JP3142689B2 - Spring with excellent fatigue strength - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spring having excellent fatigue resistance, and this spring is particularly useful for a valve spring of an automobile engine requiring high fatigue strength.

[0002]

2. Description of the Related Art In recent years, as automobiles have become lighter and higher in power, springs such as valve springs and suspension springs used for engines and suspensions have been designed to have high stress design, and load stress has been reduced. In order to cope with the increase, a material excellent in fatigue resistance and sag resistance is strongly desired.

In particular, valve springs are required to have particularly high fatigue strength, and SWOSC-V (JIS) having excellent fatigue strength among conventional spring steels specified in JIS.
G3566). In order to cope with such a problem, there has been proposed a method of increasing the fatigue strength by reducing the size of nonmetallic inclusions in addition to increasing the strength of the material (for example, JP-A-63-216951, JP-A-60-1985). 247453).

[0004] In general, fatigue of a steel material starts from a defect such as an inclusion existing on the surface or inside of the material and causes cracks to be generated and propagated, leading to breakage. Here, by increasing the surface hardness by increasing the strength of the material or reducing the size of the nonmetallic inclusions, a spring having a higher fatigue strength than before has been obtained. In recent years, in particular, the amount of inclusions in spring steel has been significantly reduced due to the improvement of steelmaking technology, and the occurrence of fatigue fracture originating from inclusions has become rare. ing. Therefore, it is necessary to strengthen the surface in order to further improve the fatigue strength. However, even if the strength of the material is increased by increasing the amount of the alloy element, there is a limit in terms of manufacturing the spring wire. Therefore, a method of strengthening only the surface layer by surface treatment such as nitriding has been studied, and a method of manufacturing a spring in which the nitriding conditions are specified has been proposed (for example, Japanese Patent Laid-Open No. 52-1982).
No. 10833, JP-A-63-176430, etc.).

[0005]

However, according to various studies conducted by the present inventors, it is not always possible to obtain a significant improvement in fatigue strength by simply performing a nitriding treatment or the like. Even if the conditions are the same, the surface hardness after nitriding is not always the same, and the required fatigue characteristics may not be obtained. Therefore, in order to obtain a spring product of stable quality in fatigue resistance and sag resistance, it is necessary to clarify the chemical composition of the steel material and the spring characteristics after nitriding which affect these characteristics.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to specify the chemical composition of a steel material to be used, clarify the spring characteristics after nitriding treatment, and thereby achieve excellent resistance to steel. An object of the present invention is to provide a spring capable of stably exhibiting fatigue resistance and sag resistance.

[0007]

Means for Solving the Problems The structure of the spring according to the present invention which can solve the above-mentioned problems is as follows: C: 0.4 to 0.8% Si: 0.8 to 4.0% Mn by weight ratio : 0.2-2.0% Cr: 0.4-3.0%, V: 0.05-0.50% Nb: 0.05-0.50% Ni: 0.2- 2% Mo: A steel containing at least one element selected from the group consisting of 0.1 to 1.0%, and the balance consisting of Fe and unavoidable impurities is subjected to nitriding treatment. 0.5 to 2%, the nitrogen diffusion depth is 0.07 mm or more, and the internal hardness is Vickers hardness of 470 or more. In the above spring,
When the relationship between the surface roughness (Rmax) and the amount of nitrogen (Ns) within 10 μm from the surface is Rmax ≦ 15 / Ns, more excellent performance in fatigue resistance is exhibited.

[0008]

As described above, in the present invention, the chemical composition of the steel material to be used is specified, and the nitrogen content, the nitrogen diffusion depth and the internal hardness of the surface layer after the nitriding treatment are specified. By specifying the relationship between the roughness and the amount of nitrogen in the surface layer portion, stable fatigue resistance and sag resistance are secured, and the reasons for setting them will be described in detail below.

First, the reason for determining the chemical composition of the steel material will be described. C is an indispensable element for imparting sufficient strength as a spring steel to which high stress is applied, and 0.4%
Must be contained. However, if it exceeds 0.8%, the toughness and ductility are extremely deteriorated, and it is easy to break during spring molding. Therefore, the C content is set to 0.4 to 0.8%. Si is
This element is necessary for deoxidation during steelmaking, and is also necessary for increasing the softening resistance in the nitriding treatment and ensuring the internal hardness of the spring. However, if it exceeds 4.0%, the toughness is significantly reduced, so the Si content was set to 0.8 to 4.0%.

[0010] Mn is also effective in deoxidizing steel and is an element necessary for enhancing the hardenability and securing the internal hardness of the wire. However, if it is less than 0.2%, the effect is insufficient. On the other hand, if it exceeds 2.0%, toughness and ductility deteriorate, so the Mn content is set in the range of 0.2 to 2.0%. Cr is an element that easily forms nitrides, is effective in increasing the surface hardness by nitriding, and is indispensable to increase the softening resistance in the nitriding treatment and secure the internal hardness of the spring. 4
%, These effects cannot be sufficiently exerted. However, if it exceeds 3.0%, the toughness deteriorates, so the range was set to 0.4 to 3.0%. The steel material used in the present invention needs to contain at least one element selected from the group consisting of V, Nb, Ni and Mo in addition to the above four elements to provide the following characteristics.

V and Nb refine crystal grains in heat treatment such as quenching and tempering, and contribute to improvement in toughness and ductility. Furthermore, in addition to secondary precipitation strengthening in the nitriding treatment step, the internal hardness is increased, and since it is an element that easily forms a nitride, these elements are added. Each of these effects is effectively contained by containing 0.05% or more. Be demonstrated. But each 0.5
%, Excessive carbides and nitrides are liable to precipitate at the stage of the steel material, which adversely affects toughness and ductility.

Ni is an element effective for improving the toughness and ductility after quenching and tempering, and also has the effect of increasing the internal hardness of the strand by the effect of improving quenching properties, and the effect is 0.2%. Effectively exhibited by containing the above. However, these effects are saturated at 2.0%, and furthermore, a bainite or martensite structure is liable to be generated at the time of hot rolling, leading to deterioration of toughness and ductility. Therefore, further addition is disadvantageous.

Mo also has the effect of strengthening the internal hardness by secondary precipitation strengthening during the nitriding treatment, and it is effective to contain Mo in an amount of 0.1% or more. However, if it exceeds 1%, a bainite or martensite structure is liable to be formed during heat treatment, and it becomes difficult to produce a strand.

Next, the reasons for limiting the characteristics of the spring after nitriding will be described. In view of the problems pointed out in the above prior art, the present inventors have repeatedly studied the relationship between the internal characteristics and the fatigue characteristics of a nitrided spring using a steel material satisfying the above component composition under a higher load stress than before. As a result, the following findings were obtained. (1) Nitriding increases the hardness and compressive residual stress of the surface layer, which correspond to the amount of nitrogen in the surface layer.
There is an appropriate nitrogen content range for fatigue properties.

When the amount of nitrogen within 10 μm from the surface layer of the nitrided material is 0.5% or more, the hardness of the surface layer increases greatly and the fatigue life is remarkably improved. However, 2%
With the above, the surface hardness is further increased, but the formation of hard and brittle nitride on the surface becomes remarkable, so that the fatigue life is conversely reduced. Therefore, in the present invention, 1
The range of the amount of nitrogen within 0 μm was determined to be 0.5 to 2%. FIG. 1 is a graph showing the relationship between the amount of nitrogen (Ns) within 10 μm from the surface and the internal hardness, surface hardness, diffusion depth of nitrogen, and fatigue life. It is understood that a high fatigue life can be secured by setting the amount (Ns) to 0.5 to 2%. However, the test method of the fatigue life is as described later.

When the nitrogen diffusion depth is 0.07 mm or more, the compressive residual stress in the surface layer increases remarkably, and the effect of improving the fatigue life appears remarkably. Therefore, the diffusion depth of nitrogen is set to 0.07 mm or more, but the effect is saturated at about 0.5 mm. Therefore, considering the productivity of the nitriding treatment, the upper limit is preferably 0.5 mm. When the surface layer is strengthened as described above, fatigue fracture is likely to occur from the inside,
In order to suppress this and secure a high level of fatigue resistance as a whole, it is necessary to set the internal hardness at a depth of 0.5 mm or more from the surface to Hv470 or more.

(2) Although the residual stress and surface roughness vary depending on the conditions of the shot peening treatment performed after the nitriding treatment, in order to further improve the fatigue resistance,
It is desirable that the surface layer has a compressive residual stress of 880 N / mm ² or more. However, the present inventors have confirmed that, when excessive shot peening is performed with the aim of increasing the compressive residual stress, the surface roughness increases and the fatigue strength decreases. And it was found that the degree of this decrease increased as the amount of nitrogen increased, and that an appropriate surface roughness was present in accordance with the amount of nitrogen. From the results of various experiments, the relationship between the surface roughness (Rmax) and the amount of nitrogen (Ns) within 10 μm from the surface was arranged, and the results shown in FIG. 2 were obtained. By setting Rmax (μm) ≦ 15 / Ns, It was confirmed that a high level of fatigue life was more stably obtained.

[0018]

EXAMPLES Hereinafter, the structure and operation and effect of the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and can conform to the spirit of the preceding and following examples. Of course, the present invention can be carried out while being changed within the scope, and all of them are included in the technical scope of the present invention.

Example 1 A steel having a chemical composition shown in Table 1 was hot-rolled into a wire having a diameter of 7 mm, and a wire for a spring having a diameter of 3.2 mm was produced in the following steps. A spring was manufactured. Rolled wire rod → annealing → shaving → patenting → wire drawing → quenching and tempering → spring forming

[0020]

[Table 1]

As is evident from Table 1, all of the steels A1 to A5 satisfying the requirements of the present invention could be formed into a spring shape without any trouble, but the comparative steel B3 suffered frequent breakage during the spring forming. In Comparative Steel B4, wire breakage occurred frequently during wire drawing, so that spring forming could not be performed. A spring for a fatigue test was prepared from the molded spring by the following steps, and a fatigue test was performed under the conditions shown in Table 2, and the distribution of nitrogen content and the hardness distribution in the cross section of the spring element wire were measured. The nitriding conditions and shot peening conditions were the same for all samples. Table 3 shows the results. Spring → strain relief annealing → descaling → nitriding treatment → shot peening → low temperature annealing

[0022]

[Table 2]

[0023]

[Table 3]

As is clear from Table 3, the comparative steel B1
Has a low internal hardness and B2 has a low nitrogen content, so that the fatigue strength is inferior. On the other hand, each of the springs according to the present invention has an extremely excellent fatigue life, and does not break even at 1 × 10 ⁷ times.

Example 2 Using the spring manufactured from the A3 steel shown in Example 1, samples a1 to a3 and b1 to b4 were manufactured under different nitriding conditions and subjected to a spring fatigue test. The manufacturing process of the fatigue test spring is the same as described above. Table 4 shows the characteristics of the test springs, and FIG. 1 shows the results of the fatigue test.

[0026]

[Table 4]

As is clear from Table 4 and FIG. 1, the nitrogen content (Ns) and the nitrogen diffusion depth (Nd) are insufficient.
While the fatigue life of B2 and b3 is low, the springs a1 to a3 of the present invention satisfying both Ns and Nd show excellent fatigue life. In addition, Ns exceeding 2% (b4)
Also, the fatigue life is clearly reduced.

Example 3 Samples a4 to a4 obtained by using a spring manufactured from steel A3 shown in Table 1 above and changing the nitriding conditions and the shot peening conditions to change the nitrogen amount and surface roughness (Rmax). a14 was manufactured and subjected to a fatigue test. The results are as shown in FIG. 2 and Table 5, and the surface roughness (Rma
x) and the nitrogen content (Ns) of the surface layer portion are "Rmax ≦ 15 / N
It can be seen that those that are "s" can exhibit excellent fatigue life stably.

[0029]

[Table 5]

[0030]

The present invention is configured as described above and specifies the chemical composition of the steel material, specifies the amount of nitrogen on the surface layer after nitriding, the nitrogen diffusion depth and the internal hardness, and determines the surface and By providing internal characteristics that suppress fatigue fracture from both inside, it is possible to achieve higher fatigue strength than conventional materials, and to provide a highly reliable spring.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of nitrogen within 10 μm from the surface and the surface hardness or fatigue life.

FIG. 2 is a graph showing the effect of the amount of nitrogen within 10 μm from the surface and the surface roughness on fatigue life.

──────────────────────────────────────────────────の Continued on the front page (72) Inventor Yasunobu Kawaguchi 2 Nadahama-Higashi-cho, Nada-ku, Kobe-shi, Hyogo Co., Ltd. Inside Kobe Works, Ltd. In-company (72) Inventor Masaru Terashita 14 Suncall, Umezu-Nishiura-cho, Ukyo-ku, Kyoto-shi (72) Inventor Yuji Isono 14 Suncall, Umezu-Nishiura-cho, Ukyo-ku, Kyoto-shi (58) Investigation area (Int .Cl. ⁷ , DB name) C23C 8/26

Claims (2)

(57) [Claims] 1. C: 0.4 to 0.8% Si: 0.8 to 4.0% Mn: 0.2 to 2.0% Cr: 0.4 to 3.0% by weight V: 0.05 to 0.50% Nb: 0.05 to 0.50% Ni: 0.2 to 2% Mo: at least one selected from the group consisting of 0.1 to 1.0% Of steel, the balance of Fe and unavoidable impurities is nitrified, the amount of nitrogen within 10 μm from the surface is 0.5 to 2%, the nitrogen diffusion depth is 0.07 mm or more, and the internal hardness is Vickers A spring excellent in fatigue strength characterized by having a hardness of 470 or more. 2. Surface roughness (Rmax) and 10
The relationship between the amount of nitrogen (Ns) within μm is Rmax (μm)
The spring having excellent fatigue strength according to claim 1, wherein ≤ 15 / Ns.