Enhanced Mechanical Properties of a Low‐Carbon Martensitic Steel by Thermally Stable Ni‐Rich Austenite

Abstract

A multistep quench−lamellarization−tempering treatment (QLT) is designed for a low‐carbon high‐Ni martensitic steel to obtain extra high strength, low yield‐to‐tensile ratio, and extra high cryogenic toughness. A mixed microstructure composed of intercritical ferrite, tempered martensite, and Ni‐rich retained austenite (RA) is obtained under the QLT treatment process. The volume fraction of Ni‐rich RA is 13.6% in the QL655T sample. The QL655T sample has a yield strength of 915 MPa and tensile strength of 1080 MPa, leading to a lower yield‐to‐tensile ratio of 0.85. In addition, a total elongation to failure of 19.9% and Charpy impact toughness of 190 J at −196 °C are encouraging in comparison with the conventional counterparts. During tensile deforming, the blocky RA grains can transform easily with increasing strain, while the strain leads to rotations of the film‐like RA grains before the transformation. The RA grains with high thermal stability are attributed to the enrichment of Ni and the effect of size, which makes the ≈11.6% RA grains remain at −196 °C in the QL655T sample. During impact, the transformation‐induced plasticity (TRIP) effect of RA leads to the excellent cryogenic toughness of the QL655T sample.