Square rolling slabs from start of casting - the elimination of butt swell

Abstract

The thickening of the bottom part of DC cast rolling slabs, butt-swell, is a well known problem in the aluminium business. The slabs are scalped, normally in the rolling plant, prior to rolling to a absolute rectangular shape. The rolling mill therefore prefers rolling slabs with geometry close to rectangular from the casthouse. Due to geometry specifications from the rolling mills the supplier casthouse has to cut off the bottom part of the slab before shipping. The bottom cut length is typically 0.3-0.6m (1-2') depending on the thickness and casting speed. Extensive research has been done on the basic mechanisms of butt-swell and the phenomenon is today well understood. The thickening in the bottom is due to the absence of pull-in which is a thickness reduction phenomenon during the stationary phase of DC casting of rolling slabs. It is impossible to avoid butt-swell when using moulds with fixed geometry. To eliminate the butt-swell flexible moulds are developed. In this technology the mould shape is gradually changed during the initial stages of casting in such manner that the rolling slabs will be rectangular from start to end of cast. This technology has been in industrial use since 1997 within Hydro casthouses. In addition to eliminating the butt-swell this technology allows for a large degree of freedom when choosing casting speed. Conventional moulds are designed to give rectangular slabs for a given casting speed, while the technology described in this paper may be adapted to any relevant casting speed. This paper describes the basic principles behind the technology, a description of the equipment, and the operational experience so far. For 600mm thick ingots flexible moulds typically reduce the total scrap rate in the casthouse with more than 5% compared to conventional moulds with fixed geometry.