RH insert pipe lining is mostly built with high-quality direct-bonded magnesia-chrome refractory bricks with good thermal shock resistance, and the outer layer is made of a new generation of ultra-low cement-bonded Al2O3-MgO castable with excellent performance.
Direct-bonded magnesia-chrome bricks generally refer to products made of chromium ore with low impurity content and relatively pure magnesia sand, fired at a temperature above 1700℃, and the refractory grains are mostly in direct contact [29~34]. The chemical composition of direct-bonded magnesia-chrome bricks has few impurities and a high direct bonding rate between refractory grains, so they have good slag resistance and high temperature performance. Its slag resistance is specifically manifested in that Cr2O3 has a high melting point (about 2400℃), and can form solid solutions, high melting point compounds or low eutectic with very high melting temperatures with many oxides, which increases the viscosity of the infiltrated slag and inhibits further penetration of the slag. At the same time, the direct bonded magnesia-chrome brick has excellent thermal shock resistance, and can be cooled by water at 1100℃ for 4 to 10 times.
However, magnesia-chrome refractory bricks also have fatal shortcomings. The problem of magnesia-chrome bricks is mainly the pollution of the environment by hexavalent chromium. In the presence of oxidizing atmosphere or strong alkaline oxides such as Na2O, K2O and CaO, the Cr3+ in refractory materials containing Cr2O3 can be converted into Cr6+. Hexavalent chromium is a carcinogen announced by the International Cancer Research Center and the American Toxicology Organization, and has obvious carcinogenic effects. Hexavalent chromium can cause damage to the human skin, respiratory tract, eyes and gastrointestinal tract. At the same time, CrO3 can also exist in the form of gas phase, polluting the environment. In addition, magnesia-chrome bricks are shaped products, not integrally cast, and are more likely to slag during use than castables, causing pollution to the molten steel.
Some people have also applied MgO-C refractory bricks on RH furnaces and achieved good results. Japan uses MgO-C bricks in the lower tank and the inner surface of the immersion tube of the RH furnace, and the furnace life has reached 417 times and 94 times respectively. If the RH furnace is not for refining ultra-low carbon steel, the RH insert tube lining uses low-carbon magnesia-carbon bricks or low-carbon magnesia-calcium-carbon bricks, which are suitable in terms of thermal shock resistance, slag penetration resistance, and alkaline slag erosion resistance. This is mainly due to the poor wettability between carbon-containing refractory materials and slag, which makes them have good resistance to slag penetration and structural spalling. At the same time, carbon has high thermal conductivity and low thermal expansion coefficient, and thermal stress can be released in time after heating, so that carbon-containing materials have good thermal shock stability.
Carbon-containing materials are not conducive to the smelting of ultra-low carbon steel and clean steel. Carbon has a high solubility in iron and is easily soluble in molten steel, which will cause pollution to the molten steel. While the RH furnace is blowing oxygen to decarburize the molten steel, the carbon in the magnesia-carbon bricks is also easily oxidized, causing damage to the refractory. At the same time, the cost of carbon-containing materials is also increasing. Therefore, developing environmentally friendly and energy-saving refractory materials to replace magnesium-chromium materials is an important task for the steel industry and the refractory industry.
