Physicalchemistry of formation of oxysulfide inclusions in steels

Recently a new process has been developed to control the gamma·alpha transformaion of steels using MnS precipitates on the oxide inclusions as nuclei.In order to optimize the composition of inclusions，phase relations for the MnO-SiO_2-MnS system have been investigated at temperatures ranging from1473K to1723K.The solubility of MnS increased with an increase in MnO/(MnO SiO_2)ratio and showed a drastic increase at the ratio of 0.6，which was consistent with the maximum precipitation ratio of MnS in the oxide metallurgy process.An oxysulfide compound composed of 8MnO·3SiO_2·MnS was newly found as a stable solid phase at lower temperature than1573K.It has been considered that this phase is formed via the peritectic reaction between2MnO·SiO_2 and liquid phase.Deliberating the compositions of liquids saturated with various compounds at several temperatures，the phase diagram for the MnO·SiO_2·MnS system has been estimated。Finally，the mechanism of the MnS precipitation in the oxide inclusions has been discussed.The relationship between the equilibrium compositions of Fe-Ti-Mn melts and those of coexisting oxides were also investigated by employing a cold crucible at 1873K.The oxygen content decreased with increasing the contents of manganese and titanium，the effect of the latter being significant。The metals which had the same contents of manganese，regardless of their titanium contents，were equilibrated wiht molten oxides having a constant activity ratio of MnO and FetO.Using the metal compositions and Gibbs energies of the formation of component oxides，iso-activity contours of MnO，and TiO_&lt；1.5&gt；for the MnO-FetO-TiO_&lt；1.5&gt；melts 1873K were evaluated.The optimal conditions for practical deoxidation processes with Ti-Mn alloys were discussed.

Recently a new process has been developed to control the gamma・alpha transformaion of steels using MnS precipitates on the oxide inclusions as nuclei. In order to optimize the composition of inclusions, phase relations for the MnO-SiO_2-MnS system have been investigated at temperatures ranging from 1473K to 1723K.The solubility of MnS increased with an increase in MnO/(MnO+SiO_2) ratio and showed a drastic increase at the ratio of 0.6, which was consistent with the maximum precipitation ratio of MnS in the oxide metallurgy process. An oxysulfide compound composed of 8MnO・3SiO_2・MnS was newly found as a stable solid phase at lower temperature than 1573 K.It has been considered that this phase is formed via the peritectic reaction between 2MnO・SiO_2 and liquid phase. Deliberating the compositions of liquids saturated with various compounds at several temperatures, the phase diagram for the MnO・SiO_2・MnS system has been estimated. Finally, the mechanism of the MnS precipitation in the oxide inclusions has been discussed.The relationship between the equilibrium compositions of Fe-Ti-Mn melts and those of coexisting oxides were also investigated by employing a cold crucible at 1873 K.The oxygen content decreased with increasing the contents of manganese and titanium, the effect of the latter being significant. The metals which had the same contents of manganese, regardless of their titanium contents, were equilibrated wiht molten oxides having a constant activity ratio of MnO and FetO.Using the metal compositions and Gibbs energies of the formation of component oxides, iso-activity contours of MnO,FetO,and TiO_<1.5> for the MnO-FetO-TiO_<1.5> melts at 1873K were evaluated. The optimal conditions for practical deoxidation processes with Ti-Mn alloys were discussed.