Migration of slab defects during hot rolling

Abstract

Blisters and slivers are serious defects that can lead to downgrading and rejection of interstitial free (IF) steels used for automotive applications as they affect the painted appearance, structural integrity, durability and performance. It is therefore important to understand the origin of these defects and to follow their morphological changes during all stages of manufacturing. This paper presents a novel method to study continuously cast slab defects (inclusions, pores, gas entrapments) in terms of their migration towards the surface during hot rolling. The method adopted is a combination of experimental and numerical techniques. The experimental method involves a 'step machining' process where the slab surface is machined gradually to find out the nature and location of defects. These defects are then correlated in terms of their ability to cause blisters or slivers on the final hot rolled coil surface. The numerical method involves finite element simulation of multipass hot rolling with defects embedded at different depths from the slab surface. The experimental findings with respect to the migration of defects are in general agreement with the numerical predictions. This analysis provides a guidance and direction towards producing defect free IF coils as well as improvements in productivity and yield.