EFFECTS OF TENSILE-TESTING TEMPERATURE ON DEFORMATION AND TRANSFORMATION BEHAVIOR OF RETAINED AUSTENITE IN A 0.14C-1.2SI-1.5MN STEEL WITH FERRITE BAINITE AUSTENITE STRUCTURE

A 0.14% C-1.21% Si-1.57% Mn steel was processed by intercritical annea ling followed by isothermal transformation to produce a structure of 7 5% ferrite, 13% bainite and 12% retained austenite. The role of retain ed austenite was studied by directly observing the deformation and tra nsformation behavior of the retained austenite. The stability of the r etained austenite was changed by varying tensile-testing temperature f rom -80-degrees-C to 120-degrees-C. With increasing temperature, the s tability of the retained austenite with deformation increased. Further more, the austenite stability increased with a decrease in the austeni te particle size. For samples tested at both 20-degrees-C and 120-degr ees-C, retained austenite was observed to remain in the deformed struc ture at high strain. The deformed retained austenite improved ductilit y by suppressing void formation at the interface between retained aust enite and either ferrite or bainite. In contrast, for samples deformed at -80-degrees-C, void formation preferentially occurred by interface decohesion of martensite-ferrite or martensite-bainite. The results o f this study indicate that the ductility in low-carbon steel with high contents of retained austenite can be altered by control of the auste nite stability.