Special steel is the basic material of advanced equipment manufacturing industry and an important guarantee for the development of high-end manufacturing industry. However, the demand for special steel is small, the variety is large and the production technology content is high, which affects the development of high-end products of special steel in China. With the increasing awareness of global environmental protection, the introduction of automobile electrification and wind power generation is accelerating, and bearings need to cope with these new changes. In addition, the durability of bearings used in harsh environments such as steel equipment needs to be improved. This year, bearing steel and related fields have continued to explore, research and innovate around these new demands and trends.
Bearing steel
Main Development Trend of Electric Vehicle
Electrification of automobile is the direction of automobile development, but it changes the driving mode of automobile, which is different from the driving mode of traditional automobile relying on internal combustion engine. In the new generation of automobile, hybrid electric vehicle (HEV) can restrain CO2, NOx and other gas emissions by using motor in driving, and electric vehicle (EV) can achieve zero emissions.
Nevertheless, IEA (International Energy Agency) predicts that by 2050, more than 50% of fuel vehicles will still be available. Therefore, for internal combustion engines, research has also been carried out on engines equipped with idling stop mechanisms and small turbochargers to promote the improvement of thermal efficiency.
In the future, if motor drive becomes the mainstream due to automobile electrification, a power transmission system with a simpler structure than the current internal combustion engine will be formed, and the number of bearings and the amount of steel used in each vehicle will inevitably decrease. In addition, from the perspective of small size and light weight, improvement of transmission efficiency (low torque), and response to motorization (high-speed rotation required by motor drive), the use environment of bearings will be worse in the future.
Small and lightweight
Due to the limitation of improving fuel efficiency and carrying space, small and lightweight components and bearings are required. Because of the miniaturization, the bearing load capacity becomes smaller, so it is necessary to take countermeasures against the bearing longevity and static strength decline.
Improved transmission efficiency
In order to improve the power transmission efficiency, from the point of view of reducing the mechanical loss of HEV · EV components, it is required to reduce the bearing torque (reduce the loss), and it is necessary to take measures to reduce rolling friction, sliding friction, mixing loss, etc.
High speed rotation
In order to improve the efficiency, the maximum speed when driving, and the miniaturization and high-speed rotation of the propulsion motor, high-speed rotation is required under the condition of exceeding the allowable number of revolutions in the past.
Problems and measures
In the face of the development trend of long-life, high-speed rotation and low torque of bearings for power transmission and drive transmission, there will be a series of problems. The latest research and development progress of related technologies to solve these problems is introduced below.
Longevity
Due to the high speed rotation of HEV · EV power transmission device, when the oil film between the ferrule and the ball is insufficient due to the deterioration of the lubrication environment of the ball bearing, the surface properties become worse, and the problem of peeling off of the starting point of the surface will occur. As a countermeasure to surface deterioration, a long-life ball bearing with strengthened rolling element was developed. Specifically, the steel added 0.3mass% - 0.4mass% Si on the basis of SAE52100 steel (equivalent to JIS standard steel SUJ2) is subject to special carburizing and nitriding treatment to precipitate high hardness Si · Mn nitride strengthened steel balls, even under the harsh lubrication conditions with low EHL oil film parameter λ (λ=0.3, λ=hmin (minimum oil film thickness)/ σ (composite roughness of two contact surfaces), the roughness of the steel ball surface is still not easy to reduce. Built in test of 6206 deep groove ball bearing has been carried out, and it is confirmed that it can achieve the effect of long service life.
The planetary gear shaft used in the planetary gear part (as the shaft of the inner wheel track surface) is specially carburized and nitrided for SUJ2 steel（ γ）、 Carbonitride and residual compressive stress increase the endurance fatigue strength; Special high-frequency quenching is carried out for SUJ2 to reduce the residual austenite in the center, inhibit the thermoplastic bending under high temperature environment, and improve the durability fatigue strength. In addition, an extra long life planetary gear shaft with high chromium quenched steel has been developed to further improve the durability and fatigue strength.
Bearing for transmission must be resistant to foreign matters because it is used in the oil lubrication environment containing hard foreign matters such as wear powder during gear engagement. Therefore, high concentration carburizing, quenching and tempering were carried out for the development steel with adjusted Si, Mn and Cr contents, and carbides (average 0.2 μ m. The area ratio is about 10%).
Dealing with high-speed rotation
In order to achieve high power, compact HEV, EV drive components, it is necessary to promote the high-speed motor. For problems such as reducing lubrication viscosity for high-speed rotation, heating and wear caused by high temperature lubrication, the sliding between the ball and the ring is reduced by optimizing the groove curvature, ball diameter, pitch diameter, etc. of the ring. In addition, in order to prevent the cage from deforming due to the increase of centrifugal force, and then friction with the ball, a double column cage is designed.
Low torque
With the development of automobile electrification and the increase of drive system options, it can be predicted that the transmission mechanism of power transmission will change. The bearing used in the transmission plays a role in smoothly supporting the gear shaft that conducts the rotation and torque of the engine and motor. Therefore, there is a problem of reducing the rotation torque. The tapered roller bearings with high load capacity, high rigidity and space saving are mostly used for the bearings of speed change engines under the radial and axial loads when the gears are engaged, but there is a problem of large torque loss. All bearing manufacturers have realized low torque by locking the source of torque generation and handling it. For example, the loss can be reduced by reducing the mixing resistance of lubricating oil and restraining the amount of lubricating oil flowing into the bearing. In addition, since the torque can be reduced by replacing the roller bearing with the ball bearing, the ball bearing which can be used for the differential drive gear shaft support and the position bearing large axial load in the transmission has been developed.
In addition, a large number of sliding bearings used in the engine need oil pumps to generate high hydraulic pressure to provide lubricating oil to the bearing. However, high pressure cannot be formed in the low-speed rotating area, so the oil supply is insufficient, and there is a problem of large sliding friction resistance. Therefore, replacing the sliding bearing with the rolling bearing does not require high hydraulic pressure, which can realize the miniaturization of the oil pump and low torque starting, and is easy to use the idle stop mechanism, which is conducive to reducing fuel consumption.
In a word, with the requirements of high efficiency and high-speed rotation of electric vehicles, it can be predicted that the service environment of bearings will become increasingly harsh. Therefore, it is necessary to meet the requirements of small size and light weight of bearings (small diameter, thin reduction to achieve small standardization). In order to cope with the low viscosity lubricating oil environment conducive to low torque and reduce friction, ball bearings are used instead of roller bearings. The requirements for high reliability and long life of bearing steel are put forward. At the same time, for the problem of metal contact (sliding) between the rolling element and the race caused by the high temperature viscosity drop of the lubricating oil caused by high-speed rotation, and the problem of component size change due to the decomposition of residual austenite, corresponding countermeasures should be taken according to the service environment of the bearing.
Expansion of wind power generation
In recent years, wind power has been developed as a clean energy that does not emit CO2 and has little impact on the environment, and the cumulative power generation capacity has increased year by year.
At the initial stage, windmills installed on land are gradually developed offshore and become larger due to the reduction of installation sites or in order to improve power generation efficiency and equipment operation rate.
As the windmill installed on the sea is not easy to maintain, the bearings used are required to be reliable for a long time.
The rolling bearing for windmill gearbox has the problem of early damage, and one of its damage forms is internal starting point peeling. This kind of peeling was studied. The research result is that hydrogen intrusion into steel, hydrogen embrittlement and peeling damage occur due to the decomposition of lubricating oil or dry oil and the moisture in lubricating oil. Sliding, vibration and static electricity are all involved in the generation of hydrogen.
Rolling fatigue tests were carried out using deep groove ball bearings with different PV values (P: face pressure, V: sliding speed) that change the groove curvature of the bearing inner race. In the test, it was found that the peeling did not occur at the trench bottom of the track surface with the largest contact surface pressure, but near the end of the track with large sliding. When PV value is above threshold value, such peeling occurs. Although the gear box bearing of the windmill adopts roller bearing and tapered roller bearing, and the sliding between the ring and the rolling element is relatively small, large sliding may occur when the rotating shaft accelerates or decelerates rapidly.
In addition, in recent years, another different damage pattern has been observed, that is, axial cracks of bearing rings in rolling bearings. The researchers conducted rolling fatigue test by invading hydrogen into the outer ring of roller bearing made in SUJ2, and speculated that the cause was a form of white tissue damage.
In order to solve the problem of peeling γ A study was conducted. The compressive stress and residual stress were confirmed γ The content has great significance for SUJ2 ordinary quenching material, and can improve the service life of carburizing and nitriding materials. It is speculated that the reason is that, under the effect of compressive stress, the crack development is delayed, and the enrichment of hydrogen shear stress region is delayed by the austenite with slower hydrogen diffusion rate compared with the martensite structure.
In addition, from the perspective of preventing hydrogen intrusion, the formation of oxide film on the track surface is also an effective means. When materials are added to the lubricant to form an oxide film on the newly formed metal surface due to tribochemical reaction, the chemical stability of the rail surface can be ensured. From the steel composition, it is confirmed that the addition of Cr can effectively inhibit the peeling of white structure, which is due to the prevention of hydrogen diffusion to small defects in the area of maximum shear stress.
In a word, in order to prevent early stripping failure of bearing for wind power generation, new steel is researched and developed from the perspective of restraining hydrogen intrusion or mitigating hydrogen intrusion.
With the development of wind power devices towards large-scale, the bearing of wind power devices will also become large-scale. Therefore, the heat treatment problem of large bearings arises. Therefore, it is required to develop bearing steel with good hardenability and small quenching deformation.
In addition, since the bearing of wind power plant is a bearing used for a long time, the amount of retained austenite in steel should be controlled.
Severe environment application
The durability of bearings used in steel equipment mixed with water and foreign matters or used under harsh conditions such as thin lubrication is required to be improved.
When used in steel equipment, the self-aligning roller bearing is used in water or thermal environment under very low speed rotation (about 10 rpm, rotation speed 0.01 m/s) and high load conditions. Especially in the environment where dry oil is used, the problem of thinner lubricating film is more likely to occur than that of base oil. There are studies on improving the oil film formation state at low speed through urea based dry oil.
In addition, the rolling mill bearings in iron and steel enterprises also have the problem of indentation caused by water or foreign matters, and it is strongly required to improve the service life of bearings in this environment. The report of fatigue process shows that, as the starting point of surface inclusions, cracks occur inwards in the environment mixed with water, and this kind of crack preferentially propagates at the grain boundary. In order to suppress such cracks and propagation, the effect of longevity of vacuum refining steel has been confirmed. High cleanliness steel can be used as bearing steel, or the element Ni that can strengthen grain boundaries can be added to inhibit crack propagation.
From the perspective of high efficiency, for rolling bearings, in order to achieve low torque, it is more and more inclined to use low viscosity lubricating oil for lubrication, resulting in thin oil film and easy metal contact (sliding). In this case, the hydrogen related structural change type or metal contact type bearing damage mode may become more obvious, and the corresponding countermeasures need to be studied in the future.