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Hydraulic cylinder steel is mainly used to manufacture rolling elements and rings of rolling hydraulic cylinders.


Because the hydraulic cylinder should have the characteristics of long service life, high precision, low calorific value, high speed, high rigidity, low noise and high wear resistance, the hydraulic cylinder steel is required to have: high hardness, uniform hardness, high elastic limit, high contact fatigue strength, necessary toughness, certain hardenability and corrosion resistance in atmospheric lubricant. In order to meet the above performance requirements, the uniformity of chemical composition, content and type of non-metallic inclusions, particle size and distribution of carbides and decarburization of hydraulic cylinder steel are strictly required.


Hydraulic cylinder steel is generally developing in the direction of high quality, high performance and multiple varieties. According to the characteristics and application environment, the steel for hydraulic cylinder is divided into: high carbon chromium hydraulic cylinder steel, carburized hydraulic cylinder steel, high temperature hydraulic cylinder steel, stainless hydraulic cylinder steel and special hydraulic cylinder materials.


In order to meet the requirements of high temperature, high speed, high load, corrosion resistance and radiation resistance, a series of new hydraulic cylinder steels with special properties need to be developed. In order to reduce the oxygen content of hydraulic cylinder steel, the smelting technologies of hydraulic cylinder steel such as vacuum smelting, electroslag remelting and electron beam remelting have been developed. The smelting of large quantities of hydraulic cylinder steel has developed from electric arc furnace smelting to various types of primary smelting furnace and external refining.


At present, hydraulic cylinder steel is produced by primary smelting furnace + LF / VD or Rh + continuous casting + continuous rolling process with a capacity of more than 60 tons, so as to achieve the purpose 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 maximum length of continuous heat treatment furnace is 150m. The spheroidized structure of hydraulic cylinder steel is stable and uniform, with small decarburization layer and low energy consumption.


Since the 1970s, with the economic development and industrial technological progress, the application scope of hydraulic cylinder has expanded; The development of international trade has promoted the internationalization of hydraulic cylinder 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 came into being. Japan and Germany have built high cleanliness and high-quality hydraulic cylinder steel production lines, which has rapidly increased the output of steel and greatly improved the quality and fatigue life of steel. The oxygen content of hydraulic cylinder steel produced in Japan and Sweden is reduced to less than 10ppm. In the late 1980s, the advanced level of Japan's Shanyang special steel company was 5.4ppm, which reached the level of vacuum remelting hydraulic cylinder steel.


The contact fatigue life of hydraulic cylinder is very sensitive to the uniformity of steel structure. Improving cleanliness (reducing the content of impurity elements and inclusions in steel) and promoting the fine and uniform distribution of non-metallic inclusions and carbides in steel can improve the contact fatigue life of hydraulic cylinder steel. The microstructure of hydraulic cylinder steel in service state should be that fine carbide particles are evenly distributed on the tempered martensite matrix, which can give the required properties of hydraulic cylinder steel. The main alloy elements in high carbon hydraulic cylinder steel are carbon, chromium, silicon, manganese, vanadium and so on.


How to obtain spheroidized structure is an important problem in the production of hydraulic cylinder steel. Controlled rolling and cooling is an important production process of advanced hydraulic cylinder steel. Through controlled rolling or rapid cooling after rolling, the network carbide is eliminated and the appropriate preliminary structure is obtained, which can shorten the spheroidizing annealing time of hydraulic cylinder steel, refine the carbide and improve the fatigue life.


In recent years, Russia and Japan have adopted low-temperature controlled rolling (below 800 ℃ ~ 850 ℃), air cooling and short-time annealing after rolling, or completely canceling the spheroidizing annealing process, and qualified hydraulic cylinder steel structure can be obtained. 650 ℃ warm machining of hydraulic cylinder steel is also a new technology. If eutectoid steel or high carbon steel has fine grain structure before hot working 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 650 ℃ warm working test of 52100 steel carried out by the US Naval Research Institute (NSP) shows that there is no fracture under the true strain of 2.5 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 processes, save energy and improve quality.


In terms 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, twice microstructure annealing is adopted in wire rod production, and the recrystallization annealing after drawing is changed from 720 ℃ ~ 730 ℃ to 760 ℃. In this way, the microstructure with low hardness, good spheroidization and no network carbide can be obtained. The key is to ensure the surface reduction rate of intermediate drawing & Ge; 14％。 This process increases the efficiency of heat treatment furnace by 25% ~ 30%. Continuous spheroidizing annealing heat treatment technology is the development direction of heat treatment of hydraulic cylinder steel.


All countries are researching and developing new hydraulic cylinder steel to expand the application and replace the traditional hydraulic cylinder steel. For example, the rapid carburizing hydraulic cylinder steel can improve the carburizing speed by changing the chemical composition, in which the carbon content is increased from the traditional 0.08% ~ 0.20% to about 0.45%, and the carburizing time is shortened from 7 hours to 30 minutes. High frequency quenching hydraulic cylinder steel is developed. Ordinary medium carbon steel or medium carbon manganese and chromium steel is used to replace ordinary hydraulic cylinder steel by high frequency heating quenching, which not only simplifies the production process, reduces the cost, but also improves the service life.


Fatigue life ratio of gcr465 and scm465 developed in Japan SUJ & mdash; 2 ~ 2 times higher. As more and more hydraulic cylinders are used in the environment of high temperature, corrosion and poor lubrication conditions, the hydraulic cylinder steels such as M50 (crmo4v) and 440C (9Cr18Mo) used in the past can not meet the use requirements. There is an urgent need to develop hydraulic cylinder steels with good processing performance, low cost, long fatigue life and suitable for different purposes and uses, such as high-temperature carburized steel m50nil, easy processing Stainless hydraulic cylinder steel 50x18m and ceramic hydraulic cylinder materials. Aiming at the weakness of low hardenability of GCr15SiMn steel, China has developed high hardenability and hardenability hydraulic cylinder steel gcr15simo, whose hardenability is HRC & Ge; 60, hardenability J60 & Ge; 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 hydraulic cylinder made of g015simo steel is twice that of gcr15simo steel. In recent years, China has also developed GCr4 hydraulic cylinder steel which can save energy, resources and impact resistance. 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 hydraulic cylinder adopts high temperature heating & mdash; Surface quenching heat treatment process. Compared with the fully quenched GCr15 steel hydraulic cylinder, the service life of GCr4 steel hydraulic cylinder is significantly improved and can be used for the hydraulic cylinder of heavy haul high-speed train. In the future, hydraulic cylinder steel will mainly develop in two directions: high cleanliness and diversified performance. Improving the cleanliness of hydraulic cylinder steel, especially reducing the oxygen content in steel, can significantly prolong the service life of hydraulic cylinder. When the oxygen content is reduced from 28ppm to 5ppm, the fatigue life can be extended by one order of magnitude. In order to prolong the service life of hydraulic cylinder 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 lowest oxygen content in hydraulic cylinder steel has been reduced from 28 ppm in the 1960s to 5 ppm in the 1990s. At present, the lowest oxygen content in hydraulic cylinder steel can be controlled at about 10ppm in China. The change of hydraulic cylinder service environment requires that hydraulic cylinder steel must have diversified properties. For example, the increase of equipment speed requires quasi high temperature (below 200 ℃) hydraulic cylinder steel (the method of increasing Si content and adding V and Nb on the basis of SUJ2 steel is usually adopted to achieve the purpose of anti softening and stabilizing size); Corrosion applications require the development of stainless hydraulic cylinder steel; In order to simplify the process, high frequency quenching hydraulic cylinder steel and short-time carburizing hydraulic cylinder steel should be developed; In order to meet the needs of aerospace, high temperature hydraulic cylinder steel should be developed.