Back-Annealing of Hot Dip Galvanised Strip Steel Microalloyed with Vanadium

Abstract

Back-annealing during the hot dip galvanising (HDG) process was applied to heavily cold-worked low C-microalloyed VN steels in an attempt to achieve a yield strength of 700 MPa and a minimum A80 elongation of 10 percent. HDG simulations were performed to compare recovery and recrystallisation kinetics in VN steel with that experienced in plain low C and Nb-Ti grades. Based on these results, an industrial melt was subjected to conventional hot rolling, cold rolling and back-annealing cycles during HDG. Precipitation in both VN and Nb-Ti steels extended the recovery window by about 75 degrees Celsius when compared to that found in low CMn steel. A temperature-time parameter M was used to compare the rates of softening. The Mrs (recrystallisation start) in both VN and Nb-Ti steels was 20.1 compared to 18 in a low CMn grade. The above properties were achieved by subjecting low C-microalloyed VN steel to low finishing and low coiling temperatures followed by back-annealing heavily cold-worked strip to a maximum temperature to prevent full recrystallisation. The softening rate during annealing is higher in both microalloyed steels than the CMn grade, with recrystallisation being completed more rapidly in the VN steel. Laboratory results implied insensitivity of restoration behaviour to hot rolling parameters whilst industrial results suggest that they are effective.