Effect of Argon Gas Injection
Argon is injected into the nozzle to reduce clogging as well as to influence the flow pattern in the mold. Thus, argon flow should be adjusted with other casting parameters in order to optimize the flow pattern. The injected gas heats quickly to steel temperature and expands, and thus the volume fraction of gas bubbles becomes significant. Those bubbles which are carried down the nozzle into the mold cavity tend to create a strong upward force on the steel jet flowing from the nozzle, owing to their buoyancy. Without gas injection, the jet typically hits the narrow face and is directed upward and back along the top surface towards the SEN. The maximum velocity near the center of the top surface can reach almost 0.2 m/s. With optimal argon, top surface velocities are greatly reduced.
Adding argon lowers the density of the steel jet, increasing its buoyancy and making the jet bend upward. when too much argon is used, the jet may bend upward to impinge first on the top surface, and then flow along with this interface towards the narrow face. If the jet reaches the top surface of the mold before it impinges on the narrow face, increased surface turbulence is likely to result. Increasing argon flow to above a critical level tends to reverse the surface flow direction, as the single-roll flow pattern carries molten steel immediately to the surface, raising the liquid flux level near the SEN. This can result in recirculation in the upper mold to get reversed and there are no longer separate recirculation zones above and below the jet. It also makes the flow pattern more variable, as it alternates between single and double-roll. This critical level is less for lower steel throughputs because the influence of gas buoyancy increases with slower-moving steel. Excessive argon injection is expensive and can generate a transient variation of the jets entering the mold. Because steel does not wet the inside of the nozzle, a gas sheet may form and separate from the steel at high gas flows. The gas leaves the ports intermittently and introduces a significant asymmetry in the mold cavity.
The average volume fraction of argon gas in the nozzle, fg, is related to the liquid steel flow rate via:
where Qg represents the gas injection flow rate at 25°C and 1 atm, Q represents the liquid steel flow rate, β represents the gas volume expansion factor. The liquid flow rate may be calculated by multiplying the casting speed by the cross-sectional area of the strand. The factor of gas volume expansion due to the temperature increase and pressure change, β, is about 5.6.
Too little argon may allow nozzle clogging. With low gas flow, the double-roll flow pattern has a steeper standing wave near the narrow face, with flow moving back across the top surface toward the SEN. The critical level is also less for wider molds, where the jet has more time to bend upward from the buoyancy before it reaches the narrow face. The critical level likely also depends on submergence depth, nozzle geometry, and strand thickness. Adjusting argon flow during operation is a good way to accommodate other changes in the casting conditions in order to maintain the flow pattern. To maintain a stable double-roll flow pattern, which is often optimal, the argon should be kept safely below the critical level.
Electromagnetic forces
Electromagnetic forces can be applied to the molten metal in a number of ways to substantially alter the flow pattern in the strand. Induction forces are applied to the molten steel by passing current through electromagnetic coils positioned adjacent to the mold or strand. The two different methods are 1.to apply direct current to create constant forces that tend to slow down or “brake” the flow and 2. to apply alternating current for “stirring” of the liquid.
A strong DC magnetic field can be applied through the mold thickness, which induces eddy currents in the metal. The resulting interaction helps to create a “braking” force which slows down the fluid in the flow direction perpendicular to the imposed field. In electromagnetic “braking,” electromagnetic forces are generally applied across the entire mold width in zones both above and below the jet inlet. These electromagnetic forces increase in proportion to the steel velocity to decrease that flow component. Careful slowing of the flow across the top surface can compensate for high casting speed and lessen the extent of level fluctuations and variations. Slowing the jets entering the lower recirculation zone can slow down the penetration of inclusions and gas bubbles and improve internal cleanliness. In addition, these forces can help to stabilize the flow pattern below the mold, thereby reducing transient fluctuations. Significant coupling between the electromagnetic field and the flow field may occur for DC braking, which then requires iteration between the magnetic field and flow calculations Care must be taken not to slow down the flow too much, or the result is the same as angling the ports to direct the jet too steeply downward resulting in defects associated with meniscus freezing. Slower flow has several potential benefits such as slower, more uniform fluid velocities along the top surface, more uniform temperature and less inclusion entrapment in the solidifying shell below the mold.
A rotating magnetic field is induced by passing an electrical current through coils placed around the mold. This results in electromagnetic “stirring” of the liquid in the horizontal plane of the strand. In electromagnetic “stirring,” the forces can be applied at the meniscus, low in the mold and/or near the region of final solidification low in the strand. Applying low-frequency electromagnetic pressure to the meniscus is a novel method to stabilize initial solidification and reduce the size of oscillation marks.
Mold stirring in the bottom half of the mold may help to remove superheat, to encourage nucleation, to stabilize the flow pattern and to help reduce inclusion entrapment. Final stirring low in the strand mixes the liquid in the region just before the final solidification point. The latter two procedures can increase the size of the central equiaxed zone and help to reduce centerline segregation in the final product. Caution must be used, however, as white bands are an undesirable byproduct of the stirring.
Finally, electromagnetic stirring both in and below the mold is reported to reduce centerline macrosegregation, presumably due to the flow effects on heat transfer and nucleation.
Electromagnetic stirring is modeled by solving Maxwell’s equations and then applying the calculated electromagnetic force field as a body force per unit volume in the steel flow equations.
Transient Flow Behavior: The transient surge in the steel jets leaving the nozzle may cause asymmetric flow, which can lead to waves in the molten pool. Jet oscillations are periodic in nature and increase in violence with casting speed, which can be a concern for thin slab casting. According to Huang, a sudden change in inlet velocity creates a large transient flow structure, that appears to be a large vortex shed into the lower region of the liquid cavity.
