Production Status and Future Development Direction of Gear Steel in China

Special steel is a key material for major equipment manufacturing and national key project construction. It is a high-tech product in iron and steel materials. Its production and application represent a country's level of industrialization. 1 Current status and development direction of bearing steel Bearings are widely used in mining machinery, precision machine tools, metallurgical equipment, heavy equipment and high-grade cars and other major equipment areas and wind power generation, high-speed rail cars and aerospace and other emerging industries. Bearings produced in China are mainly middle and low end bearings and small and medium sized bearings, which are characterized by low-end surplus and high-end shortage. Compared with foreign countries, there is a big gap between high-end bearings and large bearings. The bearings for the special high-speed railway passenger trains in China all need to be imported from abroad. In the key bearings used in aerospace, high-speed railways, high-end cars and other industrial fields, there is a big gap between China's bearings and the advanced level in terms of service life, reliability, Dn value and bearing capacity. For example, the service life of foreign automotive transmission bearings is a minimum of 500,000 kilometers, while the life of similar domestic bearings is approximately 100,000 kilometers, and the reliability and stability are poor. As a key component of the aerospace engine, aviation is developing second-generation aeroengine bearings with a thrust ratio of 15-20, and is expected to be installed in the fifth generation fighter aircraft around 2020. In the past 10 years, the United States has developed the second generation of bearing steels for aircraft engines. The representative steels are the high-strength, corrosion-resistant bearing steel CSS-42L that withstands 500°C and the high-nitrogen stainless bearing steel bearing X30 (Cronidur30) that withstands 350°C, China. The second generation of aircraft engine bearings was developed.

Automobiles For automobile wheel bearings, the 1st and 2nd generation hub bearings (ball bearings) are widely used in China, and 3rd generation hub bearings are widely used in Europe. The main advantages of the 3rd generation hub bearings are their reliability, short payload spacing, easy installation, no adjustment, and compact construction. At present, most of the imported vehicles in China use such lightweight and integrated wheel bearings. In the area of ​​railway vehicles, China's railway heavy-duty train bearings are made of domestically-made electroslag remelted G20CrNi2MoA carburized steel, while vacuum degassing smelting technology and inclusion homogenization technology of ultrahigh-purity bearing steel (EP steel) have been developed abroad ( (IQ steel), ultra-long life steel technology (TF steel), fine heat treatment technology, surface super-hardening treatment technology and advanced sealing lubrication technology are applied to the production and manufacture of bearings, thereby greatly increasing the life and reliability of the bearings. Sex. China's electric slag bearing steel is not only of low quality, but also costs more than 2000-3000 yuan/ton more than vacuum degassed steel. In the future, China needs to develop ultra-high purity, fine quality, uniform and quality-stable vacuum degassed bearing steel to replace the current Electroslag bearing steel used. For wind power energy, wind turbine bearings currently cannot produce high-tech spindle bearings and speed-increasing bearings. Basically, they rely on imports. Localization of the supporting bearings for wind turbines larger than 3 MW has not yet been resolved. In order to improve the strength, toughness and service life of wind turbine bearings in foreign countries, a new type of special heat-treated steel SHX (40CrSiMo) is used. For yaw and pitch bearings, the depth of hardened layer, surface hardness, and the width of soft ribbon are controlled by surface induction hardening heat treatment. Surface cracks; carbonitriding is used for the gearbox bearing and the spindle bearing, so that the surface of the part gets more stable residual austenite volume fraction (30% - 35%) and a large amount of fine carbides and carbonitrides, improving the bearing Service life under contaminated lubrication conditions. In order to improve the service life and running accuracy of rolling mill bearings, the development of high-purity vacuum degassing smelting of bearing steels such as GCr15SiMn and G20Cr2Ni4 for rolling mills and large austenitic bearing heat treatment for bearing surfaces is required in the future. Japan NSK and NTN Bearing Co., Ltd. respectively developed a surface austenitic strengthening technology, that is, by increasing the austenite content of the surface, developed TF bearings and WTF bearings, thereby increasing the bearing life by 6-10 times. In the future, the research and development direction of China's bearing steel is mainly reflected in four aspects: First, economic cleanliness: In the premise of considering economy, the cleanliness of steel is further improved, and the content of oxygen and titanium in steel is reduced to achieve oxygen in bearing steel. The mass fraction of titanium is less than the level of 6×10-6 and 15×10-6, respectively, which reduces the content and size of inclusions in the steel and improves the uniformity of distribution. The second is the refinement and homogenization of the structure: through the application of alloying design and controlled rolling and cooling technology, the uniformity of inclusions and carbides is further improved, mesh and ribbon carbides are reduced and eliminated, and the average size and the largest particles are reduced. Size, to achieve the goal of the average size of carbide is less than 1μm; further increase the grain size of the matrix structure, so that the grain size of the bearing steel is further refined. The third is to reduce the low-fold organizational defects: to further reduce the central looseness in the bearing steel, the central shrinkage hole and the center component segregation, and improve the uniformity of low-power organizations. Fourth, the high toughness of bearing steel: Through the new alloying, hot rolling process optimization and heat treatment process research, improve the toughness of bearing steel. 2 Status and development direction of spring steel Spring steel is mainly used in automobile, engine manufacturing and railway industries. At present, the problems in China's spring steel products are the surplus of low-end and middle-end products, and the lack of high-end and special varieties. China's spring steel has a large gap with foreign countries in terms of cleanliness, fatigue resistance, surface quality, and quality stability. Satisfy the spring steel performance requirements of high-end passenger car suspension springs, valve springs, railways and special trucks for heavy-duty trucks. China's high-grade and deep-processed spring steels still rely on imports. The imported varieties mainly include spring steel for cars, spring round steel for railways, and spring wire for oil pump valves. Although reducing the content of oxygen and inclusions in steel is a way to obtain clean steel, it is difficult to obtain zero-inclusion spring steel. For this reason, researchers have proposed oxide metallurgy technology, which is an effective grain refinement. The method of chemical transformation is the most effective way to increase the strength and toughness of steel materials. It utilizes small, high-melting non-metallic inclusions dispersed in the steel, mainly oxides, sulphides and nitrides, as the nucleation core of the intragranular ferrite, which acts to refine the grain. A systematic study of Ti and Zr oxide systems has been conducted at home and abroad, and it is believed that titanium-containing oxides are the most desirable. In the austenite grains, the oxidation point of titanium becomes an effective nucleation site of acicular ferrite and promotes the formation of intragranular ferrite. However, due to the limitation of steel composition, the promotion of titanium oxide metallurgy has been limited. In recent years, researches on rare earth elements have begun. The strong deoxidation capability of rare earth elements and the high melting point of the products can be used to study the effect of rare earth oxides on the properties of steel. The automotive industry is increasingly demanding the suspension spring strength, and the design stress has been increased to 1100 to 1200 MPa. For this reason, Japan has developed steels with added alloys to increase strength and improve corrosion fatigue strength. China Spring Steel cannot meet the performance requirements of high-end passenger vehicle suspension spring steels. Spring steels with strengths of 1200 MPa and above are all dependent on imports. However, in recent years, in order to avoid resource risks, reduce costs, and realize global supply of raw materials, it is strongly demanded to use standard steel (SAE9254) to maintain high strength, and it is strongly demanded to increase the toughness of steel. Therefore, shot peening is increasingly used. Treatment replaces the costly surface hardening heat treatment. Shot peening treatment applies compressive residual stress to the surface, which can increase the fatigue strength and reduce the influence of surface defects. Therefore, it has been regarded as an indispensable technology for surface treatment in recent years. With the development of surface hardening technology, the design stress of the suspension spring has reached 1200 MPa. It is expected that the requirements for the strength, toughness, corrosion resistance, and durability of high-strength suspension steels will increase in the future. In the future, with lightweight vehicles, the development of high-strength, excellent anti-embarkation and anti-fatigue properties of spring steel for automotive suspension is an inevitable trend to improve the self-supporting capabilities of high-end equipment components in China and effectively replace imports. Among all the spring products, valve springs require the most stringent materials, especially high-stress and profiled cross-section valve springs require nearly stringent material requirements. For example, the required tensile strength is 2000 MPa; the requirements for inclusions of oxides and sulfides are all up to 0; special profile materials have special requirements for curvature, length and short axis. At present, the production of special spring steel for foreign valve springs is mainly concentrated in Japan, South Korea and Sweden. The production companies include Japan Suzuki, Sanxing, Sumitomo, Kobelco Steel Wire, South Korea KisWire, and Sweden Garphyttan, which almost monopolize all shaped sections and heights in China. Stress valve spring steel market. Since 2000, with the development of new engines, the requirements for the rotation speed, lighter weight, and compactness of the engine have become higher and higher. Therefore, the use of 2100-2200 MPa OT wire has begun in Japan. In this case, not only the alloy composition needs to be adjusted, but also the existing manufacturing process is improved, and low-temperature dispersion hardening becomes an indispensable process. However, the shape of the spring after the low-temperature dispersion hardening has changed, and in order to improve the control accuracy of the shape and the size, the technique of controlling the shape change in the entire manufacturing process has begun to attract attention. In the future, in order to meet the development needs of localization of high-end spring basic components, high-performance spring steel products should be continuously developed. On the one hand, development is proceeding in the direction of high strength, and fatigue life and relaxation resistance are required to be improved at the same time under high stress; on the other hand, It is to develop in the functional direction. According to different uses, it is required to have corrosion resistance, non-magnetic, electrical conductivity, abrasion resistance, heat resistance, and the like.

Gear steel status and development direction Gears are subjected to a variety of stresses, such as impact force, contact stress, pulsating bending stress, and friction force, which are subjected to variable loads for a long period of time. They are also subject to machining accuracy, assembly accuracy, grinding of foreign hard spots, etc. The influence of various factors is extremely easy to damage parts, so the gear steel is required to have high toughness, fatigue strength and wear resistance. In order to produce high-quality gear steel, on the one hand, steel mills are required to provide users with gear steel products that have a hardenability and meet user's technological requirements. On the other hand, gear plants must also optimize existing technologies and introduce new technologies to improve the quality of gears. Compared with gear steels produced in Japan, Germany, and the United States, the gaps in China's gear steels are mainly due to the fact that steel grades are not serialized and product standards are lagging behind; the hardenability of steel is wider, and the hardenability of foreign steels is greater. Has reached 4HRC, while China is around 6-8HRC, and is not stable enough; the purity of steel is low, and the oxygen content of gear steel imported from Japan, Germany, Austria and other countries fluctuates (7-18)×10-6. China is around (15-25)×10-6, and the dispersion of non-metallic inclusions is not enough, the distribution is uneven, and there are more large-particle inclusions; grain size requirements are different, and China's gear steel grain size level generally requires 5 Grade -8, while Japan particularly emphasizes that the grain size of carburized gear steel should not be thicker than 6; Japan has developed a series of low silicon grain boundary oxidation carburizing steel, which can reduce the grain boundary oxide layer to ≤ 5μm, while SCM420H Such as Cr-Mo steel 15-20μm; average life expectancy, energy consumption per unit of product, labor productivity is low. In addition, how to ensure that the low-order defects such as porosity are small and within the core during the rolling process is also an area that China has not studied, because low-magnification defects will have many adverse effects on subsequent processing of parts and deformation of heat treatment. At present, the main steel type of China's automotive gear steel is still 20CrMnTi. This type of steel usually adopts a gas carburizing process. Due to the presence of oxidizing gases in the carburizing atmosphere, the elements Si and Mn that have a higher affinity for oxygen in the permeation layer are caused. The oxidation of Cr occurs at the grain boundary to form a grain boundary oxide layer. The occurrence of the grain boundary oxide layer causes the solid solution content of the alloying elements such as Si, Mn, and Cr to decrease, and the hardenability of the permeation layer is reduced, thereby reducing the hardness of the permeation layer and resulting in the generation of non-martensitic structure, thereby significantly reducing Gear fatigue performance. To solve this problem, two methods can be used: 1) Use a special heat treatment process. Vacuum carburizing can reduce the oxygen potential in the carburizing atmosphere, which can effectively reduce the occurrence of grain boundary oxidation in the carburized layer; the rare earth carburizing process can also reduce the degree of grain boundary oxidation, because the rare earth is preferentially enriched on the workpiece surface. It also preferentially diffuses along the grain boundary of steel, and its affinity for oxygen is much higher than that of Si, Mn, and Cr. It will preferentially combine with oxygen and hinder the inward diffusion of oxygen atoms, thus helping to reduce non-martensite. The organization's production. 2) Development of grain boundary-oxidized gear steels through alloy design. Ni and Mo have strong anti-oxidation energy, followed by Cr element, and Mn have weak antioxidant capacity, while Si has the weakest anti-oxidation ability (Si oxidation tendency is 10 times that of Cr and Mn). Therefore, in order to reduce grain boundary oxidation and ensure hardenability, when the composition design of gear steel, the content of easily oxidized elements should be properly reduced, especially the content of Si, so as to increase the contents of Ni and Mo, which are difficult to oxidize. It has been reported that controlling the Si, Mn, and Cr to 0.05%, 0.35%, and 0.01%, respectively, can completely suppress surface structural abnormalities, and that even at 1000° C., there is little occurrence of grain boundary oxidation. In order to meet the high-performance and lightweight development requirements of the automotive industry, the future should focus on the development of: hardenability with narrow gear steel, ultra-low oxygen carburizing steel, low grain boundary oxide carburized steel, ultra-fine grain carburized steel , improve high temperature hardness and high temperature anti-softened carburized steel, easy-cutting gear steel, cold forged gear steel.