JPH06172847A - Production of high strength spring steel - Google Patents

Abstract

PURPOSE:To produce a high strength spring steel capable of being subjected to both of ordinary hot spring forming stage and cold spring forming stage and excellent in fatigue strength and settling resistance. CONSTITUTION:Oil quenching is applied to a steel which has a composition consisting of 0.30-0.75% C, 1.0-4.0% Si, 0.5-1.5% Mn, 0.1-2.0% Cr, <=2.0% Ni, and the balance Fe with inevitable impurities and further containing, if necessary, 0.05-0.5% V and/or 0.05-2.0% Mo. After the amount of retained austenite after oil quenching is regulated to <10%, tempering is done. By this method, the high strength spring steel excellent in fatigue strength and settling resistance can be obtained. By controlling the contents of C, Si, and Ni so that 35XC(%)+2XSi(%)+Ni(%)<23% is satisfied, the amount of retained austenite after oil quenching can easily be controlled to <10% by means of ordinary spring forming stage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing spring steel, and more particularly to a method for manufacturing high strength spring steel suitable for suspension coil springs for automobiles and the like.

[0002]

2. Description of the Related Art Spring steels such as valve springs and suspension springs used in internal combustion engines of automobiles and the like are required to have high strength with the demand for weight reduction and speedup. As a result, it has been desired to develop a high strength spring steel having excellent fatigue strength and sag resistance.

In order to manufacture a spring using this kind of spring steel, usually, in the case of hot forming, hot coiling is carried out, followed by quenching and tempering, and shot peening for setting. In the case of cold forming, quenching and tempering are performed to adjust the temperature, then cold coiling and shot peening are used for setting. In this way, quenching and tempering processes are always included in the spring forming process of cold forming springs and hot forming springs, so if the addition amount of alloying elements such as Ni is increased with the aim of achieving high strength and toughness, it will remain. Austenite remains, which is harmful to fatigue strength.

In this respect, as the applicant has previously proposed,
(JP-A-60-89553), in the case of a cold-formed spring, the amount of Ni added is increased to intentionally leave ductile leaving residual austenite by quenching, and using this, cold coiling is performed after quenching. After that, there is a method to eliminate residual austenite by tempering. However, this method is different from the normal manufacturing process of cold forming springs,
Also, it cannot be applied to hot formed springs.

The present invention can be applied to both a normal hot spring forming process and a cold spring forming process, and a method for producing a high strength spring steel excellent in fatigue strength and sag resistance. It is intended to provide.

[0006]

In order to solve the above-mentioned problems, the inventor of the present invention has earnestly studied to produce a high-strength spring steel which can be applied to both normal hot and cold spring forming processes. As a result of repeating the above, in order to obtain high-strength spring steel with excellent fatigue strength by the normal spring forming process, oil quenching and tempering should be performed under the appropriate chemical composition adjustments. The present invention has been made based on the finding that this is possible by limiting the amount of retained austenite generated to less than 10%.

That is, according to the present invention, C: 0.30 to 0.7.
5%, Si: 1.0 to 4.0%, Mn: 0.5 to 1.5%, C
r: 0.1 to 2.0% and Ni ≤ 2.0%, and V: 0.05 to 0.5% and Mo: 0.05 to 2.0 as required.
% Of steel, the balance of which is Fe and unavoidable impurities, is subjected to oil quenching to reduce the amount of retained austenite generated after oil quenching to less than 10%, and then tempering. The gist is a method for producing a high-strength spring steel excellent in fatigue strength and fatigue resistance, which is characterized by being applied.

[0008]

The present invention will be described in more detail below. First, the reasons for limiting the chemical composition of the steel of the present invention will be shown.

C is an element effective for increasing the strength, but if it is less than 0.30%, the strength required as a high strength spring cannot be secured, but if it exceeds 0.75%, reticulated cementite appears. Since it becomes easy and the fatigue strength of the spring is impaired, the range is made 0.30 to 0.75%.

Si is an element effective as a solid solution in ferrite to increase the strength and improve the sag resistance of the spring, and for that purpose, 1.0% or more is required. But,
If it exceeds 4.0%, the toughness deteriorates and free carbon may be generated by heat treatment, so the content is made 1.0 to 4.0%.

Mn is an element effective as a deoxidizing element and for improving hardenability, and for that purpose, 0.5% or more is necessary. However, if it exceeds 1.5%, the hardenability becomes excessive and the toughness deteriorates, and at the same time, it deforms at the time of oil quenching, so the range is 0.5 to 1.5%.

Ni is an element necessary for improving the toughness after oil quenching and tempering, but if added in excess of 2%, the amount of retained austenite generated after oil quenching increases and fatigue strength decreases. Therefore, it is added at 2% or less.

In addition to the above essential components, one or two kinds of V and Mo can be added in appropriate amounts, if necessary, to improve the spring characteristics.

In particular, V is an element which has a great effect of refining crystal grains during low temperature rolling, is expected to improve spring characteristics and reliability, and also contributes to precipitation hardening during oil quenching and tempering. Mo is an element effective in improving the sag resistance, and when added, V: 0.05 to 0.5%, M
o: The range is 0.05 to 2.0%. If V is increased beyond the upper limit, toughness is deteriorated and spring characteristics are deteriorated.
Further, if Mo is increased beyond the upper limit, complex carbides that are not dissolved in austenite are formed, and if the amount increases and becomes large lumps, it causes the same damage as non-metallic inclusions, thus reducing fatigue strength. There is a fear that it is not preferable.

In addition, unavoidable impurities [O],
It is preferable that [N] etc. be as small as possible. Particularly, [O] forms oxide-based inclusions, which easily become the starting point of fatigue fracture, so it is desirable to limit the content to 0.0010% or less. Since TiN-based inclusions form and lead to a decrease in fatigue strength, it is desirable to limit the content to 0.005% or less.

The spring steel having the above chemical composition is subjected to a normal hot or cold spring forming process including an oil quenching / tempering process to form a spring. At that time, in order to obtain a high-strength spring having excellent spring characteristics such as fatigue strength, the amount of retained austenite generated after oil quenching must be less than 10%, and 10%
If it is less than the above, the fatigue strength is not substantially affected.

The production conditions of the oil quenching / tempering process are not particularly limited. For example, when the amount of retained austenite in the oil-quenched state is 10% or more, after the oil-quenching, the sub-zero treatment is performed to reduce the amount of retained austenite
Although it is possible to reduce the amount to less than%, it is not preferable considering the complexity of the mass production process of the spring. In a preferred embodiment, the content of C, Si and Ni is regulated to 35 × C (%) + 2 × Si (%) + Ni (%) <23% so that the residual after oil quenching in a normal spring forming process. The amount of austenite generated can easily be made less than 10%.

Next, an embodiment of the present invention will be shown.

[0019]

Example For steels having the chemical composition (wt%) shown in Table 1, a tensile test piece, a fatigue test piece and a fatigue test piece were cut out from a 16 mmφ rolled wire rod manufactured by a conventional method.
After oil quenching at ℃ × 30 min oil cooling, tempering was performed at 350 ° C × 0.1 hr to finish processing. In addition, all the test pieces were conditioned so as to be HRC55. As a result, the Y value and the durability limit are shown in Table 1 together, and the sag resistance is shown in FIG. The amount of residual austenite generated after oil quenching and the amount of residual shear strain were also investigated, and are also shown in Table 1.

The sag resistance is shown in FIG.
A weight-type torsion creep tester (maximum torque 25 kgf ・ m)
Using a test piece with the dimensions and shape shown in Fig. 4, Test temperature: 80 ℃ Test time: 72hr Load stress: 110kgf / mm²  Shear prestrain: 0.1% Hardness: Tested under the test conditions of HRC55. In FIG. 3, 1 is a test piece, 2
Is a specimen holder, 3 is a load arm, 4 is a dial game
D, 5 is a weight, and 6 is a jack.

FIG. 1 is a plot of the relationship between the endurance limit shown in Table 1 and the amount of retained austenite produced after oil quenching for each test steel. From this, the amount of retained austenite produced after oil quenching is plotted. It was found that the durability limit was remarkably lowered when the ratio was 10% or more, and all of the steels of the present invention had an excellent fatigue strength of less than 10%.

The relationship between the contents of different impurities [O] and [N] and the durability strength of a part of the test steel was examined, and the test steel No. 2 shown in Table 1 was [O]: 0.0020
%, [N]: 0.0100%, the durability limit was 75 kgf / mm ² , whereas [O]: 0.0006%, [N]: 0.0
The durability limit was improved to 79 kgf / mm ² at 045%, and similarly, No. 17 shown in Table 1 was [O]: 0.0018%, [N]:
While endurance limit was 84kgf / mm ² at 0.0100%, [O]: 0.0007%, [N]: the endurance limit at 0.0050% was improved to 88 kgf / mm ^2.

A suspension spring, which has a great weight in design due to its sag resistance, has recently been attracting attention because of its warm sag characteristics. Therefore, a torsion creep test was conducted under the above test conditions. As shown in Table 1 and FIG. 2, the steel of the present invention has a shear leap strain of 72 hours after the current J
Much less than IS SUP7 material and showed excellent sag resistance.

[0024]

[Table 1]

[0025]

As described in detail above, according to the present invention,
Since the chemical composition of high-strength spring steel is properly adjusted and the amount of retained austenite generated after quenching in the oil quenching / tempering process is regulated to less than 10%, it is used for normal hot and cold spring forming processes. High strength springs such as valve springs and suspension springs can be manufactured, mass production is possible, and excellent fatigue strength and sag resistance can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a durability limit and an amount of retained austenite generated after quenching.

FIG. 2 is a diagram showing the shear creep strain of the steel of the present invention in comparison with the current material.

FIG. 3 is a diagram showing an outline of a weight-type torsion creep testing machine.

4 (a) and 4 (b) are diagrams showing the shape dimension (mm) of a torsion creep test piece, FIG. 4 (a) is a side view, and FIG. 4 (b) is a sectional view.

Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location C22C 38/40 38/46

Claims (3)

[Claims] 1. In% by weight (hereinafter the same), C: 0.30 to.
0.75%, Si: 1.0-4.0%, Mn: 0.5-1.5
%, Cr: 0.1 to 2.0% and Ni ≦ 2.0%, the balance of which is Fe and unavoidable impurities, the oil is subjected to oil quenching, and the amount of residual austenite generated after the oil quenching is 10 %, And then tempering is performed, and a method for producing a high-strength spring steel excellent in fatigue strength and fatigue resistance. 2. The amount of retained austenite is 35 × C
The method according to claim 1, wherein the amount is less than 10% by regulating (%) + 2 × Si (%) + Ni (%) <23%. 3. The method according to claim 1, wherein the steel further contains one or two of V: 0.05 to 0.5% and Mo: 0.05 to 2.0%. .