Abstract: Two heat treatment processes were adopted to obtain 800 MPa grade dual phase steels with different matrix microstructure, and the microstructure morphology characteristics of the matrix and its effect on the mechanism of strength and ductility were studied systematically. The results show that the matrix microstructure has a significant indigenous effect on the plastic deformation mechanism of 800 MPa grade dual phase steels, resulting in differences in the properties. The (F+M) dual phase steel is composed of polygonal ferrite and about 28% second-phase martensite, with yield ratio of 0.540 and elongation of 23.3%. The (BF+γ) dual phase steel is composed of bainite ferrite matrix structure and about 24% second-phase retained austenite, with yield ratio of 0.702 and hole expansion ratio of 56%. During the plastic deformation process of (BF+γ) dual phase steel, the stress concentration at the crack tip can be effectively decomposed by the γ phase with a thickness of about 60-150 nm, and the crack propagation energy is also consumed. At the same time, the cracks generated by microcracks can be bridged by the volume expansion caused by the induced martensite transformation of residual austenite. Under the coordinated deformation mechanism of α phase BF and residual austenite phase, it is beneficial to improve the strength, plasticity and stretch-flange property. In addition, the proportion of large angle grain boundaries in (BF+γ) dual phase steel increases to 63.1%, and higher dislocation density exists in the matrix, the driving energy required for micro-crack propagation is weakened effectively, and the energy required for further propagation is also increased, the stress concentration generated during deformation or hole expansion is alleviated.

Key words: dual phase steel, matrix microstructure, large-angle grain boundary, hole-expansion performance, mechanism of strength and ductility