Phosphorus is generally known as harmful element in steel. During the solidification, P strongly segregates in the interdendritic liquid leading to a significant drop of the solidus temperature. Depending on the solidification sequence in the peritectic range of the Fe-C based, multicomponent phase diagram the segregation tendency increases with higher amount of carbon. As a consequence, even small amounts of P favour the hot tear formation in technological solidification processes. However, in recent steel design, P is added as alloying element to improve the physico-mechanical properties of advanced steel grades. In so called �rephosphorized� steels, the maximum P content exceeds 0.10 mass percent and is far higher than the typical concentration of P < 0.010% in high-quality steels or P < 0.05 % in construction steels. In the continuous casting process, mechanical stresses are exerted on the solidifying shell due to bulging, bending/straightening and thermal gradients. In combination with the increased hot tear sensitivity of highly P-alloyed steels, the tensile stresses along the solidification front may cause serious internal defects and special care has to be taken in optimizing the process parameters for casting rephosphorized steels. However, beside the precise knowledge of the actual process conditions, accurate phase diagram data and thermodynamic properties of the steel are required for a successful process control during casting. In the present PhD thesis, the effect of phosphorus on high-temperatures phase transformations with respect to the continuous casting process was therefore studied by investigating the binary Fe-P and ternary Fe-C-P key-systems. In the experimental part various in-situ techniques of differential scanning calorimetry (DSC), high-temperature laser scanning confocal microscopy (HT-LSCM), high-temperature X-ray diffraction (HT-XRD) and dilatometry were applied to characterize the phase transformation temperatures and phase stabilities. In addition to classical DSC analysis of high-temperature phase equilibria, a novel approach of coupling DSC analysis and HT-LSCM observations was developed to characterize the �?-loop� in the Fe-P system. CALPHAD-type (Calculation of Phase Diagrams) thermodynamic optimizations of the Fe-P and Fe-C-P systems were carried out considering the newly gained phase diagram data. For the first time, the modified quasichemical model (MQM) was applied for the liquid phase in this alloying system. Finally, the experimental data and the improved thermodynamic assessment were used to further develop a microsegregation model for calculation of solidification close to continuous casting conditions with particular focus on rephosphorized steels.