Ferrosilicon-Based Calcium Treatment of Aluminum-Killed and Silicomanganese-Killed Steels
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e presence of metallic calcium in commercial grades of ferrosilicon can affect oxide inclusions in liquid steel.[1,2] Calcium in high-Si ferroalloys (especially FeSi75) can be detrimental by causing the formation of solid CaS that can clog steelmaking nozzles during casting[2–4] but can also be an effective inclusion modifier fully substituting the addition of Ca cored wire, converting alumina and spinel inclusions to liquid calcium aluminates.[5] Previous studies considered fundamental aspects of calcium treatment. Larsen and Fruehan summarized the relative stability of calcium aluminates and calcium sulfide in Al-killed steel.[6] Lu et al.[7,8] studied the reaction kinetics of Ca-bearing cored wire with steel, concluding that calcium readily reacts with steel, potentially forming CaS and/or CaO inclusions that are partially retained in the melt; after injection, Ca removal from the melt (by inclusion flotation) follows first-order reaction kinetics. Verma et al.[9,10] demonstrated that the transformation of alumina and spinels into CaSTEPHANO P. T. PIVA and PETRUS CHRISTIAAN PISTORIUS are with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213. Contact e-mail: : [email protected]. Manuscript submitted October 18, 2019; Accepted October 18, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
aluminates often involve CaS as intermediate product that persists for several minutes in the steel melt. Under plant conditions, the yield of calcium is typically 10 to 15 pct, if calcium is added by injecting wire that contains metallic calcium or calcium silicide. In laboratory steel melts (of around half a kilogram), the yield is much lower when adding calcium silicide, around 1 pct.[11] In contrast, the description of a patented approach that instead uses Ca-bearing ferrosilicon for calcium treatment implies that the calcium yield is approximately 100 pct.[5] The work presented here tested whether the improved calcium yield from Cabearing ferrosilicon is also observed in small laboratory melts, also characterizing the evolution of composition and morphology of the oxide phases that form after calcium treatment. Ca-containing FeSi (containing 75 pct Si and 2 pct Ca, balance Fe) was prepared by melting (in an RFinduction furnace, using argon as protective atmosphere) 2.4 g (0.0024 kg) of CaSi2, 36 g (0.036 kg) of pure Si, and 11.3 g (0.0113 kg) of electrolytic iron, in a graphite crucible, reaching a maximum temperature of 1450 C (1723 K). Based on analysis by energy-dispersive spectrometry (EDS) in a scanning electron microscope (SEM), the calcium yield was close to 100 pct. The solidification microstructure of the alloy (Figure 1) included primary Si, Fe0.92Si2, and CaSi2; the latter was evenly distributed throughout the matrix in interdendritic spaces. These phases agree with the predicted solidification products (FactSage 8.0, FSstel database),[12] which are primary Si, followed by Fe3Si7 and finally CaSi2 . Calcium tre
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