Fundmental Research On The Secondary Refining Of Silicomanganese Ferroalloy

The current silicomanganese ferroalloy ?SiMn? products in China have much higher impurities contents ?such as P, A1, Ti and O?, which can not meet the technical requirements of some grade steels and the development of new steel grades. The impurities contents could be reduced through secondary refining, for example, reducing dephosphorization ?de-P? and the removal of Al and Ti through blowing N2. The aim of present paper was to investigate the fundamental problems encountered during the secondary refining process of SiMn.Firstly, thermodynamic data of Mn-based melts were systematically collected. The Unified Interaction Parameter Model ?UIPM? was used to evaluate the Mn-Fe-Si-C-Ca-P system. The model could be used to calculate the activities of Mn, Si, Fe, C, Ca and P, the C and Ca solubilities and the equilibrium between Ca and P Mn-based melts. The model is also useful for understanding the process of ferromanganese and SiMn.The oxygen solubilities in Mn-Si-Fe ternary melts were determined. The inclusions were extracted from the Medium Carbon SiMn and Low carbon SiMn by electrolytic extraction SiO₂ was the predominanat nonmetallic inclusion in Low carbon SiMn, while the main inclusions included SiO₂ and silicate containing manganese oxide and iron oxides in Medium Carbon SiMn It could be concluded that almost all the oxygen in SiMn products existed in the form of oxide inclusions and the amount of oxide inclusions accounted for 0.075-0.253% of SiMn Most of the oxide inclusions in SiMn were exogenous inclusions. The total oxygen content could be reduced to less than 50 ppm in theory through properly extending the killed time of melts in the ladle and blowing Ar into the melts.Nitrogen solubility and the concentration products of A1N and TiN of Mn-Si-Fe-?Csat? melts were measured by the gas-liquid metal equilibration technique. It was found that nitrogen solubility increased as the increase of temperature but decreased as the increase of silicon content The nitrogen partial pressure dependence of nitrogen solubility in Mn68Sil8Fe and Mn60Si30Fe melts follows Sievert's law. The Mn60Si30Fe melt was saturated with silicon nitride. The concentration products of [%A1][%N] and [%Ti][%N] in Mn-Si-Fe?-Csat? melts increased as the increase of temperature but decreased as the increase of Si content. Both Al and Ti contents became smaller as the increase of Si content and decrease of temperature in Mn-Si-Fe?-Csat? melts at twofold saturation that the melts were both nitrogen and AN or TiN saturated. The Al and Ti contents could be theoretically reduced to 0.002% and 0.011% respectively for Mn60Si30FeCsat melts by blowing N2, which could almost meet the technical requirements of 90 grade hardwire steels.Reducing de-P of SiMn was carried out by using CaO saturated CaO-CaF2 slags. It was found that P could be removed no matter in air and Ar atmosphere. The oxygen potential at slag-metal interface was controlled by the silicon in SiMn melts. De-P efficiency increased as increasing the silicon content and decreasing temperature. It was not fit for the reducing de-p by using CaO saturated CaO-CaF2 slags for SiMn containing less than 20% Si. Rephosphorization occurred during the reducing de-P of SiMn. It was the key factor to select the proper drossing time for the practice of reducing de-p. Al₂O₃ and Na2O deteriorated the de-P. Carbon crucible was unfavorable for reducing de-P.The viscosities of CaO-MgO-Al₂O₃-SiO₂ melts containing SiC or Si₃N₄ particles were investigated by the rotating-cylinder method. It was found that temperature dependence of viscosity could be always described by the Arrhenius law with or without solid particles addition. The activation energies of the liquid-solid mixtures were determined by the liquid phase. Temperature had little influence on the relative viscosity. Viscosity and relative viscosity increased as decreasing rotation speed and increasing volume fraction of SiC and Si₃N₄ solid particles. For the same volume fraction of solid particles, relative viscosity was affected by the liquid slag compositions. The relative viscosity became smaller when the liquid slag has a larger ratio of CaO to SiO₂ or MgO to Al₂O₃.