JPH05331597A - High fatigue strength coil spring - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a coil spring which is used in a valve spring for an automobile engine and has excellent fatigue resistance and sag resistance. [Structure] C: 0.5-0.8% by weight, Si:
1.2-2.5%, Mn: 0.4-0.8%, Cr:
0.7-1.0%, N: 0.005-0.030%, and V: 0.1-0.6%, Mo: 0.05-
0.50%, W: 0.05 to 0.50% of 2 or 3 types, the balance of Fe and unavoidable impurities, Al content of the unavoidable impurities is 0.005% or less ,
A coil spring obtained by subjecting a wire having the same Ti content of 0.005% or less to patenting, wire drawing, and quenching and tempering, and spring-forming the obtained steel wire.
The maximum is 10 μm or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coil spring having excellent fatigue resistance, which is used for a valve spring of an automobile engine.

[0002]

2. Description of the Related Art Conventionally, as a steel wire for a valve spring used in an engine of an automobile or the like, a carbon steel oil temper wire for a valve spring (JIS SWO-V), a chrome vanadium steel oil temper wire for a valve spring (JIS SWOCV-V) and silicon chrome steel oil tempered wire for valve springs (JIS SWOSC-V) are used. Above all, the silicon chrome steel oil tempered wire for valve springs is widely used because of its excellent fatigue resistance and heat resistance. However, it cannot be said that it has sufficiently coped with the recent high performance of the engine, and based on this, the level of inclusions is further reduced to improve fatigue resistance.

Further, nitriding has been widely used as a method for increasing fatigue strength. This method is carried out in a salt bath or in a gas atmosphere, and the higher the temperature, the greater the effect. But in the case of valve springs,
When treated at high temperature, strength is reduced. Currently, even the most heat-resistant silicon chrome oil tempered wire for valve springs can be processed only at 380 to 420 ° C, and nitriding at a temperature higher than this lowers the strength, resulting in fatigue resistance and sag resistance. Cause decline.

The present invention has been made based on such a technical background, and is excellent in fatigue resistance and sag resistance so as to cope with high output of automobile engines and high temperature of atmosphere in which springs are used. It is intended to provide a coil spring.

[0005]

To achieve this object, the coil spring of the present invention has a weight ratio of C: 0.5 to.
0.8%, Si: 1.2 to 2.5%, Mn: 0.4 to
0.8%, Cr: 0.7-1.0%, N: 0.005-
0.030% and V: 0.1-0.6%, M
o: 0.05 to 0.50%, W: 0.05 to 0.50%
2 or 3 or more and the balance consists of Fe and unavoidable impurities, and the Al content of the unavoidable impurities is 0.
A wire rod having a Ti content of 005% or less and a Ti content of 0.005% or less is subjected to patenting, wire drawing and quenching and tempering treatment, and the obtained steel wire is spring-formed. The surface roughness is Rmax 10 μm or less. Is characterized by

The reasons for limiting the composition of the coil spring of the present invention will be described in detail below.

C: 0.5 to 0.8 wt% C is an essential element for increasing the strength of the steel wire,
If less than 0.5%, sufficient strength cannot be obtained, and conversely 0.8%
If it exceeds 1.0, the toughness decreases, and the susceptibility of the steel wire to defects increases, so that the reliability decreases.

Si: 1.2 to 2.5 wt% Si is an element effective for improving the strength of ferrite and improving the sag resistance. This is because if it is less than 1.2%, there is no sufficient effect, and if it exceeds 2.5%, cold workability is deteriorated and decarburization by hot working or heat treatment is promoted.

Mn: 0.4 to 0.8 wt% Mn improves the hardenability of steel and fixes S in the steel to prevent its damage, but if it is less than 0.4%, it has no effect. On the contrary, if it exceeds 0.8%, the toughness decreases.

Cr: 0.7 to 1.0 wt% Cr, like Mn, improves the hardenability of steel, imparts toughness by patenting treatment after hot rolling, and softens during tempering after quenching. It is an element effective in increasing resistance and increasing strength. If it is less than 0.7%, its effect is small. On the contrary, if it exceeds 1.0%, solid solution of carbides is suppressed, strength is lowered, and hardenability is excessively increased, resulting in lower toughness. Is.

N: 0.005 to 0.030 wt% N combines with Al to contribute to the refinement of crystal grains and acts as a solid solution strengthening element for ferrite, but if less than 0.005%, the effect is insufficient. This is because if it exceeds 0.030%, the toughness is lowered.

V: 0.1 to 0.6 wt% V forms carbides in steel and refines austenite crystal grains to improve durability, but if less than 0.1%, this effect cannot be obtained. . On the other hand, if it exceeds 0.6%, the solid solution of carbide tends to be suppressed, which adversely affects the heat treatment.

Mo: 0.05 to 0.50 wt% Mo is an element effective in improving the sag resistance of the spring, and also enhances the temper softening resistance and imparts durability. However, if it is less than 0.05%, its effect is small, and if it exceeds 0.50%, the wire drawability is deteriorated.

W: 0.05 to 0.50 wt% W combines with C to form a carbide, which makes the crystal grains finer and also increases the resistance to temper softening and imparts durability. However, if it is less than 0.05%, the effect is small, and conversely, if it exceeds 0.50%, the improvement of the effect cannot be expected.

Al, Ti: 0.005 wt% or less All of these are high melting point inclusions of Al ₂ O ₃ and Ti.
Generate O. These inclusions are hard and, if present immediately below the steel wire, significantly reduce the fatigue strength. Therefore, although they are unavoidable impurities, the content of each of them is set to 0.005% or less. A raw material having a low concentration of these impurities may be used.

The coil spring is formed in the following steps, for example. First, a metal having the above composition is rolled into a wire rod, which is subjected to patenting treatment and wire drawing. Next, it is heated to a temperature of A ₃ point or higher to form austenite, and is subjected to quenching treatment to transform into martensite. After that, tempering is performed at an appropriate temperature and spring forming is performed. Further, after the spring forming, nitriding treatment or shot peening treatment can be performed to further improve fatigue resistance.

Regarding Surface Roughness of Coil Spring If the surface roughness of the coil spring is smooth, the fatigue strength can be further improved. From this perspective,
It was found that when the Rmax is 10 μm or less, the fatigue strength is remarkably improved. The surface roughness of the steel wire before quenching and tempering is adjusted by adjusting the workability in wire drawing, and mechanical polishing may be performed after wire drawing if necessary. For adjusting the surface roughness of the coil spring, methods such as mechanical polishing, shot peening, pickling, and electrolytic polishing are used.

[0018]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below. Each sample shown in Table 1 was prepared, melted in an induction melting furnace, and hot-rolled into a wire having a diameter of 6.5 mm. Here, F, G, and H are comparative examples, and F = JIS SWOS
CV, G = JIS SWOCV-V, H = JIS SWO-V. After this wire is heat treated, it is peeled off and the diameter is 6.2 mm.
And processed into a diameter of 3.8 mm by cold drawing. Further, quenching and tempering treatments were performed to obtain steel wires having the mechanical properties shown in Table 2.

[0019]

[Table 1]

[0020]

[Table 2]

Such a steel wire was formed into a spring having the specifications shown in Table 3, strain relief annealing was carried out at 420 ° C. for 30 minutes, and nitriding treatment was carried out at 460 ° C. for 6 hours. Subsequently, a shot wire with a diameter of 0.7 mm and a steel ball with a diameter of 0.3 mm were used for shot peening treatment for 30 minutes each. Further, low temperature annealing was performed at 200 ° C. for 20 minutes. The surface roughness of the coil springs here was in the range of 4 to 8 μm in Rmax. Then, a fatigue test was performed on the obtained coil spring using a star fatigue tester.

[0022]

[Table 3]

The test conditions were such that the average stress was 637 MPa, the stress amplitude was changed and repeated stress was applied up to 5 × 10 ⁷ times, and the stress amplitude that did not break was the fatigue limit. The test results are shown in Table 4.

[0024]

[Table 4]

As shown in the table, the embodiments of the present invention (A to
It was confirmed that all of E) had excellent fatigue properties as compared with Comparative Examples (F to H). Further, the same coil spring was used to examine the amount of sag when a torsional stress of 686 MPa was applied at a temperature of 230 ° C. for 96 hours. The results are shown in Table 5 as the residual shear strain amount.

[0026]

[Table 5]

As shown in the table, it was confirmed that the examples of the present invention had less residual shear strain amount than any of the comparative examples and had excellent heat resistance.

(Embodiment 2) Next, among the above samples, A
And the surface roughness of the steel wire before quenching and tempering was adjusted using the steel wire having the composition shown in D, and then the same quenching, tempering treatment, spring forming, nitriding treatment and shot peening treatment as in Example 1 were performed. A coil spring having a surface roughness of about 4 to 18 μm was produced. Then, a fatigue test similar to that in Example 1 was performed for each of these. The results are shown in Table 6.

[0029]

[Table 6]

From the above results, it was confirmed that if the surface roughness of the coil spring was 10 μm or less in Rmax, the fatigue characteristics were excellent.

[0031]

As described above, the coil spring having excellent fatigue characteristics can be obtained by limiting the composition and the surface roughness. Further, heat resistance is also improved, nitriding treatment can be performed at high temperature, and fatigue characteristics can be further improved. Therefore, it can be expected to be effectively used for valve springs of automobile engines, which have been improved in performance in recent years.

Claims (1)

[Claims] 1. 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%, N: 0.005 to 0.030%
And V: 0.1 to 0.6%, Mo: 0.05
~ 0.50%, W: 0.05 to 0.50% of 2 or 3
A wire rod containing at least one species and the balance consisting of Fe and unavoidable impurities and having an Al content of the unavoidable impurities of 0.005% or less and a Ti content of 0.005% or less is patented and stretched. The steel wire obtained by wire processing and quenching and tempering is spring-formed and has a surface roughness of R.
A high fatigue strength coil spring having a maximum of 10 μm or less.