2. Automotive bearing material and performance requirements

At present, the automobile is developing towards light, comfortable, solid, high power, compact structure, durability, good acceleration and high reliability. In addition, due to the progress of electronic control technology, the automobile is also developing towards the direction of low energy consumption, simple and convenient, strong operability and other practical performance, so the rolling bearing, as an important part of the automobile support, must adapt to this trend.

During the service process of automobile bearing, the unit area of the rolling body and the ring surface should bear great pressure, which can reach up to 5000N/mm2 by calculation. When the bearing is running, it is not only rolling but also sliding, which is subject to high-frequency and alternating contact stress, but also affected by centrifugal force, as shown in figure 6. According to the literature records, the main failure modes of automotive bearings are peeling, pitting, adhesion, strain, fracture, loss of precision, vibration noise exceeding the standard, etc., therefore, the bearing steel performance has the following requirements: (1) high purity. (2) low oxygen content. High hardness and wear resistance. (4) good dimensional stability. (5) adequate compressive strength and deformation resistance. 6. Good technical performance.

As one of the important special steel varieties, the quality and performance of bearing steel largely reflect the metallurgical level of a country. Of course, the life and reliability of automobile bearings are related to the initial design, manufacturing, lubrication, installation and maintenance, but the raw materials are the key.

By, in auto bearing assembly of several major components, in addition to the cage from the original stamping steel to popularize the nylon material PA66 + 30% GF (such as injection molding) and seal is more often used rubber material (such as ACM + + SUS430 (SPCC), NBR), China's auto bearing roller and the extensive use of the inside and outside the circle is such with wC = 0.95% ~ 1.05%, wCr = 1.40% ~ 1.65% of high carbon chromium bearing steel, to improve the hardenability to meet the needs of the part thickness change, increase content of Mo can develop a series of high hardenability high carbon chromium bearing steel: Cr6, 100, 100 crmo such as Germany, Sweden SKF2, SKF3, 52100.3, 52100.4, in the United States, Japan's SUJ2, SUJ3, SUJ4, SUJ5 etc., applicable to martensitic hardening, also suitable for super thick wall bearing parts of bainite hardening, the chemical composition has a slight difference between them, as the contrast table 1, these can be as a variant of GCr15 steel.

表1  GCr15与100Cr6化学成分（质量分数）对比表（%）

Although the variety is relatively single, it is also the most demanding quality of structural steel (gear steel, bearing steel, spring steel, non-quenched steel, cold heading steel). Strict is the need to improve the purity of steel to strictly reduce O, S, Ca, N, Ti and other trace elements content, also need to control smelting, casting, rolling, forging and other metallurgical process may cause defects, such as vacuum degassing, electroslag remelting, refining furnace smelting process. It is an indisputable fact that reducing oxygen content can significantly prolong the fatigue life of bearings in the industry. FIG. 7 shows the relation curve between oxygen content and the relative life of bearings. In GB/T 18254 -- 2002, the O content of high carbon chromium bearing steel is clearly stipulated: die cast steel ≤15ppm, continuous cast steel ≤12ppm, in actual production, with the development of metallurgical equipment and process more strict control, the lowest level can reach 5ppm. In addition, the technical standards or technical agreement for smelting method, non-metallic inclusion, segregation, decarburization layer, macrostructure, microstructure, carbide inhomogeneity, surface quality and dimension tolerance limit are involved, such as steel mills and the bearing production enterprises in the steel factory, must strictly testing and strict management, shall not be careless.

Need to be added, auto bearing besides most of rolling bearing, only the crankshaft bearing, connecting rod, piston bushing, shock absorber with guidance, gearbox reverse gear bushing, etc as the sliding bearing, they don't involve the heat treatment, rely mainly on powder metallurgy sintering + rolling forming, material is generally on the wC 0.15% or less steel manufacturing (e.g., 08 Al), spindle watts for alloy layer (such as Al - Sn20Cu or common tin, lead, copper or aluminum bearing alloy). The heat treatment discussed later is also for automobile rolling bearings.

3. Heat treatment technology of automobile bearing

The main role of main alloy element chromium in automotive bearing steel is to improve the hardenability of steel, so that parts in quenching, tempering on the whole section to obtain more uniform structure. Chromium can form alloy cementite (Fe, Cr) 3 c, the austenitic grain refinement, when heated to reduce steel overheating sensitivity, improve the wear resistance, and can make the steel quenching when get fine acicular or cryptocrystalline martensite, increase on the basis of high strength steel toughness, generally do not need to cryogenic treatment of auto bearing, unless the parts size stability and the content of residual austenite have special requirements.

(1) production process route of automobile bearing parts

The general production route of steel ball rolling body is: bar material → ball billet hot pier forming → smooth ball (file) → soft grinding → heat treatment → hard grinding → fine grinding → fine grinding (polishing).

The general production route of the inner and outer rings is: pipe material (cold milling) → annealing → turning → soft grinding → heat treatment → grinding → finishing.

The general production route of the cage is: solid cage (nylon) : blank → turning → drawing, drilling window → surface treatment.

Stamping cage (metal) : strip or sheet material → forming → bottom cutting → impact window → pressure slope → expansion → surface treatment.

Auto bearing, outer ring, inner ring rolling body, wheel hub bearing two or three generations of the flange should be proper heat treatment, in order to give full play to the potential of material itself, to obtain the expected performance and improve the service life of the assembly parts, mainly include the spheroidizing annealing, quenching + tempering at low temperature, chemical heat treatment, induction heat treatment and so on.

(2) heat treatment profile of key bearing parts of automobile

The following is a brief introduction of heat treatment equipment, process examples, technical requirements, and development forecasts.

Spheroidal annealing equipment usually USES a protective atmosphere furnace to make the surface of annealed parts less, no oxidation, and can improve the material utilization of bearing parts. The heating temperature of bearing material is 835~850℃, and the spherification temperature is 750~760℃, as shown in FIG. 8.

After the ball rolling body has been forged, the continuous nitrogen base protective atmosphere annealing furnace with aixielin roller bottom is used, as shown in FIG. 9a. The spheroidal annealing temperature was 760℃, oxygen potential ≥900, dew point ≤20℃, propane flow rate 0.18±0.02m3/h, furnace pressure 150~300MPa, hardness ≤210HBW, decarburization layer less than or equal to 0.25mm, mesh carbide ≤CN4.2, the hardness of the steel ball after the whole quenching and tempering requirements of 60HRC above.

Quenching and tempering equipment usually also USES protective atmosphere furnace, such as casting chain furnace, mesh belt furnace or push-rod furnace, as shown in FIG. 9b, with high production efficiency and low energy consumption. The common domestic manufacturers include jiangsu fengdong, north industry, hangzhou jinzhou and so on.

Use on inner and outer protective atmosphere furnace quenching, continuously to reduce the deformation, use KR468G special bearing quenching oil and adjust the heating parameters such as temperature, time, speed of conveyor belt running or can also be used in martensite gradation of salt bath quenching and process curve is shown in figure 10, 63 ~ 64.5 HRC hardness after quenching, after 180 plus or minus 10 ℃ tempering hardness 61 ~ 63 HRC, check to taper ring around 0.05 mm, 0.15 mm or less of the cylindricity.

The second and third generation of automobile hub bearings are equipped with flanges. The material is S55C, and the surface hardness of the quenched area is required to reach more than 60HRC. It is advisable to use automatic induction quenching production line, as shown in FIG. 9c. The cooling medium is water-based quenching solution with adjustable concentration. The pressure of the quenching solution is 0.2-0.6mpa.

Quenching and tempering equipment usually also USES protective atmosphere furnace, such as casting chain furnace, mesh belt furnace or push-rod furnace, as shown in FIG. 9b, with high production efficiency and low energy consumption. The common domestic manufacturers include jiangsu fengdong, north industry, hangzhou jinzhou and so on.

Use on inner and outer protective atmosphere furnace quenching, continuously to reduce the deformation, use KR468G special bearing quenching oil and adjust the heating parameters such as temperature, time, speed of conveyor belt running or can also be used in martensite gradation of salt bath quenching and process curve is shown in figure 10, 63 ~ 64.5 HRC hardness after quenching, after 180 plus or minus 10 ℃ tempering hardness 61 ~ 63 HRC, check to taper ring around 0.05 mm, 0.15 mm or less of the cylindricity.

The second and third generation of automobile hub bearings are equipped with flanges. The material is S55C, and the surface hardness of the quenched area is required to reach more than 60HRC. It is advisable to use automatic induction quenching production line, as shown in FIG. 9c. The cooling medium is water-based quenching solution with adjustable concentration. The pressure of the quenching solution is 0.2-0.6mpa.

With the continuous improvement of domestic heat treatment equipment and production technology, the application in the production and processing of automotive bearing parts has been basically mature. The original box furnace, well furnace, salt bath furnace, drum furnace, ordinary air heating furnace and other equipment has been basically eliminated, now a large number of popular is the protective atmosphere equipment assisted by pressure swing adsorption, film nitrogen production and other technologies, with roller mesh belt furnace as the mainstream of the overall quench return line and automatic induction quenching return line has become the mainstream. In addition, the transition from protective atmosphere to controllable atmosphere and the furnace control system from single-wire computer control to cluster computer control will gradually develop into two major trends.