influences of non-metal impurities in steel production

Sulfur is derived from steel-making ore and fuel coke, and is a harmful element in steel. Sulfur exists in the steel in the form of iron sulfide (FeS). FeS and Fe form a low melting point (985°C) compound, and the hot working temperature of steel is generally between 1150-1200°C. Therefore, when the steel is hot processed, the FeS compound will Premature melting causes cracking of the workpiece. This phenomenon is called "hot brittleness". The higher the sulfur content, the more serious the hot embrittlement phenomenon, so the sulfur content in the steel must be controlled. High-quality steel S<0.02-0.03%, high-quality steel S<0.03-0.045%, and ordinary steel S<0.055%-0.07%. In individual cases, there may be cases where sulfur needs to be added. For example, adding 0.08-0.20% sulfur to steel can improve machinability, usually called free-cutting steel. Sulfur is also detrimental to welding performance and reduces corrosion resistance. The S content can be adjusted by adding pyrite or desulfurizer.

Phosphorus is brought into steel by ore. Generally speaking, phosphorus is also a harmful element. Although phosphorus can increase the strength and hardness of steel, it causes a significant decrease in plasticity and impact toughness. Especially at low temperatures, it makes the steel significantly brittle. This phenomenon is called "cold brittleness". Cold brittleness deteriorates the cold working and weldability of steel. The higher the phosphorus content, the greater the cold brittleness, so the phosphorus content in the steel is strictly controlled. By adding dephosphorizer or ferro phosphorus to adjust P content of the steel or iron. High-quality steel: P<0.025%; high-quality steel: P<0.04%; ordinary steel: P<0.085%.

The ability of ferrite to dissolve nitrogen is very low. When supersaturated nitrogen is dissolved in the steel, after a long period of time or subsequent heating at 200-300℃, nitrogen will precipitate in the form of nitrides, which will increase the hardness and strength of the steel, decrease the plasticity, and cause aging. . Adding Al, Ti or V to the molten steel for nitrogen fixation, so that nitrogen is fixed in AlN, TiN or VN, which can eliminate the tendency of aging.

Oxygen is a harmful element in steel. It naturally enters the steel during the steelmaking process. Although manganese, silicon, iron and aluminum are added for deoxidation at the end of the steelmaking process, it is impossible to remove it completely. Oxygen in the steel takes the form of FeO, MnO, SiO2, Al2O3 and other inclusions, which reduces the strength and plasticity of the steel. In particular, it has a serious impact on fatigue strength and impact toughness.