Damage mechanism of permeable bricks in steel ladles
The main causes of damage to permeable bricks are fourfold: thermal stress, mechanical wear, and chemical corrosion.
In actual production, the ladle drying temperature is 1000℃, and the molten steel temperature reaches 1600℃. The temperature difference between the two when they come into contact generates significant thermal stress, leading to substantial thermal shock. The ladle operates in a cyclical environment, and the frequent temperature changes of the permeable bricks during contact with molten steel and ladle drying are a major cause of their damage. The key factor in generating thermal stress in permeable bricks is temperature variation, and the temperature changes differ across different parts of the permeable brick.
When permeable bricks are subjected to strong thermal shock, layered spalling near the working surface causes damage.
The scouring effect of the swirling flow on the permeable bricks can be roughly divided into three situations:
(1) When the permeable brick is higher than the base brick, the main cause of damage is the swirling flow formed by the molten steel in the ladle, which generates a huge shearing and scouring effect on the sides of the permeable brick. Therefore, scouring force is one of the reasons for the great damage to the seat brick of the permeable brick. In normal working environment, the permeable seat brick is used only once, and the part above the seat brick is scoured away after one use.
(2) When the permeable brick and the seat brick are level, the seat brick plays a protective role for the permeable brick. The swirling flow formed by molten steel first scours the seat brick, causing the shear force of the permeable brick to decrease accordingly.
(3) When the permeable brick is lower than the seat brick, cold steel is easily accumulated on the working surface during normal operation. Since the viscosity of the remaining cold steel is relatively high, it has a certain obstruction effect on the blowing. Therefore, in order to meet the refining requirements, the blowing pressure needs to be increased. In this way, the scouring force of the airflow on the permeable brick is increased accordingly, and it is subjected to greater shear and impact strength, making it more prone to damage.
During use, the working surface of the permeable bricks at the bottom of the ladle is in direct contact with the high-temperature molten steel. Under cyclical use, the temperature inside the ladle is constantly changing. The temperature difference between the high and low temperatures of the refractory material at the bottom of the ladle, and the difference in the expansion coefficients between the primary and modified layers of the refractory material, subject the permeable bricks to shear stress, leading to transverse cracks and even breakage.
During normal production, the working layer of the permeable bricks is in prolonged contact with steel slag and molten steel, allowing molten slag to continuously erode and penetrate into the permeable bricks. The oxides such as MnO, MgO, SiO2, FeO, and Fe2O3 in the molten steel and slag react with the refractory material in the permeable bricks. Physical corrosion caused by melting and spalling of the working layer by the molten steel, and chemical corrosion caused by the continuous penetration of steel slag and molten steel during the smelting process, are the primary causes of damage to the permeable bricks.
Common damage conditions of breathable bricks
1.Peeling the core of breathable bricks
When the ladle is placed into the converter after hot repair, the temperature inside the ladle is approximately 900℃. The final temperature of the converter is approximately 1630℃. The rapid cooling and heating caused by the temperature difference leads to the peeling of the core of the permeable bricks. The thickness of the peeled layer is generally 10-20mm. Moreover, because the tapered peeled part does not fall off after the core peeling, it affects the permeability of the molten steel in the ladle.
2.Breathable brick base brick fracture
The extreme temperature fluctuations caused by temperature differences can also lead to the fracture of the base bricks in permeable bricks, with fracture heights ranging from 100 to 200 mm. The brick core, lacking the protection of the base bricks, will also fracture due to erosion from the molten steel. Such fractures are extremely dangerous, especially in the later stages of the permeable brick’s service life, when the remaining bricks are shorter, making it prone to accidents such as steel leakage at the bottom.
3.Low strength and poor corrosion resistance
When cleaning permeable bricks with an oxygen lance during hot repair, the normal cleaning rate is 9-12 mm/furnace. If the cleaning strength is insufficient, the brick core height will decrease significantly with each cleaning, and the permeable bricks will be removed from the production line before their specified normal service life, affecting the normal turnover of the ladle and causing production instability.
4.Steel molten steel erosion damage
When inert gas is sprayed through the permeable brick, the airflow back impacts the exposed permeable brick. The high-speed flowing molten steel and airflow interact to form a vortex. When the permeable brick is higher than the permeable seat brick, its protruding part is scoured and sheared by the vortex, forming annular grooves.
5.Excessive oxygen burning damage
If the ladle is not refilled in time after continuous casting, cold steel will form at the bottom of the ladle. It is necessary to use an oxygen lance to burn oxygen on the core of the permeable brick to clean it. However, burning oxygen on the core of the permeable brick will cause it to burn off quickly.
6.The core of the breathable brick was fired unevenly
When cleaning breathable bricks, using an oxygen gun to press against the brick core concentrates the oxygen at one point, easily causing uneven burning of the brick core. This results in the brick core surface being sloping instead of flat, with a height difference exceeding 50mm. When burned unevenly, the breathable brick has poor air permeability, and each cleaning will exacerbate the unevenness, causing it to be removed from production prematurely.
7.Low backflush gas pressure
The backflushing gas is nitrogen at a pressure of 0.5 MPa, while the oxygen pressure of the oxygen lance is 0.8 MPa. During oxygen cleaning, molten steel is blown into the narrow gaps of the permeable bricks, and after cooling, it blocks the gaps, causing poor ventilation. Subsequent hot repair cleaning will exacerbate the burn damage to the permeable brick core.
8.The corundum material around the breathable brick and the brick detachment from the bottom
During the construction of the base brick, a 50-100mm wide layer of corundum self-flowing material is poured around the permeable brick base brick to protect it. Due to insufficient strength of the corundum self-flowing material or construction problems, the corundum material around the permeable brick base brick may fall off, allowing molten steel to seep in and causing the permeable brick base brick to break; or the base brick around the permeable brick base brick may break, allowing molten steel to seep in and causing the surrounding corundum material and the permeable brick base brick to break.
