The role of alloying elements(Niobium/Columbium(Nb/Cb), Zirconium(Zr), Silicon(Si), Nitrogen(N))
There are many types of alloying elements. Although they are similar, many different places that can make incredible changes in steel. Let's take a look Niobium/Columbium, zirconium, silicon and nitrogen.
(1)Niobium/Columbium(Nb/Cb)
Niobium and Columbium are often symbiotic with Tantalum, and their role in steel is similar. The Niobium and Tantalum are partially dissolved in the solid solution to form a solid solution strengthening effect. When the austenite is dissolved, the hardenability of the steel is remarkably improved. However, in the form of carbides and oxide particles, the grains are refined and the hardenability of the steel is lowered. It can increase the tempering stability of steel and has a secondary hardening effect.A small amount of niobium can increase the strength of the steel without affecting the ductility or toughness of the steel. Due to the effect of refining the grains, the impact toughness of the steel can be improved and the brittle transition temperature can be lowered. When the content is more than 8 times that of carbon, almost all the carbon in the steel can be fixed, so that the steel has good hydrogen resistance. In the austenitic steel, the intergranular corrosion of the steel by the oxidizing medium can be prevented. Due to the fixed carbon and precipitation hardening effect, the high-temperature performance of the heat-strength steel can be improved. Such as creep strength and so on.
Niobium can improve the yield strength and impact toughness in ordinary low alloy steel for construction, and reduce the brittle transition temperature and beneficial weldability. In the carburizing and quenching and tempering alloy structural steel while increasing the hardenability. Improve the toughness and low-temperature properties of steel. It can reduce the air hardenability of low carbon martensitic heat-resistant stainless steel, avoid hardening and temper brittleness, and improve creep strength.
(2)Zirconium(Zr)
Zirconium is a strong carbide forming element and its role in steel is similar to that of Niobium, Tantalum and Vanadium. Adding a small amount of zirconium has the effect of degassing, purifying and refining crystal grains. This is beneficial to the low-temperature performance of steel and improves the punching performance. It is commonly used in the manufacture of ultra-high strength steels and nickel-based superalloys for gas engines and ballistic missile structures.
(3)Silicon(Si)
Silicon is soluble in ferrite and austenite to increase the hardness and strength of steel. Its role is second only to phosphorus, stronger than manganese, nickel, chromium, tungsten, molybdenum, vanadium and other elements. However, when the silicon content exceeds 3%, the plasticity and toughness of the steel are significantly reduced. Silicon can increase the elastic limit, yield strength and yield ratio (σs/σb) of steel, as well as fatigue strength and fatigue ratio (σ-1/σb). This is because silicon or silicon manganese steel can be used as a spring steel.
Silicon can reduce the density, thermal conductivity and electrical conductivity of steel. It can promote the coarsening of ferrite grains and reduce the coercive force. There is a tendency to reduce the anisotropy of the crystal, making the magnetization easy, and the magnetic resistance is reduced. It can be used to produce electrical steel, so the silicon steel sheet has a lower magnetic resistance loss. Silicon can increase the magnetic permeability of ferrite, so that the steel sheet has a higher magnetic induction strength under a weaker magnetic field. However, silicon reduces the magnetic induction strength of steel under strong magnetic fields. Due to the strong deoxidation of silicon, the magnetic aging effect of iron is reduced. When the silicon-containing steel is heated in an oxidizing atmosphere, a SiO2 film is formed on the surface, thereby improving the oxidation resistance of the steel at a high temperature. Silicon promotes the growth of columnar crystals in cast steel and reduces plasticity. If the silicon steel cools faster when heated, the internal and external temperature difference of the steel is large due to the low thermal conductivity, and thus it is broken.
Silicon can reduce the weldability of steel. Because silicon in the ability to combine with oxygen is stronger than iron, it is easy to form low-melting silicate during welding, which increases the fluidity of slag and molten metal, causing splashing and affecting the quality of welding. Silicon is a good deoxidizer. Adding a certain amount of silicon as appropriate when deoxidizing with aluminum can significantly improve the deoxidation rate. Silicon has a certain residual in steel, which is brought into the raw material during ironmaking steelmaking. In boiling steel, silicon is limited to <0.07%. When intentionally added, a ferrosilicon alloy is added during steel making.
(4)Nitrogen(N)
Part of the nitrogen energy is used in iron, which has the effect of solid solution strengthening and hardenability improvement, but it is not significant. Since the nitride precipitates on the grain boundary, the high temperature strength of the grain boundary can be increased, and the creep strength of the steel is increased. It combines with other elements in steel and has precipitation hardening. The corrosion resistance of steel is not significant. However, after the surface of the steel is nitrided, it not only increases its hardness and wear resistance, but also significantly improves the corrosion resistance. Residual nitrogen in low carbon steel causes ageing brittleness.
(1)Niobium/Columbium(Nb/Cb)
Niobium and Columbium are often symbiotic with Tantalum, and their role in steel is similar. The Niobium and Tantalum are partially dissolved in the solid solution to form a solid solution strengthening effect. When the austenite is dissolved, the hardenability of the steel is remarkably improved. However, in the form of carbides and oxide particles, the grains are refined and the hardenability of the steel is lowered. It can increase the tempering stability of steel and has a secondary hardening effect.A small amount of niobium can increase the strength of the steel without affecting the ductility or toughness of the steel. Due to the effect of refining the grains, the impact toughness of the steel can be improved and the brittle transition temperature can be lowered. When the content is more than 8 times that of carbon, almost all the carbon in the steel can be fixed, so that the steel has good hydrogen resistance. In the austenitic steel, the intergranular corrosion of the steel by the oxidizing medium can be prevented. Due to the fixed carbon and precipitation hardening effect, the high-temperature performance of the heat-strength steel can be improved. Such as creep strength and so on.
Niobium can improve the yield strength and impact toughness in ordinary low alloy steel for construction, and reduce the brittle transition temperature and beneficial weldability. In the carburizing and quenching and tempering alloy structural steel while increasing the hardenability. Improve the toughness and low-temperature properties of steel. It can reduce the air hardenability of low carbon martensitic heat-resistant stainless steel, avoid hardening and temper brittleness, and improve creep strength.
(2)Zirconium(Zr)
Zirconium is a strong carbide forming element and its role in steel is similar to that of Niobium, Tantalum and Vanadium. Adding a small amount of zirconium has the effect of degassing, purifying and refining crystal grains. This is beneficial to the low-temperature performance of steel and improves the punching performance. It is commonly used in the manufacture of ultra-high strength steels and nickel-based superalloys for gas engines and ballistic missile structures.
(3)Silicon(Si)
Silicon is soluble in ferrite and austenite to increase the hardness and strength of steel. Its role is second only to phosphorus, stronger than manganese, nickel, chromium, tungsten, molybdenum, vanadium and other elements. However, when the silicon content exceeds 3%, the plasticity and toughness of the steel are significantly reduced. Silicon can increase the elastic limit, yield strength and yield ratio (σs/σb) of steel, as well as fatigue strength and fatigue ratio (σ-1/σb). This is because silicon or silicon manganese steel can be used as a spring steel.
Silicon can reduce the weldability of steel. Because silicon in the ability to combine with oxygen is stronger than iron, it is easy to form low-melting silicate during welding, which increases the fluidity of slag and molten metal, causing splashing and affecting the quality of welding. Silicon is a good deoxidizer. Adding a certain amount of silicon as appropriate when deoxidizing with aluminum can significantly improve the deoxidation rate. Silicon has a certain residual in steel, which is brought into the raw material during ironmaking steelmaking. In boiling steel, silicon is limited to <0.07%. When intentionally added, a ferrosilicon alloy is added during steel making.
(4)Nitrogen(N)
Part of the nitrogen energy is used in iron, which has the effect of solid solution strengthening and hardenability improvement, but it is not significant. Since the nitride precipitates on the grain boundary, the high temperature strength of the grain boundary can be increased, and the creep strength of the steel is increased. It combines with other elements in steel and has precipitation hardening. The corrosion resistance of steel is not significant. However, after the surface of the steel is nitrided, it not only increases its hardness and wear resistance, but also significantly improves the corrosion resistance. Residual nitrogen in low carbon steel causes ageing brittleness.
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