Bearing steel is mainly used to manufacture rolling elements and ferrules for rolling bearings.
Since the bearing should have long life, high precision, low heat generation, high speed, high rigidity, low noise, high wear resistance, etc., it is required that the bearing steel should have: high hardness, uniform hardness, high elastic limit, high contact fatigue Strength, necessary toughness, certain hardenability, and corrosion resistance in atmospheric lubricants. In order to achieve the above performance requirements, the chemical composition uniformity of the bearing steel, the content and type of non-metallic inclusions, the particle size and distribution of carbides, and decarburization are strictly required.
Bearing steels generally develop in the direction of high quality, high performance and many varieties. The bearing steel is divided into high carbon chromium bearing steel, carburized bearing steel, high temperature bearing steel, stainless bearing steel and special special bearing materials according to the characteristics and application environment.
In order to meet the requirements of high temperature, high speed, high load, corrosion resistance and radiation resistance, it is necessary to develop a series of new bearing steels with special properties. In order to reduce the oxygen content of bearing steel, the smelting technology of bearing steel such as vacuum smelting, electroslag remelting and electron beam remelting has been developed. The smelting of large-volume bearing steels is smelted by electric arc furnaces and developed into various types of primary refining furnaces and refining outside the furnace.
At present, the bearing steel is produced by the primary furnace + LF / VD or RH + continuous casting + continuous rolling process with a capacity of more than 60 tons to achieve high quality, high efficiency and low energy consumption. In the heat treatment process, the car bottom furnace and the hood furnace are developed into a continuous controlled atmosphere annealing furnace for heat treatment. At present, the continuous heat treatment furnace has a maximum length of 150 m, and the spheroidized structure of the processed bearing steel is stable and uniform, the decarburization layer is small, and the energy consumption is low.
Since the 1970s, with the development of economy and industrial technology, the application range of bearings has expanded; and the development of international trade has promoted the development and application of bearing steel standard internationalization and new technologies, new processes and new equipment. High-quality, high-quality, low-cost supporting technologies and process equipment came into being. Both Japan and Germany have built high-purity, high-quality bearing steel production lines, which have led to a rapid increase in steel production and a significant increase in steel quality and fatigue life. The bearing steel produced in Japan and Sweden has an oxygen content of less than 10 ppm. In the late 1980s, the advanced level of Japan's Shanyang Special Steel Co., Ltd. was 5.4ppm, reaching the level of vacuum remelted bearing steel.
The contact fatigue life of the bearing is very sensitive to the uniformity of the steel structure. Improve the cleanliness (reduce the content of impurities and inclusions in the steel), promote the uniform distribution of non-metallic inclusions and carbides in the steel, and improve the contact fatigue life of the bearing steel. The structure of the bearing steel in use should be a uniform distribution of fine carbide particles on the tempered martensite matrix. Such a structure can impart the required properties to the bearing steel. The main alloying elements in high carbon bearing steel are carbon, chromium, silicon, manganese, vanadium and the like.
How to obtain spheroidized structure is an important issue in the production of bearing steel. Controlled rolling and controlled cooling is an important production process for advanced bearing steel. The meshed carbide is eliminated by controlled rolling or rapid cooling after rolling, and a suitable preparatory structure is obtained, which can shorten the spheroidizing annealing time of the bearing steel, refine the carbide and improve the fatigue life.
In recent years, Russia and Japan have adopted low-temperature controlled rolling (800 ° C ~ 850 ° C or less), after rolling using air cooling plus short time annealing, or completely cancel the spheroidizing annealing process, you can get qualified bearing steel structure. 650 ° C temperature processing of bearing steel is also a new technology. If the eutectoid steel or high carbon steel has fine grain structure before hot working or can form fine grains during processing, it will exhibit superplasticity at a certain strain rate in the melting range of (0.4~0.6). .
The US Naval Research Institute (NSP) conducted a 650 ° C temperature processing test on 5 2100 steel, which showed that the true strain 2.5 did not break at 650 ° C. Therefore, it is possible to replace the high temperature processing with a 650 ° C temperature processing and combine it with the spheroidizing annealing process, which is important for simplifying equipment and processes, saving energy, and improving quality.
In terms of heat treatment, progress has been made in improving the quality of spheroidizing annealing, obtaining fine, uniform, spherical carbides, and shortening the annealing time or eliminating the spheroidizing annealing process, that is, the wire rod production adopts two tissue annealing, which will be drawn. After the 720 ° C ~ 730 ° C recrystallization annealing to 760 ° C tissue annealing. In this way, a structure with low hardness, good spheroidization and no reticulated carbide can be obtained, and the key is to ensure that the intermediate drawing reduction rate is ≥14%. This process increases the efficiency of the heat treatment furnace by 25% to 30%. Continuous spheroidizing annealing heat treatment technology is the development direction of bearing steel heat treatment.
Countries are researching and developing new bearing steels, expanding applications and replacing traditional bearing steels. For example, fast carburizing bearing steel increases the carburizing speed by changing the chemical composition, wherein the carbon content is increased from the conventional 0.08% to 0.20% to about 0.45%, and the carburizing time is shortened from 7 hours to 30 minutes. The high-frequency quenching bearing steel was developed, and ordinary medium carbon steel or medium carbon manganese and chrome steel were used to replace the ordinary bearing steel by high-frequency heating quenching, which not only simplifies the production process but also reduces the cost and improves the service life.
The fatigue life of GCr465 and SCM465 developed in Japan is 2~4 times higher than that of SUJ-2. Due to the increasing use of bearings in high temperature, corrosive and lubricated environments, the bearing steels such as M50 (CrMo4V) and 440C (9Cr18Mo) ​​used in the past can no longer meet the requirements of use. It is urgent to develop good processing performance, low cost and fatigue. Bearing steel with long life and suitable for different purposes and applications, such as high temperature carburized steel M50NiL, easy to process stainless bearing steel 50X18M and ceramic bearing materials.
Aiming at the weak point of low hardenability of GCr15SiMn steel, China has developed high hardenability and hardenability bearing steel GCr15SiMo, which has hardenability HRC≥60 and hardenability J60≥25mm. The contact fatigue life L10 and L50 of GCr15SiMo are 73% and 68% higher than that of GCr15Si Mn, respectively. Under the same conditions of use, the bearing life of G015SiMo steel is twice that of GCr15SiMo steel.
In recent years, China has also developed GCr4 bearing steel that can save energy, save resources and resist impact. Compared with GCr15, the impact value of GCr4 increased by 66%~104%, the fracture toughness increased by 67%, and the contact fatigue life L10 increased by 12%. GCr4 steel bearings use high temperature heating - surface quenching heat treatment process. Compared with fully hardened GCr15 steel bearings, the life of GCr4 steel bearings is significantly improved and can be used for heavy-duty high-speed train bearings.
In the future, bearing steel will mainly develop in two directions: high cleanliness and diversified performance. Increasing the cleanliness of the bearing steel, especially reducing the oxygen content in the steel, can significantly extend the life of the bearing. The oxygen content is reduced from 28 ppm to 5 ppm and the fatigue life can be extended by an order of magnitude. In order to extend the life of bearing steel, people have been developing refining technology for many years to reduce the oxygen content in steel.
Through unremitting efforts, the minimum oxygen content in bearing steel has been reduced from 28 ppm in the 1960s to 5 ppm in the 1990s. At present, China can control the minimum oxygen content in bearing steel to about 10ppm. Changes in the environment in which the bearing is used require that the bearing steel must have a variety of properties. If the speed of the equipment is increased, the bearing steel for quasi-high temperature (below 200 °C) is required (usually the method of increasing Si content and adding V and Nb on the basis of SUJ2 steel to achieve softening and stable size); corrosion application It is necessary to develop stainless bearing steel; in order to simplify the process, high-frequency hardened bearing steel and short-time carburized bearing steel should be developed; in order to meet the needs of aerospace, high-temperature bearing steel should be developed.
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