Year

2012

Degree Name

Master of Engineering (Mechanical)

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

The behaviour of the surface defects on continuously casting slabs during hot rolling is investigated by using commercial FE-code LS-DYNA3D. The objective of this study is to find out if surface cracks will disappear or be minimised after rolling.

The material used in the simulation models is austenitic stainless steel 304, which is a commercially used material in the manufacturing industry. The coupled thermomechanical method for simulation was implemented in this study. The main surface defects studied are edge cracks, transverse cracks, V-shaped transverse cracks and longitudinal cracks.

The simulation results show that it is hard for edge cracks to close after hot rolling, and the situation actually deteriorates. Edge cracks near the bottom of the slab are enlarged much more than those near the top surface of the slab. For transverse and longitudinal cracks, the two surfaces will come into contact with each other, causing the defects to be healed to some extent. Due to the closure of some of the crack area, the oxidation of these areas will be minimised. However, because of the non-uniformity of the flow stress around the cracks on the top surface of the slab, it is highly possible that it will form folder layers.

The simulated models also indicate the temperature distribution for the global strip and for the crack areas. By considering contact heat transfer, convection and radiation, it is found that contact heat transfer is the dominant factor affecting the temperature difference of both the work piece and the work roll. Heat generated from the plastic deformation of the strip only accounts for a small proportion of the temperature difference and would not compensate for the heat loss through the ambient air and through the strip/roll contact.

By analysing the temperature of the cracks and the slab, it is becoming possible to predict the formation of the chemical conditions which are likely to cause oxidation and defects.

It is recommended that the oxidation in the crack areas could be simulated to examine its effect on the crack evolution behaviour during hot rolling process. Experiments should be carried out to compare the simulation results with the experimental ones. By investigating the microstructure of the crack behaviour it becomes possible to take effective steps to avoid crack propagation and to ensure the surface quality of the final products.

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