Using torsion tests, residual stress (Δσ) and critical rolling temperatures (T nr , A r3 , A r1 ) have been determined for a low Nb content microalloyed steel by means of simulation of rolling cycles and subsequent representation of mean flow stress versus the inverse of the temperature. The above magnitudes were determined as a function of interpass time for two strains applied in each pass (0.20, 0.35), respectively. Among the results found, it is notable that Δσ decreases with longer interpass times until it reaches zero, and is greater the smaller the strain applied. With regard to the cooling transformation temperatures A r3 and A r1 , these were found to be practically independent of the interpass time and were higher for smaller applied strains. Cooling transformation temperatures A r3 and A r1 are, together with the no-recrystallisation temperature T nr , considered to be critical in hot rolling. The parameter T nr , defined as the temperature at which static recrystallisation starts to be inhibited in hot rolling, is of great importance as it influences the austenite microstructure at the end of rolling, completely or partially recrystallised or completely strengthened. The phase transformation temperatures A r3 and A r1 are also important since their values indicate the temperature limits between which rolling will be performed, either in the austenitic region or in the intercritical region. The parameter T nr can be determined by simulation of several rolling passes and subsequent graphic representation of mean flow stress (MFS) versus the inverse of the temperature for each pass [1…4]. A r3 and A r1 can be determined simultaneously with T nr using the same method, i.e. from the graphs that represent MFS versus 1/T, [5…9] or by other methods such as the dilatometry technique. The latter is a classic method which offers very good precision. On the other hand, it is well known that the value of T nr depends on the chemical composition of the steel, the equivalent strain applied in each pass, the strain rate and the interpass time (Δt). [3; 5] For their part, temperatures A r3 and A r1 are dependent on the chemical composition of the steel and the austenite microstructure (grain size, strengthening) before the start of the γ→α transformation, and are easier to determine the lower the carbon content, especially when the technique used is representation of MFS versus 1/T. Finally both the accumulated strain and accumulated stress, referred to in this work as residual stress (Δσ) by allusion to the stress accumulated just before the temperature A r3 is reached, represent the appropriate magnitude for relating the state of the austenite microstructure before the start of the γ→α transformation with the final fer
