C element
The higher the carbon content, the higher the hardness of the steel, but the worse its plasticity and toughness.
When the carbon content exceeds 0.23%, the weldability of the steel deteriorates, so the low-alloy structural steel used for welding generally has a carbon content of not more than 0.20%. The high carbon content will also reduce the atmospheric corrosion resistance of the steel, and the high carbon steel in the open yard will be easily corroded; in addition, carbon can increase the cold brittleness and age sensitivity of the steel.
Si element
1. It can increase the hardness of steel, but the plasticity and toughness decrease. The steel used in electrical engineering contains a certain amount of silicon, which can improve the soft magnetic properties.
2. Silicon is also an element added to steel as a deoxidizer during steelmaking. Silicon and molten steel in molten steel can be removed by forming a less dense silicate slag, so silicon is a beneficial element. Dissolving silicon in the ferrite in steel increases the strength and hardness of the steel, and reduces the ductility and toughness. The silicon content in the killed steel is usually from 0.1% to 0.37%, and the boiling steel contains only 0.03% to 0.07%. Since the silicon content in the steel generally does not exceed 0.5%, it has little effect on the properties of the steel.
3. Silicon is added as a reducing agent and deoxidizer during the steel making process, so the killed steel contains 0.15-0.30% of silicon. If the silicon content in the steel exceeds 0.50â€0.60%, silicon is an alloying element. Silicon can be used as a spring steel because it can significantly increase the elastic limit, yield point and tensile strength of steel.
4. Add 1.0-1.2% silicon to the quenched and tempered structural steel, and the strength can be increased by 15-20%. Silicon combines with molybdenum, tungsten, chromium, etc. to improve corrosion resistance and oxidation resistance, and to produce heat-resistant steel. Low carbon steel with silicon content of 1 to 4%, with high magnetic permeability, used in the electrical industry as a steel sheet.
5. The increase in the amount of silicon will reduce the welding performance of the steel.
6. Increasing the strength of the solid solution in the steel and the degree of cold work hardening reduce the toughness and plasticity of the steel.
7. Silicon can significantly increase the steel's elastic limit, yield limit and yield ratio, which is common spring steel.
8. Corrosion resistance. High silicon cast iron with a mass fraction of 15%-20% silicon is a good acid resistant material. The steel containing silicon is heated in an oxidizing atmosphere,
A SiO2 film will also be formed on the surface to improve the oxidation resistance of the steel at high temperatures.Mn element
1. It can improve the strength of steel, can weaken and eliminate the adverse effects of sulfur, and can improve the hardenability of steel. High alloy steel (high manganese steel) with high manganese content has good wear resistance and other Physical properties.
2. Manganese is added to steel as a deoxidizer during steel making. Since manganese can form a high melting point (1600 ° C) MnS with sulfur, the harmful effects of sulfur are eliminated to some extent. Manganese has a good deoxidizing ability and can become MnO into the slag with FeO in the steel, thereby improving the quality of the steel, especially reducing the brittleness of the steel and improving the strength and hardness of the steel. Therefore, manganese is a beneficial element in steel. It is generally considered that when the amount of manganese in the steel is 0.5% to 0.8% or less, manganese is regarded as an ordinary impurity. The technical conditions stipulate that the normal manganese content in high-quality carbon structural steel is 0.5% to 0.8%; and in the higher manganese content structural steel, the amount can reach 0.7% to 1.2%.
3. In the steelmaking process, manganese is a good deoxidizer and desulfurizer, and the general steel contains 0.30-0.50% of manganese. When adding more than 0.70% of carbon steel, even if it is "manganese steel", it has more toughness than ordinary steel, and has higher strength and hardness, improves the quenchability of steel, and improves the hot workability of steel. For example, 16Mn steel is 40% higher than the A3 yield point. 11-14% manganese containing steel has extremely high wear resistance, used in excavator buckets, ball mill liners, etc.
4. Manganese is also an important element in steel. Its effects and effects are as follows: a. At an appropriate amount, an increase in the amount of manganese increases the maximum strength and hardness of the steel. b. Manganese has deoxidation and de-solubility, so manganese can be used for forging and plasticity. c. Manganese has a high content in steel, which can reduce the quenching temperature of steel. d. It can improve the hardening depth of steel, especially in oil-hard manganese steel with high carbon content.
5. The increase of manganese amount reduces the corrosion resistance of steel and reduces the welding performance. (1) Manganese improves the hardenability of steel.
(2) Manganese has a significant effect on improving the strength of low carbon and medium carbon pearlitic steel.
(3) The high temperature instantaneous strength of manganese to steel is improved.The main disadvantage of manganese steel is that
1 When the manganese content is high, there is obvious temper brittleness;
2 Manganese has the effect of promoting grain growth, so manganese steel is sensitive to overheating, and must be paid attention to in the heat treatment process. This shortcoming can be overcome by adding fine crystal elements such as molybdenum, vanadium, titanium, etc.:
3 When the mass fraction of manganese exceeds 1%, the weldability of the steel deteriorates;
4 Manganese will reduce the rust resistance of steel.6. Lower the lower critical point of steel, increase the degree of subcooling during austenite cooling, refine the pearlite structure to improve its mechanical properties, and be an important alloying element of low alloy steel, which can significantly improve the hardenability of steel, but There is an unfavorable tendency to increase grain coarsening and temper brittleness.
7. The beneficial effects of Mn are: high strength and wear resistance, hardened carburizing, cold work hardening
14% (high wear-resistant steel), 17-19% (guard ring steel)
1 As a deoxidizer for steelmaking, since Mn is generally contained in steel, the amount is ≤0.7%.
2 Mn and S action counteract the red-brittle effect of S on iron.
3 The effect of Mn on various types of steel is:
Pearlitic Mn steel: It can improve its strength and wear resistance, and its plasticity is also good. So it can refine the pearlite structure. (For steels with a high carbon content, Mn↑ has a slight decrease in plasticity. For low carbon steels, it contains Mn↑ and toughness.
Austenitic Mn steel: high enough plasticity and high wear resistance. Therefore, Mn can increase the stability of austenite and enlarge the γ phase to obtain austenite. Reduce the critical cooling rate during quenching. Reducing the critical point of steel (A1 and A3) is 25 to 30 °C lower than that of carbon steel, so it can improve the hardenability of steel and the deformation during quenching, so it is suitable for making large sections and complicated parts. When Mn = 5%, Mn drops to 0 °C.
Martensite Mn steel: easy to make it brittle and quenched. Mn is easily soluble in ferrite, and weak carbides are not stable. Therefore, it is easy to completely dissolve into the austenite during the heating process, and the critical point is low, so the grain is easily roughened and easily quenched. For this reason, the quenching heating temperature and the holding time should be strictly controlled, generally with oil. Cooling in quenched or flowing air is preferred, and only simple shapes are used for water quenching. Quenched and tempered steel: it will reduce its plasticity (the effect of temper brittleness).
Carburized steel: The presence of Mn can promote carburization, so it can greatly improve the surface hardness and wear resistance of steel. Especially valuable is that the surface soft spots are less when carburizing, and the tendency of excessive carbon addition is not changed. (Manganese steel after carburizing should be subjected to a normalizing or annealing treatment before final quenching to eliminate the overheating of the heart caused by prolonged carburization).Structural steel: will promote its temper brittleness. Tool steel: Adding about 1% Mn reduces the volumetric deformation during quenching, which is important for precision tools and long tools. (such as CrMn, CrWMn steel, etc.).
4 Mn can improve the weldability and low temperature performance of steel, and can also slow the decarburization of steel.
5 The right amount of Mn can also improve the cutting performance of steel.
6 For some steels, the effect of Mn can be Ni, which can expand the γ phase region to obtain austenite, such as die steel (enhanced hardenability), austenitic steel and so on.
7 High manganese steel is sensitive to cold hardening and can improve the strength and wear resistance of steel. (Mn = 10 to 14%, and C = 1 to 1.4%)
8 Chromium-manganese austenitic steel has good heat strength, even exceeding Cr and Ni steel, and adding 4% Cr and Ni red heat wear resistance is better. Mn is cheap.The adverse effects of manganese are:
1 Increase the heat sensitivity of the steel (coarse grain): This is because the stability of the Mn-containing cementite is not strong and it is easily dissolved in the austenite during heating. In addition, the critical point of Mn steel is also low, so it is easy to coarse crystal. For this purpose, the heating temperature and holding time are strictly controlled for forging and heat treatment heating. Among all the alloying elements, Mn is an element which does not reduce the tendency of austenite grain growth, and instead causes coarse crystals.
2 Reinforce the sensitivity of steel to white spots, so it should be slow. (When C>0.3%, the effect is larger)
3 enhance temper brittleness, and easy to form ribbon and fibrous tissue. Therefore, the longitudinal and lateral performance differences are large (Mn>2.4% elongation ↓↓)
4 High manganese steel has a low melting point (Mn13～14%, T melt 1350～1400°C), and the average linear expansion coefficient is large (equivalent to 1.9 times that of steel bismuth steel), and the thermal conductivity is small (about 1/3 of the same type of strontium steel) 1/4), hot processing is a little harder.
5 When the cooling rate is not high, the high manganese steel tends to form massive carbides along the grain boundary, which makes the steel brittle. When water quenching and rapid cooling, the carbides can not be precipitated, and the uniform austenite structure is obtained, and the performance is improved. However, because the amount of Mn is high, the thermal conductivity is poor, and the rapid cooling stress is large and easy to be quenched, so the number of quenching is not suitable.(3) Classification of Mn-containing steels 1 Carbon steel: a, normal Mn content carbon steel Mn=0.25-0.8% B, higher Mn content carbon steel Mn=0.7-1.0% and 0.9-1.2%
2 manganese steel: Mn = 1.1 ~ 1.8% a few ~ 2.4%
3 high manganese steel: Mn = 13 ~ 14% (C = 1.0 ~ 1.3%) Note: Mn <1.2% for steelmaking deoxidation and slightly change the properties of steel, for general steel. Mn = 1.1 to 1.8% or 2.4% is used for high plasticity, wear resistance, and strength. Mn = 2.4 to 13% is coarse crystals and is extremely brittle and not usable. Mn = 13 to 14% is cold hardened to become a high wear resistant steel.
Cr element1. It can improve the hardenability and wear resistance of steel, and can improve the corrosion resistance and oxidation resistance of steel.
2. The role of chromium in steel is diverse and important. It forms stable and hard carbides and is corrosion resistant. Its main functions are:
a. Improve the hardening energy and carburizing effect of steel. b. The steel still has high strength at high temperatures. c. Can increase wear resistance.d. Increase the quenching temperature of steel. f. Can improve the corrosion resistance of steel.
3. Chromium In structural steels and tool steels, chromium can significantly increase strength, hardness and wear resistance, but at the same time reduce plasticity and toughness. Chromium can improve the oxidation resistance and corrosion resistance of steel, and is therefore an important alloying element for stainless steel and heat-resistant steel.
(1) Chromium can increase the strength and hardness of steel. (2) Chromium can improve the high temperature mechanical properties of steel. (3) Make steel have good corrosion resistance and oxidation resistance (4) Prevent graphitization (5) Improve hardenability.
Disadvantages: 1 Chromium is a significant increase in the brittle transition temperature of steel 2 Chromium can promote the temper brittleness of steel.
4. 1) The beneficial effect of chromium: It has many valuable properties: high hardness, high strength, yield point, high wear resistance, little effect on plasticity and toughness, high oxidation resistance, corrosion resistance, It can also increase resistance and permeability, and so on.
1) Cr is a medium carbide forming element, among all the various carbides, chromium carbide is the finest one, which is evenly distributed in the steel volume.
Therefore, it has high strength, hardness, yield point and high wear resistance. Because it can refine the structure and distribute it evenly, it has good plasticity and toughness, which is especially valuable for tool steel.2) The carbide of Cr is also difficult to dissolve, and it hinders the growth of crystal grains in a short time of heating, and the carburization is also coarse-grained for a long time. Therefore, the overheat sensitive effect can be reduced.
3) Cr can slow down the decomposition rate of the A body and reduce the critical cooling rate during quenching, thus contributing to the formation of the M body and improving the stability of the M body, so the Cr steel has excellent hardenability and less quenching deformation. . Note: Cr is a ferrite forming element that shrinks the gamma region, so high Cr steel will be ferrite in the absence of A-formation elements.
4) The combination of Cr and W or Mo can increase the retained austenite in the quenched steel and help to obtain a carbide phase requiring a degree of pulverization.
5) Cr can greatly improve the strength and plasticity of structural steel, and this effect is particularly remarkable in steels in which Cr and Ni are combined. Such as 12CrNi3N and so on.
6) When Cr≥12%, it has good corrosion resistance, and adding 8~9% of Ni, the corrosion resistance will be greatly improved. The effect of Cr on improving corrosion resistance decreases as the carbon content increases, since Cr does not work when combined with C.
7) When Cr≥25～30%, it has good oxidation resistance. For example, when Cr=27-28%, it can be used as a protective cover for a thermocouple thermometer of 1300 °C. When Cr is combined with Si and Al, even Cr is relatively small and the oxidation resistance is also high. For example, Cr 6 to 10% + Si 2 to 3% have high heat resistance and oxidation resistance.
8) Cr, Al bonding (1Cr17AL5, Cr13AL4, etc.) and Cr, Ni bonding (such as Cr15Ni60, Cr20Ni80, etc.) have high electrical resistance.
9) Cr can increase the coercivity of steel and prevent the aging of steel, so Cr steel is used to make permanent magnets.
10) The price of Cr is lower.
11) Since Cr can form stable carbides, slow down the diffusion of carbon and form a tight oxide film, decarburization can be reduced.
12) Multi-element alloy steel containing Cr > 2.5%. (18Cr3MoWVA, 20Cr3MoWVA, etc.) is a good anti-hydrogen corrosion steel.
Containing Cr <0.08% This is a requirement for graphite steel, so Cr is an element that prevents graphitization.
Cr≯1.2% low alloy high strength steel (generally Mn steel and SiMn steel)
Containing Cr=0.5～1.65% for bearing steel (C≈1%)——It has low alloy content and low cost, but has high strength, high wear resistance, good fatigue resistance and hardenability, and heat treatment. Simple.The effect of Cr on the strength and toughness of steel containing Cr=3~10% is the most significant when the steel is gradually increased to 2% when Cr<2%. However, exceeding this limit will damage its heat. The most obvious when 3 to 10%, when Cr>12%, the strength increases again. However, when the Cr content is increased to 3%, the martensite tempering stability is remarkably increased. Therefore, it has high hardness and wear resistance and is used for the mold. When the amount of Cr is increased to 3%, it is also the best in combination with the magnetic content containing C1%. Therefore, it is used as a magnetic steel. Contain
Cr4% combined with 18% W and 1% V can get good red hardness (620 °C HRC=60), so it is widely used in high speed tool steel. Cr5% steel (including C1%) can be used in air. Quenching. Antimony-chromium steel containing 5 to 6% Cr and 6 to 10% Cr and containing 2 to 3% of Si, although not strong in strength, has sufficient heat resistance and oxidation resistance for use in gas valves and petroleum and chemistry. Industrial (ammonia synthesis equipment, etc.).Steels containing Cr=12~14% are the most typical stainless steels (1Cr13～4Cr13). They all have high corrosion resistance and good strength. Surfaces such as Cr12 and Cr12Mo are typical with high hardenability and high resistance. Grinding mold steel. (This type is mostly martensitic steel)
Some steels containing Cr=16~18% have only single phase (ferrite), and some have two phases (M body-ferrite). The corrosion resistance of such single-phase Cr steel is better than Cr=12. ~14% of the steel is still high. Such as Cr17, 9Cr18, etc., such as adding 8 ~ 9% of Ni, its corrosion resistance will be greatly increased. For example, 1Cr18Ni9, 1Cr18Ni9Ti, etc. are all typical stainless steel and have good acid resistance. The disadvantage of CrNi steel is intergranular corrosion, and Ti and Nb can be improved.
The steel containing Cr=23~32% has good corrosion resistance, high oxidation resistance, and can resist the corrosion of concentrated nitric acid, concentrated phosphoric acid and concentrated sulfuric acid even at ordinary temperature.
Steel containing 27 to 28% Cr can be used as a shield for thermocouple thermometers at 1300 °C. This type of steel is pure ferritic steel, so its microstructure and properties cannot be changed by heat treatment. Moreover, the recrystallization temperature is low, the coarse crystal action is strong, and the brittleness is high, so it cannot be used for vibration and striking parts. Adding Mo, W, and V can appropriately change the performance. Reduce the Cr content,
The amount of Si added can increase its thermal strength such as Cr9Si2, Cr10Si2Mo and the like. Ni is also added, such as Cr20Ni14Si, Cr25Ni20, Cr18Ni25Si, Cr14Mn14W, Cr18Ni6Mn5 and the like.Cr is different from Mn and Ni, which is an alloying element that reduces the γ region. (It has a body-centered cubic lattice with α-Fe, and the melting point is 1849 ° C to absolute 0 oK, pure chromium is the same lattice), so with the increase of Cr content, Ac3 also decreases from 910 ° C, However, the rate is very slow, while Ac4 is rapidly reduced from 1400 ° C, and the lowest ac is 850 ° C when the chromium content is 8%. When Cr is added, Ac3 rises rapidly. When the content of Cr is 13%, Ac3 and Ac4 will merge into one point, and the γ region will be closed. Therefore, when Cr>13%, it will become pure ferrite phase, no change will occur, and the grain can not be changed by heat treatment. size. - that is ferritic steel. When the amount of CCr continues to increase, about 25 to 60%, especially 45 to
In the 48% region, when the temperature is lower than 950 ° C (more at 820 ° C), cold cooling will precipitate a non-magnetic brittle component - σ phase. These will be freely precipitated after the secondary heating, so that a large volume change in the solid solution causes considerable stress and is extremely brittle. However, when quenching below 950 ° C, the σ phase may not be precipitated in the solid solution, and the influence is small. δ phase problem: It has been pointed out that when the Cr and C contents are matched, especially in the case of containing the left and right, it will easily form free ferrite, ie, δ phase, which will reduce the process performance and heat resistance of the steel, so it is very It is noted that when Cr=0.11% is contained, Cr content = 10.9% can minimize the δ phase amount.Cr is advantageous in improving corrosion resistance, but the effect on creep is more complicated. Because heat-resistant steel should be noted, when Cr = 1%, the creep strength is the highest. Contain
Cr↑ appears, Cr>C3 cubic lattice, until Cr=7% creep strength is reduced to the lowest point, when Cr is increased to 12%, Cr23C6 will replace Cr7C3, anti-creep (heat resistance) can be a little Increasing, adding V, Nb, Ti can obtain a very fine dispersion phase, which is extremely advantageous against the improvement of creep (heat resistance).
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