RH vacuum degassing furnace was originally used as a degassing device. At that time, the refractory lining mainly used clay bricks and high-alumina bricks. Now, the function of RH furnace has been expanded to oxygen blowing and powder spraying. The applicable conditions of lining refractory materials have become more stringent, so high-grade refractory materials are selected. In particular, with the increase in the output of high-grade special steel, the method of increasing the circulation flow and blowing a large amount of gas is being vigorously promoted to achieve stable production and high-speed processing of ultra-low carbon steel. Increasing the circulation flow increases the wear of the refractory lining; increasing the amount of cold air blowing causes high-temperature spalling; and increasing the amount of ladle slag suction increases structural spalling and erosion. All these factors will lead to increased damage to the lining material. Therefore, the current RH/RH-OB lining is dominated by directly bonded corundum bricks, while semi-bonded or re-bonded corundum bricks are used around the oxygen blowing port of the RH/RH-OB lining, and part of it uses magnesia carbon bricks.
The refractory materials used in the top and upper troughs of the RH device are generally less damaged than the lower part because they are not in direct contact with molten steel and slag. The refractory lining of the middle part is damaged due to contact with molten steel and slag corrosion or high temperature spalling. The refractory lining of the lower tank including the immersion tube is a high corrosion area of the RH device, which often determines the service life of the RH furnace. Therefore, the lining of the lower tank should be made of high-temperature fired direct bonded corundum bricks. The most seriously damaged part of the lower part of the furnace body is the circulation tube, because the structure of the lining limits its thickness, and the refractory products of complex shapes need to be heated twice, so no refractory material has a sufficient service life. In addition, in the RH-OB furnace, OB also has an important influence on the use of refractory materials. When the upper spray gun method is used, the refractory material is eroded by the oxides generated by the blown oxygen and the iron element in the molten steel and the high-temperature reaction gas. In particular, the generated oxides will quickly erode the working surface of the refractory material. Therefore, MgO-Cr2O3 bricks with high Cr2O3 content need to be selected to have a higher service life, and MgO-Cr2O3 bricks with low Cr2O3 content exposed to the lower part of the high-temperature gas will have better comprehensive performance.
Corundum refractory bricks for RH furnace
Corundum bricks fired at high temperature (such as direct bonded, rebonded, and semi-rebonded corundum bricks) have been widely used in refining furnace linings due to their strong resistance to low-alkalinity slag corrosion. There are many different varieties of magnesia-chrome bricks, and the production process, organizational structure, and performance of the bricks are also very different. Magnesia-chrome bricks can be divided into corundum bricks (Cr2O3 content of 5~20%), chrome-magnesia bricks (Cr2O3 content of 20~35%), and chrome bricks (Cr2O3 content of more than 35%) according to the Cr2O3 content, and can be divided into sintered bricks and cast bricks according to the production process.
Since there are many varieties of corundum bricks, some literature has classified and summarized them:
(1) Unfired corundum bricks (or chemically bonded magnesia-chrome). Chemically bonded unfired corundum bricks are generally made of magnesia sand and chromium ore as brick-making raw materials, and corundum bricks pressed with sodium polyphosphate or sodium hexametaphosphate or water glass as binders. No high-temperature firing is required, only baking at a temperature of about 200°C. Since it is not fired at a high temperature, the magnesia will be hydrated and cannot be stored for a long time.
(2) Silicate-bonded corundum brick (ordinary fired corundum brick). This brick is made of common chrome ore with a high content of impurities (SiO2 and CaO) and brick-making magnesia, and the firing temperature is about 1550°C. The structural characteristics are: the refractory grains are bonded by silicates, the apparent porosity is high, the resistance to slag erosion is poor, and the high-temperature volume stability is poor.
(3) Cast magnesia-chrome product brick. Magnesia and chromium ore are used as the main raw materials, and a small amount of additives are added. After mixing, pressing and biscuit firing, the bricks are broken into blocks, melted in an electric arc furnace, injected into a mold, annealed, and produced into mother bricks. The mother bricks are cold-processed by cutting, grinding, etc. to make products of various specific shapes. The structural characteristics of fused-cast corundum bricks are uniform distribution of components, direct contact between refractory grains, and the existence of silicates in isolated islands. The bricks are particularly resistant to melt erosion, penetration and erosion, but have poor thermal shock stability.
(4) Co-sintered corundum bricks (also known as fully synthetic magnesia-chrome). The corundum bricks produced by using 100% sintered synthetic magnesia-chrome sand as the raw material for brick making and then firing at high temperature are co-sintered magnesia-chrome. Its characteristics are good corrosion resistance and good high-temperature volume stability.
(5) Directly bonded corundum bricks. Directly bonded magnesia-chrome refractory materials are made of chromium concentrate with low impurity content and relatively pure magnesia sand, and the firing temperature is above 1700℃. Its structural characteristics are: the refractory grains are mostly in direct contact, and the direct bonding degree of periclase (solid solution)-periclite (solid solution) and periclase (solid solution)-spinel (solid solution) in the bricks is high. Therefore, its high-temperature performance, good resistance to slag corrosion, and high-temperature volume stability are better than ordinary corundum bricks.
(6) Molten rebonded corundum brick (electrolytic rebonded magnesia-chromium). Magnesia sand and chromium ore (lightly burned magnesia powder or magnesite and chromium ore) are fully and evenly reacted by electrofusion to synthesize more ideal periclase solid solution and spinel solid solution magnesia-chromium raw materials. Rebonded corundum bricks are made of this raw material and are called molten rebonded corundum bricks. Since the raw materials for brick making are relatively pure, they need to be fired at a high temperature or ultra-high temperature above 1750℃. Its microstructural characteristics are uniform distribution of components such as spinel, low porosity, direct contact between refractory grains, high compressive strength, good corrosion resistance, high high temperature strength, etc., but the disadvantage is poor thermal shock stability.
(7) Semi-rebonded corundum bricks. Corundum bricks with artificial synthetic raw materials as particles and chromium concentrate and magnesia sand as fine powder should be called semi-rebonded corundum bricks. In China, the corundum bricks made of fused magnesia-chromium materials as particles and co-sintered materials as fine powder or mixed fine powder of chromium concentrate and magnesia powder are called semi-recombined corundum bricks. The firing temperature is above 1700℃, and the refractory grains in the bricks are mainly directly bonded. The advantages are good thermal shock resistance, erosion resistance and scouring resistance.
(8) Pre-reacted corundum bricks. Magnesia (lightly burned magnesium powder) and chromium ore are co-grinded and pressed into billets and fired in a kiln. Synthetic magnesia-chromium sand is used as raw material to make bricks to form "pre-reacted corundum bricks". Pre-reacted corundum bricks are an improved version of silicate-bonded corundum bricks.
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