Hot-rolled steel sheet and method for producing same

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

C: 0.030% to 0.10%;

Mn: 0.5% to 2.5%;

Si+Al: 0.100% to 2.5%;

P: 0.04% or less;

S: 0.01% or less;

N: 0.01% or less;

Nb: 0% to 0.06%;

Ti: 0% to 0.20%;

V: 0% to 0.20%;

W: 0% to 0.5%;

Mo: 0% to 0.40%;

Cr: 0% to 1.0%;

Cu: 0% to 1.2%;

Ni: 0% to 0.6%;

B: 0% to 0.005%;

REM: 0% to 0.01%;

Ca: 0% to 0.01%; and

a remainder consisting of Fe and impurities,

wherein the steel sheet has a microstructure including, by area fraction, ferrite: 80% or more and 87.5% or less, martensite: 3% to 15.0%, and pearlite: less than 3.0%, in which a number density of martensite having an equivalent circle diameter of 3 μm or more at a position which is at a depth of ¼ of the steel sheet thickness from the surface of the steel sheet, is 5.0 pieces/10000 μm 2 or less, and the following Expression (1) is satisfied,

R/D M 2 ≥1.00  Expression (1),

here, R is an average martensite interval (μm) defined by the following Expression (2), and D M is a martensite average diameter (μm),

R={ 12.5×(π/6 V M ) 0.5 −⅔) 0.5 }×D M   Expression (2),

here, V M is a martensite area fraction (%) and D M is the martensite average diameter (μm), and

wherein the hot-rolled steel sheet has a tensile strength of 630 MPa or more.

2. A method for producing a hot-rolled steel sheet comprising:

heating a slab having the chemical composition according to claim 1 , to 1150° C. to 1300° C., then subjecting the slab to multipass rough rolling and rolling the slab with four or more final passes of rolling in a temperature range of 1000° C. to 1050° C. at a total reduction of 30% or more to form a rough bar;

starting rolling on the rough bar 32 seconds to 60 seconds after completing the rough rolling and subjecting the rough bar to finish rolling to complete rolling in a temperature range of 850° C. to 950° C. to obtain a finish-rolled steel sheet; and

after cooling the finish-rolled steel sheet to a temperature range of 600° C. to 750° C. at an average cooling rate of 50° C./s or more and air-cooling the steel sheet for 5 seconds to 10 seconds, cooling the steel sheet to a temperature range of 400° C. or lower at an average cooling rate of 30° C./s or more and coiling the steel sheet to obtain a hot-rolled steel sheet.

3. The hot-rolled steel sheet according to claim 1 , further comprising, as a chemical composition, by mass %,

at least one of Nb: 0.005% to 0.06% and Ti: 0.02% to 0.20%.

4. A method for producing a hot-rolled steel sheet comprising:

heating a slab having the chemical composition according to claim 3 , to 1150° C. to 1300° C., then subjecting the slab to multipass rough rolling and rolling the slab with four or more final passes of rolling in a temperature range of 1000° C. to 1050° C. at a total reduction of 30% or more to form a rough bar;

starting rolling on the rough bar 32 seconds to 60 seconds after completing the rough rolling and subjecting the rough bar to finish rolling to complete rolling in a temperature range of 850° C. to 950° C. to obtain a finish-rolled steel sheet; and

after cooling the finish-rolled steel sheet to a temperature range of 600° C. to 750° C. at an average cooling rate of 50° C./s or more and air-cooling the steel sheet for 5 seconds to 10 seconds, cooling the steel sheet to a temperature range of 400° C. or lower at an average cooling rate of 30° C./s or more and coiling the steel sheet to obtain a hot-rolled steel sheet.

5. The hot-rolled steel sheet according to claim 1 , further comprising, as a chemical composition, by mass %,

at least one of V: 0.02% to 0.20%, W: 0.1% to 0.5%, and Mo: 0.05% to 0.40%.

6. A method for producing a hot-rolled steel sheet comprising:

heating a slab having the chemical composition according to claim 5 , to 1150° C. to 1300° C., then subjecting the slab to multipass rough rolling and rolling the slab with four or more final passes of rolling in a temperature range of 1000° C. to 1050° C. at a total reduction of 30% or more to form a rough bar;

starting rolling on the rough bar 32 seconds to 60 seconds after completing the rough rolling and subjecting the rough bar to finish rolling to complete rolling in a temperature range of 850° C. to 950° C. to obtain a finish-rolled steel sheet; and

after cooling the finish-rolled steel sheet to a temperature range of 600° C. to 750° C. at an average cooling rate of 50° C./s or more and air-cooling the steel sheet for 5 seconds to 10 seconds, cooling the steel sheet to a temperature range of 400° C. or lower at an average cooling rate of 30° C./s or more and coiling the steel sheet to obtain a hot-rolled steel sheet.

7. The hot-rolled steel sheet according to claim 1 , further comprising, as a chemical composition, by mass %,

at least one of Cr: 0.01% to 1.0%, Cu: 0.1% to 1.2%, Ni: 0.05% to 0.6%, and B: 0.0001% to 0.005%.

8. A method for producing a hot-rolled steel sheet comprising:

heating a slab having the chemical composition according to claim 7 , to 1150° C. to 1300° C., then subjecting the slab to multipass rough rolling and rolling the slab with four or more final passes of rolling in a temperature range of 1000° C. to 1050° C. at a total reduction of 30% or more to form a rough bar;

starting rolling on the rough bar 32 seconds to 60 seconds after completing the rough rolling and subjecting the rough bar to finish rolling to complete rolling in a temperature range of 850° C. to 950° C. to obtain a finish-rolled steel sheet; and

after cooling the finish-rolled steel sheet to a temperature range of 600° C. to 750° C. at an average cooling rate of 50° C./s or more and air-cooling the steel sheet for 5 seconds to 10 seconds, cooling the steel sheet to a temperature range of 400° C. or lower at an average cooling rate of 30° C./s or more and coiling the steel sheet to obtain a hot-rolled steel sheet.

9. The hot-rolled steel sheet according to claim 1 , further comprising, as a chemical composition, by mass %,

at least one of REM: 0.0005% to 0.01% and Ca: 0.0005% to 0.01%.

10. A method for producing a hot-rolled steel sheet comprising:

heating a slab having the chemical composition according to claim 9 , to 1150° C. to 1300° C., then subjecting the slab to multipass rough rolling and rolling the slab with four or more final passes of rolling in a temperature range of 1000° C. to 1050° C. at a total reduction of 30% or more to form a rough bar;

starting rolling on the rough bar 32 seconds to 60 seconds after completing the rough rolling and subjecting the rough bar to finish rolling to complete rolling in a temperature range of 850° C. to 950° C. to obtain a finish-rolled steel sheet; and

after cooling the finish-rolled steel sheet to a temperature range of 600° C. to 750° C. at an average cooling rate of 50° C./s or more and air-cooling the steel sheet for 5 seconds to 10 seconds, cooling the steel sheet to a temperature range of 400° C. or lower at an average cooling rate of 30° C./s or more and coiling the steel sheet to obtain a hot-rolled steel sheet.

11. The hot-rolled steel sheet according to claim 1 , further comprising, as a chemical composition, by mass %,

at least one of Nb: 0.005% to 0.06%, Ti: 0.02% to 0.20%, V: 0.02% to 0.20%, W: 0.1% to 0.5%, Mo: 0.05% to 0.40%, Cr: 0.01% to 1.0%, Cu: 0.1% to 1.2%, Ni: 0.05% to 0.6%, B: 0.0001% to 0.005%, REM: 0.0005% to 0.01%, and Ca: 0.0005% to 0.01%.

12. A method for producing a hot-rolled steel sheet comprising:

heating a slab having the chemical composition according to claim 11 , to 1150° C. to 1300° C., then subjecting the slab to multipass rough rolling and rolling the slab with four or more final passes of rolling in a temperature range of 1000° C. to 1050° C. at a total reduction of 30% or more to form a rough bar;

starting rolling on the rough bar 32 seconds to 60 seconds after completing the rough rolling and subjecting the rough bar to finish rolling to complete rolling in a temperature range of 850° C. to 950° C. to obtain a finish-rolled steel sheet; and

after cooling the finish-rolled steel sheet to a temperature range of 600° C. to 750° C. at an average cooling rate of 50° C./s or more and air-cooling the steel sheet for 5 seconds to 10 seconds, cooling the steel sheet to a temperature range of 400° C. or lower at an average cooling rate of 30° C./s or more and coiling the steel sheet to obtain a hot-rolled steel sheet.

13. A hot-rolled steel sheet comprising, as a chemical composition, by mass %:

C: 0.030% to 0.10%;

Mn: 0.5% to 2.5%;

Si+Al: 0.100% to 2.5%;

P: 0.04% or less;

S: 0.01% or less;

N: 0.01% or less;

Nb: 0% to 0.06%;

Ti: 0% to 0.20%;

V: 0% to 0.20%;

W: 0% to 0.5%;

Mo: 0% to 0.40%;

Cr: 0% to 1.0%;

Cu: 0% to 1.2%;

Ni: 0% to 0.6%;

B: 0% to 0.005%;

REM: 0% to 0.01%;

Ca: 0% to 0.01%; and

a remainder comprising Fe and impurities,

wherein the steel sheet has a microstructure including, by area fraction, ferrite: 80% or more and 87.5% or less, martensite: 3% to 15.0%, and pearlite: less than 3.0%, in which a number density of martensite having an equivalent circle diameter of 3 μm or more at a position which is at a depth of ¼ of the steel sheet thickness from the surface of the steel sheet, is 5.0 pieces/10000 μm 2 or less, and the following Expression (1) is satisfied,

R/D M 2 ≥1.00  Expression (1),

here, R is an average martensite interval (μm) defined by the following Expression (2), and D M is a martensite average diameter (μm),

R={ 12.5×(π/6 V M ) 0.5 −(⅔) 0.5 }×D M   Expression (2),

here, V M is a martensite area fraction (%) and D M is the martensite average diameter (μm), and

wherein the hot-rolled steel sheet has a tensile strength of 630 MPa or more.