The control of flow through the tundish is done by choosing the appropriate size of the opening by means of metering nozzle, slide gate system or stopper rod mechanism.
Metering Nozzles: These are basically alumina graphite body mix with zirconia in bore walls.
The open-stream pouring practices relate to producing inferior quality steel (where some air entrainment can be tolerated) and is used for smaller mold cross-section less than 125 mm sq. Air entrainment can be reduced by surrounding the stream with argon gas but cannot be completely eliminated.
Variations of the liquid level in the tundish will produce variations in flow rate. The speed of the strand depending on the flow rate changes by means of auto mold level PLC controlled speed regulator. In case of small adjustments to be done in speed due to change in the superheat, the tundish level is varied to bring about the desired casting speed. However, this is not a recommended practice because heavy fluctuations in bath level can lead to slag entrapment, re-oxidation, temperature loss from steel and thermal shock to the refractory lining. This type of occurrence can take place during the delay in connecting sequence heat and assisted ladle opening.
The condition of the stream is very sensitive to the flow pattern in the tundish. Any imperfections in the nozzle shape, such as oxide buildup, metal build-up or notches in the outside of the nozzle lip can disturb the flow pattern of the stream. The condition of the stream directly affects the quality of the final steel product. The turbulent and scattered stream leads to metal jam formation on the top walls of the mold which can result in sticker type of breakouts if allowed to grow and not removed periodically.
The formula for calculating the flow through a nozzle:
G= nozzle throughput in kg/min
α=factor, 0.9-1.0.
F=area of the nozzle bore in mm2
H= total ferrostatic height,
ᵞ= density of steel at 1550 deg C.
A rough, turbulent stream must be avoided because its irregular shape entrains much more air which leads to the formation of more and larger oxide inclusions in the product. Such stream is responsible for meniscus level fluctuation in the mold and can entrap mold slag.
Slide Gate Flow Control
In case of the three-plate slide gate system, the central plate is moved hydraulically to adjust the opening between the upper and lower stationary plates. In case of the two-plate slide gate system the middle plate is missing and the lower plate has the collector nozzle where the SEN sits. The lower plate serves as the moving plate and travels to adjust the opening. This has the disadvantage of continuous variation in the alignment of the nozzle relative to the strand centerline. In both systems, the joints are sealed by means of low-pressure inert gas (argon) to protect against air entrainment.
This opening size may be quantified in several different ways. Two popular measures are “area opening fraction,” fA, defined by the ratios of the shaded area to the total bore area, and “linear opening fraction,” fL, defined as the ratio of the distances S to T.
For equal sized openings, these different measures of opening fraction are related by:
fA = gate opening (area fraction)
fL = gate opening (linear fraction)
L = length of opening (m)
D = nozzle diameter (m)
R = offset distance from nozzle bore to reference line used to measure S and T (m) and
The flow rate depends on the height of molten steel in the tundish and the pressure drop across the slide gate. Flow rate automatically increases with increasing slide gate opening position and with increasing tundish liquid pool height. The flow rate is also influenced by the nozzle bore size, the amount of gas injection, the constriction of the ports, and the extent of clogging and wear.
“Nozzle clogging factor”- a factor used to determine the extent of clogging is derived by comparing the theoretical flow rate produced under ideal conditions to the measured flow rate.
Stopper Rod Flow Control
Flow control with a stopper rod is difficult than with slide gates because the stopper must be manipulated through the entire depth of the molten steel in the tundish, and the area of the annular opening that controls the flow is more sensitive to displacement.
In case of running stopper emergency blank plate is fired which cuts off any trickling metal flow and shears off the SEN as well.
The stopper rod offers several significant advantages over slide gates:
1. Prevention of molten steel from entering the upper tundish well block and freezing prior to startup of casting.
2. Prevention of vortex formation above the tundish well and slag entrainment into the nozzle when the liquid level is low especially during tundish draining, delay in connecting sequence heat, etc.
3. Easier sealing to avoid air entrainment due to the reduced number of moving surfaces.
4. More uniform distribution of flow to both the ports allowing the flow in the mold to be more symmetrical.
5.It offers flexibility to the operator by giving him the freedom to start the strand at higher tundish weight depending on the situation.
6. The use of start-up flow control device is not required.
Flow rate based on the measured stopper rod opening position and other parameters are given by:
A SEN is the SEN inner bore cross-section area; hsen_sub is the submergence depth of SEN, htundish is the total height of the tundish; ftundish is the tundish weight fraction; LSEN is the total length of SEN; DSEN is the SEN inner bore diameter; hSRO is the stopper rod opening. The three adjustable coefficients represent different pressure head losses: C1 for friction, C2 for the stopper rod gap and C3 for clogging.
