Study on Surface Linear Defects of Non-oriented Electrical Steel Strip

The surface "dark line" and longitudinal hair crack in W600 non-oriented electrical steel strip are observed by SEM.The cause of surface "dark line" defects found in the after coated strip is that foreign substances are embedded into the scratches of strip surface caused by the graphite bottom roll build-ups in the soaking furnace（SF） section of the continuous annealing line and then covered by the coating in the coating process;the cause of surface longitudinal hair crack defects found in both of the cold rolled strip and after coated strip is that the complex aluminate inclusions in the slab are broken due to deformation and then extruded out from the inside layer during hot and cold rolling processes.In order to eliminate the "black line",the graphite bottom rolls should be changed or cleaned at regular intervals,and sections for feeding wet H₂+N₂+H₂O gas should be reduced from the original 6 to 4 and the furnace temperature in these four sections should be controlled under 880 ℃.In order to eliminate the longitudinal crack,inclusions in steel should be strictly controlled in refining and continuous casting processes to ensure the liquid level fluctuation in crystallizer kept at ±3 mm~± 5 mm.