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Metallurgical and Materials Transactions B

, Volume 50, Issue 1, pp 459–470 | Cite as

Formation of Plastic Inclusions in U71Mnk High-Speed Heavy-Rail Steel Refined by CaO-SiO2-Al2O3-MgO Slag

  • Hua Zhang
  • Chengsong LiuEmail author
  • Qiang Lin
  • Bao Wang
  • Xiaoqin Liu
  • Qing Fang
Article
  • 142 Downloads

Abstract

To precisely control the characteristics of nonmetallic inclusions with high plasticity in U71Mnk high-speed heavy-rail steel deoxidized by Si and Mn, the chemical composition of CaO-SiO2-Al2O3-MgO slag in the ladle furnace (LF) refining process was optimized by thermodynamic calculations. Relationships among the refining slag, nonmetallic inclusions, and elemental concentrations in the steels were analyzed to obtain the conditions for determining the optimal composition of refining slag. The optimal compositions and the component activities of CaO-SiO2-Al2O3-MgO refining slag were determined and discussed according to calculations using FactSage 7.0. Finally, ten industrial experiments were performed. Variations in the compositions of LF refining slag, inclusions, and steel, as well as the sizes of the inclusions, and metallographic classification assessment of the U71Mnk steel product, were investigated and discussed to verify the feasibility and effectiveness of the optimized LF refining slag. Kinetics of steel–slag and steel-inclusion reactions during LF refining was studied and compared. The results show that to obtain low [O], [Al], and [S] contents and inclusions with high plasticity in molten steel, the optimal composition of the LF refining slag should be CaO: 44.2 to 51.7 wt pct, SiO2: 41.4 to 47.0 wt pct, Al2O3: 0 to 4.7 wt pct and MgO 5.0 to 7.0 wt pct, where the basicity R is above 1.00 and the C/A ratio is above 9.00. After the LF refining process, the contents of total oxygen (T.O.), [Al], and [S] gradually stabilized in the range of 0.0008 to 0.0012 wt pct, 0.002 to 0.0028 wt pct and 0.0026 to 0.0037 wt pct, respectively. The densities of the three kinds of typical inclusions (CaO-SiO2-Al2O3-MgO, MnS, and CaO-SiO2-Al2O3-MgO-MnS) also decreased and reached approximately 0.25/mm2, and no inclusion larger than 6.0 μm was found. The composition of the inclusions gradually changed into the optimal range in the CaO-SiO2-Al2O3-MgO slag system during the LF refining process.

Notes

Acknowledgments

The current study was supported by the National Natural Science Foundation of China (Grant Nos. 51774217 and 51604201), and the scholarship from China Scholarship Council (CSC) under the Grant CSC No. 201708420228. The authors also gratefully acknowledge the fruitful discussions on deoxidation with Professor G.Q. Li from the Wuhan University of Science and Technology.

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Copyright information

© The Minerals, Metals & Materials Society and ASM International 2018

Authors and Affiliations

  • Hua Zhang
    • 1
    • 2
  • Chengsong Liu
    • 1
    • 3
    Email author
  • Qiang Lin
    • 4
  • Bao Wang
    • 1
  • Xiaoqin Liu
    • 5
  • Qing Fang
    • 1
  1. 1.The State Key Laboratory of Refractories and MetallurgyWuhan University of Science and TechnologyWuhanP.R. China
  2. 2.Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of EducationWuhan University of Science and TechnologyWuhanP.R. China
  3. 3.Department of Materials Science and EngineeringCarnegie Mellon UniversityPittsburghUSA
  4. 4.Forging Business DepartmentHubei Xinyegang Steel Co., Ltd.HuangshiP.R. China
  5. 5.Engineering Training CenterWuhan University of Science and TechnologyWuhanP.R. China

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