Abstract: The continuous development of industrial manufacturing in China in recent years has posed new challenges to the R&D and manufacturing of bearing steel in China. The R&D and production of high-quality bearing steel has become one of the urgent problems to be solved in the bearing industry in China. The research progress of bearing steel at home and abroad is comprehensively discussed from the aspects of the classification of bearing steel, the main influencing factors of its properties and the development of heat treatment technology, and the methods of process optimization, the control of inclusions and carbides, the lack of testing equipment and technical evaluation indexes to be solved in the development of domestic high quality bearing steel are put forward. In order to realize the quantitative production and application of high quality bearing steel in China.

Key words: bearing steel; Quality control; Inclusion; Carbide; Heat treatment; Development trend

With the rapid development of the current society, bearing steel has been widely used in industrial machinery, equipment manufacturing, transportation, aerospace and other fields. After decades of development, the domestic bearing and bearing steel manufacturing industry has gradually formed a relatively perfect industrial system. The domestic research and development and production of bearings and bearing steel, as well as the steady improvement of the quality of bearing steel products, make China gradually become a major bearing manufacturing country in the world, and steadily progress towards the goal of becoming a powerful manufacturing country. The annual sales volume of the bearing industry in China has reached more than 200 billion yuan, and the annual growth rate is 12% ~ 15%. Bearing steel products produced by some large domestic steel enterprises, such as Xingcheng Special Steel and Baowu Steel, have been recognized by SKF, FAG and other famous foreign bearing steel production enterprises. Although the domestic bearing industry has developed rapidly in recent years, the problems still cannot be ignored. The bearing steel produced in China is mainly concentrated in the middle and low-end bearing products, and there is even excess capacity in the low-end bearing field. The aerospace, high-speed railway and other fields require bearings to have good strength and toughness, quality and stability, and longer service life. At present, it is difficult to make an obvious breakthrough in the research and development of high-end bearings in China, so that the domestic high-quality bearings are heavily dependent on imports, which has become one of the main factors restricting the development of China's high-end equipment manufacturing and strategic emerging industries. At present, the research and development level of China's high-end bearing steel products has a large gap with that of Sweden, Germany, Japan and other bearing manufacturing powers. Factors such as low cleanliness of bearing steel, unstable oxide inclusion, carbide and uneven distribution greatly reduce the life and reliability of domestic bearing steel, and seriously affect the development of domestic high-end equipment manufacturing and strategic emerging industries. Therefore, the development of high quality bearing steel is currently one of the key research points of China's scientific research institutes, universities and bearing steel manufacturing industry. This paper mainly discusses the development status of bearing steel in recent years from the aspects of bearing steel classification, performance influencing factors, heat treatment technology and development direction.

1. Classification of bearing steel

The main role of bearings in industrial equipment is to support the rotating body, reduce its friction factor in the process of equipment operation, to ensure the running accuracy of mechanical equipment, so the working state of bearings is generally line contact or point contact. Due to the small contact area, the bearing needs to withstand 1500 ~ 5000MPa compressive stress during operation. The working environment of bearing puts forward higher requirements on the strength toughness, wear resistance and service life of bearing steel, which makes bearing steel become one of the most stringent steel in all alloy steel production requirements. In 1976, the International Organization for Standardization ISO divided bearing steel into surface hardened bearing steel, fully quenched bearing steel, high temperature bearing steel and stainless bearing steel and other four categories. The bearing steel classification in China is basically the same as the ISO classification, which is respectively high carbon chromium bearing steel, carburized bearing steel, high temperature bearing steel and stainless bearing steel. The corresponding standard number is GB/T18254, GB/T3203, GB/T1205, GB/T3086 and YB/T688.

1.1 High carbon chromium bearing steel

GCr15 high carbon chromium bearing steel as the world's first generation of bearing steel, after more than 100 years of development, its chemical composition has not changed. According to statistics, the output of this kind of steel still accounts for more than 80% of the total annual output of bearing steel. However, the increase of bearing design size and bearing load require higher strength, better dimensional stability and longer fatigue life of bearing steel.

Because a certain amount of Si and Mn elements are added to the bearing steel, the hardenability of the steel is improved, the strength and hardness of the bearing steel are improved, and the wear resistance of the steel is effectively improved. When Mo co-exists with Cr and Mn elements, it can improve the tempering brittleness of steel and enhance the stability of carbide, which is very important for the improvement of the performance of bearing steel. Therefore, high carbon chromium bearing steel represented by chrome-silico-molybdenum, chrome-mann-molybdenum bearing steel is gradually formed. At present, most of the high-carbon chromium bearing steels widely used in various fields are obtained by further optimizing the alloying element content on the basis of GCr15. The brands of high-carbon chromium bearing steels and their main chemical compositions are shown in Table 1.

1.2 Carburized bearing steel

The high carbon content of high-carbon chromium bearing steel reduces the impact toughness of steel. Because the impact resistance and toughness of bearing steel have higher requirements on the working environment, carburized bearing steel is developed on the basis of low-alloy structural steel. Reduce the carbon content of bearing steel, carburize the steel surface, and finally develop the carburized bearing steel with higher surface hardness and good wear resistance, and the steel core with good toughness. The representative steel types of carburized bearing steel are G20CrMo, G20CrNiMo, G20CrNi2Mo, G20Cr2Ni4, G20Cr2Mn2Mo, etc.

Zhao Guodian et al. studied the influence of heat treatment process on the microstructure and properties of carburized bearing steel, and the results showed that the gradient distribution of carbon content from the surface to the core of steel directly affects the gradient distribution of steel hardness. Due to the difference in the specific volume of austenite and martensite, compressive stress exists on the surface of the carburized layer, which improves the fatigue life of steel. FIG. 1 shows the morphology of carburizing layer of bearing steel after carburizing heat treatment.

Zhang Fucheng's research team of Yanshan University developed a kind of carburized nano-bainite bearing steel. The surface structure of the steel is shown in Figure 2. The surface structure of the steel is high-carbon nano-bainite, and the core structure is low-carbon martensite. The experimental results show that the contact fatigue life of the nano-bainitic carburized steel is more than twice as long as that of the traditional martensitic carburized steel, which has a good application prospect in the field of carburized bearing steel.

1.3 Stainless bearing steel

Typical stainless bearing steels include martensitic stainless steel (such as 9Cr18, 9Cr18Mo, Cr15Mo4), austenitic stainless steel (such as 1Cr18Ni9Ti), etc. The brands and chemical compositions of the main high-carbon chromium stainless bearing steels at home and abroad are shown in Table 2. 9Cr18, 9Cr18Mo and other high carbon high chromium martensitic stainless steel has high hardness and wear resistance, at the same time, in the Marine environment, some acid solution, salt solution has excellent corrosion resistance, often used in the use of corrosion resistance and wear resistance requirements. Austenitic stainless bearing steel is often used in bearing parts that require higher corrosion resistance but lower impact load. The steel shows good corrosion resistance in most working environments, but because of the characteristics of austenitic structure, the bearing steel cannot be applied in working environments with higher requirements for strength load and wear resistance.

In recent years, researchers have developed a kind of stainless bearing steel with high corrosion resistance G30Cr15MoN, and compared the microstructure and properties of martensitic stainless steel 440C and stainless bearing steel G30Cr15MoN. The SEM microstructure of G30Cr15MoN and 440C stainless bearing steel is shown in FIG. 3. The research shows that there is composite strengthening between the fine dispersed carbonitride Cr2(C,N) and M23C6 carbide in G30Cr15MoN, which makes G30Cr15MoN steel with excellent performance of high hardness and corrosion resistance.

1.4 High temperature bearing steel

The working temperature of high temperature bearing steel is generally above 150℃. With the continuous improvement of the power of large bearing equipment such as aeroengines and gas turbines, the working temperature of bearings can reach above 350 ℃, which requires high temperature bearing steel to have high strength, high hardness, oxidation resistance, dimensional stability and corrosion resistance in high temperature environment.

In recent years, in order to meet the performance requirements of bearing steel under high temperature environment, high-speed steel has been gradually applied to the research and development of bearing steel. For example, 8Cr4Mo4V is a kind of high-speed steel with a certain amount of Mo and V elements added. Mo element can improve the hardness at high temperature, improve the wear resistance and corrosion resistance of steel; V is a strong carbide forming element, VC carbide formed with C in steel can effectively improve the wear resistance of steel; At the same time, W can also improve the hardenability and wear resistance of steel. Based on the isothermal quenching process of high temperature bearing steel 8Cr4Mo4V bainite, Zhao Kaili et al. conducted solution treatment at different temperatures on 8Cr4Mo4V steel, followed by salt bath treatment at 260℃, and studied and analyzed the influence of different solution temperatures on the microstructure transformation of 8Cr4Mo4V steel. The bainite microstructure of 8Cr4Mo4V high temperature bearing steel treated at different solution temperatures and tempered is shown in Figure 4. The results show that, at the solution temperatures of 1050 and 1065℃, the bainite microstructure in 8Cr4Mo4V steel is finer and more uniform, and its impact toughness is also better.

2. Influencing factors of bearing steel properties

2.1 Cleanliness

The mass fraction of N, H and O in steel is an important index to measure the cleanliness of bearing steel. Nitrogen mainly exists in the form of TiN and AlN, oxygen generally exists in the form of oxide inclusion, and hydrogen is easy to cause hydrogen embrittlement and internal cracks in steel. In order to facilitate the analysis of the influence of inclusions on steel, the inclusions are usually divided into A -- sulfide, B -- Al2O3, C -- silicate, D -- spot (spherical) non-deforming inclusions. The existence of inclusions damages the continuity and uniformity of steel matrix structure. Under the influence of alternating loads, the parts with inclusions are prone to stress concentration, leading to fatigue spalling of bearings. When the stress concentration is more serious, micro-cracks will be generated, and the continuous expansion of cracks will lead to bearing failure.

Oxide inclusion is the most harmful defect of non-metallic inclusion to the fatigue life of bearings. The increase of oxygen mass fraction in steel leads to the obvious increase of oxide inclusion. The quantity and size of oxide inclusion are important factors affecting the fatigue life of bearings. Japan Shanyang Special Steel, Sweden SKF Company and other famous bearing manufacturing enterprises in recent years to improve the smelting process, the quality fraction of oxygen in steel has been stable control at 0.0004% ~ 0.0006%, the quality stability and service life of products have been greatly improved. In recent years, although the domestic control of oxygen mass fraction in steel has gradually approached the European and American standards, due to the large fluctuation of oxygen mass fraction in steel, the quality stability of the product is insufficient. In addition, because Ti is a strong carbon and nitride forming element, if not properly controlled, Ti is easy to form titanium nitride and titanium carbon nitride inclusions with C, N, O and other elements in steel, seriously reducing the fatigue life of bearing steel. Bearing steel manufacturers at home and abroad have strict requirements on the control of Ti quality fraction in steel. SKF production standards stipulate that the residual Ti quality fraction in bearing steel should not be higher than 0.0012%, and Shanyang Special Steel requires that the Ti quality fraction should be controlled to 0.0014 ~ 0.0015%.

2.2 Carbide and microstructure defects

The influence of carbide size and uniformity on bearing fatigue life is more significant with the improvement of bearing steel cleanliness. The results show that the larger the size and the lower the uniformity of carbide in steel, the greater the probability of bearing failure. The lower the content of carbide, the longer the fatigue life of bearing. Bearing steel contains more alloy carbide forming elements, such as Mn, Cr, Mo and other elements, which is easy to cause the segregation of carbide, resulting in uneven distribution of carbide, and thus reducing the fatigue life of bearing. The existence forms of carbides in steel mainly include three kinds: carbides liquid out, network carbides and banded carbides. With the improvement of smelting quality, carbide liquid out is eliminated. The network carbide needs to be eliminated by a certain hot rolling process (such as controlled rolling and cooling). For medium carbon bearing steel or carburized bearing steel, because of its low carbon content, it is mainly controlled by strip carbide. Spheroidizing treatment is needed to refine carbide and improve the uniformity of carbide in steel.

In addition to the fine control of carbide in steel, high-end bearing steel also has extremely strict requirements for low microstructure, and the microstructure defects such as loose center, shrinkage cavity and segregation need to be controlled at an extremely low level to achieve the product quality standard of high-quality bearing steel. At present, there is a large gap between domestic bearing steel production enterprises and Sweden, Japan and other bearing manufacturing power in terms of the control of organizational defects, dimensional accuracy and so on. While controlling the size and uniformity of carbide and inclusions, it is also necessary to strictly control the loose level to avoid the appearance of organizational defects such as white spots and shrinkage holes in the organization, so as to improve the stability of product quality.

2.3 Smelting process

In recent years, with the continuous improvement of smelting process, the bearing quality of domestic large bearing steel manufacturing enterprises such as Xingcheng and Daye has gradually been recognized by foreign advanced bearing enterprises. The current domestic smelting process has gradually been in line with the international advanced special steel production enterprises, which generally adopts the smelting process flow of scrap steel + blast furnace hot metal → ultra-high power arc furnace smelting →LF refining →VD/RH vacuum degassing →IC/CC. The smelting process flow of domestic advanced bearing steel production enterprises is shown in Table 3.

Based on the current smelting process and advanced smelting equipment, the oxygen mass fraction of domestic bearing steel can be basically controlled as 0.0006%-0.0008%, and the titanium mass fraction control of some advanced steel enterprises can even reach 0.0015%, and the control level of inclusions in steel has been significantly improved. However, the quality and stability of domestic bearing steel products cannot be guaranteed due to the refined control in the smelting process, such as the lack of composition and process parameter control and the lack of detection technology. The performance of steel produced in different batches differs greatly, especially the quality of high-end bearing steel products cannot be effectively guaranteed. Compared with the international manufacturing level, there is still a large room for improvement in the distribution of carbide in bearing steel and the control of low structure defects. In view of this, foreign advanced smelting processes such as vacuum induction + vacuum self-consumption (VIM+VAR) can be introduced in China to improve the cleanliness of bearing steel and the size and distribution uniformity of inclusions and carbides in steel.

3.Heat treatment process

The improvement of heat treatment process can not only refine grain, but also refine carbide and improve the uniformity of carbide distribution. The high hardness martensite in traditional high carbon chromium bearing steel makes the steel have good wear resistance and excellent contact fatigue resistance. However, due to poor toughness, residual tensile stress is easy to exist on the workpiece surface during quenching, and its high sensitivity to hydrogen embrittleness, the service life of high carbon chromium bearing steel in the working environment with high performance requirements is greatly reduced. Therefore, on the basis of traditional martensite quenching and tempering process, new heat treatment processes such as bainite isothermal quenching and martensite-bainite multiphase quenching have been developed.

Taking GCr15 high carbon chromium bearing steel as an example, Figure 5 shows three common heat treatment processes for this steel. Process 1 is conventional martensite quenching and tempering process; Process 2 is bainite isothermal quenching heat treatment process; Process 3 is the combined heat treatment process of martensie-bainite. FIG. 6 shows the microstructure morphology of GCr15 bearing steel after heat treatment under martensitic quenching and tempering, bainite isothermal quenching, martensitic - bainite multiphase quenching and bainite variable temperature quenching.

3.1 Spheroidizing annealing

The conventional heat treatment of bearings includes spheroidizing annealing and quenching + low temperature tempering. On the one hand, spheroidizing annealing makes spherical carbide particles evenly distributed on the ferritic matrix, thus obtaining granular pearlite structure. On the other hand, spheroidizing annealing can reduce the hardness of materials, so that the steel can obtain good machining properties, and prepare the organization for the final treatment (quenching + low temperature tempering).

Yang Hongbo et al. treated GCr15 bearing steel at different spheroidizing annealing times and carried out martensite-bainite multiphase quenching process. They found that, with the increasing of spheroidizing annealing time, bainitic ferrite sheets tend to gradually widen. The SEM microstructure morphology of GCr15 bearing steel after isothermal quenching at different spheroidizing annealing times is shown in Figure 7. When the annealing time was 150min, the bainitic ferrite strip became narrower. The analysis shows that when the spheroidizing annealing time is 150min, a large number of undissolved carbides hinder the growth of bainitic ferrite strips during the lower bainite transformation process, resulting in the increase of distortion energy and the generation of new bainitic ferrite strips. Therefore, the spheroidizing annealing time directly affects the distribution of carbides in steel. The precise control of spheroidizing annealing time can refine the size of bainitic ferrite strip, thus effectively improving the comprehensive performance of bearing steel.

Conventional spheroidizing annealing cooling speed is generally 10 ~ 25℃/h, too fast cooling rate, will be unfavorable to cutting; Too slow and the size of the carbide will increase. Therefore, the isothermal spheroidizing annealing process was developed, and the processing time of the spheroidizing annealing process can be shortened to 10 ~ 18h generally. James M et al. put forward another periodical spheroidizing annealing process which can shorten the processing time, but the heat treatment control steps of this process are more complicated, which is limited in practical production and application.

3.2 Martensite quenching and tempering process

In order to improve the strength, hardness, wear resistance and fatigue life of bearing steel, the pretreatment (spheroidizing annealing) and quenching steel were tempered at low temperature. The heating temperature of final treatment (quenching + low temperature tempering) is generally Ac1 ~ Accm. Taking GCr15 bearing steel as an example, the quenching temperature is generally 820~860℃. Too high or too low temperature will affect the comprehensive performance of steel. Low temperature tempering temperature is generally 150~170℃, in order to obtain a relatively stable tempering structure, as far as possible to eliminate the internal stress in the structure, should ensure a long tempering time. The microstructure morphology of GCr15 bearing steel after martensite quenching and tempering is shown in Figure 8. After quenching treatment at 850℃ for 30min and low temperature tempering treatment at 160℃ for 3h, the tempering structure consisting of tempered martensite, a small amount of residual austenite and carbide is finally obtained.

3.3 Bainite isothermal quenching process

With the continuous improvement of bearing steel performance requirements, the hardening - tempering process of bearing steel is also improving. The final microstructure of high carbon chromium bearing steel is obtained by isothermal quenching of lower bainite, martensite, residual austenite and carbide. Zhang Zengqi et al. showed that the impact toughness of GCr15 bearing steel after bainite isothermal quenching was more than double that of the steel after conventional martensite quenching and tempering.

ZHAOJ et al. isothermal quenching of high-carbon chromium bearing steel at 200℃ for 2, 6, 12 and 72h respectively, and acicular bainite microstructure with volume fractions of 2.1%, 40.8%, 60.2% and 86.6% is obtained. FIG. 9 shows the transmission electron microscopy images of the two samples at 200℃ after isothermal for 6h and 12h respectively. The final microstructure composed of lamellar bainitic ferrite, thin film residual austenite and ε-carbide can be observed. The data measured and corrected by mean line intercept method show that the microstructure is nano-bainitic lath with an average thickness of about 50nm±15nm.

Zhang Fucheng et al. from Yanshan University found that compared with the conventional heat treatment process, the heat treatment process of nano-bainite bearing steel has a lower isothermal temperature, which increases the driving force of phase transformation and increases the nucleation point of bainite. However, the shortcoming of this process is that it prolongs the isothermal time in the process of bainite transformation and increases the production cost. Therefore, for the thermal treatment process of nano-bainite, the composition design should be further optimized, the heat treatment process should be improved, so as to shorten the formation time in the process of bainite transformation and reduce the manufacturing cost.

4.Development direction of bearing steel

With the continuous development of domestic industrial manufacturing, China has made the national strategic deployment of "Made in China 2025" based on the great situation of international industrial transformation. As one of the indispensable key components in the field of equipment manufacturing, bearings are related to the future development direction of high-end manufacturing in the domestic industrial field and determine China's stride towards the goal of becoming a world manufacturing power. On the whole, China's bearing industry has been significantly improved in recent decades of development, but in the field of high-end bearings, due to the lack of fine control of the size and distribution of inclusions and carbide in steel, as well as low times of microstructure defects, the quality stability of high-quality bearing products can not be guaranteed. In view of this, in order to meet the requirements of long life and high reliability of high-end bearing products in the future, the research direction of domestic bearing industry can be considered from the following aspects.

(1) Process improvement. Based on the conventional martensite quenching and tempering treatment of bearing steel, new heat treatment processes such as bainite isothermal quenching, martensite-bainite isothermal quenching and variable temperature bainite quenching have been developed successively. For the bainite bearing steel which has been studied more at present, firstly, attention should be paid to the applicability of bainite isothermal quenching process, and the selection of heat treatment process should be determined according to the working environment and actual performance requirements of bearings. Secondly, for the improvement of bainite isothermal quenching medium, the future should try to avoid excessive use of toxic nitrate, and develop more environmentally friendly quenching medium; Thirdly, due to the low isothermal temperature of bainite, the processing time of the whole heat treatment process is too long, which undoubtedly increases the manufacturing cost of enterprises. Therefore, the reduction of bainite transition time should be one of the future research focuses. In addition, although the smelting process of domestic bearing steel is basically in line with the international level, the smelting of domestic scrap steel accounts for a large proportion, the vacuum degree in the refining process is not up to standard, and the oxygen content in steel fluctuates greatly, resulting in the control of non-metallic inclusions and carbides is difficult to reach the rating standard. Finally, foreign advanced smelting processes such as vacuum degassing and inclusion homogenization should be used for reference to realize the localization of ultra-clean and ultra-long life bearing steel.

(2) Internal quality control. First, there should be more fine detection and control standards for the control of oxygen mass fraction and the distribution uniformity of inclusions in steel. In the future, the mass fraction of oxygen in steel should be stabilized below 0.0006%, and the mass fraction of titanium should be less than 0.0015%, so as to reduce or eliminate fatigue spalling and fracture caused by hard and brittle inclusions in steel. Minimize the influence of inclusions on steel quality; Secondly, in view of the prominent carbide instability or even excessive problem of bearing steel in China, advanced processes such as controlled rolling and controlled cooling, periodic spheroidizing annealing and cyclic induction spheroidizing annealing should be adopted to eliminate carbide segregation in steel as far as possible, improve the uniformity of carbide distribution and achieve microstructure refinement and homogenization. Thirdly, the continuous casting process in the smelting process is optimized to reduce the defects of low microstructure in steel, reduce the loose center and shrinkage hole of casting billet, strictly control the component segregation, and improve the quality of continuous casting billet.

(3) Surface modification. In view of the increasingly complex working environment, as far as possible based on surface carburizing, carbonitriding and other original surface treatment processes, combined with surface coating, cladding and other new surface modification technologies, to achieve the optimization of material surface properties, extend the fatigue life of bearings, develop bearing steel products that adapt to different working environments, realize the characteristic development of bearing steel from single to diversified.

(4) Testing equipment and technical evaluation standards. First of all, the domestic bearing industry production concentration is low, the quality of the bearing manufacturing enterprises is uneven, due to the lack of high precision testing equipment in most small and medium-sized enterprises, such as microscopic inclusions, mesh carbide, surface defects and so on are difficult to be detected, ultimately leading to unqualified products into the market; Secondly, the domestic bearing industry standard has no rating requirements for the content of some harmful elements, inclusions and the size and distribution of carbides, and the control of the decarbonization layer and dimensional accuracy is not strict enough. In addition, at present, there are relevant control standards for the detection and evaluation of residual austenite and residual stress in steel in foreign countries, while the detection and analysis of residual stress in steel in China has not been included in the control index. In the future, a unified technical evaluation system and complete detection and rating standards should be formulated based on the actual development situation of bearing industry at home and abroad to strictly control product quality. Improve the quality and stability of domestic bearing products.