The formation of non-metallic inclusions and their impact on the quality of steel pipes

During the steelmaking process, a small amount of slag, refractory materials, and reaction products in smelting may enter the molten steel to form non-metallic inclusions. They will reduce the mechanical properties of steel, especially the plasticity, toughness, and fatigue limit. In severe cases, it will also cause cracks in hot working and heat treatment or sudden brittle fracture during use. Non-metallic inclusions also cause steel to form a hot-working fiber structure and banded structure, making the material anisotropic. A class of components with non-metallic properties contained in the materials of seamless steel pipes and precision steel pipes. They are produced during the smelting and solidification of metals and alloys and undergo a series of changes in the subsequent hot and cold working processes, which have a multi-faceted impact on the properties of metals and alloys. According to the source of non-metallic inclusions (hereinafter referred to as inclusions), inclusions are usually divided into two categories: exogenous and endogenous. Lining refractory materials or slag particles mixed into the metal (including those just introduced or those that have undergone chemical reactions with the molten metal and have undergone considerable changes in composition and structure) are foreign inclusions; during the smelting and solidification process, the chemical reaction products of the chemical elements contained in the molten metal are not removed in time and remain in the solid metal, which is called endogenous inclusions.


Classification of non-metallic inclusions in steel
Non-metallic inclusions can be classified according to chemical composition or mechanical properties. Classification of non-metallic inclusions in steel. 

Classification by chemical composition of inclusions:
① Simple oxides such as FeO, MnO, Cr2O3, Al2O3, SiO2, and oxides of titanium, vanadium, and niobium.
② Complex oxides Among them, spinel inclusions are represented by the chemical formula AO·B2O3 (in the chemical formula, A represents a divalent metal, such as magnesium, manganese, iron, etc.; B represents a trivalent metal, such as iron, chromium, aluminum, etc.). This type of compound has a spinel MgO·Al2O3 structure, hence the name.
Spinel inclusions are a large class of oxides, such as MnO·Al2O3, MnO·Cr2O3, MnO·Fe2O3, FeO·Al2O3, FeO·Cr2O3, FeO·Fe2O3 (Fe3O4), MgO·Al2O3, MgO·Cr2O3, MgO·Fe2O3, etc. These compounds have a fairly wide range of composition variables; the spinel inclusions encountered are often multi-component. This type of oxide is more common in industrial steel. Calcium aluminates such as CaO·Al2O3 and CaO·2Al2O3 are also complex oxides. FeO·Fe2O3(Fe3O4), MgO·Al2O3, MgO·Cr2O3, MgO·Fe2O3, etc. Calcium aluminates such as CaO·Al2O3 and CaO·2Al2O3 are also complex oxides. However they do not have a spinel structure, so they do not belong to spinel oxides. However they do not have a spinel structure, so they do not belong to spinel oxides.
③ Silicates and silicate glasses The general chemical formula can be written as ιFeO·mnO·nAl2O3·pSiO2. They generally have a multi-component form. It can be either a single phase or a multi-phase. In the case of a single phase, it is generally in a glassy state. Various silicates such as iron silicates, iron manganese silicates, iron manganese aluminum silicates, etc. appear depending on the deoxidation situation. The above three types of inclusions are collectively referred to as oxide inclusions.
④ Sulfides are mainly FeS and MnS; in addition, CaS, TiS, rare earth sulfides, etc. may appear depending on the situation. According to the composition of the molten steel, especially the degree of deoxidation of the molten steel, the sulfides formed have different forms in the cast state: Type I is a composite sulfide (oxysulfide), Type II is a sulfide formed by eutectic reaction, and Type III is a sulfide with geometric shape and random distribution.
⑤ Nitrides such as VN, TiN, AlN, ZrN, NbN, etc. Classification by the Mechanical Properties of Inclusions Non-metallic inclusions destroys the continuity of the metal matrix. When metal products are subjected to loads, inclusions will cause stress concentration and make the material prone to cracking. In metals that have undergone deformation processing, the shape of non-metallic inclusions depends on the degree of deformation of the inclusions relative to the metal matrix, which varies with the composition of the inclusions and the deformation temperature of the metal (steel).

According to the deformation of the inclusions, the inclusions can be divided into four categories:
① Brittle inclusions: refers to those simple oxides and complex oxides and nitrides that are not plastic; when the steel is deformed by hot working, the shape and size of this type of inclusions do not change, but the distribution of the inclusions changes. Both oxide and nitride inclusions can be arranged in a string along the extension direction of the steel, in the form of a point chain. This category includes Al2O3, Cr2O3, spinel oxides, nitrides of vanadium, titanium, cobalt, and some other high melting point inclusions.
② Plastic inclusions: This type of inclusion has good plasticity when the steel is subjected to processing deformation and extends into strips along the rheological direction of the steel. This category includes sulfides, iron manganese silicates with a low SiO2 content (40-60%), and calcium silicates and magnesium silicates in which FeO, MnO, and Al2O3 are dissolved.
③ Spherical (or point-shaped) non-deformable inclusions: spherical in cast steel; after deformation processing, the inclusions remain spherical. This category includes SiO2, silicates with high SiO2 content (>70%), calcium aluminates, pure calcium silicate pure aluminum silicate, etc.
④ Semi-molded inclusions: refers to various multi-phase aluminum silicate inclusions. Among them, the inclusions (aluminosilicate glass) as the base generally have plasticity when the steel is heated; but the precipitated phase crystals (such as Al2O3, spinel oxides) distributed on this base have very poor plasticity. After the steel is thermally deformed, the plastic inclusion phase (base) extends more or less with the deformation of the steel, while the brittle inclusion phase does not deform and still maintains its original shape, but the distance between them is lengthened.