U.S. patent application number 10/385656 was filed with the patent office on 2004-04-29 for oil tempered wire for cold forming coil springs.
This patent application is currently assigned to Chuo Hatsujo Kabushiki Kaisha. Invention is credited to Nakano, Tomohiro, Sakakibara, Takayuji, Wakita, Masami, Yoshikawa, Hidetoshi.
Application Number | 20040079067 10/385656 |
Document ID | / |
Family ID | 32104885 |
Filed Date | 2004-04-29 |
United States Patent
Application |
20040079067 |
Kind Code |
A1 |
Yoshikawa, Hidetoshi ; et
al. |
April 29, 2004 |
Oil tempered wire for cold forming coil springs
Abstract
[OBJECT] An oil-tempered wire for a cold-formed coil spring
having a quality equivalent of or higher than a hot-formed coil
spring is provided. A cold-formed coil spring made from the
oil-tempered wire is also provided. [MEANS FOR SOLVING THE PROBLEM]
Material used is the steel which contains, in weight percentage,
0.35 to 0.55% C, 1.8 to 3.0% Si, 0.5 to 1.5% Mn, 0.5 to 3.0% Ni,
and 0.1 to 1.5% Cr. The ferrite fraction in the microscopic
structure of this material is set to 50% or less. Hot rolled wire
is cold drawn with a predetermined reduction of area, and a heat
treatment using high frequency induction heating is conducted. It
is preferable to set the maximum heating temperature between
900.degree. C. to 1020.degree. C. (favorably 950.degree. C.) and
the holding time between 5 to 20 seconds. It is preferable to make
the oil-tempered material to have the grain size number of 9 or
more, and the tensile strength from 1830 to 1980 MPa.
Inventors: |
Yoshikawa, Hidetoshi;
(Nagoya-shi, JP) ; Nakano, Tomohiro; (Nagoya-shi,
JP) ; Sakakibara, Takayuji; (Nagoya-shi, JP) ;
Wakita, Masami; (Nagoya-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
Chuo Hatsujo Kabushiki
Kaisha
Nagoya-shi
JP
458-0835
|
Family ID: |
32104885 |
Appl. No.: |
10/385656 |
Filed: |
March 12, 2003 |
Current U.S.
Class: |
57/200 ; 148/580;
148/595 |
Current CPC
Class: |
C21D 8/065 20130101;
C21D 9/02 20130101; C21D 9/525 20130101; Y02P 10/253 20151101; C21D
8/06 20130101; Y02P 10/25 20151101; C21D 9/60 20130101 |
Class at
Publication: |
057/200 ;
148/580; 148/595 |
International
Class: |
D02G 003/02; C21D
009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 2002 |
JP |
2002-074142 |
Claims
1. An oil-tempered wire for a cold-formed coil spring characterized
by being made from steel comprising, in weight percentage, 0.35 to
0.55% C, 1.8 to 3.0% Si, 0.5 to 1.5% Mn, 0.5 to 3.0% Ni, and 0.1 to
1.5% Cr, and by being heat treated with high frequency induction
heating.
2. The oil-tempered wire for a cold-formed coil spring according to
claim 1, wherein the material is hot-rolled to a wire with a
predetermined diameter, the wire is cold drawn with a predetermined
reduction of area, and then the drawn wire is heat treated with a
high frequency induction heating.
3. The oil-tempered wire for a cold-formed coil spring according to
claim 1, wherein the ferrite fraction in the microscopic structure
of the steel before the heat treatment with a high frequency
induction heating is 50% or less.
4. The oil-tempered wire for a cold-formed coil spring according to
claim 1, wherein the maximum heating temperature in the high
frequency induction heating is 1020.degree. C. or lower.
5. The oil-tempered wire for a cold-formed coil spring according to
claim 1, wherein the maximum heating temperature in the high
frequency induction heating is 950.degree. C. or lower.
6. The oil-tempered wire for a cold-formed coil spring according to
claim 1, wherein the maximum heating temperature in the high
frequency induction heating is 940.degree. C. or higher.
7. The oil-tempered wire for a cold-formed coil spring according to
claim 1, wherein the maximum heating temperature in the high
frequency induction heating is 900.degree. C. or higher.
8. The oil-tempered wire for a cold-formed coil spring according to
claim 1, wherein a holding time at the maximum heating temperature
in the high frequency induction heating is 5 seconds or more.
9. The oil-tempered wire for a cold-formed coil spring according to
claim 1, wherein a holding time at the maximum heating temperature
in the high frequency induction heating is 20 seconds or less.
10. The oil-tempered wire for a cold-formed coil spring according
to claim 1, wherein the grain size number after the heat treatment
is set to be 9 or more.
11. The oil-tempered wire for a cold-formed coil spring according
to claim 1, wherein the tensile strength after the heat treatment
is set to be from 1830 to 1980 MPa.
12. The oil-tempered wire for a cold-formed coil spring according
to claim 1, wherein the material further comprises 0.01 to 0.025% N
and 0.05 to 0.5% V, and the P content is restricted to 0.01% or
less, and the S content is restricted to 0.01% or less.
13. A cold-formed coil spring made from the oil-tempered wire
according to claim 1.
Description
TECHNICAL FIELD TO WHICH THE INVENTION PERTAINS
[0001] The present invention relates to an oil-tempered wire used
for a cold-formed coil spring, and a cold-formed coil spring made
of oil-tempered wire.
PRIOR ART
[0002] A demand for better fuel efficiency in automobiles is rising
these days from the viewpoint of conservation of natural resources,
as well as environmental concerns. One of the targets under
scrutiny is weight reduction of the suspension springs which are
relatively heavy among automotive parts. The weight reduction has
continuously been demanded, and has mainly been achieved by
increasing the working strength of the spring. For increasing the
working strength of the spring, it is necessary to strengthen or
harden the material. Hence, hardening of the material has been the
major strategy for the weight reduction.
[0003] This strategy, however, might create the negative effect on
corrosion fatigue. Therefore, a high-strength hot-formed coil
spring of reduced weight and having good corrosion fatigue strength
has been developed and is used. One example is a spring made from
steel with a lower carbon content and a slightly higher silicon
content (Publication No. 11-241143 of Japanese unexamined patent
application).
[0004] At the same time, for the material of cold-formed coil
springs, mainly small springs, SAE (Society of Automotive
Engineers) 9254 steel has been used, where it underwent a high
frequency induction heating process (hereafter called "short period
heat treatment"). The short period heat treatment is characterized
by quick and short time heating by means of direct heating (self
heating) using high frequency induction heating, which provides
advantages such as fine structures and grains as well as minimum
decarburization.
PROBLEMS TO BE SOLVED BY THE INVENTION
[0005] The above-mentioned steel with lower carbon content and
higher silicon content demonstrates high-performance in a
hot-formed coil spring when an appropriate heat treatment is
conducted. However, it is difficult to conduct a sufficient heat
treatment with a short period heat treatment using induction
heating. Therefore, it is difficult for the spring to demonstrate
equivalent performance of a hot-formed coil spring.
[0006] After high-committed investigations, we have found
conditions of high frequency induction heating suitable for steel
with lower carbon content and higher silicon content. These
conditions make it possible that an oil-tempered wire for a
cold-formed coil spring, and a cold-formed coil spring made from an
oil-tempered wire demonstrate higher performance than a hot-formed
coil spring.
MEANS FOR SOLVING THE PROBLEMS
[0007] An oil-tempered wire for a cold-formed coil spring according
to the present invention is made from steel containing, in weight
percentage, 0.35 to 0.55% C, 1.8 to 3.0% Si, 0.5 to 1.5% Mn, 0.5 to
3.0% Ni, 0.1 to 1.5% Cr, and is characterized by being processed
with a heat treatment using high frequency induction heating.
[0008] In addition, the steel may further contain, in weight
percentage, 0.01 to 0.025% N and 0.05 to 0.5% V, and P content is
limited to 0.01% or less P, and S content to 0.01% or less.
[0009] Before conducting the above-mentioned high frequency
induction heating, it is preferable to conduct wire drawing at a
predetermined reduction of area by cold-forming on a wire
manufactured with hot-formed rolling. By doing this, it is
preferable to restrict the ferrite fraction in the micro-structure
of the steel to 50% or less. In addition, by conducting the
heating, it is preferable to set the maximum heating temperature to
be within the range of 900 to 1020.degree. C. It is preferable to
set the holding time at the maximum heating temperature to be
within 5 to 20 seconds. And it is preferable to make the grain size
number of the oil-tempered material to be 9 or more, and set the
tensile strength from 1830 to 1980 MPa.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a table showing the composition range of the
material of the present invention, and conventional materials for
oil-tempered wire and cold-formed coil spring.
[0011] FIG. 2 is a table showing the chemical compositions of the
material of the present invention and the comparative steel used in
tests.
[0012] FIG. 3 is a diagram showing the shape and dimensions of a
specimen used in basic heat treatment tests.
[0013] FIG. 4 is a diagram showing the heating pattern of a heat
treatment conducted in basic heat treatment tests.
[0014] FIG. 5 is a TTA diagram showing the results of basic heat
treatment tests using the material of the present invention.
[0015] FIG. 6 is a TTA diagram showing the results of basic heat
treatment tests using the comparative material.
[0016] FIG. 7 is a graph showing the internal hardness distribution
of the material of the present invention austenitized under the
severest condition.
[0017] FIG. 8 is a graph showing the relationship between the
maximum heating temperature and the internal hardness with
different ferrite fractions before heat treatments.
[0018] FIG. 9 is a graph showing the relationship between the
maximum heating temperature and the grain size number with
different ferrite fractions before heat treatments.
[0019] FIG. 10 is a manufacturing process chart of sample
oil-tempered wires and cold-formed coil springs.
[0020] FIG. 11 is a table of the dimensions of sample springs.
[0021] FIG. 12 is a graph showing the surface hardness distribution
of the material of the present invention and the comparative
material in the state of oil-tempered wire.
[0022] FIG. 13 is a graph showing the surface compression residual
stress distribution of the material of the present invention and
the comparative material in the state of coil spring.
[0023] FIG. 14 is a graph showing the results of fatigue tests on
the material of the present invention and the comparative
material.
[0024] FIG. 15 is a graph showing the results of artificial pit
corrosion fatigue tests on the material of the present invention
and the comparative material.
[0025] FIG. 16 is a graph showing the results of clamping tests on
the material of the present invention and the comparative
material.
[0026] FIG. 17 is a graph showing the relationship between maximum
heating temperatures and ferrite decarburizing depths.
[0027] FIG. 18 are photos of the surface microscopic structure
showing the relationship between the maximum heating temperatures
and the ferrite decarburizing depths.
MODE FOR CARRYING OUT THE INVENTION
[0028] Steel used as the material of the present invention is
almost the same as that described in the above-mentioned
Publication No. 11-241143 of Japanese unexamined patent
application. The basic idea of the design of the ingredients is, as
mentioned in the patent publication, to improve the corrosion
fatigue strength.
[0029] In general, sag effectively decreases when the hardness of
the material is increased. And under ideal conditions, although
there is a certain limit, an increase in the hardness of the
material leads to an enhancement of the fatigue resistance.
However, since automotive suspension springs, for example, are
installed at such places where water or mud easily attaches to, the
problem of corrosion must be considered first for the actual use.
This is because corrosion generates pits (micro-pits) on the spring
surface, and they become the origin of fatigue fracture.
[0030] The main causes of fracture by corrosion fatigue are: (1)
the delayed-fracture phenomenon of steel, (2) generation of surface
pits (micro-pits) by corrosion, and (3) a decrease in the residual
stress due to long-term use.
[0031] The delayed-fracture, which is peculiar to high-strength
steels, is a phenomenon where hydrogen atoms from moisture on the
surface or vapor in the air enter into the steel, and are
accumulated at the irregular part of the grain boundary or the
boundary between precipitates and matrix, which increases the
pressure, resulting in generating a micro crack and finally
fracture. Materials used for various springs have been strengthened
especially in recent years, and are subjected to a higher working
stress than conventional materials. They are also used under
situations that, as mentioned above, moisture or other forms of
water easily attaches to it. Therefore the delayed-fracture
property of the material has to be considered first when an
improved corrosion fatigue strength is sought.
[0032] Stress is concentrated on a surface pit generated by
corrosion, which greatly decreases the fatigue resistance. One way
to avoid this is not to generate pits as possible, or to generate a
pit in a form in which the stress concentration is minimized. At
the same time it is important to adopt measures in the material to
produce strength against cracking in the presence of pits.
[0033] Residual stress of a spring is endowed by a shot peening
operation. In detail, when the surface is deformed by the shot
peening, the difference in the deformation between the deformed
surface layer and the undeformed subsurface layer causes strain and
residual compression stress on the surface. When the surface layer
is removed by corrosion, or when a micro crack is generated on the
surface, the strain and the residual stress decrease.
[0034] The ranges of the compositions were determined considering
the circumstances mentioned above, and the lower and upper limits
of the respective composition range were specified for the
following reasons.
[0035] C content is set lower than that of the JIS-SUP 7 steel,
which is the most common material used for hot-formed coil springs,
or the material of various oil-tempered wires. This is because,
with the same hardness (strength), the toughness of material with
lower C content and higher alloying element content is better than
that of material with higher C content. When the toughness is
improved, the occurrence of fatigue cracks from corrosion pits is
decreased, the growing speed of the cracks is lowered, and the
corrosion fatigue strength is improved, which is an object of the
present invention. The lower limit of C content is set to 0.35%
because, with less content, it is difficult to obtain the
above-mentioned hardness after the heat treatment even when other
alloying elements are maximized. The upper limit of C content is
set to 0.55% because, with more content, the toughness of the
material greatly deteriorates.
[0036] Si is known to be effective to enhance the sag resistance.
Therefore, the upper limit of Si content is set higher than that of
conventional steels to improve sag resistance. On the other hand,
Si promotes the surface decarburization of steels. If Si content is
set to more than 3.00%, decarburization by a heat treatment becomes
significant. In such a case it is difficult to obtain the
above-mentioned hardness and the residual stress on the surface.
Therefore the upper limit is set to 3.00%.
[0037] Mn is effective in improving hardenability. Exercising a
thorough quenching and tampering all the way to the center of a
spring is crucial in obtaining the full effect of alloying
elements, such as Ni, in improving the toughness. The lower limit
is set to 0.5%, because an adequate hardening cannot be obtained on
a spring with a large diameter when the Mn content is less than
0.5%. However, if the Mn content is set to more than 1.5%, the
hardenability enhancing effect is saturated and the toughness tends
to decrease on normal size springs. Therefore the upper limit is
set to 1.5%.
[0038] Ni is effective to improve toughness and to suppress steel
corrosion. As mentioned above, suppression of corrosion enhances
the corrosion fatigue strength by blocking the occurrence of
corrosion pits and preventing a decrease in the residual stress.
This effect of Ni can only be obtained when the Ni content is 0.5%
or more. However, if the Ni content is set to more than 3%, the
effect of improving toughness is saturated. In addition, since Ni
is an austenite-stabilizing element, the amount of residual
austenite increases after quenching, which means that the
transformation to martensite is incomplete. Moreover, it increases
the cost of a spring because the element is expensive. Therefore
the upper limit is set to 3.0%.
[0039] Cr is effective in improving hardenability like Mn, and it
is also effective in suppressing surface decarburization. The lower
limit is set to 0.1% because such effects are hardly obtained with
a Cr content of under 0.1%. On the other hand, if the Cr content is
set to more than 1.5%, the effect is saturated, and an adverse
effect arises that it brings about a heterogeneous microstructure
of the steel after tempering. Therefore the upper limit is set to
1.5%.
[0040] N bonds to Al in steel to become AlN, which precipitates
into steel as fine particles. Because this prevents grains from
growing, N is very effective in reducing the size of (or refining)
grains in steel. To obtain this effect, it is necessary that the N
content is set to 0.01% or more. However, if the N content is
excessive, the quality of steel deteriorates since N generates
N.sub.2 gas when the steel is manufactured (solidified and cooled).
Therefore the upper limit is set to 0.025%.
[0041] V bonds to C to become VC (vanadium carbide), which
precipitates into steel as fine particles. It increases the
toughness of the steel by refining grains as in the case of AlN.
Dispersing these fine carbide particles in steel prevents H
(hydrogen) entering from outside from accumulating at certain
limited locations, and prevents the above-mentioned
delayed-fracture from occurring. To obtain this effect, it is
necessary that the V content is set to 0.05% or more. However, when
the V content is set to more than 0.5%, the effect is not obtained
because the VC only grows without increasing the number of VC
precipitation. Therefore, the upper limit is set to 0.5%.
[0042] P decreases toughness of steel. Therefore, by limiting the
content to 0.01% or less, the toughness of the material is
improved, and the corrosion fatigue strength of the spring
according to the present invention increases. Because the present
invention relates to a cold-formed coil spring, it is very
important to increase toughness.
[0043] S bonds to Mn to become MnS, which is insoluble in steels.
MnS is easy to deform, and is drawn and elongated by rolling, etc.
The elongated MnS tends to be the origin of fracture by mechanical
impacts or fatigue. Therefore the upper limit of S content is set
to 0.01% in the present invention. This brings about a toughness
and fatigue resistance of the steel at a higher hardness equivalent
to those of conventional material.
[0044] FIG. 1 shows the composition range of: the oil-tempered
chrome vanadium alloy steel wires for valve springs specified in
the JIS (Japanese Industrial Standard) (SWOCV-V: JIS G3565), the
oil-tempered silicon chrome steel wire for valve springs (SWOSC-V:
JIS G3566), the SAE (Society of Automotive Engineers) 9254 steel
which has been widely used as a material of cold-formed coil
springs for a small spring, and the material of the present
invention. As is obvious from the table, an oil-tempered wire
according to the present invention contains a lower carbon content
and a remarkably higher silicon content compared to the
conventional oil-tempered wire or cold-formed coil spring steel.
This makes the austenitic transformation temperature (Ac3
temperature) of the steel higher. Therefore it is necessary to set
appropriate conditions to a high frequency induction heat treatment
which is generally short period heating.
[0045] For the above reason, we decided to conduct wire drawing
with a predetermined reduction of area before the material is heat
treated, and to set the ferrite fraction in the microstructure to
50% or less. Owing to these treatments, sufficient austenitizing is
done even with the high frequency induction heating, and it is
possible to obtain the equivalent performance to the
above-mentioned hot-formed coil spring.
[0046] Sufficient austenitization with a short period heat
treatment can be achieved by increasing the heating temperature.
However, a too high heating temperature coarsens the austenitic
grain size, and may decrease the steel toughness. Therefore, in the
present invention, the maximum heating temperature in the high
frequency induction heating was controlled to 1020.degree. C. or
lower, or preferably 950.degree. C. or lower. At a temperature of
900.degree. C. or lower, sufficient austenitization may not be
obtained. Based on the results of the below-mentioned basic tests,
the holding time at the maximum heating temperature, which greatly
affects the austenitizing and coarsening of the grain size, is set
within the range of 5 to 20 seconds in the present invention.
[0047] By conducting a heat treatment on the steel whose component
range is mentioned above, the coarsening of grain size can be
decreased, and by setting the grain size number to 9 or more, the
quality of a cold-formed coil spring is assured (especially to
corrosion fatigue resistance).
[0048] On the one hand, there is a case that a ferrite decarburized
layer exists on the surface of the material before heating. This
ferrite-decarburized surface layer of the material is usually
passed onto the spring, which greatly decreases the fatigue
resistance. Hence, when the ferrite decarburized layer exists on
the surface of the material before heating, it is preferable to set
the maximum heating temperature in the high frequency induction
heating to 940.degree. C. or more. This decreases the depth of the
surface decarburization layer of material, or avoids it
totally.
[0049] Tensile strength is set to 1830 to 1980 MPa, because with
less strength it does not meet the durability requirement for
suspension springs, and with more strength the toughness greatly
decreases.
[0050] [Embodiments]
[0051] The results of basic tests conducted to determine the
condition of a heat treatment is described. The SAE9254 steel
conventionally used for cold-formed coil springs was included in
the basic tests as the comparative material. Steel with the
compositions shown in FIG. 2 is melted, and small specimens as
shown in FIG. 3 are prepared. Heat treatment is conducted with the
heating pattern shown in FIG. 4 which simulates quenching.
[0052] First, heat treatments according to the heating pattern of
FIG. 4 are conducted with varying maximum heating temperatures
within the range of 900.degree. C. to 980.degree. C. with an
increment of 20.degree. C. The holding time at the maximum
temperature is set to 5, 10 and 20 seconds. After the heat
treatments, the internal hardness (Hv 20 kg) and the austenitic
grain size number (JIS-G0551) of the specimens were measured. The
result is shown in the TTA (Time-Temperature-Austenitizing) diagram
of FIG. 5.
[0053] In FIG. 5, no marked difference can be found in the internal
hardness and austenitic grain size number with the variation of the
maximum-temperature holding time within the range of 5 to 20
seconds. This indicates that, within such range of
maximum-temperature holding time, the holding time has little
effect on the short period heat treatment.
[0054] As to the heating temperature, on the other hand, the
internal hardness is not affected so much by the rise in the
heating temperature, but the grain size number is revealed to
decrease (the grains coarsen) as the heating temperature rises.
[0055] FIG. 6 shows similar results of SAE9254 steel, or the
comparative material, derived from Kawasaki, et al., "Heat
Treatment", The Japan Society for Heat Treatment 20, 1980, pp
281-288. The heating speed of the two diagrams differs, and the
change in the austenitic transformation temperature (Ac3
temperature) due to the heating speed difference is estimated to be
about 10.degree. C., where the comparative material with the larger
heating speed has a higher Ac3 temperature. Taking this into
account, the austenitic grain size number of the material of the
present invention is larger (or finer) by 2 points. This effect can
be ascribed to the high Ac3 temperature of the material of the
present invention and the pinning effect of fine vanadium carbides
included in the material of the present invention.
[0056] FIG. 7 shows the hardness distribution obtained under the
maximum heating temperature of 900.degree. C. and the holding time
of 5 seconds, which is the severest austenitizing condition in the
TTA diagram of FIG. 5. Under this severest condition, a uniform
internal hardness is obtained in the material of the present
invention. It was also confirmed that the microscopic structure
showed normal martensite structure all the way to the center.
[0057] In order to assess the effect of the microscopic structure
before a high frequency induction heating (especially the ferrite
fraction) on a short period heat treatment, specimens with 30%
ferrite fraction and 35% ferrite fraction are prepared from the
material of the present invention by giving them appropriate heat
treatments. After conducting heat treatments according to the
pattern shown in FIG. 4 on these specimens with the maximum heating
temperature of 900.degree. C. to 980.degree. C. and holding time of
5 seconds, the internal hardness and austenitic grain sizes were
measured. As shown in FIGS. 8 and 9, it is confirmed that the
microscopic structure of specimens with 50% or less ferrite
fraction before heat treatment have little effect.
[0058] In order to assess the relationship between the surface
ferrite decarburization and the heating temperature of high
frequency induction heating, specimens are prepared from the
material of the present invention with the surface
ferrite-decarburized layer of 0.03 mm. After conducting heat
treatments according to the pattern shown in FIG. 4 on these
specimens with the maximum heating temperatures of 900.degree. C.
to 1000.degree. C. and the holding time of 17.5 seconds, the depths
of surface ferrite decarburized layer were measured. As shown in
FIGS. 17 and 18, the surface ferrite decarburized layer existed
before heating remained at the heating temperature of 940.degree.
C., fell by half to 0.015 mm at 970.degree. C., and almost
disappeared at 1000.degree. C.
[0059] This is explained as follows. Although surface ferrite
decarburized layer exists in the material before heating,
conducting a short period heat treatment at a higher temperature
than usual makes the carbon in the material diffuse and dissolve
into the surface ferrite layer, and then makes the surface ferrite
decarburized layer thin or disappear. High frequency induction
heating has been known to have the advantage of causing less
surface decarburization owing to its quick and short period
heating. The inventors confirmed that by conducting the heating
under the condition specified by the present invention, already
existing decarburization can disappear and even carbon restoration
in the layer is possible.
[0060] Based on these basic tests, fatigue resistance tests on coil
springs were conducted. From the material of the present invention,
an oil-tempered wire was made by high frequency heating using the
process shown in FIG. 10(a). Then from the oil-tempered wire, coil
springs were produced using the process shown in FIG. 10(b).
Coiling was conducted by cold-forming. The dimensions of the
produced coil springs are shown in FIG. 11. From the comparative
material, an oil-tempered wire was made by a furnace heating, and
from the oil-tempered wire, coil springs of the same dimensions
were produced by hot-forming.
[0061] After the process of FIG. 10(a), the surface hardness
distributions of the oil-tempered wires were measured. As shown in
FIG. 12, the decrease in the hardness due to surface
decarburization is minimized in the material of the present
invention on which high frequency heating was conducted.
[0062] Distribution of the surface compression residual stress of
the coil springs produced by the process of FIG. 10(b) is shown in
FIG. 13. The residual stress of the material of the present
invention is larger at any depths by about 100 to 200 MPa than that
of the comparative material. This suggests the effect of the
surface decarburization shown in FIG. 12.
[0063] Fatigue resistance tests on the coil springs made from the
material of the present invention and coil springs made from the
comparative material were conducted under the condition of the
average stress of .tau.m=735 MPa, and the stress amplitude of
.tau.a=550 MPa. As shown in FIG. 14, it is confirmed that the
material of the present invention has the durability of 300,000
times, which is nearly equivalent to the comparative material which
has the durability of 280,000 times.
[0064] Next, corrosion fatigue tests were conducted. Pits of 0.4 mm
were formed on the surface of the springs, and then the springs was
subjected to corrosion by salt water. The fatigue test was
conducted under the condition of the average stress of .tau.m=735
MPa, and the stress amplitude of .tau.a=196 MPa. As shown in FIG.
15, it is confirmed that the material of the present invention has
nearly the equivalent corrosion fatigue properties to the
comparative material.
[0065] Finally, sag resistance tests were conducted. Sample coil
springs were clamped to yield the maximum shear stress of 1200 MPa
on the surface, and were placed in the temperature of 80.degree. C.
for 96 hours, whereby sag is caused. The residual shear strain on
the surface is calculated from the difference in the free height
before and after the sag resistance tests. FIG. 16 shows the
results. The material of the present invention showed slightly
better result than the comparative material as for the sag
resistance. This suggests that the higher silicon content, as well
as the controlled microscopic structure before heat treatment, has
brought about the result.
[0066] Thus, the material for cold-formed coil springs having
equivalent comparable quality to materials for hot-formed coil
spring.
* * * * *