On the influence of aluminium content on the stability of retained austenite in multiphase TRIP-assisted steels

Abstract

TRIP-assisted multiphase 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 and heat-treatment. It has been shown recently that aluminium could be substituted to silicon, whose effect has been mainly studied in the literature so far, for this austenite retention. In this work, the influence of aluminium content and heat-treating conditions on the retention of carbon-enriched austenite is investigated in two 0.12 wt. % C - 1.5 wt. % Mn steels with 0.51 wt. % Al and 1.16 wt. % Al respectively. Special attention is given to the effect of aluminium on the phenomena developing during bainitic holding. The bainitic transformation kinetics is followed by dilatometry. Coupled with a characterization of the microstructures by X-ray diffraction, scanning electron microscopy and image analysis, these dilatometry experiments enabled us to draw transformation maps giving the volume fractions of the different phases. The retarding effect of aluminium on carbide precipitation during the bainitic transformation is highlighted, although Al appears less efficient than Si.