DESCRIPTION
RESISTANCE SPOT WELDING METHOD AND WELDED STRUCTURE
TECHNICAL FIELD
[0001] The present invention relates to a resistance spot welding method suitable to join
three or more steel plates, and a welded structure obtained by the resistance spot welding
method.
BACKGROUND ART
[0002] In recent years, in a welding step of welding automotive components, a
resistance spot welding method is widely used in which a plurality of steel plates are
overlapped and then sandwiched by a pair of electrodes, and the overlapped body is then
energized while being pressed so as to form a molten pool (usually called "nugget") at the
steel plate interface and thereby to join the steel plates.
[0003] For example, a door opening portion of an automobile includes, as structural
members, a pillar and a roof rail. A pillar 20 includes an overlapped body 21 in which,
for example, three steel plates respectively constituting an outer steel plate, a reinforced
steel plate and an inner steel plate are overlapped (hereinafter referred to as "steel plates"
including the overlapped body). As shown in Figure 4, the overlapped body 21 is joined
by forming weld portions 23 at a predetermined interval by spot welding in a flange 22
of the overlapped body 2 1.
[0004] As the overlapped body described above, various combinations (hereinafter
referred to as "plate combinations") can be conceived by selecting the number of steel
plates and the material of the steel plates, but for some plate combinations, it may be
difficult to find appropriate welding conditions for obtaining a sound nugget (hereinafter,
such combinations will be referred to as "difficult-to-weld plate combinations"). As
used herein, "sound nugget" refers to a nugget having a sufficiently large molten pool and
exhibiting a sufficient joint strength in a tensile test or the like using a joined body (joint).
As the size of a weld portion, for example, generally, a value of 4dt or more (where t is
the thickness (rnm) of the thinner one of the steel plates forming the steel plate interface)
is used as the reference. A typical example of a difficult-to-weld plate combination is
an overlapped configuration of three steel plates including a thin steel plate, a thick steel
plate 1 and a thick steel plate 2 as used in the door opening portion of the body of an
automobile described above. Generally, for these members, a mild steel plate having a
thickness of 1.0 mrn or less is used as the thin steel plate, and high-tensile strength steel
plates having a thickness of 1.2 mm or more and a tensile strength of 340 MPa or more
are used as the thick steel plate 1 and the thick steel plate 2.
[0005] The reason that it is difficult to find appropriate welding conditions for obtaining
a sound nugget for the difficult-to-weld plate combination will be described below.
[0006] Specifically, if the amount of energization is too large with respect to the contact
electric resistance (hereinafter referred to simply as "contact resistance") at an interface
between steel plates that are in contact with each other, erosion called expulsion and
surface flash (also called spatter, splash or the like) caused as a result of the steel plate
interface being overheated due to resistive heating is likely to occur. If, on the other
hand, the amount of energization is too small, the amount of resistive heating at the steel
plate interface is reduced, and thus the molten pool cannot be made sufficiently large. In
the case of, for example, the overlapped body composed of three steel plates, there are
two steel plate interfaces, but the amount of resistive heating at a steel plate interface
between the thin steel plate and the thick steel plate 1 is relatively smaller than the amount
of resistive heating at a steel plate interface between the thick steel plate 1 and the thick
steel plate 2. For this reason, if welding is performed at one of the steel plate interfaces
by using the amount of energization that is preferable to obtain a sound nugget without
causing the expulsion and surface flash, the amount of resistive heating at the other steel
plate interface will inevitably be too large or small. As a result, the expulsion and
surface flash will be generated at the interface between the thick steel plate 1 and the thick
steel plate 2, or the nugget formed at the interface between the thin steel plate and the
thick steel plate 1 will be insufficient. For this reason, generally, the amount of
energization is set to be relatively high although the expulsion and surface flash occurs at
the interface between the thick steel plate 1 and the thick steel plate 2.
[0007] Patent Document 1 discloses a method for making a spot welded joint including
a nugget having a required size without causing the expulsion and surface flash in a
difficult-to-weld plate combination as described above. According to the invention
disclosed in Patent Document 1, a two-stage welding process including a first stage
welding process and a second stage welding process is performed, wherein the second
stage welding process is performed with a higher welding pressure, a lower current or the
same current, and a longer or the same energization time with respect to the first stage
welding process.
[0008] Other than the above, Patent Document 2 discloses a method for performing
welding in a difficult-to-weld plate combination, wherein the welding pressure of an
electrode chip on a workpiece is changed between the front surface and the back surface.
Patent Document 3 discloses a method in which the cooling effect of cooling water that
circulates through an electrode is alleviated by interposing a welding aid between the thin
steel plate and the electrode.
[0009] In a welding step of welding automotive components or the like with the use of
resistance spot welding, generally, a plurality of welding spots are provided consecutively
at locations required from the design point of view. Accordingly, when resistance
welding is performed at a given location, if there already is a welding spot near the
location (hereinafter referred to as "existing welding spot"), a branch current that flows
through the existing welding spot as an energization path is generated. Another case is
also conceived in which, an energization path is formed at a location other than the
existing welding spot and a branch current is generated depending on the geometric shape
of members and the arrangement of space with another member. As described above, if
the welding current is branched at the time of welding, formation of the molten pool is
delayed and thus a sound nugget cannot be obtained. The branch current is also called
reactive current, and various investigations have been made on the method for limiting
the influence thereof.
[0010] For example, Patent Document 4 discloses an invention in which a reactive
current is calculated and a current that is increased by an amount corresponding to the
calculated reactive current is set as a welding current. Patent Document 5 discloses a
method for obtaining a sound nugget by forming a slit so as to reduce the influence of
reactive current. Patent Documents 6 and 7 disclose inventions in which an auxiliary
electrode is provided near an electrode for the purpose of adjusting the branched state of
the welding current so as to give a proper heating condition near the welding spot.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENTS
100 1 11 Patent Document 1 : JP 2005-262259A
Patent Document 2: JP 2003-25 1469A
Patent Document 3: JP 2009-291 827A
Patent Document 4: JP H09-99379A
Patent Document 5: JP 2009-279597A
Patent Document 6: JP 2012-1 1398A
Patent Document 7: JP 20 12-55896A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0012] The invention of Patent Document 1 is problematic in that not only the
configuration of the welding apparatus and the welding conditions are complex, but also
due to the welding pressure used in the first stage welding process being low, if there is a
certain amount of gap at each interface between overlapped steel plates, it may not be
able to bring all of the interfaces of overlapped steel plates into contact by the welding
pressure of the first stage welding process, in which case the energization path is not
formed and welding cannot be performed at all. For this reason, there is a need for a
method which welding can be performed on a difficult-to-weld plate combination
irrespective of the presence or absence of a gap between steel plates.
[0013] The invention of Patent Document 2 requires special control to be performed
because it is necessary to change the welding pressure of each electrode based on the
contact resistance at each steel plate interface. The invention of Patent Document 3 can
alleviate the cooling effect of electrodes, but does not overcome the difficulties of welding
caused by the difference in contact resistance between steel plate interfaces.
[0014] Patent Documents 4 and 5 merely disclose methods for cancelling the reactive
current basically for a plate combination composed of two plates, and contain no
disclosure regarding a method with which welding can be performed on a difficult-toweld
plate combination.
Patent Documents 6 and 7 both disclose that the inventions thereof are
advantageous in that as a result of providing an auxiliary electrode near an electrode, a
branch current is generated, and thus the resistive heating can be increased in the steel
plate where the auxiliary' electrode is provided, and the molten pool can be made large.
[0015] However, the inventions of the above patent documents require an auxiliary
electrode to be provided near an electrode, which makes the operation mechanism of the
welder complex and the size of the electrode becomes larger than ordinary electrodes due
to the presence of the auxiliary electrode, which make it difficult to perform welding on,
for example, a narrow area such as a flange. In particular, these methods are intended
to increase the resistive heating of the steel plate where the auxiliary electrode is provided,
and thus a problem arises in that when spot welding is performed on, for example, an
overlapped body that is composed of three or more steel plates and in which at least one
steel plate interface has a contact resistance different from the contact resistance of
another steel plate interface, the molten pool cannot grow as large as intended unless a
steel plate interface having a small contact resistance is provided near the auxiliary
electrode. For this reason, there is a need for a technique for uniforming the amounts of
heat generation at a plurality of steel plate interfaces having different contact resistances
by using only a pair of opposing electrodes as with regular resistance spot welding,
without using an auxiliary electrode as described above.
[00 161 The present invention has been made to solve the problems described above, and
it is an object of the present invention to provide a resistance spot welding method with
which it is possible to form molten pools having a sufficiently large size in an overlapped
body that is composed of three or more steel plates and in which at least one steel plate
interface has a contact resistance different from that of another steel plate interface, while
suppressing the generation of the expulsion and surface flash, and a welded structure
obtained by the resistance spot welding method.
MEANS FOR SOLVING THE PROBLEMS
[OO 171 As shown in Figure 1 (a), when spot welding is performed on an overlapped body
10 that is composed of at least three steel plates la, lb and lc and in which at least one
steel plate interface 2a has a contact resistance different from the contact resistance of
another steel plate interface 2b so as to form molten pools 4a and 4b at the steel plate
interfaces 2a and 2b by energizing (see the solid white arrow) the overlapped body 10
while sandwiching and pressing the overlapped body 10 by a pair of electrodes 3a and 3b,
if the steel plate la is a mild steel plate and the steel plates lb and lc are high-tensile
strength steel plates, the steel plate interface 2a has a small contact resistance, and the
steel plate interface 2b has a large contact resistance. At this time, the amount of
resistive heating is larger at the steel plate interface 2b than at the steel plate interface 2a,
and thus the molten pool 4b at the steel plate interface 2b is formed faster than the molten
pool 4a formed at the steel plate interface 2a. Thus, as shown in Figure l(b), if
energization is performed based on the heated state of the steel plate interface 2b where
resistive heating is large, it is difficult to form a molten pool having a sufficiently large
size at the steel plate interface 2a where resistive heating is small. If, on the other hand,
the steel plate interface 2a where resistive heating is small is sufficiently heated, the steel
plate interface 2b where resistive heating is large is overheated, which causes a problem
in that the expulsion and surface flash is likely to occur at the steel plate interface.
[0018] To address this, the present inventors conducted intensive studies and
accomplished the present invention in which unlike a conventional technique that
generates a branch current at the outermost surface of the steel plates, a portion having a
small energization resistance (hereinafter referred to as "energization point") is formed at
a steel plate interface having a large contact resistance so as to generate a branch current
at the steel plate interface, whereby it is possible to make it difficult to generate the
expulsion and surface flash at the steel plate interface where resistive heating is large and
easily form a molten pool having a sufficiently large size at the steel plate interface where
resistive heating is small.
[0019] The present invention is intended to provide a resistance spot welding method
as described below and a welded structure obtained by the resistance spot welding method.
[0020] (1) A resistance spot welding method in which in an overlapped body that
includes three or more steel plates and in which at least one steel plate interface has a
contact resistance different from a contact resistance of another steel plate interface, a
molten pool is formed at the steel plate interfaces so as to join the steel plates, the
method including: a preliminary welding step of forming an energization point at a steel
plate interface having the largest contact resistance; and a main welding step of
performing initial spot welding under a condition in which a branch current is generated
in the energization point.
[0021] (2) The resistance spot welding method according to (1) described above, in
which in the main welding step, the initial spot welding is performed such that the
molten pool is formed at a position at a horizontal distance of 30 mrn or less from the
energization point.
[0022] (3) The resistance spot welding method according to (1) or (2) described
above, in which in the preliminary welding step, the energization point is formed at the
steel plate interface having the largest contact resistance by energizing the overlapped
body while sandwiching and pressing the overlapped body by a pair of electrodes.
[0023] (4) The resistance spot welding method according to any one of (1) to (3)
described above, in which in the main welding step, spot welding is further performed
repeatedly under a condition in which a branch current is generated in the energization
point or the molten pool.
[0024] (5) The resistance spot welding method according to any one of (1) to (4)
described above, in which in the main welding step, spot welding is further performed
repeatedly such that a molten pool is formed at a position at a horizontal distance of 30
mrn or less from the energization point or the molten pool.
[0025] (6) The resistance spot welding method according to any one of (1) to (5)
described above, in which the overlapped body consists of one mild steel plate and two
high-tensile strength steel plates.
[0026] (7) A welded structure obtained by a method according to any one of (1) to (6)
described above.
ADVANTAGEOUS EFFECTS OF THE INVENTION
LO0271 According to the present invention, even when spot welding is performed on an
overlapped body that is composed of three or more steel plates and in which at least one
steel plate interface has a contact resistance different from the contact resistance of
another steel plate interface, it is possible to make it difficult to generate the expulsion
and surface flash at a steel plate interface where resistive heating is large and form a
molten pool having a sufficiently large size at a steel plate interface where resistive
heating is small. In addition, if the overlapped body is, for example, a long automotive
structural member, with the present invention in which weld portions are consecutively
formed in the lengthwise direction, it is possible to obtain a high-strength structural
member.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] [Figure 11 Figure 1 is a conceptual diagram showing, as an example, a resistance
spot welding method according to a conventional technique.
[Figure 21 Figure 2 is a conceptual diagram showing, as an example, a resistance
spot welding method according to the present invention.
[Figure 31 Figure 3 is a conceptual diagram showing another example of a
resistance spot welding method according to the present invention.
[Figure 41 Figure 4 is a diagram showing the structural members of a door
opening portion of an automobile.
MODE FOR CARRYING OUT THE INVENTION
[0029] As shown in Figure 2(a), a resistance spot welding method according to the
present invention is a resistance spot welding method in which an overlapped body 10
that includes, for example, at least three steel plates la, lb and lc and in which at least
one steel plate interface 2a has a contact resistance different from the contact resistance
of another steel plate interface 2b is energized (see a solid white arrow C) while being
sandwiched and pressed by a pair of dome-shaped electrodes 3a and 3b so as to form
molten pools at the steel plate interfaces and thereby to join the steel plates la, lb and lc.
And, it is important to, in a preliminary step, form an energization point 5 at the steel
plate interface 2b having the highest contact resistance. Hereinafter, an example will be
described in which the energization point 5 has a circular or generally circular shape on
the steel plate interface. Also, the shape of the contact region between a steel plate
against which the tip of a dome-shaped electrode is pressed and the electrode is
substantially circular or generally circular.
[0030] In a main welding step, when initial spot welding is performed under a condition
in which a branch current is generated in the energization point 5, as shown in Figure 2(b),
a welding current C is branched (C2 = C - Cl), and thus it is possible to delay the
formation of the molten pool 4b at the steel plate interface 2b where resistive heating is
large without impeding the formation of the molten pool 4a at the steel plate interface 2a
where resistive heating is small. Since the present invention has a configuration as
described above, as shown in Figure 2(c), it is possible to form a sufficiently large molten
pool 4 at the steel plate interface 2a where resistive heating is small and the steel plate
interface 2b where resistive heating is large.
LO03 11 It is preferable that the initial spot welding is performed such that a molten pool
is formed at a position at a horizontal distance W 1 of 30 mm or less fiom the energization
point 5 (the distance between the center of the energization point and the center of the
weld portion). In particular, when it is particularly difficult to obtain heat generation in
the steel plate of the smallest thickness such as when the ratio between the smallest
thickness in the thicknesses of a plurality of steel plates overlapped and the total thickness
of all of the steel plates (hereinafter referred to as "thickness ratio") is large, the horizontal
distance W1 fiom the energization point 5 is preferably set to be 25 mm or less, more
preferably 20 mm or less, and even more preferably 15 rnrn or less. However, if the
distance is too short, the amount of branch current will be too large, causing a problem in
that the amount of resistive heating at the steel plate interface 2b where resistive heating
is large will be smaller than that at the steel plate interface 2a. Accordingly, the distance
is preferably set to 10 mm or more. The center of the energization point and the center
of the weld portion refer to the center of the contact region of the steel plate surface
pressed by the electrode.
[0032] Ln the example shown in Figure 2, a description has been given by taking an
overlapped body composed of three steel plates as an example, but the resistance spot
welding method according to the present invention is applicable to an overlapped body
that is composed of four or more steel plates and in which at least one steel plate interface
has a contact resistance different from that of the other steel plate interfaces. This is
because in the case of the overlapped body composed of four or more steel plates as well,
the molten pools at the steel plate interfaces where resistive heating is large are more
easily formed than the molten pools at the steel plate interface where resistive heating is
small, and thus by providing energization points as described above at the steel plate
interfaces having a large contact resistance, the formation of molten pools at the steel
plate interfaces can be delayed.
[0033] The size of the energization point 5 may be smaller or larger than a circle having
a diameter required for the molten pool 4 from the design point of view (for example, 4dt,
where t is the thickness (mm) of the thinner one of the steel plates forming the steel plate
interface). However, if the energization point 5 is too small, the influence of so-called
"constriction resistance" will be large, and a branch current flowing through the
energization point 4 may not be obtained sufficiently. Accordingly, the size of the
energization point 5 is preferably a size corresponding to a circle having a diameter of 1
mm or more.
[0034] In the embodiment described above, a description has been given of an example
of forming the energization point 5 at which the interface is fusion joined, but it is also
possible to form an energization point at which the interface is not fused and is thus in a
press contact state. In the case of forming the energization point in a press contact state,
it is preferable that the size of a press contact portion between the steel plate lb and the
steel plate lc (the portion serving as an energization point at the steel plate interface) is a
size corresponding to a circle having a diameter of 1 mm or more. Furthermore, in the
embodiment described above, an example has been described in which the shape of the
weld portions and energization point at the steel plate interfaces is circular or generally
circular, but the shape may be polygonal such as triangular or rectangular other than
circular or generally circular as long as constriction resistance is not generated. At this
time, the horizontal distance W1 and a horizontal distance W2 described later can be
measured assuming the centroids of the weld portion and the energization point as the
centers thereof.
[0035] In the example shown in Figure 2, both preliminary welding and main welding
may be performed in a state in which three steel plates are overlapped, but it is also
possible to, for example, perform preliminary welding in a state in which two steel plates
constituting a steel plate interface having the largest contact resistance are overlapped in
advance, and thereafter having another steel plate overlapped to the two steel plates to
perform main welding. From the viewpoint of production efficiency, it is preferable to
use a method in which both preliminary welding and main welding are performed in a
state in which three steel plates are overlapped.
100361 In the case of welding four or more steel plates (or in other words, in the case
where there are three or more steel plate interfaces) as well, both preliminary welding and
main welding may be performed in a state in which all of the steel plates are overlapped,
but it is also possible to form an energization point and a molten pool (it may be an
energization point) in an overlapped body made of three steel plates by any one of the
methods described above, and thereafter having another steel plate overlapped to the three
steel plates to perform main welding. The same applies to an overlapped body in which
five steel plates are overlapped. That is, in the case where N (where N > 3) steel plates
are overlapped, or in other words, in the case where the number of steel plate interfaces
is N - 1, preliminary welding is performed N - 2 times.
[0037] The resistance spot welding method according to the present invention is suitable
to perform welding on, in particular, an overlapped body composed of one mild steel plate
and two high-tensile strength steel plates. Such a plate combination is widely used in
automotive components as described above. In the case of the pillar shown in Figure 4,
a plate combination composed of a 0.5 to 0.8 mm mild steel plate (having a tensile
strength of, for example, 270 MPa grade) as the outer steel plate, a 1.2 to 2.0 mm hightensile
strength steel plate having a tensile strength of 980 to 1500 MPa grade as the
reinforced steel plate, and a 1.2 to 2.0 mm high-tensile strength steel having a tensile
strength of 340 to 780 grade is used. As used herein, "mild steel plate" refers to a steel
plate typically having a tensile strength of 270 MPa or more and less than 340 MPa, and
can be, for example, JAC270D (galvannealed steel plate having a tensile strength of 270
MPa grade) or the like. "High-tensile strength steel plate" refers to a steel plate typically
having a tensile strength of 340 MPa or more, and can be, for example, JSC590R,
JSC980DP or the like.
[0038] At this time, the steel plate la in the diagram is a mild steel plate, the steel plates
lb and lc are high-tensile strength steel plates, and the interface 2a between the mild the
steel plate 1 a and the high-tensile strength steel plate 1 b is a steel plate interface having a
small contact resistance, and the interface 2b between the high-tensile strength the steel
plates lb and lc is a steel plate interface having a large contact resistance.
[0039] In a preliminary welding step, the energization point 5 can be formed by
resistance spot welding under a condition of a smaller amount of resistive heating than
regular welding when the overlapped body 10 is energized while being sandwiched and
pressed by the pair of electrodes 3a and 3b so as to form molten pools at the steel plate
interfaces as with regular welding. The resistance spot welding is usually performed by
setting the amount of energization and the pressing force according to the material to be
welded, but the condition of smaller amount of resistive heating than regular welding
means to perform resistance spot welding under a condition in which the amount of
energization is set to be smaller or the pressing force is set to be larger than regular
welding. It is sufficient that the energization point has a resistance small enough to
obtain sufficient branch currents, and the energization point may be an energization point
at which the interface is fusion joined or an energization point at which the interface is
not fused and is thus in a press contact state.
[0040] For example, in the case of an overlapped body composed of a 0.6 mrn thick
mild steel plate (JSC270F), a 1.6 mm thick high-tensile strength steel plate (JSC980Y)
and a 1.6 mm thick high-tensile strength steel plate (JSC980Y), the main welding step is
performed by setting the pressing force to 3.43 kN, the current to 6.0 kA and the
energization time to 18 cycles (300 ms). However, if energization (main welding) is
immediately performed under the above-described conditions in a state in which there is
no energization point in the vicinity thereof, the expulsion and surface flash is very highly
likely to be generated. To address this, prior to the main welding step, a preliminary
welding step is performed in which energization is performed by setting the pressing force
to 3.43 kN, the current to 5.0 kA and the energization time to 6 cycles (100 ms), it is
thereby possible to form an energization point at the interface between the high-tensile
strength steel plates.
[0041] In the main welding step, as shown in Figure 2(d), furthermore, by performing
the initial spot welding such that a molten pool 6 is formed under a condition in which a
branch current is generated in the energization point 5 or the already-formed molten pool
4, the welding current C is branched (C2, C3) in the same manner as described above.
Here, in Figure 2(d), a branch current indicated by C3 is generated in the steel plate la.
As the amounts of energization flowing through the interfaces, as in Figure 2(c), the
amount of energization at the interface 2a is C1 + C2, but the amount of energization at
the interface 2b is C1 only, and thus the interface 2a has a larger amount of energization
than the interface 2b. Accordingly, the formation of a molten pool at the steel plate
interface 2b where resistive heating is large can be delayed without impeding the
formation of a molten pool at the steel plate interface 2a where resistive heating is small,
and thus it is possible to form the molten pool 6 having a sufficiently large size at the steel
plate interface 2a where resistive heating is small and the steel plate interface 2b where
resistive heating is large.
COO421 It is preferable that the molten pool 6 is formed at a position at a horizontal
distance W2 of 30 rnm or less from the molten pool 4 (the distance between the center of
the molten pool 4 and the center of the weld portion 6). Furthermore, spot welding may
be repeatedly performed at positions close to the weld portions 4 and 6 that have already
been formed so as to perform spot welding consecutively at a plurality of spots. For
example, the automotive structural member shown in Figure 4 can be produced by
repeating spot welding in an area within 30 mm from the molten pools 4 and 6 that were
formed earlier.
[0043] It is also possible to, as shown in Figure 3, form a molten pool 4 near two or
more energization points 5 by performing main welding under a condition in which a
branch current is generated. At this time, it is preferable to set horizontal distances W3
and W4 from the energization points to be 30 mm or less. A part or all of the
energization points may be made as another weld portion (not shown).
EXAMPLE 1
[0044] In order to verify the effects of the present invention, spot welding was
performed on one JAC270D steel plate (thickness: 0.7 mm, tensile strength: 270 MPa)
and two JSC590DP steel plates (thickness: 2.0 mm, tensile strength: 590 MPa) under
conditions shown in Table 1 (tests Nos. 1 to 5). In each example, main welding was
performed at a current value of 4.0 kA or more with an increment of 0.25 kA until the
expulsion and surface flash was generated, so as to investigate the minimum current value
at which all molten pools formed at each interface had a diameter o f 4 4 (t = 0.7 mm and
2.0 rnm) or more and the maximum current value at which the expulsion and surface flash
was not generated. The results are shown in Table 1, with a circle (0) indicating that the
difference between the maximum current value and the minimum current value was 1.0
kA or more, and a cross (x) indicating that the difference between the maximum current
value and the minimum current value was less than 1.0 kA. In the examples in which
preliminary welding was carried out, an energization point was formed at the interface
between two JSC590DP steel plates (the steel plate interface having the largest contact
resistance) in a state in which three steel plates were overlapped.
Note: 1 cycle = 1/60 second
Test
1
2
3
4
5
Main welding ste
Interval of spot Pressing Energintion Lention time Proper current
range @A)
o
o
o
X
x
Preliminary
(cycle)
10
10
10
10
I0
Pressing
force (k~)
3.43
3.43
3.43
3.43
welding ste
time (cycle)
20
20
20
20
20
positions (mm)
20 .
25
30
35
Current
1
7.8
7.8
7.8
7.8
Energintion
time (cycle)
20
20
20
20
force (kN)
3.43
3.43
3.43
3.43
3.43
Retention time
(cycle)
10
10
10
10
COO461 As shown in Table 1, in tests Nos. 1 to 3 in which preliminary welding was
carried out to form an energization point in advance and thereafter main welding was
carried out, a branch current was generated at the energization point during main welding,
and thus a proper current range of 1.0 kA or more was attained. It can be seen from this
that by using these conditions, it is possible to make it difficult to generate the expulsion
and surface flash at the steel plate interface where resistive heating is large and easily
form a molten pool having a sufficiently large size at the steel plate interface where
resistive heating is small.
[0047] In test No. 4 in which main welding was carried out without carrying out
preliminary welding, the proper current range was small. In test No. 5, although
preliminary welding was carried out, the position at which the molten pool was formed
by main welding was too far from the energization point, and thus a branch current was
not generated, as a result of which the proper current range was as small as less than 1.0
kA. Accordingly, if a molten pool having a sufficiently large size is attempted to be
formed at the steel plate interface where resistive heating is small by using these
conditions, the expulsion and surface flash will be easily generated at the steel plate
interface where resistive heating is large, which makes it dificult to perform management
in the actual operation.
EXAMPLE 2
[0048] Next, spot welding was performed on one JAC270D steel plate (thickness: 0.7
mrn, tensile strength: 270 MPa) and two high-tensile strength steel plates of 1180 MPa
grade (thickness: 1.6 mm, tensile strength: 11 80 MPa) under various conditions (tests Nos.
6 to 8). In each example, main welding was performed at a current value of 4.0 kA or
more with an increment of 0.25 kA until the expulsion and surface flash was generated,
so as to investigate the minimum current value at which all molten pools formed at each
interface had a diameter of 4Jt (t = 0.7 mm and 1.6 mm) or more and the maximum
current value at which the expulsion and surface flash was not generated.
[0049] (Test No. 6)
Main welding was carried out, without carrying out preliminary welding, under
the following conditions so as to form a molten pool:
Pressing force: 3.43 kN;
Current: 4.0 kA or more (with an increment of 0.25 kA);
Energization time: 20 cycles; and
Retention time: 10 cycles.
[0050] In test No. 6, the minimum current value was 6.0 kA, the maximum current value
was about 6.8 kA, and the proper current range was about 0.8 kA.
[0051] (Test No. 7)
Preliminary welding was carried out in a state in which three steel plates were
overlapped under the following conditions so as to form an energization point at the
interface between two high-tensile strength steel plates of 11 80 MPa grade (the steel plate
interface having the largest contact resistance):
Pressing force: 3.43 kN;
Current: 5.0 M;
Energization time: 20 cycles; and
Retention time: 10 cycles.
After that, main welding was carried out under the same conditions as those used
in test No. 6 so as to form a molten pool at a position corresponding to a horizontal
direction from the energization point of 15 mm. The conditions for main welding were
the same as those used in test No. 6.
[0052] In test No. 7, the minimum current value was 6.5 kA, the maximum current value
was 8.0 kA, and the proper current range was increased to 1.5 kA. Even in the case of
a plate combination including higher tensile strength steel plates, by performing
preliminary welding prior to main welding so as to form an energization point and then
performing the initial spot welding under a condition in which a branch current is
generated in the formed energization point, it is possible to make it difficult to generate
the expulsion and surface flash at the steel plate interface where resistive heating is large
and easily form a molten pool having a sufficiently large size at the steel plate interface
where resistive heating is small.
100531 (Test No. 8)
Preliminary welding was carried out in a state in which three steel plates were
overlapped under the following conditions so as to form an energization point:
Pressing force: 3.43 kN;
Current: 5 .O kA;
Energization time: 20 cycles; and
Retention time: 10 cycles.
After that, main welding was carried out under the following conditions so as to
form a first molten pool at a position corresponding to a horizontal distance from the
energization point of 15 mrn:
Pressing force: 3.43 kN;
Current: 7.5 kA;
Energization time: 20 cycles; and
Retention time: 10 cycles.
After that, furthermore, main welding was carried out under the following
conditions so as to form a second molten pool at a position corresponding to a horizontal
distance from the first molten pool of 15 mm:
Pressing force: 3.43 kN;
Current: 4.0 kA or more (with an increment of 0.25 kA);
Energization time: 20 cycles; and
Retention time: 10 cycles.
[0054] In test No. 8, the minimum current value was 7.0 kA, the maximum current value
was about 8.8 k . , and the proper current range was about 1.8 kA. In this way, by
performing spot welding under a condition in which a branch current is generated not
only in the energization point but also in the molten pool that has already been formed, it
is possible to make it difficult to generate the expulsion and surface flash at the steel plate
interface where resistive heating is large and easily form a molten pool having a
sufficiently large size at the steel plate interface where resistive heating is small.
INDUSTRLAL APPLICABILITY
COO551 According to the present invention, even when spot welding is perfonned on an
overlapped body that is composed of three or more steel plates and in which at least one
steel plate interface has a contact resistance different from that of another steel plate
interface, it is possible to make it difficult to generate the expulsion and surface flash at
the steel plate interface where resistive heating is large and easily form a molten pool
having a sufficiently large size at the steel plate interface where resistive heating is small.
Accordingly, the present invention is optimal in spot welding for producing, for example,
a plate combination or the like in which one mild steel plate and two high-tensile strength
steel plates are overlapped, in particular, an automotive structural member.
DESCRLPTION OF REFERENCE SIGNS
[0056] la, lb, lc Steel plate
2a, 2b Steel plate interface
3a, 3b Electrode
4,4a, 4b Molten pool
5 Energization point
6 Molten pool
10 Overlapped body
20 Pillar
2 1 Overlapped body
22 Flange
23 Weld portion
C Current
C 1, C2, C3 Current (branch current)
We claim:
1. A resistance spot welding method in which in an overlapped body that includes
three or more steel plates and in which at least one steel plate interface has a contact
resistance different from a contact resistance of another steel plate interface, a molten
pool is formed at the steel plate interfaces so as to join the steel plates, the method
comprising:
a preliminary welding step of forming an energization point at a steel plate
interface having the largest contact resistance; and
a main welding step of performing initial spot welding under a condition in
which a branch current is generated in the energization point.
2. The resistance spot welding method according to claim 1,
wherein in the main welding step, the initial spot welding is performed such
that the molten pool is formed at a position at a horizontal distance of 30 rnm or less
from the energization point.
3. The resistance spot welding method according to claim 1 or 2,
wherein in the preliminary welding step, the energization point is formed at the
steel plate interface having the largest contact resistance by energizing the overlapped
body while sandwiching and pressing the overlapped body by a pair of electrodes.
4. The resistance spot welding method according to any one of claims 1 to 3,
wherein in the main welding step, spot welding is further performed repeatedly
under a condition in which a branch current is generated in the energization point or the
molten pool.
5. The resistance spot welding method according to any one of claims 1 to 4,
wherein in the main welding step, spot welding is further performed repeatedly
such that a molten pool is formed at a position at a horizontal distance of 30 mrn or less
from the energization point or the molten pool.
6. The resistance spot welding method according to any one of claims 1 to 5,
wherein the overlapped body consists of one mild steel plate and two hightensile
strength steel plates.
7. A welded structure obtained by a method according to any one of claims 1 to 6.