Formation and evolution of magnesia-alumina spinel in Si-Mn deoxidized 55SiCr spring steel
ZENG Yi-bin1, BAO Yan-ping1, ZHAO Jia-qi1,2, WANG Min1
1. State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China; 2. Steelmaking and Continuous Casting Laboratory, Institute of Research of Iron and Steel, Shasteel, Zhangjiagang 215625, Jiangsu, China
Abstract：The 55SiCr spring steel produced by a steel mill adopts silicon-manganese deoxidization process, but there are a lot of spinel inclusions in its smelting process, which is very unfavorable to the final product performance. Hard and brittle inclusions are one of the main factors during fatigue fracture of spring in service, so in order to clarify the source of spinel inclusions in spring steel and then control and remove non-metallic inclusions in steel, the evolution of inclusions and formation mechanism of main inclusions in 55SiCr spring steel during smelting were analyzed by means of automatic inclusion analysis, scanning electron microscope and energy spectrum analysis, combined with FactSage thermodynamic calculation. The analysis results show that the number of inclusions in the steel after LF refining increases greatly, and its average composition tends to the high melting point region in the ternary phase diagram of SiO2-Al2O3-CaO. The inclusions are mainly SiO2·Al2O3·CaO·MgO, mostly in the form of calcium aluminate-wrapped or semi-wrapped spinel composite inclusions, and a small amount of single spinel inclusions exist in steel. Thermodynamic calculation of formation and evolution for above inclusions shows that increase of Mg and Al content in molten steel will lead to the precipitation of a large number of spinel inclusions in steel and combine with liquid inclusions to form Mg-containing multiphase inclusions. At the same time, the change of molten steel composition will also lead to the substantial increase of MgO and Al2O3 contents in SiO2·Al2O3·CaO·MgO inclusions generated during refining, and the spinel phase will be precipitated in the composite inclusions. Therefore, in order to reduce the formation of spinel-like hard and brittle inclusions in Si-Mn deoxidized spring steel, it is necessary to control the contents of Mg and Al in the steel strictly and reduce the contents of MgO and Al2O3 in the inclusions as much as possible, so as to realize the plastic control of non-metallic inclusions in spring steel.
曾溢彬, 包燕平, 赵家七, 王敏. 硅锰脱氧55SiCr弹簧钢中镁铝尖晶石的形成及演变[J]. 钢铁, 2022, 57(8): 69-77.
ZENG Yi-bin, BAO Yan-ping, ZHAO Jia-qi, WANG Min. Formation and evolution of magnesia-alumina spinel in Si-Mn deoxidized 55SiCr spring steel[J]. Iron and Steel, 2022, 57(8): 69-77.
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2022, Vol. 57 
