Research On The Microstructure And Properties Of New Types Of Purging Plug Refractories In The System Al₂O₃-MgO-CaO

Steel ladle purging plug as the one of most important functional refractories in the secondary metallurgy, its performance direnctly influences the effect of ladle metallurgy. The quality of steel liquid, alloy element homogenization and the steel ladle production rate depend on the purging plug's performance regarding the purging stability, flow intensity, and service life. The service life of purging plug is normall short especially for the steel ladle which is subjected to secondary metallurgy like RH. VOD, LF, et al, which has a service life of 20-50 heats. This is significantly lower than that of other refractories for the steel ladle, like Mg-C for slag line, ladle wall linings and bottom linings. So the maintanence or exchange of the steel ladle refractories mainly depends on the condition of purging plug. The 1st minor maintanence on the steel ladle refractory is normally to check the purging plug. Therefore to improve the performance of steel ladle purging plug is very important for the steel metallurgy.The present thesis firstly studied the post-motem purging plug. The wear mechanism of the purging plug is multifold. The service life of the purging plug was greatly affected by the steel liquid abrasion, corrosion, infiltration, spalling and cracking by the thermal stress, super high temperature and severe oxidization by the oxygen lance. A large quantity of magnesia aluminate spinel solid solution (Mg_0.15Fe_0.96).Al₂O₄ was formed in the iron infiltration area. This was due to the infiltration of slag and liquid steel into the slit of purging plug, and also due to the super high temperature of oxgen lancing. Iron oxide and magnesia and corundum can react with each other to form spinel solid solution. Meanwhile, the spinel and CA₆ phases were found stable in the used purging plug, which meaned such two phases will be suitable for the recipe design for purging plug.The modelling on purging plug temperature field and thermal stress field, illustrated the attack on the purging plug by the temperature and thermal stress. The tensile stress in the vertical direction, which can be as high as 350MPa, is the root cause of the horizontal cracks and pellings.Theoretical analysis on Al₂O₃-MgO-CaO ternary phase diagram showed in the present system the peritectic temperature was 1850? with the existing phases of corundum, magnesia aluminate spinel and CA₆. However, in the high spinel/high cement system, the eutectic point decreased to 1730? due to the CA₂ phase involed. Therefore, too much cement was not suggested to add into the castables. Iron oxide had a severe corrosion effect to the CaO containing refractory at 1700?. The corrosion mechanism of Fe₂O₃ to the system of Al₂O₃-MgO-CaO and the system of Al₂O₃-MgO was studied by the thermodynamic software FactSage. Results showed the iron oxide had great corrosion effect to the CaO containing system at 1700?. The starting temperature of liquid phase apprearance of no-CaO system was 91? higher than the CaO containing system. And the liquid phase content of the no-CaO system was 25.5% lower than the CaO containing system. No-CaO system had a better corrosion resistance to the iron-rich slag than the CaO containing system. The crucible slag corrosion test verified the above results, which showed no-cement system has a clear better corrosion resistance than the cement bonded system.The spinel effect to the low cement system and super low cement castable were studied. Results showed by increasing the spinel content, no significant difference on the cold physical properties. But the HMoR was greatly increased. As the SEM and EDS results showed, there was ternary phase C₂M₂A₁₄ in the high spinel recipe. Such ternary phase cross placed in the matrix and interlocked each other, which enhaced the stiffness and increased the hot strength.The binding system on the Al₂O₃-MgO-CaO was analized. Calcium alumina cement and hydratable alumina ?-Al₂O₃ were used as the binder. Results showed by increasing the cement, the porosities increased, bulk density decreased and HMoR decreased. This is mainly due to lower refractoriness of the higher cement system according to the thermodynamic study on one hand. On the other hand, the microstructure was becoming loose caused by the formation of too much CA₂ and CA₆. Considering the practical application of purging plug, the cemenl dosage is normally limited to below 5%. By increasing the content of ?-Al₂O₃, the porosities of the castable will be greatly increased and bulk density will be decreased. The optimum dosage of ?-Al₂O₃ is 4%. The HMoR was not affected by the content of ?-Al₂O₃, but it's much lower than that of super low cement castable C2S26-Microstructure analysis showed, no-cement castable was difficult to sinter compared to cement bonded castable. The erosion resistance against steel liquid of the cement bonded castable will be better than the no-cement system.The thermal stress resistance factor R"" was calculated by the wedge-spitting' method in Leoben University. Thermal shock resistance was evaluated by the R"" Meanwhile, the traditional method of water quench was compared with'wedge-splitting'. The results were quite comparable. The R' was 100 for the ?-Al₂O₃ bonded recipe A4S26 after 1250? tempered, which was 60-70% higher than the cement bonded system. Low temperature treated samples had better thermal shock resistance than the high temperature treated samples. After 3 times water quench tests, MOR of all samples dereased greately. ?-Al₂O₃ bonded samples exhibited best thermal shock resistance, no matter low temperature treated or high temperature treated.The present program thoroughly studied the purging plug refractories, in order to improve the performance of the purging plug. Wear mechanism was studied through the post-motem analysis. Modelling on temperature field and thermal stress was conducted. Variables on spinel and the binder systems were studied, by which an optimized recipe can be designed. The thermal shock resistance research provided a new direction to balance the properties of the purging plug. No-cement system showed much balanced properties than the cement bonded system.This PhI) program optimized the traditional purging plug refractories in the system Al₂O₃-MgO-CaO, and also studied the new system Al₂O₃-MgO. The research provided a scientific basis for the design of highest performance refractory formulations which combined the desired hot properties, wear resistance characteristics and the required high thermal shock resistance properties. Thereby the upgrading of the high performance functional refractories will be promoted. And the steel making process will be optimized.