Abstract: High carbon chromium bearing steel GCr15 is the most typical bearing steel. Through the analysis of the chemical composition design of GCr15 steel, the technical concept and basic principles laid down when it was invented were sorted out and extracted – “use performance, processability and economy”. For the research and development of new bearing steel in the recent stage, the cases of foreign countries following the guidelines are cited, and some domestic phenomena that deviate from the guidelines are pointed out. It is reiterated that in the research and development of bearing steel and even new materials, it is the correct direction and fundamental way to bear in mind and uphold this principle.
High carbon chromium bearing steel GCr15 (100Cr6 in Germany, 52100 in the United States, SUJ2 in Japan, etc.) is a very excellent steel grade. Since it was invented by Stribeck in 1901, it has spanned more than a century of history, and its chemical composition has remained basically unchanged. , is still used as the main steel for rolling bearings, and has made outstanding contributions. With the continuous advancement of science and technology and industry, the requirements for bearings to work in special working conditions and harsh environments are more diverse. The American third-generation high-temperature bearing steel CSS-42L, the German high-nitrogen stainless steel Cronidur30, and the Japanese medium-carbon surface hardening steel SHX are all typical representatives. In the recent period, China has also attached great importance to the research and development of bearing steel, especially high-performance bearing steel, and has formed an unprecedented upsurge in different levels of “production, learning and research”. However, the research and development of new steel types at home and abroad (especially domestic) Among them, there are some phenomena worthy of attention and consideration. For example, some of the basic requirements of “oriented to engineering applications” have always been followed, some seem to have been forgotten, and some are completely deviated from: Therefore, reviewing the composition of GCr15 steel more than a hundred years ago The concept and criteria of the design undoubtedly have strong practical reference and guiding significance.
At the beginning of the birth of the modern bearing industry, bearings were mainly made of carbon steel  and carburized steel . The smelting technology of carbon steel is mature and the price is low, but its comprehensive performance cannot meet the requirements of the bearing’s service characteristics. Carburized steel is hard on the outside and tough on the inside, and has gradient properties suitable for complex loads. However, Thrust Roller Bearing due to the limitations of the solid carburizing method used at that time, the individual soft spots caused by uneven carburizing greatly reduced the bearing life, and the heat treatment process was complicated. Production The cycle is long, the efficiency is low and the cost is high. Therefore, research and development of special steel grades suitable for mass production of bearings has become an urgent need. The invention of GCr15 steel is a major breakthrough based on carbon steel. The main chemical composition (mass fraction, the same below) of GCr15 and equivalent grades of bearing steel is shown in Table 1. The chemical composition is mainly carbon and chromium. Except carbon, the mass fraction of other alloying elements is less than 3%, which is high carbon and low chromium. , low alloy, hypereutectoid steel.
Bearing steel must first meet the performance requirements of its target product, that is, bearings. The basic performance of bearings includes:
1) Fatigue resistance. Since the rolling elements and the raceway in the bearing are in high-pair (point/line) contact, the contact area is small and the contact stress is high (usually 1 000-2 000 MPa for general bearings), the normal failure mode of the bearing is mainly rolling contact fatigue. When the carbon content (mass fraction, the same below) is within the range of 0.8% to 1.0%, the fatigue life of the bearing is basically the same; when it exceeds 1%, the fatigue life decreases. Adding an appropriate amount of molybdenum, silicon and other elements can enhance the tempering stability and also help to improve the fatigue life of the bearing.
2) Wear resistance. During the operation of the bearing, the wear of the working parts (rolling, especially the sliding contact) is inevitable, which will affect the rotation accuracy, motion stability, vibration and noise of the bearing, so the bearing steel should have higher hardness (the hardness is usually is a proxy for wear resistance).
①Carbon—The hardness of steel is mainly determined by the hardness of martensite and the number of undissolved carbides. In the range where the carbon content is less than 0.9% (some research conclusions are 1.0% or 1.2%), the higher the carbon content, the more supersaturated carbon and undissolved carbides in the martensitic structure after quenching, and the higher the hardness; When the carbon content is exceeded, the hardness begins to decrease due to the increase of retained austenite. In addition, under the same hardness conditions, there are uniform and fine undissolved carbides precipitated on the martensite matrix, which has higher wear resistance than purely tempered martensite. Therefore, in order to form a sufficient amount of carbides, the carbon content should tend to the upper limit.
② Chromium——Chromium is a medium carbide-forming element. A high chromium content can greatly enhance the hardenability of steel, and achieve high hardness, high strength and good wear resistance; but when the chromium content exceeds 1.65%, it will Increase the heterogeneity of carbides and retained austenite, reduce impact toughness and fatigue strength.
③Molybdenum and manganese——Molybdenum and manganese are medium and weak carbide forming elements respectively, adding an appropriate amount can further improve the hardenability of steel. If the molybdenum content is too high (more than 0.5%), very stable coarse carbides will be produced; manganese tends to promote the coarsening of austenite grains, and too much is not conducive to enhancing toughness. When molybdenum, manganese and chromium coexist and cooperate, it can reduce or inhibit the temper brittleness caused by other elements, improve the temper stability, and strengthen the hardness and strength retention.
3) High strength. The high pair contact in the bearing is easy to produce plastic deformation or indentation; the bearing bears heavy load, vibration shock, high-speed centrifugal and other additional loads will produce high tensile and compressive stress; some bearings require a large interference fit during installation. Significant hoop stress: Therefore, bearing steel must have excellent mechanical strength, mainly including yield strength, tensile (compressive) strength and impact toughness. The strength of steel is directly proportional to the hardness, and generally the tensile strength is 0.33 to 0.36 times of the Brinell hardness. Although the hardness increases with the increase of carbon content, when the carbon content is greater than 0.9%, due to the increase in the number of brittle secondary cementite and the formation of a network, the strength of the steel decreases, and the plasticity and toughness are also poor. In addition to carbon, alloying elements such as chromium, manganese, molybdenum, and silicon are beneficial to improving the strength of steel.
4) Good dimensional stability. Bearings are precision mechanical parts, which require good dimensional stability (that is, precision retention) during processing, storage, transportation and use. High carbon and related alloying elements reduce the initial temperature Ms of martensitic transformation (the degree of reducing Ms of related alloying elements in GCr15 steel is arranged from strong to weak to manganese, chromium, and molybdenum), resulting in an increase in retained austenite and stable dimensions Sex is hard to guarantee. Therefore, in the case of high carbon and chromium content, it is necessary to reduce the content of manganese, molybdenum and other elements as much as possible, so that the retained austenite after quenching and tempering can be kept at a low level.
5) Corrosion resistance. Bearings are widely used, and it is inevitable to work in humid, muddy water and other environments. The added chromium content is slightly higher to have a certain anti-rust and corrosion resistance. Molybdenum enhances the rust resistance of chromium (the effect of molybdenum on the rust resistance is equivalent to 3 times that of chromium, but it cannot replace the role of chromium, and must be dominated by chromium). The combination of silicon with molybdenum, chromium and other elements also has the effect of improving corrosion resistance and oxidation resistance.
Under the premise of meeting the performance requirements, the bearing steel must also have good process performance, which is convenient for specific cold and hot processing of bearing parts.
1) forging. After forging and heating, the austenite grains are fine, have certain thermoplasticity, and have good forging performance.
2) Heat treatment and machining. Conventional heat treatment is adopted, the process is simple, and it is ready for subsequent processing procedures such as turning and grinding.
①Spheroidizing annealing——When the carbon content is about 1%, the time of spheroidizing annealing can be accelerated, and the quality is better. The microstructure and lower hardness of spherical fine carbides are easy to carry out subsequent ring turning or rolling elements Cold heading and ready for good quenching and tempering quality. A certain amount of manganese can also reduce or eliminate the adverse effects of sulfur and form MnS, which is beneficial to turning.
② Martensitic quenching and tempering – the heat treatment equipment and process are simple, the time is short, the hardenability is good, the deformation of the workpiece is small, and high hardness can be obtained (usually required to reach 58 HRC or more), which is conducive to the high grinding process of bearing parts Precision and high efficiency.
③ Grinding superfinishing – the high chromium content makes the structure and carbides finer, and it is easy to obtain a lower surface roughness during ring grinding and rolling body grinding.
Bearing steel must be suitable for the mass production of bearings and have good economical efficiency before it has the value of promotion and application.
1) The alloying elements are simple and the mass fraction is small, and the mainly used chromium and manganese are common rare elements and the cost is low.
2) The conventional smelting method (the electric arc furnace smelting method at that time) is enough, and there is no need to adopt complicated special smelting methods.
3) The spheroidizing annealing time should be as short as possible; after quenching, tempering at low temperature (such as 150-180 ℃) is enough.
4) Grinding can not only achieve high precision, but also take into account high efficiency.
To sum up, the carbon content is about 1.0%, the chromium content is about 1.5%, supplemented by a small amount of elements such as manganese, molybdenum, and silicon, and the content of impurity elements such as phosphorus and sulfur is strictly controlled to ensure that it reaches the level of high-quality steel. Heat treatment, GCr15 steel with comprehensive and balanced performance in all aspects has established the basic framework for chemical composition design. After testing, application and promotion, it was popularized in the bearing industry all over the world in the 1920s, and it still occupies about 80% of the bearing steel market. In terms of performance, no other steel can replace and surpass, “has the supreme status”.
2 Cases following the invention guidelines of GCr15 steel
The research and development guidelines that bearing steel should follow, GCr15 steel as a pioneering work has given an excellent demonstration.
In the research and development of bearing steel in foreign countries, most of them strictly adhere to the three principles of performance, manufacturability and economy, especially in Japan.
1) Based on GCr15 steel, Japan and Europe mainly developed a series of steel grades by adjusting the different contents of silicon, manganese, chromium, molybdenum, etc., for example, Japan’s SUJ series has a total of 4 steel grades  (formerly SUJ1-SUJ5, a total of 5 Steel number, later canceled SUJ1), see Table 2; Sweden’s OVAKO company only has 8 steel numbers in the 803 (ie GCr15) series, and there are more than a dozen steel numbers in other derivative products.
2) Japanese bearing steel manufacturers generally focus on continuous improvement in conventional smelting methods, such as the technological progress of Sanyang Special Steel in vacuum degassing bearing steel: atmospheric smelting → ladle refining LF → circular vacuum degassing RH → continuous casting CC → Eccentric furnace bottom tapping EBT → ultra-pure steel SNRP → ultra-pure steel SURP, always pursuing high cost performance, its high-quality vacuum degassed steel can be used in important bearing product fields such as railways and wind power; unlike some Research and development ideas always prefer to seek solutions on special smelting methods (electroslag remelting ESR, vacuum induction melting VIM or vacuum arc remelting VAR or even “double vacuum” VIM+VAR).
3) The anti-surface fatigue NSJ2 steel  developed by NSK Corporation of Japan increases the silicon content and controls the optimal chromium content. The surface hardness, wear resistance and seizure resistance are comprehensively evaluated by multiple indicators, and the orthogonal test level is as many as 80. At the same time, the mechanical properties, hardenability, turning and grinding properties related to its processability are considered to prove that it is compared with SUJ2 steel , can reduce early failure to a minimum, thus greatly extending the service life.
4) Japan’s NTN company and Datong Special Steel Co., Ltd. jointly developed a bearing steel that can withstand a working temperature of 250 °C. Although the mature aero-engine spindle bearing steel M50 can be used to solve the problem at this temperature, the steel contains about 10% Cr, Mo, V and other elements are high-alloy steels, with high material costs and complex heat treatment processes, making them difficult to popularize. Therefore, on the basis of SUJ2, by increasing the silicon content to prevent the reduction of hardness at high temperature; by adding nickel to maintain the rolling fatigue characteristics; and considering the heat treatment and cold working performance, STJ2 high temperature bearing steel was successfully developed. The test life of bearings using STJ2 steel is 3.5 times that of SUJ2 steel at room temperature, and 15 times that of SUJ2 steel at a high temperature of 200 °C, forming a series of high-temperature, long-life bearing products, which can be extended to automobiles, steel, paper, etc. fields.
GCr15 steel is a classic and benchmark in the history of bearing steel development. Paying homage to classics and benchmarking against benchmarks are the awe and basic principles that should be upheld in the research and development of new steel types. Among the new steel types introduced abroad, most of the “three major principles” established by GCr15 steel have been fully reflected, and some have deviated from them. This should be screened to avoid blindly following the trend. In China’s recent research on bearing steel, the phenomenon of “projects for projects, development for research, and performance for performance” is still relatively common and prominent. Ultra-high carbon has become the mainstream research direction (the research and development of special bearing steel is understandable, if you try to replace GCr15 steel, you will go the wrong way); only use fatigue life or a certain (some) special performance as an evaluation index, and other core indicators that affect the basic performance of the bearing (such as the influence of retained austenite on dimensional stability, the influence of high hardness on crack sensitivity or fracture toughness, etc.) seldom do multi-dimensional and multi-level comprehensive research; performance and life test results are often selectively announced , unfavorable indicators are either avoided or ambiguous; in terms of craftsmanship, there is a lack of systematic and in-depth cold and hot processing process tests and process applications; there is little analysis and little consideration of economics: these practices , fundamentally deviated from the starting point and foothold of bearing steel as an engineering material research and development, making it difficult for many project results to meet the technical characteristics of bearing products and the industrial characteristics of the bearing industry. Take stock of the so-called engineering and industrialization goals for several years. , very few are good examples.
Therefore, it is necessary to reiterate that in the research and development of bearing steel and even all engineering materials, the technical concepts and principles that must always be borne in mind and firmly adhered to are performance, manufacturability and economy. This is the correct direction and fundamental way.