Characterization and quantification methods of complex BCC matrix microstructures in advanced high strength steels

The development of comprehensive and reliable microstructure characterization tools is very necessary for the understanding and modelling of both the formation of different phases during processing and the relationship between microstructure and mechanical properties. A series of samples containing different phases with a BCC structure have been characterised using different techniques including optical microscopy (OM), field emission gun scanning electron microscopy (FEG-SEM) with secondary electron (SE) and back scattered electron (BSE) modes, electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). It is shown that it is difficult to distinguish ferrite from bainite especially granular bainitic ferrite using conventional OM and FEG-SEM (SE) techniques, whereas FEG-SEM (BSE) and EBSD are the most suitable techniques to differentiate them. A new EBSD method has been developed to dissociate and quantify ferrite, bainite and martensite in multi-phase AHSS steels. This method has been proven to be pertinent and has been validated using reference specimens.