The metastable austenite transformation in Mo–Nb–Cu–B low carbon steel

Abstract

The continuous cooling and isothermal transformation microstructure evolution in low carbon Mo–Nb–Cu–B microalloying steel were investigated. It was found that there are three kinds of mainly intermediate isothermal transformation structures: the grain boundary ferrite, acicular ferrite and lath-like bainitic ferrite sheaves. The growth of reconstructive ferrite was delayed or restrictive below 580 °C. The upper limiting temperature of the low “c”-curve is about 560 °C. The acicular ferrite can be transformed independently at the lower portion of metastable state region of overcooling austenite (550–500 °C). From the results of the continuous cooling test, the multi-phase of intermediate transformation products such as quasi-polygonal ferrite, granular bainite, acicular ferrite, lath bainite and M/A would appear. The fraction of acicular ferrite can be controlled by the interrupted cooling process. The acicular ferrite plates grew up independently in the prior-austenite grain and distributed randomly, the optimum amount of acicular ferrite can divide the prior-austenite grain and results in an ultra fine multi-phase microstructure.