Abstract
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.
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