Breathable Components
It has become an important means of modern metallurgy and an important part of off-furnace refining technology to use gas supply element (also known as breathable brick) to spray inert gas into steel making vessel and mix molten steel. Its function is to strengthen the metallurgical process, promote the metallurgical reaction, shorten the smelting time, even the temperature and composition of molten steel, make the inclusion in molten steel float up with the gas, reduce the non-metallic inclusion in molten steel, clean the molten steel, improve the quality of steel, widely used in converter, electric steelmaking and refining ladle. The trend is to apply to the whole process of smelting, including tundish, torpedo tank and other metallurgical containers.
The gas supply element has become an important functional refractory material in the process of steelmaking. At present, there are two main types: one is for converter and electric furnace and the other is for ladle. The two have obvious differences in the use conditions and requirements, and have their own characteristics in material selection, structure form and production technology.
Ventilating element for converter and electric furnace
(1) Bessemer bottom blast brick is an important refractory material in the top-bottom blast steelmaking process and plays an important role in strengthening the smelting process. The breathable element for top - bottom bof is magnesium carbon and is installed at the bottom of the bof. There used to be porous diffusion type in the structure -- there were pores about 0.15mm in diameter in the diffusion distribution of the brick, but the density was poor, the strength was low, the gas resistance was large, and the durability was eliminated. Combination - by a number of firebrick combination of a variety of brick seam ventilation, but poor distribution of airflow. At present, the main use is isostatic pressure molding magnesium carbon breathable brick, the vent adopts sectional combination: embedded heat resistant steel tube, 1.5~4mm channel diameter. This kind of structure is the main form of bof bottom breather brick at present, channel diameter design should be reasonable, hour resistance is big, when big seepage steel. Fusible metal wire is also useful: it is embedded in the fusible metal wire, which melts when burned to form a ventilation channel with a diameter of 0.5~3mm or adopts the chip type to form a ventilation channel, which has the characteristics of small expansion coefficient, good flaking resistance and long service life. The performance indexes of the bof bottom blowing plugs are: mgo73%-75%, C17%-19%, D= 2.75%-2.88g /cm3. The working conditions are: the steel outlet temperature is 1660-1750 ℃, the blowing volume is 30-300m3 /h, the gas pressure is 0.4-1.2mpa, and the whole process is blowing. Different gas sources are selected according to the needs of the metallurgical process: Ar,N2,CO2,O2, etc.
(2) there are two types of breathable element systems for the bottom of the electric furnace. One is the direct mixing system breathable brick, which is similar to the magnesium carbon high-pressure sectional composite type for the converter. The other is the air supply element for the indirect mixing system. The plug is not in direct contact with the molten steel. Comparison of the two structure forms: direct mixing system ventilation brick, service life of 300~500 furnace, flow rate of 3~5h(m3/h, 0.1mpa), is characterized by low service life, requires a small flow of ventilation brick; The gas supply component of the indirect mixing system, with a service life of 4000~5000 units and a flow rate of 5~7 (m3/h,0.1MPa), is characterized by a high service life and requires a large flow rate of the air plug. Table 15-25 lists the performance indexes of gas plugs for converter and electric furnace.
Air permeable element for ladle
Argon blowing at the bottom of ladle has been widely used in off-furnace refining. Inert gas is injected into molten steel by means of the air plug installed at the bottom of ladle, the molten steel is stirred, and the alloy, deoxidizer and desulfurizer added to molten steel are rapidly dispersed and melted, so as to float the harmful inclusions and gases and achieve the purpose of refining. Bottom blowing air brick is the key to the process function components, due to the use condition is very harsh, liquid steel agitation scour, oxygen blowing cleaning, etc., require multiple high performance: good permeability, with the passage of the wetting Angle, breathable steel have larger permeability less steel, intermittent operation requires good thermal shock resistance and heat shock resistance to spalling, face oxygen cleaning, better oxidation resistance.
(1) structure, material, performance and production process. There are two types of air permeable elements in ladle structure due to the different types of air permeable plugs: diffused air permeable elements and air permeable elements with directional air permeable elements. Diffused plug is formed by hydraulic press and fired at high temperature. It is characterized by low gas pressure, high porosity, low density, low strength, poor durability and poor stirring effect of molten steel. It is used in Japan due to limited gas pressure, while in China it is mainly used with directional vent. In the directional vent plug, many vertical channels are preset in the straight hole, which has good bottom blowing effect and high service life. At present, it mainly adopts slit type air plug, which is prearranged or combined into slit arranged in a certain direction, with large gas flow, good stirring effect on molten steel, not easy to be blocked, high blowing rate and long service life. Slit slit type vent plug generally formed by embedded burn lost, also have the core board combination - core plate pressure molding, high temperature firing, core board assembled after outsourcing castable to form slit, this technology can buffer the thermal stress, in use is not easy to break, conducive to life and blowing rate increase, corundum, corundum, spinel, the core material and the oxide combined with just the quality of the jade. The slit parameters are related to the gas flow, flow rate and penetration depth of molten steel into the brick, and the slit width is generally 0.2mm suitable.
In terms of the installation mode, the ladle breathable elements can be divided into external type and internal type. The external breathable element consists of a block block, a cover block and a plug, which are laid together with the bottom of the bag. The cover block and the plug are pre-assembled and then installed in the block block. External breathable elements are suitable for use in harsh refining furnaces, such as LF, lf-vd, high gas flow requirements, high blow-through rate requirements. Due to the harsh conditions of use, the external breathable brick can be damaged quickly and its service life and age are out of step with each other. The advantage is that when the permeability is bad or the erosion is too fast, the cover brick and the breathable plug can be removed from the outside of the ladle and replaced from the outside of the furnace, which can facilitate the hot replacement, accelerate the ladle turnover and improve the production efficiency. The built - in (integral) type breathable element consists of a plug and a block that are first assembled together and then used in the bottom of the bag. The built-in breathable element is safe to use, and the air volume is generally 100~700L/min (0.4mpa). It is used for the ladle with less harsh conditions, which is built together with the bottom of the ladle. The service life is synchronous with the lining of the ladle or with the bottom of the ladle. Two types of breathable element installations are shown in figure 15-7.
Ladle with breathable components using strict, as a very important function in the process of refining outside the furnace components in service life, use effect (ventilation and blow into rate), the use of safety and reliability and other aspects have higher requirements, vent plug and block of material selection, production technology, product performance indicators have close relationship with breathable components use level. Breathable plug material is usually made of rigid jade, the raw materials used are white corundum and plate-shaped corundum, pure calcium aluminate cement, silicon powder, additives such as chromium oxide, zirconium, spinel, etc. Adding Cr2O3 and MgO into the matrix based on corundum can improve the corrosion resistance and permeability. The former is conducive to the formation of solid solution, and the chrome-containing glass phase can improve the viscosity after contact with the slag and prevent penetration. The latter can absorb Fe2O3 and MgO in the slag and form a dense spinel layer under the reaction layer, which plays the same role. The final material is corundum, chrome corundum or corundum spinel material, corundum also abroad - mullite. Matching block is just jade commonly.
The manufacturing process of the slit plug is a combination of amorphous refractory technology, casting molding and high temperature firing. Its production process includes: batching, mixing, pouring, curing, drying, firing, ironclad, drying, permeability test, finished products. The size, permeability and service life of the plug vary with the capacity of the refining equipment and the refining process. The height of the plug ranges from 250 to 450mm, the service conditions are: the steel outlet temperature is 1650 to 1750℃, the stirring time is 20 to 30min per furnace, the air velocity is 100 to 1500NL/min, usually 300 to 800NL/min, the pressure is up to 1.5mpa, usually 0.3 to 1.0mpa, the service life is from several to hundreds of times. Table 15-26~ table 15-28 show some performance indicators of plug and block.
(2) damage factors of breathable elements. High corrosion resistance, safe operation and reliable air permeability under different pressures are required for ladle used. Two very important indicators are repeated blowing rate and service life. The failure of blowing is caused by the blockage of the airway by oozing steel and oozing slag. The life depends on the anti-damage capability of the material in the use process and the control level of the field operation process.
The use of air permeable elements for ladle is characterized by intermittent operation, which includes receiving steel, blowing refining, pouring, dumping slag and residual steel, cleaning of air permeable plug, and no blowing when receiving steel and casting, that is, the air permeable brick is in a complicated working condition during the use. Air brick damage is the result of multiple factors, have been high temperature liquid steel erosion wear and erosion of permeability, and the thermal stress caused by blowing out cold air, turn over the package when slag oxygen after cleaning the face of high, moderate and strong oxidation recycled heat shock caused by thermal spalling effect, slag, etc.
1) scour and wear. The turbulence caused by mixing gas flow with molten steel during ladle blowing has a strong shear, scour and wear effect on the plug and aggravates the erosion effect of molten steel on the plug, which is the main reason for the damage of the plug. The extent of damage depends on argon blowing pressure, total time, and gas flow. When the ladle has a large capacity, the static pressure of the molten steel is high, and the gas pressure required for blowing is high. When the ladle is transported far away, the turnover time is long, the temperature of the molten steel drops, the viscosity of the molten steel increases, and the blowing pressure must increase, which also affects the service life of the plug.
2) thermal stress. On the one hand, the refractory material around the outlet slit is in contact with molten steel up to 1600~1700℃; On the other hand, there is the effect of continuous cold air blowing, which will produce high thermal stress in the plug, leading to cracks. Breathable brick also belongs to the refractory material which is used many times. It is prone to crack and spalling near the working face.
3) oxygen blowing. After ladle pouring and slag dumping, the plug shall be cleaned by blowing oxygen to clean the residual steel and residue on the working surface, so as to ensure that the working surface is clean and the gas passage is smooth. However, over cleaning during oxygen blowing can burn the plug, which is extremely hot (above 2000℃) and highly oxidized. The damage has a lot to do with operating conditions.
4) erosion. It is mainly caused by corrosion of molten steel and permeation of molten steel and slag to form reaction metamorphic layer on the working face of porous plug, and then the structure spalling. Severe penetration can clog the airway, reducing blowout rate and service life.
Measures to improve and ensure the blowing rate: first, the ventilation passage should be designed carefully according to the use conditions, the airway structure should be optimized, and the masonry method should be improved, such as installing two ventilation blocks; The second is careful operation and maintenance, such as after each pouring slag and residual steel should be poured clean, and immediately clean the surface of the plug with oxygen or other gas cold steel, blowing through the ventilation pipe. The whole operation: steel extraction, refining, pouring, air plug cleaning should be continuous and stable, as far as possible to shorten the time between each other. In addition, in the actual operation process, the air brick mushroom head technology, improve the blowing rate, has also been made in reducing the speed of air brick pitting and prolonging the service life of the air brick effect, namely air brick surface after burn, will vent brick gunning material spray gun spray directly to the air brick surface, using the residual heat of gas supply brick and sintering, the sintering layer formed by honeycomb mushroom head, not only ensure the air brick of permeability, and prevent the penetration of liquid steel.
(3) matching block. Block and breathable plug, the use of the same harsh conditions: high temperature operation, low alkalinity slag penetration - high temperature corrosion resistance, forced agitation - wear resistance, intermittent operation, temperature change, thermal peeling and structure peeling - peeling resistance. Its quality not only has an impact on the service life of the breathable element, but also has a direct impact on the service life of the bottom working layer. Often dig repair block or replace block, can not get high age. The requirements for the block are high resistance to erosion and erosion, good resistance to lamellar peeling and fracture properties, high quality block can ensure the overall life of the argon blowing brick or gas core hot replacement. The selection of brick materials also requires high performance, such as low cement corundum castable, low cement corundum spinel castable, high temperature fired chrome corundum, etc., with good resistance to high temperature corrosion, peeling and fracture.