Patent ID: 11885025
Assignee: NIPPON STEEL CORPORATION
Field: Materials, metallurgy (Chemistry)
Classification: CPC C | IPC C

Claim 8:
9. A manufacturing method of the high-strength steel sheet excellent in formability and impact resistance according to claim 1, the method comprising: providing a steel sheet for heat treatment by performing:
a hot rolling process of heating a cast slab comprising components according to claim 1 to a temperature in a range from 1080 degrees C. to 1300 degrees C., and subsequently subjecting the cast slab to hot rolling, in which hot rolling conditions in a temperature region from a maximum heating temperature to 1000 degrees C. satisfy a formula (A) and a hot rolling completion temperature falls in a range from 975 degrees C. to 850 degrees C.;
a cooling process in which cooling conditions applied from the completion of the hot rolling to 600 degrees C. satisfy a formula (2) that represents a sum of transformation progress degrees in 15 temperature regions obtained by equally dividing a temperature region ranging from the hot rolling completion temperature to 600 degrees C., and a temperature history that is measured by every 20 degrees C. from a time when 600 degrees C. is reached to a time when an intermediate heat treatment below is started satisfies a formula (3);
a cold rolling process of cold rolling at a rolling reduction of 80% or less; and
an intermediate heat treatment process comprising: heating the cold-rolled cast slab to a temperature in a range from (Ac3−30) degrees C. to (Ac3+100) degrees C. at an average heating rate of at least 30 degrees C. per second in a temperature region ranging from 650 degrees C. to (Ac3−40) degrees C.; limiting a dwell time in a temperature region ranging from the heating temperature to (maximum heating temperature −10) degrees C. to 100 seconds or less; and subsequently cooling the cast slab from the heating temperature at an average cooling rate of at least 30 degrees C. per second in a temperature region ranging from 750 degrees C. to 450 degrees C.; and
performing a main heat treatment process comprising:
heating the steel sheet for heat treatment to a temperature ranging from (Ac1+25) degrees C. to an Ac3 point so that a temperature history from 450 degrees C. to 650 degrees C. satisfies a formula (B) below and subsequently a temperature history from 650 degrees C. to 750 degrees C. satisfies a formula (C) below;
retaining the steel sheet for heat treatment for 150 seconds or less at the heating temperature;
cooling the steel sheet for heat treatment from the heating retention temperature to a temperature region ranging from 550 degrees C. to 300 degrees C. at an average cooling rate of at least 10 degrees C. per second in a temperature region from 700 degrees C. to 550 degrees C.;
setting a dwell time in the temperature region from 550 degrees C. to 300 degrees C. to 1000 seconds or less; and
setting dwell conditions in the temperature region from 550 degrees C. to 300 degrees C. to satisfy a formula (4) below,, ∑
            
              i
              =
              1
            
            n
          
          ⁢
          
            [
            
              A
              ·
              
                
                  
                    h
                    i
                  
                  -
                  
                    h
                    
                      i
                      -
                      1
                    
                  
                
                
                  h
                  i
                
              
              ·
              
                exp
                ⁡
                
                  (
                  
                    -
                    
                      B
                      
                        
                          T
                          i
                        
                        +
                        
                          2
                          ⁢
                          7
                          ⁢
                          3
                        
                      
                    
                  
                  )
                
              
              ·
              
                t
                
                  0
                  .
                  5
                
              
            
            ]
          
        
        ≧
        
          
            1
            .
            0
          
          ⁢
          0
        
      
    
    
      
        (
        A
        ), n: rolling pass number up to 1000 degrees C. after removal from a heating furnace, hi: finishing sheet thickness [mm] after i pass, Ti: rolling temperature [degrees C.] at the i pass, ti: elapsed time [second] after the rolling at the i pass to an (i+1) pass

A=9.11×107,B=2.72×104:constant value, (
            
              
                ∑
                
                  n
                  =
                  1
                
                
                  1
                  ⁢
                  5
                
              
              ⁢
              
                [
                
                  
                    
                      
                        
                          1
                          .
                          8
                        
                        ⁢
                        8
                        ×
                        1
                        ⁢
                        
                          0
                          2
                        
                      
                      
                        1
                        +
                        
                          17
                          ⁢
                          Ti
                        
                        +
                        
                          51
                          ⁢
                          Nb
                        
                        +
                        
                          3.3
                          ⁢
                          
                            Mo
                          
                        
                        +
                        
                          3
                          ⁢
                          5
                          ⁢
                          
                            B
                          
                        
                      
                    
                    ·
                    exp
                  
                  ⁢
                  
                    
                      {
                      
                        
                          3
                          ⁢
                          
                            6
                            .
                            1
                          
                        
                        -
                        
                          
                            (
                            
                              
                                
                                  0
                                  .
                                  0
                                
                                ⁢
                                4
                                ⁢
                                2
                                ⁢
                                4
                              
                              -
                              
                                
                                  0
                                  .
                                  0
                                
                                ⁢
                                0
                                ⁢
                                2
                                ⁢
                                7
                                ⁢
                                n
                              
                            
                            )
                          
                          ⁢
                          T
                          ⁢
                          f
                        
                        -
                        
                          
                            1
                            .
                            6
                          
                          ⁢
                          4
                          ⁢
                          n
                        
                        -
                        
                          14.4
                          ⁢
                          C
                        
                        +
                        
                          0.62
                          ⁢
                          Si
                        
                        -
                        
                          1.36
                          ⁢
                          Mn
                        
                        +
                        
                          0.82
                          ⁢
                          Al
                        
                        -
                        
                          0.62
                          ⁢
                          Cr
                        
                        -
                        
                          0.62
                          ⁢
                          Ni
                        
                        -
                        
                          

                        
                        ⁢
                        
                          
                        
                        ⁢
                        
                          
                            2.85
                            ×
                            
                              10
                              4
                            
                          
                          
                            253
                            +
                            
                              
                                (
                                
                                  
                                    
                                      1
                                      .
                                      0
                                    
                                    ⁢
                                    3
                                    ⁢
                                    3
                                  
                                  -
                                  
                                    
                                      0
                                      .
                                      0
                                    
                                    ⁢
                                    6
                                    ⁢
                                    7
                                    ⁢
                                    n
                                  
                                
                                )
                              
                              ⁢
                              T
                              ⁢
                              f
                            
                            +
                            
                              4
                              ⁢
                              0
                              ⁢
                              n
                            
                          
                        
                      
                      }
                    
                    ·
                    
                      
                        t
                        ⁡
                        
                          (
                          n
                          )
                        
                      
                      0.25
                    
                  
                
                ]
              
            
            )
          
          
            
              0
              .
              3
            
            ⁢
            3
            ⁢
            3
          
        
        ≤
        1.00
      
    
    
      
        (
        2
        ), t(n): dwell time [second] in the n-th temperature region, element symbol: mass % of the element, Tf: hot rolling completion temperature [degrees C.], ⁢
        
          
            1.00
            ≤
            
              
                [
                
                  
                    
                      T
                      n
                    
                    ·
                    
                      {
                      
                        
                          
                            log
                            
                              1
                              ⁢
                              0
                            
                          
                          ⁡
                          
                            (
                            
                              t
                              n
                            
                            )
                          
                        
                        +
                        C
                      
                      }
                    
                  
                  
                    
                      1
                      .
                      5
                    
                    ⁢
                    0
                    ×
                    
                      10
                      4
                    
                  
                
                ]
              
              2
            
            ≤
            
              
                1
                .
                5
              
              ⁢
              0
            
          
          ⁢
          
            

          
          ⁢
          
            
          
          ⁢
          
            
              t
              1
            
            =
            
              Δ
              ⁢
              
                t
                1
              
              ⁢
              
                
              
              ⁢
              
                (
                
                  n
                  =
                  1
                
                )
              
            
          
          ⁢
          
            

          
          ⁢
          
            
          
          ⁢
          
            
              t
              n
            
            =
            
              
                Δ
                ⁢
                
                  t
                  n
                
              
              +
              
                
                  
                    
                      T
                      
                        n
                        -
                        1
                      
                    
                    
                      T
                      n
                    
                  
                  ·
                  
                    {
                    
                      
                        
                          log
                          
                            1
                            ⁢
                            0
                          
                        
                        ⁡
                        
                          (
                          
                            t
                            
                              n
                              -
                              1
                            
                          
                          )
                        
                      
                      +
                      C
                    
                    }
                  
                
                ⁢
                
                  
                
                ⁢
                
                  (
                  
                    n
                    >
                    1
                  
                  )
                
              
            
          
          ⁢
          
            

          
          ⁢
          
            C
            =
            
              
                2
                ⁢
                
                  0
                  .
                  0
                
                ⁢
                0
              
              -
              
                
                  1
                  .
                  2
                
                ⁢
                
                  8
                  ·
                  
                    Si
                    
                      0
                      .
                      5
                    
                  
                
              
              -
              
                
                  0
                  .
                  1
                
                ⁢
                
                  3
                  ·
                  
                    Mn
                    
                      0
                      .
                      5
                    
                  
                
              
              -
              
                
                  0
                  .
                  4
                
                ⁢
                
                  7
                  ·
                  
                    Al
                    
                      0
                      .
                      5
                    
                  
                
              
              -
              
                1.20
                ·
                Ti
              
              -
              
                2.50
                ·
                Nb
              
              -
              
                0.82
                ·
                
                  Cr
                  
                    0
                    .
                    5
                  
                
              
              -
              
                
                  1
                  .
                  7
                
                ⁢
                
                  0
                  ·
                  
                    Mo
                    0.5
                  
                
              
            
          
        
      
    
    
      
        (
        3
        )
      
    
  

Tn: an average steel sheet temperature [degrees C.] from the (n−1)th calculation time point to the n-th calculation time point
tn: effective total time [hour] for carbide growth at the n-th calculation
Δtn: an elapsed time [hour] from the (n−1)th calculation time point to the n-th calculation time point
C: parameters related to a growth rate of carbides (element symbol: mass % of element), ⁢
        
          
            
              a
              0
            
            =
            
              
                1
                .
                0
              
              ⁢
              0
            
          
          ⁢
          
            

          
          ⁢
          
            
          
          ⁢
          
            
              a
              n
            
            =
            
              
                
                  F
                  
                    C
                    n
                  
                
                ·
                
                  
                    t
                    n
                  
                  
                    (
                    
                      1
                      K
                    
                    )
                  
                
              
              +
              
                10
                
                  (
                  
                    
                      
                        
                          354
                          +
                          
                            5
                            ⁢
                            n
                          
                        
                        
                          359
                          +
                          
                            5
                            ⁢
                            n
                          
                        
                      
                      ·
                      
                        log
                        10
                      
                    
                    ⁢
                    
                      
                    
                    ⁢
                    
                      a
                      
                        n
                        -
                        1
                      
                    
                  
                  )
                
              
            
          
          ⁢
          
            

          
          ⁢
          
            
          
          ⁢
          
            
              K
              +
              
                
                  log
                  
                    1
                    ⁢
                    0
                  
                
                ⁢
                
                  
                
                ⁢
                
                  a
                  
                    2
                    ⁢
                    0
                  
                
              
            
            ≤
            3.20
          
          ⁢
          
            

          
          ⁢
          
            
              
                
                  
                    C
                    n
                  
                  :
                
              
              ⁢
              
                
              
              ⁢
              
                
                  {
                  
                    1.28
                    +
                    
                      34
                      ·
                      
                        
                          (
                          
                            1
                            -
                            
                              
                                
                                  8
                                  ⁢
                                  9
                                
                                +
                                
                                  2
                                  ⁢
                                  n
                                
                              
                              
                                1
                                ⁢
                                3
                                ⁢
                                0
                              
                            
                          
                          )
                        
                        2
                      
                    
                  
                  }
                
                ·
                
                  Si
                  0.5
                
              
            
            +
            
              0.13
              ·
              
                Mn
                0.5
              
            
            +
            
              0.47
              ·
              
                Al
                0.5
              
            
            +
            
              0.82
              ·
              
                Cr
                0.5
              
            
            +
            
              1.70
              ·
              
                Mo
                0.5
              
            
          
        
      
    
    
      
        (
        B
        ), each element of the chemical composition represents an added amount [mass %],, F: constant value, 2.57, tn: elapsed time [second] from (440+10n) degrees C. to (450+10n) degrees C., K: a value of a middle side of the formula (3), 1.00
        ≤
        
          
            ∑
            
              n
              =
              1
            
            
              1
              ⁢
              0
            
          
          ⁢
          
            
              M
              
                N
                +
                P
              
            
            ·
            
              exp
              ⁡
              
                (
                
                  -
                  
                    Q
                    
                      
                        9
                        ⁢
                        1
                        ⁢
                        8
                      
                      +
                      
                        1
                        ⁢
                        0
                        ⁢
                        n
                      
                    
                  
                
                )
              
            
            ·
            
              
                t
                n
              
              0.5
            
          
        
        ≤
        5.00
      
    
    
      
        (
        C
        ), M: constant value, 5.47×1010, N: a value of the left side of the formula (B), P: 0.38Si+0.64Cr+0.34Mo
each element of the chemical composition represents an added amount [mass %],, Q: 2.43×104, tn: elapsed time [second] from (640+10n) degrees C. to (650+10n) degrees C., [
              
                
                  ∑
                  
                    n
                    =
                    1
                  
                  
                    1
                    ⁢
                    0
                  
                
                ⁢
                
                  
                    1
                    .
                    2
                  
                  ⁢
                  9
                  ×
                  1
                  ⁢
                  
                    
                      0
                      2
                    
                    ·
                    
                      {
                      
                        Si
                        +
                        
                          0.9
                          ⁢
                          
                            Al
                            ·
                            
                              
                                (
                                
                                  
                                    T
                                    ⁡
                                    
                                      (
                                      n
                                      )
                                    
                                  
                                  
                                    5
                                    ⁢
                                    5
                                    ⁢
                                    0
                                  
                                
                                )
                              
                              2
                            
                          
                        
                        +
                        
                          0.3
                          ⁢
                          
                            
                              (
                              
                                Cr
                                +
                                
                                  1.5
                                  ⁢
                                  Mo
                                
                              
                              )
                            
                            ·
                            
                              
                                T
                                ⁡
                                
                                  (
                                  n
                                  )
                                
                              
                              
                                5
                                ⁢
                                5
                                ⁢
                                0
                              
                            
                          
                        
                      
                      }
                    
                    ·
                  
                
              
               
            
            ⁢
            
              
            
             
          
          ⁢
          
            
               
              
                
                  
                    (
                    
                      
                        B
                        s
                      
                      -
                      
                        T
                        ⁡
                        
                          (
                          n
                          )
                        
                      
                    
                    )
                  
                  3
                
                ·
                
                  exp
                  ⁡
                  
                    (
                    
                      -
                      
                        
                          1.44
                          ×
                          
                            10
                            4
                          
                        
                        
                          
                            T
                            ⁡
                            
                              (
                              n
                              )
                            
                          
                          +
                          273
                        
                      
                    
                    )
                  
                
                ·
                
                  t
                  0.5
                
              
              ]
            
            
              -
              1
            
          
        
        ≤
        1.00
      
    
    
      
        (
        4
        )
      
    
  

T(n): an average temperature of the steel sheet in an n-th time zone obtained by equally dividing the dwell time into 10 parts
Bs point (degrees C.)=611−33[Mn]−17[Cr]−17[Ni]−21[Mo]
−11[Si]+30[Al]+(24[Cr]+15[Mo]
+5500[B]+240[Nb])/(8[C])
[element]: mass % of each element,
at Bs<T(n), (Bs−T(n))=0
t: total [seconds] of a dwell time in the temperature region from 550 degrees C. to 300 degrees C.