To improve some properties of steel and make it obtain some special properties, the elements that are intentionally added in the smelting process are called alloying elements. Commonly used alloying elements are chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, zirconium, cobalt, silicon, manganese, aluminum, copper, boron, and rare earth. Phosphorus, sulfur, nitrogen, etc. also act as alloys in some cases.
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(1) Cr
Chromium can increase the hardenability of steel and has the effect of secondary hardening, which can improve the hardness and wear resistance of carbon steel without making the steel brittle. When the content exceeds 12%, the steel has good high-temperature oxidation resistance and oxidation corrosion resistance and also increases the thermal strength of the steel. Chromium is the main alloying element of stainless acid-resistant steel and heat-resistant steel.
Chromium can improve the strength and hardness of carbon steel in the rolled state, and reduce the elongation and reduction of area. When the chromium content exceeds 15%, the strength and hardness will decrease, and the elongation and reduction of the area will increase accordingly. Chromium-containing steel parts are easy to obtain high surface finish quality by grinding.
The main function of chromium in the quenched and tempered structure is to improve the hardenability so that the steel has better comprehensive mechanical properties after quenching and tempering. In the carburized steel, chromium-containing carbides can also be formed, thereby improving the surface resistance of the material. Abrasiveness. Chromium-containing spring steel is not easily decarburized during heat treatment. Chromium can improve the wear resistance, hardness, and red hardness of tool steel, and has good tempering stability. In electrothermal alloys, chromium can improve the oxidation resistance, resistance, and strength of the alloy.
(2) Ni
Nickel strengthens ferrite and refines pearlite in steel. The overall effect is to increase the strength, and the effect on plasticity is not significant. Generally speaking, for low carbon steel used in a rolled, normalized, or annealed state without quenching and tempering treatment, a certain nickel content can increase the strength of the steel without significantly reducing its toughness. According to statistics, every 1% increase in nickel can increase the strength by about 29.4Pa. With the increase of nickel content, the yield degree of steel increases faster than the tensile strength, so the ratio of nickel-containing steel can be higher than that of ordinary carbon steel.
While improving the strength of steel, nickel has less damage to the toughness, plasticity, and other process properties of steel than other alloying elements. For medium carbon steel, because nickel reduces the pearlite transformation temperature, the pearlite becomes thinner; and because nickel reduces the carbon content of the eutectoid point, the number of pearlite is larger than that of carbon steel with the same carbon content. The strength of nickel-containing pearlitic ferritic steel is higher than that of carbon steel with the same carbon content. On the contrary, if the strength of the steel is the same, the carbon content of the nickel-containing steel can be appropriately reduced, so that the toughness and plasticity of the steel can be improved.
Nickel can increase the resistance of steel to fatigue and reduce the sensitivity of steel to notch. Nickel reduces the low-temperature brittle transition temperature of steel, which is of great significance for low-temperature steel. Steel with 3.5% nickel can be used at -100℃, and steel with 9% nickel can work at -196℃. Nickel does not increase the resistance of steel to creep, so it is generally not used as a strengthening element for thermally strong steel.
The coefficient of linear expansion of iron-nickel alloys with high nickel content changes significantly with the increase or decrease of nickel content. Using this characteristic, precision alloys and bimetallic materials with extremely low or certain linear expansion coefficients can be designed and produced.
In addition, nickel added to steel can not only resist acid, but also resist alkali, and has corrosion resistance to atmosphere and salt. Nickel is one of the important elements in stainless acid-resistant steel.
(3) Mo
Molybdenum can improve hardenability and thermal strength in steel, prevent temper brittleness, and increase remanence and coercivity, and corrosion resistance in certain media.
In quenched and tempered steel, molybdenum can harden and harden parts with larger sections, improve the tempering resistance or tempering stability of the steel, and enable the parts to be tempered at a higher temperature, thereby more effectively eliminating ( or reducing) residual stress and increase plasticity.
In addition to the above functions, molybdenum in carburized steel can also reduce the tendency of carbides to form a continuous network on the grain boundary in the carburized layer, reducing the residual austenite in the carburized layer, and relatively increase the surface layer. wear resistance.
In forging die steel, molybdenum can also maintain a relatively stable hardness of the steel and increase the resistance to deformation. Resistance to cracking and abrasion, etc.
In stainless acid-resistant steel, molybdenum can further improve the corrosion resistance to organic acids (such as formic acid, acetic acid, oxalic acid, etc.) and hydrogen peroxide, sulfuric acid, sulfurous acid, sulfate, acid dye, bleaching powder, etc. Especially due to the addition of molybdenum, the tendency to pitting corrosion caused by the presence of chloride ions is prevented.
W12Cr4V4Mo high-speed steel containing about 1% molybdenum has wear resistance, tempering hardness, and red hardness.
(4) W
In addition to forming carbides in steel, tungsten partially dissolves into iron to form a solid solution. Its effect is similar to that of molybdenum. Calculated by a mass fraction, the general effect is not as significant as that of molybdenum. The main pattern of tungsten in steel is to increase tempering stability, red hardness, thermal strength, and increased wear resistance due to the formation of carbides. Therefore, it is mainly used for tool steel, such as high-speed steel, steel for hot forging dies, etc.
Tungsten forms refractory carbides in high-quality spring steel. When tempered at a higher temperature, it can ease the aggregation process of carbides and maintain high-temperature strength. Tungsten can also reduce the thermal sensitivity of steel, increase hardenability and increase hardness. 65SiMnWA spring steel has high hardness after air cooling after hot rolling. Spring steel with a cross-section of 50mm2 can be hardened in oil and can be used as an important spring that can withstand large loads, heat resistance (not more than 350 ℃), and shock. 30W4Cr2VA high-strength, heat-resistant, and high-quality spring steel with great hardenability, quenched at 1050-1100℃, and tempered at 550-650℃, the tensile strength reaches 1470-1666MPa. It is mainly used to manufacture springs used at high temperatures (not more than 500℃).
Since the addition of tungsten can significantly improve the wear resistance and machinability of steel, tungsten is the main element of alloy tool steel.
(5) V
Vanadium has a strong affinity with carbon, ammonia, and oxygen, and forms corresponding stable compounds with it. Vanadium exists mainly in the form of carbides in steel. Its main function is to refine the structure and grain of the steel and reduce its strength and toughness of the steel. When it is dissolved into a solid solution at a high temperature, it increases the hardenability; on the contrary, when it exists in the form of carbide, it reduces the hardenability. Vanadium increases the tempering stability of hardened steel and produces a secondary hardening effect. The vanadium content in steel, except for high-speed tool steel, is generally not more than 0.5%.
Vanadium can refine grains in ordinary low carbon alloy steel, improve the strength and yield ratio after normalizing and low-temperature characteristics, and improve the welding performance of steel.
Vanadium in alloy structural steel is often used in combination with elements such as manganese, chromium, molybdenum, and tungsten in structural steel because it will reduce the hardenability under general heat treatment conditions. Vanadium is mainly used in quenched and tempered steel to improve the strength and yield ratio of steel, refined grains, and pick up overheating sensitivity. In the case of carburizing steel, the grain can be refined, so that the steel can be directly quenched after carburizing without secondary quenching.
In spring steel and bearing steel, vanadium can improve the strength and yield ratio, especially the proportional limit and elastic limit, and reduce the decarburization sensitivity during heat treatment, thereby improving the surface quality. The bearing steel containing pentachrome and vanadium has high carbonization dispersion and good performance.
Vanadium refines grains in tool steels, reduces overheating sensitivity, and increases tempering stability and wear resistance, thereby extending tool life.
(6) Ti
Titanium has a strong affinity with nitrogen, oxygen, and carbon, and has a stronger affinity with sulfur than iron. Therefore, it is a good deoxidizer and an effective element for fixing nitrogen and carbon. Although titanium is a strong carbide-forming element, it does not combine with other elements to form complex compounds. Titanium carbide has strong binding force, and stability, and is not easy to decompose. Only when it is heated to above 1000 °C in steel can it slowly dissolve into a solid solution. Before being dissolved, the titanium carbide particles have the effect of preventing grain growth. Since the affinity between titanium and carbon is much greater than that between chromium and carbon, titanium is often used to fix carbon in stainless steel to eliminate the depletion of chromium at the grain boundary, thereby eliminating or reducing intergranular corrosion of steel.
Titanium is also one of the strong ferrite-forming elements, which strongly increases the A1 and A3 temperatures of the steel. Titanium can improve plasticity and toughness in ordinary low alloy steel. The strength of the steel is increased as titanium fixes nitrogen and sulfur and forms titanium carbide. After normalizing, the grains are refined, and the precipitation to form carbides can significantly improve the plasticity and impact toughness of the steel. The alloy structural steel containing titanium has good mechanical properties and process properties, but the main disadvantage is that the hardenability is slightly poor.
Titanium with a content of about 5 times carbon is usually added to high chromium stainless steel, which can not only improve the corrosion resistance (mainly anti-intergranular corrosion) and toughness of the steel but also organize the grain growth tendency of the steel at high temperature and improve Weldability of steel.
(7) Nb/Cb
Niobium and columbium often coexist with tantalum, and their role in steel is similar. Niobium and tantalum partially dissolve into solid solution and play a role in solid solution strengthening. When dissolved in austenite, the hardenability of steel is significantly improved. However, in the form of carbides and oxide particles, it refines the grains and reduces the hardenability of the steel. It can increase the tempering stability of steel and has a secondary hardening effect. Trace amounts of niobium can increase the strength of steel without affecting its ductility or toughness. Due to the effect of grain refinement, it can improve the impact toughness of steel and reduce its brittle transition temperature. 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 austenitic steels, it can prevent intergranular corrosion of steel by oxidizing media. Due to fixed carbon and precipitation hardening, it can improve the high-temperature properties of thermal strength steel, such as creep strength.
Niobium can improve the yield strength and impact toughness of ordinary low-alloy steel for construction, and reduce the brittle transition temperature, which is beneficial to welding performance. In carburizing and quenched and tempered alloy structural steel while increasing the hardenability. Improve toughness and low-temperature properties of steel. It can reduce the air hardenability of low carbon martensitic heat-resistant stainless steel, avoid hardening and tempering brittleness, and improve creep strength.
(8) Zr
Zirconium is a strong carbide former, 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 grains, which is beneficial to the low-temperature performance of steel and improves stamping performance.
(9) Co
Cobalt is mostly used in special steels and alloys. Cobalt-containing high-speed steel has high high-temperature hardness. Adding molybdenum to maraging steel at the same time can obtain ultra-high hardness and good comprehensive mechanical properties. In addition, cobalt is also an important alloying element in thermally strong steels and magnetic materials.
Cobalt reduces the hardenability of steel, so adding it to carbon steel alone will reduce the comprehensive mechanical properties after quenching and tempering. Cobalt can strengthen ferrite, and when added to carbon steel, it can improve the hardness, yield point, and tensile strength of steel in the annealed or normalized state. decreased with increasing cobalt content. Due to its anti-oxidation properties, cobalt is used in heat-resistant steels and heat-resistant alloys. It shows its unique role in cobalt-based alloy gas turbines.
(10) Si
Silicon can dissolve 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 plasticity and toughness of the 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 because silicon or silicon-manganese steel can be used as spring steel.
Silicon reduces the density, thermal conductivity, and electrical conductivity of steel. It can promote the coarsening of ferrite grains and reduce coercivity. There is a tendency to reduce the anisotropy of the crystal, making the magnetization easy and reducing the magnetoresistance, which can be used to produce electrical steel, so the magnetoresistance loss of the silicon steel sheet is low. Silicon can improve the magnetic permeability of ferrite so that the steel sheet has a higher magnetic induction in a weaker magnetic field. But silicon reduces the magnetic induction of steel under strong magnetic fields. Silicon has strong deoxidizing power, thereby reducing the magnetic aging effect of iron.
When the silicon-containing steel is heated in an oxidizing atmosphere, a layer of SiO2 film will be formed on the surface, thereby improving the oxidation resistance of the steel at high temperatures. Silicon can promote the growth of columnar crystals in cast steel and reduce plasticity. If the silicon steel cools quickly when heated, due to the low thermal conductivity, the temperature difference between the inside and outside of the steel is large, so it will break.
Silicon can reduce the weldability of steel. Because silicon has a stronger binding ability with oxygen than iron, it is easy to generate low-melting silicate during welding, which increases the fluidity of slag and molten metal, causes splashing, and affects welding quality. Silicon is a good deoxidizer. When deoxidizing with aluminum, adding a certain amount of silicon as appropriate can significantly improve the rate of deoxidation. There is a certain amount of residual silicon in steel, which is brought in as a raw material during iron and steel making. In boiling steel, silicon is limited to <0.07%, and when intentionally added, ferrosilicon is added during steelmaking.
(11) Mn
Manganese is a good deoxidizer and desulfurized. Steel generally contains a certain amount of manganese, which can eliminate or weaken the hot brittleness of steel caused by sulfur, thereby improving the hot workability of steel.
The solid solution formed by manganese and iron increases the hardness and strength of ferrite and austenite in the steel; at the same time, it is an element formed by carbides, and it enters the cementite to replace part of the iron atoms. Manganese reduces the critical transformation temperature in steel. It plays the role of refining pearlite and indirectly improves the strength of pearlite steel. Manganese is second only to nickel in its ability to stabilize austenite and also strongly increases the hardenability of steel. A variety of alloy steels have been made of manganese with a content of not more than 2% and other elements.
Manganese has the characteristics of abundant resources and various performances and has been widely used, such as carbon structural steel and spring steel with high manganese content.
In high-carbon and high-manganese wear-resistant steel, the manganese content can reach 10% to 14%, and it has good toughness after solution treatment. When it is deformed by impact, the surface layer will be strengthened due to deformation, and it has high resistance to Abrasiveness.
Manganese and sulfur form MnS with a higher melting point, which can prevent hot embrittlement caused by FeS. Manganese tends to increase steel grain coarsening and sensitivity to temper brittleness. Improper cooling after smelting, casting, and forging will easily cause white spots to occur in the steel.
(12) Al
Aluminum is mainly used for deoxidation and grain refinement. In nitrided steel, it promotes the formation of a hard, corrosion-resistant nitrided layer. Aluminum can inhibit the aging of low carbon steel and improve the toughness of steel at low temperatures. When the content is high, the oxidation resistance of steel and the corrosion resistance in oxidizing acid and H2S gas can be improved, and the electrical and magnetic properties of steel can be improved. Aluminum has a great solid solution strengthening effect in steel, which improves the wear resistance, fatigue strength, and core mechanical properties of carburized steel.
In refractory alloys, aluminum and nickel form compounds, thereby improving the metallurgical strength. Aluminum-containing iron-chromium-aluminum alloys have near-constant resistance characteristics and excellent oxidation resistance at high temperatures and are suitable for electro-metallurgical alloy materials and chromium-aluminum alloys. Resistance wire.
When some steel is deoxidized, if the amount of aluminum is too much, the steel will produce abnormal microstructure and tend to promote the graphitization of the steel. In ferritic and pearlitic steels, when the aluminum content is high, its high-temperature strength and toughness will be reduced, and it will bring some difficulties to smelting and casting.
(13) Cu
The prominent role of copper in steel is to improve the atmospheric corrosion resistance of ordinary low-alloy steel, especially when used in combination with phosphorus, adding copper can also improve the strength and yield ratio of steel without adversely affecting the welding performance. Rail steel (U-Cu) contains 0.20% to 0.50% copper, in addition, to wear resistance, its corrosion resistance life is 2 to 5 times that of ordinary carbon steel rails.
When the copper content exceeds 0.75%, the aging strengthening effect can be produced after solution treatment and aging. When the content is low, its effect is similar to that of nickel, but it is weaker. When the content is high, it is unfavorable for hot deformation processing, which leads to copper embrittlement during hot deformation processing. 2% to 3% copper in austenitic stainless steel can have corrosion resistance to sulfuric acid, phosphoric acid, and hydrochloric acid and stability to stress corrosion.
(14) B
The main function of boron in steel is to increase the hardenability of steel, thereby saving other rarer metals, such as 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 be replaced by boron because molybdenum can prevent or reduce temper brittleness, while boron has a slight tendency to promote temper brittleness, so it cannot be used. Boron completely replaces molybdenum.
Adding boron to medium carbon steel can greatly improve the properties of steel with a thickness of more than 20mm after quenching and tempering due to the improved hardenability. Therefore, 40B and 40MnB steel can be used instead of 40Cr, and 20Mn2TiB steel can be used instead of 20CrMnTi carburized steel. However, since the effect of boron weakens or even disappears with the increase of carbon content in the steel when selecting boron-containing carburized steel, it must be considered that after the parts are carburized, the hardenability of the carburized layer will be lower than that of the core. This feature of permeability.
Spring steel generally requires complete hardening, and usually the spring area is not large, so it is advantageous to use boron-containing steel. The effect of boron on high silicon spring steel fluctuates greatly, which is inconvenient to use.
Boron has a strong affinity with nitrogen and oxygen. Adding 0.007% boron to boiling steel can eliminate the aging phenomenon of steel.
(15) RE
Generally speaking, rare earth elements refer to the lanthanide elements (15) with atomic numbers from 57 to 71 in the periodic table, plus 21 scandium and 39 yttrium, a total of 17 elements. They are close and cannot be easily separated. Unseparated mixed rare earth elements are relatively cheap, and rare earth elements can improve the plasticity and impact toughness of forged 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 and corrosion resistance of steel. The effect of oxidation resistance exceeds that of elements such as silicon, aluminum, and titanium. It can improve the fluidity of steel, reduce non-metallic inclusions, and make the structure of steel dense and pure.
Adding appropriate rare earth elements to ordinary low alloy steel has a good deoxidation and desulfurization effect, improves impact toughness (especially low-temperature toughness), and improves anisotropic properties. Rare earth elements increase the oxidation resistance of the alloy in Fe-Cr-Al alloys, maintain the fine grains of the steel at high temperatures, and improve the high-temperature strength, thus significantly improving the life of the electrothermal alloy.
(16) N
Nitrogen can be partially used in iron, and it has the effect of solid solution strengthening and hardenability improvement, but it is not significant. Due to the precipitation of nitrides on the grain boundaries, the high-temperature strength of the grain boundaries can be improved, and the creep strength of the steel can be increased. Combined with other elements in steel, it has a precipitation hardening effect. The corrosion resistance of steel is not significant, but after nitriding the surface of the steel, it not only increases its hardness and wear resistance but also significantly improves corrosion resistance. Residual nitrogen in mild steel can cause age brittleness.
(17) S
increases the content of sulfur and manganese, which can improve the machinability of steel. In free-cutting steel, sulfur is added as a beneficial element. Sulfur segregates seriously in steel. Deteriorating the quality of the steel, at high temperatures, reducing the plasticity of the steel, is a harmful element that exists in the form of FeS with a lower melting point. The melting point of FeS alone is only 1190 °C, while the eutectic temperature that forms eutectic with iron in steel is even lower, only 988 °C. When the steel solidifies, iron sulfide gathers at the primary grain boundary. When the steel is rolled at 1100-1200 °C, FeS on the grain boundary will melt, which greatly weakens the bonding force between grains, resulting in hot embrittlement of the steel, so sulfur should be strictly controlled. Generally controlled at 0.020% to 0.050%. To prevent brittleness due to sulfur, enough manganese should be added to form MnS with a higher melting point. If the flow rate in the steel is too high, pores and porosity will be formed in the welded metal due to the generation of SO2 during welding.
(18)P
Phosphorus has strong solid solution strengthening and cold work hardening effects in steel. Adding it as an alloying element to low-alloy structural steel can improve its strength and atmospheric corrosion resistance of steel, but reduce its cold stamping performance. The combined use of phosphorus, sulfur, and manganese can increase the cutting performance of steel and increase the surface quality of the workpiece. It is used for free-cutting steel, so free-cutting steel contains relatively high phosphorus. Phosphorus is used in ferrite. Although it can improve the strength and hardness of steel, the biggest harm is that the segregation is serious, which increases temper brittleness, significantly increases the plasticity and toughness of steel, and causes steel to be easily brittle during cold working. brittle” phenomenon. Phosphorus also adversely affects weldability. Phosphorus is a harmful element and should be strictly controlled, and the general content is not more than 0.03% to 0.04%.
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