Mechanism of ferrite grain refinement during warm deformation of a low carbon Nb-microalloyed steel

Warm torsion testing was carried out on a low carbon Nb-microalloyed steel to study the mechanism of ferrite grain refinement during warm deformation. Deformation characteristics of ferrite were studied by means of optical microscopy and electron back-scattering diffraction. The results show an extended flow softening of ferrite during warm deformation. With increasing strain, the new fine ferrite grains surrounded by high-angle boundaries start generating at the initial boundaries. The new grains develop as a result of gradual increase in the misorientations between the subgrains that are caused by warm deformation. With further increasing strain, the grain sizes continuous to decrease and the number of newly generated ultrafine grains increased. Furthermore, it was seen that there is no evidence of discontinuous dynamic recrystallization. It was concluded that the mechanism, which is responsible for the both dynamic flow softening of ferrite and formation of new fine grains, is continuous dynamic recrystallization.