JP3405391B2 - Oil tempered wire for spring and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oil temper wire, and more particularly to an oil temper wire for a high strength and high fatigue resistance spring used in an automobile engine valve spring, a transmission and a clutch.

[0002]

2. Description of the Related Art In response to the reduction in fuel consumption of automobiles, the size and weight of automobile engines and transmissions have been reduced in recent years, and the stress applied to engine valve springs and springs used in transmissions has become severer year by year. Become,
The spring material used is required to have higher strength. These engine valve springs and transmission springs are mainly made of silicon chrome steel oil tempered wire for valve springs and high strength silicon chrome steel oil tempered wire in which the carbon content is increased or a carbide forming element is added. .

The oil tempered wire secures strength and toughness by making the metal structure into tempered martensite in the quenching and tempering process (oil tempering process). The temperature history in this quenching and tempering process is as shown in FIG. 1, but during this quenching, a martensite transformation occurs from the high temperature austenite phase to produce a martensite phase. At that time, the oil tempered wire for spring has a characteristic of its component value,
Even if all the austenite phase is not transformed into the martensite phase, even if it is cooled to a room temperature state (A region in FIG. 1), a few percent of the austenite phase by volume remains inevitably. And this austenite phase is called retained austenite.

When the stress acts on the retained austenite itself, stress-induced martensitic transformation is caused by the stress, and the amount of deformation may be increased by that amount, so that the toughness is not directly adversely affected. Rather, for example,
As shown in 7-322266, JP-A-3-162550, Spring Technical Research Group 1997 Spring Lecture Lecture Proceedings P13-16, the amount of retained austenite is positively increased to induce stress-induced transformation of retained austenite. It has been proposed to improve toughness by utilizing it.

Further, it is known that a part of the retained austenite which is inevitably or intentionally generated during quenching is decomposed into martensite during tempering. That is, FIG.
In the region B, part of the retained austenite decomposes into martensite. However, since these martensites are tempered at the same time they are decomposed, they do not adversely affect the toughness.

[0006]

However, the inventors have found that retained austenite decomposes during cooling after tempering (region C in FIG. 1) to produce martensite of 0.5% or more in volume ratio. It was found that this martensite adversely affects the toughness because it is not tempered.

Until now, for example, as shown in Kadoma and Sudo, "Steel Materials and Their Heat Treatments" (Japan Institute of Metals), P134-136, steels with a low martensite transformation start temperature (Ms point) were formed during quenching. It is known that part of retained austenite undergoes martensitic transformation during cooling after tempering. However, this relates to steel types having a carbon content of 0.95% or more or Cr or Ni of 2% or more, such as high speed steel and die steel. Different composition and Ms
It is not known that residual austenite undergoes martensitic transformation during cooling after tempering in spring steels whose points are not low and untempered martensite exists.

Further, as a method of reducing the amount of retained austenite, there is a method of rapidly quenching with water after oil quenching as shown in JP-A No. 4-311529, but this method also produces during quenching after tempering. The amount of martensite cannot be 0.5% or less by volume.

Therefore, the main object of the present invention is to provide untempered martensite present in an oil tempered wire in a volume ratio of 0.5.
% Or less, and to provide an oil tempered wire for springs having high strength and high toughness.

[0010]

The present invention was made on the basis of the above findings, and is characterized in that the amount of martensite existing in the untempered state is 0.5% or less by volume.

This oil-tempered wire has a weight ratio of C: 0.
5 to 0.9%, Si: 0.8 to 3.0%, Mn: 0.4 to 1.0%, C
r: A wire rod containing 0.4 to 1.0% and subjected to quenching and tempering treatment.

Further, as a chemical component, V: 0.05 by weight%.
~ 0.6%, Mo: 0.05% to 0.5% and Nb: 0.05 to 0.2
You may add at least 1 sort (s) selected from%.

In the method for producing the oil tempered wire of the present invention, C: 0.5-0.9%, Si: 0.8-3.0%, M by weight%.
A method for manufacturing a spring oil-tempered wire, in which a wire containing n: 0.4 to 1.0% and Cr: 0.4 to 1.0% is quenched and tempered, and the wire is cooled to -50 ° C or less before tempering. It is characterized by doing.

As a concrete procedure for cooling the wire rod to -50 ° C or lower, a cooling step is provided between the quenching at room temperature and the tempering to cool the wire rod to -50 ° C or lower. It is possible to cool the wire to -50 ° C or lower by controlling the temperature to 50 ° C or lower.

Still another manufacturing method is a method for manufacturing an oil tempered wire for spring, in which a wire having the same chemical composition as described above is subjected to quenching and tempering treatment. Is performed, and the martensite produced by the decomposition of residual austenite during cooling after the quenching and tempering treatment is tempered.

The reasons for limiting the steel composition and the reasons for specifying the manufacturing conditions in the steel wire of the present invention and the manufacturing method thereof will be described below. <C: 0.5 to 0.9 wt%> C is an essential element for increasing the strength of the steel wire, but if it is less than 0.5%, sufficient strength cannot be obtained, and conversely if it exceeds 0.9%, cementite is formed in the grain boundaries. This is because they precipitate and significantly reduce toughness.

<Si: 0.8 to 3.0% by weight> Si is a solid solution element in the steel as a substitutional element and is an element effective for increasing the strength and heat resistance of the steel. This is because if it is less than 0.8%, the effect is not sufficient, and if it exceeds 3.0%, the solid solution cannot be completely dissolved and the cold workability is significantly impaired.

<Mn: 0.4 to 1.0% by weight> Mn improves the hardenability of steel and fixes S in the steel to prevent its harm, but less than 0.4% has almost no effect and exceeds 1.0%. And toughness decreases.

<Cr: 0.4-1.0%> Cr is an element effective for enhancing the hardenability as well as Mn, enhancing the softening resistance during tempering, and enhancing the strength. This is because if it is less than 0.4%, there is no effect, and if it exceeds 1.0%, solid solution of carbide is suppressed and strength is lowered.

<V: 0.05 to 0.6%> V is an element that forms carbides during quenching and tempering to increase the softening resistance, but
If it is less than 05%, the effect is extremely limited, and if it exceeds 0.6%, the carbide formed becomes coarse and the toughness is reduced.

<Mo: 0.05-0.5%> Mo is an element that forms carbides similarly to V and increases temper softening resistance.
This is because if it is less than 0.05%, there is no effect, and if it exceeds 0.6%, the formed carbide becomes coarse and the toughness is remarkably lowered.

<Nb: 0.05 to 0.2%> Nb is an element that forms carbides and increases temper softening resistance, but 0.05%
If it is less than 0.2%, the effect is hardly present, and if it exceeds 0.2%, the carbide formed is coarsened and the toughness is significantly lowered.

<Cooling of wire rod to −50 ° C. or lower> The amount of retained austenite is decreased by lowering the temperature of the wire rod during quenching or after tempering until tempering, and the amount of martensite generated during cooling after tempering is reduced. Can be reduced, but the temperature reached by the wire rod is -50
If the temperature is higher than 0 ° C, the amount of retained austenite is not sufficiently reduced, and martensite exceeding 0.5% by volume is generated during cooling after tempering.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION A sample having the chemical composition shown in Table 1 was melted, rolled, heat-treated and drawn into a wire having a wire diameter of 3.0 mmφ, and then quenched and tempered under the conditions shown in Table 2. -70 to-for the cooling medium for quenching and the cooling medium for cooling after quenching.
A liquid mixture of ethyl alcohol and dry ice was used at 30 ° C, and liquid nitrogen was used at -196 ° C. Cooling after quenching was performed by quenching to room temperature and then providing a cooling step before the first tempering. The second tempering corresponds to the "second tempering" of claim 4.

For each sample, the tensile strength was measured, the amount of retained austenite was measured by X-ray diffraction,
The amount of untempered martensite in the steel wire was measured by a transmission electron microscope. Further, as a toughness evaluation, a flaw having a depth of 30 μm and a tip portion of 0.2 mmR was formed on the surface of the wire, and a Charpy impact test was performed with the flaw on the outside to measure the absorbed energy. The results are shown in Table 3.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

As shown in Table 3, all the examples (other than the comparative material) have the same strength level as the comparative material, but the untempered martensite content is 0.5% or less by volume. And Charpy impact value absorbed energy is 30kg ・ m / c
It is m ² or more, and clearly shows excellent toughness.

[0030]

As described above, according to the present invention, it is possible to provide an oil tempered wire for a spring having high strength and high toughness.

[Brief description of drawings]

FIG. 1 is a graph showing a temperature history in a quenching and tempering process.

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Claims (4)

(57) [Claims] 1. C: 0.5-0.9% by weight%, Si: 0.8-
An oil-tempered spring wire made by quenching and tempering a wire rod containing 3.0%, Mn: 0.4 to 1.0%, and Cr: 0.4 to 1.0%. Tempered from a portion of retained austenite formed during quenching. An oil tempered wire for a spring, characterized in that the amount of untempered martensite produced during the subsequent cooling is 0.5% or less by volume. 2. V: 0.05-0.6% by weight%, M
The oil tempered wire for a spring according to claim 1, wherein at least one selected from o: 0.05% to 0.5% and Nb: 0.05 to 0.2% is added. 3. C: 0.5-0.9%, Si: 0.8-by weight%
A method for producing an oil tempered wire for a spring, in which a wire material containing 3.0%, Mn: 0.4 to 1.0%, and Cr: 0.4 to 1.0% is subjected to quenching and tempering treatment, wherein the wire material is -50 ° C before tempering treatment. A method for manufacturing an oil tempered wire for a spring, which comprises cooling as follows. 4. C: 0.5-0.9%, Si: 0.8-by weight%.
A method for producing an oil tempered wire for spring, which comprises subjecting a wire containing 3.0%, Mn: 0.4 to 1.0%, Cr: 0.4 to 1.0% to quenching and tempering treatment, the method comprising: The method for producing an oil-tempered wire for a spring, which comprises performing the tempering treatment of 1. and tempering martensite generated by decomposition of retained austenite during cooling after the quenching and tempering treatment.