After a short introduction to the theoretical background of thermally activated glide of dislocations, a constitutive model is presented, which describes the temperature and strain-rate dependence of the flow stress. The properties of this constitutive equation were estimated for several plain carbon steels in normalized conditions, for quenched and tempered low-alloy steels, as well as for some high-strength low-alloy (HSLA) steels based on the temperature dependence and strain-rate sensitivity of the flow stress at temperatures 81 K≤T≤398 K and strain rates 5 · 10−5 s−1≤ε≤1 · 10−2 s−1. The constitutive equation enables the extrapolation of flow-stress data to higher strain rates (ε≲10+4 s−1), which are in good agreement with the results obtained from high strain-rate deformation tests. The influence of solute-alloying elements on the thermal stress, the activation enthalpy, and the constitutive parameters will be discussed.