High strength thin steel sheet having high hydrogen embrittlement resisting property and high workability
The present invention provides a high strength thin steel sheet that has high hydrogen embrittlement resisting property and high workability. The high strength thin steel sheet having high hydrogen embrittlement resisting property has a metallurgical structure after stretch forming process to elongate 3% comprises: (i) 1% or more residual austenite; 80% or more in total of bainitic ferrite and martensite; and 9% or less (may be 0%) in total of ferrite and pearlite in terms of proportion of area to the entire structure, wherein the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher, or (ii) 1% or more residual austenite in terms of proportion of area to the entire structure; mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher; mean length of minor axes of the residual austenite grains is 1 μm or less; minimum distance between the residual austenite grains is 1 μm or less; and the steel has tensile strength of 1180 MPa or higher.
1 . A high strength thin steel sheet having high hydrogen embrittlement resisting property and high workability, which comprises:
C: higher than 0.25 up to 0.60%;
Si: 1.0 to 3.0%;
Mn: 1.0 to 3.5%;
P: 0.15% or less;
S: 0.02% or less; and
Al: 1.5% or less (higher than 0%) in terms of percentage by weight, with balance of iron and inevitable impurities;
wherein
the metal structure after stretch forming operation with elongation of 3% comprises:
residual austenite; 1% by area or more in proportion to the entire structure;
bainitic ferrite and martensite: 80% or more in total; and
ferrite and pearlite: 9% or less (may be 0%) in total, while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher, and
the steel has tensile strength of 1180 MPa or higher.
2 . The high strength thin steel sheet according to claim 1 , wherein the metal structure after said stretch forming operation with elongation of 3% further satisfies the requirements that:
mean length of minor axes of said residual austenite grains is 1 μm or less; and
minimum distance between said residual austenite grains is 1 μm or less.
3 . The high strength thin steel sheet according to claim 1 , wherein 0.5% or less (higher than 0%) by weight of Al is contained.
4 . The high strength thin steel sheet according to claim 1 , wherein
0.003 to 0.5% of Cu and/or
0.003 to 1.0% of Ni
in terms of percentage by weight are further contained.
5 . The high strength thin steel sheet according to claim 1 , wherein
0.003 to 1.0% of Ti and/or V
in terms of percentage by weight are further contained.
6 . The high strength thin steel sheet according to claim 1 , wherein
1.0% or less (higher than 0%) of Mo and
0.1% or less (higher than 0%) of Nb
in terms of percentage by weight are further contained.
7 . The high strength thin steel sheet according to claim 1 , wherein 0.0002 to 0.01% of B in terms of percentage by weight is further contained.
8 . The high strength thin steel sheet according to claim 1 , wherein at least one element selected from the group consisting of:
0.0005 to 0.005% of Ca;
0.0005 to 0.01% of Mg; and
0.0005 to 0.01% of REM
in terms of percentage by weight is further contained.
9 . A high strength thin steel sheet having high hydrogen embrittlement resisting property and high workability, which comprises:
C: higher than 0.25 up to 0.60%;
Si: 1.0 to 3.0%;
Mn: 1.0 to 3.5%;
P: 0.15% or less;
S: 0.02% or less; and
Al: 1.5% or less (higher than 0%) in terms of percentage by weight,
with balance of iron and inevitable impurities, wherein the metal structure after stretch forming operation with elongation of 3% comprises:
residual austenite; 1% by area or more in proportion to the entire structure;
while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher;
mean length of minor axes of said residual austenite grains is 1 μm or less; and
minimum distance between said residual austenite grains is 1 μm or less; and
the steel has tensile strength of 1180 MPa or higher.
10 . The high strength thin steel sheet according to claim 9 , wherein 0.5% or less (higher than 0%) by weight of Al is contained.
11 . The high strength thin steel sheet according to claim 9 , wherein
0.003 to 0.5% of Cu and/or
0.003 to 1.0% of Ni
in terms of percentage by weight are further contained.
12 . The high strength thin steel sheet according to claim 9 , wherein
0.003 to 1.0% of Ti and/or V
in terms of percentage by weight are further contained.
13 . The high strength thin steel sheet according to claim 9 , wherein
1.0% or less (higher than 0%) of Mo and
0.1% or less (higher than 0%) of Nb
in terms of percentage by weight are further contained.
14 . The high strength thin steel sheet according to claim 9 , wherein 0.0002 to 0.01% of B in terms of percentage by weight is further contained.
15 . The high strength thin steel sheet according to claim 9 , wherein at least one element selected from the group consisting of:
0.0005 to 0.005% of Ca;
0.0005 to 0.01% of Mg; and
0.0005 to 0.01% of REM
in terms of percentage by weight is further contained.