High-speed steel type cast iron rolls (HSS-rolls) used in the hot rolling process of steel are composite rolls manufactured by the CPC method or the centrifugal casting method (CCM). Comparing the two, the centrifugal cast HSS-roll is advantageous in terms of production cost, and since the shaft material is ductile cast iron, the thermal crown during rolling is small, and the plate threading property is excellent. For this reason, Centrifugal cast HSS-rolls are often used as standard type HSS-rolls, and some of them are also used in the later stands of the hot strip mills. On the other hand, forged steel is used for the shaft material of CPC-HSS-rolls, and the strength of the shaft material is superior to that of ductile cast iron, so there is very little risk of fracture accident (thermal breakage) from the center. The thermal breakage of the roll is defect that occurs when the tensile residual stress of the shaft and the thermal stress generated during rolling exceed the material strength of the shaft. Therefore, in order to prevent thermal breakage, it is considered extremely effective to suppress the tensile residual stress of the shaft portion. Therefore, in this study, an attempt was made to significantly reduce the tensile residual stress of the shaft by improving the heat treatment conditions. Furthermore, the influence of heat treatment conditions on the compressive residual stress of the outer layer was also investigated. As a result, it was confirmed that by applying the surface rapid heating method to heating conditions before quenching, the tensile residual stress inside the roll can be reduced by about 30% compared to the conventional uniform heating method. Effective results were obtained by deciding the manufacturing conditions for significantly improving safety against thermal breakage in centrifugal cast HSS-rolls.

This Paper was Originally Published in Japanese in J. JFS 92 (2020) 144–150. Figures 10 and 11 were slightly changed.