A two dimensional model was developed to predict the grain structure (Equiaxed to Columnar Transformation (ECT) and Columnar to Equiaxed Transformation (CET) in the continuous casting of steel. The processes of nucleation, growth and impingement of the grains are modelled as follows: (I) the nucleation is modeled through a continuous dependency of the nucleation density on temperature by the Gaussian distribution. Different nucleation parameters are used at the boundary and in the bulk region. (II) The growth and impingement are modeled by the Kurz, Giovanola, Trivedi (KGT) model. The Cellular Automata (CA) technique is used to solve the model. The CA method is based on the Nastac’s and simplified neighborhoods. Calculations are shown for square billets of the dimension 140 mm. Fixed input parameter of the model represents the macroscopic temperature field obtained from the Štore Steel billet simulation system . All other grain structure physical model parameters are varied, such as: the surface and the bulk area, mean nucleation undercooling, standard deviation of undercooling, maximum density of nuclei. The computational parameters, such as the micro mesh size and the time step are varied as well. The influence of the variation of different parameters on calculated grain structure is shown. Finally, the model parameters are adjusted in order to obtain the experimentally determined actual billet ECT and CET positions for 51CrV4+Mo spring steel (Al: 0.02, Cr: 1.05, Cu: 0.125, Mn: 0.9,Mo: 0.025, Ni: 0.1, Si: 0.275, V: 0.155, C: 0.51, P: 0.0125, S: 0.0275 wt%). A systematic procedure is outlined for adjusting of the model data with the experiment.