There are many steel grades for gears used in China's heavy-duty trucks, mainly to meet the requirements of foreign advanced automotive technologies introduced at that time. In the 1950s, China imported from the former Soviet Union Rihabov Automobile Factory the production technology of the then Soviet medium-sized truck (the “Liberation†brand original model). At the same time, it also imported 20CrMnTi steel from the former Soviet Union.
After the reform and opening up, with the rapid development of China's economic construction, in order to meet the rapid development of China's transportation, from the 1980s, China has planned to introduce various types of advanced models of industrialized countries, and various types of foreign advanced medium and heavy loads. Cars are also constantly introduced. At the same time, China's large automobile plant cooperates with famous foreign car companies to introduce foreign advanced automobile production technologies, including the production technology of automotive gears. At the same time, the level of iron and steel smelting technology in China has also been continuously improved. Advanced refining techniques such as secondary smelting of steel ladle and fine-tuning of components and continuous casting and rolling are used to enable steel mills to produce high purity and hardenability with narrowing. The use of steel for gears has enabled the localization of the introduction of steel for automobile gears, which has brought the level of production of gear steel in China to a new level. In recent years, nickel-containing high hardenability steels for domestic heavy-duty automobile gears suitable for China's national conditions have also been developed and applied, and have achieved good results. The heat treatment technology of automotive gears has also been developed from the well-type gas carburizing protection in the original 50-60 years to the current widespread use of computer-controlled continuous gas carburizing automatic lines and box type multi-purpose furnaces and automatic production lines (including low pressure (vacuum) carburizing Technology), gear carburizing pre-oxidation treatment technology, gear quenching control cooling technology (due to the use of special quenching oil and quenching cooling technology), gear forging billet isothermal normalizing technology, etc. The adoption of these technologies not only enables gear carburizing quenching and distortion to be effectively controlled, but also improves the machining accuracy of gears, prolongs the service life, and satisfies the mass production needs of modern heat treatment of gears.
Relevant literature points out that the life of automobile gears is mainly assessed by two major indicators: one is the contact fatigue strength of gears, and the other is the bending fatigue strength of gears. The former is mainly determined by the quality of carburizing and quenching, and the latter is mainly determined by the gear material. For this reason, it is necessary to have a more comprehensive understanding of the requirements, properties and heat treatment characteristics of carburized steel for automotive gears.
Chromium manganese titanium steel and boron steel
For a long time, the most common type of steel used for truck gear in China is 20CrMnTi. This is the 18XTr type of steel gear (ie, 20CrMnTi steel) imported from the former Soviet Union in the 1950s. The grain of the steel is fine, the grain growth tends to be small when carburizing, has good carburizing and quenching properties, and can be directly quenched after carburizing. The literature points out that before 1980, the carburized alloy structural steel (including 20CrbinTi steel) in China only ensured the chemical composition of the steel and the mechanical properties measured by the sample when the steel was shipped from the factory. However, the chemical composition and mechanical properties often occur during automobile production. Qualified steel, due to the excessive fluctuation range of the hardenability, affects the quality of the product. For example, if the hardenability of 20CrMnTi carburizing steel is too low, the hardness of the core after the gear carburizing and quenching is lower than the value specified in the technical specifications, and the fatigue life of the gear is reduced by half during the fatigue test; if the hardenability is over High, the gear shrinkage after the carburizing quenching is too large and affects the gear assembly.
As the hardenability of the steel has a significant impact on the hardness and distortion of the gear tooth's core, in 1985 the Ministry of Metallurgy promulgated the technical conditions (GB5216-85) for ensuring the hardenability of the structural steel in China, and included in this technical condition. The chemical composition and hardenability data of 10 carburized steels, including 20CxMnTiH and 20MnVBH steels. The standard stipulates that the hardenability index of 20CrMnTi steel used for making gears is 30-42HRC from the water cooled end. After that, the problem of excessively low hardness and excessive distortion in the tooth center of gears produced with 20CrMnTi steel was basically solved. However, it is obviously unreasonable to use the same steel 20CrMnTi steel regardless of the size of the gear module and the thickness of the steel. In recent years, due to the improvement of China's steel smelting technology and the improvement of the supply of alloy structural steel, it has been a condition to further narrow the hardenability of gear steel, and according to different products (such as transmission gears and rear axle gears, etc.) Requires the development of new steel grades to meet their requirements.
Through negotiations with the steel mills, Changchun FAW has successively signed an agreement with the manufacturer of the gear steel mills to supply the hardenability of the 20CrMnTi steels in 1997. For example, the "Liberation" brand 5t trucks are used to manufacture cross-sectional dimensions. The small transmission first shaft, the countershaft gears and the rear axle main and driven bevel gears with larger cross-sectional dimensions are 20CMnTiH steels. The hardenability performance groups are I and II respectively, and the corresponding hardenability performances are J9:30-36HRC respectively. J9=36 to 42HRC.
Around 1960, due to the tight supply of nickel and chromium steel in China, the production of nickel and chromium steel in China was affected. At that time, China’s automobile industry was a technology imported from the former Soviet Union, and the Soviet Union applied a large number of nickel and chromium steels. Therefore, at that time, China's auto industry vigorously developed the development and research work of boron steel, using 20MnVB and 20Mn2TiB steel instead of 20CrMnTi carburizing steel to make gears. This is because the addition of trace boron (0.0001%-0.0035%) to the structural steel can significantly improve the hardenability of the steel. Therefore, adding a trace of boron to the steel can replace a certain amount of precious alloy elements such as manganese, nickel, chromium, and molybdenum. Therefore, boron steel has been widely used. Changchun FAW once used 20MnTiB and 20Mn2TiB steel in the production of “liberation†brand automobile gears.
The “Dongfeng†brand 5 produced by Dongfeng Motor Company is manufactured from 20CrMnTi and 20MnVB steel respectively for the truck transmission and rear axle gear. Similarly, agreements have been signed with steel mills to narrow the hardenability of steel and supply it in segments. Transmission and rear axle main and driven bevel gear steels are respectively 20CrMnTiH(3) and 20MnVBH(2), 20MnVBH(3), corresponding hardenability performances are J9=32～39HRC and J9=37～44HRC, J9=34 respectively. ~42HRC.
China's Minjiang Gear Factory has introduced the German company's heavy-duty automotive transmission gear production technology. In the country, according to the German III company's standard trial production of the company's Cr-Mn-B series of boron-containing gear steel was successful. The hardenability of the gear material is J10=31~39HRC
Of course, boron-containing steels such as 20CrMnTi steel, 20MnTiB steel, and 20MVB steel are also insufficient. It is generally believed that 20CrMnTi iso-carburizing steel is an intrinsically fine grain steel, and the crystal grains will not be rented after carburizing, and can be directly quenched. However, due to the influence of the smelting quality of steel, grain coarsening often occurs under normal conditions. For the actual grain size test of a number of batches of materials, it was found that a considerable part of the actual grain size was only grade 2-3 (at 930°C for 3 h). According to the literature, 20CrMnTi has high Ti content and many TiN inclusions in steel. Especially, large TiN inclusions are fatigue sources for gear fatigue, and its presence will reduce the contact fatigue performance of gears. This inclusion has a cubic structure, and it easily undergoes cleavage and cracking when subjected to stress, which results in early failure of the gear. Another problem is that the hardenability of this steel is limited, and it cannot meet the requirements for large-diameter and large-modulus gears. Both the depth of hardened carburized layers and the hardness of the core cannot meet the requirements for heavy-duty gears. In addition, 20CrMnTi steel tends to produce internal oxidation and non-martensitic structure during heat treatment to reduce the gear fatigue life. However, at present, there is no steel in gear cement steel in China that has 20CrMnTi steel mature and reliable in the carburizing process. Therefore, it is still the most commonly used carburizing steel in China. Boron steels such as 20MnVB, 20MnTiB, and 20Mn2TiB also have some drawbacks, such as the effect of boron on the deoxidation and denitrification during the smelting process, which does not increase the hardenability. Therefore, the performance of the boron steel is unstable, and after carburizing and quenching The gear distortion increases and affects the quality of the product. At the same time, mixed crystals and grains are prone to coarseness, deformation is not easy to control and toughness is poor, and boron steel gear roots are prone to produce black meshes and black belts of tortoise body tissues and carbonitriding gears. Therefore, many plants are currently suspending the use of this steel. However, it must not be concluded that boron steel is not suitable for gear carburizing steel. Boron-containing carburizing steels are used abroad. For example, the famous German IV gear factory has been using the retained steel ZF7 developed by its factory, which is a boron-containing low-carbon chromium manganese steel. The main chemical composition of the steel (mass fraction, %) is 0.15 to 0.20C, 0.15 to 0.40 S, 1.0 to 1.3 Cr, 1.0 to 1.3 Mn, and 0.001 to 0.003 B. Some American automotive transmission gears and rear axle main and driven gears also use boron-containing carburizing steels such as 50B15, 43BVl4 and 94B17. Therefore, as long as steel mill smelting techniques keep up, the above problems of boron steel can be solved.
The 20CrMnTiH, 20MnVBH, and 20MnTiBH steel gear forgings are processed in a continuous isothermal normalizing furnace to ensure uniform distribution of flaky pearlite + ferrite. This can greatly reduce the heat treatment distortion of gears, improve the accuracy of the gears, and extend the service life.
The gear forging billet has an isothermal normalizing hardness of 156 to 207 HB.
Chromium-manganese-molybdenum steel
22CrMnMo, 20CrMnMoH and 20CrMoH steels are used for medium-sized automotive gears due to their high hardenability. Such steel can be directly quenched after carburizing. Since chromium, manganese-molybdenum steels and chromium-molybdenum steels contain elements such as chromium and molybdenum that form carbides, the carburizing process will promote the increase of the carbon content on the surfaces of gear teeth, and it will easily cause a large amount of carbides in the carburized layer to cause carburizing. Layer performance deteriorated. Therefore, when the gears are carburized with chromium-manganese-molybdenum steel and chromium-molybdenum steel, a weak carburizing atmosphere should be adopted to prevent excessive carbide formation. 22CrMnMo and 20CrMnMoH gear forgings after normalizing at 650 ~ 670 °C after high temperature tempering, the metallographic structure of fine pearlite + a small amount of ferrite, hardness of 171 ~ 229HB. 20CrMnH gear forging billet is best handled in a continuous isothermal normalizing furnace, 935 ~ 945 °C heating, 640 ~ 650 °C after the first pre-cooling after isothermal, get uniform ferrite + pearlite organization, hardness 156 ~ 207HB. The literature points out that the 20CrMoH steel smelting process is stable, the hardenability zone is narrow and easy to control, compared with the 20CrMnTi steel gear, has a small heat treatment distortion; the penetration layer has a good, stable hardenability; metallurgical organization, after carburizing and quenching The hardness of the surface and the core can meet the technical requirements better; the fatigue performance is good, and it is more suitable for small and medium-sized gears in automobiles. Comprehensive consideration of the service conditions of the gears not only ensures the fatigue life of the gears, but also reduces the heat treatment distortion of the gears. It should be J9=30~36HRC for the gearbox gears and J9=37~ for the rear axle gears. 42HRC.
Foreign advanced domestic steel for automotive gears
With the introduction of foreign advanced models, the localization of various gear steels has made China's gear steel a new level. At present, Germany's Cr-Mn steel, Japan's Cr-Mo steel, and the United States' SAE86 steel meet the medium and small module gear steel. Some domestically-made automobile gears use the American steel grade SAE8822H steel, such as 8t and 10t bridge bevel gears adopt SAE8822H, the main chemical composition of the steel (mass fraction, %) is 0.19-0.25C, 0.70-1.05Mn, 0.15-0.35Si 0.35-0.75Ni, 0.35-0.65Cr, 0.30-0.40Mo. According to the literature, controlling hardenability is the key to solving the problem of gear distortion. In order to reduce the distortion, Jominy's hardenability shall be selected for H steels with a bandwidth below 4HRC. The precision of H-steel gear heat treatment (contact zone) is 70%~80% higher than that of ordinary steel and the service life is extended. Therefore, industrialized countries have successively provided hardenability belts for carburizing alloy structural steels. The hardenability zone is limited to a narrow range (4 to 5 HRC) as required. 1) When ordering in Germany, the hardenability of the steel may be required to be within a given range, and the hardenability of the steel may also be required. 17CrNiM06 is very suitable for the manufacture of large module heavy-duty automotive gears, the main chemical composition (mass fraction,%) of the steel is 0.15 ~ 0.20C, 0.40 ~ 0.60Mn, 1.50 ~ 1.80Cr, 0.25 ~ 0.35Mo, 1.40 ~ 1.70Ni. This steel has already begun production and use in our country. According to the literature, during the carburizing process of 17CrNiM06 steel gears, the cooling rate after carburizing was accelerated while the carbon potential at the later stage of carburizing was properly reduced. The cooling rate was changed from air cooling to air cooling to prevent the formation of large carbides. Then the temperature was elevated at 630 cC. Tempering, in order to precipitate part of the alloy carbides, in order to reduce the amount of retained austenite in the secondary heat quenching at 820 °C, and finally get a better metallurgical structure. 2) The Austrian "Styer" heavy-duty automobile plant requires a hardenability bandwidth of 7 HRC. 3) Japan's heavy-duty trucks, such as the "Hino" brand KB222 9t truck and the "Nissan" brand CKL20DD 8t car transmission gear and rear axle gear widely used Cr-Mo steel, such as SCM420H and SCM822H steel, quite Made in China 20CrMnMoH and 22CrMoH steel.
This type of steel has a high hardenability. Within a certain range, the bending fatigue life of gears increases with the increase of hardenability. The literature points out that when Changchun FAW began producing "liberated" brand 9t truck rear axle gears, 20CrMnTiH steel was used, even though the hardenability of the group II steel (J9=36 to 42HRC) was used, and the heat-treated gear wheel teeth were used. The hardness of the part is only 22-24 HRC, which cannot meet the requirements of the gear technical requirements. When the car is in use, early damage occurs on the rear axle active and driven bevel gears. Therefore, Ct-Mo steel with higher hardenability has to be selected. The main components of the steel are referenced to Japan's SCM822H gear steel. The main chemical composition (mass fraction, %) of the steel is: 0.19 to 0.25C, 0.55 to 0.90 Mn, 0.15 ~0.35Si, 0.85 to 1.25Cr, 0.35 to 0.45Mo. After consultation with the steel plant, the new 22CrMoH steel made in China has been produced. Its hardenability performance index is J9=36 to 42HRC, which satisfies the requirements for automotive gears. However, the process performance of the steel is poor, and the gear forgings must be subjected to isothermal annealing before cutting. The hardness is 156 to 207 HB. The metallographic structure is pre-eutectoid ferrite+pseudo-eutectoid pearlite. The hardenability of this steel is relatively high. Ordinary normalizing tends to produce granular bainite, and the appearance of granular bainite is extremely unfavorable to the cutting process. This not only reduces the service life of the tool, but also accompanies the appearance of abnormal microstructures. The inhomogeneity of the metallurgical structure eventually leads to an increase in the distortion of the heat treatment of the gears. 4) In recent years, American automobile manufacturers have tried to reduce production costs and improve the reliability and durability of parts. This requires a high degree of consistency in the geometric dimensions and mechanical properties of the products. For heat-treated parts to improve the consistency of product performance, the degree of dispersion of hardness after quenching of the parts must be reduced, which is directly related to the width of the hardenability band of the steel. The consistency of the hardness of the gear core will reduce the distortion of the heat treatment, thereby improving the accuracy of the gear and making the distribution of the residual compressive stress of the teeth more uniform. Some of the US truck gears and rear axle drive bevel gears are made of SAE 8620 steel and SAFA 820 steel. US grades such as SAE8620H and SAE8822H have also begun production in China (such as Baosteel Group's Shanghai No. 5 Steel Plant) and used for medium-duty truck transmission gears and rear axle bevel gears.
Domestic heavy-duty automotive gear steel
At present, China's gear steel basically meets the requirements for the localization of the use and introduction of technical processes, while heavy-duty vehicle transmission gears and rear axle gear steels for medium and heavy-duty vehicles have yet to be developed and produced. According to the analysis of the status quo of domestic heavy-duty vehicle use technology, the two problems of overloaded use and poor road conditions are more serious and can not be overcome in the short term, which makes the gears often subject to large overload impact loads. The overload impact load is between fatigue and fracture stress. It has a great influence on the service life of the gear and often causes early failure of the gear. From this point of view, large-modulus heavy-duty automotive gears should be selected from Cr-Ni or Cr-Ni-Mo steels, such as Germany's 17CrNiM06 steel, and domestically produced 20CrNi3H, 20CrNiMoH steel. The advent of high-power engines has promoted the development and application of new Cr-Ni-Mo series gear steels. Such as new gear steel 20CrNi2Mo, 17CrNiM06. A car modification company of FAW Group has developed a new type of cargo vehicle bridge, which features a large power matching engine. In order to ensure the service life of gears, there is a higher requirement for the material and quality of the gears. The rear axle active bevel gears originally made of 22CrMoH steel appeared early failure during use, and even broken teeth occurred in severe cases. In terms of heat treatment, due to the sometimes unstable heat treatment process of the gear material, insufficient effective hardened layers of some gears, and low hardness of the gear core and surface, these are the main reasons for the early failure of gears. Moreover, Cr tends to form intercrystalline carbides and impairs the mechanical properties of the layer. The analysis found that when the hardness of the gear tooth center is low, the plastic deformation of the transition layer will cause excessive stress on the carburized layer, which will lead to the formation of cracks in the carburized layer, and finally cause the entire tooth to break. For this reason, according to the good driving effect of 20CrNi3H steel on the active bevel gear of the rear axle of the “Steyr†auto bridge, the effective hardened layer depth of the gear should be 1.8~2.2mm, and the hardness of the gear tooth core should be 38~45HRC. The surface hardness of the gears is between 60 and 64 HRC, carbides are between 1 and 3, martensite and residual austenite are between 1 and 4, which can increase the service life of gears by 30% to 40%.