Owing to essentially turbulent nature, many important aspects of flow in continuous casting are transient and difficult to control. The flow characteristics in the mold are determined by the condition of the jet entering the mold cavity which is again affected by the amount of gas injection. The time-averaged flow pattern gets affected by the section size, nozzle geometry, submergence depth, casting speed, electromagnetic forces, etc.
Surface turbulence is caused by insufficient nozzle submergence, type of pouring practice, amount of argon flow, etc. Shallow submergence can cause high surface velocity which can sweep off the top mold slag deep into the liquid pool. Open stream pouring from the tundish causes surface turbulence with high-velocity flow directed towards the meniscus especially for the turbulent or rough stream which increases the probability for slag entrapment. But for the straight-through nozzle, stream penetration is very deep, and recirculating flow travels long distance beneath the surface before flowing upward to the meniscus corners. The surface turbulence is substantially reduced thereby minimizing slag entrainment chances but the meniscus might become too cold and stagnant.
The last liquid processing step takes place in the mold. Proper flow control in the mold can help in improving steel quality whereas improper flow patterns can be even more detrimental to the steel quality. The problem associated are entrainment of mold slag causing surface defects and breakouts, the entrapment of air argon bubbles, heavy level fluctuations leading to breakouts, inadequate liquid slag layer coverage, re-oxidation, meniscus stagnation, jet impingement and entrapment of inclusion particles.
Air Entrainment
Open-stream pouring leads to air entrainment problems especially for rough and turbulent which can be minimized by controlling the flow in the tundish and employing inert gas curtain around the stream. The design of the metering nozzle can be useful in producing a smooth stream. High-speed flow in the tundish can be controlled across the exit nozzles by the installation of flow modifiers and the shape of the tundish near the nozzle exit. Castellated metered nozzles which, have grooves that introduce controlled roughness into the stream in order to avoid inconsistent severe roughness.
Steel can be prevented from coming in contact with air by the use of the submerged entry nozzle. But entrainment can take place if sealing is inadequate between the nozzle joints or due to the presence of leaks or cracks. If the internal pressure in the nozzle drops below atmospheric pressure, air will aspirate through any of these pathways into the nozzle. Pressure in the nozzle is lowest just below the flow-control device, due to the venturi effect of the metal stream. Air infiltration through the joints can be reduced by sealing with inert argon gas and adding argon gas can raise the minimum pressure in the nozzle above ambient. This occurs because the slide gate must open up to accommodate the gas in addition to the pressurizing effect of the gas.
When the bore opening is either very small or very large, the pressure never drops below one atmosphere, so gas injection is not needed to prevent aspiration. Thus less gas is required at low casting speed and low tundish level when the pressure drops are lower and injection of high gas flow rate at these times could disrupt the liquid flow in the mold which can be detrimental to steel quality.
Design of the nozzle and flow control geometry should promote smooth flow with minimal recirculation, in order to both minimize the pressure drops that allow reoxidation and to discourage clogging. The presence of a thick slag layer over the surface of the steel in the mold due to the melting of mold flux prevents reoxidation. Finally, flow in the mold should be controlled to avoid surface turbulence which could deplete the slag cover and entrain air.
Surface Entrapment
The origin of the most surface defects in cast product is in the mold and the source is the capture of foreign particles such as the argon bubbles and the oxide inclusions.
Argon bubbles have a tendency to float out but sometimes could not penetrate through into the slag layer, especially if they are small. In case of unstable meniscus level, the bubbles may get entrapped below the surface. If the meniscus is unstable, stagnant or has a “hook,” bubbles may be captured, forming pinholes just beneath the surface of the solidified strand. Inclusion particles if not removed by scale formation or during scarfing can lead to line defects or “slivers” in the final product.
Internal Entrapment
Although most bubbles and inclusions get circulated on the top surface and float out into the slag layer, some are carried deep into the liquid pool which gets entrapped in the solidifying strand. The argon gas bubbles traveling with the incoming jet can sometimes get carried into the lower recirculation zone and eventually get entrained.
It has been observed that the inner radius of a curved caster has a higher inclusion density due to the presence of a curve mold where large particles in the lower recirculation zone will spiral toward the inner radius and get trapped in the solidifying shell.
The chances of entrapment are more during a casting at high speeds, with wide slabs, asymmetric flow conditions in case of nozzle clogging, or sudden increase or decrease of the casting speed. Entrapped solid oxide inclusions lead to line defects or surface slivers or internal defects, which act as stress concentration sites to reduce fatigue and toughness properties of the final product. Most particles are captured 1–3 m below the meniscus, independent of casting speed.
Gas bubbles entrapped can cause blister defects such as pencil pipe, which appear as streaks in the final rolled product. These elongate at the time of rolling and the layer of metal separating them from the surface becomes thinner. Later during annealing, they can expand to raise the surface of the sheet locally, especially if the steel is weak such as ultra-low-carbon grades, or if hydrogen is present.
The problem can be reduced by lowering the liquid mass flow rate and by the use of straight mold and the straight top section of about 2.5-m thus avoiding the spiraling entrapment mechanism. Alumina inclusions or mold slag entrapment can also result in such sort of defect.
