[TECHNICAL PROBLEM]
[0005]
2
For working steel sheet, in addition to the bending such as described in PTL 1, there is also
cutting, etc. As a specific example of the cutting, typically shearing is known. In shearing of
steel sheet, in general, the steel sheet workpiece is placed between a punch and die and these are
used to cut the steel sheet by the action of the shear force. In such a work process, sometimes
5 tensile residual stress is caused at the sheared edge of the steel sheet. There is the problem that if
the tensile residual stress becomes greater, the risk will rise of hydrogen embrittlement cracking
occurring due to the hydrogen penetrating the steel from the outside environment.
[0006]
The present invention was made in consideration of such a situation, and an object of the
10 present invention is to provide a steel sheet able to reduce tensile residual stress occurring at a
sheared edge at the time of shearing by a novel constitution.
[SOLUTION TO PROBLEM]
[0007]
15 To achieve the above object, the inventors studied the constitution of a steel sheet able to
reduce the tensile residual stress occurring at a sheared edge at the time of shearing. As a result,
the inventors discovered that by providing the two sides of steel sheet with surface layer softened
parts having hardnesses lower than the sheet thickness center part of the steel sheet and further
providing a difference in hardness between these surface layer softened parts, it is possible to
20 reduce the tensile residual stress occurring at the sheared edge, and thereby completed the
present invention.
[0008]
The steel sheet for achieving the above object is as follows:
[1] A steel sheet comprising a sheet thickness center part and a first surface layer softened
25 part and a second surface layer softened part respectively arranged at two sides of the sheet
thickness center part, wherein
the first surface layer softened part and second surface layer softened part have 10 m or
more average thicknesses and have average Vickers hardnesses of 0.90 time or less of the
average Vickers hardness of a sheet thickness 1/2 position, and
30 the first surface layer softened part has an average Vickers hardness of 1.05 times or more
the average Vickers hardness of the second surface layer softened part.
[2] The steel sheet according to [1], wherein the first surface layer softened part has an
average Vickers hardness of 2.00 times or more the average Vickers hardness of the second
surface layer softened part.
35 [3] The steel sheet according to [1] or [2], wherein the first surface layer softened part has
an average thickness smaller than the average thickness of the second surface layer softened part.
3
[4] The steel sheet according to [3], wherein the first surface layer softened part has an
average thickness of 1/2 or less of the average thickness of the second surface layer softened
part.
[5] The steel sheet according to any one of [1] to [4], wherein at least one of the first surface
5 layer softened part and second surface layer softened part comprises an internal oxide layer.
[6] The steel sheet according to any one of [1] to [5], wherein the tensile strength is 980
MPa or more.
[7] The steel sheet according to [6], wherein the tensile strength is 1470 MPa or more.
[8] The steel sheet according to any one of [1] to [7], wherein the sheet thickness center
10 part has a chemical composition comprising, by mass%,
C: 0.050 to 0.800%,
Si: 0.01 to 3.00%,
Mn: 0.01 to 10.00%,
Al: 0.001 to 0.500%,
15 P: 0.100% or less,
S: 0.050% or less,
N: 0.010% or less,
Cr: 0 to 3.000%,
Mo: 0 to 1.000%,
20 B: 0 to 0.0100%,
Ti: 0 to 0.500%,
Nb: 0 to 0.500%,
V: 0 to 0.500%,
Cu: 0 to 0.50%,
25 Ni: 0 to 0.50%,
O: 0 to 0.020%,
W: 0 to 0.100%,
Ta: 0 to 0.10%,
Co: 0 to 0.50%,
30 Sn: 0 to 0.050%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Mg: 0 to 0.050%,
Ca: 0 to 0.050%,
35 Y: 0 to 0.050%,
Zr: 0 to 0.050%,
4
La: 0 to 0.050%,
Ce: 0 to 0.050%, and
balance: Fe and impurities.
[9] The steel sheet according to [8], wherein the chemical composition comprises, by
5 mass%, at least one selected from the group consisting of:
Cr: 0.001 to 3.000%,
Mo: 0.001 to 1.000%,
B: 0.0001 to 0.0100%,
Ti: 0.001 to 0.500%,
10 Nb: 0.001 to 0.500%,
V: 0.001 to 0.500%,
Cu: 0.001 to 0.50%,
Ni: 0.001 to 0.50%,
O: 0.0001 to 0.020%,
15 W: 0.001 to 0.100%,
Ta: 0.001 to 0.10%,
Co: 0.001 to 0.50%,
Sn: 0.001 to 0.050%,
Sb: 0.001 to 0.050%,
20 As: 0.001 to 0.050%,
Mg: 0.0001 to 0.050%,
Ca: 0.001 to 0.050%,
Y: 0.001 to 0.050%,
Zr: 0.001 to 0.050%,
25 La: 0.001 to 0.050%, and
Ce: 0.001 to 0.050%.
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0009]
30 According to the present invention, it is possible to provide a steel sheet able to reduce
tensile residual stress occurring at a sheared edge at the time of shearing.
BRIEF DESCRIPTION OF DRAWINGS
[0010]
35 FIG. 1 is a schematic view for explaining one example of a mechanism of formation of a
sheared edge in the case of shearing a steel sheet.

CLAIMS
[Claim 1]
A steel sheet comprising a sheet thickness center part and a first surface layer softened part
and a second surface layer softened part respectively arranged at two sides of the sheet thickness
5 center part, wherein
the first surface layer softened part and second surface layer softened part have 10 m or
more average thicknesses and have average Vickers hardnesses of 0.90 time or less of the
average Vickers hardness of a sheet thickness 1/2 position, and
the first surface layer softened part has an average Vickers hardness of 1.05 times or more
10 the average Vickers hardness of the second surface layer softened part.
[Claim 2]
The steel sheet according to claim 1, wherein the first surface layer softened part has an
average Vickers hardness of 2.00 times or more the average Vickers hardness of the second
15 surface layer softened part.
[Claim 3]
The steel sheet according to claim 1 or 2, wherein the first surface layer softened part has an
average thickness smaller than the average thickness of the second surface layer softened part.
20
[Claim 4]
The steel sheet according to claim 3, wherein the first surface layer softened part has an
average thickness of 1/2 or less of the average thickness of the second surface layer softened
part.
25
[Claim 5]
The steel sheet according to any one of claim 1 to 4, wherein at least one of the first surface
layer softened part and second surface layer softened part comprises an internal oxide layer.
30 [Claim 6]
The steel sheet according to any one of claims 1 to 5, wherein the tensile strength is 980
MPa or more.
[Claim 7]
35 The steel sheet according to claim 6, wherein the tensile strength is 1470 MPa or more.
37
[Claim 8]
The steel sheet according to any one of claims 1 to 7, wherein the sheet thickness center
part has a chemical composition comprising, by mass%,
C: 0.050 to 0.800%,
5 Si: 0.01 to 3.00%,
Mn: 0.01 to 10.00%,
Al: 0.001 to 0.500%,
P: 0.100% or less,
S: 0.050% or less,
10 N: 0.010% or less,
Cr: 0 to 3.000%,
Mo: 0 to 1.000%,
B: 0 to 0.0100%,
Ti: 0 to 0.500%,
15 Nb: 0 to 0.500%,
V: 0 to 0.500%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
O: 0 to 0.020%,
20 W: 0 to 0.100%,
Ta: 0 to 0.10%,
Co: 0 to 0.50%,
Sn: 0 to 0.050%,
Sb: 0 to 0.050%,
25 As: 0 to 0.050%,
Mg: 0 to 0.050%,
Ca: 0 to 0.050%,
Y: 0 to 0.050%,
Zr: 0 to 0.050%,
30 La: 0 to 0.050%,
Ce: 0 to 0.050%, and
balance: Fe and impurities.
[Claim 9]
35 The steel sheet according to claim 8, wherein the chemical composition comprises, by
mass%, at least one selected from the group consisting of:
38
Cr: 0.001 to 3.000%,
Mo: 0.001 to 1.000%,
B: 0.0001 to 0.0100%,
Ti: 0.001 to 0.500%,
5 Nb: 0.001 to 0.500%,
V: 0.001 to 0.500%,
Cu: 0.001 to 0.50%,
Ni: 0.001 to 0.50%,
O: 0.0001 to 0.020%,
10 W: 0.001 to 0.100%,
Ta: 0.001 to 0.10%,
Co: 0.001 to 0.50%,
Sn: 0.001 to 0.050%,
Sb: 0.001 to 0.050%,
15 As: 0.001 to 0.050%,
Mg: 0.0001 to 0.050%,
Ca: 0.001 to 0.050%,
Y: 0.001 to 0.050%,
Zr: 0.001 to 0.050%,
20 La: 0.001 to 0.050%, and
Ce: 0.001 to 0.050%