EBSD Study on Proeutectoid Ferrite and Eutectoid Ferrite Refinement Mechanism of D2 Wheel Steel Under a Rolling Condition

This study used scanning electron microscopy with an electron backscatter diffraction system to study the refinement mechanism of proeutectoid ferrite (Pro-F) and eutectoid ferrite (Eut-F) in a D2 wheel steel under rolling contact conditions. The results indicate that as the shear strain increased (γ < 0.21), the dislocation density in the proeutectoid ferrite increased continuously, and the formed dislocation cells were distributed uniformly in the grains. Subsequently, the dislocation cell boundaries were changed into low-angle boundaries (LABs); these LABs gradually became high-angle boundaries (HABs), and the average grain size was refined from 8 μm to 710 nm. Under a shear strain of 0.21 ≤ γ ≤ 0.84, dislocation pile-up occurred at the ferrite side of the eutectoid ferrite–cementite interface, and the spatial misorientation between the two adjacent eutectoid ferrites increased gradually. The ferrite lamellar was divided into bamboo-like grains by the LABs, and the proeutectoid ferrite was refined gradually into equiaxed grains. When the shear strain was at 0.84 < γ < 3.314, the number of HABs inside the eutectoid ferrite lamellar increased, and they were refined into bamboo-like grains. The two kinds of ferrite grains are refined repeatly by the equiaxial grains in an “elongation-bamboo like refinement-elongation” process, which gradually reduced the size difference. As the shear strain further increased, the two ferrites were completely mixed into the same morphology, the dislocation density was reduced dramatically, and ultra-fine equiaxed grains of approximately 110 nm were formed.