The paper reports the results of numerical simulation of the flow of liquid steel with the use of a multiphase model. The facility under study was a mould designed for continuous casting of steel slabs. The geometry of the facility, along with the computational grid, was generated within the program Ansys-MeshingR. Numerical computations were performed in the programs: COMSOL Multiphysics¯ and Ansys-Fluent¯. The use of the multiphase model enabled the determination of the behavior of the liquid slag layer on the metal bath surface. From the performed computer simulations, the fields of liquid steel motion and liquid steel turbulence kinetic energy distribution in the mould's symmetry plane have been presented. Based on the values recorded at selected measurement points located on the slag surface, a diagram illustrating the variation of the slag layer position during continuous steel casting has been plotted.