OBSERVATION OF DEFORMATION AND TRANSFORMATION BEHAVIOR OF RETAINED AUSTENITE IN A 0.14C-1.2SI-1.5MN STEEL WITH FERRITE BAINITE AUSTENITE STRUCTURE

A 0.14wt.%C-1.21wt.%Si-1.57wt.%Mn steel was intercritically annealed at 770-degrees-C for 5 min and then isothermally held at 400-degrees-C for 4 min followed by oil quenching to room temperature. The as-annealed microstructure consisted of 75% ferrite, 13% bainite, and 12% retained austenite. The steel was used to establish a microscopic method to identify retained austenite, and to observe the deformation and transformation behavior of the retained austenite with tensile deformation. Submicron-sized particles of retained austenite could be distinguished from ferrite, bainite and martensite in light microscopy when etched with 5% nital followed by etching in a 10% sodium metabisulphite solution. The retained austenite particles were observed in three types of locations: type I at ferrite grain boundaries always in contact with bainite; type II at ferrite grain boundaries out of contact with bainite; and type III within ferrite grains. It was also observed that the larger retained-austenite particles transformed more easily to martensite during deformation at room temperature. Even after deformation to a tensile strain greater than 32%, about 4% of the retained austenite remained, and the retained-austenite particles were observed in the deformed structure.