Study on the ferrite grain refinement during intercritical deformation of a microalloyed steel

In the present research, the ferrite grain refinement during intercritical deformation of a low carbon microalloyed steel within the two phase (α + γ) region was investigated using hot torsion testing. The physical processes that occurred during intercritical deformation were discussed by observing the optical microstructure and analyzing the flow curve responses. The shape of the flow curve suggests that the certain dynamic softening mechanisms take place during deformation. Dynamic softening mechanisms compensate for the hardening effect of deformation and gradually keep balance with it. This flow softening is the result of deformation-induced ferrite transformation and continuous dynamic recrystallization of ferrite. Strain increasing promotes both of the softening mechanisms. Consequently, ultrafine ferrite grains continuously nucleate not only at grain boundaries but also inside austenite and pre-eutectoid ferrite. As a result, ultrafine ferrite with average grain size of ∼1.8 μm achieved. It is concluded that with strain increasing, in addition to deformation-induced ferrite transformation, continuous dynamic recrystallization of ferrite contributes to the further ferrite grain refinement.