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 annealing followed by isothermal transformation to produce a structure of 75% ferrite, 13% bainite and 12% retained austenite. The role of retained austenite was studied by directly observing the deformation and transformation behavior of the retained austenite. The stability of the retained austenite was changed by varying tensile-testing temperature from -80-degrees-C to 120-degrees-C. With increasing temperature, the stability of the retained austenite with deformation increased. Furthermore, the austenite stability increased with a decrease in the austenite particle size. For samples tested at both 20-degrees-C and 120-degrees-C, retained austenite was observed to remain in the deformed structure at high strain. The deformed retained austenite improved ductility by suppressing void formation at the interface between retained austenite 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 of this study indicate that the ductility in low-carbon steel with high contents of retained austenite can be altered by control of the austenite stability.