JP2003055741A - Oil tempered steel wire for cold formed coil spring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oil tempered steel wire for a cold formed coil spring which has performance equal to or above that of a hot formed coil spring, and to provide a cold formed spring using the same wire. SOLUTION: Steel containing, by weight, 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 is used as the stock. The fraction of ferrite in the steel sheet of the stock is controlled to <=50%, and the hot rolled wire rod is subjected to cold wire drawing at a prescribed reduction of area. After that, heat treatment by high-frequency induction heating is performed thereto. Desirably, the maximum heating temperature is controlled to 900 to 1,020 deg.C (more desirably, to 950 deg.C), and the holding time is controlled to 5 to 20 sec. As the stock after oil tempering treatment, desirably, the crystal grain size number is controlled to >=9, and its tensile strength is controlled to 1,830 to 1,980 MPa.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oil tempered wire used as a material for a cold-formed coil spring, and a cold-formed coil spring manufactured using the oil-tempered wire.

[0002]

2. Description of the Related Art In recent years, demands for improving fuel efficiency of automobiles have been increasing in view of resource and environmental problems. with this,
As for the suspension spring, which is a relatively heavy automobile component as a single unit, the demand for weight reduction is particularly strong. Needless to say, such a demand for weight reduction has been constantly made, and in the past, such demand has been met mainly by increasing the working stress of the spring (increasing the stress). In order to increase the working stress of the spring, it is necessary to increase the strength (hardness) of the material. Therefore, as a measure for reducing the weight of the conventional springs, increasing the hardness of the material has been the main focus.

However, this may have a bad influence on corrosion fatigue. Therefore, high-strength springs (hereinafter referred to as hot springs) have been developed and used by hot forming with emphasis on weight reduction and corrosion fatigue strength. One of them is made of steel having a low carbon content and a slightly high silicon content (for example, Japanese Patent Laid-Open No. 11-241143).

On the other hand, conventionally, as a cold coil spring (hereinafter referred to as a cold spring) material centered on a small spring, mainly 9254 steel of SAE (American Society of Automotive Engineers) standard is used as a material, and high frequency short A material that has been subjected to time induction heat treatment (hereinafter referred to as short-time heat treatment) is used. This short-time heat treatment is a heat treatment method characterized by rapid short-time heating by direct heating (self-heating) using high-frequency induction heating, and has the advantage that a fine structure and crystal grains can be obtained and surface decarburization is small. There is.

[0005]

The above-mentioned low carbon / high silicon steel can obtain high performance as a hot-formed spring by sufficient heat treatment, but high-frequency short-time induction heating treatment is sufficient. It is difficult to secure sufficient heat treatment, and it is difficult to obtain the same performance as the hot-formed spring.

As a result of intensive studies, the inventors of the present invention found a condition for high frequency induction heating suitable for the above low carbon / high silicon steel material. Thus, an oil tempered wire for a cold-formed coil spring having a performance equal to or higher than that of the hot-formed coil spring, and a cold-formed spring using the oil-tempered wire.

[0007]

The oil tempered wire for a cold-formed coil spring according to the present invention has a weight ratio of C: 0.
35-0.55%, Si: 1.8-3.0%, Mn:
0.5-1.5%, Ni: 0.5-3.0%, Cr:
A steel containing 0.1 to 1.5% is used as a raw material, and heat treatment by high frequency induction heating is performed.

This material is further N: 0.01-0.
0: 25%, V: 0.05-0.5%, P: 0.
It may be 01% or less and S: 0.01% or less.

Prior to the above-mentioned high frequency induction heating, it is desirable that the wire rod manufactured by hot rolling is drawn by cold working at a predetermined area reduction rate. At that time, it is desirable that the ferrite fraction in the steel structure is 50% or less.
Further, in the high frequency induction heating, it is desirable that the maximum heating temperature is in the range of 900 ° C to 1020 ° C. The holding time at the maximum heating temperature is preferably 5 to 20 seconds. And, as the material after the oil tempering treatment under such conditions, the grain size number is 9
The tensile strength is 1830-19.
It is desirable to set it to 80 MPa.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The steel used as a raw material in the present invention is
This is almost the same as that described in the above-mentioned JP-A No. 11-241143. The basic idea of the component design is to improve the corrosion fatigue strength as described in the publication.

That is, regarding the sagging, generally, the sagging can be effectively reduced by increasing the hardness of the material. Also, under ideal conditions, although there is a limit, an increase in material hardness leads to an improvement in fatigue resistance. However, for example, springs for automobile suspension are mounted on the parts of the automobile body where water, mud, etc. are most likely to adhere, so considering actual use, the problem of corrosion must be considered first. I won't. This is because the corrosion forms pits (micro holes) on the surface of the spring and causes fatigue fracture starting from this.

The main causes of damage due to corrosion fatigue are:
Possible causes are (1) delayed fracture phenomenon of steel, (2) generation of surface pits (micro holes) due to corrosion, and (3) reduction of residual stress value due to long-term use.

Delayed fracture is a phenomenon peculiar to high strength steel,
When stress is applied to steel, hydrogen enters the steel from moisture adhering to the surface or water vapor in the atmosphere and accumulates at grain boundaries and irregular parts such as the boundary between the precipitate and the base material and pressure Is increased, leading to micro-cracks and finally rupture. In recent years, the strength of the materials used for various springs has been particularly high, and when used, they are subject to higher stresses than before, and as described above, they are used in an environment where moisture easily adheres. In order to improve the corrosion fatigue strength, it is necessary to fully consider the delayed fracture characteristics of the material.

Surface pits due to corrosion serve as a stress concentration source, which significantly reduces fatigue strength. On the other hand, one of the measures is to prevent the corrosion pits from being generated as much as possible, or to generate them in such a form that the stress concentration is as small as possible even if they are generated. Even so, it is important to take measures on the material side so that cracks are less likely to occur from there.

In the case of a spring, the residual stress is imparted by shot peening. To explain it in detail, when the surface is plastically deformed by shot peening, it is deformed between the lower layer and a portion which is not plastically deformed. There is a difference in degree, and the resulting strain produces a compressive residual stress on the surface. Therefore, when the surface layer is removed by corrosion or a microcrack is generated on the surface, the strain becomes small and the residual stress value decreases.

The above-mentioned component ranges have been determined in view of the above points, and the reason why the lower limit value and the upper limit value of each chemical component range are set as described above is as follows.

First, the C content was set to a range lower than that of JIS-SUP7 steel, which is the most commonly used steel for hot forming springs, or the material steel of various oil tempered wires. This is because when the hardness (strength) is the same, the toughness is improved when the C content is decreased and the alloy element content is increased rather than when the C content is increased. The improvement in toughness greatly contributes to the improvement in corrosion fatigue strength, which is the object of the present invention, by reducing the generation and growth rate of fatigue cracks from corrosion pits. The lower limit of the C content is set to 0.35% because it is difficult to obtain the above hardness after the heat treatment even if other alloy elements are added to the maximum if the lower limit is set to 0.35%. The upper limit is set to 0.55% because the toughness of the material remarkably deteriorates if it is contained more.

It is known that Si has the effect of improving the sag resistance. Therefore, in order to further improve the sag resistance, the upper limit of the Si content in the present invention is set to a value higher than that of the conventional steel. However, Si is an element that promotes surface decarburization of steel, and if it is contained in excess of 3.00%, decarburization during heat treatment cannot be ignored. In this case, it becomes difficult to obtain the hardness and the residual stress value on the surface, so the upper limit is defined in this way.

Mn is an element effective in improving the hardenability. Sufficient quenching and tempering to the center of the spring is an essential condition for fully exerting the toughness improving effect of the material by the alloying elements such as Ni described below. Mn is 0.5
If it is less than 0.1%, in the case of a spring having a large diameter, sufficient quenching to the center cannot be obtained, so the lower limit was made 0.5%. But 1.5
Even if it is contained in excess of%, the hardenability improving effect saturates and the toughness deteriorates in a normally used spring, so the upper limit was made 1.5%.

Ni has the effect of improving the toughness of the steel and the effect of suppressing the corrosion of the steel. Suppression of corrosion is
As described above, the corrosion fatigue strength of the spring is improved from both aspects of prevention of corrosion pit formation and reduction of residual stress. Such an effect of Ni cannot be obtained unless the content of Ni is 0.5% or more. However, even if the content exceeds 3%, the toughness improving effect is saturated, while conversely, since it is an austenite stabilizing element, austenite remains during quenching and the transformation to martensite is incomplete. There is a risk. Further, since it is expensive, it also causes a significant increase in the cost of the spring. Therefore, the upper limit is set to 3%.

Like Mn, Cr has an effect of improving hardenability and an effect of suppressing surface decarburization. 0.1
If it is less than%, such an effect can hardly be expected, so the lower limit was made 0.1%. However, even if the content exceeds 1.5%, such an effect is saturated, and the tempering structure becomes nonuniform. Therefore, the upper limit is set to 1.5%.

N combines with Al in steel to form AlN,
Precipitates in the steel as fine particles. This hinders the growth of crystal grains, so N has a great effect on refining the crystal grains of steel. To obtain such an effect, it is necessary to contain 0.01% or more of N. But,
If the N content is too large, it is generated as N ₂ gas in the steel during steel production (solidification / cooling), deteriorating the internal quality of the steel.
Therefore, the upper limit is set to 0.025%.

V combines with C and precipitates in the steel as fine VC (vanadium carbide), and like the above AlN, crystal grains are refined to enhance the toughness of the steel. Further, by dispersing a large number of such fine carbides in the steel, it is possible to disperse the places where H (hydrogen) that has entered from the outside accumulates and suppress the generation of the delayed fracture. In order to obtain such an effect, it is necessary to contain V in an amount of 0.05% or more. However, if the content exceeds 0.5%, VC
VC does not increase in number without increasing the number of precipitation sites, and such an effect cannot be obtained. Therefore, the upper limit was made 0.5%.

P reduces the toughness of steel. Therefore, by setting the content to 0.01% or less, the effect of improving the toughness of the material and, in turn, improving the corrosion fatigue strength of the spring according to the present invention can be obtained. In particular, since the present invention is intended for cold-formed springs, the improvement of toughness is particularly important.

S is Mn which is insoluble in the steel by combining with Mn in the steel.
It becomes S. Since MnS is easily plastically deformed, it is easily stretched by rolling or the like and becomes a starting point of fracture due to impact or fatigue. Therefore, in the present invention, the upper limit of S is set to 0.01% so that the toughness and the fatigue resistance when the hardness is increased become equal to the conventional values.

FIG. 1 shows a chrome vanadium steel oil temper wire for valve springs (SWOCV-V: JIS G3565) and a silicon chrome steel oil temper wire for valve springs (SWOSC-V :) specified in Japanese Industrial Standards (JIS). JIS G3566), and the chemical composition range of SAE (American Society of Automotive Engineers) 9254 steel, which has been widely used as a cold coil spring material mainly for small springs, is compared with the composition range of the present invention. As is clear from this table, the oil-tempered wire according to the present invention has a low carbon content as a whole even when compared with the conventional oil-tempered wire and steel for cold forming springs, while the silicon content is very high. It's getting higher. Therefore, the austenite (A _c3 ) transformation point of steel becomes high, and it is necessary to set appropriate conditions for the high frequency induction heat treatment, which is generally a short time heating.

This is the reason why it is decided to perform wire drawing at a predetermined area reduction ratio before the high frequency induction heating treatment and to set the ferrite fraction in the pretreatment structure to 50% or less.
By these treatments, sufficient austenitization is achieved even by the high frequency induction heating treatment, and it becomes possible to secure the same performance as that of the hot-formed spring.

In order to achieve sufficient austenitization even if the heating time is short, there is a method of raising the heating temperature. However, excessive temperature rise may cause coarsening of austenite crystal grains and impair the toughness of steel. Therefore, in the present invention, the maximum heating temperature is also regulated,
The maximum heating temperature during high-frequency induction heat treatment was set to 1020 ° C or lower. This maximum heating temperature is preferably 95
It shall be 0 ° C or lower. However, at 900 ° C. or lower, austenitization may be insufficient. Further, since the holding time at the maximum heating temperature also has a great influence on the austenitization and the coarsening of crystal grains, the time is set to 5 to 20 seconds in the present invention based on the result of the basic experiment described later.

By subjecting the steel having the above-mentioned compositional range to heat treatment under such conditions, coarsening of the crystal grains is naturally suppressed, but when the grain size is 9 or more, the performance as a cold-formed spring (particularly Corrosion fatigue resistance) will be guaranteed more reliably.

On the other hand, a ferrite decarburized layer may exist on the surface of the material before heating. Such a ferrite decarburized layer usually transfers to the surface of the spring as it is, and the durability (fatigue resistance) of the spring is significantly impaired. Therefore, when the ferrite decarburized layer is present on the surface of the raw material before heating, it is desirable to set the maximum heating temperature during the high frequency induction heat treatment to 940 ° C or higher. As a result, as will be described later, the surface decarburized layer depth of the material is reduced or completely eliminated.

The reason why the tensile strength is set to 1830 to 1980 MPa is that the strength less than this range does not satisfy the durability required as a suspension spring, and if it exceeds this range, the adverse effect of deterioration of toughness becomes remarkable. This is because.

[0032]

EXAMPLES First, the results of basic experiments conducted to determine the heat treatment conditions will be described. The basic experiment was conducted using the conventional cold-formed spring steel, SAE9254 steel, as a comparative material. After the steel having the components shown in FIG. 2 was melted, a small test piece as shown in FIG. 3 was prepared, and heat treatment was performed in a heat pattern as shown in FIG. 4 in which quenching was simulated.

First, the maximum heating temperature T _max is set to 900 to 980 ° C.
Varied in five stages at 20 ° C. increments between, also by changing the heating and holding time t _h in three stages of 10, 20 seconds was heat-treated in the pattern of Figure 4, the test piece under each condition Internal hardness of (Hv
20kg) and grain size number of old austenite crystal grains (JIS-G0
551) was measured. The result (Time-Temperature-Austen
Itizing = TTA diagram) is shown in FIG.

Focusing on the heating and holding time in FIG. 5,
No significant difference was observed between the internal hardness and the former austenite grain size when the heating and holding time t _h was 5 to 20 seconds. Within this heating and holding time range, the heating and holding time for the short-time heat treatment was It can be seen that the impact is small.

On the other hand, focusing on the heating temperature, it can be seen that the internal hardness does not change much even if the heating temperature rises, but the grain size number is smaller (the crystal grains are larger). .

A similar graph of SAE9254 steel, which is a comparative material, is shown in FIG. 6 (Kawasaki et al., Japan Heat Treatment Technical Association “Heat Treatment” 20
(1980), pp. 281-288). Although the heating rates of the two are different, the change in the austenite transformation temperature (A _c3 point) due to it is expected to be about 10 ° C (A _c3 point is higher in the comparative material with a higher heating rate), so consider that Even if it is put in, the material of the present invention is finer by about 2 in the grain size number. This is considered to be due to the fact that the material of the present invention has a higher A _c3 point and the pinning effect by the V fine carbide contained in the material of the present invention.

From the TTA diagram of FIG. 5, the maximum heating temperature T _max = 900 ° C., which is the most severe condition in terms of austenitization,
FIG. 7 shows the result of measuring the hardness distribution from the surface by performing heat treatment under the heat treatment condition of heat holding time t _h = 5 seconds.
It can be seen that even under such severe heat treatment conditions, the material of the present invention can obtain uniform hardness even inside. It was confirmed by observation of the microscopic structure that a normal martensite structure was formed in the central part.

Next, in order to confirm the effect of the structure (particularly the ferrite fraction) before the high frequency heat treatment on the short time heat treatment, the ferrite fraction of the material of the present invention was changed by the heat treatment.
A 30% sample and a 35% sample were prepared. After subjecting them to the heat treatment of the heat pattern of FIG. 4 under the conditions of the maximum heating temperature T _max = 900 to 980 ° C. and the heating holding time t _h = 5 seconds, the internal hardness and the former austenite grain size were measured. As a result, as shown in FIGS. 8 and 9, it was confirmed that when the ferrite fraction was 50% or less, there was almost no influence of the pretreatment structure.

Further, in order to confirm the relationship between the ferrite decarburized layer on the surface of the material before the induction heat treatment and the induction heat treatment temperature, a sample having a ferrite decarburized layer of 0.03 mm on the surface was prepared for the material of the present invention. After heat-treating the heat pattern of FIG. 4 under the conditions of the maximum heating temperature T _max = 900 to 1000 ° C. and the heating holding time t _h = 17.5 seconds, the ferrite decarburization depth of the surface was measured. As a result, as shown in FIG. 17 and FIG. 18, the ferrite decarburized layer that was present before heating was still present up to the heating temperature of 940 ° C.
By changing the temperature to ℃, it becomes half, 0.015mm, and the heating temperature is 1
When it was raised to 000 ℃, it almost disappeared.

This is because even if there is a ferrite decarburized layer on the surface of the material before heating, by performing high-frequency induction heating at a temperature higher than usual for a short time, carbon inside diffuses and the ferrite part on the surface is heated. It is considered that the ferrite decarburized layer was dissolved or dissolved in the steel and the ferrite decarburized layer was reduced or eliminated. Conventionally, high-frequency induction heating is known to have an advantage that surface decarburization is small because it is rapid and short-time heating. However, the present inventors have performed heating under the conditions according to the present invention. We were able to confirm that it is even possible to eliminate decarburization and reconstitute.

Based on the results of the above basic experiments, experiments such as durability with coil springs were conducted. Regarding the material of the present invention, an oil tempered wire was first produced by high frequency heating in the step shown in FIG. 10 (a), and a coil spring was produced from the oil tempered wire in the step shown in FIG. 10 (b). In addition,
The coiling was of course cold. The specifications of the manufactured coil spring are as shown in FIG. As for the comparative material, an oil tempered wire was manufactured by heating in a furnace, and a coil spring having the same specifications was manufactured by hot forming.

First, after the step shown in FIG. 10A, that is, the surface hardness distribution in the state of the oil temper wire was measured. The results are shown in FIG. 12, and the material of the present invention by high-frequency heating also has a minimal decrease in hardness due to surface decarburization.

FIG. 13 shows the results of measuring the distribution of the surface compressive residual stress after the coil spring was manufactured in the step shown in FIG. 10 (b). The material of the present invention has a residual stress larger than that of the comparative material by 100 to 200 MPa at any depth. This seems to be due to the effect of surface decarburization shown in FIG.

The invention material coil spring and the comparative material coil spring were subjected to a fatigue endurance test under stress conditions of an average stress τm = 735 MPa and a stress amplitude τa = 550 MPa. As a result, as shown in FIG. 14, it was confirmed that the material of the present invention has a durable life of 300,000 times and has a durability almost equal to that of the comparative material of 280,000 times.

Next, a corrosion fatigue test was conducted. After forming 0.4 mm pits on the outer surface of the coil spring and corroding them with salt water, a fatigue endurance test was performed under the stress conditions of average stress τm = 735 MPa and stress amplitude τa = 196 MPa. As a result, as shown in FIG. 15, it was confirmed that the material of the present invention has substantially the same performance as the comparative material also in the corrosion fatigue property.

Finally, a sag resistance test was conducted. The test coil spring was tightened so that the maximum shear stress on the surface was 1200 MPa, and the coil spring was placed in an environment of 80 ° C. for 96 hours to cause sagging. FIG. 16 shows the result of calculating the residual shear strain on the surface from the difference in free height before and after the test. In terms of sag resistance, the material of the present invention gives slightly better results than the comparative material. This seems to be due to the fact that the control of the structure before heat treatment is effective in addition to the high silicon content of steel. Thus, a cold coil spring material having a performance comparable to that of the hot coil spring material can be provided.

[Brief description of drawings]

FIG. 1 is a table showing the composition range of the material steel of the present invention and the conventional oil tempered wire and steel for cold forming coil springs.

FIG. 2 is a table of chemical compositions of the material steels of the present invention material and the comparative material which were tested.

FIG. 3 is a shape and dimension diagram of a test piece used in a heat treatment basic experiment.

FIG. 4 is a diagram of a heat pattern of heat treatment performed in a heat treatment basic experiment.

FIG. 5: TTA showing the result of the basic heat treatment experiment for the material of the present invention
Fig.

FIG. 6 is a TTA diagram of the comparative material.

FIG. 7 is a graph showing the internal hardness distribution when the material of the present invention is austenitized under the most severe conditions.

FIG. 8 is a graph showing the relationship between the maximum heating temperature and internal hardness, with the ferrite fraction before heat treatment as a parameter.

FIG. 9 is a graph showing the relationship between the maximum heating temperature and the grain size number, with the ferrite fraction before heat treatment as a parameter.

FIG. 10 is a process drawing showing the manufacturing process of the oil tempered wire and the cold-formed coil spring which have been tested.

FIG. 11 is a table of specifications of tested coil springs.

FIG. 12 is a graph of surface hardness distributions of the material of the present invention and the comparative material in the state of oil tempered wire.

FIG. 13 is a graph of surface compressive residual stress distributions of the material of the present invention and the comparative material in the state of a coil spring.

FIG. 14 is a diagram showing the results of a durability fatigue test of the material of the present invention and the comparative material.

FIG. 15 is a view showing results of an artificial pit corrosion fatigue test of the material of the present invention and the comparative material.

FIG. 16 is a diagram showing the results of a settling test of the material of the present invention and the comparative material.

FIG. 17 is a graph showing the relationship between the maximum heating temperature and the ferrite decarburization depth.

FIG. 18 is a surface structure micrograph showing the relationship between the maximum heating temperature and the ferrite decarburization depth.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) // C21D 8/06 C21D 8/06 A (72) Inventor Takayuki Sakakibara Kamishiota, Narumi-cho, Midori-ku, Nagoya No. 68 Chuo Joujo Co., Ltd. (72) Inventor Masami Wakita 68 Kamishiota, Narumi-cho, Midori-ku, Nagoya AA11 AA12 AA16 AA23 AA24 AA32 BA02 CH04 CH05 CH06 4K043 AA02 AB04 AB05 AB10 AB11 AB15 AB20 AB22 AB23 AB25 AB26 AB28 AB30 CA04 DA01 FA03

Claims (13)

[Claims] 1. A weight ratio of C: 0.35 to 0.55.
%, Si: 1.8 to 3.0%, Mn: 0.5 to 1.5
%, Ni: 0.5 to 3.0%, Cr: 0.1 to 1.5%
An oil tempered wire for a cold-formed coil spring, which is made of a steel containing iron and is heat-treated by high-frequency induction heating. 2. The method according to claim 1, wherein the material is hot-rolled into a wire having a predetermined diameter, drawn by cold-working at a predetermined surface-reduction rate, and then heat-treated by high-frequency induction heating. 1. An oil tempered wire for a cold-formed coil spring according to 1. 3. The oil tempered wire for a cold-formed coil spring according to claim 1, wherein the ferrite fraction in the steel structure before heat treatment by high-frequency induction heating is 50% or less. 4. The maximum heating temperature of the high frequency induction heating is 1
The oil tempered wire for cold forming coil springs according to any one of claims 1 to 3, wherein the temperature is 020 ° C or lower. 5. The maximum heating temperature of the high frequency induction heating is 9
The oil tempered wire for a cold-formed coil spring according to any one of claims 1 to 4, which has a temperature of 50 ° C or less. 6. The maximum heating temperature of the high frequency induction heating is 9
It is 40 degreeC or more, The oil tempered wire for cold forming coil springs in any one of Claims 1-4 characterized by the above-mentioned. 7. The maximum heating temperature of the high frequency induction heating is 9
The oil tempered wire for a cold-formed coil spring according to any one of claims 1 to 6, which has a temperature of 00 ° 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 of 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 of the high frequency induction heating is 20 seconds or less. 10. The oil tempered wire for cold forming coil spring according to claim 1, wherein the grain size number after heat treatment is 9 or more. 11. The tensile strength after heat treatment is 1830-19.
The oil tempered wire for cold forming coil spring according to any one of claims 1 to 10, wherein the oil tempered wire is 80 MPa. 12. The material further comprises N: 0.01-0.
0: 25%, V: 0.05-0.5%, P: 0.
The oil tempered wire for a cold-formed coil spring according to any one of claims 1 to 11, wherein the oil tempered wire is 01% or less and S: 0.01% or less. 13. A cold-formed coil spring manufactured from the oil-tempered wire according to any one of claims 1 to 12.