The shape of small billet after continuous casting is an important parameter for determining the surface quality of the billet and the production level of a steel plant. When the shape of the billet after continuous casting is not ideal, the subsequent rolling process may have defects such as cold shuts and surface cracks. In order to control the shape of the billet after continuous casting, it is necessary to understand the twisting phenomenon of the small billet.
Twisting occurs when the billet in the continuous casting process after solidification is subjected to unequal cooling and thermal stresses. Due to the effect of centrifugal force, molten steel flows around the billet in the mould, causing non-uniform solidification and the subsequent generation of secondary dendritic structure. When the internal and external temperature differences of the billet increase, the shrinkage of the external side is greater than the internal side, resulting in unequal thermal stresses. These stresses cause the billet to undergo plastic deformation and twist.
The tilting of small billet after continuous casting can be effectively prevented by controlling both the design of the mould and the process of continuous casting. To control the mould parameters, the standard design parameters should be strictly followed. The positioning of the nozzle and the taphole, the diameter of the mould, and the depth of the pouring cavity should be appropriate to ensure the smooth flow of molten steel and the uniform shrinkage of the billet during solidification. The process parameters in continuous casting also affect the shape of the small billet. The casting speed should be set properly and kept constant to eliminate the fluctuation of the thickness of the billet, and the preheating of the mould and billet before pouring should avoid excessive cooling.
Besides process parameter control, the shape of small billet after continuous casting can be improved by reasonable additives. Adding reasonable amount of zinc powder, aluminum powder and other nodulizers can refine the spheroidization of the dendritic structure in the billet, so as to eliminate the uneven shrinkage in cooling, thus reducing the intensity of plastic deformation and twisting. In addition, reasonable control of the level of sulfur and phosphorus in molten steel can reduce the internal stress of billet during solidification, thus reducing the twisting phenomenon.
To sum up, reasonable control of the design, process parameters, and reasonable addition of additives can effectively reduce the twisting of small billet after continuous casting. With the deepening of research, the phenomenon of twisting of small billet after continuous casting can be completely eliminated in the near future.
