Purpose: The purpose of this research is to analyse the steel refining technology in a ladle furnace including refining slag formation in the ladle. The research was conducted using chromium-nickel- molybdenum steel intended for heat treatment in order to produce crankshafts. Design/methodology/approach: This research was conducted in a ladle furnace with the capacity of 65 Mg of steel. Data from 10 melts were analysed. Samples of refining slag and metal were taken to analyse their chemical composition. On their basis researchers analysed the process of desulphurisation of steel at the ladle furnace workstation. Calculations were made concerning the equilibrium chemical composition of slag to determine the share of gas phase, liquid phase and solid precipitations. The calculations were made using a thermodynamic software FactSage 6.2. Findings: Analysing the process of desulphurisation of steel under slag with the average chemical composition of: CaO - 54.0%, Al2O3 - 30.2%, SiO2 9.0%, MgO - 8.2%, and FeO content at the level of 0.94% drew attention to a high degree of desulphurisation at the level of 86%. Due to this fact it was possible to obtain high basicity of slag: V=CaO/SiO2 at the level of 6.21. Mannesmann index, which characterizes the ability of slag to remove sulphur and non-metallic inclusions from steel and is defined as M=(CaO/SiO2)/Al2O3, amounted to 0.21%-1 and was too low in comparison to the required one ranging from 0.35-0.45%-1. The calculations concerning the equilibrium chemical composition of slag, which determined the share of gas phase, liquid phase and solid precipitations, were performed using the thermodynamic software FactSage 6.2. These calculations showed that there were no solid precipitations present. The refining process under slag used in this research proved to be extremely efficient in terms of desulphurisation. Research limitations/implications: It was concluded that the slag forming technology in the ladle is correct in terms of materials used, both as far as their type and amount are concerned. Deoxidation is efficient. Practical implications: It is important to ensure that the furnace slag does not enter the ladle. As a result, it will be possible to improve the quality of the refining slag. Using the thermodynamic software FactSage 6.2 can contribute to modifying the chemical composition of the refining slag and result in an improvement of refinement as well as a better protection of the refractory lining of the ladle. Originality/value: In order to calculate the equilibrium chemical composition of slag researchers used the thermodynamic software FactSage 6.2. Its use in practice can help select slag forming materials that are characterized by high refining parameters.