November 3, 2022
(9) Cobalt
Cobalt is mostly used in special steels and alloys. High speed steel containing cobalt has high high temperature hardness, and molybdenum can be added to martensitic aging steel at the same time to obtain ultra-high hardness and good comprehensive mechanical properties. In addition, cobalt is an important alloying element in hot strength steels and magnetic materials.
Cobalt reduces the hardenability of steel, so adding carbon steel alone will reduce the overall mechanical properties of tempered steel. Cobalt can strengthen ferrite, and when added to carbon steel, it can improve the hardness, yield point and tensile strength of steel in annealed or normalized state, and has an adverse effect on elongation and section shrinkage. Impact toughness decreases with the increase of cobalt content. Cobalt is used in heat resistant steels and alloys because of its oxidation resistance. Cobalt-based alloy gas turbines show its unique role.
(10) Silicon (Si)
Silicon can be dissolved in ferrite and austenite to improve the hardness and strength of steel, its role is second only to phosphorus, and stronger than manganese, nickel, chromium, tungsten, molybdenum, vanadium and other elements. However, when the silicon content exceeds 3%, the ductility and toughness of steel will be significantly reduced. Silicon can improve the elastic limit, yield strength and yield ratio (σs/σb), and fatigue strength and fatigue ratio (σ-1/σb) of steel. This is the reason why silicon or silicon manganese steel can be used as spring steel.
Silicon can reduce the density, thermal conductivity and electrical conductivity of steel. Can promote ferrite grain coarsening, reduce coercivity. It has the tendency to reduce the anisotropy of the crystal, so that the magnetization is easy, the reluctance is reduced, and it can be used to produce electrical steel, so the magnetic block loss of the silicon steel sheet is low. Silicon can improve the magnetic conductivity of ferrite, so that the steel sheet has a higher magnetic sensitivity under a weak magnetic field. However, silicon reduces the magnetic sensitivity of steel under strong magnetic field. Silicon has a strong deoxidation, which reduces the magnetic aging effect of iron.
When the steel containing silicon is heated in the oxidizing atmosphere, a layer of SiO2 film will be formed on the surface, thus improving the oxidation resistance of the steel at high temperature.
Silicon can promote columnar crystal growth and reduce plasticity in cast steel. If the silicon steel cools faster when heated, the temperature difference between the inside and outside of the steel is large because of the low thermal conductivity, and thus the fracture.
Silicon can reduce the weldability of steel. Because silicon is stronger than iron in combining with oxygen, it is easy to generate low melting point silicate in welding, which increases the fluidity of molten slag and melted metal, causing spatter phenomenon and affecting welding quality. Silicon is a good deoxidizer. When using aluminum deoxidation, a certain amount of silicon is added, which can significantly improve the deoxidation rate. There is a certain amount of silicon in steel, which is introduced as a raw material in the process of making iron and steel. In boiling steel, silicon is limited to < 0.07%, when it is intended to be added, ferrosilicon alloy is added in steelmaking.
(11) Manganese (Mn)
Manganese is a good deoxidizer and desulfurizer. Steel generally contains a certain amount of manganese, which can eliminate or weaken the hot brittleness of steel caused by sulfur, so as to improve the hot working performance of steel.
The solid solution formed by manganese and iron improves the hardness and strength of ferrite and austenite in steel. At the same time, it is an element formed by carbides and enters the cementite to replace some iron atoms. Manganese plays a role in refining pearlite and indirectly improving the strength of pearlite steel by reducing the critical transition temperature in steel. Manganese is second only to nickel in its ability to stabilize austenitic structures and also strongly increases the hardenability of steel. A variety of alloy steels have been made with manganese containing less than 2% and other elements.
Manganese has the characteristics of rich resources and diverse efficiency, and has been widely used, such as carbon structural steel with high manganese content, spring steel.
In high-carbon and high-manganese wear-resisting steel, the manganese content can reach 10% ~ 14%. After solid solution treatment, it has good toughness. When it is deformed by impact, the surface layer will be strengthened due to deformation and has high wear resistance.
Manganese and sulfur form MnS with higher melting point, which can prevent the hot brittle phenomenon caused by FeS. Manganese has the tendency to increase grain coarsening and temper brittleness sensitivity of steel. If the cooling after smelting and forging is not proper, it is easy to produce white spots.
(12) Aluminum (Al)
Aluminum is mainly used to deoxidize and refine grains. Promote the formation of a hard corrosion resistant nitriding layer in nitriding steel. Aluminum can inhibit the aging of low carbon steel and improve the toughness of steel at low temperature. When the content is high, the oxidation resistance and corrosion resistance in oxidizing acid and H2S gas of steel can be improved, and the electrical and magnetic properties of steel can be improved. Aluminum plays a great role in solution strengthening in steel, improving the wear resistance, fatigue strength and core mechanical properties of carburized steel.
In refractory alloys, aluminum and nickel form compounds to improve the strength of smelting. Fe-cr aluminum alloy containing aluminum has the characteristics of near constant resistance and excellent oxidation resistance at high temperature, which is suitable to be used as electrometallurgical alloy materials and chromium aluminum resistance wire.
When some steels are deoxidized, if the amount of aluminum is too much, it will produce abnormal microstructure and promote the graphitization tendency of the steel. In ferritic and pearlite steels, high aluminum content will reduce the high temperature strength and toughness, and bring some difficulties to smelting, pouring and other aspects.
(13) Copper (Cu)
The outstanding role of copper in steel is to improve the atmospheric corrosion resistance of ordinary low-alloy steel, especially when used with phosphorus, the addition of copper can also improve the strength and yield ratio of steel, but has no adverse effect on the welding performance. The corrosion resistance life of the rail steel (U-Cu) containing 0.20% ~ 0.50% copper is 2-5 times that of the ordinary carbon rail in addition to wear resistance.
When the copper content is more than 0.75%, it can produce aging strengthening effect after solution treatment and aging. At low content, its effect is similar to nickel, but weaker. When the content is higher, it is unfavorable to the hot deformation processing and leads to the copper brittleness during the hot deformation processing. The 2%-3% copper in austenitic stainless steel can resist the corrosion of sulfuric acid, phosphoric acid and hydrochloric acid and the stability of stress corrosion.
(14) Boron (B)
The main function of boron in steel is to increase the hardenability of steel, thus saving other rarer metals, and nickel, chromium, molybdenum, etc. For this purpose, its content is generally specified in the range of 0.001% to 0.005%. It can replace 1.6% nickel, 0.3% chromium or 0.2% molybdenum. It should be noted that molybdenum can prevent or reduce tempering brittleness, while boron slightly promotes the tendency of tempering brittleness, so it can not be completely replaced by boron.
Medium carbon carbon steel with boron, due to the improvement of hardenability, can make the thickness of more than 20mm steel after tempering performance is greatly improved, therefore, 40B and 40MnB steel can be used instead of 40Cr, 20Mn2TiB steel can be used instead of 20CrMnTi carburized steel. But because the role of boron with the increase of the content of carbon in steel and weaken, or even disappear, in the selection of boronized carbon steel, must take into account the parts after carburizing, carburizing layer hardenability will be lower than the core of the hardenability of this feature.
Spring steel is generally required to be fully quenched, usually the spring area is not large, the use of boron-containing steel is advantageous. The effect of boron on high silicon spring steel fluctuates greatly, so it is inconvenient to use.
Boron has a strong affinity with nitrogen and oxygen. Adding 0.007% boron to the boiling steel can eliminate the aging phenomenon of the steel.
(15) Rare Earth (Re)
Generally speaking, rare earth elements refer to the periodic table of elements with atomic numbers from 57 to 71 (15 lanthanides) plus 21 scandium and 39 yttrium, a total of 17 elements. They are close in nature and cannot easily be separated. The unseparated ones, called mixed rare earths, are cheaper and can improve the plasticity and impact toughness of forged rolled steel, especially in cast steel. It can improve the creep resistance of heat-resistant steel electrothermal alloys and superalloys.
Rare earth elements can also improve the oxidation resistance and corrosion resistance of steel. The antioxidant effect is more than silicon, aluminum, titanium and other elements. It can improve the fluidity of steel, reduce non-metallic inclusions, and make the steel structure compact and pure.
Ordinary low-alloy steel with proper rare earth elements has good deoxidization and sulfur removal effect, improves impact toughness (especially low temperature toughness), and improves anisotropy properties.
Rare earth elements in Fe-Cr aluminum alloy increase the antioxidant capacity of the alloy, maintain the fine grain of steel at high temperature, improve the high temperature strength, so that the life of electric heating alloy is significantly increased.
(16) Nitrogen (N)
Nitrogen energy is partly used in iron, which has the effect of solid solution strengthening and improving hardenability, but it is not significant. Due to the precipitation of nitrides on grain boundaries, the high temperature strength of grain boundaries can be increased and the creep strength of steel can be increased. Combined with other elements in steel, precipitation hardening effect. The corrosion resistance of steel is not significant, but the surface nitriding of steel not only increases the hardness and wear resistance, but also significantly improves the corrosion resistance. Residual nitrogen in low carbon steel can lead to aging brittleness.
(17) Sulfur (S)
The machinability of steel can be improved by increasing the content of sulfur and manganese. In the easily machinable steel, sulfur is added as a beneficial element. Sulfur is highly segregated in steel. Deterioration of steel quality, at high temperatures, reduces the plasticity of steel, is a harmful element, which exists in the form of FeS with lower melting point. FeS alone has a melting point of only 1190℃, while the eutectic temperature of the eutectic crystal formed with iron in steel is even lower, only 988℃. When the steel solidifies, the iron sulfide coalesces at the primary grain boundary. When the steel is rolled at 1100 ~ 1200℃, the FeS on the grain boundary will melt, which greatly weakens the binding force between the grains and leads to the hot brittle phenomenon of the steel, so the sulfur should be strictly controlled. Generally, it is controlled between 0.020% and 0.050%. To prevent brittleness caused by sulfur, enough manganese should be added to form MnS with a higher melting point. If the steel contains a high flow rate, welding due to the generation of SO2, will form pores and loose in the welding metal.
(18) Phosphorus (P)
Phosphorus has strong effect on solid solution strengthening and cold hardening in steel. The addition of low alloy structural steel as alloying element can improve its strength and atmospheric corrosion resistance, but reduce its cold stamping performance. Phosphorus combined with sulfur and manganese, can increase the cutting performance of steel, increase the surface quality of the workpiece, for easy cutting steel, so easy cutting steel is also relatively high phosphorus content. Phosphorus used in ferrite, although it can improve the strength and hardness of steel, the biggest harm is that the segregation is serious, increase the tempered brittleness, significantly increase the plasticity and toughness of steel, resulting in steel in cold processing is easy to crack, namely the so-called "cold brittleness" phenomenon. Phosphorus also has an adverse effect on weldability. Phosphorus is a harmful element, should be strictly controlled, the general content is not more than 0.03% ~ 0.04%.
