Oxidation behaviour of roughing mill work rolls and effect on thermomechanical fatigueWednesday (26.09.2018) 17:30 - 17:45 S1/03 - 123 Part of:
Due to the growth of high-strength steel production, rolls for hot rolling mills are facing higher mechanical loads and increasing contact temperatures. Therefore, continuous improvements of wear behaviour and process safety are required. The rolling process itself depends strongly on the entire tribological system, giving rise to abrasive and adhesive wear, rolling fatigue and tribochemical reactions. So a roll material improvement needs to consider all these aspects. Work rolls are in contact with the hot strip and therefore their surfaces are exposed to high alternating temperatures. In combination with water vapour, caused by roll cooling, this leads to an accelerated high-temperature corrosion of the material. In state of the art four-high rolling stands the work roll is also in contact with the backup roll. Hence, the strip-induced alternating temperature occurs in combination with backup roll-induced alternating stresses. Consequently, thermomechanical fatigue might be a suitable approach for a process simulation. In roughing mills, where several high deformation passes at highest temperatures are carried out, this approach becomes even more valid. Also, the impact of oxidation on the thermomechanical fatigue is of particular importance.
The objective of the study presented is to improve the understanding of damage mechanisms and damage evolution in rolling processes. By means of the finite element method the periodic variation of temperature and stress at the roll surface needs to be calculated. Based on the results obtained, thermomechanical fatigue experiments must be carried out on a typical roughing work roll alloy. In order to understand the effect of oxidation processes, the high temperature corrosion behaviour of the alloy has been investigated first. This was done applying isothermal and thermally cyclic conditions in both, dry air and air mixed with water vapour. The oxidation kinetics were determined by means of continuous thermogravimetric analysis. The microstructure as well as the oxides formed were characterized by X-ray diffraction and scanning electron microscopy combined with energy dispersive X-ray spectroscopy. Furthermore, samples from worn out work rolls were taken and analysed, in order to compare real damage mechanisms with those occurring under laboratory conditions. The results of the microstructure analyses and the observed oxidation behaviour of the alloy will be presented and discussed in this contribution.