Behaviour of longitudinal surface cracks in the hot rolling of steel slabs

The behaviour of a longitudinal V-shaped crack, on the surface of a continu ously cast steel slab, is studied during hot rolling. The analysis is carri ed out by means of the commercial FE-code LS-DYNA3D. Process parameters obt ained from industry are used as a reference. The slab of initial width 1000 mm and thickness 220 mm is rolled down to 30 mm. It is assumed that the ma terial can be treated as rigid-perfectly plastic and that the cracks do not propagate. The latter assumption is in agreement with industrial observati ons for a steel grade similar to that analysed here. The aim of the study i s to investigate the possibility of controlling the plastic deformation so that the cracks disappear or so that their deteriorating effects are minimi sed. The analysis is focused upon the influence of friction, roll radius an d rolling schedule on the change in the shape of a crack of initial depth 2 0 mm and a crack angle of 6 degrees. The reliability of the simulations is checked by pilot-plant experiments using aluminium as the model material fo r steel. The results indicate that it is not possible to prevent the bottom side sur faces of the crack from coming into contact, especially not for small reduc tions/pass and small roll radii. The influence of friction was found to be marginal. Contact between the crack surfaces is found already at the beginn ing of the rolling, as the V-shaped crack is being changed to Y-shape. Cons idering the upper part of the crack, this remained open during the majority of the schedules studied. However for heavy reduction/pass and a large rol l radius, this part of the crack is closed also, but not before the final p asses. If the bottom side surfaces of the crack are in complete contact, th ey are prevented from further oxidisation. It is assumed that for such cond itions the deteriorating influence of the bottom part of the defect decreas es during subsequent rolling. During the elongation of the workpiece, the o xide flake of the Y-crack bottom is broken into splinters with oxide free m aterial in between, making the creation of a high performance weld possible . Provided that this supposition is correct, the best results should be obt ained for Light reductions/pass at the beginning of the rolling, resulting in an early closure of the crack bottom, followed by heavy reductions/pass enabling also the closure and oxide splintering of the upper part of the fo lded crack. (C) 1999 Published by Elsevier Science S.A. All rights reserved .