The effects of the initial microstructure on microstructural evolution, mechanical properties and reversed austenite stability of intercritically annealed Fe-6.1Mn-1.5Si-0.12C steel

A typical medium Mn steel with nominal chemical composition of Fe-6.1Mn-1.47Si-0.12 C (wt%) was intercritically annealed from different initial microstructures, i.e., cold-rolled martensite and as-quenched martensite (hereinafter referred as CR and AQ sample, respectively). The CR sample is mainly composed of equiaxed sub-micron grains owing to nearly full recrystallization of deformed martensitic matrix, while the AQ sample still presents martensite lath structure in general. Reversed austenite grains in the CR sample are almost entirely granular. In the AQ samples, there are two kinds of microscopic morphology of reversed austenite, i.e., acicular and granular austenite, and the nucleation and growth of them are analyzed individually. Additionally, Reversed austenite fraction is not affected by initial microstructure irrespective of deformed and as-quenched martensite. Tensile strength and yield strength of the CR sample are about 80 MPa and 166 MPa higher than that of the AQ sample, respectively, but the PSE value of both CR and AQ sample is a little higher than 30 GPa%. Metastable austenite in the CR sample is more sensitive to the increasing strain, i.e., lower mechanical stability, which can be attributed to the higher nucleation rate of strain-induced martensitic transformation.