Black oxide layer formation and banding in high chromium rolls

Abstract

In order to improve the surface quality of strip steels it is essential that surface oxide adheres to the roll surface. To understand the mechanism of banding (the detachment of oxide from the roll surface), black oxide layers on a roll surface and on a slab surface were analysed using SEM, optical microscopy, microhardness testing, energy dispersive spectrometry, and X-ray diffraction. A mathematical model for black oxide layer thickness of a high chromium iron roll was established on the basis of the oxidation mechanism and oxide layer structure. It is shown that the black oxide layers, formed on the roll surface, induce the formation of cracks which propagate easily along M3 C and M7 C3 carbides resulting in their final separation from the roll surface during the rolling process.