Abstract: In order to study the segregation behavior of elements in the banded structure of cold-rolled duplex steel DP780 and its effect on mechanical properties, OM, SEM, EMPA, EBSD and other methods were used to characterize and analyze the morphology and element distribution characteristics of the banded structure, tensile properties and microstructure change during the tensile process, fracture mechanism, and so on. The research results show that the macroscopic segregation formed by redistribution of C, Mn, Al and Si solute elements during the solidification process leads to the formation of banded structure, which is mostly distributed parallel to the rolling direction at 1/2 and 1/4 of the plate thickness, the grain size distribution is obviously nonuniform, and the small angle grain boundary density of the banded structure is significantly higher than that of the equiaxed structure. Meanwhile, the properties and microstructure change in tensile process indicate that after 10% tensile pre-deformation, the fibrosis is more obvious in the banded structure specimen, increasing the resistance of dislocation movement during the deformation process and improving the strength. The banded structure can also cause dislocation pile-up and lattice distortion, deteriorating the ability to deform. The banded structure reduces the coordinated deformation ability of the matrix structure, resulting in prominent anisotropy. Further, the observation of fracture morphology shows that the initial crack initiation occurs at the M/F phase interface, and a tear ridge is formed at the M/F interface during fracture, while the location and size of the tear ridge are consistent with that of the banded structure. There is a martensite band region with poor plastic deformation ability between the tear ridges, and the ferrite side of the M/F interface has significant plastic deformation; the specimen with equiaxed structure exhibits uniform axial deformation and better fracture plasticity.

Key words: banded structure, dual phase steel DP780, fracture mechanism, macroscopic segregation, coordinate deformation