Modelling the Behaviour of Oxide Scale in Hot Rolling

Abstract

In the hot strip mill, the slab is brought to temperature in the reheating furnace and discharged for rolling. To break the primary scale, the slab is passed through a slab descaler before the reversing roughing mill. A secondary oxide scale is inevitably formed between successive rolling passes during hot rolling, which is further removed by high pressure water jets before the subsequent passes during reversing rolling or before the strip enters the tandem finishing mill. Whether the oxide scale is deformed or fractures, it will inevitably affect the interaction between tool and workpiece for several reasons: the thermal conductivity of the oxide scale is significantly less than that of the steel 1); fractured scale can enable direct contact of hot metal with the cold tool 2,3) ; because of the possibility of sliding of the fractured scale raft due to weakness of the scale/metal interface at high temperatures 4) ; and because the location of the plane of sliding is determined by the cohesive strength at the different interfaces within the multilayer oxide/metal system and by the stress distribution when delamination within the scale takes place. 5)The oxide scale on the surface of the hot metal is important for the control of friction and heat transfer when in contact with the roll, as well as influencing the rate of heat loss between rolling deformations under air or water sprays. During multi-pass hot rolling of long products, the magnitude of the coefficient of friction within the roll bite is variable due to the complex pressure-slip variations. A new Coulomb-Norton type friction model for long products and bar sections has been developed recently at Swinden Technology Centre (Corus RD&T UK).6) Among other assumptions, the model takes into consideration some complex interactions at the stock-roll interface due to the presence of secondary oxide scale, as mentioned above. Hence, the different modes of scale failure, such as the through-thickness cracking and the sliding mode, depending on the temperature and steel composition, have been implemented into the mathematical model. The friction force occurs either between the roll surface and oxide scale or between the roll surface, the oxide scale fragments and eventually fresh steel extruded through the scale gaps depending on the relative magnitude of the shear stresses inside the scale layer and at the oxide scale/stock interface. An additional point of consideration is the demand for increasingly small final thicknesses of the hot rolled steel strip, approximately 1.2 mm during conventional rolling and 0.8-1.0 mm for ultra-thin hot rolled strips produced on mini-mills using endless rolling technology.7) The scale related imperfections formed on upper and lower surfaces of the thin strip affect deformation of subsurface layers of the material during rolling which become relatively more pronounced during rolling of the very thin strips.Perhaps of greater and growing importance is the role played by the oxide scale in determining the quality of the surface of t...

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