Papers by Author: V. Andrade-Carozzo

Abstract: Fe and Fe-C based alloys present the exceptional feature that the processing route can be adapted to lead to various phases that present antagonist mechanical properties ranging from soft ferrite to high strength martensite. Among the different deformation mechanisms that can be exhibited by these phases, the TRIP effect brings about large enhancements of the work-hardening rate. The current TRIP-assisted multiphase steels present a ferrite-based matrix with a distribution of islands of bainite and retained austenite obtained at the end of specific thermal or thermomechanical treatments. The present study aims at characterising the interactions occurring between ferrite recrystallisation and austenite formation during the intercritical annealing of cold rolled Nb-added TRIP-aided steels. It is shown that the addition of niobium retards the ferrite recrystallisation during heating. As a consequence, ferrite may not be completely recrystallised before the nucleation and growth of the austenite grains. Strong interactions between these phenomena can then be observed, i.e. a strong hindering of the ferrite recrystallisation due to the austenite formation. Furthermore, the heating rate from room temperature to the intercritical temperature range influences the thermodynamic conditions prevailing at the ferrite / austenite interface and dictates the phase proportions.

Abstract: It seems completely clear that the combination within the same microstructure of various steel phases (ferrite, bainite, martensite) and the existence of the TRIP effect (i.e. a mechanically induced martensitic transformation) improves in a large way the properties of strength and ductility of low-alloy steels. TRIP-aided multiphase steels have been studied for several years and arouse a growing industrial interest. This project deals with the influence of a large Nb addition on the microstructure and mechanical properties of TRIP-assisted multiphase steels. It is shown that Nb influences in a large way the different phase transformations as well as the recrystallisation of the ferrite matrix occurring during the heat-treatment of cold-rolled TRIP-aided steels. As a consequence, the mechanical properties also depend on the parameters of the thermomechanical process and on the content and stability of the retained austenite.