Jan Mahieu

Galvanisability of silicon free CMnAl TRIP steels

Abstract Transformation induced plasticity (TRIP) steels are a promising solution for the production of cars with low body mass because of the combination of high strength and large uniform elongation that they offer. However, conventional CMnSi TRIP steels with more than 1 wt-%Si have the drawback of poor Zn coating quality after continuous galvanising. This problem is due to the presence of complex Si–Mn oxides on the strip surface. The present research work therefore focused on the full substitution of Si by Al in TRIP steels and the detailed analysis of the galvanising behaviours of these Si free CMnAl TRIP steels. If the hot dipping is done after a combination of intercritical annealing and isothermal bainitic transformation in a furnace atmosphere with a high dewpoint, the wetting of the strip by the liquid Zn is improved significantly. However, the improvement is limited and not enough to avoid bare spots and coating defects cannot be avoided on conventional CMnSi TRIP steel. In contrast, the Si free CMnAl TRIP steel has a much better wettability when annealed at a low dewpoint. The surface segregation of the elements, which have a high affinity for oxygen, i.e. Si, Al, and Mn, was studied in detail and this revealed that Si was much more readily enriched on the surface than Al during the annealing in the low dewpoint atmosphere. The difference in the surface segregation between Si and Al resulted in a clear difference in the galvanisability. The limited presence of Al on the strip surface is due to the fact that Al can be oxidised internally during hot rolling. As a result, an Al depleted surface region is formed owing to selective internal oxidation of Al before the continuous galvanising.

Empirical microstructure prediction method for combined intercritical annealing and bainitic transformation of TRIP steel

Abstract The processing of cold rolled intercritically annealed steels which exhibit transformation induced plasticity (TRIP) requires the precise determination of equilibrium parameters, the kinetics of the intercritical γ phase formation and the kinetics of the isothermal bainitic transformation. In addition, the aging phenomena associated with the bainitic transformation must be taken into account. A detailed analysis of the kinetics of the combination of both transformations carried out in succession has hitherto not been reported. The present contribution proposes an empirical method to study these transformations in low C intercritically annealed TRIP steels: a standard CMnSi TRIP steel and a CMnAlSi TRIP steel in which part of the Si is replaced by Al. The latter TRIP steel is more likely to be used in continuous galvanising lines. Dilatometry was used to determine the soaking time necessary to obtain the equilibrium phase distribution during the intercritical annealing. Furthermore, the transformation kinetics and the evolution of the C content in the retained austenite and the bainitic ferrite were evaluated. The results show clearly that the kinetics of the intercritical austenite decomposition during the bainitic transformation cannot be fitted to a single transformation mechanism owing to the formation of carbides. It is shown that during intercritical annealing local equilibrium conditions are achieved at the phase boundaries for the substitutional solutes. This results in an inhomogeneous γ phase composition, which was observed in the Al alloyed TRIP steels. In addition, it was found that whereas equilibrium thermodynamic calculations can be used to predict phase boundaries reliably, the C content of the retained austenite was much larger than the calculated C content based on the free energy of ferrite=free energy of austenite condition.

Mechanical Properties of Low Alloy Intercritically Annealed Cold Rolled Trip Sheet Steel Containing Retained Austenite

Abstract Key factors influencing the mechanical properties of low alloy intercritically annealed sheet steel containing high C retained austenite are discussed. Ferrous microstructures containing retained austenite are fundamentally different from most current automotive sheet steel products both in terms of their asproduced properties and application related performance (formability, bake hardening and high strain rate deformation). The properties are mainly determined by the C distribution and the latter is influenced by the composition, in particular the Si, Al and P contents, the main processing parameters (the intercritical and isothermal bainitic transformation temperatures and times) and the type of deformation. The properties of the retained austenite islands control the static strain hardening, the bake hardening and the high strain rate dependence of the transformation induced plasticity (TRIP) effect. The short term technical solution of fundamental questions related to low alloy intercritically annealed sheet steel containing high C retained austenite which are the influence of the distribution of substitutional solutes, the C distribution and the strain induced transformation of these steels are reviewed On discute des facteurs-clés influençant les propriétés mécaniques de tôle d’acier faiblement allié, à recuit intercritique, contenant de l’austénite résiduelle à haute teneur en carbone. Les microstructures de fer contenant de l’austénite résiduelle sont fondamentalement différentes des produits les plus communs de tôle d’acier pour automobiles, tant en termes de leurs propriétés à l’état brut que du rendement relié à l’application, i.e. formabilité, durcissement par cuisson et vitesse de déformation élevée. Les propriétés sont déterminées principalement par la distribution de C et cette dernière est influencée par la composition, en particulier la teneur en Si, Al et P, par les principaux paramètres de traitement, i.e. par les températures et les durées de transformation intercritique et bainitique isotherme ainsi que par le type de déformation. Les propriétés des îles d’austénite résiduelle contrôlent l’écrouissage statique, le durcissement par cuisson et la dépendance de la vitesse de déformation élevée de l’effet TRIP. On examine la solution technique à court terme des questions fondamentales reliées à la tôle d’acier faiblement allié à recuit intercritique, contenant de l’austénite résiduelle à haute teneur en C, i.e. l’influence de la distribution des solutés de substitution, la distribution du C et la transformation induite par la déformation de ces aciers.