How to improve internal quality of Slab? ----center segregation3. Improvement of the cooling effect of the secondary cooling zoneSecondary cooling is the coolin

How to improve internal quality of Slab? ----center segregation

3. Improvement of the cooling effect of the secondary cooling zone

Secondary cooling is the cooling that the cast slab undergoes after exiting the mold. The internal quality of the cast slab is mainly controlled in the secondary cooling zone. The cooling uniformity of the secondary cooling zone and the cooling system of each zone have a greater impact on the center segregation of the cast slab. In September 2016, during a detailed inspection of the secondary cooling nozzles, it was found that the average number of nozzle blockages per pouring time reached 35, and the number of individual pouring times exceeded 50, which seriously affected the uniformity of cooling in the secondary cooling zone. Through follow-up investigations, it was found that the water quality of the secondary cold turbid ring water was the root cause of large-area blockage of the nozzles. The chemical degreaser of the core equipment of the secondary cold water treatment was modified and the water treatment chemical dosage was adjusted. At present, the number of blocked nozzles has been controlled at about 15 per pouring time. At the same time, the inspection and replacement of blocked nozzles during the shutdown process have been strengthened. The current secondary cooling conditions have been significantly improved.

After the improvement of the secondary cooling conditions, the segregation at 1/2 of the slab width is slight, but the serious segregation at 1/4 of the slab width is more frequent. By trying to change the electromagnetic stirring parameters and soft reduction parameters, no obvious results have been achieved. . Through the analysis of the on-site situation, it is found that the arrangement of the secondary cooling nozzles and the water supply mode in each area lead to stronger cooling in the middle of the slab than at the edges. When the central area of the slab is completely solidified, there is still a certain amount of unsolidified liquid phase at 1/4 , Which makes the solidification end present a "W" shape, which is the main cause of serious segregation at 1/4. In this regard, the steelmaking plant has optimized the target temperature of the secondary cooling zone and the nozzle model at the edge. The purpose is to appropriately increase the cooling intensity, reduce the transverse temperature gradient of the slab and the bulge of the slab in the arc section, and reduce the level of the slab. The temperature of the section rises to achieve the purpose of improving the center segregation of the cast slab.

4. Superheat and constant speed control

The superheat of molten steel is the main measure to control the equiaxed crystal zone in the center of the slab. When the degree of superheat is low, the central equiaxed crystal region is wide, so that the solute elements are dispersed between the dendrites in a wide range, reducing the aggregation of the central elements and reducing the central segregation. It can be seen that the implementation of low superheat casting is an important means to improve the center segregation of the cast slab. Through the implementation of stable converter blowing operation technology, the tapping temperature hit rate is improved; the ladle baking is strengthened, so that the on-line temperature of the ladle reaches 900 ℃, and the turnover frequency of the ladle is accelerated, which can reduce the temperature drop of the ladle; standardize the operation of argon blowing at the bottom of the ladle, and stabilize the blowing The temperature drops; measures such as strengthening the heat preservation of the ladle and the tundish, the current overheating of the tundish is stable at 18-25 ℃, accounting for more than 90%.

Through the implementation of the above-mentioned series of measures to stabilize the temperature, the current pass rate of constant drawing speed has reached more than 90%.