The main aim of this thesis was to optimize the Ca-treatment at voestalpine Stahl GmbH by analyzing the most important impacts along the production process—from Ca-treatment up to the heavy plate slab. The main focus was on measuring the transformation of oxide und sulfide micro inclusions during secondary metallurgy treatment and continuous casting to secure the source service properties of the heavy plates and the pipes made of them. Basic scope of the work was therefore to research the basic influence of Ca-treatment by CaSi-wire injection on the modification of micro particles in steel. Therefore, it was necessary to characterize the influence of different types, size distributions and chemical analysis of inclusions on source-service-tested heavy plate samples (with and without Ca-treatment). The application of different appropriated measurement systems and a combination of those results in order to get a consistent understanding and a maximum of detailed information concerning steel cleanliness are essential elements. The informational value and limitations of the inspection methods applied were critically analyzed. Thermodynamic calculations with different model databases and variations of initial parameters completed this research task and were done in order to match calculated results with the measured phenomena at samples of each important process step. All results have been discussed and summarized to get coherent conclusions about the behavior of calcium, sulfur and magnesium in steel when calcium-treated.