Work-hardening and ductility enhancement mechanism of cold rolled multiphase TRIP steels

The work hardening behavior of a high strength sheet steel has been studied in relation to the deformation induced martensitic transformation (DIMT) of retained austenite (gamma(R)). The work hardening rate was observed to decrease gradually in a continuously oscillating manner, leading to a significant ductility enhancement in these multiphase steels, in contrast to specimens without gamma(R). Ductility enhancement has been found to depend on the austenite stability (beta), with the largest tensile elongation of similar to 35 % exhibited in the case with the minimum value of beta. The DIMT has thus been interpreted in this study as the successive processes of accumulation and relaxation of internal strain energy produced during inelastic deformation, as has been prescribed by an internal variable theory. This process appears to be responsible for the locally fluctuating work hardening behavior. A new ductility enhancement mechanism is finally proposed in this connection to account for the observed work hardening as well as for the transformation behavior.