U.S. patent application number 14/903975 was filed with the patent office on 2016-06-09 for coil spring, and method for manufacturing same.
This patent application is currently assigned to NHK Spring Co., Ltd.. The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO(KOBE STEEL, LTD., NHK SPRING CO., LTD., SHINKO WIRE CO., LTD.. Invention is credited to Tetsuo JINBO, Toshio MAEHATA, Kei MASUMOTO, Hiroshi OURA, Kengo TSURUGAI, Fumio YAMAMOTO, Yoshiharu YAMAMOTO, Nao YOSHIHARA.
Application Number | 20160160306 14/903975 |
Document ID | / |
Family ID | 52279991 |
Filed Date | 2016-06-09 |
United States Patent
Application |
20160160306 |
Kind Code |
A1 |
YAMAMOTO; Fumio ; et
al. |
June 9, 2016 |
COIL SPRING, AND METHOD FOR MANUFACTURING SAME
Abstract
To provide a coil spring having excellent fatigue resistance.
Disclosed is a coil spring made of steel, including (in % by mass,
the same shall apply for a chemical composition): C: 0.40 to 0.70%;
Si: 1.50 to 3.50%; Mn: 0.30 to 1.50%; Cr: 0.10 to 1.50%; V: 0.50 to
1.00%, and Al: 0.01% or less (excluding 0%), with the balance being
iron and inevitable impurities, wherein an average crystal grain
size number of prior austenite crystals in a depth of 0.3 mm from a
surface is 11.0 or more, while a difference in grain size number
between the respective prior austenite crystals is in a range of
less than 3 from a grain size number observed at the maximum
frequency, and wherein a carburized layer is provided in a depth of
0.30 to 1.00 mm from the surface, while an average Vickers hardness
is 600 or higher at a position in a depth of (1/4).times.diameter
in the depth direction from the surface.
Inventors: |
YAMAMOTO; Fumio;
(Komagane-shi, JP) ; TSURUGAI; Kengo;
(Aikawa-machi, JP) ; YOSHIHARA; Nao; (Kobe-shi,
JP) ; MASUMOTO; Kei; (Kobe-shi, JP) ; OURA;
Hiroshi; (Kobe-shi, JP) ; JINBO; Tetsuo;
(Amagasaki-shi, JP) ; MAEHATA; Toshio;
(Amagasaki-shi, JP) ; YAMAMOTO; Yoshiharu;
(Amagasaki-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NHK SPRING CO., LTD.
KABUSHIKI KAISHA KOBE SEIKO SHO(KOBE STEEL, LTD.
SHINKO WIRE CO., LTD. |
Yokohama-shi, Kanagawa
Kobe-shi, Hyogo
Amagasaki-shi, Hyogo |
|
JP
JP
JP |
|
|
Assignee: |
NHK Spring Co., Ltd.
Yokohama-shi, Kanagawa
JP
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
Shinko Wire Co., Ltd.
Amagasaki-shi, Hyogo
JP
|
Family ID: |
52279991 |
Appl. No.: |
14/903975 |
Filed: |
July 8, 2014 |
PCT Filed: |
July 8, 2014 |
PCT NO: |
PCT/JP2014/068123 |
371 Date: |
January 8, 2016 |
Current U.S.
Class: |
148/223 ;
148/319 |
Current CPC
Class: |
C21D 6/004 20130101;
C22C 38/06 20130101; C22C 38/00 20130101; C21D 1/06 20130101; C22C
38/34 20130101; C22C 38/24 20130101; C21D 1/773 20130101; C21D
6/008 20130101; C22C 38/26 20130101; C22C 38/04 20130101; C22C
38/48 20130101; C21D 6/005 20130101; C23C 8/22 20130101; C22C 38/46
20130101; C22C 38/002 20130101; C21D 9/02 20130101 |
International
Class: |
C21D 9/02 20060101
C21D009/02; C21D 1/773 20060101 C21D001/773; C21D 6/00 20060101
C21D006/00; C22C 38/46 20060101 C22C038/46; C22C 38/00 20060101
C22C038/00; C22C 38/26 20060101 C22C038/26; C22C 38/24 20060101
C22C038/24; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C23C 8/22 20060101 C23C008/22; C22C 38/34 20060101
C22C038/34 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 9, 2013 |
JP |
2013-143514 |
Claims
1. A coil spring made of steel, comprising: C: 0.40 to 0.70% by
mass; Si: 1.50 to 3.50% by mass; Mn: 0.30 to 1.50% by mass; Cr:
0.10 to 1.50% by mass; V: 0.50 to 1.00% by mass; Al: 0.01% by mass
or less (excluding 0%), with the balance being iron; and inevitable
impurities, wherein: an average crystal grain size number of prior
austenite crystals in a depth of 0.3 mm from a surface is 11.0 or
more, while a difference in grain size number between the
respective prior austenite crystals is in a range of less than 3
from a grain size number observed at the maximum frequency; and a
carburized layer is provided in a depth of 0.30 to 1.00 mm from the
surface, while an average Vickers hardness is 600 or higher at a
position in a depth of (1/4).times.diameter in the depth direction
from the surface.
2. The coil spring according to claim 1, further comprising: Ni:
1.50% by mass or less (excluding 0%), and/or Nb: 0.50% by mass or
less (excluding 0%).
3. A method for manufacturing the coil spring of claim 1, the
method comprising performing a vacuum carburization process at
1,000.degree. C. or higher.
4. A method for manufacturing the coil spring of claim 2, the
method comprising performing a vacuum carburization process at
1,000.degree. C. or higher.
Description
TECHNICAL FIELD
[0001] The present invention relates to a coil spring and a method
for manufacturing same, and more particularly to a coil spring with
excellent fatigue resistance and a method for manufacturing
same.
BACKGROUND ART
[0002] Coil springs are used as a valve spring, a clutch spring, a
suspension spring, etc., in the engine, clutch and suspension of an
automobile and the like. A coil spring is repeatedly used under a
high stress over a long period of time and thus required to have a
high level of fatigue resistance.
[0003] Wires for the valve spring in the engine as specified by JIS
include, for example, an oil-tempered wire for a valve spring
(SWO-V: JIS G 3561), a chrome-vanadium steel oil-tempered wire for
a valve spring (SWOCV-V: JIS G 3565), and a silicon-chromium steel
oil-tempered wire for a valve spring (SWOSC-V: JIS G 3566). In
particular, SWOSC-V has been mainly used because of its excellent
fatigue resistance.
[0004] These wires are produced by drawing rolled wire rod,
followed by quenching and tempering, resulting in the desired
strength. A general method for manufacturing a valve spring
involves coiling such a wire into a spring having a required shape,
followed by nitriding, shot peening, tempering, setting, and the
like, thereby producing a spring with excellent fatigue
resistance.
[0005] From the viewpoint of conservation of the environment and
natural resources, there have been increasing demands for cleaning
exhaust gas from automobiles and improving the fuel efficiency
thereof. The reduction in weight of automobiles significantly
contributes to these demands, and thus many attempts have been
continuously undertaken to reduce the weight of components included
in the vehicle body.
[0006] The valve spring is modified to improve its fatigue
resistance, making the valve spring more compact, which can further
contribute to the reduction in weight of the engine. For this
reason, some proposals have been presented to improve the fatigue
resistance of the valve spring.
[0007] For example, Patent Document 1 discloses the technical
feature in which a coil spring is designed to have on its surface a
carburized layer (of 0.05 to 1.00 mm in depth) with a predetermined
composition and also to exhibit the hardness in a depth of 0.02 mm
from its surface within a predetermined range (of 650 to 1,000 HV),
thereby improving the fatigue resistance.
CONVENTIONAL ART DOCUMENT
Patent Document
[0008] Patent Document 1: JP 2012-77367 A
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0009] The coil spring mentioned in Patent Document 1 has a fatigue
resistance of a level of fifty million times. In recent years,
along with the progress in reducing the weight and enhancing the
output of the vehicle, coil springs have been required to exhibit
more excellent fatigue resistance.
[0010] The present invention has been made in light of the
foregoing circumstance, and it is an object of the present
invention to provide a coil spring with excellent fatigue
resistance and a method for manufacturing such a coil spring with
the excellent fatigue resistance.
Means for Solving the Problems
[0011] The present invention that can solve the foregoing problems
provides a coil spring made of steel, including (in % by mass, the
same shall apply for a chemical composition): C: 0.40 to 0.70%; Si:
1.50 to 3.50%; Mn: 0.30 to 1.50%; Cr: 0.10 to 1.50%; V: 0.50 to
1.00%, and Al: 0.01% or less (excluding 0%), with the balance being
iron and inevitable impurities, wherein an average crystal grain
size number of prior austenite crystals in a depth of 0.3 mm from a
surface is 11.0 or more, while a difference in grain size number
between the respective prior austenite crystals is in a range of
less than 3 from a grain size number observed at the maximum
frequency, and wherein a carburized layer is provided in a depth of
0.30 to 1.00 mm from the surface, while an average Vickers hardness
is 600 or more at a position in a depth of (1/4).times.diameter in
the depth direction from the surface.
[0012] Further, in a preferred embodiment, the chemical composition
of the above-mentioned coil spring comprises: Ni: 1.50% or less
(excluding 0%) , and/or Nb: 0.50% or less (excluding 0%).
[0013] To manufacture the coil spring with excellent fatigue
resistance as mentioned above, it is recommended to perform the
vacuum carburization process at 1,000.degree. C. or higher.
Effects of the Invention
[0014] Accordingly, the present invention can control the
carburizing depth of the surface of the coil spring and the Vickers
hardness thereof as appropriate, while properly controlling the
chemical composition and the prior austenite crystal grain size,
thereby producing the coil spring with excellent fatigue
resistance. Furthermore, the method according to the present
invention can provide the coil spring with excellent fatigue
resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic explanatory diagram of measurement
positions of a carburized layer in a coil spring as well as
measurement positions for Vickers hardness located at a distance of
1/4.times.the diameter.
[0016] FIG. 2 is a schematic explanatory diagram of measurement
positions for prior austenite crystal grain size of the coil
spring.
MODE FOR CARRYING OUT THE INVENTION
[0017] The inventors have studied from various perspectives to
provide a coil spring that has improved fatigue resistance,
compared to the related art, and exhibits excellent fatigue
resistance to achieve the result of a fracture-lifetime test that
exceeds sixty million times in examples to be mentioned later. In
the technique mentioned in Patent Document 1, a metallographic
structure is controlled while increasing the amount of added C.
However, only this technique did not attain a level of sixty
million times of the fracture lifetime (see Example 4 mentioned in
Patent Document 1 as well as a test specimen No. 8 in Table 2
obtained by simulating this example).
[0018] Thus, to achieve the more excellent fatigue resistance, the
inventors have studied chemical compositions, metallographic
structures, and the like. As a result of intensive studies, it has
been found that as the toughness and strength of the coil spring
affect fatigue fracture of the coil spring in use, these factors
can be controlled as appropriate to enable drastic improvement of
the fatigue resistance of the coil spring.
[0019] First, to enhance the strength of the coil spring, it is
necessary to ensure some carburized layer depth from the surface of
steel configuring the coil spring (hereinafter simply referred to
as a "surface"), as well as the adequate Vickers hardness of the
inside of steel (which can sometimes be represented by
"1/4.times.D", which means multiplication of a diameter D of a
steel wire forming the coil spring by 1/4). To sufficiently ensure
the depth of the carburized layer as well as the Vickers hardness,
it is required to enhance the temperature of the carburization
process. Only the carburization process at a high temperature was
not able to improve the fracture lifetime of the coil spring. This
is because the carburization process at high temperatures coarsens
the crystal grains of the prior austenite, or leads to variations
in crystal grain size of the prior austenite (which means the
presence of a difference in crystal grain number; hereinafter
referred to as duplexed grains), drastically reducing the toughness
of the coil spring, which might degrade the fracture lifetime.
[0020] Regarding these problems, the inventors have diligently
investigated and, as a result, found that the chemical composition
of the steel can be appropriately controlled to solve the
above-mentioned problems. In particular, it has been revealed that
by increasing the V content in the chemical composition, the
crystal grain of prior austenite is prevented from being coarsened
even after the carburization process at a high temperature, further
suppressing generation of duplex-grains.
[0021] The present invention has been made based on the findings
that, on the assumption that the following chemical composition is
satisfied, the carburized layer depth, the Vickers hardness, and
the prior austenite crystal grain size are appropriately controlled
to enable keeping balance between the strength and toughness
required for improving the fatigue resistance, thereby providing a
coil spring with the excellent fatigue resistance mentioned
above.
[0022] The chemical composition of the coil spring in the present
invention will be described below.
C: 0.40 to 0.70%
[0023] Carbon (C) is an element that is effective in ensuring the
adequate strength of a coil spring used under a high load and the
Vickers hardness of the coil spring in the position of 1/4.times.D.
To exhibit these effects, the C content is 0.40% or more,
preferably 0.45% or more, and more preferably 0.50% or more. Any
excessive C content, however, degrades the toughness of the coil
spring and increases surface flaws of the coil spring, resulting in
reduced fatigue resistance. Accordingly, the C content should be
0.70% or less, preferably 0.65% or less, and more preferably 0.60%
or less.
Si: 1.50 to 3.50%
[0024] Silicon (Si) is an element that is effective in ensuring the
adequate Vickers hardness, similar as C. Further, Si is also
effective in improving the strength of the coil spring, the fatigue
resistance and the sagging resistance. To exhibit these effects,
the Si content is 1.50% or more, preferably 1.80% or more, and more
preferably 2.10% or more. Any excessive Si content, however,
degrades the toughness of the coil spring and reduces the cold
workability and the hot workability during the manufacturing
procedure for the coil spring, which leads to poor yield of
products and assists in decarburization due to a heat treatment,
thus degrading the fatigue resistance. Accordingly, the Si content
should be 3.50% or less, preferably 3.30% or less, and more
preferably 3.10% or less.
Mn: 0.30 to 1.50%
[0025] Manganese (Mn) is an element that is effective in improving
the strength of the coil spring by enhancing the quenching
properties. Further, Mn serves to fix, in the steel, sulfur (S)
that would adversely affect the fatigue resistance, to thereby
convert it into MnS, which reduces the above-mentioned
disadvantage. To exhibit these effects, the Mn content is 0.30% or
more, preferably 0.40% or more, and more preferably 0.50% or more.
Any excessive Mn content, however, degrades the toughness of the
coil spring and also reduces the cold workability and the fatigue
strength. Accordingly, the Mn content should be 1.50% or less,
preferably 1.20% or less, and more preferably 0.90% or less.
Cr: 0.10 to 1.50%
[0026] Chromium (Cr) is an element that is effective in improving
the strength of the coil spring by enhancing the quenching
properties, similar as Mn. Cr also has the effects of reducing
activity of C to prevent the decarburization in the hot-rolling
process or the heat treatment. To exhibit these effects, the Cr
content is 0.10% or more, preferably 0.15% or more, and more
preferably 0.20% or more. Any excessive Cr content, however,
drastically decreases a C diffusion coefficient in a vacuum
carburization process, making it difficult to form a desired
carburized layer, resulting in reduced fatigue resistance. When the
carburization temperature is increased to ensure the desired
carburized layer, the prior austenite crystals are coarsened while
generating duplex-grains, thus degrading the fatigue resistance of
the coil spring. Accordingly, the Cr content should be 1.50% or
less, preferably 1.20% or less, and more preferably 0.90% or
less.
V: 0.50 to 1.00%
[0027] Vanadium (V) is an element that is effective in making the
prior austenite crystal grains finer. Especially, V is the element
that is also effective in suppressing the coarsening of the prior
austenite crystal grains and generation of duplex-grains, which are
problems in the related art when the carburization temperature is
increased to ensure the desired carburized layer. To exhibit these
effects, the V content is 0.50% or more, preferably 0.53% or more,
and more preferably 0.56% or more. Any excessive V content,
however, forms a large amount of V carbide, degrading the
ductility, the cold workability, and the resistance to the fatigue
of the coil spring. Accordingly, the V content should be 1.00% or
less, preferably 0.90% or less, and more preferably 0.80% or
less.
Al: 0.01% or less (excluding 0%)
[0028] Aluminum (Al) is a deoxidizing element but any excessive Al
content forms inclusions, such as AlN. These inclusions drastically
degrade the fatigue resistance of the coil spring. Accordingly, the
Al content needs to be reduced to 0.01% or less, preferably 0.008%
or less, and more preferably 0.006% or less.
[0029] The basic chemical composition of the steel configuring the
coil spring in the present invention has been mentioned above, with
the balance being substantially iron. Here, the term
"substantially" as used herein means that the present invention
allows, without departing from the feature of the invention, the
contamination of a very small amount of elements present in a steel
raw material, including scraps, and which would inevitably occur
during an iron manufacture process, a steel manufacture process,
further, a steel-manufacture preliminary treatment process, and the
like. For example, exemplary inevitable impurities include P
(preferably, of 0.016% or less, and more preferably 0.015% or
less), and S (of 0.015% or less).
[0030] The invention may contain both or either of Ni and Nb in the
following ranges as other elements, as needed. The characteristics
of the coil spring are further improved depending on the kinds of
contained elements.
Ni: 1.50% or less (excluding 0%)
[0031] Nickel (Ni) is an element that is effective in improving the
toughness of the coil spring that increased its strength by C. To
exhibit these effects, the Ni content is preferably 0.05% or more,
and more preferably 0.10% or more. Any excessive Ni content,
however, generates residual austenite in an excessively amount,
which degrades the fatigue resistance of the coil spring.
Accordingly, the Ni content is preferably 1.50% or less, more
preferably 1.20% or less, and much more preferably 0.90% or
less.
Nb: 0.50% or less (excluding 0%)
[0032] Niobium (Nb) has the effect of making the crystal grains
finer in the hot-rolling process as well as the
quenching-and-tempering process, thereby improving the ductility of
the coil spring. To exhibit these effects, the Nb content is
preferably 0.01% or more, and more preferably 0.02% or more. Any
excessive Nb content, however, generates the V carbides in an
excessive amount to thereby degrade the ductility of the coil
spring, reducing the cold workability and the fatigue strength.
Accordingly, the Nb content is preferably 0.50% or less, more
preferably 0.40% or less, and much more preferably 0.30% or
less.
[0033] To improve the fatigue resistance, it is important to
appropriately control not only the chemical composition as
mentioned above, but also the metallographic structure (control of
the prior austenite crystals), the carburized layer and the Vickers
hardness of the steel of the coil spring.
[0034] Average crystal grain size number of the prior austenite
crystals: 11.0 or more
[0035] The crystal grain size of the prior austenite crystals in a
depth of 0.3 mm from the surface of the coil spring can be made
finer to increase its crystal grain size number, thereby enhancing
the toughness thereof to drastically improve the fatigue resistance
of the coil spring. To exhibit these effects, the average crystal
grain size number of the prior austenite crystal is 11.0 or more,
preferably 12.0 or more, and more preferably 13.0 or more. From the
viewpoint of improving the toughness, the upper limit of the
average crystal grain size number of the prior austenite crystal is
not specifically limited. However, in terms of the easiness of
manufacturing and the cost of alloys, the average crystal grain
size number is preferably approximately 15.0 or less, and more
preferably 14.0 or less.
[0036] Difference in grain size number between the prior austenite
crystals: within a range of less than 3 from the grain size number
observed at the maximum frequency
[0037] When variations in crystal grain size number of the prior
austenite crystals measured in a depth of 0.3 mm from the surface
are large, the toughness of the coil spring is significantly
degraded even though the prior austenite crystals in the steel of
the coil spring satisfy the above-mentioned average grain size
number, which makes the cold workability and the fatigue resistance
worse. Therefore, in the present invention, the measured crystal
grain size number of each prior austenite crystal needs to be
within a difference of less than 3, preferably 2 or less, and more
preferably 1 or less from the grain size number observed at the
maximum frequency. Note that in the present invention, the state in
which such a condition for the difference in grain size number is
satisfied is referred to as "no duplex-grain".
[0038] In the present invention, the austenite crystal grains in
the steel wire of the coil spring satisfy the above-mentioned
average crystal grain size number, and further the formation of
duplex-grains is suppressed, whereby the fatigue resistance can be
improved.
[0039] Carburized layer: in a depth of 0.30 to 1.00 mm from the
surface of the coil spring
[0040] The appropriate carburized layer is effective in improving
the fatigue resistance of the coil spring. That is, the surface
side of the coil spring is sufficiently hardened, which can
suppress the occurrence of fracture starting from the surface of
the spring when the coil spring is repeatedly used under the high
load stress. To exhibit these effects, the carburized layer needs
to be formed in at least a depth of 0.30 mm or more, preferably
0.40 mm or more, and more preferably 0.50 mm or more, from the
surface of the coil spring. However, when the rate of the
carburized layer in the steel wire of the coil spring becomes
excessive, coarsened carbides are precipitated, which might degrade
the fatigue resistance of the coil spring. Therefore, the
carburized layer needs to be formed in a depth of 1.00 mm or less,
preferably 0.90 mm or less, and more preferably 0.80 mm or less
from the surface of the coil spring.
[0041] Average Vickers hardness in the position of
(1/4).times.diameter D in the depth direction from the surface: 600
or more
[0042] The coil spring formed of steel, the inside of which has an
appropriate Vickers hardness (Hv), is effective in improving the
fatigue resistance of the coil spring. Specifically, when the inner
hardness of the coil spring is low, in repeatedly use of the coil
spring under a high load stress, plastic deformation occurs in the
coil spring even though the stress applied to the spring is below a
limit of elasticity. As a result, the required spring stress cannot
be exhibited, degrading the fatigue resistance of the coil spring.
Thus, from the viewpoint of improving the fatigue resistance, an
average Vickers hardness at least in the depth (1/4).times.D from
the surface of the coil spring is 600 or more, preferably 670 or
more, and more preferably 690 or more. The upper limit of the
average Vickers hardness is not specifically limited. However, when
the Vickers hardness is too high, the toughness of the coil spring
would be reduced, thus degrading the fatigue resistance.
Accordingly, the above-mentioned average Vickers hardness is
preferably 750 or less, and more preferably 730 or less.
[0043] When manufacturing the coil spring with the excellent
fatigue resistance as mentioned above, manufacturing conditions
therefor can be desirably controlled as appropriate. In particular,
to ensure the above-mentioned predetermined carburizing depth and
Vickers hardness (average thereof), it is effective to control the
temperature of the vacuum carburization process. Preferable
conditions for manufacturing the coil spring in the present
invention will be described below.
[0044] The coil spring in the present invention can be manufactured
by subjecting a steel material satisfying the above predetermined
chemical composition to melting, hot forging, and hot rolling into
a wire rod having a desired wire diameter, followed by shaving,
patenting, wire-drawing and oil tempering of the wire rod, and
thereafter forming the obtained wire into a spring, which is then
subjected to vacuum carburization process. Thereafter, to further
improve the fatigue properties, shot peening, setting, or the like
may be performed as needed.
[0045] The conditions for the aforesaid melting, hot forging and
hot rolling are not specifically limited, and thus may be
conventional manufacturing conditions. For example, a steel ingot
satisfying the above predetermined chemical composition is
manufactured through melting in a blast furnace, and the steel
ingot is subjected to blooming to produce a billet with a
predetermined size. In order to suppress the deformation resistance
that would affect the workability as well as the coarsening of the
prior austenite crystal grains, the billet might be heated, for
example, to approximately 900.degree. C. to 1100.degree. C., and
then hot-rolled at a desired rolling reduction to form a wire rod
with a desired shape property. Thereafter, a deoxidized layer
formed on the surface of the wire rod is removed by being shaved by
a desired thickness. To remove the processed hardened layer
generated by the shaving process and to obtain desired micro
structure (for example, pearlite) with excellent drawability, the
patenting process, or a soft annealing process or the like in an IH
(induction heating) equipment is performed.
[0046] Thereafter, the wire rod is drawn into one with a desired
wire diameter, followed by the oil tempering process to thereby
form a wire for a spring. Then, the wire is formed into a spring
with the desired coil diameter, free height and number of turns.
The reason why the wire is formed into the spring shape before the
carburization process is that after the carburization quenching and
tempering for forming the carburized layer, the surface part of the
steel (carburized layer) becomes hard and the ductility of the wire
is degraded, making it difficult to form the coil spring.
[0047] After forming the spring shape, the vacuum carburization
process is performed. However, in the present invention, to attain
the predetermined carburizing depth and Vickers hardness, the
vacuum carburization process needs to be performed at a high
carburization temperature of 1,000.degree. C. or higher. When the
carburization temperature is lower than 1,000.degree. C., the
desired carburized layer and Vickers hardness cannot be obtained,
which degrades the fatigue resistance. The carburization
temperature is preferably 1,020.degree. C. or higher, and more
preferably 1,040.degree. C. or higher. When the carburization
temperature becomes too high, however, the carbides are coarsened
and precipitated, so that the coil spring becomes excessively hard,
reducing its toughness, which may lead to degradation of the
fatigue resistance. The carburization temperature is preferably
1,100.degree. C. or lower, and more preferably 1,080.degree. C. or
lower.
[0048] Then, the carburization process is applied to the coil
spring. As the degree of decarburization is increased during the
carburization process, or as variation in processing temperature
becomes larger, the fatigue strength of the coil spring is
degraded. Thus, in the present invention, the vacuum carburization
process is performed from the viewpoint of suppressing the
decarburization and temperature variation. The vacuum carburization
process is performed at a temperature of 1,000.degree. C. or
higher, whereby the carburized layer can be uniformly formed in the
desired thickness. The carburization time and the diffusion time
are not specifically limited and may be any adequate times that
form the desired carburized layer. For example, the carburization
time may be set at 1 to 10 minutes, and the diffusion time may be
set at 1 to 10 minutes.
[0049] After the carburization process, gas cooling or oil
quenching is continuously performed down to a temperature of the
A.sub.l transformation temperature or lower. Then, a re-heating
process (for example, at a temperature of 830.degree. C. to
850.degree. C. for 10 minutes to 30 minutes) is desirably
performed, whereby the prior austenite crystal grains can be made
much finer. The tempering process may be performed to improve the
toughness and ductility as needed.
[0050] The obtained coil spring may be subjected to the
conventional shot peening and setting as appropriate for the
purpose of further improving the fatigue resistance of the coil
spring.
[0051] When manufacturing the coil spring of the present invention,
any conditions other than the above-mentioned ones are not
specifically limited, and general manufacturing conditions may be
applied.
[0052] The coil spring obtained in this way can be used as the coil
spring with excellent fatigue resistance in various applications,
including a valve spring for an engine, a spring for a
transmission, and the like, as mentioned above.
[0053] This application is based upon and claims the benefit of
priority from Japanese Patent Application No. 2013-143514, filed on
Jul. 9, 2013, the entire contents of which are incorporated herein
by reference.
EXAMPLES
[0054] The present invention will be described in more detail below
by way of Examples. It should be noted that, however, these
examples are never construed to limit the scope of the invention;
various modifications and changes may be made without departing
from the scope and spirit of the invention and should be considered
to be within the scope of the invention.
[0055] A steel material was melted in a vacuum melting furnace to
form steels A to H having the chemical compositions shown in Table
1 below (with the balance being iron and inevitable impurities) and
subjected to hot forging, thereby fabricating billets of 155 mm
square. Each billet was heated at 1,000.degree. C. and hot-rolled
to produce a wire rod for a spring having a diameter of 8.0 mm. The
wire rod for a spring was subjected to the soft annealing (while
being kept at 660.degree. C. for 2 hours), and then a surface part
of the wire rod for a spring was shaved by 0.15 mm to thereby
remove a decarburized layer. Thereafter, the wire rod for a spring
was heated at a temperature of 900.degree. C. or higher in a
neutral gas atmosphere to thereby be austenitized. Then, a
lead-patenting process (heating temperature: 980.degree. C., lead
furnace temperature: 620.degree. C.) was performed on the wire rod
for a spring to take place pearlite transformation. Thereafter, the
wire rod for a spring was subjected to the cold drawing into a wire
having a diameter of 4.1 mm, and then to the oil tempering process
under the conditions appropriate for the respective components of
the wire (heating temperature: 900.degree. C. to 1,000.degree. C.,
quenching oil temperature: 60.degree. C., tempering temperature:
400 to 500.degree. C.), thus fabricating the wire for a spring. The
wire for a spring was used to be cold-formed, thereby producing a
spring (average coil diameter: 24.60 mm, free height: 46.55 mm, the
effective number of turns: 5.75).
[0056] Then, the thus-obtained spring was heated to the
"carburization temperature" mentioned in Table 2 below and
subjected to the vacuum carburization process (carburization time:
5 minutes, diffusion time: 3 minutes). Thereafter, the spring was
kept at 950.degree. C. for 15 minutes, and immersed in oil kept at
50.degree. C. to be quenched, followed by being tempered (at
350.degree. C., for 90 minutes). The thus-obtained spring was
subjected to three-stage shot peening (by gradually decreasing a
diameter of a shot particle from the first stage), and subsequently
hot setting (at 230.degree. C., T.sub.max=1,600 MPa-equivalent).
The following measuring and tests were performed on the coil
springs obtained in this way (test samples Nos. 1 to 13).
(Carburized Layer Depth)
[0057] The carburized layer depth in each test sample was
determined by measuring the carbon content in the coil spring.
Specifically, as shown in FIG. 1, four lines were drawn at
intervals of 90 degrees from the center of the cross section of the
steel wire forming the coil spring in each test sample. Then, the
depth on each line in which the carbon (C) content (%) became
substantially equal to that of carbon added to the steel was
measured in the sample. Measurements in this test were shown in the
"carburized layer depth" column in the table. In the present
invention, the samples having the carburized layer depth in a range
from 0.30 mm to 1.00 mm were rated as "pass".
(Vickers Hardness in 1/4.times.D Position)
[0058] The hardness (Hv) of the coil spring in each test sample was
measured using the Vickers hardness tester. Specifically, as shown
in FIG. 1, the hardness of the coil spring in each sample was
measured on the four lines drawn at intervals of 90 degrees from
the positions at 1/4.times.diameter D (d/4) of the cross section of
the steel wire that forms the coil spring in each test sample (test
load: 10 kgf). Then, an average of these measurements was
determined. The average Vickers hardness was shown in the "Vickers
hardness" column. In the present invention, the samples having the
Vickers hardness of 600 or more were rated as "pass".
(Average Crystal Grain Size Number of Prior Austenite Crystals)
[0059] A method for measuring a crystal grain size of the prior
austenite crystals in the coil spring was as follows. Specifically,
first, as shown in FIG. 2, the cross section of the coil spring in
each sample was partitioned at intervals 45 degrees from the center
thereof into eight regions. In the respective regions, the crystal
grain size of the prior austenite crystal in the depth of 0.3 mm
from the surface of the steel wire forming the coil spring in the
direction to the center thereof was observed and measured in each
test sample with an optical microscope (at 400-fold magnification)
in accordance with JIS G 0551 (size of the field of view: 250
.mu.m.times.200 .mu.m). The average of measurements was shown in
the "average crystal grain size number of prior .gamma.-crystal"
column. In the present invention, the samples having the average
crystal grain size number of the prior austenite crystals of 11.0
or more were rated as pass.
(Difference in Grain Size Number between Prior Austenite
Crystals)
[0060] A method for determining a difference in grain size number
between the prior austenite crystals in the coil spring was as
follows. Some samples had the above-mentioned measured crystal
grain size number of the prior austenite crystal that differed by
three or more from the grain size number observed at the maximum
frequency. These samples were determined to contain duplex-grains.
The samples where the duplex-grain was present were defined as
"present" in the "Duplex-grain" column of the table, while the
samples where the duplex-grain was not present were defined as "not
present".
(Fatigue Resistance: Fatigue Test)
[0061] A shear stress with the maximum shear stress (T.sub.max) of
588.+-.441 MPa was applied to each test sample obtained in the
above way, and the test sample was subjected to the fatigue test up
to sixty million times. The test samples to which the shear stress
could be applied sixty million times (that is, which were not
broken) were defined as the "A" determination (which means
excellent fatigue resistance), and then these samples were shown as
">6000" in the table. The test samples to which the shear stress
could not be applied sixty million times (that is, when the test
sample was broken midway) were defined as the "F" determination
(which means the failure of the test, or inferior fatigue
resistance), and then the number of application of the shear stress
that caused the breakage of the sample was recorded in the
table.
TABLE-US-00001 TABLE 1 Chemical composition (% by mass) Steel type
C Si Mn Cr V Al Ni Nb P S A 0.47 2.96 0.53 0.62 0.56 0.003 0.00
0.00 0.010 0.006 B 0.55 3.03 0.52 0.60 0.73 0.003 0.00 0.00 0.012
0.008 C 0.55 3.04 0.52 0.21 0.72 0.006 0.00 0.00 0.008 0.010 D 0.46
2.21 0.71 0.30 0.66 0.002 0.02 0.00 0.016 0.009 E 0.60 1.98 0.55
0.62 0.75 0.003 0.00 0.02 0.011 0.008 F 0.60 2.10 0.50 1.76 0.26
0.004 0.20 0.00 0.013 0.006 G 0.59 2.10 0.51 1.30 0.44 0.003 0.20
0.00 0.011 0.007 H 0.37 1.20 0.35 0.70 0.63 0.004 0.00 0.00 0.009
0.009
TABLE-US-00002 TABLE 2 Average crystal grain size Fracture
Carburization Carburized Vickers number of lifetime Steel
temperature layer depth hardness prior [.times.10.sup.4
Fracture-lifetime No. type [.degree. C.] [mm] Hv .gamma.-crystal
Duplex-grain times] determination 1 A 1,050 0.80 679 11.1 Not
present >6,000 A 2 B 1,050 0.65 718 13.6 Not present >6,000 A
3 C 1,050 0.70 690 13.1 Not present >6,000 A 4 D 1,050 0.65 688
13.6 Not present >6,000 A 5 E 1,050 0.75 691 13.6 Not present
>6,000 A 6 E 1,030 0.70 685 13.6 Not present >6,000 A 7 E
1,070 0.80 693 13.1 Not present >6,000 A 8 F 850 0.27 681 13.0
Not present 5,000 F 9 F 1,050 0.41 705 10.1 Present 2,000 F 10 G
1,050 0.50 705 11.6 Present 1,000 F 11 H 950 0.43 568 12.1 Not
present 2,000 F 12 A 900 0.25 680 12.6 Not present 1,500 F 13 B 900
0.27 708 13.6 Not present 2,000 F
[0062] These results can be explained by the following
consideration. Samples Nos. 1 to 7 are examples that met the
requirements defined by the present invention (chemical
composition, crystal grain size, carburized layer depth, and
Vickers hardness). All the coil springs of samples Nos. 1 to 7 are
found to have a long fracture lifetime with a high load applied
thereto (A determination) and an excellent fatigue resistance.
[0063] In contrast, samples Nos. 8 to 13 did not satisfy the
requirements defined by the present invention, including the
chemical composition and the preferable manufacturing conditions,
and thus could not ensure the predetermined crystal grain size,
carburizing depth, Vickers hardness, and the like, leading to the
result of inferior fatigue resistance (F determination).
[0064] Samples Nos. 8 and 9 are examples in which the same type of
steel was used. These are the examples simulating Example No. 4
disclosed in Patent Document 1 (steel type of A and carburization
condition L in Patent Document 1). Samples Nos. 8 and 9 are the
examples in which the amount of added V was small, and the amount
of added Cr was large. Since the diffusion coefficient of C was
drastically reduced, the carburized layer was shallow. In
particular, in sample No. 8, the carburization temperature was low,
so that the adequate carburizing depth could not be obtained,
resulting in worse fatigue resistance. Although in sample No. 9,
the processing was performed at the carburization temperature
recommended by the present invention, the amount of added V was
small, which could not exhibit the sufficient effect of making the
crystal grains of the prior austenite crystals finer. As a result,
the duplex-grains were generated to degrade the fatigue
resistance.
[0065] In sample No. 10, since the amount of added V was small, the
processing at the predetermined carburization temperature generated
duplex-grains, thus degrading the fatigue resistance.
[0066] Sample No. 11 is an example in which the amounts of added C
and Si were small, and thus the carburization temperature was low.
In this example, the predetermined Vickers hardness was not
obtained, thereby degrading the fatigue resistance.
[0067] In samples Nos. 12 and 13, the carburization temperature was
low, whereby the predetermined carburizing depth was not obtained,
degrading the fatigue resistance.
* * * * *