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.