**Control of Brittle Inclusions in GCr15 Bearing Steel**
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Source: China Bearing Network | Time: March 10, 2014
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Brittle inclusions in bearing steel can significantly reduce the fatigue life and mechanical properties of the material, negatively impacting its overall quality. Microscopically, GCr15 bearing steel contains various types of inclusions, including D-type (O-Al-Ca-S), O-Al-Mg-Ca-(S), and Ti-N-Cr, as well as B-type inclusions such as O-Al-Ca and O-Al-Mg-Ca. These inclusions are primarily influenced by the slag system, refractory materials, and vacuum treatment processes.
To effectively control these brittle inclusions, the following strategies should be implemented:
1. **Slag System Optimization**:
Choosing an appropriate slag system is crucial. A CaO (40-60%) - Al₂O₃ (20-40%) - SiO₂ (4-10%) - MgO (5-12%) composition is recommended for domestic bearing steels. Two main types exist: high-alkalinity (CaO/SiO₂ > 3) and low-alkalinity (CaO/SiO₂ ≈ 2). High-alkalinity slag systems are preferred because they result in lower residual impurities and better slag fluidity. This improves the ability of the slag to absorb non-metallic inclusions, leading to a more stable and less brittle final product.
2. **Refractory Material Selection**:
Since the steel temperature during processing can reach up to 1600°C, refractory materials must withstand extreme conditions. For high-alkalinity operations, high-magnesium oxide-based refractories are used for ladles, tundishes, and nozzles. Low-alkalinity operations benefit from chromium oxide or high-alumina materials. Zirconia-containing materials are often used for continuous casting nozzles to ensure durability and minimize contamination.
3. **Vacuum Treatment**:
Under VD (vacuum degassing) conditions, extending the argon stirring time helps complete the reaction between the slag and molten steel, allowing non-metallic inclusions to float more easily. The stirring intensity should be carefully controlled to avoid damaging the refractory lining or introducing new contaminants. Proper vacuum treatment ensures that the molten steel remains free from oxidation and that inclusions are efficiently removed.
In addition, maintaining a low total oxygen content in the steel ([O] ≤ 8 × 10â»â¶) is essential for improving the purity of the molten steel. By implementing these methods, Chinese steel producers like Xingcheng Special Steel have successfully controlled brittle inclusions in GCr15 bearing steel, meeting industrial production requirements.
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This article was originally published on China Bearing Network.
Please visit [China Bearing Network](http://www.chinabearing.net) for more information.
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