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Process routes

From hot strip to packaging steel

Hot rolled strip is the base material for the production of packaging steel. By cold rolling, it is brought to the required thickness and then finished by applying a coating of tin or chromium by means of an electrolytic process.


The exact process route can be seen from the diagram. To obtain detailed information on the various production steps, please click on the graphic. A window will pop up where you will be able follow the various stages of the process with the mouse.

Vom Warmband zum Weißblech
Hot rolled strip

Hot rolled steel strip is the raw material for the production of tinplate. It is supplied to the Rasselstein works harbor at Andernach in large coils weighing up to 27 tons. The coiled strip, also referred to as hot rolled strip or hot wide strip, measure about 2 - 3 mm in thickness.

Pickling in an acid bath

Processing begins with the pickling of the hot rolled strip. The aim is to remove the iron oxide layer formed during hot rolling. To achieve this, the material is temporarily joined by welding into one endless ribbon. In this form it passes a succession of four tanks containing sulfuric acid. After pickling, the strip is rinsed, dried, edge-trimmed, oiled and rewound into coils weighing up to 27 tons.

Cold rolling on a tandem mill

Here the pre-treated hot rolled strip is rolled to the final thicknesses of between 0.12 and 0.49 mm in one of the two tandem cold rolling mills. Each of these tandem mills consists of several stands equipped with a four-high arrangement of rolls - one large backup roll at the top and bottom and two smaller work rolls in the middle between which the strip passes. The roll gap is lubricated with a palm oil/water mixture to permit a reduction by more than 90 percent from the original hot strip thickness. In addition, the strip and the rolls must be cooled with large amounts of water. The water consumtion is minimized by a closed-loop system.

Cleaning and degreasing

After cold rolling the strip is initially cleaned of impurities and lubricant residue. In an electrolytic degreasing cycle it is then passed through an alkaline bath to which an electric current is applied. After this, the strip is scrubbed, rinsed, dried and rewound into coils weighing up to 23 tons.

Continuous annealing

Cold rolling makes the strip hard and brittle and, in this condition, it is not suitable for use as packaging material. Recrystallization annealing of the degreased strip restores the necessary ductility. In continuous annealing the strip undergoes an annealing cycle of about 2 minutes' duration, passing through the furnace in vertical loops. A controlled (gas) atmosphere in the furnace prevents oxidation of the strip surface. Strip thus annealed in a continuous furnace for a short time at up to 880°C is harder and more resilient than batch-annealed material.

Batch annealing

The second option for restoring the ductility of steel strip after cold rolling consists in the batch-annealing method. This three-day process restores the crystal structure of the strip that was previously destroyed during cold-rolling. Up to four coils are stacked on top of each other and placed under a system of protective (inner) and furnace (outer) covers. The coils are heated to a temperature between 600 and 800°C. Reoxidation of the strip surface is prevented by an oxygen-free controlled atmosphere. Compared with the continuous annealing method, this type of recrystallization annealing yields a product of lower hardness.

Temper rolling

Although the annealing process restores the crystal structure of the steel strip, the annealed product is not yet suitable for fabrication into tinplate packaging. An additional temper-rolling step is necessary to prevent severe kinking and irregular deformation behavior in downstream processing. Temper rolling involves a thickness reduction by up to 40 percent. In addition, it produces the appropriate surface roughness and strip flatness. The resulting blackplate is ready for shipping to the customer . However, by far the greater part is coated before leaving the tinplate works.

Tin/chromium coating

The difference between tinplate and blackplate lies in the coating. Joined once again into an endless strip by welding, the material is passed through an electrolytic strip coating line where a thin layer of thin or chromium is applied to its surface. After tin coating the strip must be heated briefly to a temperature above 232°C (melting point of tin) to acquire its typical tinplate luster. It is then quenched in a water bath. The weight of the tin layer is usually in the range between 1.0 and 5.6 g/m². In electrolytic chromium coating the amount of coating applied is much smaller, i.e., between 0.05 and 0.1g/m². Chromium coated blackplate is also referred to as ECCS (Electrolytically Chromium Coated Steel) and must be lacquered for further processing.

Film laminating

For certain applications, a plastic film made of polyethylene terephtalate (PET) or polypropylene is laminated onto the tinplate. While PET is preferred for chromium coated surfaces, PP is used on both tin and chromium. The resulting film-laminated product offers many benefits. It provides outstanding metalforming properties, high visual appeal, and full anti-corrosion protection.

Slitting

After coating the tinplate can be shipped to the processor in coils or various cut-to-size formats.

- Straight-edged sheets for rectangular sheetmetal parts such as body sections

- Scroll-cut sheets for round sheetmetal parts such as ends or cups

- Narrow strip for semi-finished product manufacturing

Cutting

After coating the tinplate can be shipped to the processor in coils or various cut-to-size formats.

- Straight-edged sheets for rectangular sheetmetal parts such as body sections

- Scroll-cut sheets for round sheetmetal parts such as ends or cups

- Narrow strip for semi-finished product manufacturing

Printing/lacquering

Many tinplate or chromium-coated products achieve their perfect looks and anti-corrosion properties only through lacquering. Various color shades and decorative printing techniques can be used to impart a stylish appearance to the product. Standard applications include three-part cans (e.g., food cans) crown caps, or decorative boxes in sophisticated printed designs.

Storage of finished products

All products are carefully packed before shipment. The favorable location of our Andernach plants allows us to deliver tinplate and blackplate quickly to any destination in the world.

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