C-Mn-Cr-Mo dual phase steel was piloted in laboratory. OM, SEM, tensile tests and XRD were used to characterize the microstructures, mechanical properties and to determine the texture. The results indicate that when annealed at 860 °C, the tensile strength of the tested steel is 440 MPa, elongation and r value exceed 35% and 1.4, respectively. With the temperature increases from 820°C to 860°C, the average grain size of ferrite increases and ferrite trends to be uniform gradually, therefore, the r value increase gradually. When the temperature is higher than 860°C, bainite forming results in reducing of r value. Al could weaken the tendency of heterogeneous deformation and modified the annealing texture by refining hot-rolling grain size. Mo-based carbides existed in hot-rolled sheet developed {111} texture in the stage of ferrite recrystallization, then re-dissolved in intercritial annealing stage so as to improve hardenability.
Two kinds of high strength cold rolled dual-phase steel with different Si content were trial-produced in the laboratory. Tensile strength and elongation of the two steels exceed 1000MPa and 15%, respectively. The phase transformation behaviors of both steels were compared and investigated in continuous cooling process by thermal dilatometer. The effects of Si on the mechanical properties and microstructures of dual-phase steel were studied by tensile testing, OM and SEM observation. The results show that the two phase region (α+γ) is enlarged, the precipitation and growth of pro-eutectoid ferrite is promoted, and the morphology and distribution patterns of martensite are improved for high Si steel. Both steels show two-stage strain hardening characteristics. StageⅠ (ε<0.05), the solution of Si hinder the movement of dislocations, make the work hardening exponent of high Si steel is higher than that low Si steel. At stage Ⅱ (ε>0.05), the compatibility and delivery between hard phase and soft phase eliminate the differences between the two steels.
Deep drawing dual phase steel was designed and trial-produced in the laboratory. The microstructure and recrystallization texture was studied using OM,TEM and XRD techniques. The results show that texture components typically in DP steels are γ (<111>//ND) and α (<110>//RD) fibers, which is partly attributed to the precipitation of Mo-based carbides in hot-rolled plate. Besides, {554}<225> and {332}<113> texture components are also founded in deep drawing DP steels, which are beneficial to improvement of drawability. With the increasing of annealing temperature, the tensile strength and elongation of DP steels are increased and decreased, respectively, and the r value is reduced slightly.
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