Formation Mechanism of Al2O3-Containing Inclusions in Al-Deoxidized Spring Steel
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The source, characteristics, and mechanism of Al2O3-containing inclusions in Al-deoxidized spring steel were investigated using electron-probe X-ray microanalysis (EPMA). Spring samples were collected during vacuum degassing (VD) refining, in a tundish ladle, and after hot rolling, respectively. Based on primary inclusion components, seven types of inclusions were observed through the manufacturing process: Al2O3, Al2O3-SiO2, Al2O3-CaO, Al2O3-MgO, Al2O3-MgO-CaO, Al2O3-SiO2-MnO, and Al2O3-SiO2-CaO. The Al2O3 and Al2O3-SiO2 inclusions were mainly attributed to deoxidization products, less than 15 μm in diameter and with liquidus temperatures exceeding 1600 °C. For Al2O3-CaO inclusions, which were considered to be formed by the reduction of entrapped slag by Al dissolved in the steel, the Al2O3/CaO ratio obviously decreased with the increase of inclusion sizes. Most Al2O3-SiO2-CaO inclusions were less than 15 μm in diameter, with their composition close to that of the refining slag and a liquidus temperature near 1500 °C. The Al2O3-MgO and Al2O3-SiO2-MnO inclusions originated from inherent reactions between dissolved [Al], [Si], [Mn], [Mg], and [O] in the steel. Al2O3-MgO-CaO inclusions resulted from coalescence between Al2O3-MgO and Al2O3-CaO inclusions.
This work was supported by the Australian Research Council and Baosteel Australia Research and Development Centre. The University of Queensland International Research Tuition Award and China Scholarship Council provided scholarships for Mr. Sha Lyu. The Australian Microscopy & Microanalysis Research Facility is acknowledged for providing characterization facilities. Technical support for the EPMA facility from Mr. Ron Rasch and Ms. Ying Yu, Centre for Microscopy and Microanalysis, University of Queensland, is gratefully acknowledged.
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