Retained austenite characteristics in thermomechanically processed Si-Mn transformation-induced plasticity steels

It is well known that a significant amount of retained austenite can be obtained in steels containing high additions (>1 pet) of Si, where bainite is the predominant microconstituent. Furthermore, retained austenite with optimum characteristics (volume fraction, composition, morphology, size, and distribution), when present in ferrite plus bainite microstructures, can potentially increase strength and ductility, such that formability and final properties are greatly improved. These beneficial properties can be obtained largely by transformation-induced plasticity (TRIP). In this work, the effect of a microalloy addition (0.035 pet Nb) in a 0.22 pet C-1.55 pet Si-1.55 pet Mn TRIP steel was investigated. Niobium was added to enable the steel to be processed by a variety of thermomechanical processing (TMP) routes, thus allowing the effects of prior austenite grain size, austenite recrystallization temperature, Nb in austenite solid solution, and Nb as a precipitate to be studied. The results, which were compared with those of the same steel without Nb, indicate that the retained austenite volume fraction is strongly influenced by both prior austenite grain size and the state of Nb in austenite. Promoting Nb(CN) precipitation by the change in TMP conditions resulted in a decrease in the V-RA. These findings are rationalized by considering the effects of changes in the TMP conditions on the subsequent transformation characteristics of the parent austenite.