In addition to steel containing iron, carbon and alloying elements, in smelting, impurities such as manganese, silicon, sulfur, phosphorus, non-metallic impurities and certain gases such as nitrogen, hydrogen, oxygen, etc. are unavoidable. These impurities affect the quality of steel. Have a great impact.
A. Non-metallic inclusions During the steelmaking process, a small amount of slag, refractory materials and reaction products during smelting may enter the molten steel to form non-metallic inclusions. For example, oxides, sulfides, silicates, nitrides, etc. They will reduce the mechanical properties of steel, especially plasticity, toughness and fatigue limit. In severe cases, it will also cause steel to crack during hot working and heat treatment or sudden brittle fracture during use. Non-metallic inclusions also promote the formation of thermally processed fibrous structure and band-like structure in steel, which makes the material anisotropic. In severe cases, the transverse plasticity is only half of the longitudinal direction, and the impact toughness is greatly reduced. Therefore, for important steels (such as rolling bearing steel, spring steel, etc.), check the number, shape, size and distribution of non-metallic inclusions. In addition, steel is in contact with air throughout the smelting process, so some gases such as nitrogen, oxygen, and hydrogen are always absorbed in the molten steel. They also have a negative impact on the quality of steel.
B. Phosphorus can be brought into steel from pig iron. Under normal circumstances, all phosphorus in steel can be dissolved in ferrite. Phosphorus has a strong solid solution strengthening effect, which increases the strength and hardness of steel, but the plasticity and toughness are significantly reduced. This embrittlement phenomenon is more serious at low temperatures, so it is called cold brittleness. It is generally desirable that the cold brittle transition temperature is lower than the working temperature of the workpiece to avoid cold brittleness. In the process of crystallization of phosphorus, intragranular segregation is easy to occur, so that the phosphorus content in local areas is higher, which leads to an increase in the cold brittle transition temperature, and thus cold brittleness occurs. Cold brittleness is a serious hazard to structural parts that work in alpine regions and other low temperature conditions. In addition, the segregation of phosphorus also causes the steel to form a banded structure after hot rolling. Therefore, under normal circumstances, phosphorus is also a harmful impurity. The phosphorus content in steel should also be strictly controlled. However, when the phosphorus content is large, the brittleness is large, which is advantageous in manufacturing shell steel and improving the machinability of steel.
C. When silicon silicon exists as an impurity in steel, it is generally less than 0.4%. It also comes from pig iron and deoxidizer. Silicon is soluble in ferrite at room temperature and has a certain strengthening effect on steel. However, when silicon exists as a small amount of impurities, its effect on the performance of steel is not significant.
D. When manganese exists as an impurity in steel, it is generally less than 0.8%. It comes from pig iron used as a raw material for steelmaking and ferromanganese as a deoxidizer. Manganese has good deoxidizing ability and can also form MnS with sulfur to eliminate the harmful effects of sulfur. Most of these reaction products enter the slag to be removed, and a small part remains in the steel as non-metallic inclusions. In addition, manganese is soluble in ferrite at room temperature and has a certain strengthening effect on steel. Manganese can also be dissolved in cementite to form alloy cementite. But when manganese exists as a small amount of impurities, it has no significant effect on the performance of steel.
E. Sulfur Sulfur is an impurity brought into steel by pig iron and fuel. In the solid state, sulfur has very little solubility in iron, but exists in steel in the form of FeS. Due to the poor plasticity of FeS, steel with more sulfur is more brittle. More seriously, FeS and Fe can form a low melting point (985°C) eutectic, which is distributed on the grain boundaries of austenite. When the steel is heated to about 1200°C for hot press processing, the eutectic on the grain boundary has melted, and the inter-grain bond is broken, causing the steel to crack along the grain boundary during processing. This phenomenon is called hot brittleness. In order to eliminate the harmful effects of sulfur, the manganese content in steel must be increased. Manganese and sulfur preferentially form high melting point (1620°C) manganese sulfide, which is distributed in the crystal grains in a granular form. It has certain moldability at high temperatures, thereby avoiding hot brittleness. Sulfides are non-metallic inclusions, which will reduce the mechanical properties of steel and form thermally processed fibrous structures during the rolling process. Therefore, under normal circumstances, sulfur is a harmful impurity. Strictly limit the sulfur content in steel. However, steel with a higher sulfur content can form more MnS. In the cutting process, MnS can play a role in chip breaking and improve the machinability of the steel. This is a favorable aspect of sulfur.