High-strength hot-rolled steel sheet and method for producing the same

1. A high-strength hot-rolled steel sheet comprising: by mass %,

C: 0.050 to 0.200%;

Si: 0.01 to 1.5%;

Mn: 1.0 to 3.0%;

B: 0.0002 to 0.0030%;

Ti: 0.03 to 0.20%;

P: limited to 0.05% or less;

S: limited to 0.005% or less;

Al: limited to 0.5% or less;

N: limited to 0.009% or less; and

one or more of Nb: 0.01 to 0.20%, V: 0.01 to 0.20%, and Mo: 0.01 to 0.20%,

with the balance being composed of Fe and inevitable impurities,

wherein a ratio of a length of small-angle crystal grain boundaries that are boundaries having a crystal orientation angle of 5° or more but less than 15° to a length of large-angle crystal grain boundaries that are boundaries having a crystal orientation angle of 15° or more is 1:1 to 1:4,

a total segregation amount of C and B in the large-angle grain boundaries is 4 to 20 atoms/nm 2 ,

tensile strength is 850 MPa or higher, and

a hole expansion ratio is 25% or more.

2. The high-strength hot-rolled steel sheet according to claim 1 , wherein the content of P is limited to 0.02% or less by mass %, and

the segregation amount of P in the large-angle grain boundaries is 1 atoms/nm 2 or less.

3. The method for producing a high-strength hot-rolled steel sheet according to claim 1 , the method comprising:

with respect to a steel slab containing by mass %,

C: 0.050 to 0.200%,

Si: 0.01 to 1.5%,

Mn: 1.0 to 3.0%,

B: 0.0002 to 0.0030%,

Ti: 0.03 to 0.20%,

P: limited to 0.05% or less,

S: limited to 0.005% or less,

Al: limited to 0.5% or less,

N: limited to 0.009% or less, and

one or more of Nb: 0.01 to 0.20%, V: 0.01 to 0.20%, and Mo: 0.01 to 0.20%,

with the balance being composed of Fe and inevitable impurities,

heating the steel slab to 1200° C. or higher;

completing finish rolling at a temperature of 910° C. or higher;

performing air cooling for 0.5 to 7 seconds after completing the finish rolling;

subjecting to primary cooling up to a temperature of 550 to 450° C. at a cooling rate of 40° C./s or more;

subjecting to holding or air cooling at a temperature that is not higher than a stop temperature of the primary cooling but not lower than 450° C. for 7.5 to 30 seconds;

subsequently subjecting to secondary cooling up to a temperature of 200° C. or lower at a cooling rate of 15° C./s or more; and

subjecting to coiling.

4. The method for producing the high-strength hot-rolled steel sheet according to claim 3 , wherein the content of P is limited to 0.02% or less, by mass %, in the steel slab.