JP4000049B2

JP4000049B2 - Manufacturing method of steel plate with excellent fatigue crack propagation resistance

Info

Publication number: JP4000049B2
Application number: JP2002326411A
Authority: JP
Inventors: 敏彦小関; 忠石川; 清孝中島; 俊永長谷川
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2002-11-11
Filing date: 2002-11-11
Publication date: 2007-10-31
Anticipated expiration: 2022-11-11
Also published as: JP2004162085A

Description

【０００１】
【発明の属する技術分野】
本発明は、船舶、橋梁、建築、海洋構造物、建設機械などの大型構造物に使用される鋼板に関するもので、繰り返し荷重により生じる疲労き裂の板厚方向の伝播抵抗に優れた鋼板の製造方法に関するものである。
【０００２】
【従来の技術】
構造物の軽量化、大容量化の要求に答え、構造用鋼板の高強度化が急速に進んでいる。しかしながら、繰り返し荷重を受ける構造物では、降伏強度のみならず疲労強度を考慮しなければならず、高強度化のニーズに応えることが出来ない場合があり、疲労強度の向上が切望されている。
【０００３】
溶接部の疲労破壊は一般に応力集中の大きな溶接止端部から発生する。溶接構造物の疲労強度は、主として溶接部の止端部形状によって支配されることが知られており、溶接止端部への応力集中を軽減する設計的な対策や止端部の形状処理等の疲労強度向上策が適用されることがある。しかしながら、これらは構造物の建造工数を増大させるばかりでなく、溶接部位によっては止端部処理が実施できない場合も多い。そのため、鋼材面から疲労特性の向上が切望されている。
【０００４】
溶接継手部の疲労破壊は一般に応力集中の大きな溶接止端部から発生するため、発生特性は溶接止端部形状の影響が大きく、鋼材組成、組織による改善が難しい。そこで、鋼材組織を制御して疲労特性を向上させるためには、止端部で発生した疲労き裂の板厚方向への伝播を遅延させることが有効である。
【０００５】
従来、鋼板中の疲労き裂の伝播抵抗を向上させる方法として、下記特許文献１〜特許文献４などが開示されている。
特許文献１では、特定成分の鋼で、組織がフェライト、パーライト、ベイナイトの１種または２種以上で主に構成され、フェライトの平均粒径が８μｍ以下であり、さらに、平均存在間隔２０μｍ以下、平均偏平比５以上の島状マルテンサイトを体積率が０．５〜５％の割合で存在する、耐疲労伝播特性とアレスト性（脆性き裂を停止する特性）の優れた鋼板の発明が記載されている。
【０００６】
また特許文献２では、特定成分の鋼で、面積率で６０〜９０％のフェライト母相と硬質第二相からなる組織を有し、母相と第二相の硬さが特定の関係式を満足し、特定の第二相の形態（アスペクト比が３．４２超、必要に応じて第二相の間隔が２５μｍ以下等）を有し、必要に応じて、さらにフェライトの集合組織、フェライトの粒径（平均粒径４μｍ以下、最大粒径１２．５μｍ以下）等を規定した、疲労き裂伝播特性の優れた鋼材の発明が記載されている。
【０００７】
また特許文献３では、特定組成の鋼で、鋼板組織が、マルテンサイト、ベイナイト、パーライト、疑似パーライトおよび焼戻しマルテンサイトの１種以上からなる硬質部の素地に、フェライトからなる軟質部が分散した組織であり、これら２部分の硬度差がビッカース硬度Ｈｖで１５０以上であり、必要に応じて軟質部の平均粒径を５０μｍ以下、硬質部の平均間隔を５０μｍ以下と規定した、疲労き裂進展抑制効果を有する鋼板の発明が開示されている。
【０００８】
また特許文献４では、特定組成の鋼で、フェライト面積率８０％以上、厚さ３０〜１００μｍの組織層と、マルテンサイト面積率２０％以上、厚さ１０〜３０μｍの組織層とを板圧方向に交互に層状に配置した、板厚方向の疲労き裂伝伝播特性に優れた厚鋼板の発明が開示されている。
【０００９】
【特許文献１】
特開平６−２７１９８４号公報
【特許文献２】
特開平１１−１７４２号公報
【特許文献３】
特開平７−２４２９９２号公報
【特許文献４】
特開平８−７３９８０号公報
【００１０】
【発明が解決しようとする課題】
しかしながら、特許文献１に記載の発明では、耐疲労伝播特性とアレスト性とを両立させるために硬質相の含有率が低く、後述のように疲労き裂の遅延効果は小さく、疲労特性改善材として実用上のメリットは十分ではない。また特許文献２に記載の発明では、き裂伝播抑制を図るためのフェライトと硬質第二相からなる組織は、二相域からフェライト単相域の低温で累積圧下率を大きくすることで達成されるため、生産性が低く、鋼板形状確保が容易でないなどの課題がある。
【００１１】
また、特許文献３で開示されている発明のような硬質相とフェライト地の硬さの差がＨｖで１５０以上必要とする要件や、特許文献４で開示されている発明のような硬質相とフェライト地とを層状に交互に配置する要件は、本発明者らの詳細な検討では、後述のように必ずしも必要十分な条件ではなく、用途によっては要求特性を満たさない場合も散見されるという課題がある。
【００１２】
そこで本発明は、溶接構造部材に用いられる引張強さが４００Ｎ／ｍｍ²級以上の厚鋼板であって、特殊なあるいは高価な合金元素の多量添加や生産性の劣る製造方法によらずに、溶接継手あるいは母材の疲労特性を大幅に向上させることのできる、疲労き裂伝播抵抗に優れた鋼板の製造方法を提供することを目的とするものである。
【００１３】
【課題を解決するための手段】
上記課題を解決するために、本発明者らは種々の鋼材において疲労き裂の伝播挙動と鋼材ミクロ組織の関係を解析し、さらに疲労き裂伝播抵抗に及ぼすフェライト地と第二相の影響を詳細に検討した結果、疲労き裂の伝播抵抗の向上手段を見出し、本発明をなしたものである。すなわち、溶接継手の止端部から疲労き裂が発生し、き裂が母材の板厚方向に突入した場合、フェライト地に硬質組織を分散することにより、疲労き裂はフェライトと硬質相の界面もしくは界面近傍で停留、分岐したり、また硬質相を迂回したりすることで、マクロなき裂の伝播が著しく抑制されることを明らかにし、さらにその効果を実用的に有効ならしめる鋼板組織の適正化と工業的製造方法を確立した。
【００１６】
すなわち、本発明の要旨は以下の通りである。
（１）質量％で、
Ｃ：０．０１〜０．１８％、Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．１〜３．０％、Ａｌ：０．００１〜０．２％、
Ｐ：０．０５％以下、Ｓ：０．０１％以下、
Ｎ：０．００１％〜０．０２％
を含有し、残部が鉄および不可避的不純物からなる鋼片を、Ａｃ3 以上の温度に加熱後、累積圧下率５０〜９８％、圧延終了温度Ａｒ3 以上、Ａｒ3 ＋１００℃以下で所定の板厚まで熱間圧延し、Ａｒ3 以下、Ａｒ3 −１５０℃以上の温度域まで放冷し、該温度域から１〜１００℃／ｓの冷却速度で５００℃以下まで加速冷却した後、さらに、Ａｃ1 以上、Ａｃ3 −２０℃以下の温度範囲に再加熱し、その後直ぐに、またはその再加熱温度で保持した後、１〜１００℃／ｓの冷却速度で５００℃以下まで焼き入れることにより、板厚方向の任意の断面組織上において、主にマルテンサイトおよびベイナイトから成り、平均ビッカース硬さ２３０以上、板厚方向の平均厚みが２０μｍ以下かつ板厚方向の平均間隔が５０μｍ以下かつ鋼板長手方向長さと板厚方向厚みの平均比が１０以上の硬質組織を平均面積率で１０〜６０％含み、残部は平均粒径４０μｍ以下のフェライトを主体とする組織とすることを特徴とする、疲労き裂伝播抵抗に優れた鋼板の製造方法。
【００１７】
（２）質量％で、
Ｃ：０．０１〜０．１８％、Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．１〜３．０％、Ａｌ：０．００１〜０．２％、
Ｐ：０．０５％以下、Ｓ：０．０１％以下、
Ｎ：０．００１％〜０．０２％
を含有し、残部が鉄および不可避的不純物からなる鋼片を、Ａｃ3 以上の温度に加熱後、累積圧下率５０〜９８％、圧延終了温度Ａｒ3 以上で所定の板厚まで熱間圧延し、５００℃以下まで冷却した後、Ａｃ1 以上、Ａｃ3 −２０℃以下の温度範囲に再加熱し、その後直ぐに、またはその再加熱温度で保持した後、１〜１００℃／ｓの冷却速度で５００℃以下まで焼き入れることにより、板厚方向の任意の断面組織上において、主にマルテンサイトおよびベイナイトから成り、平均ビッカース硬さ２３０以上、板厚方向の平均厚みが２０μｍ以下かつ板厚方向の平均間隔が５０μｍ以下かつ鋼板長手方向長さと板厚方向厚みの平均比が１０以上の硬質組織を平均面積率で１０〜６０％含み、残部は平均粒径４０μｍ以下のフェライトを主体とする組織とすることを特徴とする、疲労き裂伝播抵抗に優れた鋼板の製造方法。
【００１８】
（３）前記鋼片が、さらに質量％で、
Ｎｉ：０．０５〜３％、Ｃｕ：０．０５〜３％、
Ｃｒ：０．０５〜３％、Ｍｏ：０．０５〜２％、
Ｎｂ：０．００５〜０．１％、Ｔｉ：０．００５〜０．１％、
Ｔａ：０．００５〜０．１％、Ｖ：０．００５〜０．４％、
Ｂ：０．０００２〜０．００４％
の１種または２種以上を含有することを特徴とする、上記（１）または（２）に記載の疲労き裂伝播抵抗に優れた鋼板の製造方法。
（４）前記鋼片が、さらに質量％で、
Ｃａ：０．０００５〜０．０３％、Ｍｇ：０．０００５〜０．０３％、
ＲＥＭ：０．０００５〜０．３％
の１種または２種以上を含有することを特徴とする、上記（１）ないし（３）のいずれか１項に記載の疲労き裂伝播抵抗に優れた鋼板の製造方法。
【００１９】
【発明の実施の形態】
溶接部止端部から発生した疲労き裂が母材に至り、母材板厚方向に進展する際にその伝播抵抗を高めるためには、フェライトを主体としたマトリックス中のき裂伝播方向に、き裂の迂回や停留、分岐を誘起しうる硬質相を適切に配置することが重要である。すなわち、き裂が伝播する方向に対する硬質相の存在頻度と、伝播抵抗力を最大限にするための形態、さらに硬質相の分率が、より重要な因子である。
【００２０】
先ず、硬質相の頻度については、き裂伝播方向である板厚方向の間隔が狭まるほどき裂と硬質相の衝突頻度が増すが、特にき裂伝播方向の平均間隔が５０μｍ以下の場合、その伝播に対する抵抗が顕著となることを見出した。き裂先端が硬質相に衝突する事により、き裂先端の塑性変形が拘束され、き裂先端の分岐・屈曲が促進されるが、その効果をき裂の先端の全線に渡って生じさせてマクロな効果につなげるには硬質相との衝突頻度の確保が重要であり、逆に平均間隔がそれより大きい場合には、マクロ的にき裂の遅延効果にはつながらないことが実験から明らかとなった。
【００２１】
また、それと同時に、硬質相の形状は粒状であるより板長手方向に対して扁平であることが、三次元的なき裂と第二相の衝突頻度向上の観点から重要である事も詳細なき裂伝播の観察から判った。すなわち、硬質相が粒状である場合はその硬質相間を抜けてき裂が進展するのが観察されるのに対し、扁平であればその板厚方向の重なりも手伝って仮にき裂がフェライト地を優先的に進展しようとすればき裂の迂回が余儀なくされ、実効的なき裂進展の遅延につながる。
【００２２】
この観点から板厚方向の断面組織の画像解析結果と疲労き裂の進展挙動の解析を行い、第二相の鋼板長手方向長さと板厚方向厚みの平均比が１０以上のときき裂伝播抵抗として効果的に働くことが見出された。これを満たせば硬質相は必ずしも従来言われているような層状である必要はなく、主として扁平型の硬質相がき裂伝播方向の平均間隔を満たして分散されればよい。この場合、硬質相間の平均間隔と相俟って、板厚方向に投影した場合の硬質相の重なり頻度がき裂伝播抵抗の向上に大きく作用するが、鋼板長手方向長さと板厚方向厚みの平均比が１０未満になると、板厚方向を伝播する疲労き裂が硬質相の間を選択して伝播する挙動とともに、き裂と硬質相の衝突によるき裂の迂回・分岐による遅延も不十分となり、結果としてマクロなき裂遅延効果にはつながらない。
【００２３】
さらに、硬質相の形状ではそのサイズも重要である。ΔＫ（き裂先端の応力拡大係数の最大と最小の差）が小さい場合、すなわちき裂長さが小さい初期や応力レベルが小さい場合は硬質相のサイズの影響は小さいが、ΔＫが増すと硬質相にき裂が衝突した場合にき裂の進展の抑制につながらず、壁開的に割れてき裂の異常進展を助長するケースが見られる。それを抑えるためには硬質相の板厚方向への厚みを平均で２０μｍ以下にすることが重要であり、これによって安定して疲労き裂の伝播抵抗となる。
【００２４】
なお、ここで硬質相としては、基本的にはマルテンサイト、ベイナイトを主体とし、平均硬さがビッカース硬さで２３０以上であることが必要である。この点はフェライトと第二相の硬さの比や差を要件とした従来技術と異なる点であり、第二相自身の硬さが所定以上であればミクロ的にき裂の遅延効果が発揮される。逆に第二相の平均ビッカース硬さが２３０未満の場合は疲労き裂先端での塑性変形の拘束が不十分で硬質相分散の効果が得られない。
また、平均硬さ２３０以上はパーライトや擬似パーライトによっても達成可能であるが、それらはフェライトとセメンタイとの層状あるいは混在組織であるため脆く、自身がき裂先端で割れて有効に作用しない場合があり、マルテンサイトやベイナイトを主体とした第二相より効果は小さい。硬質第二相による疲労き裂の進展の遅延効果がマクロな疲労特性の向上につながるためには、硬質相は少なくとも、板厚方向の断面上の面積率で１０％以上必要であり、上記の形態、分散を維持する限り第二相は多いほど有効であるが、他方、硬質相の分率が過大になると鋼材の靭性が低下するため上限を６０％とした。
【００２５】
以上のように本発明が目的としている鋼材の疲労特性の向上には上述のような第二相の形態や分散を制御した導入が必要であるが、加えて化学組成についても具体的に限定する必要がある。以下に化学組成の限定理由を述べる。
Ｃは、鋼の強度を向上する有効な成分として０．０１％以上添加するが、０．１８％を超える過剰な添加では、溶接部の靭性を著しく劣化させるので、０．１８％以下に規制する。
【００２６】
Ｓｉは、溶鋼の脱酸元素として必要であり、また強度増加元素として有用であるが、２．０％を超えて過剰に添加すると、鋼の加工性を低下させ、溶接部の靭性を劣化させる。また０．０１％未満では脱酸効果が不十分なため、添加範囲を０．０１〜２．０％に規定する。
【００２７】
Ｍｎも、脱酸成分元素として必要であり、０．１％未満では鋼の清浄度を低下させ、加工性を害する。また鋼材の強度を向上する成分として０．１％以上の添加が必要である。しかし、過剰の添加は焼入性を過剰にしてフェライト相を得難くするので、３．０％を上限とする。
【００２８】
Ｐ，Ｓは、不純物元素で、延性、靭性を劣化させる元素であり、極力低減することが好ましいが、材質劣化が大きくなく、許容できる量として、Ｐの上限を０．０５％、Ｓの上限を０．０１％に限定する。
【００２９】
Ａｌは、脱酸元素として有用であるとともに、Ｎと結びついて窒化物を形成し鋼の微細化に有効な働きする。０．００１％以上の含有が必要であるが、０．２％を超えて過剰に含有すると、脱酸生成物である酸化物が粗大化し延性を極端に劣化させるため、０．００１％〜０．２％の範囲に限定する。
【００３０】
Ｎは、鋼中に単独固溶状態では鋼板靭性に有害であるが、工業的に鋼中のＮを完全に除去することは不可能であり、必要以上に低減することは製造工程に過大な負荷をかけるため好ましくない。そのため、工業的に制御が可能で、製造工程への負荷が許容できる範囲として下限を０．００１％とする。しかしながら過剰に含有すると、固溶Ｎが増加し延性や靭性に悪影響を及ぼすため、許容できる範囲として上限を０．０２％とする。
【００３１】
また、本発明の鋼板では強度確保および靭性確保のための組織制御に、Ｎｉ，Ｃｕ，Ｃｒ，Ｍｏ，Ｎｂ，Ｔｉ，Ｔａ，Ｖ，Ｂの一種または二種以上を含有させることができる。
Ｎｉは、鋼材の靭性を向上させる元素であり、特に硬質第二相を比較的多く含む場合の靭性確保に有効である。他方、Ｎｉは高価な合金元素であり、さらに含有量が多くなると加工熱処理によって鋼材を製造する場合の焼入れ性が過剰となるため、範囲を０．０５％〜３％とする。
【００３２】
Ｃｕは、焼入れ性向上、固溶強化、析出強化に有効な元素であるが、明瞭な効果を生じるためには０．０５％以上必要であり、一方、３％超では熱間加工性に問題を生じるので、効果を発揮し、かつ熱間加工性等の問題を生じない範囲として、本発明においては０．０５〜３％の範囲に限定する。
【００３３】
Ｃｒは、焼入れ性向上、析出強化により母材の強度向上に有効な元素であるが、明瞭な効果を生じるためには０．０５％以上必要であり、一方、３％を超えて添加すると、靭性及び溶接性が劣化する傾向を有するため、０．０５〜３％の範囲とする。
【００３４】
Ｍｏは、焼入れ性向上、強度向上、耐焼戻し脆化、耐ＳＲ脆化に有効な元素であるが、明瞭な効果を生じるためには０．０５％以上必要であり、２％を超える添加では逆に靭性、溶接性が劣化するため、０．０５〜２％に限定する。
【００３５】
Ｎｂは、オーステナイト相中に固溶及び析出状態で、オーステナイトの再結晶を抑制するために、また、変態時あるいは焼戻し時にＮｂ（Ｃ，Ｎ）を形成することで、強度の向上に有効な元素であるが、明瞭な効果を生じるためには０．００５％以上必要である。ただし、０．１％を超える過剰の含有では析出脆化により靭性が劣化する。従って、靭性の劣化を招かずに効果を発揮できる範囲として、０．００５〜０．１％の範囲に限定する。
【００３６】
Ｔｉは、析出強化により母材強度向上に寄与するとともに、高温でも安定なＴｉＮの形成により加熱オーステナイト粒径微細化にも有効な元素であり、加工熱処理を基本とする本発明においては必須の元素である。効果を発揮するためには０．００５％以上の含有が必要である。一方、０．１％を超えると、粗大な析出物、介在物を形成して靭性や延性を劣化させるため、０．００５〜０．１％とする。
【００３７】
Ｔａ，Ｖもまた、Ｎｂ，Ｔｉと同様の効果を持つ元素であり、それぞれ靭性の劣化なしに効果を発揮できる範囲として０．００５〜０．１％、０．００５〜０．４％と規定した。
【００３８】
Ｂは、靭性に有害なＮを固定して無害化するとともに、固溶状態でオーステナイト粒界に偏析することで再結晶抑制にも有効である。Ｎの無害化、再結晶抑制に効果を発揮するためには０．０００２％以上の添加が必要であるが、一方、０．００４％を超える過剰の添加では、ＢＮ、Ｆｅ₂₃（Ｃ，Ｂ）₆等の粗大な析出物を生じて靭性が劣化するため、０．０００２〜０．００４％に限定する。
【００３９】
さらに本発明では、脱酸・脱硫元素としてＣａ，Ｍｇ，ＲＥＭの一種または二種以上を含有して、鋼材およびその溶接部の組織制御を更に進め靭性を向上させることが可能である。すなわち、これらの元素による微細酸化物及び硫化物は組織の微細化に有効である。
Ｃａは、鋼中最大の脱酸力・脱硫力を有する元素である。溶鋼中で酸化物及び硫化物を形成するには０．０００５％以上含有させる必要があるが、０．０３％を超えて過剰に添加すると酸化物の液状化を促進し、合体により粗大化させるので、範囲を０．０００５〜０．０３％とする。
【００４０】
Ｍｇは、強力な脱酸元素であり、多様な元素と複合酸化物を作り有効である。溶鋼中で酸化物を形成するには０．０００５％以上含有させる必要があるが、０．０３％を超えて含有させると蒸発蒸気圧が高まり製鋼工程では危険である。したがって０．０００５〜０．０３％とする。
【００４１】
ＲＥＭもまた、強力な脱酸力を有し複合酸化物の生成を促進する。溶鋼中で酸化物を形成するには０．０００５％以上含有させる必要があるが、高価であるため、０．３％を超える添加は鋼板のコストを大幅に引き上げるとともに、脱酸元素としての効果は飽和するので、範囲を０．０００５〜０．３％とする。
【００４２】
次に製造プロセス条件の限定理由を述べる。
上記の化学組成を有する鋼片を、Ａｃ3 以上の温度に加熱後、累積圧下率５０〜９８％、圧延終了温度Ａｒ3 以上、Ａｒ3 ＋１００℃以下で所定の板厚まで熱間圧延し、更にＡｒ3 以下、Ａｒ3 −１５０℃以上の温度域まで放冷し、該温度域から加速冷却した後、さらに、Ａｃ 1 以上、Ａｃ 3 −２０℃以下の温度範囲に再加熱し、その後直ぐに、またはその再加熱温度で保持した後、１〜１００℃／ｓの冷却速度で５００℃以下まで焼き入れることにより、標記の組織は達成することができる。
【００４３】
圧延終了温度がＡｒ3 未満の低温になると、二相組織は達成されるものの集合組織の発達による異方性が生じ、実用上、構造物の安全性確保に用いられる超音波探傷が適用できない、シャルピー吸収エネルギー値が低下する、などの問題が生じるとともに、製造上は圧延負荷が高まる、圧延生産性が落ちるなどの問題も生じるため、圧延終了温度はＡｒ3 以上とした。
他方、圧延終了温度がＡｒ3 ＋１００℃超の場合は、硬質相の長手方向平均長さと板厚方向平均厚みの比が１０以上確保できないため、仕上げ温度の上限をＡｒ3 ＋１００℃以下とした。
【００４４】
また、冷却開始温度が低すぎると硬質相の所定の分率が確保できなくなるため、冷却開始の下限温度はＡｒ3 −１５０℃とした。
更に、加速冷却後、再度Ａｃ1 以上、Ａｃ3 −２０℃以下の温度範囲に加熱し、焼入れることにより、硬質相の硬度、分率を精度よく制御することが可能となるが、加熱温度がＡｃ3 −２０℃を超えると、硬質相硬度が低下し分率が上昇するので、加熱上限をＡｃ3 −２０℃とした。
【００４５】
一方、加速冷却を用いなくても本発明組織の達成は可能であり、その場合は、本発明の成分系を有する鋼片を、Ａｃ3 以上の温度に加熱後、累積圧下率５０〜９８％、圧延終了温度Ａｒ3 以上で所定の板厚まで熱間圧延し、５００℃以下まで冷却した後、Ａｃ1 以上、Ａｃ3 −２０℃以下の温度範囲に再加熱し、焼入れる。ただし、この場合も、硬質相の硬度、分率の制御の観点から、加熱上限温度はＡｃ3 −２０℃以下とする。
【００４６】
Ａｃ1 以上、Ａｃ3 −２０℃以下に再加熱の場合、その後の焼入れは、加熱温度到達直後から可能であるが、加熱温度到達後１０min 〜１２０min 保持することが望ましい。また、その後の焼入れは、１〜１００℃／ｓの冷却速度で５００℃以下まで急冷する必要であるが、冷却速度が１℃／ｓより遅い場合は、硬質相の所定の硬度が得られず、１００℃／ｓより速い冷却速度は現状の技術では経済的でない。また、焼入れ停止温度が５００℃を超える場合は硬質相の所定の分率が達成できないので、停止温度を５００℃以下にする。
【００４７】
【実施例】
以上が、本発明の要件についての説明であるが、さらに、実施例に基づいて本発明の効果を示す。
表１に示すＡ〜Ｉ、９種の化学組成の鋼を実機溶製し、実機連続鋳造にてスラブを作製した。このうち、Ａ〜Ｆは本発明の化学組成を満足しており、Ｇ〜Ｉは本発明の化学組成範囲を満足していない。
【００４８】
これらのスラブを表２に示した条件で熱間圧延、熱処理を行い鋼板＃７〜１９を作成した。鋼板＃７〜＃１２は本発明を満たすものである。また＃１３〜１９は本発明を満たさないもので、＃１３〜１４は化学成分を、＃１５は化学成分・鋼板製造条件とも、＃１６〜１９は鋼板製造条件をそれぞれ満たしていない。
それら鋼板の組織の解析結果、および廻し溶接継手の曲げ疲労試験により評価した疲労寿命、ならびに２ｍｍＶノッチシャルピー衝撃試験により評価した５０％破面遷移温度を表３に示す。
【００４９】
疲労試験は、構造物の溶接止端部から疲労き裂が発生し母材部を伝播する場合の疲労特性を評価するために、図１に示す廻し溶接継手について行った。試験片は鋼板から鋼板長手方向に長さ３００ｍｍ、幅８０ｍｍ、板厚２５ｍｍ（２５ｍｍ厚材については全厚、その他は表層から採取）のサイズの試験片を作製し、幅１０ｍｍ、長さ３０ｍｍ、高さ３０ｍｍのリブ板を炭酸ガス溶接により試験板の中央に廻し溶接で溶接した。
【００５０】
この際、炭酸ガス溶接は、化学組成がＣ：０．０６mass％、Ｓｉ：０．５mass％、Ｍｎ：１．４mass％である１．４ｍｍ径の溶接ワイヤを用いて、電流２７０Ａ、電圧３０Ｖ、溶接速度２０ｃｍ／min で行った。疲労試験は、荷重支点スパンを、下スパン７０ｍｍ、上スパン２２０ｍｍとする四点曲げで行い、最大荷重５５ｋＮ、応力比０．１の繰返し応力負荷を加えて、破断までの疲労寿命を測定した。またシャルピー試験は、試験片を板厚ｔ／４部から圧延方向に直角に採取して行った。
【００５１】
表３から明らかなように、本発明範囲により作成した鋼板は、いずれも疲労寿命で１００万cycle 超と、従来の鋼の数倍以上の優れた特性を示した。これは、前述のように硬質相が板厚方向に緻密に配置されたことによるものであり、破断後の破面観察、およびクラックパスの観察では、硬質相でのき裂の分岐、迂回、停留などが頻繁に観察され、これらに起因して疲労き裂伝播抵抗が高く、疲労寿命が著しく向上したことが確認された。
【００５２】
一方、比較鋼に関しては、＃１３は本発明範囲よりＣが高く、そのため硬質相の分率が本発明範囲を超えている。硬質相によって従来鋼より幾分疲労特性は向上するものの、本発明鋼には及ばない。また疲労試験後の破面状には硬質相の劈開によるとみられる脆性的な様相もみられ、硬質相の効果が十分でないことが示唆された。さらに高度の高い硬質相の分率が高いことで、靭性が著しく低下し、実用に耐えない。
【００５３】
次に＃１４は、本発明範囲よりＭｎが高く、これも＃１３同様に硬質相の量の増大と過剰な硬化により、靭性が実用鋼として不適である。
＃１５は、成分的には本発明範囲よりＰが高く、かつプロセス的には圧延終了温度、熱処理の再加熱温度とも本発明を満たしていない。実際、８８０℃という高温圧延によって組織はかなり等軸であり、さらに、６２０℃の熱処理で焼戻されたことで、硬質相はパーライト以外なく、疲労特性も従来鋼並みに低いものである。さらに、高Ｐのため靭性的にも実用的でない。
【００５４】
一方＃１６は、鋳片Ａを用いて化学成分的には本発明を満たすが、仕上げ板厚を８０ｍｍとしたため、累計圧下率が本発明範囲に至らず、組織的には硬質相の厚み方向の厚さが厚く、かつ扁平度も本発明が最適と考える所に至っていない。その結果、疲労特性も同鋳片を用いた本発明鋼、例えば＃１と比べて劣るものとなっている。
＃１７は、鋳片Ｂを用いているが、熱処理温度がＡｃ1 未満であり、硬質相の造り込みができず疲労特性も低い。熱処理による組織改善と疲労特性の向上は＃１７と＃９を比較すると明確である。
【００５５】
＃１８は、制御冷却の停止温度が本発明範囲より高い例である。これにより十分な硬質相硬さが得られず疲労特性も改善していない。同鋳片を用いた＃４と比較するとその差は明確である。
最後に＃１９は、Ｆ鋳片を用いて製造したもので、圧延仕上げ温度、制御冷却開始温度とも本発明を満たしていない。組織は鋼材組成の焼き入れ性の高さと相まって１００％のＢ＋Ｍ組織となり、疲労特性の向上が十分でないとともに、靭性的にも実用に耐えないレベルとなった。
これらの実施例より、本発明の有効性は明らかであり、本発明により構造用鋼として十分高い靭性を有しながら、疲労き裂伝播抵抗を有する鋼板が可能となった。
【００５６】
【表１】

【００５７】
【表２】

【００５８】
【表３】

【００５９】
【発明の効果】
本発明は、疲労特性が必要とされる溶接構造部材に用いられる、引張強度４００Ｎ／ｍｍ²級以上の厚鋼板において、従来溶接部では向上が困難とされてきた継手疲労特性の向上を特殊な合金元素や複雑な製造プロセスに頼ることなく製造できる点で、産業上の有用性は極めて大きい。
【図面の簡単な説明】
【図１】実施例における溶接継手疲労特性評価のための、廻し溶接四点曲げ試験片の概略図である。[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a steel plate used for large structures such as ships, bridges, buildings, marine structures, construction machinery, etc., and is excellent in propagation resistance in the thickness direction of fatigue cracks caused by repeated loads.Steel sheetIt relates to a manufacturing method.
[0002]
[Prior art]
In response to the demand for lighter structures and larger capacities, structural steel plates are rapidly becoming stronger. However, in structures subjected to repeated loads, not only the yield strength but also the fatigue strength must be taken into account, and there are cases where it is not possible to meet the need for higher strength, and an improvement in fatigue strength is eagerly desired.
[0003]
Fatigue fracture of welds generally occurs at the weld toe where stress concentration is large. It is known that the fatigue strength of welded structures is mainly governed by the shape of the toe of the welded part. Design measures to reduce stress concentration on the welded toe, shape processing of the toe, etc. Fatigue strength improvement measures may be applied. However, these not only increase the number of man-hours for constructing the structure, but there are many cases where the toe portion cannot be processed depending on the welded part. Therefore, improvement of fatigue characteristics is eagerly desired from the steel surface.
[0004]
Fatigue fracture of welded joints generally occurs from the weld toe where the stress concentration is large, so the generation characteristics are greatly affected by the shape of the weld toe and it is difficult to improve it by the steel composition and structure. Therefore, in order to improve the fatigue characteristics by controlling the steel structure, it is effective to delay the propagation of the fatigue crack generated at the toe portion in the thickness direction.
[0005]
Conventionally, Patent Documents 1 to 4 listed below have been disclosed as methods for improving the propagation resistance of fatigue cracks in steel plates.
In Patent Document 1, the steel is a specific component, and the structure is mainly composed of one or more of ferrite, pearlite, and bainite, the average particle diameter of ferrite is 8 μm or less, and the average existence interval is 20 μm or less. An invention of a steel sheet having excellent fatigue propagation resistance and arrestability (property to stop brittle cracks) is present, in which island-shaped martensite having an average flatness ratio of 5 or more is present at a volume ratio of 0.5 to 5%. Has been.
[0006]
Moreover, in patent document 2, it has the structure which consists of a ferrite mother phase of 60 to 90% and a hard second phase by area ratio steel, and the hardness of a mother phase and a second phase has a specific relational expression. Satisfied, and has a specific second phase form (aspect ratio is over 3.42; second phase spacing is 25 μm or less, etc., if necessary). An invention of a steel material having excellent fatigue crack propagation characteristics in which the particle size (average particle size of 4 μm or less, maximum particle size of 12.5 μm or less) is specified is described.
[0007]
Further, in Patent Document 3, a steel composition having a specific composition, in which a steel sheet structure is a hard part made of at least one of martensite, bainite, pearlite, pseudo pearlite, and tempered martensite, and a soft part made of ferrite is dispersed. The difference in hardness between these two parts is 150 or more in terms of Vickers hardness Hv, and if necessary, the average particle size of the soft part is defined as 50 μm or less, and the average interval between the hard parts is defined as 50 μm or less. An invention of a steel plate having an effect is disclosed.
[0008]
Further, in Patent Document 4, a steel layer having a specific composition and having a ferrite area ratio of 80% or more and a thickness of 30 to 100 μm and a martensite area ratio of 20% or more and a thickness of 10 to 30 μm are obtained in the plate pressure direction. An invention of a thick steel plate having excellent fatigue crack propagation characteristics in the thickness direction, alternately arranged in layers, is disclosed.
[0009]
[Patent Document 1]
JP-A-6-271984
[Patent Document 2]
JP-A-11-1742
[Patent Document 3]
Japanese Patent Laid-Open No. 7-242992
[Patent Document 4]
JP-A-8-73980
[0010]
[Problems to be solved by the invention]
However, in the invention described in Patent Literature 1, the content of the hard phase is low in order to achieve both fatigue propagation resistance and arrestability, and the fatigue crack delay effect is small as will be described later. The practical merit is not enough. In the invention described in Patent Document 2, the structure composed of ferrite and a hard second phase for suppressing crack propagation is achieved by increasing the cumulative rolling reduction at a low temperature from the two-phase region to the ferrite single-phase region. Therefore, there are problems such as low productivity and difficulty in securing the shape of the steel sheet.
[0011]
Further, the requirement that the difference in hardness between the hard phase and the ferrite ground as in the invention disclosed in Patent Document 3 is 150 or more in Hv, and the hard phase as in the invention disclosed in Patent Document 4 The requirement for alternately arranging the ferrite ground in a layered manner is not necessarily a necessary and sufficient condition as will be described later in the detailed study by the present inventors. There is.
[0012]
  Therefore, the present invention has a tensile strength of 400 N / mm used for welded structural members.²It is a heavy steel plate of grade or better, and it can greatly improve the fatigue characteristics of welded joints or base metals, regardless of the addition of large amounts of special or expensive alloying elements or inferior productivity. Excellent crack propagation resistanceSteel sheetThe object is to provide a manufacturing method.
[0013]
[Means for Solving the Problems]
In order to solve the above problems, the present inventors analyzed the relationship between fatigue crack propagation behavior and steel microstructure in various steel materials, and further examined the effects of ferrite ground and second phase on fatigue crack propagation resistance. As a result of detailed studies, the present inventors have found a means for improving the propagation resistance of fatigue cracks and have made the present invention. That is, when a fatigue crack occurs from the toe of the welded joint and the crack enters the thickness direction of the base metal, the fatigue crack is formed between the ferrite and the hard phase by dispersing the hard structure in the ferrite base. It is clarified that the propagation of macro cracks is remarkably suppressed by stopping, branching at the interface or in the vicinity of the interface, or by bypassing the hard phase. Optimization and industrial production methods were established.
[0016]
  That is, the gist of the present invention is as follows.
(1)% By mass
    C: 0.01 to 0.18%, Si: 0.01 to 2.0%,
    Mn: 0.1 to 3.0%, Al: 0.001 to 0.2%,
    P: 0.05% or less, S: 0.01% or less,
    N: 0.001% to 0.02%
After heating the steel slab comprising iron and the inevitable impurities to a temperature of Ac3 or higher, heat to a predetermined plate thickness at a cumulative reduction of 50 to 98%, rolling end temperature Ar3 or higher and Ar3 + 100 ° C or lower. Cold-rolled, allowed to cool to a temperature range of Ar3 or lower, Ar3 -150 ° C or higher, and accelerated cooling from the temperature range to 500 ° C or lower at a cooling rate of 1 to 100 ° C / s.And thenReheat to a temperature range of Ac1 or more and Ac3 to -20 ° C or less, and then immediately or after holding at the reheating temperature, quench to 500 ° C or less at a cooling rate of 1 to 100 ° C / s.Thus, on an arbitrary cross-sectional structure in the thickness direction, mainly composed of martensite and bainite, the average Vickers hardness is 230 or more, the average thickness in the thickness direction is 20 μm or less, and the average interval in the thickness direction is 50 μm or less. A hard structure having an average ratio of the length in the longitudinal direction of the steel sheet to the thickness in the thickness direction of 10 or more includes an average area ratio of 10 to 60%, and the balance is a structure mainly composed of ferrite having an average grain size of 40 μm or lessIt is characterized by,A method for producing a steel sheet having excellent fatigue crack propagation resistance.
[0017]
(2)% By mass
    C: 0.01 to 0.18%, Si: 0.01 to 2.0%,
    Mn: 0.1 to 3.0%, Al: 0.001 to 0.2%,
    P: 0.05% or less, S: 0.01% or less,
    N: 0.001% to 0.02%
A steel slab comprising iron and the inevitable impurities in the balance is heated to a temperature of Ac3 or higher, and then hot-rolled to a predetermined plate thickness at a cumulative reduction of 50 to 98% and a rolling end temperature Ar3 or higher, 500 After cooling to below ℃, reheat to the temperature range of Ac1 or more and Ac3 -20 ℃ or less, and then hold immediately or at the reheating temperature, then to 500 ℃ or less at a cooling rate of 1-100 ℃ / s QuenchThus, on an arbitrary cross-sectional structure in the thickness direction, mainly composed of martensite and bainite, the average Vickers hardness is 230 or more, the average thickness in the thickness direction is 20 μm or less, and the average interval in the thickness direction is 50 μm or less. A hard structure having an average ratio of the length in the longitudinal direction of the steel sheet to the thickness in the thickness direction of 10 or more includes an average area ratio of 10 to 60%, and the balance is a structure mainly composed of ferrite having an average grain size of 40 μm or lessA method for producing a steel sheet having excellent fatigue crack propagation resistance.
[0018]
(3) The steel slab is further mass%,
    Ni: 0.05-3%, Cu: 0.05-3%,
    Cr: 0.05-3%, Mo: 0.05-2%,
    Nb: 0.005-0.1%, Ti: 0.005-0.1%,
    Ta: 0.005 to 0.1%, V: 0.005 to 0.4%,
    B: 0.0002 to 0.004%
1 type or 2 types or more of the above, characterized in that(1) or (2)The manufacturing method of the steel plate excellent in the fatigue crack propagation resistance as described in 2.
(4) The billet is further mass%,
    Ca: 0.0005 to 0.03%, Mg: 0.0005 to 0.03%,
    REM: 0.0005 to 0.3%
1 type or 2 types or more of the above, characterized in that(1) to (3)The manufacturing method of the steel plate excellent in the fatigue crack propagation resistance of any one of these.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
In order to increase the propagation resistance when the fatigue crack generated from the weld toe reaches the base material and propagates in the thickness direction of the base material, in the crack propagation direction in the matrix mainly composed of ferrite, It is important to properly place a hard phase that can induce crack detouring, retention, and branching. That is, the presence frequency of the hard phase in the direction in which the crack propagates, the form for maximizing the propagation resistance, and the fraction of the hard phase are more important factors.
[0020]
First, regarding the frequency of the hard phase, the frequency of collision between the crack and the hard phase increases as the interval in the plate thickness direction, which is the crack propagation direction, becomes narrower. Especially, when the average interval in the crack propagation direction is 50 μm or less, We found that the resistance to propagation is significant. When the crack tip collides with the hard phase, the plastic deformation of the crack tip is constrained and the branching and bending of the crack tip is promoted, but the effect is caused over the entire line of the crack tip. It is important to ensure the collision frequency with the hard phase to connect to the macro effect, and conversely, if the average interval is larger than that, it will be clear from the experiment that it does not lead to the crack delay effect macroscopically. It was.
[0021]
At the same time, it is important that the shape of the hard phase be flat with respect to the longitudinal direction of the plate rather than being granular in terms of improving the collision frequency of the three-dimensional crack and the second phase. It was found from observation of propagation. In other words, when the hard phase is granular, cracks are observed to propagate between the hard phases, whereas if flat, the cracks preferentially give priority to the ferrite ground, helping the overlap in the thickness direction. If you try to propagate the crack, you will be forced to bypass the crack, leading to a delay in the effective crack propagation.
[0022]
From this point of view, the image analysis results of the cross-sectional structure in the plate thickness direction and the analysis of the fatigue crack propagation behavior are conducted. When the average ratio of the length of the second phase steel plate to the plate thickness is 10 or more, the crack propagation resistance Was found to work as effectively. If this is satisfied, the hard phase does not necessarily have a layered structure as conventionally known, and the flat hard phase only needs to be dispersed so as to satisfy the average interval in the crack propagation direction. In this case, combined with the average interval between the hard phases, the overlapping frequency of the hard phases when projected in the plate thickness direction greatly affects the improvement of crack propagation resistance. If the ratio is less than 10, the fatigue crack propagating in the thickness direction will be selected and propagated between the hard phases, and the delay due to crack detour and branching due to the collision between the crack and the hard phase will be insufficient. As a result, it does not lead to a macro crack delay effect.
[0023]
Furthermore, the size of the hard phase is also important. When ΔK (the difference between the maximum and minimum stress intensity factors at the crack tip) is small, that is, when the crack length is small or when the stress level is small, the effect of the size of the hard phase is small, but when ΔK increases, the hard phase In some cases, cracks do not lead to suppression of crack growth when cracks collide, but cracks open and promote abnormal crack growth. In order to suppress this, it is important that the thickness of the hard phase in the plate thickness direction is 20 μm or less on average, thereby stably providing resistance to propagation of fatigue cracks.
[0024]
Here, as the hard phase, it is basically required that martensite and bainite are mainly used, and the average hardness is 230 or more in terms of Vickers hardness. This point is different from the conventional technology that requires the hardness ratio and difference between the ferrite and the second phase. Is done. Conversely, if the average Vickers hardness of the second phase is less than 230, the plastic deformation at the tip of the fatigue crack is not sufficiently restricted, and the effect of dispersing the hard phase cannot be obtained.
An average hardness of 230 or more can also be achieved by pearlite or pseudo-pearlite, but they are brittle because they are a layered or mixed structure of ferrite and cementite, and may not work effectively because they crack at the crack tip. The effect is smaller than the second phase mainly composed of martensite and bainite. In order for the delay effect of fatigue crack growth by the hard second phase to lead to an improvement in macro fatigue properties, the hard phase needs to be at least 10% in terms of the area ratio on the cross section in the plate thickness direction. As long as the morphology and dispersion are maintained, the more the second phase is, the more effective it is. On the other hand, if the fraction of the hard phase is excessive, the toughness of the steel material decreases, so the upper limit was made 60%.
[0025]
As described above, in order to improve the fatigue characteristics of the steel material which is the object of the present invention, the introduction of the above-described second phase morphology and dispersion is required, but the chemical composition is also specifically limited. There is a need. The reasons for limiting the chemical composition will be described below.
C is added in an amount of 0.01% or more as an effective component for improving the strength of the steel, but excessive addition exceeding 0.18% significantly deteriorates the toughness of the welded portion, so it is regulated to 0.18% or less. To do.
[0026]
Si is necessary as a deoxidizing element for molten steel and is useful as an element for increasing the strength. However, if it is added in excess of 2.0%, the workability of the steel is lowered and the toughness of the weld is deteriorated. . Moreover, since the deoxidation effect is inadequate if it is less than 0.01%, the addition range is specified to be 0.01 to 2.0%.
[0027]
Mn is also necessary as a deoxidizing component element. If it is less than 0.1%, the cleanliness of the steel is lowered and the workability is impaired. Further, it is necessary to add 0.1% or more as a component for improving the strength of the steel material. However, excessive addition makes the hardenability excessive and makes it difficult to obtain a ferrite phase, so 3.0% is made the upper limit.
[0028]
P and S are impurity elements which are elements that deteriorate ductility and toughness, and it is preferable to reduce them as much as possible. However, the material deterioration is not so large that the upper limit of P is 0.05% and the upper limit of S is acceptable. Is limited to 0.01%.
[0029]
Al is useful as a deoxidizing element, and also works effectively with refinement of steel by forming a nitride in combination with N. The content of 0.001% or more is necessary. However, if the content exceeds 0.2% excessively, the oxide which is a deoxidation product is coarsened and the ductility is extremely deteriorated. Limited to a range of 2%.
[0030]
N is harmful to the toughness of the steel sheet in a single solid solution state in steel, but it is impossible to completely remove N in steel industrially, and reducing it more than necessary is excessive in the production process. This is not preferable because it imposes a load. Therefore, the lower limit is set to 0.001% as a range that can be industrially controlled and the load on the manufacturing process is allowable. However, if contained in excess, the solute N increases and adversely affects ductility and toughness, so the upper limit is made 0.02% as an acceptable range.
[0031]
In the steel sheet of the present invention, one, two or more of Ni, Cu, Cr, Mo, Nb, Ti, Ta, V, and B can be contained in the structure control for ensuring strength and toughness.
Ni is an element that improves the toughness of the steel material, and is particularly effective in securing toughness when a relatively large amount of the hard second phase is contained. On the other hand, Ni is an expensive alloy element, and if the content is further increased, the hardenability in the case of producing a steel material by thermomechanical processing becomes excessive, so the range is made 0.05% to 3%.
[0032]
Cu is an element effective for improving hardenability, solid solution strengthening, and precipitation strengthening, but 0.05% or more is necessary for producing a clear effect, whereas if it exceeds 3%, there is a problem in hot workability. Therefore, in the present invention, it is limited to a range of 0.05 to 3% as a range that exhibits the effect and does not cause problems such as hot workability.
[0033]
Cr is an element effective for improving the hardenability and improving the strength of the base metal by precipitation strengthening, but 0.05% or more is necessary to produce a clear effect, while adding over 3%, Since the toughness and weldability tend to deteriorate, the range is set to 0.05 to 3%.
[0034]
Mo is an element effective for improving hardenability, improving strength, tempering embrittlement resistance, and SR embrittlement resistance, but 0.05% or more is necessary for producing a clear effect. On the contrary, toughness and weldability deteriorate, so it is limited to 0.05 to 2%.
[0035]
Nb is an element effective for improving the strength by suppressing recrystallization of austenite in a solid solution and precipitated state in the austenite phase, and by forming Nb (C, N) during transformation or tempering. However, 0.005% or more is necessary to produce a clear effect. However, if the content exceeds 0.1%, the toughness deteriorates due to precipitation embrittlement. Therefore, the range in which the effect can be exhibited without causing deterioration of toughness is limited to the range of 0.005 to 0.1%.
[0036]
Ti is an element that contributes to improving the strength of the base metal by precipitation strengthening and is also effective for refining the heated austenite grain size by forming TiN that is stable even at high temperatures, and is an essential element in the present invention based on thermomechanical processing. It is. In order to exert the effect, the content of 0.005% or more is necessary. On the other hand, if it exceeds 0.1%, coarse precipitates and inclusions are formed to deteriorate toughness and ductility, so 0.005 to 0.1%.
[0037]
Ta and V are also elements having the same effect as Nb and Ti, and are specified as 0.005 to 0.1% and 0.005 to 0.4% as ranges where the effect can be exhibited without deterioration of toughness, respectively. did.
[0038]
B fixes and detoxifies N, which is harmful to toughness, and is also effective in suppressing recrystallization by segregating at the austenite grain boundary in a solid solution state. In order to exert an effect on detoxification of N and suppression of recrystallization, addition of 0.0002% or more is necessary. On the other hand, if excessive addition exceeds 0.004%, BN, Fe_{twenty three}(C, B)₆Since coarse precipitates such as the above are produced and the toughness deteriorates, the content is limited to 0.0002 to 0.004%.
[0039]
Furthermore, in the present invention, it is possible to contain one or more of Ca, Mg, and REM as deoxidation / desulfurization elements, and to further improve the toughness by further controlling the structure of the steel material and its welded portion. That is, fine oxides and sulfides based on these elements are effective for refining the structure.
Ca is an element having the maximum deoxidizing power / desulfurizing power in steel. In order to form oxides and sulfides in molten steel, it is necessary to contain 0.0005% or more, but adding excessively exceeding 0.03% promotes liquefaction of oxides and coarsens them by coalescence. Therefore, the range is made 0.0005 to 0.03%.
[0040]
Mg is a powerful deoxidizing element and is effective in forming complex oxides with various elements. In order to form an oxide in molten steel, it is necessary to contain 0.0005% or more. However, if it exceeds 0.03%, the vapor pressure of vapor increases, which is dangerous in the steelmaking process. Therefore, the content is made 0.0005 to 0.03%.
[0041]
REM also has a strong deoxidizing power and promotes the formation of complex oxides. In order to form an oxide in molten steel, it is necessary to contain 0.0005% or more. However, since it is expensive, the addition exceeding 0.3% greatly increases the cost of the steel sheet and is effective as a deoxidizing element. Is saturated, so the range is 0.0005 to 0.3%.
[0042]
  Next, the reasons for limiting the manufacturing process conditions will be described.
  The steel slab having the above chemical composition is heated to a temperature of Ac3 or higher, then hot-rolled to a predetermined sheet thickness at a cumulative reduction of 50 to 98%, a rolling end temperature of Ar3 or higher and Ar3 + 100 ° C or lower, and further Ar3 or lower. , Ar3 -150 ° C or higher, and cooled from this temperature rangeAnd then Ac 1 Above, Ac Three Reheat to a temperature range of −20 ° C. or lower, and then immediately or after holding at the reheat temperature, quench to 500 ° C. or lower at a cooling rate of 1 to 100 ° C./s.Thus, the title structure can be achieved.
[0043]
When the rolling finish temperature is lower than Ar3, a two-phase structure is achieved, but anisotropy occurs due to the development of the texture. In practice, ultrasonic flaw detection used to ensure the safety of structures cannot be applied. In addition to problems such as a decrease in the absorbed energy value and problems such as an increase in rolling load and a reduction in rolling productivity in production, the rolling end temperature is set to Ar3 or higher.
On the other hand, when the rolling end temperature is higher than Ar3 + 100 ° C, the ratio of the average length in the longitudinal direction of the hard phase to the average thickness in the thickness direction cannot be ensured to 10 or more, so the upper limit of the finishing temperature is set to Ar3 + 100 ° C or lower.
[0044]
Further, if the cooling start temperature is too low, a predetermined fraction of the hard phase cannot be secured, so the lower limit temperature for starting cooling is Ar3 -150 ° C.
Furthermore, after accelerated cooling, it is possible to accurately control the hardness and fraction of the hard phase by heating again to a temperature range of Ac1 to Ac3-20 ° C. and quenching, but the heating temperature is Ac3. If it exceeds -20 ° C, the hardness of the hard phase decreases and the fraction increases, so the upper limit of heating was set to Ac3 -20 ° C.
[0045]
On the other hand, it is possible to achieve the structure of the present invention without using accelerated cooling. In that case, after the steel slab having the component system of the present invention is heated to a temperature of Ac3 or higher, the cumulative rolling reduction is 50 to 98%, The steel sheet is hot-rolled to a predetermined thickness at a rolling end temperature Ar3 or higher, cooled to 500 ° C. or lower, reheated to a temperature range of Ac1 to Ac3 −20 ° C. and quenched. However, also in this case, from the viewpoint of controlling the hardness and fraction of the hard phase, the heating upper limit temperature is set to Ac3 -20 ° C or lower.
[0046]
In the case of reheating to Ac1 or higher and Ac3 to -20 ° C or lower, the subsequent quenching can be performed immediately after reaching the heating temperature, but it is desirable to hold for 10 min to 120 min after reaching the heating temperature. Further, the subsequent quenching requires rapid cooling to 500 ° C. or less at a cooling rate of 1 to 100 ° C./s. However, when the cooling rate is slower than 1 ° C./s, the predetermined hardness of the hard phase cannot be obtained. Cooling rates faster than 100 ° C./s are not economical with current technology. Moreover, since the predetermined fraction of a hard phase cannot be achieved when quenching stop temperature exceeds 500 degreeC, stop temperature shall be 500 degrees C or less.
[0047]
【Example】
The above is an explanation of the requirements of the present invention. Further, the effects of the present invention are shown based on examples.
A to I and 9 types of chemical compositions of steel shown in Table 1 were melted in actual machines, and slabs were prepared by continuous casting of actual machines. Among these, A to F satisfy the chemical composition of the present invention, and GI does not satisfy the chemical composition range of the present invention.
[0048]
  These slabs were hot rolled and heat treated under the conditions shown in Table 27~ 19 were made. steel sheet#7~ # 12 meets the present inventionis there. Further, # 13 to 19 do not satisfy the present invention, # 13 to 14 are chemical components, # 15 is neither chemical components nor steel plate manufacturing conditions, and # 16 to 19 does not satisfy steel plate manufacturing conditions.
  Table 3 shows the analysis results of the structures of these steel plates, the fatigue life evaluated by the bending fatigue test of the turn welded joint, and the 50% fracture surface transition temperature evaluated by the 2 mm V notch Charpy impact test.
[0049]
The fatigue test was performed on the turn welded joint shown in FIG. 1 in order to evaluate the fatigue characteristics when a fatigue crack occurs from the weld toe of the structure and propagates through the base metal. The test piece is a test piece having a size of 300 mm in length from the steel plate in the longitudinal direction of the steel plate, a width of 80 mm, and a thickness of 25 mm (total thickness for the 25 mm thick material, other samples are taken from the surface layer), a width of 10 mm, a length of 30 mm, A rib plate with a height of 30 mm was turned to the center of the test plate by carbon dioxide welding and welded by welding.
[0050]
At this time, carbon dioxide welding uses a 1.4 mm diameter welding wire whose chemical composition is C: 0.06 mass%, Si: 0.5 mass%, and Mn: 1.4 mass%, with a current of 270 A, a voltage of 30 V, The welding speed was 20 cm / min. The fatigue test was performed by four-point bending with a load fulcrum span of 70 mm for the lower span and 220 mm for the upper span, a repeated stress load with a maximum load of 55 kN and a stress ratio of 0.1 was applied, and the fatigue life until fracture was measured. In addition, the Charpy test was performed by collecting a test piece from a thickness t / 4 part at a right angle to the rolling direction.
[0051]
As is apparent from Table 3, all the steel plates prepared according to the scope of the present invention exhibited fatigue properties of over 1 million cycles, which were superior to several times that of conventional steel. This is due to the fact that the hard phase is densely arranged in the plate thickness direction as described above, and in the fracture surface observation after fracture and the crack path observation, crack branching in the hard phase, detouring, Residues and the like were frequently observed, and it was confirmed that the fatigue crack propagation resistance was high due to these, and the fatigue life was remarkably improved.
[0052]
On the other hand, for the comparative steel, # 13 has a C higher than the range of the present invention, and therefore the fraction of the hard phase exceeds the range of the present invention. Although the fatigue properties are somewhat improved by the hard phase compared to the conventional steel, it is not as good as the steel of the present invention. In addition, the fracture surface after the fatigue test also showed a brittle appearance that could be attributed to cleavage of the hard phase, suggesting that the effect of the hard phase was not sufficient. Furthermore, since the fraction of the hard phase having a high degree is high, the toughness is remarkably lowered and cannot be put into practical use.
[0053]
Next, # 14 has a higher Mn than the scope of the present invention, which is also unsuitable as a practical steel due to an increase in the amount of hard phase and excessive hardening as in # 13.
In # 15, P is higher than the range of the present invention in terms of components, and the rolling finish temperature and the reheating temperature of heat treatment do not satisfy the present invention in terms of process. In fact, the structure is fairly equiaxed by high-temperature rolling at 880 ° C., and further, tempered by heat treatment at 620 ° C., the hard phase is not pearlite, and the fatigue properties are as low as conventional steel. Furthermore, because of the high P, it is not practical in terms of toughness.
[0054]
On the other hand, # 16 uses the slab A to satisfy the present invention in terms of chemical composition, but the finished plate thickness is set to 80 mm, so the cumulative rolling reduction does not reach the range of the present invention, and structurally, the thickness direction of the hard phase However, the flatness has not reached the point where the present invention is considered optimal. As a result, the fatigue properties are inferior to the steel of the present invention using the cast slab, for example, # 1.
# 17 uses the slab B, but the heat treatment temperature is less than Ac1, the hard phase cannot be built, and the fatigue characteristics are low. The improvement of the structure and the improvement of fatigue properties by heat treatment are clear when comparing # 17 and # 9.
[0055]
# 18 is an example in which the control cooling stop temperature is higher than the range of the present invention. As a result, sufficient hard phase hardness cannot be obtained, and fatigue characteristics are not improved. The difference is clear when compared with # 4 using the slab.
Finally, # 19 was manufactured using an F slab, and neither the rolling finishing temperature nor the controlled cooling start temperature satisfied the present invention. Combined with the high hardenability of the steel composition, the structure became a 100% B + M structure, and the fatigue characteristics were not sufficiently improved, and the toughness was at a level that could not withstand practical use.
From these examples, the effectiveness of the present invention is clear, and the present invention enables a steel sheet having fatigue crack propagation resistance while having sufficiently high toughness as a structural steel.
[0056]
[Table 1]

[0057]
[Table 2]

[0058]
[Table 3]

[0059]
【The invention's effect】
The present invention is used for welded structural members that require fatigue characteristics, and has a tensile strength of 400 N / mm.²In steel plates of grades or better, the industrial usefulness is extremely high in that it can be manufactured without relying on special alloying elements or complicated manufacturing processes to improve joint fatigue properties, which has been difficult to improve with conventional welds. .
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic view of a turn welded four-point bending test piece for evaluating fatigue characteristics of welded joints in Examples.

Claims

% By mass
C: 0.01 to 0.18%,
Si: 0.01 to 2.0%,
Mn: 0.1 to 3.0%
Al: 0.001 to 0.2%,
P: 0.05% or less,
S: 0.01% or less,
N: 0.001% to 0.02%
After heating the steel slab comprising iron and the inevitable impurities to a temperature of Ac3 or higher, heat to a predetermined plate thickness at a cumulative reduction of 50 to 98%, rolling end temperature Ar3 or higher and Ar3 + 100 ° C or lower. After cold rolling, the mixture is allowed to cool to a temperature range of Ar 3 or less and Ar 3 to 150 ° C. or more, accelerated from the temperature range to 500 ° C. or less at a cooling rate of 1 to 100 ° C./s , and further Ac 1 or more, Ac 3) Reheating to a temperature range of -20 ° C or less, and then holding it immediately or at the reheating temperature, and then quenching it to 500 ° C or less at a cooling rate of 1 to 100 ° C / s. In the cross-sectional structure of the steel sheet, it is mainly composed of martensite and bainite, has an average Vickers hardness of 230 or more, an average thickness in the thickness direction of 20 μm or less, an average interval in the thickness direction of 50 μm or less, and a length in the longitudinal direction of the steel plate. And wherein 10% to 60% on average ratio mean area ratio of 10 or more hard tissue in the thickness direction thickness, the remainder is characterized by a structure mainly the following average ferrite grain diameter of 40 [mu] m, the fatigue crack A method of manufacturing a steel sheet with excellent propagation resistance.

% By mass
C: 0.01 to 0.18%,
Si: 0.01 to 2.0%,
Mn: 0.1 to 3.0%
Al: 0.001 to 0.2%,
P: 0.05% or less,
S: 0.01% or less,
N: 0.001% to 0.02%
A steel slab comprising iron and the inevitable impurities in the balance is heated to a temperature of Ac3 or higher, and then hot-rolled to a predetermined plate thickness at a cumulative reduction of 50 to 98% and a rolling end temperature Ar3 or higher, 500 After cooling to below ℃, reheat to the temperature range of Ac1 or more and Ac3 -20 ℃ or less, and then hold immediately or at the reheating temperature, then to 500 ℃ or less at a cooling rate of 1-100 ℃ / s By quenching, it consists mainly of martensite and bainite on an arbitrary cross-sectional structure in the thickness direction, the average Vickers hardness is 230 or more, the average thickness in the thickness direction is 20 μm or less, and the average interval in the thickness direction is 50 μm. In the following, a hard structure having an average ratio between the length in the longitudinal direction of the steel sheet and the thickness in the thickness direction of 10 or more is contained in an average area ratio of 10 to 60%, and the balance is mainly composed of ferrite having an average grain size of 40 μm or less. Characterized by a woven, a manufacturing method of steel sheet excellent in fatigue crack propagation resistance.

The steel slab is further mass%,
Ni: 0.05-3%,
Cu: 0.05-3%,
Cr: 0.05-3%,
Mo: 0.05-2%,
Nb: 0.005 to 0.1%,
Ti: 0.005 to 0.1%,
Ta: 0.005 to 0.1%,
V: 0.005 to 0.4%,
B: 0.0002 to 0.004%
1 or 2 types or more of these are contained, The manufacturing method of the steel plate excellent in the fatigue crack propagation resistance of Claim 1 or 2 characterized by the above-mentioned.

The steel slab is further mass%,
Ca: 0.0005 to 0.03%,
Mg: 0.0005 to 0.03%,
REM: 0.0005 to 0.3%
The method for producing a steel sheet excellent in fatigue crack propagation resistance according to any one of claims 1 to 3 , wherein the steel sheet contains one or more of the following.