A new way to evaluate steel sheet surfaces

This paper introduces a new assessment technique to monitor the quality of coated steel sheet surfaces, mainly used in the construction industry, e.g. as cladding for commercial buildings, external surface composite panels or even domestic applications, so clearly the visual appearance is critical. Normally, these types of steel sheets are coil coated onto steel strips. After curing the coating is soft, when it has cooled to approximated 60 C it can be embossed with a pattern by application of pressure by a roller with the appropriately pattered surface. After embossing and further cooling the coated strip can be coiled and stored. However during the storing process the weight of the coil (of the order of several tonnes), can damage the soft surface by creating visible pressure marks. Sometimes the surface will relax back to the original embossed shape some time after the pressure has been relieved or with the help of heat treatment. On occasion however the surface does not recover and a visual defect remains, which leads to customer rejection. In this paper, a new way to characterize coated surface of steel sheets, which combines wavelet filtering [1] with decomposed numerical analysis techniques [2] is presented. The wavelet filtering is based on complex wavelet, and it has very good metrological characteristics in the scale domain and provides shift-invariance and directions sensitive properties. In this research, this wavelet model is used to extract the information of structured texture of coated steel surfaces within the original emboss. The decomposed numerical description is used to qualify the variation phenomena of surface texture during storage and transportation. The results can be feedback to steel industry to significantly improve their coating techniques, storage and transportation.