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Langenfeld, October 01, 2019

Integrated strip stabilization system in galvanizing lines

With the aid of magnetic fields, FOEN DEMCOJET® reduces the distance between air knife and strip in galvanizing lines, and thus attains an optimum coating result.

FOEN® air knives rank among the most advanced air knife systems in hot-dip galvanizing lines for coating all steel grades used in the automotive industry which places the highest demands on surface quality. The first integrated solution combining air knife and strip stabilization system was the FOEN DEMCO® system. Due to changes in design and constant improvement, it was advanced to the FOEN DEMCOJET® system. Thanks to the very compact design, the distance between air knife and the stabilization point of the electromagnet could be reduced to about 500 millimeters providing the basis to achieve an optimum coating result. Up to now, twelve FOEN DEMCOJET® systems for galvanizing lines have already been sold. The system was installed at reputed steel companies like Tata Steel (Netherlands), US Steel Kosice and Salzgitter and will soon be supplied to voestalpine, US Steel Protec, Ternium Pesqueria, Tata Steel UK, Stelco CA, Nucor, Hickman and CSN Galvasud. Further orders were awarded by HBIS Laoting for CGL 1 and 2.

Electromagnetic strip stabilization systems generally feature several electromagnets each nozzle arranged on both sides of the strip at the same height above the air knives. A position measuring sensor is installed below each magnet to continuously measure the distance of the magnet to the strip in a non-contact manner. The strip position determined by this sensor is compared to the target position. Through current variations in the coil it is possible to control the magnetic attraction acting on the steel strip and thus to equalize the strip profile and minimize strip movements.

Strip stabilization systems are a necessary feature in modern hot-dip galvanizing lines, since the requirements on process and surface quality are continuously growing due to various reasons. New industrial standards require closer tolerances, especially for the automotive industry. For example, the new European standard for the automotive industry (VDA 239-100) demands that the coil must not fall short of the minimum coating thickness value at no point. This makes the new standard stricter, since small deviations have a stronger impact on the evaluation. The trend towards higher- quality and more homogeneous surfaces requires a very stable strip run in the area of the air knives. 

Minimization of strip-to-nozzle distance

The key issue of strip stabilization is a minimization of the distance between air knife and strip to use the jet core zone, since only in this zone there is a stable and homogeneous flow pattern without turbulence. A short distance will lead to many benefits for the galvanizing process and thus fulfill all aforementioned requirements. The main condition to be met is a stable strip run in the air knife area. Only then, the distance can be shortened systematically without the risk of scratching the strip surface and contaminating the nozzles. To make the strip running stably between the nozzles, three operating parameters have to be optimized: strip movements and vibrations must be reduced, the strip cross-bow be compensated, and the distance between strip stabilizer and nozzle gaps be minimized.

Operation results showed that with the FOEN DEMCOJET® it was possible to ensure a substantially improved and stabilized strip run. And the highly dynamic control system working with a response time of just one millisecond, effectively reduced strip vibrations by more than 50 percent. The FOEN DEMCOJET® magnets can be positioned by motor power and almost compensate and eliminate the strip cross-bow. This means, the main objective was reached and the minimum distance between nozzle and strip shortened by at least three millimeters, which was sufficient to reach the jet core zone and ensure homogeneous coating.

The installation of the FOEN DEMCOJET® resulted in many benefits in terms of quality, productivity and operation. Coating uniformity improved, which led to lower zinc consumption. The yield of high-quality material was increased, since strip losses due to cut-outs at welds could be reduced significantly. Processing speed could be raised by up to 20 percent. In addition, it was possible to lower pressure which was accompanied by energy savings and a decrease in dross formation. Another benefit was the reduction of the correcting roll immersion depth. This went along with less wear of bath rolls and associated bearings.

Patented system

Based on the operational experience, FOEN® is now continuously optimizing the DEMCOJET® system to further increase its performance. The technological lead is broadly protected by several international patents.

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