Microstructure and Properties of Mo Microalloyed Cold Rolled DP1000 Steels

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.