Dual refinement effect of Rare Earth addition on solidification structure of TRIP steels

A novel concept involving dual-refinement of solidification structure (referring to high-temperature ferrites δ-Fe and proeutectoid ferrites α-Fe) followed by dual-homogenization of solute atoms has been proposed accounting for the crystallization process of Nb/V-alloyed TRIP steels with Rare Earth (RE) addition. M(Nb,V)C carbides were predicted and observed easily precipitating on RE₂O₂S. Both RE₂O₂S and RE₂O₂S-MC can serve as heterogeneous nucleation sites for δ-Fe, and induce oriented epitaxy of α-Fe following certain orientation relationships. In above processes, RE segregation ahead of liquid-solid interfaces significantly hinders the movement of these interfaces thus promoting the intragranular dissolution of solute atoms. Furthermore, extensive precipitations of α-Fe from both austenite boundaries and grain interiors can also facilitate C/Mn homogenization. Such effects can be similarly reproduced in Nb/V-free TRIP steels. This study provides a strategy for microstructure tailoring of peritectic steels using RE-metamorphic products, and offers profound insights into alloy designing of RE-Nb(V) co-microalloying or replacing Nb(V) by RE.