JP2004323912A - High strength coil spring, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coil spring (a high strength coil spring) of excellent set-in resistance and durability which is suitably applied to a valve spring for an automobile sustained under progressively high stress level in recent years. <P>SOLUTION: A wire of the tensile strength of ≤ 1,600 MPa is formed of steel as a material, the wire is cold-formed into a coil, and hardened/tempered so that residual austenite amount is ≤ 3%. Steel containing, by mass, 0.50-0.90% C and 0.80-2.10% Si is preferably used for the material. The heat treatment is preferably induction hardening and induction tempering. Quenching is preferable even after tempering. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength coil spring having a relatively small wire diameter (about 5 mm or less in wire diameter). Specifically, such as a valve spring used in an automobile engine, a high-strength coil spring having high sag resistance and fatigue resistance in a situation where it is used while repeatedly receiving stress under a high temperature environment, The present invention relates to a manufacturing method and spring steel used as the material.
[0002]
[Prior art]
Coil springs used in engine valve springs, etc., must have both sag resistance and fatigue resistance in order to achieve higher output of automobile engines and lighter vehicle bodies. Highly required.
[0003]
In order to obtain a coil spring having both high sag resistance and fatigue resistance, it is necessary to select an appropriate material as spring steel, select an appropriate process, and sequentially perform the steps. is there.
[0004]
To form a coil spring, there are hot forming in which coiling is performed by heating at a high temperature, and cold forming in which coiling is performed at room temperature. Hot forming is easy to process, but cannot improve the accuracy of dimensions and shapes. Cold forming is easy to obtain the desired shape and dimensions, but is difficult to process. In general, in consideration of workability, a material having a small wire diameter is formed cold, and a material having a large wire diameter is formed hot.
[0005]
In addition, a high-strength coil spring such as a valve spring requires a microstructure having a required hardness and heat treatment (quenching / tempering) is indispensable. Depending on the order of coil processing and heat treatment, the following two methods are used. is there.
(1) A method in which heat treatment is performed after coil formation. As described above, coil forming includes hot and cold.
(2) A method of performing coil forming after heat treatment. In this case, since the heat treatment has been performed, the coil forming is performed only in a cold state.
[0006]
The method (1) is somewhat difficult to obtain an accurate spring shape and dimensions, but has the advantage that a high strength spring can be obtained by heat treatment. On the other hand, in the method (2), a spring having a precise shape and dimensions can be formed, but it is difficult to obtain a high-strength spring.
[0007]
[Problems to be solved by the invention]
As described above, when the main purpose is high strength, the method (1) of performing heat treatment after coil forming is desirable, but in the case of a spring, retained austenite is a major problem. That is, the heat treatment of quenching steel means heating the steel to the austenite range and rapidly cooling it to transform it into hard martensite.However, when the cooling rate is insufficient or depending on the steel composition, the austenite Some of them do not transform into martensite and remain after quenching. This is called retained austenite. In the case of a spring, this has a bad effect such as a decrease in fatigue strength and an increase in set during use.
[0008]
The retained austenite is transformed into martensite by processing and disappears. In the method (2), since the coil is formed after the heat treatment, the retained austenite disappears by the coil forming process. However, in the method (1), the heat treatment is performed after the coil is formed. Therefore, although the retained austenite generated by the quenching is partially decomposed by the tempering, most of the retained austenite remains as it is in the product spring, resulting in durability. It is a major cause of deterioration and sag (Non-Patent Document 1, Patent Document 1).
[0009]
[Non-patent document 1]
Spring Transactions, No. 43 (1998) pp. 1-12 “Effect of retained austenite on fatigue strength of oil-tempered wire”
[Patent Document 1]
JP-A-9-143621
At present, the design stress (operating stress) of the spring is gradually increasing to reduce the weight, and it is desirable to eliminate as much as possible the factors that reduce the sag resistance and durability.
[0011]
The present invention has been made in order to solve such a problem, and an object of the present invention is to provide an anti-sagging which can be suitably used particularly for a valve spring of an automobile in which high stress has recently been progressing. An object of the present invention is to provide a coil spring (high-strength coil spring) having excellent properties and durability.
[0012]
[Means for Solving the Problems]
The method for manufacturing a high-strength coil spring according to the present invention, which has been made to solve the above-described problem, is to produce a wire having a tensile strength of 1600 MPa or less by drawing a wire using steel as a raw material. And then quenching and tempering so that the amount of retained austenite is 3% or less.
[0013]
As the material, it is desirable to use steel containing 0.50 to 0.90% of C and 0.80 to 2.10% of Si in mass ratio. Further, it is desirable to use high-frequency heating for heating during quenching and tempering. In addition, it is desirable to rapidly cool after tempering (of course, after quenching).
[0014]
After the heat treatment, it is desirable to perform shot peening, particularly, two or more shot peening.
[0015]
When performing shot peening in two or more stages, it is desirable to use hard type shot grains in at least one stage of shot peening.
[0016]
The internal hardness of the high-strength coil spring manufactured by the above method is desirably HV600 to 650.
[0017]
And it is desirable to set a fracture toughness value to 70 to 130 kgf / mm3 / ² .
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
The reasons for setting the upper and lower limits of the carbon and silicon contents of steel, which is the material of the high-strength coil spring according to the present invention, are as follows.
[0019]
First, the carbon content was set to 0.50 to 0.90%.
Carbon is the element that has the greatest effect on the strength of steel, and must be contained at least 0.50% in order to provide sufficient strength to set and have sufficient sag resistance and durability as a valve spring. Must. However, when the content exceeds 0.90%, cold forming becomes difficult. In addition, a decrease in durability due to a decrease in toughness also poses a problem.
[0020]
Next, the silicon content was set to 0.80 to 2.10%.
Silicon, like carbon, is an element that gives strength to steel, but in the case of a spring, the effect of imparting sag resistance is important. If the silicon content is less than 0.80%, sufficient set resistance cannot be imparted to the spring. However, silicon is also an element that promotes surface decarburization during heating. In the case of springs that are used with maximum stress on the surface, the most attention must be paid to the generation of surface decarburization during heat treatment. If the silicon content exceeds 2.10%, the possibility of surface decarburization cannot be excluded when heat treatment (quenching and heating) is performed under the above conditions.
[0021]
In the method of manufacturing a high-strength coil spring according to the present invention, a steel rod having a tensile strength of 1600 MPa or less is first manufactured using steel having the carbon content and the silicon content set in appropriate ranges as described above. The reason for setting the tensile strength to 1600 MPa or less is to facilitate the subsequent cold coil forming. At this time, in order to make the structure dense, it is desirable to perform wire drawing at a surface reduction rate of 50% or more.
[0022]
After the wire is cold-formed into a coil, quenching and tempering are performed. At this time, heat treatment is performed so that the amount of retained austenite is 3% or less. The heat treatment conditions (quenching temperature / time and tempering temperature / time) for this purpose are appropriately determined according to the composition of the material steel by conducting experiments in advance.
[0023]
The quenching and / or tempering is desirably performed by high-frequency heating for the following reasons. Since the high-frequency heating can raise the temperature quickly, it has the effect of minimizing the generation of surface decarburization and not allowing the crystal grains to grow inside. Further, there is an advantage that temperature control is relatively easy and heating can be performed with high accuracy. As a result, the residual austenite content after heat treatment can be reduced to 3% or less for most of the material steels satisfying the above component ranges.
[0024]
These effects and advantages are particularly useful in quenching, but also in tempering, by shortening the time and slightly raising the temperature in order to obtain the same effect (tempering hardness), the set resistance in the coil spring is reduced. This leads to a useful effect that the property can be improved.
[0025]
The quenching and tempering conditions are determined so that the internal hardness is HV600 to 650. If the internal hardness is less than HV600, the fatigue resistance required for a valve spring cannot be secured. On the other hand, if the heat treatment is performed under the condition that the internal hardness exceeds HV650, the residual austenite increases and the sag resistance decreases as described later. In addition, the toughness is reduced and even a small number of flaws easily cause breakage, so that the fatigue resistance is similarly reduced.
[0026]
Therefore, from the viewpoint of securing the toughness, it is desirable to set the fracture toughness value to 70 to 130 kgf / mm ^3/2 .
[0027]
After quenching and tempering in this way, shot peening is performed. In particular, by performing it in two or more steps, it becomes possible to apply a large compressive residual stress to the spring surface. Note that shot peening may be performed cold (room temperature) or warm (about 250 to 340 ° C.).
[0028]
As shot grains used for this shot peening, it is desirable to use hard type shot grains (hardness Hv750 or more) harder than normal grains in at least one stage. As a result, a larger compressive residual stress can be applied, and the surface roughness due to the heat treatment can be more reliably repaired.
[0029]
【The invention's effect】
The high-strength coil spring according to the present invention sets the carbon content and the silicon content in appropriate ranges, specifies the wire drawing conditions, and performs heat treatment so that the retained austenite does not exceed 3%. As a result, both the desired high sag resistance and fatigue resistance can be obtained.
[0030]
Therefore, it is extremely effective to use such a high-strength coil spring for a valve spring of an automobile engine used under repeated stress especially in a high temperature region.
【Example】
[0031]
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, in order to specify the optimal component content range of steel used as a material of the high-strength coil spring according to the present invention, the sag resistance of steel having different carbon and silicon contents was examined. The results are shown in FIGS. Residual shear strain was used as an index for evaluating sag resistance.
[0032]
As can be seen from FIG. 1, when the carbon content is in the range of C: 0.50 to 0.90%, the residual shear strain is 10 × 10 ⁻⁴ or less, and sufficient sag resistance is secured. I have.
[0033]
Further, as can be seen from FIG. 2, when the silicon content is in the range of 0.80 to 2.10% of Si, the residual shear strain is 10 × 10 ⁻⁴ or less. Rust is ensured.
[0034]
Next, using a steel having the chemical composition shown in FIG. 3 as a raw material, wire drawing was performed under the conditions of a surface reduction rate of 50% or more and a tensile strength of 1600 MPa or less.
[0035]
The wire drawn under such conditions was cold-formed into a coil, and then heat-treated by high-frequency heating. The heat treatment conditions are
Quenching: 900 ° C. × 10 seconds Tempering: 210 to 420 ° C. × 10 seconds.
[0036]
FIG. 4A shows a micrograph of the material before heat treatment (after drawing), and FIG. 4B shows a micrograph after quenching and tempering.
[0037]
Next, the internal hardness was variously changed by changing the heat treatment conditions, and the relationship between sag resistance and the amount of retained austenite at each hardness level was examined. The result is shown in FIG. As an index for evaluating sag resistance, residual shear strain was used as described above.
[0038]
As is clear from FIG. 5, when the retained austenite increases, the residual shear strain also increases. Therefore, in order to reduce the residual shear strain to 10 × 10 ⁻⁴ or less, the amount of retained austenite must be suppressed to 3% or less.
[0039]
Springs having various hardnesses were manufactured by using the same conditions as those in FIG. 5, and the results of the respective durability tests are shown in FIG. The test stress was 686 ± 530 MPa. As can be seen from FIG. 6, when the internal hardness is HV650 or more, breakage starts from inclusions due to a decrease in toughness.
[0040]
Next, the effect of shot peening on durability was examined. FIG. 7 shows the result. The test stress was also 686 ± 530 MPa. In FIG. 7, ● represents the result when shot peening was performed only once using normal shot grains having a diameter of 0.7 mm. Except for ●, the results in the case where two shots were performed using hard shot grains having a diameter of 0.6 mm in the first stage and a diameter of 0.3 mm in the second stage.
As can be seen from Figure 7, in order to ensure endurance over ¹⁰⁷ times performs shot peening two or more stages, it is desirable to use a hard grains that time (in particular the last stage).
[0041]
FIG. 8 shows the fracture toughness value at each hardness. As can be seen from FIG. 8, the drawing toughness improves the fracture toughness after quenching and tempering.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the carbon content of spring steel and residual shear strain.
FIG. 2 is a graph showing the relationship between the silicon content of spring steel and residual shear strain.
FIG. 3 is a table showing chemical components of a test material steel.
FIG. 4 is a micrograph (a) showing a microstructure of a test material steel before heat treatment (after drawing) and a micrograph (b) showing a microstructure after quenching and tempering.
FIG. 5 is a graph showing a relationship between residual austenite amount and residual shear strain.
FIG. 6 is a graph showing the relationship between internal hardness and the number of times of repeated use.
FIG. 7 is a graph showing a relationship between internal hardness and endurance count when shot peening conditions are changed.
FIG. 8 is a graph showing a fracture toughness value of a test material steel.

Claims (11)

A wire having a tensile strength of 1600 MPa or less is produced from a steel material by wire drawing, and the wire is cold-formed into a coil, and then quenched and tempered so that the residual austenite amount is 3% or less. A method for manufacturing a high-strength coil spring, comprising: 2. A high-strength coil spring according to claim 1, wherein the steel is a steel containing a mass ratio of C: 0.50 to 0.90% and Si: 0.80 to 2.10%. Method. The method for manufacturing a high-strength coil spring according to claim 1, wherein the quenching and / or tempering is performed using high-frequency heating. The method for manufacturing a high-strength coil spring according to any one of claims 1 to 3, wherein shot peening is performed after the quenching / tempering. The method for manufacturing a high-strength coil spring according to any one of claims 1 to 4, wherein the shot peening is performed in two or more stages. The method for manufacturing a high-strength coil spring according to claim 5, wherein hard type shot grains are used in at least one stage of shot peening. By using a steel containing a mass ratio of C: 0.50 to 0.90% and Si: 0.80 to 2.10% as a material, the tensile strength is increased by performing wire drawing at a surface reduction rate of 50% or more. A method for producing a high-strength coil spring, comprising: preparing a wire having a pressure of 1600 MPa or less, forming a coil of the wire in a cold state, and then quenching and tempering the residual austenite to 3% or less. By using a steel containing a mass ratio of C: 0.50 to 0.90% and Si: 0.80 to 2.10% as a material, the tensile strength is increased by performing wire drawing at a surface reduction rate of 50% or more. A high-strength coil spring characterized in that a wire rod of 1600 MPa or less is quenched and tempered so as to reduce residual austenite to 3% or less after cold coil forming. The high-strength coil spring according to claim 8, wherein the internal hardness is HV600 to 650. The high-strength coil spring according to claim 8 or 9, wherein the fracture toughness value is 70 to 130 kgf / mm3 / ² . Cold-formed high-frequency heating and tempering high-strength coil spring steel containing C: 0.50 to 0.90% and Si: 0.8 to 2.10% by mass ratio.

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