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Products and solutions, 2005-09-12, 11:04 AM

Production-ready and cost-effective: Prepainted automotive steel direct from coil

After introducing its concept for preprimed and prefilled steel sheet at the IAA Frankfurt Motor Show 2003, ThyssenKrupp Stahl AG is now demonstrating that the material is ready for production use in the auto industry. At IAA 2005, ThyssenKrupp Stahl is presenting a convertible tonneau cover made from steel that is preprimed and prefilled direct from coil. The part is identical to a component from a current production vehicle and was stamped with the same die used to make the production part.

Using its own coil coating lines the steel manufacturer is thus able to apply half the paint system needed by body outer panels to a substrate of galvanized chromium-free pretreated steel. The paint system on a car usually consists of a primer coat (electrocoat), filler coat, color top coat and a final clear coat. The primer provides corrosion protection while the filler offers mechanical protection against stone chipping for example. Customers using coil coated sheet steel from ThyssenKrupp Stahl can significantly shorten the process chain for painting body outer panels, especially as preliminary operations such as cleaning and phosphating are no longer needed. In addition, capital expenditures for paint facilities are reduced, as are running costs.

The idea of preprimed and prefilled steel direct from coil has been pursued by steel producers, paint manufacturers, car makers and component suppliers over the last two years in a joint project called Prepainted Steel on Wheels (PSW). One of the aims was to find paints which could survive the stamping operation in the die undamaged. Because precoated steel is not suitable for welding, the project team also wanted to show what other joining techniques could be used for the material. In addition, the PSW team demonstrated the overall cost efficiency of the concept and developed a process description for fabrication.

When sheet steel is formed into auto components, the stamping forces reach several thousand kilonewtons. The PSW team has developed formulae for paints which suffer neither cracking nor surface damage under these conditions. An additional advantage is that unlike conventional uncoated material the preprimed and prefilled steel does not have to be oiled before stamping. Without the use of additional lubricants, the organic coating prevents excessive friction buildup
between die and sheet and avoids damage to the part. Users therefore also save the cost of subsequent part degreasing.

To prove that the new paints can be applied to steel on an industrial scale, ThyssenKrupp Stahl ran tests on its coil coating lines in Duisburg and the Siegerland area. As well as the tonneau cover, several other parts such as door inner panels, brake disk covers, trim panels and front modules were fabricated from the test coils in production dies.

Part of the tonneau cover on display at IAA is coated with the full metallic-blue paint system, demonstrating that precoated sheets offer a reliable basis for a top-class appearance of the finished part. The cover also shows how organic coated sheets can be reliably joined without the need for welding. It consists of an outer and inner panel which are joined by adhesive bonding and clinching. Clinching is a commonly used process in which steel sheets are tacked together using press-stud-like fasteners.

ThyssenKrupp Stahl offers organic coated sheet for automotive applications in two versions. The preprimed version (electrocoat replacement) for corrosion protection has a 20 to 25 micrometer thick coat on each side. The prefilled version with a 25 to 35 micrometer thick filler coat on the exposed side offers protection against chipping and a surface finish suitable for body outer panels. The material is targeted particularly at automotive suppliers, for whom capital outlay on in-house paint shops is usually not worthwhile. Currently the work is carried out in many cases by contract coaters, adding substantial logistical costs.

ThyssenKrupp Stahl is exhibiting at IAA in hall 4.1, booth C50.

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