DESCRIPTION
TITLE OF INVENTION
ROLLED STEEL MATERIAL FOR FRACTURE SPLITTING CONNECTING
ROD
TECHNICAL FIELD
[0001]
The present invention relates to steel materials, and more particularly relates
to a rolled steel material for fracture splitting connecting rods.
BACKGROUND ART
[0002]
Connecting rods are used in"engines of, for example, automobiles. The
connecting rod couples a piston to a crankshaft to convert the vertical motion of the
piston to the rotational motion of the crankshaft.
[0003]
FIG. I is a front view of a conventional connecting rod 1. As illustrated in
FIG. 1, the conventional connecting rod 1 includes a big end portion 10, a rod
portion 20, and a small end portion 30. The big end portion 10 is disposed at one
end of the rod portion 20 and the small end portion 30 is disposed at the other end of
the rod portion 20. The big end portion l0 is coupled to a crank pin. The small
end portion 30 is coupled to a piston.
[0004]
The conventional connecting rod 1 includes two parts (a cap 40 and a rod 50).
The cap 40 and one end ofthe rod 50 correspond to the big end portion 10. The
other portions than the one end ofthe rod 50 correspond to the rod portion 20 and the
small end portion 30.
[000s]
The big end porlion 10 and the small end portion 30 are formed by machining.
Thus, the connecting rod 1 needs to exhibit high machinability.
[0006]
Fufthermore, during operation of the engine, the connecting rod I is subjected
to loading from nearby components. Furthermore, for fuel saving, there have been
needs in recent years for size reduction ofthe connecting rod I and an increase in
cylinder pressure within the cylinder. Accordingly, there is a need for the
connecting rod I to have a thinner rod portion 20 and at the same time be able to
exhibit high buckling strength sufficient to withstand the explosive loading
transmitted from the piston. The buckling strength heavily depends on the yield
strength of the material. Thus, connecting rods need to exhibit high yield strength
as well as high machinability.
[0007]
In the conventional connecting rod 1, the cap 40 and the rod 50 are separately
produced as described above. Thus, for positioning ofthe cap 40 and the rod 50, a
dowel pinning process is performed. Furthermore, a machining process is applied
to the mating surfaces of the cap 40'and the rod 50. In view of this, fracture
splitting connecting rods, which make it possible to eliminate these processes, are
increasingly being employed.
[0008]
A fracture splitting connecting rod is formed by forming a one-piece
connecting rod and then fracturing the big end portion thereof into two parts
(corresponding to the cap 40 and the rod 50). When mounting it to an engine, the
split two parts are joined together. Thus, the dowel pinning process and the
machining process are not performed. This results in reduced production cost.
[000e]
Technologies relating to a steel material for such a fracture splitting
connecting rod and a method for producing such a fracture splitting connecting rod
are disclosed in U.S. PatentNo. 5135587 (Patent Literature l), Japanese Patent
Application Publication No. 201 0- 1 80473 (Patent Literature 2), Japanese Patent
Application Publication No. 2004-301324 (Patent Literature 3), International
Application Publication No. 'WO 20121164710 (Patent Literature 4), Japanese Patent
Application Publication No. 201 1-084767 (Patent Literature 5), and International
Application Publication No. WO 20121157455 (Patent Literature 6).
[0010]
(, 5
{
Patent Literature I discloses the following. A steel for fracture splitting
connecting rods contains, in weight Vo, C: 0.6 to 0.75Yo, Mn: 0.25 to 0.50%, and S:
0.04 to 0.12yo, the balance being Fe and up to 1,2Yo of impurities. Mn/S is 3.0 or
more. The steel has a 100% pearlitic structure and a grain size of 3 to 8 ASTM per
Specification 8112-88.
[001 1]
Patent Literature 2 discloses the following. A steel for fractLrre splitting
connecting rods is a non-heat treated steel made up of ferrite and pearlite and
containing 0.20 to 0.60% of C in mass 0%. The rod portion is subjected to a coining
process. The steel for fracture splitting connecting rods contains C, N, Ti, Mn, and
Cr as essential elements and contains Si, P, S, V, Pb, Te, Ca, and Bi as optional
elements. The essential elements include, in mass yo,0.30to 1.50Yo of Mn,0.05 to
1.00% of Cr,0.005 to 0,030% ofN, and 0.20% or less of Ti. The formula, Ti >
3.4N + 0.02, is satisfied. The 0.2Yo proof stress of the big end portion is lower than
650 MPa. Further, the 0.2Yo proof stress of the rod poftion, which has been
subjected to the coining process, is higher than 700 MPa.
[00r 2]
Patent Literature 3 discloses the following. A non-heat treated connecting
rod contains, in mass %o,C:0.25 fo 0.35o/o, Si: 0.50 to 0.70Yo, Mn: 0.60 to 0.90%, P:
0.040to 0.070y0, S: 0.040 to0.130o/o, Cr: 0.l0Io0.20Yo,V: 0.15 to0.20Yo, Ti: 0.15
to 0.20%o, and N: 0.002 to 0.020yo, the balance being Fe and impurities. The Ceq
value defined by Formula (1) is less than 0.80. The structure of the big end portion
is made up of ferrite and pearlite. The total hardness of the big end portion ranges
from 255 fo 320 on the Vickers hardness scale. Further, the hardness of the ferrite
of the big end portion is 250 or more on the Vickers hardness scale. Fudher, the
hardness of the ferrite relative to the total hardness of the big end portion is 0.80 or
more.
Ceq:C+(Si/I0)+(Mn/5) +(5Crl22) + 1.6sV- (sS/7) (1)
[00 r 3]
Patent Literature 4 discloses the following. A non-heat treated steel bar for
connecting rods contains, in mass Yo,C:0.25 to 0.35Yo, Si: 0.40 to 030%o, Mn: more
than0.65Yoto 0.90%o or less, P: 0.040 to 0.070Yo, S: 0.040 to 0.130%, Cr: 0.10 to
+
0.30o/o, Cu: 0,05 To 0.40Vo, Ni: 0.05 to 0.30Y0, Mo: 0,01 to 0.15o/o, V: 0.12 to 0.200/o,
Ti: more than 0.1 50 to 0.200% or less, Al: 0.002 to 0.100%, and N: 0.020 or less, the
balance being Fe and impurities. Fn1, defined by the fonnula below, ranges from
0.60 to 0.80, and Fn 2, defined by the formula below is 7 or more. In the structure
ofthe non-heat treated connecting rod steel, the ferrite and pearlite structure accounts
for 90%o or more. The proportion of the ferrite in the ferrite and pearlite structure is
40o/o or mote.
Fn1 : C + (Si/l0) + (Mn/5) + (5Crl22) + 1.65V - (5S/7) + (Cu/33) + (1.{i/20) +
(Mo/10)
Pn2: (Mn + TiyS
[00r 4]
Patent Literature 5 discloses the following. A method for producing a
fracture splitting connecting rod includes: a step of providing a steel material; a step
of heating the steel material to a temperature ranging from 1200'C to 1300"C; a step
of hot forging the steel material into a rough forged body, the step being carried out
by applying compression to the steel material at at least a predetermined portion
thereof at a temperature of 1 000"C or more and at a working ratio of 5 00lo or more;
and a step of cooling the rough forged body at at least 5oC/s or less to form a ferrite
and pearlite structure therein. The resulting fracture splitting connecting rod
contains, inmass Yo,C:0.16to0.35Yo, Si:0.1 tol.jYo, Mn: 0.3 to7.ÙYo, P: 0.040to
0.070%o, S: 0.080 to 0.130%, V: 0.10 to 0.35%o, and Ti: 0.08 to 0.20%. The
hardness of the predetermined portion is at least 250 HV or more.
[00 ] s]
Further, Patent Literature 6 discloses a non-heat treated steel having a low V
content. Specifically, Patent Literature 6 discloses the following. The non-heat
treated steel contains, in mass Vo, C: 0.27 to 0.40%0, Si: 0.1 5 to 0.7jYo, Mn: 0.55 to
1.50Yo, P: 0.010 to 0.070%, S: 0.05 to 0.15%o, Cr: 0.10 to 0.60Yo, V: 0.030% or more
to less than 0.150yo, Ti: more than 0.100% to 0.200% or less, Al: 0.002 to 0.050%,
and N: 0.002 to 0.020y0, the balance being Fe and irnpurities. Et, defined by the
formula below, is less than 0. Ceq, defined by the formula below, is more than 0,60
to less than 0.80.
Er: [ri] - 3.4 [N] - l.s [S]
5
Ceq : [C] + ([Si]/10) + ([Mn]/s) + (s [cr]/22) + (33 lv)tzj) - (s tsl/7)
[0016]
The steel for fracture splitting connecting rods of Patent Literature t has been
widely commercialized in Europe. However, the steel for fracture splitting
connecting rods of Patent Literature I may have low yield strength and machinability
in some cases.
[00 r 7]
The steel for fracture splitting connecting rods disclosed in Patent Literature2
has high yield strength, However, it may have low fracture splittability in some
cases.
[00 r 8]
Furthermore, production conditions for hot forging, e.g., the heating
temperature prior to hot forging, may vary from production site to production site.
If a fracture splitting connecting rod is produced using any of the steel materials and
the production methods disclosed in Patent Literatures 1 to 6 with the heating
temperatures prior to hot forging being non-uniform, the fracture splitting connecting
rod, in some cases, has a low fracture splittability, low yield strength, or low
machinability.
SUMMARY OF INVENTION
[001e]
An object of the present invention is to provide a rolled steel material for
fracture splitting connecting rods which has high fracture splittability, high yield
strength and high machinability after hot forging even if the heating temperatures for
the hot forging are non-uniform.
[0020]
A rolled steel material for fracture splitting connecting rods according to the
present embodiment has a chemical composition consisting of, in mass %o, C: 0.30 to
0.40Yo, Si: 0.60 to 1 .00%, Mn: 0,50 to l.00Yo, P: 0.04 to 0.07Yo, S: 0.04 to 0.130/0,
Cr: 0.l0 to 0.30%, V: 0.05 to 0.14Yo, Ti: more than 0.1 5Yo Io 0.20%o or less, N: 0.002
to 0.020%o, Cu: 0 to 0.40yo, Nii 0 to 0.30V0, Mo: 0 to 0.10Yo, Pb: 0 to 0.30%o, Te: 0 to
0.30yo, Ca: 0 to 0.010%, and Bi: 0 to 0.30%, the balance being Fe and impurities,
Ç
wherein fn1, defined by Formula (1), ranges from 0.65 to 0.80. Relative to the V
content in the rolled steel material for fracture splitting connecting rods, a V content
in coarse precipitates having a particle size of 200 nm or more is 7 }Yo or less.
Relative to the Ti content in the rolled steel material for fracture splitting connecting
rods, a Ti content in the coarse precipitates is 50% or more.
fnl : C + Si/l0 + Mn/5 + 5Crl22 +(Cu+Niy20 +Molz+33V120 - ssll ...
Formula (1)
where each element symbol in Formula (l) is substituted by the content
(mass %) of a corresponding element or is substituted by "0" in a case where the
corresponding element is not present.
[0021]
The rolled steel material for fracture splitting connecting rods according to the
present embodiment exhibits high fracture splittability, high yield strength and high
machinability after hot forging even if the heating temperatures for the hot forging
are non-uniform.
BRIEF DESCRIPTION OF DRAWINGS
100221
[FIG. 1] FIG. 1 is a side view of a conventional connecting rod.
DESCRIPTION OF EMBODIMENTS
[0023]
A rolled steel material for fracture splitting connecting rods according to the
present embodiment has a chemical composition consisting of, in mass Vo, C: 0.30 to
0.40y0, Si: 0.60 to 1.00%, Mn: 0.50 to 1.00%, P: 0.04 to 0.07%o, S: 0.04 to 0.13o/o,
Cr: 0.10 to 0.30%o, V: 0.05 to 0.14%0, Ti: more than 0.15o/oto 0.20%o or less, N: 0.002
to 0.0200/o, Cu: 0 to 0.400/0, Ni: 0 to 0.30Yo, Mo: 0 to 0.10Yo, Pb: 0 to 0.30yo, Te: 0 to
0.30y0, Ca: 0 to 0.010%, and Bi: 0 to 0.30%, the balance being Fe and impurities,
wherein fn1, defined by Formula (l), ranges from 0.65 to 0.80. Relative to the V
content in the rolled steel material for fracture splitting connecting rods, a V content
in coarse precipitates having a particle size of 200 nm or more is 700lo or less.
+
Relative to the Ti content in the rolled steel material for fracture splitting connecting
rods, a Ti content in the coarse precipitates is 50% or more.
fnl : c + si/l0 + Mn/5 + 5crl22+ (cu +Niy20 + Molz + 33V120 - 5511 ...
Formula (1)
where each element symbol in Formula (1) is substituted by the content
(mass %) of the corresponding element or is substituted by "0" in the case where the
corresponding element is not present.
[0024]
In the rolled steel material for fracture splitting connecting rods according to
the present embodiment, fn 1 , which is defined by Fonnula ( I ), is within the range of
0.65 to 0.80. As a result, excellent yield strength and machinability are achieved.
[002s]
Furthermore, relative to the V content in the rolled steel material for fracture
splitting connecting rods, a V content in coarse precipitates having a particle size of
200 nm or more is70% or less. In such a case, fine V precipitates (V-containing
precipitates) having a particle size of less than 200 nm are present in large amounts
in the rolled steel material for fracture splitting connecting rods. Fine V precipitates
readily dissolve during heating in the hot forging process. Thus, even if the heating
temperature in the hot forging process is low (e.g., approximately 1000'C), V readily
dissolves by heating. The dissolved V precipitates as carbides in the cooling
process of the hot forging. As a result, the hot forged steel material exhibits
consistently excellent yield strength even if the heating temperatures in the hot
forging process are non-uniform.
[0026]
Furthermore, relative to the Ti content in the rolled steel material for fracture
splitting connecting rods, a Ti content in the coarse precipitates is 50% or more. In
the present embodiment, Ti forms sulfides and carbo-sulfides to increase the
machinability of the steel. Furthermore, Ti partially dissolves in the steel during
heating in the hot forging process. The dissolved Ti forms carbides dr"rring
subsequent cooling to embrittle the ferrite and thereby increase the fracture
splittability. However, if Ti dissolves in excessive amounts during heating in the
hot forging process, the steel material after being cooled will have a bainite structure.
8
This results in a decrease in the fracture splittabilify. In addition, if Ti dissolves in
excessive amounts, the steel material will have excessively high tensile strength and
therefore have decreased machinability. Thus, it is preferred that excessive
dissolution of the Ti precipitates (Ti-containing precipitates) during heating in the hot
forging process be inhibited. When the relative Ti content in the coarse precipitates
is not less than 50Y0, fine Ti precipitates are present in the steel in sufficiently small
amounts. As a result, even if the heating temperature in the hot forginþ process is
high (e.g., 1280'C), the Ti precipitates do not readily dissolve (i.e., Ti does not
readily dissolve) and therefore decreases in fracture splittability and machinability
are inhibited.
[00271
As a result of the above, the rolled steel material for fracture splitting
connecting rods according to the present embodiment exhibits high fracture
splittability, high yield strength and.high machinability after hot forging even if the
heating temperatures for the hot forging are non-uniform.
[0028]
The chemical composition mentioned above may contain one or more
selected from the group consisting of, Cu: 0.01 to 0.40%o, Ni: 0.01 to 0.300/o, and Mo:
0,01 to 0.10%. Furthermore, the chemical composition mentioned above may
contain one or more selected from the group consisting of, Pb: 0.05 to 0.30%o,Te:
0.0003 to 0.30Yo, Ca: 0.0003 to 0.010V0, and Bi: 0.0003 to 0.30%o.
[002e]
A rolled steel material for fracture splitting connecting rods according to the
present embodiment will be described in detail below. "Percent" used for the
contents of the elements means "mass percent".
[0030]
[Chemical Composition]
The chemical composition of the rolled steel material for fracture splitting
connecting rods according to the present embodiment contains the following
elements.
[0031]
C: 0.30 to 0.40Yo
I
Carbon (C) increases the strength of the steel, If the C content is too low,
this advantageous effect cannot be produced. On the other hand, ifthe C content is
too high, the hardness of the steelmaterialwill increase, which willresult in a
decrease in machinability. Accordingly, the C content ranges from 0.30 fo 0.40%.
The lower limit of the C content is preferably more than 0.30yo, more preferably
0.31%o, and even more preferably 032%. The upper limit of the C content is
preferably less than 0.40y0, more preferably 0.39Y0, and even more preferably 0.38%.
[0032]
Si: 0.60 to 1.00%
Silicon (Si) deoxidizes the steel, In addition, Si dissolves in the steel and
thereby increases the strength of the steel. If the Si content is too low, this
advantageous effect cannot be produced. On the other hand, ifthe Si content is too
high, the above advantageous effects reach saturation, ln addition, ifthe Si content
is too high, the hot workability of the steel will decrease and the cost of producing
the steel material will increase. Accordingly, the Si content ranges from 0.60 to
1.00%. The lower limit of the Si content is preferably more than 0.60%0, more
preferably 0.62Vo, and even more preferably 0.65Vo. The upper limit of the Si
content is preferably less than l.00Vo, more preferably 0.95Y0, and even more
preferably 0.90%.
[0033]
Mn: 0.50 fo 1.00Yo
Manganese (Mn) deoxidizes the steel. In addition, Mn increases the strength
of the steel. If the Mn content is too low, these advantageous effects cannot be
produced. On the other hand, if the Mn content is too high, the hot workability of
the steel will decrease. In addition, if the Mn content is too high, the hardenability
will increase and bainite will form in the structure of the steel. This results in a
decrease in the fracture splittability of the steel. Accordingly, the Mn content
ranges from 0.50 to 1.00%. The lower limit of the Mn content is preferably more
tlran 0.50%, more preferably 0.60Yo, and even more preferably 0.65%. The upper
limit of the Mn content is preferably less than 1.00yo, more preferably 0.95%0, and
even more preferably 0.90%.
[0034]
to
P: 0.04 Io 0.01%o
Phosphorus (P) segregates at the grain boundaries and embriftles the steel.
As a result, the fracture surfaces of the fracture splitting connecting rod after being
fractured and split are smooth. This results in increased accuracy in assembling the
fracture splitting connecting rod after being fractured and split. If the P content is
too low, this advantageous effect cannot be produced. On the other hand, if the P
content is too high, the hot workability of the steel will decrease. Accôrdingly, the
P content ranges from 0.04 to 0.07Y0. The lower limit of the P content is preferably
more than 0.04yo, more preferably 0.042Yu and even more preferably 0.045%. The
upper limit of the P content is preferably less than 0.070/0, more preferably 0.0680/o,
and even more preferably 0.065%.
[003s]
S: 0.04 to 0.13%o
Sulfur (S) combines with Mn and Ti to form sulfides and thereby increases
the machinability of the steel. If the S content is too low, this advantageous effect
cannot be produced. On the other hand, ifthe S content is too high, the hot
workability of the steel will decrease. Accordingly, the S content ranges from 0.04
to 0.130/0. The lower limit of the S content is preferably more than 0.04%0, more
preferably 0.0450/0, and even more preferably 0.05%. The upper limit of the S
content is preferably less than 0.13yo, more preferably 0.125%, and even more
preferably 0.12%.
[0036]
Cr: 0.10 to 0.30%
Chromium (Cr) increases the strength of the steel. If the Cr content is too
low, this advantageous effect cannot be produced. On the other hand, if the Cr
content is too high, the hardenability of the steel will increase and bainite will form
in the structure of the steel. This results in a decrease in the fracture splittability of
the steel. In addition, if the Cr content is too high, the production cost will increase.
Accordingly, the Cr content ranges from 0.10 to 0.30%. The lower limit of the Cr
content is preferably more than 0.l}yo, more preferably 0,llYo, and even more
preferably 0.12%. The upper limit of the Cr content is preferably less than 0.30%0,
more preferably 0.25%o, and even more preferably 0.20%o.
lt
/
[0037]
V: 0.05 r.o 0.14Yo
Vanadium (V) precipitates in the ferrite as carbides in the cooling process
after hot forging and thereby increases the yield strength of the steel. In addition,
V, when included together with Ti, increases the fracture splittability of the steel. If
the V content is too low, these advantageous effects cannot be produced. On the
other hand, if the V content is too high, the cost of producing the steel will extremely
increase, and in addition, the machinability will decrease. Accordingly, the V
content ranges from 0.05 to 0.140/o. The lower limit of the V content is preferably
more than 0.05y0, more preferably 0.060/o, and even more preferably 0.07%. The
upper lirnit of the V content is preferably less than 0.14yo, more preferably 0.13%0,
and even more preferably less than0.l3Yo.
[0038]
Ti: more than 0.15%oto 0.20p/o or less
Titanium (Ti) precipitates as carbides or nitrides in the steel and thereby
increases the strength of the steel. In addition, Ti forms sulfides or carbo-sulfides
and thereby increases the machinability of the steel.
[003e]
When the rolled steel material for fracture splitting connecting rods is heated
prior to hot forging, part of Ti in the Ti sulfides and Ti carbo-sulfides dissolves,
Furthermore, when the steel material is allowed to cool in air after hot forging, the
part of Ti remains dissolved until the ferrite transformation begins. When the
ferrite transformation has begun, the dissolved Ti precipitates together with V in the
ferrite as carbides and thereby increases the yield strength and tensile strength ofthe
steel. In addition, the Ti carbides, which formed during the ferrite transformation,
embrittles the ferrite to increase the fracture splittability of the steel. If the Ti
content is too low, these advantageous effects cannot be produced. On the other
hand, if the Ti content is too high, excessive amounts of Ti will dissolve prior to hot
forging. In such a case, the hardenabilify of the steel will increase and bainite will
form therein. Furthermore, an excessively large number of Ti carbides will
precipitate, which will result in an excessively high tensile strength. This results in
a decrease in the machinabilify of the steel. Accordingly, the Ti content ranges
IL
from more than 0.15%oto 0.20% or less. The upper limit of the Ti content is
preferably less than 0.200/0, and more preferably 0.19%.
[0040]
N: 0.002 fo 0.0200/o
Nitrogen (N) combines with Ti to form nitrides and thereby increases the
strength of the steel. If the N content is too low, this advantageous effect cannot be
produced. On the other hand, ifthe N content is too high, this advantageous effect
reaches saturation. Accordingly, the N content ranges from 0.002 to 0.020Yo. The
lower limit of the N content is preferably more than 0.002yo, more preferably
0.003yo, and even more preferably 0.004%. The upper limit of the N content is
preferably less than 0.020Yo, more preferably 0.019%, and even more preferably
0.018%.
[0041]
The balance of the chemical-composition of the rolled steel material for
fracture splitting connecting rods according to the present embodiment is made up of
Fe and impurities. Herein, the irnpurities refers to impurities that are incidentally
included in the steel material, during its industrial production, from raw materials
such as ores and scrap or from the production environment for example, and which
are allowable within arange that does not adversely affect the steel material of the
present embodiment.
100421
The chemical composition of the rolled steel material for fracture splitting
connecting rods according to the present embodiment may further contain, as a
partial replacement for Fe, one or more selected from the group consisting of Cu, Ni,
and Mo. These elements are optional elements and each increase the strength of the
steel,
[0043]
Cu: 0 to 0.40%
Copper (Cu) is an optional element and may not be contained. When
contained, Cu dissolves in the steel and thereby increases the strength of the steel.
However, if the CLr content is too high, the cost of producing the steel will increase,
and in addition, the machinability will decrease. Accordingly, the Cu content
r3
yI
ranges from 0 to 0.400/o. The lower limit of the Cu content is preferably 0.01yo,
more preferably 0.05V0, and even more preferably 0.10%. The upper limit of the
Cu content is preferably less than 0.40y0, more preferably 0.35%o, and even more
preferably 0.30%.
[0044]
Ni: 0 to 0.30%
Nickel (Ni) is an optional element and may not be contained. When
contained, Ni dissolves in the steel and thereby increases the strength of the steel.
However, if the Ni content is too high, the production cost will increase, and in
addition, the Charpy impact value will increase and thus the fracture splittability will
decrease. Accordingly, the Ni content ranges from 0 to 0.30-%. The lower limit of
the Ni content is preferably 0.01y0, more preferably 0.02Yo, and even more
preferably 0.05%. The upper limit of the Ni content is preferably less than 0.300/0,
more preferably 0.28%o, and even rnore preferably 0.25%.
[004s]
Mo: 0 to 0.10%
Molybdenum (Mo) is an optional element and may not be contained. When
contained, Mo dissolves in the steel and thereby increases the strength of the steel,
In addition, Mo forms carbides in the steel and thereby increases the strength of the
steel. However, if the Mo content is too high, the hardenabilify will increase and
bainite will form after hot forging. This results in a decrease in the fracture
splittability of the steel. Accordingly, the Mo content ranges from 0 to 0.10%.
The lower limit of the Mo content is preferably 0.0I%. The upper limit of the Mo
content is preferably less than 0.l}yo, more preferably 0.09%o, and even more
preferably 0.08%.
[0046]
The chemical composition of the rolled steel material for fracture splitting
connecting rods according to the present embodiment may further contain, as a
partial replacement for Fe, one or more selected from the group consisting of Pb, Te,
Ca, and Bi. These elements are optional elements and each increase the
machinability of the steel.
[0047]
f?
Pb: 0 to 0.30%
Lead (Pb) is an optional element and may not be contained. When
contained, Pb increases the machinability of the steel. However, if the Pb content is
too high, the hot workability of the steel will decrease. Accordingly, the Pb content
ranges from 0 to 0.30Vo. The lower limit of the Pb content is preferably 0.050/o, and
more preferably 0.10%o. The upper limit of the Pb content is preferably less than
0.30yo, more preferably 0.25%0, and even more preferably 0.20%.
[0048]
Te: 0 to 0.30%
Tellurium (Te) is an optional element and may not be contained, When
contained, Te increases the machinability of the steel. However, if the Te content is
too high, the hot workability of the steel will decrease. Accordingly, the Te content
ranges from 0 to 0.30%. The lower limit of the Te content is preferably 0.0003%,
more preferably 0.0005%o, and even more preferably 0.0010%. The upper limit of
the Te content is preferably less than 0.30yo, more preferably 0,25Y0, and even more
preferably 0.20%.
[004e]
Ca: 0 to 0.010%
Calcium (Ca) is an optional element and may not be contained. When
contained, Ca increases the machinability of the steel. However, if the Ca content is
too high, the hot workability of the steel will decrease. Accordingly, the Ca content
ranges from 0 to 0.010%. The lower limit of the Ca content is preferably 0.0003%,
more preferably 0.0005%o, and even more preferably 0.0010%, The upper limit of
the Ca content is preferably less than 0.010Vo, more preferably 0.008%, and even
more preferably 0.005%.
Bi: 0 to 030%
Bismuth (Bi) is an optional element and may not be contained. When
contained, Bi increases the machinability of the steel. However, if the Bi content is
too high, the hot workability of the steel will decrease. Accordingly, the Bi content
ranges from 0 to 0.30Yo. The lower limit of the Bi content is preferably 0.0003%,
more preferably 0.0005%o, and even more preferably 0.0010%. The upper limit of
t5
the Bi content is preferably less than 0.30yo, more preferably 0.20Yo, and even more
preferably 0.10%.
[00s0]
[Formula (l)]
Furlhennore, in the chemical composition of the steel material of the present
embodiment, fn1, which is defined by Formula (1), ranges from 0.65 to 0.80.
fnl : c + si/l0 + Mf/5 + 5crl22+ (cu +Niy20 +Mol2 + 33V120 - 5517,..
(1)
The element symbols in Formula (1) are each substituted by the content
(mass %) of the corresponding element. In the case where the element
corresponding to the element symbol in Formula (1) is not present, the element
symbol is substituted by "0".
[00s 1]
There is a positive correlation between fnl and the tensile strength of the steel
after being hot forged. If fnl is more than 0.80, the steel will have excessively high
tensile strength and therefore decreased machinability. Furthermore, there is also a
positive correlation befween fnl and the yield strength of the steel. Thus, if fnl is
less than 0.65, the steel willhave decreased strength. When fn1 is 0.65 to 0.80, the
steel exhibits excellent strength and machinability. The lower limit of fn1 is
preferably more than 0.65, more preferably 0.66, and even more preferably 0.67.
The upper limit of fnl is preferably less than 0.80, more preferably 0.79, and even
more preferably 0.78.
[00s2]
[V Content and Ti content in Precipitates]
Furthermore, according to the present embodiment, relative to the V content
in the rolled steel material for fracture splitting connecting rods, a V content in
coarse precipitates having a particle size of 200 nm or more is 70% or less.
Fufthermore, relative to the Ti content in the rolled steel material for fracture
splitting connecting rods, a Ti content in the coarse precipitates is 50% or more.
This will be described in detail below.
[00s3]
[V Content in Precipitates]
It
In the present embodiment, V precipitates as carbides. More specifically, V
dissolves in the heating step prior to hot forging, and then, during cooling after hot
forging, it precipitates as carbides at the austenite-ferrite interphase boundaries under
phase transformation (interphase boundary precipitation). The interphase boundary
precipitation of V carbides results in increased yield strength of the hot forged steel
material. In order to produce this effect, it is preferred that V dissolve in the
austenite in the steel material prior to hot forging.
[00s4]
An effective way to promote the dissolution of V-containing precipitates
(hereinafter referred to as V precipitates) is to refine the V precipitates prior to hot
forging to increase the total surface area of the V precipitates. That is, fineness of
the V precipitates in the rolled steel material for fracture splitting connecting rods
assists in dissolution of V, This is because, when the V precipitates are fine and
have a large totalsurface area, sufficient amounts of V dissolve in the austenite
during heating, even if the heating temperature for hot forging is low (e.g., 1000'C).
[00ss]
The V content in the entire rolled steel material for fracture splitting
connecting rods is denoted as Vm (mass %) and the V content in coarse precipitates
in the entire steel material is denoted as Vp (mass %). Here, when a V fraction Rv,
which is defined by Formula (2), is not more than 70yo,V precipitates in the rolled
steel material for fracture splitting connecting rods are sufficiently fine. As a result,
suffìcient amounts of V dissolve during heating for hot forging. As a result, fine V
carbides precipitate in the cooling process after hot forging, which results in high
strength of the hot forged steel material,
Rv: Vp/Vm x 100 (2)
[00s6]
Vm and Vp are measured in the following manner. A cylindrical specimen
of 8 mm diameter and 12 mm length is obtained from any one of R/2 regions of the
rolled steel material for fracture splitting connecting rods in round bar form (R/2
region refers to a region, in the cross section of the steel material, including a point
that bisects the length between the central axis of the steel material and the outer
t+
peripheral surface of the steel material), The length of the cylindrical specimen is
parallel to the axial direction of the steel material.
[00s7]
Using the cylindrical specimen, extraction residue analysis by an electrolytic
process is carried out. Specifically, the outer layer of the cylindrical specimen is
removed from the surface to a depth of 200 pm by adjusting the electrolysis time
while maintaining a constant current. This removes impurities that have deposited
on the surface of the cylindrical specimen. After the surface layer has been
removed, the electrolyte solution is replaced with a new electrolyte solution, Both
electrolyte solutions are AA type electrolyte solutions (electrolyte solutions
containing 10 vol% acetyl acetone and 1 vol%io tetrarnethylammonium chloride with
the balance being methanol).
[00s8]
Using the new electrolyte solution, electrolysis is perforrned on the cylindrical
specimen. In the electrolysis, while the current is maintained constant at 1000 mA,
the electrolysis time is adjusted so that the cylindrical specimen, subjected to the
electrolysis, has a volume of 0.5 cm3. The electrolyte solution after the electrolysis
is filtered through a filter having a mesh size of 200 nm to obtain the residue. The
obtained residue corresponds to the coarse precipitates.
[00se]
Inductively coupled plasma (lCP) emission spectroscopy is performed on the
obtained residue to determine Yp (%), the V content in the coarse precipitates.
Specifically, Vp is determined by the following formula.
Vp: V content(mg) in coarse precipitates in 0.5 cm3 steel material/mass(mg)
of 0.5 cm3 steelmaterial x 100
[0060]
The V content in the rolled steel material for fracture splitting connecting rods
is measured in the following manner using the cylindrical specimen after being
subjected to the electrolysis. Machined chips are obtained from the cylindrical
specimen. The machined chips can be obtained by machining the cylindrical
specimen with a lathe, for example. ICP emission spectroscopy is performed on the
(8
,f
machined chips to determine the V content Vm(%). Using the determined Vp and
Vm, the V fraction Rv(%) is determined by Formula (2).
[0061 ]
[Ti Content in Precipitates]
In the present embodiment, Ti precipitates as Ti carbides or Ti nitrides and Ti
sulfides or Ti carbo-sulfides. Ti sulfrdes and Ti carbo-sulfldes increase the fracture
splittability of the steel material. However, if excessive amounts of Ti sulfides and
Ti carbo-sulfides dissolve during heating for hot forging, the amount of Ti dissolved
in the austenite increases, and this is not preferred. If the heating temperature for
hot forging is high (e.g., 1280'C) and excessive amounts of Ti dissolve in the
austenite, Ti carbides precipitate in excessive amounts in the cooling process after
hot forging. This results in excessively high strength of the hot forged steel
material and therefore a decrease in the rnachinability thereof.
[0062]
Furthermore, if the amount of dissolved Ti in the austenite is excessive,
bainite will form during cooling. Bainite increases the Charpy impact value of the
steel material excessively. This results in a decrease in the fracture splittability of
the steel material.
[0063]
Thus, it is preferred that Ti sulfides and Ti carbo-sulfides do not dissolve in
large amounts during heating for hot forging. An effective way to inhibit an
excessive dissolution of Ti is to coarsen Ti-containing precipitates (hereinafter
referred to as Ti precipitates) prior to hot forging to reduce the surface area of the Ti
precipitates. This is because, when Ti precipitates are coarse and their total surface
area is small, Ti does not readily dissolve in the austenite during heating even if the
heating temperature for hot forging is high (e.g., 1280'C).
[0064]
The Ti content in the rolled steel material for fracture splitting connecting
rods is denoted as Tim (%) and the Ti content in the coarse precipitates is denoted as
Tip (%). Here, when a Ti fraction Rti, which is defined by Formula (3), is not less
than 50Yo, the Ti precipitates in the rolled steel material for fracfure splitting
connecting rods are sufficiently coarse. As a result, an excessive dissolution of Ti
t1
during heating for hot forging can be sufficiently inhibited. As a result, the hot
forged steel material exhibits high machinability and fracture splittability.
Rti: Tip/Tim x 100 (3)
[006s]
Tim and Tip are measured in the following manner. A cylindrical specimen
is obtained in the same manner as that for the case of determining Vm and Vp.
Then, electrolysis is performed under the same conditions as those for the case of
determining Vm and Vp to thereby obtain the residue (coarse precipitates). ICP
emission spectroscopy is performed on the residue under the same conditions as
those for the case of determining Vp to determine Tip (%), the Ti content in the
coarse precipitates. Specifically, Tip is determined by the following formula.
Tip : Ti content (mg) in coarse precipitates in 0.5 cm3 steel material/mass
(mg) of 0.5 cm3 steel material x 100
[0066]
Furthermore, machined chips are obtained in the same manner as that for the
case of determining Vm. ICP emission spectroscopy is performed on the obtained
machined chips under the same conditions as those for the case of determining Vm to
determine Tim (%), the Ti content in the steel material. The Ti fraction Rti (%) is
determined by Forrnula (3) using the determined Tip and Tim.
[0067]
The Ti fraction Rti is preferably more than 500/0, more preferably not less than
600/o,and even more preferably not less ThanT0o/o.
[0068]
[Production Method]
Described below is an exemplary method for producing the above-described
rolled steel material for fracture splitting connecting rods.
[006e]
A molten steel having the chemical composition mentioned above is produced
by a well-known method. The produced molten steel is subjected to continuous
casting to produce a continuously cast material (slab or bloom). The molten steel
may be subjected to an ingot-making process to produce an ingot. A billet may be
produced by continuous casting.
LO
,y
[0070]
The produced continuously cast material or ingot is subjected to hot working
to produce a billet. The hot working is, for example, hot rolling. The hot rolling is
carried out using, for example, a billeting machine and a continuous rolling mill in
which a plurality of stands are arranged in a line.
[007 r ]
A steel bar (rolled steel material for fracture splitting connecting rods) is
produced from the billet. Specifically, the billet is heated in a reheating furnace
(heating step). After being heated, the billet is hot rolled using a continuous mill to
be formed into a rolled steel material for fracture splitting connecting rods in bar
form (hot rolling step). These steps will be described below.
100121
[Heating Step]
In the heating step, the billet is heated to 1 000 to 1 I 00oC. If the heating
temperature, Tf, is too low, V precipitates in the billet do not readily dissolve. As a
result, coarse V precipitates that were present in the billet are retained even after hot
rolling, resulting in large amounts of coarse V precipitates in the hot rolled steel
material. As a result, the V fraction Rv will exceed 70Yo. Furthermore, if the
heating temperature Tf is too low, Ti precipitates do not agglomerate and grow
during heating and therefore do not readily become coarse. As a result, in the rolled
steel material, coarse Ti precipitates will be present in small amounts, and therefore
the Ti fraction Rtiwill fall below 50%.
[0073]
When the heating temperature Tf is increased, Ti precipitates agglomerate and
grow. However, if the heating temperature Tf is excessively high, excessive
amounts of Ti precipitates will dissolve during heating. The dissolved Ti finely
precipitates as carbides during rolling or during cooling. As a result, the Ti fraction
Rtiwill fallbelow 50%.
10074)
When the heating temperature Tf ranges from 1000 to 1 100oC, V precipitates
dissolve suitably and the Ti precipitates agglomerate and grow during heating to
become coarse. When the below-described conditions for hot rolling step are also
LI
/
satisfied, the rolled steel material for fracture splitting connecting rods, after being
rolled, have the V fraction Rv of not more than 70%o and the Ti fraction Rti of not
less than 50%.
[007s]
[Hot Rolling Step]
The heated billet is hot rolled using a continuous mill to produce the rolled
steel material for fracture splitting connecting rods.
[0076]
The continuous mill includes a plurality of sets of rolls. Each set of rolls
includes a pair of rolls or three or more rolls disposed around the rolling axis (pass
line). The rolling axis means a line along which the billet to be rolled is passed.
The plurality of sets of rolls are arranged in a line. Each set of rolls is
accommodated in a corresponding stand.
100111
ln the hot rolling step, the rolling rate, Vr, ranges from 5 to 20 m/second.
The rolling rate Vr is defined as follows, A time t0 (second) is measured, which is
a length of time from when the leading end of the billet is rolled by the first set of
rolls, among the plurality of sets of rolls of the continuous mill, to when it is rolled
by the last set of rolls among the sets to be used for the rolling. The time t0 can be
measured by finding the load applied to the first rolls and the load applied to the last
rolls. The rolling rate Vr (m/second) is determined by Formula (4) using the time
t0.
Vr = distance along the rolling axis from the center of the first set of rolls to
the center of the last set of rolls/tO (4).
[0078]
In short, the rolling rate Vr means a rolling rate throughout the hot rolling. If
the rolling rate Vr is too slow, work-induced heat due to hot rolling is less likely to
occur. As a result, during the rolling, the temperature of the workpiece decreases.
In such a case, Ti precipitates do not readily agglomerate and grow during the
rolling. Consequently, the Ti fraction Rti will fall below 50%.
[007e]
92
On the other hand, if the rolling rate Vr is too fast, excessive work-induced
heat is more likely to occur in the workpiece being rolled. In such a case, V
carbides that precipitate during rolling will be coarser. As a result, large amounts of
coarse V precipitates will form. Consequently, the V fraction Rv will exceed'70%o.
[0080]
Furthermore, water cooling is performed for 1 to 3 seconds on the workpiece
being rolled at a reduction ofarea of 50 to 7\Yo. The reduction ofarea is defined as
follows. A cross-sectional area A0 (mm2) of the starting material, i.e., the billet, for
the hot rolling process (the area ofthe cross section perpendicular to the central axis
of the billet) is determined. Next, a cross-sectional area Al (mm2) of the workpiece
after having been passed through a selected one ofthe sets of-rolls in the continuous
mill is determined. The cross-sectional area A1 can be calculated from the groove
of the selected one of the sets of rolls. Alternatively, the cross-sectional area Al
rnay be detennined by actually rolling the workpiece through the selected one of the
sets of rolls.
The reduction of area (%) is determined by Formula (5) using A0 and 41.
Reduction of area: (40 - A1)/40 x 100 (5)
[0081]
Water cooling is performed for I to 3 seconds on the workpiece being rolled,
at a location where the reduction of area reaches 50 to 7 0%. For example, water
cooling equipment (water cooling zone) is provided between sets of rolls (between
stands) where the reduction of area reaches 50 to 7ÙYo. The workpiece is water
cooled when it is being passed through the water cooling equipment. The amount
of water for the water cooling is 100 to 300 liters/second.
[0082]
If the water cooling time, tw, is too short, the temperature of the workpiece
will become excessively high because of work-induced heat. In such a case, V
carbides that precipitate during rolling will be coarser. As a result, large amounts of
coarse V precipitates will form. Consequently, the V fraction Rv will exceed 70%0.
[0083]
On the other hand, if the water cooling time tw is too long, the temperature of
the workpiece will become excessively low. In such a case, Ti precipitates do not
2_g
agglomerate and grow during the rolling and therefore not readily become coarse.
Consequently, the Ti fraction Rti will fall below 50%.
[0084]
When the heating temperature Tf, rolling rate Vr, and water cooling time tw
fall within the ranges described above, the steel material after being rolled has the V
fraction Rv of not more than ljYo and the Ti fraction Rti of not less than 50%.
[008s]
[Connecting Rod Production Step]
Described below is an exemplary method for producing a fracture splitting
connecting rod from the rolled steel material for fracture splitting connecting rods.
Firstly, the steel material is heated in a reheating furnace. The heated steel material
is subjected to hot forging to produce a fracture splitting connecting rod.
Preferably, the degree of deformation in the hot forging is not less than 0.22.
Herein, the degree of deformation is the value of the maximum logarithmic strain
that occurs in the material excluding flash in the forging process.
[0086]
The hot forged fracture splitting connecting rod is allowed to cool to room
temperature. The fracture splitting connecting rod after cooling is subjected, as
necessary, to machining. Through the steps described above, the fracture splitting
connecting rod is produced.
[0087]
When the rolled steel material for fracture splitting connecting rods of the
present embodiment is employed, the resulting fracture splitting connecting rod
exhibits excellent fracture splittability, excellent machinabilify, and excellent yield
strength as long as the heating temperature for hot forging is within the range of 1000
to 1280'C,
EXAMPLES
[0088]
A molten steel having the chemical composition shown in Table 1 was
produced.
[008e]
L+
[Table 1]
TABLE 1
L5
Steel
A
B
C
C
0.31
D
0.38
E
Si
0.32
F
0.6s
0.34
G
0.6 r
0.38
Mn
H
0.71
0.36
I
0.73
0.9s
0.36
J
0.62
0.37
0.78
K
P
0.86
0.37
0.6 r
L
0.0s
0.83
0.60
0.37
Chemical composition (in mass %o, the balance being Fe and impurities)
M
0.05
0.84
0.6
0.31
N
S
0.05
0.74
0.34
0.6
0.096
o
0.0'7
0.75
0.32
0.6
0.1 l8
0.0s
P
0.76
0.7
0.36
Cr
o
0.090
0.05
0.7s
0.35
0.9s
0
0.098
R
0.05
0.62
0.78
0.35
5
0.
0.088
S
0.05
0.86
0.65
0.36
7
0
T
0.101
0.05
0.83
0.1 08
0.39
0.63
7
0.
0.095
U
0.05
0.84
0.1 08
0.3 r
0.63
0
4
0.098
0.0s
0.74
0.1 l8
Ti
0.64
0.37
0.20
I
w
0.092
0.07
0.7s
0.074
0
0.73
0.33
0.19
0.118
X
0.05
0.7s
0.128
70
0
0.34
0.67
0.18
0.090
Y
0.05
0.74
66
0.098
0
0.88
N
* 0.41
0.38
0.18
0.098
Z
0.05
0.86
55
0.005
0
0.1 00
0.69
AA
0.17
0.088
0.05
0.58
75
0.003
0
0.72
0.099
0.38
AB
0.t7
0
0.05
0.88
68
Cu
0.098
0
0.78
0.006
0.35
01
0.14
0
0.05
l) Symbol "*" indicates that the value falls outside the range specified by the present embodiment.
90
0.78
0.1 08
0.012
0
0.64
* 0.70
0.32
03
0.1 I
0
0.07
0.65
88
0.118
0.
0.62
0.013
05
0.20
0
0.07
Ni
0.72
89
0
0.074
0.009
0.88
0l
0.19
0.086
0.06
0.78
92
0
0.008
0.128
* 0.20
0.74
0.20
0.1 r4
0.05
0.72
58
0.098
0.006
0
0.19
0.1 09
Mo
0.06
0.76
65
0.100
0
0.008
0.15
0.1 02
0.05
0.74
8s
0.003
0
0.101
0.12
0.099
0.07
0.s3
68
0.1 00
0.006
0.
* 0.045
0.18
Pb
0.110
0.06
75
0
0.012
0.15
0.092
* 0.01
0.07
78
0.013
0.112
0.20
0
0
0.088
.14
0.1 28
77
0.
Te
0.009
0.10
0.29
0.17
0.102
11
0
0.1 03
0.0r r
0.22
0.1 l6
70
0
0.072
0. l0
0.38
0.010
0
0.060
52
Ca
0.010
0.
0.1 l9
0.25
0.20
+0
3
0.
63
0.092
0.004
0.08
0.24
-)
0
0.21
38
0.03
0.003
0
0.28
0.096
0
* 0.045
8
0
.25
Bi
98
0.004
0.06
0
0.
0.25
2
0.r0
70
0.066
0.003
5
0
0.
*0
+ 0.029
58
fnl
0.02
0.002
6
0
0.23
0.06
32
0
6
0.004
0.
0.66
74
0.0s
0.006
0
5
*
0.70
63
0.0s
0.10
0.003
0.73
0.06
0.014
0.25
0.003
0.20
0.68
0.012
0.20
0.80
0.08
0.015
0.29
2Ê
0.70
0.1 5
0.02
0.04
n
0.09
0.25
;tI
0.23
0.72
0.02
0.20
0.09
0.72
0.06
0.06
0.72
0.04
0.20
0.78
0.06
0.004
0.06
0.74
0.10
+ 0.19
0.78
0.76
0.02
0.74
0.'/5
0.76
0.68
+ 0.62 * 0.81
0.69
* 0.64
* 0.81
0.80
0.77
0.68
0.73
+ 0.87
[00e0]
With reference to Table 1, Steels A to Q each had an appropriate chemical
composition and their fn1s, defined by Formula (1), were within the range of 0.65 to
0.80. On the other hand, as for Steels R to AB, either an element content in the
chemical composition or fn1 was inappropriate. The chemical cornposition of Steel
AB was within the range of the chemical composition of the steel disclosed in Patent
Literature 1.
[00e 1]
Steels A and B were produced in a70 ton converter and Steels C to AB were
produced in a 3 ton laboratory furnace. A bloom or an ingot was produced from the
produced molten steels. The produced bloom or ingot was subjected to billeting to
produce billets. The temperature to which the steel material was heated for billeting
was 1 100oC. The cross section of the billet (cross section perpendicular to the axial
direction of the billeÐ had a rectangular shape of 180 mm x 180 mm. The steel
grade of the billet used in each number of test was as shown in the "starting material"
column in Table 2,
[00e2]
The billets were subjected to hot rolling using a continuous mill to produce
rolled steel materials for fracture splitting connecting rods of Test Nos. I to 42. For
the production, the heating temperatures Tf, rolling rates Vr, and water cooling times
fw were as shown in Table 2. Water cooling was applied to the workpiece (billet)
when the reduction of area reached 65Yo. The amount of water was 200
liters/second.
[00e3]
fTable 2]
TABLE 2
L+
,/
I ) Symbol "#" indicates that the chemical composition falls outside the range specifìed by the present
embodiment.
2) Symbol "*" indicates that the value falls outside the range specified by the present embodiment.
Test
No.
Staning
material
Heating temperature
Tf
Rolling rate
Vr
Water cooling time
tw
Rv Rri
Steel A 000'c 0m/s a^ 63% 97%
2 Steel B 000.c 0m/s 1^ 68o/o 92o/o
3 Steel C 000.c 0m/s 1^ 64% 98%
4 Steel D 000.c 0m/s 59% 98%
5 Steel E 000.c 0m/s La5^ 52% 82%
6 Steel F 000.c 0m/s 1¿^ò 630/o 99%
7 Steel G 000'c 0m/s a^ 61% 93%
I Steel H 000.c 0m/s 2s 68% 97o/o
9 Steel I 0000c 0m/s 1^ 560/o 88%
0 Steel J 000"c 0m/s 2s s8% 81%
Steel K 000.c 0mis 1^ 69% 90%
2 Steel L 000'c 0m/s 48% 82%
J Steel M 000.c 0m/s 2s 67% 85Y.
4 Steel N 000'c 0m/s 1^ 61% 98%
5 Steel O 000'c 0m/s a^ 66% 92%
6 Steel P 000.c 0m/s a^ 66% 94%
Steel a 000.c 0m/s 1^ 65% 92%
8 Steel A 100.c 0m/s 69% 99o/o
9 Steel B I 00.c 0m/s 2s 69% 97%
20 # Steel R 000.c 0m/s 2s 66% 97o/o
2t # Steel S 000'c 0m/s LI 64o/o 94o/o
22 # Steel T 000.c 0m/s a^ 66% 88%
23 # Steel U 0000c 0m/s a^ ¿S 56% 83%
24 # Steel V 000'c 0m/s 2s 63% 88%
25 # Steel W 000.c 0m/s 2s 62% 86%
26 # Steel X 000"c 0m/s a^ 66% 84%
27 # Steel Y 000"c 0m/s 2s 6t% 9t%
28 # Steel Z 000'c 0m/s a^ 55% 89Vr
29 # Steel AA 000'c 0m/s a^ LS 67% 95%
30 Steel A 900.c 0m/s 2s * 84%;o * 48Yo
3l Steel A 000.c 0m/s 0.5s * 82o/o 97%
32 Steel A 0000c 0m/s 5s 64% * 47o/o
JJ Steel A 000'c m/s 2s 62% * 44o/o
34 Steel A 0000c 5m/s 2s * 78Yo 82%
35 Steel A 200.c 0m/s 2s 620/o * 42Yo
36 Steel B 900'c 0m/s 2s * 78Yo * 46Yo
37 Steel B 000'c 0m/s 0.5s * 86Yo 96%
38 Steel B 000'c 0m/s 5s 68% * 48o/o
39 Steel B 000.c m/s a^ 6s% * 46Yo
40 Steel B 000'c l5m/s 2s ', 82Yo 84%
4l Steel B 200'c 0m/s 2s 67% * 39Yo
42 # Steel AB 000.c 0m/s 2s
LV
[00e4]
The rolled steel materials for fracture splitting connecting rods of all test
numbers were round bars having a diameter of 35 mm.
[00es]
[Experiment for Measuring V Fraction Rv and Ti Fraction Rti]
Using the measurement methods described above, Vm (%), Yp (%),Tim (%),
and Tip (%) of each test number were determined. Furthermore, the V fraction Rv
and the Ti fraction Rti were determined using Formula (2) and Formula (3). The
determined V fractions Rv and Ti fractions Rti are shown in Table 2.
[00e6]
[Production of Simulated Forged Product]
From the round bars of Test Nos. 1 to 4l , small round bar specimens and
large round bar specimens were obtained, The smallround bar specimens were22
mm in diameter and 50 mm in length. The central axis of each small round bar
specimen conformed to the central axis of the round bar, which had a diameter of 35
mm, of the corresponding test number. The large round bar specimens were 32 mm
in diameter and 50 mm in length. The centralaxis of each large round bar specimen
conformed to the central axis of the round bar, which had a diameter of 35 mm, of
the corresponding test number.
[00e7]
Each small round bar specimen was heated and held at 1000'C for 5 minutes,
Thereafter, it was subjected to forward extrusion to produce a round bar having a
diameter of 20 mm. The extruded round bar was allowed to cool in air. The
reduction of area in the forward extrusion was 20Vo. Hereinafter, the round bar
produced from a small round bar specimen is referred to as "low temperature
simulated forged product".
[00e8]
Each large round bar specimen was heated and held at 1280'C for 5 minutes.
Thereafter, it was subjected to forward extrusion to produce a round bar having a
diameter of 20 mm. The extruded round bar was allowed to cool in air. The
reduction of area in the forward extrusion was 600/o. Hereinafter, the round bar
Lq
produced from a large round bar specimen is referred to as "high temperature
simulated forged product".
[00ee]
[Production of Reference Forged Product]
From the round bar of Test No. 42, a plurality of large round bar specimens
were obtained. The large round bar specimens were heated and held at 1250'C for
5 minutes. Thereafter, they were subjected to forward extrusion to produce round
bars having a diameter of 20 mm. Hereinafter, the simulated forged products of
Test No. 42 are referred to as "reference product".
[0r 00]
IM icrostructure Observation Experirnent]
A microstructure observation experiment was conducted using the low
temperature simulated forged products, high temperature simulated forged products,
and reference products of the respective test numbers. Specifically, samples were
obtained from the forged products (low temperature simulated forged products, high
temperature simulated forged products, and reference products) so that each sample
included anP.V2 region in the cross section of the forged product. A surface of each
sample (hereinafter referred to as observation surface) was polished and etched with
a nital etching reagent, the surface corresponding to the cross section including an
R/2 region. After etching, the microstructure of the observation surface was
observed with an optical microscope at a magnification of 400x.
[0101]
[Fracture Splittability Evaluation Test]
A Charpy impact test was conducted on each forged product to evaluate the
fracture splittability. Specifically, a V-notch test specimen (No. 4 test specimen)
specified in JIS Z 2202 (2012) was obtained from a central portion of each forged
product. Using the test specimens, a Charpy impact test was conducted in air at
room temperature (25oC) to determine the impact value (J/cm2). Impact values of
not more than 10 J/cm2 were evaluated as excellent fracture splittability.
[0102]
[Yield Strength and Tensile Strength Evaluation Test]
Zo
A JIS No. 144 test specimen was obtained from anPJ2 region of each forged
product. Using the obtained test specimens, a tensile test was conducted in air at
room temperature (25oC) to detennine the yield strength YS (MPa) and tensile
strength TS (MPa).
[0103]
With regard to the yield strengths YS (MPa) of Test Nos. 1 to 41, the relative
values Rys thereof (inYo,hereinafter referred to as relative yield strength) to the
yield strength YS (MPa) of the reference product were determined. Furthermore,
with regard to the tensile strengths TS (MPa) of Test Nos. 1 to 41, the relative values
Rts thereof (in o/0, hereinafter refen'ed to as relative tensile strength) to the tensile
strength TS (MPa) of the reference product were determined.
[0104]
Relative yield strengths Rys of not less than I l0% were evaluated as excellent
yield strength. Furthermore, relative tensile strengths Rts of not more than 100%
were evaluated as excellent machinability.
[010s]
fTest Results]
The test results are shown in Table 3. In Table 3, "F" in the "miçrostructure"
column means ferrite was observed. "P" means pearlite was observed. "8" means
bainite was observed.
[0106]
fTable 3]
TABLE 3
3t
Test
No. Structure
3
4
Low
l- +P
5
t-+P
6
temÞe
Chamv irnoact value lJlcm2)
F+P
7
F+P
rature simulated forged oroduct
8
F+P
9
F+P
0
F+P
1
F+P
2
F+P
3.2
F+P
{
74
4
F+P
4.2
5
F.+P
5.t
6
F+P
5.1
Rvs lulnì
7
F+P
4.6
8
F+P
4)
I
F+P
4.8
tt
20
F+P
Kts (Yo)
l5
2l
5.4
t9
F.+P
4
//.
8
l6
F+P
3.2
23
x')
30
F+P
4.
24
87
I
Structure
F+P
th
90
F+P
t'7
4.8
26
85
F+P
l7
5.t
27
Hish temoerature simulated forsed n¡oduct
97
F+P
r8
t+P
28
42
F+P
x/
l. +P
Chamv imnact value lJlcmz)
t9
4.5
29
F+P
¡t9
29
F+P
** l4.g
89
\.6
F+P
{t I
** F+P+B
??
F+P
88
J5
lt
F+P
F+P
t7
J¿
3.8
90
F+P
22
tt
95
62
F+P
24
t-+P
** 15.6
s.8
\4
(Jl)
F+P
21
F+P
35
95
F+P
24
F+P
** 14.6
9t
{fì
F+P
14
F+P
tl
54
93
\7
F+P
3.6
t-+P
l9
4.3
38
90
F+P
4.0
0l
F+P
92
39
F+P
Rvs lul^ì
5.0
03
t- +P
5.4
40
Xfi
F+P
4.9
F+P
22
4t
92
F+P
5.2
F+P
l0
4)
75
ì5
It5
4.3
F+P
** 101
ôÁ
F+P
124
3.3
F+P
4.5
t6
Rfs lul^l
F+P
25
4.1
127
F+P
5.0
l) Symbol "**" indicates failure to neet the target.
29
F+P
4
117
,14
F+P
4.6
** 102
5
t6
136
5.1
XI
F+P
87
** 105
ì0
02
5.2
12
91
F+P
Structure
l8
0
124
L)
F+P
94
5.2
ô?
t25
4.8
90
IJ ¡I
F+P
4.7
05
4.5
125
99
F+P
5.1
13
Chamv imnact value (J/cmz)
5.2
r2l
9)
F+P
4.3
t4
3.2
9l
F+P
4.4
04
J
4.3
78
t30
93
++ F+P+R
F+P
Reference product
14
t2
132
** F+P+R
83
F+P
9l
3.6
oÁ
129
t( ¡t
93
5.2
02
t2'7
74
T* F+P+B
qR
46
t5
** F+p+B
131
F+P
5.6
** 15 0
l4
98
14.
/x
02
R
72
9
+* F+P+R
r* 103
96
I tq
46
i5
t26
88
** F+P+R
95
F+P
4.3
Kvs (Yo)
85
96
3.6
l
't2
r+ F+p+B
gÁ
*+ 15.8
05
** lôÁ
127
F+P
76
ql
5.2
** 16.1
n2
96
45.2
F+P+B
Kts (70)
++ F+P+R
** 106
78
16.
t29
F+P
)
79
3.6
** 19.3
t32
** 106
t20
** 109
++ 13.2
Xfi
** 107
5.2
xfl
** 16.8
t2
I 4
I
2
I
89
4.8
2
80
arâ
ó'¿-
0
9il
I
I
** l0l
5.6
02
I
95
** 104
13.
{
I
2
** lo5
9
** 104
98
5
4
** 102
I
97
+* 105
3
99
I 07
t'+P 9 ** lnn I f)f)
[01 07]
With reference to Table 3, in Test Nos, I to 19, the chemical compositions
were appropriate and the fnl values were appropriate. Furthemore, the V fractions
Rv and Ti fractions Rti were appropriate. Furthermore, the microstructures were
made up of ferrite and pearlite with no bainite observed. As a result, both the low
temperature simulated forged products and high temperature simulated forged
products had Charpy impact values of not more than 10 Jlcm2, relative yield
strengths Rys of not less than 1l0o/0, and relative tensile strengths Rts of not more
than 100%.
[01 08]
On the other hand, in Test Nos. 20 and28, the V contents of the steels were
too low. As a result, the low temperature simulated forged products and high
temperature simulated forged products all had relative yield strengths Rys of less
than llÙ%io.
[010e]
In Test Nos. 21 and 24, the contents of the elements in the steels were
appropriate but fn 1s were less than 0.65. As a result, the low temperature simulated
forged products and high temperature simulated forged products all had relative yield
strengths Rys of less than 110%.
[01 r 0]
In Test Nos. 22 and25, the contents of the elements were appropriate but fnls
were more than 0.80. As a result, the low temperature simulated forged products
and high temperature simulated forged products all had relative tensile strengths Rts
of more than 100%.
[01 1 1]
In Test Nos. 23 and 27 , the Ti contents in the steels were too low, As a
result, the low temperature simulated forged products and high temperature
simulated forged products had Charpy impact values of more than 10 J/cm2 and
therefore had low fracture splittabilities.
[0112]
)n,9
7r:
In Test No. 26, the C content was too high. As a result, the low temperature
simulated forged product and high temperature simulated forged product had relative
tensile strengths Rts of more than 100% and therefore had low machinability.
[0] i3]
In Test No. 29, the Mo content was too high. As a result, bainite was
observed in the microstructure. Furthermore, very small amounts of ferrite and
pearlite were observed. [n Test No. 29, the low temperature simulated forged
product and high temperature simulated forged product had Charpy impact values of
more than 10 J/cm2 and therefore had low fracture splittability.
[0114]
In Test Nos. 30 and 36, the chemical compositions were appropriate and the
fn1 values were within the range of 0.65 to 0.80. However, the heating
temperatures Tf were too low. As a result, the V fractions Rv were too high and the
Ti fractions Rti were too low. Consequently, the low temperature simulated forged
products had excessively low relative yield strengths Rys. Furthermore, in the
microstructures of the high temperature simulated forged products, bainite was
observed. As a result, the Charpy impact values were more than 10 J/cm2 and
therefore the fracture splittabilities were low. Furthermore, the relative tensile
strengths Rts were more than 100/o and therefore the machinabilities were low.
[0r 1s]
In Test Nos. 31 and 37, the chemical compositions were appropriate and the
fnl values were within the range of 0.65 to 0.80. However, the water cooling times
tw were too short. As a result, the V fractions Rv were too high. Consequently,
the low temperature forged products had low relative yield strengths Rys.
[0116]
In Test Nos. 32 and 38, the chemical compositions were appropriate and the
fnl values were within the range of 0.65 to 0.80. However, the water cooling times
tw were too long. As a result, the Ti fractions Rti were too low. Furthermore, in
the microstructures of the high temperature simulated forged products, bainite was
observed. As a result, the Charpy impact values were more than l0 J/cm2 and
therefore the fracture splittabilities were low, Furthermore, the relative tensile
strengths Rts were more than 100% and therefore the machinabilities were low.
3+
/
[01 r 7]
In Test Nos. 33 and 39, the chemical compositions were appropriate and the
fnl values were within the range of 0.65 to 0.80. However, the rolling rates Vr
were too slow. As a result, the Ti fractions Rti were too low. Furthermore, in the
microstructures of the high temperature simulated forged products, bainite was
observed. As a result, the Charpy impact values were more than 10 Jlcm2 and
therefore the fracture splittabilities were low. Furthermore, the relative tensile
strengths Rts were more than 100%o and therefore the machinabilities were low.
[01 1 8]
In Test Nos. 34 and 40, the chemical compositions were appropriate and the
fnl values were within the range of 0.65 to 0,80. However, the rolling rates Vr
were too fast. As a result, the V fractions Rv were too high. Consequently, the
low temperature forged products had low relative yield strengths Rys.
[01 re]
In Test Nos. 35 and 41, the chemical compositions were appropriate and the
fnl values were within the range of 0.65 to 0.80. However, the heating
temperatures Tf were too high. As a result, the Ti fractions Rti were too low.
Consequently, the low temperature simulated forged products had excessively low
relative yield strengths Rys. Furthermore, in the microstructures of the high
temperature simulated forged products, bainite was observed. As a result, the
Charpy impact values were more than 1 0 Jlcmz and therefore the fracture
splittabilities were low.
[0120]
In the foregoing specification, an embodiment of the present invention has
been described. However, the embodiment described above is merely an example
for implementing the present invention. Thus, the present invention is not limited
to the embodiment described above, and modifications of the embodiment described
above may be made appropriately for the implementation without departing from the
scope of the invention.

We claim:
l. A rolled steel material for fracture splitting connecting rocls, the rolled steel
rnaterial comprising a chemical composition consisting of, in mass 0/0,
C: 0.30 to 0.400/o,
Si: 0.60 to 1.0070,
Mn: 0.50 to 1.000á,
P: 0.04 ta0.07%o,
S: 0.04 to 0.130/o,
Cr: 0.10 to 0.30Yo,
V: 0.05 to 0.14o/o,
Ti: more than 0.15%o fo 0.20% or less,
N: 0.002 to 0.020Yo,
Cu: 0 to 0.40Vo,
Ni: 0 ro 0.30Yo,
Mo: 0 to 0. i 0%,
Pb: 0 to 0.3OYo,
Te: 0 to 0.30Yo,
Ca: 0 to 0.010o/o, and
Bi: 0 to 0.30Yo,
the balance being Fe and impurities,
wherein fnl, defined by Formula (1), ranges from 0.65 to 0.80,
wherein a V content in coarse precipitates having a particle size of 200 nm or
more is 70Yo or less relative to the V content in the rolled steel material for fracture
splitting connecting rods, and
wherein a Ti content in the coarse precipitates is 50% or more relative to the
Ti content in the rolled steel material for fracture splitting connecting rods:
fnl : c + si/I0 + Mn/5 + 5crl22+ (cu + Niy20 +Mtol2+ 33V/20- 5S/7 ...
Formula (1)
where each element symbol in Formula (l) is substituted by the content
(mass o/o) of a corresponding element or is substituted by "0" in a case where the
corresponding element is not present.
3L
2. The rollecl steel rraterial for fracture splitting connecting rods accot'ding to
clairn l, wherein the chemical composition contains one or more selected from the
group consisting of,
Cu: 0.01 ro 0.40Yo,
Ni: 0.01 to 0.300%, and
Mo: 0.01 to 0.10%.
3. The rolled steel material for fracture splitting connecting rods accoiding to
claim I or 2, wherein the chemical composition contains one or more selected from
the group consisting of,
Pb: 0.05 to 0.300/0,
Te: 0.0003 to 0.30Yo,
Ca: 0.0003 to 0.010%o,and
Bi: 0.0003 to 0.300/0.