Year

2011

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering, Faculty of Engineering

Abstract

Edge cracks affect the quality of rolled strip and the efficiency of production. The objective of this study is to improve our understanding of the evolution of edge cracks in cold rolling of thin strip. Experimental research and mechanics analysis were developed to investigate the effects of rolling parameters and initial microstructures on the evolution of edge cracks during rolling. Low carbon steel was prepared for experiments on a Hille 100 experimental rolling mill to examine the characteristics of edge cracks during cold rolling. The characteristics of edge cracks were evaluated by specific instruments, such as the Atomic Force Microscope (AFM), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and surface profile-meters. An Electron Backscattered Diffraction (EBSD) technique was also used to analyse the features of rolled strips with edge cracks. The results obtained from the experiments show that rolling parameters such as thickness reduction, rolling speed and lubrication have significant effects on the initiation and propagation of edge cracks. A crack on the break area produced by a worn trimmed edge is much larger than that on the cut area, and the surface quality of the break area affects the growth of the crack. The crack increases with the ratio of edge drop, which indicates that the edge drop has a significant effect on edge cracks. When the total reduction in thickness exceeds 85 % (for this experiment, the thickness is about 0.15 mm), the crack increases significantly and the reduction distribution also has a significant effect on the edge cracks. AFM and SEM observations have confirmed that the edge cracks originate from the porosity and the grooves of rougher surface. Rougher strip surface increases the concentration of stress and acts as a source of crack initiation during edge cracking. The initial microstructure of the rolled strip affects the initiation and propagation of edge cracks. Edge cracking was found to be sensitive to the size of the grain in the microstructure. The grain boundary is a barrier for edge crack initiation. A finer grain structure can increase the range of threshold stress intensity and also decrease the probability of crack initiation. A coarse microstructure resists crack propagation better due to blunted crack tip. Sheared edges and lubrication can delay the initiation and propagation of edge cracks. TEM shows that there are high density dislocations around the tip of edge crack and the inhomogeneous structure in a non-cracked area is a potential crack propagation direction. The stress intensity factor (SIF) solution to edge cracks has been investigated. The effective range of the stress intensity factor is of importance in defining crack growth rate. The efficiency and reliability of SIF analytical modelling has been demonstrated and the weight function was also applied in edge crack analysis. Three dimensional finite element method (FEM) was proposed to simulate the propagation of edge cracks in cold rolling of thin strip. The increasing pressure around the edge of the strip accelerates the propagation of edge cracks. More reduction causes more inhomogeneous deformation between the centre and the edge of rolled strip, which extends the edge cracks significantly. Light pass reduction and lubrication to reduce the propagation of edge cracks are recommended. The simulation provided further important information for improving the edge quality of rolled strip. The optimum condition to eliminate edge crack has been discussed and the proposed method for predicting edge crack can be utilised to make defect free products from rolling processes. It is recommended that using more lubrication could clarify how rolling conditions affect the evolution of edge cracks. The influence of the diameter of work rolls should be investigated, together with the testing of a wide range of materials including various chemical compositions and other steels. Further research about the effect of surface roughness and oxidation on edge cracks and the Crystal Plastic (CP) application in the analysis of the influence mechanism of microstructure on crack formation is also recommended.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.