ELD
[0001]
The present invention relates to a steel sheet.
10 BACKGROUND
[0002]
Steel sheet is generally assembled into various structures and supplied to their respective
applications through cutting, bending, and other working steps, welding and other joining steps,
and coating and other finishing steps. In the working steps, according to the working operations,
15 various stresses may act upon the steel sheet or the quality of the steel sheet may change. Due to
these, sometimes cracking or embrittlement occur. Therefore, steel sheet is generally required to
have excellent characteristics for the specific working operations applied.
[0003]
PTL 1 describes tensile strength 800 MPa or more high strength steel sheet comprising a
20 sheet thickness center part and a surface layer softened part arranged on one side or both sides of
the sheet thickness center part, wherein each surface layer softened part has a thickness of from
more than 10 m to 30% or less of the sheet thickness, an average Vickers hardness of each
surface layer softened part is 0.60 time or less the average Vickers hardness of a sheet thickness
1/2 position, and a standard deviation of nanohardness of each surface layer softened part is 0.8
25 or less. Further, PTL 1 teaches that the bendability is improved by providing the surface layer
softened part on one side or both sides of the steel sheet and suppressing variation in the
microhardness of each surface layer softened part.
[CITATIONS LIST]
30 [PATENT LITERATURE]
[0004]
[PTL 1] WO2018/151331
SUMMARY
35 [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 parts
having hardnesses different from the sheet thickness center part of the steel sheet and further
providing a difference in hardness at the two sides of the steel sheet, it is possible to reduce the
20 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 part and
a second surface layer part respectively arranged at two sides of the sheet thickness center part,
25 wherein
the first surface layer part and second surface layer part respectively independently have
thicknesses of more than 10 m to 30% or less of the sheet thickness,
the first surface layer part and second surface layer part have average Vickers hardnesses
different from the average Vickers hardness of a sheet thickness 1/2 position, and
30 a first hardness cumulative value at a region from a surface of the first surface layer part
side to 30% of the sheet thickness is 1.05 times or more of a second hardness cumulative value at
a region from a surface of the second surface layer part side to 30% of the sheet thickness.
[2] The steel sheet according to [1], wherein a ratio P (Hh i g h /Hl o w ) of a hardness
cumulative value Hh i g h at a region from a surface of the first surface layer part side to X% of
35 the sheet thickness and a hardness cumulative value Hl o w at a region from a surface of the
second surface layer part side to X% of the sheet thickness satisfies the following formula 1:
3
P0.00035(X-30)2
+1.05 ･･･ formula 1
where, 0
The steel sheet according to an embodiment of the present invention comprises a sheet
thickness center part and a first surface layer part and a second surface layer part respectively
arranged at two sides of the sheet thickness center part, wherein
the first surface layer part and second surface layer part respectively independently have
20 thicknesses of more than 10 m to 30% or less of the sheet thickness,
the first surface layer part and second surface layer part have average Vickers hardnesses
different from the average Vickers hardness of a sheet thickness 1/2 position, and
a first hardness cumulative value at a region from a surface of the first surface layer part
side to 30% of the sheet thickness is 1.05 times or more of a second hardness cumulative value at
25 a region from a surface of the second surface layer part side to 30% of the sheet thickness.
[0012]
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. In shearing of a steel sheet, generally, as
shown in FIG. 1(A), a first surface 10a of a workpiece comprised of the steel sheet 5 is pressed
30 by a cutting edge of a first blade 21 (for example, a punch). In the process until the cutting edge
of the first blade 21 cuts into the steel sheet 5, a droop 1a is formed on the first surface 10a side.
Next, in the process of the cutting edge of the first blade 21 cutting into the steel sheet 5, the
sheared surface 1e (see FIG. 3) is formed. After the droop 1a and the sheared surface 1e are
formed, as shown in FIG. 1(B), the first crack 1dx is generated from the first blade 21 side
35 toward the second blade 22 (for example the die) side. On the other hand, at the second blade 22
side as well, in the same way, the second surface 10b of the steel sheet 5 is cut into by the cutting
6
edge of the second blade 22, whereby the second crack 1dy is generated from the second blade
22 side toward the first blade 21 side. Finally, as shown in FIG. 1(C), the first crack 1dx and the
second crack 1dy grow and merge with each other, whereupon the fracture surface 1b is formed.
Further, by making the first blade 21 and the second blade 22 further move, the steel sheet 5 is
5 separated into scrap 15 and the target product workpiece 10. At that time, as shown in FIG. 1(C),
in the sheared edge 1 of the workpiece 10, a burr 1c is generally formed at the edge at the second
blade 22 side.
[0013]
In the sheared edge 1 formed in the above way, due to the damage and warping, etc., caused
10 by the shearing, compressive residual stress and tensile residual stress can be formed. If there is a
large tensile residual stress at the sheared edge 1, sometimes the hydrogen embrittlement
resistance of the sheared edge 1 falls and hydrogen embrittlement cracking occurs. “Hydrogen
embrittlement cracking” means the phenomenon of a steel member suddenly fracturing due to
hydrogen penetrating the steel from the outside environment and is also referred to as delayed
15 fracture, etc. Hydrogen embrittlement cracking generally easily occurs at locations where stress
concentrates, in particular is known to easily occur at a location where tensile residual stress is
present. Further, in shearing, generally, in the sheared edge 1, the highest tensile strength tends
to occur at the fracture surface 1b. Therefore, to avoid in advance the problem of hydrogen
embrittlement cracking due to shearing, it is important in particular to reduce the tensile residual
20 stress occurring at the fracture surface 1b in the tensile residual stress occurring at the sheared
edge 1.
[0014]
In relation to this, the inventors engaged in numerous experiments and analyses repeatedly
regarding the relationship between the conditions of shearing of the steel sheet 5 and the
25 properties of the sheared edge 1 formed by the shearing and as a result obtained the following
new discovery.
[0015]
FIG. 2 is a schematic view for explaining a new discovery obtained by the inventors. In the
figure, “○” means a small tensile residual stress, “” means a medium extent tensile residual
30 stress, and “” means a large tensile residual stress. As shown in FIG. 2(A) to (C), the case of
stamping one part 11 of the steel sheet 5 by the first blade 21 and stamping another part 12 of the
steel sheet 5 by the second blade 22 will be explained. In this case, as shown in FIG. 2(A), if a
crack preferentially grows from the first blade 21 side, the tensile residual stress at the sheared
edge of the one part 11 becomes larger. On the other hand, the tensile residual stress at the
35 sheared edge of the other part 12 becomes smaller. In other words, the one part 11 can be made
scrap 15, while the other part 12 can be suitably employed as the product (workpiece 10).
7
Further, as shown in FIG. 2(B), if cracks equivalently grow from both the first blade 21 side and
the second blade 22 side, equivalent tensile residual stress can be generated at the sheared edges
of both of the one part 11 and other part 12. In other words, variation of characteristics at the one
part 11 and the other part 12 can be suppressed. Therefore, it can be said to be suitable when
5 employing both the one part 11 and other part 12 as the product. Furthermore, as shown in FIG.
2(C), if a crack preferentially grows from the second blade 22 side, the tensile residual stress at
the sheared edge of the other part 12 becomes larger. On the other hand, the tensile residual
stress at the sheared edge of the one part 11 becomes smaller. In other words, the other part 12
can be made scrap 15, while the one part 11 can be suitably employed as the product (workpiece
10 10).
[0016]
FIG. 3 is a schematic enlarged view showing one example of the sheared edge 1 part of the
workpiece 10 (other part 12) corresponding to FIG. 2(A). Referring to FIG. 3, the droop 1a, the
fracture surface 1b, the burr 1c, and the sheared surface 1e are formed on the sheared edge 1. The
15 fracture surface 1b includes a first part 1bx and a second part 1by. The first part 1bx is formed by
the first crack 1dx growing from the droop 1a side to the burr 1c side, while the second part 1by
is formed by the second crack 1dy proceeding from the burr 1c side to the droop 1a side. At the
sheared edge 1 of FIG. 3, due in part to the preferential crack growth from the first blade 21 side,
the area ratio of the first part 1bx in the fracture surface 1b becomes larger than the area ratio of
20 the second part 1by in the fracture surface 1b.
[0017]
From the above, the following (1) to (3) can be said:
(1) In the sheared edge 1, the tensile residual stress occurring at the fracture surface 1b changes
depending on the directions of growth and lengths of the cracks 1dx, 1dy forming the fracture
25 surface 1b.
(2) At the fracture surface 1b, the longer the crack 1dx growing from the droop 1a side, the
smaller the tensile residual stress of the fracture surface 1b of the workpiece 10 and the larger the
tensile residual stress of the fracture surface of the scrap 15.
(3) In other words, if, at the fracture surface 1b of the workpiece 10, the area ratio of the part
30 derived from the first crack 1dx growing from the droop 1a side is larger than the area ratio of
the part derived from the second crack 1dy growing from the burr 1c side, compared with when
the area ratio of the part derived from the first crack 1dx growing from the droop 1a side is
smaller than the area ratio of the part derived from the second crack 1dy growing from the burr
1c side, it is possible to relatively reduce the tensile residual stress of the fracture surface 1b.
35 [0018]
Based on the discoveries of the above (1) to (3), the inventors further studied the
8
configuration of a steel sheet 5 able to reduce the tensile residual stress occurring at the sheared
edge 1 of the workpiece 10, in particular, the fracture surface 1b, obtained by preferentially
growing the crack from the first surface 10a of the steel sheet 5 corresponding to the first blade
21 (generally, the punch) side. As a result, the inventors discovered that by providing a first
5 surface layer part (for example the first surface 10a side) and a second surface layer part (for
example the second surface 10b side) having different hardnesses from the sheet thickness center
part of the steel sheet 5 at the two sides of the steel sheet 5 and further providing a difference in
hardness at the two sides of the steel sheet 5, more specifically by making the first hardness
cumulative value at the region from the surface of the first surface layer part side to 30% of the
10 sheet thickness 1.05 times or more of the second hardness cumulative value at the region from
the surface of the second surface layer part side to 30% of the sheet thickness, it is possible to
preferentially grow a crack from the first surface layer part side with a relatively large hardness
when shearing the steel sheet 5 and thereby possible to remarkably reduce the tensile residual
stress occurring at the sheared edge 1 of the workpiece 10 (steel sheet 5), in particular the
15 fracture surface 1b. Therefore, according to the steel sheet of the present invention, it becomes
possible to remarkably improve the hydrogen embrittlement resistance of the sheared edge at the
workpiece obtained by the shearing.
[0019]
Below, steel sheets according to embodiments of the present invention will be explained in
20 detail, but the explanation of these is intended to just illustrate preferred embodiments of the
present invention and is not intended to limit the present invention to such specific embodiments.
[0020]
(First Surface Layer Part and Second Surface Layer Part)
According to this embodiment of the present invention, the first surface layer part and the
25 second surface layer part respectively independently have thicknesses of more than 10 m to
30% or less of the sheet thickness and have average Vickers hardnesses different from the
average Vickers hardness of the sheet thickness 1/2 position. By having thicknesses of more than
10 m to 30% or less of the sheet thickness, it is possible to sufficiently obtain the effect of
provision of surface layer parts at the two sides of the steel sheet. So long as satisfying the
30 requirement, explained later in detail, of the first hardness cumulative value being 1.05 times or
more of the second hardness cumulative value, the thicknesses of the first surface layer part and
the second surface layer part may respectively be any values in ranges from more than 10 m to
30% or less of the sheet thickness. The values of the two may be the same or may be different.
For example, the thicknesses of the first surface layer part and second surface layer part may
35 respectively independently be 15 m or more, 30 m or more, 50 m or more, 100 m or more,
150 m or more, or 200 m or more and/or may be 25% or less, 20% or less, 15% or less, or
9
10% or less of the sheet thickness. If illustrating the upper limits not by ratios with respect to the
sheet thickness, but by specific thicknesses, for example, the thicknesses of the first surface layer
part and second surface layer part may respectively independently be 1800 m or less, 1200 m
or less, 800 m or less, 600 m or less, 500 m or less, 470 m or less, 450 m or less, 430 m
5 or less, 400 m or less, 350 m or less, or 300 m or less. If the thickness of the first surface
layer part and the thickness of the second surface layer part differ, the absolute value of the
difference between the thickness of the first surface layer part and the thickness of the second
surface layer part may be 5 m or more, 10 m or more, 20 m or more, 30 m or more, 40 m
or more, or 50 m or more and/or may be 400 m or less, 300 m or less, 200 m or less, 150
10 m or less, or 100 m or less. For example, even if the first surface layer part and the second
surface layer part have the same chemical compositions and same average Vickers hardnesses,
by providing a suitable difference to the thicknesses of the same, it becomes possible to
relatively easily control the hardness cumulative value of the first surface layer part side of the
steel sheet to become 1.05 times or more of the hardness cumulative value of the second surface
15 layer part side. Therefore, from the viewpoint of streamlining the process of production of the
steel sheet, the thicknesses of the first surface layer part and the second surface layer part
preferably differ. For example, in an embodiment where the average Vickers hardnesses of the
first surface layer part and second surface layer part are lower than the average Vickers hardness
of the sheet thickness 1/2 position, the thicker the first surface layer part and second surface layer
20 part, the more the strength of the steel sheet tends to fall, but hydrogen embrittlement cracking
generally becomes harder to occur. Therefore, the thicknesses of the first surface layer part and
the second surface layer part are preferably suitably selected in the ranges of more than 10 m to
30% or less of the sheet thickness considering the relationship with the sheet thickness center
part, the balance of the strength and hydrogen embrittlement resistance of the steel sheet, etc.
25 [0021]
Similarly, so long as satisfying the requirement that the first hardness cumulative value be
1.05 times or more of the second hardness cumulative value, the average Vickers hardnesses of
the first surface layer part and the second surface layer part may respectively be any average
Vickers hardnesses different from the average Vickers hardness of the sheet thickness 1/2
30 position, i.e., may be lower than or may be higher than the average Vickers hardness of the sheet
thickness 1/2 position. From the viewpoint of improving the bendability of the steel sheet, the
average Vickers hardnesses of the first surface layer part and the second surface layer part are
preferably lower than the average Vickers hardness of the sheet thickness 1/2 position. On the
other hand, from the viewpoint of improving the wear resistance and fatigue characteristics of
35 the steel sheet, the average Vickers hardnesses of the first surface layer part and the second
surface layer part are preferably higher than the average Vickers hardness of the sheet thickness
10
1/2 position. Furthermore, the average Vickers hardness of the first surface layer part may be
higher than the average Vickers hardness of the sheet thickness 1/2 position, and the average
Vickers hardness of the second surface layer part may be lower than the average Vickers
hardness of the sheet thickness 1/2 position. For example, the average Vickers hardnesses of the
5 first surface layer part and the second surface layer part may respectively independently be 0.90
time or less, 0.80 time or less, or 0.60 time or less of the average Vickers hardness of the sheet
thickness 1/2 position and/or may be 1.10 times or more, 1.20 times or more, or 1.40 times or
more of the average Vickers hardness of the sheet thickness 1/2 position. Further, so long as
satisfying the requirement of the first hardness cumulative value being 1.05 times or more of the
10 second hardness cumulative value and being different from the average Vickers hardness of the
sheet thickness 1/2 position, the average Vickers hardnesses of the first surface layer part and the
second surface layer part may be the same or may be different.

CLAIMS
[Claim 1]
A steel sheet comprising a sheet thickness center part and a first surface layer part and a
second surface layer part respectively arranged at two sides of the sheet thickness center part,
wherein
the first surface layer part and second surface layer part respectively independently have
thicknesses of more than 10 m to 30% or less of the sheet thickness,
the first surface layer part and second surface layer part have average Vickers hardnesses
different from the average Vickers hardness of a sheet thickness 1/2 position, and
a first hardness cumulative value at a region from a surface of the first surface layer part
side to 30% of the sheet thickness is 1.05 times or more of a second hardness cumulative value at
a region from a surface of the second surface layer part side to 30% of the sheet thickness.
[Claim 2]
The steel sheet according to claim 1, wherein a ratio P (Hh i g h /Hl o w ) of a hardness
cumulative value Hh i g h at a region from a surface of the first surface layer part side to X% of
the sheet thickness and a hardness cumulative value Hl o w at a region from a surface of the
second surface layer part side to X% of the sheet thickness satisfies the following formula 1:
P0.00035(X-30)2
+1.05 ･･･ formula 1
where, 0