Non-metallic phases that are produced or mixed during smelting, casting, and solidification of molten steel are referred to as non-metallic inclusions.

The non-metal phase is a compound in which some metal elements and Si are combined with non-metal elements, such as oxides, nitrides, sulfides, and the like. Due to the presence of inclusions, the continuity of the steel matrix is destroyed, resulting in unevenness of the steel structure, affecting the mechanical properties and processing properties of the steel.

However, non-metallic inclusions also have a favorable effect on steel, such as controlling the fine grains, precipitation hardening, promoting grain orientation, and improving the cutting performance of steel.