High-strength steel sheet, hot-dipped steel sheet, and alloy hot-dipped steel sheet that have excellent fatigue, elongation, and collision characteristics, and manufacturing method for said steel sheets

1. A high-strength steel sheet having excellent fatigue properties, elongation and collision properties, comprising: in terms of percent by mass,

0.03 to 0.10% of C;

0.01 to 1.5% of Si;

1.0 to 2.5% of Mn;

0.1% or less of P;

0.02% or less of S;

0.01 to 1.2% of Al;

0.06 to 0.092% of Ti;

0.01% or less of N; and

a balance of iron and inevitable impurities,

wherein

a tensile strength is in a range of 590 MPa or more,

a ratio of a yield strength to the tensile strength is in a range of 0.80 or more,

a microstructure comprises bainite at an area ratio of 40% or more and a balance of at least one of ferrite and martensite,

a density of Ti(C,N) precipitates having sizes of 10 nm or smaller is in a range of 10 10 precipitates/mm 3 or more, and

a ratio Hvs/Hvc of a hardness, Hvs, at a depth of 20 μm from a surface to a hardness, Hvc, at a center of a sheet thickness, of 0.92 or more.

2. The high-strength steel sheet according to claim 1 , wherein a fatigue strength ratio is in a range of 0.45 or more.

3. The high-strength steel sheet according to claim 1 , wherein an average dislocation density is in a range of 1×10 14 m −2 or less.

4. The high-strength steel sheet according to claim 1 , wherein the high-strength steel sheet further comprises one or more elements selected from the group consisting of: in terms of percent by mass,

0.005 to 0.1% of Nb;

0.005 to 0.2% of Mo;

0.005 to 0.2% of V;

0.0005 to 0.005% of Ca;

0.0005 to 0.005% of Mg; and

0.0005 to 0.005% of B.

5. A hot-dipped steel sheet having excellent fatigue properties, elongation and collision properties, comprising:

the high-strength steel sheet according to claim 1 ; and

a hot-dipped layer on a surface of the high-strength steel sheet.

6. The hot-dipped steel sheet according to claim 5 , wherein the hot-dipped layer consists of Zn.

7. An alloyed hot-dipped steel sheet having excellent fatigue properties, elongation and collision properties, comprising:

the high-strength steel sheet according to claim 1 ; and

an alloyed hot-dipped layer on a surface of the high-strength steel sheet.

8. A method for producing the high-strength steel sheet having excellent fatigue properties, elongation and collision properties according to claim 1 , the method comprising:

heating a slab comprising: in terms of percent by mass %, 0.03 to 0.10% of C; 0.01 to 1.5% of Si; 1.0 to 2.5% of Mn; 0.1% or less of P; 0.02% or less of S; 0.01 to 1.2% of Al; 0.06 to 0.092% of Ti; 0.01% or less of N; and a balance of iron and inevitable impurities, at a temperature of 1,150° C. to 1,280° C.,

hot rolling the heated slab under conditions where a finish rolling is finished at a temperature of not less than an Ar 3 point, thereby obtaining a hot-rolled material;

coiling the hot-rolled material at a temperature of 600° C. or less, thereby obtaining a hot-rolled steel sheet;

acid pickling the hot-rolled steel sheet;

subjecting the pickled hot-rolled steel sheet to a first skin pass rolling at an elongation rate of 0.1 to 5.0%;

annealing the hot-rolled steel sheet under conditions where a maximum heating temperature (Tmax° C.) is in a range of 600° C. to 750° C. and a holding time (t seconds) in a temperature range of 600° C. or higher fulfills expressions (1) and (2) as follows; and

subjecting the annealed hot-rolled steel sheet to a second skin pass rolling,

530−0.7 ×T max≦ t≦ 3,600−3.9 ×T max  (1)

t> 0  (2).

9. The method for producing the high-strength steel sheet according to claim 8 , wherein an elongation rate of 0.2 to 2.0% is set in the second skin pass rolling.

10. The method for producing the high-strength steel sheet according to claim 8 , wherein ½ or more of the amount of Ti contained in the hot-rolled steel sheet after the coiling exists in a solid-solution state.

11. A method for producing the hot-dipped steel sheet having excellent fatigue properties, elongation and collision properties according to claim 5 , the method comprising:

heating a slab comprising: in terms of percent by mass %, 0.03 to 0.10% of C; 0.01 to 1.5% of Si; 1.0 to 2.5% of Mn; 0.1% or less of P; 0.02% or less of S; 0.01 to 1.2% of Al; 0.06 to 0.092% of Ti; 0.01% or less of N; and a balance of iron and inevitable impurities at a temperature of 1,150° C. to 1,280° C.;

hot rolling the heated slab under conditions where a finish rolling is finished at a temperature of not less than an Ar 3 point, thereby obtaining a hot-rolled material;

coiling the hot-rolled material at a temperature of 600° C. or less, thereby obtaining a hot-rolled steel sheet;

acid pickling the hot-rolled steel sheet;

subjecting the pickled hot-rolled steel sheet to a first skin pass rolling at an elongation rate of 0.1 to 5.0%;

annealing the hot-rolled steel sheet under conditions where a maximum heating temperature (Tmax° C.) is in a range of 600° C. to 750° C. and a holding time (t seconds) in a temperature range of 600° C. or higher and fulfills expressions (1) and (2) as follows,

hot dipping the annealed hot-rolled steel sheet to form a hot-dipped layer on a surface of the hot-rolled steel sheet, thereby obtaining a hot-dipped steel sheet; and

subjecting the hot-dipped steel sheet to a second skin pass rolling,

530−0.7 ×T max≦ t≦ 3,600−3.9 ×T max  (1)

t> 0  (2).

12. The method for producing the hot-dipped steel sheet according to claim 11 , wherein an elongation rate of 0.2 to 2.0% is set in the second skin pass rolling.

13. A method for producing the alloyed hot-dipped steel sheet having excellent fatigue properties, elongation and collision properties according to claim 7 , the method comprising:

heating a slab comprising: in terms of percent by mass %, 0.03 to 0.10% of C; 0.01 to 1.5% of Si; 1.0 to 2.5% of Mn; 0.1% or less of P; 0.02% or less of S; 0.01 to 1.2% of Al; 0.06 to 0.092% of Ti; 0.01% or less of N; and a balance of iron and inevitable impurities, at a temperature in a range of 1,150° C. to 1,280° C.;

hot rolling the heated slab under conditions where a finish rolling is finished at a temperature of not less than an Ar 3 point, thereby obtaining a hot-rolled material;

coiling the hot-rolled material at a temperature of 600° C. or less, thereby obtaining a hot-rolled steel sheet;

acid pickling the hot-rolled steel sheet;

subjecting the pickled hot-rolled steel sheet to a first skin pass rolling at an elongation rate of 0.1 to 5.0%;

annealing the hot-rolled steel sheet under conditions where a maximum heating temperature (Tmax° C.) is in a range of 600° C. to 750° C. and a holding time (t seconds) in a temperature range of 600° C. or higher and fulfills expressions (1) and (2) as follows,

hot dipping the annealed hot-rolled steel sheet to form a hot-dipped layer on a surface of the hot-rolled steel sheet so as to obtain a hot-dipped steel sheet,

subjecting the hot-dipped steel sheet to an alloying treatment to convert the hot-dipped layer into an alloyed hot-dipped layer; and

subjecting the hot-dipped steel sheet on which the alloying treatment is performed to a second skin pass rolling,

530−0.7 ×T max≦ t≦ 3,600−3.9 ×T max  (1)

t> 0  (2).

14. The method for producing the alloyed hot-dipped steel sheet according to claim 13 , wherein an elongation rate of 0.2 to 2.0% is set in the second skin pass rolling.