This master thesis deals with the non metallic inclusions behaviour in steel melts during their treatment in the secondary metallurgy. Therefore in the course of secondary metallurgy samples were taken of a heat-treated-steel 37CrS4 and a rail steel R260. The sampling started with the arrival of the melt at the ladle furnace and ended at the tundish of the continuous caster. The taken samples include total oxygen, disk and slag samples. The inclusions in total oxygen samples and disk samples have been analysed with a scanning electron microscope to detect the influence of the different steel deoxidisers, of the length of the steel treatment, of the melt heating and of the slags chemical analyses. Within this thesis the reproducibility of total oxygen samples and disk samples concerning the size distribution of the non metallic inclusions and their chemical analyses has been investigated.