JP3918587B2 - Spring steel for cold forming - Google Patents
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Description
【０００１】
【発明の属する技術分野】
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鋼に関する。
【０００２】
【従来の技術】
近年、自動車は燃費向上のため、軽量化が強く要求され、懸架用コイルばねにおいても一層の軽量化が進められている。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ためにはばねの設計応力を高める必要があり、ばね素線にはより高い材料強度が要求されるようになった。このような要求に対処して、従来のばね用鋼の化学成分を調整しさらに特殊元素を配合することによって２００ｋｇｆ／ｍｍ2（ＨＲＣ：５３）以上の材料強度を有する高強度ばね用鋼が開発されている（例えば特許第３０６４６７２号公報）。
【０００３】
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。鋼線を熱間加工によってばね形状に成形したのち、焼入れ焼もどしを行ってばね強度に調整する熱間成形法に対して、あらかじめ所要のばね強度に調整された鋼線を用い、冷間加工によってばね形状に成形する冷間成形法は、ばねに成形したのちに焼入れなどの高温処理を要せず、焼入れ変形の修正作業がなくなるなど、ばね製造における設備面・工程面での簡素化効果が高い経済的なばね製造方法であり、自動車懸架用ばねの製造にも広く採用されている。
【０００４】
冷間成形に用いるばね鋼線は、適当な組成を有する鋼の圧延線材に対して、所定の線径まで伸線加工を施し、その後オイルテンパー処理、高周波連続焼入れ処理などの連続加熱処理を施して製造される。
ばね鋼線としては、前記のように高強度化が望まれているが、一般に鋼の強度を高めると靭延性が低下する。一方、冷間成形法によってばねを成形する上では、ばね鋼線は適当な靭延性を有することが求められる。また鋼線の製造過程では圧延線材に対して伸線加工を施すので、熱間圧延後の線材は伸線加工に耐える延性を備えることも必要である。
【０００５】
従来、ばね鋼としては、強度向上の観点から比較的Ｃ含有率の高い鋼が使用されている。そのため、熱間圧延によって製造される圧延線材の熱間圧延後の硬さが高まり、その後の引抜きや伸線加工において割れやカッピー状の断線が発生するとか、焼入れ時に焼割れを生じるという問題もあった。
【０００６】
【発明が解決しようとする課題】
本発明は、圧延線材の状態での伸線加工性、オイルテンパー処理・高周波連続焼入れ処理における熱処理対応性などのばね鋼線の製造性に優れ、かつ２００ｋｇｆ／ｍｍ2（ＨＲＣ：５３）以上という高強度を有する冷間成形用ばね鋼線を製造するに適した鋼を提供することを目的とする。
【０００７】
【課題を解決するための手段】
上記の課題を解決するために、本発明の冷間成形用ばね鋼は、
（１）質量％で、Ｃ：０．３８〜０．５３％、Ｓｉ：１．４〜２．４％、Ｍｎ：０．４〜１．４％、Ｐ：０．０１５％以下、Ｓ：０．０１０％以下、Ｃｕ：０．０５〜０．３５％、Ｎｉ：０．０５〜０．４０％、Ｃｒ：０．０５〜０．５５％を含み、かつ、１．２％≦Ｃ％＋Ｍｎ％＋Ｃｒ％≦２．０％、および１．４％≦（Ｓｉ％）/３＋（Ｃｒ％）/２＋Ｍｎ％≦２．１％の関係を満足し、残余、Ｆｅおよび不可避的不純物よりなることを特徴とする。
【０００８】
また、
（２）上記（１）記載の化学成分に加えて、Ｂ：０．０００５〜０．００３０％、Ｎ：０．０２０％以下を含み、かつ、Ｎｂ：０．０２０〜０．０５０％、Ｔｉ：０．０２０〜０．１００％、Ａｌ：０．００５〜０．０４０％のうちいずれか1種以上を含み、さらに１．２％≦Ｃ％＋Ｍｎ％＋Ｃｒ％≦２．０％、および１．４％≦（Ｓｉ％）/３＋（Ｃｒ％）/２＋Ｍｎ％＋３５３×Ｂ％≦２．１％の関係を満足することを特徴とする。
（３）上記（２）記載の化学成分に加えて、さらにＭｏ：０．０１〜０．３０％を含有することを特徴とする。
【０００９】
【作用】
本発明の冷間成形用ばね鋼は、鋼材の化学組成分を特定することによって、圧延線材の状態での伸線加工性に優れ、オイルテンパー処理・高周波連続焼入れ処理において焼割れなどの障害を生じることなく十分に焼入れができるなど、ばね鋼線の製造性に優れ、かつ２００ｋｇｆ／ｍｍ2（ＨＲＣ：５３）以上の高強度を具備する冷間成形用ばね鋼線を製造するに適した鋼を得ることに成功したものである。以下、各化学成分の含有率を定めた理由について説明する。
【００１０】
Ｃ：０．３８〜０．５３％
Ｃは、焼入れ焼もどし後の鋼の強度を高めるために必須の元素である。その含有率が０．３８％未満では焼入れによって十分な硬さが得られないのでＣ含有率の下限を０．３８％とする。０．５３％を超えてＣを含有すると焼入れ時に割れを生じたり、焼入れ焼もどし後の鋼の靭性が低下したりするうえ、疲労強度、耐遅れ破壊性が劣化する。また、熱間圧延後の鋼の硬さが高くなりすぎ、その後の引抜きや伸線加工工程において割れやカッピー状の断線が発生するなどの障害をもたらす。それゆえＣ含有率の上限を０．５３％とする。
【００１１】
Ｓｉ：１．４〜２．４％
Ｓｉは、固溶強化元素として鋼の強度および耐へたり性を向上するために添加するが、その効果を十分に発揮するためには１．４％以上を含有する必要がある。しかし、過剰に含有すると製造工程中の高温加熱時に鋼表面に脱炭を生じ疲労強度の低下を招くのでＳｉ含有率の上限を２．４％とする。
【００１２】
Ｍｎ：０．４〜１．４％
鋼溶製時の脱酸を促し、ＭｎＳ形成による赤熱脆性を防止し、また、鋼の焼入れ性を高めるために０．４％以上含有せしめる。しかし過剰に含有すると熱間圧延後の鋼の硬さを高め、また焼入れ時には焼割れを生じるなど鋼の製造性を損なうので含有率の上限を１．４％とする。
【００１３】
Ｐ：０．０１５％以下
Ｐは、鋼の結晶粒界に偏析して結晶粒界を脆弱化させ、遅れ破壊の発生を助長するので、その含有量は少ないほど好ましいが、経済性を考慮して含有率の上限を０．０１５％とする。
【００１４】
Ｓ：０．０１０％以下
Ｓは、鋼中でＭｎと結合してＭｎＳを形成し非金属介在物となり鋼の疲労強度を低下するので、その含有量は極力低減することが好ましいが、経済性を考慮して含有率の上限を０．０１０％とする。
【００１５】
Ｃｕ：０．０５〜０．３５％
Ｃｕは、鋼の耐食性の向上に対して有効であり、またフェライト脱炭の防止効果を有する。その効果を発揮するためには０．０５％以上を含有させる必要がある。しかし過剰に含有すると鋼の熱間加工性を損なうので含有率の上限を０．３５％とする。
【００１６】
Ｎｉ：０．０５〜０．４０％
Ｎｉは、鋼の耐食性を高め、またフェライト脱炭の防止効果を有するので０．０５％以上を含有させる。しかし過剰に含有すると鋼のコストを高めるので含有率の上限を０．４０％とする。
【００１７】
Ｃｒ：０．０５〜０．５５％
Ｃｒは、鋼の焼入れ性を高めるために０．０５％以上を含有させる。しかし過剰に含有すると熱間圧延後の鋼の硬さが高くなりすぎ、伸線加工において割れを生じるなどの弊害をもたらすので、Ｃｒ含有率の上限を０．５５％とする。
【００１８】
Ｂ：０．０００５〜０．００３０％
Ｂは、鋼の結晶粒界に優先析出し、Ｐ、Ｓの結晶粒界偏析を防止して鋼の遅れ破壊強度を向上させる。この効果を得るためには０．０００５％以上のＢを含有する必要がある。しかし過剰に含有すると結晶粒界にＢ構成物を形成して鋼の焼入れ性を低減し、鋼の靭性を損なうので、Ｂ含有率の上限は０．００３０％とする。
【００１９】
Ｎ：０．０２０％以下
Ｎは、鋼中で窒化物あるいは炭窒化物を形成し、非金属介在物として鋼の疲労強度を低下させるので、その含有率は低いほど好ましいが、経済性を考慮して含有率の上限を０．０２０％とする。
【００２０】
Ｎｂ：０．０２０〜０．０５０％、Ｔｉ：０．０２０〜０．１００％、Ａｌ：０．００５〜０．０４０％のうちいずれか１種以上
これらの元素は、いずれも鋼のオーステナイト結晶粒の微細化効果を有し鋼の靭性向上に寄与するので、これらの元素のうちいずれか１種以上を添加する。特に、高周波誘導加熱による焼入れを行う場合に優れた結晶粒微細化効果が得られ、高強度とした鋼の靭延性の向上に著しく寄与する。このような効果を得るために、それぞれＴｉは０．０２％以上、Ｎｂは０．０２％以上、Ａｌは０．００５％以上を含有する必要がある。
【００２２】
Ｎｂは、鋼中で炭窒化物を形成し、さらに析出硬化に寄与してばねの耐へたり性を向上する。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および冷間加工性を低下させるので、Ｎｂ含有率の上限を０．０５０％とする。
【００２３】
Ｔｉは、Ｎｂと同様に鋼中で炭窒化物を形成し、オーステナイト結晶粒を微細化するとともに析出硬化に寄与する。しかし過剰にＴｉを含有すると、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、Ｔｉ含有率の上限を０．１００％とする。
【００２４】
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、鋼中の酸素（Ｏ）含有率の上限は１２ｐｐｍとすることが好ましい。
【００２５】
Ｍｏ：０．０１〜０．３０％
Ｍｏは、鋼の焼入れ性を向上するとともに焼もどし軟化抵抗性を高める元素であり、高強度における靭性や遅れ破壊特性の改善に効果がある。これらの効果を発揮するためにはＭｏ含有率を０．０１％以上とする必要がある。しかし過剰に含有すると上記の効果は飽和するばかりでなく、熱間圧延後の鋼の硬さを上昇させ、伸線加工を困難にするので、Ｍｏ含有率の上限を０．３０％とする。
【００２６】
１．２％≦Ｃ％＋Ｍｎ％＋Ｃｒ％≦２．０％
種々研究の結果、焼入れ硬化した状態で２００ｋｇｆ/ｍｍ2（ＨＲＣ５３）以上の材料強度を確保するためにはＣ、Ｍｎ、Ｃｒの含有率がそれぞれ本発明の特定する範囲にあるほか、さらに（Ｃ％＋Ｍｎ％＋Ｃｒ％）の値が１．２％以上である必要があることが判った。しかし（Ｃ％＋Ｍｎ％＋Ｃｒ％）の値が２．０％を超え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を生じる。それゆえ（Ｃ％＋Ｍｎ％＋Ｃｒ％）の値は１．２〜２．０％とする。
【００２７】
１．４％≦（Ｓｉ％）/３＋（Ｃｒ％）/２＋Ｍｎ％＋３５３×Ｂ％≦２．１％種々検討の結果、鋼の焼入れ後２００ｋｇｆ/ｍｍ2（ＨＲＣ５３）以上の材料強度を確保するためにはＳｉ、Ｃｒ、Ｍｎ、Ｂの含有率がそれぞれ本発明の特定する範囲にあるほか、さらに（（Ｓｉ％）/３＋（Ｃｒ％）/２＋Ｍｎ％＋３５３×Ｂ％）の値が１．４％以上である必要があることが判った。しかしその値が２．１％を超え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ることがある。それゆえ（（Ｓｉ％）/３＋（Ｃｒ％）/２＋Ｍｎ％＋３５３×Ｂ％）の値は１．４〜２．１％とする。
【００２８】
本発明の冷間成形用ばね鋼は、上述のように化学成分を特定し、化学組成を調整することによって、熱間圧延後の線材の硬さを制御し、焼なましなどの熱処理を施すことなく引抜き加工あるいは伸線加工を行える。さらに、焼入れ焼もどしを施すことによって２００ｋｇｆ/ｍｍ2（ＨＲＣ５３）以上の材料強度を示し、かつ、前記材料強度において冷間成形法によってコイルばねに成形することができる十分な靱延性と、優れた耐へたり性、耐食性、耐疲労性、遅れ破壊特性を兼備する。
【００２９】
なお、本発明の請求項２および請求項３に提示する鋼については、２００ｋｇｆ/ｍｍ2（ＨＲＣ５３）以上の材料強度を得るために行う焼入れとして、高周波誘導加熱焼入れを行うことにより、一層優れた強靭化の効果を得ることができる。
【００３０】
【実施例】
表１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１，１００℃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。
【００３１】
【表１】
前記圧延線材の横断面について金属組織を観察し、フェライト脱炭層の厚さを測定した。その結果を「脱炭深さ」として表２に示す。また、該横断面について硬さを測定した。ロックウエルＣスケール硬さを３０点測定し、（平均値＋６σ）の値を「圧延線材の最大硬さ」として表２に示す。
【００３３】
【表２】
前記圧延線材にボンダ皮膜処理を施し、伸線加工してφ１２ｍｍの伸線材とした。前記伸線加工における破断の発生の有無を「伸線時の破断」として表２に示す。
前記伸線材を高周波コイル中に通して周波数３０ｋＨｚ、３００Ａの条件で加熱し、加熱後直ちに水冷して焼入れを行い、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２に示す。また、焼入れ材の横断面についてロックウエルＣスケール硬さを２０点測定し、その平均値を「焼入れ硬さ」として表２に示す。
【００３５】
前記焼入れ材を焼もどしして硬さＨＲＣ５３の焼もどし材とした。該焼もどし材から試験片を切り出し、重錘式ねじりクリープ試験機を用い、負荷応力１０５０ＭＰａとし、７０℃で９６ｈ負荷後、残留する歪を測定した。その結果を「残留せん断歪」として表２に示す。
【００３６】
前記焼もど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、３５℃で５％ＮａＣｌ水溶液を２ｈ噴霧、相対湿度５０％で６０℃の環境において４ｈ乾燥、相対湿度９５％で３５℃の環境において２ｈ保持のサイクルを６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い、破断繰り返し数を測定した。比較例８（ＪＩＳ ＳＵＰ７相当鋼）の破断繰り返し数を１として各試験片の破断繰り返し数との比を求め、「腐食強度比」として表２に示す。
【００３７】
前記焼もどし材から直径６ｍｍ、長さ８０ｍｍで、中央部に深さ１ｍｍで先端部半径０．１ｍｍのＶ型切欠きを有する遅れ破壊試験片を切り出した。該遅れ破壊試験片に所定の曲げモーメントを加えつつ、切欠き部にｐＨ２の塩酸水溶液（試験液）を滴下して破断にいたるまでの時間を測定し、３０ｈで破断しない最大の曲げ応力（遅れ破壊強度）を求めた。他方、同形の試験片について試験液を滴下することなく漸増曲げモーメントを加えて破断した時の最大曲げ応力（曲げ破断強度）を求めた。（遅れ破壊強度）/（曲げ破断強度）の比の値を遅れ破壊強度比として表２に示す。
【００３８】
表２から明らかなように、本発明が特定する範囲を超えてＳｉ含有率の高い比較例２はフェライト脱炭を生じているが、本発明の実施例は、いずれも脱炭深さが０であり、フェライト脱炭が認められない。
【００３９】
（Ｃ％＋Ｍｎ％＋Ｃｒ％）の値が本発明の特定する範囲を超えて高い比較例４および９〜１４は、圧延材の最大硬さがＨＲＣ３７以上と高く、伸線時に破断を生じているが、本発明の実施例においては圧延材の最大硬さはいずれもＨＲＣ３３以下となり、φ１３ｍｍの圧延線材からφ１２ｍｍに伸線加工した結果においても伸線時の破断は発生していない。本発明材では焼なましなどの軟化処理を行わなくても十分に冷間伸線によって加工可能であることを示している。
【００４０】
（（Ｓｉ％）/３＋（Ｃｒ％）/２＋Ｍｎ％＋３５３×Ｂ％）の値が本発明の特定する範囲を超えて高い比較例４および５では高周波焼入れ工程で焼割れが発生しているが、本発明の実施例では高周波焼入れにおいても焼割れの発生は認められず、焼入れ硬さもＨＲＣ５５以上の高い値を示し、焼もどし後にも十分ＨＲＣ５３の硬さを維持することを示している。
【００４１】
残留せん断歪は材料の耐へたり性の指標となり、その値が小さい程耐へたり性が高いことを示す。表２より明らかなように、本発明の実施例はいずれも比較例に比べて耐へたり性が優れていることを示している。
腐食強度比は、腐食作用を蒙った鋼の疲労強度の指標となるもので、標準的なばね鋼であるＪＩＳ ＳＵＰ７を基準として比較している。表２から明らかなように、本発明の実施例はいずれも腐食後の疲労強度がＪＩＳ ＳＵＰ７より優れていることを示している。
【００４２】
遅れ破壊強度比は、鋼の遅れ破壊に対する感受性を示す指標であって、その数値が高いほど遅れ破壊を生じにくいことを示す。表２から判るように、本発明の実施例は、いずれも比較例より遅れ破壊感受性が低く、遅れ破壊に対して安定であることを示している。
【００４３】
以上実施例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、熱間圧延による脱炭もなく、圧延材の伸線加工性にも優れ、焼入れにおいても割れを生じることがなく、製造製に優れている。また、焼入れ材の強度もＨＲＣ５３（２００ｋｇｆ/ｍｍ2）を超えることが明らかである。このような高強度を保持しつつ、耐へたり性、疲労強度、遅れ破壊特性など、ばね鋼として所用の諸特性も優れている。
【００４４】
【発明の効果】
以上に説明したように、本発明の冷間成形用ばね鋼は、圧延線材の状態での伸線加工性、オイルテンパー処理・高周波連続焼入れ処理における熱処理対応性などのばね鋼線の製造性に優れ、かつ、２００ｋｇｆ／ｍｍ2（ＨＲＣ：５３）以上という高強度を保持しつつ、耐へたり性、疲労強度、遅れ破壊特性など、優れたばね特性を示す冷間成形用ばね鋼線を製造を可能とするもので、その経済効果は極めて大きいといえる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a spring steel suitable for forming into a coil spring by a cold forming method.
[0002]
[Prior art]
In recent years, automobiles are strongly required to be lighter in order to improve fuel efficiency, and further weight reduction is being promoted in suspension coil springs. 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, it is necessary to increase the design stress of the spring, and higher material strength is required for the spring element wire. In response to these demands, a high strength spring steel having a material strength of 200 kgf / mm 2 (HRC: 53) or more has been developed by adjusting the chemical composition of conventional spring steel and blending special elements. (For example, Japanese Patent No. 3064672).
[0003]
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 steel wire, which has been adjusted to the required spring strength in advance, is used for the hot forming method in which the steel wire is formed into a spring shape by hot working and then quenched and tempered to adjust to the spring strength. The cold forming method of forming into a spring shape by using a spring does not require high-temperature treatment such as quenching after molding into a spring, eliminating the need for work to correct quenching deformation and simplifying the facilities and processes in spring production Is a highly economical spring manufacturing method, and is widely used for manufacturing automobile suspension springs.
[0004]
Spring steel wire used for cold forming is subjected to wire drawing to a predetermined wire diameter on a rolled steel wire having an appropriate composition, and then subjected to continuous heat treatment such as oil tempering and induction hardening. Manufactured.
The spring steel wire is desired to have high strength as described above, but generally the toughness of the steel decreases when the strength of the steel is increased. On the other hand, when forming a spring by the cold forming method, the spring steel wire is required to have an appropriate toughness. Further, since the wire drawing is performed on the rolled wire in the manufacturing process of the steel wire, the wire after hot rolling needs to have ductility to withstand the wire drawing.
[0005]
Conventionally, as spring steel, steel having a relatively high C content is used from the viewpoint of strength improvement. Therefore, the hardness after hot rolling of the rolled wire manufactured by hot rolling is increased, and there is a problem that cracking or capty-like disconnection occurs in the subsequent drawing or wire drawing processing, or that cracking occurs during quenching. there were.
[0006]
[Problems to be solved by the invention]
The present invention is excellent in manufacturability of spring steel wires such as wire drawing workability in the state of a rolled wire, heat treatment compatibility in oil temper treatment / high frequency continuous quenching treatment, and 200 kgf / mm 2 (HRC: 53) or more. It aims at providing the steel suitable for manufacturing the spring steel wire for cold forming which has high intensity | strength.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, the spring steel for cold forming of the present invention is
(1) By mass%, C: 0.38 to 0.53%, Si: 1.4 to 2.4%, Mn: 0.4 to 1.4%, P: 0.015% or less, S: 0.010% or less, Cu: 0.05 to 0.35%, Ni: 0.05 to 0.40%, Cr: 0.05 to 0.55%, and 1.2% ≦ C% + Mn% + Cr% ≦ 2.0% and 1.4% ≦ (Si%) / 3+ (Cr%) / 2 + Mn% ≦ 2.1%, satisfying the relationship of the balance, Fe and inevitable impurities It is characterized by.
[0008]
Also,
(2) In addition to the chemical component described in (1) above, B: 0.0005 to 0.0030%, N: 0.020% or less, and Nb: 0.020 to 0.050%, Ti : 0.020 to 0.100%, Al: any one or more of 0.005 to 0.040%, 1.2% ≦ C% + Mn% + Cr% ≦ 2.0%, and 1 4% ≦ (Si%) / 3+ (Cr%) / 2 + Mn% + 353 × B% ≦ 2.1% is satisfied.
(3) In addition to the chemical component described in (2) above, Mo: 0.01 to 0.30% is further contained.
[0009]
[Action]
The spring steel for cold forming of the present invention has excellent wire drawing workability in the state of a rolled wire by specifying the chemical composition of the steel material, and has problems such as cracking in oil tempering and high-frequency continuous quenching. Steel suitable for manufacturing cold forming spring steel wires having excellent strength of spring steel wires, such as being able to be sufficiently hardened without being generated, and having high strength of 200 kgf / mm 2 (HRC: 53) or more. Is a success. Hereinafter, the reason for determining the content of each chemical component will be described.
[0010]
C: 0.38 to 0.53%
C is an essential element for increasing the strength of the steel after quenching and tempering. If the content is less than 0.38%, sufficient hardness cannot be obtained by quenching, so the lower limit of the C content is set to 0.38%. If C exceeds 0.53%, cracking occurs during quenching, the toughness of the steel after quenching and tempering decreases, and fatigue strength and delayed fracture resistance deteriorate. In addition, the hardness of the steel after hot rolling becomes too high, which causes problems such as the occurrence of cracks and broken cuts in the subsequent drawing and wire drawing processes. Therefore, the upper limit of the C content is 0.53%.
[0011]
Si: 1.4-2.4%
Si is added as a solid solution strengthening element in order to improve the strength and sag resistance of the steel, but in order to fully exhibit its effects, it is necessary to contain 1.4% or more. However, if contained excessively, the steel surface is decarburized during high-temperature heating during the production process, leading to a decrease in fatigue strength, so the upper limit of the Si content is set to 2.4%.
[0012]
Mn: 0.4 to 1.4%
In order to promote deoxidation at the time of steel melting, to prevent red heat brittleness due to MnS formation, and to increase the hardenability of steel, 0.4% or more is included. However, if the content is excessive, the hardness of the steel after hot rolling is increased, and the steel manufacturability is impaired by quenching at the time of quenching, so the upper limit of the content is set to 1.4%.
[0013]
P: 0.015% or less P segregates at the grain boundaries of steel and weakens the grain boundaries and promotes the occurrence of delayed fracture. Therefore, the lower the content, the better. Therefore, the upper limit of the content is set to 0.015%.
[0014]
S: 0.010% or less S combines with Mn in the steel to form MnS and becomes a non-metallic inclusion, thereby reducing the fatigue strength of the steel. Therefore, the content is preferably reduced as much as possible. In consideration of the above, the upper limit of the content is set to 0.010%.
[0015]
Cu: 0.05 to 0.35%
Cu is effective for improving the corrosion resistance of steel and has an effect of preventing ferrite decarburization. In order to exhibit the effect, it is necessary to contain 0.05% or more. However, excessive content impairs the hot workability of the steel, so the upper limit of the content is set to 0.35%.
[0016]
Ni: 0.05-0.40%
Ni increases the corrosion resistance of the steel and has an effect of preventing ferrite decarburization, so 0.05% or more is contained. However, excessive content increases the cost of steel, so the upper limit of the content is set to 0.40%.
[0017]
Cr: 0.05-0.55%
Cr contains 0.05% or more in order to improve the hardenability of the steel. However, if it is contained excessively, the hardness of the steel after hot rolling becomes too high, and there is a detrimental effect such as cracking in the wire drawing, so the upper limit of the Cr content is made 0.55%.
[0018]
B: 0.0005 to 0.0030%
B preferentially precipitates at the grain boundaries of the steel, prevents segregation of P and S grain boundaries, and improves the delayed fracture strength of the steel. In order to acquire this effect, it is necessary to contain 0.0005% or more of B. However, if contained in excess, B constituents are formed at the grain boundaries to reduce the hardenability of the steel and impair the toughness of the steel, so the upper limit of the B content is set to 0.0030%.
[0019]
N: 0.020% or less N forms nitrides or carbonitrides in steel and reduces the fatigue strength of steel as a non-metallic inclusion, so its content is preferably as low as possible, but economics are considered. Thus, the upper limit of the content is set to 0.020%.
[0020]
Any one or more of Nb: 0.020 to 0.050%, Ti: 0.020 to 0.100%, and Al: 0.005 to 0.040% are all austenite crystals of steel. One of these elements is added because it has a grain refinement effect and contributes to improved toughness of the steel. In particular, when performing quenching by high frequency induction heating, an excellent grain refinement effect is obtained, which contributes significantly to improving the toughness of steel with high strength. In order to obtain such an effect, it is necessary that Ti is 0.02% or more, Nb is 0.02% or more, and Al is 0.005% or more.
[0022]
Nb forms carbonitrides in steel and further contributes to precipitation hardening to improve spring sag resistance. 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 of the Nb content is made 0.050%.
[0023]
Ti, like Nb, forms carbonitrides in steel, refines austenite crystal grains and contributes to precipitation hardening. However, if Ti is excessively contained, it remains as an undissolved compound during the quenching and heating of the steel, and its size is relatively large. Therefore, it becomes a starting point of fracture and causes a decrease in fatigue strength. 100%.
[0024]
Al contributes to the refinement of austenite crystal grains by forming nitrides in steel, but since it is an element that has a strong tendency to form oxides, the inclusion of a large amount increases the amount of oxide inclusions in the steel, and the cleanliness of the steel. Damage. Therefore, the upper limit of the Al content is set to 0.040%. Moreover, it is preferable that the upper limit of oxygen (O) content rate in steel shall be 12 ppm.
[0025]
Mo: 0.01-0.30%
Mo is an element that improves the hardenability of the steel and increases the tempering softening resistance, and is effective in improving toughness and delayed fracture characteristics at high strength. In order to exert these effects, the Mo content needs to be 0.01% or more. However, if the content is excessive, the above effect is not only saturated, but also the hardness of the steel after hot rolling is increased and wire drawing is difficult, so the upper limit of the Mo content is set to 0.30%.
[0026]
1.2% ≦ C% + Mn% + Cr% ≦ 2.0%
As a result of various studies, in order to ensure a material strength of 200 kgf / mm 2 (HRC53) or more in the state of quench hardening, the contents of C, Mn, and Cr are within the ranges specified by the present invention, respectively, % + Mn% + Cr%) was found to be 1.2% or more. However, if the value of (C% + Mn% + Cr%) exceeds 2.0%, a capty crack or fracture occurs during the drawing process of the rolled material. Therefore, the value of (C% + Mn% + Cr%) is set to 1.2 to 2.0%.
[0027]
1.4% ≦ (Si%) / 3+ (Cr%) / 2 + Mn% + 353 × B% ≦ 2.1% As a result of various investigations, a material strength of 200 kgf / mm 2 (HRC53) or more is ensured after quenching of steel. For this purpose, the contents of Si, Cr, Mn, and B are within the ranges specified by the present invention, and the value of ((Si%) / 3+ (Cr%) / 2 + Mn% + 353 × B%) is 1. It was found that it should be 4% or more. However, if the value exceeds 2.1%, the hardenability of the steel becomes excessive, and there are cases in which quenching cracks occur when the steel is quenched. Therefore, the value of ((Si%) / 3+ (Cr%) / 2 + Mn% + 353 × B%) is set to 1.4 to 2.1%.
[0028]
The spring steel for cold forming according to the present invention controls the hardness of the wire after hot rolling by specifying the chemical components and adjusting the chemical composition as described above, and performs heat treatment such as annealing. Without drawing or drawing. Furthermore, the material strength of 200 kgf / mm 2 (HRC53) or higher is exhibited by quenching and tempering, and sufficient tough ductility that can be formed into a coil spring by a cold forming method in the material strength and excellent Combines sag resistance, corrosion resistance, fatigue resistance, and delayed fracture characteristics.
[0029]
In addition, about the steel shown to Claim 2 and Claim 3 of this invention, it was much more excellent by performing high frequency induction heating hardening as hardening performed in order to obtain material strength more than 200 kgf / mm < 2 > (HRC53). A toughening effect can be obtained.
[0030]
【Example】
A steel ingot obtained by melting the steel having the chemical composition shown in Table 1 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 1,100 ° C. to a rolling end temperature of 869 ° C. After rolling, air cooling was performed.
[0031]
[Table 1]
The metal structure was observed about the cross section of the rolled wire rod, and the thickness of the ferrite decarburized layer was measured. The results are shown in Table 2 as “Decarburization depth”. Moreover, hardness was measured about this cross section. The Rockwell C scale hardness was measured at 30 points, and the value of (average value + 6σ) is shown in Table 2 as “maximum hardness of rolled wire”.
[0033]
[Table 2]
The rolled wire rod was subjected to a bonder film treatment and drawn to obtain a wire rod having a diameter of 12 mm. The presence or absence of breakage in the wire drawing process is shown in Table 2 as “break during wire drawing”.
The wire drawing material was passed through a high frequency coil and heated under the conditions of a frequency of 30 kHz and 300 A, and immediately after the heating, it was cooled with water and quenched to obtain a quenched material. The quenching material was examined for the presence or absence of cracks, and the results are shown in Table 2 as “quenched cracks”. Further, the Rockwell C scale hardness was measured at 20 points on the cross section of the quenched material, and the average value is shown in Table 2 as “quenched hardness”.
[0035]
The quenching material was tempered to obtain a tempered material having a hardness of HRC53. A test piece was cut out from the tempered material, and the residual strain was measured after applying a load stress of 1050 MPa and loading at 70 ° C. for 96 h using a weight-type torsion creep tester. The results are shown in Table 2 as “residual shear strain”.
[0036]
Using a salt spray tester on a specimen collected from the tempered material, a 5% NaCl aqueous solution is sprayed for 2 hours at 35 ° C., dried for 4 hours in an environment of 60 ° C. at a relative humidity of 50%, and 35% at a relative humidity of 95%. After repeating the 2h holding cycle in an environment of ° C. six times, a double-twisted torsional fatigue test was conducted with a stress amplitude of 700 MPa, and the number of repetitions of fracture was measured. The ratio of the number of repetitions of breakage of each test piece was determined with the number of repetitions of breakage of Comparative Example 8 (JIS SUP7 equivalent steel) being 1, and is shown in Table 2 as “corrosion strength ratio”.
[0037]
A delayed fracture specimen having a V-shaped notch having a diameter of 6 mm, a length of 80 mm, a depth of 1 mm and a tip radius of 0.1 mm was cut out from the tempered material. While applying a predetermined bending moment to the delayed fracture test piece, measure the time from dropping a hydrochloric acid aqueous solution (test solution) with a pH of 2 to the notch until it breaks. The breaking strength was determined. On the other hand, the maximum bending stress (bending rupture strength) when the test piece having the same shape was broken by applying an increasing bending moment without dropping the test solution was obtained. Table 2 shows the ratio of (delayed fracture strength) / (bending fracture strength) as the delayed fracture strength ratio.
[0038]
As is clear from Table 2, Comparative Example 2 having a high Si content exceeding the range specified by the present invention produced ferrite decarburization, but all of the examples of the present invention had a decarburization depth of 0. And ferrite decarburization is not observed.
[0039]
In Comparative Examples 4 and 9 to 14, in which the value of (C% + Mn% + Cr%) is higher than the range specified by the present invention, the maximum hardness of the rolled material is as high as HRC37 or more, and breakage occurs during wire drawing. However, in the examples of the present invention, the maximum hardness of the rolled material is HRC33 or less, and no breakage during wire drawing occurs even in the result of drawing from φ13 mm rolled wire to φ12 mm. It shows that the material of the present invention can be sufficiently processed by cold drawing without performing a softening treatment such as annealing.
[0040]
In Comparative Examples 4 and 5 in which the value of ((Si%) / 3+ (Cr%) / 2 + Mn% + 353 × B%) is higher than the range specified by the present invention, quench cracking occurs in the induction hardening process. In the examples of the present invention, the occurrence of quenching cracks was not observed even in induction hardening, and the quenching hardness also showed a high value of HRC55 or higher, indicating that the hardness of HRC53 is sufficiently maintained even after tempering.
[0041]
Residual shear strain is an index of sag resistance of the material, and the smaller the value, the higher the sag resistance. As is apparent from Table 2, all of the examples of the present invention have better sag resistance than the comparative examples.
The corrosion strength ratio is an index of the fatigue strength of steel subjected to a corrosive action, and is compared based on JIS SUP7, which is a standard spring steel. As is apparent from Table 2, all the examples of the present invention show that the fatigue strength after corrosion is superior to JIS SUP7.
[0042]
The delayed fracture strength ratio is an index indicating the sensitivity of steel to delayed fracture, and the higher the value, the less likely that delayed fracture occurs. As can be seen from Table 2, all of the examples of the present invention are less susceptible to delayed fracture than the comparative examples, and are stable against delayed fracture.
[0043]
As shown in the above examples, the steel having the chemical composition specified by the present invention is free from decarburization by hot rolling, excellent in the wire drawing workability of the rolled material, and does not cause cracking even in quenching, and is manufactured. Excellent in making. It is also clear that the strength of the hardened material exceeds HRC53 (200 kgf / mm 2 ). While maintaining such high strength, various characteristics as spring steel such as sag resistance, fatigue strength and delayed fracture characteristics are also excellent.
[0044]
【The invention's effect】
As explained above, the spring steel for cold forming according to the present invention is suitable for the production of spring steel wires such as wire drawing workability in the state of rolled wire, heat treatment compatibility in oil temper treatment and high-frequency continuous quenching treatment. Produces a spring steel wire for cold forming that has excellent spring characteristics such as sag resistance, fatigue strength, delayed fracture characteristics, etc. while maintaining excellent strength of 200 kgf / mm 2 (HRC: 53) or higher. It can be said that the economic effect is extremely large.
Claims (3)
Ｃ ：０．３８〜０．５３％、
Ｓｉ：１．４〜２．４％、
Ｍｎ：０．４〜１．４％、
Ｐ ：０．０１５％以下、
Ｓ ：０．０１０％以下、
Ｃｕ：０．０５〜０．３５％、
Ｎｉ：０．０５〜０．４０％、
Ｃｒ：０．０５〜０．５５％
を含み、かつ、
１．２％≦Ｃ％＋Ｍｎ％＋Ｃｒ％≦２．０％、および
１．４％≦（Ｓｉ％）/３＋（Ｃｒ％）/２＋Ｍｎ％≦２．１％
の関係を満足し、残余、Ｆｅおよび不可避的不純物よりなることを特徴とする冷間成形用ばね鋼。% By mass
C: 0.38 to 0.53%,
Si: 1.4 to 2.4%,
Mn: 0.4 to 1.4%
P: 0.015% or less,
S: 0.010% or less,
Cu: 0.05 to 0.35%,
Ni: 0.05-0.40%,
Cr: 0.05-0.55%
Including, and
1.2% ≦ C% + Mn% + Cr% ≦ 2.0%, and 1.4% ≦ (Si%) / 3+ (Cr%) / 2 + Mn% ≦ 2.1%
A spring steel for cold forming, characterized by satisfying the above relationship and comprising the balance, Fe and inevitable impurities.
Ｃ ：０．３８〜０．５３％、
Ｓｉ：１．４〜２．４％、
Ｍｎ：０．４〜１．４％、
Ｐ ：０．０１５％以下、
Ｓ ：０．０１０％以下、
Ｃｕ：０．０５〜０．３５％、
Ｎｉ：０．０５〜０．４０％、
Ｃｒ：０．０５〜０．５５％、
Ｂ ：０．０００５〜０．００３０％、
Ｎ ：０．０２０％以下
を含み、かつ、
Ｎｂ：０．０２０〜０．０５０％
Ｔｉ：０．０２０〜０．１００％、
Ａｌ：０．００５〜０．０４０％
のうちいずれか1種以上を含み、さらに
１．２％≦Ｃ％＋Ｍｎ％＋Ｃｒ％≦２．０％、および
１．４％≦（Ｓｉ％）/３＋（Ｃｒ％）/２＋Ｍｎ％＋３５３×Ｂ％≦２．１％
の関係を満足し、残余Ｆｅおよび不可避的不純物よりなることを特徴とする冷間成形用ばね鋼。% By mass
C: 0.38 to 0.53%,
Si: 1.4 to 2.4%,
Mn: 0.4 to 1.4%
P: 0.015% or less,
S: 0.010% or less,
Cu: 0.05 to 0.35%,
Ni: 0.05-0.40%,
Cr: 0.05 to 0.55%,
B: 0.0005 to 0.0030%,
N: 0.020% or less, and
Nb: 0.020 to 0.050%
Ti: 0.020 to 0.100%,
Al: 0.005-0.040%
Any one or more of the following: 1.2% ≦ C% + Mn% + Cr% ≦ 2.0%, and 1.4% ≦ (Si%) / 3+ (Cr%) / 2 + Mn% + 353 × B % ≦ 2.1%
A spring steel for cold forming, characterized by satisfying the above relationship and consisting of residual Fe and unavoidable impurities.
Ｍｏ：０．０１〜０．３０％
を含むことを特徴とする請求項１または２記載の冷間成形用ばね鋼。% By mass
Mo: 0.01-0.30%
The spring steel for cold forming according to claim 1 or 2, characterized by comprising:
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
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| JP2002061805A JP3918587B2 (en) | 2002-03-07 | 2002-03-07 | Spring steel for cold forming |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002061805A JP3918587B2 (en) | 2002-03-07 | 2002-03-07 | Spring steel for cold forming |
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| Publication Number | Publication Date |
|---|---|
| JP2003253391A JP2003253391A (en) | 2003-09-10 |
| JP3918587B2 true JP3918587B2 (en) | 2007-05-23 |
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| JP2002061805A Expired - Fee Related JP3918587B2 (en) | 2002-03-07 | 2002-03-07 | Spring steel for cold forming |
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Families Citing this family (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP4588030B2 (en) * | 2004-08-26 | 2010-11-24 | 大同特殊鋼株式会社 | Steel for high strength spring, high strength spring and method for producing the same |
| JP4393467B2 (en) | 2006-02-28 | 2010-01-06 | 株式会社神戸製鋼所 | Hot rolled wire rod for strong wire drawing and manufacturing method thereof |
| JP5121360B2 (en) | 2007-09-10 | 2013-01-16 | 株式会社神戸製鋼所 | Spring steel wire rod excellent in decarburization resistance and wire drawing workability, and method for producing the same |
| KR101289132B1 (en) * | 2009-12-28 | 2013-07-23 | 주식회사 포스코 | High strength and toughness spring steel wire having excellent fatigue life, spring for the same and method for manufacturing thereof |
| JP5250609B2 (en) * | 2010-11-11 | 2013-07-31 | 日本発條株式会社 | Steel for high strength spring, method for producing high strength spring, and high strength spring |
| KR20140117505A (en) * | 2012-02-14 | 2014-10-07 | 제이에프이 스틸 가부시키가이샤 | Spring steel |
| JP5796781B2 (en) * | 2012-03-07 | 2015-10-21 | 株式会社神戸製鋼所 | Steel wire for high strength spring excellent in spring workability, manufacturing method thereof, and high strength spring |
| CN105308201A (en) * | 2013-06-04 | 2016-02-03 | 中央发条株式会社 | Steel for springs, spring, and spring production method |
| JP6212473B2 (en) * | 2013-12-27 | 2017-10-11 | 株式会社神戸製鋼所 | Rolled material for high-strength spring and high-strength spring wire using the same |
| CN114959486A (en) * | 2022-06-13 | 2022-08-30 | 杭州兴发弹簧有限公司 | 42SiCrV6 spring steel for large-wire-diameter hot-rolling spring on large excavator |
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2002
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| JP2003253391A (en) | 2003-09-10 |
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