Talking about the smelting requirements of steel for rolling bearings
The service life and reliability of rolling bearings are closely related to the quality of the steel used for bearing. Due to the characteristics of bearing steel, the quality of smelting is much stricter than that of general industrial steel, such as the chemical composition, purity, organization and uniformity of steel.
Strict chemical composition requirements
Generally, the steel for bearing is mainly high carbon chromium bearing steel, that is, hypereutectoid steel containing about 1% of carbon, adding about 1.5% of chromium, and containing a small amount of manganese and silicon. Chromium can improve heat treatment performance, improve hardenability, microstructure uniformity, tempering stability, and improve the rust resistance and grinding performance of steel. However, when the chromium content exceeds 1.65%, the retained austenite in the steel is increased after quenching, the hardness and dimensional stability are lowered, the carbide non-uniformity is increased, and the impact toughness and fatigue strength of the steel are lowered. For this reason, the chromium content in high carbon chromium bearing steel is generally controlled below 1,65%. Only by strictly controlling the chemical composition of the bearing steel can the microstructure and hardness satisfying the bearing performance be obtained through the heat treatment process.
Second, high precision size requirements
Steel for rolling bearings requires high dimensional accuracy of the steel because most of the bearing parts are pressure molded. In order to save materials and increase labor productivity, most of the bearing rings are forged, the steel balls are cold-rolled or hot-rolled, and the small-sized rollers are also cold-formed. If the dimensional accuracy of the steel is not high, the size and weight of the blank cannot be accurately calculated, and the product quality of the bearing parts cannot be guaranteed, and the damage of the equipment and the mold is easily caused.
Third, the particularly strict purity requirements
The purity of steel refers to the amount of non-metallic inclusions contained in the steel. The higher the purity, the less non-metallic inclusions in the steel. Harmful inclusions such as oxides and silicates in bearing steel are the main causes of early fatigue spalling of bearings and significant reduction in bearing life. In particular, brittle inclusions are the most harmful, and they are easily peeled 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, it is necessary to reduce the content of inclusions in the bearing steel.
Fourth, strict low-fold tissue and microscopic (high-fold tissue requirements
The low-magnification structure of bearing steel refers to general looseness, center looseness and flatness, and microscopic (high-fold structure including steel annealed structure, carbide network, band and liquid analysis, etc.. Carbide liquid is hard and brittle, its The hazard is the same as the brittle inclusion. The reticulated carbide reduces the impact toughness of the steel and makes it unevenly organized, and is easily deformed and cracked during quenching. The banded carbide affects the annealing and quenching and tempering structure and the contact fatigue strength. The advantages and disadvantages of high-magnification organization have a great influence on the performance and service life of rolling bearings. Therefore, there are strict requirements for low and high-fold organization in bearing material standards.
5. Particularly strict surface defects and internal defect requirements
For bearing steel, surface defects include cracks, slag inclusions, burrs, crusting, oxide scales, etc. Internal defects include shrinkage cavities, bubbles, white spots, severe looseness and segregation. These defects have a great influence on the processing of the bearing, the performance and the life of the bearing, and it is clearly stated in the bearing material standard that these defects are not allowed.
Strict chemical composition requirements
Generally, the steel for bearing is mainly high carbon chromium bearing steel, that is, hypereutectoid steel containing about 1% of carbon, adding about 1.5% of chromium, and containing a small amount of manganese and silicon. Chromium can improve heat treatment performance, improve hardenability, microstructure uniformity, tempering stability, and improve the rust resistance and grinding performance of steel. However, when the chromium content exceeds 1.65%, the retained austenite in the steel is increased after quenching, the hardness and dimensional stability are lowered, the carbide non-uniformity is increased, and the impact toughness and fatigue strength of the steel are lowered. For this reason, the chromium content in high carbon chromium bearing steel is generally controlled below 1,65%. Only by strictly controlling the chemical composition of the bearing steel can the microstructure and hardness satisfying the bearing performance be obtained through the heat treatment process.
Second, high precision size requirements
Steel for rolling bearings requires high dimensional accuracy of the steel because most of the bearing parts are pressure molded. In order to save materials and increase labor productivity, most of the bearing rings are forged, the steel balls are cold-rolled or hot-rolled, and the small-sized rollers are also cold-formed. If the dimensional accuracy of the steel is not high, the size and weight of the blank cannot be accurately calculated, and the product quality of the bearing parts cannot be guaranteed, and the damage of the equipment and the mold is easily caused.
Third, the particularly strict purity requirements
The purity of steel refers to the amount of non-metallic inclusions contained in the steel. The higher the purity, the less non-metallic inclusions in the steel. Harmful inclusions such as oxides and silicates in bearing steel are the main causes of early fatigue spalling of bearings and significant reduction in bearing life. In particular, brittle inclusions are the most harmful, and they are easily peeled 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, it is necessary to reduce the content of inclusions in the bearing steel.
Fourth, strict low-fold tissue and microscopic (high-fold tissue requirements
The low-magnification structure of bearing steel refers to general looseness, center looseness and flatness, and microscopic (high-fold structure including steel annealed structure, carbide network, band and liquid analysis, etc.. Carbide liquid is hard and brittle, its The hazard is the same as the brittle inclusion. The reticulated carbide reduces the impact toughness of the steel and makes it unevenly organized, and is easily deformed and cracked during quenching. The banded carbide affects the annealing and quenching and tempering structure and the contact fatigue strength. The advantages and disadvantages of high-magnification organization have a great influence on the performance and service life of rolling bearings. Therefore, there are strict requirements for low and high-fold organization in bearing material standards.
5. Particularly strict surface defects and internal defect requirements
For bearing steel, surface defects include cracks, slag inclusions, burrs, crusting, oxide scales, etc. Internal defects include shrinkage cavities, bubbles, white spots, severe looseness and segregation. These defects have a great influence on the processing of the bearing, the performance and the life of the bearing, and it is clearly stated in the bearing material standard that these defects are not allowed.
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