RETAINED AUSTENITE CHARACTERISTICS IN THERMOMECHANICALLY PROCESSED SI-MN TRANSFORMATION-INDUCED PLASTICITY STEELS

It is well known that a significant amount of retained austenite can b e obtained in steels containing high additions (>1 pet) of Si, where b ainite is the predominant microconstituent. Furthermore, retained aust enite with optimum characteristics (volume fraction, composition, morp hology, 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 ben eficial properties can be obtained largely by transformation-induced p lasticity (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 in vestigated. Niobium was added to enable the steel to be processed by a variety of thermomechanical processing (TMP) routes, thus allowing th e effects of prior austenite grain size, austenite recrystallization t emperature, Nb in austenite solid solution, and Nb as a precipitate to be studied. The results, which were compared with those of the same s teel without Nb, indicate that the retained austenite volume fraction is strongly influenced by both prior austenite grain size and the stat e 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 conditio ns on the subsequent transformation characteristics of the parent aust enite.