Quenching apparatus, quenching method, and method of manufacturing metal sheet

Abstract

A metal-sheet quenching apparatus cools a metal sheet while conveying the metal sheet and includes a cooling tank in which a cooling fluid is to be stored and the metal sheet is immersed in the cooling fluid to be cooled; restraining rolls that are installed inside the cooling tank and convey the metal sheet that has been cooled in the cooling tank while restraining the metal sheet in the thickness direction; a water-level adjustor that adjusts the height of a fluid surface of the cooling fluid inside the cooling tank; and a position control device that adjusts the height of the fluid surface of the cooling fluid inside the cooling tank by controlling the operation of the water-level adjustor.

1 . A metal-sheet quenching apparatus configured to cool a metal sheet while conveying the metal sheet, the metal-sheet quenching apparatus comprising:
 a cooling tank configured to store a cooling fluid and to cool the metal sheet when the metal sheet is immersed in the cooling fluid;   a restraining roll that is installed inside the cooling tank and that is configured to convey the metal sheet cooled in the cooling tank while restraining the metal sheet in a thickness direction;   a water-level adjustor configured to adjust a height of a fluid surface of the cooling fluid inside the cooling tank, the fluid surface being a cooling start position of the metal sheet; and   a position control device configured to control the height of the fluid surface of the cooling fluid inside the cooling tank by controlling an operation of the water-level adjustor.   
     
     
         2 . The metal-sheet quenching apparatus according to  claim 1 , further comprising a plurality of nozzles configured to jet the cooling fluid to the metal sheet to cool the metal sheet, the plurality of nozzles being installed inside the cooling tank. 
     
     
         3 . The metal-sheet quenching apparatus according to  claim 1 , wherein:
 the water-level adjustor includes:
 an adjustment tank that is configured to store the cooling fluid and that is connected to the cooling tank, 
 a supply source configured to supply the cooling fluid to the adjustment tank, and 
 a weir configured to control discharge of the cooling fluid from the adjustment tank, and 
   the water-level adjustor is configured to adjust the height of the fluid surface of the cooling fluid inside the cooling tank by adjusting a stored amount of the cooling fluid inside the adjustment tank.   
     
     
         4 . The metal-sheet quenching apparatus according to  claim 1 , wherein the position control device is configured to control the height of the fluid surface of the cooling fluid inside the cooling tank such that the restraining roll restrains the metal sheet at a position at which the metal sheet has a target temperature. 
     
     
         5 . The metal-sheet quenching apparatus according to  claim 4 , wherein the target temperature is in a temperature range of (TMs+150) (° C.) to (TMf−150) (° C.), where TMs (° C.) is a Ms temperature at which a martensitic transformation of the metal sheet starts and TMf (° C.) is a Mf temperature at which the martensitic transformation ends. 
     
     
         6 . The metal-sheet quenching apparatus according to  claim 4 , wherein:
 the position control device is further configured to set a distance from the cooling start position to the restraining roll based on a line speed of the metal sheet, a cooling start temperature of the metal sheet at a time when cooling in the cooling tank is started, the target temperature, and a cooling rate of cooling of the metal sheet, and   the position control device is configured to control the height of the fluid surface of the cooling fluid inside the cooling tank such that the set distance is achieved.   
     
     
         7 . The metal-sheet quenching apparatus according to  claim 6 , wherein the position control device is further configured to obtain a distance d (mm) from the cooling start position to the restraining roll by Formula (1) below: 
       
         
           
             
               
                 
                   
                     
                       d 
                       = 
                       
                         
                           ( 
                           
                             
                               T 
                               ⁢ 
                               1 
                             
                             - 
                             
                               T 
                               ⁢ 
                               2 
                             
                           
                           ) 
                         
                         × 
                         v 
                         / 
                         CV 
                       
                     
                     , 
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         where v (mm/s) is the line speed of the metal sheet, T1(C) is the cooling start temperature, T2(° C.) is the target temperature, and CV(° C./s) is the cooling rate of cooling of the metal sheet in the cooling tank. 
       
     
     
         8 . The metal-sheet quenching apparatus according to  claim 7 , wherein the position control device is further configured to set, based on a sheet thickness t of the metal sheet and a coefficient α indicating a condition of cooling of the metal sheet, the cooling rate CV as CV=α/t. 
     
     
         9 . The metal-sheet quenching apparatus according to  claim 2 , wherein a distance between the fluid surface of the cooling fluid inside the cooling tank and a hitting position on the metal sheet at which a fluid jet stream from each of the nozzles hits the metal sheet is more than or equal to 30 mm and less than or equal to 2000 mm. 
     
     
         10 . A metal-sheet quenching method in which a metal sheet is cooled while being conveyed, the method comprising:
 immersing the metal sheet in a cooling fluid that is stored in a cooling tank and cooling the metal sheet with a cooling start position set at a height of a fluid surface of the cooling fluid inside the cooling tank,   wherein the height of the fluid surface of the cooling fluid inside the cooling tank is adjusted such that the metal sheet is restrained by a restraining roll at a position at which the metal sheet has a target temperature.   
     
     
         11 . The metal-sheet quenching method according to  claim 10 , wherein the target temperature is in a temperature range of (TMs+150) (° C.) to (TMf−150) (° C.), where TMs(° C.) is a Ms temperature at which a martensitic transformation of the metal sheet starts and TMf(° C.) is a Mf temperature at which the martensitic transformation ends. 
     
     
         12 . The metal-sheet quenching method according to  claim 10 , further comprising:
 adjusting the height of the fluid surface of the cooling fluid to achieve a set distance from the cooling start position to the restraining roll,   wherein the set distance is based on a line speed of the metal sheet, a cooling start temperature of the metal sheet at a time when cooling is started, the target temperature, and a cooling rate of cooling of the metal sheet.   
     
     
         13 . The metal-sheet quenching method according to  claim 12 , wherein the set distance from the cooling start position to the restraining roll is a distance d (mm) obtained by Formula (1) below: 
       
         
           
             
               
                 
                   
                     
                       d 
                       = 
                       
                         
                           ( 
                           
                             
                               T 
                               ⁢ 
                               1 
                             
                             - 
                             
                               T 
                               ⁢ 
                               2 
                             
                           
                           ) 
                         
                         × 
                         v 
                         / 
                         CV 
                       
                     
                     , 
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         where v (mm/s) is the line speed of the metal sheet, T1(° C.) is the cooling start temperature, T2(° C.) is the target temperature, and CV(° C./s) is the cooling rate of cooling of the metal sheet. 
       
     
     
         14 . The metal-sheet quenching method according to  claim 13 , wherein the cooling rate CV is based on a sheet thickness t of the metal sheet and a coefficient α indicating a condition of cooling of the metal sheet according to the formula CV=α/t. 
     
     
         15 . A method of manufacturing a metal sheet, the method comprising the metal-sheet quenching method according to  claim 10 . 
     
     
         16 . A method of manufacturing a metal sheet, the method comprising performing any of a hot-dip galvanizing treatment, an electro-galvanizing treatment, or a hot-dip galvannealing treatment on a metal sheet obtained by the method according to  claim 15 . 
     
     
         17 . The metal-sheet quenching method according to  claim 10 , further comprising:
 jetting the cooling fluid, from a nozzle that is installed inside the cooling tank, to the metal sheet to cool the metal sheet, and   wherein a distance between the fluid surface of the cooling fluid inside the cooling tank and a hitting position on the metal sheet at which a fluid jet stream from the nozzle hits the metal sheet is more than or equal to 30 mm and less than or equal to 2000 mm.