Exploitation of the TRIP Effect for the Development of Formable, Fracture and Fatigue Resistant Steels for Automotive Applications

Abstract

The present paper summarizes recent and on-going work on the exploitation of TRansformation-Induced Plasticity (TRIP) in an effort to develop automotive steels which would possess high-strength combined with high formability while these steels could also exhibit high fracture and fatigue resistance. Especially for the automotive industry, the driving force for these developments is the vehicle weight reduction, which could eventually lead to lower fuel consumption combined with reduced greenhouse gas emissions.The discussion starts with the modelling and characterization of the retained austenite stability (the transforming phase) as well as the modelling of the transformation kinetics, i.e. evolution of transformation with plastic strain.Enhancement of formability is discussed next. Constitutive micromechanical modelling has been employed 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.Fracture resistance can be considerably increased by the TRIP effect. A review is made of the "transformation toughening" arising from metastable austenitic dispersions in ultrahigh strength steels and the high fracture toughness achieved in this class of materials. The potential of the TRIP effect in increasing fatigue strength has not yet received considerable attention. However once we understand the fatigue behaviour of these materials, new applications, e.g. long products made of TRIP steel, might emerge for automotive applications.