JP3899014B2

JP3899014B2 - Tensile strength 570 to 800 MPa class high strength steel plate excellent in toughness of base metal and weld heat affected zone and method for producing the same

Info

Publication number: JP3899014B2
Application number: JP2002322443A
Authority: JP
Inventors: 仁志古谷; 達也熊谷; 政昭藤岡
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2002-11-06
Filing date: 2002-11-06
Publication date: 2007-03-28
Anticipated expiration: 2022-11-06
Also published as: JP2004156095A

Description

【０００１】
【発明の属する技術分野】
本発明は、母材及び溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法に関するものである。この製法で製造した鋼材は、造船、橋梁、建築、海洋構造物、圧力容器、ラインパイプなどの溶接構造物一般に用いることができるが、各構造物のうち母材あるいは溶接熱影響部の靱性確保が困難となる板厚４０ｍｍ以上の鋼板を適用する部位での使用において特に有効である。
【０００２】
【従来の技術】
近年の構造物の大型化に伴い、強度や板厚の要求水準は高くなる一方であり、高強度かつ厚手、しかも母材靱性や溶接熱影響部靱性、さらには溶接性にも優れる鋼材が求められている。母材靱性や溶接熱影響部靱性を考慮した場合、合金元素の添加量は極力控えた上で強度の確保を図る必要があり、これを達成するための手法は、例えば下記の特許文献１〜４などに開示されている。これらは、主として製造条件の規定により組織を微細化し、靱性の向上を図ったものである。
【０００３】
【特許文献１】
特開昭５９−２００７２３号公報
【０００４】
【特許文献２】
特開平２−２０５６２７号公報
【０００５】
【特許文献３】
特開平６−９３３３２号公報
【０００６】
【特許文献４】
特開平１０−１５８７７８号公報
【０００７】
前記特許文献１には、未再結晶域での圧下量を５０％以上として、さらに圧延パス間時間や圧延終了後の冷却開始時間を制御することで微細なフェライト組織を得ると記述されている。このように未再結晶域で導入される加工とその後の回復過程を制御することは組織微細化に対して非常に有効であるものの、この手法で製造した鋼板の組織はフェライトが主体となるため、引張強さの高い鋼板、例えば６００ＭＰａ超の引張強さの鋼板を製造することは困難である。
【０００８】
一方、前記特許文献２，４及び３は、いずれもベイナイトを含む組織の靱性を向上する方法を開示している。
特許文献２に記載の発明は、Ｂ（ホウ素）添加による焼入性向上効果の活用を規定している。しかし、焼入性低下を抑制するために圧下率を３０％以下としており、組織微細化による靱性向上の程度は小さい。
【０００９】
特許文献４に記載の発明では、未再結晶域での圧下量を５０％以上とし、冷却速度を制御することで微細なベイナイトとフェライトの混合組織を生成させ、靱性向上をはかっている。しかし、未再結晶温度域で導入された加工下部組織からのベイナイトの核生成促進、加工下部組織によるベイナイトの成長抑制、フェライト生成によるベイナイト分断などを通じた最終組織の大幅な微細化は、未再結晶温度域での圧下量と圧延後の冷却速度を主たる制御指針とするのみでは不十分であり、最大の組織微細化効果は得られない。
【００１０】
また、同様の視点による前記特許文献３に記載の発明では、未再結晶温度域での圧下量や冷却速度の制御のみでなく、圧延後の冷却開始までの時間を「直ちに」と規定しており、圧延最終パス時点での加工下部組織は回復が抑制された状態で加速冷却されるため、組織微細化には有利と考えられる。しかし、組織微細化効果を得るための「直ちに」の定量的指標、すなわち圧延終了後何秒以内に加速冷却を開始すべきかが明確でなく、効果発現の信頼性を疑問視せざるを得ない。さらに、未再結晶温度域での圧下率が５０％以上となっているため、圧下率の確保が困難となる厚手材ではこの手法を使用することはできない。
【００１１】
以上挙げたように、ベイナイトを含む組織の微細化を考える場合には、ベイナイトの核生成サイト増大やベイナイトラスの成長抑制、フェライト生成によるベイナイトの分断等を通じた組織微細化に寄与する加工下部組織の導入量や不均一性、さらに逐次進行する回復や再結晶の制御が非常に重要であり、上記に述べたような指針のみでは最大限の効果を得るには不十分である。しかもこれら技術の根幹となっている未再結晶域での大圧下率は板厚の大きい鋼板を製造する上では大きな障害となり、かつ生産性を大きく阻害する。
【００１２】
【発明が解決しようとする課題】
本発明は、上記の問題点を解消し、母材及び溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板及びその製造方法を提供することを課題としている。
【００１３】
【課題を解決するための手段】
発明者らは、種々の製造条件による組織微細化の検討を進めた結果、鋼中の析出物の制御や圧延前、圧延中、圧延後の幅広い範囲での冷却制御が重要であることを新たに知見し、この有効な範囲を限定するに至り、本発明を完成したもので、その要旨とするところは以下の通りである。
【００１４】
（１）鋼が、質量％で、
Ｃ：０．００５〜０．２％、Ｓｉ：０．０１〜１％、
Ｍｎ：０．１〜２％、Ｐ：０．０２％以下、
Ｓ：０．０２％以下、Ｎ：０．０００１〜０．０１％、
Ｂ：０．０００５〜０．００５％、Ａｌ：０．００１〜０．１％
を含有し、残部がＦｅ及び不可避的不純物からなる鋼組成とし、Ｂを含有する炭化物、窒化物、炭窒化物のなかで円相当直径の寸法が５０ｎｍ以上１００００ｎｍ以下のものの個数密度が５．０×１０⁷個／ｍｍ²未満であり、該個数密度をＢの添加量で除した値が２．０×１０¹⁰個／（ｍｍ²・質量％）未満であり、ベイナイトとマルテンサイトの体積率の和が６０％以上のベイナイト主体組織であり、さらにベイナイト中で隣接組織との結晶方位差が１５度以上となる領域の平均寸法が２０μｍ以下であることを特徴とする、母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板。
（２）質量％で、さらに、
Ｎｂ：０．００１〜０．１％、Ｔｉ：０．００１〜０．２％、
Ｖ：０．００１〜０．２％
の１種または２種以上を含有することを特徴とする、前記（１）に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板。
（３）質量％で、さらに、
Ｃｕ：０．００５〜１％、Ｎｉ：０．０１〜２％、
Ｃｒ：０．０１〜１％、Ｍｏ：０．０１〜１％
の１種または２種以上を含有することを特徴とする、前記（１）または（２）のに記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板。
（４）質量％で、さらに、
Ｃａ：０．０００５〜０．０２％、Ｍｇ：０．０００５〜０．０２％、
ＲＥＭ：０．００１〜０．１％
の１種または２種以上を含有することを特徴とする、前記（１）ないし（３）のいずれか１項に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板。
【００１５】
（５）質量％で、
Ｃ：０．００５〜０．２％、Ｓｉ：０．０１〜１％、
Ｍｎ：０．１〜２％、Ｐ：０．０２％以下、
Ｓ：０．０２％以下、Ｎ：０．０００１〜０．０１％、
Ｂ：０．０００５〜０．００５％、Ａｌ：０．００１〜０．１％
を含有し、残部がＦｅ及び不可避的不純物からなる鋼組成を有する鋼片または鋳片を、１０５０℃以上１３５０℃以下の温度域に加熱し、該温度域に２０分以上保持をした後に圧延を開始し、圧延をＡｒ3 点以上９５０℃以下で終了した後、２０秒以内に鋼板表面の平均冷却速度が０．５℃／ｓ以上の冷却を行い、３５０℃以上で冷却を終了した後空冷することを特徴とする、Ｂを含有する炭化物、窒化物、炭窒化物のなかで円相当直径の寸法が５０ｎｍ以上１００００ｎｍ以下のものの個数密度が５．０×１０ ⁷ 個／ｍｍ ² 未満であり、該個数密度をＢの添加量で除した値が２．０×１０ ¹⁰ 個／（ｍｍ ² ・質量％）未満であり、ベイナイトとマルテンサイトの体積率の和が６０％以上のベイナイト主体組織であり、さらにベイナイト中で隣接組織との結晶方位差が１５度以上となる領域の平均寸法が２０μｍ以下である母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
（６）質量％で、
Ｃ：０．００５〜０．２％、Ｓｉ：０．０１〜１％、
Ｍｎ：０．１〜２％、Ｐ：０．０２％以下、
Ｓ：０．０２％以下、Ｎ：０．０００１〜０．０１％、
Ｂ：０．０００５〜０．００５％、Ａｌ：０．００１〜０．１％
を含有し、残部がＦｅ及び不可避的不純物からなる鋼組成を有する鋼片または鋳片を、１０５０℃以上１３５０℃以下の温度域に加熱し、該温度域に２０分以上保持をした後に圧延を開始し、圧延をＡｒ3 点以上９５０℃以下で終了した後、２０秒以内に鋼板表面の平均冷却速度が０．５℃／ｓ以上の冷却を行い、３５０℃未満で冷却を終了した後空冷し、さらにその後に９００℃以下の温度で焼き戻しを行うことを特徴とする、Ｂを含有する炭化物、窒化物、炭窒化物のなかで円相当直径の寸法が５０ｎｍ以上１００００ｎｍ以下のものの個数密度が５．０×１０ ⁷ 個／ｍｍ ² 未満であり、該個数密度をＢの添加量で除した値が２．０×１０ ¹⁰ 個／（ｍｍ ² ・質量％）未満であり、ベイナイトとマルテンサイトの体積率の和が６０％以上のベイナイト主体組織であり、さらにベイナイト中で隣接組織との結晶方位差が１５度以上となる領域の平均寸法が２０μｍ以下である母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
（７）質量％で、さらに、
Ｎｂ：０．００１〜０．１％、Ｔｉ：０．００１〜０．２％、
Ｖ：０．００１〜０．２％
の１種または２種以上を含有することを特徴とする、前記（５）または（６）に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
（８）質量％で、さらに、
Ｃｕ：０．００５〜１％、Ｎｉ：０．０１〜２％、
Ｃｒ：０．０１〜１％、Ｍｏ：０．０１〜１％
の１種または２種以上を含有することを特徴とする、前記（５）ないし（７）のいずれか１項に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
（９）質量％で、さらに、
Ｃａ：０．０００５〜０．０２％、Ｍｇ：０．０００５〜０．０２％、
ＲＥＭ：０．００１〜０．１％
の１種または２種以上を含有することを特徴とする、前記（５）ないし（８）のいずれか１項に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
【００１６】
（１０）加熱後の鋼片または鋳片を、鋼塊表面の平均冷却速度が２℃／ｓ以上で１０５０℃以下Ａｒ3 点以上の温度まで冷却し、復熱後に圧延を開始することを特徴とする、前記（５）ないし（９）のいずれか１項に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
（１１）再結晶温度未満７００℃以上で実施する圧延の圧下率を７０％以上とすることを特徴とする、前記（５）ないし（１０）のいずれか１項に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
（１２）全ての圧延パスにおける圧下率を平均した値が２５％以上であることを特徴とする、前記（５）ないし（１１）のいずれか１項に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
（１３）圧延最終パスの圧下率が２５％以上であることを特徴とする、前記（５）ないし（１２）に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
（１４）圧延パス間の一部あるいは全てにおいて、鋼板表面の平均冷却速度が２℃／ｓ以上の冷却を行い、復熱後に次の圧延パスを実施することを特徴とする、前記（５）ないし（１３）のいずれか１項に記載の母材および溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰａ級高強度鋼板の製造方法。
【００１７】
【発明の実施の形態】
本発明を詳細に説明する。
発明者らは、圧延プロセスにおける冷却の効果を最大限に活用するために、様々な冷却過程を取り入れた圧延実験を行い、組織微細化に対する影響を調べた。強度レベルは５７０ＭＰａ級以上８００ＭＰａ級までを対象とした。この結果、母材及び溶接熱影響部の靱性確保に関して、以下の手法が非常に有効であることを見いだした。
【００１８】
最も重要な点は、Ｂの存在状態を規定することである。具体的には、Ｂを含有する炭化物、窒化物、炭窒化物の中で５０ｎｍ以上１００００ｎｍ以下のものの個数密度が５．０×１０⁷個／ｍｍ²未満であり、該個数密度をＢの添加量で除した値が２．０×１０¹⁰個／（ｍｍ²・質量％）未満と規定する。これは、５０ｎｍ以上１００００ｎｍの範囲にある粗大析出物が上記範囲にある場合には、焼入性を高める固溶Ｂが変態温度域で多量に残存し、変態強化による強度増大が顕著に現れることを新たに知見したためである。
Ｂを含む炭化物、窒化物、炭窒化物の個数密度が５．０×１０⁷個／ｍｍ² 以上の場合は、特に強度増大量が低下し、さらに母材靭性も低下するため、Ｂを含有する炭化物、窒化物、炭窒化物の個数密度を５．０×１０ ⁷ 個／ｍｍ²未満と限定する。
【００１９】
また、Ｂを含有する炭化物、窒化物、炭窒化物の絶対的個数はＢの添加量増大とともに大きくなるため、その添加量に対して個数密度の上限値を規定する必要がある。Ｂを含有する炭化物、窒化物、炭窒化物のなかで５０ｎｍ以上１００００ｎｍ以下のものの個数密度をＢの添加量で除した値が２．０×１０¹⁰個／（ｍｍ²・質量％）以上の場合は、たとえ個数密度が５．０×１０⁷個／ｍｍ²未満であっても強度増大効果が小さくなるため、Ｂを含有する炭化物、窒化物、炭窒化物のなかで５０ｎｍ以上１００００ｎｍ以下のものの個数密度をＢの添加量で除した値を２．０×１０¹⁰個／（ｍｍ²・質量％）未満と規定した。
【００２０】
なお、ここでＢを含有する炭化物、窒化物、炭窒化物というのは、例えばＢＮなどである。また、前記の炭化物、窒化物、炭窒化物が他の酸化物、硫化物、炭化物、窒化物、炭窒化物、例えばＡｌ₂Ｏ₃、ＭｎＳ、ＡｌＮなどと複合して析出した場合にもこれを含めるものとする。
【００２１】
本発明ではＢを含有する炭化物、窒化物、炭窒化物の寸法が５０ｎｍ以上１００００ｎｍ以下のものを規定しているが、これら炭化物、窒化物、炭窒化物の寸法を測定する方法は以下の通りである。
鋼材を下記の非特許文献１に示すような選択的腐食技術により腐食し、この後下記の非特許文献２に示すような抽出レプリカ法により電子顕微鏡試料を作製し、透過型電子顕微鏡で倍率１万倍の明視野像の観察を行い、個々の粒子の面積から算出した円相当直径をもってその寸法とする。
【００２２】
【非特許文献１】
「防食技術」第３７号、１９８８年１２月、腐食防食協会発行、７７６〜７７８頁、
【００２３】
【非特許文献２】
「電子顕微鏡Ｑ＆Ａ」、１９９６年１２月１５日、アグネ承風社発行、１７４頁
【００２４】
また、Ｂを含有する炭化物、窒化物、炭窒化物の個数密度は、同じく透過型電子顕微鏡で撮影した倍率１万倍の明視野写真をその総面積が１０００μｍ²以上となる枚数だけ撮影し、この領域内に存在するＢを含有する炭化物、窒化物、炭窒化物のうちその寸法が５０ｎｍ以上１００００ｎｍ以下であるものの個数を測定し、これを撮影した領域の面積で除した値とする。さらに、個数密度を添加量で除した値を算出する際の添加量は、鋼材の成分分析により得られた全Ｂ量を質量％で表した値とする。
【００２５】
また、本発明が対象とする５７０ＭＰａ以上の高強度と母材および溶接熱影響部の高靱性を両立させるためには、以下の点が重要である。母材の靱性を向上させるためには、一定の方位差を有する組織単位を微細化することが重要となる。特にベイナイト中で隣接組織との結晶方位差が１５度以上となる領域を考えた場合、この領域の平均寸法が２０μｍ以下である場合に、靱性が非常に高くなることを知見した。このことから、ベイナイト中で隣接組織との方位差が１５度以上となる領域の平均寸法を２０μｍ以下と規定する。
【００２６】
一方、溶接熱影響部の靱性を高めるためには合金元素の添加量を極力低くすることが望ましい。一定の強度で合金元素の添加量を低減するためには、ベイナイトやマルテンサイトの体積率を一定量以上確保することが有効であり、特にベイナイトとマルテンサイトの体積率の和が６０％以上である場合に合金元素の添加量低減効果が大となるため、本発明におけるベイナイトとマルテンサイトの体積率の和を６０％以上と規定する。
なお、ベイナイト中で隣接組織との方位差が１５度以上となる領域の平均寸法については、ＥＢＳＰにより１．０×１０⁵μｍ²の領域について解析した結果得られる平均面積より円相当の平均直径を計算するものとする。
【００２７】
以下に、合金成分の添加量を規定した理由を述べる。
Ｃは、強度確保に必須の元素であるため、その添加量を０．００５％以上とする。しかし、一方でＣ量の増大は粗大析出物の生成による母材靱性や溶接性の低下を招くためその上限を０．２％とする。
【００２８】
Ｎは、Ｂの窒化物形成を通じて焼入性を低下させるため、極力低減することが望ましく、その上限を０．０１％とする。一方下限値は、コストを勘案して０．０００１％とする。
【００２９】
Ｂは、焼入性の増大に有効な元素であり、その添加量を０．０００５％以上とする。しかし、一方でＢ量の増大は粗大析出物の生成により母材靭性の低下を招くためその上限を０．００５％とする。
【００３０】
Ｓｉ、Ｍｎは強度確保の観点から必要に応じて添加する。
Ｓｉは、強度確保及び脱酸に必要な元素であるため、その添加量を０．０１％以上とする。しかし、一方でＳｉ量の増大は溶接性を低下させるためその上限を１％とする。
【００３１】
Ｍｎは強化元素として有用であるが、過剰な添加は溶接性を低下させるため、その範囲を０．１％以上２％以下とする。
【００３２】
Ａｌは、脱酸材として添加される他、窒化物の形成により固溶Ｂ量を増大する効果のある元素であり、その添加量を０．００１％以上とする。しかし、一方でＡｌ量の増大は母材靭性の低下を招くためその上限を０．１％とする。
【００３３】
Ｐは不純物元素であり低い方が望ましく、０．０２％以下とする。特に、フェライトに固溶したＰは母材の延性を低下させるため、望ましくは０．０１５％以下とする。
【００３４】
Ｓは、不純物元素であり低い方が望ましく、０．０２％以下とする。ＳはＭｎＳの生成により母材靱性を低下させるため、望ましくは０．０１％以下とする。
【００３５】
Ｔｉ、Ｎｂ、Ｖは固溶Ｎ量の低減や析出強化のため必要に応じて添加する。
Ｔｉは、窒化物の形成による固溶Ｎ量の低減を通じてＢの焼入性を向上させることや、微細析出物による強度確保に有効な元素であるため、その添加量を０．００１％以上とする。しかし、一方でＴｉ量の増大は粗大析出物の生成による母材靭性や溶接性の低下を招くためその上限を０．２％とする。
【００３６】
Ｎｂは、窒化物の形成による固溶Ｎ量の低減を通じてＢの焼入性を向上させることや、微細析出物による強度確保に有効な元素であるため、その添加量を０．００１％以上とする。しかし、一方でＮｂ量の増大は粗大析出物の生成による母材靭性や溶接性の低下を招くためその上限を０．１％とする。
【００３７】
Ｖは、窒化物の形成による固溶Ｎ量の低減を通じてＢの焼入性を向上させることや、微細析出物による強度確保に有効な元素であるため、その添加量を０．００１％以上とする。しかし、一方でＶ量の増大は粗大析出物の生成による母材靭性の低下を招くためその上限を０．２％とする。
【００３８】
Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏは、強度確保の観点から必要に応じて添加する。
Ｃｕは、強度確保のため必要に応じて添加する。０．００５％未満の添加ではその効果は小さく、一方、１％を超える添加は溶接性を低下させるため、その範囲を０．０１〜１％とする。
【００３９】
Ｎｉは、強度確保のために必要に応じて添加する。０．０１％未満の添加ではその効果は小さく、一方、２％を超える添加は溶接性を低下させるため、その範囲を０．０１〜２％とする。
【００４０】
Ｃｒは、強度確保のために必要に応じて添加する。０．０１％未満の添加ではその効果は小さく、一方、１％を超える添加は母材靱性や溶接性を低下させるため、その範囲を０．０１〜１％とする。
【００４１】
Ｍｏは、強度確保のために必要に応じて添加する。０．０１％未満の添加ではその効果が小さく、一方、１％を超える添加は母材靱性や溶接性を低下させるため、その範囲を０．０１〜１％とする。
【００４２】
また、Ｃａ，Ｍｇ，ＲＥＭの１種または２種以上の添加により、母材介在物制御、溶接熱影響部の加熱オーステナイトの微細化や粒内からの変態核生成を通じて母材靱性及び溶接熱影響部靱性を高めることができる。この効果を発揮するためには、Ｃａ及びＭｇはそれぞれ０．０００５％以上、ＲＥＭは０．００１％以上の添加が必要である。一方、過剰に添加すると硫化物や酸化物が粗大化して母材靱性や延性の低下をもたらすため、その上限値をＣａ，Ｍｇで０．０２％、ＲＥＭで０．１％とする。
【００４３】
次に、上記の鋼材を得るための製造方法について規定する。最も重要な点は、圧延前の加熱、圧延終了温度、圧延最終パス後の冷却の条件をそれぞれ規定することである。
本発明の鋼組成を有する鋼片または鋳片を加熱する条件は、凝固時に析出した析出物を十分に固溶するために１０５０℃以上に加熱の上、この温度域に２０分以上保持する必要がある。また、１３５０℃を超える温度まで加熱したのちに２０分以上保持することは、オーステナイトの粗大化による最終組織の粗大化を通じて母材靭性の低下をもたらすため、加熱温度の上限を１３５０℃とした。
なお、保持時間については、設定加熱温度に達した後設定温度±５０℃以内にある時間を指す。また、加熱温度は鋼板表面で測定した値とし、その測定精度を高めるため３箇所以上の測定の平均値とすることが望ましい。
【００４４】
実際の鋳造から圧延に至る過程では、鋳造後の鋼塊を常温まで冷却することなく直接圧延を開始する場合がある。この場合においても、鋳造後の鋼塊の温度が１０５０℃未満の場合には析出物を固溶させるため、１０５０℃以上１３５０℃未満に加熱の上２０分以上保持する必要があるが、１０５０℃以上の場合にはその時点でも固溶量が大きいためそのまま圧延を開始することが可能である。
【００４５】
圧延の仕上温度に関しては、これを９００℃超とした場合には、オーステナイト中の加工下部組織が回復や再結晶により減少し、ベイナイトやマルテンサイトを微細化する効果が低下する。一方、圧延の仕上温度をＡｒ3 点未満とした場合には、フェライトが加工され靭性が大幅に低下する。よって、圧延の仕上げ温度は９００℃以下Ａｒ3 点以上とする。
圧延の仕上げ温度とは、圧延の最終パスの直前温度、たとえば圧延機直前に取り付けられた温度計にて測定した鋼板表面の温度を指し、測定精度を高めるためには３点以上の平均値を採用することが望ましい。Ａｒ3 点は、たとえば圧延を想定した熱加工履歴を与え、種々の温度から急冷を行った組織を観察することにより、変態が開始する温度として推定が可能である。
【００４６】
圧延を仕上げた後に加速冷却を実施するための条件は本発明で最も重要な製造上の要件である。具体的には、圧延を仕上げた後２０秒以内に鋼板表面の平均冷却速度が０．５℃／ｓ以上の冷却を行う。これには２つの重要な理由がある。
一つは、圧延時に導入された加工下部組織の回復を極力抑制した状態で加速冷却を実施し、該下部組織をベイナイトの変態核やベイナイト成長に対する抵抗、さらには微細なフェライトの生成核として最大限有効活用し、ベイナイトやマルテンサイトの組織を大幅に微細化するためである。
【００４７】
もう一つが、圧延時に導入された加工下部組織にＢの窒化物、炭窒化物が析出する以前に加速冷却を開始することで充分な焼入性を確保することである。
圧延終了後２０秒超で加速冷却を開始した場合には、既に進行した回復過程により組織微細化効果が低減し、さらにＢの炭化物や炭窒化物の生成により焼入性が低下するため、これを２０秒以内とした。なお、圧延終了後加速冷却を開始するまでの時間が１０秒以内の場合には組織微細化の効果が一層顕著となるため、望ましくはこれを１０秒以内とする。
【００４８】
また、圧延後２０秒以内に実施する冷却の速度は０．５℃／ｓ以上とする。この冷却速度はベイナイトやマルテンサイト主体組織を得るために必須であり、０．５℃／ｓ未満ではフェライト主体の組織となり強度が不足する。冷却速度は速いほどよいが、設備コストなどの観点から１００℃／ｓを上限とすることが好ましい。なお、圧延を仕上げた後２０秒以内とは、圧延の最終パス圧下を受けた鋼板の一部位が０．５℃／ｓ以上の平均冷却速度で冷却が開始されるまでの時間を意味する。鋼板表面の平均冷却速度とは、例えば放射温度計により測定された鋼板表面温度の単位時間当たりの変化量を意味し、測定精度を高めるため３箇所以上を測定してその平均値を採用することが望ましい。
【００４９】
以上の手法により、合金成分量を極力抑制した上で微細なベイナイト主体の組織を製造することができ、高い強度を確保するとともに母材の靱性を大幅に高め、さらに、同一の強度を達成するために必要な合金元素量が大幅に低減されるため、溶接熱影響部靱性も向上させることができる。
【００５０】
なお、本発明ではＴｉ、Ｎｂ、Ｖは必要に応じて添加することとしているが、圧延仕上げ後２０秒以内に鋼板表面の冷却速度が０．５℃／ｓ以上で冷却を行うことにより、オーステナイト中での粗大析出物の生成が抑制され、変態時あるいは変態後に微細析出するため、従来にくらべ格段に高く強度を増大させることができる。この効果は本発明の主要な構成要件ではないものの、副次的な溶接性向上要件として必要に応じて使用することが望ましい。
【００５１】
圧延終了後２０秒以内に鋼板表面の冷却速度が０．５℃／ｓ以上で冷却を開始した後は、必要に応じて焼き戻しを行うことができる。冷却終了温度が３５０℃以上の場合や焼き戻し温度が９００℃を超える場合には強度が低下することから、圧延終了後２０秒以内に鋼板表面の冷却速度が０．５℃／ｓ以上で冷却を開始した後は、冷却終了温度を３５０℃未満とし、焼き戻し温度は９００℃以下とする。また、焼き戻しを行わない場合には、冷却終了温度を３５０℃未満とした場合には母材靱性が大幅に低下するため、冷却終了温度を３５０℃以上と規定する。
【００５２】
本発明で規定した鋼材を得るための製造方法としては、上記の条件とあわせて下記の要件を満足することが望ましい。
粗圧延については、母材靱性と生産性を低下させない条件で所定の圧下を加えることが重要である。粗圧延の開始温度が９５０℃未満となる場合には、加熱後に圧延を開始するまでの時間が長くなり生産性が大きく低下するため、一方開始温度が１１００℃超ではオーステナイトが粗大化して最終組織の粗大化を通じて母材靱性が低下するため、粗圧延の開始温度は１１００℃以下９５０℃以上とする。
粗圧延の終了温度が９００℃未満となる場合には、仕上圧延時の圧延温度が低くなり圧延能率が低下して生産性が低下するため、一方終了温度が１１００℃を超えるとオーステナイトが粗大化して最終組織の粗大化を通じて母材靱性が低下するため、粗圧延の終了温度は１１００℃以下９００℃以上とする。
【００５３】
また、粗圧延での圧下率が９０％を超える場合には、圧延温度が上記の範囲にある場合でもＢを含む粗大炭化物、粗大窒化物、粗大炭窒化物の析出量が増大するため、粗圧延での圧下率は９０％以下とする。
また、粗圧延での圧下率が２０％未満の場合にはオーステナイトが十分に微細化せず母材靭性が低下するため、粗圧延での圧下率は２０％以上とする。
【００５４】
なお、ここでの粗圧延とは、制御圧延による種々の効果を発現させるために行う高温側、低温側２段階の圧延のうち高温側での圧延を指し、低音側での圧延は仕上げ圧延と呼ばれる。低温側での圧延の開始にあたっては、所定の開始温度までの時間待ちが生じるのが一般的であり、低温側圧延の開始温度という管理指標の存在により粗圧延、仕上げ圧延を区別するものとする。しかし、全圧延過程の途中で圧延開始温度の管理指標が存在しない場合には、粗圧延を９００℃以上における圧延とする。また、粗圧延の開始温度や終了温度は圧延機の直前直後に取り付けられた温度計により測定された鋼板表面の温度とし、測定精度を高めるため３点以上の平均値を採用することが望ましい。また圧下率とは、圧延前の板厚から圧延後の板厚を引いた値を圧延前の板厚で除した値の百分率表示である。
【００５５】
仕上圧延は組織の微細化効果を充分に得るために１０００℃以下で開始し、フェライトの加工により母材靱性が低下するのを抑制するためこれをＡｒ3 点以上とする。また仕上圧延の圧下率は、充分な組織微細化効果を得るために４０％以上とすることが望ましい。なお、上記で規定した圧下率とは、圧延前の鋼板の厚さから圧延後の厚さを引き、圧延前の鋼板厚さで除した値の百分率表示である。
【００５６】
本発明では、以下に述べる製造方法を必要に応じて組み合わせることで、鋼板の特性を一層向上することが可能である。
第一に、圧延開始前の冷却の制御が有効である。具体的には、加熱後の鋼片、鋳片を、圧延開始前に鋼板表面の平均冷却速度が２℃／ｓ以上でＡｒ3 点以上１０５０℃以下の温度まで冷却し、復熱後に圧延を開始する。圧延開始前にこの冷却を実施するのは、一端圧延を開始した後は圧延最終パスまでの間の温度待ち時間を低減できるよう予め温度を低下させるためである。これにより、未再結晶温度域での加工組織の導入が回復や再結晶の影響を極力受けずに可能となり、組織の微細化に対して有効である。
【００５７】
この方法は従来圧下量の増大が困難であった厚手材で特に有効であり、圧延開始温度の低温化によりこれまで以上に未再結晶温度域の圧下率を確保することが可能であり、さらに圧延に先立つ水冷と復熱過程により鋼板内部と表面に温度差が生じ、相対的に変形抵抗の小さい板厚中心部での再結晶オーステナイトの微細化や加工下部組織の導入が促進され、母材全体に加え板厚中心部の靱性が大きく向上する。この効果は２℃／ｓ未満では小さいため、これを２℃／ｓ以上とした。冷却速度は速いほどよいが、設備コストなどの観点から１００℃／ｓを上限とすることが好ましい。
【００５８】
なお、圧延前の冷却温度をＡｒ3 点以上１０５０℃以下としたのは、１０５０℃超では未再結晶域での圧下量が減少して十分な組織微細化効果が得られないためであり、Ａｒ3 以上としたのは、Ａｒ3 未満の温度まで冷却すると鋼板表層部近傍ではオーステナイトとフェライトの二相域圧延となり、圧延による加工フェライトが鋼中に残存することで靱性低下を招くためである。
なお、ここにいう復熱とは、冷却された表面と未冷却の内部との温度勾配が時間と共に緩和される結果、表面温度が上昇する現象であり、本発明においては、鋼板中心部の温度と表面温度の差が５０℃未満になった時点、あるいは冷却後２０秒以上経過した時点を指す。
【００５９】
第二に、再結晶温度未満７００℃以上の圧下率を７０％以上とすることが有効である。これは、未再結晶域での圧延により粒内への加工組織導入が進み、特に低温変態の場合には格段の組織微細化効果が得られ、母材靭性と溶接性の一層の向上が可能となる。上限は圧延負荷の観点から１５０％とすることが望ましい。
なお、ここでの圧下率は再結晶温度未満で圧延を行う直前の板厚と最終板厚の差を再結晶温度未満で圧延を行う直前の板厚で除した値の百分率表示である。
【００６０】
第三に、圧延１パス当たりの平均圧下率を２５％以上とすることが有効である。平均圧下率を２５％以上とすることで、圧延により導入された加工下部組織の局所的集中を強め、組織微細化効果が一層顕著となり、母材靭性が向上する。しかも、強度を一定とした場合には合金元素の添加量を削減可能であるため、溶接熱影響部の靭性も向上する。平均圧下率が２５％未満では有意な効果が得られないため、これを２５％以上とした。上限は圧延負荷の観点から５０％とすることが好ましい。なお、平均圧下率とは各圧延パスの圧下率、すなわち圧延パス前の板厚と圧延パス後の板厚の差を圧延パス前の板厚で除した値の百分率表示の平均値である。
【００６１】
第四に、圧延最終パスの圧下率を２５％以上とすることが有効である。圧延最終パスの圧下率を高めることで、特に低温変態の場合に大きな析出強化量増大効果と組織微細化効果が得られる。この値が２５％未満では効果が小さいため、圧延最終パスの圧下率を２５％以上とした。なお、上限は圧延負荷の観点から９５％とすることが好ましい。
【００６２】
第五に、圧延パス間の一部あるいは全てにおいて、鋼板表面の平均冷却速度が２℃／ｓ以上の冷却を行い、復熱後に次の圧延パスを実施することが有効である。圧延パスの間に前記の冷却を実施するのは、圧延パス間での加工下部組織の回復を抑制し、かつ温度を低下させることで制御圧延効果を増大させることができる。さらに、鋼板表層部と中心部に温度差をつけることで内部の変形抵抗を相対的に小さくして、板厚中心部への加工下部組織の導入を促進し、これにより組織を微細化して板厚中心部の靱性を高める効果もある。パス間冷却を行うことにより圧延開始から終了までの時間が短縮され、生産性の向上にも大きく寄与する。パス間の冷却による上記効果は、圧延パス間の鋼板表面における平均冷却速度が２℃／ｓ未満では効果が小さいため、２℃／ｓ以上とする必要がある。冷却速度は速いほどよいが設備コストなどの観点から１００℃／ｓを上限とすることが好ましい。圧延パス間の冷却については必ずしも全圧延パス間について実施する必要はなく、要求される靱性に応じてそれを選択することができる。
【００６３】
圧延パス間の冷却開始温度及び停止温度は、この圧延パス間冷却がオーステナイトの再結晶温度域及び未再結晶温度域の両方で効果を有することから、上限を圧延開始温度とすることが好ましい。一方、Ａｒ3 点以下に冷却された場合には、引き続く圧延によりフェライトが加工され靱性が低下するため、圧延パス間冷却の停止温度の下限を鋼板表面温度でＡｒ3 点以上とすることが好ましい。
なお、ここにいう復熱とは、冷却された表面と未冷却の内部との温度勾配が時間と共に緩和される結果、表面温度が上昇する現象であり、本発明においては、鋼板中心部の温度と表面温度の差が１００℃未満になった時点、あるいは冷却後５秒以上経過した時点を指す。
【００６４】
【実施例】
種々の化学成分の供試鋼材を用いて、種々の製造条件で製造した板厚２０〜８０ｍｍの鋼板について、母材の引張強さ、靱性および溶接熱影響部靱性を評価した。鋼板の化学成分とＣｅｑ．、Ｐｃｍ、Ｂを含有する炭化物、炭窒化物、窒化物のなかで５０ｎｍ以上１００００ｎｍ以下のものの個数密度、個数密度をＢの添加量で除した値、ベイナイトとマルテンサイトの体積率、結晶方位差が１５度以上となる領域の平均寸法を表１（表１−１、表１−２）に、製造条件を表２（表２−１、表２−２）に、母材の引張強さ、靱性および溶接熱影響部靱性を表３に示す。
【００６５】
引張強さは、鋼板の１／４ｔ部（板厚中心と表面との中間）から圧延方向に垂直に採取したＪＩＳ４号サブサイズ引張試験片を用いて常温試験により測定した。母材靱性は、鋼板の１／４ｔ部から圧延方向と試験片の長手方向が垂直になるように、さらにノッチが板厚貫通方向となるように採取したＪＩＳ４号シャルピー試験片を用い、−２０℃で試験を実施し、衝撃吸収エネルギーを測定した。
なお、引張強さについては同一条件で実施した２本の試験結果の平均値を採用し、母材靱性については同一温度で３本の試験を実施し、その平均値を採用した。溶接熱影響部靱性は、鋼板に入熱１０ｋＪ／ｍｍのサブマージアーク溶接を実施し、ノッチ底部位が１／４ｔ、ボンドに対応するように採取したシャルピー試験片により−２０℃での衝撃吸収エネルギーとして測定した。試験は同一温度で３本実施し、その平均値を採用した。
【００６６】
発明例１は、０．３１という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が大きいため、強度は６００ＭＰａを超え、さらに低Ｃｅｑ．かつ組織が微細化しているため母材靱性、溶接熱影響部靱性に優れ、低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成して変態強化量が小さく低強度であり、発明例１に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。また比較例２は、仕上圧延後に加速冷却を開始するまでの時間は２０秒以内であるものの、Ｃ量が本発明の成分範囲を外れているため、母材靱性、溶接熱影響部靱性が大幅に低下しており、発明例１に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００６７】
発明例２は、０．４０という非常に低いＣｅｑ．で引張強さ７００ＭＰａ程度を達成するため、Ｂを添加し、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が大きいため、強度は７００ＭＰａを超え、さらに低Ｃｅｑ．かつ組織が微細化しているため母材靱性、溶接熱影響部靱性に優れ、低Ｐｃｍであることから溶接性にも優れている。
一方、比較例３は仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成して変態強化量が小さく低強度であり、発明例２に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。また比較例４は、仕上圧延後に加速冷却を開始するまでの時間は２０秒以内であるものの、Ｎ量が本発明の成分範囲を外れているため、母材靱性、溶接熱影響部靱性が大幅に劣化しており、発明例２に比べて母材強度と母材靱性および溶接熱影響部靱性のバランスが大きく劣っている。
【００６８】
発明例３は、０．４４という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が大きいため、強度は８００ＭＰａを超え、さらに低Ｃｅｑ．かつ組織が微細化しているため母材靱性、溶接熱影響部靱性に優れ、低Ｐｃｍであることから溶接性にも優れている。
一方、比較例５は仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成して変態強化量が小さく低強度であり、発明例３に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。また比較例６は、仕上圧延後に加速冷却を開始するまでの時間は２０秒以内であるものの、Ｂ量が成分範囲を外れているため、母材靱性、溶接熱影響部靱性が大幅に低下しており、発明例３に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００６９】
発明例４は、０．３１という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が大きいため、強度は６００ＭＰａを超え、さらに低Ｃｅｑ．、組織微細化、Ｃａ、Ｍｇ添加のため母材靱性、溶接熱影響部靱性に優れ、低Ｐｃｍであることから溶接性にも優れている。
一方、比較例７は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成して変態強化量が小さく低強度であり、発明例４に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００７０】
発明例５は、０．４４という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が大きいため、強度は８００ＭＰａを超え、さらに低Ｃｅｑ．、組織微細化、ＲＥＭ添加のため母材靱性、溶接熱影響部靱性に優れ、低Ｐｃｍであることから溶接性にも優れている。一方、比較例８は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成して変態強化量が小さく低強度であり、発明例５に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００７１】
発明例６は、０．２８という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、さらに仕上圧延後１０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は６００ＭＰａを超え、さらに低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性に優れ、低Ｐｃｍであることから溶接性にも優れている。
一方、比較例９は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成して変態強化量が小さく低強度であり、さらに加熱温度が本発明の範囲よりも高いために母材靱性が大幅に低下しており、発明例６に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００７２】
発明例７は、０．４４という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、さらに仕上圧延後１０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は８００ＭＰａを超え、さらに低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性に優れ、低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１０は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、強度が低く、さらに圧延の仕上温度が本発明の範囲よりも高いため、母材靱性が大幅に低下しており、発明例７に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００７３】
発明例８は、０．３０という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、さらに圧延前に２℃／ｓ以上の冷却を実施し、仕上圧延後２０秒以内に加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は６００ＭＰａを超え、さらに低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性に優れ、低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１１は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているために粗大析出物が多量に生成し、強度が低く、発明例８に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００７４】
発明例９は、０．４０という非常に低いＣｅｑ．で引張強さ７００ＭＰａ程度を達成するため、Ｂを添加し、さらに仕上圧延中に２℃／ｓ以上のパス間冷却を実施し、仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は７００ＭＰａを超え、さらに低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１２は、仕上圧延中に２℃／ｓ以上のパス間冷却を実施しているものの、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているために粗大析出物が多量に生成し、強度が低く、しかもＢ添加量が本発明の範囲を超えているために粗大析出物が多量に生成し、母材および溶接熱影響部の靱性が低下し、発明例９に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００７５】
発明例１０は、０．４５という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、さらに粗圧延、仕上圧延中に２℃／ｓ以上のパス間冷却を実施し、未再結晶温度域での圧下率を７０％以上とし、仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は８００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１３は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、強度が低く、発明例１０に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００７６】
発明例１１は、０．３１という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、さらに圧延最終パスの圧下率を２５％以上とし、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は６００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１４は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、強度が低く、さらに仕上圧延の終了温度がＡｒ3 点よりも低くなっているために母材靱性が大幅に低下し、発明例１１に比べて母材強度と母材靱性および溶接熱影響部接性のバランスが大きく劣っている。
【００７７】
発明例１２は、０．３９という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、さらに圧延の平均圧下率を２５％以上とし、さらに仕上圧延後１０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は８００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１５は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、強度が低く、発明例１２に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００７８】
発明例１３は、０．２８という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、圧延前に２℃／ｓ以上の冷却を実施し、未再結晶温度域での圧下率を７０％以上とし、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は６００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１６は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、さらにＣ量が本発明の範囲を超えているために母材と溶接熱影響部の靱性が大幅に低下し、発明例１３に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００７９】
発明例１４は、０．２８という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、圧延前に２℃／ｓ以上の冷却を実施し、未再結晶温度域での圧下率を７０％以上とし、圧延最終パスの圧下率を２５％以上とし、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は６００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１７は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、強度が低く、さらにＮ量が本発明の範囲を超えているために母材と溶接熱影響部の靱性が大幅に低下し、発明例１４に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８０】
発明例１５は、０．２９という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、圧延平均および最終パスの圧下率を２５％以上とし、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は６００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１８は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、強度が低く、さらにＢ量が本発明の範囲を超えているために母材と溶接熱影響部の靱性が大幅に低下し、発明例１５に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８１】
発明例１６は、０．２９という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、圧延平均の圧下率を２５％以上とし、圧延中に２℃／ｓ以上のパス間冷却を実施し、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は６００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例１９は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、強度が低く、さらに加熱温度が本発明の範囲を超えているために母材靱性が大幅に低下し、発明例１６に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８２】
発明例１７は、０．２７という非常に低いＣｅｑ．で引張強さ６００ＭＰａ程度を達成するため、Ｂを添加し、未再結晶温度域での圧下率を７０％以上とし、圧延平均および最終の圧下率を２５％以上とし、圧延中に２℃／ｓ以上のパス間冷却を実施し、さらに仕上圧延後１０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は６００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化、Ｍｇ添加のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例２０は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、強度が低く、さらに仕上圧延の終了温度が本発明の範囲より低くなっているために母材靱性が大幅に低下し、発明例１７に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８３】
発明例１８は、０．４０という非常に低いＣｅｑ．で引張強さ７００ＭＰａ程度を達成するため、Ｂを添加し、未再結晶温度域での圧下率を７０％以上とし、圧延最終の圧下率を２５％以上とし、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は７００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例２１は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、さらにＭｎ量が本発明の範囲を超えているために母材靱性が大幅に低下し、発明例１８に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８４】
発明例１９は、０．４０という非常に低いＣｅｑ．で引張強さ７００ＭＰａ程度を達成するため、Ｂを添加し、圧延前に冷却速度２℃／ｓ以上の冷却を実施し、未再結晶温度域での圧下率を７０％以上とし、圧延平均および最終の圧下率を２５％以上とし、圧延中に２℃／ｓ以上の加速冷却を実施し、さらに仕上圧延後１０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は７００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化、Ｃａ添加のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例２２は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、強度が低く、さらに加熱温度が本発明の範囲を超えているために母材靱性が大幅に低下し、発明例１９に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８５】
発明例２０は、０．４０という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、圧延前に冷却速度２℃／ｓ以上の冷却を実施し、未再結晶温度域での圧下率を７０％以上とし、圧延中に２℃／ｓ以上の加速冷却を実施し、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は８００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例２３は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、さらにＣ量が本発明の範囲を超えているために母材靱性、溶接熱影響部靱性が大幅に低下し、発明例２０に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８６】
発明例２１は、０．４０という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、圧延前に冷却速度２℃／ｓ以上の冷却を実施し、未再結晶温度域での圧下率を７０％以上とし、圧延平均の圧下率を２５％以上とし、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は８００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例２４は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、さらにＮ量が本発明の範囲を超えているために母材靱性、溶接熱影響部靱性が大幅に低下し、発明例２１に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８７】
発明例２２は、０．４０という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、圧延最終パスの圧下率を２５％以上とし、圧延中に冷却速度２℃／ｓ以上の加速冷却を実施し、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は８００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例２５は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、さらにＢ量が本発明の範囲を超えているために母材靱性、溶接熱影響部靱性が大幅に低下し、発明例２２に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８８】
発明例２３は、０．４０という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、圧延前に冷却速度２℃／ｓ以上の加速冷却を実施し、未再結晶温度域の圧下率を７０％以上とし、圧延平均および最終パスの圧下率を２５％以上とし、圧延中に冷却速度２℃／ｓ以上の加速冷却を実施し、さらに仕上圧延後２０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は８００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化、Ｃａ、ＲＥＭ添加のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例２６は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、さらに加熱温度が本発明の範囲を超えているために母材靱性が大幅に低下し、発明例２３に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００８９】
発明例２４は、０．３７という非常に低いＣｅｑ．で引張強さ８００ＭＰａ程度を達成するため、Ｂを添加し、圧延前に冷却速度２℃／ｓ以上の加速冷却を実施し、未再結晶温度域の圧下率を７０％以上とし、圧延平均および最終パスの圧下率を２５％以上とし、圧延中に冷却速度２℃／ｓ以上の加速冷却を実施し、さらに仕上圧延後１０秒以内の加速冷却を実施したものである。粗大析出物の生成量が少なく、変態強化量が極めて大きいため、強度は８００ＭＰａを超え、かつ低Ｃｅｑ．、組織微細化、Ｍｇ添加のため母材靱性、溶接熱影響部靱性が非常に優れ、さらに低Ｐｃｍであることから溶接性にも優れている。
一方、比較例２７は、仕上圧延後に加速冷却を開始するまでの時間が２０秒を超えているため、粗大析出物が多量に生成し、さらに圧延仕上温度が本発明の範囲を下回っているために母材靱性が大幅に低下し、発明例２４に比べて母材強度と母材および溶接熱影響部の靱性のバランスが大きく劣っている。
【００９０】
以上の実施例から、本発明により製造された鋼材である発明例１〜２４の鋼板は、同一の強度で比較した場合の母材靱性や溶接熱影響部靱性に極めて優れた鋼材であることは明白である。
【００９１】
【表１】

【００９２】
【表２】

【００９３】
【表３】

【００９４】
【表４】

【００９５】
【表５】

【００９６】
【発明の効果】
本発明によれば、幅広い強度範囲で母材及び溶接熱影響部の靱性に優れた引張強さ５７０ないし８００ＭＰＡ級高強度鋼板およびその製造方法を提供することが可能であり、産業上の価値の高い発明であるといえる。[0001]
BACKGROUND OF THE INVENTION
  The present invention is excellent in the toughness of the base material and the weld heat affected zone.Tensile strength 570 to 800 MPa class high strengthThe present invention relates to a method for manufacturing a steel sheet. Steel materials produced by this method can be used for general welded structures such as shipbuilding, bridges, buildings, marine structures, pressure vessels, line pipes, etc., but the toughness of the base material or weld heat affected zone of each structure is ensured. This is particularly effective when used in a portion where a steel plate having a thickness of 40 mm or more is applied.
[0002]
[Prior art]
With the recent increase in size of structures, the required level of strength and plate thickness is increasing, and there is a need for steel materials that are high in strength and thick, and have excellent base material toughness, weld heat affected zone toughness, and excellent weldability. It has been. When considering base material toughness and weld heat-affected zone toughness, it is necessary to ensure the strength while keeping the addition amount of alloying elements as small as possible, and methods for achieving this include, for example, the following Patent Documents 1 to 4 and the like. These are intended to refine toughness and improve the toughness mainly by the definition of manufacturing conditions.
[0003]
[Patent Document 1]
JP 59-200723 A
[0004]
[Patent Document 2]
JP-A-2-205627
[0005]
[Patent Document 3]
JP-A-6-93332
[0006]
[Patent Document 4]
JP-A-10-158778
[0007]
Patent Document 1 describes that a fine ferrite structure is obtained by controlling the time between rolling passes and the cooling start time after the end of rolling, with the reduction amount in the non-recrystallized region being 50% or more. . Although controlling the processing introduced in the non-recrystallized region and the subsequent recovery process is very effective for refining the structure, the structure of the steel sheet manufactured by this method is mainly composed of ferrite. It is difficult to produce a steel plate having a high tensile strength, for example, a steel plate having a tensile strength exceeding 600 MPa.
[0008]
On the other hand, Patent Documents 2, 4 and 3 each disclose a method for improving the toughness of a structure containing bainite.
The invention described in Patent Document 2 defines the use of the effect of improving hardenability by adding B (boron). However, the rolling reduction is set to 30% or less in order to suppress the hardenability deterioration, and the degree of improvement in toughness due to the refinement of the structure is small.
[0009]
In the invention described in Patent Document 4, the reduction amount in the non-recrystallized region is set to 50% or more, and a fine mixed structure of bainite and ferrite is generated by controlling the cooling rate, thereby improving toughness. However, significant refinement of the final structure through the promotion of bainite nucleation from the processed substructure introduced in the non-recrystallization temperature range, the suppression of bainite growth by the processed substructure, and the bainite fragmentation by ferrite formation has not been repeated. It is not sufficient to use only the amount of reduction in the crystallization temperature range and the cooling rate after rolling as the main control guideline, and the maximum structure refinement effect cannot be obtained.
[0010]
Further, in the invention described in Patent Document 3 from the same viewpoint, not only the reduction amount and the cooling rate in the non-recrystallization temperature range but also the time until the start of cooling after rolling is defined as “immediately”. In addition, the processing substructure at the time of the final rolling pass is accelerated and cooled in a state in which recovery is suppressed, which is considered advantageous for refining the structure. However, it is not clear how quickly "immediate" quantitative index to obtain the microstructure refinement effect, that is, how many seconds after the end of rolling should start accelerated cooling, and the reliability of the effect must be questioned. . Furthermore, since the rolling reduction in the non-recrystallization temperature region is 50% or more, this method cannot be used for thick materials that make it difficult to secure the rolling reduction.
[0011]
As mentioned above, when considering refinement of a structure containing bainite, the processed substructure that contributes to refinement of the structure through bainite nucleation site increase, bainite lath growth inhibition, bainite fragmentation by ferrite formation, etc. The amount of introduction and non-uniformity, and the recovery and recrystallization control that progress sequentially are very important, and the guidelines as described above are insufficient to obtain the maximum effect. Moreover, the large rolling reduction in the non-recrystallized region, which is the basis of these technologies, is a major obstacle to manufacturing a steel plate having a large thickness, and greatly impedes productivity.
[0012]
[Problems to be solved by the invention]
  The present invention has solved the above problems and has excellent toughness of the base material and the weld heat affected zone.Tensile strength 570 to 800 MPa class high strengthIt is an object to provide a steel plate and a manufacturing method thereof.
[0013]
[Means for Solving the Problems]
As a result of studying the refinement of the structure under various production conditions, the inventors have newly found that control of precipitates in steel and cooling control in a wide range before, during and after rolling are important. As a result, the present invention has been completed by limiting the effective range, and the gist of the present invention is as follows.
[0014]
(1) Steel is mass%,
    C: 0.005 to 0.2%, Si: 0.01 to 1%,
    Mn: 0.1 to 2%, P: 0.02% or less,
    S: 0.02% or less, N: 0.0001-0.01%,
    B: 0.0005 to 0.005%, Al: 0.001 to 0.1%
And the balance is Fe and inevitable impurities, and the number density of B-containing carbides, nitrides and carbonitrides having a circle equivalent diameter of 50 nm to 10,000 nm is 5.0. × 10⁷Piece / mm²The value obtained by dividing the number density by the amount of addition of B is 2.0 × 10^TenPieces / (mm²・ Mass%) and the sum of volume fractions of bainite and martensite is 60% or more.Bainite main organizationFurthermore, the toughness of the base material and the weld heat-affected zone is excellent, characterized in that the average dimension of the region where the crystal orientation difference with the adjacent structure is 15 degrees or more in bainite is 20 μm or less.Tensile strength 570 to 800 MPa class high strengthsteel sheet.
(2) In mass%,
    Nb: 0.001 to 0.1%, Ti: 0.001 to 0.2%,
    V: 0.001 to 0.2%
It is excellent in the toughness of the base material and the weld heat affected zone as described in (1) above, characterized by containing one or more ofTensile strength 570 to 800 MPa class high strengthsteel sheet.
(3) In mass%,
    Cu: 0.005 to 1%, Ni: 0.01 to 2%,
    Cr: 0.01-1%, Mo: 0.01-1%
It is excellent in the toughness of the base material and the weld heat affected zone according to (1) or (2), characterized by containing one or more ofTensile strength 570 to 800 MPa class high strengthsteel sheet.
(4) In mass%,
    Ca: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%,
    REM: 0.001 to 0.1%
The tensile strength of 570 to 800 MPa excellent in toughness of the base material and the weld heat-affected zone according to any one of (1) to (3), characterized by containing one or more of Grade high strength steel sheet.
[0015]
(5) In mass%,
    C: 0.005 to 0.2%, Si: 0.01 to 1%,
    Mn: 0.1 to 2%, P: 0.02% or less,
    S: 0.02% or less, N: 0.0001-0.01%,
    B: 0.0005 to 0.005%, Al: 0.001 to 0.1%
A steel slab or slab having a steel composition consisting of Fe and inevitable impurities in the balance is heated to a temperature range of 1050 ° C. or higher and 1350 ° C. or lower, and after being held in the temperature range for 20 minutes or longer, rolling is performed. Start and finish rolling at Ar3 point or more and 950 ° C or less, then cool the steel sheet with an average cooling rate of 0.5 ° C / s or more within 20 seconds, and finish cooling at 350 ° C or more and then air-cool. It is characterized byAmong the carbides, nitrides, and carbonitrides containing B, the number density of a circle equivalent diameter of 50 nm to 10,000 nm is 5.0 × 10 ⁷ Piece / mm ² The value obtained by dividing the number density by the amount of addition of B is 2.0 × 10 ^Ten Pieces / (mm ² The average dimension of the region where the sum of the volume fractions of bainite and martensite is 60% or more, and the crystal orientation difference with the adjacent structure is 15 degrees or more in bainite. 20 μm or lessExcellent toughness of base metal and weld heat affected zoneTensile strength 570 to 800 MPa class high strengthA method of manufacturing a steel sheet.
(6) In mass%,
    C: 0.005 to 0.2%, Si: 0.01 to 1%,
    Mn: 0.1 to 2%, P: 0.02% or less,
    S: 0.02% or less, N: 0.0001-0.01%,
    B: 0.0005 to 0.005%, Al: 0.001 to 0.1%
A steel slab or slab having a steel composition consisting of Fe and inevitable impurities in the balance is heated to a temperature range of 1050 ° C. or higher and 1350 ° C. or lower, and after being held in the temperature range for 20 minutes or longer, rolling is performed. After starting and rolling at an Ar3 point or more and 950 ° C or less, the steel sheet is cooled at an average cooling rate of 0.5 ° C / s or more within 20 seconds, and after cooling at less than 350 ° C, air cooling is performed. And then tempering at a temperature of 900 ° C. or lower.Among the carbides, nitrides, and carbonitrides containing B, the number density of a circle equivalent diameter of 50 nm to 10,000 nm is 5.0 × 10 ⁷ Piece / mm ² The value obtained by dividing the number density by the amount of addition of B is 2.0 × 10 ^Ten Pieces / (mm ² The average dimension of the region where the sum of the volume fractions of bainite and martensite is 60% or more, and the crystal orientation difference with the adjacent structure is 15 degrees or more in bainite. 20 μm or lessExcellent toughness of base metal and weld heat affected zoneTensile strength 570 to 800 MPa class high strengthA method of manufacturing a steel sheet.
(7) In mass%,
    Nb: 0.001 to 0.1%, Ti: 0.001 to 0.2%,
    V: 0.001 to 0.2%
It is excellent in the toughness of the base material and the weld heat affected zone according to (5) or (6), characterized by containing one or more ofTensile strength 570 to 800 MPa class high strengthA method of manufacturing a steel sheet.
(8) In mass%,
    Cu: 0.005 to 1%, Ni: 0.01 to 2%,
    Cr: 0.01-1%, Mo: 0.01-1%
The base material according to any one of the above (5) to (7) and excellent toughness of the weld heat affected zone, characterized by containing one or more ofTensile strength 570 to 800 MPa class high strengthA method of manufacturing a steel sheet.
(9) In mass%,
    Ca: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%,
    REM: 0.001 to 0.1%
The base material according to any one of the above (5) to (8) and excellent toughness of the weld heat affected zone, characterized by containing one or more ofTensile strength 570 to 800 MPa Class high strengthA method of manufacturing a steel sheet.
[0016]
(10) The steel slab or slab after heating is cooled to a temperature of 1050 ° C. or less Ar 3 point or more at an average cooling rate of the steel ingot surface of 2 ° C./s or more, and rolling is started after reheating. The manufacturing method of the tensile strength 570 thru | or 800 MPa class high strength steel plate excellent in the toughness of the base material and welding heat affected zone of any one of said (5) thru | or (9).
(11) The base material and welding heat described in any one of (5) to (10) above, wherein the rolling reduction rate of the rolling performed at a temperature lower than the recrystallization temperature of 700 ° C. or higher is 70% or higher. A method for producing a high-strength steel sheet having a tensile strength of 570 to 800 MPa that is excellent in toughness of the affected part.
(12) The average value of the rolling reduction ratios in all rolling passes is 25% or more, wherein the base material and the weld heat affected zone according to any one of (5) to (11) above Excellent toughnessTensile strength 570 to 800 MPa class high strengthA method of manufacturing a steel sheet.
(13) The rolling reduction of the final pass of the rolling is 25% or more, and is excellent in the toughness of the base material and the weld heat affected zone according to (5) to (12) aboveTensile strength 570 to 800 MPa class high strengthA method of manufacturing a steel sheet.
(14) In the part or all of the rolling passes, cooling is performed such that the average cooling rate of the steel sheet surface is 2 ° C./s or more, and the next rolling pass is performed after reheating, (5) Thru | or the toughness of the base material and welding heat affected zone of any one of (13)Tensile strength 570 to 800 MPa class high strengthA method of manufacturing a steel sheet.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
  The present invention will be described in detail.
  The inventors conducted rolling experiments incorporating various cooling processes in order to make maximum use of the cooling effect in the rolling process, and investigated the influence on the refinement of the structure. Strength level is 570 MPaClassmore thanUp to 800MPa classTargeted. As a result, it has been found that the following method is very effective for securing the toughness of the base material and the weld heat affected zone.
[0018]
  The most important point is to define the existence state of B. Specifically, the number density of carbides, nitrides, and carbonitrides containing B that are 50 nm or more and 10,000 nm or less is 5.0 × 10 5.⁷Piece / mm²The value obtained by dividing the number density by the amount of addition of B is 2.0 × 10^TenPieces / (mm²(Mass%) This is because, when coarse precipitates in the range of 50 nm to 10000 nm are in the above range, a large amount of solid solution B that enhances hardenability remains in the transformation temperature range, and the increase in strength due to transformation strengthening appears remarkably. This is because of the new knowledge.
  The number density of carbides, nitrides, and carbonitrides containing B is 5.0 × 10⁷Piece / mm² More thanIn this case, particularly, the amount of increase in strength is decreased, and the base material toughness is also decreased. Therefore, the number density of carbide, nitride, and carbonitride containing B is set to 5.0 ×.10 ⁷ Pieces/ Mm²Limited to less than.
[0019]
  In addition, since the absolute number of carbides, nitrides, and carbonitrides containing B increases as the addition amount of B increases, it is necessary to define an upper limit value of the number density for the addition amount. The value obtained by dividing the number density of carbides, nitrides, and carbonitrides containing B by 50 nm to 10,000 nm by the amount of addition of B is 2.0 × 10^TenPieces / (mm²·mass%)More thanIf the number density is 5.0 × 10⁷Piece / mm²Since the effect of increasing the strength is reduced even if it is less than 1, the value obtained by dividing the number density of B-containing carbides, nitrides, carbonitrides from 50 nm to 10000 nm by the addition amount of B is 2.0 ×. 10^TenPieces / (mm²(Mass%)
[0020]
In addition, the carbide | carbonized_material, nitride, and carbonitride containing B here are BN etc., for example. In addition, the above-mentioned carbide, nitride, carbonitride may be other oxides, sulfides, carbides, nitrides, carbonitrides such as Al.₂O_Three, MnS, AlN and the like are also included in the case of precipitation.
[0021]
In the present invention, B-containing carbides, nitrides, and carbonitrides have dimensions of 50 nm to 10000 nm, and methods for measuring the dimensions of these carbides, nitrides, and carbonitrides are as follows. It is.
The steel material is corroded by a selective corrosion technique as shown in Non-Patent Document 1 below, and then an electron microscope sample is prepared by an extraction replica method as shown in Non-Patent Document 2 below, and the magnification is 1 with a transmission electron microscope. Observe a 10,000 times bright field image, and use the equivalent circle diameter calculated from the area of each particle as the dimension.
[0022]
[Non-Patent Document 1]
"Anti-corrosion technology" No. 37, December 1988, issued by the Corrosion and Anti-corrosion Association, pages 776-778,
[0023]
[Non-Patent Document 2]
“Electron Microscope Q & A”, December 15, 1996, published by Agne Jofusha, p. 174
[0024]
The number density of carbides, nitrides, and carbonitrides containing B is the same as that of a bright-field photograph taken with a transmission electron microscope at a magnification of 10,000 times with a total area of 1000 μm.²Photographed as many as the above, and measured the number of carbides, nitrides, and carbonitrides containing B existing in this region whose dimensions were 50 nm or more and 10,000 nm or less, and the area of the region in which this was photographed The value divided by. Furthermore, the addition amount at the time of calculating the value obtained by dividing the number density by the addition amount is a value representing the total B amount obtained by the component analysis of the steel material in mass%.
[0025]
  Moreover, the following points are important in order to achieve both the high strength of 570 MPa or more and the high toughness of the base material and the weld heat affected zone, which are the objects of the present invention. In order to improve the toughness of the base material, it is important to refine the structure unit having a certain orientation difference. In particularIn bainiteConsidering a region where the crystal orientation difference with the adjacent structure is 15 degrees or more, it was found that the toughness becomes very high when the average dimension of this region is 20 μm or less. From this,In bainiteThe average dimension of the region where the orientation difference with the adjacent tissue is 15 degrees or more is defined as 20 μm or less.
[0026]
  On the other hand, in order to increase the toughness of the weld heat affected zone, it is desirable to reduce the addition amount of the alloy element as much as possible. In order to reduce the addition amount of the alloy element with a constant strength, it is effective to secure a volume ratio of bainite and martensite of a certain amount or more, especially when the sum of the volume ratio of bainite and martensite is 60% or more. In some cases, the effect of reducing the addition amount of the alloying element becomes large, so the sum of the volume fractions of bainite and martensite in the present invention is defined as 60% or more.
  In addition,In bainiteAbout the average dimension of the area | region where an azimuth | direction difference with an adjacent structure | tissue is 15 degree | times or more, it is 1.0x10 by EBSP.^Fiveμm²The average diameter corresponding to the circle is calculated from the average area obtained as a result of analyzing the region.
[0027]
The reason why the addition amount of the alloy component is specified will be described below.
Since C is an element essential for ensuring the strength, its addition amount is set to 0.005% or more. However, on the other hand, an increase in the amount of C causes a decrease in base metal toughness and weldability due to the formation of coarse precipitates, so the upper limit is made 0.2%.
[0028]
N decreases the hardenability through the formation of nitride of B, so it is desirable to reduce it as much as possible, and the upper limit is made 0.01%. On the other hand, the lower limit is set to 0.0001% in consideration of cost.
[0029]
B is an element effective for increasing hardenability, and its addition amount is set to 0.0005% or more. However, on the other hand, an increase in the amount of B causes a decrease in the base material toughness due to the formation of coarse precipitates, so the upper limit is made 0.005%.
[0030]
  Si, Mn is added as necessary from the viewpoint of securing strength.
  Since Si is an element necessary for securing strength and deoxidation, its addition amount is set to 0.01% or more. However, on the other hand, an increase in the amount of Si decreases weldability, so the upper limit is made 1%.
[0031]
Mn is useful as a strengthening element, but excessive addition reduces weldability, so the range is made 0.1% to 2%.
[0032]
In addition to being added as a deoxidizing material, Al is an element that has an effect of increasing the amount of dissolved B due to the formation of nitrides, and the amount added is 0.001% or more. However, on the other hand, an increase in the amount of Al causes a decrease in the base material toughness, so the upper limit is made 0.1%.
[0033]
P is an impurity element and is preferably as low as 0.02% or less. In particular, P dissolved in ferrite reduces the ductility of the base material, so it is preferably made 0.015% or less.
[0034]
S is an impurity element and is preferably low, and is 0.02% or less. S is desirably 0.01% or less because S lowers the toughness of the base material due to the generation of MnS.
[0035]
  Ti, Nb, and V are added as necessary to reduce the amount of dissolved N and strengthen the precipitation.
  Ti is hardenability of B by reducing the amount of dissolved N due to the formation of nitrides.ImproveAnd is an element effective for securing the strength due to fine precipitates, so the addition amount is made 0.001% or more. However, on the other hand, an increase in the amount of Ti causes a decrease in base metal toughness and weldability due to the formation of coarse precipitates, so the upper limit is made 0.2%.
[0036]
  Nb is a hardenability of B by reducing the amount of dissolved N due to the formation of nitrides.ImproveAnd is an element effective for securing the strength due to fine precipitates, so the addition amount is made 0.001% or more. However, on the other hand, an increase in the amount of Nb causes a decrease in base metal toughness and weldability due to the formation of coarse precipitates, so the upper limit is made 0.1%.
[0037]
  V is the hardenability of B by reducing the amount of dissolved N due to the formation of nitrides.ImproveAnd is an element effective for securing the strength due to fine precipitates, so the addition amount is made 0.001% or more. However, on the other hand, an increase in the amount of V causes a decrease in the base material toughness due to the formation of coarse precipitates, so the upper limit is made 0.2%.
[0038]
Cu, Ni, Cr, and Mo are added as necessary from the viewpoint of securing strength.
Cu is added as necessary to ensure strength. If the addition is less than 0.005%, the effect is small. On the other hand, the addition exceeding 1% lowers the weldability, so the range is made 0.01 to 1%.
[0039]
Ni is added as necessary to ensure strength. If the addition is less than 0.01%, the effect is small. On the other hand, the addition exceeding 2% lowers the weldability, so the range is made 0.01 to 2%.
[0040]
Cr is added as necessary to ensure strength. If the addition is less than 0.01%, the effect is small. On the other hand, the addition exceeding 1% lowers the base metal toughness and weldability, so the range is made 0.01 to 1%.
[0041]
Mo is added as needed to ensure strength. If the addition is less than 0.01%, the effect is small. On the other hand, the addition exceeding 1% lowers the base metal toughness and weldability, so the range is made 0.01 to 1%.
[0042]
In addition, by adding one or more of Ca, Mg, and REM, base metal toughness and welding heat influence through control of base material inclusions, refinement of heated austenite in weld heat affected zone and generation of transformation nuclei from within grains. The toughness of the part can be increased. In order to exhibit this effect, it is necessary to add 0.0005% or more of Ca and Mg and 0.001% or more of REM, respectively. On the other hand, if it is added excessively, sulfides and oxides are coarsened and the base material toughness and ductility are reduced. Therefore, the upper limit is set to 0.02% for Ca and Mg and 0.1% for REM.
[0043]
Next, a manufacturing method for obtaining the above steel material will be defined. The most important point is to define heating conditions before rolling, rolling finishing temperature, and cooling conditions after the final rolling pass, respectively.
The condition for heating the steel slab or slab having the steel composition of the present invention is to heat at 1050 ° C. or higher and keep it in this temperature range for 20 minutes or more in order to sufficiently dissolve the precipitate deposited at the time of solidification. There is. Further, holding for 20 minutes or more after heating to a temperature exceeding 1350 ° C. brings about a decrease in the base material toughness through the coarsening of the final structure due to the austenite coarsening, so the upper limit of the heating temperature was set to 1350 ° C.
In addition, about holding time, after reaching preset heating temperature, the time which exists in preset temperature +/- 50 degreeC is pointed out. Further, the heating temperature is a value measured on the surface of the steel sheet, and it is desirable to set the average value of three or more measurements in order to increase the measurement accuracy.
[0044]
In the process from actual casting to rolling, rolling may be started directly without cooling the ingot after casting to room temperature. Even in this case, when the temperature of the steel ingot after casting is less than 1050 ° C., the precipitate is dissolved, and it is necessary to maintain the temperature at 1050 ° C. or more and less than 1350 ° C. for 20 minutes or more. In the above case, since the amount of the solid solution is large even at that time, the rolling can be started as it is.
[0045]
Regarding the finishing temperature of rolling, when this is over 900 ° C., the processed substructure in austenite is reduced by recovery and recrystallization, and the effect of refining bainite and martensite is reduced. On the other hand, when the finishing temperature of rolling is less than the Ar3 point, ferrite is processed and the toughness is greatly reduced. Therefore, the rolling finishing temperature is 900 ° C. or lower and Ar 3 point or higher.
The finishing temperature of rolling refers to the temperature immediately before the final pass of rolling, for example, the temperature of the steel sheet surface measured with a thermometer attached immediately before the rolling mill, and an average value of three or more points is required to increase measurement accuracy. It is desirable to adopt. The Ar3 point can be estimated as the temperature at which transformation starts, for example, by giving a thermal processing history assuming rolling and observing the structure that has been quenched from various temperatures.
[0046]
The conditions for implementing accelerated cooling after finishing the rolling are the most important manufacturing requirements in the present invention. Specifically, cooling is performed at an average cooling rate of the steel sheet surface of 0.5 ° C./s or more within 20 seconds after finishing the rolling. There are two important reasons for this.
One is that accelerated cooling is performed in a state where the recovery of the processed substructure introduced during rolling is suppressed as much as possible, and the substructure is the maximum resistance to bainite transformation nuclei, bainite growth, and fine ferrite nuclei. This is to make the most effective use and to make the structure of bainite and martensite significantly finer.
[0047]
The other is to ensure sufficient hardenability by starting accelerated cooling before B nitride and carbonitride are precipitated in the working substructure introduced during rolling.
When accelerated cooling is started over 20 seconds after the end of rolling, the effect of refining the structure is reduced by the recovery process that has already progressed, and the hardenability is reduced by the formation of B carbide and carbonitride. Was within 20 seconds. In addition, since the effect of refinement | miniaturization becomes more remarkable when the time from the completion of rolling to the start of accelerated cooling is within 10 seconds, this is preferably within 10 seconds.
[0048]
  The cooling rate performed within 20 seconds after rolling is 0.5 ° C./s or more. This cooling rate is indispensable for obtaining a bainite or martensite main structure, and if it is less than 0.5 ° C./s, it becomes a ferrite-based structure and the strength is insufficient. The higher the cooling rate, the better. However, it is preferable that the upper limit is 100 ° C./s from the viewpoint of equipment cost. In addition, within 20 seconds after finishing the rolling, one part of the steel plate subjected to the final pass pressure of the rolling is cooled at an average cooling rate of 0.5 ° C./s or more.StartsIt means time to be done. The average cooling rate of the steel sheet surface means the amount of change per unit time of the steel sheet surface temperature measured by, for example, a radiation thermometer, and measures three points or more to increase the measurement accuracy and adopt the average value. Is desirable.
[0049]
By the above method, it is possible to produce a fine bainite-based structure while suppressing the amount of alloy components as much as possible, ensuring high strength, greatly increasing the toughness of the base material, and achieving the same strength Therefore, the amount of alloying elements necessary for this is greatly reduced, so that the weld heat affected zone toughness can also be improved.
[0050]
In the present invention, Ti, Nb, and V are added as necessary, but austenite is obtained by cooling the steel sheet surface at a cooling rate of 0.5 ° C./s or more within 20 seconds after rolling finish. The formation of coarse precipitates therein is suppressed, and fine precipitates are produced during or after transformation, so that the strength can be increased significantly compared to the prior art. Although this effect is not a main constituent requirement of the present invention, it is desirable to use it as a secondary weldability improvement requirement if necessary.
[0051]
  After starting the cooling at a cooling rate of 0.5 ° C./s or more within 20 seconds after the end of rolling, tempering can be performed as necessary. Cooling end temperature is 350 ° CMore thanIn this case or when the tempering temperature exceeds 900 ° C., the strength decreases, so after starting the cooling at a cooling rate of 0.5 ° C./s or more within 20 seconds after the end of rolling, the cooling is finished. The temperature is less than 350 ° C., and the tempering temperature is 900 ° C. or less. In the case where tempering is not performed, the base metal toughness is significantly reduced when the cooling end temperature is less than 350 ° C., so the cooling end temperature is defined as 350 ° C. or higher.
[0052]
  As a production method for obtaining the steel material defined in the present invention, it is desirable to satisfy the following requirements in addition to the above conditions.
  For rough rolling, it is important to apply a predetermined reduction under conditions that do not reduce the base material toughness and productivity. When the starting temperature of the rough rolling is less than 950 ° C., the time until the rolling starts after heating becomes longer and the productivity is greatly reduced. On the other hand, the starting temperature is 1100 ° C.SuperSince the austenite coarsens and the base material toughness decreases through coarsening of the final structure, the rough rolling start temperature is set to 1100 ° C. or lower and 950 ° C. or higher.
  When the finish temperature of the rough rolling is less than 900 ° C., the rolling temperature at the finish rolling is lowered and the rolling efficiency is lowered to lower the productivity. On the other hand, when the finish temperature exceeds 1100 ° C., the austenite becomes coarse. Since the base material toughness decreases through the coarsening of the final structure, the end temperature of the rough rolling is set to 1100 ° C. or lower and 900 ° C. or higher.
[0053]
If the rolling reduction in rough rolling exceeds 90%, the amount of precipitation of coarse carbide, coarse nitride, coarse carbonitride containing B increases even when the rolling temperature is in the above range. The rolling reduction in rolling is 90% or less.
Further, when the rolling reduction in rough rolling is less than 20%, austenite is not sufficiently refined and the base metal toughness is lowered, so the rolling reduction in rough rolling is set to 20% or more.
[0054]
In addition, rough rolling here refers to rolling on the high temperature side among high-temperature side and low-temperature side two-stage rolling performed in order to express various effects by controlled rolling, and rolling on the low-frequency side is finish rolling. be called. At the start of rolling on the low temperature side, it is common to wait for a time until a predetermined starting temperature, and rough rolling and finish rolling are distinguished by the presence of a management index called the starting temperature of the low temperature side rolling. . However, if there is no management index for the rolling start temperature during the entire rolling process, rough rolling is performed at 900 ° C. or higher. Further, it is desirable that the rough rolling start temperature and end temperature be the temperature of the steel sheet surface measured by a thermometer attached immediately before and after the rolling mill, and an average value of three or more points be adopted in order to improve measurement accuracy. The rolling reduction is a percentage display of a value obtained by dividing a value obtained by subtracting a thickness after rolling from a thickness before rolling by a thickness before rolling.
[0055]
Finish rolling starts at 1000 ° C. or less in order to sufficiently obtain the effect of refining the structure, and this is set to the Ar3 point or more in order to prevent the base metal toughness from being lowered due to the processing of ferrite. Further, the rolling reduction of finish rolling is desirably 40% or more in order to obtain a sufficient structure refinement effect. In addition, the rolling reduction prescribed | regulated above is a percentage display of the value which pulled the thickness after rolling from the thickness of the steel plate before rolling, and remove | divided by the steel plate thickness before rolling.
[0056]
In the present invention, the characteristics of the steel sheet can be further improved by combining the production methods described below as necessary.
First, control of cooling before the start of rolling is effective. Specifically, the steel slab and slab after heating are cooled to a temperature at which the average cooling rate on the steel sheet surface is 2 ° C./s or higher and Ar 3 point or higher and 1050 ° C. or lower before rolling, and rolling is started after reheating. To do. The reason why this cooling is performed before the start of rolling is to reduce the temperature in advance so that the temperature waiting time until the final rolling pass can be reduced after rolling is started. As a result, the introduction of the processed structure in the non-recrystallization temperature range becomes possible without being affected by recovery and recrystallization as much as possible, which is effective for refinement of the structure.
[0057]
This method is particularly effective for thick materials where it has been difficult to increase the amount of rolling reduction conventionally, and it is possible to secure a rolling reduction rate in an unrecrystallized temperature region more than before by lowering the rolling start temperature. Due to the water cooling and recuperation process prior to rolling, a temperature difference occurs between the inside and the surface of the steel sheet. In addition to the whole, the toughness at the center of the plate thickness is greatly improved. Since this effect is small at less than 2 ° C./s, this is set to 2 ° C./s or more. The higher the cooling rate, the better. However, it is preferable that the upper limit is 100 ° C./s from the viewpoint of equipment cost.
[0058]
The reason why the cooling temperature before rolling was set to the Ar3 point or higher and 1050 ° C or lower is that if it exceeds 1050 ° C, the amount of reduction in the non-recrystallized region is reduced and a sufficient structure refinement effect cannot be obtained. The reason for this is that when cooled to a temperature lower than Ar3, two-phase rolling of austenite and ferrite occurs in the vicinity of the surface layer portion of the steel sheet, and the work ferrite due to rolling remains in the steel, leading to a decrease in toughness.
The recuperation here is a phenomenon in which the surface temperature rises as a result of the temperature gradient between the cooled surface and the uncooled interior being relaxed over time. In the present invention, the temperature at the center of the steel sheet And when the difference between the surface temperature is less than 50 ° C. or when 20 seconds or more have passed after cooling.
[0059]
  Second, the recrystallization temperatureLess thanIt is effective to set the rolling reduction at 700 ° C. or more to 70% or more. This is because the processing structure is introduced into the grains by rolling in the non-recrystallized region, and in particular in the case of low-temperature transformation, a remarkable microstructure refining effect can be obtained, and the base metal toughness and weldability can be further improved. It becomes. The upper limit is desirably 150% from the viewpoint of rolling load.
  The rolling reduction here is the recrystallization temperature.Less thanThe recrystallization temperature is the difference between the plate thickness just before rolling and the final plate thickness.Less thanIs a percentage display of the value divided by the plate thickness just before rolling.
[0060]
Third, it is effective to set the average rolling reduction per rolling pass to 25% or more. By setting the average reduction ratio to 25% or more, the local concentration of the processed substructure introduced by rolling is strengthened, the effect of refining the structure becomes more remarkable, and the base material toughness is improved. In addition, when the strength is constant, the additive amount of the alloy element can be reduced, so that the toughness of the weld heat affected zone is also improved. If the average rolling reduction is less than 25%, a significant effect cannot be obtained, so this was set to 25% or more. The upper limit is preferably 50% from the viewpoint of rolling load. The average rolling reduction is the rolling reduction of each rolling pass, that is, an average value expressed as a percentage of a value obtained by dividing the difference between the plate thickness before the rolling pass and the plate thickness after the rolling pass by the plate thickness before the rolling pass.
[0061]
Fourth, it is effective to set the rolling reduction of the final rolling pass to 25% or more. By increasing the rolling reduction of the final rolling pass, a large precipitation strengthening effect and a fine structure effect can be obtained particularly in the case of low temperature transformation. Since the effect is small if this value is less than 25%, the rolling reduction of the final rolling pass is set to 25% or more. The upper limit is preferably 95% from the viewpoint of rolling load.
[0062]
Fifth, it is effective to perform cooling at an average cooling rate of the steel sheet surface of 2 ° C./s or more in part or all between rolling passes, and to carry out the next rolling pass after reheating. Implementing the cooling during the rolling pass can suppress the recovery of the working substructure between the rolling passes and increase the controlled rolling effect by lowering the temperature. Furthermore, the internal deformation resistance is made relatively small by making a temperature difference between the steel sheet surface layer and the central part, and the introduction of the processed substructure to the central part of the plate thickness is promoted, thereby making the structure finer and There is also an effect of increasing the toughness of the thickness center portion. By performing the inter-pass cooling, the time from the start to the end of rolling is shortened, which greatly contributes to the improvement of productivity. The above effect due to the cooling between passes needs to be 2 ° C./s or higher because the effect is small when the average cooling rate on the steel sheet surface between rolling passes is less than 2 ° C./s. The higher the cooling rate, the better, but it is preferable to set the upper limit at 100 ° C./s from the viewpoint of equipment costs. The cooling between rolling passes does not necessarily need to be performed between all rolling passes, and can be selected according to the required toughness.
[0063]
The upper limit of the cooling start temperature and the stop temperature between rolling passes is preferably set to the rolling start temperature because the cooling between the rolling passes has an effect in both the austenite recrystallization temperature region and the non-recrystallization temperature region. On the other hand, when cooled to the Ar3 point or lower, ferrite is processed by subsequent rolling and the toughness is lowered. Therefore, the lower limit of the cooling temperature between rolling passes is preferably set to the Ar3 point or higher at the steel sheet surface temperature.
The recuperation here is a phenomenon in which the surface temperature rises as a result of the temperature gradient between the cooled surface and the uncooled interior being relaxed over time. In the present invention, the temperature at the center of the steel sheet When the difference between the surface temperature and the surface temperature is less than 100 ° C., or when 5 seconds or more have elapsed after cooling.
[0064]
【Example】
The tensile strength, toughness, and weld heat affected zone toughness of the base material were evaluated for steel plates having a thickness of 20 to 80 mm manufactured under various manufacturing conditions using test steel materials having various chemical components. Chemical composition of steel sheet and Ceq. , Pcm, B-containing carbides, carbonitrides, nitrides having a number density of 50 nm or more and 10000 nm or less, a value obtained by dividing the number density by the amount of addition of B, a volume ratio of bainite and martensite, a difference in crystal orientation Table 1 (Table 1-1, Table 1-2) shows the average dimensions of the regions where the angle is 15 degrees or more, Table 2 (Table 2-1, Table 2-2) shows the manufacturing conditions, and the tensile strength of the base material. Table 3 shows the toughness and weld heat-affected zone toughness.
[0065]
Tensile strength was measured by a normal temperature test using a JIS No. 4 subsize tensile specimen taken perpendicularly to the rolling direction from a 1/4 t portion (intermediate between the center of the plate thickness and the surface) of the steel plate. The base material toughness was measured using a JIS No. 4 Charpy test piece sampled so that the rolling direction and the longitudinal direction of the test piece were perpendicular to each other from the 1/4 t part of the steel plate, and the notch was in the plate thickness penetration direction. The test was conducted at 0 ° C. and the impact absorption energy was measured.
In addition, about the tensile strength, the average value of the two test results implemented on the same conditions was employ | adopted, and about the base material toughness, three tests were implemented at the same temperature, and the average value was employ | adopted. The weld heat-affected zone toughness was determined by impact absorption energy at -20 ° C by Charpy test specimens that were subjected to submerged arc welding with a heat input of 10 kJ / mm on the steel sheet and the notch bottom part was 1/4 t, corresponding to the bond. As measured. Three tests were performed at the same temperature, and the average value was adopted.
[0066]
Invention Example 1 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is large, the strength exceeds 600 MPa, and further, low Ceq. Moreover, since the structure is refined, the base material toughness and the weld heat affected zone toughness are excellent, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 1, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, the transformation strengthening amount is small, and the strength is low. In comparison, the balance between the strength of the base metal and the toughness of the base metal and the weld heat affected zone is greatly inferior. Further, in Comparative Example 2, although the time until the start of accelerated cooling after finish rolling is within 20 seconds, the C amount is outside the component range of the present invention, so that the base material toughness and the weld heat affected zone toughness are greatly increased. Compared with Invention Example 1, the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior.
[0067]
Invention Example 2 has a very low Ceq. In order to achieve a tensile strength of about 700 MPa, B is added, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is large, the strength exceeds 700 MPa, and further, low Ceq. Moreover, since the structure is refined, the base material toughness and the weld heat affected zone toughness are excellent, and the weldability is also excellent because of the low Pcm.
On the other hand, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds in Comparative Example 3, coarse precipitates are generated in large quantities, the transformation strengthening amount is small, and the strength is low. Therefore, the balance between the base metal strength and the toughness of the base metal and the weld heat affected zone is greatly inferior. Further, in Comparative Example 4, although the time until the start of accelerated cooling after finish rolling is within 20 seconds, the N amount is outside the component range of the present invention, so that the base material toughness and the weld heat affected zone toughness are greatly increased. Compared with Invention Example 2, the balance between the base material strength, the base material toughness, and the weld heat affected zone toughness is greatly inferior.
[0068]
Invention Example 3 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is large, the strength exceeds 800 MPa, and further, low Ceq. Moreover, since the structure is refined, the base material toughness and the weld heat affected zone toughness are excellent, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 5, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, the transformation strengthening amount is small, and the strength is low. Therefore, the balance between the base metal strength and the toughness of the base metal and the weld heat affected zone is greatly inferior. Further, in Comparative Example 6, although the time until the start of accelerated cooling after finish rolling is within 20 seconds, the B amount is out of the component range, so that the base material toughness and the weld heat affected zone toughness are greatly reduced. Compared with Invention Example 3, the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior.
[0069]
Invention Example 4 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is large, the strength exceeds 600 MPa, and further, low Ceq. Further, because of the refinement of the structure and the addition of Ca and Mg, the base material toughness and the weld heat affected zone toughness are excellent, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 7, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, the transformation strengthening amount is small, and the strength is low. In comparison, the balance between the strength of the base metal and the toughness of the base metal and the weld heat affected zone is greatly inferior.
[0070]
Invention Example 5 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is large, the strength exceeds 800 MPa, and further, low Ceq. Further, because of the refinement of the structure and the addition of REM, the base material toughness and the weld heat affected zone toughness are excellent, and the weldability is also excellent because of the low Pcm. On the other hand, in Comparative Example 8, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, the transformation strengthening amount is small, and the strength is low. In comparison, the balance between the strength of the base metal and the toughness of the base metal and the weld heat affected zone is greatly inferior.
[0071]
Invention Example 6 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added and accelerated cooling is performed within 10 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 600 MPa, and further, low Ceq. In addition, because of the refinement of the structure, the base material toughness and the weld heat affected zone toughness are excellent, and because of the low Pcm, the weldability is also excellent.
On the other hand, in Comparative Example 9, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, the transformation strengthening amount is small, the strength is low, and the heating temperature is further increased. Since it is higher than the range of the present invention, the toughness of the base material is greatly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 6.
[0072]
Invention Example 7 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added and accelerated cooling is performed within 10 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 800 MPa, and further, a low Ceq. In addition, because of the refinement of the structure, the base material toughness and the weld heat affected zone toughness are excellent, and because of the low Pcm, the weldability is also excellent.
On the other hand, in Comparative Example 10, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, the strength is low, and the finishing temperature of rolling is within the range of the present invention. Therefore, the toughness of the base material is greatly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 7.
[0073]
Invention Example 8 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added, cooling at 2 ° C./s or more is performed before rolling, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 600 MPa, and further, low Ceq. In addition, because of the refinement of the structure, the base material toughness and the weld heat affected zone toughness are excellent, and because of the low Pcm, the weldability is also excellent.
On the other hand, in Comparative Example 11, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated and the strength is low. The balance of the toughness of the base metal and the weld heat affected zone is greatly inferior.
[0074]
Invention Example 9 has a very low Ceq. In order to achieve a tensile strength of about 700 MPa, B is added, cooling between passes of 2 ° C./s or more is performed during finish rolling, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 700 MPa, and further, low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 12, although the inter-pass cooling of 2 ° C./s or more is performed during finish rolling, the time until start of accelerated cooling after finish rolling exceeds 20 seconds. Is produced in a large amount, the strength is low, and the addition amount of B exceeds the range of the present invention, so that a large amount of coarse precipitates are produced, and the toughness of the base material and the weld heat affected zone is lowered. Compared with, the balance between the base metal strength and the toughness of the base metal and the weld heat affected zone is greatly inferior.
[0075]
Invention Example 10 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added, and further, cooling between passes of 2 ° C./s or more is performed during rough rolling and finish rolling, and the reduction rate in the non-recrystallization temperature range is 70% or more. And accelerated cooling within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 800 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 13, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated and the strength is low. The balance of the toughness of the base metal and the weld heat affected zone is greatly inferior.
[0076]
Invention Example 11 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added, the rolling reduction of the final rolling pass is set to 25% or more, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 600 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 14, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are formed, the strength is low, and the finish rolling finish temperature is higher than the Ar3 point. Therefore, the toughness of the base metal is greatly reduced, and the balance between the base material strength, the base material toughness, and the weld heat affected zone contact property is greatly inferior to that of Invention Example 11.
[0077]
Invention Example 12 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added, the rolling average rolling reduction is 25% or more, and accelerated cooling is performed within 10 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 800 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 15, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated and the strength is low. The balance of the toughness of the base metal and the weld heat affected zone is greatly inferior.
[0078]
Invention Example 13 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added, cooling at 2 ° C./s or more is performed before rolling, the reduction rate in the non-recrystallization temperature region is set to 70% or more, and after finish rolling, 20%. Accelerated cooling is performed within seconds. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 600 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 16, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, and the amount of C exceeds the range of the present invention. The toughness of the base material and the weld heat affected zone is greatly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 13.
[0079]
Inventive Example 14 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added, cooling at 2 ° C./s or higher is performed before rolling, the rolling reduction in the non-recrystallization temperature region is set to 70% or higher, and rolling is reduced in the final pass of rolling. The rate is 25% or more, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 600 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 17, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, the strength is low, and the N amount exceeds the range of the present invention. For this reason, the toughness of the base material and the weld heat affected zone is significantly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 14.
[0080]
Invention Example 15 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added, the rolling average and the rolling reduction of the final pass are set to 25% or more, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 600 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 18, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, the strength is low, and the B amount exceeds the range of the present invention. Therefore, the toughness of the base material and the weld heat affected zone is greatly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 15.
[0081]
Invention Example 16 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added, the rolling average rolling reduction is 25% or more, cooling between passes of 2 ° C./s or more is performed during rolling, and within 20 seconds after finish rolling. Accelerated cooling was performed. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 600 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 19, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are formed, the strength is low, and the heating temperature exceeds the range of the present invention. Therefore, the base material toughness is greatly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 16.
[0082]
Invention Example 17 has a very low Ceq. In order to achieve a tensile strength of about 600 MPa, B is added, the rolling reduction in the non-recrystallization temperature range is 70% or more, the rolling average and the final rolling reduction are 25% or more, and 2 ° C. / The cooling between passes more than s was implemented, and also the accelerated cooling within 10 second after finish rolling was implemented. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 600 MPa, and the low Ceq. Further, due to the refinement of the structure and the addition of Mg, the base material toughness and the weld heat affected zone toughness are excellent, and further, the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 20, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are formed, the strength is low, and the finish rolling finish temperature is the present invention. Since it is lower than the range, the toughness of the base material is greatly lowered, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 17.
[0083]
Invention Example 18 has a very low Ceq. In order to achieve a tensile strength of about 700 MPa, B is added, the rolling reduction in the non-recrystallization temperature region is 70% or more, the rolling reduction at the final rolling is 25% or more, and within 20 seconds after finish rolling. Accelerated cooling is performed. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 700 MPa and low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 21, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, and the amount of Mn exceeds the range of the present invention. The base material toughness is significantly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 18.
[0084]
Inventive Example 19 has a very low Ceq. In order to achieve a tensile strength of about 700 MPa, B is added, cooling is performed at a cooling rate of 2 ° C./s or more before rolling, the rolling reduction in the non-recrystallization temperature region is set to 70% or more, the rolling average and The final rolling reduction is 25% or more, accelerated cooling of 2 ° C./s or more is performed during rolling, and accelerated cooling is performed within 10 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 700 MPa and low Ceq. Further, due to the refinement of the structure and the addition of Ca, the base material toughness and the weld heat affected zone toughness are excellent, and further, the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 22, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, the strength is low, and the heating temperature exceeds the range of the present invention. Therefore, the base material toughness is greatly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 19.
[0085]
Inventive Example 20 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added, cooling is performed at a cooling rate of 2 ° C./s or more before rolling, the rolling reduction in the non-recrystallization temperature region is set to 70% or more, and rolling is performed. Accelerated cooling of 2 ° C./s or more is performed, and further accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 800 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 23, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, and the amount of C exceeds the range of the present invention. The base material toughness and the weld heat affected zone toughness are greatly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 20.
[0086]
Invention Example 21 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added, cooling is performed at a cooling rate of 2 ° C./s or more before rolling, the rolling reduction in the non-recrystallization temperature region is set to 70% or more, and the rolling average The rolling reduction is 25% or more, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 800 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 24, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, and the N amount exceeds the range of the present invention. The base material toughness and the weld heat affected zone toughness are significantly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 21.
[0087]
Inventive Example 22 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added, the rolling reduction of the final rolling pass is set to 25% or more, accelerated cooling at a cooling rate of 2 ° C./s or more is performed during rolling, and after finishing rolling, 20 Accelerated cooling is performed within seconds. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 800 MPa, and the low Ceq. The base material toughness and the weld heat-affected zone toughness are extremely excellent due to the refinement of the structure, and the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 25, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, and the amount of B exceeds the range of the present invention. The base material toughness and the weld heat affected zone toughness are greatly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 22.
[0088]
Invention Example 23 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added, accelerated cooling at a cooling rate of 2 ° C./s or higher is performed before rolling, the rolling reduction in the non-recrystallization temperature region is set to 70% or higher, the rolling average and The rolling reduction of the final pass is 25% or more, accelerated cooling at a cooling rate of 2 ° C./s or more is performed during rolling, and accelerated cooling is performed within 20 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 800 MPa, and the low Ceq. Further, due to the refinement of the structure and the addition of Ca and REM, the base material toughness and the weld heat affected zone toughness are extremely excellent, and further, the weldability is also excellent due to the low Pcm.
On the other hand, in Comparative Example 26, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, and the heating temperature exceeds the range of the present invention. The base material toughness is significantly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 23.
[0089]
Invention Example 24 has a very low Ceq. In order to achieve a tensile strength of about 800 MPa, B is added, accelerated cooling at a cooling rate of 2 ° C./s or higher is performed before rolling, the rolling reduction in the non-recrystallization temperature region is set to 70% or higher, the rolling average and The rolling reduction in the final pass is 25% or more, accelerated cooling at a cooling rate of 2 ° C./s or more is performed during rolling, and accelerated cooling is performed within 10 seconds after finish rolling. Since the generation amount of coarse precipitates is small and the transformation strengthening amount is extremely large, the strength exceeds 800 MPa, and the low Ceq. Further, due to the refinement of the structure and the addition of Mg, the base material toughness and the weld heat affected zone toughness are excellent, and further, the weldability is also excellent because of the low Pcm.
On the other hand, in Comparative Example 27, since the time until the start of accelerated cooling after finish rolling exceeds 20 seconds, a large amount of coarse precipitates are generated, and the rolling finish temperature is below the range of the present invention. The base material toughness is greatly reduced, and the balance between the base material strength and the toughness of the base material and the weld heat affected zone is greatly inferior to that of Invention Example 24.
[0090]
From the above examples, the steel sheets of Invention Examples 1 to 24, which are steel materials manufactured according to the present invention, are steel materials that are extremely excellent in base material toughness and weld heat affected zone toughness when compared with the same strength. It is obvious.
[0091]
[Table 1]

[0092]
[Table 2]

[0093]
[Table 3]

[0094]
[Table 4]

[0095]
[Table 5]

[0096]
【The invention's effect】
According to the present invention, the toughness of the base metal and the weld heat affected zone is excellent in a wide strength range.Tensile strength 570 to 800MPA class high strengthIt is possible to provide a steel plate and a manufacturing method thereof, and it can be said that the invention has high industrial value.

Claims

Steel is mass%
C: 0.005-0.2%,
Si: 0.01 to 1%,
Mn: 0.1 to 2%,
P: 0.02% or less,
S: 0.02% or less,
N: 0.0001 to 0.01%
B: 0.0005 to 0.005%,
Al: 0.001 to 0.1%
And the balance is Fe and inevitable impurities, and the number density of B-containing carbides, nitrides and carbonitrides having a circle equivalent diameter of 50 nm to 10,000 nm is 5.0. a × 10 below ⁷ / mm ^2, the value obtained by dividing the number of the coefficients density amount of B is 2.0 × 10 ¹⁰ pieces / less than (mm ² · wt%), the volume ratio of bainite and martensite Is a bainite-based structure having a sum of 60% or more, and an average dimension of a region in which a crystal orientation difference with an adjacent structure is 15 degrees or more in bainite is 20 μm or less, and a base material and welding heat Tensile strength 570 to 800 MPa class high strength steel plate with excellent toughness of the affected area.

In mass%,
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.2%,
V: 0.001 to 0.2%
The tensile strength 570 thru | or 800 MPa class high strength steel plate excellent in the toughness of the base material and a welding heat affected zone of Claim 1 characterized by including 1 type, or 2 or more types of these.

In mass%,
Cu: 0.005 to 1%
Ni: 0.01-2%,
Cr: 0.01-1%,
Mo: 0.01 to 1%
A high strength steel sheet having a tensile strength of 570 to 800 MPa class excellent in toughness of the base material and the weld heat-affected zone according to claim 1 or 2, characterized by containing one or more of the following .

In mass%,
Ca: 0.0005 to 0.02%,
Mg: 0.0005 to 0.02%,
REM: 0.001 to 0.1%
The tensile strength 570 to 800 MPa class high strength excellent in the toughness of the base material and welding heat-affected zone according to any one of claims 1 to 3, characterized by containing one or more of steel sheet.

% By mass
C: 0.005-0.2%,
Si: 0.01 to 1%,
Mn: 0.1 to 2%,
P: 0.02% or less,
S: 0.02% or less,
N: 0.0001 to 0.01%
B: 0.0005 to 0.005%,
Al: 0.001 to 0.1%
A steel slab or slab having a steel composition consisting of Fe and inevitable impurities in the balance is heated to a temperature range of 1050 ° C. or higher and 1350 ° C. or lower, and after being held in the temperature range for 20 minutes or longer, rolling is performed. After starting and rolling at an Ar3 point or higher and 900 ° C or lower, the steel sheet is cooled at an average cooling rate of 0.5 ° C / s or higher within 20 seconds, and after cooling at 350 ° C or higher, air cooling is performed. Among the carbides, nitrides, and carbonitrides containing B, the number density of the equivalent circle diameter of 50 nm to 10,000 nm is less than 5.0 × 10 ⁷ pieces / mm ² , A value obtained by dividing the number density by the amount of addition of B is less than 2.0 × 10 ¹⁰ pieces / (mm ² ·% by mass), and the bainite main structure has a volume ratio of bainite and martensite of 60% or more. Yes, and adjacent pairs in bainite Method for producing a crystal misorientation of 15 ° or more to become the average size of the area to 570 no excellent tensile strength in toughness of the base metal and weld heat affected zone is 20μm or less 800MPa grade high strength steel with.

% By mass
C: 0.005-0.2%,
Si: 0.01 to 1%,
Mn: 0.1 to 2%,
P: 0.02% or less,
S: 0.02% or less,
N: 0.0001 to 0.01%
B: 0.0005 to 0.005%,
Al: 0.001 to 0.1%
A steel slab or slab having a steel composition consisting of Fe and inevitable impurities in the balance is heated to a temperature range of 1050 ° C. or higher and 1350 ° C. or lower, and after being held in the temperature range for 20 minutes or longer, rolling is performed. After starting and rolling at an Ar3 point or more and 900 ° C or less, the steel sheet is cooled at an average cooling rate of 0.5 ° C / s or more within 20 seconds, and after cooling at less than 350 ° C, air cooling is performed. Further, tempering is performed at a temperature of 900 ° C. or lower, and among the carbides, nitrides, and carbonitrides containing B, the number density of the circle-equivalent diameter is 50 nm to 10,000 nm. It is less than 5.0 × 10 ⁷ pieces / mm ² , and the value obtained by dividing the number density by the amount of addition of B is less than 2.0 × 10 ¹⁰ pieces / (mm ² ·% by mass), and bainite and martensite Bays with a volume ratio of 60% or more Ito is mainly tissue, to further crystal orientation differences average size of the regions to be more than 15 degrees strength 570 no tensile excellent toughness of the base metal and weld heat affected zone is 20μm or less and adjacent tissue in bainite 800MPa Of manufacturing high-grade steel sheets.

In mass%,
Nb: 0.001 to 0.1%,
Ti: 0.001 to 0.2%,
V: 0.001 to 0.2%
The method for producing a high strength steel sheet having a tensile strength of 570 to 800 MPa class excellent in toughness of the base material and the weld heat-affected zone according to claim 5 or 6, characterized by containing one or more of the following.

In mass%,
Cu: 0.005 to 1%
Ni: 0.01-2%,
Cr: 0.01-1%,
Mo: 0.01 to 1%
The tensile strength 570 to 800 MPa class high strength excellent in the toughness of the base material and welding heat-affected zone according to any one of claims 5 to 7, characterized by containing one or more of A method of manufacturing a steel sheet.

In mass%,
Ca: 0.0005 to 0.02%,
Mg: 0.0005 to 0.02%,
REM: 0.001 to 0.1%
The tensile strength 570 to 800 MPa class high strength excellent in the toughness of the base material and welding heat-affected zone according to any one of claims 5 to 8, characterized by containing one or more of A method of manufacturing a steel sheet.

The steel slab or slab after heating is cooled to a temperature at which the average cooling rate on the surface of the steel ingot is 2 ° C./s or more and 1050 ° C. or less and Ar 3 point or more, and rolling is started after reheating. Item 10. A method for producing a high strength steel sheet having a tensile strength of 570 to 800 MPa class excellent in toughness of the base material and the weld heat affected zone according to any one of Items 5 to 9.

It is excellent in the toughness of the base material and welding heat-affected zone according to any one of claims 5 to 10, characterized in that the rolling reduction of rolling performed at a temperature lower than the recrystallization temperature of 700 ° C or higher is 70% or higher. A method for producing a high strength steel sheet having a tensile strength of 570 to 800 MPa .

The tensile strength excellent in the toughness of the base material and the weld heat affected zone according to any one of claims 5 to 11, wherein an average value of rolling reduction in all rolling passes is 25% or more. 570 to 800 MPa class high strength steel plate manufacturing method.

13. The production of a high strength steel sheet having a tensile strength of 570 to 800 MPa class excellent in toughness of the base material and the weld heat affected zone according to claim 5, wherein the rolling reduction of the final rolling pass is 25% or more. Method.

14. The cooling method according to claim 5, wherein an average cooling rate of the steel sheet surface is cooled to 2 ° C./s or more in a part or all of the rolling passes, and the next rolling pass is performed after reheating. A method for producing a high-strength steel sheet having a tensile strength of 570 to 800 MPa, which is excellent in toughness of the base material and the weld heat-affected zone according to claim 1.