DESCRIPTION
RESISTANCE SPOT WELDING METHOD AND WELDED STRUCTURE
TECHNICAL FELD
[0001] The present invention relates to a resistance spot welding method suitable to
join two 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 or the like,
a resistance spot welding method is widely used in which two 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 an interface between the steel plates 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 (see, for example, Figure 4) includes an
overlapped body 21 in which, for example, two steel plates are overlapped. 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 21.
[0004] As the overlapped body described above, various combinations (hereinafter
referred to as "plate combinations") can be conceived by selecting the material of the
two steel plates. At the time of performing resistance spot welding, the welding
pressure (pressing force) and the amount of energization are set so as to be appropriate
for each plate combination. With some plate combinations, it may be difficult to attain
a sufficient range (hereinafter referred to as "proper current range") of welding current
(hereinafter referred to as "proper current") that can provide a sound nugget without
causing erosion called expulsion and surface flash (also called spatter, splash or the like).
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). "Sufficiently large molten pool" refers to a molten pool having a
diameter larger than, for example, 4 4 (rnm) (where t is the thickness (mm) of the
thinner one of the two steel plates constituting a plate combination). The proper
current range is determined based on the conditions determined by equipment and
production constraints or the like (combined conditions of welding pressure and
energization time), and can be determined as the difference between the upper limit
value of the proper current (hereinafter referred to as "upper limit current") and the
lower limit value of the proper current (hereinafter referred to as "lower limit current").
It is generally believed that the proper current range is desirably wide because stable
welding quality can be attained even if disturbance (current fluctuations and the wear of
electrode tip or the like) occurs during welding.
[0005] Generation of the expulsion and surface flash during welding will degrade the
work environment and will cause a reduction in product quality with adhesion of
spatters onto the product surface. Furthermore, if an excessively large amount of the
expulsion and surface flash is generated, the volume of a fusion joined portion will
decrease, which significantly reduces the joint strength in the joined portion. For the
reasons given above, it is considered desirable to suppress the generation of the
expulsion and surface flash to the extent possible.
[0006] Under the circumstances, Patent Documents 1, 2, 3 and 4 disclose techniques
for increasing the diameter of nuggets while suppressing the generation of the expulsion
and surface flash by improving the conformance (contact state) of the contact plane
between steel plates so as to attain a sufficient contact area during energization. These
techniques can be construed as techniques that can raise the upper limit current.
[0007] Patent Document 5 discloses a technique for increasing the area of a corona
bond (a ring shaped portion formed around a nugget by solid state welding, see JIS Z
3001-6 2013) by pressing the periphery of a contact portion between a spot welding
electrode and a material to be welded by an insulation indenter. Patent Document 5
teaches that as a result of increasing the area of a corona bond, the same effects as those
obtained by increasing the nugget diameter can be obtained. This technique can be
construed as a technique that can prevent the generation of the expulsion and surface
flash by suppressing the welding current to a low level.
[0008] 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.
[0009] For example, Patent Document 6 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 7 discloses a
method for obtaining a sound nugget by forming a slit so as to reduce the influence of
reactive current.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENTS
[OO 101 Patent Document 1 : JP HI1 - 104849A
Patent Document 2: JP 2003-236674A
Patent Document 3: JP 20 10-207909A
Patent Document 4: JP 20 10-2472 15A
Patent Document 5: JP H07-178563A
Patent Document 6: JP H09-99379A
Patent Document 7: JP 2009-279597A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0011] All of the techniques disclosed in Patent Documents 1 to 4 require the welding
pressure andfor the amount of energization used at the time of welding to be changed
twice or more during welding, and thus are problematic in that the setting and
management of the proper current range are complex. The technique disclosed in
Patent Document 5 is problematic in that it requires an insulation indenter in addition to
spot welding electrodes, which makes the structure of the welder complex.
[0012] In the case of performing spot welding on a plate combination composed of
two mild steel plates, a sufficient contact area between steel plates can be attained
without performing a special step. Accordingly, the generation of the expulsion and
surface flash as described above is unlikely to occur. In contrast, in the case of
performing spot welding on a plate combination including high-tensile strength steel
plates, the probability of generation of the expulsion and surface flash increases. The
. .
present inventors contemplated the cause of such a phenomenon as follows.
Specifically, in a plate combination including high-tensile strength steel plates, it may
not be possible to attain a sufficient contact area at the contact interface between steel
plates during welding. In this case, resistive heating becomes excessively large at the
contact interface between steel plates, and the expulsion and surface flash is likely to be
generated. With this phenomenon, in the plate combination including high-tensile
strength steel plates, it may become difficult to sufficiently increase the value of the
current in which the expulsion and surface flash has been generated (upper limit
current) with respect to the lower limit current. In other words, it may become
difficult to attain a sufficient proper current range. In this case, it is difficult to form a
molten pool having a sufficiently large size while suppressing the generation of the
expulsion and surface flash. In this regard, the present inventors further conducted
detailed investigations and found out that it is difficult to attain a sufficient proper
current range when spot welding is performed on a plate combination that is composed
of two steel plates (hereinafter referred to as "first steel plate" and "second steel plate")
and that satisfies the following equation (i) while sandwiching the plate combination by
a pair of electrodes,
(TS1 x tl +TS2 x t2)/(tl + t2)>_440-(i),
In the equation (i), TS1 represents the tensile strength (MPa) of the first steel
plate, tl represents the thickness (mm) of the first steel plate, TS2 represents the tensile
strength (MPa) of the second steel plate, and t2 represents the thickness (rnm) of the
second steel plate.
[0013] Patent Documents 6 and 7 disclose methods for cancelling the reactive current.
However, these patent documents do not disclose conditions for attaining a sufficient
proper current range when spot welding is performed on a plate combination that
satisfies the above equation (i) while sandwiching the plate combination by a pair of
electrodes. Patent Document 7 shows an example in which spot welding was
performed on a SPCC and a 60k precipitation steel plate in Table 1, and an example in
which spot welding was performed on 60k precipitation steel plates in Table 2.
However, the example of Table 1 described above is not an example in which welding
was performed on a plate combination that satisfies the above equation (i). Likewise,
the example of Table 2 described above is an example in which welding was performed
on a plate combination by performing spot welding on one side thereof, and thus is not
an example in which spot welding was performed by sandwiching the plate combination
by a pair of electrodes.
[0014] The present invention has been made to solve the problems described above.
Specifically, it is an object of the present invention to provide a resistance spot welding
method with which it is possible to form a molten pool having a sufficiently large size
while suppressing the generation of the expulsion and surface flash at the time of
welding steel plates without requiring the welding pressure andlor the amount of
energization used at the time of welding to be changed twice or more during welding
and without making the structure of the welder complex, and a welded structure
obtained by the resistance spot welding method.
MEANS FOR SOLVING THE PROBLEMS
[0015] The present invention is intended to provide a resistance spot welding method
and a welded structure that are described below.
[0016] (1) A resistance spot welding method in which an overlapped body that consists
of a first steel plate and a second steel plate and that satisfies the following equation (i)
is energized while being sandwiched and pressed by a pair of electrodes so as to form a
molten pool at an interface between the steel plates and thereby to join the steel plates,
the method including: a preliminary step of forming an energization point at the
interface between the steel plates by energizing the overlapped body while sandwiching
and pressing the overlapped body by the pair of electrodes; and a welding step of
performing spot welding so as to form the molten pool at a position at a horizontal
distance of 20 mm or less fi-om the energization point,
(TS1 x tl +TS2 x t2)/(tl +t2)2440...(i)
where TS1 represents a tensile strength (MPa) of the first steel plate, tl
represents a thickness (mm) of the first steel plate, TS2 represents a tensile strength
(MPa) of the second steel plate, and t2 represents a thickness (mm) of the second steel
plate.
[0017] It is sufficient that the energization point has a small resistance enough to
obtain a sufficient branch current, 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. As used herein, "press
contact" does not mean "pressure welding", but rather means a state in which steel
plates are in contact with each other while being pressed against each other.
[0018] (2) The resistance spot welding method according to (1) described above,
further including a subsequent step of repeatedly performing spot welding such that a
new molten pool is formed at a position at a horizontal distance of 20 mm or less fi-om
the energization point or the molten pool.
[0019] (3) A welded structure obtained by the method according to (1) or (2) described
above.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0020] According to the present invention, when resistance spot welding is performed
on an overlapped body composed of two steel plates, it is possible to form a molten pool
having a sufficiently large size while suppressing the generation of the expulsion and
surface flash at the interface between the steel plates without changing the welding
pressure andfor the amount of energization used at the time of welding twice or more
during welding and without making the structure of the welder complex. 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
[0021] [Figure 11 Figure 1 is a diagram illustrating a resistance spot welding method
according to an embodiment of the present invention.
[Figure 21 Figure 2 is a diagram illustrating the resistance spot welding method
according to an embodiment of the present invention.
[Figure 31 Figures 3 is a diagram illustrating a resistance spot welding method
according to a reference example.
[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
[0022] Hereinafter, a resistance spot welding method according to the present
invention will be described. The resistance spot welding method according to the
present invention is used when an overlapped body that consists of a first steel plate and
a second steel plate and that satisfies the following equation (i) is spot welded,
(TS1 x tl + TS2 x t2) / (tl + t2) 2 440 ... (i).
In the equation (i), TS1 represents the tensile strength (MPa) of the first steel
plate, tl represents the thickness (mm) of the first steel plate, TS2 represents the tensile
strength (MPa) of the second steel plate, and t2 represents the thickness (mm) of the
second steel plate.
[0023] Figure 1 is a diagram illustrating a resistance spot welding method according to
an embodiment of the present invention, with Figure l(a) being a conceptual diagram
showing a preliminary step and Figure l(b) being a conceptual diagram showing a
welding step. As shown in Figure l(a), in the preliminary step of the resistance spot
welding method according to the present embodiment, first, an overlapped body 10
composed of a first steel plate la (hereinafter referred to as "steel plate la") having a
thickness tl and a second steel plate lb (hereinafter referred to as "steel plate lb")
having a thickness t2 is sandwiched by a pair of electrodes 2a and 2b of a welder. The
electrodes 2a and 2b are disposed so as to oppose to each other. Then, energization
(see a solid white arrow C) is performed across the electrodes 2a and 2b so as to
perform resistance spot welding. Through the preliminary step, an energization point
4a is formed at an interface 4 (hereinafter referred to as "steel plate interface 4")
between the steel plates la and lb. In the present embodiment, a molten pool is
formed as the energization point 4a. As the electrodes 2a and 2b, for example,
DR-type electrodes (DR40) having a tip diameter of 6 mm can be used.
COO241 Next, as shown in Figure l(b), as a welding step, spot welding is performed by
performing energization (see a solid white arrow C) across the electrodes 2a and 2b
such that a molten pool 4b is formed at a position away in the horizontal direction from
the energization point 4a by a distance W. At this time, a welding current C is
branched between the electrodes 2a and 2b, and a part of the welding current C flows
through the energization point 4a (see a solid white arrow C2, C2 = C - Cl). To be
specific, a current C1 traveling fi-om the electrode 2a directly to the electrode 2b and a
current C2 traveling from the electrode 2a through the energization point 4a to the
electrode 2b flow between the electrodes 2a and 2b. At this time, the molten pool 4b is
formed, by the current C1, at the steel plate interface 4 located between the electrodes
2a and 2b, and the steel plates la and lb are joined.
[0025] As a result of a part (current C2) of the welding current C passing through the
energization point 4a between the electrodes 2a and 2b as described above, heat
generation near the molten pool 4b can be facilitated, and the steel plates la and lb near
the molten pool 4b can be softened. For example, even in the case where the steel
plates 1 a and lb are high-strength steel plates, the hardness of the steel plates 1 a and lb
near the molten pool 4b can be lowered to a hardness similar to that of mild steel during
welding. Consequently, the steel plate interface 4 near the molten pool 4b can be
softened. To rephrase it, the conformance of the steel plate interface 4 near the molten
pool 4b can be improved (contact area can be enlarged). Accordingly, it is possible to
suppress the generation of the expulsion and surface flash at the time of welding steel
plates while forming the molten pool 4b having a sufficiently large size. In addition,
according to the present embodiment, it is possible to form the energization point 4a by
spot welding by the electrodes 2a and 2b. Accordingly, it is unnecessary to provide a
separate member (for example, an indenter or the like) for forming the energization
point 4a. That is, with the resistance spot welding method according to the present
embodiment, the molten pool 4b having a sufficient size can be formed while
suppressing the generation of the expulsion and surface flash at the time of welding
steel plates without changing the welding pressure andlor the amount of energization
used at the time of welding twice or more during welding and without making the
structure of the welder complex. Also, the generation of the expulsion and surface
flash during welding is suppressed, and thus the upper limit current (the maximum
value of a welding current that can provide a sound nugget without causing the
expulsion and surface flash) can be set sufficiently larger than the lower limit current
(the minimum value of the welding current). In other words, a proper current range
can be attained sufficiently.
[0026] In the present embodiment, the molten pool 4b is formed at a position at a
horizontal distance W of 20 mm or less from the energization point 4a (the distance
between the center of the energization point and the center of the molten pool). By
setting the distance W in this way, heat generation near the molten pool 4b can be
sufficiently facilitated, and the steel plates 1 a and 1 b can be efficiently softened near the
molten pool 4b.
[0027] In order to efficiently soften the steel plates la and lb near the molten pool 4b,
it is preferable to form the molten pool 4b at a position at a horizontal distance W of 15
mm or less from the energization point 4a. However, if the distance W is too short, the
current C2 flowing through the energization point 4a will be excessively large and the
molten pool 4b will be small. Accordingly, the horizontal distance W is preferably set
to 10 mm or more.
COO281 The welding conditions are adjusted as appropriate according to the thickness,
strength and the like of the steel plates la and lb. If the steel plates la and lb are 1.4
mm thick high-tensile strength steel plates of 590 MPa grade, for example, in the
preliminary step, the pressing force (welding pressure) of the electrodes 2a and 2b is set
to 3.5 kN, the welding current C flowing across the electrodes 2a and 2b is set to 3.0 kA
to 4.0 kA, the energization time is set to 16 cycles (267 msec), and the retention time
after energization is set to 10 cycles (167 msec). Likewise, in the welding step, the
pressing force (welding pressure) of the electrodes 2a and 2b is set to 3.5 kN, the
welding current C flowing across the electrodes 2a and 2b is set to 5.9 kA to 9.4 kA, the
energization time is set to 16 cycles (267 msec), and the retention time is set to 10
cycles (167 msec).
[0029] The diameter of the energization point 4a may be smaller or larger than the
diameter required for the molten pool 4b fiom the design point of view (for example,
4dt, where t is the thickness (mm) of the thinner one of the two steel plates constituting
a plate combination). However, if the energization point 4a is too small, the influence
of so-called "constriction resistance" will be large, and a branch current flowing through
the energization point 4a may not be obtained sufficiently. For this reason, the
diameter of the energization point 4a is preferably set to 1 mm or more.
[0030] If resistance spot welding is repeated, as a subsequent step following the
welding step described above, as shown in Figure 2, spot welding can be performed by
using, as an energization point, the molten pool 4b that has already been formed without
forming a new energization point. To be specific, the electrodes 2a and 2b are
disposed and energized such that a new molten pool 4c is formed at a position at a
horizontal distance W of 20 mm or less (preferably 10 mm or more and 15 mm or less)
fiom the molten pool 4b. In this case, a part (current C2) of the welding current C
flows through the molten pool 4b located between the electrodes 2a and 2b, and thus
heat generation near the molten pool 4c can be facilitated, and the steel plates la and lb
near the molten pool 4c can be softened. It is thereby possible to reduce the contact
resistance at the steel plate interface 4 near the molten pool 4c and prevent heat
generation in the molten pool 4c from being excessively large. As a result, the
generation of the expulsion and surface flash at the time of welding steel plates can be
prevented. In the subsequent step, spot welding can be further performed by using the
molten pool 4c as an energization point so as to form a new molten pool (not shown) at
a position at a horizontal distance W of 20 mrn or less (preferably, 10 mm or more and
15 mm or less) fiom the molten pool 4c. In the manner as described above, a plurality
of molten pools can be consecutively formed. For example, the automotive structural
member shown in Figure 4 can be produced by repeating resistance spot welding at a
position within a range of 20 mm from a molten pool formed earlier. In the subsequent
step, by using the energization point 4a that has already been formed, new molten pools
may be formed consecutively around the energization point 4a. It is also possible to
repeat resistance spot welding while forming a new energization point 4a, without
executing the subsequent step.
[0031] In the embodiment described above, a description has been given of an
example of forming the energization point 4a 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 as well, it is preferable to set the diameter of the energization point to 1
rnm or more as with the energization point 4a described above. To rephrase it, it is
preferable that the size of a press contact portion between the steel plate la and the steel
plate lb (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.
[0032]
Hereinafter, another example of a resistance spot welding method for joining a
plate combination by fonning energization points, which is different fiom the
embodiment of the present invention, will be described as a reference example. Figure
3 is a diagram illustrating a resistance spot welding method according to the reference
example. In the reference example, for example, as shown in Figure 3(a), a pair of
protruding portions 6a and 6b are provided on the underside of a steel plate la, and an
overlapped body 10 is sandwiched by electrodes 2a and 2b, with the protruding portions
6a and 6b being in contact with the upper surface of a steel plate lb. Next, as shown in
Figure 3@), as a preliminary step, the overlapped body 10 is pressed by the pair of
electrodes 2a and 2b, and as a welding step, energization (see a solid white arrow C) is
performed across the electrodes 2a and 2b while the overlapped body 10 is pressed by
the electrodes 2a and 2b so as to form a molten pool 4d at a steel plate interface 4. In
this way, the steel plates 1 a and 1 b can be joined.
[0033] In the reference example, the protruding portions 6a and 6b are brought into
press contact with the steel plate lb, and thus energization points 5a and 5b are formed
at contact portions between the protruding portions 6a and 6b and the steel plate lb.
As a result, a welding current C is branched between the electrodes 2a and 2b, and a
part (see solid white arrows C2 and C3) of the welding current C flows through the
energization points 5a and 5b. To be specific, a current C1 traveling from the electrode
2a directly to the electrode 2b, a current C2 traveling from the electrode 2a through the
energization point 5a to the electrode 2b, and a current C3 traveling from the electrode
2a through the energization point 5b to the electrode 2b flow between the electrodes 2a
and 2b. The currents C2 and C3 flow through the energization points 5a and 5b as
described above, and thus heat generation near the molten pool 4d can be facilitated,
and the steel plates la and lb near the molten pool 4d can be softened. It is thereby
possible to reduce the contact resistance at the steel plate interface 4 near the molten
pool 4d and prevent heat generation in the molten pool 4d fiom being excessively large.
As a result, it is possible to form the molten pool 4d having a sufficiently large size
while suppressing the generation of the expulsion and surface flash at the time of
welding steel plates. In the reference example as well, by executing a subsequent step
as described above, spot welding can be performed consecutively in an area in which
the protruding portions 6a and 6b are not formed.
[0034] In the reference example as well, the horizontal distance W between the
energization point 5a, 5b and the molten pool 4d (the distance between the center of the
energization point and the center of the molten pool) is set to 20 rnm or less, as with the
resistance spot welding method described with reference to Figures 1 and 2. By
setting the distance W in this way, heat generation near the molten pool 4d can be
sufficiently facilitated, and the steel plates la and lb near the molten pool 4d can be
efficiently softened. In the reference example, a horizontal distance between an axis of
the electrodes 2a and 2b and a tip portion (a contact portion in contact with the steel
plate lb) of the protruding portion 6a and a horizontal distance between the axis of the
electrodes 2a and 2b and a tip portion (a contact portion in contact with the steel plate
lb) of the protruding portion 6b are set to 20 mm or less. The molten pool 4d is
preferably formed at a position at a horizontal distance W of 10 mm or more and 15 mrn
or less &om the energization point 5a, 5b.
COO351 The reference example given above has been described taking an example in
which the protruding portions 6a and 6b are provided on the steel plate la, but it is
sufficient that the protruding portions are provided on at least one steel plate.
Accordingly, the protruding portions may be provided on the steel plate lb, instead of
the steel plate 1 a. The protruding portions may also be provided on the steel plates la
and lb. The number of protruding portions is not limited to that used in the
above-described example. It is possible to provide only one protruding portion, or
provide three or more protruding portions.
[0036] The press contact state of the press contact portions of the protruding portions
6a and 6b with respect to the steel plate lb may be changed to a fusion joined state
during welding. In this case as well, the function of the energization points 5a and 5b
is maintained. Also, instead of the protruding portions 6a and 6b, spacers having the
same size as the protruding portions 6a and 6b may be disposed and brought into press
contact with the steel plates la and lb so as to form energization points.
[0037] The resistance spot welding method according to the present invention is
suitable for use in welding an overlapped body that is composed of two high-tensile
strength steel plates and that satisfies the equation (i) given above, and also suitable for
use in welding an overlapped body (for example, automotive steel plate) that is
composed of a mild steel plate and a high-tensile strength steel plate and that satisfies
the equation (i) given above.
EXAMPLES
[0038] In order to verify the effects of the present invention, overlapped bodies having
various configurations were prepared, resistance spot welding was performed by using
the method described with reference to Figure 1, and a range of current value (proper
current range) that can form a molten pool having a sufficiently large size without
generating the expulsion and surface flash was measured. As the electrodes 2a and 2b,
DR-type electrodes (DR40) having a tip diameter of 6 mm were used. In the following
description, "molten pool having a sufficiently large size" refers to a molten pool having
a nugget diameter of 4Jt (rnm) or more (where t is the minimum value (mm) among the
thickness of the first steel plate and the thickness of the second steel plate).
[0039] In this example, as shown in Table 1 given below, four different types of
overlapped bodies (overlapped bodies Nos. 1 to 4) having different values on the left
side of the equation (i) given above were prepared. Then, resistance spot welding was
performed on each overlapped body by setting the distance W (see Figure l(b)) to 15
mm, 20 rnm, 25 mrn and 30 mm, and the proper current range was measured. In
addition, the proper current range when single-~ointw elding (conventional resistance
spot welding without performing preliminary welding) was performed on each
overlapped body was also measured. Table 1 below shows the configuration of each
overlapped body, welding conditions and the measured results of the proper current
range. Note that the proper current ranges under various conditions shown in Table 1
are normalized with the proper current range obtained at the time of single-point
welding being set to 1.00. In this example, it was decided that a sufficient proper
current range was attained when the proper current range was increased by 20% or more
with respect to that obtained at the time of single-point welding.
[0040]
[Table 1 ]
[0041] As shown in Table 1, in the overlapped bodies, by setting the distance W
between energization point and molten pool to 20 mm or less, the proper current range
was increased by 20% or more with respect to that obtained at the time of single-point
welding. It can be seen from this that according to the present invention, one of the
features of which is to set the distance between energization point and molten pool to 20
mm or less, the proper current range can be sufficiently increased with respect to that
obtained at the time of single-point welding. That is, it can be seen that according to
the present invention, it is possible to easily form a molten pool having a sufficiently
large size while suppressing the generation of the expulsion and surface flash.
100421 In the examples described above, a single energization method is used as an
energization condition, but the energization condition of the resistance spot welding
method according to the present invention is not limited to a single-stage energization
method. Also, the plate combinations (overlapped bodies) in which the present
invention is applied are not limited to plate combinations composed only of high-tensile
strength steel plates. That is, according to the present invention, the effect of
increasing the proper current range can be obtained in any plate combination that
satisfies the equation (i) given above.
INDUSTRIAL APPLICABILITY
[0043] According to the present invention, when resistance spot welding is performed
on an overlapped body composed of two steel plates, it is possible to sufficiently attain a
proper current range without changing the welding pressure andlor the amount of
energization used at the time of welding twice or more during welding and without
making the structure of the welder complex. Accordingly, the present invention is
optimal in resistance spot welding for producing a plate combination in which two steel
plates are overlapped, in particular an automotive structural member.
DESCRIPTION OF REFERENCE SIGNS
[0044] la, lb Steel plate
2a, 2b Electrode
4 Steel plate interface
4a, 5a, 5b Energization point
4b, 4c, 4d Molten pool
6a, 6b Protruding portion
10 Overlapped body
20 Pillar
2 1 Overlapped body
22 Flange
23 Weld portion
C Welding current
C1, C2, C3 Current
W Distance between energization point and molten pool
tl, t2 Thickness of steel plate
We claim:
1. A resistance spot welding method in which an overlapped body that consists of
a first steel plate and a second steel plate and that satisfies the following equation (i) is
energized while being sandwiched and pressed by a pair of electrodes so as to form a
molten pool at an interface between the steel plates and thereby to join the steel plates,
the method comprising:
a preliminary step of forming an energization point at the interface between the
steel plates by energizing the overlapped body while sandwiching and pressing the
overlapped body by the pair of electrodes; and
a welding step of performing spot welding so as to form the molten pool at a
position at a horizontal distance of 20 mm or less fiom the energization point,
(TSlxtl+TS2xt2)/(tl+t2)Z440-.(i) ,
where TS 1 represents a tensile strength (MPa) of the first steel plate, t 1
represents a thickness (mm) of the first steel plate, TS2 represents a tensile strength
(MPa) of the second steel plate, and t2 represents a thickness (rnm) of the second steel
. plate.
2. The resistance spot welding method according to claim' 1, further comprising
a subsequent step of repeatedly performing spot welding such that a new
molten pool is formed at a position at a horizontal distance of 20 mm or less fiom the
energization point or the molten pool.
3. A welded structure obtained by the method according to claim 1 or 2.