Resistance spot welding method

A resistance spot welding method to join parts to be welded by squeezing and pressing the parts between a pair of electrodes and passing current, the parts being a plurality of overlapping metal sheets, the method comprising:

• dividing a current pattern into two or more steps for welding;
• before actual welding, performing test welding to store, for each step as a target value, a time variation 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 electrodes when forming an appropriate nugget by passing current with constant current control;
• and
• subsequently, as actual welding, starting welding using, as a standard, a time variation curve of the instantaneous amount of heat generated per unit volume obtained by the test welding, and when a time variation amount of an instantaneous amount of heat generated deviates during any step from the time variation curve by a difference, performing adaptive control welding to control a current passage amount in order to compensate for the difference during a remaining welding time in the step so as to match a cumulative amount of heat generated in the actual welding to the cumulative amount of heat generated that is determined in advance in the test welding, wherein
• the two or more steps for welding comprise a step of securing a current path between the sheets directly below the electrodes and a subsequent step of forming a nugget having a predetermined diameter, and
• a welding interval time is provided between the step of securing a current path between the sheets directly below the electrodes and the subsequent step of forming a nugget having a predetermined diameter.