They are impurities that occur in steel that are usually composed of different types of oxides. The most common inclusions in the steel are silicates, aluminas, globular oxides and sulphides.
Inclusions may be caused by the reaction between the different elements from the mineral or the scrap and the added ones in the process of melting and refining of the steel, and contamination of refractory particles of the troughs used in the process of melting.
Non-metallic inclusions are detrimental to the quality of the steel, since they can be the source of cracks and defects that can lead to loss of mechanical characteristics or fractures. As an exception we have the manganese sulphides that are generated to improve the machinability of steel.