The influence of rolling temperature on roll wear, investigated in a new high temperature test rig

Abstract

Roll consumption represents a significant part of rolling cost. Investigations have shown that thermal deterioration of the rolls is more severe than other wear mechanisms. Cast-iron rolls show as fire-crack pattern after the rolling of only a few bars, whilst such defects occur in steel rolls, and especially in cemented-carbide rolls, only after a very long run. The temperature field and thermal stress in the rolls are influenced by the thermal and mechanical properties of the roll material. Cemented-carbide rolls are outstanding with respect to thermal stresses due to their excellent heat conduction, but the cooling problem is much more severe, since more heat is conducted into the roll body. A simple heat-transfer model is derived by means of successive stationary solutions of the heat conduction equation, enabling the thermal stress to be calculated easily if the oxide scale situation is determined. The results of experiments carried out using a high-temperature test rig have shown that the main wear mechanism in hot-work tool-steel rolls is abrasion, the wear rate increasing considerably when the rolling temperature is reduced to 900°C. However, further reduction of the rolling temperature, to below 800°C, causes the abrasive wear to be reduced to a level that is lower than at 1000°C. By an analysis of the thermal situation in the area of contact between the test roller and the hot disc, from consideration of the properties of the oxide layer present and by further development of a material-related abrasive-wear model, it is become clear why the abrasive wear is most severe at 900°C, thus vindicating the experimental results. The experimental results, and the analysis of the roll-wear situation, show that steel rolls are not suitable in rolling at 900°C. In this area conventional roll grades that are less sensitive to abrasion are better, if the rolling velocity is low. At high rolling velocity, on the other hand, cemented-carbide rolls are suitable. However, the roll-grade optimisation problem is unique for evrey rolling mill, since the temperature path and rolling velocity are entirely different, these factors being most important in the choice of roll material.