The choice of annealing heat cycle of annealing furnace and its temperature annealing knowledge

       The choice of annealing heat cycle of annealing furnace and its temperature annealing knowledge

       For cold-rolled strip steel, the faster the heating speed during annealing, the shorter the residence time in different temperature regions, and the higher the required recrystallization temperature; on the contrary, the slower the heating speed of strip steel during annealing, that is, the slower the heating speed during annealing. The longer the residence time in the hot zone, the lower the recrystallization temperature. This produces a high recrystallization temperature annealing heat cycle and a low recrystallization temperature annealing heat cycle.

       The high recrystallization temperature annealing cycle of the Suzhou annealing furnace is to heat the strip to the peak temperature and then immediately cool it down (sometimes it is slow cooling for a short time before the rapid cooling to obtain a stable metal structure). Its advantage is that it can be applied to A short, uncomplicated (low investment) production line; the disadvantage is that the strip is heated to a higher temperature and is wasted in the cooling process, so it consumes more energy.

      The low recrystallization temperature annealing heat cycle means that the annealing temperature of the strip is lower than the peak temperature. In order to compensate for the energy, the strip is soaked for a long time and then quickly cooled. The advantage is that the strip steel is heated at a lower temperature, consumes less energy, and has less strict requirements for the cooling process; the disadvantage is that the soaking time is longer, and it is generally used in a longer (high investment) production line.

      The annealing furnace uses a low recrystallization temperature annealing heat cycle to reflect the design requirements for lower energy consumption.

      ①Level 1 waste heat utilization, that is, the strip steel in the preheating section obtains energy from the waste flue gas generated in the direct combustion section at the beginning, and the waste flue gas heats the cold strip steel in the furnace in the form of natural heat exchange.

      ②Level 2 waste heat utilization, that is, at a position 10m away from the outlet of the preheating section, an air injection pipe is installed twice, and the strip temperature at this position is 275C under normal production conditions. During normal production, the flue gas produced in the direct combustion section is above the auto-ignition temperature (760°C). In the entire preheating section, the sprayed air is mixed with reducing flue gas (including unburned fuel in the direct combustion section) and burned to heat the steel strip.

      ③Level 3 waste heat utilization, that is, under normal operation conditions, the temperature of the flue gas generated by the combustion leaving the preheating section is still very high (800℃), so a convection heat exchanger is designed at the flue gas outlet to use the waste heat of the waste flue gas to preheat The combustion-supporting air in the hot direct combustion section improves the heat utilization rate.

      ④The fourth-level waste heat utilization of the annealing furnace, that is, the strip steel in the direct combustion section is directly heated by gas combustion, and there are three heating methods: heat conduction, convection, and radiation, so the direct combustion section has a high thermal efficiency. In the heating section of the radiant tube, indirect heating is used to prevent the protective gas from being polluted. Therefore, the gas is burned in a special Ni-Cr alloy radiant tube. The alloy tube is heated by the gas, and then the heat is radiated to the strip steel. This method is an indirect heating method with low thermal efficiency.