Electrolytic tinning and chromium plating lines

Transcription

1 W7/304E Strip Processing Lines Electrolytic tinning and chromium plating lines

2 Electrolytic tinning and chromium plating lines They have to offer the utmost in productivity, product quality and eco-friendly operation today's electrolytic tinning and chromium plating lines. These two methods of surface treatment preserve and ensure a longer service life of products. Everywhere in the world, there is a growing market for durable, easy-care goods for industrial and private use. That explains the increasing demand for corrosion-resistant and simultaneously inexpensive steel strip for numerous applications. What steel strip processors require is a sustained, constant quality level with exactly defined properties of coatings and carrier materials. Furthermore, they expect quality optimization they can promise their customers as part of their marketing claims. To meet these high demands, SMS Demag has specifically improved its process and plant technology for manufacturing top-quality products. We supply electrolytic tinning and chromium plating lines that correspond to the highest standards of quality, precision and flexibility. Exceptionally large, our product range also ensures consistent quality. 2 3

3 Different plant concepts for different requirements of electrolytically tinned or chromium plated steel Whatever the process whether electrolytic tinning or chrome plating SMS Demag provides plant and process technology custom-made for each special application. This ensures that a large number of product variations can be manufactured at a high rate in on-line operations. The entire production process is monitored by inspection devices and geared to safe operation. Included here is continuous operation which guarantees high plant performance and product quality. Pay off reels Electrolytic tinning line Pay off reels Double cut shear Double cut shear Electrolytic chrome plating line Welding machine Entry bridle unit Common to the various types of plant is the fact that they can be extended in stages to adapt to the development of the market.these extension stages are already taken into account during initial planning. Seeing that extensive process technology is involved in electrolytic coating, we give priority to ecofriendly and energy-saving operation of the installations, to recycling and multiple use of media, to responsible waste disposal and economical use of energy. Side trimmer Welding machine Entry bridle unit Side trimmer Entry loop tower Entry loop tower There is a wealth of advice and practical assistance we offer our customers throughout planning, installation and production start-up. Whenever you require, we will advise you right from the very beginning, in planning production for the new plant, as well as informing you about alternative processes. It goes without saying that our experts team up with your personnel to take care of commissioning the entire system along with the various auxiliary equipment. Cleaning section Tension leveller Cleaning section Pickling section Tension leveller Tinning section Pickling section Strip marker Induction reflow unit Chrome plating section Post treatment section Electrolytic tinning lines These lines process steel strip made from soft, unalloyed, lowcarbon steel. First it is coldrolled, then the steel strip is cleaned, annealed and skinpassed to give it the necessary mechanical properties. To protect the material against corrosion it is electrolytically tinplated. Known as tinplate, this is a leading packaging material due to its many positive qualities. It is used for food and beverage cans, cans for chemical Oiler Exit loop tower Oiler Exit loop tower products, bottle caps and jar lids. Tinplate is very eco-friendly because it can be 100 % recycled. Depending on requirements, it is possible to produce steel strip in thicknesses of 0.12 to 0.6 mm and widths of 500 to 1,250 mm in various types of finish. Just like electrolytic galvanization, tin plating can provide all the tin layer thicknesses necessary for the various applications, either with equal or different thicknesses on the two strip sides. The range of plating weights is 1.1 to Surface inspection Surface inspection Exit bridle unit Instrument stand Exit bridle unit Instrument stand Snip shear Snip shear Tension reels Tension reels 15.1 g/m²/side. There is a large range within which the performance of the tinning plant to be installed can be adjusted to the market for tinplate. So far, SMS Demag has supplied electrolytic tinning plant with a production capacity of 350,000 tons/year and plant speeds of up to 660 m/minute. Electrolytic chromium plating lines They also process steel strip made from soft, unalloyed, lowcarbon steel. Here again, after cold rolling, the steel strip is cleaned, annealed and skinpassed to provide it with the required mechanical properties. The final product is manufactured in all kinds of hardnesses and tensile strengths depending on the type of application envisaged. It can be produced in a thickness range of 0.1 to 0.5 mm and widths of 500 to 1,250 mm in various types of treatment. To protect the strip against corrosion it is treated with a special electrolytic chromium plating process. Available here are chromium metal plating, with layer thicknesses between 30 and 140 mg/m²/side, and chromium oxide plating, applied in layers 7 to 27 mg/m²/side thick. There have already been several instances where packaging strip made of specially chromium plated material has replaced tinplate. It is possible to achieve plant speeds of up to 450 m/minute. 4 5

4 Combined electrolytic tinning and chromium plating line Economic considerations such as plant investments and overheads have led to a combination of tinning and chromium plating lines with two completely different plating processes united in one plant. This is achieved using one of two different arrangements, with the tinning process either in front of or behind the chromium plating process. Whichever process area is not required is bypassed and the material only passes through the process stages of the plant which are equally necessary for both processes e.g. precleaning and pickling. Electrolytic tinning and chromium plating Here are the processes used for electrolytic tinning, along with the appropriate electrolytes: Ferrostan electrolyte, PSA (phenolsulfonic acid) Ronastan electrolyte, MSA (methanesulfonic acid) Halogen electrolyte Stannous fluoborate eletrolyte Line speed [m/min] Tinning speed curves Line parameter No. of cells 10 Total current capacity 180,000 A Max. current density 40 A/dm 2 Anode length 1,500 mm Limited max. current density 40 A/dm 2 Coating weight 1,12/1,12 g/m 2 + 2,8/2,8 g/m 2 Coating weight 5,6 / 5,6 Coating weight 8,4 / 8,4 Coating weight 11,2/11,2 g/m Strip width [mm] Most systems use the Ferrostan process with PSA electrolytes, but due to its advantages the MSA electrolyte, available on the market for only a few years, is gaining in importance. What makes this electrolyte more attractive is its higher conductivity. This results in lower energy costs and a flex- Sectional view of tinning tank Tin anodes Conductor rolls Electrolyte Sink roll ible operating mode through a larger current density area. Largely bio-degradable, the electrolyte can also be disposed of in an eco-friendly way. There is a difference between one and two-stage chromium plating processes. Both create a metal chromium layer with a Steel strip Tin anode - + Enlarged detail layer of chromium oxide on top. Using the two-stage process enables manufacturers to vary the relationship between the two layers. This process is usually used for thicker platings. Depending on the application purpose of the material, various chromium electrolytes with fluoride or sulfate additives are used. Vertical tinning cells or tanks have become the preferred types for tinning plants using acidic electrolytes. It is usual to fit each tank with 4,500 AMP to 10,000 AMP rectifiers for each strip side. Plants with high rates of production (approx. 250,000 tons/ year) require 10 to 15 tanks. The tinning speed curves show the attainable line speed (m/min) The strip is conveyed vertically through the tank over a conductor roll with a depending on the desired coating weight (g/m 2 ) and the hold-down roll and a rubber-coated sink roll. strip width (mm). Each tank contains four adjustable anode stands with soluble anodes, while separate rectifiers make it possible to plate the strip differently at the same time. Pay off reels Double cut shear Welding machine Entry bridle unit Entry loop tower Tension leveller By pass for chrome plating section Strip marker Oiler Exit loop tower Surface inspection Instrument stand Side trimmer Cleaning section Pickling section Tinning section Induction reflow unit Post treatment section Chrome plating section Exit bridle unit Snip shear Tension reels Combined electrolytic tinning and chromium plating line 6 7

5 Individual plant components combined to create successful overall systems Electrolytic tinning and chromium plating lines Pay off and tensions reel units Loop accumulators Tension leveler Essential for continuous operations, two coilers are provided at the feed and discharge ends. The coils are wound with or without sleeves, and lifting cars lift them onto the mandrel of the coiler. Depending on the weight of the coils, both coilers are equipped with or without mandrel support bearings. A changing device can be used to put on or take off the sleeves. To ensure continuous strip travel in the process section even while the coils are being changed, vertical loop accumulators are provided. They are fitted with guide rolls and steering rolls, some of them driven, which keep the strip perfectly on track. To meet today's demands on the flatness of the finished product, a tension leveler is necessary. an extra alkaline treatment. Electrolysis then takes place with higher current densities. Degreasing is followed by cascade rinsing to prepare the strip for subsequent pickling. Pickling section with rinse SMS Demag manufactures tension levelers for single- and doublereduced material. They ensure excellent strip flatness through the arrangement of the bending and leveling units. Arranged in cassette form, the bending and leveling units can be changed quickly during operation. Moreover, there are bridle roll sets with a compact drive in front of and behind the tension leveler, which increase and decrease the tension of the strip. Side trimmer Recommended for rapid width and program changing as well as blade changing during operation is a set of rotary side trimmer with two pairs of knife heads. 8 SMS Demag makes trimming shears with driven top and bottom knives for a minimum strip thickness of 0.1 mm, especially for tin plate. There is a choice between a scrap press and a scrap baller to deal with the trimming scrap. Both devices work automatically. Electrolytic pickling at room temperature is carried out to remove any corrosion residues which may be present as well as any additives from the degreasing process. That also activates the strip ready for subsequent plating. Depending on the plating process, both hydrochloric and sulfuric acid are used. The type of acid is matched to the coating electrolytes. It is advisable, for instance, to use sulfuric acid with PSA or MSA electrolytes. Emerging from the pickling process, the strip is then washed in a cascade rinse. Cleaning section with rinse The single-reduced material is degreased electrolytically in an alkaline solution. To clean double-reduced material, this stage is preceded by 9 10

6 Tinning section Now the pretreated strip is introduced into a vertical tinning section. SMS Demag is expert in applying the technology for tinning processes using soluble anodes for Ferrostan, fluoborate and methane sulfone electrolytes. The anodes can be handled manually or with an automated anode shifting device. Ideal for ultrathin plating is the parallel anode arrangement SMS Demag builds. This ensures an optimal, even layer distribution. To provide the electrolysis current, one rectifier for each anode stand is used in order to achieve an exact layer thickness. The rectifiers are connected to the tin anodes and the current rolls. Adjusting the current, which is calculated using Faraday's Law, regulates the deposit of tin depending on the strip width and speed. It is an added advantage that both strip surfaces can be plated with equal or different layer thicknesses. There are recovery tanks arranged after the tinning process to remove electrolyte residues from the strip. This is effected by evaporation, so hardly any electrolyte is lost. To finish this stage, the strip is dried and marked according to the layer thickness. SMS Demag performs this process at high strip speeds of up to 660 m/minute. Chromium plating section An alternative to tin plating is chromium plating the strip. Typical of combined plant, our machines are designed in such a way that the material bypasses the tinning section to be chromium plated. They can also be built exclusively as a chromium plating line, with chromium plating taking place after strip cleaning and pickling. Then the pretreated strip is fed into the chromium plating section which also works with vertical cells. To provide the electrolysis current, rectifiers connected to the nonsoluble anodes and the current rolls are used. Adjusting the current regulates the chromium deposits that consist of chromium metal and oxide, depending on the strip width and speed. Both sides receive the same chromium plating thickness. Emerging from the chromium plating tanks, the strip is cleaned of chromium residues in the cascade rinser. Finally, the strip is dried and fed into the electrostatic oiling stage. Reflow unit To achieve a shiny surface the layer of tin is melted on, which also affects the iron-tin alloy layer. Melting is performed with electric current in one of two ways, or using a combination of both: Resistance melting The current is transferred through the two conductor rolls which are supplied with alternating current. The constant power necessary for an even melting effect is achieved by controlling the voltage. Inductive melting Melting with high-frequency induction coils offers the advantages of better efficiency and therefore lower energy costs, prevention of surface imperfections (woodgrain) and flexible operation when layer thickness and strip speed change. The strip is quenched in a water tank after melting. For some products melting is not required

7 Instrument stand Now available are a strip thickness measuring device and a hole searching device to check and ensure an excellent surface quality. This is achieved, even at high process speeds, thanks to the automatic surface inspection system. Added to this is a camera system installed along the inspection route. Posttreatment To protect the tinned surface, the strip is treated electrolytically or in an immersion process with a sodium dichromate solution. Called passivation, this stage is followed by washing and drying. Shearing line Now the measuring devices inspect the strip to determine its thickness and to detect any tiny holes. Then dividing shears or rotary shears cut the strip into panels, ready for stacking. The sorting and stacking system is fitted with three to four stacking units in which the various grades can be stored and forwarded. However, if the entire material is to be processed into panels, the shearing line can be coupled to the preceding coating line. That makes the exit accumulator and winding group in the preceding line superfluous, but it also means the shearing line limits the speed that can be achieved in the process section

8 Worldwide references 16 17

9 SMS Demag Aktiengesellschaft Strip Processing Lines Division Postfach Düsseldorf Germany Tel. ( ) Fax ( ) bewe@sms-demag.de Internet: /07/2000 A Printed in Germany