Abstract Two medium carbon microalloyed steels, one with vanadium and the other with vanadium and titanium, have been studied by means of high-temperature torsion tests. The evolution of the austenitic grain size with deformation has been determined together with the rheological characteristics. On the basis of these data, the apparent activation energy for deformation and the Zener-Hollomon parameter have been calculated. The dependence of the final austenitic grain size on the initial one and on the thermomechanical treatment conditions (true strain, true strain rate and temperature) have been analyzed. Two mathematical expressions are proposed to described the evolution of the austenitic grain size due to static or dynamic recrystallizaton. It was found that the strain rate can be as important as the strain to influence the final microstructure and therefore the mechanical properties of the final product.