Discussion on the Cleanliness of GCr15 Bearing Steel Produced by the Converter Process
The production process of GCr15 bearing steel using the converter, LF, RH, and continuous casting route was studied. Systematic sampling and analysis were conducted to investigate the total oxygen (T[O]) content and non-metallic inclusions at various stages—converter, LF, RH, tundish, and slab. The results showed that the average T[O] level in the slab was 12×10-6, while in the final wire rod it dropped to 9×10-6. The amount of macro-inclusions in the slab was found to be 2.68 mg/10kg. These findings indicate that with a properly optimized process, the converter process can produce high-purity GCr15 bearing steel.
Keywords: Converter; Bearing Steel; Inclusions; Cleanliness
Authors: JIA Nan1, WU Huajie1, BAO Yanping1, XUE Zhengxue2, WU Xun1, MA Fuping2
1 University of Science and Technology Beijing, 2 Xingtai Iron & Steel Co., Ltd.
This research focused on the cleanliness of GCr15 bearing steel produced through the BOF-LF-RH-CC process. A systematic study of the total oxygen content and the behavior of non-metallic inclusions was carried out. It was found that the average total oxygen content in the billet is 12×10-6, while in the wire rod it is 9×10-6. The average amount of macro-inclusions in the casting billet is 2.68 mg/10kg. The results demonstrate that high-cleanliness GCr15 bearing steel can indeed be produced via the converter process.
Key words: BOF; Bearing Steel; Inclusion; Cleanliness
1. Introduction
The development of modern technology has increased the demand for bearings with longer lifespan, higher stability, and greater reliability. This necessitates minimizing impurities in bearing steel. One of the key factors affecting bearing life is the cleanliness of the steel. Cleanliness primarily refers to the oxygen content and the quantity, size, and distribution of impurities in the steel. Due to the presence of various elements in molten iron, achieving high cleanliness requires careful control throughout the production process.
2. Production Process and Research Methods
The process for producing bearing steel includes: blast furnace → top and bottom combined blowing converter → LF → RH → continuous casting. Molten iron from the blast furnace is smelted with scrap steel in the converter, then tapped. Deoxidation and alloying are performed. The LF process uses white slag, and the steel is vacuum-degassed in the RH furnace. The continuous casting machine is a four-stream curved caster with electromagnetic stirring in the mold.
Samples were taken from six different stages using a bucket sampler. Metallographic samples were ground, polished, and analyzed under an optical microscope. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to analyze the composition of inclusions. Large sample electrolysis was applied to study inclusions larger than 50μm in the casting billet.
3. Results and Analysis
3.1 Total Oxygen and Nitrogen Content in Each Stage
The total oxygen content decreased significantly after the converter tapping. The average T[O] value at the end of the converter was 56×10-6, which reduced to 16×10-6 after the LF process. Further reduction occurred in the RH stage, where T[O] was 11×10-6. During the tundish stage, T[O] slightly increased to 13×10-6, but the final wire rod had T[O] at 9×10-6, below the national standard requirement of 12×10-6.
Nitrogen content was lowest at the end of the converter (20×10-6). After tapping, nitrogen increased by 25% during the transport to LF, and further rose by 36% in the LF process. However, the RH process helped reduce nitrogen by 9×10-6. The final wire rod had [N] at 30×10-6.
3.2 Non-Metallic Inclusions in the Process
Four types of inclusions were identified: Al2O3, MnS, calcium aluminate, and calcium aluminate + sulfide composites. Before LF treatment, Al2O3 was dominant, while after LF, it transformed into massive Al2O3. Inclusions were mainly between 0–10 μm, with 30.6% in the 0–2 μm range.
3.3 Inclusions in the Slab
Three main types of inclusions were found in the slab: calcium aluminate, calcium aluminate + sulfide, and Ti-rich inclusions. Most inclusions were smaller than 5 μm, but some exceeded 10 μm. SEM and EDS confirmed their composition. Proper control of the casting process can help reduce these large inclusions and improve overall cleanliness.
4. Conclusion
1) The converter → LF → RH → continuous casting process is effective in producing high-cleanliness GCr15 bearing steel, with T[O] levels as low as 9×10-6.
2) The main impurities include Al2O3, MnS, calcium aluminate, and calcium aluminate + sulfide. Their formation is related to deoxidizers and residual elements in the steel.
3) Three types of inclusions were identified in the slab, and optimizing the casting process can further enhance the cleanliness of the steel.
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