JP3879365B2 - Manufacturing method of steel material with excellent fatigue crack growth resistance - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、疲労亀裂進展抵抗性に優れた鋼材の製造方法に関する。より詳しくは、船舶、海洋構造物、橋梁、建築物、タンクなど各種構造物に用いられ、繰返し荷重を受けた場合でも良好な耐疲労亀裂進展性を示す構造用鋼材の製造方法に関する。
【０００２】
【従来の技術】
船舶、海洋構造物、橋梁、建築物、タンクなど各種の構造物に使用される鋼材には、強度、靱性などの機械的性質や溶接性に優れていることが要求される。上記構造物に対して構造上の安全性を確保させるためには、機械的性質の中でも特に耐疲労特性を高めることが極めて重要である。
【０００３】
構造物が疲労損傷で破壊する過程については、従来、応力集中部における疲労亀裂の発生及びその後の疲労亀裂進展の２つの過程に大きく分類した検討がなされてきた。
【０００４】
構造物のうちでも特に溶接構造物の場合には、応力集中部となり得る溶接止端部が多数存在するので、疲労亀裂の発生を工業的規模で完全に防止することは技術的に不可能に近い。又、疲労亀裂の発生を完全に防止することは検査費用の著しい上昇を招くため経済的にも得策ではない。そこで、亀裂の進展寿命をいかに長寿命化するかが重要となり、設計面からの工夫として、荷重経路を多重化させる構造を採用することがある。これは、荷重経路を多重化した場合には、構造物中のある箇所に亀裂が発生して相当な長さにまで成長しても、剛性の低下した亀裂を含む部分が分担していた荷重を、他の部分が分担し合うことになり、その結果、亀裂を含む部分での亀裂進展は抑制され、亀裂の発生・進展直後に致命的な破壊に至ることを防ぐことができるからである。
【０００５】
一方、現実問題としては、通常の構造物では経済性を優先させるため強度上の冗長度を多くとることには自ずと制約があり、上述の荷重経路の多重化という設計手法による対応では効果に限界がある。したがって、産業界からは鋼材自身の疲労亀裂進展抵抗性を増すことに対する要望が大きい。
【０００６】
鋼材の疲労亀裂進展抵抗性の向上に関する技術が、例えば、特開平４−３３７０３７号公報、特開平６−２７１９８５号公報や特開平６−２９９２３８号公報に開示されている。
【０００７】
このうち特開平４−３３７０３７号公報には、特定の化学組成を有する鋼を特定の条件で圧延、冷却、巻取りして特定の最終組織を有する熱延鋼板を得る「疲労強度と亀裂伝播抵抗の優れた良成形性熱延鋼板の製造方法」が開示されている。しかし、この公報で提案された技術の場合、改善の対象としている「疲労亀裂伝播抵抗」は「亀裂進展下限界特性」に限定されたものであり、したがって、「疲労亀裂伝播停止特性」を記述する力学パラメータはΔＫｔｈのみで、亀裂の安定進展領域は対象にはされていないので、外力が増すなどして疲労亀裂先端での応力拡大係数範囲ΔＫがΔＫｔｈを超えて疲労亀裂が進展し始めると、従来鋼と同じ疲労亀裂進展速度で亀裂が成長してしまう。
【０００８】
特開平６−２７１９８５号公報には、特定の組織を有する鋼板及びその鋼板を得るために鋼の化学組成と圧延や冷却の条件を規定した「耐疲労伝播特性の優れた鋼板およびその製造方法」が開示されている。しかし、この公報で提案された技術の場合、島状マルテンサイトの平均存在間隔、平均偏平比及び体積率を規定して疲労亀裂進展速度の抑制を実現しており、特に島状マルテンサイトの体積率を０．５〜５％と規定している。したがって、島状マルテンサイトを起点とする脆性破壊が容易に発生し、疲労亀裂進展特性と破壊靱性とを両立させることが困難である。
特開平６−２９９２３８号公報には、鋼の化学組成と圧延や冷却の条件を規定した「耐疲労伝播特性と溶接熱影響部靱性の優れた鋼板の製造方法」が開示されている。しかし、この公報で提案された技術の場合、Ｔｉで脱酸した後にＡｌを添加するという複雑な製鋼工程が必須であり、したがって、製鋼コストの上昇や他鋼種の製造工程への悪影響を避け難い。
【０００９】
【発明が解決しようとする課題】
本発明は、上記現状に鑑みなされたもので、その目的は、構造物の亀裂進展寿命を延伸させて構造上の安全性を確保させるために、繰返し荷重を受けた場合でも良好な耐疲労亀裂進展性を示す構造用鋼材の製造方法を提供することである。
【００１０】
【課題を解決するための手段】
本発明の要旨は、下記（１）、（２）に示す疲労亀裂進展抵抗性に優れた鋼材の製造方法にある。
【００１１】
（１）質量％で、
Ｃ：０．０６〜０．２５％、
Ｓｉ：０．０３〜０．６％、
Ｍｎ：０．３０〜２．０％、
Ａｌ：０．０１０〜０．１０％、
Ｎｂ：０．０１０〜０．１０％、
Ｔｉ：０．０１０〜０．１０％
を含有し、下記(1)式で表されるＦＣＧ１の値が４．０以下を満たし、残部がＦｅ及び不純物からなる鋼片を、１０５０℃以上に加熱して１０４０〜７４０℃の圧延仕上げ温度で熱間圧延し、その後、（Ｔｓ１＋５０）〜（Ｔｓ１−２５）℃の温度から５〜５０℃／秒の冷却速度で少なくとも５５０℃まで冷却することを特徴とする疲労亀裂進展抵抗性に優れた鋼材の製造方法。ここで、Ｔｓ１は下記(2)式から計算される温度である。
【００１２】
ＦＣＧ１＝（０．７＋Ｃ）／｛（Ｓｉ／２５）＋（Ｍｎ／５）＋（Ｎｂ／２５）＋Ｔｉ｝・・・・・(1)、
Ｔｓ１＝（８２０−２００Ｃ−６０Ｓｉ−６００Ｎｂ＋２０００Ｔｉ）・・・・・(2)。
なお、各式における元素記号はその元素の質量％での含有量を示す。
【００１３】
（２）質量％で、
Ｃ：０．０２〜０．２５％、
Ｓｉ：０．０３〜０．６％、
Ｍｎ：０．３０〜２．０％、
Ａｌ：０．０１０〜０．１０％、
Ｎｂ：０．０１０〜０．１０％、
Ｔｉ：０．０１２〜０．１０％
を含有し、更に、
第１群：Ｃｕ：０．０２〜０．２１％、Ｃｒ：０．０３〜２．０％のうちの１種以上、
第２群：Ｎｉ：０．０５〜１．０％、Ｍｏ：０．０５〜１．０％のうちの１種以上、
第３群：Ｖ：０．０１〜０．５％
の１群以上をも含み、下記(3)式で表されるＦＣＧ２の値が４．０以下を満たし、残部がＦｅ及び不純物からなる鋼片を、１０５０℃以上に加熱して１０４０〜７４０℃の圧延仕上げ温度で熱間圧延し、その後、（Ｔｓ２＋５０）〜（Ｔｓ２−２５）℃の温度から５〜５０℃／秒の冷却速度で少なくとも５５０℃まで冷却することを特徴とする疲労亀裂進展抵抗性に優れた鋼材の製造方法。ここで、Ｔｓ２は下記(4)式から計算される温度である。
【００１４】
ＦＣＧ２＝（０．７＋Ｃ）／｛（Ｓｉ／２５）＋（Ｍｎ／５）＋（Ｃｕ／５０）＋（Ｎｉ／５０）＋（Ｃｒ／２５）＋（Ｍｏ／２５）＋（Ｖ／５）＋（Ｎｂ／２５）＋Ｔｉ｝・・・・・(3)、
Ｔｓ２＝（１＋０．１Ｖ）（８２０−２００Ｃ−６０Ｓｉ＋２００Ｃｕ＋５０Ｎｉ−１００Ｃｒ＋６００Ｍｏ−６００Ｎｂ＋２０００Ｔｉ）・・・・・(4)。
なお、各式における元素記号はその元素の質量％での含有量を示す。
以下、上記（１）及び（２）の疲労亀裂進展抵抗性に優れた鋼材の製造方法に係る発明を、それぞれ、「本発明（１）」および「本発明（２）」という。また、総称して「本発明」ということがある。
【００１５】
本発明者らは、前記した課題を達成するために鋼材の疲労亀裂進展挙動に及ぼす鋼の化学組成、圧延条件、冷却条件の影響を検討し、下記の知見を得た。
【００１６】
（ａ）疲労亀裂進展速度を抑制し、疲労亀裂進展寿命を延伸させるためには、▲１▼鋼材内の微視的な領域で硬度差を設け、疲労亀裂伝播経路を微視的に屈曲させて巨視亀裂進展速度を抑制させる方法、▲２▼鋼材に繰返し歪みが負荷された時の応力応答として繰返し軟化を生ずる鋼材を準備し、疲労亀裂進展時の亀裂先端での応力を緩和させることにより実質的な亀裂先端の応力を低めて疲労亀裂進展速度を抑制させる方法、▲３▼鋼材の降伏強度に対する引張強度の比（降伏比）を低く押さえることにより、疲労亀裂先端に形成される塑性変形領域をより広くして亀裂先端の局所的負荷を軽減し、結果として疲労亀裂進展速度を抑制する方法があるが、これらのいずれの方法の場合にも、前記 (1)式で表されるＦＣＧ１の値又は前記 (3)式で表されるＦＣＧ２の値を小さくすることが有効である。なお、以下の説明においては、前記 (1)式で表されるＦＣＧ１及び前記 (3)式で表されるＦＣＧ２をまとめてＦＣＧということもある。
【００１７】
（ｂ）熱間圧延後の冷却開始温度は、鋼材の疲労亀裂進展抵抗性に大きく影響し、疲労亀裂進展速度が最小になる最適な冷却開始温度領域が存在する。
【００１８】
そこで、本発明者らは、種々の化学組成を有する鋼材の疲労亀裂進展抵抗性と熱間圧延後の冷却開始温度、冷却速度との関係について更に詳細な検討を加えた。その結果、次の事項が明らかになった。
【００１９】
（ｃ）いずれの鋼材においても熱間圧延後の冷却開始温度を低温側から徐々に上昇させると、疲労亀裂進展速度は漸減して行くが、その漸減割合は冷却開始温度が高くなるにつれて段々と少なくなる。そして、更に冷却開始温度を上昇させると却って疲労亀裂進展速度が漸増して行く。つまり、疲労亀裂進展速度が最小になる最適な冷却開始温度が存在する。
【００２０】
（ｄ）上記の疲労亀裂進展速度漸増の割合は、その漸減時の割合よりも緩やかであり、最適冷却開始温度に対し疲労亀裂進展抵抗性がほぼ同等である冷却開始温度は、最適冷却開始温度に対し、低温側に２５℃、高温側に５０℃の７５℃の幅の領域である。
（ｅ）上記（ｃ）の疲労亀裂進展速度が最小になる最適な冷却開始温度は、前記 (2)式で表されるＴｓ１又は前記 (4)式で表されるＴｓ２と関係する。なお、以下の説明においては、前記 (2)式で表されるＴｓ１及び前記 (4)式で表されるＴｓ２をまとめてＴｓということもある。
【００２１】
（ｆ）熱間圧延後の鋼材冷却速度の適正化も、疲労亀裂進展速度を抑制し、疲労亀裂進展寿命を延ばすのに有効である。
本発明は、上記の知見に基づいて完成されたものである。
【００２２】
【発明の実施の形態】
以下、本発明の各要件について詳しく説明する。なお、化学成分の含有量の「％」は「質量％」を意味する。
（Ａ）鋼材の素材鋼の化学組成
Ｃ：０．０６〜０．２５％（本発明（１））、Ｃ：０．０２〜０．２５％（本発明（２））
Ｃは、構造部材の強度確保に有効な元素である。しかし、本発明（１）の場合、その含有量が０．０６％未満では鋼材の組織に占めるフェライト相の割合が極めて多く強度向上効果が得難い。また、本発明（２）の場合、その含有量が０．０２％未満では鋼材の組織に占めるフェライト相の割合が極めて多く強度向上効果が得難い。一方、本発明（１）及び本発明（２）のいずれの場合にも、Ｃの含有量が０．２５％を超えると溶接性が低下するので溶接施工が困難となり、構造用鋼材としての使用領域が著しく限定されてしまう。したがって、Ｃの含有量を、本発明（１）の場合には０．０６〜０．２５％とし、また、本発明（２）の場合には０．０２〜０．２５％とした。なお、鋼材の組織に占めるフェライト相の割合を抑えて大きな強度を確保するとともに良好な溶接性をも確保するために、Ｃの含有量は、本発明（１）の場合には０．０６〜０．１２％とし、また、本発明（２）の場合には０．０４〜０．１２％とすることが望ましい。
【００２３】
Ｓｉ：０．０３〜０．６％
Ｓｉは、脱酸作用を有する。しかし、その含有量が０．０３％未満では添加効果に乏しい。一方、Ｓｉの含有量が０．６％を超えると破壊靱性が低下するようになる。したがって、Ｓｉの含有量を０．０３〜０．６％とした。なお、Ｓｉの含有量は０．２５〜０．５％とすることが望ましい。
【００２４】
Ｍｎ：０．３０〜２．０％
Ｍｎは、強度の確保に有効な元素である。しかし、その含有量が０．３０％未満では添加効果に乏しい。一方、Ｍｎの含有量が２．０％を超えると溶接性が低下するので溶接施工が困難となり、構造用鋼材としての使用領域が著しく限定されてしまう。したがって、Ｍｎの含有量を０．３０〜２．０％とした。なお、大きな強度を確保するとともに良好な溶接性をも確保するために、Ｍｎの含有量は０．５〜１．８％とすることが望ましい。
【００２５】
Ａｌ：０．０１０〜０．１０％
Ａｌは、脱酸作用を有する。しかし、その含有量が０．０１０％未満では添加効果に乏しく、一方、０．１０％を超えると破壊靱性が低下するし、鋼の清浄性も悪くなる。したがって、Ａｌの含有量を０．０１０〜０．１０％とした。なお、Ａｌ含有量は０．０１０〜０．０５％とすることが望ましい。
【００２６】
Ｎｂ：０．０１０〜０．１０％
Ｎｂは、破壊靱性を確保するのに有効な元素であり、構造材料として用いるために０．０１０％以上含有させる必要がある。しかし、その含有量が０．１０％を超えると却って破壊靱性が低下するようになる。したがって、Ｎｂの含有量を０．０１０〜０．１０％とした。なお、Ｎｂの含有量は０．０２０〜０．０５％とすることが望ましい。
【００２７】
Ｔｉ：０．０１０〜０．１０％（本発明（１））、Ｔｉ：０．０１２〜０．１０％（本発明（２））
Ｔｉは、破壊靱性を確保するのに有効な元素であり、構造材料として用いるために本発明（１）の場合０．０１０％以上、また、本発明（２）の場合０．０１２％以上含有させる必要がある。しかし、本発明（１）及び本発明（２）のいずれの場合にも、その含有量が０．１０％を超えると却って破壊靱性が低下するようになる。したがって、Ｔｉの含有量を、本発明（１）の場合には０．０１０〜０．１０％とし、また、本発明（２）の場合には０．０１２〜０．１０％とした。た。なお、本発明（１）及び本発明（２）のいずれの場合にも、Ｔｉの含有量は０．０１５〜０．０５％とすることが望ましい。
【００２８】
ＦＣＧ１：４．０以下
前記(1)式で表されるＦＣＧ１の値が４．０以下の場合に、疲労亀裂進展速度を抑制して疲労亀裂進展寿命を延伸させることができる。したがって、本発明（１）においては、ＦＣＧ１の値を４．０以下とした。このＦＣＧ１の値は３．０以下とすることが好ましい。なお、ＦＧＣ１の下限値は、Ｃ、Ｓｉ、Ｍｎ、Ｎｂ、Ｔｉの含有量がそれぞれ０．０６％、０．６％、２．０％、０．１％、０．１％の場合の１．４４である。
【００２９】
なお、本発明（１）の疲労亀裂進展抵抗性に優れた鋼材の製造方法に用いられる素材鋼は、上記ＣからＴｉまでの各成分元素を含み、前記 (1) 式で表されるＦＣＧ１の値が４．０以下を満たし、残部がＦｅ及び不純物からなる鋼であるが、本発明（２）の疲労亀裂進展抵抗性に優れた鋼材の製造方法に用いられる素材鋼は、上記ＣからＴｉの各成分元素に加えて更に、前記第１群〜第３群のうちの１群以上を含み、前記 (3) 式で表されるＦＣＧ２の値が４．０以下を満たし、残部がＦｅ及び不純物からなる鋼である。前記第１群〜第３群の合金元素の作用効果と望ましい含有量は下記のとおりである。
【００３０】
Ｃｕ：０．０２〜０．２１％、Ｃｒ：０．０３〜２．０％
Ｃｕ及びＣｒには耐食性を高める作用があるので、鋼材が腐食環境下で使用される場合等に耐食性を確保する目的で含有させるが、Ｃｕ含有量が０．０２％未満、Ｃｒ含有量が０．０３％未満ではその効果が得難い。一方、Ｃｕを０．２１％を超えて、又、Ｃｒを２．０％を超えて含有させると溶接性が低下するので溶接施工が困難となり、構造用鋼材としての使用領域が著しく限定されてしまう。更に、Ｃｕを０．２１％を超えて含有させた場合には、熱間加工性が低下して圧延で鋼材表面に疵が生じたり、鋼材の表面や内部に割れが発生することがある。したがってＣｕ、Ｃｒの１種以上を添加する場合には、Ｃｕの含有量を０．０２〜０．２１％、Ｃｒの含有量を０．０３〜２．０％とするのがよい。なお、Ｃｕ、Ｃｒの１種以上を添加する場合、Ｃｕの含有量は０．１〜０．２１％、Ｃｒの含有量は０．２〜１．５％とするのが好ましい。
Ｎｉ：０．０５〜１．０％、Ｍｏ：０．０５〜１．０％
Ｎｉ及びＭｏには鋼の破壊靱性を高めるとともに焼入れ性を高める作用があるので、破壊靱性を確保したり鋼材が大型構造物に使用される場合の焼入れ性を確保したりする目的で含有させるが、Ｎｉ含有量が０．０５％未満、Ｍｏ含有量が０．０５％未満ではその効果が得難い。一方、Ｎｉを１．０％を超えて、又、Ｍｏを１．０％を超えて含有させてもその効果は飽和しコストが嵩むばかりである。したがってＮｉ、Ｍｏの１種以上を添加する場合には、Ｎｉの含有量を０．０５〜１．０％、Ｍｏの含有量を０．０５〜１．０％とするのがよい。
Ｖ：０．０１〜０．５％
Ｖには強度を高める作用があるので、構造物に大きな強度を確保する目的で含有させるが、その含有量が０．０１％未満ではその効果に乏しく、一方、０．５％を超えて含有させてもその効果は飽和しコストが嵩むばかりである。したがってＶを添加する場合には、０．０１〜０．５％の含有量とするのがよい。
【００３１】
ＦＣＧ２：４．０以下
前記(3)式で表されるＦＣＧ２の値が４．０以下の場合に、疲労亀裂進展速度を抑制して疲労亀裂進展寿命を延伸させることができる。したがって、本発明（２）においては、ＦＣＧ２の値を４．０以下とした。このＦＣＧ２の値は３．０以下とすることが好ましい。なお、ＦＧＣ２の下限値は、Ｃ、Ｓｉ、Ｍｎ、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖ、Ｎｂ、Ｔｉの含有量がそれぞれ０．０２％、０．６％、２．０％、１．０％、１．０％、２．０％、１．０％、０．５％、０．１％、０．１％の場合の０．９１である。
（Ｂ）鋼材の製造条件
（Ｂ−１）熱間圧延前の加熱温度：１０５０℃以上
圧延前組織を整え、圧延加工が容易に行えるようにするために、加熱温度は１０５０℃以上とする必要がある。したがって、上記（Ａ）に記載した化学組成を有する鋼片の熱間圧延前の加熱温度を１０５０℃以上とした。なお、加熱温度は１１００〜１２００℃とすることが好ましい。
（Ｂ−２）圧延仕上げ温度：１０４０〜７４０℃
組織を粗大化させないために圧延仕上げ温度は１０４０℃以下とする必要がある。しかし、圧延仕上げ温度が７４０℃を下回ると、被圧延材の表面に加工が集中して、鋼材の厚さ方向の機械的性質が不均一になってしまう。したがって、熱間圧延仕上げ温度を１０４０〜７４０℃とした。なお、圧延仕上げ温度は９５０〜８５０℃とすることが好ましい。
（Ｂ−３）熱間圧延後の冷却
疲労亀裂進展速度を抑制し、疲労亀裂進展寿命を延伸させるためは、熱間圧延後の冷却開始を（Ｔｓ＋５０）〜（Ｔｓ−２５）℃の温度から行う必要がある。冷却開始温度が（Ｔｓ＋５０）℃を上回ったり、（Ｔｓ−２５）℃を下回れば疲労亀裂進展速度が大きくなってしまう。なお、上記のＴｓは鋼材の化学組成に応じて前記(2)式で表されるＴｓ１（本発明（１）の場合）又は前記(4)式で表されるＴｓ２（本発明（２）の場合）を指すものである。
更に、上記温度領域からの冷却に際し、その冷却速度を５〜５０℃／秒として少なくとも５５０℃まで冷却する必要がある。冷却速度が５℃／秒未満の場合には、鋼材内の微視的な領域で硬度差を設け、疲労亀裂伝播経路を微視的に屈曲させて巨視亀裂進展速度を抑制させることができず、一方、冷却速度が５０℃／秒を超えると、鋼材の板厚方向に極めて大きな残留応力分布が発生し、引張残留応力が形成される板厚中心部での疲労亀裂進展速度が加速する。又、冷却終了温度が５５０℃を超えると、鋼材の繰返し軟化特性が発現せず、亀裂先端での応力が緩和されないので、結果として亀裂進展速度を抑制できない。
【００３２】
したがって、熱間圧延後、（Ｔｓ＋５０）〜（Ｔｓ−２５）℃の温度から５〜５０℃／秒の冷却速度で少なくとも５５０℃まで冷却することとした。
【００３３】
次に、実施例により本発明を更に詳しく説明する。
【００３４】
【実施例】
（実施例１）
表１〜３に示す化学組成を有する鋼を通常の方法によって試験炉溶製した。なお、表１〜３には化学組成から計算されるＦＣＧ（つまり、ＦＣＧ１とＦＣＧ２）の値及びＴｓ（つまり、Ｔｓ１とＴｓ２）の値も示した。
【００３５】
【表１】
このようにして得た板厚２５ｍｍの鋼板から各種試験片を採取し、溶接性、清浄度、破壊靱性及び引張強度を測定した。
すなわち、上記板厚２５ｍｍの鋼板の板厚中心部を主に評価できるように、ｙ型溶接割れ試験片、ミクロ試験片、JIS Z 2202(1998)に記載のＶノッチ衝撃試験片、JIS Z 2201(1998)に記載の４号引張試験片を採取し、次の各条件で溶接性、清浄度、破壊靱性及び引張強度を調査した。
【００３６】
ｙ型溶接割れ試験は、通常のサブマージアーク溶接用の溶接材料を用いて、試験片予熱温度が２５℃、５０℃、７５℃、１００℃、１２５℃の各場合について、温度３０℃、湿度８０％の雰囲気で実施した。なお、前記溶接材料を温度３０℃、湿度８０％の環境に１時間放置してから試験を行った。各予熱温度に対する試験数は２である。
【００３７】
清浄度の評価は、JIS G 0555(1998)に則って顕微鏡の倍率を４００倍とし、視野数６０で行った。
【００３８】
破壊靱性試験は、板厚中心部から試験片の長手方向が圧延方向に一致し、亀裂進展方向が鋼材の幅方向に一致するように採取した前記JIS Z 2202(1998)に記載のＶノッチ衝撃試験片を用い、各試験温度における試験数を３としてシャルピー衝撃試験を行い、脆性−延性の破面遷移温度（ｖＴｒｓ）を求めた。
引張試験は、板厚中心部から試験片の長手方向が圧延方向に一致するように採取したJIS Z 2201(1998)に記載の４号引張試験片を用いて室温大気中で行った。なお、試験数は２とした。
【００３９】
表４に上記の各試験結果を示す。なお、表２におけるマーク「○」と「×」はそれぞれ次の区分に基づくものである。
【００４０】
すなわち、「溶接性」は予熱温度２５℃で割れを生じない場合を「○」、割れを生じないために５０℃以上の予熱を必要とした場合を「×」とした。「清浄度」は、現状のＡ級の船級規格鋼を基準に判断し、これと同等以上の清浄度である場合を「○」、それを下回る場合を「×」とした。「破壊靱性」は破面遷移温度（ｖＴｒｓ）が−２０℃未満の場合を「○」、−２０℃以上の場合を「×」とした。又、「引張強度」は４００ＭＰａ以上の公称引張強度が得られた場合を「○」、４００ＭＰａを下回る場合を「×」とした。
【００４１】
【表４】
【００４２】
次いで、溶接性、清浄度、破壊靱性、引張強度のいずれもが目標に達した鋼１〜１５、鋼１７〜２１及び鋼２３〜２６の板厚２５ｍｍの鋼板の全厚での疲労亀裂進展特性を評価するために、ＬＴ方向でＣＴ試験片を採取し、負荷条件として繰返し速度２５Ｈｚ、応力比０．１の下、室温大気中でＡＳＴＭ規格（Ｅ６４７）に則って疲労亀裂進展試験を行った。疲労亀裂進展速度は、亀裂先端における応力拡大係数範囲ΔＫが２０ＭＰａ・ｍ1/2における進展速度で代表させて求めた。なお、一般鋼材における疲労亀裂進展速度のレベルを参照して、目標とする疲労亀裂進展速度はその上限を４．０×１０-5ｍｍ／サイクルとした。
表５に、疲労亀裂進展試験の結果を示す。
【００４３】
【表５】
（実施例２）
前記実施例１で溶製した鋼の一部を用いて、疲労亀裂進展速度に及ぼす冷却開始温度の影響を詳細に調査した。
【００４４】
すなわち、鋼６、鋼７及び鋼２０について、通常の方法で熱間鍛造して作製した厚さ１５０ｍｍの鋼片を、１１００℃に加熱してから熱間圧延した。なお、熱間圧延は、圧延仕上げ温度を９５０〜７５０℃として、板厚２５ｍｍに仕上げた。熱間圧延仕上げ後は、鋼の化学組成に応じて表６に記載の各種の温度から４０℃／秒の冷却速度で５００℃まで冷却した。
【００４５】
【表６】
疲労亀裂進展試験の結果は表６に示したとおりである。
【００４６】
表６から、鋼材に含まれる各元素が本発明で規定する含有量の範囲内にあり、しかもＦＧＣの値が本発明で規定する範囲内にあっても、冷却開始温度が本発明で規定する範囲（すなわち（Ｔｓ＋５０）〜（Ｔｓ−２５）℃）から外れた場合には、疲労亀裂進展速度が目標とする値を超え、疲労亀裂進展抵抗性に劣ることが明らかである。
【００４７】
【発明の効果】
本発明によれば、疲労亀裂進展抵抗性に優れた鋼材が得られるので、船舶、海洋構造物、橋梁、建築物、タンクなど各種構造物に用いることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a steel material having excellent fatigue crack growth resistance. More specifically, the present invention relates to a method for producing a structural steel material that is used in various structures such as ships, offshore structures, bridges, buildings, and tanks and exhibits good fatigue crack growth resistance even when subjected to repeated loads.
[0002]
[Prior art]
Steel materials used in various structures such as ships, offshore structures, bridges, buildings, tanks, etc. are required to have excellent mechanical properties such as strength and toughness and weldability. In order to ensure structural safety for the above structure, it is extremely important to improve fatigue resistance among mechanical properties.
[0003]
Conventionally, the process of fracture of a structure due to fatigue damage has been broadly classified into two processes, namely, the generation of a fatigue crack in a stress concentration portion and the subsequent fatigue crack growth.
[0004]
Among the structures, especially in the case of welded structures, there are many weld toes that can become stress concentrated parts, so it is technically impossible to completely prevent the occurrence of fatigue cracks on an industrial scale. close. Further, it is not economically advantageous to completely prevent the occurrence of fatigue cracks because it causes a significant increase in inspection costs. Therefore, it is important how to extend the life of cracks, and a structure that multiplexes load paths may be adopted as a device in terms of design. This is because when a load path is multiplexed, even if a crack occurs in a part of the structure and grows to a considerable length, the load that was shared by the part containing the crack with reduced rigidity was shared. This is because other parts share each other, and as a result, the crack propagation in the part including the crack is suppressed, and it is possible to prevent a fatal fracture immediately after the occurrence and propagation of the crack. .
[0005]
On the other hand, as a practical problem, there is a limitation to increase the redundancy in strength in order to give priority to economic efficiency in ordinary structures, and there is a limit to the effect in the response by the design method of load path multiplexing described above. There is. Therefore, there is a great demand from the industry for increasing the resistance to fatigue crack growth of the steel material itself.
[0006]
Techniques relating to the improvement of fatigue crack propagation resistance of steel materials are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 4-337037, 6-271985, and 6-299238.
[0007]
Among them, JP-A-4-337037 discloses that a steel having a specific chemical composition is rolled, cooled, and wound under specific conditions to obtain a hot-rolled steel sheet having a specific final structure “fatigue strength and crack propagation resistance. The manufacturing method of the excellent formable hot-rolled steel sheet is disclosed. However, in the case of the technology proposed in this publication, the "fatigue crack propagation resistance" that is the object of improvement is limited to the "crack growth lower limit characteristic", and therefore describes the "fatigue crack propagation stop characteristic". Since the only dynamic parameter is ΔKth, and the stable growth region of the crack is not targeted, when the external force increases, the stress intensity factor range ΔK at the fatigue crack tip exceeds ΔKth and the fatigue crack begins to progress Cracks grow at the same fatigue crack growth rate as conventional steel.
[0008]
Japanese Patent Application Laid-Open No. 6-271985 discloses a steel sheet having a specific structure and a steel sheet with excellent fatigue propagation resistance and a method for producing the same, in which the chemical composition of the steel and the rolling and cooling conditions are defined in order to obtain the steel sheet. Is disclosed. However, in the case of the technique proposed in this publication, it is possible to control the fatigue crack growth rate by defining the average existence interval, average flatness ratio, and volume ratio of island martensite, especially the volume of island martensite. The rate is specified as 0.5 to 5%. Therefore, brittle fracture starting from island martensite easily occurs, and it is difficult to achieve both fatigue crack growth characteristics and fracture toughness.
Japanese Patent Application Laid-Open No. 6-299238 discloses a “method for producing a steel sheet having excellent fatigue propagation resistance and weld heat affected zone toughness” that defines the chemical composition of steel and the conditions for rolling and cooling. However, in the case of the technique proposed in this publication, a complicated steelmaking process in which Al is added after deoxidation with Ti is indispensable. Therefore, it is difficult to avoid an increase in steelmaking cost and an adverse effect on the production process of other steel types. .
[0009]
[Problems to be solved by the invention]
The present invention has been made in view of the above situation, and its purpose is to provide good fatigue crack resistance even when subjected to repeated loads in order to extend the crack propagation life of the structure and ensure structural safety. It is providing the manufacturing method of the structural steel material which shows progress.
[0010]
[Means for Solving the Problems]
The gist of the present invention resides in a method for producing a steel material having excellent fatigue crack growth resistance as shown in the following (1) and (2).
[0011]
(1) In mass%,
C: 0.06 to 0.25%,
Si: 0.03 to 0.6%,
Mn: 0.30 to 2.0%,
Al: 0.010 to 0.10%,
Nb: 0.010 to 0.10%,
Ti: 0.010 to 0.10%
A steel slab comprising FCG1 represented by the following formula (1) satisfying a value of 4.0 or less and the balance of Fe and impurities being heated to 1050 ° C. or higher to a rolling finish temperature of 1040 to 740 ° C. It was excellent in fatigue crack growth resistance characterized by being hot-rolled at a temperature of (Ts1 + 50) to (Ts1-25) ° C and then cooled to at least 550 ° C at a cooling rate of 5 to 50 ° C / sec. Steel manufacturing method. Here, Ts1 is a temperature calculated from the following equation (2).
[0012]
FCG1 = (0.7 + C) / {(Si / 25) + (Mn / 5) + (Nb / 25) + Ti} ····· (1),
Ts1 = (820-200C-60Si-600Nb + 2000Ti) (2) .
In addition, the element symbol in each formula shows the content in the mass% of the element.
[0013]
(2) In mass%,
C: 0.02 to 0.25%,
Si: 0.03 to 0.6%,
Mn: 0.30 to 2.0%,
Al: 0.010 to 0.10%,
Nb: 0.010 to 0.10%,
Ti: 0.012-0.10%
It contains, in further,
Group 1: Cu: 0.02~ 0.21%, Cr : 0.03~2.0% 1 or more of,
Second group: Ni: 0.05 to 1.0%, Mo: one or more of 0.05 to 1.0%,
Third group: V: 0.01 to 0.5%
The steel slab comprising FCG2 represented by the following formula (3) satisfying 4.0 or less and the balance consisting of Fe and impurities is heated to 1050 ° C. or higher to 1040 to 740 ° C. Fatigue crack growth resistance, characterized by being hot-rolled at a rolling finish temperature of (Ts2 + 50) to (Ts2-25) ° C. and then cooled to at least 550 ° C. at a cooling rate of 5 to 50 ° C./sec. A method for producing steel with excellent properties. Here, Ts2 is a temperature calculated from the following equation (4).
[0014]
FCG2 = (0.7 + C) / {(Si / 25) + (Mn / 5) + (Cu / 50) + (Ni / 50) + (Cr / 25) + (Mo / 25) + (V / 5) + (Nb / 25) + Ti } ····· (3),
Ts2 = (1 + 0.1V) (820-200C-60Si + 200Cu + 50Ni-100Cr + 600Mo-600Nb + 2000Ti) (4) .
In addition, the element symbol in each formula shows the content in the mass% of the element.
Hereinafter, the inventions related to the method for producing a steel material having excellent fatigue crack growth resistance (1) and (2) are referred to as “present invention (1)” and “present invention (2)”, respectively. Also, it may be collectively referred to as “the present invention”.
[0015]
In order to achieve the above-described problems, the present inventors have studied the effects of the chemical composition of steel, rolling conditions, and cooling conditions on the fatigue crack propagation behavior of steel materials, and obtained the following knowledge.
[0016]
(A) In order to suppress the fatigue crack growth rate and extend the fatigue crack growth life, (1) a hardness difference is provided in a microscopic region in the steel material, and the fatigue crack propagation path is microscopically bent. (2) By preparing a steel material that repeatedly softens as a stress response when repeated strain is applied to the steel material, and by relaxing the stress at the crack tip during fatigue crack growth A method of suppressing the fatigue crack growth rate by lowering the stress at the crack tip, and (3) plastic deformation formed at the fatigue crack tip by keeping the ratio of the tensile strength to the yield strength (yield ratio) of the steel material low. There is a method of reducing the local load at the crack tip by widening the region and consequently suppressing the fatigue crack growth rate. In any of these methods, FCG1 represented by the above formula (1) is used. Or the above equation (3) It is effective to reduce the value of FCG2 represented. In the following description, FCG1 represented by the above formula (1) and FCG2 represented by the above formula (3) may be collectively referred to as FCG.
[0017]
(B) The cooling start temperature after hot rolling greatly affects the fatigue crack growth resistance of the steel material, and there is an optimum cooling start temperature region in which the fatigue crack growth rate is minimized.
[0018]
Therefore, the present inventors made further detailed studies on the relationship between the fatigue crack propagation resistance of steel materials having various chemical compositions, the cooling start temperature after hot rolling, and the cooling rate. As a result, the following matters became clear.
[0019]
(C) In any steel material, when the cooling start temperature after hot rolling is gradually increased from the low temperature side, the fatigue crack growth rate gradually decreases, but the gradually decreasing rate gradually increases as the cooling start temperature increases. Less. When the cooling start temperature is further increased, the fatigue crack growth rate gradually increases. That is, there is an optimum cooling start temperature at which the fatigue crack growth rate is minimized.
[0020]
(D) The rate at which the fatigue crack growth rate gradually increases is slower than the rate at which the fatigue crack growth rate gradually decreases, and the cooling start temperature at which fatigue crack growth resistance is approximately equal to the optimum cooling start temperature is the optimum cooling start temperature. On the other hand, it is a region having a width of 75 ° C., 25 ° C. on the low temperature side and 50 ° C. on the high temperature side.
(E) The optimum cooling start temperature at which the fatigue crack growth rate in (c) is minimized is related to Ts1 expressed by the above formula (2) or Ts2 expressed by the above formula (4). In the following description, Ts1 represented by the above formula (2) and Ts2 represented by the above formula (4) may be collectively referred to as Ts.
[0021]
(F) Optimization of the steel material cooling rate after hot rolling is also effective in suppressing the fatigue crack growth rate and extending the fatigue crack growth life.
The present invention has been completed based on the above findings.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.
(A) Chemical composition of steel material steel C: 0.06 to 0.25% (present invention (1)), C: 0.02 to 0.25% (present invention (2))
C is an element effective for securing the strength of the structural member. However, in the case of the present invention (1), if the content is less than 0.06%, the ratio of the ferrite phase in the structure of the steel material is extremely large, and it is difficult to obtain the strength improvement effect. In the case of the present invention (2), if the content is less than 0.02%, the proportion of the ferrite phase in the structure of the steel material is extremely large, and it is difficult to obtain the strength improvement effect. On the other hand, in both cases of the present invention (1) and the present invention (2), if the C content exceeds 0.25%, the weldability deteriorates, so that the welding work becomes difficult and it is used as a structural steel material. The area is significantly limited. Accordingly, the C content is 0.06 to 0.25% in the case of the present invention (1), and 0.02 to 0.25% in the case of the present invention (2) . In addition, in order to secure a large strength and to secure a good weldability by suppressing the ratio of the ferrite phase in the steel structure, the C content is 0.06 to 0.12%, and, in the case of the present invention (2) is preferably set to from 0.04 to 0.12%.
[0023]
Si: 0.03-0.6%
Si has a deoxidizing action. However, if the content is less than 0.03%, the effect of addition is poor. On the other hand, when the Si content exceeds 0.6%, the fracture toughness decreases. Therefore, the Si content is set to 0.03 to 0.6%. Note that the Si content is desirably 0.25 to 0.5%.
[0024]
Mn: 0.30 to 2.0%
Mn is an element effective for securing strength. However, if the content is less than 0.30%, the effect of addition is poor. On the other hand, if the Mn content exceeds 2.0%, the weldability is deteriorated, so that welding is difficult, and the use area as a structural steel material is remarkably limited. Therefore, the content of Mn is set to 0.30 to 2.0%. Note that the Mn content is preferably 0.5 to 1.8% in order to ensure high strength and good weldability.
[0025]
Al: 0.010 to 0.10%
Al has a deoxidizing action. However, if the content is less than 0.010%, the effect of addition is poor. On the other hand, if the content exceeds 0.10%, the fracture toughness decreases and the cleanliness of the steel also deteriorates. Therefore, the content of Al is set to 0.010 to 0.10%. In addition, it is desirable that the Al content is 0.010 to 0.05%.
[0026]
Nb: 0.010 to 0.10%
Nb is an element effective for ensuring fracture toughness, and needs to be contained in an amount of 0.010% or more in order to be used as a structural material. However, if the content exceeds 0.10%, the fracture toughness decreases. Therefore, the Nb content is set to 0.010 to 0.10%. The Nb content is preferably 0.020 to 0.05%.
[0027]
Ti: 0.010 to 0.10% (present invention (1)), Ti: 0.012 to 0.10% (present invention (2))
Ti is an element effective for ensuring fracture toughness, and is 0.010% or more in the case of the present invention (1) and 0.012% or more in the case of the present invention (2) for use as a structural material. It is necessary to let However, in both cases of the present invention (1) and the present invention (2), if the content exceeds 0.10%, the fracture toughness is lowered. Therefore, the Ti content is set to 0.010 to 0.10% in the case of the present invention (1), and is set to 0.012 to 0.10% in the case of the present invention (2) . It was. In both cases of the present invention (1) and the present invention (2), the Ti content is preferably 0.015 to 0.05%.
[0028]
FCG1: 4.0 or less When the value of FCG1 represented by the formula (1) is 4.0 or less, the fatigue crack growth rate can be suppressed and the fatigue crack growth life can be extended. Therefore, in the present invention (1), the value of FCG1 is set to 4.0 or less. The value of FCG1 is preferably 3.0 or less. The lower limit of FGC1 is, C, Si, Mn, Nb, content 0.06% each Ti, 0.6%, 2.0%, 0.1%, 1 in the case of 0.1% it is .44.
[0029]
In addition, the raw material steel used for the manufacturing method of the steel material excellent in fatigue crack growth resistance of the present invention (1) includes each component element from C to Ti , and is the FCG1 represented by the above formula (1) . Although the value is 4.0 or less and the balance is steel composed of Fe and impurities, the material steel used in the method for producing a steel material excellent in fatigue crack growth resistance of the present invention (2) is the above-mentioned C to Ti in addition to the component elements of, look containing one or more groups of the first group to third group, wherein (3) the value of FCG2 of the formula satisfies the 4.0 or less, the balance being Fe And steel made of impurities. The effects and desirable contents of the alloy elements of the first group to the third group are as follows.
[0030]
Cu: 0.02~ 0.21%, Cr: 0.03~2.0%
Since Cu and Cr have the effect of enhancing the corrosion resistance, they are contained for the purpose of ensuring the corrosion resistance when the steel material is used in a corrosive environment. However, the Cu content is less than 0.02% and the Cr content is 0. If it is less than 0.03%, it is difficult to obtain the effect. On the other hand, if Cu exceeds 0.21 % and Cr exceeds 2.0%, the weldability deteriorates, so that welding work becomes difficult, and the use area as a structural steel material is remarkably limited. End up. Furthermore, when Cu is contained exceeding 0.21 %, the hot workability is lowered, and rolling may cause flaws on the steel material surface, or cracks may occur on the surface or inside of the steel material. Therefore Cu, when adding one or more Cr is from 0.02 to 0.21% content of Cu, the content of Cr preferably set to 0.03 to 2.0 percent. Incidentally, Cu, when adding one or more Cr, the Cu content from 0.1 to 0.21%, the content of Cr is preferably set to 0.2 to 1.5 percent.
Ni: 0.05-1.0%, Mo: 0.05-1.0%
Ni and Mo have the effect of increasing the fracture toughness of the steel and enhancing the hardenability, so it is included for the purpose of ensuring the fracture toughness and the hardenability when the steel material is used in a large structure. If the Ni content is less than 0.05% and the Mo content is less than 0.05%, it is difficult to obtain the effect. On the other hand, even if Ni is contained in excess of 1.0% and Mo is contained in excess of 1.0%, the effect is saturated and the cost is increased. Therefore, when adding 1 or more types of Ni and Mo, it is good to make content of Ni 0.05-1.0% and content of Mo 0.05-1.0%.
V: 0.01 to 0.5%
V has the effect of increasing the strength, so it is included in the structure for the purpose of securing a large strength. However, if its content is less than 0.01%, its effect is poor, while it exceeds 0.5%. Even if it is made, the effect is saturated and the cost is increased. Therefore, when adding V, it is good to set it as 0.01 to 0.5% of content.
[0031]
FCG2: 4.0 or less When the value of FCG2 represented by the above formula (3) is 4.0 or less, the fatigue crack growth rate can be suppressed and the fatigue crack growth life can be extended. Accordingly, in the present invention (2), it was less than 4.0 the value of FCG2. The value of FCG2 is preferably 3.0 or less. The lower limit values of FGC2 are C, Si, Mn, Cu, Ni, Cr, Mo, V, Nb, and Ti contents of 0.02%, 0.6%, 2.0%, and 1.0, respectively. %, 1.0%, 2.0%, 1.0%, 0.5%, 0.1%, and 0.1%.
(B) Manufacturing conditions for steel materials (B-1) Heating temperature before hot rolling: 1050 ° C. or higher Heating temperature needs to be 1050 ° C. or higher in order to prepare a pre-rolling structure and facilitate rolling. There is. Therefore, the heating temperature before hot rolling of the steel slab having the chemical composition described in (A) above was set to 1050 ° C. or higher. In addition, it is preferable that heating temperature shall be 1100-1200 degreeC.
(B-2) Rolling finishing temperature: 1040 to 740 ° C
In order not to coarsen the structure, the rolling finishing temperature needs to be 1040 ° C. or lower. However, when the rolling finishing temperature is lower than 740 ° C., the processing is concentrated on the surface of the material to be rolled, and the mechanical properties in the thickness direction of the steel material become non-uniform. Therefore, the hot rolling finishing temperature was set to 1040 to 740 ° C. The rolling finishing temperature is preferably 950 to 850 ° C.
(B-3) Cooling after hot rolling In order to suppress the fatigue crack growth rate and extend the fatigue crack growth life, start cooling after hot rolling from a temperature of (Ts + 50) to (Ts-25) ° C. There is a need to do. If the cooling start temperature is higher than (Ts + 50) ° C. or lower than (Ts−25) ° C., the fatigue crack growth rate is increased. In addition, said Ts is Ts1 (in the case of this invention (1)) represented by the said Formula (2) according to the chemical composition of steel materials, or Ts2 (in this invention (2) of the said (4) ) Case) .
Further, when cooling from the above temperature range, it is necessary to cool to at least 550 ° C. at a cooling rate of 5 to 50 ° C./second. When the cooling rate is less than 5 ° C./second, a hardness difference is provided in a microscopic region in the steel material, and the fatigue crack propagation path can be microscopically bent to suppress the macrocrack growth rate. On the other hand, if the cooling rate exceeds 50 ° C./second, a very large residual stress distribution is generated in the thickness direction of the steel material, and the fatigue crack growth rate at the thickness center portion where the tensile residual stress is formed is accelerated. On the other hand, when the cooling end temperature exceeds 550 ° C., the repeated softening characteristics of the steel material are not exhibited, and the stress at the crack tip is not relaxed. As a result, the crack growth rate cannot be suppressed.
[0032]
Therefore, after hot rolling, it was decided to cool from a temperature of (Ts + 50) to (Ts-25) ° C. to at least 550 ° C. at a cooling rate of 5 to 50 ° C./second.
[0033]
Next, the present invention will be described in more detail with reference to examples.
[0034]
【Example】
Example 1
Steels having chemical compositions shown in Tables 1 to 3 were melted in a test furnace by an ordinary method. Tables 1 to 3 also show the values of FCG (that is, FCG1 and FCG2) and Ts (that is, Ts1 and Ts2) calculated from the chemical composition.
[0035]
[Table 1]
Various test pieces were collected from the steel plate having a thickness of 25 mm thus obtained and measured for weldability, cleanliness, fracture toughness and tensile strength.
That is, the y-type weld crack test piece, micro test piece, V-notch impact test piece described in JIS Z 2202 (1998), JIS Z 2201 (1998) No. 4 tensile test specimen was collected, and weldability, cleanliness, fracture toughness and tensile strength were investigated under the following conditions.
[0036]
The y-type weld cracking test uses a welding material for normal submerged arc welding, and the specimen preheating temperature is 25 ° C., 50 ° C., 75 ° C., 100 ° C., and 125 ° C., and the temperature is 30 ° C. and the humidity is 80 ° C. % Atmosphere. The test was performed after the welding material was left in an environment of a temperature of 30 ° C. and a humidity of 80% for 1 hour. The number of tests for each preheating temperature is two.
[0037]
The cleanliness was evaluated in accordance with JIS G 0555 (1998) with a microscope magnification of 400 times and a field number of 60.
[0038]
Fracture toughness test was conducted using the V-notch impact described in JIS Z 2202 (1998) taken from the center of the plate thickness so that the longitudinal direction of the specimen coincides with the rolling direction and the crack propagation direction coincides with the width direction of the steel material. Using the test piece, a Charpy impact test was performed with the number of tests at each test temperature being 3, and the brittle-ductile fracture surface transition temperature (vTrs) was determined.
The tensile test was performed in the atmosphere at room temperature using a No. 4 tensile test piece described in JIS Z 2201 (1998) collected so that the longitudinal direction of the test piece coincided with the rolling direction from the center of the plate thickness. The number of tests was 2.
[0039]
Table 4 shows the results of the above tests. The marks “◯” and “x” in Table 2 are based on the following categories, respectively.
[0040]
That is, the “weldability” was “◯” when no cracking occurred at a preheating temperature of 25 ° C., and “X” when preheating at 50 ° C. or more was required to prevent cracking. “Cleanliness” was determined based on the current class A ship class standard steel, “◯” when the cleanliness was equivalent to or better than this, and “X” when below the cleanliness. “Fracture toughness” was “◯” when the fracture surface transition temperature (vTrs) was less than −20 ° C., and “x” when the fracture surface temperature was −20 ° C. or higher. The “tensile strength” was “◯” when a nominal tensile strength of 400 MPa or more was obtained, and “x” when it was below 400 MPa.
[0041]
[Table 4]
[0042]
Next, fatigue crack growth characteristics of steels 1 to 15, steels 17 to 21, and steels 23 to 26, each having a weld thickness, cleanliness, fracture toughness, and tensile strength, of 25 mm thick steel plate. In order to evaluate the above, a CT specimen was taken in the LT direction, and a fatigue crack growth test was performed in accordance with the ASTM standard (E647) in a room temperature atmosphere under a repetition rate of 25 Hz and a stress ratio of 0.1 as a load condition. . The fatigue crack growth rate was determined by representing the growth rate when the stress intensity factor range ΔK at the crack tip was 20 MPa · m 1/2 . Note that the upper limit of the target fatigue crack growth rate was 4.0 × 10 −5 mm / cycle with reference to the level of fatigue crack growth rate in general steel materials.
Table 5 shows the results of the fatigue crack growth test.
[0043]
[Table 5]
(Example 2)
Using a part of the steel melted in Example 1, the influence of the cooling start temperature on the fatigue crack growth rate was investigated in detail.
[0044]
That is, about the steel 6, the steel 7, and the steel 20, the steel piece of thickness 150mm produced by carrying out the hot forging by a normal method was heated to 1100 degreeC, and was hot-rolled. In the hot rolling, the rolling finishing temperature was set to 950 to 750 ° C. and finished to a plate thickness of 25 mm. After the hot rolling finish, the steel was cooled from various temperatures shown in Table 6 to 500 ° C. at a cooling rate of 40 ° C./second according to the chemical composition of the steel.
[0045]
[Table 6]
The results of the fatigue crack growth test are as shown in Table 6.
[0046]
From Table 6, even if each element contained in the steel material is within the range of the content defined by the present invention, and the FGC value is within the range defined by the present invention, the cooling start temperature is defined by the present invention. When out of the range (that is, (Ts + 50) to (Ts-25) ° C.), it is clear that the fatigue crack growth rate exceeds the target value and the fatigue crack growth resistance is inferior.
[0047]
【The invention's effect】
According to the present invention, since a steel material excellent in fatigue crack growth resistance can be obtained, it can be used for various structures such as ships, marine structures, bridges, buildings, and tanks.
Claims (2)
Ｃ：０．０６〜０．２５％、
Ｓｉ：０．０３〜０．６％、
Ｍｎ：０．３０〜２．０％、
Ａｌ：０．０１０〜０．１０％、
Ｎｂ：０．０１０〜０．１０％、
Ｔｉ：０．０１０〜０．１０％
を含有し、下記(1)式で表されるＦＣＧ１の値が４．０以下を満たし、残部がＦｅ及び不純物からなる鋼片を、１０５０℃以上に加熱して１０４０〜７４０℃の圧延仕上げ温度で熱間圧延し、その後、（Ｔｓ１＋５０）〜（Ｔｓ１−２５）℃の温度から５〜５０℃／秒の冷却速度で少なくとも５５０℃まで冷却することを特徴とする疲労亀裂進展抵抗性に優れた鋼材の製造方法。
ここで、Ｔｓ１は下記(2)式から計算される温度である。
ＦＣＧ１＝（０．７＋Ｃ）／｛（Ｓｉ／２５）＋（Ｍｎ／５）＋（Ｎｂ／２５）＋Ｔｉ｝・・・・・(1)
Ｔｓ１＝（８２０−２００Ｃ−６０Ｓｉ−６００Ｎｂ＋２０００Ｔｉ）・・・・・(2)
なお、各式における元素記号はその元素の質量％での含有量を示す。% By mass
C: 0.06 to 0.25%,
Si: 0.03 to 0.6%,
Mn: 0.30 to 2.0%,
Al: 0.010 to 0.10%,
Nb: 0.010 to 0.10%,
Ti: 0.010 to 0.10%
A steel slab comprising FCG1 represented by the following formula (1) satisfying a value of 4.0 or less and the balance of Fe and impurities being heated to 1050 ° C. or higher to a rolling finish temperature of 1040 to 740 ° C. It was excellent in fatigue crack growth resistance characterized by being hot-rolled at a temperature of (Ts1 + 50) to (Ts1-25) ° C and then cooled to at least 550 ° C at a cooling rate of 5 to 50 ° C / sec. Steel manufacturing method.
Here, Ts1 is a temperature calculated from the following equation (2).
FCG1 = (0.7 + C) / {(Si / 25) + (Mn / 5) + (Nb / 25) + Ti} (1)
Ts1 = (820-200C-60Si-600Nb + 2000Ti) (2)
In addition, the element symbol in each formula shows the content in the mass% of the element.
Ｃ：０．０２〜０．２５％、
Ｓｉ：０．０３〜０．６％、
Ｍｎ：０．３０〜２．０％、
Ａｌ：０．０１０〜０．１０％、
Ｎｂ：０．０１０〜０．１０％、
Ｔｉ：０．０１２〜０．１０％
を含有し、更に、
第１群：Ｃｕ：０．０２〜０．２１％、Ｃｒ：０．０３〜２．０％のうちの１種以上、
第２群：Ｎｉ：０．０５〜１．０％、Ｍｏ：０．０５〜１．０％のうちの１種以上、
第３群：Ｖ：０．０１〜０．５％
の１群以上をも含み、下記(3)式で表されるＦＣＧ２の値が４．０以下を満たし、残部がＦｅ及び不純物からなる鋼片を、１０５０℃以上に加熱して１０４０〜７４０℃の圧延仕上げ温度で熱間圧延し、その後、（Ｔｓ２＋５０）〜（Ｔｓ２−２５）℃の温度から５〜５０℃／秒の冷却速度で少なくとも５５０℃まで冷却することを特徴とする疲労亀裂進展抵抗性に優れた鋼材の製造方法。
ここで、Ｔｓ２は下記(4)式から計算される温度である。
ＦＣＧ２＝（０．７＋Ｃ）／｛（Ｓｉ／２５）＋（Ｍｎ／５）＋（Ｃｕ／５０）＋（Ｎｉ／５０）＋（Ｃｒ／２５）＋（Ｍｏ／２５）＋（Ｖ／５）＋（Ｎｂ／２５）＋Ｔｉ｝・・・・・(3)
Ｔｓ２＝（１＋０．１Ｖ）（８２０−２００Ｃ−６０Ｓｉ＋２００Ｃｕ＋５０Ｎｉ−１００Ｃｒ＋６００Ｍｏ−６００Ｎｂ＋２０００Ｔｉ）・・・・・(4)
なお、各式における元素記号はその元素の質量％での含有量を示す。 % By mass
C: 0.02 to 0.25%,
Si: 0.03 to 0.6%,
Mn: 0.30 to 2.0%,
Al: 0.010 to 0.10%,
Nb: 0.010 to 0.10%,
Ti: 0.012-0.10%
It contains, in further,
Group 1: Cu: 0.02~ 0.21%, Cr : 0.03~2.0% 1 or more of,
Second group: Ni: 0.05 to 1.0%, Mo: one or more of 0.05 to 1.0%,
Third group: V: 0.01 to 0.5%
The steel slab comprising FCG2 represented by the following formula (3) satisfying 4.0 or less and the balance consisting of Fe and impurities is heated to 1050 ° C. or higher to 1040 to 740 ° C. Fatigue crack growth resistance, characterized by being hot-rolled at a rolling finish temperature of (Ts2 + 50) to (Ts2-25) ° C. and then cooled to at least 550 ° C. at a cooling rate of 5 to 50 ° C./sec. A method for producing steel with excellent properties.
Here, Ts2 is a temperature calculated from the following equation (4).
FCG2 = (0.7 + C) / {(Si / 25) + (Mn / 5) + (Cu / 50) + (Ni / 50) + (Cr / 25) + (Mo / 25) + (V / 5) + (Nb / 25) + Ti} (3)
Ts2 = (1 + 0.1V) (820-200C-60Si + 200Cu + 50Ni-100Cr + 600Mo-600Nb + 2000Ti) (4)
In addition, the element symbol in each formula shows the content in the mass% of the element.
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