Abstract: The forming limit of high strength dual phase steel DP980 specimens with 1.4 mm thichness and different widths was tested. The strain degree of forming limit fracture and necking were studied, and the effect factors of microstructure on necking and cracking were analyzed. The microstructure and properties of the steel were analyzed by means of scanning electron microscope, electron backscatter diffraction, transmission electron microscope and tensile machine. The results show that there are multiple stress limit points in the forming limit test of the steel, where the FLD₀ value at the necking location is 0.14, while that for the rupture forming limit is 0.20. The tensile fracture results show that the Nb and Ti precipitates existing in the high strength dual phase steel DP980 make the fracture characteristics after necking change from mixed fracture to ductile. The TEM results show that the lath martensite hinders the dislocation movement in the steel DP980, and greatly increases the strength of the steel. The KAM results show that the massive dislocations pile up in the ferrite grains after necking leads to the formation of dislocation walls, which makes the material produce stronger work hardening.

Key words: DP980 steel, forming limit, necking, microstructure