용강/정련 슬래그간 Ti과 친환경원소 Se의 열역학적 거동

Abstract

Titanium is added in many grade steels as an alloying element to improve mechanical properties via the grain refinement during hot rolling. Also, Ti is an important alloying element in stainless steels to stabilize C and N, and hence to improve the inter-granular corrosion resistance of the weld. Meanwhile, Ti alloyed steels can have severe nozzle clogging problems, which are detrimental to slab surface quality. The selenium added steels have been concerned to replace the conventionally lead added machinability steels because of environmental friendliness. Since the yield of Se addition into liquid steel is quite low due to its high vapor pressure at steelmaking temperatures, the improvement of Se yield becomes essential for its successful application to plant. In this thesis, in order to decrease the deleterious inclusions resulting in the nozzle clogging and increase the Ti yield, the effect of Ti addition between melt and refractory will be investigated. In case of Se, thermodynamics on the behavior of selenium based on dissolution reactions in liquid iron as well as selenium-containing slags will be investigated as well. In chapter II, the thermodynamic role of Ti in steels and the effect of Ca and Ti addition to the molten stainless steel deoxidized with Al on the formation of (Al2O3-xMgO) spinel inclusions were investigated. High melting point inclusions such as spinel(Al2O3-xMgO) are known to promote clogging of the SEN nozzle in a continuous caster mold. In particular, Ti alloyed steels can have severe nozzle clogging problems, which are detrimental to slab surface quality. The sequence of Ca and Ti additions after Al deoxidation was also investigated. The inclusion chemistry and morphology according to the order of Ca and Ti are discussed from the standpoint of spinel formation.The thermodynamic interaction parameter of Mg with respect to the Ti alloying element was determined. The element of Ti in steels could contribute to enhancing the spinel formation, because Ti accelerates Mg dissolution from the MgO containing refractory walls or slags due to its high thermodynamic affinity for Mg. Even though Ti induces also Ca dissolution from the CaO containing refractory walls or slags owing to its thermodynamic affinity for Ca, dissolved Ca plays a role in favoring the formation of calcium aluminate inclusions, which are more stable thermodynamically in an Al deoxized steel.The inclusion content of steel samples was analyzed to better understand fundamentals of (Al2O3-xMgO) spinel inclusion formation. The optimum processing conditions for Ca treatment and Ti addition in austenitic stainless steel melts to achieve the minimized spinel formation and the maximized Ti alloying yield is discussed.In chapter III, the standard free energy change of the dissolution reaction of Se vapor in liquid iron at steelmaking temperatures was determined .The Henrian activity constant of Se in Fe-Se system at 1873K was also assessed. The self interaction parameter of Se was obtained as -0.045 by assuming the regular solution model in Fe-Se system using the values of the Gibbs free energy and the Herian constant. The interaction parameters between Se and second element i such as O, Cr, Ni, Mn, N, Mo, Si, Al, S, Bi and Te were also determined. The solute elements of O, Ni, Cr and Mn significantly increase the selenium solubility in liquid iron. The determined interaction parameters between Se and second elements in liquid iron at 1873K will be addressed in terms of the economical and feasible ways of Se addition as well as the improvement of machinability owing to MnSe formation during the solidification stage.Using these values of interaction parameters, the further studies employing the refining slags and melt equilibrium reaction system need to be done to address a feasible processing way to produce an environmentally friendly free-cutting steels. However, there has not been much research on these slags system for optimization of the behavior of Se. Therefore, it is necessary to develop the processing methods to increase Se yield in steels enough to complete against the production cost of Pb containing steel grade. In order to increase the Se yield, the slag system of CaO-SiO2-Al2O3-MgO relevant to refining Se-containing steels such as free cutting steel is thermodynamically assessed at 1873K. The distribution ratio of Se has been determined in slag/melt system. Selenium capacity due to the basicity and oxygen potential was also researched. Besides, the effect of substitution of each slag component on the selenium capacity and the relationship between optical basicity and selenium capacity were researched as well. This thermodynamic assessment can be performed to understand the behavior of Se in slag/metal system and get the data of optimization for these slag systems. In chapter IV, based on the results from chapter II and III, this thesis suggested an application method of the present results to actual refining processes, in order to decrease the deleterious inclusions such as spinel(Al2O3-xMgO) as well as increase Se contents in steel melt.