Hot-rolled steel sheet and manufacturing method thereof

1. A hot-rolled steel sheet comprising:

a chemical composition comprising: in mass %,

C: 0.01 to 0.2%;

Si: 0.001 to 2.5%;

Mn: 0.10 to 4.0%;

P: 0.10% or less;

S: 0.030% or less;

Al: 0.001 to 2.0%;

N: 0.01% or less;

Ti: (0.005+48/14[N]+48/32[S])%≤Ti≤0.3%;

Nb: 0 to 0.06%;

Cu: 0 to 1.2%;

Ni: 0 to 0.6%;

Mo: 0 to 1%;

V: 0 to 0.2%;

Cr: 0 to 2%;

Mg: 0 to 0.01%;

Ca: 0 to 0.01%;

REM: 0 to 0.1%; and

B: 0 to 0.002%,

with a balance being composed of Fe and impurities;

an texture in which, at a central portion of a sheet thickness that is a steel sheet portion sectioned at a ⅜ thickness position and a ⅝ thickness position of the sheet thickness from a surface of the steel sheet, an average value of X-ray random intensity ratios of a group of {100}<011> to {223}<110> orientations of a sheet plane is 6.5 or less and an X-ray random intensity ratio of a {332}<113> crystal orientation is 5.0 or less; and

a microstructure in which a total area ratio of tempered martensite, martensite and lower bainite is more than 85%, and an average crystal grain diameter is 12.0 μm or less.

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

the chemical composition contains one or two or more selected from a group consisting of: in mass %,

Nb: 0.005 to 0.06%;

Cu: 0.02 to 1.2%;

Ni: 0.01 to 0.6%;

Mo: 0.01 to 1%;

V: 0.01 to 0.2%; and

Cr: 0.01 to 2%.

3. The hot-rolled steel sheet according to claim 1 , wherein the chemical composition contains one or two or more selected from a group consisting of: in mass %, Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and REM: 0.0005 to 0.1%.

4. The hot-rolled steel sheet according to claim 1 , wherein the chemical composition contains, in mass %, B: 0.0002 to 0.002%.

5. The hot-rolled steel sheet according to claim 1 , comprising the microstructure in which assuming that an average value of hardness is E (HV0.01) and a standard deviation is σ (HV0.01) when measuring the Vickers hardness at 100 points or more with a load of 0.098 N, σ (HV0.01)/E (HV0.01) is 0.08 or less.

6. The hot-rolled steel sheet according to claim 1 , comprising mechanical properties that an r value (rC) in a direction perpendicular to a rolling direction is 0.70 or more, and an r value (r30) in a direction 30° from the rolling direction is 1.10 or less.

7. The hot-rolled steel sheet according to claim 1 , comprising mechanical properties that an r value (rL) in a rolling direction is 0.70 or more and an r value (r60) in a direction 60° from the rolling direction is 1.10 or less.

8. The hot-rolled steel sheet according to claim 1 , comprising a plating layer provided on the surface of the steel sheet.

9. A manufacturing method of a hot-rolled steel sheet according to claim 1 , which comprises: sequentially performing rough hot rolling, finish hot rolling, primary cooling and secondary cooling on a slab comprising a chemical composition comprising in mass %

C: 0.01 to 0.2%;

Si: 0.001 to 2.5%;

Mn: 0.10 to 4.0%;

P: 0.10% or less;

S: 0.030% or less;

Al: 0.001 to 2.0%;

N: 0.01% or less;

Ti: (0.005+48/14[N]+48/32[S])%≤Ti≤0.3%;

Nb: 0 to 0.06%;

Cu: 0 to 1.2%;

Ni: 0 to 0.6%;

Mo: 0 to 1%;

V: 0 to 0.2%;

Cr: 0 to 2%;

Mg: 0 to 0.01%;

Ca: 0 to 0.01%;

REM: 0 to 0.1%; and

B: 0 to 0.002%,

with a balance being composed of Fe and impurities;

and coiling a resultant slab into the hot-rolled steel sheet, wherein:

the finish hot rolling is hot rolling in which with respect to a temperature T1 defined in a following expression (1), a maximum reduction ratio per pass in a first temperature region of 1004° C. or higher and (T1+200)° C. or lower is 41% or more, a total reduction ratio in the first temperature region is 50% or more, a total reduction ratio in a second temperature region of T1° C. or higher and lower than (T1+30)° C. is 0 to 30%, and the rolling is completed in the first temperature region or the second temperature region;

the primary cooling is water cooling that satisfies a following expression (2) and achieves a cooling amount of 40° C. or higher and 140° C. or lower;

the secondary cooling is water cooling that is started within three seconds after the primary cooling and performs cooling at an average cooling rate of 30° C./sec or higher; and

the coiling is to coil the slab at a temperature CT satisfying a following expression (3),

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

1≤ t/t 1≤2.5  (2)

CT (° C.)≤max[Ms,350]  (3)

t 1=0.001×{( Tf−T 1)× P 1/100} 2 −0.109×{( Tf−T 1)× P 1/100}+3.1   (4)

Ms(° C.)=561−474×C−33×Mn−17×Ni−21×Mo   (5)

where in the expression (1) and the expression (5), a symbol of each element is a content (mass %) of the element in the steel,

in the expression (2), t is a time period (sec) from a final reduction in the reduction in one pass at 30% or more in the first temperature region to start of the primary cooling, and t1 is a time period (sec) decided by the above expression (4),

in the expression (3), max[ ] is a function of returning a maximum value among arguments, and Ms is a temperature decided by the above expression (5), and

in the expression (4), Tf and P1 are a steel sheet temperature and a reduction ratio (%) in the final reduction in the reduction in one pass at 30% or more in the first temperature region respectively.

10. The manufacturing method of the hot-rolled steel sheet according to claim 9 , wherein the rough hot rolling achieves a maximum reduction ratio per pass in a temperature region of 1000° C. or higher and 1200° C. or lower of 40% or more, and an austenite average grain diameter of 200 μm or less.

11. The manufacturing method of the hot-rolled steel sheet according to claim 9 , wherein a maximum heat generation due to plastic deformation in a temperature region of (T1+30)° C. or higher and (T1+150)° C. or lower of the finish hot rolling is 18° C. or lower.

12. A manufacturing method of a hot-rolled steel sheet comprising: performing a plating treatment on the surface of the hot-rolled steel sheet obtained by the manufacturing method of the hot-rolled steel sheet according to claim 9 .