JP2001049337A - Production of high strength spring excellent in fatigue strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize the increase of the stress of a spring by executing tempering at a specified temp. after quenching and executing shot peening in the subsequent stage in such a manner that the maximum value of compressive residual stress in the surface layer part is controlled to the specified valve or above. SOLUTION: Relating to the steel to be used, the average of cross-sectional hardness (core part hardness) other than that of a hardened region by shot peening is >=53.5 HRC. As to this steel, the temp. in tempering executed successively to quenching is controlled to <=340 deg.C. By controlling the temp. in the tempering to the relatively low one in this way, movable dislocations caused at the time of the quenching are largely left even after the tempering, which relax the concentration of strains on the circumferences of nonmetallic inclusions to suppress the generation of fatigue cracking. The tempering temp. at this time is preferably controlled to <=260 deg.C, by which the effect is moreover improved. Then, the subsequent shot peening is executed in such a manner that the maximum value of the compressive residual stress in the surface layer part is controlled to >=80 kgf/mm2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high-strength spring such as a valve spring or a suspension spring of an internal combustion engine. A useful method for obtaining a high-strength spring excellent in fatigue strength by achieving high stress in a spring whose average is used with HRC of 53.5 or more and whose fatigue breakage due to inclusions determines the spring life. It is about.

[0002]

2. Description of the Related Art In many high-strength springs typified by valve springs and suspension springs of internal combustion engines, further higher stress is desired from the viewpoint of weight reduction. Important factors in increasing the stress of a spring include fatigue life and sag resistance, and it is generally known that any of these can be improved by increasing the material hardness. Further, it is known that the fatigue life can be improved by applying a compressive residual stress to the surface layer by performing shot peening, and is widely used industrially.

However, in a spring in which both the material strength and the surface residual compressive stress are increased, fatigue fracture starting from non-metallic inclusions inside (excluding the hardened region of the surface layer) becomes remarkable, and further. Even if the hardness is increased or the compressive residual stress is increased, the fatigue strength is not improved or is rather lowered.

[0004] Such a phenomenon will be described with reference to the drawings. The present inventors conducted a rotational bending fatigue test using steel materials tempered to various hardnesses and investigated fatigue limit stress. The result is shown in FIG. That is, FIG. 1 is a graph showing the effect of the presence or absence of shot peening on the fatigue strength. In this figure, the hardness of the steel material not subjected to shot peening means the average of the entire cross-sectional hardness, and the hardness of the steel material subjected to shot peening means the cross-sectional hardness excluding the hardened region by shot peening. (Hereinafter sometimes referred to as “core hardness”).

[0005] As is apparent from the results, in the sample not subjected to shot peening, the surface starting point was destroyed in all regions of HRC 50 to 60, and the fatigue limit stress (10 ⁽⁶ hour strength) increases linearly, but in the sample subjected to shot peening, breakage due to inclusions becomes remarkable near HRC 53.5 or more, and the fatigue limit stress increases in accordance with the increase in hardness. You can see that it is no longer visible.

[0006] For these reasons, various measures have been studied in the steelmaking stage to reduce nonmetallic inclusions in steel, but it is difficult to completely eliminate them by the usual steelmaking method. Actually hinders further increase in stress in the high-strength spring.

[0007]

DISCLOSURE OF THE INVENTION The present invention has been made under such a circumstance, and it is an object of the present invention to use an HRC having an average cross-sectional hardness of 53.5 or more, especially excluding a work hardened area. It is an object of the present invention to provide a useful method for realizing a high-stress spring having excellent fatigue strength by realizing a high stress of the spring such that fatigue breakage due to inclusions determines the spring life.

[0008]

According to the method of the present invention which can achieve the above object, C: 0.36 to 0.48%, Si: 3%
Or less (including 0%), Mn: 2% or less (including 0%),
Cr: 3.0 or less (including 0%), Ni: 2% or less (0
%, And the average of the cross-sectional hardness excluding the hardened region by shot peening is HRC 53.5.
In manufacturing the tempered spring described above, the tempering temperature performed after the quenching treatment is set to 340 ° C. or lower, and the compressive residual stress in the surface layer in the process after the tempering treatment is 80 kgf / mm at maximum. ^The point is that the shot peening process is performed so as to be ² or more.

In the method of the present invention, the tempering temperature is preferably 260 ° C. or lower, and the Si content of the steel used is preferably 1.4 to 2.4%.

In the method of the present invention, (a) Ti: 0.03 to 0.15%, (b) C
u: 0.18 to 0.5%, (c) V: 0.05 to 0.2
5% and / or Nb: 0.01 to 0.1%, (d)
It is also useful to use a steel material containing B: 0.0005 to 0.01%.

[0011]

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for improving the fatigue life of non-metallic inclusion starting points in a high-strength spring.
It has the function of suppressing the occurrence of cracks from non-metallic inclusions, and as a result can withstand higher repetitive stresses than conventional springs having the same hardness.

In order to manufacture a spring exhibiting the above-described characteristics, the tempering temperature performed after quenching is set to 3
It needs to be 40 ° C. or lower. Thus, by setting the tempering temperature relatively low, it was possible to suppress the occurrence of fatigue cracks from non-metallic inclusions. The reason why such an effect is exhibited by setting the firing temperature to a relatively low temperature as described above could be considered as follows.

That is, by performing the treatment at a relatively low tempering temperature, a large amount of mobile dislocations generated during the quenching remain even after the tempering, which alleviates the concentration of strain around the nonmetallic inclusions, thereby causing fatigue cracking. It is presumed to be suppressed. The tempering temperature at this time is preferably set to 260 ° C. or lower, and by setting such low temperature conditions, the above-described functions and effects are further achieved. However, if the tempering temperature is too low, the strain generated during quenching is insufficiently removed and the toughness becomes too low.
The temperature is preferably set to 20 ° C. or higher.

In the present invention, the average of the cross-sectional hardness (core hardness) excluding the hardened region by shot peening is H
As shown in FIG. 1, it is intended to target steel materials having an RC hardness of 53.5 or more. This is because there is no significant difference in the service life between the two cases, and the effect of the method of the present invention is reduced.

The most characteristic of the present invention is to suppress the occurrence of fatigue cracks in steel by appropriately defining the tempering temperature as described above. It is necessary to appropriately adjust the chemical components such as C in (spring steel). In order to obtain a steel material having an average core hardness of 53.5 or more, it is necessary to appropriately adjust the chemical composition. The reasons for limiting the range of the chemical component composition defined from such a viewpoint are as follows.

C: 0.36 to 0.48% C is an effective and economical element for increasing the strength of the steel material. However, if the C content exceeds 0.48% and becomes excessive, distortion generated during quenching is made. In addition, since low-temperature tempering is performed, such distortion cannot be sufficiently removed during tempering, which adversely affects fatigue life. Also, C
When the content is less than 0.36%, it is difficult to obtain high hardness (an average of core hardness is HRC 53.5 or more), and a high-stress spring intended in the present invention cannot be obtained. still,
A preferred lower limit of the C content is 0.38%, and a preferred upper limit is 0.45%.

Si: 3% or less (including 0%) In the present invention, since a steel material having a relatively low C content is used, there is a surface on which high hardness is difficult to be obtained. It is effective to add Si which has a solid solution strengthening action and high tempering softening resistance. These effects increase as their content increases,
The content is preferably 1.4% or more, more preferably 1.7% or more. However, S
If the i content is excessive, an abnormal structure such as grain boundary oxidation or decarburization is generated, and fatigue at the surface origin is induced. Note that a preferable upper limit of the Si content is about 2.4%.

Mn: 2% or less (including 0%), Cr: 3
% Or less (including 0%) Mn and Cr are elements that increase the hardenability of the steel and help increase the hardness by solid solution strengthening. However, when it is excessive, the Ms point is lowered to obtain a sufficient hardened structure. Therefore, it is necessary to reduce Mn to 2% or less and Cr to 3% or less. In order to exhibit the above effects, it is preferable that Mn is contained at 0.1% or more and Cr is contained at 0.5% or more.

Ni: 2% or less (including 0%) Ni is an element that enhances the toughness particularly in a low temperature range and is also an element effective in improving defect sensitivity.
As with n and Cr, a decrease in the Ms point is detrimental to obtaining a sufficient quenched structure. From such a viewpoint, the Ni content needs to be 2% or less. Further, in order to exhibit the above effects, it is preferable that Ni is contained at 0.3% or more.

The basic chemical composition of the steel material used in the present invention is as described above, and the balance is composed of Fe and unavoidable impurities. If necessary, the steel material may further comprise (a ) Ti: 0.03-0.
15%, (b) Cu: 0.18 to 0.5%, (c) V:
0.05 to 0.25% and / or Nb: 0.01 to
It is also useful to use a steel material containing 0.1%, (d) B: 0.0005 to 0.01%, etc., whereby the characteristics as a high-strength spring can be further improved. The reasons for limiting the range of these elements are as follows. It should be noted that the steel material used in the present invention may include a trace component that does not affect the properties as a spring product, and such a spring product is also included in the technical scope of the present invention.

Ti: 0.03 to 0.15% When a spring is used in an environment in which hydrogen particularly penetrates into steel, hydrogen accumulates at stress concentration portions around inclusions, and There is a risk of promoting cracking. Ti has been found to have a hydrogen trapping action in the form of precipitates, and the addition of Ti is particularly effective in improving the fatigue life in a hydrogen environment.
In order to exhibit such an effect, the content of Ti is set to 0.1.
The content is preferably not less than 03%. However, if the Ti content is excessive, coarse inclusions are formed and the fatigue life is rather reduced, so that the content should be set to 0.15% or less.

Cu: 0.18 to 0.5% In a corrosive environment, hydrogen is generated by a corrosion reaction.
It is effective to include Cu that suppresses the corrosion reaction by densifying rust. To achieve these effects, C
Preferably, u is contained at 0.18% or more, more preferably 0.22% or more. However, the effect is saturated when the Cu content is excessive, so that the content is preferably 0.5% or less.

V: 0.05-0.25% and / or
Nb: 0.01 to 0.1% V and Nb are effective in reducing the crystal grain size, increasing the proof stress ratio, and improving the sag resistance characteristics. In order to effectively exhibit such effects, it is preferable to contain V in an amount of 0.05% or more and Nb in an amount of 0.01% or more. However, when the content of these elements is excessive, coarse carbonitrides are formed during quenching and heating, and the fatigue life is deteriorated. Therefore, the content of V is set to 0.25% or less and the content of Nb is set to 0.1% or less. Should.

B: 0.0005 to 0.01% B is an element effective for improving hardenability. To exert such an effect, it is preferable to contain 0.0005% or more. The effect is saturated even if added, so the content should be 0.01% or less.

As described above, the characteristics required for a high-strength spring include not only fatigue life but also sag resistance.
In the method of the present invention, since the tempering temperature is relatively low, the sag may occur more easily than a conventional spring having the same hardness. As a countermeasure against such a tendency, in addition to the addition of V and Nb as described above, conventional setting is effective, but more preferably hot setting is performed at a high temperature. However, if hot setting is performed at an excessively high temperature, the effect of lowering the tempering temperature disappears. Therefore, the heating temperature at the time of hot setting is preferably 340 ° C. or lower, more preferably 260 ° C. or lower. Should.

In the method of the present invention, shot peening is performed so that the maximum value of the compressive residual stress in the surface layer portion is 80 kgf / mm ² or more. It is defined from the viewpoint of giving. That is, the maximum value of the compressive residual stress is 80
If it is less than kgf / mm ² , fatigue cut off from the surface origin is caused, and the effect intended by the present invention of increasing the resistance to internal inclusions cannot be obtained. Further, the “surface portion” means a hardened region subjected to shot peening,
This is a portion having a depth of approximately 100 μm from the outermost surface.

In carrying out the method of the present invention, the spring forming method (spring forming time) is not particularly limited, and (1) a hot winding method in which a polishing rod is heated and hot-rolled and quenched; 2) Various methods such as cold winding method of cold bending an oil-tempered wire, and (3) piece hardening method of cold winding a wire before tempering and then tempering the whole. Is applicable. However,
When the cold winding method is applied, the strain relief annealing after the cold winding is preferably performed at 340 ° C. or less, more preferably at 260 ° C. or less, from the above viewpoint.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention, and any design change characterized by the above and following points is not limited to this example. It is within the technical scope of the invention.

[0029]

EXAMPLES Various steel materials A to A having the chemical composition shown in Table 1 below.
H was melted in a vacuum melting furnace. The ingot obtained at this time contains nonmetallic inclusions such as ordinary commercial spring steel. These were hot forged into billets having a cross section of 155 × 155 (mm) and then hot rolled to have a diameter of 1 mm.
A 3.0 mm wire was produced. These wire rods were processed into a rough shape of a rotary bending test piece, further quenched in oil after holding at 920 ° C. for 10 minutes, and tempered at each temperature shown in Table 1 below for 60 minutes.

After finishing these, the diameter of the parallel portion is 8
mm smooth Ono-type rotating bending fatigue test piece. afterwards,
In order to apply compressive residual stress to all samples,
Two-stage shot peening was performed. The shot grains used at this time are steel balls equivalent to HRC60, and the first stage has a diameter of:
The second stage used had a diameter of 0.3 mm.

The distribution of the residual stress in the axial direction in the depth direction was measured for each sample by the X-ray diffraction method. As a result, 0-100 of the shot-peened sample
The maximum value of the compressive residual stress at a depth of μm is 90 to 100 k.
gf / mm ² , and the maximum value was not lower than 80 kgf / mm ² . Also, for each sample, it was evaluated fatigue strength at rupture life 10 ⁶ times the amplitude stress when the (number of cycles) (10 ⁶ times time-intensity).

The results of the obtained fatigue strength are shown in Table 1 below together with the hardness (core hardness) inside the surface layer. FIG. 2 (relationship between fatigue strength and core hardness) shows the result of organizing the fatigue strength by core hardness. The cycle number is 1
0 ⁶ times around or more samples broken at the number of cycles was inclusions broken STARTING FROM from inside all.

[0033]

[Table 1]

From these results, the following can be considered.
The hardness of the core part satisfies the requirements stipulated in the present invention with respect to the HRC 53.5, and there is a large difference in the service life between those not satisfying the requirements. It shows that the fatigue strength is higher in the high hardness region than in the comparative example, and the difference is more remarkable as the hardness is higher.

On the other hand, no. 10 to 22 are defined in the present invention (hardness of core, tempering temperature,
C content etc.), and excellent fatigue strength is not obtained. First, the wire rod No. 10 satisfies the requirements specified in the present invention except that the core hardness is low, but the core hardness is also low and the component (C content) is also in the range specified in the present invention. No.
18, 21 and 22, it is understood that the effect of low-temperature tempering and component adjustment is small in the low hardness range. In addition, No. In the case of samples 11 to 13, since the tempering temperature is higher than the range specified in the present invention, it can be seen that the fatigue strength is low while the other requirements satisfy the range specified in the present invention. This is because the same steel material was tempered in the temperature range specified in the present invention.
o. It is clear when compared with the results of 2-9. No. 1
In the samples of Nos. 4 to 21, since the C content is larger than the amount specified in the present invention, the fatigue life is deteriorated. In addition, No. 2
Sample No. 2 had a core hardness not exceeding HRC 53.5 or more even after tempering at 200 ° C. due to low C content.

[0036]

The present invention is constructed as described above. In particular, the average of the cross-sectional hardness excluding the hardened region is HRC 53.5 or more, and the fatigue breakage due to inclusions determines the spring life. Thus, a useful method for obtaining a high-strength spring having excellent fatigue strength by realizing a high stress of the spring was realized.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the presence or absence of shot peening on fatigue strength.

FIG. 2 is a graph showing the relationship between fatigue strength and core hardness.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K032 AA01 AA02 AA05 AA11 AA12 AA14 AA16 AA22 AA23 AA24 AA32 AA35 AA36 BA02 CF01 CM01

Claims (7)

[Claims] 1. C: 0.36 to 0.48% (meaning by mass%, the same applies hereinafter), Si: 3% or less (including 0%), M
n: 2% or less (including 0%), Cr: 3.0 or less (0%
Quenching when producing a tempered spring having an average cross-sectional hardness excluding a hardened region by shot peening of HRC 53.5 or more using steel materials each containing Ni: 2% or less (including 0%). The temperature of the tempering performed subsequent to the treatment is set to 340 ° C. or lower, and the compressive residual stress in the surface layer in the process after the tempering treatment is 8
A method for producing a high-strength spring excellent in fatigue strength, wherein a shot peening treatment is performed so as to be 0 kgf / mm ² or more. 2. The method according to claim 1, wherein the tempering temperature is 260 ° C. or lower. 3. The method according to claim 1, wherein a steel material having a Si content of 1.4 to 2.4% is used. 4. As another component, Ti: 0.03 to
The steel material containing 0.15% is used.
4. The production method according to any one of claims 1 to 3. 5. The composition according to claim 1, further comprising Cu: 0.18 to
A steel material containing 0.5% is used.
4. The production method according to any one of 4. 6. The composition according to claim 5, wherein V: 0.05 to
The method according to any one of claims 1 to 5, wherein a steel material containing 0.25% and / or Nb: 0.01 to 0.1% is used. 7. As another component, B: 0.0005
The production method according to any one of claims 1 to 6, wherein a steel material containing 0.1% to 0.01% is used.