Decoding the role of pre-existing austenite in transformation pathways during slow heating of cold-rolled medium-Mn steel for process design

• A new transformation pathway is revealed during slow heating of cold-rolled medium-Mn steel. • Pre-existing austenite decomposes into fine (Fe,Mn)-rich carbides that nucleate new austenite. • Controlled decomposition–reconstruction enables bimodal austenite with tailored chemistry. • A retained-austenite fraction of 55% is achieved at 780 °C without isothermal holding. • The two-step annealing route provides an efficient strategy to stabilize austenite and cut alloying costs. This study reveals a previously unreported transformation pathway governing austenite evolution during slow continuous heating of cold-rolled medium-Mn steel (Fe–0.4C–6Mn–2Al–1Si–0.05Nb, wt.%). Using a correlative approach combining in-situ high-energy X-ray diffraction, electron microscopy, atom probe tomography, and DICTRA simulations, we demonstrate that introducing a considerable amount of pre-existing austenite ( γ pre ) at the onset of the intercritical annealing region is beneficial for microstructural control which acts as both a solute reservoir and a structural template for the formation of newly formed austenite ( γ new ). Upon heating between 500 and 800 °C, fine (Fe, Mn)-rich carbides precipitate from γ pre , serving as heterogeneous nucleation sites for γ new while concurrently depleting C and Mn from γ pre . The subsequent dissolution of these carbides above ∼ 660 °C releases solutes into the surrounding matrix, driving localized partitioning and stabilizing γ new . This process results in a bimodal microstructure comprising elongated, low-alloy γ pre and blocky, high-alloy γ new embedded in a submicron ferritic matrix. At higher temperatures, solute dilution reduces austenite stability. A retained austenite of 55 vol% was achieved at 780 °C, consistent with thermodynamic predictions without prolonged isothermal holding. These findings establish the controlled decomposition–reconstruction of γ pre into γ new as a viable strategy for tailoring austenite morphology and chemistry, enabling efficient stabilization of high retained austenite amounts in medium-Mn steels while minimizing alloying requirements.