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Unstable mold flow could induce surface velocity and level fluctuations, and entrain slag, leading to surface defects during continuous casting of steel. Both argon gas injection and Electro-Magnetic Braking (EMBr) greatly affect transient mold flow and stability. Part I of this two-part article investigates transient flow of steel and argon in the nozzle and mold during nominally steady-state casting using both plant measurements and computational modeling. Nail board dipping measurements are employed to quantify transient surface level, surface velocity, flow direction, and slag depth. Transient flow in the nozzle and strand is modeled using Large Eddy Simulation (LES) coupled with the Lagrangian Discrete Phase Model (DPM) for argon gas injection. The surface level of the molten steel fluctuates due to sloshing and shows greater fluctuations near the nozzle. The slag level fluctuates with time according to the lifting force of the molten steel motion below. Surface flow shows a classic double roll pattern with transient cross-flow between the Inside Radius (IR) and the Outside Radius (OR), and varies with fluctuations up to ~50% of the average velocity magnitude. The LES results suggest that these transient phenomena at the surface are induced by up-and-down jet wobbling caused by transient swirl in the slide-gate nozzle. The jet wobbling influences transient argon gas distribution and the location of jet impingement on the Narrow Face (NF), resulting in variations of surface level and velocity. A power-spectrum analysis of the predicted jet velocity revealed strong peaks at several characteristic frequencies from 0.5-2 Hz (0.5-2 sec).

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Unstable meniscus flow leads to slab surface defects during continuous casting of steel, due to level fluctuations and vortex formation, which causes entrapment of argon bubbles and mold flux. Applying electromagnetic fields across the liquid steel pool, such as the "doubleruler" or "FC-mold" braking system, has been commercialized to stabilize meniscus flow.Plant measurements were performed using nail boards to quantify meniscus flow in a typical steel slab-casting mold with a slide gate system. The shape of the meniscus level, the surface velocity, and the direction of meniscus flow, are all quantified with time and location by analyzing the shape of the skull of solidified steel that encases each dipped nail. The results reveal interesting insights into time-variations of the flow pattern, which cannot and should not be detected with a standard mold-level sensor used for flow control. Further, the effect of applying the electromagnetic braking field on the flow pattern is revealed.