voestalpine Böhler special steel is a manufacturer of highly alloyed steel grades, besides corrosion-resistant steels also many tool steels are available in their portfolio. The casting of large ingots is still state of the art for special steels. The solidification of such steel grades can be improved by promoting an earlier columnar to equiaxed transition and a fine primary grain structure. Segregation, porosities, and shrinkage can be minimized through such measures. This thesis examines the solidification of the corrosion resistant super-austenitic steel X1CrNiMoCuN20-18-7 and the high-speed steel HS2-9-1-8. The change from columnar to equiaxed and fine-grained solidification is the main objective for the super-austenitic steel, while for the high-speed steel the refinement of the eutectic carbide structure is most important. The literature review engages in the necessary basics to discuss the physicochemical processes during solidification. In addition, the possibilities to apply endogenous particles as heterogeneous nucleation sites are described in detail. To utilize such inclusions as the nucleus for the primary grain structure is the main objective of this thesis. Especially AlCeO3 is a very promising agent for grain refinement of austenitic steels. The most considerable influence on the eutectic carbides is exerted by the chemical composition of the particular high-speed steel grade and the cooling conditions. For the simulation of the AlCeO3 formation, the reliability of the database FToxid, which is included in FactSage, is tried and approved. The calculation represents a useful tool to forecast the inclusion population. Grain refinement experiments with the super-austenitic steel show that at a total oxygen content of 140 ppm and with 40 AlCeO3 particles per mm², the ratio of equiaxed solidification is increased from 23 to 64 %. Additionally, the number of grains rises from 1.7 to 41 per cm². Deformation and recrystallization tests result in slower grain growth of the refined steel in comparison to industrial material. This means that the advantage of the fine cast structure prevails even after the experiment. The influence of grain refinement and solidification and casting conditions on the eutectic carbide structure is investigated for the high-speed steel. The application of titanium leads to an improvement of the primary structure, but the ratio of coarse carbides is not altered. The casting and solidification conditions showed a distinct impact on the eutectic carbides structure. High cooling rates and low superheating temperatures pose an advantage to decrease the ratio of coarse carbides. The total carbide content stays unaffected.