Breathable bricks are important functional components in steelmaking production and play a very key role in production. By analyzing the reasons for the damage of the breathable bricks, measures can be taken to improve the performance during the production of the breathable bricks, and the operations should be standardized during on-site production to achieve the purpose of increasing the length of the remaining bricks to ensure breathability.
Keywords: breathable bricks; damage; oxygen burning; measures
The refining ladle of No. 1 Steel Rolling Plant uses integral breathable bricks. During use, there are phenomena such as peeling, steel clamping in slits, and failure to blow open. The length of the core of the remaining bricks is basically about 180 mm, and the monthly air permeability is 99.2%. . By analyzing the reasons for the damage of the breathable bricks, corresponding measures can be formulated to increase the core length of the breathable bricks and improve the air permeability.
Analysis of causes of damage to breathable bricks
Peeling the core of breathable bricks
When the ladle is placed in the converter after hot repair, the temperature inside the ladle is about 900°C, and the end temperature of the converter is basically 1630°C. The shock and heat caused by the temperature difference cause the core of the breathable brick to peel off. Generally, the thickness of the peeling layer is 10 to 20mm. . Moreover, after the brick core is peeled off, the tapered part of the peeled layer will not fall off, affecting the air permeability of the molten steel in this package.
The breathable brick seat brick is broken
The cooling and heating caused by the temperature difference can also cause the breathable brick seat bricks to break, and the height of the fault is 100~200mm. The brick core will break due to the erosion of molten steel because it is not protected by the base brick. This kind of fault is extremely harmful, especially in the later period of the use of breathable bricks, because the remaining bricks are short, accidents of steel leakage at the bottom of the package are prone to occur.
Low strength and poor corrosion resistance
When using an oxygen gun to clean the breathable bricks in hot repair, the normal rate is 9 to 12 mm/furnace. If the strength is not enough, the height of the brick core will drop significantly during each cleaning, and the breathable bricks will be offline before the specified normal service life, affecting the quality of the ladle. Normal turnover causes instability in production.
Damage caused by liquid steel erosion and erosion
When inert gas is sprayed through the breathable bricks, the airflow counterattack will produce a certain impact on the exposed breathable bricks. The high-speed flowing molten steel and the air flow interact to form a plume. When the breathable brick is higher than the breathable seat brick, its protruding parts will be affected by the plume. scouring and shearing, forming annular grooves.
Excessive oxygen damage
At the end of the continuous casting, the ladle was not poured back in time, and the cold steel was condensed at the bottom of the ladle. It was necessary to use an oxygen gun to clean the core of the breathable bricks by burning oxygen. Burning oxygen will cause the core of the breathable bricks to be burned faster.
The core of breathable bricks is burned
When cleaning breathable bricks, use an oxygen gun to hold the brick core. At this time, the burning oxygen is concentrated at one point, which will easily cause the brick core to burn sideways, causing the surface of the brick core to be not flat but inclined, with a height difference of more than 50 mm. The breathable bricks do not have good ventilation when burned out, and every time they are cleaned, the degree of deflection will be aggravated, causing early offline production.
Backflush gas pressure is low
The backflush gas is nitrogen with a pressure of 0.5 MPa, and the oxygen pressure of the oxygen gun is 0.8 MPa. During oxygen cleaning, the molten steel will be blown into the slits of the breathable bricks. After cooling, the slits will be blocked, causing poor ventilation. Hot repair and cleaning will aggravate the burning damage of the breathable brick core.
1.8 Corundum material and bottom bricks around the breathable bricks fall off
When laying the bottom, pour corundum self-flowing material with a width of 50 to 100 mm around the breathable brick base bricks to protect the breathable base bricks. Due to the insufficient strength of the corundum self-flowing material or construction problems, the corundum material around the breathable brick seat falls off, and the penetration of molten steel causes the breathable seat brick to break; or the bottom brick around the breathable seat brick breaks, and the penetration of molten steel causes the surrounding corundum material and The breathable seat brick is broken.
Measures to improve the quality of breathable bricks
Adding zirconium-based materials and tabular corundum to improve the spalling resistance of breathable bricks
Adding pre-synthesized zirconium-based materials to the breathable bricks can modify the aluminum-chromium breathable bricks. The thermal shock stability of breathable bricks can be significantly improved by utilizing the toughening effect of ZrO2. The high strength of plate corundum gives the breathable brick excellent thermal volume stability and good thermal shock resistance.
Improve the corrosion resistance and strength of breathable bricks
It is necessary to reduce the impurity content in raw materials as much as possible and improve the high temperature resistance of breathable bricks. Adding α-Al2O3 micro powder and controlling the amount of spinel at 16% to 20% with 1 to 0 mm particles can improve the strength of the breathable bricks and help enhance the resistance to oxygen cleaning.
Standardize on-site operations
Return the remaining water in time
If the remaining steel is not poured back in time after continuous casting, the cold steel will condense on the bottom of the package, and the steel slag will encrust, making it difficult to clean the breathable bricks and affecting the air permeability. Long-term oxygen burning cleaning will also cause Accelerated damage of breathable bricks. Therefore, it is necessary to stipulate the time from pouring steel in continuous casting to returning the pouring residue. According to on-site statistics, the pouring residue will not solidify if it does not exceed 10 minutes, but it is also related to the refining exit temperature, steel type, and ladle condition.
It is strictly prohibited to operate the oxygen gun against the breathable bricks
When using an oxygen gun to clean the breathable bricks, do not use an oxygen tube to burn the brick core to burn oxygen. This will accelerate the damage of the breathable bricks and make them easy to burn. Therefore, there should be a certain distance between the front end of the oxygen gun and the brick core, usually about 50 mm.
Install the flow mete
Another factor in cleaning breathable bricks due to excessive oxygen burning is that the operator judges based on experience. The black core of the breathable bricks accompanied by gas noise proves that the breathable bricks are well ventilated. In order to prevent excessive burning and solve the problem of operating based on experience, a flow meter is installed at the hot repair position of the ladle, and a certain alarm value is set. The alarm value is the value at which the air permeability of the breathable brick can meet the requirements of refining argon blowing, using sound and light. Call the police. In this way, the operator stops the operation after the breathable brick reaches the flow alarm, which not only reduces the artificial burning damage to the breathable brick, but also avoids the deviation caused by artificial judgment of the breathability of the breathable brick. Figure 1 shows the flow curve of the flow meter.
Figure 1 Breathable flow curve
Increase backflush gas pressure
The backflush gas was changed from low-pressure nitrogen to high-pressure nitrogen, and the pressure was increased from 0.5 MPa to 1.2 MPa to avoid the phenomenon of molten steel entering the slit. At the same time, the backflush time of the backflush gas was adjusted. The backflush started as soon as the ladle was turned over and the backflush started when the nozzle was cleaned and the breathable bricks were about to be cleaned. This shortened the backflush time and avoided the temperature drop of the brick core during the long backflush time. The hazards of easy peeling.
Slag control
Before adding drainage sand during hot repair of the ladle, operate the ladle turning machine to lower the ladle mouth to about 45°, and perform ladle slag control operations to avoid backflow of thin slag and blocking the breathable bricks after the ladle is erected, causing poor air permeability and affecting production.
Bottom-covering masonry
First, use a pneumatic pick to remove the loose bottom bricks. If there is steel sandwiched between the layers of the bottom or there are cracks in the bottom bricks, the refractory materials must be removed until there are no hidden dangers. Secondly, the permanent layer around the seat bricks falls off due to mechanical vibration or damage during the demolition operation. All suspended bottom bricks must be removed until the fully sintered permanent layer is seen. Finally, measure the thickness of the remaining bricks. The remaining thickness of the bottom bricks around the breathable bricks and nozzle seat bricks should be no less than 200 mm. All bottom bricks less than 200 mm should be removed with a pneumatic pick.
Corundum material construction
Use a mixer to mix materials. The weight ratio of corundum material to water is 1: 0.05~0.08 (1:0.05~0.07 in winter). Use a special measuring bucket to hold water. The wet mixing time is 5~10 minutes. Use clean water and not sewage. After being reconciled, the water and materials will not separate into layers and will have fluidity. Pour the reconciled corundum self-flowing material into the gap between the breathable seat brick and the bottom brick, and use a vibrator to vibrate around the breathable seat brick for 2 weeks for 1.5 to 2 minutes. Through the improvement of the quality of the breathable bricks and further standardization of on-site operations, the erosion rate of the breathable bricks has been significantly reduced, and the lifespan and length of the remaining bricks have been improved. See Table 1 for details.
Table 1 Comparison of erosion rate of breathable bricks and length of remaining bricks before and after
| Project | Average service life/furnace | Remaining brick length/mm | Erosion amount per furnace/mm |
| Before improvement | 19.8 | 195 | 12.3 |
| After improvement | 22.1 | 213 | 10.4 |
Conclusion
Through the implementation of various measures, the air permeability of the ladle has been increased to 99.9%, ensuring the quality of molten steel and the stability of production. At the same time, the brick core length of the remaining bricks reaches 196 mm, laying the foundation for the next step to increase the service life of the breathable bricks and increase the package age.