Non-metallic inclusions are chemical compounds and nonmetals that are present in steel and other alloys. They are the product of chemical reactions, physical effects, and contamination that occurs during the melting and pouring process. These inclusions are categorized by origin as either endogenous or exogenous. Endogenous inclusions, also known as indigenous, occur within the metal and are the result of chemical reactions. These products precipitate during cooling and are typically very small. Exogenous inclusions are caused by the entrapment of nonmetals. Their size varies greatly and their source can include slag, dross, flux residues, and pieces of the mold. Sources of inclusions formation Non-metallic inclusions arise because of many physical-chemical effects that occur in molten and consolidated metal during production. Non-metallic inclusions that arise because of different reactions during metal production are called natural or indigenous. They include oxides, sulfides, nitrides and phosphides. Apart from natural inclusions there are also parts of slag, refractories, material of a casting mould (the material the metal contacts during production) in the metal. Such non-metallic inclusions are called foreign, accidental or exogenous. Most inclusions in the reduction smelting of metal formed because of admixture dissolubility decreasing during cooling and consolidation. The present-day level of steel production technology allows the elimination of most natural and foreign inclusions from the metal. However its general content in different steels can vary between wide limits and has a big influence on the metal properties. Classification of non-metallic inclusions Non-metallic inclusions, the presence of which defines purity of steel, are classified by chemical and mineralogical content, by stability and by origin. By chemical content non-metallic inclusions are divided into the following groups: sulfides (simple — FeS, MnS, Al2S3, CaS, MgS, Zr2S3 and others; compound — FeS·FeO, MnS·MnO and others); nitrides (simple — ZrN, TiN, AlN, CeN and others; compound — Nb(C, N), V(c, N) and others), which can be found in alloyed steel and has strong nitride-generative elements in its content: titanium, aluminium, vanadium, cerium and others; silicates oxides (simple — FeO, MnO, Cr2O3, SiO2, Al2O3, TiO2 and others; compound — FeO·Fe2O3, FeO·Al2O3, FeO·Cr2O3, MgO·Al2O3, 2FeO·SiO2 and others; The majority of inclusions in metals are oxides and sulfides since the content of phosphorus is very small. Silicates are very detrimental to steels, especially if it has to undergo heat treatment at a later stage. Usually nitrides are present in special steels that contain an element with a high affinity