Hot-rolled steel sheet and manufacturing method thereof
There is provided a hot-rolled steel sheet in which a composition contains: in mass %, C: 0.01% to 0.2%; Si: 2.5% or less; Mn: 4.0% or less; P: 0.10% or less; S: 0.03% or less; Al: 0.001% to 2.0%; N: 0.01% or less; O: 0.01% or less; Ti: 0.01 to 0.30%; and the balance being composed of iron and impurities and a structure is composed of by volume fraction, 90% or more of tempered martensite with an average aspect ratio of 2 or less, or 90% or more in total of both tempered martensite and lower bainite.
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.