Modelling of transformations in TRIP steels

Industrial processing of low-alloy Transformation Induced Plasticity (TRIP) steels involves various stages of heat-treating, such as Intercritical Annealing (IA) and Bainitic Isothermal Treatment (BIT), in order to produce a dispersion of retained austenite (gamma(R)) particles and bainite (alpha(B)) in a ferritic matrix (alpha). Retained austenite then transforms to martensite (alpha') during forming processes undergone by the steel. In the present work an effort was made to model these stages of processing, i.e. IA, BIT and the gamma(R)-->alpha' strain-induced transformation. Simulation of heat-treatment stages was implemented using computational kinetics methods. Investigation of the strain-induced alpha(R)-->alpha' transformation kinetics was performed by means of a simple analytical model. Simulation of IA and comparison with available experimental data showed that the amount of austenite (gamma) forming during IA reaches the values predicted by thermodynamic equilibrium only at high annealing temperatures (>825degreesC). It was also observed that kinetic and thermodynamic predictions set a lower and an upper limit, respectively, within which the actual amount of austenite experimentally observed is contained. Results from the simulation of the BIT indicated considerable carbon enrichment, and thus stabilization of gamma(R), in agreement with recent experimental observations. As regards the strain-induced gamma(R)-->alpha' transformation, the analytical model employed in the present work was fitted to available experimental results, showing reasonably good adaptation to the kinetic behaviour of the microstructure during plastic deformation