Abstract: The magnetic properties of the middle grade nonoriented silicon steel of  1.3 %-2.0 %  Si were improved under the same testing standard when the coiling temperature increased from  630 ℃  to  720 ℃ , but the black line defect were appeared on the steel’s surface after rolling.The main reason for this was that the increase of coiling temperature changed the structure of the strip surface, and too much residual iron oxide was appeared on the surface of the strip after pickling, which caused the black line defects on the surface of the strip after cold rolling.The practice proved that the black line defects on the rolled surface could be eliminated by means of rewinding and secondary pickling, which could improve the magnetic properties and ensure the surface quality.

Key words: non oriented silicon steel, cold rolling, black line defect, surface quality