Bainite-containing-type high-strength hot-rolled steel sheet having excellent isotropic workability and manufacturing method thereof

1. A manufacturing method of a bainite-containing-type hot-rolled steel sheet comprising: on a slab containing: in mass%,

C: greater than 0.07 to 0.2%;

Si: 0.001 to 2.5%;

Mn: 0.01 to 4%;

P: 0.15% or less (not including 0%);

S: 0.03% or less (not including 0%);

N: 0.01% or less (not including 0%);

Al: 0.001 to 2%; and

a balance being composed of Fe and inevitable impurities,

performing first hot rolling on the slab 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.;

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 T1+30° C. nor higher than T1 +200° C. determined by Expression (1) below; and

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 a manner that a waiting time period 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.;

within three seconds after completion of the primary cooling, performing secondary cooling in which cooling is performed at an average cooling rate of 15° C./second or more; and

after completion of the secondary cooling, performing air cooling for 1 to 20 seconds in a temperature region of lower than an Ar3 transformation point temperature and an Ar1 transformation point temperature or higher and next performing coiling at 450° C. or higher and lower than 550° C.,

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

wherein, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the element (mass%).

t≦ 2.5 ×t 1  (2)

wherein, 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)

wherein, in Expression (3) above, Tf represents the temperature of the slab 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.

2. The manufacturing method of the bainite-containing-hot-rolled steel sheet according to claim 1 , wherein

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

3. The manufacturing method of the bainite-containing-hot-rolled steel sheet according to claim 1 , wherein

heat generation by working between respective passes in the temperature region of not lower than T1+30° C. nor higher than T1+200° C. in the second hot rolling is 18° C. or lower.

4. The manufacturing method of the bainite-containing-hot-rolled steel sheet according to claim 1 , wherein

the waiting time period t second further satisfies Expression (4) below,

t<t 1  (4).

5. The manufacturing method of the bainite-containing-hot-rolled steel sheet according to claim 1 , wherein

the waiting time period t second further satisfies Expression (5) below,

t 1≦ t≦t 1×2.5  (5).

6. The manufacturing method of the bainite-containing-hot-rolled steel sheet according to claim 1 , wherein

the primary cooling is started between rolling stands.