Effects of Cooling Rate and Vanadium on δ→γ Phase Transformation in Low Carbon Steel

The phase transformation of delta-ferrite to austenite (δ→γ) in low carbon steel and vanadium micro-alloyed low carbon steel was in-situ observed utilizing confocal scanning laser microscope at three different cooling rates (0.17, 1, 10 ℃·s-1). The characteristics of δ→γ phase transformation in vanadium micro-alloyed low carbon steel at various cooling rates and the effects of cooling rate and vanadium were analyzed. The results show that the undercooling rose and the starting and ending temperatures dropped with the increase of cooling rate. At a low cooling rate, the γ phase nucleated prior at the triple points of δ-Fe, then at δ-Fe grain boundaries, finally nucleated in grains. At a high cooling rate, γ phase nucleated prior at the triple points of δ-Fe and δ-Fe grain boundaries almost simultaneously, then nucleated in grains quickly. The element vanadium can inhibit the movement of the triple points of δ-Fe, expand temperature range and prolong duration of δ→γ phase transformation.