HOT DEFORMATION CHARACTERISTICS OF SI-MN TRIP STEELS WITH AND WITHOUT NB MICROALLOY ADDITIONS

It has been long recognized that steels with high formability and strength can be produced with the aid of the deformation induced transformation of retained austenite to martensite. Selecting the steel composition and microstructure to obtain significant amounts of retained austenite has been the first step in designing TRIP steels. Thus, it is now well established that a significant amount of retained austenite can be obtained in steels containing high concentrations (> 1 %) of Si, where bainite is one of the main microconstituents. The effects of different intercritical annealing conditions on various aspects of Si-Mn TRIP steels have been extensively studied in other investigations. The present work was, however, conducted to investigate the influence of thermomechanical processing on the general behaviour of two Si-Mn TRIP steels, including a microalloyed Nb-bearing grade. The effects of microstructural features produced by dynamic recrystallization and pancaking of austenite, along with that of finishing temperature below the T-nr (the austenite no-recrystallization temperature) on the subsequent transformations of the parent austenite, and the state of retained austenite al room temperature, were examined. It was found that, for microstructures compised of polygonal ferrite, bainite and retained austenite, the variation of the retained austenite volume fraction with strain in the no-recrystallization region exhibits a maximum. Furthermore, the dynamically recrystallized austenite substructure, when retained to transformation temperatures, tends to retain more austenite.