Improving mechanical properties and corrosion resistance of 0.5 wt.% C TRIP steel by adjusting retained austenite stability and microstructural constituents

Abstract

Here the effect of retained austenite stability and martensite-austenite island/fresh martensite on mechanical properties, fracture behavior, and corrosion resistance of 0.5 wt.% C TRIP steel was studied. The volume fraction of retained austenite is distinctly high at ∼ 30% in 0.5 wt.% C TRIP steel compared to the conventional TRIP steels (10–20%) while the morphologies of retained austenite are mainly film-like, mixed film-like and small blocky (0.3–0.7 μm), and large blocky (0.7–1.5 μm), respectively, in the steels after bainitic holding at 360 °C, 400 °C and 450 °C. In addition, the volume fraction of bainite decreases from 53 vol.% to 44 vol.% and 32 vol.%, and that of martensite-austenite island/fresh martensite content increases from 2 vol.% to 9 vol.% and 21 vol.% with increasing temperatures. The two-stage mechanical stability of retained austenite with large volume fraction and mixed film-like and blocky morphologies ensures stable and continuous TRIP effect. As a result, after bainitic holding at 400 °C for 30 min, the superiorly comprehensive mechanical properties are obtained for the steel, showing ultrahigh tensile strength of 1220 MPa and retaining good ductility of 44%. Moreover, the effects of multiphase on strength and ductility were discussed and the presences of martensite-austenite island/fresh martensite and blocky retained austenite lead to premature crack initiation. Crack initiation and propagation mainly happen along the interface between deformation-induced martensite and the ferritic matrix and that between M-A island/FM and the ferritic matrix. The decreasing carbon content in RA and the occurrence as well as increase of M-A island/FM can deteriorate corrosion resistance of 0.5 wt.% C TRIP steels.