Transversal cracks and their behaviour in the hot rolling of steel slabs

The behaviour of transversal cracks in reverse rolling of steel slabs is an alysed. For this purpose a commercial FE-program, LS-Dyna3D has been utilis ed. Process parameters from an industrial rolling mill have been used as a reference. One basic assumption is that the cracks do nor propagate for the steel considered. This is in agreement with observations in industry. Furt hermore the material is treated as elastic-plastic. The aim of the study is to investigate the possibility of controlling the plastic deformation so t hat the cracks disappear at the same time as their boundary surfaces are pr evented from getting into contact with each other. If they should get into contract, a defect in the form of an oxide flake should be created. The opt imum situations are assumed to be found when the depth of the crack decreas es more rapidly than the height of the slab at the same time as the crack w idth increases. For such rolling conditions it should be possible to find t he crack bottom on the workpiece surface without any trace let by the origi nal crack. During rolling two phenomena are present, counteracting each other. At the entrance to the roll gap, the crack widens, which is regarded as beneficial . The explanation to the phenomenon is that when the front edge of the crac k gets into contact with the roil it is dragged towards the gap at the same time as its velocity is rapidly changed from horizontal to become parallel to the tangential velocity of the roll. If this phenomenon did not occur, the risk for the creation of a permanent oxide Rake should be big. During t he passage of the gap the widened crack angle is decreased. The decrease of the angle is explained by the high hydrostatic pressure that is associated with the roll/workpiece contact surface. At the exit the angle is slightly increased once again which might be explained by the fact that the back ed ge of the crack is still in contact with the roll and its movement is retar ded because of friction. Thus the final result depends on which of the two phenomena, widening or contraction that has been dominating. The analysis shows that light reductions/pass, small roll radius and high f riction are preferred. A deep crack of V-shape is easily transformed to Y-s hape. This means that a remaining defect should be formed at the bottom of the crack. Reverse rolling is found to be beneficial because this way of ro lling implies that the crack does nor tilt so easily which should result in the formation of an oxide flake. (C) 2000 Published by Elsevier Science S. A. All rights reserved.