JP6437415B2 - Method of hot bending steel pipe - Google Patents

Method of hot bending steel pipe Download PDF

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JP6437415B2
JP6437415B2 JP2015196061A JP2015196061A JP6437415B2 JP 6437415 B2 JP6437415 B2 JP 6437415B2 JP 2015196061 A JP2015196061 A JP 2015196061A JP 2015196061 A JP2015196061 A JP 2015196061A JP 6437415 B2 JP6437415 B2 JP 6437415B2
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steel pipe
heating
copper
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zinc
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JP2017064771A (en
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理生 鈴森
理生 鈴森
義之 永山
義之 永山
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Toyoda Iron Works Co Ltd
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Description

本発明は鋼管を熱間曲げ加工する工法に関するものであり、詳しくは表面に亜鉛や銅の不純物が付着した鋼管を、溶融金属脆化を抑えながら熱間曲げ加工する方法に関するものである。   The present invention relates to a method of hot-bending a steel pipe, and more particularly to a method of hot-bending a steel pipe having zinc or copper impurities adhered to its surface while suppressing molten metal embrittlement.

本来延性を示すはずの固体金属にある種の液体金属を接触させると突然脆性を示すようになる液体金属脆化という現象が古くから知られている。特に母材の金属が多結晶固体である場合は、その固体金属の結晶粒界に液体金属原子が急速に浸透していくことがこの脆化現象のひとつの原因であると考えられている。結晶粒界に液体金属が浸み込むと材料の強度が著しく低下するため、例えば自動車の足回り部分に使われる鋼鉄製部材など、繰り返しの大きな曲げ荷重にさらされる機械部材ではこのような脆化が起きないようにすることが重要である。材料の脆化を抑えることができれば製品を薄く軽くできるため、設計の自由度が高まる。   A phenomenon known as liquid metal embrittlement that suddenly becomes brittle when a certain type of liquid metal is brought into contact with a solid metal that should originally exhibit ductility has long been known. In particular, when the base metal is a polycrystalline solid, it is considered that one of the causes of this embrittlement phenomenon is that liquid metal atoms rapidly penetrate into the crystal grain boundaries of the solid metal. When liquid metal penetrates into the grain boundaries, the strength of the material decreases significantly.For example, steel members used in the undercarriage parts of automobiles, such as mechanical parts exposed to repeated large bending loads, are brittle. It is important to prevent this from happening. If the embrittlement of the material can be suppressed, the product can be made thinner and lighter, which increases the degree of freedom in design.

特許文献１では、ステンレス鋼管を曲げる際に割れが発生するのを防止するため、鋼管を加熱する前に鋼管の表面に付着している銅、亜鉛、アルミニウムなどの低融点金属を研磨によって除去するとともに表面を滑らかにする工程を設けた方法が記載されている。しかしながら熱処理の前に研磨を行うのは時間が掛かるうえに特別な設備を要するため費用も掛かり好ましくない。   In Patent Document 1, in order to prevent cracks from occurring when a stainless steel pipe is bent, low melting point metals such as copper, zinc, and aluminum adhering to the surface of the steel pipe are removed by polishing before the steel pipe is heated. A method of providing a process for smoothing the surface is also described. However, it is not preferable to perform the polishing before the heat treatment because it takes time and requires special equipment.

特許文献２では、亜鉛めっき鋼板を加熱炉内で９００°Ｃ程度で４分間ほど加熱した後に熱間プレス成形する方法が記載されている。このプレス成形前の加熱によりめっき層の鉄亜鉛固溶体や酸化亜鉛以外の亜鉛を蒸発させ、熱間プレス成形時には液体亜鉛が残らず母材鋼板の表面に脆化による割れが全く発生しないようにしている。   Patent Document 2 describes a method of hot press forming after heating a galvanized steel sheet at about 900 ° C. for about 4 minutes in a heating furnace. This heating before press forming evaporates zinc other than the iron zinc solid solution and zinc oxide in the plating layer so that no liquid zinc remains during hot press forming so that no cracks due to embrittlement occur on the surface of the base steel plate. Yes.

特開昭５４−０６２１６２号公報JP 54-061622 A 特開２０１３−１８４２２１号公報JP 2013-184221 A

鉄鋼素材の表面には生産の段階から微細な割れ（クラック）が自然に一定の割合で存在し、これをあらかじめ完全になくすことは困難である。この自然な割れは素材を曲げ加工するときに曲がりの外側で広がる傾向にあるが、大きく開口した割れがあるとそこに溶融金属が入り込んで新たな表面に接触するため上述の溶融金属脆化がさらに起きやすくなる。実験によれば、表面にある一定の値以上の引っ張り方向の歪みが生じたときに溶融金属の浸入が起きやすくなることがわかっている。したがって、素材を熱間曲げ加工する必要がある場合には、曲げの時点で脆化を起こすような液体金属が素材表面になるべく存在しないことが望ましい。しかし特許文献１にはこのように曲げによって生じうる問題について何も触れていない。また、特に鋼管素材の場合、特許文献１に記載されているような研磨を鋼管の内側表面に施すのは難しい。   Fine cracks are naturally present on the surface of the steel material from the production stage, and it is difficult to eliminate them completely in advance. This natural crack tends to spread outside the bend when bending the material, but if there is a large open crack, the molten metal enters there and contacts the new surface, so the above-mentioned molten metal embrittlement occurs. It becomes easier to get up. Experiments have shown that molten metal intrusion is more likely to occur when a tensile strain greater than a certain value occurs on the surface. Therefore, when it is necessary to hot-bend the material, it is desirable that a liquid metal that causes embrittlement at the time of bending does not exist on the material surface as much as possible. However, Patent Document 1 does not mention anything about problems that can occur due to bending. In particular, in the case of a steel pipe material, it is difficult to polish the inner surface of the steel pipe as described in Patent Document 1.

本発明では、鉄鋼素材の製造過程で付着しやすくかつ素材の脆化を起こす可能性のある不純物金属として、特に亜鉛と銅に着目している。亜鉛は沸点が約９０６°Ｃと低いため、上記特許文献２で示唆されているように鉄鋼素材を焼き入れ温度まで加熱することにより、蒸発させて表面から除去できる。一方、銅は融点が約１０８３°Ｃ以上と高く、通常の焼き入れ温度では溶融しないため、脆化には寄与しないと考えられた。しかし、銅とともに亜鉛が存在する場合、亜鉛が銅の中に固溶したり銅と合金（真鍮など）を形成したりして融点が例えば８４０°Ｃ程度までは下がる可能性があるため、銅も液体状態となって脆化に影響しうると本発明者は考えた。本発明は以上のような問題の少なくともひとつを抑制することを課題とする。   In the present invention, attention is particularly paid to zinc and copper as impurity metals that are likely to adhere during the manufacturing process of steel materials and that may cause embrittlement of the materials. Since zinc has a low boiling point of about 906 ° C., it is possible to evaporate and remove it from the surface by heating the steel material to the quenching temperature as suggested in Patent Document 2 above. On the other hand, copper has a high melting point of about 1083 ° C. or higher and does not melt at a normal quenching temperature, so it was considered that it does not contribute to embrittlement. However, when zinc is present together with copper, zinc may be dissolved in copper or may form an alloy with copper (such as brass), and the melting point may be lowered to, for example, about 840 ° C. The present inventor has thought that the liquid state can also be affected by embrittlement. An object of the present invention is to suppress at least one of the above problems.

本発明のひとつの形態としての方法は、鋼管を熱間曲げ加工する工法であって、表面に亜鉛と銅を含む不純物が付着した鋼管を曲げることなく亜鉛の沸点以上の温度に加熱した後、この鋼管を一時的に冷却し、その後焼き入れ可能な温度以上かつ銅の融点未満の温度でその鋼管を局所的に加熱しながら曲げ、この曲げられた鋼管を冷却して焼入れする。   The method as one embodiment of the present invention is a method of hot bending a steel pipe, and after heating the steel pipe with impurities including zinc and copper attached to the surface to a temperature not lower than the boiling point of zinc, The steel pipe is temporarily cooled, then bent while locally heating the steel pipe at a temperature higher than the quenchable temperature and lower than the melting point of copper, and the bent steel pipe is cooled and quenched.

好ましい実施例として、鋼管がめっきや塗布により表面被膜が形成されていない裸材とすることもできる。   As a preferred embodiment, the steel pipe may be a bare material in which a surface coating is not formed by plating or coating.

また好ましい実施例として、２回目の加熱を誘導加熱により行うこともできる。さらに１回目の加熱も誘導加熱により行い、間隔を空けて配置された各誘導コイルの中に鋼管を連続的に通すことによりそれぞれの加熱を行うこともできる。   In a preferred embodiment, the second heating can be performed by induction heating. Further, the first heating can also be performed by induction heating, and the respective heating can be performed by continuously passing the steel pipe through the induction coils arranged at intervals.

また好ましい実施例として、鋼管の断面を長径と短径をもつ非円形断面とし、曲げる工程において長径側に曲げるようにすることもできる。   As a preferred embodiment, the cross section of the steel pipe may be a non-circular cross section having a major axis and a minor axis, and bent in the major axis side in the bending step.

本発明によれば鋼管の表面に付着した亜鉛や銅やその合金が鋼管の熱間曲げの際に脆化を引き起こすのを抑制することができる。   ADVANTAGE OF THE INVENTION According to this invention, it can suppress that zinc, copper, and its alloy adhering to the surface of a steel pipe cause embrittlement in the case of hot bending of a steel pipe.

本発明の方法を適用できる鋼管の形状の例と鋼管の表面に自然に存在する微細な割れを示す断面図である。It is sectional drawing which shows the example of the shape of the steel pipe which can apply the method of this invention, and the fine crack which exists naturally on the surface of a steel pipe. 鋼管を曲げたときに表面の微細な割れが開く様子を誇張して示す断面図である。It is sectional drawing which exaggerates and shows a mode that the fine crack of the surface opens when a steel pipe is bent. 本発明の方法の流れを不純物金属の変化とともに示す模式図である。It is a schematic diagram which shows the flow of the method of this invention with the change of an impurity metal.

以下、本発明の各種実施例について図面を参照しながら説明する。本発明の方法は、鋼管を曲げて一箇所以上の曲がりを含む形状とする製造工程に適用することができる。鋼管の材料は焼き入れ可能な（焼き入れ硬化させて用いることを前提とした）炭素鋼であればいかなるものでもよい。鋼管の表面に亜鉛と銅を含む不純物が付着している場合、本発明の方法によれば溶融金属が微細な表面の割れに浸入することによる鋼管の脆化を抑えることができる。鋼管はめっきや塗布により表面に被膜が形成されていない裸材であるのが好ましい。裸材であると不純物金属が直接鋼管の素地に触れているため、溶融金属脆化が起きやすくなる。しかしめっき材であってもめっき層とその下の母材表面との間に亜鉛や銅が存在すればやはり脆化の問題は生じる可能性があると考えられる。   Various embodiments of the present invention will be described below with reference to the drawings. The method of the present invention can be applied to a manufacturing process in which a steel pipe is bent into a shape including one or more bends. The material of the steel pipe may be any carbon steel that can be hardened (assuming that it is used after being hardened by quenching). When impurities including zinc and copper are adhered to the surface of the steel pipe, according to the method of the present invention, it is possible to suppress embrittlement of the steel pipe due to the molten metal entering the cracks on the fine surface. The steel pipe is preferably a bare material whose surface is not formed by plating or coating. In the case of a bare material, since the impurity metal directly touches the base of the steel pipe, molten metal embrittlement tends to occur. However, even in the case of a plating material, it is considered that the problem of embrittlement may still occur if zinc or copper is present between the plating layer and the surface of the base material therebelow.

図１に示すように、鋼管１０の断面は任意の様々な形状とすることができ、何ら限定しない。例えば、一定の径をもつ円形断面１２のほか、長径ａと短径ｂをもつ長円形断面１４などの非円形断面（異形管ともいう）とすることもできる。あるいは正方形断面や長方形断面などの角管（図示せず）を用いることもできる。図１には、長円形断面１４の一例として、両側部に平行な平面状の壁を有する長円形断面の鋼管を示している。   As shown in FIG. 1, the cross section of the steel pipe 10 can be made into various various shapes, and is not limited at all. For example, in addition to the circular cross section 12 having a constant diameter, a noncircular cross section (also referred to as a deformed tube) such as an oval cross section 14 having a major axis a and a minor axis b can be used. Alternatively, a square tube (not shown) such as a square cross section or a rectangular cross section can be used. In FIG. 1, as an example of the oval cross section 14, a steel pipe having an oval cross section having planar walls parallel to both sides is shown.

図２に示すように、鋼管の表面には微細な割れ１６が自然に存在する。この微細な割れ１６は曲げの中立面を境として外側（引っ張り側）では広がる傾向にあるため、溶融金属が存在すると微細な割れ１６の奥まで浸入し脆化を引き起こす可能性が大きくなる。一方、曲げの内側では圧縮されるため、割れ１６の開口が閉じて液体金属の浸入の影響は小さい。特に、鋼管１０が長円形断面１４あるいは長方形の断面であって長径ａ側に（つまり長径方向を含む平面内で）曲げる場合など、曲げる方向の径が大きいほど曲げの外側の表面に生じる歪みが大きくなるため、口が開いた割れ１６に溶融金属が浸入するのを抑える必要性は大きくなる。   As shown in FIG. 2, fine cracks 16 naturally exist on the surface of the steel pipe. Since the fine crack 16 tends to spread on the outer side (tensile side) with respect to the neutral plane of bending, the presence of molten metal increases the possibility of entering the depth of the fine crack 16 and causing embrittlement. On the other hand, since it is compressed inside the bend, the opening of the crack 16 is closed and the influence of liquid metal intrusion is small. In particular, when the steel pipe 10 has an oval cross section 14 or a rectangular cross section and is bent toward the major axis a (that is, within a plane including the major axis direction), the larger the diameter in the bending direction, the greater the distortion that occurs on the outer surface of the bend. Therefore, the necessity to suppress the molten metal from entering the crack 16 having an open mouth is increased.

以下、図３を参照し、本発明の方法の実施例を説明する。まず、鋼管１０に変形を加えることなく鋼管１０を亜鉛の沸点（約９０６°Ｃ）以上の温度で加熱する。鋼管１０の加熱は様々な方法で行うことができる。例えば、後に述べるような誘導コイルを用いる誘導加熱や、赤外線加熱など外部の発熱体を利用する加熱を用いることもできる。   Hereinafter, an embodiment of the method of the present invention will be described with reference to FIG. First, the steel pipe 10 is heated at a temperature equal to or higher than the boiling point of zinc (about 906 ° C.) without deforming the steel pipe 10. The steel pipe 10 can be heated by various methods. For example, induction heating using an induction coil, which will be described later, or heating using an external heating element such as infrared heating may be used.

鋼管１０が加熱されると、表面に付着している亜鉛２０は液体状態を経て空気中に蒸発し、最終的に取り除かれる。加熱の温度は基本的に銅の融点よりも低いため鋼管の表面にある銅２２は固体のままである。また、銅は単体の形態ではなく鋼管の表面で亜鉛と固溶体や合金（真鍮など）２４を形成している場合も考えられる。この場合、固溶体ないし合金２４の融点は単体銅の融点から亜鉛の含有量に応じて下がり、亜鉛と銅の組成によっては約８４０°Ｃとなる。したがって加熱の温度が単体銅の融点より低くても鋼管表面には液体状態の銅成分が存在する可能性がある。このように加熱中には鋼管の表面に何らかの液体金属が存在する可能性があるため、鋼管には曲げなどの変形を何も加えないことが重要である。   When the steel pipe 10 is heated, the zinc 20 adhering to the surface evaporates into the air through a liquid state and is finally removed. Since the heating temperature is basically lower than the melting point of copper, the copper 22 on the surface of the steel pipe remains solid. In addition, copper may be formed in the form of a solid solution or alloy (such as brass) 24 with zinc on the surface of a steel pipe instead of a single form. In this case, the melting point of the solid solution or alloy 24 decreases from the melting point of the single copper according to the zinc content, and is about 840 ° C. depending on the composition of zinc and copper. Therefore, even if the heating temperature is lower than the melting point of single copper, there is a possibility that a liquid copper component exists on the surface of the steel pipe. As described above, since some liquid metal may be present on the surface of the steel pipe during heating, it is important that the steel pipe is not subjected to any deformation such as bending.

次に、熱せられた鋼管１０は一時的に冷却する。鋼管１０の冷却は上記の加熱工程で溶融していた銅２２の成分が確実に再固化する温度になるまで行うのが好ましく、例えば亜鉛と銅の特定の中間化合物の融点である約８４０°Ｃ以下の温度まで冷却する。この冷却は周囲の室温空気による放冷や冷却水を掛ける水冷など、任意の方法により行うことができる。この冷却では鋼管を焼き入れ硬化させる必要はないが、目的に応じて完全あるいは不完全に焼き入れ硬化させてもよい。   Next, the heated steel pipe 10 is temporarily cooled. The cooling of the steel pipe 10 is preferably performed until the temperature of the component of the copper 22 melted in the heating step is surely resolidified. For example, the melting point of a specific intermediate compound of zinc and copper is about 840 ° C. Cool to the following temperature. This cooling can be performed by an arbitrary method such as cooling by ambient room temperature air or water cooling with cooling water. In this cooling, it is not necessary to quench and harden the steel pipe, but it may be completely or incompletely hardened depending on the purpose.

次に、鋼管１０は焼き入れ可能な温度以上かつ銅の融点未満の温度（例えば約１０３０°Ｃ）で局所的に加熱しながら曲げる熱間曲げを行う。鋼管１０の熱間曲げは任意の公知の方法を用いることができる。好ましい例としては、高周波の交流電流を流したコイルの中に鋼管１０を通していき、電磁的に鋼管材料内に渦電流を誘導することによって熱する誘導加熱により行うとよい。誘導加熱は鋼管１０を短時間で高温（例えば１０００°Ｃ）まで昇温することができるとともに、コイルの幅に対応する鋼管１０の短い区間を局所的に加熱することができる。したがって加熱中に鋼管１０の端に力を加えると高温となっている特定の部位で鋼管１０を曲げることができ、変形を制御することが容易である。このような熱間曲げ方法は例えば国際公開第２００８／１２３５０５号に説明されている。   Next, the steel pipe 10 is subjected to hot bending in which the steel pipe 10 is bent while being locally heated at a temperature that is higher than the quenching temperature and lower than the melting point of copper (for example, about 1030 ° C.). Any known method can be used for hot bending of the steel pipe 10. As a preferred example, it may be performed by induction heating in which a steel pipe 10 is passed through a coil through which a high-frequency alternating current is passed and heated by electromagnetically inducing eddy currents in the steel pipe material. Induction heating can raise the temperature of the steel pipe 10 to a high temperature (for example, 1000 ° C.) in a short time, and can locally heat a short section of the steel pipe 10 corresponding to the width of the coil. Therefore, when a force is applied to the end of the steel pipe 10 during heating, the steel pipe 10 can be bent at a specific part that is at a high temperature, and deformation can be easily controlled. Such a hot bending method is described in, for example, International Publication No. 2008/123505.

曲げ角度と曲げ半径も用途に応じて様々に変えることができる。曲げ角度は小さいほど曲げの外側の表面に生じる歪みが大きくなるため、口が開いた割れ１６に溶融金属が浸入するのを抑える必要性が増す。   The bending angle and the bending radius can also be changed variously according to the application. The smaller the bend angle, the greater the strain generated on the outer surface of the bend, which increases the need to prevent molten metal from entering the cracks 16 with open mouths.

次に、曲げられた鋼管１０を冷却して焼入れする。冷却速度は鋼鉄材料の焼き入れ性、素材の寸法、目的の用途に応じて様々に変えることができる。一つの実施例としては、前述の加熱工程のための誘導コイルのすぐ直後に冷却水を供給する冷却ユニットを配置することにより簡便に冷却することができる。   Next, the bent steel pipe 10 is cooled and quenched. The cooling rate can be varied depending on the hardenability of the steel material, the dimensions of the material and the intended application. As one embodiment, the cooling can be easily performed by arranging a cooling unit for supplying cooling water immediately after the induction coil for the heating process described above.

最初の加熱工程と２回目の加熱工程はひとつの製造ラインで連続的に行うことができる。例えば、一つの実施例として、各加熱工程に対応する誘導コイルを備えた２個の加熱ユニットを適当な距離（例えば５００ｍｍほど）だけ離して設置し、その中に鋼管１０を通していく。最初の加熱ユニットを通り抜けたところで周囲の室温空気により一時的な冷却を行い、２番目の加熱ユニットを通っている間に曲げ加工を行う。しかし、別の実施例として、別個のラインでそれぞれの加熱を行うこともできる。   The first heating step and the second heating step can be continuously performed on one production line. For example, as one embodiment, two heating units each provided with an induction coil corresponding to each heating step are set apart by an appropriate distance (for example, about 500 mm), and the steel pipe 10 is passed through them. After passing through the first heating unit, it is temporarily cooled by ambient room temperature air and bent while passing through the second heating unit. However, as another example, each heating can be performed in a separate line.

以上の方法によって曲げた鋼管１０は様々な用途に用いることができるが、一つの実施例としては自動車など陸上車両のサスペンションのアーム部材として用いるのが適する。サスペンションのアームは他の骨格部材の合間を縫って配置されるため複雑な曲がり形状を必要とするうえに、車両の荷重を支えるため素材の脆化をなるべく抑える必要もある。しかしながら本発明を適用して曲げた鋼管１０は車両以外にも屈曲形状を有する様々な機械構造部材に用いることができる。   The steel pipe 10 bent by the above method can be used for various applications, but as one embodiment, it is suitable to be used as an arm member of a suspension of a land vehicle such as an automobile. Since the arm of the suspension is sewn between the other skeleton members, it requires a complicated bent shape, and it is also necessary to suppress the embrittlement of the material as much as possible to support the load of the vehicle. However, the steel pipe 10 bent by applying the present invention can be used for various machine structural members having a bent shape in addition to the vehicle.

以上、本発明の実施例を具体的な態様に触れて説明したが、当業者であれば本発明の目的から逸脱することなく様々な置換、改良、変更を施すことが可能なことは明らかである。すなわち、本発明の実施例には添付した特許請求の範囲の真意と目的に適うあらゆる置換、改良、変更が含まれる。   Although the embodiments of the present invention have been described with reference to specific embodiments, it is obvious that those skilled in the art can make various substitutions, improvements, and changes without departing from the object of the present invention. is there. In other words, the embodiments of the present invention include all substitutions, improvements, and modifications suitable for the purpose and purpose of the appended claims.

試験例：２本の同一の長円形断面をもった鋼管素材を用意し、一方は従来の方法で熱間曲げ加工し、他方は本発明の方法を用いて一回目の加熱をした後に一旦冷却し、その後再び加熱しながら曲げ加工することにより試料を作成した。曲げられた２つの鋼管試料にはそれぞれ曲げの外側の表面に一定の応力を繰り返し加える試験を行った。従来の方法で曲げた試料は多くの繰り返し負荷を与えていくとある時点で曲げの外側から深い亀裂が入り破断したが、本発明の方法を適用して曲げた試料は従来の試料が破断したときの回数の約４．５倍の回数の繰り返し負荷を経てもなお破断しなかった。   Test example: Two steel pipe materials having the same oval cross section are prepared, one is hot bent by a conventional method, and the other is temporarily cooled after the first heating using the method of the present invention. Then, a sample was prepared by bending while heating again. Each of the two bent steel pipe samples was subjected to a test in which a constant stress was repeatedly applied to the outer surface of the bend. When a sample bent by the conventional method was subjected to many repeated loads, a deep crack started to break from the outside of the bend at a certain point, but the sample bent by applying the method of the present invention broke the conventional sample. Even after repeated loading of about 4.5 times the number of times, it did not break.

１０ 鋼管
１２ 円形断面
１４ 長円形断面
２０ 亜鉛
２２ 銅
２４ 合金

10 Steel pipe 12 Circular section 14 Oval section 20 Zinc 22 Copper 24 Alloy

Claims (5)

鋼管を熱間曲げ加工する工法であって、表面に亜鉛と銅を含む不純物が付着した鋼管を曲げることなく亜鉛の沸点以上の温度に加熱した後、この鋼管を一時的に冷却し、その後焼き入れ可能な温度以上かつ銅の融点未満の温度でその鋼管を局所的に加熱しながら曲げ、この曲げられた鋼管を冷却して焼入れすることを特徴とする工法。   A method of hot-bending a steel pipe. After heating a steel pipe with impurities including zinc and copper on its surface to a temperature above the boiling point of zinc without bending, this steel pipe is temporarily cooled and then baked. A method characterized in that the steel pipe is bent while locally heated at a temperature not lower than the melting point and lower than the melting point of copper, and the bent steel pipe is cooled and quenched. 請求項１に記載した工法であって、鋼管がめっきや塗布により表面被膜が形成されていない裸材であることを特徴とする工法。   The method according to claim 1, wherein the steel pipe is a bare material on which a surface film is not formed by plating or coating. 請求項１または請求項２に記載した工法であって、２回目の加熱を誘導加熱により行うことを特徴とする工法。   The method according to claim 1 or 2, wherein the second heating is performed by induction heating. 請求項３に記載した工法であって、１回目の加熱も誘導加熱により行い、間隔を空けて配置された各誘導コイルの中に鋼管を連続的に通すことによりそれぞれの加熱を行うことを特徴とする工法。   The method according to claim 3, wherein the first heating is also performed by induction heating, and each heating is performed by continuously passing a steel pipe through each induction coil arranged at intervals. The construction method. 請求項１から請求項４に記載した工法であって、鋼管の断面が長径と短径をもつ非円形断面であり、曲げ工程において鋼管を長径側に曲げることを特徴とする工法。
5. The method according to claim 1, wherein the cross section of the steel pipe is a non-circular cross section having a major axis and a minor axis, and the steel pipe is bent to the major axis side in a bending step.
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