QUENCHED STEEL PIPE MEMBER, VEHICLE AXLE BEAM USING
QUENCHED STEEL PIPE MEMBER, AND METHOD FOR MANUFACTURING
QUENCHED STEEL PIPE MEMBER
[Teclmical Field of the Invention]
[OOOl]
The present invelltion relates to a quenched steel pipe member, a vehicle axle
beam using a quenched steel pipe membel; and a method for manufacturing a
quenched steel pipe member.
Priority is claimed on Japanese Patent Application No. 2012-207249, filed on
September 20,2012, the content of which is incorporated herein by reference.
[Related Art]
[0002]
A vehicle axle beam is a member that couples the right axle and the left axle.
Since a load is repeatedly applied to the vehicle axle beam during the travellirlg of the
vehicle, in~provedf atigue properties are required.
[0003]
Therefore, there has been proposed a method for manufacturing a vehicle axle
beam in which a steel pipe is press-formed and then quenched so as to increase the
strength and ensure fatigue properties as described in Patent Document 1.
However, in this method, there have been problenls in that the quenching ia a
furnace itlvolves long heating time, the surface of the outermost layer of the member is
decarburized and thus softened, and sufficient fatigue properties cannot be obtained.
[0004]
In addition, to suppress the softening of the surface layer, there has been
proposed a technique in vvhicli a steel pipe is heated after galvanization is carried out
on the surface of the steel pipe, whereby a carbon-enriched layer is formed on tlie steel
surface, and tlie quenched surface layer is hardened as described in Patent Docoment 2.
However, in this metliod as well, tlie heating in a furnace requires long
heating, and zinc is volatilized during the heating. Therefore, there have been
problems in that it is necessary to sparely prepare zinc in consideration of the amount
volatilized, and considerable costs are required.
[OOOS]
Patelit Documelit 3 discloses an axle beatn having excellent fatigue properties
which is obtained by press-forming a steel pipe under predetermined pressing
conditions so as to obtain a V-like cross-section.
However, Patent Document 3 is intended to provide an axle beam capable of
exhibiting excellent fatigue properties even when a heat treatment such as quenching is
not carried out, and does not describe anytlling about the decarburization of the
outern~ost surface through the above-described heat treatment.
[Prior Art Document]
[Patent Document]
[0006]
[Patent Doculiient 11 Japanese Unexamined Patent Application, First
Publicatio~Ni o. 2005-171337
[Patent Document 21 Japanese Unexamined Patent Application, First
PublicationNo. 2006-45592
[Patent Docunient 31 Japanese Unexamined Patent Application, First
PublicationNo. 2009-274077
[Disclosure of the I~ive~ition]
[Problems to be Solved by the Invention]
[0007]
An object of the present invention is to solve the above-described probleni of
the related art and provide a low-cost quenched steel pipe member having excellent
fatigue properties, a vehicle axle beam, and a method for manufacturing a quenched
steel pipe melnber.
[Means for Solvi~igti le Probleni]
[OOOS]
The overview of the present invention is as described below.
(1) According to a first aspect of the present invention, there is provided a
quenched steel pipe member in ~vliichth e quenched steel pipe member is formed of a
GI galvanized steel pipe, in a middle sectioti in a longitudinal direction of the GI
galvanized steel pipe, a cross-section perpendicular to the longitudinal direction has a
substantially V shape including a contact section at \vllicli opposite parts of an inner
circumferential surface of the GI galvanized steel pipe come into contact with each
other, the contact section is bonded using a Fe-Zn alloy phase, and a micro Vickers
hardness at a location 50 p~idie ep from a base material surface layer is 95% or Inore of
a ~nicroV ickcrs hardness at a location 200 11111d ecp fiom the base material surface
layer.
(2) The quenched steel pipe lne~nbear ccording to tlie above-described (I), in
which the micro Vickers hardness at the location 50 pm deep from the base material
surface layer of the GI galvanized steel pipe may be 500 Hv or more.
(3) The quencl~eds teel pipe menlber according to the above-described (1) 01
(2), in which tlie contact section may be formed tlnougho~rta length that is 50% or
more of a total length of the GI galvanized steel pipe.
(4) According to a second aspect of the present invention, there is provided a
vehicle axle bean1 in which the quenched steel pipe tnenlber according to any one of
the above-described (1) to (3) is used.
(5) According to a third aspect of the present invention, there is provided a
method for manufacturing a quenched steel pipe member including press-forniing a GI
galvanized steel pipe so that, in a middle section in a longitudinal direction of the GI
galvanized steel pipe, a cross-section perpendicular to the longitudinal direction has a
substantially V shape including a contact section at which opposite parts of an inner
circulnferential surface of the GI galvanized steel pipe come into contact with each
other; heating and holding the press-formed GI galvanized steel pipe under conditions
in which a plated zinc a~nounAt (g/m2), a maxinluln heating temperature T (OC) that is
850°C or higher, and a holding time t (hr) at the maxi~nunh~ea ting temperature satisfy
tlie following fornlula (I); and cooling tlie heated and held GI galvanized steel pipe
through water cooling, thereby bonding the contact section using a Fe-ZII alloy phase,
(T+273.15)x(logt+20)/A<340 ... Formula (1).
(6) The method for nlanufacturing a quenched steel pipe lnetnber according to
the above-described (5), in which, in the cooling, the heated and held GI galvanized
. steel pipe may be water-cooled to 200°C or lower at a cooling rate of 30 "CIS or more.
(7) The tnethod for nlanufacluring a quenched steel pipe rnelnber according to
tlie above-described (5) or (6), in which the plated zinc amount Anlay be 60 g/m2 or
more.
(8) The method for manufacturing a quenched steel pipe nlenlber according to
any one of the above-described (5) to (7), in which, in the press-forming, the GI
galvanized steel pipe may be press-formed so that the contact section is formed
tlxougliout a length that is 50% or illore of a total length of the GI galvanized steel
pipe.
(9) The method for manufacturing a quenched steel pipe member according to
any one of the above-described (5) to (8), in which, in the heating and holding, the GI
galvanized steel pipe may be electrically heated.
(10) The method for manufacturing a quenched steel pipe member according
to any one of the above-described (5) to (9), in v\rhicl~i,n the heating and holding, the
press-formed GI galvanized steel pipe may be electrically heated so as to be held in a
temperature range of an Ac3 point of steel or higher for 3 seconds to 30 seconds.
(1 1) The method for matmfacturing a quenched steel pipe member according
to any one of the above-described (5) to (lo), in which the GI galvanized steel pipe
may have chemical con~positionh aving an Ac3 point of 850°C or lower.
[Effects of the Invention]
[0009]
According to the above-described quenched steel pipe member, since the
quenched steel pipe member is formed of the GI galvanized steel pipe, the
decarburization of the surface layer is suppressed due to the galvanization, and
therefore a high surface layer hardness can be ensured, and the fatigue properties
improve.
[OO lo]
In addition, since the contact section at which opposite parts of the inner
circumferential surface of the GI galvanized steel pipe come into contact wit11 each
other is bonded using the Fe-Zn alloy phase, it is possible to suppress a decrease in the
fatigue life caused by the friction of the contact section, and the fatigue properties
improve. Therefore, thickness reduction and weight reduction beconle possible, and
it is possible to significantly decrease the costs.
I001 11
In addition, according to the above-described method for manufacturing a
quenched steel pipe member, when the GI galvanized steel pipe is heated and held
under conditions satisfiing Formula (I), it is possible to suppress the decarburization of
the surface layer with a minimum plated zinc amount, or to suppress the
decarburizatiotl of the surface layer by adjusting the plated zinc amount in accordance
with a heating and holding facility. Therefore, it is possible to significantly decrease
the costs.
[Brief Description of the Drawing]
100 121
FIG. 1A is a plan view of a vehicle axle beatn according to a present
embodiment.
FIG. 1B is a perspective view of the vehicle axle beam.
FIG. 1D is a cross-sectional view in the direction of ID-ID in FIG. 1A.
FIG. 2 is a step explanatory view for manufacturing a quenched steel pipe
memnber according to the present enlbodiment.
FIG. 3 is a phase diagram of a Fe-Zn alloy.
[Embodiments of the Inventio~i]
[0013]
Hereinafter, a a~eliiclea xle bean1 according to a first emboditnent of the
present invention (hereinafter, referred to as the axle beam) will be described in detail.
In the following description, the axle beam will be described as a specific example of a
quenched steel pipe metnber, but a quenched steel pipe member according to the
present invention is not limited thereto, and examples thereof include a variety of
quenched steel pipe members requiring itnproved fatigue properties such as structural
members for industrial machinery and structural menlbers for const~~~ction.
[0014]
FIGS. 1A and 1B are a plan view and a perspective view illustrating at1 axle
beam 1 according to the present enlbodiment. FIG. 1C is a cross-sectional view in
the direction of IC-IC in FIG. lA, and FIG. 1D is a cross-sectional view in the
direction of ID-ID in FIG. 1A.
As illustrated in FIGS. 1A and lB, the axle beam 1 according to the present
embodiment is formed by press-forming a GI galvanized steel pipe 10 so that a crosssection
perpendicular to a longitudinal direction of the axle beam (hereinafter, referred
to as the perpendicular cross-section) has a substantially V shape.
[0015]
In addition, as illustrated in FIG. lC, the axle beam 1 according to the present
circumferential surface cotne into contact with each other in a central part of the axle
beat11 in the longitudinal direction.
The contact section 11 is bonded using a Fe-Zn alloy phase by carrying out a
quenching treatment in a state in which opposite [tarts of a zinc plate on the inner
circumferential surface of the GI galvanized steel pipe 10 are in contact with each other
In addition to an effect that improves the stiffness of the axle beam 1, the
above-described configuration is capable of suppressing a decrease in the fatigue life
caused by the friction between tlie opposite parts of the inner circun~ferentiaslu rface of
tlie GI galvanized steel pipe 10, and imnproving the fatigue properties.
[0016]
In addition, as illustrated in FIG. ID, the opposite parts of the inner
circumferential surface may riot be in contact with each other in po~-tionsa way from
the central part in the longitudi~ladl irection. That is, the opposite paits of the contact
section 11 need to be in contact with each other only in the central part of the GI
galvanized steel pipe 10 in the longitudinal direction.
However, to more preferably exhibit the effect that improves the fatigue
properties, the contact section 11 is preferably for~nedth roughout a length that is 50%
or inore of tlie total length of the GI galvanized steel pipe 10, and more preferably
formed tlxoughout a length that is 70% or Inore of the total length.
[0017]
In addition, the axle bean1 1 according to the present embodiment is obtained
by carrying out a que~ichingtr eatlnent after the press-forlning of the GI galvanized
steel pipe 10, and therefore it is possible to carry out the quenching treatmetit while
suppressing decarburization from the surface layer using the effect of the galvanizatioli,
detarburization is suppressed in the surface part of the axle beam 1, it is possible to
ensure the same hardness as that at the central part in the thickness direction, and
consequently, improve the fatigue properties.
[0018]
More specifically, in the axle beam 1 accorciing to tlie present embodinlent,
when tlie micro Vickers hardness at a locatio1l200 pm deep from the base ~naterial
surface layer is represented by X, and the micro Vickers hardness at a location 50 pm
deep from the base material surface layer is represented by Y, the value of (Y/X)x100
is set to 95 or more. Meanwhile, the micro Vickers harduess is nleasured at a load of
50 g.
When the value of (Y/X)x100 is less tliau 95, there is a concern over a
decrease in the fatigue life due to fatigue cracks from the surface layer. The value of
(Y/X)x 100 is preferably 96 or more, and more peferably 97 or more.
[00 191
In addition, the nlicro Vickers hardness Y at a location 50 pm deep froill the
base material surface layer is preferably set to 500 Hv or more, and more preferably set
to 540 I-Iv or more in terms of the micro Vickers hardness since improved fatigue
properties are ensured.
[0020]
As described above, according to tlie axle beam 1 of the present en~bodiment,
since tlie axle beam is formed of the GI galvanized steel pipe 10, the decarburization of
the surface layer is suppressed due to tlie effect of the galvanization, and an effect that
increases the hardness of the surface layer and thus improves the fatigue propei-ties is
obtained. Since the contact section 11 at which opposite pai-ts of the inner
circu~nferentials urface of the GI galvanized steel pipe 10 come into contact with each
other is bonded using tlie Fe-Zn alloy phase, it is possible to suppress a decrease in the
fatigue life caused by tlie friction between opposite parts of the inner circumferential
surface, and to synergistically improve the fatigue propel-ties. Therefore, thickness
reduction and weight reduction become possible, and it is possible to significantly
decrease the costs.
[0021]
In the present invention, there is no particular limitation regarding the
clien~icacl omponents of steel for the GI galvanized steel pipe 10, but a preferred
component cornposition will be described. Hereinafter, regarding the amounts of the
chemical components, '%' indicates 'mass%'.
[0022]
The steel for the GI galvanized stcel pipe 10 may include, as the chetnical
components, C, Si, Mn, Ti, and B in the following ranges.
[0023]
C: 0.15% to 0.30%
C is an element that determines the strength of the axle beam 1. To ensure a
strength for I~avings ufficient fatigue properties, the amount of C is preferably set to
0.15% or tnore, and more preferably set to 0.20% or tnore. To set the hardness Hv to
500 or more, the amount of C is preferably set to 0.24% or more. In addition, to
suppress the occurrence of quenching cracks, the amount of C is preferably set to
0.30% or less, and tnore preferably set to 0.25% or less.
[0024]
Si: 0.05% to 0.35%
Si is a deoxidizing element, and also contributes to solid solution
strengthening. To obtain the above-described effects, 0.05% or more of Si is
preferably included. In addition, when the amount of Si is set to 0.35% or less, it is
possible to ensure toughness. The lower limit of the amount of Si is no re preferably
0.20%, and the upper litnit of the amount of Si is more preferably 0.30%.
[0025]
Mn: 0.5% to 2.0%
Mn is an element that improves hardenability, and the amount of Mn is
preferably set to 0.5% or more since an effect that improves hardenability can be
sufficiently ensured. In addition, the amount of Mn is preferably set to 2.0% or less
since the deterioration of the delayed fracture properties can be suppressed, the
precipitation of MnS can be suppressed, and a decrease in the fatigue strength in the
vicinity of an electric resistance welded section can be avoided.
The lower limit of the amount of Mn is more preferably 1.0%, and the upper
litnit of tlle amount of Mn is more preferably less than 1.7%.
100261
Ti: 0.005% to 0.05%
Ti acts to stably and effectively improve hardenability obtained by the
addition of B by fixing N in steel in a TiN form so as to suppress the precipitation of
BN. Therefore, based on tlie stoichionletry of TiN, Ti is preferably added in an
amount that is 3.42 times or more the amount of N, and the preferred range of the
amount of Ti is also automatically determined from the range of the amount of N.
However, since some of the Ti is precipitated in a carbide form, the atnount of
Ti is preferably set in a range of 0.005% to 0.05% which is higher than tlie theoretical
value to more reliably fix N. The amount of Ti is more preferably in a range of
0.01% to 0.02%.
B is an element that significantly improves the hardenability of the steel by
the addition of a small amount. The amount of B is preferably set to 0.0005% or
more, and more preferably set to 0.001% or more since an effect that improves
liardenability is preferably obtained.
In addition, the atnount of B is preferably set to 0.005% or less, and more
preferably set to 0.002% or less since the generation of a coarse B-containing
precipitate can be suppressed, and ernbrittle~nelltc an be suppressed.
[0028]
In addition, tlie steel for the GI galvanized steel pipe 10 may contain, as
clietiiical components, Al, P, S, N, and 0 in the following limited ranges.
[0029]
Al: 0.08% or less
Al is an effective element as a deoxidizing material of molten steel, and
0.01% or more of Al is preferably added. In addition, since A1 is also an element that
fixes N, the amount ofAl has a significant influence on the grain size or nlechanical
properties. The amount ofAl is preferably set to 0.08% or less since it is possible to
suppress the generation of surface defects in a product caused by non-metallic
inclusions. The amount ofAl is more preferably 0.05% or less.
[0030]
P: 0.05% or less
P is at1 element having an adverse influence on welding cracking resistance
and toughness, and thus the amount of P is preferably set to 0.05% or less, and Inore
preferably set to 0.03% or less.
[003 11
S: less than 0.0030%
S deteriorates toughness, and causes the precipitation of MnS wllich leads to a
decrease in the fatigue strength in the vicinity of an electric resistance welded section,
and therefore the amount of S is preferably set to less than 0.0030%, and more
preferably set to 0.0026% or less.
In addition, to suppress the precipitation of MnS, it is preferable to suppress
the amount of S in consideration of the amount of Mn rather than to suppress only the
amount of S, and specificall): the value of the product of the amount of Mn and the
amount of S is preferably set to 0.0025 or less. When the value of the product of the
amount of Mn and the amount of S is set to 0.0025 or less, it is possible to sufficiently
ensure the fatigue strength in the vicinity of an electric resistance welded section.
100331
N: 0.006% or less
N is an element having an effect that increases strength by precipitating a
nitride or a carbonitride. However, in B-added steel, there are problems of the
degradation of liardenability due to tlie precipitation of BN, the degradation of hot
workability or fatigue strength due to the precipitation of TiN which is caused by Ti
being added to prevent the precipitation of BN as described above, and tlie degradation
of toughness. Meanwhile, TiN also has an effect tliat suppresses the coarsening of a y
grain diameter at a high tetnperatt~rea nd improves toughness. Therefore, to obtain
the optimal balance among hot workability, fatigue strength, and toughness, the
aniount of N is preferably set to 0.006% or less. Meanwhile, the amount of N is more
preferably in a range of 0.001% to 0.005%, and still more preferably in a range of
0.002% to 0.004%.
[0034]
0 : 0.004% or less
0 is an element tliat forms Ca0 and thus impairs the effect of the addition of
Ca, and therefore the amount of 0 is preferably limited to 0.004% or less.
[0035]
In addition, the steel for the GI galvanized steel pipe 10 may contain, as
cheliiical colilponents and selective eleme~itso, ne or more of Mo, Cr, Nb, V, and Ni as
necessary in tlie following ranges.
[0036]
Mo: 0.05% to 0.5%
Mo is an element having an effect that itliproves hardenability. When the
amount of Mo is less than 0.05%, the above-described effect cannot be suficiently
expected, and on the other hand, when the amount of Mo exceeds 0.5%, the alloy cost
increases, and therefore the amount of Mo is preferably set in a range of 0.05% to
0.5%.
[0037]
Cr: 0.05% to 1.0%
Cr is not an essential additive element, but is an eletnent added for the
purpose of improving hardenability. To sufficiently obtain the hardenabilityiniproving
effect, the amount of Cr is preferably set to 0.05% or more, and more
preferably set to 0.10% or more. In addition, the amount of Cr is preferably set to
1 .O% or less, and more preferably set to 0.8% or less since the generation of defects
during electric resistance welding is suppressed.
[0038]
Nb: 0.01% to 0.1%
Nb has an effect that decreases the grain size of the steel and improves
touglmess in addition to an effect of precipitation strengthening from a Nb carbonitride.
When the amount of Nb is 0.01% or more, an effect that improves strength and
toughness is sufficiently obtained. On the other hand, even when the amount of Nb
exceeds 0.1%, it is not possible to expect an additional effect that f~irtheirn lproves
strength and toughness, and only the cost is increased, and therefore the amount of Nb
is preferably set in a range of 0.01% to 0.1%.
[0039]
V: 0.01% to 0.1%
V is an element having an effect of precipitation strengthening fiorn a V
carbonitride. The amount of V is preferably set to 0.01% or more since the abovedescribed
effect can be preferably exhibited. On the other hand, even wIie11 the
amount of V exceeds 0.1%, it is not possible to expect a better effect of precipitation
strengthening, and only the alloy cost is increased, and therefore the amount of V is
preferably set to 0.1% or less.
[0040]
Ni: 0.1% to 1.0%
Ni is an element having an effect that improves hardenability and toughness.
The amount of Ni is preferably set to 0.1% or more since the above-described effect
can be preferably exhibited. On the other hand, even when the amount of Ni exceeds
1.0%, the alloy cost is increased, and therefore the amount of Ni is preferably set to
1.0% or less.
[0041]
That is, the steel for tlie GI galvanized steel pipe 10 contains, as chemical
components, C, Si, Mn, Ti, arid B in tlie above-described ranges, Al, P, S, N, and O in
the above-described limited ranges, and contains, as selective elements, one or more of
Mo, Cr, Nb, V, and Ni as necessary in tlie above-described ranges, and the balance is
consisting of Fe and unavoidable impurities.
[0042]
111 additioa, in the present inveiltion, to obtain a martensite structure of tlie
axle beam 1 tluough quenching, it is necessary to sufficiently ensure tlie liardellability
of a material. As an index of hardenability, for exatnple, the critical cooling rate
Vc90 ('CIS) which has been thus far known by "Iron and steel, 74 (1988) P.1073" is
preferably used. This is an index represented by tlie following (Fommla A), and
refers to a cooling rate at which the volu~nera tio of niartensite reaches 90% or more.
Therefore, tlie hardenability improves as Vc90 decreases, and a niartensite structure
can be obtained even when tlie cooling rate becomes slow.
[0043]
logVc90=2.94-0.75P ... (Formula A)
Here, P=~.~C+O.~S~+M~+O.~C~+~.OMO+O.~~N~.
In a case in \vliicli~h boron (B) is not included, (Formula A) is changed to
(Formula A').
logVc90=2.94-0.75(p'-1) ... (Formula A')
Here, ~'=~.~C+O.~S~+MI~+O.~CI+MO+O.~~N~.
[0044]
Next, a method for manufacturing the axle beam 1 will be described in detail.
[0045]
As illustrated in the flow diagram of FIG. 2, the method for manufacturing the
axle beam 1 according to the present embodiment includes at least a press-fomiing step,
be described in detail.
[0046]
(Press-forming step)
I11 tlie press-forming step, first, tlie GI galvanized steel pipe 10 is pressfornled
into a substantially V shape by su~pplyinga n inward displacenlent in the
longitudinal direction, thereby producing a shape of the axle bean1 1. That is, tlie GI
galvanized steel pipe 10 is press-formed so that a cross-section perpendicular to the
longitudinal direction in the middle section in the longitudinal direction has a
substantially V shape including the contact section 11 at which opposite parts of an
inner circumferential surface of the GI galvanized steel pipe 10 come into contact mith
each other.
The specific shape is as illustrated in FIGS. 1A to ID, and the contact section
11 at which opposite parts of the inner circnmferential surface of the steel pipe come
into contact with each other is formed in the middle section in the longitudinal
direction as illustrated in FIG. 1C. Both end sections have a shape in which the steel
pipe is flatly crushed.
[0047]
(Heating and holding step)
In the heating and holding step, the GI galvanized steel pipe 10 press-formed
as described above is heated and held under conditions satisfying the follo~ving
fornlula (1).
(T+273.15)x(logt+20)/A5340 ... Fornlula (1)
[0048]
I11 Formula (I), A represents the plated zinc amount (g/m2) of the GI
850°C or higher, and t represents the holding time t (llr) at the maximum heating
temperature . In the present specification, (T+273,15)x(logt+20) regarding the
heating conditions will be called a heat treatment parameter B.
When the plated zinc amount A and the heat treatment parameter B are
designed so as to satisfl~F onnula (1), it is possible to achieve the value of (Y/X)x100
to be 95 or more in which X represents the micro Vickers hardness at a location 200
[Im deep from the base material surface layer, and Y represents the micro Vickers
hardness at a location 50 pm deep fiom the base material surface layer.
Since the maximum heating te~nperatureis set to 850°C or highe~;i t becomes
possible to obtain a martensite as quenched structure. On the other hand, when the
maximunl heating temperature is lower than 850°C, the te~nperatureis in a two-phase
region, and the steel pipe is not fully quenched, which leads to the abrupt degradation
of the fatigue properties.
That is, when the GI galvanized steel pipe 10 is heated and held under
conditions satisfying Formula (I), it becomes possible to s~~nergisticalilmy prove the
fatigue properties from an effect that improves the fatigue properties by the
suppression of surface layer decarburization and an effect that improves the fatigue
propel-ties by the alloying of the contact section 11.
[0049]
The plated zinc amount A of the GI galvanized steel pipe 10 is preferably set
to 60 dm2 or more since surface layer decarburization can be more reliably suppressed
in the heating and holding step.
[0050]
Examples of the heating method in the heating and holding step include
into account, electrical heating is more preferred.
[0051]
The upper limit of the maxinlum heating temperature is not pa~-ticularly
specified; however, when the upper limit is an extremely high temperature, there is a
concern over the volatilization of zinc from the surface of the steel pipe. Therefore,
the upper limit is set to llOO°C to more reliably suppress surface layer decarburization.
[0052]
The liolding time is preferably set to 3 seconds or longer in a temperature
range of at1 Ac3 point or higher. When the holding titlie is 3 seconds or loagel;
temperature variation is more reliably suppressed, and therefore it is possible to
decrease hardness variation after quenching. In addition, since it is possible to
reliably diffiise iron into a galvanized layes, the coritact section 11 can be stably
bonded using an Fe-Zn alloy phase.
In addition, the holding time is preferably set to 30 seconds or shorter. This
is because, when the holding time is set to 30 seconds or shorter, it is possible to
suppress the excessive diffusion of iron into the galvanized la ye^
Mea~iwliile,w hen the GI galvanized steel pipe 10 is heated through electrical
heating, there is a concern that a currellt may drift a~idth us cause temperature
variatioa; however, it could be coilfirmed that, wlien the holding time is set in a range
of 3 seconds to 30 seconds, the steel pipe can be lieated in a temperature range in
which the steel pipe is uniformly quencl~ed.
[0053]
As described above, in the method for ma~iufacturillgth e axle beam 1
according to tlie present embodiment, since the steel pipe is heated and held at the
of the steel broadens the temperature ratige in which tlie steel pipe car1 be held, and
facilitates the production, and therefore it is preferable to use a steel pipe having
chemical composition having anAc3 point of 85OoC or lower.
TlieAc3 point call be cotliputed fiotn the following formula (2).
[0054]
~c3=910-203(~'~)-15.2x~i+44.7x~i+l04x~+31.5x~o+l...3 F.o1rxm~u la
(2)
[0055]
Meanwhile, chemical con~positiono f the steel in which the hardenability
(Vc90) reaches 70 OC/second or lower can be expressed by the following formulae (3)
and (4). Vc90 indicates the cooling rate at which martellsite accounts for 90% or
Illore of the structure.
[0056]
1 OU<70 ... Formula (3)
u=2.94-0.7Sx(2.7xC+O.4xSi+Mn+0.45xNi+0.8Cr2Mo..). F ormula (4)
[0057]
When the GI galvanized steel pipe 10 is held under the above-described
conditions, the galvanized phase in the surface of the steel pipe belongs to the hatched
area in the phase diagram of FIG. 3, and when the steel pipe is rapidly cooled from this
area, the phase turns into a Fe-Zn alloy phase, and the contact section 11 in a portion
having a V-like cross-sectional shape is bonded. As a result, it is possible to suppress
a decrease in the fatigue life caused by the fiiction between the opposite parts of the
inner circulnfere~ltiasl urface of the GI galvanized steel pipe 10, and to inlprove the
fatigue strength. As described above, when the diffusion of iron into a plated phase is
an iron-zinc alloy phase and iron is formed even after rapid cooling, which is not
preferred.
[OOS8]
(Cooling step)
In the coolirig step, the heated and held G1 galvanized steel pipe 10 is watercooled,
and therefore the contact section 11 is bonded using the Fe-Zn alloy phase.
In the cooling step, it is preferable to cool the steel pipe to 200°C or lower at a
cooling rate of 30 "CIS or inore, since the fatigue properties can be filrther itnproved
due to the structure tunling into martensite. The cooling rate is more preferably set to
50 "CIS or more.
Exanlples of the cooli~lgm ethod include spray cooling, immersion cooling,
mist (air-water) cooling, and the like, and when productivity is taken into account,
spray cooling is preferred.
[0059]
In the axle beam 1 according to the present embodiment obtained in the
above-described manner, the contact section 11 is bonded using the Fe-ZII alloy phase,
and the micro Vickers hardness at a location 50 ptn deep fiotn the base material surface
layer is 95% or niore of the micro Vickers hardness at a location 200 pm deep from the
base material surface layel; and therefore it becomes possible to synergistically
improve the fatigue properties fro111 an effect that improves the fatigue propei-ties by
the suppression of surface layer decarburization and an effect that improves the fatigue
propei-ties by the alloying of the contact section 11.
[0060]
The reason for using GI (Galvanized Iron) instead of GA (Galvannealed) for
after s1101-t heating, GA is into the same state as GI that has been heated for a long time,
a rnultiphase of an iron-zinc alloy phase and iron is formed even after cooling, and the
effect that bonds the contact section 11 using the Fe-Zn alloy phase becomes
insufficient.
[Exat~iplc]
[0061]
Hereinaftel; exa~nplesw ill be described.
As Invention Examples 1 to 6 and Comparative Examples 1 to 3, axle beams
were mal~ufacturedb y carrying out electric resistance welding GI steel sheets having
components of 0.24% of C, 0.2% of Si, 1.2% of Mn, 0.02% of Ti, and 10 ppm of B,
pressing, electrical heating, and spray cooling on, and the micro Vickers hardness at
the locatio1l50 ptn deep from the base material surface layer, the micro Vickers
hardness at the location 200 lun deep from the base material surface layer, and the
fatigue properties were measured. In addition, for Invention Examples 1 to 6 and
Comparative Examples 1 and 2, the contact sections were bonded through alloying.
Table 1 describes a variety of setting conditions and measurement results. A
represents the plating atnouttt (g/m2), T represents the maximum heating temperature
PC), t represents the holding time (1x1, B represents the heat treatment parametel; X
represents the micm Vickers hardness at a location 200 pm deep from the base material
surface layel; and Y represents the micro Vickers hardness at a locatiotl50 p n ~de ep
from the base material surface layer.

[0063]
In Invention Examples 1 to 6 in which the values of BIA satisfied the range of
the invention, it was possible to achieve the values of Y/X to be 95% or Illore from an
effect of the suppressioll of decarburization from the surface layer, and it was possible
to obtain improved fatigue properties from a synergistic effect with an effect that
itnproved the fatigue properties through the alloyirig of the contact sections. Atnong
Inventioli Examples 1 to 6, in Invention Examples 1 to 5 in which the micro Vickers
hardness at a locatioll50 {tnl deep from the base material surface layer was 500 Hv or
more, more improved fatigue properties could be obtained compared with Invention
Exanlple 6.
[0064]
On the other hand, in Co~nparativeE xamples 1 and 2 in which the values of
B1A failed to satisfy the range of the invention, the hardness of the surface part was
the effect that ilnproved the fatigue propel-ties through the alloying of the contact
sections could not be obtained, and improved fatigue properties could not be obtained.
[0065]
In addition, in Colnparative Exat~~p3le in wliich plating was not carried out,
not only was the hardness of the surface part decreased due to decarburizatioti from the
surface layer, but the effect that improved the fatigue properties tlrough the alloying of
the contact sections could not be obtained, and therefore, naturally, ilnproved fatigue
properties could not be obtained.
[0066]
As is evident from the above-described results, according to the present
invention, it is possible to obtain a low-cost axle beat11 having excellerit fatigue
properties.
[Industrial Applicability]
[0067]
According to the present invention, it is possible to provide a low-cost
quenched steel pipe meliiber having excelle~ifta tigue properties, a vehicle axle beam,
and a method for lilaliufacturing a quenched steel pipe niember.
[Brief Desc~iptiono f the Reference Symbols]
[0068]
1: AXLE BEAM
10: GI GALVANIZED STEEL PIPE
11 : CONTACT SECTION
A: ZINC PLATING AMOUNT
B: HEAT TREATMENT PARAMETER
t: HOLDING TIME
T: MAXIMUM HEATING TEMPERATURE
X: MICRO VICKERS HARDNESS AT LOCATION 200 pm DEEP FROM
BASE MATERIAL SURFACE LAYER
Y: MICRO VICKERS HARDNESS AT LOCATION 50 11m DEEP FROM
BASE MAIEIUAL SURFACE LAYER

CLAIMS
[Claim 1]
A quenched steel pipe member,
wherein the quenched steel pipe member is formed of a GI galvanized steel
pipe,
in a middle section in a longitudinal direction of the GI galvanized steel pipe,
a cross-section perpendicular to the longitudinal direction has a substantially V shape
including a contact section at which opposite parts of an inner circumferential surface
of the GI galvanized steel pipe come into contact with each other,
the contact section is bonded using a Fe-Zn alloy phase, and
a micro Vickers hardness at a location 50 p n ~de ep fiom a base material
surface layer is 95% or more of a micro Vickers hardness at a location 200 pln deep
[Claim 2]
The quenched steel pipe member according to Claim 1,
wherein the micro Vickers hardness at the locatioll50 ptn deep from the base
material surface layer of the GI galvanized steel pipe is 500 I-Iv or more.
[Claim 3]
The quenched steel pipe member according to Claim 1 or 2,
wherein the contact section is formed throughout a length that is 50% or more
of a total length of the GI galvanized steel pipe.
[Claim 4]
A vehicle axle beam,
wherein the quetlched steel pipe member according to any one of Clainls 1 to
3 is used.
[Claim 5]
A method for manufacturing a quenched steel pipe member, the metllod
comprising:
press-forming a GI galvanized steel pipe so that, in a middle section in a
longitudinal direction of the GI galvanized steel pipe, a cross-section perpendicular to
the longitudinal direction has a substantially V shape including a contact section at
which opposite parts of an i~lnerc ircumferential surface of the GI galvanized steel pipe
come into contact with each other;
heating and holding the press-forl~led GI galvanized steel pipe under
conditions in wliicli a plated zinc amount A (!g/n12), a maximum heating temperature T
("C) that is 850°C or higher, and a holding time t (h) at the maximum heating
temperature satisfy the following formula (1); and
thereby bonding the contact section using a Fe-Zn alloy phase,
(T+273.15)x(logt+20)/Ai340 ... Formula (1).
[Claim 6]
The metliod for inanufacturing a quenched steel pipe member according to
Claim 5,
~vliereini, n the cooling, the heated and held GI galvanized steel pipe is watercooled
to 200°C or lower at a cooling rate of 30 "CIS or more.
[Claim 7]
The method for manufacturing a quenched steel pipe member according to
Claim 5 or 6,
wherein the plated zinc amount A is 60 g/~1~or2 m ore.
[Claim 8]
The method for manufacturing a quenched steel pipe member according to
any one of Claims 5 to 7,
wherein, in the press-forming, the GI galvanized steel pipe is press-formed so
that the contact section is formed throughout a length that is 50% or more of a total
length of the GI galvanized steel pipe.
[Claim 9]
The method for manufacturing a quenched steel pipe mernber according to
any one of Claims 5 to 8,
ivherein, in the heating and holding, the GI galvanized steel pipe is electrically :
heated.
[Claim 10]
The pethod for maliufacturing a quenched steel pipe member according to
any one of Clainls 5 t
wherein, in the heating and holding, the press-formed GI galvanized steel pipe
is electrically heated so as to be held in a temperature range of anAc3 point of steel or . -
higher for 3 seconds to 30:secoads.
[Clainl 11]
The method for manufacturing a quenched steel pipe luernber according to
any one of Claims 5 to 10, I
wherein the GI galvanized steel pipe has cllemical cotnposition having an Ac3
a point of 850°C or lowel.