Argon blowing is usually to build one or several breathable bricks at the bottom of the pouring ladle or brick ladle, and blow argon gas through the breathable bricks after tapping to cause agitation of the molten steel in the ladle. Argon blowing can promote the floating of emulsified slag droplets and inclusions in the steel, and can partially remove the dissolved gas and uniform components in the steel. In the case of continuous casting, the ladle blowing of argon can play a role in adjusting the temperature of the molten steel. Ladle argon blowing is an important steelmaking process, and breathable bricks are an important functional element of this process.
Type and structure of breathable brick
According to the gas channel form, there are three main types of air-permeable bricks, namely, the diffuse type, the straight-through directional type, and the slit-oriented type. The diffuse structure has fine pores, high porosity, low density and strength, and poor durability; the straight-through directional type is made of unequal numbers of thin steel pipes embedded in bricks, and there are also special pore-forming technologies without the orientation of thin steel pipes. The pore size is generally between 0.6mm and 1.0mm. The flow and distribution of gas in the straight-through directional ventilating brick are reasonable, the bottom blowing effect is good, and the service life is long, but it is easy to block in the later stage, which affects the blowing rate; The directional air-permeable brick is integrally formed with the inlay material during the forming process. During the high-temperature firing process, the inlay material is heated to melt and volatilize to form a slit, and the gas enters the ladle molten pool through the slit. Proper slit width can ensure low argon blowing intensity, control and adjust the flow rate, and avoid the danger of permeation and blockage. The slit width of the LF-VD refining ladle breathable brick at Maanshan Iron and Steel No. 1 Rolling Plant is selected at about 0.18mm, and the breathable brick can better meet the requirements of refining production.
s conditions that the argon blowing time is low and the life of the ventilating brick can be synchronized with the life of the ladle. External ventilation bricks are easy to replace, but due to the installation equipment, the structure is relatively complex, and the quality of the on-site installation of the brick core is relatively high, which is likely to cause man-made operation accidents. Because the externally mounted ventilating brick can quickly replace the brick core, it is beneficial to the turnover of the ladle and the improvement of the age of the ladle. It is suitable for the process conditions where the argon blowing time is long and the ventilating brick is frequently replaced, especially the refined ladle. At present, the slit directional air-permeable bricks are widely used on ladle.
Material and performance of breathable brick
The materials of permeable bricks are mainly sintered magnesia, magnesia chromium, high aluminum and corundum. Some data show that the stability order of several refractory oxides under high temperature and vacuum is as follows: Al2O3>CaO>MgO>Cr2O3, and the order of the wetting angle of molten steel is as follows: Cr2O3>Al2O3>MgO. Based on the above two considerations, corundum is a better choice as the main crystal phase of the breathable brick. At the same time, the melting point of Cr2O3 is 2275°C, which is higher than the melting point of Al2O3 (2050°C). Alumina and chromium oxide can form a continuous solid solution, and the solid solution formed by Al2O3-Cr2O3 is significantly more resistant to corrosion by iron oxide or slag. . Adding a small amount of Cr2O3 can inhibit the excessive growth of alumina crystals, thereby reducing the internal stress of the crystals and improving the physical properties of the material. However, if Cr2O3 is added too much, the growth rate of corundum grains will be seriously affected, thereby reducing the physical properties of the material. Therefore, proper introduction of Cr2O3 can improve the thermal shock stability, erosion resistance and erosion resistance of the material.
There are also many researchers who have conducted research and experiments on non-oxide bonded corundum breathable bricks. Non-oxide has the advantages of high high temperature strength, good thermal shock resistance and difficult to be wetted by metal melt and slag, such as Si3N4, β- Si AION et al. In the actual field use results, it is found that the non-oxide bonded corundum breathable brick has the characteristics of good thermal shock resistance, non-wetting with molten steel, high blow-through rate during use, easy to clean by oxygen blowing, and low corrosion rate. At the same time, the service life has also been significantly improved.
The choice of the installation position of the breathable brick
When selecting the position of the blowing element at the bottom of the ladle, it should be determined according to the purpose of ladle processing. The air-permeable bricks are installed at the center of the bottom of the ladle and deviated from the center of the ladle (the blowing point is at a radius of 1/2-1/3 from the center of the bottom of the ladle). The blowing effect on the molten steel is different. Blowing argon at the bottom of the ladle center is beneficial to the ladle slag. The reaction between gold is conducive to the desulfurization reaction of the top slag; while the eccentric bottom blowing of argon is conducive to the mixing and temperature homogenization in the ladle and the floating of inclusions.
Process requirements for the use of breathable bricks
- Good air permeability. Air permeability is one of the important parameters to measure the quality of air-permeable bricks. Studies have shown that the stirring energy of molten steel is directly proportional to the flow rate of the blowing gas; stirring can directly affect the stirring efficiency of molten steel, and only sufficient stirring energy can make molten steel have a good stirring effect. When the amount of argon blowing is constant, the more argon bubbles blown out, the more beneficial it is to the degassing and stirring of molten steel.
High temperature corrosion resistance. Refining ladle has very strict requirements in terms of temperature and time. The maximum temperature is often above 1750℃, and the refining time sometimes reaches tens of minutes. During the refining operation, the alkalinity of the slag has a great influence on the life of the breathable brick. Therefore, the air-permeable brick will be corroded by the alkaline slag which is very permeable at high temperature, and the damage rate will be fast.
High temperature wear resistance. When argon is blown at the bottom of the refining ladle, due to the argon blowing at the bottom, the flow of molten steel in the ladle is very fast, and the erosion and abrasion of molten steel on the lining material, bottom breathable bricks and seat bricks increases significantly. During the hot repair of the ladle, in order to remove the residual steel and residue on the surface of the ventilating brick and restore the venting function of the ventilating brick, it is necessary to clean the surface of the ventilating brick by blowing oxygen to melt the steel slag attached to the surface of the ventilating brick. The gas blows away the slag. During the cleaning process, the air-permeable brick is scoured by the high-speed airflow, so the air-permeable brick is required to have good high temperature wear resistance.
Good thermal shock resistance. Due to the intermittent operation of the ladle, when the ladle is poured into the molten steel, the end of the breathable brick is affected by the high temperature molten steel, and the temperature rises sharply. When the argon gas is blown, it is cooled by the cold airflow, and a great thermal stress is generated inside the material. At the same time, when molten steel is injected into the empty ladle, a large temperature change will also occur, so the use conditions of the ventilating brick are very harsh, and it is easy to cause thermal spalling and structural spalling.
The installation is simple, safe and reliable. The breathable brick is installed inside the seat brick at the bottom of the ladle, and the working conditions are extremely harsh. The life of the ladle breathable brick cannot be synchronized with the life of the entire ladle, so the breathable brick needs to be replaced. Therefore, the installation and operation are required to be simple, safe and reliable to use, and to avoid the occurrence of steel seepage and steel breakout incidents.
Damage mechanism of air-permeable bricks
There are four main reasons for the damage of air-permeable bricks: thermal stress, mechanical abrasion, mechanical stress, and chemical erosion. Figure 2 shows the external effects of the air-permeable bricks during use.
In actual production, the baking temperature of the ladle is 1000°C, and the temperature of molten steel reaches 1600°C. The temperature difference between the two contacts will cause a large thermal stress, which will cause a very large thermal shock. The ladle is in a repetitive working environment, and the thermal stress caused by the frequent temperature changes of the air-permeable brick when it comes into contact with molten steel and the baking ladle is one of the reasons for the damage of the air-permeable brick. The key factor for the thermal stress of the air-permeable brick is the change of temperature, and there are differences in the temperature change of different parts of the air-permeable brick.
When the air-permeable brick is subjected to strong thermal shock, it causes the layered peeling near the working surface to cause the air-permeable brick to be damaged.
The scouring of the ventilating brick by the plume is roughly divided into three situations:
When the breathable brick is higher than the seat brick, the main reason for the damage of the breathable brick is that the molten steel in the ladle produces a huge shearing effect and scouring effect on the side of the breathable brick. Therefore, the scouring force is one of the reasons for the great damage of the seat brick of the ventilating brick. In a normal working environment, the ventilating seat brick is used only once, and the part higher than the seat brick is washed away after one time.
When the breathable brick and the seat brick are flat, the seat brick has a protective effect on the breathable brick, and the plume formed by molten steel first washes the seat brick, so that the shear force of the breathable brick is correspondingly reduced.
When the air-permeable brick is lower than the seat brick, it is easy to accumulate cold steel on the working surface during normal work. Because the remaining cold steel has a relatively large viscosity, it has a certain obstructive effect on blowing, so it needs to be added to meet the refining requirements. Big blow pressure. In this way, the scouring force of the airflow on the ventilating brick is correspondingly increased, and the shearing and impact strength are high, and it is more vulnerable to damage.
During use, the working surface of the air-permeable brick at the bottom of the ladle is in direct contact with the high-temperature molten steel. Under the condition of recycling, the temperature in the ladle is always in a state of change. The temperature difference from high to low temperature in the use of refractory at the bottom of the ladle will result in a temperature difference. There is a difference between the expansion coefficient of the original layer and the metamorphic layer, which causes the ventilating brick to be subjected to shear stress, causing the ventilating brick to appear transverse cracks or even breaks.
In the normal production process, the working layer of the air-permeable brick and the steel slag and molten steel are in contact with each other for a long time, and the molten slag continues to corrode and penetrate into the air-permeable brick. The MnO, MgO, SiO2, FeO, Fe2O3 and other oxides in the molten steel and slag react with the refractory materials in the breathable bricks. The molten steel melts and peels off the working layer of the breathable bricks. The chemical corrosion caused by the continuous penetration of molten steel on the breathable brick is the main reason for the damage of the breathable brick.
Common damaged state of breathable brick
1) Breathable brick core peeling layer
After the ladle is hot repaired, the temperature inside the ladle is about 900°C, and the end temperature of the converter is basically 1630°C. The chilling heat caused by the temperature difference makes the core of the air-permeable brick peel off, and the thickness of the peeled layer is generally 10-20mm. . Moreover, after the brick core is peeled off, the tapered peeled part will not fall off, which affects the air permeability of the molten steel in this ladle.
2) Breathable brick base brick breaks
The chill and heat caused by the temperature difference can also cause the ventilating brick to break, and the height of the fault is 100-200mm. The brick core will also break due to corrosion by molten steel because it is not protected by a brick. This kind of fault is extremely harmful, especially in the later period of the use of the air-permeable brick, because the residual brick is relatively short, it is easy to cause the leakage of the bottom of the package.
3）Low strength and poor corrosion resistance
When using an oxygen lance to clean the air-permeable bricks for hot repair, the normal rate is 9-12mm/furnace. If the strength is not enough, the height of the brick core will decrease obviously every time it is cleaned. Turnover, resulting in instability of production.
4）Erosion and damage of molten steel
When inert gas is sprayed through the ventilating brick, the air blowback will have a certain impact on the exposed ventilating brick. The high-speed flowing molten steel interacts with the airflow to form a vortex. When the ventilating brick is higher than the ventilating seat brick, its protruding parts will be subject to the turbulence. The scouring and shearing, forming an annular groove.
5) Overburning oxygen damage
After continuous casting is finished, the ladle is not poured in time, and the bottom of the ladle is cold steel. It is necessary to use an oxygen lance against the core part of the ventilated brick to perform oxygen burning. The burning of the ventilating brick core is faster.
6）Permeable brick core burning side
When cleaning the air-permeable bricks, use an oxygen lance against the brick core. At this time, the burning oxygen is concentrated at one point, which is easy to burn the brick core. The surface of the brick core is not flat but inclined, and the height difference is more than 50mm. The ventilation bricks are not well ventilated when they are burnt, and the degree of deviation will be aggravated every time they are cleaned, resulting in premature off-line.
7）Low backflush gas pressure
The backflushing gas is nitrogen, the pressure is 0.5MPa, and the oxygen pressure of the oxygen burning gun is 0.8MPa. The molten steel during oxygen burning will blow into the slits of the ventilating bricks, and the slits will be blocked after cooling, resulting in poor ventilation. Hot repair and cleaning will increase the burning loss of the ventilated brick core.
8）The corundum material around the air-permeable brick and the bottom of the bag are removed
When laying the bottom of the package, the corundum self-flowing material with a width of 50-100mm is poured around the ventilating brick base brick to protect the ventilating base brick. Due to insufficient strength of the corundum self-flowing material or construction problems, the corundum material around the air-permeable block bricks falls off, and the infiltration of molten steel causes the air-permeable block brick to break; or the bottom bricks around the air-permeable block are broken, and the infiltration of molten steel causes the surrounding corundum material and The breathable seat brick is broken.
Based on the above analysis of the causes of damage, we concluded that the damage of the breathable brick is not only related to the material, shape and structure of the refractory material itself, but also closely related to the use environment, operation and refining process. Regarding refractory materials, scholars at home and abroad are constantly conducting research and experiments, including choosing different additives, aggregates, micropowders, etc., so that refractory materials can form a solid solution phase in high temperature environments, increasing and improving the resistance of materials. Thermal shock, slag resistance, etc. At an offline academic conference last year, some units also conducted research and trial of corundum spinel non-fired ventilating bricks. Tomorrow, they will share the test of this non-fired ventilating brick.