Nucleation of Proeutectoid Ferrite on Complex Precipitates in Austenite

Effect of various intragranular inclusions or precipitates (MnS, VC and V(C,N)) on the microstructure and kinetics of intragranular ferrite transformation at the temperatures between 973 and 823 K was studied using various Fe-2Mn-(0.13,0.2)C(mass%) alloys with the small addition of sulfur, vanadium, and nitrogen. In Fe-2Mn-0.13C-50ppmS and Fe-2Mn-0.2C-470ppmS alloys, MnS particles, mostly incoherent in austenite, do not act as effective nucleation sites of ferrite. V addition slightly improves the potency of MnS as ferrite nucleation site by forming MnS+VC complex precipitates. The addition of both V and N largely enhances the intragranular nucleation of ferrite idiomorph on the MnS+V(C,N) complex precipitate. It is considered that two factors, i.e., (1) the advantage in the balance of interphase boundary energy and (2) the increase in the fraction of V(C,N) precipitate by the addition of nitrogen, are mainly responsible for the promotion of intragranular ferrite formation on the MnS+V(C,N) complex precipitate.