5th International High Manganese Steel Conference - HMNS 2022

Excellent combinations of strength and ductility have been observed in complex phase medium Mn steels, due to the activation of TRIP and TWIP in retained austenite, stabilized at room temperature via C and Mn partitioning. Limited attempts have been made in the past to model the deformation behavior and transformation kinetics of medium Mn steels. Previous studies relied on fitting the Olson-Cohen equation to phase fraction measurements in order to determine the kinetics of strain induced martensite or mechanical twinning, and then to calculate the ensuing mechanical response. In the present study a constitutive model describing the evolution of the flow stress upon loading as well as the kinetics of stress assisted and strain induced ε-, α΄-Martensite and mechanical twinning in a physical framework is presented. The elastoplastic response of each constituent phase is calculated as a function of the dislocation density evolution according to a Kocks-Mecking-Estrin type model and the macroscopic flow stress is calculated considering a rule of mixtures and the iso-work principle. The nucleation kinetics of ε-, α΄-Martensite and twin embryos and the subsequent growth of shear bands, as a function of the plastic strain facilitated in austenite were modeled in terms of the stacking fault energy of austenite. The nucleation of α΄-Martensite from shear band intersections and the consumption of ε and twins as α΄ grows, is considered, effectively modeling the evolution of phase fractions and the 𝛾 → 𝜀 → 𝛼΄, 𝛾 → 𝑡 → α΄ and 𝛾 → 𝛼΄ transformations upon loading, in fully austenitic and complex phase steels. The model could be used to accelerate the design of medium Mn steels with desired microstructural and mechanical properties by providing insight into the transformation kinetics and deformation behavior of the material.