Relationship Between Dissolved Calcium and Total Calcium in Al-Killed Steels After Calcium Treatment
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alcium treatment has been widely used in the secondary refining process to prevent nozzle clogging during continuous casting[1–4] or to modify the shape of inclusions.[5,6] The calcium includes dissolved calcium in steel and undissolved calcium in CaS and CaO.[7] Actually, it is the dissolved calcium that reacts with solid alumina inclusions to form liquid calcium alumina to improve the castability.[8] Hence, the variation of [Ca] content during calcium treatment is very worthy of being investigated. Since there is uncertainty regarding the concentration of dissolved calcium in the steel, some researchers considered that T.Ca and [Ca] were approximately equal.[9–11] As reported elsewhere, the [Ca] content is estimated to be a few parts per million at the most with the achievement of thermodynamic equilibrium,[12,13] which was verified based on the measured and calculated results.[14] Yang et al.[15] proposed that ln[Ca] linearly decreased with time from 21.9 ppm at first to lower than 1 ppm. Based on the equation, calculated [Ca] contents in samples taken at 10 and 20 minutes in a vacuum induction furnace were around 1 ppm, which
YANG LIU and LIFENG ZHANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, P. R. China. Contact e-mail: [email protected] Manuscript submitted October 10, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
agreed with Verma’s results.[7,12,14,15] However, the effect of T.S, [Al]s, and temperature on [Ca] contents was not evaluated in their experiments. Although numerous investigations were performed on thermodynamics[14,16–21] and kinetics[22,23] during calcium treatment, the influential factors on [Ca] contents were not adequately clarified. In the current study, laboratory experiments and thermodynamic calculation were performed to investigate the dissolved calcium in the melt during calcium treatment in pipeline steels. A comparison between the calculated and predicted [Ca] contents using Factsage was performed. The effects of T.Ca, T.O., T.S, [Al]s, and temperature on dissolved calcium were discussed from a thermodynamic perspective. In the current study, two experiments were performed with two pure Al2O3 crucibles with different apparent porosities, 13 pct and lower than 1 pct, respectively.[24] A 20-kW vacuum induction furnace was employed, as shown schematically in Figure 1. The furnace chamber was evacuated with a mechanical pump to achieve a vacuum pressure of 10 Pa, and then backfilled with a high-purity argon gas. One thousand gram pipeline steel was melted with no slag addition with chemical composition as shown in Table I. After being melted in 1873 K (1600 °C), the molten steel was held for 5 min for homogenization, and then 7 g of CaSi powders was wrapped with an iron foil and inserted into the molten steel. Samples were taken with tube samplers after the addition of the alloy at 1, 5, 10, and 15 minutes, and then the melt was poured into an iron mold at 20 minutes. For each sample, a cross secti
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