Study on Surface Defects of Coated Sheets Caused by the Surface Condition of A Hot Roughing Mill Roll

by Katsuhiro Yamamoto

Publisher - Kobe Steel, Ltd., Japan

Category - General Novel

Galvannealed steel sheets require very strict surface quality (especially for the exposed panels of automobiles). Sometimes these defects look like repeated white line “white line defects” appearing on the bottom of the sheet. We found that these defects were created in the hot rolling process after detecting the scales under the defects by SEM analysis. Further, the percentage of “white line defects” increased with the amount of rolling at the roughing mill. Experiments revealed the following. 1. The width of the heat cracks of the roughing mill roll increased with the amount of rolling. 2. The width of the heat cracks was too wide for “white line defects” to occur. 3. Heavier grinding reduces the growth of the heat cracks. As a result, we discovered the optimum grinding conditions for roll consumption and the surface quality of galvannealed steel sheets by managing the width of heat cracks, which create “white line defects”.

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