Intercritical annealing behaviour of an ultrafine grained C-Mn steel obtained by hot torsion deformation

Several studies concerning ferrite grain refinement have been developed in recent the last years due to the recognised influence of such microstructures on steels properties. This work was focused on the evaluation of the microstructure and mechanical properties of an ultrafine grained C-Mn steel obtained by hot torsion deformation and intercritical annealing. After 5 min soaking at 900 and 1200 degrees C, the samples of low carbon steel were quenched and then reheated. Hot torsion deformation was conducted at temperatures of 700 or 740 degrees C. The torsion schedule consisted of 7 isothermal passes leading to a total true strain of approximate to 1 and generating an ultrafine and inhomogeneous microstructure with grain sizes of the order of 1 mu m, formed by strain-induced dynamic transformation (SIDT). The samples were heated up to 800 degrees C and held for 1, 2 and 3 h. A more homogeneous microstructure and ferrite grain size were obtained after annealing The microhardness tests showed the reduction in hardness with the increase in annealing time. They also highlighted the effects of the ferrite grain size and the volume fractions of the microstructure constituents.