Hot torsion tests are carried on three plain carbon steels and a Nb microalloyed steel of increasing C concentrations. The tests are performed at strain rates up to 4 s−1 and over the temperature range 743–917°C. The onsets of dynamic transformation (DT) and dynamic recrystallization (DRX) are detected using the double-differentiation method. Both mechanisms are initiated under all testing conditions but one. The critical strain for DT increases with temperature while the reverse dependency is exhibited by the critical strain for DRX.