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In this paper the thermo-visco-plastic behaviour of six high strength steels is analyzed in a wide range of strain rates and temperatures. The steels belong to two well-known versions of steels: three ferritic steels: HSLA-65, DH-36 and Weldox 460-E, and three austenitic steels: Nitronic-50, Uranus B66 and AL-6XN.Micro-mechanism of plastic deformation in the case of ferritic steels is dominated by dislocation's gliding. In the case of austenitic steels the twinning appears that modifies the material's plastic properties. Twins appear as a consequence of the low value of the stacking fault energy (SFE) of the austenitic steels, and they act as obstacles for the dislocations glide. These deformation micro-mechanisms provide that the austenitic steels exhibit a high work hardening and large ductility. In the present paper the RK (Rusinek&-Klepaczko) constitutive relation is evaluated for modelling of the plastic behaviour of six steels which represent these two different deformation modes. Satisfactory agreement between experimental data and RK predictions within a wide range of strain rates and temperatures have been found for all materials considered.