Increasing requirements regarding steel quality, involve a better understanding of the interaction between microstructure and steel cleanness. The specific use of non – metallic inclusions as nuclei for acicular ferrite can modify the steel microstructure, resulting in better mechanical properties. Because of its fine and chaotic shape, acicular ferrite enables this kind of modification in low alloyed high strength steels. The aim of the master thesis was to investigate non – metallic inclusions in HSLA (High Strength Low Alloyed) and rail steels regarding their potential for acicular ferrite. Furthermore, the effects of alloying elements and crucible material on acicular ferrite were evaluated. The first part includes a literature study and describes the acicular ferrite nucleation potential of selected inclusion types and the influence of alloying elements like aluminum, silicon, manganese or boron on the nucleation behavior. In the second part the results of practical tests verify the literature and reply unanswered questions, like how different steel grades can influence the acicular ferrite formation. The HSLA reference samples transformed to over 70 % acicular ferritic and the inclusions with the highest potential for acicular ferrite nucleation were (Ti,Al,Mn)OxSy and (Ti,Al,Mn)Ox particles. Alloying chromium and nickel suppresses the acicular ferrite formation completely. The reduction of manganese and the substitution of manganese with nickel decreased the acicular ferrite amount below 10 %. By using a rail steel composition with different titanium contents no acicular ferrite formed. A magnesium modification by using a MgO – crucible produced (Ti,Al,Mn,Mg)OxSy and (Ti,Al,Mg)Ox inclusions which showed a high potential for acicular ferrite formation. The master thesis contributes to a better understanding of the use of non – metallic inclusions as nucleation sites, which may lead to a better controllable acicular ferrite formation in future.