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Behaviour of longitudinal surface cracks in the hot rolling of steel slabs
KTH, Superseded Departments, Materials Processing.
KTH, Superseded Departments, Materials Processing.
1999 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, Vol. 94, no 2-3, 141-150 p.Article in journal (Refereed) Published
Abstract [en]

The behaviour of a longitudinal V-shaped crack, on the surface of a continuously cast steel slab, is studied during hot rolling. The analysis is carried out by means of the commercial FE-code LS-DYNA3D. Process parameters obtained 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 material can be treated as rigid-perfectly plastic and that the cracks do not propagate. The latter assumption is in agreement with industrial observations for a steel grade similar to that analysed here. The aim of the study is to investigate the possibility of controlling the plastic deformation so that the cracks disappear or so that their deteriorating effects are minimised. The analysis is focused upon the influence of friction, roll radius and rolling schedule on the change in the shape of a crack of initial depth 20 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 for steel.

The results indicate that it is not possible to prevent the bottom side surfaces of the crack from coming into contact, especially not for small reductions/pass and small roll radii. The influence of friction was found to be marginal. Contact between the crack surfaces is found already at the beginning of the rolling, as the V-shaped crack is being changed to Y-shape. Considering the upper part of the crack, this remained open during the majority of the schedules studied. However for heavy reduction/pass and a large roll radius, this part of the crack is closed also, but not before the final passes. If the bottom side surfaces of the crack are in complete contact, they are prevented from further oxidisation. It is assumed that for such conditions the deteriorating influence of the bottom part of the defect decreases during subsequent rolling. During the elongation of the workpiece, the oxide flake of the Y-crack bottom is broken into splinters with oxide free material in between, making the creation of a high performance weld possible. Provided that this supposition is correct, the best results should be obtained 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 folded crack.

Place, publisher, year, edition, pages
1999. Vol. 94, no 2-3, 141-150 p.
Keyword [en]
longitudinal surface cracks; hot rolling; steel slabs
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-8195DOI: 10.1016/S0924-0136(99)00090-4ISI: 000083300600010OAI: oai:DiVA.org:kth-8195DiVA: diva2:13451
Note
QC 20100720Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-07-20Bibliographically approved
In thesis
1. Closed-die forging and slab hot rolling with focus on material yield: some industrial problems analysed by FEM
Open this publication in new window or tab >>Closed-die forging and slab hot rolling with focus on material yield: some industrial problems analysed by FEM
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The thesis is focussed on improving the material yield in closed-die forging and rolling. The former is restricted to the manufacturing of heavy crown wheels and front axle beams and the latter to the hot rolling of steel slabs. To enhance the yield the commercial FE-codes Form2D and Dyna3D are used. Results from forging simulations are strengthened by full scale experiments. The research is carried out in near contact with the Swedish steel and engineering industry. In closed die forging, two bulk forming problems are treated: How to improve the tool design and how to change the pre-form geometry for decreasing the amount of material exiting the flash gap? In slab rolling, two problems related to material defects are considered: How to eliminate existing surface cracks and how to prevent the formation of voids around macro-inclusions embedded in the steel matrix? Internal voids might be the reason for scrapping the whole workpiece.

Considering the forging of crown wheels, a new concept is proposed. For making the central hole of the product, the traditional method was forging a thin circular plate which was then sheared off and scrapped. Using the new technique this operation is replaced by forging a conical tap in the centre, which is then discarded. Doing so, the inner scrap material decreased with about 15%. The idea has been used in production for seven years. – Regarding the forging of front axle beams, a quasi-3D method is used comprising full scale measurements of the axial material flow. Here the theoretical material yield increased 2-7%.

Regarding rolling, the initial surface cracks in the simulations are V-shaped with a crack angle of 6o and of depth 5-20 mm. The inclusions are cylindrical and either three times harder or three times softer than the matrix. The behaviour of the cracks and the inclusions are studied as influenced by process parameters. Current industrial input data are used as a reference. – It is concluded that longitudinal cracks cannot be totally eliminated. During rolling their V-shape gradually changes to Y-shape and a remaining oxide flake separates the crack bottom surfaces. For minimizing the detrimental influence of the entrapped oxide, an early closure of the crack bottom is important. Throughout the remaining rolling schedule the entrapped oxide is then torn to pieces with large areas of virgin metal in between ensuring a strong bond. The following recommendations are given for the longitudinal cracks: Light drafts/pass at the beginning of the rolling schedule followed by heavy ones. – Contradictory to longitudinal cracks it is concluded that transversal cracks are possible to eliminate. When eliminated, the initial bottom of the crack coincides with the slab surface. No folds are formed. For the transversal cracks light drafts/pass are proposed together with reversal rolling, the latter for avoiding crack folding. – Voids are easiest formed around hard macro-inclusions in the centre of the slab. Large rolls and heavy drafts are recommended to avoid this.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. 41 p.
Series
Trita-IIP, ISSN 1650-1888 ; 08-01
Keyword
Material yield, Closed-die forging, Hot rolling, Cracks, Macro-inclusions, Steel, FEM
National Category
Production Engineering, Human Work Science and Ergonomics
Identifiers
urn:nbn:se:kth:diva-4690 (URN)978-91-7178-871-9 (ISBN)
Public defence
2008-04-25, Sal M311, KTH, Brinellvägen 68, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100720Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2010-07-20Bibliographically approved

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