High strain rate behaviour of multiphase transformation induced plasticity (TRIP) steels

TRansformation Induced Plasticity (TRIP) is an important phenomenon during which metastable retained austenite transforms to martensite as a result of imposed stress or strain. This phenomenon was highlighted in fully austenitic stainless steels during the 1970s and thoroughly studied for two decades to understand its complexity. More recently the same strengthening phenomenon has been used in low alloy TRIP assisted multiphase steels resulting in a simultaneous increase in strength and ductility. The full potential of this class of steels is yet to be uncovered due to the complexity of the TRIP transformation and its dependence on various physical conditions and their interactions.;A phenomenological constitutive model for predicting the quasi-static and high strain rate mechanical response of these steels is proposed. The model incorporates the effects of strain, strain rate, stress state and temperature, and is based on the Perzyna type viscoplastic model. The viscoplastic function is a coupling of two proposed hardening functions accounting for the strain hardening and the effect of the martensitic transformation. The model successfully represents the four TRIP steels under the prescribed conditions using the determined parameters.;In this work, the metallurgical and mechanical properties of four TRIP assisted multiphase steels are characterised under physical conditions emulating those faced during automotive crash. This is done by studying the effects of the strain, strain rate, stress state and temperature on the transformation kinetics and mechanical properties of these steels.