Method for heating steel sheet, method for producing coated steel sheet, direct fired furnace, and continuous hot-dip galvanizing facility

Abstract

A method for heating a steel sheet includes heating a front surface side and a rear surface side of a steel sheet that is passing through a direct fired furnace that has an oxidation zone where operation is conducted at an air ratio of 1 or more and a reduction zone where operation is conducted at an air ratio of less than 1, in which the heating is carried out with flames injected from at least one slit burner while the steel sheet passes through at least the oxidation zone.

1 . A method for heating a steel sheet, the method comprising heating a front surface side and a rear surface side of a steel sheet that is passing through a direct fired furnace that has an oxidation zone where operation is conducted at an air ratio of 1 or more and a reduction zone where operation is conducted at an air ratio of less than 1, wherein the heating is carried out with flames injected from at least one slit burner while the steel sheet passes through at least the oxidation zone. 
     
     
         2 . The method for heating a steel sheet according to  claim 1 , wherein the direct fired furnace conveys the steel sheet in a vertical direction and suctions combustion exhaust gas from an exhaust port installed under the slit burner. 
     
     
         3 . The method for heating a steel sheet according to  claim 1 , wherein the air ratio in the oxidation zone is controlled to 1.00 or more and less than 1.50, and
 the air ratio in the reduction zone is controlled to 0.70 or more and less than 1.00.   
     
     
         4 . The method for heating a steel sheet according to  claim 1 ,
 wherein the steel sheet is heated with the at least one slit burner in such a range that a temperature of the steel sheet passing the oxidation zone is 400° C. or higher.   
     
     
         5 . A method for producing a coated steel sheet, the method comprising heat-treating a cold rolled steel sheet by the heating method according to  claim 1 , and subjecting the cold rolled steel sheet to a coating treatment. 
     
     
         6 . The method for producing a coated steel sheet according to  claim 5 , wherein the coating treatment uses one of an electrogalvanizing treatment, a hot-dip galvanizing treatment, and a galvannealing treatment. 
     
     
         7 . A direct fired furnace comprising:
 an oxidation zone where operation is conducted at an air ratio of 1 or more, a reduction zone where operation is conducted at an air ratio of less than 1, and a control device capable of controlling the air ratios in the oxidation zone and the reduction zone,   wherein at least one portion of the oxidation zone is equipped with:   at least one slit burner disposed on a front surface side of the steel sheet and at least one slit burner disposed on a rear surface side of the steel sheet, each of the slit burners extending in a width direction of the steel sheet and injecting flames toward the steel sheet passing through the oxidation zone and the reduction zone.   
     
     
         8 . The direct fired furnace according to  claim 7 , wherein the steel sheet is conveyed in a vertical direction, and combustion exhaust gas is suctioned from exhaust ports installed under the slit burners. 
     
     
         9 . The direct fired furnace according to  claim 7 ,
 wherein the air ratio in the oxidation zone is controlled to 1.00 or more and less than 1.50, and   the air ratio in the reduction zone is controlled to 0.70 or more and less than 1.00.   
     
     
         10 . The direct fired furnace according to  claim 7 , wherein at least one of the slit burners is installed in a range where a temperature of the steel sheet passing through the oxidation zone is 400° C. or higher. 
     
     
         11 . The direct fired furnace according to  claim 7 , wherein the oxidation zone includes a burner group having two or more slit burners for which the air ratio and a combustion rate are independently controllable. 
     
     
         12 . A continuous hot-dip galvanizing facility comprising the direct fired furnace according to  claim 7 . 
     
     
         13 . The continuous hot-dip galvanizing facility according to  claim 12 , further comprising an alloying facility that alloys hot-dip galvanized coatings.