[0001]The present invention relates to a steel sheet and a member.
Priority is claimed on Japanese Patent Application No. 2019-057608, filed
March 26, 2019, the content of which is incorporated herein by reference.
[Background Art]
10 [0002]
When a steel sheet is subjected to various types of processing to form a blank
having a predetermined shape, the steel sheet may be subjected to various treatments to
adjust properties and functions of the steel sheet.
[0003]
15 The following Patent Document 1 describes a technology for shearing a steel
sheet to cut out a blank having a predetermined shape.
[Citation List]
[Patent Document]
[0004]
20 [Patent Document 1]
PCT International Publication No. WO 2015/170707
[Summary of the Invention]
[Problems to be Solved by the Invention]
[0005]
25 However, the technology described in Patent Document 1 is based on the
2
premise that a single steel sheet is subjected to shearing. Therefore, there is a problem
that the influence caused when a steel sheet formed by welding a plurality of sheet
materials is sheared in a range including a welded part is not considered. In particular,
there is a problem that the influence on productivity in shearing of the welded steel sheet
5 is not considered.
[0006]
The present invention has been made in view of the above problems, and an
object of the present invention is to provide a novel and improved steel sheet and
member that can improve productivity including that of a shearing process for a welded
10 steel sheet.
[Means for Solving the Problem]
[0007]
(1) According to an aspect of the present invention, there is provided a steel sheet formed
by causing end surfaces of a first sheet material and a second sheet material to abut each
15 other in an in-plane direction and welding the first sheet material and the second sheet
material via a strip-shaped welded part, in which a softened part that is softened more
than other parts in the welded part is formed in at least a part of the welded part, in
which, on a first end surface of the steel sheet in which an end part of the welded part in a
longitudinal direction is formed, a region in which the softened part is not formed is
20 provided in at least a part of the end part of the welded part in the longitudinal direction,
and in which a maximum value of a depth of the softened part in a sheet thickness
direction, as a ratio to a sheet thickness of the steel sheet, is 50% or less.
[0008]
(2) In the steel sheet according to (1), at least one of the first sheet material and the
25 second sheet material may be a steel material having a tensile strength of 1,180 MPa or
3
more.
[0009]
(3) In the steel sheet according to (1), at least one of the first sheet material and the
second sheet material may be a steel material having a tensile strength of 1,310 MPa or
5 more.
[0010]
(4) In the steel sheet according to any one of (1) to (3), a Vickers hardness of the softened
part may be 90% or less with respect to a Vickers hardness of other parts in the welded
part.
10 [0011]
(5) In the steel sheet according to any one of (1) to (4), the maximum value of the depth
of the softened part in the sheet thickness direction, as a ratio to a sheet thickness of the
steel sheet, may be 30% or less.
[0012]
15 (6) In the steel sheet according to any one of (1) to (5), the maximum value of the depth
of the softened part in the sheet thickness direction, as a ratio to a sheet thickness of the
steel sheet, may be 10% or less.
[0013]
(7) In the steel sheet according to any one of (1) to (6), the softened parts may be
20 provided on the side of a first surface of the steel sheet and on the side of a second
surface opposite to the first surface.
[0014]
(8) In the steel sheet according to any one of (1) to (7), the softened parts may be
provided such that they are side by side with each other, and a first position at which the
25 depth of the softened part provided on the side of the first surface of the steel sheet in the
4
sheet thickness direction is a maximum and a second position at which the depth of the
softened part provided on the side of the second surface of the steel sheet in the sheet
thickness direction is a maximum may be different from each other in a direction
orthogonal to the sheet thickness direction of the softened part on a sheet surface of the
5 steel sheet.
[0015]
(9) According to another aspect of the present invention, there is provided a member,
including: a first part; a second part; and a welded part in which the first part and the
second part are caused to abut in an in-plane direction and welded, in which a softened
10 part that is softened more than other parts in the welded part is formed in at least a part of
the welded part, in which a region in which the softened part is not formed is provided in
at least a part of an end part of the welded part of the member in a longitudinal direction,
and in which, on a second end surface in which an end part of the welded part in the
longitudinal direction is formed, an average value of Vickers hardnesses at a distance of
15 80 μm from the second end surface is a value that is higher than an average value of
Vickers hardnesses at a distance of 300 μm from the second end surface by at least 10%.
[Effects of the Invention]
[0016]
As described above, according to the present invention, there is provided a novel
20 and improved steel sheet that can improve productivity including that of a shearing
process for a welded steel sheet.
[Brief Description of Drawings]
[0017]
Fig. 1 is a perspective view showing a steel sheet according to one embodiment
25 of the present invention.
5
Fig. 2 is a partial cross-sectional view showing an example of a state of shearing
on a steel sheet according to the same embodiment.
Fig. 3 is a partial cross-sectional view showing an example of a part in which a
softened part according to the same embodiment is formed.
Fig. 4A is a plan view of a steel shee 5 t before shearing is performed on a welded
part according to the same embodiment.
Fig. 4B is a side view of a steel sheet before shearing is performed on a welded
part according to the same embodiment.
Fig. 5A is a plan view illustrating a shape of a welded part according to the same
10 embodiment after shearing.
Fig. 5B is a side view illustrating a shape of a welded part according to the same
embodiment after shearing.
Fig. 6 is a cross-sectional view for illustrating a cross-sectional shape of a
welded part according to the same embodiment after shearing.
15 Fig. 7 is a diagram illustrating an example of a method of producing a steel sheet
according to the same embodiment.
Fig. 8A is a plan view showing another example of the steel sheet according to
the same embodiment.
Fig. 8B is a side view showing another example of the steel sheet according to
20 the same embodiment.
Fig. 9A is a diagram for illustrating a test piece as a comparative example.
Fig. 9B is a diagram for illustrating a test piece as a comparative example.
Fig. 9C is a diagram for illustrating a test piece as an example.
Fig. 10 is a graph showing the number of shots until defects occurs in a shearing
25 tool due to shearing.
6
Fig. 11 is a graph showing a scrap discharge success rate after shearing.
Fig. 12 is a graph showing the number of shots until tool damage occurred and
the formability with respect to a maximum value of a depth of a softened part in a sheet
thickness direction.
5 [Embodiment(s) of the Invention]
[0018]
Hereinafter, preferable embodiments of the present invention will be described
in detail with reference to the appended drawings. Here, in this specification and
drawings, components having substantially the same functional configuration are denoted
10 with the same reference numerals and redundant descriptions thereof are omitted.
[0019]
<>
[Schematic configuration of steel sheet]
First, with reference to Fig. 1 and Fig. 2, a schematic configuration of a steel
15 sheet according to one embodiment of the present invention is described. Fig. 1 is a
perspective view showing an example of a steel sheet 100 according to the present
embodiment. Fig. 2 is a partial cross-sectional view showing an example of a state of
shearing on the steel sheet 100 according to the present embodiment. As shown in Fig.
1, the steel sheet 100 is a flat member. When the steel sheet 100 is sheared, a blank
20 (corresponding to a blank 100’ illustrated in Fig. 7), which is a sheet material having a
predetermined shape, is cut out from the steel sheet 100. The blank is additionally
subjected to various types of processing and treatments and formed into a formed product
having a predetermined shape.
[0020]
25 As shown in Fig. 1, the steel sheet 100 is formed by causing end surfaces of a
7
first sheet material 111 and a second sheet material 113 to abut each other in the in-plane
direction and welding the first sheet material 111 and the second sheet material 113 via a
strip-shaped welded part 115. The steel sheet 100 is an example of a so-called tailored
blank (TB) or tailored welded blank (TWB).
5 [0021]
The welded part 115 is a strip-shaped region formed by melting and solidifying
the first sheet material 111 and the second sheet material 113 with each other. The end
part of the strip-shaped welded part 115 in the longitudinal direction is formed on the end
surface (corresponding to a first end surface 117 to be described below in Fig. 4A and
10 Fig. 4B) of the steel sheet 100. That is, the welded part 115 is formed over the steel
sheet 100 in the width direction (X direction in the Fig. 1). The welded part 115 may be
formed by a known welding technique such as laser welding and arc welding, and the
forming method is not particularly limited.
[0022]
15 At least one of the first sheet material 111 and the second sheet material 113 may
be a steel material having a tensile strength of 1,180 MPa or more (or a steel material
having a tensile strength of 1,180 MPa grade or more). In addition, at least one of the
first sheet material 111 and the second sheet material 113 may be a steel material having
a tensile strength of 1,310 MPa or more (or a steel material having a tensile strength of
20 1,310 MPa grade or more). In addition, the sheet thickness t of the first sheet material
111 or the second sheet material 113 may be, for example, 2 mm or less. In particular,
the sheet thickness t of the first sheet material 111 or the second sheet material 113 may
be, for example, about 1.6 mm.
[0023]
25 In shearing on the steel sheet 100 according to the present embodiment, as
8
shown in Fig. 1, along a predetermined scheduled cutting line α (refer to the dotted line
in Fig. 1), the shearing tool comes in contact with the steel sheet 100. In this case, an
end part of the shearing tool enters the steel sheet 100, and thus the load during shearing
is likely to be concentrated on the end part. In particular, in the welded part 115, due to
melting and solidification during 5 welding, a structure which is mainly composed of a
martensite structure and which is brittle and has relatively high strength may be formed.
As a result, in the steel sheet 100, when a region including the welded part 115 is
sheared, defects may occur in a part of the shearing tool. Specifically, during shearing
on the region including the welded part 115, cracks in the welded part 115 may progress,
10 and fracture may occur in the welded part 115 according to the rapid progress of cracks.
Therefore, the energy required for shearing is rapidly released from the broken part, and
the impact on the shearing tool during processing tends to increase. As a result, defects
may occur in the shearing tool.
[0024]
15 Here, the inventors conducted extensive studies, and as a result, found that in a
case that at least a part of the welded part 115 of the steel sheet 100 is softened, the
influence on the shearing tool during shearing is reduced. Hereinafter, the steel sheet
100 according to the present embodiment will be described in detail.
[0025]
20 In the steel sheet 100 according to the present embodiment, at least a part of the
welded part 115 has a softened part 120. The softened part 120 is a region that is
partially formed in the welded part 115 of the steel sheet 100 and more softened than
other parts of the welded part 115.
[0026]
25 [Softened part]
9
The softened part 120 is formed in the welded part 115 so that it includes at least
a part of the position of the surface of the steel sheet 100 with which the end part of the
shearing tool comes in contact in the sheet thickness direction during shearing. For
example, as shown in Fig. 2, the steel sheet 100 is sheared by a pair of upper and lower
shearing tools (a punch A and a 5 die B). In this case, as shown in Fig. 2, the softened
part 120 is formed so that it includes the position of the surface of the steel sheet 100
with which ends (edges) of the punch A and the die B come in contact. The distance
(clearance) between the punch A and the die B may be, for example, 10% or less of the
sheet thickness of the steel sheet 100.
10 [0027]
In addition, as shown in Fig. 1, the softened part 120 is formed so that it
includes at least a part of the welded part 115. In particular, the softened part 120 is
formed so that it includes a part in which the welded part 115 and the scheduled cutting
line α intersect.
15 [0028]
The Vickers hardness of the softened part 120 is 90% or less of the Vickers
hardness of the other region (a region excluding the softened part 120) of the welded part
115. When the Vickers hardness of the softened part 120 is 90% or less of the Vickers
hardness of the other part, it is possible to reduce defects of the shearing tool.
20 [0029]
As an example of the hardness of the welded part 115, a Vickers hardness of
about 463 Hv may be exemplified. On the other hand, as an example of the hardness of
the softened part 120, a Vickers hardness of about 180 Hv may be exemplified.
[0030]
25 Hardness measurement conditions are as follows. In the welded part 115 of the
10
steel sheet 100, a sample including the softened part 120 is collected, and a sample of the
measurement surface is prepared, and subjected to a Vickers hardness test. The
measurement surface is a surface parallel to the steel sheet 100 in the sheet thickness
direction. A method of preparing a measurement surface is performed according to JIS
Z 2244:2009. After the measurement sur 5 face is polished using #600 to #1500 silicon
carbide paper, mirror finishing is performed using a liquid in which diamond powder
particles having a particle size of 1 μm to 6 μm are dispersed in a diluting solution such
as an alcohol or pure water. The Vickers hardness test is performed according to the
method described in JIS Z 2244:2009. For the sample for which the measurement
10 surface is prepared, the Vickers hardness is measured using a micro Vickers hardness
tester at a test load of 0.98 N and a pitch of 0.05 mm.
[0031]
Examples of methods of forming the softened part 120 include a method of
performing softening by partially tempering using a known partial heating technique such
15 as laser heating and high frequency heating. In addition, as another example, softening
may be performed by partial tempering using a hot forming technique such as partial hot
forming. The method of forming the softened part 120 may be a method other than
tempering by heating as long as the hardness can be partially reduced. For example, a
method of forming the softened part 120 may be a method such as partial
20 decarburization.
[0032]
The softened part 120 may be partially formed in the welded part 115, and the
shape and size of the steel sheet 100 in a plan view are not particularly limited. For
example, in a plan view, it may have a circular shape, an oval shape, a polygonal shape, a
25 rounded square shape or the like. In addition, in a plan view of the steel sheet 100, the
11
width (length in the short direction) of the softened part 120 may be 40 mm or less.
[0033]
Subsequently, with reference to Fig. 1 to Fig. 3, the softened part 120 will be
described. Fig. 3 is a partial cross-sectional view showing an example of a part in
which the softened part 120 is 5 formed. As shown in Fig. 3, the softened part 120 may
be formed so that it has a predetermined depth D in the steel sheet 100 in the sheet
thickness direction. The depth D of the softened part 120 is determined as a distance
from the outermost surface of the steel sheet 100 in a region in which the hardness
measured under the above hardness measurement conditions is a predetermined value
10 with respect to the other part of the welded part 115.
[0034]
Specifically, the maximum value of the depth of the softened part 120 in the
sheet thickness direction may be set to be as a ratio to the sheet thickness t of the steel
sheet 100 that is 50% or less. In particular, the maximum value of the depth of the
15 softened part 120 in the sheet thickness direction may be set to be as a ratio to the sheet
thickness t of the steel sheet 100 that is 30% or less. In addition, in particular, the
maximum value of the depth of the softened part 120 in the sheet thickness direction may
be set to be as a ratio to the sheet thickness t of the steel sheet 100 that is 10% or less.
[0035]
20 When the softened part 120 has a predetermined depth, since the region of the
softened part 120 is secured in the sheet thickness direction of the steel sheet 100, it is
possible to reduce defects of the tool during shearing and it is possible to sufficiently
secure the formability and collision characteristics of the member after shearing.
[0036]
25 Here, the softened part 120 may be formed over the sheet thickness direction.
12
Thereby, the softened part 120 is sufficiently secured and thus defects of the tool during
shearing are reduced.
[0037]
In addition, as shown in Fig. 2, the softened parts 120 may be provided on the
side of a first surface 101 of 5 the steel sheet 100 and on the side of a second surface 103
provided on the side opposite to the first surface respectively. Since the shearing tool
comes in contact with both the first surface 101 and the second surface 103 of the steel
sheet 100, the softened parts 120 are provided on both sides and thus defects of the tool
during shearing are further reduced.
10 [0038]
In addition, as shown in Fig. 3, in a cross-sectional view of the softened part 120
in the width direction, the softened part 120 has a position at which the depth D in the
steel sheet 100 in the sheet thickness direction is a maximum value. In this case, as
shown in Fig. 2, in the softened part 120 provided on the side of the first surface 101 of
15 the steel sheet 100, the position at which the depth D in the sheet thickness direction is a
maximum value is defined as a first position Y1. In addition, in the softened part 120
provided on the side of the second surface 103 of the steel sheet 100, the position at
which the depth D in the sheet thickness direction is a maximum value is defined as a
second position Y2. In addition, the softened part 120 on the side of the first surface
20 101 and the softened part 120 on the side of the second surface 103 are provided such
that they are side by side with each other. In this case, as shown in Fig. 2, the first
position Y1 and the second position Y2 are different positions in the longitudinal direction
of the welded part 115. That is, the first position Y1 and the second position Y2 are
provided at positions different from each other in the direction orthogonal to the sheet
25 thickness direction of the steel sheet 100.
13
[0039]
The first position Y1 and the second position Y2 of the softened parts 120 on the
side of the first surface 101 and on the side of the second surface 103 are positions
different from each other in the longitudinal direction of the welded part 115, and thus
shearing is smoothly performed. As a result, de 5 fects of the shearing tool are further
reduced.
[0040]
[Relationship between softened part and sheared part]
Next, with reference to Fig. 4A and Fig. 4B, the state of shearing in the welded
10 part 115 in which the softened part 120 is formed will be described. Fig. 4A is a plan
view of the steel sheet 100 before shearing is performed on the welded part 115
according to the present embodiment. In addition, Fig. 4B is a side view of the steel
sheet 100 before shearing is performed on the welded part 115 according to the present
embodiment. As shown in Fig. 4A, the softened part 120 is formed in a part of the
15 welded part 115 of the steel sheet 100. In addition, the softened part 120 is formed so
that it includes a surface of a part in which the scheduled cutting line α for shearing the
steel sheet 100 and the welded part 115 intersect.
[0041]
In addition, as shown in Fig. 4B, the end part of the welded part 115 in the
20 longitudinal direction (X direction) is formed on the first end surface 117 of the steel
sheet 100. In a side view of the first end surface 117, on the first end surface 117, in a
part in which the end part of the welded part 115 in the longitudinal direction is formed,
the softened part 120 is not formed. Although details will be described below, on the
first end surface 117 in which the end part of the welded part 115 of the steel sheet 100 is
25 formed, there is a region in which the softened part 120 is not formed, and thus the
14
productivity of shearing on the steel sheet 100 is further improved.
[0042]
Next, with reference to Fig. 5A, Fig. 5B, and Fig. 6, a second end surface 119
after shearing on the steel sheet 100 will be described. Fig. 5A is a plan view
illustrating the shape of the welded part 1 5 15 according to the present embodiment after
shearing. Fig. 5B is a side view illustrating the shape of the welded part 115 according
to the present embodiment after shearing. Fig. 6 is a cross-sectional view for
illustrating a cross-sectional shape of the welded part 115 according to the present
embodiment after shearing. As shown in Fig. 5A, the steel sheet 100 is sheared along
10 the scheduled cutting line α and the blank 100’ is formed. As shown in Fig. 5B, the
softened part 120 is formed so that it includes the position of the surface of the welded
part 115. Thereby, during shearing, the shearing tool comes in contact with the softened
part 120, and thus concentration of the load applied to the shearing tool is restrained. In
addition, since the softened part 120 is formed in the welded part 115, the progress of
15 cracks during shearing is inhibited, and rapid release of energy is inhibited. As a result,
defects of the tool due to the impact on the shearing tool are reduced, the burden of
maintenance and inspection operations such as tool replacement is reduced, and the
productivity in the shearing process is improved.
[0043]
20 Through the shearing as described above, the steel sheet 100 is formed as the
blank 100’, and then the blank 100’ is subjected to various types of processing, and
formed into a member 200 having a predetermined shape. As shown in Fig. 6, the
blank 100’, or the member 200 formed from the steel sheet 100 according to the present
embodiment has a predetermined hardness change in the vicinity of the second end
25 surface 119 (cut surface after shearing) in which the end part of the welded part 115 in
15
the longitudinal direction is formed. That is, the steel sheet 100 has the softened part
120 so that it includes the welded part 115. Therefore, in the shearing on the welded
part 115, shearing is smoothly performed, work hardening effectively occurs at the
second end surface 119, and the hardness in the vicinity of the second end surface 119 is
5 higher than that of the inside of the blank 100’ or the member 200.
[0044]
Specifically, as shown in Fig. 6, in the cross section of the steel sheet 100
formed in the blank 100’ or the member 200 viewed in the width direction (Y direction in
Fig. 6), the hardness in the vicinity of the second end surface 119 along the
10 predetermined line L1 is measured. The start point of the line L1 is a position at a
distance W1=80 μm from the side of the first surface 101’ of the blank 100’ (or a first
surface 201 of the member 200) in the sheet thickness direction and a distance W2=80
μm from the second end surface 119. The line passing a position of 80 μm from the
second end surface 119 along the second end surface 119 from the start point is defined
15 as L1. The end point of the line L1 is a position at a distance of 80 μm from the side of
the second surface 103’ of the blank 100’ (or a second surface 203 of the member 200) in
the sheet thickness direction and a distance W2=80 μm from the second end surface 119.
[0045]
In addition, in the cross section of the steel sheet 100 viewed in the width
20 direction, the hardness is measured along the line L2 inside the blank 100’ or the member
200. The start point of the line L2 is a position at a distance W1=80 μm from the side
of the first surface 101’ of the blank 100’ (or the first surface 201 of the member 200) in
the sheet thickness direction and a distance W3=300 μm from the second end surface
119. The line passing a position of 300 μm from the second end surface 119 along the
25 second end surface 119 from the start point is defined as L2. The end point of the line
16
L2 is a position at a distance of 80 μm from the side of the second surface 103’ of the
blank 100’ (or the second surface 203 of the member 200) in the sheet thickness direction
and a distance W3=300 μm from the second end surface 119.
[0046]
In this case, conditions for measuring the hardness 5 along the line L1 and the line
L2 are as follows. On the cut surface of the welded part 115 of the steel sheet 100
exemplified in Fig. 6, a sample including the softened part 120 is collected, and a sample
of the measurement surface is prepared, and subjected to a Vickers hardness test. A
method of preparing a measurement surface is performed according to JIS Z 2244:2009.
10 After the measurement surface is polished using #600 to #1500 silicon carbide paper,
mirror finishing is performed using a liquid in which diamond powder particles having a
particle size of 1 μm to 6 μm are dispersed in a diluting solution such as an alcohol or
pure water. The Vickers hardness test is performed according to the method described
in JIS Z 2244:2009. For the sample for which the measurement surface is prepared, the
15 Vickers hardness is measured using a micro Vickers hardness tester at a test load of 0.98
N and a pitch of 0.1 mm.
[0047]
The average value of the hardness measurement results along the line L1 in the
vicinity of the second end surface 119 is higher than the average value of the hardness
20 measurement results along the line L2 on the inside of the blank 100’ or the member 200.
Specifically, as an example, the average value of the hardness measurement results along
the line L1 in the vicinity of the second end surface 119 is a Vickers hardness of about
217 Hv. In addition, as an example, the average value of the hardness measurement
results along the line L2 on the inside of the blank 100’ or the member 200 is a Vickers
25 hardness of about 181 Hv. That is, the Vickers hardness in the vicinity of the second
17
end surface 119 (position at a distance W2 of 80 μm from the second end surface 119) is
a value that is higher than the Vickers hardness on the inside of the blank 100’ or the
member 200 (position at a distance W3 of 300 μm from the second end surface 119) by at
least 10%.
5 [0048]
In this manner, the hardness in the vicinity of the second end surface 119 is set
higher than the hardness on the inside of the blank 100’ or the member 200. Thereby,
after the steel sheet 100 is formed as the blank 100’ or the member 200, the strength of
the second end surface 119 can be sufficiently maintained, and deformation and fracture
10 starting from the vicinity of the second end surface 119 can be suppressed. As a result,
deformation at the second end surface 119 in the production process using the steel sheet
100 and the like are suppressed, the yield is improved, and the productivity including that
of the shearing process is improved.
[0049]
15 Specifically, an example in which the steel sheet 100 is formed into a member
and a member having a hat shape in a cross-sectional view is obtained is exemplified.
In addition, an example in which the member is applied as a framework member A pillar
or B pillar is exemplified. In addition, the member can be applied as a framework
member formed of the steel sheet 100 having the welded part 115.
20 [0050]
[Method of producing steel sheet]
Next, with reference to Fig. 7, an example of a method of producing the steel
sheet 100 according to the present embodiment will be described. Fig. 7 is a diagram
illustrating an example of a method of producing the steel sheet 100 according to the
25 present embodiment. As shown in Fig. 7, first, a high-tensile steel sheet is prepared as
18
the first sheet material 111 and a high-tensile steel sheet is prepared as the second sheet
material 113. Subsequently, as a result of welding the first sheet material 111 and the
second sheet material 113 by laser welding or the like, the first sheet material 111 and the
second sheet material 113 are welded via the welded part 115, and the steel sheet 100 is
formed. Subsequently, as a result of softening 5 the steel sheet 100 by laser heating or the
like, the softened part 120 is formed in at least a part of the welded part 115. In
particular, the softened part 120 is formed so that it includes a part in which the welded
part 115 and the scheduled cutting line α intersect. Then, the steel sheet 100 is sheared.
That is, a first shearing tool A and a second shearing tool B come in contact with the
10 softened part 120, the part is cut, and the blank 100’ having a predetermined shape is cut
out from the steel sheet 100. Then, as necessary, the blank 100’ is additionally
subjected to a cutting process or a forming process, and finally processed into a member
having a predetermined shape. The method of producing the steel sheet 100 according
to the present embodiment has been described above.
15 [0051]
According to the present embodiment, in the steel sheet 100 having the welded
part 115, the softened part 120 is formed in at least a part of the welded part 115, and in
the first end surface 117 of the steel sheet 100 in which the end part of the welded part
115 is formed, the softened part 120 is not formed at the end part of the welded part 115
20 in the longitudinal direction, and the maximum value of the depth of the steel sheet 100
in the sheet thickness direction of the softened part 120 is, as a ratio to the sheet thickness
of the steel sheet 100, 50% or less. Thereby, the influence of the welded part 115
having a relatively high strength and brittle structure on the shearing tool can be reduced,
defects of the shearing tool can be reduced, the burden of maintenance and inspection
25 operations such as shearing tool replacement can be reduced and the formability and
19
collision characteristics of the member after shearing can be sufficiently secured. In
addition, in the first end surface 117 of the steel sheet 100, since there is a region in
which the softened part 120 is not formed, the strength of the outer peripheral edge
surface of the steel sheet 100 is improved. Therefore, after shearing is performed, the
shape of the scrap is stable, and the scrap is effectively 5 discharged. As a result, the
productivity in the shearing process for the steel sheet 100 is improved.
[0052]
In addition, according to the present embodiment, in the blank 100’ or the
member 200 formed from the steel sheet 100 having a first part (for example, a part
10 obtained by shearing the first sheet material 111), a second part (for example, a part
obtained by shearing the second sheet material 113), and a welded part in which the first
part and the second part are caused to abut in the in-plane direction and welded, a
softened part that is softened more than other parts in the welded part is formed in at least
a part of the welded part, and there is a region in which the softened part is not formed in
15 at least a part of the end part of the welded part in the longitudinal direction, and
hardness in the vicinity of the second end surface 119 is a value that is higher than the
hardness on the inside of the blank 100’ or the member 200 by at least 10%. Thereby,
after the steel sheet 100 is formed as the blank 100’ or the member 200, the strength of
the second end surface 119 can be sufficiently maintained, and deformation and fracture
20 starting from the vicinity of the second end surface 119 can be suppressed. In addition,
according to the softened part 120 formed in the welded part 115 of the steel sheet 100,
the productivity including that of the shearing process is improved. The steel sheet 100
according to one embodiment of the present invention has been described above.
[0053]
25 (Modification example)
20
Subsequently, a modification example of the steel sheet 100 according to one
embodiment of the present invention will be described with reference to Fig. 8A and Fig.
8B. Fig. 8A is a plan view showing the steel sheet 100 according to the present
modification example. Fig. 8B is a side view of the steel sheet 100 according to the
present modification example. The present m 5 odification example is different from the
above embodiment in that, in the first end surface 117 in which the end part of the
welded part 115 of the steel sheet 100 is formed, there are a region in which the softened
part 120 is formed and a region in which the softened part 120 is not formed. In the
description of the present modification example, for content common to the description
10 of the above embodiment, descriptions thereof may be omitted.
[0054]
As shown in Fig. 8A, in the steel sheet 100 according to the present modification
example, the softened part 120 is formed in a part of the welded part 115. In addition,
the softened part 120 is formed so that it includes a surface of a part in which the
15 scheduled cutting line α for shearing the steel sheet 100 and the welded part 115
intersect. In addition, a part of the outer edge of the softened part 120 in a plan view
extends on the first end surface 117 of the steel sheet 100.
[0055]
In addition, as shown in Fig. 8B, the end part of the welded part 115 in the
20 longitudinal direction (X direction) is formed on the first end surface 117 of the steel
sheet 100. In a side view of the first end surface 117, on the first end surface 117, in at
least a part of the part in which the end part of the welded part 115 in the longitudinal
direction is formed, there is a region β in which the softened part 120 is not formed. As
shown in Fig. 8B, in a side view of the first end surface 117, on the side of the first
25 surface 101, the softened part 120 is formed. On the other hand, in a side view of the
21
first end surface 117, on the side of the second surface 103, there is a region β in which
the softened part 120 is not formed. That is, the outer edge of the softened part 120 in
the width direction (X direction) reaches the first end surface 117 only on the side of the
first surface 101. On the other hand, at the center part in the sheet thickness direction (Z
direction) and on the side of the second sur 5 face 103, the outer edge of the softened part
120 does not reach the first end surface 117. In this manner, in at least a part of the first
end surface 117 in which the end part of the welded part 115 is formed, there is a region
β in which the softened part 120 is not formed, and thus the productivity of shearing on
the steel sheet 100 is further improved.
10 [0056]
That is, when the steel sheet 100 is sheared, the blank 100’ is formed and parts
other than the blank 100’ are scrapes. In the scraps, the first end surface 117 of the steel
sheet 100 remains without change. In this case, as described above, if the softened part
120 is formed in the entire region in the sheet thickness direction in a part in which the
15 end part of the welded part 115 is formed within the outer peripheral edge surface of the
scrap, it may be difficult to discharge the scrap. That is, since there are many softened
parts on the end surface in which the end part of the welded part 115 of the scrap is
formed, the shape of the scrap is not stable during shearing or after processing, and a
discharge defect after shearing is likely to occur. As a result, the productivity of the
20 shearing process for the steel sheet 100 is lowered because, for example, a separate
manual discharging operation is required. Therefore, on the first end surface 117 of the
steel sheet 100, in at least a part of the part in which the end part of the welded part 115
in the longitudinal direction is formed, there is a region β in which the softened part 120
is not formed, and thus the scrap is effectively discharged, and improvement in the
25 productivity of the shearing process is realized. The modification example of the steel
22
sheet 100 according to one embodiment of the present invention has been described
above.
[Examples]
[0057]
5 (Experimental Example 1)
In order to evaluate the performance of the steel sheet 100 according to the
present embodiment, as an example, a shearing experiment and evaluation were
performed with the steel sheet 100 in which the softened part 120 was formed. With
reference to Figs. 9A to 9C, Fig. 10, and Fig. 11, the experiment and the evaluation result
10 will be described. Figs. 9A and 9B are diagrams for illustrating test pieces S1 and S2 as
comparative examples. Fig. 9C is a diagram for illustrating a test piece S3 as an
example. Fig. 10 is a graph showing the number of shots until defects occurred in the
shearing tool due to shearing. Fig. 11 is a graph showing a scrap discharge success rate
after shearing.
15 [0058]
As shown in Figs. 9A to 9C, the test pieces S1 to S3 in a plan view were sheet
materials having a rectangular shape. The test pieces S1 to S3 had a size with a width a
of 100 mm and a length b of 140 mm. In addition, the scheduled cutting line α was
parallel to the first end surface 117 and a distance c from the first end surface 117 was set
20 to 20 mm.
[0059]
The substance of the first sheet material 111 was a steel material having a tensile
strength of 780 MPa grade. In addition, the substance of the second sheet material 113
was a steel material having a tensile strength of 1,180 MPa grade. The first sheet
25 material 111 and the second sheet material 113 both had a sheet thickness of 1.6 mm.
23
The first sheet material 111 and the second sheet material 113 were caused to abut on the
end surfaces and welded by laser welding.
[0060]
In the test piece S2 as Comparative Example 2, the softened part 120 was
formed by laser heating 5 from the position at which the welded part 115 and the scheduled
cutting line α intersected to the first end surface 117. The softened part 120 in a plan
view had a rounded square shape extending in the extension direction of the welded part
115. The round part had a semicircular shape with a radius R of 5 mm. The softened
part 120 in a side view was formed over the entire region in the sheet thickness direction
10 on the end surface of the first end surface 117. The width d of the softened part 120 was
10 mm.
[0061]
In the test piece S3 as an example, the softened part 120 was formed by laser
heating at a position at which the welded part 115 and the scheduled cutting line α
15 intersected. The softened part 120 in a plan view had a circular shape with a diameter φ
of 10 mm.
[0062]
In shearing conditions, the punching clearance was set to 10% of the sheet
thickness of the test pieces S1 to S3. The test pieces S1 to S3 corresponded to
20 Comparative Example 1, Comparative Example 2 and an example to be described below.
[0063]
As shown in Fig. 10, in Comparative Example 1 having no softened part 120,
defects occurred in the shearing tool by shearing with the number of shots of about 3,000.
On the other hand, in Comparative Example 2 and the example having the softened part
25 120, defects occurred in the shearing tool with the number of shots of about 10,000.
24
[0064]
As shown in Fig. 11, in Comparative Example 1 in which the softened part 120
was not formed, the scrap discharge success rate after shearing was about 100%. On the
other hand, in Comparative Example 2 in which the softened part 120 was formed over
the entire region of the first end surface 5 117 in the sheet thickness direction, the scrap
discharge success rate was about 80%. In addition, in the example in which the
softened part 120 was formed and the softened part 120 was not formed in the entire
region of the first end surface 117 in the sheet thickness direction, the scrap discharge
success rate was about 100%.
10 [0065]
(Experimental Example 2)
In order to evaluate the performance of the steel sheet according to the present
embodiment, the maximum value of the depth of the softened part in the sheet thickness
direction with respect to the steel sheet in the sheet thickness was changed, and
15 evaluation was performed according to the shearing and the tensile test.
[0066]
The test piece had a size with a width a of 100 mm and a length b of 140 mm.
The substance of the first sheet material was a steel material having a tensile strength of
780 MPa grade. In addition, the substance of the second sheet material was a steel
20 material having a tensile strength of 1,180 MPa grade. The first sheet material and the
second sheet material both had a sheet thickness of 1.6 mm. The first sheet material and
the second sheet material were caused to abut on the end surfaces and welded by laser
welding.
[0067]
25 On each test piece, the softened part was formed by laser heating at a position at
25
which the welded part and the scheduled cutting line α intersected. Test pieces in which
the maximum value of the depth of the softened part in the sheet thickness direction with
respect to the sheet thickness of the steel sheet differed by 10% from 10% to 100% were
prepared. In addition, a test piece in which no softened part was formed was also
5 prepared.
[0068]
In shearing conditions, the punching clearance was set to 10% of the sheet
thickness of the test piece. In the same manner as in Experimental Example 1, in each
test piece, the number of shots until the tool was damaged was measured.
10 [0069]
In addition, each test piece was cut out into the shape of a JIS No. 5 test piece.
The welded part was positioned at the center of the JIS No. 5 test piece in the tensile
direction and the longitudinal direction and the tensile direction of the welded part were
orthogonal to each other. Then, the tensile test was performed according to the method
15 described in ISO 6892. According to the tensile test, the breaking elongation of each
test piece was measured, and the breaking elongation (breaking elongation after the heat
treatment) of each test piece in which the softened part was formed with respect to the
breaking elongation before the heat treatment (breaking elongation of the test piece in
which no softened part was formed) was calculated.
20 [0070]
Fig. 12 shows the number of shots until the tool was damaged and the
formability with respect to the maximum value of the depth of the softened part in the
sheet thickness direction. In the graph in Fig. 12, the horizontal axis represents the
maximum value (%) of the depth of the softened part in the sheet thickness direction and
25 the vertical axis represents the number of shots (times) until the tool was damaged and
26
the formability (breaking elongation after the heat treatment/breaking elongation before
the heat treatment). Square plots indicate the number of shots and round plots indicate
the formability.
[0071]
5 [Number of shots]
As shown in Fig. 12, in the test piece having no softened part, defects occurred
in the shearing tool by shearing with the number of shots of about 3,000. On the other
hand, in the test piece having a softened part of 10% or more, the number of shots until
defects occurred in the shearing tool exceeded about 10,000. That is, it can be
10 understood that defects of the tool were reduced by forming an appropriate softened part
in the welded part. Here, when the maximum value of the depth of the softened part in
the sheet thickness direction with respect to the steel sheet in the sheet thickness
exceeded 50%, no clear difference was observed in the number of shots.
[0072]
15 [Formability]
In the vicinity of the welded part, there was a HAZ softened region having a
lower hardness than a base material. Since such a HAZ softened region had a relatively
low hardness, strain due to deformation was likely to be concentrated, and the region was
likely to be a starting point of breakage. When the heat treatment is performed to form
20 a softened part in the welded part, the HAZ softened region was additionally softened.
That is, when a steel sheet having a large region of the softened part formed in the
welded part was deformed, breakage in the HAZ softened region was more likely to
occur.
[0073]
25 Based on the result of this experimental example, it can be understood that,
27
when the maximum value of the depth of the softened part in the sheet thickness
direction with respect to the steel sheet in the sheet thickness was 50% or less, the
formability (breaking elongation after the heat treatment/breaking elongation before the
heat treatment) based on the test piece in which no softened part was formed was 0.8 or
more. When the formability was within this range, it 5 was possible to sufficiently secure
the formability and collision characteristics of the member. However, it can be
understood that, when the maximum value of the depth of the softened part in the sheet
thickness direction with respect to the steel sheet in the sheet thickness exceeded 50%,
the formability deteriorated. This is thought to be caused by the fact that the above
10 HAZ softened region was additionally softened.
[0074]
Based on the results of this experimental example, it can be understood that, in
order to avoid breakage in the HAZ softened region, it was desirable to adjust heat
treatment conditions and set the maximum value of the depth of the softened part in the
15 sheet thickness direction to be a predetermined value or less.
[0075]
In this manner, according to Experimental Example 1, it was shown that, when
the softened part was provided, defects of the shearing tool were reduced. In addition, it
was found that, in a side view of the first end surface of the steel sheet when the softened
20 part was not formed over the entire region in the sheet thickness direction, the scrap
discharge success rate was improved. Therefore, according to Experimental Example 1,
it was found that, in the shearing process of the steel sheet according to the present
embodiment, since the scrap discharge success rate was improved while reducing defects
of the shearing tool, the productivity of the shearing process was further improved.
25 [0076]
28
In addition, according to Experimental Example 2, when the maximum value of
the depth of the softened part in the sheet thickness direction with respect to the steel
sheet in the sheet thickness was 50% or less, the number of shots until the tool was
damaged increased, but when the maximum value of the depth of the softened part in the
sheet thickness direction with respect to the steel s 5 heet in the sheet thickness exceeded
50%, no significant change was observed. In addition, in order to secure the formability
and collision characteristics of the member, it was important to inhibit softening of the
HAZ softened region. Based on the results of Experimental Example 2, it can be
understood that, in the steel sheet of the present invention, when the maximum value of
10 the depth of the softened part in the sheet thickness direction with respect to the steel
sheet in the sheet thickness was 50% or less, it was possible to sufficiently secure the
formability and collision characteristics of the member while reducing damage on the
tool.
[0077]
15 While preferable embodiments of the present invention have been described
above in detail with reference to the appended drawings, the present invention is not
limited to these examples. It can be clearly understood by any person with ordinary
knowledge in the field of technology to which the present invention belongs that various
alternations or modifications can be made within the scope of the technical idea
20 described in the scope of the claims, and these also naturally belong to the technical
scope of the present invention.
[0078]
For example, in the above embodiment, the welded part 115 had a straight strip
shape in a plan view of the steel sheet 100, but the present invention is not limited to
25 such an example. For example, in a plan view of the steel sheet 100, the welded part
29
115 may have a zigzag shape or a curved shape.
[Industrial Applicability]
[0079]
Since the present invention can provide a novel and improved steel sheet and
member that can improve productivity including that of 5 a shearing process for a welded
steel sheet, it is industrially useful.
[Brief Description of the Reference Symbols]
[0080]
100 Steel sheet
10 101 First surface
103 Second surface
111 First sheet material
113 Second sheet material
115 Welded part
15 117 First end surface
119 Second end surface
120 Softened part
200 Member

WE CLAIMS

1. A steel sheet formed by causing end surfaces of a first sheet material and a second
sheet material to abut each other in an in-plane direction and welding the first sheet
material and the second sheet material 5 via a strip-shaped welded part,
wherein a softened part that is softened more than other parts in the welded part
is formed in at least a part of the welded part,
wherein, on a first end surface of the steel sheet in which an end part of the
welded part in a longitudinal direction is formed, a region in which the softened part is
10 not formed is provided in at least a part of the end part of the welded part in the
longitudinal direction, and
wherein a maximum value of a depth of the softened part in a sheet thickness
direction is, as a ratio to a sheet thickness of the steel sheet, 50% or less.
15 2. The steel sheet according to claim 1,
wherein at least one of the first sheet material and the second sheet material is a
steel material having a tensile strength of 1,180 MPa or more.
3. The steel sheet according to claim 1,
20 wherein at least one of the first sheet material and the second sheet material is a
steel material having a tensile strength of 1,310 MPa or more.
4. The steel sheet according to any one of claims 1 to 3,
wherein a Vickers hardness of the softened part is 90% or less with respect to a
25 Vickers hardness of other parts in the welded part.
31
5. The steel sheet according to any one of claims 1 to 4,
wherein the maximum value of the depth of the softened part in the sheet
thickness direction is, as a ratio to a sheet thickness of the steel sheet, 30% or less.
5
6. The steel sheet according to any one of claims 1 to 5,
wherein the maximum value of the depth of the softened part in the sheet
thickness direction is, as a ratio to a sheet thickness of the steel sheet, 10% or less.
10 7. The steel sheet according to any one of claims 1 to 6,
wherein the softened parts are provided on a side of a first surface of the steel
sheet and on a side of a second surface opposite to the first surface.
8. The steel sheet according to claim 7,
15 wherein the softened parts are provided side by side with each other, and
wherein a first position at which the depth of the softened part provided on the
side of the first surface of the steel sheet in the sheet thickness direction is a maximum
and a second position at which the depth of the softened part provided on the side of the
second surface of the steel sheet in the sheet thickness direction is a maximum are
20 different from each other in a direction orthogonal to the sheet thickness direction of the
softened part on a sheet surface of the steel sheet.
9. A member, comprising:
a first part;
25 a second part; and
32
a welded part in which the first part and the second part are caused to abut in an
in-plane direction and welded,
wherein a softened part that is softened more than other parts in the welded part
is formed in at least a part of the welded part,
wherein a region in whic 5 h the softened part is not formed is provided in at least
a part of an end part of the welded part of the member in a longitudinal direction, and
wherein, on a second end surface in which an end part of the welded part in the
longitudinal direction is formed, an average value of Vickers hardnesses at a distance of
80 μm from the second end surface is a value that is higher than an average value of
10 Vickers hardnesses at a distance of 300 μm from the second end surface by at least 10%.