Steel sheet and producing method therefor

Abstract

A steel sheet including a chemical composition in mass %: C: 0.14-0.60%, Si+Al≤3.00, P 0.030%, S≤0.0050%, N 0.015%, B≤0.0050%, C×Mn≤0.80, Mn+Ni+Cu+1.3Cr+4(Mo+W)≥0.80, 0.003≤Ti+Zr+Hf+V+Nb+Ta+Sc+Y≤0.20, Sn+As+Sb+Bi≤0.020, Mg: 0 to 0.005%, Ca: 0 to 0.005%, REM: 0 to 0.005%, with the balance: Fe and impurities, and satisfying Ms=546 ×exp(−1.362 x C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)≥200.

1 . A steel sheet having a chemical composition consisting of, in mass %:
 C: 0.14 to 0.60%,   Si: more than 0% to less than 3.00%,   Al: more than 0% to less than 3.00%,   Mn: 5.00% or less,   P: 0.030% or less,   S: 0.0050% or less,   N: 0.015% or less,   B: 0 to 0.0050%,   Ni: 0 to 5.00%,   Cu: 0 to 5.00%,   Cr: 0 to 5.00%,   Mo: 0 to 1.00%,   W: 0 to 1.00%,   Ti: 0 to 0.20%,   Zr: 0 to 0.20%,   Hf: 0 to 0.20%,   V: 0 to 0.20%,   Nb: 0 to 0.20%,   Ta: 0 to 0.20%,   Sc: 0 to 0.20%,   Y: 0 to 0.20%,   Sn: 0 to 0.020%,   As: 0 to 0.020%,   Sb: 0 to 0.020%,   Bi: 0 to 0.020%,   Mg: 0 to 0.005%,   Ca: 0 to 0.005%, and   REM: 0 to 0.005%,   with the balance: Fe and impurities, and   satisfying following formulas (i) to (v), wherein   a value of Ms expressed by a following formula (vi) is 200 or more,   a steel micro-structure contains, in volume %:   martensite: 85% or more, and   retained austenite: 15% or less,   with the balance: bainite,   an average block size of martensite and bainite: 3.0 μm or less,   an average axial ratio of martensite and bainite:  1 . 0004  to  1 . 0100 , and   a yield stress is 1000 MPa or more:
   Si+Al≤3.00  (i)
 
   C×Mn 0.80  (ii)
 
   Mn+Ni+Cu+1.3Cr+4(Mo+W)≥0.80  (iii)
 
   0.003≤Ti+Zr+Hf+V+Nb+Ta+Sc+Y≤0.20  (iv)
 
   Sn+As+Sb+Bi≤0.020  (v)
 
   Ms=546 ×exp(−1.362 x C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi)
 
   
       where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol. 
     
     
         2 . The steel sheet according to  claim 1 , wherein an average particle size of iron carbides included in the steel micro-structure is 0.005 to 0.20 μm. 
     
     
         3 . The steel sheet according to  claim 1 , wherein the steel sheet includes a plating layer on a surface of the steel sheet. 
     
     
         4 . A method for producing the steel sheet according to  claim 1 , wherein
 a cast piece having the chemical composition according to  claim 1  is subjected to a hot-rolling step, a cold-rolling step, an annealing step, and a heat treatment step in this order,   in the hot-rolling step, the steel sheet is cooled to room temperature at an average cooling rate for a range from a rolling finish temperature to 650° C. set at 8° C./s or more,   in the annealing step, the steel sheet is held within a temperature range from an Ac 3  point to (Ac 3  point+100°) C for 3 to 90 s, and   an average cooling rate for a range from 700° C. to (Ms point - 50°) C is set at 10° C./s or more, and   in the heat treatment step,   in a case where the Ms point is 250° C. or more,   a holding time for a temperature range from (Ms point+50) to 250° C. is set at 100 to 10000 s, and   in a case where the Ms point is less than 250° C.,   a holding time for a temperature range from (Ms point+80) to 100° C. is set at 100 to 50000 s,   where the Ms point (° C.) and the Ac 3  point (° C.) are expressed by following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol:
   Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr−8(Mo+W)  (vi)
 
   Ac 3= 910−203 ×C 0.5 +44.7(Si+Al)− 30  ×Mn+700 ×P−15.2 ×Ni−26 ×Cu−11 ×Cr+31.5 ×Mo  (vii).
 
   
     
     
         5 . A method for producing the steel sheet according to  claim 1 , wherein
 a cast piece having the chemical composition according to  claim 1  is subjected to a hot-rolling step, an annealing step, and a heat treatment step in this order,   in the hot-rolling step, the steel sheet is cooled to room temperature at an average cooling rate for a range from a rolling finish temperature to 650° C. set at 8° C./s or more,   in the annealing step, the steel sheet is held within a temperature range from an Ac 3  to (Ac 3+100 )° C. for 3 to 90 s, and   an average cooling rate for a range from 700° C. to (Ms−50°) C is set at 10° C./s or more, and   in the heat treatment step,   in a case where the Ms point is 250° C. or more,   a holding time for a temperature range from (Ms+50) to 250° C. is set at 100 to 10000 s, and   in a case where the Ms point is less than 250° C.,   a holding time for a temperature range from (Ms+80) to 100° C. is set at 100 to 50000 s,   where the Ms point (° C.) and the Ac 3  point (° C.) are expressed by following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol:
   Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr−8(Mo+W)  (vi)
 
   Ac 3= 910−203 ×C 0.5 +44.7(Si+Al)−30 ×Mn+700 ×P−15.2 ×Ni−26 ×Cu−11 ×Cr+31.5 ×Mo  (vii).
 
   
     
     
         6 . A method for producing the steel sheet according to  claim 1 , wherein
 a cast piece having the chemical composition according to  claim 1  is subjected to a hot-rolling step and a heat treatment step in this order,   in the hot-rolling step, a rolling finish temperature is set at a Ar 3  point or more, and   an average cooling rate for a range from a rolling finish temperature to (Ms - 50°) C is set at 10° C./s or more, and   in the heat treatment step,   in a case where the Ms point is 250° C. or more,   a holding time for a temperature range from (Ms+50) to 250° C. is set at 100 to 10000 s, and   in a case where the Ms point is less than 250° C.,   a holding time for a temperature range from (Ms+80) to 100° C. is set at 100 to 50000 s,   where the Ms point (° C.) and the Ar 3  point (° C.) are expressed by following formulas, where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol:
   Ms=546 ×exp(−1.362 ×C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi)
 
   Ar 3= 910−310 ×C+33 ×Si−80 xMn−55 xNi−20 ×Cu−15 ×Cr−80 ×Mo  (viii).
 
   
     
     
         7 . The steel sheet according to  claim 2 , wherein the steel sheet includes a plating layer on a surface of the steel sheet. 
     
     
         8 . A steel sheet having a chemical composition comprising, in mass %:
 C: 0.14 to 0.60%,   Si: more than 0% to less than 3.00%,   Al: more than 0% to less than 3.00%,   Mn:  5 . 0   0 % or less,   P: 0.030% or less,   S: 0.0050% or less,   N: 0.015% or less,   B: 0 to 0.0050%,   Ni: 0 to 5.00%,   Cu: 0 to 5.00%,   Cr: 0 to 5.00%,   Mo: 0 to 1.00%,   W: 0 to 1.00%,   Ti: 0 to 0.20%,   Zr: 0 to 0.20%,   Hf: 0 to 0.20%,   V: 0 to 0.20%,   Nb: 0 to 0.20%,   Ta: 0 to 0.20%,   Sc:  0  to  0 . 2   0 %,   Y: 0 to 0.20%,   Sn: 0 to 0.020%,   As: 0 to 0.020%,   Sb: 0 to 0.020%,   Bi: 0 to 0.020%,   Mg: 0 to 0.005%,   Ca: 0 to 0.005%, and   REM: 0 to 0.005%,   with the balance: Fe and impurities, and   satisfying following formulas (i) to (v), wherein   a value of Ms expressed by a following formula (vi) is 200 or more,   a steel micro-structure contains, in volume %:   martensite: 85% or more, and   retained austenite: 15% or less,   with the balance: bainite,   an average block size of martensite and bainite: 3.0 μm or less,   an average axial ratio of martensite and bainite:  1 . 0004  to  1 . 0100 , and   a yield stress is 1000 MPa or more:
   Si+Al≤3.00  (i)
 
   C×Mn 0.80  (ii)
 
   Mn+Ni+Cu+1.3Cr+4(Mo+W)≥0.80  (iii)
 
   0.003≤Ti+Zr+Hf+V+Nb+Ta+Sc+Y≤0.20  (iv)
 
   Sn+As+Sb+Bi≤0.020  (v)
 
   Ms=546 ×exp(−1.362 x C)−11 ×Si−30 ×Mn−18 ×Ni−20 ×Cu−12×Cr −8(Mo+W)  (vi)
 
   
       where symbols of elements represent contents (mass %) of the elements in the steel sheet, and in a case where an element is not contained, zero is assigned to its symbol.