XEOL and EPMA–CL for On-Line and On-Site Analysis of Inclusions in Steel 1145


observation and elemental analysis of the model sample were also carried out using a SEM (Mighty-8DXL, TECHNEX, Tokyo, Japan) equipped with a silicon drift X-ray detector (AXAS-D, KETEK GmbH, Munich, Germany) at the acceleration voltage of 17 kV.


RESULTS AND DISCUSSION


Figure 2. Schematic illustrations of the portable (a)cathodo- luminescence spectrometer and (b) electron probe microanalyzer.


The LiTaO3 crystal was heated to 180°C for more than 2 min and then cooled to 0°C by reversing the DC power supply polarity. The acceleration voltage of the electron beam was ~10 kV. A CL image of the model sample was captured during the cooling cycle with the same digital single-lens reflex camera as used for capturing the XEOL image through the quartz viewport. The exposure time was 80 s. A platinum–iridium alloy (Pt0.8Ir0.2) wire with a sharp tip and its holder were attached on the −z plane of the LiTaO3 single crystal for elemental analysis of the model sample using the portable EPMA as shown in Figure 2b. The details of the preparation for the Pt0.8Ir0.2 wire with a sharp tip and its holder were reported in our previous paper (Imashuku & Wagatsuma, 2017). The analyzed positions of the sample were controlled with a linear transition manual stage connected to the brass metal rod and monitored using the digital single-lens reflex camera. The LiTaO3 crystal was heated to 130°C for more than 2 min and then cooled to room temperature by disconnecting the DC power supply. X-ray spectra were measured for 60 s of the cooling cycle with a silicon drift X-ray detector (RES-Labo Co., Osaka, Japan). The pressure of the sample chamber was maintained at 1 Pa during the measurements using a portable rotary pump for both the CL spectrometer and EPMA. Surface


XEOL Analysis On-line identification of nonmetallic inclusions in steel contributes to the reduction of time for the analysis and the improvement of the production efficiency of steel because nonmetallic inclusions are normally analyzed off-line. XEOL analysis is a promising method for the on-line identification of these inclusions because XEOL analysis can be conducted in air. We thus attempted to identify particles of MgAl2O4 spinel and Al2O3, which cause serious problems in steel, in the model sample containing these particles by XEOL analysis. XEOL images of the model sample are shown in Figures 3b, 3c which is an enlarged image of Figure 3b, with the photograph of the sample before irradiation with X-rays (Fig. 3a) and the corresponding SEM image (Fig. 3d) of the same area as is shown in Figure 3c. Particles producing green or blue luminescence were observed in the XEOL images. The illuminated area in Figure 3c corresponded to particles observed in the SEM image. The particles producing green and blue luminescencewere confirmed to be MgAl2O4 spinel and Al2O3, respectively, by energy-dispersive X-ray (EDX) point analysis of the five particles (particles 1–5 in Fig. 3d) in the SEM image. We also confirmed the luminescent colors of MgAl2O4 spinel and Al2O3 by capturing XEOL images of their respective reagent powders. The results, therefore, indicate thatwe can identifyMgAl2O4 spinel and Al2O3 from their luminescent colors in air by irradiating X-rays on the model sample. Comparing MgAl2O4 spinel and Al2O3 par- ticles with similar size, such as particles 1 and 4, it could be shown that the XEOL intensity of MgAl2O4 spinel is higher than that of Al2O3. This trend is consistent with the results that we have previously obtained by investigating the CL spectra of a single particle with the size of approximately 100 μm consisting of MgAl2O4 spinel and Al2O3 phases; the CL intensity of MgAl2O4 spinel was ~10 times higher than that of Al2O3 (Imashuku & Wagatsuma, 2017). Thus, when we used the digital single-lens reflex camera, the number of particles producing green luminescence was larger than that of particles producing blue luminescence, as shown in Figure 2b, although almost the same amounts of MgAl2O4 spinel and Al2O3 were present in the model sample. The XEOL intensities of the particles increased with increasing size for both MgAl2O4 spinel and Al2O3 particles. MgAl2O4 spinel with a size of 30 μm(particle 3 in Fig. 3d) was detected, whereas an MgAl2O4 spinel with a size of 15 μm(particle 6 in Fig. 3d), which was confirmed to be MgAl2O4 spinel by EDX point analysis, was not detected by XEOL under these experimental conditions. In addition, XEOL intensities of the MgAl2O4 spinel and Al2O3 drastically decreased by


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