JP2003213372A - Steel wire for spring and spring - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel wire for a spring in which the fatigue resistance of the material itself is improved without addition of expensive elements and expensive treatment, to provide a production method therefor and a spring. <P>SOLUTION: The steel wire for a spring contains, by mass, 0.50 to 0.90% C, 1.0 to 3.0% Si, 0.5 to 1.5% Mn and 0.1 to 5.0% Cr, and has a metallic structure mainly consisting of tempered martensite. The old austenite grain size (JIS G 0551) of tempered martensite is #11 to 15, and the maximum grain size is ≤8 μm. By the refining of the grain size, the density of the grain boundaries is increased, so that the strength of the material itself is improved, and its fatigue resistance is improved. <P>COPYRIGHT: (C)2003,JPO

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 used in automobiles and the like, and a steel wire for a spring applied to its manufacture.

[0002]

2. Description of the Related Art A valve spring used in an automobile engine is used under high stress and high rotation, and is one of the springs in the most severe operating environment. As the weight of automobiles has been reduced in recent years, the spring steel wire is required to have higher strength and longer life, that is, higher fatigue resistance. Generally, chrome vanadium steel oil tempered wire for valve springs and silicon chrome steel oil tempered wire for valve springs are used as materials for valve springs, and their fatigue resistance has been advanced.

JP-A-62-177152 (Patent No. 2650225)
No.), an element such as Mo is added, and a nitriding treatment is further performed to increase the strength. Further, in Japanese Patent Laid-Open No. 5-320827 (Patent No. 2898472), Mo, Nb
By increasing the size and composition of inclusions by adding such elements as above, the strength is increased. Furthermore, JP-A-02
-247354 in the former austenite crystal grain size, JP 11-6033 JP in the old austenite crystal grain size and undissolved carbide, JP 9-71843 JP by limiting the residual γ amount fatigue characteristics We are trying to improve.

[0004]

However, the above-mentioned technique has a problem in that expensive treatments and additional elements are added in spite of strong demand for cost reduction in recent years. In order to carry out wire drawing, it is necessary to make the structure a pearlite structure, but the cooling rate for transforming to a pearlite structure by adding Mo must be controlled to a slow cooling side compared to conventional materials, which is industrially Is also not suitable.

Further, it is important to control and reduce the portions that may be the starting point of fatigue fracture, such as undissolved carbides, residual γ amount, and inclusions, but further improvement in fatigue resistance is desired.

Therefore, a main object of the present invention is to provide a spring steel wire and a spring which can improve the fatigue resistance of the material itself without adding expensive elements and performing expensive treatment as much as possible.

[0007]

A first feature of the present invention is that C: 0.50 to 0.90, Si: 1.0 to 3.0, M in mass%.
n: 0.5 to 1.5, Cr: 0.1 to 5.0, a steel wire for a spring, the metal structure of which is mainly tempered martensite,
The former austenite grain size number (JIS G O551) of tempered martensite is # 11 to # 15, and the maximum grain size is 8
It is to be less than μm.

Here, the phrase "mainly composed of tempered martensite as the metal structure" means that tempered martensite is 90
Percentage of more than% is included. Further as a chemical component, in mass% Mo: 0.05 to 0.50, V: 0.05 to 0.50, W: 0.05 to
0.15, Nb: 0.05 to 0.15, Ti: 0.01 to 0.20, Ni: 0.02 to 1.
One or more of 00, Co: 0.02 to 1.00 and Cu: 0.02 to 1.00 may be contained. Other than these chemical components, those composed of Fe and inevitable impurities are preferable.

In order to improve the fatigue resistance of the material itself, it is possible to reduce the crystal grain size. Because the grain boundaries of crystals are stronger than those inside the grains,
This is because the strength can be improved by increasing the density of crystal grain boundaries due to the miniaturization.

That is, if the grain size is made coarse, the toughness deteriorates, and it is necessary to make it # 11 or more in order to obtain sufficient toughness. In addition, in order to miniaturize, it is necessary to lower the heating temperature during quenching, or to heat in a short time,
In this case, undissolved carbides are generated and the toughness is reduced.
Specified below 15.

In addition to the grain size number, the maximum grain size is 8
The reason why it is specified to be μm or less is that if there is even one large crystal, stress concentrates on the large crystal grain and becomes a starting point of fatigue fracture. The grain size numbers # 11 to # 15 mean that the grain size is 6.2 μm to 1.6 μm, but this is the average grain size and does not specify the maximum grain size.
The present invention also defines the maximum particle size.

Such a refinement of the crystal grain size can be made under the conditions that the heating temperature during quenching is 800 to 1100 ° C. and the heating time is 1 to 10 s. The heating time is the time to maintain the heating temperature during quenching. In order to obtain a structure in which no large crystals locally exist, it can be produced by lowering the heating temperature and heating uniformly for a short time. Further, the fatigue-resistant property of the spring-formed spring steel wire can be improved without performing expensive nitriding treatment, but it is considered that the fatigue resistance property is further improved by performing a surface hardening method such as nitriding treatment.

A second feature of the present invention is that the maximum grain size of the block in martensite is specified to be 600 nm or less, in addition to the grain size and the maximum grain size specified above.

The third feature of the present invention is that the yield ratio is 0.97 or more, the tensile strength is 1800 to 2300 N / mm ² , and the wire surface hardness is HmV 600 or more. Is.

Next, the reasons for limiting the constituents of the present invention will be described.

<C: 0.50 to 0.90 mass%> C is an essential element for increasing the strength of steel, but if it is less than 0.50%, sufficient strength cannot be obtained, and if it exceeds 0.90%, the grain boundary is increased. Cementite precipitates in the and the toughness decreases.

<Si: 1.0 to 3.0% by Mass> Si is an element effective in improving the strength of the ferrite and the sag resistance by forming a solid solution in the ferrite. 1.
If it is less than 0%, sufficient sag resistance cannot be obtained. On the contrary, if it exceeds 3.0%, the solid solution is not completely dissolved and the toughness is lowered, and the spring workability is lowered.

<Mn: 0.5 to 1.5% by mass> Mn is an element effective in improving the hardenability of steel. If it is less than 0.5%, its effect is small, and if it exceeds 1.5%, toughness is lowered.

<Cr: 0.1 to 5.0 mass%> Like Mn, Cr is an element effective for improving the hardenability of steel and enhancing the softening resistance at the time of tempering to enhance the strength. If it is less than 0.1%, its effect is small, and if it exceeds 5.0%, the hardenability is excessively increased and the toughness is lowered.

<Mo: 0.05 to 0.50 mass%> Mo is an element that forms carbides during tempering and increases the softening resistance. 0.
If it is less than 05%, its effect is small, and if it exceeds 0.50%, the wire drawability is deteriorated.

<V: 0.05 to 0.50, W: 0.05 to 0.15, Nb: 0.
05-0.15 mass%> W, Nb, and V also have the effect of forming carbides in the steel during tempering and increasing the softening resistance. However, in all cases, if it is less than 0.05%, the effect cannot be exhibited. Conversely, V
Over 0.50%, and for W and Nb over 0.15%, both form a large amount of carbide during quenching and heating, reducing toughness.

<Ti: 0.01 to 0.20% by mass> Ti also has the effect of forming carbides in the steel during tempering and increasing the softening resistance. However, Ti produces TiO which is a refractory nonmetallic inclusion. Therefore, it is important to set the conditions for refining. The amount that can be expected to improve the softening resistance is 0.01% or more, and is 0.20% or less in consideration of deterioration of toughness due to excessive increase of carbides and inclusions.

<Ni: 0.02 to 1.00, Co: 0.02 to 1.00, Cu:
0.02 to 1.00% by mass> Ni, Co, and Cu are austenite-forming elements, and are materials that greatly reduce the Ms point by adding Ni, Co, and Cu and easily generate retained austenite. Increasing the retained austenite has the effect of reducing the hardness of the steel wire, but conversely solid solution strengthening with Si and Mo, W, Nb, V, Ti
Has the effect of imparting toughness to the steel wire reinforced by carbide precipitation elements such as. In addition, Ni is Cl in a corrosive environment.
It also has the role of blocking the entry of elements. 0.02% as the minimum limit that has the effect of improving toughness, and as the upper limit that does not cause hardness decrease
It was 1.00%.

<Maximum thickness of block in martensite: 600 nm or less> The former austenite grain size number of tempered martensite is an effective evaluation method of martensite structure. However, the structure of tempered martensite is composed of prior austenite, packets, blocks, and laths, and strictly speaking, the thickness of blocks composed of laths having the same crystal orientation greatly affects strength and toughness. Therefore, in order to obtain sufficient toughness, the maximum block thickness was specified to be 600 nm or less.

<Yield ratio: 0.97 or more> In order to further improve the fatigue resistance of the material itself, it is necessary to strengthen the inside of the grains. In general, the fatigue fracture of high strength material is caused by the occurrence of cracks due to the concentration of slip bands. Therefore, it is necessary to have a high elastic limit in order to reduce the occurrence of slip bands, and the yield ratio was specified as 0.97 or higher.

<Tensile Strength: 1800 to 2300 N / mm ² > When the tensile strength is less than 1800 N / mm ² , sufficient strength as a spring cannot be obtained and sag resistance is also deteriorated. On the other hand, 2300N /
If it exceeds mm ² , toughness is lowered and the workability of the spring is lowered.

<Hardness of wire surface: HmV 600 or more> When the surface of the steel wire of the present invention is hardened, further improvement in fatigue resistance can be expected. Nitriding treatment, carburizing treatment, and the like are available as hardening methods, but these surface treatments are easily affected by the mechanical properties of the base material. That is, it is possible to expect a remarkable improvement in the fatigue limit by combining a spring manufactured using the steel wire of the present invention with these conventional surface hardening methods. In the present invention, HmV600 is set as the lower limit of the surface hardness that can be expected to improve the fatigue limit.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. (Example 1) Melting a sample steel having the chemical composition shown in Table 1,
Wire diameter by rolling, heat treatment (patenting), and wire drawing
The wire rod is from 8.0 mm to 3.5 mm. This wire is heated at the heating temperature and heating time shown in Tables 3 and 3 and then tempered to prepare a sample, and then based on the former austenite grain size number, maximum grain size, maximum block thickness measurement and tensile test. The yield ratio was calculated. In addition to the elements listed in Table 1, the test steel consists of Fe and the unavoidable impurities in the balance. The patenting conditions were a holding temperature of 600 ° C. and a holding time of 40 seconds. The tempering conditions were a heating temperature of 480 ° C. and a holding time of 1 second. The grain size number was determined based on JIS G O551. Maximum grain size,
The maximum block thickness was obtained by enlarging the sample with a microscope.

Further, the samples prepared by processing a spring in the spring specifications shown in Table 4, was subjected to shot peening, performed fatigue test by a spring fatigue tester, in a non-breakage amplitude number 1 × 10 ⁷ times A certain amplitude stress was measured. The average stress in the fatigue test is 700 MPa. The test results are shown in Tables 2 and 3.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Table 4]

As described above, A1 and a1, B1 and b1, respectively.
From the result of comparison between C1 and c1, it can be seen that the steel of the present invention has large amplitude stress and increases by about 60 MPa. Also, B1 and C1
From the results, it can be seen that the addition of Mo or Ni does not improve the amplitude stress.

From the results of b3, B1, B6 and B7, it can be seen that the larger the grain size number, the higher the amplitude stress.
From the results of b2, B1 and B8, it can be seen that the amplitude stress increases as the maximum grain size decreases. From the results of B1, B2, and B9, it can be seen that the amplitude stress increases as the maximum thickness of the block decreases. From the results of B1, B3, and B10, it can be seen that the amplitude stress increases as the yield ratio increases.

Similar results were obtained for spring steel wires having tensile strengths of 1800 MPa and 2250 MPa.

Example 2 With respect to the springs of the invention steel B1 and the comparative steel b1 in Table 3 above, the surface hardness was increased by nitriding treatment and the like, and a spring fatigue test was conducted in the same manner as in Example 1. Table 5 shows the nitriding conditions, surface hardness, and fatigue test results.

[0038]

[Table 5]

As described above, in the steel of the present invention, the amplitude stress is remarkably improved as the surface hardness becomes higher than that of the comparative steel.

[0040]

As described above, according to the present invention, the fatigue resistance can be improved without any special treatment. As a result, the diameter of the spring can be reduced, and the automobile engine can be reduced in size and weight.

Further, it is industrially effective to reduce the raw material cost by reducing the diameter and reduce the processing cost by omitting the surface hardening method.

Further, by combining conventional surface hardening methods, further improvement in fatigue resistance can be obtained.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kento Yamao             Sumitomo, 1-1 1-1 Koyokita, Itami City, Hyogo Prefecture             Electric Industry Co., Ltd. Itami Works (72) Inventor Teruyuki Murai             Sumitomo, 1-1 1-1 Koyokita, Itami City, Hyogo Prefecture             Electric Industry Co., Ltd. Itami Works F-term (reference) 3J059 AB11 BA01 BA31 BC02 EA09                       GA08

Claims (7)

[Claims] 1. The chemical composition in% by mass, C: 0.50 to 0.90, S
i: 1.0 to 3.0, Mn: 0.5 to 1.5, Cr: 0.1 to 5.0, which is a steel wire for spring mainly composed of tempered martensite. The former austenite grain size number (JIS G O551 ) Is # 11 to # 15, and the maximum grain size is
Steel wire for springs characterized by having a diameter of 8 μm or less. 2. Further, in mass%, Mo: 0.05 to 0.50, V: 0.0
5 to 0.50, W: 0.05 to 0.15, Nb: 0.05 to 0.15, Ti: 0.01 to
0.20, Ni: 0.02-1.00, Co: 0.02-1.00, Cu: 0.02-1.
2. One or more of 00 are contained, The claim 1 characterized by the above-mentioned.
The steel wire for spring according to. 3. The maximum thickness of the block in martensite is 600 nm or less.
Alternatively, the steel wire for spring according to item 2. 4. The steel wire for spring according to claim 1, which has a yield ratio of 0.97 or more. 5. The steel wire for spring according to claim 1, wherein the tensile strength is 1800 to 2300 N / mm ² . 6. A spring manufactured by using the steel wire for spring according to claim 1. Description: 7. The spring according to claim 6, wherein the hardness of the surface is HmV600 or more.