The quantitative measurement of the degree of clogging helps the caster know about the severity of the clog, its buildup rate, and the effective solution. The study of real-time data and analyzing trends in the positions of the slide gate or stopper rod helps in assessing the degree of clogging. In some cases, clogging severity is measured by downgrades or lost production due to countermeasures such as rodding or purging of the clog. The study of the position of the slide gate, stopper rod, and mold level fluctuations can help to understand the severity. However, it is difficult to consistently delineate the effect of plugging on these signals from effects of routine casting events, such as changes in speed or ferrostatic pressure changes due to a fluctuating tundish level.
When nozzle clogging becomes really problematic, the operator will use countermeasures like rodding or increasing inert gas purging to clear the nozzle. A casting speed change is found to be helpful. A sudden speed change is required since the typical response required by the slide gate to compensate for decreased flow is nonlinear which helps to clear off the agglomerated mass. However, the sudden speed change also can cause mold turbulence leading to a quality defect. Sometimes commands given to change the initial throttle positions in the start of casting at the time of casting throttles the slide gate thereby releasing the buildup.
In certain cases the lump can get dislodged from the nozzle causing a momentary increase in mold level which can be counteracted by the increase in speed. Several producers and steel industry suppliers have developed operator interface and data archiving systems to assist in the work of nozzle clogging reduction by quantitative assessment.
A term known as “nozzle clogging factor,” or NCF, in real time is being introduced which compares the actual steel throughput to the theoretical throughput determined by the slide gate (or stopper rod) opening and the tundish level.
The on-line NCF measurement system helps to determine where in the casting system the clogging occurs. For example, if an SEN change reduces the clogging factor from 18% to 2%, the plugging occurred in the SEN, and the cast may be continued safely without enhanced risk of defects. But if the clogging factor remained unchanged by an SEN and/or slide gate plate change, then the constriction to casting is in the tundish well nozzle, and a tundish fly or caster turnaround would be recommended.
Mimic of the ladle, tundish, nozzles and associated data are presented on the computer screen in real time, and the operator is thus informed of several mostly strand-specific factors such as ladle weight, tundish weight, mold dimensions, casting speed, mold level, NCF (%), rate of change of percent clogging, as well as flow rates and back-pressures for all argon input locations. The automatic processing of the signals helps in the generations of alarms when excessive clogging requires action and for impending casting speed slowdowns.
Accumulation and interpretation of these data helps in:
1. planned or gradual casting speed changes can be made as and when required.
2. The operator can accurately assess the parameters that affect nozzle clogging.
3. There is the potential for real-time feedback to caster and ladle station operators.
4. A basis exists for a quantitative approach to remedy clogging and downgrades.
5. Any existing calcium addition programs can be optimized.
6. Decision-making functions may be automated.
This type of nozzle clogging indicator is also applicable to stopper-rod flow-control systems, although with some minor modifications as the shape of the stopper rod is subjected to change during casting due to accretions or tip erosion. Thus, a heuristic algorithm is required to estimate the NCF based on a comparison between expected and actual response to stopper movement.
A decrease in the slide gate position means opening of slide gate to compensate for decreasing throughput and abrupt change present represent severe chocking. Chocking can be limited to a particular strand which can be due to sporadic local increase in the production rate of inclusions due to a processing issue. Most common sporadic problems are related to reoxidation in the tundish, aspiration through a slide gate, variable ladle metallurgy practices leading to variable incoming steel cleanliness and problems with refractory integrity. However an increase in the stopper rod position compared to the theoretical value represents a clogging nozzle.
The first step in a program aimed at the reduction of nozzle clogging is to implement a real-time quantitative analysis that includes a nozzle clogging factor, rate of change of clogging, maximum possible casting speed, and also incorporate defect or impending defect indicators. The next step in the anti-clogging program is to identify the clog material through macroscopic, microscopic and chemical analysis, and then categorize the type of clogging. Remedial procedures must then be implemented using the real-time clogging monitor as a measure of effectiveness.
