1. Introduction

When metals are made by casting processes, voids are generally formed in the workpiece due to material shrinkage or gas entrapment during solidification. If any void remains in the final metal product, it will have an unfavorable effect on the performance of the material in service. Thus, it is highly desirable to use the rolling process to eliminate the voids.

A number of papers have been published discussing the closure of voids during hot rolling of slabs. For example, Wallero (Wallero, 1985) analyzed the closure of a central longitudinal pore during hot rolling process with a square cross-sectional workpiece in experiments. His work showed the rolling mills with large rolls were advantageous in pore closure. Keife et al. (1980) studied the closure of a void in homogeneous plane strain forging between parallel dies with the upper bound approach and carried out experiments when voids of the same size and square shapes were homogeneously and squarely distributed throughout the specimen. Stahlberg (1986) has proposed a three-dimensional theoretical model of upper-bound solutions for the closure of a central longitudinal hole during hot rolling between flat parallel rolls. The above papers concluded that large roll radii and heavy reductions per pass were favorable for the closure of a central longitudinal hole. Wang et al. (Wang et al. 1996) used a FE software "ABAQUS" to investigate the effects...