Evolution of intermediate transformation microstructures in Mn-Mo-Nb-B low carbon microalloyed steel

Morphologyies and crystallographic orientations of grains, and nano-mechanical properties of continuous transformation structures in a low carbon Mn-Mo-Nb-B microalloyed steel were studied by using OM, EBSD and nano indentation. The microstructures of the specimen cooled continuously at a rate of 1 degrees C/s to ambient temperature consisted of granular bainite, bainitic ferrite and M/A constituent. While, the microstructure in the specimen cooled continuously to 560 degrees C at rate of 1 degrees C Is then quenched consisted of acicular ferrite and martensite/bainete. The misorientation angles between some nearby acicular ferrites are much low and the nanohardness of granular bainite (2.31 GPa) is rather close to that of acicular ferrite (2.22 GPa). Hereby, the formation of granular bainite should be through coalescence of several acicular ferrites with the almost same orientation, which formed during continuously cooling at 1 degrees C/s.