Method of lowering dew point of amibient gas within annealing furnace, device thereof, and method of producing cold-rolled annealed steel sheet

Abstract

Part of an ambient gas in a heating zone and/or a soaking zone is sucked out and is cooled through a high-temperature gas passage of a heat exchanger by heat exchange with a gas in a low-temperature gas passage, further cooled by mixing with part of an ambient gas in a cooling zone, further cooled through a gas cooler, dehumidified to a dew point of −45° C. or less in a dryer, heated through the low-temperature gas passage of the heat exchanger by heat exchange with a gas in the high-temperature gas passage, and returned to the heating zone and/or the soaking zone.

1 - 5 . (canceled) 
     
     
         6 . A method of lowering the dew point of a furnace ambient gas in a continuous annealing furnace that anneals a metal strip in a reducing atmosphere by passing the metal strip through a heating zone and a cooling zone in this order or through a heating zone, a soaking zone, and a cooling zone in this order, comprising:
 (a) providing a circulator that includes a heat exchanger for heat exchange between a low-temperature gas and a high-temperature gas, a gas cooler for cooling a gas, and a dryer for dehumidifying a gas to a dew point of −45° C. or less;   (b) sucking part of the ambient gas from the heating zone and/or the soaking zone;   (c) passing the sucked part of the ambient gas through a high-temperature gas passage of the heat exchanger and decreasing the temperature of the sucked part of the ambient gas by heat exchange with a gas in a low-temperature gas passage;   (d) mixing the part of the ambient gas having a decreased temperature with part of an ambient gas sucked from the cooling zone to further decrease the temperature of the part of the ambient gas;   (e) passing the part of the ambient gas further cooled by mixing with the part of the ambient gas sucked from the cooling zone through the gas cooler to further cooling the part of the ambient gas;   (f) dehumidifying the part of the ambient gas further cooled through the gas cooler to a dew point of −45° C. or less in the dryer;   (g) passing the dehumidified part of the ambient gas through the low-temperature gas passage of the heat exchanger to increase the temperature of the dehumidified part of the ambient gas by heat exchange with a gas in the high-temperature gas passage; and   (h) returning the part of the ambient gas having an increased temperature to the heating zone and/or the soaking zone.   
     
     
         7 . The method according to  claim 6 , wherein the part of gas flowing from the dryer toward the low-temperature gas passage of the heat exchanger is returned directly to the cooling zone without passing through the heat exchanger. 
     
     
         8 . A device that lowers the dew point of an ambient gas in a continuous annealing furnace that anneals a metal strip in a reducing atmosphere by passing the metal strip through a heating zone and a cooling zone in this order or through a heating zone, a soaking zone, and a cooling zone in this order, comprising:
 a gas passage including a heat exchanger that exchanges heat between a low-temperature gas and a high-temperature gas, a gas cooler that cools a gas, a dryer that dehumidifies a gas to a dew point of −45° C. or less, and a gas mixer,   a gas passage extending from the heating zone and/or the soaking zone through a gas passage to a high-temperature gas passage of the heat exchanger and through the gas cooler to the dryer,   a second gas passage extending from the dryer to a low-temperature gas passage of the heat exchanger and from the heat exchanger to the heating zone and/or the soaking zone, and   a third gas passage extending from the cooling zone, the third gas passage being connected to a fourth gas passage extending from the gas cooler to the dryer in the gas mixer.   
     
     
         9 . The device according to  claim 8 , further comprising a fifth gas passage that returns part of gas flowing from the dryer toward the low-temperature gas passage of the heat exchanger directly to the cooling zone through a gas distributor but without passing through the heat exchanger. 
     
     
         10 . A method of producing a cold-rolled and annealed steel sheet, comprising continuously annealing a cold-rolled steel strip, wherein
 the dew point of an ambient gas in a continuous annealing furnace is reduced by the method according to  claim 6  during the continuous annealing.   
     
     
         11 . A method of producing a cold-rolled and annealed steel sheet, comprising continuously annealing a cold-rolled steel strip, wherein
 the dew point of an ambient gas in a continuous annealing furnace is reduced by the method according to  claim 7  during the continuous annealing.