Hot torsion tests were used to simulate hot rolling of a Nb-microalloyed steel. Subsequent graphic representation of Mean Flow Stress (MFS) versus the inverse of absolute temperature for each pass allowed to know the critical rolling temperatures (T-nr, A(r3), A(r1)) and residual stress accumulated in austenite just before austenite to ferrite phase transformation. It has been found that, as successive rolling passes are applied at temperatures below T-nr, mean precipitate size decreases as a result of deformation applied and hardening by incomplete recrystallization of austenite.