Influence of spread and stress on the closure of a central longitudinal hole in the hot rolling of steel

Abstract

Voids are often found in steel in the as-cast condition, being mainly due to material shrinkage during solidification. Usually the highest void fraction is located at the centre of the ingot or continuously cast strand and quite often is so high that a central longitudinal hole is formed. This unsoundness has to be eliminated in subsequent hot working to ensure high yield and full strength and ductility of the product. In the present work a three-dimensional theoretical model is presented for the closure of a central longitudinal hole by means of hot rolling between flat parallel rolls. The analysis is carried out for a workpiece of initially square cross-section, rotation of the workpiece not being taken into account. According to the model, rolling conditions resulting in large spread are favourable. If the pressure at the centre of the slab is sufficiently high it will cause a mechanism favourable to hole closure, i.e. a rapid decrease in hole fraction at the beginning of the rolling schedule comprising no extension of the hole in the ‘spread’ direction. Once established, this mechanism will be maintained for small negative pressure values also. The results are presented in terms of rolling geometry and other rolling conditions defining the process.