There are a number of different types of continuous process lines, such as pickling, annealing, galvanizing and color coating. Flatness is important in most of these lines for various reasons.
A strip is usually under tension; hence care has to be taken when measuring flatness optical measurement systems on strips. Tension “hides” flatness problems since the tension extends the shortest parts of the strip most. If the tension is too high, the strip will look totally flat, and contact-based methods must be used to measure the latent flatness. However, flatness can often be measured accurately in low-tension areas. In addition, crossbow is measurable also in high-tension areas, such as galvanization or hardening applications. Optical flatness measurement can also be useful in quench and temper lines for high yield strength or special steel.
In many lines, the strip can be stopped for some seconds and tension released. This enables a possibility to scan the strip and sample the flatness under zero tension, which can be done many times for a single strip.