Microstructure and Properties of Mo Microalloyed Cold Rolled DP1000 Steels Original Research Article

Two kinds of ultra-high strength cold rolled dual phase steels have been developed by designing C-Si-Mn-Cr and C-Si-Mn-Cr-Mo alloy systems. Tensile strength and elongation of both steels exceed 1100 MPa and 10%, respectively. The microstructures of both steels consist of massive martensite and ferrite. And the massive martensite of Mo-free steel disperses in the ferrite with volume fraction of 64%. However, the massive martensite of Mo-containing steel is connected or closed by small martensite islands each other, and martensite volume fraction is 69%. As to Mo-free steel, the yield strength, yield ratio, and work hardening exponent n are 548 MPa, 0.49, and 0.26, respectively. As for Mo-containing one, the yield strength, yield ratio, and n value are 746 MPa, 0.66, and 0.33, respectively. Besides, the ferrite of Mo-free steel is deformed at the initial stage of plastic deformation. However, for Mo-containing one, Mo solution strengthened ferrite and small overaged martensite islands are deformed preferentially at small strain, which causes the yield strength to reach approximately 200 MPa higher than that of Mo-free steel.