The fluid flow in the mold is influenced by the position of the flow control device (slide gate or stopper rod), nozzle clogging, casting speed, section size, argon gas injection rate, submergence depth, and electromagnetic forces.
In slab casting, the mold flow pattern varies between the two extremes. With an upward-directed jet exiting the nozzle or a large amount of argon gas injection, the flow will quickly reach the top surface and travel away from the nozzle toward the narrow faces before being turned downward. This “single-roll” flow pattern is formed with multi-port nozzles, or a bifurcated nozzle with small, upward-directed ports. It is also encouraged by shallow nozzle submergence, lower casting speed or large mold widths. This can lead to higher surface velocities and level fluctuations resulting in higher chances of mold slag entrainment which can be the cause for surface defects in the final product.
With the other typical mold flow pattern, the steel jet enters the mold cavity from a more deeply submerged nozzle with larger or downward-angled entry ports of a bifurcated nozzle. The submerged jet then travels across the width of the mold to impinge on the narrow faces. The jet then splits and some of the flow travels upward toward the meniscus and back across the top surface toward the nozzle. The rest of the jet flows down the narrow faces deep into the liquid pool. Two large recirculating regions are formed in each symmetric half of the caster, so this flow pattern is termed as “double roll.” Often, the flow pattern alternates between the single- and double-roll arche types or it may attain some intermediate condition. In general, the relative size of the side and bottom ports can be adjusted to optimize the flow condition and thereby avoid defects.
Flow Control device Position: The flow rate of steel entering the mold is governed by the pressure head generated by the difference in steel levels between the tundish and the mold. Stopper rod or slide gate system controls the overall flow rate and at the same time influences the flow pattern in the mold, by affecting the symmetry of the flow.
When a slide gate operates parallel to the wide face (0° orientation), the flow from the opposing two symmetrical ports in the bifurcated nozzle is very non-symmetrical. The increased flow down the side of the nozzle beneath the gate opening tends to increase the flow rate leaving from the port on the opposite side. The increase can exceed 150% of the flow through the other port. The increased flow rate is accompanied by a much shallower angle into the mold (as little as half the downward angle from 25° to 12.5° for the opposite ports).
Opening the slide gate perpendicular to the direction of the ports (90° orientation) avoids this asymmetry problem and introduces a consistent rotational component or “swirl” into the jet restricting the flow to less than 100%. This configuration tends to induce a consistent horizontal angle to the jet and to steepen the vertical jet angle. Whereas a 45° slide gate orientation has both the left-to-right asymmetry and the swirl-based asymmetries,
Stopper rods avoid the steady-state asymmetry of slide gates, but are prone to transient fluctuations, especially when the stopper is misaligned slightly or has any erosion or clogging on or near to its critical control surfaces.
Nozzle Clogging: Inclusion particles that get stick to the nozzle walls can decrease the throughput but at the same time disrupt the flow pattern in the mold. Flow through the nozzle and its ports depending on the cross-sectional area of the opening at the flow control and on the sharpness of the edges there. Clogging affects both of these. If clogging and erosion round off the edges near the slide gate, the consequent streamlining of the flow reduces the total pressure and encourages consistent swirl at the ports. Clogging of the stagnant regions around the slide gate further affects the pressure drop and causes reversals in the direction of the swirl. Slight changes in the shape of clogging cause significant changes to the mass flow and direction of the jets exiting the ports. In order to compensate for the changing pressure drop, the flow control position must change. This produces further changes in the outlet jet properties. The consequence of these inlet flow transients is manifested inflow at the steel/slag interface in the mold. Increased level fluctuations have been correlated with clogging.
Casting Speed: Increasing casting speed tends to increase all the velocities proportionally and produces little qualitative change in the time-averaged flow pattern in the mold, as long as other conditions are constant and there is no gas injection. However, increasing casting speed tends to increase transient turbulent fluctuations and also worsens the extent of flow pattern asymmetries, which oscillate between the two flow patterns. This, in turn, worsens detrimental surface turbulence and level fluctuations. The period of the flow pattern oscillations corresponds to the residence time of a fluid particle in the upper recirculation zone, which is typically 5–30 sec.
In addition, increasing casting speed increases the height of the standing wave on the top surface, which is highest where the jet momentum impacts the steel/slag interface. For the standard double-roll flow pattern, for example, higher casting speed increases the interface height next to the narrow face, where flux feeding into the gap becomes a chronic problem. The wave height increases with increasing casting speed. At very high casting speed, surface-level fluctuation problems may suddenly increase greatly when the surface flow velocity exceeds the critical value for wave instability.
The surface quality problems associated with high casting speed can be addressed by adjusting the nozzle geometry, increasing the submergence depth and by the application of electromagnetic forces which can decrease the intensity of surface directed flows. Improving internal cleanliness requires lower casting speed.
Strand Width and Thickness: For the same liquid steel flow rate, increasing the strand width or decreasing the strand thickness increases the tendency for transient variations in the flow pattern. Thin-slab casting machines are therefore more prone to level fluctuation problems than conventional casters because a higher casting speed is needed for a given flow rate. Increasing strand width tends to increase the single-roll flow pattern. In addition, and more importantly, the time oscillations in jet position are particularly severe in wide strands, where velocities are higher for a given casting speed. Especially in very wide strands, (e.g., 2 m) the jet position may become so unstable that it sometimes impinges on the narrow face wall (with a conventional double-roll flow pattern), and at other times impinges first on the top surface. These variations are caused by the phenomenon of “vortex shedding”, as the walls of the strand are too far away to constrain the jet in the vertical plane. Overcoming this problem may even require a second inlet nozzle.
Submergence Depth The depth of nozzle submergence, is measured as the distance between the slag/steel interface/meniscus to the nozzle port and is often changed during operation in order to accommodate nozzle erosion at that interface. Increasing submergence depth naturally shifts the flow pattern downward, which tends to encourage the double-roll flow pattern, discourages jet impingement on the steel/slag interface, and lowers the intensity of surface flow velocities. This decreases the amplitude of surface waves, which lessens fluctuation and instability of the steel/slag interface. Thus, deeper submergence lessens slag entrainment and encourages uniform slag feeding into the mold/strand gap, which tends to improve surface quality.
Increasing submergence depth too much, however, can cause quality problems in other ways. Deeper submergence may send more particles deep into the lower recirculation zones, where a greater fraction may become entrapped. Deeper submergence also risks surface defects due to the fluid flowing across the surface being too slow and too cold. As a consequence, very deep submergence produces insufficient mixing and lowers heat transfer across the slag layer. This may lead to inadequate liquid slag layer thickness, meniscus freezing, and problems feeding slag into the interfacial gap, especially near the SEN. This increases problems with longitudinal cracks and transverse depressions. The optimum submergence depth changes with casting parameters. Steeper jet angles from the nozzle, wider strands, electromagnetic braking across the surface, lower superheat, and higher viscosity slag all tend to require shallower submergence. Deeper submergence is required for higher casting speed and increased argon gas fraction.
