Mn-V-O 산화물계와 MnV2O4 스피넬 고용체에 관한 열역학적 연구

Abstract

Manganese and vanadium are often added as alloying elements in various kinds of steel. Due to their relatively high affinity with oxygen dissolved in the molten steel, the complex oxide inclusions could be formed in solidification. To understand the behavior of those oxide inclusions, thermodynamic information of the Mn-V-O system is required but there is insufficient information available, in particular under the steelmaking condition.As the first step of thermodynamic investigation on the Mn-V-O oxide system, phase equilibria of the Mn-V-O systems under controlled atmosphere have been experimentally investigated in the temperature range from 1200 to 1600°C and in the range of logpO2 from -5.81 (PCO2/PH2 = 9) to -12.9 (PCO/PCO2 = 9). High temperature equilibration, quenching technique, and electron probe microanalysis (EPMA) were conducted to identify equilibrium compositions in equilibrium phases. XRD analysis was also employed in order to confirm oxide phases. Oxygen partial pressure was controlled using CO/CO2 or H2/CO2 gas mixtures of different ratio to figure out the effect of oxygen partial pressure on the phase diagram. The following phases have been observed

molten oxide, manganosite (MnO (s.s.)), spinel (MnV2O4 (s.s.)), and vanadium trioxide (V2O3 (s.s.)). Liquidus of manganosite and spinel were measured under CO-CO2 gas mixture condition (PCO/PCO2 = 1). It was found in the present study that oxygen partial pressure has an effect on sub-solidus phase equilibria, in particular for solubility limit of intermediate phase (e.g., MnV2O4 spinel).Once the phase equlibria in the Mn-V-O system was obtained, thermodynamic investigation for the MnV2O4 spinel phase saturated with either MnO or V2O3 was followed to measure the standard Gibbs energy of formation of the spinel phase. High temperature equilibriation, water quenching technigue, and several methods such as ICP-AES and LECO combustion analysis were conducted to obtain equilibrium compositions in liquid metal in equilibrium with MnV2O4 crucibles. Also XRD analysis and electron probe microanalysis (EPMA) were introduced to confirm that the oxide phases in the crucible. Controlling the atmosphere with H2/CO2 gas mixture (PH2/PCO2 = 9) and purified Ar gas, the standard Gibbs free energy of formation of the MnV2O4 spinel phase was calculated from the equilibrium compositions of elements in liquid metal, and thermodynamic parameters available in the literatures. From the information obtained in the present study, an inclusion stability diagram of Fe-Mn-V-O system was constructed, which can be used as a guide of alloying design of steels.