ORIGINAL
Description
Title of Invention:
STEEL FOR INDUCTION HARDENING AND CRANKSHAFT MANUFACTURED
BY USING THE SAME
Technical Field
[OOOlI
The present invention relates to a steel for
induction hardening and a crankshaft manufactured by
using the steel for induction hardening.
Background Art
[00021
Engine parts such as a crankshaft are required to
have high wear resistance and high fatigue strength. To
enhance the wear resistance and fatigue strength,
induction hardening may be performed for engine parts.
Consequently, a steel for induction hardening is used for
engine parts. The steel for induction hardening have
been disclosed in, for example, JP2009-41046A, JP2010-
144226A, and JP9-235654A.
[00031
In induction hardening, quenching cracks
attributable to residual stress may occur. Accordingly,
the steel for induction hardening is required to have
quenching crack resistance.
[00041
Techniques for suppressing cracking of the steel for
induction hardening have been proposed in JP5-25546A,
JP2004-76086A, and JP2005-256134A.
[00051
JP5-25546A describes a method for manufacturing a
part that has an excellent torsional strength with
quenching cracks being prevented. Specifically, it
describes that, among others, the ratio t/r of effective
hardened depth t on account of induction hardening -
tempering to part radius r is made 0.4 to 0.8, and a
cross-sectional average hardness HVa is made 550 or
higher.
[00061
JP2004-76086A describes a high-strength steel part
capable of reliably improving the delayed fracture
characteristics even if the steel part has a wide
chemical composition. Specifically, it describes that,
for example, the content of fine Tic having a grain size
of 0.1 pm or smaller is 0.01%, and the ratio TiC/Ti of
the content of the fine Tic to the content of total Ti is
0.4 or higher.
[00071
JP2005-256134A describes a steel for induction
hardening in which grinding cracks are not produced even
if grinding is performed after induction hardening or
low-temperature tempering has been carried out and a
crankshaft by using this steel for induction hardening.
Specifically, it describes a steel for induction
hardening in which the number of MnS in steel in the
longitudinal cross section after rolling is 300/mm2 or
smaller, and the longitudinal shrinkage amount in
differential thermal expansion test is 15 pm or smaller,
and the like.
Disclosure of the Invention
[00081
JP5-25546A describes that the ratio t/r of effective
hardened depth t on account of induction hardening -
tempering to part radius r is made 0.8 or lower to
prevent quenching cracks. It is, however, more desirable
to have a technique capable of improving the quenching
crack resistance without restricting the ratio of
effective hardened depth t to part radius r.
[00091
JP2004-76086A assumes the good use of Tic formed by
high-temperature tempering. Therefore, this technique
cannot be applied to a general induction hardened part
subjected to low-temperature tempering.
[00101
The steel material described in JP2005-256134A aims
at the suppression of grinding cracks. Specifically, the
heat generated by grinding after induction hardening -
tempering is taken into consideration, and the shrinkage
amount in that temperature range is reduced. The
grinding cracks and the quenching cracks are fracture
modes in different stress states. Therefore, it is
unknown whether or not the steel material described in
JP2005-256134A has an excellent quenching crack
resistance.
[00111
Of the crankshafts, a large-sized crankshaft used
for trucks and the like is required to have further high
wear resistance and fatigue strength as compared with a
crankshaft having an ordinary size used for passenger
cars and the like. Therefore, the quench hardened layer
of the large-sized crankshaft is formed deeper as
compared with the crankshaft having an ordinary size used
for passenger cars and the like. In order to deepen the
quench hardened layer, the large-sized crankshaft is
heated for a long period of time with an output higher
than the ordinary one.
[00121
Therefore, in the case of the steel for induction
hardening used for such a large-sized crankshaft, it is
rather desirable that the occurrence of quenching cracks
is suppressed even if induction hardening, in which
heating is performed for a long period of time with a
high output, is carried out.
[00131
An objective of the present invention is to provide
a steel for induction hardening excellent in quenching
crack resistance and a crankshaft manufactured by using
the steel for induction hardening.
[00141
The steel for induction hardening in accordance with
one embodiment of the present invention comprising, by
mass percent, C: 0.35 to 0.6%, Si: at least 0.01% and
less than 0.40%, Mn: 1.0 to 2.0%, S: more than 0.010% and
at most 0.05%, Cr: 0.01 to 0.5%, Al: 0.001 to 0.05%, N:
Ti/3.4 to 0.02%, and Ti: 0.005 to 0.05%, the balance
being Fe and impurities, and satisfies formula (1) :
where, into each element symbol in formula (I), the
content (mass%) of the corresponding element is
substituted.
In the above-described steel for induction hardening,
in place of some of Fe, Ca: at most 0.005% may be
contained.
[0016]
The crankshaft in accordance with one embodiment of
the present invention is manufactured by induction
hardening the above-described steel for induction
hardening.
[00171
According to the present invention, there can be
provided a steel for induction hardening excellent in
quenching crack resistance and a crankshaft manufactured
by using the steel for induction hardening.
Brief Description of Drawings
[00181
[Figure 11 Figure 1 is a graph showing the relationship
between the value of the parameter 2s-3Ti specified in
the embodiment of the present invention and the crack
critical stress defined in the embodiment of the present
invention.
[Figure 21 Figure 2 is a schematic view showing the test
condition of crack critical stress measurement.
Description of Embodiments
[00191
An embodiment of the present invention will now be
described in detail. Hereunder, 11%" representing the
content of each element means "mass percent".
[0020]
The present inventors conducted examinations and
studies to improve the quenching crack resistance of the
steel for induction hardening. As the result, the
present inventors obtained the following findings:
[00211
(A) The steel for induction hardening is required to
have high machinability. Such a steel for induction
hardening has a high content of sulfur (S) to enhance the
machinability. Sulfur forms sulfide-base inclusions such
as MnS among others, thereby enhancing the machinability
of steel. However, the sulfide-base inclusions are
softer than the base metal (matrix). For this reason,
the sulfide-base inclusion is more likely to be the
starting point of quenching crack. Therefore, the
quenching crack resistance is improved with the decrease
in S content.
[0022]
(B) As described above, in order to deepen the
quench hardened layer of a large-sized crankshaft for
trucks and the like, it is preferable that the output of
high frequency be increased, and the heating time be
lengthened. However, if the output of high frequency is
increased and the heating time is lengthened, a portion
having a low heat capacity of the crankshaft is
overheated, and the crystal grains in this portion are
coarsened. If the crystal grains are coarsened, the
quenching crack resistance decreases.
[00231
In order to restrain the coarsening of crystal
grains, titanium (Ti) is effective. Titanium forms
nitrides and/or carbo-nitrides, and restrains the
coarsening of crystal grains by means of the pinning
effect. The Ti nitrides and/or Ti carbo-nitrides remain
in the steel even at high temperatures. Therefore, the
pinning effect can be achieved at high induction
hardening temperatures.
[00241
In the case where the induction hardening
temperature is low, vanadium (V) also forms VC and brings
about the pinning effect. However, in the case where the
steel for induction hardening is overheated, especially
in the case where the induction hardening temperature is
- 8 -
1000°C or higher, VC dissolves in the steel. Therefore,
the pinning effect brought about by VC is not maintained.
On the other hand, the Ti nitrides and/or Ti carbonitrides
are not dissolved in the steel even if the
induction hardening temperature becomes 1000°C or higher,
and maintain the pinning effect. For the steel for
induction hardening used for large-sized crankshafts, the
induction hardening temperature is high, and overheating
occurs easily. Therefore, Ti is more liable to maintain
the pinning effect as compared with V, and is effective
in enhancing the quenching crack resistance.
[00251
(C) As described above, the Ti nitrides and/or Ti
carbo-nitrides make the crystal grains fine by means of
the pinning effect. However, if the content of nitrogen
(N) runs short relative to the Ti content, excessive Ti
combines with carbon to form Tic. The Tic decreases the
quenching crack resistance of steel. Therefore, N of an
amount equal or larger than that of Ti is preferably
contained. Specifically, the N content is preferably
Ti/3.4 or higher.
[0026]
(D) Further, when the S content and the Ti content
satisfy formula (I), the quenching crack resistance
enhances remarkably:
2s - 3Ti < 0.040 ... (1)
where, into each element symbol in formula (I), the
content (mass%) of the corresponding element is
substituted.
[00271
Figure 1 is a graph showing the relationship between
the value on the left-hand side member 2s-3Ti of formula
(1) and the crack critical stress defined below. Figure
1 was obtained by the method described below.
[0028]
Fifty kilograms of each of steels having various
chemical compositions was melted in a vacuum induction
heating furnace. From the molten steel, a 100-mm
diameter ingot was produced. The ingot was heated to
1250°C. The heated ingot was hot-forged to produce a 60-
mm diameter round bar. The forging finishing temperature
was 1000°C. The round bar after being hot-forged was
allowed to cool to room temperature in the atmosphere.
[00291
From the middle position (~/2po sition) of the
distance R between the central axis and the surface of
the round bar after being allowed to cool (that is, the
radius), a test specimen was sampled. The size of the
test specimen was 10.0 mm x 2.0 mm x 75.0 mm. The
lengthwise direction of the test specimen was parallel to
the lengthwise direction of the round bar.
[00301
The test specimen was subjected to induction
hardening. Specifically, the test specimen was subjected
- 10 -
to high-frequency heating at an output of 40 kW and at a
frequency of 200 kHz. The hardening temperature was set
at 1000°C. The heating time was about 30 seconds. After
the heating time had elapsed, the test specimen was
cooled rapidly.
[0031]
As shown in Figure 2, a bending stress was applied
while the induction hardened test specimen was supported
at four points. The distance sl between two supporting
points on the upper surface of test specimen was set to
10 mm, and the distance s2 between two supporting points
on the lower surface thereof was set to 60 mm. The
stress was measured by affixing a strain gage in the
center of test specimen, and stress was applied until the
stress reaches a predetermined value. The test specimen
having been subject to bending stress was immersed in a
hydrochloric acid aqueous solution of 0.3 mol/liter for
24 hours. Thereafter, the test specimen was taken out of
the hydrochloric acid aqueous solution, and the presence
of cracks was checked.
[0032]
The test was conducted with a plurality of levels of
bending stresses, and the maximum bending stress at which
no crack was generated was defined as a crack critical
stress. Based on the obtained crack critical stress and
the parameter 2s-3Ti, Figure 1 was prepared.
[0033]
As shown in Figure 1, with the decrease in the value
of 2s-3Ti, the crack critical stress increases. In
particular, when the value of 2s-3Ti is not higher than
0.040, the crack critical stress increases suddenly. On
the other hand, when the value of 2s-3Ti is not lower
than 0.040, the crack critical stress does not increase
so much even if the value of 2s-3Ti decreases. In other
words, the crack critical stress is a monotone decreasing
function of the variable 2s-3Ti, and has an inflection
point in the vicinity of the point at which the value of
2s-3Ti is 0.040.
[00341
Based on the above-described findings, the present
inventors completed the steel for induction hardening in
accordance with this embodiment. In the following, the
steel for induction hardening in accordance with this
embodiment is described in detail.
[0035]
[Chemical composition]
The steel for induction hardening in accordance with
this embodiment has the chemical composition described
below.
[00361
C: 0.35 to 0.6%
Carbon (C) martensitizes the outer layer of steel by
means of induction hardening, and increases the hardness
of outer layer. On the other hand, if C is contained
excessively, the steel hardens excessively, and the
- 12 -
machinability of steel decreases. Therefore, the C
content is 0.35 to 0.6%. The preferable lower limit of
the C content is higher than 0.35%. The upper limit of
the C content is preferably less than 0.6%, further
preferably 0.5% or less.
[ 0 0 3 7 I
Si: at least 0.01% and less than 0.40%
Silicon (Si) deoxidizes the steel. Further, Si
strengthens the ferrite. On the other hand, if Si is
contained excessively, the machinability of steel
decreases. Therefore, the Si content is at least 0.01%
and less than 0.40%. The lower limit of the Si content
is preferably higher than 0.01%, further preferably at
least 0.05%. The preferable upper limit of the Si
content is at most 0.30%.
[0038]
Mn: 1.0 to 2.0%
Manganese (Mn) enhances the hardenability, and
increases the strength and hardness of steel. On the
other hand, if Mn is contained excessively, austenite is
liable to be retained when hardening is performed. If
the retained austenite exists, the mechanical properties
of steel degrade. Therefore, the Mn content is 1.0 to
2.0%. The lower limit of the Mn content is preferably
higher than 1.0%, further preferably at least 1.2%. The
upper limit of the Mn content is preferably less than
2.0%, further preferably at most 1.7%.
[00391
S: more than 0.010% and at most 0.05%
Sulfur (S) forms sulfide-base inclusions such as MnS
among others, thereby enhancing the machinability of
steel. On the other hand, if S is contained excessively,
a large number of coarse sulfide-base inclusions are
formed. The coarse sulfide-base inclusion becomes the
starting point of quenching crack. Therefore, the S
content is more than 0.010% and at most 0.05%. The
preferable upper limit of the S content is less than
0.05%.
[00401
Cr: 0.01 to 0.5%
Chromium (Cr) increases the hardness of steel.
Further, Cr enhances the hardenability of steel. On the
other hand, if Cr is contained excessively, bainite is
produced. If bainite is produced, the machinability of
steel decreases. Therefore, the Cr content is 0.01 to
0.5%. The lower limit of the Cr content is preferably
higher than 0.01%, further preferably at least 0.05%.
The upper limit of the Cr content is preferably less than
0.5%, further preferably at most 0.35%.
[0041]
Ti: 0.005 to 0.05%
Titanium (Ti) deoxidizes the steel. Further, Ti
combines with N to form Ti nitrides and/or Ti carbonitrides.
The Ti nitrides and/or Ti carbo-nitrides make
the crystal grains fine due to the pinning effect. If
the crystal grains are made fine, the ductility and
toughness of steel enhance. For this reason, the
quenching crack resistance enhances. On the other hand,
if Ti is contained excessively, coarse Ti nitrides, Ti
carbo-nitrides, and Ti carbides are formed, and the
machinability of steel decreases. Therefore, the Ti
content is 0.005 to 0.05%. The lower limit of the Ti
content is preferably higher than 0.005%, further
preferably at least 0.008%. The upper limit of the Ti
content is preferably less than 0.05%, further preferably
at most 0.04%.
[0042]
Al: 0.001 to 0.05%
Aluminum (Al) deoxidizes the steel. On the other
hand, if A1 is contained excessively, alumina-base
inclusions are formed. The alumina-base inclusions
decrease the machinability of steel. Therefore, the A1
content is 0.001 to 0.05%. The preferable lower limit of
the A1 content is higher than 0.001%. The upper limit of
the A1 content is preferably less than 0.05%, further
preferably at most 0.04%.
[00431
N: Ti/3.4 to 0.02%
Nitrogen (N) combines with Ti to form Ti nitrides
and/or Ti carbo-nitrides. As described above, the Ti
nitrides and/or Ti carbo-nitrides make the crystal grains
fine due to the pinning effect, thereby enhancing the
quenching crack resistance of steel. If the N content
runs short relative to the Ti content, excessive Ti
- 15 -
combines with carbon to form Tic. The Tic decreases the
machinability of steel. Therefore, N of an amount equal
or larger than that of Ti is preferably contained. On
the other hand, if N is contained excessively, defects
such as voids are easily produced in the steel.
Therefore, the N content is Ti/3.4 to 0.02%. Into I1Ti1'
in the 11~i/. 43" , the Ti content is substituted. The
value 3.4 is the mass ratio between Ti and N. The
preferable lower limit of the N content is higher than
~i/3.4. The preferable upper limit of the N content is
less than 0.02%.
[00441
The balance of the chemical composition of the steel
for induction hardening in accordance with this
embodiment consists of Fe and impurities. The impurities
in this description mean elements that mixedly enter from
ore and scrap used as the raw materials of steel,
environments in the production process, or the like.
[00451
In this embodiment, vanadium (V) is an impurity.
Vanadium combines with C to form VC that has the pinning
effect. However, in the case where the induction
hardening temperature is high, VC dissolves in the steel.
For this reason, the pinning effect due to VC is not
achieved. Further, V decreases the machinability of
steel. Therefore, in the steel for induction hardening
in accordance with this embodiment, V is an impurity.
[0046]
In this embodiment, boron (B) is an impurity. Boron
combines with N to form B nitrides. The B nitrides
decrease the cold workability of steel. Therefore, in
the steel for induction hardening in accordance with this
embodiment, B is an impurity.
[00471
[Concerning formula (1) 1
The chemical composition of the steel for induction
hardening in accordance with this embodiment further
satisfies the following formula (1) :
2s - 3Ti < 0.040 . . . (1)
where, into each element symbol in formula (I), the
content (mass%) of the corresponding element is
substituted.
[0048]
As shown in Figure 1, with the increase in the ratio
of Ti content to S content, the crack critical stress
increases gradually, and is increased remarkably by the
satisfaction of formula (1). Therefore, the quenching
crack resistance of steel is enhanced.
[0049]
[Concerning crystal grain size No.]
The steel for induction hardening in accordance with
this embodiment contains Ti and N as described above.
Therefore, the coarsening of crystal grains is restrained,
and excellent quenching crack resistance is attained.
The preferable crystal grain size No. of the steel for
induction hardening is 5.5 or higher. The crystal grain
- 17 -
size No. is defined as described below. A test specimen
is sampled from the steel for induction hardening. Of
the surface of the sampled test specimen, five arbitrary
visual fields are selected. By using the "Reference
Chart of Austenite Grain Size for Steeln in JIS G0551,
the austenite grain size Nos. in the selected five visual
fields are determined. The mean value of the austenite
grain size Nos. determined in the five visual fields is
defined as the crystal grain size No. of that test
specimen.
[ 0 0 5 0 I
In the steel for induction hardening in accordance
with this embodiment, in place of some of Fe, Ca may be
contained.
[0051]
Ca: at most 0.005%
Calcium (Ca) deoxidizes the steel. Also, Ca
spheroidizes inclusions. If inclusions are spheroidized,
the stress concentration created by the notch effect is
relaxed. For this reason, the quenching crack resistance
of steel enhances. On the other hand, if Ca is contained
excessively, coarse inclusions are formed, and thereby
the quenching crack resistance of steel is decreased.
Therefore, the Ca content is at most 0.005%. The
preferable upper limit of the Ca content is less than
0.005%.
[0052]
[Manufacturing method]
Explanation is given of one example of the steel for
induction hardening in accordance with this embodiment
and the method for manufacturing the crankshaft using the
steel for induction hardening.
[00531
A molten steel having the above-described chemical
composition is produced. The molten steel is formed into
cast pieces by the continuous casting process. The
molten steel may be formed into an ingot by the ingotmaking
process. The cast piece or the ingot may be hotworked
into a billet or a steel bar.
[00541
Next, by hot-forging the cast piece, ingot, billet,
or steel bar, an intermediate product having the rough
shape of the crankshaft is produced. The produced
intermediate product is allowed to cool in the atmosphere
The intermediate product is subjected to induction
hardening. As described above, the steel for induction
hardening in accordance with this embodiment can be used
for a large-sized crankshaft. In the large-sized
crankshaft, the quench hardened layer is formed deep.
For example, the thickness of the quench hardened layer
is lmm or larger. For the large-sized crankshaft, the
hardening temperature is as high as 950°C as compared
with the crankshaft having the ordinary size used for
general passenger cars. Even if induction hardening is
performed under such a hardening condition (hardening
temperature), the steel for induction hardening in
accordance with this embodiment is less liable to be
subjected to quenching cracks.
[0055]
The intermediate product having been induction
hardened is subjected to tempering. The tempering
process may be omitted. The hardness of the outer layer
(the quench hardened layer) of the intermediate product
is preferably 600 HV or higher in Vickers hardness.
[0056]
The intermediate product having been induction hardened
(and tempered) is ground into a predetermined shape by
machining. By the above-described processes, the
crankshaft is manufactured.
Examples
[ 0 0 5 7 I
Steel bars were produced by hot-forging the steel
for induction hardening having various chemical
compositions. By using each of the steel bars, the
cutting resistance was measured to evaluate the
machinability of the induction hardened steel. A test
specimen was sampled from the steel bar, and the test
specimen was induction hardened. By using the test
specimen, the crack critical stress, hardness, and
crystal grain size No. were measured to evaluate the
quenching crack resistance, hardness, and machinability
of the steel for induction hardening, respectively.
[00581
[Preparation of test specimen]
Fifty kilograms of each of steels of samples 1 to 5
and samples a to i having the chemical compositions given
in Table 1 was melted in a vacuum induction heating
furnace. From the melted steel, a 100-mm diameter ingot
was produced.
[00591
[Table 11
V
m
-0 Sample
1
Chemical composition (unit: mass%, balance being Fe an
C
0.39
Si
0.1 4
Mn
1.49
S I Cr Ca
0.045 0.1 4 -
V
-
In each element (C, Si, Mn, S, Cr, Ca, V, Ti, Al, N)
column in Table 1, the content (mass%) of the
corresponding element in the chemical composition of each
sample is described. The balance excluding the abovedescribed
elements in the chemical composition of each
sample is Fe and impurities. The symbol " - " in Table 1
indicates that the content of the corresponding element
is at an impurity level. In the I1Ti/3. 411 column, the
value obtained by dividing the Ti content by 3.4 is
described. In the "2s-3TiI1 column, the value on the
left-hand side of formula (1) is described.
[ 0 0 6 1 I
As shown in Table 1, the chemical compositions of
samples 1 to 5 were within the range of the chemical
composition of the steel for induction hardening in
accordance with this embodiment, and satisfied formula
(1).
[0062]
On the other hand, the chemical compositions of
samples a to i did not satisfy at least either one of the
chemical composition and formula (1) of the steel for
induction hardening in accordance with this embodiment.
The symbol "*" described at the right-hand side of the
numerical value in Table 1 indicates that the content
value is out of the definition range of the steel for
induction hardening in accordance with this embodiment.
[0063]
After having been heated to 1250°C, the ingot was
hot-forged to produce a 60-mm diameter round bar. The
forging finishing temperature was 1000°C. The round bar
after having been hot-forged was allowed to cool to room
temperature in the atmosphere.
[0064]
From the middle position (R/2 position) of the
distance R between the central axis and the surface of
the round bar, a test specimen was sampled. The size of
the test specimen was 10.0 mm x 2.0 mm x 75.0 mm. The
lengthwise direction of the test specimen was parallel to
the lengthwise direction of the round bar. From the
steel of each sample, a plurality of test specimens were
prepared.
[00651
Each of the test specimens was subjected to
induction hardening. Specifically, the test specimen was
subjected to high-frequency heating at an output of 40 kW
and at a frequency of 200 kHz. The hardening temperature
was set at 1000°C. The heating time was about 30 seconds.
After the heating time had elapsed, the test specimen was
cooled rapidly.
[0066]
By using the round bar produced as described above
and the test specimen, the cutting resistance, crack
critical stress, hardness, and crystal grain size No.
were measured.
[0067]
[Cutting resistance]
The cutting resistance (N) was measured by using the
round bar before being induction hardened. For the
measurement of cutting resistance, a multicomponent tool
dynamometer was used. By using a 6-mm diameter carbide
coating drill, cutting was performed perpendicularly to
the axial direction of the round bar. The
circumferential speed was 65 m/min, and the feed speed
was 0.22 mm/rev.
[0068]
[Crack critical stress]
The crack critical stress (MPa) was determined by
using the induction hardened test specimen. Specifically,
the test specimen of each sample was tested under the
same conditions as those in the case where Figure 1 was
prepared.
[0069]
[Hardness]
The hardness was measured by using the induction
hardened test specimen. Specifically, the test specimen
was cut perpendicularly to the major axis direction
thereof. The cut surface was mirror polished. The
Vickers hardness (HV) based on JIS 22244 was measured at
three arbitrary points at a 1-mm depth from the surface
of the cut surface having been polished, that is, at
three arbitrary points in the central portion of the
thickness of 2 mm. The test force was 98N. The mean
value of the three obtained Vickers hardnesses was
defined as the hardness (HV) of each test specimen.
[00701
[Crystal grain size No.]
The induction hardened test specimen was cut
perpendicularly to the major axis thereof in the central
portion thereof. Five arbitrary visual fields at a 1-mm
depth from the surface within the cut surface, that is,
in the central portion of the thickness of 2 mm were
selected. The austenite grain size Nos. in the five
selected visual fields were determined by using the
"Reference Chart of Austenite Grain Size for SteelM in
JIS G0551. A region surrounded by the prior-austenite
grain boundary appearing on account of corrosion produced
by a picric acid saturated aqueous solution was
recognized as one austenite grain. The mean value of the
austenite grain size Nos. determined in the five visual
fields was defined as the crystal grain size No. of that
test specimen.
[00711
[Test results]
Table 2 gives the test results. In the "Crack
critical stressu column in Table 2, the crack critical
stress (MPa) is described. The crack critical stress not
higher than 250 MPa was marked with I1#l1. In the
"HardnessH column, the hardness (HV) is described. In
the IrCrystal grain size No." column, the crystal grain
size No. is described. In the "Cutting resistancen
- 26 -
column, the cutting resistance (N) is described. The
cutting resistance not lower than 990N was marked with
[Table 21
Cutting resistance
Crystal grain sire No. I IN) Sample
[0073]
As described above, each sample was subjected to
induction hardening. Therefore, as shown in Table 2, all
hardnesses of samples 1 to 5 and samples a to i exceeded
600 HV.
[0074]
The chemical compositions of samples 1 to 5 were
within the range of this embodiment, and satisfied
formula (1). Therefore, for samples 1 to 5, the crack
critical stress exceeded 250 MPa, and excellent quenching
- 27 -
Crack critical stress
(MPa)
Hardness
(HV)
crack resistance was exhibited. Further, the crystal
grain size Nos. of samples 1 to 5 were 5.5 or higher. It
is thought that excellent quenching crack resistance was
exhibited because the coarsening of crystal grains was
restrained by Ti nitrides and/or Ti carbo-nitrides, and
formula (1) was satisfied. Further, the cutting
resistances of samples 1 to 5 were lower than 990N, and
excellent machinability was exhibited.
[00751
Because containing Ca, sample 4 exhibited a crack
critical stress much higher than that of sample 2 having
almost the same chemical composition.
[00761
On the other hand, for samples a to h, the quenching
crack resistance or the machinability was low because the
chemical composition and/or the parameter 2s-3Ti for the
steel for induction hardening of this embodiment was not
satisfied. Specifically, the S content of sample a was
too high, and the Ti content thereof was too low.
Further, sample a did not satisfy formula (1) . Therefore,
the crack critical stress was not higher than 250 MPa.
Further, the crystal grain size No. was lower than 5.5.
The reason for this result is thought to be that the Ti
content was too low.
[00771
For sample b, the S content was too high, and the Ti
content was too low. Further, sample b did not satisfy
formula (1). Therefore, the crack critical stress was
- 28 -
not higher than 250 MPa, and the crystal grain size No.
was lower than 5.5. Further, sample b contained V.
Therefore, the cutting resistance was not lower than 990N.
[00781
The S content of sample c was too high. Further,
sample c did not satisfy formula (1). Therefore, the
crack critical stress was not higher than 250 MPa.
Further, since sample c contained V, the cutting
resistance thereof was not lower than 990N.
[0079]
The Si content and the S content of sample d were
too high. Further, sample d did not satisfy formula (1) .
Therefore, the crack critical stress was not higher than
250 MPa.
[0080]
The Ti content of sample e was too low. Further,
sample e did not satisfy formula (1). Therefore, the
crack critical stress was not higher than 250 MPa, and
the crystal grain size No. was lower than 5.5.
[00811
The S content of sample f was too high. Further,
sample f did not satisfy formula (1). Therefore, the
crack critical stress was not higher than 250 MPa.
[0082]
The chemical composition of sample g was within the
range of the chemical composition of the steel for
induction hardening in accordance with this embodiment.
However, sample g did not satisfy formula (1) . Therefore,
the crack critical stress was not higher than 250 MPa.
[00831
For sample h, the Ti content was too high, and the N
content was too low. Therefore, the cutting resistance
was not lower than 990N. The reason for this is thought
to be that Tic was formed.
[00841
The N content of sample i was too low. Therefore,
the crack critical stress was not higher than 250 MPa.
Also, the crystal grain size No. of sample i was lower
than 5.5. The reason for this is thought to be that the
N content was too low, and sufficient TiN was not formed.
[0085]
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 modified
as appropriate without departing from the spirit and
scope of the present invention.
Industrial Applicability
[00861
The steel for induction hardening in accordance with
this embodiment can be used widely for steel materials to
be induction hardened. Specifically, it can be used for
- 30 -
automotive engine parts and the like. In particular, it
can be used for large-sized crankshafts for trucks or the
like.

We Claim:
[Claim 11
A steel for induction hardening comprising, by mass
percent, C: 0.35 to 0.6%, Si: at least 0.01% and less
than 0.40%, Mn: 1.0 to 2.0%, S: more than 0.010% and at
most 0.05%, Cr: 0.01 to 0.5%, Al: 0.001 to 0.05%, N:
Ti/3.4 to 0.02%, and Ti: 0.005 to 0.05%, the balance
being Fe and impurities, and satisfying the following
formula (1) :
2s - 3Ti < 0.040 . . . (1)
where, into each element symbol in formula (I), the
content (mass%) of the corresponding element is
substituted.
[Claim 21
The steel for induction hardening according to claim
1, further comprising: in place of some of Fe, Ca: at
most 0.005%.
[Claim 31
A crankshaft manufactured by induction hardening the
steel for induction hardening described in claim 1 or 2.
Dated this lgth day of January, 2014.
& Sumitomo Metal Corporation
Suresh A. Shroff & Co.
Attorneys for the Applicant