抄録
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 (Trg) 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 Trg and growth rate by the P addition. The decrease of Trg 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.
