Effects of surface preparation on tribological behaviour of a ferritic stainless steel in hot rolling

Some defects on the surface of carbon steel do not need to be removed before hot rolling because the surface will be vigorously oxidised in a reheating environment. Thus the defects can be minimised by oxidising and then removed by the de-scaling process. The defects on the surface of ferritic stainless steels, however, are not easily removed by oxidation when a high chromium concentration is used. In this paper, a ferritic stainless steel grade 445 was selected as a research material. The effects of different surface features on oxidation and tribological behaviour in the hot rolling process were investigated. Three surface states were prepared, namely, smooth surface, surface with 45° grinding marks and surface with oscillation marks. The samples were put into an electric furnace at 1100°C for reheating. Hot rolling tests were carried out on a 2-high Hille 100 experimental rolling mill. Rolling forces were measured, and the coefficient of friction was calculated and compared under various rolling parameters. It was found that the original surface profiles with grinding marks were still maintained during oxidation. The original oxide scale on the surface with oscillation marks caused the formation of irregular oxide nodules and the spallation of the oxide scale. Surface morphology and the reduction in thickness had a significant impact on the oxide scale integrity and coefficient of friction in the hot rolling process.