The present communication describes the development of a novel incremental formulation for the determination of the flow stress of austenite, as a function of microstructure and deformation conditions. The proposed constitutive description is validated by employing the flow stress curves derived from axisymmetric compression tests performed with samples of a SAE 5120 steel, both under constant and transient deformation conditions. The hot-working response of this structural steel grade has been analyzed in the temperature range of 850°C to 1200°C, at strain rates in the range of 0.01 to 10 s−1. Under most deformation conditions dynamic recrystallization occurs and the evolution of the volume fraction recrystallized has been determined by an appropriate incremental form of the Johnson-Mehl-Avrami-Kolmogorov equation. The work-hardening rate of the material in the work-hardening and dynamic recovery transient has been described by an equivalent incremental form of the original differential equation advanced by Estrin and Mecking. It has been shown that this novel approach allows a very precise description of the experimental values of the flow stress of the material during deformation under constant deformation conditions, as well as the trend followed by this parameter under transient loading conditions. The latter analysis is conducted on the basis of the H parameter indicated by Abbod and co-workers. It has been determined that hot working of this steel grade at low temperatures in the austenitic range, followed by cooling to room temperature at moderate rates, leads to a significant refinement of the ferrite-pearlite microstructure, with ferrite grain sizes of less than 10 µm.