The role and significance of martensite on the mechanical properties of TRIP-assisted multiphase steels

TRIP-assisted multiphase steels exhibit enhanced strength and ductility properties. However, such high-strength formable steels do not seem adequate for applications in which good toughness properties are required. These properties result from the unique combination of various work-hardening and damage mechanisms taking place in a multiphase microstructure consisting of an intercritical ferrite matrix with a dispersion of bainite+martensite+metastable retained austenite grains. Martensite plays a crucial role in these mechanisms, particularly in the TRIP effect (i.e. the mechanically-induced martensitic transformation) for the improvement of plastic properties, but also in the damaging process. This study aims at establishing how martensitic transformation influences the mechanical properties (plasticity and fracture) when occurring in a multiphase microstructure. On the one hand, the dislocation density generated within ferrite by the martensitic transformation is shown to lead to a high work-hardening rate. On the other hand, it is shown how martensite strongly influences the appearance of microvoids in these steels and deteriorates the resulting fracture toughness.Anglai