Stability and constitutive modelling in multiphase TRIP steels

Multiphase TRIP steels are a relatively new class of steels exhibiting excellent combinations of strength and cold formability, a fact that renders them particularly attractive for automotive applications. The present work reports models regarding the prediction of the stability of retained austenite, the optimisation of the heat-treatment stages necessary for austenite stabilization in the microstructure, as well as the mechanical behaviour of these steels under deformation. Austenite stability against mechanically-induced transformation to martensite depends on chemical composition, austenite particle size, strength of the matrix and stress state. The stability of retained austenite is characterized by the M-S(sigma) temperature, which can be expressed as a function of the aforementioned parameters by an appropriate model presented in this work. Besides stability, the mechanical behaviour of TRIP steels also depends on the amount of retained austenite present in the microstructure. This amount is determined by the combinations of temperature and temporal duration of the heat-treatment stages undergone by the steel. Maximum amounts of retained austenite require optimisation of the heat-treatment conditions. A physical model is presented in this work, which is based on the interactions between bainite and austenite during the heat-treatment of multiphase TRIP steels, and which allows for the selection of treatment conditions leading to the maximization of retained austenite in the final microstructure. Finally, a constitutive micromechanical model is presented, which describes the mechanical behaviour of multiphase TRIP steels under deformation, taking into account the different plastic behaviour of the individual phases, as well as the evolution of the microstructure itself during plastic deformation. This constitutive micromechanical model is subsequently used for the calculation of forming limit diagrams (FLD) for these complex steels, an issue of great practical importance for the optimisation of stretch-forming and deep-drawing operations