JPH11246914A - High strength valve spring and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a valve spring having fatigue strength more improved than that of the conventional one by selecting an optimum stock and properly combining subsequent spring manufacturing stages. SOLUTION: A steel which has a composition containing, by weight ratio, 0.5-0.8% C, 1.2-2.5% Si, 0.4-0.8% Mn, 0.7-1.0% Cr <=0.005% Al and <=0.005% Ti, and in which the maximum nonmetallic inclusion has 15 μm, is used as a stock. At the time of oil tempering treatment, hardening temperature is made to 950 to 1,100 deg.C. After coiling, shot peening is applied at least twice by using shots of >=Hv 720 to regulate compressive residual stress to >=85 kgf/mm<2> .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength valve spring mainly used as a valve spring for an internal combustion engine of an automobile and having excellent fatigue resistance, sag resistance and delayed fracture resistance, and a method of manufacturing the same.

[0002]

2. Description of the Related Art As a wire rod for a valve spring of an internal combustion engine, JIS
Oil-tempered wire for valve spring (SWO-V: JIS G3561)
Chrome vanadium steel oil-tempered wire for valve springs (SWOCV-
V: JISG3565) and silicone chrome steel oil-tempered wire for valve springs (SWOSC-V: JIS G3566), etc., but conventionally, SWOSC- which has excellent fatigue resistance and sag resistance
V has been mainly used.

On the other hand, from the viewpoint of environmental protection and resource protection,
Efforts to purify the exhaust and improve fuel efficiency are constantly being demanded of automobiles, but the major contribution to this is the weight reduction of the vehicle body, and the components that make up the vehicle body are also being reduced in weight. Effort is continuing.

[0004] For this reason, various proposals have been made to further increase the fatigue strength and reduce the sag of a valve spring wire. For example, in Japanese Patent Application Laid-Open No. 8-176730, C: 0.5 to 0.8% by weight, Si: 1.2 to
2.5%, Mn: 0.4-0.8%, Cr: 0.7-
Quenching heating temperature to steel containing 1.0%, the balance being Fe and unavoidable impurities, the Al content being unavoidable impurities being 0.005% or less, and the Ti content being 0.005% or less. An oil-tempered wire quenched and tempered at a temperature of 950 ° C. or more and 1100 ° C. or less has been proposed for a high-strength valve spring (claim 1). In this publication, furthermore, an oil-tempered wire containing V: 0.05 to 0.15% in the material steel (Claim 2), and in addition, Mo: 0.05 to 0.1%
0.5%, W: 0.05 to 0.15%, Nb: 0.05
There has been proposed an oil-tempered wire containing at least one of 0.15% to 0.15% (claim 3). Also,
Japanese Patent Application Laid-Open No. 9-71843 by the same applicant discloses a high toughness spring oil temper using steel containing the same component amount and having a volume ratio of residual γ (austenite) after quenching and tempering of 1 to 5%. The line is proposed (claim 1,
2). According to this publication, after quenching and tempering, the in-structure density of carbide having a particle size of 0.05 μm or more is
An oil-tempered wire having a thickness of 5 or less / μm ² or less on a tissue observation photograph has also been proposed (claims 3 and 4), and a combination thereof may be used (claims 5 and 6). As a specific manufacturing method, in the case of claims 1, 2, 5, and 6, the tempering in the quenching and tempering step is performed at a heating rate of 150.
Heating to 450 to 600 ° C. at a rate of not less than 15 ° C./sec, and the time from the start of heating to the start of cooling using a refrigerant such as water is within 15 seconds. T (° C.) = 500 + 750 · C (carbon content%) + 50 at quenching heating at a heating rate of 150 ° C./sec or more and 1100 ° C. or less.
It discloses that heating is performed to a temperature not lower than a temperature determined by 0 · V (amount of vanadium) and the time from the start of heating to the start of cooling with water or oil is within 15 seconds.

[0005]

However, most of the conventionally proposed materials are steel as a raw material, or at most, up to the above-mentioned wire (oil-tempered wire) stage, and manufacture a valve spring as a final product. Until this stage, no measures were specified to achieve high fatigue strength and sag resistance. However, no matter how good the material is, if the subsequent manufacturing process is inappropriate, not only the performance of the material will not be sufficiently exhibited, but also the manufacture of the valve spring itself will be difficult, and in some cases, fatigue strength and There is a risk of deteriorating sag resistance.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and it is an object of the present invention to select an optimum material and to perform a subsequent spring manufacturing process in an appropriate manner according to the material. Accordingly, it is an object of the present invention to provide a valve spring having a fatigue strength equal to or higher than that of a conventional one, and in particular, having reduced sag. Specifically, a valve spring having high set resistance such that the residual shear strain γ when kept at 120 ° C. for 48 hours at the maximum shear stress τmax of the surface = 90 kgf / mm ² is 5 × 10 ⁻⁴ or less. Is provided. Also,
In the present invention, its characteristics are also improved in consideration of resistance to delayed fracture.

[0007]

Means for Solving the Problems A high-strength valve spring according to the present invention, which has been made to solve the above-mentioned problems, comprises: i) a weight ratio of C: 0.5 to 0.8%; 2
-2.5%, Mn: 0.4-0.8%, Cr: 0.7-
1.0%, the balance being Fe and inevitable impurities, the inevitable impurities Al content is 0.005% or less, the Ti content is 0.005% or less, Ii) Using a tempered oil tempered wire with a quenching heating temperature of 950 ° C or higher and 1100 ° C or less, as a material, iii) Shot peening using a high hardness shot ball after coiling Which is characterized by the following.

[0008] Here, the material steel of the above i) further comprises V:
0.05-0.15%, Mo: 0.05-0.5%,
W: 0.05 to 0.15%, Nb: 0.05 to 0.15
% May be contained.

In the oil tempered wire after the quenching and tempering of the above ii), the retained austenite has a volume ratio of 1%.
It is good to be set to ５5%.

Similarly, in the oil-tempered wire after the quenching and tempering of ii), the density of the carbide having a particle diameter of 0.05 μm or more in the structure is 5 / μm on the structure observation photograph.
^It is better to be ² or less.

In the above iii), the hardness of the shot ball is H
v600 or higher, preferably Hv720 or higher.

The shot peening treatment may be performed only once, but is performed twice or more to reduce the residual stress near the surface to 85 kgf / mm.
By setting it to ² or more, the fatigue strength and the like can be further improved.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the high strength valve spring of the present invention, first,
The silicon content of the material steel is higher than the conventional silicon chrome steel oil tempered wire (SWOSC-V) for valve springs,
２．５2.5%. Silicon has a function of forming a solid solution in ferrite and martensite, strengthening them, and delaying the decomposition of the martensite phase into [ferrite + carbide] during tempering. That is, since the phase decomposition temperature at the time of oil tempering is shifted to a higher temperature side, the tempering temperature for obtaining the same tensile strength can be made higher than before. Raising the tempering temperature promotes dislocation recovery and stabilizes the structure. This increases the fatigue strength over time and raises the fatigue limit, since the starting point of the fatigue crack is hardly generated. It also improves the delayed fracture strength.

An increase in the tempering temperature prevents a structural change due to an increase in temperature during use of the valve spring, and makes it difficult to move dislocations. This greatly contributes to improvement in sag resistance.

In the high-strength valve spring according to the present invention, as described above, the tempering temperature is raised by utilizing the strengthening action of silicon, and the tempering temperature is slightly lowered, so that the valve spring has a higher hardness than before. It is also possible to use as. As a result, fatigue strength and sag resistance are improved. Of course, by performing shot peening using a shot ball having higher hardness than before, the compressive residual stress on the surface is increased, and the fatigue strength is further improved. It is also possible.

Under the above basic concept, the present invention employs the oil-tempered wire proposed in Japanese Patent Application Laid-Open Nos. 8-176730 and 9-71843 as a material. Therefore, the components and the tissue limiting conditions will be described below with reference to the description of the publication.

C: 0.5-0.8% by weight C is an essential element for increasing the strength of a steel wire.
If it is less than 0.5%, sufficient strength cannot be obtained, and conversely, 0.8%
If the ratio exceeds 1, the toughness decreases, the flaw sensitivity of the steel wire further increases, and the reliability decreases.

Si: 1.2 to 2.5% by weight Si is an element effective for improving the strength of ferrite and martensite and improving sag resistance as described above. If it is less than 1.2%, the effect is not sufficient, and if it exceeds 2.5%, on the other hand, the cold workability is lowered and the hot workability and decarburization by heat treatment are promoted.

Mn: 0.4 to 0.8% by weight Mn improves the hardenability of steel and fixes S in the steel to prevent its harm. On the contrary, if it exceeds 0.8%, the toughness is reduced.

Cr: 0.7 to 1.0% by weight Cr, like Mn, improves the hardenability of steel, imparts toughness by patenting after hot rolling, hardens, and then softens during tempering. It is an element effective for increasing resistance and increasing strength. If it is less than 0.7%, the effect is small, and if it exceeds 1.0%, on the other hand, solid solution of carbides is suppressed, strength is reduced, and hardenability is excessively increased, resulting in toughness. .

V: 0.05 to 0.15% by weight V is an element which forms carbides during tempering and increases the softening resistance. However, if it is less than 0.05%, the effect is small.
On the other hand, when the content exceeds 0.15%, a large amount of carbide is formed during quenching and heating, leading to a decrease in toughness.

Mo: 0.05-0.5% by weight Mo is an element that forms carbides during tempering and increases the softening resistance. If it is less than 0.05%, its effect is small, and 0.5% If the temperature exceeds the above range, a large amount of carbides will be formed during quenching and heating, leading to a decrease in toughness.

Nb: 0.05 to 0.15% by weight Nb is an element that forms carbides during tempering and increases the softening resistance, but less than 0.05% has little effect. On the other hand, when the content exceeds 0.15%, a large amount of carbide is formed during quenching and heating, leading to a decrease in toughness.

Al, Ti: 0.005% by weight or less All of these are high melting point non-metallic inclusions such as Al ₂ O ₃ ,
Form TiO. These inclusions are hard and, when present immediately below the surface of the steel wire, significantly reduce fatigue strength. For this reason, although they are unavoidable impurities, all of them are 0.005.
% Or less. As the raw material, those having a low impurity concentration may be used.

[0025] Quenching heating temperature: 950 ° C or higher and 1100 ° C
Hereinafter, the amount of solid solution such as V during quenching is determined by the quenching heating temperature, and the higher the temperature, the greater the amount of solid solution. If the temperature is lower than 950 ° C., the amount of solid solution such as V becomes small, and a large amount of carbide precipitates. Further, it is considered that most of V, W, and Nb in the present invention at 1100 ° C. are dissolved in Fe. Therefore, even when the temperature exceeds 1100 ° C., no improvement in toughness and no increase in softening resistance is observed. is there.

Shot Peening: Use of Shot Ball of Hv600 or More Shot peening substantially reduces the maximum shear stress acting on the surface of the valve spring when the valve spring is used by applying a compressive residual stress to the surface of the valve spring, thereby reducing fatigue strength. Significant improvement. However, conventionally, as a result of sufficiently improving the shot peening treatment, the magnitude of the obtained compressive residual stress is almost at a maximum, and even if a shot ball of higher hardness is used, Even if shot peening was performed twice or more, a larger compressive residual stress could not be obtained. As described above, in the high-strength valve spring according to the present invention, since high silicon steel is used as a material, the tempering temperature for obtaining the same hardness as the conventional can be increased, and as a result, the structure at that time Is more stable than the conventional one. Therefore, by using a shot ball of Hv600 or higher having a higher hardness than in the past, it is possible to obtain a correspondingly higher compressive residual stress.

Further, as described above, in the present invention, the tempering temperature is set to be slightly lower than the conventional one, and the oil-tempered wire having a higher hardness than the conventional one is used to improve the fatigue strength and the sag resistance. Can also. When the material is harder than the conventional material, the shot ball is made of Hv720 or higher accordingly.
A sufficient compressive residual stress can be obtained.

In any of the above cases, a higher compressive residual stress can be obtained by performing the shot peening treatment two or more times, whereby a higher fatigue strength can be obtained. Specifically, the residual stress near the surface is reduced to 85
kgf / mm ² or more is desirable. It is also effective in preventing delayed fracture from the surface.

Retained austenite (γ): 1 to 5% by volume% The retained austenite phase present in the tempered martensite improves the toughness of the steel. This is because the amount of set may increase due to martensitic transformation during use of the spring.

In-tissue density of carbide having a particle diameter of 0.05 μm or more: 5 / μm ² or less If carbide having a particle diameter of 0.05 μm or more is present in the tissue, it can be a starting point of fracture during spring molding or the like. This is because if the existing density exceeds 5 / μm ² on a microscopic observation photograph, the toughness is significantly reduced.

It is desirable that the definition of the amount of retained austenite and / or the amount of carbide is realized by the following heat treatment method.

Regarding the quenching heating in the quenching and tempering step, the heating rate is 150 ° C./sec or more and 1100 ° C.
T (° C.) = 500 + 750 · C (carbon content%) + 5
Heating is performed at a temperature not lower than the temperature determined by 00 · V (amount of vanadium) (at least 950 ° C.), and the time from the start of heating to the start of cooling with water or oil is set to 15 seconds or less.

If the time until the start of cooling is not shorter than 15 seconds, the crystal grains become coarse, the toughness is deteriorated, and the heating rate is reduced to 15 seconds.
If the temperature is 0 ° C./sec or less, a sufficient solid solution of carbide cannot be obtained in 15 seconds until the start of cooling. When the heating temperature is 110
If it is 0 ° C. or more, toughness deterioration and decarburization occur due to coarsening of crystal grains, and if T (° C.) = 500 + 750 · C + 500 · V or less, a sufficient solid solution of carbide cannot be obtained.

Regarding the tempering heating in the quenching and tempering step, the heating rate is not less than 450 ° C. at a heating rate of 150 ° C./sec or more.
Heat to 600 ° C., and set the time from the start of heating to the start of cooling using a coolant such as water to be within 15 seconds.

Unless the heating rate is set to 150 ° C./sec and the time until the start of cooling is set within 15 seconds, the residual austenite phase disappears to a volume fraction of less than 1%.

[0036]

EXAMPLES The characteristics of the high-strength valve spring according to the present invention were experimentally tested using, as a comparative material, a steel obtained by adding a small amount of vanadium to a conventionally used silicon chrome steel oil-tempered wire (SWOSC-V). Clarify by result. FIG. 1 shows the chemical compositions of the inventive material and the comparative material used in the experiment.

Both of these test materials were melted in a vacuum melting furnace, and then hot forged and hot rolled into a 6.5 mm diameter wire (element wire). The steps required to obtain a φ3.2 mm oil-tempered wire from this wire are as shown in FIG. The oil tempering treatment differs depending on each test material, and was performed under the conditions shown in FIG. FIG. 4 shows the tensile strength and drawing of both test materials in the state of the oil-tempered wire.

From the oil-tempered wire thus obtained, FIG.
A valve spring having the following specifications was manufactured. After forming the valve spring,
Shot peening was performed under the conditions shown in FIG. As shown in FIG. 4, the material of the present invention has a higher hardness in the state after the heat treatment than the conventional material. FIG. 7 shows the result of measuring the residual stress distribution on the surface after shot peening. Compared with the comparative material, the material of the present invention showed a higher compressive residual stress value than the comparative material even after one shot peening, and further increased the compressive residual stress after the second shot peening to 85 kgf / mm
It is understood that the condition of ² or more is sufficiently satisfied.

The results of tests on the valve spring thus manufactured for set resistance and delayed fracture strength will be described with reference to FIGS. 8 and 9. FIG.

FIG. 8 shows that the maximum shear stress on the surface is τ = 90 kg.
FIG. 6 is a graph showing the results of measuring the residual shear strain γ after placing the valve spring in an atmosphere of 120 ° C. and allowing it to stand for 48 hours while applying a fixed load such as f / mm ² to the test valve spring. . The material of the present invention shows much better sag resistance than the comparative material, and its residual shear strain sufficiently satisfies the target value of τ <5 × 10 ⁻⁴ .

FIG. 9 shows the results of a test for evaluating delayed fracture characteristics. That is, it is a graph showing the result of variously changing the value of the residual stress generated after coiling the valve spring and measuring the time until a crack occurs at each stress value. It can be seen that the inventive material has a much longer crack generation time than the comparative material.

[0042]

In the high-strength valve spring according to the present invention, the silicon content of the material steel is increased to strengthen the solid solution of ferrite and martensite, and [ferrite + carbide] during tempering of the martensite phase. Utilizing the action of delaying the decomposition into water. This shifts the phase decomposition temperature during oil tempering to a higher temperature side and promotes the recovery of dislocations, so that the structure of the material is stabilized and the toughness is improved. Also,
Structural change due to temperature rise during use of the valve spring is prevented, making it difficult to move dislocations. As a result, the valve spring according to the present invention has better sag resistance than the conventional one. In addition, the stabilization of the structure effectively acts on the delayed fracture resistance, and from this aspect, the improvement of the durability in actual use can be expected.

[Brief description of the drawings]

FIG. 1 Chemical composition of test materials.

FIG. 2 shows a manufacturing process up to an oil-tempered wire.

FIG. 3 shows oil tempering conditions.

FIG. 4 shows tensile properties of an oil-tempered wire.

FIG. 5 shows valve spring specifications.

FIG. 6 shows shot peening conditions.

FIG. 7 shows residual stress distribution after shot peening.

FIG. 8 shows the results of a hot tightening test.

FIG. 9 shows the results of a delayed fracture test.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/18 C22C 38/18

Claims (14)

[Claims] 1. A weight ratio of C: 0.5-0.8%, S
i: 1.2 to 2.5%, Mn: 0.4 to 0.8%, C
r: 0.7 to 1.0%, the balance being Fe and inevitable impurities, the inevitable impurities Al content is 0.005% or less, and the Ti content is 0.005% or less. Then, a steel having a maximum non-metallic inclusion of 15 μm, using a quenched and tempered oil-tempered wire with a quenching heating temperature of 950 ° C. or more and 1100 ° C. or less as a material, and after coiling, using a high-hardness shot ball A high strength valve spring characterized by shot peening. 2. The steel material further comprises V: 0.05 to 0.15.
%, Mo: 0.05-0.5%, W: 0.05-0.1
The high-strength valve spring according to claim 1, which contains at least one of 5% and Nb: 0.05 to 0.15%. 3. The high-strength valve spring according to claim 1, wherein the retained austenite has a volume ratio of 1 to 5% in the oil-tempered wire after quenching and tempering. 4. The oil-tempered wire after quenching and tempering, wherein the density of the carbide having a particle diameter of 0.05 μm or more in the structure is 5 / μm ² or less on a structure observation photograph. A high-strength valve spring as described in Crab. 5. The high-strength valve spring according to claim 1, wherein shot peening is performed with a shot ball having a hardness of Hv600 or more. 6. The high-strength valve spring according to claim 1, wherein shot peening is performed with a shot ball having a hardness of Hv720 or more. 7. The high-strength valve spring according to claim 5, wherein the shot peening is performed at least twice so that a residual stress near the surface is 85 kgf / mm ² or more. 8. A weight ratio of C: 0.5 to 0.8%, S
i: 1.2 to 2.5%, Mn: 0.4 to 0.8%, C
r: 0.7 to 1.0%, the balance being Fe and inevitable impurities, the inevitable impurities Al content is 0.005% or less, and the Ti content is 0.005% or less. Then, after coiling a quenched and tempered oil-tempered wire at a quenching heating temperature of 950 ° C. or more and 1100 ° C. or less, steel having a maximum non-metallic inclusion of 15 μm is subjected to shot peening using a high-hardness shot ball. A method for manufacturing a high-strength valve spring, comprising: 9. The steel material further comprises V: 0.05 to 0.15.
%, Mo: 0.05-0.5%, W: 0.05-0.1
The method for producing a high-strength valve spring according to claim 8, comprising one or more of 5% and Nb: 0.05 to 0.15%. 10. A heating rate of 150 in the quenching process.
T (° C.) = 500 ° C./sec or more and 1100 ° C. or less
+ 750.C (carbon content%) + 500.V (vanadium content%) Heating to the temperature range or more (however, 950 ° C or more), and the time from the start of heating to the start of cooling with water or oil within 15 seconds The method for manufacturing a high-strength valve spring according to claim 8. 11. A heating rate of 150 in the tempering process.
At 450 ° C./sec or more, heating to 450 ° C. to 600 ° C., and the time from the start of heating to the start of cooling using a refrigerant such as water is 15 minutes.
The method for manufacturing a high-strength spring according to any one of claims 8 to 10, wherein the time is within seconds. 12. The method for manufacturing a high-strength valve spring according to claim 8, wherein shot peening is performed with a shot ball having a hardness of Hv600 or more. 13. The method for manufacturing a high-strength valve spring according to claim 8, wherein shot peening is performed with a shot ball having a hardness of Hv720 or more. 14. The method of claim 1 wherein said shot peening is performed for at least
The method for manufacturing a high-strength valve spring according to claim 12 or 13, wherein the valve spring is applied.