Significant austenite decomposition during slow heating of cold-rolled medium Mn steel with a high fraction of pre-existing austenite

In-depth analysis of phase transformations during continuous heating (2.4 °C/s) of 50 % cold-rolled Fe-6Mn-2Al-1Si-0.4C (wt.%) steel with 32 % pre-existing austenite was conducted using in-situ high-energy synchrotron X-ray diffraction and microstructural characterisation. Significant austenite decomposition was found to occur in two stages between 226 and 530 °C and 530–600 °C, involving bainitic transformation and cementite precipitation, respectively, reducing the austenite fraction to 17.5 %. Lattice parameter changes suggested C enrichment in austenite during bainite formation and C and Mn depletion during precipitation. The austenite fraction increased around 650 °C, yielding 8 % new austenite by 680 °C in ∼40 s and 7 % during isothermal holding at 680 °C for 600 s. A further 3 % decrease during cooling, presumably due to pearlite formation, resulted in a final austenite fraction of 29.5 %, slightly below the initial level but further enriched in C and Mn. These findings provide insights into austenite evolution during thermal processing of cold-rolled medium Mn steels.