Tribological properties of magnetite precipitate from oxide scale in hot-rolled microalloyed steel

Nano-magnetite (Fe3O4) particles have a potential to lead to the formation of lubrication tribofilm that reduces the friction and wear in hot steel strip rolling. In this paper, an attempt to fabricate the oxide film with magnetite precipitates from thermally-grown wustite (Fe1−xO) layer during isothermal cooling of low carbon microalloyed steel, was obtained. The precipitation behaviors were investigated on Gleeble 3500 thermo-mechanical simulator under the humid air with water vapour content of 19.5 vol%. Several types of magnetite precipitates were examined using scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis. The tribological properties of magnetite precipitates were investigated in pin-on-disc configuration. It was found that the dispersed magnetite particles originate from either the pro-eutectoid precipitation above 570 °C or the partial decomposition of wustite below 570 °C. The oxide film on the presence of free particles during eutectoid precipitation could be a lubricant and consequently resist wear, particularly for the oxide scale with a typical thickness in the range of 8 to 11 μm in dry air and moisture atmosphere. Furthermore, characterisation and precipitation process of the oxide scale are discussed, with respect to a probable mechanism to explain the lubricated properties has been proposed.