Steel sheet and method of manufacturing the same

1 . A steel sheet comprising, as a chemical composition, by mass %:

C: 0.030% to 0.180%;

Si: 0.030% to 1.400%;

Mn: 1.60% to 3.00%;

Al: 0.010% to 0.700%;

P: 0.0800% or less;

S: 0.0100% or less;

N: 0.0050% or less;

Ti: 0.020% to 0.180%;

Nb: 0.010% to 0.050%;

Mo: 0% to 0.600%;

V: 0% to 0.300%;

a sum of Ti, Nb, Mo, and V: 0.100% to 1.130%;

B: 0% to 0.0030%;

Cr: 0% to 0.500%; and

a remainder consisting of Fe and impurities,

wherein a microstructure of the steel sheet contains, by area %,

bainite: 80.0% or more,

a sum of fresh martensite and tempered martensite: 20.0% or less, and

a sum of pearlite, ferrite, and austenite: 20.0% or less,

a number density of alloy carbides present at grain boundaries and having a major axis of 10 to 100 nm is 1.0×10 8 to 1.0×10 11 /cm 2 ,

a number density of alloy carbides present in grains and having a major axis of 10 nm or less is 1.0×10 16 to 1.0×10 19 /cm 3 , and

a tensile strength of the steel sheet is 1,030 MPa or more.

2 . The steel sheet according to claim 1 ,

wherein a proportion of an area ratio of the tempered martensite in a sum of area ratios of the fresh martensite and the tempered martensite is 80.0% or more.

3 . The steel sheet according to claim 1 ,

wherein the steel sheet contains, as the chemical composition, by mass %, one or more of

Mo: 0.001% to 0.600%,

V: 0.010% to 0.300%,

B: 0.0001% to 0.0030%, and

Cr: 0.001% to 0.500%.

4 . A method of manufacturing the steel sheet according to claim 1 , comprising:

heating a slab having the chemical composition according to claim 1 and performing rough rolling of four passes or more in a temperature range of 1,000° C. to 1,300° C.;

performing finish rolling after the rough rolling so that a final rolling reduction is 24% to 60% and a finish rolling temperature is in a temperature range of 960° C. to 1,060° C.;

performing cooling after the finish rolling so that an average cooling rate in a temperature range of 900° C. to 650° C. is 30° C./sec or faster;

performing coiling in a temperature range of 400° C. to 580° C. after the cooling; and

after the coiling, performing heating to a temperature range of 600° C. to 750° C. at an average heating rate of 0.2 to 5.0° C./sec, performing holding in the temperature range of 600° C. to 750° C. for 60 to 3,010 seconds, and performing cooling so that an average cooling rate in a temperature range of 500° C. to 700° C. is 10° C./sec or faster,

wherein, in the rough rolling,

a temperature difference between a final pass and a pass one pass before the final pass is set to 50° C. or less,

a rolling reduction in first to third passes is set to 10% to 30%, and

a rolling reduction in fourth and subsequent passes is set to 15% to 50%.

5 . The steel sheet according to claim 2 ,

wherein the steel sheet contains, as the chemical composition, by mass %, one or more of

Mo: 0.001% to 0.600%,

V: 0.010% to 0.300%,

B: 0.0001% to 0.0030%, and

Cr: 0.001% to 0.500%.

6 . A steel sheet comprising, as a chemical composition, by mass %:

C: 0.030% to 0.180%;

Si: 0.030% to 1.400%;

Mn: 1.60% to 3.00%;

Al: 0.010% to 0.700%;

P: 0.0800% or less;

S: 0.0100% or less;

N: 0.0050% or less;

Ti: 0.020% to 0.180%;

Nb: 0.010% to 0.050%;

Mo: 0% to 0.600%;

V: 0% to 0.300%;

a sum of Ti, Nb, Mo, and V: 0.100% to 1.130%;

B: 0% to 0.0030%;

Cr: 0% to 0.500%; and

a remainder comprising Fe and impurities,

wherein a microstructure of the steel sheet contains, by area %,

bainite: 80.0% or more,

a sum of fresh martensite and tempered martensite: 20.0% or less, and

a sum of pearlite, ferrite, and austenite: 20.0% or less,

a number density of alloy carbides present at grain boundaries and having a major axis of 10 to 100 nm is 1.0×10 8 to 1.0×10 11 /cm 2 ,

a number density of alloy carbides present in grains and having a major axis of 10 nm or less is 1.0×10 16 to 1.0×10 19 /cm 3 , and

a tensile strength of the steel sheet is 1,030 MPa or more.