On the influence of aluminium and silicon contents on the kinetics of bainite transformation of intercritical austenite

TRIP-assisted steels show enhanced mechanical properties thanks to the strain-induced transformation of retained austenite to martensite (TRIP effect). Stabilization of austenite is made possible by the combination of appropriate chemical composition (containing classically high levels of silicon, or, as developed more recently, aluminium) and of a two-steps heat treatment comprising an intercritical annealing followed by an isothermal holding in the bainite formation range. A detailed understanding of the effects of both elements on the bainite transformation kinetics is thus of primary importance for the optimization of austenite retention. The present study aims at a better knowledge of the influence of aluminium levels on the kinetics of the bainite transformation of intercritical austenite, as compared to the effect of silicon. The bainite transformation kinetics was followed by dilatometry in a set of 6 steel grades with varying aluminium and silicon contents. These dilatometry experiments were coupled with a characterization of the microstructures by X-ray diffraction, SEM and image analysis in order to assess the occurrence of an incomplete reaction phenomenon. It was shown that aluminium has a less pronounced effect than silicon in retarding the bainite transformation. A model was proposed to account for this retarding effect as a function of aluminium or silicon contents.