Technical Field
[0001] The present disclosure relates to a welded member
having excellent welded portion fatigue strength and method
of manufacturing the same.
5
Background Art
[0002] In the automobile field, research into lightweight
technology for car bodies and parts is emerging as a major
issue due to fuel efficiency regulation policies, in
10 accordance with environmental protection for issues such as
global warming. Chassis parts, important for automobile
driving performance, also require the application of a highstrength steel material for weight reduction, in accordance
with this principle. In order to achieve the weight reduction
15 of such parts, it is essential to increase the strength of
materials, and it is an important factor to guarantee
durability of parts made of high-strength steel materials in
an environment in which repeated fatigue loads are applied.
In the case of arc welding, which is mainly used to secure
20 strength when assembling automobile chassis parts, since
overlap joint welding is performed between parts by welding
of a welding wire, it is inevitable to provide a joint
portion with a geometric shape. However, since this acts as
a repetitive fatigue stress concentration portion (notch
25 effect) and becomes a fracture initiation point, resulting
in deterioration of durability performance of the parts,
there is a limitation in which an advantage of applying high-
3
strength steel materials is lost. As described above, for
fatigue characteristics of a welded portion, the most
important thing is to reduce an angle (toe angle) of an end
portion of a bead, which is mainly a stress concentration
5 portion, and it has been reported that there is no direct
correlation with softening of a heat-affected zone (HAZ) due
to heat input from welding.
[0003] Meanwhile, as a representative technique for solving
such a problem, there is provided Patent Document 1. In
10 Patent Document 1, a concept of material control for each
temperature section of a toe portion of a weld bead, that
is, a heat-affected zone (HAZ), in order to improve fatigue
characteristics of an arc welded portion of a steel material
having a plate thickness of 5 mm or less and tensile strength
15 of 780 MPa or more(for example, a position of minimum
hardness at a depth of 0.1 mm on a surface must be at least
0.3 mm away from a melting line), but the details of a
specific welding method that can improve the fatigue
characteristics by reducing a weld bead toe angle are
20 insufficient.
[0004] As another technique, there are provided Patent
Documents 2 and 3. Patent Document 2 suggests that fatigue
characteristics may be improved by applying compressive
stress by continuously hitting an end portion of a weld bead
25 with a chipper (striking pin) to form a plastic deformation
region, and Patent Document 3 discloses a re-melting
treatment method of the end portion of the weld bead through
4
a plasma heat source after welding in order to reduce a toe
angle of an arc weld bead between a sub-frame and a bracket,
which are chassis parts for automobiles. However, the aboveproposed methods have an unavoidable limitation in that a
5 process cost may increase when manufacturing parts because
a post-welding process is added.
[0005] As another technique, there is provided Patent
Document 4. Patent Document 4 suggests a welded member that
can secure excellent fatigue strength without special post10 processing treatment such as laser re-melting after welding,
but has a disadvantage that a level of fatigue strength is
merely at a level of up to 285 MPa.
[0006] [Prior art Document]
15 [0007] (Patent Document 1) Japanese Patent Laid-Open No.
2013-220431
[0008] (Patent Document 2) Japanese Patent Laid-Open No.
2014-014831
[0009] (Patent Document 3) Japanese Patent Laid-Open No.
20 2014-004609
[0010] (Patent Document 4) Korean Patent Publication No. 10-
2019-0103244
Summary of Invention
25 Technical Problem
[0011] An aspect of the present disclosure is to provide
a welded member having excellent welded portion fatigue
5
strength and method of manufacturing the same.
Solution to Problem
[0012] According to an aspect of the present disclosure, a
5 welded member is obtained by overlapping a portion of two
base materials and performing fillet welding using a welding
material, the welded member having excellent welded portion
fatigue strength thereof, the welded member including: a
base material, a weld bead and reinforcing welding metal of
10 a root portion, wherein the base material has a tensile
strength of 780 MPa or more, the weld bead has a toe angle
of 160° or more, and the weld bead and reinforcing welding
metal in the root portion have an average Vickers hardness
of 280 to 320 Hv and an average fatigue strength of 350 MPa
15 or more.
[0013] According to another aspect of the present disclosure,
a method for manufacturing a welded member obtained by
overlapping a portion of two base materials and performing
fillet welding using a welding material, the welded member
20 having excellent welded portion fatigue strength thereof, in
the method, wherein the base material has a tensile strength
of 780 MPa or more, during the welding, a protective gas
containing, by volume %, 5 to 10% of CO2 and a remainder of
Ar is used, during the welding, a welding heat input (Q)
25 defined by the following [Equation 1] satisfies 1.15t ≤ Q ≤
1.6t (where, t is a thickness of a base material (mm), and
an unit of Q is kJ/cm), the welding material has specific
6
resistance (R) defined by the following [Equation 2]
satisfies 0.5 ≤ R ≤ 1.1, and X defined by the following
[Equation 3] satisfies 0.6 ≤ X ≤ 3.4,
[Equation 1] Q = (I × E) × 0.048 /υ
5 [Equation 2] R = [Si] + 0.25 × ([Mn] + [Cr])
[Equation 3] X = 28 × [Si] / [Mn]2 - [Cr] / 3 + 4 ×
[Mo]
(however, in the [Equation 1], I, E, and υ represent a
welding current [A], a welding voltage [V], and a welding
10 speed (cm/min), respectively, and in the [Equation 2] and
[Equation 3], and [Si], [Mn], [Cr] and [Mo] represent each
element content (wt%)).
Advantageous Effects of Invention
15 [0014] According to an aspect of the present disclosure, it
is possible to provide a welded member having excellent
welded portion fatigue strength and method of manufacturing
the same.
20 Brief Description of Drawings
[0015] FIG. 1 is a photograph captured with an optical
microscope after etching a cross-sectional structure of a
welded member according to an embodiment of the present
disclosure with a nital solution, FIG. 1(a) is a photograph
25 of Inventive Example 1, and FIG. 1(b) is a photograph of
Comparative Example 5.
[0016] FIG. 2 illustrates hardness distribution of a welded
7
member according to an embodiment of the present disclosure,
FIG. 2(a) illustrates hardness distribution of Inventive
Example 1, and FIG. 2(b) illustrates hardness distribution
of Comparative Example 5.
5 [0017] FIG. 3 is an Image Quality (IQ) and an Inverse Pole
Figure (IPF) photograph of Inventive Example 1 according to
an embodiment of the present disclosure observed with EBSD.
[0018] FIG. 4 is an Image Quality (IQ) and an Inverse Pole
Figure (IPF) photograph of Comparative Example 5 according
10 to an embodiment of the present disclosure observed with
EBSD.
Best Mode for Invention
[0019] Hereinafter, a welded member having excellent welded
15 portion fatigue strength according to an embodiment of the
present disclosure will be described.
[0020] The welded member of the present disclosure may be
obtained by overlapping a portion of two base materials and
performing fillet welding using a welding material. In this
20 case, the welded member may include a base material, a weld
bead, and reinforcing welding metal of a root portion. The
reinforcing welding metal in the root portion refers to an
additional welding metal formed according to penetrability
characteristics in which molten metal smoothly penetrates
25 between an upper plate and a lower plate of the overlap joint
portion during gas shield arc welding. The root portion
reinforcing welding metal is present between a rear end
8
portion of the weld bead and the overlapping portion of the
base material. By forming the reinforcing welding metal in
the root portion in this region, it is possible to
effectively prevent a decrease in fatigue strength due to
5 stress concentration at the weld root portion in a normal
fatigue environment.
[0021] The base material preferably has a tensile strength
of 780 MPa or more. As described above, by using the highstrength base material, it is possible to achieve weight
10 reduction when applied to portions of car bodies used in the
automobile field. Meanwhile, in the present disclosure, as
long as it is a steel type having a high strength of 780 MPa
or more as described above, the type is not particularly
limited. However, it may preferably have an alloy composition
15 similar to the alloy composition of the welding material
applied to the present disclosure. For example, the base
material may include, by weight%, 0.02 to 0.08% of C; 0.01
to 0.5% of Si; 0.8 to 1.8% of Mn; 0.01 to 0.1% of Al; 0.001
to 0.02% of P; 0.001 to 0.01% of S; 0.001 to 0.01% of N;
20 0.01 to 0.12% of Ti; 0.01 to 0.05% of Nb, and a remainder of
Fe and unavoidable impurities. In addition, the base material
may further include at least one of Mo, Cr, V, Ni, and B so
that a total amount thereof is 1.5% by weight or less.
[0022] The base material may have a thickness of 1.0 to 2.0
25 mm. When the thickness of the base material is less than 1.0
mm, there may be a disadvantage in that it is difficult to
exhibit sufficient arc force to form reinforcing welding
9
metal of a root portion as well as being sensitive to melting
during normal gas shield arc welding. On the other hand,
when the thickness thereof exceeds 2.0 mm, a step of a
thickness of the overlapping joint portion becomes excessive,
5 so that it may be difficult to secure a weld bead toe angle
for securing excellent fatigue strength
[0023] An interval of the overlapping portion between the
two base materials (interval between upper and lower plates
to be welded) may be 0.5 mm or less (including 0 mm). When
10 the interval of the overlapping portion between the two base
materials exceeds 0.5 mm, it may be difficult to secure a
weld bead toe angle to achieve excellent fatigue strength
within the appropriate base material thickness range. The
interval of the overlapping portion between the two base
15 materials refers to an interval between the upper and lower
plates to be welded.
[0024] The weld bead toe angle is preferably 160° or more.
The weld bead toe angle refers to an angle between the weld
bead and a base material positioned in a lower portion of
20 the two base materials at an end portion of the weld
bead. A reason for controlling the weld bead toe angle is to
relieve stress concentration of the weld part in a normal
fatigue environment. That is, by controlling the weld bead
toe angle to be high, an effect of significantly improving
25 the fatigue strength compared to a conventional welded
portion can be obtained, and if the weld bead toe angle is
less than 160°, it may be difficult to sufficiently obtain
10
the effect.
[0025] The weld bead may include a microstructure of at least
one of acicular ferrite and bainite, and the acicular ferrite
and bainite may have an average effective grain size of 5 μm
5 or less. The acicular ferrite and bainite are microstructures
advantageous for securing the strength and toughness of the
welding metal, that is, the weld bead. In addition, in the
present disclosure, it is possible to obtain an effect of
simultaneously securing sufficient strength and toughness of
10 the weld bead and the reinforcing welding metal in the root
portion by refining the crystal grains of the acicular
ferrite and bainite. If the average effective grain size of
the acicular ferrite and bainite exceeds 5 μm, it is
difficult to simultaneously secure sufficient strength and
15 toughness of the welding metal as described above. Meanwhile,
the above-mentioned average effective grain size refers to
an average size of the grains converted from the number of
grains per unit area.
[0026] Meanwhile, the welding material used during the
20 welding, may include, by weight%, C: 0.06 to 0.1%, Si: 0.04
to 0.2%, Mn: 1.6 to 1.9%, Cr: 0.5 to 1.6%, Mo: 0.1 to 0.6%,
a remainder of Fe, and other unavoidable impurities.
[0027] Carbon (C): 0.06 to 0.1%
25 [0028] Carbon (C) is a beneficial element for an action of
stabilizing arc to atomize a volume. When a content of C is
less than 0.06%, the volume becomes coarse and the arc
11
becomes unstable, and an amount of spatter generation
increases, and it may be difficult to secure sufficient
strength of welding metal, which are disadvantages. On the
other hand, when the content of C exceeds 0.1%, there may be
5 a disadvantage in that viscosity of molten metal is lowered,
resulting in a poor bead shape, as well as excessive
hardening of the welding metal, thereby reducing toughness.
A lower limit of the content of C is more preferably 0.062%,
is even more preferably 0.065%, and is most preferably 0.07%.
10 An upper limit of the content of C is more preferably 0.095%,
is even more preferably 0.09%, and is most preferably 0.085%.
[0029] Silicon (Si): 0.04 to 0.2%
[0030] Silicon (Si) is an element (a deoxidation element)
15 promoting deoxidation of molten metal during arc welding,
and is effective in suppressing occurrence of blowholes.
When a content of Si is less than 0.04%, there may be a
disadvantage in that deoxidation becomes insufficient and
blowholes is easily generated, and when the content of Si
20 exceeds 0.2%, there may be a disadvantage in that nonconductive slag is generated remarkably, paint defects in a
welded portion are caused and penetrability of molten metal
is lowered due to a lack of surface activation of the welded
portion due to excessive deoxidation. A lower limit of the
25 content of Si is more preferably 0.045%, is even more
preferably 0.05%, and is most preferably 0.06%. An upper
limit of the content of Si is more preferably 0.15%, is even
12
more preferably 0.1%, and is most preferably 0.08%.
[0031] Manganese (Mn): 1.6 to 1.9%
[0032] Manganese (Mn) is a deoxidizing element and is an
5 element of promoting deoxidation of molten metal during arc
welding and suppressing generation of blowholes. When a
content of Mn is less than 1.6%, there may be a disadvantage
in that deoxidation becomes insufficient within an
appropriate range of the above-described content of Si, and
10 blowholes are likely to be generated. When the content of Mn
exceeds 1.9%, there may be a disadvantage in that viscosity
of molten metal becomes excessively high, and a welding speed
is high, the molten metal cannot flow properly into a welded
site, resulting in a humped bead, which is likely to cause
15 a poor bead shape. A lower limit of the content of Mn is
more preferably 1.65%, is even more preferably 1.7%, and is
most preferably 1.75%. An upper limit of the content of Mn
is more preferably 1.87%, is even more preferably 1.85%, and
is most preferably 1.8%.
20
[0033] Chromium (Cr): 0.5 to 1.6%
[0034] Chromium (Cr) is a ferrite stabilizing element, and
is an element advantageous for securing hardenability to
improve strength of welding metal. When a content of Cr is
25 less than 0.5%, there may be a disadvantage in that it is
difficult to secure sufficient strength of the welding metal,
and when the content of Cr exceeds 1.6%, there may be a
13
disadvantage in that brittleness of the welding metal is
unnecessarily increased in some cases, making it difficult
to sufficient toughness. A lower limit of the content of Cr
is more preferably 0.6%, is even more preferably 0.7%, and
5 is most preferably 0.8%. An upper limit of the content of Cr
is more preferably 1.55%, is even more preferably 1.5%, and
is most preferably 1.45%.
[0035] Molybdenum (Mo): 0.1 to 0.6%
10 [0036] Molybdenum (Mo) is a ferrite stabilizing element, and
is an element advantageous for securing hardenability to
improve strength of welding metal. When a content of Mo is
less than 0.1%, there may be a disadvantage in that it is
difficult to secure sufficient strength of welding metal
15 within the above-described appropriate component range, and
when the content of Mo exceeds 0.6%, there may be a
disadvantage in that toughness of the welding metal is
lowered in some cases. A lower limit of the content of Mo is
more preferably 0.15%, is even more preferably 0.2%, and is
20 most preferably 0.25%. An upper limit of the content of Mo
is more preferably 0.55%, is even more preferably 0.52%, and
is most preferably 0.5%.
[0037] In addition, the welding material may further include
0.015% or less of P, 0.005% or less of S, 0.10% or less of
25 Ni, 0.25% or less of Cu, and 0.10% or less of Al.
[0038] Phosphorus (P): 0.015% or less
14
[0039] Phosphorus (P) is an element that is generally
incorporated as an unavoidable impurity in steel, and is
also an element included as a normal impurity in a solid
wire for arc welding. When a content of P exceeds 0.015%,
5 there may be a disadvantage in that high-temperature cracking
of welding metal becomes excessive. The content of P is more
preferably 0.014% or less, is even more preferably 0.012% or
less, and is most preferably 0.01% or less.
10 [0040] Sulfur (S): 0.015% or less
[0041] Sulfur (S) is also generally incorporated as an
unavoidable impurity in steel, and is an element usually
included as a normal impurity in a solid wire for arc welding
as well. When a content of S exceeds 0.01%, there may be a
15 disadvantage in that toughness of welding metal deteriorates
in some cases, and surface tension of molten metal is
insufficient during welding , so that a molten portion flows
downwardly excessively due to gravity during high-speed
welding, resulting in a poor shape of the weld bead. The
20 content of S is more preferably 0.008% or less, is even more
preferably 0.006% or less, and is most preferably 0.005% or
less.
[0042] Nickel (Ni): 0.40% or less
25 [0043] Nickel (Ni) is an element capable of improving
strength and toughness of welding metal. However, when a
content of Ni exceeds 0.40%, there may be a disadvantage in
15
that it becomes sensitive to cracks within the abovedescribed appropriate component range. The content of Ni is
more preferably 0.30% or less, is even more preferably 0.20%
or less, and is most preferably 0.10% or less.
5
[0044] Copper (Cu): 0.50% or less
[0045] Copper (Cu) is generally contained in about 0.02% as
an impurity in steel constituting a wire, and in a solid
wire for arc welding, a content of Cu may be determined
10 mainly due to copper plating performed on a surface of the
wire. Cu is an element capable of stabilizing feedability
and conductivity of the wire. However, when the content of
Cu exceeds 0.50%, there may be a disadvantage in that crack
susceptibility of the welding metal is increased. The content
15 of Cu is more preferably 0.45% or less, is even more
preferably 0.40% or less, and is most preferably 0.30% or
less.
[0046] Aluminum (Al): 0.20% or less
20 [0047] Aluminum (Al)is a deoxidizing element and is an
element of promoting deoxidation of molten metal during arc
welding and improving strength of welding metal. When a
content of Al exceeds 0.20%, generation of Al-based oxides
increase, so that there may be a disadvantage strength and
25 toughness of the welding metal are deteriorated in the abovementioned appropriate component range in some cases, and
electrodeposition paint defects of a welded portion become
16
sensitive due to non-conductive oxides. The content of Al is
more preferably 0.15% or less, is even more preferably 0.12%
or less, and is most preferably 0.10% or less.
5 [0048] Meanwhile, in the present disclosure, the type of the
welding material is not particularly limited, but a solid
wire or a metal cored wire may be preferably used. More
preferably, a solid wire is used, and the solid wire is more
advantageous for securing wire rigidity, so that it is
10 possible to obtain an effect of improving penetrability of
molten metal through securing excellent feedability and
straightness of the wire during welding.
[0049] In the welded member of the present disclosure
provided as described above, the weld bead and the
15 reinforcing welding metal in the root portion may have an
average Vickers hardness of 280 to 320 Hv and an average
fatigue strength of 350 MPa or more, so that very excellent
fatigue strength in the welded portion can be secured.
20 [0050] Hereinafter, a method for manufacturing a welded
member having excellent welded portion fatigue strength
according to an embodiment of the present disclosure will be
described.
[0051] According to the present disclosure, the method for
25 manufacturing the same may be performed by overlapping a
portion of two base materials and performing fillet welding
using a welding material. It is preferable to use gas
17
shielded arc welding during the fillet welding.
[0052] In this case, it is preferable use a protective gas
containing, by volume %, 5 to 10% of CO2 and a balance of Ar
during the welding. The CO2 is a gas advantageous for
5 securing penetrability of molten metal due to arc pinch force
and surface activation by generating arc contraction by a
dissociation reaction during arc welding. When the fraction
of CO2 is less than 5%, there is a disadvantage that a volume
transfer of a wire is unstable during arc welding and
10 penetrability of the molten metal may be poor, and when the
fraction of CO2 exceeds 10%, there is a disadvantage that
the arc contraction increases and the penetrability
increases, but it is difficult to secure a sufficient toe
angle to ensure excellent fatigue characteristics of the
15 welded portion.
[0053] In addition, it is preferable that a welding heat
input (Q) defined by the following [Equation 1] during the
welding satisfies 1.15t ≤ Q ≤ 1.6t (where, t is a thickness
of a base material (mm), and an unit of Q is kJ/cm). If the
20 welding heat input (Q) is less than 1.15t, there is a concern
that strength and toughness of welding metal and a coarse
grained heat-affected zone may be insufficient, and if the
welding heat input (Q) exceeds 1.6t, there is a problem that
not only insufficient strength of the welding metal and a
25 decrease in strength of a welding heat-affected zone becomes
excessive, but also back beads and melting easily occur in
the welded portion, resulting in a defect.
18
[0054] [Equation 1] Q = (I × E) × 0.048 /υ 56
[0055] (However, in the [Equation 1], I, E, and υ represent
a welding current [A], a welding voltage [V], and a welding
speed (cm/min), respectively).
5 [0056] The welded member of the present disclosure is a
welded member having a welded portion obtained by welding
two or more base materials using a welding material, in the
welded member, it is possible to improve penetrability of
molten metal by increasing arc pinch force by activating the
10 surface of the welded portion by controlling deoxidation
according to a chemical component and reducing the specific
resistance of the welding wire. In particular, it is possible
to prevent excessive deoxidation during arc welding by
controlling the content of Si, which is a major deoxidation
15 element among alloying components of the welding material.
Meanwhile, since conventional gas-shielded arc welding is
controlled by a constant voltage method, as the specific
resistance of the welding wire, serving as a positive
electrode of the flow of the arc current, decreases, the
20 welding current capable of increasing the penetrability of
molten metal, that is, the arc pinch force, increases.
Accordingly, it is preferable that the welding material used
during welding has a specific resistance (R) defined by the
following [Equation 2] satisfies 0.5 ≤ R ≤ 1.1. Meanwhile,
25 when the specific resistance (R) is less than 0.5, not only
the deoxidation of the molten metal during welding is
insufficient, but also the specific resistance of the welding
19
wire is too low, so that it has a disadvantage in that it is
difficult to obtain a good weld bead due to a poor volume
transfer at a tip of the wire, and when the specific
resistance (R) exceeds 1.1, there is a disadvantage in that
5 a sufficient arc pinch force is not exerted according to the
above-described principle, and thus the penetrability of the
molten metal is insufficient.
[0057] [Equation 2] R = [Si] + 0.25 × ([Mn] + [Cr])
[0058] (However, in the [Equation 2], [Si], [Mn] and [Cr]
10 represent each element content (wt%)).
[0059] Furthermore, it is preferable that X defined by the
following [Equation 3] satisfies 0.6 ≤ X ≤ 3.4. During arc
welding of a thin plate targeted in the present disclosure,
a phase transformation structure according to continuous
15 cooling of a welded metal portion changes rapidly according
to the above-mentioned value of X, and in this case, it is
possible to secure sufficient strength and toughness of the
welded metal portion by securing the acicular ferrite and
bainite microstructures, which are typical low-temperature
20 transformation phases generated according to the
transformation caused by lattice deformation without
diffusion. Accordingly, it is possible to realize excellent
fatigue strength in the welded portion by securing a dense
microstructure of the welded metal part together with a
25 welded toe portion and reinforcing welding metal of a root
portion formed smoothly as described above. In this case,
nucleation of acicular ferrite transformation starts from
20
complex oxides generated from micro metal elements contained
in the welding base material and welding material, and in
order to promote the transformation of the acicular ferrite
phase, it is more effective when the CO2 fraction of the
5 welding shielding gas is 5-10 vol%. When an amount of oxygen
generated according to a dissociation reaction of CO2 during
arc welding is excessive than the above-mentioned
appropriate range, the number of oxides increases, but it
cannot reach a critical oxide size for nucleation, so it is
10 not easy to generate acicular ferrite phase transformation.
Otherwise, the transformation of grain boundary ferrite,
which is unfavorable to securing toughness, increases.
Conversely, when the amount of oxygen is insufficient than
the appropriate range, the hardenability increases due to
15 the reduction of oxidation of hardenable elements contained
in the steel material, which is a welding base material, and
the welding wire, so that low-temperature transformation
such as bainite and martensite occur predominantly rather
than acicular ferrite transformation. In order to obtain all
20 the effects of increasing the weld bead toe angle and
increasing a phase fraction of acicular ferrite, a C02
fraction is preferably close to 5% by volume. In addition,
as described above, by appropriately controlling the content
of Si, which is a strong deoxidation element, among the
25 chemical components of the welding material, it can be
helpful for forming the composite oxide. In addition, when
the Vickers hardness (Hv, load of 500gf, measured at 0.2 mm
21
intervals) of the welded metal part including reinforcing
welding metal of the root portion becomes 280 or more, it is
possible to significantly improve the fatigue strength in
the welded portion. Meanwhile, when the value of X is less
5 than 0.6, there is a disadvantage that the martensitic phase
transformation is mainly promoted and the brittleness of the
welding metal increases. When the value of X exceeds 3.4,
there is a disadvantage in that the strength of the welding
metal is lowered due to lack of hardenability.
10 [0060] [Equation 3] X = 28 × [Si] / [Mn]2 - [Cr] / 3 + 4 ×
[Mo]
[0061] (However, in the [Equation 3], [Si], [Mn], [Cr] and
[Mo] represent each element content (wt%)).
15 Mode for Invention
[0062] Hereinafter, the present disclosure will be described
in detail through Examples. However, the following Examples
are only examples for explaining the present disclosure in
more detail, and do not limit the scope of the present
20 disclosure.
[0063] (Example 1)
[0064] After dissolving an ingot having an alloy composition
illustrated in Table 1 below, a welding wire was prepared by
annealing after being drawn at room temperature through hot
25 rolling. Then, a Cu plating layer was formed on a surface of
the wire, and at this time, it was plated so that a copper
content is in a range of 0.12 to 0.50%, by mass% with respect
22
to a total wire including a plating layer. Then, the copperplated wire was drawn, and manufactured as a solid wire for
welding having a diameter of 0.9 to 1.2 mm.
[0065] Using the solid wire for welding manufactured as
5 described above, two Pickled & Oiled (PO) steel sheets having
the alloy composition illustrated in Table 2 below were
fillet welded (overlap joint welding) using the welding
conditions illustrated in Table 3 below. In this case,
tensile strength of the PO steel sheet was 780 MPa, average
10 hardness thereof was 260 Hv, and a thickness thereof was 2.0
mm. During the welding, an interval between overlapping parts
between the two base materials was fixed with by clamping to
be 0.5 mm or less, and pulse MAG welding was performed under
conditions of a wire protrusion length of 15 mm, a welding
15 speed of 80cm/min, as an overlap joint portion.
[0066] For a welded member manufactured through the abovedescribed welding, whether or not reinforcing welding metal
in a root portion is formed, a weld bead toe angle, an
average Vickers hardness and an average fatigue strength of
20 the weld bead and the reinforcing welding metal of the root
portion, and a microstructure and an average effective grain
size thereof, were measured, and then results thereof were
shown in Table 4 below.
[0067] Whether or not reinforcing welding metal in the root
25 portion is formed was determined by the presence of
additional welding metal between a rear end portion of the
weld bead and an overlapping portion of the base material,
23
that is, in a region beyond a molten boundary line (a
boundary between the welding metal and the heat-affected
zone).
[0068] The weld bead toe angle was measured as an outer angle
5 formed by a lower plate reference plane of the welding base
material, in contact with a normal line to a curved surface
of the toe portion of the weld bead.
[0069] The average Vickers hardness of the weld bead and the
reinforcing welding metal in the root portion was measured
10 at an interval of 0.2 mm in a width direction using a Vickers
hardness meter under a condition that a load was 500 gf, and
then an average value thereof was measured.
[0070] The average fatigue strength of the weld bead and the
reinforcing welding metal in the root portion was measured
15 by taking a specimen from the welded portion, and then
performing a fatigue test, so that a maximum additional load
in which a fatigue life satisfies 2 × 106 Cycles as fatigue
strength. In this case, the fatigue strength was described
as the average value for three specimens. In the fatigue
20 test, fatigue life (Cycles) of the welded portion was
measured using a tensile-tensile high cycle fatigue test for
each load, and in this case, a ratio of a minimum load and
a maximum load was 0.1, a repetitive load frequency was 15Hz,
and in addition, a fatigue life corresponding to converted
25 strength Mpa was derived by dividing load kN by an area
according to a width and a thickness of each specimen.
[0071] The microstructure was observed with an optical
24
microscope after micro-polishing a cross-sectional structure
of the welded member and etching with a nital solution. In
addition, Kikuchi patterns were analyzed through electron
backscattered diffraction (EBD), so that image quality (IQ)
5 and inverse pole figure (IPF) maps were obtained visualizing
grain boundaries and grain orientation information.

WE CLAIM:
1. A welded member obtained by overlapping a portion
of two base materials and performing fillet welding using a
welding material, the welded member having excellent welded
5 portion fatigue strength thereof,
the welded member, comprising:
a base material, a weld bead and reinforcing welding
metal of a root portion,
wherein the base material has a tensile strength of
10 780MPa or more,
the weld bead has a toe angle of 160° or more, and
the weld bead and the reinforcing welding metal in the
root portion have an average Vickers hardness of 280 to 320
Hv and an average fatigue strength of 350 MPa or more.
15
2. The welded member having excellent welded portion
fatigue strength of claim 1,
wherein the base material comprises, by wt%,
0.02 to 0.08% of C, 0.01 to 0.5% of Si, 0.8 to 1.8% of
20 Mn, 0.01 to 0.1% of Al, 0.001 to 0.02% of P, 0.001 to 0.01%
of S, 0.001 to 0.01% of N, 0.01 to 0.12% of Ti, 0.01 to 0.05%
of Nb, and a remainder of Fe and other unavoidable impurities.
3. The welded member having excellent welded portion
25 fatigue strength of claim 2,
wherein the base material further comprises,
at least one of Mo, Cr, V, Ni, and B so that a total
34
amount thereof is 1.5% by weight or less.
4. The welded member having excellent welded portion
fatigue strength of claim 1, wherein the base material has
5 a thickness of 1.0 to 2.0 mm.
5. The welded member having excellent welded portion
fatigue strength of claim 1, wherein an interval of an
overlapping portion between the two base materials is 0.5 mm
10 or less (including 0 mm).
6. The welded member having excellent welded portion
fatigue strength of claim 1, wherein the weld bead comprises
a microstructure of at least one of acicular ferrite and
15 bainite, the acicular ferrite and bainite having an average
effective grain size of 5 μm or less.
7. The welded member having excellent welded portion
fatigue strength of claim 1,
20 wherein the welding material comprises, by weight%,
0.06 to 0.1% of C, 0.04 to 0.2% of Si, 1.6 to 1.9% of
Mn, 0.5 to 1.6% of Cr, 0.1 to 0.6% of Mo, and a remainder of
Fe and other unavoidable impurities.
25 8. The welded member having excellent welded portion
fatigue strength of claim 7,
wherein the welding material further comprises, by
35
weight %,
0.015% or less of P, 0.01% or less of S, 0.40% or less
of Ni, 0.50% or less of Cu, and 0.20% or less Al.
5 9. The welded member having excellent welded portion
fatigue strength of claim 1,
wherein the welding material is a solid wire or a metal
cored wire.
10 10. A method for manufacturing a welded member
obtained by overlapping a portion of two base materials and
performing fillet welding using a welding material, the
welded member having excellent welded portion fatigue
strength thereof,
15 in the method,
wherein the base material has a tensile strength of 780
MPa or more,
during the welding, a protective gas containing, by
volume %, 5 to 10% of CO2 and a remainder of Ar is used,
20 during the welding, a welding heat input (Q) defined
by the following [Equation 1] satisfies 1.15t ≤ Q ≤ 1.6t
(where, t is a thickness of a base material (mm), and
an unit of Q is kJ/cm), and
for the welding material, a specific resistance (R)
25 defined by the following [Equation 2] satisfies 0.5 ≤ R ≤
1.1, and X defined by the following [Equation 3] satisfies
0.6 ≤ X ≤ 3.4,
36
[Equation 1] Q = (I × E) × 0.048 /υ
[Equation 2] R = [Si] + 0.25 × ([Mn] + [Cr])
[Equation 3] X = 28 × [Si] / [Mn]2 - [Cr] / 3 + 4 ×
[Mo]
5 (However, in the [Equation 1], I, E, and υ represent a
welding current [A], a welding voltage [V], and a welding
speed (cm/min), respectively, and in the [Equation 2] and
[Equation 3], [Si], [Mn], [Cr] and [Mo] represent each
element content (by wt%)).
10
11. The method for manufacturing a welded member
having excellent welded portion fatigue strength of claim
10,
wherein the base material comprises, by weight %,
15 0.02 to 0.08% of C, 0.01 to 0.5% of Si, 0.8 to 1.8% of
Mn, 0.01 to 0.1% of Al, 0.001 to 0.02% of P, 0.001 to 0.01%
of S, 0.001 to 0.01% of N, 0.01 to 0.12% of Ti, 0.01 to 0.05%
of Nb, and a remainder of Fe and other unavoidable impurities.
20 12. The method for manufacturing a welded member
having excellent welded portion fatigue strength of claim
11, wherein the base material further comprises,
at least one of Mo, Cr, V, Ni, and B so that a total
amount thereof is 1.5% by weight or less.
25
13. The method for manufacturing a welded member
having excellent welded portion fatigue strength of claim
37
10, wherein the welding material comprises, by weight %,
0.06 to 0.1% of C, 0.04 to 0.2% of Si, 1.6 to 1.9% of
Mn, 0.5 to 1.6% of Cr, 0.1 to 0.6% of Mo, and a remainder of
Fe and other unavoidable impurities.
5
14. The method for manufacturing a welded member
having excellent welded portion fatigue strength of claim
13,
wherein the welding material further comprises,
10 0.015% or less of P, 0.01% or less of S, 0.40% or less
of Ni, 0.50% or less of Cu, and 0.20% or less of Al.
15. The method for manufacturing a welded member
having excellent welded portion fatigue strength of claim
15 10,
wherein the welding material is a solid wire or a metal
cored wire.