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Magnesia-carbon bricks are a new refractory material that emerged in the 1970s. It is a non-burning refractory material made of high-temperature sintered magnesia or fused magnesia and carbon materials and various carbon binders. Magnesia-carbon bricks That is to say, it maintains the advantages of carbonic refractory materials, and at the same time completely changes the inherent shortcomings of previous alkaline refractories that have poor spalling resistance and are easy to absorb slag.
So far, magnesia-carbon bricks are still widely used in the steel and metallurgical industry. The application of magnesia-carbon bricks has improved various technical and economic indicators of converters and reduced the consumption of refractory materials. In addition, as a non-fired product, compared with traditional fired magnesia dolomite bricks, the fuel consumption of magnesia carbon bricks is saved by at least 80%.
1. Properties of magnesia carbon bricks
Magnesia-carbon bricks are composed of magnesium oxide and carbon, both of which have high melting points, and the two components do not solidify each other. Therefore, magnesia-carbon bricks have high melting resistance.
Magnesia-carbon bricks are a composite structure. The main part is made of magnesium oxide clinker, which has strong resistance to alkaline slag corrosion, and carbon, which has poor wettability with molten slag. Therefore, it has excellent resistance to slag corrosion. In particular, it has strong resistance to the penetration of molten slag. Compared with the old fired alkaline bricks, the penetration layer of magnesia carbon bricks is much shallower.
Dry graphite has excellent thermal shock resistance. Therefore, magnesia-carbon bricks, which inherit the excellent properties of graphite, have high thermal conductivity, relatively small linear expansion coefficient and elastic modulus, and relatively large high-temperature strength. They avoid Tissue damage and peeling caused by cracking during use.
In addition to the above-mentioned excellent characteristics, magnesia-carbon bricks also have good thermal creep resistance. Compared with other ceramic-bonded bricks, magnesia-carbon bricks show particularly good resistance to deformation.
2. Application of magnesia carbon bricks in converter steelmaking
Since the advent of oxygen converters, furnace lining materials have experienced three stages of evolution: tar dolomite bricks – fired alkaline oil-immersed bricks – and magnesia carbon bricks. Since the 1980s, steelmaking converters and their lining refractory materials have made great progress. As far as steelmaking converters are concerned, they have completed the process of large-scale and automated development. At present, it is developing towards double blowing and high temperature.
During the smelting process, the use conditions and damage conditions of each part of the converter are different. The refractory materials used in each part of the converter under different use conditions are also different.
Furnace mouth: Due to the drastic temperature changes at the furnace mouth, the erosion of slag and high-temperature exhaust gas is harmful, and the furnace door is hit when scrap steel is removed and materials are added, so the refractory materials used for the furnace mouth must have high thermal shock resistance. It has good properties and slag resistance, must be resistant to the erosion of molten slag and high-temperature exhaust gas, and must be difficult to hang on steel and easy to clean.
Furnace cap: The furnace cap is the most severely corroded part by slag. It is also affected by temperature changes, oxidation of carbon, and erosion of dusty exhaust gas. Therefore, magnesia carbon bricks with strong slag resistance and thermal shock resistance are needed.
Charging side: The splashing effect of slag and molten steel during blowing can easily cause chemical erosion, wear, and erosion on the charging side. The charging side is also directly impacted and eroded by the loaded scrap steel and molten iron, causing serious mechanical damage. Therefore, magnesia carbon bricks are required to have high slag resistance and high-temperature strength. Very good thermal shock resistance, usually high-strength magnesia carbon bricks with added antioxidants.
Tapping side: The tapping side is not affected by mechanical damage during charging, and the impact of thermal shock is also small. However, due to the thermal shock and erosion of molten steel during tapping, the damage speed is much smaller than that of the charging side. When the material of the charging side is the same, to maintain the balanced life of the converter lining, the masonry structure is thinner than that of the charging side.
Slag line location: The slag line is the location where the furnace lining has been in contact with molten slag for a long time and is seriously corroded by slag. On the tapping side, since the position of the slag changes with tapping time, it is mostly not obvious. On the slag discharge side, due to the combined influence of strong slag corrosion and other effects on the furnace belly during the blowing process, the damage was serious. Therefore, it is necessary to build magnesia carbon bricks with excellent slag resistance.
Both sides of the trunnion: In addition to being damaged during blowing, the surface of both sides of the trunnion is not covered by a protective layer and is not easy to repair. Therefore, the carbon in the furnace lining material is easily oxidized, so the damage is serious, and the slag resistance should be built. Excellent, high-grade magnesia carbon brick with strong oxidation resistance.
Hearth and furnace bottom: These parts are violently eroded by molten steel during blowing, but compared with other parts, the damage is generally lighter. Magnesia carbon bricks with low carbon content can be used, or tar dolomite bricks can be used. When high-speed blowing is used and the molten pool is shallow, the damage to the center of the furnace bottom may be serious. In addition, when bottom blowing is used, the damage to these parts may be aggravated, and the same material as the charging side of the furnace body should be used.
At present, to improve the technical and economic indicators of converters, comprehensive masonry is generally used.
3. Use of magnesia carbon bricks on electric furnaces
At present, the walls of electric furnaces are almost entirely built with magnesia-carbon bricks. Therefore, the service life of magnesia-carbon bricks determines the service life of the electric furnace. The main factors that determine the quality of magnesia-carbon bricks for electric furnaces include the purity of the magnesia of the MgO source, the type of impurities, and periclase. The bonding state and grain size of the grains; the purity, crystallization degree, and flake size of the flake graphite as the source of carbon introduction; the thermosetting phenolic resin is usually used as the binder, and the main influencing factors are the amount added and the amount of residual carbon. It has now been proven that adding antioxidants to magnesia-carbon bricks can change and improve their matrix structure. However, when used under normal operating conditions of electric furnaces, antioxidants are not necessary raw materials for magnesia-carbon bricks but are only used in arcs with high FeOn slag. Furnaces, such as directly reduced iron or parts with irregular oxidation and hot spots of electric furnaces, can become an important part of magnesia carbon bricks by adding various metal antioxidants.
The corrosion behavior of magnesia-carbon bricks used in the slag line area is manifested by the formation of an obvious reaction dense layer and a decarburized loose layer. The reaction dense zone is also called the slag invasion zone, which is the erosion area where high-temperature liquid slag penetrates the interior of the brick after the decarburization of magnesia-carbon bricks forms a large number of pores. In this area, FeOn in the slag is reduced to metallic iron, and even the desolvated phase and intergranular Fe2O3 solidly dissolved in MgO are reduced to metallic iron. The depth of slag penetration into bricks is mainly determined by the thickness of the decarburized loose layer and usually ends where graphite remains. Under normal circumstances, the decarburization layer in magnesia carbon bricks is relatively thin due to the presence of graphite.
There are two ways of tapping the steel outlet of the electric furnace: tilting the tapping trough and tapping the furnace bottom. When the tapping trough is tilted to tap steel, magnesia carbon bricks are not used, but Al2O3 or ZrO2 is selected, and non-oxygen substances such as C, SiC, and Si3N4 are added. When bottom tapping is used, the tapping port is composed of outer jacket bricks and inner tube bricks. The tap hole at the bottom of the furnace is made of magnesia-carbon brick pipe bricks. The hole diameter of the pipe brick is determined according to factors such as furnace capacity and tapping time. The general inner diameter is 140~260mm.
The electric furnace of a steel plant uses mid-range and low-range magnesia-carbon bricks at the tap hole. The two sides of the copper tap hole replace the original sintered magnesia bricks and initially achieve good results. The furnace life is increased from about 60 furnaces to more than 1 time. After use, the magnesia carbon bricks at the slag line remain intact and do not stick to the slag. There is no need to repair the furnace at the slag line, which not only reduces labor intensity but also improves the purity and productivity of molten steel.
4. The use of alumina-magnesia carbon bricks on ladles
When MgO-C bricks are used in refining ladle furnaces and ladles, they are mainly used in clearance and slag lines. According to the operating conditions, the refractory materials used in these parts must be resistant to high temperature, thermal shock, and mechanical corrosion caused by slag erosion. Therefore, in the past, magnesia-chromium refractory materials were used in these parts. However, considering that chromium pollutes the environment, its dosage has been reduced. Now magnesia-carbon bricks are used.
Since the magnesia carbon bricks in the new ladle will be severely damaged during the preheating process, the loose decarburization layer can reach 30~60mm thick. This layer is washed away during the injection of molten steel and the magnesia particles are brought into the molten slag. Preventing the carbon in magnesia carbon bricks from being burned during preheating has become one of the important steps to improve the service life of magnesia carbon bricks in the ladle clearance and slag line areas. The technical measure, in addition to adding composite antioxidants to the magnesia carbon bricks, is that after lining, the surface of the magnesia carbon bricks should be covered with an alkali-containing low-melting glass phase liquid to protect the magnesia carbon bricks. The carbon is not burned during the preheating process of the ladle.
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