Research

Quenched-Partitioned Silicon-Bearing High Strength Steels

In the development of third generation of advanced high strength steels with high strength-ductility combination, thin strip casting and thin-slab casting are viewed as attractive and cost-effective options. In one instance, an approach that is being increasingly considered derives inspiration from transformation-induced plasticity (TRIP) steels that are characterized by significant fraction of austenite, which on deformation transforms to martensite, contributing to increased levels of strain hardening. This aspect constitutes one of the focal themes toward the futuristic development of advanced high strength steels with significantly high volume fraction of austenite, without adversely affecting the fabrication of structural components including welding.

In the aforementioned regard, processing routes that can enable stabilization of austenite through diffusion of carbon from martensite to austenite, in a manner that transitional carbide formation is inhibited is a viable approach. During straining, the higher elongation is connected to gradual and progressive transformation of austenite to martensite at higher levels of strain depending on the metastablity of the alloy in question, and the grain size will be of lesser importance, as is the case with nanograined/ultrafine-grained austenitic stainless steel. This would enable the work hardening rate to increase and with twinning and dislocation glide being the possible primary deformation mechanisms. The challenge, however, is the degree to which stabilization can be accomplished. In the above regard, the effects of direct quenching and partitioning (DQ&P) process on the evolution of microstructure and consequent changes in hardness in a set of medium and high carbon steels containing varying contents of chromium, manganese, and silicon are being examined. Initial studies have indicated that higher partitioning of carbon from martensite to retained austenite and stabilization of austenite occurs when martensite has a higher supersaturation of carbon after quenching, which is obtained at lower quench temperature.