Rolling of hot strip and strip defects Jan14

Rolling of hot strip and strip defects...

Rolling of hot strip and strip defects Rolling is one of the most important industrial metal forming operations. Hot Rolling is employed for rolling of steel slabs to hot strips in hot strip mills.  Like any other hot rolling operations, rolling of hot strip is also a plastic deformation of the material of the slab caused by compressive force applied through a set of rolls. The cross section of the slab is reduced by the hot rolling process. The material gets squeezed between a pair of rolls, as a result of which the thickness gets reduced and the length gets increased. Rolling of hot strip is done at high temperature because of requirement of large deformations. Hot rolling results in residual stress free strip. Normally, oxide scaling is accompanied with the hot rolling, due to which dimensional accuracy is inferior when compared with the dimensional accuracy of the cold rolled strips. For rolling of hot strip, the slabs are heated initially at 1100 deg C to 1300 deg C. The temperature in the last finishing stand of the hot strip mill varies from 700 deg C to 900 deg C. It is always to be above the upper critical temperature to produce uniform equiaxed ferrite grains. The objective during the rolling of hot strip is to decrease the thickness of the slab with an increase in the length and with very little increase in the width. The material in the centre of the strip is constrained in the direction across the width of the strip and the constraints of undeformed shoulders of material on each side of the rolls prevent extension of the strip in the direction of the width. This condition is known as plane strain. The material therefore gets longer and not...