JP2003073776A - Steel wire with high fatigue strength for spring, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for a spring, which contains a reduced amount of oxide-based inclusions in the steel causing reduction of fatigue strength, as much as possible, and hence has a high level of fatigue characteristics, and provide a method for stably manufacturing such a steel of high quality for the spring. SOLUTION: The steel wire with high fatigue strength for the spring is a rolled steel wire including 0.4-1.3% C, 0.1-2.5% Si, 0.1-1.2% Mn, and 0.1% or less Al, and contains oxide-based inclusions with width direction sizes of 5 μm or more, of 2 pieces/100 mm<2> or less, out of those observed in a longitudinal section. The manufacturing method is also disclosed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel wire for a spring, which is used as a valve spring for an automobile engine or the like and has an improved fatigue strength, and a manufacturing method thereof.

Recently, valve springs and the like tend to be smaller in size from the viewpoints of weight reduction and high output of engines, and they have come to be used under more severe conditions, that is, under higher stress load. ing. The present invention deals with such a situation, and can withstand harsh conditions of use.
By reducing the diameter and reducing the number of oxide-based inclusions present in the steel, we will provide spring steel wires with higher levels of fatigue characteristics and ensure such high-performance spring steel wires. It provides a manufacturing method that can be obtained from.

[0003]

2. Description of the Related Art As a general method for producing steel used for springs, after performing converter refining and electric furnace refining, outside refining, for example, slag refining in a molten steel transport container, The method of sending to a continuous casting process or an ingot making process and casting is adopted. In this case, in order to obtain excellent fatigue properties, it is desirable to reduce oxide inclusions that are the starting points of fatigue fracture as much as possible.

By the way, regarding suppression of alumina-based inclusions that are particularly prone to fatigue fracture, for example, Japanese Patent Publication No. 6-
No. 104844, Japanese Examined Patent Publication No. 7-103416, JP-A-6
As disclosed in JP-A-212237, a method is known in which the amount of A1 added is suppressed to a certain amount or less and the inclusion of A1 is reduced as much as possible to reduce the amount of alumina inclusions. When this method is adopted, Al is not used for deoxidizing molten steel in order to suppress mixing of Al, and deoxidation is generally performed with Si or Mn. Silica-based inclusions generated in molten steel by this deoxidation treatment are modified into CaO-SiO ₂ -based inclusions by slag refining using CaO-containing slag, and the inclusions are made to have a composition that is easily expanded during rolling. By doing so, the damage is suppressed as much as possible.

Further, in Japanese Patent Laid-Open No. 11-199982, the number of oxide-based inclusions contained in steel is 20/1.
A high-cleanliness rolled steel material having improved fatigue properties is disclosed by suppressing it to 000 mm ² or less. However, in the case of a general converter / electric furnace → slag refining outside the furnace → casting, mixing of oxides from the refractory of the processing furnace and by-product slag into the steel is unavoidable, and the oxides are stable. It is difficult to suppress the number of system inclusions.

As another technique for reducing inclusions, Japanese Patent Laid-Open No. Sho 60
JP-A-177139 discloses refining by vacuum induction furnace melting (hereinafter referred to as "VIM") or argon-oxygen decarburization (hereinafter referred to as "AOD") under A1 and A1 ₂ O ₃ regulations, and subsequently. After performing electroslag remelting refining (hereinafter referred to as "ESR"), vacuum arc remelting (hereinafter referred to as "VAR") processing,
A method of reducing alumina-based inclusions to the extreme is disclosed.

Since these ESR method and VAR method are refining methods that do not use refractory materials, it is possible to reduce alumina-based inclusions to the limit. However, in the ESR method, since CaO—CaF ₂ slag is usually used, a part of the slag is mixed in the steel. Moreover, CaO is difficult to be reduced and decomposed even by the VAR method and remains as oxide inclusions in the steel until the end. Therefore, in spring steels that require a higher level of fatigue strength, the inclusions cause fatigue fracture. However, there is a problem in providing the valve spring steel for automobiles used under a high stress load environment.

On the other hand, the VAR method is a refining method using neither refractory nor slag, for example, Japanese Patent Application No. 11-1642.
Japanese Patent Publication No. 05 discloses a method of obtaining a high-cleanliness steel having excellent fatigue properties only by the VAR method. However, according to the method disclosed in the publication, A1 in steel ingot after VAR refining
Since the content is limited to 2.3 to 6 ppm, VAR
It is pointed out that the degree of freedom in the selection of steel materials used in the production is extremely narrowed.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to maximize the amount of oxide inclusions present in steel that causes a decrease in fatigue strength. The present invention aims to provide a spring steel having a high level of fatigue characteristics, and a manufacturing method capable of reliably obtaining such a high-quality spring steel.

[0010]

The steel wire for high fatigue strength spring according to the present invention, which has solved the above-mentioned problems, is C: 0.4-1.
3%, Si: 0.1 to 2.5%, Mn: 0.1 to 1.2
%, And A1 is 0.10% or less, and among the oxide inclusions observed in the longitudinal section, 2/100 of the oxide-based inclusions having a size in the width direction of 5 μm or more.
The gist exists when it is less than or equal to mm ² .

In the spring steel, as a selective element,
Ni: 1% or less, Cu: 1% or less, Cr: 2.5% or less, Mo: 1% or less, and V: containing at least one element selected from the group consisting of 0.5% or less, In addition to high fatigue properties, the strength, toughness and ductility are also excellent, which is preferable.

The manufacturing method according to the present invention is positioned as a method capable of reliably obtaining the above-mentioned spring steel wire having excellent fatigue characteristics. The mass% of C: 0.4-
1.3%, Si: 0.1 to 2.5%, Mn: 0.1
A steel material containing 1.2%, A1 of 0.10% or less, and having a reduced CaO content in inclusions in steel is melted and refined at least once by a vacuum arc remelting method, cast, and rolled. It has a gist where it does.

In carrying out this method, the average C in the inclusions contained in the steel material subjected to the melting and refining is averaged.
When the aO content is 25% or less, the diameter and the number of oxide inclusions in the obtained spring steel wire can be further reduced, and as a result, the fatigue properties can be more reliably enhanced. It is preferable because it is possible. Further, as the above steel material, N
i: 1% or less, Cu: 1% or less, Cr: 2.5% or less,
By using one containing at least one selected from the group consisting of Mo: 1% or less and V: 0.5% or less, a spring steel wire excellent in strength, toughness and ductility in addition to fatigue properties can be obtained. Since it can be obtained, it is recommended as a preferred embodiment of the present invention.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Under the circumstances as described above, the present inventors examined the relationship between the number of inclusions contained in a steel material and the fatigue life. As a result, as shown in FIG.
By carrying out the VAR refining, the widthwise size appearing in the longitudinal section of the steel wire rolled to a diameter of 3 to 10 mm is 5
It was confirmed that if the number of oxide inclusions with a size of μm or more is suppressed to 2 or less per 100 mm ² , the breakage rate in the Nakamura-type rotary bending fatigue test will be significantly reduced and excellent fatigue strength will be exhibited. Was done. Here, the diameter of the rolled wire is 3 to
The range of 10 mm is generally such that the diameter of the steel wire rod used for the spring is within this range, and the performance evaluation as the steel wire for spring can be accurately evaluated before the coiling process to the spring. Because.

The steel wire for spring having the above-mentioned characteristics has a C: 0.
4 to 1.3%, Si: 0.1 to 2.5%, Mn: 0.1
.About.1.2% as a basic component, Al is suppressed within a range not exceeding 0.10%, and the average CaO concentration in inclusions is set to 25.
% Or less, preferably 20% or less, more preferably 15%
It can be obtained by using a steel suppressed to the following and rolling a steel ingot obtained by melting and refining the steel once or more by the VAR method. As the steel material used here, Ni: 1% or less, Cu: 1% or less, Cr: 2.5% or less, Mo: 1%
If a steel material containing at least one selected from the group consisting of the following and V: 0.5% or less is used as a selective element, the spring steel wire obtained has not only fatigue characteristics but also strength, toughness and ductility. It is preferable because it is excellent.

First, the reason why the basic composition of the steel material is determined in the present invention will be explained.

C: 0.4 to 1.3% C is an element useful for improving the strength, and in order to effectively exert its action as spring steel, it is 0.4% or more, more preferably 0.1%. It is desirable to contain 5% or more. However, if the amount of C becomes too large, the steel becomes brittle and the toughness is impaired, so it is preferable to keep it to 1.3% or less, more preferably 1.0% or less.

Si: 0.1 to 2.5% Si is an element necessary as a deoxidizing agent, and is at least 0.
It is preferable to contain 1% or more, preferably 0.2% or more. However, if the Si content is too large, the amount of SiO ₂ produced as a deoxidation product becomes too large and becomes the starting point of fatigue fracture. Therefore, it is possible to suppress it to 2.5% or less, more preferably 2.2% or less. desirable.

Mn: 0.1 to 1.2% Mn effectively acts on deoxidation and is an important element for fixing S in steel which adversely affects fatigue properties and preventing its adverse effect. 0.1% or more, more preferably 0.2
% Or more is preferably contained. However, if the Mn content is too large, the hardenability during hot rolling increases, the possibility that the metal structure becomes bainite or martensite structure lacking toughness / ductility increases, and not only the wire drawability deteriorates, but also the spring steel. Since the toughness and ductility of the wire are also reduced, it is desirable to suppress it to 1.2% or less, more preferably 1.0% or less.

Next, the reason for setting the upper limit of the amount of A1 contained in the base steel material to be subjected to the VAR treatment is as follows. Usually, the main component of the oxide-based inclusions contained in the steel material to which A1 is not added as a deoxidizing agent is SiO ₂ , while the main component of the inclusions in the steel material using A1 as a deoxidizing agent is A1
_Although it is ₂ O ₃ , when VAR refining using these steel materials as a base material, both SiO ₂ and A1 ₂ O ₃ are reduced by the following reaction under high pressure reduction (for example, about 10 pa) by VAR treatment.

SiO ₂ + 2C → Si + 2CO ... (1) A1 ₂ O ₃ + 3C → 2A1 + 3CO ... (2) The present inventors have made a steel material not added with A1 and a steel material deoxidized by adding Al. The reduction ratio of SiO ₂ , A1 ₂ O ₃ and CaO was investigated by performing VAR refining as a base material and measuring the area ratio of inclusions and the composition of inclusions appearing in the unit cross section of the steel ingot before and after the VAR treatment. The results shown in FIG. 2 were obtained. The reduction rate was calculated by the following equation (3).

[0022]   Reduction rate (%) = 100−100 × [inclusion area after VAR × (in inclusions     SiO₂, A1₂O₃Or the concentration of CaO)] / [inclusions in the base metal before VAR]     Area x (SiO in inclusions₂, A1₂O₃Or the concentration of CaO)] (3) As is clear from FIG. 2, do not add A1 as a deoxidizer.
The Al content of both the steel materials containing A1 and those containing A1
If less, SiO before and after VAR treatment ₂Return of
There is almost no difference in the rate, and most are reduced
Can be confirmed. However, Al₂O₃Regarding the content
When the direction is slightly different and the Al content in the steel is relatively low
Is Al₂O₃Most of is reduced
However, when the Al content in the steel material becomes relatively large, a considerable amount of
Al₂O₃Remains in the steel in an unreduced state.

From the results shown in FIG. 2, if the Al content in the steel material subjected to the VAR treatment is suppressed to a level of 0.10% or less, the Al ₂ O ₃ reduction rate during the VAR treatment can be made sufficiently high. It is judged that the amount of Al ₂ O ₃ can be maintained and the amount of Al ₂ O ₃ after the treatment can be sufficiently reduced. Therefore, in carrying out the present invention, it is desirable to suppress the Al content in the base steel material subjected to the VAR treatment to 0.10% or less.

Although the VAR refining can sufficiently fulfill its purpose by being carried out only once, the above (1),
It is also effective to carry out the reaction of the formula (2) two or more times in order to further advance the reaction to the right and promote the reduction of the oxide inclusions SiO ₂ , Al ₂ O ₃ and CaO.

Next, the reason for determining the average CaO concentration in the inclusions present in the base steel material subjected to the VAR treatment is as follows. First, as is clear from FIG. 2, the first reason is that CaO is SiO ₂ , A1 ₂ O in oxide inclusions.
_The reduction rate is lower than that of _3, and some of them are likely to remain as an oxide even after the VAR treatment. Therefore, for CaO, which is less likely to be reduced by VAR refining, it is desirable to reduce the CaO content in the inclusions from the base steel material to be treated itself.

On the other hand, the present inventors have confirmed separately that the average CaO concentration in the inclusions contained in the base steel used for VAR refining and the area of the inclusions generated in the steel wire rod with and without VAR refining. It was confirmed that there is a relationship as shown in FIG. 3 between the reduction rates. That is, as is clear from this figure, the area reduction rate of inclusions due to the presence or absence of VAR refining is the average Ca in the inclusions contained in the VAR-treated mother steel.
The average CaO concentration is 25
%, The area reduction rate of inclusions by VAR refining is less than 85%, but if the average CaO concentration is reduced to 25% level or less, the area reduction rate of inclusions becomes stable. It can be seen that the above area reduction rate by VAR refining can secure a high value of 95% or more if the concentration can be increased to 85% or more and the same concentration is suppressed to 20% or less, more preferably 15% or less.

Thus, the CaO concentration in the inclusions causes V
The area reduction rate of inclusions significantly changes depending on the presence or absence of AR refining because CaO in the inclusions is SiO ₂ or Al ₂ O _3.
Also affects the reduction of VAR and increases CaO concentration
It is considered that this is because the reduction efficiency during refining is lowered, and the effect of reducing oxide inclusions is significantly impaired.

From the above, according to the present invention, in order to reduce the amount of oxide inclusions after VAR refining as much as possible and enhance fatigue properties, oxide inclusions contained in the base steel material to be treated to be subjected to VAR refining It is desirable to reduce the average CaO concentration therein as much as possible, preferably 25% or less, more preferably 20% or less, and particularly preferably 15% or less. The means for reducing the average CaO concentration in the inclusions is not particularly limited, but as a general method, a method of decreasing the CaO concentration in the slag during molten steel refining, using Al for molten steel deoxidation Examples include a method of increasing the Al ₂ O ₃ concentration in the inclusions.

Next, the reasons for defining the types and content rates of the selective elements that are allowed to be contained in the steel material will be described.

Ni: 1% or less, Cu: 1% or less, Cr:
One or more Ni selected from the group consisting of 2.5% or less, Mo: 1% or less, and V: 0.5% or less does not contribute so much to the strength increase of the steel wire for springs, but has an effect of enhancing toughness. have. However, its effect saturates at about 1%, so increasing the content beyond that would only be economically disadvantageous.

Cu is an element that contributes to the strengthening of the steel wire by the precipitation hardening action. However, if added excessively, it segregates at the grain boundaries and causes cracks and scratches in the hot rolling process of the steel material, so it should be suppressed to 1% or less, more preferably 0.8% or less.

Cr reduces the activity of C and effectively acts to prevent decarburization during heat treatment. However, if contained too much, the hardenability during heat treatment increases and the toughness and
Since it deteriorates the ductility, it should be suppressed to 2.5% or less, more preferably 2.0% or less.

Mo is an element that enhances the hardenability and contributes to the high strength of the spring steel, but if it is too much, the toughness and ductility are extremely deteriorated, so 1% or less, more preferably 0.8.
% Should be kept below.

V contributes to refinement of crystal grains during heat treatment such as quenching and tempering, and effectively acts to improve toughness and ductility. However, if it is too large, coarse carbides are generated during quenching and heating, which rather reduces toughness and ductility and adversely affects fatigue properties, so 0.5% or less, more preferably 0.4% or less. I have to hold back.

The balance component according to the present invention is substantially Fe, but it is of course permissible to mix inevitable impurities such as P and S, and if necessary, within the range that does not impair the above-mentioned effects of the present invention. In addition, a small amount of Co, which has the effect of improving ductility, or Ti, Nb, which has the effect of refining crystal grains, may be contained.

[0036]

EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples, and may be appropriately applied within the scope of the above and the following points. Modifications can be made, and all of them are included in the technical scope of the present invention.

Example As a steel material to be subjected to VAR refining, a steel ingot produced for a spring by a generally known converter → ladle heating refining → continuous casting process was used. The composition of the steel ingot was adjusted at the ladle refining stage, and the average CaO concentration in the inclusions was adjusted by increasing or decreasing the CaO concentration of the CaO—SiO ₂ slag used during the ladle heating and refining. Specifically, when wollastenite (CaO.SiO ₂ ) is used as the basic slag composition and the CaO concentration is increased, the main component is C
In the case of adding calcined lime which is aO, and in the case of lowering the concentration of CaO, silica stone whose main component is SiO ₂ was added. Table 1
The average CaO concentration in the components and inclusions of the steel material before being subjected to VAR refining is shown in FIG.

[0038]

[Table 1]

A part of each of the above steel materials is machined and the consumable electrode for VAR is 1.5 to 2.1 tons (diameter 340).
˜360 mm × length 2300 to 2700 mm) was prepared, and VAR refining was performed for each, and the obtained VAR ingots were hot-rolled to a steel wire rod having a diameter of 5.5 mm, and low temperature annealing was performed at 660 ° C. And then the diameter is 4.8 mm
Cold drawn up to. For comparison, a steel material not subjected to VAR refining was similarly subjected to hot rolling, low temperature annealing and cold wire drawing to obtain a steel wire material having a diameter of 4.8 mm. A sample for observing cross-sectional inclusions was taken from each of the obtained wire rods having a diameter of 4.8 mm.

Incidentally, "EPMA-8705" manufactured by Shimadzu Corporation was used to quantify the composition of the above inclusions, and an accelerating voltage of 20 was used.
Quantitative analysis of the central portion of the inclusions was performed by wavelength dispersive spectroscopy of characteristic X-rays at kV and a sample current of 0.01 μA. The size and number of inclusions were observed by a scanning electron microscope attached to the EPMA at the same time as determining the composition, and the number of inclusions having a widthwise size of 5 μm or more existing per unit cross section was determined. The elements to be quantified are Al, Mn, Si, Mg,
Ca, Zr, O and S.

A substance having a known concentration of each element is used as a standard sample, and the relationship between the X-ray intensity and the element concentration is prepared in advance as a calibration curve, and the concentration of each element is determined from the X-ray intensity obtained from the inclusion to be observed. Was quantified. Further, each element is A1 ₂ O ₃ , Mn
Assuming that they exist in the form of O, SiO ₂ , MgO, CaO, ZrO ₂ , and S, from the concentration of each element analyzed by EPMA, A 1 ₂ O ₃ , MnO, SiO ₂ , and Mg in the inclusions are obtained.
The composition ratio of O, CaO, ZrO ₂ , and S was calculated.

Each of the steel wires having a diameter of 4.8 mm obtained above was subjected to oil tempering treatment → strain relief annealing → shot peening treatment → strain relief annealing again, and then a Nakamura rotary bending fatigue tester was used. The breakage rate was evaluated. The conditions of the fatigue test are: test piece length: 650 mm, number of test pieces: 3
0 to 50 pieces, test load: 95.8 kgf / mm ² , rotation speed: 4500 rpm, test stop rotation speed: 2 × 10 ⁷ times. The breakage rate was calculated by the following equation (4).

Damage rate = [Number of broken pieces / Number of all test pieces] × 100 (%) (4) In Table 2, the components of the steel wire rod obtained by VAR refining, and the length observed in the cross section of 5 μm or more The number of inclusions and the fracture rate in the Nakamura-type rotating bending fatigue test are shown. Table 1 shows the number of inclusions and the breakage rate of steel wire rods obtained in the same manner as above using an untreated steel ingot not subjected to VAR refining.

[0044]

[Table 2]

The following can be considered from Tables 1 and 2. No. Examples 1 to 7 are examples satisfying the requirements of the present invention, and the number of coarse oxide inclusions was reduced by carrying out VAR refining, and excellent fatigue strength was obtained. For these, No. In Nos. 8 to 11, one of the constituents of the base steel material and the average CaO concentration of the inclusions in the base steel material is out of the specified requirements of the present invention, and the number of oxide-based inclusions is reduced by performing VAR refining. However, the fracture rate is relatively high and the fatigue strength is poor.

[0046]

The present invention is constituted as described above, and A
By providing a steel material in which 1 concentration and the average CaO concentration of inclusions are specified for VAR refining, oxide inclusions in the steel wire material can be minimized, and a spring steel wire having excellent fatigue strength is provided. I was able to do it.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between the number of oxide-based inclusions having a widthwise size of 5 μm or more, which appears in a vertical cross section of a spring steel wire, and a breakage rate in a rotary bending fatigue test.

FIG. 2 SiO ₂ obtained from the area ratio of inclusions and the composition of inclusions appearing in the unit cross section of the steel ingot before and after the VAR treatment,
It is a graph which shows the reduction rate of A1 ₂ O ₃ and CaO.

FIG. 3 is a graph showing the relationship between the average CaO concentration in the inclusions contained in the base steel material subjected to VAR refining and the area reduction rate of the inclusions generated in the steel wire rod with and without VAR refining.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/46 C22C 38/46 F16F 1/02 F16F 1/02 A (72) Inventor Shinya Muranaka Hyogo 2-3-1 Shinhama, Arai-cho, Takasago-shi Kobe Steel Works Takasago Works (72) Inventor Nobuhiko Ibaraki No. 2 Nadahama Higashi-cho, Nada-ku, Kobe-shi Kobe Steel Works Kobe Steel Works (72) Inventor Sue Sumie Kobe-shi 2 Nadahama Higashi-cho, Nada-ku Kobe Steel Works, Ltd. Kobe Steel Works F-term (reference) 3J059 AB11 BC02 BC19 EA02 GA08 4K001 AA10 EA02 FA10 GA16 4K013 AA09 BA07 BA14 CD02 CE00

Claims (5)

[Claims] 1. C: 0.4 to 1.3%, Si: 0.1
2.5%, Mn: 0.1-1.2%, and
A1 is 0.10% or less rolled steel wire, the oxide-based inclusions observed in the longitudinal section, the size of the width direction is 5μm or more 2 / 100mm ² or less High fatigue strength steel wire for springs. 2. Steel as another element, Ni: 1% or less, Cu: 1% or less, Cr: 2.5% or less, Mo: 1%
The steel wire for spring according to claim 1, which contains at least one element selected from the group consisting of the following and V: 0.5% or less. 3. In mass%, C: 0.4 to 1.3%, S
i: 0.1 to 2.5%, Mn: 0.1 to 1.2%, A1 is 0.10% or less, Ca in inclusions in steel
A method for producing a steel wire for high fatigue strength spring, which comprises melting and refining a steel material having a reduced O content at least once by a vacuum arc remelting method, casting and rolling. 4. The production method according to claim 3, wherein the average CaO content in inclusions contained in the steel material to be melt-refined is 25% or less. 5. The steel contains, as other elements, Ni: 1% or less, Cu: 1% or less, Cr: 2.5% or less, Mo: 1%.
The method according to claim 3 or 4, which comprises at least one element selected from the group consisting of the following and V: 0.5% or less.