Medium-Mn steels usually require long intercritical annealing to achieve sufficient Mn partitioning and metastable austenite, which compromises energy efficiency and scalability. Here we elucidate a solidification–phase transformation coupling mechanism in a strip-cast 9Mn steel, emphasizing how solidification-preset Mn micro-segregation governs subsequent austenite reverse transformation and lamellar microstructure inheritance. Sub-rapid solidification generates periodic Mn-enriched interdendritic bands ( \({{k}^{^{\prime}}}_{\text{M}\text{n}}\) ≈ 1.45 − 1.55) and stabilizes a fraction of pre-existing austenite at room temperature, establishing a macroscopically homogeneous yet microscopically heterogeneous chemical–structural template. In-situ high-temperature observations reveal that the inherited Mn heterogeneity markedly accelerates austenite reversion, lowering the transformation onset temperature and increasing nucleation density relative to a homogenized counterpart, thereby producing refined and spatially registered austenite morphologies. During intercritical rolling, the Mn-enriched bands are geometrically elongated and refined; diffusion-length estimates based on lattice diffusion predict negligible Mn redistribution, whereas experiments indicate pronounced Mn re-patterning, implying dominant short-circuit diffusion along deformation-induced defects and phase boundaries. This coupled “template inheritance + fast-channel redistribution” mechanism is summarized schematically and explains the emergence of a refined lamellar γ/α′ heterostructure through a simplified processing route. Tensile results are consistent with the microstructure refinement and retained heterogeneity, demonstrating that exploiting solidification-preset chemical templates provides an energy-efficient pathway for microstructure control in strip-cast medium-Mn steels.
