The steady flow stress also has the same change law. When the deformation temperature is further lowered, or the deformation rate is further increased, the deformed austenite changes from a fully dynamic recrystallization state to a partial dynamic recrystallization to a final work hardening state.
The microstructure of Mn-Cr gear steel at 950 °C and 1 s-1 deformation 016 is indicated under different original austenite grain sizes. When the grain size is 140 μm, the original austenite grains are significantly elongated in the direction perpendicular to the compression axis. At the same time, the prior austenite grain boundaries form many fine serrations. In some local regions, a very small amount of fine equiaxed recrystallized grains begin to form on the prior austenite grain boundaries, as shown in (b). The above experimental results show that the deformed austenite is basically in the critical state of recrystallization, and the critical strain is about 016. When the austenite grain is reduced to 100 μm, a large amount of recrystallized grains are formed, indicating deformed austenite. It is in a partially dynamic recrystallization state, and its structure is as shown in (c); when the crystal grain is reduced to 70 μm, the structure is completely composed of fine equiaxed grains, as shown in (d). This indicates that complete dynamic recrystallization has occurred. It can be seen that the original austenite grain size strongly affects the dynamic recrystallization behavior, and the fine grained austenite is prone to dynamic recrystallization and has a fast recrystallization kinetics. This is because when the fine austenite grains are deformed, the dislocation accumulation rate is fast, and the grain boundary area which can be used as the recrystallized nucleation site is large.
Stress-strain curves of Mn-Cr gear steel at different deformation temperatures and deformation rates In (b), grain boundary protrusions were observed at several locations in the prior austenite grain boundaries, as indicated by the arrows in the figure. Literature [2] pointed out that dynamic recrystallization of J crystals is nucleated by the prominent mechanism of austenite grain boundaries. In the early stages of the deformation process, local grain boundary shearing occurs frequently, causing the austenite grain boundaries to assume a sawtooth morphology. At the same time, a strain-induced subgrain boundary with medium and high angular mismatch is formed by dynamic recovery. During the grain boundary shearing process, the local strain gradient and the dislocation substructure are not uniform along the grain boundary. The local grain boundary is moved in a direction in which the dislocation density is high, that is, the grain boundary is protruded. At the same time, the strain-induced subgrain boundary with medium and high angular mismatch will act as a grain boundary to separate the protrusion from the parent. Thus, an independent recrystallized grain is formed on the prior austenite grain boundary.
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