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, high-speed rail and aerospace. The bearings produced in China are mainly medium and low-end bearings and small and medium-sized bearings, which are characterized by low-end surplus and high-end lack. Compared with foreign countries, there is a big gap between high-end bearings and large bearings. China's high-speed railway passenger car special wheelset bearings need to be imported from abroad. In the key bearings used in aerospace, high-speed railway, high-end cars and other industrial fields, there is a big gap between China's bearings in terms of service life, reliability, Dn value and carrying capacity. For example, the service life of foreign automobile gearbox bearings is at least 500,000 kilometers, while the domestic similar bearing life is about 100,000 kilometers, and the reliability and stability are poor.
As a key component of the aero engine, the second generation of aerospace engine bearings with a thrust ratio of 15-20 is being developed abroad, and is ready to be assembled into the 5th generation aircraft by 2020. In the past 10 years, the United States has developed the second generation of bearing steel for aerospace engines. The representative steel grades are high-strength corrosion-resistant bearing steel CSS-42L resistant to 500 °C and high-nitrogen stainless steel bearing bearing X30 (Cronidur30) resistant to 350 °C. The development of bearings for the second generation of aerospace engines is underway.
For automotive wheel bearings, the first and second generation of hub bearings (ball bearings) are widely used in China, and the third generation of hub bearings has been widely used in Europe. The main advantages of the 3rd generation of 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 lightweight and integrated structural wheel bearing.
At present, China's railway heavy-duty train bearings are made of domestic electroslag remelting G20CrNi2MoA carburized steel, while foreign high-purity bearing steel (EP steel) vacuum degassing smelting technology, inclusion homogenization technology (IQ steel) 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, which greatly improves the life and reliability of bearings. 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, fine-grained, homogenized and stable quality vacuum degassing bearing steel to replace the current Electroslag bearing steel used.
Wind power energy
For wind power bearings, China is currently unable to produce spindle bearings and speed increase bearings with high technical content, basically relying on imports, and the localization of bearings for wind turbines above 3MW has not been solved. In order to improve the strength, toughness and service life of wind power bearings, a new special heat-treated steel SHX (40CrSiMo) was adopted. For yaw and pitch bearings, the surface hardness, surface hardness, soft belt width and width of the hardened layer were controlled by surface induction hardening heat treatment. Surface crack; carbonitriding for the gearbox bearings and spindle bearings, resulting in more stable residual austenite volume fraction (30%-35%) and a large number of fine carbides and carbonitrides on the surface of the part, improving the bearing Service life under contaminated lubrication conditions.
In order to improve the service life and running accuracy of the rolling mill bearings, it is necessary to carry out research and development of ultra-high-purity vacuum degassing and smelting of bearing steels such as GCr15SiMn and G20Cr2Ni4 for rolling mills and large austenite volume control heat treatment of bearing surfaces. Japan's NSK and NTN Bearing Company have developed surface austenite strengthening technology, which has developed TF bearings and WTF bearings by increasing the surface austenite content, 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, the economic cleanliness: under the premise of considering economics, further improve the cleanliness of steel, reduce the oxygen and titanium content in the steel, and achieve the mass fraction of oxygen and titanium in the bearing steel are less than 6×10-6 and 15 respectively. The level of ×10-6 reduces the content and size of inclusions in the steel and improves the uniformity of distribution.
Second, organizational refinement and homogenization: through the application of alloying design and controlled rolling and controlled cooling process, further improve the uniformity of inclusions and carbides, reduce and eliminate network and ribbon carbides, reduce average size and maximum particle size. Dimensions, reaching the target of an average carbide size of less than 1 μm; further increasing the grain size of the matrix structure to further refine the grain size of the bearing steel.
The third is to reduce the low-fold tissue defects: further reduce the center looseness in the bearing steel, the center shrinkage hole and the center component segregation, and improve the uniformity of the low-fold structure.
The fourth is 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.