Stretch-flangeability of Ultra High-strength Low Alloy TRIP-aided Sheet Steels with Mixed Structure Matrix of Bainitic Ferrite and Martensite

Abstract

The microstructure, retained austenite characteristics, tensile properties and stretch-flangeability of ultra high-strength 0.2%C–1.5%Si–1.5%Mn (mass%) TRIP-aided bainitic ferrite cold-rolled sheet steel, “TBF steel”, were investigated for automotive applications. When isothermally held at temperatures less than martensite-start temperature for 300–3000 s after annealing or austenitizing, the TBF steel possessed mixed matrix structure of bainitic ferrite and martensite, with retained austenite of about 4 vol%. The TBF steel achieved the tensile strength higher than 1400 MPa and hole-expanding ratio of 40%. The good combination of tensile strength and hole-expanding ratio was mainly caused by highly carbon–enriched retained austenite and softened matrix structure composing of bainitic ferrite and martensite.