Hot-rolled steel sheet and manufacturing method thereof

1. A hot-rolled steel sheet, comprising:

a structure that is a composition containing, in mass %,

C: 0.01 to 0.2%;

Si: 2.5% or less (not including “0” (zero));

Mn: 4.0% or less (not including “0” (zero));

P: 0.10% or less;

S: 0.03% or less;

Al: 0.001 to 2.0%;

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

O: 0.01% or less (not including “0” (zero));

Ti: 0.03 to 0.30%;

Nb: “0” (zero) to 0.30%;

Cu: “0” (zero) to 2.0%;

Ni: “0” (zero) to 2.0%;

Mo: “0” (zero) to 1.0%;

V: “0” (zero) to 0.3%;

Cr: “0” (zero) to 2.0%;

Mg: “0” (zero) to 0.01%;

Ca: “0” (zero) to 0.01%;

REM: “0” (zero) to 0.1%;

B: “0” (zero) to 0.01%; and

the balance comprising iron and impurities; and in which by volume fraction, 90% or more of grain-shaped tempered martensite transformed from recrystallized austenite, or by volume fraction, 90% or more in total of both grain-shaped tempered martensite transformed from recrystallized austenite and lower bainite transformed from recrystallized austenite is contained and an average aspect ratio of the tempered martensite and the lower bainite is 2 or less, and

wherein thickness of the hot-rolled steel sheet is less than 10 mm.

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

an effective crystal grain diameter of the tempered martensite and the lower bainite is 10μm or less.

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

1×10 6 (pieces/mm 2 ) or more of iron-based carbides exist in the tempered martensite and the lower bainite.

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

1×10 6 (pieces/mm 2 ) or more of iron-based carbides exist in the tempered martensite and the lower bainite.

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

in mass%,

Nb: 0.01 to 0.30%.

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

in mass %,

one type or two or more types selected from the group consisting of

Cu: 0.01 to 2.0%;

Ni: 0.01 to 2.0%;

Mo: 0.01 to 1.0%;

V: 0.01 to 0.3%; and

Cr: 0.01 to 2.0%.

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

in mass%,

one type or two or more types selected from the group consisting of

Mg: 0.0005 to 0.01%;

Ca: 0.0005 to 0.01%; and

REM: 0.0005 to 0.1%.

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

in mass%,

B: 0.0002 to 0.01%.

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

a galvanized layer or an alloyed galvanized layer is included on a surface of the hot-rolled steel sheet.