Hot-rolled steel sheet
View Patent ↗A hot-rolled steel sheet has a composition containing: 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; and O: 0.01% or less, and one kind or a total of two kinds of Ti and Nb for 0.01 to 0.30%. An average effective crystal grain diameter at a sheet thickness ¼ part is 10 μm or less, and an average effective crystal grain diameter at a part of a range of 50 μm from a surface is 6 μm or less. A structure of the steel sheet is tempered martensite or lower bainite, and a volume fraction thereof is 90% or more as a total.
1. A hot-rolled steel sheet, comprising:
a composition containing in mass %,
C: 0.01 to 0.2%;
Si: 2.5% or less (excluding “0” (zero));
Mn: 4.0% or less (excluding “0” (zero));
P: 0.10% or less;
S: 0.03% or less;
Al: 0.001 to 2.0%;
N: 0.01% or less (excluding “0” (zero));
O: 0.01% or less (excluding “0” (zero));
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
Ti+Nb: 0.01 to 0.30%, and the balance made up of iron and impurities, and
a structure comprises lower bainite generated at less than 400° C., a volume fraction of lower bainite generated at less than 400° C. is 5% or more, and
a total of volume fractions of both of tempered martensite and lower bainite generated at less than 400° C. is 90% or more,
wherein an average effective crystal grain diameter of effective crystal grains in a region between a surface of the steel sheet and a position at a depth of ¼ of a sheet thickness from the surface, in the whole steel sheet, is 10 μm or less, and
wherein an average effective crystal grain diameter in a region from the surface of the steel sheet to a position at a depth of 50 μm from the surface is 6 μm or less.
2. The hot-rolled steel sheet according to claim 1 ,
wherein iron-based carbides existing in the either one or both of tempered martensite and lower bainite are 1×10 6 (pieces/mm 2 ) or more, and an average aspect ratio of the effective crystal grains of the either one or both of tempered martensite and lower bainite is two or less.
3. The hot-rolled steel sheet according to claim 1 , further comprising, in mass %,
one kind or two or more kinds selected from a group made up 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%.
4. The hot-rolled steel sheet according to claim 1 , further comprising, in mass %,
one kind or two or more kinds selected from a group made up of:
Mg: 0.0005 to 0.01%;
Ca: 0.0005 to 0.01%; and
REM: 0.0005 to 0.1%.
5. The hot-rolled steel sheet according to claim 1 , further comprising, in mass %,
B: 0.0002 to 0.01%.
6. The hot-rolled steel sheet according to claim 1 ,
wherein a galvanized layer or an alloyed galvanized layer is included at a surface of the steel sheet.