JPH0713269B2 - High fatigue strength spring manufacturing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for improving fatigue characteristics of a spring, and more specifically, in manufacturing a valve spring for an engine, a leaf spring for a vehicle, or a suspension spring, performance of use, particularly The present invention relates to a spring manufacturing method capable of improving fatigue strength.

(Prior Art) High fatigue strength and sag resistance are important for springs used in automobiles and the like. It is now common sense to perform shot peening treatment on fatigue strength. this is,
This is to apply compressive residual stress to the surface layer of the spring by shot peening to reduce the effective value of the external load. Since the sag is a kind of relaxation phenomenon, it is attempted to improve the sag resistance by preloading the spring by overloading the spring in advance, that is, by carrying out setting.

Recently, automotive springs have been required to have higher performance. Therefore, there is an idea to use steel that is stronger than the spring steel specified in JIS to cope with this requirement. For example, “Iikubo, et al .: Electric steelmaking, 57 (19
86) No. 1, 33 ”and so on. That is, it is intended to use a steel having a higher alloy than the JIS spring steel as a high fatigue strength spring steel. In this type of high alloy steel, the higher the alloy, the more retained austenite, as described in the same document. If this amount is small, it does not seem to affect the fatigue strength of the spring.However, if a high alloy is used to strengthen the steel, the retained austenite will increase and the fatigue strength of the spring will be affected. become. Further, when the strength of the spring is increased in this way, the spring becomes hard and it becomes difficult to obtain a sufficient shot peening effect.

When mechanical parts are carburized, carbonitrided and hardened or high C steel is simply hardened, austenite remaining only on the surface layer is subjected to sub-zero treatment (immersion in liquid nitrogen for 30 minutes) to transform into martensite. Japanese Patent Application No. 1-274014 describes that the compressive residual stress generated by the above is used as a means for improving the fatigue strength of mechanical parts for carburizing or carbonitriding. In this case, a special means of sub-zero is required, and when it is applied to a spring, it is difficult to satisfy the condition of only the surface layer.

(Problems to be Solved by the Invention) The present invention also aims to increase the fatigue strength of springs, and as a means to increase the strength of springs, it does not differ from the conventional idea, but the steel material used for that purpose The high alloying of Al and the martensite conversion of retained austenite have the above new problems. The present invention intends to provide a method for improving fatigue characteristics of a spring that overcomes these problems.

(Means for Solving the Problem) The gist of the present invention is that C: 0.4 to 0.65%, Si: 1.0
~ 2.5%, Mn: 0.3-1.5%, Cr: 0.2-1.5%, Ni:
2.5% or less, Mo: 0.6% or less, Cu: 0.6% or less, V: 0.6% or less, Nb: 0.06% or less or Al: 0.05% or less, and one or more of them are added, and the balance is Fe and inevitable impurities. Spring is formed by using the steel which is made of austenite, quenched from the austenite state into a coolant at room temperature to 200 ° C, and martensite and 7
% Of retained austenite, the structure is set at room temperature to 400 ° C., and shot peening is performed if necessary.

(Operation) In order to realize the high fatigue strength of the spring, the present inventors formed a spring using alloy steel and focused on positively utilizing the unavoidable austenite remaining after quenching, Various studies were conducted.

The present invention relates to the above-mentioned Japanese Patent Application No. 1-274014, in which an appropriate amount of austenite is retained in a quenched spring, and the compressive residual stress generated when the austenite is transformed into martensite improves the fatigue strength of the spring. Use it as a means. But,
Since the spring has a relatively small wall thickness, it is difficult to martensite only the retained austenite in the surface layer by the subzero treatment. On the other hand, since a spring requires a settling treatment peculiar to the spring in order to improve the sag resistance, it was considered to transform the retained austenite into martensite during this treatment.

In order to embody the above idea, the formed spring must have a martensitic structure containing 7% or more of retained austenite at the time of quenching. In addition, a spring product requires a minimum required toughness. These two
From this point, the chemical composition and quenching conditions of the applied steel material are limited. These limiting conditions will be described below.

The important chemical components for forming the necessary retained austenite are C, Si, Mn, Cr and Ni, but appropriate amounts of these elements must be combined depending on the target strength and toughness.

If C is less than 0.4%, sufficient strength and retained austenite cannot be obtained, but if it exceeds 0.65%, toughness cannot be obtained.

If Si, Mn, and Cr are less than 1.0%, 0.3%, and less than 0.2, respectively, sufficient strength and retained austenite cannot be obtained, but if they exceed 2.5%, 1.5%, and 1.5%, respectively, toughness cannot be obtained. .

Ni is effective to retain the toughness and austenite, so 2.5% or less is added if necessary.

Mo and Cu are added in an amount of not more than 0.6%, if necessary, in order to increase the retained austenite and improve the corrosion resistance as well as contributing to the strength.

V is an element useful for refining the structure together with strength, and Nb and Al are elements useful for refining the structure, and are 0.6% or less, 0.06% or less, and 0.0%, respectively, as necessary.
Add up to 5%.

Incidentally, one or more of Ni, Mo, Cu, V, Nb, and Al are selectively added for the above purpose, if necessary.

A steel having such a chemical composition is quenched from an austenite state in a coolant at room temperature to 200 ° C. here,
The normal temperature to 200 ° C. is necessary to secure the necessary amount of retained austenite, and a large amount of C, Ni, etc. may be the normal temperature coolant, but if these elements are small, the temperature of the coolant is Need to increase. If the temperature exceeds 200 ° C, the amount of retained austenite may be unnecessarily high.
The upper limit was 200 ° C.

Of the retained austenite generated under these conditions, the retained austenite present on the surface layer of the spring is martensiticized during the setting at room temperature to 400 ° C,
Apply compressive residual stress. At this time, if the initial retained austenite amount is less than 7%, the effect is not sufficient. The amount of retained austenite is measured by the X-ray diffraction method. Further, the setting temperature is set to room temperature to 400 ° C. because the retained austenite existing in the surface layer of the spring is converted to martensite as much as possible, and depending on the C content of the material, the function of the conventional tempering treatment can be combined with the setting process. This is to improve the original sag resistance of the treatment, but 40
When the temperature exceeds 0 ° C, the relaxation of the added compressive residual stress becomes remarkable.

In the spring manufactured through the above steps, if sufficient compressive residual stress exists in the surface layer of the spring, the shot peening treatment is not always necessary. However, since there is no guarantee that the retained austenite during quenching is uniformly distributed in the outermost layer and that it is uniformly martensitic, it is desirable to carry out the shot peening treatment as an auxiliary also in the present invention.

Hereinafter, the effects of the present invention will be described more specifically by way of examples.

(Example) A steel spring having a chemical composition shown in Table 1 was used to manufacture a coil spring having a spring diameter of 36 mm, a height of 55 mm, and an effective winding number of 4.5, using a stripped wire rod having a diameter of 4 mm. After austenitizing for 10 minutes, the steel was quenched in oil with various oil temperatures (Table 2) changed for each steel type and set at each temperature (Table 2). After that, a shot peening treatment was also performed on some of them. For these, a fatigue test was carried out at τ = 80 ± 50 kgf / mm ² , and the fatigue life was investigated. The results are shown in Table 2. The comparative springs shown in the table have the same shape based on the conventional manufacturing method using SAE9254 steel.

From the above results, it is understood that according to the present invention, a spring having an excellent fatigue life can be obtained as compared with the fatigue lives of the comparative spring and the conventional spring.

(Effects of the invention) As described above, according to the present invention, the chemical composition of the steel material and the cooling conditions during quenching are limited so as to secure the retained austenite in advance, and an appropriate amount of austenite is retained,
This is converted to martensite during the setting process, and the remarkable compressive residual stress generated at that time can significantly improve the fatigue characteristics of the spring, so the present invention is extremely useful industrially.

Claims (2)

[Claims] 1. C: 0.4 to 0.65%, Si: 1.0 to 2.5%, Mn: 0.3 to
1.5%, Cr: 0.2-1.5%, Ni: 2.5% or less, Mo: 0.6
% Or less, Cu: 0.6% or less, V: 0.6% or less, Nb: 0.06% or less, or Al: 0.05% or less, or one or more of them are added, and the balance is Fe and inevitable impurities. Is formed, quenched from austenite state in a coolant at room temperature to 200 ° C, and made into a structure containing martensite and 7% or more of retained austenite, which is characterized by being set at room temperature to 400 ° C. Fatigue strength spring manufacturing method. 2. C: 0.4 to 0.65%, Si: 1.0 to 2.5%, Mn: 0.3 to
1.5%, Cr: 0.2-1.5%, Ni: 2.5% or less, Mo: 0.6
% Or less, Cu: 0.6% or less, V: 0.6% or less, Nb: 0.06% or less, or Al: 0.05% or less, or one or more of them is added, and a spring is formed by using steel composed of the balance Fe and unavoidable impurities. Molding, quenching from an austenite state into a coolant at room temperature to 200 ° C, and forming a structure containing martensite and 7% or more of retained austenite at room temperature to 400 ° C, and then shot peening A method for manufacturing a high fatigue strength spring characterized by: