Flow curves were determined in torsion at a series of temperatures on four plain C steels and a Nb microalloyed steel of increasing C concentration. The mean flow stresses (MFSs) pertaining to each experimental condition were calculated from the flow curves by integration. These are plotted against inverse absolute temperature in the form of Boratto diagrams. The stress drop temperatures, normally defined as the upper critical temperature , were determined from these diagrams. These are shown to be about 40°C above the paraequilibrium and about 20°C above the orthoequilibrium temperatures. This type of behavior is ascribed to the occurrence of dynamic transformation (DT) during deformation. The general characteristics of the DT of austenite to ferrite are reviewed. It is suggested that some of the unexpected load drops that have been reported to take place above the Ae3 temperature in strip mills may be attributable to this phenomenon.