JP4362219B2 - Steel excellent in high temperature strength and method for producing the same - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、例えば、火災など異常時において高温にさらされてもなお十分耐力(強度)を維持し得る鋼およびその製造方法に関するもので、鉄鋼業においては厚板、形鋼、ホットストリップミルなどに適用できる。なお、用途としては、建築分野のみならず、土木、海洋構造物、造船、各種の貯槽タンクなどの一般的な構造用鋼として広範な用途に適用できる。
【０００２】
【従来の技術】
高温強度の保証を目的とした建築用途でのいわゆる耐火鋼は、特開平２−７７５２３号公報他多くの公開公報で、含Ｍｏ鋼の製造方法が開示されている。しかし、Ｍｏは鋼の焼入性を顕著に高めるとともに、Ｃとの相互作用が極めて強いために、材質変化が製造条件の変動に敏感で、常温での強度−靭性バランスやそのばらつき、常温強度と高温強度のバランスを考慮した場合、高温強度上は有効であるが、必ずしも使いやすい元素とは言えない。また、高温強度を維持する程度の比較的多いＭｏの添加は、溶接性の顕著な劣化に加え、母材および溶接部の靭性も著しく劣化させるという問題があった。
【０００３】
【発明が解決しようとする課題】
本発明は、上述した従来技術の問題点を解決すべく、優れた高温強度とともに、靭性や溶接性にも優れる鋼を得るため、Ｃｕの固溶・析出を利用することで、Ｍｏは必要に応じて少量添加する程度に止め、さらに製造方法を限定することで、上述した複合特性を有する鋼、および該鋼を工業的に安定して供給可能な方法を提供するものである。
【０００４】
【課題を解決するための手段】
本発明の最大のポイントは、Ｃｕを比較的多く添加し、６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加する程度に、Ｃｕが常温で固溶状態で存在させること、あるいはさらにそのような状態を得るよう製造条件を限定することである。
【０００５】
これはすなわち、６００℃に加熱されることで、常温では固溶状態にあったＣｕが析出し、析出強化として高温強度を発現させることを意味している。この結果、常温強度と高温強度をバランス良く向上することができる。
【０００６】
そのために鋼成分をはじめ製造方法を本発明の通り限定したものであるが、その要旨は以下に示す通りである。
【０００７】
（１） 鋼成分が質量％で、
Ｃ：０．０２８％以下、
Ｓｉ：０．６％以下、
Ｍｎ：０．５１〜１．６％、
Ｐ：０．０２％以下、
Ｓ：０．０１％以下、
Ｃｕ：０．６〜２．０％、
Ｎｉ：Ｃｕ添加量の１／２〜１．０％、
Ａｌ：０．０１８〜０．０６％、
Ｎ：０．００６％以下、
かつ、
Ｎｂ：０．００５〜０．１％、
Ｖ：０．０１〜０．２％、
Ｔｉ：０．００５〜０．１％
の範囲で少なくとも１種以上を含有し、残部が鉄および不可避的不純物からなり、６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加することを特徴とする高温強度に優れた鋼。
【０００８】
（２） 上記鋼成分に加え、質量％で、
Ｂ：０．０００２〜０．００３％、
Ｍｇ：０．０００２〜０．００５％
の範囲で１種または２種を含有することを特徴とする上記（１）に記載の高温強度に優れた鋼。
【０００９】
（３） 質量％で、
Ｃａ：０．０００５〜０．００４％、
を含有することを特徴とする上記（１）または（２）に記載の高温強度に優れた鋼。
【００１０】
（４） 上記（１）〜（３）のいずれか１項に記載の鋼成分からなる鋼片または鋳片を１０００〜１２５０℃の温度範囲に再加熱後、１０００℃以下での累積圧下量を３０％以上として７５０℃以上の温度で圧延を終了し、その後７００℃以上の温度から強制冷却で４００℃以下の任意の温度まで加速冷却することを特徴とする、６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加する高温強度に優れた鋼の製造方法。
【００１１】
（５） 上記（１）〜（３）のいずれか１項に記載の鋼成分からなる鋼片または鋳片を熱間圧延後、Ａｃ3以上９５０℃以下の温度に再加熱後、強制冷却で加速冷却することを特徴とする、６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加する高温強度に優れた鋼の製造方法。
【００１２】
【発明の実施の形態】
以下に、本発明を詳細に説明する。
【００１３】
本発明が、請求項の通りに鋼組成および製造方法を限定した理由について説明する。
【００１４】
Ｃは、鋼の焼入性を最も顕著に増大させるため、多く添加することは好ましくない。特に本発明においては、Ｃｕを６００℃での時効析出処理を行えば降伏強さが３０Ｎ／ｍｍ2以上増加する程度に固溶状態で存在させることが最大のポイントとなっているため、後述するように、圧延後、Ｃｕが全量析出してしまわないよう放冷相当を超える冷速で加速冷却する必要がある。この際、必要以上に焼きが入らないようＣ量は低いレベルに抑える必要があり、許容できるＣ量の上限は０．０３％である。Ｃ量の下限は特性上の理由からは制約されるものではなく特に限定しないが、製鋼能力やコストなどにより自ずと制限されるものである。
【００１５】
Ｓｉは、脱酸上鋼に含まれる元素であるが、多く添加すると溶接性、ＨＡＺ靭性が劣化するため、上限を０．６％に限定した。鋼の脱酸はＴｉ、Ａｌのみでも十分可能であり、ＨＡＺ靭性、焼入性などの観点から低いほど好ましく、必ずしも添加する必要はない。
【００１６】
Ｍｎは、母材の強度、靭性を確保する上で不可欠な元素である。置換型の固溶強化元素であるＭｎは、特に６００℃超の高温強度にはあまり大きな改善効果はないが、常温強度確保の観点から０．５１％以上の添加を必須とする。上限については、多すぎる添加は連続鋳造スラブの中心偏析を助長したり、溶接性を劣化させるため１．６％に限定する。
【００１７】
Ｐは、本発明鋼においては不純物であり、Ｐ量の低減はＨＡＺにおける粒界破壊を減少させる傾向があるため、少ないほど好ましい。含有量が多いと母材、溶接部の低温靭性を劣化させるため上限を０．０２％とした。
【００１８】
Ｓは、Ｐと同様本発明鋼においては不純物であり、母材の低温靭性の観点からは少ないほど好ましい。含有量が多いと母材、溶接部の低温靭性を劣化させるため上限を０．０１％とした。
【００１９】
Ｃｕは、本発明においては不可欠の元素で、「Ｃｕを６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加する程度に固溶状態で存在させること」のため、０．６％以上の添加が必須である。上限については、本来、目的とする常温および高温強度によって変えるべき性質のものであるが、本発明が意図する「Ｃｕを６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加する程度に固溶状態で存在させること」を達する上での制約はなく、むしろ溶接性、さらには熱間圧延時のＣｕ−クラックの観点から２．０％に限定した。
【００２０】
なお、「Ｃｕを６００℃での時効析出処理を行えば降伏強さが３０Ｎ／ｍｍ2以上増加すること」を本発明の特徴とするが、常温状態におけるＣｕが固溶か析出かを判定するのは必ずしも容易ではなく、「６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加すること」をその判定基準とする。
【００２１】
Ｎｉは、比較的多いＣｕ添加必須とする本発明においては必須元素である。なぜならば、熱間圧延時のＣｕ−クラックを防止するためであり、そのためにＣｕ添加量の１／２以上とする必要がある。Ｎｉは、過剰に添加しなければ、溶接性、ＨＡＺ靭性に悪影響を及ぼすことなく母材の強度、靭性を向上させるが、過剰な添加は高価なだけでなく、溶接性にも好ましくないため、上限を１．０％とした。
【００２２】
なお、これら、Ｃｕ、Ｎｉの比較的多い添加は、耐候性にも有利に作用する。
【００２３】
Ａｌは、一般に脱酸上鋼に含まれる元素であるが、本発明鋼においては、その下限は０．０１８％とした。しかし、Ａｌ量が多くなると鋼の清浄度が悪くなるだけでなく、溶接金属の靭性が劣化するので、上限を０．０６％とした。
【００２４】
Ｎは、不可避的不純物として鋼中に含まれるものであるが、後述するＴｉ、Ｎｂ、Ｖを少なくとも１種以上添加する本発明鋼においては、ＴｉＮを形成して鋼の性質を高めたり、Ｎｂ、Ｖと結合して炭窒化物を形成して強度を増加させる。この目的のためには、Ｎ量として最低０．００１％含有することが望ましい。しかしながら、Ｎ量の増加はＨＡＺ靭性、溶接性に極めて有害であり、本発明鋼においてはその上限は０．００６％である。
【００２５】
本発明の優れた特徴を損なうことなく、溶接構造用鋼として基本特性をさらに向上させるため、上述した元素に加え、Ｎｂ、Ｖ、Ｔｉのうち少なくとも１種以上の添加を必須とする。
【００２６】
Ｎｂは、まず、一般的な効果として、オーステナイトの再結晶温度を上昇させ、熱間圧延時の制御圧延の効果を最大限に発揮する上で有用な元素で、最低０．００５％の添加が必要である。また、圧延に先立つ再加熱や圧延後の熱処理時の加熱オーステナイトの細粒化にも寄与する。さらに、析出硬化として強度向上効果を有し、高温強度向上にも寄与する。しかし、過剰な添加は、溶接部の靭性劣化を招くため上限を０．１％とした。
【００２７】
Ｖは、Ｎｂとほぼ同様の作用を有するものであるが、Ｎｂに比べてその効果は小さい。また、Ｖは焼入性にも影響を及ぼし、高温強度向上にも寄与する。Ｎｂと同様の効果は０．０１％未満では効果が少なく、上限は０．２％まで許容できる。
【００２８】
Ｔｉは、母材および溶接部靭性に対する要求が厳しい場合には、添加することが好ましい。なぜならばＴｉは、Ａｌ量が少ないとき（例えば０．００３％以下）、Ｏと結合してＴｉ2Ｏ3を主成分とする析出物を形成、粒内変態フェライト生成の核となり溶接部靭性を向上させる。また、ＴｉはＮと結合してＴｉＮとしてスラブ中に微細析出し、加熱時のγ粒の粗大化を抑え圧延組織の細粒化に有効であり、また鋼板中に存在する微細ＴｉＮは、溶接時に溶接熱影響部組織を細粒化するためである。これらの効果を得るためには、Ｔｉは最低０．００５％必要である。しかし多すぎるとＴｉＣを多量に形成し、低温靭性や溶接性を劣化させるので、その上限は０．１％である。
【００２９】
次に、必要に応じて含有することができるＢ、Ｍｇの添加理由について説明する。
【００３０】
基本となる成分に、さらにこれらの元素を添加する主たる目的は、本発明鋼の優れた特徴を損なうことなく、強度、靭性などの特性を向上させるためである。したがって、その添加量は自ずと制限されるべき性質のものである。
【００３３】
Ｂは、オーステナイト粒界に偏析し、フェライトの生成を抑制することを介して、焼入性を向上させ、強度向上に寄与する。この効果を享受するため、最低０．０００２％以上必要である。しかし、多すぎる添加は焼入性向上効果が飽和するだけでなく、靭性上有害となるＢ析出物を形成する可能性もあるため、上限を０．００３％とした。なお、タンク用鋼などとして、応力腐食割れが懸念されるケースでは、母材および溶接熱影響部の硬さの低減がポイントとなることが多く（例えば、硫化物応力腐食割れ（ＳＣＣ）防止のためにはＨＲＣ≦２２（ＨＶ≦２４８）が必須とされる）、そのようなケースでは焼入性を増大させるＢ添加は好ましくない。
【００３４】
Ｍｇは、溶接熱影響部においてオーステナイト粒の成長を抑制し、細粒化する作用があり、溶接部の強靭化が図れる。このような効果を享受するためには、Ｍｇは０．０００２％以上必要である。一方、添加量が増えると添加量に対する効果代が小さくなるため、コスト上得策ではないので上限は０．００５％とした。
【００３５】
さらに、Ｃａは、ＭｎＳの形態を制御し、母材の低温靭性を向上させるほか、湿潤硫化水素環境下での水素誘起割れ（ＨＩＣ、ＳＳＣ、ＳＯＨＩＣ）感受性を低減させる。これらの効果を発揮するためには、最低０．０００５％必要である。しかし、多すぎる添加は、鋼の清浄度を逆に高め、母材靭性や湿潤硫化水素環境下での水素誘起割れ（ＨＩＣ、ＳＳＣ、ＳＯＨＩＣ）感受性を高めるため、添加量の上限は０．００４％に限定した。
【００３６】
次に、本発明の請求項４および５に規定する製造条件およびその限定理由について説明する。
【００３７】
まず、本発明の請求項４にかかる圧延に先立つ加熱温度を１０００〜１２５０℃に限定した理由は、加熱時のオーステナイト粒を小さく保ち、圧延組織の微細化を図るためである。１２５０℃は加熱時のオーステナイトが極端に粗大化しない上限温度であり、加熱温度がこれを超えるとオーステナイト粒が粗大混粒化し、変態後の組織も粗大化するため鋼の靭性が著しく劣化する。一方、加熱温度が低すぎると、後述する圧延終了温度（７５０℃以上）の確保が困難となるばかりでなく、Ｎｂを添加した場合、オーステナイトの再結晶温度を上昇させ、熱間圧延時の制御圧延の効果を最大限に発揮させたり、析出効果を発現させるためのＮｂの液体化の観点から下限を１０００℃に限定した。なお、Ｎｂを添加しない場合は、その溶体化を考慮する必要がないため、加熱オーステナイトを必要以上に粗大化させない観点から１１５０℃以下の温度で加熱することが好ましい。
【００３８】
前記温度範囲に再加熱した鋳片または鋼片を、圧延では１０００℃以下での累積圧下量を３０％以上として７５０℃以上で熱間圧延を終了する必要がある。１０００℃以下での累積圧下量が少ない場合、圧延オーステナイトの細粒化が不十分となり、靭性確保が困難なためである。また、圧延終了温度が７５０℃を下回ると、Ｃ量が比較的低い本発明鋼においては、変態が一部開始する可能性が高まり、最終組織に加工（圧延）組織を残す恐れがあり、靭性上好ましくないばかりでなく、降伏比の上昇を招き、建築用途などとして低降伏比が求められた場合、製造が困難となるため、圧延終了温度は７５０℃以上に限定する。
【００３９】
圧延後は、７００℃以上の温度から強制冷却、つまり放冷以外の冷却速度で４００℃以下の任意の温度まで加速冷却する。これらはいずれも「Ｃｕを６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加する程度に固溶状態で存在させること」のためである。いずれか１つでも上記限定条件を逸脱すると、「Ｃｕを６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加する程度に固溶状態で存在させること」が困難となる。
【００４０】
なお、加速冷却時の冷速は、鋼成分や意図する材質（強度、靭性）レベルによっても変わるため一概には言えないが、板厚１／４厚位置の加速冷却開始温度から停止温度までの平均冷速で、少なくとも３℃／秒以上とすることが望ましい。
【００４１】
次に、本発明の請求項５にかかる製造方法について説明する。
【００４２】
本発明が限定する成分を有する鋼を熱間圧延後、本発明が限定する熱処理を行っても、本発明鋼材の優れた特性を損なうものではない。むしろ、鋼材の組織や結果として材質が均質化するため、目的によっては好ましい場合もある。ただし、この場合でも、組織の微細化が鋼材の強度、靭性を同時に向上させるポイントの１つであるため、熱処理時の再加熱温度はＡｃ3以上９５０℃以下の温度とする必要がある。下限はＣｕの再固溶と組織の均質化のため、また上限は、再加熱時のオーステナイト粒径を必要以上に大きくしないためである。冷却は、請求項４にかかる圧延後の加速冷却と同様「Ｃｕを６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加する程度に固溶状態で存在させること」のため、放冷相当を超える冷速で加速冷却する必要がある。この加速冷却は、４００℃以下まで行えば良いことは、請求項４にかかる説明で述べた通りである。なお、本熱処理を行う場合、圧延後の鋼材は加速冷却を行う必要はなく、圧延後放冷された鋼材であってもよい。
【００４３】
【実施例】
転炉−連続鋳造−厚板工程で種々の鋼成分の鋼板（厚さ２０〜１００ｍ）を製造し、その機械的性質を調査した。
【００４４】
表１に比較鋼とともに本発明鋼の鋼成分を、表２に鋼板の製造条件および諸特性の調査結果を示す。なお、表２中、熱処理を行ったものは所定の温度への再加熱後、４００℃以下まで加速冷却を行っているのは言うまでもない。
【００４５】
本発明法に則った成分、組織および製造方法による鋼板（本発明鋼）は、すべて良好な特性を有する。これに対し、本発明の限定範囲を逸脱する比較鋼は、靭性が劣っていたり、特に本発明が規定するＣｕの固溶程度を表すところの６００℃での時効析出後の降伏強さの上昇代が少なく、結果として６００℃での高温強度（ＹＳ）が明らかに劣っている。
【００４６】
すなわち、比較例２１では、Ｃ量が高いため焼入性が高くなり加速冷却後の強度が高く、Ｃｕ量は適正であるが６００℃での時効析出後の降伏強さは焼入組織の焼戻し効果が優ってむしろ低下し、常温ＹＳに対する６００℃ＹＳの比が低い。比較例２２は、Ｃｕ添加量が少ないのに加え、１０００℃以下の累積圧下量が少なく、加速冷却停止温度も高いため、靭性に劣るとともに、６００℃での時効析出後の降伏強さの上昇代が少なく、結果として６００℃での高温強度（ＹＳ）が劣る。比較例２３は、圧延後に加速冷却されておらず、放冷中にＣｕの析出が進み、６００℃での時効析出後の降伏強さが上昇代が少なく、結果として６００℃での高温強度（ＹＳ）が劣る。また、Ｃｕ添加量に対してＮｉ添加量が低いため、熱間圧延時にクラックが生じ、製造が困難となった。比較例２４では、Ｎｂ、Ｖ、Ｔｉのいずれもが添加されておらず、また製造条件の上でも、圧延終了温度が低く加速冷却開始温度も低いため、フェライトが加工を受け靭性に劣り、加速冷却開始までの間にＣｕ析出も促進されて６００℃での時効析出後の降伏強さの上昇代が少なく、結果として６００℃での高温強度（ＹＳ）が劣る。
【００４７】
【表１】
【表２】
【発明の効果】
本発明により、Ｍｏを多く添加することなく高温強度に優れた鋼の提供が可能となった。その結果、溶接性や靭性を損なうことなく、溶接構造用鋼としての各種用途向けに優れた高温強度を有する鋼材が大量かつ安価に供給できるようになった。このような鋼材を用いることにより、火災時などの高温での強度を維持でき、各種の溶接鋼構造物の安全性を一段と向上させることが可能となった。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel that can maintain sufficient proof stress (strength) even when exposed to high temperatures in the event of an abnormality such as a fire, and a method for producing the same, and in the steel industry, a thick plate, a section steel, a hot strip mill, etc. Applicable to. In addition to the construction field, it can be used in a wide range of applications as general structural steel such as civil engineering, offshore structures, shipbuilding, and various storage tanks.
[0002]
[Prior art]
As a so-called fireproof steel for architectural use for the purpose of guaranteeing high temperature strength, a method for producing Mo-containing steel is disclosed in JP-A-2-77523 and many other published publications. However, Mo remarkably enhances the hardenability of steel and has an extremely strong interaction with C. Therefore, material changes are sensitive to fluctuations in manufacturing conditions, and the strength-toughness balance at normal temperature and its variation, normal temperature strength. In consideration of the balance between high-temperature strength and high-temperature strength, it is effective in terms of high-temperature strength, but is not necessarily an easy-to-use element. In addition, the addition of relatively large amount of Mo to maintain the high temperature strength has a problem that the toughness of the base metal and the welded portion is significantly deteriorated in addition to the remarkable deterioration of the weldability.
[0003]
[Problems to be solved by the invention]
In order to solve the above-mentioned problems of the prior art, the present invention obtains steel having excellent high-temperature strength as well as toughness and weldability, so that Mo is required by utilizing solid solution / precipitation of Cu. Accordingly, it is possible to provide a steel having the above-mentioned composite characteristics and a method capable of supplying the steel in an industrially stable manner by limiting the amount of addition to a small amount and further limiting the production method.
[0004]
[Means for Solving the Problems]
The most important point of the present invention is that Cu is present in a solid solution state at room temperature to the extent that the yield strength increases by 30 N / mm 2 or more when a relatively large amount of Cu is added and aging precipitation treatment is performed at 600 ° C. Or to further limit the manufacturing conditions to obtain such a state.
[0005]
This means that when heated to 600 ° C., Cu that was in a solid solution state at room temperature is precipitated, and high-temperature strength is developed as precipitation strengthening. As a result, the normal temperature strength and the high temperature strength can be improved with a good balance.
[0006]
Therefore, the manufacturing method including the steel components is limited as in the present invention, and the gist thereof is as follows.
[0007]
(1) The steel component is mass%,
C: 0.028% or less,
Si: 0.6% or less,
Mn: 0.51 to 1.6%
P: 0.02% or less,
S: 0.01% or less,
Cu: 0.6 to 2.0%,
Ni: 1/2 to 1.0% of Cu addition amount,
Al: 0.018-0.06%,
N: 0.006% or less,
And,
Nb: 0.005 to 0.1%,
V: 0.01-0.2%
Ti: 0.005 to 0.1%
High-temperature strength characterized by containing at least one or more in the range of iron, the balance being iron and inevitable impurities, and yield strength increasing by 30 N / mm 2 or more when subjected to aging precipitation treatment at 600 ° C. Excellent steel.
[0008]
(2) In addition to the above steel components ,
B: 0.0002 to 0.003%,
Mg: 0.0002 to 0.005%
The steel having excellent high-temperature strength as described in (1) above, which contains 1 type or 2 types within the range described above.
[0009]
(3) In mass%,
Ca: 0.0005 to 0.004% ,
The steel excellent in high temperature strength as described in said (1) or (2) characterized by containing.
[0010]
(4) After reheating the steel slab or slab comprising the steel component according to any one of (1) to (3) above to a temperature range of 1000 to 1250 ° C, the cumulative reduction amount at 1000 ° C or less is set. An aging precipitation treatment at 600 ° C., characterized in that rolling is completed at a temperature of 750 ° C. or higher as 30% or higher, and then accelerated cooling from a temperature of 700 ° C. or higher to an arbitrary temperature of 400 ° C. or lower by forced cooling. A method for producing steel with excellent high-temperature strength that yield strength increases by 30 N / mm 2 or more when performed.
[0011]
(5) After hot rolling the steel slab or slab comprising the steel component according to any one of (1) to (3) above, reheating to a temperature of Ac 3 to 950 ° C. and then forced cooling A method for producing steel excellent in high-temperature strength, wherein yield strength increases by 30 N / mm 2 or more when aging precipitation treatment is performed at 600 ° C., characterized by accelerated cooling.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
[0013]
The reason why the present invention limited the steel composition and the manufacturing method as described in the claims will be described.
[0014]
Since C most significantly increases the hardenability of steel, it is not preferable to add a large amount of C. In particular, in the present invention, if Cu is subjected to an aging precipitation treatment at 600 ° C., the most important point is that the yield strength is increased to 30 N / mm 2 or more so that it is present in a solid solution state. Thus, after rolling, it is necessary to perform accelerated cooling at a cooling speed exceeding that of cooling so that the entire amount of Cu does not precipitate. At this time, it is necessary to suppress the amount of C to a low level so that baking does not occur more than necessary, and the upper limit of the allowable amount of C is 0.03%. The lower limit of the amount of C is not limited and is not particularly limited for reasons of characteristics, but is naturally limited by steelmaking capacity and cost.
[0015]
Si is an element contained in the deoxidized upper steel, but if added in a large amount, weldability and HAZ toughness deteriorate, so the upper limit was limited to 0.6%. Deoxidation of steel can be sufficiently performed only with Ti and Al, and is preferably as low as possible from the viewpoints of HAZ toughness, hardenability, and the like, and it is not always necessary to add them.
[0016]
Mn is an essential element for ensuring the strength and toughness of the base material. Although Mn, which is a substitutional solid solution strengthening element, does not have a significant improvement effect especially at high temperature strength exceeding 600 ° C., addition of 0.51 % or more is essential from the viewpoint of securing normal temperature strength. As for the upper limit, too much addition is limited to 1.6% in order to promote the center segregation of the continuously cast slab or deteriorate the weldability.
[0017]
P is an impurity in the steel of the present invention, and a reduction in the amount of P tends to reduce the grain boundary fracture in the HAZ, so the smaller the better. If the content is large, the low temperature toughness of the base metal and the welded portion is deteriorated, so the upper limit was made 0.02%.
[0018]
S, like P, is an impurity in the steel of the present invention, and is preferably as small as possible from the viewpoint of the low temperature toughness of the base material. If the content is large, the low temperature toughness of the base metal and the welded portion is deteriorated, so the upper limit was made 0.01%.
[0019]
Since Cu is an indispensable element in the present invention, “when Cu is subjected to an aging precipitation treatment at 600 ° C., the yield strength should be present in a solid solution so as to increase by 30 N / mm 2 or more”. Addition of 0.6% or more is essential. The upper limit is originally a property that should be changed depending on the intended normal temperature and high-temperature strength, but the “intended yield strength is 30 N / mm 2 when Cu is subjected to aging precipitation treatment at 600 ° C.” as intended by the present invention. There is no restriction on reaching “to be present in a solid solution state to such an extent that it is increased above”. Rather, it is limited to 2.0% from the viewpoint of weldability and Cu-cracks during hot rolling.
[0020]
In addition, although it is a feature of the present invention that “the yield strength increases by 30 N / mm 2 or more if Cu is subjected to aging precipitation treatment at 600 ° C.”, it is determined whether Cu is dissolved or precipitated at room temperature. However, it is not always easy, and the determination criterion is that “the yield strength increases by 30 N / mm 2 or more when aging precipitation treatment is performed at 600 ° C.”.
[0021]
Ni is an essential element in the present invention in which a relatively large amount of Cu is essential. This is to prevent Cu-cracks during hot rolling, and for that purpose, it is necessary to set the amount of Cu added to ½ or more. If Ni is not added excessively, it improves the strength and toughness of the base material without adversely affecting the weldability and HAZ toughness, but excessive addition is not only expensive, but also undesirable for weldability. The upper limit was 1.0%.
[0022]
In addition, these comparatively large additions of Cu and Ni also have an advantageous effect on the weather resistance.
[0023]
Al is an element generally contained in deoxidized upper steel, but in the steel of the present invention, the lower limit was made 0.018% . However, when the amount of Al increases, not only the cleanliness of the steel deteriorates but also the toughness of the weld metal deteriorates, so the upper limit was made 0.06%.
[0024]
N is contained in the steel as an unavoidable impurity. However, in the steel of the present invention to which at least one of Ti, Nb, and V described later is added, TiN is formed to improve the properties of the steel, , Combined with V to form a carbonitride to increase the strength. For this purpose, it is desirable that the N content is at least 0.001%. However, the increase in the amount of N is extremely harmful to the HAZ toughness and weldability, and the upper limit of the steel of the present invention is 0.006%.
[0025]
In order to further improve the basic characteristics as a welded structural steel without impairing the excellent features of the present invention, at least one of Nb, V, and Ti must be added in addition to the elements described above.
[0026]
First, as a general effect, Nb is an element useful for raising the recrystallization temperature of austenite and maximizing the effect of controlled rolling during hot rolling, and at least 0.005% is added. is necessary. Moreover, it contributes also to the refinement | miniaturization of the heating austenite at the time of the reheating prior to rolling and the heat processing after rolling. Furthermore, it has a strength improving effect as precipitation hardening, and contributes to an increase in high temperature strength. However, excessive addition causes toughness deterioration of the weld zone, so the upper limit was made 0.1%.
[0027]
V has substantially the same action as Nb, but its effect is smaller than that of Nb. V also affects the hardenability and contributes to improving the high temperature strength. The effect similar to Nb is less if it is less than 0.01%, and the upper limit is acceptable up to 0.2%.
[0028]
Ti is preferably added when the requirements for the base material and weld toughness are severe. This is because when Ti has a small amount of Al (for example, 0.003% or less), it combines with O to form precipitates mainly composed of Ti 2 O 3 , and becomes the nucleus of intragranular transformation ferrite formation, resulting in weld toughness. Improve. Ti is combined with N and finely precipitated in the slab as TiN, which suppresses the coarsening of γ grains during heating and is effective for refining the rolled structure. The fine TiN present in the steel sheet is welded. This is to sometimes refine the weld heat affected zone structure. In order to obtain these effects, Ti needs to be at least 0.005%. However, if it is too much, a large amount of TiC is formed and the low temperature toughness and weldability are deteriorated, so the upper limit is 0.1%.
[0029]
Next, the reason for the addition of B and Mg that can be contained as necessary will be described.
[0030]
The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is naturally limited.
[0033]
B segregates at austenite grain boundaries and suppresses the formation of ferrite, thereby improving hardenability and contributing to strength improvement. In order to enjoy this effect, at least 0.0002% is necessary. However, too much addition not only saturates the effect of improving hardenability but also may form B precipitates that are harmful to toughness, so the upper limit was made 0.003%. In cases where stress corrosion cracking is a concern, such as for tank steel, reduction of the hardness of the base metal and the weld heat affected zone is often the point (for example, prevention of sulfide stress corrosion cracking (SCC)). Therefore, HRC ≦ 22 (HV ≦ 248) is essential), and in such a case, B addition for increasing the hardenability is not preferable.
[0034]
Mg suppresses the growth of austenite grains in the weld heat-affected zone and has the effect of making the grains finer, so that the weld zone can be strengthened. In order to enjoy such an effect, Mg needs to be 0.0002% or more. On the other hand, since the effect cost for the added amount decreases as the added amount increases, the upper limit is set to 0.005% because this is not a cost effective measure.
[0035]
Further, Ca controls the morphology of MnS, improves the low temperature toughness of the base material, and reduces the susceptibility to hydrogen induced cracking (HIC, SSC, SOHIC) in a wet hydrogen sulfide environment. In order to exert these effects, 0.0005% is necessary at least. However, too much addition increases the cleanliness of the steel on the contrary, and increases the base metal toughness and susceptibility to hydrogen induced cracking (HIC, SSC, SOHIC) in a wet hydrogen sulfide environment, so the upper limit of the addition amount is 0.004. % .
[0036]
Next, the manufacturing conditions specified in claims 4 and 5 of the present invention and the reasons for the limitation will be described.
[0037]
First, the reason why the heating temperature prior to rolling according to claim 4 of the present invention is limited to 1000 to 1250 ° C. is to keep the austenite grains during heating small and to refine the rolling structure. 1250 ° C. is an upper limit temperature at which the austenite during heating is not excessively coarsened. When the heating temperature is exceeded, the austenite grains are coarsely mixed and the structure after transformation is also coarsened, so that the toughness of the steel is remarkably deteriorated. On the other hand, if the heating temperature is too low, it becomes difficult to secure the rolling end temperature (750 ° C. or higher) described later, and when Nb is added, the recrystallization temperature of austenite is increased, and control during hot rolling is performed. The lower limit was limited to 1000 ° C. from the viewpoint of liquefaction of Nb for maximizing the effect of rolling or expressing the precipitation effect. When Nb is not added, since it is not necessary to consider the solution, it is preferable to heat at a temperature of 1150 ° C. or less from the viewpoint of not coarsening the heated austenite more than necessary.
[0038]
When rolling a slab or steel slab reheated to the above temperature range, it is necessary to end the hot rolling at 750 ° C. or more by setting the cumulative reduction amount at 1000 ° C. or less to 30% or more. This is because when the cumulative amount of rolling at 1000 ° C. or less is small, the austenite of the rolled austenite is insufficiently refined and it is difficult to ensure toughness. Further, when the rolling end temperature is lower than 750 ° C., in the steel of the present invention having a relatively low C content, there is a possibility that a part of transformation starts, and there is a possibility that a processed (rolled) structure is left in the final structure, and toughness In addition to being unfavorable, if the yield ratio is increased and a low yield ratio is required for architectural use, the production becomes difficult, so the rolling end temperature is limited to 750 ° C. or higher.
[0039]
After rolling, forced cooling from a temperature of 700 ° C. or higher, that is, accelerated cooling to an arbitrary temperature of 400 ° C. or lower at a cooling rate other than cooling. These are all because “the Cu should be present in a solid solution so that the yield strength increases by 30 N / mm 2 or more when aging precipitation treatment is performed at 600 ° C.”. If any one of the above conditions deviates, it is difficult to “exist Cu in a solid solution so that the yield strength increases by 30 N / mm 2 or more when aging precipitation treatment is performed at 600 ° C.”. Become.
[0040]
Although the cooling speed during accelerated cooling varies depending on the steel composition and the intended material (strength, toughness) level, it cannot be generally stated, but from the accelerated cooling start temperature to the stop temperature at the 1/4 thickness position. The average cooling rate is preferably at least 3 ° C./second or more.
[0041]
Next, the manufacturing method concerning Claim 5 of this invention is demonstrated.
[0042]
Even if the steel having the components limited by the present invention is hot-rolled and then subjected to the heat treatment limited by the present invention, the excellent properties of the steel of the present invention are not impaired. Rather, it may be preferable depending on the purpose because the structure of the steel material and the resulting material are homogenized. However, even in this case, since the refinement of the structure is one of the points that simultaneously improve the strength and toughness of the steel material, the reheating temperature during the heat treatment needs to be a temperature of Ac 3 or higher and 950 ° C. or lower. The lower limit is for re-dissolution of Cu and homogenization of the structure, and the upper limit is for not making the austenite grain size during reheating larger than necessary. The cooling is the same as the accelerated cooling after rolling according to claim 4 "Cu should be present in a solid solution so that the yield strength increases by 30 N / mm 2 or more when aging precipitation treatment is performed at 600 ° C" Therefore, it is necessary to perform accelerated cooling at a cooling speed exceeding that of cooling. As described in the description of claim 4, the accelerated cooling may be performed to 400 ° C. or lower. In addition, when performing this heat processing, the steel material after rolling does not need to perform accelerated cooling, and may be the steel material cooled after rolling.
[0043]
【Example】
Steel sheets (thickness 20 to 100 m) having various steel components were manufactured by a converter-continuous casting-thick plate process, and the mechanical properties thereof were investigated.
[0044]
Table 1 shows the steel components of the steel of the present invention together with the comparative steel, and Table 2 shows the results of the investigation of the manufacturing conditions and various properties of the steel sheet. In Table 2, it goes without saying that those subjected to heat treatment are subjected to accelerated cooling to 400 ° C. or lower after reheating to a predetermined temperature.
[0045]
The steel sheets (invention steels) according to the components, structures and production methods according to the invention method all have good characteristics. On the other hand, the comparative steels that depart from the limited range of the present invention are inferior in toughness or increase in yield strength after aging precipitation at 600 ° C., which expresses the degree of solid solution of Cu specified by the present invention. As a result, the high temperature strength (YS) at 600 ° C. is clearly inferior.
[0046]
That is, in Comparative Example 21, since the amount of C is high, the hardenability is high and the strength after accelerated cooling is high. The amount of Cu is appropriate, but the yield strength after aging precipitation at 600 ° C. is tempered of the quenched structure. The effect is rather low and the ratio of 600 ° C. YS to room temperature YS is low. In Comparative Example 22, in addition to the small amount of Cu added, the cumulative rolling amount below 1000 ° C. is small and the accelerated cooling stop temperature is high, so that the toughness is inferior and the yield strength increases after aging precipitation at 600 ° C. As a result, the high temperature strength (YS) at 600 ° C. is inferior. Comparative Example 23 was not acceleratedly cooled after rolling, and Cu precipitation progressed during cooling, and the yield strength after aging precipitation at 600 ° C. was small, resulting in high temperature strength at 600 ° C. ( YS) is inferior. Moreover, since Ni addition amount was low with respect to Cu addition amount, the crack generate | occur | produced at the time of hot rolling, and manufacture became difficult. In Comparative Example 24, none of Nb, V, and Ti was added, and even on the manufacturing conditions, since the rolling end temperature was low and the accelerated cooling start temperature was low, the ferrite was processed and inferior in toughness and accelerated. Cu precipitation is also promoted before the start of cooling, and there is little increase in yield strength after aging precipitation at 600 ° C., resulting in poor high-temperature strength (YS) at 600 ° C.
[0047]
[Table 1]
[Table 2]
【The invention's effect】
According to the present invention, it is possible to provide steel having excellent high-temperature strength without adding much Mo. As a result, it has become possible to supply a large amount and a low cost of a steel material having excellent high-temperature strength for various uses as welded structural steel without impairing weldability and toughness. By using such a steel material, it is possible to maintain the strength at a high temperature such as in a fire, and it is possible to further improve the safety of various welded steel structures.
Claims (5)
Ｃ：０．０２８％以下、
Ｓｉ：０．６％以下、
Ｍｎ：０．５１〜１．６％、
Ｐ：０．０２％以下、
Ｓ：０．０１％以下、
Ｃｕ：０．６〜２．０％、
Ｎｉ：Ｃｕ添加量の１／２〜１．０％、
Ａｌ：０．０１８〜０．０６％、
Ｎ：０．００６％以下、
かつ、
Ｎｂ：０．００５〜０．１％、
Ｖ：０．０１〜０．２％、
Ｔｉ：０．００５〜０．１％
の範囲で少なくとも１種以上を含有し、残部が鉄および不可避的不純物からなり、６００℃での時効析出処理を行った場合降伏強さが３０Ｎ／ｍｍ2以上増加することを特徴とする高温強度に優れた鋼。Steel component is mass%,
C: 0.028% or less,
Si: 0.6% or less,
Mn: 0.51 to 1.6%
P: 0.02% or less,
S: 0.01% or less,
Cu: 0.6 to 2.0%,
Ni: 1/2 to 1.0% of Cu addition amount,
Al: 0.018-0.06%,
N: 0.006% or less,
And,
Nb: 0.005 to 0.1%,
V: 0.01-0.2%
Ti: 0.005 to 0.1%
High-temperature strength characterized by containing at least one or more in the range of iron, the balance being iron and inevitable impurities, and yield strength increasing by 30 N / mm 2 or more when subjected to aging precipitation treatment at 600 ° C. Excellent steel.
Ｂ：０．０００２〜０．００３％、
Ｍｇ：０．０００２〜０．００５％
の範囲で１種または２種を含有することを特徴とする請求項１に記載の高温強度に優れた鋼。In addition to the above steel components ,
B: 0.0002 to 0.003%,
Mg: 0.0002 to 0.005%
The steel having excellent high-temperature strength according to claim 1, wherein the steel contains 1 type or 2 types within the range.
Ｃａ：０．０００５〜０．００４％、
を含有することを特徴とする請求項１または２に記載の高温強度に優れた鋼。% By mass
Ca: 0.0005 to 0.004% ,
The steel excellent in high-temperature strength according to claim 1 or 2, characterized by comprising:
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000311216A JP4362219B2 (en) | 2000-10-11 | 2000-10-11 | Steel excellent in high temperature strength and method for producing the same |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000311216A JP4362219B2 (en) | 2000-10-11 | 2000-10-11 | Steel excellent in high temperature strength and method for producing the same |
Publications (2)
| Publication Number | Publication Date |
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| JP5098210B2 (en) * | 2005-05-02 | 2012-12-12 | 新日鐵住金株式会社 | Refractory steel and method for producing the same |
| JP4482527B2 (en) * | 2005-05-17 | 2010-06-16 | 新日本製鐵株式会社 | High-strength ultra-thick H-shaped steel with excellent fire resistance and method for producing the same |
| JP4656417B2 (en) * | 2006-01-18 | 2011-03-23 | 株式会社神戸製鋼所 | Low yield ratio refractory steel |
| CN101379209A (en) * | 2006-02-08 | 2009-03-04 | 新日本制铁株式会社 | Fire-resistant high-strength rolled steel material and method for production thereof |
| JP4072191B1 (en) | 2006-09-04 | 2008-04-09 | 新日本製鐵株式会社 | Refractory steel material excellent in high temperature strength, toughness and reheat embrittlement resistance, and production method thereof |
| JP4673822B2 (en) | 2006-11-14 | 2011-04-20 | 新日本製鐵株式会社 | Refractory steel material excellent in toughness of welded joint and method for producing the same |
| WO2008126910A1 (en) * | 2007-04-06 | 2008-10-23 | Nippon Steel Corporation | Steel material having excellent high temperature properties and excellent toughness, and method for production thereof |
| CN101657555B (en) | 2007-04-11 | 2011-08-03 | 新日本制铁株式会社 | Steel material having excellent high-temperature strength and toughness, and method for production thereof |
| JP4638956B2 (en) | 2008-03-31 | 2011-02-23 | 新日本製鐵株式会社 | Refractory steel material excellent in reheat embrittlement resistance and toughness of welded joint and method for producing the same |
| CN103789630B (en) * | 2014-01-24 | 2016-01-20 | 李露青 | A kind of carbon steel work-piece production method for speed skate blade |
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