Microstructural comparison of the thermomechanically treated and cold deformed Nb-microalloyed trip steel

The work deals with a microstructural comparison of the thermomechanically processed and subsequently cold deformed Nb-microalloyed Si-Al-type multiphase steel, showing a TRIP effect. The newly developed steel was subjected to the thermomechanical rolling and controlled cooling under the conditions allowing us to obtain a fine-grained ferritic-bainitic microstructure with a large fraction of the retained austenite. Subsequently, the thermomechanically rolled sheet samples were subjected to a 10 % elongation in uniaxial tension. The comparison of the multiphase microstructures and, especially, the identification of the strain-induced martensite were carried out using light microscopy, electron transmission microscopy and electron scanning microscopy equipped with EBSD (electron backscatter diffraction). Morphological details influencing the mechanical stability of the retained austenite were indicated.