The quality of steel used for bearings is the most stringent and inspected steel among all alloy steels. The world recognized level of bearing steels is a symbol of the metallurgical level of a country. Bearing has high requirements for its materials, which must have high hardness, wear resistance, contact fatigue strength, elastic limit, good impact toughness, fracture toughness, dimensional stability, rust resistance and cold and hot processing properties. In order to guarantee the performance requirements, the quality of bearing steels metallurgy must ensure that its strict uniformity of the chemical composition and chemical composition, high purity, low oxygen content and residual element content, low power and high power organizations strictly, strict carbide homogeneity, strict surface decarburization layer and internal loose, segregation, and microscopic pore, Surface and internal defects such as cracks, slag inclusion, burrs, folds, scarring, oxide scale, shrinkage holes, bubbles, white spots and overfiring are not allowed. China’s special steel enterprises in 2007 bearing steels output of 2.45 million tons.
At present, there are five main categories of bearing steels used in the world , namely, high carbon chromium bearing steels, carburized bearing steels, stainless bearing steels, high temperature bearing steels and medium carbon bearing steels.
1) High carbon chromium bearing steels. High carbon chromium bearing steels is a high carbon alloy steel containing 0.6% ~ 1.5% chromium. At present, there are five grades GCr15, GCr6, GCr9, GCr9SiMn and GCr15SiMn in China standard. High carbon chromium bearing steels has been 100 years of history, the production and use of long-term proved that this kind of steel can basically meet the requirements of general working condition of bearing, has good abrasion resistance and contact fatigue resistance, have ideal rust resistance and certain elasticity, toughness, processing performance is good, price is cheaper, so the most widely used. Due to the difference in hardenability, GCr15 and GCr15SiMn are the most widely used, while GCr6 and GCr9 have rarely been used.
2) carburized bearing steels. Carburized bearing steel is essentially a high-quality carburized structural steel. At present, there are six grades in China’s standard, G20CrMo, G20CrNiMo, G20Cr2Ni4, G20CrNi2Mo, G10CrNi3Mo, and G20Cr2Mn2Mo. This kind of steel after carburizing, quenching, tempering and other heat treatment procedures, the surface has a high hardness and certain wear resistance and high contact fatigue strength, and its core has good toughness, a certain strength and hardness. It is mainly used for bearing bearing impact load and extra-large bearing, such as railway, rolling mill bearing, etc.
3) stainless bearing steels. There are martensitic stainless steel and austenitic stainless steel, currently in China’s standard using three grades, 9Cr18, 9Cr18Mo, Cr14Mo4. Martensitic stainless steel 9Cr18 is the most used. After heat treatment, all or most of the martensitic structure can be obtained, with high strength, good wear resistance and toughness, especially strong corrosion resistance in seawater, acid and organic brine solutions. The disadvantage of these bearing steels is that they have high carbon content. Carbon and chromium form coarse primary carbides, which reduce the mechanical properties of the materials and reduce the contribution of chromium to anti-corrosion. 1Cr18Ni9Ti austenitic stainless steel, after heat treatment can get a single austenitic structure, with good toughness and good corrosion resistance. But because of its low strength and hardness, it is only suitable for manufacturing low load and low speed bearings.
4) High temperature bearing steels. High temperature bearing steels has certain high temperature hardness and high temperature wear resistance. High temperature contact fatigue strength, oxidation resistance, impact resistance, high temperature dimensional stability and so on. This type of steel utilizes some steel grades of high speed steel and high chromium martensitic stainless steel. The main high temperature bearing steels currently used in China are: 9Cr18Mo (260 ℃), Cr4Mo4V (315 ℃), Cr14Mo4 (480 ℃), W6Mo5Cr4V2 (480 ℃), W9Cr4V2Mo (520 ℃), W18Cr4V (560 ℃), W12Cr4V5Co5 (590 ℃), The most commonly used is Cr4Mo4V. The tempering temperature of high carbon chromium bearing steel is 160 ~ 180 ℃, so the use temperature generally does not exceed 120 ℃. If the use temperature exceeds 120 ℃, the strength of steel drops sharply, the size is unstable, and the fatigue strength of bearings is reduced. Therefore, high temperature bearing steel can only be used instead of high carbon chromium bearing steel. For example, with the increasing power of turbojet engines for aircraft, the spindle bearing of compressors and turbines requires high temperature resistance. Its operating temperature is generally 55 ~ 260 ℃, and the highest one can reach 370 ℃.
5) Medium carbon bearing steels. Medium carbon bearing steel is medium carbon alloy steel. At present, there are mainly 55, 50MnA, 70Mn, 5CrMnMo, 60CrMnMoA, 55SiMoVA and 50SiMo in China. The last two steel numbers are impact bearing steels developed in China. This steel is suitable for making extra-large size bearings used in excavation, lifting, large machine tools and other heavy equipment. Generally, the speed is not high, but it bears large axial and radial loads and bending stresses.
The smelting level of steel is the prerequisite for the inherent quality of bearing steels. The oxygen content, component segregation, the number and distribution of inclusions and the condition of large carbides in steel are important metallurgical factors affecting the intrinsic quality of bearing steels. In recent years, the smelting technology of bearing steels is developed successively around these key factors. The oxygen content in steel can be stabilized at ≤ (10 ~ 15) ×10-6, and can be controlled at (5 ~ 8) ×10-6 in some advanced enterprises.
Inclusions in steel can be classified into four types: A (sulfide), B (oxide), C (silicate) and D (point non-deforming inclusion). Different types of inclusions have different effects on bearing life, and the size and position of inclusions in steel also have different effects on bearing life. The lower the oxygen content of steel, the higher the service life of bearing steels. The last three of the above four inclusions are composed of oxides, and their harmful effects on bearing life have been proved by a large number of facts. Almost all the oxygen content in steel (except dissolved oxygen) is provided by them. Therefore, in recent years, oxygen content is regarded as a very important index to measure the quality of bearing steels.
No matter in converter or electric furnace process, secondary refining is a key process in bearing steels production. The main refining equipment is LF or ASEA-SKF equipped with VD or RH, which can ensure the inherent quality.
The continuous casting problem of bearing steels has always been the focus of people’s attention. For a long time, the serious problem of loose center and segregation of continuous casting billet has not been fundamentally solved, at least in China is still a “difficult problem”. Even if electromagnetic stirring technology and light pressure down technology are used to improve the porosity and segregation, the center porosity will remain in the spherical polar region, and a new “white bright band” defect will be brought. However, at present, the continuous casting process of bearing steels has been adopted by many special steel mills in the world, and except for some rolling bearing steels, the vast majority of bearing steels is produced by continuous casting. At present, in order to solve the quality problems of continuous casting billets of bearing steels, the following work is being actively carried out: In addition to strengthening smelting technology, reduce the oxygen content in steel and harmful impurities, is also used in the continuous casting process tundish heating, electromagnetic stirring, mould level control, strengthening the secondary cooling and liquid core of pressing technology, adopt the protection of the submerged nozzle in protecting slag casting technology, casting billet section to achieve greater compression ratio increasing hot rolling bearing steels, etc.
In the rolling production of bearing steel bar, it has been agreed that the surface defect detection and grinding of the blank should be carried out to realize the rolling of fine blank, and the heating process should be strictly observed to control the degree of decarburization. Adopt controlled rolling, controlled cooling technology, step heating furnace, continuous rolling mill, continuous annealing furnace, on-line quality control and other technologies.
The most famous bearing steel manufacturers in the world are SKF Company in Sweden, Shanyang Special Steel Plant in Japan, and Timken Company in the United States.
The production process of bar line of Japan Shanyang company: UHP furnace -LF refining furnace -RH refining furnace – continuous casting or die casting – initial rolling – three high planetary mill or large rolling mill – non-destructive testing – repairing grinding – continuous rolling mill – non-destructive testing – continuous furnace spherodizing annealing – non-destructive testing – inspection storage.
SKF OVaK0 Steel Mill rod line production process: electric furnace -LFV refining furnace – die casting – soaking furnace – initial rolling – flame cleaning -150×150 billet – inspection, shot peening, grinding – heating – descaling – rolling – cooling – shear – packing – delivery. The annual output of 450,000 tons of steel, all using die casting, rolling, but not continuous casting. OVaK0 also believes that continuous casting bearing steel is no longer a technical problem, but their main concern is economics, as the coverage of product specifications does not match the cost of investment with the scale of production. They have made a series of investment to improve the quality of products.