Kinetics of Silicothermic Reduction of Manganese Oxide for Advanced High-Strength Steel Production
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MANGANESE has become an important alloying element in advanced high-strength steels (AHSS) with over 20 wt pct being proposed for certain grades of twinning-induced plasticity (TWIP) steel, and values between 4 and 11 pct being of interest in the third-generation AHSS.[1] Reduction of manganese oxide dissolved in the slag has been proposed as a possible method of adding manganese to the second- and third-generation steels in order to improve the economics of alloy addition.[2,3] The current study seeks to advance the fundamental understanding of the kinetics and mechanism of silicothermic reduction of manganese oxide from slags. The thermic reduction process promotes solute elements with high oxygen affinities to reduce slag components into the metal. There are two thermodynamically attainable pathways for metallic silicon to reduce manganese oxide. The reduction of manganese in the slag requires the supply of electrons from silicon, either by the formation of tetravalent silicon or, if the supply of Mn2+ is inadequate, by the formation of divalent silicon; the former results in silicon being incorporated into the slag as silicate, while the latter requires the net transfer of one O2 ion from the slag to form SiO gas with the divalent silicon. While the authors do not make any claims to the detailed mechanism, conceptually and B.J. JAMIESON and K.S. COLEY are with the McMaster Steel Research Centre, Department of Materials Science and Engineering, McMaster University, Hamilton, L8S 4L8, Canada. Contact e-mail: [email protected] Manuscript submitted September 30, 2016. Article published online March 28, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
stoichiometrically, these reactions can be written as Eqs. [1] and [2]. Bracket notation of [metal], (slag), and {gas} is used in this study. The free energies for these reactions were taken from FactSage Reaction Module and the FTDemo database,[4] which take the following data from the NIST JANAF Tables.[5] At 1873 K (1600 °C, Eq. [1] yields 103 kJ/mol, while Eq. [2] yields 14.7 kJ/mol; under standard conditions, it is clear that while SiO2 may be more favorable, SiO can still form. ½Si þ 2ðMnOÞ Ð 2½Mn þ ðSiO2 Þ DG ¼ 235733 þ 70:639T ½J/mol
½Si þ ðMnOÞ Ð ½Mn þ fSiOg DG ¼ 186596 107:468T [J/mol]
½1
½2
Some of the earliest quantitative studies regarding the reduction of MnO using Si in nongraphite crucibles have come from Daines and Pehlke.[6] Their study showed that manganese mass transport in the metal was the rate-limiting step, where the mass-transfer coefficient (km) was equal to 7 9 106 m/s. The system showed significant rate enhancement with stirring, confirming mass transport control. The system underwent a twostage reaction, with the first stage being faster than the second. Shibata et al.[7] performed a series of experiments involving multiple simultaneous reactions. MnO was reduced by silicon and carbon simultaneously; those authors concluded that MnO reduction was more likely to be controlled by mass transport in the metal than in the slag. The
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