Convertor automation: the next step
Tapping becomes slag free…
In the BOF vessel, scrap and hot metal are converted into liquid steel by blowing pure oxygen onto the hot metal bath. In the process, carbon and impurities are burnt, which then turn into slag floating on top of the liquid steel. The BOF vessel is tilted into casting position and the steel is poured into a steel ladle through a tap hole. Afterwards, the convertor is tilted to the other side, and the slag is poured into the slag pot via the vessel mouth.
During the first stage of the process, when the vessel is being loaded with scrap and hot metal, a mathematical model indicates to the crane driver what quantities of scrap and hot metal need to be charged. These calculations to a large extent steer the blowing process. By also automating the tapping process, ArcelorMittal Gent sets a new and important step towards higher efficiency and increased productivity.
Where tapping is concerned, the challenge the steel shop faces is to separate slag from steel as much as possible. Basically, steel is tapped into a steel ladle, and slag into a slag pot. However, to improve the process, the steel shop now also uses a reststeel pot, which is placed next to the steel ladle on the transfer car underneath the BOF vessel.
… and automatic!
How does slag free tapping work? The BOF vessel, which contains liquid steel and slag, is tipped over by means of a stick. First, the upper layer of slag is poured into the reststeel pot. As soon as the angle of the vessel reaches 80° and steel is being tapped from the vessel, the transfer car automatically moves so that the centre of the steel ladle is positioned under the steel flow. When the scales on the transfer car indicate a weight of 270 tonnes, the transfer car moves so that the steel flows into the steel ladle at its brim. Shortly afterwards, at a moment fixed by a mathematical model, the transfer car moves until the tap hole is located above the reststeel pot, which catches a mixture of slag and steel. Finally, when only slag is left in the vessel, the vessel is tilted to the other side to pour the slag into the slag pot via the vessel mouth.
Slag free tapping offers many advantages, as it is no longer necessary to remove slag afterwards. This shortens the lead time, reduces energy and material losses and diminishes crane occupation.
The tapping process is controlled by a PLC (Programmable Logic Controller), a processor that uses input from sensors, measurements, data from a processing computer etc. In particular, the PLC in the steel shop receives input from:
- The controls operated by the vessel operator
- The angle of the vessel
- The position of the steel transfer car
- The weight of the steel ladle and the reststeel pot
- Ferrotron measurements
- ConFloWa data
- Temperature data from a pyrometer
Basing itself on these data, the PLC takes care that the steel ladle moves along with the angle of the vessel, that the angle of the vessel is optimal at all times and that all data are transferred to a WinCC touch screen for the operator.
When the liquid steel is being poured from the BOF vessel into the steel ladle, it should end up right in the middle of the steel ladle. So as to position the steel transfer car (upon which the steel ladle is placed) automatically, two coordinates are required: the angle of the vessel and the position of the transfer car.
The ideal vessel angle
As the steel is tapped, the BOF vessel is tilted more and more.
When a vessel is tilted, a difference in height is created between the vessel mouth and the level of steel and slag. This difference in height is called freeboard. Tapping comes down to finding a compromise: on the one hand, a small freeboard enables swift tapping, on the other hand, we then risk losing slag or steel via the vessel mouth. A larger freeboard could avoid this problem, but this has a negative influence on efficiency and productivity, since after tapping, some steel may still be left in the BOF vessel.
Cross section of the BOF vessel during tapping, with a view on the freeboard.
To determine the ideal angle of the vessel, the steel shop uses a mobile laser, called Ferrotron, which calculates the content of the vessel in three dimensions. This is done on a daily basis, since the inside shape of the vessel continually changes as the refractory layer is subject to wear.
During tapping, the Ferrotron generates this curve:
The actual converter angle is nicely in between the minimum and maximum curves.
An and-and story
The lower green line, the maximum curve, indicates the angle that entails minimal freeboard and, consequently, maximal ferrostatic pressure. However, it is not recommended to continually follow this curve: if the vessel is being tilted constantly and progressively, this might overburden the engines. Therefore, pauses between tilting movements are inserted, the minimal angle being determined by the upper green curve. On a touch screen, the operator can follow these curves along with other information about tonnages.
Still, in case of extra wear in the vessel or exceptionally thick or light slag, it is not entirely excluded that some slag or steel may flow over the vessel mouth. Therefore, the industrial automation and measuring technique department has developed the ConFloWa programme (Converter Flow Watcher), which detects whether or not slag or steel is flowing over the vessel mouth. On a camera image of the vessel mouth, the number of bright pixels is counted. The result is passed on to the PLC, which tilts the converter up again as soon as the number of bright pixels is too high.
Another system that detects this problem uses a pyrometer, which measures the temperature of the surface it is aimed at. If no slag or steel is flowing over the vessel mouth, the pyrometer measures the temperature of the wall underneath the vessel, which is low. If slag or steel is poured over the mouth, the pyrometer measures the temperature of the steel or slag, which is high. These data are also transferred to the PLC, which enables immediate intervention.
Converter automation offers more than one advantage. Both the efficiency and the productivity of the steel shop are increased, since the process is controlled more precisely. Moreover, safety gains are substantial. During the oxygen blowing process, the liquid steel may reach a temperature of more than 1,600 degrees. Thanks to the automation of the process, in the future, employees will not need to come near the vessel very often.