JP2003293085A - Hard drawn spring having excellent fatigue strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hard drawn spring which exhibits fatigue strength equal to or greater than that of a spring obtained by using an oil tempered steel wire. <P>SOLUTION: The hard drawn spring has a composition containing 0.5 to 0.7% C, 1.0 to 2.0% Si, 0.5 to 1.5% Mn and 0.5 to 1.5% Cr, and the balance Fe with inevitable impurities. The difference between the surface residual stress at the inside of the spring (R<SB>+</SB>) and the surface residual stress at the outside of the spring (R<SB>-</SB>), i.e., [(R<SB>+</SB>)-(R<SB>-</SB>)], is ≤500 MPa. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spring (working spring) used after being subjected to a strong cold drawing process, and particularly exhibits excellent fatigue strength without being quenched and tempered as drawn. The present invention relates to a hard tension spring.

[0002]

2. Description of the Related Art With the weight reduction and higher output of automobiles and the like, higher stress is also directed toward valve springs and suspension springs used in engines and suspensions. Further, as the load stress on the spring increases, a spring having excellent fatigue strength is required.

In recent years, most of valve springs, suspension springs, etc. are generally manufactured by quenching and tempering steel wire called oil temper wire, which is spring-wound at room temperature.

Since the oil tempered wire as described above has a tempered martensite structure, it is convenient for obtaining high strength and has the advantage of being excellent in fatigue strength and settling resistance, but it is hardened and tempered. There is a drawback that the heat treatment of requires large-scale equipment and treatment cost.

On the other hand, for some springs designed to have a relatively low load stress, a wire rod ("hard drawn wire") having a strength increased by wire-drawing a carbon steel of (ferrite + pearlite) structure or pearlite structure It is called)) and is spring-rolled at room temperature. As such a spring,
According to the JIS standard, among piano wires (JIS G3522), especially for "valve springs or springs equivalent thereto",
"Piano wire SWP-V type" is defined.

The spring manufactured by the above-described hard-drawing wire (hereinafter, this spring is referred to as "hard-pulling spring") is
Since there is no need for heat treatment, there is an advantage of low cost. However, a spring manufactured with such a hard-drawn wire has a drawback that its fatigue strength is low, and a high-stress spring which has been increasingly demanded in recent years cannot be realized.

Various techniques for increasing the stress in a hard tension spring, which has the advantage that it can be manufactured at low cost, have been studied, and as such a technique, for example, Japanese Patent Laid-Open No. 11-1
99981 proposes a method of obtaining a specific cementite shape by devising a wire drawing method for eutectoid to hypereutectoid steel pearlite as "piano wire having characteristics equivalent to oil tempered wire". ing. However, even in such a method, an increase in manufacturing cost due to complication of the process such as switching the drawing direction is inevitable.

[0008]

SUMMARY OF THE INVENTION The present invention has been made under these circumstances, and an object thereof is to provide a hard-pull spring which exhibits fatigue strength equal to or higher than that of a spring using an oil temper wire. It is in.

[0009]

Means for Solving the Problems The hard tension spring of the present invention capable of achieving the above object is C: 0.5 to 0.7%, S.
i: 1.0 to 2.0%, Mn: 0.5 to 1.5%, C
r: 0.5 to 1.5% each, each of which is a spring-formed steel wire with the balance being Fe and unavoidable impurities,
A surface residual stress in the spring interior (R _+), the surface residual stress in the spring outside _- the difference _{(R) [(R +)} - (R -)]
Has a gist of 500 MPa or less. In the steel wire used in the present invention, (a) Ni:
0.05-0.5%, (b) Mo: 0.3% or less (0%
It is also effective to include such).

Further, the spring of the present invention preferably satisfies the following requirements (1) to (5). (1) The surface is shot peened two or more times. (2) The difference [(R _{s +} ) − (R _s− )] between the surface residual stress (R _{s +} ) inside the spring and the surface residual stress (R _s− ) outside the spring after the above shot peening is 300 MPa or less. . (3) The surface roughness is 10 μm or less at the maximum height Ry. (4) The surface is nitrided. (5) The ratio (D / d) of the spring diameter D and the wire diameter d is 9.0 or less.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied from various angles in order to realize a hard tension spring that can achieve the above object. As a result, while appropriately adjusting the chemical composition, the difference in residual stress between the spring inner side and the spring outer side (after coiling) (hereinafter sometimes simply referred to as "residual stress difference") is 500 MPa or less. It was found that excellent fatigue strength can be achieved by controlling, and the present invention has been completed.

In the hard tension spring of the present invention, it is necessary to properly adjust the chemical composition of the steel wire (drawn material) to be used, but the reason for limiting the range is as follows.

C: 0.5 to 0.7% C is an element useful for increasing the tensile strength of a wire drawing material and ensuring fatigue strength and sag resistance, and 0.8 for ordinary piano wire. %, But in the high-strength wire drawing material intended in the present invention, the content of C is 0.
If it exceeds 7%, the susceptibility to defects is increased, and cracks from surface flaws and inclusions occur and the fatigue life deteriorates.
It was set to 0.7% or less. However, if the C content becomes too small, not only the tensile strength required for a high stress spring cannot be secured, but also the fatigue strength and sag resistance will deteriorate, so the C content must be 0.5% or more. There is.
The preferred lower limit of the C content is 0.63%, and the preferred upper limit is 0.68%.

Si: 1.0 to 2.0% Si is an element necessary as a deoxidizing agent during steel making, and is solid-dissolved in ferrite to increase temper softening resistance and improve sag resistance. It is an important element above. In order to exert such effects, it is necessary to contain 1.0% or more. However, if the Si content exceeds 2.0% and becomes excessive, not only the toughness and ductility deteriorate, but also the surface decarburization, flaws and the like increase, and the fatigue resistance deteriorates. Incidentally, S
The preferable lower limit of the i content is about 1.2%, and the preferable upper limit is about 1.6%.

Mn: 0.5 to 1.5% Mn is an element effective for deoxidation during steel making, and is an element that makes the pearlite structure dense and orderly and contributes to improvement of fatigue properties. In order to exert such effects, Mn
Must be contained at least 0.5%. But,
If it is contained in excess, a supercooled structure such as bainite is likely to be formed during hot rolling or patenting treatment, and wire drawability is significantly deteriorated. Therefore, the content should be 1.5% or less.
The preferable lower limit of the Mn content is about 0.6%,
A preferable upper limit is about 1.0%.

Cr: 0.5 to 1.5% Cr is an element useful for reducing the pearlite lamella spacing, increasing the strength after rolling or heat treatment, and improving the sag resistance. In order to exert such effects, the Cr content needs to be 0.5% or more.
However, if the Cr content is excessive, a bainite structure is likely to be generated during patenting, coarse carbides are likely to be precipitated, and fatigue strength and sag resistance are deteriorated. There is a need to. Incidentally, C
The preferable lower limit of the r content is about 0.7%, and the preferable upper limit is about 1.0%.

The basic chemical composition of the steel wire used in the present invention is as described above, and the balance consists essentially of Fe, but it is also effective to include a predetermined amount of Ni or Mo if necessary. Is. The reason for limiting the range when these are contained is as follows.

Ni: 0.05 to 0.5% Ni is an element effective for improving hardenability and toughness, suppressing breakage troubles during spring processing, and improving fatigue strength. In order to exert such effects, the Ni content is preferably 0.05% or more. However, if it is contained excessively, a bainite structure is generated during hot rolling or patenting, and wire drawability is significantly deteriorated. Therefore, its upper limit is preferably 0.5%.

Mo: 0.3% or less (not including 0%) Mo is an element effective for improving the sag resistance by ensuring the hardenability and improving the softening resistance. Such an effect increases as the content thereof increases, but if the content is excessive, the patenting treatment time becomes too long and the ductility deteriorates, so the upper limit is preferably set to 0.3%.

The steel wire used in the present invention may contain, in addition to the above-mentioned various components, trace amounts of components that do not impair the characteristics of the hard spring, and such steel wires are also included in the scope of the present invention. Is. The trace components include impurities, especially inevitable impurities such as P, S, As, Sb and Sn.

In the hard tension spring of the present invention, it is also an important requirement to control the residual stress difference to 500 MPa or less as described above, but the reason for defining this requirement is as follows. Since the residual stress imparted by the spring forming (coiling) is balanced between the inside and the outside of the spring, the larger the residual stress difference after coiling, the higher the tensile residual stress inside. When the tensile residual stress (tensile residual stress) is high, the occurrence and propagation of fatigue cracks are promoted, and the fatigue strength is reduced. In addition, the compression residue applied by shot peening becomes small.

Based on the above findings, the present inventor investigated the relationship between the residual stress difference [(R ₊ ) − (R ₋ )] inside and outside the spring and the fatigue strength, and found that the difference was 500 MPa or less. It was found that the fatigue strength was remarkably improved by the above.

By the way, when a spring is molded, residual stress in the tensile direction (tensile residual stress) is generated inside the spring, and tensile residual stress is generated outside the spring depending on the manufacturing conditions. Residual stress (compressive residual stress) may occur. Therefore, when measuring the residual stress difference in the present invention, it is necessary to consider these points as well. That is, when the surface residual stress on both sides is tensile residual stress, the difference may be simply measured,
When the residual stress (R ₋ ) on the outer side of the spring is a compressive residual stress, the residual stress has a value minus the residual stress. For example, the tensile residual stress inside the spring is 150MP
a, when the outer compressive residual stress is 50 MPa,
The residual stress difference [(R ₊ ) − (R ₋ )] is (150)
-(-50) = 200 MPa.

As described above, according to the present invention, by setting the residual stress difference between the spring inner side and the spring outer side after coiling to 500 MPa or less, the fatigue strength of the hard-pull spring can be improved. The reason why the residual stress difference is defined as an index is as follows. The stress applied to the spring (shear stress) is not the same on the inside and the outside, and the stress on the inside of the spring is also high on the outside. For example,
If the ratio of the spring diameter D and the wire diameter d (D / d: hereinafter referred to as “spring index”) is 2.0 to 9.0, the correction coefficient A _{1 of the} whirl shown by the following equation (1) is The stress becomes 1.16 to 2.06, and a stress that is 1.16 to 2.06 times as high as that is applied (for example, “spring”, edited by Spring Technology Research Group, published by Maruzen). A ₁ = [(4c-1) / (4c-4)] + [0.615 / c] (1) where c: spring index (D / d) On the other hand, the spring correction coefficient A ₂ for the outside of the spring is According to this equation, the stress applied to the outside of the spring is 0.44 when the spring index is 2.0.
It triples. A ₂ = [(4c + 1) / (4c + 4)] + [0.615 / c] (2) where c: spring index (D / d)

As described above, if a large shear stress is applied to the inside of the spring and the tensile residual stress is high, the spring characteristics are further deteriorated. From this point of view, it suffices to specify the residual stress inside the spring, but the surface has tensile residual stress even in the drawn state, and its value depends on the drawing conditions and the material. Since it changes, the tensile residual stress of the surface changes due to the addition effect even after the spring coiling, and it is difficult to define it as the residual stress. Therefore, in the present invention, the difference in residual stress between the inner side of the spring and the outer side of the spring is defined as an index of fatigue strength.

As a condition for making the difference in residual stress 500 MPa or less, for example, the strain relief annealing temperature after coiling may be controlled to be 400 ° C. or more. In the conventional piano wire, when the strain relief annealing temperature is performed at 400 ° C. or higher, the strength is lowered, and the fatigue strength and the sagging resistance are lowered. Since a steel wire containing a large amount of Si that gives good results on the property is used as a material, even if strain relief annealing is performed at 400 ° C. or higher, there is almost no decrease in strength and the coiling strain can be removed.

The hard tension spring of the present invention can achieve excellent fatigue strength by defining the chemical composition and residual stress difference, but in order to exert such effects more effectively, It is effective to subject the surface to shot peening twice or more. Valve springs and high-stress springs equivalent thereto are usually used in the state where compressive residual stress is applied to the surface layer by shot peening. This shot peening is an effective means for projecting high hardness hard spheres (shot particles) onto the surface of the material to be treated at high speed to give residual stress for compression, suppressing the occurrence of surface cracks and improving fatigue strength. is there. In particular, it is effective to perform shot peening twice or more in a component used under high stress.

Shot peening as described above is effective in imparting compressive residual stress to the spring surface and suppressing the development of fatigue cracks. Since springs that perform shot peening are used with particularly high stress, higher compressive residual stress is required, and the above residual stress difference must be controlled more strictly. For that purpose, the residual stress difference material is preferably 300 MPa or less.

By the way, when the surface roughness of the spring is large, fatigue fracture is likely to occur starting from this, and the fatigue strength is reduced. Therefore, from the viewpoint of improving fatigue strength, the surface roughness Ry (maximum height:
It is preferable to set JIS B 0601) to 10 μm or less. For example, when high-strength shot peening is performed twice or more as described above, the surface may be deformed to increase the surface roughness. In particular, in the case of a material such as a hard-drawn wire, the ferrite in the weakest part may be deformed more and the surface roughness may be increased. The means for adjusting the surface roughness as described above is not limited,
This can be achieved, for example, by properly controlling the shot peening conditions.

Considering such control of the surface roughness Ry,
The preferable shot peening conditions are as follows.
Particle size in the first shot peening: 1.0
~ 0.3mm shot particles are used, particle speed: 30 ~ 100
m / sec, projection time: 20 to 200 minutes. The hardness of the shot particles used at this time is Vickers hardness (H
In v), it is preferably 500 or more.

Next, in the second and subsequent shot peening, shot grains smaller than those in the first shot are used. The size of the shot particles used at this time is preferably 1/10 or less of the particle size of the first shot. The time is about 10 to 200 minutes. By such second and subsequent shot peening, the surface roughness can be reduced, the compressive residual stress on the surface can be increased, and the fatigue strength can be further improved. still,
The inventor of the present invention has confirmed that with the hard-pull spring, the effect of shot peening from the second time onward is greater than that from the tempered / tempered oil tempered wire.

In the hard tension spring of the present invention, it is also effective to subject the surface thereof to nitriding treatment when it is expected to be used under particularly severe stress conditions. By performing such a nitriding treatment, the fatigue strength can be further improved. Regarding such a nitriding treatment, the treatment has been conventionally performed for the valve spring manufactured from the oil tempered wire, but not for the hard tension spring at all. This is because it was thought that the effect could not be expected even if nitriding was performed with the chemical composition of ordinary hard drawn wire, and the strain introduced during wire drawing during nitriding was released and the strength became extremely strong. The reason is that it was thought to decrease.

On the other hand, when the steel wire having the chemical composition defined in the present invention is hard-drawn and then subjected to the nitriding treatment,
The fatigue life of the spring will be further improved. The reason why these effects are exhibited was as follows. That is, in the steel wire used in the present invention, ferrite is added to S
Since the strength of the wire material depends on the strength of the ferrite itself by strengthening it with an alloying element such as i, Cr, etc., increasing the strength of the ferrite by nitriding leads to a direct improvement of the fatigue strength. Conceivable. still,
The hardness of the spring surface produced by the nitriding treatment is 0.0
Vickers hardness (HV) of 600 at 2mm depth
Or more, preferably 700 or more,
Depending on the required fatigue strength, it may be about Hv500-600.

The method for carrying out the above nitriding treatment is not particularly limited, and gas nitriding, liquid (salt bath) nitriding, ion nitriding and the like can be adopted. For example, preferable conditions for gas nitriding are as follows. On the street. That is, the nitriding treatment may be performed under the conditions of 350 to 470 ° C. for 1 to 6 hours in an atmosphere of 100% ammonia gas, or in an atmosphere mainly composed of ammonia gas and 50% or less of nitrogen gas and 10% or less of carbon dioxide gas. .

The effect of the present invention is that the spring index (D / d) is
Is more exerted when applied to a small diameter spring having a value of 9.0 or less. In a spring, the above-mentioned D / d indicates a spring index, but in a spring having such a ratio (D / d), the difference in stress between the inside and outside of the spring when a desired load response is obtained is large and the inside of the spring is large. High stress is applied. Even under such a high-stress use environment, the spring of the present invention can maintain its function. Further, the smaller the value of (D / d), the greater the effect. However, if it is less than 2.0, the effect of surface processing such as shot peening becomes difficult to obtain, so the lower limit thereof is 2.0. It is preferable.

The hard tension spring of the present invention achieves its purpose by coiling a steel wire having the above chemical composition to satisfy the above requirements. The steel wire to be used may be one that satisfies the requirements (a) and / or (b) below. By satisfying these requirements, the fatigue strength and sag resistance of the spring can be further improved, which is preferable. (A) The number of carbides having a size of 0.1 μm or more (circle equivalent diameter) present in the steel wire is 5 or less per 100 μm ^{2 of} the observation field (b) The melting point of the oxide-based inclusions present in the steel wire A (℃)
Satisfies the following expression (3): A ≦ 2 × (TS / 100) ² −1.2 × TS + 3200 (3)

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and any modification of the design of the present invention can be made without departing from the spirit of the preceding and the following. Are included in the technical scope of.

[0038]

EXAMPLES Example 1 Steels (A to J) having the chemical composition shown in Table 1 below were melted,
It hot-rolled and produced the wire material of diameter (wire diameter): 8.0 mm. After that, the steel wire having a wire diameter of 3.1 mm was subjected to skinning, patenting treatment and wire drawing treatment. In the patenting at this time, the austenitizing temperature was set to 910 ° C., and isothermal transformation was performed in a lead bath at 550 to 650 ° C. depending on each steel type.

[0039]

[Table 1]

After that, the wire drawn materials of the steel types A, B and C were spring-formed (spring index: 6.81), strain relief annealing (350, 380, 410 ° C. × 20 minutes), seat polishing and cold setting. I went.

Each spring thus obtained was subjected to a fatigue test under a load stress of 588 ± 441 MPa to measure the rupture life, and the residual stress inside the spring (R ₊ ) and the residual stress outside the spring (R ₋ ) were measured as X. The residual stress difference [(R ₊ ) − (R ₋ )] was obtained by measurement by line diffraction. In addition, the tensile strength of the drawn wire material (after drawing and after strain relief annealing) was measured, and its surface roughness Ry was also measured. The results are shown in Table 2 below together with the strain relief annealing temperature.

[0042]

[Table 2]

From these results, it is clear that excellent fatigue strength is achieved when the residual stress difference is 500 MPa or less (No. 3 to 5). On the other hand, if the residual stress difference exceeds 500 MPa (N
o. 1) and 2), the fatigue strength is remarkably deteriorated.

Example 2 Each drawn wire material obtained in the same manner as in Example 1 (steel types A to H)
For various types, springs with various spring indexes were formed, strain relief annealing (350, 380, 410 ° C x 20 minutes), spot polishing, two-step shot peening, low temperature annealing (230 ° C x 20 minutes) and cold setting were performed. It was At this time, with respect to the steel type C, after the spot polishing, the nitriding treatment was performed in an atmosphere of NH ₃ 80% + N ₂ 20% under the conditions of 420 ° C. × 2 hours, two-step shot peening, low temperature annealing (230 ° C. × 20 minutes). ) And cold-set (No. 11 in Table 3 below) were also prepared.

A fatigue test was performed on each of the obtained springs under a load stress of 588 ± 441 MPa to measure the fracture life, and the residual stress (R ₊ ) on the inner side of the spring after spring forming (before shot peening) and the outer side of the spring were measured. the residual stress (R _-), and shot peening the spring inside the residual stress after (Rs ₊₎ and spring outside the residual stress (R _s-)
Was measured by X-ray diffraction, and respective residual stress differences [(R ₊ )-(R ₋ )] and [(R _{s +} ) − (R _s −)] were obtained. Further, in the same manner as in Example 1, the tensile strength of the drawn wire material (after drawing and after strain relief annealing) was measured, and the surface roughness Ry thereof was also measured. The results are shown in Table 3 below together with the spring index and the strain relief annealing temperature.

[0046]

[Table 3]

From this result, the following can be considered. First, No. It is understood that those of Nos. 8 to 13 satisfy all the requirements specified in the present invention, and have excellent fatigue strength.

On the other hand, No. Nos. 6, 7, and 14 to 16 lack any of the requirements defined in the present invention, and any one of the characteristics is deteriorated.
That is, No. In Nos. 6 and 7, the difference in residual stress between the inside of the spring and the outside of the spring (after spring molding and after shot peening) is large, so the fatigue strength is significantly deteriorated.

No. No. 14 has a high C content, and thus has a high defect susceptibility.
Since the i content is low, sufficient strength cannot be obtained after strain relief annealing and the fatigue life is shortened.

No. No. 15 had a low C content and had a low strength after patenting.
Sufficient strength cannot be obtained after wire drawing and fatigue life is shortened.

No. No. 16 had a high Cr content, so bainite was generated during patenting and wire breakage occurred during wire drawing.

[0052]

The present invention is constructed as described above, and it is possible to realize a hard-pull spring that exhibits fatigue strength equal to or higher than that of a spring using an oil-tempered wire as drawn.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F16F 1/02 F16F 1/02 B 1/06 1/06 A (72) Inventor Nobuhiko Ibaraki Nada Ward, Kobe City Nadahama Higashimachi No. 2 Kobe Steel Works, Ltd. Kobe Steel Works (72) Inventor Nao Yoshihara Nadahama Higashicho, Nada-ku, Kobe City Kobe Steel Works Ltd. Kobe Steel Works (72) Inventor Shigeji Yoshida Umezu Nishiura-cho, Ukyo-ku, Kyoto 14th Suncole Co., Ltd. (72) Inventor Koji Harada 14th Umezu Nishiuramachi, Ukyo-ku, Kyoto City F-Term in Suncole Co., Ltd. (reference) 3J059 AB11 AD05 BA01 BC02 BC19 EA08 EA17

Claims (8)

[Claims] 1. C: 0.5 to 0.7% (meaning mass%,
The same shall apply hereinafter), Si: 1.0 to 2.0%, Mn: 0.5 to
A steel wire containing 1.5% and Cr: 0.5 to 1.5%, the balance of which is Fe and unavoidable impurities, is spring-formed. The surface residual stress (R ₊ ) inside the spring is
And a difference in surface residual stress (R ₋ ) on the outer side of the spring [(R ₊ ) − (R ₋ )] is 500 MPa or less, a hard-pull spring excellent in fatigue strength. 2. The steel wire further comprises Ni: 0.05-
The steel wire for a hard spring according to claim 1, which contains 0.5%. 3. The steel wire for a hard spring according to claim 1, wherein the steel wire further contains Mo: 0.3% or less (not including 0%). 4. The hard tension spring according to claim 1, wherein the surface has been subjected to shot peening twice or more. 5. The difference [(R _{s +} ) − (R _s− )] between the surface residual stress (R _{s +} ) inside the spring and the surface residual stress (R _s− ) outside the spring after shot peening is 30.
The hard tension spring according to claim 4, which has a pressure of 0 MPa or less. 6. The hard tension spring according to claim 1, wherein the surface roughness has a maximum height Ry of 10 μm or less. 7. The hardened spring according to claim 1, wherein the surface thereof is subjected to a nitriding treatment. 8. The hard tension spring according to claim 1, wherein a ratio (D / d) of the spring diameter D and the wire diameter d is 9.0 or less.