The structure and properties of mill scale in relation to easy removal.

Oxide scale must be removed before cold drawing wire, otherwise it will cause
bad surface quality, inferior die life and many wire ruptures. The nature of oxide scales
and the methods of scale removal are reviewed, with particular emphasis on mechanical
descaling. This is the major concern of the research, therefore a cantilever bending test
has been developed to assess the ease of removal of the scale on commercial steel rod
surface in the laboratory. A scanner method and a Finite Element model have been
developed to evaluate the critical strain for scale cracking and removal after cantilever
bending. Scanning electron microscopy (SEM), together with electron backscattered
diffraction (EBSD), energy diffraction spectrum (EDS) and X-ray element mapping
analysis, was used to characterize the scale before and/or after bending test.
The effects of laying temperature, cooling conditions, ageing time, relative
humidity and temperature, and coil positions on scale cracking and removal behaviour
were studied. It was found that laying temperature has a larger effect on descalability
than cooling conditions. The effect of relative humidity and temperature on
descalability depended on a critical holding time. Beyond it, relative humidity and
temperature had no further effect on descalability. The higher the environmental
temperature, the less the critical holding time. Ageing time had an effect on
descalability, but the effect was relatively small.
Failure in tension started with first cracks formed at the places with high stress
concentration. As tensile strain increased, new cracks formed midway between the
existing cracks. Crack spacing stayed uniform but decreased until the scale segments
spalled off the rod surface. The crack spacing increased with scale thickness and
decreased with strain applied. Scale cracking and spallation mechanisms in
compression depended on the relative shear strength of the oxide, the buckling stability
of the layer and the relative shear strength of the interface. Spallation always required
the propagation of a crack at the interface.
The residual sub-layer left on the rod surface of EAF steel after the bending test
was identified as magnetite. On the same sample, copper enrichment was found at the
scale/metal interface, but within the metal side, and silicon enrichment was found at the
scale/metal interface, but within the scale side.