Austenite transformation associated with δ-ferrite phase in a medium-Mn steel after cold-rolling and intercritical annealing

Abstract

By adjusting the initial δ-ferrite (δ) microstructure based on the cold-rolling, two types of unusual austenite (γ) transformation were investigated in a medium-Mn steel during intercritical annealing. First, δ/α' (martensite) phase interfaces are considered advantageous nucleation sites for γ transformation. When the cold-rolling reduction ratio reaches up to 70% (CR70 sample), δ phase morphology evolves from fibrous structure to small pieces, resulting in more δ/α' (martensite) phase interfaces. The results reveal that an increase in δ/α' phase interfaces promotes γ formation ranging from 15.7% to 17.9% (in volume fraction) for samples subjected to quenching and annealing. Furthermore, the γ nuclei are found to grow into both phases, accompanied by a gradual shrinking of local α' and δ phases due to γ transformation. Secondly, precipitation of carbide particles from δ phase in cold-rolled samples also creates a condition for subsequent γ nucleation within δ grains, and then the dissolution of the carbides accelerates γ development. Besides, the thin piece structure of δ phase in the CR70 sample can transform into γ phase at an elevated degree of austenitization. This study innovatively sheds light on the effect of δ phase on γ transformation in medium-Mn steels.