[Document Type] Specification
[Title of the Invention] JOINT COMPONENT AND MANUFACTURING
METHOD THEREOF
[Technical Field of the Invention]
[0001]
The present invention relates to a joint component and a manufacturing
method thereof.
Priority is claimed on Japanese Patent Application No. 2020-022754, filed
February 13, 2020, the content of which is incorporated herein by reference.
[Related Art]
[0002]
In the automotive field, in order to improve both fuel consumption and
colli sion safety against the background of recent stringent environmental regulations
and collision safety standards, the application of a steel sheet having high tensile
strength (high-strength steel sheet) has expanded. However, the press formability of
the steel sheet decreases with high-strengthening, thereby making it difficult to
manufacture the steel sheet into a product having a complex shape.
[0003]
Specifically, the ductility of the steel sheet decreases with high-strengthening,
and the steel sheet is fractured at a highly processed portion when the steel sheet is
processed into a complex shape, which is a problem. Furthermore, with the highstrengthening
of the steel sheet, the residual stress after processing causes springback
and wall warpage, and the dimensional accuracy deteriorates, which is a problem.
Therefore, it is not easy to process a steel sheet having high strength, patticularly a
tensile strength of 780 MPa or more, into a product having a complex shape by press-
- 1 -
forming. Roll forming makes it easier to process a high-strength steel sheet than
press forming, but is limited to being applied to components each having a uniform
cross section in a longitudinal direction.
[0004]
Therefore, in recent years, for example, as disclosed in Patent Documents 1 to
3, a hot stamping technique has been adopted as a technique of press-forming a
material that is difficult to form, for example, a high-strength steel sheet. The hot
stamping technique is a hot forming technique of heating a material provided for
forming and then of forming the material.
[0005]
In this technique, the material is heated and then formed. Therefore, during
forming, the steel is soft and has good formability. Accordingly, even a steel sheet
having high strength can be accurately formed into a complex shape. Furthermore, in
the hot stamping technique, since quenching is performed simultaneously with forming
by a press die, a steel member obtained after forming has sufficient strength.
[0006]
For example, Patent Document 1 discloses that a steel member having a
tensile strength of 1 ,400 MPa or more can be obtained after forming by the hot
stamping technique.
[0007]
In recent years, countries around the world have set higher C02 reduction
targets, and each vehicle manufacturer has progressed in reducing fuel consumption in
consideration of collision safety. Not only gasoline vehicles but also electric vehicles
that are under rapid progress require, as its materials, high-strength materials that
protect not only passengers but also batteries from collision and that cancel out the
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amount of an increase in weight. For example, in a steel member that is in use for
vehicles and the like, a hot stamping member that has a higher strength than a strength
that is commonly used as a steel member formed by hot stamping at present, that is,
more than 1.5 GPa (1500 MPa), is required.
[0008]
However, most of metal materials deteriorate in various properties with highstrengthening
and particularly, the hydrogen embrittlement susceptibility increases. It
is known that the hydrogen embrittlement susceptibility increases when the tensile
strength of a steel member is 1.2 GPa or more, and there is a case of hydrogen
embrittlement cracking in bolt steel for which high-strengthening has been progressed
ahead of the automotive field. Therefore, in the hot stamping member having a
tensile strength of more than 1.5 GPa, a further increase in the hydrogen embrittlement
susceptibility is concerned.
[0009]
In steel members that are in use for vehicles, there is a risk that hydrogen
embrittlement cracking may be caused due to hydrogen that is generated from the
corrosion of a steel while vehicles are in operation. As described above, since the
hydrogen embrittlement susceptibility of a steel extremely increases particularly in a
strength range of more than 1.5 GPa, it is considered that the steel may hydrogenembrittle
due to a trace amount of hydrogen generated by slight corrosion. However,
vehicle design that completely prevents corrosion of a steel is difficult. Therefore, in
order to apply the hot stamping member having a strength of more than 1.5 GPa to the
vehicle body for a further reduction in the weight of the vehicle body, a risk of
hydrogen embrittlement cracking needs to be sufficiently reduced.
[0010]
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A point where, particularly, hydrogen embrittlement is concerned while
vehicles are in operation is a spot-welded portion. There are three main reasons for
the spot-welded portion to be particularly prone to hydrogen embrittlement.
Specifically, the spot-welded portion is likely to hydrogen-embrittle due to the facts
that (i) corrosion is likely to progress in the spot-welded portion, (ii) stress is generated
in the spot-welded portion when a component having poor dimensional accuracy is
welded or the like, and (iii) the structure of a melted and solidified portion such as the
spot-welded portion is coarse and likely to embrittle. That is, in the spot-welded
portion, all of the generation of hydrogen, the application of stress, and the limit of the
material, which are the causes of hydrogen embrittlement, are under stricter conditions
than those in stationary portions of the base metal.
As a supplement to the reason (i), since the effect of a chemical conversion
treatment and painting is unlikely to reach a portion where steel sheets (or members)
are overlapped and welded, and the presence of a gap caused by dimensional defects
makes corrosion progress locally, a large amount of hydrogen is generated (gap
corrosion reaction).
[0011]
Regarding a high-strength steel having a tensile strength of more than 1.5 GPa,
for example, Patent Document 2 discloses a press-formed article that has excellent
toughness and a tensile strength of 1.8 GPa or more and that is hot press-formed.
Patent Document 3 discloses a steel having an extremely high tensile strength of 2.0
GPa or more and, furthermore, having good toughness and ductility. Patent
Document 4 discloses a steel having a high tensile strength of 1.8 GPa or more and,
furthermore, having good toughness. Patent Document 5 discloses a steel having an
extremely high tensile strength of 2.0 GPa or more and, furthermore, having good
- 4 -
toughness.
However, in Patent Documents 2 to 5, regarding hydrogen embrittlement
resistance, measures against hydrogen embrittlement in a spot-welded portion where
embrittlement is concerned particularly in a corrosive environment are not sufficient.
Therefore, the steels of Patent Documents 2 to 5 have a tensile strength of more than
1.5 GPa, but do not sufficiently satisfy safer requirements in some cases when used as
vehicle members.
[0012]
Regarding high-strength steels having a spot-welded portion, for example,
Patent Documents 6 to 8 disclose spot welding methods in which electrode
contamination or welding dust generation in an aluminum-plated steel sheet is
suppressed.
However, in all of the patent documents, measures against hydrogen
embrittlement in the spot -welded portion of the high-strength steel are not sufficient,
and a requirement for higher safety may not be sufficiently satisfied in the application
of the high-strength steel having a tensile strength of more than 1.5 GPa to vehicle
members.
[Prior Art Document]
[Patent Document]
[0013]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2002-102980
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2012-180594
[Patent Document 3] Japanese Unexamined Patent Application, First
- 5 -
Publication No. 2012-1802
[Patent Document 4] PCT International Publication No. W02015/182596
[Patent Document 5] PCT International Publication No. W02015/182591
[Patent Document 6] Japanese Unexamined Patent Application, First
Publication No. 2006-212649
[Patent Document 7] Japanese Unexamined Patent Application, First
Publication No. 2011-167742
[Patent Document 8] Japanese Unexamined Patent Application, First
Publication No. 2004-2932
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0014]
The present invention has been made to solve the above problems, and an
object of the present invention is to provide a joint component having a spot-welded
portion having excellent hydrogen embrittlement resistance in a corrosive environment
and a manufacturing method thereof.
[Means for Solving the Problem]
[0015]
The gist of the present invention is the following joint component and
manufacturing method thereof.
(1) A joint component according to one aspect of the present invention is a
joint component including a first steel member, a second steel member, and a spotwelded
portion that joins the first steel member and the second steel member, in which
the first steel member includes a steel sheet substrate containing, as a chemical
composition, by mass%, C: 0.25% to 0.65%, Si: 0.10% to 1.00%, Mn: 0.30% to 1.50%,
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P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:
0.0005% to 0.0100%, Mo: 0% to 1.00%, Cu: 0% to 1.00%, Ni: 0% to 1.00%, Cr: 0%
to 1.00%, Nb: 0% to 0.10%, V: 0% to 1.00%, Ca: 0% to 0.010%, Al: 0% to 1.00%, Sn:
0% to 1.00%, W: 0% to 1.00%, Sb: 0% to 1.00%, Zr: 0% to 1.00%, REM: 0% to
0.30%, and a remainder of Fe and an impurity and a coating that is formed on a surface
of the steel sheet substrate, contains Al and Fe, and has a thickness of 25 )lm or more,
in a cross section in a thickness direction of the first steel member and the second steel
member including the spot-welded portion, a filled metal containing Aland Fe is
present in a gap between the first steel member and the second steel member in a
periphery of the spot-welded portion, in the cross section, the filled metal has a crosssectional
area of 3.0 x 104 )lm2 or more and has a filling ratio of 80% or more in the
gap in a range of 100 )lm from an end portion of a corona bond formed in the periphery
of the spot-welded portion, and the filled metal includes a first region containing, by
mass%, Al: 15% or more and less than 35%, Fe: 55% or more and 75% or less, and Si:
4% or more and 9% or less and a second region containing, by mass%, Al: 35% or
more and 55% or less, Fe: 40% or more and less than 55%, and Si: 1% or more and
less than 4%.
(2) In the joint component according to ( 1) above, the steel sheet substrate of
the first steel member may contain, as the chemical composition, by mass%, one or
more ofMo: 0.10% to 1.00%, Cu: 0.10% to 1.00%, and Ni: 0.10% to 1.00%, the first
region may further contain one or more ofMo, Cu, and Ni in a total content of0.25%
or more, and the second region may further contain one or more of Mo, Cu, and Ni in a
total content of0.15% or more.
(3) In the joint component according to (2) above, an average of Feret
diameters of the second region may be set to 30 )lm or less.
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(4) A manufacturing method of a joint component according to another aspect
of the present invention includes a heat treatment step of heating a coated steel sheet
including a steel sheet containing, as a chemical composition, by mass%, C: 0.25% to
0.65%, Si: 0.10% to 1.00%, Mn: 0.30% to 1.50%, P: 0.050% or less, S: 0.0100% or
less, N: 0.010% or less, Ti: 0.010% to 0.100%, B: 0.0005% to 0.0100%, Mo: 0% to
1.00%, Cu: 0% to 1.00%, Ni: 0% to 1.00%, Cr: 0% to 1.00%, Nb: 0% to 0.10%, V: 0%
to 1.00%, Ca: 0% to 0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb:
0% to 1.00%, Zr: 0% to 1.00%, REM: 0% to 0.30%, and a remainder of Fe and an
impurity and a coating that is formed on a surface of the steel sheet, contains Al, and
has an adhesion amount of 50 g/m2 or more to an Ac3 point to (Ac3 point + 300)°C at
a temperature rising rate of 1.0 to 1,000 °C/s and cooling the coated steel sheet to an
Ms point or lower at an upper critical cooling rate or faster to obtain a steel member
and a spot-welding step of joining the steel member after the heat treatment step and a
second steel member that serves as an opposite material by spot welding, in which, in
the spot-welding step, at least at a position where an energizing electrode is pressed,
the steel member and the second steel member are disposed so as to overlap each other
with a gap of 50 )liD to 500 )liD therebetween, the energizing electrode is pressed
against the steel member and the second steel member such that a contact angle is 15
degrees or less and an electrode force is 300 kgf or more, 5 or more cycles of upslope
in which an energizing amount is gradually increased is imparted with a 50 Hz or 60
Hz alternating source, and then a weld nugget is formed to join the steel member and
the second steel member.
(5) In the manufacturing method of a joint components according to (4) above,
the chemical composition of the steel sheet may contain, by mass%, one or more of
Mo: 0.10% to 1.00%, Cu: 0.10% to 1.00%, and Ni: 0.10% to 1.00%.
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(6) In the manufacturing method of a joint component according to (5) above,
in the spot-welding step, an average cooling rate from 800°C to 500°C may be set to
500 °C/s or faster.
[Effects of the Invention]
[0016]
According to the aspect of the present invention, it is possible to provide a
joint component having a spot-welded portion having excellent hydrogen
embrittlement resistance in a corrosive environment and a manufacturing method
thereof.
The joint component according to the aspect of the present invention has high
strength and excellent hydrogen embrittlement resistance and thus contributes to
improvement in fuel consumption and collision safety when being applied to a vehicle
component.
[Brief Description of the Drawings]
[0017]
FIG. 1 is a schematic view showing one example of a joint component
according to the present embodiment.
[Embodiments of the Invention]
[0018]
In order to obtain a joint component having a spot-welded portion having high
tensile strength and excellent hydrogen embrittlement resistance in a corrosive
environment, the present inventors investigated the influences of the structure of a
weld or a steel that serves as a material on these properties. As a result, the following
findings were obtained.
[0019]
- 9 -
Most of materials to be used for hot stamping members that are commonly
manufactured are coated steel sheets of which a surface is subjected to an aluminum
plating having excellent corrosion resistance. When hot stamping is performed on
this coated steel sheet, an alloying reaction between Alina plating layer on the surface
and Fe in the steel sheet progresses during heating, and a steel member including a
coating containing Aland Fe (coated steel member) (hereinafter, referred to as the AlFe-
based coating in some cases) is obtained. Most of commonly used steel sheets
showing a tensile strength of about 1.5 GPa after hot stamping contain about 0.20
mass% of C, and the strength after the hot stamping is secured due to C. This steel
member is joined to another member by spot welding, whereby a joint component can
be obtained.
[0020]
(a) In order to achieve a further reduction in the weight of the vehicle body,
the present inventors conducted a detailed study for obtaining a high strength member
that has a tensile strength of more than 1.5 GPa (1500 MPa) after hot stamping by
means of an increase in the C content. As a result, it was found that, in terms of
tensile strength, an ultrahigh strength of more than 1.5 GPa could be obtained after a
heat treatment including quenching such as hot stamping by setting the C content to
0.25 mass% or more. On the other hand, there was a concern about a risk that
hydrogen embrittlement susceptibility increased with ultrahigh-strengthening to a
tensile strength of more than 1.5 GPa and hydrogen embrittlement cracking was caused
by hydrogen generated in a corrosive environment while vehicles were in operation.
In particular, when a joint component was produced using this coated steel member,
since a spot-welded portion was melted once, corrosion resistance by aluminum plating
could not be guaranteed, and a risk of hydrogen embrittlement was concerned.
- 10 -
[0021]
(b) The present inventors studied a method for suppressing hydrogen
embrittlement by preventing corrosion of a spot-welded portion, which acts as a
starting point of embrittlement, in a joint component made of a coated steel member
having a high strength of more than 1.5 GPa and an Al-Fe-based coating. As a result,
it was found that corrosion can be sufficiently prevented by covering the periphery of a
weld with an alloy containing Aland Fe.
[0022]
(c) The present inventors further investigated the hydrogen embrittlement
resistance of a coated steel member having a tensile strength of more than 1.5 GPa and
found component design or structure design that was excellent in terms of hydrogen
embrittlement resistance.
[0023]
Based on the above findings, the present inventors developed a joint
component made of a high-strength coated steel member having a tensile strength of
more than 1.5 GPa in which the hydrogen embrittlement resistance in a corrosive
environment is significantly improved by preventing corrosion of a spot-welded
portion, reducing the amount of hydrogen intrusion, and improving the hydrogen
embrittlement resistance of a steel. Such a joint component has a high strength and a
low risk of hydrogen embrittlement and thus can be applied to vehicle bodies more
safely.
[0024]
Hereinafter, each requirement of a joint component according to one
embodiment of the present invention (the joint component according to the present
embodiment) and a manufacturing method thereof will be described in detail.
- 11 -
[0025]
(A) Joint Component
As shown in FIG. 1, a joint component 1 according to the present embodiment
includes a first steel member 11, a second steel member 12, and a spot-welded portion
21 that joins the first steel member 11 and the second steel member 12. This first
steel member 11 is a coated steel member having a steel sheet substrate 111 having a
predetermined chemical composition and a coating (Al-Fe-based coating) 112 that is
formed on the surface of the steel sheet substrate 111 and contains Al and Fe.
Furthermore, in the joint component 1 according to the present embodiment,
in a cross section in the thickness direction of the first steel member 11 and the second
steel member 12 including the spot-welded portion 21, a filled metal31 containing Al
and Fe is present in a gap g between the first steel member 11 and the second steel
member 12 in the periphery of the spot-welded portion 21. The filled metal 31
includes a first region containing, by mass%, Al: 15% or more and less than 35%, Fe:
55% or more and 75% or less, and Si: 4% or more and 9% or less and a second region
containing, by mass%, Al: 35% or more and 55% or less, Fe: 40% or more and less
than 55%, and Si: 1% or more and less than 4%.
Furthermore, in the cross section, the filled metal 31 has a cross-sectional area
of 3.0 x 104 11m2 or more and has a filling ratio of 80% or more in the gap g in a range
of 100 11m from the end portion of a corona bond formed in the peripher y of the spotwelded
portion 21.
Hereinafter, each will be described below.
[0026]
(A1) First Steel Member
As described above, the first steel member 11 included in the j oint component
- 12 -
1 according to the present embodiment has the steel sheet substrate 111 and the coating
(Al-Fe-based coating) 112 that is formed on the surface of the steel sheet substrate 111
and contains Al and Fe.
As described below, the first steel member 11 is obtained by performing a heat
treatment accompanying quenching such as hot stamping on a coated steel sheet
having a steel sheet substrate and anAl-based coating.
[0027]
(Al-l) Steel Sheet Substrate
The steel sheet substrate 111 of the first steel member 11 included in the joint
component 1 according to the present embodiment has a predetermined chemical
composition. Specifically, the steel sheet substrate 111 has a chemical composition
containing, by mass%, C: 0.25% to 0.65%, Si: 0.10% to 1.00%, Mn: 0.30% to 1.50%,
P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010% to 0.100%, B:
0.0005% to 0.0100%, Mo: 0% to 1.00%, Cu: 0% to 1.00%, Ni: 0% to 1.00%, Cr: 0%
to 1.00%, Nb: 0% to 0.10%, V: 0% to 1.00%, Ca: 0% to 0.010%, Al: 0% to 1.00%, Sn:
0% to 1.00%, W: 0% to 1.00%, Sb: 0% to 1.00%, Zr: 0% to 1.00%, REM: 0% to
0.30%, and a remainder of Fe and an impurity.
The reasons for limiting each element are as follows. Here, the chemical
composition of the steel sheet substrate 111 refers to the chemical composition of a
portion of the first steel member 11 excluding the Al-Fe-based coating 112 on the
surface (for example, a 1/4 position of the thickness from the surface of the steel sheet
substrate). Hereinafter, % regarding the content is mass% unless otherwise specified.
[0028]
C: 0.25% to 0.65%
C is an element that enhances the hardenability of steel and increases the
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strength of the steel member that is obtained after quenching such as hot stamping.
When the C content is less than 0.25%, it becomes difficult to secure sufficient
strength (more than 1.5 GPa) in the steel member after quenching. Therefore, the C
content is set to 0.25% or more. The C content is preferably 0.28% or more.
On the other hand, when the C content is more than 0.65%, the strength of the
steel member after quenching becomes too high, and deterioration of the hydrogen
embrittlement resistance becomes significant. Therefore, the C content is set to
0.65% or less. The C content is preferably 0.60% or less.
[0029]
Si: 0.10% to 1.00%
Si is an element that is effective in enhancing the hardenability of steel and in
stably securing the strength of the steel member after quenching. In order to obtain
this effect, the Si content needs to be set to 0.10% or more. The Si content is
preferably 0.35% or more.
On the other hand, when the Si content in steel is more than 1.00%, a heating
temperature required for austenitic transformation becomes significantly high during
the heat treatment (quenching). Accordingly, the cost required for the heat treatment
may increase, or ferrite may remain during heating and the strength of the steel
member decreases. Therefore, the Si content is set to 1.00% or less. The Si content
is preferably 0.60% or less.
[0030]
Mn: 0.30% to 1.50%
Mn is an element that is very effective in enhancing the hardenability of steel
and in stably securing the strength after quenching. Furthermore, Mn is an element
that lowers an Ac3 point and promotes the lowering of the quenching treatment
- 14 -
temperature. However, when the Mn content is less than 0.30%, the effect is not
sufficient. Therefore, the Mn content is set to 0.30% or more. The Mn content is
preferably 0.40% or more.
On the other hand, when the Mn content is more than 1.50%, the hydrogen
embrittlement resistance of the steel member after quenching deteriorates. Therefore,
the Mn content is set to 1.50% or less. The Mn content is preferably 1.30% or less
and more preferably 1.10% or less.
[0031]
P: 0.050% or less
Pis an element that degrades the hydrogen embrittlement resistance of the
steel member after quenching. In particular, when the P content is more than 0.050%,
deterioration of the hydrogen embrittlement resistance becomes significant.
Therefore, the P content is limited to 0.050% or less. The P content is preferably
limited to 0.005% or less.
Since it is preferable that the P content is small, the P content may be 0%.
However, the P content may be set to 0.001% or more from the viewpoint of cost.
[0032]
S: 0.0100% or less
Sis an element that degrades the hydrogen embrittlement resistance of the
steel member after quenching. In particular, when the S content is more than
0.0100%, deterioration of the hydrogen embrittlement resistance becomes significant.
Therefore, the S content is limited to 0.0100% or less. The S content is preferably
limited to 0.0050% or less. Since it is preferable that the S content is small, the S
content may be 0%. However, the S content may be set to 0.0001% or more from the
viewpoint of cost.
- 15 -
[0033]
N: 0.010% or less
N is an element that degrades the hydrogen embrittlement resistance of the
steel member after quenching. In particular, when theN content is more than 0.010%,
coarse nitrides are formed in steel, and the hydrogen embrittlement resistance
significantly deteriorates. Therefore, theN content is set to 0.010% or less. A lower
limit of theN content does not have to be particularly limited and may be 0%.
However, setting theN content to less than 0.0002% leads to an increase in
steelmaking cost and is economically undesirable. Therefore, theN content may be
set to 0.0002% or more or 0.0008% or more.
[0034]
Ti: 0.010% to 0.100%
Ti is an element having an action of refining austenite grains by suppressing
recrystallization and by suppressing grain growth by means of the formation of fine
carbides when the steel sheet is subjected to a heat treatment by being heated to a
temperature of the Ac3 point or higher. Therefore, an effect of increasing the
hydrogen embrittlement resistance of the steel member can be obtained by containing
Ti. Furthermore, Ti is an element that is preferentially bonded toN in the steel to
suppress the consumption of B caused by the precipitation of BN and to promote an
effect of enhancing the hardenability induced by B to be described below. When the
Ti content is less than 0.010%, the above effects cannot be sufficiently obtained.
Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably
0.015% or more.
On the other hand, when the Ti content is more than 0.100%, the amount of
precipitation of TiC increases and C is consumed, so that the strength of the steel
- 16 -
member after quenching decreases. Therefore, the Ti content is set to 0.100% or less.
The Ti content is preferably 0.080% or less.
[0035]
B: 0.0005% to 0.0100%
B is an important element having an action of dramatically enhancing the
hardenability of steel even with a trace amount. Furthermore, B is an element that is
segregated at grain boundaries to strengthen the grain boundaries and to improve the
hydrogen embrittlement resistance, and that suppresses the growth of austenite grains
when the steel sheet is heated. When the B content is less than 0.0005%, the above
effects may not be sufficiently obtainable. Therefore, the B content is set to 0.0005%
or more. The B content is preferably 0.0010% or more.
On the other hand, when the B content is more than 0.0100%, a large amount
of coarse compounds are precipitated, and the hydrogen embrittlement resistance of the
steel member deteriorates. Therefore, the B content is set to 0.0100% or less. The B
content is preferably 0.0080% or less.
[0036]
In the chemical composition of the steel sheet substrate 111 included in the
first steel member 11 included in the joint component of the present embodiment,
elements other than the above elements, that is, the remainder may be Fe and an
impurity, but one or more elements selected from the group consisting of Mo, Cu, Ni,
Cr, Nb, V, Ca, Al, Sn, W, Sb, Zr, and REM may be contained within ranges described
below in order to improve various properties (hardenability, strength, hydrogen
embrittlement resistance, deoxidation properties, corrosion resistance, and the like) of
the steel member and the joint component including this steel member. These
elements are optional elements and do not necessarily have to be contained.
- 17 -
Therefore, the lower limit thereof is 0%.
[0037]
Mo: 0% to 1.00%
Moisan element that is very effective in enhancing the hardenability of steel
and in stably securing the strength of the steel member after quenching. In particular,
a synergistic effect of improving the hardenability can be obtained by containing Mo
and B simultaneously. Furthermore, Mo is capable of further improving the corrosion
resistance by being contained in a filled metal (Al-Fe-based filled metal) that is formed
in the periphery of the spot-welded portion. Therefore, Mo is preferably contained.
When the Mo content is less than 0.10%, since these effects are not sufficient, the Mo
content is preferably set to 0.10% or more and more preferably set to 0.20% or more.
On the other hand, Mo has an action of stabilizing iron carbides. When the
Mo content is more than 1.00%, coarse iron carbides may remain undissolved when
the steel sheet is heated, and the hydrogen embrittlement resistance of the steel
member after quenching may deteriorate. In addition, the cost increase is significant.
Therefore, in the case of containing Mo, the Mo content is set to 1.00% or less. The
Mo content is preferably 0.80% or less.
[0038]
Cu: 0% to 1.00%
Cu is an element that is effective in enhancing the hardenability of steel and in
stably securing the strength of the steel member after quenching. Furthermore, Cu is
an element that further improves the corrosion resistance by being contained in anAlFe-
based filled metal that is formed in the periphery of the spot-welded portion, which
will be described below. Therefore, Cu is preferably contained. When the Cu
content is less than 0.1 0%, since these effects are not sufficient, in the case of
- 18 -
containing Cu, the Cu content is preferably set to 0.10% or more. The Cu content is
more preferably 0.20% or more.

[Document Type] CLAIMS
member,
1. A joint component comprising:
a first steel member;
a second steel member; and
a spot-welded portion that joins the first steel member and the second steel
wherein the first steel member includes a steel sheet substrate containing, as a
chemical composition, by mass%,
C: 0.25% to 0.65%,
Si: 0.10% to 1.00%,
Mn: 0.30% to 1.50%,
P: 0.050% or less,
S: 0.0100% or less,
N: 0.010% or less,
Ti: 0.010% to 0.100%,
B: 0.0005% to 0.0100%,
Mo: 0% to 1.00%,
Cu: 0% to 1.00%,
Ni: 0% to 1.00%,
Cr: 0% to 1.00%,
Nb: 0% to 0.10%,
V: 0% to 1.00%,
Ca: 0% to 0.010%,
Al: 0% to 1.00%,
Sn: 0% to 1.00%,
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W: 0% to 1.00%,
Sb: 0% to 1.00%,
Zr: 0% to 1.00%,
REM: 0% to 0.30%, and
a remainder of Fe and an impurity; and
a coating that is formed on a surface of the steel sheet substrate, contains Al
and Fe, and has a thickness of 25 J.lm or more,
in a cross section in a thickness direction of the first steel member and the
second steel member including the spot-welded portion, a filled metal containing Al
and Fe is present in a gap between the first steel member and the second steel member
in a periphery of the spot-welded portion,
in the cross section, the filled metal has a cross-sectional area of 3.0 x 104
J.lm2 or more, and a filling ratio of 80% or more in the gap in a range of 100 J.lm from
an end portion of a corona bond formed in the periphery of the spot-welded portion,
and
the filled metal includes a first region containing, by mass%, Al: 15% or more
and less than 35%, Fe: 55% or more and 75% or less, and Si: 4% or more and 9% or
less and a second region containing, by mass%, Al: 35% or more and 55% or less, Fe:
40% or more and less than 55%, and Si: 1% or more and less than 4%.
2. The joint component according to claim 1,
wherein the steel sheet substrate of the first steel member contains, as the
chemical composition, by mass%,
one or more ofMo: 0.10% to 1.00%, Cu: 0.10% to 1.00%, and Ni: 0.10% to
1.00%,
the first region further contains one or more of Mo, Cu, and Ni in a total
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content of 0.25% or more, and
the second region further contains one or more of Mo, Cu, and Ni in a total
content of 0.15% or more.
3. The joint component according to claim 2,
wherein an average ofFeret diameters of the second region is 30 J.lm or less.
4. A manufacturing method of a joint component, comprising:
a heat treatment step of heating a coated steel sheet including a steel sheet
containing, as a chemical composition, by mass%, C: 0.25% to 0.65%, Si: 0.10% to
1.00%, Mn: 0.30% to 1.50%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less,
Ti: 0.010% to 0.100%, B: 0.0005% to 0.0100%, Mo: 0% to 1.00%, Cu: 0% to 1.00%,
Ni: 0% to 1.00%, Cr: 0% to 1.00%, Nb: 0% to 0.10%, V: 0% to 1.00%, Ca: 0% to
0.010%, Al: 0% to 1.00%, Sn: 0% to 1.00%, W: 0% to 1.00%, Sb: 0% to 1.00%, Zr:
0% to 1.00%, REM: 0% to 0.30%, and a remainder of Fe and an impurity and a
coating that is formed on a surface of the steel sheet, contains Al, and has an adhesion
amount of 50 g/m2 or more to an Ac3 point to (Ac3 point + 300)°C at a temperature
rising rate of 1.0 to 1,000 ac/s and cooling the coated steel sheet to an Ms point or
lower at an upper critical cooling rate or faster to obtain a steel member; and
a spot-welding step of joining the steel member after the heat treatment step
and a second steel member that serves as an opposite material by spot welding,
wherein, in the spot-welding step,
at least at a position where an energizing electrode is pressed, the steel
member and the second steel member are disposed so as to overlap each other with a
gap of 50 J.lm to 500 J.lm therebetween, and
the energizing electrode is pressed against the steel member and the second
steel member such that a contact angle is 15 degrees or less and an electrode force is
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300 kgf or more, 5 or more cycles of upslope in which an energizing amount is
gradually increased is imparted with a 50 Hz or 60Hz alternating source, and then a
weld nugget is formed to join the steel member and the second steel member.
5. The manufacturing method of a joint component according to claim 4,
wherein the steel sheet contains, as the chemical composition, by mass%, one
or more ofMo: 0.10% to 1.00%, Cu: 0.10% to 1.00%, and Ni: 0.10% to 1.00%.
6. The manufacturing method of a joint component according to claim 5,
wherein, in the spot-welding step, an average cooling rate from 800°C to
500°C is set to 500 °C/s or faster.
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[Document Type] Abstract
This joint component is a joint component including a first steel member, a
second steel member, and a spot-welded portion that joins the first steel member and
the second steel member, in which the first steel member includes a steel sheet
substrate having a predetermined chemical composition and a coating that is formed on
a surface of the steel sheet substrate, contains Al and Fe, and has a thickness of 25 )liD
or more, in a cross section in a thickness direction of the first steel member and the
second steel member including the spot-welded portion, a filled metal containing Al
and Fe is present in a gap between the first steel member and the second steel member
in a periphery of the spot-welded portion, in the cross section, the filled metal has a
cross-sectional area of 3.0 x 104 )lm2 or more, and has a filling ratio of 80% or more in
the gap in a range of 100 )liD from an end portion of a corona bond formed in the
periphery of the spot-welded portion, and includes a first region and a second region.