1. The purity of bearing steel The fatigue spalling of bearing parts is related to non-metallic inclusions in steel, especially the influence of oxides is greater.This is because when the rolling bearing rotates, the contact surface of the bearing parts is under the action of impulse load, and the non-metallic inclusions cause the local destruction of the continuity of the steel matrix structure, resulting in the concentration of internal stress, the plastic deformation around the non-metallic inclusions, and the formation of micro-cracks.
In addition, the content of gas (oxygen, nitrogen, hydrogen) in bearing steel is also an important indicator to measure purity.Dissolved oxygen precipitation decrease with temperature in the steel and aluminum, calcium, silicon, manganese and other elements to form the oxide inclusion, nitrogen in the form very diffuse aluminum nitride inclusions in steel and the thick titanium and carbon nitride, titanium nitride inclusions after the gathering of hydrogen atoms into molecular hydrogen produced tremendous pressure, once more than the ultimate strength of steel can cause inside crack, form a white dot.Reducing gas content is an important link to improve the purity of bearing steel.
In recent years, most of the major bearing steel manufacturers in China have successively built and put into operation ultra-high power arc furnaces. By adopting advanced eccentric bottom extraction technology, the oxygen content in the steel is reduced to about 10×10-6, and the minimum oxygen content in some steel is reduced to (4~3)×10-6.The secondary refining ratio also reached 100%.Therefore, the number of inclusions in steel is well controlled.
2. Uniformity of bearing steel
The uniformity of bearing steel refers to the uniformity of chemical composition and carbide.The structure, weight, casting temperature and casting method of ingot affect the distribution of chemical components in steel.The heating process of ingot and billet before hot working, the end temperature of hot working and subsequent cooling method of steel, spheroidizing annealing process affect the uniformity of carbide.The shape of the microstructure according to the carbide inhomogeneity in bearing steel.It can be divided into liquid out carbides, banded carbides and netted carbides. The harm of liquid out carbides is equivalent to that of inclusions in steel, and the fatigue life of steel can be reduced by 30% if the banded carbides are rated as grade 3~4.When the mesh carbide increases L level, the bearing life can be reduced by 1/3, and the carbide particle size directly or indirectly affects the bearing life.The grade of liquid chromatography carbides, banded carbides, and mesh carbides is an indicator of the uniformity of carbides.
3. Dimensional accuracy and surface quality of steel
Improving steel dimensional accuracy not only increases the utilization rate of bearing materials, but also reduces unnecessary cutting.Some steel mills adopt short stress line equihigh stiffness rolling mills, and the diameter deviation of steel section is greatly reduced. For round steel with diameter below 30mm, the diameter deviation can reach about 0.3mm.
The life and reliability of bearings are related to the structural design of bearings, materials used, cold and hot processing technology and equipment, installation and use conditions and other factors, among which, materials and heat treatment are one of the main factors affecting the life and feasibility of bearings.Due to the influence of materials and heat treatment process, bearing damage is easily caused by factors such as low material strength, poor purity, unreasonable fiber organization orientation of metal materials, low hardness, improper forging ratio, large residual stress and so on.The influence of foreign steel quality on bearing life was about 50% in the 1950s ~ 1960s, and it was reduced to 1/3 in the 1980s.At present, due to the influence of technology and equipment, the quality of bearing steel accounts for about 60% of the bearing life. Therefore, for bearing production enterprises, choosing qualified and high-quality bearing materials is also one of the key factors to improve the bearing life.