JP3823906B2 - Manufacturing method of ERW steel pipe for high-strength line pipe with excellent hydrogen cracking resistance and toughness - Google Patents
Manufacturing method of ERW steel pipe for high-strength line pipe with excellent hydrogen cracking resistance and toughness Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は、耐水素割れ特性および靭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API X60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および低温靭性を効率的に付与しうる耐水素割れ特性および靭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A3 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A3 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および靭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】
本発明者らは、UO鋼管と同じ鋼成分組成の造管ままの電縫鋼管をAc1 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、上記目的が達成できるのではないかと考え、次のような実験を行なった。
C:0.05% 、Si:0.25% 、Mn:1.2%、P:0.009%、S:0.002%、Al:0.033%、Nb:0.045%、Ti:0.009%、N:0.036%およびO:0.0018% 、Ca:0.0022% の鋼成分組成のスラブを、1200℃に加熱後、再結晶域圧延に次いで70% の未再結晶域圧延を施し、570 ℃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24inch(×25.4mm/inch )×肉厚12.7mmの電縫鋼管を造管し、図2に示す3種類の熱処理を施して、その材料特性(強度、低温靭性)を調べた。短時間加熱は外表面から高周波誘導加熱にて行い、鋼管の外表面の温度を測定して加熱温度とした。
【0008】
熱処理後の鋼管の機械的性質を図1に示す。図1から明らかなように、短時間加熱では雰囲気炉を用いたバッチ熱処理による比較的長時間の加熱とは明らかに異なる特性を示した。すなわち、短時間加熱‐水冷(: A)することによって、通常(C:バッチ熱処理)なら強度低下してしまうような温度域への加熱でも強度をあまり変化させることなく、造管歪による低温靭性劣化を補って余りある靭性向上が得られることが判った。一方、短時間加熱しても冷却を空冷で行なう(:B)と、強度の低下が比較的低温側の加熱温度域で起こってしまうばかりか、歪時効と思われる時効により降伏伸びが発生してYR(降伏比)が劣化してしまう。
【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、短時間加熱の故に、造管歪や転位の緩和および組織変化と、侵入型固溶元素および置換型固溶元素の拡散などの時差が複雑に関係しあい、このような特性変化になったものと思われる。また、これら熱処理材の耐水素割れ特性を調査した結果、造管歪緩和などにより造管まま材よりも向上していることも判った。
【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:0.005%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、Ar3 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連続的に3℃/s以上の加熱速度で650 ℃以上850 ℃以下でかつAc1 点以上ではγ分率が20% 以下となる温度へ加熱し、60s 以下の保持の後、冷却速度5〜30℃/sの冷却を施すことを特徴とする耐水素割れ特性および靭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】
まず、本発明における鋼成分組成の限定理由を以下に述べる。
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。API X60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 〜0.07% 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:0.005%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0.005%以下、好ましくは0.003%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時効を抑制する。また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、Bが鋼中NによりBNとして析出固定されるが、その効果が抑制されないように添加する。0.005%未満では効果なく、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を劣化するため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、1.0%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1.0%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。
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。
【0024】
次に、工程条件の限定理由を以下に述べる。
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Ar3 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。
【0025】
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。
【0026】
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短時間加熱‐冷却処理を行なう。加熱速度はひずみ緩和と侵入型固溶元素および置換型固溶元素拡散による組織変化などの時差を有効に活用すべく、3℃/s以上の加熱速度を必要とする。高速加熱を実施するために必然的に高周波加熱が必要となる。この平均加熱速度で特性改善を目的に650 ℃以上、850 ℃以下に60s以内の均熱時間で加熱する。650 ℃未満では靭性の向上が得られないばかりかYRの劣化が起こりAPI規格を満たすことができない。一方、850 ℃超では強度が低下してしまうため、上限を850 ℃とする。ただし、850 ℃がAc1 点以上の場合は、加熱時のγ(オーステナイト)分率が体積率で20% 以下となる温度を上限とする。これは、γ分率が20% 超では、その後の急冷により著しく靭性が劣化するためγ分率20% 以下の温度を上限とした。高周波による加熱の方法は特に問わないが、生産性の観点から外面よりの一方向加熱でも良い。この場合、高速加熱ゆえに加熱時に鋼管の内外面に必然的に温度差を生じるが、冷却開始時には温度差が50℃以内であることが材料特性上好ましい。
【0027】
加熱後の鋼管は5〜30℃/sの冷却速度で冷却する必要がある。冷却速度が5℃/s未満では強度の低下とYRの上昇が起こるばかりか、靭性の向上代も小さい。また冷却速度30℃/s超の冷却は設備的に多大な費用を必要とし、また均一冷却も困難であるため30℃/sを上限とした。
また、本発明によって得られるラインパイプは、施工のためのメッキ処理など、通常行なわれる表面改質などを施しても、その特徴を損ねることはないのでかまわない。
【0028】
【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、下記の試験要領によりYS(降伏強度)、TS(引張強度)、YR(降伏比=YS/TS )、DWTT85%FATT (DWTT85% 延性破面遷移温度)、CLR(耐水素割れ特性指標である割れ長さ率)を測定した。
【0029】
YS,TS,YR:API 5Lによる試験方法にて測定
DWTT85%FATT :API RP 5L3による試験方法にて測定
CLR:NACE TM−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%H2S
試験温度:25℃± 3℃
試験時間:96時間
結果を表3に示す。
【0030】
【表1】
【0031】
【表2】
【0032】
【表3】
【0033】
【表4】
【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、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および靭性を示した。
【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および靭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】電縫溶接後の熱処理条件と強度、靭性の関係の例を示すグラフである。
【図2】熱処理条件の例を示す温度パターン図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing an ERW steel pipe for high-strength line pipe excellent in hydrogen cracking resistance and toughness, and more specifically, excellent in an ERW steel pipe for API X60 grade or higher high-strength line pipe having a wall thickness of 25 mm or less. The present invention relates to a method for producing an ERW steel pipe for a high-strength line pipe excellent in hydrogen cracking resistance and toughness capable of efficiently imparting hydrogen cracking resistance and low temperature toughness.
[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]
To overcome this disadvantage by applying heat treatment to ERW pipes, the entire ERW pipe should be heated to 800 ° C or higher in order to simultaneously give sour resistance, low temperature toughness, and low yield ratio to ERW pipes. A method of heating and then quenching the steel pipe has been proposed (see Patent Document 1). In this method, the low carbon steel has a composition of C ≦ 0.12%, Mn: 0.5 to 1.4%, Si: 0.10 to 0.25%, P ≦ 0.015%, S ≦ 0.0020%, Ca: 0.0010 to 0.0060%. the ERW steel pipe, and the austenitic state of the above a 3 transformation point or higher 800 ° C., to remove the cold distortion, it then quenching, the acicular ferrite or low carbon bainitic structure, that tempering is not performed It is a feature.
[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, since 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 a high-strength linepipe ERW steel pipe excellent in crack characteristics and toughness.
[0007]
[Means for Solving the Problems]
The inventors of the present invention can achieve the above object by rapidly heating, holding, and rapidly cooling an electric resistance welded steel pipe having the same steel composition as that of the UO steel pipe in a temperature range around Ac 1 point. The following experiment was conducted.
C: 0.05%, Si: 0.25%, Mn: 1.2%, P: 0.009%, S: 0.002%, Al: 0.033%, Nb: 0.045%, Ti: 0.009%, N: 0.036% and O: 0.0018% Ca: A slab having a steel composition of 0.0022% was heated to 1200 ° C, then subjected to 70% non-recrystallization zone rolling after recrystallization zone rolling, and wound at 570 ° C to obtain a hot rolled coil. Using this hot-rolled coil as a raw material, an ERW steel pipe with an outer diameter of 24 inches (× 25.4 mm / inch) × wall thickness of 12.7 mm is made and subjected to the three types of heat treatment shown in FIG. Toughness) was investigated. Short-time heating was performed by high-frequency induction heating from the outer surface, and the temperature of the outer surface of the steel pipe was measured as the heating temperature.
[0008]
FIG. 1 shows the mechanical properties of the steel pipe after the heat treatment. As is apparent from FIG. 1, the short-time heating showed distinctly different characteristics from the relatively long-time heating by batch heat treatment using an atmospheric furnace. In other words, low-temperature toughness due to tube-forming strain without changing the strength so much even when heated to a temperature range where strength is reduced by normal heating (C: batch heat treatment) by short-time heating-water cooling (A). It has been found that a toughness improvement that can compensate for the deterioration can be obtained. On the other hand, when the air is cooled even if heated for a short time (: B), not only does the strength decrease in the heating temperature range on the relatively low temperature side, but yield elongation occurs due to aging that seems to be strain aging. YR (yield ratio) deteriorates.
[0009]
The details of the difference in these characteristics are not clear, but due to short-time heating, there are time differences such as tube-forming strain and dislocation relaxation and structural changes, and diffusion of interstitial and substitutional solid solution elements. It seems to have become such a characteristic change related to each other. Moreover, as a result of investigating the hydrogen cracking resistance characteristics of these heat-treated materials, it was found that the heat-treated materials were improved over the as-formed materials due to the relaxation of tube-forming strain.
[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: 0.005% or less, Al: 0.005 to 0.050%, N: 0.0050% or less, O: 0.0030% or less, and Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, Ti: 0.005 Steel material containing one or more of ~ 0.1%, Mo: 0.05-0.5%, B: 0.0001-0.0030%, Ca: 0.0005-0.0060%, the balance Fe and unavoidable impurities at 1100 ° C or higher Heated, hot rolled to a reduction rate of 50% or more in the non-recrystallized area of Ar 3 or higher, wound at 600 ° C or lower to form a coil, and then made into a steel pipe in the ERW steel pipe process. This steel pipe is continuously heated by high frequency heating at a heating rate of 3 ° C / s or higher to a temperature of 650 ° C or higher and 850 ° C or lower, and at a point of Ac 1 or higher, the γ fraction is 20% or lower. After holding, cooling In the manufacturing method of the resistance to hydrogen cracking properties and high strength line ERW pipe superior in toughness and characterized by applying cooling in degrees 5 to 30 ° C. / s.
[0011]
In the present invention, the steel material may further contain one or more of Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, and Cr: 0.05 to 1.0% by mass%.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
First, the reasons for limiting the steel component composition in the present invention will be described below.
C: 0.01-0.10%
C needs to be contained in an amount of 0.01% or more for securing the strength. On the other hand, if it exceeds 0.10%, both hydrogen cracking resistance and toughness are lowered. In order to achieve high strength, hydrogen cracking resistance and toughness with API X60 or higher in a well-balanced manner, it is particularly preferably 0.025 to 0.07%.
[0013]
Si: 0.05-0.5%
Si requires a minimum of 0.05% as a deoxidizer and a strength securing element, but if added in excess, HAZ (welding heat affected zone) toughness is lowered, which is undesirable for welding, so the upper limit was made 0.5%.
Mn: 0.5 to 2.0%
Mn is an element necessary for increasing strength, and 0.5% or more is added. On the other hand, if it exceeds 2.0%, not only the toughness of the base metal deteriorates, but also hard segregation phase is formed and the hydrogen cracking resistance is significantly deteriorated. Therefore, it is limited to the range of 0.5 to 2.0%. In order to achieve particularly excellent hydrogen cracking resistance, the content is preferably 1.2% or less.
[0014]
P: 0.03% or less P is an element that segregates at the grain boundary and lowers the grain boundary strength. To lower the toughness of the base metal and the welded part, 0.03% was made the upper limit to prevent grain boundary cracking. In particular, when high toughness is required, the content is preferably 0.015% or less.
S: 0.005% or less S is present in steel as sulfides such as MnS, and is an element that significantly deteriorates the resistance to hydrogen cracking and toughness. In order to suppress the influence, 0.005% or less, preferably 0.003% or less, It is necessary to.
[0015]
Al: 0.005 to 0.050%
Al is necessary for deoxidation and N fixation, and it is necessary to add 0.005% or more. On the other hand, if it exceeds 0.050%, alumina inclusions increase and the hydrogen cracking resistance and toughness are impaired, so 0.050% was made the upper limit.
N: 0.0050% or less If N exceeds 0.0050%, coarse nitrides are formed to deteriorate the hydrogen cracking resistance and toughness, so the content was made 0.0050% or less.
[0016]
O: 0.0030% or less O is present as an inclusion, and when it is agglomerated and coarsened, it works as a starting point for hydrogen cracking. Therefore, it is preferable to reduce the amount as much as possible. . When particularly excellent hydrogen cracking resistance is required, the content is preferably 0.0020% 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 to 0.1%
Nb forms fine carbonitrides to increase the strength, and favors the accumulation of strain in hot controlled rolling, and improves the toughness by refining the structure. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.1%, the weld toughness is unfavorably affected, so it is limited to 0.005 to 0.1%.
[0018]
V: 0.005 to 0.1%
V is an element having almost the same effect as Nb, but the precipitation hardening ability is slightly inferior to Nb. If it is less than 0.005%, the curability is poor, and if it exceeds 0.1%, the toughness of the welded part is deteriorated, so 0.005 to 0.1%.
Ti: 0.005 to 0.1%
Ti is a strong nitriding element, and N aging is suppressed by addition of about N equivalent (N% × (48/14)). Further, when added, fine carbide is formed to increase the strength, and B is precipitated and fixed as BN by N in the steel, but is added so that the effect is not suppressed. If it is less than 0.005%, there is no effect, and it is particularly preferable to add (N% × (48/14)) or more. On the other hand, if added over 0.1%, coarse nitrides are easily formed and the toughness is deteriorated, so the content is made 0.1% or less.
[0019]
Mo: 0.05-0.5%
Mo dissolves or forms carbides and has the effect of increasing strength without significant toughness degradation, but exceeding 1.0% not only saturates the effect, but also increases the cost, so 1.0% or less You may add in the range. In order to exhibit the effect of increasing the strength, it is preferable to add 0.05% or more.
[0020]
B: 0.0001-0.0030%
B, like Nb, is important for controlling the structure of the rolled material, and 0.0001% or more must be added to exert its effect. 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, if added excessively, the effect is not only saturated, but the toughness of the weld is deteriorated, so 0.0030% is made the upper limit.
[0021]
Ca: 0.0005 to 0.0060%
Ca is added for the purpose of controlling the shape of inclusions, which are the starting point of hydrogen cracking, in a spherical shape, but 0.0005% or more is necessary to exert its effect, whereas when it exceeds 0.0060%, the effect is only saturated. Alternatively, coarse inclusions are formed, so the range is 0.0005 to 0.0060%.
[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 even if it is added excessively, the effect is saturated, so the content is limited to 0.05 to 1.0%. 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 to improve strength and hydrogen cracking resistance, but it is necessary to add 0.05% or more in order to exert its effect. On the other hand, if it exceeds 1.0%, it tends to cause hot embrittlement and toughness. Since it falls, it is made 0.05 to 1.0% of range.
Cr: 0.05-1.0%
Cr is effective in increasing the strength, but if added excessively, the toughness is reduced, so it may be added in a range of 1.0% or less. However, if it is less than 0.05%, the effect is not exhibited, so 0.05% or more is preferably added.
[0024]
Next, the reasons for limiting the process conditions will be described below.
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.
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. However, as the controlled rolling, non-recrystallization zone rolling with a rolling reduction temperature of 50% or more and a rolling finishing temperature of Ar 3 point or higher is required. At this time, the cast slab can be continuously rolled without being cooled to less than 1100 ° C, or it can be heated to 1100 ° C or higher from the course of cooling from 1100 ° C to room temperature, and then rolled after soaking. Will not be damaged.
[0025]
Furthermore, it is necessary to coil at a coiling temperature of 600 ° C. or lower after rolling in order to effectively exert 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, making the structure single phase and hydrogen resistance 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.
[0026]
The formed steel pipe is subjected to a short-time heating-cooling process, which is a feature of the present invention, continuously or appropriately after that in the line. The heating rate requires a heating rate of 3 ° C./s or more in order to effectively utilize time differences such as strain relaxation and interstitial solid solution element and structural change caused by substitutional solid solution element diffusion. In order to perform high-speed heating, high-frequency heating is inevitably required. In order to improve the characteristics at this average heating rate, heating is performed at 650 ° C or higher and 850 ° C or lower with a soaking time within 60 seconds. If it is less than 650 ° C., not only the toughness cannot be improved, but also YR deterioration occurs and the API standard cannot be satisfied. On the other hand, if the temperature exceeds 850 ° C, the strength decreases, so the upper limit is set to 850 ° C. However, when 850 ° C. is at least Ac 1 point, the upper limit is the temperature at which the γ (austenite) fraction during heating is 20% or less by volume. When the γ fraction exceeds 20%, the toughness is remarkably deteriorated by subsequent rapid cooling, so the temperature is set to 20% or less. The method of heating by high frequency is not particularly limited, but unidirectional heating from the outer surface may be used from the viewpoint of productivity. In this case, because of high-speed heating, a temperature difference inevitably occurs on the inner and outer surfaces of the steel pipe during heating, but it is preferable in terms of material properties that the temperature difference is within 50 ° C. at the start of cooling.
[0027]
The heated steel pipe needs to be cooled at a cooling rate of 5 to 30 ° C./s. When the cooling rate is less than 5 ° C./s, not only the strength is decreased and the YR is increased, but also the margin for improving toughness is small. Cooling at a cooling rate exceeding 30 ° C / s requires a large amount of equipment, and uniform cooling is difficult, so 30 ° C / s was set as the upper limit.
Further, the line pipe obtained by the present invention may be subjected to surface modification that is usually performed, such as plating for construction, because the characteristics thereof are not impaired.
[0028]
【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), DWTT85% FATT (DWTT85% ductile fracture surface transition temperature) and CLR (crack length ratio which is a hydrogen cracking resistance characteristic index) were measured.
[0029]
YS, TS, YR: Measured by test method using API 5L
DWTT85% FATT: Measured by a test method using API RP 5L3 CLR: Conducted 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 ℃ ± 3 ℃
Test time: The results for 96 hours are shown in Table 3.
[0030]
[Table 1]
[0031]
[Table 2]
[0032]
[Table 3]
[0033]
[Table 4]
[0034]
All the steel pipes (invention examples) manufactured by the manufacturing method satisfying the requirements of the present invention exhibited excellent hydrogen cracking resistance and toughness while satisfying the required strength characteristics of APIX60 to X80.
[0035]
【The invention's effect】
According to the present invention, a high-strength line pipe excellent in hydrogen cracking resistance and toughness that can easily solve the problem of material deterioration during pipe making while effectively utilizing a uniform and fine hot-rolled steel sheet structure before pipe making There is an excellent effect that an electric resistance welded steel pipe can be obtained.
[Brief description of the drawings]
FIG. 1 is a graph showing an example of the relationship between heat treatment conditions, strength, and toughness after ERW welding.
FIG. 2 is a temperature pattern diagram showing an example of heat treatment conditions.
Claims (2)
C:0.01〜0.10% 、Si:0.05〜0.5%、Mn:0.5 〜2.0%、P:0.03% 以下、S:0.005%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、Ar3 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連続的に3℃/s以上の加熱速度で650 ℃以上850 ℃以下でかつAc1 点以上ではγ分率が20% 以下となる温度へ加熱し、60s 以下の保持の後、冷却速度5〜30℃/sの冷却を施すことを特徴とする耐水素割れ特性および靭性に優れる高強度ラインパイプ用電縫鋼管の製造方法。% 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: 0.005% or less, Al: 0.005 to 0.050%, N: 0.0050% or less, O: 0.0030% Including and
Nb: 0.005 to 0.1%, V: 0.005 to 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 containing one or more of the above, the balance Fe and unavoidable impurities is 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 the ERW steel pipe process, and then this steel pipe is continuously heated by high frequency heating at a heating rate of 3 ° C / s or more at 650 ° C or more and 850 ° C Heating to a temperature at which the γ fraction is 20% or less when the temperature is 1 ° C. or lower and the temperature is 1 point or higher, and holding at 60 seconds or lower, followed by cooling at a cooling rate of 5 to 30 ° C./s A method for producing ERW steel pipes for high-strength line pipes with excellent properties and toughness.
Ni:0.05〜1.0%、Cu:0.05〜1.0%、Cr:0.05〜1.0%
の1種または2種以上を含むことを特徴とする耐水素割れ特性および靭性に優れる請求項1記載の高強度ラインパイプ用電縫鋼管の製造方法。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 high-strength line pipes according to claim 1, which is excellent in hydrogen cracking resistance and toughness, characterized by comprising one or more of the following.
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JP5124988B2 (en) * | 2005-05-30 | 2013-01-23 | Jfeスチール株式会社 | High-tensile steel plate with excellent delayed fracture resistance and tensile strength of 900 MPa or more and method for producing the same |
JP2007307566A (en) * | 2006-05-16 | 2007-11-29 | Jfe Steel Kk | Method for producing electric resistance welded tube for high-strength thick line pipe having excellent weld zone toughness |
JP5246280B2 (en) * | 2011-02-15 | 2013-07-24 | 新日鐵住金株式会社 | Steel sheet for high strength steel pipe and high strength steel pipe |
JP5679114B2 (en) * | 2011-02-24 | 2015-03-04 | Jfeスチール株式会社 | Low yield ratio high strength hot rolled steel sheet with excellent low temperature toughness and method for producing the same |
JP5516659B2 (en) * | 2012-06-28 | 2014-06-11 | Jfeスチール株式会社 | High-strength ERW pipe excellent in long-term softening resistance in the medium temperature range and its manufacturing method |
CN107406940B (en) * | 2015-03-06 | 2019-05-07 | 杰富意钢铁株式会社 | High-strength electric resistance welded steel pipe and its manufacturing method |
CN109023091B (en) * | 2018-09-10 | 2019-11-01 | 武汉科技大学 | A kind of good hot rolling ultra-high strength steel plate of processability and preparation method thereof |
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