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Optimizing sheet-steel surfaces.

At the simplest form of production, all steel-based products receive some type of surface coating treatment to protect against corrosion. However, at the more sophisticated levels, finishing treatments become an integral element of end-user appeal, especially in the automotive, furniture, and domestic markets.

To determine the most efficient production specifications for coating compatible sheet steel, OCAS, a Belgian steel industry research center, has implemented an innovative research program focusing on the analysis of sheet-steel surface texture. The surface of sheet steel plays a pivotal role in end-result quality by affecting such properties as formability during pressing, paint adhesion, lacquering, gloss, and coating adhesion (Zn, ZnNi, organic, etc).

Established in 1988, OCAS was founded as an integral part of a total quality initiative being undertaken by a European steel manufacturer group: Sidmar and ALZ of Belgium and Arbed of Luxembourg. OCAS's objective is to assist industries that process sheet steel and to develop new steel grades, especially for automotive and appliance stampings.

It is imperative that the surface texture of sheet steel possess a particular size of micro cavities in order for the various stages of rolling and manufacturing to produce a high quality product. However, production roughness or texturing requirements can often conflict. On one hand, cavities are needed to hold oil and to help build paint. On the other hand, high-gloss paint finishes are hard to achieve if the surface has been over textured. Therein lies the need to identify controllable and quantifiable parameters to define texture within the context of particular applications. Commercial negotiations are much easier if each side can quantify its needs and measure achievement against specification in the end product.

To measure their progress, OCAS selected a Form Talysurf Series 120mm Interferometric System for the production of a 3D representation of the steel surfaces, with the objective of researching and developing optimized parameters for ranges and applications of sheet steel. The Form Talysurf Series was selected because of its wide measurement range, straightness accuracy, high resolution, low stylus load, and high-speed processing. The 2 ||micro~meter~ stylus tip is able to resolve surface details without damaging the surface under analysis, and stylus loading can be maintained at 70 mg to 100 mg force.

One of OCAS's early projects has been the measurement and analysis of roll texturing techniques. Currently, four techniques are used for texturing work rollers in cold strip rolls: shot blasting, electron erosion, laser texturing, and electron-beam texturing.

Of the four, electron-beam texturing, a technique developed in 1992 at OCAS and sponsored by the EEC, has been shown to offer the best and most repeatable surface. Electron-beam texturing places rollers in a low-pressure chamber where the surface is textured with an electron gun. Electron beam texturing produces highly reproducible, uniform parabolic craters over the entire roller surface. Also, because it is developed under vacuum, oxidation of molten metal is avoided and the crater rims are not as fragile as those in laser-textured rollers.

Trials of electron-beam textured rollers in mills at Sidmar show them to offer a life expectancy of three to six times that of rollers textured using other methods. OCAS believes that the high degree of controllability and flexibility electron-beam texturing offers, together with its ability to achieve defined optimized parameters for sheet steel across a wide variety of situations, provides a solution to the search for well-controlled and constant-strip surface textures.

OCAS also believes the use of Form Talysurf equipment gives the steel industry the opportunity, for the first time, to accurately measure and optimize production innovation. In a market that is perhaps the most competitive in the world, the ability to understand and control surface texture in the context of final applications will confer a real competitive advantage.
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Title Annotation:Technology Update
Publication:Tooling & Production
Date:Sep 1, 1993
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