Effect of O2 in Heating Atmosphere on Hydraulic Descaling in Hot Rolling of Low Carbon Steel

Low carbon steel was oxidized over the temperature range 1050–1250°C in O2–CO2–H2O–N2 gas mixtures. The oxidation times were 15–120 min, and the scales were 130–2000 μm thick. The experimental parameters were chosen to approximate scale formation under conditions similar to those of reheating furnaces in hot strip mills. In the hydraulic descaling tests, two modes of scale removal were observed. In the first mode, observed in classical three-layer scales that developed an inner porous layer with low or medium porosity, the horizontal undercutting occurred at the boundary of the inner porous layer and dense scale. The second mode was observed in classical three-layer scales that developed an inner porous layer with high porosity, and in crystalline scales. In the second mode, the horizontal undercutting occurred at the first plane of large pores relative to the scale/steel interface. A rise in the concentration of free oxygen resulted in an increase in thickness of the residual scale at 1050°C. At 1100°C and 1150°C, the increase in residual scale thickness with increasing free oxygen was significant in the initial 60 min of oxidation; after this period of time the differences in the thickness of the residual scale became smaller. At higher temperatures, the influence of free oxygen in the residual scale thickness was small at 1200°C and negligible at 1250°C. In general, the experiments showed that scale morphology controlled the removability of scale.