An investigation of the conditions leading to strip adhesion in industrial steel coils
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Date
2001-08
Authors
Wakaba, Lulama Velile
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
During the production of steel strip, a significant amount of work hardening takes place
when the steel is rolled into thin strips, which are stored in a coiled form. These steel
coils are batch annealed in order to reduce the hardness and restore formability, before
further processing takes place. The development of diffusion welds between spirals of
steel coils, during batch annealing, is of particular interest because it prevents the coils
from being unwound for further use. This problem is often referred to as strip adhesion or
stickering.
In order for strip adhesion to develop, it is essential for some coil spirals to be in contact,
while the inter-diffusion between spirals takes place. Furthermore, high temperatures also
aid in the diffusion process. It is therefore useful to study the temperature and resulting
thermal stress distributions in the coil, during batch annealing. The temperature
distribution allows for the calculation of thermal stress, which is the driving force for
establishing contact between spirals, and in addition to this, the temperature distribution
also provides some clues regarding the likelihood of inter-diffusion.
In this study, models of temperature and stress are presented. A two-dimensional finite
difference model for temperature is presented and confirmed by an analytical solution of
the same problem. Analysis of a three-dimensional temperature model in the third chapter
shows that, as far as heat transfer is concerned, a cylindrical coil can be well
approximated by a solid cylinder with a concentric hole. All the temperature modeling
was done for the interior of a coil. Further discussion in later chapters shows that a
cylindrical coil can also be treated as a solid cylinder for thermal stress modeling.
A long cylinder stress model in the fifth chapter provides some useful insight, as far as
strip adhesion is concerned, even though it is a one-dimensional model that does not
consider the effect of axial heat transfer. The radial compressive stress during the cooling
stage of the batch annealing process was identified as a paramount ingredient to strip
adhesion. The thermal stress calculations are later extended to include a linear cooling temperature
ramp and these results are arguably the most important findings of this study. According
to these results, when the a cylinder is cooled in such a way that the outer edge lags
behind the cooling of the inner edge, by a few hours, the compressive radial stress is
greatly reduced. Consequently, the contact pressure between spirals at the most critical
stage of batch annealing (where strip adhesion occurs) is decreased and the chance of
strip adhesion developing is reduced.
Description
Keywords
Adhesion, Thermal stresses, Heat -- Transmission, Dissertation (M.Sc. (Physics))--University of the Free State, 2001