Steel sheet, member, and methods for manufacturing the same

1 . A steel sheet comprising a chemical composition containing, in mass %,

C: 0.12% or more and 0.40% or less,

Si: 1.5% or less,

Mn: 1.8% or more and 4.0% or less,

P: 0.03% or less,

S: less than 0.0023%,

sol. Al: 0.20% or less,

N: 0.005% or less,

B: 0.0100% or less, and

one or more of Nb and Ti with a total content of 0.005% or more and 0.080% or less,

with the balance being Fe and incidental impurities,

wherein the steel sheet has a microstructure in which an area fraction of martensite relative to the entire microstructure is 95% or more and 100% or less, with the balance being one or more of bainite, ferrite, and retained austenite,

prior-austenite grains have an average grain size of 18 μm or less,

90 mass % or more of the total content of Nb and Ti contained is present as a carbonitride having an equivalent circular diameter of 100 nm or more,

a Nb carbonitride and a Ti carbonitride, having an equivalent circular diameter of 1.0 μm or more, are present at a rate of 5 pieces/mm 2 or more and 800 pieces/mm 2 or less in total, and

the steel sheet has a tensile strength of 1310 MPa or more, and

optionally wherein a coating layer is disposed on a surface of the steel sheet.

2 . The steel sheet according to claim 1 , wherein the prior-austenite grains have an average grain size of 10 μm or less.

3 . The steel sheet according to claim 2 , wherein a fracture stress go before immersion in a solution containing a 10 mass % aqueous ammonium thiocyanate solution and a McIlvaine buffer solution having a pH of 3,

a fracture stress σ 1 after immersion in the solution, and

the tensile strength satisfy (A), (B), or (C) below:

(A) Tensile strength: 1310 MPa or more and less than 1500 MPa, and σ 1 /σ 0 is 0.80 or more,

(B) Tensile strength: 1500 MPa or more and less than 1800 MPa, and σ 1 /σ 0 is 0.50 or more,

(C) Tensile strength: 1800 MPa or more, and σ 1 /σ 0 is 0.35 or more.

4 . The steel sheet according to claim 3 , wherein the chemical composition contains, in mass %, one or two or more selected from the following groups A to E:

group A:

S: less than 0.0010%,

group B:

one or two selected from Cu: 1.0% or less and Ni: 1.0% or less,

group C:

one or two or more selected from Cr: 1.0% or less, Mo: less than 0.3%, V: 0.5% or less, Zr: 0.2% or less, and W: 0.2% or less,

group D:

one or two or more selected from Ca: 0.0030% or less, Ce: 0.0030% or less, La: 0.0030% or less, REM (excluding Ce and La): 0.0030% or less, and Mg: 0.0030% or less,

group E:

one or two selected from Sb: 0.1% or less and Sn: 0.1% or less.

5 . The steel sheet according to claim 2 , wherein the chemical composition contains, in mass %, one or two or more selected from the following groups A to E:

group A:

S: less than 0.0010%,

group B:

one or two selected from Cu: 1.0% or less and Ni: 1.0% or less,

group C:

one or two or more selected from Cr: 1.0% or less, Mo: less than 0.3%, V: 0.5% or less, Zr: 0.2% or less, and W: 0.2% or less,

group D:

one or two or more selected from Ca: 0.0030% or less, Ce: 0.0030% or less, La: 0.0030% or less, REM (excluding Ce and La): 0.0030% or less, and Mg: 0.0030% or less,

group E:

one or two selected from Sb: 0.1% or less and Sn: 0.1% or less.

6 . The steel sheet according to claim 1 , wherein a fracture stress do before immersion in a solution containing a 10 mass % aqueous ammonium thiocyanate solution and a McIlvaine buffer solution having a pH of 3,

a fracture stress σ 1 after immersion in the solution, and

the tensile strength satisfy (A), (B), or (C) below:

(A) Tensile strength: 1310 MPa or more and less than 1500 MPa, and σ 1 /σ 0 is 0.80 or more,

(B) Tensile strength: 1500 MPa or more and less than 1800 MPa, and σ 1 /σ 0 is 0.50 or more,

(C) Tensile strength: 1800 MPa or more, and σ 1 /σ 0 is 0.35 or more.

7 . The steel sheet according to claim 6 , wherein the chemical composition contains, in mass %, one or two or more selected from the following groups A to E:

group A:

S: less than 0.0010%,

group B:

one or two selected from Cu: 1.0% or less and Ni: 1.0% or less,

group C:

one or two or more selected from Cr: 1.0% or less, Mo: less than 0.3%, V: 0.5% or less, Zr: 0.2% or less, and W: 0.2% or less,

group D:

one or two or more selected from Ca: 0.0030% or less, Ce: 0.0030% or less, La: 0.0030% or less, REM (excluding Ce and La): 0.0030% or less, and Mg: 0.0030% or less,

group E:

one or two selected from Sb: 0.1% or less and Sn: 0.1% or less.

8 . The steel sheet according to claim 1 , wherein the chemical composition contains, in mass %, one or two or more selected from the following groups A to E:

group A:

S: less than 0.0010%,

group B:

one or two selected from Cu: 1.0% or less and Ni: 1.0% or less,

group C:

one or two or more selected from Cr: 1.0% or less, Mo: less than 0.3%, V: 0.5% or less, Zr: 0.2% or less, and W: 0.2% or less,

group D:

one or two or more selected from Ca: 0.0030% or less, Ce: 0.0030% or less, La: 0.0030% or less, REM (excluding Ce and La): 0.0030% or less, and Mg: 0.0030% or less,

group E:

one or two selected from Sb: 0.1% or less and Sn: 0.1% or less.

9 . A member obtained by subjecting the steel sheet according to claim 1 to at least one of forming and welding.

10 . A member obtained by subjecting the steel sheet according to claim 2 to at least one of forming and welding.

11 . A member obtained by subjecting the steel sheet according to claim 6 to at least one of forming and welding.

12 . A member obtained by subjecting the steel sheet according to claim 3 to at least one of forming and welding.

13 . A member obtained by subjecting the steel sheet according to claim 8 to at least one of forming and welding.

14 . A member obtained by subjecting the steel sheet according to claim 5 to at least one of forming and welding.

15 . A member obtained by subjecting the steel sheet according to claim 7 to at least one of forming and welding.

16 . A member obtained by subjecting the steel sheet according to claim 4 to at least one of forming and welding.

17 . A method for manufacturing the steel sheet according to claim 1 , comprising:

heating a steel slab having the chemical composition from 1000° C. to a heat holding temperature of 1250° C. or higher in terms of slab surface temperature at an average heating rate of 10° C./min or less and holding the steel slab at the heat holding temperature for 30 minutes or more,

then performing hot finish rolling at a finish rolling temperature not lower than an Ar 3 temperature,

performing cooling at an average cooling rate of 40° C./s or more in a range from the finish rolling temperature to 650° C.,

then performing cooling and coiling at a coiling temperature of 600° C. or lower to obtain a hot rolled steel sheet,

cold rolling the hot rolled steel sheet at a rolling reduction of 40% or more to obtain a cold rolled steel sheet, and

performing continuous annealing including:

heating the cold rolled steel sheet from 700° C. to an annealing temperature set to 800° C. to 950° C. at an average heating rate of 0.4° C./s or more,

performing holding at the annealing temperature for 600 seconds or less,

performing cooling at a first average cooling rate of 2° C./s or more from the annealing temperature to 420° C.,

performing cooling at a second average cooling rate of 10° C./s or more from 420° C. to a cooling stop temperature of 280° C. or lower, and

then performing holding at a holding temperature of 120° C. to 260° C. for 20 to 1500 seconds,

and optionally further comprising performing coating treatment on a steel sheet surface after the continuous annealing.

18 . A method for manufacturing the steel sheet according to claim 8 , comprising:

heating a steel slab having the chemical composition from 1000° C. to a heat holding temperature of 1250° C. or higher in terms of slab surface temperature at an average heating rate of 10° C./min or less and holding the steel slab at the heat holding temperature for 30 minutes or more,

then performing hot finish rolling at a finish rolling temperature not lower than an Ar 3 temperature,

performing cooling at an average cooling rate of 40° C./s or more in a range from the finish rolling temperature to 650° C.,

then performing cooling and coiling at a coiling temperature of 600° C. or lower to obtain a hot rolled steel sheet,

cold rolling the hot rolled steel sheet at a rolling reduction of 40% or more to obtain a cold rolled steel sheet, and

performing continuous annealing including:

heating the cold rolled steel sheet from 700° C. to an annealing temperature set to 800° C. to 950° C. at an average heating rate of 0.4° C./s or more,

performing holding at the annealing temperature for 600 seconds or less,

performing cooling at a first average cooling rate of 2° C./s or more from the annealing temperature to 420° C.,

performing cooling at a second average cooling rate of 10° C./s or more from 420° C. to a cooling stop temperature of 280° C. or lower, and

then performing holding at a holding temperature of 120° C. to 260° C. for 20 to 1500 seconds,

and optionally further comprising performing coating treatment on a steel sheet surface after the continuous annealing.

19 . A method for manufacturing a member, comprising a step of subjecting a steel sheet manufactured by the method for manufacturing a steel sheet according to claim 17 to at least one of forming and welding.

20 . A method for manufacturing a member, comprising a step of subjecting a steel sheet manufactured by the method for manufacturing a steel sheet according to claim 18 to at least one of forming and welding.