Bearing steel is used to make balls,rollers and bearing rings.Bearing bears great pressure and friction when working,so bearing steel is required to have high and uniform hardness,wear resistance and high elastic limit.The requirements for the uniformity of chemical composition,the content and distribution of non-metallic inclusions,and the distribution of carbides of bearing steel are very strict.It is one of the steel grades with strict requirements in all steel production.In 1976,the International Organization for Standardization(ISO)incorporated some general bearing steel grades into the international standards,and divided the bearing steel into four types,namely,fully hardened bearing steel,surface hardened bearing steel,stainless bearing steel,and high temperature bearing steel,with 17 steel grades.Some countries add a category of bearing steel or alloy for special purposes.The classification method of bearing steel that has been included in the standard in China is similar to that of ISO,which corresponds to four categories:high carbon chromium bearing steel,carburized bearing steel,stainless corrosion resistant bearing steel and high temperature bearing steel.In the past 50 years,China has also made great progress in bearing steel grades and bearing materials,such as chrome-free bearing steel,medium-carbon bearing steel,special purpose bearing steel and alloy,and cermet.High-carbon chromium bearing steel GCr15 is the largest bearing steel in the world,with a carbon content of about 1%and a chromium content of about 1.5%.Since its birth in 1901,its main composition has remained unchanged for more than 100 years.With the progress of science and technology,the research work continues,and the product quality continues to improve,accounting for more than 80%of the world's total bearing steel production.

　　So now,if there is no special description for the bearing steel,it means GCr15.Bearing steel is mainly used for manufacturing rolling bodies and rings of rolling bearings.Since the bearing should have the characteristics of 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,corrosion resistance in atmospheric lubricants.In order to meet the above performance requirements,the uniformity of chemical composition,the content and type of non-metallic inclusions,the size and distribution of carbides,and decarburization of bearing steel are strictly required.Bearing steel is generally developing towards high quality,high performance and multiple varieties.Bearing steel is divided into high carbon chromium bearing steel,carburized bearing steel,high temperature bearing steel,stainless bearing steel and special bearing materials according to its 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,vacuum smelting,electroslag remelting,electron beam remelting and other smelting technologies of bearing steel have been developed.The smelting of large quantities of bearing steel has developed from electric arc furnace smelting to various types of primary smelting furnace plus external refining.At present,bearing steel is produced by the process of primary smelting furnace+LF/VD or RH+continuous casting+continuous rolling with a capacity of more than 60 tons,in order to achieve the goal of high quality,high efficiency and low energy consumption.In terms of heat treatment process,the car bottom furnace and bell furnace have been developed into continuous controllable atmosphere annealing furnace for heat treatment.At present,the continuous heat treatment furnace type**is 150m long,the spheroidized structure of bearing steel processed and produced 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;The development of international trade has also promoted the internationalization of bearing steel standards and the development and application of new technologies,new processes and new equipment.Supporting technologies and process equipment with high efficiency,high quality and low cost have emerged as the times require.Japan and Germany have built bearing steel production lines with high cleanliness and high quality,which has rapidly increased the output of steel and greatly improved the quality and fatigue life of steel.The oxygen content of bearing steel produced in Japan and Sweden decreased to below 10 ppm.At the end of the 1980s,the advanced level of Japan's Shanyang Special Steel Company was 5.4 ppm,reaching the level of vacuum remelting bearing steel.The contact fatigue life of bearing is very sensitive to the uniformity of steel structure.The contact fatigue life of bearing steel can be improved by improving the cleanliness(reducing the content of impurity elements and inclusions in the steel)and promoting the fine and uniform distribution of non-metallic inclusions and carbides in the steel.The microstructure of bearing steel under service condition should be that fine carbide particles are uniformly distributed on the tempered martensite matrix,which can endow the bearing steel with the required properties.

　　The main alloying elements in high carbon bearing steel include carbon,chromium,silicon,manganese,vanadium,etc.How to obtain spheroidized structure is an important problem in the production of bearing steel,and controlled rolling and controlled cooling is an important production process of advanced bearing steel.The network carbide is eliminated by controlled rolling or rapid cooling after rolling,and the proper preparation structure is obtained,which can shorten the spheroidizing annealing time of bearing steel,refine the carbide and improve the fatigue life.In recent years,Russia and Japan have adopted low temperature controlled rolling(below 800℃~850℃),followed by air cooling and short time annealing,or completely eliminating the spheroidizing annealing process,to obtain qualified bearing steel structure.The processing of bearing steel at 650℃is also a new technology.If eutectoid steel or high carbon steel has fine grain structure before hot processing or can form fine grains during processing,it will show superplasticity at a certain strain rate within the melting temperature range of(0.4~0.6).The United States Naval Research Institute(NSP)conducted a 650℃warm working test on 52100 steel,which showed that the true strain 2.5 did not break at 650℃.Therefore,it is possible to replace high temperature processing with 650℃temperature processing and combine it with spheroidizing annealing process,which is of great significance to simplify equipment and procedures,save energy and improve quality.In the aspect of heat treatment,progress has been made in improving the quality of spheroidizing annealing,obtaining fine,uniform and spherical carbides,shortening the annealing time or canceling the spheroidizing annealing process.That is,the wire rod production adopts two times of structure annealing,changing the recrystallization annealing at 720℃~730℃after drawing to the structure annealing at 760℃.In this way,the structure with low hardness,good spheroidization and no network carbide can be obtained.The key is to ensure that the reduction ratio of intermediate drawing is≥14%.This process improves the efficiency of heat treatment furnace by 25%~30%.Continuous spheroidizing annealing heat treatment technology is the development direction of bearing steel heat treatment.

　　All countries are researching and developing new bearing steel,expanding the application and replacing the traditional bearing steel.For example,the rapid carburizing bearing steel can improve the carburizing speed by changing the chemical composition,in which the carbon content is increased from 0.08%to 0.20%to about 0.45%,and the carburizing time is shortened from 7 hours to 30 minutes.High frequency quenched bearing steel has been developed.Ordinary medium carbon steel or medium carbon manganese and chromium steel has been used to replace 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 ratio of GCr465 and SCM465 developed in Japan is SUJ?2.2-4 times higher.As more and more bearings are used in the environment of high temperature,corrosion and poor lubrication conditions,the bearing steel M50(CrMo4V),440C(9Cr18Mo)and other bearing steels used in the past can no longer meet the use requirements.It is urgent to develop bearing steels with good processing performance,low cost,long fatigue life,and suitable for different purposes and purposes,such as high-temperature carburized steel M50NiL,easy-to-process stainless bearing steel 50X18M,and ceramic bearing materials.In view of the weakness of low hardenability of GCr15SiMn steel,China has developed high hardenability and hardenability bearing steel GCr15SiMo,with hardenability HRC≥60 and hardenability J60≥25mm.The contact fatigue life L10 and L50 of GCr15SiMo are 73%and 68%higher than that of GCr15SiMn,respectively.Under the same service conditions,the service life of bearings made of G015SiMo steel is twice that of GCr15SiMo steel.In recent years,China has also developed GCr4 bearing steel which can save energy,resources and impact.Compared with GCr15,the impact value of GCr4 is increased by 66%~104%,the fracture toughness is increased by 67%,and the contact fatigue life L10 is increased by 12%.GCr4 steel bearing adopts high temperature heating?Surface quenching heat treatment process.Compared with the fully hardened GCr15 steel bearing,the service life of GCr4 steel bearing is significantly improved,and it can be used for the bearing of heavy-load high-speed trains.In the future,bearing steel will mainly develop in the direction of high cleanliness and performance diversification.Improving the cleanliness of bearing steel,especially reducing the oxygen content in steel,can significantly prolong the bearing life.When the oxygen content is reduced from 28 ppm to 5 ppm,the fatigue life can be extended by one order of magnitude.In order to prolong the service life of bearing steel,people have been committed to developing and applying refining technology to reduce the oxygen content in steel for many years.Through unremitting efforts,the low 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 low oxygen content of***in bearing steel to about 10ppm.The change of bearing service environment requires that the bearing steel must have a variety of properties.If the speed of the equipment is increased,it is necessary to use bearing steel(below 200℃)at quasi-high temperature(usually the method of increasing Si content and adding V and Nb on the basis of SUJ2 steel to achieve the purpose of softening resistance and size stability);Corrosive applications require the development of stainless bearing steel;In order to simplify the process,high frequency quenched 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.Bearing steel is used to make balls,rollers and bearing rings.Bearing bears great pressure and friction when working,so bearing steel is required to have high and uniform hardness,wear resistance and high elastic limit.The requirements for the uniformity of chemical composition,the content and distribution of non-metallic inclusions,and the distribution of carbides of bearing steel are very strict.It is one of the steel grades with strict requirements in all steel production.In 1976,the International Organization for Standardization(ISO)incorporated some general bearing steel grades into the international standards,and divided the bearing steel into four types,namely,fully hardened bearing steel,surface hardened bearing steel,stainless bearing steel,and high temperature bearing steel,with 17 steel grades.Some countries add a category of bearing steel or alloy for special purposes.The classification method of bearing steel that has been included in the standard in China is similar to that of ISO,which corresponds to four categories:high carbon chromium bearing steel,carburized bearing steel,stainless corrosion resistant bearing steel and high temperature bearing steel.In the past 50 years,China has also made great progress in bearing steel grades and bearing materials,such as chrome-free bearing steel,medium-carbon bearing steel,special purpose bearing steel and alloy,and cermet.High-carbon chromium bearing steel GCr15 is the largest bearing steel in the world,with a carbon content of about 1%and a chromium content of about 1.5%.Since its birth in 1901,its main composition has remained unchanged for more than 100 years.With the progress of science and technology,the research work continues,and the product quality continues to improve,accounting for more than 80%of the world's total bearing steel production.So if there is no special description for the bearing steel,it means GCr15.