PHASE RELATIONS AND ACTIVITY MEASUREMENTS FOR THE Al_2O_3-SiO_2-TiO_x SYSTEM

摘要

Inclusions in steels have been considered to be harmful to quality of final product of steels, and extensive studies on removal of inclusions or making them harmless have been focused. The alumina formed by aluminum deoxidation in a secondary steel refining process is known to cause a defect like a flaw and a crack. Therefore, a modification of simple deoxidation by aluminum is required. On the other hand, an innovative attempt called "oxide inclusion metallurgy" has been made to improve steel properties by controlling the size, composition and distribution of non-metallic inclusions in steels. In such a process, an appropriate oxide will act as a precipitation site of MnS that is a transformation site of intragranular ferrite from austenite. If the oxides were finely dispersed in steels, MnS formed on the oxide should make the ferrite grains so fine that the toughness of steels could be enhanced. Sawai et al. have reported that the complex deoxidization by strong and weak deoxidizing agents is effective for the fine dispersion of deoxidation product, and they have also reported that an oxide with high sulfide capacity was preferable for the MnS precipitation. Moreover, Ogibayashi reported that low liquidus temperature of the oxide would promote the MnS precipitation. Thus, thermodynamic properties of the MnO-SiO_2-TiO_x and the MnO-Al_2O_3-TiO_2 systems that are considered to be suitable for such a process have been studied. However, in the practical process, TiO_2 is not the equilibrium state as titanium oxide because of the low partial pressure of oxygen. Ti~(3+) and Ti~(4+) are reported to coexist in oxide melts under strongly reducing conditions. In addition, aluminum is a strong deoxidant enough to reduce MnO in the oxide phase. Therefore, the Al_2O_3-SiO_2-TiO_x oxide system will be formed as a practical deoxidation product in a secondary refining process. In this study, phase relations of the Al_2O_3-SiO_2-TiO_x system as well as the activities of AlO_(1.5) and SiO_2 have been investigated. Isothermal phase relations for the oxide system have been determined by equilibrating various solid oxides under strongly reducing conditions, achieved by equilibrium between graphite and CO gas, Po_2=4.56xlO~(-16) atm at 1873K. Activities of AlO_(1.5) and SiO_2 were measured by the Knudsen effusion method. Activities of SiO_2 were also measured by the chemical equilibrium between the oxide and the Fe-Si-C alloy. Activities of TiO_(1.5) and TiO_2 were also estimated from the composition of the Si-Al-Ti alloy in equilibrium with the oxide assuming that the alloy conformed to a regular solution. In addition, steel composition equilibrated with the oxide of arbitrary composition has been calculated by the activity of each component of the oxide system obtained in the present study.