Nonmetallic inclusions in steel cause problems in steel products and steel production. In particular, an analysis of Al2O3 and MgO·Al2O3 spinel inclusions is important since they are one of the most harmful inclusions. A rapid and simple analysis of nonmetallic inclusions is required as the conventional analytical methods for nonmetallic inclusions are time-consuming. In this study, we propose a simple method to identify Al2O3 and MgO·Al2O3 spinel inclusions in steels. X-ray-excited optical luminescence (XEOL) analysis was selected as a promising method because it can rapidly identify sizes, shapes, and compositions of nonmetallic inclusions and can be performed in air. A model steel sample prepared by heating a mixture of Fe, Al, and MgO powders at 1550°C in argon atmosphere was used. XEOL images of the model steel sample showed that Al2O3 inclusions emitted blue and red luminescences. Using a filter attached to the camera, blocking light in the wavelength region above 650 nm, only the blue luminescence of the Al2O3 inclusions was observed. This was implemented as the luminescences of the Al2O3 inclusions appeared in both blue and red regions at wavelengths of 350, 485, 695, and 750 nm; consequently, the luminescences at 695 nm and 750 nm were blocked by the filter. In contrast, MgO·Al2O3 spinel inclusions emitted green luminescence (peak at 520 nm), unaffected by the filter. This indicates that we can simply identify Al2O3 and MgO·Al2O3 spinel inclusions by an XEOL image in the wavelength range of 420–650 nm.
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