Cathodoluminescence Analysis of Nonmetallic Inclusions in Steel Deoxidized and Desulfurized by Rare-Earth Metals (La, Ce
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INTRODUCTION
DEOXIDIZATION and desulfurization are highly important processes for producing high-quality steels. In general, aluminum (Al), silicon (Si), and manganese (Mn) are used as deoxidizers, whereas calcium oxide (CaO), magnesium (Mg), calcium carbide (CaC2), and sodium carbonate (Na2CO3) are used as desulfurizers. The oxides and sulfides that form during deoxidization and desulfurization, such as alumina (Al2O3), spinel (MgAl2O4), silica (SiO2), calcium aluminates, manganese sulfide (MnS), and calcium sulfide (CaS), become nonmetallic inclusions in steel. Such inclusions may induce quality degradations in steel, such as surface flaws, hydrogen-induced cracking, decreased resistance to corrosion, fatigue failure, reduced in ductility, and low-temperature embrittlement.[1–5] The contribution of nonmetallic inclusions to the abovementioned problems can be relieved by reducing the size of nonmetallic inclusions[1,2,6] (below ca. 10 lm[3,4]), dispersing nonmetallic inclusions,[1,2,6] and removing harmful SUSUMU IMASHUKU and KAZUAKI WAGATSUMA are with the Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Contact e-mail: [email protected] Manuscript submitted April 16, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
nonmetallic inclusions,[1,6,7] such as Al2O3, MgAl2O4, and MnS. When misch metal, which primarily consists of the rare-earth metals of lanthanum (La), cerium (Ce), and neodymium (Nd), is used as a deoxidizer and desulfurizer, the size of nonmetallic inclusions can be reduced to below 5 lm[8–12]; moreover, the nonmetallic inclusions exhibit a disperse distribution,[10,13] and the formation of MnS inclusions, which act as initiation sites for pitting corrosion and create cracks in stainless steels, is suppressed by the formation of rare-earth oxysulfides.[9,11,12,14,15] Additionally, rare-earth metals can reduce the amount of oxygen and sulfur in steel to a magnitude of 10 4 mass pct.[13] Therefore, misch metal is a promising deoxidizer and desulfurizer for producing high-quality steel. Oxides, oxysulfides, and sulfides containing rare-earth elements form as nonmetallic inclusions in steel when misch metal is used as a deoxidizer and desulfurizer. The identification of these compounds is vital to investigate whether the rare-earth elements in misch metal effectively suppress the formation of harmful nonmetallic inclusions and reduce oxygen and sulfur contents in steel. Electron probe microanalyzer (EPMA) is a widely used instrument for performing compositional analyses of nonmetallic inclusions in steel.[16,17] This analytical method can provide precise chemical compositions of nonmetallic inclusions. However, when multiple chemical compounds, such as oxides, oxysulfides, and sulfides, are distributed in an analyzed area, it is difficult
for EPMA to identify these chemical compounds. In this study, we focused on cathodoluminescence (CL) analysis, which is used to obtain images and spectra based on light emission from materials induced by electron
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