Characteristics of Welds of High Strength Steel Sheets with Ultra-fine Grained Microstructure Welded by Resistance Spot Heating Method.

Resistance spot heating which has a cooling rate of approximately 5000 K/s was carried out for high strength steel sheets with ultra-fine grained microstructure to investigate the characteristics of the welds. The steel sheets have a low carbon equivalent in order to retain good weldability and realize high strength by means of their ultra-fine grained microstructure. Though ferrite grains in the HAZ coarsened, the HAZ softening was restrained due to the formation of tiny dispersed MA constituents from γ phase at the boundary of the ferrite grains. Though the constitution of the nugget was martensite, the nugget hardness was restrained due to the low carbon content. Therefore the resistance spot welding joint of the steel sheets might have good tensile shear strength and good cross tension strength without the temper treatment. The prior austenite grain size was also examined for the steel sheets (the initial ferrite grain size was approximately 0.8 μm) and annealed steel sheets (the initial ferrite grain size was approximately 30 μm). The results indicate that the prior austenite grain size depends upon the initial ferrite grain size. The prior austenite grain size at the fusion boundary also decreased as the initial ferrite grain size decreased.