On the structure-property relationship in a novel 1000 MPa hot-rolled TRIP steel with strain-assisted ferrite transformation

Abstract

We describe here structure-property relationship in a novel 1000 MPa hot-rolled TRIP steel with strain-assisted ferrite processed by thermo-mechanical controlled processing (TMCP) and tuning of the coiling process. A wide range of experimental material characterization techniques, notably, electron microscopy, EBSD, TEM, and nanoindentation were uniquely combined to understand the structure-property relationship. The ultrafine ferrite (average size of ~2.2–3.4 μm) was obtained by strain-assisted ferrite transformation and carbon was enriched in untransformed austenite, which led to increase in the content of retained austenite. The results indicated that ferrite delayed granular bainite transformation, and a high content of ferrite resulted in high austenite hardenability and facilitated the formation of lath bainite and martensite. The volume fraction of retained austenite was initially increased with the decrease of rolling temperature, followed by a decrease. Both granular bainite and lath bainite contained blocky retained austenite, while film-like retained austenite mainly existed in lath bainite. A good combination of strength and plasticity in the experimental steel was obtained because of strong TRIP effect. The product of tensile strength and % elongation (PSE) of the experimental steel approached 20.4 GPa·%, which is significantly greater as compared to hot-rolled ferrite-free TRIP steels.