DESCRIPTION
FORGED CRANKSHAFT
Technical Field
[0001]
The present invention relates to a crankshaft, and
more particularly to a forged crankshaft which is used
without a thermal refining treatment nor a surface
hardening heat treatment after hot forging.
Background Art
[0002]
Recently, forged crankshafts for which thermal
refining treatment is omitted have been provided.
Thermal refining treatment is a quenching and tempering
treatment for improving mechanical properties of steel
such as strength. Hereafter, a forged crankshaft for
which thermal refining treatment is omitted is also
referred to as a "non-heat treated forged crankshaft."
[0003]
The steel material constituting a non-heat treated
forged crankshaft generally contains vanadium (V). A
non-heat treated forged crankshaft is produced by hot
forging the non-heat treated steel and allowing it to
cool in the atmosphere. The microstructure of the steel
material constituting the non-heat treated forged
2
-/-
crankshaft is a ferrite-pearlite structure. Vanadium (V)
remains in steel in the form of fine carbides and
increases the strength and hardness of steel. In short,
even without thermal refining treatment, a non-heat
treated forged crankshaft containing vanadium has
excellent strength and hardness. However, since vanadium
is expensive, the manufacturing cost of non-heat treated
forged crankshaft becomes high. Therefore, there is a
need for a non-heat treated forged crankshaft having
excellent strength and hardness even without containing
vanadium.
[0004]
Further, wear resistance is required for the surface
of the forged crankshaft. A crank pin of the forged
crankshaft is inserted into a large-end portion of a
connecting rod. As the crankshaft rotates, the crank pin
rotates via the inner surface of the large-end portion of
the connecting rod and a slide bearing. Therefore, the
surface of the crank pin is required to have excellent
wear resistance.
[0005]
JP2000-328l93A and JP2002-256384A disclose a nonheat
treated steel which is intended for improving wear
resistance without addition of vanadium.
[0006]
The non-heat treated steel for hot forging disclosed
in JP2000-328l93A has a ferrite-pearlite microstructure.
- ;;/-
Further, in the non-heat treated steel for hot forging
disclosed in Patent Literature 1, Si and Mn dissolve in
ferrite thereby strengthening the ferrite. This is
intended for improving wear resistance.
[0007]
The non-heat treated steel for crankshaft disclosed
in JP2002-256384 has a microstructure dominantly made up
of pearlite with a pro-eutectoid ferrite fraction of less
than 3%, and contains sulfides inclusions having a
thickness of not more than 20 pm. Further, Si content is
not more than 0.60%, and Al content is less than 0.005%.
This is intended for improving wear resistance and
machinability.
[0008]
Meanwhile, to improve the wear resistance of a
forged crankshaft, in general, a forged crankshaft is
subjected to a surface hardening heat treatment. The
surface hardening heat treatment includes, for example,
an induction hardening treatment and a nitriding
treatment. By an induction hardening treatment, a
quenched layer is formed on the surface of a forged
crankshaft. Moreover, by a nit riding treatment, a
nitrided layer is formed on the surface of a forged
crankshaft. The quenched layer and the nitrided layer
have high hardness. Therefore, the wear resistance of
the surface of the forged crankshaft is improved.
[0009]
4-
-~
However, performing surface hardening heat treatment
will result in an increase of manufacturing cost.
Therefore, there is a need for a non-heat treated forged
crankshaft which has excellent wear resistance even if
vanadium is not contained and the surface hardening heat
treatment is omitted.
[0010]
A forged crankshaft manufactured by using a non-heat
treated steel disclosed in JP2000-328193 and JP2002256384
may exhibit a decline in wear resistance when the
surface hardening heat treatment is omitted.
Disclosure of the Invention
[0011]
It is an objective of the present invention to
provide a non-heat treated forged crankshaft which has
excellent wear resistance even if it is used without a
thermal refining treatment nor a surface hardening heat
treatment after hot forging.
[0012]
A forged crankshaft according to an embodiment of
the present invention includes a non-heat treated steel
material, in which the non-heat treated steel material
has a matrix consisting of a ferrite-pearlite
microstructure in which an area ratio of pro-eutectoid
ferrite is less than 10% or a pearlite microstructure,
and the non-heat treated steel material has a chemical
/
-~/ - ;/-
composition containing, by mass%, C: 0.45 to 0.70%, Si:
0.75 to 1.30%, Mn: 1.00 to 2.00%, S: 0.03 to 0.30%, Cr:
0.05 to 0.30%, AI: 0.005 to 0.050%, and N: 0.005 to
0.020%, the balance being Fe and impurities, and which
satisfies the following Formula (1):
1.1C + Mn + 0.2Cr > 2.0 (1)
Where, a symbol of each element in Formula (1) is
substituted by the content (mass%) of the each element.
[0013]
In this case, the forged crankshaft does not contain
vanadium (V) and has excellent wear resistance even if
the surface hardening heat treatment such as an induction
quenching treatment and a nitriding treatment is omitted.
[0014]
The chemical composition of the non-heat treated
steel material composes the above described forged
crankshaft may contain Ti: not more than 0.03% in place
of part of Fe.
[0015]
Preferably, the forged crankshaft includes a crank
pin for which the surface hardening heat treatment is
omitted.
Brief Description of the Drawings
[0016]
I
.~-{. -y-
Figure 1 is a diagram showing principal parts of a
forged crankshaft according to an embodiment of the
present invention; and
Figure 2 is a diagram showing a relationship between
the amounts of C, Mn, and Cr of a non-heat treated steel
material that makes up the forged crankshaft shown in
Figure 1, and an area ratio of pro-eutectoid ferrite.
Description of Embodiment
[0017]
Hereafter, with reference to the drawings, an
embodiment of the present invention will be described in
detail. Like or corresponding parts in the figures are
given like reference symbols, and description thereof
will not be repeated.
[0018]
[Outline of forged crankshaft according to the present
embodiment]
The present inventors have conducted investigation
and analysis to improve the wear resistance, strength and
hardness of a non-heat treated forged crankshaft in which
a surface hardening heat treatment is omitted. As a
result, the present inventors have obtained the following
findings.
[0019]
(A) The matrix of non-heat treated steel material
constituting a forged crankshaft consists of a ferrite-/-
pearlite microstructure in which the area ratio of proeutectoid
ferrite is less than 10%, or a pearlite
structure. Here, the "pro-eutectoid ferrite" means
ferrite that precipitates from austenite prior to the
eutectic transformation when the steel is cooled.
Further, the "ferrite-pearlite microstructure" means a
microstructure consisting of pro-eutectoid ferrite and
pearlite, where the "pearlite microstructure" means
substantially a single-phase microstructure of pearlite
in which the area ratio of pro-eutectoid ferrite is 0%.
In the description below, the area ratio of pro-eutectoid
ferrite is referred to as a "pro-eutectoid ferrite
ratio."
[0020]
Pro-eutectoid ferrite is soft compared with pearlite,
and the wear resistance of pro-eutectoid ferrite is low.
If the pro-eutectoid ferrite ratio is less than 10%, a
forged crankshaft has excellent wear resistance even if
the surface hardening heat treatment is omitted.
[0021]
(B) To obtain a ferrite-pearlite microstructure in
which pro-eutectoid ferrite ratio is less than 10%, or a
pearlite microstructure, the chemical composition of nonheat
treated steel material constituting a forged
crankshaft needs to satisfy Formula (1) below:
1.lC + Mn + 0.2Cr > 2.0 (1)
8
-~-
Where, the symbol of each element in Formula (1) is
substituted by the content (mass%) of the each element.
[0022]
Any of carbon (C), manganese (Mn) and chromium (Cr)
suppresses the precipitation of pro-eutectoid ferrite.
If the contents of these elements satisfy Formula (1),
the pro-eutectoid ferrite ratio of a forged crankshaft
which is cooled after hot forging will be less than 10%,
and an excellent wear resistance wilL be obtained.
[0023]
(C) Si content is made to be 0.75 to 1.30%. Si
strengthens ferrite within pearlite. Therefore, the
strength and hardness of non-heat treated forged
crankshaft will be improved even if vanadium (V) is not
contained.
[0024]
Based on the findings described so far, the present
inventors have completed a forged crankshaft according to
the present embodiment. Hereafter, the forged crankshaft
according to the present embodiment will be described in
detail.
[0025]
[Configuration of forged crankshaft]
Figure 1 is a diagram showing principal parts of a
forged crankshaft 1 according to the present embodiment.
The forged crankshaft 1 includes a crank pin 2, a crank
journal 3, and a crank arm 4. The crank arm 4 is
/ -y- -~
disposed between the crank pin 2 and the crank journal 3,
and is linked to the crank pin 2 and the crank journal 3.
The forged crankshaft 1 further includes a fillet portion
5. The fillet portion 5 corresponds to a joint portion
between the crank pin 2 and the crank arm 4.
[0026]
The crank pin 2 is rotatably attached with a
connecting rod (not shown). The crank pin 2 is disposed
to be shifted from the rotational axis of the forged
crankshaft 1. The crank journal 3 is disposed coaxial
with the rotational axis of the forged crankshaft 1.
[0027]
The crank pin 2 rotates via the inner surface of the
large-end portion of the connecting rod and a slide
bearing. Therefore, the surface of the crank pin 2 is
required to have the wear resistance.
[0028]
As described above, an ordinary forged crankshaft is
subjected to a surface hardening heat treatment. The
surface hardening heat treatment includes, for example,
induction quenching treatment and nitriding treatment. A
surface hardening heat treatment hardens the surface of
the crank pin thereby improving wear resistance thereof.
[0029]
However, the forged crankshaft 1 according to the
present embodiment includes a crank pin 2 for which the
surface hardening heat treatment is omitted. That is,
ID
/ -~-
the crank pin 2 is not subjected to the surface hardening
heat treatment. This will reduce the manufacturing cost.
Further, along with for the crank pin 2, the surface
hardening heat treatment may be omitted for the crank
journal 3 as well, or the surface hardening heat
treatment may be omitted for the entire forged crankshaft
l.
[0030]
The forged crankshaft 1 is produced by hot forging
the non-heat treated steel material described below.
This allows the forged crankshaft 1 to exhibit excellent
wear resistance even if a surface hardening heat
treatment thereof is omitted. Further, even if vanadium
(V) is not contained, the forged crankshaft 1 has
excellent strength and hardness. Hereafter, the chemical
composition and microstructure of a non-heat treated
steel material constituting the forged crankshaft 1, and
a manufacturing method of the non-heat treated steel
material will be described in detail.
[0031]
[Chemical composition]
The non-heat treated steel material constituting the
forged crankshaft 1 has the following chemical
composition. Hereafter, percentage relating to elements
means mass%.
[0032]
c: 0.45 to 0.70%
II
-;1-
Carbon (C) reduces pro-eutectoid ferrite ratio in
steel and increases the area ratio of pearlite in steel.
This increases the strength and hardness of steel, and
improves wear resistance thereof as well. When the C
content is too low, the pro-eutectoid ferrite ratio
exceeds 10% in the microstructure of steel. On the other
hand, when the C content is too high, the steel is
excessively hardened, there..b. y reducing the machinability
of steel. Therefore, the C content is 0.45 to 0.70%. A
preferable C content is 0.48 to 0.60%, and a more
preferable one is 0.50 to 0.58%.
[0033]
Si: 0.75 to 1.30%
Silicon (Si) dissolves in ferrite within pearlite,
and strengthens the ferrite. Si further dissolves in
pro-eutectoid ferrite as well, and strengthens the proeutectoid
ferrite. Therefore, Si increases the strength
and hardness of steel. Si also deoxidizes steel. When
the Si content is too low, the strength and hardness of
steel declines. On the other hand, when the Si content
is too high, the steel is decarburized during hot forging.
Therefore, the Si content is 0.75 to 1.30%. A preferable
Si content is 0.90 to 1.20%.
[0034]
Mn: 1.00 to 2.00%
Manganese (Mn) dissolves in steel and increases the
strength and hardness of the steel. Mn further
12
-/-
suppresses the production of pro-eutectoid ferrite. When
the Mn content is too low, the pro-eutectoid ferrite
ratio exceeds 10%. On the other hand, when the Mn
content is too high, bainite is produced. Bainite
reduces the wear resistance and machinability of steel.
For this reason, the production of bainite is not
preferable. Therefore, the Mn content is 1.00 to 2.00.
A preferable Mn content is 1.20 to 1.70%, and a more
preferable one is 1.30 to 1.50%.
[0035]
S: 0.03 to 0.30%
Sulfur (S) produces sulfides such as MnS, thereby
improving the machinability of steel. On the other hand,
when the S content is too high, the hot workability of
)
steel deteriorates. Therefore, the S content is 0.03 to
0.30%. A preferable S content is 0.04 to 0.06%.
[0036]
Cr: 0.05 to 0.30%
Chromium (Cr) increases the strength and hardness of
steel. Cr further suppresses the production of proeutectoid
ferrite in steel. When the Cr content is too
low, the pro-eutectoid ferrite ratio exceeds 10%. On the
other hand, when the Cr content is too high, bainite is
produced. Since bainite reduces the wear resistance and
machinability of steel, the production of bainite is not
preferable. Therefore, the Cr content is 0.05 to 0.30%.
A preferable Cr content is 0.08 to 0.20%.
13
- )/-
[0037]
AI: 0.005 to 0.050%
Aluminum (AI) deoxidizes steel. Al further produces
nitrides and refines grains. As the result of grain
refining, the strength, hardness, and toughness of steel
increase. On the other hand, when the Al content is too
high, A1203 inclusions are produced. The A1203 inclusions
deteriorate the machinability of steel. Therefore, the
Al content is 0.005 to 0.050%. A preferable Al content
is 0.010 to 0.040%. The Al content in the present
embodiment is the content of acid-soluble Al (Sol. AI).
[0038]
N: 0.005 to 0.020%
Nitrogen (N) produces nitrides and carbo-nitrides.
Nitrides and carbo-nitrides refine grains, and further
increase the strength, hardness and toughness of steel by
precipitation strengthening. On the other hand, when the
N content is too high, defects such as voids become more
likely to occur in steel. Therefore, the N content is
0.005 to 0.020%. A preferable N content is 0.008 to
0.020%, and more preferable one is 0.012 to 0.018%.
[0039]
The balance of the chemical composition of non-heat
treated steel material constituting the forged crankshaft
1 consists of Fe and impurities. Here, the impurities
refer to ores and scraps which are used as raw materials
for steel, or elements which are mixed from the
l4-
-;-( -
environments of production process. The impurities are,
for example, phosphorus (P), oxygen (0), and the like.
[0040]
The non-heat treated steel material constituting
forged crankshaft 1 of the present invention may further
contain Ti in place of part of Fe.
[0041]
Ti: not more than 0.03% (not including 0%)
Ti forms nitrides and carbo-nitrides and refines
grains of steel. Therefore, Ti increases the strength,
toughness and fatigue strength of steel. On the other
hand, when the Ti content is too high, a large amount of
nitrides is produced, thereby deteriorating machinability
of steel. Therefore, the Ti content is not more than
0.03% (not including 0%). When the Ti content is not
less than 0.005%, the above described effects is
noticeably obtained.
[0042]
The non-heat treated steel material constituting the
forged crankshaft 1 does not substantially contain
vanadium (V). In short, V is an impurity in the non-heat
treated steel material constituting the forged crankshaft
1, and preferably V is not contained. Even if V is not
contained, since the Si content is high, the strength and
hardness of the non-heat treated steel material is high.
[0043]
[Microstructure]
-/-
The chemical composition of the non-heat treated
steel material constituting the forged crankshaft 1
further satisfies Formula (1).
1.1C + Mn + 0.2Cr > 2.0 (1)
Where, the symbol of each element in Formula (1) is
substituted by the content (mass%) of the each element.
[0044]
As described above, any of C, Mn and Cr suppresses
the production of pro-eutectoid ferrite. As the result
of the chemical composition satisfying Formula (1), the
matrix of the non-heat treated steel material
constituting the forged crankshaft 1 becomes a ferritepearlite
microstructure in which pro-eutectoid ferrite
ratio is less than 10%, or a pearlite microstructure.
[0045]
The pro-eutectoid ferrite ratio of the
microstructure of the non-heat treated steel constituting
the forged crankshaft 1 may be 0%, that is, a singlephase
microstructure of pearlite. Hereafter, the
relationship between Formula (1) and pro-eutectoid
ferrite ratio will be described.
[0046]
Figure 2 is a diagram showing the relationship
between the F-value shown by Formula (2) and proeutectoid
ferrite ratio.
F = 1.1C + Mn + 0.2Cr (2)
IG
- ;;£-
Formula (2) is a mathematical expression among the C
content, Mn content, and Cr content in the non-heat
treated steel material, and is the left-hand side of
Formula (1). Figure 2 has been obtained by the below
described method.
[0047]
A plurality of steels each having a different C
content, Mn content and Cr content were melt to produce
ingots of 50 kg. The plurality of produced ingots were
heated to 1250°C. The plurality of heated ingots were
subjected to hot forging to produce a plurality of roundbar
forged products having an outer diameter of 50 mm.
Hereafter, a round-bar forged product is simply referred
to as a "round bar."
[0048]
In each of the round bars produced, a specimen for
microstructure observation (hereafter, referred to as a
"micro-specimen") was sampled from an intermediate
position (hereafter, referred to as R/2) between the
central axis and the surface. Using the sampled microspecimen,
the microstructure of each round bar was
observed. More specifically, the surface of each microspecimen
was mirror-polished. Then, the surface of the
polished micro-specimen was etched with a Nital etch
solution. The surface of the etched micro-specimen was
observed with an optical microscope.
[0049]
1':(-
-y-
Any of produced round bars had a matrix of ferritepearlite
microstructure, or a pearlite microstructure.
Then, the pro-eutectoid ferrite ratio of each round bar
was measured. More specifically, a respective region of
0.03 mm2 (150 ~x 200 pm/field of view) was observed for
arbitrary 20 fields of view by an optical microscope with
a magnification of 400 times. The microphotograph (a
magnification of 400 times) of each region was image
processed to determine the area ratio of pro-eutectoid
ferrite which constitutes each region. An average value
of the determined 20 area ratios of pro-eutectoid ferrite
was defined as the area ratio of pro-eutectoid ferrite
(that is, the pro-eutectoid ratio in units of percent) .
[0050 ]
Based on the pro-eutectoid ferrite ratio obtained by
the method described above, Figure 2 was obtained. The
abscissa of Figure 2 shows the F-value defined by Formula
(2). The ordinate of Figure 2 shows the pro-eutectoid
ferrite ratio (%). Referring to Figure 2, when the Fvalue
is more than 1.4 and not more than 2.0, the proeutectoid
ferrite ratio is not less than 10%. Moreover,
even if the F-value increases, the pro-eutectoid ferrite
ratio does not greatly change. On the other hand, if the
F-value exceeds 2.0, the pro-eutectoid ferrite ratio
becomes less than 10%. Then, as the F-value increases,
the pro-eutectoid ferrite ratio rapidly declines.
Further, if the F-value exceeds 2.25%, the pro-eutectoid
re
/
.~.
, -yferrite
ratio becomes 0%. In short, a curve "e" in
Figure 2 has an inflection point in the vicinity of the
F-value = 2.0.
[0051]
Therefore, if the chemical composition of non-heat
treated steel material satisfies Formula (1), the matrix
of the non-heat treated steel material becomes a ferritepearlite
microstructure in which the pro-eutectoid
ferrite ratio is less than 10%, or a pearlite
microstructure. Since the forged crankshaft 1 is
produced by being hot forged, the matrix of the forged
crankshaft 1 will become a ferrite-pearlite
microstructure in which the pro-eutectoid ferrite ratio
is less than 10%, or a pearlite microstructure. A
preferable F-value is not less than 2.05.
[0052]
Making the microstructure of the non-heat treated
steel material be such a microstructure allows the forged
crankshaft 1 to have excellent wear resistance.
Therefore, even if a surface hardening heat treatment to
the surface of the crank pin 2 is omitted, the wear
resistance of the crank pin 2 will be high.
[0053]
Further, the non-heat treated steel material that
makes up the forged crankshaft 1 contains no vanadium (V),
and thermal refining treatment is omitted. However,
having the above described chemical composition and
-y"-
satisfying Formula (1) allow the forged crankshaft 1 to
have high strength and hardness.
[0054 ]
[Production method]
An example of the method for producing a forged
crankshaft will be described.
[0055]
Molten steel of the above described chemical
composition is produced. The molten steel is cast into a
cast piece by a continuous casting process. The molten
steel may be formed into an ingot by an ingot-making
process. The cast piece or ingot may be subjected to hot
working to form a billet or a steel bar.
[0056]
Next, the cast piece, ingot, billet, or steel bar is
hot forged and cooled in the atmosphere to produce an
intermediate product having a rough shape of crankshaft.
The intermediate product of crankshaft will not be
subjected to thermal refining treatment. That is, the
intermediate product of crankshaft is not thermally
refined. The intermediate product of crankshaft is cut
into a predetermined shape by machining to produce a
forged crankshaft 1.
[0057]
In the produced forged crankshaft 1, at least the
crank pin 2 is omitted from the surface hardening heat
treatment. That is, at least the surface of the crank
-~-
pin 2 is neither subjected to an induction quenching
treatment, nor to a nitriding treatment. Further, the
fillet portion 5 may be subjected to a fillet rolling
processing to improve the surface hardness of the fillet
portion 5 by work hardening. In the fillet rolling
processing, a roller is pressed against the surface of
the fillet portion 5 while the forged crankshaft 1 is
rotated. As the result of this, the surface of the
fillet portion 5 is subjected to plastic working, thereby
being work hardened. The fillet portion 5 may not be
subjected to fillet rolling processing.
[0058]
The matrix of the non-heat treated steel material
constituting the forged crankshaft 1 produced by the
above described processes consists of a ferrite-pearlite
microstructure in which the pro-eutectoid ferrite ratio
is less than 10%, or a pearlite microstructure.
Therefore, even if vanadium (V) is not contained and the
thermal refining treatment and the surface hardening heat
treatment are omitted, the surface of the forged
crankshaft 1 has excellent wear resistance, and the
forged crankshaft 1 has excellent strength and hardness.
[Examples]
[0059]
Steels of Nos. 1 to 10 having chemical compositions
shown in Table 1 were melted in a vacuum induction
heating furnace to obtain molten steel. The molten steel
21
-rwas
subjected to an ingot-making process to produce a
columnar ingot. The produced ingot had a weight of 50 kg,
and an outer diameter of 100 rom.
[Table 1]
Table 1
-1-
it
~"
\".)
oJ
No.
Chemical Composition (in units of mass%, and the balance is Fe and impurities)
F-value
C Si Mn Cr Ti AI S N V
1 0.49 0.94 1.50 0.10 - 0.012 0.064 0.0072 - 2.06
2 0.54 0.99 1.49 0.28 - 0.035 0.057 0.0088 - 2.14
3 0.54 1.00 1.50 0.09 0.024 0.034 0.056 0.0097 - 2.11
4 0.70 0.93 1.50 0.09 - 0.013 0.061 0.0070 - 2.29
5 0.48 1.01 1.22 0.10 - 0.020 0.062 0.0122 - 1.77
6 0.47 0.54 0.90 0.12 - 0.039 0.054 0.0092 0.084 1.44
7 0.38 0.70 1.40 0.18 0.003 0.005 0.050 0.0034 0.120 1.85
8 0.39 0.58 1.48 0.12 0.003 0.003 0.067 0.0191 - 1.93
9 0.43 0.27 1.23 0.17 0.004 0.002 0.048 0.0095 0.100 1.74
10 0.38 0.33 0.86 1.19 - 0.040 0.012 0.0082 - 1.52
-y-
[0060]
The "F-value" column in Table 1 shows an F-value of
steel of each number which is calculated based on Formula
(2). Referring to Table 1, the chemical compositions of
steel No. 1 to No. 4 were within the range of the
chemical composition of the non-heat treated steel
material for crankshaft according to the present
embodiment. Moreover, none of the chemical compositions
No. 1 to No. 4 contained vanadium (V). The chemical
compositions of No. 1 to No. 4 satisfied Formula (1).
That is, F-values of No.1 to No.4 exceeded 2.0.
[0061]
The composition of steel No. 5 was within the range
of the chemical composition of the non-heat treated steel
material according to the present embodiment. However,
the chemical composition of steel No. 5 did not satisfy
Formula (1). That is, the F-value of No. 5 was less than
2.0.
Further, the chemical compositions of steels Nos. 6
to 10 were out of the range of the chemical composition
of the non-heat treated steel material according to the
present embodiment, and did not satisfy Formula (1).
[0062]
Ingots of each number were hot forged to produce
forged products. To be specific, each ingot was heated
to 1250°C by a well-known heating furnace. The heated
ingot was hot forged to produce a forged product of round
24-
-ybar
having an outer diameter of 50 mm (hereafter, simply
referred to as a round bar). The finishing temperature
during hot forging was 1000°C. After hot forging, the
round bar was cooled in the atmosphere. After cooling,
the round bar was not subjected to thermal refining
treatment. Further, the surface of the round bar was not
subjected to a surface hardening heat treatment such as
induction quenching treatment and nitriding treatment.
[0063]
[Microstructure investigation]
A micro-specimen was sampled from a round bar of
each number, and the microstructure was observed. A
micro-specimen for microstructure observation was sampled
from the R/2 position of the round bar. The surface of
the sampled micro-specimen was mirror polished. After
polishing, the surface of the micro-specimen was etched
with a Nital etch solution. After etching, the
microstructure of the etched surface was observed with an
optical microscope of a magnification of 400 times.
[0064]
As a result of observation, any of the matrices of
the round bars No. 1 to No. 9 was a ferrite-pearlite
microstructure or a pearlite microstructure. On the
other hand, the matrix of No. 10 was substantially a
single phase of martensite. Then, the pro-eutectoid
ferrite ratios in the microstructure of the round bars No.
1 to 10 were determined based on the above described
()5
method. Specifically, a respective region of 0.03 rom2
(150 pm x 200 pm/field of view) was observed for
arbitrary 20 fields of view by an optical microscope with
a magnification of 400 times. The microphotograph
(magnification of 400 times) of each region was image
processed to determine the area ratio of pro-eutectoid
ferrite which constitutes each region. An average value
of the determined 20 area ratios of pro-eutectoid ferrite
was defined as the pro-eutectoid ferrite ratio (%).
[0065 ]
[Surface hardness investigation]
The surface hardness was investigated on the round
bars No. 1 to No. 10. Specifically, a Rockwell hardness
test using the C scale was conducted according to JIS
Z2245.
[0066]
[Wear resistance investigation]
A pin-on-disk test was conducted to investigate the
wear resistance of round bars No. 1 to No. 10.
Specifically, a specimen of 1.5 rom x 2.0 rom x 3.7 rom was
sampled from each round bar. The surface of 2.0 rom x 3.7
rom (hereafter, referred to as a "principal plane") of
each specimen was parallel with a cross section of the
round bar. That is, the normal line to the principal
plane of each specimen was parallel with the central axis
of round bar. The weight of specimen before test was
measured.
-~-
[0067]
An emery paper of grit 800 was stuck on the surface
of the rotary disk of the pin-on-disk tester. Then, the
rotary disk was rotated at a circumferential speed of
39.6 m/min for 50 minutes while the principal plane of
specimen was pressed against the emery paper at an
interfacial pressure of 26 gf/mm2
•
[0068 ]
After rotating for 50 minutes, the weight of
specimen was measured. Then, the difference in weight
between before and after testing was defined as the
amount of wear (g).
[0069]
[Investigation results]
Investigation results are shown in Table 2.
- ;p1-
[Table 2]
Table 2
Pro-eutectoid Amount of
No. Microstructure ferrite ratio HRC wear
(%) (Il)
1 Ferrite-Pearlite 5 23.4 0.0085
2 Ferrite-Pearlite 3 26.2 0.0083
3 Ferrite-Pearlite 1 25.5 0.0081
4 Pearlite 0 31.2 0.0081
5 Ferrite-Pearlite 12 21.3 0.0125
6 Ferrite-Pearlite 11 22.0 0.0112
7 F, errite-Pearlite 13 24.3 0.0130
8 Ferrite-Pearlite 10 20.6 0.0105
9 Ferrite-Pearlite 11 23.3 0.0110
10 Martensite 0 32.4 0.0119
[0070]
The "Microstructure" column in Table 2 shows the
microstructure of each number. The "Pro-eutectoid
ferrite ratio (%)" shows a pro-eutectoid ferrite ratio
(%) of each number obtained by the microstructure testing.
The "HRC" column shows the surface hardness (HRC) of each
number obtained by Rockwell Hardness testing using scale
C. The "Amount of wear (g)" column shows the amount of
wear (g) of each number obtained by the pin-on-disk test.
[0071]
Referring to Table 2, as described above, the matrix
of any of round bars Nos. 1 to 3, and Nos. 5 to 9 was
ferrite-pearlite microstructure, and the matrix of No. 4
was pearlite microstructure. On the other hand, since
-~-
the round bar No. 10 had a high Cr content, the matrix of
No. 10 consisted of a martensite microstructure.
[0072]
The chemical compositions of Nos. 1 to 4 were within
the range of the chemical composition of the non-heat
treated steel material according to the present
embodiment, and satisfied Formula (1). Therefore, the
pro-eutectoid ferrite ratio was less than 10%. The
amount of wear of any of No. 1 to No. 4 was less than
0.0100 g, thus exhibiting excellent wear resistance.
Further, the Rockwell hardnesses HRCs of Nos. 1 to 4 were
not less than 23.0, and Nos. 1 to 4 each had high
strength and hardness.
[0073]
On the other hand, although the chemical composition
of No. 5 was within the range of the chemical composition
of the non-heat treated steel material according to the
present embodiment, it did not satisfy Formula (1).
Therefore, the pro-eutectoid ferrite ratio was not less
than 10%. Moreover, the Rockwell hardness HRC of No. 5
was less than 23.0 and the amount of wear exceeded 0.0100
g.
The chemical compositions of Nos. 6 to 9 were out of
the range of chemical composition of the non-heat treated
steel material according to the present embodiment, and
moreover, did not satisfy Formula (1). As a result, the
pro-eutectoid ferrite ratios in the microstructures of
-~-
Nos. 6 to 9 were not less than 10%. Further, the amounts
of wear of Nos. 6 to 9 exceeded 0.0100 g.
[0074]
Since the matrix of the round bar No. 10 consisted
of a martensite microstructure, the Rockwell hardness HRC
was high. However, the amount of wear exceeded 0.0100 g.
[0075]
Although, the embodiment of the present invention
has been described so far, the above described embodiment
is merely exemplification for carrying out the present
invention. Therefore, the present invention will not be
limited to the above described embodiment and can be
carried out by appropriately modifying the above
described embodiment within a range not departing from
the spirit thereof.

-~ -
,.,(' We claim:
1. A forged crankshaft comprising a non-heat treated
steel material, wherein
the non-heat treated steel material has a chemical
composition which contains, by mass%, C: 0.45 to 0.70%,
Si: 0.75 to 1.30%, Mn: 1.00 to 2.00%, S: 0.03 to 0.30%,
Cr: 0.05 to 0.30%, AI: 0.005 to 0.050%, and N: 0.005 to
0.020%, the balance being Fe and impurities, and
satisfies Formula (1), and wherein
a matrix of the non-heat treated steel material is
one of a ferrite-pearlite microstructure in which an area
ratio of pro-eutectoid ferrite is less than 10% or a
pearlite microstructure:
I.IC + Mn + 0.2Cr > 2.0 ( 1)
where a symbol of each element in Formula (1) is
substituted by the content (mass%) of the each element.
2. The forged crankshaft according to claim 1, wherein
the chemical composition of the non-heat treated
steel material contains Ti: no more than 0.03% in place
of part of Fe.
3. The forged crankshaft according to one of claims 1
and 2, wherein
the forged crankshaft includes a crank pin for which
a surface hardening heat treatment is omitted.