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.