Turn-Up and Turn-Down in Hot Rolling: A Study on a Model Mill Using Plasticine

Abstract

Turn-up and turn-down is a phenomenon which occurs in the hot rolling of flat products when the nose of a slab, or the whole slab, curls up or down upon emerging from the rolls and remains deformed unless subsequently straightened. Turn-up and turn-down can be troublesome to production, since it can cause damage to the mill rolls, housing, and run-out tables and can also cause production delays due to the difficulty of re-rolling a deformed slab. The object of this paper is to determine the main factors governing the phenomenon, to obtain quantitative relationships between causes and effects, and to suggest means of preventing curvature occurring. The best approach was considered to be a scale model using plasticine to simulate the hot metal. As a basis for the work, several dimensionless ratios are formed which have to be the same in both model and full size mill, if the model is to be both dynamically and geometrically similar. In the more important dimensionless ratios, account is taken of the plastic yield characteristics of the material, including the rate-of-strain component, of the stresses due to surface friction and gravity, and finally of the ratio of roll speeds. The results are compared with some previous work by others on steel slabs using a full size mill. In all respects, the results agree closely. The most significant factor affecting turn-up and turn-down is the ratio of surface speeds of the two work-rolls. Curvature is proportional to this speed ratio, but slipping at the rolls may modify this relationship. For a given angle of entry, the ratio of torque upon each roll is a function of the speed ratio. Angle of entry has negligible effect on turn-up and turn-down, but has a powerful effect upon the torque ratio. Slip is very undesirable and it appears that the roll surface finish should not be better than necessary if slip is to be avoided. To produce a flat slab, a speed ratio of about 0·97 is required in the model. In practice, this may be achieved either directly by controlling the speed ratio, or indirectly by controlling the torque ratio, provided the angle of entry is constant.