Quantitative Crystallographic Characterization of Boundaries in Ferrite-Bainite/Martensite Dual-Phase Steels

In this study, two dual-phase steels with different ferrite-bainite/martensite ratios were obtained by rolling in two-phase region and setting the relaxation time after rolling. The tested steel with smaller ferrite content obtained higher yield strength and tensile strength, greater total elongation and lower ductile-brittle transition temperature; while the steel with higher ferrite content obtained higher uniform elongation and lower yield strength ratio. The EBSD characterization of the two steels shows that for the ferrite-ferrite boundaries and ferrite-bainite/martensite boundaries, if the interface has a large overall misorientation angle, it usually has a large cleavage plane misorientation angle and large slip plane misorientation angle; but for the variant-variant boundaries within bainite or martensite, if the interface has a large overall misorientation angle, it usually has a large cleavage plane misorientation angle, but not necessarily has a large slip plane misorientation angle, and this phenomenon is more significant in martensite microstructure. The ductility of dual-phase steel is not only affected by the proportion of the two phases, but also influenced by the grain refinement of the two phases. Therefore, in order to improve the comprehensive mechanical properties of the dual phase steel, it is necessary to refine the dual phase micro-structure from the view of effective slip unit and the effective cleavage unit.