In the continuous casting operation of the continuous casting machine, the stable use of the ladle shroud, tundish stopper and immersion nozzle is the key to high-reliability continuous casting. The use of the tundish stopper mainly includes the stopper tip. The problem of adhesion of inclusions and stopper rod erosion at the location can be effectively solved by measures such as slagging process optimization and calcium treatment. Therefore, the problem of stopper rod erosion has become the key to stable continuous casting operations. The related literature mainly studies the causes and control of the erosion of the stopper rod tip, and there are few research reports on the slag line of the stopper rod. Aiming at the problem of slag line erosion in the production process, this paper analyzes the influence factors of stopper slag line erosion in the production process of aluminum-containing steel in combination with related literature research and inspection and analysis methods, and proposes relevant control measures.
Analysis of Causes of Stopper Corrosion
1.1 Stopper rod material and production steel type
All stopper rods currently used by Xing Steel are made of aluminum carbon (Al2O3-C), which is prone to corrosion of the slag line of the stopper when producing low-silicon aluminum killed steel, especially the carbon content of finished products such as ML08Al and XGM6-1. The frequency of slag line erosion in low-carbon, low-silicon, aluminum-killed steel below 0.10% is higher. In severe cases, the slag line erosion rate of the stopper rod reaches 80%, and the stopper rod breaks from the slag line, causing production to be interrupted.
1.2 Corrosion reaction mechanism at the slag line
The Marangoni effect plays an important role in the local corrosion of refractory materials at the steel slag interface. In the actual production process, the slag line of the carbon-containing refractory material fluctuates at the slag-steel interface due to the effect of interfacial tension, resulting in localized slag line materials. erosion. Since the stopper rod itself is continuously reciprocating up and down in the tundish, it will further aggravate the erosion of the slag line.
In the tundish, in order to avoid direct contact between molten steel and air and prevent secondary oxidation of molten steel, a covering agent is added to the surface of molten steel for protection. At this time, a temperature gradient is generated in the tundish, resulting in convection of molten steel and slag at the slag line, which increases the erosion of the slag line of the stopper. This micro-circulation caused by convection at the slag-steel interface will increase the resistance to resistance. Erosion of wood.
1.3 Corrosion of the stopper rod by molten steel
When producing low-carbon, low-silicon, aluminum-killed steel with a carbon content of less than 0.10% in molten steel, because aluminum is used for deoxidation kill, the molten steel will be treated with calcium and then cast on the machine. At the same time, the calcium treatment in the molten steel will cause the CaO in the molten steel to increase significantly. Except for the Al2O3 denaturation in the molten steel, the excess [Ca] and [CaO] will form a large amount of 12CaO·7Al2O3, CaO with the Al2O3 in the plug matrix. ·Al2O3 and other low-melting calcium aluminates flow into molten steel and slag to form corrosion.
In the actual production process, when the Al content of molten steel is controlled at 0.045%, and the calcium content is controlled at 0.010%, corrosion still occurs. Through field tracking research, it is found that the main erosion of the slag line at this time is the slag layer in the casting area of the tundish. The middle CaO component reacts with the Al2O3 in the stopper matrix to produce the same erosion condition.
1.4 Corrosion of the slag line by the temperature of the tundish
The XGM6-1 ultra-low carbon steel stopper slag line corrosion problem produced by Xing Steel is the most serious. The corresponding relationship between the tundish temperature and the slag line erosion is calculated. The average tundish temperature of the first three pouring times is controlled at 1567~1575℃, and the stopper slag The line erosion is relatively light, and no erosion has occurred. The average tundish temperature of the last five pouring times was controlled at 1577~1583℃, and the stopper rods were eroded and broken.
Improvements
2.1 Strictly control the slag from the big bag
The main sources of slag components in the casting area of the tundish are ladle refining slag, tundish covering agent and molten steel inclusions floating up into the slag layer. Among them, the low silicon aluminum killed steel refining slag, which is more severely corroded by the stopper rod, is a high basicity refining slag system, and the CaO content in the slag is controlled at 55%-65%. The large ladle slag of each furnace will form a refined slag enrichment in the injection point area of the tundish. During the subcontracting process and when the pouring steel stream impacts the slag surface of the injection point area, it will cause the refined slag to enter the casting area and cause erosion of the stopper. .
Therefore, it is necessary to strictly control the slagging of the big ladle, and use the automatic control of the slag detection to avoid a large amount of slagging at the end of pouring. At the same time, the tundish slagging operation should be adopted. When the large ladle is continuously pouring 5 to 7 furnaces of molten steel, the tundish level slagging operation should be carried out to control the thickness of the slag layer in the injection point area.
2.2 Control the superheat of the package
The liquidus line of the molten steel of XGM6-1 steel grade is 1535℃, and the superheat is controlled at 25~45℃. From the actual production process, when the average superheat of the tundish reaches 45℃ (the temperature of the tundish is 1580℃), all slag lines appear Erosion off the situation. The average superheat of the tundish is reduced by 15℃, and the actual control average tundish temperature is reduced to about 1560~1565℃. The erosion of the slag line has been significantly improved, and the erosion rate of the stopper slag line can be stably controlled within 20%.
2.3 Optimizing the composition of the covering agent for tundish molten steel
In view of the reaction situation between the tundish slag and the stopper slag line, it is impossible to completely avoid the problem of refined slag entering the casting area and high superheat of the tundish in the actual production process. Therefore, the composition of the covering agent for the tundish molten steel is optimized for different steel grades. The temperature condition of the package increases the MgO content in the covering agent, and forms the Mg-Ca-Al-Si multi-element compound in the slag layer of the middle cladding. The melting point is above 1600℃. A protective layer is formed at the slag line of the stopper to slow down the damage of the slag. The rod body is resistant to corrosion.
The control of the MgO content in the covering agent needs to be adjusted according to the actual control range of the molten steel ladle. When the MgO content exceeds 15%, the melting point of the tundish slag will increase significantly. The slag layer in the ladle pouring area is crusted, which affects the normal stopper control. The added amount of the tundish covering agent is controlled to keep the tundish molten steel liquid surface black.
By optimizing the composition of the tundish covering agent, a coating layer of high-melting compound mainly composed of MgO is formed at the slag line of the stopper rod, which prevents the slag-steel interface reaction from corroding the refractory material at the slag line and effectively improves the service life of the stopper.
in conclusion
(1) By reducing the superheat of molten steel at 15℃, the slag line erosion rate of XGM6-1 steel stopper can be stably controlled within 20%.
(2) Strictly control the slag under the big ladle, adopt the tundish raising liquid level slag discharge operation to discharge the enriched refining slag in the injection point area, reduce the refining slag to enter the pouring area, and reduce the source of CaO in the tundish slag.
(3) By increasing the MgO content in the tundish molten steel covering agent to more than 10%, the corrosion of the refractory material at the slag line can be slowed down, and the MgO content can be adjusted to more than 80% to prevent the corrosion of the slag line of the stopper rod and increase the service life of the stopper rod.
