This article analyzes the reasons for the steelmaking ladle nozzle and proposes improvement measures.
Key words: ladle fermented steel; cause analysis; improvement measures
The steel ladle in the steelmaking plant is a metal container containing high-temperature molten steel. After receiving the molten steel from the converter during the production process, it is hoisted by the crane to the continuous casting area for steel pouring. There is a slide mechanism at the bottom of the ladle, which is composed of steel structures and refractory materials. On the top of the continuous casting tundish, manually open the sliding plate mechanism, and the molten steel in the ladle automatically flows into the tundish to complete the transfer of molten steel between containers. At the bottom of the ladle, the liquid steel seal relies on the tight fit between the slide plates. The slide plate mechanism is driven by hydraulic pressure. The periphery of the slide plate is composed of supporting refractory materials such as the upper nozzle and the lower nozzle. The slide plate mechanism at the bottom of the ladle is shown in Figure 1.
Figure 1 Sliding plate mechanism at the bottom of the ladle
A steel mill uses smart ladles (the slag detection induction coil is used in the mechanism, and ordinary ladles do not have this equipment). During the use of the ladle, there have been many accidents of steel ladle penetration between the upper nozzle and the block, and between the upper nozzle and the upper sliding plate. Through continuous analysis and improvement by technicians and on-site construction and maintenance personnel, the ladle is finally running stably and safely.
Analysis of the location and cause of ladle fertilization steel
Zigang position
The position of the Zigang is between the upper nozzle of the ladle and the upper sliding plate, as shown in Figure 2 and Figure 3.
Figure 2 Ladle ladle piercing position
Figure 3 Schematic diagram of the location of Zigang breakout
Analysis on the Causes of Intelligent Ladle Zinc Steel
Aiming at multiple leaking ladle accidents of smart steel ladles, professional technicians combined with the design department, user unit, and maintenance department to conduct multiple follow-ups and discussions, and found the following design defects.
(1) The nozzle wrapping strength exposed outside the nozzle block is low.
The length of the upper nozzle of the intelligent ladle wrapped inside the seat brick is 140 mm, and the length outside the seat brick is 115 mm. The length of the nozzle and the seat brick is only 140 mm when they are bonded together by refractory cement. The sealing and bonding between the nozzle and the steel structure rely on the sealing cement refractory material. The two are not of the same material and cannot be fused together. At high temperatures, the expansion coefficients of the two are quite different, which will form cavities. The overall strength of the nozzle and the seat brick is poor. The on-site installation diagram is shown in Figure 4.
Figure 4 nozzle installation
(2) The slag detection coil is added in the mounting plate, resulting in non-tight fit.
The sliding plate mechanism is fixed by the mounting plate. The mounting plate and the bottom of the bag are welded and fixed by 4 bolts. The total thickness of the mounting plate is 70 mm, and the slag detection coil is added, with a thickness of about 35 mm. The comparison of the installation boards of the smart package and the conventional package is shown in Figure 5.
Figure 5 Comparison of Ladle Mounting Plates
During the installation process of the nozzle on the ordinary bag, the nozzle and the nozzle block are closely matched. Figure 6 shows the installation of the nozzle on the ordinary bag.
Figure 6. Conventional ordinary water outlet and seat brick installation
During the installation process of the intelligent ladle nozzle, the nozzle and the nozzle seat brick are not closely matched due to the detection coil in the cavity of the installation plate, as shown in Figure 7.
Figure 7 Smart bag nozzle and seat brick installation
(3) The design material of the sliding bar of the mechanism is 45# steel, which has low strength at high temperature.
During the operation of the slide plate mechanism, it mainly relies on the slide bar to compress the spring. The material of the slide bar is designed to be 45# steel, which has weak strength and toughness under high temperature conditions. During the operation process, there are different degrees of wear and tear, and the surface is uneven, which leads to steel and zipper between the upper slide plate and the nozzle, and finally burns out the mechanism, resulting in a steel leakage accident.
(4)The skateboard of the smart bag is originally designed as a drop shape, and it is fixed by four-corner blocks, also known as top tensioners (the contact edge with the skateboard is linear). The line contact between the blocks and the skateboard is insufficient for wrapping. The on-site skateboard and its fixing method are shown in Figure 8.
Figure 8 Smart bag skateboard fixation
(5) The design size of the nozzle top platform inside the nozzle block is small. In the middle and later stages of use, the inner surface of the inner cavity of the seat brick and the top platform of the nozzle (made of aluminum-magnesium-carbon material) were oxidized, corroded, and peeled off, and they were forced to go off the production line in advance.
The upper nozzle of the ladle is installed inside the nozzle seat brick, and the seat brick is integrated with the bottom of the ladle. During each ladle off-line inspection and sand addition process, it is necessary to clean the impurities inside the nozzle with an oxygen tube, also known as the cleaning nozzle. The nozzle seat brick is made of aluminum-magnesium carbon, and there is a top platform inside, which is mainly used to fix the front end of the upper nozzle. In the process of cleaning the nozzle, oxygen will gradually erode the surface of the inner cavity of the seat brick and the top platform at the front end, causing the surface of the inner cavity of the seat brick to oxidize, peel and fall off, and the front top platform is completely eroded. After the new nozzle is installed, the nozzle loses its supporting force and will be displaced inward during the manual installation process. At the same time, due to the oxidation of the inner cavity of the block, the mud joint between the nozzle and the block increases, and the wrapping strength weakens, as shown in Figure 9.
Figure 9 Erosion of block bricks during use
In addition, there are still some problems in actual field operation.
① In the process of replacing the upper nozzle and the upper sliding plate, it is necessary to manually use external force to install the upper nozzle. During installation, due to the short sintering time of the sealing cement between the upper nozzle and the nozzle seat brick, the overall bond strength is weak, and the upper nozzle will move toward the interior of the seat brick (after the inner roof of the nozzle seat brick is eroded), resulting in the increase of the gap between the upper nozzle and the snap button of the upper skateboard; at the same time, the strong pressure on the surface of the skateboard and the pulling force of the oil cylinder lead to the loosening of the refractory mortar between the upper slider and the upper nozzle, resulting in steel rust. From Zigang’s position, they are all in the same direction of skateboarding.
② The operating pressure between the slides is 7 MPa. In order to reduce the clamping steel and the steel between the plates, the pressure on the slide surface is adjusted to 11 MPa by adjusting the spring and the pressure gear. When the production is started, pull the lower slide (the upper slide and the upper nozzle are fixed together), strong extrusion force is transmitted to the upper slide, the upper slide and the upper nozzle are stressed, and the nozzle may move toward the inside of the block (the inner roof of the block is eroded, and the surface of the inner cavity of the block is oxidized and peeled off). At the same time, under the strong pulling force of the oil cylinder, the upper slide plate produces a lateral displacement, and the refractory mortar between the upper slide plate and the upper nozzle becomes loose, and steel cracks appear.
Follow-up improvement measures
Research and analysis were carried out on the defects exposed during the use of the above intelligent ladle, and some rectifications were carried out in combination with the site.
(1) Transformation of the slag detection coil embedded in the smart bag mounting plate
The slag detection coil is embedded inside the mounting plate of the mechanism, and forms a cavity due to the size deviation of the mounting plate hole. In order to eliminate this hidden danger, a protective cover is made and embedded outside the coil, which not only fills the cavity position, eliminates the staggered position, but also increases the protection of the coil. The on-site transformation is shown in Figure 10.
Figure 10 Protective cover for slag detection coil
(2) Transformation of the upper and lower boards of the smart bag
In the actual production process, the sliding plate moves under the drive of the sliding plate mechanism, and the upper sliding plate is always in a static state, so that the holes of the upper and lower sliding plates are aligned, and the molten steel flows and closes. In order to ensure that the upper slide will not be displaced during use, the fixed blocks of the slide (connected to the entire mounting plate) are not changed, but the upper and lower slides are modified from the original drop shape to an octagon, as shown in Figure 11. In this way, the fixing method of the slider is changed from line contact to surface contact, which increases the contact area and increases the wrapping strength.
(3) Improvement of the slider material in the skateboard mechanism
Because the sliding bar in the sliding plate mechanism is subjected to repeated action of high temperature and friction during the movement of the sliding plate mechanism, the sliding bar is worn to varying degrees. Later, the slider material was changed from 45# steel to 40Cr, and the annealing process was carried out at the same time, which increased the wear resistance and toughness of the slider, prolonged the service life of the slider, reduced the amount of wear, and the symptoms of steel sandwiched between the sliders disappeared.
Figure 11 The modified skateboard
(4) Renovation of the top platform inside the nozzle seat brick
During the use of the ladle nozzle seat brick, the inner top platform is continuously eroded due to the use of oxygen to clean the nozzle, and the nozzle will move inward during use, causing the sealing glue between the upper nozzle and the upper sliding plate to loosen, resulting in steel seepage and steel growth, and in severe cases, steel leakage accidents will occur. In response to this design defect, technicians combined with on-site personnel to improve the top platform and inner diameter inside the block. The specific improvement size is shown in Figure 12, that is, the original size is increased by 20 mm, which increases the contact area between the nozzle and the top platform, thereby reducing the erosion rate of the seat brick, prolonging the service life, slowing down the probability of the nozzle moving inward during use, and eliminating the accident of steel seepage and fertilization between the upper nozzle and the upper slide. At the same time, for the transformation of the nozzle seat brick, the corresponding upper nozzle has also been modified, and the specific dimensions are shown in Figure 13.
Figure 12 Renovation of nozzle seat brick
Figure 13 Retrofit dimensions of the upper nozzle
(5) According to the actual situation on site, sum up the operation experience and formulate the operation standard operation.
① Due to the cumbersome operation and long time in the process of replacing the nozzle and the sliding plate by the intelligent mechanism, in order to ensure the safety of the mud hole between the upper sliding plate and the upper nozzle, the filling fire clay (sealing cement) mixed manually on site is changed to a molding clay cake. On the one hand, the sintering time of the mud mouth is effectively prolonged, and on the other hand, the amount of the sealing glue mud is effectively controlled, so that the amount of mud between the mud mouths is full and the force between the upper and lower plates is uniform.
② Regularly replace all mechanism tracks, slide bars, springs, track wheels, and wheel axles to achieve a balanced force between the slides.
③Regularly replace the drain sleeves and trolleys of all mechanisms, which reduces the impact of spare parts on the centering of the aperture and solves the problem that the drain cannot be locked.
④ Formulate replacement standards for slide mechanism and refractory materials, implement standardized operations, conduct on-site operation process training from time to time, and strengthen workers’ operating concepts and operating habits.
After the practice, exploration and summary of the use of the new intelligent ladle in the steelmaking plant, and through the careful analysis and research of many accidents, a series of transformations were finally carried out on the ladle sliding mechanism. Currently, the ladle sliding mechanism is relatively stable. According to on-site data collection and statistics, there were 9 ladle accidents in 2019 and 6 ladle accidents in 2020, reducing the annual accident rate by 25%. In the subsequent use process, professional and technical personnel need to continue to track, constantly adjust and improve, in order to achieve on-site safety production.