| The center segregation usually appears in high carbon billets during casting. And it shows that the effective way to solve such problem is increasing the equiaxed crystal ratio by the researches on technology. There are two measures to increase the equiaxed crystal ratio, one is reducing the superheat of steel and the other is decreasing the temperature gradients in the solidification front. But it is proved difficult to apply these two techniques into industrial practice in continuous casting process control field.There are still no reports found on the application of these two methods. In this thesis, aiming at controlling the superheat, a circular seam cooling shroud was designed and developed. Based on that, the cooling control model of the circular seam cooling shroud was studied. And by using the control model, online control of superheat came true which made the solidification structure of high carbon billets under control.This research is a part of a sub project "basic research on homogenizing the solidification structure of steel", which is from the "973" project "Metallurgical basical study for increasing steel quality and service time". In the project, the control of superheat and low superheat casting based on circular seam cooling shroud and related technology researches in continuous casting process are one of the main contents.The main research work and innovations are as follows:(1) Developement of circular seam cooling shroud apparatusTo decrease and stablize the superheat of the molten steel and to increase the equiaxed crystal ratio, a study on superheat control was conducted. According to the industrial application, the shroud apparatus with refractories was optimized, and a circular seam cooling shroud apparatus was designed and developed. By the numerical simulation of cooling shroud heat-transfer and test in laboratory, the cooling shroud proved effective to increase the heat conductivity, decrease the superheat and prevent the freeze or scaffold formation. The final determined results are as follows:the external diameter is115mm, the inner diameter is87mm, the height of apparatus is160mm, the width of water slot was chosen to be3mm, the air pressure is0.18MPa, and the water flux is2-3.25L/min.(2) Establishment of control model for circular seam cooling shroud apparatusBased on the analysis with thermodynamic and kinetic for metallurgical process, a flow model and heat transfer model for the shroud were developed.After discretizing the equation, calculation scheme and convergence standard criterion were determined. By using the software CFX, the heat transfer process of circular seam cooling shroud was calculated and a superheat control model was established.The numerical analysis results show that:on the aspect of flow field, when the steel was prouded into the shroud, the velocity decreased and the circumfluence could be formed at the corner. The circumfluence area was also the area where velocity is minimum in the shroud, and the steel was nearly still and easy to solidify there. At central zone of the shroud, the velocity was also small. But at the exit of the shroud, the velocity of steel became bigger because of the shrinkage of caliber and disappearance of the solidification shell. When the casting speed was at1.8-2.2m/min, the superheat of steel was varied between30～50℃, the change of flow field was little. On the aspect of heat transfer and solidification, the effect of the cooling system to the molten steel in the shroud was obviously, and the superheat of steel at exit also could be controlled at about10℃. Nearby the inner wall, local solidification occurred, but the solidification shell thickness was quite thin that it would not block the shroud. At the same time, the control model of water flux was also established by numerical calculation when the air pressure was kept at0.18MPa. In the model, the parameters were designed:W1=0, W2=6, W3=12,k1=0.04, k2=0.06,k3=0.1.(3) Establishment of online controlling system for circular seam cooling shroudAfter determining the demands, and also based on the above model, an online control system for the circular seam cooling shroud apparatus was developed, which realized the reliable and steady online control of the molten steel superheat.(4) The application research on superheat control based on circular seam cooling shroudThe industrial production trials of the superheat control system based on circular seam cooling shroud was conducted in Huai Steel. These trials indicated that the heat transfer coefficient increased to12W/m·℃after optimizing the design of shroud. The control system realized the automatic control for superheat and the closed-loop control for the water flux of the cooling shroud. The apparatus worked reliably for four hours under normal continuous casting conditions and during the period about400ton casting steel was producted. The trial process proved trouble-free and the system had highly maneuverability and good cooling effect. The cooling shroud can decrease the superheat of steel by14℃, and the equiaxed crystal ratio can be improved by more than10%. The center porosity degree is also improved. At same time, the thermo-analysis and test of AlN and BN competition precipitation was conducted in aluminum killed steels. The purity of steel and inner properties can be improved by adding boron (B) into the molten steel at the same pouring temperature. |
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