SPOT WELDING METHOD ~-
Tecl~nicaFl ield
[0001] The present invention relates to a spot welding method employing a 11igh strength
steel sheet having a tensile strength of 980 MPa or greatel; used in autotnotive fields and the
like.
Background Art
[0002] Recently, in the auton~otivefi elds, there is demand for more lightweight vehicle
bodies in order to achieve lower fuel consutnption and a reduction in COZe n~issionsa, nd
there is also demand for higher vehicle body rigidity in order to improve collision safety. In
order to meet these demands, the need to e~nployh igh strength steel sheets in vehicle bodies,
components, and the like is on the rise.
Processes in vehicle body assembly, colllpotlent attachtnent, and the like
predominatltly employ spot welding. However, there have liitherto been issues regarding the
tensile strength ofjoitlts, particularly when spot welding is used on high strength steel sheets
having a high tensile strength.
[0003] There is a large amount of carbon etc. contained in the base material of a high
strength steel sheet in order to attain its strength. Moreover, in spot welding, rapid cooling is
performed imnlediately after heating the weld portion, such that the weld portion adopts a
martensite structure, increasing the hardness of the weld portion and a heat affected zone, and
reducing ductility.
[0004] 111 spot welding of high strengtli steel sheets, methods using a two-stage welding, in
which a post heating welding is perfomled after a main welding, exist as methods to improve
spot weld portion ductility and secure joint strength.
For exan~plei,n Japanese Patent Application Laid-Open (P-A) No. 2002-103048, a
tempering welding is performed once a fixed duration has elapsed after a spot welding has
ended. The spot weld portion (a nugget portion and a heat affected zone) is annealed,
reducing the hardness of the weld portion. JP-ANo. 2010-115706 describes a method in
which, after for~ningth e nugget with a tnait~w elding, a post heating welding is performed
with a current value of the main welding current value or greater
SUMMARY OF INVENTION
Tecl~nicaPl rob1e111
[0005] Recently, issues of delayed fracture (hydrogen embrittlement) are emerging as
problems when a high strength steel sheet having a high tensile strength has been spot welded.
A high strength steel sheet having a carbon content of 0.15% by mass or greater and a tensile
1
strength of 980 Mpa or greater includes a large atnount of quenching elenletlts in addition to
C, such as Si atid Mti, such that quenching and hardening occur wlien the spot weld portions
are heated and cooled during welding. Moreovel; in a cooling process with thernial
contraction, the weld portion is pulled from its periphery, such that there is a residual tensile
stress distribution at root11 temperature.
[0006] Three major factors governing delayed fkacture are the hardness of the steel sheet,
residual stress, and the hydrogen content of the steel. Spot weld portions in the high strength
steel sheet have high hardness and residual tensile stress distributions as described, above, and
are therefore locations vulnerable to delayed fracture should t~ydrogenp enetration occur.
However, conventional two-stage welding neth hods do not give any consideration to
improving the delayed fracture resistance characteristics of the weld portion.
[0007] Accordingly, an object of the present invention is a spot welding nietliod for a high
strength steel sheet having a carbon content of 0.15% by Inass or greater and a tensile strength
of 980 MPa or greater, in which stable high delayed fracture resistance cliaracteristics are
obtained while suppressing variation in hardness reduction by tempering.
Solution to Probleni
[0008] hi order to improve the delayed fiacture resistance cliaracteristics of spot welded
joints, the inventors considered it very important to perform softening between a steel sheet
press contact portion (also referred to as a corona bond), and an end portion of tlie nugget,
where fractures develop and propagate. Due to this consideration, investigation was
accordingly carried out into softening conditions from the steel sheet press contact portion to
the nugget end portion, and into improving delayed fracture resistance cliaracteristics by using
a two-stage welding in which a post heating welding is performed after a main welding.
As a result, it was found that welded joints with iti~provedd elayed fracture resistance
cliaracteristics can be obtained by setting suitable conditions for the pressure up to a main
welding, the pressure after the maiti welding, a cooling time, and a post heating welding.
[0009] The outline of the present invention arising fioni such investigation is as follows.
[I] A spot welding method in whicli liigli strength steel sheets each having a carbon content
of 0.15% by mass or greater and a tensile strength of 980 MPa or greater are overlapped and
spot welded to obtain a spot welded joint, the spot welding method including: a spot welding
process that is split into three processes: coniprising a first welding process that fornls a
nugget; a cooling process that follows tlie first welding process and during whicli a current of
welding is zero; and a second welding process that follows the cooling process and in which
the nugget is softened, and wherein during the spot welding, 12/11 is set to fiom0.5 to 0.8
wherein 11 is a current in the first welding process and 12 is a current in the second welding
2
process, a titlie tc (sec) of the cooling process is set within a range of koni 0.8 x tniin to 2.5 x
tmin wherein tmin is calculated nsing Equation (1) below according to a sheet thickness H
(nitn) of the steel sheets, a welding time t2 (sec) of the second welding process is set within a
range of from 0.7 x tniin to 2.5 x tmin, and pressure applied by electrodes from the cooling
process onward is set to greater than a pressure applied by the electrodes until tlie first
welding process
tniin = 0.2 x I-I~. ...( 1)
[2] The spot welding method of [I], wherein the high strength steel slieets are plated steel
sheets.
Advantageous Effects of Invention
[0010] The present invention is a spot welding method for a high strength steel sheet having
a carbon content of 0.15% by mass or greater and a tensile strength of 980 MPa or greatel;
enabling variation to be suppressed in hardness reduction by tempering, and enabling stable
high delayed fracture resistance characteristics to be obtained, while reducing welding titne.
BRIEF DESCRIPTION OF DRAWINGS
[0011] Fig. 1 is a graph illustrating examples of change in Vtckers hardness in spot weld
portions using a one-stage welding and a two-stage welding.
Fig. 2 is a diagram to explain the basic concept of a welding and pressure application pattern
in spot welding.
. Fig. 3 is a diagrani to explain a range for Vickers hardness testing at nugget end portions
positioned at tlie front and rear of a line of fusion of a spot weld portion, and on a steel sheet
press contact portion.
Fig. 4 is a line graph illustrating relationship between sheet thickness, and tmin and 2.5 x
tmin.
DESCRIPTION OF EI\/LBODIMENTS
[0012] Explanation follows regarding an exemplary etnbodin~ento f the present invention,
with reference to the attached drawings.
W11et1 performing spot welding on high stretigtl~s teel sheets each having a carbon
content of 0.15% or greater by mass and tensile strength of 980 MPa or above, a spot weld
portion configured by a nugget and a heat affected zone is hardened by quenching in heating
and cooling processes of welding. Moreover, in tliermal contraction in the cooling process,
the weld portion is pulled from its periphery, such that there is a residual tensile stress
distribution at room temperature. For vehicle bodies, for example, hydrogen may penetrate
3
the weld portions and cause delayed fiacture (hydrogen embrittlenient crackitlg) during
vellicle body manufacture, or when running in corrosive environnients.
[0013] 111 spot welding with a two-stage welding, the inventors attenipted to improve
delayed fracture resistance cliaracteristics by using post welding for tempering a martensite
structure in the vicinity of a boundary (line of fusiotl) between a nugget fornled by tlie nlain
welding and a base material, to for111 tempered martensite. Specifically, the i~ivetltors
produced nunierous test samples by variously adjusting the ratio of the currertt amount of the
post welding to the current amount of the main welding in the two-stage welding, adjusting
the cooling time after welding, and adjusting the welding time of the post welding. As
illustrated in Fig. 3, the inventors then tested the Vickers hardness along overlapping faces of
the steel sheets, at nugget end pottio~lsp ositioned at the front and rear of the line of fusion
and at a steel sheet press contact portion, and investigated the relationship between the
welding conditions and the Vickers hardness. The inventors also investigated the
relationship between the Vickers hardness and the delayed fracture resistatlce characteristics
at the front and rear of the line of hsion.
[0014] As a result, it was found that achieving a tempered mattensite structure in a range on
either side of the front and rear of the line of hsion, and making the Vickers liardt~ess4 00 or
lower, enables welded joints with excellent delayed fracture resistance cl~aracteristicsto be
achieved.
[0015] Fig. 1 illustrates an example of results of such testing. In the exatnple of Fig. 1,
spot welding was perforriled eti~ployiogs teel sheets having C: 0.22% by mass, a tensile
stret~gtho f 1510 MPa, and a sheet thicktless of 2 mm. The origin 0 of the horizotltal axis in
Fig. 1 indicates an intersectioll between overlapping faces of the steel sheets that form a steel
sheet press cotitact portion, and the line of hsiotl of the nugget. The positive side indicates
tlie distance from the origin toward the overlapping faces, and the negative side indicates the
distance from the origin toward the nugget. The black circles refer to an example in which
only a main welding was performed, and a post welding was not performed, atid the white
circles and the triangles refer to exanlples using the two-stage welding, in which a post
welding was performed after the main welding wit11 a coolitlg period sandwitched
tlierebetween. The white circles indicate at1 example in which welding was perfornled under
suitable conditions, and the triangles indicate an exatnple it1 which welding was perfortued
under unsuitable conditiotls.
[0016] As illustrated in Fig. 2, the two-stage welding enlployed a welding pattern it1 which
the mait1 welding (a first melding process) is performed at a current 11, tlleti cooling is
performed for a cooling time tc, during ~v11ic11a current of welding is zero, and then the post
4
welding (a second welding process) is performed at a current 12 for a welding time of t2.
Welding conditions A in the case of the triangles were I ~ I0.:6, tc: 0.6 sec, t2: 2.0 sec, and
welding conditions B in the case of the white circles were 12/11: 0.6, tc: 1.2 sec, t2: 1.2 sec.
[0017] It can be seen fiom Fig. 1 that when spot welding is performed using a welding
pattern in which oilly a main welding is performed, and a post welding is not petformed, the
Vckers hardness has a high value it1 the vicinity of the nugget end; llowever the hardness in
the vicinity of the nugget end decreases when welding uses the two-stage welding. Even
when employing two-stage welding, the amount of softening varies according to the welding
conditions. There is a snlall alnount of softening in the case of the unsuitable welding
conditions 4 with little difference to when no post welding is employed. However, there is
a large amount of softening in the case of the suitable welding conditions B, and it can be seen
that the Vickers hardness is greatly reduced in the vicinity of the nugget end.
[0018] As a result of investigating the delayed fracture resistance characteristics ofjoints
welded under the welding conditions A and B using a hydrochloric acid pickling test,
described later, cracking was observed.in the weld portion under the welding conditions A that
gave a small amount of softening in the vicinity of the nugget end. However, under the
welding conditions B that gave a large atnount of softening, cracking was not observed in the
weld portion.
[0019] Based on the above results, the inventors then produced numerous test sarnples with
different atnounts of softening, by adjusting the welding conditions and pressure, and
investigated the relationship between hardness in the vicinity of the nugget end, and the metal
structure and delayed fiacture resistance characteristics. As a result, it was found that the
metal structure had a tempered tnartensite structure in a range from -L to +L from the nugget
end, where L is the length of the steel sheet press contact portion, and the average value of the
Vickers hardness in this range was 400 or lower, enabling spot welded joints with excellent
delayed fracture resistance characteristics to be obtained.
[0020] The present invention was arrived at based on such investigative results.
Explanation follows in sequence regarding necessary conditior~so f the present invention.
[0021] Target Steel Sheet
The spot welded joint target of the present invention is a joint formed by spot
welding high strength steel sheets liavitig a carbon content of 0.15% by mass or greatel; and
having a tensile strength of 980 MPa or greater.
This is due to the fact that some or all of the base material structure of such a steel
sheet has a tnartensite structure. After spot welding, the entire nugget and heat affected zone
in the vicinity of the t~nggeth ave a martensite structure, giving rise to issues of delayed
5
&actuse of the joint.
Note that there is no particular upper limit to tlie tensile strength; however, regarding
spot weldability, the upper tensile strength limit is currently approximately 2000 MPa.
100221 Metal Structure and Hardness in tlie Vicinity of the Nugget End
In order to prevent delayed fracture of the spot weld portions, there is a need to
reduce hardness and residual tensile stress in tlie vicinity of the nugget end. In patticular, it
is very important to perform softening between tlie steel slieet press-contact portion and a
nugget end pottion, where fractures develop atid propagate.
[0023] In the present invention, as a result of investigation such as described above, it was
found tliat the tiietal structure in a range frotii -L to +L had a tempered martensite structure in
a cross-section of the welded joint including the nugget, wherein the origin (0 point) is the
intersection between tlie steel sheet press-contact portion and tlie line of fusion of the nugget,
and L is tlie length of the steel sheet press-contact portiot~, as illustrated in Fig. 3. Moreover,
tlie Vickers hardtiess within this range had an average value of 400 or lower, enabling a
marked improvement in delayed fracture resistance characteristics.
[0024] In the present invention, it is thought that using the post welding that follows tlie
main welding to heat the nugget and tlie welding heat affected zone to reduce the hardness in
the vicinity of tlie nugget end over the range described above, enabled a reductiot~in residual
tensile stress to be achieved at the same time, and thereby improved tlie delayed fracture
resistance characteristics.
[0025] Note tliat the Vickers hardness may be tested using a method such as the following.
Firstly, a spot welded joint formed by spot welding is cut to give a cross-section that
is orthogonal to tlie sheet surface and tliat passes through center of tlie nugget. A sample
piece including tlie nugget is cut out froni tlie cut piece, is embedded in a resin or tlie like, and
tlie face of the cross-section is polished. Then, as illustrated in Fig. 3, tlie Vickers hardness
Hv is tested froni tlie inside of tlie nugget to a position tliat has undergone virtually no
softening, along a line running parallel to tlie overlapping faces of the steel sheet
press-contact portion, at a displacetiletit of 0.2 mni from the overlapping faces.
[0026] Chet~iicalC oniposition of the Steel Sheet
The chetiiical coniposition of tlie steel sheet is not particularly limited other than in
carbon content, and a known hot stamp material, a tliiti steel slieet referred to as super Iligli
tensile, or a plated thin steel slieet is employed. As a more specific example, a steel sheet
having a chemical co~iipositiotis uch as below may be employed.
For example, a steel sheet tiiay be employed that is composed of, in percentage by
mass, C: from 0.15% to 0.50%, Si: from 0.01% to 2.50%, Mn: from 1.0% to 3.0%, P: 0.03%
6
or less, S: 0.01% or less, N: 0.0100% or less, 0: 0.007% or less, and Al: 1.00% or less, witli
the remainder being Fe and utiavoidable impurities, and hrther including eletiients selected
fiom tlie group consistilig of (a) to (c) below, if required:
(a) One, or two, or more, out of Ti: from 0.005% to 0.10%, Nb: from 0.005% to 0.10%, and
V: from 0.005% to 0.10%
(b) One, or two, or more, selected from out ofB: from 0.0001% to 0.01%, Cr: fro111 0.01%
to 2.0%, Ni: from 0.01% to 2.0%, Cu: from 0.01% to 2.0%, Mo: from 0.01% to 0.8%.
(c) At least one out of tlie group consisting of Ca, Ce, Mg, and the rare earth metals (REM),
up to a total of fro111 0.0001% to 0.5%.
[0027] Spot Welded Joint Manufacturing Method
Spot welding is performed as follows to fort11 the spot welded joint described above.
Fig. 2 illustrates an example of a welding pattern in a spot welding process. In this
current pattern, firstly, a first welding process is performed to form the nugget with a primary
current 11 as the main welding, while applying a specific pressure PI. Then, a pressure P2,
this being larger than the pressure that was applied until the first welding process, is applied,
welding is conipletely stopped, and after a cooling process of cooling time tc has elapsed, a
second welding process is performed to sofieti the nugget and tlie periphery thereof with a
post welditig at a secondaty current 12 for a welding time t2. Then, after completitig the
welding of tlie second welding process, welding electrodes are removed from tlie steel sheets
at the point when a specific holding time has elapsed, and the pressure is released. P2 - P1 ?
50 kgf (490N). The difference (P2-PI) between tlie pressures does not have a particular
specified upper limit, but is set within a range in which there are no obvious dents
(indentations) in the steel sheet.
[0028] In the above, the welding conditions are set witliin the following ranges.
Iz/Il: fioni 0.5 to 0.8
tc: fro111 0.8 x ttnin to 2.5 x tniiti
t2: fro111 0.7 x ttniti to 2.5 x tniin
Note that tmiti witli respect to the sheet thickness H (mni) is expressed by tlie following
Equation (1).
ttiiiti = 0.2 x H'
The reasons for these welding conditions are explaiued below.
[0029] 12/21: Frorii 0.5 10 0.8
I11 order to softell the weld pottioli with the post welding and achieve a hardness
distribution with an average Vickers hardness value of 400 or lower in the rauge from -L to
+L, it is necessary to set the current 12 in the post welding within a range of (0.5 to 0.8) x 11
7
with respect to the current 11.
[0030] If I?/ll is below 0.5, the heating temperature during the post welding is low, and tlie
degree of softening at the weld portion is itisufficient, or the region that is softened does not
cover tlie entire range from -L to tL. The delayed fracture resistance cl~aracteristic
improving effect is accordingly small. However, if 1~A1e xceeds 0.8, then tlie nugget formed
by the main welding and tlie heat affected zone (HAZ) is re-heated to tlie austenite phase
region, and quenching occurs in tlie subsequent cooling process. Since softening therefore
cannot be achieved, an improvement in the delayed fracture resistance characteristics cannot
be achieved.
Even if 12/11 exceeds 0.8, the weld portion can sometimes be brought to the
temperature range in which softening occurs if t2 is set to a very short duration. However, if
variation occurs in the gap between the steel sheets, or in tlie contact state between the
electrodes and the steel sheets, sonietinies the weld portion strays from the correct
temperature and softening cannot be achieved. Namely, stable softening is difficult to
achieve.
12/11 is preferably set within a range of from 0.55 to 0.75, this being desirable from
the viewpoint achieving stable and adequate softening.
Note that 11 is determined according to the desired nugget diameter.
[003 11 fc:fr.or~0r. 8 x firri?~f o2 .5 x frrrirl
In order to soften the weld portion in the post welding and achieve the required
hardness distribution, it is necessary to set the cooling time tc after the main welding to within
a range of fro111 0.8 x tmin to 2.5 x tmin, calculated according to the steel sheet thickness H
using the equation (0.2 x H').
Iftc is below 0.8 x tmin, the temperature drop during the cooling process after the
main welding is insufficient, and there is little or no ~nartensiteg eneration within the nugget
and the HAZ (namely, most or all is still present as austenite). Softening (tempering) during
the post welding is accordingly sonieti~ilesu nacl~ievable.
However, if tc is above 2.5 x tmin, as well as entailing reduced productivity,
tempering is also insufficient utiless the duration of the subsequent post welding is made very
long, and sometimes delayed fracture cannot be prevented. The range of tc is accordingly
set to fro111 0.8 x to 2.5 x tmin.
Note that in cases in which the sheet thickness of tlie steel sheet for spot welding
varies, the average sheet thickness is taken as H.
[0032] 12:fr'01101. 7 X Nrrirl fo2 .5 X firtirr
In order to soften the weld portion and achieve the required l~ardtiessd istribution in
8
tlie post welding, it is necessary to set tlie time t2 of tlie post welding after the cooling process
to fiotii 0.7 x tmin to 2.5 x tmin.
If t2 is less tlian 0.7 x tniin, the temperature iticrease during tlie post welding is
insufficient, atid sometimes softening (tempering) of the martensite within the nugget and the
HAZ is unachievable. However, sitice softetiitig by tenipering depends more heavily on the
temperature tlian on tlie duration for which it is maintained, even if tc exceeds 2.5 x tmin, the
temperature distribution of the weld portion attains a steady state, with little change to the
hardness distribution of the weld portion, and with a fiirther reduction in productivity. The
range of t2 is accordingly set to from 0.7 x to 2.5 x tmin.
[0033] Fig. 4 illustrates a relationsliip between slieet thickness, and tniin and 2.5 x tmin.
The slieet thickness range corresponds to, for example, a range of sheet thicknesses in
automotive steel slieet.
[0034] The present invetition is configured as described above. Further explanation
follows regarding implementability and advantageous effects of the present invention using
examples.
Examples
[0035] Example 1
Cold-rolled steel sheets having C: 0.22% by mass, slieet thickness: 2 mm, and tensile
strength: 1510 MPa were prepared. The steel sheets were overlapped and spot welding was
performed under the conditions listed in Table 1 using a servo-guu welding machine, to
produce sample spot welded joints (number n=30) for Vickers hardness testing. Tlie main
welding conditions were kept constant, A squeezing time between applying pressure and the
welding, and a pressure holding titiie after tlie post welding, were also kept constant. The
pressure until tlie main welding process was set to 450 kgf (4410N). Tlie pressure from tlie
cooling process onward was set to the three values of 450 kgf (4420N), 500 kgf (4900N), and
650 kgf (6370N).
Note that preliminary testing was performed prior to spot welding, atid, based on the
preliminary testing results, the current amount of the main welding in spot welding was set
such that the nugget diameter attained a value of 4.5 tinies tlie square root of tlie sheet
thickness of a single steel slieet (4.5dt).
[0036] As illustrated in Fig. 3, the Vickers liardness within tlie nugget atid in tlie heat
affected zone was tested at a pitch of 500 pni along a joint interface of a cross-section cut in
tlie slieet thickness direction at tlie plate width center of the satiiple piece produced above.
Note that Vickers hardtiess testing was performed based on JIS-Z2244 under a load of 200 gf
(1.96133N).
9
[0037] For hydrochloric acid pickling testing, a steel sheet having a sheet thickness of 1.4
n~nwi as inserted between both edges of the cold-rolled steel sheet mentioned above to open a
gap at a central spot welding portion, and spot welding was performed sinlilarly to the above
at the central portion in a restrained state of both edges, so as to produce test pieces in a state
with stress applied to the weld portion. The presence or absence of cracking was
iuvestigated after pickling the test pieces in 0.15 N (normalities) hydrochloric acid for 100
hours. The presence or absence of cracking was confrrtned by cutting to give a cross-section
of the spot welded joint formed by spot welding orthogonal to the sheet surface and passing
tlxough the center of the nugget, cutting out a sarllple piece including the tlugget fiotn the cut
piece, embedding the satnple piece in resin or the like, polisliit~gt he cross-sectiotl face, and
i~lspectir~thge polished cross-section using an optical microscope.
[0038] The test results and evaluation results thereof are also listed in Table 1. Note that L
was 0.6 mm.
As demonstrated in Table 1, in the examples of the present invention, cracking did
not occur in hydrochloric acid pickling testing, and spot welded joints with excellet~dt elayed
fiacture resistance characteristics were obtained.
However, in a comparative exatllple that did not satisfy the conditions of the present
invention regarding the pressure in the cooling process onward, cracking was observed in
hydrochloric acid pickling testing, and spot welded joints with excellet~dt elayed fracture
resistance characteristics were not obtained.
[0039] Table 1
Pressure
until main
welding
process
(kg0
Pressure
cooling
process
onward
Main
Main
welding
welding
current
time t 1 (s)
11 (kA)
I
Cooling
time tc
(3)
tmin
(s)
-
0.8
Post
welding
current I2
Post
welding
time t2 (s)
2.5 x
tmin (s)
2
Average
hardness in
-L to +L
range
(Hv)
370
Hydrochloric
acid pickling
testing result
(number of
cracks, n=30)
0130
Comments
Example of the
present invention
Example of the
present invention
Comparative
example
[0040] Example 2
-- Cold-rolled steel sheets having C: 0.22% by masqsheet thickness: 2 mm, and tensile
strength: 1510 MPa were prepared. The steel sheets were overlapped atid spot welding wes
performed sitililarly to in Exanlple 1, under the conditions listed it1 Table 2. Vickers
hardness testitig atid l~ydrochlorica cid pickli~igte sting were performed. Tlie pressure until
the ~ilaitwl elding process was set to 450 kgf (4410N). Tlie pressure fro111 the cooli~lg
process onward was set to 550 kgf(5390N).
[0041] The test results and evaluatio~rle sults thereof are also listed i11 Table 2. Note that L
was 0.8 mm.
As demonstrated it1 Table 2, in the exatilples of the presetit itlvention, cracking did
not occur in hydrochloric acid pickling testitig, and spot welded joints with excelle~idt elayed
fracture resistance characteristics were obtained.
However, for co~nparativee xamples it1 which the post welding was not perfor~iled,o r
the cooling titile and post welding time did not satisfy the cotiditions of the present invention,
cracking was observed in iiydrochloric acid pickling testing, and spot welded joints with
excellelit delayed fracture resistance characteristics were not obtained.
[0042] Table 2
Sheet
thickness
(mm)
2
-7
Base
material
strength
(MPa)
1510
1510
B Comparative
2 1510 450 550 8.0 0.36 -3 0.8 0.60 0.8 2 410
(cracking present) example
Pressure
until
main
welding
process
@sf,
450
450
2
2
2
2
Pressure
from
cooling
process
onward
@sf,
550
550
1510
1510
1510
1510
450
450
~~~~~
450
450
Main
welding
current
I1 (U)
8.0
8.0
550
550
550
550
Main
welding
time tl
6)
0.36
0.36
tmin
6)
0.8
0.8
Hydrochloric acid
pickling testing
result
B
(cracking present)
B
(cracking present)
Cooling
time tc
6)
-0.6
8.0
---------
8.0
8.0
8.0
Comments
Comparative
example
Comparative
example
Post
welding
current
12 w.1
4.8
0.36
0.36
I2Al
0.60
1.2
1.2
Post
welding
time t2
6)
2
0.36
0.36
0.8
0.8
2.5 x
tmin
(9
2
2
1.2
1.2
Average
hardness
in from
-L to +L
"WF
(Hv)
485
450
-3
- 0.8
0.8
0.50
4.8
4.8
5.2
3
0.60
0.60
0.65
2
-0.j
1.2
1.2
435
2
2
2
B
(cracking present)
Comparative
example
420
370
365
B
(cracking present)
G
(no cracking)
G
(no cracking)
Comparative
example
Example of
the present
invention
Example of
the present
invention
[0043] Example 3
Cold-rolled steel sheets having C: 0.21% by Inass, sheet thickness: 1.2 mm, and
tensile strength: 1486 MPa were prepared (11utliber n = 3). The steel sheets were overlapped
and spot welding was perfornied similarly to it1 Example 1, under the conditiotis listed in
Table 3. Vickers hardness testing and I~ydrocl~loraicci d pickling testing were perfor~i~ed.
The pressure until the main welding process was set to 350 kgf (3430N). The pressure fro111
the cooling process onward was set to 450 kgf (4410N).
The test results and evaluation results thereof are also listed in Table 3. Note that L
was 0.5 mm.
As de~ilonstratedin Table 3, in the present Example, in the examples of the present
invention, cracking did not occur in liydrochloric acid pickling testing, and spot welded joints
with excelletit delayed fracture resistance characteristics were obtained.
However, for co~iiparativee xamples in which the post welding was not performed, or
the cooling time and the current conditions of the post welding did not satisfy the conditions
of the present invention, cracking was observed it1 liydrochloric acid pickling testing, and spot
welded joints with excellent delayed fracture resistance characteristics were not obtained.
sheet
thickness
(mm)
material
process
Pressure
from
cooling
process
onward
F&
450
-
Main
weldin
g
current
11 (kA)
Main
welding
time tl
(S)
Cooling
time tc
(5)
Post welding
tmin
current I2
(s)
W)
Post
welding
time t2
(S)
Average
hardness
in from
-L to +L
range
(Hv)
Hydrochloric acid
Comments
pickling testing
result
B Comparative
(cracking present) example
B Comparative
(cracking present) example
B C y i v e
(cracking present) example
B Comparative
(cracking present) example
B Comparative
(cracking present) example
Example of
the present
(no cracking)
invention i
the present
(no cracking)
invention
[0045] As can be seen from tlie evaluation results of the above Examples, in spot welding,
when the pressure P2 applied by tlie electrodes from the cooling process onward is set greater
than the pressure P1 applied by the electrodes until tlie first welding process, tlie contact
surface between the electrodes and the steel sheets increases, iticreasirig the effect of heat
extraction from tlie steel sheets. The tirile until the te~iiperaturef alls to an Ms point (tlie
temperature at which martensite appears), namely the required duration of the cooling process,
is thereby reduced. Moreover, variation can be suppressed in the reduction of solidificatioti
segregation and reduction in hardness by tetiipering, enabling variation in delayed fracture
evaluation results to be sin~ilarlys uppressed. This thereby enables stable suppression of
delayed fracture (lydrogen embrittle~nenct racking).
Reducing the time required for the coolitig process enables a reductioti it1 the overall
titile required for spot welding. Moreover, the time saved in the cooling process can be
allocated to the second welding process, and tenyering time call be increased, enabling stable
reduction it1 hardness. The expulsion of lydrogen from the spot weld portion is also
promoted.
[0046] The entire contents of the disclosure of Japanese Patent Application No. 2013-86837,
filed on April 17, 2013, are i~icorporatedb y reference in the present specification.
All publicatiotis, patent applications and technical standards mentioned it1 the present
specificatioti are incorporated by reference in the presetit specification to tlie sanie extent as
tlie individual publication, patent application, or teclinical standard is specifically and
individually indicated to be incorporated by reference.

CLAIMS
1. Aspot welding method in which high strength steel sheets each liaving a carbon content
of 0.15% by Illass or greater and a tensile strengtli of 980 MPa or greater are overlapped and
spot welded to obtain a spot welded joint, the method coniprising:
a spot welding process that is split into three processes, comprising:
a fi rst welding process that forms a nugget;
a cooling process that follows tlie first welding process and during which a
current of weldi~~isg z ero; and
a second welding process that follows the cooling process and in which tlie
nugget is softened,
wherein during the spot welding process,
12/11 is set to from 0.5 to 0.8 wherein 11 is a current in tlie first welding
process and 12 is a current in the second welding process,
a time tc (sec) of tlie cooling process is set within a range of fiom 0.8 x tmin
to 2.5 x tn~irwi herein tmin is calculated usilig Equation (1) below accor-dingt o a sheet
thickness H (mm) of the steel sheets,
an welding time t2 (sec) of the second welding process is set within a range
of from 0.7 x trnin to 2.5 x trnin, and
a pressure applied by electrodes from the cooling process onward is set to
greater than a pressure applied by the electrodes until the first welding process
tmin = 0.2 x 13'. . . . (1)
2. The spot welding method of claim 1, wherein the high strength steel sheets are plated
steel sheets.