the foregoing simulated state of a disturbance.

[0096] Actual Welding

[0097] After the test welding, the actual welding is performed.

[0098] In the actual welding, electrode force application to the parts to be welded is performed under each of the same conditions as in the test welding before the start of current passage, an electrode force parameter from when the electrode force application starts to when the set electrode force is reached and the electrode force parameter stored in the test welding are compared to determine the difference therebetween for each of the welding conditions of the test welding, and a time variation curve of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume (hereafter also referred to as time variation curve, etc.) in main current passage in the test welding that are stored for the welding condition corresponding to the smallest difference are set as the target in main current passage in the actual welding.

[0099] We consider the reason why the use of the electrode force parameter enables selecting, as the target, the optimal one of the plurality of time variation curves of the instantaneous amounts of heat generated, etc. based on the effect of a disturbance, as follows.

[0100] For example, in the case where there is a gap between the mating surfaces of the metal sheets, it is necessary to deform the metal sheets to eliminate the gap and bring the metal sheets into contact with each other at the welding position, in order to perform current passage. Here, for example, since the force applied by the upper electrode first causes the deformation of the metal sheets, the force applied by the upper electrode does not sufficiently act on the lower electrode.

[0101] Therefore, the electrode force parameter such as the time T.sub.F from when the electrode force application starts to when the set electrode force F is reached or the electrode force change rate F from when the electrode force application starts to when the set electrode force is reached changes depending on the degree of a disturbance such as a gap between the mating surfaces of the metal sheets.

[0102] Thus, the use of the electrode force parameter enables selecting, as the target, the optimal time variation curve, etc. from the plurality of time variation curves, etc. based on the effect of a disturbance.

[0103] Performing electrode force application to the parts to be welded under the same condition as in the test welding means that in the actual welding, electrode force application is performed to the parts to be welded of the same condition (the same steel type and thickness) as in the test welding, with the set electrode force at the start of current passage being the same as the set electrode force at the start of current passage in the test welding.

[0104] A typical resistance spot welding device is configured to switch the control method (position control torque control) when a given electrode force (hereafter also referred to as switching set value) between the start of electrode force application to the time at which a set electrode force is reached is reached. Generally, electrode force application using the same electrode force application means and the same switching set value can be regarded as electrode force application under the same condition (assuming that the set electrode force at the start of current passage is the same).

[0105] Then, in the actual welding, adaptive control welding is performed to control the current passage amount according to the target set as described above.

[0106] For example, in the adaptive control welding, welding is performed with the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume being set as the target. If the amount of time variation of the instantaneous amount of heat generated per unit volume follows the time variation curve, the welding is continued without change and completed. If the amount of time variation of the instantaneous amount of heat generated per unit volume differs from the time variation curve, the current passage amount is controlled in order to compensate for the difference within a remaining welding time so that the cumulative amount of heat generated per unit volume in the actual welding matches the cumulative amount of heat generated per unit volume set as the target.

[0107] The method of calculating the amount of heat generated is not limited. PTL 5 describes an example of the method, which may be used herein. The following is the procedure of calculating the amount q of heat generated per unit volume and per unit time and the cumulative amount Q of heat generated per unit volume according to this method.

[0108] Let t be the total thickness of the parts to be welded, r be the electrical resistivity of the parts to be welded, V be the voltage between electrodes, I be the welding current, and S be the contact area of the electrodes and the parts to be welded. In this case, the welding current passes through a columnar portion whose cross-sectional area is S and thickness is t, to generate heat by resistance. The amount q of heat generated per unit volume and per unit time in the columnar portion is given by the following Equation (1):

q=(V.Math.I)/(S.Math.t)(1).

[0109] The electrical resistance R of the columnar portion is given by the following Equation (2):

R=(r.Math.t)/S(2).

[0110] Solving Equation (2) for S and substituting the solution into Equation (1) yields the amount q of heat generated as indicated by the following Equation (3):

q=(V.Math.I.Math.R)/(r.Math.t.sup.2)=(V.sup.2)/(r.Math.t.sup.2)(3).

[0111] As is clear from Equation (3), the amount q of heat generated per unit volume and per unit time can be calculated from the voltage between electrodes V, the total thickness t of the parts to be welded, and the electrical resistivity r of the parts to be welded, and is not affected by the contact area S of the electrodes and the parts to be welded. Although the amount of heat generated is calculated from the voltage between electrodes V in Equation (3), the amount q of heat generated may be calculated from the interelectrode current I. The contact area S of the electrodes and the parts to be welded need not be used in this case, either. By cumulating the amount q of heat generated per unit volume and per unit time for the welding time, the cumulative amount Q of heat generated per unit volume for the welding is obtained. As is clear from Equation (3), the cumulative amount Q of heat generated per unit volume can also be calculated without using the contact area S of the electrodes and the parts to be welded.

[0112] Although the above describes the case of calculating the cumulative amount Q of heat generated by the method described in PTL 5, the cumulative amount Q may be calculated by any other method.

[0113] In the case where the current pattern in the test welding is divided into two or more steps and the target of adaptive control welding is set based on the data stored in the test welding for the welding condition divided into the two or more steps as mentioned above, the current pattern in the actual welding is preferably divided into two or more steps as with the current pattern in the test welding and the adaptive control welding is preferably performed for each step in the actual welding.

[0114] In the adaptive control welding for each step, if the amount of time variation of the instantaneous amount of heat generated per unit volume in the step differs from the time variation curve, the current passage amount is controlled in order to compensate for the difference within a remaining welding time in the step so that the cumulative amount of heat generated per unit volume in the step matches the cumulative amount of heat generated per unit volume in the step in the test welding.

[0115] Particularly when the current path changes greatly between the test welding and the actual welding due to current shunting to an existing weld, with adaptive control welding of one step, the welding current may rapidly change in a short time in an attempt to achieve the target cumulative amount of heat generated.

[0116] Thus, with adaptive control welding of one step, there is a concern that, due to a failure to respond to the change of the amount of heat generated or a significant change of the heat generation pattern from the test welding, expulsion occurs or the target nugget diameter cannot be obtained.

[0117] By dividing the current pattern into two or more steps, the target cumulative amount of heat generated can be optimized for each step. Consequently, even in the case where variations in disturbance, particularly current shunting to an existing weld, are significant, the target nugget diameter can be obtained in response to the variations in disturbance.

[0118] The parts to be welded that are used are not limited. The resistance spot welding method may be used for welding of steel sheets and coated steel sheets having various strengths from mild steel to ultra high tensile strength steel and light metal sheets of aluminum alloys and the like. The resistance spot welding method may also be used for a sheet combination of three or more overlapping steel sheets.

[0119] Moreover, a subsequent current may be applied to heat-treat the weld after the current for nugget formation. The current passage condition in this case is not limited, and the magnitude relationships with the welding currents in the preceding steps are not limited. The electrode force in the current passage may be constant, or be changed as appropriate.

[0120] By joining a plurality of overlapping metal sheets by the resistance spot welding method described above, various weld members, in particular weld members of automotive parts and the like, are produced while stably ensuring a desired nugget diameter by effectively responding to variations in the disturbance state.

Examples

[0121] Test welding was performed under the conditions listed in Table 1 for each sheet combination of two or three overlapping metal sheets listed in Table 1, and then actual welding was performed under the conditions listed in Table 2 for each sheet combination of two or three overlapping metal sheets listed in Table 2, to produce a weld joint (weld member).

[0122] The test welding and the actual welding were performed in a state of no disturbance as illustrated in each of FIGS. 1 and 2, and in a simulated state of a disturbance as illustrated in each of FIGS. 3 to 6. In the drawings, reference signs 11, 12, and 13 are each a metal sheet, 14 is an electrode, 15 is a spacer, and 16 is an existing weld. In FIGS. 3 and 4, spacers 15 were inserted between the metal sheets 11 and 12 and between the metal sheets 12 and 13, and the sheet combination was clamped from above and below (not illustrated), to create a sheet gap of any of various sheet gap thicknesses tg. The sheet gap distance was 40 mm in all cases. As illustrated in FIGS. 5 and 6, there were two existing welds 16, and the welding position (the center between the electrodes) was adjusted to be at a midpoint between the existing welds (i.e. the same distance L from each existing weld).

[0123] In the test welding, the time T.sub.F from when electrode force application started to when the set electrode force F was reached was stored. Moreover, current passage was performed by constant current control for each welding condition in the current pattern illustrated in FIG. 7 or 8, and the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume were stored.

[0124] In the actual welding, electrode force application to the parts to be welded was performed under each of the same conditions as in the test welding before the start of current passage, the time T.sub.F from when the electrode force application started to when the set electrode force F was reached in the actual welding and the time T.sub.F stored in the test welding were compared to determine the difference therebetween for each welding condition (compared test welding No. in Table 2), and the time variation curve of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume stored for the welding condition corresponding to the smallest difference were set as the target when performing adaptive control welding in the actual welding. Adaptive control welding was performed to control the current passage amount according to the target.

[0125] An inverter DC resistance spot welder was used as the welder, and chromium copper electrodes with 6 mm face diameter DR-shaped tips were used as the electrodes.

[0126] For each obtained joint, the weld was cut and etched in section, and then observed with an optical microscope. Whether the nugget diameter between the metal sheets was not less than 4.5t as a target diameter (t: the sheet thickness (mm) of the thinner metal sheet of adjacent two metal sheets) was determined, and evaluation was conducted according to the following criteria. For a sheet combination of three overlapping sheets, evaluation was conducted based on the nugget diameter between the thinnest outer metal sheet and the metal sheet adjacent to it.

[0127] A (pass, excellent): the target nugget diameter was ensured and no expulsion occurred under all conditions regardless of the disturbance.

[0128] B (pass, good): the target nugget diameter was ensured and no expulsion occurred under all conditions except in the case where the effect of current shunting was very significant (the distance L from an existing weld=10 mm).

[0129] C (pass): the target nugget diameter was ensured and no expulsion occurred under all conditions except in the case where the effect of a disturbance was very significant (the distance L from an existing weld=10 mm and the sheet gap thickness tg=2.0 mm).

[0130] F (fail): the target nugget diameter was not ensured and/or expulsion occurred under at least one of the following conditions: the distance L from an existing weld=20 mm; the distance L from an existing weld=40 mm; the sheet gap thickness tg=1.0 mm; the sheet gap thickness tg=0.5 mm; and no disturbance.

TABLE-US-00001 TABLE 1 Test welding condition Time to reach Elec- Sheet combination Set set trode First step Second step Steel sheet Steel sheet Steel sheet elec- elec- force Weld- Weld- Weld- Weld- Test of reference of reference of reference trode trode change ing ing ing ing weld- sign 11 sign 12 sign 13 force force rate current time current time Distur- ing in the in the in the F T.sub.F F I1 T1 I2 T2 bance No. drawings drawings drawings (kN) (ms) (kN/ms) (kA) (ms) (KA) (ms) state A 980MPa-grade 980MPa-grade 5.0 185 0.0270 7.5 320 None cold- cold- rolled steel rolled steel sheet (sheet sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) B 980MPa-grade 980MPa-grade 5.0 250 0.0200 7.3 320 Sheet cold- cold- gap rolled steel rolled steel tg = sheet (sheet sheet (sheet 1.0 thickness: thickness: mm 1.6 mm) 1.6 mm) C 980MPa-grade 980MPa-grade 5.0 285 0.0175 6.9 320 Sheet cold- cold- gap rolled steel rolled steel tg = sheet (sheet sheet (sheet 2.0 thickness: thickness: mm 1.6 mm) 1.6 mm) D 980MPa-grade 980MPa-grade 5.0 185 0.0270 4.0 140 7.5 280 None cold- cold-rolled steel rolled steel sheet (sheet sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) E 980MPa-grade 980MPa-grade 5.0 250 0.0200 4.0 140 7.3 280 Sheet cold-rolled steel cold-rolled steel gap sheet (sheet sheet (sheet tg = thickness: thickness: 1.0 1.6 mm) 1.6 mm) mm F 980MPa-grade 980MPa-grade 5.0 285 0.0175 4.0 140 6.9 280 Sheet cold-rolled steel cold-rolled steel gap sheet (sheet sheet (sheet tg = thickness: thickness: 2.0 1.6 mm) 1.6 mm) mm G 1180MPa-grade 1180MPa-grade 5.0 190 0.0263 5.0 120 8 240 None GA steel GA steel sheet (sheet sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) H 1180MPa-grade 1180MPa-grade 5.0 255 0.0196 4.5 140 7.5 280 Sheet GA steel GA steel gap sheet (sheet sheet (sheet tg = thickness: thickness: 1.0 1.6 mm) 1.6 mm) mm I 1180MPa-grade 1180MPa-grade 5.0 295 0.0169 4.5 160 7 320 Sheet GA steel GA steel gap sheet (sheet sheet (sheet tg = thickness: thickness: 2.0 1.6 mm) 1.6 mm) mm J 1470MPa-grade 1470MPa-grade 5.0 140 0.0357 8.0 320 None cold- cold- rolled steel rolled steel sheet (sheet sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) K 1470MPa-grade 1470MPa-grade 5.0 180 0.0278 9.5 320 Sheet cold-rolled steel cold-rolled steel gap sheet (sheet sheet (sheet tg = thickness: thickness: 1.0 1.6 mm) 1.6 mm) mm L 1470MPa-grade 1470MPa-grade 5.0 230 0.0217 7.5 400 Sheet cold- cold- gap rolled steel rolled steel tg = sheet (sheet sheet (sheet 2.0 thickness: thickness: mm 1.6 mm) 1.6 mm) M 270MPa-grade 980MPa-grade 980MPa-grade 5.0 160 0.0313 5.0 120 8.0 260 None GA steel GA steel GA steel sheet (sheet sheet (sheet sheet (sheet thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.2 mm) N 270MPa-grade 980MPa-grade 980MPa-grade 5.0 205 0.0244 5.0 120 7.8 260 Sheet GA steel GA steel GA steel gap sheet (sheet sheet (sheet sheet (sheet tg = thickness: thickness: thickness: 1.0 0.7 mm) 1.4 mm) 1.2 mm) mm O 270MPa-grade 980MPa-grade 980MPa-grade 5.0 237 0.0211 5.0 120 7.5 260 Sheet GA steel GA steel GA steel gap sheet (sheet sheet (sheet sheet (sheet tg = thickness: thickness: thickness: 1.6 0.7 mm) 1.4 mm) 1.2 mm) mm P 1470MPa-grade 1470MPa-grade 4.0 151 0.0265 5.0 100 8.0 140 None GA steel GA steel sheet (sheet sheet (sheet thickness: thickness: 1.0 mm) 1.0 mm) Q 1470MPa-grade 1470MPa-grade 4.0 186 0.0215 4.5 100 8.0 140 Sheet GA steel GA steel gap sheet (sheet sheet (sheet tg = thickness: thickness: 1.0 1.0 mm) 1.0 mm) mm R 1470MPa-grade 1470MPa-grade 4.0 224 0.0179 4.5 100 7.5 140 Sheet GA steel GA steel gap sheet (sheet sheet (sheet tg = thickness: thickness: 1.6 1.0 mm) 1.0 mm) mm S 1800MPa-grade 1800MPa-grade 5.0 300 0.0167 4.0 120 6.0 260 None Zn-Ni- Zn-Ni- coated hot coated hot stamp steel stamp steel sheet (sheet sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) T 1800MPa-grade 1800MPa-grade 5.0 332 0.0151 4.0 120 5.9 260 Sheet Zn-Ni- Zn-Ni- gap coated hot coated hot tg = stamp steel stamp steel 1.0 sheet (sheet sheet (sheet mm thickness: thickness: 1.6 mm) 1.6 mm) U 1800MPa-grade 1800MPa-grade 5.0 367 0.0136 3.5 200 5.7 260 Sheet Zn-Ni- Zn-Ni- gap coated hot coated hot tg = stamp steel stamp steel 2.0 sheet (sheet sheet (sheet mm thickness: thickness: 1.6 mm) 1.6 mm) V 270MPa-grade 2000MPa-grade 1180MPa- 6.0 101 0.0594 4.5 180 6.5 280 None GA steel Zn-Ni- grade sheet coated hot GA steel (sheet stamp steel sheet (sheet thickness: sheet (sheet thickness: 0.7 mm) thickness: 1.6 mm) 1.4 mm) W 270MPa- 2000MPa-grade 1180MPa- 6.0 152 0.0395 4.5 180 6.4 280 Sheet grade GA Zn-Ni- grade gap steel sheet coated hot GA steel tg = (sheet stamp steel sheet (sheet 1.0 thickness: sheet (sheet thickness: mm 0.7 mm) thickness: 1.6 mm) 1.4 mm) X 270MPa-grade 2000MPa-grade 1180MPa- 6.0 188 0.0319 4.0 240 6.0 280 Sheet GA Zn-Ni- grade GA gap steel sheet coated hot steel sheet tg = (sheet stamp steel (sheet 1.6 thickness: sheet (sheet thickness: mm 0.7 mm) thickness: 1.6 mm) 1.4 mm)

TABLE-US-00002 TABLE 2 Actual welding condition Test weld- ing Time No. to Elec- fol- Sheet combination reach trode lowed Com- Steel set force in pared sheet of Set elec- change adapt- Nug- test Steel sheet of Steel sheet of reference elec- trode rate ive get weld- reference reference sign sign 13 trode force F Distur- control dia- Joint ing sign 11 in 12 in the force T.sub.F (kN/ bance weld- meter Expul- Evalu- Re- No, No. No. the drawings in the drawings drawings (kN) (ms) ms) state ing (mm) sion ation marks 1 1-1 A,B 980MPa-grade 980MPa-grade 5.0 184 0.0272 None A 6.2 None C Ex. cold-rolled cold-rolled steel sheet steel (sheet sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 1-2 980MPa-grade 980MPa-grade 5.0 201 0.0249 Sheet A 6.4 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 0.5 mm thickness: thickness: 1.6 mm) 1.6 mm) 1-3 980MPa-grade 980MPa-grade 5.0 252 0.0198 Sheet gap B 6.1 None cold- cold-rolled tg = rolled steel steel sheet 1.0 mm sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 1-4 980MPa-grade 980MPa-grade 5.0 285 0.0175 Sheet gap B 4.5 Oc- cold-rolled cold-rolled tg = curred steel sheet steel sheet 2.0 mm (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 1-5 980MPa-grade 980MPa-grade 5.0 186 0.0269 Existing A 6.1 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 40 mm thickness: thickness: 1.6 mm) 1.6 mm) 1-6 980MPa-grade 980MPa-grade 5.0 187 0.0267 Existing A 5.9 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 20 mm thickness: thickness: 1.6 mm) 1.6 mm) 1-7 980MPa-grade 980MPa-grade 5.0 187 0.0267 Existing A 4.9 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 10 mm thickness: thickness: 1.6 mm) 1.6 mm) 2 2-1 A,B,C 980MPa-grade 980MPa-grade 5.0 184 0.0272 None A 6.2 None B Ex. cold-rolled cold-rolled steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 2-2 980MPa-grade 980MPa-grade 5.0 201 0.0249 Sheet A 6.4 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 0.5 mm thickness: thickness: 1.6 mm) 1.6 mm) 2-3 980MPa-grade 980MPa-grade 5.0 252 0.0198 Sheet B 6.1 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 1.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 2-4 980MPa-grade 980MPa-grade 5.0 285 0.0175 Sheet C 5.9 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 2.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 2-5 980MPa-grade 980MPa-grade 5.0 186 0.0269 Existing A 6.1 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 40 mm thickness: thickness: 1.6 mm) 1.6 mm) 2-6 980MPa-grade 980MPa-grade 5.0 187 0.0267 Existing A 5.9 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 20 mm thickness: thickness: 1.6 mm) 1.6 mm) 2-7 980MPa-grade 980MPa-grade 5.0 187 0.0267 Existing A 4.9 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 10 mm thickness: thickness: 1.6 mm) 1.6 mm) 3 3-1 B,C,D 980MPa-grade 980MPa-grade 5.0 184 0.0272 None D 6.3 None A Ex. cold-rolled cold-rolled steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 3-2 980MPa-grade 980MPa-grade 5.0 201 0.0249 Sheet D 6.3 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 0.5 mm thickness: thickness: 1.6 mm) 1.6 mm) 3-3 980MPa-grade 980MPa-grade 5.0 252 0.0198 Sheet B 6.1 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 1.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 3-4 980MPa-grade 980MPa-grade 5.0 285 0.0175 Sheet C 5.9 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 2.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 3-5 980MPa-grade 980MPa-grade 5.0 186 0.0269 Existing D 6.2 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 40 mm thickness: thickness: 1.6 mm) 1.6 mm) 3-6 980MPa-grade 980MPa-grade 5.0 187 0.0267 L = D 6.1 None cold-rolled cold-rolled 20 mm steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 3-7 980MPa-grade 980MPa-grade 5.0 187 0.0267 Existing D 6.3 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 10 mm thickness: thickness: 1.6 mm) 1.6 mm) 4 4-1 D,E,F 980MPa-grade 980MPa-grade 5.0 185 0.0270 None D 6.3 None A Ex. cold-rolled cold-rolled steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 4-2 980MPa-grade 980MPa-grade 5.0 202 0.0248 Sheet D 6.3 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 0.5 mm thickness: thickness: 1.6 mm) 1.6 mm) 4-3 980MPa-grade 980MPa-grade 5.0 248 0.0202 Sheet E 6.2 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 1.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 4-4 980MPa-grade 980MPa-grade 5.0 290 0.0172 Sheet F 6.3 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 2.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 4-5 980MPa-grade 980MPa-grade 5.0 182 0.0275 Existing D 6.2 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 40 mm thickness: thickness: 1.6 mm) 1.6 mm) 4-6 980MPa-grade 980MPa-grade 5.0 187 0.0267 Existing D 6.1 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 20 mm thickness: thickness: 1.6 mm) 1.6 mm) 4-7 980MPa-grade 980MPa-grade 5.0 188 0.0266 Existing D 6.3 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 10 mm thickness: thickness: 1.6 mm) 1.6 mm) 5 5-1 A 980MPa-grade 980MPa-grade 5.0 184 0.0272 None A 6.2 None F Comp. cold-rolled cold-rolled Ex. steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 5-2 980MPa-grade 980MPa-grade 5.0 201 0.0249 Sheet A 6.4 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 0.5 mm thickness: thickness: 1.6 mm) 1.6 mm) 5-3 980MPa-grade 980MPa-grade 5.0 252 0.0198 Sheet A 4.5 Oc- cold-rolled cold-rolled gap curred steel sheet steel sheet tg = (sheet (sheet 1.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 5-4 980MPa-grade 980MPa-grade 5.0 285 0.0175 Sheet A 4.2 Oc- cold-rolled cold-rolled gap curred steel sheet steel sheet tg = (sheet (sheet 2.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 5-5 980MPa-grade 980MPa-grade 5.0 186 0.0269 Existing A 6.1 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 40 mm thickness: thickness: 1.6 mm) 1.6 mm) 5-6 980MPa-grade 980MPa-grade 5.0 187 0.0267 Existing A 5.9 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 20 mm thickness: thickness: 1.6 mm) 1.6 mm) 5-7 980MPa-grade 980MPa-grade 5.0 187 0.0267 Existing A 4.9 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 10 mm thickness: thickness: 1.6 mm) 1.6 mm) 6 6-1 G,H,I 1180MPa- 1180a-grade 5.0 190 0.0263 None G 6.1 None A Ex. grade GA GA steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 6-2 1180MPa- 1180MPa- 5.0 205 0.0244 Sheet gap G 6.2 None grade GA grade GA tg = steel sheet steel sheet 0.5 mm (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 6-3 1180MPa- 1180MPa- 5.0 252 0.0198 Sheet gap H 6.2 None grade GA grade GA tg = steel sheet steel sheet 1.0 mm (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 6-4 1180MPa- 1180MPa- 5.0 294 0.0170 Sheet gap I 6.0 None grade GA grade GA tg = steel sheet steel sheet 2.0 mm (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 6-5 1180MPa- 1180MPa- 5.0 191 0.0262 Existing G 5.9 None grade GA grade GA weld steel sheet steel sheet L = (sheet (sheet 40 mm thickness: thickness: 1.6 mm) 1.6 mm) 6-6 1180MPa- 1180MPa- 5.0 190 0.0263 Existing G 5.9 None grade GA grade GA weld steel sheet steel sheet L = (sheet (sheet 20 mm thickness: thickness: 1.6 mm) 1.6 mm) 6-7 1180MPa- 1180MPa- 5.0 188 0.0266 Existing G 6.3 None grade GA grade GA weld steel sheet steel sheet L = (sheet (sheet 10 mm thickness: thickness: 1.6 mm) 1.6 mm) 7 7-1 J,K,L 1470MPa-grade 1470MPa-grade 5.0 141 0.0355 None J 6.2 None B Ex. cold-rolled cold-rolled steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 7-2 1470MPa-grade 1470MPa-grade 5.0 151 0.0331 Sheet J 6.3 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 0.5 mm thickness: thickness: 1.6 mm) 1.6 mm) 7-3 1470MPa-grade 1470MPa-grade 5.0 178 0.0281 Sheet K 6.0 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 1.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 7-4 1470MPa-grade 1470MPa-grade 5.0 230 0.0217 Sheet L 5.9 None cold-rolled cold-rolled gap steel sheet steel sheet tg = (sheet (sheet 2.0 mm thickness: thickness: 1.6 mm) 1.6 mm) 7-5 1470MPa-grade 1470MPa-grade 5.0 140 0.0357 Existing J 5.9 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 40 mm thickness: thickness: 1.6 mm) 1.6 mm) 7-6 1470MPa-grade 1470MPa-grade 5.0 140 0.0357 Existing J 5.8 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 20 mm thickness: thickness: 1.6 mm) 1.6 mm) 7-7 1470MPa-grade 1470MPa-grade 5.0 139 0.0360 Existing J 4.7 None cold-rolled cold-rolled weld steel sheet steel sheet L = (sheet (sheet 10 mm thickness: thickness: 1.6 mm) 1.6 mm) 8 8-1 M,N,O 270MPa- 980MPa- 980MPa- grade GA grade GA grade GA 5.0 164 0.0305 None M 4.0 None A Ex. steel sheet steel sheet steel sheet (sheet (sheet (sheet thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.2 mm) 8-2 270MPa- 980MPa- 980MPa- 5.0 179 0.0279 Sheet M 4.2 None grade GA grade GA grade GA gap steel sheet steel sheet steel sheet tg = (sheet (sheet (sheet 0.5 mm thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.2 mm) 8-3 270MPa- 980MPa- 980MPa- 5.0 210 0.0238 Sheet N 4.1 None grade GA grade GA grade GA gap steel sheet steel sheet steel sheet tg = (sheet (sheet (sheet 1.0 mm thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.2 mm) 8-4 270MPa- 980MPa- 980MPa- 5.0 237 0.0211 Sheetgap O 4.0 None grade GA grade GA grade GA tg = steel sheet steel sheet steel sheet 1.6 mm (sheet (sheet (sheet thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.2 mm) 8-5 270MPa- 980MPa- 980MPa- 5.0 162 0.0309 Existing M 4.1 None grade GA grade GA grade GA weld steel sheet steel sheet steel sheet L = (sheet (sheet (sheet 40 mm thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.2 mm) 8-6 270MPa- 980MPa- 980MPa- 5.0 161 0.0311 Existing M 4.3 None grade GA grade GA grade GA weld steel sheet steel sheet steel sheet L = (sheet (sheet (sheet 20 mm thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.2 mm) 8-7 270MPa- 980MPa- 980MPa- 5.0 162 0.0309 Existing M 4.4 None grade GA grade GA grade GA weld steel sheet steel sheet steel sheet L = (sheet (sheet (sheet 10 mm thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.2 mm) 9 9-1 P,Q,R 1470MPa- 1470MPa- 4.0 150 0.0267 None P 5.1 None A Ex. grade GA grade GA steel sheet steel sheet (sheet (sheet thickness: thickness: 1.0 mm) 1.0 mm) 9-2 1470MPa- 1470MPa- 4.0 160 0.0250 Sheet gap P 5.0 None grade GA grade GA tg = steel sheet steel sheet 0.5 mm (sheet (sheet thickness: thickness: 1.0 mm) 1.0 mm) 9-3 1470MPa- 1470MPa- 4.0 186 0.0215 Sheet gap Q 5.1 None grade GA grade GA tg = steel sheet steel sheet 1.0 mm (sheet (sheet thickness: thickness: 1.0 mm) 1.0 mm) 9-4 1470MPa- 1470MPa- 4.0 222 0.0180 Sheet gap R 5.4 None grade GA grade GA tg = steel sheet steel sheet 1.6 mm (sheet (sheet thickness: thickness: 1.0 mm) 1.0 mm) 9-5 1470MPa- 1470MPa- 4.0 148 0.0270 Existing P 5.0 None grade GA grade GA weld steel sheet steel sheet L = (sheet (sheet 40 mm thickness: thickness: 1.0 mm) 1.0 mm) 9-6 1470MPa- 1470MPa- 4.0 147 0.0272 Existing P 4.8 None grade GA grade GA weld steel sheet steel sheet L = (sheet (sheet 20 mm thickness: thickness: 1.0 mm) 1.0 mm) 9-7 1470MPa- 1470MPa- 4.0 149 0.0268 Existing P 5.2 None grade GA grade GA weld steel sheet steel sheet L = (sheet (sheet 10 mm thickness: thickness: 1.0 mm) 1.0 mm) 10 10-1 S,T,U 1800MPa-grade 1800MPa-grade 5.0 305 0.0164 None S 6.2 None A Ex. Zn-Ni-coated Zn-Ni-coated hot stamp hot stamp steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 10-2 1800MPa-grade 1800MPa-grade 5.0 312 0.0160 Sheet gap S 6.4 None Zn-Ni-coated Zn-Ni-coated tg = hot stamp hot stamp 0.5 mm steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 10-3 1800MPa-grade 1800MPa-grade 5.0 333 0.0150 Sheet gap T 5.9 None Zn-Ni-coated Zn-Ni-coated tg = hot stamp hot stamp 1.0 mm steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 10-4 1800MPa-grade 1800MPa-grade 5.0 370 0.0135 Sheet gap U 5.8 None Zn-Ni-coated Zn-Ni-coated tg = hot stamp hot stamp 2.0 mm steel sheet steel sheet (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 10-5 1800MPa-grade 1800MPa-grade 5.0 299 0.0167 Existing S 6.2 None Zn-Ni-coated Zn-Ni-coated weld hot stamp hot stamp L = steel sheet steel sheet 40 mm (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 10-6 1800MPa-grade 1800MPa-grade 5.0 302 0.0166 Existing S 6.1 None Zn-Ni-coated Zn-Ni-coated weld hot stamp hot stamp L = steel sheet steel sheet 20 mm (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 10-7 1800MPa-grade 1800MPa-grade 5.0 305 0.0164 Existing S 6.4 None Zn-Ni-coated Zn-Ni-coated weld hot stamp hot stamp L = steel sheet steel sheet 10 mm (sheet (sheet thickness: thickness: 1.6 mm) 1.6 mm) 11 11-1 V,W,X 270MPa- 2000MPa-grade 1180MPa- 6.0 102 0.0588 None V 4.2 None A Ex. grade GA Zn-Ni-coated grade GA steel sheet hot stamp steel sheet (sheet steel sheet (sheet thickness (sheet thickness 0.7 mm) thickness: 1.6 mm) 1.4 mm) 11-2 270MPa-grade 2000MPa-grade 1180MPa- 6.0 120 0.0500 Sheet gap V 4.3 None GA steel Zn-Ni- grade GA tg = sheet coated hot steel sheet 0.5 mm (sheet stamp steel (sheet thickness sheet (sheet thickness 0.7 mm) thickness: 1.6 mm) 1.4 mm) 11-3 270MPa- 2000MPa-grade 1180MPa- 6.0 152 0.0395 Sheet gap W 4.2 None grade GA Zn-Ni- grade tg = steel sheet coated hot GA steel 1.0 mm (sheet stamp steel sheet thickness sheet (sheet (sheet 0.7 mm) thickness: thickness 1.4 mm) 1.6 mm) 11-4 270MPa-grade 2000MPa-grade 1180MPa- 6.0 190 0.0316 Sheet gap X 4.0 None GA steel Zn-Ni-coated grade GA tg = sheet hot stamp steel sheet 1.6 mm (sheet steel sheet (sheet thickness (sheet thickness 0.7 mm) thickness: 1.6 mm) 1.4 mm) 11-5 270MPa-grade 2000MPa-grade 1180MPa- 6.0 101 0.0594 Existing V 4.4 None GA Zn-Ni-coated hot grade GA weld steel sheet stamp steel steel sheet L = (sheet sheet (sheet (sheet 40 mm thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.6 mm) 11-6 270MPa-grade 2000MPa-grade 1180MPa- 6.0 100 0.0600 Existing V 4.3 None GA Zn-Ni-coated grade GA weld steel sheet hot stamp steel steel sheet L = (sheet sheet (sheet (sheet 20 mm thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.6 mm) 11-7 270MPa-grade 2000MPa-grade 1180MPa- 6.0 105 0.0571 Existing V 4.5 None GA Zn-Ni-coated grade GA weld steel sheet hot stamp steel steel sheet L = (sheet sheet (sheet (sheet 10 mm thickness: thickness: thickness: 0.7 mm) 1.4 mm) 1.6 mm)

[0131] In all Examples (Ex.), the target nugget diameter was obtained with no expulsion as a result of effectively responding to most of variations in disturbance. Particularly in Nos. 3, 4, 6, and 8 to 11, the target nugget diameter was obtained with no expulsion as a result of effectively responding to variations in disturbance, regardless of the type or degree of disturbance.

[0132] In Comparative Examples (Comp. Ex.), expulsion occurred or a nugget with a sufficient diameter was not formed in the case where the effect of a disturbance was particularly significant, without effectively responding to variations in disturbance.

[0133] The same results as above were obtained in the case where the electrode force change rate F from when the electrode force application started to when the set electrode force was reached was stored in the test welding and the target of the time variation curve of the instantaneous amount of heat generated and the cumulative amount of heat generated when performing adaptive control welding in the actual welding was set using F.

REFERENCE SIGNS LIST

[0134] 11, 12 metal sheet [0135] 14 electrode [0136] 15 spacer [0137] 16 existing weld