Effect of Prior Austenite Grain Size and Solidification Segregation on the Joint Strength in Resistance Spot Welds

Recently, high-strength steels have been widely used in automobile bodies to improve collision safety and fuel efficiency. However, for high-strength steels with strengths of at least 780 and 1500 MPa, the cross-tension strength (CTS) and tensile-shear strength (TSS) of resistance spot-weld joints decrease, respectively, with increasing steel strength. This result may be attributed to hardening of the nugget or segregation of impurity elements such as P and S. Methods of improving CTS include post-heating, which tempers or diffuses solidification segregation elements in the nugget, improving its toughness, while TSS can be improved via nugget hardening. In this study, resistance spot welded joints fabricated from lab-melted 1.5-GPa-class steel were heat-treated to change the state of solidification segregation and prior γ grain size, and the effects of these on joint strength were investigated. TSS and CTS were expected to vary with solidification segregation and prior γ grain size. TSS improved upon increasing Vickers hardness owing to solidification segregation of P and S and under uniform Vickers hardness because of relaxation of solidification segregation. CTS improved upon increasing toughness owing to relaxation of the solidification segregation of P and S and refinement of the prior γ grain size.