Japanese bearing steel research and development
The role of rolling bearing steel in automobiles and various industrial machinery is to smooth the rotary motion and reduce power loss. Since the bearing often works in harsh environments, the raw material bearing steel of the bearing is one of the most stringent steels in the special steel. Bearing steel requirements are high hardness, high static, dynamic strength and high toughness microstructure, and internal defects as a source of rolling fatigue. In addition, in the long-term use of bearings, it is also important that no structural changes and dimensional changes occur under various stresses.
The most used in bearing steel is JIS SUJ2 for betting high carbon chromium bearing steel. Some types of bearings also use carburized steel and high frequency quenched medium carbon steel, but in terms of superb, bearing steel generally refers to high carbon chromium. Bearing steel.
Development of high carbon chromium bearing steel
High carbon chromium bearing steel as special steel has four steel grades in JIS G 4805 (2008): SUJ2-SUJ5. Due to the different hardenability of each steel grade, it is used for bearings of different specifications. Among them, SUJ2 accounts for more than 90% of the output. Although its composition series is relatively simple, it is 1% C-1.5%Cr, but it can satisfy various bearings. Characteristic requirements, but also has the advantage of making production stable and efficient. Therefore, the basic composition of 1% C-1.5% Cr bearing steel has not changed since the development in Europe for a century, and it has been used up to now.
Of course, the quality of bearing steel has improved greatly during this period. In the late 1950s, it was known that the service life of bearings was greatly affected by non-metallic inclusions such as oxides in steel. Therefore, the oxygen content in the steel is used as the quality index of the bearing steel, and efforts are made to improve the cleanliness of the steel. It can be seen from the relevant data that the rolling fatigue life of the bearing steel is greatly improved due to the use of refining equipment such as vacuum degassing and the improvement of operating conditions, and the rolling fatigue life can be several tens of times that of the non-degassing steel.
In recent years, with increasing attention to environmental issues, improving vehicle fuel efficiency has become an important issue at any time in the past. The small size and weight of mechanical equipment as a whole have a great effect on improving fuel efficiency, so the small size and weight reduction of bearing components is an inevitable trend. For this reason, bearing steels of higher cleanliness have been developed, and the composition, morphology and distribution of non-metallic inclusions have been basically controlled in low oxygen content.
At the same time as the development of sorghum clarity bearing steel, the method for evaluating the cleanliness of steel has also developed. The conventional JIS evaluation method (JIS G 0555-2003) and the ASTM evaluation method (ASTM E45-05) have not been able to evaluate the advantages and disadvantages of today's bearing steel. The maximum non-metallic inclusion size prediction value in the specified area obtained by the combination of optical microscopy and extremum statistics can be used not only as an indicator of cleanliness, but also as an indicator for judging fatigue life. A method for predicting the maximum size of non-metallic inclusions using extreme value statistics is also specified in the ASTM standard in 2008 (ASTM E 2283-08). Large non-metallic inclusions that cause sudden short-lived accidents are rarely present in steel, so they are difficult to find with optical microscopes. In this case, it is effective to use an ultrasonic flaw detection method capable of performing large volume detection. In addition, inspection methods suitable for bearings requiring higher reliability are under development.
Development direction of high carbon chromium bearing steel
This year, the use environment of bearing steel will change greatly. The requirements for bearing steel will be polarized, that is, bearing steel with high reliability and bearing steel with low demand. In the case of high-reliability bearing steel, further measures should be taken to improve the fatigue life with non-metallic inclusions as indicators. For this reason, the basic observational research on the rolling fatigue mechanism is now carried out using the latest observation and analysis equipment and the latest experimental methods. Through joint research with bearing users, based on the new concept, a new type of bearing steel with environmentally friendly and international competitiveness will be developed.