China's Refractories

A ladle primarily is a container used to transport molten steel from the steelmaking unit to the casting facility. The essential requirements are heat resistant, insulative and strong enough to hold molten steel up to 300 t, with a structure of multilayered refractory lining in a steel shell vessel. When a ladle is assembled with three-phase graphite electrodes, a ladle furnace forms, starting steel refining process with/without the vacuum tank degasser and RH circulating degasser, in order to meet the growing demand on high purity and higher quality steel. The working lining is under aggressive conditions that comprise the chemical reaction with molten steel, the severe corrosion of liquid slag, the disaggregation through oxidation, and the strong stress due to the impacting of the melts and gases and the effects of thermomechanical behaviors. Magnesia-carbon brick is one of the major materials, being indispensable for the slagline of a ladle furnace. Alumina-magnesia-carbon brick has played an important role to substantially increase the service life in the metal zone, from the early lining materials of high alumina and doloma bricks. The permanent lining must assume sufficient responsibility to allow finishing the process of the engaged charge in case of a failure in the working lining. The insulation layer must be as thin as possible in order to maximize the ladle’s capacity, and reduce the shell temperature for saving energy. In this issue, several integrant refractories are reviewed or investigated in order to compile a lining overview, and to contribute a prolonged service life under aggressive working conditions of the ladle furnace.

The manufacturing techniques of magnesia-carbon bricks in China have been documented from raw materials, production (process and facilities) to performance and wear issues in the ladle. Magnesia-carbon bricks made of ordinary fused magnesia is the prevailing material used in the slagline of the ladle, but its service life is substantially lower than the bricks based on large-periclase-crystal fused magnesia. In two types of fused magnesia, the average values of periclase crystal size are in double for their difference. It is suggested that large-periclase-crystal fused magnesia should be used for manufacturing magnesia-carbon bricks for the slagline of the ladle by abandoning ordinary fused magnesia, in order to have a prolonged service life, increase the availability of the ladle and reduce the number of downtimes of the ladle. Free phenol in resin produced in China should be as low as that of resin made in Europe, to improve production environment and reduce smoke emission during the ladle preheating. There are large spaces to promote the productivity of magnesia-carbon bricks in China, with high intensity mixers and hydraulic presses. Expansion controlled magnesia-carbon bricks in the ladle depend on the optimized combination of sintered magnesia, Carbores and antioxidants as the matrix, to minimize the premature wear of vertical cracks and joint opening formed in the ladle lining.

The manufacture of clean steel needs high performance carbon-free bricks for ladle lining. Based on long term application experiences of the prefab Al₂O₃-MgO blocks, unburnt Al₂O₃-MgO brick has been developed by pressing a mix of alumina, spinel and magnesia as major raw materials and Al₂O₃-MgO gel powder as binder. In addition to low open porosity and high strength, the unburnt Al₂O₃-MgO brick shows superior corrosion resistance and thermal shock resistance to the prefab block. Field trials in a 300t steel ladle have indicated the residual lining of unburnt Al₂O₃-MgO bricks showed smooth surface without large spalling and seldom steel infiltration, consequently longer service life than the prefab blocks. The reliable, unburnt Al₂O₃-MgO bricks have more advantages of high efficiency, energy-saving and eco-friendly production. It will be one of the best options for ladle lining in the manufacture of high-purity steel.

Calcium magnesium aluminate, with the commercial name of MagArmour, is a synthetic material consisting of 70 mass% Al₂O₃, 20 mass% MgO and 10 mass% CaO, approximately. It is characterized by porous microstructure, intergranular aluminates phases and micro-crystalline spinel. Since globally launched in 2017, MagArmour has been successfully applied to various carbon-containing refractories serving for steel refining process. Lots of cases have demonstrated the role of MagArmour in enhancing the service life of carbon containing bricks for ladle lining. The benefits embody in formation of protective coating on hot surface, relief of drilling corrosion in joint positions, and elimination of grooves or cracks caused by mechanical stress concentration. In addition, MagArmour is effective in protecting graphite from deep oxidization so as to be capable of replacing the metallic or carbide anti-oxidants in carbon-containing bricks entirely. By means of chemical analysis and microstructural dissection, postmortem investigations on the used MgO-C bricks from both metal and slag zones of 120t steel refining ladle were conducted to clarify the working mechanism of MagArmour. The formation of protective coating on hot face is attributed to the dissolution of micro-crystalline spinel into contacting slag, which changes the slag chemistry so as to enhance viscosity. The improvement in corrosion/erosion resistance is highly related to the porous microstructure and dispersive aluminates. As well known, evaporation of Mg, Al and SiO, and/or internal migration, occurs in MgO-C bricks at elevated temperatures. The gaseous phases are absorbed by MagArmour particles due to the high surface area of porous microstructure and condense as corresponding oxides. These oxides react with the intergranular calcium aluminates forming liquid phase. With increasing temperature, the liquid phase seeps into the matrix under capillary force. The increased liquid amount improves the flexibility of the matrix and thus releases the internal stresses concentration resulting from mechanical stress and temperature gradient. Meanwhile, densification of the matrix microstructure occurs under the static pressure generated by liquid steel and molten slag, which blocks the channels of oxygen infiltration.

A ladle shroud is one of the functional refractories for continuous casting, which undergoes severe thermal shock by molten steel when used without pre-heating. The composite ladle shroud with an insulating liner presents excellent thermal shock resistance. Finite element simulation is an effective method to explore the maximum thermal stress for predicting the thermal shock resistance of ladle shrouds. In this paper, the influence of the lining materials and the structure of ladle shrouds on the thermal stress distribution is systematically researched. The working mechanism of the lining material on the body material is also presented. Lining materials with low thermal expansion, elastic modulus and thermal conductivity are helpful to improve the thermal shock resistance and an optimum lining thickness is suggested. The lining material can both serve as thermal resistance for the body material to buffer the thermal stress, and apply a strain load to the body material by the thermal strain to increase the stress.

Minimizing of heat loss of liquid steel through the ladle lining led to the integration of increasing insulation function of the permanent lining and high-quality insulating layer between the permanent lining and the ladle steel shell. In this paper, a study was made on physical and thermomechanical properties and corrosion resistance of calcium hexaluminate based castables. The chemical and physical properties of CA₆ based castables fabricated in this work can fulfill general requirements of the permanent lining in ladle. The permanent linings of CA₆ based castables offer superb thermal insulation and corrosion resistance, compared to conventional refractory systems of 300t ladle in Baosteel. Both functions of insulation and safety work effectively in whole process of ladle operation.

The residual expansion of in-situ spinel formation in using of alumina-magnesia-carbon (AMC) bricks monolizes the lining of steel-making ladles with the closure of their joints, which has been an effective solution avoiding washing out of the joints in ladle lining by the reduction of the penetration of liquid slag and molten steel. Alumina-magnesia-carbon refractories are overall reviewed, in terms of major raw materials, thermal evolution, corrosion and oxidation, and thermomechanical behavior, as well as type, addition and fraction of magnesia used. General commercial products contain 5%-10% MgO and 5%-10% C with a certain amount of metallic aluminum powder, which is believed to facilitate spinel formation at early stage of heating-up, although high magnesia containing AMC bricks are studied and used sometimes. With low ratio of Al₂O₃/C=12.9 and the carbon content of 6.4% C, AMC brick exhibits the highest corrosion resistance. It is important to determine the type, addition and fraction of magnesia used in AMC refractories for demonstrating high corrosion resistance and superior thermomechanical behavior.