Patent ID: 11866800
Assignee: NIPPON STEEL CORPORATION
Field: Materials, metallurgy (Chemistry)
Classification: CPC C  B  Y | IPC B  C

Claim 6:
7. A method of manufacturing the steel sheet according to claim 1, comprising:
a hot-rolling process of heating a steel piece having the composition according to claim 1 to 1200° C. to 1320° C., completing hot rolling such that a hot rolling completion temperature is 880° C. or higher, and cooling the steel piece to obtain a hot-rolled steel sheet such that an average cooling rate in a temperature range of the hot rolling completion temperature to 500° C. is 20° C./s or faster;
a reheating process of heating the hot-rolled steel sheet to a temperature range of 500° C. to 700° C.;
a cooling process of cooling the hot-rolled steel sheet to room temperature;
a cold rolling process of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet such that a total rolling reduction is 60% to 90% and a cold rolling completion temperature is 250° C. or lower;
an annealing process of heating the cold-rolled steel sheet to an annealing temperature of 700° C. to 850° C. and cooling to a temperature range of 80° C. or lower; and
a temper rolling process of performing temper rolling on the cold-rolled steel sheet such that a total rolling reduction is 0.05% to 2.00%,
wherein in the reheating process,
Expression (3) is satisfied in a temperature range of 500° C. to 700° C., and
in the annealing process,
Expression (4) is satisfied in a temperature range of 700° C. to the annealing temperature during heating to the annealing temperature, and
Expression (5) is satisfied in the temperature range of 500° C. to 700° C. during cooling from the annealing temperature, and
bending is performed while applying a tension of 20 MPa or higher in a temperature range of 80° C. to 500° C.,, t
       n
      
      =
      
       
        10
        ⁢
        
         
          
           
            T
            
             n
             -
             1
            
           
           +
           273
          
          
           
            T
            n
           
           +
           273
          
         
         ·
         
          log
          10
         
        
        ⁢
        
         t
         
          n
          -
          1
         
        
       
       -
       
        
         (
         
          1
          -
          
           
            
             T
             
              n
              -
              1
             
            
            +
            273
           
           
            
             T
             n
            
            +
            273
           
          
         
         )
        
        ·
        21
        ·
        
         (
         
          1
          +
          
           2.5
           ·
           
            
             C
             ·
             
              (
              
               Nb
               +
               
                4
                ⁢
                Ti
               
              
              )
             
            
           
          
         
         )
        
       
       +
       
        Δ
        ⁢
        
         t
         K
        
       
      
     
     ⁢
     

     
      
       K
       n
      
      =
      
       
        (
        
         
          T
          n
         
         +
         273
        
        )
       
       ·
       
        {
        
         
          
           log
           10
          
          ⁢
          
           t
           n
          
         
         +
         
          21
          ·
          
           (
           
            1
            +
            
             2.5
             ·
             
              
               C
               ·
               
                (
                
                 Nb
                 +
                 
                  4
                  ⁢
                  Ti
                 
                
                )
               
              
             
            
           
           )
          
         
        
        }
       
      
     
     ⁢
     

                      
     
      
       K
       20
      
      ≥
      
       1.5
       ×
       
        10
        4
       
      
     
    
   
   
    
     Expression
     ⁢
        
     
      (
      3
      )
     
    
   
  
 

in Expression (3), K20 represents an index representing a degree of progress of precipitation of a Ti and/or Nb carbonitride in a 20th period when a temperature history in the temperature range of 500° C. to 700° C. of the reheating process is divided into 20 periods with respect to time,
tn and Kn are calculated when the temperature history in the temperature range of 500° C. to 700° C. of the reheating process is divided into 20 periods with respect to time and an average temperature in an n-th period is represented by Tn[° C.],
ΔtK represents a time [hr·] in one of 20 periods into which a total residence time in the same temperature range is divided,
each of C, Nb, and Ti represents a content [mass %] of the element, and t1=ΔtK,, R
       m
      
      =
      
       A
       ·
       
        
         Δ
         ⁢
         
          t
          R
          0.5
         
        
        
         K
         20
        
       
       ·
       
        exp
        ⁡
        (
        
         -
         
          B
          
           T
           m
          
         
        
        )
       
      
     
     ⁢
     

     
      1.
      ≤
      
       
        ∑
        
         i
         =
         1
        
        10
       
        
       
        R
        i
       
      
      ≤
      15.
     
    
   
   
    
     Expression
     ⁢
        
     
      (
      4
      )
     
    
   
  
 

in Expression (4), Ri represents an index representing a degree of progress of recrystallization in the temperature range of 700° C. to the annealing temperature and a degree of progress of diffusion of C from a Ti and/or Nb carbonitride present in a grain boundary into crystal grains,
Rm is calculated when a temperature history of the steel sheet from 700° C. to the annealing temperature during heating in the annealing process is divided into 10 periods with respect to time and an average temperature in an m-th period is represented by Tm[° C.],
ΔtR represents a time [s] in one of 10 periods into which a total residence time in the temperature range of 700° C. to the annealing temperature is divided,
K20 is a value obtained by Expression (3), and
A and B represent constant terms, A represents 9.67×109, and B represents 1.25×104, and, P
       k
      
      =
      
       
        D
        ·
        
         {
         
          
           E
           ·
           
            
             (
             
              700
              -
              
               T
               k
              
             
             )
            
            1.5
           
          
          +
          
           (
           
            700
            -
            
             T
             k
            
           
           )
          
         
         }
        
        ·
        
         exp
         ⁡
         (
         
          -
          
           F
           
            T
            k
           
          
         
         )
        
        ·
        Δ
       
       ⁢
       
        t
        p
        0.5
       
      
     
     ⁢
     

                      
     
      1.
      ≤
      
       
        R
        10
       
       ·
       
        
         ∑
         
          j
          =
          1
         
         10
        
         
        
         p
         j
        
       
      
      ≤
      15.
     
    
   
   
    
     Expression
     ⁢
        
     
      (
      5
      )
     
    
   
  
 

in Expression (5), Pj represents an index representing a degree of progress of precipitation of C in a temperature range of 700° C. to 500° C.,
Pk is calculated when a temperature history of the steel sheet from 700° C. to 500° C. during cooling in the annealing process is divided into 10 periods with respect to time and an average temperature in a k-th period is represented by Tk [° C.],
Δtp represents a time [s] in one of 10 periods into which a total residence time in the same temperature range is divided,
R10 represents a value obtained by substituting 10 into m of Rm in Expression (4), and
D, E, and F represent constant terms, D represents 4.47×104, E represents 2.11×100, and F represents 1.25×104.