Transformation behavior of bainite during two-step isothermal process in an ultrafine bainite steel

Four different isothermal treatment processes were designed to investigate the effect of isothermal treatment parameters on the comprehensive property in an ultrafine bainite steel. Four different isothermal treatment processes, including two kinds of one-step bainite transformation procedures at temperature above MS and below MS, respectively, and two kinds of two-step isothermal processes, were employed in this study. The results indicate that compared with the one-step bainite transformation at temperature above MS, the comprehensive property of ultrafine bainite steel was not improved by austempering at temperature below MS. However, it was improved by the two-step isothermal process. In addition, at the condition that the total transformation time of multi-step isothermal process was the same, the product of tensile strength and elongation (PSE) of sample increased with the decrease of transformation time at the first stage. At the same time, the transformation kinetics of the second stage transformation was significantly accelerated with the decrease of transformation time at the first stage.