Effects of prior austenite grain size and phase transformation temperature on bainitic ferrite formation in multi-constituent microstructures of a strong ultra-low-carbon steel

Bainitic ferrite is a key microstructural constituent in High-Strength Low-Alloy (HSLA) steels, influenced by the prior austenite grain size and heat treatment conditions. This study investigates the decomposition of coarse- and fine-grained austenite under rapid step quenching heat treatments around the martensite-start (MS) temperature in an ultra-low-carbon HSLA steel. An isothermal-to-nonisothermal transition occurred in the austenite phase transformation behavior after the grain size refinement from -69 mu m to -10 mu m resulting in the formation of a complex combination of various ferritic products consisting of quasi-polygonal ferrite, granular bainite, bainitic ferrite, and martensite in the microstructure of the below-MS-treated samples. Fine bainitic ferrite laths with an average thickness of -150 nm formed, which were about 70% thinner than those of the above-MS-treated samples, contributing in part to an ultimate tensile strength of 791 MPa along with a uniform elongation of 6.4% for the below-MS-treated samples. The results can give insight into the methodologies for transferring the concept of nanoscale bainite to ultra-low-carbon steels.