High-strength steel sheet and method of manufacturing the same

1. A high-strength steel sheet, comprising:

a chemical composition represented by, in mass %:

C: 0.03% to 0.35%;

Si: 0.01% to 2.0%;

Mn: 0.3% to 4.0%;

Al: 0.01% to 2.0%;

P: 0.10% or less;

S: 0.05% or less;

N: 0.010% or less;

Cr: 0.0% to 3.0%;

Mo: 0.0% to 1.0%;

Ni: 0.0% to 3.0%;

Cu: 0.0% to 3.0%;

Nb: 0.0% to 0.3%;

Ti: 0.0% to 0.3%;

V: 0.0% to 0.5%;

B: 0.0% to 0.1%;

Ca: 0.00% to 0.01%;

Mg: 0.00% to 0.01%;

Zr: 0.00% to 0.01%;

REM: 0.00% to 0.01%; and

a balance: Fe and impurities, and

a microstructure represented by, in area %,

martensite: 5% or more;

ferrite: 20% or more; and

pearlite: 5% or less,

wherein

an average diameter of each martensite grain is 4 μm or less in equivalent circle diameter,

a ratio of a number of bulging type martensite grains to a number of martensite grains on grain boundary triple points of a matrix is 70% or more, wherein:

the bulging type martensite grains are on one of the grain boundary triple points of the matrix; and

at least one of all grain boundaries of the bulging type martensite grains, which are grain boundaries connecting two adjacent grain boundary triple points of the bulging type martensite grains and grains of the matrix, has a convex curvature to an outer side with respect to line segments connecting the two adjacent grain boundary triple points, and

an area ratio represented by VM/A 0 is 1.0 or more, wherein:

VM denotes a total area of the martensite grains on the grain boundary triple points of the matrix; and

A 0 denotes a total area of polygons comprising the line segments connecting two adjacent grain boundary triple points of the martensite grains.

2. The high-strength steel sheet according to claim 1 , wherein an average diameter D S of ferrite in a surface layer portion from a surface of the high-strength steel sheet to a depth 4×D 0 is not more than twice an average diameter D 0 , wherein the average diameter D 0 is an average diameter of ferrite in a region where a depth from the surface of the high-strength steel sheet is ¼ of a thickness of the high-strength steel sheet.

3. The high-strength steel sheet according to claim 1 , wherein an area fraction of unrecrystallized ferrite is 10% or less in the microstructure.

4. The high-strength steel sheet according to claim 1 , wherein, in the chemical composition,

Cr: 0.05% to 3.0%,

Mo: 0.05% to 1.0%,

Ni: 0.05% to 3.0%, or

Cu: 0.05% to 3.0%,

or any combination thereof is satisfied.

5. The high-strength steel sheet according to claim 1 , wherein, in the chemical composition,

Nb: 0.005% to 0.3%,

Ti: 0.005% to 0.3%, or

V: 0.01% to 0.5%,

or any combination thereof is satisfied.

6. The high-strength steel sheet according to claim 1 , wherein, in the chemical composition, B: 0.0001% to 0.1% is satisfied.

7. The high-strength steel sheet according to claim 1 , wherein, in the chemical composition,

Ca: 0.0005% to 0.01%,

Mg: 0.0005% to 0.01%,

Zr: 0.0005% to 0.01%, or

REM: 0.0005% to 0.01%,

or any combination thereof is satisfied.

8. A high-strength steel sheet according to claim 4 , further comprising a hot-dip galvanized layer.

9. A method of manufacturing the high-strength steel sheet according to claim 4 , comprising the steps of:

preparing a steel sheet;

reheating the steel sheet to a first temperature of 770° C. to 820° C. at an average heating rate of 3° C./s to 120° C./s; and

then, cooling the steel sheet to a second temperature of 300° C. or less at an average cooling rate of 60° C./s or more,

wherein

an area fraction of pearlite is 10% or less, an area fraction of unrecrystallized ferrite is 10% or less, and an average diameter of pearlite grain is 10 μm or less in the steel sheet,

an average diameter D S of ferrite in a surface layer portion from a surface of the steel sheet to a depth 4×D 0 is not more than twice an average diameter D 0 , wherein the average diameter D 0 is an average diameter of ferrite in a region where a depth from the surface of the steel sheet is ¼ of a thickness of the steel sheet,

the cooling to the second temperature is started within 8 seconds once the temperature of the steel sheet reaches the first temperature, and

the steel sheet comprises a chemical composition represented by, in mass %:

C: 003% to 0.35%;

Si: 0.01% to 2.0%;

Mn: 0.3% to 4.0%;

Al: 0.01% to 2.0%;

P: 0.10% or less;

S: 0.05% or less;

N: 0.010% or less;

Cr: 0.0% to 3.0%;

Mo: 0.0% to 1.0%;

Ni: 0.0% to 3.0%;

Cu: 0.0% to 3.0%;

Nb: 0.0% to 0.3%;

Ti: 0.0% to 0.3%;

V: 0.0% to 0.5%;

B: 0.0% to 0.1%;

Ca: 0.00% to 0.01%;

Mg: 0.00% to 0.01%;

Zr: 0.00% to 0.01%;

REM: 0.00% to 0.01%; and

the balance: Fe and impurities, and

a microstructure represented by, in area %,

martensite: 5% or more;

ferrite: 20% or more; and

perlite: 5% or less,

wherein

an average diameter of martensite grain is 4 μm or less in equivalent circle diameter,

a ratio of the number of bulging type martensite grains to the number of martensite grains on grain boundary triple points of a matrix is 70% or more, wherein:

the bulging type martensite grain is on one of the grain boundary triple points of the matrix; and

at least one of grain boundaries of the bulging type martensite grain, the grain boundaries connecting two adjacent grain boundary triple points of the bulging type martensite grain and grains of the matrix; has a convex curvature to an outer side with respect to line segments connecting the two adjacent grain boundary triple points, and an area ratio represented by VM/A 0 is 1.0 or more, wherein:

VM denotes a total area of the martensite grains on the grain boundary triple points of the matrix; and

A 0 denotes a total area of polygons composed of the line segments connecting two adjacent grain boundary triple points of the martensite grains.

10. The method of manufacturing the high-strength steel sheet according to claim 9 , wherein the step of preparing the steel sheet comprises the step of hot-rolling and cooling a slab.

11. The method of manufacturing the high-strength steel sheet according to claim 10 , wherein

a rolling temperature is “Ar3 point+10° C.” to 1000° C., and a total reduction ratio is 15% or more in last two stands of finish rolling in the hot rolling, and

a cooling stop temperature is 550° C. or lower of the cooling in the step of preparing the steel sheet.

12. The method of manufacturing the high-strength steel sheet according to claim 9 , wherein the step of preparing the steel sheet comprises the steps of:

hot rolling a slab to obtain a hot-rolled steel sheet; and

cold rolling, annealing and cooling the hot-rolled steel sheet.

13. The method of manufacturing the high-strength steel sheet according to claim 12 , wherein

a reduction ratio in the cold rolling is 30% or more,

a temperature of the annealing is 130° C. to 900° C., and

an average cooling rate from the temperature of the annealing to 600° C. is 1.0° C./s to 20° C./second in cooling in the step of preparing the steel sheet.

14. The method of manufacturing the high-strength steel sheet according to claim 9 , wherein, in the chemical composition,

Cr: 0.05% to 3.0%,

Mo: 0.05% to 1.0%,

Ni: 0.05% to 3.0%, or

Cu: 0.05% to 3.0%,

or any combination thereof is satisfied.

15. The method of manufacturing the high-strength steel sheet according to claim 9 , wherein, in the chemical composition,

Nb: 0.005% to 0.3%,

Ti: 0.005% to 0.3%, or

V: 0.01% to 0.5%,

or any combination thereof is satisfied.

16. The method of manufacturing the high-strength steel sheet according to claim 9 , wherein, in the chemical composition, B: 0,0001% to 0.1% is satisfied.

17. The method of manufacturing the high-strength steel sheet according to claim 9 , wherein, in the chemical composition,

Ca: 0.0005% to 0.01%,

Mg: 0.0005% to 0.01%,

Zr: 0.0005% to 0.01%, or

REM: 0.0005% to 0.01%,

or any combination thereof is satisfied.