A special class
Coating thickness control in combination with EMG eMASS®
In hot-dip galvanising lines, the winding of the strip around the sink pot roll results in flatness defects, which are most often referred to as "crossbow". These deviations influence the zinc layer distribution by varying the nozzle distance across the strip width and require a significant increase in the coating target if an undercoating of parts of the strip is to be prevented. Automatic coating control systems can compensate for the skew of the strip between the nozzle lips, but they cannot rule out coating defects due to crossbow errors.
ArcelorMittal Florange reported for the first time at METEC & 2nd ESTAD 2015 on a technique in which the online coating measurement (cold measurement) was used to determine the cross profile of the coating between the air knife lips and to achieve an absolutely flat strip between the nozzle lips by overcompensation of the strip shape between the EMG eMASS® magnets. The basic principle is based on dividing for each scan of the coating measurement (approx. 120 m after the coating process) the coating cross section profile of the top and bottom side of the strip into two linear and non-linear components. The linear component is used to correct the skew error by re-aligning the nozzle with respect to the strip while the non-linear component is used to deliberately deform the strip in the area of EMG eMASS® so that the strip is held flat further down between the air knife nozzles. The coupling of EMG eMASS® with the jet machine requires robust coating control to avoid interference between the three control loops: air jet pressure, nozzle position and electromagnetic strip stabilisation.
And the result?
In the project described by ArcelorMittal, the crossbow was reduced from 8.2 mm to 4.7 mm for the Alusi® coating and from 3.1 mm to 0.6 mm for zinc. So again a drastic improvement between 50 and 80 % of already very good values!
[Reference: Cross-strip coating weight control at the hot dip galvanizing line of ArcelorMittal
Florange, N. Guelton et. al., METEC & 2nd ESTAD, 2015]
These results underline the fact that excellent results can be achieved by combining EMG eMASS® with intelligent control approaches using information from online coating layer measurement. This requires close cooperation between the line operator and the automation suppliers at the line in question. EMG provides in all cases the necessary data from the strip stabilisation for the corresponding interfaces.
The solution described here is particularly suitable for very modern, technologically advanced automobile lines such as ArcelorMittal Florange. The results are then comparable to the possibilities offered by an integrated EMG eMASS® / DUMA-BANDZINK solution.
Please note: The closer the strip stabilisation can be brought to the level of the nozzle lips, the smaller the need for a theoretically possible "overcompensation". The results attainable in such an arrangement (typical for the majority of current applications) are described in detail in the article:
For other successful EMG eMASS® technology application scenarios, please read also:
- Producing thin layers – massive zinc savings with EMG eMASS®
- On the way to a successful EMG eMASS® project
- Update: The integrated solution EMG eMASS®/ DUMA-BANDZINK JetPro®
It is generally valid, what such considerations mean for your plant or projects in individual cases, can only be estimated after taking a look at the situation on site and knowing the concrete production data. We will also be happy to provide you with appropriate calculation aids and advise you in detail.
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