OVERLAP-WELDED MEMBER, AUTOMOBILE PART, METHOD OF
WELDING OVERLAPPED POKITON, AND METHOD OF
MANUFACTURING OVERLAP-WELDED MEMBER
Technical Field
[OOOl]
The present invention relates to an overlap-welded member obtained
by joining an overlapped portion including plural steel sheet melnbers at a
spot-welded portion, an autoniobile part including the overlap-welded member,
a method of welding the overlapped portion, and a method of nlanufacturing
the overlap-welded member.
The present application clainls priority based on Japanese Patent
Application No. 2012-178691 filed in Japan on August 10, 2012, the contents
of which are incorporated herein by reference.
Background Art
[0002]
In recent years, in the automobile field, high-tensile steel sheets have
been increasingly used in order to reduce the weight of a vehicle and improve
safety against collision.
Furtherlnore, the level of strength of high-tensile steel sheets has
increased year by year, and for example, hot-stamped members having tensile
strength of 1500 MPa or higher have been practically used. The hot-stamped
tnen~bera s used herein is a rneniber obtained by applying press working in a
state where a steel sheet is heated to approximately 900°C to be softened, and
at the same tinie, quenching and strengthening the steel sheet using a cooling
effect (contact cooling) due to contact with a die, thereby achieving the
tensile strength of a 1500 MPa class as described above and favorable
diniensional accuracy.
[0003]
Furthermore, for example, in the case of assembling a vehicle body, a
resistance spot welding is frequently used in which two or inore steel sheet
tnembers formed by steel sheets are overlapped, and energization is applied
while pressure is being applied with electrodes.
With this resistance spot welding, a nlelted and solidified portion
having an ellipse shape, in other words, a nugget is formed in the overlapped
portion through energization and heating, whereby it is possible to join the
plural steel sheet tnembers.
[0004]
For example, FIG. 1 is a diagram schematically illustrating
distribution of hardness in a spot-welded portion 10 in the case where
conventional energization conditions are applied to two
transformation-induced plasticity (TRIP) members S 11 and S12.
More specifically, (a) in FIG. 1 is a sectional view schematically
illustrating the vicinity of the spot-welded portion 10 in which the vertical
direction on tlie paper is set to the thickness direction (in other words, a
direction in wvhicli pressure is applied with tlie electrodes) of the TRIP
tnembers S11 and S12. Note that, in the following descriptions in the
specification of the present application, a diagram illustrating a cross-section
of two overlapped nlelnbers when viewed in a similar manner to that in (a) in
FIG. 1 is also referred to as a "sectional view illustrating althe spot-welded
portion."
Furthermore, (b) in FIG. 1 is a graph schematically illustrating
distribution of Vickers hardness so as to correspond to (a) in FIG. 1.
A molten metal generated through resistance spot welding is cooled at
a high cooling rate, and hence, martensite is more likely to form in a nugget
12. As a result, the nugget 12 has a structure harder than a base ~itetal
portion. Note that, in the case where the strength of the base metal is high,
the carbon equivalent is generally higli, so that Vickers hardness of the nugget
is high.
As illustrated in FIG. 1, the spot-welded portion 10 includes the
nugget 12 and a HAZ 14. The HAZ 14 includes a HAZ hardened portion 14H
located close to the nugget 12, and a HAZ softening zone 14T formed in tlie
vicinity of the HAZ hardened portion 14H. Furtl~ermore,t he softest zone
14L in HA2 exists at an inner peripheral edge of the HAZ softening zone 14T.
[OOOS]
The quality of the spot-welded portion is often evaluated on tlie basis
of a tensile shear strength and a cross-tension strength (the strength of joint in
a peel direction), and it is known that the tensile shear strength increases with
an increase in the strength of the base metal
However, in the case where the base metal has a tensile strength
higher than a 780 MPa class, the peel strength, typified by the cross-tension
strength, tends to decrease with an increase in strength of the base metal.
[0006]
Below, a cross-tension test based on JIS 23 137 (1999), which is
designed for measuring the cross-tension strength, will be schematically
described with reference to FIG. 2A.
As illustrated in FIG. 2A, in the cross-tension test, two test pieces S21
and S22 formed by steel sheets are orthogonally arranged, and are joined by
forming the spot-welded portion 10 including the nugget 12 through resistance
spot welding.
Then, the test pieces S2 1 and S22 are pulled in a direction in which
they are peeled, and the peel strength is measured until the spot-welded
portion 10 is fractured.
[0007]
A fracture mode with the cross-tension test can be divided into the
following:
(a) interface fracture in which an interface between sheets in the nugget
fractures;
(b) partial plug fracture in which, as illustrated in FIG. 2B, a crack propagates
\vitliin the nugget 12 (inner side than a nugget end 12E) and then, fracture
advances in the thickness direction; and
(c) plug fracture in which, as illustrated in FIG. 2C, the nugget 12 does not
break, and the outer peripheral portion of the nugget 12 fractures in the
thickness direction.
[OOOS]
FIG. 2D is a diagram illustrating an example of a correlation between
a base metal tensile strength and a cross-tension strength.
In FIG. 20, "black dots" represent the plug fracture, and "blank
circles" represent the partial plug fracture.
As illustrated in FIG. 2D, the cross-tension strength is approximately
9 kN in tlie case of a hot-statnped Inember of a 1500 MPa class (a steel sheet
member obtained by hot-stamping a steel sheet for hot-stamping whose tensile
strength becomes a 1500 MPa class by being hot-stamped), and is
approxilnately 4 kN in the case of a hot-statnping member of an 1800 MPa
class (a steel sheet n~enlbero btained by hot-stamping a steel sheet for
hot-stamping whose tensile strength becomes an 1800 MPa class by being
hot-stamped).
On the other hand, tlie cross-tension strength of a high-strength steel
sheet of a 980 MPa class or lower falls in the range of approximately 8 kN to
14 kN.
In other words, the cross-tension strength of the hot-stamped member
of a 1500 MPa class or higher is significantly lower than that of the
high-strength steel sheet of 980 MPa class or lower.
Furthermore, as for the fracture mode through the cross-tension test,
the high-strength steel sheet of a 980 MPa class or lower is fractured mainly
in relation to the plug fracture in which the outside of the nugget 12 fractures,
whereas the hot-stamp nie~nbero f a 1500 MPa class or the hot-stamped
menlber of an 1800 MPa class is fractured mainly in relation to the partial
plug fracture.
This shows that, in the case of the hot-stamped member of a 1500 MPa
class or higher, a crack is more likely to occur in the nugget because the
toughness is small in the nugget.
[0009]
As described above, in the case of the spot welding of the
high-strength steel sheet, it is considered that the peel strength reduces mainly
because the toughness reduces with an increase in hardness of the nugget, and
thus, fracture (partial plug fracture) is more likely to occur in the nugget.
[OO lo]
In general, with an increase in the diameter of the nugget, the fracture
mode is more likely to be the plug fracture rather than the partial plug fracture,
and the strength of the spot-welded portion increases.
Thus, in order to inlprove the peel strength of the spot-welded portion
of the high-tensile steel sheet, it is effective, for example, to increase the
diameter of the nugget.
[OOll]
However, in the case where the high-tensile steel sheet is subjected to
resistance spot welding, spattering of niolten steel called splash is more likely
to occur as compared with a case where mild steel is subjected to resistance
spot welding, possibly making it difficult to increase the diameter of the
nugget.
In order to suppress the occurrence of splash, it is effective, for
example, to increase the compression force with the electrodes. I-Iowever,
there is a restriction resulting from equipment such as a limitation of a
welding gun in terms of stiffness.
[OO 121
Furthermore, it can be considered that, by increasing the number of
spots in spot welding, it is possible to reduce the load stress per spot in spot
welding. However, deterioration in productivity is inevitable.
Furthermore, if the distance between spots in spot welding is reduced,
electric current is diverted to the spot-welded portions that have been already
formed, causing a problem in which nuggets cannot be formed in a stable
manner.
[O013]
In other words, a desirable technique is one that can improve the
strength of an overlap-welded tnennber with resistance spot welding without
changing the dianleter of the nugget from the conventional one.
As for the technique described above, a subsequent energization
nlethod is disclosed in which a nugget is formed with main energization, and
after the nugget is cooled, energization is performed again (see, for example,
Non-Patent Document 1).
[0014]
With the subsequent energization method, as illustrated, for example,
in FIG. 3, in a state where a predetermined compression force is applied with
electrodes in resistance spot welding,
(A) a nugget is formed by applying first energization (main energization)
under conventional normal conditions;
(B) a predetermined suspension time is set to cool until martensite is formed
in the vicinity of the nugget; and
(C) second energization (subsequent energization) is applied, thereby
tempering the martensite.
With the subsequent energization nlethod as described above, each
heat-affected zone (hereinafter, referred to as a HAZ) of the nugget and the
spot-welded portion is tempered, whereby toughness is improved.
Furthermore, the HAZ is softened and is easily deformed, whereby stress in a
nugget end portion area is alleviated at the time of peeling. Thus, it is
considered that the peel strength can be improved.
[00 151
With the resistance spot welding enlploying the subsequent
energization, after the nugget is fornled through the main energization, the
molten metal is rapidly cooled through an Ms point to an Mf point or lower,
and martensite is formed.
The martensite thus forlned becomes tempered tnartensite by
controlling the electric current conditions and the like used in the subsequent
energization to adjust a heat-inputted amount so as to raise temperatures to
fall in an appropriate temperature range (in other \vords, not less than
approximately 550 to 600°C and not Inore than an Acl point as illustrated in
FIG. 3) in which tempering is possible, and being cooled after the subsequent
er~ergizationi s completed.
[00 161
FIG. 4 is a diagram schematically illustrating distribution of hardness
in a spot-welded portion 10 after the spot-welded portior~ 10 is formed by
overlapping test pieces S3 1 and S32, which are dual phase (DP) lnelnbers or
TRIP members, under norlnal conditions used in the conventional resistance
spot welding illustrated in FIG. 3, and applying the subsequent energization.
More specifically, (a) in FIG. 4 is a sectional view illustrating a
spot-welded portion, and (b) in FIG. 4 is a graph schematically showing
distribution of Vickers hardness in which each position corresponds to that in
(a) in FIG. 4.
[00 171
In the case where the overlapped portion is welded through resistance
spot welding using the subsequent energization as illustrated in FIG. 3, the
spot-welded portion 10 is first forlned through main energization.
At this point in time, as illustrated in (b) in FIG. 1, the spot-welded
portion 10 includes the nugget 12 and the HA2 14, and the HAZ 14 includes a
HAZ hardened portion 14H proximate to the nugget 12, and a HAZ softening
zone 14T formed in the vicinity of the HAZ hardened portion 14H.
Furthermore, the softest zone 14L in HAZ exists at an inner peripheral edge of
the HAZ softening zone 14T.
[0018]
Then, by applying the subsequent energization to the spot-welded
portion 10, the nugget 12 and the HAZ hardened portion 14H are tempered as
illustrated in FIG. 4, and the hardness of the nugget 12 and the HAZ hardened
portion 141-1 is decreased.
I-Iowever, hard portions 14P locally remain in the HAZ hardened
portion 14H. Thus, at the time of peeling, the hard portions in the HAZ 14
are not deformed, and deformation concentrates on the vicinity of the nugget
end 12E. As a result, stress concentration on the nngget end 12E is not
sufficiently improved.
[0019]
Furthermore, FIG. 5 is a diagram schematically illustrating changes of
a HAZ 14 in a spot-welded portion 10 in the case where resistance spot
welding according to conventional normal conditions is applied to test pieces
S41 and S42, which are hot-stamped member, to form the spot-welded portion
10, and the spot-welded portion 10 is subjected to subsequent energization.
More specifically, (a) in FIG. 5 is a sectional view illustrating a
spot-welded portion including the nugget 12 formed through single
energization applied to the test pieces S41 and S42, and (b) in FIG. 5 is a
graph schetnatically showing distribution of Vickers hardness in which each
position corresponds to that in (a) in FIG. 5.
Furthermore, (c) in FIG. 5 is a sectional view illustrating a
spot-welded portion including the nugget 12 after the subsequent energization,
and (d) in FIG. 5 is a graph schetnatically showing distribution of Vickers
hardness in which each position corresponds to that in (c) in FIG. 5.
It should be noted that the long dashed double-short dashed line
illustrated in (d) in FIG. 5 illustrates the distribution of Vickers hardness after
the main energization and before the subsequent energization.'
[0020]
In the case where the subsequent energization is performed under
appropriate conditions, a large area including the nugget 12 and the HAZ
hardened portion 14H is tempered as illustrated in (d) in FIG. 5. However,
terlipering cannot be sufficiently performed between the nugget end 12E and
the softest zone 14L in HAZ, and portions 14P having high Vickers hardness
locally reniain.
In other words, an effect of iniproving toughness through tempering
cannot be sufficiently obtained, and hence, it is not easy to sufficiently secure
peel strength of the spot-welded portion 10.
[0021]
Furthermore, in the case where heat inputted is excessive during the
subsequent energization, the HAZ hardened portion 14H is tempered.
However, the nugget 12 is quenched again. Thus, although the HAZ
hardened portion 14H is tempered, the nugget 12 is quenched again, and hence
the nugget 12 becomes hardened.
As a result, the toughness of the nugget 12 is deteriorated, and the peel
strength of the spot-welded portion 10 is reduced.
[0022]
As described above, with the conventional subsequent energization
method, it is not easy to sufficiently obtain the effect of improving the
toughness of the spot-welded portion, and there is a problem in which welding
time increases, which leads to a notion that this conventional subsequent
energization method is not practical. In order to solve these problems,
various techniques have been disclosed.
[0023]
Patent Doculllent 1 discloses an invention in which conditions for
subsequent energization are deternlined accordilig to sheet sets through
nunlerical calculation.
[0024]
Patent Doculllent 2 discloses an invention in which subsequent
energization is applied at least once for a short period of time under high
electric current conditions to effectively heat a portion that is to be a starting
point of fracture, thereby reducing the welding time, and furthermore, the
itlvelition has a wide range of appropriate conditions.
[0025]
Patent Document 3 discloses an invention that improves the fracture
strength of the joined portion by increasing the width of the HAZ softening
zone in the vicinity of the nugget tlirougli subsequent energization, and
making the structure fine while nlaintaining the hardness of the nugget.
[0026]
Patent Document 4 discloses an invention related to spot welding that
can secure excellent tensile strength when applied to a higli-tensile steel sheet,
by forming tlie nlaximuln point of hardness in a HAZ portion while
maintaining the hardness of a nugget through spot welding with a simple
two-step energization type formed by cotnbining main energization and
tempering energization.
Related Art Documents
Patent Document
LO0271
Patent Document 1: Japanese Unexamined Patent Application, First
Publication No. 2002-103054
Patent Document 2: Japanese Unexamined Patent Application, First
Publication No. 201 0-1 15706
Patent Document 3: Japanese Utiexatnined Patent Application, First
Publication No. 2012-1 87617
Patent Document 4: Japanese Unexamined Patent Application, First
Publication No. 2008-229720
Non-Patent Docutnent
[0028]
Non-Patent Document 1 : "Tetsu-to-Hagane" Vol. 68, No.9,
P1444-1451
Disclosure of the Invention
Problems to be Solved by the Invention
[0029]
According to the technique disclosed in Patent Docutnent 1, an object
thereof is to inlprove peel strength and fatigue strength of the spot-welded
portion, and it is possible to optimize conditions for subsequent energization.
However, the effect obtained therefrom is limited because this technique
utilizes residual stress.
[0030]
According to the technique disclosed in Patent Document 2, it is
possible to soften the nugget and the hardened portion of the HAZ serving as a
fracture starting point to improve the toughness, by optimizing subsequent
energization after welding.
However, it does not specifically indicate the state of softness.
Furthermore, although the cross-tension strength is improved, the mechanism
thereof is not clear, and the peel strength is not necessarily sufficiently
improved.
[003 11
According to the technique disclosed in Patent Document 3, although
it argues that the fracture strength can be improved by increasing the width of
the HA2 portion, the position of I-IAZ softening, rather than the width of the
HAZ portion, is more itnportant to alleviate strain concentration as will be
described later, and hence, there is a possibility that the strain concentration
on the nugget end portion cannot be sufficiently alleviated.
[0032]
According to the technique disclosed in Patent Document 4, excellent
tensile strength is obtained by changing the distribution of the hardened
portion in the HAZ portion. However, this technique improves the strength
of a joint by distributing the strain concentrated on the HAZ portion. Thus,
there is a possibility that an effect is less likely to be obtained in the case
where fracture occurs within the nugget.
Furthermore, the techniques disclosed in Patent Documents 3 and 4 are
techniques by which the effects cannot be obtained, for example, in the case
of hot-stamped members of a 1500 MPa class or higher, to which the present
invention is directed.
[0033]
As described above, in the case of the high-strength steel sheet
i~icludingm artensite, it is difficult to ilnprove the peel strength of the
spot-welded portion through subsequent energization, and effective
subsequent energization methods are desired. Furthermore, in place of the
subsequent energization having poor productivity because of longer weldi~ig
time, there has been a demand for a technique that can in~proveth e peel
strength of the spot-welded portion through single energization.
[0034]
The present invention has been made in view of the situations as
described above, and an object of the present invention is to provide an
overlap-welded member, an automobile part including the overlap-welded
member, a niethod of welding an overlapped portion, and a method of
manufacturing the overlap-welded member, which can improve the peel
strength of the spot-welded portion.
Means for Solving the Problem
[0035]
Each aspect of the present invention is as follows:
( 1 ) A first aspect of the present invention provides an overlap-welded
member in which an overlapped portion including plural steel sheet members
is joined at a spot-welded portion, in which at least one o f the plural steel
sheet members contains martensite; and the spot-welded portion includes: a
nugget formed through spot welding; a heat-affected zone formed in the
vicinity of the nugget; a softest zone having the lowest Vickers hardness in the
heat-affected zone; and a tempered area formed between a central portion of
the nugget and the softest zone and made out of tempered martensite having
Vickers hardness of not more than 120% in a case where Viekers hardness of
the softest zone is 100%.
(2) A second aspect of the present invention provides an overlap-welded
member in which an overlapped portion including plural steel sheet members
is joined at a spot-welded portion, it1 which at least one of the plural steel
sheet members contains inartensite; the spot-welded portion includes a nugget
formed through resistance spot welding a heat-affected zone formed in the
vicinity of the nugget, and a softest zone having the lowest Vickers hardness
in the heat-affected zone; and Equation (1) described below is satisfied, where
D (mm) is a distance from a nielting boundary portion of the nugget to the
softest zone, and if there is only one steel sheet member having the highest
tensile strength of the plural steel sheet members, t (mm) is a thickness of this
steel sheet member, whereas, if there are plural steel sheet niembers having
the highest tensile strength, t (mm) is a thickness of a steel sheet member
having the thinnest thickness of these steel sheet members.
D 5 to.' ... Equation ( 1 )
(3) In the overlap-welded member according to (1) or (2) described above,
the plural steel sheet members may include a hot-stamped member.
(4) A third aspect of the present invention provides an automobile part
including the overlap-welded member according to any one of (1) to (3)
described above.
(5) A fourth aspect of the present invention provides a method of welding
an overlapped portion, including a resistance spot welding process in which a
spot-welded portion is formed through resistance spot welding in an
overlapped portion including a plurality of steel sheet members, the
spot-welded portion including: a nugget; a heat-affected zone formed in the
vicinity of the nugget; and a softest zone having the lowest Vickers hardness
in the heat-affected zone, and a tempering process of forming, between a
central portion of the nugget and the softest zone, a tempered area made out of
tempered martensite having Vickers hardness of not more than 120% in a case
where Vickers hardness of the softest zone is 100%.
(6) In the method of welding an overlapped portion according to (5)
described above, in the resistance spot welding process, energization may be
performed so as to satisfy Equation (2) described below, where: T (second) is
an energization time in the resistance spot welding; if there is only one steel
sheet member having the highest tensile strength of the plural steel sheet
members, t (mm) is a thickness of this steel sheet member, whereas, if there
are plural steel sheet members having the highest tensile strength, t (mm) is a
thickness of a steel sheet member having the thinnest thickness of these steel
sheet members; and cyc (second) is a period of time for one cycle of
energization in the resistance spot welding.
5t x cyc < T 5 (5t + 4) x cyc ... Equation (2)
(7) In the method of welding an overlapped portion according to (5)
described above, it may be possible that the method further include, before the
resistance spot welding process, applying a preheat electric current I (kA) to
the overlapped portion in a state where an energization time TI (second), a
period of time cyc (second) for one cycle of energization, and a thickness t
(mm) satisfy Equatioti (3) described below; as for the thickness t (mtn), if
there is only one steel sheet tnelnber having the highest tensile strength of the
plurality of steel sheet members, a thickness of this steel sheet member is
used, whereas, if there are a plurality of steel sheet lnelnbers having the
highest tensile strength, a thickness of a steel sheet Inember having the
thinnest thickness of these steel sheet members is used; in the resistance spot
welding process, a welding electric current I. (kA) not more than a splash
occurring current is applied to the overlapped portion in a state where
Equation (4) described below is satisfied, where T2 (second) is an
energization time, and cyc (second) is a period of time for one cycle of
energization in the resistance spot welding; and the preheat electric current I
(kA) and the welding electric current I. (kA) satisfy Equation (5) described
below.
5t x cyc 5 TI < (5t + 8) x cyc ... Equation (3)
5t x cyc 5 T2 5 (5t + 4) x cyc ... Equation (4)
0.310 5 I 5 0.710 ... Equation (5)
(8) In the method of welding an overlapped portion according to (5)
described above, it may be possible that, in the resistance spot welding
process, the resistance spot welding be performed so as to satisfy Equation (6)
described below, where D (mm) is a distance from a melting boundary portion
of the nugget to the softest zone, and if there is only one steel sheet member
having the highest tensile strength of the plural steel sheet members, t (mm) is
a thickness of this steel sheet member, whereas, if there are plural steel sheet
members having the highest tensile strength, t (mm) is a thickness of a steel
sheet member having the thinnest thickness of these steel sheet members, and
the tempering process is a subsequent energization process in which the
tempered area is fornied through subsequent energization.
D I to.' ... Equation ( 6 )
(9) In the lnetliod of welding an overlapped portion according to (8)
described above, in the resistance spot welding process, energization may be
applied so as to satisfy Equation (7) described below, where T (second) is an
energization time in the resistance spot welding, and cyc (second) is a period
of time for one cycle of energization in the resistance spot welding.
5t x cyc I T I (5t + 4) x cyc ... Equation (7)
(10) In the method of welding an overlapped portion according to (8)
described above, it tilay be possible that the method further include, before the
resistance spot welding process: applying a preheat electric current I (kA) to
the overlapped portion in a state where an energization time TI (second), a
period of time cyc (second) for one cycle of energization, and the thickness t
(mm) satisfy Equation (8) described below; in the resistance spot welding
process, a welding electric current Io (kA) not more than a splash occurring
current is applied to the overlapped portion in a state where Equation (9)
described below is satisfied, where T2 (second) is an energization time, and
cyc (second) is a period of time for one cycle of energization; and
the preheat electric current I (kA) and the welding electric current 10
(kA) satisfy Equation (1 0) described below.
St x cyc I Tl 5 (5t + 8) x cyc ... Equation (8)
5t x cyc < T2 5 (St + 4) x cyc ... Equation (9)
0.310 5 I 5 0.710 ... Equation (10)
(11) A fifth aspect of tlle present invention provides a method of welding
an overlapped portion irlcluding a resistance spot welding process in which a
spot-welded portion is fornled in an overlapped portion including a plurality
of steel sheet members, the spot-welded portion including: a nugget; a
heat-affected zone formed in the vicinity of the nugget; and a softest zone
having the lowest Vickers hardness in the heat-affected zone, and in the
resistance spot welding process, resistance spot welding is performed so as to
satisfy Equation (11) described below, where D (mm) is a distance from a
nlelti~lgb oundary portion of the nugget to the softest zone, and if there is only
one steel sheet member havitig the highest tensile strength of the plural steel
sheet members, t (mm) is a thickness of this steel sheet member, whereas, if
there are plural steel sheet members having the highest tensile strength, t
(mm) is a thickness of a steel sheet member having the thin~lesth ickness of
these steel sheet members.
D 5 ... Equation (1 1)
(12) In the ~llethodo f welding an overlapped portion according to (1 1)
described above, in the resistance spot welding process, energization may be
applied so as to satyisfy Equation (12) described below, where T (second) is
an energization time in the resistance spot welding, and cyc (second) is a
period of time for one cycle of energization it1 the resistance spot welding.
5t x cyc 5 T 5 (5t + 4) x cyc ... Equation (12)
(13) In the method of welding an overlapped portion according to (11)
described above, it may be possible that the method further include, before the
resistance spot welding process, applying a preheat electric current I (kA) to
tlle overlapped portion in a state where an energization time TI (second), a
period of time cyc (second) for one cycle of energization, and the thickness t
(mm) satisfy Equation (13) described below; in the resistance spot welding
process, a welding electric current lo (kA) not more than a splash occurring
current is applied to the overlapped portion in a state where Equation (14)
described below is satisfied, where T2 (second) is an eliergization time, and
cyc (secoad) is a period of time for one cycle of energization; and the preheat
electric current I (kA) and the welding electric current 10 (kA) satisfy
Equation (1 5) described below.
5t x cyc 5 TI 5 (St + 8) x cyc ... Equation (13)
5t x cyc 5 Tz 5 (St + 4) x cyc ... Equation (14)
0.310 5 I < 0.710 ... Equation (15)
(14) A sixth aspect of the present invention provides a method of
manufacturing an overlap-welded member in which an overlapped portion
including plural steel sheet members is joined at a spot-welded portion, the
method including: overlapping the plural steel sheet members at a position of
the overlapped portion; and welding the overlapped portion through the
method of welding an overlapped portion according to any one of (5) to (13)
described above.
[0036]
It should be noted that, in this specification, "cyc" represents one
cycle (llfrequency) (second) of power supply used for energization it1 the
resistance spot welding. In the case of 60 Hz, 1 x cyc is (1160) (second), and
in the case of 50 Hz, 1 x cyc is (1150) (second).
Effects of the Invention
100371
According to the overlap-welded member, the automobile part
including the overlap-welded member, the method of welding an overlapped
portion, and the method of liialiufacturing the overlap-welded member of the
present invention, it is possible to improve the peel strength of the
spot-welded portion.
Brief Description of the Drawings
[0038]
FIG. 1 is a diagram schematically illustrating distribution of hardness
in a spot-welded portion in the case where conventional energization
conditions are applied to a TRIP member.
FIG. 2A is a perspective view schelnatically illustrating a
cross-tension test.
FIG. 2B is a diagram illustrating a fracture mode concerning a
spot-welded portion with a cross-tension test, and is a sectional view
illustrating a partial plug fracture.
FIG. 2C is a diagram illustrating a fracture mode concerning a
spot-welded portion with a cross-section test, and is a sectional view
illustrating a plug fracture.
FIG. 2D is a diagram illustrating an example of a correlation between
tensile strength of a base metal and a cross-tension strength.
FIG. 3 is a diagram schematically illustrating a subsequent
energization method.
FIG. 4 is a diagram schematically illustrating distribution of hardness
in a spot-welded portion after the spot-welded portion is formed by
overlapping test pieces according to a subsequent energization neth hod
illustrated in FIG. 3, and applying subsequent energization.
FIG. 5 is a diagram schematically illustrating changes of a HAZ in a
spot-welded portion in the case where the spot-welded portion formed on a
hot-stamped member is subjected to subsequent energization.
FIG. 6 is a diagram illustrating a schematic configuration of a
spot-welded portion including a nugget according to an embodiment of the
present invention.
FIG. 7 is a diagram illustrating a schematic configuration of a nugget
and a HAZ in the same spot-welded portion.
FIG. 8 is a diagranl explaining energization conditions in resistance
spot welding according to an embodiment of the present invention.
FIG. 9A is a diagram scliematically illustrating portions of a
spot-welded portion according to an embodinlent of the present invention
where hardness is measured.
FIG. 9B is a graph showing a relationship between distances (mm)
from a nielting boundary of a nugget and Vickers hardness.
FIG. 10A is a diagram illustrating an analysis nlodel under a
short-time energization condition with a distance from a nugget end to tlie
softest zone in HAZ being set to 0.75 tnm.
FIG. 1013 is a diagram illustrating an analysis model under a nornial
condition with a distance from a nugget end to the softest zone in HAZ being
set to 1.5 mm.
FIG. 11 is a graph related to an analysis model of each spot-welded
portion under "(a) short-time energization condition," "(b) normal condition,"
and "(c) no HAZ softening," and illustrating equivalent plastic strain at
Position I illustrated in FIG. 10A.
FIG. 12 is a graph related to an analysis model of each spot-welded
portion under "(a) short-time energization condition," "(b) normal condition,"
and "(c) no HAZ softening," and illustrating equivalent plastic strain at
Position 2 illustrated in FIG. 10A.
FIG. 13A is a diagram illustrating a relationship between a thickness t
and a distance D from a melting boundary of a nugget to a HAZ softening
zone.
FIG. 13B is a diagram illustrating a relationship between cross-tension
strength and distances D from the niclting boundary of a nugget to a HAZ
softening zone.
FIG. 14 is a graph showing behavior of nugget growth in the case
where a short-time energization condition, a normal condition, and a two-step
energization condition are applied to a hot-stamped Inember of an 1800 MPa
class having a thickness of 1.6 mm.
FIG. 15 is a graph showing distribution of hardness in a spot-welded
portion formed under tlie conditions shown in FIG. 14.
FIG. 16 is a diagram schematically illustrating changes of HAZ in
spot-welded portions after single energization and after subsequent
energization in the case where a short-time energization condition according
to an embodiment of the present invention is applied to a hot-stamped
member.
FIG. 17 is a diagram schematically illustrating changes in Vickers
hardness in spot-welded portions after single energization and after
subsequent energization in the case where a short-time energization condition
accordirlg to an embodi~liento f the present invention is applied to a
hot-stamped member.
FIG. 18 is a graph showing distribution of hardness in spot-welded
portions after single energization in the case where a short-time energization
condition according to an embodiment of the present invention and a normal
energization condition are applied to a hot-stamped member of an 1800 MPa
class having a thickness of 1.8 mm.
FIG. 19 is a graph showing distribution of hardness in spot-welded
portions after subsequent energization in the case where a short-time
energization condition according to an e~nbodimento f the present invention
and a normal energization condition are applied to a hot-stamped member of
an 1800 MPa having a thickness of 1.8 mm.
FIG. 20 is a diagram schelnatically illustrating changes of distribution
of hardness in a spot-welded portion obtained by forming the spot-welded
portion under a short-time energization condition according to an e~nbodi~nent
of the present invention and a normal energization condition, and then,
applying subsequent energization.
FIG. 21 is a perspective view illustrating an L-sllaped test.
Ernboditnents of the Invention
[0039]
The present inventors carried out thorough investigations on
improving the peel strength in the case where plural steel sheet members
including at least one steel sheet member containing martensite are joined at a
spot-welded portion in an overlapped portion. As a result, it was found that,
by applying single energization (short-time single energization) under a
short-time energization condition in which an electric current value is
increased and an energization period of time is shorter than conventional one,
the HAZ hardened portion is reduccd, and a distance between a nugget end
and the softest zone in HAZ is reduced.
Furthermore, it was also found that, with the reduction in the distance
between the nugget end and the softest zone in HAZ, the stress acting at the
time of load applied on a nugget end portion area in the peeling direction is
alleviated, and the peel strength largely improves.
On the basis of the findings described above, in place of the
subsequent energization with the conventional type that needs longer time, the
present inventors developed a method that can improve the strength with
single energization.
[0040]
Furthermore, it was also found that, by reducing the distance between
the nugget end and the softest zone in HAZ, and applying the subsequent
energization, the nugget and the HAZ hardened portion are tempered, whereby
it is possible to suppress hard portions being locally formed between the
nugget end and the softest zone in HAZ. Th~ust, he peel strength of the
spot-welded portion is improved as compared with the subsequent
energization with the collventional type.
[0041]
Below, the present invention made on the basis of the findings
described above will be described in detail with reference to the drawings.
FIG. 6 is a sectional view illustrating a spot-welded portion, which
illustrates the schematic configuration of a spot-welded portion 10 formed in
an overlap-welded member used, for example, as an automobile part,
according to an embodiment of the present invention.
The overlap-welded member according to this embodiment is formed
by joining steel sheet members S1 and S2 through a spot-welded portion 10 as
illustrated in FIG. 6.
As illustrated in FIG. 6, a nugget 12 is formed in an overlapped
portion of the steel sheet members S1 and S2 through energizatioti applied
from a pair of electrodes 50, 50, which are used for resistance spot welding
and to squeeze the steel sheet members S1 and S2 in the thickness direction
between the pair of electrodes with the central line CL of electrodes 50 being
the center.
[0042]
As for molten metal generated through the energization, solidification
grows in an area in the vicinity of the central line CL and in contact with the
electrodes 50 toward the thickness direction due to heat dissipated to the
electrodes 50, whereas, in an area distant from the central line CL of the
electrodes 50, solidification grows toward the central direction of the nugget
(toward the central line CL of the electrodes) in addition to toward the
thickness direction.
As a result, the nugget 12 includes an area 12A where dendrite grows
in the thickness direction, and an area 12B where dendrite grows so as to
intersect the thickness direction.
[0043]
In this specification, when the overlapped portion is viewed from the
thickness direction, the nugget end 12E represents the outertnost boundary (in
other words, nielting boundary portion of the nugget 12) that nielts when the
nugget 12 is formed, and the nugget end portion area 12B represents an area
from a meeting portion 12C between the area 12A and the area 12B to the
nugget end 12E.
[0044]
FIG. 7 is a sectional view illustrating a spot-welded portion, which
illustrates the spot-welded portion 10 where the overlapped portion is welded.
The spot-welded portion 10 includes the nugget 12 formed through spot
welding, and the I-IAZ 14 formed in the vicinity of this nugget 12 through spot
welding.
The HAZ 14 includes a I-IAZ hardened portion 14H formed next to the
nugget 12, and a HAZ softening zone 14T formed around the HA2 hardened
portion 141-1.
Furthermore, the softest zone 14L in HAZ having the lowest Vickers
hardness is formed in the vicinity of the inner peripheral portion in the HAZ
softening zone 14T.
The reference character D illustrated in FIG. 7 represents a distance
between the nugget end 12E and the softest zone 14L in HAZ.
[0045]
FIG. 8 is a diagram illustrating energization conditions in resistance
spot welding according to this embodiment.
As illustrated in FIG. 8, in the case of a short-time energization
condition C11 according to this embodiment, resistance spot welding is
performed by first applying single energization in which an energization
electric current I1 I, which is higher than an energization electric current I21
under a normal energization condition C21, is applied for an energization time
T11, which is shorter than a conventional normal energization time T21.
The broken line in FIG. 8 indicates the first energization C21 (an
electric current value 121 and an energization time T21) under a normal
condition, where the electric current value I1 1 > the electric current value 121,
and the energization time TI 1 (cyc) < the energization time T21 (cyc).
Furthermore, in FIG. 8, the reason that the short-titne energization
condition C11 is illustrated from the intermediate stage in the nornlal
energization condition C21 on the time axis is to match the conlpletion times
of energization.
[0046]
In the short-time energization condition C1 I according to this
embodiment, as illustrated in FIG. 8, the molten metal generated at the tinle of
forming the nugget 12 through energization is rapidly cooled after the single
energization is completed, and temperatures thereof pass through the Ms point
and are decreased to the Mf point or lower, so that martensite is formed.
[0047]
Furthermore, by comparing a tenlperature curve H1 of the nugget 12
under the short-time energization condition C11 with a tenlperature curve H2
of the nugget under the normal energization condition C21, the joined portion
is melted, and the nugget 12 is formed under the short-time energization
condition Cl1 in a shorter period of time than those under the normal
eliergization condition C21.
Thus, with the short-time energization condition C1 1, the excessive
heat flow to the vicinity of the nugget 12 is suppressed, the size of the HAZ
hardened portion is reduced, and the distance D between the nugget end 12E
and the softest zone 14L in HAZ is reduced.
As a result, only with the single energization described above are the
strains at the time of peeling concentrated 011 portions other than the nugget
end portion area 12B, and the stress concentrated on the nugget end portion
area 12B can be alleviated, whereby the peel strength improves.
[0048]
It should be noted that the spot-welded portion 10 formed with single
energization under the short-time energization condition C11 may be used as
it is without applying additional processing. Furthermore, after a
predetermined suspension period of time Ts elapses, it niay be possible to
apply subsequelit energization (in other words, the second energization) under
a subsequent energization condition C12 to the spot-welded portion 10 thus
formed.
[0049]
By applying the energization under the subsequent energization
condition C12 (an electric current value I12 and an energization time T12) to
the spot-welded portion 10 formed under the short-time energization condition
C11 after energization is suspended for the suspension period of time Ts, the
nugget 12 is heated to temperatures not less than a temperature
(approximately 550 to 600°C) at which telnpering is possible and not more
than Acl, and then, is gradually cooled, \vilereby tempered martensite can be
obtained without re-quenching the HAZ 14.
[0050]
As described above, in the case of the short-time energization
condition C11, the electric current value I1 1 is set so as to be larger than the
electric current value I21 under the ~lormal energization condition C21, and
the energization time TI1 is set so as to be shorter than the energization time
T21 under the normal energization condition C21. Thus, temperatures of the
nugget 12 are raised in a short period of time, and transference of the heat
generated through energization to the vicinity thereof is not developed,
whereby the HAZ 14 is less likely to become high temperature as compared
with the normal condition.
As a result, it can be considered that the width of the HAZ hardened
portion 14H is narrow, and the distance D between the nugget end 12E and the
softest zone 14L in HAZ is reduced.
[005 11
As described above, by forming the nugget 12 under the short-time
energization condition C11, the width of the HAZ hardened portion 14H can
be made narrow, whereby the nugget 12 and the HAZ hardened portion 14H
are sufficiently tempered.
Thus, it is possible to prevent high Vickers hardness portions from
being formed between the nugget end 12E and the softest zone 14L in HAZ.
More specifically, since the HAZ hardened portion 14H is softened in
a uniform manner, deformation becomes easy, and stress acting on the nugget
end portion area 12B at the time of peeling is reduced, whereby it is possible
to inlprove the peel strength.
[0052]
As described above, by employing the short-time energization
condition C11 and the subsequent energization condition C12, Vickers
hardness between the softest zone 14L in HAZ and the nugget end 12E can be
made to 120% or lower in the case where Vickers hardness in the softest zone
14L in HAZ is loo%, wliercby toughness of the spot-welded portion 10 can be
sufficiently secured.
[0053]
Below, a relationship between distances (nun) from the melting
boundary of the nugget 12 and Vickers hardness will be described with
reference to FIG. 9A and FIG. 98.
FIG. 9A and FIG. 9B are diagrams each illustrating a case where a
first energization is applied to a hot-stamped member of a 1500 MPa class
having a thickness of 1.6 mni under a "(a) short-time energization condition"
and a "(b) normal condition" according to this embodiment. FIG. 9A is a
sectional view illustrating a spot-welded portion, and FIG. 9B is a graph
sl~owingd istribution of hardness in the spot-welded portion 10.
As for measurement of distribution of hardness, as illustrated in FIG.
9A, measureilient is performed at a position located at a distance of 114 of the
thickness from the joining surface of the steel sheet members S1 and S2
toward the steel sheet illember S1 arid the inner side of the steel sheet menlber
Sl by applying a load of 9.8 N at 0.5 mtn pitches according to JIS Z 2244.
In the graph shown in FIG. 9B, the "blank diamonds" represent the
short-time energization condition, and the "blank circles" represent the
normal energization condition.
It sliould be noted that the energization time in the short-time
energization condition is set to 9 x cyc, the energization time in the normal
condition is set to 20 x cyc, and the electric current value is adjusted such that
the nugget diameter is 4dt (mm) (t represents a thickness).
From FIG. 9A and FIG. 98, it can be understood that the distance D
from the nugget end 12E to the softest zone 14L in I-IAZ is reduced by
applying the first energization under the "(a) short-time energization
condition."
[0054]
Below, equivalent strain in the case where energization is performed
under the short-time energization and the normal condition will be described
with reference to FIG. 10A, FIG. 10B, FIG. 1 1 , and FIG. 12.
Equivalent plastic strains in the case of the "(a) short-time
energization condition," the "(b) normal condition," and the "(c) no HAZ
softening" are obtained through elasto-plastic FEM analysis under the
short-time energization and the normal condition. Detailed descriptions will
be made below.
FIG. 10A is a sectional view illustrating a spot-welded portion, which
illustrates an analysis model of a test piece for single energization obtained by
applying the first energization under the " ( a ) short-time energization
condition" with the distance D from the nugget end 12E to the softest zone
14L in HAZ being set to 0.75 mm.
FIG. 10B is a sectional view illustrating a spot-welded portion, which
illustrates an analysis rilodel o f a test piece for single energization obtained by
applying the first energization under the normal condition with the distance D
froni the nugget end 12E to the softest zone 14L in HAZ being set to 1.5 mm.
[0055]
It should be noted that, for the analysis models, the distribution of
hardness in the HAZ softening zone is varied in a stepwise lilanner froni the
hardness of the softest zone to the hardness of the base metal portion on the
basis of the nleasurement results shown in FIG. 9B.
[0056]
In FIG. 10A and FIG. 10B, the Position 1 represents the softest zone
14L in HAZ, and the Position 2 represents the nugget end 12E.
For the analysis models, three patterns are used, which include a case
where the "(a) short-time energization condition" in FIG. 10A is simulated, a
case where the "(b) norrlial condition" in FIG. 10B is silnulated, and a case
where the "(c) no HAZ softening" is simulated.
[0057]
FIG. 11 is a graph showing equivalent plastic strains at the Position 1
illustrated in FIG. 10A in the case where the analysis models formed by the
spot-welded portions with the "(a) short-time energization condition," the "(b)
normal condition," and the "(c) no HAZ softening" are subjected to
cross-tension testing with a load with which the spot-welded portion in the
case of the "(b) norlnal condition" is fractured through the cross-tension
testing.
It should be noted that, in FIG. 1 1, the Position 1 in the analysis model
with the "(c) no HAZ softening" is set to the same position as that with the
"(b) nornial condition."
[OOSS]
As shown in the graph in FIG. 11, the equivalent plastic strain at the
Position 1 is approxi~nately0 .032 with the "(a) short-time energization
condition," which significantly increases as compared with 0.013 with the "(b)
normal condition" and approxi~nately0 .01 8 with the "(c) no HAZ softening."
[0059]
FIG. 12 is a graph showing equivalent plastic strains at the Position 2
illustrated in FIG. 10A in the case where the analysis models formed by the
spot-welded portions with the "(a) short-time energization condition," the "(b)
nor~nalc ondition," and the "(c) no I-IAZ softening" are subjected to
cross-tension testing with a load with which the spot-welded portion in the
case of the (b) nornlal condition is fractured through the cross-tension testing.
It should be noted that, in FIG. 12, the Position 2 in the analysis rnodel
with the "(c) 110 HAZ softening" is set to the same position as that with the
"(b) normal condition."
[0060]
Furthermore, as shown in the graph in FIG. 12, the equivalent plastic
strain at the Position 2 is approximately 0.010 with the "(a) short-time
energization condition," which decreases as compared with 0.01 15 with the
"(b) normal condition" and approximately 0.01 18 with the "(c) no HAZ
softening."
28
However, at the position of HAZ softening with tlie "(b) normal
condition," the existence or absence of the HAZ softening has a limited effect
on the equivalent plastic strain it1 the end portio~ia rea of the nugget, as
co~i~parewdi th the "(b) nor~nalc ondition" and the "(e) no HAZ softening."
[006 11
More specifically, in the ease of tlie normal condition, the HAZ
softe~iingz one 14T provides little effect as to reducing the strains to the
nugget end portion area 12B at tlie time of peeling, and since the HAZ
softening zone 14T approaches the nugget end portion area 12B, the strains
concentrate on the HAZ softening zone 14T. As a result, it was found that
the strains concentrated on the nugget end portion area 12B can be reduced.
In other words, with this effect, tlie peel strength can be increased by using
the "short-time energizatio~c~on dition."
[0062]
Below, conditions appropriate for the overlap-welded member
obtained by joining, at the spot-welded portion, overlapped portions including
plural steel sheet tnetnbers, according to this embodiment will be described
with reference to FIG. 13A and FIG. 13B.
[0063]
FIG. 13A is a diagram illustrating a relationship between the thickness
t (mm) of the overlapped portion and a distance D (mm) from a melting
boundary (end of the nugget) of the nugget to the softest zone in HAZ.
In FIG. 13A, the "blank circles" represent a DP steel of a 980 MPa
class with a conventional single energization.
Furthermore, the "blank dian~onds"r eprese~ita hot-stamp steel of a
1500 MPa class with a conventional single energization.
[0064]
Here, as for the thickness t (mm) in FIG. 13A, in the ease where there
is only one steel sheet member having the highest tensile strength of plural
steel sheet members, t (mni) is the thickness of this steel sheet member, and in
the case where there are plural steel sheet members having the highest tensile
strength, t (11lm) is the thickness of a steel sheet member having the thinnest
thickness of all the steel sheet members.
[0065]
As illustrated in FIG. 13A, in the case of the single energization wit11 a
conventional condition, the distance D between the nugget end 12E and the
softest zone 14L in HAZ on the overlapping interface between two steel sheets
is formed so as to fall in a range exceeding D (mm) = to.2 (mm). At tlle time
of the cross-tension test, in the case of a sheet set obtained by colllbining a
steel type having low joint strength and a steel type having high joint strength,
fracture tends to occur on the side of the low joint strength.
[0066]
For example, in the case where the strength of the base metal is higher
than a 780 MPa class, the cross-tension strength decreases wit11 an increase in
the strength of the base metal, and hence, fracture is more likely to occur as
the strength of the base metal increases.
Furthermore, in the case of a sheet set having the same steel type but
different thicknesses, fracture occurs on the side of tlle steel sheet having the
thinner thickness.
For the reasons described above, the thickness t of a steel sheet
member having the thinnest thicktless is employed.
[0067]
FIG. 13B is a diagram illustrating a relationship between the
cross-tension strength and the distance D from the nugget end 12E of the
spot-welded portion to the HAZ softening zone in the case where the nugget
dianieter in a hot-stamped menlber of a 1500 MPa class is 4dt.
As illustrated in FIG. 13B, by setting the distance D (mm) between the
nugget end 12E and the softest zone 14L in HAZ to (mm) or shorter, the
cross-tension strength increases to approximately 7 kN and is made stable,
which makes it possible to make the fracture mode to be the plug fracture.
Furthermore, by setting the distance D (mm) between the nugget end 12E and
the softest zone 14L in HAZ to 0.75 x (to-') (mm) or shorter, the cross-tension
strengtli increases to approximately 8 kN, and is made further stable, which
niakes the fracture niode to be the plug fracture. This is more favorable.
As described above, by reducing tlie distance D from the nugget end
12E of the spot-welded portion to the HAZ softening zone 14T, the
cross-tension strengtll improves.
[0068]
Furthermore, as for tlie hardness from the base metal toward the
nugget end portion area 12B (including the nugget end portion area 12B) on
the overlapping interface between these two steel sheet members, the hardness
gradually decreases toward the nugget end 12E in a range where the maximum
value of Vickers hardness relative to the softest zone 14L in HAZ is
approximately 115%, or the hardness is equivalent to the hardness of the
softest zone 14L in HAZ.
[0069]
According to the overlap-welded member of this embodiment, by
bringing the HAZ softening zone 141' close to tlie nugget end portion area 12B,
the stress concentration on tlie nugget end portion area 12B serving as the
starting point of the fracture within the nugget (interface fracture, partial plug
fracture) is alleviated, whereby it is possible to improve the joint strengtli.
[0070]
The effect of improving the joint strength becomes more apparent as
tlie fracture mode changes from the fracture within tlie nugget (the interface
fracture and the partial plug fracture) to tlie plug fracture.
In particular, as for a joint for which tlie plug fracture cannot be
obtained because the tough~iesso f the nugget 12 itself is not sufficient and tlie
crack propagates into the nugget even if the stress concet~trationo n the nugget
end portion area 12B is alleviated, by applying subsequent energization to this
joint in addition to optimization of the HAZ softening zone 14T, it is possible
to obtain tlie effect of improving the joint strength stronger than the
conventional one.
This mechanism has already been described above.
[0071]
As described above, i f the distance D (nun) from the nugget end 12E
to the softest zone 14L in HAZ satisfies
D 5 ... Equation ( I ) ,
it is possible to sufficiently improve the joint strength.
Thus, with the overlap-welded member according to this embodiment,
a condition is set such that the distance D from the nugget end 12E to the
softest zone 14L in HAZ satisfies Equation ( 1 ) described above.
Furthermore, by making the distance D (mm) from the nugget end 12E
to the softest zone in HAZ satisfy
D < 0.75 x ... Equation ( l A ) ,
the fracture mode can be more reliably made to be the plug fracture, which is
preferable.
[0072]
Below, a method of welding the overlap-welded portion using a
resistance spot welding process and a tempering process will be described in
detail.
[0073]
(Resistance Spot Welding Process)
In the resistance spot welding process, a spot-welded portion 10
including a nugget 12, a I-IAZ 14 formed around this nugget 12, and the softest
zone 14L having the lowest Vickers hardness in this HAZ 14 is formed
through resistance spot welding at an overlapped portion formed by plural
steel sheet members.
[0074]
(Tempering Process)
In the tempering process, a tempered area made out of tempered
martensite having the Vickers hardness of 120% or lower in the case where
the Vickers hardness of the softest zone 14L is 100% is fornled between the
central portion of the nugget 12 formed through the resistance spot welding
process and the softest zone 14L.
It is preferable to apply subsequent energization to form the tempered
area. However, this formation is not limited to through the subsequent
energization. It niay be possible to use, for example, emission of laser beam
to form the tempered area.
[0075]
With the method of welding an overlapped portion according to this
embodiment as described above, the tempered area having the Vickers
hardness of 120% or lower in the case where the Vickers hardness of the
softest zone 142, is 100% is formed between the central portion of the nugget
12 and the softest zone 14L.
[0076]
Furthermore, in the resistance spot welding process described above,
the nugget 12 may be for~nedw ith an energization time T expressed in the
following manner, where t (mm) is the thickness, and cyc (second) is a period
of time for one cycle of energization in the resistance spot welding.
5t x cyc 5 T 5 (5t + 4) x cyc ... Equation (2)
In general, in the spot welding, with an increase in the thickness, the
energization time increases, and the distance D from the nugget end 12E to the
softest zone 14L in HAZ tends to increase. However, with the satisfaction of
this Equation (2), the nugget can be stably formed, and the distance D (mm)
between the nugget end 12E and the softest zone 14L in I-IAZ can be more
reliably formed so as to be not more than to.'.
In other words, it is possible to stably improve tile peel strength at the
spot-welded portion.
It should be noted that, as for the thickness t (mm), in the case where
there is only one steel sheet member having the highest tensile strength of
plural steel sheet members, t (mm) is the thickness of this steel sheet member,
and in the case where there are plural steel sheet nielnbers having the highest
tensile strength, t (mm) is the thickness of a steel sheet member having the
thinnest thickness of all the steel sheet members.
[0077]
(Preheat Energization Process)
As described above, by applying spot welding wllile satisfying the
energization time specified in this embodiment, this spot welding is effective
fro111 the viewpoi~lt of the HAZ softening zone 14T. On the other hand, the
appropriate electric current range reduces as cotnpared with the conventional
energization condition.
In this respect, the present inventors found that it is preferable to
perform a preheat energization process before the resistance spot welding
process described above is performed, in terms of being able to bring the
softest zone in I-IAZ closer to the end portion area of the nugget as compared
with the conventiollal technique while maintaining the appropriate electric
current range (margin of electric current to the splash occurring current)
equivalent to the conventional condition.
[0078]
Here, the above-described effect obtained by performing the preheat
energization process will be described with reference to FIG. 14 and FIG. 15.
FIG. 14 is a graph showing behavior of nugget growth in the case
where the short-time energization condition (9 x cyc), the nor~nalc ondition
(20 x cyc), and a two-step energization condition (energization time in the
first step: 1 1 x cyc, welding electric current: 4 kA, and energization time in
the second step: 9 x cyc) are applied to a hot-stamped member of an 1800 MPa
class having a thickness of 1.6 mm.
Furthermore, FIG. 15 is a graph showing distribution of Vickers
hardness based on distances from the nugget end of the spot-welded portion
formed under the conditions shown in FIG. 14.
As shown in FIG. 14 and FIG. 15, by applying two-step energization
including the preheat energization and the main energization, it is possible to
bring the position of HAZ softening closer to the nugget end portion area 12B
as conlpared with the conventional technique while maintaining the
appropriate electric current range allnost equivalent to that of the
conventional technique.
[0079]
Below, energization conditions for the preheat energization process
will be described in detail.
In the preheat energization process, energization with preheat electric
current I (kA) is applied to the overlapped portion in a manner such that
energization time TI (second), a period of time cyc (second) for one cycle of
energization, and a thickness t (mm) satisfy
5t x cyc I TI 5 (5t + 8) x cyc ... (Equation 3).
Then, in the case where the preheat energization process is performed,
tlie nugget is formed by, after the preheat energization process, applying
energization wit11 welding electric current 10 (kA), which is less than or equal
to tlie splash occurring current, to the overlapped portion so as to satisfy
5t x cyc 5 Tz 5 (5t + 4)xcyc ... Equation (4),
where T2 (second) is an energization time and cyc (second) is a period of time
for one cycle of energization in the resistance spot welding.
Here, a relationship between the preheat electric current I (kA) and the
welding electric current 10 (kA) satisfies
0.310 5 1 5 0.710 ... Equation (5).
[OOXO]
In the preheat energization process described above, the energization
tinle Ti (second) is longer than or equal to 5t x cyc, and the preheat electric
current I (kA) is more than or equal to 0.310, in other words, is more than or
equal to 30% of the welding electric current 10 in the resistance spot welding
process for forming the nugget. Thus, the preheating effect is sufficient, and
it is possible to secure a desired appropriate electric current range, which is
preferable.
Furthermore, the energization time TI (second) is less than or equal to
(5t + 4) x cyc, and the preheat electric current I (kA) is less than or equal to
0.710, in other words, is less than or equal to 70% of the welding electric
current I0 in the resistance spot welding process for forming the nugget.
Thus, it is possible to reduce the distance D fronl the nugget end 12E to the
softest zone 14L in HAZ, which is preferable.
[0081]
Then, in the resistance spot welding process performed after the
preheat energization, the energization time T2 is set to be not shorter than 5t x
cyc and not longer than (5t + 4) x cyc. Thus, it is possible to sufficiently
form the nugget, and it is possible to make the distance D (mm) between the
nugget end 12E and the softest zone 14L in HAZ to be not longer than
This makes it possible to stably improve the peel strength in the spot-welded
portion.
Furthermore, by adjusting the energization time such that the D (mm)
is shorter than or equal to 0.75 x it is possible to more reliably obtain
the spot-welded portion whose fracture mode is the plug fracture, and it is
possible to improve the peel strength.
[0082]
By applying the tempering process described above (for example,
tempering with the subsequent energization) to the thus obtained
overlap-welded portion so that the nugget end portion area 12B is tempered, it
is possible to form, between the central portion of the nugget 12 and the
softest zone 14L, the tempered area formed by the tempered tnartensite having
the Vickers hardness of 120% or less in the case where the Vickers hardness of
the softest zone 14L is 100%.
Thus, it is possible to manufacture the overlap-welded member having
the nugget diameter same as the conventional one, exhibiting excellent
strength, and increased joint strength.
[0083]
In order to obtain the effect as described above, it is necessary to
adjust the short-time energization condition such that the nugget end 12E and
the softest zone 14L in HAZ are brought closer to each other, and then, apply
the tempering process to make the Vickers hardness of the tempered area to be
not more than 120% of the Vickers hardness of the softest zone 14L.
However, in order to obtain the effect in a more favorable manner, it is
preferable to make the Vickers hardness of the tempered area to be not more
than 115% of the Vickers hardness of the softest zone 14L, and it is Inore
preferable to make the Vickers hardness of the tempered area to be not more
than 110% of the Vickers hardness of the softest zone 14L.
It should be noted that the lower litnit value of the Vickers hardness of
the tempered area is not specified.
[0084]
If the tensile strength of the base material is stronger than or equal to a
980 MPa class, the interface fracture or the partial plug fracture is more likely
to occur in the overlap-welded member, and the joint strength tends to
decrease.
This embodiment is effective to the steel sheet member having the
HAZ softening made as a result of spot welding. However, it is preferable to
apply this elnbodiment to a high-tensile steel sheet having the base metal with
a tensile strength of a 980 MPa class or higher.
In particular, in the case of the hot-stamped member, the base metal is
full marteasite. Thus, the amount of softening of I-IAZ is large, and the
effect obtained by this embodiment is significant.
[0085]
Furthermore, as for the overlap-welded member according to this
embodiment, there is no limitation on the tllickness of, the type (for example,
DP, TRIP, and so on) of, and the existence or absence of plating of each steel
sheet member in the overlapped portion formed by two or more steel sheet
members.
Furthermore, in Example described later, although description will be
made of a sheet set obtained by overlapping two steel sheets with the same
type, application is not limited to this sheet set. The effect can be obtained
in the case of a sheet set with different materials, or a sheet set with three or
more sheets.
[0086]
FIG. 16 is a diagram illustrating a schematic configuration of the
spot-welded portion 10 formed with the energization condition according to
this embodiment in tlie case where hot-stamped members are used as the steel
sheet rllembers S1 and S2. More specifically, (a) in FIG. 16 is a sectional
view illustrating a spot-welded portion after a short-time energization is
applied, and (b) in FIG. 16 is a sectional view illustrating a spot-welded
portion after a subsequent energization is applied. (c) in FIG. 16 is a graph
showing distribution of Vickers hardness after the single energization and
after the subsequent energization.
Furthermore, FIG. 17 is a diagram scliematically illustrating changes
in Vickers hardness in tlie spot-welded portion after the single energization
and after the subsequent energization.
By applying the short-time energization, thc distancc D between the
nugget end 12E and the softest zone 14L in HAZ is reduced to approximately
1 mm as illustrated in (a) in FIG. 16.
Since the distance D between the nugget end 12E and the softest zone
14L in HAZ through energization under a conventional normal condition is
approxi~nately 1.5 mm, the distance is significantly reduced as compared with
the conventional one.
As a result, it is possible to alleviate the stress concentration in the
vicinity of the nugget end 12E.
[0087]
Moreover, by further applying the subsequent energization, it is
possible to sufficiently temper the HAZ hardened portion 14H in the
dot-hatched area illustrated in (b) in FIG. 16.
[0088]
As described above, in the case where the spot-welded portion 10 is
formed by applying the energization condition according to the present
invention to the hot-stamped members S1 and S2, the Vickers hardness of tlie
HAZ hardened portion 141-1 is al~iloste qual to that of the nugget 12 by
applying the short-time single energization as illustrated in (a) and (b) in FIG.
17.
[0089]
Furthennore, by applying the subsequent energization, the nugget 12
and the HAZ hardened portion 14H are sufficiently tempered as illustrated in
(c) in FIG. 17, and the hardness in terms of Vickers hardness between the
softest zone 14L in HAZ and the nugget end 12E is similar to that of the
softest zone 14L in HAZ, or the maximum value of the hardness thereof is
approximately 115% of the softest zone 14L in HAZ. Thus, the stress in the
nugget end portion area 12B is sufficiently alleviated.
As a result, it is possible to improve the peel strength of the
spot-welded portion 10 of the hot-stamped members. Note that the entire
nugget 12 does not necessarily have to be tempered, provided that the nugget
end portion area 12B is tempered.
[0090]
As described above, by tempering the HAZ hardened portion 14H, the
maximum hardness in terms of Vickers hardness between the nugget 12 and
the softest zone 14L in HAZ is made fall in a range of less than or equal to
approxin~ately 120% of the hardness of the softest zone 14L in HAZ as
illustrated in (c) in FIG. 16.
As a result, the tonghness of the nugget 12 and the HAZ hardened
portion is improved, whereby it is possible to improve the peel strength.
It should be noted that, even if the entire nugget 12 is not tempered,
the joint strength improves, provided that the ~naxi~lluVmi ckers hardness
between the nugget end 12E and the softest zone 14L in HAZ is less than or
equal to 120% of the Vickers hardness of the softest zone 14L in HAZ,
preferably is less than or equal to 115%, more preferably is less than or equal
to 110%.
[0091]
It should be noted that it is preferable that: the short-time energization
condition be adjusted; the nugget end and the softest zone in HAZ be brought
closer to each other; and the maxilnum value in ternls of Vickers hardness
between the central portion of the nugget 12 and the softest zone 14L in HAZ
after the subsequent energization is applied be made to be 11 5% relative to the
softest zone 14L in IIAZ.
Furthermore, it is Inore preferable that: the short-time energization
condition be adjusted; the nugget end and the softest zone in HAZ be brought
close to each other; and the maximum value in terms of Vickers hardness
between the nugget and the softest zone in HAZ after the subsequent
energization is applied be made to be 110% relative to the softest zone 14L in
I-IAZ.
[0092]
FIG. 18 is a graph showing distribution of hardness in the spot-welded
portion through single energization in the case where the short-time
energization condition according to this embodiment and a norrnal
energization condition are applied to a hot-stamped member of an 1800 MPa
class having a thickness of 1.8 tnm.
Furthermore, FIG. 19 is a graph showing distribution of hardness in
the spot-welded portion after tlie subsequent energization is applied in the
case where tlie short-time energization condition according to this
embodiment and a normal energization condition are applied to a hot-stamped
meniber of an 1800 MPa class having a thickness of 1.8 inm.
In FIG. 18, the "blank diamonds" represent the distribution of
hardness in a spot-welded portion in the case where the spot-welded portion is
formed with a main energization employing a short-time energization
condition with an energization time being 9 x cyc (second). Furthermore,
the "blank circles" represent the distribution of hardness in a spot-welded
portion in the case where tlie spot-welded portion is formed with a main
energization employing a normal condition with an energization time being 22
x cyc (second).
In FIG. 19, the "blank diamonds" represent the distribution of
hardness in a spot-welded portion in the case where the spot-welded portion is
formed with a main energization employing a short-time energization
condition with an energization time being 9 x cyc (second), and then,
tempering is performed through a subsequent energization, and tlie "blank
circles" represent the distribution of hardness in a spot-welded portion in the
case where the spot-welded portion is formed with a main energization
employing a normal condition with an energization time being 22 x cyc
(second), and then, tempering is performed through a subsequent energization.
[0093]
First, as shown in the graph in FIG. 18, with the short-time
energization condition indicated by the "blank diamonds," the distance from
the nugget end to the softest zone in HAZ is reduced, as compared with that of
the normal condition indicated by the "blank circles."
[0094]
As shown in the graph in FIG. 19, in tlie case where the subsequent
energization is applied, in the spot-welded portion formed by applying the
normal condition at the time of the main energization, hard portions exist
between the nugget and the softest zone (at a position approximately 1 mm
fro111 the end portion area of the nugget) because the softest zone in HAZ
formed at the time of tlie main energization is far from the nugget end portion.
[0095]
On tlie other hand, in the spot-welded portion formed by applying the
short-time energization condition at tlie time of the main energization, the
softest zone in HAZ formed at the time of the main energization is brought
close to the end portion area of the nugget, and hence, it is possible to make
the Vickers hardness of the nugget and tlie HAZ extending from the base
metal to the nugget end portion area 12B (includi~igtl ie nugget end portion
area 12B) to be 120% or lower of the hardness of tlie softest zone in HAZ.
[0096]
I11 other words, in the case where the subsequent energization is
applied, the strain concentration in the vicinity of the nugget end 12E can be
alleviated if tlie distance from the nugget end 12E to the softest zone in HAZ
forrned at the time of the main energization is reduced.
As described above, reducing the energization time for forming the
nugget 12 is effective from the viewpoint of bringing the position of the HAZ
softening closer to the end portion area of the nugget to improve the joint
strength.
[0097]
FIG. 20 is a diagram schematically illustrating changes in HAZ of the
spot-welded portion after the single energization and tlie subsequent
energization in the case where the short-time energization condition according
to this embodiment is applied to a DP member or a TRIP member.
(a) in FIG. 20 is a sectional view illustrating a spot-welded portion.
[0098]
As illustrated in (b) in FIG. 20, in the case where the DP members or
TRIP members are used as steel sheet members S1 and S2, the HAZ hardened
portion 14H has the Vickers hardness almost equal to the nugget 12 in a state
where the short-time single energization is applied.
In this case, in the nugget 12 and the I-IAZ hardened portion 14H, the
distribution of hardness is different from the hot-stamped member illustrated
in FIG. 17 in that they are significantly harder than the base material of the
DP member or TRIP member.
[0099]
Furthermore, as illustrated in (c) in FIG. 20, by applying the
subsequent energization, the nugget 12 and the HAZ hardened portion 14H are
sufficiently tempered, and the hardness in terms of Vickers hardness between
the softest zone 14L in I-IAZ and the nugget end 12E becomes approxiniately
115% of the softest zone 14L in HAZ, whereby the stress in the nugget end
portion area 12B is sufficiently alleviated.
As a result, it is possible to improve the peel strength in the
spot-welded portion 10 in the case of the DP member or TRIP mernber.
It should be noted that the entire nugget 12 does not have to be
necessarily tempered, provided that the nugget end portion area 12B is
tempered.
It should be noted that the long dashed double-short dashed line
illustrated in (c) in FIG. 20 indicates the distribution of hardness before the
subsequent energization is applied.
[Ol 001
I t should be noted that, in the embodiment described above,
descriptions have been made of a case where the HAZ hardened portion 14H is
tempered through subsequent energization. However, the nugget and the
HAZ hardened portion may be tempered, for example, by laser emission after
the spot-welded portion 10 is formed through the short-time single
energization.
Exatnples
[OlOl]
Below, it is confirmed that bringing the softest zone in HAZ close to
the nugget end portion is effective to improve the joint strength in the peeling
direction.
By reducing the energization time, it is possible to bring the softest
zone in IIAZ close to the nugget end portion. However, from the viewpoint
of formation of the nugget, i f the energization time is reduced, the appropriate
electric current range (in general, the electric current range from an electric
current value with which the nugget diameter of 4dt can be obtained, to
occurrence of splash) becomes narrow.
Then, a study was made to achieve both controlling the position of
HAZ softening and obtaining the appropriate electric current range, using a
two-step energization in which steel sheets are heated through preliminary
energization with an early-stage of energization being low electric current,
and then, a nugget is expanded with a short-time high electric current.
Investigations were made on a hot-stamped member of an 1800 MPa
class having a thickness of 1.6 mnl under welding conditio~lss hown in Table 1.
Condition (1) correspo~ldsto a short-time single energization condition, (2)
corresponds to a conventional single energization condition, and (3)
corresponds to a two-step energization condition.
As can be understood from the behavior of nugget formation
illustrated in FIG. 14 and the distribution of hardness in the spot-welded
portion illustrated in FIG. 15, with the two-step energization, it is possible to
obtain an appropriate elcctric current range equivalent to that with the
conventional single energization and bring the softest zone in HAZ close to
the nugget end portion.
[O 1021

[0 1031
The welding elcctric current was adjusted so that the nugget diameter
of 4dt (mm) can be obtained. As for the subsequent energization condition, a
condition effective in improving the peel strength, in other words, a condition
with which the end portion area of a nugget can be softened was selected.
[0 1041
By using a steel sheet of a 980 MPa class, hot-stamped menlbers of a
1500 MPa class, and hot-stamped members of an 1800 MPa class, each of
which has a thickness in a range of 1.6 mtll to 2.0 mm, the cross-tension
strength and the L-shape tension strength were investigated. Table 2 shows
welding conditions used. I. represents an electric current value in the main
energization process.
[0 1051
[Table 21

[0 1061
Condition n corresponds to a conventional single energization
condition, and condition b corresponds to a conventional subsequent
energization condition. Condition A1 corresponds to a short-time single
energization condition, and condition A2 corresponds to a two-step
energization condition. As for conditions BI and B2, subsequent
energization was performed for the conditions A1 and A2, respectively. In
each of the welding conditions, welding electric current was adjusted so that
the nugget diameter of 4dt can be obtained.
As for the joint strength, measuretnent was performed according to
cross-tension test based on JIS 23137 (1999) in the case of a cross joint, and
measurement was performed with a test illustrated by a schematic diagram of
FIG. 21 in the case of an L-shaped joint.
More specifically, in L-shaped tensile testing, bent portions of two
test pieces each formed by bending a steel sheet into an L shape were
overlapped with each other as illustrated in FIG. 21, and were joined by
forming a spot-welded portion 10 having a nugget 12 formed in the
overlapped portion through resistance spot welding; then, the overlapped
portion was pulled in a direction of peeling; and a strength of the spot-welded
portion 10 until fracture was measured as the joint strength.
[O 1071
First, by using a DP steel sheet of a 980 MPa class and hot-stamped
members of a 1500 MPa class and an 1800 MPa class, each of which has a
thickness in a range of 1.6 min to 2.0 mm, the peel strength of overlap-welded
members and fracture mode thereof were investigated. Table 4 shows
welding conditions. t is the thickness of the steel sheet, and I. was adjusted
so that the nugget diameter 4dt (mm) can be obtained in each of the sheet sets.
Note that the distance D is a distance from the nugget end portion area to the
softest zone in HAZ.
Table 3 shows effects of i~nproven~eonft joint strength under the
conditions A1 and A2. Table 3 is a table explaining Example related to the
single energization.
[OI 081
[Table 31
980 MPa I--
1500 MPa L
ioint shape
; shape
Thickness
t
I I(m)m I ( W ) I I I
1.15 a 1 1.5 ( 9.8 1 Partial plug 1 Comparative Example
/ fracture
A1 1 1.0 1 12.5 ( Partial plug / Example of present invention
tU'2 Fracture mode
(mm)
Distance Note
D
Welding
condition
1.10
/ fracture
A1 I 1.0 / 8.6 I Partial plug Example of present invention
Joint
strength
a ( 1.3 / 5.6 ( Partial plug ( Comparative Example
1.15
1 fracture
A2 1 1.0 1 3.9 / Partial plug / Example of present invention
A1
A2
a
( fracture
1.10
1 fracture
1.12 a 1 1.3 1 1.2 I Interface I Comparative Example
0.8
1 .O
1.5
a 1 1.3 1 3.2 1 Partial plug Comparative Example
1.15
1 fracture
A1 1 0.8 1 1.5 / Interface / Example of present invention
8.2
8.3
7.2
a
A1
fracture
Plug fracture
Plug fracture
Interface
1.5
1.0
Example of present invention
Example of present invention
Comparative Example
4.8
5.5
fracture
Interface
fracture
Interface
Comparative Example
ExampIe of present invention
[0109]
As shown in Table 3, with any steel type and any thickness, the
spot-welded portions formed through the short-time energization with the
condition A1 and the preli~ninarye nergization and the short-time energization
with the A2 inlprove the joint strength, as compared with the conventional
single energization with the condition (1. It can be considered that this is
because of the effect in which the stress concentration on the nugget end
portion is alleviated by bringing the position of HAZ softening closer to the
nugget end portion.
[OllO]
Furthermore, as for the L-shaped tensile strength concerning the
hot-stamped member of an 1800 MPa class, by applying the short-time
energization with the condition Al, the joint strength improves by
approximately 25%, as compared with the case of the condition a.
[Olll]
Furthermore, by using the DP steel sheet of a 980 MPa class, and
hot-stamped members of a 1500 MPa class and an 1800 MPa class, each of
which has a thickness in a range of 1.6 nlm to 2.0 mni, the peel strength of
overlap-welded members and fracture nlode thereof were investigated. Table
4 shows welding conditions. t is the thickness of the steel sheet, and 10 was
adjusted so that the nugget diameter 4dt (tnm) can be obtained in each of the
sheet sets. Note that the distance D is a distance from the nugget end portion
area to the softest zone in HAZ.
[0112]
Table 4 sho~vse ffects of in~provenlento f joint strength under the
conditions B1 and B2. Table 4 is a table explaining Example in the case
where the subsequent energization is applied.
[0113]
Steel type
980 MPa
1500 MPa
1800 MPa
1800 MPa
Joint shape
Cross
L shape
Thickness t
1 fracture I Example
B 1 I 1.0 1 18.5 1 Plug fracture I Example of present
Note
Comparative
Example
Example of present
invention
Comparative
1 fracture I Example
B 1 I 1.0 1 7.3 1 Partial plug I Example of present
tU." (mm)
1.15
1.15
1.10
1.15
Distance D
(mm)
1.5
1 .O
1.5
Welding
condition
b
B 1
b
---
b
B 1
B2
b
1.12
Joint stren,&
(m)
15.9
18.1
~~~~~~
14.5
Fracture
mode
Plug fracture
Plug fracture
Partial plug
invention
b
B 1
1.5
1 .O
1 .O
1.5
1.5
1 .O
5.7
8.7
8.7
5.2
2.2
4.7
Partial plug
fracture
Plug fracture
Plug fracture
Interface
Comparative
Example
Example of present
invention
Example of present
invention
Comparative
fracture
Interface
fracture
Plug fracture
invention
Comparative
Example
Example of present
invention
[OI 141
As shown in Table 4, with any steel type and any thickness, the joint
strength improves, as cotnpared with the conventional single energization
condition with the condition (I. It can be considered that this is not only
because the nugget end portion is tetnpered and the toughness improves as
with the conventional subsequent energization technique, but also because of
the effect in which the distribution of hardness appropriate for alleviating the
stress concentration on the nugget end portion can be obtained.
[0115]
Furthermore, as for the L-shaped tensile strength concerning the
hot-stamped member of an 1800 MPa class, by applying the short-time
energization and the subsequent energization with the condition B1, the
fracture mode changes from the interface fracture to the plug fracture, and the
joint strength iniproves by approxinlately 114%, as compared with the case of
the condition b.
Industrial Applicability
[0116]
According to the present invention, it is possible to improve the peel
strength of a spot-welded portion of an overlap-welded member in which
plural steel sheet metnbers, at least one of which contains inartensite, are
joined at an overlapped portion, and the overlapped portion is joined at the
spot-welded portion. Therefore, the present invention is industrially
applicable.
Brief Description of the Reference Symbols
[0117]
10 spot-welded portion
12 nugget
12B nugget end portion area
12C meeting portion
12E nugget end
14 HAZ
14H HAZ hardened portion
14T HAZ softening zone
14L the softest zone in I3AZ
1. An overlap-welded member in which an overlapped portion including
a plurality of steel sheet members is joined at a spot-welded portion, wherein
at least one of the plurality of steel sheet inembers contains
martensite; and
the spot-welded portion includes:
a nugget formed through spot welding;
a heat-affected zone formed in the vicinity of the nugget;
a softest zone having the lowest Vickers hardness in the
heat-affected zone; and
a tempered area formed between a central portion of the
nugget and the softest zone and made out of tempered martensite having
Vickers hardness of not more than 120% in a case where Vickers hardness of
the softest zone is 100%.
2. An overlap-welded nieniber in which an overlapped portion including
a plurality of steel sheet members is joined at a spot-welded portion, wherein
at least one of the plurality of steel sheet members contains
martensite;
the spot-welded portion includes
a nugget formed through resistance spot welding
a heat-affected zone formed in the vicinity of the nugget, and
a softest zone having the lowest Vickers hardness in the
heat-affected zone; and
Equation (1) described below is satisfied, where D (mm) is a distance
from a melting bo~indaryp ortion of the nugget to the softest zone, and if there
is only one steel sheet member having the highest tensile strength of the
plurality of steel sheet menibers, t (mm) is a thickness of this steel sheet
member, whereas, if there are a plurality of steel sheet menibers having the
highest tensile strength, t (mtn) is a thickness of a steel sheet member having
the thinnest thickness of these steel sheet members.
D 5 to.' ,.. Equation (1)
3. The overlap-welded nlember according to Claim 1 or 2, wherein
the plurality of steel sheet members include a hot-stamped member.
4. An automobile part including the overlap-welded member according to
any one of Claim 1 to Claim 3.
5. A method of welding an overlapped portion, including
a resistance spot welding process in which a spot-welded portion is
formed through resistance spot welding in an overlapped portion including a
plurality of steel sheet tnetnbers, the spot-welded portion including:
a nugget;
a heat-affected zone formed in the vicinity of the nugget; and
a softest zone having the lowest Vickers hardness in the
heat-affected zone, and
a tempering process of forming, between a central portion of the
nugget and the softest zone, a tempered area made out of tempered martensite
having Vickers hardness of not more than 120% in a case where Vickers
hardness of the softest zone is 100%.
6. The method of welding an overlapped portion according to Claim 5,
wherein
in the resistance spot welding step, energization is performed so as to
satisfy Equation (2) described below, where:
T (second) is an energization time in the resistance spot
welding;
if there is only one steel sheet member having the highest
tensile strength of the plurality of steel sheet members, t (mm) is a thickness
of this steel sheet member, whereas, if there are a plurality of steel sheet
members having the highest tensile strength, t (mm) is a thickness of a steel
sheet member having the thinnest thickt~esso f these steel sheet members; and
cyc (second) is a period of time for one cycle of energization
in the resistallce spot welding.
5t x cyc 5 T 5 (5t + 4) x cyc ... Equation (2)
7. The method of welding an overlapped portion according to Claim 5,
whereill
the method further includes, before the resistance spot weldillg step,
applying a preheat electric current I (kA) to the overlapped
portion in a state where an energization time TI (second), a period of time cyc
(second) for one cycle of energization, and a thickness t (mm) satisfy
Equation (3) described below;
as for the thickness t (mm), if there is only one steel sheet member
having the highest tensile strength of the plurality of steel sheet n~en~beras ,
thickness of this steel sheet member is used, whereas, if there are a plurality
of steel sheet ~llembersh aving the liighest tensile strength, a thickness of a
steel sheet member having the thinnest thickness of these steel sheet members
is used;
in the resistallce spot welding step, a welding electric current I. (kA)
not more than a splash occurring current is applied to the overlapped portion
in a state where Equation (4) described below is satisfied, where Tz (second)
is an energization time, and cyc (second) is a period of time for one cycle of
energization in the resistance spot welding; and
the preheat electric current I (kA) and the welding electric current 10
(kA) satisfy Equation (5) described below.
5t x cyc 5 TI i (5t + 8) x cyc ... Equation (3)
5t x eye 5 Tz 5 (5t + 4) x cyc ... Equation (4)
0.310 5 I 5 0.710 ... Equation (5)
8. The method of welding at1 overlapped portion according to Claim 5,
wherein
in the resistance spot welding step, the resistance spot welding is
performed so as to satisfy Equation (6) described below, where D (mm) is a
distance from a melting boundary portion of the nugget to the softest zone,
and i f there is only one steel sheet metilber having the highest tensile strength
of the plurality of steel sheet members, t (mm) is a thickness of this steel sheet
member, whereas, i f there are a plurality of steel sheet menibers having the
highest tensile strength, t (mm) is a thickness of a steel sheet member having
the thinnest thickness of these steel sheet members, and
the tempering process is a subsequent energization process in which
the tempered area is formed through subsequent energization.
D <- to-2. .. Equation (6)
9. The method of welding an overlapped portion according to Claim 8,
wherein
in the resistance spot welding step, energization is applied so as to
satisfy Equation (7) described below, where T (second) is an energization tirne
in the resistance spot welding, and cyc (second) is a period of tirne for one
cycle of energization in the resistance spot welding.
St x cyc 5 T 5 (St + 4) x cyc ... Equation (7)
10. The method of welding an overlapped portion according to Claim 8,
wherein
the nletliod further includes, before the resistance spot welding step:
applying a preheat electric current I (kA) to the overlapped
portion in a state where an energization time TI (second), a period of tirne cyc
(second) for one cycle of energization, and the thickness t (mm) satisfy
Equation ( 8 ) described below;
in the resistance spot welding step, a welding electric current I. (kA)
not more than a splash occurring current is applied to the overlapped portion
in a state where Equation (9) described below is satisfied, where T2 (second)
is an energization time, and cyc (second) is a period of time for one cycle of
energization; and
the preheat electric current I (kA) and the welding electric current I.
(kA) satisfy Equation (10) described below.
St x cyc 5 TI 5 (St + 8) x cyc ... Equation (8)
St x cyc < Tz 5 (5t + 4) x cyc ... Equation (9)
0.310 < 15 0.710 ... Equation (10)
11. A method of welding an overlapped portion, includi~tg
a resistance spot welding process in which a spot-welded portion is
formed in an overlapped portion including a plurality of steel sheet members,
the spot-welded portion including:
a nugget;
a heat-affected zone formed in the vicinity of the nugget; and
a softest zone having the lowest Vickers hardness in the
heat-affected zone, and
in the resistance spot welding step, resistance spot welding is
performed so as to satisfy Equation (1 1) described below, where D (mm) is a
distance from a melting boundary portion of the nugget to the softest zone,
and if there is only one steel sheet member having the highest tensile strength
of the plurality of steel sheet members, t (mm) is a thickness of this steel sheet
member, whereas, if there are a plurality of steel sheet lnembers having the
highest tensile strength, t (mm) is a thickness of a steel sheet member having
the thinnest thickness of these steel sheet melnbers.
D 5 to.' ,.. Equation (1 1)
12. The method of welding an overlapped portion according to Claim 11,
wherein
in the resistance spot welding step, energization is applied so as to
satisfy Equation (12) described below, where T (second) is an energization
time in the resistance spot welding, and cyc (second) is a period of time for
one cycle of energization in the resistance spot welding.
5t x cyc I T 5 (5t + 4) x cyc ... Equation (12)
13. The method of welding an overlapped portion according to Claim 11,
wherein
the method further includes, before the resistance spot welding step:
applying a preheat electric current I (kA) to the overlapped
portion in a state where an energization time TI (second), a period of time cyc
(second) for one cycle of energization, and the thickness t (mm) satisfy
Equation (13) described below;
in the resistance spot welding step, a welding electric current I. (kA)
not more than a splash occurring current is applied to the overlapped portion
in a state where Equation (14) described below is satisfied, where T2 (second)
is an energization time, and cyc (second) is a period of time for one cycle of
energization; and
the preheat electric current I (kA) and the welding electric current 10
(kA) satisfy Equation (1 5) described below.
St x cyc < TI < (5t + 8) x cyc ... Equation (13)
5t x cyc 5 T2 5 (5t + 4) x cyc ... Equation (14)
0.310 5 I I 0.710 ... Equation (15)
14. A method of manufacturing an overlap-welded mer~~bienr which an
overlapped portion including a plurality of steel sheet members is joined at a
spot-welded portion,
the method including:
overlapping the plurality of steel sheet members at a position
of the overlapped portion; and
welding the overlapped portion through the method of welding
an overlapped portion according to any one of Clailn 5 to Claim 13.