DESCRIPTION
STEEL FOR INDUCTION HARDENING AND CRANKSHAFT MANUFACTURED
USING THE SAME
Technical Field
[OOOl]
The present invention relates a steel for induction
hardening and a crankshaft manufactured using the same.
More particularly, the present invention relates to a
steel for induction hardening that is used for a
crankshaft manufactured by induction hardening, and a
crankshaft manufactured using the steel for induction
hardening.
Background Art
[0002]
Some crankshaft is manufactured by being subjected
to induction hardening and tempering followed by grinding.
Induction hardening induces residual stress in steel.
The residual stress causes cracks such as quenching
cracks and grinding cracks. Tempering reduces residual
stress and restrains the occurrence of cracks.
[0003]
If a tempering process can be omitted, the
manufacturing cost goes down. However, cracks
attributable to the residual stress induced at the time
of induction hardening are liable to occur. For this
reason, there has been a demand for a steel for induction
hardening in which cracks are less liable to occur even
if a tempering process is omitted in the manufacturing
process of a crankshaft.
JP61-186419A, JP2000-26933A, JP2005-256134A, and
JP2007-113063A disclose steels which are used for
manufacturing a hot forged product and a crankshaft, and
in which cracks attributable to induction hardening are
less liable to occur.
[0005]
In the method for manufacturing a driveshaft
disclosed in JP61-186419A, the occurrence of quenching
cracks attributable to induction hardening is restrained
by reducing the C content in a steel product. In JP61-
186419A, the steel product further contains B to
compensate the decrease in hardenability caused by the
reduction in the C content.
[0006]
In the steel for hot forging disclosed in JP2000-
26933A, at least 0.04 wt% of Ti is contained to change
the mode of sulfide, whereby the machinability after
forging is improved, and the occurrence of grinding
cracks at the time of grinding is restrained.
[0007]
The steel material for crankshaft disclosed in
JP2005-256134A contains at least 0.4 mass% of Si. A
large amount of Si contained reduces the shrinkage at the
time when carbides are produced by heat generated at the
time of grinding, and restrains the occurrence of
grinding cracks.
[0008]
The hot-forged part disclosed in JP2007-113063A has
a chemical composition such that a formula of fn2 = 521 -
353C - 22Si - 25Mn - 8Cu - 17Ni - 18Cr - 26Mo is at least
300. By making the formula of fn2 at least 300, the
occurrence of quenching cracks is restrained.
Disclosure of the Invention
[0009]
However, like the steel product disclosed in JP61-
186419A, in the steel for induction hardening that
contains B, the variations in hardenability are large, so
that the quality is less liable to be stabilized. The
steel for hot forging disclosed in JP2000-26933A contains
much Ti, so that the production cost is high. The steel
materials disclosed in JP2005-256134A and JP2007-113063A
contain much Si. For this reason, the amount of
formation of scale increases. Therefore, it is more
favorable that the occurrence of cracks caused by
induction hardening can be restrained by any other method
different from the methods in the above-described Patent
Documents.
[ OOlO]
Further, it is more preferable that the hardness of
crankshaft is higher. Also, the pin of crankshaft is
inserted into the large end portion of a connecting rod,
and rotates with respect to the inner surface of the
large end portion of the connecting rod via a sliding
bearing. For this reason, the surface of crankshaft is
required to have excellent seizure resistance. Therefore,
the steel for induction hardening used for manufacturing
the crankshaft is required to have high hardness and
seizure resistance.
[ OOll]
It is an objective of the present invention to
provide a steel for induction hardening in which cracks
are less liable to occur and high hardness and excellent
seizure resistance are attained even if a tempering
process after induction hardening is omitted.
[0012]
The steel for induction hardening according to one
embodiment of the present invention contains, by mass
percent, C: 0.20 to 0.34%, Si: at most 0.20%, Mn: 0.75 to
2.0%, P: at most 0.03%, S: at most 0.20%, Cr: 0.05 to
1.2%, Ti: at least 0.002% and less than 0.030%, Al: 0.005
to 0.04%, and N: 0.0040 to 0.020%, the balance being Fe
and impurities, and satisfies Formula (1):
1.20 5 Mn + Cr 52.10 (1)
where the content (mass%) of each element is substituted
for each of the symbols of elements in Formula (1).
[0013]
For the steel for induction hardening according to
one embodiment of the present invention, cracks are less
liable to occur and high hardness and excellent seizure
resistance are attained even if a tempering process after
induction hardening is omitted.
[0014]
The crankshaft according to one embodiment of the
present invention is manufactured by induction-hardening
the above-described steel for induction hardening. The
crankshaft may be manufactured by being not tempered.
Best Mode for Carrying Out the Invention
[0015]
An embodiment of the present invention will now be
described in detail. Hereinafter, symbol % concerning a
chemical element means percent by mass.
[0016]
[Outline of steel for induction hardening according to
this embodiment]
The present inventors conducted examinations and
studies to improve the cracking resistance, hardness, and
seizure resistance of the steel for induction hardening
that has not been tempered. As the result, the present
inventors obtained the findings described below.
[0017]
The cracks (quenching cracks and grinding cracks)
occurring in the steel for induction hardening when a
crankshaft is manufactured are attributable to residual
stress induced in the steel at the time of induction
hardening and grinding. In order to reduce the residual
stress, to decrease the carbon (C) content in the steel
is effective. If the C content is decreased, the change
in volume of steel caused by heat can be restrained, so
that the residual stress can be reduced. Therefore,
cracks are less liable to occur. That is, the cracking
resistance is improved. If the C content is at most
0.34%, the occurrence of cracks can be restrained even if
a tempering process is omitted.
[0018]
(2) Carbon LC) increases the hardness of steel.
Therefore, if the C content is decreased, the hardness of
the steel for induction hardening decreases. Therefore,
in place of C, manganese (Mn) and chromium (Cr)
satisfying Formula (1) are contained to increase the
hardness of steel.
[0019]
(3) If the thermal conductivity of steel decreases,
the seizure resistance of steel decreases. Silicon (Si)
decreases the thermal conductivity of steel. If the Si
content is at most 0.20%, the thermal conductivity of
steel can be kept high, and excellent seizure resistance
can be attained.
[0020]
Based on the above-described findings, the present
inventors completed the steel for induction hardening
according to this embodiment. Hereunder, the steel for
induction hardening according to this embodiment is
described in detail.
[0021]
[Chemical composition]
The steel for induction hardening according to this
embodiment has the chemical composition described below.
LO0221
C: 0.20 to 0.34%
Carbon (C) increases the strength and hardness of
steel. On the other hand, if the C content is too high,
the change in volume of steel caused by heat increases,
so that residual stress is liable to be induced in steel.
For this reason, cracks are liable to occur. Therefore,
the C content is 0.20 to 0.34%. The C content is
preferably 0.28 to 0.34, further preferably 0.30 to 0.33.
[0023]
Si: at most 0.20%
Silicon (Si) lowers the thermal conductivity of
steel, and decreases the seizure resistance of steel. On
the other hand, if the Si content is too high, the amount
of scale formed at the time of hot forging becomes large,
so that the surface texture of the forged steel becomes
rough. Therefore, it is more preferable that the Si
content is lower. The Si content is at most 0.20%. The
Si content is preferably at most 0.18%, further
preferably at most 0.10%.
[0024]
Mn: 0.75 to 2.0%
Manganese (Mn) dissolves in steel, and enhances the
strength and toughness of steel. Further, Mn increases
the hardness of the steel before being inductionhardented.
Further, Mn forms MnS, and restrains the
production of FeS. By restraining the production of FeS,
the hot ductility of steel is improved, and cracks are
made less liable to occur at the time of forging. On the
other hand, if the Mn content is too high, bainite is
produced. Bainite decreases the machinability of steel.
For this reason, the production of bainite is unfavorable
Also, if the Mn content is too high, the hardness of
steel becomes too high, and cracks are liable to occur.
Further, Mn decreases the thermal conductivity of steel.
Therefore, the Mn content is 0.75 to 2.0%. The Mn
content is preferably 1.10 to 1.70%, further preferably
1.30 to 1.60%.
[0025]
P: at most 0.03%
Phosphorus (P) is an impurity. Phosphorus decreases
the hot ductility. Further, P decreases the cracking
resistance at the time of quenching. Therefore, it is
more preferable that the P content is lower. The P
content is at most 0.03%. The P content is preferably at
most 0.020%, further preferably at most 0.010%.
[0026]
S: at most 0.20%
Sulfur ( S ) is an impurity. However, if S is
contained, MnS isformed, and the machinability of steel
is improved. On the other hand, if the S content is too
high, the hot workability of steel deteriorates. Further,
if the S content is too high, the number of sulfides in
steel is increased, and grinding cracks are liable to
occur. Therefore, the S content is at most 0.20%. In
the case where the advantageous effect of improving the
machinability of steel is achieved, the lower limit of
the S content is preferably at least 0.02%. The S content
is further preferably 0.02 to 0.07%.
LO0271
Cr: 0.05 to 1.2%
Chromium (Cr) enhances the strength and hardness of
steel. Specifically, Cr decreases the ACg transformation
point. On account of the decrease of the Ac3
transformation point, the outer layer of steel is liable
to come to be of a uniform martensitic structure in
induction hardening. Also, Cr enhances the hardness of
the steel before being induction-hardented. On the other
hand, if the Cr content is too high, bainite is produced
on the base metal before being induction-hardented.
Since bainite decreases the machinability, the production
of bainite is unfavorable. Therefore, the Cr content is
0.05 to 1.2%. The Cr content is preferably 0.10 to 0.50%,
further preferably 0.15 to 0.30%.
[0028]
Ti: at least 0.002% and less than 0.030%
Titanium (Ti) forms nitrides and carbo-nitrides, and
makes the crystal grains fine by means of pinning effect.
By making the crystal grains fine, the ductility and
toughness of steel are improved, and the cracks
attributable to induction hardening are made less liable
to occur. On the other hand, if the Ti content is too
high, coarse nitrides are formed, and the machinability
of steel is decreased. Further, the manufacturing cost
is increased. Therefore, the Ti content is at least
0.002% and less than 0.030%. The Ti content is
preferably at least 0.005% and less than 0.030%.
[0029]
Al: 0.005 to 0.04%
Aluminum (Al) deoxidizes steel. Further, A1 forms
nitrides, and makes the crystal grains fine by means of
pinning effect. By making the crystal grains fine, the
ductility and toughness of steel are improved, and cracks
attributable to induction hardening are made less liable
to occur. On the other hand, if the A1 content is too
high, the toughness of steel rather decreases. Therefore,
the A1 content is 0.005 to 0.04%. The A1 content is
preferably 0.008 to 0.030%. The A1 content in this
embodiment is the content of acid-soluble A1 (Sol.Al).
[0030]
N: 0.0040 to 0.020%
Nitrogen (N) combines with A1 and Ti to forms
nitrides and carbo-nitrides. These nitrides and carbonitrides
make the crystal grains fine by means of pinning
effect. By making the crystal grains fine, the ductility
and toughness of steel are improved, and cracks
attributable to induction hardening are made less liable
to occur. On the other hand, if the N content is too
high, defects such as voids are liable to occur in steel.
Therefore, the N content is 0.0040 to 0.020%. The N
content is preferably 0.0080 to 0.020%.
[0031]
The balance of chemical composition of the steel for
induction hardening according to this embodiment consists
of Fe and impurities. The impurities described herein
are elements that mixedly enter from ore and scrap used
as the raw materials of steel, the environment of
production process, and the like. In this embodiment,
the impurities are, for example, copper (Cu), nickel (Ni),
molybdenum (Mo) , and oxygen (0).
[0032]
The chemical composition of the steel for induction
hardening according to this embodiment satisfies Formula
(1) :
1.20 I Mn + Cr < 2.10 (1
where the content (mass%) of each element is substituted
for each of the symbols of elements in Formula (1).
[0033]
If the total of the Mn content and the Cr content is
at least 1.20%, high hardness can be attained even if the
C content is low. On the other hand, if the total of the
Mn content and the Cr content exceeds 2.10%, bainite is
produced in steel, and the machinability of steel
decreases. Further, the hardness becomes too high, and
cracks are liable to occur.
[0034]
[Micro-structure]
The micro-structure of the steel for induction
hardening according to this embodiment is a ferriticpearlitic
structure or a pearlitic structure.
[0035]
[Manufacturing method]
One example of the manufacturing method for a
crankshaft using the steel for induction hardening
according to this embodiment is explained.
[0036]
Molten steel having the above-described chemical
composition is produced. ~ 6 meol ten steel is cast into a
cast piece by the continuous casting process. The molten
steel may be cast into an ingot by the ingot-making
process. The cast piece or the ingot may be turned into
a billet or a steel bar by hot working.
[0037]
Next, the cast piece, ingot, billet, or steel bar is
hot-forged and allowed to cool in the air to produce an
intermediate product having a rough shape of crankshaft.
Then, the intermediate product of crankshaft is
induction-hardened under the well-known conditions.
[0038]
The intermediate product of crankshaft having been
induction-hardened is not tempered. That is, the
tempering process is omitted. The intermediate product
of crankshaft that is not tempered is ground into a
predetermined shape by machining, whereby a crankshaft is
manufactured.
[0039]
The crankshaft is manufactured using the steel for
induction hardening having the above-described chemical
composition. Therefore, after the induction hardening
has been performed, cracks are less liable to occur even
if the tempering process is omitted. Further, because
the Mn and Cr contents satisfy Formula (I), the steel for
induction hardening has a high hardness. Further,
because of low content of Si, the steel for induction
hardening is excellent in seizure resistance.
Examples
The micro-structure, hardness, thermal conductivity,
and cracking resistance of each of the steels for
induction hardening having various chemical compositions
were examined.
[Test method]
Each of the steels of marks A to X having the
chemical compositions given in Table 1 was melted in a
vacuum induction heating furnace to produce molten steel.
From the molten steel, a columnar ingot was produced.
The produced ingot had a weight of 50 kg and an outside

[0042]
In the "F value" column in Table 1, the value of F
expressed by following Formula (2) is described.
F = Mn + Cr (2)
in which, for each of the symbols of elements in Formula
(21, the content (mass%) of each element is substituted.
[0043]
Referring to Table 1, the chemical composition of
each of the steels of marks A to J was within the range
of the chemical composition of the steel for induction
hardening according to this embodiment, and satisfied
Formula (1) .
[0044]
On the other hand, the chemical composition of each
of the steels of marks K to X was out of the range of the
chemical composition of the steel for induction hardening
according to this embodiment, or did not satisfy Formula
(1). Specifically, the C content, the Si content, and
the A1 content of mark K, the C content and the A1
content of mark L, the Si content of mark MI the C
content of mark N, the Mn contents of marks 0 and PI the
Cr contents of marks Q and R, the S content of mark S,
the N content of mark TI the A1 content of mark U, and
the Ti content of mark V were, respectively, out of the
range of the chemical composition of the steel for
induction hardening in accordance with this embodiment.
Also, the F value of mark P was smaller than the lower
limit of Formula (I), and the F values of marks 0 and Q
each exceeded the upper limit of Formula (1).
[0045]
The chemical compositions of marks W and X each were
within the range of the chemical composition of the steel
for induction hardening according to this embodiment.
However, the F value of mark W exceeded the upper limit
of Formula (I), and the F value of mark X was smaller
than the lower limit of Formula (1).
[0046]
The ingot of each mark was hot-forged to produce a
forged product. Specifically, each ingot was heated to
1250°C in a well-known heating furnace. The heated ingot
was hot-forged to produce a round-bar shaped forged
product (hereinafter, referred simply to as a round bar)
having an outside diameter of 65 mm. The finishing
temperature at the time of hot forging was 1000°C. After
hot forging, the round bar was allowed to cool in the air
[0047]
[Structure observation test]
The round bar of each mark having been allowed to
cool was cut perpendicularly to the axial direction to
sample a test specimen. The normal of the cross
sectional surface of the test specimen was in the axial
direction of the round bar. The cross sectional surface
of the test specimen was polished. After polishing, the
cross sectional surface was corroded with a nital etching
reagent. After corroding, the micro-structure at the R/2
position (a position determined by dividing the distance
between the center point and the outer periphery of the
cross sectional surface (circular shape) into two equal
parts) of the corroded cross sectional surface was
observed under an optical microscope having a
magnification of x400.
[0048]
[Grain size measurement test]
In the structure observation test, the austenite
grain size number was further determined at optional five
visual fields at the R/2 position of the cross sectional
surface of the round bar of each mark by using the
reference chart of grain size in JIS G0551. At this time,
a region surrounded by pro-eutectoid ferrite was
recognized as one crystal grain. For each mark, the
average value of the austenite grain size numbers
determined at five visual fields was defined as the
austenite grain size number of each mark.
[0049]
[Hardness test]
Each round bar was cut perpendicularly to the axial
direction. After the cross sectional surface had been
mirror polished, the Vickers hardness (Hv) was measured
at optional three points at the R/2 position of the cross
sectional surface in conformity to JIS 22244. The test
force was 98.07N. The average value of the obtained
three Vickers hardnesses was defined as the hardness of
each mark.
[OOSO]
[Crack 'test]
The round bar of each mark was turned by the wellknown
turning method to produce a ring test specimen
(hereinafter, referred simply to as a ring) having an
outside diameter of 60 mm, an inside diameter of 40 mm,
and a width of 15 mm. The outer peripheral surface of
each ring was induction-hardened. In the induction
hardening, the outer peripheral surface of the ring was
heated for 1.2 second under the conditions of 150 kHz of
frequency and 100 kW. After heating, the ring was watercooled.
[0051]
After the induction hardening has been performed,
the outer surface of ring was ground by cylindrical
plunge grinding. A grindstone of trade name of "80A 80N
8V201" manufactured by Kure Grinding Wheel K.K. was used.
The grindstone had an outer diameter of 405 mm, an inner
diameter of 152.4 mm, and a width of 25 mm. The
grindstone circumferential speed at the time of grinding
was 60 m/s, and the infeed speed was 0.5 mm/min. The
allowance was 0.3 mm dia / cut. That is, the outer
surface of ring was ground until the outer diameter of
each ring reached 59.7 rnm.
[0052]
The outer surface of the ground ring was immersed in
4.1% hydrochloric acid for 10 minutes. After immersion,
the presence of crack was checked visually by the
fluorescent magnetic particle flaw detection test.
[0053]
[Thermal conductivity measurement test]
A test specimen having a diameter of 5 mm and a
thickness of 1 mrn was sampled from the R/2 position of
each round bar. By using the sampled test specimen, the
thermal conductivity (W/(msK)) of the test specimen of
each mark was measured by the laser flash method in
conformity to JIS R1611.
[0054]
[Test results]
The test results are given in Table 1. In the
"Micro-structure" column in Table 1, the micro-structures
observed in the structure observation test are given. In
the "Hardness" column, the hardnesses (Hv) obtained by
the hardness test are given. In the "Grain size No."
column, the grain size numbers obtained by the grain size
measurement test are given. In the "Thermal
conductivity" column, the thermal conductivities
(W/( m0K)) obtained by the thermal conductivity
measurement test are given. In the "Crack" column, the
crack test results are given. The "Absent" indicates
that no crack was confirmed. The "Present" indicates
that a crack was confirmed.
[0055]
Referring to Table 1, the chemical compositions of
marks A to J each were within the range of the chemical
composition of the steel for induction hardening
according to this embodiment, and the F values of marks A
to J each satisfied Formula (1). Therefore, the hardness
at the R/2 position of the round bar of each mark was at
least 200 Hv in Vickers hardness. Also, in each of marks
A to J, the thermal conductivity was at least 40 W/ (maK),
and an excellent seizure resistance was exhibited.
Further, the grain size number was at least 2.0, and in
the crack test, no crack was confirmed on the test
specimen of each mark. The micro-structure was a
ferritic-pearlitic structure in each of marks A to J.
[0056]
On the other hand, the C contents of marks K and L
each exceeded the upper limit of the C content of the
steel for induction hardening according to this
embodiment. Therefore, in the crack test, a crack was
confirmed. Also, the Si contents of marks K and M each
exceeded the upper limit of the Si content of the steel
for induction hardening according to this embodiment.
Therefore, the thermal conductivities of marks K and M
each were lower than 40 W/(m0K), and it was presumed that
the seizure resistance was low.
[0057]
The C content of mark N was less than the lower
limit of the C content of the steel for induction
hardening according to this embodiment. Therefore, the
Vickers hardness was lower than 200 Hv.
[0058]
The Mn content of mark 0 exceeded the upper limit of
the Mn content of the steel for induction hardening in
accordance with this embodiment. Also, the F value
exceeded the upper limit of Formula (1). Therefore, in
the structure observation, bainite was observed. Further,
the Vickers hardness was too high, being 256 Hv, a crack
was confirmed in the crack test, and the thermal
conductivity was also lower than 40 W/(m*K).
[0059]
The Mn content of mark P was less than the lower
limit of the Mn content of the steel for induction
hardening according to this embodiment, and the F value
was smaller than the lower limit of Formula (1).
Therefore, the Vickers hardness was lower than 200 Hv.
[0060]
The Cr content of mark Q exceeded the upper limit of
the Cr content of the steel for induction hardening
according to this embodiment, and the F value exceeded
the upper limit of Formula (1). Therefore, bainite was
confirmed in the structure observation. Further, a crack
was confirmed in the crack test.
[0061]
The Cr content of mark R was less than the lower
limit of the Cr content of the steel for induction
hardening according to this embodiment. Therefore, the
Vickers hardness was lower than 200 Hv.
[0062]
The S content of mark S exceeded the upper limit of
the S content of the steel for induction hardening
according to this embodiment. Therefore, a crack was
confirmed in the crack test. It is presumed that the
reason for this was the production of much sulfides.
[0063]
The N content of mark T, the A1 content of mark U,
and the Ti content of mark V were less than the lower
limits of the N content, the A1 content, and the Ti
content of the steel for induction hardening according to
this embodiment, respectively. Therefore, the austenite
grain size number was less than 2.0, and a crack was
confirmed in the crack test. Also, the Vickers
hardnesses of marks U and V each were lower than 200 Hv.
[0064]
The chemical compositions of marks W and X each were
within the range of the chemical composition of the steel
for induction hardening according to this embodiment.
However, the F value of mark W exceeded the upper limit
of Formula (I), so that a crack was confirmed in the
crack test. Also, the F value of mark X was smaller than
the lower limit of Formula (I), so that the Vickers
hardness was lower than 200 Hv.
[0065]
The above is the explanation of an embodiment of the
present invention. The above-described embodiment is
merely an illustration for carrying out the present
invention. Therefore, the present invention is not
limited to the above-described embodiment, and the abovedescribed
embodiment can be carried out by being changed
as appropriate without departing from the spirit and
scope of the present invention.

We claim:
1. A steel for induction hardening, containing, by mass
percent, C: 0.20 to 0.34%, Si: at most 0.20%, Mn: 0.75 to
2.0%, P: at most 0.03%, S: at most 0.20%, Cr: 0.05 to
1.2%, Ti: at least 0.002% and less than 0.030%, Al: 0.005
to 0.04%, and N: 0.0040 to 0.020%, the balance being Fe
and impurities, and satisfying Formula (1):
1.20 5 Mn + Cr 5 2.10 (1)
where the content (mass%) of each element is substituted
for each of the symbols of elements in Formula (1) .
2. A crankshaft manufactured by induction-hardening the
steel for induction hardening according to claim 1.
3. The crankshaft according to claim 2, which is
manufactured by being not tempered.