The production of bearing steel mainly implements the GB / T18254-2002 standard and the Laiwu Steel GCr15JD quality agreement adapted to the requirements of precision forging bearing users. The quality requirements of the GCr15JD agreement are stricter than the GB / T18254-2002 standard. The GCr15JD requires oxygen content ≤10ppm and central segregation level. Grade ≤1.0, composition control, sizing and dimensional deviation are all stricter than GB / T18254-2002 standard.

Bearings are subjected to great pressure and friction during work, so bearing steel is required to have high and uniform hardness and wear resistance, as well as high elastic limit. The requirements for the uniformity of the chemical composition of bearing steel, the content and distribution of non-metallic inclusions, and the distribution of carbides are very strict. It is one of the most stringent steel types in all steel production. In 1976, the International Organization for Standardization (ISO) included some common bearing steel numbers into international standards, and divided bearing steel into four types: total hardened bearing steel, surface-hardened bearing steel, stainless bearing steel, and high-temperature bearing steel. Steel number.

Some countries add a category of bearing steel or alloy for special purposes. China has included standard bearing steel classification methods similar to ISO, corresponding to four categories of high-carbon chromium bearing steel, carburized bearing steel, stainless corrosion-resistant bearing steel, and high-temperature bearing steel. In the past 50 years, China has also made great progress in bearing steel types and bearing materials, such as chromium-free bearing steel, medium carbon bearing steel, special-purpose bearing steel and alloys, and cermets.

basic requirements

According to the above basic requirements for bearing steel, the following basic requirements for the metallurgical quality of bearing steel are proposed;

① Strict chemical composition requirements.

The general bearing steel is mainly high-carbon chromium bearing steel, that is, a carbon content of about 1%, added about 1.5% of chromium, and contains a small amount of manganese, silicon hypereutectoid steel. Chromium can improve heat treatment performance, hardenability, microstructure uniformity, and tempering stability, and can also improve the rust prevention and grinding performance of steel.

However, when the chromium content exceeds 1.65%, the residual austenite in the steel will increase after quenching, reduce the hardness and dimensional stability, increase the heterogeneity of carbides, and reduce the impact toughness and fatigue strength of the steel. For this reason, the chromium content in high-carbon chromium bearing steels is generally controlled below 1,65%. Only by strictly controlling the chemical composition in the bearing steel can the microstructure and hardness that meet the performance of the bearing be obtained through the heat treatment process.

② Higher dimensional accuracy requirements. For hot-rolled and annealed bars forged on high-speed upset forging machines, there should be higher requirements for their dimensional accuracy.

Rolling bearing steel requires high dimensional accuracy of steel, because most bearing parts are subjected to pressure forming. In order to save material and improve labor productivity, most of the bearing rings are forged. Steel balls are cold-formed or hot-rolled. Small-sized rollers are also cold-formed. If the dimensional accuracy of the steel is not high, the size and weight of the blanking cannot be accurately calculated, the product quality of the bearing parts cannot be guaranteed, and equipment and molds are easily damaged.

③ Particularly strict purity requirements.

The purity of steel refers to the amount of non-metallic inclusions in the steel. The higher the purity, the less non-metallic inclusions in the steel. The harmful inclusions such as oxides and silicates in the bearing steel are the main reasons leading to the early fatigue spalling of the bearing and significantly reducing the bearing life. In particular, brittle inclusions are the most harmful, because they are easy to peel off from the metal substrate during processing, which seriously affects the surface quality of the bearing parts after finishing. Therefore, in order to improve the service life and reliability of the bearing, the content of inclusions in the bearing steel must be reduced.

④ Strict requirements for low magnification and microscopic (high magnification) organization.

The low magnification structure of bearing steel refers to general looseness, central looseness and segregation, and the microscopic (high magnification) structure includes annealed structure, carbide network, band shape, and liquid precipitation of steel. The carbide solution is hard and brittle, and its harm is the same as that of brittle inclusions. Reticulated carbides reduce the impact toughness of steel and make its structure non-uniform, which is easy to deform and crack during quenching. Banded carbides affect the annealed and quenched and tempered structure and contact fatigue strength. The pros and cons of low and high magnification organizations have a great impact on the performance and service life of rolling bearings. Therefore, low and high magnification organizations have strict requirements in bearing material standards.

⑤ Particularly strict requirements for surface defects and internal defects.

For bearing steel, surface defects include cracks, slag inclusions, burrs, scars, scale, etc. Internal defects include shrinkage, bubbles, white spots, severe porosity, and segregation. These defects have a great impact on bearing processing, bearing performance and life, and it is clearly stipulated in the bearing material standards that these defects are not allowed.

⑥ Strict requirements for carbide heterogeneity.

In bearing steel, if there is serious uneven distribution of carbides, it is easy to cause non-uniformity in structure and hardness during heat treatment processing, and the non-uniformity of steel structure has a greater impact on contact fatigue strength. In addition, severe carbide inhomogeneities can easily cause cracks in bearing parts during quenching and cooling, and carbide inhomogeneities can also lead to reduced bearing life. Therefore, in bearing material standards, different specifications of steel materials are clearly defined. special requirement.

⑦Strict requirements for the depth of the surface decarburized layer.

The bearing material standard has strict regulations on the surface decarburization layer of the steel. If the surface decarburization layer exceeds the scope of the standard, and it is not completely removed during the processing before heat treatment, it will be removed during the heat treatment and quenching. It is easy to produce quench cracks and cause scrapping of parts.

⑧ Other requirements.

The bearing steel material standard also has strict requirements on bearing steel smelting methods, oxygen content, annealing hardness, fractures, residual elements, spark inspection, delivery status, and labeling.

Performance requirements

In order to meet the above requirements for the performance of dynamic bearings, the following basic performance requirements are proposed for bearing steel materials:

1) high contact fatigue strength,

2) After heat treatment, it should have high hardness or a hardness that can meet the requirements of bearing performance.

3) High abrasion resistance and low coefficient of friction,

4) High elastic limit,

5) Good impact toughness and fracture toughness,

6) Good dimensional stability,

7) Good anti-rust performance,

8) Good cold and hot workability.

Manufacturing requirements

The content of inclusions is closely related to the oxygen content of steel. The higher the oxygen content, the greater the number of inclusions and the shorter the life.

The larger the particle size of the inclusions and carbides, the more uneven the distribution, and the shorter the service life. Their size and distribution are closely related to the smelting process and smelting quality used. The main processes for producing bearing steel are continuous casting and electric furnaces. Smelting + electroslag remelting process smelting, a small amount of vacuum induction + vacuum self-consumption double vacuum or + multiple vacuum self-consumption processes are used to improve the quality of bearing steel.

The smelting quality of bearing steel is very high, and the content of sulfur, phosphorus, hydrogen, and the number, size, and distribution of non-metallic inclusions and carbides need to be strictly controlled because the number, size, and distribution of non-metallic inclusions and carbides. The condition has a great impact on the service life of the bearing steel, and often the failure of the bearing is caused by the propagation of micro-cracks around large inclusions or carbides.