Abstract The paper deals with the origin of transition zones in continuously cast billets, discussing the technological procedures used for their minimisation and the main factors affecting their extent. Moreover, the article gives account of the experimental conditions applying to the numerical modelling of the extent of these zones using CFD Fluent package. The actual simulations were focused upon the evaluation of the effect of the steel bath weight in the tundish, tundish flow pattern (isothermal, non-isothermal) and dimensionless specifications of the chemical composition of the steels upon the extent of transition zones present in 150x150mm billets continuously cast at the CCM no.2 at Třinecké železárny, a.s. The results obtained from the simulations are also discussed and compared to the present operational practice.