JPH05320827A - Steel for spring excellent in fatigue property and steel wire for spring as well as spring - Google Patents

Abstract

PURPOSE:To obtain a steel for a spring having high strength and excellent in fatigue properties by specifying its compsn. constituted of C, Si, Mn, Cr, V, Nb and Fe and the properties of contained inclusions and percipitates. CONSTITUTION:In a steel contg., by weight, 0.5 to 0.8% C, 0.8 to 2.5% Si, 0.4 to 1.3% Mn and 0.4 to 2% Cr, furturemore contg., at need, 0.05 to 0.5% V and/or 0.05 to 0.5% Nb and moreover contg. 0.1 to 2% Ni and/or 0.1 to 0.5% Mo, and the balance Fe with inevitable impurities, the m.p. of oxide inclusions contained therein is regulated to <=1500 deg.C, and the size of carbide precipitates and nitride precipitates are regulated to <=15mum. In this way, the objective steel for a high strength spring excellent in fatigue properties can be obtd. This steel is subjected to hardening and tempering treatment, by which the objective high strength steel wire for a spring having crystalline grain size, of No.11, or more 3 to 20% retained austenitic amt., >=205kgf/mm<2> tensile strength and <=0.95 proof stress ratio after low temp. annealing at >=400 deg.C and the objective spring having residual stress shown by the formula and inequalities can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spring steel having excellent fatigue characteristics, a high-strength steel wire for springs manufactured by using the steel, and a spring. The spring is required for automobiles requiring extremely high fatigue strength. It is useful as a valve spring, a clutch spring, a brake spring, etc. of a commercial engine.

[0002]

2. Description of the Related Art Recently, as the demand for weight reduction and higher output of automobiles has increased, high stress design has been directed to springs such as valve springs and suspension springs used in engines and suspensions. Therefore, it is strongly desired that these springs have excellent fatigue resistance and sag resistance in order to cope with an increase in load stress. Especially, the demand for increasing the fatigue strength of the valve spring is very strong, and SWOSC-V (J
Even IS G 3566) has become difficult to deal with.

It is well known that increasing the strength of the material, refining the non-metallic inclusions, and strengthening the surface layer of the spring are effective in improving the fatigue strength. For example, the addition of an alloy element increases the strength. Steel (for example, JP-A-63-21695)
No. 1), steel having higher strength and finer inclusions (for example, Japanese Patent Laid-Open No. 62-107044) and the like have been proposed. Regarding the spring, for example, a high-strength spring that defines the maximum residual stress in the vicinity of the surface (for example, Japanese Patent Laid-Open No.
No. 4-83644) is disclosed.

[0004]

Generally, the fatigue fracture of springs is roughly classified into the case of starting from the surface and the case of starting from internal defects such as inclusions. Strengthening the spring material is effective in suppressing fatigue fracture from the surface and increasing fatigue strength, but increasing strength, on the other hand, increases the susceptibility to defects such as inclusions. It is easy for the damage to occur. Therefore, there is a limit to the improvement of fatigue strength only by increasing the strength of steel.

Therefore, as a means for obtaining high strength and excellent fatigue strength, miniaturization of inclusions is required, and various studies are being conducted. For example, it has been attempted to control the oxide inclusions to have a low melting point and a ductile composition, and to make the inclusions fine by stretching by hot working such as wire rolling. the composition of, in JP 62-107044 discloses shown example above, Al ₂ O _3: 20% or less, MnO: 10 to 8
_{0%, SiO 2: 20~60%} , MgO: 15% or less,
CaO: A substance satisfying the requirement of 50% or less is disclosed.

However, the oxide-based inclusions having these compositions include hard ones having a very high melting point, and such hard oxide-based inclusions having a high melting point are hard to be rolled. It cannot be sufficiently miniaturized by working, and the expected effect of improving fatigue properties cannot be obtained.

Further, according to the confirmation by the present inventors, as the oxide inclusions become finer to some extent, carbide precipitates and nitride precipitates (hereinafter , Which is sometimes referred to as carbon / nitride-based precipitates) causes fatigue fracture. Improving the fatigue strength by simply refining the inclusions targeting oxide-based inclusions as described above. It has become clear that there are limits.

On the other hand, even if the steel materials used have the same composition, it often happens that the fatigue properties of the final product springs differ. This is because the conventional means for improving fatigue properties is to study the chemical composition of steel and oxide inclusions at the steelmaking stage. It is considered that this is because sufficient consideration has not been given to making the best use of the performance of steel materials, taking into consideration the processing conditions until processing into a certain spring and the heat treatment conditions.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a spring steel, a spring steel wire, and a spring for which fatigue characteristics are further improved as compared with conventional materials. It is what

[0010]

The composition of the spring steel according to the present invention, which has been able to achieve the above object, is C: 0.5-0.8%, Si: 0.8-2.5%, Mn: 0.4- by weight ratio.
1.3%, Cr: 0.4-2%, and V: 0.05-0.5
% And / or Nb: 0.05 to 0.5% with the balance Fe
And steel inevitable impurities, or in addition to these, Ni: 0.1 to 2% and / or Mo: 0.1 to 0.5
%, The melting point of oxide inclusions contained in the steel is 1500 ° C. or less, and the size of carbide precipitates and nitride precipitates is 15 μm or less. is there.

Then, the spring steel satisfying these requirements is quenched and tempered to obtain a grain size of 11 or more, a retained austenite amount of 3 to 20%, and a tensile strength of 205 kgf / mm ^2.
As described above, the steel having a yield strength ratio of 0.95 or less after low temperature annealing at 400 ° C. or higher is very excellent as a spring steel wire. Further, the spring having the residual stress satisfying the requirements of the following formula [I] and having the maximum hardness (Vickers hardness) of 700 or more in the surface layer using this spring steel wire as a material is particularly excellent in fatigue characteristics. It is a thing.

[0012]

[Equation 2]

[0013]

The structure of the present invention is as described above. The main points are as follows. First, for spring steel, the type and content of alloying elements are specified to obtain high strength and high toughness after quenching and tempering. , The melting point of oxide inclusions is set to a low level so that it can be sufficiently refined in the drawing step during hot or cold working, and by making the oxide inclusions finer, a new close-up is achieved. The feature is that the size of the charcoal / nitride-based precipitates is specified in order to suppress the adverse effect on the fatigue properties of the charcoal / nitride-based precipitates.The steel wire for springs has a high temperature after quenching / tempering. In order to obtain strength and toughness and ductility that can withstand spring forming, the amount of retained austenite and the grain size are specified, and the amount of retained austenite and the yield strength ratio are specified to improve fatigue properties. Furthermore, in the final product, the spring, the residual stress distribution is specified to improve the fatigue characteristics, and in particular, the residual compressive stress in the surface layer is increased to suppress fracture from the surface and the internal tensile residual stress. In order to suppress the fracture caused by inclusions as a starting point and to suppress the fracture from the surface to enhance the fatigue strength, the characteristic is that the hardness of the surface layer portion is specified.

Hereinafter, the structure, operation and effect of the present invention will be described in detail. First, the reason for defining the composition of the spring steel according to the present invention will be described.

C: 0.5-0.8% C is an element that is indispensable for securing sufficient strength as a spring steel to which high stress is applied, but if it is too much, the toughness and ductility will be extremely deteriorated. C content is 0.5 to 0.
Must be 8%. Si: 0.8 to 2.5% Si is a necessary component as a deoxidizing agent during steel making, and also has the effect of forming a solid solution in ferrite to enhance the strength of the base material, and such an effect should be included by 0.8% or more. Is effectively demonstrated by. However, if the amount is too large, not only the toughness and ductility deteriorate, but also the surface decarburization and flaws increase and the fatigue resistance deteriorates. Therefore, it must be kept to 2.5% or less.

Mn: 0.4 to 1.3% Mn is also an element effective in deoxidizing steel and contributes to the improvement of hardenability by strengthening strength, but if too much, toughness and ductility deteriorate, so 0.4 to 1.3% Stipulated in the range of. Cr: 0.4 to 2% Cr is an element that reduces the activity of C to prevent deoxidation during rolling and heat treatment and is effective in suppressing graphitization of carbides, but if too much, toughness and ductility deteriorate. Therefore, the range is set to 0.4 to 2%.

V and / or Nb: 0.05% to 0.5%, respectively V and Nb have the effect of refining crystal grains during heat treatment such as quenching and tempering, and have the effect of improving toughness and ductility. In addition, secondary precipitation hardening occurs during quenching / tempering and strain relief annealing after spring forming, which also contributes to higher strength. However, if the amount is too large, it becomes easy to deposit huge carbides or nitrides in the ingot-making step, as described below.
It should be in the range of 0.05 to 0.5%.

Ni: 0.1 to 2.0% and / or Mo: 0.1
.About.0.5% Ni and Mo are also effective elements for enhancing the toughness and ductility after quenching and tempering, and also contribute to the strengthening by enhancing the quenchability. However, if the amount is too large, bainite or martensite structure is generated in rolling, which leads to deterioration in toughness and ductility.
I decided.

Next, the reason for determining the melting point of oxide inclusions and the size of carbon / nitride precipitates, which are features of the steel for springs according to the present invention, will be described. The melting point of the oxide-based inclusions referred to here is obtained by quantitatively analyzing the composition of the inclusions on a plane parallel to the working direction or at the fatigue starting point by EPMA, and refer to the document "Phase Equilibrium of Oxides" (1971.1.10 Gihodo) etc. It was determined from the state diagram described in 1. Also charcoal
Nitride-based precipitates are not such fine particles that precipitate during quenching and tempering processes, but a few tens of μm that precipitate during cooling of steel ingots or slabs during ingot casting or continuous casting V-based and Nb-based carbides, nitrides, and charcoal / nitrides are typical examples.

The inventors of the present invention, while proceeding with the research along the above-mentioned problems, have a melting point of oxide inclusions and a size of inclusions which become fatigue starting points when subjected to higher load stress than before. After examining the relationship between (size) and fatigue life,
The following findings were obtained. That is, if the composition of the oxide-based inclusions is a melting point of 1500 ° C. or less, one is a hot working step such as ingot casting or slab casting and rolling, and a cold working step such as wire drawing. The size of the inclusions in the direction perpendicular to the direction is significantly reduced, which contributes to the improvement of fatigue strength. Secondly, the melting point of the oxide-based inclusions is lowered to make the inclusions finer, Even if it suppresses the fatigue fracture at the starting point of inclusions, the occurrence of fatigue fracture starting from carbon / nitride-based precipitates, which has hardly appeared in the conventional fatigue starting point, becomes remarkable. It is also clear that the fatigue life is shortened even with precipitates of the same size as oxide-based inclusions, and the size of these inclusions is hardly reduced even by hot or cold working. Has become.

The reason why the melting point of oxide inclusions and the size of carbon / nitride precipitates are determined in the present invention is based on these findings.
If the temperature is 00 ° C or lower, the size of inclusions during hot working or cold working is significantly reduced, and it is effective in improving fatigue strength. Therefore, the melting point of oxide inclusions is 1500 ° C.
Defined below. In this case, the size of the inclusions when wire drawing is performed with a normal workability is about 20 μm or less.

On the other hand, the size of the carbon / nitride-based precipitates hardly changes even during hot working such as rolling or cold working such as wire drawing, and the properties thereof are also higher than those of oxide inclusions. In contrast, if the size is the same as that of oxide inclusions, the fatigue life deteriorates, so the size of carbon / nitride precipitates is 15 μm.
Below.

Next, the reasons for limiting each of the steel wire for spring and the spring will be described. First of all, regarding the steel wire for springs, the spring steel of the present invention which satisfies the above-mentioned requirements in the chemical composition shows significantly higher strength than the conventional steel by quenching and tempering treatment, but forming into a spring shape Considering the toughness, it should have excellent toughness and ductility. Generally, the drawing value, which is an index showing the ductility of the steel wire, is used as one of the evaluation criteria showing the spring formability. Therefore, as a result of examining the relationship between the drawing value and various characteristics, the austenite crystal of the steel wire If the particle size is 11 or more according to the measurement method according to JIS G 0551, a high reduction value can be obtained even in the ultra-high strength region (for example, tensile strength of 230 kgf / mm ² or more), and good spring formability can be secured. At the same time, the tensile strength is increased by refining the crystal grains, so the crystal grain size of the steel wire for spring was determined to be 11 or more.

Further, in order to obtain a high strength spring steel wire, it is necessary to reduce the amount of retained austenite after tempering, and it has been confirmed that the smaller the amount of retained austenite, the higher the fatigue strength. For example, JP-A-63-2 mentioned above.
Japanese Patent No. 16951 discloses a steel wire for spring in which the retained austenite after quenching is regulated to 10% or less to improve the fatigue strength after tempering. But,
When manufacturing springs, shot peening is often performed in any of the steps, so the relationship between the amount of retained austenite after tempering and the fatigue strength after shot peening was investigated, and an appropriate amount of retained austenite was found. It was found that the fatigue compressive stress increases with the inclusion of. This is because the retained austenite is transformed into martensite by shot peening and the residual compressive stress is increased.Therefore, the fatigue strength improving effect due to the increase in the residual compressive stress due to the increase in the amount of the retained austenite and the fatigue strength reducing effect due to the strength decrease. This means that the fatigue characteristics are determined by the balance, and it is considered that an appropriate amount of retained austenite exists in order to improve the fatigue strength.

Therefore, as a result of research on the amount of retained austenite capable of obtaining high fatigue strength while ensuring high strength, when the amount of retained austenite after tempering in the quenching / tempering process is less than 3%, residual compression after shot peening is performed. The effect of increasing the stress is small, and it becomes difficult to obtain a satisfactory fatigue strength. On the other hand, when it exceeds 20%, it becomes difficult to obtain a high strength, and the fatigue fracture at the surface starting point tends to occur. It was set in the range of 3 to 20%.

The amount of retained austenite is defined by the amount after tempering for the following reason. That is, what directly affects the fatigue strength after quenching and tempering is
It is the amount of retained austenite after tempering, and since the normal cold forming spring steel wire is subjected to quenching and tempering continuously, it is difficult to accurately measure the amount of retained austenite after quenching. That is, the measurement itself of the amount of retained austenite can be performed by collecting and quantifying the quenched steel wire, but the temperature of the steel wire at the time of measurement is usually considerably lower than that at the time of quenching. The retained austenite transforms to martensite during the temperature decrease.
Therefore, the retained austenite amount of steel containing a large amount of alloying elements, such as the steel of the present invention, is likely to have decreased considerably at the time of measurement, and the retained austenite amount after actual quenching cannot be accurately grasped. .. Therefore, in the present invention, the amount of retained austenite after tempering is defined.

The spring steel wire satisfying the above requirements has excellent toughness and ductility, and has a tensile strength of 205 after quenching and tempering.
It can be set to kgf / mm ² or more, and the fatigue strength is particularly excellent.

Next, the reason why the yield strength ratio of the spring steel wire after low temperature annealing is set to 0.95 or less will be described. The yield strength ratio here is a value obtained by dividing the 0.2% yield strength of the steel wire by the tensile strength, and the 0.2% yield strength was obtained according to the offset method defined in JIS Z 2241. .. Conventional cold-formed springs are usually coiled and then strain relief annealed (eg 4
Although low temperature annealing at a temperature of 00 ° C. or higher) is performed to remove the residual stress generated during coiling, it has been reported in various literatures that the low temperature annealing affects the fatigue characteristics of springs. For example, Spring Papers No. 33, p. 53 (198).
8 years; Spring Technology Study Group) describes the strength level and fatigue properties for ordinary spring steel, and shows the relationship between hardness and fatigue strength after low temperature annealing.

The inventors of the present invention investigated the relationship between the fatigue strength and the characteristics of the steel wire after low temperature annealing at a higher strength than usual for the above spring steel, and found that the fatigue strength ratio of the steel wire was too high. It has been found that the strength rather decreases. That is, the steel wire obtained by using the spring steel of the present invention shows much higher strength than a normal steel wire in the state after quenching and tempering, and the strength decrease after low temperature annealing is small, so that the surface due to fatigue It is easy to suppress the destruction of the starting point. On the other hand, there is a tendency for fracture to occur starting from inclusions present inside, and even in the steel of the present invention in which inclusions are miniaturized as described above, it is easy to cause fatigue fracture starting from inclusions. Become. Therefore, as a result of further studies to eliminate such defects, if the yield strength ratio after low-temperature annealing is set to 0.95 or less, fatigue fracture originating from inclusions and the like can be suppressed, and fatigue characteristics can be significantly improved. It was confirmed.

The lower limit value of the yield strength ratio is not particularly specified, but if the yield strength ratio is too low, the weakest part of the internal matrix (eg grain boundaries).
Since fatigue fracture starting from the point of is likely to occur, 0.8 or more is desired.

Next, the reasons for limiting the spring, which is the final product, will be described. As mentioned above, it is well known that the reinforcement of the spring surface greatly contributes to the improvement of fatigue strength.
A method of performing shot peening is widely used to increase the residual compressive stress hardness of the surface layer. For example, Japanese Unexamined Patent Publication No. 64-83644 mentioned above discloses a high-strength spring in which the maximum residual compressive stress near the surface layer is 85 to 110 kgf / mm ² .

On the other hand, the inventors of the present invention conducted various studies on the relationship between the residual stress and the fatigue strength of the spring steel wire of the present invention satisfying the above-mentioned requirements, and as a result, found that the residual stress distribution suitable for improving the fatigue strength was appropriate. Knew that there existed. That is,
It is well known that the higher the residual stress in the surface layer is, the better it is to suppress the fracture at the surface origin in fatigue.
When the residual compressive stress is increased, the internal residual tensile stress is increased, so that in a high strength steel wire such as the steel of the present invention, the fracture of the internal origin is likely to occur, and such a tendency becomes remarkable as the residual tensile stress increases. come. Therefore, as a result of further research to clarify the residual stress distribution capable of suppressing both the fatigue fracture at the surface and the internal origin, the residual stress distribution from the surface to the inside satisfies the requirement of the above formula [I]. He found that what he met was the best.

That is, in the formula [I], when σ _R when D> 0.05d exceeds 30 kgf / mm ² , fatigue fracture originating from internal oxide-based inclusions and carbon / nitride-based precipitates Is likely to occur, and σ _R when D = 0 to 0.03d
When the value exceeds -100 kgf / mm ² , the internal residual tensile stress becomes large and the above-mentioned internal starting point is liable to occur. On the contrary, when the value is less than -50 kgf / mm ² , the surface starting point causes the fracture. It becomes easy, and in any case, the fatigue characteristics deteriorate. On the other hand, the larger the residual compressive stress when D = 0.03 to 0.05d is, the more effective it is in suppressing fatigue fracture at the surface origin, but if it is too large, the internal residual tensile stress becomes too large. Therefore, the upper limit of σ _R is -125+ (2500) in consideration of the feasibility on an industrial scale. / d) × D (kgf / mm ² ).

Further, in the present invention, the maximum hardness of the surface layer portion is specified, but this is based on the following reason. That is, the above-mentioned residual stress distribution is determined mainly for the purpose of suppressing fatigue fracture originating from internal inclusions and the like, and the fatigue strength is almost determined by whether or not fracture originating from the surface occurs. Therefore, in the spring of the present invention, the hardness of the surface layer portion is set to 700 or more in Vickers hardness as a requirement for further improving the fatigue strength by strengthening the surface layer portion. The surface layer portion referred to here is 0.01d from the surface.
It means a range within (mm), and the hardness of this portion can be increased by shot peening, nitriding treatment, or the like.

[0035]

EXAMPLES Next, the present invention will be described more specifically by way of examples, but the present invention is not limited by the following examples. Example 1 Table 1 below shows the components contained in the sample steel, the melting point of oxide inclusions, and the size of carbon / nitride precipitates. The sizes of inclusions and precipitates were measured by measuring the longitudinal cross section of each wire rod hot-rolled at a rolling ratio of 50 or more after casting for 30 times.
Observed individually, the maximum and average values are shown in Table 1.
In the above, A1 to A5 are steels of the present invention, and B1 to B4 are comparative steels.

[0036]

[Table 1]

After scraping the surface of each of these test materials, patenting treatment and wire drawing treatment were carried out to obtain a wire diameter of 4.0.
After being made into mm, the steel wire for spring was subjected to quenching and tempering treatment. Low temperature annealing of these steel wires (400 ℃ × 20min)
Then, shot peening treatment was performed and a Nakamura-type rotary bending fatigue test was performed. The results are shown in Table 2.

[0038]

[Table 2]

As is clear from Table 2, the steels A1 to A5 of the present invention having high strength, a low melting point of oxide inclusions, and a small precipitate size were subjected to rotary bending of 1 × 10 ⁷ times. Even after application, no fracture at the surface origin or fracture at the inclusion or precipitate origin was observed, and high fatigue strength was obtained. On the other hand, in the comparative steel, B having a high melting point of oxide inclusions
No. 1, B3, B2 having a large precipitate size, B3, and B4 having a low strength all show fracture at a rotational bending of 1 × 10 ⁷ or less, and the fatigue strength is clearly inferior to the steel of the present invention.

Example 2 Using the steel A3 of the present invention shown in Table 1 above, spring steel wires a1 to a10 were produced in the same manner as in Example 1. Here, by adjusting the heating temperature during quenching, the temperature of the steel wire after quenching, the tempering temperature, etc., the austenite grain size, the amount of retained austenite, the tensile strength, etc. are adjusted,
I investigated its performance. Table 3 shows the relationship among the tensile strength after tempering, the austenite grain size number, the amount of retained austenite, and the fatigue strength. The fatigue test was carried out by Nakamura rotary bending after subjecting to shot peening treatment after tempering. Further, the relationship between the austenite grain size number, the aperture value and the breakage incidence rate according to the winding test (JIS G 3566) is shown in FIG.

[0041]

[Table 3]

As is clear from Table 3, all the examples satisfying all the specified requirements of the present invention have excellent fatigue characteristics, but those having a small amount of retained austenite have low fatigue strength. On the other hand, if the amount is too large, the tensile strength becomes low, and in either case, sufficient fatigue strength cannot be obtained. Further, as can be seen from FIG. 1, when the grain size number exceeds 11, the drawing value remarkably increases, does not break even in a winding test severer than usual coiling, and has excellent spring formability. Can be confirmed.

Example 3 Next, steels A1 and A3 of the present invention shown in Table 1 and comparative steel B3 were used.
A spring steel wire was produced in the same manner as in Example 1, and further processed into a spring. After that, low temperature annealing at 400 ° C. or higher and shot peening treatment are performed to obtain a11 to a16 and b1.
The sample springs of to b2 were produced and the spring fatigue test was conducted.
Table 4 shows the specifications and test conditions of the test spring. The proof stress ratio was adjusted by changing the tempering temperature, low temperature annealing temperature, etc. for each test spring. The results are shown in Table 5.

[0044]

[Table 4]

[0045]

[Table 5]

As is clear from Table 5, even in the case where the steel of the present invention is used, the spring having a high yield ratio is easily broken at the origin of inclusions, while the spring having a low yield ratio is excellent. Has fatigue strength. Further, in Comparative Steel B3 having a high melting point of oxide inclusions and a large precipitate size, the fatigue strength was poor regardless of the strength ratio. The yield strength ratio is 0.9
In Comparative Example a15 which is 6, the fatigue strength equivalent to that of the example is shown, but in the steel of the present invention with small inclusions and precipitates, this is due to inclusions and precipitates in a fatigue test of about 10 springs. It is thought that this is because the destruction starting from is unlikely to occur. However, in the rotating bending fatigue test, which has a high probability of fatigue fracture starting from inclusions or precipitates, the yield strength ratio is 0.
When it exceeds 95, the fatigue strength is obviously reduced. As is clear from these results, the spring steel wire of the present invention having a yield strength ratio of 0.95 or less has excellent reliability against spring fatigue.

Example 4 Using the test spring a12 shown in Table 5, test springs a17 to a23 having different residual stress distributions and surface layer hardnesses were prepared by changing the shot peening conditions, and the spring fatigue test was conducted. Carried out. The fatigue test conditions were the same as those shown in Table 4. For the same reason as in Example 3, a steel wire having a residual stress distribution and surface hardness equivalent to that of a spring was also subjected to a rotary bending fatigue test. The results are shown in Table 6, and the fatigue characteristics of the spring are not significantly different between the example and the comparative example due to the relationship of the number of tests as in the case of the example 3, but the residual in the rotary bending test. If the tensile stress exceeds 30 kgf / mm ² , fracture originating from inclusions or precipitates will be clearly recognized, and the fatigue strength will obviously decrease. In the surface layer, in both spring and rotary bending tests, if the residual compressive stress is less than 50 kgf / mm ² and the hardness is less than 700, fatigue fracture from the surface origin is likely to occur, and a decrease in fatigue strength is recognized. ..

[0048]

[Table 6]

[0049]

The present invention is configured as described above. Since the spring steel, the steel wire for spring and the spring are improved in terms of both surface and internal characteristics, fatigue fracture is suppressed. The fatigue strength can be remarkably increased as compared with the conventional material, and the reliability of the spring or its material can be remarkably improved.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between an austenite grain size number, a winding breakage rate, and a drawing of a spring steel wire according to the present invention.

Claims (4)

[Claims] 1. A weight ratio of C: 0.5 to 0.8%, Si: 0.8
-2.5%, Mn: 0.4-1.3%, Cr: 0.4-2%, and V: 0.05-0.5% and / or Nb: 0.05-0.
It consists of steel containing 5% and the balance Fe and inevitable impurities, and the melting point of oxide inclusions contained in the steel is 1
A spring steel having excellent fatigue properties, characterized in that the temperature is 500 ° C. or lower, and the size of carbide-based precipitates and nitride-based precipitates is 15 μm or less. 2. C: 0.5-0.8% by weight ratio, Si: 0.8
-2.5%, Mn: 0.4-1.3%, Cr: 0.4-2%, and V: 0.05-0.5% and / or Nb: 0.05-0.
5% and Ni: 0.1 to 2% and / or Mo: 0.1 to 0.5
%, With the balance being Fe and unavoidable impurities, and the melting point of oxide inclusions contained in the steel being 15
A spring steel having excellent fatigue properties, characterized in that the temperature is 00 ° C. or less and the size of carbide-based precipitates and nitride-based precipitates is 15 μm or less. 3. The spring steel according to claim 1 or 2, which is obtained by quenching and tempering, has a grain size of 11 or more, a retained austenite amount of 3 to 20%, and a tensile strength of 205 kgf /
mm ² or more, yield strength ratio after low temperature annealing at 400 ℃ or more is 0.9
A steel wire for a spring, characterized by being 5 or less. 4. A spring manufactured from the spring steel wire according to claim 3, wherein the residual stress satisfies the requirement of the following formula [I] and the maximum hardness (Vickers hardness) of the surface layer portion is 700.
The spring characterized by the above. [Equation 1]