Hot-rolled steel sheet
A hot-rolled steel sheet includes, by mass %, C: 0.030% or more and less than 0.075%, Si+Al: 0.08% to 0.40%, Mn: 0.5% to 2.0%, and Ti: 0.020% to 0.150%, and includes a microstructure having a ferrite and a martensite. The hot-rolled steel sheet includes a microstructure having, by area %, 90% to 98% of the ferrite, 2% to 10% of the martensite, 0% to 3% of a bainite, and 0% to 3% of a pearlite. In the martensite, the number proportion of martensite grains having a hardness of 10.0 GPa or more is 10% or less, and a ratio N1/N2 of the number N1 of martensite grains having a hardness of 8.0 GPa or more and less than 10.0 GPa to the number N2 of martensite grains having a hardness of less than 8.0 GPa is 0.8 to 1.2.
1. A hot-rolled steel sheet comprising, as a chemical composition, by mass %,
C: 0.030% or more and less than 0.075%,
Si+Al: 0.08% to 0.40%,
Mn: 0.5% to 2.0%,
Ti: 0.020% to 0.150%,
Nb: 0% to 0.06%,
Mo: 0% to 1.0%,
V: 0% to 1.00%,
W: 0% to 1.0%,
B: 0% to 0.005%,
Cu: 0% to 1.2%,
Ni: 0% to 0.80%,
Cr: 0% to 1.5%,
Ca: 0% to 0.005%,
REM: 0% to 0.050%,
P: 0% to 0.040%,
S: 0% to 0.0100%,
N: 0% to 0.0100%, and
a remainder comprising Fe and impurities,
wherein the hot-rolled steel sheet includes a microstructure having ferrite and martensite,
the microstructure contains 90% to 98% of ferrite, 2% to 10% of martensite, 0% to 3% of bainite, and 0% to 3% of a pearlite, by area %,
in the martensite, a number proportion of martensite grains having a hardness of 10.0 GPa or more is 10% or less, and
a ratio N1/N2 of a number N1 of martensite grains having a hardness of 8.0 GPa or more and less than 10.0 GPa to a number N2 of martensite grains having a hardness of less than 8.0 GPa is in a range from 0.8 to 1.2.
2. The hot-rolled steel sheet according to claim 1 , comprising, as the chemical composition, by mass %, at least one selected from the group consisting of
Nb: 0.005% to 0.06%,
Mo: 0.05% to 1.0%,
V: 0.02% to 1.0%,
W: 0.1% to 1.0%,
B: 0.0001% to 0.005%,
Cu: 0.1% to 1.2%,
Ni: 0.05% to 0.8%,
Cr: 0.01% to 1.5%,
Ca: 0.0005% to 0.0050%, and
REM: 0.0005% to 0.0500%.
3. The hot-rolled steel sheet according to claim 1 ,
wherein Ti existing as a Ti carbide is, by mass %, 40% or more of Tief calculated by Equation (1),
Tief=(Ti)−48/14×(N)−48/32×(S) (1),
where (Ti), (N), and (S) each represent a content, by mass %, of each corresponding element.
4. The hot-rolled steel sheet according to claim 3 ,
wherein a ratio of a total mass of Ti carbide having a circle equivalent grain size of 7 nm to 20 nm to a total mass of all Ti carbides is 50% or more.
5. The hot-rolled steel sheet according to claim 2 ,
wherein Ti existing as a Ti carbide is, by mass %, 40% or more of Tief calculated by Equation (1),
Tief=(Ti)−48/14×(N)−48/32×(S) (1),
where (Ti), (N), and (S) each represent a content, by mass %, of each corresponding element.
6. The hot-rolled steel sheet according to claim 5 ,
wherein a ratio of a total mass of Ti carbide having a circle equivalent grain size of 7 nm to 20 nm to a total mass of all Ti carbides is 50% or more.