Effect of Cooling Rate on High Temperature δ→γ Phase Transformation in Low Carbon Silicon-Bearing Steel

High temperature ferrite (δ)→austenite (γ) phase transformation in a low carbon silicon-bearing steel (0.98% Si) was observed by in-situ using a confocal scanning laser microscope(CSLM) at three different cooling rates(1,10,30 ℃·s-1). The effect of cooling rate on the transformation was analyzed. The results show that the undercooling increased and the starting transformation temperature lowered with increasing the cooling rate. At lower cooling rate, the γ phase nucleated only at the δ grain boundaries, but not happened inside the grains.While at higher cooling rate, the γ phase precipitated both at the δ grain boundaries and inside δ grains, and deeper undercooling was needed when inside the grains. The γ boundaries nibbled the δ matrix phase with plana-like patterns during its growth process. The migration velocity of the grain boundary increased with increasing the cooling rate.