The Study of Continuous Casting for Amorphous Silicon Steel

An amorphous structure improves the physical properties of metal and enhances material performance. Planar flow casting process (PFC) is a rapid solidification method for continuously producing microcrystalline and amorphous ribbons. In the PFC process, molten metal flows through a nozzle onto the chill wheel where the melt is frozen and a continuous ribbon is spun. First, this study reorganizes the related experimental studies and performs the operability diagrams for all kinds of metal ribbons using dimensional analysis. Based on these diagrams, a rapid method that can assist on-site operators to adjust parameters is proposed. The method is able to determine the wheel-nozzle gap, applied pressure difference and wheel speed for the designated the ribbon thickness with a preset nozzle slot breadth. The determined operating variables enable the successful production of continuous ribbon and can serve as a reference for designing and modifying the processes. Then, the puddle development and heat transfer behavior in the PFC process are simulated using the computational fluid dynamics (CFD). A two-dimensional model is developed for the puddle in which the inertial force, viscosity, surface tension, wettability, and heat transfer with phase transformation are incorporated and the volume of fluid (VOF) method is employed to characterize the behaviors of the two-phase flow including the melt and air. The effects of the nozzle shapes and wetting conditions between the puddle/nozzle and puddle/wheel on the puddle shape and ribbon thickness are evaluated and their velocity and temperature fields are examined. The result shows that the nozzle with larger gap in the downstream tends to produce a longer puddle length and a thicker ribbon and it takes more time to reach a steady state for the casting process. In addition, the puddle shape was affected significantly by the wetting condition on the nozzle surface rather than that on the wheel surface. The contact condition between the puddle and nozzle must be non-wetting in order to make the puddle reach a steady state rapidly. However, the wetting contact condition is preferable between the puddle and wheel surface to reduce the amount of air entrainment and the air-pocket frequency on the ribbon surface. At the experimental section, the effects of manufacturing parameters in the PFC process on the ribbon formation and the puddle stability of Fe78B13Si9 alloy are investigated. The ribbon morphology, surface quality, and puddle geometry are examined at different conditions and the transient evolution processes of puddle for molten metal passing through a rectangular nozzle are observed. The successful operability window for the production of Fe78B13Si9 ribbon is established and it is found the scope is different from that of Al-based alloy. The ribbon thickness is found to vary with the applied pressure across the crucible and the wheel speed to the power of 0.45 and , respectively. The formation of small air pockets could be enhanced by increasing the applied pressure difference and wheel speed, or by decreasing the nozzle-wheel gap and the jetting temperature. Next, the influences of surface roughness of chill wheel on ribbon topography and surface quality are examined and the average cooling rate during the PFC process is evaluated. The result reveals that a lower roughness tends to induce the appearance of herringbone pattern on the wheel-side ribbon surface and capture more air at the wheel-melt interface to form larger elongated air pockets on the ribbon surface. On the contrary, less air is trapped at higher roughness and the distribution of air pockets on the ribbon surface is mainly corresponding to the concave spots on the wheel surface. The roughness on the wheel-side ribbon surface increases monotonically with the wheel roughness, while a minimum roughness is observed on the opposite air-side surface exhibiting the smoothest air-side ribbon surface. A higher wheel roughness enhances the thermal contact resistance at the interface between the ribbon and the chill wheel, which reduces the average cooling rate during the casting process and results in the occurrence of crystalline structure in the ribbon formation. Finally, a high-speed imaging system only records the evolution of puddle geometry due to the glare of the melt. The layout for the simulated apparatus of flow field is carried out in order to view the fluid flow in the puddle. According to the experimental data of the PFC process, the required operating variables are calculated, the suitable fluid and seeding particles are selected and the wheel size and the motor specification are designed under the same dimensionless parameters. The whole process is that the silicon oil with seeding particles conveyed by the metering and pressurizing pump is fed through a nozzle onto the rotating wheel and is collected in the recycling tank. The result discovers that a strong clockwise vortex is formed clearly in the upstream region of the puddle. The back views of the puddle are found to vary from convex to waves distinctly with the wheel speed. Simultaneously, the system is predicted the stability by comparing with the theoretical results of the Taylor-Dean flow. The research issues in the future include the puddle stability of the PFC process and the relationship between dynamic contact angles and air pockets. The former is investigated progressively by the results of linear stability analysis for the plate flow and cylinder flow. The latter is studied by the use of the simulated apparatus of flow field and applies the linear stability analysis for the contact line to attain the aim of avoiding the air entrainment.