During forging and machining manufacturing processes, the material is subject to large strains at high strain rates which provoke local heating and microstructural changes. Modelling of these phenomena requires precise knowledge of the stress-strain constitutive equations for a large rangeof strains, strain rates and temperatures. An experimental study of the rheology of both hyper and hypo-eutectoid steels (with different microstructrues) over a temperature range from 20◦C to 1000◦C and with strain rates from 10−2 to 105 s−1, has been undertaken. These tests were performedin compression on cylindrical specimens and in shear using hat-shaped specimens. Both a GLEEBLE 3500 thermo-mechanical testing machine and a Split-Hopkinson Pressure Bar apparatus were used. From these tests, three deformation domains have been identified as a function of the material behaviour and of the changes in the deformed microstructure. Each domain was characterized by its behaviour, including: the competition between hardening and softening, strain rate sensitivity on the flow stress and the softening phenomenon (i.e. recrystallisation or recovery, etc.). Finally, based on thermodynamical considerations, the conditions of thermo-plastic instability (i.e. shear bands, twinning, heterogeneities, etc.) and microstructural changes are highlighted using process maps of the dissipated power repartition.Read less <