Effects of Alloying Elements and Cooling Rate on Austenite Grain Growth in Solidification and the Subsequent Cooling Processes of Low Alloy Steels

Abstract

Effects of alloying elements and cooling rate on γ grain growth behavior in the solidification and subsequent cooling processes of low alloy steels have been studied in relation to surface cracking of continuously cast (CC) slabs.
Grain growth of γ phase occurs rapidly below a completion temperature of transformation into γ phase (T_γ ), since strong pinning effect of the second phase such as δ-ferrite and/or liquid phase on γ grain boundary migration disappears. Thus γ grain size of as-cast steels can mainly be determined by T_γ, i.e., the maximum grain size is obtained at a peritectic composition, as expected from Fe-C phase diagram. The carbon concentration at the peritectic point is significantly shifted by the addition of alloying elements. The coefficient k_x on the shifting was determined separately for Mn, Ni, Si, Cr and S. Using the k_x for element X, carbon equivalent (C_p) for perifectic reaction of low alloy steels can be written additively as
C_p= C+Σik_xi·X_i
In the same composition, the γ grains are largely refined by increasing the cooling rate because of the effects of decrease in T_γ and of the suppress of γ grain boundary migration below T_γ.