The impact of the parameters of thermomechanical treatment on the structural changes and micro-hardness of low- and medium carbon steels under a high hydrostatic pressure at high pressure torsion (HPT) is investigated. It is established that for HPT the deformation temperature has a decisive impact on the efficiency of refinement and hardening. This is caused by the effect of dynamic strain ageing (DSA) at 250 and 400оС during HPT. At the same time the level of accumulated strain in the sample intensifies the refinement under the effect of dynamic strain ageing (DSA). It should be noted that an increase of the carbon content to 0.2 % and 0.45% in steel leads to an increase in the temperature of dynamic strain ageing (DSA) to 400оС. It is stated that HPT under the effect of dynamic strain ageing (DSA) results in the formation of a gradient structure in the samples after five rotations of the anvil (емах=6.26) at a hydrostatic pressure of 6 GPa.