High-strength hot-rolled steel sheet having excellent local deformability and manufacturing method thereof

1. A hot-rolled steel sheet comprising:

in mass %,

C: not less than 0.07% nor more than 0.20%;

Si: not less than 0.001% nor more than 2.5%;

Mn: not less than 0.01% nor more than 4.0%;

P: not less than 0.001% nor more than 0.15%;

S: not less than 0.0005% nor more than 0.03%;

Al: not less than 0.001% nor more than 2.0%;

N: not less than 0.0005% nor more than 0.01%;

O: not less than 0.0005% nor more than 0.01%; and

a balance being composed of iron and inevitable impurities, wherein

an area ratio of bainite in a metal structure is 95% or more,

at a sheet thickness center portion being a range of ⅝ to ⅜ in sheet thickness from the surface of the steel sheet, an average value of pole densities of the {100}<011> to {223}<110> orientation group represented by respective crystal orientations of {100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>, {335}<110>, and {223}<110> is 4.0 or less, and a pole density of the {332}<113> crystal orientation is 5.0 or less, and

a mean volume diameter of crystal grains in the metal structure is 10 μm or less.

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

to crystal grains of the bainite, a ratio of the crystal grains in which a ratio of a length dL in a rolling direction to a length dt in a sheet thickness direction: dL/dt is 3.0 or less is 50% or more.

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

one type or two or more types of

in mass %,

Ti: not less than 0.001% nor more than 0.20%,

Nb: not less than 0.001% nor more than 0.20%,

V: not less than 0.001% nor more than 1.0%, and

W: not less than 0.001% nor more than 1.0%.

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

one type or two or more types of

in mass %,

B: not less than 0.0001% nor more than 0.0050%,

Mo: not less than 0.001% nor more than 1.0%,

Cr: not less than 0.001% nor more than 2.0%,

Cu: not less than 0.001% nor more than 2.0%,

Ni: not less than 0.001% nor more than 2.0%,

Co: not less than 0.0001% nor more than 1.0%,

Sn: not less than 0.0001% nor more than 0.2%,

Zr: not less than 0.0001% nor more than 0.2%, and

As: not less than 0.0001% nor more than 0.50%.

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

one type or two or more types of

in mass %,

Mg: not less than 0.0001% nor more than 0.010%,

REM: not less than 0.0001% nor more than 0.1%, and

Ca: not less than 0.0001% nor more than 0.010%.

6. A manufacturing method of a hot-rolled steel sheet, comprising:

on a steel billet containing:

in mass %,

C: not less than 0.07% nor more than 0.20%;

Si: not less than 0.001% nor more than 2.5%;

Mn: not less than 0.01% nor more than 4.0%;

P: not less than 0.001% nor more than 0.15%;

S: not less than 0.0005% nor more than 0.03%;

Al: not less than 0.001% nor more than 2.0%;

N: not less than 0.0005% nor more than 0.01%;

O: not less than 0.0005% nor more than 0.01%; and

a balance being composed of iron and inevitable impurities,

performing first hot rolling in which rolling at a reduction ratio of 40% or more is performed one time or more in a temperature range of not lower than 1000° C. nor higher than 1200° C.;

setting an austenite grain diameter to 200 μm or less by the first hot rolling;

performing second hot rolling in which rolling at 30% or more is performed in one pass at least one time in a temperature region of not lower than a temperature T1+30° C. nor higher than T1+200° C. determined by Expression (1) below;

setting the total of reduction ratios in the second hot rolling to 50% or more;

performing final reduction at a reduction ratio of 30% or more in the second hot rolling and then starting primary cooling in such a manner that a waiting time t second satisfies Expression (2) below;

setting an average cooling rate in the primary cooling to 50° C./second or more and performing the primary cooling in a manner that a temperature change is in a range of not lower than 40° C. nor higher than 140° C.;

starting secondary cooling after completion of the primary cooling;

performing cooling down to a temperature region of not lower than Ae 3 —50° C. nor higher than 700° C. at an average cooling rate of 15° C./second or more in the secondary cooling; and

performing coiling at higher than 350° C. to 650° C.;

T1(° C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V  (1)

t≤ 2.5× t 1  (2)

Here, t1 is obtained by Expression (3) below;

t 1=0.001×(( Tf−T 1)× P 1/100) 2 −0.109×(( Tf−T 1)× P 1/100)+3.1  (3)

Here, in Expression (3) above, Tf represents the temperature of the steel billet obtained after the final reduction at a reduction ratio of 30% or more, and P1 represents the reduction ratio of the final reduction at 30% or more.

7. The manufacturing method of the hot-rolled steel sheet according to claim 6 , wherein

the total of reduction ratios in a temperature range of lower than T1+30° C. is 30% or less.

8. The manufacturing method of the hot-rolled steel sheet according to claim 6 , wherein

the waiting time t second further satisfies Expression (2a) below

t<t 1  (2a).

9. The manufacturing method of the hot-rolled steel sheet according to claim 6 , wherein

the waiting time t second further satisfies Expression (2b) below

t 1≤ t≤t 1×2.5  (2b).

10. The manufacturing method of the hot-rolled steel sheet according to claim 6 , wherein

the primary cooling is started between rolling stands.