DESCRIPTION
TITLE OF INVENTION: SPOT WELDING METHOD OF HIGHSTRENGTH
STEEL SHEETS EXCELLENT IN JOINT STRENGTH
TECHNICAL FIELD
[0001] The present invention relates to a spot
welding method of high-strength steel sheets
excellent in joint strength, for forming a weld by
spot welding in, for example, processes of
manufacturing automobile parts, assembling a vehicle
body, and so on.
BACKGROUND ART
[0002] In recent years, in an automobile field,
there has been an increasing need for using highstrength
steel sheets for a vehicle body, parts, and
so on in order to reduce the weight of the vehicle
body with the aim of improving fuel efficiency and
reducing an emission of carbon dioxide gas (C02) and
to improve collision safety. Meanwhile, in processes
of assembling the vehicle body, attaching the parts,
and so on, spot welding is mainly used.
[0003] As important properties of a joint formed by
the spot welding (hereinafter, referred to as a spotwelded
joint), tensile strength and fatigue strength
can be cited, but what is important first is tensile
strength. Tensile strength of the spot-welded joint
includes tensile shear strength (TSS) measured under
tensile loads applied in shear directions and cross
tensile strength (CTS) measured under tensile loads
applied in peeling directions.
[00041 Generally, as for tensile strength of the
spot-welded joint, TSS and CTS with sufficiently high
values can be obtained with a good fracture state and
little variation of strength when there is no defect
or crack in weld metal (nugget) and the weld metal
has a good property. Further, as for tensile
strength of the spot-welded joint, it is possible to
improve CTS also by sufficiently securing its nugget
diameter (joint area) and so on.
As a method of improving cross tensile strength
of a spot-welded joint formed by spot welding of
high-strength steel sheets, there has been proposed a
spot welding method in which two-stage welding, twostage
welding including cooling in the middle, or
three-stage welding is performed under predetermined
conditions at the time of welding (for example, refer
to Patent Literature 1).
[ 0 0 0 5 ] Further, as a spot welding method that gives
high joint strength in a short time, there has been
proposed a method including: a main welding step of
obtaining a nugget with a predetermined diameter; and
a post-heating welding step of repeating suspension
for predetermined cycles and short-time welding while
sandwiching steel sheets with the same pressurizing
force as that in the main welding step (for example,
refer to Patent Literatures 2, 3).
[ 0 0 0 6 ] Further, as a spot welding method that gives
high joint strength, there has been proposed a method
including: a first step of forming a nugget in main
welding; a second step of cooling a weld while
keeping steel sheets sandwiched by electrodes, after
the end of the first step; and a third step of
passing a higher welding current than that of the
main welding within a range causing no expulsion for
a short time, after the end of the second step (for
example, refer to Patent Literature 4).
[ 0 0 0 7 ] Further, as a spot welding method that stably
gives high joint strength, there has been proposed a
method in which resistance spot welding is performed
as welding composed of three first, second, and third
stages, the welding in the second stage is performed
as welding with a higher pressurizing force, a lower
current or the same current, and a longer welding
time or the same welding time as compared with the
welding in the first stage, and in the third stage,
welding with a higher current than that of the second
stage is repeated (for example, refer to Patent
Literature 5).
[ 0 0 0 8 1 Further, as a spot welding method that stably
gives high joint strength, there has been proposed a
method in which resistance spot welding is performed
as welding composed of three first, second, and third
stages, the welding in the second stage is performed
as welding with a higher pressurizing force, a lower
current or the same current, and a longer welding
time or the same welding time as compared with the
welding in the first stage, and in the third stage,
welding with a higher pressurizing force and a higher
current than those of the second stage is repeated
(for example, refer to Patent Literature 6).
[00091 Here, spot welding of high-strength steel
sheets whose tensile strength is 780 MPa or more
especially has a problem that sufficient joint
strength cannot be obtained because a stress
concentrates on weld metal when dislocation (load) is
applied to a weld and the weld metal is loxi in
ductility and toughness. Further, the high-strength
steel sheets are high in specific resistance because
they contain many alloy elements, and therefore, even
when the same welding current is passed therein as in
the case of soft steel sheets, a heat generation
amount is larger than in the case of the soft steel
sheets. Further, due to their high strength, the
high-strength steel sheets are more difficult to fit
with electrodes than the soft steel sheets, and their
contact area becomes small. In such a case, since a
fusion diameter grows to a size equal to or more than
a contact size between the electrodes and the steel
sheets during the welding, the molten metal shoots
out from an overlapping surface of the steel sheets
and what is called expulsion occurs. The occurrence
of such expulsion has a problem of causing reduction
and variation of a size of the weld metal, that is,
of a joint area, leading to deterioration also of
joint strength. Further, in actual production, the
fact itself that a proper current range with which a
predetermined weld metal size is obtained without any
occurrence of expulsion is narrow is also a problem.
Therefore, in the spot welding of the high-strength
steel sheets, there is generally adopted a method
which increases the contact size (area) between the
electrodes and the steel sheets to prevent the
occurrence of expulsion by setting a pressurizing
force by the electrodes high, and increases a proper
current range, to thereby stably secure the joint
area.
[0010] However, when the high-strength steel sheets
are spot-welded while the pressurizing force of the
electrodes is set high, concavities generally called
indentations due to plastic deformation are generated
on surfaces of the stacked high-strength steel sheets
(refer to the indentations 4 in Fig. 1 and Fig. 2).
The indentations (concavities) being thus too large
cause a problem of lowering joint strength on the
contrary even when a nugget (refer to the nugget 3 in
Fig. 1 and Fig. 2) with a large diameter is obtained.
Therefore, in the spot welding of the high-strength
steel sheets, it has been considered to appropriately
decrease the pressurizing force of the electrodes
within a range enabling to obtain a predetermined
joint area in order to suppress the generation of
large indentations.
[00111 However, when the pressurizing force at the
time of the welding is set low in order to suppress
the generation of the indentations, the contact area
between the electrodes and the steel sheets during an
- 5 -
initial period of the welding reduces and a current
density in a weld zone becomes high, so that
expulsion is likely to occur. The expulsion is a
phenomenon that part of the molten metal scatters out
from the overlapping surface of the steel sheets at
the time of the spot welding. The occurrence of this
expulsion makes it difficult to secure the nugget
diameter with a necessary size at the time of the
spot welding, and sometimes promotes the generation
of the indentations contrary to the intention. In
such a case, there are problems that deterioration
and variation of joint strength occur. Further, the
expulsion sometimes adheres to the periphery of the
weld zone, and when its removal is necessary, there
is a problem that workability lowers.
[0012] Here, as a method to suppress the occurrence
of the expulsion at the time of the spot welding,
there has been proposed a method which adopts a
welding pattern including a first step with a welding
current, a second step of passing a welding current
that is 20 to 90% of that in the first step, and a
third step of further passing a welding current
larger than that in the first step (for example,
refer to Patent Literature 7).
[00131 Further, as a method to suppress the
occurrence of the expulsion at the time of the spot
welding, there has been proposed a method that adopts
a welding pattern including a first step with a
welding current, a second step where a welding
current that is 20 to 90% of that in the first step
is passed and a pressurizing force of electrodes is
set larger than that in the first step, and a third
step where a welding current and a pressurizing force
of the electrodes are set larger than those in the
first step (for example, refer to Patent Literature
8 ) .
[0014] Further, as a method of spot-welding hightension
steel sheets whose sheet thickness is 1.8 mm
or more and tensile strength is 580 MPa or more,
there has been proposed a method in which a
pressurizing force and a welding current value at the
time of the spot welding are set according to
conditions satisfying predetermined expressions, and
the spot welding is performed while expulsion is
generated (for example, refer to Patent Literature 9).
[0015] Further, as a method of resistance spotwelding
a sheet assembly composed of two stacked
thick sheets and a steel sheet with a small sheet
thickness stacked on an upper surface of the two
thick sheets, having sheet gaps between the steel
sheets, and having a large sheet thickness ratio,
there has been proposed a method in which, in an
advance step, a high pressurizing force is applied to
crush the sheet gaps and thereafter the pressurizing
force is reduced, and in a main step, the welding is
performed for a short time with a low pressurizing
force and a high current in an initial period of the
welding, and the welding is performed with a high
pressurizing force in a latter period of the welding
(for example, refer to Patent literature 10).
CITATION LIST
PATENT LITERATURE
[00161 Patent Literature 1: Japanese Laid-open
Patent Publication No. 2009-241086
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2010-115706
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2010-149187
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2010-172946
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2010-240739
Patent Literature 6: Japanese Laid-open Patent
Publication No. 2010-240740
Patent Literature 7: Japanese Laid-open Patent
Publication No. 2010-207909
Patent Literature 8: Japanese Laid-open Patent
Publication No. 2010-247215
Patent Literature 9: Japanese Laid-open Patent
Publication No. 2009-190046
Patent Literature 10: Japanese Laid-open Patent
Publication No. 2009-241112
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0017] However, in any of the aforesaid methods, a
technique to suppress the generation of a large
indentation is not proposed at all. Further, when a
condition to lower the pressurizing force of the
electrodes is employed in these conventional arts in
order to suppress the generation of the indentation,
there has been a problem that the expulsion is more
likely to occur. In such a case, reduction and
variation of the joint area occur and further the
generation of the indentation is promoted, and thus
there has been a problem that deterioration and
variation of joint strength occur.
On the other hand, increasing the pressurizing
force of the electrodes in order to suppress the
occurrence of the expulsion has a problem of
generating a large indentation to reduce joint
strength.
[00181 The present invention was made in
consideration of the aforesaid problems, and has an
object to provide a spot welding method of highstrength
steel sheets excellent in joint strength,
the method being capable of securing a nugget
diameter and preventing the occurrence of expulsion
while suppressing the generation of an indentation
especially when high-strength steel sheets with high
tensile strength are spot-welded.
SOLUTION TO PROBLEM
[00191 As a result of studious studies for solving
the aforesaid problems, the present inventors have
found out that, especially when high-strength steel
sheets whose tensile strength is not less than 780
MPa nor more than 1850 MPa (hereinafter, written as
780 to 1850 MPa) are welded by a resistance spot
welding method, by first setting a pressurizing force
of electrodes to a proper range according to a sheet
thickness of the steel sheets and further by setting
a welding pattern to a proper range, it is possible
to secure a nugget diameter and prevent the
occurrence of expulsion while suppressing the
generation of an indentation. That is, it has been
found out that it is possible to obtain a spot-welded
joint excellent in joint strength while maintaining
good workability, by optimally controlling the
welding pattern including pre-welding and main
welding and the pressurizing force of the electrodes,
and the present invention has been completed.
Specifically, the gists of the present invention
are as follows.
[0020] [l] A spot welding method of high-strength
steel sheets excellent in joint strength, the method
applying resistance spot welding to a stack of a
plurality of steel sheets,
wherein the plural steel sheets
are two steel sheets which both have tensile
strength of not less than 780 MPa nor more than 1850
MPa and whose sheet thickness ratio = {a sum of sheet
thicknesses of the steel sheets}/(the sheet thickness
of the thinner steel sheet (when the both have the
same thickness, the sheet thickness per one sheet))
is within a range of not less than 2 nor more than 5,
or
are three steel sheets which are three steel
sheets all having tensile strength of not less than
780 MPa nor more than 1850 MPa or which are two steel
sheets both having tensile strength of not less than
780 MPa nor more than 1850 MPa and one steel sheet
provided on an outer side of the two steel sheets and
having tensile strength of less than 780 MPa, and
whose sheet thickness ratio = (a sum of sheet
thicknesses of the steel sheetsJ/{the sheet thickness
of the thinner steel sheet (when the steel sheets all
have the same thickness, the sheet thickness per one
sheet)) is within a range of not less than 3 nor more
than 6,
wherein the spot welding includes: a first
welding step being pre-welding with a pressurizing
force P1 (kN) and a welding current I1 (kA); and a
second welding step being main welding with a
pressurizing force P2 (kN) and a welding current I2
(kA),
wherein the pressurizing forces PI, P2 are set to
a fixed pressurizing force P = Pl = P2 all through
the first welding step and the second welding step,
and are set within a range expressed by the following
expression ( I ) , where t (mm) is an average sheet
thickness of the plural steel sheets,
0.5 5 P s 3.0t ( 1 / 3 ) . . . (I),
wherein the welding current I1 is set within a
range of not less than 30% nor more than 90% of the
welding current 12, and
wherein t h e s e c o n d w e l d i n g s t e p is s t a r t e d w i t h i n
0 . 1 ( s ) a f t e r t h e f i r s t welding s t e p is f i n i s h e d .
[21 A s p o t w e l d i n g method o f h i g h - s t r e n g t h s t e e l
s h e e t s e x c e l l e n t i n j o i n t s t r e n g t h , t h e method
a p p l y i n g r e s i s t a n c e s p o t welding t o a s t a c k of a
p l u r a l i t y of steel s h e e t s ,
wherein t h e p l u r a l s t e e l s h e e t s
a r e two steel s h e e t s which b o t h have t e n s i l e
s t r e n g t h of n o t less t h a n 780 MPa n o r more t h a n 1850
MPa and whose s h e e t t h i c k n e s s r a t i o = { a sum of s h e e t
t h i c k n e s s e s of t h e s t e e l s h e e t s ) / { t h e s h e e t t h i c k n e s s
o f t h e t h i n n e r s t e e l s h e e t (when t h e b o t h have t h e
same t h i c k n e s s , t h e s h e e t t h i c k n e s s p e r one s h e e t ) ]
is w i t h i n a r a n g e of n o t less t h a n 2 n o r more t h a n 5 ,
o r
a r e t h r e e s t e e l s h e e t s which a r e t h r e e steel
s h e e t s a l l h a v i n g t e n s i l e s t r e n g t h of n o t less t h a n
780 MPa n o r more t h a n 1850 MPa o r which a r e two steel
s h e e t s b o t h h a v i n g t e n s i l e s t r e n g t h of n o t less t h a n
780 MPa n o r more t h a n 1850 MPa and one s t e e l s h e e t
p r o v i d e d on an o u t e r s i d e of t h e two steel s h e e t s and
h a v i n g t e n s i l e s t r e n g t h of l e s s t h a n 780 MPa, and
whose s h e e t t h i c k n e s s r a t i o = { a sum of s h e e t
t h i c k n e s s e s of t h e s t e e l s h e e t s l / [ t h e s h e e t t h i c k n e s s
of t h e t h i n n e r s t e e l s h e e t (when t h e s t e e l s h e e t s a l l
have t h e same t h i c k n e s s , t h e s h e e t t h i c k n e s s p e r one
s h e e t ) ] i s w i t h i n a r a n g e of n o t less t h a n 3 n o r more
t h a n 6,
wherein t h e s p o t welding i n c l u d e s : a f i r s t
welding step being pre-welding with a pressurizing
force P1 (kN) and a welding current I1 (kA); and a
second welding step being main welding with a
pressurizing force P2 (kN) and a welding current I2
(kA),
wherein the pressurizing forces P1, P2 are Set
within ranges expressed by the following expression
(2), expression (3), where t (mm) is an average sheet
thickness of the plural steel sheets,
0.5 P2 3.0t'l'~' . . . (2)
1.0 x P2 < P1 s 2.0 x P2 ... (3)
wherein the welding current I1 is set within a
range of not less than 30% nor more than 90% of the
welding current 12, and
wherein the second welding step is started within
0.1 (s) after the first welding step is finished.
[3] The spot welding method of the high-strength
steel sheets excellent in joint strength according to
[I], wherein any gap between the stacked steel sheets
before the spot welding is less than 0.5 (mm).
[4] The spot welding method of the high-strength
steel sheets excellent in joint strength according to
[2], wherein at least one of gaps between the stacked
steel sheets before the spot welding is 0.5 (mm) or
more.
[5] The. spot welding method of the high-strength
steel sheets excellent in joint strength according to
[I] or [2], the method including a third welding step
being post welding after the second welding step
being the main welding,
wherein, with a welding current and a welding
time of the third welding step being represented by
I3 (kA) and T3 (s) respectively, and with a nonwelding
time between the second welding step and the
third welding step being represented by TC (s),
the welding current I3 is set within a range of
not less than 3 (kA) nor more than 15 (kA),
the non-welding time TC is within a range of not
less than 0 (s) nor more than 0.2 (s), and
a relation between the welding current I3 and the
welding time T3 is set within a range expressed by
the following expression (4).
13 X T3 5 0.7 + TC ... ( 4 )
ADVANTAGEOUS EFFECTS OF INVENTION
[0021] According to the present invention, when the
high-strength steel sheets are welded by the spot
welding method, by setting the pressurizing forces of
the electrodes within the proper range(s) according
to the sheet thickness of the steel sheets and
optimally controlling the welding pattern including
the pre-welding and the main welding, it is possible
to secure the nugget diameter and also prevent the
occurrence of expulsion while suppressing the
generation of an indentation. Consequently, it is
possible to form a spot-welded joint high in
reliability and excellent in joint strength while
maintaining good workability. Therefore, for example,
in processes of manufacturing automobile parts,
assembling a vehicle body, and so on, it is possible
to fully enjoy merits such as fuel efficiency
improvement and an emission reduction of carbon
dioxide gas (COz) owing to a weight reduction of the
whole vehicle body, and their social contribution is
immeasurable.
BRIEF DESCRIPTION OF DRAWINGS
[0022] [Fig. 11 Fig. 1 is a cross-sectional view
illustrating a state where a weld metal zone is
formed by a resistance spot welding method in a case
where two high-strength steel sheets are stacked.
[Fig. 21 Fig. 2 is a cross-sectional view
illustrating a state where a weld metal zone is
formed by a resistance spot welding method in a case
where three steel sheets including high-strength
steel sheets are stacked.
[Fig. 31 Fig. 3 is a graph illustrating an
example of a pressurizing force and a welding pattern
when a weld metal zone is formed by a resistance spot
welding method in a first embodiment.
[Fig. 41 Fig. 4 is a graph illustrating an
example of a pressurizing force and a welding pattern
when a weld metal zone is formed by a resistance spot
welding method in a second embodiment.
[Fig. 51 Fig. 5 is a graph illustrating an
example of a pressurizing force and a welding pattern
when a weld metal zone is formed by a resistance spot
welding method in a third embodiment.
[Fig. 6Al Fig. 6A is a rough view illustrating a
method of measuring cross tensile strength.
[Fig. 6Bl Fig. 6B is a rough view illustrating a
method of measuring cross tensile strength.
[Fig. 71 Fig. 7 is a chart presenting a list of
fabrication conditions and test results of test
pieces in an example 1.
[Fig. 81 Fig. 8 is a chart presenting a list of
fabrication conditions and test results of test
pieces in an example 2.
[Fig. 9 ) Fig. 9 is a chart presenting a list of
fabrication conditions and test results of test
pieces in an example 3.
[Fig. 101 Fig. 10 is a chart presenting a list of
fabrication conditions and test results of test
pieces in an example 4.
[Fig. 11A] Fig. 11A is a rough view illustrating
a state where spacer steel sheets are used in a
method of measuring cross tensile strength.
[Fig. 11B] Fig. 11B is a rough view illustrating
the state where the spacer steel sheets are used in
the method of measuring cross tensile strength.
DESCRIPTION OF EMBODIMENTS
[0023] Hereinafter, a spot welding method of highstrength
steel sheets according to the present
invention will be described with reference to Fig. I
to Fig. 6 when necessary, taking first to third
embodiments as examples. In the present invention,
the high-strength steel sheet refers to one whose
tensile strength is 780 to 1850 MPa. Note that the
embodiments are described in detail for better
understanding of the spirit of the present invention
and therefore are not intended to limit the present
invention unless otherwise mentioned.
[0024] [Resistance Spot Welding Method]
Fig. 1 and Fig. 2 are explanatory schematic views
of a resistance spot welding method used for welding
steel sheets 1.
As illustrated in Fig. 1, first, two steel sheets
1A, 1B being materials to be welded are stacked. The
two steel sheets lA, 1B are both high-strength steel
sheets. Then, a current is passed to an overlapping
part of the steel sheets lA, 1B while electrodes 2A,
2B each made of a copper alloy are pressed against
the steel sheets lA, 1B from both sides, that is, so
as to sandwich them from up and down directions as
illustrated in Fig. 1, whereby a molten metal zone is
formed between the two steel sheets 1A, 1B. After
the welding is finished, this molten metal zone is
rapidly cooled to coagulate by heat removal by the
water-cooled electrodes 2A, 2B and heat transfer to
the steel sheets lA, lB, and a nugget (weld metal
zone) 3 having an elliptical cross sectional shape is
formed between the two steel sheets lA, 1B. The
formation of such a nugget results in the welding of
the two steel sheets lA, 1B.
Alternatively, as illustrated in Fig. 2, three
steel sheets lA, lB, 1C are stacked and a current is
passed while the electrodes 2A, 28 are pressed
similarly to the above, so that a nugget (molten
metal zone) 3 is formed among the three steel sheets
1A to lC, resulting in the welding of the three steel
sheets 1A to 1C. The three steel sheets lA, lB, 1C
are all high-strength steel sheets, or two of them
are high-strength steel sheets and the outer one
(steel sheet 1A) is a low-strength steel sheet
(tensile strength less than 780 MPa).
Note that, in the description below, the steel
sheets 1A to 1C are sometimes referred to simply as
the steel sheets 1 for convenience' sake.
[0025] In the welding by the resistance spot welding
described above, pressurizing forces by the
electrodes 2A, 2B and a welding pattern of prewelding
to main welding and in addition post-welding
are stipulated within proper ranges as described
below. Consequently, it is possible to prevent the
occurrence of expulsion while suppressing the
generation of an indentation, enabling to form a
highly reliable weld having sufficiently high
strength with good workability.
[0026] [First Embodiment]
Hereinafter, a first embodiment of the spot
welding method of the high-strength steel sheets
according to the present invention will be described.
The spot welding method of the high-strength steel
sheets of the first embodiment is a method of welding
two steel sheets 1A, 1B or three steel sheets 1A to
1C by resistance spot welding in order to obtain a
spot-welded joint 10 illustrated in Fig. 1 and Fig. 2.
Concretely, when the two steel sheets lA, 1B are
spot-welded as illustrated in Fig. 1, these two steel
sheets 1A, 1B both have tensile strength of 780 to
1850 MPa. Further, a sheet thickness ratio = (the
sum of sheet thicknesses of the steel sheets)/(the
sheet thickness of the thinner steel sheet (when the
both have the same thickness, the sheet thickness per
one sheet)) is within a range of not less than 2 nor
more than 5.
When the three steel sheets 1A to 1C are spotwelded
as illustrated in Fig. 2, these three steel
sheets 1A to 1C all have tensile strength of 780 to
1850 MPa, or the two sheets both have tensile
strength of 780 to 1850 MPa and the outer one sheet
has tensile strength of less than 780 MPa. A sheet
thickness ratio = (the sum of sheet thicknesses of
the steel sheets)/{the sheet thickness of the thinner
steel sheet (when they all have the same thickness,
the sheet thickness per one sheet)} is within a range
of not less than 3 nor more than 6.
The first embodiment assumes a case where any gap
in the stacked steel sheets 1 before the spot welding
(hereinafter, referred to simply as the gap) is less
than 0.5 (mm) .
[0027] In the first embodiment, the spot welding
includes: a first welding step being pre-welding with
a pressurizing force P1 (kN) and a welding current I1
(kA); and a second welding step being main welding
with a pressurizing force P2 (kN) and a welding
current I2 (kA).
The pressurizing forces PI, P2 are set to a fixed
pressurizing force P = P1 = P2 all through the first
welding step and the second welding step, and are set
within a range expressed by the following expression
(11, where t (mm) is an average sheet thickness of
the plural steel sheets 1.
0.5 P 3.0t"'~' . . . (1)
Further, the welding current I1 is set within a
range of not less than 30% nor more than 90%
(hereinafter, written as 30 to 90%) of the welding
current 12.
Further, the second welding step is started
within 0.1 (s) after the first welding step is
finished.
[0028] "Reasons for Limiting Properties of Steel
Sheets"
Hereinafter, reasons for limiting the properties
of the steel sheets 1 (the high-strength steel sheets
lA, 1 B illustrated in Fig. 1 or the steel sheets 1A
to 1C illustrated in Fig. 2) being objects to be
welded will be described in detail.
[0029] (Tensile Strength: 750 to 1850 MPa)
The steel sheets 1 being the objects to be welded
include two high-strength steel sheets or more each
having tensile strength of 780 to 1850 MPa.
The strength of the steel sheets has a great
influence on a stress concentration state on a weld,
and thus has an influence also on fracture state
deterioration and accompanying strength variation and
strength deterioration. When tensile strength of the
steel sheets is less than 780 MPa, these problems are
difficult to occur, and when it is over 1850 MPa, it
becomes difficult to make improvements for preventing
the deterioration and variation of joint strength.
[0030] As described above, the case where the highstrength
steel sheets having tensile strength within
the range of 780 to 1850 MPa and capable of realizing
a weight reduction and improvement of collision
safety is a target. A basic property of such steel
sheets for ensuring both strength and moldability is
high strength and in some cases, a high carbon
equivalent. However, as a result, hard martensite is
formed in a weld metal zone and a heat-affected zone.
If the heat-affected zone around the weld metal zone
is hard and further strength of base metal is high,
the deformation of the heat-affected zone and the
base metal around it is difficult to occur, and
accordingly a stress concentration on the weld metal
zone is likely to occur. Since such a case has a
problem of causing fracture state deterioration,
variation and deterioration of strength, and so on of
the spot-welded joint, it is necessary to solve these
problems in the practical application.
Therefore, the properties of the steel sheets are
first stipulated to conditions described below, and
then the spot welding is performed under various
welding conditions which will be described in detail
later. Consequently, even when the high-strength
steel sheets are spot-welded, it is possible to
obtain a good fracture state of the spot-welded joint
and prevent variation and deterioration of strength
while suppressing the occurrence of an indentation,
enabling to form a highly reliable weld.
[0031] (Steel Type)
A steel type of the steel sheets 1 being the
objects to be welded is not particularly limited, and
for example, the steel sheets 1 may be of any type
such as a two-phase structure type (for example, a
structure containing martensite in ferrite, a
structure containing bainite in ferrite, or the like),
a strain-induced transformation type (a structure
containing residual austenite in ferrite), a hardened
type (martensite structure), or a microcrystalline
type (structure essentially made of ferrite). Even
when the high-strength steel sheets are of any of the
steel types, applying the present invention makes it
possible to weld the high-strength steel sheets
without impairing the properties of the steel sheets
while suppressing the generation of an indentation
and suppressing the occurrence of expulsion at the
time of the spot welding. Consequently, a highly
reliable spot-welded joint (weld) with a good
fracture state and with less variation and
deterioration of strength can be obtained.
[ 0 0 3 2 ] Further, the combination of the steel sheets
is not limited to the combination of the steel sheets
of the same type and with the same thickness, but the
combination of the steel sheets of the same type and
with different thicknesses, of different types and
with the same thickness, or of different types and
and with different thicknesses can be welded,
provided that the steel sheets satisfy the
stipulations, and further, besides the combination of
the two stacked sheets illustrated in Fig. 1, the
three stacked sheets may be combined as illustrated
in Fig. 2.
LO0331 (Plating)
Plating layers applied on surface layers of the
steel sheets 1 being the objects to be welded may be
plating layers of any kind such as, for example, a
Zn-based one, a Zn-Fe-based one, a Zn-Ni-based one, a
Zn-Al-based one, a Zn-Mg-based one, a Pb-Sn-based one,
a Sn-Zn-based one, or a Al-Si-based one. Further,
inorganic or organic coatings (for example,
lubricating coatings or the like) may be applied on
surface layers of the plating layers. Further, a
weight of.any of these plating layers is not
particularly limited, but the weight on both surfaces
is preferably 100 (g/mZ)/lOO g/m2) or less. When the
weight of the plating per one surface is over 100
(g/mZ), the plating layers may obstruct the welding.
10034 I (Sheet Thickness)
The sheet thickness of each of the steel sheets 1
being the objects to be welded is not particularly
limited, and if the steel sheet has an ordinary sheet
thickness, for example, about 0.6 to 3.2 (mm)
generally used in the field of an automobile vehicle
body and the like, the aforesaid effects can be
stably obtained by applying the present invention.
However, since the stress concentration on the weld
increases as the sheet thickness increases, the sheet
thickness of each of the steel sheets 1 is more
preferably within a 0.6 to 2.0 (mm) range.
[0035] "Reasons for Limiting Welding Conditionsr'
Hereinafter, reasons for limiting the welding
conditions at the time of the resistance spot welding
will be described in detail.
100361 First, a welding pattern to be described
later, at the time of the resistance spot welding of
the steel sheets 1 is illustrated in the graph in Fig.
3. The welding pattern illustrated in Fig. 3 is an
example of a welding pattern applicable to the spot
welding method of the present invention. Note that,
in the graph illustrated in Fig. 3, the vertical axis
represents the welding currents 11, I2 or the
pressurizing force P and the horizontal axis
represents time T.
The main welding by the second welding step is
performed within 0.1 (s) after the pre-welding is
performed in the first welding step. That is, in
this pattern, after the first welding step, the
second welding step is immediately performed or the
second welding step is performed after a welding
suspension time within 0.1 (s). At this time, the
welding current I1 of the first welding step is set
to a lower current than the welding current I2 of the
second welding step. Here, in a case of an
alternating-current power source, the current
indicates its effective value.
Further, the pressurizing force P by the
electrodes 2A, 28 is fixed all through the first
welding step and the second welding step, and further,
the pressurized state is kept also in the welding
suspension time between the first welding step and
the second welding step.
[0037] Conventionally, when steel sheets are welded
by an ordinary spot welding method, a welding pattern
presents a roughly rectangular current waveform, that
is, a current is shut off after a fixed current (I)
is passed for welding for a predetermined time ( T ) ,
and the pressurizing force P is also fixed during
this period, though detailed illustration of this is
omitted. On the other hand, the first embodiment
adopts the welding pattern illustrated in Fig. 3 in
which, prior to the second welding step being the
main welding, the first welding step being the prewelding
is provided as described above. Here, a
rising pattern of the welding current I1 when the
first welding step is started may be a pattern in
which it is increased vertically up to the set
welding current I1 as illustrated in the graph in Fig.
3, or may be an up-slope pattern (stepwise pattern in
which the current is gradually increased).
[GO381 (Pressurizing Force of Electrodes to Highstrength
Steel Sheets: P)
The pressurizing force P (kN) of the electrodes
2A, 28 to the steel sheets 1 at the time of the
welding in the first welding step and the second
welding step is stipulated within the range expressed
by the following expression (1).
0.5 s P 5 3.0t'l'" .. . (1)
In the above expression (I), P is the
pressurizing force (kN) by the electrodes ZA, 2B, and
t is an average sheet thickness (mm) of the steel
sheets 1.
[0039] The pressurizing force P of the electrodes 2A,
2B has a great influence not only on strength,
especially a change of strength in a peeling
direction, of a weld accompanying the generation of
an indentation but also on the occurrence of a defect
or a crack in weld metal (nugget), and so on.
Therefore, as expressed by the above expression ( I ) ,
an upper limit of the pressurizing force P is first
limited as above based on the average sheet thickness
t of the plural steel sheets 1 so as to prevent the
pressurizing force from becoming too high, whereby
the generation of the indentations on the surfaces of
the steel sheets 1 is suppressed. On the other hand,
when the pressurizing force P is too low, expulsion
is liable to occur at the time of the spot welding,
and therefore, a lower limit of the pressurizing
f o r c e P w i t h which t h e o c c u r r e n c e of t h e e x p u l s i o n
can be s u p p r e s s e d is s e t t o 0.5 ( k N ) .
[0040] On t h e o t h e r hand, when t h e p r e s s u r i z i n g
f o r c e P i s o v e r t h e upper l i m i t of t h e above r a n g e ,
t h e r e is a problem t h a t t h e s u r f a c e s o f t h e steel
s h e e t s 1 a r e d e n t e d by t h e e l e c t r o d e s 2A, 2B t o c a u s e
t h e g e n e r a t i o n o f l a r g e i n d e n t a t i o n s , which l o w e r s
j o i n t s t r e n g t h and i m p a i r s t h e o u t e r a p p e a r a n c e .
F u r t h e r , when t h e p r e s s u r i z i n g f o r c e P is lower t h a n
t h e lower l i m i t of t h e above r a n g e , a c o n t a c t a r e a
becomes s m a l l and a c u r r e n t d e n s i t y becomes h i g h , s o
t h a t e x p u l s i o n i s l i k e l y t o o c c u r t o c a u s e a
r e d u c t i o n and v a r i a t i o n of a nugget s i z e ( j o i n t a r e a ) ,
and a c c o r d i n g l y d e t e r i o r a t i o n and v a r i a t i o n of j o i n t
s t r e n g t h o c c u r .
[0041] I n t h e f i r s t embodiment, t h e p r e s s u r i z i n g
f o r c e P by t h e e l e c t r o d e s 2A, 2B i s s e t w i t h i n t h e
a f o r e s a i d r a n g e , and t h u s is s t i p u l a t e d w i t h i n a
lower r a n g e t h a n t h a t of an o r d i n a r y p r e s s u r i z i n g
f o r c e . F u r t h e r , t h e p a t t e r n is a d o p t e d i n which, by
e x e c u t i n g t h e f i r s t welding s t e p ( p r e - w e l d i n g ) b e f o r e
t h e second w e l d i n g s t e p (main w e l d i n g ) , t h e
e l e c t r o d e s 2A, 2B a r e made t o f i t w i t h t h e steel
s h e e t s 1 and t h e s t e e l s h e e t s 1 a r e made t o fit w i t h
each o t h e r w i t h t h e low welding c u r r e n t w h i l e t h e
o c c u r r e n c e of e x p u l s i o n is s u p p r e s s e d , and t h e r e a f t e r
t h e main welding by t h e s e c o n d w e l d i n g s t e p is
p e r f o r m e d , which w i l l be d e s c r i b e d i n d e t a i l l a t e r .
[0042] (Welding C u r r e n t s : 11, 12)
The welding current I1 (kA) in the first welding
step being the pre-welding is stipulated within the
range of 30 to 90% of the welding current I2 ( k A ) in
the second welding step being the main welding.
LO0431 As described above, by limiting the upper
limit of the pressurizing force P based on the
average sheet thickness t of the steel sheets 1, it
is possible to suppress the generation of large
indentations on the surfaces of the steel sheets 1,
but on the other hand, there is a problem that the
current density becomes high due to the reduction of
the contact area, and accordingly expulsion is likely
to occur. Therefore, the welding pattern is set to
the pattern divided into the first welding step being
the pre-welding and the second welding step being the
main welding, and the welding current I1 in the first
welding step is set lower than the welding current I2
in the second welding step. By thus performing the
pre-welding by the first welding step first, an
excessive increase of the current density is
suppressed to prevent the occurrence of expulsion
even when the initial contact area is small, and in
addition, the electrodes 2A, 28 fit with the steel
sheets I and the steel sheets 1 fit with each other.
[ 0 0 4 4 ] Further, by performing the second welding
step with the higher welding current I2 than that of
the first welding step to sufficiently melt the base
metal, it is possible to secure a sufficient joint
area. By the pre-welding with the lower current by
the first welding step, the electrodes 2A, 2B get to
fit with the steel sheets 1 and the steel sheets 1
get to fit with each other, and the contact area is
sufficiently secured, which makes it possible to
suppress the occurrence of expulsion also in the
second welding step.
[00451 When the welding current I1 in the first
welding step is less than 30% of the welding current
I2 in the second welding step, it is difficult to
obtain the effect of making the electrodes 2A, 2B fit
with the steel sheets 1 and making the steel sheets 1
fit with each other by the pre-welding. Further,
when the welding current I1 in the first welding step
is over 90% of the welding current I2 in the second
welding step, the current density in a joint portion
becomes high, which is liable to cause the occurrence
of expulsion.
(00461 (Welding Suspension Time)
The second welding step being the main welding is
started within 0.1 (s) after the first welding step
being the pre-welding is finished. That is, the
pattern can be such that, after the first welding
step, the second welding step is immediately
performed or the second welding step is performed
after the welding suspension time within 0.1 (s).
[0047] When the welding suspension time up to the
start of the second welding step after the first
welding step is finished is over 0.1 (s), a
preheating effect by the pre-welding in the first
- 29 -
welding step is lost. This necessitates a need for
increasing the welding current I2 in the second
welding step, leading to inefficient current supply
and lower productivity due to the elongated process
time. Therefore, the shorter the suspension time,
the more preferable, and more preferably, there is no
suspension time between the first welding step and
the second welding step.
[0048] Here, the upper limit 0.1 (s) of the rielding
suspension time is a 5-cycle (0.1 (s)) suspension
time when, for example, a 50 Hz commercial power
source is used as a power source of a welder, and is
a 6-cycle (0.1 (s)) suspension time when a 60Hz
commercial power source is used.
to0491 Note that, though the method in which the
second welding step is started immediately after the
first welding step or the aforesaid welding
suspension time for 0.1 (s) is provided is described,
this is not restrictive. For example, an up-slope
pattern is also possible in which the current is
gradually increased between the first welding step
and the second welding step.
[00501 (Other Welding Conditions)
Concrete numerical values of the welding currents,
the welding time, and so on are not particularly
limited, and they may be about equal to current
values and welding time which have been
conventionally adopted in a resistance spot welding
method of welding steel sheets.
[0051] Further, for example, a conventionally known
resistance spot welding facility provided with the
electrodes 2A, 2B exemplified in Fig. 1 can be
adopted without any limitation. Further, as the
electrodes 2A, 2B and so on, those having
conventionally used structures may be used.
Furthermore, a power source which supplies the
current to the electrodes 2A, 2B may be an
alternating-current power source, or besides, a
direct-current inverter or an alternating-current
inverter. Further, the size and shape of the
electrodes 2A, 2B are not particularly limited, but
in order to properly obtain a contact pressure at
electrode tips, the electrodes whose tip diameter is
about 6 to 8 (mm) are preferably used.
[0052] According to the first embodiment, as
described above, when any of the gaps between the
steel sheets 1 is less than 0.5 (mm), the
pressurizing force P of the electrodes 2A, 2B is set
according to the average thickness t of the steel
sheets 1, and further there is provided the first
welding step in which the pre-welding is performed
with the lower welding current I1 than the welding
current I2 of the second welding step being the main
welding. At this time, by setting the pressurizing
force P to a pressurizing force that is not too high,
it is possible to suppress the generation of
indentations in the steel sheets 1, and by property
setting the lower limit of the pressuring force P, it
is possible to prevent the occurrence of expulsion.
Further, by performing the pre-welding by the first
welding step under the aforesaid conditions, it is
possible to secure the nugget diameter to obtain
sufficient joint strength while suppressing the
generation of the indentations.
[0053] [Second Embodiment]
A second embodiment of the spot welding method of
the high-strength steel sheets according to the
present invention will be hereinafter described.
Note that, in the second embodiment, its structure is
described with reference to the same drawings as
those in the above-described first embodiment, and
the same structures will be denoted by the same
reference signs and detailed description thereof will
be omitted.
(00541 The spot welding method of the high-strength
steel sheets of the second embodiment is a method of
welding two steel sheets lA, 1B or three steel sheets
1A to 1C by resistance spot welding as in the first
embodiment, but is different from that of the abovedescribed
first embodiment in that, when at least one
of gaps in the stacked steel sheets before the spot
welding (hereinafter, simply referred to as the gaps)
is 0.5 (mm) or more, a pressurizing force of
electrodes 2A, 2B in a second welding step is changed
from their pressurizing force in a first welding step.
In the second embodiment, the spot welding
includes: the first welding step being pre-welding
with a pressurizing force P1 (kN) and a welding
current I1 (kA); and the second welding step being
main welding with a pressurizing force P2 ( k N ) and a
welding current I2 (kA) .
The pressurizing forces PI, P2 are set within
ranges expressed by the following expression (2),
expression (3), where t (mm) is an average sheet
thickness of the plural steel sheets 1.
0.5 P2 3.0t"'~' . . . (2)
1.0 x P2 < P1 s 2.0 X P2 . . . (3)
Further, the welding current I1 is set within a
range of 30 to 90% of the welding current 12.
Further, the second welding step is started
within 0.1 (s) after the first welding step is
finished.
LO0551 Generally, in the welding of a vehicle body
and so on of an automobile, gaps sometimes exist
between steel sheets, and a large gap over 0.5(mm)
sometimes exists. In order to fill such a large gap,
in the first welding step being the pre-welding, the
high-strength steel sheets 1 are first pressed with
the higher pressurizing force PI than that of the
second welding step being the main welding,
concretely, with the pressurizing force P1 larger
than 1.0 times of the pressurizing force P2 of the
second welding step and equal to or smaller than 2.0
times of the pressurizing force P2. The welding
current is set to the same condition as the welding
current I1 (kA) of the first welding step described
in the above-described first embodiment. In the
.second embodiment, by first performing the prewelding
of the first welding step under the aforesaid
conditions, a sufficient contact area is secured
between the steel sheets 1 between which gaps exist,
and between the electrodes 2A, 2B and the steel
sheets 1.
[0056] Then, in the second welding step, the main
welding is performed with the same pressurizing force
as the pressurizing force P of the above-described
first embodiment, that is, with the pressurizing
force P2 within the range expressed by the following
expression (0.5 P2 3.0t"'"} and with the welding
current 12 that is the same condition as above. As
described above, in the second embodiment, the
conditions of the second welding step are stipulated
to the same conditions as those of the abovedescribed
first embodiment. The second embodiment
adopts the method of performing the second welding
step after the pre-welding in the first welding step
is performed with the hiqher pressurizing force P1 as
described above. Consequently, the contact area
between the electrodes 2A, 2B and the steel sheets 1
is sufficiently secured in the first welding step,
and while the occurrence of expulsion is suppressed
in the subsequent second welding step, base metal is
sufficiently melted in this second welding step and
the contact area can be sufficiently secured.
[0057] Concretely, as illustrated in the graph in
Fig. 4, a welding pattern of the second embodiment is
the same as the weiding pattern of the abovedescribed
first embodiment. Then, as illustrated in
the graph in Fig. 4, a pressurizing force pattern is
adopted in which, in the first welding step and a
welding suspension time, the pressurizing force by
the electrodes ZA, 28 is set to the higher
pressurizing force P1 than the pressurizing force P2
of the subsequent second welding step.
[0058] Here, when the pressurizing force P1 in the
first welding step is less than the lower limit
stipulated by the above expression ( 3 ) , that is,
equal to or less than 1.0 times of the pressurizing
force P2 of the second welding step, the aforesaid
effect of securing the contact area between the steel
sheets 1 between which the gaps exist is difficult to
obtain. Further, hen the pressurizing force P1 is
over the upper limit stipulated by the above
expression ( 3 1 , that is, over 2.0 times of the
pressurizing force P2 of the second welding step, a
current density lowers due to too high a pressurizing
force in addition to the low welding current I1 of
the first welding step, and temperature does not
increase sufficiently, so that the securing of the
contact area between the electrodes 2A, 2B and the
steel sheets 1 and the securing of the contact area
between the steel sheets 1, which are the objects of
the first welding step, sometime become insufficient.
[0059] Further, setting the pressurizing force P2 in
the second welding step within the range expressed by
the above expression (2) makes the base metal
sufficiently melt in the second welding step, making
it possible to secure the contact area and a nugget
diameter. Incidentally, when the pressurizing force
P2 in the second welding step falls out of the range
stipulated by the above expression (2), the same
problems as those described in the above-described
first embodiment are liable to occur.
[00601 Incidentally, it is described that the size
of the gap between the steel sheets 1 is 0.5 (mm) or
more, and this is about the same size as that of an
ordinary gap occurring in an assembling process of a
vehicle body and the like of an automobile. Further,
an upper limit of the gap between the steel sheets 1
is not particularly limited, but the largest value of
the gap occurring in the above process is generally
about 2.0 (mm) . Incidentally, r~lhen the gap between
the steel sheets 1 is less than 0.5 (mm), by applying
the conditions in the above-described first
embodiment, it is possible to obtain a sufficient
effect of improving joint strength.
[ 0 0 6 1 ] According to the second embodiment, when at
least one of the gaps in the steel sheets 1 is 0.5
(mm) or more in the examples illustrated in Fig. 1
and Fig. 2, by making the pressurizing force of the
electrodes 2A, 28 different between the first welding
step and the second welding step under the abovedescribed
conditions, it is possible to secure the
sufficient joint area while effectively suppressing
the generation of an indentation, and therefore
excellent joint strength is obtained. Further, by
performing the spot welding under the aforesaid
conditions, it is possible to effectively prevent the
occurrence of expulsion at the time of the welding,
which makes it possible to obtain a spot-welded joint
excellent in joint strength with good workability.
[ 0 0 6 2 ] [Third Embodiment]
A third embodiment of the spot welding method of
the high-strength steel sheets according to the
present invention will be hereinafter described.
Note that, in the third embodiment, its structure is
described with reference to the same drawings as
those in the above-described first and second
embodiments, and the same structures will be denoted
by the same reference signs and detailed description
thereof will be omitted.
[ 0 0 6 3 ] The spot welding method of the high-strength
steel sheets of the third embodiment is different
from those of the above-described first and second
embodiments in that, in obtaining a spot-welded joint
10, it has a third welding step under the following
conditions after a second welding step being main
welding is finished.
Concretely, the third welding step being postwelding
is provided in the method described in the
above first embodiment or second embodiment, after
the second welding step being the main welding. In
this third welding step, a welding current is
represented by I3 (kA), a welding time is represented
by T3 (s), and a non-welding time between the second
welding step and the third welding step is
represented by TC (s), and the welding current I3 is
set within a range of not less than 3 (kA) nor more
than 15 (kA) (hereinafter, written as 3 to 15 ( k A ) ) .
Further, the non-welding time TC is set within a
range of not less than 0 (s) nor more than 0.2 (s)
(hereinafter, written as 0 to 0.2 (s) ) .
Further, a relation between the welding current
I3 and the aforesaid welding time T3 is set within a
range expressed by the following expression (4).
I3 X T3 0.7 + T C . . . (4)
[0064] "Pressurizing Forces of Electrodes"
Pressuring forces of electrodes 2A, 2B can be the
same as that of the pattern (pressurizing force P) in
the above-described first embodiment or those of the
pattern (pressurizing forces PI, P2) in the second
embodiment. Further, in adopting these patterns of
the pressurizing forces, a pressurizing force in the
third welding step can be the same as the
pressurizing force in the previous second welding
step, that is, can be the same pressurizing force as
the pressurizing force P in the first embodiment or
the pressuring force P2 in the second embodiment.
[0065] The graph in Fig.5 illustrates a relation
between the welding pattern and the pressurizing
force pattern in the third embodiment. In the
example illustrated in Fig. 5, the pressurizing force
pattern is the same as the pattern in the abovedescribed
first embodiment, that is, the pressurizing
force is set to the fixed pressurizing force P in the
first welding step, the welding suspension time, and
the second welding step, and in addition, the
pressurizing force is also set to the fixed
pressurizing force P in the non-welding time and the
third welding step. Then, as illustrated in Fig. 5,
the pattern in the first welding step, the welding
suspension time, and the second welding time is the
same as in the first embodiment and the second
embodiment, and thereafter the non-welding time (TC)
and the third welding step are provided.
100661 "Third Welding Step (Post-welding)"
By providing the third welding step (postwelding)
under the above-described conditions in
addition to the first welding step (pre-welding) and
the second welding step (main welding), it is
possible to noticeably obtain the effect of improving
joint strength by the multi-stage welding. Such
conditions of the post-welding have a great influence
on structures of a weld metal zone and a heataffected
zone and a segregation state. In the third
embodiment, the conditions of the third welding step
being the post-welding are stipulated to the
conditions described in detail below.
[ 0 0 6 7 ] (Welding Current: 13)
In the third welding step, first, the welding
current I 3 is stipulated within a range of 3 to 15
(kA). This welding current I 3 has an especially
great influence on the structures of the weld metal
zone and the heat-affected zone and the segregation
state. By setting the welding current I3 in the
third welding step within this range, it is possible
to more noticeably obtain the effect of improving
joint strength by the multi-stage welding.
When the welding current I3 in the third welding
step is less than 3 (kA), it is difficult to obtain
the effect of improving joint strength by the postwelding,
and when it is over 15 (kA), expulsion is
likely to occur, which reduces the aforesaid effect.
[0068] (Non-welding Time between Second Welding Step
and Third Welding Step:TC)
In the third welding step, the non-welding time
TC between the second welding step and the third
welding step is set within a range of 0 to 0.2 (s),
that is, it is set to a condition so that the third
welding step is started immediately after the second
welding step is finished or the third welding step is
started within 0.2 ( s ) after the second welding step
is finished.
[0069] By stipulating the non-welding time TC within
the aforesaid range, it is possible to more
noticeably obtain the effect of improving joint
strength by the post-welding. Here, when the nonwelding
time TC in the third welding step is over 0.2
(s), the process time becomes long, which not only
lowers productivity but also is liable to reduce the
effect of improving joint strength by the postwelding.
[0070] Note that, though the third embodiment
describes the pattern where, after the second welding
step is finished, the third welding step is started
immediately or the third welding step is started
within a predetermined time, this is not restrictive.
For example, the pattern between the second welding
step and the third welding step can be a down-slope
pattern in which the current is gradually reduced.
[0071] (Relation between Welding Current I3 and
Welding Time T 3 )
In the third welding step, the relation between
the welding current I3 and the welding time T3 is
stipulated within the range satisfying the relation
expressed by the following expression ( I 3 X T3 0 . 7
+ TC), that is, a range of a value obtained by
multiplying the welding current I3 and the welding
time T 3 is stipulated based on the aforesaid nonwelding
time TC.
[0072] Here, a range of an absolute value of the
welding time T3 (s) in the third welding step is not
particularly limited, but is determined by a
calculation value obtained by multipl.ying this and
the aforesaid welding current 13. Further, this
welding time T 3 , similarly to the welding current 13,
has an especially great influence on the structures
of the weld metal and the heat-affected zone and the
segregation state.
[a0731 When the value of the welding current I3
multiplied by the welding time T3 is over a value
calculated by the following expression (0.7 + TC},
expulsion is likely to occur, which reduces the
effect of improving joint strength by the postwelding.
Further, a lower limit of the value of the
welding current I3 multiplied by the welding time T3
is not particularly provided, but is more preferably
a value obtained by the following expression (0.2 +
TC) or more because the effect of improving joint
strength becomes noticeable.
[0074] In the third embodiment, the third welding
step being the post-welding is further provided in
addition to the conditions described in the first and
second embodiments as described above, which makes it
possible to more improve joint strength. Further,
the value obtained by multiplying the welding current
I3 and the welding time T3 in the third welding step
is used as an index and this value is optimally
stipulated within the aforesaid range, which makes it
possible to obtain the effect of further improving
joint strength. A reason why such an effect of
improving joint strength is obtained by the third
welding is not necessarily clear, but it is inferred
that the alleviation of local softening and
microsegregation progresses and accordingly ductility
and toughness are improved.
[0075] As described above, when the steel sheets 1
are welded by the spot welding method, by setting the
pressurizing force P of the electrodes 2A, 2B within
the proper range according to the average sheet
thickness t of the steel sheets 1 and further
optimally controlling the welding pattern including
the first welding step (pre-welding) and the second
welding step (main welding), it is possible to secure
the nugget diameter and prevent the occurrence of
expulsion while suppressing the generation of
indentations. Consequently, it is possible to form
the spot-welded joint 10 high in reliability and
excellent in joint strength while maintaining good
workability.
100761 Further, by setting the pressurizing force P1
in the first welding step being the pre-welding
higher than the pressurizing force P2 in the second
welding step being the main welding, even when at
least one of the gaps in the steel sheets 1 is 0.5
(mm) or more, it is possible to fill the gap, secure
the contact area, secure a sufficient joint area, and
effectively prevent the occurrence of expulsion at
the time of the welding. Consequently, it is
possible to obtain the spot-welded joint 10 excellent
in joint strength with good workability.
[0077] Further, by providing the third welding step
being the post-welding in which the non-welding time
TC, the welding current 1 3 , and the welding time T3
are optimized, it is possible to obtain the spotwelded
joint 10 whose joint strength is further
improved.
Therefore, for example, in processes of
manufacturing automobile parts, assembling a vehicle
body, and so on, it is possible to fully enjoy merits
such as fuel efficiency improvement and an emission
reduction of carbon dioxide gas ( C 0 2 ) owing to a
weight reduction of the whole vehicle body, and their
social contribution is immeasurable.
EXAMPLES
[00781 Hereinafter, the present invention will be
more concretely described, by presenting examples of
the spot welding method of the high-strength steel
sheets according to the present invention. However,
the present invention is not of course limited to the
examples described below, and may be implemented by
being appropriately changed within the range
conformable to the spirits described previously and
later, and these changes are all included in the
technical scope of the present invention.
[0079] [Example 11
An example 1 is to verify the first embodiment.
By using steel sheets each with a sheet thickness
and of a steel type shown in Fig. 7, test pieces for
structure observation with 40 X 40 (mm) were
fabricated. Among the steel types presented in Fig.
7, CR1470HP and CR1780HP represent hot-pressed (hotstamped)
steel sheets disclosed in Japanese Laid-open
Patent Publication No. 2000-234153 and so on.
GAll8OY represents a product conforming to the Japan
Iron and Steel Federation Standard (JAC980Y). CR980Y
and CR270D represent JSC980Y and JSC270D respectively
which are. products conforming to the Japan Iron and
Steel Federation Standard. Note that CR means a
cold-rolled steel sheet and GA means an alloyed
galvanized steel sheet.
[0080] Further, cross tensile test pieces were
fabricated based on the cross tensile testing method
of a resistance spot-welded joint (JIS 23137).
[0081] Next, two sheets or three sheets of the
aforesaid test pieces for structure observation were
stacked in the combination of the same steel type or
different steel types and were welded by a resistance
spot welding method under the conditions presented in
Fig. 7, whereby welding test pieces were fabricated.
At this time, the presence/absence of the occurrence
of expulsion from each weld was visually confirmed.
Then, microstructures of their cross sections
were observed by using an optical microscope and a
nugget diameter was measured.
Further, regarding an indentation generated on a
surface of each high-strength steel sheet, its depth
was measured in the cross section of the weld.
[00821 Further, by using the aforesaid cross tensile
test pieces, the test pieces were stacked in the
shape of a cross as illustrated in Fig. 6A and Fig.
6B based on the cross tensile testing method of the
resistance spot-welded joint (JIS 23137) and were
spot-welded under the conditions presented in Fig. 7,
whereby cross tensile test pieces were fabricated.
Note that three sets of cross tensile test pieces
were fabricated under the same condition.
Then, the cross tensile test was executed by
applying loads in peeling directions, that is,
applying a load in an upward direction to the lower
test piece and a load in a downward direction to the
lower test piece as indicated by the reference number
6 in Fig. 6A and Fig. 68 so that they peel from each
other, and then cross tensile strength (CTS) was
measured. Here, when high-strength steel sheets are
stacked and spot-welded, it can be generally said
that CTS is sufficient if cross tensile strength is
2.5 (kN) or more when a sheet thickness t of the
thinnest steel sheet is 0.7 (mm). Similarly, it can
be said that CTS is sufficient if cross tensile
strength is 5 (kN), 9 (kN), and 11 (kN) or more when
the sheet thickness t of the thinnest steel sheet is
1.0 (mm) , 1 . 6 (mm), and 2.0 (mm) respectively .
[0083] By the above-described procedure, the
resistance spot welding was performed under the
welding conditions within the ranges described in the
first embodiment, and various tests were conducted by
the above-described methods (refer to the present
invention examples in Fig. 7).
Further, by the above-described procedure, the
resistance spot welding was conducted under welding
conditions falling out of the ranges described in the
first embodiment, and various tests were conducted by
the above-described methods (refer to comparative
examples in Fig. 7).
Fig. 7 presents a list of fabrication conditions
and test results of the respective test pieces. The
example 1 is to verify the first embodiment, and in
each condition number, the pressurizing forces PI, P2
are set to the same numerical value (pressurizing
force P) .
[0084] The condition numbers 1 to 5 are examples
where two or three sheets of CR1470HP each with a 1.0
(mm) sheet thickness were stacked and spot-welded.
Among these, the condition numbers 2 to 4 are the
present invention examples where the spot welding was
performed under the spot welding conditions within
the ranges described in the first embodiment. The
condition number 4 is an examp1.e where the three
high-strength steel sheets were stacked and spotwelded.
On the other hand, the condition numbers 1,
5 are the comparative examples where the spot welding
was performed under a spot welding condition falling
out of the range described in the first embodiment.
Concretely, the pressurizing force P =P1 = P2
falls out of the range expressed by the above
expression (1) .
[0085] The condition numbers 6 to 8 are examples
where two sheets of GA118OY each with a 2.0 (mm)
sheet thickness were stacked and spot-welded. Among
these, the condition number 7 is the present
invention example where the spot welding was
performed under the spot welding conditions within
the ranges described in the first embodiment. On the
other hand, the condition numbers 6, 8 are the
comparative examples where the spot welding was
performed under a spot welding condition falling out
of the range described in the first embodiment.
Concretely, the pressurizing force P = P1 = P2 falls
out of the range expressed by the above expression
(1).
[0086] The condition numbers 9 to 11 are examples
where two sheets of CR1780HP each with a 1.0 (mm)
sheet thickness were stacked and spot-welded. Among
these, the condition number 10 is the present
invention example where the spot welding was
performed under the spot welding conditions within
the ranges described in the first embodiment. On the
other hand, the condition numbers 9, 11 are the
comparative examples where the spot welding was
performed under a spot welding condition falling out
of the range described in the first embodiment.
Concretely, these conditions fall out of the
condition that the welding current I1 is set to the
range of 30 to 90% of the welding current 12.
[0087] The condition numbers 12 to 14 are examples
where two sheets of CR980Y each with a 0.7 (mm) sheet
thickness were stacked and spot-welded. Among these,
the condition number 13 is the present invention
example where the spot welding was performed under
the spot welding conditions within the ranges
described in the first embodiment. On the other hand,
the condition numbers 12, 14 are the comparative
examples where the spot welding was performed under a
spot welding condition falling out of the range
described in the first embodiment. Concretely, the
pressurizing force P = P1 = P2 falls out of the range
expressed by the above expression (1).
Further, the condition number 15 is an example
where two sheets which were CR980Y with a 0.7 (mm)
sheet thickness and CR980Y with a 4.0 (mm) sheet
thickness were stacked and spot-welded, but joining
itself was not possible because of too large a sheet
thickness difference.
[0088] The condition numbers 16 to 18 are examples
where two sheets of CR98OY each with a 1.6 (mm) sheet
thickness and one sheet of CR270D with a 0.7 (mm)
sheet thickness on an outer side thereof were stacked
and spot-welded. Among these, the condition number
17 is the present invention example where the spot
welding was performed under the spot welding
conditions within the ranges described in the first
embodiment.
On the other hand, the condition numbers 16, 18
are the comparative examples where the spot welding
was performed under a spot welding condition falling
out of the range described in the first embodiment.
Concretely, the pressurizing force P = P1 = P2 falls
out of the range expressed by the above expression
(1).
[0089] As is seen in the results in Fig. 7, it could
be confirmed that, in the present invention examples
of the condition numbers 2, 3, 4, 7, 10, 13, 17, when
any of the steel types was used, a depth of the
indentation was suppressed to 0.2 (mm) at the largest
and a 4.2 (mm) nugget diameter or more could also be
secured in all of them. Further, in the present
invention examples, the cross tensile strength (CTS)
by the cross tensile test was 2.5 (kN) or more when
the sheet thickness t was 0.7 (mm), 5.0 (kN) or more
when the sheet thickness t was 1 (mm), 10 (kN) or
more when the sheet thickness t was 1.6 (mm), and 11
( k N ) or more when the sheet thickness t was 2.0 (mm),
and it became clear that they were excellent in joint
strength. Further, in the present invention examples,
it could be visually confirmed that no expulsion
occurred at the time of the spot welding.
[0090] On the other hand, in the comparative
examples of the condition numbers 1, 5, 6, 8, 9, 11,
12, 14, 16, 18, the tendency that the depth of the
indentation became large and on the other hand the
nugget diameter became small was confirmed. Further,
it became clear that, in the comparative examples,
the cross tensile strength (CTS) was low as compared
with the above-described present invention examples,
and thus joint strength was inferior.
Incidentally, in the condition numbers 16 to 18,
as the nugget diameter, a nugget diameter on an
interface between the sheets of CR980Y being the
high-strength steel sheets was measured. Further, as
the cross tensile strength (CTS), a value when the
sheets of CR980Y were pulled so as to separate from
each other, that is, strength of a weld between the
sheets of CR980Y was measured.
[0091] In the comparative example of the condition
number 1, since the pressurizing force P tias 0.4 (kN),
which is lower than the range stipulated in the first
embodiment, the nugget diameter was small, namely 3.1
(mm), and the depth of the indentation was large,
namely 0.4 (mm), as compared with the present
invention examples 2, 3. Accordingly, in the
condition number 1, the cross tensile strength was
2.1 (kN) and thus joint strength was low. Further,
in the condition number 1, it was visually confirmed
that the expulsion occurred at the time of the spot
welding because the pressurizing force P was low.
[0092] Further, in the comparative example of the
condition number 5, since the pressurizing force P
was 4.0 (kN), which is over the range stipulated in
the first embodiment, the depth of the indentation
was large, namely 0.3 (mm) even though the nugget
diameter was sufficient, namely 5.3 (mm), as compared
with the present invention examples 2, 3.
Accordingly, in the condition number 5, the cross
tensile strength was 3.5 (kN) and thus joint strength
was low.
[0093] Further, in the comparative example of the
condition number 6, since the pressurizing force P
was 0.4 (kN), which is below the range stipulated in
the first embodiment, the nugget diameter was small,
namely 5.5 (mrn), and the depth of the indentation was
large, namely 0.3 (mm), as compared with the present
invention example 7. Accordingly, in the condition
number 6, the cross tensile strength was 7.0 (kN) and
thus joint strength was low. Further, in the
condition number 6, the occurrence of the expulsion
was confirmed.
[0094] Further, in the comparative example of the
condition number 8, since the pressurizing force P
was 4.5 (kN), which is over the range stipulated in
the first embodiment, the depth of the indentation
was large, namely 0.3 (mm) even though the nugget
diameter was sufficient, namely 6.7 (mm), as compared
with the present invention example 7. Accordingly,
in the condition number 8, the cross tensile strength
was 6.2 (kN) and thus joint strength was low.
[0095] Further, in the comparative example of the
condition number 9, since the welding current I1 was
less than 30% of the welding current 12, the nugget
diameter was small, namely 4.4 (mm), and the depth of
the indentation was large, namely 0.3 (mm), as
compared with the present invention example 10.
Accordingly, in the condition number 9, the cross
tensile strength was 4.1 (kN) and thus joint strength
was low. Further, in the condition number 9, the
occurrence of the expulsion was confirmed.
[0096] Further, in the comparative example of the
condition number 11, since the welding current I1 was
more than 90% of the welding current 12, the depth of
the indentation was large, namely 0.3 (mm) even
though the nugget diameter was sufficient, namely 4.9
(mm), as compared with the present invention example
10. Accordingly, in the condition number 11, the
cross tensile strength was 4.2 (kN) and thus joint
strength was low.
[0097] Further, in the comparative example of the
condition number 12, since the pressurizing force P
was 0.4 (kN), which is below the range stipulated in
the first embodiment, the nugget diameter was small,
namely 3.3 (mm), and the depth of the indentation was
large, namely 0.3 (mm), as compared with the present
invention example 13. Accordingly, in the condition
number 12, the cross tensile strength was 2.0 (kN)
and thus joint strength was low. Further, in the
condition number 12, the occurrence of the expulsion
was confirmed.
[0098] Further, in the comparative example of the
condition number 14, since the pressurizing force P
was 3.5 (kN), which is over the range stipulated in
the first embodiment, the depth of the indentation
was large, namely 0.2 (mm) even though the nugget
diameter was sufficient, namely 4.4 (mm), as compared
with the present invention example 13. Accordingly,
in the condition number 14, the cross tensile
strength was 1.9 (kN) and thus joint strength was low.
[0099] Further, in the comparative example of the
condition number 16, since the pressurizing force P
was 0.3 (kN), which is below the range stipulated in
the first embodiment, the nugget diameter was small,
namely 5.2 (mm), and the depth of the indentation was
large, namely 0.3 (mm), as compared with the present
invention example 17. Accordingly, in the condition
number 16, the cross tensile strength was 6.8 (kN)
and thus joint strength was low. Further, in the
condition number 16, the occurrence of the expulsion
was con£ irmed.
[0100] Further, in the comparative example of the
condition number 18, since the pressurizing force P
was 5.0 (kN), which is over the range stipulated in
the first embodiment, the depth of the indentation
was large, namely 0.3 (mm) even though the nugget
diameter was sufficient, namely 6.4 (mm) , as compared
with the present invention example 17. Accordingly,
in the condition number 18, the cross tensile
strength was 7.3 (kN) and thus joint strength was low.
[0101] [Example 21
An example 2 is to verify the second embodiment.
By using steel sheets each with a sheet thickness
and of a steel type shown in Fig. 8, test pieces for
structure observation and cross tensile test pieces
were fabricated by the same procedure as in the
example 1, and various tests were conducted by the
same methods. Note that a gap between the stacked
steel sheets 1 was set by spacer steel sheets with a
sheet thickness corresponding to a predetermined gap
- 54 -
being sandwiched between the steel sheets 1 whose
welding was to be evaluated. Concretely, as
illustrated in Fig. 11A and Fig. 11B, spacer steel
sheets 11 were disposed at a 40 mm interval on both
outer sides of a weld which was to be evaluated,
thereby securing the predetermined gap between the
steel sheets 1.
Fig. 8 presents a list of fabrication conditions
and test results of the respective test pieces. The
example 2 is to verify the second embodiment, and the
pressurizing force PI in the first welding step (prewelding)
and the pressurizing force P2 in the second
welding step (main welding) were set to different
numerical values.
[0102] Condition numbers 21 to 26 are examples where
two or three sheets of CR1470HP each with a 1.0 (mm)
sheet thickness were stacked and spot-welded. Among
these, the condition numbers 22, 25 are present
invention examples where the spot welding was
performed under the spot welding conditions within
the ranges described in the second embodiment. On
the other hand, the condition numbers 21, 23, 24, 26
are comparative examples where the spot welding was
performed under a spot welding condition falling out
of the range described in the second embodiment.
Concretely, the relation between the pressurizing
forces PI, P2 falls out of the range expressed by the
above expression (3).
[0103] Condition numbers 27 to 29 are examples where
two sheets of GA1180Y each with a 2.0 (mm) sheet
thickness were stacked and spot-welded. Among these,
the condition number 28 is a present invention
example where the spot welding was performed under
the spot welding conditions within the ranges
described in the second embodiment. On the other
hand, the condition numbers 27, 29 are comparative
examples where the spot welding was performed under a
spot welding condition falling out of the range
described in the second embodiment. Concretely, the
relation between the pressurizing forces PI, P2 falls
out of the range expressed by the above expression
(3).
[0104] Condition numbers 30 to 32 are examples where
two sheets of CR1780HP each with a 1.0 (mm) sheet
thickness were stacked and spot-welded. Among these,
the condition number 31 is a present invention
example where the spot welding was performed under
the spot welding conditions within the ranges
described in the second embodiment. On the other
hand, the condition numbers 30, 32 are comparative
examples where the spot welding was performed under a
spot welding condition falling out of the range
described in the second embodiment. Concretely, the
relation between the pressurizing forces Pl, P2 falls
out of the range expressed by the above expression
(3).
[0105] Condition numbers 33 to 35 are examples where
two sheets of CR980Y each with a 0.7 (mm) sheet
thickness were stacked and spot-welded. Among these,
the condition number 34 is a present invention
example where the spot welding was performed under
the spot welding conditions within the ranges
described in the second embodiment. On the other
hand, the condition numbers 33, 35 are comparative
examples where the spot welding was performed under a
spot welding condition falling out of the range
described in the second embodiment. Concretely, the
relation between the pressurizing forces PI, P2 falls
out of the range expressed by the above expression
(3).
[0106] Condition numbers 36 to 38 are examples where
two sheets of CR980Y each with a 1.6 (mm) sheet
thickness and one sheet of CR270D with a 0.7 (mm)
sheet thickness on an outer side thereof were stacked
and spot-welded. Among these, the condition number
37 is a present invention example where the spot
welding was performed under the spot welding
conditions within the ranges described in the second
embodiment. On the other hand, the condition numbers
36, 38 are comparative examples where the spot
welding was performed under a spot welding condition
falling out of the range described in the second
embodiment. Concretely, the relation between the
pressurizing forces PI, P2 falls out of the range
expressed by the above expression (3).
[0107] As is seen in the results in Fig. 8, it could
be confirmed that, in the present invention examples
of the condition numbers 22, 25, 28, 31, 34, 37, when
any of the steel types was used, the depth of the
indentation was suppressed to 0.2 (mm) at the largest
and a 4.2 (mm) nugget diameter or more could be
secured in all of them. Further, in the present
invention examples, the cross tensile strength (CTS)
by the cross tensile test was 2.5 (kN) or more when
the sheet thickness t was 0.7 (mm), 5.0 (kN) or more
when the sheet thickness t was 1 (mm), 10 (kN) or
more when the sheet thickness t was 1.6 (mm), and 11
(kN) or more when the sheet thickness t was 2.0 (mm),
and it became clear that they were excellent in joint
strength. Further, in the present invention examples,
it could be visually confirmed that no expulsion
occurred at the time of the spot welding.
[0108] On the other hand, in the comparative
examples of the condition numbers 21, 23, 24, 26, 27,
29, 30, 32, 33, 35, 36, 38, the tendency that the
depth of the indentation became large and on the
other hand the nugget diameter became small was.
confirmed. Further, it became clear that, in the
comparative examples, the cross tensile strength
(CTS) was low as compared with the above-described
present invention examples, and thus joint strength
was inferior.
Incidentally, in the condition numbers 36 to 38,
as the nugget diameter, a nugget diameter on an
interface between the sheets of CR980Y being highstrength
steel sheets was measured. Further, as the
cross tensile strength (CTS), a value when the sheets
of CR980Y were pulled so as to separate from each
other, that is, strength of a weld between the sheets
of CR980Y was measured.
[0109] In the comparative example of the condition
number 21, since the pressurizing force ratio Pl/P2
was 1.0, which is below the range described in the
second embodiment, the nugget diameter was small,
namely 3.2 (mm), and the depth of the indentation was
large, namely 0.3 (mm), as compared with the present
invention example 22. Accordingly, in the condition
number 21, the cross tensile strength was 2.4 (kN)
and thus joint strength was low. Further, in the
condition number 21, the occurrence of the expulsion
was confirmed.
[0110] Further, in the comparative example of the
condition number 23, since the pressurizing force
ratio Pl/P2 was 2.4, which is over the range
described in the second embodiment, the nugget
diameter was small, namely 4.7 (mm), and the depth of
the indentation was large, namely 0.2 (mm), as
compared with the present invention example 22.
Accordingly, in the condition number 23, the cross
tensile strength was 4.2 (kN) and thus joint strength
was low. Further, in the condition number 23, the
occurrence of the expulsion was confirmed.
[0111] In the comparative example of the condition
number 24, since the pressurizing force ratio Pl/P2
was 1.0, which is below the range described in the
second embodiment, the nugget diameter was small,
namely 3.0 (mm), and the depth of the indentation was
large, namely 0.4 (mm), as compared with the present
invention example 25. Accordingly, in the condition
number 24, the cross tensile strength was 3.6 (kN)
and thus joint strength was low. Further, in the
condition number 24, the occurrence of the expulsion
was confirmed.
[0112] In the comparative example of the condition
number 26, since the pressurizing force ratio Pl/P2
was 2.2, which is over the range described in the
second embodiment, the nugget diameter was small,
namely 4.3 (mm), and the depth of the indentation was
large, namely 0.4 (mm), as compared with the present
invention example 25. Accordingly, in the condition
number 26, the cross tensile strength was 3.7 (kN)
and thus joint strength was low. Further, in the
condition number 26, the occurrence of the expulsion
was confirmed.
[ 0 1 1 3 ] In the comparative example of the condition
number 27, since the pressurizing force ratio Pl/P2
was 1.0, which is below the range described in the
second embodiment, the nugget diameter was small,
namely 3.7 (mm), and the depth of the indentation was
large, namely 0.4 (mm), as compared with the present
invention example 28. Accordingly, in the condition
number 27, the cross tensile strength was 5.5 (kN)
and thus joint strength was low. Further, in the
condition number 27, the occurrence of the expulsion
was confirmed.
[0114] Further, in the comparative example of the
condition number 29, since the pressurizing force
ratio Pl/P2 was 2.7. which is over the range
described in the second embodiment, the nugget
diameter was small, namely 5.0 (mm), and the depth of
the indentation was large, namely 0.4 (mm), as
compared with the present invention example 28.
Accordingly, in the condition number 29, the cross
tensile strength was 7.5 (kN) and thus joint strength
was low. Further, in the condition number 29, the
occurrence of the expulsion was confirmed.
[0115] In the comparative example of the condition
number 30, since the pressurizing force ratio Pl/P2
was 1.0, which is lower than the range described in
the second embodiment, the nugget diameter was small,
namely 3.9 (mm), and the depth of the indentation was
large, namely 0.4 (mm), as compared with the present
invention example 31. Accordingly, in the condition
number 30, the cross tensile strength was 4.1 (kN)
and thus joint strength was low. Further, in the
condition number 30, the occurrence of the expulsion
was con£ irmed .
[0116] Further, in the comparative example of the
condition number 32, since the pressurizing force
ratio Pl/P2 was 2.2, which is over the range
described in the second embodiment, the nugget
diameter was small, namely 4.7 (mm), and the depth of
the indentation was large, namely 0.3 (mm), as
compared with the present invention example 31.
Accordingly, in the condition number 32, the cross
tensile strength was 4.8 (kN) and thus joint strength
was low. Further, in the condition number 32, the
occurrence of the expulsion was confirmed.
[0117] In the comparative example of the condition
number 33, since the pressurizing force ratio Pl/P2
was 1.0, which is below the range described in the
second embodiment, the nugget diameter was small,
namely 3.9 (mm), and the depth of the indentation was
large, namely 0.2 (mm), as compared with the present
invention example 34. Accordingly, in the condition
number 33, the cross tensile strength was 1.9 (kN)
and thus joint strength was low. Further, in the
condition number 33, the occurrence of the expulsion
was confirmed.
[0118] In the comparative example of the condition
number 35, since the pressurizing force ratio Pl/P2
was 2.4, which is over the range described in the
second embodiment, the nugget diameter was small,
namely 3.5 (mm), and the depth of the indentation was
large, namely 0.3 (mm), as compared with the present
invention example 34. Accordingly, in the condition
number 35, the cross tensile strength was 1.7 (kN)
and thus joint strength was low. Further, in the
condition number 35, the occurrence of the expulsion
was confirmed.
[0119] In the comparative example of the condition
number 36, since the pressurizing force ratio P1/P2
was 1.0, which is below the range described in the
second embodiment, the nugget diameter was small,
namely 5.3 (mm), and the depth of the indentation was
large, namely 0.2 (mm), as compared with the present
invention example 37. Accordingly, in the condition
number 36, the cross tensile strength was 6.9 (kN)
and thus joint strength was low. Further, in the
condition number 36, the occurrence of the expulsion
was confirmed.
[0120] In the comparative example of the condition
number 38, since the pressurizing force ratio Pl/P2
was 2.2, which is over the range described in the
second embodiment, the nugget diameter was small,
namely 5.2 (mm), and the depth of the indentation was
large, namely 0.2 (mm) , as compared with the present
invention example 37. Accordingly, in the condition
number 38, the cross tensile strength was 7.2 (kN)
and thus joint strength was low. Further, in the
condition number 38, the occurrence of the expulsion
was conf irmed.
[0121] [Example 31
An example 3 is to verify the case where, in the
third embodiment, the third welding step being the
post-welding is provided after the second welding
step in the first embodiment.
By using steel sheets each with a sheet thickness
and of a steel type shown in Fig. 9, test pieces for
structure observation and cross tensile test pieces
were fabricated by the same procedure as in the
example 1, and various tests were conducted by the
same methods.
Fig. 9 presents a list of fabrication conditions
and test results of the respective test pieces. The
example 3 is to verify the third embodiment, and in
the first welding step and the second welding step,
the spot welding conditions within the ranges
described in the first embodiment are satisfied.
[0122] Condition numbers 41 to 45 are examples where
two sheets of CR1470HP each with a 1.0 (mm) sheet
thickness were stacked and spot-welded. Among these,
the condition numbers 41, 42, 45 are present
invention examples where the spot welding was
performed under the spot welding conditions within
the ranges described in the third embodiment. On the
other hand, the condition numbers 43, 44 are
comparative examples where the spot welding was
performed under a spot welding condition falling out
of the range described in the third embodiment.
Concretely, the non-welding time TC, and the welding
current I3 and the welding time T3 in the third
welding step are changed, so that the left side - the
right side in the expression (4) is over 0.
[0123] Condition numbers 46 to 49 are examples where
three sheets of CR1470HP each with a 1.0 (mm) sheet
thickness were stacked and spot-welded. Among these,
the condition numbers 46, 47 are present invention
examples where the spot welding was performed under
the spot welding conditions within the ranges
described in the third embodiment. On the other hand,
the condition numbers 48, 49 are comparative examples
where the spot welding was performed under a spot
welding condition falling out of the range described
in the third embodiment. Concretely, the non-welding
time TC, and the welding current I3 and the welding
time T3 in the third welding step are changed, so
that the left side - the right side in the expression
(4) is over 0.
[0124] As is seen in the results in Fig. 9, it
became clear that, in the present invention examples
of the condition numbers 41, 42, 45 to 47, the cross
tensile strength (CTS) by the cross tensile test was
high as compared with the comparative examples of the
condition numbers 43, 44, 48, 49.
Further, the condition numbers 41, 42, 45 and the
condition number 2 are the same conditions except for
the presence/absence of the third welding step, but
it became clear that, in the condition numbers 41, 42,
45, the cross tensile strength (CTS) by the cross
tensile test became high as compared with the
condition number 2.
[0125] [Example 41
An example 4 is to verify the case where, in the
third embodiment, the third welding step being the
post-welding is provided after the second welding
step in the second embodiment.
By using steel sheets each with a sheet thickness
and of a steel type shown in Fig. 10, test pieces for
structure observation and cross tensile test pieces
were fabricated by the same procedure as in the
example 1, and various tests were conducted by the
same methods.
Fig. 10 presents a list of fabrication conditions
and test results of the respective test pieces. The
example 4 is to verify the third embodiment, and in
the first welding step and the second welding step,
the spot welding conditions within the ranges
described in the second embodiment are satisfied.
[0126] Condition numbers 51 to 55 are examples where
two sheets of CR1470HP each with a 1.0 (mm) sheet
thickness were stacked and spot-welded. Among these,
the condition numbers 51, 52, 55 are present
invention examples where the spot welding was
performed under the spot welding conditions within
the ranges described in the third embodiment. On the
other hand, the condition numbers 53, 54 are
comparative examples where the spot welding was
performed under a spot welding condition falling out
of the range described in the third embodiment.
Concretely, the non-welding time TC, and the welding
current I3 and the welding time T3 in the third
welding step are changed, so that the left side - the
right side in the expression (4) is over 0.
[0127] Condition numbers 56 to 59 are examples where
three sheets of CR1470HP each with a 1.0 (mm) sheet
thickness were stacked and spot-welded. Among these,
the condition numbers 56, 57 are present invention
examples where the spot welding was performed under
the spot welding conditions within the ranges
described in the third embodiment. On the other hand,
the condition numbers 58, 59 are comparative examples
where the spot welding was performed under a spot
welding condition falling out of the range described
in the third embodiment. Concretely, the non-welding
time TC, and the welding current I3 and the welding
time T3 in the third welding step are changed, so
that the left side - the right side in the expression
(4) is over 0 .
[0128] As is seen in the results in Fig. 10, it
became clear that, in the present invention examples
of the condition numbers 51, 52, 55 to 57, the cross
tensile strength (CTS) by the cross tensile test was
high as compared with the comparative examples of the
condition numbers 53, 54, 58, 59.
Further, the condition numbers 51, 52, 55 and the
condition number 22 are the same conditions except
for the presence/absence of the third welding step,
but it became clear that, in the condition numbers 51,
52, 55, the cross tensile strength (CTS) by the cross
tensile test became high as compared with the
condition number 22.
Similarly, the condition numbers 56, 57 and the
condition number 25 are the same conditions except
for the presence/absence of the third welding step,
but it became clear that, in the condition numbers 56,
57, the cross tensile strength (CTS) by the the cross
tensile test became high as compared with the
condition number 25.
[0129] Incidentally, in the above-described examples
1 to 3, when experiments were further conducted with
other steel types and with the sheet thickness being
changed, and when experiments were further conducted
with the plating seed, weight, and so on being
changed, the results are also the same as above, and
it could be confirmed that it is possible to obtain
the effects of the present invention, that is, it is
possible to secure the nugget diameter and prevent
the occurrence of the expulsion while suppressing the
occurrence of the indentation, and to form a highly
reliable welded joint having sufficiently high
strength.
[0130] From the above-described results of the
examples, it has become clear that the use of the
spot welding method of the high-strength steel sheets
of the present invention makes it is possible to
secure the nugget diameter and prevent the occurrence
of the expulsion while suppressing the occurrence of
the indentation, and the highly reliable spot-welded
joint having sufficiently high strength can be
obtained with good workability, when the steel sheets
are welded by the resistance spot welding method.
[0131] Hitherto, the present invention has been
descried together with the various embodiments, but
the present invention is not limited only to these
embodiments, and changes and so on can be made within
the range of the present invention.
INDUSTRIAL APPLICABILITY
[0132] According to the present invention, when
high-strength steel sheets used in the manufacture of
automobile parts, the assembling of a vehicle body,
and so on are spot-welded, it is possible to secure a
nugget diameter and prevent the occurrence of
expulsion while suppressing the generation of an
indentation. Consequently, it is possible to obtain
a highly reliable welded joint having sufficiently
high strength with good workability. Therefore,
merits obtained by applying the high-strength steel
sheets in the automobile field and the like, such as
fuel efficiency improvement and an emission reduction
of carbon dioxide gas (C02) accompanying a weight
reduction of the whole vehicle, can be fully enjoyed,
and their social contribution is immeasurable.
CLAIMS
[Claim 1: A spot welding method of high-strength
steel sheets excellent in joint strength, the method
applying resistance spot welding to a stack of a
plurality of steel sheets,
wherein the plural steel sheets
are two steel sheets which both have tensile
strength of not less than 780 MPa nor more than 1850
MPa and whose sheet thickness ratio = {a sum of sheet
thicknesses of the steel sheetsl/{the sheet thickness
of the thinner steel sheet (when the both have the
same thickness, the sheet thickness per one sheet)]
is within a range of not less than 2 nor more than 5,
0 r
are three steel sheets which are three steel
sheets all having tensile strength of not less than
780 MPa nor more than 1850 MPa or which are two steel
sheets both having tensile strength of not less than
780 MPa nor more than 1850 MPa and one steel sheet
provided on an outer side of the two steel sheets and
having tensile strength of less than 780 MPa, and
whose sheet thickness ratio = {a sum of sheet
thicknesses of the steel sheets)/{the sheet thickness
of the thinner steel sheet (when the steel sheets all
have the same thickness, the sheet thickness per one
sheet)) is within a range of not less than 3 nor more
than 6,
wherein the spot welding includes: a first
welding step being pre-welding with a pressurizing
force P1 (kN) and a welding current I1 (kA); and a
second welding step being main welding with a
pressurizing force P2 (kN) and a welding current I2
(kA) ,
wherein the pressurizing forces PI, P2 are set to
a fixed pressurizing force P = P1 = P2 all through
the first welding step and the second welding step,
and are set within a range expressed by the following
expression (11, where t (mm) is an average sheet
thickness of the plural steel sheets,
0.5 =< P ( 3.0t"'~' ... (I),
wherein the welding current I1 is set within a
range of not less than 30% nor more than 90% of the
welding current 12, and
wherein the second welding step is started within
0.1 (s) after the first welding step is finished.
[Claim 21 A spot welding method of high-strength
steel sheets excellent in joint strength, the method
applying resistance spot welding to a stack of a
plurality of steel sheets,
wherein the plural steel sheets
are two steel sheets which both have tensile
strength of not less than 780 MPa nor more than 1850
MPa and whose sheet thickness ratio = (a sum of sheet
thicknesses of the steel sheets)/(the sheet thickness
of the thinner steel sheet (when the both have the
same thickness, the sheet thickness per one sheet)]
is within a range of not less than 2 nor more than 5,
or
a r e t h r e e s t e e l s h e e t s which a r e t h r e e s t e e l
s h e e t s a l l h a v i n g t e n s i l e s t r e n g t h of n o t less t h a n
780 MPa n o r more t h a n 1850 MPa o r which a r e two s t e e l
s h e e t s b o t h having t e n s i l e s t r e n g t h of n o t l e s s t h a n
780 MPa n o r more t h a n 1850 MPa and one s t e e l s h e e t
p r o v i d e d on an o u t e r s i d e of t h e two s t e e l s h e e t s and
h a v i n g t e n s i l e s t r e n g t h of l e s s t h a n 780 MPa, and
whose s h e e t t h i c k n e s s r a t i o = [ a sum of s h e e t
t h i c k n e s s e s of t h e s t e e l s h e e t s ) / [ t h e s h e e t t h i c k n e s s
of t h e t h i n n e r s t e e l s h e e t (when t h e steel s h e e t s a l l
have t h e same t h i c k n e s s , t h e s h e e t t h i c k n e s s p e r one
s h e e t ) ] is w i t h i n a r a n g e of n o t l e s s t h a n 3 n o r more
t h a n 6,
wherein t h e s p o t w e l d i n g i n c l u d e s : a f i r s t
welding s t e p b e i n g p r e - w e l d i n g w i t h a p r e s s u r i z i n g
f o r c e P I (kN) and a welding c u r r e n t I1 (kA); and a
s e c o n d welding s t e p b e i n g main w e l d i n g w i t h a
p r e s s u r i z i n g f o r c e P2 (kN) and a w e l d i n g c u r r e n t I 2
(kA),
wherein t h e p r e s s u r i z i n g f o r c e s P I , P2 a r e s e t
w i t h i n r a n g e s e x p r e s s e d by t h e f o l l o w i n g e x p r e s s i o n
( 2 ) , e x p r e s s i o n ( 3 ) , where t (mm) is a n a v e r a g e s h e e t
t h i c k n e s s of t h e p l u r a l s t e e l s h e e t s ,
0 . 5 =< P2 3 . 0 t ' " ~ ' . . . ( 2 )
1 . 0 x P2 < PI 5 2 . 0 X P2 . . . ( 3 )
wherein t h e welding c u r r e n t I1 is s e t w i t h i n a
r a n g e o f n o t l e s s t h a n 30% n o r more t h a n 90% of t h e
w e l d i n g c u r r e n t 12, and
wherein t h e second welding s t e p is s t a r t e d w i t h i n
0 . 1 (s) a f t e r t h e f i r s t welding s t e p is f i n i s h e d .
[Claim 31 The s p o t welding method of t h e h i g h -
s t r e n g t h s t e e l s h e e t s e x c e l l e n t i n j o i n t s t r e n g t h
a c c o r d i n g t o c l a i m 1, wherein any gap between t h e
s t a c k e d s t e e l s h e e t s b e f o r e t h e s p o t welding is l e s s
t h a n 0 . 5 ( m m ) .
[Claim 4 1 The s p o t welding method of t h e h i g h -
s t r e n g t h s t e e l s h e e t s e x c e l l e n t i n j o i n t s t r e n g t h
a c c o r d i n g t o c l a i m 2, wherein a t l e a s t one of gaps
between t h e s t a c k e d steel s h e e t s b e f o r e t h e s p o t
welding i s 0 . 5 (mm) o r more.
[Claim 51 The s p o t welding method of t h e h i g h -
s t r e n g t h s t e e l s h e e t s e x c e l l e n t i n j o i n t s t r e n g t h
a c c o r d i n g t o c l a i m 1, t h e method c o m p r i s i n g a t h i r d
welding s t e p b e i n g p o s t w e l d i n g a f t e r t h e second
welding s t e p b e i n g t h e main w e l d i n g ,
w h e r e i n , w i t h a welding c u r r e n t and a welding
t i m e of t h e t h i r d welding s t e p b e i n g r e p r e s e n t e d by
I3 (kA) and T 3 ( s ) r e s p e c t i v e l y , and w i t h a nonwelding
t i m e between t h e s e c o n d w e l d i n g s t e p and t h e
t h i r d w e l d i n g s t e p b e i n g r e p r e s e n t e d by TC ( s ) ,
t h e welding c u r r e n t I3 is set w i t h i n a r a n g e o f
n o t l e s s t h a n 3 (kA) n o r more t h a n 15 (kA),
t h e non-welding t i m e TC is s e t w i t h i n a r a n g e of
n o t l e s s t h a n 0 ( s ) nor more t h a n 0 . 2 ( s ) , and
a r e l a t i o n between t h e w e l d i n g c u r r e n t I3 and t h e
welding t i m e T3 is set w i t h i n a r a n g e e x p r e s s e d by
t h e f o l l o w i n g e x p r e s s i o n ( 4 ) .
I3 X T3 ( 0.7 t- TC . . . ( 4 )
[Claim 61 The spot welding method of the highstrength
steel sheets excellent in joint strength
according to claim 2, the method compris~ng a third
welding' step being post welding after the second
welding step being the main welding,
wherein, with a welding current and a welding
time of the third welding step being represented by
I3 (kA) and T3 (s) respectively, and with a nonwelding
time between the second welding step andthe
third welding step being represented by TC (s),
the welding current I3 is set within a range of
not less than 3 (kA) nor more than 15 (kA),
the non-welding time TC is set within a range of
not less than 0 ( s ) nor more than 0.2 (s), and
a relation between the welding current I3 and the
welding time T3 is set within a range expressed by
the following expression (4).
I3 X T3 ( 0.7 + TC . . . ( 4 )