Recent theory demonstrates that the Kocks-Mecking formulation of plasticity has a foundation in multiscale irreversible thermodynamics. The key terms in the formulation can be obtained form experiments and from fundamental calculations. This offers two advantages to materials scientists and alloy designers: the Kocks-Mecking approach goes beyond being a phenomenological approach, gaining a solid physical foundation in multiscale computational physics; the new formulation can be employed to conceive new alloys displaying complex synergistic interactions at several scales and among several phases. This approach is ideal for designing and modelling nanostructured steels. This work incorporates four concomitant strengthening effects: solid solution, Hall-Petch, dislocation forest and precipitation. The new formulation is applied to nanostructured martensitic, dual phase and twinning induced plasticity steels, describing with excellent accuracy of their stress-strain behaviour.