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AUTHORS: Matthias Kretschmer and Holger BehrensSMS Demag AG

Hot-dip galvanising lines use cold rolled steel stripas entry materials which are burdened with rolling

emulsion, abraded iron particles and other dirt, ofapproximately 500 mg/m2 per side, resulting from therolling process. Steel strip burdened in this way has tobe relieved of these residues before further processingsuch as hot dip coating.

Essential customer demands for such sheet productsare good machine process capability in secondarymanufacturing processes and long term preservation ofthe final product. These qualities are fundamentallydetermined by the functional layer applied to the metalsurface. The compound between the functional layer(e.g. zinc) and the steel strip surface is primarily basedon the adhesive strength within the boundary surface.Surface deposits as described above discourageadhesion, hence, as a consequence the layer is onlyirregularly applied or is easily removed undermechanical pressure, and the functional layer is not ableto adequately fulfil its purpose.

In order to remove these surface contaminants beforefurther processing, the strip is cleaned in a multi-stagestrip cleaning section which is located after the entrysection in a continuous galvanising line (see Fig.1). Acombination of alkaline spray cleaning and brushing forthe removal of surface deposits, electrolytic cleaning fordeep pore cleaning, and a final multi-stage rinsingoperation including a second brushing operation withfully demineralised water are used in this section.Cleaning agents are aqueous solutions mainly designedfrom an alkaline basis with tensides and phosphates.

SMS DEMAG INSTALLATIONS These have a cleaning section which has both vertical spraydegreasing and vertical electrolytic degreasing whichenables a long effective treatment time within a shortfacility length. Figure 2 shows the principle layout of a high-performance cleaning section in a galvanising line. There arefour key areas; spray cleaning, brushing, electrolytic cleaningand final rinse system. These will now be described.

Hot dip galvanising lines require the base strip to be ultra clean to ensure the coatings adhereproperly and perform well in service. Conventional high efficiency SMS DEMAG strip cleaning unitsare based on four key areas; spray cleaning, brushing, electrolytic cleaning and final rinse system. Anew development involving high pressure and ultrasonic methods to remove surface contaminantshas been developed which is cheaper and uses less energy and consumables.

Strip cleaning technology

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Spray cleaning section In the spray degreasing sectionthe strip is intensively sprayed with a hot alkali cleaningagents in order to heat it up to the required temperatureand to remove coarse contamination. The effectivemechanical cleaning effect results from the nozzlearrangement on the pipes, spray angle and heavyspraying which results in optimal energy utilisation of thespray jets. Well balanced pressurisation of the mediumonto the strip is enabled by the optimal selection of thenozzle characteristics. Nozzle blockages are prevented bythe use of tongue nozzles. The nozzle pipes are laterallyextendable and can be exchanged during operation andthe applied setting ensures that the adjusted spray angleis retained after changing. These features result in a highmechanical cleaning quality combined with lowevaporation of the fluid, low maintenance and low energylosses.

r Fig.1 Principle layout of a Hot-Dip Galvanising Line

r Fig.2 SMS DEMAG high-performance strip cleaning section

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equipment for fluid maintenance, especially the suitableinstallation of magnetic chain filters, ensures maximumefficiency of the strip cleaning.

The highlights of the SMS DEMAG strip cleaningtechnology by can be summarised as:` Spraying systems with tongue type spray nozzles to

avoid header and nozzle clogging - achieving a highmechanical cleaning effect, uniform spraying dis-tribution and reduced atomisation of the liquid

` Rugged and vibration-free brush machines ensure acalm operation with no chatter marking whichcontributes to long brush service life

` Electrolytic cleaning gives an excellent cleaningefficiency enabled by high current densities,optimised electrolyte routing, minimised processinherent foaming and optimum degassing of thehydrogen that results from the electrolysis

` Cascading of rinsing water for optimised rinsingwater consumption, reduction in the amount ofwaste water required, and continuous qualitymonitoring for controlled fresh water addition

NEW DEVELOPMENTS IN STRIP CLEANINGTECHNOLOGY The combination of good customer contacts, experiencefrom running production lines, and working with itspartner ThyssenKrupp Stahl in the DortmunderOberflächen Centrum (DOC), enables SMS DEMAG toindependently optimise existing process technologies,and develop new technologies and new productapplications. The DOC is equipped with an experimentalplant for testing forward-looking processing methodsand new processes (see Fig. 3). The potential forinnovations in conventional cleaning technologies lay in:` Energy reduction ` Minimising chemical use` Lower maintenance costs

Based on these objectives SMS DEMAG has developednew technologies in the DOC using a combination ofultrasonics and high pressure cleaning which removesthe need for cleaning brushes and electrolytic cleaningsections, with significant reductions in energy, waterand cleaning agent consumption. The system alsorequires minimal safety requirements and exhaust, andhas a simpler process tank design.

Figure 4 shows the high pressure section of the line atDOC.

Technology of high pressure cleaning Inconventional brush cleaning the mechanical contact of

Brush machine Brush degreasing removescontamination on the strip surface by means of severalrotating brushing units. The brush machines arecharacterised by their robust construction and runningsmoothness such that material damage and traces on thestrips are prevented. The service life of the brushes is high,resulting in low spare parts consumption and lowmaintenance costs. A key feature of the machines is thedesign of the brushing load system. Load is applied onthe brushing roll neck by bellow cylinders which put theload centrally onto the bearings of the brushing rolls. Thisdesign prevents the bending of the brushing rolls,guarantees smooth operation of the machines, reduceswear of bearings and thus chatter marks are avoided. Thebearings of brushes and rolls are sealed and shortchanging times (approximately 30 minutes per roll) forbrushing roll and counter roll contribute to the easymaintenance regime.

Electrolytic cleaning Electrolytic degreasing removescontamination which is deeper into the surfacetopography by means of direct blistering on the stripsurface. The system has an excellent cleaningperformance using high current density and an optimalelectrolyte guide way. The flow path of the cleaningagent from downwards to upwards through the verticalprocess tank supports the degassing of hydrogen andoxygen produced, avoids formation of gas pockets in thesystem and minimises foam production, allowingoperation of the electrolytic cleaning process with highcurrent densities up to 20 A/dm2. The construction andperformance of the electrodes enables minimum electricresistor losses and the vertical strip flow ensures astable, defined position of the strip between theelectrodes, without sagging.

Cascade rinsing section with brush machine andstrip dryer Finally a brush rinse removes the remainingsurface film and the strip surface is rinsed with hotdemineralised water in order to completely remove thecleaning solvent. The rinsing sections are arranged incascades and combine modest waste water losses andfresh water consumption with an excellent flushingeffect. The last rinsing cascade is operated withdemineralised water. The process tank is equipped withflat-jet nozzles in the spraying header and runs with aprocess pressure up to 1.0 MPa to achieve excellentrinsing.

The combination of strip edge blow-off and strip dryerafter strip cleaning guarantees a complete and costefficient drying process across the full strip width, sopreventing the spreading of fluids to the next process.

Cascade guiding of the cleaning medium and special

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the bristles with the strip surface causes wear of thebrushes, which leads to ongoing costs. The new removalprocess with high energy water headers in the highpressure cleaning section does not require directmechanical contact and is, therefore, completely wear free.

Electrolytic degreasing requires costly processcontainers and produces oxygen and hydrogen gas as aprocess by-product. With the omission of the electro-chemical reaction using ultrasonic cleaning the designof the process container is much simpler. The emission-free, mechanical removal process is not subject tospecial requirements for the container exhaust and is,therefore, not safety critical.

High pressure cleaning combines active surfacecleaning by tensides in the strip cleaning agent and themechanical removal by the high kinetic energy of thehot fluid jets (see Fig. 5). Loose surface layers simply getwashed away, whereas more stable layers initially getbroken up by the kinetic energy then also washed away.The tensides in the additive strip cleaner partly support

the refining process. The essential function of thetensides is the bonding of removed disposals in the fluidsuch that the removed film is trapped within the fluidphase and does re-contact with the strip surface, thuspreventing re-greasing or re-contamination.

The cost efficient use of high pressure cleaningrequires the continuous circulation of the cleaningmedium through a high pressure pump. In centrifugal orpiston pumps even small gas quantities in the mediumlead to cavitation in the pump chamber and cause thebreakdown of the pump within a short time. The chosenpump package is insensitive to air / foam (gas part < 10Vol. %) in the pumping fluid. The central component inthe high pressure cleaning process is the pitot pipepump (see Fig. 6). The medium enters the pump via thesuction socket and is brought to a very high rotaryvelocity in the rotating pump section. This (kinetic)rotary energy is then transformed into (potential)pressure energy in the pipe, resulting in a fluid pressureabove 10.0 MPa at the pressure socket.

Ultrasonic cleaning Ultrasonic cleaning combines thecleaning of the surface by active processes on the surfacewith the mechanical removal by kinetic energy ofimploding gas bubbles. The ultrasonic vibrations lead tolocal pressure fluctuation in the fluid chamber (see Fig. 7).In the low pressure regions tiny cavitation bubblesdevelop resulting from the pressure of the dissolved gasor the steam pressure of the fluid which then rapidly

r Fig.3 Pilot plant for metal strips at theDortmunderOberflächen Centrum(DOC)

r Fig.4 High pressure and ultrasonic cleaning at the strip pilot lineat the DOC

r Fig.5 High pressure cleaning principle r Fig.6 Pitot pipe high pressure pump

r Fig.7 Principle of ultrasonic cleaning

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implode. The shock waves which are induced by theseimplosions - especially on the strip surface - release thecontaminants from the strip surface. Chemical additiveswhich support aqueous ultrasonic cleaning are added inlow quantities, but are important in the choice ofultrasonic performance and the working frequency.Depending on the field of application ultrasonic cleaningproduces high quality and homogeneous cleaning thatcannot be achieved by any other cleaning process.

COMBINED HIGH PRESSURE AND ULTRASONICCLEANING TECHNOLOGIESThe efficiency and quality of the combined highpressure and ultrasonic cleaning technology has beenexamined in different tests at the strip pilot line of theDOC and at one HDGL at ThyssenKrupp Stahl AG.Cleaning in the pilot plant at the DOC essentiallyconsists of a pair of high pressure spraying headers andan ultrasonic cleaning tank. Detailed tests haveconfirmed that the cleaning efficiency for differentkinds of steel strip is higher than a conventional stripcleaning section.

The degree of cleaning of rolling emulsion and ironabrasion compared to conventional brush andelectrolytic cleaning is shown in figure 8. A high-strength multi-phase steel was used as substrate becausethis type of steel is hard to clean on occasions.

Highly efficient conventional SMS DEMAG cleaningsections can achieve a maximum cleaning efficiency ofup to 95 %. The test on the strip pilot line with the highpressure and ultrasonic cleaning systems have shownthat a further 3 to 4% improvement could be achieved.

High pressure cleaning provides a good coarsecleaning then contamination which is trapped deeper inthe topography of the strip surface is loosened by theultrasonic cleaning and washed away.

The efficiency of the high pressure cleaning

technology was effectively confirmed in a first industrialapplication test. The existing HDGL strip cleaninginstallation at ThyssenKrupp Stahl AG was equippedwith a high pressure module, however, it was operatedwith cold industrial water and without the addition ofcleaning media. The kinetic energy alone of the highpressure water jets was sufficient to result in asignificant improvement of the cleaning efficiency (seeFig. 9). At the same time it was confirmed that anincrease of work pressure above a certain value does notnecessarily imply an improvement in cleaning. Theincrease of jet pressure from 8.0 MPa to 10.0 MPaimplies a provable improvement of the cleaning quality,but a further rise to 12.0 MPa has no further benefits.The working pressure of the high pressure module canbe adapted by the design of the spraying header andthe choice of spraying systems optimal to theenvironmental conditions.

SUMMARYIn comparison with conventional strip cleaning designsequipped with brush and electrolytic degreasingsystems, the new line concept using ultrasonic and highpressure cleaning technology combined with the wellproven mechanical and process equipment leads to thefollowing advantages:` No abrasive cleaning required` Shortening of line length by up to 25%` Approximately 10% lower investment costs` Less energy expenditure due to omission of

electrolytic cleaning ` Uses the existing plant operating cycles

Based on the discussed test results the followingcleaning concept was developed and focused upon (seeFig. 10). The arrangement of the strip cleaning sectionstays, in general, the same but is modified with a high

r Fig.8 Cleaning efficiency of high pressure and ultrasonic cleaningr Fig.9 Cleaning efficiency of the high-pressurecleaning module in a HGDL

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With the use of the high pressure cleaning technologyas substitution for mechanical brush cleaning, operationalcosts for spare brushes ceases. The compact design of theprocess technology opens new opportunities for thedesign and construction of space-saving strip cleaningsec-tions in strip processing lines (see Fig.10).

Additionally it was shown that due to the increasedefficiency of strip cleaning with the new cleaningprocesses savings in cleaning chemicals can also beachieved. The environmentally important components(tensides, phosphates, etc.) in the cleaning agent itselfcan be significantly reduced and the cleaning of thewaste water can be operated at lower cost and energy use.

Matthias Kretschmer and Holger Behrens are with SMSDemag AG, Germany

Holger.Behrens@sms-demag.de

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pressure and ultrasonic cleaning section. The brushdegreasing and brush rinsing are substituted by highpressure spray headers and the electrolytic degreasingsection is replaced by an ultrasonic cleaning section.

Ultrasonic cleaning, in comparison to electrolyticcleaning, uses less energy to achieve good cleaningresults. The compact design of the process technologiesopens up new opportunities for the construction anddesign of space-saving high pressure cleaning systemsin strip processing lines. High pressure cleaningtechnology eliminates the costly use and maintenanceof brushes.

The results have shown that high pressure andultrasonic systems can be used for the cleaning of steelstrip before the surface refinement process with goodsuccess. The research projects conducted intominimising environmental pollution resulted inimprovement to the product and a reduction inproduction costs, adding major benefit.

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r Fig.10 SMS DEMAG conventional and high pressure/ultrasonic designs

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