Consequent Improvement of Surface Quality by Systematic Analysis of Slabs

Abstract

Conventional continuous slab casting currently represents the best method for producing material for top-grade cold strip. Steady optimisation of the metallurgy and of the casting process has made it possible to reduce the material-induced defects in cold strip considerably. The present report describes the route taken by ThyssenKrupp Stahl AG over the past few years to achieve this goal. The classic continuous casting defects, such as slivers deriving from near-surface casting powder or slag inclusions, and especially blisters typical for bow type casters, have been sustainably reduced in this respect. So-called inclusion bands beneath the slab surface have also diminished to such an extent, thanks to substantial improvements in the secondary metallurgy, that even the surface of top-grade cold strip is no longer significantly affected. In the course of the investigations it has been possible to identify a further, independent defect type: the so-called microcrack defect. These are very fine subsurface cracks at the cast slab which have frequently been detectable only after the material has undergone initial or even final rolling. This defect type has so far either remained unconsidered among the host of classic defect types or has been wrongly attributed to the hot rolling process. Confirmed by controlled high-temperature simulations in a continuous casting simulator, it has now been possible to attribute this defect to a primary solidification phenomenon, namely the formation of fine interdendritic hot cracks beneath the slab surface, which is free of defects at the moment of the cracks' formation. The understanding gained in the meantime concerning the defect's causes will enable further optimisation of the continuous casting process.