Characteristics, sources analysis of large size inclusions and technical improvement during bearing steel production
LONG Hu1, CHENG Guo-guang2, QIU Wen-sheng1, ZENG Ling-yu1, YU Da-hua1, LIU Dong3
1. Technology Center, Baowu Group Guangdong Shaoguan Iron and Steel Co. , Ltd. , Shaoguan 512123, Guangdong, China; 2. State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing,Beijing 100083, China; 3. Baosteel Special Steel Shaoguan Co. , Ltd. , Shaoguan 512123, Guangdong, China
Abstract:Based on the BOF-ARS(argon stirring)-LF-RH-CC process of bearing steel production in Shaoguan Steel Plant, the characteristics and sources of large size inclusions were explored through the method of water immersion ultrasonic test combined with metalloscope, scanning electron microscope and the systematic sampling during the metallurgical-continuous casting process, and the improved process was proposed. Results showed that there were mainly two kinds of large inclusions, one was low-melting CaO-MgO-Al2O3-SiO2 inclusion from 6% to 7% SiO2(mass percent), whose size was from 50 to 500 μm, and the other was CaO-MgO-Al2O3 without SiO2, whose size was larger than 500 μm. The source of the former was the slag entrapment caused by the uneven slagging, which was the result of the combined charge of low basicity slag with high viscosity and high melting point lime during steel tapping. The latter was induced by the charge of large bulk of calcium-aluminate slag during the refining process, which was difficult to be melted rapidly and was entrapped into steel. Therefore, the design of refining slag and the optimization of the slagging process were the key points to decrease the large size inclusions. The improved slagging technology was applied by feeding the large bulk of calcium-aluminate slag during tapping in advance, instead of the low-basicity slag, and the addition amount of other slag was reduced during LF refining process. The basicity of refining slag (w(CaO)/w(SiO2)) was controlled in the range from 5 to 9, and the mass percent of Al2O3 was from 23% to 28%. After the improvement, the fluidity of slag was good, and nozzle clogging was reduced. The main inclusions in the products were micro MgO-Al2O3 spinel and composite sulfides. The qualified rate of bearing steel products evaluated by ultrasonic test was significantly improved.
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