China’s gear steel, bearing steel, spring steel production status and future development trends

1 Current status and development direction of gear steel

When the gear is working, it is subject to various stresses such as variable load impact force, contact stress, pulsating bending stress and friction force for a long time. It is also affected by various factors such as machining accuracy, assembly accuracy, and grinding of external hard points. It is an extremely vulnerable part, so 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, the steel mill is required to provide users with gear steel products with stable hardenability and adapt to the user’s process requirements. On the other hand, the gear factory must optimize the existing process and introduce new processes to improve the quality of gears.

Compared with gear steels produced in Japan, Germany, and the United States, the main gaps in gear steels in China are: steel grades have not been serialized, and product standards are backward; steel hardenability bands are wider, and foreign steel hardenability bands Has reached 4HRC, while China is around 6-8HRC, and it 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 in (7-18)×10- 6. China is around (15-25)×10-6, and the degree of dispersion of non-metallic inclusions is not enough, uneven distribution, and there are more large-particle inclusions; the grain size requirements are different, and the grain size level of China’s gear steel is general Grade 5-8 is required, and Japan emphasizes that the grain size of carburized gear steel should not be coarser than Grade 6. Japan has developed a series of low-silicon anti-grain boundary oxidation carburized steel, which can reduce the grain boundary oxide layer to ≤5&mu ; m, while Cr-Mo steel such as SCM420H is 15-20 m; the average service life is short, the energy consumption per unit product is large, and the labor productivity is low. In addition, how to ensure that low-magnification defects such as porosity are small and within the core area during the rolling process is also an area that has not been studied in China, because low-magnification defects will have many adverse effects on the subsequent processing of parts and heat treatment deformation.

At present, the main steel grade of gear steel for automobiles in China is still 20CrMnTi. The steel grade usually adopts gas carburizing process. Due to the presence of oxidizing gas in the carburizing atmosphere, the elements Si and Mn in the carburized layer have a greater affinity for oxygen. , Cr oxidizes at the grain boundary to form a grain boundary oxide layer. The occurrence of grain boundary oxide layer will reduce the solid solution content of alloying elements such as Si, Mn, Cr, etc. in the infiltration layer, reduce the hardenability of the infiltration layer, thereby reducing the hardness of the infiltration layer and lead to the production of non-martensitic structure, which will significantly reduce Fatigue performance of gears. Two methods can be used to solve this problem:

• 1) Adopt a special heat treatment process. Vacuum carburizing can reduce the oxygen potential in the carburizing atmosphere, which can effectively reduce the degree of grain boundary oxidation of 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 surface of the workpiece It preferentially diffuses along the grain boundary of steel, and its affinity with oxygen is much higher than that of Si, Mn, and Cr. It will preferentially combine with oxygen and prevent oxygen atoms from continuing to diffuse inward, thereby helping to reduce non-martensite The emergence of the organization.
• 2) Through alloy design, develop gear steel resistant to grain boundary oxidation. Ni and Mo have strong oxidation resistance, followed by Cr, Mn has weak oxidation resistance, and Si has the weakest oxidation resistance (Si oxidation tendency is 10 times that of Cr and Mn). Therefore, in order to reduce grain boundary oxidation and ensure hardenability, the content of easy-to-oxidize elements, especially the content of Si, should be appropriately reduced when the gear steel composition is designed, and the content of difficult-to-oxidize elements Ni and Mo should be increased accordingly. According to reports, controlling Si, Mn, and Cr at 0.05%, 0.35%, and 0.01%, respectively, can completely suppress the abnormal surface structure, and there is very little grain boundary oxidation even at 1000°C.

In order to meet the development requirements of high performance and light weight in the automotive industry, the future should focus on development: gear steel with narrow hardenability band, ultra-low oxygen carburized steel, low grain boundary oxide layer carburized steel, ultra-fine grain carburized steel , Improve high-temperature hardness and high-temperature softening-resistant carburizing steel, free-cutting gear steel, cold-forged gear steel, etc.

2 Current status and development direction of bearing steel

Bearings are widely used in major equipment fields such as mining machinery, precision machine tools, metallurgical equipment, heavy equipment and high-end cars, and emerging industries such as wind power generation, high-speed trains, and aerospace. The bearings produced in China are mainly low-end bearings and small and medium-sized bearings, which are characterized by surplus of low-end and lack of high-end.

Compared with foreign countries, there is a big gap between high-end bearings and large bearings. All wheelset bearings for China’s high-speed railway passenger cars need to be imported from abroad. In key bearings used in aerospace, high-speed railways, high-end cars and other industrial fields, China’s bearings are far from the advanced level in terms of service life, reliability, Dn value and load carrying capacity. For example, the minimum service life of foreign automobile gearbox bearings is 500,000 kilometers, while the domestic similar bearings have a service life of about 100,000 kilometers, and their reliability and stability are poor. To

Aviation

As the key basic components of aero engines, foreign countries are developing second-generation aero-engine bearings with a thrust ratio of 15-20, and are preparing to be assembled into the fifth-generation fighter aircraft around 2020. In the past 10 years, the United States has developed the second generation of bearing steel for aero-engines. Its representative steel types are 500℃-resistant high-strength corrosion-resistant bearing steel CSS-42L and 350℃-resistant high-nitrogen stainless bearing steel X30 (Cronidur30), China The research and development of bearings for the second-generation aero-engine is underway. Automotive aspects For automotive wheel bearings, the first and second generation wheel bearings (ball bearings) are currently widely used in China, while the third generation of wheel bearings has been widely adopted in Europe. The main advantages of the third-generation wheel hub bearings are reliability, short payload spacing, easy installation, no adjustment, and compact structure. At present, most of the imported models in China use this kind of lightweight and integrated structure wheel bearings. In terms of railway vehicles, the bearings for heavy-duty trains in China are currently made of domestically-made electroslag remelted G20CrNi2MoA carburizing steel, while foreign countries have already used vacuum degassing smelting technology and inclusion homogenization technology for ultra-high-purity bearing steel (EP steel) ( IQ steel), ultra-long-life steel technology (TF steel), refined heat treatment technology, surface super-hardening treatment technology and advanced sealing and lubrication technology are applied to the production and manufacturing of bearings, thereby greatly improving the life and reliability of bearings sex. China’s electroslag bearing steel is not only of low quality, but also costs 2000-3000 yuan/ton higher than vacuum degassed steel. In the future, China needs to develop ultra-high purity, refined, homogenized and stable quality vacuum degassed bearing steel to replace the current one. Electroslag bearing steel used.

In terms of wind power energy

For wind turbine bearings, China is currently unable to produce high-tech spindle bearings and speed-increasing gear bearings, and basically relies on imports. The localization of supporting bearings for wind turbines above 3MW has not yet been resolved. In order to improve the strength, toughness and service life of wind turbine bearings, a new type of special heat-treated steel SHX (40CrSiMo) is used in foreign countries. For yaw and variable slurry bearings, the depth of the hardened layer, surface hardness, soft belt width and soft belt width are controlled by surface induction hardening heat treatment. Surface cracks; carbonitriding is used for speed-increasing gear bearings and spindle bearings to obtain more stable volume fraction of retained austenite (30%-35%) and a large number of fine carbides and carbonitrides on the surface of the parts, which improves the bearing The service life under contaminated lubrication conditions.

In order to improve the service life and operation accuracy of rolling mill bearings, it is necessary to carry out research and development of ultra-high-purity vacuum degassing smelting of bearing steels such as GCr15SiMn and G20Cr2Ni4 for rolling mills and heat treatment of large austenite on the bearing surface in the future. Japan’s NSK and NTN bearing companies have respectively developed surface austenite strengthening technology, that is, by increasing the surface layer austenite content, developed TF bearings and WTF bearings, thereby increasing the life of the bearing by 6-10 times. The future direction of China’s bearing steel research and development is mainly reflected in four aspects: First, economic cleanliness: Under the premise of considering economy, the cleanliness of steel is further improved, and the oxygen and titanium content in the steel is reduced to reach the oxygen in the bearing steel. The mass fraction of titanium and titanium are respectively less than 6×10-6 and 15×10-6 levels, reducing the content and size of inclusions in steel and improving 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, and the network and band carbides are reduced and eliminated, and the average size and maximum particle size are reduced. To achieve the goal that the average size of carbides is less than 1 m; to further increase the grain size of the matrix structure, and to further refine the grain size of the bearing steel. The third is to reduce macrostructure defects: further reduce the center porosity, center shrinkage and center component segregation in the bearing steel, and improve the uniformity of the macrostructure. The fourth is the high toughness of bearing steel: through new alloying, hot rolling process optimization and heat treatment process research, the toughness of bearing steel is improved. 3 Current status and development direction of spring steel Spring steel is mainly used in automobile, engine manufacturing and railway industries. At present, the problem of Chinese spring steel products is that there is a surplus of low-end products and lack of high-end and special varieties; Chinese spring steel has a large gap with foreign countries in terms of purity, fatigue resistance, surface quality and quality stability. It meets the performance requirements of high-end passenger car suspension springs, valve springs, railway and heavy-duty trucks special springs. China’s high-end and deep-processed spring steel still relies on imports. Imported varieties are mainly spring steel for cars, spring round steel for railways, and spring steel wires for oil pump valves. Although reducing the content of oxygen and inclusions in steel is a way to obtain pure steel, it is difficult to obtain spring steel with zero inclusions. For this reason, some researchers have proposed oxide metallurgy technology, which is an effective grain fineness. The method of transformation is the most effective way to double the strength and toughness of steel materials. It uses small and dispersed high-melting non-metallic inclusions in steel, mainly oxides, sulfides and nitrides, as the nucleation core of intragranular ferrite, thereby refining the grain. Systematic research has been done on Ti and Zr oxide systems at home and abroad, and it is believed that titanium-containing oxides are the most ideal. The titanium oxide point in the austenite grain becomes an effective nucleation site of acicular ferrite, which promotes the formation of intragranular ferrite. However, due to the limitation of steel composition, the promotion of titanium oxide metallurgy has been restricted. In recent years, research on rare earth elements has begun. The strong deoxidation and desulfurization ability of rare earth elements and the high melting point of the product can be used to study the effect of rare earth oxides on the properties of steel. The automobile industry has higher and higher requirements for the strength of the suspension spring, and the design stress has increased to 1100-1200MPa. For this reason, Japan has developed steels that add alloys to increase the strength and increase the corrosion fatigue strength. China’s spring steel cannot meet the performance requirements of high-end passenger car suspension spring steel, and all spring steel for suspension spring products with a strength of 1200MPa and above depends on imports.

However, in recent years, in order to avoid resource risks, reduce costs, and realize the global supply of raw materials, it is strongly required to use standard steel (SAE9254) to maintain high strength, and to improve the toughness of steel, so more and more shot peening is used. Treatment replaces the costly surface hardening heat treatment. Shot peening treatment acts on the surface with compressive residual stress, which can increase the fatigue strength and reduce the influence of surface defects. Therefore, in recent years, it has been regarded as an indispensable technology for surface treatment. With the development of surface strengthening technology, the design stress of the suspension spring has reached the 1200MPa level. It is expected that the requirements for the strength, toughness, corrosion resistance and durability of high-strength suspension spring steel will become higher and higher in the future. In the future, with the lightweight of automobiles, the development of spring steel for automobile suspensions with high strength, excellent elasticity and fatigue resistance is an inevitable trend to improve China’s independent matching capabilities of high-end equipment parts and effectively replace imports.

Among all spring products, valve springs have the most stringent requirements for materials, especially for high-stress and special-shaped cross-section valve springs. For example, the tensile strength is required to reach 2000MPa; the inclusion levels of oxides and sulfides are required to reach level 0; the special-shaped cross-section materials have special requirements for curvature, long and short axes, etc. At present, the production of special spring steel for valve springs abroad is mainly concentrated in Japan, South Korea, and Sweden. The production companies include Suzuki, Sanxing, Sumitomo, Kobelco Steel Wire, KisWire of South Korea, Garphyttan of Sweden, etc., almost monopolizing all the special-shaped cross-sections and heights in China. Stress valve spring steel market.

After 2000, with the development of new engines, the requirements for engine rotation speed and lighter weight and compactness became higher and higher. Therefore, Japan began to adopt 2100-2200MPa OT steel wires. In this case, it is not only necessary to adjust the alloy composition, but also to improve the existing manufacturing process, and low-temperature dispersion hardening becomes an indispensable process. However, the shape of the spring after low-temperature dispersion hardening changes. In order to improve the control accuracy of the shape and size, the technology 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 the localization of high-end spring basic parts, we should continue to develop high-performance spring steel products. On the one hand, it is to develop in the direction of high strength, requiring simultaneous improvement of fatigue life and relaxation resistance under high stress; on the other hand; It is developing towards functionality, and according to different uses, it is required to have corrosion resistance, non-magnetic, electrical conductivity, wear resistance, heat resistance, etc.