JP2004263247A

JP2004263247A - Steel for cold-formed spring

Info

Publication number: JP2004263247A
Application number: JP2003054696A
Authority: JP
Inventors: Kazuyoshi Kimura; 和良木村
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 2003-02-28
Filing date: 2003-02-28
Publication date: 2004-09-24
Anticipated expiration: 2023-02-28
Also published as: JP4044460B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for a cold-formed spring in which high strength and toughness and the characteristic of corrosion-resistant fatigue strength are satisfied, and cold formability and quench crack resistance are enhanced. <P>SOLUTION: The steel for the cold-formed spring consists mainly of Fe, and contains, by mass, ≥ 0.36% and ≤ 0.46% C, ≥ 1.8% and ≤ 2.6% Si, ≥ 0.2% and ≤ 1.2% Mn, ≤ 0.015% P, ≤ 0.01% S, ≥ 0.1% and ≤ 0.5% Cu, ≥ 0.1% and ≤ 1.5% Ni, ≥ 0.05% and ≤ 1% Cr, and ≥ 0.002% and ≤ 0.012% N. In the steel for the cold-formed spring, WC + WMn + WCr is ≥ 1.2% and ≤ 2%, WSi/3 + WCr/2 + WMn is ≥ 1.4% and ≤ 2.1%, and WCu + WNi is ≥ 0.4%, where WC is C content, WMn is Mn content, WCr is Cr content, WSi is Si content, WNi is Ni content. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明が属する技術分野】
本発明は、冷間成形法によってコイルばねに成形するのに適したばね用鋼に関する。
【０００２】
【従来の技術】
【特許文献１】
特許第３０６４６７２号公報
【０００３】
近年、地球温暖化の観点から二酸化炭素ガスの排出量削減が強く求められている。そのため、自動車等の車両には、燃費向上に繋がる車体の軽量化が重要となっており、懸架用コイルばねにおいても一層の軽量化が図られている。コイルばねの軽量化を達成するためには、使用するばね素線の直径を細くするか、あるいは使用するばね素線の長さを短くする必要がある。所要のばね特性を維持しつつコイルばねの軽量化を達成するには、引張強度で１９００ＭＰａ以上（ロックウェル硬さＨＲＣでは５２以上に相当）の高い材料強度が求められている。
【０００４】
ところで、懸架用コイルばねの製造方法には、大別して熱間成形法と冷間成形法の二種類がある。前者は、鋼線を熱間加工によってコイル形状に成形した後、焼入れ焼戻しを行って所要のばね強度に調整するため、成形が比較的容易である。一方、後者は、あらかじめ所要のばね強度に調整された鋼線を用い、冷間加工によってコイル形状に成形するため、ばねの製造における設備面・工程面での簡素化効果が高く経済的な方法である。
【０００５】
【発明が解決しようとする課題】
冷間加工性ばね用鋼には、焼入れ焼戻し後において上記のような高強度が求められると同時に、冷間加工に耐え得る強靭性も必要とされる。そのような鋼材を得るためには、高強度の観点からすればＣ含有量を高めることが考えられるが、その場合は強靭性の付与が困難となるため、逆にＣ含有量を低減させることにより靭性を確保しつつ、強度不足を補うための合金元素を添加させることになる。また、強靭化対策としてＶ、Ｎｂ、Ｔｉ、Ａｌ等の合金元素を添加し、微細炭窒化物による結晶粒微細化効果を利用することも有効である。しかし、合金元素を多量に添加させると熱間圧延後の強度（硬さ）が過度なものとなり、その後の引抜きや伸線加工、コイリング等の冷間加工の際に割れやカッピー状の断線が発生したり、または焼入れの際に割れが生じてしまうといった問題があった。そのため、熱間圧延後に焼鈍等の軟化処理を行い、加工性を改善したりする方法も取られるが、この場合は製造コストが増大するといった問題が生じてしまう。
【０００６】
さらに、上記のような鋼材は、Ｖ系炭化物が鋼材表面で局部電極となり腐食ピットを発生させるため、腐食疲労強度に劣るものとなってしまうという問題もあった。
【０００７】
したがって、本発明の課題は、高強度及び強靭性、耐腐食疲労強度特性を満足させ、且つ、冷間加工性及び耐焼割れ性を改善させた、冷間成形ばね用鋼を提供することにある。
【０００８】
【課題を解決するための手段及び作用・発明の効果】
上記課題を解決するため、本発明の冷間成形ばね用鋼では、
主成分としてのＦｅと、
０．３６質量％以上０．４６質量％以下のＣと、
１．８質量％以上２．６質量％以下のＳｉと、
０．２質量％以上１．２質量％以下のＭｎと、
０．０１５質量％以下のＰと、
０．０１質量％以下のＳと、
０．１質量％以上０．５質量％以下のＣｕと、
０．１質量％以上１．５質量％以下のＮｉと、
０．０５質量％以上１質量％以下のＣｒと、
０．００２質量％以上０．０１２質量％以下のＮと、
を含有し、且つ、Ｃの含有量をＷＣ、Ｍｎの含有量をＷＭｎ、Ｃｒの含有量をＷＣｒ、Ｓｉの含有量をＷＳｉ、Ｎｉの含有量をＷＮｉとしたとき、
ＷＣ＋ＷＭｎ＋ＷＣｒが１．２質量％以上２質量％以下、
及びＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎが１．４質量％以上２．１質量％以下、
及びＷＣｕ＋ＷＮｉが０．４質量％以上であることを特徴とする。
なお、本明細書にて「主成分」（「主体に」等も同様）とは、着目している材料あるいは組織において、最も質量含有率の高い成分（相も概念として含む）を意味する。
【０００９】
上記本発明の冷間成形ばね用鋼は、鋼材の化学組成分を特定することによって、高強度及び強靭性、耐腐食疲労強度特性を満足させ、且つ、冷間加工性及び耐焼割れ性を具備するとともに、特に懸架用コイルばねを製造するに適した冷間成形用ばね鋼を得ることに成功したものである。
【００１０】
以下、本発明における各数値範囲の限定理由を説明する。
本発明の冷間成形ばね用鋼の組成限定理由は以下の通りである。
Ｆｅ（鉄）：主成分
Ｆｅは、鋼を構成するのに必須の成分であるため、本発明の冷間成形ばね用鋼の主成分として含有させる。
【００１１】
Ｃ（炭素）：０．３６質量％以上０．４６質量％以下
Ｃは、焼入れ焼戻し後の鋼の強度を高めるために必須の元素である。所望のばね強度を得るには０．３６質量％以上の含有が必要であるが、過度に含有させると、焼入れ焼戻し後の靭性が低下し疲労強度及び腐食疲労強度を劣化させるばかりでなく、圧延後の硬さが硬くなり過ぎ冷間加工性を低下させてしまうので、上限を０．４６質量％とする。
【００１２】
Ｓｉ（ケイ素）：１．８質量％以上２．６質量％以下
Ｓｉは、固溶強化元素として鋼の強度及び耐へたり性を向上させるのに有効な元素である。この効果を得るには１．８質量％以上の含有が必要であるが、過度に含有させると靭性が低下し疲労特性を劣化させるばかりでなく、製造工程中の高温加熱時に鋼表面に脱炭を生じ、加工性をも悪化させてしまうので、上限を２．６質量％とする。
【００１３】
Ｍｎ（マンガン）：０．２質量％以上１．２質量％以下
Ｍｎは、鋼溶製時の脱酸材として有効であると共に、焼入れ性の向上に寄与する元素である。また、Ｓによる害を無害化する効果も有する。これらの効果を得るには０．２質量％以上の含有が必要であるが、過度に含有させると焼入れ時に粒界酸化を助長し脆化を招くだけでなく、圧延後の線材の硬さを過度に高め冷間加工性をも劣化させてしまうので、上限を１．２質量％とする。
【００１４】
Ｐ（リン）：０．０１５質量％以下
Ｐは、オーステナイト化加熱時にオーステナイト粒界に偏析して、結晶粒界を脆弱化させてしまうので、低減することが望ましい元素である。経済性を考慮して含有量の上限を０．０１５質量％以下とする。
【００１５】
Ｓ（硫黄）：０．０１質量％以下
Ｓは、Ｐと同様にオーステナイト粒界を脆化させ、またＭｎＳを形成してばねの疲労強度の劣化を招いてしまうので、低減することが望ましい元素である。経済性を考慮して含有量の上限を０．０１質量％以下とする。
【００１６】
Ｃｕ（銅）：０．１質量％以上０．５質量％以下
Ｃｕは、鋼の耐腐食性を高めるのに有効な元素で、腐食疲労強度を向上させる効果がある。また、フェライト脱炭の防止にも有効である。これらの効果を得るためには０．１質量％以上の含有が必要であるが、過度に含有させると熱間加工性が損なわれてしまうので、上限を０．５質量％とする。
【００１７】
Ｎｉ（ニッケル）：０．１質量％以上１．５質量％以下
Ｎｉは、鋼の耐腐食性を高めるのに有効な元素で、腐食疲労強度を向上させる効果がある。また、フェライト脱炭の防止に有効である。これらの効果を得るためには０．１質量％以上の含有が必要であるが、過度に含有させると鋼のコストが増加してしまうので、上限を１．５質量％とする。
【００１８】
Ｃｒ（クロム）：０．０５質量％以上１質量％以下
Ｃｒは、焼入れ性の向上に寄与する元素である。この効果を得るためには０．０５質量％以上の含有が必要であるが、過度に含有させるとＭｎと同様に圧延後の線材の硬さを過度に高め、冷間加工性を劣化させてしまうので、上限を１質量％とする。
【００１９】
Ｎ（窒素）：０．００２質量％以上０．０１２質量％以下
Ｎは、鋼中で結晶粒微細化に寄与する炭窒化物及び窒化物を形成する効果がある。この効果を得るためには０．００２質量％以上の含有が必要であるが、過剰に含有させると、粗大なＮｂ炭窒化物が生成して粒界ピン止め効果が得られず、またＴｉＮ系の介在物を生成し鋼の疲労強度を低下させるため、その上限を０．０１２質量％とする。
【００２０】
さらに、Ｃの含有量をＷＣ、Ｍｎの含有量をＷＭｎ、Ｃｒの含有量をＷＣｒ、Ｓｉの含有量をＷＳｉ、Ｎｉの含有量をＷＮｉとしたとき、本発明の冷間成形ばね用鋼は次の要件を満たす。
【００２１】
ＷＣ＋ＷＭｎ＋ＷＣｒ：１．２質量％以上２質量％以下
本発明者等が鋭意研究を重ねた結果、焼入れ硬化した状態で鋼が引張強度１９００ＭＰａ以上（ロックウェル硬さＨＲＣでは５２以上に相当）の高強度を確保するためには、Ｃ、Ｍｎ、Ｃｒのそれぞれの含有率が上記の本発明の特定する範囲にあるほか、さらにＷＣ＋ＷＭｎ＋ＷＣｒが１．２質量％以上必要であることが判った。しかし、その値が過度に大きくなると、圧延材の伸線加工時にカッピー状割れあるいは破断が生じてしまうため、上限を２質量％とする。
【００２２】
ＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎ：１．４質量％以上２．１質量％以下
また、焼入れ硬化した状態で鋼が引張強度１９００ＭＰａ以上（ロックウェル硬さＨＲＣでは５２以上に相当）の高強度を確保するためには、Ｓｉ、Ｃｒ、Ｍｎのそれぞれの含有率が上記の本発明の特定する範囲にあるほか、さらにＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎが１．４質量％以上必要であることが判った。しかし、その値が過度に大きくなると鋼の焼入れ性が過剰となり、鋼の焼入れ時に焼割れを生じてしまうことがあるため、上限を２．１質量％とする。
【００２３】
ＷＣｕ＋ＷＮｉ：０．４質量％以上
また、上記鋼が引張強度１９００ＭＰａ以上（ロックウェル硬さＨＲＣでは５２以上に相当）の高強度を有しつつ、優れた腐食疲労強度を確保するためには、Ｃｕ、Ｎｉのそれぞれの含有率が上記の本発明の特定する範囲にあるほか、さらにＷＣｕ＋ＷＮｉが０．４質量％以上必要であることが判った。
【００２４】
また、本発明の冷間成形ばね用鋼では、上記組成に加え、
０．０５質量％以上０．３質量％以下のＶ；
０．０２質量％以上０．０５質量％以下のＮｂ；
０．０２質量％以上０．０７質量％以下のＴｉ；
０．００５質量％以上０．０４質量％以下のＡｌ；
０．０００５質量％以上０．００３質量％以下のＢ；
のうちいずれか１種以上を含有し、且つ、
Ｃの含有量をＷＣ、Ｍｎの含有量をＷＭｎ、Ｃｒの含有量をＷＣｒ、Ｓｉの含有量をＷＳｉ、Ｎｉの含有量をＷＮｉ、Ｂの含有量をＷＢとしたとき、
ＷＣ＋ＷＭｎ＋ＷＣｒが１．２質量％以上２質量％以下、
及びＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎ＋３５３ＷＢが１．４質量％以上２．１質量％以下、
及びＷＣｕ＋ＷＮｉが０．４質量％以上、
とすることができる。
【００２５】
以下、各数値の限定範囲を説明する。なお、上述した限定範囲と同一であるものは省略する。
【００２６】
Ｖ（バナジウム）：０．０５質量％以上０．３質量％以下
Ｖは、鋼中で炭化物、炭窒化物を形成してオーステナイト結晶粒を微細化させたり、また析出硬化に寄与してばねの耐へたり性を向上させる効果がある。これらの効果を得るためには０．０５質量％以上の含有が必要である。Ｖの炭化物は鋼表面で局部電極となり腐食ピットを形成して亀裂破壊の起点となってしまうので、上限を０．３質量％とする。また、上限を超えて含有させると巨大な一次炭化物が晶出し、冷間加工性が劣化してしまう。
【００２７】
Ｎｂ（ニオブ）：０．０２質量％以上０．０５質量％以下
Ｎｂは、Ｖと同様に鋼中で炭化物、炭窒化物を形成してオーステナイト結晶粒を微細化させる効果がある。この効果を得るためには０．０２質量％以上の含有が必要である。しかし、過剰に含有してもその効果は飽和し、むしろ鋼の熱間加工性及び冷間加工性を低下させるので、上限を０．０５０％とする。
【００２８】
Ｔｉ（チタン）：０．０２質量％以上０．０７質量％以下
Ｔｉは、Ｎｂ、Ｖと同様に鋼中で炭化物、炭窒化物を形成してオーステナイト結晶粒を微細化させる効果がある。この効果を得るためには０．０２質量％以上の含有が必要であるが、過度に含有させると、鋼の焼入れ加熱時に未溶解化合物として残留し、そのサイズが比較的大きいため、破壊の起点となって疲労強度の低下をもたらすので、上限を０．０７質量％とする。
【００２９】
Ａｌ（アルミニウム）：０．００５質量％以上０．０４質量％以下
Ａｌは、鋼中で窒化物を形成してオーステナイト結晶粒の微細化に寄与するが、酸化物形成傾向の強い元素なので多量に含有すると鋼中の酸化物系介在物量を増し、鋼の清浄度を損なう。それゆえ、Ａｌ含有率の上限を０．０４０％とする。また、鋼中のＯ（酸素）含有率は２０ｐｐｍ以下とすることが好ましい。
【００３０】
Ｂ（ホウ素）：０．０００５質量％以上０．００３質量％以下
Ｂは、鋼の結晶粒界に優先的に偏析し、Ｐ、Ｓの結晶粒界偏析を防止して靭性を向上させる効果がある。また、線材の冷間加工性を向上させるためにＳｉ、Ｍｎ、Ｃｒなどの焼入れ性に寄与する合金元素を低減すると、素材の焼入れ性が不足し、不完全焼入れ組織を生じてしまうが、Ｂはこのような焼入れ性低下を補う効果もある。これらの効果を得るためには０．０００５質量％以上の含有が必要であるが、過剰に含有させるとＢ窒化物を形成して、鋼の靭性を損ない、また鋼の疲労特性も悪化させるので、上限を０．００３０質量％とする。
【００３１】
ＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎ＋３５３ＷＢ：１．４質量％以上２．１質量％以下本発明者等が鋭意研究を重ねた結果、焼入れ硬化した状態で鋼が引張強度１９００ＭＰａ以上（ロックウェル硬さＨＲＣでは５２以上に相当）の高強度を確保するためには、Ｓｉ、Ｃｒ、Ｍｎ、Ｂのそれぞれの含有率が上記の本発明の特定する範囲にあるほか、さらにＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎ＋３５３ＷＢが１．４質量％以上必要であることが判った。しかし、その値が過度に大きくなると鋼の焼入れ性が過剰となり、鋼の焼入れ時に焼割れを生じてしまうことがあるため、上限を２．１質量％とする。なお、Ｂが含有されていない場合は、上記のＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎが１．４質量％以上２．１質量％以下とされた範囲と同義になる。
【００３２】
また、本発明の冷間成形ばね用鋼では、上記組成に加え、０．０５質量％以上０．５質量％以下のＭｏを含有することができる。
Ｍｏは、焼入れ性の向上に寄与する効果がある。また、耐食性を高め、腐食疲労強度を向上させる効果もある。これらの効果を得るためには０．０５質量％以上の含有が必要であるが、過度に含有させると圧延後の線材にベーナイトが生成し、冷間加工性の劣化を招いてしまうので、上限を０．５質量％とする。
【００３３】
【実施例】
以下、本発明の実施例について詳細に説明する。
表１及び２に示す化学組成を有する鋼を溶製して得た鋼塊を分塊圧延し、さらに線材圧延によってφ１３ｍｍの圧延線材とした。線材圧延は鋼片を１１００℃に加熱し圧延終了温度８６９℃として行った。圧延終了後は空冷とした。なお、表中の各々の組成において、本発明で規定する組成範囲を逸脱しているものには、下限を下回る場合は下向矢印（↓）、上限を上回る場合は上向矢印（↑）を付している。
【００３４】
また、ＷＣ＋ＷＭｎ＋ＷＣｒの値、及びＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎ＋３５３ＷＢの値、及びＷＣｕ＋ＷＮｉの値は、表３及び４に示している。なお、表中にはＷＣ＋ＷＭｎ＋ＷＣｒを「Ｃ＋Ｍｎ＋Ｃｒ」と、ＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎ＋３５３ＷＢを「Ｓｉ／３＋Ｃｒ／２＋Ｍｎ＋３５３＊Ｂ」と、ＷＣｕ＋ＷＮｉを「Ｃｕ＋Ｎｉ」と表している。
【００３５】
【表１】

【００３６】
【表２】

【００３７】
前記圧延線材の切断面について硬さを測定した。ロックウェルＣスケール硬さを３０点測定し、平均硬さに６倍の標準偏差σ（バラツキ）を加えたものを「圧延後硬さ」として表３及び４に示す。該圧延後硬さはＨＲＣ３４を上限とする。また、前記圧延線材にボンダ皮膜処理を施し、冷間でφ１２ｍｍまで伸線加工し、伸線材を得た。該伸線加工の際における、破断発生の有無を「伸線時破断の有無」として表３及び４に示す。
【００３８】
前記伸線材を周波数３０ｋＨｚ、電流３００Ａの条件で高周波コイル中に通線させた後、直ちに水冷により焼入れを行い、焼入れ材を得た。該焼入れ材にについて割れの発生の有無を調べ、その結果を「焼割れ」として表３及び４に示す。
また、前記焼入れ材の断面芯部硬さを測定した。ロックウェルＣスケール硬さを２０点測定し、その平均値を「焼入れ後硬さ」として、所望の硬さ（ＨＲＣ５２）を満たしているかを評価した。その結果を表３及び４に示す。
【００３９】
前記焼入れ材を焼戻して、硬さＨＲＣ５２の焼戻し材を得た。該焼戻し材から採取した試験片に対して、塩水噴霧試験機を用いて、▲１▼３５℃で５％ＮａＣｌ水溶液を２ｈ噴霧し、▲２▼相対湿度７０％で６０℃の環境において４ｈ乾燥させ、▲３▼相対湿度９５％で３５℃の環境において２ｈ保持するサイクル▲１▼〜▲３▼を９回繰り返した後、応力振幅を７００ＭＰａとして両振りねじり疲労試験を行った。破断繰返し数を測定し、破断までの繰返し回数で腐食疲労特性を評価した。「腐食疲労強度」として表３及び４に示す。なお、破断繰返し数が１０００００回に達するか否かを良否の判断基準とした。
【００４０】
また、前記疲労試験後の腐食部の断面から、腐食ピットの深さを計４０点計測し、ピット深さの最大値（最大ピット深さ）を測定した。最大ピット深さが１００μｍ以上か否かを良否の判断基準とした。最大ピット深さの測定結果を「腐食ピット深さ」として表３及び４に示す。また、腐食ピット深さと、前記破断繰返し数との関係を図１に示す。
【００４１】
次に、表１及び２に示す化学組成を有する鋼を溶製して得た鋼塊を分塊圧延し、線材圧延によってφ１６ｍｍの圧延線材を得た。線材圧延は鋼片を１１００℃に加熱し圧延終了温度８６９℃として行った。そして圧延終了後、空冷し、更に焼入れ焼戻し処理を施して硬さＨＲＣ５２の焼戻し材を得た。その後、該焼戻し材から、ＪＩＳ３号２ｍｍＵノッチ試験片を作製し、シャルピー衝撃試験を実施した。そして、得られた値から、所望の靭性（シャルピー衝撃値で４０Ｊ／ｃｍ^２以上）を満たしているかを評価した。その結果を「シャルピー衝撃試験」として、表３及び４に示す。シャルピー衝撃値が４０Ｊ／ｃｍ^２を下回ると、腐食疲労強度が急激に劣化するため、４０Ｊ／ｃｍ^２以上か否かを良否の判断基準とする。
【００４２】
【表３】

【００４３】
【表４】

【００４４】
以下、表３及び４、並びに図１に示した評価結果について説明する。
本発明が特定する組成範囲を充足する本発明２４〜３３は、いずれの試験においても、良好な特性を示し、本発明とする腐食疲労強度に優れた高強度ばね鋼であることがわかった。
【００４５】
本発明が特定する範囲よりもＷＣ＋ＷＭｎ＋ＷＣｒ値が低い比較例１及び２は（比較例１はＣ含有量、また比較例２はＣｒ含有量及びＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎ＋３５３ＷＢ値についても不足）、焼入れ後の硬さがＨＲＣ５２を下回っており、強度及び焼入れ性が不足したものであった。
【００４６】
本発明が特定する範囲よりもＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎ＋３５３ＷＢ値が不足している比較例３〜５は（比較例３はＳｉ含有量、比較例４はＭｎ含有量についても不足）、比較例１及び２の場合と同様に、焼入れ後の硬さがＨＲＣ５２を下回っており、強度及び焼入れ性が不足したものであった。
【００４７】
本発明が特定する範囲よりもＷＳｉ／３＋ＷＣｒ／２＋ＷＭｎ＋３５３ＷＢ値が過剰な比較例６〜９は（比較例６はＳｉ含有量、比較例７はＭｎ含有量、比較例Ｃｒ含有量、比較例９はＢ含有量についても過剰）、焼入れ時に焼割れが生じた。
【００４８】
本発明が特定する範囲よりもＰ含有量が過剰な比較例１０、１２、１４、またＳ含有量が過剰な比較例１１、１３は、シャルピー衝撃試験値が４０Ｊ／ｃｍ^２を下回っており、靭性が不足したものであった。また、これらは腐食疲労強度も不足しており、中でも比較例１０、１１、１３では腐食ピット深さが１００μｍを超えていた。
【００４９】
比較例１６〜２３に関しては（比較例１６はＣ、比較例１７はＳｉ、比較例１８はＣｒ、比較例１９はＶ、比較例２０はＢの含有量が過剰なものであり、また比較例２１はＣｕ、比較例２２はＮｉの含有量が不足しており、さらに比較例２３はＷＣｕ＋ＷＮｉが不足している）、いずれも腐食疲労強度が不足しており、また腐食ピット深さも１００μｍを超えたものであった。
【００５０】
以上、本発明２４〜３３によって示すごとく、本発明が特定する化学組成を有する鋼は、圧延材の伸線加工性に優れ、焼入れにおいても割れを生じることがなく、製造性に優れている。また、焼入れ材の強度もＨＲＣ５２以上、すなわち引張強度１９００ＭＰａ以上の高強度を保持しつつ、靭性及び腐食疲労強度に優れる等、ばね鋼として所用の諸特性も有する。
【図面の簡単な説明】
【図１】腐食ピット深さと、破断繰返し回数との関係を表す図[0001]
[Technical field to which the invention belongs]
The present invention relates to a spring steel suitable for forming into a coil spring by a cold forming method.
[0002]
[Prior art]
[Patent Document 1]
Japanese Patent No. 3064672
In recent years, reduction of carbon dioxide gas emissions has been strongly demanded from the viewpoint of global warming. For this reason, for vehicles such as automobiles, it is important to reduce the weight of the vehicle body, which leads to improved fuel efficiency, and the coil springs for suspension are further reduced in weight. In order to reduce the weight of the coil spring, it is necessary to reduce the diameter of the spring wire to be used or to shorten the length of the spring wire to be used. In order to reduce the weight of the coil spring while maintaining the required spring characteristics, a high material strength of 1900 MPa or higher (corresponding to 52 or more in Rockwell hardness HRC) is required.
[0004]
By the way, the manufacturing method of the coil spring for suspension is roughly classified into two types, a hot forming method and a cold forming method. The former is relatively easy to form because the steel wire is formed into a coil shape by hot working and then subjected to quenching and tempering to adjust to the required spring strength. On the other hand, the latter uses a steel wire that has been adjusted to the required spring strength in advance and is formed into a coil shape by cold working. It is.
[0005]
[Problems to be solved by the invention]
Cold workable spring steel is required to have such high strength after quenching and tempering, and at the same time, toughness that can withstand cold working. In order to obtain such a steel material, it is conceivable to increase the C content from the viewpoint of high strength, but in that case, it becomes difficult to impart toughness, so conversely, the C content should be reduced. Therefore, an alloying element is added to make up for the lack of strength while securing toughness. It is also effective to add alloying elements such as V, Nb, Ti, Al, etc. as a countermeasure against toughening and utilize the effect of refining crystal grains by fine carbonitride. However, if a large amount of alloying element is added, the strength (hardness) after hot rolling becomes excessive, and cracks and cappy breaks occur during cold processing such as drawing, wire drawing, and coiling. There is a problem that it occurs or cracks occur during quenching. For this reason, a method of improving the workability by performing a softening process such as annealing after hot rolling can be taken, but in this case, there arises a problem that the manufacturing cost increases.
[0006]
Furthermore, the steel materials as described above also have a problem that the corrosion fatigue strength is inferior because the V-based carbide becomes a local electrode on the steel material surface and generates corrosion pits.
[0007]
Accordingly, an object of the present invention is to provide a steel for cold-formed springs that satisfies high strength, toughness and corrosion fatigue strength characteristics, and has improved cold workability and fire cracking resistance. .
[0008]
[Means for solving the problems and functions / effects of the invention]
In order to solve the above problems, in the steel for cold forming spring of the present invention,
Fe as a main component;
0.36 mass% or more and 0.46 mass% or less of C,
1.8% by mass or more and 2.6% by mass or less of Si;
0.2% by mass or more and 1.2% by mass or less of Mn,
0.015 mass% or less P,
0.01 mass% or less S,
0.1 mass% or more and 0.5 mass% or less of Cu,
0.1 mass% or more and 1.5 mass% or less of Ni;
0.05 mass% or more and 1 mass% or less of Cr,
0.002% by mass or more and 0.012% by mass or less N;
And the content of C is WC, the content of Mn is WMn, the content of Cr is WCr, the content of Si is WSi, and the content of Ni is WNi,
WC + WMn + WCr is 1.2 mass% or more and 2 mass% or less,
And WSi / 3 + WCr / 2 + WMn is not less than 1.4% by mass and not more than 2.1% by mass,
And WCu + WNi is 0.4 mass% or more.
In this specification, “main component” (same as “mainly” and the like) means a component (including phase as a concept) having the highest mass content in the material or structure of interest.
[0009]
The cold formed spring steel of the present invention satisfies the high strength and toughness and corrosion fatigue strength characteristics by specifying the chemical composition of the steel material, and has cold workability and fire cracking resistance. In addition, the present inventors have succeeded in obtaining a cold forming spring steel particularly suitable for manufacturing a suspension coil spring.
[0010]
Hereinafter, the reasons for limiting each numerical range in the present invention will be described.
The reasons for limiting the composition of the steel for cold forming springs of the present invention are as follows.
Fe (iron): The main component Fe is an essential component for constituting the steel, and is thus contained as the main component of the steel for cold forming spring of the present invention.
[0011]
C (carbon): 0.36 mass% or more and 0.46 mass% or less C is an essential element for increasing the strength of the steel after quenching and tempering. In order to obtain a desired spring strength, the content of 0.36% by mass or more is necessary. However, if excessively contained, not only the toughness after quenching and tempering is lowered, but the fatigue strength and corrosion fatigue strength are deteriorated. Since the later hardness becomes too hard and cold workability is lowered, the upper limit is made 0.46% by mass.
[0012]
Si (silicon): 1.8% by mass or more and 2.6% by mass or less Si is an element effective as a solid solution strengthening element for improving the strength and sag resistance of steel. In order to obtain this effect, it is necessary to contain 1.8% by mass or more. However, if it is excessively contained, not only the toughness is lowered and the fatigue characteristics are deteriorated, but also the steel surface is decarburized during high-temperature heating during the production process. And the workability is also deteriorated, so the upper limit is made 2.6 mass%.
[0013]
Mn (manganese): 0.2% by mass or more and 1.2% by mass or less Mn is an element that is effective as a deoxidizer during steel melting and contributes to improvement of hardenability. It also has the effect of detoxifying the harm caused by S. In order to obtain these effects, a content of 0.2% by mass or more is necessary. However, if excessively contained, not only promotes grain boundary oxidation during quenching and causes embrittlement, but also increases the hardness of the wire after rolling. The upper limit is set to 1.2% by mass because excessively high cold workability is deteriorated.
[0014]
P (phosphorus): 0.015 mass% or less P is an element that is desirably reduced because it segregates at the austenite grain boundaries and weakens the grain boundaries during austenitizing heating. In consideration of economy, the upper limit of the content is set to 0.015% by mass or less.
[0015]
S (sulfur): 0.01% by mass or less S is an element that is desirable to be reduced because it causes embrittlement of austenite grain boundaries as well as P, and also causes formation of MnS, resulting in deterioration of the fatigue strength of the spring. It is. In consideration of economy, the upper limit of the content is set to 0.01% by mass or less.
[0016]
Cu (copper): 0.1 mass% or more and 0.5 mass% or less Cu is an element effective for enhancing the corrosion resistance of steel, and has an effect of improving the corrosion fatigue strength. It is also effective in preventing ferrite decarburization. In order to obtain these effects, it is necessary to contain 0.1% by mass or more. However, if excessively contained, hot workability is impaired, so the upper limit is made 0.5% by mass.
[0017]
Ni (nickel): 0.1% by mass or more and 1.5% by mass or less Ni is an element effective for enhancing the corrosion resistance of steel, and has an effect of improving corrosion fatigue strength. It is also effective in preventing ferrite decarburization. In order to obtain these effects, the content of 0.1% by mass or more is necessary. However, if excessively contained, the cost of steel increases, so the upper limit is made 1.5% by mass.
[0018]
Cr (chromium): 0.05 mass% or more and 1 mass% or less Cr is an element contributing to improvement of hardenability. In order to obtain this effect, it is necessary to contain 0.05% by mass or more. However, if excessively contained, the hardness of the wire rod after rolling is excessively increased in the same manner as Mn, and the cold workability is deteriorated. Therefore, the upper limit is 1% by mass.
[0019]
N (nitrogen): 0.002 mass% or more and 0.012 mass% or less N has an effect of forming carbonitrides and nitrides that contribute to grain refinement in steel. In order to obtain this effect, it is necessary to contain 0.002% by mass or more. However, if it is excessively contained, coarse Nb carbonitrides are generated and the grain boundary pinning effect cannot be obtained. In order to reduce the fatigue strength of the steel, the upper limit is made 0.012% by mass.
[0020]
Furthermore, when the C content is WC, the Mn content is WMn, the Cr content is WCr, the Si content is WSi, and the Ni content is WNi, the cold formed spring steel of the present invention is Meet the following requirements:
[0021]
WC + WMn + WCr: 1.2% by mass or more and 2% by mass or less As a result of extensive research by the present inventors, the steel has a high strength with a tensile strength of 1900 MPa or more (corresponding to Rockwell hardness HRC of 52 or more) after being hardened. In order to ensure the above, it has been found that the respective contents of C, Mn, and Cr are in the range specified by the present invention, and that WC + WMn + WCr is required to be 1.2% by mass or more. However, if the value becomes excessively large, a cappy crack or breakage occurs during the drawing of the rolled material, so the upper limit is made 2% by mass.
[0022]
WSi / 3 + WCr / 2 + WMn: 1.4% by mass or more and 2.1% by mass or less In order to ensure a high strength of steel having a tensile strength of 1900 MPa or more (corresponding to 52 or more in Rockwell hardness HRC) in the quench hardening state. In addition to the above, the respective contents of Si, Cr, and Mn are in the range specified by the present invention, and it is also found that WSi / 3 + WCr / 2 + WMn is required to be 1.4 mass% or more. However, if the value becomes excessively large, the hardenability of the steel becomes excessive and may cause quenching cracks during the quenching of the steel, so the upper limit is made 2.1 mass%.
[0023]
WCu + WNi: 0.4% by mass or more In order to ensure excellent corrosion fatigue strength while the steel has a high strength of tensile strength of 1900 MPa or more (equivalent to 52 or more in Rockwell hardness HRC), Cu In addition to the content of Ni and Ni being within the range specified by the present invention, it was found that WCu + WNi should be 0.4 mass% or more.
[0024]
Further, in the cold forming spring steel of the present invention, in addition to the above composition,
0.05 mass% or more and 0.3 mass% or less V;
0.02 mass% or more and 0.05 mass% or less of Nb;
0.02 mass% or more and 0.07 mass% or less of Ti;
0.005 mass% or more and 0.04 mass% or less of Al;
0.0005 mass% or more and 0.003 mass% or less of B;
One or more of these, and
When the content of C is WC, the content of Mn is WMn, the content of Cr is WCr, the content of Si is WSi, the content of Ni is WNi, and the content of B is WB,
WC + WMn + WCr is 1.2 mass% or more and 2 mass% or less,
And WSi / 3 + WCr / 2 + WMn + 353WB is 1.4 mass% or more and 2.1 mass% or less,
And WCu + WNi is 0.4 mass% or more,
It can be.
[0025]
Hereinafter, the limited range of each numerical value will be described. In addition, the thing same as the limited range mentioned above is abbreviate | omitted.
[0026]
V (Vanadium): 0.05% by mass or more and 0.3% by mass or less V forms carbides and carbonitrides in the steel to refine the austenite crystal grains, and contributes to precipitation hardening in the spring. It has the effect of improving sag resistance. In order to acquire these effects, 0.05 mass% or more needs to be contained. Since the carbide of V becomes a local electrode on the steel surface and forms corrosion pits and becomes a starting point of crack fracture, the upper limit is made 0.3 mass%. Moreover, when it contains exceeding an upper limit, a huge primary carbide will crystallize and cold workability will deteriorate.
[0027]
Nb (niobium): 0.02% by mass or more and 0.05% by mass or less Nb, like V, has the effect of forming carbides and carbonitrides in steel to refine the austenite crystal grains. In order to acquire this effect, 0.02 mass% or more needs to be contained. However, even if contained excessively, the effect is saturated, and rather the hot workability and cold workability of the steel are lowered, so the upper limit is made 0.050%.
[0028]
Ti (titanium): 0.02% by mass or more and 0.07% by mass or less Ti, like Nb and V, has the effect of forming carbides and carbonitrides in the steel to refine the austenite crystal grains. In order to obtain this effect, the content of 0.02% by mass or more is necessary. However, if excessively contained, it remains as an undissolved compound during quenching and heating of steel, and its size is relatively large. Thus, the fatigue strength is lowered, so the upper limit is made 0.07% by mass.
[0029]
Al (aluminum): 0.005% by mass or more and 0.04% by mass or less Al forms nitrides in steel and contributes to the refinement of austenite crystal grains. If contained, the amount of oxide inclusions in the steel is increased and the cleanliness of the steel is impaired. Therefore, the upper limit of the Al content is set to 0.040%. The O (oxygen) content in the steel is preferably 20 ppm or less.
[0030]
B (boron): 0.0005 mass% or more and 0.003 mass% or less B is preferentially segregated at the grain boundaries of steel, and has the effect of preventing the segregation of P and S grain boundaries and improving toughness. is there. Moreover, if alloy elements that contribute to the hardenability such as Si, Mn, Cr, etc. are reduced in order to improve the cold workability of the wire, the hardenability of the material is insufficient and an incompletely hardened structure is produced. Has the effect of compensating for such a decrease in hardenability. In order to obtain these effects, the content of 0.0005% by mass or more is necessary. However, if excessively contained, B nitride is formed, and the toughness of the steel is impaired, and the fatigue characteristics of the steel are also deteriorated. The upper limit is 0.0030% by mass.
[0031]
WSi / 3 + WCr / 2 + WMn + 353WB: 1.4% by mass or more and 2.1% by mass or less As a result of the inventors' extensive research, the steel has a tensile strength of 1900 MPa or more in the quenched and hardened state (52 or more for Rockwell hardness HRC). In order to ensure high strength, the content of each of Si, Cr, Mn, and B is in the range specified by the present invention, and WSi / 3 + WCr / 2 + WMn + 353WB is 1.4% by mass. It turns out that the above is necessary. However, if the value becomes excessively large, the hardenability of the steel becomes excessive and may cause quenching cracks during the quenching of the steel, so the upper limit is made 2.1 mass%. In addition, when B is not contained, it becomes synonymous with the range in which said WSi / 3 + WCr / 2 + WMn was 1.4 mass% or more and 2.1 mass% or less.
[0032]
Moreover, in the steel for cold forming springs of the present invention, in addition to the above composition, 0.05 mass% or more and 0.5 mass% or less of Mo can be contained.
Mo has an effect which contributes to the improvement of hardenability. It also has the effect of enhancing corrosion resistance and improving corrosion fatigue strength. In order to obtain these effects, it is necessary to contain 0.05% by mass or more. However, if excessively contained, bainite is generated in the wire after rolling, resulting in deterioration of cold workability. Is 0.5 mass%.
[0033]
【Example】
Examples of the present invention will be described in detail below.
A steel ingot obtained by melting steel having the chemical composition shown in Tables 1 and 2 was subjected to split rolling, and further rolled into a rolled wire with a diameter of 13 mm by wire rolling. Wire rod rolling was performed by heating the steel slab to 1100 ° C. to a rolling end temperature of 869 ° C. After rolling, air cooling was performed. In addition, in each composition in the table, those that deviate from the composition range defined in the present invention are indicated by a downward arrow (↓) when the lower limit is exceeded, and an upward arrow (↑) when the upper limit is exceeded. It is attached.
[0034]
Tables 3 and 4 show the values of WC + WMn + WCr, WSi / 3 + WCr / 2 + WMn + 353WB, and WCu + WNi. In the table, WC + WMn + WCr is represented as “C + Mn + Cr”, WSi / 3 + WCr / 2 + WMn + 353WB is represented as “Si / 3 + Cr / 2 + Mn + 353 * B”, and WCu + WNi is represented as “Cu + Ni”.
[0035]
[Table 1]

[0036]
[Table 2]

[0037]
The hardness of the cut surface of the rolled wire was measured. Tables 3 and 4 show the values obtained by measuring 30 Rockwell C scale hardnesses and adding 6 times standard deviation σ (variation) to the average hardness as “hardness after rolling”. The upper limit of the hardness after rolling is HRC34. Further, the rolled wire rod was subjected to a bonder coating treatment, and was cold drawn to φ12 mm to obtain a wire drawing rod. Tables 3 and 4 show the presence or absence of occurrence of breakage during the wire drawing process as “presence or absence of breakage during wire drawing”.
[0038]
The drawn wire was passed through the high frequency coil under the conditions of a frequency of 30 kHz and a current of 300 A, and then immediately quenched by water cooling to obtain a quenched material. The hardened material was examined for the presence or absence of cracks, and the results are shown in Tables 3 and 4 as “quenched cracks”.
Moreover, the cross-sectional core part hardness of the said hardening material was measured. The Rockwell C scale hardness was measured at 20 points, and the average value was determined as “hardness after quenching” to evaluate whether the desired hardness (HRC 52) was satisfied. The results are shown in Tables 3 and 4.
[0039]
The quenched material was tempered to obtain a tempered material having a hardness of HRC52. Using a salt spray tester, the test piece collected from the tempered material is sprayed with (1) 35% of 5% NaCl aqueous solution for 2 hours, and (2) dried for 4 hours in an environment of 70% relative humidity and 60 ° C. (3) After repeating the cycles (1) to (3) for 2 hours in an environment of 95% relative humidity and 35 ° C. for 9 hours, a double torsional fatigue test was performed at a stress amplitude of 700 MPa. The number of repetitions of fracture was measured, and the corrosion fatigue characteristics were evaluated by the number of repetitions until the fracture. The “corrosion fatigue strength” is shown in Tables 3 and 4. Note that whether or not the number of repetitions of breakage reached 100,000 was used as a criterion for acceptability.
[0040]
Further, from the cross section of the corroded portion after the fatigue test, the depth of the corrosion pits was measured at a total of 40 points, and the maximum value of the pit depth (maximum pit depth) was measured. Whether or not the maximum pit depth is 100 μm or more was used as a criterion for quality. The measurement results of the maximum pit depth are shown in Tables 3 and 4 as “corrosion pit depth”. FIG. 1 shows the relationship between the corrosion pit depth and the number of repetitions of fracture.
[0041]
Next, the steel ingot obtained by melting the steel having the chemical composition shown in Tables 1 and 2 was subjected to split rolling, and a rolled wire rod having a diameter of 16 mm was obtained by wire rod rolling. Wire rod rolling was performed by heating the steel slab to 1100 ° C. to a rolling end temperature of 869 ° C. Then, after the rolling was completed, the product was air-cooled and further subjected to quenching and tempering treatment to obtain a tempered material having a hardness of HRC52. Thereafter, a JIS No. 2 mm U notch test piece was produced from the tempered material, and a Charpy impact test was performed. Then, it was evaluated from the obtained value whether the desired toughness (Charpy impact value of 40 J / cm ² or more) was satisfied. The results are shown in Tables 3 and 4 as “Charpy impact test”. When the Charpy impact value falls below 40 J / cm ^2, since the corrosion fatigue strength deteriorates rapidly, the criterion of acceptability whether 40 J / cm ² or more.
[0042]
[Table 3]

[0043]
[Table 4]

[0044]
Hereinafter, the evaluation results shown in Tables 3 and 4 and FIG. 1 will be described.
It turned out that this invention 24-33 which satisfy | fills the composition range which this invention specifies is a high strength spring steel which showed the favorable characteristic in all the tests, and was excellent in the corrosion fatigue strength made into this invention.
[0045]
Comparative Examples 1 and 2 having a WC + WMn + WCr value lower than the range specified by the present invention (Comparative Example 1 has C content, and Comparative Example 2 has insufficient Cr content and WSi / 3 + WCr / 2 + WMn + 353WB value). The hardness was lower than HRC52, and the strength and hardenability were insufficient.
[0046]
Comparative Examples 3 to 5 in which the WSi / 3 + WCr / 2 + WMn + 353WB value is insufficient from the range specified by the present invention (Comparative Example 3 is Si content, Comparative Example 4 is also insufficient for Mn content), Comparative Example 1 and As in the case of 2, the hardness after quenching was lower than HRC52, and the strength and hardenability were insufficient.
[0047]
Comparative Examples 6 to 9 in which the WSi / 3 + WCr / 2 + WMn + 353WB value is excessive from the range specified by the present invention are (Comparative Example 6 is Si content, Comparative Example 7 is Mn content, Comparative Example Cr content, Comparative Example 9 is B content was also excessive), and cracking occurred during quenching.
[0048]
Comparative Examples 10, 12, and 14 in which the P content is excessive from the range specified by the present invention, and Comparative Examples 11 and 13 in which the S content is excessive have Charpy impact test values lower than 40 J / cm ² , The toughness was insufficient. Moreover, these also have insufficient corrosion fatigue strength, and in Comparative Examples 10, 11, and 13, the depth of the corrosion pit exceeded 100 μm.
[0049]
Regarding Comparative Examples 16 to 23 (Comparative Example 16 is C, Comparative Example 17 is Si, Comparative Example 18 is Cr, Comparative Example 19 is V, and Comparative Example 20 is excessive in B content. No. 21 is Cu, Comparative Example 22 is insufficient in Ni content, and Comparative Example 23 is insufficient in WCu + WNi), both of which are insufficient in corrosion fatigue strength, and the depth of the corrosion pit exceeds 100 μm. It was.
[0050]
As described above, the steel having the chemical composition specified by the present invention, as shown by the present invention 24-33, is excellent in the wire drawing workability of the rolled material, and is excellent in manufacturability without being cracked even during quenching. Further, the strength of the hardened material also has various characteristics as a spring steel, such as excellent toughness and corrosion fatigue strength, while maintaining a high strength of HRC 52 or higher, that is, a tensile strength of 1900 MPa or higher.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between corrosion pit depth and the number of repeated fractures.

Claims

Fe as a main component;
0.36 mass% or more and 0.46 mass% or less of C,
1.8% by mass or more and 2.6% by mass or less of Si;
0.2% by mass or more and 1.2% by mass or less of Mn,
0.015 mass% or less P,
0.01 mass% or less S,
0.1 mass% or more and 0.5 mass% or less of Cu,
0.1 mass% or more and 1.5 mass% or less of Ni;
0.05 mass% or more and 1 mass% or less of Cr,
0.002% by mass or more and 0.012% by mass or less N;
And the content of C is WC, the content of Mn is WMn, the content of Cr is WCr, the content of Si is WSi, and the content of Ni is WNi,
WC + WMn + WCr is 1.2 mass% or more and 2 mass% or less,
And WSi / 3 + WCr / 2 + WMn is not less than 1.4% by mass and not more than 2.1% by mass,
And WCu + WNi is 0.4 mass% or more,
A steel for cold-formed springs.

Fe as a main component;
0.36 mass% or more and 0.46 mass% or less of C,
1.8% by mass or more and 2.6% by mass or less of Si;
0.2% by mass or more and 1.2% by mass or less of Mn,
0.015 mass% or less P,
0.01 mass% or less S,
0.1 mass% or more and 0.5 mass% or less of Cu,
0.1 mass% or more and 1.5 mass% or less of Ni;
0.05 mass% or more and 1 mass% or less of Cr,
0.002% by mass or more and 0.012% by mass or less N;
And containing
0.05 mass% or more and 0.3 mass% or less V;
0.02 mass% or more and 0.05 mass% or less of Nb;
0.02 mass% or more and 0.07 mass% or less of Ti;
0.005 mass% or more and 0.04 mass% or less of Al;
0.0005 mass% or more and 0.003 mass% or less of B;
One or more of these, and
When the content of C is WC, the content of Mn is WMn, the content of Cr is WCr, the content of Si is WSi, the content of Ni is WNi, and the content of B is WB,
WC + WMn + WCr is 1.2 mass% or more and 2 mass% or less,
And WSi / 3 + WCr / 2 + WMn + 353WB is 1.4 mass% or more and 2.1 mass% or less,
And WCu + WNi is 0.4 mass% or more,
A steel for cold-formed springs.

The steel for cold forming springs according to claim 1 or 2, characterized by containing 0.05 mass% or more and 0.5 mass% or less of Mo.