On the optimisation of the mechanical properties of two aluminium-alloyed multiphase TRIP-assisted steels

Aluminium-alloyed multiphase TRIP-assisted steels have been recently developed, so as to get rid of the disdavantages related to the high silicon contents classically used so far in TRIP-assisted steels. It has been shown that aluminium has a stronger ferritising effect than silicon and that it is slightly less efficient as far as austenite retention is concerned. A careful selection of the aluminium content and of the processing conditions is thus of primary importance in order to optimise the mechanical properties. The present study aims at determining the optimal heat treatment conditions leading to enhanced mechanical properties for two low aluminium-alloyed grades containing with mass contents of 0.12% C, 1.5% Mn and 0.5% Al or 1.0% Al, respectively. The microstructures generated during the heat treatment were assessed using SEM, X-ray diffraction and Mössbauer spectrometry. The mechanical properties were evaluated by uniaxial tensilte testing and the strain-hardening behaviour was characterised by means of an incremental strain-hardening exponent. The relations between the observed microstructures and the mechanical properties were discussed and, as a consequence, the importance of a careful control of the isothermal bainitic holding was highlighted.