Ultrahigh-strength steel sheet excellent in hydrogen embrittlement resistance and workability, and manufacturing method therefor
The ultrahigh-strength steel sheet contains, in mass %, C: 0.05-0.25%, Si: 1.00-2.5%, Mn: 2.0-4.0%, P: 0.1% or below (not inclusive of 0%), S: 0.05% or below (not inclusive of 0%), Al: 0.01-0.15%, Ti: 0.003-0.10%, N: 0.01% or below (not inclusive of 0%), the balance comprising iron with inevitable impurities, and is a composite structure steel sheet comprising 10-50 area % ferrite and 50 area % or above martensite, in which the average circle-equivalent grain diameter of ferrite grains is 2.0 μm or below, the average aspect ratio of ferrite grains is 2.0 or below, and the tensile strength is 1,100 MPa or above. Accordingly, the ultrahigh-strength steel sheet is excellent in hydrogen embrittlement resistance and workability.
1. A steel sheet, comprising
C: 0.05-0.25%;
Si: 1.00-2.5%;
Mn: 2.0-4.0%;
P: 0.1% or below;
S: 0.05% or below;
Al: 0.01-0.15%;
Ti: 0.003-0.10%;
N: 0.01% or below; and
the balance comprising iron with inevitable impurities,
wherein:
% means mass %;
the steel sheet is a composite structure steel sheet comprising ferrite and martensite;
ferrite is 10-50 area % and martensite is 50 area % or above;
the average circle-equivalent grain diameter of ferrite grains is 2.0 μm or below and the average aspect ratio of ferrite grains is 2.0 or below; and
the tensile strength is 1,100 MPa or above.
2. The steel sheet of claim 1 , further comprising at least one of:
Nb: 0.003-0.20%; and
V: 0.003-0.20%,
wherein a total content of Ti, Nb and V is 0.25% or below.
3. The steel sheet of claim 1 , further comprising at least one element selected from a group consisting of:
Cu: 0.01-1.0%;
Ni: 0.01-1.0%; and
Cr: 0.01-1.0%.
4. The steel sheet of claim 1 , further comprising at least one of:
Mo: 0.01-1.0%; and
W: 0.01-1.0%.
5. The steel of sheet of claim 1 , further comprising B: 0.0001-0.005%.
6. The steel sheet of claim 1 , further comprising at least one element selected from a group consisting of:
Ca: 0.0005-0.005%;
Mg: 0.0005-0.005%; and
REM: 0.0005-0.005%.
7. The steel sheet of claim 1 , wherein the steel sheet is hot-dip galvanized or alloyed hot-dip galvanized.
8. The steel sheet of claim 1 , comprising:
P: greater than or equal to 0.006% to less than or equal to 0.022%.
9. A method for manufacturing the steel sheet of claim 1 , the method comprising:
(a) cold rolling a hot rolled steel sheet so that X expressed by an equation (I) satisfies X≧0 and a cold rolling ratio CR(%) becomes CR<50%;
X
=
20
(
[
Ti
]
+
[
Nb
]
2
+
[
V
]
4
)
-
9
[
Si
]
+
7
[
Mn
]
+
10
-
2.7
×
2
C
R
20
;
(
I
)
and
(b) soaking after the cold rolling at (A C1 +50)° C. to 900° C.,
wherein:
in equation (I), [Ti], [Nb], [V], [Si], [Mn] respectively represent content (mass %) of each element; and
the hot rolled steel sheet comprises:
C: 0.05-0.25%;
Si: 1.00-2.5%;
Mn: 2.0-4.0%;
P: 0.1% or below;
S: 0.05% or below;
Al: 0.01-0.15%;
Ti: 0.003-0.10%;
N: 0.01% or below; and
iron with inevitable impurities, wherein % means mass %.