Special steel plays a crucial role in the manufacturing of major equipment and national infrastructure projects. It is a high-tech material within the iron and steel industry, and its production and application reflect a country's level of industrial development. Currently, bearing steel faces significant challenges. Bearings are widely used in mining machinery, precision tools, metallurgical equipment, heavy machinery, high-end cars, wind power generation, high-speed rail, and aerospace. However, Chinese bearings mainly occupy the mid-to-low end market, with small and medium-sized bearings being abundant but high-end bearings lacking. Compared to foreign counterparts, there is a noticeable gap in high-end and large bearings. For example, the bearings used in China’s high-speed rail passenger trains must be imported. In critical applications such as aerospace, high-speed railways, and premium automobiles, Chinese bearings lag behind in terms of service life, reliability, Dn value, and load capacity. While foreign automotive transmission bearings can last at least 500,000 kilometers, domestic ones only reach about 100,000 km, with poor stability and reliability.
In the automotive sector, first and second-generation hub bearings (ball bearings) are commonly used in China, while third-generation bearings are more prevalent in Europe. These third-generation bearings offer better reliability, compact design, ease of installation, and no need for adjustment. Most imported vehicles in China use these lightweight and integrated wheel bearings. In railway systems, China uses domestically produced electroslag remelted G20CrNi2MoA carburized steel for heavy-duty train bearings. However, foreign countries have developed advanced technologies such as vacuum degassing, inclusion homogenization, ultra-pure bearing steel (EP steel), ultra-long-life steel (TF steel), fine heat treatment, surface super-hardening, and advanced sealing and lubrication techniques. These technologies significantly improve bearing life and reliability. Chinese electroslag bearing steel is not only of lower quality but also costs 2,000–3,000 yuan/ton more than vacuum-degassed steel. Future development should focus on producing ultra-high-purity, fine-quality, stable-bearing steel to replace current electroslag steel.
For wind turbines, China still cannot produce high-tech spindle and speed-increasing bearings, relying largely on imports. To enhance the strength, toughness, and lifespan of wind turbine bearings, foreign manufacturers use special heat-treated steels like SHX (40CrSiMo). Surface induction hardening controls the hardened layer depth, surface hardness, and soft ribbon width to prevent cracks. Carbonitriding improves residual austenite content and carbide distribution, enhancing performance under contaminated lubrication. For rolling mill bearings, future research should focus on high-purity vacuum-degassed steels like GCr15SiMn and G20Cr2Ni4, along with advanced surface heat treatments. Japanese companies like NSK and NTN have developed surface austenitic strengthening technologies, increasing bearing life by 6–10 times.
Looking ahead, the development of bearing steel in China will focus on four areas: improving economic cleanliness by reducing oxygen and titanium content; refining and homogenizing microstructure to reduce carbide size and increase grain refinement; minimizing low-fold organizational defects; and enhancing toughness through alloying and heat treatment optimization.
Spring steel is primarily used in the automotive and railway industries. China currently faces a surplus of mid-range products but lacks high-end and specialized varieties. Cleanliness, fatigue resistance, surface quality, and stability are all areas where Chinese spring steel lags behind foreign counterparts. High-grade spring steels, such as those for luxury car suspension springs and valve springs, are still heavily reliant on imports. Recent advancements in oxide metallurgy and rare earth elements aim to refine grain structure and improve strength and toughness. The automotive industry increasingly demands higher-strength suspension springs, with design stresses reaching up to 1200 MPa. Shot peening is becoming a popular alternative to costly surface hardening treatments, offering improved fatigue strength and reduced surface defect impact. Future trends involve developing high-strength, corrosion-resistant, and fatigue-resistant spring steels tailored for specific applications.
Gear steel must withstand various stresses, including impact, contact, bending, and friction, over long periods. It requires high toughness, fatigue strength, and wear resistance. Compared to Japanese, German, and American gear steels, China’s gear steels face issues such as wider hardenability ranges, lower purity, uneven non-metallic inclusions, and less stable grain sizes. To address these, new heat treatment processes like vacuum carburizing and rare earth carburizing are being explored. Alloy design that reduces easily oxidized elements like silicon and increases nickel and molybdenum content can help minimize grain boundary oxidation. Future developments should focus on narrow hardenability, ultra-low oxygen, low-oxidation, ultra-fine grain, high-temperature resistant, easy-cutting, and cold-forged gear steels to meet the needs of the automotive industry’s lightweight and high-performance demands.
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