Year

2005

Degree Name

Doctor of Philosophy

Department

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

Abstract

In hot steel rolling the continuous exposure of metal to air produces a layer of oxide scale on the surface of the steel. This layer of scale separates the work roll from the metal substrate. Its behavior and features affect the roll bite tribology and final quality of the product. This study aims to characterize the oxide scale features and improve our understanding of the behavior of oxide scale during hot rolling. Oxide scale surface topography features and the morphology of scale layers were analyzed by SEM, X-ray diffraction, optical microscopy, and AFM. The kinetics of growth of the layer of scale of low-carbon steel and a mild-steel were carried out on a Gleeble-3500 Thermal Mechanical Simulation System to simulate a hot finishing strip mill. An investigation of scale growth on an as-supplied surface proved that the growth of the original 21μm layer of scale decreased as the steel oxidized further. The effects of hot rolling on the surface roughness and oxide layers such as reduction, roll speed, and entry temperature, were investigated to identify any significant parameters. Hot rolling experiments were carried out on a HILLE-100 experimental rolling mill. Five lubricating modes were used in the rolling tests to examine the deformation behavior of the oxide scale layer and changes to its surface roughness, these included dry rolling and water, oil/water emulsions and pure oil. The effects of the original ground sample surface roughness in analyzing the outcome of surface features after hot rolling was also taken into account. The surface profiles of the scale layer and the picked steel surface, where the scale were removed with an inhibitor-added hydrochloric acid, were analyzed. The temperature and reduction showed a significant change, i.e., the lower the temperature the greater the reduction and less surface roughness. On the other hand, rolling speed had a limited effect on surface roughness. The surface roughness of the original sample affects the product profiles when reduction is smaller than 35% and also, the effect of lubrication depends on reduction. The effect of the thickness of the oxide scale on the roughness of the pickled surface is affected by lubrication. In this present study, the deformation behavior of oxide scale was investigated on two types of scale, the primary at 330-512μm in thickness and the secondary at 41-80μm in thickness. After rolling, it was observed that the thickness of secondary scale layer reduces more than the primary scale layer, while in turn the latter reduces more than the metal substrates. AFM images and SEM results demonstrate that the secondary oxide scale layer is a little ductile. Crack developments in both primary and secondary scale were discussed according to reduction and stalled rolling, respectively. It was found in the present study that fresh metal extruded up the cracks in the secondary oxide scale but were unable to touch the roll surface. An index γ was introduced to describe the secondary oxide scale expansion property due to hot rolling. An inverse calculation of the coefficient of friction found no significant effect on the coefficient of friction from the thickness of the secondary oxide scale layers at the exit of the roll bite. Two empirical equations that correlate the coefficient of friction μ with the reduction ε (%), roll speed v (m/s) and absolute temperature T have been obtained.

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