EFFECTS OF HOT-ROLLING AND DESCALING CONDITION ON RED SCALE DEFECTS FORMATION

Red scale defects usually observed in high Si hot rolled strip were reproduced in a laboratory 3 stand tandem mill. The effects of hot rolling and descaling condition on the strip surface color and scale structure were examined. Irrespective of Si content in steel, the hot rolled strip surface became red when the scale thickness before rolling was above 20 mu m and the rolling temperature was below 900 degrees C. It was found that surface part of the scale (mainly FeO) was broken to powder by the hot rolling at the temperature below 900 degrees C. The red scale of Fe2O3 was formed by the oxidation of powdered scale during cooling. Thick scale formed during slab soaking was completely removed by hydraulic descaling before rolling in low Si steel, whereas that was not removed in high Si steel. This remained scale caused the red scale defects after rolling and cooling. The application of obtained results to the hot strip mill production of red scaleless strip was discussed.