JP3718369B2 - Steel for high strength bolt and method for producing high strength bolt - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼及び高強度ボルトの製造方法に関するものである。
【０００２】
【従来の技術】
自動車や産業機械の高性能化、軽量化、また建築構造物の大型化に伴い、引張強さが１２００ＭＰａ以上の高強度ボルト用鋼の開発が要求されてきている。
【０００３】
現在一般に高強度ボルト用鋼として使用されている鋼種はＪＩＳに規定されているＳＣＭ４３５、ＳＣＲ４３５等の低合金構造用鋼であり、焼入れ、焼もどし処理を施すことによって製造されている。しかし、これらの鋼種は、引張強さが１２００ＭＰａを超えると耐遅れ破壊特性が急激に低下し、ボルトの使用中に遅れ破壊による破断を生じる危険が増大するため、このレベル以上の高強度化は実用上不可能であった。
【０００４】
また最近、ボルトを降伏点以上に締めつける塑性域締結法の採用が拡大している。塑性域締結は従来の弾性域締結と比べて高い軸力を得ることができることに加え、軸力のバラツキが小さく、精度の高い、安定した締付けが可能となるという特徴がある。塑性域締結用のボルトに要求される特性とは、（１）降伏強さのバラツキがボルトの締結軸力のバラツキに直結するため、降伏強さのバラツキが少ないこと、（２）再締付け性の観点から、降伏点から最大軸力に至るまでの伸びが大きいこと、等が挙げられる。
【０００５】
降伏強さのバラツキは、通常の鋼種の場合、焼もどし温度のバラツキの影響が大きい。例えば、通常の工業生産炉では±２０℃程度の温度バラツキを生じる可能性があるが、通常の鋼種の場合、このバラツキによって１００ＭＰａ以上の降伏強さのバラツキを生じる可能性がある。このバラツキを低減するには通常の操業範囲よりも狭幅の温度管理が必要となり、実際の工業生産炉での生産上課題を残しているが、これに対して材料面から検討した例は見当たらない。
【０００６】
さらに伸びは、通常の鋼種の場合、高強度化するほど伸びが不可避的に低下するため、ボルト形状の変更等で対応する必要があり、塑性域締結の適用の拡大、ボルトの汎用性に課題を残している。もちろん、高強度ボルトを塑性域締結に供した場合、高い軸力が負荷されるため、従来よりも遅れ破壊特性に優れた材料である必要がある。しかしながら、塑性域締結に適した高強度ボルト用鋼を提案している例は見当たらず、材料面からの検討はほとんどされていない。
【０００７】
耐遅れ破壊特性の改善を目的とした高強度ボルト用鋼は、例えば、特開昭５８−１１７８５６号公報、特開昭６２−１９９７５１号公報、特開昭６１−２２３１６８号公報、特開平３−２４３７４５号公報、特許第２６７０９３７号公報に開示されている。しかし、これらの鋼種は塑性域締結を考慮していないため、上述したような特性を得ることができない。特開昭５８−１１７８５６号公報には、鋼中の不純物を低減して延性（伸び、絞り）、耐遅れ破壊特性を向上する方法が述べられているが、Ｃ、Ｃｒ、Ｍｏ、Ｖの量が最適化されていないため降伏強さのバラツキを低減することができないし、伸び、耐遅れ破壊特性の改善も今一歩である。特許第２６７０９３７号公報には、成分系、熱処理条件を限定することによって耐遅れ破壊特性、疲労特性を向上する方法が述べられているが、この場合も上記と同じ問題がある。
【０００８】
以上述べた通り、耐遅れ破壊特性に優れ、なおかつ塑性域締結に適した高強度ボルト用鋼は現状では見あたらない。
【０００９】
【発明が解決しようとする課題】
本発明は以上のような課題を解決し、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼及び高強度ボルトの製造方法を提供することを目的とする。詳細には、高強度においても現在１０００ＭＰａ級のボルトとして多く使われているＳＣＭ４３５よりも耐遅れ破壊特性に優れ、焼もどし温度が多少変動しても降伏強さの変化が小さく、良好な伸びを示す、引張強さ１２００ＭＰａ以上の高強度ボルト用鋼及び高強度ボルトの製造方法を提供することを目的とする。
【００１０】
【課題を解決するための手段】
本発明の要旨は、以下の通りである。
【００１１】
（１） 重量％で、
Ｃ：０．１５〜０．４５％、
Ｍｎ：０．４０超〜１．５０％、
Ｃｒ：０．３０〜２．００％、
Ｍｏ：０．１０〜０．３５％、
Ｖ：０．２０超〜０．４０％、
Ａｌ：０．０１０〜０．１００％
を含有し、かつＣｒ、Ｍｏ、Ｖを１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係を満足するバランスとし、
Ｓｉ：０．１０％以下（０％を含む）、
Ｐ：０．０１５％以下（０％を含む）、
Ｓ：０．０１５％以下（０％を含む）
に各々制限し、残部がＦｅ及び不可避的不純物よりなることを特徴とする、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼。
【００１２】
（２） 重量％で、
Ｃ：０．１５〜０．４５％、
Ｍｎ：０．４０超〜１．５０％、
Ｃｒ：０．３０〜２．００％、
Ｍｏ：０．１０〜０．３５％、
Ｖ：０．２０超〜０．４０％、
Ａｌ：０．０１０〜０．１００％
を含有し、さらに、
Ｎｂ：０．００５〜０．１００％、
Ｔｉ：０．００５〜０．１００％
のうちの１種または２種を含有し、かつＣｒ、Ｍｏ、Ｖを１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係を満足するバランスとし、
Ｓｉ：０．１０％以下（０％を含む）、
Ｐ：０．０１５％以下（０％を含む）、
Ｓ：０．０１５％以下（０％を含む）
に各々制限し、残部がＦｅ及び不可避的不純物よりなることを特徴とする、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼。
【００１３】
（３） 重量％で、
Ｃ：０．１５〜０．４５％、
Ｍｎ：０．４０超〜１．５０％、
Ｃｒ：０．３０〜２．００％、
Ｍｏ：０．１０〜０．３５％、
Ｖ：０．２０超〜０．４０％、
Ａｌ：０．０１０〜０．１００％
を含有し、さらに、
Ｂ：０．０００５〜０．００５０％
を含有し、かつＣｒ、Ｍｏ、Ｖを１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係を満足するバランスとし、
Ｓｉ：０．１０％以下（０％を含む）、
Ｐ：０．０１５％以下（０％を含む）、
Ｓ：０．０１５％以下（０％を含む）
に各々制限し、残部がＦｅ及び不可避的不純物よりなることを特徴とする、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼。
【００１４】
（４） 重量％で、
Ｃ：０．１５〜０．４５％、
Ｍｎ：０．４０超〜１．５０％、
Ｃｒ：０．３０〜２．００％、
Ｍｏ：０．１０〜０．３５％、
Ｖ：０．２０超〜０．４０％、
Ａｌ：０．０１０〜０．１００％
を含有し、さらに、
Ｎｂ：０．００５〜０．１００％、
Ｔｉ：０．００５〜０．１００％
のうちの１種または２種を含有し、さらに、
Ｂ：０．０００５〜０．００５０％
を含有し、かつＣｒ、Ｍｏ、Ｖを１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係を満足するバランスとし、
Ｓｉ：０．１０％以下（０％を含む）、
Ｐ：０．０１５％以下（０％を含む）、
Ｓ：０．０１５％以下（０％を含む）
に各々制限し、残部がＦｅ及び不可避的不純物よりなることを特徴とする、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼。
【００１５】
（５） 前記（１）〜（４）のいずれかの高強度ボルト用鋼を所望の形状に成形後、ＡC3以上の温度に加熱した後に焼入れ処理を行い、５００〜６５０℃の温度範囲で焼きもどすことを特徴とする、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルトの製造方法。
【００１６】
【発明の実施の形態】
本発明者らは、遅れ破壊特性に及ぼす各種因子について以下の知見を得た。すなわち、（１）Ｃｒ、Ｍｏ、Ｖをある成分範囲で複合添加して５００℃以上の温度域で焼きもどすことによって低温焼もどし脆性域を回避することができる。また、焼もどし時に粒界に析出するセメンタイトの形態を球状化して粒界の結合力を増加し、粒界割れを防止することができる。（２）特定量のＶを添加することによって、焼もどし時にマトリックス中に微細に析出するＶ炭窒化物が水素の粒内トラップサイトとなり、粒界に集積する水素の量を低減し、耐遅れ破壊特性が大幅に向上する。（３）粒界に偏析する不純物であるＰ、Ｓ量をある量以下に規制することによって旧オーステナイト粒界の強化が図られ、耐遅れ破壊特性が向上する。
【００１７】
また、本発明者らは塑性域締結に必要な諸特性についても材料面で以下の知見を得た。すなわち、（１）Ｃ、Ｃｒ、Ｍｏ、Ｖの添加量をある成分範囲に限定し、なおかつＣｒ、Ｍｏ、Ｖを１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係を満足するバランスとすることによって、５００℃〜６２０℃の焼もどし温度範囲で焼もどし軟化と焼もどし軟化抵抗、焼もどし二次硬化が相殺され、引張強さ、０．２％耐力（降伏強さ）の変動が非常に小さく、平坦な焼もどし性能を得ることができる。このことにより、焼もどし温度が多少変動してもボルトの降伏強さのバラツキを小さく抑えることができる。（２）Ｃｒ、Ｍｏ、Ｖを上記の範囲で複合添加し、５００℃以上の高温で焼きもどしを行なうことによって高い強度と良好な伸びを両立できる。
【００１８】
さらに、フェライトの固溶強化元素であるＳｉを極力低減することによってＣｒ、Ｍｏ、Ｖ添加による冷間鍛造性の低下を補い、ボルトの冷間鍛造性を損なうことなく高強度化を図ることができる。
【００１９】
以下、本発明について詳細に説明する。
【００２０】
Ｃ：Ｃは強度を得るために有効な元素であるため０．１５％以上添加するが、０．４５％を超えて添加すると冷間鍛造性、靭性が低下するので、０．１５〜０．４５％の範囲にする必要がある。好適範囲は０．２０〜０．４０％である。
【００２１】
Ｍｎ：Ｍｎは焼入れ性を向上するのに有効な元素であるため０．４０％を超えて添加するが、１．５０％を超えて添加すると耐遅れ破壊特性、冷間鍛造性が低下するので、０．４０超〜１．５０％の範囲にする必要がある。好適範囲は０．５０〜０．８０％である。
【００２２】
Ｃｒ：Ｃｒは焼入れ性を向上するのに有効な元素であり、かつ鋼に焼もどし軟化抵抗を付与する効果があるため０．３０％以上添加するが、２．００％を超えて添加すると冷間鍛造性が低下するので、０．３０〜２．００％の範囲にする必要がある。好適範囲は１．００〜１．４０％である。
【００２３】
Ｍｏ：Ｍｏは焼もどし時に顕著な二次硬化を起こし、高温焼もどしを可能とすることによって耐遅れ破壊特性を向上させる元素である。また、高温焼もどしによって強度−延性バランスが向上し、高い伸びを得ることができるため０．１０％以上添加するが、０．３５％を超えて添加すると冷間鍛造性が低下するとともに、二次硬化量が過大となり、平坦な焼もどし特性を得ることができない。さらに、焼入れ加熱時に合金炭化物がマトリックスに固溶し難くなり、粗大な未溶解炭化物の量が多くなることによって伸びが低下し、塑性域締結に不適となるので、０．１０〜０．３５％の範囲にする必要がある。好適範囲は０．２０〜０．３５％である。
【００２４】
Ｖ：Ｖは旧オーステナイト結晶粒を特に顕著に微細化する効果があるとともに、焼もどし時に顕著な二次硬化を起こし、高温焼もどしを可能とすることによって耐遅れ破壊特性を向上させる元素である。また、高温焼もどしによって強度−延性バランスが向上し、高い伸びを得ることができる。さらに、焼もどし時にマトリックス中に微細に析出するＶ炭窒化物が水素の粒内トラップサイトとなり、粒界に集積する水素の量を低減し、耐遅れ破壊特性を大幅に向上させる元素であるので０．２０％を超えて添加するが、０．４０％を超えて添加すると効果が飽和するのみならず冷間鍛造性が低下するとともに、二次硬化量が過大となり、平坦な焼もどし特性を得ることができない。さらに、焼入れ加熱時に合金炭化物がマトリックスに固溶し難くなり、粗大な未溶解炭化物の量が多くなることによって伸びが低下し、塑性域締結に不適となるので、０．２０超〜０．４０％の範囲にする必要がある。好適範囲は０．２５〜０．３５％である。
【００２５】
Ａｌ：Ａｌは鋼の脱酸に必要な元素であるとともに、窒化物を形成して旧オーステナイト粒を微細化する効果があるので０．０１０％以上添加するが、０．１００％を超えて添加すると効果が飽和するのみならずアルミナ系介在物が増加し、靭性が低下するので、０．０１０〜０．１００％の範囲にする必要がある。好適範囲は０．０２０〜０．０５０％である。
【００２６】
Ｎｂ：ＮｂはＡｌ、Ｔｉ、Ｖと同様に、結晶粒を微細化する効果があるとともに、耐遅れ破壊特性を向上する効果があるので０．００５％以上添加するのが好ましいが、０．１００％を超えて添加すると効果が飽和するのみならず冷間鍛造性が低下するので、添加する場合は０．００５〜０．１００％の範囲にする必要がある。好適範囲は０．０１０〜０．０５０％である。
【００２７】
Ｔｉ：ＴｉはＡｌ、Ｎｂ、Ｖと同様に、結晶粒を微細化する効果があるとともに、鋼中の固溶Ｎを窒化物として固定し、耐遅れ破壊特性を向上する効果があるので０．００５％以上添加するのが好ましいが、０．１００％を超えて添加すると効果が飽和するのみならず冷間鍛造性が低下するので、添加する場合は０．００５〜０．１００％の範囲にする必要がある。好適範囲は０．０１０〜０．０５０％である。
【００２８】
Ｂ：Ｂは少量の添加で焼入れ性を向上する効果があるとともに、旧オーステナイト粒界に偏析して粒界を強化し、耐遅れ破壊特性を向上する効果があるので０．０００５％以上添加するのが好ましいが、０．００５０％を超えて添加すると効果が飽和するので、添加する場合は０．０００５〜０．００５０％の範囲にする必要がある。好適範囲は０．００１０〜０．００３０％である。
【００２９】
Ｓｉ：Ｓｉは鋼の脱酸に必要な元素であるが、０．１０％を超えて添加すると冷間鍛造性が顕著に低下するので０．１０％以下に制限する必要がある。好適範囲は０．０８％以下である。
【００３０】
Ｐ：Ｐは旧オーステナイト粒界に偏析して粒界を脆化させ、耐遅れ破壊特性を顕著に低下させる効果があるので０．０１５％以下に制限する必要があり、極力低減すべきである。好適範囲は０．０１０％以下である。
【００３１】
Ｓ：Ｓは旧オーステナイト粒界に偏析して粒界を脆化させ、耐遅れ破壊特性を顕著に低下させる効果があるので０．０１５％以下に制限する必要があり、極力低減すべきである。好適範囲は０．０１０％以下である。
【００３２】
Ｃｒ、Ｍｏ、Ｖ添加バランス：本発明ではＣｒ、Ｍｏ、Ｖの添加量を１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係式を満足するバランスとする。上記のバランスにすることによって、５００℃〜６５０℃の焼もどし温度範囲で焼もどし軟化と焼もどし軟化抵抗、焼もどし二次硬化が相殺され、引張強さ、０．２％耐力（降伏強さ）の変動が非常に小さく、平坦な焼もどし性能を得ることができる。このことにより、焼もどし温度が多少変動してもボルトの降伏強さのバラツキを小さく抑えることができる。しかし、上記の式の値が下限より低くなると焼もどし軟化抵抗、焼もどし二次硬化が不十分となり、平坦な焼もどし性能を得ることができず、上限より高くなると焼きもどし二次硬化が過大となり、平坦な焼もどし性能を得ることができないのみならず、素材強度の上昇を招き、冷間鍛造性も低下するため、上記範囲にする必要がある。好適範囲は１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜１．９０である。
【００３３】
本発明は二次加工工程を特に規定していないが、製造工程中に冷間鍛造工程が入るものについては冷間鍛造性を向上させるため、熱間圧延後の素材に焼鈍、または球状化焼鈍処理を施しても良い。また、素材の寸法精度が必要なボルトの場合は、冷間鍛造の前に伸線を行なうのが一般的である。
【００３４】
上述した成分のボルト用鋼が最も効果を発揮するのは、以下に説明するボルトの製造方法においてである。
【００３５】
上述した成分のボルト用鋼を鍛造、切削等によって所望のボルト形状に成形した後、鋼に強度を付与するため、ＡC3点以上の温度に加熱した後、水冷または油冷によって焼入れ処理を行う。加熱温度が高すぎると結晶粒の粗大化を招き、靭性及び耐遅れ破壊特性の劣化を招き、また、操業面からは熱処理炉の炉体、及び付属部品の損傷が顕著になり、製造コストが上昇するため、あまり高い温度に加熱するのは好ましくないため、焼入れ加熱温度を８５０〜９５０℃とするのが好適である。
【００３６】
鋼に所定の強度および靱性、延性を付与するために焼入れ後に焼もどしを行う必要がある。焼もどしは、一般に１５０℃〜ＡC1点の温度範囲で行われるが、本発明では５００〜６５０℃の温度範囲に限定する必要がある。その理由は、５００℃以下では粒界に析出するセメンタイトの形態を球状化して粒界の結合力を増加することができず、また、水素のトラップサイトとなるＶ炭窒化物の析出が不十分となり、耐遅れ破壊特性の改善ができないこと、５００℃以下では塑性域締結用ボルトの必要特性である伸び値の改善ができないこと、６５０℃以上では軟化が著しく、引張強さ１２００ＭＰａ以上の高強度を得られないこと、さらに、５００〜６５０℃の温度範囲以外では平坦な焼もどし性能を得ることができず、焼もどし温度のバラツキに起因する降伏強さのバラツキを低減することができないことのためである。好適範囲は５２５〜６２５℃である。
【００３７】
【実施例】
以下に、実施例により本発明をさらに説明する。
【００３８】
表１に示す組成を有する転炉溶製鋼を連続鋳造し、必要に応じて均熱拡散処理工程、分塊圧延工程を経て１６２ｍｍ角の圧延素材とした。続いて熱間圧延によって線材形状とした。
【００３９】
【表１】
次にこれらの材料の遅れ破壊特性を調査するため、ボルトを製作した。圧延材に必要により焼鈍をまたは球状化焼鈍を施し、冷間鍛造によってボルト形状に成形した。その後所定の条件で加熱し、油槽中に焼入れ、表２、表３の条件で焼もどしを行った。上記の工程で製作したボルトから、直径８ｍｍの引張試験片、及び環状切り欠きノッチ付きの遅れ破壊試験片（平行部の直径８ｍｍ、ノッチ部の直径６ｍｍ）を機械加工によって製作し、機械的性質、及び遅れ破壊特性を調査した。
【００４１】
遅れ破壊試験はｐＨ３．０の希硫酸（液温３０℃）中で試験片に電流密度１．０ｍＡ／ｃｍ2の水素チャージを行い、定荷重を負荷して破断までの時間を測定した。試験時間は最大２００時間とし、２００時間破断しない最大の負荷応力を測定した。２００時間破断しない最大の負荷応力を大気中での破断応力で割った値を「遅れ破壊強度比」と定義し、遅れ破壊特性の指標とした。引張強さが１０００ＭＰａ級のボルトとして多く使われているＳＣＭ４３５の遅れ破壊強度比が０．５程度であることから、遅れ破壊強度比が０．５未満のものは耐遅れ破壊特性に劣ると判断した。これらの各種試験結果も表２、３にまとめて示した。遅れ破壊強度比と引張強さの関係を図１に示す。本発明例は、比較例に比べて高強度にも関わらず、良好な遅れ破壊特性を示すことが分かる。
【００４２】
引張試験では０．２％耐力（降伏強さ）、引張強さ、伸びを測定した。伸びは評点間距離を３０ｍｍとして測定した。引張強さが１０００ＭＰａ級のボルトとして多く使われているＳＣＭ４３５の伸びが１５％程度であることから、伸びが１５％未満のものは塑性域締結に不適と判断した。伸びと引張強さの関係を図２に示す。本発明例は、比較例に比べて高強度にも関わらず、良好な伸びを示し、塑性域締結に適することが分かる。焼もどし温度と０．２％耐力（降伏強さ）の関係を図３に示す。本発明鋼は、比較鋼に比べて０．２％耐力（降伏強さ）の変動が非常に小さく、平坦な焼もどし性能を得ることができることが分かる。このことにより、焼もどし温度が多少変動してもボルトの降伏強さのバラツキを小さく抑えることができ、ボルトの締結軸力の精度が向上する。
【００４３】
【表２】
【表３】
これらから明らかなように、本発明例は比較例に比べて高強度であり、耐遅れ破壊特性、伸びに優れ、焼もどし温度のバラツキに起因する降伏強さのバラツキが小さく、塑性域締結に適している。
【００４６】
【発明の効果】
本発明によれば、引張強さ１２００ＭＰａ以上の高強度であり、遅れ破壊特性に優れ、塑性域締結に適したボルトを安価に提供することが可能となり、ボルトの締結軸力の増加、サイズダウンによる軽量化、塑性域締結の一層の適用拡大等その効果は極めて大きい。
【図面の簡単な説明】
【図１】遅れ破壊強度比と引張強さの関係を示す図である。
【図２】伸びと引張強さの関係を示す図である。
【図３】焼もどし温度と０．２％耐力（降伏強さ）の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel for high-strength bolts excellent in delayed fracture resistance and suitable for fastening in a plastic region, and a method for producing a high-strength bolt.
[0002]
[Prior art]
Development of high-strength bolt steels with a tensile strength of 1200 MPa or more has been demanded as automobiles and industrial machines have become higher performance, lighter, and larger building structures.
[0003]
Currently, the steel types generally used as high-strength bolt steel are low alloy structural steels such as SCM435 and SCR435 specified by JIS, and are manufactured by quenching and tempering. However, these steel types, when the tensile strength exceeds 1200 MPa, the delayed fracture resistance characteristics are drastically reduced, and the risk of fracture due to delayed fracture increases during the use of the bolt. It was impossible in practice.
[0004]
Recently, the adoption of a plastic zone fastening method for fastening a bolt to a yield point or higher has been expanded. In addition to being able to obtain a higher axial force than the conventional elastic region fastening, the plastic region fastening has a feature that there is little variation in the axial force and high-accuracy and stable fastening is possible. The characteristics required for bolts for fastening plastic zones are as follows: (1) The variation in yield strength is directly linked to the variation in the fastening axial force of the bolt, so there is little variation in yield strength, and (2) Retightening performance From the viewpoint of the above, there is a large elongation from the yield point to the maximum axial force.
[0005]
The variation in yield strength is greatly affected by the variation in tempering temperature in the case of ordinary steel types. For example, in a normal industrial production furnace, there is a possibility that a temperature variation of about ± 20 ° C. may occur, but in the case of a normal steel type, this variation may cause a variation in yield strength of 100 MPa or more. In order to reduce this variation, temperature control that is narrower than the normal operating range is required, leaving problems in production in actual industrial production furnaces. Absent.
[0006]
Furthermore, in the case of normal steel grades, the elongation inevitably decreases as the strength increases, so it is necessary to respond by changing the bolt shape, etc. Is leaving. Of course, when a high-strength bolt is used for fastening a plastic region, a high axial force is applied, and therefore, it is necessary to use a material that is superior in delayed fracture characteristics than in the past. However, there is no example that proposes steel for high-strength bolts suitable for fastening in the plastic region, and little consideration has been given from the material aspect.
[0007]
Examples of steels for high-strength bolts intended to improve delayed fracture resistance include, for example, JP-A-58-117856, JP-A-62-199751, JP-A-61-223168, JP-A-3-223. No. 243745 and Japanese Patent No. 2670937. However, since these steel types do not consider the fastening of the plastic region, the above-described characteristics cannot be obtained. JP-A-58-117856 describes a method for improving the ductility (elongation, squeezing) and delayed fracture resistance by reducing impurities in the steel. The amount of C, Cr, Mo, V is described. Is not optimized, variation in yield strength cannot be reduced, and improvement of elongation and delayed fracture resistance is still one step. Japanese Patent No. 2670937 describes a method of improving delayed fracture resistance and fatigue characteristics by limiting the component system and heat treatment conditions, but this case also has the same problem as described above.
[0008]
As described above, there is no steel for high-strength bolts that has excellent delayed fracture resistance and is suitable for fastening in the plastic region.
[0009]
[Problems to be solved by the invention]
An object of the present invention is to solve the above-mentioned problems, and to provide a steel for high-strength bolts excellent in delayed fracture resistance and suitable for fastening a plastic region, and a method for producing a high-strength bolt. Specifically, even at high strength, it has better delayed fracture resistance than SCM435, which is currently widely used as a 1000 MPa class bolt, and even if the tempering temperature varies slightly, the change in yield strength is small and good elongation is achieved. An object of the present invention is to provide a steel for high-strength bolts having a tensile strength of 1200 MPa or more and a method for producing a high-strength bolt.
[0010]
[Means for Solving the Problems]
The gist of the present invention is as follows.
[0011]
(1) By weight%
C: 0.15-0.45%,
Mn: more than 0.40 to 1.50%,
Cr: 0.30 to 2.00%,
Mo: 0.10 to 0.35%,
V: more than 0.20 to 0.40%,
Al: 0.010 to 0.100%
And Cr, Mo and V are in a balance satisfying the relationship of 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <2.30,
Si: 0.10% or less (including 0%),
P: 0.015% or less (including 0%),
S: 0.015% or less (including 0%)
A steel for high-strength bolts that is excellent in delayed fracture resistance and suitable for fastening in the plastic region, characterized in that each of the remaining elements is made of Fe and inevitable impurities.
[0012]
(2) By weight%
C: 0.15-0.45%,
Mn: more than 0.40 to 1.50%,
Cr: 0.30 to 2.00%,
Mo: 0.10 to 0.35%,
V: more than 0.20 to 0.40%,
Al: 0.010 to 0.100%
In addition,
Nb: 0.005 to 0.100%,
Ti: 0.005 to 0.100%
And a balance satisfying the relationship of 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <2.30. ,
Si: 0.10% or less (including 0%),
P: 0.015% or less (including 0%),
S: 0.015% or less (including 0%)
A steel for high-strength bolts that is excellent in delayed fracture resistance and suitable for fastening in the plastic region, characterized in that each of the remaining elements is made of Fe and inevitable impurities.
[0013]
(3) By weight%
C: 0.15-0.45%,
Mn: more than 0.40 to 1.50%,
Cr: 0.30 to 2.00%,
Mo: 0.10 to 0.35%,
V: more than 0.20 to 0.40%,
Al: 0.010 to 0.100%
In addition,
B: 0.0005 to 0.0050%
And Cr, Mo and V are in a balance satisfying the relationship of 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <2.30,
Si: 0.10% or less (including 0%),
P: 0.015% or less (including 0%),
S: 0.015% or less (including 0%)
A steel for high-strength bolts that is excellent in delayed fracture resistance and suitable for fastening in the plastic region, characterized in that each of the remaining elements is made of Fe and inevitable impurities.
[0014]
(4)% by weight
C: 0.15-0.45%,
Mn: more than 0.40 to 1.50%,
Cr: 0.30 to 2.00%,
Mo: 0.10 to 0.35%,
V: more than 0.20 to 0.40%,
Al: 0.010 to 0.100%
In addition,
Nb: 0.005 to 0.100%,
Ti: 0.005 to 0.100%
Containing one or two of:
B: 0.0005 to 0.0050%
And Cr, Mo and V are in a balance satisfying the relationship of 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <2.30,
Si: 0.10% or less (including 0%),
P: 0.015% or less (including 0%),
S: 0.015% or less (including 0%)
A steel for high-strength bolts that is excellent in delayed fracture resistance and suitable for fastening in the plastic region, characterized in that each of the remaining elements is made of Fe and inevitable impurities.
[0015]
(5) After forming the steel for high-strength bolts according to any one of (1) to (4) above into a desired shape, the steel is heated to a temperature of AC 3 or higher, and then subjected to a quenching treatment, in a temperature range of 500 to 650 ° C A method for producing high-strength bolts that have excellent delayed fracture resistance and are suitable for fastening in plastic regions, characterized by tempering.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have obtained the following knowledge about various factors affecting delayed fracture characteristics. That is, (1) Low temperature tempering and brittleness can be avoided by adding Cr, Mo and V in a certain component range and tempering in a temperature range of 500 ° C. or higher. Further, the form of cementite that precipitates at the grain boundaries during tempering can be spheroidized to increase the bond strength of the grain boundaries and prevent grain boundary cracking. (2) By adding a specific amount of V, V carbonitrides that finely precipitate in the matrix during tempering become hydrogen intragranular trap sites, reducing the amount of hydrogen accumulated at the grain boundaries, and delay resistance Destructive properties are greatly improved. (3) By limiting the amounts of P and S, which are impurities segregated at the grain boundaries, to a certain amount or less, the prior austenite grain boundaries are strengthened, and the delayed fracture resistance is improved.
[0017]
In addition, the present inventors have obtained the following knowledge in terms of materials regarding various characteristics necessary for fastening the plastic region. That is, (1) The amount of addition of C, Cr, Mo, V is limited to a certain component range, and Cr, Mo, V is 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <By making the balance satisfying the relationship of 2.30, temper softening, temper softening resistance, and tempering secondary hardening are offset in the tempering temperature range of 500 ° C. to 620 ° C. The fluctuation of 2% yield strength (yield strength) is very small, and flat tempering performance can be obtained. As a result, even if the tempering temperature fluctuates somewhat, variations in the yield strength of the bolt can be kept small. (2) A combination of Cr, Mo, and V within the above range and tempering at a high temperature of 500 ° C. or higher can achieve both high strength and good elongation.
[0018]
Furthermore, by reducing Si as a solid solution strengthening element of ferrite as much as possible, the reduction of cold forgeability due to the addition of Cr, Mo and V can be compensated, and high strength can be achieved without impairing the cold forgeability of bolts. it can.
[0019]
Hereinafter, the present invention will be described in detail.
[0020]
C: Since C is an element effective for obtaining strength, 0.15% or more is added. However, if added over 0.45%, cold forgeability and toughness are reduced, so 0.15 to 0.005%. It needs to be in the range of 45%. The preferred range is 0.20 to 0.40%.
[0021]
Mn: Since Mn is an element effective for improving the hardenability, it is added in excess of 0.40%. However, if added in excess of 1.50%, delayed fracture resistance and cold forgeability deteriorate. Therefore, it is necessary to be within the range of more than 0.40 to 1.50%. The preferred range is 0.50 to 0.80%.
[0022]
Cr: Cr is an element effective for improving the hardenability and has the effect of tempering and imparting softening resistance to the steel, so 0.30% or more is added. Since the forgeability is lowered, it is necessary to be within the range of 0.30 to 2.00%. The preferred range is 1.00 to 1.40%.
[0023]
Mo: Mo is an element that causes remarkable secondary hardening during tempering and improves delayed fracture resistance by enabling high-temperature tempering. Further, high temperature tempering improves the strength-ductility balance, and high elongation can be obtained, so 0.10% or more is added, but if added over 0.35%, the cold forgeability decreases and The next hardening amount becomes excessive, and flat tempering characteristics cannot be obtained. Further, the alloy carbide is difficult to dissolve in the matrix during quenching heating, and the elongation decreases due to an increase in the amount of coarse undissolved carbide, making it unsuitable for fastening the plastic region. It is necessary to be in the range. The preferred range is 0.20 to 0.35%.
[0024]
V: V is an element that has an effect of remarkably refining the prior austenite crystal grains, causes significant secondary hardening during tempering, and improves delayed fracture resistance by enabling high-temperature tempering. . Moreover, high temperature tempering improves the strength-ductility balance, and high elongation can be obtained. In addition, V carbonitrides that finely precipitate in the matrix during tempering become hydrogen intragranular trap sites, reducing the amount of hydrogen that accumulates at the grain boundaries and greatly improving delayed fracture resistance. Adding over 0.20%, but adding over 0.40% not only saturates the effect but also reduces cold forgeability, excessive secondary hardening, and flat tempering characteristics. Can't get. Furthermore, the alloy carbides are difficult to dissolve in the matrix during quenching heating, and the elongation decreases due to an increase in the amount of coarse undissolved carbides, making it unsuitable for fastening the plastic region. % Range is required. The preferred range is 0.25 to 0.35%.
[0025]
Al: Al is an element necessary for deoxidation of steel and has the effect of forming nitrides to refine the prior austenite grains, so 0.010% or more is added, but more than 0.100% is added Then, not only is the effect saturated, but alumina inclusions increase and the toughness decreases, so it is necessary to set the content within a range of 0.010 to 0.100%. A preferable range is 0.020 to 0.050%.
[0026]
Nb: Nb, like Al, Ti, and V, has the effect of refining crystal grains and improves delayed fracture resistance, so 0.005% or more is preferably added. If it is added in excess of%, the effect is not only saturated but also the cold forgeability is lowered. Therefore, when it is added, it is necessary to be in the range of 0.005 to 0.100%. The preferred range is from 0.010 to 0.050%.
[0027]
Ti: Ti, like Al, Nb, and V, has the effect of refining crystal grains, and also has the effect of fixing the solid solution N in the steel as nitrides and improving the delayed fracture resistance. It is preferable to add 005% or more, but if added over 0.100%, not only the effect is saturated but also the cold forgeability is lowered, so in the case of adding in the range of 0.005 to 0.100% There is a need to. The preferred range is from 0.010 to 0.050%.
[0028]
B: B has the effect of improving hardenability when added in a small amount, and segregates at the prior austenite grain boundaries to strengthen the grain boundaries and improve delayed fracture resistance, so 0.0005% or more is added. Although it is preferable to add over 0.0050%, the effect is saturated, so when it is added, it is necessary to make it in the range of 0.0005 to 0.0050%. The preferred range is 0.0010 to 0.0030%.
[0029]
Si: Si is an element necessary for deoxidation of steel, but if added over 0.10%, the cold forgeability is remarkably lowered, so it is necessary to limit it to 0.10% or less. The preferred range is 0.08% or less.
[0030]
P: P has the effect of segregating at the prior austenite grain boundaries to embrittle the grain boundaries and significantly reduce the delayed fracture resistance, so it must be limited to 0.015% or less and should be reduced as much as possible. . The preferred range is 0.010% or less.
[0031]
S: S segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and significantly reduces the delayed fracture resistance. Therefore, it must be limited to 0.015% or less, and should be reduced as much as possible. . The preferred range is 0.010% or less.
[0032]
Cr, Mo, V addition balance: In the present invention, the addition amount of Cr, Mo, V satisfies the relational expression of 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <2.30. Balance. By making the above balance, temper softening, temper softening resistance and tempering secondary hardening are offset in the tempering temperature range of 500 ° C. to 650 ° C., and tensile strength, 0.2% yield strength (yield strength) ) Fluctuation is very small, and flat tempering performance can be obtained. As a result, even if the tempering temperature fluctuates somewhat, variations in the yield strength of the bolt can be kept small. However, if the value of the above formula is lower than the lower limit, temper softening resistance, tempering secondary curing becomes insufficient, and flat tempering performance cannot be obtained, and if it exceeds the upper limit, tempering secondary curing is excessive. Thus, not only flat tempering performance cannot be obtained, but also the strength of the material is increased, and the cold forgeability is also decreased. The preferred range is 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <1.90.
[0033]
Although the present invention does not particularly define the secondary processing step, the material after hot rolling is annealed or spheroidized in order to improve the cold forgeability in the case where the cold forging step is included in the manufacturing process. Processing may be performed. In the case of bolts that require dimensional accuracy of the material, it is common to perform wire drawing before cold forging.
[0034]
The bolt steel having the components described above is most effective in the bolt manufacturing method described below.
[0035]
After forming the steel for bolts having the above-described components into a desired bolt shape by forging, cutting, etc., in order to give strength to the steel, the steel is heated to a temperature equal to or higher than the AC3 point and then quenched by water cooling or oil cooling. . If the heating temperature is too high, it will cause coarsening of the crystal grains, leading to deterioration of toughness and delayed fracture resistance, and from the operational aspect, damage to the furnace body and accessory parts of the heat treatment furnace will be significant, and the manufacturing cost will be reduced. Since it rises, it is not preferable to heat to a very high temperature, so it is preferable to set the quenching heating temperature to 850 to 950 ° C.
[0036]
It is necessary to perform tempering after quenching in order to impart predetermined strength, toughness and ductility to the steel. Tempering is generally performed in a temperature range of 150 ° C. to AC 1 point, but in the present invention, it is necessary to limit the temperature range to 500 to 650 ° C. The reason for this is that at 500 ° C. or lower, the cementite precipitates precipitated at the grain boundaries cannot be spheroidized to increase the bond strength of the grain boundaries, and the precipitation of V carbonitrides that serve as hydrogen trap sites is insufficient. The delayed fracture resistance cannot be improved, the elongation value, which is a necessary characteristic of the bolt for fastening the plastic region, cannot be improved at 500 ° C. or lower, the softening is remarkable at 650 ° C. or higher, and the high strength of the tensile strength of 1200 MPa or higher. In addition, flat tempering performance cannot be obtained outside the temperature range of 500 to 650 ° C., and variation in yield strength due to variation in tempering temperature cannot be reduced. Because. The preferred range is 525-625 ° C.
[0037]
【Example】
The following examples further illustrate the present invention.
[0038]
Converter molten steel having the composition shown in Table 1 was continuously cast, and a 162 mm square rolled material was obtained through a soaking diffusion treatment process and a block rolling process as necessary. Subsequently, a wire shape was obtained by hot rolling.
[0039]
[Table 1]
Next, bolts were manufactured to investigate the delayed fracture characteristics of these materials. If necessary, the rolled material was annealed or spheroidized and formed into a bolt shape by cold forging. Thereafter, the mixture was heated under predetermined conditions, quenched in an oil tank, and tempered under the conditions shown in Tables 2 and 3. From the bolts manufactured in the above process, tensile test pieces with a diameter of 8 mm and delayed fracture test pieces with a circular notch notch (parallel part diameter 8 mm, notch part diameter 6 mm) are manufactured by machining, and mechanical properties , And delayed fracture characteristics were investigated.
[0041]
In the delayed fracture test, the test piece was charged with hydrogen at a current density of 1.0 mA / cm 2 in dilute sulfuric acid having a pH of 3.0 (liquid temperature: 30 ° C.), a constant load was applied, and the time until fracture was measured. The test time was a maximum of 200 hours, and the maximum load stress that did not break for 200 hours was measured. A value obtained by dividing the maximum load stress that does not break for 200 hours by the breaking stress in the atmosphere is defined as a “delayed fracture strength ratio” and is used as an index of delayed fracture characteristics. Since the delayed fracture strength ratio of SCM435, which is often used as a bolt with a tensile strength of 1000 MPa, is about 0.5, it is judged that those with a delayed fracture strength ratio of less than 0.5 are inferior in delayed fracture resistance. did. These various test results are also summarized in Tables 2 and 3. The relationship between the delayed fracture strength ratio and the tensile strength is shown in FIG. It can be seen that the inventive examples show better delayed fracture characteristics despite the high strength compared to the comparative examples.
[0042]
In the tensile test, 0.2% yield strength (yield strength), tensile strength, and elongation were measured. Elongation was measured with a distance between scores of 30 mm. Since the elongation of SCM435, which is often used as a bolt with a tensile strength of 1000 MPa, is about 15%, those with an elongation of less than 15% were judged to be unsuitable for plastic region fastening. The relationship between elongation and tensile strength is shown in FIG. It can be seen that the inventive examples show good elongation despite the high strength compared to the comparative examples, and are suitable for plastic region fastening. FIG. 3 shows the relationship between the tempering temperature and the 0.2% yield strength (yield strength). It can be seen that the steel of the present invention has a very small variation in 0.2% proof stress (yield strength) as compared with the comparative steel, and a flat tempering performance can be obtained. As a result, even if the tempering temperature fluctuates somewhat, variations in the yield strength of the bolt can be kept small, and the accuracy of the fastening axial force of the bolt is improved.
[0043]
[Table 2]
[Table 3]
As is clear from these, the inventive examples are higher in strength than the comparative examples, are excellent in delayed fracture resistance and elongation, have less variation in yield strength due to variations in tempering temperature, and are suitable for fastening in plastic areas. Are suitable.
[0046]
【The invention's effect】
According to the present invention, it is possible to provide a bolt with high tensile strength of 1200 MPa or more, excellent delayed fracture characteristics, and suitable for plastic region fastening at low cost, increasing bolt fastening axial force, and reducing size. The effects such as weight reduction by the use of plastics and further expansion of application of the plastic zone fastening are extremely great.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between delayed fracture strength ratio and tensile strength.
FIG. 2 is a diagram showing the relationship between elongation and tensile strength.
FIG. 3 is a diagram showing the relationship between tempering temperature and 0.2% yield strength (yield strength).
Claims (5)
Ｃ：０．１５〜０．４５％、
Ｍｎ：０．４０超〜１．５０％、
Ｃｒ：０．３０〜２．００％、
Ｍｏ：０．１０〜０．３５％、
Ｖ：０．２０超〜０．４０％、
Ａｌ：０．０１０〜０．１００％
を含有し、かつＣｒ、Ｍｏ、Ｖを１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係を満足するバランスとし、
Ｓｉ：０．１０％以下（０％を含む）、
Ｐ：０．０１５％以下（０％を含む）、
Ｓ：０．０１５％以下（０％を含む）
に各々制限し、残部がＦｅ及び不可避的不純物よりなることを特徴とする、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼。% By weight
C: 0.15-0.45%,
Mn: more than 0.40 to 1.50%,
Cr: 0.30 to 2.00%,
Mo: 0.10 to 0.35%,
V: more than 0.20 to 0.40%,
Al: 0.010 to 0.100%
And Cr, Mo and V are in a balance satisfying the relationship of 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <2.30,
Si: 0.10% or less (including 0%),
P: 0.015% or less (including 0%),
S: 0.015% or less (including 0%)
A steel for high-strength bolts that is excellent in delayed fracture resistance and suitable for fastening in the plastic region, characterized in that each of the remaining elements is made of Fe and inevitable impurities.
Ｃ：０．１５〜０．４５％、
Ｍｎ：０．４０超〜１．５０％、
Ｃｒ：０．３０〜２．００％、
Ｍｏ：０．１０〜０．３５％、
Ｖ：０．２０超〜０．４０％、
Ａｌ：０．０１０〜０．１００％
を含有し、さらに、
Ｎｂ：０．００５〜０．１００％、
Ｔｉ：０．００５〜０．１００％
のうちの１種または２種を含有し、かつＣｒ、Ｍｏ、Ｖを１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係を満足するバランスとし、
Ｓｉ：０．１０％以下（０％を含む）、
Ｐ：０．０１５％以下（０％を含む）、
Ｓ：０．０１５％以下（０％を含む）
に各々制限し、残部がＦｅ及び不可避的不純物よりなることを特徴とする、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼。% By weight
C: 0.15-0.45%,
Mn: more than 0.40 to 1.50%,
Cr: 0.30 to 2.00%,
Mo: 0.10 to 0.35%,
V: more than 0.20 to 0.40%,
Al: 0.010 to 0.100%
In addition,
Nb: 0.005 to 0.100%,
Ti: 0.005 to 0.100%
And a balance satisfying the relationship of 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <2.30. ,
Si: 0.10% or less (including 0%),
P: 0.015% or less (including 0%),
S: 0.015% or less (including 0%)
A steel for high-strength bolts that is excellent in delayed fracture resistance and suitable for fastening in the plastic region, characterized in that each of the remaining elements is made of Fe and inevitable impurities.
Ｃ：０．１５〜０．４５％、
Ｍｎ：０．４０超〜１．５０％、
Ｃｒ：０．３０〜２．００％、
Ｍｏ：０．１０〜０．３５％、
Ｖ：０．２０超〜０．４０％、
Ａｌ：０．０１０〜０．１００％
を含有し、さらに、
Ｂ：０．０００５〜０．００５０％
を含有し、かつＣｒ、Ｍｏ、Ｖを１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係を満足するバランスとし、
Ｓｉ：０．１０％以下（０％を含む）、
Ｐ：０．０１５％以下（０％を含む）、
Ｓ：０．０１５％以下（０％を含む）
に各々制限し、残部がＦｅ及び不可避的不純物よりなることを特徴とする、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼。% By weight
C: 0.15-0.45%,
Mn: more than 0.40 to 1.50%,
Cr: 0.30 to 2.00%,
Mo: 0.10 to 0.35%,
V: more than 0.20 to 0.40%,
Al: 0.010 to 0.100%
In addition,
B: 0.0005 to 0.0050%
And Cr, Mo and V are in a balance satisfying the relationship of 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <2.30,
Si: 0.10% or less (including 0%),
P: 0.015% or less (including 0%),
S: 0.015% or less (including 0%)
A steel for high-strength bolts that is excellent in delayed fracture resistance and suitable for fastening in the plastic region, characterized in that each of the remaining elements is made of Fe and inevitable impurities.
Ｃ：０．１５〜０．４５％、
Ｍｎ：０．４０超〜１．５０％、
Ｃｒ：０．３０〜２．００％、
Ｍｏ：０．１０〜０．３５％、
Ｖ：０．２０超〜０．４０％、
Ａｌ：０．０１０〜０．１００％
を含有し、さらに、
Ｎｂ：０．００５〜０．１００％、
Ｔｉ：０．００５〜０．１００％
のうちの１種または２種を含有し、さらに、
Ｂ：０．０００５〜０．００５０％
を含有し、かつＣｒ、Ｍｏ、Ｖを１．５０＜０．１２５×（Ｃｒ％＋Ｍｏ％）＋５．２２×Ｖ％＜２．３０の関係を満足するバランスとし、
Ｓｉ：０．１０％以下（０％を含む）、
Ｐ：０．０１５％以下（０％を含む）、
Ｓ：０．０１５％以下（０％を含む）
に各々制限し、残部がＦｅ及び不可避的不純物よりなることを特徴とする、耐遅れ破壊特性に優れ、塑性域締結に適した高強度ボルト用鋼。% By weight
C: 0.15-0.45%,
Mn: more than 0.40 to 1.50%,
Cr: 0.30 to 2.00%,
Mo: 0.10 to 0.35%,
V: more than 0.20 to 0.40%,
Al: 0.010 to 0.100%
In addition,
Nb: 0.005 to 0.100%,
Ti: 0.005 to 0.100%
Containing one or two of:
B: 0.0005 to 0.0050%
And Cr, Mo and V are in a balance satisfying the relationship of 1.50 <0.125 × (Cr% + Mo%) + 5.22 × V% <2.30,
Si: 0.10% or less (including 0%),
P: 0.015% or less (including 0%),
S: 0.015% or less (including 0%)
A steel for high-strength bolts that is excellent in delayed fracture resistance and suitable for fastening in the plastic region, characterized in that each of the remaining elements is made of Fe and inevitable impurities.
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP13318899A JP3718369B2 (en) | 1999-05-13 | 1999-05-13 | Steel for high strength bolt and method for producing high strength bolt |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP13318899A JP3718369B2 (en) | 1999-05-13 | 1999-05-13 | Steel for high strength bolt and method for producing high strength bolt |
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| JP2000328191A JP2000328191A (en) | 2000-11-28 |
| JP3718369B2 true JP3718369B2 (en) | 2005-11-24 |
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| CN107312970A (en) * | 2017-06-24 | 2017-11-03 | 武汉钢铁有限公司 | A kind of high-strength steel for fastener of super large and production method |
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| JP4142853B2 (en) * | 2001-03-22 | 2008-09-03 | 新日本製鐵株式会社 | High strength bolt with excellent delayed fracture resistance |
| JP4186684B2 (en) * | 2002-04-12 | 2008-11-26 | 住友金属工業株式会社 | Method for producing martensitic stainless steel |
| JP4381355B2 (en) * | 2005-07-22 | 2009-12-09 | 新日本製鐵株式会社 | Steel having excellent delayed fracture resistance and tensile strength of 1600 MPa class or more and method for producing the molded product thereof |
| FR2914929B1 (en) | 2007-04-12 | 2010-10-29 | Mittal Steel Gandrange | STEEL WITH GOOD HYDROGEN RESISTANCE FOR THE FORMING OF VERY HIGH CHARACTERISTIC MECHANICAL PARTS. |
| CN114807738B (en) * | 2021-01-27 | 2023-06-13 | 宝山钢铁股份有限公司 | High-strength steel for bolts and manufacturing method thereof |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN107312970A (en) * | 2017-06-24 | 2017-11-03 | 武汉钢铁有限公司 | A kind of high-strength steel for fastener of super large and production method |
| CN107312970B (en) * | 2017-06-24 | 2019-07-16 | 武汉钢铁有限公司 | A kind of high-strength steel for fastener of super large and production method |
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