Method for continuously annealing steel strip, apparatus for continuously annealing steel strip, method for manufacturing hot-dip galvanized steel strip, and apparatus for manufacturing hot-dip galvanized steel strip

A method for manufacturing a hot-dip galvanized steel strip, the method comprising:

• annealing the steel strip in a vertical annealing furnace including a heating zone and a soaking zone arranged in this order from an entry side of the furnace through which the steel strip is vertically conveyed, an atmosphere in the furnace being separated by a partition disposed within the heating zone, said partition having an opening through which the steel strip passes, a gas seal being provided at the opening in the partition to separate the atmosphere in the furnace on either side of the partition,
• wherein:
• an atmosphere gas is supplied from an outside of the furnace into the furnace to form a furnace gas that is discharged from a steel strip entrance at the entry side of the furnace;
• a part of the furnace gas is sucked and discharged into a refiner, which is disposed outside of the furnace, including a deoxygenator and a dehumidifier such that oxygen and moisture in the part of the furnace gas are removed to form a refiner gas having a lowered dew point, the annealing furnace including a plurality of gas suction ports positioned on downstream and upstream sides of the partition in both the heating zone and the soaking zone of the annealing furnace in order to take in gas from the downstream and upstream sides of the partition in the annealing furnace and a plurality of gas discharge ports disposed on the downstream and upstream sides of the partition to discharge gas from the refiner, wherein the plurality of gas suction ports are provided at different heights in a furnace height direction or the plurality of gas discharge ports are provided at different heights in the furnace height direction;
• and
• the refiner gas having the lowered dew point is returned into the furnace via the plurality of gas discharge ports;
• and
• controlling a temperature of the steel strip passing through the partition to a range of 550° C. to 700° C. so that a dew point of the atmosphere on the downstream side of the partition in the heating zone and the soaking zone of the furnace is lower than a dew point of the atmosphere on an upstream side of the partition and so that the dew point of the atmosphere on the downstream side of the partition in the heating zone and the soaking zone of the furnace is −48.8° C. or less and −54.8° C. or more at the gas suction ports.

A method for manufacturing a hot-dip galvanized steel strip, the method comprising:

• annealing the steel strip in a vertical annealing furnace including a heating zone and a soaking zone arranged in this order from an entry side of the furnace through which the steel strip is vertically conveyed, an atmosphere in the furnace being separated by a partition disposed within the heating zone,
• wherein:
• an atmosphere gas is supplied from an outside of the furnace into the furnace to form a furnace gas that is discharged from a steel strip entrance at the entry side of the furnace;
• a part of the furnace gas is sucked and discharged into a refiner, which is disposed outside of the furnace, including a deoxygenator and a dehumidifier such that oxygen and moisture in the part of the furnace gas are removed to form a refiner gas having a lowered dew point;
• and
• the refiner gas having the lowered dew point is returned into the furnace;
• and
• controlling a temperature of the steel strip passing through the partition to a range of 550° C. to 700° C. so that a dew point of the atmosphere on the downstream side of the partition in the heating zone and the soaking zone of the furnace is lower than a dew point of the atmosphere on an upstream side of the partition and so that the dew point of the atmosphere on the downstream side of the partition in the heating zone and the soaking zone of the furnace is −48.8° C. or less and −54.8° C. or more at gas suction ports positioned on the downstream side of the partition in both the heating zone and the soaking zone of the annealing furnace.