THE DEMCO SYSTEMTHE DEMCO SYSTEM Non-contact …€¦ ·  · 2013-11-22THE DEMCO SYSTEMTHE DEMCO SYSTEM Non-contact electromagnetic ... Surface defects are also reduced as a result

Hans-Georg Hartung, Pascal Fontaine, Holger Behrens, Caesar SasseSMS Siemag AG, HildenFontaine Engineering GmbH, Langenfeld

THE DEMCO SYSTEMTHE DEMCO SYSTEMNon-contact electromagneticstrip stabilisation for strip galvanising lines

SMS SIEMAG

STEELWORKS and ROLLING MILLS TECHNOLOGY

Strip processing lines

Figure 1. The DEMCO system in operation at the Wuppermann-CGL.

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A large quantity of zinc is used in conventional stripgalvanising lines. Since the coating process is subjectto fluctuation, for example strip movements, addi-tional zinc has to be applied so as to ensure the mini-mum required coating at all points. Once the strip hasbeen stabilised, the quantity of zinc to be applied canbe adjusted more precisely. The quantity of zinc thussaved enables a more economical production of high-quality galvanised steel strip. In addition, the linespeed can be increased because of the stable striptravel. Surface defects are also reduced as a result ofthe contact between strip and air knife.

Together with Fontaine Engineering (FOEN) and SMSElotherm, SMS Siemag has developed the DEMCOsystem of electromagnetic strip stabilisation for stripgalvanising lines with FOEN air knives (DEMCO –Dynamic Electro Magnetic Coating Optimizer). Bymeans of inductive measurement, the system con-trols several magnetic forces in such a way that unde-

sired movements or forms of the strip can be levelled simultaneously. This occurs in a non-contactmanner in the area above the FOEN air knives whichadjust the zinc coating. Since April 2007, the first stripstabilisation system has been successfully in opera-tion in the annealing and galvanising line at HYSCO inKorea. A second system has meanwhile been testedduring production at Wuppermann in the Netherlands,Figure 1. Orders have already been placed by MMKand TKS for further plants.

The following first shows the conventional procedureused for hot-dip galvanising with special considerationof the air-knife system. This will be followed by adetailed description of the functioning of the DEMCOsystem, including that of its individual components aswell as the process-related and economical effects.The report will also provide a summary of the operatingexperience so far of Hysco and Wuppermann andpresent a final conclusion.

THE DEMCO SYSTEMTHE DEMCO SYSTEMNon-contact electromagnetic strip stabilisation for strip galvanising lines

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Strip processing linesSMS SIEMAG

STRIP GALVANISING LINES

Galvanised steel strip

Galvanised steel is a composite material which com-bines the excellent properties of zinc, such as resist-ance to oxidation and its high-quality appearance, withthe strength of steel. Owing to these properties, gal-vanised steels are used in many applications in orderto meet the high material requirements, for example,in the automotive industry, the building industry, plantconstruction and the household goods industry.

Of the 220 million tons of cold strip produced world-wide in 2008, 113 million tons were hot-dip galvanised(Source CRU). Even in future it can be assumed thatmore than half of the total cold strip produced is hot-dip galvanised. A big advantage of galvanised steel isits extremely lasting resistance to corrosion, as a resultof which there is frequently no need for maintenance.

Conventional strip galvanising lines

In conventional hot-dip galvanising lines, the annealedsteel strip is dipped into a zinc bath (bath temperatureapprox. 460°C). When coming out of the zinc bath, thestrip entrains some metal depending on its speed andsurface structure. Since this quantity of metal consid-erably exceeds the required coating thickness, knivesat the front and rear end wipe off a part of the exces-sive zinc by means of a fine but strong air or nitrogenjet. In addition to the coating thickness, the knives alsoensure that the zinc is evenly applied on the strip. Thesteel strip coated on both sides by liquid zinc is thenconducted upwards vertically after the wipe-offprocess, Figure 2.

However, due to the strip movements and deviationsfrom the ideal strip form, more zinc than necessaryhas to be left on the strip so that the required coatingcan be attained on an average along the entire cross-section and length. The examination of the coatingthickness is regulated by a standard (e.g. DIN EN10327). At certain positions, three areas are takenfrom the strip and the coating thickness is determinedby chemically dissolving the coat. Every individual sur-face must have a specific minimum coating thicknessand the arithmetic mean of the three test results mustat least correspond to the coating characteristic value.

For a desired strip coating of 100 g/m2, for example,approx. 103 to 110 g/m2 must be left on the strip sothat the required minimum coatings are met at allpoints in case of turbulent strip travel. For continuouschecks at the manufacturer's, non-destructive testingmethods are normally used. In case of arbitration, theprocedure which describes the standard applicable tothe relevant material should be used.

Air knives

The air-knife system controls the coating thicknessafter the strip has left the molten-metal bath. Knivesarranged on both sides with an extremely fine openingblow air or nitrogen at a certain pressure onto the stripand thus systematically wipe off the superfluous zinc.The coating thickness is obtained in this manner

Figure 2. Strip after leaving the air knife in a conventional strip galvanising line without strip stabilisation.

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depending on the pressure and distance from thestrip. The FOEN air-knife system is outfitted with con-trol systems which are directed centrally onto the striptravel in order to guarantee uniform wiping as far aspossible. The system is also equipped with a "skewedposition" movement function by means of which it canbe adapted perfectly to the skewed movement of astrip, Figure 3.

GI and GA steels

It is possible to manufacture two different types ofhot-dip galvanised steels:

On the one hand, GI steel plates are manufac-tured, with GI standing for "galvanised". For theproduction of these steels, the steel strip coatedwith liquid zinc is cooled, and a high-grade zinccoating with a metallic shine emerges, Figure 4.

On the other hand, GA steel strips are manufac-tured, with GA standing for "galvannealed". In thecase of GA plates, the zinc-coated strip after havingleft the zinc bath is reheated by means of aninduction furnace to more than 500 °C. Thisannealing diffuses iron into the zinc coating, thusproducing an alloy coating of zinc and iron. Theotherwise shiny metallic surface now becomes a

matt grey coat. Compared with the pure zinc coatof the GI steels, the GA steels are characterisedby weldability and paintability.

Strip Movements

Strip movements are unavoidable in this process: Thestrip has to travel contactless between zinc pot andupper deflection roll (approx. 35 - 70 m) because of thestill liquid zinc. Thus, it cannot be stabilized by any roll.In addition slight turbulences arise in the zinc bath andthe cooling tower, which influence together with thebath rolls the strip travel. Every reduction of the stripmovement in the knives reduces overcoating. In orderto save zinc in this way, the electromagnetic strip sta-bilization system DEMCO has been developed.

Figure 3. FOEN air-knife system in a hot-dip galvanising line.

Figure 4. Entry of the strip into the cooling tower – without strip stabilisation.

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Strip processing linesSMS SIEMAG

Positioning and number of magnets

The electromagnets are arranged above the air kniveson both sides of the strip and at the same level. Theouter pairs of magnets parallel to the strip width aremovable. The middle pair of magnets cannot bemoved. With the help of quick-clamping devices, themagnets can be easily installed on the FOEN airknives. The movement functions of the air knife sys-tem are taken over by the strip stabilisation by meansof a mechanical connection. Hence the middle pair ofmagnets always remains in a central position in thecentre of the strip.

THE DEMCO SYSTEM

The principle behind the DEMCO system

Strip stabilisation takes place via several electromag-nets and inductive sensors which are arranged in pairsat the front and rear side of the strip above the air-knife system. In this way, non-contact measurementand control of the strip position is performed, and unde-sired strip movements are reduced to a minimum. Thisapplies to movements orthogonal to the longitudinalaxis of the strip, i.e. the vibrations, as well as to hori-zontal deviations from the strip form, for example theso-called cross-bow, Figure 5 and Figure 6.

Figure 5. The DEMCO system in the adjustment with 3 pairsof magnets.

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Figure 6. The DEMCO System in the adjustment with 5 pairsof magnets.

The effect of strip stabilisation decreases with thegrowing distance between the magnets and the airknives. If the distance between the air knife gap andmagnet centre is more than 1.5 times to twice thestrip width, a considerably lower effect on the stripvibration and form in the knife is attained.

Depending on the setting, the distance between mag-net and strip is between 20 and 50 mm. This distanceis to be selected in each case in such a way that an asefficient force impact as possible is ensured betweenthe strip and the magnet.

Since the DEMCO system is of modular design, threeor five pairs of magnets can be used depending on thestrip widths to be galvanised and the customerrequirements. At least three pairs of coils are requiredfor cushioning the strip vibrations and for eliminating across-bow in the strip which may arise. The two outerpairs of magnets are then positioned precisely at thestrip edges. Due to the movement functions of theFOEN air-knife system, the centre pair of magnets isalways positioned at the strip centre. This enables thelargest possible cross-bow correction.

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Strip processing linesSMS SIEMAG

Figure 9. Arrangement for < 1,300 mm strip width.

Figure 8. Arrangement for > 1,300 mm strip width.

Figure 7. Arrangement for max. 2,110 mm strip width.

For an extended solution with five pairs of magnets,the outer magnets are likewise always arranged at the strip edges. The next pairs of magnets are thenpositioned in such a way that the strip can be uni-formly controlled along the entire width and travels ina stable manner, Figure 7 and Figure 8.

For strip widths less than 1,300 mm, only three pairsof magnets are used even if five are available. Theouter magnets are not active, and the system auto-matically switches over to the three-pair arrangement,Figure 9.

Owing to the movable magnet pairs, the strip stabili-sation offers great flexibility in terms of strip form cor-rection during operation. On the whole, the movableouter magnets and the modular design guarantee thebest stabilisation for every standard strip size duringhot-dip galvanising.

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Inductive measurement and

fine adjustments

Below the magnets, a position measuring system isinstalled for each pair of magnets which continuouslymeasures the distance to the strip in a non-contactmanner, Figure 10. The strip position determined bythis sensor is compared continuously with the targetposition in a quick control loop. By means of the vari-ations in the current flowing through the stabilisationcoils, the magnetic attraction for the steel strip can becontrolled systematically in such a way that the stripmovements are equalised. A high cushioning of thestrip movements and a quiet strip travel can thus beguaranteed. The highly dynamic control system workswith a response time of 1 to 2 msec.

Unlike a large stationary magnetic unit, the movablemagnets arranged parallel to the strip width ensurethat the fine adjustments can always be made opti-mised to the strip edge also during production.

Figure 10. Below the magnet, position measuring systems are installed which measurethe distance to the strip in a non-contact manner (here in the workshop).

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Strip processing linesSMS SIEMAG

Increase in plant speed and reliability

With the use of DEMCO strip stabilisation, the line canbe operated at a higher speed. Furthermore, the sta-bilisation enhances the reliability and reduces therejects arising from surface defects. A smooth striptravel reduces the risk of contact between the stripand the air knives where the air knives could be damaged so severely that they need to be manuallycleaned or replaced.

The DEMCO system offers advantages with regard tostrip travel also in the production of GA strips. As theinduction furnace is arranged between the air knivesand the air cooling, the strip travel through the furnaceis often very turbulent. With a strip stabilisationinstalled upstream of the induction furnace, the striptravel through the furnace is smooth even at higher

speeds which results in a good quality of the GA product as a more uniform GA coating is produced atthe top and bottom side.

Saving on zinc

The amount of zinc saved when using the DEMCOsystem depends on the relevant strip width and speedas well as on the zinc coats to be applied. On average,1 to 2 g/m2 of zinc can be saved per strip side due tothe stabilisation. In case of strips on which a relativelythick zinc coat is applied, more than 3 g/m2 can in factbe saved.

By saving 1 g/m² zinc per strip side, it is possible tosave a total of 28.8 kg/h of zinc for a medium stripwidth of 1,500 mm and a medium speed of 160m/min. At an assumed production time of 6500 h per

Figure 11. Strip stabilisation on a FOEN air knife systemundergoing preassembly.

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year, the total savings is then more than 185 t.Depending on the price of zinc which has been fluctu-ating between 900 and 3500 €/t in the last five years,savings of 160,000 to 650,000 € can be attained in ayear by using the strip stabilisation.

By saving 2 g/m2 per strip side, the amount of zincsaved for these parameters is 370 t a year. Dependingon the price of zinc, this in turn amounts to savings of330,000 to more than 1,000,000 €.

Depending on the structure of the galvanising line andits implementation and depending on the price of zinc,the plant design and the zinc savings attained, thestrip stabilisation pays off between six months and 3years.

Maintenance and retrofitting

Due to the quick-clamping device, the electromagnetscan easily be removed for routine maintenance. In thisway the maintenance of the strip stabilisation in theworkshop can be performed on maintenance standsspecifically designed for this purpose. The distancebetween the magnet and the measuring sensor is alsoadjusted here. The linear movement functions of themagnets as well as the pneumatic quick-opening ofthe magnets can also be easily examined, Figure 11.

The strip stabilisation can be retrofitted for all FOEN airknives. A DEMCO strip stabilisation can therefore beintegrated in an existing galvanising line with a FOENair-knife system without any difficulty. On the basis ofprevious experienced and given the compatibility ofthe systems, excellent results can be expected incase of a retrofitting.

OPERATING EXPERIENCE AND PRO-

DUCTION RESULTS

Test phase

At first, various tests were carried out with a proto -type of the DEMCO strip stabilisation system on thepremises of SMS Elotherm in Remscheid. The desiredresults were obtained for a strip width of up to 1,100 mm at a strip speed of 60 m/min. Due to themaximum strip width of 1,100 mm, the prototype wasequipped with three pairs of magnet coils

Hysco

Since April 2007, a DEMCO prototype system hasbeen successfully in operation in the annealing andgalvanising line of HYSCO in Korea. Three pairs ofmagnets are installed here above the FOEN air knives.Depending on the strip width, the two outer magnetpairs are positioned at the strip edges while the middle magnet pair is aligned towards the strip centre.In this manner, the cross-bow in the strip is minimisedfor all strip widths and a vibration-free strip travelbetween the air knives is attained, Figure 12 and Fig-ure 13. The distance between the gap of the air knifeand the magnet centre is approx. 1,500 mm. It hasbeen proved both for GI and for GA steels that the useof electromagnetic strip stabilisation reduces over-coating.

It was possible to reduce the overcoating due to twodifferent effects. On the one hand, the strip vibrationamplitudes in the centre of the strip were reduced by40% and at the strip sides by 60%, so that the striptravel was considerably smoother. On the other hand,it was possible to eliminate the cross-bow effect dueto the optimum positioning of the outer magnets, thusenabling a much more uniform coat over the stripwidth. Consequently, for each strip the standard devi-ation for the coating was able to be reduced on averageby 1 to 3 g/m2.

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Strip processing linesSMS SIEMAG

Another positive effect that was ascertained was animproved strip travel in the induction furnace whenproducing GA strip, in particular at strip speeds of 140to 180 m/min. In this way, the use of strip stabilisationincreases the line capacity during the production ofhigh-grade GA steel strips.

Saving on zinc

To elucidate the zinc-saving potential, we wish to citethe example of the production of a GI strip in theannealing and galvanising line at Hysco where variousnon-destructive measurements of the coating thick-ness were carried out with and without strip stabilisa-tion, thus providing accurate information on the coatingthickness and the influence of strip stabilisation. Thetests were carried out during the production of a steelstrip with a width of 1,423 mm and a thickness of 0.66mm at a speed of 165 m/min. Both sides of the stripwere to be treated with a zinc coat of 140 g/m2. Thisvalue had to thus be maintained along the width andlength of the strip.

Without the help of strip stabilisation, an average of143.184 g/m2 of zinc had to be applied to the top and143.192 g/m2 to the bottom side of the strip in orderto obtain the desired coating. After the strip stabilisa-tion was activated, these values dropped to 141,860g/m2 and 141,920 g/m2 respectively. Consequently,1.324 g/m2 less of zinc were applied to the top and1.272 g/m2 less to the bottom side. All in all, the sav-ings amounted to 2.596 g/m2 of zinc.

In relation to these parameters, a savings of 0.5 kg ofzinc is obtained per ton of galvanised steel strip.

The improved coating value was measured down-stream of the coating zone using a coating weightgauge (distance of 128 m). With a time delay after acti-vating the stabilisation, the coatings on the top andbottom side showed drastically reduced amplitudesand an equalisation on both sides, Figure 14.

Figure 12. Hysco prototype line with 3-magnet strip stabilisation system – adjusted for narrow strip.

Figure 13. Hysco prototype line with 3-magnet strip stabilisation system – adjusted for wider strip.

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Wuppermann

Another DEMCO strip stabilisation is meanwhile inuse for testing purposes in the hot-strip galvanisingline at Wuppermann in the Netherlands. Here, the ver-sion with five pairs of magnets is being put into prac-tice for the first time. In this manner, extremely widestrips can also be optimised. For strip widths below1,300 mm, only the inner three pairs of magnets areused for the stabilisation, Figure 15.

This arrangement also produced excellent results.Especially for larger strip widths, five pairs of magnetscan bring about a considerable improvement by ensuring steady strip travel. Since the five pairs ofmagnet parallel to the strip width can be moved, aneffective arrangement can be adjusted for all stripwidths, and fine adjustments can be made to thisarrangement also during operation.

Figure 14. Measurement of the strip movement via inductors, of the activity of the electromagnetpairs and of the coating thickness with strip stabilisation activated and deactivated.

All in all, the positive results gained at the cold stripgalvanising line in Korea were able to be reproducedfor hot strip galvanising in the Netherlands. Due to theDEMCO strip stabilisation and the ensuing steadierstrip travel, less zinc has to be applied. Furthermore,the risk of standstills caused by turbulent strip travel isreduced. Owing to the design with 5 pairs of magnets,the travel of wide hot strip was also stabilised withpositive effects on production.

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Strip processing linesSMS SIEMAG

Figure 15. Strip stabilisation in use at Wuppermann.

CONCLUSION

The DEMCO system developed by SMS Siemag andFontaine for electromagnetic non-contact strip stabili-sation of the strip galvanising lines has proved itsworth from the design to the test phase and right upto the first operating experience. The steadier striptravel guaranteed by the system not only makes itpossible to save considerable amounts of zinc but alsoensures greater plant reliability, thereby reducing sur-face defects. Due to the quick response time of themodular structure and the movable pairs of magnets,favourable results can be obtained for all strip widthsand thicknesses both for hot strip and for cold strip.By means of a quick-clamping device, the stabilisationsystem can be easily installed on the knives and main-

tained. It can also be implemented without any diffi-culty in already existing galvanising lines with FOENair knives.

Thanks to the amount of zinc saved as a result of thestrip stabilisation, strip galvanising lines with an inte-grated DEMCO system are much more economical intheir production. Depending on the strip steel pro-duced, savings of 190 to 370 t/a of zinc are possible.This in conjunction with the zinc price produces annualsavings of 160,000 € per year with the mostfavourable zinc price of the last years to more than 1million € in the case of the highest zinc price in 2007.

“The information provided in this brochure contains a general description of the performance characteristics of the products concerned. The actual products may not always have thesecharacteristics as described and, in particular, these may change as a result of further developments of the products. The provision of this information is not intended to have and willnot have legal effect. An obligation to deliver products having particular characteristics shall only exist if expressly agreed in the terms of the contract.”

SMS SIEMAG AG

Strip processing lines division

Walder Straße 51-5340724 Hilden

Telefon: +49 (0) 211 881-5100Telefax: +49 (0) 211 881-5200

E-Mail: [email protected]: www.sms-siemag.com

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