To provide a working model of the ductile-brittle transition temperature, it is first essential to develop a reasonably complete description of alloying effects on the low-temperature deformation process. In this paper, a semi-empirical plastic flow model for Fe, Fe-Ni and Fe-Si is described. In a sequel, the plastic flow model is applied to predictions of the ductile-brittle transition. For the plasticity description, some rigor has been sacrificed in providing this but the essential features of solid-solution softening and hardening have been retained and justified in terms of detailed measurements of activation enthalpies, activation volumes and strain-rate sensitivities. It is shown that modelling of both the Peierls stress and solid solution strengthening using double-kink nucleation and kink migration approaches can be empirically accomplished with a consistent use of a single atom misfit parameter for each alloy system. For the seven Fe, Fe-Ni and Fe-Si alloys of this investigation, the low temperature deformation is well characterized by a two mechanism approach to yielding. A proposed semi-empirical model offers reasonable prediction of yielding at low temperatures in b.c.c. iron as a function of strain-rate, temperature, substitutional solute additions and grain size.