When it comes to car design, the trend is very much towards sharp lines and concave or convex forms as well as exceptionally smooth paint finishes. At the same time, components also have to be as lightweight and as easy to process as possible in order to keep overall costs as low as possible for the manufacturer. Fulfilling these requirements calls for exposed body panels that are easy to form and that present a premium appearance after painting.
Against this background Tata Steel evaluated exactly how the surface properties of the steel substrate influence the final paint appearance. Through specific modifications to the manufacturing process, the steel manufacturer collaborated with a European vehicle manufacturer to develop a premium surface finish for hot-dip galvanised steels designed specifically for an excellent paint result that can be processed on existing equipment.
Steel surface properties
When considering the properties of the steel surface, there are two competing requirements. On the one hand, a certain roughness is necessary for it to react appropriately in the press shop. The various steps involved in the forming process, such as deep drawing, call for good lubrication, whereby the oil collects in the valleys of the surface profile. On the other hand, the steel surface must appear as smooth as possible, so that the structure profile is not visible in the paint coat, thus detracting from the finish.
Image 1: Measurement of the surface topography
The exact properties of the steel surface are determined with the aid of a BMT roughness scanner. This mechanical measuring device runs along the surface with a probe tip and calculates the microscopic surface profile (image 1). A high-pass filter is then applied to these results to identify the roughness (Ra) – defined as the average depth of the measured surface profile, known as mean roughness parameter – as well as the peak count (RPc). RPc expresses the number of peaks per centimetre in the surface profile. A low-pass filter permits determination of waviness (Wa), which is defined as the average value of the waves measured and is perceived by the human eye as long-wave surface topography. A waviness profile of Wa0.8 has established itself as the industry standard, as this correlates strongly with paint waviness. It is also possible to calculate the Wsa figure – wave surface arithmetic value – using an adjusted profile filter. Wsa 1-5 mm shows an equally strong correlation with paint waviness as Wa0.8.