Method of producing hot-dip galvanized steel sheet

Abstract

A method includes: annealing a steel strip by conveying the steel strip through a heating zone, a soaking zone, and a cooling zone in this order inside an annealing furnace; and applying a hot-dip galvanized coating onto the steel strip discharged from the cooling zone. Reducing gas or non-oxidizing gas supplied into the soaking zone includes humidified gas and dry gas. While a width and a sheet passing speed of the steel strip passing through the soaking zone are constant, a variation of pressure in the annealing furnace is suppressed by adjusting a flow rate of the dry gas, and a variation range of an amount of moisture supplied into the soaking zone by the humidified gas is limited to 20% or less.

1 . A method of producing a hot-dip galvanized steel sheet using a continuous hot-dip galvanizing apparatus that includes: an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in the stated order; and a hot-dip galvanizing line adjacent to the cooling zone, the method comprising:
 annealing a steel strip by conveying the steel strip through the heating zone, the soaking zone, and the cooling zone in the stated order inside the annealing furnace; and   applying a hot-dip galvanized coating onto the steel strip discharged from the cooling zone, using the hot-dip galvanizing line,   wherein reducing gas or non-oxidizing gas is supplied into the soaking zone, the reducing gas or the non-oxidizing gas including: humidified gas humidified by a humidifying device; and dry gas not humidified by the humidifying device, and   while a width and a sheet passing speed of the steel strip passing through the soaking zone are constant, a variation of pressure in the annealing furnace is suppressed by adjusting a flow rate of the dry gas, and a variation range of an amount of moisture supplied into the soaking zone by the humidified gas is limited to 20% or less.   
     
     
         2 . The method of producing a hot-dip galvanized steel sheet according to  claim 1 ,
 wherein a flow rate and a dew point of the humidified gas are set so that an amount of moisture M supplied into the soaking zone by the humidified gas and expressed in g/min satisfies the following Formula (1):
   40+ Vf ( W− 0.9)( S+ 4)/90< M< 60+ Vf ( W− 0.9)( S+ 4)/90  (1)
 
   where Vf is a volume of the soaking zone expressed in m 3 , W is the width of the steel strip passing through the soaking zone and expressed in m, and S is the sheet passing speed of the steel strip expressed in m/s.   
     
     
         3 . The method of producing a hot-dip galvanized steel sheet according to  claim 2 ,
 wherein when at least one of the width and the sheet passing speed of the steel strip passing through the soaking zone varies, the flow rate and the dew point of the humidified gas are changed so that the amount of moisture M expressed in g/min satisfies the Formula (1).   
     
     
         4 . The method of producing a hot-dip galvanized steel sheet according to  claim 1 ,
 wherein a dew point in the soaking zone measured at a dew point measurement port provided in the soaking zone is controlled to −25° C. or more and 0° C. or less, the dew point measurement port being provided in a region of upper ½ of the soaking zone in a height direction, and located 1 m or more away from a position of a supply port of the humidified gas provided in the soaking zone and 1 m or more away from an inner wall position of the soaking zone facing the supply port.   
     
     
         5 . The method of producing a hot-dip galvanized steel sheet according to  claim 1 ,
 wherein the heating zone includes a direct fired furnace,   the continuous hot-dip galvanizing apparatus includes an alloying line adjacent to the hot-dip galvanizing line, and   the method further comprises   heat-alloying the galvanized coating applied on the steel strip, using the alloying line.   
     
     
         6 . The method of producing a hot-dip galvanized steel sheet according to  claim 2 ,
 wherein a dew point in the soaking zone measured at a dew point measurement port provided in the soaking zone is controlled to −25° C. or more and 0° C. or less, the dew point measurement port being provided in a region of upper ½ of the soaking zone in a height direction, and located 1 m or more away from a position of a supply port of the humidified gas provided in the soaking zone and 1 m or more away from an inner wall position of the soaking zone facing the supply port.   
     
     
         7 . The method of producing a hot-dip galvanized steel sheet according to  claim 3 ,
 wherein a dew point in the soaking zone measured at a dew point measurement port provided in the soaking zone is controlled to −25° C. or more and 0° C. or less, the dew point measurement port being provided in a region of upper ½ of the soaking zone in a height direction, and located 1 m or more away from a position of a supply port of the humidified gas provided in the soaking zone and 1 m or more away from an inner wall position of the soaking zone facing the supply port.