A novel multi-step intercritical heat treatment induces multi-phase microstructure with ultra-low yield ratio and high ductility in advanced high-strength steel

A significant volume fraction of 26% retained austenite was obtained via a two-step intercritical treatment in a low carbon low alloy steel with ultra-high tensile strength of similar to 1200 MPa, ultra-low yield ratio of 0.32, excellent uniform elongation of similar to 16.4% and total elongation of similar to 23%. The ultra-low yield ratio and excellent uniform elongation implied high work hardening and excellent formability. The high work hardening and ultra-high tensile strength are attributed to deformation-induced twin-martensite. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.