JP3633866B2 - Steel wire for spring, spring and manufacturing method thereof - Google Patents

Description

【０００６】
さらに、特開平９−７１８４３号公報では、金属組織中の残留オーステナイト相の体積率を低減し、焼入れ時の未固溶炭化物の組織内密度を均一に低減させることで、靭性低下抑制効果を実現しているが、材料そのものの母相を強化するものではなく、耐疲労性向上効果は少ない。
【０００７】
【発明が解決しようとする課題】
しかしながら、上記従来の技術は、いずれも結晶粒微細化や組織に散在する残留オーステナイト相や未固溶炭化物の低減を行うことで目的とする耐疲労性向上を図ったものであり、母相マルテンサイトの結晶粒の強化を積極的に図ったものではない。
【０００８】
従って、本発明の主目的は、母相マルテンサイトの結晶粒強化を行うことによって高い耐疲労性、耐腐食疲労性を得ることができるばね用鋼線とその製造方法ならびにばねを提供することにある。
【０００９】
【課題を解決するための手段】
本発明は、マルテンサイト結晶内の炭化物形状を所定の針状に特定することで上記の目的を達成する。
【００１０】
すなわち、本発明ばね用鋼線は、化学成分として、質量％でＣ：０．４〜１．０、Ｓｉ：０．１〜２．５、Ｍｎ：０．２〜１．２、Ｃｒ：０．５〜１．２を含有し、残部がＦｅ及び不可避不純物からなる成分を持ち、主に焼入れ焼戻しを行って得られる焼戻しマルテンサイト組織を有し、マルテンサイト結晶内の炭化物形状が平均アスペクト比で３．０以上であることを特徴とする。
【００１１】
ここで、マルテンサイト結晶内の炭化物の長径が０．１μｍ以上であることが好ましい。
【００１２】
さらに化学成分として、質量％でＭｏ：０．０５〜０．５０、Ｖ：０．０５〜０．５０、Ｗ：０．０５〜０．１５、Ｎｂ：０．０５〜０．１５、Ｔｉ：０．０１〜０．２０のうち１種以上を含有することが好適である。その他の化学成分として、質量％で、Ｎｉ：０．０２〜１．００、Ｃｏ：０．０２〜１．００、Ｃｕ：０．０２〜１．００を含有することも望ましい。
【００１３】
また、鋼線横断面の焼入れ後におけるオーステナイト結晶粒（旧オーステナイト結晶粒）の平均結晶粒径が１．０〜１８．０μｍとすることが好ましい。さらに好ましい平均結晶粒径の範囲は１．０〜７．０μｍである。この結晶粒径は、焼入れ後の粒径であるが、焼戻し後もほぼ同じ粒径のまま残る。
【００１４】
鋼線の引張強さは、１３００ＭＰａ以上２８００ＭＰａ以下であることが好適である。
【００１５】
さらに、本発明のばねは、上記の鋼線を用いて作製されたことを特徴とする。
【００１６】
一方、本発明ばね用鋼線の製造方法は、焼入れ時および焼戻し時の加熱を昇温速度５０〜２０００℃／ｓ（℃／秒）で行い、保持時間を０．５〜３０ｓ（秒）で行うことを特徴とする。
【００１７】
マルテンサイト中に焼戻し時析出する結晶粒内炭化物は、非常に硬いため、結晶粒を強化するのに有効である。しかし、これが球状あるいは粒状であるとき、圧縮応力あるいは引張応力またはせん断応力が繰り返しかかると、その弾性限やヤング率の違いから炭化物／母相の境界にクラックを生じ、破壊の原因となる。また、疲労によって生じたすべり帯が容易に結晶粒内に発生し、結果としてすべり帯の集中が起きやすく、疲労破壊の起点となり易い。
【００１８】
そこで本発明では、結晶粒内の炭化物を前述のように、マルテンサイトラス内に針状に析出させることで、炭化物は強化繊維の役割を果たし、その結晶粒を非常に強固で靭性を持ったものとし、耐疲労性の向上を図った。
【００１９】
以下に本発明における構成元素の選定および成分範囲を限定する理由並びに製造条件の特定理由を述べる。
【００２０】
（Ｃ：０．４〜１．０質量％）
Ｃは鋼の機械的特性を決定する重要な元素であるが、０．４％未満では十分な強度が得られず、逆に１．０％を越えると靭性が低下し、更に鋼線の疵感受性が高くなり信頼性が低下するため、Ｃ含有量を０．４〜１．０％とした。
【００２１】
（Ｓｉ：０．１〜２．５質量％）
Ｓｉは溶解精錬時の脱酸剤として使用される。またフェライト中に固溶し、強化する効果も合わせ持つ。但し、過度の添加は靭性の欠如を招き、熱間加工性の低下や熱処理による脱炭の助長、そして、ばね加工時の折損の原因となり易いため、脱酸効果を持たせるために０．１％以上、靭性欠如を防止するために２．５％以下とした。
【００２２】
（Ｍｎ：０．２〜１．２質量％）
ＭｎもＳｉ同様、溶解精錬時の脱酸剤として使用され、鋼の焼入性を向上させ、鋼中のＳを固定してその害を阻止する。但しＭｎは線材の中心偏析を生じ易くする元素でもあり、熱間圧延後のパテンティング処理時に中心偏析箇所にマルテンサイトを生じ、著しく線引き加工時の断線率を増加させる。そこで脱酸作用を持つ下限として０．２％以上、靭性劣化を招かない範囲として上限を１．２％とした。
【００２３】
（Ｃｒ：０．５〜１．２質量％）
ＣｒはＭｎ同様に鋼の焼入性を向上させ、かつ熱間圧延後のパテンティング処理により靭性を付与し、焼入れ後、焼戻し時の軟化抵抗を高め、高強度化に有効な元素である。０．５％未満ではその効果が少なく、逆に、１．２％を越えると炭化物の固溶を抑制し、強度の低下を招くとともに、焼入性の過度の増加となって靭性の低下をもたらすためである。
【００２４】
（Ｍｏ：０．０５〜０．５０質量％）
Ｍｏは焼戻し時に炭化物を形成し、軟化抵抗を増大させる元素であるが、０．０５％未満ではその効果が少なく、０．５０％を越えると伸線加工性を低下させるため、含有量をＭｏ：０．０５〜０．５０％とした。
【００２５】
（Ｗ，Ｎｂ：０．０５〜０．１５質量％、Ｖ：０．０５〜０．５０質量％）
Ｗ，Ｎｂ，Ｖも焼戻し時に鋼中に炭化物を形成し、軟化抵抗を増大させる効果がある。但し、いずれも０．０５％未満ではその効果を発揮し得ない。逆に、Ｖでは０．５０％、Ｗ，Ｎｂでは０．１５％を越えるといずれも焼入れ加熱時に炭化物を多く形成し、靭性の低下を招くため、含有量をそれぞれＶ：０．０５〜０．５０％、Ｗ：０．０５〜０．１５％、Ｎｂ：０．０５〜０．１５％と定めた。
【００２６】
（Ｔｉ：０．０１〜０．２０質量％）
Ｔｉも焼戻し時に鋼中に炭化物を形成し、軟化抵抗を増大させる効果がある。但しＴｉは高融点非金属介在物であるＴｉＯを生成する。故に精錬時の条件設定などが重要である。軟化抵抗向上効果が期待できる量として０．０１％以上、炭化物、介在物の過度の増加による靭性劣化を考慮して０．２０％以下とした。
【００２７】（Ｎｉ，Ｃｏ，Ｃｕ：０．０２〜１．００質量％）
Ｎｉ，Ｃｏ，Ｃｕはオーステナイト生成元素であり、Ｎｉ，Ｃｏ，Ｃｕ添加によってＭｓ点を大きく低下させ、残留オーステナイトを生じ易くする材料である。残留オーステナイトの増加は、鋼線の硬度を低下させる作用を持つが、逆にＳｉによる固溶強化やＭｏ，Ｗ，Ｎｂ，Ｖ，Ｔｉといった炭化物析出元素で強化された鋼線に靭性を持たせる効果を持つ。またＮｉは塩水腐食環境において、Ｃｌ元素の侵入を阻止する役割も持つ。靭性向上効果を持つ最低限度として０．０２％、硬度低下を招かない上限として１．００％とした。
【００２８】
（アスペクト比：３以上、炭化物の長径：０．１μｍ以上）
アスペクト比が３以上である細長い針状の炭化物を形成することで、炭化物は強化繊維の役割を果たし、非常に強固で靭性を持った材料とできるからである。また、炭化物の長径が０．１μｍ以上のものであれば、強化繊維の役割を効果的に果たしやすいからである。
【００２９】
（旧オーステナイト結晶粒径：１．０〜１８．０μｍ）
上記の粒内炭化物を有する鋼線は、旧オーステナイト結晶粒径が１．０〜１８．０μｍであるとき、さらに耐疲労性に優れる。これは結晶粒の強化を行い、金属組織を微細化することで高強度と高靭性を両立させた材料がはじめて得られることに起因する。結晶粒径は１８．０μｍ以下としたとき微細化効果が現れるが、更に７．０μｍ以下とするとき、その微細化による強化の効果は著しい。但し、結晶粒径１．０μｍ未満の時、熱処理による未固溶炭化物の除去が非常に困難となるため、下限を１．０μｍ以下とした。１８．０μｍを越える場合は、疲労限が低く、かつ靭性も低下しやすいという問題がある。
【００３０】
（引張強さ：１３００ＭＰａ以上２８００ＭＰａ以下）
引張り強さは、１３００ＭＰａ以上２８００ＭＰａ以下であるとき、ばね用鋼線として特に優れた性能を発揮する。この値は、ばねとしてコイリングするときに最低限必要な引張り強さとして１３００ＭＰａ以上、コイリング時に折損しない靭性を持たせるために２８００ＭＰａ以下とした。
【００３１】
（製造条件）
前述した針状粒内炭化物を持つ本発明鋼線を得るには、極めて短時間の焼入れ・焼戻し加熱が有効で、中でも短時間の焼戻し加熱が有効である。長時間の加熱は粒内炭化物の球状化、粗大化を引き起こす。また、この製造条件は、所定の旧オーステナイト結晶粒径を実現するためにも必要である。そこで、鋼線のサイズにもよるが、焼入れおよび焼戻し時の昇温速度を５０〜２０００℃／ｓとし、保持時間を０．５〜３０ｓとしたとき、粒内炭化物を効果的に針状化、微細化することができる。焼入れの好ましい加熱温度は８００〜１１５０℃程度、焼戻しの好ましい加熱温度は２５０〜５５０℃程度である。さらに、焼入れ時の昇温速度を５０〜２０００℃／ｓとすることで、酸化スケールを低減することができ、それに伴って高い疲労強度を得ることができる。
【００３２】
【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） 表１に示す実施例であるサンプルＡ，Ｂ，Ｃ，Ｄ，Ｅ，Ｆ，Ｌ，Ｍ，Ｎ，Ｏ，Ｐ，Ｑ，Ｒと、比較例であるサンプルＧ，Ｈ，Ｉ，Ｊ，Ｋについて鋼を真空溶解炉にて溶製し、熱間鍛造、熱間圧延により直径６．５ｍｍの線材を作製した。 この線材を熱処理、皮剥、冷間伸線により直径４．０ｍｍに加工した。さらにこれらに焼入れ加熱温度を１０００℃として焼入れ、４５０℃で焼戻し処理を施してオイルテンパー線を得た。 表１に得られた試料の化学成分、粒内炭化物平均アスペクト比、旧γ粒結晶粒径および室温での引張り強さを示す。
【００３３】
アスペクト比は、前記の試料から電解研磨で薄膜を作り、更にエッチングを行ってＴＥＭ（Ｔｒａｎｓｍｉｓｓｉｏｎ Ｅｌｅｃｔｒｏｎ Ｍｉｃｒｏｓｃｏｐｅ）で観察し、ＴＥＭ写真からマルテンサイト結晶粒内に析出した粒内炭化物のアスペクト比を実際に測定した。アスペクト比は粒内炭化物の長径／短径で求める。参考までに、本発明実施例であるサンプルＡのＴＥＭ観察写真を図１に、比較例であるサンプルＧのＴＥＭ観察写真を図２に示す。
【００３４】
図１、２中の黒く見える個所が炭化物を示している。図１、２からわかるように、サンプルＡのマルテンサイト結晶粒内炭化物の長径は平均で０．１μｍ以上あった。これに対して、サンプルＧのマルテンサイト結晶粒内炭化物は平均で０．１μｍ未満であり、形状もほぼ球状であった。なお、他の実施例であるサンプルＢ〜Ｆも同様にマルテンサイト結晶粒内炭化物の長径は平均で０．１μｍ以上であり、他の比較例Ｈ〜Ｋのマルテンサイト結晶粒内炭化物も平均で０．１μｍ未満であり、形状もほぼ球状であった。
【００３５】
実施例と比較例では、マルテンサイト結晶粒内に析出した粒内炭化物の平均アスペクト比が異なる。実施例では平均５．０〜７．０としたのに対し、比較例では１．０〜４．０としている。アスペクト比を変える方法としては、焼き入れおよび焼戻し加熱の昇温速度と保持時間とを制御することが挙げられる。保持時間とは、焼入れ又は焼戻しの加熱を開始してから冷却を開始するまでの時間である。
【００３６】
実施例は、焼戻し加熱を、加熱温度４５０℃、昇温速度１０００℃／ｓ、保持時間を１．０ｓとした。これに対し、比較例では、焼戻し加熱を、加熱温度４５０℃、昇温速度３０℃／ｓ、保持時間を４０ｓで行った。
【００３７】
また、結晶粒微細化の影響を調べるため、鋼線横断面における旧オーステナイト平均結晶粒径が３μｍ程度のもの（サンプルＥ）、５μｍ程度のもの（サンプルＤ，Ｆ，Ｊ）、１０μｍ程度のもの（サンプルＡ，Ｂ，Ｃ，Ｇ，Ｈ，Ｉ，Ｌ〜Ｒ）並びに２０μｍ程度のもの（サンプルＫ）の４種類を得た。各種類ごとの焼入れ時の加熱温度、昇温速度および保持時間は次のとおりである。

【００３８】
得られた線材について酸化スケールの量を調べてみた。その結果、いずれの実施例も酸化スケール量が１０ｇ／ｍ^２以下と少なくなっていることがわかった。また、いずれのサンプルも室温での引張り強さは２０００ＭＰａ程度であり、ほぼ同一の強度の材料を得た。そこで、ばね加工後のひずみ取りテンパーを想定して、４００℃×３０分のテンパーを行った。
【００３９】
いずれの試料も、光学顕微鏡観察やＳＥＭ（Ｓｃａｎｎｉｎｇ Ｅｌｅｃｔｒｏｎ Ｍｉｃｒｏｓｃｏｐｅ）で観察の結果、線表面の脱炭、酸化皮膜、金属組織内の未固溶炭化物は存在しないことを確認して評価に移った。
【００４０】
【表１】

【００４１】
次に、以上のサンプルを中村式回転曲げ疲労試験機にかけた結果を表２に示す。試験はひずみ一定で試料に応力をかけ、繰り返し回数１×１０^７回で折損のなかった振幅応力をとった（ｎ数＝８）。
【００４２】
【表２】

【００４３】
いずれの実施例も比較例に比べて疲れ強さが向上することがわかった。特に、Ｃ、Ｓｉ、Ｍｎ、Ｃｒ、Ｖ以外の化学成分の含有に伴う疲れ強さについて考察して見ると、サンプルＣ，Ｆ，ＩおよびＭ〜Ｒの比較から明らかなように、Ｍｏ，Ｖ，Ｗ，Ｎｂ，Ｔｉといった炭化物生成元素添加により析出強化が行われ、Ｎｉ，Ｃｏ，Ｃｕといったオーステナイト生成元素添加により靭性向上が図られていることがわかる。
【００４４】
（試験例２）
次に、サンプルＡ，Ｃ，Ｇ，Ｉを用いて腐食疲労試験を行った。図３は腐食疲労試験の概略説明図である。この図に示すように、まず塩水噴霧を行い、続いて回転曲げ疲労試験を行って、さらに恒温恒湿槽放置を行う。これら一連の試験を１日当たり１サイクル行い、折損するまで繰り返した。いずれも試験前にばね加工後のひずみ取りテンパーを想定して、４００℃×３０分のテンパー処理を行っている。
【００４５】
【表３】

【００４６】
表３からわかるように、粒内析出炭化物形状を制御することで、耐腐食疲労性も大きく向上することが確認できた。さらにＮｉのような耐腐食疲労性向上に寄与するといわれる元素を添加するとき、粒内析出炭化物形状を制御することで、相乗効果を持つことが確認できる。これは金属組織内の炭化物が球状化粗大化させないことで、粒界への余分な炭化物析出を制御した結果、高い耐腐食性を示したものと考えられる。
【００４７】
【発明の効果】
以上説明したように、本発明のばね用鋼線およびばねは、母相マルテンサイトの結晶粒強化を粒内析出炭化物の形状制御を行うことによって高い耐疲労性、耐腐食疲労性を得ることが可能である。更にＭｏ，Ｖ，Ｗ，Ｎｂ，Ｔｉといった炭化物生成元素添加による析出強化を行い、Ｎｉ，Ｃｏ，Ｃｕといったオーステナイト生成元素添加による靭性向上や耐食性向上を行うことで、従来鋼線では得られない高い靭性と耐食性を得ることができる。本発明の鋼線を用いることで、弁ばね、縣架ばねなどに要求される高疲労強度ばね、もしくは耐腐食疲労ばねを得ることができる。
【図面の簡単な説明】
【図１】実施例であるサンプルＡのＴＥＭ写真である。
【図２】比較例であるサンプルＧのＴＥＭ写真である。
【図３】腐食試験方法の概略説明図である。[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a spring steel wire having a tempered martensite structure used for compression, tension coil springs, wire springs and the like, which are mainly used for household appliances and automobile parts, and a method for producing the same. Is.
[0002]
[Prior art]
A high-strength oil tempered wire mainly composed of Si—Cr steel has been used as a spring component material used in an exhaust system of an automobile engine. In recent years, in order to meet the demand for lower fuel consumption and higher efficiency of engines in response to the growing global environmental problems, weight savings and space savings of valve train mechanisms and rack spring units have been performed. As a result, the spring tends to be downsized, that is, the strength of the spring steel wire is increased. Oil tempered wire has high fatigue resistance and is excellent as a steel wire for springs. However, it is necessary to further improve fatigue resistance and corrosion fatigue resistance.
[0003]
Therefore, as a method for improving fatigue resistance, for example, as in JP-A-5-320826 and JP-A-5-331597, by adding carbide-generating elements such as V, Nb, and W, quenching heating can be performed. Crystal grain coarsening suppression by carbide precipitation is performed. However, these carbides precipitate at the grain boundaries of the prior austenite grains, reducing the amount of carbides precipitated in the martensite grains during tempering and lowering the strength of the crystals. I can't. Further, the presence at the grain boundaries adversely affects the corrosion fatigue resistance.
[0004]
Similarly, in Japanese Examined Patent Publication No. 9-6981, by specifying the amount of added V and quenching conditions, the crystal grain size (JIS) is 10 or more (the crystal grain size is 12 μm on average), thereby improving fatigue resistance. However, a dramatic improvement in strength and toughness cannot be expected simply by reducing the crystal grain size. “Crystal grain size (n)” is defined as “crystal grain size (d = unit μm) according to the following formula that b crystal grains (old austenite grains in the present invention) exist within 1 mm ² . ) ”Has the following relationship.
[0005]
[Expression 1]

[0006]
Furthermore, in Japanese Patent Laid-Open No. 9-71843, the volume fraction of the retained austenite phase in the metal structure is reduced, and the in-solution density of the insoluble carbide during the quenching is uniformly reduced, thereby realizing a toughness reduction suppressing effect. However, it does not reinforce the matrix of the material itself and has little effect on improving fatigue resistance.
[0007]
[Problems to be solved by the invention]
However, all of the above conventional techniques are intended to improve the intended fatigue resistance by refining the crystal grains and reducing the residual austenite phase and insoluble carbides scattered in the structure. It is not an attempt to strengthen the crystal grains of the site.
[0008]
Accordingly, a main object of the present invention is to provide a spring steel wire, a manufacturing method thereof, and a spring, which can obtain high fatigue resistance and corrosion fatigue resistance by strengthening the crystal grains of matrix martensite. is there.
[0009]
[Means for Solving the Problems]
The present invention achieves the above object by specifying the shape of carbide in the martensite crystal in a predetermined needle shape.
[0010]
That is, the steel wire for springs of the present invention has, as chemical components, C: 0.4 to 1.0, Si: 0.1 to 2.5, Mn: 0.2 to 1.2, Cr: 0 as chemical components. 0.5 to 1.2, with the balance being composed of Fe and inevitable impurities, having a tempered martensite structure obtained mainly by quenching and tempering, and the carbide shape in the martensite crystal has an average aspect ratio It is characterized by being 3.0 or more.
[0011]
Here, it is preferable that the major axis of the carbide in the martensite crystal is 0.1 μm or more.
[0012]
Further, as chemical components, Mo: 0.05 to 0.50, V: 0.05 to 0.50, W: 0.05 to 0.15, Nb: 0.05 to 0.15, Ti: It is preferable to contain one or more of 0.01 to 0.20. As other chemical components, it is also desirable to contain Ni: 0.02 to 1.00, Co: 0.02 to 1.00, Cu: 0.02 to 1.00 by mass%.
[0013]
Moreover, it is preferable that the average crystal grain diameter of the austenite crystal grain (old austenite crystal grain) after quenching of the steel wire cross section is 1.0 to 18.0 μm. A more preferable range of the average crystal grain size is 1.0 to 7.0 μm. This crystal grain size is the grain size after quenching, but remains substantially the same grain size after tempering.
[0014]
The tensile strength of the steel wire is preferably 1300 MPa or more and 2800 MPa or less.
[0015]
Furthermore, the spring of the present invention is manufactured using the above steel wire.
[0016]
On the other hand, in the method for producing a spring steel wire according to the present invention, heating at the time of quenching and tempering is performed at a temperature rising rate of 50 to 2000 ° C./s (° C./second) and a holding time of 0.5 to 30 s (seconds). It is characterized by performing.
[0017]
The intra-crystal grain carbide that precipitates in the martensite at the time of tempering is very hard and therefore effective for strengthening the crystal grain. However, when this is spherical or granular, if compressive stress, tensile stress, or shear stress is repeatedly applied, cracks are generated at the carbide / matrix boundary due to the difference in elastic limit and Young's modulus, causing fracture. In addition, slip bands caused by fatigue are easily generated in the crystal grains, and as a result, the concentration of the slip bands is likely to occur, which is likely to be the starting point of fatigue failure.
[0018]
Therefore, in the present invention, the carbides in the crystal grains are precipitated in the martensite as needles as described above, so that the carbides play the role of reinforcing fibers, and the crystal grains are very strong and tough. And improved fatigue resistance.
[0019]
The reasons for selecting the constituent elements and limiting the component ranges in the present invention and the reasons for specifying the production conditions are described below.
[0020]
(C: 0.4-1.0 mass%)
C is an important element that determines the mechanical properties of steel. However, if it is less than 0.4%, sufficient strength cannot be obtained. Since the sensitivity increases and the reliability decreases, the C content is set to 0.4 to 1.0%.
[0021]
(Si: 0.1-2.5% by mass)
Si is used as a deoxidizer during melting and refining. It also has the effect of strengthening by dissolving in ferrite. However, excessive addition leads to lack of toughness, tends to cause deterioration of hot workability, promotion of decarburization by heat treatment, and breakage during spring processing. % Or more and 2.5% or less to prevent lack of toughness.
[0022]
(Mn: 0.2-1.2% by mass)
Mn, like Si, is used as a deoxidizer during melting and refining, improving the hardenability of steel, fixing S in the steel and preventing its damage. However, Mn is also an element that easily causes the center segregation of the wire, and martensite is generated at the center segregation site during the patenting process after hot rolling, which significantly increases the disconnection rate during the drawing process. Therefore, the lower limit of deoxidizing action is 0.2% or more, and the upper limit is 1.2% as the range not causing toughness deterioration.
[0023]
(Cr: 0.5-1.2% by mass)
Cr, like Mn, improves the hardenability of steel and imparts toughness by patenting after hot rolling, and is an element effective for increasing strength by increasing softening resistance during tempering after quenching. If the content is less than 0.5%, the effect is small. On the other hand, if the content exceeds 1.2%, solid solution of carbides is suppressed, the strength is reduced, and the hardenability is excessively increased to reduce the toughness. To bring.
[0024]
(Mo: 0.05 to 0.50 mass%)
Mo is an element that forms carbides during tempering and increases the softening resistance. However, if it is less than 0.05%, its effect is small, and if it exceeds 0.50%, wire drawing workability is lowered. : 0.05 to 0.50%.
[0025]
(W, Nb: 0.05 to 0.15 mass%, V: 0.05 to 0.50 mass%)
W, Nb and V also have the effect of forming carbides in the steel during tempering and increasing the softening resistance. However, in any case, if it is less than 0.05%, the effect cannot be exhibited. On the contrary, when V exceeds 0.50% and W and Nb exceed 0.15%, a large amount of carbides are formed during quenching heating, resulting in a decrease in toughness. .50%, W: 0.05 to 0.15%, Nb: 0.05 to 0.15%.
[0026]
(Ti: 0.01-0.20 mass%)
Ti also has the effect of forming carbides in the steel during tempering and increasing softening resistance. However, Ti produces | generates TiO which is a high melting point nonmetallic inclusion. Therefore, setting conditions for refining is important. The amount that can be expected to improve the softening resistance is 0.01% or more, and is 0.20% or less in consideration of toughness deterioration due to excessive increase of carbides and inclusions.
(Ni, Co, Cu: 0.02 to 1.00% by mass)
Ni, Co, and Cu are austenite-generating elements, and are materials that greatly reduce the Ms point by adding Ni, Co, and Cu and easily cause retained austenite. The increase in retained austenite has the effect of lowering the hardness of the steel wire, but conversely imparts toughness to the steel wire strengthened with solid solution strengthening by Si or carbide precipitation elements such as Mo, W, Nb, V, Ti. Has an effect. Ni also has a role of preventing the ingress of Cl element in a salt water corrosive environment. The minimum degree with the effect of improving toughness is 0.02%, and the upper limit that does not cause a decrease in hardness is 1.00%.
[0028]
(Aspect ratio: 3 or more, major axis of carbide: 0.1 μm or more)
This is because by forming an elongated needle-like carbide having an aspect ratio of 3 or more, the carbide serves as a reinforcing fiber and can be made a very strong and tough material. Further, if the major axis of the carbide is 0.1 μm or more, it is easy to effectively play the role of reinforcing fiber.
[0029]
(Old austenite grain size: 1.0-18.0 μm)
The steel wire having the intragranular carbide is further excellent in fatigue resistance when the prior austenite crystal grain size is 1.0 to 18.0 μm. This is because a material having both high strength and high toughness can be obtained for the first time by strengthening crystal grains and refining the metal structure. When the crystal grain size is 18.0 μm or less, the effect of miniaturization appears, but when the crystal grain size is further 7.0 μm or less, the effect of strengthening by the refinement is remarkable. However, when the crystal grain size is less than 1.0 μm, it is very difficult to remove undissolved carbide by heat treatment, so the lower limit was made 1.0 μm or less. If it exceeds 18.0 μm, there is a problem that the fatigue limit is low and the toughness tends to decrease.
[0030]
(Tensile strength: 1300 MPa to 2800 MPa)
When the tensile strength is 1300 MPa or more and 2800 MPa or less, the steel wire for a spring exhibits particularly excellent performance. This value was set to 1300 MPa or more as the minimum required tensile strength when coiling as a spring, and 2800 MPa or less to give toughness that does not break during coiling.
[0031]
(Production conditions)
In order to obtain the steel wire according to the present invention having the above-mentioned acicular intragranular carbide, quenching and tempering heating for an extremely short time is effective, and tempering heating for a short time is particularly effective. Prolonged heating causes spheroidization and coarsening of intragranular carbides. This manufacturing condition is also necessary for realizing a predetermined prior austenite crystal grain size. Therefore, although depending on the size of the steel wire, when the heating rate during quenching and tempering is 50 to 2000 ° C./s and the holding time is 0.5 to 30 s, the intragranular carbide is effectively acicularized. Can be miniaturized. A preferable heating temperature for quenching is about 800 to 1150 ° C, and a preferable heating temperature for tempering is about 250 to 550 ° C. Furthermore, an oxide scale can be reduced by making the temperature increase rate at the time of quenching 50-2000 degrees C / s, and high fatigue strength can be acquired in connection with it.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
(Test Example 1) Samples A, B, C, D, E, F, L, M, N, O, P, Q, R, which are examples shown in Table 1, and samples G, H, which are comparative examples Steels I, J, and K were melted in a vacuum melting furnace, and a wire with a diameter of 6.5 mm was produced by hot forging and hot rolling. This wire was processed to a diameter of 4.0 mm by heat treatment, peeling, and cold drawing. Further, they were quenched at a quenching heating temperature of 1000 ° C. and tempered at 450 ° C. to obtain oil tempered wires. Table 1 shows the chemical composition, the intragranular carbide average aspect ratio, the old γ grain size, and the tensile strength at room temperature of the sample obtained.
[0033]
As for the aspect ratio, a thin film is made from the above sample by electropolishing, further etched, and observed with TEM (Transmission Electron Microscope). From the TEM photograph, the aspect ratio of intragranular carbides precipitated in the martensite crystal grains is actually It was measured. The aspect ratio is determined by the major axis / minor axis of the intragranular carbide. For reference, a TEM observation photograph of sample A which is an example of the present invention is shown in FIG. 1, and a TEM observation photograph of sample G which is a comparative example is shown in FIG.
[0034]
The black portions in FIGS. 1 and 2 indicate the carbide. As can be seen from FIGS. 1 and 2, the major axis of the carbide in the martensite crystal grains of Sample A was 0.1 μm or more on average. On the other hand, the carbide in the martensite crystal grains of sample G was less than 0.1 μm on average and the shape was almost spherical. In addition, samples B to F which are other examples similarly have an average length of carbide in the martensite crystal grains of 0.1 μm or more on average, and the martensite crystal grains in other comparative examples H to K also average. It was less than 0.1 μm and the shape was almost spherical.
[0035]
The average aspect ratio of intragranular carbides precipitated in the martensite crystal grains is different between the examples and the comparative examples. In the examples, the average value is 5.0 to 7.0, whereas in the comparative example, the average value is 1.0 to 4.0. As a method of changing the aspect ratio, there is a method of controlling the heating rate and holding time of quenching and tempering heating. The holding time is the time from the start of quenching or tempering heating to the start of cooling.
[0036]
In the example, tempering was performed at a heating temperature of 450 ° C., a heating rate of 1000 ° C./s, and a holding time of 1.0 s. On the other hand, in the comparative example, tempering heating was performed at a heating temperature of 450 ° C., a heating rate of 30 ° C./s, and a holding time of 40 s.
[0037]
In order to investigate the effect of grain refinement, the average austenite grain size in the cross section of the steel wire is about 3 μm (sample E), about 5 μm (samples D, F, J), about 10 μm Four types (samples A, B, C, G, H, I, L to R) and about 20 μm (sample K) were obtained. The heating temperature, heating rate and holding time during quenching for each type are as follows.

[0038]
The amount of oxide scale was examined for the obtained wire. As a result, it was found that the oxide scale amount was reduced to 10 g / m ² or less in any of the examples. In addition, each sample had a tensile strength at room temperature of about 2000 MPa, and materials having almost the same strength were obtained. Therefore, assuming a strain-relieving temper after spring processing, a temper was performed at 400 ° C. for 30 minutes.
[0039]
As a result of observation of each sample with an optical microscope or SEM (Scanning Electron Microscope), it was confirmed that there was no decarburization on the surface of the wire, an oxide film, and undissolved carbide in the metal structure.
[0040]
[Table 1]

[0041]
Next, Table 2 shows the results of applying the above samples to a Nakamura rotary bending fatigue tester. In the test, a stress was applied to the sample with a constant strain, and an amplitude stress with no breakage was obtained at 1 × 10 ⁷ repetitions (n number = 8).
[0042]
[Table 2]

[0043]
It was found that the fatigue strength was improved in all examples as compared with the comparative example. In particular, considering the fatigue strength associated with the inclusion of chemical components other than C, Si, Mn, Cr, and V, as is apparent from the comparison of samples C, F, I and MR, Mo, V It can be seen that precipitation strengthening is performed by adding carbide-generating elements such as, W, Nb, and Ti, and toughness is improved by adding austenite-generating elements such as Ni, Co, and Cu.
[0044]
(Test Example 2)
Next, a corrosion fatigue test was performed using samples A, C, G, and I. FIG. 3 is a schematic explanatory diagram of the corrosion fatigue test. As shown in this figure, salt water spray is performed first, followed by a rotating bending fatigue test, and then left in a constant temperature and humidity chamber. A series of these tests was performed one cycle per day and repeated until breakage occurred. In all cases, a temper treatment at 400 ° C. for 30 minutes is performed assuming a strain-relieving temper after spring processing before the test.
[0045]
[Table 3]

[0046]
As can be seen from Table 3, it was confirmed that the corrosion fatigue resistance was greatly improved by controlling the intragranular precipitated carbide shape. Furthermore, when adding an element said to contribute to the improvement of corrosion fatigue resistance such as Ni, it can be confirmed that a synergistic effect is obtained by controlling the intragranular precipitated carbide shape. This is presumably because the carbides in the metal structure were not spheroidized and coarsened, and as a result of controlling excessive carbide precipitation at the grain boundaries, high corrosion resistance was exhibited.
[0047]
【The invention's effect】
As explained above, the spring steel wire and spring of the present invention can obtain high fatigue resistance and corrosion fatigue resistance by controlling the shape of the intragranular precipitated carbides and strengthening the crystal grains of the matrix martensite. Is possible. Furthermore, precipitation strengthening by adding carbide-generating elements such as Mo, V, W, Nb, and Ti, and improving toughness and corrosion resistance by adding austenite-generating elements such as Ni, Co, and Cu are not possible with conventional steel wires. Toughness and corrosion resistance can be obtained. By using the steel wire of the present invention, it is possible to obtain a high fatigue strength spring or a corrosion-resistant fatigue spring required for a valve spring, a bridge spring and the like.
[Brief description of the drawings]
FIG. 1 is a TEM photograph of sample A as an example.
FIG. 2 is a TEM photograph of Sample G, which is a comparative example.
FIG. 3 is a schematic explanatory diagram of a corrosion test method.

Claims (13)

As a chemical component, it contains C: 0.4 to 1.0, Si: 0.1 to 2.5, Mn: 0.2 to 1.2, Cr: 0.5 to 1.2 in mass%, and the balance has components composed of Fe and inevitable impurities,
It has a tempered martensite structure mainly obtained by quenching and tempering,
This tempering is heating at a heating rate of 50 to 2000 ° C./s and a holding time of 0.5 to 1.0 s .
A steel wire for a spring, wherein the carbide shape in the martensite crystal is 3.0 or more in average aspect ratio. Heating rate for the quenching 50 ~ 2000 ℃ / s Hold time 0.5 ~ 30s The spring steel wire according to claim 1, wherein the steel wire is used for heating. As a chemical component, a component containing C: 0.4 to 1.0, Si: 0.1 to 2.5, Mn: 0.2 to 1.2, Cr: 0.5 to 1.2, Co : 0.02 to 1.00 as a chemical component, with the balance being Fe and inevitable impurities Have
It has a tempered martensite structure mainly obtained by quenching and tempering,
A steel wire for springs, wherein the carbide shape in the martensite crystal is 3.0 or more in average aspect ratio. The steel wire for a spring according to any one of claims 1 to 3, wherein a major axis of the carbide in the martensite crystal is 0.1 µm or more. Furthermore, as a chemical component, it contains at least one selected from the group consisting of Mo: 0.05 to 0.50, V: 0.05 to 0.50, W: 0.05 to 0.15, Nb: 0.05 to 0.15 and Ti: 0.01 to 0.20 in terms of mass%. The steel wire for a spring according to any one of claims 1 to 4, wherein The spring steel wire according to any one of claims 1 to 5 , further comprising at least one of Ni : 0.02 to 1.00 and Cu : 0.02 to 1.00 as a chemical component in terms of mass%. The spring steel wire according to any one of claims 1 to 6 , wherein the average grain size of the austenite crystal grains (old austenite crystal grains) after quenching of the steel wire cross section is 1.0 to 18.0 µm. 8. The spring steel wire according to claim 7 , wherein the average grain size of the austenite crystal grains (old austenite crystal grains) after quenching of the steel wire cross section is 1.0 to 7.0 μm. The steel wire for a spring according to any one of claims 1 to 8, wherein a tensile strength is 1300 MPa or more and 2800 MPa or less. As a chemical component, it contains C : 0.4 to 1.0 , Si : 0.1 to 2.5 , Mn : 0.2 to 1.2 , Cr : 0.5 to 1.2 in mass%, and the remainder is quenched and tempered to a component consisting of Fe and inevitable impurities. A method of manufacturing a spring steel wire, comprising:
Method for producing a spring steel wire which is characterized in that the heating during the tempering heating rate 50 ~ 2000 ℃ / s, the holding time at 0.5 ~ 1.0 s. Heating rate during the quenching 50 ~ 2000 ℃ / s Hold time 0.5 ~ 30s Claims performed in Ten The manufacturing method of the steel wire for springs of description. As a chemical component, Co : 0.02 ~ 1.00 Claims containing mass% Ten Or 11 The manufacturing method of the steel wire for springs of description. A spring produced using a steel wire corresponding to any one of claims 1 to 9 .

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