JP2004346355A

JP2004346355A - Method for producing electroseamed steel pipe for high-strength line pipe excellent in hydrogen-crack resistance

Info

Publication number: JP2004346355A
Application number: JP2003142995A
Authority: JP
Inventors: Yoshio Yamazaki; 義男山崎; Mitsuo Kimura; 光男木村; Takaaki Toyooka; 高明豊岡
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2003-05-21
Filing date: 2003-05-21
Publication date: 2004-12-09
Anticipated expiration: 2023-05-21
Also published as: JP4165292B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an electroseamed steel pipe for a line pipe excellent in hydrogen-crack resistance and toughness, which is capable of solving the problem of quality deterioration at pipe-making and taking advantage of a uniform and fine metallic structure of hot-rolled steel sheet before the pipe-making. <P>SOLUTION: A steel raw material comprises specific amounts of C, Si, Mn, P, S, Al, N and O and a specific amount of at least one element chosen from Nb, V, Ti, Mo, B and Ca. The steel raw material is heated to ≥1,100°C, subjected to hot-rolling that yields a rolling reduction ratio of ≥50% within a non-recrystallization temperature range equal to or above Ar<SB>3</SB>, coiled at ≤600°C and processed into the steel pipe through an electroseamed steel pipe process. The steel pipe is heated by high-frequency heating so that the temperature in the coldest part falls within 500-650°C and that the temperature difference between the hottest and coldest parts in the thickness direction becomes ≥70°C, held at that temperature or annealed for ≤30 s and cooled at an average cooling rate of 5-30°C/s. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、耐水素割れ特性に優れる高強度ラインパイプ用電縫鋼管の製造方法に関し、詳しくは、肉厚２５ｍｍ以下のＡＰＩＸ６０級以上の高強度ラインパイプ用電縫鋼管に、優れた耐水素割れ特性を効率的に付与しうる耐水素割れ特性に優れる高強度ラインパイプ用電縫鋼管の製造方法に関する。
【０００２】
【従来の技術】
一般に、電縫鋼管は、ＵＯ鋼管と比べて造管方法の違いから同じ素材を用いても、耐水素割れ特性や低温靭性で劣ることから、造管ままでＵＯ鋼管と同等の特性を得るためには、素材の特性がより優れたものを使用する必要があり、不利であった。
【０００３】
この不利を、電縫鋼管に熱処理を施すことで解消しようとする技術として、電縫鋼管に耐サワー性、低温靭性、低降伏比を同時に付与するために、電縫鋼管全体を８００℃以上で加熱し、その後鋼管を焼入れするという方法が提案されている（特許文献１参照。）。この方法では、成分組成がＣ≦０．１２％、Ｍｎ：０．５〜１．４％、Ｓｉ：０．１０〜０．２５％、Ｐ≦０．０１５％、Ｓ≦０．００２０％、Ｃａ：０．００１０〜０．００６０％の範囲内にある低炭素鋼の電縫鋼管を、８００℃以上のＡ_３変態点以上のオーステナイト状態にし、冷間歪を除去し、その後焼入れすることで、アシキュラーフェライトまたは低炭素型ベイナイト組織とし、焼戻しは行わないことを特徴としている。
【０００４】
【特許文献１】
特公平６−６３０４０号公報
【０００５】
【発明が解決しようとする課題】
しかしながら、上記特許文献１所載の方法は、電縫鋼管全体を８００℃以上のＡ_３変態点以上のオーステナイト状態にするため、制御圧延‐制御冷却によって折角微細化された造管前鋼板組織を全く活かすことができないという問題がある。また、管全体を加熱するには通常雰囲気炉加熱（所謂バッチ式加熱）が用いられるが、バッチ式加熱では、炉内温度の場所によるばらつきなどがあって、管全体を一様な温度にすることが難しく、そのため、組織を均一微細に制御することが困難であるという問題がある。
【０００６】
本発明は、上記のような従来技術の問題点に鑑み、造管前の均一微細な熱延鋼板組織を有効に活用しつつ、造管時の材質劣化の問題を容易に解決しうる耐水素割れ特性に優れる高強度ラインパイプ用電縫鋼管の製造方法を提供することを目的とする。
【０００７】
【課題を解決するための手段】
発明者らは、本発明に先駆する発明として、ＵＯ鋼管と同じ鋼成分組成の造管ままの電縫鋼管をＡｃ_１点前後の温度域に急速加熱、短時間保持、急速冷却することにより、優れた靭性および耐水素割れ特性が得られることを明らかにし、特願２００２−２８１５３７を出願した。一方で、６５０℃以下の低温加熱でも耐水素割れ特性は向上したが、短時間加熱故に生じる歪時効と思われる高ＹＲ（：降伏比）化の問題があり、ＡＰＩ規格を満たすことは困難であった。そこで発明者らは、これを解決すればさらに効率のよい生産が可能であると考え、肉厚内の温度差に着目して、鋭意検討、実験を行なって、本発明の基礎となった次のような結果を得た。。
【０００８】
発明者らは、Ｃ：０．０４％、Ｓｉ：０．２４％、Ｍｎ：１．１９％、Ｐ：０．０１０％、Ｓ：０．０００７％、Ａｌ：０．０３３％、Ｎｂ：０．０４７％、Ｖ：０．０４１％、Ｔｉ：０．０１１％、Ｎ：０．００２８％およびＯ：０．００１６％、Ｃａ：０．００２１％の鋼成分組成になるスラブを、１２００℃に加熱後、再結晶域圧延に次いで圧下率７０％の未再結晶域圧延を施し、５５０℃で巻き取って熱延コイルとした。この熱延コイルを素材として外径２４ｉｎ（１ｉｎ（インチ）＝２５．４ｍｍ）×肉厚１４．４ｍｍの電縫鋼管を造管し、その外表面より高周波加熱にて内外面の温度差すなわち肉厚内温度差が変わるように種々の熱処理条件にて、内表面温度５９０℃±１０℃へ加熱し、その温度にて３秒保持後水冷した。肉厚内温度差を変化させる手段としては、高周波のパワーや周波数を変えたり、加熱途中に数回にわたり短時間高周波加熱を停止させ肉厚内均熱化を図るといった手段を採用した。前記水冷後の鋼管の材料特性調査結果を図１に示す。同図より明らかなように、肉厚内温度差を７０℃以上とすることでＹＲ≦９３％となり、ＡＰＩ規格に適合することが判った。
【０００９】
これら特性に違いが出た理由の詳細は明らかでないが、肉厚内温度差が小さい場合の高ＹＲの原因は短時間加熱の故に、一種の歪時効と考えられる降伏ピークが原因であると推察され、これが肉厚内温度差を大きくすることによって肉厚内で異なる降伏点をもつ複合材料のように働き、降伏変形が肉厚内で拘束されたため降伏ピーク現象を生じなくなり、図１のような特性変化になったものと思われる。また、加熱温度を変化させてこの熱処理材の耐水素割れ特性を調査した結果、最低温度部である内面温度を５００℃以上、すなわち肉厚内の全域を５００℃以上とすることで造管歪緩和などにより改善されることも判った。
【００１０】
本発明は、以上のような知見に基づいてなされたもので、その要旨とするところは、質量％で、Ｃ：０．０１〜０．１０％、Ｓｉ：０．０５〜０．５％、Ｍｎ：０．５〜２．０％、Ｐ：０．０３％以下、Ｓ：０．００３％以下、Ａｌ：０．００５〜０．０５０％、Ｎ：０．００５０％以下、Ｏ：０．００３０％以下を含み、かつＮｂ：０．００５〜０．１％、Ｖ：０．００５〜０．１％、Ｔｉ：０．００５〜０．１％、Ｍｏ：０．０５〜０．５％、Ｂ：０．０００１〜０．００３０％、Ｃａ：０．０００５〜０．００６０％の１種または２種以上を含み、残部Ｆｅおよび不可避的不純物からなる鋼素材を、１１００℃以上に加熱し、Ａｒ_３点以上の未再結晶域での圧下率が５０％以上になる熱間圧延を行い、６００℃以下で巻き取ってコイルとした後、電縫鋼管プロセスにて鋼管とし、続いてこの鋼管を高周波加熱にて最低温度部が５００〜６５０℃の範囲内に収まり、かつ肉厚内での最高温度部と最低温度部の温度差が７０℃以上となるように加熱し、３０秒以下の保持または徐冷の後、平均冷却速度５〜３０℃／ｓで冷却することを特徴とする耐水素割れ特性に優れる高強度ラインパイプ用電縫鋼管の製造方法にある。
【００１１】
本発明では、前記鋼素材がさらに、質量％で、Ｎｉ：０．０５〜１．０％、Ｃｕ：０．０５〜１．０％、Ｃｒ：０．０５〜１．０％の１種または２種以上を含むものであってもよい。
【００１２】
【発明の実施の形態】
以下、本発明の実施の形態について、まず鋼成分組成の限定理由から、詳しく説明する。
Ｃ：０．０１〜０．１０％
Ｃは強度確保のために０．０１％以上含有することが必要であり、一方で０．１０％を超えると耐水素割れ特性および靭性が共に低下するため０．１０％以下とする。ＡＰＩＸ６０以上の高強度化と耐水素割れ特性および靭性をバランス良く達成させるためには、特に０．０２５〜０．０６％とすることが好ましい。
【００１３】
Ｓｉ：０．０５〜０．５％
Ｓｉは脱酸剤および強度確保元素として最低０．０５％を必要とするが、過剰に添加するとＨＡＺ（溶接熱影響部）靭性を低下させ、溶接上好ましくないため上限は０．５％とした。
Ｍｎ：０．５〜２．０％
Ｍｎは高強度化のために必要な元素であり０．５％以上を添加するが、一方２．０％を超えると母材靭性が劣化するばかりか、硬質偏析相を形成して耐水素割れ特性を著しく劣化するため０．５〜２．０％の範囲に限定する。特に優れた耐水素割れ性とするためには１．２％以下とすることが好ましい。
【００１４】
Ｐ：０．０３％以下
Ｐは粒界に偏析して粒界強度を低下させる元素であり、母材および溶接部の靭性を低下させるため、粒界割れ防止のために０．０３％を上限とした。特に高靭性を必要とする場合には、０．０１５％以下とすることが好ましい。
Ｓ：０．００３％以下
ＳはＭｎＳなどの硫化物として鋼中に存在し、耐水素割れ特性および靭性を著しく劣化させる元素で、その影響を抑制するためには０．００３％以下、特に高い水準の耐水素割れ特性を得るためには好ましくは０．００１５％以下、にする必要がある。
【００１５】
Ａｌ：０．００５〜０．０５０％
Ａｌは脱酸およびＮ固定のために必要であり、０．００５％以上添加する必要がある。一方、０．０５０％を超えるとアルミナ系介在物が増え、耐水素割れ特性および靭性を損なうため０．０５０％を上限とした。
Ｎ：０．００５０％以下
Ｎは０．００５０％を超えて存在すると、粗大な窒化物を形成して耐水素割れ特性および靭性を劣化させるため０．００５０％以下とした。
【００１６】
Ｏ：０．００３０％以下
Ｏは介在物として存在し、凝集粗大化した場合は水素割れの起点として働くため極力少ない方が好ましいが、０．００３０％以下であれば凝集粗大化しにくくなるため０．００３０％以下とした。特に優れた耐水素割れ特性を必要とする場合には、０．００２０％以下とすることが好ましい。
【００１７】
さらに本発明では、以下の成分を耐水素割れ特性向上や靭性向上、強度上昇を目的に１種または２種以上添加する。
Ｎｂ：０．００５〜０．１％
Ｎｂは微細な炭窒化物を形成し強度を増加させ、また熱間制御圧延の歪蓄積に有利に働き組織微細化により靭性も向上させる。しかし、０．００５％未満ではその効果はなく、０．１％を超えると溶接部靭性に好ましくない影響があるため０．００５〜０．１％に限定する。
【００１８】
Ｖ：０．００５〜０．１％
ＶはＮｂとほぼ同じ効果をもつ元素であるが、Ｎｂに比べて析出硬化能はやや劣る。０．００５％未満では硬化能に乏しく、０．１％を超えると溶接部靭性劣化を招くため、０．００５〜０．１％とする。
Ｔｉ：０．００５〜０．１％
Ｔｉは強い窒化物形成元素であり、Ｎ当量である（Ｎ％×（４８／１４））程度の添加でＮ時効を抑制し、またＢ添加がある場合はＢが鋼中ＮによりＢＮとして析出固定され、その効果が抑制されないように添加する。さらに添加することで微細な炭化物を形成して強度を増加させる。０．００５％未満では効果はなく、とくに（Ｎ％×（４８／１４））以上添加するのが好ましい。一方、０．１％を超えて添加すると、粗大な窒化物を形成しやすくなり靭性が劣化するため０．１％以下とする。
【００１９】
Ｍｏ：０．０５〜０．５％
Ｍｏは固溶しあるいは炭化物を形成して大きな靭性劣化を伴わずに強度を上昇する効果があるが、０．５％を超えるとその効果が飽和してくるばかりか、高価となるので０．５％以下の範囲で添加しても良い。なお強度上昇効果を発揮するためには０．０５％以上添加することが好ましい。
【００２０】
Ｂ：０．０００１〜０．００３０％
ＢはＮｂと同様に圧延材の組織制御に重要であり、その効果を発揮するには０．０００１％以上の添加が必要である。とくにＮｂと併用して添加すると相乗効果を示す。また粒界強化元素として粒界割れを抑制して靭性向上に寄与する。一方、過剰に添加してもその効果は飽和するばかりか、溶接部靭性を劣化するので０．００３０％を上限とする。
【００２１】
Ｃａ：０．０００５〜０．００６０％
Ｃａは水素割れの起点となる介在物の形態を球状に制御することを目的に添加するが、その効果を発揮するには０．０００５％以上必要で、一方０．００６０％を超えるとその効果は飽和するばかりか、粗大介在物を形成するので、０．０００５〜０．００６０％の範囲とする。
【００２２】
さらに本発明では、強度上昇を主目的として以下の元素を１種または２種以上添加することも可能である。
Ｎｉ：０．０５〜１．０％
Ｎｉは強度、靭性を向上させるに有効な元素である。またＣｕを添加した場合には圧延時のＣｕ割れを防止するにも有効であるが、高価である上、過剰に添加してもその効果が飽和するため０．０５〜１．０％の範囲に限定する。特にＣｕ割れの観点からは（Ｃｕ％×０．３）以上添加するのが好ましい。
【００２３】
Ｃｕ：０．０５〜１．０％
Ｃｕは強度、耐水素割れ特性を向上させるために添加するが、その効果を発揮するには０．０５％以上添加する必要があり、一方１．０％を超えると熱間脆化を引き起こしやすく、また靭性も低下するので０．０５〜１．０％の範囲とする。
【００２４】
Ｃｒ：０．０５〜１．０％
Ｃｒは強度上昇に有効であるが過剰に添加すると靭性を低下するため１．０％以下の範囲で添加しても良い。ただし０．０５％未満ではその効果を発揮しないため０．０５％以上添加することが好ましい。
次に、工程条件の限定理由を以下に述べる。
【００２５】
まず製鋼法については、常法に従って行なえばよく、それらの条件は特に限定されないが、介在物の浮上処理や凝集抑制などの低減対策をとることが好ましい。また鋳造時の鍛圧や均熱保持炉により、中心偏析の低減を図っても良い。
圧延については、本発明の特徴である耐水素割れ特性向上や靭性向上のためには、圧延前に炭化物を固溶させるべく１１００℃以上の加熱が必要であり、再結晶域での圧延は常法によればよいが、制御圧延として圧延仕上温度をＡｒ_３点以上とする圧下率５０％以上の未再結晶域圧延を必要とする。このとき鋳造後の鋳片を１１００℃未満に冷却することなく引き続いて圧延するか、もしくは１１００℃から常温までの冷却途上から１１００℃以上に加熱‐均熱後に圧延しても、本発明の特徴を損なうことはない。
【００２６】
さらに制御圧延の効果を有効に発揮すべく圧延後に６００℃以下の巻取温度でコイル化する必要がある。このとき、圧延終了から巻取り開始までの冷却方法および冷却速度は特に限定されないが、１０℃／ｓ以上の冷却速度を確保することが靭性向上に好ましく、組織の単相化が図れて耐水素割れ特性向上も期待できる。得られた熱延コイルを常法の電縫鋼管プロセスに従い鋼管とするが、このときに必然的に生じる造管ひずみのため、熱延コイル特性に対して大きく耐水素割れ特性や靭性が劣化する。これを抑制するために、造管ひずみを低減する造管方法（例えば、ＣＢＲ法など）を用いてもかまわない。
【００２７】
造管された鋼管はそのライン内で連続もしくは然るべき後に、本発明の特徴であるＹＲ低減を目的とした肉厚内温度差を７０℃以上とした短時間加熱‐冷却処理を行なう。肉厚内温度差を大きくするためには、高速加熱が有利であり、必然的に高周波加熱を必要とする。高周波加熱での加熱方法は特に問わない。肉厚内温度差を大きくするためには片面加熱が好ましいが、目的とする肉厚内温度差が得られるなら両面加熱でもかまわない。このとき耐水素割れ特性改善のために、最低温度部が５００℃以上になる加熱を必要とする。一方、最低温度部が６５０℃超になる範囲に加熱しても、耐水素割れ特性の改善効果は飽和し、また肉厚内温度差が小さくても高ＹＲ化しないため、最低温度部での温度の上限を６５０℃とする。
【００２８】
また、加熱時の均熱などの保持もしくは放冷などの徐冷は、肉厚内温度差を小さくしてしまい、この時間が３０秒を超えると７０℃以上の肉厚内温度差を得ることが困難となるため、かかる保持又は徐冷の時間は３０秒以下とし、加熱後は速やかに冷却に入るのが好ましい。
この冷却は、開始（最低温度部が５００〜６５０℃でかつ肉厚内温度差が７０℃以上になっている状態）から終了（降温途上で鋼管肉厚内全域が２００℃に到達した状態）までの平均冷却速度が５〜３０℃／ｓになるように行う必要がある。というのは、この冷却速度が５℃／ｓ未満では十分なＹＲ低下が得られず、一方、３０℃／ｓ超では設備的に多大な費用を必要とし、均一冷却も困難となるからである。なお、時効の観点から冷却終了温度は全肉厚で１００℃以下とすることが好ましい。
【００２９】
【実施例】
表１に示す成分組成になる鋼スラブを表２に示す加熱‐圧延‐冷却‐巻取り条件で熱間圧延し、得られた熱延コイルを素材として電縫鋼管プロセスにより鋼管を造管し、造管ままの鋼管、および造管後に表３に示す条件で熱処理を施した鋼管について、下記の要領によりＹＳ（降伏強度）、ＴＳ（引張強度）、ＹＲ（降伏比＝ＹＳ／ＴＳ）、ＣＬＲ（耐水素割れ特性指標である割れ長さ率）を測定した。
【００３０】
ＹＳ，ＴＳ，ＹＲ：ＡＰＩ５Ｌによる試験方法にて測定
ＣＬＲ：ＮＡＣＥＴＭ−０２−８４に従い実施
試験溶液：ＳｏｌｕｔｉｏｎＡ（０．５％酢酸＋５％ＮａＣｌ水溶液、ｐＨ２．７±０．１）
ガス：１００％Ｈ_２Ｓ
試験温度：２５℃／ｓ± ３℃
試験時間：９６時間
結果を表３に示す。表３に記した内外面温度差は、実施加熱条件（管外面からの高周波加熱）での加熱過程の終点における最高温度部と最低温度部の温度差である。
【００３１】
【表１】

【００３２】
【表２】

【００３３】
【表３】

【００３４】
本発明要件を満たす製造方法で製造された鋼管（発明例）はいずれも低ＹＲ化が達成され、ＡＰＩＸ６０〜Ｘ８０の要求強度特性を満たしながら、優れた耐水素割れ特性を示した。
【００３５】
【発明の効果】
本発明によれば、造管前の均一微細な熱延鋼板組織を有効に活用しつつ、造管時の材質劣化の問題を容易に解決しうる耐水素割れ特性に優れる高強度ラインパイプ用電縫鋼管が得られるという優れた効果を奏する。
【図面の簡単な説明】
【図１】電縫鋼管の熱処理における肉厚内温度差条件と該熱処理後のＹＳ、ＴＳ、ＹＲの関係の例を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-strength linepipe electric resistance steel pipe having excellent hydrogen cracking resistance, and more specifically, to an API X60 grade or higher high-strength linepipe ERW steel pipe having a wall thickness of 25 mm or less. The present invention relates to a method for producing an ERW steel pipe for high-strength line pipes, which has excellent hydrogen cracking resistance that can efficiently impart cracking characteristics.
[0002]
[Prior art]
In general, ERW steel pipes are inferior in hydrogen cracking resistance and low-temperature toughness even if the same material is used due to the difference in pipe making method compared to UO steel pipes. However, it was necessary to use a material with better material characteristics, which was disadvantageous.
[0003]
As a technique to overcome this disadvantage by applying heat treatment to ERW steel pipe, in order to simultaneously give sour resistance, low temperature toughness and low yield ratio to ERW steel pipe, A method of heating and then quenching the steel pipe has been proposed (see Patent Document 1). In this method, the component composition is C ≦ 0. 12%, Mn: 0. 5-1. 4%, Si: 0. 10-0. 25%, P ≦ 0. 015%, S ≦ 0. 0020%, Ca: 0. 0010-0. The electric resistance welded steel pipe of a low carbon steel is within the range of 0060%, the austenitic state more than A ₃ transformation point or higher 800 ° C., to remove the cold distortion, it then quenching, acicular ferrite or low carbon It is characterized by a bainite structure and no tempering.
[0004]
[Patent Document 1]
Japanese Examined Patent Publication No. 6-63040
[Problems to be solved by the invention]
However, the method of Patent Document 1 Shosai, in order to the entire electric resistance welded steel pipe austenitic state more than ₃ transformation point or higher 800 ° C. A, controlled rolling - the pipe-making before steel tissue much trouble miniaturized by controlled cooling There is a problem that it cannot be utilized at all. In addition, furnace heating (so-called batch type heating) is usually used to heat the entire tube, but batch type heating has a uniform temperature throughout the tube due to variations in the temperature in the furnace. Therefore, there is a problem that it is difficult to control the structure uniformly and finely.
[0006]
In view of the problems of the prior art as described above, the present invention is a hydrogen resistant material that can easily solve the problem of material deterioration during pipe making while effectively utilizing a uniform fine hot-rolled steel sheet structure before pipe making. An object of the present invention is to provide a method for producing an electric resistance welded steel pipe for a high-strength line pipe having excellent cracking characteristics.
[0007]
[Means for Solving the Problems]
The inventors, as an invention pioneering the present invention, rapidly heat-maintained electric welded steel pipes with the same steel composition as UO steel pipes in a temperature range around Ac ₁ point, hold for a short time, and rapidly cool, Clarified that excellent toughness and hydrogen cracking resistance can be obtained, and filed Japanese Patent Application No. 2002-281537. On the other hand, although the hydrogen cracking resistance improved even at low temperature heating of 650 ° C. or lower, there is a problem of high YR (yield ratio) that seems to be strain aging caused by short-time heating, and it is difficult to satisfy the API standard. there were. Therefore, the inventors believe that if this is solved, more efficient production is possible, paying attention to the temperature difference within the wall thickness, and carrying out intensive studies and experiments, which are the basis of the present invention. The result was as follows. .
[0008]
The inventors have C: 0.04%, Si: 0.24%, Mn: 1.19%, P: 0.010%, S: 0.0007%, Al: 0.033%, Nb: 0 0.047%, V: 0.041%, Ti: 0.011%, N: 0.0028% and O: 0.0016%, Ca: 0.0021% Steel composition of the slab to 1200 ° C After heating, the recrystallization zone rolling was followed by non-recrystallization zone rolling with a rolling reduction of 70%, and wound at 550 ° C. to obtain a hot rolled coil. Using this hot-rolled coil as a raw material, an electric resistance steel pipe having an outer diameter of 24 in (1 in (inch) = 25.4 mm) × thickness of 14.4 mm is formed, and the temperature difference between the inner and outer surfaces by the high-frequency heating from the outer surface, that is, the meat Under various heat treatment conditions so as to change the temperature difference within the thickness, the inner surface temperature was heated to 590 ° C. ± 10 ° C., held at that temperature for 3 seconds, and then cooled with water. As means for changing the temperature difference in the wall thickness, means such as changing the power and frequency of the high frequency, or stopping the high-frequency heating for a short time several times during the heating to achieve uniform temperature in the wall thickness. FIG. 1 shows the results of the material property investigation of the steel pipe after water cooling. As is clear from the figure, it was found that YR ≦ 93% when the temperature difference in the wall thickness was set to 70 ° C. or more, and conformed to the API standard.
[0009]
Although the details of the reason for the difference in the characteristics are not clear, it is assumed that the cause of high YR when the temperature difference in the wall thickness is small is due to a yield peak that is considered to be a kind of strain aging due to short-time heating. This works like a composite material with different yield points within the wall thickness by increasing the temperature difference within the wall thickness, and since yield deformation is constrained within the wall thickness, the yield peak phenomenon does not occur, as shown in FIG. It seems that it became a characteristic change. In addition, as a result of investigating the hydrogen cracking resistance of this heat-treated material by changing the heating temperature, the inner surface temperature, which is the lowest temperature part, is 500 ° C or higher, that is, the entire region within the wall thickness is 500 ° C or higher. It was also found that it could be improved by mitigation.
[0010]
The present invention has been made on the basis of the above findings, and the gist thereof is mass%, C: 0.01 to 0.10%, Si: 0.05 to 0.5%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.003% or less, Al: 0.005 to 0.050%, N: 0.0050% or less, O: 0.0. Nb: 0.005-0.1%, V: 0.005-0.1%, Ti: 0.005-0.1%, Mo: 0.05-0.5% , B: 0.0001 to 0.0030%, Ca: 0.0005 to 0.0060%, or a steel material composed of Fe and unavoidable impurities is heated to 1100 ° C. or higher. , Ar Hot rolling is performed so that the reduction rate in the non-recrystallized region of ₃ or more points is 50% or more, and the coil is wound at 600 ° C. or less. After that, the ERW steel pipe process was used to make a steel pipe, and then this steel pipe was subjected to high-frequency heating so that the lowest temperature part was within the range of 500 to 650 ° C., and the highest temperature part and the lowest temperature part within the wall thickness. A high-strength line with excellent resistance to hydrogen cracking, characterized in that it is heated to a temperature difference of 70 ° C. or higher, held or gradually cooled for 30 seconds or less, and then cooled at an average cooling rate of 5 to 30 ° C./s. It exists in the manufacturing method of the ERW steel pipe for pipes.
[0011]
In the present invention, the steel material is further in mass%, Ni: 0.00. 05-1. 0%, Cu: 0. 05-1. 0%, Cr: 0. 05-1. It may contain 1% or 2 or more of 0%.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the embodiment of the present invention will be described in detail first from the reason for limiting the steel component composition.
C: 0. 01-0. 10%
C is 0. It is necessary to contain 01% or more. If it exceeds 10%, both hydrogen cracking resistance and toughness will decrease. 10% or less. In order to achieve a high strength, hydrogen cracking resistance and toughness with API X60 or higher in a well-balanced manner, it is particularly preferable to add 0. 025-0. It is preferable to set it as 06%.
[0013]
Si: 0. 05-0. 5%
Si is at least 0 as a deoxidizer and a strength securing element. 05% is required, but if added in excess, the HAZ (welding heat affected zone) toughness is lowered, and this is not preferable for welding, so the upper limit is set to 0. 5%.
Mn: 0. 5 to 2.0%
Mn is an element necessary for increasing the strength. Addition of 5% or more, on the other hand, exceeding 2.0% not only deteriorates the toughness of the base metal, but also forms a hard segregation phase and remarkably deteriorates the hydrogen cracking resistance. 5-2. Limited to a range of 0%. In order to achieve particularly excellent hydrogen cracking resistance, It is preferable to make it 2% or less.
[0014]
P: 0. P of 03% or less is an element that segregates at the grain boundary and lowers the grain boundary strength, and lowers the toughness of the base metal and the welded portion. The upper limit was 03%. In particular, when high toughness is required, 0. It is preferable to set it to 015% or less.
S: 0.003% or less S is an element that exists in steel as a sulfide such as MnS, and significantly deteriorates the hydrogen cracking resistance and toughness. In order to obtain a level of hydrogen cracking resistance, it is preferable that the content be 0.0015% or less.
[0015]
Al: 0. 005-0. 050%
Al is necessary for deoxidation and N fixation. It is necessary to add 005% or more. On the other hand, 0. If it exceeds 050%, alumina inclusions increase, and the hydrogen cracking resistance and toughness are impaired. The upper limit was set to 050%.
N: 0. 0050% or less N is 0. If the content exceeds 0050%, coarse nitrides are formed to deteriorate the hydrogen cracking resistance and toughness. 0050% or less.
[0016]
O: 0. 0030% or less O is present as an inclusion, and when it is agglomerated and coarsened, it acts as a starting point for hydrogen cracking. If it is 0030% or less, it becomes difficult to agglomerate and coarsen. 0030% or less. When particularly excellent hydrogen cracking resistance is required, the It is preferable to set it to 0020% or less.
[0017]
Furthermore, in this invention, the following components are added 1 type (s) or 2 or more types for the purpose of a hydrogen cracking-proof characteristic improvement, toughness improvement, and a strength increase.
Nb: 0. 005-0. 1%
Nb forms fine carbonitrides to increase the strength, and also favors strain accumulation in hot controlled rolling and improves toughness by refining the structure. However, 0. Less than 005% has no effect. If it exceeds 1%, the weld toughness is unfavorably affected. 005-0. Limited to 1%.
[0018]
V: 0. 005-0. 1%
V is an element having almost the same effect as Nb, but the precipitation hardening ability is slightly inferior to Nb. 0. If it is less than 005%, the curability is poor. If it exceeds 1%, the weld toughness deteriorates. 005-0. 1%.
Ti: 0. 005-0. 1%
Ti is a strong nitride forming element, and N aging is suppressed by addition of about N equivalent (N% × (48/14)), and when B is added, B precipitates as BN by N in the steel. It is fixed so that the effect is not suppressed. Furthermore, the addition increases the strength by forming fine carbides. 0. If it is less than 005%, there is no effect, and it is particularly preferable to add (N% × (48/14)) or more. On the other hand, 0. If it exceeds 1%, coarse nitrides are easily formed and the toughness is deteriorated. 1% or less.
[0019]
Mo: 0. 05-0. 5%
Mo has the effect of increasing the strength without forming a solid solution or forming a carbide to cause significant deterioration in toughness. However, if it exceeds 0.5%, the effect becomes saturated and becomes expensive. You may add in 5% or less of range. In order to exhibit the effect of increasing the strength, it is preferable to add 0. It is preferable to add 05% or more.
[0020]
B: 0. 0001-0. 0030%
B, like Nb, is important for controlling the structure of the rolled material. Addition of 0001% or more is necessary. In particular, when added in combination with Nb, a synergistic effect is exhibited. Moreover, it contributes to the improvement of toughness by suppressing grain boundary cracking as a grain boundary strengthening element. On the other hand, even if added excessively, the effect is not only saturated, but also the toughness of the welded portion is deteriorated, so that it is 0. The upper limit is 0030%.
[0021]
Ca: 0. 0005-0. 0060%
Ca is added for the purpose of controlling the shape of inclusions, which are the starting point of hydrogen cracking, to a spherical shape. 0005% or more is necessary, while 0. If it exceeds 0060%, the effect is not only saturated but also coarse inclusions are formed. 0005-0. The range is 0060%.
[0022]
Furthermore, in the present invention, one or more of the following elements can be added for the purpose of increasing strength.
Ni: 0. 05-1. 0%
Ni is an element effective for improving strength and toughness. Further, when Cu is added, it is effective in preventing Cu cracking during rolling, but it is expensive and the effect is saturated even if it is added excessively, so the range is 0.05 to 1.0%. Limited to. In particular, from the viewpoint of Cu cracking, (Cu% × 0.3) or more is preferably added.
[0023]
Cu: 0. 05-1. 0%
Cu is added in order to improve strength and hydrogen cracking resistance. It is necessary to add 05% or more. If it exceeds 0%, hot embrittlement is likely to occur, and the toughness is also reduced. 05-1. The range is 0%.
[0024]
Cr: 0. 05-1. 0%
Cr is effective in increasing the strength, but if added excessively, the toughness is reduced. You may add in 0% or less of range. However, if it is less than 0.05%, the effect is not exhibited. It is preferable to add 05% or more.
Next, the reasons for limiting the process conditions will be described below.
[0025]
First, the steel manufacturing method may be performed according to a conventional method, and the conditions are not particularly limited, but it is preferable to take reduction measures such as the floating treatment of inclusions and the suppression of aggregation. Further, the center segregation may be reduced by a forging pressure at casting or a soaking furnace.
As for rolling, in order to improve the hydrogen cracking resistance and toughness, which are the characteristics of the present invention, heating at 1100 ° C. or higher is required to dissolve the carbide before rolling, and rolling in the recrystallization region is usually performed. Although it may be based on the method, it requires non-recrystallized zone rolling with a rolling reduction of 50% or more with a rolling finishing temperature of Ar ₃ points or more as controlled rolling. At this time, the cast slab is continuously rolled without being cooled to less than 1100 ° C., or even after being heated from 1100 ° C. to room temperature and heated to 1100 ° C. or higher and rolled after soaking, the features of the present invention Will not be damaged.
[0026]
Furthermore, it is necessary to coil at a coiling temperature of 600 ° C. or lower after rolling in order to effectively exhibit the effect of controlled rolling. At this time, the cooling method and cooling rate from the end of rolling to the start of winding are not particularly limited, but securing a cooling rate of 10 ° C./s or more is preferable for improving toughness, and a single phase of the structure can be achieved and hydrogen resistance is improved. Improvement in cracking characteristics can also be expected. The obtained hot-rolled coil is made into a steel pipe according to the conventional ERW steel pipe process. However, due to the pipe-forming strain that inevitably occurs at this time, the resistance to hydrogen cracking and toughness are greatly deteriorated with respect to the hot-rolled coil characteristics. . In order to suppress this, a pipe making method (for example, a CBR method or the like) for reducing the pipe making strain may be used.
[0027]
The formed steel pipe is subjected to a short-time heating-cooling process in which the temperature difference in the wall thickness is 70 ° C. or more for the purpose of reducing the YR, which is a feature of the present invention, continuously or appropriately after that in the line. In order to increase the temperature difference within the thickness, high-speed heating is advantageous, and high-frequency heating is inevitably required. The heating method in the high frequency heating is not particularly limited. In order to increase the temperature difference within the thickness, single-sided heating is preferable, but double-sided heating may be used if the desired temperature difference within the thickness is obtained. At this time, in order to improve the hydrogen cracking resistance, heating is required so that the minimum temperature part is 500 ° C. or higher. On the other hand, even if heating to a range where the minimum temperature part exceeds 650 ° C., the effect of improving the hydrogen cracking resistance is saturated, and even if the temperature difference in the wall thickness is small, it does not become high YR. The upper limit of the temperature is 650 ° C.
[0028]
In addition, slow cooling such as soaking or soaking during heating reduces the temperature difference within the thickness, and if this time exceeds 30 seconds, a temperature difference within the thickness of 70 ° C. or more is obtained. Therefore, it is preferable that the holding or gradual cooling time is 30 seconds or less, and that the cooling starts immediately after heating.
This cooling is completed from the start (state where the minimum temperature part is 500 to 650 ° C. and the temperature difference within the wall thickness is 70 ° C. or more) (the state where the entire area within the steel pipe wall thickness has reached 200 ° C. during the temperature drop). Up to an average cooling rate of 5 to 30 ° C./s. This is because if the cooling rate is less than 5 ° C./s, a sufficient YR reduction cannot be obtained, whereas if it exceeds 30 ° C./s, a large amount of equipment is required and uniform cooling becomes difficult. . From the viewpoint of aging, the cooling end temperature is preferably 100 ° C. or less in terms of the total thickness.
[0029]
【Example】
A steel slab having the composition shown in Table 1 is hot-rolled under the heating-rolling-cooling-winding conditions shown in Table 2, and a steel pipe is formed by the ERW steel pipe process using the obtained hot-rolled coil as a raw material. For steel pipes that have been pipe-formed, and steel pipes that have been heat-treated under the conditions shown in Table 3 after pipe making, YS (yield strength), TS (tensile strength), YR (yield ratio = YS / TS), CLR are as follows. (The crack length ratio which is a hydrogen cracking resistance characteristic index) was measured.
[0030]
YS, TS, YR: Measured by a test method using API 5L. CLR: Performed according to NACE TM-02-84 Test solution: Solution A (0.5% acetic acid + 5% NaCl aqueous solution, pH 2.7 ± 0.1)
Gas: 100% H ₂ S
Test temperature: 25 ° C / s ± 3 ° C
Test time: The results for 96 hours are shown in Table 3. The temperature difference between the inner and outer surfaces shown in Table 3 is the temperature difference between the highest temperature portion and the lowest temperature portion at the end point of the heating process under the actual heating conditions (high-frequency heating from the tube outer surface).
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
[Table 3]

[0034]
All of the steel pipes (invention examples) produced by the production method satisfying the requirements of the present invention achieved low YR, and exhibited excellent hydrogen cracking resistance properties while satisfying the required strength properties of API X60 to X80.
[0035]
【The invention's effect】
According to the present invention, while effectively utilizing a uniform and fine hot-rolled steel sheet structure before pipe making, it is possible to easily solve the problem of material deterioration during pipe making. An excellent effect is obtained in that a sewn steel pipe is obtained.
[Brief description of the drawings]
FIG. 1 is a graph showing an example of the relationship between an in-thickness temperature difference condition during heat treatment of an ERW steel pipe and YS, TS, YR after the heat treatment.

Claims

% By mass
C: 0.01 to 0.10%, Si: 0.05 to 0.5%,
Mn: 0.5 to 2.0%, P: 0.03% or less,
S: 0.003% or less, Al: 0.005 to 0.050%,
N: 0.0050% or less, O: 0.0030% or less, and Nb: 0.005-0.1%, V: 0.005-0.1%,
Ti: 0.005 to 0.1%, Mo: 0.05 to 0.5%,
B: 0.0001 to 0.0030%, Ca: 0.0005 to 0.0060%
A steel material comprising one or more of the above, the balance Fe and inevitable impurities being heated to 1100 ° C. or higher, and the reduction rate in the unrecrystallized region of Ar ₃ or higher is 50% or higher. After rolling and coiling at 600 ° C. or less to make a coil, it is made into a steel pipe by an electric resistance steel pipe process. Subsequently, the steel pipe is subjected to high frequency heating so that the minimum temperature part is within a range of 500 to 650 ° C. Heating so that the temperature difference between the highest temperature part and the lowest temperature part within the thickness is 70 ° C. or more, and holding or slow cooling for 30 seconds or less, followed by cooling at an average cooling rate of 5 to 30 ° C./s. A method for producing ERW steel pipes for high-strength line pipes with excellent hydrogen cracking resistance.

The steel material is further mass%,
Ni: 0.05-1.0%, Cu: 0.05-1.0%,
Cr: 0.05-1.0%
The method for producing an ERW steel pipe for a high-strength line pipe excellent in hydrogen cracking resistance according to claim 1, comprising one or more of the following.