The main goal of the paper is modelling of phenomena accompanying the steel subjected to deformation in very high temperature. Analysis of the phenomena in solid surface part and semi-solid core is necessary due to inhomogeneity of material properties in both the zones of the sample. Experimental basis of the work were TESTS done in IMZ Gliwice using Gleeble 3800 physical simulator. The interpretation of the tests was done using Def_Semi_Solid software. This program allows the numerical analysis of steel behaviour during deformation of axial-symmetrical samples at very high temperatures. The paper presents the comparison of experimental and numerical results.