Steel sheet, plated steel sheet, and method for producing the same

1. A steel sheet comprising, by mass %:

C: 0.020% or more and 0.080% or less;

Si: 0.01% or more and 0.10% or less;

Mn: 0.80% or more and 1.80% or less;

Al: more than 0.10% and less than 0.40%;

Mo: 0% or more and 1.000% or less;

W: 0% or more and 1.000% or less;

V: 0% or more and 1.000% or less;

B: 0% or more and 0.0100% or less;

Ni: 0% or more and 1.50% or less;

Cu: 0% or more and 1.50% or less;

Cr: 0% or more and 1.50% or less;

P: limited to 0.0100% or less;

S: limited to 0.0150% or less;

N: limited to 0.0100% or less;

Nb: 0.005% or more and 0.095% or less;

Ti: 0.005% or more and 0.095% or less; and

a balance comprising Fe and unavoidable impurities,

wherein:

a total amount of Nb and Ti is 0.030% or more and 0.100% or less,

a metallographic structure of the steel sheet includes ferrite, bainite, and other phases,

the other phases include a pearlite, a residual austenite, and a martensite,

an area fraction of the ferrite is 80% or more and 95% or less,

an area fraction of the bainite is 5% or more and 20% or less,

a total fraction of the other phases is less than 3%,

an equivalent circle diameter of a cementite in the ferrite is 0.003 μm or more and 0.300 μm or less,

a number density of the cementite in the ferrite is 0.02 particles/μm 2 or more and 0.10 particles/μm 2 or less,

a tensile strength is 590 MPa or more, and

a fatigue strength ratio as a fatigue strength to the tensile strength is 0.45 or more.

2. The steel sheet according to claim 1 , comprising one or two more of, by mass %:

Mo: 0.005% or more and 1.000% or less;

W: 0.005% or more and 1.000% or less;

V: 0.005% or more and 1.000% or less;

B: 0.0005% or more and 0.0100% or less;

Ni: 0.05% or more and 1.50% or less;

Cu: 0.05% or more and 1.50% or less; and

Cr: 0.05% or more and 1.50% or less.

3. A plated steel sheet, in which a plating is provided on a surface of the steel sheet according to claim 1 .

4. A plated steel sheet, in which a plating is provided on a surface of the steel sheet according to claim 2 .

5. A method for producing the steel sheet according to claim 1 , the method comprising:

heating a slab having a chemical composition according to claim 1 to 1150° C. or higher before the slab is hot-rolled;

finishing finish rolling at a temperature of Ar 3 ° C. or higher;

pickling a hot-rolled steel sheet which is coiled within a temperature range of 400° C. or higher and 600° C. or lower;

heating the hot-rolled steel sheet within a temperature range of 600° C. or higher and Ac 1 ° C. or lower;

annealing the hot-rolled steel sheet for a holding time, in which the temperature of the hot-rolled steel sheet is within the temperature range for 10 seconds or longer and 200 seconds or shorter;

cooling the steel sheet to 350° C. or higher and 550° C. or lower; and

cooling the steel sheet after holding the steel sheet for the holding time, in which the temperature of the hot-rolled steel sheet is within a temperature range of 350° C. or higher and 550° C. or lower for 10 seconds or longer and 500 seconds or shorter,

wherein the Ar 3 ° C. and the Ac 1 ° C. are a Ar 3 transformation temperature and a Ac 1 transformation temperature, respectively, obtained from expressions 1 and 2,

Ar 3 =910−325×[C]+33×[Si]+287×[P]+40×[Al]−92×([Mn]+[Mo]+[Cu])−46×([Cr]+[Ni])  (Expression 1),

Ac 1 =761.3+212×[C]−45.8×[Mn]+16.7×[Si]  Expression 2), and

elements noted in brackets represent an amount of the elements by mass %.

6. The method for producing a steel sheet according to claim 5 , further comprising:

carrying out skin pass rolling on the steel sheet at an elongation ratio of 0.4% or more and 2.0% or less.

7. A method for producing a plated steel sheet comprising:

plating and then cooling the steel sheet after the annealing, the cooling, and the holding according to claim 5 .

8. The method for producing a plated steel sheet according to claim 7 , further comprising:

carrying out a heat treatment within a temperature range of 450° C. or higher and 600° C. or lower for 10 seconds or longer and then cooling the steel sheet after the plating.

9. A method for producing a plated steel sheet comprising:

plating and then cooling the steel sheet after the annealing, the cooling, and the holding according to claim 6 .

10. The method for producing a plated steel sheet according to claim 9 , further comprising:

carrying out a heat treatment within a temperature range of 450° C. or higher and 600° C. or lower for 10 seconds or longer and then cooling the steel sheet after the plating.