High temperature thermomechanical processes (TMP) such as rod, bar and wire rolling operate under a wide range of strain rates (0.4 - 3000 s(-1)) and interpass times (0.0015 - 2 s). The design and optimization of TMP sequence in these processes depend crucially on the characteristics of deformation and restoration behavior of austenite under prevailing operating conditions. In the present work, the stress - strain behavior of a C-Mn steel is explored under controlled deformation conditions both under low strain rate torsion tests and high strain rate Hopkinson-bar tests. Constitutive equations that describe the relationships between temperature-compensated strain rate and peak stress and peak strain are derived for both strain rate regimes. It is found that a constitutive equation with a hyperbolic sine law gives the best fit at low strain rates, as expected. However, at high strain rates, a power law showed a better correlation between experimental and predicted peak stress and peak strain rather than the conventional hyperbolic sine law.