The Influence Of Impurities On the Steel Properties

Publish Time: 06/27 2021 Author: Site Editor Visit: 15094

In addition to iron, carbon and alloy elements, some impurities (such as manganese, silicon, sulfur, phosphorus, non-metallic impurities and some gases, such as nitrogen, hydrogen, oxygen, etc.) will inevitably be brought into the actual steel in the smelting process. These impurities have a great influence on the quality of steel.

When manganese exists as impurity in steel, it is generally less than 0.8%. It comes from pig iron and deoxidizer ferromanganese as raw materials for steelmaking. Manganese has good deoxidation ability and can form MNS with sulfur to eliminate the harmful effect of sulfur. Most of these reaction products enter into the slag and are removed, and a small part remains in the steel and becomes non-metallic inclusions. In addition, manganese can be dissolved in ferrite at room temperature, which has a certain strengthening effect on steel. Manganese can also be dissolved in cementite to form alloy cementite. However, when manganese exists as a small amount of impurity, its effect on the properties of steel is not significant.
When silicon exists as impurity in steel, it is generally less than 0.4%. It also comes from pig iron and deoxidizer. At room temperature, silicon can dissolve in ferrite and has a certain strengthening effect on steel. However, when silicon exists as a small amount of impurity, its effect on the properties of steel is not significant.
Sulfur is the impurity brought into steel by pig iron and fuel. In the solid state, the solubility of sulfur in iron is very small, but it exists in the form of FES in steel. Due to the poor plasticity of FES, the steel with more sulfur is more brittle. What's more, FES and Fe can form eutectic with low melting point (985 ℃) and distribute on the grain boundary of austenite. When the steel is heated to about 1200 ℃ for hot pressure processing, the eutectic on the grain boundary has melted and the bonding between grains is destroyed, which makes the steel crack along the grain boundary during the processing. This phenomenon is called hot brittleness. In order to eliminate the harmful effect of sulfur, the content of manganese in steel must be increased. Manganese and sulfur preferentially form manganese sulfide with high melting point (1620 ℃) and distribute in grains in granular form. It has certain molding property at high temperature, thus avoiding thermal brittleness. Sulfide is a kind of non-metallic inclusion, which can reduce the mechanical properties of steel and form hot working fiber structure during rolling. Therefore, sulfur is usually a harmful impurity. The content of sulfur in steel should be strictly limited. But the steel with more sulfur content can form more MNS. In the cutting process, MNS can play the role of chip breaking and improve the machinability of steel, which is the favorable side of sulfur.
Phosphorus is carried into steel by pig iron. Generally, all phosphorus in steel can be dissolved in ferrite. Phosphorus has a strong solution strengthening effect, which increases the strength and hardness of the steel, but decreases the plasticity and toughness significantly. This embrittlement is more serious at low temperature, so it is called cold embrittlement. Generally, it is expected that the transition temperature of cold embrittlement is lower than the working temperature of the workpiece to avoid cold embrittlement. During the crystallization process of phosphorus, it is easy to produce intragranular segregation, which makes the phosphorus content in some areas higher, resulting in the increase of cold brittle transition temperature, resulting in cold brittle. Cold embrittlement is harmful to the structural parts working in the high cold zone and other low temperature conditions. In addition, the segregation of phosphorus also makes the steel form banded structure after hot rolling. Therefore, in general, phosphorus is also a harmful impurity. The content of phosphorus in steel should also be strictly controlled. However, when the phosphorus content is high, the brittleness of the shell steel is high, so it is advantageous to manufacture the shell steel and improve the machinability of the steel.
Non metallic inclusions in the process of steelmaking, a small amount of slag, refractory and reaction products may enter into the liquid steel and form non-metallic inclusions. For example, oxides, sulfides, silicates, nitrides, etc. They will reduce the mechanical properties of steel, especially the plasticity, toughness and fatigue limit. When it is serious, it will make the steel crack during hot working and heat treatment or brittle fracture suddenly. The non-metallic inclusions also promote the formation of hot working fiber structure and banded structure, which makes the material anisotropic. In severe cases, the transverse plasticity is only half of the longitudinal one, and the impact toughness is greatly reduced. Therefore, the number, shape, size and distribution of non-metallic inclusions should be checked for important steel (such as rolling bearing steel, spring steel, etc.). In addition, during the whole smelting process, the steel is in contact with air, so some gases such as nitrogen, oxygen and hydrogen are always absorbed in the liquid steel. They also have an adverse effect on the quality of steel.

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