Analysis on Refinement of Columnar γ Grain by Phosphorus in Continuously Cast 0.1 mass% Carbon Steel

Abstract

Based on a grain growth model, this paper discusses the γ grain refinement by phosphorus (P) in as cast 0.1 mass% C slabs. Two important factors, the starting temperature of the rapid grain growth (T_rg) and the grain growth rate, are especially focused in the model. Local Equilibrium Mapping (LEM), an analytical method with local equilibrium calculations in the micro-segregation maps determined by EPMA, is carried out to evaluate the local transformation temperature and the γ phase fraction. The LEM analysis shows that the micro-segregation of P extends the δ/γ transformation temperature range to a lower temperature and retains the δ phase at a lower temperature. The Classical Grain Growth Model (CGM) based on a theory by Burke (1949) and Turnbull (1951) is derived, which assumes a normal grain growth with a parabolic law. The CGM successfully evaluates the γ grain growth curve in the as cast 0.1 mass% carbon steels considering the rapid growth after the δ/γ transformation and the change of the growth rate during cooling. The CGM analysis predicts the decrease of both T_rg and growth rate by the P addition. The decrease of T_rg was determined to be 160-170 K and agrees with the extension of the δ/γ transformation temperature range evaluated by the LEM analysis. Hence, the grain growth curves can be predicted by considering the extension of the δ/γ transformation temperature range due to the P addition and its micro-segregation.