JP3557813B2

JP3557813B2 - Steel pipe manufacturing method

Info

Publication number: JP3557813B2
Application number: JP27920896A
Authority: JP
Inventors: 高明豊岡; 伸樹田中; 元晶板谷; 章依藤; 寿雄大西; 裕二橋本
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1996-10-22
Filing date: 1996-10-22
Publication date: 2004-08-25
Anticipated expiration: 2016-10-22
Also published as: JPH10128435A

Description

【０００１】
【発明の属する技術分野】
本発明は、溶接鋼管の製造方法に関し、とくに、固相圧接による鋼管の製造方法に関する。
【０００２】
【従来の技術】
溶接鋼管は、鋼板または鋼帯を管状に成形しその継目を溶接したもので、小径から大径まで各種の製造法によりつくられているが、主な製造法として、電気抵抗溶接（電縫）、鍛接、電弧溶接によるものが挙げられる。
小径〜中径鋼管用としては、高周波誘導加熱を利用した電気抵抗溶接法（電気抵抗溶接鋼管、電縫管）が主として利用されている。この方法は、連続的に帯鋼を供給し、成形ロールで管状に成形してオープン管とし、続いて高周波誘導加熱によりオープン管の両エッジ部端面を鋼の融点以上に加熱した後、スクイズロールで両エッジ部端面を衝合溶接して鋼管を製造する方法である（例えば、第３版鉄鋼便覧第ＩＩＩ巻（２）１０５６〜１０９２頁）。
【０００３】
上記した高周波誘導加熱を利用した電縫管の製造方法では、オープン管の両エッジ部端面を鋼の融点以上に加熱するため、電磁力の影響により溶鋼が流動し、生成された酸化物が衝合溶接部に噛み込まれペネトレータ等の溶接欠陥あるいは、溶鋼飛散（フラッシュ）が発生しやすいという問題があった。この問題に対し、例えば、特開平２−２９９７８２号公報には、２つの加熱装置を有する電縫鋼管の製造法が提案されている。第１の加熱装置でオープン管の両側エッジ部の温度をキュリー点以上に加熱し、第２の加熱装置で更に融点以上に加熱し、スクイズロールで両エッジ部を衝合溶接して鋼管を製造する。また、特開平２−２９９７８３号公報には、第１の加熱装置で周波数４５〜２５０ｋＨｚの電流を流し、両側エッジ部を予熱し、第２の加熱装置で更に融点以上に加熱し、スクイズロールで両エッジ部を衝合溶接して鋼管を製造する電縫管製造装置が提案されている。
【０００４】
しかしながら、これらの電縫管製造技術では、エッジ部を均一に加熱することは示唆しているものの、両エッジ部を鋼の融点以上に加熱するため、衝合溶接時に、溶融した鋼が管の内外面に排出されビード（余盛）が形成される。そのため、衝合溶接後に管内外面の溶接ビードの除去が必要であり、ほとんどがビード切削用バイトにより切削されて除去されている。
【０００５】
このようなことから、この方法では、
▲１▼ビード切削用バイトの切削量の調整で、材料と時間のロスが発生する。
▲２▼ビード切削用バイトは消耗品であり、造管速度によって異なるが、３０００〜４０００ｍのビード切削長毎にバイトを交換する必要があり、そのため、１時間程度ごとに３〜５分間のバイト交換のためのライン停止を余儀なくされる。
【０００６】
▲３▼特に造管速度が１００ｍ／ｍｉｎを超える高速造管では、ビード切削用バイトの寿命が短く、交換頻度が高い。
など、ビード切削がネックとなり、高速造管ができないため生産性が低いという問題があった。
一方、比較的小径鋼管用として極めて高い生産性を有する鍛接鋼管製造方法がある。この方法は、連続的に供給した帯鋼を加熱炉で１３００℃程度に加熱した後、成形ロールで管状に成形してオープン管とし、続いてオープン管の両エッジ部に高圧空気を吹き付けて端面のスケールオフを行った後、ウェルディングホーンにより端面に酸素を吹き付け、その酸化熱で端面を１４００℃程度に昇温させてから、鍛接ロールで両エッジ部端面を衝合させ固相接合して鋼管を製造する方法である（例えば、第３版鉄鋼便覧第ＩＩＩ巻（２）１０５６〜１０９２頁）。
【０００７】
しかし、この鍛接鋼管製造方法では、
▲１▼端面のスケールオフが完全ではないので、鍛接衝合部へのスケール噛込みが発生し、シーム部の強度が母材部に比べてかなり劣る。このため、偏平試験で、電縫鋼管なら偏平高さ比ｈ／Ｄ＝２ｔ／Ｄ（ｔ：板厚）を達成できるのに対し、鍛接鋼管では偏平高さ比ｈ／Ｄが０．５程度に劣るものとなる。
【０００８】
▲２▼帯鋼を高温に加熱するため、管表面にスケールが生成し表面肌が悪い。
など、造管速度が３００ｍ／ｍｉｎ以上と速く生産性は高いが、シーム品質及び表面肌が悪く、ＪＩＳのＳＴＫ等の強度信頼性や表面品質を要求されるものは製造できないという問題があった。
【０００９】
【発明が解決しようとする課題】
本発明は、上記問題を有利に解決し、優れたシーム品質及び表面肌を有する鋼管を高い生産性で製造することができる、高周波加熱接合方式での鋼管の製造方法を提供することを目的とする。
【００１０】
【課題を解決するための手段】
第１の本発明は、オープン管の両エッジ部を加熱し、スクイズロールにより衝合接合する鋼管の製造方法において、まずオープン管全体を２００℃〜７００℃に昇温し、ついで両エッジ部を２段階以上に分けて高周波加熱してキュリー点以上で融点未満の温度域に到達させ、さらに高周波加熱して衝合接合することを特徴とする鋼管の製造方法である。
【００１１】
第２の本発明は、第１の本発明において、２００ ℃〜７００ ℃のオープン管の両エッジ部をキュリー点以上融点未満の温度域に到達させる高周波加熱を、第２段目を１００ｋＨｚ未満かつ第１段目より低い周波数で行うことを要旨とする。
第３の本発明は、第１または第２の本発明のいずれかにおいて、両エッジ部の加熱に先立つオープン管の昇温を、オープン管を加熱して行うことを要旨とする。
【００１２】
第４の本発明は、第１または第２の本発明のいずれかにおいて、両エッジ部の加熱に先立つオープン管の昇温を、オープン管に成形する前の帯鋼を加熱して行うことを要旨とする。
第５の本発明は、第１または第２の本発明のいずれかにおいて、両エッジ部の加熱に先立つオープン管の昇温を、オープン管に成形する前の帯鋼を加熱しさらにオープン管を加熱して行うことを要旨とする。
【００１３】
【発明の実施の形態】
第１の本発明によれば、オープン管は、その全体が２００℃〜７００℃に昇温され、ついで両エッジ部が２段階以上の高周波加熱によりキュリー点以上で融点未満の温度域に昇温され、さらに高周波加熱されて衝合接合される。
オープン管全体を２００〜７００℃に昇温する（管体予熱という）のは、衝合接合時のエッジ部端面温度差を小さくするために必須の要件であり、これにより、エッジ部とエッジ部に連なる母管部との温度差が小さくなり、エッジ部から母管部への熱拡散が効果的に抑制できて、固相圧接のための周辺温度環境が整う。なお、固相圧接可能温度域は、１３００℃以上融点未満、好ましくは１３５０℃以上融点未満、より好ましくは１４００℃以上融点未満の温度域である。
【００１４】
管体予熱温度が２００℃未満のとき、エッジ部から母管部への熱拡散の抑制効果が小さく接合時の温度を固相圧接可能温度域に維持するのが困難となり、シーム品質が劣化する。他方、７００℃を超える管体予熱は、相対する接合すべきエッジ部を含む管体全面にスケールが生成・成長し、製品のシーム品質及び表面肌がともに劣化する結果を招く。なお、管体予熱において、より好ましいのは４００℃以上６５０℃以下の温度域である。
【００１５】
管体予熱で管全体が２００〜７００ ℃に保持されたオープン管の両エッジ部は、キュリー点（７７０ ℃程度）以上融点未満好ましくは１３００℃未満の温度域まで、２段階以上の高周波加熱によって昇温される。そして、そこからさらに高周波加熱されて衝合接合される。衝合接合直前のエッジ部の高周波加熱を本加熱、その前段階の２段階以上の高周波加熱を予備加熱という。
【００１６】
エッジ部の加熱方法をこのように規定する理由を以下に述べる。
図３は鋼の比透磁率の温度依存性を示す特性図である。この図に示されるように、鋼はキュリー点以上の温度域で強磁性体から常磁性体に磁気変態し、比透磁率（対真空比）が１に近い値となる。一方、電流の浸透深さは次式（１）で与えられる。
【００１７】
Ｓ＝α｛ρ／（μ_ｒｆ）｝^１／２………（１）
ここに、Ｓ：浸透深さ（ｍ）、ρ：抵抗率（Ω・ｍ）、μ_ｒ：比透磁率、ｆ：周波数（ｋＨｚ）、α：定数である。また、図４は、浸透深さの温度および周波数依存性を示す特性図である。
エッジ部を、本加熱前にキュリー点以上に予備加熱することにより、本加熱段階において、式（１）で与えられ図４にも示される浸透深さが大きくなり、被接合面（エッジ部端面）内の温度が均一化の方向に向かう。
【００１８】
しかしながら、エッジ部のキュリー点以上への予備加熱を１段階の高周波加熱で行うのは、予備加熱終了時点（すなわち本加熱開始時点）でのエッジ部端面の温度差が十分に縮まらず、本加熱における誘導加熱でそれが拡大し、衝合接合の際に、角部の過剰溶融によるビードの盛り上がり、または面部の部分的接合不良のいずれかが発生しやすくなる。
【００１９】
これに対し、キュリー点以上融点未満の温度域への予備加熱を２段階以上に分けて行うことにより、この問題は解消し、衝合接合時点でのエッジ部端面全体を固相圧接可能温度域に安定的に保持できる。
なお、予備加熱、本加熱の高周波加熱は、直接通電による方法と誘導コイルによる方法とのいずれも適用可能であるが、直接通電による方法は管体にコンタクトチップを接触させねばならず、高速造管の際スパーク疵の原因となるから、非接触である誘導コイルによる方法（誘導加熱方式）のほうが好ましい。
【００２０】
第２の本発明は、第１の本発明において、エッジ部の予備加熱を２段階の高周波加熱で行うときの好適周波数を規定したもので、第２段目を１００ｋＨｚ未満かつ第１段目より低い周波数で行うものである。これにより、予備加熱終了時のエッジ部端面内温度差とその時点で角部を目標温度に到達させるに要する電流値との最適な組合せが得られる。
【００２１】
これに対し、第２段目を１００ｋＨｚ以上の周波数で予備加熱すると、所要電流値は節減できるが端面内温度差が拡大し、また第２段目を１００ｋＨｚ未満かつ第１段目以上の周波数とすると端面内温度差の拡大はないが、所要電流値が大幅に増大する。
第３〜第５の本発明は、管体予熱の具体的加熱方法に関する。管体予熱は、予備加熱段階でのエッジ部と母管部との温度差を小さくすることを骨子とするので、予備加熱直前のオープン管全体を２００〜７００℃の温度域に保持できさえすれば、加熱はオープン管自体に施しても、オープン管に成形する前の帯鋼に施しても、またその両方に施してもよく、本発明を適用する造管ラインの事情に応じて適宜選択可能である。なお、管体予熱の加熱方式は特に限定されず、炉内加熱、誘導加熱、通電加熱等いずれの方式で行ってもよい。
【００２２】
以上に述べたように、本発明によれば、高周波加熱方式を用いてスクイズロールの直前でエッジ部端面全域を固相圧接可能温度域に安定的に保持できるから、優れたシーム品質及び表面肌を有する鋼管を高い生産性で製造することができる。
【００２３】
【実施例】
図１は本発明の実施例に用いた装置列の模式側面図である。図１において、１はオープン管、２は鋼管、３はスクイズロール、４は本加熱コイル、５Ａ、５Ｂは２段階の予備加熱コイル、６は本加熱域、７は予備加熱域、８は加熱炉、１０はエッジ部である。
【００２４】
図２は、実施例の予備加熱パターンの例を示す線図であり、実線、点線はエッジ部１０の角部、面部の温度を夫々示し、図１と同一部材には同一符号を付し説明を省略する。
オープン管１が加熱炉８で全体加熱（管体予熱）された後、順次、予備加熱域７、本加熱域６を経てスクイズロール３でエッジ部１０を固相圧接されて、鋼管２が形成されるように装置列を構成し、予備加熱域７における加熱方式は、予備加熱コイル５Ａ，５Ｂによる２段階誘導加熱方式とした。なお、特に限定されないが、予備加熱コイル５Ａ，５Ｂは水平型ワンターンコイルをタンデムに配置し、本加熱コイル４は垂直型スリーターンコイルを１段に配置した。
【００２５】
ところで、本発明は固相圧接法を採用するので電縫管のような溶接余盛部は形成されないが、圧接のアップセット量によっては圧接点において軟化部が外面余肉を生じることもあり得る。これを防ぐには、図１に示すように、スクイズロール３をそのロール面が圧接点を踏むように配置することが好ましい。
０．０８ｗｔ％炭素鋼の帯鋼を成形ロールにより成形してオープン管とし、これを図１の装置列に表１に示す種々の加熱条件で通し、両エッジ部を固相圧接して製造した寸法６０．５ｍｍＤ×６．０ｍｍｔの鋼管について行った偏平試験の結果を同表１に示す。表１中、◎は偏平高さ比２ｔ／Ｄ（密着）まで割れなし、○は４ｔ／Ｄまで割れなし、×はそれ超えの偏平高さ比で割れを生じたものである。
【００２６】
【表１】

【００２７】
いずれの鋼管も、エッジ部を予備加熱する前に加熱炉８でオープン管全体を６００℃に加熱して管体予熱を行った。この加熱による管体表面の肌荒れは認められなかった。
この状態から、鋼管Ａは予備加熱を行わずいきなり周波数４００ｋＨｚで本加熱したので、角部が融点直下に達したとき面部が１３００℃を下回り、シーム品質が劣化した。鋼管Ｂは、予備加熱域７を１本の誘導コイルでカバーして予備加熱を行ったものであるが、予備加熱終了時点での端面内温度差が１００ ℃以上となり、鋼管Ａ同様シーム品質が劣化した。
【００２８】
第１の本発明に従い予備加熱を２段階に分けて行った鋼管Ｃ〜Ｆは、いずれも予備加熱終了時点での端面内温度差１００ ℃未満で、偏平高さ比が４ｔ／Ｄ以下の良好なシーム品質が得られた。
これらのうち、２段目の周波数が１００ｋＨｚ以上で第２の本発明の規定外である鋼管Ｃ、Ｅは、端面内温度差が５０℃以上で、偏平試験において密着までに割れを生じ、２段目の周波数は１００ｋＨｚ未満であるが１段目の周波数と同じであって、第２の本発明の規定を外れる鋼管Ｆは、端面内温度差１０℃以内であり偏平試験では密着まで割れを生じなかったが、所要電流値が７０００Ａと大きかった。これに対し、第２の本発明の規定を満たす鋼管Ｄは、端面温度差１０℃以内であり偏平試験では密着まで割れを生ぜず、所要電流値も４４００Ａと適度に抑制された。
【００２９】
【発明の効果】
本発明によれば、高周波加熱方式を用いてスクイズロールの直前でエッジ部の全域を固相圧接可能温度域に安定的に保持できるから、優れたシーム品質及び表面肌を有する鋼管を高い生産性で製造することができるという格段の効果を奏する。
【図面の簡単な説明】
【図１】本発明の実施例に用いた装置列の（ａ）は模式側面図、（ｂ）はＡ−Ａ矢視図、（ｃ）はＢ−Ｂ矢視図である。
【図２】実施例の予備加熱パターンの例を示す線図である。
【図３】鋼の比透磁率の温度依存性を示す特性図である。
【図４】浸透深さの温度および周波数依存性を示す特性図である。
【符号の説明】
１オープン管
２鋼管
３スクイズロール
４本加熱コイル
５Ａ，５Ｂ予備加熱コイル
６本加熱域
７予備加熱域
８加熱炉
１０エッジ部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a welded steel pipe, and particularly to a method for manufacturing a steel pipe by solid-state pressure welding.
[0002]
[Prior art]
Welded steel pipes are formed by forming a steel plate or a steel strip into a tubular shape and welding the seams of the tube, and are manufactured by various manufacturing methods from small diameter to large diameter. The main manufacturing method is electric resistance welding (electric resistance welding). , Forge welding, and arc welding.
For small to medium diameter steel pipes, an electric resistance welding method (electric resistance welded steel pipe, electric resistance welded pipe) using high frequency induction heating is mainly used. In this method, a steel strip is continuously supplied, formed into a tubular shape by a forming roll to form an open pipe, and then the end faces of both edges of the open pipe are heated to a temperature equal to or higher than the melting point of the steel by high-frequency induction heating. And a method for producing a steel pipe by abutting the end faces of both edges (for example, 3rd Edition Iron and Steel Handbook, Vol. III (2), pp. 1056-1092).
[0003]
In the above-described method of manufacturing an electric resistance welded tube using high-frequency induction heating, since both end portions of the open tube are heated to a temperature equal to or higher than the melting point of steel, molten steel flows under the influence of electromagnetic force, and the generated oxide is impinged. There has been a problem that welding defects such as a penetrator or the like, or molten steel scatter (flash) are likely to occur due to being bitten by a joint weld. To cope with this problem, for example, Japanese Patent Application Laid-Open No. 2-299792 proposes a method for manufacturing an electric resistance welded steel pipe having two heating devices. The first heating device heats the temperature of both sides of the open pipe to the Curie point or higher, the second heating device heats it to the melting point or more, and squeeze rolls to weld both edges to produce steel pipe. I do. In Japanese Patent Application Laid-Open No. 2-299832, a current having a frequency of 45 to 250 kHz is passed by a first heating device to preheat both side edges, and further heated to a melting point or higher by a second heating device, and then squeezed by a squeeze roll. There has been proposed an electric resistance welded pipe manufacturing apparatus that manufactures a steel pipe by butting and welding both edges.
[0004]
However, although these ERW pipe manufacturing techniques suggest that the edges are heated evenly, since both edges are heated above the melting point of steel, the molten steel is It is discharged to the inner and outer surfaces to form a bead (excess). Therefore, it is necessary to remove the weld bead on the inner and outer surfaces of the pipe after the impact welding, and most of the bead is removed by cutting with a bead cutting tool.
[0005]
For this reason, in this method,
(1) Adjustment of the cutting amount of the bead cutting tool causes loss of material and time.
(2) The bead cutting tool is a consumable and depends on the pipe forming speed. However, it is necessary to change the tool every bead cutting length of 3000 to 4000 m. The line must be stopped for replacement.
[0006]
{Circle around (3)} Particularly in high-speed pipe forming in which the pipe forming speed exceeds 100 m / min, the life of the bead cutting tool is short and the frequency of replacement is high.
There was a problem that productivity was low because bead cutting became a bottleneck and high-speed pipe making was not possible.
On the other hand, there is a method for manufacturing a forged steel pipe having extremely high productivity for a relatively small diameter steel pipe. In this method, a continuously supplied steel strip is heated to about 1300 ° C. in a heating furnace, then formed into a tubular shape by a forming roll to form an open pipe, and high-pressure air is blown to both edges of the open pipe to form an end face. After the scale-off is performed, oxygen is blown to the end face by a welding horn, and the end face is heated to about 1400 ° C. by the heat of oxidation. This is a method for producing a steel pipe (for example, Third Edition Iron and Steel Handbook, Vol. III, (2), pp. 1056-1092).
[0007]
However, in this forged steel pipe manufacturing method,
{Circle around (1)} Since the scale-off of the end face is not perfect, the scale bites into the forged contact portion, and the strength of the seam portion is considerably inferior to that of the base material portion. For this reason, in the flattening test, the flattened height ratio h / D = 2t / D (t: plate thickness) can be achieved with the ERW steel pipe, while the flattened height ratio h / D is about 0.5 with the forged steel pipe. Is inferior to
[0008]
{Circle around (2)} Since the steel strip is heated to a high temperature, scale is formed on the pipe surface, and the surface skin is poor.
For example, the pipe making speed is as fast as 300 m / min or more and the productivity is high, but the seam quality and surface skin are poor, and there is a problem that products requiring strength reliability and surface quality such as JIS STK cannot be manufactured. .
[0009]
[Problems to be solved by the invention]
An object of the present invention is to provide a method for manufacturing a steel pipe by a high-frequency heating joining method, which can solve the above-described problems advantageously and can manufacture a steel pipe having excellent seam quality and surface skin with high productivity. I do.
[0010]
[Means for Solving the Problems]
According to a first aspect of the present invention, in a method for manufacturing a steel pipe in which both edges of an open pipe are heated and abutted and joined by a squeeze roll, first, the entire open pipe is heated to 200 to 700 ° C. This is a method for producing a steel pipe, characterized in that high-frequency heating is performed in two or more stages to reach a temperature range equal to or higher than the Curie point but lower than the melting point, and further high-frequency heating is performed for abutment joining.
[0011]
According to a second aspect of the present invention, in the first aspect of the present invention, the high-frequency heating for causing both edges of the open tube at 200 ° C. to 700 ° C. to reach a temperature range from the Curie point to the melting point is performed. The point is to perform at a frequency lower than the first stage.
According to a third aspect of the present invention, in any one of the first and second aspects of the present invention, the temperature of the open pipe is increased by heating the open pipe prior to heating both edges.
[0012]
According to a fourth aspect of the present invention, in any one of the first and second aspects of the invention, the temperature of the open pipe prior to the heating of both edges is increased by heating the steel strip before forming the open pipe. Make a summary.
According to a fifth aspect of the present invention, in any one of the first and second aspects of the present invention, the temperature of the open pipe prior to heating both edges is increased by heating the steel strip before forming the open pipe. The gist is that heating is performed.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
According to the first aspect of the present invention, the temperature of the entire open tube is raised to 200 ° C. to 700 ° C., and then both edges are heated to a temperature range higher than the Curie point and lower than the melting point by two or more stages of high-frequency heating. Then, it is subjected to high-frequency heating and abutment bonding.
Raising the temperature of the entire open pipe to 200 to 700 ° C. (referred to as preheating of the pipe body) is an essential requirement to reduce the difference in the temperature of the end face at the time of abutment joining. The temperature difference from the mother pipe part connected to the pipe becomes small, the heat diffusion from the edge part to the mother pipe part can be effectively suppressed, and the surrounding temperature environment for solid-phase pressure welding is prepared. The solid-state pressure-contactable temperature range is 1300 ° C. or higher and lower than the melting point, preferably 1350 ° C. or higher but lower than the melting point, more preferably 1400 ° C. or higher but lower than the melting point.
[0014]
When the tube preheating temperature is less than 200 ° C., the effect of suppressing heat diffusion from the edge portion to the mother tube portion is small, and it is difficult to maintain the temperature at the time of joining in the temperature range where solid-state pressure welding is possible, resulting in deterioration of seam quality. . On the other hand, preheating of the tube at a temperature exceeding 700 ° C. causes scale to be generated and grown on the entire surface of the tube including the opposed edges to be joined, resulting in deterioration of both the seam quality and surface skin of the product. In the tube preheating, a temperature range of 400 ° C. or more and 650 ° C. or less is more preferable.
[0015]
Both edges of the open tube whose entire tube is kept at 200 to 700 ° C. by preheating the tube are heated by two or more stages of high-frequency heating to a temperature range of not less than the Curie point (about 770 ° C.) and less than the melting point, preferably less than 1300 ° C. The temperature is raised. Then, it is further heated at a high frequency and joined by abutment. The high-frequency heating of the edge portion immediately before the abutment joining is called main heating, and the high-frequency heating of two or more stages before that is called preheating.
[0016]
The reason for defining the heating method of the edge portion in this manner will be described below.
FIG. 3 is a characteristic diagram showing the temperature dependence of the relative magnetic permeability of steel. As shown in this figure, the steel undergoes magnetic transformation from a ferromagnetic material to a paramagnetic material in a temperature range above the Curie point, and the relative magnetic permeability (vs. vacuum ratio) becomes a value close to 1. On the other hand, the penetration depth of the current is given by the following equation (1).
[0017]
S = α {ρ / (μ _rf )} ^1/2 (1)
Here, S: penetration depth (m), ρ: resistivity (Ω · m), μ _r : relative permeability, f: frequency (kHz), α: constant. FIG. 4 is a characteristic diagram showing the temperature and frequency dependence of the penetration depth.
By preheating the edge portion to a temperature higher than the Curie point before the main heating, the penetration depth given by the equation (1) shown in FIG. The temperature in parentheses) is headed for uniformity.
[0018]
However, performing the preheating to the Curie point or higher at the edge by one-stage high-frequency heating is because the temperature difference at the edge of the edge at the end of the preheating (that is, at the start of the main heating) is not sufficiently reduced. In the case of the abutment, the bead bulges due to excessive melting of the corners, or a partial bonding failure of the surface is likely to occur.
[0019]
On the other hand, this problem is solved by performing the preheating to the temperature range of the Curie point or higher and lower than the melting point in two or more stages. Can be held stably.
For the high-frequency heating of the preheating and the main heating, either a method using direct current or a method using an induction coil can be applied. The method using a non-contact induction coil (induction heating method) is more preferable because it causes spark flaws in the case of a pipe.
[0020]
According to a second aspect of the present invention, in the first aspect of the present invention, a preferable frequency for performing preheating of the edge portion by two-stage high-frequency heating is specified. This is performed at a low frequency. Thus, an optimum combination of the temperature difference in the end face of the edge portion at the end of the preheating and the current value required to reach the target temperature at the corner at that time can be obtained.
[0021]
On the other hand, if the second stage is pre-heated at a frequency of 100 kHz or more, the required current value can be reduced, but the temperature difference in the end face increases, and the second stage is less than 100 kHz and the first stage or more When the frequency is used, the temperature difference in the end face does not increase, but the required current value increases significantly.
The third to fifth aspects of the present invention relate to a specific heating method for preheating a tubular body. Since the preheating of the tube is based on reducing the temperature difference between the edge portion and the mother tube portion in the preheating stage, the entire open tube immediately before the preheating can be maintained in a temperature range of 200 to 700 ° C. For example, the heating may be applied to the open pipe itself, to the steel strip before forming into the open pipe, or to both of them, and may be appropriately selected according to the circumstances of the pipe forming line to which the present invention is applied. It is possible. In addition, the heating method of the tube preheating is not particularly limited, and may be any method such as furnace heating, induction heating, and electric heating.
[0022]
As described above, according to the present invention, the entire end face of the edge portion can be stably maintained in the solid-state pressure-contactable temperature range immediately before the squeeze roll by using the high-frequency heating method, so that excellent seam quality and surface texture can be obtained. Can be manufactured with high productivity.
[0023]
【Example】
FIG. 1 is a schematic side view of an apparatus row used in an embodiment of the present invention. In FIG. 1, 1 is an open pipe, 2 is a steel pipe, 3 is a squeeze roll, 4 is a main heating coil, 5A and 5B are two-stage preheating coils, 6 is a main heating area, 7 is a preheating area, and 8 is heating. Furnace 10 is an edge portion.
[0024]
FIG. 2 is a diagram showing an example of a preheating pattern according to the embodiment. A solid line and a dotted line show the temperatures of the corners and the surface of the edge portion 10, respectively, and the same members as those in FIG. Is omitted.
After the open pipe 1 is entirely heated (pipe preheating) in the heating furnace 8, the edge portion 10 is solid-phase pressed with the squeeze roll 3 through the preheating area 7 and the main heating area 6 to form the steel pipe 2. The heating system in the preheating zone 7 was a two-stage induction heating system using preheating coils 5A and 5B. Although not particularly limited, the preheating coils 5A and 5B have horizontal one-turn coils arranged in tandem, and the main heating coil 4 has vertical three-turn coils arranged in one stage.
[0025]
By the way, since the present invention employs the solid-state pressure welding method, a weld excess portion such as an electric resistance welded tube is not formed.However, depending on the upset amount of the pressure welding, the softened portion of the pressure contact may cause extra outer surface. . In order to prevent this, as shown in FIG. 1, it is preferable to arrange the squeeze roll 3 so that the roll surface of the squeeze roll steps on the pressure contact.
A 0.08 wt% carbon steel strip was formed by a forming roll into an open pipe, which was passed through the apparatus row of FIG. 1 under various heating conditions shown in Table 1, and both edges were solid-phase pressed to produce an open pipe. Table 1 shows the results of the flattening test performed on the steel pipe having the dimensions of 60.5 mmD × 6.0 mmt. In Table 1, ◎ indicates no cracks up to a flat height ratio of 2 t / D (adhesion), ○ indicates no cracks up to 4 t / D, and X indicates cracks occurred at a flat height ratio exceeding that.
[0026]
[Table 1]

[0027]
Before preheating the edges of the steel pipes, the entire open pipe was heated to 600 ° C. in the heating furnace 8 to preheat the pipes. No roughening of the tube surface due to this heating was observed.
From this state, since the steel pipe A was immediately pre-heated at a frequency of 400 kHz without performing pre-heating, when the corner portion reached just below the melting point, the surface portion fell below 1300 ° C., and the seam quality was deteriorated. The steel pipe B was preheated by covering the preheating area 7 with one induction coil. At the end of the preheating, the temperature difference in the end face was 100 ° C. or more, and the seam quality was the same as that of the steel pipe A. Deteriorated.
[0028]
The steel pipes C to F in which the preheating was performed in two stages according to the first invention had good temperature differences of less than 100 ° C. in the end face at the end of the preheating and a flat height ratio of 4 t / D or less. Seam quality was obtained.
Among these, the steel pipes C and E whose second-stage frequency is 100 kHz or more and which is out of the range of the second invention have a temperature difference in the end face of 50 ° C. or more, and generate cracks by the flattening test until the adhesion, The frequency of the second stage is less than 100 kHz but the same as the frequency of the first stage, and the steel pipe F which does not meet the requirements of the second invention has a temperature difference within the end face of 10 ° C. or less. Although no cracking occurred, the required current value was as large as 7000A. On the other hand, the steel pipe D satisfying the requirements of the second present invention had an end face temperature difference of 10 ° C. or less, did not crack until the adhesion in the flat test, and the required current value was moderately suppressed to 4400 A.
[0029]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, since the whole area | region of an edge part can be stably hold | maintained in the solid-state pressure welding possible temperature area just before a squeeze roll using a high frequency heating system, the steel pipe which has excellent seam quality and surface skin can be obtained with high productivity. This has a remarkable effect of being able to be manufactured by using a liquid crystal display.
[Brief description of the drawings]
FIGS. 1A and 1B are schematic side views, FIG. 1B is a view taken along the line AA, and FIG.
FIG. 2 is a diagram showing an example of a preheating pattern according to the embodiment.
FIG. 3 is a characteristic diagram showing temperature dependence of relative magnetic permeability of steel.
FIG. 4 is a characteristic diagram showing the temperature and frequency dependence of the penetration depth.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Open pipe 2 Steel pipe 3 Squeeze roll 4 Heating coil 5A, 5B Preheating coil 6 Heating area 7 Preheating area 8 Heating furnace 10 Edge part

Claims

In a method of manufacturing a steel pipe in which both edges of an open pipe are heated and joined by a squeeze roll, first the entire open pipe is heated to 200 to 700 ° C. A method for producing a steel pipe, comprising heating to reach a temperature range of not less than the Curie point and less than a melting point, and further performing high-frequency heating and abutment joining.

2. The high-frequency heating for causing both edge portions of the open tube at 200 ° C. to 700 ° C. to reach a temperature range of the Curie point or higher and lower than the melting point is performed in the second stage at a frequency lower than 100 kHz and lower than the first stage. Method.

The method according to claim 1 or 2, wherein the temperature of the open pipe is increased by heating the open pipe prior to heating both edges.

3. The method according to claim 1, wherein the temperature of the open pipe is raised before heating the edge portions by heating the steel strip before forming the open pipe. 4.

3. The method according to claim 1, wherein the temperature of the open pipe is raised before heating both edges by heating the steel strip before forming the open pipe and further heating the open pipe.