Pretex® helping to make customized surface topographies

In order to obtain high quality deep-drawing and paint characteristics, the demands placed on thin sheet surfaces are stringent, particularly in the automotive industry. The surface structure is a fundamental product feature that is relevant to the quality. It substantially influences the formability characteristics, the glue adhesion and the automotive paint’s visual properties.
The Pretex^® method uses the galvanic Topocrom® technique (Figure 1): chrome ions move from the electrolyte to the roll, where they pick up electrons, and are then deposited as chromium. As a result, the chromium accumulates as a hemi-spherical segment which is also called a characteristic Pretex^® calotte.
 

With the help of the rolls coated in the Pretex^® roll texturing method, the thin sheet surfaces are stamped with the surface structure of a textured working roll depending on the customer requirements. The skin pass roll transfers its surface structure to the fine sheet surface. Furthermore, the steel sheet is deformed by between 0.2% and 2.0% during the skin pass rolling in order to avoid the formation of a pronounced yield point. The roughness transfer from the roll to the steel sheet is between 60% and 120% and depends on the material dimensions, tensile strength, ductility, required roughness on the steel sheet, roll diameter, and rolling force.
The density, number and depth of the textured steel sheet's calottes directly influence the deep-drawing characteristics at the processor. This is because the closed calottes act as lubricant pockets. During the forming, the lubricant is released, which improves the frictional characteristics. At the same time, in order to achieve outstanding results, the painting requires low waviness levels that are described by a low W_sa-value. Pretex^®focar^® has been developed for both hot-dip zinc coated and electrolytically zinc-coated sheet for such demanding body shell requirements in the automotive industry.

The following are some of the characteristics that are determined:

• S_a (surface average) is the 3D equivalent of the 2D roughness characteristic Ra. This is calculated as an average of all measurement points in the 2D measurements. The higher the S_a level, the rougher the surface.
• S_mr(c) (surface material ratio at the depth of c µm) is also an extension of the 2D characteristic R_mr(c). The greater the S_mr(c), the more material there is at a depth of c in relationship to the measurement surface. This characteristic is used to indicate the proportion of the surface occupied by the calottes.
• C_d (Calotte Density) is the number of calottes in a 1x1 mm² normalized measurement field. A higher C_d is similar to a higher S_mr level, however the S_mr level describes the area taken up by the calottes while the C_d level indicates the number of calottes, regardless of their size. The characteristic that is more useful for the particular problem can be selected.
• C_ra (average calotte radius) is the average radius that is calculated based on the radii of all calottes.

With different Pretex^® textures, combinations of characteristics of the steel sheet surface can be selectively configured that are optimal for different applications, such as deep drawing or the paint's appearance.
Table 1 shows examples of different textures and their characteristics with

• Many small calottes (number 1 in Table 1),
• Very many calottes (number 2 in Table 1) and
• Few large calottes (number 3 in Table 1).

Table 1: Selection of roll textures and their roughness characteristics

With a confocal microscope, 2D and 3D topography characteristics of the roll and sheet surface are two-dimensionally recorded. The calculated 2D and 3D rough-ness parameters material ratio S_mr, average calotte radius C_ra, calotte density C_d, average 2D roughness S_a and peak count R_Pc allow a unique description and the development and optimization of special topographies.
These characteristics are used as the basis to enhance the roll texturing process in order to develop innovative topographies for steel strips. Various combinations of 2D and 3D surface characteristics offer advantages during deep drawing and painting and allow targeted selection of steel sheet surfaces for the most diverse range of applications.
In cooperation with a number of automakers, surfaces are consequently being optimized for even better deep-drawing characteristics and an improved paint appearance.