(I) Conditions for use of refractory materials
Equipment that uses the arc generated between the electrode end and the charge as a heat source for steelmaking. Based on the use of high power, DC electric arc furnaces are developed, with gas supply and stirring at the bottom of the furnace and steel tapping at the bottom of the furnace. The furnace top is built with magnesia-chromium and periclase spinel refractory materials, which have the most stable effect on alkaline slag and metal and silicate melts. The conditions for the use of refractory materials here are quite harsh. This is determined by the particularity of arc melting. The arc melting time is about 2 times shorter than that of an open hearth furnace. Therefore, the refractory materials are subjected to temperature changes more frequently and are in a high temperature state for a long time.
Because the furnace top is separated from the electric furnace during the charging of the charge, and the temperature at the center of the furnace top is higher than that of the surrounding area, the unevenness is increased because the working arc (the influence of the weird phase) is uneven. Therefore, the center of the furnace roof is quickly worn out. For example, the damage rate of the center of the furnace roof of a 100t electric furnace reaches 4~4.4mm/furnace, while the peripheral part is 2~2.6mm/furnace. The unevenness of the damage causes uneven bulges on the furnace roof, and bricks are sometimes found to fall off. Since the DC arc furnace adopts a single electrode structure, there is no hot spot area. In addition, the water cooling area of the furnace roof is expanded, and the use conditions of refractory materials have improved. However, with the expansion of furnace capacity and the increase of unit power, the working conditions of the furnace roof have become more stringent. The electric furnace roof is round, and fan-shaped arch methods and circular masonry are often used. Bricklaying is like not using binders and cement materials, using metal gaskets with plug tips. Electrode holes, holes for extracting gas and oxygen bricks make the furnace roof lighter. In some cases, the hole area uses a castable with high-alumina cement or phosphate bonding as a whole. At the same time, measures are taken to eliminate current short circuits. The life of the electric furnace roof with a capacity of less than 100t is about 60~120 furnaces, while that of a large electric furnace with a capacity of more than 100t is 60~80 furnaces. The total consumption of 1t steel refractory materials for an electric furnace is about 10~12kg, of which 6~7kg is consumed by the furnace roof.
(II) Selection of refractory materials for the furnace roof
The selection of refractory types for the electric arc furnace roof is still in progress. Although MgO-Cr₂O₃ series refractory materials have high slag resistance and metal resistance, the load softening temperature is relatively low. Another disadvantage of basic refractory materials is that they have considerable expansion at high temperatures, causing cracking of brick joints and deformation of the furnace roof. In order to prevent cracking of brick joints, a mixed masonry of fired and unfired products is adopted, and finally the expansion of some fired products is offset by shrinkage. Some literature introduces: fired, metal-skinned and unfired magnesium-chromium products are used in combination.
New refractory materials for the electric furnace roof tested: corundum chrome ore, mullite corundum, basic and high aluminum combination, etc. Pay attention to the application of chromium-containing refractory materials. Since chromium-containing bricks may peel off, chromium may appear in steel, and smelting of certain grades of steel is not allowed.
According to the development prospects of the metallurgical industry, due to the technology of atomic energy electric furnaces and continuous casting machines, large-scale electric arc furnace steelmaking has a considerable development. This depends on the important advantages of electric furnaces, among which electric furnaces are more than flat furnaces or converters. They bid farewell to adjusting the chemical composition of steel and can obtain steel of any composition. Increasing electric arc furnace steelmaking is beneficial to the continuous increase of economic scrap metal.
The top of my country's electric furnaces is generally built with high-aluminum bricks, and the central part of the small furnace cover and the electrode hole are generally rammed. With the development of large-scale ultra-high power electric furnaces, the service life of high-aluminum bricks has decreased. Leading to the further use of alkaline bricks.
(III) Application of water cooling technology
The development trend of modern electric smelting steel production is related to the use of high-power transformers. Ensure that the unit power during the melting period is 600~800kV·A/t, preheat the charge, blow oxygen into the molten pool, and use a gas oxygen burner to heat the furnace. Artificial cooling components should be used to replace part of the refractory masonry. When oxygen is violently supplied to the molten pool, dust (15~40g/m³) is formed, causing the optical density of the gas medium to increase, and the gas blackness becomes close to 1, which actually completely covers the molten pool and the furnace top, and reduces the furnace top temperature.
The slagging type furnace top and wall adopt various water-cooled components, and the hot spots of the wall masonry are protected by movable cooling guards. The artificial cooling components of the masonry form a new arc furnace structure, and the variable degree of masonry development (the ratio of the metal plane area to the refractory masonry surface area) may be reduced, which reduces the power consumption of steel smelting. Due to the optimal heat exchange conditions, the heat on the molten pool surface is reduced, which increases with the decrease of the masonry development degree, which depends on reducing the number of stacking or eliminating it. With a structure with a variable degree of masonry development, the horizontal water-cooled furnace top has the possibility of moving down along the water-cooled wall according to the smelting process.
With an artificially cooled steelmaking furnace, the factors of the masonry are used to remove the restrictions on the size of the heat load and the masonry temperature, and form the prerequisite for strengthening smelting. Although the arc furnace top adopts a water-cooled structure, the power consumption has increased slightly, the shutdowns that determine the overall economic efficiency have decreased, and the productivity has increased. The consumption of refractory materials has been significantly (almost to the maximum extent), the unit consumption of electrodes has been reduced (15%), and the heavy manual labor of manual bricklaying of refractory masonry has been reduced.
The experimental development of furnaces with water-cooled masonry components has enabled the energy and cost savings of related departments (refractory production, transportation, electrode production, etc.) to exceed the cost of increased power for water-cooling equipment.
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