TECHNICAL FIELD
[0001] The present invention relates to a joint
formed by overlapping a plurality of pieces of steel
plates and performing spot welding on the steel
plates.
BACKGROUND ART
[00021 In recent years, a reduction in weight of a
vehicle body has been demanded for achieving a fuel
efficiency and a C02 emission reduction in an
automobile field. Further, in order to improve a
collision safety, it has been demanded to increase a
strength of members of a vehicle body. In order to
fulfill these demands, it is effective to use a highstrength
steel plate for a vehicle body, components
and the like. When performing assembling of a
vehicle body, mounting of a component, and the like,
spot welding is mainly used. When a plurality of
pieces of steel plates including at least one piece
of steel plate whose tensile strength is 750 MPa or
more are spot-welded, a strength of a welded joint
becomes a problem.
[00031 In a joint formed by overlapping a plurality
of pieces of steel plates and performing . spot welding *.
on the steel plates (hereinafter, referred to also as
"spot-welded joint"), a tensile strength is an
important property. Such a tensile strength includes
- 1 -
a tensile shear strength (TSS) measured under a
tensile load applied in a shear direction, and a
cross tensile strength (CTS) measured under a tensile
load applied in a peeling direction. Note that
measuring methods for the tensile shear strength and
the cross tensile strength are defined in JIS Z 3136
and JIS Z 3137.
[00041 The CTS in a spot-welded joint formed of a
plurality of pieces of steel plates each having a
tensile strength of 270 MPa to 600 MPa increases, in
accordance with an increase in strength of the steel
plates. Therefore, a problem regarding a joint
strength is difficult to occur, in the spot-welded
joint formed of the steel plates each having the
tensile strength of 270 MPa to 600 MPa. However, the
CTS in a spot-welded joint formed of a plurality of
pieces of steel plates including at least one piece
of steel plate whose tensile strength is 750 MPa or
more, does not increase or reduces even if the
tensile strength of the steel plates increases.
[00051 Generally, in a spot-welded joint formed of a
plurality of pieces of steel plates including at
least one piece of steel plate whose tensile strength
is 750 MPa or more, the CTS is easily reduced. This
is because a stress concentration with respect to a
welded portion is increased due to a lowering of
ductility, and because a toughness of the welded
portion is lowered since the welded portion is
tempered. For this reason, an improvement of the CTS
- 2 -
in the spot-welded joint formed of the plurality of
pieces of steel plates including at least one piece
of steel plate whose tensile strength is 750 MPa or
more is demanded.
[0006] As a method of securing the strength and the
toughness in the spot-welded joint formed of the
plurality of pieces of steel plates including at
least one piece of steel plate whose tensile strength
is 750 MPa or more, there is a two-stage energization
method in which main energization is conducted, and
then post-energization is conducted.
Patent Literature 1 describes a method in which
main energization is finished and after a
predetermined time passes, tempering energization is
conducted, to thereby perform annealing on a spotwelded
joint (a nugget portion and a heat-affected
zone) to reduce a hardness of the joint.
[0007] However, in this method, there is a need to
cause a martensite transformation to be almost
completed before performing the tempering
energization. Accordingly, a long cooling time
becomes required after the main energization is
finished. Further, in this method, the nugget is
softened to reduce a shear force.
[0008] Further, as a method of securing the strength
and the toughness in the spot-welded joint formed of
the plurality of pieces of steel plates including at
least one piece of steel plate whose tensile strength
is 750 MPa or more, there is a method in which
- 3 -
welding is performed, and a welded portion is then
heated by a way of heating different from the welding.
Patent Literature 2 describes a method in which
welding is performed, and after that, a welded
portion is heated with high frequency to be subjected
to tempering treatment.
[0009] However, in this method, another process is
required after the welding, and thus an operating
procedure becomes complicated. Further, in this
method, a special apparatus for using the high
frequency is required. Furthermore, in this method,
a nugget is softened to reduce a shear force.
[0010] Further, Patent Literature 3 describes a
method in which a nugget is formed through main
welding, and then post-energization is performed with
a current which is equal to or greater than a main
welding current.
However, in this method, when a post-energization
time is lengthened, a nugget diameter is only
enlarged, and a structure becomes the same as that
obtained in conventional welding.
[OOll] Patent Literature 4 describes a method in
which spot welding is performed on steel plates each
having a tensile strength of 440 MPa or more. In
this method, a composition of components of the steel
plate is restricted to satisfy the following
conditions: C X P 0.0025; P: 0.015% or less; and
S: 0.01% or less. Further, after performing the
welding, heat treatment is performed on a welded
- 4 -
portion at 300°C for about 20 minutes.
However, in this method, the applicable steel
plate is limited. In addition, in this method, it
takes a long time to perform the welding, and thus
the productivity is low.
[00121 Patent Literature 5 describes a spot-welded
joint formed of high-strength steel plates (tensile
strength: 750 to 1850 MPa, carbon equivalent Ceq:
0.22 to 0.55 mass%) in which a microstructure of a
nugget outer layer zone, and an average grain
diameter and a number density of carbides in the
microstructure are defined.
However, when a fracture occurs on the outside of
the nugget, the structure of the nugget makes no
contribution, and thus the definition related to the
microstructure has no meaning.
[0013] Patent Literature 6 describes a method in
which spot welding is performed on steel plates each
having a tensile strength of 900 to 1850 MPa, and
having a plate thickness of 1.8 to 2.8 mm. In this
method, after performing the welding, postenergization
is successively performed with a current
which is 0.5 times to 0.9 times a welding current,
for a time which is 0.3 times to 0.5 times a welding
time.
However, in this method, a study regarding the
time between the main welding and the postenergization
has not been sufficiently conducted, and
thus the method does not contribute to the
- 5 -
improvement of the joint strength.
CITATION LIST
PATENT LITERATURE
[0014] Patent Literatnre 1: ~Japanese Laid-open
Patent Publication No. 2002-103048
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2009-125801
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2010-115706
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2010-059451
Patent Literature 5: International Publication
Pamphlet No. WO 2011-025015
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2011-5544
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[00151 From the backgrounds as described above, in a
spot-welded joint formed of a plurality of pieces of
steel plates including at least one piece of steel
plate whose tensile strength is 750 MPa to 2500 MPa,
the toughness becomes easily insufficient, and thus
it has been conventionally difficult to secure a
sufficiently high cross tensile strength.
Accordingly, the present invention has an object
to improve a cross tensile strength of a spot-welded
joint formed of a plurality of pieces of steel plates
including at least one piece of steel plate whose
tensile strength is 750 MPa to 2500 MPa.
- 6 -
SOLUTION TO PROBLEM
[0016] A spot-welded joint of the present invention
is a spot-welded joint formed by overlapping a
plurality of pieces of steel plates and performing
spot welding on the steel plates, including a highstrength
steel plate whose tensile strength is 750
MPa to 2500 MPa, being at least one piece of steel
plate out of the plurality of pieces of steel plates,
in which a carbon equivalent Ceq of the high-strength
steel plate represented by the following expression
(A) is 0.20 mass% to 0.55 mass%, and ten or more of
iron-based carbides in each of which a length of a
longest portion is 0.1 (pm) or more exist in a square
region whose length of one side is 10 (pm) in which a
plate thickness direction and a plate surface
direction of the steel plates are set to a vertical
direction and a horizontal direction, respectively,
being a region within a heat-affected zone of a cross
section that passes through a center of a welding
mark formed on surfaces of the steel plates by the
spot welding, and is cut along the plate thickness
direction of the steel plates, in which a position of
a center of the square region is a position, at the
cross section, separated by 100 (pm) from a position
of an end portion of a nugget in a direction
perpendicular to a tangent to a line indicating the
end portion of the nugget, at that position, in which
the position of the end portion of the nugget is a
position, out of positions on the line indicating the
- 7 -
-~
end portion of the nugget, within a range whose
center is set to a center in the plate thickness
direction of the spot-welded joint and having a
length of 1/4 times a total plate thickness being a
total value of plate thicknesses of the plurality of
pieces of steel plates along the plate thickness
direction.
Ceq = [Cl + [Sil/30 + [Mn]/20 + 2 [PI + 4 [S] ... (A)
LC], [Sil, [Mnl, [PI, and [Sl in the above
expression (A) indicate respective contents (mass%)
of C, Si, Mn, P, and S.
[00171 A first example of a spot welding method of
the present invention is a spot welding method of
overlapping a plurality of pieces of steel plates and
performing spot welding on the steel plates, in which
at least one piece of steel plate out of the
plurality of pieces of steel plates is a highstrength
steel plate whose tensile strength is 750
MPa to 2500 MPa, in which a carbon equivalent Ceq of
the high-strength steel plate represented by the
following expression (A) is 0.20 mass% to 0.55 mass%,
the spot welding method including: performing main
welding of energizing welding electrodes with a main
welding current In (kA) in a state where the
overlapped plurality of pieces of steel plates are
pressurized by the welding electrodes at a
pressurizing force FE (N) satisfying the following
expression ( B ) ; performing, after the main welding is
finished, cooling after main welding of cooling the
- 8 -
plurality of pieces of steel plates for a cooling
time after main welding ts (msec) satisfying the
following expression ( C ) while retaining the
pressurizing force FE (N) satisfying the following
expression (B) ; performing, after the cooling after
main welding is finished, post-energization of
energizing the welding electrodes with a postenergization
current Ip (kA) satisfying the following
expression (D) for a post-energization time tp (msec)
satisfying the following expression ( E ) while
retaining the pressurizing force FE (N) satisfying the
following expression (B); and retaining, after the
post-energization is finished, the pressurizing force
FE (N) satisfying the above expression (B) for a
retention time t~ (msec) satisfying the following
expression (F), and then releasing the pressurizing
at the pressurizing force FE (N).
Ceq = [Cl + [Si]/30 + [Mn]/20 + 2 [PI + 4 [S] ...
(A)
1960 X h 5 FE 3920 X h ... ( B )
7 X h + 5 2 ts 5 300 ... (C)
0.66 X Iw 5 Ip < IF, ... (D)
48/( ( I ~ / I P-~ )0.~4 4) 5 tp ... (E)
0 tH 300 ... (F)
[Cl, [Sil, [Mnl, [ P I , and [Sl in the above
expression (A) indicate respective contents (mass%)
of C, Si, Mn, P, and S, and h in the above expression
(B), and the above expression (C) indicates a plate
thickness of the steel plate (mm).
- 9 -
[0018] A second example of a spot welding method of
the present invention is a spot welding method of
overlapping a plurality of pieces of steel plates and
performing spot welding on the steel plates, in which
at least one piece of steel plate out of the
plurality of pieces of steel plates is a highstrength
steel plate whose tensile strength is 750
MPa to 2500 MPa, in which a carbon equivalent Ceq of
the high-strength steel plate represented by the
following expression (A) is 0.20 mass% to 0.55 mass%,
the spot welding method including: performing preenergization
of energizing welding electrodes with a
pre-energization current If (kA) satisfying the
following expression ( C ) for a pre-energization time
tf (msec) satisfying the following expression ( D ) , in
a state where the overlapped plurality of pieces of
steel plates are pressurized by the welding
electrodes at a pressurizing force FE (N) satisfying
the following expression (B); performing, after the
pre-energization is finished, cooling after preenergization
of cooling the plurality of pieces of
steel plates for a cooling time after preenergization
tc (msec) satisfying the following
expression ( E ) while retaining the pressurizing force
F, (N) satisfying the following expression (B);
performing, after the cooling after pre-energization
is finished, main welding of energizing the welding
electrodes with a main welding current Iw (kA) while
retaining the pressurizing force FE (N) satisfying the
- lo -
following expression (B); performing, after the main
welding is finished, cooling after main welding of
cooling the plurality of pieces of steel plates for a
cooling time after main welding ts (msec) satisfying
the following expression (F) while retaining the
pressurizing force FE (N) satisfying the following
expression (B); performing, after the cooling after
main welding is finished, post-energization of
energizing the welding electrodes with a posteneryization
current IP ( k A ) satisfying the following
expression ( G ) for a post-energization time tp (msec)
satisfying the following expression (H) while
retaining the pressurizing force FE (N) satisfying the
following expression (B); and retaining, after the
post-energization is finished, the pressurizing force
FE (N) satisfying the above expression (B) for a
retention time tH (msec) satisfying the following
expression (I), and then releasing the pressurizing
at the pressurizing force FE (N) .
Ceq = [Cl + [Sil/30 + [Mn]/20 + 2[P] + 4[S] ...
(A)
1960 X h l FE 5 3920 X h ... (B)
0.40 X Im 5 If < Iw ... (C)
20 s tf ... (D)
0 =< tc < 200 + 7 X h ... (E)
7 X h + 5 i ts 300 ... ( F )
0.66 X I Ip < 1, ... (G)
48/{ ( I ~ / I-~ ~0.)4)~ 5 t p ... (H)
0 tH 5 300 ... (I)
- 11 -
[C], [Sil, [Mn], [ P I , and [Sl in the above
expression (A) indicate respective contents (mass%)
of C, Si, Mn, P, and S, and h in the above expression
( B ) , the above expression (E) , and the above
expression (F) indicates a plate thickness of the
steel plate (mm).
ADVANTAGEOUS EFFECTS OF INVENTION
[00141 According to the present invention, it is
possible to improve a cross tensile strength of a
spot-welded joint formed of a plurality of pieces of
steel plates including at least one piece of steel
plate whose tensile strength is 750 MPa to 2500 MPa.
BRIEF DESCRIPTION OF DRAWINGS
[00201 [Fig. 11 Fig. 1 is a diagram illustrating
one example of an arrangement of two pieces of steel
plates and welding electrodes when spot welding is
started.
[Fig. 21 Fig. 2 is a diagram schematically
illustrating one example of a nugget and a heataffected
zone formed by the spot welding.
[Fig. 31 Fig. 3 is a diagram illustrating an
example of first form of an energization pattern.
[Fig. 41 Fig. 4 is a diagram schematically
illustrating one example of an appearance in the
middle of solidification of a molten zone which is
solidified to be a nugget.
[Fig. 51 Fig. 5 is a diagram illustrating one
example of a relationship between a cooling time
after main welding and a plate thickness of a steel
- 12 -
plate.
[Fig. 61 Fig. 6 is a diagram illustrating a
first example of a relationship between a postenergization
time and a square of a value obtained by
dividing a post-energization current by a main
welding current.
[Fig. 71 Fig. 7 is a diagram illustrating, in a
conceptual manner, one example of a relationship
between the post-energization time and a degree of
embrittlement of an outer peripheral portion of the
nugget and the heat-affected zone.
[Fig. 81 Fig. 8 is a diagram illustrating an
example of second form of an energization pattern.
[Fig. 91 Fig. 9 is a diagram illustrating one
example of a relationship between a cooling time
after pre-energization and a plate thickness of a
steel plate.
[Fig. 101 Fig. 10 is a diagram illustrating a
second example of a relationship between a postenergization
time and a square of a value obtained by
dividing a post-energization current by a main
welding current.
[Fig. 11Al Fig. 11A is a diagram (photograph)
illustrating one example of a structure of a heataffected
zone of a welded joint obtained by
unconventional welding.
[Fig. 11Bl Fig. 11B is a diagram (photograph)
illustrating one example of a structure of a heataffected
zone of a welded joint obtained by
- 13 -
conventional welding.
[Fig. 12Al Fig. 12A is a diagram explaining one
example of a precipitation condition of iron-based
carbides.
[Fig. 12Bl Fig. 128 is a diagram illustrating a
part of a region A in Fig. 12A in an enlarged manner.
DESCRIPTION OF EMBODIMENTS
[00211 The present inventors conducted earnest
studies, from a metallurgical point of view and a
mechanical point of view, regarding the reason why
the cross tensile strength (CTS) in the spot-welded
joint formed of the plurality of pieces of steel
plates including at least one piece of steel plate
whose tensile strength is 750 MPa to 2500 MPa cannot
be sufficiently improved by the conventional
technique in which the post-energization is performed
after the main welding. Note that in the following
description, the steel plate whose tensile strength
is 750 MPa to 2500 MPa is referred to as "highstrength
steel plate" according to need.
[0022] As a result of this, it was proved that if
the toughness in the nugget is only improved as in
the above-described conventional techniques, when
performing a cross tensile test, although it is
possible to suppress a fracture under low load which
occurs inside the nugget, a fracture under low load
which occurs in the heat-affected zone (HAZ) in the
periphery of the nugget cannot be sufficiently
suppressed.
- 1 4 -
Here, the nugget indicates a part of a steel
plate which is melted through energization between
welding electrodes and then is solidified. The heat
affected zone indicates a part of a steel plate
heated to a temperature equal to or more than the Acl
point and less than a melting temperature.
[0023] As described above, the present inventors
found out that, in order to obtain a spot-welded
joint with high reliability, it is necessary to
improve not only the fracture load inside the nugget
but also the fracture load in the peripheral portion
of the nugget. In order to achieve that, in the
present embodiment, after a solidified region is
formed in an inner periphery of a molten zone, the
solidified region and a heat-affected zone
surrounding the solidified region are retained at a
high temperature for a long time.
[0024] Hereinafter, embodiments of the present
invention will be described. Basically, the
respective embodiments to be described below are
realized by retaining a solidified region formed in
an inner periphery of a molten zone and a heataffected
zone surrounding the solidified region at a
high temperature for a long time. However, it is not
possible to obtain a welded joint with high
reliability only by performing the retention for a
long time which exceeds a conventional retention time.
[0025] [High-strength steel plate]
First, steel plates used for spot welding will be
- 15 -
described.
(Steel type)
A steel type is not particularly limited. The
steel type can employ any type such as, for example,
a two-phase structure type (for example, a structure
containing martensite in ferrite, or a structure
containing bainite in ferrite), a strain-induced
transformation type (a structure containing residual
austenite in ferrite), a hardened type (a martensite
structure), or a microcrystalline type (a structure
essentially made of ferrite).
[0026] In the present embodiment, a spot-welded
joint using the high-strength steel plate constituted
by whichever steel type, can suppress 'reduction and
fluctuation" of joint strength to realize a good
fracture appearance, so that it is possible to obtain
a welded joint with high reliability.
[0027] Note that a steel type of a steel plate to be
overlapped with the high-strength steel plate, is not
particularly limited as well. A steel plate of a
steel type different from the steel type of the highstrength
steel plate can also be employed. For
example, the steel plate to be overlapped with the
high-strength steel plate can also be set to a mild
steel plate. Further, the steel plate to be
overlapped with the high-strength steel plate can
also be a steel plate of a steel type which is the
same as the steel type of the high-strength steel
plate.
- 16 -
[0028] (Tensile strength)
A tensile strength of at least one piece of steel
plate (high-strength steel plate) out of a plurality
of pieces of overlapped steel plates is set to 750
MPa to 2500 MPa. Normally, as a tensile strength of
a high-strength steel plate increases, a high joint
strength is required. Although a cross tensile
strength ( C T S ) of a spot-welded joint increases in
proportion to a strength of a steel plate in the
class of 590 MPa to 780 MPa, it decreases in a steel
plate having a strength of 780 MPa or more.
[00291 If the tensile strength of the high-strength
steel plate is less than 750 MPa, the cross tensile
strength is high from the beginning, and further, a
load with respect to the spot-welded joint is small.
Accordingly, a problem regarding a deterioration of
fracture appearance in a welded portion and the joint
strength is hard to occur. Therefore, the tensile
strength of the high-strength steel plate is set to
750 MPa or more.
[0030] If the tensile strength of the high-strength
steel plate exceeds 2500 MPa, the suppression of
"reduction and fluctuation" of the joint strength
becomes difficult. Further, in accordance with this,
it becomes difficult to suppress the deterioration of
the fracture appearance in the welded portion, and to
suppress an occurrence of a defect or a crack inside
the nugget. Therefore, the tensile strength of the
high-strength steel plate is set to 2500 MPa or less.
- 17 -
[00311 Note that a tensile strength of a steel plate
to be overlapped with the high-strength steel plate
is not particularly limited as well. The steel plate
to be overlapped with the high-strcngth steel plate
can also be set to a high-strength steel plate whose
tensile strength is 750 MPa to 2500 MPa, and it can
also be set to a steel plate whose tensile strength
is less than 750 MPa. For example, if the steel
plate is a steel member used in the automobile field
and the like, the tensile strength thereof may be
selected in accordance with the steel member to be
used.
[0032 ] (Plate thickness)
A plate thickness of the high-strength steel
plate is not particularly limited. For example, a
plate thickness (0.5 mm to 3.2 mm) of a high-strength
steel plate used in general for a vehicle body or the
like of an automobile suffices. However, since a
stress concentration in a periphery of a nugget
increases in accordance with an increase in a plate
thickness of a high-strength steel plate, the plate
thickness of the high-strength steel plate is
preferably 2.6 mm or less.
[I30331 A plate thickness of a steel plate to be
overlapped with the high-strength steel plate is not
particularly limited. It is also possible that plate
thicknesses of a plurality of pieces of steel plates
to be overlapped are mutually different. For example,
when three pieces or more of steel plates are
- 18 -
overlapped, plate thicknesses of the respective three
pieces or more of steel plates may also be different
from one another. It is only required that at least
one piece of steel plate out of the three pieces or
more of steel plates is the high-strength steel plate,
and the other steel plates may also be mild steel
plates. Further, when three pieces or more of steel
plates are overlapped, plate thicknesses of at least
two pieces of steel plates may also be the same.
Note that generally, a thickness of a steel plate is
6 mm or less.
[0034] (Carbon equivalent Ceq)
A carbon equivalent Ceq of the high-strength
steel plate represented by the following expression
(1) is preferably within a range of 0.20 mass% to
0.55 mass%. If the carbon equivalent Ceq is less
than 0.20 mass%, it is not possible to obtain a
tensile strength of equal to or more than 750 MPa,
which is the lower limit value of the tensile
strength of the high-strength steel plate described
above. On the other hand, it is not preferable that
the carbon equivalent Ceq exceeds 0.55 mass%, since
the tensile strength exceeds 2500 MPa, which is the
upper limit value of the tensile strength of the
high-strength steel plate described above. The Ceq
of a steel plate to be overlapped with the highstrength
steel plate can take any value.
Ceq = [Cl + [Sil/30 + [Mn]/20 t 2 [PI + 4[S] ...(I)
[Cl, [Sil, [Mnl, [PI, and [Sl indicate respective
- 19 -
contents (mass%) of C, Si, Mn, P, and S.
[0035] (Composition of components)
It is only required to select a composition of
components capable of securing the tensile strength
(750 MPa to 2500 MPa) of the high-strength steel
plate described above. When considering that a steel
member after being subjected to spot welding is
mainly used in the automobile field and the like, the
composition of components of the high-strength steel
plate is preferably the following composition of
components. Note that in the description
hereinbelow, % means mass%.
[0036] ( (C: 0.07 mass% to 0.45 mass%) )
C is an element which increases a tensile
strength of steel. It is possible that the higher a
C content in the steel is, the higher a strength of a
nugget becomes. However, if the C content in the
steel is less than 0.07 mass%, it is difficult to
obtain a tensile strength of 750 MPa or more. On the
other hand, if the C content in the steel exceeds
0.45 mass%, a workability of the high-strength steel
plate is lowered. Therefore, the C content in the
high-strength steel plate is preferably 0.07 mass% to
0.45 mass%.
[0037] ((Si: 0.001 mass% to 2.50 mass%))
Si is an element which increases a strength of
steel by solid solution strengthening and structure
strengthening. However, if a Si content in the steel
exceeds 2.50 mass%, the workability of the steel is
- 20 -
-
lowered. Meanwhile, it is technically difficult to
reduce the Si content in the steel to less than 0.001
mass% industrially. Therefore, the Si content in the
high-strength steel plate is preferably 0.001 mass%
to 2.50 mass%.
[0038] ((Mn: 0.8 mass% to 5.0 mass%))
Mn is an element which increases a strength of
steel. However, if a Mn content in the steel exceeds
5.0 mass%, the workability of the steel deteriorates.
On the other hand, if the Mn content in the steel is
less than 0.8 mass%, it is difficult to obtain a
tensile strength of 750 MPa or more. Therefore, the
Mn content in the high-strength steel plate is
preferably 0.8 mass% to 5.0 mass%.
[00391 ((P: 0.03 mass% or less))
P is an element which causes embrittlement of a
nugget. If a P content in steel exceeds 0.03 mass%,
a crack in a nugget is apt to occur, and it is
difficult to obtain a sufficiently high joint
strength. Therefore, the P content in the highstrength
steel plate is preferably 0.03 mass% or less.
Note that it is not preferable, in terms of cost, to
reduce the P content in the steel to less than 0.001
mass%. Therefore, the P content in the high-strength
steel plate is preferably 0.001 mass% or more.
However, it is also possible to set the P.content in
the high-strength steel plate to less than 0.001
mass%.
[0040] ((S: 0.01 mass% or less))
- 21 -
S is an element which causes embrittlement of a
nugget. Further, S is an element which is bonded to
Mn to form coarse MnS, thereby hindering the
workability of steel. If a S content in the steel
exceeds 0.01 mass%, a crack in the nugget is apt to
occur, which makes it difficult to obtain a
sufficiently high joint strength. Further, the
workability of the steel is lowered. Therefore, the
S content in the high-strength steel plate is
preferably 0.01 mass% or less. Note that it is not
preferable, in terms of cost, to reduce the S content
in the steel to less than 0.0001 mass%. Therefore,
the S content in the high-strength steel plate is
preferably 0.0001 mass% or more. However, it is also
possible to set the S content in the high-strength
steel plate to less than 0.0001 mass%.
[00411 ((N: 0.01 mass8 or less))
N is an element which forms a coarse nitride to
deteriorate the workability of steel. Further, N is
an element which causes a generation of blowhole at a
time of welding. If a N content in the steel exceeds
0.01 mass%, the deterioration of the workability of
steel and the generation of blowhole are caused
prominently. Therefore, the N content in the highstrength
steel plate is preferably 0.01 mass% or less.
Note that it is not preferable, in terms of cost, to
reduce the N content in the steel to less than 0.0005
mass%. Therefore, the N content in the high-strength
steel plate is preferably 0.0005 mass% or more.
- 22 -
However, it is also possible to set the N content in
the high-strength steel plate to less than 0.0005
mass%.
[0042] ((0: 0.01 mass% or less))
0 is an element which forms an oxide to
deteriorate the workability of steel. If an 0
content in the steel exceeds 0.01 mass%, the
deterioration of the workability of steel is caused
prominently. Therefore, the 0 content in the highstrength
steel plate is preferably 0.01 mass% or less.
Note that it is not preferable, in terms of cost, to
reduce the 0 content in the high-strength steel plate
to less than 0.0005 mass%. Therefore, the 0 content
in the high-strength steel plate is preferably 0.0005
mass% or more. However, it is also possible to set
the 0 content in the high-strength steel plate to
less than 0.0005 mass%.
[0043] ((Al: 1.00 mass% or less))
Al is a ferrite stabilizing element and exhibits
an effect such as a suppression of precipitation of
cementite during a bainite transformation.
Accordingly, A1 is contained for controlling a steel
structure. Further, A1 also functions as a
deoxidizer. On the other hand, A1 is easily oxidized.
If an Al content exceeds 1.00 mass%, inclusions
increase, resulting in that the deterioration of the
workability of steel is apt to occur. Therefore, the
A1 content in the high-strength steel plate is
preferably 1.00 mass% or less.
- 23 -
-
[0044] The high-strength steel plate may selectively
contain the following elements according to need,
other than the above-described main elements.
LO0451 ((Ti: 0.005 mass% to 0.20 mass%))
((Nb: 0.005 mass% to 0.20mass%))
( ( V : 0.005 mass% to 0.20 mass%))
[0046] Ti, Nb, and V are elements which contribute
to an increase in a strength of steel by at least any
one of precipitation strengthening, fine grain
strengthening by a suppression of growth of a ferrite
crystal grain, and dislocation strengthening by a
suppression of recrystallization. However, if a
content of any of the elements in steel is less than
0.005 mass%, the effect of adding the elements is
difficult to be exhibited. On the other hand, if the
content of each of these elements in the steel
exceeds 0.20 mass%, the workability of the steel is
hindered. Therefore, it is preferable that the
contents of these elements in the high-strength steel
plate are respectively 0.005 mass% to 0.20 mass%.
[0047] ((B: 0.0001 mass% to 0.01 mass%))
B is an element which strengthens steel by
controlling a steel structure. However, if a B
content in the steel is less than 0.0001 mass%, the
effect of adding the element is difficult to be
exhibited. On the other hand, if the B content in
the steel exceeds 0.01 mass%, the effect of adding
the element is saturated. Therefore, the B content
in the high-strength steel plate is preferably 0.0001
- 24 -
mass% to 0.01 mass%.
[0048] ((Cr: 0.01 mass% to 2.0 mass%))
((Ni: 0.01 mass% to 2.0 mass%))
((Cu: 0.01 mass% to 2.0 mass%))
((Mo: 0.01 mass% to 0.8 mass%))
. [0049] Cr, Ni, Cu, and Mo are elements which
contribute to an improvement of strength of steel.
These elements can be used in place of a part of Mn
(strength improving element), for example. However,
if a content of any of the elements in the steel is
less than 0.01 mass%, no contribution is made for
improving the strength.
[0050] Therefore, it is preferable that the contents
of these elements in the high-strength steel plate
are respectively 0.01 mass% or more. On the other
hand, if the content of each of Cr, Ni, and Cu in the
steel exceeds 2.0 mass%, and if the Mo content in the
steel exceeds 0.8 mass%, problems sometimes occur at
a time of pickling or hot working. Therefore, it is
preferable that the content of each of Cr, Ni, and Cu
in the high-strength steel plate is 2.0 mass% or less.
Further, it is preferable that the Mo content in the
high-strength steel plate is 0.8 mass% or less.
[0051] (At least one kind of Ca, Ce, Mg, and REM
(rare earth metal): 0.0001 mass% to 1.0 mass% in
total)
[0052] Ca, Ce, Mg, and REM are elements which
contribute to an improvement of the workability of
steel by reducing a size of an oxide after
- 25 -
- -
deoxidation or a size of a sulfide existing in a hotrolled
steel plate. However, if contents of these
elements in the steel are less than 0.0001 mass% in
total, the effect of adding the elements is difficult
to be exhibited. On the other hand, if the contents
of these elements in the steel exceed 1.0 mass% in
total, the workability of the steel is reduced.
Therefore, it is preferable that the contents of
these elements in the high-strength steel plate are
0.0001 mass% to 1.0 mass% in total.
LO0531 Note that REM is an element which belongs to
a lanthanoide series, and REM and Ce can be added as
misch metals to molten steel in a stage of
steelmaking. Further, other than La and Ce, elements
of the lanthanoide series may be contained
compositely.
[00541 A balance other than the respective elements
described above in the high-strength steel plate may
be constituted of Fe and inevitable impurities. Note
that regarding any one of Cr, Ni, Cu, Mo, B, Ti, Ni,
and V described above, containing a very small amount
less than the above lower limit values as impurities
is tolerated. Further, regarding Ca, Ce, Mg, La, and
REM, containing a very small amount less than the
above lower limit values of the total amounts thereof
as impurities is tolerated.
[00551 The above is the explanation regarding the
composition of components of the high-strength steel
plate, and a composition of components of a steel
- 26 -
plate to be overlapped with the high-strength steel
plate may employ any composition of components.
[0056] (Plating)
A plating layer may be formed on a surface of the
high-strength steel plate. Further, it is also
possible that a plating layer is formed on a surface
of a steel plate to be overlapped with the highstrength
steel plate. As kinds of the plating layer,
there can be cited, for example, a Zn base, a Zn-Fe
base, a Zn-Ni base, a Zn-A1 base, a Zn-Mg base, a Pb-
Sn base, a Sn-Zn base, an Al-Si base, and the like.
[0057] As the high-strength steel plate including a
Zn-based plating layer, there can be cited, for
example, an alloyed hot-dip galvanized steel plate, a
hot-dip galvanized steel plate, an electrogalvanized
steel plate, and the like. When the plating layer is
formed on the surface of the high-strength steel
plate, a spot-welded joint exhibits an excellent
corrosion resistance. If the plating layer is a
galvanized layer alloyed on the surface of the highstrength
steel plate, an excellent corrosion
resistance is obtained, and further, an adhesiveness
of coating material becomes good.
LO0581 A weight of the plating layer is not
particularly limited as well. It is preferable to
set a weight of the plating layer on one surface of
the high-strength steel plate to 100 g/m2 or less. If
the weight of the plating layer on one surface of the
high-strength steel plate exceeds 100 g/m2, the
- 27 -
plating layer may hinder the welding. The plating
layer may be formed on only one surface or both
surfaces of the high-strength steel plate. Note that
an inorganic or organic coating film (such as, for
example, a lubricating coating film) or the like may
be formed on a surface layer of the plating layer.
Conditions same as the conditions regarding the
plating layer described above are applied to a steel
plate to be overlapped with the high-strength steel
plate.
[0059] Next, examples of a spot welding method will
be described.
[0060]
First, a first example of the spot welding method
will be described.
[Spot welding]
Fig. 1 is a diagram illustrating one example of
an arrangement of two pieces of steel plates
including at least one piece of high-strength steel
plate and welding electrodes when spot welding is
started. As illustrated in Fig. 1, steel plates 1A
and 1B are overlapped so that their plate surfaces
face each other. The overlapped steel plates 1A and
1B are sandwiched by welding electrodes 2A and 2B
from up and down directions, and by applying a
required pressurizing force, the welding electrodes
2A and 2B are energized.
[0061] Fig. 2 is a diagram schematically
illustrating one example of a nugget and a heat-
- 28 -
affected zone formed by the spot welding. Fig. 3 is
a diagram illustrating an example of first form of an
energization pattern when the energization is
performed on the welding electrodes. Note that in
this case, in order to simplify the explanation, a
case where two pieces of steel plates including at
least one piece of high-strength steel plate are
spot-welded, is cited as an example. However, as
described above, even in a case where three pieces or
more of steel plates including at least one piece of
high-strength steel plate are spot-welded, it is
possible to conduct the spot welding through a method
same as a method to be described below.
[ 0 0 6 2 1 The steel plates 1A and lB, and the welding
electrodes 2A and 2B are arranged in a manner as
illustrated in Fig. 1. Further, when energization is
performed in an energization pattern illustrated in
Fig. 3, for example, a nugget 3 is formed at a
boundary between the steel plates 1A and l B , as
illustrated in Fig. 2 . Further, a heat-affected zone
4 is formed in a periphery of the nugget 3. Note
that at least one of the steel plates 1A and 1B is
the above-described high-strength steel plate.
[ 0 0 6 3 ] The following is the explanation regarding
the energization pattern illustrated in Fig. 3. Note
that a current to be described below indicates a
current which flows between the welding electrode 2A
and the welding electrode 2B.
First, a current value is gradually increased
- 29 -
--
(up-sloped) from 0 (zero) until when it reaches a
value of a main welding current Ivl (kA) . Further,
main welding is performed under a state where the
current value is set to the value of the main welding
current Ivl (kA) . When the main welding is finished,
the current value is set to 0 (zero), and a state
where the current value is 0 (zero) is retained for a
cooling time after main welding (solidification time)
ts (msec). After the cooling time after main welding
ts (msec) passes, the current value is set to a value
of a post-energization current I p (kA), and a state
where the current value is the value of the postenergization
current Ip (kA) is retained for a postenergization
time tp (msec), thereby performing postenergization.
After the post-energization time tp
(msec) passes, the current value is set to 0 (zero).
Note that a retention time t~ (msec) indicated in
Fig. 3 corresponds to a time of retaining a
pressurizing force FE (N) after the post-energization
is finished, as will be described later.
Further, it is also possible to design such that
the current value is not gradually increased (upsloped)
from 0 (zero) until when it reaches the value
of the main welding current IT: (kA), and the current
value is immediately set to the value of the main
welding current Ivi (kA) .
[ 0 0 6 4 ] (Pressurizing force: FE)
The energization with the main welding current Iv:
is performed while pressurizing the overlapped
- 30 -
plurality of pieces of steel plates by the welding
electrodes 2A and 2B at the pressurizing force F,
satisfying the following expression (2).
1960 X h 5 F, 3920 X h ... (2)
h: plate thickness of steel plate (mm)
[0065] The pressurizing force FE of the welding
electrodes 2A and 28 with respect to the steel plates
1A and 1B, exerts a large influence on an occurrence
of a defect or a crack inside the nugget 3 and in the
heat-affected zone 4. If the pressurizing force FE is
less than "1960 x h" (N), a suppression of the
occurrence of the defect or the crack inside the
nugget 3 and in the heat-affected zone 4 becomes
difficult. As a result of this, it is not possible
to improve a fracture appearance of a spot-welded
joint, and thus it is difficult to achieve an
improvement of joint strength and a reduction in
fluctuation of joint strength.
[0066] On the other hand, if the pressurizing force
FE exceeds "3920 x h" (N), a region which is brought
into contact with each of the welding electrodes 2A
and 2B, in a region of the steel plates 1A and 1B, is
indented largely. Therefore, an external appearance
is impaired, and in addition to that, the joint
strength is lowered. Further, in order to obtain the
pressurizing force F, exceeding '3430 x h" (N), a
welding gun (a device which performs energization by
applying a pressurizing force to the welding
electrodes 2A and 2B) is required to have a robot arm
- 31 -
with a high stiffness. Therefore, in the present
embodiment, the pressurizing force FE of the welding
electrodes 2A and 2B with respect to the steel plates
1A and 1B is set to not less than "1960 x h" (N) nor
more than "3920 x h" (N).
[0067] Note that if diameters of tips of the welding
electrodes 2A and 2B become too large, surface
pressures at the tips of the welding electrodes 2A
and 2B are reduced. Accordingly, it becomes
difficult to improve the fracture appearance.
Further, it becomes difficult to achieve the
improvement of the joint strength and the reduction
in the fluctuation of the joint strength, which are
realized in accordance with the improvement of the
fracture appearance. Therefore, it is preferable
that the diameters of the tips of the welding
electrodes 2A and 2B are respectively about 6 mm to 8
[ 0 0 6 8 ] In the above expression (21, h indicates a
plate thickness of a steel plate (mm). Plate
thicknesses of two pieces of steel plates are
sometimes different (in an example illustrated in Fig.
2, plate thicknesses of the steel plates 1A and 18
are sometimes different). In this case, it is only
required to use an arithmetic average value of the
plate thicknesses of the two pieces of steel plates
(an arithmetic average value of the plate thickness
of the steel plate 1A and the plate thickness of the
steel plate lB), as 'h" in the above expression (2).
- 32 -
If spot welding is performed on a plurality of pieces
of steel plates whose number is three or more, a
total sum of plate thicknesses of the plurality of
pieces of steel plates is determined, and a value
obtained by halving the total sum may be used as "h"
in the above expression (2).
[ 00691 (Main welding current: IVI)
The welding electrodes 2A and 2B are energized
with the main welding current IVl while pressurizing
the steel plates 1A and 1B at the above-described
pressurizing force FE, to thereby perform the main
welding. The main welding current IW and a main
welding time (a time during which the main welding
current Im is flowed) are not particularly limited.
Lt is only required to employ a welding current and
an energization time which are nearly the sameaas a
welding current and an energization time
conventionally employed for stably obtaining a nugget
with a required size, as the main welding current IvT
and the main welding time.
[0070] Note that, for example, a square root of an
average value in the main welding time of values each
of which being a square of the main welding current
in the main welding time (specifically, an effective
value of the main welding current), or a maximum
value of the main welding current, can be employed as
the main welding current In.
[0071] As a spot welding equipment, a conventional
spot welding equipment commonly used can be used as
- 33 -
it is. Further, regarding welding electrodes and the
like, it is also possible to use conventional welding
electrodes as they are. A power supply is not
limited in particular as well, and an AC power supply,
a DC inverter, an AC inverter, or the like can be
used.
[0072] (Cooling time after main welding: ts)
The energization of the welding electrodes 2A and
2B with the main welding current IN is performed for a
predetermined time, and right after the energization
is finished, the energization is stopped while the
pressurizing force FE applied when performing the main
welding (when the energization with the main welding
current I vi~s conducted) is retained as it is.
Subsequently, that state is retained for a cooling
time after main welding ts (msec) satisfying the
following expression (3). Accordingly, a molten zone
is solidified from an outer periphery of the molten
zone (specifically, a boundary of the molten zone
with another region), to thereby form a shell-shaped
solidified region having an unsolidified region
remained inside thereof. Note that in the
description hereinbelow, the boundary of the molten
zone with the other region is referred to as a
melting boundary according to need.
7 X h + 5 5 ts 300 ... (3)
h: plate thickness of steel plate (mm)
[0073] Right after the finish of energization with
the main welding current I,,, the solidification of the
- 34 -
molten zone is started from the melting boundary.
Fig. 4 is a diagram schematically illustrating one
example of an appearance in the middle of
solidification of the molten zone which is solidified
to be a nugget.
When the welding electrodes 2A and 2B are
energized with the main welding current Iw while
pressurizing the steel plates 1A and 1 B at the
pressurizing force FE, a molten zone which is
solidified to be a nugget is formed. Thereafter,
when the energization is finished, the solidificat2on
is started from a melting boundary 3a, and after the
cooling time after main welding t~ passes, a
solidified region 5 is formed. At this moment, an
unsolidified region 6 remains inside the solidified
region 5. In a periphery of the solidified region 5,
a heat-affected zone 4 is formed.
[0074] The unsolidified region 6 is solidified to
form a nugget. In the present embodiment, postenergization
is started when the unsolidified region
6 exists. Specifically, the cooling time after main
welding ts determines a width (length in a plate
surface direction) of the solidified region 5 at the
time of starting the post-energization.
[0075] In a high-strength steel plate having a
relatively large carbon amount, a martensite
transformation occurs in a process of performing
cooling for the cooling time after main welding ts
after the main welding. At this time, when a prior
- 35 -
austenite grain is large, an apparent martensite
transformation temperature increases. When the
martensite transformation temperature increases,
automatic tempering (auto-temper) easily occurs.
Therefore, the toughness of the heat-affected zone 4
is improved by later-described post-energization. In
order to achieve this effect, the heat-affected zone
4 is required to be formed of an austenite single
phase. In order to realize that, the cooling time
after main welding ts has to be set to 300 (msec) or
less.
[0076] Further, if the cooling time after main
welding ts exceeds 300 (msec), a temperature is
lowered to enlarge the solidified region 5.
Therefore, the post-energization for a long time has
to be performed for obtaining an effect of postenergization
to be described later (effect of
structure improvement and segregation improvement) in
an outer peripheral portion of the nugget 3 and the
heat-affected zone 4 in the periphery of the nugget 3.
Accordingly, the productivity of the spot-welded
joint is lowered. As described above, the cooling
time after main welding ts exceeding 300 (msec) is not
realistic.
[0077] On the other hand, if the cooling time after
main welding ts is less than "7 X h + 5" (msec), the
solidification of the molten zone becomes
insufficient, resulting in that the width of the
solidified region 5 becomes narrow. Further, if the
- 36 -
cooling time after main welding ts is less than "7 X
h + 5" (msec), the prior austenite grain becomes too
large, resulting in that the toughness of the heataffected
zone 4 'is lowered, on the contrary, by the
post-energization to be described later. Therefore,
it is not possible to achieve the effect of postenergization
to be described later (effect of
structure improvement and segregation improvement),
resulting in that it becomes difficult to
sufficiently improve the joint strength. Further,
the larger the plate thickness h of the steel plate
is, the slower a cooling rate of the steel plates 1A
and 1B becomes. Generally, the larger the plate
thickness h of the steel plate is, the longer a
cooling time of the steel plates 1A and 1B becomes
exponentially. However, in a general thickness range
of a steel plate employed for a spot-welded joint
(0.5 mm to 3.2 mm, for example), a relationship
between the,cooling time of the steel plates 1A and
1B and the plate thickness h of the steel plate can
be linearly approximated. Therefore, in the present
embodiment, a lower limit value of the cooling time
after main welding ts is represented by a linear
expression using the plate thickness h of the steel
plate, as represented by the expression (3)
[ 0 0 7 8 1 Two pieces of steel plates with various plate
thicknesses each having the carbon equivalent Ceq
represented by the above expression (1) of 0.3 mass%
or more, were overlapped to be spot-welded by using a
- 37 -
servo gun-type welding machine and performing main
welding, cooling, and post-energization, in this
order, with various energization patterns.
Subsequently, the CTS (cross tensile strength) of
each spot-welded joint was measured based on a method
defined in JIS Z 3137. In the description
hereinbelow, the spot-welded joint is referred to as
a welded joint obtained by first unconventional
welding, according to need.
[ 0 0 7 9 1 Further, a spot-welded joint having a nugget
diameter same as a nugget diameter of the welded
joint obtained by first unconventional welding, was
obtained by overlapping two pieces of steel plates
each having the above-described carbon equivalent and
the above-described plate thickness, and performing
spot welding through a method same as the abovedescribed
method except for the performance of the
cooling after the main welding and the postenergization.
Subsequently, the CTS (cross tensile
strength) of each spot-welded joint was measured
based on the method defined in JIS Z 3137. In the
description hereinbelow, the spot-welded joint is
referred to as a welded joint obtained by first
conventional welding, according to need.
[ 0 0 8 0 ] The CTS in the welded joint obtained by first
unconventional welding, and the CTS in the welded
joint obtained by first conventional welding in which
no post-energization was performed, were compared.
Fig. 5 is a diagram illustrating one example of a
- 38 -
relationship between the cooling time after main
welding ts and the plate thickness h of the steel
plate.
In Fig. 5, a plot based on the cooling time after
main welding ts and the plate thickness h of the steel
plate when the CTS in the welded joint obtained by
first unconventional welding was improved by 20% or
more, when compared to the CTS in the welded joint
obtained by first conventional welding, is indicated
by 0. Further, a plot based on the cooling time
after main welding ts and the plate thickness h of the
steel plate when the CTS in the welded joint obtained
by first unconventional welding was improved but an
amount of improvement was less than 20% or when it
was not improved, when compared to the CTS in the
welded joint obtained by first conventional welding,
is indicated by A. As illustrated in Fig. 5, a
horizontal axis indicates h (mm), and a vertical axis
indicates ts (msec) .
In Fig. 5, a boundary line between 0 and A was
determined as a regression curve. From a result of
the determination, a linear expression defining the
lower limit value in the above expression (3) was
obtained.
[0081] From the above description, in the present
embodiment, the cooling time after main welding ts is
set to not less than '7 X h + 5" (msec) nor more than
300 (msec) .
Note that in order to avoid the reduction in
- 39 -
productivity of the spot-welded joint, it is more
preferable to set the cooling time after main welding
ts to not less than ' 7 X h + 5" (msec) nor more than
250 (msec). Further, in order to facilitate the
formation of the solidified region 5, it is
preferable that no energization is performed during
the cooling time after main welding ts. However, it
is also possible to energize the welding electrodes
2A and 2B with a current which is 0.5 times or less
the main welding current Ipl for the cooling time after
main welding ts for adjusting a formation speed and a
temperature of the solidified region 5.
Note that as the plate thickness h of the steel
plate in the expression ( 3 ) , a value same as the
value of the plate thickness h of the steel plate in
the above expression (2) is employed, for example
Further, it is preferable, in terms of working
efficiency, that the pressurizing force FE applied
when performing the main welding is retained as it is
during the cooling time after main welding ts.
However, it is also possible that the pressurizing
force FE during the cooling time after main welding ts
is different from the pressurizing force FE applied
when performing the main welding, within a range
satisfying the above expression (2).
[0082] (Post-energization current: IF, postenergization
time: tp)
When the cooling time after main welding ts is
passed, and right after the solidified region 5 with
- 40 -
a required width is formed, the welding electrodes 2A
and 2B are energized with a post-energization current
Ip (kA) satisfying the following expression (4) for a
post-energization time tp (msec) satisfying the
following expression (5) while retaining the
pressurizing force FE (N) applied when performing the
main welding, to thereby conduct post-energization.
[0083] 0.66 X Iv7 5 Ip < IVI ... (4)
Iv,: main welding current (kA)
48/(a2 - 0.44) tp ... (5)
a! = Ip/IyT
Therefore, the expression (5) can be rewritten as
follows.
4 8 / I P / I 2 - 0.44) S tp ... (5)
[00841 As described above, the pressuring force FE
during the post-energization time tp is set to the
pressurizing force satisfying the above expression
(2). It is preferable, in terms of working
efficiency, that this pressurizing force FE is
normally set to a pressurizing force same as the
pressurizing force FE applied when performing the main
welding (when the energization with the main welding
current Iw is performed), and when the molten zone is
solidified from the melting boundary to form the
shell-shaped solidified region 5 (during the cooling
time after main welding t,). However, as long as the
pressurizing force is within a range satisfying the
above expression (2), the pressurizing force FE during
the post-energization time t~ does not always have to
- 41 -
be the same pressurizing force as that applied when
performing these operations.
[0085] The post-energization current Ip exerts a
large influence on a structure and a segregation of
the shell-shaped solidified region 5, a structure and
a segregation of the nugget 3 formed after the
completion of solidification, and a structure and a
segregation of the heat-affected zone 4.
If the post-energization current Ip is less than
"0.66 X I,.," (kA), a heat input in the solidified
region 5 and the heat-affected zone 4 is insufficient,
resulting in that the effect of improving the
structure and the segregation (effect of structure
improvement and segregation improvement) cannot be
achieved.
[0086] On the other hand, if the post-energization
current Ip is equal to or more than the main welding
current I,., (kA), a temperature of the solidified
region 5 and the heat-affected zone 4 increases too
much. Further, the solidified region 5 is melted
again. Therefore, the effect of improving the
structure and the segregation (effect of structure
improvement and segregation improvement) cannot be
achieved.
Accordingly, in the present embodiment, the postenergization
current Ip is set to "0.66 X Iw" (kA) or
more and less than "I,.," (kA). Note that in order to
obtain the effect of improving the structure and the
segregation more securely, it is preferable to set
- 42 -
the post-energization current Ip to not less than
"0.70 X IvlU (kA) nor more than "0.98 X Ivl" (kA) .
Note that when an effective value is employed as the
main welding current Ivl, it is preferable that the
post-energization current Ip also employs an effective
value. Further, when a maximum value is employed as
the main welding current I , it is preferable that the
post-energization current Ip also employs a maximum
value.
[0087] In the spot welding method of the present
embodiment, the welding electrodes 2A and 2B are
energized with the post-energization current Ip for a
time satisfying the above expression (5) (postenergization
time tp (msec)) . Accordingly, the
structure and the segregation in the solidified
region 5 and the heat-affected zone 4 are improved,
to thereby increase the reliability of the welded
joint.
[0088] Regarding the post-energization time tp, there
is a description, in a paragraph COO871 in Patent
Literature 5, that "when the time exceeds 200 msec,
the effect of improving the joint strength and
reducing the fluctuation of joint strength becomes
small, and further, the productivity is lowered".
Specifically, Patent Literature 5 discloses that the '
post-energization time tp should be set to 200 (msec)
or less.
[0089] However, in recent years, the improvement of
CTS in high-strength steel plates is a pressing issue
- 43 -
--
Accordingly, the present inventors conducted earnest
studies regarding the method of further increasing
the joint strength of the spot-welded joint, without
sticking to conventional common sense and biased view.
[0090] Patent Literature 5 describes a structure
inside a nugget. However, no description is made
regarding an improvement plan of CTS when a plug
fracture occurs. Accordingly, the present inventors
conducted systematic experiments regarding postenergization
which further increases the CTS when the
plug fracture occurs.
[0091] As described in the section of (cooling time
after main welding: ts), two pieces of steel plates
with various plate thicknesses each having the carbon
equivalent Ceq represented by the above expression
(1) of 0.3 mass% or more, were overlapped to be spotwelded
by using a servo gun-type welding machine and
performing main welding, cooling, and postenergization,
in this order, with various
energization patterns. Subsequently, the CTS (cross
tensile strength) of each spot-welded joint was
measured based on the method defined in JIS Z 3137.
As described in the section of (cooling time after
main welding: ts), in the description hereinbelow, the
spot-welded joint is referred to as the welded joint
obtained by first unconventional welding, according
to need.
[0092] Further, as described in the section of
(cooling time after main welding: ts), a spot-welded
- 44 -
joint having a nugget diameter same as a nugget
diameter of the welded joint obtained by first
unconventional welding, was obtained by overlapping
two pieces of steel plates each having the abovedescribed
carbon equivalent and the above-described
plate thickness, and performing spot welding through
a method same as the above-described method except
for the performance of the cooling after the main
welding and the post-energization. Subsequently, the
CTS (cross tensile strength) of each spot-welded
joint was measured based on the method defined in JIS
Z 3137. As described in the section of (cooling time
after main welding: t,), in the description
hereinbelow, the spot-welded joint is referred to as
the welded joint obtained by first conventional
welding, according to need.
[ 0 0 9 3 ] The CTS in the welded joint obtained by first
unconventional welding, and the CTS in the welded
joint obtained by first conventional welding in which
no post-energization was performed, were compared.
Fig. 6 is a diagram illustrating a first example
of a relationship between the post-energization time
tP and a square of a value obtained by dividing the
post-energization current Ip by the main welding
current IW ( (IP/IvJ') ) .
In Fig. 6, a plot based on the post-energization
time tp, the post-energization current IP, and the
main welding current Iw, when the CTS in the welded
joint obtained by first unconventional welding was
- 45 -
improved by 20% or more, when compared to the CTS in
the welded joint obtained by first conventional
welding, is indicated by 0. Further, a plot based on
the post-energization time tp, the post-energization
current IP, and the main welding current Iv,, when the
CTS in the welded joint obtained by first
unconventional welding was improved but an amount of
improvement was less than 20% or when it was not
improved, when compared to the CTS in the welded
joint obtained byfirst conventional welding, is
indicated by A. As illustrated in Fig. 6, a
horizontal axis indicates (Ip/IW)2, and a vertical
axis indicates tp (ms) .
[0094] The plug fracture in the spot-welded joint
occurs in the heat-affected zone 4. Therefore, it
was estimated that a difference in plug fracture
strengths is generated by a difference in resistance
forces with respect to a propagation of crack in the
heat-affected zone 4, namely, a difference in
toughness of the heat-affected zone 4. Accordingly,
a concentration distribution of P and S exerting a
large influence on the toughness of the heat-affected
zone 4 was measured through FE-EPMA. As a result of
this, in Fig. 6, it was found out that in the heataffected
zone 4 of the welded joint obtained by first
unconventional welding obtained under the conditions
(the post-energization time tp, the post-energization
current Ip, and the main welding current IvT) indicated
by 0, the segregation of P and S is greatly reduced,
- 46 -
when compared to that in the heat-affected zone 4 of
the welded joint obtained by first conventional
welding. Specifically, it can be assumed that the
outer peripheral portion of the nugget 3 and the
heat-affected zone 4 were retained at a high
temperature by the heat generated through the postenergization,
resulting in that the segregation of P
and S was greatly reduced.
[ 0 0 9 5 1 If the solidified region 5 formed in the
inner pe'riphery of the melting boundary is melted
again by the post-energization, the segregation of P
and S increases in the resolidified resolidified
region, resulting in that the toughness of the nugget
3 is lowered. As a result of this, the fracture
occurs inside the nugget 3 even under a low load,
Therefore, the post-energization current Ip is
required to be a current of a value at which the
melting of the solidified region 5 does not occur.
Specifically, it is required to satisfy the condition
of I,, > Ip. The IP/Iv,i s an index of determining a
heat input amount when performing the postenergization
(a size of the nugget 3). Accordingly,
the Ip/Iw is expressed as a (< 1).
[ 0 0 9 6 1 The heat generated in the post-energization
is in proportion to a square of the post-energization
current Ip. Therefore, in Fig. 6, the horizontal axis
takes 1 / I 2 . Further, a part of the heat generated
in the post-energization is escaped to all over the
welding electrodes 2A and 2B and steel plates 1A and
- 47 -
1 B . A quantity of the heat to be escaped is set to 0 .
Accordingly, a heat quantity Q which acts on the
increase in temperature of the nugget 3 and the heataffected
zone 4 during the post-energization, can be
represented by the following expression (6).
Q (a2 - 8) x te ... ( 6 )
a = Ip/Iw
[00971 As described above, by determining the
cooling time after main welding ts as in the
expression (3), the automatic tempering caused by the
post-energization is apt to occur. There is a need
to perform energization with the post-energization
current Ip for a time required for improving the
toughness of the heat-affected zone 4 realized by the
automatic tempering.
[00981 Further, in order to reduce the segregation
of P and S in the outer peripheral portion of the
nugget 3 and the heat-affected zone 4 in the
periphery of the nugget 3 to sufficiently improve the
toughness of the nugget 3 and the heat-affected zone
4, a heat quantity exceeding a heat quantity A being
a predetermined quantity is required.
Therefore, a condition for improving the
toughness of the spot-welded zone is represented by
an expression ( 8 1 , which is obtained by modifying the
following expression (7).
A < (aZ - B ) x t~ ... (7)
A/(a2 - 8) < te ... (8)
[0099] In Fig. 6, a boundary line between 0 and A
- 48 -
was determined as a regression curve (specifically,
coefficients A and 6 in the expression (8) were
determined),. From a result of the determination, the
above expression (5) was obtained.
[01001 In order to realize the diffusion of P and S
to reduce the segregation of P and S in the outer
peripheral portion of the nugget 3 and the heataffected
zone 4, there is a need to heat the outer
peripheral portion of the nugget 3 and the heataffected
zone 4 to 1050°C or more. In order to
realize that, a condition satisfying a 0.66 is
required.
[01011 Fig. 7 is a diagram illustrating, in a
conceptual manner, one example of a relationship
between the post-energization time tp and a degree of
embrittlement of the outer peripheral portion of the
nugget 3 and the heat-affected zone 4. Fig. 7
illustrates, in a conceptual manner, a .sequence of
events in which the segregation of P and S is reduced
and the toughness is improved. In Fig. 7, a vertical
axis indicates a degree of embrittlement caused by
the segregation or insufficient automatic tempering.
As the value on the vertical axis is lowered, the
segregation is reduced and the automatic tempering is
sufficiently performed, resulting in that the
toughness is improved. A temperature in the outer
peripheral portion of the nugget 3 reaches a
substantially steady temperature (k melting point)
due to the main welding performed for forming the
- 49 -
welded portion, and thus is completely increased. On
the contrary, a temperature of the heat-affected zone
4 is not sufficiently increased by the main welding.
[0102] Further, when the post-energization is
started, the temperature 0% the heat-affected zone 4
is lower than the temperature of the outer peripheral
portion of the nugget 3 which is just solidified and
thus has a high temperature. For this reason, it
takes a long time to perform heat treatment by
retaining the heat-affected zone 4 at a high
temperature with the use of the post-energization,
when compared to a time required for performing heat
treatment on the outer peripheral portion of the
nugget 3. This can be estimated to be a reason why
the result in Fig. 6 can be obtained.
[0103] If the post-energization time tp is less than
200 (msec), a range of selection of the
aforementioned a = I P I becomes narrow (refer to
Fig. 6). Therefore, it is preferable to set the
post-energization time tp to 200 (msec) or more, and
it is more preferable to set the post-energization
time tp to 400 (msec) or more. Although an upper
limit value of the post-energization time tp is not
particularly defined, the upper limit value is
preferably 2000 (msec) or less, when the productivity
of the spot-welded joint is taken into consideration.
[0104] (Retention time: t ~ )
After the post-energization is performed under
the above-described conditions, the mutually
- 50 -
overlapped steel plates 1A and 1B are pressurized and
retained by the welding electrodes 2A and 2B for a
retention time tll (msec) defined by the following
expression (91, and then the pressurizing is released.
0 5 t11 5 300 ... (9)
[01051 Note that the pressurizing force FE (N)
applied when the steel plates 1A and 1B are
pressurized and retained by the welding electrodes 2A
and 2B for the retention time tH within the range
represented by the expression (91, is within a range
defined by the above expression ( 2 ) , for example.
[01061 The retention time t11 exerts an influence on
an occurrence of a defect or a crack in a structure
of the nugget 3 and the heat-affected zone 4 and
inside the nugget 3. When the retention time tH
exceeds 300 (msec), the productivity of the spotwelded
joint is lowered. Therefore, in the present
embodiment, the retention time t~ is set to 300 (msec)
or less. The retention time tH is desirably short, in
order to stably achieve a desired effect by starting
air cooling in an early stage.
E01071 Note that in an existing welding machine, an
operation of a welding gun is delayed, so that the
actual retention time tw normally becomes longer than
the set retention time tH. Therefore, there is a need
to take the above fact into consideration to set the
retention time tH.
Further, the temperature of the nugget 3 is
lowered also when the post-energization is performed.
- 51 -
Accordingly, even if the retention time t~ is
shortened, a contraction defect or a crack is
difficult to occur. Therefore, if it is possible to
immediately separate the welding electrodes 2A and 2B
from the steel plates 1A and lB, the retention time tH
may also be set to 0 (zero). When the retention time
is not set to 0 (zero), the expression (9) becomes
the following expression (9a).
0 < t~ 5 300 ... (9a)
[0108]
Next, a second example of the spot welding method
will be described. In the first example of the spot.
welding method, explanation was made by citing a case
where two times of energization including the main
welding and the post-energization are performed, as
an example. On the contrary, in the second example
of the spot welding method, explanation will be made
by citing a case where pre-energization is performed,
and then main welding and post-energization are
performed, as an example. As described above, the
present example is different from the first example
in a point that the pre-energization is added, and a
point that the above expression (5) can be changed
because of the addition of the pre-energization.
Therefore, in the explanation of the present example,
parts same as those of the first example are denoted
by reference numerals same as those in Fig. 1 to Fig.
7, to thereby omit detailed explanation.
[0109] Also in the present example, the steel plate
- 52 -
1A and the steel plate 1B are overlapped so that
their plate surfaces face each other, as illustrated
in Fig. 1. The overlapped steel plate 1A and steel
plate 1B are sandwiched by the welding electrode 2A
and the welding electrode 2B from up and down
directions, and the energization is performed by
applying a required pressurizing force. Also in the
present example, in order to simplify the explanation,
a case where two pieces of steel plates including at
least one piece of high-strength steel plate are
spot-welded, is cited as an example. However, even
in a case where three pieces or more of steel plates
including at least one piece of high-strength steel
plate are spot-welded, it is possible to conduct the
spot welding through a method same as a method to be
described below. For example, in a state where, out
of three pieces or more of steel plates including at
least two pieces of high-strength steel plates, the
at least two pieces of high-strength steel plates are
mutually overlapped, the three pieces or more of
steel plates can be spot-welded.
[01101 A high-strength steel plate generally has a
large electrical resistance, so that heat generation
is apt to occur when performing main welding.
Further, when performing main welding, a gap between
mutually adjacent two pieces of steel plates may
exist. If an internal pressure of a molten metal
exceeds an external pressure which acts on a corona
bond when performing main welding, an expulsion
- 53 -
occurs. One of objects of performing the preenergization
is to suppress the occurrence of
expulsion
[01111 Fig. 8 is a diagram illustrating an example
of second form of an energization pattern when
performing energization on welding electrodes.
First, a current value is set to a value of preenergization
current If (kA), and a state where the
current value is the value of pre-energization
current If (kA) is retained for a pre-energization
time tf (msec), to thereby perform pre-energization.
After the pre-energization time tf (msec) passes, the
current value is set to 0 (zero), and a state where
the current value is 0 (zero) is retained for a
cooling time after pre-energization tc (msec). After
the cooling time after pre-energization tc passes, the
main welding is performed under a state where the
current value is set to the value of the main welding
current It, (kA) . When the main welding is finished,
the current value is set to 0 (zero), and a state
where the current value is 0 (zero) is retained for
the cooling time after main welding (solidification
time) ts (msec) . After the cooling time after main
welding ts (msec) passes, the current value is set to
the value of the post-energization current Ip (kA),
and a state where the current value is the value of
the post-energization current Ip (kA) is retained for
the post-energization time tp (msec), thereby
performing the post-energization. After the post-
- 54 -
energization time tp (msec) passes, the current value
is set to 0 (zero). Note that the retention time tH
(msec) indicated in Fig. 8 corresponds to a time of
retaining the pressurizing force FE (N) after the
post-energization is finished, as described in the
first example. Further, it is also possible to
design such that, when the pre-energization is
started, the current value is not set to the value of
the pre-energization current If (kA) immediately, and
is gradually increased (up-sloped) from 0 (zero)
until when it reaches the value of the preenergization
current If (kA).
[0112] (Pressurizing force: FE)
The energization with the pre-energization
current If is performed while pressurizing the
overlapped plurality of pieces of steel plates by the
welding electrodes 2A and 2B at the pressurizing
force FE satisfying the above expression (2). In the
pre-energization, the overlapped plurality of pieces
of steel plates are pressurized to prevent a
generation of gap between the adjacent two pieces of
steel plates 1A and 1B. In the present embodiment, a
range of the pressurizing force FE in the preenergization
is set to a range same as the range of
the pressurizing force FE applied in the main welding
and the post-energization, thereby increasing the
working efficiency.
[0113] (Pre-energization current: If, preenergization
time: tr)
- 55 -
While pressurizing the steel plates 1A and 1B at
the pressurizing force FE described above,
energization with the pre-energization current If (kA)
satisfying the following expression (10) is performed
between the welding electrodes 2A and 2B for the preenergization
time tf (msec) satisfying the fbllowing
expression (ll), to thereby perform the preenergization.
[0114] 0.40 X I 5 Ir < 1r;i ... (10)
11.~m:a in welding current (kA)
20 i tf ( 11)
[0115] If the pre-energization current If is set to
equal to or more than the main welding current Ivl,
there is a possibility that the expulsion occurs when
performing the pre-energization. On the other hand,
if the pre-energization current If is set to less than
0.4 times the main welding current IVZa, quantity of
heat to be supplied to the steel plates 1A and 1B
becomes insufficient. Consequently, there is a
possibility that the steel plates 1A and 18 cannot be
softened, and it is not possible to sufficiently
reduce the gap between the steel plates 1A and 1B by
the above-described pressurizing, resulting in that
the expulsion occurs when performing the main welding
From the above description, in the present
embodiment, the pre-energization current If is set to
0.4 times or more the main welding current Ivr and less
than the main welding current Ivr. However, in order
to suppress the occurrence of expulsion more securely,
- 56 -
it is preferable to set the pre-energization current
If to fall within a range of 0.6 times or more the
main welding current IVTa nd 0.95 times or less the
main welding current Im.
Note that when an effective value is employed as
the main welding current IVl, it is preferable that the
pre-energization current If also employs an effective
value. Further, when a maximum value is employed as
the main welding current ITI, it is preferable that the
pre-energization current If also employs a maximum
value.
[0116] If the pre-energization time tr is less than
20 (msec), a quantity of heat to be supplied to the
steel plates 1A and 1B becomes insufficient.
Consequently, there is a possibility that the steel
plates 1A and 1B cannot be softened, and it is not
possible to sufficiently reduce the gap between the
steel plates 1A and 1B by the above-described
pressurizing, resulting in that the expulsion occurs
when performing the main welding.
When the main welding current Iri within the range
represented by the above expression (10) is employed,
even if the pre-energization time tf is long, it is
possible to suppress the occurrence of expulsion when
performing the main welding. Therefore, although an
upper limit value of the pre-energization time tf is
not particularly defined, the upper limit value is
preferably 300 (msec) or less, when the productivity
of the spot-welded joint is taken into consideration
- 57 -
-~
[0117] (Cooling time after pre-energization: tc)
When the energization with the pre-energization
current If is performed for the pre-energization time
tf, and right after the energization is finished, the
energization is stopped while retaining the
pressurizing force FE applied when performing the preenergization
(when the energization with the preenergization
current If is performed) as it is..
Subsequently, that state is retained for the cooling
time after pre-energization tc (msec) satisfying the
following expression (12).
0 5 tc < 200 + 7 X h ... (12)
h: plate thickness of steel plate (mm)
[01181 For the growth of corona bond, the cooling
time after pre-energization tc can be set to a time
exceeding 0 (zero). Note that if there is no
occurrence of expulsion when performing the preenergization,
it is possible to set the cooling time
after pre-energization tc to 0 (zero). Further, if
the cooling time after pre-energization tc becomes
"200 + 7 X h" (msec) or more, the steel plates 1A and
1B are cooled too much, resulting in that
conformability of the steel plates 1A and 1B may be
lost when performing the main welding. The larger
the plate thickness h of the steel plate is, the
slower the cooling rate of the steel plates 1A and 1B
becomes. As described in the section of (cooling
time after main welding: ts) in the first example, 'n
the general thickness range of the steel plate
- 58 -
--
employed for the spot-welded joint (0.5 mm to 3.2 mm,
for example), the relationship between the cooling
time of the steel plates 1A and 1B and the plate
thickness h of the steel plate can be linearly
approximated. Therefore, in the present embodiment,
an upper limit value of the cooling time after preenergization
tc is represented by a linear expression
using the plate thickness h of the ste.el plate, as
represented by the expression (12).
101191 Two pieces of steel plates with various plate
thicknesses each having the carbon equivalent Ceq
represented by the above expression (1) of 0.3 mass%
or more, were overlapped in which a gap was provided
to a part of region or a whole region between these
two pieces of steel plates in various appearances,
and spot-welded by using a servo gun-type welding
machine and performing pre-energization, cooling,
main welding, cooling, and post-energization, in this
order, with various energization patterns.
Subsequently, examination was made regarding whether
or not the expulsion occurs when performing the main
welding.
[01201 Fig. 9 is a diagram illustrating.one example
of a relationship between the cooling time after preenergization
tc and the plate thickness h of the steel
plate.
In Fig. 9, a plot based on the cooling time after
pre-energization tc and the plate thickness h of the
steel plate when the expulsion did not occur in the
- 59 -
aforementioned examination, is indicated by 0.
Further, a plot based on the cooling time after preenergization
tc and the plate thickness h of the steel
plate when the expulsion occurred in the
aforementioned examination, is indicated by A. As
illustrated in Fig. 9, a horizontal axis indicates h
(mm), and a vertical axis indicates tc (msec) .
In Fig. 9, a boundary line between 0 and A was
determined as a regression curve. From a result of
the determination, a linear expression defining the
upper limit value in the above expression (12) was
obtained.
[0121] Prom the above description, in the present
embodiment, the cooling time after pre-energization tc
is set to not less than 0 (zero) nor more than 200 +
7 X h" (msec) .
Note that as the plate thickness h of the steel
plate in the expression (121, a value same as the
value of the plate thickness h of the steel plate in
the above expression (2) is employed, for example.
Further, it is preferable, in terms of working
efficiency, that the pressurizing force FE applied
when performing the pre-energization is retained as
it is during the cooling time after pre-energization
tc. However, it is also possible that the
pressurizing force FE during the cooling time after
pre-energization tc is different from the pressurizing
force FE applied when performing the pre-energization,
within a range satisfying the above expression (2).
- 60 -
Further, when the cooling time after preenergization
tc is secured, the expression (12)
becomes the following expression (12a).
0 < tc < 200 + 7 X h ... (12a)
LO1221 (Main welding current: IVT)
Right after the cooling time after preenergization
tc passes, energization with the main
welding current Im is performed between the welding
electrodes 2A and 2B while retaining the pressurizing
force,FE applied when performing the pre-energization
as it is, to thereby conduct the main welding. As
explained in the first example, the main welding
current IW and the main welding time (the time during
which the energization with the main welding current
1i.i is conducted) are not particularly limited. Note
that it is preferable, in terms of working efficiency,
that the pressurizing force FE applied when performing
the pre-energization is retained as it is during the
main welding time. However, it is also possible that
the pressurizing force FE during the main welding time
is different from the pressurizing force FE applied
when performing the pre-energization, within a range
satisfying the above expression (2).
[0123] (Cooling time after main welding: ts)
When the energization with the main welding
current IF, is performed for a predetermined time, and
right after the energization is finished, the
energization is stopped while retaining the
pressurizing force FE applied when performing the pre-
- 61 -
energization and the main welding as it is. Further,
that state is retained for the cooling time after
main welding ts (msec) satisfying the above expression
( 3 ) -
A method of determining the cooling time after
main welding ts is a method same as that of the first
example. Note that as described in the first example,
in order to avoid the reduction in the productivity
of the spot-welded joint, the cooling time after main
welding ts is more preferably set to not less than "7
X h + 5" (msec) nor more than 250 (msec). Further,
in order to facilitate the formation of the
solidified region 5, it is preferable that no
energization is performed during the cooling time
after main welding ts, but, it is also possible to
perform energization with a current which is 0.5
times or less the main welding current IVFd uring the
cooling time after main welding ts for adjusting the
formation speed and the temperature of the solidified
region 5. Further, it is preferable, in terms of
working efficiency, that the pressurizing force FE
applied when performing the pre-energization and the
main welding is retained as it is during the cooling
time after main welding ts. However, it is also
possible that the pressurizing force FI: during the
cooling time after main welding ts is different from
the pressurizing force FE applied when performing the
pre-energization and the main welding, within a range
satisfying the above expression (2).
- 62 -
[0124] (Post-energization current: IP, postenergization
time: tp)
When the cooling time after main welding ts is
passed, and right after the solidified region 5 with
a required width is formed, energization with the
post-energization current Ip (kA) satisfying the
following expression (13) is performed for the postenergization
time tp (msec) satisfying the following
expression (14) while retaining the pressurizing
force FE (N) applied when performing the preenergization
and the main welding, to thereby conduct
the post-energization.
[0125] 0. 66 X I p r 5 Ip < IF{ ... (13)
I,: main welding current (kA)
48/(n2 - 0.4) tp ... (1 4 )
a: = Ip/I m
Therefore, the expression (14) can be rewritten
as follows.
48/(Ip/Ia)' - 0.4) I tp ... ( 14 )
[0126] The expression (13) is the same as the above
expression (4) . Specifically, a method of
determining the post-energization current Ip is a
method same as that of the first example. Note that
as described in the first example, the postenergization
current Ip is preferably set to not less
than "0.70 X IFlU (kA) nor more than "0.98 X I (kA) ,
in order to obtain the effect of improving the
structure and the segregation more securely. Further,
it is preferable, in terms of working efficiency,
- 63 -
that the pressurizing force FE applied when performing
the pre-energization and the main welding is retained
as it is during the post-energization time tp.
However, it is also possible that the pressurizing
force FE during the post-energization time tp is
different from the pressurizing force FE applied when
performing the pre-energization and the main welding,
within a range satisfying the above expression (2).
[0127] Two pieces of steel plates with various plate
thicknesses each having the carbon equivalent Ceq
represented by the above expression (1) of 0.3 mass%
or more, were overlapped to be spot-welded by using a
servo gun-type welding machine and performing preenergization,
cooling, main welding, cooling, and
post-energization, in this order, with various
energization patterns. Subsequently, the CTS (cross
tensile strength) of each spot-welded joint was
measured based on the method defined in JIS Z 3137.
In the description hereinbelow, the spot-welded joint
is referred to as a welded joint obtained by second
unconventional welding, according to need.
[0128] A spot-welded joint having a nugget diameter
same as a nugget diameter of the welded joint
obtained by second unconventional welding, was
obtained by overlapping two pieces of steel plates
each having the above-described carbon equivalent and
the above-described plate thickness, and performing
spot welding through a method same as the abovedescribed
method except for the performance of the
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cooling after the main welding and the postenergization.
Subsequently, the CTS (cross tensile
strength) of each spot-welded joint was measured
based on the method defined in JIS Z 3137. In the
description hereinbelow, the spot-welded joint is
referred to as a welded joint obtained by second
conventional welding, according to need.
[01291 The CTS in the welded joint obtained by
second unconventional welding, and the CTS in the
welded joint obtained by second conventional welding
in which no post-energization was performed, were
compared.
Fig. 10 is a diagram illustrating a second
example of a relationship between the postenergization
time tp and a square of a value obtained
by dividing the post-energization current Ip by the
main welding current IF, ( (Ip/Ipl') ) .
[01301 In Fig. 10, a plot based on the postenergization
time t,, the post-energization current Ip,
and the main welding current 1 when the CTS in the
welded joint obtained by second unconventional
welding was improved by 20% or more, when compared to
the CTS in the welded joint obtained by second
conventional welding, is indicated by 0. Further, a
plot based on the post-energization time tp, the postenergization
current Ip, and the main welding current
IT1, when the CTS in the welded joint obtained by
second unconventional welding was improved but an
amount of improvement was less than 20% or when it

CLAIMS
[Claim 11 A spot-welded joint formed by overlapping
a plurality of pieces of steel plates and performing
spot welding on the steel plates, the spot-welded
joint, comprising
a high-strength steel plate whose tensile
strength is 750 MPa to 2500 MPa, being at least one
piece of steel plate out of the pLurality of pieces
of steel plates, wherein:
a carbon equivalent Ceq of the high-strength
steel plate represented by the following expression
(A) is 0.20 mass% to 0.55 mass%; and
ten or more of iron-based carbides in each of
which a length of a longest portion is 0.1 (pm) or
more exist in a square region whose length of one
side is 10 (pm) in which a plate thickness direction
and a plate surface direction of the steel plates are
set to a vertical direction and a horizontal
direction, respectively, being a region within a
heat-affected zone of a cross section that passes
through a center of a welding mark formed on surfaces
of the steel plates by the spot welding, and is cut
along the plate thickness direction of the steel
plates, wherein
a position of a center of the square region is a
position, at the cross section, separated by 100 (urn)
from a position of an end portion of a nugget in a
direction perpendicular to a tangent to a line
indicating the end portion of the nugget, at that
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position, wherein
the position of the end portion of the nugget is
a position, out of positions on the line indicating
the end portion of the nugget, within a range whose
center is set to a center in the plate thickness
direction of the spot-welded joint and having a
length of 1/4 times a total plate thickness being a
total value of plate thicknesses of the plurality of
pieces of steel plates along the plate thickness
direction.
Ceq = [Cl + [Sil/30 + [Mn]/20 i 2[P] + 4[S] ... (A)
[Cl, [Sil, [Mnl, [PI, and [S] in the above
expression (A) indicate respective contents (mass%)
of C, Si, Mn, P, and S.
[Claim 21 A spot welding method of overlapping a
plurality of pieces of steel plates and performing
spot welding on the steel plates, wherein
at least one piece of steel plate out of the
plurality of pieces of steel plates is a highstrength
steel plate whose tensile strength is 750
MPa to 2500 MPa, wherein
a carbon equivalent Ceq of the high-strength
steel plate represented by the following expression
(A) is 0.20 mass% to 0.55 mass%,
the spot welding method comprising:
performing main welding of energizing welding
electrodes with a main welding current Iw (kA) in a
state where the overlapped plurality of pieces of
steel plates are pressurized by the welding
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electrodes at a pressurizing force FE (N) satisfying
the following expression (B);
performing, after the main welding is finished,
cooling after main welding of cooling the plurality
of pieces of steel plates for a cooling time after
main welding ts (msec) satisfying the following
expression (C) while retaining the pressurizing force
FE (N) satisfying the following expression (B);
performing, after the cooling after main welding
is finished, post-energization of energizing the
welding electrodes with a post-energization current Ip
(kA) satisfying the following expression (D) for a
post-energization time tp (msec) satisfying the
following expression ( E ) while retaining the
pressurizing force FE (N) satisfying the following
expression (B); and
retaining, after the post-energization is
finished, the pressurizing force FE (N) satisfying the
above expression (B) for a retention time tH (msec)
satisfying the following expression (F), and then
releasing the pressurizing at the pressurizing force
FE (N) .
Ceq = [Cl + [Sil/30 t [Mnl /20 + 2[P] + 4[S] ...
(A)
1960 X h 5 F, 5 3920 X h ... ( B )
7 X h + 5 ts 300 ... (C)
0.66 X Ivr 5 Ip < Ipl ... ( D )
48/( (1p/1v02 - 0.441 5 tp ... ( E )
0 5 tH 5 300 ... (F)
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[Cl, [Sil, [Mn], [PI, and [S] in the above
expression (A) indicate respective contents (mass%)
of C, Si, Mn, P, and S, and h in the above expression
(B), and the above expression ( C ) indicates a plate
thickness of the steel plate (mm) .
[Claim 31 A spot welding method of overlapping a
plurality of pieces of steel plates and performing
spot welding on the steel plates, wherein
at least one piece of steel plate out of the
plurality of pieces of steel plates is a highstrength
steel plate whose tensile strength is 750
MPa to 2500 MPa, wherein
a carbon equivalent Ceq of the high-strength
steel plate represented by the following expression
(A) is 0.20 mass% to 0.55 mass%,
the spot welding method comprising:
performing pre-energi zation of energizing welding
electrodes with a pre-energization current If (kA)
satisfying the following expression (C) for a preenergization
time tr (msec) satisfying the following
expression (D), in a state where the overlapped
plurality of pieces of steel plates are pressurized
by the welding electrodes at a pressurizing force FE
(N) satisfying the following expression (B) ;
performing, after the pre-energization is
finished, cooling after pre-energization of cooling
the plurality of pieces of steel plates for a cooling
time after pre-energization tc (msec) satisfying the
following expression (E) while retaining the
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pressurizing force FE (N) satisfying the following
expression (B) ;
performing, after the cooling after preenergization
is finished, main welding of energizing
the welding electrodes with a main welding current II.,
(kA) while retaining the pressurizing force FE (N)
satisfying the following expression ( B ) ; .
performing, after the main welding is finished,
cooling after main welding of cooling the plurality
of pieces of steel plates for a cooling time after
main welding ts (msec) satisfying the following
expression (F) while retaining the pressurizing force
FE (N) satisfying the following expression (B);
performing, after the cooling after main welding
is finished, post-energization of energizing the
welding electrodes with a post-energization current I~
(kA) satisfying the following expression (G) for a
post-energization time t, (msec) satisfying the
following expression ( H ) while retaining the
pressurizing force FE (N) satisfying the following
expression (B); and
retaining, after the post-energization is
finished, the pressurizing force FE (N) satisfying the
above expression (B) for a retention time t~ (msec)
satisfying the following expression (I), and then
releasing the pressurizing at the pressurizing force
FE (N) .
Ceq = [Cl + [Si]/30 + [Mn]/20 + 2 [ P I + 4 [ S ] ...
(A)
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1 9 6 0 X h FE 5 3920 X h ... (B)
0 . 4 0 X Iw 5 If < IV7.. . ( C )
20 5 tf ... (D)
0 tc < 200 + 7 X h ... (E)
7 X h + 5 5 ts 300 ... (F)
0 . 6 6 X I, 2 Ip < IP7 ... ( G )
4 / I * - 0 . 4 1 5 tp ... ( H )
0 5 t~ 300 ... (I)
[Cl, [Sil, [Mnl, [PI, and [Sl in the above
expression (A) indicate respective contents (mass%)
of C, Si, Mn, P, and S, and h in the above expression
( B ) , the above expression ( E ) , and the above
expression (F) indicates a plate thickness of the
steel plate (