In order to improve and enhance some properties of steel and make it obtain some special properties and the elements intentionally added in the smelting process are called alloying elements. Common alloying elements are chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, zirconium, cobalt, silicon, manganese, aluminum, copper, boron, rare earth and so on. Phosphorus, sulfur, nitrogen, etc., also act as alloys in some cases.
1
Cr
24
Chromium can increase the hardenability of steel and has a secondary hardening effect, 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 steel acid-resistant steel and heat-resistant steel.
Chromium can improve the strength and hardness of carbon steel in rolling state, and reduce the elongation and shrinkage of section. When the chromium content exceeds 15%, the strength and hardness will decrease, and the elongation and section shrinkage will increase accordingly. The parts containing chromium steel are easy to obtain higher surface machining quality by grinding.
The main role of chromium in the tempered structure is to improve the hardenability, so that the steel after quenching and tempering has better comprehensive mechanical properties, in the carburized steel can also form chromium carbide, so as to improve the wear resistance of the material surface.
Spring steels containing chromium are not easy to decarburize 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
28
Nickel strengthens ferrite and refines pearlite in steel, the overall effect is to increase strength, and the effect on plasticity is not significant. Generally speaking, for low carbon steel used in rolled, normalized or annealed state without tempering treatment, a certain nickel content can improve the strength of the steel without significantly reducing its toughness. According to statistics, every 1% increase in nickel can increase the strength of 29.4Pa. With the increase of nickel content, the yield of steel is 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 impact on the toughness, plasticity and other process properties of steel than other alloying elements. For medium carbon steel, the pearlite becomes thinner because nickel reduces the pearlite transition temperature. And because nickel reduces the carbon content of the eutectoid point, the pearlite quantity is more than that of carbon steel with the same carbon content, so that the strength of pearlite ferritic steel with nickel 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 improve 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 very important for low temperature steel. Steel with 3.5% nickel can be used at -100 ° C, and steel with 9% nickel can work at -196 ° C. Nickel does not increase the resistance of steel to creep, so it is generally not used as a strengthening element of heat-strength steel.
The linear expansion coefficient of Fe-Ni alloy with high nickel content changes significantly with the increase or decrease of nickel content. Using this property, precision alloys and bimetallic materials with very low or certain linear expansion coefficient can be designed and produced.
In addition, nickel added to steel is not only acid-resistant, but also alkali resistant, corrosion resistant to atmosphere and salt, nickel is one of the important elements in stainless acid-resistant steel.
3
Mo
42
Molybdenum in steel can improve hardenability and thermal strength, prevent temper brittleness, increase remanence and coercivity and corrosion resistance in some media.
In tempered steel, molybdenum can make the larger section of the parts quenched deep, quenched through, improve the tempering resistance or tempering stability of the steel, so that the parts can be tempered at higher temperatures, so as to more effectively eliminate (or reduce) residual stress, improve plasticity.
In addition to the above functions in carburized steel, molybdenum can also reduce the tendency of carbides to form a continuous network on the grain boundary in the carburizing layer, reduce the residual austenite in the carburizing layer, and relatively increase the wear resistance of the surface layer.
In the forging die, molybdenum can also maintain the steel has a relatively stable hardness, increase the deformation. Resistance to cracking and wear.
In stainless acid-resistant steel, molybdenum can further improve the corrosion resistance of organic acids (such as formic acid, acetic acid, oxalic acid, etc.) and hydrogen peroxide, sulfuric acid, sulfite, sulfate, acid dyes, bleaching powder, etc. In particular, due to the addition of molybdenum, the point corrosion tendency 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
74
In addition to forming carbides in steel, tungsten is partially dissolved into iron to form a solid solution. Its effect is similar to molybdenum, according to the mass fraction calculation, the general effect is not as significant as molybdenum. The main sample 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, hot forging die steel and so on.
Tungsten forms refractory carbides in high-quality spring steel, which can ease the accumulation process of carbides and maintain high high temperature strength when tempered at higher temperatures. Tungsten can also reduce the overheating sensitivity of steel, increase hardenability and increase hardness. The 65SiMnWA spring steel has a high hardness after air cooling after hot rolling, and the spring steel with a 50mm2 cross-section can be quenched in oil, which can be used as an important spring to withstand large loads, heat resistance (not more than 350 ° C) and impact. 30W4Cr2VA high strength heat resistant spring steel, with large hardenability, 1050 ~ 1100℃ quenching, 550 ~ 650℃ tempering tensile strength of 1470 ~ 1666Pa. It is mainly used to manufacture springs used at high temperatures (not more than 500 ° C).
Due to the addition of tungsten, it can significantly improve the wear resistance and machinability of steel, so tungsten is the main element of alloy tool steel.
5
V
23
Vanadium has a strong affinity with carbon, ammonia and oxygen, and forms corresponding stable compounds with them. Vanadium exists mainly in the form of carbide in steel. Its main function is to refine the structure and grain of steel, improve the strength and toughness of steel. When dissolved into the solid solution at high temperature, increase hardenability; On the contrary, if it exists in the form of carbide, the hardenability is reduced. 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 in ordinary low carbon alloy steel can refine the grain, improve the strength and yield ratio after normalizing and low temperature characteristics, improve the welding performance of steel.
Vanadium in alloy structural steel due to the general heat treatment conditions will reduce the hardenability, so it is often used in combination with manganese, chromium, molybdenum and tungsten in structural steel. Vanadium in tempered steel is mainly used to improve the strength and yield ratio of steel, refine the grain, and pick up the overheating sensitivity. In the carburized 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 increase the proportional limit and elastic limit, reduce the decarbonization sensitivity during heat treatment, and thus improve the surface quality. Bearing steel containing vanadium in five chromium has high carbonization dispersion and good performance.
Vanadium in tool steel refines grains, reduces overheating sensitivity, increases tempering stability and wear resistance, thereby extending the service life of tools.
6
Ti
81
Titanium and nitrogen, oxygen, carbon have a strong affinity, and the affinity with sulfur is stronger than iron. Therefore, it is a good deoxidizing degassing agent 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 binding force is strong, stable, not easy to decompose, in steel only heated to more than 1000 ° C to slowly dissolve into the solid solution. Before melting, titanium carbide particles have the effect of preventing grain growth. Since the affinity between titanium and carbon is much greater than the affinity between chromium and carbon, titanium is commonly used in stainless steel to fix the carbon in it to eliminate the depletion of chromium at the grain boundary, thereby eliminating or reducing the intergranular corrosion of steel.
Titanium is also one of the strong ferrite forming elements, which strongly increases the A1 and A3 temperatures of steel. Titanium can improve plasticity and toughness in ordinary low alloy steel. Since titanium fixes nitrogen and sulfur and forms titanium carbide, the strength of the steel is increased. By normalizing the grain refinement, precipiting carbide can make the plasticity and impact toughness of steel can be significantly improved, titanium alloy structural steel, has good mechanical properties and process properties, the main disadvantage is that the hardenability is slightly poor.
It is usually necessary to add about 5 times the carbon content of titanium in high chromium stainless steel, which can not only improve the corrosion resistance (mainly resistance to intergranular corrosion) and toughness of the steel; It can also organize the grain growth tendency of steel at high temperature and improve the welding performance of steel.
7
Nb/Cb
41
Niobium and columbium often coexist with tantalum, and their roles in steel are similar. Niobium and tantalum are partially dissolved into the solid solution to strengthen the solid solution. When dissolved into austenite, the hardenability of the steel is significantly improved. However, in the form of carbide and oxide particles, the grain is refined and the hardenability of the steel is reduced. It can increase the tempering stability of steel and has a secondary hardening effect. A small amount of niobium can improve the strength of steel without affecting its plasticity or toughness. Because of the effect of refining grain, the impact toughness of steel can be improved and the brittle transition temperature can be reduced. 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 steel, the intergranular corrosion of steel by oxidizing medium can be prevented. Due to fixed carbon and precipitation hardening, it can improve the high temperature properties of hot strength steel, such as creep strength.
Niobium can improve the yield strength and impact toughness and reduce the brittle transition temperature in the common low alloy steel used in construction. In the carburized and tempered alloy structural steel, the hardenability is increased at the same time. Improve the toughness and low temperature performance of steel. It can reduce the air hardening of low carbon martensitic heat-resistant stainless steel, avoid hardening tempering brittleness, and improve creep strength.
8
Zr
40
Zirconium is a strong carbide forming element, and its role in steel is similar to niobium, tantalum and vanadium. Adding a small amount of zirconium has the effect of degassing, purifying and refining grains, which is conducive to the low temperature performance of steel and improving the stamping performance, and it is often used in the manufacture of ultra-high strength steel and nickel-based superalloys used in gas engines and ballistic missile structures.
9
Co
27
Cobalt is mostly used in special steel and alloy, high-speed steel containing cobalt has high temperature hardness, and molybdenum can be added to maraging steel at the same time to obtain ultra-high hardness and good comprehensive mechanical properties. In addition, cobalt is also an important alloying element in heat-strong steels and magnetic materials.
Cobalt reduces the hardenability of steel, therefore, adding carbon steel alone will reduce the comprehensive mechanical properties after tempering. Cobalt can strengthen ferrite, added to carbon steel, annealed or normalized state can improve the hardness of steel, yield point and tensile strength, elongation and section shrinkage have adverse effects, impact toughness also decreases with the increase of cobalt content. Because of its oxidation resistance, cobalt is used in heat-resistant steels and heat-resistant alloys. Cobalt-based alloy gas turbines also show its unique role.
10
Si
14
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 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 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. It can promote ferrite grain coarsening and reduce coercivity. It has the tendency to reduce the anisotropy of the crystal, so that the magnetization is easy, the magnetic resistance is reduced, and it can be used to produce electrical steel, so the magnetic blocking 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 strength under a weak magnetic field. However, silicon decreases the magnetic induction strength 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 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 is cooled faster when heated, due to the low thermal conductivity, the internal and external temperature difference of the steel is large, and therefore the fracture.
Silicon can reduce the weldability of steel. Because the binding ability of silicon with oxygen is stronger than iron, it is easy to generate low melting point silicate during welding, which increases the fluidity of molten slag and melted metal, causing splashing phenomenon and affecting welding quality. Silicon is a good deoxidizer. When deoxidizing with aluminum, adding a certain amount of silicon can significantly improve the deoxidization of the rate. There is a certain amount of silicon in steel, which is brought in as a raw material when making iron and steel. In boiling steel, silicon is limited to <0.07%, and when it is intended to be added, ferrosilicon alloy is added in steelmaking.
1
11
Mn
25
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, thereby improving 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, which enters the cementite to replace a part of iron atoms. Manganese plays a role in refining pearlite by reducing the critical transition temperature in steel, and also indirectly plays a role in improving the strength of pearlite 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 of less than 2% combined with other elements.
Manganese has the characteristics of rich resources and diverse performance, and has been widely used, such as carbon structural steel with high manganese content and spring steel.
In high-carbon high-manganese wear-resistant steel, the manganese content can reach 10% to 14%, after solid solution treatment, good toughness, when the impact and deformation, the surface layer will be strengthened due to deformation, with high wear resistance.
Manganese and sulfur form MnS with a higher melting point, which can prevent thermal embrittleness 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 in steel.
12
Al
13
Aluminum is mainly used for deoxidation and grain refinement. 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, it can improve the oxidation resistance and corrosion resistance of steel in oxidizing acid and H2S gas, and improve the electrical and magnetic properties of steel. Aluminum plays a great role in solid solution strengthening in steel, improving the wear resistance, fatigue strength and core mechanical properties of carburized steel.
Aluminum and nickel form compounds in refractory alloys to improve the metallurgical strength. Fe-cr aluminum alloys containing aluminum have near-constant resistance characteristics and excellent oxidation resistance at high temperatures, and are suitable for electrometallurgical alloy materials and chrome-al resistance wires.
When some steel is deoxidized, if the amount of aluminum is too much, it will cause the steel to produce abnormal structure and promote the graphitization tendency of steel. In ferritic and pearlitic steel, when the content of aluminum is high, it will reduce its high temperature strength and toughness, and bring some difficulties to smelting, pouring and other aspects.
13
Cu
29
The prominent 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, and has no adverse effect on the welding performance. Rail steel (U-Cu) containing 0.20% to 0.50% copper, in addition to wear resistance, its corrosion resistance life is 2-5 times that of general carbon rail.
When the copper content exceeds 0.75%, the aging strengthening effect can be produced after solution treatment and aging. At low levels, its effect is similar to nickel, but weaker. When the content is higher, it is unfavorable to the hot deformation processing, resulting in copper brittleness during the hot deformation processing. 2% ~ 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
5
The main role of boron in steel is to increase the hardenability of steel, thereby saving other rarer and more expensive metals, such as nickel, chromium, molybdenum and so on. 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, with boron instead of molybdenum should be noted, because molybdenum can prevent or reduce tempering brittleness, while boron slightly promote the tendency of tempering brittleness, so can not be completely replaced with boron molybdenum.
Medium carbon carbon steel with boron, due to improved 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. However, due to the role of boron with the increase of the content of carbon in the steel and weaken, or even disappear, in the selection of boron-containing carbon carburizing steel, it is necessary to take into account the parts after carburizing, the hardenability of the carburizing layer will be lower than the hardenability of the core.
Spring steel is generally required to be fully quenched, usually the spring area is not large, and the use of boron-containing steel is advantageous. The action 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 steel.
1
15
Re
75
Generally speaking, rare earth elements refer to the lanthanides (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 in nature and not easily separated. The unseparated, called mixed rare earth, is cheaper, and rare earth elements 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 effect of oxidation resistance exceeds that of silicon, aluminum, titanium and other elements. It can improve the fluidity of steel, reduce non-metallic inclusions, and make the steel structure dense and pure.
The addition of appropriate rare earth elements in ordinary low alloy steel has good deoxidation and sulfur removal, improves impact toughness (especially low temperature toughness), and improves anisotropy properties.
Rare earth elements increase the oxygen resistance of ferrochrome aluminum 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 improved.
16
N
7
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 nitride on the grain boundary, the high temperature strength of the grain boundary can be increased and the creep strength of the steel can be increased. Combined with other elements in steel, precipitation hardening effect. The corrosion resistance of the steel is not significant, but the surface of the steel after nitriding, not only increases its hardness and wear resistance, but also significantly improves the corrosion resistance. Residual nitrogen in mild carbon steel can lead to aging brittleness.
17
S
16
Increasing the content of sulfur and manganese can improve the cutting performance of steel, and sulfur is added as a beneficial element in free-cutting steel. Sulfur is heavily segregated in steel. The deterioration of the quality of steel, at high temperatures, reduces the plasticity of steel, is a harmful element, which exists in the form of FeS with a lower melting point. FeS alone has a melting point of only 1190 ° C, while the eutectic temperature of forming a eutectic crystal with iron in steel is even lower, only 988 ° C, and when the steel solidifies, the iron sulfide will settle 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, resulting in the hot brittleness of the steel, so the sulfur should be strictly controlled. The general control is 0.020% ~ 0.050%. In order to prevent brittleness caused by sulfur, enough manganese should be added to form MnS with a higher melting point. If the flow rate in the steel is high, the porosity and porosity will be formed in the welding metal due to the generation of SO2.
18
P
15
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. The combined use of phosphorus with sulfur and manganese can increase the cutting performance of steel, increase the surface quality of the workpiece, and is used for free-cutting steel, so the phosphorus content of free-cutting steel is also relatively high. Phosphorus is used in ferrite, although it can improve the strength and hardness of steel, the biggest harm is that the segregation is serious, the tempering brittleness is increased, and the plasticity and toughness of steel are significantly increased, resulting in the steel is prone to brittleness during cold processing, that is, 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%.
