Interactions between recrystallisation and phase transformations during annealing of cold rolled Nb-added TRIP-aided steels

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.Anglai

TRIP-Assisted multiphase steels with niobium additions. Microstructures and properties

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

Microstructures resulting from the interaction between ferrite recrystallization and austenite formation in dual-phase steels

The present work investigates the interactions between ferrite recrystallization and austenite formation in dual-phase steels by experiments performed at high heating rate (100 °C/s). It was shown that both ferrite recrystallization and austenite formation are strongly coupled and interdependent. The kinetics of ferrite recrystallization is strongly affected by the formation of austenite and can be even inhibited in some cases. The microstructure is more heterogeneous and anisotropic when both the austenite formation and the ferrite recrystallization overlap. It was highlighted that the degree of anisotropy depends on the volume fraction of austenite at a given temperature. Furthermore, an unusual behavior for austenite growth was highlighted. It is characterized by a much higher volume fraction than those obtained under OrthoEquilibrium and ParaEquilibrium. The results, especially those at 715 °C close to the eutectoid plateau, at which the driving force for austenite growth is classically low, suggest a diffusionless transformation for austeniteDesign des Alliages Métalliques pour Allègement des Structure

Delaying Effect of Cementite on Recrystallization Kinetics of a Ti-Nb Microalloyed High-Formability Steel

International audienceAnnealing of cold-rolled high-strength steels leads to various microstructural changes such as recrystallization, cementite precipitation, microalloying elements precipitation and austenite formation. These transformations are expected to interact with each other. Understanding how and where austenite forms in a microstructure is of prime importance to avoid formation of banded microstructures, which are detrimental to good in-use properties. In this work, a mean-field model is used to simulate concomitant recrystallization, cementite precipitation, microalloying elements precipitation and austenite formation kinetics, as well as their interactions during 1 and 10 °C/s heating. Excellent agreement with experimental data is obtained only if cementite pinning effect on recrystallized grain boundaries is considered. It is shown that cementite exhibits a much stronger delaying effect on recrystallization kinetics than microalloying elements, leading to the formation of banded microstructures. Carbon nominal content of a steel appears to be the most important parameter to acknowledge to understand recrystallization kinetics