Microstructure evolution of advanced high-strength TRIP-aided bainitic steel

The present work is focused on monitoring the microstructure evolution of the thermomechanically rolled medium-C steel containing Si and Al to prevent the precipitation of carbides.The initial microstructure consists of carbide-free bainite, polygonal ferrite and blocky-type and interlath retained austenite.To monitor the transformation behaviour of the retained austenite transforming into strain-induced martensite, tensile tests were interrupted at particular strains (5 %, 10 %, 15 %, 18.4 %).The amount of the retained austenite determined with XRD and EBSD techniques decreased from 17 % to 6.8 % upon straining, which was related to the transformation-induced plasticity effect.The identification of the retained austenite and strain-induced martensite was carried out using an electron scanning microscope equipped with EBSD (electron backscatter diffraction).It was found that sheet samples are strongly strengthened by the transformation of the blocky retained austenite, whereas the layers and films of the g phase are over-stabilized and they do not transform into martensite.