The subject of the presented paper is the modelling of strain-stress relationship, which is the main mechanical property characteristic of the behaviour of C45 grade steel subjected to plastic deformation in semi-solid state. The methodology of yield stress curves development requires high accuracy systems of tension and compression tests simulation. Hence, the proposed testing procedure is related to dedicated hybrid FEM system, which was developed by authors of the contribution. The experimental work has been done using the Gleeble ® 3800 thermo-mechanical simulator. The testing machine allows the physical deformation of samples while solidification of their central part is still in progress. The essential goal of the simulation was the computer reconstruction of both temperature changes and strain evolution inside a specimen subjected to simultaneous deformation and solidification. In order to verify the predictive ability of the developed rheological model, a number of compression tests using Gleeble ® 3800 simulator have been done, as well. The comparison between numerical and experimental results is also a part of the presented paper.