Retained austenite in medium manganese steel with dissimilar morphology and composition

The microstructure and mechanical properties of medium manganese steel with a composition of 0.2C-5Mn-0.5Si-1.5Al (wt.%) were investigated, employing a two-stage heat treatment process. The microstructure following pretreatment comprised pearlite, martensite, and ferrite. Subsequent intercritical annealing resulted in the formation of retained austenite with varying morphology and composition. The filmy retained austenite transformed from pearlite exhibited a higher Mn content compared to the blocky retained austenite transformed from martensite, while also displaying a slower growth rate. Moreover, the filmy retained austenite exhibited superior thermal and mechanical stability. Tensile tests showed that the test steels not only obtained high strength (>1000 MPa) and high elongation (>30%), but the stress-strain curves also showed continuous yielding behaviour and a more reasonable ratio of tensile strength to yield strength.