Influence Of Hot-deformation Parameters On Ferrite Grain Refinement Of Cu-P-Cr-Ni-Mo Dual-phase Weathering Steel

Novel duplex Cu-P-Cr-Ni-Mo weathering steel provides excellent atmospheric corrosion resistance of weathering steel and favorable comprehensive mechanical properties of dual-phase steel, which was prepared via direct hot-rolling. However, the ferrite grains of 30-40 μm in size were relatively coarse, thus the ferrite grain refinement is an effective way for realizing the potential properties of tested steel. In this study, the microstructure evolution behavior and ferrite grain refinement mechanism in the dual-phase region during single pass deformation at various deformation temperatures,strain rates and strains were investigated via Gleeble-3500 simulator, optical microscopy,transmission electron microscopy(TEM) and electron backscattering diffraction(EBSD),which would provide a theoretical basis for the formulation of a controlled rolling process in dual-phase steel.The analysis of deformation at dual-phase region indicated that deformation at the temperature of duplex region, i.e. just below Ar3 and just above Ar1, can effectively refine the ferrite, and grain size of 1.77-2.69 μm can be obtained. As the temperature declined,the equiaxed ferrite volume fraction increased while the ferrite grain size reduced;moreover, the deformation induced ferrite formed not only at the original austenite grain boundaries but also the intragranular. In the process of temperature declining from 850 ?C to 750 ? C, some sub-boundaries in deformation-induced ferrite grains shifted to grain boundaries via continuously absorbed dislocations and the ferrite continuous dynamic recrystallization generated. The main mechanism for ferrite refinement at the temperature of 800 ?C-Ar3 was deformation induced ferrite transformation, yet the continuous dynamic recrystallization was the main contributor at the temperature ranging from Ar1 to 775 ?C.The flow curves of deformation at the temperature of 875 ?C at high strain rates(>1 s-1)represented bimodal type, the work-hardening and continuous dynamic recrystallization of deformation induced ferrite resulted in the appearance of the second peak. Furthermore,the ferrite grain size reduced as increase in strain rate and strain, conversely, the ferrite volume fraction increased yet with a saturation value of ~86%.Ferrite grain refinement mechanisms during deformation of Cu-P-Cr-Ni-Mo weatheringsteel were deformation induced ferrite transformation and ferrite continuous dynamic recrystallization. Deformation at the high temperature of dual-phase region, the main ferrite grain refinement mechanism was deformation induced ferrite transformation;whereas the main ferrite grain refinement at the low temperature of dual-phase region can be attributed to continuous dynamic recrystallization. Furthermore, the ferrite phase of continuous dynamic recrystallization depended on deformation temperature, i.e., the ferrite phase for continuous dynamic recrystallization at the high temperature of dual-phase region was equiaxed deformation induced ferrite, while the coarse deformation induced ferrite was the ferrite phase at the low temperature of duplex region. Meanwhile,the critical strain for deformation induced ferrite transformation reduced as deformation temperature decreased, which changed from 0.36 at 850 ?C to 0.31 at 800 ?C.