Martensite to austenite reversion in a high-Mn steel: Partitioning-dependent two-stage kinetics revealed by atom probe tomography, in-situ magnetic measurements and simulation

Austenite (gamma) reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and in situ magnetic measurements during slow continuous annealing. A splitting of the gamma-reversion into two stages was observed to be a result of strong elemental partitioning between gamma and alpha'-martensite during the low temperature stage between 390 and 575 degrees C. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed-gamma and the Mn-depleted remaining alpha'-martensite. Because of its lower Mn content, the reversion of the remaining alpha'-martensite into austenite takes place at a higher temperature range between 600 and 685 degrees C. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for gamma-nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed gamma-growth during isothermal annealing at 500 degrees C and 600 degrees C. Si partitioning to gamma was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during gamma-growth and homogenizes over time. We used the chemical composition of the remaining alpha'-martensite from APT data to calculate its Curie temperature (T-Curie) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only gamma-reversion but also the T-Curie of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. (AU)