Multi-phase field modeling in TRIP steels: Distributed vs. average stability and strain-induced transformation of retained austenite

Austenite stability and dispersion are key parameters controlling the strain-induced transformation of Retained Austenite (RA) to martensite in TRIP steels. Stabilization is achieved through a two-stage heat-treatment process, consisting of Intercritical Annealing (IA) followed by Bainitic Isothermal Treatment (BIT). In the present study, an integrated approach is developed, for the description of microstructural evolution of a TRIP700 steel during processing, using multi-phase field modeling. Two models are employed to assess RA stability, through MSσ, MS calculations, and strain-induced transformation kinetics of dispersed RA upon plastic deformation. The first model considers the grain size and composition distributions, while the second the respective average property values of RA obtained at the end of BIT. In-depth understanding of the influence of microstructural evolution on the transformation kinetics of RA can be achieved by taking into account the local properties of RA particles, presenting significant differences from that using average values. Results indicate that refined RA is more stable, transforming into martensite at late stages of deformation. Simulations of the RA particle size, stability and martensite transformation fraction resulting from uniaxial tension are validated against experiments conducted on TRIP700 steel under different BIT conditions, presenting good agreement. The proposed integrated approach can assist the design of TRIP steels by identifying optimal microstructural characteristics.