JP3539843B2 - High fatigue strength steel wire and its manufacturing method - Google Patents

Description

として、γ化加熱後の冷却速度Ｖ（℃／秒）が５８０℃以上の温度域で、
Ｖ≧−５０Ｔ＋２７５ …（式３）
を満たせばよい。
【０００９】
また、本発明鋼線の製造方法は、mass％でＣ：０．７〜１．０，Ｓｉ：０．５〜１．５を含むパーライト組織の鋼線をシェービングしてからパテンティングし、伸線することを特徴とする。さらに、伸線後に歪み取り焼鈍を３５０〜４５０℃で行うことが望ましい。なお、伸線加工度は８０％以上とすることが好適である。
【００１０】
以下、本発明の構成を上記のように限定した理由を述べる。
＜化学成分＞
Ｃ：疲労強度の観点から下限値を決め、伸線性の観点から上限値を決めた。
Ｓｉ：耐熱性の向上に必要な元素である。下限値未満では十分な耐熱性が得られず、上限値を越えると鋼線表面に傷が付きやすい。
Ｍｏ：下限値未満では耐熱性・疲労強度向上の効果が小さく、上限値を越えるとパテンティングの時間が長く生産性が劣る。
Ｍｎ：焼入れ性向上のために添加する。上限値を越えると偏析が多くなりやすく、伸線性に劣る。
Ｃｒ：上限値を越えるとパテンティングの時間が長く生産性に劣るからである。
【００１１】
＜シェービング＞
鋼線表面の低硬度層の除去が目的である。鋼線の内部の硬度よりもマイクロビッカース硬度で５０以上硬度の低い層を除去することで疲労特性を改善する。
【００１２】
＜歪み取り焼鈍＞
ばねの疲労特性向上のため３５０〜４５０℃で行う。下限値未満では疲労特性向上の効果が少なく、上限値を越えるとワイヤの強度，疲労強度も下がる。この焼鈍の時間は２０分程度が効果と生産性の点で好ましい。
【００１３】
＜粒界フェライト＞
本発明鋼線のようにＳｉの高い材料は粒界フェライトが析出しやすいことが特徴であり、疲労特性に対しては欠点である。この粒界フェライトが５vol ％以下であると疲労特性および耐熱性に優れる。
【００１４】
＜セメンタイト形状＞
セメンタイト形状についても同様に疲労・耐熱性にとって重要である。従来パラレルワイヤでの４５０℃以上の耐熱性とは異なり、１００〜２００℃の温度域での疲労・耐熱性にとっては前述の式１を満たすことが好ましいからである。
【００１５】
＜化学成分と冷却速度との関係＞
化学成分とγ化加熱後の冷却速度との関係が前述の式２，式３を満たすことで、前記の粒界フェライトやセメンタイト形状の組織を得ることができるからである。
【００１６】
【発明の実施の形態】
以下、本発明の実施の形態を説明する。
（試験例１）
Ｃ＝０．８２，Ｓｉ＝１．０５，Ｍｎ＝０．５１，Ｃｒ＝０．０９mass％の成分のインゴット１００ｋｇを真空溶解設備で溶解鋳造し、熱間鍛造，圧延により１１mmφと１０mmφの線材を製造した。
【００１７】
これらの線材のうち、１１mmφのものは１０mmφまでシェービングにより表面層を除去してから、またシェービングしない１０mmφの線材はそのままで下記のパテンティング，伸線，歪み取り焼鈍を行ってパーライト組織の鋼線を得た。
パテンティング：９５０→５８０℃鉛浴
伸線：１０mmφ→４mmφ
歪み取り焼鈍：３００，３５０，４００，４５０，５００℃で各２０分間
【００１８】
さらに比較例として、Ｃ：０．６〜０．８，Ｓｉ：０．１５〜０．３５，Ｍｎ：０．３〜０．９mass％を含む鋼種を溶解鋳造→圧延→パテンティング→伸線（減面率８４％）→歪み取り焼鈍（３００±３０℃）の工程で製造される４mmφの鋼線を得た。
【００１９】
そして、上記の各鋼線に中村式の回転曲げ疲労試験機で疲労試験を行い、その際の疲労限を１０⁷ 回とした。その結果を図１に示す。また、各鋼線の横断面における硬度分布も調べた。その結果を図２に示す。
【００２０】
図１に示すように、シェービングを行ったものは行わないものに比べて疲労限界振幅応力が大きく、疲労強度に優れることがわかる。特に、シェービングを行ったものでは歪み取り焼鈍の温度が３５０〜４５０℃の場合に好結果となっており、シェービングを行わないものでも歪み取り焼鈍の温度が３５０〜４００℃の場合に比較例よりも好結果となっている。
【００２１】
また、図２に示すように、シェービングを施していない線材では表面の硬度が低下しているが、シェービングを施した線材は断面の中心から表面にわたってほぼ均等な硬度分布を示している。そして、表面から５０μmにおける平均硬度と内部の平均硬度との差がマイクロビッカース硬度で５０以内の場合に疲労強度の改善に効果的であった。
【００２２】
なお、各鋼線の引張強度は次の通りであった。
シェービングありの線材：２１３０Ｎ／mm²
シェービングなしの線材：２１１０Ｎ／mm²
比較例 ：１９００Ｎ／mm²
【００２３】
（試験例２）
次に、表１に示す化学成分の鋼種について試験例１と同様に真空溶解でメルトにした後、試験例１と同様の工程で鍛造，圧延，シェービング，伸線，歪み取り焼鈍（３５０℃×２０分）を行った。
【００２４】
【表１】

【００２５】
そして、これらの鋼線について耐熱性の評価と回転曲げ疲労試験とを行った。耐熱性は７００MPa の捻じり応力，１５０℃，１時間保持後のへたり量として残留剪断歪を求めることで評価した。具体的な評価基準は、残留剪断歪が従来のピアノ線（試験例１の比較例と同等の鋼線）の１／２以下となる０．０７５％以下、疲労限界振幅が従来のピアノ線に対し２０％以上の向上を示す５５０MPa 以上である。評価結果を図３に示す。
【００２６】
図３に示すように、Ｃの含有量が多い試料番号1-5,Ｓｉの含有量が多い同2-4,Ｍｏが多い同3-2 はパテンティングでマルテンサイトが発生したか表面傷が多発したため、鋼線として不十分であった。また、Ｃの含有量が少ない試料番号1-1 ，Ｓｉの含有量が少ない同2-1 は疲労強度，耐熱性の点で不十分であることがわかる。これに対し、試料番号1-2〜1-4,2-2,2-3,3-1 はいずれも疲労強度と耐熱性について好結果である。特にＭｏを適量添加した試料番号3-1 は高い疲労強度と耐熱性を示している。
【００２７】
（試験例３）
試験例１で用いた１１mm線材を用いて試験例１と同様の工程で４mmの鋼線を得た。ただし、パテンティング時のγ化から恒温変態するまでの冷却速度を変化させて、それぞれの金属組織とばね特性（疲労限界振幅応力と残留剪断歪）の関係を評価した。
【００２８】
＜金属組織とばね特性の関係＞
セメンタイト形状｛厚さｔ（μｍ），長さＬ（μｍ）｝Ｌ／ｔと粒界フェライト量α（vol.％）の違いによるばね特性評価の結果を図４のグラフに示す。なお、このグラフの評価基準は表２に示す通りである。
【００２９】
【表２】

【００３０】
図４のグラフから明らかなように、セメンタイト形状がＬ／ｔ≧５で、粒界フェライト量αが≦５の場合に良好なばね特性を示すことがわかる。なお、Ｌ／ｔ≧５となるセメンタイト比率が８０％以上であると、ばね特性、特にへたりで安定性が増す。
【００３１】
＜製造条件と組織の関係＞
前記表１における試料番号1-2〜1-4,2-2,2-3,3-1 の６鋼種を用いて、下記の式により求められるＴとγ化加熱後の冷却速度Ｖの違いが組織に与える影響について調べた。各試料のＴを表３に、試験結果を図５のグラフに示す。
Ｔ＝１０×｛Ｃ（mass％）−０．７６｝−Ｓｉ（mass％）＋５×Ｃｒ（mass％）
【００３２】
【表３】

【００３３】
図５のグラフから明らかなように、V≧−50T＋275を満たす条件において良好な組織が得られていることがわかる。
【００３４】
【発明の効果】
以上説明したように、本発明鋼線は高い耐熱性と疲労強度を具えており、ばね用鋼線に最適である。また、本発明鋼線の製造方法は、焼入れ・焼戻し処理を行うことなく本発明鋼線を製造することができ、低コストで耐熱性と疲労強度を兼ね具えた鋼線を製造することができる。
【図面の簡単な説明】
【図１】歪み取り焼鈍温度と疲労限界振幅応力との関係を示すグラフである。
【図２】線材断面の硬度分布を示すグラフである。
【図３】化学成分の異なる鋼線の残留剪断歪みおよび疲労限界振幅との関係を示すグラフである。
【図４】セメンタイト形状Ｌ／ｔと粒界フェライト量αの違いによるばね特性評価の結果を示すグラフである。
【図５】Ｔとγ化加熱後の冷却速度Ｖの違いが組織に与える影響について示すグフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel wire excellent in heat resistance and fatigue strength and a method for producing the same.
[0002]
[Prior art]
As a spring steel wire, a wire containing C: 0.6 to 0.8, Si: 0.15 to 0.35, and Mn: 0.3 to 0.9 mass% is known. This spring steel wire is manufactured through the steps of rolling → patenting (gamma heating → constant temperature transformation) → drawing → (coiling) → strain relief annealing (300 ± 30 ° C.).
[0003]
[Problems to be solved by the invention]
However, the above-mentioned spring steel wire is not sufficient in both heat resistance and fatigue strength. On the other hand, it is known in parallel wires and other steel wires that heat resistance is improved by increasing the content of Si. However, the aim of heat resistance is various, and the original aim of the heat resistance of the parallel wire is that the change in TS after hot-dip galvanizing (450 ° C. × 30 seconds) is small. However, in the case of a spring around an engine of an automobile using the steel wire of the present invention, it is important that the sag in a temperature range of 100 to 200 ° C. is small and that the steel wire also has fatigue characteristics. For this reason, simply applying the chemical composition of the parallel wire to the spring does not provide sufficient properties as a spring material. That is, although there is a report that the fatigue characteristics are improved by adding Si with a parallel wire, these are repeated fatigues of tensile force, and the required characteristics are essentially different from those of spring materials. It was found that even if the surface hardness of the parallel wire was reduced, the effect on the fatigue properties was small, but the spring steel wire with a high Si content had a large effect on the fatigue properties.
[0004]
It is known that a heat treatment (quenching / tempering) is performed in the final step of steel wire production to obtain a steel wire excellent in both heat resistance and fatigue strength. However, when quenching / tempering is performed, the cost increases. There is a problem.
[0005]
Therefore, a main object of the present invention is to provide a steel wire which does not perform quenching and tempering, that is, has excellent heat resistance and fatigue strength obtained by wire drawing, and a method for producing the same.
[0006]
[Means for Solving the Problems]
The present invention solves the above-mentioned problems, and is characterized by a steel wire having a pearlite structure containing C: 0.7 to 1.0 and Si: 0.5 to 1.5 in mass%. In the cross section of the line, the difference between the average of the hardness from the surface to 50 μm and the average of the internal hardness is set to within 50 micro Vickers hardness. Preferably, the steel wire further contains Mo in an amount of 0.03 to 0.1 mass%. Further, Mn: 0.3 to 0.9 mass% and Cr: 0.2 mass% or less may be contained. The tensile strength of the steel wire is preferably 1800 N / mm ² or more in order to obtain sufficient fatigue strength.
[0007]
Here, as the metal structure of the steel wire, it is preferable that grain boundary ferrite is 5 vol% or less. Further, as the cementite shape constituting the pearlite structure, when the thickness of the cementite is t and the length is l,
1 / t ≧ 5 (Equation 1)
Is desirably 80 vol% or more.
[0008]
To get such an organization,

In a temperature range where the cooling rate V (° C./sec) after γ-heating is 580 ° C.
V ≧ −50T + 275 (Equation 3)
Should be satisfied.
[0009]
Further, in the method for producing a steel wire of the present invention, a steel wire having a pearlite structure including C: 0.7 to 1.0 and Si: 0.5 to 1.5 in mass% is subjected to shaving, patenting, and elongation. It is characterized by line drawing. Furthermore, it is desirable to perform the strain relief annealing at 350 to 450 ° C. after drawing. The degree of wire drawing is preferably set to 80% or more.
[0010]
Hereinafter, the reason why the configuration of the present invention is limited as described above will be described.
<Chemical components>
C: The lower limit was determined from the viewpoint of fatigue strength, and the upper limit was determined from the viewpoint of drawability.
Si: an element necessary for improving heat resistance. If it is less than the lower limit, sufficient heat resistance cannot be obtained, and if it exceeds the upper limit, the surface of the steel wire is easily damaged.
Mo: If less than the lower limit, the effect of improving heat resistance and fatigue strength is small, and if it exceeds the upper limit, the patenting time is long and productivity is poor.
Mn: added for improving hardenability. If it exceeds the upper limit, segregation tends to increase, resulting in poor drawability.
Cr: If it exceeds the upper limit, the patenting time is long and the productivity is poor.
[0011]
<Shaving>
The purpose is to remove the low hardness layer on the steel wire surface. Fatigue properties are improved by removing a layer having a hardness not less than 50 in micro Vickers hardness than the hardness inside the steel wire.
[0012]
<Strain relief annealing>
This is performed at 350 to 450 ° C. to improve the fatigue characteristics of the spring. If it is less than the lower limit, the effect of improving the fatigue properties is small, and if it exceeds the upper limit, the strength and fatigue strength of the wire also decrease. The annealing time is preferably about 20 minutes in view of the effect and the productivity.
[0013]
<Grain boundary ferrite>
A material having a high Si like the steel wire of the present invention is characterized in that grain boundary ferrite is easily precipitated, and has a disadvantage in fatigue characteristics. If the grain boundary ferrite is at most 5 vol%, the fatigue properties and heat resistance will be excellent.
[0014]
<Cementite shape>
The cementite shape is also important for fatigue and heat resistance. This is because, unlike the conventional parallel wire having a heat resistance of 450 ° C. or higher, it is preferable to satisfy the above-described expression 1 for fatigue and heat resistance in a temperature range of 100 to 200 ° C.
[0015]
<Relationship between chemical composition and cooling rate>
When the relationship between the chemical component and the cooling rate after the γ-formation heating satisfies Expressions 2 and 3, the grain boundary ferrite or cementite-shaped structure can be obtained.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
(Test Example 1)
100 kg of an ingot having a composition of C = 0.82, Si = 1.05, Mn = 0.51, and Cr = 0.09 mass% is melted and cast in a vacuum melting facility, and hot forging and rolling are performed to form 11 mmφ and 10 mmφ wires. Manufactured.
[0017]
Of these wire rods, the surface layer was removed by shaving to 11 mmφ by shaving to 10 mmφ, and the following 10 mmφ wire rods without shaving were subjected to the following patenting, drawing, and strain relief annealing to obtain a pearlite steel wire. Got.
Patenting: 950 → 580 ° C Lead bath wire drawing: 10mmφ → 4mmφ
Strain relief annealing: 300, 350, 400, 450 and 500 ° C. for 20 minutes each.
Further, as a comparative example, a steel type containing C: 0.6 to 0.8, Si: 0.15 to 0.35, and Mn: 0.3 to 0.9 mass% is melt-cast → rolling → patenting → wire drawing ( A steel wire of 4 mmφ manufactured in the process of reducing the area (84%) → strain annealing (300 ± 30 ° C.) was obtained.
[0019]
Then, a fatigue test in rotating bending fatigue tester of Nakamura in the steel wire above and the fatigue limit at that time with 10 ⁷ times. The result is shown in FIG. The hardness distribution in the cross section of each steel wire was also examined. The result is shown in FIG.
[0020]
As shown in FIG. 1, it can be seen that the one subjected to shaving has a larger fatigue limit amplitude stress than the one not subjected to shaving, and is superior in fatigue strength. In particular, in the case where shaving was performed, good results were obtained when the temperature of the strain relief annealing was 350 to 450 ° C, and even when the temperature of the strain relief annealing was 350 to 400 ° C even in the case where the shaving was not performed, Have also been successful.
[0021]
As shown in FIG. 2, the hardness of the surface of the wire without shaving is reduced, but the hardness of the wire with shaving shows a substantially uniform hardness distribution from the center of the cross section to the surface. When the difference between the average hardness at 50 μm from the surface and the average hardness inside was within 50 micro Vickers hardness, it was effective in improving fatigue strength.
[0022]
The tensile strength of each steel wire was as follows.
Wire with shaving: 2130 N / mm ²
Wire without shaving: 2110 N / mm ²
Comparative example: 1900 N / mm ²
[0023]
(Test Example 2)
Next, the steel types having the chemical components shown in Table 1 were melted by vacuum melting in the same manner as in Test Example 1, and then forged, rolled, shaved, drawn, and strain-relieved annealing (350 ° C. × 20 minutes).
[0024]
[Table 1]

[0025]
Then, evaluation of heat resistance and rotational bending fatigue test were performed on these steel wires. The heat resistance was evaluated by determining the residual shear strain as a set amount after holding at a torsion stress of 700 MPa and 150 ° C. for 1 hour. The specific evaluation criteria are 0.075% or less, in which the residual shear strain is １ ／ or less of the conventional piano wire (a steel wire equivalent to the comparative example of Test Example 1), and the fatigue limit amplitude is less than that of the conventional piano wire. On the other hand, it is 550 MPa or more which shows an improvement of 20% or more. FIG. 3 shows the evaluation results.
[0026]
As shown in FIG. 3, Sample No. 1-5 having a high C content, Sample No. 2-4 having a high Si content, and Sample No. 3-2 having a high Mo content had martensite generated by patenting or surface scratches. Because it occurred frequently, it was insufficient as a steel wire. Further, it can be seen that Sample No. 1-1 having a low C content and Sample No. 2-1 having a low Si content are insufficient in terms of fatigue strength and heat resistance. On the other hand, sample numbers 1-2 to 1-4, 2-2, 2-3, and 3-1 all have good results in terms of fatigue strength and heat resistance. In particular, Sample No. 3-1 to which an appropriate amount of Mo was added exhibited high fatigue strength and heat resistance.
[0027]
(Test Example 3)
Using the 11 mm wire rod used in Test Example 1, a 4 mm steel wire was obtained in the same process as in Test Example 1. However, the relationship between the respective metal structures and the spring properties (fatigue limit amplitude stress and residual shear strain) was evaluated by changing the cooling rate from gamma transformation during patenting to constant temperature transformation.
[0028]
<Relationship between metal structure and spring characteristics>
FIG. 4 is a graph showing the results of spring characteristic evaluation based on the difference between the cementite shape {thickness t (μm), length L (μm)} L / t and the amount of grain boundary ferrite α (vol.%). The evaluation criteria of this graph are as shown in Table 2.
[0029]
[Table 2]

[0030]
As is clear from the graph of FIG. 4, when the cementite shape is L / t ≧ 5 and the amount of grain boundary ferrite α is ≦ 5, good spring characteristics are exhibited. In addition, when the cementite ratio that satisfies L / t ≧ 5 is 80% or more, the spring characteristics, particularly the set, increase the stability.
[0031]
<Relationship between manufacturing conditions and organization>
Difference between T obtained by the following equation and cooling rate V after gamma heating using six steel types of sample numbers 1-2 to 1-4, 2-2, 2-3 and 3-1 in Table 1 above. We investigated the effect of sucrose on the tissue. Table 3 shows the T of each sample, and the test results are shown in the graph of FIG.
T = 10 × {C (mass%) − 0.76} −Si (mass%) + 5 × Cr (mass%)
[0032]
[Table 3]

[0033]
As is clear from the graph of FIG. 5, it is understood that a good structure is obtained under the condition of V ≧ −50 T +275.
[0034]
【The invention's effect】
As described above, the steel wire of the present invention has high heat resistance and fatigue strength, and is optimal for a spring steel wire. In addition, the method for producing a steel wire of the present invention can produce the steel wire of the present invention without performing quenching and tempering treatment, and can produce a steel wire having both heat resistance and fatigue strength at low cost. .
[Brief description of the drawings]
FIG. 1 is a graph showing a relationship between a strain relief annealing temperature and a fatigue limit amplitude stress.
FIG. 2 is a graph showing a hardness distribution of a cross section of a wire rod.
FIG. 3 is a graph showing the relationship between residual shear strain and fatigue limit amplitude of steel wires having different chemical components.
FIG. 4 is a graph showing a result of spring characteristic evaluation based on a difference between a cementite shape L / t and a grain boundary ferrite amount α.
FIG. 5 is a graph showing the effect of the difference between T and the cooling rate V after gamma heating on the structure.

Claims (5)

C in mass%: 0.7~1.0, Si: 0.5~1.5 , Mn: 0.3 ~ 0.9, Cr: 0.2 or less (but not including 0) seen including, steel wire pearlite structure the balance being Fe and unavoidable impurities A high fatigue strength steel wire, wherein a difference between an average hardness from the surface to 50 μm from the surface and an average hardness inside the steel wire cross section is 50 or less in micro Vickers hardness. The high fatigue strength steel wire according to claim 1, further comprising 0.03 to 0.1 mass% of Mo. 3. The high fatigue strength steel wire according to claim 1, wherein the tensile strength is 1800 N / mm ² or more. C in mass%: 0.7~1.0, Si: 0.5~1.5 , Mn: 0.3 ~ 0.9, Cr: 0.2 see contains below (but not including 0), and shaving a steel wire and the balance being Fe and unavoidable impurities A step of removing a layer having a hardness of 50 or more lower than the hardness inside the steel wire with a micro Vickers hardness ,
Patenting the shaved steel wire to a pearlite structure ,
And a step of drawing a patented steel wire. The method for producing a high fatigue strength steel wire according to claim 4, wherein the strain relief annealing is performed at 350 to 450 ° C after drawing.

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