ZrO2 is an important member of the advanced ceramic material family and widely used in many industrial fields. As a refractory, ZrO2 has extremely excellent properties, such as high melting point, high specific heat and thermal conductivity, good chemical stability, low high temperature steam pressure, difficult to be wetted and dissolved by molten metal, and strong resistance to acid corrosion. It is an ideal refractory for steel continuous casting system. At present, ZrO2 refractory materials have developed rapidly and have become one of the important materials in the field of refractory materials. For example, zircon bricks or zircon castables, as well as MgO-ZrSiO4 castables, are used for the slag line of the steel drum; ZrO2 lining for sliding nozzle holes; ZrO2 fixed diameter nozzle for horizontal continuous casting; Steel liquid filters are made of materials such as ZrO2 or ZrO2-Al2O3, ZrO2-CaO, ZrO2-MgO, etc. In addition, the submerged nozzle bricks have also developed from Al2O3-C to Al2O3-ZrO2-C.

The introduction of ZrO2 in refractory materials usually includes direct introduction method and indirect introduction method of pre synthesized ZrO2 containing clinker. Due to the crystal phase transformation and accompanying volume changes of ZrO2, when producing refractory materials that introduce ZrO2, when the amount of ZrO2 added is within a relatively small range, the direct addition method can be used, and the cast products are not limited by this range. The method of pre synthesizing synthetic materials containing ZrO2 is applicable to the entire range, and both sintering and electric melting processes can be used to pre synthesize ZrO2 composite raw materials.

1. Properties of fused zirconia corundum

Fused zirconia corundum is one of the important refractory raw materials containing zirconia. It is made by adding zirconia (or desilicated zirconia) and zircon sand to industrial alumina or bauxite and melting them in an electric arc furnace. The main crystal phase of fused zirconia corundum is α- Al2O3, the sub crystal phase is baddeleyite, and there is a small amount of glass phase, and the appearance is generally grayish brown. According to the different content of ZrO2, fused zirconia corundum can be divided into low fused zirconia corundum (about 10%~15% ZrO2), medium fused zirconia corundum (about 25% ZrO2), and high fused zirconia corundum (about 40% ZrO2).

Zirconia corundum brick

Gao Zhenxin et al. prepared six samples containing 25%, 30%, 35%, 40%, 45%, and 48% ZrO2 in the Al2O3-ZrO2 system through experiments, and quickly cooled the samples after melting in an electric furnace. The microstructure of the samples was studied using SEM-EDS, and the following conclusions were obtained: 1. The composition of primary crystalline corundum and fine egg like eutectic phase can be observed in the samples containing ZrO2 with a mass fraction of 25% to 40%; 2. The initial crystal ZrO2 phase and fine egg like eutectic microstructure can be observed in the samples containing ZrO2 with a mass fraction of 45%~48%; 3. The eutectic composition of two samples containing 40% and 45% ZrO2 was determined using EDS, and the result showed that the mass fraction of ZrO2 was 43.3 ± 0.4. Eutectic is defined according to the principle of phase equilibrium: homogeneous, delicate, two-phase (multiphase) composition without crystallization sequence. It is pointed out that Al2O3-ZrO2 system has no compound, and its eutectic is a two-phase combination of corundum and baddeleyite (m-ZrO2). However, in reality, it is often the coexistence of m-ZrO2 and t-ZrO2 (or m-ZrO2 and c-ZrO2), which is a binary system of three-phase eutectic. Only when studying the composition of eutectic, m -; T - and c-3 phases are considered as one ZrO2 phase. The characteristic of this eutectic structure is that due to the rapid growth rate of the crystal surface of corundum, a self normalized basal crystal (shape) of a cubic crystal system is often formed, in which ZrO2 particles are densely arranged and oriented.

2. Research on the application of fused zirconia corundum in refractory materials

Academician Zhong Xiangchong once gave an outlook on the future development direction of refractory materials in China, pointing out that a series of distinctive high-quality synthetic materials, including homogeneous materials, modified materials, and transition materials, should be developed using China's abundant resources. For example, high alumina bauxite can remove most impurities (subtraction) through the electric melting process, and adding an appropriate amount of beneficial oxides (such as zirconia) can further improve its performance (addition). The high-quality fused zirconia corundum mullite series modified materials made in this way have a lower cost and suitable price, and the performance of this series of bauxite based fused materials prepared from high alumina bauxite and zircon sand is comparable to that of the same series of alumina based synthetic materials.

Chen Fang et al. studied the effects of replacing graphite in aluminum carbon materials with 3%, 6%, 9%, and 12% alumina based zirconia corundum in equal amounts on their room temperature physical properties, thermal strength, and oxidation resistance. SEM was used to analyze the microstructure of the sintered samples. The following conclusions have been drawn: with the increase of the amount of alumina based zirconia corundum added, the apparent porosity of the sample decreases, the bulk density increases, the bending strength at room and high temperatures increases, and the antioxidant capacity increases; When the addition amount of alumina based zirconia corundum exceeds 6%, the thermal shock resistance of the sample significantly decreases; The amount of carbon replaced by alumina based zirconia corundum should not exceed 6%.

With the increasing demand for high-quality steel varieties in China, new requirements are constantly being put forward for both quality and quantity of molten steel. To adapt to this situation, inert gas is used to forcibly stir the molten steel to improve productivity and cleanliness. As the main equipment for secondary refining of molten steel, the use conditions of refractory materials for the inner lining of the ladle are becoming increasingly stringent. At present, aluminum magnesium castable is widely used as refractory material for steel ladle at home and abroad. However, for the blowing part of the ladle that requires extremely high thermal shock stability, the thermal shock stability of the self bonded aluminum magnesium casting material can be improved by adding zirconium corundum.

On large and medium-sized steel ladles, high-purity Al2O3-MgO castables and Al2O3 spinel castables are widely used. However, Al2O3-MgO castables are prone to cracks, and slag penetrates into the interior of the castables along the cracks, accelerating erosion and reducing the service life of the ladle lining. Although the composite oxide formed by ZrO2 and Al2O3 has the advantages of good corrosion resistance, thermal shock resistance, and high strength, the high cost of pure ZrO2 limits its promotion and application. Zhang Yanli et al. used newly developed bauxite based fused zirconia corundum as fine powder to study the effect of bauxite based fused zirconia corundum on the properties of Al2O3-MgO ladle castables. The following conclusions were obtained: (1) bauxite based fused zirconia corundum can promote the sintering of Al2O3-MgO castables, thereby improving the physical properties of the castables at room temperature; (2) The introduction of alumina based fused zirconia corundum has little effect on the thermal shock resistance of Al2O3-MgO castable; (3) The introduction of bauxite based fused zirconia corundum has a slight negative impact on the high-temperature flexural strength and slag corrosion resistance of Al2O3-MgO castables, mainly due to the formation of liquid phase impurities in bauxite based fused zirconia corundum at high temperatures.