[Document Name] Description
[Title of Invention] STEEL BAR FOR NON-HEAT-TREATED CONNECTING
ROD
[Technical Field]
[00011
The present invention relates to a steel bar for a non-heat-treated
connecting rod containing Cu, Ni, and Mo. Specifically, the present invention
relates to a steel bar which can be readily cut and hot-forged, and is used in
starting material of a non-heat-treated connecting rod for an automobile
engine in which preferable machinability, fracture splitting performance, and
fatigue resistance property are required.
[Background Art]
[00021
A connecting rod 1 for an automobile engine or the like shown in Figure
1 as an example thereof is an engine part for coupling a piston and a
crankshaft, and serves for transmitting explosive power to a driving shaft.
Hence, high fatigue strength is required in the connecting rod. Particularly,
because of recent increase in high output power of engines, the fatigue
strength required in connecting rods become more and more increased.
[00031
A bolt hole 9, and a portion for connecting the piston and the crankshaft
of the connecting rod are formed through cutting. Hence, preferable
machinability is required in the starting material of the connecting rod.
[00041
Carbon steel material for mechanical structures specified by JIS, such as
S48C, is subjected to treatment of quenching and tempering (referred to as
"heat treatment", hereinafter) so as to secure a stable ratio of fatigue limit
(value obtained by dividing fatigue strength by tensile strength). For this
reason, conventional connecting rods are produced by heat-treating the carbon
steel materials for mechanical structures, such as S48C.
[00051
Reflecting the current severe economic situation, various efforts for
reducing production cost of various automobile components have been
enthusiastically exerted, and this trend is no exception to the connecting rods
as an engine part. Hence, it has been increasingly desired to realize nonheat-
treated connecting rods having the ratio of fatigue limit equivalent to
that of the above-described heat-treated carbon steel for mechanical structures,
without carrying out the heat treatment that increases the production cost.
In some types of automobiles, such non-heat-treated connecting rods have been
employed that use steel including a basic chemical composition containing
0.35% of C, 0.4% Si, 0.95% of Mn, 0.04% of S, 0.5% of Cr, and 0.1% of V in
mass% as the starting material.
[OOO~]
Meanwhile, the connecting rod 1 is assembled in such a conventional
manner that each of a rod 2 and a cap 3 that are separately hot-forged in
different processes is separately subjected to forming and finishing processes
of a bolt hole 9 through cutting, and thereafter the crankshaft (not shown) is
held between the rod 2 and the cap 3, and the rod 2 and the cap 3 are then
coupled with bolts 4. In this way, the heat treatment can be omitted, but
reduction in cost is not attained as sufficiently as in the case of a "cracking
connecting rod" described below.
[OOO~]
Because of this, in addition to producing non-heat-treated steel as the
starting material, so-called "cracking connecting rods" have been employed in
some types of automobiles for the sake of further reduction in production cost;
and in this cracking connecting rod, the rod 2 and the cap 3 are integrally
formed through hot-forging, and a bolt hole 9 is then formed, and thereafter
this integrally formed member is cracked (fracture-split) into the rod 2 and the
cap 3 in a big end 5.
~00081
A method employed in the above cracking that inserts a jig into a portion
desired to be split, which is located in a hole 7 of the big end of the integrally
formed member (portion N in Figure 1, for example) so as to fracture the
integrally formed member by applying stress.
[00091
When the cracked connecting rod 1 (i.e., "cracking connecting rod") is to
be connected with a crankshaft and if it has fracture surfaces of embrittled
fracture surfaces created when it is cracked, the rod 2 and the cap 3 can be
coupled with each other by simply holding the crankshaft therebetween,
matching the fracture surfaces of the rod 2 and the cap 3 to each other, and
then coupling the rod 2 and the cap 3 through bolts 4.
[00101
Consequently, if the fracture surfaces of the cracking connecting rod are
embrittled fracture surfaces, a cutting processing onto the matching surfaces
of the portions for holding the crankshaft therebetween is eliminated,
resulting in reduction in production cost, and the coupling is accomplished
between the fracture surfaces, and thus this cracking connecting rod is
excellent in coupling toughness (i.e. strength).
[oo 111
A cracking connecting rod disclosed in Patent Document 1, in which non- e heat-treated steel containing C (carbon) of approximately 0.7% in mass% is
used as the starting material, has already been practically used in Europe as
the above cracking connecting rod. Unfortunately, the non-heat-treated
cracking connecting rod disclosed in Patent Document 1 includes a greater C
content, and is poorer in machinability compared to conventional connecting
rods using heat-treated carbon steel material for mechanical structures.
Consequently, such connecting rods do not sufficiently satisfy needs of the
industry that desire enhancement of machinability in the bolt hole forming.
The non-heat-treated cracking connecting rod of Patent Document 1 has poorer
fatigue strength than fatigue strength of connecting rods using heat-treated
carbon steel for mechanical structures, and fatigue strength of non-heattreated
connecting rods including a basic chemical composition containing
0.35% of C, 0.4% of Si, 0.95% of Mn, 0.04% of S, 0.5% of Cr, and 0.1% of V in
mass%.
[00121
Accordingly, it has been strongly desired to realize non-heat-treated
connecting rods having fracture splitting performance (referred to as "cracking
performance", hereinafter) equal to or more than the fracture splitting
performance of Patent Document 1, and having fatigue strength equivalent to
or more than fatigue strength of non-heat-treated connecting rods including a
basic chemical composition containing 0.35% of C, 0.4% of Si, 0.95% of Mn,
0.04% of S, 0.5% of Cr, and 0.1% of V in mass%, and is also excellent in
machinability.
[OO13 1
In order to satisfy the above needs of the industry, the present inventors
have suggested a non-heat-treated connection rod using Pb-free steel as the
starting material which is excellent in machinability, cracking performance,
and fatigue resistance property, and a producing method of the same.
[00141
Based on the technique disclosed in Patent Document 2, a non-heattreated
connecting rod, which is produced by melting steel with a "blast
furnace-converter" method, hot-rolling the melted steel to produce a steel bar,
and hot-forging this steel bar, is excellent in machinability, cracking
performance, and fatigue resistance property, which satisfies the needs of the
industry. For this reason, the non-heat-treated connecting rod of Patent
Document 2 has been employed in "cracking connecting rods" of some types of
automobiles, and has been practically used in Japan.
[OO 151
As demands on the cracking connecting rods become increased, however,
needs of producing non-heat-treated connecting rods with further cost
reduction have been increased by using, as the starting material, steel melted
with an "electric furnace" method that uses scraps as raw material.
[00161
Different from the "blast furnace-converter" method using iron ores as
the main raw material, the "electric furnace" method mainly uses raw material
of scraps containing Cu, Ni, and Mo as impurities. Hence, it is concerned that
increase in content of Cu, Ni, and Mo in the steel bars that are the raw
material causes deterioration of the hot forging performance during the steel
bars are processed into a connecting rod shape (deterioration of ductility
during hot-forging), and deterioration of machinability when forming bolt holes.
[00171
The non-heat-treated connecting rods can secure desired strength while
retaining the steel bars that are the starting material as they are formed in a
predetermined shape through the hot forging, and as aforementioned, the steel
bars as the starting material are produced by hot-rolling the melted steel. e Hence, the steel bars as the starting material also become hard if they are left
as they are hot-rolled. For the sake of hot-forging the connecting rods, each
steel bar is cut in a predetermined length, and hard steel bars cause reduction
in durability life of a cutting tool, such as a cutting blade, a cutting-off wheel,
and shears. It is concerned that increase in content of Cu, Ni, and Mo in the
steel bars as the starting material also reduces the durability life of the cutting
tool for cutting the steel bars.
[ooial
Techniques of using non-heat-treated steel containing Cu, Ni, and Mo in
the raw material of the "cracking connecting rods" are disclosed in Patent
Document 3 and Patent Document 4.
[00191
Patent Document 3 discloses "high-strength non-heat-treated steel"
containing a certain amount of C, Si, Mn, S, P, Cu, Ni, Cr, V, Ti, sol-Al, and N,
further containing one or more of Pb, Te, Ca, Bi, Nb, Zr, and B if necessary, the
balance being unavoidable impurities and Fe, and also having a ferritepearlite
microstructure. Unfortunately, the non-heat-treated steel suggested
in Patent Document 3 is accomplished without sufficient consideration of hot
forging performance.
[oo2ol
Patent Document 4 discloses "easy fracture-splittable non-heat-treated
steel for hot forging" containing a certain amount of C, Si, Mn,Cr, S, P, V, sol-
Al, N, and 0, further containing one or more of Pb, Te, Ca, Bi, Cu, Ni, Mo, Ti,
I
Nb, and Zr if necessary, the balance being Fe and impurities. Unfortunately,
I
the non-heat-treated steel suggested in Patent Document 4 includes no Ti as
an essential element, so that no strengthening of ferrite is attained with Ti
carbide, and thus this steel is poor in cracking performance. The non-heat-
-x-
7
treated steel suggested in Patent Document 4 may include Ti as an optional
element, but the disclosed Ti content in the steel is small, so that
strengthening of ferrite attained with Ti carbide becomes insufficient, and thus
this steel is poor in cracking performance.
[Citation List]
[Patent Document]
[00211
[Patent Document 11 US 5135587A
[Patent Document 21 JP 2004-301324A
[Patent Document 31 JP 2004-277817A
[Patent Document 41 JP 2002-256394A
[Non-Patent Document]
Coo221
[Non-patent Document 11 Z. Iida et al.: Review of Automotive Engineering, 27
(2006) pp. 439-443
[Summary of Invention]
[Technical Problem]
Lo0231
An object of the present invention, which has been made in order to
solve the problems according to the conventional art, is to provide a steel bar
for a non-heat-treated connecting rod for use in a non-heat-treated connecting
rod of an automobile engine, which can be easily cut and hot-forged, and in
which preferable machinability, cracking performance, and fatigue resistance
property are required even if Cu, Ni, and Mo are contained because of using
scraps as raw material for steel production.
[Solution to Problem]
[00241
The gist of the present invention is described by a steel bar for a nonheat-
treated connecting rod as set forth in the following (1) and (2).
[00251
(1) A steel bar for a non-heat-treated connecting rod including
a chemical composition containing:
in mass%
C: 0.25 to 0.35%;
Si: 0.40 to 0.70%;
Mn: more than 0.65% to 0.90 or less;
P: 0.040 to 0.070%;
S: 0.040 to 0.130%;
Cr: 0.10 to 0.30%;
Cu: 0.05 to 0.40%;
Ni: 0.05 to 0.30%;
Mo: 0.01 to 0.15%;
V: 0.12 to 0.20%;
Ti: more than 0.150% to 0.200% or less
Al: 0.002 to 0.100%; and
N: 0.020% or less;
the balance being Fe and impurities,
wherein
Fnl represented by Formula : Fnl = C + (SiIlO) + (Mn15) + (5Cr122) +
1.65V - (5S17) + (Cu133) + (Ni120) + (MoIlO) satisfies 0.60 to 0.80,
Fn2 represented by Formula <2>: Fn2 = (Mn + Ti)/S satisfies 7 or more,
where a symbol of each element in Formula and in Formula <2>
denotes a content (mass%) of the element in the steel,
and
a microstructure of the steel bar includes a ferrite-perlite microstructure
of 90% or more, and a ratio of ferrite in the ferrite-perlite microstructure is
40% or more.
Lo0261
(2) The steel bar for a non-heat-treated connecting rod set forth in (I),
wherein the chemical composition contains, in mass%, one or more selected
from
Pb: 0.30% or less, and
Te: 0.30% or less,
in lieu of part of Fe.
Lo0271
The "impurities" of the "Fe and impurities" as the balance denote
substances mixed from raw material, or through an environment of a
manufacturing process and the like during industrial production of steel
material.
[00281
The "ferrite-perlite microstructure" denotes a mixed microstructure of
ferrite and perlite. Each phase thereof can be identified through observation
using an optical microscope or an electron microscope.
[Advantageous Effects of Invention]
[00291
The steel bar for a non-heat-treated connecting rod of the present
invention can readily be cut and hot-forged, and is suitable as starting
material for a non-heat-treated connecting rod for use in an automobile engine
in which preferable machinability, cracking performance, and fatigue
resistance property are required.
[Brief Description of Drawings]
[00301
[Figure 11 Figure 1 is a drawing showing an example of a connecting rod.
[ ~ i ~ u r21e Figure 2 is a drawing explaining dimensional measurement
positions for obtaining an amount of fracture strain of the connecting rod.
The reference character "a" in the drawing indicates an inner diameter of a
hole 7 in a big end of the connecting rod when this hole 7 is measured in a
longitudinal direction of the connecting rod, and reference characters "b" and
"c" indicate an inner diameter of the hole 7 when the hole 7 is measured a t an
angle of 87" relative to "a", and an inner diameter of the hole 7 when the hole 7
is measured at an angle of 93" relative to "a", respectively.
[Figure 31 Figure 3 is a drawing showing a shape of a fatigue test specimen
used in Example. Numerical values in the drawing show dimensions (unit:
mm).
[Description of Embodiment]
[00311
Each feature of the present invention will be described in detail,
hereinafter.
[00321
(A) Chemical composition of steel bar for non-heat-treated connecting
rod:
A symbol "%" indicating a content of each element denotes "mass%" in
the following description.
[00331
C: 0.25 to 0.35%
C serves for enhancing strength of steel, the C content of 0.25% or more
@ attains this effect. The C content of more than 0.35% rather increases the
strength too much, resulting in deterioration of machinability. Accordingly,
the C content is defined to be 0.25 to 0.35%. The C content is preferably
0.30% or less.
Lo0341
Si: 0.40 to 0.70%
Si is effective for deoxidation of the steel, and has effect of enhancing the
strength of the steel through solid-solution strengthening. The Si content of
less than 0.40% attains insufficient effect. On the other hand, the Si content
of more than 0.70% rather saturates the above effect, only resulting in increase
in cost. This also deteriorates hot forging performance of the steel, as well.
Accordingly, the Si content is defined to be 0.40 to 0.70%. It is preferable to
define the Si content to be 0.50% or more.
[00351
Mn: more than 0.65% to 0.90% or less
Mn has effect of deoxidation of the steel, and also has effect to enhance
the strength of the steel. In order to attain the above effect, it is necessary to
contain the amount of Mn of more than 0.65%. The Mn content of more than
0.90% rather deteriorates the hot forging performance of the steel. In
addition, hardenability becomes too high, which generates a bainitic
microstructure, and causes deterioration of cracking performance and
machinability. Accordingly, the Mn content is defined to be more than 0.65%
to 0.90% or less. It is preferable to define the Mn content to be 0.70% or more,
and 0.85% or less. It is more preferable to define the Mn content to be 0.80%
or less.
[00361
P: 0.040 to 0.070%
P segregates at grain boundaries and embrittles the steel, and has effect
to allow fracture surfaces to be embrittled fracture surfaces at the time of
cracking the connecting rod. In order to attain this effect, it is necessary to
define the P content to be 0.040% or more. The P content of more than
0.070% deteriorates the hot forging performance of the steel. Accordingly, the
P content is defined to be 0.040 to 0.070%.
100371
S: 0.040 to 0.130% A
S is one of the important elements in the present invention, and has
effect to generate sulfide along with Mn and Ti, and to enhance the
machinability of the steel. In order to attain this effect, it is necessary to
define the S content to be 0.040% or more. The S content of more than
0.130% rather deteriorates the hot forging performance of the steel.
Accordingly, the S content is defined to be 0.040 to 0.130%. For securing
excellent machinability, it is preferable to define the S content to be more than
0.070%.
[0038]
It is known that the more the S content becomes increased, the more the
hot ductility becomes deteriorated. The present invention has an aim to
attain further reduction in cost of starting material by using scraps as raw
material. In this sense, it must be avoided that deterioration of the hot
ductility due to Cu, Ni, and Mo contained in the scraps, and deterioration of
the hot ductility due to the large content of S are caused at the same time.
Accordingly, in the present invention as described later, it is configured to
adjust a balance between the amount of "Mn + Ti" that is a sum of the contents
of Mn and Ti and the content of S.
[00391
Cr: 0.10 to 0.30%
Cr serves for enhancing strength. In order to sufficiently attain this
effect, it is necessary to define the Cr content to be 0.10% or more. The Cr
content of more than 0.30% rather causes the bainitic microstructure, which
deteriorates the cracking performance and the machinability. Accordingly,
the Cr content is defined to be 0.10 to 0.30%. The preferable Cr content is
0.20% or less.
[OO~O]
V: 0.12 to 0.20%
V is one of the important elements in the present invention.
Specifically, V precipitates as carbide in ferrite, thereby enhancing the
strength. In order to attain this effect, the V content should be 0.12% or more.
The V content of more than 0.20% attains almost no enhancement of the above
effect, which rather causes significant increase in cost. Accordingly, the V
content is defined to be 0.12 to 0.20%. It is preferable to define the V content
to be 0.13% or more, and 0.18% or less.
[00411
Ti: more than 0.150% to 0.200% or less
The present invention has a feature of containing Ti along with V, and
Ti is one of the important elements in the present invention. Specifically, as
similar to V, Ti precipitates as carbide in the ferrite, thereby enhancing the
strength, and also serves for significantly strengthening the ferrite by
containing Ti and V in combination. This strengthening of the ferrite can
secure the preferable cracking performance in the ferrite-perlite
microstructure. In addition, the strengthening of the ferrite reduces
occurrence of fatigue cracks, thereby enhancing the fatigue strength. Ti also
serves for generating sulfide so as to improve the machinability, and also to
enhance the hot ductility. Ti preferentially combines with N to generate
nitride (TIN), and in order to attain the above effect, it is essential to contain a
large amount of free Ti in the steel after the nitride is generated. Hence, the
Ti content is necessary to be more than 0.150%. The Ti content of more than
0.200% rather deteriorates the hot forging performance. Accordingly, the Ti
content is defined to be more than 0.150% to 0.200% or less.
Lo0421
Al: 0.002 to 0.100%
Al is an effective element as deoxidizer of the steel. One of the features
of the present invention is to contain Ti and V in combination, as
aforementioned. Ti preferentially combines with N to generate TIN, as
described above. Ti has a deoxidizing action that is strong enough to generate
oxide. Hence it is concerned that the ratio of Ti for generating carbide is
relatively decreased, which reduces the yield rate of Ti effective for
strengthening the ferrite along with V, resulting in increase in production cost.
In the present invention, it is configured to contain Al so as to stabilize
deoxidation by deoxidizing the steel with Al, and also to secure Ti that is
effective for generating carbide, thereby strengthening the ferrite. The Al
content of less than 0.002% attains no desired effect. The Al content of more
than 0.10% rather saturates the above effect, which increases the cost.
Accordingly, the Al content is defined to be 0.002 to 0.100%. It is preferable
to define the Al content to be 0.050% or less.
[00431
In order to stabilize deoxidation of the steel by deoxidizing the steel with
Al, and to secure the effect of strengthening the ferrite with the combination of
Ti and V at the same time, it is preferable that Ti is added after the steel is
sufficiently deoxidized with Al, that is, a predetermined amount of A1 and Ti
are added in the order of A1 and Ti.
[00441
N: 0.020% or less
As described above, one of the features of the present invention is to
contain Ti and V in combination so as to generate precipitation of carbide,
thereby significantly strengthening the ferrite. In the state of containing Ti
and V together, N preferentially combines with Ti to generate nitride. In
order to attain significant strengthening of the ferrite through the
precipitation of carbide, it is essential that a large amount of free Ti exists in
the steel after the nitride is generated. Accordingly, the upper limit of the N
content is defined to be 0.020% or less. It is preferable to define the N content
to be 0.012% or less, and the N content is preferably as small as possible.
[00451
Cu: 0.05 to 0.40%
Ni: 0.05 to 0.30%
Mo: 0.01 to 0.15%
In the present invention, for the purpose of satisfying the needs of the
industry that desires further reduction in cost, it is assumed to use scraps as
raw material. The scraps contain Cu, Ni, and Mo as impurities. The large
contents of Cu, Ni, and Mo as the impurities deteriorate the hot forging
performance in processing of the steel bar into the shape of the connecting rod
as well as the machinability in forming of the bolt hole, and others.
Accordingly the upper limits of the Cu, Ni, and Mo contents are defined to be
0.40% or less, 0.30% or less, and 0.15% or less, respectively. On the other
hand, strict control of the lower limit of each content of Cu, Ni, and Mo causes
necessity of using expensive scraps, which rather increases the production cost.
Accordingly, the lower limits of the Cu, Ni, and Mo contents are defined to be
0.05% or more, 0.05% or more, and 0.01% or more, respectively.
[00461
Fnl: 0.60 to 0.80
In the steel bar of the present invention, the content of each element is
required to be within the above range, and Fnl represented by the following
Formula should satisfy 0.60 to 0.80.
Fnl = C + (Sitlo) + (Mn15) + (5CrI22) + 1.65V - (537) + (CuI33) + (Nit20)
+ (Motlo) ...< l>
[00471
If Fnl is more than 0.80, the machinability of the connecting rod
becomes deteriorated, and on the other hand, if Fnl is less than 0.60, the
strength becomes deteriorated, resulting in decrease in fatigue strength.
[00481
As aforementioned, a symbol of each element in Formula denotes a
content (mass%) of the element in the steel.
[00491
Fn2: 7 or more
In the present invention that enhances the machinability by including
the S content of 0.040 to 0.130%, the more the S content becomes increased,
the more the hot ductility becomes deteriorated. The scraps used as the raw
material contain Cu, Ni, and Mo, and these elements deteriorate the hot
ductility. Hence, simply adjusting the content of each element and the above
Fnl to satisfy the respective predetermined ranges Cannot secure the
preferable hot forging performance because the deterioration of the hot
ductility due to containing of S, and deterioration of the hot ductility due to Cu,
Ni, and Mo are caused at the same time. Accordingly, in the steel bar of the
present invention, Fn2 represented by the following Formula <2> is also
required to satisfy 7 or more. Satisfying this condition can secure
machinability improving effect with S, as well as can attain the preferable hot
forging performance, thereby easily forming the steel bar into the
predetermined shape of the connecting rod.
Fn2 = (Mn + Ti)/S ...< 2>.
[00501
As aforementioned, a symbol of each element in the Formula <2>
denotes a content (mass%) of the element in the steel.
[00511
It is preferable that Fn2 represented by Formula <2> is 22 or less.
[00521
One steel bar of the present invention, in addition to the above elements,
includes a chemical composition in which the balance is Fe and impurities, and
Fnl represented by the above Formula satisfies 0.60 to 0.80, and Fn2
represented by the above Formula <2> satisfies 7 or more.
[00531
Another steel bar of the present invention includes a chemical
composition that contains one or more of elements selected from Pb and Te, in
lieu of part of Fe of the steel bar.
[00541
Hereinafter, description will be provided on operational effects of Pb and
Te that are optional elements, and the reason for limiting the contents of these
elements.
Loo551
Pb: 0.30% or less
Pb serves for enhancing the machinability of the steel, so that Pb may be
contained for attaining this effect. The Pb content of more than 0.30% rather
deteriorates the hot forging performance. Accordingly, the Pb content is
defined to be 0.30% or less if contained.
[00561
On the other hand, in order to stably attain the aforementioned
machinability enhancing effect of Pb, it is preferable to define the Pb content
to be 0.02% or more.
roo571
If usage of Pb is restricted because of global environment preservation or
the like, no Pb may be contained.
[00581
Te: 0.30% or less
Te serves for enhancing the machinability of the steel, so that Te may be
contained in order to attain this effect. The Te content of more than 0.30%
rather deteriorates the hot forging performance. Accordingly, the Te content
is defined to be 0.30% or less if contained. It is preferable to define the Te
content to be 0.10% or less.
[00591
On the other hand, in order to stably attain the machinability enhancing
effect of Te, it is preferable to define the Te content to be 0.002% or more.
[0060]
(B) Microstructure of steel bar for non-heat-treated connecting rod:
In the steel bar of the present invention, the microstructure of the steel
bar should include the ferrite-perlite microstructure of 90% or more, and the
ratio of the ferrite in this ferrite-perlite microstructure should be 40% or more.
This facilitates cutting of the steel bar, which is to be the starting material in
the hot-forging of the connecting rod, into a predetermined length, and
increases the durability life of a cutting tool, such as a cutting blade, a cuttingoff
wheel, and shears.
[00611
If the microstructure includes bainite andlor martensite, the steel bar
becomes hard and difficult to be cut. Specifically, when cutting the steel bar
with the above cutting tool, damages and/or wear are caused to the tool, so
that deterioration of productivity is caused, or it becomes difficult to cut the
steel bar in a predetermined length. Accordingly, it is necessary to include
the ferrite-perlite microstructure of 90% or more in the microstructure of the
steel bar so that the bainite microstructure and/or the martensite
microstructure are reduced to a substantially negligible level.
Lo0621
Even if the microstructure of the steel bar includes the ferrite-perlite
microstructure of 90% or more, the ratio of hard perlite of 60% or more makes
it difficult to cut the steel bar into a predetermined length because the perlite
includes cementite. For easy cutting of the steel bar into the predetermined
length, it is required to reduce the ratio of the perlite in the ferrite-perlite
microstructure to 60% or less, that is, it is required to define the ratio of the
ferrite to be 40% or more.
roo631
The steel bar of the non-heat-treated connecting rod according to the
present invention may be produced through the following steps (a) to (c), for
example.
LOO641
(a) Scraps are used as raw material to be melted, and steel having the
chemical composition set forth in the above (A) is melted into ingots or cast
pieces. These ingots and cast pieces are bloomed into slabs if necessary.
[0065]
(b) Each ingot, cast piece, or slab is heated at a temperature of 1000 to
1300°C, and is subjected to hot rolling into a steel bar (round bar or square
bar) having a predetermined size.
[00661
(c) After finishing each steel bar into a predetermined size through the
hot rolling, the steel bar is cooled at an average cooling rate of O.S°C/second or
less in temperature range of 800 to 500°C.
[0067]
The "temperature" and the "average cooling rate" in the steps (b) and (c)
denote a temperature and an average cooling rate on the surface of a
processing-target material in each step.
[00681
In the step (b), the ingot, cast piece, or slab may be heated at a
temperature of 1000°C or more for the sake of processing the target material
in the austenite zone so as to enhance efficiency of the hot rolling. The above
heating temperature is preferably 1100°C or more. An excessively high
heating temperature encourages growth of austenite grains, so that an area of
grain boundaries serving as a nucleus of generating the ferrite becomes
decreased, which reduces an area ratio of the ferrite. Accordingly, the upper
limit of the heating temperature for the ingot, cast piece, or slab may be
1300°C.
[0069]
In the step (c), after each steel bar is finished into a predetermined size
through the hot rolling, the steel bar is cooled at an average cooling rate of
O.B°C/second or less in temperature range of 800 to 500°C where the ferriteperlite
transformation is generated, thereby easily reducing generation of
bainite and martensite. In addition, the ratio of the perlite in the ferriteperlite
microstructure can be reduced to 60% or less. It is more preferable to
define the average cooling rate in the above temperature range to be
O.G°C/second or less. In an industrial mass-production process, it is
preferable to define the average cooling speed in the above temperature range
to be O.B°Clsecond or more in the light of productivity.
[00701
The ferrite-perlite transformation is substantially completed at 500°C.
Hence, any cooling pattern may be used in the cooling in the temperature
range of less than 500°C.
[007 11
The steel bar of the present invention used as the starting material
makes it possible to easily produce the non-heat-treated connecting rod having
the preferable machinability, cracking performance, and fatigue resistance
property with an ordinary hot forging method.
Lo0721
Hereinafter, the present invention will be more specifically described
based on Examples, but the present invention is not limited to these Examples.
[Example]
[00731
The steel 1 to the steel 22 having chemical compositions shown in Table
1 were melted to be produced into ingots through a 3-ton electric furnace with
an ordinary method.
Lo0741
In Table 1, the steel 3 to the steel 8, and the steel 22 satisfy the chemical
composition defined by the present invention. Meanwhile, the steel 1, the
steel 2, and the steel 9 to the steel 21 are steels of Comparative Examples
having chemical compositions deviating from the chemical composition defined
by the present invention.
[00751
In Comparative Examples, the steel 1 is equivalent to the steel for a
cracking connecting rod which is disclosed in Patent Document 1, and has
already been practically used in Europe, and the steel 2 is a steel produced by
adding Pb and Ca for the purpose of improving machinability to a steel
including a basic chemical composition containing 0.35% of C, 0.4% of Si,
0.95% of Mn, 0.04% of S, 0.5% of Cr, and 0.1% of V, which is for use in the nonheat-
treated connecting rod employed in some types of automobiles.
[00761
[Table 11
Table 1
Steel
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
An asterisk (*) indicates deviation from the range defined by the present invention.
Fn2
9.09
20.67
9.20
12.00
11.58
' 8.95
8.54
8.95
10.89
13.79
*6.58
*6.41
8.28
14.50
86.75
*6.43
7.94
8.78
13.30
9.73
8.82
9.88
Chemical
C
*0.70
0.35
0.30
0.26
0.32
0.34
0.26
0.27
0.30
0.33
0.28
0.28
0.29
0.32
0.32
0.33
0.35
0.34
*0.18
*0.48
0.26
0.29
Fnl = C +
Si
*0.19
0.42
0.55
0.50
0.60
0.45
0.55
0.52
0.59
0.65
0.50
0.42
0.60
0.50
0.46
0.66
0.44
*0.99
0.70
0.60
0.51
0.55
(SiflO)
Te
-
-
0.03
0.05
Composition
Mn
*0.50
*0.93
0.75
0.80
0.89
0.68
0.82
0.71
0.80
0.88
0.68
*0.55
0.72
0.81
0.88
0.71
0.80
0.90
0.77
0.89
0.79
0.86
+ (Mn15)
Ca
-
*0.0011
-
-
-
-
Fnl
*0.869
*0.837
0.762
0.723
0.774
0.729
0.685
0.721
*0.822
*0.843
0.734
0.714
0.794
0.789
0.772
0.793
0.782
*0.850
0.700
*0.984
*0.570
0.780
(mass%)
P
*0.012
*0.015
0.060
0.050
0.055
0.044
0.054
0.067
0.050
0.043
0.063
0.070
0.060
0.060
0.040
0.055
*0.140
0.068
0.050
0.042
0.045
0.057
+ (5CrJ22)
Balance:
S
0.055
0.045
0.100
0.080
0.090
0.093
0.118
0.098
0.090
0.075
0.128
0.1 10
0.087
0.060
*0.012
*0.140
0.120
0.120
0.070
0.111
0.110
0.105
+1.65V
Fe and
Cu
0.10
0.13
0.20
0.30
0.25
0.18
0.10
0.23
0.30
0.22
0.28
0.09
0.14
0.30
0.12
0.16
0.19
0.33
0.08
0.21
0.28
0.21
- (5Sl7)
Cr
0.13
*0.51
0.15
0.18
0.20
0.13
0.14
0.15
0.20
0.17
0.20
0.11
0.20
0.15
0.12
0.12
0.19
0.19
0.11
0.18
0.12
0.12
+
Impurities
Ni
0.08
0.09
0.10
0.20
0.18
0.06
0.09
0.13
0.19
0.13
0.22
0.08
0.10
0.12
0.08
0.11
0.15
0.21
0.07
0.19
0.20
0.08
+ (Cu133)
Mo
0.03
0.02
0.03
0.03
0.04
0.04
0.03
0.06
0.06
0.02
0.04
0.02
0.02
0.03
0.01
0.04
0.06
0.06
0.02
0.08
0.02
0.03
(Ni120) +
Ti
*-
*-
0.170
0.160
0.152
0.152
0.188
0.167
0.180
0.154
0.162
0.155
*-
*0.060
0.161
0.190
0.153
0.154
0.161
0.190
0.180
0.177
(MoJlO),
V
*0.030
*0.100
0.170
0.150
0.130
0.141
0.150
0.166
0.180
0.165
0.176
0.198
0.185
0.150
0.123
0.190
0.152
0.150
0.190
0.170
*0.080
0.180
Fn2 = (Mn
Pb
-
0.11
-
-
-
0.19
-
0.20
-
-
-
-
-
-
-
-
-
-
-
-
-
-
A1
0.010
0.008
0.030
0.040
0.020
0.012
0.050
0.040
0.025
0.012
0.031
0.022
0.033
0.044
0.029
0.022
0.045
0.055
0.050
0.022
0.009
0.013
+ Ti)/S
N
0.009
0.010
0.007
0.007
0.008
0.007
0.010
0.012
0.010
0.006
0.009
0.011
0.008
0.020
0.005
0.011
0.010
0.120
0.007
0.010
0.009
0.009
Lo0771
Each of the above ingots was heated at a temperature of 1250°C, and
then hot-rolled into a slab of 180 mm x 180 mm. Subsequently, it was
assumed to produce the starting material for the hot forging, so that the slab
was hot-rolled into a round bar of 30 mm in diameter under each condition of
Table 2.
[00781
Table 2 specifically shows the heating temperature for the hot rolling,
the finishing temperature for the hot rolling, and the average cooling rate in
the temperature range of 800 to 500°C after the finishing through the hot
rolling. The average cooling rate in the above temperature range was
adjusted by changing the air cooling condition. The cooling in the
temperature range of less than 500°C was carried out by cooling in the
atmosphere.
[Table 21 a Table 2
[00801
Various test specimens were collected from the above-produced steel
bars of 30 mm in diameter, and these test specimens were examined on the
microstructure, Rockwell C hardness (referred to as "HRC", hereinafter), and
the hot forging performance. The steel bars of 30 mm in diameter were also
subjected to the cutting test.
Test No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2 1
22
23
An asterisk
defined by the present invention.
Steel
* 1
* 2
3
4
5
6
7
8
* 9
*10
*11
* 12
*I3
* 14
"15
"16
"17
* 18
"19
"20
*21
22
22
(*) indicates
Heating
Temperature of
Hot Rolling
("c)
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1350
1200
a steel whose chemical
Finishing
Temperature of
Hot Rolling
("c)
1000
1000
1050
1050
1050
1050
1050
1050
1000
1000
1000
1050
1050
1050
1050
1050
1050
1050
1050
1050
1050
1300
1050
composition deviates
Average Cooling
Rate in Temperature
Range of 800 to
500°C ("Clsecond)
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
2.5
from the condition
[008 11
From each of the above-described steel bars of 30 mm in diameter, a
microscopic test specimen, in which an entire surface vertical to the
longitudinal direction of the steel bar was used as an observation surface, was
cut-out, and the observation surface was mirror-polished, and subjected to
nital etching, and thereafter, an observation with an optical microscope of 400
magnifications was conducted at five visual fields in the vicinity of the surface
(at a position of 3 mm from the surface), and at five visual fields in the vicinity
of R/2 ("R denotes a radius of the steel bar) (at a position of 7.5 mm from the
surface), that is, at 10 visual fields in total so as to determine each phase in
the microstructure. The microstructures at the same visual fields were
photographed, and these photographed images were analyzed by using a
conventional method so as to measure the ratio of the ferrite-perlite
microstructure, and the ratio of the ferrite in the ferrite-perlite microstructure.
An arithmetic mean value at the ten visual fields was found.
[00821
Each test specimen, in which a surface vertical to the longitudinal
direction of the steel bar was used as a test surface, was cut out and polished,
and thereafter the HRC was measured at four points in the R/2 portion with
intervals of 90°, and an arithmetic mean value at the four points was found.
[0083]
The cutting test was conducted by cutting each steel bar of 30 mm in
diameter with a cutting-off wheel of a high speed cutter. Specifically, the
cutting test was carried out by using a conventional automatic high speed
cutter having a rotational frequency of a cutting-off wheel of 1650 rpm,
equipped with an ordinary cutting-off wheel of 455mm in outer diameter, 30
mm in inner diameter, and 3.5 mm in thickness.
[0084]
As already described, the steel 2 was produced on the basis of the steel
for the non-heat-treated connecting rod employed in some types of automobiles.
In this steel 2, 500 cuttings were attained with a single cutting-off wheel.
Hence, it was evaluated that steels having a durability life of 500 cuttings If:
10% are equivalent on the industrial basis. Specifically, a test specimen of
each test number was evaluated based on the following criteria: a test
specimen of interest having a durability life of the cutting-off wheel of less
than 450 cuttings (90%) was harder to be cut than the steel 2, a test specimen
of interest having 450 to 550 cuttings (90 to 110%) was equivalent to the steel
2, and a test specimen of interest having more than 550 cuttings (110%) was
easier to be cut than the steel 2.
[0085l
A cylindrical test specimen of 15 mm in diameter, and of 22.5 mm in
length was collected from a central portion of each steel bar of 30 mm in
diameter in parallel to the longitudinal direction, and each test specimen was
examined on the hot forging performance. Specifically, each cylindrical test
specimen was heated at a temperature of 1250°C, cooled down to a
temperature of 1100°C in the atmosphere, and at this temperature, the test
specimen was subjected to hot upsetting until the length of the test specimen
becomes 30% (6.75 mm) in the axial direction. After the above upsetting,
visual observation was conducted on the surface of each test specimen
regarding existence of cracks. The above examination was conducted by five
times for a test specimen of each test number, and a frequency of cracking
occurrence thereof was indicated in percentage.
[OOB~I
In the above steel 2, the frequency of cracking occurrence was 60%
(specifically, cracking was observed in three of five counts of the hot upsetting),
and thus the "frequency of cracking occurrence of 60%" was defined as the
evaluation criteria of the hot forging performance. A case having the
frequency of cracking occurrence of more than 60% was evaluated as poor in
hot forging performance, and was determined to be inappropriate as the
starting material of the non-heat-treated connecting rod for use in an
automobile engine. Consequently, an examination carried out by producing
an actual connecting rod described later was omitted in this case. For use as
the evaluation criteria of the cracking performance, a connecting rod for the
examination was produced by using the steel 1 substantially equivalent to the
steel for the cracking connecting rod in Patent Document 1.
[00871
For each test specimen whose frequency of cracking occurrence was 60%
or less in the examination of the hot forging performance, an actual connecting
rod was produced so as to determine whether or not the required properties
could be attained.
[00881
Each steel bar of 30 mm in diameter, and of 300 mm in length was
heated at a temperature of 1250°C, and was hot-forged so as to integrally
produce the rod 2 and the cap 3 having a total length of 170 mm as shown in
Figure 1. The cooling after the hot-forging was carried out by cooling in the
atmosphere.
[00891
A cut-out having a stress concentration factor of 3 was formed in the
portion N of a big end 5.
[00901
The integrally formed members each having such a cut-out were
examined on the Vickers Hardness (referred to as "HV", hereinafter), the
fatigue resistance property, the machinability, and the cracking performance.
[00911
, Specifically, a surface vertical to the longitudinal direction of each steel
bar having a diameter of 30 mm as the starting material was taken from the
portion N (portion where the cut-out was formed) of the big end 5 of each
integrally formed member, and this surface was mirror-polished, and then the
HV was measured on this surface with a test force of 98.07 N.
[00921
A fatigue test specimen having a shape that includes a parallel portion
of 3 mm in diameter and of 11 mm in length, and a grip portion of 6 mm in
diameter was cut from a stick portion 6 (middle portion between the big end 5
and a small end 8) of the rod 2 of each integrally formed member, and was
subjected to a fatigue test using load-controlled tensile/compression at a room
temperature in the atmosphere in a stress ratio of -1, and at a repetitive speed
of 10 to 20 Hz with an electro-hydraulic servo type fatigue tester so as to
measure the fatigue strength (referred to as "ow", hereinafter). The
maximum strength that caused no fracture in repetitive counts of 107 was
defined as the fatigue strength.
[00931
The ow value (375 MPa) of the test number 2 using the steel 2 as the
starting material was defined as the reference performance, and the case
having a value equal to or more than this ow value was determined to be
preferable in fatigue resistance property.
[00941
The machinability was evaluated based on the bolt hole forming.
Specifically, the bolt hole 9 (through hole) shown in Figure 1 was pierced in the
big end 5 of each integrally formed member with a drill, and the amount of
corner wear of the drill (amount of wear at the outermost peripheral portion of
the drill) was measured and evaluated after 300 counts of drilling. The
amount of corner wear in the test number 2 using the steel 2 as the starting
material was defined as the reference amount of corner wear, and a case
having the amount of wear of 110% or less of the reference value was
evaluated to have "preferable" machinability, and a case having the amount of
wear of more than 110% was evaluated to be "poor" in machinability. This is
because it is unlikely considered that machinability of the steel containing no
Pb and no Ca that are free cutting elements is more excellent than
machinability of the steel 2 containing Pb and Ca, but it was determined that
the amount of wear approximately 10% poorer than the above reference value
yields equivalent productivity in the industrial scale. In the steel of the
present invention containing Pb and/or Te, the amount of wear may become
less than 90% of the reference value in some cases. In this case, the
machinability thereof was determined to be "excellent."
[00951
The piercing condition is as follows.
Drill: straight shank drill of P20 cemented carbide having a diameter of
8 mm,
Rotational frequency: 1200 rpm,
Feed: 0.15 mmlrev,
Lubricant: water-soluble lubricant.
[00961
The cracking test was carried out by using an apparatus shown in
Figure 6 of non-Patent Document 1 in the same manner. Specifically, a jig
was inserted into a hole 7 of the big end of the integrally formed member
shown in Figure 1. The jig was moved so as to apply impact of tensile load in
the direction of an arrow "a" of Figure 2, thereby cracking the integrally
formed member into the rod 2 and the cap 3 at the cut-out formed in the
portion N. No cracking into the rod 2 and the cap 3 could be obtained in the
integrally formed member of the test number 2 using the steel 2 as the
starting material, and in the integrally formed member of the test number 13
using the steel 13 containing no Ti as the starting material.
[00971
After the cracking, the fracture surfaces were observed, and the amount
of fracture strain was measured for evaluation of the cracking performance;
and such a case was evaluated to be preferable that had fracture surfaces
substantially equal to those of the test number 1 using the steel 1 as the
starting material, which is equivalent to the steel for the cracking connecting
rods already practically used in Europe, and also had the amount of fracture
strain smaller than the amount of fracture strain (0.15 mm) of the test number
1. The above amount of fracture strain was defined by subtracting a value of
[a - {(b + c)/2)1 before the cracking from a value of [a - {(b + c)/2)1 after the
cracking where values a to c shown in Figure 2 were measured in a state in
which the rod 2 and the cap 3 were matched at their fracture surfaces with
each other after the cracking. The value "a" denotes an inner diameter of the
hole 7 in the big end of the connecting rod measured in the longitudinal
direction of the connecting rod, and the values "b" and "c" denote an inner
diameter measured at an angle of 87" relative to "a", and an inner diameter
measured at an angle of 93" relative to "a", respectively.
[00981
Table 3 comprehensively shows the above test results.
[00991
[Table 31
Table 3
Test No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2 1
22
23
"F + P" in
A symbol "-" in the Properties of Connecting Rod column indicates that no connection rod was produced.
"Brittleness + Ductility" in the Fracture Surface State column indicates that a ductile fracture surface was partially mixed.
A symbol $ indicates that no cracking into the connecting rod body and the connecting rod cap could be obtained.
A symbol # indicates a reference value for determination.
A symbol * indicates deviation from the condition defined by the present invention.
Steel
* 1
*2
3
4
5
6
7
8
*9
*I0
*11
*12
*I3
*14
*I5
*I6
*17
$18
*I9
*20
*2 1
22
22
the
HV
300
265
300
260
300
310
290
280
330
330
290
280
270
220
350
220
265
265
the ferrite,
Properties of Steel
HRC
27
24
24
22
24
25
23
24
27
2 7
24
24
24
25
24
25
25
27
18
29
17
29
30
a ratio
F+P
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
90%
100%
90%
100%
90%
90%
"Microstructure"
ow
( m a )
350
#375
420
400
410
400
400
410
320
430
350
340
390
340
430
350
420
420
and "P" denotes
Microstructure
Ratio of F
*O%
45%
55%
60%
50%
45%
60%
55%
*35%
*30%
50%
50%
50%
45%
50%
50%
40%
*35%
80%
*IS%
60%
*15%
*IS%
column indicates
Bar of 30 mm in
Durability Life of
Wheel
(%)
75
#I00
98
102
95
115
120
115
85
80
95
105
105
105
92
105
102
75
120
70
120
75
70
of the ferrite-perlite
Diameter
Frequency of
Cracking Occurrence
(%)
80
#60
40
40
40
60
60
60
40
60
80
80
20
40
20
80
80
80
20
60
20
40
40
microstructure. "F" denotes
Fracture Strain
Amount
(mm)
#O. 15
$
0.10
0.13
0.10
0.10
0.13
0.10
0.10
0.05
$
0.50
0.13
0.15
0.05
0.20
0.10
0.10
"ratio of F" in "F + P."
Properties of
Machinability
Poor
#
Preferable
Preferable
Preferable
Excellent
Excellent
Excellent
Poor
Poor
Preferable
Preferable
Poor
Preferable
Poor
Preferable
Preferable
Preferable
the perlite.
Connecting Rod
Fracture Surface State
Brittleness + Ductility
$
Brittleness
Brittleness
Brittleness
Brittleness
Brittleness
Brittleness
Brittleness
Brittleness
$
Brittleness + Ductility
Brittleness
Brittleness
Brittleness
Brittleness + Ductility
Brittleness
Brittleness
"Ratio of F" indicates a
[01001
As apparent from Table 3, the steel bar including the chemical
composition and the microstructure defined by the present invention can
readily be cut and hot-forged.
[01011
It is also apparent that the connecting rod produced by hot-forging the
steel bar including the chemical composition and the microstructure defined by
the present invention has preferable machinability, cracking performance, and
fatigue resistance property.
[Industrial Applicability]
[01021
The steel bar for the non-heat-treated connecting rod of the present
invention can readily be cut and hot-forged, and is suitable as the starting
material for the non-heat-treated connecting rod for use in an automobile
engine or the like in which preferable machinability, cracking performance,
and fatigue resistance property are required.
[Reference Signs List]
[01031
1: Connecting rod
2: Rod
3: Cap
4: Bolt
5: Big end
6: Stick portion
7: Hole of big end
8: Small end
9: Bolt hole