Rapid Identification of Calcium Aluminate Inclusions in Steels Using Cathodoluminescence Analysis
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INTRODUCTION
IT is of vital importance to analyze non-metallic inclusions in steels, such as alumina (Al2O3), spinel (MgAl2O4), silica (SiO2), manganese sulfide (MnS), calcium sulfide (CaS), boron nitride (BN), and titanium nitride (TiN) because they can cause serious problems to steel products, such as breakage of steel wire during drawing, hydrogen induced cracking, fatigue failure, surface flaws, and low-temperature embrittlement.[1–4] Such inclusions may also be problematic for the steel production process. For example, Al2O3 inclusions can lead to nozzle clogging in the continuous casting of aluminum (Al)-killed steel (Figure 1)[1,2,4] because Al2O3 solidifies at the casting temperature. Al2O3 inclusions are formed by Al deoxidation, which is a process to reduce the oxygen content in steel by adding aluminum metal into molten steel in the ladle (Figure 1). To prevent nozzle clogging, calcium (Ca) is injected into the molten steel in the forms of Ca metal, ferrocalcium, Ca-Si alloy, or Mg-Ca alloy, which is termed as Ca treatment. Ca treatment is effective because solid Al2O3
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 17, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
inclusions transform into liquid calcium aluminates, which do not clog the nozzle, at the casting temperature.[5–7] However, the addition of excess Ca causes the stopper rod, which controls the flow rate of the molten steel into the mold (Figure 1), to melt because the stopper rod is normally made from alumina-graphite.[8–10] It has been reported that the compositions of the calcium aluminate inclusions in Ca-treated Al-killed steels have to be CaAl2O4 and Ca3Al10O18 to prevent both nozzle clogging and melting of the stopper rod.[9,10] Therefore, we can detect the symptoms of nozzle clogging and melting of the stopper rod by the compositional analysis of the calcium aluminate inclusions in the steels. The compositional analysis of non-metallic inclusions in steels is usually performed using electron probe microanalyzers (EPMA).[11] This analytical process is a time-consuming step in steelmaking[2,3] because it takes approximately 1 week to complete the analysis for a single sample. Analytical techniques, such as optical emission spectrometry with pulse distribution analysis (OES-PDA),[12,13] electron beam (EB) melting,[14] cold crucible-X-ray fluorescence,[15,16] fractional thermal decomposition (FTD),[17] laser-induced breakdown spectroscopy (LIBS),[18] and cathodoluminescence (CL) analysis,[19,20] have been proposed for the rapid compositional analysis of non-metallic inclusions in steels.[21] Among these techniques, we have recently focused on CL analysis[22–25] because CL analysis can simultaneously identify the size, shape, and composition of
non-metallic inclusions, which are essential for the analysis of non-metallic inclusions in steels. We have previously
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