JP2004010965A - High strength spring steel having excellent corrosion fatigue strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high strength spring steel having excellent corrosion fatigue strength which has satisfactory settling resistance, and is free from the problem of ferrite decarburization. <P>SOLUTION: The spring steel has a composition comprising, by weight, 0.38 to 0.48% C, 1.6 to 2.8% Si, ≥4.0 Si/C, 0.6 to 1.2% Mn, ≤0.015% P, ≤0.005% S, 0.15 to 0.45% Cu, 0.05 to 0.30% Ni, ≥0.20% Cu+Ni, 0.10 to 0.30% Cr and ≤0.0012% O, and the balance substantially Fe. <P>COPYRIGHT: (C)2004,JPO

Description

【００３０】
また腐食疲労強度試験は以下に示すように塩水噴霧，大気中加振，恒温恒湿槽放置を順に行ってこれを１サイクルとし、そしてこれを繰り返して試験体が破断した時点の大気中加振回数の合計を求めて評価を行った。
但し評価は表１の従来鋼の大気中加振回数を１．０として、これに対する比率で表した。
【００３１】
＜腐食疲労強度試験＞
▲１▼塩水噴霧
（５％ＮａＣｌ，３５℃×３０ｍｉｎ）
↓
▲２▼大気中加振
（３０００回，３０ｍｉｎ）
↓
▲３▼恒温恒湿槽放置
（２６℃，９５％×２３ｈｒ）
【００３２】
一方耐へたり特性（残留剪断歪み）の測定は、図１（Ｂ）に示す試験体１０を以下の条件で製造し、これを図１（Ａ）に示す重錘式捩りクリープ試験機１２を用いて測定することにより行った。
【００３３】
直径２０ｍｍの棒材から、図１（Ｂ）に示す形状の試験体１０を切り出し、試験体の硬さがＨＲＣ５２となるように焼入れ焼戻し処理を施して、試験体１０とした。
【００３４】
即ち８０℃の温度条件の下でアーム１４の基端に試験体１０の一端を固定する一方、アーム１４の先端に重錘１６を吊り下げ、７２時間後のアーム１４の先端の変位をダイヤルゲージ１８で計測することにより行った。
尚図中２０は試験片保持台であり、２２はジャッキである。
【００３５】
また表層部脱炭（フェライト脱炭）の測定は、次の条件で製造した試験体について、以下のようにＥＰＭＡ装置で線分析を行い評価した。
【００３６】
圧延用素材を加熱温度１２００℃，終止温度９５０℃，圧延後の冷却速度１℃／ｓｅｃの条件にて直径１３ｍｍの線材に圧延し、そしてこれよりサンプルを切り出して試験体とした。
そしてその横断面を鏡面研磨し、表層部分のＣ濃度分布をＥＰＭＡ装置にて測定した。
【００３７】
これらの結果が表１及び図２〜図４に併せて示してある。
尚、図２は表１で得られた結果に基づいてＣ量或いはＳｉ／Ｃ比と腐食疲労強度比との関係を、図３は同じく表１で得られた結果に基づいてＳｉ含有量或いはＳｉ／Ｃ比と耐へたり特性（残留剪断歪み）との関係を、更に図４はＣｕ＋Ｎｉ含有量と脱炭深さとの関係をそれぞれ表したものである。
【００３８】
図２（Ａ）及び表１に表れているように、Ｃ量については本発明の上限値である０．４８％超では腐食疲労強度比が１．０より低く、腐食疲労強度が低下するのに対し、Ｃ量を０．４８％以下に抑えることで、腐食疲労強度比が目標値である１．０以上となり、腐食疲労強度が良好であることが分る。
【００３９】
但し図２（Ｂ）に表れているように、Ｓｉ／Ｃ比が４．０より小さいと腐食疲労強度比は１．０よりも小さく十分でないこと、逆にＳｉ／Ｃ比を４．０以上にすることで腐食疲労強度比が１．０以上となり、腐食疲労強度が良好となることが分る。
【００４０】
一方耐へたり特性については、図３（Ａ）からＳｉ含有量を本発明の下限値である１．６％以上とすることで１．０×１０^−３よりも良好となること、更にこの場合においてもＳｉ／Ｃ比が重要で（（Ｂ）参照）、Ｓｉ／Ｃ比を４．０以上とすることで、耐へたり特性が良好となることが分る。
【００４１】
図４は脱炭深さに対するＣｕ＋Ｎｉ含有量の影響を示したもので、この図４に表れているように、Ｃｕ＋Ｎｉ含有量を多くすることで脱炭深さは小さくなっている。即ちフェライト脱炭が抑制される傾向にあることが分る。
【００４２】
但しＳｉ含有量が本発明の上限値である２．８％よりも多い３．０％のものの場合、Ｃｕ＋Ｎｉ含有量の増加につれて脱炭深さは小さくなっているものの、全体のレベルは従来鋼のもの（４８μｍ）に比べて脱炭深さが深く、不十分なものとなっている。
【００４３】
これに対しＳｉ含有量が本発明の範囲内にあるものについては、Ｃｕ＋Ｎｉ含有量を０．２０％以上とすることで脱炭深さが従来鋼のそれに比べて小さく、またその脱炭深さはＣｕ＋Ｎｉ含有量を０．２０％よりも多くしてもその効果はほぼ０．２０％で飽和し、Ｃｕ＋Ｎｉ含有量が０．２０％以上の範囲内で脱炭深さが何れも良好な値を示していることが分る。
【００４４】
また表１の発明例Ｎｏ．１９〜２３に示しているように、Ｖ，Ｎｂ，Ｔｉ，Ａｌ，Ｎ，Ｂ等を本発明の範囲内で含有させることで、ばね鋼として必要な良好な特性が得られることが分る。
【００４５】
以上本発明の実施例を詳述したがこれはあくまで一例示であり、本発明はその主旨を逸脱しない範囲において種々変更を加えた態様で実施可能である。
【図面の簡単な説明】
【図１】本発明の実施例のばね鋼の耐へたり特性の測定に用いた試験機を試験体とともに示す説明図である。
【図２】本発明の実施例において得られたＣ含有量，Ｓｉ／Ｃ比と腐食疲労強度比との関係を表した図である。
【図３】本発明の実施例において得られたＳｉ含有量，Ｓｉ／Ｃ比と残留剪断歪みとの関係を表した図である。
【図４】本発明の実施例において得られたＣｕ＋Ｎｉ含有量と脱炭深さとの関係を表した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a spring steel, and more particularly to a high-strength spring steel having excellent corrosion fatigue strength.
[0002]
[Prior art and problems to be solved by the invention]
In recent years, there has been a strong demand for improving the fuel efficiency of automobiles, and in connection with this, weight reduction of automobiles has been demanded.
As part of this, weight reduction of suspension springs and the like in automobiles is required, and various measures are taken for that purpose.
[0003]
Here, in order to achieve weight reduction of the suspension spring or the like, it is necessary to reduce the diameter of the spring, reduce the number of turns, etc., but in this case, the load stress applied to the spring increases.
Conventionally, SUP7 has been used as a typical material for this type of suspension spring and the like, but it is necessary to further increase the strength of the spring steel as the load stress increases.
[0004]
Therefore, the strength can be increased by increasing the C content of the spring steel. In this case, however, there is a problem that the corrosion fatigue strength is decreased by increasing the C content.
In this kind of spring steel, it is important not to lower the corrosion resistance, particularly pitting corrosion resistance. However, if the C content is increased, the corrosion resistance is lowered.
On the other hand, when the C content is lowered, not only high strength cannot be realized, but also the sag resistance characteristics deteriorate.
[0005]
In this case, the sag resistance can be improved by increasing the Si content, but if the Si content is increased, ferrite decarburization is likely to occur in the manufacturing process.
Thus, when ferritic decarburization occurs, the strength as a spring steel is adversely affected. In addition to the deterioration of manufacturability, there is a problem that fatigue characteristics in the atmosphere (atmospheric fatigue characteristics) are also deteriorated.
[0006]
[Means for Solving the Problems]
The high-strength spring steel excellent in corrosion fatigue strength of the present invention has been devised to solve such problems.
Thus, in the first aspect, the composition of the spring steel is, by weight, C: 0.38 to 0.48%, Si: 1.6 to 2.8%, Si / C: ≧ 4.0, Mn: 0.6 to 1.2%, P: ≦ 0.015%, S: ≦ 0.005%, Cu: 0.15 to 0.45%, Ni: 0.05 to 0.30%, Cu + Ni : ≧ 0.20%, Cr: 0.10 to 0.30%, O: ≦ 0.0012%, the balance being substantially composed of Fe.
[0007]
Further, according to claim 2, in claim 1, one or more of V, Nb, Ti and Al are further contained in the following ranges in terms of% by weight, V: ≦ 0.20%, Nb: 0.020˜ 0.050%, Ti: 0.030-0.070%, Al: 0.010-0.040% and N: ≦ 0.012%, B: 0.0005-0.0030% It is characterized by that.
[0008]
[Operation and effect of the invention]
The present invention as described above reduces the C content by reducing the C content to 0.48% or less, while reducing the C content by adjusting the Si content to 1.6 to 2.8%. The deterioration of the sag resistance due to the above is improved by adjusting the Si content so as to ensure the desired sag resistance.
[0009]
However, the ratio of Si / C is important in the present invention, and if this Si / C ratio is less than 4.0, sufficient corrosion fatigue strength cannot be obtained even if the C content is reduced to 0.48% or less. Also, the desired sag characteristics cannot be obtained.
[0010]
In the present invention, the corrosion fatigue strength is increased by reducing the C content, and the sag resistance is enhanced by adjusting the Si content. However, when the Si content is increased, the above-described ferrite decarburization is performed. It tends to occur.
Therefore, in the present invention, a predetermined amount of Cu + Ni is added as a decarburization suppressing component.
[0011]
As described above, according to the present invention, it is possible to obtain a spring steel having high strength, strong corrosion fatigue strength, good sag resistance, good ferrite decarburization, and good manufacturability.
[0012]
In the present invention, V, Nb, Ti, Al, N, and B can be contained as necessary, and the characteristics of the spring steel can be further improved.
[0013]
Next, the reasons for limiting each chemical component in the present invention will be described in detail below.
C: 0.38 to 0.48%
If C is less than 0.38%, the desired spring strength cannot be obtained.
On the other hand, if it exceeds 0.48%, the toughness after quenching and tempering decreases, and the corrosion fatigue strength and delayed fracture resistance deteriorate. On the other hand, if it exceeds 0.48%, the hardness after rolling becomes too hard and the productivity is lowered. Therefore, in the present invention, the upper limit is 0.48%.
[0014]
Si: 1.6 to 2.8%
If Si is less than 1.6%, the strength and sag resistance required for the spring cannot be ensured, so 1.6% or more.
On the other hand, the content of 2.8% or less is to prevent embrittlement, and to prevent deterioration of machinability and workability associated with surface layer decarburization, the upper limit is 2.8%.
[0015]
Mn: 0.6 to 1.2%
Mn is made 0.6% or more in order to prevent deoxidation and damage caused by S, and to secure hardenability and prevent strength reduction.
On the other hand, the content of 1.2% or less is to prevent embrittlement and deterioration of workability.
[0016]
P: ≦ 0.015%
P segregates at the crystal grain boundary, weakens the crystal grain boundary, and causes delayed fracture.
[0017]
S: ≦ 0.005%
When S is present in steel, it takes the form of inclusions of MnS, and it is desirable to reduce it as much as possible in order to reduce atmospheric fatigue characteristics and corrosion fatigue strength. Therefore, the upper limit is made 0.005% in order to reduce the influence.
[0018]
Cu: 0.15-0.45%
Cu is effective in enhancing corrosion resistance, and is also effective in suppressing ferrite decarburization, so 0.15% or more is added to obtain an effect of improving corrosion resistance and an effect of suppressing decarburization.
On the other hand, if adding more than 0.45%, hot workability is impaired, so the upper limit is made 0.45%.
[0019]
Ni: 0.05-0.30%
Ni is effective in enhancing the corrosion resistance, and is also effective in suppressing ferrite decarburization, so 0.05% or more is added to obtain an effect of improving corrosion resistance and an effect of suppressing decarburization.
On the other hand, if the addition is more than 0.30%, the cost increases, so the upper limit is made 0.30%.
Further, by adding Cu and Ni in combination (Cu + Ni ≧ 0.20%), ferrite decarburization is suppressed, and not only the intended effect of the present invention is obtained, but also the effect of suppressing delayed fracture strength deterioration is obtained. It is done.
[0020]
Cr: 0.10 to 0.30%
Cr is effective for adjusting the hardenability, but if it is less than 0.10%, the effect of improving the hardenability cannot be obtained, so the content is made 0.10% or more.
On the other hand, if it exceeds 0.30%, it becomes too hard after rolling, and the workability is impaired, so the upper limit is made 0.30%.
[0021]
O: ≦ 0.0012%
If O is contained in a large amount, oxide-based inclusions are generated, and the upper limit is made 0.0012% in order to reduce atmospheric fatigue characteristics and corrosion fatigue strength.
[0022]
V: ≦ 0.20%
V is effective for crystal grain refinement, contributes to precipitation hardening, and improves sag resistance. However, the carbide of V becomes a local electrode on the steel surface, forms corrosion pits, and becomes a starting point of crack fracture, so it is made 0.20% or less. On the other hand, if it exceeds 0.20%, it becomes too hard after rolling and the workability is impaired.
[0023]
Nb: 0.020 to 0.050%
Nb is effective for refining crystal grains, contributes to precipitation hardening, and improves sag resistance. In order to make the crystal grains fine and improve sag resistance, 0.020% or more is contained. On the other hand, if it exceeds 0.050%, the effect is not only saturated, but also the hot and cold workability is lowered, so the upper limit is made 0.050%.
[0024]
Ti: 0.030 to 0.070%
Al: 0.010 to 0.040%
Like Nb, Ti and Al are effective for refining crystal grains, contribute to precipitation hardening, and improve sag resistance. In order to refine the crystal grains and improve the sag resistance, Ti is contained by 0.030% or more, and Al is contained by 0.010% or more.
On the other hand, when Ti exceeds 0.070% and Al exceeds 0.040%, not only the effect is saturated, but also the hot and cold workability is lowered. 0.040%.
In order to generate oxide inclusions, oxygen (O) is desirably 12 ppm or less.
[0025]
N: ≦ 0.012%
N forms TiN-based inclusions and decreases the atmospheric fatigue characteristics and corrosion fatigue strength of steel to 0.012% or less.
[0026]
B: 0.0005 to 0.0030%
B preferentially precipitates at the grain boundaries of the steel and prevents segregation of P and S crystal grains and improves delayed fracture strength. In order to obtain this effect, 0.0005% or more is necessary.
On the other hand, if it exceeds 0.0030%, a B component is formed at the crystal grain boundary, the hardenability is reduced and the toughness is impaired, so the upper limit is made 0.0030%.
[0027]
【Example】
Next, examples of the present invention will be described in detail below.
Steels having the chemical composition shown in Table 1 were melted to produce specimens for corrosion fatigue strength test, sag resistance measurement, and decarburization measurement (Comparative Example No. 33 in the table is SUP7).
The test body for the corrosion fatigue strength test was a solid spring and was manufactured under the following conditions and in the following shape.
[0028]
A wire spring was drawn from a material produced by rolling to a wire diameter of 12.5 mm, and then a coil spring having the following shape was formed hot.
Wire diameter: 12.5mm
Coil diameter: 110.0mm
Free height: 382mm
Effective number of turns: 5.39, and after forming the spring, quenching and tempering are performed so that the hardness of each coil spring is HRC52, and shot peening and sag resistance are ensured to further improve fatigue strength. A solid spring was manufactured by setting the material to obtain a test body.
[0029]
[Table 1]

[0030]
As shown below, the corrosion fatigue strength test is carried out in the order of salt spray, atmospheric vibration, and standing in a constant temperature and humidity chamber to make this a cycle, and this is repeated in the atmosphere when the specimen breaks. The total number of times was calculated and evaluated.
However, the evaluation was expressed as a ratio with respect to 1.0 in which the number of vibrations of the conventional steel in Table 1 in the atmosphere was 1.0.
[0031]
<Corrosion fatigue strength test>
(1) Salt spray (5% NaCl, 35 ° C. × 30 min)
↓
(2) Excitation in the atmosphere (3000 times, 30 min)
↓
(3) Leave in a constant temperature and humidity chamber (26 ° C, 95% x 23 hours)
[0032]
On the other hand, the measurement of the sag resistance (residual shear strain) is performed by manufacturing the test body 10 shown in FIG. 1B under the following conditions and using the weight-type torsional creep test machine 12 shown in FIG. It was performed by using and measuring.
[0033]
A test body 10 having a shape shown in FIG. 1B was cut out from a rod having a diameter of 20 mm, and subjected to quenching and tempering treatment so that the hardness of the test body was HRC52.
[0034]
That is, one end of the test body 10 is fixed to the base end of the arm 14 under a temperature condition of 80 ° C., while the weight 16 is suspended from the tip of the arm 14, and the displacement of the tip of the arm 14 after 72 hours is measured with a dial gauge. This was done by measuring at 18.
In the figure, 20 is a specimen holder and 22 is a jack.
[0035]
Moreover, the measurement of surface part decarburization (ferrite decarburization) evaluated the test body manufactured on the following conditions by performing a line analysis with an EPMA apparatus as follows.
[0036]
The rolling material was rolled into a wire having a diameter of 13 mm under the conditions of a heating temperature of 1200 ° C., an end temperature of 950 ° C., and a cooling rate after rolling of 1 ° C./sec.
And the cross section was mirror-polished and C concentration distribution of the surface layer part was measured with the EPMA apparatus.
[0037]
These results are shown together in Table 1 and FIGS.
2 shows the relationship between the C amount or Si / C ratio and the corrosion fatigue strength ratio based on the results obtained in Table 1, and FIG. 3 shows the Si content or the relationship between the results obtained in Table 1. FIG. 4 shows the relationship between the Si / C ratio and sag resistance (residual shear strain), and FIG. 4 shows the relationship between the Cu + Ni content and the decarburization depth.
[0038]
As shown in FIG. 2 (A) and Table 1, when the amount of C exceeds 0.48% which is the upper limit of the present invention, the corrosion fatigue strength ratio is lower than 1.0, and the corrosion fatigue strength decreases. On the other hand, by suppressing the C content to 0.48% or less, it can be seen that the corrosion fatigue strength ratio becomes 1.0 or more, which is the target value, and the corrosion fatigue strength is good.
[0039]
However, as shown in FIG. 2B, if the Si / C ratio is less than 4.0, the corrosion fatigue strength ratio is less than 1.0 and is not sufficient, and conversely, the Si / C ratio is 4.0 or more. It can be seen that the corrosion fatigue strength ratio is 1.0 or more, and the corrosion fatigue strength is improved.
[0040]
On the other hand, with respect to sag resistance, it is better than 1.0 × 10 ⁻³ by setting the Si content to 1.6% or more, which is the lower limit of the present invention, from FIG. Even in this case, the Si / C ratio is important (see (B)), and it can be seen that when the Si / C ratio is 4.0 or more, the sag resistance is improved.
[0041]
FIG. 4 shows the influence of the Cu + Ni content on the decarburization depth. As shown in FIG. 4, the decarburization depth is reduced by increasing the Cu + Ni content. That is, it can be seen that ferrite decarburization tends to be suppressed.
[0042]
However, in the case where the Si content is 3.0%, which is higher than the upper limit of 2.8% of the present invention, the decarburization depth decreases as the Cu + Ni content increases, but the overall level is the conventional steel. Decarburization depth is deeper than that (48 μm), which is insufficient.
[0043]
On the other hand, in the case where the Si content is within the range of the present invention, the decarburization depth is smaller than that of the conventional steel by setting the Cu + Ni content to 0.20% or more, and the decarburization depth. Even if the Cu + Ni content is more than 0.20%, the effect is saturated at about 0.20%, and the decarburization depth is good in the range where the Cu + Ni content is 0.20% or more. It can be seen that
[0044]
Inventive example Nos. As shown to 19-23, it turns out that the favorable characteristic required as spring steel is acquired by containing V, Nb, Ti, Al, N, B, etc. within the scope of the present invention.
[0045]
Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented in a mode in which various changes are made without departing from the gist of the present invention.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing a test machine used for measuring the sag resistance characteristics of a spring steel according to an embodiment of the present invention together with a specimen.
FIG. 2 is a diagram showing the relationship between C content, Si / C ratio and corrosion fatigue strength ratio obtained in an example of the present invention.
FIG. 3 is a graph showing the relationship between Si content, Si / C ratio and residual shear strain obtained in an example of the present invention.
FIG. 4 is a graph showing the relationship between the Cu + Ni content and the decarburization depth obtained in the examples of the present invention.

Claims (2)

% By weight
C: 0.38 to 0.48%
Si: 1.6 to 2.8%
Si / C: ≧ 4.0
Mn: 0.6 to 1.2%
P: ≦ 0.015%
S: ≦ 0.005%
Cu: 0.15-0.45%
Ni: 0.05-0.30%
Cu + Ni: ≧ 0.20%
Cr: 0.10 to 0.30%
O: ≦ 0.0012%
A high-strength spring steel excellent in corrosion fatigue strength, characterized in that the balance is substantially made of Fe. In claim 1, further, one or more of V, Nb, Ti, and Al, in the following range by weight% V: ≤ 0.20%
Nb: 0.020 to 0.050%
Ti: 0.030 to 0.070%
Al: 0.010 to 0.040%
And N: ≦ 0.012%
B: 0.0005 to 0.0030%
A high-strength spring steel with excellent corrosion fatigue strength.

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