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

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 degrees C. The upper limiting temperature of the low "c"-curve is about 560 degrees C. The acicular ferrite can be transformed independently at the lower portion of metastable state region of overcooling austenite (550-500 degrees 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. (c) 2006 Elsevier B.V. All rights reserved.