[0001] The present disclosure relates to a welded joint manufacturing method, a welded joint,
a tempering device, and a welding apparatus.
Background Art
[0002] For example, when high-strength sheet steels are spot welded, insufficient joint
strength may cause welds to fracture, such that members fail to perform as designed.
Examples of high-strength sheet steels include sheet steel with a comparatively high carbon
(C) content and tensile strength of 440 MPa or greater.
[0003] Various methods, such as post heat methods and two-stage energization methods are
being investigated as joining processes for modifying weld properties. For example,
Japanese Patent No. 5714537 (Patent Document 1) discloses technology for spot welding two
or more overlapped high-strength steel sheets, in which welding is followed by an appropriate
rest period before performing post heat for a short time. In Patent Document 1, the
properties of a nugget region and a heat affected zone are modified by a tempering effect and
an effect of slowing the rate of cooling (what is referred to as an auto-tempering effect),
thereby enabling the joint strength to be improved. Note that a nugget or nugget region is a
region where metal has melted.
SUMMARY OF INVENTION
Technical Problem
[0004] However, in post heat methods or in two-stage energization methods such as that
proposed in Patent Document 1, the range of conditions under which the desired effects can
be obtained are narrow, and are liable to be affected by various external factors encountered at
an actual manufacturing site. Application of such approaches at an actual manufacturing site
therefore remains difficult. Hereafter, the quality of maintaining stable behavior under
various external factors encountered at a manufacturing site is referred to as having high
robustness, whereas conversely, being unable to maintain stable behavior is referred to as
having low robustness.
[0005] Namely, many conventional post heat methods and two-stage energization methods
are often methods in which energization is performed at least twice during a single cycle from
when a sheet grouping is applied with pressure by a pair of electrodes until the electrodes are
2
retracted after welding, as a technique for tempering the weld (nugget). However, with such
methods, suitable ranges of conditions (temperature conditions and the like) in which the
desired effects can be obtained are narrow. Moreover, at actual manufacturing sites, various
external factors including debris, electrode wear, electrode core misalignment, and inter-steel
sheet gaps may be encountered.
[0006] Since the current density during post heat varies due to the influence of such external
factors, various issues exist, such as a tendency for uneven current density and difficulties in
controlling the current density. Out of the various conditions that enable the desired effects
to be obtained, the current passing through a weld being liable to stray from its suitable range
in particular reduces robustness, making application of post heat methods and two-stage
energization methods such as that in Patent Document 1 to actual manufacturing sites difficult.
As a specific example, productivity of manufactured components suffers. There is
accordingly a risk that the desired modified properties (tempering effect) in spot welded joints
may be difficult to obtain.
[0007] An object of the present disclosure is to provide a welded joint manufacturing
method, a welded joint, a tempering device, and a welding apparatus for a welded joint with
excellent robustness that is not liable to be affected by external factors during a post heat
process.
Solution to Problem
[0008] Specific aspects of the present disclosure are as follows.
A welded joint manufacturing method according to the present disclosure includes:
preparing a welded joint including a first steel sheet, a second steel sheet overlapped with the
first steel sheet, and a quenched nugget joining the first steel sheet and the second steel sheet
together; abutting a first electrode against the first steel sheet at a site A, which is a location at
an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first
steel sheet of the welded joint; abutting a second electrode against the second steel sheet at a
site B, which is a location at an outer side of the nugget in a sheet-plane direction in a plane
running parallel to the first steel sheet of the welded joint, and positioned on an opposite side
of the nugget from the site A; and passing a current through the welded joint between the first
electrode and the second electrode.
[0009] A tempering device according to the present disclosure includes a first electrode, and
a second electrode. Approach and retract directions of the first electrode and approach and
retract directions of the second electrode are mutually opposing directions to each other. An
inter-electrode distance between the first electrode and the second electrode is at least 6 mm
in a flat plane orthogonal to the approach and retract directions.
3
[0010] A welding apparatus according to the present disclosure includes the tempering
device according to the present disclosure, a robot arm to which the tempering device is
attached, a welding machine configured to form a nugget, and a position controller. The
position controller is configured to control the robot arm so as to move an intermediate point
between a tip of the first electrode and a tip of the second electrode to a location that has been
welded as the nugget by the welding machine, and to dispose the first electrode and the
second electrode at an outer side of the location that has been welded.
[0011] A welded joint according to the present disclosure includes a first steel sheet, a
second steel sheet overlapped with the first steel sheet, and a quenched nugget joining the first
steel sheet and the second steel sheet together. Tensile strength of the first steel sheet and the
second steel sheet is at least 1180 MPa. A contact mark from a first electrode is formed on
the first steel sheet at a site A, which is a location at an outer side of the nugget in a
sheet-plane direction in a plane running parallel to the first steel sheet of the welded joint. A
contact mark from a second electrode is formed on the second steel sheet at a site B, which is
a location at an outer side of the nugget in a sheet-plane direction in a plane running parallel
to the first steel sheet of the welded joint, and positioned on an opposite side of the nugget
from the site A. A softened structure having Vickers hardness lower than Vickers hardness
of the first steel sheet and Vickers hardness of the second steel sheet by at least 10 HV is
continuously present between the contact mark from the first electrode and the contact mark
from the second electrode.
Advantageous Effects of Invention
[0012] The present disclosure is able to provide a welded joint manufacturing method, a
welded joint, a tempering device, and a welding apparatus for a welded joint with excellent
robustness that is not liable to be affected by external factors during a post heat process.
BRIEF DESCRIPTION OF DRAWINGS
[0013] Fig. 1 is a cross-section illustrating the periphery of electrodes of a welding machine
employed to form a nugget as employed in a tempering process of a welded joint
manufacturing method according to an exemplary embodiment of the present disclosure.
Fig. 2 is a cross-section illustrating the periphery of electrodes of a tempering device
included in a spot welding apparatus as employed in a tempering process of a welded joint
manufacturing method according to an exemplary embodiment of the present disclosure.
Fig. 3 is a cross-section illustrating the periphery of electrodes of a tempering device
included in another example of a spot welding apparatus as employed in a tempering process
of a welded joint manufacturing method according to an exemplary embodiment of the
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present disclosure.
Fig. 4 is a plan view illustrating a welded joint according to an exemplary
embodiment of the present disclosure.
Fig. 5 is a cross-section illustrating measurement positions for hardness distribution
in a welded joint.
Fig. 6A is a diagram illustrating a hardness distribution in a welded joint subjected to
a conventional tempering process.
Fig. 6B is a diagram illustrating a hardness distribution in a welded joint subjected to
a conventional tempering process.
Fig. 6C is a diagram illustrating a hardness distribution in a welded joint subjected to
a conventional tempering process.
Fig. 6D is a diagram illustrating a hardness distribution in a welded joint subjected to
a tempering process of the present disclosure.
Fig. 6E is a diagram illustrating a hardness distribution in a welded joint subjected to
a tempering process of the present disclosure.
Fig. 7 is a graph to explain a relationship between heating duration and temperature
with respect to a welded joint during a tempering process of a welded joint manufacturing
method according to an exemplary embodiment of the present disclosure.
Fig. 8 is a perspective view to assist explanation of a welded joint obtained through a
welded joint manufacturing method according to an exemplary embodiment of the present
disclosure.
Fig. 9 is a cross-section illustrating the periphery of electrodes included in a spot
welding apparatus employed in a tempering process of a welded joint manufacturing method
according to a first modified example.
Fig. 10 is a cross-section illustrating the periphery of electrodes included in a spot
welding apparatus employed in a tempering process of a welded joint manufacturing method
according to a second modified example.
Fig. 11 is a cross-section of another welded joint of the present disclosure.
DESCRIPTION OF EMBODIMENTS
[0014] Detailed explanation follows regarding a favorable exemplary embodiment of a
welded joint manufacturing method of the present disclosure, with reference to the drawings.
A welded joint of the present disclosure is also referred to as a "spot welded joint". In the
drawings, portions that are the same or similar to one another are allocated the same or similar
reference numerals. Note that the relationships between thickness and plan view dimensions,
5
as well as thickness proportions and the like in the various devices and various members in
the drawings may differ from those in reality. Accordingly, specific thicknesses and
dimensions should be ascertained with reference to the forthcoming explanation. Moreover,
dimensional relationships and proportions may also differ between the respective drawings.
[0015] < Spot Welded joint Manufacturing Method >
A spot welded joint manufacturing method of the present disclosure is a
manufacturing method including a welding process of post-heating plural mutually
overlapped steel sheets to form a nugget, a cooling process of cooling at least the nugget, and
a tempering process of post-heating the plural steel sheets in an oblique direction relative to a
sheet thickness direction of the steel sheets in order to temper at least the nugget.
[0016] Note that in the present specification, an "oblique direction relative to a sheet
thickness direction" is also referred to simply as an "oblique direction".
[0017] In the manufacturing method of the present disclosure, during a post heat process,
this being a tempering process separate to the welding process to form the nugget, the nugget
is tempered by post-heating the plural steel sheets in an oblique direction. In other words,
when tempering the nugget, a pair of electrodes are disposed at an outer side of the nugget
and the length of an energization path is increased such that the pair of electrodes on either
side of the plural steel sheets do not overlap in a sheet thickness direction. As a result, a
broad region including not only the nugget but also a heat affected zone (HAZ) peripheral to
the nugget is gently heated, enabling tempering to occur. So doing enables a broad range of
suitable current conditions to be secured in which a tempering effect (namely, an effect of
improving toughness) can be obtained.
[0018] The manufacturing method of the present disclosure is thus capable of securing
excellent robustness and is not liable to be affected by external factors during the post heat
process.
[0019] Detailed explanation follows regarding each process of the manufacturing method of
the present disclosure, with reference to the drawings. In the present disclosure, a welding
apparatus is referred to as a "spot welding apparatus". As illustrated in Fig. 1, a spot
welding apparatus 1 serving as a welding apparatus includes a welding machine 101 used to
form the nugget in the welding process. Moreover, as illustrated in Fig. 2, the spot welding
apparatus 1 serving as a welding apparatus also includes a tempering device 102 used to
temper a formed nugget N during the tempering process. Note that the tempering device 102
may be a separate device independent of the spot welding apparatus 1. Explanation follows
regarding each of these processes.
[0020] [Welding Process]
6
The welding process of the present disclosure is a process of post-heating plural
overlapped steel sheets to form a nugget.
[0021] The welding process may employ a similar process to a process performed during
normal spot welding, as long as a nugget can be formed at mutually superimposed faces of the
plural overlapped steel sheets and in a region in the vicinity of these mutually superimposed
faces. An example of such a process is a process in which the plural overlapped steel sheets
are interposed between a pair of electrodes, and then applied with a pressurizing force while
being energized in a sheet thickness direction with a predetermined weld current for a
predetermined weld time, in order to melt the mutually superimposed faces of the plural steel
sheets and the region in the vicinity of these mutually superimposed faces and thereby form a
nugget.
[0022] Fig. 1 illustrates an example in which a sheet grouping configured by overlapping an
upper sheet 8 with a lower sheet 9 has been disposed inside the welding machine 101 in order
to form a nugget. The upper sheet 8 of the sheet grouping is contacted by an upper electrode
2A provided to an upper holder member 6A. The lower sheet 9 of the sheet grouping is
contacted by a lower electrode 3A provided to a lower holder member 7A. Note that in this
explanation, "upper" and "lower" are used to refer to the upper and lower sides as they appear
in the drawings, and are not used to indicate positional relationships in a vertical direction in
practice. Similarly, in this explanation, "left" and "right" are used to refer to the left and
right as they appear in the drawings, and are not used to indicate positional relationships in a
horizontal direction in practice. This applies throughout the explanation.
[0023] The upper electrode 2A and the lower electrode 3A each have a substantially circular
cylinder shape, and have substantially the same dimensions as each other. Tips of the upper
electrode 2A and the lower electrode 3A on the sheet grouping side have a reduced diameter,
and outer edges of the tips as viewed along the sheet thickness direction are substantially
circular in shape. In the welding process, the nugget is internally formed in the sheet
grouping between the upper electrode 2A and the lower electrode 3A.
[0024] Note that predetermined weld conditions corresponding to a desired nugget diameter
or the like may be employed as weld conditions during the welding process. Weld
conditions may include a weld current, a weld time, and a pressurizing force to be applied by
the electrodes.
[0025] A pair of electrodes such as those employed in normal spot welding may be
employed as the pair of electrodes employed in the welding process, as long as they are
capable of forming a nugget of a predetermined size at the mutually superimposed faces of the
plural steel sheets and in a region in the vicinity of these mutually superimposed faces.
7
[0026] [Cooling Process]
The cooling process of the present disclosure is a process of cooling at least the
nugget formed during the welding process described above.
[0027] The cooling process may employ a similar process to a cooling process performed
during normal spot welding, as long as it is capable of transforming the nugget formed during
the welding process into martensite. An example of such a process is a process in which the
pair of electrodes are not retracted after the welding process and the pair of electrodes
continue to hold the plural steel sheets in a non-energized state so as to allow heat from the
steel sheets to dissipate into the electrodes. Another example of such a process is a process
in which the pair of electrodes are retracted after the welding process so as to allow heat from
the steel sheets to dissipate in air while the plural steel sheets are being conveyed to the spot
welding apparatus employed in the tempering process.
[0028] Note that in the latter process, since the nugget cools in a state in which the pair of
electrodes have been retracted, any thinning of the nugget can be suppressed since the nugget
is not being applied with pressure from the electrodes. This is advantageous due to enabling
a high joint strength to be stably achieved. Moreover, as a result of the process to retract the
pair of electrodes, the plural steel sheets or welded joint cool while being conveyed after the
welding process. This is advantageous in terms of productivity since it enables concurrent
implementation of the welding process and the tempering process at separate weld locations.
[0029] Examples of cooling conditions employed during the cooling process include cooling
duration or holding duration, as well as cooling temperature. Such a cooling condition may
be that the nugget reaches a temperature of the MS point or below, this being the temperature
at which the nugget transforms into martensite, after the welding process. A cooling
condition such that the nugget reaches a temperature of the Mf point or below is preferable.
[0030] [Tempering Process]
The tempering process of the present disclosure is a process of tempering at least the
nugget by post-heating in a direction that is oblique to the sheet thickness direction of the
plural steel sheets (namely, an oblique direction) after cooling.
[0031] The tempering process is a tempering process in which the plural steel sheets are
energized in the oblique direction after cooling so as to temper martensite structures in at least
the nugget, and in particular martensite structures in the nugget and the heat affected zone.
Apart from post-heating in the oblique direction, the tempering process may be performed in a
similar manner to a post heat process or a tempering process employed in a normal post heat
method or two-stage energization method.
[0032] Note that whether or not the nugget has been tempered by such a post heat method or
8
two-stage energization method may be confirmed by measuring a hardness distribution of the
nugget. Tempering is confirmed when partial or full softening of the hardness is present
after the post heat method or two-stage energization method. Although not illustrated in the
drawings, during the tempering process, some or all of the upper electrode 2, the lower
electrode 3, the upper fixing member 4, or the lower fixing member 5 may be employed to
apply pressure to the sheet grouping. Such pressure application enables the upper electrode
2 and the lower electrode 3 to make more reliable contact with the sheet grouping.
[0033] Specific explanation follows regarding the tempering process, with reference to an
exemplary embodiment of the present disclosure.
[0034] In the manufacturing method according to an exemplary embodiment of the present
disclosure, the tempering process employs the tempering device 102 included in the spot
welding apparatus 1 illustrated in Fig. 2. Note that Fig. 2 illustrates only the surroundings of
the electrodes of the tempering device 102 of the spot welding apparatus 1. As illustrated in
Fig. 2, two steel sheets, these being the upper sheet 8 serving as a first steel sheet and the
lower sheet 9 serving as a second steel sheet, are overlapped as a sheet grouping, and the
overlapped sheet grouping can be interposed between the spot welding apparatus 1 in a sheet
thickness direction DT.
[0035] The tempering device 102 of the spot welding apparatus 1 includes the upper
electrode 2, the lower electrode 3, the upper fixing member 4, the lower fixing member 5, an
upper holder member 6, and a lower holder member 7 as principal configuration members.
The upper electrode 2 is disposed at an upper side of the sheet grouping, and the lower
electrode 3 is disposed at a lower side of the sheet grouping.
[0036] The upper electrode 2 corresponds to a first electrode of the present disclosure. The
lower electrode 3 corresponds to a second electrode of the present disclosure. The upper
fixing member 4 corresponds to a first fixing member of the present disclosure. The lower
fixing member 5 corresponds to a second fixing member of the present disclosure. Note that
Fig. 2 illustrates an example in which the lower fixing member 5 is disposed substantially
coaxially to the upper electrode 2 along an up-down direction so as to correspond to the upper
electrode 2. Moreover, Fig. 2 illustrates an example in which the lower fixing member 5 is
disposed substantially coaxially to the lower electrode 3 along the up-down direction so as to
correspond to the lower electrode 3. However, the present disclosure is not limited thereto,
and the first fixing member and the second fixing member do not need to be disposed
coaxially with the electrodes across the sheet grouping. The first fixing member and the
second fixing member may be disposed further toward the inside, namely further toward the
nugget N side in Fig. 2 than the corresponding electrodes, or may be disposed further toward
9
the outer side, namely further away from the nugget N in Fig. 2, than the corresponding
electrodes.
[0037] The upper fixing member 4 and the lower fixing member 5 are both fixing members.
The upper fixing member 4 is disposed at the upper side of the sheet grouping, and the lower
fixing member 5 is disposed at the lower side of the sheet grouping. The sheet grouping can
be interposed between the upper fixing member 4 and the lower fixing member 5. The upper
holder member 6 holds the upper electrode 2 and the upper fixing member 4, and is capable of
moving in the up-down direction. The lower holder member 7 holds the lower electrode 3
and the lower fixing member 5.
[0038] As illustrated in Fig. 2, the upper electrode 2 and the lower electrode 3 of the
tempering device 102 of the spot welding apparatus 1 are disposed at positions on mutually
opposing sides of the nugget N formed during the welding process in a sheet-plane direction
Dh that is orthogonal to the sheet thickness direction DT. This enables the tempering device
102 of the spot welding apparatus 1 to easily implement post heat of the two steel sheets in
the oblique direction after cooling.
[0039] Note that the upper electrode 2 and the lower electrode 3 illustrated in Fig. 2 are
respectively configured by circular rod shaped conductive members. For example, these
conductive members are each configured by a circular rod made of a Cu-Cr alloy. Each of
the conductive members extends in a direction orthogonal to both the sheet thickness direction
DT and the sheet-plane direction in which the two electrodes are arranged. Namely, each of
the conductive members extends in a direction orthogonal to a length direction of the two
steel sheets. The upper electrode 2 and the lower electrode 3 each make linear contact with a
surface of the corresponding steel sheet (namely the upper sheet 8 or the lower sheet 9).
[0040] Moreover, in the tempering device 102 of the spot welding apparatus 1, as illustrated
in Fig. 3, the upper fixing member 4 and the lower fixing member 5 are disposed positioned
on mutually opposing sides of the nugget N formed in the welding process in the sheet-plane
direction Dh. As a result, the upper fixing member 4 is positioned on the opposite side of the
nugget N from the upper electrode 2 in the sheet-plane direction Dh. The lower fixing
member 5 is positioned on the opposite side of the nugget N from the lower electrode 3 in the
sheet-plane direction Dh. The tempering device 102 of the spot welding apparatus 1 is thus
capable of fixing the two steel sheets even more reliably when post-heating the two steel
sheets in an oblique direction after cooling. This thereby enables the occurrence of external
factors such as positional misalignment of the steel sheet or space between the steel sheets and
the electrodes to be made more unlikely.
[0041] Note that although an inter-sheet separation (gap) between the upper sheet 8 and the
10
lower sheet 9 is formed at both ends in the left-right direction in Fig. 2 of the sheet grouping
in the example illustrated in Fig. 2, there is no limitation thereto. As illustrated in Fig. 3,
such an inter-sheet separation need not be formed in the present disclosure.
[0042] Moreover, in the present exemplary embodiment, as illustrated in Fig. 4 as viewed in
plan view, namely as viewed along the sheet thickness direction, each of the upper electrode 2
and the lower electrode 3 has a constant width W along the up-down direction in Fig. 4.
Each of the upper electrode 2 and the lower electrode 3 has substantially the same width W,
and an indentation (pressing mark) H on the nugget N has a diameter of at least ϕ as measured
in a sheet-plane direction. As illustrated by solid lines as an example in Fig. 4, the shape of
a contact region M1 of the upper electrode 2 on the left side of the upper sheet 8 is a
rectangular shape. As illustrated by dashed lines as an example in Fig. 4, the shape of a
contact region M2 of the lower electrode 3 on the right side of the upper sheet 8 is also a
rectangular shape. In the present exemplary embodiment, the contact region M1 of the
upper electrode 2 and the contact region M2 of the lower electrode 3 are formed
symmetrically to one another about the nugget N in plan view.
[0043] Note that an example of a state in which the upper electrode 2 with the constant
width W along the up-down direction is separated from the upper sheet 8 is illustrated on the
upper left side in Fig. 4. An example of a state in which the lower electrode 3 with the
constant width W along the up-down direction is separated from the lower sheet 9 is
illustrated on the lower right side in Fig. 4. Illustrated as an example. Configuration of the
tempering device 102 other than the upper electrode 2 and the lower electrode 3 is omitted
from illustration in the interests of simplicity.
[0044] As illustrated in Fig. 4, in the present exemplary embodiment, a circular outer edge H
of the indentation, this being a pressing mark formed by the upper electrode 2A illustrated in
Fig. 1, may be considered to correspond to the circular outer edge of the nugget N in plan
view. The circular outer edge H of the indentation from the upper electrode 2A can be seen
when viewing the exterior of a welded joint 10. Note that since the shapes of the tips of the
upper electrode 2A and the lower electrode 3A are substantially the same as each other, a
circular outer edge of an indentation from the lower electrode 3A may also be considered to
correspond to the circular outer edge of the nugget N in plan view.
[0045] In plan view, the "circular" shape of the outer edge of the indentation and the
"circular" shape of the nugget N are not limited to perfectly circular shapes, and may be
considered to be circular overall even when some local distortions are present, and treated as
if circular. Moreover, the shape of the outer edge of the indentation and the shape of the
nugget N in plan view are not limited to circular shapes, and for example elliptical shapes or
11
other geometric shapes may be adopted therefor as appropriate.
[0046] Note that the upper fixing member 4 and the lower fixing member 5 illustrated in Fig.
2 are configured by circular rod shaped electrically insulating members extending in the same
direction as the upper electrode 2 and the lower electrode 3. These electrically insulating
members may, for example, be circular ceramic rods. When a sheet grouping is interposed
between the upper fixing member 4 and the lower fixing member 5 and between the upper
electrode 2 and the lower electrode 3, the upper fixing member 4 and the lower fixing member
5 make linear contact with the surface of the corresponding steel sheet (namely the upper
sheet 8 or the lower sheet 9). Configuring the fixing members from such electrically
insulating members enables any effect on the oblique direction post heat performed by the
upper electrode and the lower electrode to be prevented.
[0047] In the present disclosure, the tempering device of the spot welding apparatus
employed in the tempering process is not limited to a configuration such as that of the
tempering device 102 of the spot welding apparatus 1 illustrated in Fig. 3, as long as it is
capable of post-heating plural steel sheets in the oblique direction after cooling. Namely, the
tempering device of the spot welding apparatus employed in the tempering process may
include three or more electrodes disposed so as to enable oblique direction post heat of the
plural steel sheets after cooling. Alternatively, the tempering device of the spot welding
apparatus may include a single or three or more fixing members disposed so as to be capable
of fixing the plural steel sheets during post heat.
[0048] Note that in cases in which the placement of the electrodes or the like is such that
there is little concern of movement or misalignment of the plural steel sheets during post heat,
fixing members such as those described above may be omitted from the spot welding
apparatus.
[0049] In the manufacturing method of the present exemplary embodiment, the tempering
process is specifically formed in the following manner using the tempering device 102 of the
spot welding apparatus 1 described above.
[0050] First, a welded joint 10 is prepared by the welding process and the cooling process
described above. As illustrated in Fig. 2, the welded joint 10 includes the upper sheet 8, the
lower sheet 9 overlapped with the upper sheet 8, and a quenched nugget N joining the upper
sheet 8 and the lower sheet 9 together. The upper electrode 2 is abutted against the upper
sheet 8 at a site A, this being a location at an outer side of the nugget N in every plane running
parallel to the upper sheet 8 of the welded joint 10. In order to achieve stable post heat, this
is preferably interposed between the upper electrode 2 provided on the upper sheet 8 side and
the upper fixing member 4 provided on the lower sheet 9 side. To describe the site A in
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other words, the site A runs through the upper sheet 8 and the lower sheet 9 in a direction
perpendicular to the upper sheet 8, and projections of the site A and the nugget N do not
overlap when the welded joint 10 is viewed along the direction perpendicular to the upper
sheet 8.
[0051] Moreover, the lower electrode 3 is abutted against the lower sheet 9 at a site B, this
being a location at an outer side of the nugget N in every plane running parallel to the upper
sheet 8 of the welded joint 10 and positioned on the opposite side of the nugget N from the
site A. In order to achieve stable post heat, this is preferably interposed between the lower
electrode 3 provided on the lower sheet 9 side and the lower fixing member 5 provided on the
upper sheet 8 side. To describe the site B in other words, the site B runs through the lower
sheet 9 and the upper sheet 8 in a direction perpendicular to the upper sheet 8, and projections
of the site B and the nugget N do not overlap when the welded joint 10 is viewed along the
direction perpendicular to the upper sheet 8. The site A and the site B are positioned with
axial symmetry to each other about a central axis passing through a center E of the nugget N
in a direction perpendicular to the upper sheet 8.
[0052] Namely, after the cooling process, the two steel sheets are energized in an oblique
direction by abutting the upper electrode 2 and abutting the lower electrode 3 of the tempering
device 102 of the spot welding apparatus 1 illustrated in Fig. 2 against the upper sheet 8 at the
site A of the welded joint 10 and against the lower sheet 9 at the site B of the welded joint 10
respectively, and passing a current through the welded joint 10 between the upper electrode 2
and the lower electrode 3. Preferably, the welded joint 10 is interposed in the sheet thickness
direction DT between the upper electrode 2 and upper fixing member 4, and the lower
electrode 3 and lower fixing member 5, and the current is passed through the welded joint 10
between the upper electrode 2 and the lower electrode 3 while applying a pressurizing force
using the upper electrode 2 and the lower electrode 3 when post-heating the two steel sheets
in the oblique direction.
[0053] Moreover, when the current is passed through the welded joint 10, as illustrated in
Fig. 4, the upper electrode 2 has the constant width W in a plane running parallel to the upper
sheet 8, and the width W of the upper electrode 2 is at least the maximum diameter ϕ of the
nugget N in a plane running parallel to the upper sheet 8. Similarly, the lower electrode 3
has the constant width W in a plane running parallel to the lower sheet 9, and the width W of
the lower electrode 3 is at least the maximum diameter ϕ of the nugget N in a plane running
parallel to the lower sheet 9.
[0054] When this is performed, as illustrated in Fig. 2, a energization path CP through the
two steel sheets runs across a broad region including the nugget N and the periphery of the
13
nugget N. Since the energization path CP runs across such a broad region, the current
density is reduced. Thus, this broad region including not only the nugget N but also the heat
affected zone peripheral to the nugget N is gently heated, enabling tempering to occur. This
enables a broad range of suitable current conditions to be secured in which a tempering effect
can be obtained.
[0055] Figs. 6 are diagrams illustrating hardness (Vickers hardness; HV) distribution in
welded joints that have been subjected to a conventional tempering process and welded joints
that have been subjected to the tempering process of the present disclosure.
[0056] Note that of Figs. 6, the distribution diagram Fig. 6A illustrates a hardness
distribution for a welded joint that has been subjected only to a welding process and a cooling
process, and has not been subjected to a tempering process. The distribution diagram Fig.
6B illustrates a hardness distribution for a welded joint for which a cool time (cooling
duration) after a welding process is set to 99 cyc, and a tempering process is performed under
conventional tempering conditions (post heat current: 3.9 kA, post heat time: 99 cyc, post heat
direction: sheet thickness direction). Note that in the present exemplary embodiment, one
second corresponds to 60 cyc.
[0057] The distribution diagram Fig. 6C illustrates a hardness distribution for a welded joint
that has been subjected to a tempering process under the same conventional tempering
conditions as those of the welded joint of the distribution diagram Fig. 6B, with the exception
that the post heat current of the tempering process was set to 4.3 kA. The distribution
diagram Fig. 6D and the distribution diagram Fig. 6E illustrate hardness distributions for
welded joints that have been subjected to the tempering process of the present disclosure with
the post heat current set to 7.0 kA and 8.0 kA respectively. Note that in the distribution
diagram Fig. 6D and the distribution diagram Fig. 3E, the post heat time during the tempering
process was set to 99 cyc in both cases. In Figs. 6, the nugget region is indicated by the
bidirectional arrows labeled N at the left-right direction center of the respective diagrams.
[0058] As illustrated in Figs. 6, in the welded joint corresponding to the distribution diagram
Fig. 6A that was not subjected to a tempering process, the hardness of the nugget region and
the hardness of the heat affected zone peripheral to the nugget are both hard (namely, exhibit
low toughness). The heat affected zone peripheral to the nugget that has become hard is
referred to hereafter as the "HAZ hardened region". The HAZ hardened region in Fig. 6A is
susceptible to internal fracturing of the nugget (namely, interfacial fracture or partial plug
fracturing).
[0059] On the other hand, in the welded joint corresponding to the distribution diagram Fig.
6B that has been subjected to a tempering process under conventional tempering conditions,
14
hardness within the nugget is lowered (namely, toughness is improved) by the tempering.
However, the hardness in the vicinity of end portions of the nugget N and the hardness of the
HAZ hardened region are not sufficiently lowered. Accordingly, were there to be even slight
misalignment in the softening position, if stress in a separating direction were to act on the
welded joint, a nugget end, this being a location where cracking readily propagates, and a site
of high hardness may align, causing the nugget to be susceptible to internal fracturing. Note
that in the present exemplary embodiment, the nugget ends refer to the two ends of the nugget
N illustrated in Fig. 6B.
[0060] The welded joint corresponding to the distribution diagram Fig. 6C is subjected to
the tempering process under conventional tempering conditions, with the exception that the
current value employed has been raised somewhat. In the welded joint corresponding to the
distribution diagram Fig. 6C, the hardness in the vicinity of the nugget end portions and the
hardness of the HAZ hardened region are lowered by the tempering. However, a central
portion of the nugget reaches a temperature of the A3 point or higher and consequently reverts
back to martensite, thus increasing the hardness. Accordingly, were there to be even slight
misalignment in the softening position, the nugget end and a site of high hardness may align,
causing the nugget to be susceptible to internal fracturing.
[0061] In cases in which such spot welding is performed during mass production, the
possibility of a site of high hardness aligning with a nugget end increases due to wear of the
electrode tips, variations in thermal conductivity of the electrodes due to alloys forming
between the electrodes and plating of the steel sheets, and so on. Moreover, the possibility
of a site of high hardness aligning with a nugget end also increases due to variation in
inter-steel sheet gaps, variation in the strike angle between the electrodes and the steel sheets,
and so on. In particular, the width of inter-sheet gaps is liable to increase the higher the
strength of the sheet steel employed. Namely, robustness is reduced as a result of the current
passing through the weld being more liable to stray from a suitable range.
[0062] Accordingly, with tempering under conventional tempering conditions, since rapid
heating occurs from the center of the nugget, it is very difficult to achieve uniform softening
over a broad region spanning both the nugget and the heat affected zone.
[0063] By contrast, as described previously, the tempering process of the present disclosure
enables the broad region including not only the nugget but also the heat affected zone
peripheral to the nugget to be gently heated, thereby enabling the broad region spanning both
the nugget and the heat affected zone to be uniformly softened, as seen in the hardness
distributions illustrated by the distribution diagram Fig. 6D and the distribution diagram Fig.
6E. Note that in the distribution diagram Fig. 6D, portions of the heat affected zone in the
15
vicinity of the nugget where the hardness is lowered are softened HAZ regions that are
formed during the welding process.
[0064] Hardness measurement positions in the various welded joints illustrated in Figs. 6 are
set as follows.
[0065] First, as illustrated in Fig. 5, as referenced against the mutually superimposed faces
of the upper sheet 8 and the lower sheet 9, a point where a sheet-plane direction position PM at
a depth of 1/4 of the sheet thickness t of the upper sheet intersects with one side end portion of
the nugget N is set as a reference point PS. The hardness of the welded joint 10 is then
measured at a predetermined pitch over a sheet-plane direction range spanning 10 mm from
the reference point PS in a + direction and a sheet-plane direction range spanning 5 mm from
the reference point PS in a – direction. Note that the horizontal axis in the hardness
distribution diagrams of Figs. 6 indicates a distance dh (mm) in the sheet-plane direction from
the reference point PS.
[0066] The two steel sheets employed in the respective welded joints 10 were hot stamped
members with 1.8 GPa grade tensile strength in each case. The sheet thickness t of the hot
stamped members was 1.6 mm.
[0067] Note that in the present disclosure, with the exception of the post heat direction, the
post heat conditions employed in the tempering process (post heat current, post heat time,
electrode pressurizing force, and the like) may be set to any predetermined post heat
conditions according to the desired joint strength and the like, as long as such conditions are
capable of tempering at least the nugget. As an example of such post heat conditions, the
tempering temperature is preferably set to a temperature condition within a range from 500°C
to the Ac3 point, and is more preferably set to a temperature condition within a range from
600°C to the Ac1 point. Setting the tempering temperature within this range facilitates
lowering of the hardness (namely, facilitates improving toughness) due to ensuring sufficient
heat input, while not being susceptible to re-quenching which would cause re-hardening.
[0068] Moreover, the electrodes employed in the tempering process are not limited to
circular rod shaped conductive members such as the upper electrode 2 and the lower electrode
3 described above. Electrodes such as those employed in normal spot welding may be
employed as long as they are capable of post-heating the plural steel sheets in an oblique
direction and of tempering at least the nugget. An example of electrodes employed in
normal spot welding are DR-type electrodes that make point contact with steel sheets.
[0069] Note that adopting circular rod shaped conductive members such as the upper
electrode 2 and the lower electrode 3 described above as the electrodes employed in the
tempering process enables a broader region to be gently tempered, and thereby enables the
16
advantageous effects of the present disclosure to be obtained to an even greater degree.
[0070] Moreover, the upper electrode and the lower electrode employed in the tempering
process are preferably set such that an inter-electrode distance de between the two electrodes
in the sheet-plane direction is at least twice the diameter ϕ of the nugget. The site A and the
site B in Fig. 2 are respectively separated from the center E of the nugget N by at least the
diameter ϕ of the nugget N. The diameter ϕ of the nugget N is a length measured in a plane
running parallel to the upper sheet 8 (a length along the left-right direction in Fig. 2).
[0071] Note that, for example, in cases in which the shape of the nugget N is a true circle,
the diameter thereof may be adopted as the diameter ϕ of the nugget N. In cases in which
the shape of the nugget N is not a true circle, such as an elliptical shape as illustrated in Fig. 2
or a circular shape with some distortion, the major axis thereof may be adopted as the
maximum diameter ϕ of the nugget.
[0072] In cases in which the nugget N has an elliptical shape as illustrated in Fig. 2, the
inter-electrode distance de between the upper electrode 2 and the lower electrode 3 as
measured along the left-right direction is at least twice the diameter ϕ of the nugget N.
Setting the inter-electrode distance de between the upper electrode and the lower electrode to
at least twice the diameter ϕ of the nugget in this manner enables an even broader region
including the nugget and the heat affected zone to be gently tempered, and thereby enables the
advantageous effects of the present disclosure to be obtained even more reliably.
[0073] Note that as illustrated in Fig. 2, the inter-electrode distance de between the upper
electrode and the lower electrode in the sheet-plane direction refers to the sheet-plane
direction distance between center axes of the respective electrodes, extending in the up-down
direction. Approach and retract directions of the upper electrode 2 and approach and retract
directions of the lower electrode 3 are mutually opposing directions to each other, and the
inter-electrode distance de configuring the spacing between the upper electrode 2 and the
lower electrode 3 is at least 6 mm in a plane orthogonal to these respective approach and
retract directions.
[0074] Note that the approach direction of the upper electrode 2 with respect to the sheet
grouping is a direction heading from the upper side toward the lower side in Fig. 2, and the
retract direction of the upper electrode 2 is a direction heading from the lower side toward the
upper side in Fig. 2. The approach direction of the lower electrode 3 with respect to the
sheet grouping is a direction heading from the lower side toward the upper side in Fig. 2, and
the retract direction of the lower electrode 3 is a direction heading from the upper side toward
the lower side in Fig. 2. In the example in Fig. 2, a flat plane in a direction orthogonal to
these approach and retract directions is illustrated by a horizontal plane at the boundary
17
between the upper sheet 8 and the lower sheet 9.
[0075] Moreover, the upper electrode and the lower electrode are preferably set such that a
distance between the upper electrode and the nugget in the sheet-plane direction sheet-plane
direction is equal to a distance between the lower electrode and the nugget in the sheet-plane
direction. Namely, in Fig. 2, a distance between the site A and the center E of the nugget N
is equal to a distance between the site B and the center E of the nugget N. When the
distances between the upper electrode and the lower electrode and the nugget are in an equal
relationship in this manner, a broad region including the nugget can be more uniformly heated
in order to achieve tempering, thereby enabling the advantageous effects of the present
disclosure to be obtained even more reliably.
[0076] Note that the distances between the upper electrode and the lower electrode and the
nugget refer to the sheet-plane direction distances between central axes running through the
respective electrodes in the up-down direction and the nugget center.
[0077] Moreover, the fixing members employed in the tempering process are not limited to
circular rod shaped electrically insulating members such as the upper fixing member 4 and the
lower fixing member 5 described previously. For example, electrically insulating members
of any predetermined shape may be employed according to the desired manner of holding,
ease of holding, and so on, as long as they are capable of fixing the plural steel sheets so as to
not move or become misaligned when the plural steel sheets are energized in an oblique
direction.
[0078] However, employing circular rod shaped electrically insulating members such as the
upper fixing member 4 and the lower fixing member 5 described previously as the fixing
members employed in the tempering process enables the plural steel sheets to be fixed more
stably. This enables the advantageous effects of the present disclosure to be obtained even
more reliably.
[0079] Moreover, in the present disclosure, in cases in which plural welding target locations
are present on a single sheet grouping, when performing the cooling process for one welding
target location, at least one process out of the welding process or the tempering process is
preferably performed concurrently at another of the welding target locations. Performing the
welding process through to the tempering process concurrently at the plural welding target
locations in this manner enables welded joint productivity to be further improved. Note that
similar also applies in cases in which spot welding is performed in succession while
conveying plural sheet groupings.
[0080] Note that in the present disclosure, processes to perform predetermined processing or
the like may be provided before and/or after each process from the welding process through to
18
the tempering process.
[0081] Next, explanation follows regarding the sheet steel employed in the manufacturing
method of the present disclosure.
[0082] < Sheet Steel >
In the present disclosure, sheet steel having predetermined tensile strength and sheet
thickness corresponding to the desired joint strength and the purpose of the joint (for example,
for use in automobile components) may be employed for the plural steel sheets configuring a
welding target. An example of such sheet steel is sheet steel with tensile strength from 270
MPa grade to 3000 MPa grade. Such sheet steel may be sheet steel plated with zinc or the
like (namely, plated sheet steel).
[0083] Of such sheet steel, high-strength sheet steel with tensile strength of 780 MPa or
higher becomes brittle at the welds and is susceptible to fracturing after welding.
Accordingly, the present disclosure is particularly advantageous when applied to a sheet
grouping of which at least one steel sheet is a high-strength sheet steel with tensile strength of
780 MPa or higher. In the present exemplary embodiment, the upper sheet 8 and the lower
sheet 9 are configured from high-strength sheet steel for automotive use.
[0084] Note that in the present disclosure, all of the plural steel sheets may be configured
from the same type of sheet steel, or some of the plural steel sheets may be configured from
the same type of sheet steel. Alternatively, all of the steel sheets may be configured from
different types of sheet steel.
[0085] Moreover, the number of steel sheets is not particularly limited, and two or more
thereof may be employed according to the purpose of the welded joint. Moreover, although
the sheet thickness of the steel sheets is not particularly limited, the thickness of a single sheet
thereof is preferably between 0.5 mm and 3.2 mm.
[0086] Moreover, in the present disclosure, the shapes of the steel sheets are not particularly
limited, as long as at least a welding target location of a steel sheet has a specific sheet shaped
structure to be overlapped with a welding target location of another steel sheet along the sheet
thickness direction. Namely, the steel sheets employed in the present disclosure may, for
example, be flat plate shaped steel sheets, or steel sheets with a flattened sheet shaped
structure overall. Alternatively, for example, a steel sheet employed in the present disclosure
may have a locally sheet shaped structure at a portion including a welding target location, and
a bent structure or the like at portions other than the sheet shaped structure, such as an
L-shaped steel sheet or a hat-shaped steel sheet.
[0087] The manufacturing method of the present disclosure is not limited to the exemplary
embodiment described above nor to Examples, described below. Various combinations,
19
substitutions, and modifications may be implemented within a range not departing from the
objective and spirit of the present disclosure.
Examples
[0088] More specific explanation follows regarding the present disclosure, with reference to
Examples and Comparative Examples. Note that the present disclosure is not limited to such
Examples.
[0089] < Examples >
(Welding Process)
A sheet grouping configured by overlapping two steel sheets, namely an upper sheet
and a lower sheet, was interposed between a pair of electrodes of a spot welding apparatus.
Each of the two steel sheets was configured by an un-plated hot stamped steel sheet with 1.5
GPa grade tensile strength and a sheet thickness of 1.2 mm. The pair of electrodes were both
DR-type 40-16 electrodes with a diameter ϕ of 6 mm.
[0090] This sheet grouping was then energized along the sheet thickness direction,
employing a weld current of 5.5 kA and a weld time of 16 cyc while applying a pressurizing
force of 400 kgf (approximately 3923 N) to the sheet grouping, thereby forming a nugget at
the mutually superimposed faces of the two steel sheets and in a region in the vicinity of the
mutually superimposed faces. The weld by the electrodes was then stopped, and the sheet
grouping was maintained in a state applied with the pressurizing force from the electrodes for
a holding duration of 10 cyc.
[0091] (Cooling Process)
After welding, the sheet grouping was then air-cooled until reaching room
temperature (namely, the Mf point or below).
[0092] (Tempering Process)
The sheet grouping was then moved to a tempering device of a spot welding
apparatus configured similarly to the tempering device 102 of the spot welding apparatus 1
illustrated in Fig. 2, and the sheet grouping was interposed in the sheet thickness direction
between the upper electrode and upper fixing member and the lower electrode and lower
fixing member. A region including the nugget and the heat affected zone was then tempered
by being energized in an oblique direction with an post heat current ranging from 5.0 to 10.0
kA as given in Table 1 below for an post heat time of 60 cyc, while applying a pressurizing
force of 300 kgf (approximately 2942 N).
[0093] Next, the post heat of the electrodes was stopped, and the sheet grouping was
maintained in an interposed state for a holding duration of 10 cyc in order to obtain a welded
joint in which the two steel sheets were joined together. Note that a total of 11 Example
20
welded joints were produced, one for each of the tempering process post heat currents given
in Table 1.
[0094] Note that the tempering device of the spot welding apparatus employed in the
tempering process was provided with Cu-Cr alloy circular rod electrodes as the upper
electrode and the lower electrode. The upper electrode and the lower electrode each had a
length of 50 mm and a diameter ϕ of 10 mm. The inter-electrode distance de between the
upper electrode and the lower electrode in the sheet-plane direction was 50 mm. The spot
welding apparatus employed in the tempering process was provided with ceramic circular
rods as the upper fixing member and the lower fixing member. The upper fixing member
and the lower fixing member each had a length of 50 mm and a diameter ϕ of 10 mm.
[0095] < Comparative Example 1 >
A welded joint of a Comparative Example 1 was produced similarly to the Examples,
with the exception that a tempering process was not performed.
[0096] < Comparative Examples 2 >
Welded joints of Comparative Examples 2 were produced similarly to the Examples,
with the exception that the cooling process used a cool time of 60 cyc and the tempering
process was performed using the pair of electrodes of the spot welding apparatus employed in
the welding process. Namely, in these Comparative Examples, post heat was performed
along the sheet thickness direction instead of in an oblique direction.
[0097] The welded joints of the Examples, the Comparative Example 1, and the
Comparative Examples 2 produced as described above were fractured by chisel testing in
order to confirm the fracture modes thereof. Table 1 gives confirmation results for the
fracture modes.
[0098] Note that the welded joints of the Examples, the Comparative Example 1, and the
Comparative Examples 2 each had the same nugget diameter ϕ, this being approximately 4√t,
and specifically approximately 4.4 mm.
[0099] Table 1
Comparative Example 1 Comparative Examples 2 Examples
No tempering process
Tempering process
(post heat in sheet thickness
direction)
Tempering process
(post heat in oblique direction)
Welding process
weld current
(for reference)
Fracture mode
Tempering
process post
heat current
Fracture mode
Tempering
process post
heat current
Fracture mode
5.5 kA
Interfacial
fracture
4.0 kA
Interfacial
fracture
5.0 kA
Interfacial
fracture
21
- - 4.5 kA Plug fracture 5.5 kA
Interfacial
fracture
- - 5.0 kA Plug fracture 6.0 kA
Partial plug
fracture
- - 5.5 kA Plug fracture 6.5 kA Plug fracture
- - 6.0 kA
Interfacial
fracture
7.0 kA Plug fracture
- - - - 7.5 kA Plug fracture
- - - - 8.0 kA Plug fracture
- - - - 8.5 kA Plug fracture
- - - - 9.0 kA Plug fracture
- - - - 9.5 kA Plug fracture
- - - - 10.0 kA Plug fracture
[0100] As illustrated by the Examples appearing within a bold frame in Table 1, it can be
seen that the welded joints of the Examples could be made to exhibit the fracture mode of
"plug fracture" across a wide range of tempering process conditions (namely an post heat
current range) spanning from 6.5 kA to 10.0 kA. The fracture mode of "plug fracture" is
indicative of high joint strength. Accordingly, it can be seen that the Examples demonstrate
an excellent degree of robustness.
[0101] By contrast, as illustrated in Table 1, the welded joint of the Comparative Example 1
exhibited a fracture mode of "interfacial fracture". The fracture mode of "interfacial
fracture" is indicative of low joint strength. Moreover, as illustrated by the Comparative
Examples 2 appearing within a bold frame in Table 1, although the welded joints of the
Comparative Examples 2 did exhibit a fracture mode of "plug fracture", the range of
tempering process conditions across which "plug fracture" was obtained was very narrow,
namely from 4.5 kA to 5.5 kA. Namely, it can be seen that Comparative Example 1 and
Comparative Examples 2 demonstrate poor robustness.
[0102] It is thought that the wide range of tempering process conditions for the Examples is
due to the gentle change in temperature as the temperature rises in response to the current.
Specifically, as illustrated in Fig. 7, in the case of the Examples, the temperature rises more
gradually than in Comparative Example 1 from a start timing T0 to an end timing T1 of the
post heat process.
[0103] This is thought to be since the energization path in the tempering process of the
Examples is broader, in other words the current density is lower, than in conventional
tempering such as that employed for the Comparative Examples 2. This may also be
22
inferred from the fact that the Examples are capable of obtaining the fracture mode of plug
fracture at higher currents than the Comparative Examples 2.
[0104] (Operation and Advantageous Effects)
In the manufacturing method of the welded joint 10 according to the present
exemplary embodiment, the pair of electrodes are separated from each other such that the pair
of electrodes on either side of the steel sheets do not overlap each other in the thickness
direction. Accordingly, when passing a tempering current between the pair of electrodes, the
length of the energization path CP is longer than it would be were the electrodes to overlap
each other. In other words, the nugget is tempered by being energized in the oblique
direction.
[0105] Note that "the length of the energization path CP is longer" encompasses a state in
which the area of the energization path CP is larger in plan view. Namely, the present
disclosure encompasses cases in which the shapes of the respective regions of the steel sheets
contacted by the pair of electrodes are planar shapes instead of points in plan view.
[0106] Increasing the length of the energization path CP enables the current density per unit
area of the energization path CP to be lowered. Since this enables the passage of current to
be suppressed from becoming excessive, the current can be easily controlled when performing
tempering in the post heat process, for example without the need to introduce a new current
control mechanism when employing welding equipment provided with an existing current
control mechanism. This makes the post heat process less liable to be affected by external
factors, enabling a manufacturing method of a welded joint with excellent robustness to be
provided.
[0107] Moreover, in the manufacturing method of the welded joint 10 according to the
present exemplary embodiment, the site A and the site B are each separated from the center E
of the nugget N by at least the maximum diameter of the nugget N. This enables tempering
to be performed gently over an even broader region not limited to the nugget N but including
both the nugget N and the heat affected zone.
[0108] Moreover, in the manufacturing method of the welded joint 10 according to the
present exemplary embodiment, the distance between the site A and the center E of the nugget
N is equal to the distance between the site B and the center E of the nugget N. This enables
more uniform heating and tempering of a broad region including the nugget N.
[0109] Moreover, in the manufacturing method of the welded joint 10 according to the
present exemplary embodiment, the site A is interposed between the upper electrode 2 and the
upper fixing member 4, thereby eliminating a gap between the upper sheet 8 and the lower
sheet 9 at the position of the site A. Moreover, the site B is interposed between the lower
23
electrode 3 and the lower fixing member 5, thereby eliminating a gap between the upper sheet
8 and the lower sheet 9 at the position of the site B. Gaps in regions around the nugget N are
thus eliminated, placing the respective steel sheets in close contact with each other. As a
result, a broad region including not only the nugget N but also the heat affected zone
peripheral to the nugget N can be gently heated. This enables uniform melting to be
achieved over the broad region spanning both the nugget N and the heat affected zone.
[0110] In the manufacturing method of the welded joint 10 according to the present
exemplary embodiment, the widths W of the upper electrode 2 and the lower electrode 3 are
set to at least the maximum diameter ϕ of the nugget N, enabling the nugget N to be more
reliably energized. Note that although the widths W of the upper electrode 2 and the lower
electrode 3 may be the same as the maximum diameter ϕ of the nugget N, setting the widths
W to at least the maximum diameter ϕ further increases the length of the energization path CP.
This enables burn-through and the like to be prevented, and is thus advantageous from the
perspective of increasing robustness.
[0111] Moreover, in the manufacturing method of the welded joint 10 according to the
present exemplary embodiment, when current is passed through the welded joint 10 between
the upper electrode 2 and the lower electrode 3, the current is passed through the inside of the
welded joint 10 in a region outside of the nugget N, thus increasing the length of the
energization path CP. This enables the current density per unit area to be further lowered on
the energization path CP.
[0112] Moreover, the manufacturing method of the welded joint 10 according to the present
exemplary embodiment is not liable to be affected by external factors during the post heat
process, enabling a welded joint 10 with excellent robustness to be realized. Note that in the
welded joint 10 illustrated in the example of Fig. 8, the site A, this being a location at the
outer side of the nugget N in a plane running parallel to the upper sheet 8, is formed with a
contact mark X where the upper electrode 2 made contact in order to perform post heat.
Moreover, the site B, this being a location at the outer side of the nugget N in a plane running
parallel to the lower sheet 9 and positioned on the opposite side of the nugget N from the site
A, is formed with a contact mark Y where the lower electrode 3 made contact in order to
perform post heat. Note that the contact mark X and the contact mark Y may have a
different color to the surrounding regions due to the post heat.
[0113] Fig. 8 illustrates an example in which a softened structure Z is continuously present
between the contact mark X and the contact mark Y, as indicated by dotted shading. For
example, for sheet steel having tensile strength of 1180 MPa or greater, for which welded
joint fracture issues are particularly pronounced, the structure is controlled, resulting in higher
24
strength. High strength structures are softened by post heat. For example, martensite
structures become tempered martensite structures. Moreover, for example, post heat reduces
migration in structures that have been subjected to hardening processing. The softened
structure Z can be confirmed to have Vickers hardness (HV) lower than the hardness of a base
metal outside of the softened structure Z by at least 10 HV. For example, measuring the
Vickers hardness (HV) of the softened structure Z as illustrated in Fig. 6D and Fig. 6E enables
the presence of the softened structure to be determined since lower values are obtained than
for other structures where tempering has not occurred.
[0114] In the tempering device 102 according to the present exemplary embodiment, the
inter-electrode distance de between the upper electrode 2 and the lower electrode 3 is set to at
least 6 mm. This is advantageous when forming a nugget N with a width of 6 mm, this
being a commonly employed width in such welded joints 10.
[0115] In the tempering device 102 according to the present exemplary embodiment, the
upper fixing member 4 is provided on an approach/retract direction axis of the upper electrode
2. Moreover, the lower fixing member 5 is provided on an approach/retract direction axis of
the lower electrode 3. This enables the post heat process to be implemented more stably.
[0116] Moreover, in the present exemplary embodiment, the tensile strength of the upper
sheet 8 and the tensile strength of the lower sheet 9 are both at least 440 MPa. Accordingly,
in cases in which high-strength sheet steel for automotive use is employed for the upper sheet
8 and the lower sheet 9, a desirable welded joint 10 for automotive use can be obtained.
Note that in the present disclosure, either the upper sheet 8 or the lower sheet 9 may be
configured from sheet steel with tensile strength of at least 440 MPa. Moreover, in cases in
which a welded joint is configured from three or more steel sheets, the tensile strength of at
least one of the steel sheets should be at least 440 MPa.
[0117] Moreover, in the present exemplary embodiment, when performing tempering, the
upper electrode 2 and the lower electrode 3 are separated from indentations of the nugget N.
Namely, even if a step has been formed on the upper sheet 8 by such an indentation, the upper
electrode 2 does not contact this step. Similarly, the lower electrode 3 does not contact such
a step on the lower sheet 9. This enables the upper electrode 2 to contact the upper sheet 8
smoothly, and enables the lower electrode 3 to contact the lower sheet 9 smoothly.
[0118] < First Modified Example >
In a tempering device 102A according to a first modified example illustrated in Fig. 9,
the upper fixing member 4 is a third electrode. Moreover, the lower fixing member 5 is a
fourth electrode. Namely, the upper fixing member 4 and the lower fixing member 5 both
have a post heat function, and function as a tempering device. Note that conversely,
25
electrodes may be employed to fix the welded joint 10 in the present disclosure.
[0119] The tempering device 102A according to the first modified example includes an
energization controller 20 configured to alternately execute energization between the upper
electrode 2 and the lower electrode 3 and energization between the upper fixing member 4
serving as a third electrode and the lower fixing member 5 serving as a fourth electrode.
Accordingly, in the post heat process, the four electrodes that are the upper electrode 2, the
lower electrode 3, the upper fixing member 4, and the lower fixing member 5 are able to
execute post heat with energization paths that form an X shape.
[0120] (Operation and Advantageous Effects)
The tempering device 102A according to the first modified example alternately
executes energization between the upper electrode 2 and the lower electrode 3 and
energization between the third electrode and the fourth electrode, thus enabling the post heat
process to be implemented with greater efficiency.
[0121] < Second Modified Example >
A welding apparatus according to a second modified example illustrated in Fig. 10
includes the tempering device 102, a pair of robot arms 12, 13 provided to the tempering
device 102, and a position controller 14 that controls operation of the robot arms 12, 13.
Alternatively configuration may be made such that gripper fingers 12, 13 are attached to a
leading end of a single robot arm and the position controller 14 controls operation of the robot
arm and the grippers. The configuration in which grippers are provided to a leading end of a
single robot arm may be adopted in cases in which a nugget is provided at an end portion of
steel sheets. The configuration including two robot arms may be adopted in cases in which a
nugget is provided at a central portion of the steel sheets.
[0122] The robot arm 12 illustrated on the upper side in Fig. 10 is capable of making the
upper electrode 2 independently approach and retract from the upper sheet 8. The robot arm
13 illustrated on the lower side in Fig. 10 is capable of making the lower electrode 3
independently approach and retract from the lower sheet 9. The positions and orientations of
the electrodes 2, 3, attached to the pair of robot arms 12, 13 or the electrodes 2, 3, attached to
the gripper fingers 12, 13 attached to the leading end of a single robot arm are controlled by
the position controller 14.
[0123] The robot arms 12, 13, or the gripper fingers 12, 13 attached to the leading end of a
robot arm, move an intermediate point between the tip of the upper electrode 2 and the tip of
the lower electrode 3 to a location that has been welded as the nugget by the welding machine
101, and dispose the upper electrode 2 and the lower electrode 3 at an outer side of this
welded location. Note that the welding machine that forms the nugget in the second
26
modified example is similar to the welding machine 101 illustrated in Fig. 1. The welding
machine is omitted from illustration in Fig. 10.
[0124] (Operation and Advantageous Effects)
Similarly to the case described in the present exemplary embodiment, the welding
apparatus according to the second modified example is not liable to be affected by external
factors during the post heat process, and is able to provide a welding apparatus capable of
manufacturing the welded joint 10 with excellent robustness.
[0125] < Other Exemplary Embodiments >
A welded joint of the present disclosure may be configured by three or more
overlapped steel sheets. Fig. 11 illustrates an example of a welded joint in which a middle
sheet 21 is interposed between the upper sheet 8 and a lower sheet 9A. Note that the upper
sheet 8 and the middle sheet 21 have substantially the same thickness as each other, whereas
the lower sheet 9A is thinner than the upper sheet 8 and the middle sheet 21. In the present
disclosure, the thicknesses of the plural steel sheets may differ from each other as in the
welded joint illustrated in Fig. 11.
[0126] For example, in welded joints for automotive use, an outer side steel sheet may be
thinner than an inner side steel sheet. Accordingly, a welded joint according to the present
disclosure that includes steel sheets having different thicknesses to each other may be
advantageous as a welded joint for automotive use. Moreover, high-strength sheet steel may
be employed in welded joints for automotive use. In the present disclosure, at least one of
the steel sheets out of the plural steel sheets included in the welded joint should be configured
by high-strength sheet steel.
[0127] The present disclosure may be configured using a combination of elements of the
respective configurations illustrated in Fig. 1 to Fig. 11. The present disclosure encompasses
various exemplary embodiments other than those described above. The technology scope of
the present disclosure is determined only by the features in the invention in the scope of the
patent claims as supported by the foregoing explanation.
Industrial Applicability
[0128] The manufacturing method of the present disclosure is not liable to be affected by
external factors and is able to secure excellent robustness during a post heat process, and is
thus favorably applied to spot welding employing high-strength sheet steel such as sheet steel
for automotive use.
[0129] The present disclosure accordingly has a high level of industrial applicability.
[0130]
1 spot welding apparatus (welding apparatus)
27
2 upper electrode (first electrode)
3 lower electrode (second electrode)
4 upper fixing member (first fixing member)
5 lower fixing member (second fixing member)
6 upper holder member
7 lower holder member
8 upper sheet (first steel sheet)
9, 9A lower sheet (second steel sheet)
10 spot welded joint (welded joint)
12, 13 robot arm or gripper finger
14 position controller
20 energization controller
101 welding machine
102, 102A tempering device
N nugget
CP energization path
X, Y contact mark
Z softened structure
ϕ maximum diameter
[0131] << Supplement >>
The present specification conceptualizes the following aspects.
[0132] Specifically, a first aspect is a welded joint manufacturing method including:
preparing a welded joint including a first steel sheet, a second steel sheet overlapped
with the first steel sheet, and a quenched nugget joining the first steel sheet and the second
steel sheet together;
abutting a first electrode against the first steel sheet at a site A, which is a location at
an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first
steel sheet of the welded joint;
abutting a second electrode against the second steel sheet at a site B, which is a
location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to
the first steel sheet of the welded joint, and positioned on an opposite side of the nugget from
the site A; and
passing a current through the welded joint between the first electrode and the second
electrode.
[0133] A second aspect is the welded joint manufacturing method of the first aspect, wherein
28
the site A and the site B are each separated from a center of the nugget in the plane running
parallel to the first steel sheet by at least a maximum diameter of the nugget.
[0134] A third aspect is the welded joint manufacturing method of the second aspect,
wherein a distance between the site A and the center of the nugget and a distance between the
site B and the center of the nugget are equal to each other.
[0135] A fourth aspect is the welded joint manufacturing method of any one of the first
aspect to the third aspect, further including:
abutting the first electrode against the first steel sheet such that the site A is
interposed between the first electrode and a first fixing member provided on a second steel
sheet side; and
abutting the second electrode against the second steel sheet such that the site B is
interposed between the second electrode and a second fixing member provided on a first steel
sheet side.
[0136] A fifth aspect is the welded joint manufacturing method of any one of claim 1 to
claim 4, wherein:
any gap between the first steel sheet and the second steel sheet at a position of the
site A is eliminated by interposing the site A between the first electrode and the first fixing
member; and
any gap between the first steel sheet and the second steel sheet at a position of the
site B is eliminated by interposing the site B between the second electrode and the second
fixing member.
[0137] A sixth aspect is the welded joint manufacturing method of any one of the first aspect
to the fifth aspect, wherein:
each of the first electrode and the second electrode has a constant width in the plane
running parallel to the first steel sheet when the current is passed through the welded joint;
and
the width of the first electrode and the width of the second electrode are at least a
maximum diameter of the nugget in the plane running parallel to the first steel sheet.
[0138] A seventh aspect is the welded joint manufacturing method of any one of the first
aspect to the sixth aspect, wherein when passing the current through the welded joint between
the first electrode and the second electrode, an energization path is increased in length by
passing the current through a region inside the welded joint other than the nugget.
[0139] An eighth aspect is a tempering device including:
a first electrode; and
a second electrode, wherein:
29
approach and retract directions of the first electrode and approach and retract
directions of the second electrode are mutually opposing directions to each other; and
an inter-electrode distance between the first electrode and the second electrode is at
least 6 mm in a flat plane orthogonal to the approach and retract directions.
[0140] A ninth aspect is the tempering device of the eighth aspect, further including:
a first fixing member provided coaxially with the approach and retract directions of
the first electrode; and
a second fixing member provided coaxially with the approach and retract directions
of the second electrode.
[0141] A tenth aspect is the tempering device of the ninth aspect, wherein:
the first fixing member is a third electrode;
the second fixing member is a fourth electrode; and
the tempering device further includes an energization controller configured to
alternately execute energization between the first electrode and the second electrode and
energization between the third electrode and the fourth electrode.
[0142] An eleventh aspect is a welding apparatus including:
the tempering device of any one of the eighth aspect to the tenth aspect;
a robot arm to which the tempering device is attached;
a welding machine configured to form a nugget; and
a position controller configured to control the robot arm so as to move an
intermediate point between a tip of the first electrode and a tip of the second electrode to a
location that has been welded as the nugget by the welding machine, and to dispose the first
electrode and the second electrode at an outer side of the location that has been welded.
[0143] A twelfth aspect is a welded joint including:
a first steel sheet;
a second steel sheet overlapped with the first steel sheet; and
a quenched nugget joining the first steel sheet and the second steel sheet together;
wherein:
tensile strength of the first steel sheet and the second steel sheet is at least 1180 MPa;
a contact mark from a first electrode is formed on the first steel sheet at a site A,
which is a location at an outer side of the nugget in a sheet-plane direction in a plane running
parallel to the first steel sheet of the welded joint;
a contact mark from a second electrode is formed on the second steel sheet at a site B,
which is a location at an outer side of the nugget in a sheet-plane direction in a plane running
parallel to the first steel sheet of the welded joint, and positioned on an opposite side of the
30
nugget from the site A; and
a softened structure having Vickers hardness lower than Vickers hardness of the first
steel sheet and Vickers hardness of the second steel sheet by at least 10 HV is continuously
present between the contact mark from the first electrode and the contact mark from the
second electrode.
[0144] Alternative Aspects
The present specification further conceptualizes the following alternative aspects.
[0145] Specifically, a first alternative aspect is a manufacturing method of a spot welded
joint configured by plural steel sheets joined together by spot welding, the manufacturing
method including:
a welding process of post-heating the plural mutually overlapped steel sheets to form
a nugget;
a cooling process of cooling at least the nugget; and
a tempering process of post-heating the plural steel sheets in an oblique direction
relative to a sheet thickness direction in order to temper at least the nugget.
[0146] A second alternative aspect is the spot welded joint manufacturing method of the first
alternative aspect, wherein in the tempering process, an upper electrode and a lower electrode
are disposed such that the plural steel sheets are interposed therebetween and also disposed at
positions on mutually opposing sides of the nugget in a horizontal direction that is orthogonal
to the sheet thickness direction, and the upper electrode and the lower electrode are employed
to energize the plural steel sheets in the oblique direction.
[0147] A third alternative aspect is the spot welded joint manufacturing method of the
second alternative aspect, wherein an inter-electrode distance between the upper electrode and
the lower electrode in the horizontal direction is at least twice a diameter of the nugget.
[0148] A fourth alternative aspect is the spot welded joint manufacturing method of the
second alternative aspect or the third alternative aspect, wherein a distance between the upper
electrode and the nugget in the horizontal direction is equal to a distance between the lower
electrode and the nugget in the horizontal direction.
[0149] A fifth alternative aspect is the spot welded joint manufacturing method of any of the
second alternative aspect to the fourth alternative aspect, wherein in the tempering process, a
fixing member is employed to fix the plural steel sheets during post heat.
[0150] A sixth alternative aspect is the spot welded joint manufacturing method of the fifth
alternative aspect, wherein:
the fixing member includes an upper fixing member and a lower fixing member
disposed such that the plural steel sheets are interposed therebetween; and
31
the upper fixing member is positioned on the opposite side of the nugget from the
upper electrode in the horizontal direction; and
the lower fixing member is positioned on the opposite side of the nugget from the
lower electrode in the horizontal direction.
[0151] The alternative aspects described above exhibit the following operation and
advantageous effects.
[0152] In the spot welded joint manufacturing method according to the alternative aspects,
the nugget is tempered by post-heating the plural steel sheets in the oblique direction with
respect to the sheet thickness direction during the tempering process, this being a separate
process to the welding process in which the nugget is formed. This approach is less liable to
be affected by external factors, enabling excellent robustness to be secured.
[0153] The disclosure of Japanese Patent Application No. 2019-047020, filed on March 14,
2019, is incorporated in its entirety by reference herein.
[0154] All cited documents, patent applications, and technical standards mentioned in the
present specification are incorporated by reference in the present specification to the same
extent as if each individual cited document, patent application, or technical standard was
specifically and individually indicated to be incorporated by reference.

CLAIMS

1. A welded joint manufacturing method comprising:
preparing a welded joint including a first steel sheet, a second steel sheet overlapped
with the first steel sheet, and a quenched nugget joining the first steel sheet and the second
steel sheet together;
abutting a first electrode against the first steel sheet at a site A, which is a location at
an outer side of the nugget in a sheet-plane direction in a plane running parallel to the first
steel sheet of the welded joint;
abutting a second electrode against the second steel sheet at a site B, which is a
location at an outer side of the nugget in a sheet-plane direction in a plane running parallel to
the first steel sheet of the welded joint, and positioned on an opposite side of the nugget from
the site A; and
passing a current through the welded joint between the first electrode and the second
electrode.
2. The welded joint manufacturing method of claim 1, wherein the site A and the site B are
each separated from a center of the nugget in the plane running parallel to the first steel sheet
by at least a maximum diameter of the nugget.
3. The welded joint manufacturing method of claim 2, wherein a distance between the site A
and the center of the nugget and a distance between the site B and the center of the nugget are
equal to each other.
4. The welded joint manufacturing method of any one of claim 1 to claim 3, further
comprising:
abutting the first electrode against the first steel sheet such that the site A is
interposed between the first electrode and a first fixing member provided on a second steel
sheet side; and
abutting the second electrode against the second steel sheet such that the site B is
interposed between the second electrode and a second fixing member provided on a first steel
sheet side.
5. The welded joint manufacturing method of claim 4, wherein:
any gap between the first steel sheet and the second steel sheet at a position of the
33
site A is eliminated by interposing the site A between the first electrode and the first fixing
member; and
any gap between the first steel sheet and the second steel sheet at a position of the
site B is eliminated by interposing the site B between the second electrode and the second
fixing member.
6. The welded joint manufacturing method of any one of claims 1 to 5, wherein:
each of the first electrode and the second electrode has a constant width in the plane
running parallel to the first steel sheet when the current is passed through the welded joint;
and
the width of the first electrode and the width of the second electrode are at least a
maximum diameter of the nugget in the plane running parallel to the first steel sheet.
7. The welded joint manufacturing method of any one of claims 1 to 6, wherein when
passing the current through the welded joint between the first electrode and the second
electrode, an energization path is increased in length by passing the current through a region
inside the welded joint other than the nugget.
8. A tempering device comprising:
a first electrode; and
a second electrode, wherein:
approach and retract directions of the first electrode and approach and retract
directions of the second electrode are mutually opposing directions to each other; and
an inter-electrode distance between the first electrode and the second electrode is at
least 6 mm in a flat plane orthogonal to the approach and retract directions.
9. The tempering device of claim 8, further comprising:
a first fixing member provided coaxially with the approach and retract directions of
the first electrode; and
a second fixing member provided coaxially with the approach and retract directions
of the second electrode.
10. The tempering device of claim 9, wherein:
the first fixing member is a third electrode;
the second fixing member is a fourth electrode; and
34
the tempering device further comprises an energization controller configured to
alternately execute energization between the first electrode and the second electrode and
energization between the third electrode and the fourth electrode.
11. A welding apparatus comprising:
the tempering device of any one of claims 8 to 10;
a robot arm to which the tempering device is attached;
a welding machine configured to form a nugget; and
a position controller configured to control the robot arm so as to move an
intermediate point between a tip of the first electrode and a tip of the second electrode to a
location that has been welded as the nugget by the welding machine, and to dispose the first
electrode and the second electrode at an outer side of the location that has been welded.
12. A welded joint comprising:
a first steel sheet;
a second steel sheet overlapped with the first steel sheet; and
a quenched nugget joining the first steel sheet and the second steel sheet together;
wherein:
tensile strength of the first steel sheet and the second steel sheet is at least 1180 MPa;
a contact mark from a first electrode is formed on the first steel sheet at a site A,
which is a location at an outer side of the nugget in a sheet-plane direction in a plane running
parallel to the first steel sheet of the welded joint;
a contact mark from a second electrode is formed on the second steel sheet at a site B,
which is a location at an outer side of the nugget in a sheet-plane direction in a plane running
parallel to the first steel sheet of the welded joint, and positioned on an opposite side of the
nugget from the site A; and
a softened structure having Vickers hardness lower than Vickers hardness of the first
steel sheet and Vickers hardness of the second steel sheet by at least 10 HV is continuously
present between the contact mark from the first electrode and the contact mark from the
second electrode.