Steel sheet and method of producing the same

1. A steel sheet comprising, by mass %,

C: 0.04% or more and 0.15% or less,

Si: 0.3% or more and 0.7% or less,

Mn: 1.0% or more and 3.0% or less,

Al: 0.005% or more and 0.10% or less,

P: limited to 0.03% or less,

S: limited to 0.01% or less,

N: limited to 0.01% or less, and

a remainder Fe and unavoidable impurities,

wherein the steel sheet has a steel structure obtained by performing a soaking for a soaking time of 15 seconds or longer and 35 seconds or shorter at a dual phase region temperature of Ac1 temperature or higher and lower than Ac3 temperature, next, performing a primary cooling to a temperature range of 250° C. or higher and 380° C. or less at a cooling rate of 0.5° C./s or more and 30° C./s or less within 3 seconds, and performing a retention in a temperature range of 260° C. or higher and 370° C. or lower for 180 seconds or longer and 540 seconds or shorter after the primary cooling,

the steel structure contains, by an area fraction, a total fraction of bainite and martensite of 5% or more and 9% or less, a residual austenite of 1% or more and 3% or less, and a remainder consisting of a ferrite,

a yield ratio is 55% or more and 65% or less and tensile strength is 590 MPa or more, and

the Ac1 temperature is a temperature expressed by a following Expression (1) in units of ° C., and the Ac3 temperature is a temperature expressed by a following Expression (2) in units of ° C.,

Ac1=732−26.6×[C]+17.6×[Si]−11.6×[Mn]  (1),

Ac3=924+56.1×[Si]−19.7×[Mn]−436.5×[C]  (2),

here, [C], [Si], and [Mn] represent a C content, an Si content, and an Mn content respectively, and a unit thereof is mass %.

2. The steel sheet according to claim 1 ,

wherein in the steel structure, by an area fraction, the bainite that is limited to 1% or less.

3. The steel sheet according to claim 1 , further comprising, by mass %,

one or two or more of Nb, Ti, and V of 0.005% or more and 0.05% or less in total.

4. The steel sheet according to claim 1 , further comprising, by mass %, any one or two or more of

Cr: 0.01% or more and 0.5% or less,

Mo: 0.01% or more and 0.5% or less, and

B: 0.0005% or more and 0.005% or less,

wherein the Ac1 temperature is a temperature expressed by a following Expression (4) in units of ° C., and the Ac3 temperature is a temperature expressed by a following Expression (5) in units of ° C.,

Ac1=732−26.6×[C]+17.6×[Si]−11.6×[Mn]+24.1×[Cr]  (4),

Ac3=924+56.1×[Si]−19.7×[Mn]−4.9×[Cr]−436.5×[C]  (5),

here, [C], [Si], [Mn], and [Cr] represent a C content, an Si content, an Mn content, and a Cr content respectively, and a unit thereof is mass %.

5. The steel sheet according to claim 4 , further comprising, by mass %,

one or two or more of Nb, Ti, and V of 0.005% or more and 0.05% or less in total.

6. The steel sheet according to claim 1 ,

wherein y which is a product of a retention temperature and a retention time in the retention and x which is the cooling rate in the primary cooling satisfy a following Expression (3),

y≤ 796700× x (−0.971)   (3).

7. The steel sheet according to claim 6 , further comprising, by mass %, any one or two or more of

Cr: 0.01% or more and 0.5% or less,

Mo: 0.01% or more and 0.5% or less, and

B: 0.0005% or more and 0.005% or less,

wherein the Ac1 temperature is a temperature expressed by a following Expression (4) in units of ° C., and the Ac3 temperature is a temperature expressed by a following Expression (5) in units of ° C.,

Ac1=732−26.6×[C]+17.6×[Si]−11.6×[Mn]+24.1×[Cr]  (4),

Ac3=924+56.1×[Si]−19.7×[Mn]−4.9×[Cr]−436.5×[C]  (5),

here, [C], [Si], [Mn], and [Cr] represent a C content, an Si content, an Mn content, and a Cr content respectively, and a unit thereof is mass %.

8. The steel sheet according to claim 6 , further comprising, by mass %,

one or two or more of Nb, Ti, and V of 0.005% or more and 0.05% or less in total.

9. The steel sheet according to claim 1 ,

wherein the cooling rate is 0.5° C./s or more and 15° C./s or less.

10. The steel sheet according to claim 9 , further comprising, by mass %, any one or two or more of

Cr: 0.01% or more and 0.5% or less,

Mo: 0.01% or more and 0.5% or less, and

B: 0.0005% or more and 0.005% or less,

wherein the Ac1 temperature is a temperature expressed by a following Expression (4) in units of ° C., and the Ac3 temperature is a temperature expressed by a following Expression (5) in units of ° C.,

Ac1=732−26.6×[C]+17.6×[Si]−11.6×[Mn]+24.1×[Cr]  (4),

Ac3=924+56.1×[Si]−19.7×[Mn]−4.9×[Cr]−436.5×[C]  (5),

here, [C], [Si], [Mn], and [Cr] represent a C content, an Si content, an Mn content, and a Cr content respectively, and a unit thereof is mass %.

11. The steel sheet according to claim 9 , further comprising, by mass %,

one or two or more of Nb, Ti, and V of 0.005% or more and 0.05% or less in total.