JP2003105498A - High strength spring, and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel wire for a high strength spring which can combinedly have high strength and satisfactory cold spring formability. SOLUTION: The high strength spring has a composition containing, by mass, 0.55 to 0.65% C, 1.2 to 2.5% Si, 0.3 to 0.6% Mn, and 0.4 to 2.0% Cr, and in which the content of P is limited to <=0.015%, and S to <=0.015%, and if required, containing one or two metals selected from 0.05 to 2.0% Mo and 0.05 to 0.3% V (wherein, the total content of Mn and V is <=0.6%), and the balance iron with inevitable impurities. The dimensions of nonmetallic inclusions are <=15 μm, and its tensile strength is >=1,960 MPa, and its yield ratio (σ0.2/σB) is 0.8 to 0.9, or its yield ratio is >0.9, and also, the content of retained austenite is <=6%. Further, old austenite grain size number is >=11, its surface roughness Rmax is <=11, and the compressive residual stress of the surface part is >=600 MPa. Further, the steel has a durability of 1×10<7> times when mean stress τm=600 MPa, and amplitude stress τa=514 MPa.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength spring and a manufacturing method thereof.

[0002]

2. Description of the Related Art Since springs are often subjected to high loads repeatedly, high strength, sag resistance and durability are required.
In particular, engine valve springs (valve springs)
Fatigue strength and sag characteristics as spring performance are particularly important because they are used in severe environments at high temperatures. Cold coiling is generally used for thin wire diameters such as valve springs of automobile engines despite their high strength.Recently, cold coiling is also used for thick wire diameters such as suspension springs. Is increasing. JIS for cold coil springs
G-4801, Si-Mn system and Si-
An oil tempered wire made of Cr spring steel has been used. Further, in order to further increase the strength, JP-A-1-83
As described in Japanese Patent Laid-Open No. 644 and Japanese Laid-Open Patent Application No. 2-57637, steel wires have been developed in which an alloying element such as Mo or V is added to the above-described spring steel as a base and subjected to oil tempering.

It is generally known that if the tensile strength or hardness of a spring material is increased, the fatigue resistance and the sag resistance are improved. However, in a high-strength spring having a tensile strength exceeding 1960 MPa, the frequency of occurrence of fracture such as fatigue fracture originating from nonmetallic inclusions and grain boundary fracture, which cannot be seen in the conventionally used low-strength materials, increases. Further, in a spring that is cold-formed, the workability (spring formability) of the oil-tempered wire that is the material is an important factor. That is, when a coil spring is formed by cold forming using an oil tempered wire, the breaking strain is small when the tensile strength of the oil tempered wire is high, so that the coil breaks during coiling.

In order to achieve both high strength and good coiling property, Japanese Patent Laid-Open No. 4-247824 discloses that warm coiling is effective. However, there are problems in productivity and workability as compared with the commonly used cold coiling method. Further, JP-A-3-162550 claims that retained austenite can be used to release strain by work-induced transformation by coiling and prevent breakage. However, the elongation value in the tensile test increases with the increase in the retained austenite amount, but in the bending angle measurement result in the bending test with the notched test piece, the retained austenite amount does not affect or rather decreases. The results are shown, and the effect of the amount of retained austenite was not clear.

The present invention has been made in view of such circumstances. That is, it is an object of the present invention to provide a high-strength spring having high strength, excellent durability, and the like, and a method of manufacturing the same.

[0006]

Therefore, the present inventor has diligently studied in order to solve this problem, and reduces the size of inclusions which are the starting points of fatigue fracture and cleans the grain boundaries to improve the grain boundary strength. It is expected that springs with excellent properties can be obtained by adopting spring steel that comprehensively satisfies the requirements of improving, making the austenite grain size of the microstructure that affects the fatigue properties fine, and so on. The present invention has been completed under the heading. Further, in order to obtain higher strength, nitriding treatment was adopted and the tempering softening resistance of the obtained nitrided layer was increased.

That is, the high strength spring of the present invention has a mass%
C: 0.55 to 0.65%, Si: 1.2 to 2.5
%, Mn: 0.3 to 0.6%, Cr: 0.4 to 2.0%
In addition, Mo: 0.05 to 2.0% and V: 0.0
5 to 0.3% of 1 or 2 types, and Mn +
V is 0.6% or less, P: 0.015% or less, S:
The content is limited to 0.015% or less, and the balance of iron and unavoidable impurities is included, and the size of non-metallic inclusions is 15 μm.
Hereafter, the tensile strength is 1960 MPa or more, the yield ratio (σ0.2 / σB) is 0.8 or more and 0.9 or less, or the yield ratio is more than 0.9 and the amount of retained austenite is 6% or less. It is formed of a steel wire having a grain size number of 11 or more, a surface roughness Rmax of 11 or less, a compressive residual stress of the surface portion of 600 MPa or more, and a fatigue strength of an average stress τm = 600 MPa and an amplitude stress τa = 51
It is characterized by having durability of 1 × 10 ⁷ times or more at 4 MPa. The high strength spring of the present invention has extremely high fatigue strength.

The high-strength spring of the present invention further has a surface roughness Rmax of 8.3 or less and a nitride layer having a compressive residual stress of 1200 MPa or more at the surface portion, and the fatigue strength is an average stress τm = Amplitude stress τa = 59 at 700 MPa
It can have a durability of 1 × 10 ⁶ times or more at 0 MPa. The manufacturing method of the high strength spring of the present invention,
After coiling the steel wire, low temperature annealing at 400 to 500 ° C., end surface grinding, shot peening, low temperature annealing, and setting are sequentially performed. In the method for manufacturing a high-strength spring of the present invention, it is preferable to carry out descaling and gas nitriding at 480 ° C. or lower after end face grinding and before shot peening.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The spring steel wire used in the high-strength spring of the present invention is C: 0.55 to 0.65%, S in mass%.
i: 1.2 to 2.5%, Mn: 0.3 to 0.6%, C
r: 0.4-2.0%, and Mo: 0.05-
2.0% and V: V-containing 0.05% to 0.3% of 1 or 2 types, Mn + V is 0.6% or less, and P:
0.015% or less, S: 0.015% or less, including the balance iron and unavoidable impurities, the size of non-metallic inclusions is 15 μm or less, tensile strength is 1960MP
a and the yield ratio (σ0.2 / σB) is 0.8 or more and 0.
9 or less, or the yield ratio is more than 0.9 and the residual austenite amount is 6% or less, and the former austenite grain size number is 1
It is steel wire that is No. 1 or more.

C is an element having a great influence on the basic strength of the spring steel wire, and is 0.55 to 0.5% in order to obtain sufficient strength.
It was set to 0.65%. If it is less than 0.55%, the tempering temperature will be low, and it will be difficult to obtain high tensile strength by the method of industrial mass production. If it exceeds 0.65%, it will be close to hyper-eutectoid and V, Mo, etc. The upper limit was set to 0.65% because it is easy to form carbides due to binding.

Si is an element necessary to secure the strength, hardness and sag resistance of the spring, and when the amount is small, the necessary strength and sag resistance are insufficient, so 1.2% was made the lower limit. . Further, if too much is added, not only the material is hardened but also embrittled. In particular, breakage is likely to occur during coiling after oil tempering. Therefore, in order to prevent embrittlement after quenching and tempering, 2.5% is made the upper limit.

Mn has a lower limit of 0.3% in order to obtain sufficient hardness, and to fix S existing in steel as MnS and suppress the strength reduction. The upper limit of Mn is 0.6%
The reason is that if the Mn content is large, a local supercooled structure is likely to occur even during rolling before wire drawing. Normally, rolling is performed carefully so as not to cause local supercooling, but M
If n is included in a large amount, it is highly possible that it suddenly occurs due to the influence of microsegregation. Such a supercooled structure causes a disconnection in the subsequent wire drawing process. Further, Mn accelerates the generation of surface martensite due to processing heat in the peeling process (shaving or peeling process) before wire drawing. Further, Mn is an element that has a large effect on the residual amount of retained austenite, and when produced by the production method described below, a large amount cannot be added because the retained austenite is suppressed to 6% or less after oil tempering. In the present invention, since S is limited, the amount of Mn added is limited to the minimum that can secure mechanical properties.

Cr improves hardenability and imparts temper softening resistance. Further, in the case of steel to be nitrided, it combines with N to form a nitride and hardens the steel. 0.4%
If it is less than 2.0%, the effect is not remarkable, and if it exceeds 2.0%, Cr-based carbides are formed and the fracture characteristics are deteriorated. Therefore, 0.4% was defined as the lower limit and 2.0% as the upper limit. Although P hardens the steel, it also causes segregation and embrittles the material. In particular, it lowers the grain boundary strength and causes a delayed fracture due to a reduced impact value and hydrogen intrusion. Therefore, the smaller the better. Therefore, it is limited to 0.015% or less at which the embrittlement tendency becomes remarkable.

Like S, if S is present in the steel, it makes the steel brittle. Although the effect can be minimized by Mn,
Since MnS also takes the form of inclusions, the fracture characteristics deteriorate. Further, in order to minimize the adverse effect of Mn addition, it is necessary to limit the S content and suppress the Mn addition amount to the minimum. Therefore, it is desirable to reduce S as much as possible, and the upper limit is set to 0.015% at which the adverse effect becomes remarkable.

The softening resistance can be increased by adding V. In particular, recently, nitriding treatment of springs is often applied as a method for obtaining high-strength springs, and the nitriding temperature in this case is 3
A high temperature of 80-580 ° C is applied. V is an effective element as an element that prevents a decrease in hardness when subjected to such a high temperature heat treatment. But the effect is 0.0 for V
If it is less than 5%, almost no effect is observed, and if it exceeds 0.3%, coarse undissolved inclusions are formed, and the toughness is reduced. V, like Mn, is an element that affects the formation of retained austenite. Therefore, the total amount of Mn and V added is 0.6%.
If it exceeds, the amount of retained austenite cannot be reduced to 6% or less. Therefore, Mn + V is limited to 0.6% or less.

Mo is an element that gives the softening resistance after quenching and tempering, and can suppress the softening of steel treated at a high temperature such as nitriding and give the required strength. Mo is 0.0
If it is less than 5%, its effect is small, and if it exceeds 2.0%, carbides may be formed in the steel to deteriorate the fracture characteristics. Therefore, the lower limit of the Mo content is 0.05% and the upper limit is 2.0%.

With respect to non-metallic inclusions, that is, hard oxides, nitrides, and sulfides, the fatigue strength is adversely affected if the size thereof is increased. 1960M that is the object of the present invention
With a high strength of Pa, even small inclusions become the starting point of fracture. Therefore, the upper limit of the size of the non-metallic inclusions that does not adversely affect the strength level of the present invention is 15 μm, so this is defined as the upper limit. The measurement method for non-metallic inclusions is 2000 mm ² using an image processing device in which a longitudinal section of a spring steel wire sampled from a random position is attached to an optical microscope.
The inclusions are observed over the entire length, and the largest circle equivalent diameter of the non-metallic inclusions recognized is the size of the non-metallic inclusions defined in the present invention.

Regarding the strength of the spring steel wire, the tensile strength of the spring steel wire must be 1960 MPa or more in order to provide a high strength spring. Below this, the performance of the spring after coiling will be no different from that using the conventional spring steel wire. However, as described above, it is necessary to pay attention to the yield point in terms of spring formability in coiling. That is, in cold forming, the spring is formed by plastic deformation near room temperature, so that a material whose plastic deformation start stress and rupture stress are close to each other is formed in a stress load state just before rupture. In such a situation, slight fluctuations in manufacturing,
Due to factors such as scratches, the probability of breakage becomes extremely high, and the coiling characteristics deteriorate.

Therefore, it is considered that a material having a large difference between the plastic deformation start stress and the fracture stress has better coiling characteristics. From such a viewpoint, the yield ratio is used as an index showing the difference between the plastic deformation start stress and the fracture stress, and the tensile strength is 196
In the case of 0 MPa, it was found that the yield ratio should be 0.9 or less. Here, the yield ratio is a ratio (σ0.2 / σB) of 0.2% proof stress (σ0.2) measured by a spring steel wire offset method and a breaking stress (σB) in a tensile test. On the contrary, if the yield ratio is less than 0.8, sufficient sag characteristics cannot be exhibited. Therefore, the yield ratio was set to 0.8 or more from the viewpoint of settling. However, since this regulation also changes depending on the amount of retained austenite, cold coiling is possible even if the yield ratio exceeds 0.9 when the amount of retained austenite is 6% or less.

The reason why the amount of retained austenite is 6% or less will be described. Retained austenite often stays near the segregation part and the former austenite grain boundary. It was found that the retained austenite becomes martensite due to the work-induced transformation, but when the transformation is induced during the spring forming, a locally high hardness portion is generated in the material and rather the coiling property as a spring is deteriorated. In addition, recent springs strengthen the surface by plastic deformation such as shot peening and setting,
In the case of having a manufacturing process including a plurality of processes for applying plastic deformation as described above, the work-induced martensite generated at an early stage reduces the fracture strain, and the workability and the fracture property of the spring in use are degraded. Further, even when an industrially unavoidable deformation such as a scratch is introduced, it is easily broken during coiling. Therefore, the workability is improved by reducing the retained austenite as much as possible and suppressing the formation of the work-induced martensite.

In order to reduce the amount of retained austenite to 6% or less, it is necessary to control the temperature of the cooling medium at the time of quenching so as not to rise above 60 ° C. and to thoroughly quench it. You need to be careful.

The oil-tempered wire is manufactured by continuously performing three steps of heating, quenching and tempering from the wire drawing material to austenitization. The generation of residual austenite is caused by the solid volume of alloying elements and the wire at the time of quenching. Temperature and tempering conditions. That is, among the alloying elements, carbon, which is an austenite stabilizing element, M
When elements such as n, Ni and Mo form a solid solution in austenite, residual austenite is likely to occur. When alloying elements are added, the martensitic transformation start temperature (M
s point), the martensite transformation end temperature (Mf point) is lowered, and at the quenching temperature with a general quenching agent, the temperature does not fall below the Mf point, martensite cannot be completely formed, and retained austenite easily occurs.

The generated retained austenite decomposes in the subsequent tempering step, but if the tempering temperature is low or the tempering time is short in order to obtain high strength, the decomposition is not completed and remains in the steel wire. Become. If the addition of alloying elements is small, the amount of retained austenite generated can be easily reduced, but the additive elements defined in claim 1 or 3 are essential from the viewpoint of increasing the softening resistance of steel and obtaining high strength.
In order to reduce the retained austenite in the steel having the chemical composition according to claim 1 or 3 to 6% or less in the oil temper treatment, it is important to make the quenching temperature as low as possible and to sufficiently cool it. Good results are obtained by the following.

If the yield ratio is made appropriate, coiling is possible even if the amount of retained austenite exceeds 6%, but the retained austenite gradually decomposes as work-induced martensite during use as a spring, and its total length changes. Therefore, it is preferably as low as possible from the viewpoint of the fatigue of the spring.

As described above, the smaller the prior austenite grain size, the better the workability as a spring and the spring fatigue strength. In the high-strength spring of the present invention, the fatigue strength is inferior unless the grain size number of the former austenite grain size reaches 11. Therefore, it has been added to the regulation that the fine particles have a grain size of 11 or more in the former austenite.

The high strength spring of the present invention has a surface roughness Rm.
If ax is 11 or less, the compressive residual stress on the surface is 600MP
a or more, and the fatigue strength is an average stress τm = 600
1 × 10 ⁷ when amplitude stress τa = 514 MPa at MPa
Has durability of more than one time. Such surface roughness, residual compressive stress and fatigue strength can be obtained by coiling the spring steel wire according to the present invention, followed by low temperature annealing at 400 to 500 ° C. and appropriate shot peening.

Further, it has a nitride layer having a surface roughness Rmax of 8.3 or less and a compressive residual stress of 1200 MPa or more on the surface portion, and its fatigue strength is 1 × 10 ⁶ when the average stress τm = 700 MPa and the amplitude stress τa = 590 MPa. The high-strength spring having durability of not less than one time, after coiling the spring steel wire according to the present invention, low temperature annealing is performed at 400 to 500 ° C., and then gas nitriding is performed at 480 ° C. or less to form a surface layer as a nitride layer. , And then by performing appropriate shot peening.

Shot peening, which is carried out in the method for manufacturing a high strength spring of the present invention, imparts a compressive residual stress to the surface of the spring, which results in fatigue resistance (fatigue strength) of the spring.
Is significantly improved. The shot used in shot peening can be cut wire (CW) or steel ball (S
B) may be used, and the projection method may be an air injection method or a centrifugal method. Furthermore, the shot peening process may have multiple stages. That is, the second shot peening using the small diameter shot may be performed after the first shot peening using the large diameter shot.

The nitriding treatment carried out by the method for manufacturing a high strength spring of the present invention is effective in increasing the hardness of the surface layer of the spring and adding a compressive residual stress to improve fatigue resistance. In this nitriding treatment, for example, a predetermined nitriding layer can be formed by treating in an ammonia atmosphere at 500 ° C. or lower for 1 to 24 hours as in the conventional case. The hardness of the obtained nitride layer near the surface is Vickers hardness of 700 or more.

For a coil spring having a constant load direction, it is preferable to perform setting after shot peening. This is because the elastic limit (proportional limit) is significantly improved by giving plastic strain. Furthermore, by performing low temperature annealing after this, the elastic limit can be further improved. This is due to strain aging and is preferable for improving the sag resistance. Usually, low temperature annealing is performed at 150 to 300 ° C. By performing warm setting at 200 to 400 ° C., the sag resistance can be remarkably improved, which is preferable for suspension coil springs of automobiles, valve springs for engines, etc., where large stress is applied.

[0031]

EXAMPLES (Spring Steel) Table 1 shows the chemical composition of the spring steel according to the present invention and the chemical composition of the comparative example. The spring steel examples and comparative examples according to the present invention are manufactured by melting the chemical components shown in Table 1,
After the wire rod having a diameter of 8 mm was formed by hot rolling, each processing of patenting-peeling-drawing-annealing-oil temper was performed to prepare an oil-tempered wire having a diameter of 3.2 mm. Including the invention examples, no problems such as disconnection occurred during the wire drawing process.

Table 2 shows heat treatment conditions and mechanical properties of the spring steel according to the invention and the oil tempered wire of the comparative example. The strength of the oil-tempered wire of the spring steel according to the invention has a tensile strength of 19 from the viewpoint of fatigue resistance and sag resistance.
It was set to 60 MPa or more. The comparative examples were basically made to have the same strength except for a part, but the chemical composition and the like were out of the specified range of the spring steel according to the present invention, and the former austenite grain size was out of the specified range. Even if the chemical composition was within the specified range, the former austenite grain size number was changed by changing the heat treatment conditions.

The spring steel according to the present invention has a higher heating temperature than before in order to avoid undissolved carbides such as V and Mo. Usually, undissolved carbides are reduced by increasing the heating temperature, but at the same time, it also coarsens the austenite grain size. Therefore, in order to make the austenite grain size fine, the heating time was set to a short level, and advanced control was performed to keep the old austenite grain size fine while avoiding undissolved carbides. Further, the quenching temperature was set to 45 ° C. or lower in order to suppress the amount of retained austenite. Further, by increasing the tempering temperature, the decomposition of the generated retained austenite was promoted, and the amount thereof was controlled to 6% or less. Also,
The yield ratio is 0.8 to avoid breakage during spring forming.
It was adjusted to about 0.9. On the other hand, in the comparative example, in addition to the steel wire whose chemical composition is not specified, even if the chemical composition is within the specified range of the spring steel according to the present invention, the microstructure and strength of the steel wire such as the amount of retained austenite and the former austenite grain size number. This is an example outside the specified range.

The higher the strength of the oil tempered wire, the higher the notch sensitivity, and the more likely it is that the oil tempered wire breaks from the fine flaws as starting points during the spring forming process. As a method of evaluating this spring formability, prior to spring forming, a high alloy tip is pressed against the oil temper wire to make a notch with a depth of 25 μm, and then the notch is placed opposite the notch so that tensile stress is applied. A three-point bending process was applied to the side with a punch having a radius of 6.5 mm, and a notch bending test was performed to measure the bending angle until breakage. The outline is as shown in FIG. 1, and the bending angle θ until the breakage was measured.

The amount of retained austenite is shown by mass% which is determined from the magnitude of the integrated intensity of the peak using an X-ray diffractometer. According to this method, it is said that the measurement can be performed accurately if the amount of retained austenite is 2% or more in mass%. The old austenite grain size number was measured according to JIS, and the mirror-polished steel wire cross section was measured in 7 fields of view, and the average was used as the old austenite grain size number of each example.

From these relationships, Table 2 shows the oil temper treatment conditions, yield ratio, residual austenite amount, former austenite grain size number, spring formability, fatigue property and sag resistance in each component system. In Table 2, the formability represents the breakage probability at the time of spring forming, and is in the case of ◯: 0.001% or less and ×: more than 0.001%. Furthermore, the fatigue property is the average load stress of 68 when repeated 5 × 10 ⁷ times.
It is expressed by the presence or absence of spring breakage at a stress amplitude from 6 MPa. When the amplitude is 450 MPa or more, the evaluation is ◯: good, 450
When MPa or less x: Shown as defective. It can be seen that the oil tempered wire according to the present invention has excellent workability by the notch bending test as described above, despite the tensile strength of 1960 MPa or more.

Table 3 shows the specifications of the spring used for the evaluation. Two types of springs were used to evaluate spring formability, fatigue resistance and sag resistance. Spring specification 1 is for evaluating fatigue resistance and sag resistance, and spring specification 2 is for evaluating spring formability in cold. Table 2 shows the evaluation results. The spring of the spring specification 1 was subjected to a nitriding treatment and shot peening and then subjected to the test. Oil tempered wire made of conventional steel has excellent spring formability and is inferior in fatigue strength and sag resistance, whereas oil tempered wire made of spring steel according to the present invention has no breakage during spring forming and fatigue resistance characteristics. In terms of sag resistance, it was equal to or higher than that of the comparative steel. In particular, a spring having a fine austenite grain size could be easily processed into a coil spring and was excellent in fatigue strength as a spring.

Even if the steel wire or the chemical composition produced by a method which is extremely industrially unreasonable by lowering the quenching temperature is within the specifications of the spring steel according to the present invention, it contains non-metallic inclusions having a size exceeding the specifications. When a steel wire or a steel wire having a former austenite grain size number smaller than the regulation and a large grain size was used, spring working was possible, but the fatigue property as a spring was poor.

FIG. 2 shows the relationship between the prior austenite grain size number and the stress amplitude at the time strength of 5 × 10 ⁷ times in the fatigue property evaluation test. Examples 1 to 6 (Invention Examples) and Examples 13 to 15 (Comparative Examples) in which the prior austenite grain size was intentionally increased by the specified chemical composition of the spring steel according to the present invention.
The results of fatigue strength evaluation of It can be seen that the prior austenite grain size number has an effect on fatigue strength. It can be seen that when the grain size number exceeds 11, the fatigue strength becomes almost the same and the effect is saturated. When the yield ratio and the amount of retained austenite were out of the specified range, the probability of breakage during coiling was high, and it was judged that industrial production was impossible.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

(High-strength spring) In this example, the invention steel grade is one in which the alloy composition is close to the median value, and the carbon content is 0.64.
Mass% (hereinafter,% means mass% unless otherwise specified), silicon 2.02%, manganese 0.30%,
Phosphorus 0.010%, sulfur 0.005%, chromium 0.86
%, Molybdenum 0.10%, vanadium 0.10%, and the balance being substantially iron. Using this spring steel material, after each of scratching, hot rolling, peeling, and annealing, cold drawing and oil tempering are performed to obtain a spring steel of 3.2 mm in diameter according to the present invention. Got the line. The tensile strength σB of the spring steel wire according to the present invention is 21.
It was 48 MPa.

As the spring steel wire of the comparative example, carbon 0.64%, silicon 1.46%, manganese 0.72%, phosphorus 0.010%, sulfur 0.006%, chromium 0.63, vanadium. A spring steel material of alloy steel consisting of 0.12% and the balance substantially iron was used. Then, similarly, after each of the scratching, hot rolling, peeling, and annealing treatments, cold drawing and oil tempering treatment are performed to obtain a spring steel wire (oil temper) of a comparative example having a diameter of 3.2 mm. Line (SWOSC-
VHV) was obtained. Tensile strength σ of the steel wire for spring of this comparative example
B was 2144 MPa.

Next, using these two types of spring steel wires,
Performed cold coiling forming, wire diameter φ3.2mm, coil center diameter φ20.0mm (coil outer diameter φ23.2mm),
A coil spring having a total number of turns of 6.0 and an effective number of turns of 4.0 was obtained. The obtained two kinds of coil springs were processed under the processing conditions shown in Table 4 and No. 11-No. No. 17 and 7 types of high strength springs of the present invention; Eighteen types of one comparative spring were manufactured. Specifically, first, the low temperature annealing condition shown in Table 4 was performed in the furnace for 30 minutes. After that, grind both bearing surfaces,
A coil spring having a free length of 47.0 mm was used.

[0046]

[Table 4]

Next, No. 11-No. The gas nitriding treatment was performed on 15 kinds of the high-strength springs of the present invention of 15 kinds. The nitriding treatment was performed in an ammonia atmosphere at 465 ° C. or 475 ° C. for 3 hours. As a result, a nitride layer was formed on the surface of the spring. In addition, No. 16 and No. 16 No. 17, two types of high-strength springs of the present invention; No nitriding treatment was performed on the 18 comparative springs. After that, shot peening treatment was performed with the shots and the treatment times shown in Table 4. After the shot peening treatment, low temperature annealing at 225 ° C. for 15 minutes or more was performed at 225 ° C., 1350 MPa,
Setting for 10 seconds, and No. 11-No. 17
7 types of high-strength springs according to the present invention; Eighteen types of one comparative spring were manufactured.

The eight types of high-strength springs thus obtained were evaluated. For evaluation, the surface roughness of the spring was measured, the hardness of the surface portion was measured, the residual stress was measured by X-ray, and the fatigue strength was measured. Table 5 shows the measurement results of the surface roughness, the hardness of the surface portion and the residual stress by X-ray, and Table 6 shows the measurement conditions and the measurement results of the fatigue strength.

[0049]

[Table 5]

[0050]

[Table 6]

The hardness was measured by cutting the spring used in the test and measuring the hardness on the cut surface with a Vicas hardness meter. The residual stress was measured by the SIN ² ψ-side tilt method using X-ray. Fatigue strength was determined by the SN method. Fatigue strength of Table 5 700 ± 590
Is the amplitude stress τa when the average stress τm is 700 MPa.
Indicates that it is 590 MPa, 10 ⁶ and 10 ⁷ indicate the number of times the compressive stress has acted, and the mark ◯ indicates the number of times that all eight test springs did not break.

No. 1 according to the embodiment of the present invention. 11-No.
Seven kinds of springs No. 17 are No. 17 springs. The fatigue strength is extremely higher than that of the springs of 18 comparative examples. Especially No. 11-No. 1
The spring subjected to the nitriding treatment of No. 5 can withstand a load of 700 ± 500 MPa nearly 10 ⁷ times. This high fatigue strength suppresses the surface roughness of Rmax to 8.3 or less by nitriding treatment and shot blasting treatment.
This is considered to be the result of applying a high compressive residual stress of 200 MPa.

No. The fatigue strength of the spring of No. 11 is No. 12
Extremely high compared to the fatigue strength of springs. This high fatigue strength corresponds to a high maximum compressive residual stress. No. 14
From the fatigue strength of the spring No. 1, the nitriding temperature was 465 ° C. and No.
It can be seen that high fatigue strength can be obtained even when the nitriding temperature of the spring 11 is lower by 10 ° C.

[0054]

The high strength spring of the present invention has extremely high fatigue strength. This is because a high strength oil tempered wire of 1960 MPa or more that can be cold-formed is used, and the surface roughness Rmax of the spring is 11 or less and the compressive residual stress of the surface portion is 600 MPa or more. The high-strength spring of the present invention has an amplitude stress τa ≧ 5 when the fatigue strength of 1 × 10 ⁷ times by the SN method is an average stress τm = 600 MPa.
It becomes 10 MPa or more.

[Brief description of drawings]

FIG. 1 is a diagram showing a notch bending test method.

FIG. 2 is a diagram showing the relationship between fatigue strength and prior austenite grain size number.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F16F 1/02 F16F 1/02 A B 1/06 1/06 A (72) Inventor Satoshi Kondo Aichi prefecture Aichi Togo-cho, Gunzai, Haruki-ji, Tobi Seisakusho Co., Ltd. 1 (72) Inventor Motohide Mori 1 Toyota-cho, Toyota-shi, Aichi Prefecture Toyota-motor Co., Ltd. (72) Go, Kawamoto 1 Toyota-cho, Toyota-shi, Aichi Address Toyota Motor Vehicle Co., Ltd. F-term (reference) 3J059 AB11 AD05 BA01 BB01 BC02 EA08 4K032 AA06 AA11 AA12 AA16 AA19 AA20 AA27 AA29 AA32 AA36 BA02 4K042 AA01 BA01 BA05 CA06 CA08 CA13 DA03 DA06 DC02

Claims (4)

[Claims] 1. In mass%, C: 0.55 to 0.65%,
Si: 1.2 to 2.5%, Mn: 0.3 to 0.6%, C
r: 0.4-2.0%, and Mo: 0.05-
2.0% and V: V-containing 0.05% to 0.3% of 1 or 2 types, Mn + V is 0.6% or less, and P:
0.015% or less, S: 0.015% or less, including the balance iron and unavoidable impurities, the size of non-metallic inclusions is 15 μm or less, tensile strength is 1960MP
a and the yield ratio (σ0.2 / σB) is 0.8 or more and 0.
9 or less, or the yield ratio is more than 0.9 and the residual austenite amount is 6% or less, and the former austenite grain size number is 1
It is made of steel wire that is No. 1 or more and has a surface roughness Rmax of 1
1 or less, the compressive residual stress of the surface portion is 600 MPa or more, and the fatigue strength is an average stress τm = 600 MPa.
A high-strength spring characterized by having a durability of 1 × 10 ⁷ times or more when the amplitude stress τa = 514 MPa. 2. A nitride layer having a surface roughness Rmax of 8.3 or less and a compressive residual stress of the surface portion of 1200 MPa or more, and the fatigue strength having an average stress τm = 700 MP.
The high-strength spring according to claim 1, which has a durability of 1 × 10 ⁶ times or more when the amplitude stress τa = 590 MPa at a. 3. In mass%, C: 0.55 to 0.65%,
Si: 1.2 to 2.5%, Mn: 0.3 to 0.6%, C
r: 0.4-2.0%, and Mo: 0.05-
2.0% and V: V-containing 0.05% to 0.3% of 1 or 2 types, Mn + V is 0.6% or less, and P:
0.015% or less, S: 0.015% or less, including the balance iron and unavoidable impurities, the size of non-metallic inclusions is 15 μm or less, tensile strength is 1960MP
a and the yield ratio (σ0.2 / σB) is 0.8 or more and 0.
9 or less, or the yield ratio is more than 0.9 and the residual austenite amount is 6% or less, and the former austenite grain size number is 1
400 ~ 50 after coiling the steel wire which is more than 1
A method for manufacturing a high-strength spring, which comprises sequentially performing low-temperature annealing at 0 ° C., end surface grinding, shot peening, low-temperature annealing, and setting. 4. The method for manufacturing a high-strength spring according to claim 3, wherein descaling and gas nitriding at 480 ° C. or lower are carried out after the end surface grinding and before the shot peening.