Effects of roll surface deteriorations on scale defect in hot rolling

Roll surface deteriorations and scale deformation behavior in hot rolling are investigated in order to clarify a mechanism of rolling load increase and scale defect formation on using high speed steel roll. Several experiments are conducted; a rolling/sliding friction test at elevated temperature and model rolling tests for the sheet coil and plate by using high Cr cast iron and high speed steel as roll material. The results obtained are as follows; (1) Worn surface of high Cr cast iron is flat and smooth in hot rolling. On the other hand, fine asperities are formed on worn surface of high speed steel. it is found that this peculiar topography of high speed steel consists of MC carbides which are left by prior wear of matrix in roll microstructure and black oxide film which prevent MC carbides from removing from worn surface, which leads higher friction coefficient and higher rolling load. (2) Fine asperities cause scale heterogeneous deformation of the steel products in roll bite, resulting in scale defect. (3) Heterogeneous scale deformation is easily occurs under conditions with the large oxide film on the steel before rolling and the large reduction in thickness and the high rolling temperature.