Investigation of the formation, constitution and properties of scale formed during the finishing, rolling, cooling and coiling of thin hot strips

The properties of tertiary scale on hot-rolled steel are receiving increasing attention, because customers of hot-strip mills place increasingly stringent requirements on the properties of unpickled hot-rolled steel strip, e.g. easy-to-pickle, minimised powdering during deformation, or minimised oxide dust formation during laser cutting. On the other hand, the possibilities to influence the tertiary scale characteristics have increased over the years, due to the continuous improvement of hot-rolling technology. For a group of unalloyed and microalloyed ULC, ELC LC and HSLA steels as weIl as DP steels, the relationship between scala adherence, scale characteristics and hot-rolling process parameter was investigated. Suitable scale adherence tests had been modified. Several microscopical and analytical investigation techniques were used to evaluate the scale layer and the interface steel/scale in terms of morphology, phase and chemical composition. From shop trials and laboratory experiments including laboratory hot rolling, guidelines were defined to get a well-adherent or easy-to-pickle scale. Scale composition, however, was found to be more important for scale integrity in terms of adherence and coherence than scale thickness. Completely decomposed wüstite within the scale gives the best results. Partially retained wüstite or layers of magnetite and hematite lead to poor scale integrity. The influence of enrichments of chemical elements (e.g. silicon) at the interface steel/scale is much less than assumed. There seems to be a broad optimal coiling temperature range for wüstite decomposing around 600 °C. Although low finishing temperatures, particularly in the ferritic range, result in a thin oxide scale, they do not guarantee a good scale integrity. High levels of elements like Mn, Si, V, Nb in HSLA grades decrease the scale adherence. Wüstite containing scale was more easily pickled than decomposed wüstite. Direct galvanising of unpickled hot-rolled strips has yielded some samples with surprisingly good coating qualities. The findings concerning scale adherence were successfully verified by industrial applications.