1. Purpose of ladle capping
In recent years, due to the financial crisis of overcapacity in steel enterprises, the profits of large steel enterprises have gradually decreased, and even the situation of loss year by year. With the development of continuous casting process, the structure optimization of production process and energy saving and consumption reduction have become the development trend, how to improve the operation level and reduce the temperature drop during ladle pouring (reduce the temperature of steel) has become an important topic of recent research of various steel enterprises. Controlling the temperature drop of molten steel in the process of transportation, refining and pouring has increasingly become a key factor in the recent stage of steelmaking industry innovation. The change of ladle's thermal state is also one of the important factors in establishing the temperature compensation system of molten steel in converter.
In the process of ladle transportation, refining and pouring, there are two main ways of heat loss: one is through the heat conduction of ladle lining material, and the other is the heat conduction and heat radiation of the upper molten steel in contact with the air. After the ladle cover is added to the ladle during the turnover process, it plays a good role in protecting the heat dissipation of ladle, and also makes the thermal state of ladle more stable during the turnover process, creating conditions for accurately controlling the temperature and temperature drop of ladle, and further reducing the heat loss of ladle during the turnover process.
2. The development of ladle covering the whole process in China
Ladle covering technology is used in some large steelmaking enterprises in China, such as Handan Steel, Tang Steel, Masteel, Xing Steel, Lian Steel and so on. By taking technical measures such as ladle covering, improving ladle turnover and casting at low temperature and high pulling speed, the temperature drop of steel in the second rolling mill of Tangsteel is reduced by 20℃, 11.1℃ and 25.9℃ respectively after blowing ammonia from the steel drawing to the bottom of ladle, during ladle lifting and casting. Through the ladle capping process, the average temperature of the converter is reduced by 10 to 30℃, and the pass rate of the ladle temperature of the continuous casting machine is increased by 8%. Compared with ladle baking for 8h after capping, the ladle lining temperature is increased by 8℃, which creates conditions for converter production and energy saving.
However, for a long time, domestic design units and ladle cover manufacturers used to use heavy castable structure, castable fiber blanket composite structure of insulation, fiber blanket structure. These methods solve the problem of falling steel water temperature to a certain extent, but it is difficult to fundamentally solve the problem of lightweight longevity of ladle cover, which brings obstacles to the promotion and innovation of ladle cover. The main reasons are as follows:
1) The density of heavy cover castable is relatively heavy (2.8g/cm³). When unloading at high temperature, the working face is affected by the pulling force on the back side, and the mechanical properties of the metal frame are low at high temperature, and the heavy castable has low ability to withstand the pulling force. When unloading at high temperature, the outer bulge of the cover causes the cover plate to bulge, thus affecting the service life of the cover. In addition, the thermal shock resistance of heavy castable is poor, and the thermal conductivity is large, the heat loss of molten steel radiation and conduction is large during the use of molten steel, and the energy-saving effect of the whole ladle capping is not very ideal, limiting the development of ladle capping.
2) The energy saving effect of the use of fiber folding blocks on the entire ladle cover is shown, and the density of fiber folding blocks is small, but under the high temperature state of molten steel radiation, the grain boundary of refractory fiber is continuously filled with similar atoms (particles) and solid solution occurs during the grain growth, and the impurity enrichment at the sample boundary is increasing. The impurities have irregular arrangement, small binding force and strong sensitivity to temperature change. As a result, the strength at the grain boundary is much weaker than the interlattice bond, causing the stress to penetrate the grain boundary at the reduced diameter, causing the fiber rod to break (i.e., powder). Large pieces of fiber folding block fell off, and the high temperature of molten steel radiation through the falling fiber folding block burned the iron sheet of the cover, and also limited the development of the whole process of ladle capping.