JPH07188852A - Steel for nitrided spring excellent in fatigue strength and nitrided spring - Google Patents

Abstract

PURPOSE:To produce a steel for nitrided spring excellent in fatigue strength and the spring by subjecting a low alloy steel material, in which the size of non-metallic inclusions in the cross section in a rolling direction is specified, to nitriding treatment. CONSTITUTION:A steel material, which has a composition containing, by weight, 0.3-0.7% C, 0.8-4% Si, 0.2-1.5% Mn, 0.4-3% Cr, 0.02-0.7% sol.Al, and <20ppm O and further containing one or more kinds among 0.05-0.5% V, 0.05-0.5% Nb, 0.05-0.5% Mo, and 0.1-3% Ni, is hot-rolled and wiredrawn. The resulting wire is formed into spring state, and, after formed into spring state having a structure where the size of non-metallic inclusions in 3600mm<2> cross section in a rolling direction is regulated to <=15mum, the resulting spring is nitrided. By this method, the nitricled spring, in which hardness in the part between the surface and a position at a depth of <=10mum from the surface is regulated to <=900 Vickers hardness Hs and also Vickers hardness Hi in the inner part is regulated to 450-570 and which has excellent fatigue strength, can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for obtaining a nitrided spring having excellent fatigue resistance and a nitrided spring using this steel material. The nitrided spring is extremely high, such as a valve spring for an automobile engine. It is useful as a spring material that requires fatigue strength.

[0002]

2. Description of the Related Art In recent years, along with the trend toward lighter weight and higher output of automobiles, springs such as valve springs and suspension springs used in engines, suspensions, etc. are also designed for high stress. Therefore, it is strongly desired that those springs have excellent fatigue resistance and sag resistance in order to cope with an increase in load stress. In particular, valve springs are required to have high fatigue characteristics, and in order to meet such demands, the strength of the material is increased by increasing the amount of alloying elements and adding them to the steel type JIS SWOSC-V (JIS G3566). A steel material aiming at this has been proposed (for example, JP-A-63-21).
6951 publication).

However, the recent demand for high fatigue strength is becoming more and more severe, and it is not possible to meet those demands only by increasing the strength of the material as described above. Therefore, in addition to increasing the strength of the material, surface hardening treatment such as nitriding treatment aimed at significantly improving the hardness of the spring surface layer has been studied in the field of valve springs, and some results have been obtained (for example, research on spring technology). Meeting Autumn 1987 and '9
(Abstracts of Autumn Year 0 Lectures).

However, even in the modification technique applying the nitriding treatment, the surface hardness is at most about Hv860 or less and the spring fatigue property is improved as compared with the conventional material, but for example, the stress 70 ± 50 kgf / mm ² is repeated. 2 when affected
Breaks after about 10 ⁷ times or less. Further, increasing the surface hardness is effective for further improving fatigue characteristics, and Al can be considered as an effective additive element. JIS SACM6 widely used for machine structural parts as nitriding steel
Even for 45 etc., from the same purpose as this, 0.70 to 1.2%
Al is contained to some extent.

However, in the valve spring, the addition of Al, which is the source of Al ₂ O ₃ inclusions, should be suppressed as much as possible in order to prevent fatigue fracture due to non-metallic inclusions. Therefore, as a deoxidizing material during steelmaking, Si or Mn is used. In this case, although a method of controlling the inclusions to have a low melting point composition and refining the inclusions in the subsequent hot working has been attempted,
To control the composition of inclusions, a certain amount of oxygen is necessary (usually about 20 to 50 ppm), so the absolute number of inclusions contained in the steel material is the steel deoxidized by Al (usually 20 pp).
more than m) and fractures due to inclusions are often experienced.

[0006]

SUMMARY OF THE INVENTION The present invention has been made by paying attention to the problems as described above, and an object thereof is a steel for nitrided spring, which has far more excellent fatigue strength than conventional materials, Another object of the present invention is to provide a nitride spring having high fatigue characteristics using the steel material.

[0007]

The composition of the steel for nitrided spring according to the present invention, which has been able to solve the above-mentioned problems, has a composition of C: 0.
3 to 0.7% Si: 0.8 to 4% Mn: 0.2 to 1.5% Cr: 0.4 to 3% sol. While containing Al: 0.02-0.7%,
Oxygen content is 20ppm or less, and V: 0.05-
0.5% Nb: 0.05 to 0.5% Mo: 0.05 to 0.5% Ni: 0.1 to 3%, containing at least one element selected from the group consisting of balance Fe And the unavoidable impurities, and the gist is that the size of the nonmetallic inclusions in the rolling direction cross section of 3600 mm ² including the center of the steel material is 15 μm or less. Then, a steel spring satisfying the above requirements is subjected to nitriding treatment, and the Vickers hardness Hs within 10 μm from the surface is 900 or more and the internal Vickers hardness Hi is 450 to 570, and the fatigue strength is very excellent. It becomes a nitriding spring.

[0008]

As described above, according to the present invention, C, Si, Mn, Cr,
sol. The content of each of Al and oxygen is specified, and the steel is made of a steel material containing an appropriate amount of one or more elements selected from the group consisting of V, Nb, Mo and Ni, and in a rolling direction cross section of 3600 mm ² . A steel for nitrided spring having high fatigue characteristics in which the size of non-metallic inclusions is specified to be 15 μm or less, and a Vickers hardness Hs of 900 or more within 10 μm from the surface, which is obtained by nitriding the steel. A nitride spring having a hardness Hi of 450 to 570 is provided, and this nitride spring has an extremely excellent fatigue property, and has an extremely excellent performance as a valve spring for an internal combustion engine for automobiles and the like. It is something to demonstrate. First, the reason for defining the component composition of the steel material used in the present invention will be described.

C: 0.3 to 0.7% This element is indispensable for ensuring sufficient strength as a spring steel material to which high stress is applied, and it must be contained at least 0.3% or more. . However, if the amount is too large, the toughness is extremely deteriorated and the spring is apt to be broken during spring forming, and it is necessary to suppress the internal hardness to 0.8% or less for the reason described below.

Si: 0.8-4% This is an essential component for improving the sag resistance of the spring after nitriding treatment, and it must be contained at least 0.8% or more. However, if the amount is too large, the toughness is remarkably deteriorated, so it should be suppressed to 4% or less.

Mn: 0.2-1.5% An element that effectively acts on deoxidation during steelmaking and improvement of toughness.
In order to effectively exhibit these effects, the content must be 0.2% or more. However, if the content exceeds 1.5% and is excessively large, a supercooled structure such as bainite is likely to be generated in the heat treatment step during steel making, and wire drawability is significantly deteriorated.

Cr: 0.4 to 3% This element is an element that easily forms a nitride and is an element that is indispensable for improving the surface hardness by nitriding treatment. The effect is obtained by adding 0.4% or more. Effectively demonstrated. However, if the content exceeds 3% and is excessively contained, the toughness and ductility is deteriorated and it becomes difficult to process the wire rod.

Sol. Al: 0.02 to 0.7% As described above, it has been confirmed that Al becomes a generation source of metal inclusions and adversely affects fatigue characteristics, and it is considered preferable to reduce the amount of Al as much as possible. It was However, according to the results of various studies by the present inventors, s
ol. Al exhibits an excellent effect of improving the surface hardness by the nitriding treatment, and is an essential component for achieving the purpose of the present invention. And, in order to effectively exert such action and effect, sol. Although Al must be contained in an amount of 0.02% or more, on the other hand, if the content is too large, it becomes difficult to deepen the nitriding layer at the time of nitriding treatment, and the surface hardening effect rather deteriorates. It must be kept below 0.7%.

V: 0.05 to 5%, Nb: 0.05 to
0.5%, Mo: 0.05 to 0.5%, Ni: 0.1
At least one element selected from the group consisting of 3% should be contained in order to increase the toughness and ductility after heat treatment such as quenching and tempering. However, if the content of V, Nb, Mo exceeds the upper limit value, huge carbides and nitrides are likely to be formed, which significantly deteriorates the fatigue properties, and if the Ni content exceeds the upper limit value, during hot rolling. Since a bainite structure and a martensite structure are likely to be formed and the toughness and ductility are deteriorated, they must be suppressed to the respective upper limits or less.

The essential constituent elements of the steel material for a valve spring according to the present invention are as described above, and the balance is composed of iron and unavoidable impurities. Regarding oxygen mixed in as unavoidable impurities, its content is An essential requirement is to keep it below 20 ppm. And the oxygen content is 20ppm
This is because if the content exceeds the above range, the amount of oxide-based inclusions increases and fatigue fracture due to the inclusions is likely to occur, and the above-mentioned object of the present invention cannot be achieved.

Further, in the present invention, as another requirement for enhancing the fatigue resistance, it is necessary to reduce the size of non-metallic inclusions in the steel which is the starting point of fatigue fracture as much as possible. As a result, it is essential that the size of the non-metallic inclusions in the rolling direction cross section of 3600 mm ² including the center of the steel wire rod satisfying the requirements of the above composition be 15 μm or less. Coarse non-metallic inclusions having a size of more than 15 μm then become a starting point of fatigue fracture and cause breakage when subjected to repeated stress.

It should be noted that fine non-metallic inclusions of 15 μm or less rarely become the starting point of fatigue fracture, but if the absolute number is too large, it cannot be denied that the toughness is adversely affected.
Preferably, within the same cross section of 3600 mm ² , 5 to 15
It is desirable to suppress the total number of non-metallic inclusions having a size of μm to 50 or less.

The nitrided spring according to the present invention has a Vickers hardness Hs within 10 μm from the surface of 900 or more by subjecting a steel wire satisfying the above requirements to a nitriding treatment according to a conventional method to intensively harden the surface layer portion. And the internal Vickers hardness Hi in the range of 450 to 570. If the Vickers hardness Hs within 10 μm from the surface is less than 900, fatigue fracture starting from the surface matrix is likely to occur, and the internal hardness is Hv45.
If it is less than 0, not only fatigue fracture starting from the internal matrix tends to occur, but also the sag resistance deteriorates,
On the other hand, if it exceeds Hv570, breakage of the origin of the inclusions is likely to occur inside, and any satisfactory fatigue life cannot be obtained. Although the depth of the nitride layer is not particularly limited, it is desirable that the depth of the nitride layer be 40 μm or more in order to suppress the variation of the fatigue life at the surface and the internal origin.

[0019]

EXAMPLES Next, examples of the present invention will be shown, but the present invention is not limited by the following examples, and may be carried out with appropriate modifications within a range compatible with the gist of the preceding and following description. Of course, it is possible, and all of them are included in the technical scope of the present invention.

Example 1 Steels having the chemical compositions shown in Table 1 were melted and hot-rolled into a wire having a diameter of 7 mm, which was then annealed → skinned → patenting → wire drawing → quenching and tempering → spring forming → nitriding. Through each treatment in order, a spring wire having a diameter of 3.2 mm was produced, and a spring having the specifications shown in Table 2 was produced. Of these, comparative steel No. No. 8 had many breakages during spring forming, and Ni
Or comparative steel No. 3 having a high Cr content. Nos. 9 and 10 were frequently broken during wire drawing and could not be spring formed.
For those that could be spring-formed, a shot peening treatment was performed and then a spring fatigue test was performed and the hardness distribution of the spring wire was measured. Table 3 collectively shows the results and the results of measuring the size of non-metallic inclusions as wire rods.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

As is clear from Table 3, each of the springs of the examples satisfying the specified requirements of the present invention does not break even when subjected to repeated stress of 5 × 10 ⁷ times, but coarse inclusions. Comparative steel No. including 4 and comparative steel No. 4 having a high oxygen content. 5 is 3 × 10 ⁷ due to the fracture of the origin of inclusions.
Breakage occurs less than the number of times. In addition, the comparative steel No. In Comparative Steel No. 6, Comparative Example No. 6 in which early fracture occurred due to the destruction of the surface starting point, on the other hand, was excessively contained Al. 7 and comparative steel No. 7 having a low Si content. In No. 11, breakage of the internal matrix causes breakage of the fish eye.

Example 2 No. 1 shown in Table 1 Using the spring manufactured from the steel material of No. 1 and changing only the nitriding conditions, the springs shown in Table 4 were manufactured, and the fatigue test was performed for each of them in the same manner as in Example 1.
The results are shown in Table 4 together with the surface hardness of the spring.

[0026]

[Table 4]

As is clear from Table 4, No. 1 having a low Hs. No. 1c has a short fatigue life due to surface breakage, and Hi is too high. 1d is 2 × 10 ⁷ due to the destruction of the origin of inclusions.
Breakage occurs less than the number of times. In addition, No. 1
In the case of e, fatigue fracture has occurred from the internal matrix and the life is also short. On the other hand, No. 1 that satisfies all of the prescribed requirements of the present invention. 70 × 55 kg for 1a and 1b
It has a fatigue life of 5 × 10 ⁷ times or more at a stress of f / mm ² , and it can be seen that the fatigue strength is remarkably improved as compared with the conventional material.

[0028]

The present invention is constituted as described above, and by specifying the composition of the steel material to be used and the size of the non-metallic inclusions in the cross section in the rolling direction, the nitrided spring having high fatigue strength is obtained. You can get the steel material that gives
By nitriding this steel material, it is possible to provide a nitriding spring having very excellent fatigue characteristics as a valve spring for an internal combustion engine.

Claims (2)

[Claims] 1. C: 0.3 to 0.7% (meaning weight%; the same applies hereinafter) Si: 0.8 to 4% Mn: 0.2 to 1.5% Cr: 0.4 to 3 % Sol. While containing Al: 0.02-0.7%,
Oxygen content is 20ppm or less, and V: 0.05-
0.5% Nb: 0.05 to 0.5% Mo: 0.05 to 0.5% Ni: 1 to 3 selected from the group consisting of 0.1 to 3%
Contains at least one species and consists of the balance Fe and inevitable impurities,
A steel for nitrided spring having excellent fatigue strength, characterized in that the size of the non-metallic inclusions in the rolling direction cross section of 3600 mm ² including the center of the steel material is 15 μm or less. 2. A steel spring satisfying the requirements of claim 1 is nitrided, and the Vickers hardness Hs within 10 μm from the surface is 900 or more, and the internal Vickers hardness Hi is 450 to 570. A nitride spring with excellent fatigue strength.