DESCRIPTION
TITLE OF INVENTION: LASER WELDING METHOD
TECHNICAL FIELD
[0001] The present invention relates to a laserwelding
method suitably used for laser-welding a
plurality of stacked members including a high-tensile
steelsheet._^
BACKGROUND ART
t
[0002] In recent years, in order to meet a demand
for enhancement of fuel efficiency and improvement of
•safety in automobiles, high-strength thin steel
sheets have come to be used for automobile bodies,
and it is required to weld these steel sheets by
using laser welding. Further, in'a method of welding
the high-strength stacked thin,steel sheets, a laser
welding method with which stable strength of joint
portions can be obtained is desired.
[0003] The laser welding uses a laser beam as a heat
source and therefore can surely and easily control an
input heat amount as compared with arc welding such
as TIG welding and MIG welding. Accordingly, it is
possible to reduce thermal deformation by
appropriately setting welding conditions such as a
welding speed and radiation output of the laser beam
and further a flow rate of shielding gas and so on.
Further, the laser welding is capableof welding from
one side and thus is suitable for assembly welding of
" • complicated members such as automobile bodies.
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J
\ / "'
[0004] Actually, the laser welding is often adopted
' for welding members molded from thin steel sheets in
the automobile manufacturing industry, the electric
equipment manufacturing industry, and other fields.
Further, relating to this, there has been proposed a
laser welding method of a lap joint excellent in weld
joint strength.
[0005] Patent Literature 1, for instance, discloses
a method which realizes quality improvement by
«
tempering a first bead by heat of a second bead to
prevent the beads from being easily fractured at the
•time of the molding, thereby improving moldability.
CITATION LIST • *
PATENT LITERATURE
[0006] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2009-000721
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007] In accordance with an increase in strength of
steel sheets, to improve strength of a weld zone has
become an issue. Especially, in lap welding of hightensile
steel sheets whose tensile strength is 780
MPa or more and whose carbon content is 0.07 mass% or
more, strength of the weld zone is sometimes
insufficient in conventional arts.
[0008] The present inventors have studiously studied
a laser welding method for enhancing strength of a
weld zone. As a result, it has been found out that
in welding where a plurality of beads are formed, by
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. . . / z
appropriately forming the beads and making average
' Vickers hardness of the first bead lower than average
Vickers hardness of the second and subsequent beads,
it is possible to obtain a laser-welded joint
excellent in joint strength.
[0009] It is an object of the present invention to
make it possible to enhance strength of a weld zone
and suppress welding deformation.
SOLUTION TO PROBLEM
[0010] A laser welding method of the present
invention is a laser welding method in which at a
•plurality of welding positions in an overlap portion
of a plurality of members including a high-tensile
steel sheet whose carbon content is 0.07 weight% or
more, first beads in a closed loop shape or a closed
loop-like shape and second beads in a closed loop
shape or a closed loop-like shape on inner sides of
the first beads are formed by remote laser welding
for joining, the method including: a procedure for
successively forming the plural first beads at all or
part of the plural welding positions; and a procedure
for successively forming the plural second beads for
the plural formed first beads, wherein, in both of
the cases of the procedure for successively forming
the plural first beads and the procedure for
successively forming the plural second beads, the
beads are each formed at a position except the
closest welding position among the plural welding
pos itions.
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Further, another characteristic of the laser
' welding method of the present invention is that, in
the procedure for successively forming the plural
second beads, the second beads are each formed for
the first bead whose maximum temperature has become
equal to or lower than an Ms point — 50°C.
Further, another characteristic of the laser
welding method of the present invention is that, in
the procedure for successively forming the plural
second beads, the second beads are each formed so
that a temperature of the first bead becomes not
•lower than 400 degrees centigrade nor higher than an
Acl point + 50°C.
Further, another characteristic of the laser
welding method of the present invention is that the
first beads each have a circular shape and the second
beads each have a circular shape concentric with the
first bead; and an angle made by a line connecting a
center of the beads and starting and terminating ends
of the first bead and a line connecting the center
• and starting and terminating ends of the second bead
is 10° or more.
ADVANTAGEOUS EFFECTS OF INVENTION
[0011] According to the present invention, it is
possible to enhance strength of a weld zone and to
suppress welding deformation.
BRIEF DESCRIPTION OF DRAWINGS
[0012] [Fig. 1] Fig. 1 is an explanatory view of an
outline of a laser-welded joint according to an
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embodiment.
' [Fig. 2A] Fig. 2A is an explanatory perspective
, view of an outline of another example of the laserwelded
joint according to the embodiment.
[Fig. 2B] Fig. 2B is a cross-sectional view
taken along the I-I line in Fig. 2A.
[Fig. 3A] Fig. 3A is a view showing an example of
a c l o s e d l o q p - l i k e b e a d s h a p e.
[Fig. 3B] Fig. 33 is a view showing an example of
the closed loop-like bead shape.
[Fig. 3C] Fig. 3C is a view showing an example of
•the closed loop-like bead shape.
[Fig. 3D] Fig. 3D is a view showing an example of
the closed loop-like bead shape.
[Fig. 3E]. Fig. 3E is a view showing an example of
the closed loop-like bead shape.
[Fig. 4A] Fig. 4A is a perspective view showing
an example of a structure member in which weld zones
each including a plurality of beads are formed at a '
plurality of welding positions.
[Fig. 4B] Fig. 4B is a plane view showing the
example of the structure member in which the weld
zones each including the plural beads are formed at
the plural welding positions.
[Fig. 5] Fig. 5 is a view showing an example of a
procedure for forming weld zones each including a
plurality of beads at a plurality of welding
positions of flange portions of a hat member by
remote laser welding.
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i
[Fig. 6A] Fig. 6A is a conceptual view showing a
' remote laser welding system whose laser spot position
^ can be moved at a high speed and is a view showing a
light collecting optical system.
[Fig. 6B] Fig. 6B is a conceptual view showing
the remote laser welding system whose laser spot
position can be moved at a high speed and is a view
showing how the laser spot position moves.
[Fig. 7] Fig. 7 is a chart showing results of an
example 1.
[Fig. 8A] Fig. 8A is a front view showing a one-
• sided hat member of an example 2.
[Fig. 8B] Fig. 8B is a plane view showing the
one-sided hat member of the example 2.
[Fig. 9] ,Fig. 9 is a front view showing a onesided
hat member of an example 3.
[Fig. 10] Fig. 10 is a front view showing a both-,
sided hat member of an example 4.
DESCRIPTION OF EMBODIMENTS
[0013] Hereinafter, suitable embodiments of the
present invention will be described with reference to
the attached drawings. Note that in the below, where
"%" is simply written, it represents "mass%".
(First Embodiment)
A laser-welded joint 1 according to this
embodiment is composed of a plurality of stacked
high-tens lie steel sheets 10 which are joined.
Generally, about two to four high-tensile sheets are
stacked and joined, but the number of the high-
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tensile sheets is not limited. Fig. 1 is an example
.' where two high-tensile sheets are stacked.
[0G14] As the high-tensile steel sheets 10, steel
sheets whose carbon content is 0.07% or more are used,
and when having a 0.07% carbon content or more, they
have high hardenability to harden in a wide heat
input range and have high hardness when quenched. As
a result, it is difficult to ensure joint strength,
especially, strength against a load in an exfoliation
direction. '
[0015] The present invention is an art to
•manufacture a weld joint having sufficient strength
even when such high-tensile steel sheets are welded,
and its target is a high-tensile steel sheet whose
carbon content is 0.07% or more. "An overlap portion
of a plurality of members including a high-tensile'
steel sheet" includes a case where a soft steel sheet,
is stacked on a further outer side of the overlap
portion of the high-tensile steel sheet so that the •
overlap portion is formed by the soft steel sheet +
the high-tensile steel sheet + the high-tensile steel
sheet or by the soft steel sheet + the high-tensile
steel sheet + the high-tensile steel sheet + the soft
steel sheet, and this case is also a target of the
present invention. Further, a case where the overlap
portion is formed by the soft steel sheet + the hightensile
steel sheet is also included, and this case
is also a target of the present invention.
[0016] The weld zone of the weld joint is composed
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of a plurality of beads. Here, let an arbitrary
> point on a toe on a side receiving a higher stress at
, the time of load application among toes of the first
bead 11 on a steel sheet surface be an origin 0 and
let a direction of a toe on a s'ide receiving a lower
stress at the time of the load application and
closest to the origin among the toes of the first
bead 11 on the steel sheet surface be a positive
direction when seen from the origin 0, the second and
ft
subsequent beads are formed more on the positive
direction side than the first bead.
• [0017] A position x of a toe closer to the origin,
of the second and subsequent beads is located within
a range of 0 < x ^ 1.2 W, where W is an average bead
width of the first bead in a sheet thickness
direction. Fig. 1 shows only the second bead 12.
[0018] The first bead is tempered by heat when the
second bead is formed. When a load is applied to the
weld zone, a stress concentrates especially on the
vicinity of a place where a weld line of the first
bead and an overlap portion of the steel sheets
intersect with each other, which is likely to cause a
fracture.
[0019] In the weld joint according to this
embodiment, tempering the first bead improves
ductility of a bead bond section and also can
alleviate the stress concentration on the overlap
portion at the time of the load application, leading
to enhanced resistance against the load application.
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Therefore, the second bead is formed more on the side
' receiving a lower stress at the time of the load
application than the first bead.
[0020] If X is 0 or less, a temperature of the toe,
of the first bead, on the side receiving a higher
stress and the overlap portion becomes an Ac3 point
or higher at the time of the formation of the second
and subsequent beads, and because quenching is done
again, joint strength does not improve. If x is
larger than 1.2 W, heat when the second and
subsequent beads are formed is not transferred to the
• first bead sufficiently, and thus the first bead
cannot be tempe.red and the joint strength is not
improved.
[0021] Forming the beads in such a positional
relation makes it possible to obtain a laser-welded
joint excellent in joint strength, with, average
Vickers hardness of the first bead being lower than
average Vickers hardness of the second and subsequent
beads.
[0022] Increasing a joint area by increasing the
number of the beads to 3, 4, or the like can enhance
shear strength as well. At this time, the third bead
is formed at the same position as that of the second
bead or at a position more on the plus side than the
second bead, and the fourth bead is formed at the
same position as that" of the third bead or at a
position more on the plus side than the third bead.
[0023] If the third and fourth beads are formed at
_ 9 _
the same position as that of the second bead, the
> tempering of the first bead further progresses and a
load in the exfoliation direction can be improved.
However, since the joint area changes little, the
shear strength does not improve.
[0024] On the other hand, when the third bead is
formed more on the plus side than the second bead, or
when the fourth bead is formed more on the plus side
than the third bead, it is possible to achieve the
tempering of the first bead and an increase in the
joint area, enabling improvement both in the strength
•in the exfoliation direction and the strength in a
shear direction.
[0025] In the laser-welded joint according to this
embodiment, the beads have a substantially linear
shape in the sheet thickness direction. Incidentally,
under the condition that the bead shape.is
substantially linear in the sheet thickness direction,
a width of the bead surface that can be confirmed on'
the steel sheet surface or an average width of bead
front and rear surfaces that can be confirmed on
front and rear surfaces of the steel sheet may be
regarded as as a typical bead width, instead of the
average bead width W.
[0026] The weld line of the laser-welded joint may
be linear, but when it is in a closed loop shape or
in a closed loop-like shape, stress concentration on
starting and terminating ends can be alleviated,
which makes it possible to further improve the joint
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strength. Fig. 2A and Fig. 2B show an outline of a
,> laser-welded joint fabricated with a closed loop
where starting and terminating ends overlap.
[0027] The closed loop refers to a shape where
starting and terminating ends of a circle/ an ellipse,
or the like overlap and also includes' a shape partly
having a curve with a different curvature and shapes
such as a triangle, a quadrangle, and the like.
[0028] The closed loop-like shape refers to a shape
t
which has one opening portion 15 or more where the
bead 11 (or 12) is not formed and in which the total
•length of the opening portions 15 is 3/4 of a
circumcircle-equivalent diameter of the bead 11 or
less as shown in Fig. 3A to Fig. BE, for instance.
Fig. 3A to Fig. 3E show examples of a bead shape
having the opening portion(s) 15, the solid line
representing the bead 11 (or 12) and the broken line
representing the opening portion 15. The bead shapes
shown in Fig. 3A to Fig. 3E are examples and the
closed loop-like bead shape is not limited to these.
[0029] The closed loop-like bead shape having the
opening portion is effective for, for example, the
welding of stacked galvanized steel sheets, and the
like. In the case of the welding of the galvanized
steel sheets or the like, when plating between the
steel sheets reaches a boiling point to evaporate and
its volume rapidly increases, a pressure between the
steel sheets increases to blow away a molten pool
during the welding and a flaw occurs in the bead
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unless there is a path of the plating which has
' turned into vapor or gas in an area surrounded by the
bead. Forming the bead in the closed loop-like shape
having the opening portion can prevent this.
[0030] As the laser-welded joint, a high-tensile
steel sheet whose sheet thickness is within a range
of 0.5 to 3.0 mm is suitably used. Even when the
sheet thickness is less than 0.5 mm, a strength
improvement effect of the weld zone can be obtained,
t
but since the strength ofthe joint is governed by
the sheet thickness, a strength improvement effect of
•the entire joint becomes small and an application
range of the member is limited. Further, even when
the sheet thickness is over 3.0 mm, the strength
improvement effect of the weld zone can be obtained,
but in view of weight reduction of the member, an
application range of the member is limited.
[0031] Next, a laser welding method according to
this embodiment will be described. As an apparatus
used for manufacturing the laser-welded joint, the
same one as a conventional apparatus for
manufacturing a laser-welded joint is usable.
[0032] In the manufacture of the laser-welded joint,
after the first bead is formed by laser welding, it
is waited for the temperature of the first bead to
reach equal to or lower than an Ms point — 50°C (Ms
point: martensite transformation starting
. temperature), and thereafter, the formation of the
second and subsequent beads is started.
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[0033] Setting the temperature of the first bead
.' equal to or lower than the Ms point — 50°C causes the
generation of a certain amount or more of martensite
in the steel sheets. Thereafter, due to the heating
for forming the second bead, the aforesaid martensite
is softened by tempering, leading to an increase in
the joint strength.
[0034] If the formation of the second and subsequent
beads is started while the temperature of the first
ft
bead is higher than the Ms point — 50°C, the
martensite is not sufficiently generated and
•accordingly a volume of the martensite tempered by
the formation of the second bead is limited, and
residual austenite transforms into martensite in a
cooling process after the formation of the second
bead and is hardened, resulting in an insufficient
effect of the tempering.
[0035] In order to lower the hardness of a bead, it
is preferable to lower a cooling speed to precipitate
a soft texture such as bainite and perlite, but this
is difficult to realize by laser welding having a
sufficiently high cooling speed.
[0036] A lower limit of the temperature of the first
bead when the formation of the second and subsequent
beads is started is not particularly limited, but is
preferably equal to or higher than the Ms point —
250°C. This is because an ordinary steel sheet
finishes its martensite transformation at the Ms
point — 250°C. Waiting until the temperature becomes
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lower than the Ms point — 250°C does not have any
} special merit and increases tact time, resulting in
an increase in production cost.
[0037] As the temperature of the bead, a temperature
measured on a toe on the steel sheet surface on the
side receiving a higher stress at the time of the
load application can be used as a typical value.
Incidentally^, the temperature can be measured by
^ using a radiation thermometer or a thermocouple.
Alternatively, when the direct measurement is -
difficult, the temperature can be estimated by finite
•element analysis software available on the market
such as Quickwelder. Further, it is possible to
estimate the Ms point from components of the steel
sheets by the,following expression.
Ms (°C) = 550 - 361 X (%C) - 39 X (%Mn) - 35 X
• (%V)
-20 X (%Cr) - 17(%Ni) - 10 X (%Cu)
-5 X (%Mo + %W) + 15 X (%Co) + 30 X (%A1)
{%C) and the like are values representing the
contents of the elements in the steel sheet in mass%.
[0038] Further, after the first bead is formed, the
second and subsequent beads are formed under a
condition capable of heating so that a reheating
temperature of the first bead becomes not lower than
400°C nor higher than the Acl point + 50°C. As
described above, the temperature of the bead can be
directly measured.by the thermocouple or the
radiation thermometer or can be estimated by the
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finite element analysis software. Therefore, the
> second and subsequent beads can be formed at a target
temperature range.
[0039] If an average temperature of the first bead
is lower than 400°C when the second and subsequent
beads are formed, the first bead is not sufficiently
tempered and is not softened, and accordingly
sufficient joint strength cannot be obtained. If the
temperature of the first bead is over the Acl point°C
+ 50°C, a ratio of austenite generated in the texture
in the first bead increases, the martensite
•transformation occurs due to re-quenching during the
cooling, so that softening does not take place, and
therefore sufficient joint strength cannot be
obtained. A more preferable temperature range is
equal to or higher than 400°C and lower than the Acl
point .
[0040] The Acl point can be estimated from the
components of the steel sheet by:
Acl (°C) = 723 - 10.7 X (%Mn) - 16.9 X (%Ni)
+ 29.1 X (%Si) + 16.9 X (%Cr) + 290 X (%As)
+ 6.38 X • (%W) .
(%C) and so on are values representing the contents
of the elements in the steel sheet in mass%.
[0041] Further, in the manufacture of the laserwelded
joint, as x/W is smaller, it is necessary to
increase v2/vl, that is, a ratio of a welding speed
vl when the first bead is formed by welding and a
welding speed v2 when the second and subsequent beads
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are formed by welding, thereby suppressing a heat
I transfer amount to the first bead. When x/W is large,
it is necessary to reduce v2/vl to increase the heat
transfer amount to the first bead.
[0042] When v2/vl becomes small, the maximum
temperature of the first bead becomes beyond the Acl
point and re-quenching occurs to increase the
hardness, and thus the joint strength does not become
high. Further, when v2/vl is extremely small, heat
«
input becomes too large and burn-through of the bead
sometimes occurs.
.[0043] When v2/vl becomes large, the maximum
temperature of the first bead'becomes low and there
is a tendency that the softening by tempering is not
possible and thus the joint strength does not become
high.
[0044] An optimum range of v2/vl depends on x/W, and
as a result of studies by the present inventor,' good
joint strength is obtained when this is within a
range of 1.2/exp(x/W) ^ v2/vl ^ 4/exp(x/W).
[0045] A power density of laser is preferably not
less than 0.5 MW/cm^ nor more than 500 MW/cm^. When
the power density is not less than 0.5 MW/cm^ nor more
than 500 MW/cm^, the tempering of the bead is possible
at a wide welding speed range.
[0046] When the power density is lower than 0.5
MW/cm^, the tempering of the bead cannot be.realized
unless a moving spee'd of a laser beam, that is, the
welding speed is greatly lowered, which is
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disadvantageous in actual production. On the other
hand, when the power density is higher than 500 MW/cm^,
it is necessary to extremely increase the moving
speed of the beam in order to temper the bead at a
predetermined temperature or lower, which limits
facility ability and makes it difficult to stably
obtain the effect of the tempering.
[0047] Incidentally, it is possible to calculate the
power density of the laser beam by dividing an output
of the laser beam by a beam area, and further it is
possible to find the beam area by using a beam radius
• (a distance from a center of the beam to a point
where intensity reduces to 1/e^ of intensity of the
center of the beam (radius)).
[0048] When the bead shape is the closed loop and
the starting end and the terminating end overlap,
heat of the starting end is superimposed on the
terminating end, resulting in over-heating, which
sometimes causes the molten steel to drop or blow off.
Further, if the first bead is put close to positions
of the starting and terminating ends of the second
and subsequent beads, the second and subsequent beads
sometimes further promote the dropping and blowing of
the molten steel.
[0049] The occurrence of the dropping of the molten
steel and the like leads to deterioration of the
joint strength. Therefore, in order to suppress this,
the positions of the starting and terminating ends of
the first bead are preferably deviated from those of
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-<• X •" '-
the second and subsequent beads.
' [0050] For example, when the first bead is circular
, and the second bead has a circular shape concentric
with the first bead, the beads are preferably formed
so that an angle made by a line connecting the center
of the beads and the starting and terminating ends of
the first bead and a line connecting the center of
the beads and the starting and terminating ends of
the second and subsequent beads becomes 10° or more.
[0051] (Second Embodiment)
In the above-described first embodiment, the
•structure of the weld zone composed of the plural
beads is described. The second embodiment describes
an example, as its application example, where at a
plurality of welding positions in an overlap portion
of a plurality of members including a high-tensile
steel sheet whose carbon content is 0.0 7 weight% or
more, first beads in a closed loop shape or a closed
loop-like shape and second beads in a closed loop
shape or a closed loop-like shape on inner sides of
the first beads are formed by remote laser welding^
for joining. In structure members which are large as
compared with the beads formed in the closed loop
shape or the closed loop-like shape, in order to
improve joining strength (exfoliation strength or
shear strength) of the members, weld zones 51 each
composed of the plural beads are sometimes formed at
a plurality of welding positions of the structure
members 50 as shown in Fig. 4A and Fig. 4B.
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[0052] In welding such members, in a method of
' fixing welding places sequentially one by one in such
a manner that after the formation of the first bead
and the waiting until the temperature o f t h e f i r st
bead becomes equal to or lower than the Ms point —
50°C, the second and subsequent beads are formed, and
thereafter the next first bead is formed, the total
welding time becomes long and the tact time increases.
[0053] In order to avoid this, when the plural beads
are formed, the first beads are formed successively
at a plurality of welding positions by using mirrors
• 61 in a light collecting optical system 60 as shown
in,Fig. 6A and Fig. 6B and using remote laser welding
requiring a very short time for the movement of a
spot position,of laser, whereby a waiting time until
the second bead is formed can be effectively used.
Note that inthe drawings, the reference sign 62
denotes a laser beam, the reference sign 63 denotes a
laser radiatable area, 64 denotes the beads, and 65 •
denotes a high-tensile steel sheet.
[0054] Thereafter, when the second bead is formed by
the remote laser welding for a bead whose maximum
point has become equal to or lower than the Ms point
— 50°C among the first beads, the waiting time during
which no radiation of the laser takes place becomes
short, and as a result, the total welding time
reduces.
[0055] The welding order of the second beads only
needs to be an order so that welding deformation
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becomes small and the order is not particularly
' limited. The welding order so as to reduce the
welding deformation can be easily analyzed by using a
finite element method.
[0056] Further, forming the plural beads by the
above-described method makes it possible to fix the
members before the welding deformation starts due to
the occurrence of a residual stress or before the
welding deformation ends, since the first beads are
formed at the plural places in a short time. As a
result, it is possible to minimize the deformation of
• the structure members after the welding and improve
shape accuracy.
[0057] In forming the third beads, the third beads
may be formed.for the second beads in the same manner
as in the formation of the second beads for the first
beads. Forming the fourth and subsequent beads in
the same manner can shorten the' total welding time
and suppress the welding deformation.
[0058] Fig. 5 is a view showing an example of a
procedure for forming weld zones each composed of a
plurality of beads by remote laser welding at a
plurality of welding positions of flange portions 50a
of a hat member. First, first beads 31 to 36 in a
closed loop shape or a closed loop-like shape are
successively formed in order of the number. Then,
after the maximum point of the first beads becomes
equal to or lower than the Ms point — 50°C, second
beads 41 to 46 in a closed loop shape or a closed
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loop-like shape are successively formed on inner
> sides of the respective first beads 31 to 36 in order
of the number. Here, successively means that the
operation of "bead formation -^ movement to another
welding position -^ bead formation ..." is performed
as a series of operations and does not mean that the
laser is continuously radiated without interruption.
Rather, at the time of the movement to another
welding position, it is necessary to stop the
radiation of the laser so as not to .give heat input
to an unnecessary place of the member.
. [0059] When the procedure for successively forming
the plural first beads and the procedure for
successively forming the plural second beads for the
plural first beads that have been formed are thus
executed, the beads are each formed at a position
except the closest welding position in both of the
procedures. This can suppress the welding
deformation.
[0060] Here, "the closest welding position" means a
position to which a distance along the shape of the
steel sheet forming the structure member 50 is the
closest. A reason why the closest welding position
is avoided is because heat transfer is deeply
involved in the welding deformation and the position
to which the distance along the shape of the steel
sheet, that is, the distance along which heat
transfers, is the closest is avoided. For example,
as shown in Fig. 4A and Fig. 4B, after the bead is
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formed at a welding position A, it is considered at.
> which of welding positions B, C, the bead is to be
formed. In this case, spatial distances (distances
in a plane view) of A-B and A-C are equal, but since
a distance along the shape of the steel sheet is
shorter for A-C, C being the closest welding position
is avoided.
[0061] In the example in Fig. 5, the example where
the first beads are formed at all the welding
e
positions (six places), and next the second beads are
formed at all the welding positions (six places) is
•described, but this is not restrictive. Another
possible form may be, for example, to first form the
first beads at three places and next form the second
beads at these three places, and thereafter form the
remaining first beads at three places and then form
the second beads at these three places.
[0062] Further, in linearly arranging the weld zones
each composed of the plural beads as shown in Fig. 5>
if an interval between the beads (gravity centers
thereof) is considered in a structure member having
undergone welding at many places, shear tensile
strength and exfoliation strength proportional to the
number of the welding places (the number of beads)
can be obtained as the welded structure. Therefore,
the interval is preferably a value (average value)
equal to "the number enabling to obtain the strength
that the structure requires" divided by "length of a
flange portion". However, the interval between the
- 22 -
welding places is preferably short partially or at a
* portion locally requiring strength in the member.
[0063] Forming the weld zones as described above
makes it possible to manufacture a weld joint having
good joint strength.
EXAMPLES •
[0064] (Example 1)
Two high-tensile steel sheets whose sheet
thickness was 1.0 mm and whose main components were
C: 0.12 weight%. Si: 0.5 weight%, Mn: 2.0 weight%, P:
0.01 weight%, and S: 0.003 weight% were stacked and
•joined by laser welding, whereby a joint was
fabricated. The shape of beads of a weld zone was
circular closed loop, and two weld beads were formed.
[0065] At this time, a diameter of the weld zone was
defined by a size of the first bead located on the
utmost outer side, that is, a diameter'of the bead on
a sheet surface to which laser was radiated was
measured and the diameter was defined as constant 6 '
mm.
[0066] An angle 9 made by a line connecting a center
of the beads and starting and terminating ends of the
first bead and a line connecting the center and
starting and terminating ends of the second bead was
0° or 15° .
[0067] From the components of the steel sheets, the
Ms point and the Acl point are estimated to be 429°C
and716°C respectively.
[0068] A plurality of kinds of the laser-welded
- 23 -
1
joints were fabricated, with a width of the weld bead,
> a bead position, and a welding temperature being
varied as shown in Table 1. The welding was
performed, with the other welding conditions being
set such that a laser out was 3.5 kW, a focus
position was on a surface of the upper steel sheet,
and a diameter of a beam spot at the focus position
was^ 0 . 5 mm.
[0069] Regarding the fabricated laser-welded joints,
cross tensile strength and hardness of the first bead
(in a cross section of the weld zone, hardness of the
•weld metal was measured at five points in a sheet
thickness direction based on a point which' is 0.1 mm
apart toward the weld metal from a point where an
overlap surface and a weld line intersect with each
other, and the obtained values were averaged) were
measured.
[0070] A method of measuring the cross tensile
strength and the shape of the joint were based on JIS
Z 3137 which is the definition regarding the spotweld
joint. A cross joint was fabricated by laser
welding, and a tensile test was conducted under a
constant tensile speed- of 10 mm/min by using a
predetermined tensile jig, and the maximum load at
this time was defined as the cross tensile strength.
[0071] The temperature of the first bead was
measured by a thermocouple pasted near a toe on a
side receiving a lower stress on the surface of the
steel sheet. The measured temperature was defined as
- 24 -
a typical temperature of the first bead.
1 [0072] Results of these are shown in Table 1. With
the cross tensile strength when only one bead was
formed (No. 5) being a reference, a case where the
cross tensile strength was 1.2 times or more of the
reference was assessed as good, and a case where a
ratio of the cross tensile strength was less than 1.2
times was assessed as defective. Note that the
underlines in the table means that the conditions
described in the first embodiment are not satisfied.
[0073] Further, Fig. 7 shows an influence that x/W
•and v2/vl have on the ratio of the cross tensile
strength. In Fig. 7, O represents the results of
this example and X represents the results of a
comparative example.
[0074] When v2/vl was in the range described in the
first embodiment, good cross tensile strength was
obtained. When v2/vl was too low, the temperature of
the first bead increased too much and the bead did
not soften or the bead melted down. On the other
hand, when v2/vl was too high, the tempering of the
first bead was insufficient and the cross tensile
strength did not improve.
[0075]
[Table 1]
- 25 -
to
to V
' -^ di4-io'S's XT) n ^ iT) i n c o r o C i n r - -
. ~ \ V O (J - I H r H < M r ^ r -l
01 0]
C H f l ^ o o o c n m r O L T j i r i o r - rH
£ 0
0) (Ti -P 0
• H ' C + j g o ' ^ - ° ; t ! ^ ' « o 0 0 0 0 0 °
( d C J M £ „ m i ^ „ ^ ^ ^ ^c ro co oi >x) ^ 0 0 0 0 0 0 0 ^ 0 00
0)
^ X \ ^ . . "^ I . . . . . .
^ ( D X I M i H * ( M C N C N J C M C M C M '^
— ° CNJ
C N ^ ^ . . i ^ ^ . r - r ^ r - r - r - r -
^ " " ^ ^ 0 0 ° 0 0 0 0 0 0
^ ^ ^1 iH rH tH| 1 tH • tH * • • •
C] CNJI tH o| C\]|
> •
S LO ^ L O - L O L O t n L O L O.
\ 0| • ,-1 • I . • • • • •
X O r-< 0 0 0 0 0 0
c
o -d X
„, - H c J - ^ ^ L n r ~ - l o m L o i r i i r i Ln
o . ^ o o « ) g ° - o o
" ^ m c j o j — o " ^ 0 0 0 0 0 0
o to XJ
g ^
JS p ® __
P ^ ^ t O — ^ ^ L D L O L O L D L O L O L O t n L O L n Ln
• H ~ ^ - H ' , ^ 0 0 0 0 0 0 0 0 0 0 0
S '^ ^ ;
to ^ - ^ O
to -rH j ^ -H r- ro c»
O t n r " , P rM • • ^ ^ v ^ ^ ^ r f - t M r M rM
M a r^ Hi ^ rH ^
O OJ ^ i-l
^ to
CD 0) 0) 0) 0) 0) 0)
•MO) O 0) O x >x ax ax ax ax >x >x ax ax
g 0 d go)
O -H -H O O O O -rH -H O O
O O Q U O O U
2 I I I I I I I I I 1 1
^
[0076] As is seen from the results in Table 1, it is
' possible to obtain a laser-welded joint excellent in
joint strength.
[0077] (Example 2)
As shown in Fig. 8A and Fig. 8B, a one-sided hat
member 80 was fabricated in such a manner that a flat
sheet 82 was laser-welded so as to be suspended
between flange portions 81a on both sides of a hat
member 81. A height of the one-sided hat member 80
is 61.2 mm, a distance between outer end portions of
theflange portions 81a is 102 mm, and a length of
•the one-sided hat member 80 (that is, a length of the
flange portions 81a) is 600 mm. Note that hightensile
steel sheets being the hat member 81 and the
flat sheet 82,whose sheet thickness was 1.2 mm and
whose main components were C: 0.12 weight%. Si: 0.5
weight%, Mn: 2.0 weight%, P: 0.01 weight%, and S:
0.003 weight% were stacked and joined by laser
welding. '
[0078] The welding was performed under welding
conditions that a laser output was 4.5 kW, a focus
position was on a surface of the upper steel sheet,
and a diameter of a beam spot at the focus position
was 0.5 mm. As shown in Fig. 8B, circular beads with
a 0.5 mm bead width and a 6 mm diameter were formed
in order of 1 —* 2 -^ 3 ^ 4, that is, they were formed
in order from an end to an end of one of the flange
portions 81a on both sides and next were formed in
order from an end to an end of the other. The second
- 27 -
beads were circular beads concentric with the first
> beads and had a 5.5 mm diameter, and were formed,
after the formation of the first beads, in order of 1
- ^ 2 - ^ 3 - ^ 4 similarly to the first beads. An
interval between positions of the beads (gravity
centers thereof) was 20 mm. In this case, a twist
angle was about 20° and was to such a degree as to
become a problem when this' one-sided hat member 80
was assembled to another member to be welded or fixed.
Here, the twist angle refers to an angle made by a
line connecting a highest height of one end portion
•80R and a lowest height of another end portion SOL
and a line connecting a lowest height of the one end
portion 80R and a highest height of the other end
portion SOL when seen in the direction shown in Fig.
8A.
[0079] On the other hand, in a similar.one-sided hat ,
member 80, both when a plurality of first beads were
successively formed and when a plurality of second
beads were successively formed, the beads were each
formed at a position except the closest welding
position. In this case, welding deformation was
suppressed and the twist angle was reduced to less
than 1° , and improvement was made to such accuracy as
to cause no problem when this one-sided hat member 80
was assembled to another member to be welded or fixed.
[0080] (Example 3)
As shown in Fig. 9, a one-sided hat member 90
whose flange portions were composed of three stacked
- 28 -
steel sheets was fabricated in such a manner that a
> flat sheet 93 being a high-tensile steel sheet was
laser-welded so as to be suspended between flange
portions 91a, 92a on both sides of a hat member 91
being a soft steel sheet and a hat member 92 being a
high-tensile steel sheet. A height of the one-sided
hat member 90 is 66.2 mm, a distance between outer
end portions of the flange portions 91a, 92a is 102
mm, and a length of the one-sided hat member 9 0 (that
is, a length of the flange portions 91a, 92a) is 600
mm. As for the soft steel sheet, a sheet thickness
.is 1.2 mm and main components of the steel sheet are
C: 0.041 weight%. Si: 0.007 weight%, Mn: 0.16 weight%,
P: 0.009 weight%, and S: 0.01 weight%, and as for the
high-tensile steel sheet, a sheet thickness is 1.2 mm
and main components of the steel sheet are C: 0.12
weight%. Si: 0.5 weight%, Mn: 2.0 weight%, P: 0.01
weight%, and S: 0.003 weight%. '
Note that the soft steel sheet is a steel sheet '
called as the standards such as SPHC, SPHD, SPHE,
SPCC, SPCD, SPCE, SPCCT, SPCEN, and the like in JIS.
The soft steel sheet mentioned in the present
application is not limited to the soft steel sheets
defined by JIS and may be considered as a steel sheet
whose strength is lower than that of a high-tensile
steel sheet whose carbon content is 0.07% or more.
[0081] The welding was perforro.ed under welding
conditions that a laser output was 4.5 kW, a focus
position was on a surface of the upper steel sheet,
- 29 -
and a diameter of a beam spot at the focus position
i was 0.5 mm. As in the example 2, as shown in Fig. 8B,
circular beads with a 0.5 mm bead width and a 6 mm
diameter were formed in order of 1 —> 2 -^ 3 ^ 4, that
is, they were formed in order from an end to an end
of one of the flange portions on both sides and next
were formed in order from an end to an end of the
other. The. second beads were circular beads
concentric with the first beads and had a 5.5 mm
diameter, and were formed, after the formation of the
first beads, in order o f l — * 2 — * 3 — * 4 similarly to
•the first beads. An interval between positions of
the beads (gravity centers thereof) was 20 mm. In
this case, a twist angle was about 18° and was to
such a degree,as to become a problem when this onesided
hat member 90 was assembled to another member
to be welded or fixed. Note that the definition of
the twist angle is the same as that described in the
example 2.
[0082] On the other hand, in a similar one-sided hat
member 90, both when a plurality of first beads were
successively formed and when a plurality of second
beads were successively formed, the beads were each
formed at a position except the closest welding
position. In this case, welding deformation was
suppressed and the twist angle was reduced to less
than 1° , and improvement was made to such accuracy as
to cause no problem when this one-sided hat member 90
was assembled to another member to be welded or fixed.
- 30 -
[0083] (Example 4)
» As shown in Fig. 10, a both-sided hat member 100
whose flange portions were composed of four stacked
steel sheets was fabricated in such a manner that a
flat sheet 103 being a high-tensile steel sheet was
laser-welded so as to be suspended between flange
portions 101a, 102a, 104a on both sides of hat
members 101, 104 being soft steel sheets and a hat
member 102 being a high-tensile steel sheet. A
height of the both-sided hat member 100 is 86.2 mm, a
distance between outer end portions of the flange
•portions 101a, 102a, 104a is 102 mm, and a length of
the both-sided hat member 100 (that is, a length of
the flange portions 101a, 102a, 104a) is 600 mm. As
for the soft steel sheets, a sheet thickness is 1.2
mm and main components of the steel sheets are C:
0.041 weight%, Si: 0.007 ,weight%, Mn: 0.16 weight%,
P: 0.009 weight%, and S': 0.01 weight%, and as for the
high-tensile steel sheet, a sheet thickness is 1.2 mm
and main components of the steel sheet are C: 0.12
weight%. Si: 0.5 weight%, Mn: 2.0 weight%, P: 0.01
weight%, and S: 0.003 weight%.
[0084] The welding was performed under welding
conditions that a laser output was 5.0 kW, a focus
position was on a surface of the upper steel sheet,
and a diameter of a beam spot at the focus position,
was 0.5 mm. As in the examp_le 2, as shown in Fig. 8B,
circular beads with a 0.5 mm bead width and a 6 mm
diameter were formed in order of 1 ^ 2 —* 3 -^ 4, that
- 31 -
)
^
is, they were formed in order from an end to an end
> of one of the flange portions on both sides and next
were formed in order from an end to an end of the
other. The second beads were circular beads
concentric with the first beads and had a 5.5 mm
diameter, and were formed, after the formation of the
first beads, in order of 1 ^ 2 -^ 3 -^ 4 similarly to
the first beads. An interval between positions of
the beads (gravity centers thereof) was 2 0 mm. In
this case, a twist angle was about 18° and was to
such a degree as to become a problem when this both-
• sided hat member 100 was assembled to another member
to be welded or fixed. Note that the definition of
the twist angle is the same as that described in the
example 2. ,
[0085] On the other hand, in a similar both-sidedhat
member 100, both when a plurality of first beads
were successively formed and when a plurality of
second beads were successively formed, the beads were
each formed at a position except the closest welding
position. In this case, welding deformation was
suppressed and the twist angle was reduced to less
than 1° , and improvement was made to such accuracy as
to cause no problem when this both-sided hat member
100 was assembled to another member to be welded or
fixed.
[0086] In the foregoing, the present invention is
described with various embodiments, but the present
invention is not limited to these embodiments and
- 32 -
modifications and the like can be made within the
1 scope of the present invention.
INDUSTRIAL APPLICABILITY
[0087] According to the present invention, since a.
laser-welded joint superior in joint strength to
conventional ones is obtained and it is applicable to
an automobile member or the like, its industrial
applicability is great.
- 33 -

CLAIMS
> [Claim 1] A laser welding method in which at a
plurality of welding positions in an overlap portion
of a plurality of members including a high-tensile
steel sheet whose carbon content is 0.07 weight! or
more, first beads in a closed loop shape or a closed
loop-like shape and second beads in a closed loop
shape or a closed loop-like shape on inner sides of
the first beads are formed by remote laser welding
for joining, the method comprising: •
a procedure for successively-forming the plural
..first beads at all or part of the plural welding
positions; and
a procedure for successively forming the plural
second beads for the plural formed first beads,
wherein, '
in both of the cases of the procedure for
successively forming the plural first beads and the
procedure for successively forming the plural second'
beads, the beads are each formed at a position except
the closest welding position among the plural welding
positions.
[Claim 2] The laser welding method according to
claim 1, wherein, in the procedure for successively
forming the plural second beads, the second beads are
each formed for the first bead whose maximum
temperature has become equal to or lower than an Ms
point — 50 °C.'
[Claim 3] The laser welding method according to
- 34 -
claim 2, wherein, in the procedure for successively
' forming the plural second beads, the' second beads are
each formsd so that a tem.perature of the first bead
becomes not lower than 400 degrees centigrade nor
higher than an Acl point + 50°C.
[Claim 4] The laser welding method according to
claim 1, wherein:
the first beads each have- a circular shape and
the second beads each have a circular shape
concentric wi -^h thefirstbead;and'
an angle made by a line connecting a center of
•the beads and starting and terminating ends of the
first bead and a line connecting the center and'
starting and terminating ends of the second bead is
10° or more . .