FEM analysis of stress on roll surface black oxide layers exfoliation in hot strip rolling

To understand the mechanism of formation and exfoliation, black oxide layers were investigated, and the effects of stress on the exfoliation were analyzed by finite element method (FEM). The roll surface on which black oxide layers form is composed mainly of Fe3O4, which is caused by the oxidation of the roll material itself. Cracks form and are easily propagated along M3C- and M7C3-type carbides, which leads to further cracking along M3C and M7C3 carbides as a result of contact stress fatigue produced by cyclic mechanical stresses that normally occur during the rolling process. Thermal fatigue of the roll surface is produced by the thermal cycles created alternately by contact with the hot strip and the cooling water on the roll. The generation and propagation of cracks in the black oxide layers during rolling is promoted by circumference compressive stress at roll surface. Under this stress, the exfoliation of the black oxide layers happens on the roll surface.