JP2002180198A - High strength steel wire for spring - Google Patents
High strength steel wire for springInfo
- Publication number
- JP2002180198A JP2002180198A JP2000386902A JP2000386902A JP2002180198A JP 2002180198 A JP2002180198 A JP 2002180198A JP 2000386902 A JP2000386902 A JP 2000386902A JP 2000386902 A JP2000386902 A JP 2000386902A JP 2002180198 A JP2002180198 A JP 2002180198A
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- Prior art keywords
- steel wire
- spring
- less
- strength
- carbide
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- Heat Treatment Of Steel (AREA)
- Heat Treatment Of Strip Materials And Filament Materials (AREA)
Abstract
Description
【０００１】[0001]
【発明の属する技術分野】本発明は冷間でコイリングさ
れ、高強度かつ高靱性を有するばね用鋼線に関するもの
である。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spring steel wire which is cold coiled and has high strength and high toughness.
【０００２】[0002]
【従来の技術】自動車の軽量化、高性能化に伴い、ばね
も高強度化され、熱処理後に引張強度１５００ＭＰａを
超えるような高強度鋼がばねに供されている。近年では
引張強度１９００ＭＰａを超える鋼線も要求されてい
る。それはばね製造時のひずみ取り焼鈍や窒化処理など
の加熱によって少々軟化してもばねとして支障のない材
料硬度を確保するためである。2. Description of the Related Art As automobiles are becoming lighter and more sophisticated, springs are becoming stronger. High-strength steels having a tensile strength exceeding 1500 MPa after heat treatment are used for springs. In recent years, steel wires exceeding 1900 MPa in tensile strength have also been required. This is to ensure a material hardness that does not hinder the spring even if it is softened a little by heating such as strain relief annealing or nitriding during spring production.
【０００３】その手法としては特開昭５７−３２３５３
号公報ではＶ、Ｎｂ、Ｍｏ等の元素を添加することで焼
入れで固溶し、焼戻しで析出する微細炭化物を生成さ
せ、それによって転位の動きを制限し、耐へたり特性を
向上させるとしている。[0003] The method is disclosed in Japanese Patent Laid-Open No. 57-32353.
In the publication, elements such as V, Nb, and Mo are added to form a solid carbide formed by quenching and precipitate by tempering, thereby restricting the movement of dislocations and improving sag resistance. .
【０００４】一方、鋼のコイルばねの製造方法では鋼の
オーステナイト域まで加熱してコイリングし、その後、
焼入れ焼戻しを行う熱間コイリングとあらかじめ鋼に焼
入れ焼戻しを施した高強度鋼線を冷間にてコイリングす
る冷間コイリングがある。冷間コイリングでは鋼線の製
造時に急速加熱急速冷却が可能なオイルテンパー処理や
高周波処理などを用いることができるため、ばね材の旧
オーステナイト粒径を小さくすることが可能で、結果と
して破壊特性に優れたばねを製造できる。またばね製造
ラインにおける加熱炉などの設備を簡略化できるため、
ばねメーカーにとっても設備コストの低減につながるな
どの利点があり、最近ではばねの冷間化が進められてい
る。On the other hand, in a method of manufacturing a coil spring made of steel, the steel is heated to the austenite region and coiled.
There are hot coiling in which quenching and tempering is performed and cold coiling in which high-strength steel wire in which steel has been quenched and tempered in advance is cold-coiled. In cold coiling, oil tempering or high-frequency treatment that allows rapid heating and rapid cooling can be used during the production of steel wire, so the former austenite grain size of the spring material can be reduced, and as a result, fracture characteristics are reduced. An excellent spring can be manufactured. Also, facilities such as heating furnaces in the spring production line can be simplified,
Spring manufacturers also have advantages such as reduction in equipment cost, and recently, cold springs have been promoted.
【０００５】しかし冷間コイリングばね用鋼線の強度が
大きくなると、冷間コイリング時に折損し、ばね形状に
成形できない場合も多い。強度と加工性が両立しないた
めに工業的には不利ともいえる方法でコイリングせざる
を得なかった。通常、弁ばねの場合、オンラインでの焼
入れ焼戻し処理、いわゆるオイルテンパー処理した鋼線
を冷間でコイリングするが、例えば特開平０５−１７９
３４８号公報では９００〜１０５０℃に加熱してコイリ
ングし、その後４２５〜５５０℃で焼戻し処理するな
ど、コイリング時の折損を防止するためにコイリング時
に線材を加熱して変形を容易な温度でコイリングし、そ
の後、高強度を得るためにコイリング後の調質処理を行
っている。このようなコイリング時の加熱とコイリング
後の調質処理はばね寸法の熱処理ばらつきの原因になっ
たり、処理能率が極端に低下したりするため、コスト、
精度の点で冷間コイリングされたばねに比べ劣る。[0005] However, when the strength of the steel wire for a cold coiling spring is increased, it often breaks during cold coiling and cannot be formed into a spring shape. Since strength and workability are not compatible, coiling has to be carried out by a method which is industrially disadvantageous. Usually, in the case of a valve spring, a steel wire subjected to online quenching and tempering treatment, that is, so-called oil-tempered treatment, is cold-coiled.
According to Japanese Patent No. 348, the wire is heated at 900 to 1050 ° C. and then tempered at 425 to 550 ° C. to prevent breakage at the time of coiling. After that, tempering treatment after coiling is performed to obtain high strength. Such heating during coiling and tempering after coiling may cause variations in the heat treatment of the spring dimensions, or may significantly reduce the processing efficiency, resulting in cost,
It is inferior to a cold coiled spring in accuracy.
【０００６】また炭化物の粒径に関しては例えば特開平
１０−２５１８０４号公報のようにＮｂ、Ｖ系の炭化物
の平均粒径に注目した発明がなされているが、Ｖ、Ｎｂ
系炭化物の平均粒径の制御だけでは不十分であることを
示している。この先行技術では圧延中の冷却水によって
異常組織が生じることを懸念する記述があり（段落００
１５）、実質的には乾式圧延を推奨している。このこと
は工業的には非定常作業であり、通常の圧延と明らかに
異なることが推定され、たとえ平均粒径を制御しても周
辺マトリックス組織に不均一を生じると圧延トラブルを
生じることを示唆している。As for the particle size of carbides, for example, Japanese Patent Application Laid-Open No. 10-251804 discloses an invention which focuses on the average particle size of Nb and V-based carbides.
This shows that controlling the average particle size of the system carbide is not sufficient. In this prior art, there is a description that the cooling water during rolling may cause an abnormal structure (paragraph 00).
15) Practically, dry rolling is recommended. This is a non-stationary operation industrially, and is presumed to be clearly different from normal rolling, suggesting that even if the average grain size is controlled, unevenness in the surrounding matrix structure causes rolling trouble. are doing.
【０００７】[0007]
【発明が解決しようとする課題】本発明は冷間でコイリ
ングされ、十分な大気強度とコイリング加工性を両立で
きる引張強度１９００ＭＰａ以上のばね用鋼線を提供す
ることを課題としている。SUMMARY OF THE INVENTION It is an object of the present invention to provide a spring steel wire having a tensile strength of not less than 1900 MPa, which is coiled in a cold state and can achieve both sufficient atmospheric strength and coiling workability.
【０００８】[0008]
【課題を解決するための手段】発明者らは従来のばね鋼
線では注目されていなかった鋼中炭化物、特にセメンタ
イトの大きさを制限することで高強度とコイリング性を
両立させたばね用鋼線を開発するに至った。Means for Solving the Problems The present inventors have limited the size of carbides in steel, particularly cementite, which has not been noticed in conventional spring steel wires, and have achieved both high strength and coilability by limiting the size of cementite. Has led to the development.
【０００９】すなわち本発明は次に示す鋼材を要旨とす
る。That is, the present invention provides the following steel materials.
【００１０】（１） 質量％で、Ｃ：０．４〜１．０
％、Ｓｉ：０．９〜３．０％、Ｍｎ：０．１〜２．０
％、Ｃｒ：２．５％以下（０％含む）、Ｐ：０．０１５
％以下、Ｓ：０．０１５％以下、Ｎ：０．００１〜０．
００７％、残部鉄および不可避的不純物を含み、引張強
度ＴＳが１９００ＭＰａ以上、かつ検鏡面に占めるセメ
ンタイト系球状炭化物に関して円相当径０．２μｍ以上
の占有面積率が７％以下、円相当径０．２〜３μｍの存
在密度が１個／μｍ2以下、円相当径３μｍ超の存在密
度が０．００１個／μｍ2以下を満たし、かつ旧オース
テナイト粒径番号が１０番以上、最大炭化物径が１５μ
ｍ以下かつ最大酸化物径が１５μｍ以下であることを特
長とするばね用熱処理鋼線。(1) In mass%, C: 0.4 to 1.0
%, Si: 0.9 to 3.0%, Mn: 0.1 to 2.0
%, Cr: 2.5% or less (including 0%), P: 0.015
%, S: 0.015% or less, N: 0.001-0.
007%, the balance containing iron and unavoidable impurities, the tensile strength TS is 1900 MPa or more, and the occupied area ratio of the cementitious spherical carbide occupying 0.2 μm or more in the microscopic surface is 7% or less, and the circle equivalent diameter is 0. The existing density of 2 to 3 μm is 1 / μm 2 or less, the existing density of more than 3 μm of equivalent circle diameter is 0.001 / μm 2 or less, and the prior austenite particle number is 10 or more and the maximum carbide diameter is 15 μ
A heat-treated steel wire for a spring, characterized in that the maximum oxide diameter is 15 μm or less.
【００１１】（２） さらに質量％で、Ｗ：０．０５〜
１．０％、Ｃｏ：０．０５〜３．０％の１種または２種
を含むことを特徴とする上記（１）記載の、ばね用熱処
理鋼線。(2) Further, in mass%, W: 0.05 to
The heat-treated steel wire for a spring according to the above (1), wherein the heat-treated steel wire contains one or two of 1.0% and Co: 0.05 to 3.0%.
【００１２】（３） さらに質量％で、Ｍｇ：０．００
０２〜０．０１％を含むことを特徴とする上記（１）ま
たは（２）のいずれかに記載の、ばね用熱処理鋼線。(3) Further, in mass%, Mg: 0.00
The heat-treated steel wire for a spring according to any one of the above (1) or (2), wherein the heat-treated steel wire contains 02 to 0.01%.
【００１３】（４） さらに質量％で、Ｔｉ：０．００
５〜０．１％、Ｍｏ：０．０５〜１．０％、Ｖ：０．０
５〜０．７％、Ｎｂ：０．０１〜０．０５％の１種また
は２種以上を含むことを特徴とする上記（１）〜（３）
のいずれかに記載の、ばね用熱処理鋼線。(4) Further, in mass%, Ti: 0.00
5 to 0.1%, Mo: 0.05 to 1.0%, V: 0.0
(1) to (3), wherein one or more of Nb: 0.01 to 0.05% is contained.
The heat-treated steel wire for a spring according to any one of the above.
【００１４】（５） さらに質量％で、Ｂ：０．０００
５〜０．００６％を含むことを特徴とする上記（１）〜
（４）のいずれかに記載の、ばね用熱処理鋼線。(5) Further, in mass%, B: 0.000
The above (1) to (10), wherein the content is 5 to 0.006%.
The heat-treated steel wire for a spring according to any one of (4).
【００１５】（６） さらに質量％で、Ｎｉ：０．０５
〜３．０％、Ｃｕ：０．０５〜０．５％の１種または２
種を含むことを特徴とする上記（１）〜（５）のいずれ
かに記載の、ばね用熱処理鋼線。(6) Further, in mass%, Ni: 0.05
-3.0%, Cu: 0.05-0.5%, 1 or 2
The heat-treated steel wire for a spring according to any one of the above (1) to (5), comprising a seed.
【００１６】[0016]
【発明の実施の形態】発明者は高強度を得るために化学
成分を規定しつつ、熱処理によって鋼中炭化物形状を制
御することで、ばねを製造するに十分なコイリング特性
を確保した鋼線を発明するに至った。BEST MODE FOR CARRYING OUT THE INVENTION The inventor of the present invention has determined the chemical composition to obtain high strength and controlled the shape of carbide in steel by heat treatment to obtain a steel wire having sufficient coiling characteristics for manufacturing a spring. Invented.
【００１７】その詳細を以下に示す。まず、鋼の化学成
分を規定した理由について説明する。The details will be described below. First, the reason for defining the chemical composition of steel will be described.
【００１８】Ｃは鋼材の基本強度に大きな影響を及ぼす
元素であり、十分な強度を得るために０．４〜１．０％
とした。０．４％未満では十分な強度を得られず、他の
合金元素をさらに多量に投入せざるを得ず、１．０％超
では過共析となり、粗大セメンタイトを多量に析出する
ため、靱性を著しく低下させる。このことは同時にコイ
リング特性を低下させる。C is an element that has a great effect on the basic strength of the steel material.
And If it is less than 0.4%, sufficient strength cannot be obtained, and other alloying elements must be added in a larger amount. If it exceeds 1.0%, hypereutectoid occurs, and a large amount of coarse cementite is precipitated. Is significantly reduced. This at the same time reduces the coiling properties.
【００１９】Ｓｉはばねの強度、硬度と耐へたり性を確
保するために必要な元素であり、少ない場合、必要な強
度、耐へたり性が不足するため、０．９％を下限とし
た。またＳｉは粒界の炭化物系析出物を球状化、微細化
する効果があり、積極的に添加することで粒界析出物の
粒界占有面積率を小さくする効果がある。しかし多量に
添加しすぎると、材料を硬化させるだけでなく、脆化す
る。そこで焼入れ焼戻し後の脆化を防ぐために３．０％
を上限とした。Si is an element necessary for securing the strength, hardness and sag resistance of the spring. If the amount is small, the necessary strength and sag resistance are insufficient, so the lower limit is 0.9%. . Si has the effect of spheroidizing and refining the carbide-based precipitates at the grain boundaries, and has the effect of reducing the area occupied by the grain boundary precipitates by actively adding them. However, when added in a large amount, the material not only hardens, but also becomes brittle. Therefore, to prevent embrittlement after quenching and tempering, 3.0%
Was set as the upper limit.
【００２０】Ｍｎは硬度を十分に得るため、また鋼中に
存在するＳをＭｎＳとして固定し、強度低下を抑制する
ために０．１％を下限とする。またＭｎによる脆化を防
止するために上限を２．０％とした。Mn has a lower limit of 0.1% in order to obtain sufficient hardness, to fix S present in steel as MnS, and to suppress a decrease in strength. The upper limit is set to 2.0% in order to prevent embrittlement due to Mn.
【００２１】Ｎは鋼中マトリックスを硬化させるが、Ｔ
ｉ、Ｖなどの合金元素が添加されている場合には窒化物
として存在し、鋼線の性質に影響を与える。Ｔｉ、Ｎ
ｂ、Ｖを添加した鋼では炭窒化物の生成が容易になり、
オーステナイト粒微細化のピン止め粒子となる炭化物、
窒化物および炭窒化物の析出サイトになりやすい。その
ためばね製造までに施される様々な熱処理条件で安定的
にピン止め粒子を生成することができ、鋼線のオーステ
ナイト粒径を微細に制御することができる。このような
目的から０．００１％以上のＮを添加させる。一方、過
剰なＮは窒化物および窒化物を核として生成した炭窒化
物および炭化物の粗大化を招く。例えばＴｉを添加する
場合には粗大なＴｉＮを析出したり、Ｂを添加するとＢ
Ｎを析出し、破壊特性を損なう。そこでそのような弊害
の伴わない０．００７％を上限とする。N hardens the matrix in steel, but T
When alloying elements such as i and V are added, they are present as nitrides and affect the properties of the steel wire. Ti, N
b, V-added steel facilitates the formation of carbonitrides,
Carbide that becomes pinned particles for austenite grain refinement,
It is likely to be a precipitation site for nitride and carbonitride. Therefore, pinning particles can be stably generated under various heat treatment conditions applied before the spring is manufactured, and the austenite particle size of the steel wire can be finely controlled. For this purpose, 0.001% or more of N is added. On the other hand, excessive N causes coarsening of nitrides and carbonitrides and carbides generated with the nitrides as nuclei. For example, when Ti is added, coarse TiN is precipitated, and when B is added, B
Precipitates N and impairs fracture characteristics. Therefore, the upper limit is 0.007% which does not involve such adverse effects.
【００２２】Ｐは鋼を硬化させるが、さらに偏析を生
じ、材料を脆化させる。特にオーステナイト粒界に偏析
したＰは衝撃値の低下や水素の侵入により遅れ破壊など
を引き起こす。そのため少ない方がよい。そこで脆化傾
向が顕著となる０．０１５％以下に制限した。P hardens the steel, but also causes segregation and embrittles the material. In particular, P segregated at the austenite grain boundary causes a delayed fracture due to a decrease in impact value or penetration of hydrogen. Therefore, the smaller the better. Therefore, the content is limited to 0.015% or less, at which the embrittlement tendency becomes remarkable.
【００２３】ＳもＰと同様に鋼中に存在すると鋼を脆化
させる。Ｍｎによって極力その影響を小さくするが、Ｍ
ｎＳも介在物の形態を取るため、破壊特性は低下する。
特に高強度鋼のでは微量のＭｎＳから破壊を生じること
もあり、Ｓも極力少なくすることが望ましい。その悪影
響が顕著となる０．０１５％を上限とした。S also embrittles the steel when it is present in the steel, like P. The effect is minimized by Mn.
Since nS also takes the form of inclusions, the destruction characteristics are degraded.
In particular, in the case of high-strength steel, a small amount of MnS may cause destruction, and it is desirable to reduce S as much as possible. The upper limit is set to 0.015% at which the adverse effect becomes significant.
【００２４】Ｃｒは焼入れ性および焼戻し軟化抵抗を向
上させるために有効な元素であるが、添加量が多いとコ
スト増を招くだけでなく、焼入れ焼戻し後に見られるセ
メンタイトを粗大化させる。結果として線材は脆化する
ためにコイリング時に折損を生じやすくする。そこで脆
化が顕著となる２．５％を上限とした。Ｃｒの添加量に
関しては好ましくは０．７％以下であり、特にＣが０．
６％以上の場合にはＣｒ量を抑制した方が粗大炭化物生
成を抑制でき、強度とコイリング性を両立しやすい。一
方、窒化処理を行う場合にはＣｒが添加されている方が
窒化による硬化層を深くできる。従ってこの場合には
０．３〜０．５％程度が好ましい。Cr is an element effective for improving the quenchability and the tempering softening resistance. However, when added in a large amount, not only the cost is increased but also the cementite observed after quenching and tempering is coarsened. As a result, the wire becomes brittle, so that it is likely to break during coiling. Therefore, the upper limit is set to 2.5% at which embrittlement becomes remarkable. The amount of Cr added is preferably 0.7% or less, and in particular, when C is 0.1% or less.
In the case of 6% or more, suppressing the amount of Cr can suppress generation of coarse carbides, and can easily achieve both strength and coilability. On the other hand, when the nitriding treatment is performed, the hardened layer by the nitriding can be deepened by adding Cr. Therefore, in this case, about 0.3 to 0.5% is preferable.
【００２５】Ｗは焼入れ性を向上させるとともに、鋼中
で炭化物を生成し、強度を高める働きがある。従って極
力添加する方が好ましい。Ｗの特徴は他の元素とは異な
り、セメンタイトを含む炭化物の形状を微細にすること
である。その添加量が０．０５％未満では効果は見られ
ず、１．０％を超えると粗大な炭化物を生じ、かえって
延性などの機械的性質を損なう恐れがあるのでＷの添加
量を０．０５〜１．０％とした。W has the function of improving the hardenability and generating carbides in the steel to increase the strength. Therefore, it is preferable to add as much as possible. The feature of W, unlike other elements, is to make the shape of carbide containing cementite fine. If the addition amount is less than 0.05%, no effect is observed, and if it exceeds 1.0%, coarse carbides are formed, and mechanical properties such as ductility may be impaired. To 1.0%.
【００２６】Ｃｏは焼入れ性を低下させるものの、高温
における強度を確保できる。また炭化物の生成を阻害す
るため、本発明で問題となる粗大な炭化物の生成を抑制
する働きがある。従ってセメンタイトを含む炭化物の粗
大化を抑制できる。従って、極力添加することが好まし
い。添加する場合、０．０５％未満ではその効果が小さ
く、３．０％を超えるとではその効果が飽和するため、
０．０５〜３．０％とした。Although Co reduces the hardenability, it can ensure the strength at high temperatures. In addition, since it inhibits the formation of carbides, it has a function of suppressing the formation of coarse carbides which is a problem in the present invention. Therefore, coarsening of carbide containing cementite can be suppressed. Therefore, it is preferable to add as much as possible. When the addition is less than 0.05%, the effect is small, and when it exceeds 3.0%, the effect is saturated.
0.05-3.0%.
【００２７】Ｗ、Ｃｏは鋼中での挙動こそ異なるもの
の、両者とも粗大なセメンタイトの生成を抑制する働き
があると考えられる。すなわちＣｏは炭化物生成そのも
のを抑制し、Ｗはセメンタイトの成長を抑制し、粗大化
を抑制すると考えられる。Although W and Co have different behaviors in steel, it is considered that both have a function of suppressing the formation of coarse cementite. That is, Co suppresses carbide formation itself, and W suppresses the growth of cementite and suppresses coarsening.
【００２８】Ｍｇは酸化物生成元素であり、溶鋼中では
酸化物を生成する。その温度域はＭｎＳの生成温度より
も高く、ＭｎＳ生成時には既に溶鋼中に存在している。
従ってＭｎＳの析出核として用いることができ、これに
よりＭｎＳの分布を制御できることを見出した。すなわ
ちＭｇ系酸化物は従来鋼に多く見られるＳｉ、Ａｌ系酸
化物より微細に溶鋼中に分散するため、Ｍｇ系酸化物を
核としたＭｎＳは鋼中に微細に分散することとなる。従
って同じＳ含有量であってもＭｇの有無によってＭｎＳ
分布が異なり、それらを添加する方がＭｎＳ粒径はより
微細になる。その効果は微量でも十分得られ、Ｍｇ０．
０００２％以上であればＭｎＳは微細化する。しかし
０．０１％を超えては溶鋼中に残留しにくいため、工業
的には０．０１％が上限と考えられる。そこでＭｇ添加
量を０．０００２〜０．０１％とした。このＭｇはＭｎ
Ｓ分布等の効果により、耐食性、遅れ破壊の向上および
圧延割れ防止などに効果があり、極力添加する方が望ま
しく、好ましい添加量は、０．０００５〜０．０１％で
ある。Mg is an oxide forming element, and forms an oxide in molten steel. The temperature range is higher than the MnS formation temperature, and is already present in the molten steel at the time of MnS formation.
Therefore, it has been found that it can be used as a precipitation nucleus of MnS, whereby the distribution of MnS can be controlled. That is, since Mg-based oxides are more finely dispersed in molten steel than Si and Al-based oxides often found in conventional steels, MnS with Mg-based oxides as nuclei is finely dispersed in steel. Therefore, even with the same S content, MnS
The distributions are different and the MnS particle size becomes finer when they are added. The effect can be sufficiently obtained even in a trace amount.
If it is 0002% or more, MnS becomes finer. However, if it exceeds 0.01%, it hardly remains in the molten steel, and therefore, industrially, 0.01% is considered to be the upper limit. Therefore, the added amount of Mg is set to 0.0002 to 0.01%. This Mg is Mn
The effect of S distribution and the like is effective in improving corrosion resistance, delayed fracture, and preventing rolling cracks. It is desirable to add as much as possible, and the preferable addition amount is 0.0005 to 0.01%.
【００２９】Ｔｉ、Ｍｏ、ＶおよびＮｂは鋼中で窒化
物、炭化物、炭窒化物として析出する。従ってこれらの
元素を１種または２種以上を添加すれば、これら析出物
を生成し、焼戻し軟化抵抗を得ることができ、高温での
焼戻しや工程で入れられるひずみ取り焼鈍や窒化などの
熱処理を経ても軟化せず高強度を発揮させることができ
る。このことは窒化後のばね内部硬度の低下を抑制した
り、ホットセッチングやひずみ取り焼鈍を容易にするた
め、最終的なばねの疲労特性を向上させることとなる。
しかしＴｉ、Ｍｏ、ＶおよびＮｂは添加量が多すぎる
と、それらの析出物が大きくなりすぎ、鋼中炭素と結び
ついて粗大炭化物を生成する。このことは鋼線の高強度
化に寄与すべきＣ量を減少させ、添加したＣ量相当の強
度が得られなくなる。さらに粗大炭化物が応力集中源と
なるためコイリング中の変形で折損しやすくなる。Ti, Mo, V and Nb precipitate as nitrides, carbides and carbonitrides in steel. Therefore, if one or more of these elements are added, these precipitates are formed, and the tempering softening resistance can be obtained. High strength can be exhibited without softening even after passing through. This suppresses a decrease in the internal hardness of the spring after nitriding and facilitates hot setting and strain relief annealing, thereby improving the fatigue properties of the final spring.
However, if Ti, Mo, V and Nb are added in too large amounts, their precipitates become too large and combine with carbon in the steel to form coarse carbides. This reduces the amount of C that should contribute to the increase in the strength of the steel wire, and the strength corresponding to the added amount of C cannot be obtained. Further, since coarse carbides are a source of stress concentration, they are easily broken by deformation during coiling.
【００３０】Ｔｉについては窒化物の析出温度は高く、
溶鋼中で既に析出している。またその結合力は強いの
で、鋼中のＮを固定する場合にも用いる。Ｂを添加する
場合にはＢをＢＮとさせないためにも、Ｎを十分に固定
できるだけ添加する必要がある。そこでＴｉによってＮ
を固定することが好ましい。Ｔｉの添加量はオーステナ
イト粒径が微細化できる最低限の必要添加量０．００５
％を下限とし、析出物寸法が破壊特性に悪影響を及ぼさ
ない最大量０．１％を上限とした。For Ti, the nitride precipitation temperature is high,
Already precipitated in molten steel. Further, since the bonding force is strong, it is also used for fixing N in steel. When B is added, it is necessary to add N as much as possible in order to prevent B from becoming BN. So Ti by N
Is preferably fixed. The addition amount of Ti is the minimum necessary addition amount 0.005 which can make the austenite particle size fine.
% As the lower limit, and the upper limit of 0.1%, at which the precipitate size does not adversely affect the fracture characteristics, was set as the upper limit.
【００３１】Ｍｏは０．０５〜１．０％を添加すること
で焼入れ性を向上させるとともに、焼戻し軟化抵抗を与
えることができる。すなわち強度を制御する際の焼戻し
温度を高温化させることができる。この点は粒界炭化物
の粒界占有面積率を低下させるのに有利である。すなわ
ちフィルム状に析出する粒界炭化物を高温で焼戻すこと
で球状化させ、粒界面積率を低減することに効果があ
る。またＭｏは鋼中ではセメンタイトとは別にＭｏ系炭
化物を生成する。特にＶ等に比べその析出温度が低いの
で炭化物の粗大化を抑制する効果がある。その添加量は
０．０５％未満では効果が認められず、１．０％を超え
ると効果が飽和する。Mo can be added in an amount of 0.05 to 1.0% to improve the hardenability and to provide temper softening resistance. That is, the tempering temperature for controlling the strength can be increased. This is advantageous in reducing the area occupied by the grain boundary carbides. That is, it is effective in tempering the grain boundary carbide precipitated in the form of a film at a high temperature to make the grain boundary spherical, thereby reducing the grain boundary area ratio. Mo generates Mo-based carbides separately from cementite in steel. In particular, since the deposition temperature is lower than that of V or the like, there is an effect of suppressing coarsening of carbide. If the amount is less than 0.05%, the effect is not recognized, and if it exceeds 1.0%, the effect is saturated.
【００３２】またＶについては窒化物、炭化物、炭窒化
物の生成によるオーステナイト粒径の粗大化抑制のほか
に焼戻し温度での鋼線の硬化や窒化時の表層の硬化に利
用することもできる。その添加量は０．０５％未満では
添加した効果がほとんど認められず、０．７％を超える
と粗大な未固溶介在物を生成し、靱性を低下させる。V can be used for hardening a steel wire at a tempering temperature and hardening a surface layer at the time of nitriding, in addition to suppressing coarsening of austenite grain size by forming nitrides, carbides and carbonitrides. If the addition amount is less than 0.05%, the effect of the addition is hardly recognized, and if it exceeds 0.7%, coarse undissolved inclusions are formed, and the toughness is reduced.
【００３３】Ｎｂも同様に窒化物、炭化物、炭窒化物の
生成によるオーステナイト粒径の粗大化抑制のほかに焼
戻し温度での鋼線の硬化や窒化時の表層の硬化に利用す
ることもできる。ＮｂはＶ、Ｍｏ等よりも高温でも微細
炭化物を生成するため、その添加量が微量であっても熱
処理鋼線製造時のオーステナイト粒径微細化にも効果が
大きく非常に有効な元素である。０．０１％未満では効
果がほとんど認められず、０．０５％を超えると粗大な
未固溶介在物を生成し、靱性を低下させる。Similarly, Nb can be used for hardening a steel wire at a tempering temperature or hardening a surface layer at the time of nitriding, in addition to suppressing coarsening of austenite grain size by forming nitrides, carbides, and carbonitrides. Since Nb generates fine carbides even at higher temperatures than V, Mo, etc., Nb is a very effective element that has a large effect on the reduction of the austenite grain size during the production of heat-treated steel wires, even if its addition amount is very small. If it is less than 0.01%, almost no effect is observed, and if it exceeds 0.05%, coarse undissolved inclusions are formed, and the toughness is reduced.
【００３４】Ｂは焼入れ性向上元素として知られてい
る。さらにオーステナイト粒界の清浄化に効果がある。
すなわち、粒界に偏析して靱性を低下させるＰ、Ｓ等の
元素をＢを添加することで無害化し、破壊特性を向上さ
せる。その際、ＢがＮと結合してＢＮを生成するとその
効果は失われる。添加量はその効果が明確になる０．０
００５％を下限とし、効果が飽和する０．００６％を上
限とした。B is known as a hardenability improving element. Further, it is effective for cleaning austenite grain boundaries.
That is, the addition of B, such as P and S, which segregates at the grain boundaries and lowers the toughness, renders them harmless and improves the fracture characteristics. At that time, if B combines with N to form BN, the effect is lost. The amount of addition is 0.0
The lower limit was 005%, and the upper limit was 0.006% at which the effect was saturated.
【００３５】Ｎｉは焼入れ性を向上させ、熱処理によっ
て安定して高強度化することができる。またマトリック
スの延性を向上させてコイリング性を向上させる。しか
し焼入れ焼戻しでは残留オーステナイトを増加させるの
で、ばね成形後にへたりや材質の均一性の点で劣る。そ
の添加量は０．０５％未満では高強度化や延性向上に効
果が認められず、３．０％を超えると効果が飽和し、コ
スト等の点で不利になる。Ni improves the hardenability and can stably increase the strength by heat treatment. Further, the ductility of the matrix is improved to improve the coilability. However, since quenching and tempering increase retained austenite, it is inferior in set and material uniformity after spring forming. If the added amount is less than 0.05%, no effect is observed in increasing the strength and the ductility, and if it is more than 3.0%, the effect is saturated and disadvantageous in cost and the like.
【００３６】Ｃｕについては、Ｃｕを添加することで脱
炭を防止できる。脱炭層はばね加工後に疲労寿命を低下
させるため、極力少なくする努力が成されている。また
脱炭層が深くなった場合にはピーリングとよばれる皮む
き加工によって表層を除去する。またＮｉと同様に耐食
性を向上させる効果もある。脱炭層を抑制することでば
ねの疲労寿命向上やピーリング工程の省略することがで
きる。Ｃｕの脱炭抑制効果や耐食性向上効果は０．０５
％以上で発揮することができ、後述するようにＮｉを添
加したとしても０．５％を超えると脆化により圧延きず
の原因となりやすい。そこで下限を０．０５％、上限を
０．５％とした。Ｃｕ添加によって室温における機械的
性質を損なうことはほとんどないが、Ｃｕを０．３％を
超えて添加する場合には熱間延性を劣化させるために圧
延時にビレット表面に割れを生じる場合がある。そのた
め圧延時の割れを防止するＮｉ添加量をＣｕの添加量に
応じて［Ｃｕ％］＜［Ｎｉ％］とすることが好ましい。
Ｃｕ０．３％以下の範囲では圧延きずが生じないことか
ら、圧延きず防止を目的としてＮｉ添加量を規制する必
要がない。Regarding Cu, decarburization can be prevented by adding Cu. Efforts have been made to reduce the decarburized layer as much as possible to reduce the fatigue life after spring processing. When the decarburized layer becomes deep, the surface layer is removed by peeling called peeling. It also has the effect of improving the corrosion resistance, similarly to Ni. By suppressing the decarburized layer, the fatigue life of the spring can be improved and the peeling step can be omitted. Cu decarburization suppression effect and corrosion resistance improvement effect are 0.05
%, And even if Ni is added, as will be described later, if it exceeds 0.5%, it is likely to cause rolling flaws due to embrittlement. Therefore, the lower limit is set to 0.05% and the upper limit is set to 0.5%. The addition of Cu hardly impairs the mechanical properties at room temperature, but when Cu is added in excess of 0.3%, the hot ductility is degraded, so that the billet surface may crack during rolling. Therefore, it is preferable that the amount of Ni added to prevent cracking during rolling is [Cu%] <[Ni%] according to the amount of Cu added.
Rolling flaws do not occur in the Cu range of 0.3% or less, so there is no need to regulate the amount of Ni to prevent rolling flaws.
【００３７】炭化物規定に関して説明する。強度と加工
性の両立には鋼中の炭化物の形態が重要になってくる。
ここでいう鋼中炭化物とは鋼中に熱処理後に鋼中に認め
られるセメンタイトおよびおよびそれに合金元素の固溶
した炭化物、（以後、両者を総じてセメンタイトと記
す）およびＮｂ、Ｖ、Ｔｉ等の合金元素の炭化物および
炭窒化物のことである。これら炭化物は鋼線を鏡面研磨
し、エッチングすることで観察することができる。A description will be given of the carbide specification. The form of carbides in steel is important for achieving both strength and workability.
The carbide in the steel mentioned here is cementite found in the steel after heat treatment in the steel and carbide dissolved in the alloy element with the cementite (hereinafter, both are collectively referred to as cementite) and alloy elements such as Nb, V and Ti. Carbide and carbonitride. These carbides can be observed by mirror polishing and etching a steel wire.
【００３８】図１に焼入れ焼戻し組織の典型的な例の顕
微鏡写真を示す。これによると鋼中には針状と球状の２
種の炭化物が認められる。一般に鋼は焼入れによって、
マルテンサイトの針状組織を形成し、焼戻しによって炭
化物を生成させることで強度と靱性を両立させることが
知られている。しかし本発明では図１にあるように必ず
しも針状組織だけではなく、球状炭化物１も多く残留し
ていることに注目し、この球状の炭化物の分布がばね用
鋼線の性能に大きく影響することを見出した。この球状
の炭化物はオイルテンパー処理や高周波処理による焼入
れ焼戻しにおいて、十分に固溶されず、焼入れ焼戻し工
程で球状化かつ成長または縮小した炭化物と考えられ
る。この寸法の炭化物は焼入れ焼戻しによる強度と靱性
には全く寄与しない。そのため、鋼中Ｃを固定して単に
添加Ｃを浪費しているだけでなく、応力集中源にもなる
ため、鋼線の機械的性質を低下させる要因となることを
見出した。FIG. 1 shows a micrograph of a typical example of a quenched and tempered structure. According to this, needle-shaped and spherical 2
Some carbides are found. Generally, steel is hardened by
It is known that a martensitic needle-like structure is formed and a carbide is formed by tempering to achieve both strength and toughness. However, in the present invention, as shown in FIG. 1, it is noted that not only the needle-like structure but also a large amount of the spherical carbide 1 remains, and the distribution of the spherical carbide greatly affects the performance of the spring steel wire. Was found. This spherical carbide is not sufficiently dissolved in quenching and tempering by oil tempering or high-frequency treatment, and is considered to be spheroidized and grown or reduced in the quenching and tempering step. Carbides of this size do not contribute at all to the strength and toughness due to quenching and tempering. Therefore, it has been found that not only the addition of C is wasted by fixing C in the steel, but also a source of stress concentration, which is a factor of deteriorating the mechanical properties of the steel wire.
【００３９】本材料のように鋼を焼入れ焼戻ししてから
冷間コイリングする場合、炭化物がそのコイリング特
性、すなわち破断までの曲げ特性に影響する。これまで
高強度を得るためにＣだけでなく、Ｃｒ、Ｖ等の合金元
素を多量に添加することが一般的であったが、強度が高
すぎて、変形能が不足してがコイリング特性を劣化させ
る弊害があった。その原因に鋼中に析出している粗大な
炭化物が考えられる。When steel is quenched and tempered and then cold coiled as in the case of the present material, carbide affects its coiling properties, that is, bending properties up to fracture. Until now, in order to obtain high strength, not only C but also a large amount of alloying elements such as Cr and V were generally added, but the strength was too high and the deformability was insufficient, but the coiling characteristics were poor. There was an adverse effect of deterioration. The cause may be coarse carbides precipitated in the steel.
【００４０】図２（ａ）、（ｂ）にＳＥＭに取り付けた
ＥＤＸによる解析例を示す。この結果は透過電子顕微鏡
でのレプリカ法でも同様の解析結果が得られる。従来の
発明はＶ、Ｎｂ等の合金元素系の炭化物だけに注目して
おり、その一例が図２（ａ）であり、炭化物中にＦｅピ
ークが非常に小さいことが特徴である。しかし本発明で
は従来の合金元素系炭化物だけでなく、図２（ｂ）に示
すように、円相当径３μｍ以下のＦｅ3Ｃとそれに合金
元素がわずかに固溶した、いわゆるセメンタイト系炭化
物の析出形態が重要であることを見出した。本発明のよ
うに従来鋼線以上の高強度と加工性の両立を達成する場
合には３μｍ以下のセメンタイト系球状炭化物が多い
と、加工性が大きく損なわれる。以後、図２（ｂ）に示
したようなＦｅとＣを主成分とする炭化物をセメンタイ
ト系炭化物、更に形状が球状の場合をセメンタイト系球
状炭化物と記す。FIGS. 2 (a) and 2 (b) show examples of analysis by EDX attached to the SEM. The same analysis result can be obtained by the replica method using a transmission electron microscope. The conventional invention pays attention only to carbides of alloying elements such as V and Nb, one example of which is shown in FIG. 2A, which is characterized by an extremely small Fe peak in the carbide. However, in the present invention, as shown in FIG. 2 (b), not only conventional alloying element carbides but also precipitation of Fe 3 C having an equivalent circle diameter of 3 μm or less and so-called cementite carbides in which the alloying elements are slightly dissolved. The morphology was found to be important. In the case of achieving both high strength and workability higher than conventional steel wires as in the present invention, if there are many cementite-based spherical carbides of 3 μm or less, workability is greatly impaired. Hereinafter, a carbide mainly composed of Fe and C as shown in FIG. 2B is referred to as a cementite-based carbide, and a carbide having a spherical shape is referred to as a cementite-based spherical carbide.
【００４１】これらの鋼中炭化物は鏡面研磨したサンプ
ルにピクラールなどのエッチングを施すことで観察可能
であるが、その寸法などの詳細な観察評価には走査型電
子顕微鏡により３０００倍以上の高倍率で観察する必要
があり、ここで対象とするセメンタイト系球状炭化物は
円相当径０．２〜３μｍである。通常、鋼中炭化物は鋼
の強度、焼戻し軟化抵抗を確保する上で不可欠ではある
が、その有効な粒径は０．１μｍ以下で、逆に１μｍを
超えるとむしろ強度やオーステナイト粒径微細化への貢
献はなく、単に変形特性を劣化させるだけである。しか
し従来技術ではこの重要性がそれほど認識されず、Ｖ、
Ｎｂなどの合金系炭化物にのみ注目し、円相当径３μｍ
以下の炭化物、特にセメンタイト系球状炭化物は無害と
考えられ、本発明で主に対象としている０．１〜５μｍ
程度の炭化物に関しては検討された例は見当たらない。These carbides in steel can be observed by subjecting the mirror-polished sample to etching such as picral, but for detailed observation and evaluation of its dimensions and the like, a scanning electron microscope uses a scanning electron microscope at a high magnification of 3000 times or more. It is necessary to observe, and the cementite-based spherical carbide to be treated here has a circle equivalent diameter of 0.2 to 3 μm. Normally, carbides in steel are indispensable for ensuring the strength and tempering softening resistance of steel, but the effective grain size is 0.1 μm or less. Conversely, if it exceeds 1 μm, the strength and austenite grain size are reduced. Does not contribute, but merely degrades the deformation characteristics. However, the prior art is less aware of this importance, V,
Focusing only on alloy-based carbides such as Nb, equivalent circle diameter of 3 μm
The following carbides, especially cementite-based spherical carbides, are considered to be harmless and are mainly used in the present invention.
No examples have been found for the degree of carbides.
【００４２】また本発明で対象としているこ３μｍ以下
のセメンタイト系球状炭化物の場合には寸法だけでな
く、数も大きな要因となる。従ってその両者を考慮して
本発明範囲を規定した。すなわち円相当径が０．２〜３
μｍと小さくとも、その数が非常に多く、検鏡面におけ
る存在密度が１個／μｍ2を超えるとコイリング特性の
劣化が顕著になるのでこれを上限とする。In the case of cementite-based spherical carbides having a size of 3 μm or less, which are the objects of the present invention, not only the size but also the number is a major factor. Therefore, the scope of the present invention has been defined in consideration of both of them. That is, the equivalent circle diameter is 0.2 to 3
Even if it is as small as μm, the number is extremely large, and if the existence density on the speculum surface exceeds 1 piece / μm 2 , the coiling characteristics will be significantly deteriorated.
【００４３】さらに炭化物の寸法が３μｍを超えると寸
法の影響がより大きくなるため、検鏡面における存在密
度が０．００１個／μｍ2を超えるとコイリング特性の
劣化が顕著になる。従って炭化物円相当径３μｍ超の炭
化物の検鏡面における存在密度０．００１個／μｍ2を
上限とし、本発明の範囲をそれ以下とした。Further, when the size of the carbide exceeds 3 μm, the influence of the size becomes greater, and when the existence density on the speculum surface exceeds 0.001 / μm 2 , the coiling characteristics are significantly deteriorated. Therefore, the upper limit was set to 0.001 particles / μm 2 in the microscopic surface of carbides having a carbide circle equivalent diameter of more than 3 μm, and the range of the present invention was set to less.
【００４４】またセメンタイト系球状炭化物の寸法に関
わらず、その検鏡面における占有面積が７％を超えると
コイリング特性の劣化が顕著になり、コイリングできな
くなる。そこで本発明では検鏡面における占有面積を７
％以下と規定した。Regardless of the size of the cementite-based spherical carbide, if the area occupied by the speculum exceeds 7%, the coiling characteristics deteriorate remarkably and coiling cannot be performed. Therefore, in the present invention, the occupied area on the speculum surface is 7
% Or less.
【００４５】一方、旧オーステナイト粒径は炭化物と並
んで鋼線の基本的性質に大きな影響をもつ。すなわち旧
オーステナイト粒径が小さい方が疲労特性やコイリング
性に優れる。しかしいくらオーステナイト粒径が小さく
とも上記炭化物が規定以上に多く含まれていると、その
効果は少ない。一般にオーステナイト粒径を小さくする
には加熱温度を低くすることが有効であるが、そのこと
は逆に上記炭化物を増加させることになる。従って炭化
物量と旧オーステナイト粒径のバランスのとれた鋼線に
仕上げることが重要である。ここで炭化物が上記規定を
満たしている場合について旧オーステナイト粒径番号が
１０番未満であると十分な疲労特性を得られれないので
旧オーステナイト粒径番号を１０番以上と規定した。On the other hand, the prior austenite grain size has a great influence on the basic properties of the steel wire along with the carbide. That is, the smaller the prior austenite grain size, the better the fatigue characteristics and coilability. However, no matter how small the austenite grain size, the effect is small if the above-mentioned carbides are contained more than specified. Generally, it is effective to lower the heating temperature in order to reduce the austenite grain size. However, this leads to an increase in the amount of the carbide. Therefore, it is important to finish the steel wire with a balance between the amount of carbide and the prior austenite grain size. Here, when the carbide satisfies the above rule, if the prior austenite grain size number is less than 10, sufficient fatigue properties cannot be obtained, so the prior austenite grain size number is defined as 10 or more.
【００４６】また合金元素系炭化物等を含む全炭化物の
最大炭化物径および最大酸化物径はともに１５μｍを超
えると疲労特性を低下させるため、これを１５μｍを上
限として制限した。If the maximum carbide diameter and the maximum oxide diameter of all the carbides including the alloying element-based carbides, etc., both exceed 15 μm, the fatigue characteristics are deteriorated. Therefore, the upper limit is set to 15 μm.
【００４７】一般にばね鋼は連続鋳造後にビレット圧
延、線材圧延を経て伸線され、冷間コイリングばねでは
オイルテンパー処理や高周波処理によって強度を付与す
る。セメンタイト系球状炭化物を抑制するにはオイルテ
ンパー処理や高周波処理などの鋼線の強度を決定する最
終熱処理だけでなく、伸線に先立つ圧延時にも注意を払
う必要がある。すなわちセメンタイト系球状炭化物は圧
延などでの未溶解のセメンタイトや合金炭化物が核とな
って成長したと考えられることから、圧延などの各加熱
工程において十分成分を固溶させることが重要である。
本発明では圧延においても十分に高揚できる高温に加熱
して圧延し、伸線に供することが重要である。In general, spring steel is drawn through billet rolling and wire rod rolling after continuous casting, and a cold coiling spring is given strength by oil tempering or high frequency processing. In order to suppress the cementite-based spherical carbides, it is necessary to pay attention not only to the final heat treatment for determining the strength of the steel wire such as oil tempering treatment or high-frequency treatment but also to the rolling before the drawing. That is, it is considered that the cementite-based spherical carbide grows with the undissolved cementite or alloy carbide in rolling or the like serving as a nucleus. Therefore, it is important to sufficiently dissolve the components in each heating step such as rolling.
In the present invention, it is important that the steel sheet is heated to a high temperature at which the material can be sufficiently elevated even in the rolling, and is then subjected to wire drawing.
【００４８】[0048]
【実施例】以下に実施例により本発明の効果を説明す
る。EXAMPLES The effects of the present invention will be described below with reference to examples.
【００４９】表１にφ４ｍｍで処理した場合の本発明と
比較鋼の化学成分、熱処理方法、セメンタイト系球状炭
化物の占有面積率、円相当径０．２〜３μｍのセメンタ
イト系球状炭化物存在密度、円相当径３μｍ超のセメン
タイト系球状炭化物存在密度、最大炭化物径、最大酸化
物径、旧オーステナイト粒度番号、引張強度、コイリン
グ特性（ノッチ曲げ角度）および平均疲労強度（回転曲
げ）を示す。Table 1 shows the chemical composition of the present invention and the comparative steel when treated at φ4 mm, the heat treatment method, the occupied area ratio of the cementite-based spherical carbide, the density of the cementite-based spherical carbide having an equivalent circle diameter of 0.2 to 3 μm, and the circle. The cementite spherical carbide existing density having an equivalent diameter of more than 3 μm, the maximum carbide diameter, the maximum oxide diameter, the prior austenite grain size number, the tensile strength, the coiling characteristics (notch bending angle), and the average fatigue strength (rotating bending) are shown.
【００５０】本発明の実施例１、１８は２５０ｔ転炉に
よって精錬したものを連続鋳造によってビレットを作成
した。またそのほかの実施例は２ｔ−真空溶解炉で溶製
後、圧延によってビレットを作成した。その際、発明例
では１２００℃以上の高温に一定時間保定した。その後
いずれの場合もビレットからφ８ｍｍに圧延し、伸線に
よってφ４ｍｍとした。一方、比較例は通常の圧延条件
で圧延され伸線に供した。In Examples 1 and 18 of the present invention, billets were produced by continuous casting from those refined by a 250 t converter. In other examples, billets were produced by rolling after melting in a 2t-vacuum melting furnace. At that time, in the invention example, the temperature was kept at a high temperature of 1200 ° C. or more for a certain time. Thereafter, in each case, the billet was rolled to φ8 mm and drawn to φ4 mm. On the other hand, the comparative example was rolled under normal rolling conditions and subjected to wire drawing.
【００５１】ここで熱処理方法ＯＴ、ＩＱＴ、Ｆはそれ
ぞれオイルテンパー処理、高周波焼入れ焼戻しおよびオ
フラインによるバッチ炉（輻射炉）を用いた焼入れ焼戻
し処理を表している。Here, the heat treatment methods OT, IQT, and F represent oil tempering, induction hardening and tempering, and quenching and tempering using an off-line batch furnace (radiant furnace), respectively.
【００５２】化学成分によって炭化物量、強度は異なっ
てくるが、本発明については引張強度２１００〜２２０
０ＭＰａ程度かつ請求項に示す規定を満たすように化学
成分にあわせて熱処理した。一方、比較例に関しては単
に引張強度をあわせるように熱処理した。Although the amount and strength of the carbides vary depending on the chemical components, the present invention has a tensile strength of 2100 to 220.
Heat treatment was performed in accordance with the chemical components so as to satisfy about 0 MPa and satisfy the requirements described in the claims. On the other hand, the comparative example was heat-treated so as to simply adjust the tensile strength.
【００５３】バッチ炉処理では１ｍの試験片を矯直後、
加熱炉に投入して加熱し、６０℃のオイル槽に投入して
焼き入れた。加熱時間は３０ｍｉｎで、熱間コイリング
して製造する熱間ばねの温度履歴と対応するようにし
た。その後、再度加熱炉に投入して焼戻し、大気雰囲気
における引張強度を調整した。焼入れおよび焼戻し時の
加熱温度およびその結果得られた大気雰囲気での引張強
度は表１中に明記したとおりである。In batch furnace treatment, immediately after straightening a 1 m test piece,
It was placed in a heating furnace and heated, then placed in a 60 ° C. oil bath and quenched. The heating time was 30 minutes, which corresponded to the temperature history of the hot spring manufactured by hot coiling. Then, it was put into a heating furnace again and tempered, and the tensile strength in the air atmosphere was adjusted. The heating temperatures during quenching and tempering and the resulting tensile strengths in the air atmosphere are as specified in Table 1.
【００５４】オイルテンパー処理では伸線材を連続的に
加熱炉を通過させ、鋼内部温度が十分に加熱されるよ
う、加熱炉通過時間を設定した。本実施例ではでは加熱
温度９５０℃、加熱時間１５０ｓｅｃ、焼入れ温度５０
℃（オイル槽）とした。さらに焼戻し温度４００〜５５
０℃、焼戻し時間１ｍｉｎで焼戻し、強度を調整した。
焼入れおよび焼戻し時の加熱温度およびその結果得られ
た大気雰囲気での引張強度は表１中に明記したとおりで
ある。In the oil tempering treatment, the wire drawing material was continuously passed through the heating furnace, and the heating furnace passage time was set so that the steel internal temperature was sufficiently heated. In this embodiment, the heating temperature is 950 ° C., the heating time is 150 sec, and the quenching temperature is 50.
° C (oil tank). Furthermore, tempering temperature 400-55
Tempering was performed at 0 ° C. for a tempering time of 1 min to adjust the strength.
The heating temperatures during quenching and tempering and the resulting tensile strengths in the air atmosphere are as specified in Table 1.
【００５５】高周波熱処理ではコイル内部を通線するこ
とで加熱し、コイル通過後即座に水冷した。加熱温度９
９０℃、加熱時間１５ｓｅｃ、焼入れは水冷（室温）で
ある。さらに焼戻し温度４３０〜６００℃で再度コイル
内を通過させて焼戻し処理を行った。その結果得られた
大気雰囲気での引張強度は表１中に明記したとおりであ
る。In the high-frequency heat treatment, the coil was heated by passing through the inside of the coil, and was cooled immediately after passing through the coil. Heating temperature 9
90 ° C., heating time 15 sec, quenching is water cooling (room temperature). Furthermore, it passed through the coil again at a tempering temperature of 430 to 600 ° C. to perform a tempering treatment. The resulting tensile strength in the air atmosphere is as specified in Table 1.
【００５６】得られた鋼線はそのまま炭化物の評価、引
張特性、ノッチ曲げ試験に供した。一方、疲労特性評価
に関しては表面にばね製作時の歪取り焼鈍を模した熱処
理４００℃×２０ｍｉｎを施しのち、ショットピーニン
グ処理（カットワイヤーφ０．６ｍｍ×２０ｍｉｎ）を
行い、さらに低温歪取り１８０℃×２０ｍｉｎを施して
疲労試験片とした。The obtained steel wire was directly subjected to evaluation of carbide, tensile properties and notch bending test. On the other hand, regarding the evaluation of fatigue characteristics, the surface was subjected to a heat treatment at 400 ° C. for 20 minutes to simulate the strain relief annealing at the time of spring production, followed by a shot peening treatment (cut wire φ0.6 mm × 20 min), and a low-temperature strain relief at 180 ° C. × The specimen was subjected to a fatigue test for 20 minutes.
【００５７】炭化物の寸法および数の評価は熱処理まま
の鋼線の長手方向断面に鏡面まで研磨し、さらにピクリ
ン酸によってわずかにエッチングして炭化物を浮き出さ
せた。光学顕微鏡レベルでは炭化物の寸法測定は困難な
ため、鋼線の１／２Ｒ部を走査型電子顕微鏡で倍率×５
０００倍にて無作為に１０視野の写真を撮影した。走査
型電子顕微鏡に取り付けたＸ線マイクロアナライザーに
てその球状炭化物がセメンタイト系球状炭化物であるこ
とを確認しつつ、その写真から球状炭化物を画像処理装
置を用いて２値化することで、その寸法、数、占有面積
を測定した。全測定面積は３０８８．８μｍである。Evaluation of the size and number of carbides was carried out by polishing a longitudinal section of the as-heat-treated steel wire to a mirror surface and further etching slightly with picric acid to lift out the carbides. Since it is difficult to measure the dimensions of carbides at the optical microscope level, a 1/2 × section of the steel wire was magnified by a scanning electron microscope at a magnification of × 5.
Photographs of 10 visual fields were randomly taken at 000 ×. Using an X-ray microanalyzer attached to a scanning electron microscope, confirming that the spherical carbide is a cementite-based spherical carbide, and binarizing the spherical carbide from the photograph using an image processing device to obtain its dimensions , Number and occupied area were measured. The total measured area is 3088.8 μm.
【００５８】引張特性はＪＩＳ Ｚ ２２０１ ９号試
験片によりＪＩＳ Ｚ ２２４１に準拠して行い、その
破断荷重から引張強度を算出した。The tensile properties were measured using JIS Z 22019 test pieces in accordance with JIS Z 2241, and the tensile strength was calculated from the breaking load.
【００５９】ノッチ曲げ試験の概要を図３に示す。また
以下のような手順で行った。図３（ａ）に示すように先
端半径５０μｍのポンチによって鋼線の長手方向に直角
に最大深さ３０μｍの溝（ノッチ）２を付け、その溝部
に最大引張応力が負荷させるように両側を支持し、中央
に荷重３を加えて変形する３点曲げ変形を加えた。ノッ
チ部から破断するまで曲げ変形を加え続け、破断時の曲
げ角度を測定した。測定角度は図３（ｂ）に示すとおり
で、測定角度（θ）が大きいほどコイリング特性が良好
である。経験的にはφ４ｍｍの鋼線においてノッチ曲げ
角度２５゜以下ではコイリングは困難である。FIG. 3 shows the outline of the notch bending test. The procedure was as follows. As shown in FIG. 3A, a groove (notch) 2 having a maximum depth of 30 μm is formed at right angles to the longitudinal direction of the steel wire by a punch having a tip radius of 50 μm, and both sides are supported so that the maximum tensile stress is applied to the groove. Then, a three-point bending deformation in which a load of 3 was applied to the center and deformed was applied. The bending deformation was continued to be applied until breaking from the notch, and the bending angle at the time of breaking was measured. The measurement angle is as shown in FIG. 3B, and the larger the measurement angle (θ), the better the coiling characteristics. Empirically, coiling is difficult at a notch bending angle of 25 ° or less in a φ4 mm steel wire.
【００６０】疲労試験は中村式回転曲げ疲労試験であ
り、１０本のサンプルが５０％以上の確率で１０7サイ
クル以上の寿命を示す最大負荷応力を平均疲労強度とし
た。The fatigue test is a Nakamura-type rotary bending fatigue test. The maximum load stress at which 10 samples exhibit a life of 10 7 cycles or more with a probability of 50% or more was defined as the average fatigue strength.
【００６１】さらに表２にφ１２ｍｍで処理した場合の
本発明と比較鋼の化学成分、熱処理方法、セメンタイト
系球状炭化物の占有面積率、円相当径０．２〜３μｍの
セメンタイト系球状炭化物存在密度、円相当径３μｍ超
のセメンタイト系球状炭化物存在密度、旧オーステナイ
ト粒度番号、引張強度、コイリング特性（引張試験にお
ける絞り）、疲労強度および遅れ破壊強度を示す。Further, Table 2 shows the chemical composition of the present invention and the comparative steel when treated at φ12 mm, the heat treatment method, the occupied area ratio of the cementite-based spherical carbide, the density of the cementite-based spherical carbide having a circle equivalent diameter of 0.2 to 3 μm, The density of cementite-based spherical carbide having an equivalent circle diameter of more than 3 μm, the prior austenite particle size number, tensile strength, coiling characteristics (drawing in tensile test), fatigue strength and delayed fracture strength are shown.
【００６２】これらの実施例は２ｔ−真空溶解炉で溶製
後、圧延によってビレットを作成した。その後いずれの
場合もビレットからφ１４ｍｍに圧延し、その後、φ１
２ｍｍまで伸線した。In these examples, billets were prepared by rolling after melting in a 2t-vacuum melting furnace. Thereafter, in any case, the billet was rolled to φ14 mm, and then
The wire was drawn to 2 mm.
【００６３】この場合、φ４ｍｍの試験に比べ太径であ
ることからコイリング性の指標として引張試験における
絞りを用いた。In this case, the diameter in the tensile test was used as an index of the coilability since the diameter was larger than that in the test of φ4 mm.
【００６４】また疲労強度は小野式回転曲げ疲労試験に
よって評価し、疲労限を疲労強度とした。The fatigue strength was evaluated by an Ono-type rotary bending fatigue test, and the fatigue limit was defined as the fatigue strength.
【００６５】図４に遅れ破壊強度評価試験方法を示す。
図４（ａ）は試験片４の形状を示す。図４（ｂ）に示す
遅れ破壊試験装置を用いれば円周ノッチ付き試験片４に
容器内で水素チャージをしながら荷重５を負荷し、その
際の破断時間を計測できる。試験片４は、バンドヒータ
ーにより３０℃とされた溶液（ｐＨ３．０、Ｈ2ＳＯ4）
７中に保持され、定電流電源８（電流密度１．０ｍＡ／
ｃｍ2）で、試験片をカソード、白金電極をアノード９
として試験される。そのような試験において負荷荷重を
変化させた場合、負荷時間２００ｈｒ関係化後も破断し
ない最大荷重Ｗが計測できる。それをノッチ底断面積Ｓ
で除した公称応力（すなわちＷ／Ｓ）を遅れ破壊強度と
した。FIG. 4 shows a test method for evaluating delayed fracture strength.
FIG. 4A shows the shape of the test piece 4. By using the delayed fracture test apparatus shown in FIG. 4 (b), a load 5 is applied to the circumferentially notched test piece 4 while charging hydrogen in the container, and the rupture time at that time can be measured. The test piece 4 was a solution (pH 3.0, H 2 SO 4 ) adjusted to 30 ° C. by a band heater.
7 and a constant current power supply 8 (current density 1.0 mA /
cm 2 ), the test piece was a cathode and the platinum electrode was an anode 9
Tested as When the applied load is changed in such a test, the maximum load W that does not break even after the relationship of the applied time of 200 hours can be measured. Notch bottom cross section S
The nominal stress (i.e., W / S) divided by the above was defined as the delayed fracture strength.
【００６６】φ１２ｍｍで処理した場合には疲労試験片
および遅れ破壊試験片には歪取り焼鈍を模した４００℃
×２０ｍｉｎの焼鈍を施したのみで、φ４ｍｍサンプル
に施したようなショットピーニング処理とその後の歪取
り焼鈍を省略した。In the case of processing at φ12 mm, the fatigue test piece and the delayed fracture test piece were subjected to 400 ° C. simulating strain relief annealing.
The shot peening treatment and the subsequent strain relief annealing as performed on the φ4 mm sample were omitted only by performing the annealing for × 20 min.
【００６７】表１に示すとおり、φ４ｍｍの鋼線に関し
ては成分は規定範囲内であってもセメンタイト系球状炭
化物の占有面積率、セメンタイト系球状炭化物の存在密
度が本規定範囲外にある比較材はコイリング性の指標と
なるノッチ曲げ試験における曲げ角度が小さく、コイリ
ングできないことがわかる。一方、炭化物に関する規定
を満たしても強度が不足していると疲労強度が不足し、
高強度ばねには使用できない。As shown in Table 1, with respect to the steel wire of φ4 mm, even if the composition is within the specified range, the comparative material having the occupied area ratio of the cementite type spherical carbide and the existing density of the cementite type spherical carbide outside the specified range is as follows. It can be seen that the bending angle in the notch bending test, which is an index of the coilability, is small and coiling cannot be performed. On the other hand, if the strength is insufficient even if the regulations regarding carbides are satisfied, the fatigue strength will be insufficient,
Cannot be used for high strength springs.
【００６８】また表２に示すφ１２ｍｍの鋼線に関する
評価結果から成分は規定範囲内であってもセメンタイト
系球状炭化物の占有面積率、セメンタイト系球状炭化物
の存在密度が本規定範囲外にある比較材はコイリング性
の指標となる絞りが小さく、またそれを改善するために
強度を低下させると疲労強度が低下する。さらにオース
テナイト粒径は疲労特性と遅れ破壊特性に影響するが、
それが大きくなるとたとえ炭化物に関する規定を満たし
ていても疲労特性と遅れ破壊特性の点で不十分であっ
た。From the evaluation results of the steel wire having a diameter of 12 mm shown in Table 2, even if the components were within the specified ranges, the comparative material having the occupied area ratio of the cementite-based spherical carbides and the existing density of the cementite-based spherical carbides outside the specified range was used. Has a small drawing which is an index of the coilability, and if the strength is reduced to improve it, the fatigue strength is reduced. Furthermore, austenite grain size affects fatigue properties and delayed fracture properties,
When it becomes large, even if the requirements for carbides are satisfied, the fatigue properties and delayed fracture properties are insufficient.
【００６９】ただしオーステナイト粒径を微細にするに
は焼入れ加熱温度を低温にする、加熱時間を短縮するな
どの手法があるが、それらは逆に未溶解炭化物を多く残
留させることになるため、本規定を満たすのは困難にな
る。従ってオイルテンパーまたは高周波処理のように炭
化物を溶解させる短時間による高温加熱を可能とする技
術の導入が重要で、表１および２に見られるようにバッ
チ炉による拙速な処理では鋼線の強度とコイリング性の
両立は困難である。このことはばねの高強度化も困難に
なることを意味する。However, there are techniques such as lowering the quenching heating temperature and shortening the heating time in order to make the austenite grain size fine. However, these methods conversely cause a large amount of undissolved carbide to remain. It will be difficult to meet the regulations. Therefore, it is important to introduce a technology that enables high-temperature heating in a short time to dissolve carbides, such as oil tempering or high-frequency treatment. As shown in Tables 1 and 2, in a rapid treatment in a batch furnace, the strength of steel wire and It is difficult to achieve good coilability. This means that it is difficult to increase the strength of the spring.
【００７０】[0070]
【表１】 [Table 1]
【００７１】[0071]
【表２】 [Table 2]
【００７２】[0072]
【発明の効果】本発明鋼は、冷間コイリングばね用鋼線
中のセメンタイトを含む球状炭化物の占有面積率、存在
密度、オーステナイト粒径を小さくすることで、強度を
１９００ＭＰａ以上に高強度化するとともに、コイリン
グ性を確保し高強度かつ破壊特性に優れたばねを製造可
能になる。According to the present invention, the strength of the steel wire for a cold coiling spring can be increased to 1900 MPa or more by reducing the occupied area ratio, the existing density, and the austenite particle size of the spherical carbide containing cementite in the steel wire for a cold coiling spring. At the same time, it becomes possible to manufacture a spring that secures coiling properties and has high strength and excellent breaking characteristics.
【図１】焼入れ焼戻し組織を示す鋼の顕微鏡写真であ
る。FIG. 1 is a micrograph of steel showing a quenched and tempered structure.
【図２】球状炭化物分析例を示すグラフである。FIG. 2 is a graph showing an example of spherical carbide analysis.
【図３】ノッチ曲げ試験方法を示す図である。FIG. 3 is a view showing a notch bending test method.
【図４】遅れ破壊試験方法を示す図である。FIG. 4 is a diagram showing a delayed fracture test method.
１ 球状炭化物 ２ 溝（ノッチ） ３ 荷重 ４ 試験片 ５ 荷重 ６ バンドヒーター ７ 溶液 ８ 定電流電源 ９ アノード θ 測定角度 Reference Signs List 1 spherical carbide 2 groove (notch) 3 load 4 test piece 5 load 6 band heater 7 solution 8 constant current power supply 9 anode θ measurement angle
Claims (6)
ｉ：０．９〜３．０％、Ｍｎ：０．１〜２．０％、Ｃ
ｒ：２．５％以下（０％含む）、Ｐ：０．０１５％以
下、Ｓ：０．０１５％以下、Ｎ：０．００１〜０．００
７％、残部鉄および不可避的不純物を含み、引張強度Ｔ
Ｓが１９００ＭＰａ以上、かつ検鏡面に占めるセメンタ
イト系球状炭化物に関して円相当径０．２μｍ以上の占
有面積率が７％以下、円相当径０．２〜３μｍの存在密
度が１個／μｍ2以下、円相当径３μｍ超の存在密度が
０．００１個／μｍ2以下を満たし、かつ旧オーステナ
イト粒径番号が１０番以上、最大炭化物径が１５μｍ以
下かつ最大酸化物径が１５μｍ以下であることを特長と
するばね用熱処理鋼線。C. 0.4 to 1.0% by mass, S
i: 0.9 to 3.0%, Mn: 0.1 to 2.0%, C
r: 2.5% or less (including 0%), P: 0.015% or less, S: 0.015% or less, N: 0.001 to 0.00
7%, including balance iron and unavoidable impurities, tensile strength T
S is 1900MPa or more, and 7% equivalent circle diameter 0.2μm or more occupied area ratio with respect to cementite spherical carbides occupying the detection mirror below, the density of the circle equivalent diameter 0.2~3μm is one / [mu] m 2 or less, The feature is that the existence density of the circle equivalent diameter exceeding 3 μm satisfies 0.001 / μm 2 or less, and the former austenite particle size number is 10 or more, the maximum carbide diameter is 15 μm or less, and the maximum oxide diameter is 15 μm or less. And heat treated steel wire for spring.
％、Ｃｏ：０．０５〜３．０％の１種または２種を含む
ことを特徴とする請求項１記載の、ばね用熱処理鋼線。2. W: 0.05 to 1.0 in mass%.
The heat-treated steel wire for a spring according to claim 1, wherein the heat-treated steel wire contains one or two of Co, 0.05 to 3.0%.
０．０１％を含むことを特徴とする請求項１または２の
いずれかに記載の、ばね用熱処理鋼線。3. The composition according to claim 1, wherein the content of Mg is 0.0002
The heat-treated steel wire for a spring according to claim 1, wherein the steel wire contains 0.01%.
０．１％、Ｍｏ：０．０５〜１．０％、Ｖ：０．０５〜
０．７％、Ｎｂ：０．０１〜０．０５％の１種または２
種以上を含むことを特徴とする請求項１〜３のいずれか
に記載の、ばね用熱処理鋼線。4. Further, in mass%, Ti: 0.005 to
0.1%, Mo: 0.05-1.0%, V: 0.05-
0.7%, Nb: one or two of 0.01 to 0.05%
The heat-treated steel wire for a spring according to any one of claims 1 to 3, comprising at least one kind.
０．００６％を含むことを特徴とする請求項１〜４のい
ずれかに記載の、ばね用熱処理鋼線。5. B: 0.0005 to 5% by mass
The heat-treated steel wire for a spring according to any one of claims 1 to 4, comprising 0.006%.
０％、Ｃｕ：０．０５〜０．５％の１種または２種を含
むことを特徴とする請求項１〜５のいずれかに記載の、
ばね用熱処理鋼線。6. Ni: 0.05 to 3.% by mass.
The composition according to any one of claims 1 to 5, wherein the composition contains one or two of 0% and Cu: 0.05 to 0.5%.
Heat treated steel wire for spring.
Priority Applications (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000386902A JP3971571B2 (en) | 2000-12-20 | 2000-12-20 | Steel wire for high strength spring |
| KR10-2002-7012197A KR100514120B1 (en) | 2000-12-20 | 2001-12-20 | High-strength spring steel and spring steel wire |
| PCT/JP2001/011216 WO2002050327A1 (en) | 2000-12-20 | 2001-12-20 | High-strength spring steel and spring steel wire |
| DE60131294T DE60131294T2 (en) | 2000-12-20 | 2001-12-20 | HIGH STRENGTH SPRING STEEL AND SPRING STEEL WIRE |
| EP01271133A EP1347069B1 (en) | 2000-12-20 | 2001-12-20 | High-strength spring steel and spring steel wire |
| US10/362,651 US7789974B2 (en) | 2000-12-20 | 2001-12-20 | High-strength spring steel wire |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000386902A JP3971571B2 (en) | 2000-12-20 | 2000-12-20 | Steel wire for high strength spring |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JP2002180198A true JP2002180198A (en) | 2002-06-26 |
| JP3971571B2 JP3971571B2 (en) | 2007-09-05 |
Family
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|---|---|---|---|
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