Role of Mn-Enriched Cementite Formed during Isothermal Treatment in Retaining Reversed Austenite in C–Mn Eutectoid Steels

To better understand the retention behavior of reversed austenite induced by Mn enrichment in cementite, Mn partitioning among austenite, ferrite, and cementite was investigated in near-eutectoid steels containing 0.6%C–2.0%Mn and 0.6%C–3.0%Mn (mass%) at pearlitic transformation temperatures. In addition, the Mn enrichment in cementite during subsequent isothermal holding and its effect on the retention of reversed austenite were evaluated. With increasing Mn content and transformation temperature, the pearlitic transformation mode shifted from non-partitioning local equilibrium (NPLE) to partitioning local equilibrium (PLE), resulting in significant Mn enrichment in lamellar cementite. This transition from NPLE to PLE was interpreted using local equilibrium theory, and the Mn concentration after pearlitic transformation was estimated through thermodynamic calculations. Furthermore, isothermal holding promoted Mn redistribution between pearlitic ferrite and cementite, leading to further Mn enrichment in spheroidized cementite. Following austenitization and quenching, a fraction of the reversed austenite was retained, and its volume fraction increased with prolonged annealing due to Mn enrichment. Kinetic analysis based on diffusion equations revealed that Mn remains localized in regions where cementite dissolves at austenitizing temperatures, stabilizing austenite locally and thereby promoting its retention at ambient temperature. These findings demonstrate that Mn enrichment in cementite plays a crucial role in reversed austenite retention, not only during the initial pearlitic transformation but also throughout the subsequent annealing process.