The ladle sliding nozzle is an important system to control the flow of molten steel in the ladle. If the system has a steel-piercing accident during use, it will lead to vicious production accidents such as the breakage of the casting machine and the burning of the continuous casting machine equipment, which restricts the stable and smooth production and seriously threatens the safety of people and equipment. With the further acceleration of the steelmaking rhythm, higher requirements are put forward for speeding up the ladle turnover and reducing production accidents, and ensuring the safe operation of the sliding nozzle is a prerequisite. Generally, the number of times of steel leakage at the sliding nozzle of the small ladle is relatively high, so reducing or eliminating steel leakage accidents at the sliding nozzle is of great significance for steelworks whose production rhythm is getting faster and faster.
Ladle sliding nozzle – Composition and service conditions of ladle sliding nozzle
The ladle sliding nozzle is generally composed of a driving device, a mechanical part and a refractory part. The working principle of the sliding nozzle is to stagger the upper and lower sliding bricks through the sliding mechanism, thereby driving the opening and closing of the flow steel hole to adjust the flow of molten steel. Generally, the steel leakage of the sliding nozzle mainly occurs at the joints of the refractory bricks of the sliding nozzle, and also in the middle of a single brick.
Sliding nozzle of ladle – analysis of steel leakage of sliding nozzle of ladle
Influence of Mechanical Part of Sliding Nozzle on Breakout
1) The upper and lower sliding boards are not parallel or deformed during use, resulting in uneven surface pressure between the two sliding boards. The force on one side is larger, and the force on one side is smaller. When the pressure of molten steel exceeds the surface pressure between the two slides, the molten steel will penetrate between the two slides, causing the slides to clamp steel or leak steel.
2) The skateboard has tiny cracks, which were not checked before use. When pouring is started, the sliding plate is affected by the pulling force of the driving device and the thermal stress, and suddenly breaks, and the pressure on one side of the sliding brick suddenly disappears. Under the action of the static pressure of the molten steel in the ladle, a gap is formed between the sliding bricks, and the molten steel will immediately pass through the middle of the two sliding bricks.
3) The processing size deviation of the skateboard is large. The groove depth of the fixed skateboard brick in the skateboard is larger than the size of the skateboard brick. The skateboard and the skateboard brick cannot be effectively matched, resulting in a gap between the two skateboard bricks, and molten steel penetrates between the two skateboard bricks. It will cause steel clamping or steel leakage on the sliding plate. In addition, it will also affect the surface pressure between the sliding plate brick and the drain brick, causing steel seepage or steel leakage at this part.
Influence of Sliding Nozzle Refractory Material on Breakout Steel
The refractory part of the sliding nozzle is the core part of the sliding nozzle. It is a key component that directly contacts and controls the molten steel. Its physical and chemical properties are the key factors that determine whether the sliding nozzle can be used normally.
1) The high-temperature strength of the nozzle seat brick is low. When using the air pick to heat-replace the ladle upper nozzle brick, the air pick often breaks the seat brick, causing the seat brick to expand in diameter. The gap between the seat brick and the upper nozzle brick is large. When the molten steel is poured , The molten steel forms an eddy current at the seat brick, and is affected by the scouring effect of the molten steel, and the molten steel is easy to penetrate into the gap, and a steel leakage accident around the upper nozzle brick occurs.
2) The thermal stability of the nozzle brick on the ladle is poor. During the use of the upper nozzle brick, with the turnover of the ladle, the temperature difference between pouring steel and hot repair of the empty ladle is relatively large. Affected by rapid cooling and rapid heat, cracking is prone to occur during use. If it is not found in time, molten steel will seep out from the mud joint between the upper nozzle brick and the seat brick through the blast joint of the upper nozzle brick.
3) The sliding bricks are not resistant to the erosion and erosion of molten steel. When the sliding nozzle is poured in half flow, the sliding bricks are strongly eroded by the molten steel due to the interception of the molten steel, and grooves will be formed on the sliding bricks for a long time. When closing the ladle, the molten steel condenses in the groove, causing steel to be sandwiched between the two slides, the sliding nozzle cannot be opened and closed, and the injection is out of control.
4) The drain bricks burst when used, and when the drain bricks are subjected to steel, transverse cracks will appear under the influence of thermal stress. Under the action of molten steel pressure, the cracks gradually expand into cracks, and molten steel seeps out of them. In severe cases, the refractory material at the lower part of the crack is broken by its own gravity and the pressure of molten steel.
5) The refractory mud is not resistant to the erosion and erosion of molten steel, causing the joints to seep or form steel during use. If it is not found in time, it will cause steel leakage between the upper nozzle brick and the seat brick and between the upper nozzle brick and the upper slide brick.
Influence of Operation on Sliding Nozzle Breakout
1) The consistency of the refractory mud is not properly controlled during the water distribution construction. If the refractory mud is too thin, it will be squeezed out, which will not support the sliding bricks. At the same time, it is also not resistant to the erosion of molten steel, and it is easy to form steel at this part. If it is too thick, the refractory mud cannot be spread evenly, and there is a gap between the upper nozzle brick and the upper sliding plate, and molten steel will drill out of the gap.
2) The pins are loose. When the pins for locking the slides are not tightened, the surface pressure between the two slides is small, which is lower than the pressure of the molten steel in the nozzle, and the molten steel passes through between the two slides.
3) The base of the sliding nozzle is loose. When the sliding nozzle is opened and closed, the pin moves up and down with the base, and as the number of opening and closing increases, the pin withdraws in the opening direction, resulting in loosening of the pin.
4) If there is less mud plastering between the upper nozzle brick and the upper slide, the upper part of the upper slide will lose its support, and it will not be able to maintain a close fit with the lower slide, and molten steel will pass through between the two slides.
5) Ladle pouring operators make mistakes. When the molten steel cannot be poured automatically and needs to be poured with oxygen, the operator did not fully open the sliding nozzle, and used the oxygen tube to burn out or burn through the sliding surface of the upper slide, causing steel to be clamped or pierced between the slides.
6) There is steel slag on the sliding surface of the skateboard, which was not cleaned up during the packing operation, resulting in a gap between the two skateboard mechanisms.
7) During the process of hanging the bag to the ladle car seat, the sliding mechanism hits the ladle car or other objects, resulting in damage to the sliding mechanism.
8) When pouring steel into the tundish, due to excessive pouring or the slagging cover of the tundish, the molten steel turns up and splashes on the sliding mechanism, resulting in the sliding nozzle mechanism not being able to open and close, and the pouring out of control.
Influence of Molten Steel Smelting on Steel Breakout of Sliding Nozzle
1) When pouring calcium-treated molten steel, Ca and CaO in molten steel will react with AL2O3 and SiO2 in aluminum-carbon or aluminum-zirconium carbon sliding bricks to form substances such as 12CaO.7SiO2 and 2CaO.SiO2.AL2O3 with low melting points. These low-melting materials are washed away by molten steel, so that the content of AL2O3 and SiO2 on the surface of the sliding bricks is rapidly reduced, so that the working surface of the sliding bricks is excessively corroded, resulting in steel clamping or steel leakage between the sliding bricks. Its reaction formula [1] is as follows:
2[Ca]＋SiO2─→2CaO＋[Si] (1) 3[Ca]＋AL2O3─→3CaO＋2[AL] (2)
nCaO＋mSiO2─→nCaO·mSiO2(n、m≥1) (3) 2CaO＋SiO2+AL2O3─→2CaO·SiO2·AL2O3 (4)
12CaO＋7SiO2─→12CaO·7SiO2 (5)
2) When the oxygen content in the molten steel is high, the carbon and graphite in the slide will be oxidized to form a decarburized layer, resulting in an increase in porosity and a decrease in strength on the working surface of the slide. It intensifies the mechanical scour and chemical corrosion of the molten steel on the skateboard bricks. Its reaction formula [2] is as follows:
2C(s)＋O2(g)=2CO(g)↑ (6) C(s)＋O2(g)=CO2(g)↑ (7) FeO(s)＋C(s)=Fe＋CO(g)↑ (8)
Fe2O3＋3C(s)=2Fe＋3CO(g)↑ (9）
3) The temperature of the molten steel in the ladle after tapping is too high, and some heats are above 1660°C. In the absence of refining outside the furnace to adjust the temperature, the thermomechanical damage and thermochemical erosion of the refractory material of the sliding nozzle are intensified, resulting in the bursting of the sliding nozzle Penetration and corrosion leakage.
Ladle sliding nozzle – precautionary measures
Strengthen the acceptance and point inspection of the mechanical part of the sliding gate
1) For the key component of the sliding nozzle, the slide plate should be carefully measured before use to ensure that the dimensional tolerance of each part meets the specified requirements.
2) The sliding mechanism must be carefully inspected every time the packaging operation is performed, the deformed and cracked parts should be replaced, and the residual steel residue in the sliding box should be cleaned.
Selection of high-quality refractory materials
1) The refractory material used for the sliding nozzle should have high high temperature strength, especially the ladle seat brick, which can withstand the mechanical damage and the erosion of molten steel when replacing the nozzle brick, and should be synchronized with the service life of the lining;
2) The thermal shock stability is good, and it will not burst when used;
3) High softening point under load, good oxidation resistance, no pore diameter deformation, no dents, pockmarks, and roughness during use
Ladle sliding nozzle – optimized operation
1) Do a good job of process control and carefully inspect the refractory materials to ensure the appearance and internal quality of the refractory materials.
2) Clean up the residual steel and mud in the inner cavity of the upper nozzle brick and seat brick, fill it with drainage sand, increase the automatic pouring rate of molten steel, and avoid burning oxygen to start pouring.
3) After the molten steel in the ladle is poured, close the sliding nozzle in time to prevent the slag in the ladle from flowing into the nozzle.
4) Adjust the hardness and dosage of refractory mud in time according to the expansion of the seat brick and the upper nozzle brick and the depth of the upper nozzle brick recessed into the base reference plate.
5) Speed up the turnover of the ladle and reduce the thermal shock damage to the refractory material of the ladle.
6) Do a good job in the control of the smelting process, ensure the temperature of the molten steel at the end point, and strictly control the oxygen and calcium content at the end point of the molten steel.
Implement process transformation
1) Add anti-return pins to the locking pins, and apply a certain amount of refractory clay to prevent the pins from loosening during use.
2) A protective plate is added under the sliding mechanism to reduce the direct radiation and splash of molten steel and the adhesion of molten steel and steel slag, which improves the life and safety of the mechanism.
3) Implement the online hot-state overall replacement technology of the upper nozzle of the ladle. Without the use of pneumatic picks, the upper nozzle can be quickly removed online, which reduces the mechanical damage to the seat brick when changing the water outlet, and prevents the premature subcontracting caused by the diameter expansion of the seat brick. Moreover, the heat loss of the lining is greatly reduced.
Ladle sliding nozzle – implementation effect
After the implementation of preventive measures, the rate of steel leakage of the sliding nozzle has been greatly reduced, and the annual accident loss can be reduced by RMB 120,000. The reduction of ladle turnover time creates conditions for reducing the tapping temperature, improves the comprehensive service life of the ladle lining, and reduces the production cost by more than ￥820,000 per year.
By studying the causes of steel leakage at the sliding nozzle and formulating targeted measures, it is possible to effectively reduce the accidents of sliding steel through steel, reduce the labor intensity of workers, speed up the turnover of ladles, and promote the stability of production.
