JP3635208B2

JP3635208B2 - Low yield ratio fireproof steel plate and steel pipe excellent in toughness and method for producing the same

Info

Publication number: JP3635208B2
Application number: JP08717699A
Authority: JP
Inventors: 学高橋; 正芳末廣; 淳伊丹
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1999-03-29
Filing date: 1999-03-29
Publication date: 2005-04-06
Anticipated expiration: 2019-03-29
Also published as: JP2000282167A

Description

【０００１】
【発明の属する技術分野】
本発明は、建築分野で使用される、常温で低降伏比を持ち、高温強度特性と靭性に優れた低降伏比型耐火用鋼板と鋼管及びその製造方法に関するものである。
【０００２】
【従来の技術】
近年、建築物の耐震性の観点から、常温での降伏比を下げ、靱性を向上させることが強く望まれるようになってきた。この様な要求に対して、例えば特開平９−４１０３５号公報には、ＮｂとＴｉの内１種または２種を（Ｔｉ＋Ｎｂ／２）Ｃ≧４を満足して含有させ、ミクロ組織をベイナイティックフェライトを含まない１００％ポリゴナルフェライトとすることで、低ＹＲと高靱性を両立する鋼板を製造できるという発明が開示されている。
【０００３】
また、昭和６２年の建築基準法の改正等により、建築用鋼材に対して高温において十分な強度が確保できれば、鋼材の温度上昇を抑えるために通常構造部表面に実施されるロックウール等の耐火被覆を必ずしも施す必要が無くなった。この様な状況に対応して、成分を調整することによって高温強度を確保する鋼材に関する発明が提案されてきた。例えば、特開平２−２８２４１９号公報に開示されている発明では、高温強度を確保するために炭化物形成元素であるＮｂ，Ｍｏ等を添加し、これらの元素の高温における微細炭化物の析出による析出強化を利用している。
【０００４】
【発明が解決しようとする課題】
建築物の耐震性の観点からの常温での低降伏比化の要請に対して、前記特開平２−２８２４１９号公報記載の発明では、Ｎｂ，Ｍｏ等の添加物が熱間圧延後の巻取段階で析出して、常温での降伏強度が、従って結果として降伏比が上昇するために、低降伏比の鋼板を得ることが困難であった。特に、実際に建築構造物として使用されるまでに鋼材に冷間加工が加えられる場合には、冷間加工によって導入される歪みによって、鋼材の降伏強度が製造時よりも高くなり、実際の使用環境での低降伏比が達成できなくなる。したがって、実使用前に冷間加工（例えば円形や角形等の閉断面への加工）が加わる場合には、製造完了段階でより低い降伏強度、すなわち低い降伏比を達成する必要がある。
【０００５】
また、前記特開平２−２０５６２５号公報記載の発明では、高価なＮｉも同時に添加する必要があり、安価な建築構造部品用鋼材を提供することはできない。
更に、前記特開平５−２２２４８４号公報記載の発明による鋼板でも、造管時の降伏強度の上昇が大きく、造管後に十分な低降伏比が得られないという問題があった。
【０００６】
また、前記特開平９−４１０３５号公報記載の発明による鋼板は、ミクロ組織を１００％ポリゴナルフェライトとしているため、常温での低ＹＲと高い靱性の確保は達成しているものの、加工後の靱性や高温での強度が十分とはいえないという問題があった。
このような技術の状況に鑑みて、本発明の具体的な目的は、ｖＴｒｓが−４０℃以下で更に０℃での吸収エネルギーが４７Ｊ以上で、同時に常温でのＹＲが７５％以下でかつ相当歪みで５％の冷間加工後のＹＲが９０％以下であるという極めて高い靱性と低いＹＲ及び高い高温強度を持った鋼板及び、造管後常温でのＹＲが９０％以下であり、同時にｖＴｒｓが−４０℃以下で更に−４０℃での吸収エネルギーが４７Ｊ以上という極めて高い靱性と低いＹＲを有する鋼管及び、６００℃での降伏応力が１９７ＭＰａ以上という高い高温強度を兼ね備えた鋼管、並びにこれらの製造方法を提供することにある。
【０００７】
【課題を解決するための手段】
本発明者らは、種々の実験、研究を重ねた結果、鋼材に含有されるＣを低減し、Ｎｂを固溶の状態で存在させ、更に鋼材のミクロ組織を適切に制御することによって、常温で降伏比が低く、靱性に優れかつ高温での強度特性に優れる鋼材が得られることを見いだした。すなわち、本発明の要旨とするところは下記の通りである。
（１）重量％で、
Ｃ：０．０３％以下、Ｓｉ：１％以下、
Ｍｎ：０．１〜２％、Ｓ：０．０２％以下、
Ａｌ：０．０１〜０．１％、Ｎｂ：０．０４〜１％、
Ｂ：０．０００１〜０．０１％
を含有し、残部がＦｅ及び不可避的不純物からなり、かつＮｂ含有量が下記（２）式を満足し、ミクロ組織がポリゴナルフェライトとベイナイトを含む低温生成相との混合組織であって、板厚方向平均のベイナイトを含む低温生成相の面積率が５％超９０％未満であり、ｖＴｒｓが−４０℃以下で、かつ０℃での吸収エネルギーが４７Ｊ以上であり、６００℃と常温での降伏強度の比が０．６以上で、常温でのＹＲが７５％以下で、更に相当歪みで５％の冷間加工後における常温でのＹＲが９０％以下で、６００℃での降伏強度が１９７ＭＰａ以上であることを特徴とする靭性に優れた低降伏比型耐火用鋼板。
Ｎｂ≧0.1+7.74C-1.94Ti+6.63(N-1.30B)・・・（２）式
（２）鋼成分として、さらに重量％で、
Ｔｉ：０．２５％以下
を含有することを特徴とする前記（１）記載の靭性に優れた低降伏比型耐火用鋼板。
（３）鋼成分として、さらに重量％で、
Ｃｕ：２％以下、Ｎｉ：１．５％以下、
Ｍｏ：１％以下、Ｖ：０．２５％以下、
Ｃｒ：１％以下
の１種若しくは２種以上を合計で２．５重量％以下含むことを特徴とする前記（１）又は（２）に記載の靭性に優れた低降伏比型耐火用鋼板。
（４）鋼成分として、さらに重量％で、
Ｃａ：０．０００５〜０．００５％、Ｒｅｍ：０．００１〜０．０２％
の１種若しくは２種を含むことを特徴とする前記（１）〜（３）のいずれか１項に記載の靭性に優れた低降伏比型耐火用鋼板。
（５）ポリゴナルフェライトの平均粒径が１０μｍ以下であることを特徴とする前記（１）〜（４）のいずれか１項に記載の靭性に優れた低降伏比型耐火用鋼板。
（６）前記（１）〜（５）のいずれか１項に記載の鋼板を製造する方法であって、鋳造後の鋼片を鋳造まま若しくは一旦Ａｒ３変態点以下まで冷却した後に再び加熱し熱間加工により所定の形状に加工するに際して、加工終了温度をＡｒ３−５０℃以上とし、その後０．１℃/sec以上８０℃/sec以下の冷却速度で７７０℃以下まで冷却することを特徴とする靭性に優れた低降伏比型耐火用鋼板の製造方法。
（７）冷却した後、さらに熱間加工鋼板を冷間にて加工することを特徴とする前記（６）記載の靭性に優れた低降伏比型耐火用鋼板の製造方法。
（８）冷間加工後、さらに４５０℃以上９５０℃以下の温度で熱処理を行うことを特徴とする前記（７）記載の靭性に優れた低降伏比型耐火用鋼板の製造方法。
（９）前記（１）〜（５）のいずれか１項に記載の鋼板が鋼管であることを特徴とする靭性に優れた低降伏比型耐火用鋼管。
（１０）前記（９）記載の鋼管を製造する方法であって、鋳造後の鋼片を、鋳造まま若しくは一旦Ａｒ３変態点以下まで冷却した後に再び加熱してから、加工終了温度がＡｒ３−５０℃以上の熱間加工によって造管し、その後０．１℃/sec以上８０℃/sec以下の冷却速度で７７０℃以下まで冷却することを特徴とする靭性に優れた低降伏比型耐火用鋼管の製造方法。
（１１）前記（９）記載の鋼管を製造する方法であって、鋳造後の鋼片を、鋳造まま若しくは一旦Ａｒ３変態点以下まで冷却した後に再び加熱してから、加工終了温度がＡｒ３−５０℃以上の熱間加工で所定の形状とし、その後０．１℃/sec以上８０℃/sec以下の冷却速度で７７０℃以下まで冷却することを特徴とする靭性に優れた低降伏比型耐火用鋼管の製造方法。
（１２）前記（９）記載の鋼管を製造する方法であって、鋳造後の鋼片を鋳造まま若しくは一旦Ａｒ３変態点以下まで冷却した後に再び加熱してから、加工終了温度がＡｒ３−５０℃以上の熱間加工を施し、その後０．１℃/sec以上８０℃/sec以下の冷却速度で７７０℃以下まで冷却し、得られた熱間圧延鋼板を溶接して造管することを特徴とする靭性に優れた低降伏比型耐火用鋼管の製造方法。
（１３）溶接により造管した後、さらに成形のための冷間加工を施すことを特徴とする前記（１２）記載の靭性に優れた低降伏比型耐火用鋼管の製造方法。
（１４）溶接により円形断面の鋼管に造管した後、さらにロール成形によって角形断面に成形することを特徴とする前記（１２）記載の靭性に優れた低降伏比型耐火用鋼管の製造方法。
【０００８】
【発明の実施の形態】
以下、本発明について詳細に説明する。
本発明の鋼板及び鋼管は、固溶Ｎｂによって高温での転位との相互作用により高温強度を上昇させると同時に、適正な成分添加によって常温での固溶Ｃや固溶Ｎを低減させ、製造プロセスで導入されている初期可動転位の固着を回避することによって、常温での低い降伏強度すなわち低いＹＲを達成し、同時にミクロ組織に鉄炭化物をほとんど含まないベイナイトを適量導入することで、良好な靱性を達成したものである。
【０００９】
まず、鋼板の化学成分の限定理由について述べる。
Ｃは他の添加元素と結合して析出することで常温及び高温での鋼材の強度を高めることができるが、Ｃが０．０３重量％を超えると、後述する様に、固溶Ｎｂを確保するための添加元素量が不必要に多くなり、鋼板の常温強度を必要以上に高めるのみならず、常温でのＹＲ（ＹＲ＝引張り試験の降伏強度ＹＳ／最大強度ＴＳ×１００）を７５％以下に保つことが困難となり、更には経済的な不利益も生じる。また、Ｃが０．０３重量％を超えると、靱性向上のために重要な役割を果たすベイナイト中に鉄炭化物であるセメンタイトが析出し、靭性を劣化させることから、Ｃの添加量を０．０３重量％以下とした。特に、製造完了後実使用の前に冷間加工が加えられる場合には、冷間加工後のＹＲを下げる為にＣを０．０１５重量％以下とすることが望ましい。また、Ｃの下限については特に限定しないが、Ｃを０．０００５重量％以下にすることは、製鋼での脱ガス等の工程に大きな負荷をかけることとなり、製造コスト上昇を招くことから、Ｃは０．０００５重量％超とすることが望ましい。
【００１０】
Ｓｉは脱酸剤として利用されると共に、固溶強化元素であり、比較的安価に鋼材の強度を上昇させることができるが、多量の添加は常温での降伏強度を上昇させ、ＹＲを高くする。Ｓｉ添加量が１重量％を超えると、常温でのＹＲを７５％以下に保ち、相当歪みで５％の冷間加工後のＹＲを９０％以下に保つことが困難になることから、添加量を１重量％以下とする。また、特に高い表面品位や、溶融めっき時のめっき割れ回避等が強く要求される場合には、Ｓｉ添加量を０．０２重量％以下とすることが望ましい。
【００１１】
ＭｎはＳｉと同様に比較的安価な固溶強化元素である。Ｍｎ添加量を０．１重量％未満とすることはプロセス上コスト上昇につながることから、０．１重量％をＭｎ添加量の下限とした。また、Ｍｎ添加量が２重量％を超えると、鋼の焼き入れ性が必要以上に高くなり、所定のミクロ組織が得られなくなり、常温でのＹＲを高めるために、その添加量の上限を２重量％とする。
【００１２】
Ｓは不可避的に含まれる元素であり、加工性劣化や靱性の劣化の要因となるため、極力低減することが望ましい。しかしながら、その添加量を０．０２重量％以下とすることで、上記の材質劣化の傾向が飽和するために、その添加量の上限を０．０２重量％とする。なお、特に厳しい冷間での加工性が要求される場合には、Ｓの添加量を０．０１重量％以下とすることが望ましい。
【００１３】
ＡｌはＳｉと共に脱酸剤として使用されるが、この効果を発揮させるためには鋼中に０．０１重量％以上含有させることが必要である。−方、Ａｌの添加量が０．１重量％を超えると、鋼中におけるＡｌ系の酸化物の量が増加し、特に靱性を劣化させることから、その添加量の上限を０．１重量％とする。
【００１４】
Ｎｂは本発明において最も重要な添加元素である。Ｎｂは通常析出強化元素として添加される場合が多く、この際にはＣやＮと結合することでほぼすべてのＮｂ元素が析出物の形態をとるように成分設計がなされるのが一般的である。しかしながら、この様な鋼の強度を上昇させる析出物は、同時に常温での降伏強度を上昇させ、低いＹＲを得ることが困難となる。
【００１５】
−方、本発明者らの研究によると、固溶の状態で存在するＮｂは、常温での靱性を著しく向上させるのみでなく、冷間加工後の靭性の劣化を抑制することが判明した。更に、高温での変形の際に転位と有効に相互作用し、転位の移動や、粒界の移動を効果的に抑制することで、高い高温強度を達成する。従って、添加したＮｂをＣやＮとの結合で消費してしまわないよう、その添加量を調整する必要がある。高温での強度上昇のためにはＮｂ添加量を増加することが望ましいが、その添加量が１重量％を超えると、靱性向上や高温強度上昇の効果が飽和すると共に、製造コストの上昇を招くことから、Ｎｂの添加量を１重量％以下とした。Ｎｂ添加量が０．０４重量％未満となった場合には、他の添加元素の調整を行っても、高い靱性や高温強度が得られないために、添加量の下限を０．０４重量％とした。
【００１６】
選択的に添加するＴｉは、Ｎｂと結合する可能性のあるＣやＮと結合し、靱性の向上と高温での強度上昇に有効な固溶Ｎｂの浪費を少なくすることで、靱性を向上させたり高温強度を上げる働きがあるので、０．００２重量％以上添加することが望ましい。−方、その添加量が０．２５重量％を超えると、加工性の劣化や、鋼材強度の不必要な上昇及び、常温でのＹＲの上昇を招くことから、その添加量の上限を０．２５重量％とした。
【００１７】
Ｂは、Ｎｂと結合する可能性のあるＮと結合し、靱性や高温での強度上昇に有効な固溶Ｎｂの浪費を少なくするばかりでなく、Ｎｂとの複合添加により、ミクロ組織の最適化や転位の移動度を低下させることによって靱性や高温強度に対する固溶Ｎｂの効果を助長する。Ｂの添加量が０．０００１重量％未満ではその効果は認められず、また、添加量が０．０１重量％超では効果が飽和することから、添加量を０．０００１〜０．０１重量％とする。
【００１８】
以上の成分に加えて、選択的に添加するＣｕ，Ｎｉ，Ｍｏ，Ｖ，Ｃｒは、それぞれ有効な鋼板の強化元素であり、同時に靱性や疲労強度、高温強度を高める働きもあることから、それぞれ、０．００５重量％、０．００５重量％、０．００５重量％、０．００２重量％、０．００５重量％以上添加することが望ましい。しかしながら、これらの元素の添加量で、Ｃｕ添加量が２重量％超、Ｎｉ添加量が１．５重量％超、Ｍｏ添加量が１重量％超、Ｖ添加量が０．２５重量％超、及びＣｒ添加量が１重量％超となった場合には、常温での降伏強度の上昇量が大きくなり、常温でのＹＲを７５％以下に保ち、相当歪みで５％の冷間加工後のＹＲを９０％以下に保つことが困難になることから、これらを各元素の添加量の上限とした。
【００１９】
また、この範囲であっても、これらの１種若しくは２種以上の添加量合計が２．５重量％を超えると、常温での降伏強度の上昇量が大きくなり、常温でのＹＲを７５％以下に保ち、相当歪みで５％の冷間加工後のＹＲを９０％以下に保つことが困難になることから、これを上記元素の１種または２種以上の添加量合計の上限とした。これらの元素は積極的な添加を行っても良く、また、スクラップ等からの混入を有効に利用しても良い。
【００２０】
選択的に添加されるＣａ，Ｒｅｍは、いずれも硫化物の形態を制御することで、耐サワー特性や靱性、溶接性等を向上させる元素である。しかしながら、Ｃａが０．０００５重量％未満、Ｒｅｍが０．００１重量％未満の場合にはその効果が発揮されず、また、Ｃａが０．００５重量％超、Ｒｅｍが０．０２重量％超ではこれらの効果が飽和するばかりでなく逆に酸化物起因で靱性を劣化させるために、これらを各々の添加量の上限、下限とした。
【００２１】
以上の限定範囲で成分が調整されるに際して、上記のごとく、靱性や高温での強度を上昇させる機能は固溶Ｎｂにあるために、固溶Ｎｂを一定量以上確保することが必要である。冷間での加工（相当歪みで５％の冷間加工に相当）の有無に関わらず、Ｎｂの含有量が下記（２）式を満足する場合に常温での低いＹＲと高い靱性が両立する。
Ｎｂ≧0.1+7.74C-1.94Ti+6.63(N-1.30B)・・・（２）式
但し、Ｔｉ含有量、Ｂ含有量が０の場合には上式でＴｉ＝０とする。
結果として、Ｎｂ含有量が上式を満足する場合に、相当歪みで５％の冷間加工後のＹＲが図１に示すように９０％以下となる。
【００２２】
図１は、本発明に規定する成分からなる鋼を鋳造後、本発明の範囲内の製造条件で熱延鋼板とし、熱延まま及び、５％の冷間圧延（角形鋼管への造管によって鋼管平坦部に導入される歪みの代表値として選択した。）後の常温でのＹＲを測定し、５％冷間圧延後のＹＲを図１の縦軸にし、計算固溶Ｎｂ量＝上式の左辺−右辺＝Ｎｂ−｛０．１＋７．７４Ｃ−１．９４Ｔｉ＋６．６３（Ｎ−１．３０Ｂ）｝の値を横軸にしてプロットした。また、Ｎｂ添加量が（２）式を満足する場合には、図２に示すように、５％の冷間加工の有無に関わらず、常温と６００度での降伏応力（０．２％耐力）の比が０．６以上という高い値となり、高温での強度にも優れることが分かる。
【００２３】
この様な材質の鋼板を得るためには鋼板のミクロ組織を適切に制御することが不可欠である。本発明の鋼板のミクロ組織は、ポリゴナルフェライト（熱間圧延のＬ断面相当位置での観察で、明瞭な粒界を有し、Ｌ方向の粒径と板厚方向の粒径の比である軸比が２．５以下の粒をポリゴナルフェライトと定義する。）以外にベイナイトを含む低温生成相（ここで、ポリゴナルフェライト以外の低温生成相は、ベイナイト、マッシブフェライト及びマルテンサイト等が含まれる。）の混合組織である。これらのミクロ組織写真の例を図３に示す。
【００２４】
Ｎｂ等の炭化物形成元素によって、ほとんどのＣが析出物の形で固定された鋼ではベイナイト、マッシブフェライト及びマルテンサイトは良く似た組織形状を呈する。本発明範囲内の鋼においては、光学顕微鏡で判断されるポリゴナルフェライト以外の組織のほとんどはベイナイトとマッシブフェライトであり、図３に示すように、軸比が２．５超の場合や、軸比がこれ以下であっても粒界が直線的でなかったり、粒内の欠陥起因と思われる濃く腐食される場合がある。また、少なくとも光学顕微鏡５００倍以下のレベルでは、ピクリン酸腐食した後でもセメンタイトと認識される粒は観察されない。電子顕微鏡レベルの倍率で観察すると、ベイナイト粒の内部に非常に微細な炭化物を含む場合もあり、更に、マルテンサイトと一部オーステナイトを含むような島状の組織が観察されることもある。
【００２５】
鋼の成分が上記のすべての要件を満たす場合で、熱間圧延のＬ断面相当位置での観察で、板厚方向の平均でベイナイトを含む低温生成相の面積率が５％以下の場合には、十分に高い高温強度を得ることができず、特に冷間加工が行われる場合には、６００℃でのＹＳと常温でのＹＳの比が低くなり、常温での低ＹＲと高温での高い降伏強度の両立が困難となるために、これを板厚方向の平均でのベイナイトを含む低温生成相面積率の下限とした。
【００２６】
また、ベイナイトを含む低温生成相の面積率が増加すると、鋼の強度は上昇するが、同時に常温での降伏強度が上昇する。特に板厚方向の平均でのベイナイトを含む低温生成相面積率が９０％以上の場合には、常温での降伏強度の上昇量が大きくなり、常温でのＹＲを７５％以下に保ち、相当歪みで５％の冷間加工後のＹＲを９０％以下に保つことが困難になることから、これを板厚方向平均でのベイナイトを含む低温生成相面積率の上限とした。
【００２７】
この様な範囲のミクロ組織を得るためには、適正な成分と製造条件の組み合わせが必要となる。特にＮｂ，Ｍｎ，Ｂはベイナイトを含む低温生成相を得るのに有効な元素であり、それぞれが本発明の範囲であれば、以下に示す様な広い製造条件の範囲で、ベイナイトを含む低温生成相の面積率を５％超、９５％未満に制限することができる。
【００２８】
この様な面積率のベイナイトを含む低温生成相が存在するときに優れた靱性を示す理由については現在のところ明確にはなっていないが、固溶Ｎｂによって転位の移動が妨げられることや、従来の鋼で良く観察される粒界炭化物が減少したこと、更にはポリゴナルフェライトとベイナイトを含む低温生成相の複合組織化が有効に働いているものと考えている。また、高温強度、特に冷間加工後の高温強度が高くなる理由については、現在のところ明確では無いが、固溶Ｎｂ若しくはクラスター状のＮｂ及びＮｂとＣの集合体がベイナイトを含む低温生成相の粒内、及びこれらの低温生成相生成に伴ってその周囲のフェライト粒内に導入された変態に伴う転位と相互作用することにより、冷間加工によって導入される転位を高温でも回復しにくくしていると考えている。
【００２９】
また、本発明の鋼は、初期から適量のベイナイトを含む低温生成相を含むと同時に、鋼中の固溶Ｃ，Ｎを低減していることから、溶接によって導入される急速な熱履歴を受けても溶接部、ＨＡＺ部の硬度変化が少なく、結果として溶接部の種々の材質劣化（靱性、疲労特性、加工性等）が抑制される。
【００３０】
ミクロ組織の定量は熱間圧延鋼板のＬ断面相当の位置で、板厚表面から板厚中心方向に向かって、板厚の０．１倍、０．２５倍、板厚中心の３カ所を光学顕微鏡で観察し、写真撮影後にポイントカウント法を用いて各位置での各組織の面積率を測定し、平均した。この時、組織の面積率の測定は画像処理等の他の方法を用いて行っても良い。
【００３１】
この様な鋼板を製造する場合には、上記成分の鋼を鋳造し、得られた鋼片を直接若しくは一旦室温を含むＡｒ３変態点以下まで冷却した後に再加熱し熱間加工を行う。また、この時の再加熱温度は特に制限しないが、生産性及び製造コストを考慮すると、１０００℃〜１３００℃の範囲が望ましい。
【００３２】
熱間加工が板状で行われる場合には、厚板の様な板状圧延でも、また連続熱延でも良く、更に、連続熱延の仕上げ熱延入り口で複数のスラブを接続して連続的に熱延しても良い。
【００３３】
また、熱間加工は鋼管の製造工程でも良く、また継ぎ目無しの鋼管製造や熱間押し出し等でも良い。
【００３４】
この時の熱間加工の終了温度は、鋼の成分で決まるＡｒ３変態点−５０℃以上とする。熱間加工完了温度がこれ未満となった場合には、鋼板にフェライトの加工組織が残留し、冷間での加工性が劣化するばかりでなく、常温でのＹＲが上昇するので、これを熱間加工終了温度の下限値とする。熱間加工開始温度の上、下限及び終了温度の上限に関しては特に制限しないが、生産性の観点とスケール起因の表面品位の観点からは、熱間加工終了温度は１０００℃以下が望ましい。
【００３５】
熱間加工後、鋼板は室温まで冷却されるが、この時、熱間加工後冷却終了温度までの平均冷却速度が０．１℃/sec未満の場合には冷却中にミクロ組織の粗大化が進み、鋼板の強度を不必要に低下させると共に、鋼板の靱性を劣化させるため、これを冷却速度の下限とした。また、冷却終了温度までの平均冷却速度が８０℃/sec超では、ベイナイト等の低温生成相の面積率の増大を招き、適正なミクロ組織が得られないことから、これを平均冷却速度の上限とした。この冷却の終了温度が７７０℃超の場合にはミクロ組織が粗大化し、靱性を劣化させるために、これを上限とした。最終的な鋼板表面のスケールを薄くして、表面品位を高める目的からは、冷却終了温度を６５０℃以下に限定することが望ましい。冷却終了温度の下限は特に定める必要は無いが、冷却終了温度が１００℃以下となる場合には、限られた製造工程長さの中で必要以上に高い冷却速度を必要とし、経済的なデメリットを生じるために、１００℃超が望ましい。
【００３６】
また、冷却終了後、連続熱延の場合には巻取処理、その他の場合には空冷されるが、鋼板内での材質バラツキを小さくしたい場合には、冷却停止温度を４００℃以上とすることが望ましい。
【００３７】
また、この様な鋼板を熱間加工後冷間加工によって所定の形状とする際には、上記方法によって製造された熱間加工鋼板に所定の冷間加工（冷間圧延、冷間曲げ、冷間プレス、冷間鍛造、冷間での拡管等）を施し、そのまま若しくはその後熱処理を行って製品とする。この時、熱処理温度は４５０℃未満では常温でのＹＲ低下が十分でなく、９５０℃超では製造コストの上昇を招くためにこれらを上、下限とした。
【００３８】
この様な鋼板を用いて鋼管を製造する場合には、熱間での加工（高温での圧延、ロール成形、鍛造、曲げ加工、拡管等を含む）によって所定の断面形状の鋼管とする際に、上記の熱間加工の条件に従う方法で良い。また、上記製造方法で製造された鋼板を冷間加工後、溶接によって鋼管とする場合も、また、溶接によって一旦鋼管の形状とした後、更に冷間加工によって所定の断面形状に加工する場合も、また、溶接によって円形鋼管とした後、ロール成形で角形の鋼管とする場合も含まれる。ここで冷間加工とは、冷間でのロール成形、冷間でのプレス成形、冷間での鍛造や冷間での拡管等を含む。また、これらの鋼板、鋼管はそのままの使用でも、溶融亜鉛めっき等の表面処理を施した後の使用でも良い。本発明の成分を満たす鋼を熱間圧延鋼板とし、その後溶接によって円形鋼管にし、冷間のロール成形によって角型鋼管とした後にその鋼管の平坦部からサンプルを採取し、調査した機械的性質を示したところ、板厚と角型鋼管の外径との比が３．６〜６．４に変化しても、本発明例である鋼は造管後平坦部のＹＲが９０％以下で、平坦部のｖＴｒｓが−４０℃以下でかつ−４０℃での吸収エネルギーが４７Ｊ以上という優れた靭性を持ち、かつ６００℃での平坦部の降伏強度が１９７ＭＰａ以上という優れた高温強度を持つ。
【００３９】
【実施例】
表１に示す各成分の鋼を鋳造し、表２に示した条件で熱延を完了した鋼板の、常温及び６００℃での機械的特性を調査した結果を同表に示した。相当歪みで５％の冷間加工は、鋼板を角形鋼管に製造する際に鋼管の平坦部に導入される歪みの代表として選択した。実際に導入される歪みは相当歪みで２．５％〜６％程度と考えられるが、鋼板の特性を代表させるためには５％程度の相当歪み量での評価が適当であることが判明したために、ここでは５％の冷間加工を選択した。冷間加工の付与方法は、実際の角形鋼管製造に近い、曲げ加工、プレス加工、冷間圧延を比較したが、常温でのＹＲ、靱性、６００℃での降伏強度共に、これらの加工形態で差が無かったことから、ここでは５％の冷間圧延によって代表させた。
【００４０】
【表１】

【００４１】
【表２】

【００４２】
本発明の成分範囲である鋼１２〜１９は、表２に示すように、製造条件が本発明の範囲内である場合には、製造ままの鋼板の常温でのＹＲ（表２中で無加工材ＹＲ（％）と表示）が７５％以下で、かつ、相当歪みで５％の冷間加工後のＹＲ（表２中で５％予加工材ＹＲ（％）と表示）が９０％以下で、更に、ｖＴｒｓが−４０℃以下でかつ０℃での吸収エネルギーが４７Ｊ以上という優れた靱性を持ち、５％予加工材の６００℃での高温強度が１９７ＭＰａという高い高温強度を持つことが分かる。
【００４３】
一方、表１の鋼２０〜２６はいずれかの成分が本発明の範囲外であり、その結果、表２に示すように、製造条件が本発明の範囲内であっても、製造ままの鋼板の常温でのＹＲか、相当歪みで５％の冷間加工後のＹＲか、ｖＴｒｓのいずれかが本発明の範囲外となる。
【００４４】
なお、Ａｒ３温度（℃）はＡｒ３＝９０１−３２５×％Ｃ＋３３×％Ｓｉ−９２×（％Ｍｎ＋％Ｎｉ／２＋％Ｃｒ／２＋％Ｃｕ／２＋％Ｍｏ／２＋％Ｎｂ／２）を用いて計算した。
【００４５】
（削除）
【００４６】
（削除）
【００４７】
本実験においては、常温での機械試験はＪＩＳ５号試験片を用い、ＪＩＳＺ２２４１に従って行い、６００℃での引張り試験はＪＩＳＧ０５６７に従って行った。衝撃試験はＪＩＳＺ２２０２に従って行った。
【００４８】
【発明の効果】
以上に述べたように、本発明によれば、製造まま及び相当歪みで５％の冷間加工後の常温でのＹＲが低く、かつ高温での降伏応力の高い靭性に優れた鋼板、及び造管後のＹＲが低く、高靱性でかつ高温での降伏応力の高い鋼管の製造が可能となり、これらの特性が要求される土木、建築分野において優れた効果を発現する。
【図面の簡単な説明】
【図１】鋼板の計算固溶Ｎｂ量＝Nb-{0.1+7.74-1.94Ti+6.63(N-1.30B)}（重量％）と、相当歪みで５％冷間加工後（図中には５％予加工後と表示）の常温でのＹＲとの関係を示す図である。
【図２】鋼板の計算固溶Ｎｂ量＝Nb-{0.1+7.74-1.94Ti+6.63(N-1.30B)}（重量％）と、製造まま（図中に無加工材と表示）、及び相当歪みで５％の冷間加工後（図中に５％予加工材と表示）の６００℃での降伏強度（ＹＳ）と常温の降伏強度の比の関係を示した図である。
【図３】本発明に規定する成分からなる鋼を、本発明の範囲内の製造条件で熱間圧延して得られた熱延鋼板の表層から板厚方向１／４の部分におけるミクロ組織の顕微鏡写真である。但し、（ａ）は２００倍で撮影した顕微鏡写真、（ｂ）は５００倍で撮影した顕微鏡写真である。[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a low yield ratio type fire-resistant steel plate and steel pipe having a low yield ratio at room temperature and excellent in high temperature strength characteristics and toughness, and a method for producing the same, which are used in the construction field.
[0002]
[Prior art]
  In recent years, from the viewpoint of earthquake resistance of buildings, it has been strongly desired to lower the yield ratio at room temperature and improve toughness. In response to such a requirement, for example, Japanese Patent Laid-Open No. 9-41035 discloses that one or two of Nb and Ti are contained satisfying (Ti + Nb / 2) C ≧ 4, and the microstructure is bainai. An invention is disclosed in which a steel sheet having both low YR and high toughness can be manufactured by using 100% polygonal ferrite not containing tick ferrite.
[0003]
  In addition, if sufficient strength can be secured at high temperatures for steel materials for construction due to revisions to the Building Standards Act of 1987, fire resistance such as rock wool that is usually applied to the surface of structural parts in order to suppress the temperature rise of steel materials It is no longer necessary to apply the coating. In response to such a situation, inventions relating to steel materials that secure high temperature strength by adjusting components have been proposed. For example, in the invention disclosed in JP-A-2-282419, Nb, Mo and the like which are carbide forming elements are added to ensure high temperature strength, and precipitation strengthening by precipitation of fine carbides of these elements at high temperatures. Is used.
[0004]
[Problems to be solved by the invention]
  In response to a request for a low yield ratio at room temperature from the viewpoint of earthquake resistance of a building, in the invention described in JP-A-2-282419, additives such as Nb and Mo are wound after hot rolling. It is difficult to obtain a steel plate with a low yield ratio because it precipitates in stages and the yield strength at room temperature and thus the yield ratio increases as a result. In particular, when cold working is applied to steel before it is actually used as a building structure, the yield strength of the steel becomes higher than that during manufacturing due to the strain introduced by cold working, and the actual use A low yield ratio in the environment cannot be achieved. Therefore, when cold working (for example, machining to a closed cross section such as a circle or a square) is applied before actual use, it is necessary to achieve a lower yield strength, that is, a lower yield ratio at the stage of completion of manufacture.
[0005]
  In the invention described in JP-A-2-205625, it is necessary to add expensive Ni at the same time, and it is not possible to provide an inexpensive steel material for building structural parts.
  Furthermore, even the steel sheet according to the invention described in JP-A-5-222484 has a problem that the yield strength at the time of pipe forming is greatly increased, and a sufficiently low yield ratio cannot be obtained after pipe forming.
[0006]
  Further, the steel sheet according to the invention described in JP-A-9-41035 has a microstructure of 100% polygonal ferrite, so that low YR and high toughness at room temperature have been achieved, but toughness after processing. There was also a problem that the strength at high temperatures was not sufficient.
  In view of the state of the art, the specific object of the present invention is that vTrs is −40 ° C. or less, the absorbed energy at 0 ° C. is 47 J or more, and at the same time, the YR at room temperature is 75% or less and is equivalent. A steel sheet with extremely high toughness, low YR and high high-temperature strength of YR after cold working of 5% due to strain, and YR at room temperature after pipe forming is 90% or less, and at the same time vTrs A steel pipe having an extremely high toughness with an absorption energy of 47 J or higher at -40 ° C. or lower and a low YR, a steel pipe having a high high temperature strength with a yield stress at 600 ° C. of 197 MPa or higher, and these It is to provide a manufacturing method.
[0007]
[Means for Solving the Problems]
  As a result of various experiments and researches, the present inventors have reduced C contained in the steel material, caused Nb to exist in a solid solution state, and further appropriately controlled the microstructure of the steel material. It was found that a steel material having a low yield ratio, excellent toughness and excellent strength properties at high temperatures was obtained. That is, the gist of the present invention is as follows.
(1) By weight%
    C: 0.03% or less, Si: 1% or less,
    Mn: 0.1 to 2%, S: 0.02% or less,
    Al: 0.01 to 0.1%, Nb: 0.04 to 1%,
    B: 0.0001 to 0.01%
The balance is Fe and inevitable impurities, the Nb content satisfies the following formula (2), and the microstructure is a mixed structure of polygonal ferrite and a low-temperature generation phase containing bainite, The area ratio of the low-temperature generation phase including bainite averaged in the thickness direction is more than 5% and less than 90%, vTrs is −40 ° C. or less, the absorbed energy at 0 ° C. is 47 J or more, and 600 ° C. at room temperature. The yield strength ratio is 0.6 or more, the YR at room temperature is 75% or less, and the YR at room temperature after cold working of 5% with a corresponding strain is 90% or less, and the yield strength at 600 ° C. is A low-yield-ratio refractory steel sheet excellent in toughness, characterized by being 197 MPa or more.
    Nb ≧ 0.1 + 7.74C-1.94Ti + 6.63 (N-1.30B) ・・・ (2) Formula
(2) As a steel component, further in weight%,
        Ti: 0.25% or less
Containing the above (1) Low yield ratio fireproof steel sheet with excellent toughness as described.
(3) As a steel component, further in weight%,
    Cu: 2% or less, Ni: 1.5% or less,
    Mo: 1% or less, V: 0.25% or less,
    Cr: 1% or less
1 type or 2 types or more in total containing 2.5% by weight or less (1)Or(2The low yield ratio type fireproof steel plate with excellent toughness as described in (1).
(4) As a steel component, further in weight%,
    Ca: 0.0005 to 0.005%, Rem: 0.001 to 0.02%
(1) to (1) characterized by containing one or two of3The low yield ratio type steel sheet for fireproofing excellent in toughness according to any one of the above.
(5The average particle size of polygonal ferrite is 10 μm or less (1) to (1)4The low yield ratio type steel sheet for fireproofing excellent in toughness according to any one of the above.
(6) (1) to (5) In which the steel slab after casting is cast or is once cooled to the Ar3 transformation point or lower and then heated again to be processed into a predetermined shape by hot working. The low-yield ratio type refractory excellent in toughness, characterized in that the processing end temperature is Ar3-50 ° C. or higher and then cooled to 770 ° C. or lower at a cooling rate of 0.1 ° C./sec to 80 ° C./sec. A method of manufacturing a steel sheet.
(7) After cooling, the hot-worked steel sheet is further processed in the cold (6) A method for producing a low-yield ratio type steel sheet for fireproofing with excellent toughness.
(8) After the cold working, heat treatment is further performed at a temperature of 450 ° C. or higher and 950 ° C. or lower.7) A method for producing a low-yield ratio type steel sheet for fireproofing with excellent toughness.
(9) (1) to (5A low yield ratio type fireproof steel pipe excellent in toughness, characterized in that the steel sheet according to any one of the above) is a steel pipe.
(10) Said (9) In which the steel slab after casting is heated as it is after it is cast or once cooled to the Ar3 transformation point or less, and then the hot working with an end temperature of Ar3-50 ° C. or higher. And then cooling to 770 ° C. or less at a cooling rate of 0.1 ° C./sec or more and 80 ° C./sec or less, and a method for producing a low yield ratio type fire-resistant steel pipe excellent in toughness.
(11) Said (9) In which the steel slab after casting is heated as it is after it is cast or once cooled to the Ar3 transformation point or less, and then the hot working with an end temperature of Ar3-50 ° C. or higher. And then cooling to 770 ° C. or less at a cooling rate of 0.1 ° C./sec or more and 80 ° C./sec or less, and a method for producing a low yield ratio type fireproof steel pipe excellent in toughness.
(12) Said (9) The method of manufacturing the steel pipe according to claim 1, wherein after the cast steel piece is cast or once cooled to below the Ar3 transformation point, it is heated again, and then hot working with an end temperature of Ar3-50 ° C or higher is performed. And then cooled to 770 ° C. or less at a cooling rate of 0.1 ° C./sec or more and 80 ° C./sec or less, and the obtained hot-rolled steel plate is welded to form a pipe. Yield ratio type fireproof steel pipe manufacturing method.
(13) After forming the pipe by welding, it is further subjected to cold working for forming (12) A method for producing a low yield ratio type refractory steel pipe excellent in toughness.
(14) After forming a steel pipe having a circular cross section by welding, it is further formed into a square cross section by roll forming.12) A method for producing a low yield ratio type refractory steel pipe excellent in toughness.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, the present invention will be described in detail.
  The steel sheet and steel pipe of the present invention increase the high-temperature strength by interaction with dislocations at high temperature by solute Nb, and at the same time reduce the solute C and solute N at normal temperature by adding appropriate components, and the manufacturing process By achieving the low yield strength at room temperature, that is, low YR, by avoiding the sticking of the initial movable dislocation introduced in the above, good toughness can be achieved by simultaneously introducing an appropriate amount of bainite containing almost no iron carbide in the microstructure. Is achieved.
[0009]
  First, the reasons for limiting the chemical components of the steel sheet will be described.
  C combines with other additive elements and precipitates to increase the strength of the steel at room temperature and high temperature. However, if C exceeds 0.03% by weight, as will be described later, solid solution Nb is secured. The amount of added elements is unnecessarily increased, and the room temperature strength of the steel sheet is increased more than necessary, and the YR at room temperature (YR = yield strength YS of tensile test / maximum strength TS × 100) is 75% or less. It is difficult to keep the balance at the same time, and further, there is an economic disadvantage. Further, if C exceeds 0.03% by weight, cementite which is an iron carbide precipitates in bainite which plays an important role for improving toughness, and deteriorates toughness. It was made into the weight% or less. In particular, when cold working is applied after the completion of manufacturing and before actual use, it is desirable to set C to 0.015% by weight or less in order to reduce YR after cold working. Further, the lower limit of C is not particularly limited. However, if C is made 0.0005% by weight or less, a large load is applied to a process such as degassing in steelmaking, which causes an increase in manufacturing cost. Is preferably more than 0.0005% by weight.
[0010]
  Si is used as a deoxidizer and is a solid solution strengthening element, and can increase the strength of steel materials relatively inexpensively. However, a large amount of addition increases the yield strength at room temperature and increases YR. . If the Si addition amount exceeds 1% by weight, it is difficult to keep the YR at room temperature to 75% or less, and to maintain the YR after cold working of 5% with an equivalent strain to 90% or less. Is 1% by weight or less. In addition, when a particularly high surface quality or avoidance of plating cracks during hot dipping is strongly required, it is desirable that the Si addition amount be 0.02% by weight or less.
[0011]
  Mn, like Si, is a relatively inexpensive solid solution strengthening element. Setting the amount of Mn to be less than 0.1% by weight leads to a cost increase in the process, so 0.1% by weight was set as the lower limit of the amount of Mn added. On the other hand, if the amount of Mn added exceeds 2% by weight, the hardenability of the steel becomes unnecessarily high, a predetermined microstructure cannot be obtained, and the upper limit of the amount added is 2 in order to increase the YR at room temperature. Weight%.
[0012]
  S is an element inevitably contained, and causes deterioration of workability and toughness, so it is desirable to reduce it as much as possible. However, by setting the addition amount to 0.02% by weight or less, the above-described tendency of material deterioration is saturated, so the upper limit of the addition amount is set to 0.02% by weight. When particularly severe cold workability is required, it is desirable that the amount of S be 0.01% by weight or less.
[0013]
  Al is used as a deoxidizing agent together with Si. In order to exert this effect, it is necessary to contain 0.01% by weight or more in steel. -Way, AlWhen the amount of addition exceeds 0.1% by weight, the amount of Al-based oxide in the steel increases, and particularly the toughness is deteriorated. Therefore, the upper limit of the amount of addition is set to 0.1% by weight..
[0014]
  Nb is the most important additive element in the present invention. In many cases, Nb is usually added as a precipitation strengthening element, and in this case, it is common to design the components so that almost all Nb elements take the form of precipitates by combining with C or N. is there. However, precipitates that increase the strength of such steels simultaneously increase the yield strength at room temperature, making it difficult to obtain a low YR.
[0015]
  -On the other hand, according to research by the present inventors, it has been found that Nb existing in a solid solution state not only significantly improves the toughness at room temperature but also suppresses the deterioration of toughness after cold working. Furthermore, high-temperature strength is achieved by effectively interacting with dislocations during deformation at high temperatures and effectively suppressing dislocation movements and grain boundary movements. Therefore, it is necessary to adjust the added amount so that the added Nb is not consumed due to the bond with C or N. In order to increase the strength at high temperatures, it is desirable to increase the amount of Nb added. However, if the amount exceeds 1% by weight, the effects of improving toughness and increasing the high-temperature strength are saturated and the manufacturing cost is increased. Therefore, the amount of Nb added was set to 1% by weight or less. When the Nb addition amount is less than 0.04% by weight, even if other additive elements are adjusted, high toughness and high-temperature strength cannot be obtained, so the lower limit of the addition amount is 0.04% by weight. It was.
[0016]
  Ti that is selectively added binds to C and N, which may bind to Nb, and improves toughness by reducing the waste of solid solution Nb effective in improving toughness and increasing strength at high temperatures. Therefore, it is desirable to add 0.002% by weight or more. On the other hand, if the amount added exceeds 0.25 wt%, the workability is deteriorated, the steel material strength is unnecessarily increased, and the YR is increased at room temperature. It was 25% by weight.
[0017]
  B isIn addition to reducing the waste of solid solution Nb effective in increasing toughness and strength at high temperatures, it is possible to combine with Nb, which may combine with Nb. The effect of solid solution Nb on toughness and high temperature strength is promoted by lowering the mobility. If the addition amount of B is less than 0.0001% by weight, the effect is not recognized, and if the addition amount exceeds 0.01% by weight, the effect is saturated, so the addition amount is 0.0001 to 0.01% by weight. And
[0018]
  In addition to the above components, Cu, Ni, Mo, V, and Cr that are selectively added are effective steel-strengthening elements, and at the same time, they also have an effect of increasing toughness, fatigue strength, and high-temperature strength. 0.005 wt%, 0.005 wt%, 0.005 wt%, 0.002 wt%, 0.005 wt% or more is desirable. However, with the addition amount of these elements, the Cu addition amount exceeds 2% by weight, the Ni addition amount exceeds 1.5% by weight, the Mo addition amount exceeds 1% by weight, the V addition amount exceeds 0.25% by weight, When the amount of Cr added exceeds 1% by weight, the amount of increase in yield strength at room temperature increases, YR at room temperature is maintained at 75% or less, and the equivalent strain is 5% after cold working. Since it becomes difficult to keep YR at 90% or less, these are set as the upper limit of the addition amount of each element.
[0019]
  Even within this range, if the total amount of one or more of these additives exceeds 2.5% by weight, the amount of increase in yield strength at room temperature increases, and the YR at room temperature is 75%. It is difficult to keep the YR after cold working of 5% with an equivalent strain at 90% or less, and this is set as the upper limit of the total amount of one or more of the above elements. These elements may be positively added, and mixing from scraps or the like may be used effectively.
[0020]
  Ca and Rem that are selectively added are elements that improve the sour resistance, toughness, weldability, and the like by controlling the form of the sulfide. However, when Ca is less than 0.0005% by weight and Rem is less than 0.001% by weight, the effect is not exhibited, and when Ca is more than 0.005% by weight and Rem is more than 0.02% by weight, In order not only to saturate these effects but also to deteriorate the toughness due to oxides, these were made the upper limit and the lower limit of each addition amount.
[0021]
  When the components are adjusted within the above limited range, as described above, the solid solution Nb has the function of increasing the toughness and the strength at high temperature. Therefore, it is necessary to secure a certain amount or more of the solid solution Nb. Regardless of the presence or absence of cold working (equivalent to 5% cold working with equivalent strain), when the Nb content satisfies the following formula (2), both low YR at normal temperature and high toughness are compatible. .
    Nb ≧ 0.1 + 7.74C-1.94Ti + 6.63 (N-1.30B) ・・・ (2) Formula
  However, when Ti content and B content are 0, Ti = 0 in the above formulaTo.
As a result, when the Nb content satisfies the above equation, the YR after cold working of 5% with considerable strain is 90% or less as shown in FIG.
[0022]
  FIG.Stipulated in the present inventionAfter casting the steel composed of the components, the steel sheet is made into a hot-rolled steel sheet under the production conditions within the scope of the present invention. (Selected as representative value) YR at normal temperature after the measurement was measured, and YR after 5% cold rolling was taken as the vertical axis in FIG. .1 + 7.74C-1.94Ti + 6.63 (N-1.30B)} is plotted on the horizontal axis. Further, when the Nb addition amount satisfies the formula (2), as shown in FIG. 2, the yield stress (0.2% yield strength) at normal temperature and 600 degrees, regardless of whether or not 5% cold work is performed. ) Ratio is a high value of 0.6 or more, and it is understood that the strength at high temperature is also excellent.
[0023]
  In order to obtain a steel plate having such a material, it is essential to appropriately control the microstructure of the steel plate. The microstructure of the steel sheet of the present invention is polygonal ferrite (having a clear grain boundary as observed at the position corresponding to the L section of hot rolling, and is the ratio of the grain size in the L direction to the grain size in the plate thickness direction. Grains with an axial ratio of 2.5 or less are defined as polygonal ferrite.) Low-temperature formation phase containing bainite (Here, low-temperature formation phase other than polygonal ferrite includes bainite, massive ferrite, martensite, etc.) It is a mixed tissue. Examples of these microstructure photographs are shown in FIG.
[0024]
  In steels in which most of C is fixed in the form of precipitates by carbide-forming elements such as Nb, bainite, massive ferrite, and martensite exhibit a similar structure. In steels within the scope of the present invention, most of the structures other than polygonal ferrite judged by an optical microscope are bainite and massive ferrite, and as shown in FIG. Even if the ratio is less than this, the grain boundary may not be linear, or it may corrode deeply, which may be caused by defects in the grain. In addition, at least at a level of 500 times or less of the optical microscope, grains recognized as cementite are not observed even after picric acid corrosion. When observed at a magnification of the electron microscope level, very fine carbides may be contained inside the bainite grains, and further, an island-like structure containing martensite and a part of austenite may be observed.
[0025]
  In the case where the steel components satisfy all the above requirements, and when the area ratio of the low-temperature formation phase including bainite is 5% or less on the average in the sheet thickness direction in the observation at the position corresponding to the L section of hot rolling When sufficiently high temperature strength cannot be obtained, especially when cold working is performed, the ratio of YS at 600 ° C. to YS at room temperature is low, and low YR at room temperature and high at high temperature. Since it is difficult to achieve both yield strengths, this is set as the lower limit of the area ratio of the low-temperature generation phase including bainite on the average in the thickness direction.
[0026]
  Moreover, when the area ratio of the low-temperature production | generation phase containing a bainite increases, the intensity | strength of steel will raise, but the yield strength in normal temperature will raise simultaneously. In particular, when the area ratio of low-temperature formation phase including bainite in the thickness direction average is 90% or more, the amount of increase in yield strength at room temperature increases, and the YR at room temperature is kept at 75% or less, corresponding strain. Therefore, it is difficult to keep the YR after cold working of 5% at 90% or less, and this is set as the upper limit of the area ratio of the low-temperature generation phase including bainite in the plate thickness direction average.
[0027]
  In order to obtain a microstructure in such a range, a combination of appropriate components and manufacturing conditions is required. In particular, Nb, Mn, and B are effective elements for obtaining a low-temperature production phase containing bainite. If each of them is within the scope of the present invention, low-temperature production containing bainite is performed within a wide range of production conditions as shown below. The phase area ratio can be limited to more than 5% and less than 95%.
[0028]
  The reason for exhibiting excellent toughness when a low-temperature generation phase containing bainite having such an area ratio exists is not clarified at present, but dislocation migration is hindered by solute Nb, It is considered that the grain boundary carbides often observed in the steels of No. 1 are reduced, and that the composite structure of the low-temperature formation phase containing polygonal ferrite and bainite works effectively. The reason why the high-temperature strength, particularly the high-temperature strength after cold working is increased is not clear at present, but a low-temperature formation phase in which a solid solution Nb or a cluster-like Nb and an aggregate of Nb and C contains bainite. And dislocations introduced by cold working are less likely to recover even at high temperatures by interacting with dislocations associated with transformations introduced into the surrounding ferrite grains with the formation of these low-temperature formation phases. I think.
[0029]
  In addition, since the steel of the present invention includes a low-temperature generation phase containing an appropriate amount of bainite from the beginning and at the same time reduces the solid solution C and N in the steel, it receives a rapid thermal history introduced by welding. However, there is little change in the hardness of the welded part and the HAZ part, and as a result, various material deteriorations (toughness, fatigue characteristics, workability, etc.) of the welded part are suppressed.
[0030]
  The microstructure is determined at the position corresponding to the L cross section of the hot-rolled steel sheet, from the surface of the sheet thickness toward the center of the sheet thickness. It observed with the microscope, the area ratio of each structure | tissue in each position was measured and averaged using the point count method after photography. At this time, the area ratio of the tissue may be measured using other methods such as image processing.
[0031]
  When manufacturing such a steel plate, the steel having the above components is cast, and the obtained steel piece is directly or once cooled to below the Ar3 transformation point including room temperature, and then reheated to perform hot working. Further, the reheating temperature at this time is not particularly limited, but is preferably in a range of 1000 ° C. to 1300 ° C. in consideration of productivity and manufacturing cost.
[0032]
  When hot working is performed in the form of a plate, it may be a plate-like rolling like a thick plate or continuous hot rolling. Furthermore, a plurality of slabs are connected continuously at the finishing hot rolling entrance of continuous hot rolling. It may be hot rolled.
[0033]
  The hot working may be a steel pipe manufacturing process, or may be seamless steel pipe manufacturing or hot extrusion.
[0034]
  The end temperature of the hot working at this time is set to Ar3 transformation point -50 ° C. or higher determined by the steel components. If the hot working completion temperature is lower than this, the ferrite work structure remains on the steel sheet, not only the cold workability is deteriorated, but also the YR at room temperature is increased. Set to the lower limit of the end-of-work temperature. The upper and lower limits of the hot working start temperature and the upper limit of the end temperature are not particularly limited, but the hot working end temperature is preferably 1000 ° C. or less from the viewpoint of productivity and the surface quality due to the scale.
[0035]
  After hot working, the steel sheet is cooled to room temperature. At this time, if the average cooling rate to the cooling end temperature after hot working is less than 0.1 ° C / sec, the microstructure becomes coarse during cooling. In order to lower the strength of the steel sheet unnecessarily and deteriorate the toughness of the steel sheet, this was made the lower limit of the cooling rate. In addition, if the average cooling rate up to the cooling end temperature exceeds 80 ° C / sec, an increase in the area ratio of the low-temperature formation phase such as bainite is caused, and an appropriate microstructure cannot be obtained. It was. When the end temperature of this cooling is over 770 ° C., the microstructure becomes coarse and deteriorates toughness, so this was made the upper limit. For the purpose of reducing the final scale of the steel sheet surface and improving the surface quality, it is desirable to limit the cooling end temperature to 650 ° C. or lower. There is no need to set the lower limit of the cooling end temperature, but if the cooling end temperature is 100 ° C or less, a cooling rate higher than necessary is required within the limited manufacturing process length, which is an economic disadvantage. In order to produce the above, it is desirable to exceed 100 ° C.
[0036]
  In addition, after completion of cooling, in the case of continuous hot rolling, it is wound up, and in other cases it is air-cooled. However, if it is desired to reduce the material variation in the steel sheet, the cooling stop temperature should be 400 ° C or higher. Is desirable.
[0037]
  In addition, when such a steel sheet is made into a predetermined shape by cold working after hot working, predetermined cold working (cold rolling, cold bending, cooling) is applied to the hot worked steel sheet manufactured by the above method. Hot pressing, cold forging, cold pipe expansion, etc.) and heat treatment is performed as it is or after that to obtain a product. At this time, if the heat treatment temperature is less than 450 ° C., the YR drop at room temperature is not sufficient, and if it exceeds 950 ° C., the production cost is increased.
[0038]
  When manufacturing a steel pipe using such a steel plate, when making a steel pipe of a predetermined cross-sectional shape by hot working (including rolling at high temperature, roll forming, forging, bending, pipe expansion, etc.) The method according to the above hot working conditions may be used. In addition, when the steel plate manufactured by the above manufacturing method is cold worked and then made into a steel pipe by welding, the steel pipe may be once shaped by welding and then further processed into a predetermined cross-sectional shape by cold working. Moreover, the case where a round steel pipe is formed by welding and then a square steel pipe is formed by roll forming is also included. Here, the cold working includes cold roll forming, cold press forming, cold forging, cold pipe expansion, and the like. Moreover, these steel plates and steel pipes may be used as they are or after being subjected to a surface treatment such as hot dip galvanization.The steel satisfying the components of the present invention is a hot-rolled steel sheet, then rounded by welding, and then formed into a square steel pipe by cold roll forming. As shown, even if the ratio between the plate thickness and the outer diameter of the square steel pipe changes to 3.6 to 6.4, the steel according to the present invention has a YR of 90% or less after the pipe making, It has excellent toughness where the vTrs of the flat portion is −40 ° C. or less and the absorbed energy at −40 ° C. is 47 J or more, and the yield strength of the flat portion at 600 ° C. is excellent high temperature strength of 197 MPa or more.
[0039]
【Example】
  The results of investigating the mechanical properties at room temperature and 600 ° C. of steel plates cast from steel having the respective components shown in Table 1 and completed hot rolling under the conditions shown in Table 2 are shown in the same table. A cold work of 5% at a considerable strain was selected as a representative strain introduced into the flat portion of the steel pipe when the steel sheet was produced into a square steel pipe. Although the strain actually introduced is considered to be about 2.5% to 6% in terms of equivalent strain, it has been found that evaluation with an equivalent strain amount of about 5% is appropriate for representing the characteristics of the steel sheet. In addition, 5% cold working was selected here. The method of providing cold working was compared to actual square steel pipe manufacturing, bending, pressing, and cold rolling, but YR at normal temperature, toughness, and yield strength at 600 ° C were used in these working forms. Since there was no difference, it was represented here by 5% cold rolling.
[0040]
[Table 1]

[0041]
[Table 2]

[0042]
  Steel which is the component range of the present invention12As shown in Table 2, as shown in Table 2, when the production conditions are within the scope of the present invention, the YR at room temperature of the steel sheet as produced (indicated as unprocessed material YR (%) in Table 2) is 75% or less, and YR after cold working (corresponding to 5% pre-processed material YR (%) in Table 2) of 90% or less, and vTrs of −40 ° C. or less. In addition, it can be seen that the absorbed energy at 0 ° C. has excellent toughness of 47 J or more, and the 5% pre-processed material has a high temperature strength at 600 ° C. of 197 MPa.
[0043]
  On the other hand, any of the components of Steels 20 to 26 in Table 1 is outside the scope of the present invention. As a result, as shown in Table 2, even if the production conditions are within the scope of the present invention, the as-manufactured steel sheets. Any of YR at room temperature, YR after 5% cold working with equivalent strain, or vTrs falls outside the scope of the present invention..
[0044]
  In addition, Ar3 temperature (° C.) was calculated using Ar3 = 901-325 ×% C + 33 ×% Si-92 × (% Mn +% Ni / 2 +% Cr / 2 +% Cu / 2 +% Mo / 2 +% Nb / 2) .
[0045]
  (Delete)
[0046]
  (Delete)
[0047]
  In this experiment, a mechanical test at room temperature was performed according to JISZ2241 using a JIS No. 5 test piece, and a tensile test at 600 ° C. was performed according to JISG0567. The impact test was conducted according to JISZ2202.
[0048]
【The invention's effect】
  As described above, according to the present invention, a steel sheet excellent in toughness with a low YR at room temperature after cold working of 5% as-manufactured and equivalent strain and high yield stress at high temperature, and It is possible to produce a steel pipe having a low YR after pipe, high toughness and high yield stress at high temperature, and exhibit excellent effects in the civil engineering and construction fields where these characteristics are required.
[Brief description of the drawings]
[Fig. 1] Calculated solid solution Nb content of steel sheet = Nb- {0.1 + 7.74-1.94Ti + 6.63 (N-1.30B)} (wt%), after 5% cold working with equivalent strain (in the figure It is a figure which shows the relationship with YR in normal temperature of 5% pre-processing and display).
[Figure 2] Calculated solid solution Nb amount of steel sheet = Nb- {0.1 + 7.74-1.94Ti + 6.63 (N-1.30B)} (% by weight), as manufactured (indicated as unprocessed material in the figure), and It is the figure which showed the relationship between the yield strength (YS) in 600 degreeC after the cold work of 5% by a considerable distortion (indicated in the figure as 5% pre-processed material), and the yield strength at normal temperature.
FIG. 3Steel composed of the components specified inIs a micrograph of the microstructure in the thickness direction 1/4 portion from the surface layer of the hot-rolled steel sheet obtained by hot rolling under the production conditions within the scope of the present invention. However, (a) is a photomicrograph taken at 200x, and (b) is a photomicrograph taken at 500x.

Claims

% By weight
C: 0.03% or less,
Si: 1% or less,
Mn: 0.1 to 2%,
S: 0.02% or less,
Al: 0.01 to 0.1%,
Nb: 0.04 to 1%
B: 0.0001 to 0.01%
The balance is Fe and inevitable impurities, the Nb content satisfies the following formula (2), and the microstructure is a mixed structure of polygonal ferrite and a low-temperature generation phase containing bainite, The area ratio of the low-temperature generation phase including bainite averaged in the thickness direction is more than 5% and less than 90%, vTrs is −40 ° C. or less, the absorbed energy at 0 ° C. is 47 J or more, and 600 ° C. at room temperature. The yield strength ratio is 0.6 or more, the YR at room temperature is 75% or less, and the YR at room temperature after cold working of 5% with a corresponding strain is 90% or less, and the yield strength at 600 ° C. is A low-yield-ratio refractory steel sheet excellent in toughness, characterized by being 197 MPa or more.
Nb ≧ 0.1 + 7.74C-1.94Ti + 6.63 (N-1.30B) ・・・ (2) Formula

As a steel component, in addition,
Ti: 0.25% or less, The low yield ratio type steel sheet for fire resistance excellent in toughness according to claim 1 .

As a steel component, in addition,
Cu: 2% or less,
Ni: 1.5% or less,
Mo: 1% or less,
V: 0.25% or less,
The low yield ratio type steel sheet for fire resistance with excellent toughness according to claim 1 or 2 , characterized by containing not more than 2.5% by weight of Cr: 1% or less.

As a steel component, in addition,
Ca: 0.0005 to 0.005%,
Rem: 0.001 to 0.02%
The low yield ratio type steel sheet for fire resistance excellent in toughness according to any one of claims 1 to 3 , characterized by comprising one or two of the following.

The low yield ratio type steel sheet for fireproofing with excellent toughness according to any one of claims 1 to 4 , wherein the average grain size of polygonal ferrite is 10 µm or less.

It is a method of manufacturing the steel plate of any one of Claims 1-5 , Comprising: The steel slab after casting is cast again or once it cools to below Ar3 transformation point, it heats again, and predetermined shape is carried out by hot processing When processing into a low toughness with excellent toughness, characterized in that the processing end temperature is Ar 3-50 ° C. or higher and then cooling to 770 ° C. or lower at a cooling rate of 0.1 ° C./sec to 80 ° C./sec. A method for producing a specific steel sheet for fireproofing.

The method for producing a low yield ratio type fire-resistant steel sheet excellent in toughness according to claim 6 , wherein the hot-worked steel sheet is further cold-worked after cooling.

The method for producing a low-yield ratio type steel sheet for fireproofing with excellent toughness according to claim 7 , wherein heat treatment is further performed at a temperature of 450 ° C or higher and 950 ° C or lower after cold working.

A steel plate according to any one of claims 1 to 5 , wherein the steel plate is a steel pipe and has a low yield ratio type fireproof steel pipe excellent in toughness.

A method for producing a steel pipe according to claim 9 , wherein the cast steel slab is heated as it is after it is cast or once cooled to the Ar3 transformation point or less, and then the processing end temperature is Ar3-50 ° C or higher. A method for producing a low-yield ratio type refractory steel pipe excellent in toughness, which is formed by hot working and then cooled to 770 ° C. or less at a cooling rate of 0.1 ° C./sec to 80 ° C./sec.

A method for producing a steel pipe according to claim 9 , wherein the cast steel slab is heated as it is after it is cast or once cooled to the Ar3 transformation point or less, and then the processing end temperature is Ar3-50 ° C or higher. A method for producing a low yield ratio type refractory steel pipe excellent in toughness, characterized by forming into a predetermined shape by hot working and then cooling to 770 ° C. or less at a cooling rate of 0.1 ° C./sec to 80 ° C./sec .

It is a method of manufacturing the steel pipe according to claim 9, wherein after the cast steel piece is cast or once cooled to the Ar3 transformation point or less, it is heated again, and then the processing end temperature is Ar3-50 ° C or higher. It is excellent in toughness characterized by being processed and then cooled to 770 ° C. or less at a cooling rate of 0.1 ° C./sec or more and 80 ° C./sec or less, and welding the obtained hot-rolled steel plate A low yield ratio type fireproof steel pipe manufacturing method.

13. The method for producing a low yield ratio type fireproof steel pipe excellent in toughness according to claim 12 , further comprising performing cold working for forming after pipe forming by welding.

13. The method of manufacturing a low yield ratio type fireproof steel pipe excellent in toughness according to claim 12 , wherein the steel pipe having a circular cross section is formed by welding and then formed into a square cross section by roll forming.