EAF steelmaking is a complex high-temperature process, with high humidity in the furnace (up to 1700 ~ 1800 ℃), large atmosphere change and short smelting cycle. The high-temperature environment and erosion mechanism of refractory materials used in various parts of EAF are different. Therefore, the study on the damage mechanism of refractory materials used in various parts of EAF plays an important role in improving the service cycle of EAF.
- Lining damage mechanism:
The lining refractory of electric arc furnace is often in the state of high temperature, slag erosion and rapid cooling and heating, and the working conditions are extremely bad. In the melting period and oxidation period of steelmaking, the furnace lining will not be damaged by high temperature, but in the reduction period, the temperature of liquid steel is quite high and is directly radiated by electric arc, so the furnace lining is easy to be damaged. In addition, when the steel is discharged and charged, the temperature of the furnace lining suddenly decreases, the charging starts to send electricity, and the temperature gradually increases. This rapid cooling and heating temperature change is often the direct cause of the peeling of the furnace lining.
The erosion of slag on the lining determines its composition and fluidity. The higher the SiO2 content in slag, the more serious the erosion of alkaline furnace lining. In addition to SiO2, Al2O3, Fe2O3 and CaF2 in the slag also erode the furnace lining. The fluidity of slag also affects the lining. The thin slag has a serious scouring effect on the lining, and has a strong ability to reflect the arc, which will increase the heat load of the lining refractory; It is difficult to thicken the molten slag (i.e. increase the heating time of the molten slag). These will accelerate the damage of furnace lining.
In many cases, the damage of furnace lining is caused by the impact of mechanical force, such as unreasonable distribution, no small scrap protection at the furnace bottom, and too high charging tank. In these cases, the furnace bottom may be hit into pits by large pieces of scrap steel. The mechanical vibration caused by the lifting of furnace cover, opening of furnace body, tilting furnace and steel tapping is also the cause of damage to furnace lining. In addition, improper burden ratio and incorrect smelting operation, such as high-temperature oxygen flow directly touching the furnace lining during oxygen blowing, will damage the furnace lining.
Therefore, the refractory materials for electric arc furnace should have high fire resistance and load softening point, good thermal stability and slag resistance, sufficient high-temperature mechanical strength and low thermal conductivity. Silicon brick is generally used for laying acid electric arc furnace, while magnesium brick is used for alkaline electric arc furnace, but high alumina brick is mostly used for its top. Silica brick roof has been rarely used.
- Damage mechanism of furnace top:
The furnace top is made of refractory materials. The furnace top, also known as the furnace cover, is in a circular arch shape and can be moved. The outer ring is a water jacket steel structure. In the smelting process, the furnace top (especially the central part of the furnace top, around the electrode hole and dust removal hole) is in a high temperature state for a long time, and is often affected by sudden temperature change, chemical erosion of furnace gas and slag forming powder, radiation of electrode arc and erosion of smoke and dust. The dust on the furnace top also generates pressure and hinders effective heat dissipation. The furnace top is also affected by mechanical vibration during lifting and rotation, and the working conditions are very poor, It is the weak link of the whole furnace lining. Therefore, the furnace life of electric furnace refers to the service cycle of electric furnace top.
Due to the single electrode structure of DC electric arc furnace, there is no hot spot area. In addition, the water-cooling area on the top of the furnace is expanded, and the service conditions of refractories are significantly improved. After entering the 1980s, with the expansion of the capacity of steelmaking electric arc furnace and the improvement of unit power, the working conditions of the furnace top have become more stringent, and the refractory materials used have also changed. The traditional refractory used for the furnace top is silica brick. Silica brick has poor thermal shock resistance in the low temperature range, does not adapt to sudden temperature changes, and it is impossible to greatly improve the fire resistance due to the limitation of raw materials. Under high temperature, not only the fluidity of silicon slag, but also the service life of refining furnace is seriously affected. In addition, the use of oil injection and oxygen blowing to strengthen smelting increases the smelting temperature and shortens the smelting cycle, and the silica brick can not meet the needs of smelting. Although measures such as adding a small amount of chromium oxide or asphalt impregnation into silica bricks have been taken, the effect is not obvious. Therefore, high alumina bricks, alkaline bricks or corresponding refractory castables or ramming materials are gradually used to build the furnace top.
The service life of silicon brick on the top of electric arc furnace is very low, which is mainly due to the high smelting temperature. In addition to the effect of slag and smoke, the peeling phenomenon is also one of the main causes of damage. Due to the appearance of cracks and the peeling of refractory bricks, the contact between bricks and slag is increased, which accelerates the penetration of slag into the interior of bricks and forms many channels, that is, the so-called deep damage.
When high alumina bricks are used on the top of electric arc furnace, in addition to melting damage, there are still small pieces of bricks peeling off in the smelting process, which is caused by temperature change, cooling shrinkage and crack formation.
The service life (i.e. furnace life) of silicon brick is generally only dozens of heats, while that of high alumina brick can reach 80 ~ 200 heats, and that of small electric arc furnace can reach 500 heats. At the same time, due to the high fire resistance of high alumina brick, the smelting strength can also be improved.
- Damage mechanism of furnace wall and bottom:
Magnesia bricks are widely used to build the furnace wall and bottom of alkaline electric arc furnace. In order to balance the damage of the furnace wall and prolong the service life, the furnace wall also adopts the measures of inlaying water-cooling box or water-cooling jacket. A layer of refractory coating is sprayed on the inner surface to form a protective layer, which can effectively reduce the unit consumption of refractory materials, but the energy consumption is relatively increased. The furnace bottom is made of refractory materials. The furnace bottom and embankment slope form a molten pool, which is the place where the furnace charge and liquid steel are collected.
The bottom refractory is in direct contact with liquid steel and slag, which is mainly subject to chemical erosion and scouring, and is covered and quenched by the furnace charge. When the furnace bottom lining reacts with the molten slag to form a metamorphic layer, it can also become loose due to the reduction of Mao dietong during the reduction period, which often causes floating due to the intrusion of liquid steel. The masonry or knotting lining in this part is required to have strong overall performance, strict prevention of liquid steel penetration, good high-temperature performance, sufficient strength, corrosion resistance, erosion resistance, good thermal shock resistance and volume stability, and get good sintering in the process of use.
Magnesia bricks are widely used to build the furnace wall and bottom of alkaline electric arc furnace. The magnesia brick absorbs the molten silicic acid and iron oxide in the smelting room during use and is strongly eroded by the slag, which is the main cause of damage. The spalling of magnesia brick surface is the result of internal stress.