A study of the conditions which promote rolled-in-scale in the hot flat rolling of steel

An investigation into the conditions that cause the scale layer that is present during hot rolling, to be rolled into the surface of the substrate metal is undertaken. The phenomenon is widely known as rolled in tertiary scale, in the flat rolled primary steel-making industry.; The conditions needed in the laboratory to simulate the oxide layer thickness that occurs in the finishing stands of a hot strip mill are investigated, using plain carbon steel samples.; It is found that this scale layer must be a mono-layer less than 30 μm thick; that is to say that it has not blistered and delaminated from the growing oxide layer beneath it, resulting in two or three loosely attached layers that can become heavily cracked. It is also found that these cracked sections are separated in the roll bite deformation zone as the metal beneath is reduced in thickness producing an elongated substrate for the scale layers to be pressed into.; Samples of porous iron oxide, FeO are compressed at high temperature. It is observed that even up to a 30% reduction the material plastically distorts. This allows for the oxide layer to behave plastically rather than as a brittle-like material in the roll bite at finishing mill rolling temperatures.; Lead with a thin mono-oxide layer is rolled at room temperature. Again it is found that the lead oxide layer does not crack, but rather elongates evenly with the lead substrate. The thin lead oxide does not behave like a brittle material either.; The effect of this thin oxide layer on heat transfer conditions in the roll bite interface is examined, using a roll with thermocouples embedded in its surface. A thick cracked porous layer decreases heat transfer to the roll; the roll surface temperature is cooler than it is when a mono-oxide layer is present.; The effect of the thin oxide layer on interface friction is examined. It is found that a lubricant directly reduces friction if it is a mono-layer. Otherwise the thicker, porous oxide acts like a lubricant itself.; It is concluded that rolled-in-scale results from processing a strip with an oxide layer that is thicker than ∼30μm before entering the finishing stands. This is observed in industry because it is the general case. This investigation has described the conditions that are not generally experienced, except when rolled-in-scale is reported, and how to avoid thick scale conditions: the time between descaling and rolling must be such that the oxide does not grow more than ∼30μm thick.