Transient Evolution of Nonmetallic Inclusions During Calcium Treatment of Molten Steel
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INTRODUCTION
THE production of clean steels involves the control, elimination, or modification of nonmetallic inclusions.[1] For Al-killed steels, calcium treatment is commonly used to modify solid alumina-based oxide inclusions into fully or partially liquid calcium aluminates in order to improve steel castability.[2] Calcium treatment plays an important role to diminish the clogging of the submerged entry nozzle during continuous casting,[2–4] to preform deep desulfurization,[5] to control inclusions,[6–8] and to improve the machinability of free cutting steels.[9] However, either insufficient or excessive addition of calcium alloys is detrimental since incomplete or superfluous calcium can lead to the formation of solid calcium alumina, CaS, CaO, and other unfavorable inclusions deteriorating the quality of the steel.[10–12] Therefore, the evolution of inclusions during calcium treatment is very much worthy of investigation.
LIFENG ZHANG, YANG LIU, YING ZHANG, WEN YANG, and WEI CHEN, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, P.R. China. Contact email: [email protected] Manuscript submitted July 4, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Extensive studies were performed to investigate thermodynamics[8,11–40] and kinetics[9,19,29,41–47] for the formation and modification of inclusions during calcium treatment to reveal the reaction mechanism under both laboratory scale experiments and industrial trials, as summarized in Table I. Thermodynamic models were established to predict the formation of inclusions according to the temperature and the composition of the steel, such as Al-S equilibrium,[11,17–19,47] activities of CaO and Al2O3 in calcium alumina,[17–19,36,37] modification of inclusions,[13,22,23,25–32,36,37,47] liquid windows of inclusions for the additional amount of calcium,[24,39,40] precipitation-area diagrams of oxide-sulfide duplex inclusions,[11,34,38] and precipitation of CaS inclusions.[21,26–28,34,35,38,39] From the aspect of kinetics, modification mechanisms of inclusions were established, for example, the unreacted core model[19,43,45,47] and the influence of calcium evaporation on inclusions were also considered.[9,42,44,46] Mass-transfer coefficients of alloy elements in molten metal were measured to perform accurate kinetic calculation, as shown in Table II.[9,42,48–53] Vacuum induction furnaces are widely used to study inclusions in steel, as summarized in Table III,[27,28,36,37,54–61] in order to achieve a favorable protective atmosphere, easy operation for alloy addition, easy sampling, and quick melting of steel. Typical research work using vacuum induction furnaces includes the investigation of inclusions in Al-Ti-killed steels,[55–58] inclusions in steel during reoxidation,[59] and calcium treatment.[27,28,36,37]
Table I. Authors Watanabe et al. Koyanagi et al. Ototani et al. Ikeda et al.
Summary
Main Work
kinetics and thermodynamics thermodynamics
discuss the relationship between the s
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