Constitutive modeling and finite element analysis of the formability of TRIP steels

The semiphysics constitutive model developed by Cherkaoui (2006, "From Micro to Macroscopic Description of Martensitic Transformation in Steels: A Viscoplastic Model," Philos. Mag., in press; Philos. Mag. Lett. in press) has been implemented in the user's material subroutine of the finite element code ABAQUS/EXPLICIT (2006, Version 6.6 Manuals, Dassault Systemes) to predict the thermomechanical behavior of unstable transformation induced plasticity (TRIP) steel sheets under conditions of forming, which are essentially composite materials with evolving volume fractions of the individual phases. These steels undergoing alpha(') martensitic phase transformation exhibit an additional inelastic strain resulting from the phase transformation itself and from the plastic accommodation in parent (austenite) and product (martensite) phases due to different sources of internal stresses. This inelastic strain known as the TRIP strain enhances ductility at an appropriate strength level due to the typical properties of the martensite. A numerical analysis of the effects of the martensitic phase transformation on formability is performed. A validation of the stress-strain behavior and the volume fraction of the martensite are carried out in the case of multiaxial paths at various temperatures. The effects of the stress state and of the kinetics of the martensite phase transformation are analyzed in the case of the cup drawing test. Finally, the numerical predictions are compared to experimental tests on type AISI304 austenitic stainless steels and TRIP800 multiphase industrial steels.