IMPROVEMENT OF PICKLING CHARACTERISTICS OF HOT-ROLLED STRIP THROUGH HEAT TREATMENT OF THE OXIDE SCALE

摘要

The oxide scale at the centre regions of hot-rolled strip can be successfully modified under continuous heating and cooling conditions. By heating the scale, either in air or in nitrogen, at a rate of 600 deg C/min to a target temperature in the range of 680 deg to 700 deg C, followed by rapid cooling at a rate of about 1200 deg C/min to 580 deg C, a substantial amount of the scale is converted to wustite. The wustite phase thus formed can be retained to room temperature at a cooling rate of 30 deg C/min from 580 deg C. If the target temperature increases to above 720 deg C, the entire scale layer is transformed to wustite if the heat treatment is conducted in nitrogen. An outer layer of magnetite, and presumably an extremely thin surface layer of hematite, forms when the heat treatment is conducted in air. When the heat treatment is conducted in nitrogen with a target temperature of above 700 deg C, there is an improvement in pickling rate of 20-25 percent. When the heat treatment is conducted in air, the improvement is about 10-15 percent when the target temperature is 680 deg -700 deg C, but the beneficial effect is reduced by the additional formation of oxide during the treatment when the target temperature is too high (e.g. 780 deg C).