The present invention relates to a welding method
comprising superposing and spot welding tensile strength 750 to
2500 MPa high strength steel sheets used in the automobile + field etc. and a spot welded joint formed by that welding
method.
[BACKGROUND ART]
[OOOZ]
In recent years, in the automotive field, car bodies have
been reduced in weight to reduce fuel consumption and cut C02
emissions and have been improved in collision.safety by use of
high strength steel sheet for the car bodies and their parts.
For assembly of car bodies and attachment of parts etc., spot
welding is mainly being used. In spot welding of high strength
steel sheets, the strength of the welded joint becomes an
lssue.
[0003]
In a joint formed by superposing and spot welding steel
sheets (below referred to as a "spot welded joint"), the
tensile strength is an important characteristic. Normally, the
mechanical characteristics of a spot welded joint are evaluated
by the tensile shear strength (TSS) measured by applying a
tensile load in the direction shearing the steel sheets and by
the cross tensile strength (CTS) measured by applying a tensile
load in a direction peeling apart the steel sheets. The methods
of measuring the tensile shear strength and the cross tensile
strength are defined in JIS Z 3136 and JIS Z 3137.
[0004]
In tensile strength 270 to 600 MPa steel sheets, the CTS
of the spot welded joint increases along with an increase in
the steel sheet strength, so there is little chance of a
problem arising relating to the joint strength. However, in
tensile strength 750 MPa or more steel sheets, even if the
tensile strength of the steel sheets increases, the CTS does
not increase, but conversely decreases.
[0005]
In general, in the case of high strength steel sheets, the
concentration of stress in a weld increases due to the drop in
the deformation ability. Further, the weld is easily hardened
and the fracture toughness of the weld falls and therefore the
CTS falls. For this reason, improvement of the CTS in the spot
welded joint of 750 MPa or more high strength steel sheets is
sought.
[0006]
As the method of securing strength in a spot welded joint
of high strength steel sheets, a two-stage conduction method of
main conduction, then post heating and conduction has been
proposed. For example, PLT 1 discloses a method of running
current for tempering after a certain time elapses from the end
of the main conduction and annealing the spot welded joint
(nugget part and heat affected zone) to lower the hardness.
[0007]
However, in this method, there is the problem that to
cause the martensite transformation to be substantially
completed before the operation of running current for
tempering, a long cooling time becomes necessary.
[OOOE]
Further, as a method of securing strength in a spot welded
joint of high strength steel sheets, a method of heating a weld
by another heating means after welding has been proposed. For
example, PLT 2 discloses a method of heating and tempering a
weld at a high frequency after welding.
[0009]
However, in this method, there are the problems that (i) a
separate step becomes required after welding and the work
procedure becomes complicated and (ii) a special apparatus
becomes required for utilizing the high frequency.
[OOlO]
Further, PLT 3 discloses a method of formlng a nugget by
main welding, then running a current of the main welding
current or more as post-conduction. However, in this method, if
making the post-conduction time longer, the nugget size becomes
larger and, further, the nugget structure becomes the same as
L
?, the structure of a usual weld.
i
!, [OOll]
5 PLT 4 discloses a method of spot welding tensile strength
440 MPa or more steel sheets comprising restricting the
chemical composition of the steel sheets to CxP10:.0025, P:
0.015% or less, and S: 0.01% or less, welding the sheets, then
i
!
k heat treating the weld by 300°Cx20 minutes or so to 5 raise the
t strength in the peeling direction of the joint. However, in $ 1 this method, (i) the steel sheets which can be used are limited
and (ii) a long time is required for welding, so the
productivity is low.
[0012]
PLT 5 discloses a spot welded joint of high strength steel
sheets (tensile strength: 750 to 1850 MPa, carbon equivalent
Ceq: 0.22 to 0.55 mass%) which defines the microstructure of
the outside layer region of the nugget and the average particle
size and number density of carbides in the microstructure.
However, with just this provision relating to the carbides, it
is not possible to obtain a highly reliable joint strength.
[0013]
PLT 6 discloses a spot welding method giving mechanical
vibration to a scheduled welding location to refine the
structure of the weld when the scheduled welding location
changes from a molten state to solidified structures. However,
in this method, there is little propagation of mechanical
vibration to the melt zone. To obtain the above effect, there
is the problem that the apparatus ends up becoming large in
size.
[CITATION LIST]
[PATENT LITERATURE]
[0014]
[PLT 11 Japanese Patent Publication No. 2002-103048A
[PLT 21 Japanese Patent Publication No. 2009-125801A
[PLT 31 Japanese Patent Publication No. 2010-115706A
[PLT 41 Japanese Patent Publication No. 2010-059451A
[PLT 51 International Patent Publication No. 2011/025015A
[PLT 61 Japanese Patent Publication No. 2011r194411A
[SUMMARY OF INVENTION]
[TECHNICAL PROBLEM]
[0015]
The present invention has as its problem to raise the
fracture toughness of the spot welded metal and improve the
spot welded joint strength and as as its object to provide a
spot welded joint and spot welding method solving this probem.
[SOLUTION TO PROBLEM]
[0016]
The inventors et al. engaged in in-depth studies on a
technique to solve such a problem. As a result, the inventors
et al. discovered that in the process of solidification of the
melt zone, if applying electromagnetic vibration of the
required frequency to the melt zone, the solidified structures
become more refined and the fracture toughness of the spot
welded metal is remarkably improved.
[0017]
The present invention was made based on the above
discovery and has as its gist the following:
[0018]
(1) A spot welded joint, said spot welded joint
characterized in that at least there are 375 or more crystal
grains of solidified structures in a 0.5 mmx0.5 mm region of a
nugget end.
[0019]
(2) The spot welded joint according to (I), wherein in
said spot welded joint, a ratio (=CTS/TSS) of a cross tensile
strength CTS and a tensile shear strength TSS is 0.33 or more.
I
[0020]
1 (3) A spot welding method comprising, said spot welding
,I
method characterized by performing the main conduction, then
I lowering the input heat to cause the melt zone to solidify
i
$ during which applying electromagnetic vibration to the melt
zone.
[0021]
(4) The spot welding method according to (3) characterized
by preparing a spot welded joint according to (1) or (2).
[0022]
(5) The spot welding method according to (3) characterized
in that a frequency f, of electromagnetic vibration according
to (3) is a frequency where an indicator A deEined by the
following formula (1) satisfies the following formula (2):
A=vs/ (hD .fv) ... (1)
fv: frequency
I ~ vs: solidification speed
!i .I ID: arm interval of dendrites
A24.0 ... (2)
[0023]
(6) The spot welding method according to (5) characterized
by preparing a spot welded joint according to (1) or (2).
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0024]
According to the present invention, the toughness of the
:: spot weld can be remarkably raised, so it is possible to
il
I provide a highly reliable spot welded joint.
d
!!II [0025]
I1
:! [BRIEF DESCRIPTION OF DRAWINGS]
[FIG. 11 is a view schematically showing a current pattern
at the time of spot welding.
[FIG. 21 is a view schematically showing a mode where a
1 solidifying melt zone is subjected to electromagnetic
!
I vibration.
[FIGS. 31 are views schematically showing modes of growth
and split of dendrites, (a) shows the mode of growth of
dendrites while (b) shows the mode of split of dendrites.
[DESCRIPTION OF EMBODIMENTS]
[0026]
The spot welded joint of the present invention (below,
sometimes referred to as "the present invention welded zone")
is a spot welded joint characterized in that at least there are
375 or more crystal grains (1500/ rnrnz) of solidified structures
in a 0.5 mmx0.5 mm region of a nugget end.
[0027]
Further, the spot welding method of the present invention
(below, sometimes referred to as the "welding method of the
present invention") is a spot welding method characterized by
performing the main conduction, then lowering the input heat to
cause the melt zone to solidify during which applying
electromagnetic vibration to the melt zone.
[0028]
First, a welding method of the present invention will be
explained.
[0029]
First, current is run for securing the necessary nugget
size. Any form of conduction for forming this melt diameter may
be selected so long as the solidification is started.
Multistage conduction is also possible. At the stage where the
melt diameter has been secured, a current value lower than the
current value for maintaining the melt diameter is run and
electromagnetic vibration is applied while causing
solidification.
[0030]
FIG. 1 is a view schematically showing a current pattern
at the time of spot welding in the simplest case in the welding
method of the present invention.
[0031]
In the welding method of the present invention, the melt
I
I diame-ter is secured by the time tW(=t2-tl) and current value I,,
then, after that, the melt zone is made to solidify while
running a current Iv lower than the current holding the molten
diameter of the melt zone for the time tv(=t3-t2). At that time,
an AC current is applied whereby an electromagnetic vibration
of a frequency of two times the conduction frequency is
generated. That is, in the solidification process from the
times t2 to t3, electromagnetic vibration is applied to the
melt zone while solidifying.
[0032]
FIG. 2 is a view schematically showing a mode where a
solidifying melt zone is subjected to electromagnetic
vibration. The steel sheet la and the steel sheet lb are
superposed and the main conduction is performed at the current
value I, for the time t, (see FIG. 1) to form the melt zone 2.
At the time t2 after the elapse of t,, the current value is
lowered to I, (see FIG. 1) and solidification of the melt zone
is started.
[0033]
In this solidification, dendrites grow from the
surroundings of the melt zone toward the center of the melt
zone to end solidification. In the welding method of the
present invention, in the solidification process,
electromagnetic vibration 4 is given to the dendrites in their
growth direction 3 whereby the growth of the dendrites is split
and the solidified structures of the melt zone are made finer.
[0034]
The frequency of the electromagnetic vibration is not
limited to a specific frequency so long as the dendrites can be
split, but to reliably split the growth of the dendrites and
make the solidified structures finer grains, it is preferable
to make the frequency one where the indicator A defined by the
following formula (1) satisfies the following formula (2):
A=vs/.(hD.fv).. . (1)
fv: frequency
vs: solidification speed
hD: arm interval of dendrites
A24.O ... (2)
[GO351
F I G S . 3 are views schematically showing modes of growth
and split of dendrites. F I G 3(a) shows the mode of growth of
dendrites while F I G . 3(b) shows the mode of split of dendrites.
[GO361
In a usual method, as shown in F I G . 3(a), the dendrites 3a
grow by a solidification speed vs while maintaining the arm
interval hD then the melt zone finishes solidifying.
[GO371
The present inventors engaged in in-depth research with
the idea that, in the solidification process where dendrites
are formed, the progress of solidification is disturbed by the
electromagnetic vibration and the heat generated by conduction
at the time of the electromagnetic vibration, the growth of
dendrites is split, and equiaxed crystal shaped solidified
structures such as shown in F I G . 3(b) are obtained.
[0038]
As a result, they discovered that if applying
electromagnetic vibration of the frequency fv satisfying the
formula (2) to dendrites grown by a dendrite arm interval hi,
and solidification speed vs, due to the electromagneLic
vibration and the heat generated by conduction at the time of
electromagnetic vibration inside of the melt zone in the
solidification process, disturbances in solidification in the
solidification process occur at intervals of positions of the
following formula (3) :
where Tv: period of electromagnetic vibration (=l/fv)
[0039]
In the solidification step of the melt zone, due to the
periodic disturbances in the melt zone, as shown in F I G . 3(b),
the dendrite growth is split resulting in fine grains 3b and
the solidified s-tructures of the melt zone can be made equiaxed
crystal shaped solidified structures.
[0040]
An indicator A defined by the inventors et al. is an
indicator showing the extent of split of the dendrites. That is
it is the dendrite split indicator. If the indicator A
satisfies the formula (2), the desired solidified structures
can be obtained in the melt zone.
[0041]
When the indicator A is "I", the solidified,structures
become equiaxed crystal structures comprised of dendrites of
arm intervals h, split at the arm intervals AD. However, the
toughness of the welded joint may be improved due to the
refinement of the solidified structures, so the indicator A was
given an extra margin and was made 4.0 or less.
[0042]
From the viewpoint of refinement, A is preferably smaller,
but to make the indicator less than 0.3, it is necessary to
raise the electromagnetic vibration frequency. The inductance
loss at the time of conduction for electromagnetic vibration
becomes larger, so the power supply ends up becoming larger in
capacitance, so 0.3 or more is preferable.
[0043]
On the other hand, if the indicator A is over 4.0, the
intervals of disturbing solidification in the solidification
process become wider, the solidified structures become
columnar, and the toughness improving effect is reduced.
Therefore, the indicator A is made 4.0 or less. Preferably, it
is 3.0 or less.
[0044]
In the welding method of the present invention, the
current of the present conduction is not particularly limited.
The current may be the alternating current or direct current of
the commercial frequency. The solidification speed at the melt
I
zone (speed of progression of solidification interface) depends
on the welding conditions, the combination of the steel sheets,
the dimensions of the welded members, etc., but is about (10 to
50)x10-~m /sec. The arm interval of the dendrites depends on
the welding conditions, the combination of the steel sheets,
the dimensions of the welded members, the heat input by
electromagnetic vibration, etc., but is about 5 to 30x10-~m .
[0045]
For example, if the solidification speed is 20x10-~ m/sec,
if running a frequency 500 to lOOOHz AC current, the frequency
fv of the electromagnetic vibration becomes 1000 to 2000Hz
(= (500 to 1000Hz) x2) .
[0046]
The period Tv (=l/fv) at which electromagnetic vibration
occurs once becomes (0.5 to 1.0)x10-~ sec. Each time
:: electromagnetic vibration is applied, the solidification
interface proceeds (10 to 20)x10-~m . That is, disturbance in
solidification due to electromagnetic vibration occurs at a
position substantially equal to the arm interval of the
dendrites. As a result, the solidified structures become
equiaxed crystal shaped solidified structures.
[0047]
In the welded joint of the present invention formed with
an indicator A of 4.0 or less, in the solidified structures of
at least the nugget end, the structures become not extremely
long columnar structures, but equiaxed crystal shape structures
(including equiaxed crystal structures).
[0048]
The inventors employed the number of crystal grains as the
indicator regarding the degree close to the equiaxed crystals
of the solidified structures of the nugget end. Further, in
weld metal with a high fracture toughness of the present
invention, it is understood that there are 375 or more crystal
grains of the solidified structures in a 0.5 mrnx0.5 mm region
of the nugget end.
[0049]
Therefore, in the welded joint of the present invention,
there are 375 or more crystal grains of solidified structures
in a 0.5 mmx0.5 rnm region of the nugget end.
[0050]
If there are less than 375 crystal grains of solidified
structures in a 0.5 mmx0.5 mm region of the nugget end, the
equiaxed crystal shapes of the solidified structures are not
achieved, the effect of improving the toughness of the weld
metal is small, and the strength of the welded jdint is not
improved, so the number of crystal grains is made 375 or more.
Preferably, it is 500 or more.
[0051]
The spot welded joint of the present invention includes
375 or more crystal grains of solidified structures in a 0.5
mmx0.5 mm region of the nugget end, so the fracture toughness
of the weld metal is improved and the mechanical properties of
the joint (in particular the joint strength) are improved.
Regarding the joint strength, the cross tensile strength CTS
and the tensile shear strength TSS are important indicators.
However, to objectively evaluate the mechanical properties of
the weld even if the strength and/or thickness of the steel
sheets used for the spot welding changes, the indicator Z
defined by the following formula (4) was employed.
Z=CTS/TSS . . (4)
CTS: cross tensile strength
TSS: tensile shear strength
[0052]
TSS does not change by the thickness of the steel sheet,
but CTS changes by the thickness of the steel sheet, so the
indicator Z was used to evaluate the mechanical properties of
the welded joint. Further, in the joint of the present
invention, Z is preferably 0.33 or more.
[0053]
If Z is less than 0.33, the difference in the yield
strength in the load direction to the joint is too large and
the properties of the members are liable to become.unstable.
Therefore, Z is preferably 0.33 or more. It is more preferably
0.4 or more. The upper limit of Z is not particularly set.
[EXAMPLES]
[0054]
Next, examples of the present invention will be explained.
The conditions in the examples are an illustration of
conditions employed for confirming the workability and effects
of the present invention. The present invention is not limited
to this illustration of conditions. The present invention can
employ various conditions so long as not deviating from the
gist of the present invention and achieving the object of the
present invention.
[0055]
(Example 1)
The steel sheets shown in Table 1 were prepared and spot
welded by the combinations shown in Table 2. The welding
conditions are shown together in Table 2.
[0056]
[Table 11
Table 1
thickness
[0057]
Carbon equivalent=Carbon amount (mass%) + silicon amount
(mass%)/30 + manganese amount (mass%)/20 + phosphorus amount
(mass%) x 4 + sulfur amount (mass%) x 4
[ O O S E ]
[Table 21
Table 2
[0059]
In the welded joint, the solidified structure of the
nugget end was observed by an optical microscope, and the
number of crystal grains was measured. Further, the cross
tensile strength CTS of the welded joint was measured in
accordance with JIS Z 3137 while the tensile shear stress TSS
was measured in accordance with JIS Z 3136. The results are
shown in Table 3.
[Table 31
Table 3
Number of solidified structures counted in 0.5 mm x 0.5 mm
region of nugget end
I
i
[INDUSTRIAL APPLICABILITY]
> [0061]
As explained above, according to the present invention, it
is possible to remarkably raise the strength of a spot welded
joint, so it is possible to provide a highly reliable spot
welded joint. Accordingly, the present invention has a high
applicability in industries using welding for assembling
structures, for example, the auto industry.
[REFERENCE SIGNS LIST]
[0062]
la, lb. steel sheets
2. melt zone
3. growth direction of dendrite
3a. dendrite
3b. fine grain
4. electromagnetic vibration

[NAME OF DOCUMENT] Claims
[Claim 11
A spot welded joint, said spot welded joint characterized
in that at least there are 375 or more crystal grains of
solidifiedstructures in a 0.5 mmx0.5 mm region of a nugget
end.
[Claim 21
The spot welded joint according to claim 1, wherein in
said spot welded joint, a ratio (=CTS/TSS) of a cross tensile
strength CTS and a tensile shear strength TSS is 0.33 or more.
[Claim 31
A spot welding method comprising, said spot welding method
characterized by performing the main conduction, then lowering
the input heat to cause the melt zone to solidify during which
applying electromagnetic vibration to the melt zone.
[Claim 41
The spot welding method according to claim 3 characterized
by preparing a spot welded joint according to claim 1 or 2.
[Claim 51
The spot welding method according to claim 3 characterized
in that a frequency fv of electromagnetic vibration according
to claim 3 is a frequency where an indicator A defined by the
following formula (1) satisfies the following formula (2):
A=vs/ (hD.fv) ... (1)
fv: frequency
vs: solidification speed
hD: arm interval of dendrites
AS4.O ... (2)
[Claim 61
The spot welding method according to claim 5 characterized
by preparing a spot welded joint according to claim 1 or 2.
[NAME OF DOCUMENT] 'Abstract
[ABSTRACT]
[PROBLEM] To raise the strength of the spot welded joint.
[SOLUTION] A spot welded joint, said spot welded joint
characterized in that at least there are 375 or more crystal
grains of solidified structures in a 0.5 mmx0.5 mm region of a
nugget end.