Generation Mechanism of Surface Defects on Steel Sheet by Laboratory Experiment

To understand the generation mechanism of surface defects appeared on the final sheet product, the defects found in cast production slabs were investigated. Defects located in a cast production slab of ultra-low carbon steel were detected by using an ultrasonic test. After that, using laboratory mill, hot rolling, cold rolling and galvanized coating were applied to the samples to simulate actual production conditions. And cross section of the sample was observed at the location corresponding to the original location of defects in the cast slab. Some samples showed similar defect shapes like defects of scabs in the steel sheet. And foreign materials which consist of Al, Fe, and O were observed at the location of the scab-like defects. Various defects are shown by the diameter and depth from the surface to the top of defects in cast slabs with their reproduction. It is clearly divided by a boundary related to the diameter and depth into two regions. Based on the information on scabs observed in steel sheet product, the size and depth of defects from the surface existed in the cast slab were estimated. Boundary plotted by the diameter and depth of the defects estimated from the surface defects of sheet products also shows the similar tendency of the threshold of possibilities of the generation of defects in the reproduction experiment.