Austenite Evolution and Solute Partitioning during Thermal Cycling in the Intercritical Range of a Medium-Mn Steel

Abstract

The evolution of austenite fraction and the associated solute partitioning during the intercritical annealing of medium-Mn steels are of great importance for austenite stabilization and the mechanical performance of this class of steels. In the present work, a 4.5Mn steel is subjected to a cyclic treatment and the evolution of the austenite fraction is measured with dilatometry. The evolution of austenite fraction and solute partitioning are simulated for a case where the starting time of the cyclic treatment is well before the equilibrium fractions have been established in the respective isothermal intercritical treatment. The evolution of austenite during thermal cycling in the intercritical range comprises of forward, inverse, and stagnant stages. The fraction of austenite formed decreases in each successive cycle while the kinetics of the evolution of austenite is controlled by the Mn diffusion in ferrite. Partitioning of Mn and C takes place from ferrite to austenite during the cyclic transformation. Due to the low diffusivity in austenite, wells form in the composition profiles in austenite of both Mn and C. These wells are the locus of the interfacial compositions of austenite, corresponding to the variation of the local equilibrium conditions during the thermal cycle. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim