Steel thixoforging process combines the advantages of performing parts having highly complex shapes and good mechanical properties. These advantages result from the shear thinning flow behaviour of semi-solids due to microstructure changes during the material deformation. Many parameters such as steel grade, raw speed, slug temperature, tool temperature influence the steel thixoforging process. In this work, we are interested in analysing the influence of thermal conditions as well as ram speed on the flow behaviour and the microstructure of the final parts. To do so, thixo-extrusion tests on a high speed hydraulic press were performed on C38 steels using different ram speeds and different temperatures for the tools. The temperature is measured in the die close to the tool-material interface during the whole process. Correlations between the temperature measurements, the flow profile and the microstructure are discussed. Using the Finite Element code Forge2005, these thixo-extrusion tests are simulated. Using an inverse method, some parameters used in the thermal exchange modelling are identified by successive comparisons between calculated and measured temperatures within the die.