Abstract: Purity of steel is vital important to the surface quality of cold rolled sheet. Samples of tundish steel and slab were analyzed by automatic feature analysis equipped on Aspex and LECO TCH600 in present paper to solve the problem of high surface defects occurring rate. T.O mass fraction of IF steel in tundish is in the range of 17.5×10-6 to 35.7×10-6, which means high purity. But about 60 t of steel become unqualified because of the reoxidation at the start of casting. Macro inclusions in the slab mainly are cluster Al2O3 inclusions and “bubble + Al2O3 inclusion” structures. The quantity density of inclusions over 50 〖WT〗μ〖WT9.5B4〗m is in the range of 0.29/cm2 to 0.63/cm2. Moreover, a new gathering area which is different from the one just blow the surface of macro “bubble + Al2O3inclusion” structures is found 20.0mm away from the surface. Measures such as lowering the sulphur content of steel, prudently lowering the argon amount from the stopper, and optimize the flow condition in the mold, are proposed to reduce the gathering phenomenon.

Key words: IF steel, purity, inclusions, bubbles