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.

Optical measurement systems on strips – The VeriFlat – system

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.

VeriFlat is our most cost-effective solution for material up to 2 m width. A standard sensor, yet powerful for most applications in strip process and cut-to-length lines. Pre-calibrated and extremely easy to install.

The material entering a strip process line usually comes from cold or hot rolling. In these lines, accurate flatness measurement is difficult due to high temperatures or high tension and temperature variations. Strip shape is also affected by cooling and stretching effects in coils as well as the coiling process itself. The shape at the entry side of a strip process line is therefore often unknown or differs from the measurement in up-stream process steps

Bad incoming shape can result in damages to the strip or equipment in the lines, which can often be avoided if the shape is known, a priori, but a good incoming shape is always to prefer.

By measuring incoming shape, there are two benefits:

  • The “true” shape can be used to fine-tune rolling for a better target shape profile, so the flatness is improved over time when the knowledge increases.
  • Strip speed, guiding and leveling can be adjusted depending on the incoming shape to produce better material and avoid damages to furnaces etc.

Shapeline has a product that is very well suited for this, the VeriFlat. A  VeriFlat can be used for a fixed installation, but also to do tests and adjustments. Once the tests are finished, the sensor can be moved to the next line.

For instance, galvanized material often suffers from crossbow arising from coiling, skin-pass milling or tension leveling. Once measured, crossbow is usually straightforward to remove either by bending in the tension leveler or the skin-pass mill (using an anti-crossbow roll). However, it must be measured accurately, which can be performed with a Shapeline gauge.  In the figure below, you can clearly see how the crossbow can be minimized through tension leveler adjustments.

Each triangle shows a change in the strip bending after the exit cassette in a tension leveler. This process can also be automated. The principle has been used successfully in both hot-dip and electro-galvanizing lines. In electro galvanizing, flatness measurement can save electrodes through a warning about large crossbow or edge defects.

More and more annealing lines are equipped with in-line quench to meet the demands for high-yield strength material to the automotive industry. Quenching can be a real challenge since Martensitic steel has a different density than other steel grades. Rapid cooling may also result in plastic deformation when the cooling distribution is uneven. Parameters like nozzle distribution and angle as well as turbulence over the surface are important factors. Clogged or worn-out nozzles will also cause flatness issues.

During the installation of a new in-line quench, a VeriFlex or VeriFlat gauge can pay for itself by shortening tuning time and verify the result. Over time, the flatness quality can be further improved and the gauge will also serve as a guardian and detect flaws and malfunctions in the line.

Since tension always affects the flatness to some extent, a perfect flatness measurement must be done without tension. One possibility for this is usually at the end of the line, after the exit looper. In many lines there is an inspection table here which can be used if a scanning VeriFlat–gauge is installed on top of the table.

When the strip is stopped and tension released, a VeriFlat-sensor can scan a strip section (typically 2-3 meters). The result will be a sample measurement under perfect conditions. Any number of measurements can be done as long as the time permits. A scanning takes 5-10 seconds.

The sensor can also measure the whole strip under tension. The result will be a complete strip measurement with a number of high quality sample measurements

An EGL has a number of sensitive anodes. In most lines, the anodes can be moved to open up a larger gap for the strip to pass.

The cells are opened of two reasons, either a very bad strip flatness/shape or that there is a coil-change weld going through. Here the Shapeline flatness data can play a vital role.

  • All of Shapeline’s references have automatic emergency opening of the anodes in case the flatness/crossbow becomes too bad, i.e. goes over a safety threshold, thus avoiding the risk for short circuit when the strip touches the anodes.
  • Some customers also have automatic tension leveler control based on crossbow data to control the flatness of the strip before the galvanizing cells.
  • The process operators all use the Shapeline’s crossbow/flatness information on a screen in the operator’s room to make manual decisions to optimize the anode distances – thus improving coating efficiency.

Hardened and Tempered Steel Strips are produced by two heat treatment stages with cooling in between. The flatness is significantly affected by several factors, such as line speed, pass-line, temperatures and air-cooling angles.

Shapeline has a long reference list of special steel producers, starting with the very first Shapeline flatness gauge from 1997. The gauges are used both for quality assurance and process parameter tuning.

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