JP3776507B2 - Manufacturing method of high-strength stainless steel bolts - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、耐食性に優れた高強度ボルトの製造方法に関する。
【０００２】
【従来の技術】
近来、建築用その他の鋼構造接合用ボルトとして、高強度で、かつ、耐食性を有するボルトに対する要求が高まっている。このような用途には、比較的低廉であり、また、太径ボルトも製造可能なＪＩＳ ＳＵＳ６３０のようなマルテンサイト系析出硬化型ステンレス鋼からなるボルトが使用されている。
【０００３】
ボルトの製造工程には、線材に鍛造加工を施して所定のボルト頭部形状を有するボルト素形材に成形加工する工程が含まれている。前記の鍛造加工方法としては、熱間鍛造加工と冷間鍛造加工とある。熱間加工法による場合、成形加工は容易であるが、寸法精度、表面品質の維持が困難であるという問題がある。他方、冷間鍛造加工は、寸法精度、表面品質の優れた製品を大量生産するのに適するが、冷間加工性のよい素材に限られる。
【０００４】
ところで、マルテンサイト系析出硬化型ステンレス鋼は固溶化熱処理状態ではマルテンサイト組織を呈し、一般に硬さが高く、例えば、ＳＵＳ６３０では固溶化熱処理後の硬さがＨＲＣ３５程度となり、冷間加工性が劣る。ＳＵＳ６３０では、鋼を６２０〜８００℃の温度で焼鈍し、オーステナイト相を多量に含んだ金属組織状態とすれば、冷間鍛造によってボルト頭部等の成形を行うことができる。この場合には、高強度ボルトとするために、ボルト成形後に固溶化熱処理を施し、しかる後析出硬化熱処理して高強度化する必要がある。製品の表面品質を維持するには、前記固溶化熱処理を保護雰囲気中で行う必要があり、そのためコストが高くなるきらいがあった。
【０００５】
ＳＵＳ６３０において、Ｃ、Ｎの含有率を低く押える等の鋼成分調整によって冷間加工性を高めることが考えられる。このように成分調整した鋼を用いることにより、ボルト形状の成形は可能となった。しかし、これによっても、なお、ボルト頭部に内部割れが生じるものがあり、この内部われを完全に防止することができなかった。
【０００６】
【発明が解決しようとする課題】
本発明は、上記の現状に鑑みてなされたもので、その目的とするところは、マルテンサイト系析出硬化型ステンレス鋼をボルト形状に成形したときにボルト頭部の内部に生じる割れの発生を防止することによって、表面品質の優れた高強度ステンレスボルトを安価に供給できる製造方法を提供することにある。
【０００７】
【課題を解決するための手段】
上記目的を達成するために、本発明の高強度ステンレスボルトの製造方法は、（１）９００〜１１００℃で固溶化熱処理して、体積百分率で、マルテンサイト相９０％以上としたマルテンサイト系析出硬化型ステンレス鋼を、冷間鍛造または２００℃以下の温度で加熱して行う温間鍛造によってボルト素形材に成形した後、該ボルト素形材の表面温度が１５０℃以下となるまで徐冷却し、その後４５０〜６５０℃の温度で析出硬化熱処理することを特徴とする。
（２）９００〜１１００℃で固溶化熱処理して、体積百分率で、マルテンサイト相９０％以上としたマルテンサイト系析出硬化型ステンレス鋼を、冷間鍛造または２００℃以下の温度で加熱して行う温間鍛造によってボルト素形材に成形した後に冷却し、この冷却過程において該ボルト素形材の表面温度が１００℃より低い温度に達する以前に加熱し、４５０〜６５０℃の温度で析出硬化熱処理することを特徴とする。
（３）上記（１）および（２）の何れか１項記載の高強度ステンレスボルトの製造方法において、マルテンサイト系析出硬化型ステンレス鋼が、以下の化学組成を有することを特徴とする。
【０００８】
質量％で、
Ｃ ：０．０１５％以下、
Ｎ ：０．０１５〜０．０５０％、
Ｓｉ：１．０％以下、
Ｍｎ：１．０％以下、
Ｐ ：０．０４０％以下、
Ｓ ：０．０３０％以下、
Ｃｕ：１．５〜５．０％、
Ｎｉ：３．０〜８．０％、
Ｃｒ：１３．０〜１６．５％、
Ｍｏ：１．５％以下、
Ｎｂ：０．１〜０．５％、
残余Ｆｅおよび不可避的不純物元素。
【０００９】
【発明の実施の形態】
本発明の高強度ステンレスボルトの製造方法では、予め固溶化熱処理を施したマルテンサイト系析出硬化型ステンレス鋼を冷間鍛造してボルト素形材に成形する。そのため、従来のようにボルト成形後に固溶化熱処理を行う必要がないという大きな効果がある。
【００１０】
ここに、マルテンサイト系析出硬化型ステンレス鋼としては、固溶化熱処理によって、鋼マトリックスがマルテンサイト化して高強度となるとともに、析出硬化元素を固溶し、その後に行われる析出硬化熱処理によって、さらに強度が上昇する鋼であって、好ましい鋼としては、ＪＩＳ ＳＵＳ６３０が挙げられる。
さらに好ましくは、質量％で、Ｃ：０．０１５％以下、Ｎ：０．０１５〜０．０５０％、Ｓｉ：１．０％以下、Ｍｎ：１．０％以下、Ｐ：０．０４０％以下、Ｓ：０．０３０％以下、Ｃｕ：１．５〜５．０％、Ｎｉ：３．０〜８．０％、Ｃｒ：１３．０〜１６．５％、Ｍｏ：１．５％以下、Ｎｂ：０．１〜０．５％を含み残余Ｆｅおよび不可避的不純物元素からなる鋼とする。
【００１１】
以下に、本発明を適用する好ましい鋼の化学組成を限定した理由について述べる。
Ｃ：０．０１５％以下
Ｃは、固溶化熱処理時の硬さ、変形能に大きく影響する元素で、その含有率が低いほど硬さは低く、変形能は増加する。本発明の鋼においては、Ｃ含有率は低いほど好ましいが、製造コストを考慮して上限を０．０１５％とする。好ましくは、Ｃ含有率は０．００８％以下とする。
【００１２】
Ｎ：０．０１５〜０．０５０％
Ｎは、鋼の固溶化熱処理時の硬さ、変形能に及ぼす影響はＣと同様の傾向を示すが、その影響の程度はＣに比べて遥かに少ない。ピーク時効時の靭延性および過時効時の強度保持効果を有するので０．０１５％以上含有させる。好ましくは０．０３０％以上とする。しかし、過剰にＮを含有すると固溶化熱処理状態の鋼の硬さを高め、また、必要以上にγ相を安定として固溶化熱処理時に多量の不安定γ相が残留するほか、過時効時に多量の逆変態γ相を生じて強度の低下をきたすので、Ｎ含有率の上限を０．０５０％とする。
【００１３】
Ｓｉ：１．０％以下
Ｓｉは鋼の脱酸剤として添加する。しかし、含有量が過大となると固溶化熱処理後の硬さが高くなる。また、δ−フェライトの形成量が過大となり、鋼の熱間加工性を損うので含有率の上限を１．０％とする。
Ｍｎ：１．０％以下
Ｍｎは、鋼の脱酸剤として作用するほか、高価なＮｉの代替元素としても有効なので添加する。しかし、Ｍｎ含有率が過大となると鋼のＭs 点を低下し、また、過時効時の強度を低下するので含有率の上限を１．０％とする。
【００１４】
Ｐ：０．０４０％以下
Ｐは鋼の結晶粒界に偏析しやすく、熱間加工性を害する。また、冷間加工性をも害するのでＰ含有率は低いほど好ましいが、製造コストを考慮して許容し得るＰ含有率の上限を０．０４０％とする。
Ｓ：０．０３０％以下
Ｓは、耐食性を著しく損い、また冷間加工性をも害するので含有率を低くすることが必要である。許容限を０．０３０％とするが、好ましくは含有率を０．０１０％以下とする。
【００１５】
Ｃｕ：１．５〜５．０％
Ｃｕは、時効加熱時にε相を形成して鋼を硬化するために添加する。前記効果を発揮するためには１．５％の含有を必要とするが、過大に含有すれば、鋼の熱間加工性を損うので、含有率の上限を５．０％とする。
Ｎｉ：３．０〜８．０％
Ｎｉは、強力なオーステナイト形成元素で、δ−フェライトの生成を抑制し、鋼の耐食性を向上するので３．０％以上を含有させる。しかし、過大に含有すれば、鋼のＭs 点を低下して残留オーステナイト量を増し、析出硬化熱処理後の強度を損うので含有率の上限を８．０％とする。
【００１６】
Ｃｒ：１３．０〜１６．５％
Ｃｒは耐食性を確保するために１３．０％以上添加する。しかし過大に含有すれば、δ−フェライトを多量に生成し、熱間加工性を害するとともに、鋼の強度を著しく低下するので含有率の上限を１６．５％とする。
Ｍｏ：１．５％以下
Ｍｏは耐食性を向上するために添加してもよい。しかし、フェライト安定化元素であって、多量に含有するとδ−フェライトを生成し、鋼の熱間加工性を損うので含有率の上限を１．５％とする。
【００１７】
Ｎｂ：０．１〜０．５％
Ｎｂは炭窒化物を形成し、鋼の結晶粒を微細化して固溶化熱処理後の鋼の硬さを低め、冷間加工性を向上する。また、析出硬化処理後の靭延性、特にピーク時効処理後の靭延性を高めるために添加する。前記効果を得るためには、０．１％以上のＮｂ含有率が必要である。しかしＮｂを多量に含有するとδ−フェライト量を増し鋼の熱間加工性を損うので含有率の上限を０．５％とする。
【００１８】
本発明の高強度ステンレスボルトの製造方法において、固溶化熱処理は、鋼組織を強度の高いマルテンサイトとするとともに、析出硬化元素Ｃｕを鋼マトリックスに固溶するために行う。固溶化熱処理の温度が９００℃未満では、析出硬化元素の固溶が十分に行われないか、または析出硬化元素は固溶するが炭化物、窒化物、炭窒化物等の固溶が十分に行われないため、強度の高いマルテンサイトが得られない。また、固溶化熱処理の温度が１１００℃を超えると、炭化物、窒化物、炭窒化物等の固溶が進みすぎて、鋼の結晶粒が粗大化したり、固溶化熱処理後の鋼組織がフェライト相の多いものとなって、鋼強度が低下したりする。それゆえ、固溶化熱処理の温度は、９００〜１１００℃とする。
【００１９】
本発明の高強度ステンレスボルトの製造方法においては、前述のマルテンサイト系析出硬化型ステンレス鋼に固溶化熱処理を施して、体積率で９０％以上のマルテンサイト相を含む金属組織とする。前記固溶化熱処理の後に該鋼に含まれる主な金属組織はオーステナイトとフェライトであるが、このオーステナイトとフェライトはマルテンサイトに比べて強度が低い。それゆえ、鋼の強度を維持するために、マルテンサイトが９０体積％以上とすることが必要である。
【００２０】
本発明の高強度ステンレスボルトの製造方法においては、前記鋼を冷間鍛造してボルト素形材に成形する。鋼の再結晶温度以下の温度に加熱して冷間加工する温間鍛造によれば、鍛造加工の際の変形抵抗を減ずることができるが、この鋼の場合、２００℃を超える温度に加熱して鍛造するとボルト頭部に割れを生じるので、温間鍛造の際の加熱温度は２００℃以下とする必要がある。
【００２１】
上記の鍛造加工に際して、被加工材の内部ではひずみエネルギーによって、また、表層部では被加工材とダイス、パンチなどの加工工具との摩擦によって発熱し、昇温する。本発明の高強度ステンレスボルトの製造方法の第１の実施態様においては、前記の鍛造加工によって昇温したボルト素形材を、該鍛造加工後少なくとも該ボルト素形材の表面温度が１５０℃以下となるまで徐冷却する。その後、４５０〜６５０℃の温度に加熱して析出硬化熱処理する。
【００２２】
ここに、徐冷却とは、発熱剤を用いるか、または断熱材、保温媒体によって鍛造加工後の前記ボルト素形材を覆う等の方法によって、積極的に該ボルト素形材の冷却を遅延する手段を講じて、放冷よりは遅い冷却速度で冷却することである。該ボルト素形材を、適度に加熱した空気、油等の浴槽に浸漬して冷却するのは好ましい方法である。
【００２３】
上記のように、鍛造加工後、昇温したボルト素形材を１５０℃以下の温度まで徐冷却することにより、ボルト素形材の頭部に発生する内部割れを防止することができる。
上記本発明の第１の実施態様による場合には、鍛造加工後のボルト素形材を、いったん室温まで冷却した後、ねじ切り加工等の後加工を行うことができる利点がある。
【００２４】
本発明の高強度ステンレスボルトの製造方法の第２の実施態様においては、前記の鍛造加工によって昇温したボルト素形材を、１００℃より低い温度に達する以前に加熱して、４５０〜６５０℃の析出硬化熱処理温度に持ちきたす。鍛造加工後のボルト素形材の冷却の下限温度を１００℃としたのは、１００℃よりも低い温度まで冷却するとボルト素形材の頭部に内部割れが発生するからである。
【００２５】
上記本発明の第２の実施態様による場合には、１００℃以上の温度における冷却速度には制限がないので、鍛造加工後のボルト素形材を短時間で冷却できる利点がある。
本発明の高強度ステンレスボルトの製造方法に用いるマルテンサイト系析出硬化型ステンレス鋼では、析出硬化熱処理の温度が、４５０℃以下では析出硬化に長時間を要し、また６５０℃以上では過時効となって軟化してしまう。４５０〜６５０℃の温度で析出硬化熱処理することによって高い強度を得ることができる。
【００２６】
【実施例】
（実験１）
表１に示す化学組成の鋼１および鋼２を溶製し、熱間圧延により直径１０ｍｍの線材コイルに加工後、１０４０℃、または９５０℃で固溶化熱処理し、直径９．８ｍｍに引抜き加工して素材とした。該素材を鍛造加工によって図１に示す４工程でＭ１０六角ボルト素形材に成形した。鍛造加工としては、冷間鍛造、または素材を１５０℃に加熱し温間鍛造によって行った。
【００２７】
【表１】
鍛造加工したボルト素形材は、直ちに徐冷バックまたはオイルバスに投入して徐冷却した。徐冷バッックは鉄箱に断熱材を内張りされたものであり、ボルト素材を装入する前に熱風にてあらかじめ加熱した。また、オイルバスは１２０℃に保持した焼入油の油槽である。徐冷却開始後適当な時間保持してから徐冷装置から取り出し、ボルト素形材の頭部表面の温度を接触温度計によって測定し、そのまま空冷して室温まで冷却した。比較のために、鍛造加工後そのまま空冷したボルト素形材を作製した。これらのボルト素形材に所定の析出硬化熱処理を施して供試材とした。
【００２９】
前記供試材について、超音波探傷試験によるボルト素形材の頭部の内部割れの有無、ボルト素形材の頭部のロックウエル硬さを調べた。また、ボルト素形材状態での引張強さを調べた。その結果を表２に示す。
【００３０】
【表２】
（実験２）
表１に示す鋼２の化学組成を有する鋼を、熱間圧延して直径１７．５ｍｍの線材コイルとした。これを１０４０℃で固溶化熱処理後、切断して長さ８１ｍｍのスラグとし、冷間鍛造によって、図２に示す４工程でＭ１６高力ボルト素形材に加工した。
【００３２】
冷間鍛造したボルト素形材は、直ちに実験１の場合と同様の徐冷バックに投入して徐冷却した。冷間鍛造による成形終了後の時間経過に伴うボルト素形材頭部の表面温度を測定した。成形終了後１５分経過したときのボルト素形材頭部の表面温度を表３に示す。徐冷却終了時のボルト素形材頭部表面の温度を測定した後、室温まで空冷した。比較のために鍛造加工後そのまま空冷したボルト素形材を作製した。転造によってＭ１６、ピッチ２ｍｍのねじを切り、その後５９５℃で析出硬化熱処理を行ってＭ１６高力ボルトを製作し、供試材とした。
【００３３】
前記供試材について、超音波探傷試験によるボルト素形材の頭部の内部割れの有無、ボルト素形材の頭部のロックウエル硬さを調べた。また、ボルト素形材状態での引張強さを調べた。その結果を表３に示す。
【００３４】
【表３】
（実験３）
表１に示す鋼３の化学組成を有する鋼を、熱間圧延して直径１６．５ｍｍの線材コイルとした。これを１０４０℃または９５０℃で固溶化熱処理後、引抜き加工して直径１６．２ｍｍの素材とした。該素材を、鍛造加工によって、図３に示す４工程でＭ１６ボルト素形材に加工した。鍛造加工としては、冷間鍛造、または素材を１５０℃に加熱して温間鍛造によって行った。
【００３６】
鍛造加工したボルト素形材は、直ちに実験１の場合と同様の徐冷バックまたはオイルバスに投入して徐冷却した。鍛造加工による成形終了後の時間経過に伴うボルト素形材頭部の表面温度を測定した。成形終了後１５分経過したときのボルト素形材頭部の表面温度を表４に示す。徐冷却終了時のボルト素形材頭部表面の温度を測定した後、室温まで空冷した。比較のために鍛造加工後そのまま空冷したボルト素形材を作成した。転造によってＭ１６、ピッチ２ｍｍのねじを切り、その後５９５℃で析出硬化熱処理を行ってＭ１６ボルトを製作し、供試材とした。
【００３７】
前記供試材について、超音波探傷試験によるボルト素形材の頭部の内部割れの有無、ボルト素形材の頭部のロックウエル硬さを調べた。また、供試材からの削り出し試験片によって引張強さを調べた。その結果を表４に示す。
【００３８】
【表４】
冷間鍛造または２００℃以下の温度に加熱して行う温間鍛造の鍛造加工によってボルト素形材に成形し、その終了後１５分経過したときのボルト素形材頭部の表面温度を測定した結果を表３および表４に示す。この結果によれば、本実施例における徐冷バックまたはオイルバスを使用した場合は、前記鍛造加工終了後空冷した場合に比べて、冷却速度が遅くなっていることが明らかである。
【００４０】
表１〜４から判るように、冷間鍛造または温間鍛造後に空冷するか、あるいは冷間鍛造後に徐冷却しても、徐冷却の終了温度が１５０℃よりも高い温度から放冷した比較例１のような場合には、ボルト素形材頭部に内部割れが検出されるものがあった。これに対して、本発明の実施例においては、析出硬化熱処理後のボルト素形材の頭部の内部割れは全く検出されない。また、本発明の実施例においては、高強度ステンレスボルトとして十分な硬さと引張強さとを示している。
（実験４）
表１に示す鋼１および鋼２の化学組成を有する鋼を、熱間圧延して直径１０ｍｍの線材コイルとした。これを１０４０℃または９５０℃で固溶化熱処理後、引抜き加工して直径９．８ｍｍの素材とした。該素材を、鍛造加工によって、図１に示す４工程でＭ１０ボルト素形材に加工した。鍛造加工としては、冷間鍛造、または前記素材を１５０℃に加熱して温間鍛造によって行った。
【００４１】
ボルト素形材は、鍛造加工終了後放冷し、所定の温度に達したら、予め所定の析出硬化熱処理温度に調整した熱処理炉に装入して析出硬化熱処理を施し、供試材とした。
前記供試材について、超音波探傷試験によるボルト素形材の頭部の内部割れの有無、ボルト素形材の頭部のロックウエル硬さを調べた。また、供試材からの削り出し試験片によって引張強さを調べた。その結果を表５に示す。
【００４２】
【表５】
（実験５）
表１に示す鋼２の化学組成を有する鋼を、熱間圧延して直径１７．５ｍｍの線材コイルとした。これを１０４０℃で固溶化熱処理後、切断して長さ８１ｍｍのスラグとし、冷間鍛造によって、図２に示す４工程でＭ１６高力ボルト素形材に加工した。
【００４４】
前記ボルト素形材は、冷間鍛造終了後放冷し、所定の温度に達したら、予め所定の析出硬化熱処理温度に調整した熱処理炉に装入して析出硬化熱処理を施して供試材とした。
前記供試材について、超音波探傷試験によるボルト素形材の頭部の内部割れの有無、ボルト素形材の頭部のロックウエル硬さを調べた。また、供試材からの削り出し試験片によって引張強さを調べた。その結果を表６に示す。
【００４５】
【表６】
表５および表６から判るように、冷間鍛造または温間鍛造後に１００℃よりも低い温度まで冷却した比較例においては、ボルト素形材頭部に内部割れが検出されるものがあった。これに対して、鍛造加工後１００℃以上の温度から析出硬化熱処理温度に移した本発明の実施例においては、析出硬化熱処理後のボルト素形材には頭部の内部割れは全く検出されない。また、本発明の実施例においては、高強度ステンレスボルトとして十分な硬さと引張強さとを示している。
【００４７】
【発明の効果】
以上のように、本発明によれば、予め固溶化熱処理を施したマルテンサイト系析出硬化型ステンレス鋼を冷間鍛造または温間鍛造によってボルト成形するとき、ボルト頭部の内部に生じる割れの発生を防止することができ、表面品質の優れた高強度ステンレスボルトを安価製造する方法を提供することができる。
【図面の簡単な説明】
【図１】本発明の実施例におけるボルト素形材の成形過程を示す工程図である。
【図２】本発明の実施例におけるボルト素形材の成形過程を示す第２の工程図である。
【図３】本発明の実施例におけるボルト素形材の成形過程を示す第３の工程図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-strength bolt excellent in corrosion resistance.
[0002]
[Prior art]
Recently, there is an increasing demand for bolts having high strength and corrosion resistance as bolts for joining steel structures for construction and other purposes. For such applications, bolts made of martensite precipitation hardening stainless steel such as JIS SUS630, which is relatively inexpensive and can also produce large diameter bolts, are used.
[0003]
The manufacturing process of the bolt includes a step of forging the wire and forming it into a bolt body having a predetermined bolt head shape. The forging methods include hot forging and cold forging. In the case of the hot working method, the forming process is easy, but there is a problem that it is difficult to maintain the dimensional accuracy and the surface quality. On the other hand, cold forging is suitable for mass production of products with excellent dimensional accuracy and surface quality, but is limited to materials with good cold workability.
[0004]
By the way, martensitic precipitation hardening stainless steel exhibits a martensitic structure in a solution heat treatment state and generally has a high hardness. For example, in SUS630, the hardness after solution heat treatment is about HRC35, and cold workability is inferior. . In SUS630, if the steel is annealed at a temperature of 620 to 800 ° C. to form a metal structure containing a large amount of austenite phase, a bolt head or the like can be formed by cold forging. In this case, in order to obtain a high-strength bolt, it is necessary to perform a solution heat treatment after forming the bolt, and then increase the strength by performing precipitation hardening heat treatment. In order to maintain the surface quality of the product, it is necessary to perform the solution heat treatment in a protective atmosphere, and therefore, the cost tends to increase.
[0005]
In SUS630, it is conceivable to improve cold workability by adjusting steel components such as keeping the C and N content low. By using the steel whose components are adjusted in this way, the bolt shape can be formed. However, even in this case, there is a case where an internal crack occurs in the bolt head, and this internal crack cannot be completely prevented.
[0006]
[Problems to be solved by the invention]
The present invention has been made in view of the above-mentioned present situation, and its purpose is to prevent the occurrence of cracks generated in the bolt head when martensitic precipitation hardening stainless steel is formed into a bolt shape. Thus, an object of the present invention is to provide a manufacturing method capable of supplying high-strength stainless steel bolts having excellent surface quality at low cost.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the method for producing a high-strength stainless steel bolt according to the present invention comprises (1) martensitic precipitation at a volume percentage of 90% or more by performing a solution heat treatment at 900 to 1100 ° C. After hardening hardened stainless steel is formed into a bolt shape material by cold forging or warm forging performed at a temperature of 200 ° C. or less, it is gradually cooled until the surface temperature of the bolt shape material becomes 150 ° C. or less. Thereafter, precipitation hardening heat treatment is performed at a temperature of 450 to 650 ° C.
(2) Perform a solution heat treatment at 900 to 1100 ° C. and heat the martensite precipitation hardening stainless steel with a volume percentage of 90% or more by cold forging or at a temperature of 200 ° C. or less. It is cooled after being formed into a bolt shape material by warm forging , and is heated before the surface temperature of the bolt shape material reaches a temperature lower than 100 ° C. in this cooling process, and precipitation hardening is performed at a temperature of 450 to 650 ° C. It is characterized by heat treatment.
(3) The method for producing a high-strength stainless steel bolt according to any one of (1) and (2) above, wherein the martensite precipitation hardening stainless steel has the following chemical composition.
[0008]
% By mass
C: 0.015% or less,
N: 0.015-0.050%,
Si: 1.0% or less,
Mn: 1.0% or less,
P: 0.040% or less,
S: 0.030% or less,
Cu: 1.5 to 5.0%,
Ni: 3.0-8.0%,
Cr: 13.0 to 16.5%,
Mo: 1.5% or less,
Nb: 0.1 to 0.5%
Residual Fe and inevitable impurity elements.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In the method for producing a high-strength stainless steel bolt according to the present invention, a martensitic precipitation hardening stainless steel that has been previously subjected to solution heat treatment is cold-forged and formed into a bolt shape material. Therefore, there is a great effect that it is not necessary to perform a solution heat treatment after bolt forming as in the prior art.
[0010]
Here, as a martensitic precipitation hardening type stainless steel, the steel matrix becomes martensite by solid solution heat treatment and becomes high strength, and the precipitation hardening element is dissolved in a solid solution. JIS SUS630 is mentioned as steel with which intensity | strength rises and it is preferable.
More preferably, by mass, C: 0.015% or less, N: 0.015 to 0.050%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.040% or less S: 0.030% or less, Cu: 1.5 to 5.0%, Ni: 3.0 to 8.0%, Cr: 13.0 to 16.5%, Mo: 1.5% or less, Nb: steel containing 0.1 to 0.5% and remaining Fe and inevitable impurity elements.
[0011]
The reason why the chemical composition of the preferred steel to which the present invention is applied will be described below.
C: 0.015% or less C is an element that greatly affects the hardness and deformability during solution heat treatment. The lower the content, the lower the hardness and the more deformable. In the steel of the present invention, the C content is preferably as low as possible, but the upper limit is made 0.015% in consideration of the manufacturing cost. Preferably, the C content is 0.008% or less.
[0012]
N: 0.015 to 0.050%
N has the same tendency as C as to the influence on the hardness and deformability during the solution heat treatment of steel, but the degree of the influence is much smaller than that of C. It has a toughness at peak aging and a strength retention effect at overaging, so it is contained in an amount of 0.015% or more. Preferably it is 0.030% or more. However, when N is contained excessively, the hardness of the steel in the solution heat treatment state is increased, and the γ phase is stabilized more than necessary, so that a large amount of unstable γ phase remains during the solution heat treatment, and a large amount during overaging. Since the reverse transformation γ phase is generated and the strength is lowered, the upper limit of the N content is set to 0.050%.
[0013]
Si: 1.0% or less Si is added as a deoxidizer for steel. However, when the content is excessive, the hardness after the solution heat treatment is increased. In addition, the amount of δ-ferrite is excessive and the hot workability of the steel is impaired, so the upper limit of the content is set to 1.0%.
Mn: 1.0% or less Mn acts as a deoxidizer for steel and is also effective as an alternative element for expensive Ni. However, if the Mn content is excessive, the Ms point of the steel is lowered and the strength at the time of overaging is lowered, so the upper limit of the content is made 1.0%.
[0014]
P: 0.040% or less P is liable to segregate at the grain boundaries of steel and impairs hot workability. Moreover, since cold workability is also impaired, it is preferable that the P content is low. However, the upper limit of the P content that can be allowed in consideration of the manufacturing cost is set to 0.040%.
S: 0.030% or less S needs to reduce the content because it significantly impairs corrosion resistance and also harms cold workability. The allowable limit is 0.030%, but the content is preferably 0.010% or less.
[0015]
Cu: 1.5-5.0%
Cu is added to harden the steel by forming an ε phase during aging heating. In order to exert the above effect, the content of 1.5% is required. However, if it is excessively contained, the hot workability of the steel is impaired, so the upper limit of the content is set to 5.0%.
Ni: 3.0-8.0%
Ni is a strong austenite-forming element, suppresses the formation of δ-ferrite, and improves the corrosion resistance of steel, so 3.0% or more is contained. However, if contained excessively, the Ms point of the steel is lowered to increase the amount of retained austenite and the strength after precipitation hardening heat treatment is impaired, so the upper limit of the content is made 8.0%.
[0016]
Cr: 13.0 to 16.5%
Cr is added in an amount of 13.0% or more to ensure corrosion resistance. However, if it is excessively contained, a large amount of δ-ferrite is generated, hot workability is impaired, and the strength of the steel is remarkably reduced, so the upper limit of the content is set to 16.5%.
Mo: 1.5% or less Mo may be added to improve the corrosion resistance. However, it is a ferrite stabilizing element, and if it is contained in a large amount, δ-ferrite is formed and the hot workability of steel is impaired, so the upper limit of the content is made 1.5%.
[0017]
Nb: 0.1 to 0.5%
Nb forms carbonitrides, refines the crystal grains of the steel, lowers the hardness of the steel after solution heat treatment, and improves cold workability. Moreover, it adds in order to improve the toughness after precipitation hardening process, especially the toughness after peak aging treatment. In order to acquire the said effect, 0.1% or more of Nb content rate is required. However, if Nb is contained in a large amount, the amount of δ-ferrite is increased and the hot workability of the steel is impaired, so the upper limit of the content is set to 0.5%.
[0018]
In the method for producing a high-strength stainless steel bolt according to the present invention, the solution heat treatment is performed so that the steel structure is martensite having high strength and the precipitation hardening element Cu is dissolved in the steel matrix. If the temperature of the solution heat treatment is less than 900 ° C., the precipitation hardening element is not sufficiently dissolved, or the precipitation hardening element is dissolved, but the carbide, nitride, carbonitride, etc. are sufficiently dissolved. Therefore, high strength martensite cannot be obtained. Further, when the temperature of the solution heat treatment exceeds 1100 ° C., the solid solution of carbide, nitride, carbonitride, etc. proceeds too much and the steel crystal grains become coarse, or the steel structure after the solution heat treatment becomes a ferrite phase. As a result, the strength of the steel decreases. Therefore, the temperature of the solution heat treatment is set to 900 to 1100 ° C.
[0019]
In the method for producing a high-strength stainless steel bolt of the present invention, the martensitic precipitation hardening stainless steel is subjected to a solution heat treatment to obtain a metal structure containing a martensite phase having a volume ratio of 90% or more. The main metal structures contained in the steel after the solution heat treatment are austenite and ferrite, but the austenite and ferrite have lower strength than martensite. Therefore, in order to maintain the strength of the steel, the martensite needs to be 90% by volume or more.
[0020]
In the method for producing a high-strength stainless steel bolt of the present invention, the steel is cold-forged and formed into a bolt shape material. According to warm forging in which cold working is performed by heating to a temperature lower than the recrystallization temperature of steel, deformation resistance during forging can be reduced. In the case of this steel, heating to a temperature exceeding 200 ° C. When forging, the bolt head cracks, so the heating temperature during warm forging needs to be 200 ° C. or less.
[0021]
In the forging process described above, heat is generated due to strain energy inside the work material and at the surface layer part due to friction between the work material and a work tool such as a die or a punch to raise the temperature. In the first embodiment of the method for producing a high-strength stainless steel bolt of the present invention, the bolt shape material that has been heated by the forging process has a surface temperature of at least 150 ° C. or less after the forging process. Slowly cool until Then, precipitation hardening heat processing is performed by heating to a temperature of 450-650 degreeC.
[0022]
Here, the slow cooling means that the cooling of the bolt shape material is positively delayed by using a heat generating agent, or by covering the bolt shape material after forging with a heat insulating material or a heat retaining medium. Take measures and cool at a cooling rate slower than that of cooling. It is a preferable method to cool the bolt shaped material by immersing it in a suitably heated air or oil bath.
[0023]
As described above, after the forging process, the internal temperature generated at the head of the bolt shape material can be prevented by gradually cooling the heated temperature of the bolt shape material to a temperature of 150 ° C. or less.
In the case of the first embodiment of the present invention described above, there is an advantage that after the forging process, the bolt shaped material is once cooled to room temperature and then subjected to post-processing such as threading.
[0024]
In the second embodiment of the method for producing a high-strength stainless steel bolt of the present invention, the bolt shape material that has been heated by the forging process is heated before reaching a temperature lower than 100 ° C., and 450 to 650 ° C. Bring to the precipitation hardening heat treatment temperature. The reason why the lower limit temperature for cooling the bolt shaped material after forging is set to 100 ° C. is that internal cracking occurs at the head of the bolt shaped material when cooled to a temperature lower than 100 ° C.
[0025]
In the case of the second embodiment of the present invention, there is no limitation on the cooling rate at a temperature of 100 ° C. or higher, so that there is an advantage that the bolt shaped material after forging can be cooled in a short time.
In the martensitic precipitation hardening stainless steel used in the method for producing a high-strength stainless steel bolt of the present invention, the precipitation hardening heat treatment requires a long time for precipitation hardening when the temperature of the precipitation hardening heat treatment is 450 ° C. or less, and is overaged at 650 ° C. or more. It becomes soft. High strength can be obtained by performing precipitation hardening heat treatment at a temperature of 450 to 650 ° C.
[0026]
【Example】
(Experiment 1)
Steel 1 and steel 2 having the chemical composition shown in Table 1 are melted, processed into a wire coil having a diameter of 10 mm by hot rolling, and then subjected to a solution heat treatment at 1040 ° C. or 950 ° C., and drawn to a diameter of 9.8 mm. And made the material. The material was formed into an M10 hexagon bolt shaped material by forging process in four steps shown in FIG. As the forging process, cold forging or warm forging was performed by heating the material to 150 ° C.
[0027]
[Table 1]
The forged bolt material was immediately put into a slow cooling bag or oil bath and gradually cooled. The slow cooling bag is a steel box lined with a heat insulating material, and heated beforehand with hot air before inserting the bolt material. The oil bath is a hardened oil tank maintained at 120 ° C. After the start of gradual cooling, after holding for an appropriate time, it was taken out from the gradual cooling apparatus, the temperature of the head surface of the bolt shaped material was measured with a contact thermometer, and then cooled by air to room temperature. For comparison, a bolt shape material that was air-cooled as it was after forging was produced. Predetermined precipitation hardening heat treatments were applied to these bolt shaped materials to obtain test materials.
[0029]
About the said test material, the presence or absence of the internal crack of the head of a bolt shape material by the ultrasonic flaw test and the Rockwell hardness of the head of a bolt shape material were investigated. In addition, the tensile strength in the bolt shape was examined. The results are shown in Table 2.
[0030]
[Table 2]
(Experiment 2)
Steel having the chemical composition of steel 2 shown in Table 1 was hot-rolled to form a wire coil having a diameter of 17.5 mm. This was subjected to a solution heat treatment at 1040 ° C. and then cut into an 81 mm long slag, which was processed into a M16 high-strength bolt base material by cold forging in four steps shown in FIG.
[0032]
The cold forged bolt base material was immediately put into the same slow cooling bag as in Experiment 1 and gradually cooled. The surface temperature of the head portion of the bolt shape member with the passage of time after the end of forming by cold forging was measured. Table 3 shows the surface temperature of the bolt shape head when 15 minutes have elapsed after the completion of molding. After measuring the temperature of the head of the bolt shape member at the end of the slow cooling, it was cooled to room temperature. For comparison, a bolt shape material that was air-cooled as it was after forging was prepared. A M16 thread with a pitch of 2 mm was cut by rolling, followed by precipitation hardening heat treatment at 595 ° C. to produce an M16 high-strength bolt as a test material.
[0033]
About the said test material, the presence or absence of the internal crack of the head of a bolt shape material by the ultrasonic flaw test and the Rockwell hardness of the head of a bolt shape material were investigated. In addition, the tensile strength in the bolt shape was examined. The results are shown in Table 3.
[0034]
[Table 3]
(Experiment 3)
Steel having the chemical composition of steel 3 shown in Table 1 was hot-rolled to form a wire coil having a diameter of 16.5 mm. This was subjected to a solution heat treatment at 1040 ° C. or 950 ° C. and then drawn to obtain a material having a diameter of 16.2 mm. The raw material was processed into an M16 bolt shape material by forging process in four steps shown in FIG. As forging, cold forging or warm forging was performed by heating the material to 150 ° C.
[0036]
The forged bolt shaped material was immediately put into the same slow cooling bag or oil bath as in Experiment 1 and gradually cooled. The surface temperature of the head part of the bolt shape member with the passage of time after completion of the forging process was measured. Table 4 shows the surface temperature of the bolt shape head when 15 minutes have elapsed after the completion of molding. After measuring the temperature of the head of the bolt shape member at the end of the slow cooling, it was cooled to room temperature. For comparison, a bolt shape material that was air-cooled as it was after forging was prepared. M16 bolts with a pitch of 2 mm were cut by rolling, followed by precipitation hardening heat treatment at 595 ° C. to produce M16 bolts, which were used as test materials.
[0037]
About the said test material, the presence or absence of the internal crack of the head of a bolt shape material by the ultrasonic flaw test and the Rockwell hardness of the head of a bolt shape material were investigated. Moreover, the tensile strength was investigated with the test piece cut out from the test material. The results are shown in Table 4.
[0038]
[Table 4]
A bolt forging was formed by cold forging or forging by warm forging performed by heating to a temperature of 200 ° C. or less, and the surface temperature of the bolt shape head was measured when 15 minutes had elapsed after the completion. The results are shown in Table 3 and Table 4. According to this result, it is clear that when the slow cooling bag or the oil bath in this example is used, the cooling rate is slower than when the air cooling is performed after the forging process is completed.
[0040]
As can be seen from Tables 1 to 4, even after cold forging or warm forging, even after cooling forcibly after cold forging, a comparative example in which the end temperature of slow cooling was allowed to cool from a temperature higher than 150 ° C. In the case of 1, there was one in which an internal crack was detected at the head of the bolt base material. On the other hand, in the embodiment of the present invention, no internal cracks are detected in the head portion of the bolt body after the precipitation hardening heat treatment. Moreover, in the Example of this invention, sufficient hardness and tensile strength are shown as a high intensity | strength stainless steel volt | bolt.
(Experiment 4)
Steel having the chemical composition of steel 1 and steel 2 shown in Table 1 was hot-rolled to form a wire coil having a diameter of 10 mm. This was subjected to a solution heat treatment at 1040 ° C. or 950 ° C. and then drawn to obtain a material having a diameter of 9.8 mm. The material was processed into an M10 bolt shape material by four processes shown in FIG. 1 by forging. The forging process was performed by cold forging or warm forging by heating the material to 150 ° C.
[0041]
The bolt shape material was allowed to cool after completion of the forging process, and when it reached a predetermined temperature, it was placed in a heat treatment furnace adjusted in advance to a predetermined precipitation hardening heat treatment temperature and subjected to precipitation hardening heat treatment to obtain a test material.
About the said test material, the presence or absence of the internal crack of the head of a bolt shape material by the ultrasonic flaw test and the Rockwell hardness of the head of a bolt shape material were investigated. Moreover, the tensile strength was investigated with the test piece cut out from the test material. The results are shown in Table 5.
[0042]
[Table 5]
(Experiment 5)
Steel having the chemical composition of steel 2 shown in Table 1 was hot-rolled to form a wire coil having a diameter of 17.5 mm. This was subjected to a solution heat treatment at 1040 ° C. and then cut into an 81 mm long slag, which was processed into a M16 high-strength bolt base material by cold forging in four steps shown in FIG.
[0044]
The bolt shape material is allowed to cool after the end of cold forging, and when the temperature reaches a predetermined temperature, the bolt shape material is placed in a heat treatment furnace adjusted to a predetermined precipitation hardening heat treatment temperature in advance and subjected to precipitation hardening heat treatment. did.
About the said test material, the presence or absence of the internal crack of the head of a bolt shape material by the ultrasonic flaw test and the Rockwell hardness of the head of a bolt shape material were investigated. Moreover, the tensile strength was investigated with the test piece cut out from the test material. The results are shown in Table 6.
[0045]
[Table 6]
As can be seen from Tables 5 and 6, in the comparative example in which cooling was performed to a temperature lower than 100 ° C. after cold forging or warm forging, there was a case where an internal crack was detected in the head portion of the bolt shape material. On the other hand, in the example of the present invention which was shifted from the temperature of 100 ° C. or higher after the forging to the precipitation hardening heat treatment temperature, no internal cracks in the head were detected in the bolt shaped material after the precipitation hardening heat treatment. Moreover, in the Example of this invention, sufficient hardness and tensile strength are shown as a high intensity | strength stainless steel volt | bolt.
[0047]
【The invention's effect】
As described above, according to the present invention, when martensitic precipitation hardened stainless steel that has been subjected to solution heat treatment in advance is bolt-formed by cold forging or warm forging, occurrence of cracks that occur inside the bolt head Thus, it is possible to provide a method for inexpensively manufacturing a high-strength stainless steel bolt having excellent surface quality.
[Brief description of the drawings]
FIG. 1 is a process diagram showing a process of forming a bolt shape material in an embodiment of the present invention.
FIG. 2 is a second process diagram showing a process of forming a bolt shape material in the embodiment of the present invention.
FIG. 3 is a third process diagram showing a process of forming a bolt shape material in the embodiment of the present invention.
Claims (3)
質量％で、
Ｃ ：０．０１５％以下、
Ｎ ：０．０１５〜０．０５０％、
Ｓｉ：１．０％以下、
Ｍｎ：１．０％以下、
Ｐ ：０．０４０％以下、
Ｓ ：０．０３０％以下、
Ｃｕ：１．５〜５．０％、
Ｎｉ：３．０〜８．０％、
Ｃｒ：１３．０〜１６．５％、
Ｍｏ：１．５％以下、
Ｎｂ：０．１〜０．５％、
残余Ｆｅおよび不可避的不純物元素。The method for producing a high-strength stainless steel bolt according to any one of claims 1 and 2, wherein the martensite precipitation hardening stainless steel has the following chemical composition.
% By mass
C: 0.015% or less,
N: 0.015-0.050%,
Si: 1.0% or less,
Mn: 1.0% or less,
P: 0.040% or less,
S: 0.030% or less,
Cu: 1.5 to 5.0%,
Ni: 3.0-8.0%,
Cr: 13.0 to 16.5%,
Mo: 1.5% or less,
Nb: 0.1 to 0.5%
Residual Fe and inevitable impurity elements.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP12815596A JP3776507B2 (en) | 1996-05-23 | 1996-05-23 | Manufacturing method of high-strength stainless steel bolts |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP12815596A JP3776507B2 (en) | 1996-05-23 | 1996-05-23 | Manufacturing method of high-strength stainless steel bolts |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH09314276A JPH09314276A (en) | 1997-12-09 |
| JP3776507B2 true JP3776507B2 (en) | 2006-05-17 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP12815596A Expired - Fee Related JP3776507B2 (en) | 1996-05-23 | 1996-05-23 | Manufacturing method of high-strength stainless steel bolts |
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Families Citing this family (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6109851A (en) * | 1999-01-13 | 2000-08-29 | Illinois Tool Works Inc. | Screws having selected heat treatment and hardening |
| TWI394848B (en) | 2007-10-10 | 2013-05-01 | Nippon Steel & Sumikin Sst | Two-phase stainless steel wire rod, steel wire, bolt and manufacturing method thereof |
| JP5541575B2 (en) * | 2010-03-18 | 2014-07-09 | 新日鐵住金ステンレス株式会社 | Stainless steel wire rod for warm forging and its plastic working method |
| JP6091046B2 (en) | 2010-11-10 | 2017-03-08 | 株式会社トープラ | Aluminum alloy bolt manufacturing method and aluminum alloy bolt |
| CN103866198B (en) * | 2012-12-17 | 2015-10-14 | 中国科学院金属研究所 | A kind of surgical operation precipitation hardening of martensitic stainless steel and thermal treatment process thereof |
| CN103398067B (en) * | 2013-07-17 | 2015-08-05 | 春雨(东莞)五金制品有限公司 | A kind of screw bolt manufacture process |
| JP7320936B2 (en) * | 2018-11-16 | 2023-08-04 | 日鉄ステンレス株式会社 | bar steel |
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1996
- 1996-05-23 JP JP12815596A patent/JP3776507B2/en not_active Expired - Fee Related
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| JPH09314276A (en) | 1997-12-09 |
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