Steel strip with perfect surface Casting determines the surface of the steel strip
ArcelorMittal Gent's clients want a steel strip with a perfect surface. This means: a surface without scratches, indentations or lines. Some surface defects originate during the strip rolling. But the surface itself is formed while casting. Therefore, a surface without any defects originates during the steel solidification.
ArcelorMittal Gent casts continuous steel in the form of slabs. The steel solidifies in the mould. This is a copper box without bottom into which the liquid steel is poured at the top. The copper walls are cooled with water. In doing so, sufficient heat is removed to have the outer steel layer solidify.
Figure 1: cross-section of the mould (front view)
The first millimetres of solidified steel form the surface of the slab, and therefore of the subsequent steel strip. The way in which these first millimetres solidify is therefore extremely important. One of the critical factors is the stability of the bath level in the mould. Small fluctuations in the bath level cause uneven solidification, and where solidification is uneven the surface blemishes of the steel strip are more likely to increase.
The bath level is measured every 50 milliseconds by an electromagnetic field. The interaction of the liquid steel with this field allows us to exactly determine the steel level. These measurements are sent to a control mechanism that opens and closes the stopper rod. This is how we control the steel flow entering the mould.
The liquid steel flows into the mould at a rate that can run up to 6 tonnes a minute. It is quite a task to keep the bath level in the mould stable with these flow rates. To achieve this, we must therefore use the most refined control techniques.
Different types of fluctuation
We make a distinction between different kinds of fluctuations of the mould level. We have standing waves, bellies and sputters. We can prevent each of these phenomena by using adapted techniques.
- A standing wave is a regularly reoccurring pattern in the mould level. The width of the mould determines the frequency of the wave. Filtering at these specific frequencies allows us to detect the origin of a standing wave. As soon as one is detected, advanced filters are used to suppress the phenomenon.
- A so-called belly is a fluctuation caused several metres under the mould. The force of the column of liquid steel causes the slab to become thicker here and there between the rolls of the moulding machine. This expansion causes a minimal movement of liquid steel measurable in the mould. The casting speed and the distance between the rolls determine the frequency of the fluctuation.
Figure 2: frequency spectrum of the steel bath level measuring signal. The characteristic frequency corresponds to the expansion of the strand under the mould
- A sputter is a sudden peak in the bath level. Sputters are not characterised by a specific frequency. We use fuzzy logic to detect sputters. We continuously monitor a number of characteristics of the mould level signal. This is combined with a fuzzy variable that represents the extent to which the sputter phenomenon is present. This result is plotted in an online SPC card (Statistical Process Control). In this way, we can send specific alarms to the operators and take action when necessary.
Figure 3: characteristic pattern of a "sputter"
Figure 4: OSPC card sputters
The result of all this is a stable bath level in the mould. For the majority of the production we succeed in limiting the fluctuations to less than 10 mm. It does, however, remain a constant aim to continuously improve these results.