Hot-dip galvanising lines use cold rolled steel strip

Strip cleaning technology Hot dip galvanising lines require the base strip to be ultra clean to ensure the coatings adhere properly and perform well in service. Conventional high efficiency SMS DEMAG strip cleaning units are based on four key areas; spray cleaning, brushing, electrolytic cleaning and final rinse system. A new development involving high pressure and ultrasonic methods to remove surface contaminants has been developed which is cheaper and uses less energy and consumables. AUTHORS: Matthias Kretschmer and Holger Behrens SMS Demag AG Hot-dip galvanising lines use cold rolled steel strip as entry materials which are burdened with rolling emulsion, abraded iron particles and other dirt, of approximately 500 mg/m 2 per side, resulting from the rolling process. Steel strip burdened in this way has to be relieved of these residues before further processing such as hot dip coating. Essential customer demands for such sheet products are good machine process capability in secondary manufacturing processes and long term preservation of the final product. These qualities are fundamentally determined by the functional layer applied to the metal surface. The compound between the functional layer (e.g. zinc) and the steel strip surface is primarily based on the adhesive strength within the boundary surface. Surface deposits as described above discourage adhesion, hence, as a consequence the layer is only irregularly applied or is easily removed under mechanical pressure, and the functional layer is not able to adequately fulfil its purpose. In order to remove these surface contaminants before further processing, the strip is cleaned in a multi-stage strip cleaning section which is located after the entry section in a continuous galvanising line (see Fig.1). A combination of alkaline spray cleaning and brushing for the removal of surface deposits, electrolytic cleaning for deep pore cleaning, and a final multi-stage rinsing operation including a second brushing operation with fully demineralised water are used in this section. Cleaning agents are aqueous solutions mainly designed from an alkaline basis with tensides and phosphates. r Fig.1 Principle layout of a Hot-Dip Galvanising Line Spray cleaning section In the spray degreasing section the strip is intensively sprayed with a hot alkali cleaning agents in order to heat it up to the required temperature and to remove coarse contamination. The effective mechanical cleaning effect results from the nozzle arrangement on the pipes, spray angle and heavy spraying which results in optimal energy utilisation of the spray jets. Well balanced pressurisation of the medium onto the strip is enabled by the optimal selection of the nozzle characteristics. Nozzle blockages are prevented by the use of tongue nozzles. The nozzle pipes are laterally extendable and can be exchanged during operation and the applied setting ensures that the adjusted spray angle is retained after changing. These features result in a high mechanical cleaning quality combined with low evaporation of the fluid, low maintenance and low energy losses. a SMS DEMAG INSTALLATIONS These have a cleaning section which has both vertical spray degreasing and vertical electrolytic degreasing which enables a long effective treatment time within a short facility length. Figure 2 shows the principle layout of a highperformance cleaning section in a galvanising line. There are four key areas; spray cleaning, brushing, electrolytic cleaning and final rinse system. These will now be described. r Fig.2 SMS DEMAG high-performance strip cleaning section 191

Brush machine Brush degreasing removes contamination on the strip surface by means of several rotating brushing units. The brush machines are characterised by their robust construction and running smoothness such that material damage and traces on the strips are prevented. The service life of the brushes is high, resulting in low spare parts consumption and low maintenance costs. A key feature of the machines is the design of the brushing load system. Load is applied on the brushing roll neck by bellow cylinders which put the load centrally onto the bearings of the brushing rolls. This design prevents the bending of the brushing rolls, guarantees smooth operation of the machines, reduces wear of bearings and thus chatter marks are avoided. The bearings of brushes and rolls are sealed and short changing times (approximately 30 minutes per roll) for brushing roll and counter roll contribute to the easy maintenance regime. Electrolytic cleaning Electrolytic degreasing removes contamination which is deeper into the surface topography by means of direct blistering on the strip surface. The system has an excellent cleaning performance using high current density and an optimal electrolyte guide way. The flow path of the cleaning agent from downwards to upwards through the vertical process tank supports the degassing of hydrogen and oxygen produced, avoids formation of gas pockets in the system and minimises foam production, allowing operation of the electrolytic cleaning process with high current densities up to 20 A/dm 2. The construction and performance of the electrodes enables minimum electric resistor losses and the vertical strip flow ensures a stable, defined position of the strip between the electrodes, without sagging. Cascade rinsing section with brush machine and strip dryer Finally a brush rinse removes the remaining surface film and the strip surface is rinsed with hot demineralised water in order to completely remove the cleaning solvent. The rinsing sections are arranged in cascades and combine modest waste water losses and fresh water consumption with an excellent flushing effect. The last rinsing cascade is operated with demineralised water. The process tank is equipped with flat-jet nozzles in the spraying header and runs with a process pressure up to 1.0 MPa to achieve excellent rinsing. The combination of strip edge blow-off and strip dryer after strip cleaning guarantees a complete and cost efficient drying process across the full strip width, so preventing the spreading of fluids to the next process. Cascade guiding of the cleaning medium and special equipment for fluid maintenance, especially the suitable installation of magnetic chain filters, ensures maximum efficiency of the strip cleaning. The highlights of the SMS DEMAG strip cleaning technology by can be summarised as: ` Spraying systems with tongue type spray nozzles to avoid header and nozzle clogging - achieving a high mechanical cleaning effect, uniform spraying distribution and reduced atomisation of the liquid ` Rugged and vibration-free brush machines ensure a calm operation with no chatter marking which contributes to long brush service life ` Electrolytic cleaning gives an excellent cleaning efficiency enabled by high current densities, optimised electrolyte routing, minimised process inherent foaming and optimum degassing of the hydrogen that results from the electrolysis ` Cascading of rinsing water for optimised rinsing water consumption, reduction in the amount of waste water required, and continuous quality monitoring for controlled fresh water addition NEW DEVELOPMENTS IN STRIP CLEANING TECHNOLOGY The combination of good customer contacts, experience from running production lines, and working with its partner ThyssenKrupp Stahl in the Dortmunder Oberflächen Centrum (DOC), enables SMS DEMAG to independently optimise existing process technologies, and develop new technologies and new product applications. The DOC is equipped with an experimental plant for testing forward-looking processing methods and new processes (see Fig. 3). The potential for innovations in conventional cleaning technologies lay in: ` Energy reduction ` Minimising chemical use ` Lower maintenance costs Based on these objectives SMS DEMAG has developed new technologies in the DOC using a combination of ultrasonics and high pressure cleaning which removes the need for cleaning brushes and electrolytic cleaning sections, with significant reductions in energy, water and cleaning agent consumption. The system also requires minimal safety requirements and exhaust, and has a simpler process tank design. Figure 4 shows the high pressure section of the line at DOC. Technology of high pressure cleaning In conventional brush cleaning the mechanical contact of 192

r Fig.3 Pilot plant for metal strips at the DortmunderOberflächen Centrum(DOC) r Fig.4 High pressure and ultrasonic cleaning at the strip pilot line at the DOC r Fig.5 High pressure cleaning principle r Fig.6 Pitot pipe high pressure pump r Fig.7 Principle of ultrasonic cleaning the bristles with the strip surface causes wear of the brushes, which leads to ongoing costs. The new removal process with high energy water headers in the high pressure cleaning section does not require direct mechanical contact and is, therefore, completely wear free. Electrolytic degreasing requires costly process containers and produces oxygen and hydrogen gas as a process by-product. With the omission of the electrochemical reaction using ultrasonic cleaning the design of the process container is much simpler. The emissionfree, mechanical removal process is not subject to special requirements for the container exhaust and is, therefore, not safety critical. High pressure cleaning combines active surface cleaning by tensides in the strip cleaning agent and the mechanical removal by the high kinetic energy of the hot fluid jets (see Fig. 5). Loose surface layers simply get washed away, whereas more stable layers initially get broken 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 the tensides is the bonding of removed disposals in the fluid such that the removed film is trapped within the fluid phase and does re-contact with the strip surface, thus preventing re-greasing or re-contamination. The cost efficient use of high pressure cleaning requires the continuous circulation of the cleaning medium through a high pressure pump. In centrifugal or piston pumps even small gas quantities in the medium lead to cavitation in the pump chamber and cause the breakdown of the pump within a short time. The chosen pump package is insensitive to air / foam (gas part < 10 Vol. %) in the pumping fluid. The central component in the high pressure cleaning process is the pitot pipe pump (see Fig. 6). The medium enters the pump via the suction socket and is brought to a very high rotary velocity in the rotating pump section. This (kinetic) rotary energy is then transformed into (potential) pressure energy in the pipe, resulting in a fluid pressure above 10.0 MPa at the pressure socket. Ultrasonic cleaning Ultrasonic cleaning combines the cleaning of the surface by active processes on the surface with the mechanical removal by kinetic energy of imploding gas bubbles. The ultrasonic vibrations lead to local pressure fluctuation in the fluid chamber (see Fig. 7). In the low pressure regions tiny cavitation bubbles develop resulting from the pressure of the dissolved gas or the steam pressure of the fluid which then rapidly a 193

r Fig.8 Cleaning efficiency of high pressure and ultrasonic cleaning r Fig.9 Cleaning efficiency of the high-pressure cleaning module in a HGDL implode. The shock waves which are induced by these implosions - especially on the strip surface - release the contaminants from the strip surface. Chemical additives which support aqueous ultrasonic cleaning are added in low quantities, but are important in the choice of ultrasonic performance and the working frequency. Depending on the field of application ultrasonic cleaning produces high quality and homogeneous cleaning that cannot be achieved by any other cleaning process. COMBINED HIGH PRESSURE AND ULTRASONIC CLEANING TECHNOLOGIES The efficiency and quality of the combined high pressure and ultrasonic cleaning technology has been examined in different tests at the strip pilot line of the DOC and at one HDGL at ThyssenKrupp Stahl AG. Cleaning in the pilot plant at the DOC essentially consists of a pair of high pressure spraying headers and an ultrasonic cleaning tank. Detailed tests have confirmed that the cleaning efficiency for different kinds of steel strip is higher than a conventional strip cleaning section. The degree of cleaning of rolling emulsion and iron abrasion compared to conventional brush and electrolytic cleaning is shown in figure 8. A highstrength multi-phase steel was used as substrate because this type of steel is hard to clean on occasions. Highly efficient conventional SMS DEMAG cleaning sections can achieve a maximum cleaning efficiency of up to 95 %. The test on the strip pilot line with the high pressure and ultrasonic cleaning systems have shown that a further 3 to 4% improvement could be achieved. High pressure cleaning provides a good coarse cleaning then contamination which is trapped deeper in the topography of the strip surface is loosened by the ultrasonic cleaning and washed away. The efficiency of the high pressure cleaning technology was effectively confirmed in a first industrial application test. The existing HDGL strip cleaning installation at ThyssenKrupp Stahl AG was equipped with a high pressure module, however, it was operated with cold industrial water and without the addition of cleaning media. The kinetic energy alone of the high pressure water jets was sufficient to result in a significant improvement of the cleaning efficiency (see Fig. 9). At the same time it was confirmed that an increase of work pressure above a certain value does not necessarily imply an improvement in cleaning. The increase of jet pressure from 8.0 MPa to 10.0 MPa implies 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 can be adapted by the design of the spraying header and the choice of spraying systems optimal to the environmental conditions. SUMMARY In comparison with conventional strip cleaning designs equipped with brush and electrolytic degreasing systems, the new line concept using ultrasonic and high pressure cleaning technology combined with the well proven mechanical and process equipment leads to the following 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 following cleaning concept was developed and focused upon (see Fig. 10). The arrangement of the strip cleaning section stays, in general, the same but is modified with a high 194

r Fig.10 SMS DEMAG conventional and high pressure/ultrasonic designs pressure and ultrasonic cleaning section. The brush degreasing and brush rinsing are substituted by high pressure spray headers and the electrolytic degreasing section is replaced by an ultrasonic cleaning section. Ultrasonic cleaning, in comparison to electrolytic cleaning, uses less energy to achieve good cleaning results. The compact design of the process technologies opens up new opportunities for the construction and design of space-saving high pressure cleaning systems in strip processing lines. High pressure cleaning technology eliminates the costly use and maintenance of brushes. The results have shown that high pressure and ultrasonic systems can be used for the cleaning of steel strip before the surface refinement process with good success. The research projects conducted into minimising environmental pollution resulted in improvement to the product and a reduction in production costs, adding major benefit. With the use of the high pressure cleaning technology as substitution for mechanical brush cleaning, operational costs for spare brushes ceases. The compact design of the process technology opens new opportunities for the design and construction of space-saving strip cleaning sec-tions in strip processing lines (see Fig.10). Additionally it was shown that due to the increased efficiency of strip cleaning with the new cleaning processes savings in cleaning chemicals can also be achieved. The environmentally important components (tensides, phosphates, etc.) in the cleaning agent itself can be significantly reduced and the cleaning of the waste water can be operated at lower cost and energy use. Matthias Kretschmer and Holger Behrens are with SMS Demag AG, Germany Holger.Behrens@sms-demag.de 195