The developments of cold-rolled TRIP-assisted multiphase steels. Al-alloyed TRIP-assisted multiphase steels

The influence of heat-treating conditions on the retention of carbon-enrich ed austenite of TRIP-assisted multiphase steel grades containing different amounts of silicon and/or aluminium is investigated. The ensuing mechanical properties resulting from the TRIP effect are also scrutinised. The bainit e transformation kinetics was followed by dilatometry whereas a detailed ch aracterisation of the microstructures led to the construction of transforma tion maps giving the volume fractions of the different phases and the carbo n content of austenite. The role of silicon and aluminium additions (i) on the retention of austenite by partial bainite transformation and (ii) on th e mechanical properties is enlightened. A strong influence of the solid-sol ution strengthening effect of silicon is highlighted. Aluminium seems to be an effective alloying element for the retention of austenite in TRIP-aided steels even if lower strength levels can be attained. A mixed Al-Si TRIP-a ided steel seems to be a very good compromise between the processing needs, the required mechanical properties and the industrial constraints.