Hot-rolled steel sheet and method for producing same

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

C: 0.01% to 0.4%,

Si: 0.001% to 2.5%,

Mn: 0.001% to 4.0%,

Al: 0.001% to 2.0%,

P: limited to 0.15% or less,

S: limited to 0.03% or less,

N: limited to 0.01% or less,

O: limited to 0.01% or less, and

a balance comprising Fe and unavoidable impurities, wherein:

an average pole density of an orientation group of {100}<011>to {223}<110>, which is a pole density represented by an arithmetic average of pole densities of each crystal orientation {100}<011>, {116}<110>, {114}<110>, {112}<110>, and {223}<110>, is 1.0 to 5.0, and a pole density of a crystal orientation {332}<113>is 1.0 to 4.0 in a thickness central portion, which is a thickness range of ⅝ to ⅜ based on a surface of the steel sheet;

the steel sheet includes, as a metallographic structure, plural grains, and includes, by area %, 30% to 99% in total of a ferrite and a bainite, and 1% to 70% of a martensite; and

an area fraction of the martensite is defined as fM in unit of area %, an average size of the martensite is defined as dia in unit of μm, an average distance between the martensite is defined as dis in unit of μm, and a tensile strength of the steel sheet is defined as TS in unit of MPa, and a following Expression 1 and a following Expression 2 are satisfied:

dia≤13 μm  (Expression 1)

TS/fM×dis/dia≥500  (Expression 2).

2. The hot-rolled steel sheet according to claim 1 , further comprising, as the chemical composition, by mass %, at least one selected from the group consisting of:

Mo: 0.001% to 1.0%,

Cr: 0.001% to 2.0%,

Ni: 0.001% to 2.0%,

Cu: 0.001% to 2.0%,

B: 0.0001% to 0.005%,

Nb: 0.001% to 0.2%,

Ti: 0.001% to 0.2%,

V: 0.001% to 1.0%,

W: 0.001% to 1.0%,

Ca: 0.0001% to 0.01%,

Mg: 0.0001% to 0.01%,

Zr: 0.0001% to 0.2%,

Rare Earth Metal: 0.0001% to 0.1%,

As: 0.0001% to 0.5%,

Co: 0.0001% to 1.0%,

Sn: 0.0001% to 0.2%,

Pb: 0.0001% to 0.2%,

Y: 0.0001% to 0.2%, and

Hf: 0.0001% to 0.2%.

3. The hot-rolled steel sheet according to claim 1 ,

wherein a volume average diameter of the grains is 5 μm to 30 μm.

4. The hot-rolled steel sheet according to claim 1 ,

wherein the average pole density of the orientation group of {100}<011>to {223}<110>is 1.0 to 4.0, and the pole density of the crystal orientation {332}<113>is 1.0 to 3.0.

5. The hot-rolled steel sheet according to claim 1 ,

wherein a major axis of the martensite is defined as La, a minor axis of the martensite is defined as Lb, and an area fraction of the martensite satisfying a following Expression 3 is 50% to 100% as compared with the area fraction fM of the martensite:

La/Lb≤ 5.0  (Expression 3).

6. The hot-rolled steel sheet according to claim 1 ,

wherein the steel sheet includes, as the metallographic structure, by area %, 30% to 99% of the ferrite.

7. The hot-rolled steel sheet according to claim 1 ,

wherein the steel sheet includes, as the metallographic structure, by area %, 5% to 80% of the bainite.

8. The hot-rolled steel sheet according to claim 1 ,

wherein the steel sheet includes a tempered martensite in the martensite.

9. The hot-rolled steel sheet according to claim 1 ,

wherein an area fraction of coarse grains having a grain size of more than 35 μm is 0% to 10% among the grains in the metallographic structure of the steel sheet.

10. The hot-rolled steel sheet according to claim 1 ,

wherein a hardness H of the ferrite under a load of 1 mN in a nano-indenter satisfies a following Expression 4:

H<200+30×[Si]+21×[Mn]+270×[P]+78×[Nb] 1/2 +108×[Ti] 1/2   (Expression 4).

11. The hot-rolled steel sheet according to claim 1 ,

wherein a hardness of the ferrite or the bainite which is a primary phase is measured at 100 points or more under a load of 1 mN in a nano-indenter, and a value dividing a standard deviation of the hardness by an average of the hardness is 0.2 or less.

12. The hot-rolled steel sheet according to claim 2 , wherein a volume average diameter of the grains is 5 μm to 30 μm.

13. The hot-rolled steel sheet according to claim 2 ,

wherein the average pole density of the orientation group of {100}<011>to {223 } <110>is 1.0 to 4.0, and the pole density of the crystal orientation {332}<113>is 1.0 to 3.0.

14. The hot-rolled steel sheet according to claim 2 ,

wherein a major axis of the martensite is defined as La, a minor axis of the martensite is defined as Lb, and an area fraction of the martensite satisfying a following Expression 3 is 50% to 100% as compared with the area fraction fM of the martensite:

La/Lb≤ 5.0  (Expression 3).

15. The hot-rolled steel sheet according to claim 2 ,

wherein the steel sheet includes, as the metallographic structure, by area %, 30% to 99% of the ferrite.

16. The hot-rolled steel sheet according to claim 2 ,

wherein the steel sheet includes, as the metallographic structure, by area %, 5% to 80% of the bainite.

17. The hot-rolled steel sheet according to claim 2 ,

wherein the steel sheet includes a tempered martensite in the martensite.

18. The hot-rolled steel sheet according to claim 2 ,

wherein an area fraction of coarse grains having a grain size of more than 35 μm is 0% to 10% among the grains in the metallographic structure of the steel sheet.

19. The hot-rolled steel sheet according to claim 2 , wherein a hardness H of the ferrite under a load of 1 mN in a nano-indenter satisfies a following Expression 4:

H<200+30×[Si]+21×[Mn]+270×[P]+78×[Nb] 1/2 +108×[Ti] 1/2   (Expression 4).

20. The hot-rolled steel sheet according to claim 2 ,

wherein a hardness of the ferrite or the bainite which is a primary phase is measured at 100 points or more under a load of 1 mN in a nano-indenter, and a value dividing a standard deviation of the hardness by an average of the hardness is 0.2 or less.