An inclusion in steel is a particle or material that is trapped in the steel during its formation. Inclusions can be solid, liquid, or gaseous, and can have a variety of shapes and sizes. They are usually made up of non-metallic substances such as oxides, sulfides, or silicates, and can have a negative impact on the mechanical properties of the steel, such as reducing its ductility, toughness, and fatigue strength. Inclusions can also contribute to the formation of defects in the steel, such as cracks and voids, and can affect its surface finish and appearance. Therefore, controlling the size and distribution of inclusions in steel is an important aspect of steel production and processing.