Abstract
Recently, high-strength steel sheets are used to automotive structural components in order to reduce the body weight and improve the fuel efficiency. Resistance spot welding is used as method in jointing steel sheets. However, there is a problem that the joint strength decreases with improving the base metal strength. Therefore, in order to solve the problem, it is effort to make to control the weld hardness and improve the joint strength. However, there is few researches of detail study. In this study, focusing on the hardness of HAZ, the effects of HAZ hardness on joint strength characteristics of resistance spot welded high-strength steel sheets were investigated by experiment and numerical analysis. Numerical analysis was performed by changing the HAZ hardness to HV480, HV320 or HV160. First, in results of cross tension test and tensile shear test, cross tension strength and tensile shear strength were improved by decreasing HAZ hardness. Second, in results of numerical analysis, stress intensity factor at edge of nugget was deceased by decreasing HAZ hardness in cross tension analysis. Equivalent plastic strain at edge of nugget decreased in HV320 of HAZ hardness but it increased in HV480 of HAZ hardness. Therefore, it was suggested that the improvement of cross tension strength is affected by the blunting of the crack shape and decreasing in stress intensity factor. Meanwhile, it was suggested that improvement of tensile shear strength is affected by decreasing in equivalent plastic strain. However, in the case of the degree of HAZ softening is excessive, equivalent plastic strain increase and tensile shear strength decrease.
