Effect of Sheet Surface Roughness on Clarity of Painted Sheet Steel

The influence of base metal surface roughness parameters on the clarity of painted automotive cold rolled sheet steels was investigated. The sheet steels were dull finished by shot blasting, electro-discharge texturing or laser dull texturing. It was found that the painted clarity improved with decreasing of Ra, while it scarcely depended on PPI. Characteristic of the surface profiles of base metal was the waviness with wavelength of hundreds micron. Whose trace remained after coating and deteriorated the clarity. In shot blasting, the dendritic microstructure and the statistical fluctuation of grit collision generated the waviness and made it difficult to control its wavelength. And, in electro-discharge texturing, it is suggested, controlling the waviness is difficult similarly. In laser dull texturing, it was possible to reduce the wavelength of waviness to less than hundreds micron and then to improve the clarity without deteriorating the press formability. A larger fraction of flat portion in laser dull texture was also responsible for the improvement of clarity.