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

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

Automated summary

By adjusting the initial delta-ferrite microstructure based on cold-rolling, two types of unusual austenite transformation were investigated in a medium-Mn steel during intercritical annealing. Firstly, an increase in delta/alpha' phase interfaces promotes gamma formation. Secondly, precipitation of carbide particles from delta phase in cold-rolled samples creates a condition for subsequent gamma nucleation.