자동차 차체용 TRIP강판의 저항 점용접부 Partial Interfacial Fracture 특성에 관한 연구

Resistance spot welding of TRIP780 steels was investigated to enhance understanding of weld fracture mode after tensile shear testing (TST) and L-shape tensile testing (LTT). The main failure mode for spot welds of TRIP780 steels was partial interfacial fracture (PIF). Although PIF does not satisfy the minimum button diameter (4√t) for acceptable welds, it shows enough load carrying capacity of resistance spot welds for advanced high strength steels. In the analysis of displacement controlled L-shape tensile test results, cracks initiated at the notch of the faying surface and propagated through the interface of weldments, and finally, cracks change path into the sheet thickness direction. Use of the ductility ratio and CE analysis suggested that the occurrence of PIF is closely related to high hardness and brittle welds, which are caused by fast cooling rates and high chemical compositions of TRIP steels. Analysis of the hold time and weld time in a welding schedule demonstrated that careful control of the cooling rate and the size of a weld nugget and the HAZ zone can reduce the occurrence of PIF, which leads to sound welds with button fractures (BFs).