Microstructural characterization of a warm-deformed microalloyed steel

A warm deformation process using torsion testing was carried out on a low carbon Nb-microalloyed steel. The physical processes that occur during deformation were studied by analyzing the warm flow curves. The mechanisms of fine ferrite grain formation were studied by means of optical microscopy and an electron back-scattering diffraction technique. The results show that warm flow curves of ferrite are similar to those affected only by dynamic softening events. Microstructural analysis shows that, with increasing strain, the new fine equiaxed ferrite grains surrounded by high-angle boundaries are generated at the initial boundaries. During the early stages of deformation, as strain increases the grain size decreases and the grain aspect ratio rapidly increases. A further increase of the strain also leads to continuous decreases of both the grain size and the grain aspect ratio. The dynamic softening mechanism and dynamical formation of new fine grains, observed during warm deformation, were verified to be due to continuous dynamic recrystallization.