In the whole process of hot dip galvanization, the other important process is the in-line annealing process of strip steel.In the continuous hot galvanizing process of strip steel, the strip steel should achieve the following two purposes after continuous annealing:
(1) complete the recrystallization annealing process of strip steel, eliminate the hardening phenomenon in the process of cold rolling, and restore its process plasticity for further cold processing. At the same time, as the final product heat treatment, it is necessary to control the properties of the finished product, and obtain the combination of different strength and plasticity.,
(2) cool the strip to the temperature of the zinc pot, so that the strip has a clean active surface free of oxide, and seal the strip into the zinc pot for galvanizing.The whole annealing furnace is mainly divided into heating section, heat balance section and jet cooling section.
1. The heating strip of strip steel will be heated to recrystallization temperature in the heating section.The heating section is divided into 5 zones along the direction of furnace length, each zone can be single
Control the size of gas flow.There are two thermocouples in each zone. One is used to measure the furnace temperature in this area, and the other is used to make the overtemperature alarm. When the furnace temperature is too high, the load is automatically reduced to prevent damage to the furnace equipment.The operator can adjust the gas supply of the area by setting the furnace and temperature in each area, that is, adjusting the load of the area.Due to the strip in the furnace temperature is low, in the process of running from one area to 5 area temperature rising, as a result, from 1 to 5 areas furnace temperature control from low to high, in 1 area due to the strip temperature minimum, so the temperature control to the lowest, and then, in turn, increases, thus ensuring strip annealing treatment according to above annealing curve.In operation, attention should be paid to balance the load of each area, and try to control the load of each area as much as possible.Have radiant panel temperature meter in heating period of export, can measure the strip steel plate temperature, annealing process is the key process parameters, for CO, plate temperature must be achieved above 730 ℃, otherwise, the strip have fallen below the recrystallization temperature, the mechanical properties of the final product to reach requirements.When the plate temperature does not reach the requirement, the operator should raise the furnace temperature of each area appropriately until the requirement is reached. However, if the plate temperature is too high, it will cause energy waste. At this time, the operator should reduce the furnace temperature of each area according to the situation.At the same time, due to the high thermal hysteresis of the furnace, it is necessary to pay attention to avoid sharp rise and fall of the furnace temperature, and maintain a certain rate of rise and drop of temperature according to the actual situation, otherwise the temperature of the plate will fluctuate greatly, which will affect the stability of mechanical properties of the strip steel.
2. Uniform heat of strip steel
After the resetting temperature of the strip reaches 1, it is necessary to maintain the recrystallization temperature sequence in the heating section for some time to complete the resetting process.The heat balance section is a zone where the operator can control the furnace temperature according to the thermocouple of temperature measurement. The final requirement is to maintain the plate temperature of the steel strip.There is also a thermocouple in the area for overtemperature alarm
3. Cooling of strip steel
In complete after annealing, the strip will need to be cooled to galvanized temperature 460-520 ℃, the temperature too high or too low will affect the quality of strip steel galvanized and zinc pot temperature control.There are 5 cooling units in the cooling section, each of which can control the load separately.A plate thermometer is installed at the outlet of the cooling section, and the operator controls the plate temperature of the strip by adjusting the load of each section.
4. Pressure and atmosphere in the furnace,
In the annealing furnace, when the annealing process is completed, the oxide layer on the surface of the strip steel shall be reduced, and the active surface shall be obtained before galvanizing, so that the band steel can be well combined with the zinc layer when galvanizing.Therefore, the protective gas should be continuously injected into the furnace during production.The main components of the protective gases are nitrogen and hydrogen, with an average hydrogen content of 15%.Proportion because hydrogen is lighter, hydrogen concentration in the annealing furnace is at the top of the stove, this will make under the strip surface reduction is affected, so in a horizontal furnace hydrogen, content, type due to the strip moves up and down, in the steel strip surface can up and down with a good contact with protective atmosphere, so the hydrogen content can be lower, generally in 5-10%. The annealing furnace is the sealing structure, but it can't be completely sealed, inevitably there is gas leakage.In order to prevent the oxygen in the air from entering the furnace, the furnace should maintain a small positive pressure, which should be controlled at about 2mbar.The operator can control the amount of protective gas injected into the furnace.
There are two key parameters to reflect the protective atmosphere in the furnace. One is the oxygen content, which requires less than 1000ppm.Another is the dew point, requires less than 20 ℃, the dew point is too high, the strip will be oxidized.In normal operation, if the dew point or oxygen content is found to be too high, first of all, the protective gas supplied by the protective gas station should be checked to see if the body meets the technical requirements. If the protective gas is normal, the leakage point of the furnace must be found out and repaired