The product portfolio of a modern steel plant is manifold. In order to meet quality requirements, a specific adaptation of slag chemistry is needed during secondary metallurgy. In the course of this work ultra low carbon (ULC) steels as well as micro alloyed structural steels characterized by a sulfur content below 30 ppm are investigated. These steel grades are observed throughout the production process and the corresponding slag compositions are investigated. In the literature part of this work the importance and the possibilities of secondary metallurgical treatment are described. The formation of ladle slag as well as the parameters promoting an effective desulfurization are highlighted. Also the influence of the MgO-saturation on the wear of the refractory is characterized. To describe the formation of the ladle slag a mass balance is generated for both steel grades. The amount of carry over slag from the basic oxygen furnace, the synthetic slag formers which are added during converter tapping, deoxidation products and slag former additions in secondary metallurgy are determined as the main impacts regarding slag composition. Furthermore, the lime and magnesia saturation is examined, which are found to be at an adequate level. The slags of the ULC steels are analyzed regarding a relationship of the (FeO+MnO)-content and the aluminum loss. For the micro alloyed steel grades the desulfurization is thoroughly investigated. Finally, it is shown that slags of both investigated steel grades are very well suited for the production of high quality steels.