Non-Metallic Inclusions in Steel

Non-Metallic Inclusions in Steel is a general term for all kinds of non-metallic particles entrained in steel. Steel contains elements such as oxygen, nitrogen, and sulfur. Their solubility in steel is high at high temperatures, but very low at room temperature. It precipitates when the steel cools and solidifies and combines with iron and other metals to form various compounds Called non-metallic inclusions.



Early literature once referred to non-metallic inclusions in steel as "slag inclusions". This name is easily misunderstood, thinking that non-metallic inclusions are slag mixed into steel. At present, all kinds of materials mixed into steel are generally called foreign inclusions, and their shapes are irregular. The inclusions generated by internal physical and chemical reactions are called endogenous inclusions. Their typical characteristics are small size and number. Many, evenly distributed. The process of forming inclusions in steel is roughly as follows: after the deoxidizer is added to the molten steel, the deoxidizing element and oxygen react chemically to form oxides that are insoluble in the steel; some deoxidizing elements can also combine with sulfur and nitride to form sulfide and nitride . Such compounds are called nascent inclusions. Except for a few small inclusions, most of the primary inclusions can float out of the molten steel and enter the slag. When the molten steel cools and solidifies, due to the decreased solubility and segregation of oxygen, sulfur, etc., oxides and sulfides are produced during the solidification process, called secondary inclusions. Secondary inclusions are difficult to exclude from the steel and remain between the dendrites or finally precipitate out on the grain boundaries. After the molten steel is deoxidized, it continues to be exposed to air or other oxides such as refractory materials, etc., so that the molten steel reabsorbs oxygen, that is, secondary oxidation occurs. Secondary oxidation is an important source of non-metallic inclusions in the finished steel.

The presence of non-metallic inclusions in the steel destroys the continuity of the metal matrix and deteriorates the quality of the steel. In special cases, some inclusions are conducive to certain properties of steel (such as machinability), but this is only under special conditions. Generally speaking, non-metallic inclusions have considerable harm to the mechanical, physical and chemical properties of steel. In layman's terms, steel with many inclusions is "dirty", and pure steel contains very few inclusions. However, pure steel is a relative concept. The cleanliness of steel is related to its use, as well as the shape, particle size and plasticity of inclusions. Inclusions with a small number but large particles tend to be more harmful than inclusions with a large number but small size; irregular shapes are more harmful than spherical inclusions. For precision small parts, tiny inclusions on the surface are also harmful. So for non-metallic inclusions, not only to study how to reduce its content, but also to study its morphology and distribution. Types of non-metallic inclusions can be classified from different viewing angles. According to the chemical composition, it can be divided into oxide inclusions, sulfide inclusions and nitride inclusions; according to the plasticity of the inclusions, it can be divided into normal inclusions, brittle inclusions and non-deformable inclusions; according to the size of the inclusions, it can be divided into large inclusions and microinclusions, etc. . The nature and morphology of inclusions are related to its composition.

Because non-metallic inclusions have many effects on the performance of steel, and the sources of inclusions are various, it is difficult to accurately determine, the study of non-metallic inclusions in steel has always been an important issue in steelmaking. The analysis and identification technology of non-metallic inclusions in steel is also constantly developing with the advancement of microscopic analysis instruments.