JP4140419B2 - Manufacturing method of high strength steel pipe with excellent composite secondary workability - Google Patents
Manufacturing method of high strength steel pipe with excellent composite secondary workability Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は、複合二次加工性に優れた高張力鋼管の製造方法に関し、詳しくは、曲げ加工、縮径加工、管端扁平加工の組み合わせよりなる複合二次加工において、表面および/または端部に亀裂を生じることなく加工を完了しうる性質である複合二次加工性に優れた引張強度550MPa以上の高張力鋼管の製造方法に関する。
【0002】
【従来の技術】
溶接鋼管を母管とした熱間縮径圧延による鋼管製造に関する従来の技術としては、電縫溶接鋼管を用いて、 熱間絞り圧延してコイル状鋼管を製造する際に、 コイル状鋼管の内面ビード部に発生した内面シワ疵による冷間加工時の割れを防止するために、該電縫溶接鋼管の内面ビード部を切削又は/及び研削してビード高さを-200〜+20 μm とし、 続いて熱間絞り圧延し、 コイル状に巻き取りコイル状鋼管を製造することを特徴とするコイル状鋼管の製造方法(特許文献1)や、焼鈍することなく二次加工でき、しかも、管内面にスケールのない長尺のコイル状鋼管を製造するために、鋼管をストレッチレデューサにより熱間絞り圧延し、該圧延をA1 点以上の温度で終了させ、ついでコイルに巻取り放冷することを特徴とするコイル状鋼管の製造方法(特許文献2)や、製管工程、 加熱工程、そして圧延仕上げ工程を備えた熱間電縫管の製造方法において、接合部の局部腐食(溝状腐食)のない熱間電縫管の安定製造を目的として、製管後、得られた母管の圧延仕上げに先立って、燃焼加熱炉そして誘導加熱装置によって前記母管を加熱して、 所定の圧延仕上げ温度を確保することを特徴とする熱間電縫管の製造方法(特許文献3)や、ハイドロフォーム成形性に優れた鋼管を得るために、電縫管製造工程で製造した鋼管を、電縫溶接部を中心として両側、周方向に肉厚の5倍の範囲において最小肉厚が円周方向全体の肉厚の平均値より小さくならないようにビード切削を行い、この素管をAc3点−30℃以上の温度に加熱し、オーステナイト、またはオーステナイト+フェライト2相組織となる温度で、かつ 700℃以上の温度域で縮径圧延を行い、その後冷却することを特徴とする成形性に優れた鋼管の製造方法(特許文献4)などが知られている。
【0003】
【特許文献1】
特開平6−238488号公報
【特許文献2】
特公昭63−53248号公報
【特許文献3】
特許第3031233号公報
【特許文献4】
特開2002−115029号公報
【0004】
【発明が解決しようとする課題】
一方、例えば、自動車用構造用鋼(例えば足回り部品等)などの用途分野では、引張強度(TSと記す)550MPa以上の高張力鋼管に、複合二次加工(:曲げ加工、縮径加工、管端扁平加工を2種以上組み合わせてなる二次加工)を施して複雑な形状に成形したものが使用されるようになってきており、そのため、相当に大きい加工量の複合二次加工を受けても割れを生じない(すなわち複合二次加工性に優れた)高張力鋼管の要求が強まっている。しかし、上記従来の技術では、複合二次加工性が考慮されておらず、必ずしも十分な複合二次加工性を有する高張力鋼管を得ることはできなかった。そこで、本発明は、TS:550MPa以上を有し複合二次加工を受けても割れを生じない鋼管を安定して製造しうる複合二次加工性に優れた高張力鋼管の製造方法を提供することを目的とする。
【0005】
【課題を解決するための手段】
発明者らは、前記目的を達成するために鋭意検討した結果、特定の組成(=化学組成)の帯鋼を電縫溶接してなる鋼管を特定の縮径率で縮径圧延するものとし、かつ、この縮径圧延を前段と後段に分けてその中間で再加熱を行い、前段圧延ではγ域ないしγ+αの2相域で圧延し、中間再加熱ではγ域に昇温し、後段圧延ではγ域で圧延することにより、複合二次加工性に優れた高張力鋼管を安定製造しうることを見出し、本発明をなした。
【0006】
すなわち、本発明は、質量%で、C:0.01〜0.60%、Si:0.01〜2.0 %、Mn:0.01〜3.0 %、Al:0.005 〜0.10%、S:0.0050%以下、P:0.1 %以下を含み、あるいはさらに、Cu:1%以下、Ni:1%以下、Cr:2%以下、Mo:1%以下、Nb:0.1 %以下、V:0.5 %以下、Ti:0.2 %以下、B:0.005 %以下、REM :0.02%以下、Ca:0.01%以下のうち1種または2種以上む組成の帯鋼を電縫溶接してなる鋼管を、縮径率:25%以上の条件で縮径圧延するにあたり、まず、縮径率:10%以上、圧延開始温度:下記〔式1〕で定義されるA3 点以上、圧延終了温度:(下記〔式2〕で定義されるA1 点−50℃)以上前記A3 点以下の条件で縮径圧延し、次いで前記A3 点以上に再加熱し、次いで縮径率:5%以上、圧延終了温度:前記A3 点以上の条件で縮径圧延することを特徴とする複合二次加工性に優れた高張力鋼管の製造方法である。
【0007】
記
〔式1〕
A3 点(℃)=910 −203 ×√(%C)−15.2×(%Ni)+44.7×(%Si)+104 ×(%V)+31.5×(%Mo)−〔30×(%Mn)+11×(%Cr)+20×(%Cu)−700 ×(%P)−400 ×(%Al)−400 ×(%Ti)〕
〔式2〕
A1 点(℃)=723 −10.7×(%Mn)−16.9×(%Ni)+29.1×(%Si)+16.9 ×(%Cr)
【0008】
【発明の実施の形態】
まず、本発明において、帯鋼の組成(=化学組成)を上記範囲に限定した理由について説明する。
C:0.01〜0.60%
Cは、基地中に固溶しあるいは炭化物として析出し、鋼の強度を増加させる元素であり、また、硬質な第2相(第2相粒子または第2相組織)として析出したセメンタイト、 パーライト、 ベイナイト、 マルテンサイトが高強度化と延性向上に寄与する。所望の強度を確保し、第2相として析出したセメンタイト等による延性向上の効果を得るためには、Cは、0.01%以上、好ましくは0.04%以上の含有を必要とするが、0.60%を超えて含有すると延性が劣化する。このため、Cは0.01〜0.60%の範囲に限定した。
【0009】
Si:0.01〜2.0 %
Siは、脱酸剤として作用するとともに、基地中に固溶し鋼の強度を増加させる。この効果は、0.01%以上、好ましくは0.1 %以上の含有で認められるが、2.0 %を超える含有は延性を劣化させる。このことから、Siは0.01〜2.0 %の範囲に限定した。なお、好ましくは、強度延性バランスの点から0.10〜1.5 %の範囲である。
【0010】
Mn:0.01〜3.0 %
Mnは、鋼の強度を増加させる元素であり第2相としてのセメンタイトの微細析出、あるいはマルテンサイト、ベイナイトの析出を促進させる。このような効果は、0.01%以上の含有で認められるが、3.0 %を超える含有は延性を劣化させる。このため、Mnは0.01〜3.0 %の範囲に限定した。なお、強度延性バランスの観点から、Mnは0.2 〜1.3 %の範囲が好ましく、より好ましくは0.6 〜1.3 %の範囲である。
【0011】
Al:0.005 〜0.10%
Alは、結晶粒を微細化する作用を有している。これにより、素材鋼管段階における第2相組織の分散を微細分散とし、本発明の効果をより大きくする。このためには少なくとも0.005 %以上の含有を必要とするが、0.10%を超えると酸化物系介在物量が増加し清浄度が劣化する。このため、Alは0.005 〜0.10%の範囲に限定した。なお、好ましくは0.015 〜0.06%である。
【0012】
S:0.0050%以下
Sは、硫化物を増加し清浄度を劣化させるのでできるだけ低減するのが望ましいが、とくに0.0050%を超えて含有すると複合二次加工性を著しく損なう。よって、Sは0.0050%以下とする。
P:0.1 %以下
Pは、微量の含有で鋼の強度を増加させるが、過度に含有すると粒界に偏析して複合二次加工性を著しく劣化させるので、0.1 %以下とする。
【0013】
上記した基本組成に加えて、次に述べる合金元素群(第1群〜第3群)を単独あるいは複合して添加してもよい。すなわち、Cu:1%以下、Ni:1%以下、Cr:2%以下、Mo:1%以下の1種または2種以上からなる第1群、Nb:0.1 %以下、V:0.5 %以下、Ti:0.2 %以下、B:0.005 %以下の1種または2種以上からなる第2群、REM :0.02%以下、Ca:0.01%以下の1種または2種からなる第3群である。
【0014】
第1群の元素Cu、Ni、Cr、Moは、いずれも強度を増加させる元素であり、必要に応じ1種または2種以上を添加できる。これら元素は、変態点を低下させ、フェライト粒あるいは第2相組織を微細化する効果を有している。しかし、Cuは多量添加すると熱間加工性が劣化するため1%を上限とした。Niは強度増加とともに靭性をも改善するが1%を超えて添加してもコスト高に比して効果が飽和してくるため、1%を上限とした。Cr、Moは多量添加すると溶接性、延性が劣化するうえコスト高となるため、それぞれ2%、1%を上限とした。なお、好ましくはCu:0.1 〜0.6 %、Ni:0.1 〜0.7 %、Cr:0.1 〜1.5 %、Mo:0.05〜0.5 %である。
【0015】
第2群の元素Nb、V、Ti、Bは、いずれも炭化物、窒化物または炭窒化物として析出し、結晶粒の微細化ひいては第2相組織の微細分散化に寄与するとともに、高強度化に寄与する元素であり、特に高温に加熱される接合部を有する鋼管では、接合時の加熱過程での粒微細化の効果に加え、冷却過程でのフェライトの析出核として作用し、接合部の硬化を防止する効果もあり、必要に応じ1種または2種以上添加できる。しかし多量添加すると、溶接性、靭性とも劣化するため、Nbは0.1 %、Vは0.5 %、Tiは0.2 %、Bは0.005 %をそれぞれ上限とした。なお、好ましくはNb:0.005 〜0.05%、V:0.05〜0.1 %、Ti:0.005 〜0.10%、B:0.0005〜0.002 %である。
【0016】
第3群の元素REM 、Caは、いずれも介在物の形状を調整し、加工性を向上させる作用を有しており、さらに、硫化物、酸化物、または酸硫化物として析出し、接合部を有する鋼管での接合部の硬化を防止する作用をも有し、必要に応じ1種または2種添加できる。しかし、REM が0.02%を超え、あるいは、Caが0.01%を超えると介在物が多くなりすぎ清浄度が低下し、延性が劣化するので、REM は0.02%、Caは0.01%をそれぞれ上限とした。なお、REM が0.004 %未満、Caが0.001 %未満では前記作用による効果が少ないため、REM :0.004 %以上、Ca:0.001 %以上とするのが好ましい。
【0017】
上に説明した成分元素以外の組成部分(残部)は、Feおよび不可避的不純物からなる。本発明では、不可避的不純物として、N:0.010 %以下、O:0.006 %以下が許容される。これら不可避的不純物元素について述べると、Nは、Alと結合して結晶粒を微細化ひいては第2相組織を微細化するに効果ある量0.010 %までは許容できるが、0.010 %超の含有は延性を劣化させるため、0.010 %以下に低減するのが好ましく、より好ましくは、0.002 〜0.006 %である。また、Oは、酸化物として清浄度を劣化させるため、できるだけ低減するのが好ましいが、0.006 %までは許容できる。
【0018】
次に、本発明の鋼管製造方法について説明する。
本発明では、上記組成を有する帯鋼を電縫溶接して鋼管(電縫鋼管)にする。電縫溶接方法はとくに限定されず、冷間、 熱間のいずれで行ってもよい。そして、得られた電縫鋼管を、ストレッチレデューサなどを用いて縮径圧延(=絞り圧延)し、所望の外径の鋼管にする。この縮径圧延全体での縮径率(全縮径率;Rと記す)は、製造効率を確保する観点から25%以上とする。
【0019】
本発明では、縮径圧延を前段圧延と後段圧延の2段階に分けて行い、前段圧延と後段圧延との間で中間加熱を行う。前段圧延ではγ域から圧延開始してα域の高温側〜α+γ域で圧延終了することで、フェライト組織ないしフェライト+オーステナイト混合組織に十分に圧延歪を導入し、第2相を微細に分散させ、圧延集合組織の発達を促す。さらに中間加熱でγ域に加熱し、後段圧延でγ域のみで圧延することで、管断面全域に再結晶を起こさせて、混粒組織の形成を回避し、微細かつ均一な整粒オーステナイト組織とする。これにより、圧延後の冷却変態組織を、第2相が微細に分散しかつ延性に好適な集合組織(例えば、{110 }面が管径方向に直交しかつ<111 >方向が管軸方向に沿う結晶方位をもつもの)が発達したフェライト主体の微細粒組織にすることができ、高強度でかつ複合二次加工性に優れた鋼管が得られる。
【0020】
本発明で採用した上記前段圧延、中間加熱および後段圧延に係る操業条件の限定理由を以下に述べる。
前段圧延の縮径率(r1 と記す):10%以上
r1 が10%未満では、材料(:被圧延鋼管)に圧延歪が十分に導入されず、第2相の微細分散化が不十分となるうえ、圧延集合組織の形成も不十分となり、その後の工程による集合組織の発達がなされなくなるので、r1 は10%以上とする。なお、好ましくは20%以上である。
【0021】
前段圧延の圧延開始温度(T0 と記す):A3 点以上
T0 がA3 点未満では、後工程の中間加熱‐後段圧延で消し去るのが困難な程の混粒組織が生じやすいので、T0 はA3 点以上とする。なお、さらなる結晶粒微細化の観点からはA3 点〜A3 点+250 ℃の範囲が好ましい。
前段圧延の圧延終了温度(T1 と記す):(A1 点−50℃)以上A3 点以下
T1 が(A1 点−50℃)未満では、表面性状の悪化や形状の乱れが生起しやすくなり、一方、T1 がA3 点超では前段圧延がγ域のみで行われ、次の中間加熱でα→γ変態が起こらず、該変態の核生成による細粒化効果が期待できないので、T1 は(A1 点−50℃)以上A3 点以下とする。
【0022】
中間加熱の加熱温度(TIHと記す):A3 点以上
TIHがA3 点未満では、次の後段圧延がγ+α域で行われ、均一微細な再結晶γ整粒組織が得られなくなるので、TIHはA3 点以上とする。なお、さらなる結晶粒微細化の観点からはA3 点〜A3 点+250 ℃の範囲が好ましい。
後段圧延の縮径率(r2 と記す):5%以上
r2 が5%未満であると、管断面全域を均一に再結晶させることが困難で、混粒組織となりやすいので、r2 は5%以上とする。なお、前段圧延および後段圧延の縮径率r1 およびr2 と全縮径率Rとは、式:R=1−(1−r1 )×(1−r2 )で関係づけられる。
【0023】
後段圧延の圧延終了温度(T2 と記す):A3 点以上
T2 がA3 点未満であると、圧延中にγ→α変態が起こり、変態生成したフェライトが圧延されて加工硬化し、延性が劣化するので、T2 はA3 点以上とする。
後段圧延した後は、常法に従って冷却すればよい。この冷却は空冷および水冷のいずれで行ってもよい。
【0024】
【実施例】
表1に示す組成になる板厚2.5mm の帯鋼(熱延鋼板を条切りしたもの)を常法により冷間にて電縫溶接し、次いでビード切削を行って、外径146.0mm の素管とし、該素管を表2に示す種々の条件で絞り圧延し、表2に示すサイズの製品管を得た。絞り圧延ではタンデム配置の4ロール式レデューサを使用した。前段圧延前の加熱は、高周波誘導コイルを用いて行った。中間加熱は高周波誘導コイルを用いて行った。なお、後段圧延後の冷却は空冷とした。
【0025】
得られた製品管について、TSおよび複合加工性を調査した。TSは管軸方向を試験方向として採取したJIS 12号A試験片(円弧型試験片、平行部幅19mm、標点間距離50mm)の引張試験により測定した。複合二次加工性は、各製品管から10本ずつ切り出した200mm 長さの鋼管試験片について、押し抜きダイスにより30%縮径後、30°曲げ加工および管端から100mm 長さ部分の扁平加工を施して、割れ発生比率(割れ発生本数/10と表記)を調査し、その結果を用いて評価した。これらの調査結果を表2に示す。表2より、本発明に則って製造された製品管は高引張強度でかつ優れた複合二次加工性を有することがわかる。
【0026】
【表1】
【0027】
【表2】
【0028】
【発明の効果】
本発明によれば、TS:550MPa以上の高引張強度を有し、かつ複合二次加工を受けたときに割れを生じない、複合二次加工性に優れた高張力鋼管を安定製造できるようになるという優れた効果を奏する。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-strength steel pipe excellent in composite secondary workability, and more specifically, in composite secondary processing comprising a combination of bending processing, diameter reduction processing, and tube end flat processing, the surface and / or the end portion. The present invention relates to a method for producing a high-tensile steel pipe having a tensile strength of 550 MPa or more and excellent in composite secondary workability, which is a property capable of completing the processing without causing cracks.
[0002]
[Prior art]
The conventional technology related to the manufacture of steel pipes by hot shrinking using a welded steel pipe as the main pipe is to use the ERW welded steel pipe to produce a coiled steel pipe by hot drawing and rolling. In order to prevent cracking during cold working due to inner wrinkles generated in the bead part, the inner bead part of the ERW welded pipe is cut or / and ground to a bead height of −200 to +20 μm, Subsequently, it is hot-drawn and coiled to produce a coiled steel pipe. A method for producing a coiled steel pipe (Patent Document 1), or secondary processing without annealing, and the inner surface of the pipe In order to produce a long coiled steel pipe having no scale, the steel pipe is hot drawn and rolled by a stretch reducer, and the rolling is finished at a temperature of A 1 point or more, and then the coil is wound and allowed to cool. Made of coiled steel pipe In a method for manufacturing a hot ERW pipe having a method (Patent Document 2), a pipe making process, a heating process, and a rolling finishing process, For the purpose of stable production, after the pipe making, prior to the rolling finish of the obtained mother pipe, the mother pipe is heated by a combustion heating furnace and an induction heating device to ensure a predetermined rolling finishing temperature. In order to obtain a hot-water-welded pipe manufacturing method (Patent Document 3) and a steel pipe excellent in hydroformability, the steel pipe manufactured in the electric-welded pipe manufacturing process is In the direction of 5 times the wall thickness in the direction, bead cutting is performed so that the minimum wall thickness does not become smaller than the average value of the entire wall thickness in the circumferential direction, and this tube is heated to a temperature of Ac 3 point -30 ° C or higher. , Austenite, or austenite + ferrite 2 phase At a temperature comprised between weaving and subjected to condensation 径圧 rolling in a temperature range of not lower than 700 ° C., then steel pipe excellent manufacturing method of moldability, characterized by cooling (Patent Document 4) is known.
[0003]
[Patent Document 1]
JP-A-6-238488 [Patent Document 2]
Japanese Patent Publication No. 63-53248 [Patent Document 3]
Japanese Patent No. 3031233 [Patent Document 4]
Japanese Patent Laid-Open No. 2002-115029
[Problems to be solved by the invention]
On the other hand, for example, in application fields such as structural steel for automobiles (such as undercarriage parts), composite secondary processing (: bending processing, diameter reduction processing, (Secondary processing is a combination of two or more types of tube end flattening) and is formed into a complicated shape. Therefore, it has been subjected to complex secondary processing with a considerably large processing amount. However, there is an increasing demand for high-strength steel pipes that do not crack even (that is, excellent in composite secondary workability). However, the above-mentioned conventional technology does not take into account composite secondary workability, and it has not always been possible to obtain a high-tensile steel pipe having sufficient composite secondary workability. Therefore, the present invention provides a method for producing a high-tensile steel pipe excellent in composite secondary workability that can stably produce a steel pipe having TS: 550 MPa or more and that does not crack even when subjected to composite secondary work. For the purpose.
[0005]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above-mentioned object, the inventors of the present invention are to reduce the diameter of a steel pipe formed by electro-welding a steel strip having a specific composition (= chemical composition) at a specific reduction ratio, In addition, this reduced diameter rolling is divided into a first stage and a second stage, and reheating is performed in the middle. In the first stage rolling, rolling is performed in a two-phase region of γ region or γ + α, and in the second stage rolling, the temperature is increased to the γ region. It was found that by rolling in the γ region, a high-tensile steel pipe excellent in composite secondary workability can be stably produced, and the present invention was made.
[0006]
That is, the present invention provides, in mass%, C: 0.01 to 0.60%, Si: 0.01 to 2.0%, Mn: 0.01 to 3.0%, Al: 0.005 to 0.10%, S: 0.0050% or less, and P: 0.1% or less. Cu: 1% or less, Ni: 1% or less, Cr: 2% or less, Mo: 1% or less, Nb: 0.1% or less, V: 0.5% or less, Ti: 0.2% or less, B: 0.005 % Or less, REM: 0.02% or less, Ca: 0.01% or less, steel pipes made by ERW welding of steel strips with a composition containing one or more of them are reduced in diameter at a reduction ratio of 25% or more. Upon that, first, radial contraction rate: 10% or more, the rolling start temperature: the following [equation 1] being defined A 3 points or more in, A 1 point, which is defined by the rolling end temperature :( following [equation 2] -50 ° C.) or higher the a rolled condensation径圧at three points following conditions, then reheated to above the a 3 point, then radial contraction rate: 5% or more, the rolling end temperature: diameter at the a 3 point or more conditions Features rolling A method for producing a high-tensile steel pipe having excellent composite-fabricating the.
[0007]
[Formula 1]
A 3 points (° C.) = 910 −203 × √ (% C) −15.2 × (% Ni) + 44.7 × (% Si) + 104 × (% V) + 31.5 × (% Mo) − [30 × ( % Mn) + 11 × (% Cr) + 20 × (% Cu) −700 × (% P) −400 × (% Al) −400 × (% Ti)]
[Formula 2]
A 1 point (° C.) = 723 −10.7 × (% Mn) −16.9 × (% Ni) + 29.1 × (% Si) + 16.9 × (% Cr)
[0008]
DETAILED DESCRIPTION OF THE INVENTION
First, the reason why the composition (= chemical composition) of the steel strip is limited to the above range in the present invention will be described.
C: 0.01-0.60%
C is an element that solid-solutions or precipitates as carbide in the matrix and increases the strength of the steel, and also cementite, pearlite, precipitated as a hard second phase (second phase particle or second phase structure). Bainite and martensite contribute to high strength and ductility. In order to secure the desired strength and obtain the effect of improving ductility by cementite and the like precipitated as the second phase, C needs to be contained in an amount of 0.01% or more, preferably 0.04% or more, but it exceeds 0.60%. If included, ductility deteriorates. For this reason, C was limited to the range of 0.01 to 0.60%.
[0009]
Si: 0.01-2.0%
Si acts as a deoxidizer and dissolves in the matrix to increase the strength of the steel. This effect is observed when the content is 0.01% or more, preferably 0.1% or more, but the content exceeding 2.0% deteriorates the ductility. From this, Si was limited to the range of 0.01 to 2.0%. In addition, Preferably, it is the range of 0.10 to 1.5% from the point of intensity | strength ductility balance.
[0010]
Mn: 0.01-3.0%
Mn is an element that increases the strength of steel and promotes the fine precipitation of cementite as the second phase, or the precipitation of martensite and bainite. Such an effect is recognized at a content of 0.01% or more, but a content exceeding 3.0% degrades the ductility. For this reason, Mn was limited to the range of 0.01 to 3.0%. From the viewpoint of balance between strength and ductility, Mn is preferably in the range of 0.2 to 1.3%, more preferably in the range of 0.6 to 1.3%.
[0011]
Al: 0.005 to 0.10%
Al has an effect of refining crystal grains. Thereby, the dispersion | distribution of the 2nd phase structure | tissue in a raw material steel pipe stage is made into fine dispersion, and the effect of this invention is enlarged more. For this purpose, a content of at least 0.005% is required, but if it exceeds 0.10%, the amount of oxide inclusions increases and the cleanliness deteriorates. For this reason, Al was limited to the range of 0.005 to 0.10%. In addition, Preferably it is 0.015 to 0.06%.
[0012]
S: 0.0050% or less S is desirable to reduce S as much as possible because it increases sulfide and deteriorates cleanliness. However, when it exceeds 0.0050%, composite secondary workability is significantly impaired. Therefore, S is set to 0.0050% or less.
P: 0.1% or less P increases the strength of the steel when contained in a small amount, but if contained excessively, it segregates at the grain boundary and significantly deteriorates the composite secondary workability, so it is made 0.1% or less.
[0013]
In addition to the basic composition described above, the following alloy element groups (first group to third group) may be added alone or in combination. That is, Cu: 1% or less, Ni: 1% or less, Cr: 2% or less, Mo: 1% or less, the first group consisting of one or more, Nb: 0.1% or less, V: 0.5% or less, A second group consisting of one or more of Ti: 0.2% or less, B: 0.005% or less, a third group consisting of one or two of REM: 0.02% or less and Ca: 0.01% or less.
[0014]
The first group elements Cu, Ni, Cr, and Mo are all elements that increase the strength, and one or more of them can be added as necessary. These elements have the effect of lowering the transformation point and refining the ferrite grains or the second phase structure. However, when Cu is added in a large amount, hot workability deteriorates, so 1% was made the upper limit. Ni improves the toughness as the strength increases, but even if added in excess of 1%, the effect is saturated compared to the high cost, so 1% was made the upper limit. When Cr and Mo are added in a large amount, the weldability and ductility deteriorate and the cost increases. In addition, Preferably, they are Cu: 0.1-0.6%, Ni: 0.1-0.7%, Cr: 0.1-1.5%, Mo: 0.05-0.5%.
[0015]
The second group elements Nb, V, Ti, and B are all precipitated as carbides, nitrides, or carbonitrides, contributing to the refinement of crystal grains and, in turn, to the fine dispersion of the second phase structure, and the increase in strength. In steel pipes with joints that are heated to high temperatures, in addition to the effect of grain refinement during the heating process during joining, they act as ferrite precipitation nuclei during the cooling process. There is also an effect of preventing curing, and one or more can be added as necessary. However, when a large amount is added, both weldability and toughness deteriorate, so Nb was set to 0.1%, V was set to 0.5%, Ti was set to 0.2%, and B was set to 0.005%. Preferably, Nb is 0.005 to 0.05%, V is 0.05 to 0.1%, Ti is 0.005 to 0.10%, and B is 0.0005 to 0.002%.
[0016]
The third group of elements REM and Ca both have the effect of adjusting the shape of inclusions and improving workability, and further precipitate as sulfides, oxides, or oxysulfides, and join portions. It has the effect | action which prevents the hardening of the junction part in the steel pipe which has this, and can add 1 type or 2 types as needed. However, if REM exceeds 0.02% or Ca exceeds 0.01%, the amount of inclusions increases and the cleanliness decreases and ductility deteriorates, so the upper limit is 0.02% for REM and 0.01% for Ca. . When REM is less than 0.004% and Ca is less than 0.001%, the effect of the above action is small. Therefore, REM: 0.004% or more and Ca: 0.001% or more are preferable.
[0017]
The composition part (remainder) other than the constituent elements described above is composed of Fe and inevitable impurities. In the present invention, N: 0.010% or less and O: 0.006% or less are allowed as inevitable impurities. As for these inevitable impurity elements, N is acceptable up to an amount of 0.010% which is effective for bonding with Al and refining the crystal grains and thus the second phase structure, but the content exceeding 0.010% is ductile. In order to deteriorate the content, it is preferably reduced to 0.010% or less, more preferably 0.002 to 0.006%. O is preferably reduced as much as possible because it deteriorates cleanliness as an oxide, but it is acceptable up to 0.006%.
[0018]
Next, the steel pipe manufacturing method of the present invention will be described.
In the present invention, the steel strip having the above composition is subjected to electric resistance welding to form a steel pipe (electric resistance welded steel pipe). There is no particular limitation on the method of electric resistance welding, and it may be performed either cold or hot. Then, the obtained electric resistance welded steel pipe is subjected to diameter reduction rolling (= drawing rolling) using a stretch reducer or the like to obtain a steel pipe having a desired outer diameter. The overall diameter reduction ratio (total diameter reduction ratio; denoted as R) is 25% or more from the viewpoint of ensuring manufacturing efficiency.
[0019]
In the present invention, the diameter reduction rolling is performed in two stages of pre-stage rolling and post-stage rolling, and intermediate heating is performed between the pre-stage rolling and the post-stage rolling. In the pre-stage rolling, rolling starts from the γ region and ends at the high temperature side of the α region to the α + γ region, so that sufficient rolling strain is introduced into the ferrite structure or ferrite + austenite mixed structure, and the second phase is finely dispersed. Promote the development of rolling texture. Furthermore, by heating in the γ region by intermediate heating and rolling only in the γ region by subsequent rolling, recrystallization occurs throughout the entire cross section of the tube, avoiding the formation of a mixed grain structure, and a fine and uniform sized austenite structure And As a result, the cooled transformation structure after rolling is a texture in which the second phase is finely dispersed and suitable for ductility (for example, the {110} plane is perpendicular to the pipe radial direction and the <111> direction is in the pipe axis direction. A fine-grained structure mainly composed of ferrite with developed crystal orientation), and a steel pipe having high strength and excellent composite secondary workability can be obtained.
[0020]
The reasons for limiting the operating conditions relating to the above-mentioned pre-rolling, intermediate heating and post-rolling employed in the present invention will be described below.
Reduced diameter ratio of pre-rolling (denoted as r 1 ): When 10% or more and r 1 is less than 10%, rolling strain is not sufficiently introduced into the material (rolled steel pipe) and the second phase is not finely dispersed. In addition, the formation of the rolling texture becomes insufficient, and the texture does not develop in the subsequent process. Therefore, r 1 is set to 10% or more. In addition, Preferably it is 20% or more.
[0021]
Rolling start temperature of pre-rolling (denoted as T 0 ): When A 3 point or more and T 0 is less than A 3 point, a mixed grain structure that is difficult to be erased by intermediate heating in the post-process and post-rolling tends to occur. , T 0 is A 3 or more. From the viewpoint of further crystal grain refinement, the range of A 3 point to A 3 point + 250 ° C. is preferable.
Rolling end temperature of pre-rolling (denoted as T 1 ): (A 1 point −50 ° C.) or more and A 3 point or less If T 1 is less than (A 1 point −50 ° C.), deterioration of surface properties and shape disturbance occur. tends to, whereas, T 1 is the previous stage rolling is performed only gamma zone at three points than a, does not occur alpha → gamma transformation in the next intermediate heating, grain refining effect by nucleation of the transformation can not be expected Therefore, T 1 is set to (A 1 point−50 ° C.) or more and A 3 point or less.
[0022]
The heating temperature of the intermediate heating (T referred to as the IH): The A to less than 3 points T the IH is A 3 points, the next subsequent rolling is performed in gamma + alpha region, because uniform fine recrystallized gamma grain structure can not be obtained , T IH shall be A 3 points or more. From the viewpoint of further crystal grain refinement, the range of A 3 point to A 3 point + 250 ° C. is preferable.
Subsequent rolling of the radial contraction rate (referred to as r 2): When more than 5% r 2 is less than 5%, it is difficult to uniformly recrystallized tube section throughout, so tends to mixed grain structure, r 2 is 5% or more. In addition, the diameter reduction ratios r 1 and r 2 and the total diameter reduction ratio R of the former stage rolling and the latter stage rolling are related by the formula: R = 1− (1−r 1 ) × (1−r 2 ).
[0023]
Rolling end temperature of latter-stage rolling (denoted as T 2 ): When A 3 point or more and T 2 is less than A 3 point, γ → α transformation occurs during rolling, and the ferrite produced by transformation is rolled and work hardened, Since ductility deteriorates, T 2 is set to A 3 or more.
What is necessary is just to cool in accordance with a conventional method after back | latter stage rolling. This cooling may be performed by either air cooling or water cooling.
[0024]
【Example】
A 2.5 mm thick steel strip (cut from a hot-rolled steel plate) having the composition shown in Table 1 is cold-welded and welded in a conventional manner, followed by bead cutting, and the outer diameter is 146.0 mm. The tube was drawn and rolled under various conditions shown in Table 2 to obtain product tubes having the sizes shown in Table 2. A tandem 4-roll reducer was used in the drawing rolling. Heating before pre-rolling was performed using a high frequency induction coil. Intermediate heating was performed using a high frequency induction coil. The cooling after the latter rolling was air cooling.
[0025]
The obtained product tube was examined for TS and composite processability. TS was measured by a tensile test of a JIS 12A test piece (arc-shaped test piece, parallel part width 19 mm, distance between gauge points 50 mm) taken with the tube axis direction as the test direction. Composite secondary workability is as follows: 10 mm steel pipe specimens cut from 10 pieces from each product pipe, 30% diameter reduction by punching dies, 30 ° bending and flat machining of 100 mm length from pipe end The crack occurrence ratio (number of crack occurrences / 10) was investigated and evaluated using the results. Table 2 shows the results of these investigations. Table 2 shows that the product tube manufactured according to the present invention has high tensile strength and excellent composite secondary workability.
[0026]
[Table 1]
[0027]
[Table 2]
[0028]
【The invention's effect】
According to the present invention, it is possible to stably produce a high-tensile steel pipe having a high tensile strength of TS: 550 MPa or more and not generating cracks when subjected to the composite secondary processing and excellent in the composite secondary workability. It has an excellent effect of becoming.
Claims (2)
記
〔式1〕
A3 点(℃)=910 −203 ×√(%C)−15.2×(%Ni)+44.7×(%Si)+104 ×(%V)+31.5×(%Mo)−〔30×(%Mn)+11×(%Cr)+20×(%Cu)−700 ×(%P)−400 ×(%Al)−400 ×(%Ti)〕
〔式2〕
A1 点(℃)=723 −10.7×(%Mn)−16.9×(%Ni)+29.1×(%Si)+16.9 ×(%Cr)
ここに、右辺の(%元素記号)は当該元素の鋼中成分含有量(質量%)であり、含有されない元素については0が代入される。A steel strip having a composition containing, in mass%, C: 0.01 to 0.60%, Si: 0.01 to 2.0%, Mn: 0.01 to 3.0%, Al: 0.005 to 0.10%, S: 0.0050% or less, and P: 0.1% or less. In reducing the diameter of a steel pipe welded by electric resistance welding under the condition of a reduction ratio of 25% or more, first, the reduction ratio: 10% or more, rolling start temperature: A 3 defined by the following [Formula 1] More than the point, rolling end temperature: (A 1 point-50 ° C. defined in the following [Formula 2]) or more rolling under the condition of the A 3 point or less, then reheated to the A 3 point or more, then radial contraction rate: 5% or more, the finish rolling temperature method of manufacturing a high-tensile steel pipe having excellent composite-fabricating, characterized in that the diameter reduction rolling at the a 3 point or more.
[Formula 1]
A 3 points (° C.) = 910 −203 × √ (% C) −15.2 × (% Ni) + 44.7 × (% Si) + 104 × (% V) + 31.5 × (% Mo) − [30 × ( % Mn) + 11 × (% Cr) + 20 × (% Cu) −700 × (% P) −400 × (% Al) −400 × (% Ti)]
[Formula 2]
A 1 point (° C.) = 723 −10.7 × (% Mn) −16.9 × (% Ni) + 29.1 × (% Si) + 16.9 × (% Cr)
Here, (% element symbol) on the right side is the content (mass%) of the element in steel of the element, and 0 is substituted for an element not contained.
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WO2010110490A1 (en) * | 2009-03-25 | 2010-09-30 | 新日本製鐵株式会社 | Electric resistance welded steel pipe having excellent workability and excellent post-quenching fatigue properties |
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