Non-metallic phases that cannot be eliminated after processing or heat treatment, such as oxides, sulfides, nitrides, etc., which are produced or mixed in the smelting and casting process and have no connection with the steel matrix and exist independently, are collectively referred to as non-metallic inclusions, also referred to as inclusions. Although Fe, Mn, Cr, Al, Ti, etc. are metal elements, they lose their metallic properties after forming compounds with O, S, N, etc. Non-metallic inclusions in steel mainly refer to such compounds. Due to their existence, the continuity of the steel matrix is destroyed, the structure of the steel is uneven, and the various properties of the steel are affected. However, it has its advantages in controlling the intrinsic grain size, producing precipitation hardening, and improving cutting performance.
Sources of non-metallic inclusions
The sources of non-metallic inclusions in steel are mainly the following:
- Pollutants in raw materials. Pollutants in raw materials mainly refer to impurities in steel or ferroalloys, gangue in ores, etc.
- Reaction products during smelting and casting. The reaction products of molten steel in the furnace, ladle and pouring process are the main sources of non-metallic inclusions in steel.
- Inclusions brought in by refractory materials. Refractory materials used for steelmaking contain oxides of Mg, Ca, Si, Al and Fe. From smelting in the furnace, tapping to pouring, molten steel must contact refractory materials, so there is high temperature and chemical erosion and mechanical scouring of refractory materials. Generally, MgO in steel accounts for more than 5% of the total inclusions, mainly from this.
- Emulsified slag drop inclusions. In addition to eccentric or furnace bottom tapping, slag and steel mixing during tapping is a common occurrence. Sometimes, slag and steel mixing is also desired for further deoxidation and desulfurization; if the adhesion work is large, slag and steel are not easy to separate, and emulsified slag drop inclusions are easy to appear in the steel.
- Other sources of inclusions in steel. During tapping and pouring, the top of the furnace cover, the tapping trough or the ladle and the pouring system are not cleaned, and the mechanical mixing of various dust particles is also very easy to become inclusions in the cylinder. In addition, the oxide film or protective slag that appears on the surface of the molten steel during the pouring process has a high density, a high melting point, or a weak ability to capture and adsorb inclusions, which will also increase the large particle inclusions in the steel. Therefore, in the production process of electric furnace steel, in order to reduce the content of inclusions in the steel, we should try our best to do a good job of cleaning in all aspects.
Classification of non-metallic inclusions
Classification by the composition of inclusions
- Oxidation inclusions. Simple oxides include FeO, Fe₂O₃, MnO, SiO₂, Al₂O₃, Cr₂O₃ and TiO₂; complex oxides include spinels and various calcium aluminates.
- Sulfide inclusions. Mainly refers to iron sulfide FeS and manganese sulfide MnS and their solid solutions (Mn·Fe)S, in addition, there are calcium sulfide CaS, etc.; when rare earth elements are added to steel, corresponding rare earth sulfides may be generated; in alloy steel, sulfides containing alloy elements of this steel are sometimes found.
- Nitriding inclusions. When elements such as Al, Ti, Nb, V, and Th with high affinity to nitrogen are added to the steel, corresponding nitrides are generated in the steel. People usually regard nitrides that are insoluble or almost insoluble in austenite and form their own fixed forms in steel as nitride inclusions, among which TiN is the most common, while AlN is fine and dispersed, and has many good effects in steel, and is generally not considered as an inclusion.
- Phosphate inclusions. Phosphorus has a high solubility in iron, but the phosphorus content in electric furnace steel is very low, and phosphorus-containing inclusions are generally not seen, and only occasionally appear in high-manganese steel.
