Compositions, Structure And Properties Of Alumina-Zirconia-Carbon Sliding Gate Refractories

Sliding gate plate is the key component to directly control molten steel flux and flowing speed,and stabilize the liquid level as it is the most important part in whole sliding gate system. Currently, Al₂O₃-ZrO₂-C sliding gate plates are widely used on medium or large size ladles and tundish in steel-making industries all over the world. With the development of the continuous casting technology with higher efficiency and steel grades, the field conditions for sliding gate plates are becoming harsher. Therefore, it is very important to continuously improve the properties of Al₂O₃-ZrO₂-C sliding gate plates. Metal aluminum and silicon powder are usually added into Al₂O₃-ZrO₂-C refractories to form ceramic phases such as Al₄C₃?SiC?AlN or Si3N4,etc, due to in-situ reaction with carbon in the matrix or the CO?N2 in surrounding atmosphere, which contributes to the excellent properties. From the thermodynamic analysis of Al-Si-SiO₂ ternary system with carbon under nitrogen atmosphere, quite a lot of reactions present, and all related resulting products have excellent physical and chemical properties, which provide different approaches to improve the refractories properties by the materials selection and microstructure design. Up to now, there are very few reports about the influence of Al, Si, SiO₂ content and manufacturing process on microstructure and mechanical properties Al₂O₃-ZrO₂-C refractories. Therefore, it is of great importance to have the whole research about the influence of amount and types of metal Al, Si, SiO₂, and firing atmospheres on Al₂O₃-ZrO₂-C refractories properties.In this thesis, focusing on Al₂O₃-ZrO₂-C refractories, the thermodynamic simulation of reactions among carbon and Al, Si under different temperatures and their micro-structure characterization were carried out. The influence of ratio of Al/Si, additional amount and types of SiO₂ powder and firing atmospheres on Al₂O₃-ZrO₂-C refractories microstructure and properties were studied. The relationship between material microstructure and mechanical properties were characterized by means of mercury analyzer and scanning electron microscopy. A new generation of aluminum zirconium carbon sliding gate material was developed, and the large-scale production and field trials were carried out. The main results are as follows:(1). The new phases from the reactions among metal Al, Si and SiO₂ powder with the mixture of graphite and carbon black are in line with those results from the thermodynamic simulation. The formation temperatures and morphology of new phases depend on the types of additions, additional ratios, and firing temperatures and atmospheres. Another remarkable influence of new ceramic phases on the morphology is from the carbon source.(2). The additional ratios of metal Al and Si play a main role on the microstructure and properties of Al₂O₃-ZrO₂-C refractories. From 800?to 1000?, the CCS, CMOR, elastic moduls, and toughness of samples increase as the metal Al content increases, but when firing temperature is 1300 or 1400?, those mechnical properties show the dropping as the metal Al content increases. The average pore sizes present opposite trends against the mechanical properties. The strength change of samples depends on their microstructure. The mechnical properties of Al₂O₃-ZrO₂-C refractories can be optimized by the additional ratio of metal Al and Si under reasonable firing temperature.(3). The different mechanical properties of Al₂O₃-ZrO₂-C refractories are contributed to the addition, types of micro/nano size SiO₂ powder and atmosphere. Addition of micro / nano size SiO₂ powder in the matrixes is beneficial for the improvement of hydration resistance, and less strength sensitivity on firing temperature for those sliding gate refractories containing metal Al. Compared with coke-bed atmosphere, samples fired under nitrogen atmosphere present better properties.(4). Based on the work above, code A21 formulation with 2% Al, 3% Si and 1% SiO₂ powder was chose to run the industrial production. Those new sliding gate plates present good microstructure with finer and well distributed whiskers after fired under 1200?in industrial production line, showed excellent physical properties, and achieved good field performance on large/medium size ladles.