The influence of the austempering temperature on the transformation behavior and properties of ultra-high-strength TRIP-aided bainitic–ferritic sheet steel

The transformation behavior, mechanical properties and stretch-flangeability of an ultra-high-strength TRIP-assisted bainitic–ferritic sheet steel 0.2C–2Si–1.8Mn were investigated at different austempering temperatures for automotive applications. Increasing the austempering temperature was found to partially transform the uniform fine lath structure matrix and film-type metastable retained austenite into blocky martensite or austenite. The investigated steel exhibits good stretch-formability, particularly when austempered at temperatures lower than or slightly higher than the Ms point, accompanied by an excellent combination of strength and ductility. The good stretch-flangeability is related to a certain number of film-type metastable retained austenite structures. Blocky martensites, which blocky austenite transforms into during the cooling process or subsequent tensile test, deteriorate the stretch-flangeability.