HOT DEFORMATION CHARACTERISTICS OF SI-MN TRIP STEELS WITH AND WITHOUTNB MICROALLOY ADDITIONS

It has been long recognized that steels with high formability and stre ngth can be produced with the aid of the deformation induced transform ation of retained austenite to martensite. Selecting the steel composi tion and microstructure to obtain significant amounts of retained aust enite has been the first step in designing TRIP steels. Thus, it is no w well established that a significant amount of retained austenite can be obtained in steels containing high concentrations (> 1 %) of Si, w here bainite is one of the main microconstituents. The effects of diff erent intercritical annealing conditions on various aspects of Si-Mn T RIP steels have been extensively studied in other investigations. The present work was, however, conducted to investigate the influence of t hermomechanical processing on the general behaviour of two Si-Mn TRIP steels, including a microalloyed Nb-bearing grade. The effects of micr ostructural features produced by dynamic recrystallization and pancaki ng 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 aus tenite al room temperature, were examined. It was found that, for micr ostructures compised of polygonal ferrite, bainite and retained austen ite, the variation of the retained austenite volume fraction with stra in in the no-recrystallization region exhibits a maximum. Furthermore, the dynamically recrystallized austenite substructure, when retained to transformation temperatures, tends to retain more austenite.