JPH1017980A - Welded steel pipe with low yield ratio, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a welded steel pipe with low yield ratio, having sound weld zone, by constituting the steel pipe of a low alloy steel of specific composition containing specific amounts of Cr, providing a structure composed of dual- phase structure of hard phase and soft phase, and regulating the yield ratio to specific values according to the value of (wall thickness)/(outside diameter). SOLUTION: A steel, having a composition consisting of, by weight, 0.03-0.3% C, 0.05-1% Si, 0.3-2% Mn, <=0.03% P, <=0.02% S, 1-3% Cr, 0.005-0.1% sol-Al, 0-1.5% Mo, 0-0.1% Nb, 0-0.5% V, 0-0.1% Ti, 0-2% Cu, 0-2% Ni, 0-0.005% B, and the balance Fe with inevitable impurities, is hot-rolled. At this time, rolling and cooling conditions are regulated to form a dual-phase structure containing soft ferritic phase and hard bainitic or martensitic phase. The resultant hot rolled steel plate is formed into open pipe shape, and both end faces are subjected to pipe welding by laser beam. Then, post-heating treatment is applied to the weld metal zone and the weld heat-affected zone, and the welded steel pipe having yield ratio YR satisfying inequalities can be obtained.

Description

【発明の詳細な説明】DETAILED DESCRIPTION OF THE INVENTION

【０００１】[0001]

【発明の属する技術分野】本発明は、引張強さＴＳに対
する降伏応力ＹＳの比（ＹＳ／ＴＳ）である降伏比ＹＲ
（％）の低いことが望ましい、原油や天然ガスの輸送用
ラインパイプや、建材用鋼管として用いて好適な低降伏
比溶接鋼管とその製造方法に関する。The present invention relates to a yield ratio YR which is a ratio of a yield stress YS to a tensile strength TS (YS / TS).
The present invention relates to a low yield ratio welded steel pipe suitable for use as a line pipe for transporting crude oil or natural gas or a steel pipe for building materials, and a method for producing the same.

【０００２】[0002]

【従来の技術】例えば、ラインパイプは破裂事故防止の
観点から、また建築用鋼管は耐震性の観点から、いずれ
も低降伏比であることが要求される。これは、上記の降
伏比ＹＲ（％）が低ければ低いほど、外力により、万
一、ラインパイプや建築用鋼管が降伏しても破断には至
り難く、それ故、その構造物が破壊に至らないと考えら
れるからである。2. Description of the Related Art For example, a line pipe is required to have a low yield ratio from the viewpoint of preventing a burst accident, and a building steel pipe is required to have a low yield ratio from the viewpoint of earthquake resistance. This is because the lower the above-mentioned yield ratio YR (%), the more difficult it is for a line pipe or a steel pipe for a building to yield even if it yields due to external force, and therefore, the structure may be damaged. It is considered that there is not.

【０００３】従って、これまで、低降伏比の鋼およびそ
の熱延鋼板や厚鋼板を製造するための発明が数多くなさ
れ、提案されている（例えば、特開昭５８−７１３３７
号公報、特開昭６１−２６６５２号公報、特開昭６３−
１１８０１２号公報、特開昭６３−１７９０１９号公報
など）。[0003] Accordingly, a number of inventions have been made and proposed for producing steels having a low yield ratio and hot-rolled steel sheets and thick steel sheets thereof (for example, Japanese Patent Application Laid-Open No. 58-71337).
JP, JP-A-61-26652, JP-A-63-662-
No. 118012, JP-A-63-179019).

【０００４】これらの公報に示される発明の基本的な技
術思想は、炭素鋼あるいは低合金鋼製の素材鋼を熱間圧
延し、その熱間圧延後に加速冷却処理を施して、柔らか
いフェライト相と硬いベイナイト相あるいはマルテンサ
イト相の複合組織を得るというものである。つまり、柔
らかいフェライト相によって降伏応力を低く保ち、硬い
ベイナイト相あるいはマルテンサイト相で高い引張強さ
を得、これによって低い降伏比を得るというものであ
る。[0004] The basic technical idea of the invention disclosed in these publications is that a material steel made of carbon steel or low alloy steel is hot-rolled, accelerated cooling treatment is performed after the hot rolling, and a soft ferrite phase is formed. This is to obtain a composite structure of a hard bainite phase or a martensite phase. That is, the yield stress is kept low by the soft ferrite phase, and the high tensile strength is obtained by the hard bainite phase or the martensite phase, thereby obtaining a low yield ratio.

【０００５】ここで、加速冷却処理時の管理ポイントと
しては、フェライトを一部析出させた後に、加速冷却を
施してベイナイトあるいはマルテンサイト変態するだけ
の冷却速度と冷却停止温度を確保することである。Here, the control points during the accelerated cooling treatment are to secure a cooling rate and a cooling stop temperature sufficient to transform bainite or martensite by performing accelerated cooling after partially depositing ferrite. .

【０００６】そして、低降伏比の鋼管は、上記のように
して得られた低降伏比の鋼板を管状に成形してシーム溶
接することによって製造されることが多い。このシーム
溶接は、通常、鋼板が熱延鋼帯の場合には電気抵抗溶接
法（以下、ＥＲＷ法という）が、厚鋼板の場合にはサブ
マージドアーク溶接法（以下、ＳＡＷ法という）が適用
される。A steel pipe having a low yield ratio is often manufactured by forming the steel plate having a low yield ratio obtained as described above into a tubular shape and performing seam welding. This seam welding is generally applied by the electric resistance welding method (hereinafter referred to as ERW method) when the steel sheet is a hot-rolled steel strip, and by the submerged arc welding method (hereinafter referred to as SAW method) when the steel sheet is a thick steel sheet. Is done.

【０００７】しかしながら、溶接鋼管は、製管時の曲げ
や拡管、さらには絞りなどの冷間加工の影響を受ける。
このため、得られた溶接鋼管は、母材である鋼板ほどの
低降伏比のものが得られない場合が多い。However, the welded steel pipe is affected by cold working such as bending and expansion at the time of pipe making, and drawing.
For this reason, in many cases, the obtained welded steel pipe cannot have a yield ratio as low as that of the steel sheet as the base material.

【０００８】また、用いる鋼は、溶接性を確保する必要
から、成分組成の制約を受ける。例えば、ＥＲＷ法で
は、ＣｒやＭｎあるいＳｉのような酸化しやすい成分の
量が多くなると、溶接時にペネトレータと称される酸化
物系介在物による溶接欠陥が生じる。また、ＳＡＷ法で
は、炭素当量が高くなると、溶接低温割れが起こりやす
くなる。[0008] Further, the steel to be used is subject to restrictions on the composition of the components because it is necessary to ensure weldability. For example, in the ERW method, when the amount of easily oxidizable components such as Cr, Mn, and Si increases, welding defects due to oxide inclusions called penetrators occur during welding. Further, in the SAW method, when the carbon equivalent is high, low-temperature cracking of the weld is likely to occur.

【０００９】一方、上記のような成分の含有量を増やす
ことによってベイナイト相やマルテンサイト相の硬質相
の強度を高め、結果として低降伏比とすることができ
る。しかし、先に述べたような理由から、その含有量に
はおのずと制約がある。On the other hand, by increasing the content of the above components, the strength of the hard phase such as the bainite phase or the martensite phase is increased, and as a result, a low yield ratio can be obtained. However, for the reasons described above, the content is naturally limited.

【００１０】すなわち、十分な低降伏比を有する母材鋼
板を得ることはできるものの、これを溶接鋼管にした場
合にその特性を十分に活用しきれていないのが現状であ
る。That is, although it is possible to obtain a base steel sheet having a sufficiently low yield ratio, at present, it is not possible to make full use of its characteristics when a welded steel pipe is used.

【００１１】そこで、製管溶接後の管全体に後熱処理を
施し、先に述べたような複合組織となすことによって低
降伏比を得る方法もあるが、この場合は生産性が低下す
る。すなわち、そもそも溶接鋼管、特にＥＲＷ法による
溶接鋼管は、高い生産性のゆえに安価であることがメリ
ットであり、生産性を落として低降伏比の鋼管を製造す
ると、そのメリットが失われる。Therefore, there is a method in which a post-heat treatment is applied to the entire pipe after pipe welding to obtain a composite structure as described above to obtain a low yield ratio, but in this case, productivity is reduced. That is, in the first place, a welded steel pipe, particularly a welded steel pipe formed by the ERW method is advantageous in that it is inexpensive due to high productivity. If the productivity is reduced and a steel pipe having a low yield ratio is manufactured, the advantage is lost.

【００１２】ところで、最近、ＳＡＷ法などのアーク溶
接法と同等の溶接部の性能を有する溶接鋼管を、ＥＲＷ
法と同等の溶接速度で得ることを目的とした製管溶接法
として、溶接熱源にレーザ、具体的には炭酸ガスレーザ
を使用する方法の実用化が検討されるようになり、多く
の発明が提案されている。By the way, recently, a welded steel pipe having a weld performance equivalent to that of an arc welding method such as the SAW method has been used for ERW.
As a pipe welding method aimed at obtaining the same welding speed as the welding method, practical use of a method using a laser as a welding heat source, specifically a carbon dioxide laser, has been studied, and many inventions have been proposed. Have been.

【００１３】例えば、特開平２−７０３７９号公報に
は、ＥＲＷ法の高周波加熱に引き続いてレーザビームに
より溶接を行う製管溶接方法（以下、高周波加熱併用レ
ーザ溶接法という）が示されている。この高周波加熱併
用レーザ溶接法は、レーザ単独溶接法に比べて溶接速度
が速いのみならず、その溶接がＥＲＷ法とは本質的に異
なる溶融溶接であるために溶接部での欠陥発生が大幅に
抑制される。しかし、その溶接速度は、レーザ単独溶接
法の高々２倍程度でしかない。For example, Japanese Patent Application Laid-Open No. 2-70379 discloses a pipe welding method in which welding is performed by a laser beam following high-frequency heating by the ERW method (hereinafter referred to as laser welding combined with high-frequency heating). This laser welding method combined with high-frequency heating not only has a higher welding speed than the laser-only welding method, but also has a large number of defects at the weld because the welding is a fusion welding that is essentially different from the ERW method. Is suppressed. However, the welding speed is at most about twice that of the laser-only welding method.

【００１４】そこで、本発明者らは、ＥＲＷ法にほぼ匹
敵する溶接速度での溶接が可能であり、ＥＲＷ法で製造
可能なサイズと同様サイズの耐ＨＩＣ性と耐ＳＳＣ性に
優れるラインパイプ用の溶接鋼管の得られる方法を開発
し、先に提案した（特開平５−２２８６６０号公報）。
すなわち、その方法は、オープンパイプ状に曲成された
鋼板の両縁端面が相互に当接する突き合わせ部に、管外
面側に開脚する所定の寸法と形状を有するＶ溝を形成さ
せ、このＶ溝の溝底部にレーザビームを照射して溶融衝
合溶接を行う方法である。Therefore, the present inventors have developed a line pipe for a line pipe which is capable of welding at a welding speed almost equal to that of the ERW method, and which is excellent in HIC resistance and SSC resistance similar to the size that can be manufactured by the ERW method. A method for obtaining a welded steel pipe was developed and proposed earlier (Japanese Patent Laid-Open No. 5-228660).
That is, according to the method, a V-groove having a predetermined size and shape is formed at an abutting portion where both edge end surfaces of a steel plate bent into an open pipe shape abut on the outer surface of the pipe. This is a method of irradiating a laser beam to the groove bottom of the groove to perform fusion butting welding.

【００１５】しかし、従来のレーザ出力は最大でも５ｋ
Ｗ程度でしかなかなく、高周波加熱による予熱を併用し
たとしても、レーザ出力に換算してせいぜい１０ｋＷに
相当する程度の効果しか得られなかった。このため、よ
り一層の高速化溶接を可能ならしめるべく、例えば２５
ｋＷというような大出力のレーザを用いての高周波加熱
併用レーザ溶接法の開発が望まれており、本発明者らは
その方法を開発し、先に提案した（特開平８−１１８０
５０号公報および特開平８−１２０３４６号公報）。However, the conventional laser output is at most 5 k.
It was only about W, and even if preheating by high-frequency heating was also used, an effect equivalent to a laser output of at most 10 kW was obtained. For this reason, for example, 25%
It is desired to develop a laser welding method combined with high-frequency heating using a laser having a large output such as kW, and the present inventors have developed such a method and have previously proposed it (Japanese Patent Application Laid-Open No. H08-1180).
No. 50 and JP-A-8-120346).

【００１６】すなわち、その方法は、母材鋼板の肉厚ｔ
（ｍｍ）に対して、レーザ出力をＰ（ｋＷ）、溶接速度
をＶ（ｍ／ｍｉｎ）、鋼板端面の予熱温度をＴ（℃）、
室温をＴ0 （℃）としたとき、下記の（１）式と（２）
式を同時に満たす条件で溶接する方法であり、この方法
によれば高速無欠陥溶接が可能である。That is, the method uses the thickness t of the base steel sheet.
(Mm), laser output is P (kW), welding speed is V (m / min), preheating temperature of steel plate end face is T (° C),
When the room temperature is T0 (° C.), the following equation (1) and (2)
This is a method of welding under conditions that simultaneously satisfy the formula, and according to this method, high-speed defect-free welding is possible.

【００１７】 Ｖ≧２ ・・・・（１） Ｐ≧０．４Ｖｔ／ｅ^a(T-T0) ・・・・（２） ただし、ａ＝０．０００６V ≧ 2 (1) P ≧ 0.4 Vt / ea ^(T−T0) (2) where a = 0.0006

【００１８】[0018]

【発明が解決しようとする課題】しかし、本発明者らが
先に提案した方法で製造された溶接鋼管は、溶接部を含
めた耐ＨＩＣ性と耐ＳＳＣ性に優れるものの、上記の降
伏比ＹＲが比較的高く、その降伏比ＹＲは、鋼管の外径
をＤ（ｍｍ）、肉厚をｔ（ｍｍ）としたとき、ｔ／Ｄが
２以下のもので８０％超８５％以下、ｔ／Ｄが２超３以
下のもので８５％超８８％以下、ｔ／Ｄが３超のもので
８８％超９１％以下、のものしか得られなかった。However, the welded steel pipe manufactured by the method proposed by the present inventors is excellent in HIC resistance and SSC resistance including the welded portion, but has the above-mentioned yield ratio YR. Is relatively high, and the yield ratio YR is, when the outer diameter of the steel pipe is D (mm) and the wall thickness is t (mm), when t / D is 2 or less, more than 80% and 85% or less, Only those with D of more than 2 and 3 or less and more than 85% and 88% or less, and those with t / D of more than 3 and more than 88% and 91% or less were obtained.

【００１９】ところが、上記ラインパイプや建材用鋼管
としては、上記従来以上に低降伏比の溶接鋼管が強く望
まれているのが実情である。However, as the above-mentioned line pipes and steel pipes for building materials, there is a strong demand for welded steel pipes having a lower yield ratio than the above-mentioned conventional ones.

【００２０】本発明は、上記の実情に鑑みてなされたも
ので、その課題は従来にも増してより低降伏比、具体的
にはその降伏比ＹＲが上記ｔ／Ｄに応じて表１に示す値
以下である溶接鋼管とその製造方法を提供することにあ
る。The present invention has been made in view of the above-mentioned circumstances, and an object thereof is to provide a lower yield ratio than in the prior art, specifically, the yield ratio YR is shown in Table 1 according to the above t / D. An object of the present invention is to provide a welded steel pipe having a value not more than the indicated value and a method for producing the same.

【００２１】[0021]

【表１】 [Table 1]

【００２２】なお、表１に示す値は、ＡＰＩ（米国石油
協会）規格に規定されたＸ５２〜Ｘ８０級のラインパイ
プ用鋼管を想定した値である。The values shown in Table 1 assume values of X52 to X80 grade line pipe steel pipes specified in the API (American Petroleum Institute) standards.

【００２３】[0023]

【課題を解決するための手段】本発明の要旨は、次の
（１）および（２）の低降伏比溶接鋼管とその製造方法
にある。The gist of the present invention resides in the following (1) and (2) low yield ratio welded steel pipes and a method of manufacturing the same.

【００２４】（１）重量％で、Ｃ：０．０３〜０．３
％、Ｓｉ：０．０５〜１％、Ｍｎ：０．３〜２％、Ｐ：
０．０３％以下、Ｓ：０．０２％以下、Ｃｒ：１〜３
％、ｓｏｌ−Ａｌ：０．００５〜０．１％、Ｍｏ：０〜
１．５％、Ｎｂ：０〜０．１％、Ｖ：０〜０．５％、Ｔ
ｉ：０〜０．１％、Ｃｕ：０〜２％、Ｎｉ：０〜２％お
よびＢ：０〜０．００５％を含み、残部Ｆｅおよび不可
避的不純物からなり、その組織が軟質のフェライト相と
硬質のベイナイトあるいはマルテンサイト相を含む複合
組織である溶接鋼管であって、その降伏比ＹＲ（％）が
下記の条件を満たすことを特徴とする低降伏比溶接鋼
管。(1) C: 0.03-0.3% by weight
%, Si: 0.05-1%, Mn: 0.3-2%, P:
0.03% or less, S: 0.02% or less, Cr: 1 to 3
%, Sol-Al: 0.005 to 0.1%, Mo: 0 to 0%
1.5%, Nb: 0 to 0.1%, V: 0 to 0.5%, T
i: 0 to 0.1%, Cu: 0 to 2%, Ni: 0 to 2%, and B: 0 to 0.005%, the balance being Fe and unavoidable impurities, the structure of which is a soft ferrite phase A welded steel pipe having a composite structure containing a hard bainite or martensite phase and a yield ratio YR (%) satisfying the following condition.

【００２５】ｔ／Ｄ≦２のとき、ＹＲ≦８０％ ２＜ｔ／Ｄ≦３のとき、ＹＲ≦８５％ ｔ／Ｄ＞３のとき、ＹＲ≦８８％ ただし、 ｔ：鋼管の肉厚（ｍｍ） Ｄ：鋼管の外径（ｍｍ） （２）重量％で、Ｃ：０．０３〜０．３％、Ｓｉ：０．
０５〜１％、Ｍｎ：０．３〜２％、Ｐ：０．０３％以
下、Ｓ：０．０２％以下、Ｃｒ：１〜３％、ｓｏｌ−Ａ
ｌ：０．００５〜０．１％、Ｍｏ：０〜１．５％、Ｎ
ｂ：０〜０．１％、Ｖ：０〜０．５％、Ｔｉ：０〜０．
１％、Ｃｕ：０〜２％、Ｎｉ：０〜２％およびＢ：０〜
０．００５％を含み、残部Ｆｅおよび不可避的不純物か
らなり、その組織が軟質のフェライト相と硬質のベイナ
イトあるいはマルテンサイト相を含む複合組織を有する
熱延鋼板をオープンパイプ状に成形し、オープンパイプ
状に成形された熱延鋼板に相対向する両端面が当接する
接合点の近傍にレーザビームを照射して製管溶接した
後、少なくとも溶接金属部と溶接熱影響部を含む部分に
後熱処理を施すことを特徴とする請求項１に記載の低降
伏比溶接鋼管の製造方法。When t / D ≦ 2, YR ≦ 80% 2 <t / D ≦ 3, YR ≦ 85% When t / D> 3, YR ≦ 88% where t: wall thickness of steel pipe ( mm) D: Outer diameter of steel pipe (mm) (2) By weight%, C: 0.03-0.3%, Si: 0.
05-1%, Mn: 0.3-2%, P: 0.03% or less, S: 0.02% or less, Cr: 1-3%, sol-A
l: 0.005 to 0.1%, Mo: 0 to 1.5%, N
b: 0 to 0.1%, V: 0 to 0.5%, Ti: 0 to 0.
1%, Cu: 0 to 2%, Ni: 0 to 2%, and B: 0 to 0%
A hot rolled steel sheet containing 0.005%, the balance being Fe and inevitable impurities, and having a composite structure including a soft ferrite phase and a hard bainite or martensite phase, is formed into an open pipe shape. After the laser beam is irradiated near the joint where the opposite end faces of the hot-rolled steel sheet formed in the shape contact each other and the pipe is welded, post-heat treatment is performed on at least the portion including the weld metal part and the weld heat-affected zone. The method for producing a low-yield-ratio welded steel pipe according to claim 1, wherein the method is performed.

【００２６】本発明者らは、上記の課題を達成すべく、
その生産性をも考慮しつつ種々の実験研究を行った結
果、次のことを知見し、本発明をなすにいたった。The present inventors have set out the following in order to achieve the above object.
As a result of conducting various experimental studies in consideration of the productivity, the following was found, and the present invention was accomplished.

【００２７】鋼板を母材とする製管溶接時には、母材鋼
板が大なり小なり冷間加工の影響を受けるので、その影
響を受けても十分な低降伏比の溶接鋼管が得られるよう
に、その母材鋼板で十分な低降伏比を確保することが必
要である。At the time of pipe welding using a steel sheet as a base material, the base material steel sheet is affected by cold working to a greater or lesser degree, so that a welded steel pipe having a sufficiently low yield ratio can be obtained even under the influence. However, it is necessary to ensure a sufficient low yield ratio with the base steel sheet.

【００２８】そこで、従来にも増して低い降伏比の熱延
鋼板を得るのに、安価でかつその効果の大きい成分とそ
の含有を見いだすべく数多くの実験を行ったところ、必
須成分として１〜３重量％のＣｒを含有する鋼を素材鋼
として用い、その組織を従来の知見通りに軟質のフェラ
イト相と硬質のベイナイトあるいはマルテンサイト相を
含む複合組織とした場合に、最も低い降伏比が得られる
ことを知見した。Therefore, in order to obtain a hot-rolled steel sheet having a lower yield ratio than ever before, a number of experiments were conducted to find a component which is inexpensive and has a large effect and its content. The lowest yield ratio can be obtained when a steel containing 1 wt% Cr is used as a material steel and its structure is a composite structure containing a soft ferrite phase and a hard bainite or martensite phase as conventionally known. I found that.

【００２９】また、熱延鋼板の製造時に高い生産性を確
保するためには、可能な限り高い仕上温度と高い巻取温
度とするのが望ましい。そして、焼入性を高める成分と
しては、Ｃ、Ｃｒ、Ｍｎ、Ｎｉなど数多くあるが、これ
らの成分のうち、Ａr₃点（オーステナイト相からフェラ
イト変態が始まる温度）をそれほど低下させず、高い仕
上温度を確保するのに最も適した成分がＣｒであること
も知見した。In order to ensure high productivity during the production of a hot-rolled steel sheet, it is desirable to set the finishing temperature and the winding temperature as high as possible. There are many components that enhance hardenability, such as C, Cr, Mn, and Ni. Of these components, the Ar ₃ point (the temperature at which ferrite transformation starts from the austenite phase) does not decrease so much and a high finish is obtained. It has also been found that the most suitable component for securing the temperature is Cr.

【００３０】なお、焼入性の高いＣｒの含有量を増加さ
せ、ベイナイト相あるいはマルテンサイト相の硬さを高
めることによって低降伏比を得るという技術思想自体
は、冶金的知識を有する者であれば比較的容易に思いつ
く事項かもしれない。しかし、従来は、製管溶接上の制
約、特にＥＲＷ法においてはＣｒ酸化物が溶接欠陥の主
原因になるとの制約からその含有量を抑制することとさ
れ、Ｃｒ含有量を多くする方向の検討はほとんどされる
ことがなかった。The technical concept itself of obtaining a low yield ratio by increasing the content of Cr having high hardenability and increasing the hardness of the bainite phase or the martensite phase does not apply to those who have metallurgical knowledge. It might be relatively easy to come up with. However, in the past, it has been determined that the content of Cr oxide is suppressed due to the restriction on pipe welding, particularly in the ERW method, because the Cr oxide is a main cause of welding defects, and a study on the direction of increasing the Cr content has been made. Was almost never done.

【００３１】さらに、上記１〜３重量％のＣｒを含有
し、その組織が軟質のフェライト相と硬質のベイナイト
あるいはマルテンサイト相を含む複合組織である鋼板を
母材とし、これをレーザ溶接、特に従来のＥＲＷ法とほ
ぼ同等の溶接速度での溶接が可能な大出力の高周波加熱
併用レーザ溶接法を用いて製管溶接して得られた溶接鋼
管の溶接金属部には、酸化物に起因した溶接欠陥や高温
溶接割れ（凝固割れ）が発生しないことを確認した。こ
れは、主たる溶接手段が大出力レーザによる溶融衝合溶
接であり、溶接端面間から酸化物がほとんど排出され、
鋼の成分組成に影響されないためであることを確認し
た。Further, the base material is a steel sheet containing the above 1 to 3% by weight of Cr and having a composite structure including a soft ferrite phase and a hard bainite or martensite phase. The weld metal of a welded steel pipe obtained by pipe welding using high-power laser welding combined with high-frequency heating, which enables welding at a welding speed almost equal to that of the conventional ERW method, was caused by oxides. It was confirmed that welding defects and high-temperature welding cracks (solidification cracks) did not occur. In this method, the main welding means is fusion butting welding with a high-power laser, and almost all oxides are discharged from between the welding end faces,
It was confirmed that it was not affected by the composition of the steel.

【００３２】ただし、その溶接金属部分は、レーザ溶接
に特有の急熱急冷組織となって靭性に乏しいので、少な
くとも溶接金属部と溶接熱影響部に後熱処理を施す必要
のあることを確認した。この後熱処理は、従来のＥＲＷ
法によった場合でも必須であり、この場合、一般に後熱
処理を施した部分の強度が低下する。従って、用いる素
材鋼としては、溶接部の強度確保を前提にして成分設計
したものを用いる必要がある。また、溶接部の著しい硬
度分布は、歪集中の観点からも望ましいものではない。However, since the weld metal portion had a rapid heat quenching structure peculiar to laser welding and had poor toughness, it was confirmed that it was necessary to perform post heat treatment on at least the weld metal portion and the weld heat affected zone. After this, the heat treatment is carried out according to the conventional ERW.
The method is indispensable even when the method is used, and in this case, generally, the strength of the portion subjected to the post heat treatment is reduced. Therefore, it is necessary to use a material steel whose composition is designed on the assumption that the strength of the welded portion is ensured. Further, the remarkable hardness distribution of the welded portion is not desirable from the viewpoint of strain concentration.

【００３３】しかし、１〜３重量％のＣｒを含有し、そ
の組織が上記複合組織である鋼板を母材する本発明の溶
接鋼管の溶接部は、前述したように、後熱処理を施す必
要があるものの、高Ｃｒ化によって焼入性が高まった分
だけ後熱処理による強度低下が抑制され、かつ後熱処理
後の溶接部の硬度分布も可及的に均一となり、さらには
降伏比も母材のそれにほぼ維持されることを確認した。However, as described above, the welded portion of the welded steel pipe of the present invention containing 1 to 3% by weight of Cr and having a structure of the above-described composite structure as a base material needs to be subjected to post heat treatment. However, the decrease in strength due to post-heat treatment is suppressed by the increase in hardenability due to the increase in Cr, the hardness distribution of the welded part after post-heat treatment becomes as uniform as possible, and the yield ratio and base metal It confirmed that it was almost maintained.

【００３４】すなわち、１〜３重量％のＣｒを含有する
素材鋼からなり、その組織が軟質のフェライト相と硬質
のベイナイトあるいはマルテンサイト相を含む複合組織
である鋼板を母材とし、母材鋼板の肉厚にもよるが、２
５ｋＷというような大出力のレーザ溶接法を用いて製管
溶接する場合には、従来にも増して低降伏比であり、か
つ母材部と溶接部の強度と硬度の均一性に優れた、低降
伏比の溶接鋼管が得られることを知見した。That is, the base material is a steel plate made of a material steel containing 1 to 3% by weight of Cr and having a composite structure including a soft ferrite phase and a hard bainite or martensite phase. It depends on the thickness of the
When pipe welding is performed using a laser welding method with a large output such as 5 kW, the yield ratio is lower than ever, and the strength and hardness of the base material and the weld are excellent in uniformity. It has been found that a welded steel pipe with a low yield ratio can be obtained.

【００３５】[0035]

【発明の実施の形態】以下、本発明の実施態様につい
て、詳細に説明する。BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail.

【００３６】先ず、本発明で用いる素材鋼の成分組成の
限定理由について説明する。なお、以下の説明におい
て、「％」は「重量％」を意味する。First, the reasons for limiting the component composition of the raw steel used in the present invention will be described. In the following description, “%” means “% by weight”.

【００３７】Ｃ：０．０３〜０．３％ Ｃは、所定の強度、具体的には前述のＡＰＩ−Ｘ４２級
以上の強度を確保し、ベイナイト相あるいはマルテンサ
イト相の硬質相を生成させて低降伏比を得るために必要
な成分である。しかし、その含有量が０．０３％未満で
あると上記の硬質相がほとんど形成されず、目標とする
低降伏比が得られない。一方、その含有量が０．３％を
超えると、靭性が著しく劣化する。よって、Ｃ含有量
は、０．０３〜０．３％とした。なお、望ましい範囲
は、０．０６〜０．２％であり、この場合には適正なバ
ランスの強度と靭性を確保することができる。C: 0.03 to 0.3% C secures a predetermined strength, specifically, a strength of API-X42 class or higher, and forms a hard phase of bainite phase or martensite phase. It is a component necessary to obtain a low yield ratio. However, if the content is less than 0.03%, the above hard phase is hardly formed, and a target low yield ratio cannot be obtained. On the other hand, if the content exceeds 0.3%, the toughness is significantly deteriorated. Therefore, the C content is set to 0.03 to 0.3%. Note that a desirable range is 0.06 to 0.2%, and in this case, appropriate balance of strength and toughness can be secured.

【００３８】Ｓｉ：０．０５〜１％ Ｓｉは、鋼の脱酸のために必要な成分であり、その含有
量が０．０５％未満ではその効果が得られない。一方、
その含有量が１％を超えると、母材部の靭性が劣化する
のみならず、後熱処理を施した部分の焼戻脆化を招く。
よって、Ｓｉ含有量は、０．０５〜１％とした。望まし
い範囲は、０．１〜０．３％である。Si: 0.05-1% Si is a component necessary for deoxidizing steel, and if its content is less than 0.05%, its effect cannot be obtained. on the other hand,
If the content exceeds 1%, not only the toughness of the base material portion deteriorates, but also tempering embrittlement of the portion subjected to the post-heat treatment is caused.
Therefore, the Si content is set to 0.05 to 1%. A desirable range is 0.1-0.3%.

【００３９】Ｍｎ：０．３〜２％ Ｍｎは、上記した所定の強度を安価に確保するこのとで
きる成分である。しかし、その含有量が０．３％未満で
は所望の強度を確保することができない。一方、その含
有量が２％を超えると、鋼のＡr₃点を低下されるのに加
え、焼入性を著しく上昇させ、熱延鋼板において所望の
低降伏比を得ることが極めて困難になる。また、母材の
成分偏析部分のＭｎ濃度が高くなりすぎ、得られた溶接
鋼管相互をその管軸長方向に突き合わせて溶接接合する
周溶接施工時に溶接低温割れが発生する場合がある。よ
って、Ｍｎ含有量は、０．３〜２％とした。望ましい範
囲は、０．６〜１．８％である。Mn: 0.3 to 2% Mn is a component which can secure the above-mentioned predetermined strength at low cost. However, if the content is less than 0.3%, desired strength cannot be secured. On the other hand, when the content exceeds 2%, in addition to lowering the Ar ₃ point of the steel, hardenability is significantly increased, and it is extremely difficult to obtain a desired low yield ratio in a hot-rolled steel sheet. . In addition, the Mn concentration in the component segregated portion of the base metal becomes too high, and low-temperature cracking may occur during peripheral welding in which the obtained welded steel pipes are welded to each other in a longitudinal direction of the pipes. Therefore, the Mn content is set to 0.3 to 2%. A desirable range is 0.6 to 1.8%.

【００４０】Ｐ：上限０．０３％ Ｐは、鋼中に不可避的に含まれる不純物であり、その含
有量は低ければ低い方が望ましい。特に、その含有量が
０．０３％を超えると、母材の成分偏析部分のＣ、Ｍｎ
およびＰなどの合金成分の濃度が高くなりすぎ、上記同
様に、周溶接施工時に溶接低温割れが発生する場合があ
る。また、靭性劣化や焼戻脆化の原因となる。よって、
Ｐ含有量は、その上限を０．０３％とした。望ましい上
限は、０．０２％である。P: upper limit 0.03% P is an impurity unavoidably contained in steel, and the lower the content, the better. In particular, if the content exceeds 0.03%, C, Mn in the component segregation portion of the base material
, And the concentration of alloy components such as P become too high, so that low-temperature cracking may occur at the time of girth welding as described above. Moreover, it causes toughness degradation and tempering embrittlement. Therefore,
The upper limit of the P content was 0.03%. A desirable upper limit is 0.02%.

【００４１】Ｓ：上限０．０２％ Ｓは、上記のＰと同様に、鋼中に不可避的に含まれる不
純物であり、その含有量は低ければ低い方が望ましい。
特に、製品の加工性、なかでも伸び、延性、曲げ性さら
には管端の一体フランジ成形性などを確保する観点から
は、その含有量を０．０２％以下とするのが好ましい。
よって、Ｓ含有量は、その上限を０．０２％とした。望
ましい上限は、０．０１％である。S: upper limit 0.02% S is an impurity unavoidably contained in steel, as in the case of P described above, and the lower the content, the better.
In particular, the content is preferably 0.02% or less from the viewpoint of ensuring the workability of the product, especially elongation, ductility, bendability, and the integral flange formability of the pipe end.
Therefore, the upper limit of the S content is set to 0.02%. A desirable upper limit is 0.01%.

【００４２】ｓｏｌ−Ａｌ：０．００５〜０．１％ Ａｌは、上記のＳｉと同様に、鋼の脱酸のために必要な
成分であり、ｓｏｌ−Ａｌの含有量が０．００５％未満
ではその効果が得られない。一方、ｓｏｌ−Ａｌの含有
量が０．１％を超えると、鋼の清浄度が低下して靭性が
劣化する。よって、ｓｏｌ−Ａｌ含有量は、０．００５
〜０．１％とした。望ましい範囲は、０．００５〜０．
０５％である。Sol-Al: 0.005 to 0.1% Al is a component necessary for deoxidizing steel similarly to the above-mentioned Si, and the content of sol-Al is less than 0.005%. Then, the effect cannot be obtained. On the other hand, when the content of sol-Al exceeds 0.1%, the cleanliness of the steel decreases, and the toughness deteriorates. Therefore, the sol-Al content is 0.005
To 0.1%. A desirable range is from 0.005 to 0.
05%.

【００４３】Ｃｒ：１〜３％ Ｃｒは、本発明において最も重要な成分であり、Ｃおよ
びＭｎと同じく、ベイナイト相あるいはマルテンサイト
相の硬質相を生成させて低降伏比と高強度を同時に得る
ために必要不可欠な必須の成分である。しかも、Ｃおよ
びＭｎとは異なり、Ａr₃点をそれほど低下させないの
で、母材となる熱延鋼板の製造時に仕上圧延温度を高く
することができ、高い生産性を維持できる。Cr: 1 to 3% Cr is the most important component in the present invention and, like C and Mn, forms a hard phase of bainite or martensite to obtain a low yield ratio and high strength simultaneously. It is an essential component that is indispensable. In addition, unlike C and Mn, the Ar ₃ point is not reduced so much, so that the finish rolling temperature can be increased during the production of a hot-rolled steel sheet as a base material, and high productivity can be maintained.

【００４４】しかし、その含有量が１％未満であると、
上記硬質相の強度上昇が十分でなく、所望の低降伏比を
確保することが困難になる。一方、その含有量が３％を
超えると、上記の効果が飽和するばかりか、所望の低降
伏比を得るために必要な熱延条件の適正範囲が狭まくな
り、操業性が損なわれる。また、溶接性、延性および靭
性の劣化を招く。However, if the content is less than 1%,
The strength of the hard phase is not sufficiently increased, and it is difficult to secure a desired low yield ratio. On the other hand, if the content exceeds 3%, not only the above effects are saturated, but also the appropriate range of the hot rolling conditions necessary for obtaining a desired low yield ratio becomes narrow, and the operability is impaired. In addition, it deteriorates weldability, ductility and toughness.

【００４５】よって、Ｃｒ含有量は、１〜３％とした。
なお、望ましい範囲は、１．２％超え２％以下であり、
この場合には適正なバランスの強度と靭性を確保するこ
とができる。Therefore, the Cr content was set to 1 to 3%.
The desirable range is more than 1.2% and 2% or less,
In this case, appropriate balance of strength and toughness can be ensured.

【００４６】本発明の素材鋼は、上記の成分以外に、Ｃ
ｕ、Ｎｉ、Ｍｏ、Ｎｂ、Ｖ、ＴｉおよびＢのうちの１種
または２種以上を、下記の範囲内で含有するものであっ
てもよい。The material steel of the present invention contains, in addition to the above components,
One or more of u, Ni, Mo, Nb, V, Ti and B may be contained within the following range.

【００４７】Ｃｕ、Ｎｉ、Ｍｏ、Ｎｂ、Ｖ、Ｔｉおよび
Ｂ：これらの成分、いずれも鋼の強度および靭性を向上
させる作用を有している。従って、これらの効果を得た
い場合には、そのうちから１種または２種以上を添加含
有させることができる。上記の成分のうち、特に、Ｍ
ｏ、ＮｂおよびＶは高温強度を向上させる作用を有して
いる。このため、耐火性の要求される鋼管、特に建築用
鋼管には、上記成分のうちのＭｏ、ＮｂおよびＶのうち
の１種または２種以上を添加含有させるのが好ましい。Cu, Ni, Mo, Nb, V, Ti and B: All of these components have an effect of improving the strength and toughness of steel. Therefore, when it is desired to obtain these effects, one or more of them can be added and contained. Among the above components, in particular, M
o, Nb and V have an effect of improving high-temperature strength. For this reason, it is preferable to add and contain one or more of Mo, Nb, and V among the above components to a steel pipe that requires fire resistance, particularly a steel pipe for construction.

【００４８】しかし、これらの成分を添加含有させる場
合の含有量は、以下に示す含有量にする必要がある。以
下に、その理由を述べる。However, when these components are added and contained, the contents must be as shown below. The reason is described below.

【００４９】Ｃｕ、Ｎｉ：いずれの成分も、その含有量
が０．０５％未満では上記の効果が得られない。すなわ
ち、強度および靭性は向上しない。一方、いずれの成分
もその含有量が２％を超えると、熱間加工性が低下し、
母材となる熱延鋼板の製造が困難となる。よって、Ｃｕ
およびＮｉを添加含有させる場合の含有量は、いずれも
０．０５〜２％とするのが好ましい。より好ましい範囲
は、いずれも０．１〜０．５％である。Cu, Ni: If the content of each component is less than 0.05%, the above effects cannot be obtained. That is, strength and toughness are not improved. On the other hand, when the content of each component exceeds 2%, the hot workability decreases,
It becomes difficult to manufacture a hot-rolled steel sheet as a base material. Therefore, Cu
When Ni and Ni are added and contained, the content is preferably 0.05 to 2%. A more preferable range is 0.1 to 0.5% in all cases.

【００５０】Ｎｂ、Ｔｉ：いずれの成分も、その含有量
が０．０１％未満では上記の効果が得られない。すなわ
ち、強度および靭性は勿論、耐火性（高温強度）は向上
しない。一方、いずれの成分もその含有量が０．１％を
超えると、靭性の低下を招く。Nb, Ti: If any of the components is less than 0.01%, the above effects cannot be obtained. That is, the fire resistance (high temperature strength) as well as the strength and toughness are not improved. On the other hand, when the content of each component exceeds 0.1%, the toughness is reduced.

【００５１】よって、ＮｂおよびＴｉを添加含有させる
場合の含有量は、いずれも０．０１〜０．１％とするの
が好ましい。より好ましい範囲は、いずれも０．０１〜
０．０５％である。Therefore, when Nb and Ti are added and contained, the content of each is preferably 0.01 to 0.1%. More preferred ranges are all 0.01 to
0.05%.

【００５２】Ｍｏ：その含有量が０．０５％未満では、
上記の効果が得られない。すなわち、強度および靭性、
並びに耐火性（高温強度）は向上しない。一方、その含
有量が１．５％を超えると、上記の効果が飽和するばか
りか、靭性の低下を招くことがある。よって、Ｍｏを添
加含有させる場合の含有量は、０．０５〜１．５％とす
るのが好ましい。より好ましい範囲は、０．１〜０．５
％である。Mo: When the content is less than 0.05%,
The above effects cannot be obtained. That is, strength and toughness,
In addition, fire resistance (high-temperature strength) is not improved. On the other hand, when the content exceeds 1.5%, not only the above effects are saturated, but also the toughness may be reduced. Therefore, when Mo is added and contained, the content is preferably set to 0.05 to 1.5%. A more preferred range is from 0.1 to 0.5.
%.

【００５３】Ｖ：その含有量が０．０５％未満では、上
記の効果が得られない。すなわち、強度および靭性、並
びに耐火性（高温強度）は向上しない。一方、その含有
量が０．５％を超えると、上記の効果が飽和するばかり
か、靭性の低下を招くことがある。よって、Ｖを添加含
有させる場合の含有量は、０．０５〜０．５％とするの
が好ましい。より好ましい範囲は、０．０５〜０．１％
である。V: If the content is less than 0.05%, the above effects cannot be obtained. That is, strength, toughness, and fire resistance (high-temperature strength) are not improved. On the other hand, when the content exceeds 0.5%, not only the above effects are saturated, but also the toughness may be reduced. Therefore, when V is added and contained, the content is preferably set to 0.05 to 0.5%. A more preferred range is 0.05-0.1%
It is.

【００５４】Ｂ:その含有量が０．０００５％未満で
は、上記の効果が得られない。すなわち、強度および靭
性は向上しない。一方、その含有量が０．００５％を超
えると、上記の効果が飽和するばかりか、靭性が低下す
る。よって、Ｂを添加含有させる場合の含有量は、０．
０００５〜０．００５％とするのが好ましい。より好ま
しい範囲は、０．０００５〜０．００２％である。B: If the content is less than 0.0005%, the above effects cannot be obtained. That is, strength and toughness are not improved. On the other hand, when the content exceeds 0.005%, not only the above effects are saturated, but also the toughness is reduced. Therefore, when B is added and contained, the content is 0.1%.
It is preferably set to 0005 to 0.005%. A more preferred range is 0.0005 to 0.002%.

【００５５】本発明の溶接鋼管は、上記の成分組成から
なり、その組織が軟質のフェライト相と硬質のベイナイ
トあるいはマルテンサイト相を含む複合組織を有するも
のである。そして、その母材となる鋼板、具体的には熱
延鋼板は、その組織が軟質のフェライト相と硬質のベイ
ナイトあるいはマルテンサイト相を含む複合組織を有す
るものであればよく、その製造履歴については特に限定
されない。しかし、その熱延鋼板は、従来にも増して低
降伏比の溶接鋼管を確実に得る観点からは下記の条件で
製造されたものを用いるのが望ましい。The welded steel pipe of the present invention has the above-mentioned composition and has a composite structure including a soft ferrite phase and a hard bainite or martensite phase. Then, the steel sheet as the base material, specifically, the hot-rolled steel sheet, may have any structure as long as its structure has a composite structure including a soft ferrite phase and a hard bainite or martensite phase. There is no particular limitation. However, it is desirable to use a hot-rolled steel sheet manufactured under the following conditions from the viewpoint of reliably obtaining a welded steel pipe having a lower yield ratio than ever before.

【００５６】スラブの加熱温度；スラブの加熱温度は、
以下の熱延が可能であれば特に制限されない。しかし、
その加熱温度は、１１００〜１２５０℃の範囲とするの
が好ましい。すなわち、スラブの加熱温度が１１００℃
未満であると、熱延後巻取りまでの水冷開始温度を確保
するのが困難になりやすい。逆に、スラブの加熱温度が
１２５０℃を超えると、粗粒組織となって靱性が低下す
るからである。The heating temperature of the slab;
There is no particular limitation as long as the following hot rolling is possible. But,
The heating temperature is preferably in the range of 1100 to 1250 ° C. That is, the slab heating temperature is 1100 ° C.
If it is less than 30 ° C., it tends to be difficult to secure a water cooling start temperature from hot rolling to winding. Conversely, if the heating temperature of the slab exceeds 1250 ° C., the slab becomes a coarse-grained structure and the toughness is reduced.

【００５７】熱延仕上温度:熱延仕上温度は、所望の低
降伏比を得るためには、Ａr₃点以上の温度域で熱延を終
了し、次に示す条件での加速冷却処が確実に実施できる
ようにするのが望ましい。なお、この熱延仕上温度は、
より優れた靭性を確保する観点からは、Ａr₃点〜（Ａr₃
点＋１００）℃とするのが好ましい。Hot-rolling finishing temperature: In order to obtain a desired low yield ratio, hot-rolling finishing temperature is set such that hot-rolling is finished in a temperature range of Ar ₃ points or more, and accelerated cooling under the following conditions is ensured. It is desirable to be able to implement it. The hot rolling finish temperature is
From the viewpoint of securing superior toughness, Ar ₃ points to (Ar ₃
(Point +100) ° C. is preferable.

【００５８】加速冷却処理；加速冷却処理は、熱延終了
後の鋼板を、その温度が６００℃前後になるまで緩冷却
する前段冷却と、その後巻取温度までに急冷却する後段
冷却の２段階に分けて行われる。しかし、熱延終了後の
冷却開始温度が（Ａr₃点＋３０）℃よりも高く、かつ前
段冷却での冷却速度が１０℃／ｓよりも速いと、軟質相
であるフェライトが十分成長せず、所望の低降伏比が得
難くなる。このため、冷却開始温度は（Ａr₃点＋３０）
℃以下、前段冷却での冷却速度は１０℃／ｓ以下とする
のが望ましい。Accelerated cooling treatment: The accelerated cooling treatment is performed in two stages: first cooling, in which the steel sheet after hot rolling is gradually cooled until its temperature becomes about 600 ° C., and second cooling, in which it is rapidly cooled to the winding temperature. It is performed separately. However, if the cooling start temperature after the end of hot rolling is higher than (Ar ₃ points + 30) ° C. and the cooling rate in the pre-stage cooling is higher than 10 ° C./s, ferrite as a soft phase does not grow sufficiently, It becomes difficult to obtain a desired low yield ratio. Therefore, the cooling start temperature is (Ar ₃ points + 30)
It is desirable that the cooling rate in the pre-stage cooling be 10 ° C./s or less.

【００５９】また、ベイナイト相あるいはマルテンサイ
ト相の硬さを確保して低降伏比とするためには、後段冷
却での冷却速度を速くして、低温まで冷却する必要があ
る。しかし、後段冷却での冷却速度が１０℃／ｓ未満で
あり、かつ冷却停止温度が５５０℃を超えると、十分に
硬さの硬いベイナイト相あるいはマルテンサイト相が生
成せず、所望の低降伏比が得難くなる。このため、冷却
停止温度は５５０℃以下、後段冷却での冷却速度は１０
℃／ｓ以上とするのが望ましい。Further, in order to secure the hardness of the bainite phase or the martensite phase and to obtain a low yield ratio, it is necessary to increase the cooling rate in the post-stage cooling to cool to a low temperature. However, if the cooling rate in the post-stage cooling is less than 10 ° C./s and the cooling stop temperature exceeds 550 ° C., a sufficiently hard bainite phase or martensite phase is not formed, and a desired low yield ratio is obtained. Is difficult to obtain. For this reason, the cooling stop temperature is 550 ° C. or less, and the cooling rate in the post-stage cooling is 10
C./s or more is desirable.

【００６０】なお、後段冷却での冷却速度は、１０℃／
ｓ以上であれば速ければ速いほどよく、その上限は特に
制限されない。しかし、良好な平坦度を有する熱延鋼板
を高い生産性をもって製造するためには、その冷却速度
の上限は５０℃／ｓ未満とするのが好ましい。The cooling rate in the post-stage cooling is 10 ° C. /
As long as s or more, the faster, the better, the upper limit is not particularly limited. However, in order to produce a hot-rolled steel sheet having good flatness with high productivity, the upper limit of the cooling rate is preferably less than 50 ° C./s.

【００６１】すなわち、例えば前述した特開昭５８−７
１３３７号公報に示されるように、Ｃｒを含有しない
か、含有したとしてもその含有量の少ない素材鋼を用い
る従来技術では、後段冷却を５０℃／ｓ以上というよう
な高速の冷却速度で行い、かつ冷却停止温度を可及的に
低くして３００℃以下で巻取る必要がある。従って、こ
の場合には、巻取り温度が低温であるので、生産性が低
下するのに加え、得られた熱延鋼板の平坦度が劣化す
る。That is, for example, the above-mentioned JP-A-58-7
As shown in Japanese Patent No. 1337, in the prior art using a material steel containing no Cr or containing a small amount of Cr even if it is contained, post-stage cooling is performed at a high cooling rate such as 50 ° C./s or more. In addition, it is necessary to lower the cooling stop temperature as much as possible and wind the film at 300 ° C. or lower. Accordingly, in this case, since the winding temperature is low, the productivity is lowered and the flatness of the obtained hot-rolled steel sheet is deteriorated.

【００６２】これに対し、本発明で用いる素材鋼は、１
〜３％のＣｒを含有しているために焼入性がよく、高強
度な硬質相が容易に得られる。このため、上記したよう
に、後段冷却の冷却速度を低くできるのに加え、冷却停
止温度を可及的に高くすることができる。この結果、本
発明では、良好な平坦度を有する熱延鋼板を高い生産性
をもって製造することが可能になるのである。しかし、
後段冷却での冷却速度を５０℃／ｓ超にすると、その製
造ラインの構造上、巻取り温度が低温となって平坦度が
劣化する。このことから、後段冷却での冷却速度の上限
は、５０℃／ｓ未満とするのが好ましいのである。On the other hand, the material steel used in the present invention is 1
Since it contains ３3% of Cr, it has good hardenability and a high-strength hard phase can be easily obtained. For this reason, as described above, in addition to being able to lower the cooling rate of the post-stage cooling, the cooling stop temperature can be made as high as possible. As a result, in the present invention, a hot-rolled steel sheet having good flatness can be manufactured with high productivity. But,
If the cooling rate in the post-stage cooling exceeds 50 ° C./s, the winding temperature will be low and the flatness will deteriorate due to the structure of the production line. From this, it is preferable that the upper limit of the cooling rate in the post-stage cooling be less than 50 ° C./s.

【００６３】上記各条件のもとで熱延を行って得られ、
その組織が軟質のフェライト相と硬質のベイナイトある
いはマルテンサイト相を含む複合組織で、従来の熱延鋼
板に比べてより低降伏比の熱延鋼板は、高出力の高周波
加熱併用レーザ溶接法により製管溶接されて溶接鋼管と
される。Obtained by hot rolling under the above-mentioned conditions;
The structure is a composite structure containing a soft ferrite phase and a hard bainite or martensite phase.Hot-rolled steel sheets with a lower yield ratio than conventional hot-rolled steel sheets are manufactured by high-power laser welding combined with high-frequency high-frequency heating. The pipe is welded to form a welded steel pipe.

【００６４】製管溶接；製管溶接は、得られた低降伏比
の熱延鋼板を常法によってオープンパイプ状に連続的に
成形し、オープンパイプ状に成形された熱延鋼板の両端
部の端面が相互に当接する突き合わせ部に、レーザビー
ムを照射して溶融衝合溶接される。この時、溶接部には
靱性の改善や耐食性の向上を目的に、溶接材料を用いて
Ａｌ、Ｔｉ、ＣｕおよびＮｉなどの成分を添加する必要
は特にないが、これらの成分を添加してもよい。Pipe welding: In the pipe welding, the obtained hot-rolled steel sheet having a low yield ratio is continuously formed into an open pipe shape by a conventional method, and both ends of the hot-rolled steel sheet formed into an open pipe shape are formed. A laser beam is applied to the butted portions where the end faces abut each other, and fusion welding is performed. At this time, it is not particularly necessary to add components such as Al, Ti, Cu and Ni using a welding material for the purpose of improving toughness and corrosion resistance in the welded portion. Good.

【００６５】なお、製管溶接は、レーザ溶接法を用いる
限りその条件は特に制限されない。しかし、微小な溶接
欠陥の発生防止して溶接部の耐ＨＩＣ性と耐ＳＳＣ性の
向上並びに高靭性化を図る一方、ＥＲＷ法と同等の能率
で製管溶接する観点からは、本発明者らが先に開発提案
した前述の特開平８−１１８０５０号公報および特開平
８−１２０３４６号公報に示される方法、すなわち高周
波加熱併用レーザ溶接法を用いて製管溶接するのが望ま
しい。The conditions for pipe welding are not particularly limited as long as a laser welding method is used. However, from the viewpoint of improving the HIC resistance and SSC resistance and increasing the toughness of the welded portion by preventing the generation of minute welding defects and increasing the toughness, from the viewpoint of pipe welding with the same efficiency as the ERW method, the present inventors have found that It is desirable to carry out tube welding using the method disclosed in the above-mentioned Japanese Patent Application Laid-Open Nos. 8-118050 and 8-120346, that is, laser welding combined with high-frequency heating.

【００６６】すなわち、母材鋼板の肉厚をｔ（ｍｍ）、
レーザ出力をＰ（ｋＷ）、溶接速度をＶ（ｍ／ｍｉ
ｎ）、鋼板端面の予熱温度をＴ（℃）、室温をＴ0
（℃）としたとき、下記（１）式と（２）式を同時に満
たす条件で製管溶接することである。That is, the thickness of the base steel sheet is t (mm),
Laser output is P (kW) and welding speed is V (m / mi)
n), the preheating temperature of the steel plate end face is T (° C), and the room temperature is T0.
(° C.) means that pipe welding is performed under conditions that simultaneously satisfy the following expressions (1) and (2).

【００６７】 Ｖ≧２ ・・・・（１） Ｐ≧０．４Ｖｔ／ｅ^a(T-T0) ・・・・（２） ただし、ａ＝０．０００６ この場合、レーザビームを照射する溶接部は、ＨｅやＡ
ｒなどの不活性ガスを用いてシールドすることでプラズ
マの発生を防止することが肝要である。V ≧ 2 (1) P ≧ 0.4 Vt / ea ^(T−T0) (2) where a = 0.0006 In this case, the welded portion irradiated with the laser beam Is He or A
It is important to prevent generation of plasma by shielding with an inert gas such as r.

【００６８】上記の（１）式と（２）式を同時に満たす
条件のもとに製管溶接する場合は、これも本発明者らが
先に開発提案した特開平５−２２８６６０号公報に示さ
れる方法、すなわち合わせ部に管外面側に開脚するＶ溝
を形成させか否かにかかわらず、正常な溶け込み形状を
有する靱性に優れた溶接部を得ることができる。When pipe welding is performed under conditions that simultaneously satisfy the above equations (1) and (2), this is also disclosed in Japanese Patent Application Laid-Open No. 5-228660, which was previously developed and proposed by the present inventors. Irrespective of the method used, that is, whether or not a V-groove is formed in the mating portion on the outer surface of the tube, a weld having a normal penetration shape and excellent toughness can be obtained.

【００６９】上記の（１）式は、溶接速度Ｖが２ｍ／ｍ
ｉｎ未満の場合、上記の特開平５−２２８６６０号公報
に示されるＶ溝の形成有無、レーザ出力Ｐおよび母材の
肉厚ｔとはほぼ無関係に、溶融金属部の溶接後の冷却速
度が低下し、溶接部の結晶粒が粗大化して肉厚方向に延
在する柱状晶組織となって溶接部のとけ込み形状がワイ
ンカップ状になることを示している。そして、溶接部の
溶け込み形状がワインカップ状になると、製管溶接後に
その溶接部にたとえ後熱処理を施しても、溶接部の靭性
が回復しない場合がある。The above equation (1) indicates that the welding speed V is 2 m / m
In the case of less than "in", the cooling rate of the molten metal portion after welding is reduced substantially irrespective of the presence or absence of the V-groove, the laser output P and the thickness t of the base material described in JP-A-5-228660. However, it is shown that the crystal grains of the welded portion are coarsened to have a columnar crystal structure extending in the thickness direction, and the welded portion has a wine cup shape. If the penetration shape of the welded portion becomes a wine cup shape, the toughness of the welded portion may not be restored even if post-heat treatment is performed on the welded portion after pipe-forming welding.

【００７０】なお、高周波誘導加熱手段を用い、溶接部
に対してワインカップ状に対応した幅広の加熱域をもっ
て後熱処理を施すと、溶接金属中心部の温度が高くなり
すぎて組織が粗粒化し、溶接部の靱性が低下する。When the high frequency induction heating means is used to perform post-heat treatment on the weld with a wide heating area corresponding to the shape of a wine cup, the temperature of the central portion of the weld metal becomes too high and the structure becomes coarse. In addition, the toughness of the weld decreases.

【００７１】また、上記（２）式は、突き合わせ部に照
射すべきレーザビームの出力Ｐが不十分であると、無欠
陥溶接が不可能になることを示しており、そのレーザ出
力Ｐは、母材の肉厚ｔ、溶接速度Ｖおよび予熱温度Ｔで
決定される一定値以上の出力が必要であることを示して
いる。The above equation (2) indicates that if the output P of the laser beam to be applied to the butted portion is insufficient, the defect-free welding cannot be performed. This indicates that an output of a certain value or more determined by the thickness t of the base material, the welding speed V and the preheating temperature T is required.

【００７２】なお、上記（２）式を満たすレーザ出力Ｐ
を確実に確保するには、その出力が５ｋＷ程度である従
来のレーザ発振器では不十分で、少なくとも２５ｋＷ級
の大出力レーザ発振器が必要である。The laser output P satisfying the above equation (2)
In order to ensure this, a conventional laser oscillator having an output of about 5 kW is insufficient, and a high-power laser oscillator of at least 25 kW class is required.

【００７３】上記した条件のもとに製管溶接する場合に
は、ＥＲＷ法と同等の溶接速度でもって正常な溶け込み
形状を有し、溶接欠陥のない溶接部を備えた低降伏比の
溶接鋼管をえることができる。In the case of pipe welding under the above conditions, a welded steel pipe having a normal penetration shape at a welding speed equivalent to that of the ERW method and having a welded portion having no welding defect and having a weld defect is provided. Can be obtained.

【００７４】なお、その突き合わせ部に管外面側に開脚
するＶ溝を形成させ、このＶ溝低部にさらなる大出力の
レーザビームを照射して溶融衝合溶接する場合には、よ
り一層高速での製管溶接が可能になることはいうまでも
ない。In the case where a V-groove is formed in the butted portion on the outer surface side of the tube, and the lower portion of the V-groove is irradiated with a laser beam having a higher output to perform the melt-butt welding, a higher speed is achieved. Needless to say, it is possible to perform pipe welding at the same time.

【００７５】シーム熱処理；上記のようにして製管溶接
された溶接鋼管の溶接金属部と溶接熱影響部には、後熱
処理を施す必要がある。その理由は、レーザ溶接の特徴
である急速冷却のため、溶接ままでは溶接金属部と溶接
熱影響部が粗粒かつマルテンサイトやベイナイト組織に
なっていて靱性が不芳であるので、その靱性回復を図る
ためである。Seam heat treatment: It is necessary to perform a post heat treatment on the weld metal part and the weld heat affected part of the welded steel pipe welded as described above. The reason is that due to the rapid cooling characteristic of laser welding, the as-welded part of the weld metal and the weld heat-affected zone is coarse-grained and has a martensite or bainite structure with poor toughness. It is for planning.

【００７６】しかし、その際、加熱温度がＡc₃点未満で
あると、その部分の組織がオーステナイト単相になら
ず、その粗粒組織を十分には破壊して所望の細粒組織を
得ることができない。一方、加熱温度が１１００℃を超
えると結晶粒が再び粗粒化してしまう。特に、Ｃの含有
量が０．１２％を超えると鋼では、高温加熱による靭性
劣化が著しくなる。このため、その加熱温度は、Ａc₃点
〜１１００℃とするのが望ましい。However, at this time, if the heating temperature is lower than Ac ₃ points, the structure at that portion does not become a single austenite phase, and the coarse structure is sufficiently broken to obtain a desired fine structure. Can not. On the other hand, when the heating temperature exceeds 1100 ° C., the crystal grains are coarsened again. In particular, when the content of C exceeds 0.12%, in steel, the toughness is significantly reduced by high-temperature heating. For this reason, the heating temperature is desirably set to _three points of Ac to 1100 ° C.

【００７７】そして、上記温度域への加熱後、直ちに急
冷する。この急冷により、溶接部の強度低下が防止さ
れ、また所望の低降伏比を確保するのに適した複合組織
が得られる。なお、上記の急冷は、加速冷却を適用する
のが好ましく、その加速冷却条件としては、上記熱延鋼
板製造時における加速冷却条件と類したものであること
が望ましい。具体的には、６００℃前後までの前段冷却
を１０℃／ｓ未満の冷却速度で、６００℃前後から５０
０℃以下、望ましくは４００℃前後までの後段冷却を１
０℃／ｓ以上の冷却速度で行うことである。Then, immediately after the heating to the above temperature range, the material is rapidly cooled. This quenching prevents a decrease in the strength of the welded portion and provides a composite structure suitable for ensuring a desired low yield ratio. In addition, it is preferable to apply accelerated cooling to the rapid cooling, and the accelerated cooling conditions are desirably similar to the accelerated cooling conditions at the time of manufacturing the hot-rolled steel sheet. Specifically, the pre-stage cooling to about 600 ° C. is performed at a cooling rate of less than 10 ° C./s from about 600 ° C. to 50 ° C.
Post-stage cooling to 0 ° C or less, preferably around 400 ° C
It is performed at a cooling rate of 0 ° C./s or more.

【００７８】さらに、上記の加速冷却後、必要に応じて
焼戻処理を施してもよい。この焼戻処理を施す場合は、
溶接部の軟化を図ることができ、溶接部の靭性がより一
層改善される。ただし、焼戻温度が５００℃未満である
と、材料が軟化せず、焼戻処理の効果が得られない。、
一方、焼戻温度が７５０℃を超えると、部分的にオース
テナイト変態が生じ、所望の強度が得られなくなる。こ
のため、加速冷却後に焼戻処理を施す場合の加熱温度
は、５００〜７５０℃とするのが望ましい。Further, after the above-mentioned accelerated cooling, a tempering treatment may be performed if necessary. When performing this tempering process,
The weld can be softened, and the toughness of the weld is further improved. However, if the tempering temperature is lower than 500 ° C., the material does not soften, and the effect of the tempering treatment cannot be obtained. ,
On the other hand, when the tempering temperature exceeds 750 ° C., austenite transformation occurs partially, and the desired strength cannot be obtained. For this reason, when performing tempering after accelerated cooling, the heating temperature is desirably 500 to 750 ° C.

【００７９】なお、硬度の均一化とより一層の低降伏比
を得ることを優先したい場合は、焼戻処理を省略するの
が好ましい。When it is desired to give priority to uniform hardness and to obtain a still lower yield ratio, it is preferable to omit the tempering process.

【００８０】[0080]

【実施例】Ｃｒ含有量の種々異なる表２に示す化学成分
を有する１４種類の素材鋼を溶製し、これらを連続鋳造
して厚さ２１２ｍｍ、幅１６００ｍｍ、長さ５０００ｍ
ｍの熱延用のスラブを得た。次いで、得られた各スラブ
を、表３に示す種々の製造条件で熱延し、種々異なる肉
厚（５．６ｍｍ〜１６．１ｍ）の熱延鋼板を製作準備し
た。EXAMPLES Fourteen types of material steels having different Cr contents and having the chemical components shown in Table 2 were smelted, and continuously cast to produce 212 mm in thickness, 1600 mm in width, and 5000 m in length.
m of slab for hot rolling was obtained. Next, each of the obtained slabs was hot-rolled under various manufacturing conditions shown in Table 3 to prepare hot-rolled steel sheets having various thicknesses (5.6 mm to 16.1 m).

【００８１】なお、熱延鋼板の製造条件のほとんどは、
低降伏比を得るために前段冷却での冷却速度を遅くし、
後段冷却での冷却速度を速くして、軟質のフェライト相
と硬質のベイナイトあるいはマルテンサイト相を含む複
合組織が確実に得られるようにした。また、一部の素材
鋼製のスラブについては、比較のため、前段冷却での冷
却速度を速くするか、もしくは後段冷却の冷却速度を遅
くし、加速冷却での冷却速度の影響を調べた。Most of the manufacturing conditions for hot-rolled steel sheets are as follows:
In order to obtain a low yield ratio, slow down the cooling rate in the pre-cooling,
The cooling rate in the post-stage cooling was increased so that a composite structure containing a soft ferrite phase and a hard bainite or martensite phase was reliably obtained. For some steel slabs, for comparison, the cooling rate in the former cooling was increased, or the cooling rate in the latter cooling was reduced, and the effect of the cooling rate in the accelerated cooling was examined.

【００８２】得られた熱延鋼板の組織と機械的性質を、
表３に、熱延鋼板の製造条件と併せて示した。The structure and mechanical properties of the obtained hot rolled steel sheet were
Table 3 also shows the manufacturing conditions of the hot-rolled steel sheet.

【００８３】[0083]

【表２】 [Table 2]

【００８４】[0084]

【表３】 [Table 3]

【００８５】表３に示すように、得られた熱延鋼板の降
伏比は、Ｃｒを除く他の成分含有量がほぼ同じである熱
延鋼板（鋼板No. Ａ１〜Ｂ１とＣ１〜Ｄ１、Ｅ１とＦ１
〜Ｇ１およびＨ１とＩ１）を対比すると、Ｃｒ含有量が
１％未満である従来の熱延鋼板よりも、本発明で規定す
る範囲内のＣｒを含有する熱延鋼板の方が低降伏比であ
ることがわかる。As shown in Table 3, the yield ratio of the obtained hot-rolled steel sheets was the same as that of the hot-rolled steel sheets (steel Nos. A1 to B1 and C1 to D1, E1 And F1
-G1 and H1 and I1), the hot-rolled steel sheet containing Cr within the range specified in the present invention has a lower yield ratio than the conventional hot-rolled steel sheet having a Cr content of less than 1%. You can see that there is.

【００８６】また、鋼板No. Ｌ１とＮ１との対比から明
らかなように、本発明で規定する範囲内のＣｒを含有す
る素材鋼は、後段冷却での冷却速度を遅くして巻取温度
を高くしても、低降伏比の熱延鋼板が得られることがわ
かる。As is clear from the comparison between the steel sheets No. L1 and N1, the material steel containing Cr within the range specified in the present invention has a lower cooling rate in the post-stage cooling to reduce the winding temperature. It can be seen that a hot rolled steel sheet having a low yield ratio can be obtained even when the height is increased.

【００８７】ただし、後段冷却での冷却速度が遅すぎた
り、前段冷却での冷却速度が速すぎると、パーライトの
析出した組織やマルテンサイト単相に近い組織になり、
低降伏比の熱延鋼板が得られないことがわかる（鋼板N
o. Ｃ２およびＦ２参照）。However, if the cooling rate in the post-stage cooling is too slow or the cooling speed in the pre-stage cooling is too fast, the structure becomes a structure in which pearlite is precipitated or a structure close to a martensite single phase.
It can be seen that a hot-rolled steel sheet with a low yield ratio cannot be obtained (Steel N
o. See C2 and F2).

【００８８】次いで、これらの熱延鋼板を適宜な幅にス
リットし、スリット後の熱延鋼板を母材にして製管溶接
を行って肉厚ｔ（ｍｍ）と外径Ｄ（ｍｍ）との比ｔ／Ｄ
が種々異なる溶接鋼管を製造した。Next, these hot-rolled steel sheets are slit to an appropriate width, and the hot-rolled steel sheets after the slits are used as base materials to perform pipe welding to determine the thickness t (mm) and the outer diameter D (mm). Ratio t / D
Manufactured different types of welded steel pipes.

【００８９】製管溶接は、母材の熱延鋼板を常法に従っ
てオープンパイプ状に成形し、オープンパイプ状に成形
された熱延鋼板の両端面が相互に当接する突き合わせ部
の管外面に、その上方からレーザビームを垂直に照射す
ることにより行った。In the pipe welding, a hot rolled steel sheet as a base material is formed into an open pipe shape according to a conventional method, and the pipe outer surface of a butt portion at which both end faces of the hot rolled steel sheet contact each other is formed. This was performed by vertically irradiating a laser beam from above.

【００９０】この際、溶接部は、Ｈｅガスを用いてシー
ルドした。また、レーザ発振器としては、集光前のビー
ム径が５０．８ｍｍ、ミラー（放物面鏡）の焦点距離が
３８１ｍｍである最大出力２５ｋＷの炭酸ガスレーザ発
振器と、集光前のビーム径が３０ｍｍ、ミラー（放物面
鏡）の焦点距離ｆが１５１ｍｍである最大出力５ｋＷの
炭酸ガスレーザ発振器との２種類を用い、熱延鋼板の肉
厚ｔ（ｍｍ）に応じて使い分けた。そして、集光後のレ
ーザビームの焦点は、いずれの場合も、突合せ部の管外
表面に設定した。さらに、オープンパイプ状に成形され
た熱延鋼板の両縁部は、高周波加熱手段を用いて種々の
温度に予熱した。At this time, the weld was shielded using He gas. As the laser oscillator, a carbon dioxide laser oscillator having a maximum output of 25 kW having a beam diameter of 50.8 mm before focusing and a focal length of a mirror (parabolic mirror) of 381 mm, a beam diameter of 30 mm before focusing, Two types of carbon dioxide laser oscillators having a maximum output of 5 kW and a focal length f of a mirror (parabolic mirror) of 151 mm were used depending on the thickness t (mm) of the hot-rolled steel sheet. In each case, the focal point of the condensed laser beam was set on the outer tube surface of the butted portion. Further, both edges of the hot-rolled steel sheet formed into an open pipe shape were preheated to various temperatures using a high-frequency heating means.

【００９１】なお、一部の熱延鋼板については、比較の
ために窒素ガスで溶接部をシールドしつつＥＲＷ法によ
って製管溶接した。For some of the hot rolled steel sheets, pipe welding was performed by the ERW method while shielding the welded portion with nitrogen gas for comparison.

【００９２】製管溶接後の溶接鋼管の溶接金属部と溶接
熱影響部には、後熱処理として、１０５０℃加熱→水冷
→７００℃加熱→空冷の焼入れ焼戻し処理（シームＱＴ
…）もしくは、９５０℃加熱→水冷の焼入れ処理（シ
ームＱ…）を、高周波誘導方式のシームアニーラを用
いて施した。なお、いずれの熱処理も、加熱時の昇温速
度は６０℃／ｓ、水冷時の冷却速度は３０℃／ｓ、空冷
時の冷却速度は６℃／ｓとした。As the post heat treatment, the quenching and tempering treatment of 1050 ° C. heating → water cooling → 700 ° C. → air cooling (seam QT
..) Or a quenching treatment of heating at 950 ° C. → water cooling (seam Q...) Using a high-frequency induction type seam annealer. In each of the heat treatments, the heating rate during heating was 60 ° C./s, the cooling rate during water cooling was 30 ° C./s, and the cooling rate during air cooling was 6 ° C./s.

【００９３】そして、得られた溶接鋼管の溶接部の健全
性を調べるため、各溶接鋼管の軸長方向の一部から中央
に溶接部が位置する円弧状の試験片を採取し、この試験
片の両端片を半径方向に折りたたむ偏平試験を行い、溶
接部に脆性破壊が認められた場合を「不良」、認められ
なかった場合を「良好」として評価した。Then, in order to examine the soundness of the welded portion of the obtained welded steel pipe, an arc-shaped test piece having a welded portion located at the center from a part in the axial direction of each welded steel pipe was sampled, and this test piece was obtained. A flat test in which both end pieces were folded in the radial direction was performed, and a case where brittle fracture was observed in the welded portion was evaluated as “poor”, and a case where no brittle fracture was observed was evaluated as “good”.

【００９４】また、上記の偏平試験結果が「良好」であ
った溶接鋼管については、その母材部と溶接部から引張
試験片を切り出し採取し、引張試験を行って引張強さＴ
Ｓ（ＭＰａ）と降伏応力ＹＳ（ＭＰａ）を調べて降伏比
ＹＲ（％）を求めた。For the welded steel pipe having the above-mentioned flat test result of “good”, a tensile test piece was cut out from the base metal part and the welded part, sampled and subjected to a tensile test to obtain a tensile strength T.
The yield ratio YR (%) was determined by examining S (MPa) and yield stress YS (MPa).

【００９５】引張試験は、ラインパイプのスペックとし
て一般的なＡＰＩ（米国石油協会）規格の５Ｌに規定さ
れたＳｔｒｉｐ試験片を用い、当該規格の規定に準拠し
て行った。なお、ＡＰＩ規格の５Ｌには、溶接部につい
ては一般に引張強さのみを調べる規定であるが、本実施
例においては降伏応力をも調べ、溶接部の降伏比を求め
た。The tensile test was carried out in accordance with the specifications of a general pipe (American Petroleum Institute) standard 5L Strip test specimen as a line pipe specification. In addition, although 5L of the API standard specifies that only the tensile strength is generally examined for the welded portion, in this example, the yield stress was also examined and the yield ratio of the welded portion was determined.

【００９６】これらの結果を、表４に、製管条件と併せ
て示した。The results are shown in Table 4 together with the tube production conditions.

【００９７】[0097]

【表４】 [Table 4]

【００９８】表４中、試番３、４、８、９、１３、１
５、１６および１７が本発明例、試番試番５、６、１
０、１１、１４および１８〜２０が本発明の比較例であ
る。また、試番１、２、６、１２および２１〜２４は従
来例で、そのうちの試番１、２、６および１２は高周波
加熱併用レーザ溶接法によったもの、試番２１〜２４は
ＥＲＷ法によったものである。In Table 4, test numbers 3, 4, 8, 9, 13, 1
5, 16 and 17 are examples of the present invention, trial numbers 5, 6, 1
0, 11, 14 and 18 to 20 are comparative examples of the present invention. Test numbers 1, 2, 6, 12 and 21 to 24 are conventional examples. Among them, test numbers 1, 2, 6, and 12 are obtained by laser welding combined with high-frequency heating, and test numbers 21 to 24 are ERW. According to the law.

【００９９】本発明の溶接鋼管（試番３、４、８、９、
１３、１５、１６および１７）は、いずれの鋼管も、偏
平試験結果が良好で健全な溶接部を備えるとともに、前
述した表１に示す本発明で得ようとする目標値を満たす
低降伏比を有していた。The welded steel pipe of the present invention (test numbers 3, 4, 8, 9,
13, 15, 16 and 17) have a low yield ratio that satisfies the target values to be obtained in the present invention shown in Table 1 described above, in which all the steel pipes are provided with sound welds having good flatness test results and sound. Had.

【０１００】これに対し、Ｃｒ含有が１％未満である素
材鋼製の熱延鋼板を用い、本発明の溶接鋼管と同一条件
で製造した従来の溶接鋼管（試番１、２、７および１
２）は、いずれの鋼管も、偏平試験結果が良好で健全な
溶接部を備えるものの、本発明で得ようとする表１に示
す目標値よりも高い降伏比であった。On the other hand, conventional welded steel pipes (test numbers 1, 2, 7 and 1) were manufactured using hot-rolled steel sheets made of material steel containing less than 1% of Cr and manufactured under the same conditions as the welded steel pipes of the present invention.
In the case of 2), all the steel pipes had good flattening test results and were provided with sound welds, but had higher yield ratios than the target values shown in Table 1 to be obtained in the present invention.

【０１０１】また、本発明の溶接鋼管と同じ熱延鋼板を
用い、ＥＲＷ法によって製造した比較例の溶接鋼管（試
番５、６、１０、１１および１４）は、いずれの鋼管
も、偏平試験で溶接に脆性破壊が発生し、健全な溶接部
が得られなかった。これは、素材鋼のＣｒ含有量が高い
ために、溶接部にＣｒ酸化物を主体とした溶接欠陥が多
く発生したためである。Further, the welded steel pipes of the comparative examples (test numbers 5, 6, 10, 11 and 14) manufactured by the ERW method using the same hot-rolled steel sheets as the welded steel pipes of the present invention were all subjected to the flatness test. As a result, brittle fracture occurred in the welding, and a sound weld was not obtained. This is because a large amount of welding defects mainly composed of Cr oxide occurred in the welded portion due to the high Cr content of the base steel.

【０１０２】さらに、本発明の溶接鋼管と同じ熱延鋼板
を用い、大出力の高周波加熱併用レーザ溶接法で製管溶
接した溶接鋼管であっても、溶接金属部と溶接熱影響部
に後熱処理を施さない比較例の溶接鋼管（試番１８〜２
０）は、ＥＲＷ法で製造した溶接鋼管と同様に、偏平試
験で溶接部に脆性破壊が発生し、健全な溶接部が得られ
なかった。これは、溶接部の組織が靱性に乏しい粗粒の
マルテンサイト組織になったためである。Further, even in the case of a welded steel pipe made by using the same hot-rolled steel sheet as the welded steel pipe of the present invention and using a high-power laser welding method combined with high-frequency heating, the post-heat treatment is applied to the weld metal part and the weld heat-affected zone. Comparative Example Welded Steel Pipe (No. 18-2)
In 0), as in the case of the welded steel pipe manufactured by the ERW method, brittle fracture occurred in the welded portion in the flattening test, and a sound welded portion was not obtained. This is because the structure of the weld became a coarse-grained martensite structure with poor toughness.

【０１０３】また更に、Ｃｒを含有しない素材鋼製の熱
延鋼板（鋼板No. Ｍ１とＮ１）を母材とし、ＥＲＷ法に
よって製造した従来の溶接鋼管（試番２１〜２４）は、
いずれの鋼管も、偏平試験結果が良好で、かつ母材部の
降伏比は低いものの、溶接部の降伏比が本発明で得よう
とする表１に示す目標値よりも高かった。これは、後熱
処理によって溶接部の硬度と引張強さが著しく低下する
のみならず、管円周方向の強度の均一性が損なわれるた
めである。Further, conventional welded steel pipes (sample numbers 21 to 24) manufactured by the ERW method using a hot-rolled steel sheet (steel No. M1 and N1) made of a material steel containing no Cr as a base material,
In all the steel pipes, the flattening test results were good and the yield ratio of the base material was low, but the yield ratio of the welded portion was higher than the target value shown in Table 1 to be obtained in the present invention. This is because not only the hardness and tensile strength of the welded portion are significantly reduced by the post-heat treatment, but also the uniformity of strength in the circumferential direction of the pipe is impaired.

【０１０４】なお、試番２１〜２４の溶接鋼管は、デー
タの記載は省略するが、その溶接金属部と溶接熱影響部
に後熱処理を施さなくても、素材鋼がＣｒを含有せず焼
入れ性に乏しいので、溶接部の組織が比較的細粒とな
り、靱性が良好なために偏平試験結果は良好であった。
しかし、母材部と溶接部の降伏比は、いずれも本発明で
得ようとする目標値を満足しなかった。[0104] For the welded steel pipes of test numbers 21 to 24, although the description of the data is omitted, the material steel does not contain Cr and is quenched without post-heat treatment on the weld metal part and the weld heat affected zone. Because of poor weldability, the structure of the weld was relatively fine and the flatness test results were good due to good toughness.
However, none of the yield ratios of the base metal portion and the welded portion satisfied the target values to be obtained in the present invention.

【０１０５】上記の結果からわかるように、１〜３％の
Ｃｒを含有し、その組織が軟質のフェライト相と硬質の
ベイナイトあるいはマルテンサイト相を含む複合組織で
ある熱延鋼板を母材とし、これを大出力の高周波予熱レ
ーザ溶接法を用いて製管溶接するとともに、その溶接金
属部と溶接熱影響部に後熱処理を施す場合には、従来に
も増して低降伏比を有する溶接鋼管が得られること明ら
かである。As can be seen from the above results, a hot rolled steel sheet containing 1 to 3% of Cr and having a composite structure including a soft ferrite phase and a hard bainite or martensite phase was used as a base material. When this is welded using a high-power high-frequency preheating laser welding method, and the post-heat treatment is performed on the weld metal and the weld heat affected zone, a welded steel pipe with a lower yield ratio than before is required. Obviously what you get.

【０１０６】[0106]

【発明の効果】本発明によれば、健全な溶接部を有し、
母材部と溶接部との降伏比がほぼ同じである従来の溶接
鋼管にも増してより低降伏比の溶接鋼管を提供すること
が可能である。従って、本発明の溶接鋼管を用いてパイ
プラインあるいは建築構造物を構築する場合には、その
安全性が一段と向上する。According to the present invention, a sound welding portion is provided,
It is possible to provide a welded steel pipe having a lower yield ratio than conventional welded steel pipes in which the yield ratio between the base metal part and the welded part is substantially the same. Therefore, when constructing a pipeline or a building structure using the welded steel pipe of the present invention, the safety is further improved.

【０１０７】また、本発明の製造方法によれば、上記の
低降伏比溶接鋼管を高能率に製造できるので、その製造
コストの低減を図ることが可能である。Further, according to the manufacturing method of the present invention, since the above-described low yield ratio welded steel pipe can be manufactured with high efficiency, the manufacturing cost can be reduced.

───────────────────────────────────────────────────── フロントページの続き (72)発明者 藤原 知哉 大阪府大阪市中央区北浜４丁目５番33号住 友金属工業株式会社内 ──────────────────────────────────────────────────続 き Continued from the front page (72) Inventor Tomoya Fujiwara 4-5-33 Kitahama, Chuo-ku, Osaka City, Osaka Sumitomo Metal Industries, Ltd.

Claims (2)

【特許請求の範囲】[Claims] 【請求項１】重量％で、Ｃ：０．０３〜０．３％、Ｓ
ｉ：０．０５〜１％、Ｍｎ：０．３〜２％、Ｐ：０．０
３％以下、Ｓ：０．０２％以下、Ｃｒ：１〜３％、ｓｏ
ｌ−Ａｌ：０．００５〜０．１％、Ｍｏ：０〜１．５
％、Ｎｂ：０〜０．１％、Ｖ：０〜０．５％、Ｔｉ：０
〜０．１％、Ｃｕ：０〜２％、Ｎｉ：０〜２％および
Ｂ：０〜０．００５％を含み、残部Ｆｅおよび不可避的
不純物からなり、その組織が軟質のフェライト相と硬質
のベイナイトあるいはマルテンサイト相を含む複合組織
である溶接鋼管であって、その降伏比ＹＲ（％）が下記
の条件を満たすことを特徴とする低降伏比溶接鋼管。 ｔ／Ｄ≦２のとき、ＹＲ≦８０％ ２＜ｔ／Ｄ≦３のとき、ＹＲ≦８５％ ｔ／Ｄ＞３のとき、ＹＲ≦８８％ ただし、 ｔ：鋼管の肉厚（ｍｍ） Ｄ：鋼管の外径（ｍｍ）(1) C: 0.03-0.3% by weight, S
i: 0.05-1%, Mn: 0.3-2%, P: 0.0
3% or less, S: 0.02% or less, Cr: 1 to 3%, so
1-Al: 0.005 to 0.1%, Mo: 0 to 1.5
%, Nb: 0 to 0.1%, V: 0 to 0.5%, Ti: 0
-0.1%, Cu: 0-2%, Ni: 0-2%, and B: 0-0.005%, the balance being Fe and unavoidable impurities, the structure of which is a soft ferrite phase and a hard ferrite phase. A welded steel pipe having a composite structure containing a bainite or martensite phase, wherein the yield ratio YR (%) satisfies the following condition. YR ≦ 80% when t / D ≦ 2, YR ≦ 85% when 2 <t / D ≦ 3, YR ≦ 88% when t / D> 3, where t: wall thickness of steel pipe (mm) D : Outer diameter of steel pipe (mm) 【請求項２】重量％で、Ｃ：０．０３〜０．３％、Ｓ
ｉ：０．０５〜１％、Ｍｎ：０．３〜２％、Ｐ：０．０
３％以下、Ｓ：０．０２％以下、Ｃｒ：１〜３％、ｓｏ
ｌ−Ａｌ：０．００５〜０．１％、Ｍｏ：０〜１．５
％、Ｎｂ：０〜０．１％、Ｖ：０〜０．５％、Ｔｉ：０
〜０．１％、Ｃｕ：０〜２％、Ｎｉ：０〜２％および
Ｂ：０〜０．００５％を含み、残部Ｆｅおよび不可避的
不純物からなり、その組織が軟質のフェライト相と硬質
のベイナイトあるいはマルテンサイト相を含む複合組織
を有する熱延鋼板をオープンパイプ状に成形し、オープ
ンパイプ状に成形された熱延鋼板に相対向する両端面が
当接する接合点の近傍にレーザビームを照射して製管溶
接した後、少なくとも溶接金属部と溶接熱影響部を含む
部分に後熱処理を施すことを特徴とする請求項１に記載
の低降伏比溶接鋼管の製造方法。2. C: 0.03 to 0.3% by weight, S
i: 0.05-1%, Mn: 0.3-2%, P: 0.0
3% or less, S: 0.02% or less, Cr: 1 to 3%, so
1-Al: 0.005 to 0.1%, Mo: 0 to 1.5
%, Nb: 0 to 0.1%, V: 0 to 0.5%, Ti: 0
-0.1%, Cu: 0-2%, Ni: 0-2%, and B: 0-0.005%, the balance being Fe and unavoidable impurities, the structure of which is a soft ferrite phase and a hard ferrite phase. A hot-rolled steel sheet with a composite structure containing bainite or martensite phase is formed into an open pipe shape, and a laser beam is applied to the vicinity of the joint where the opposite end faces abut the hot-rolled steel sheet formed into an open pipe shape The method for producing a low yield ratio welded steel pipe according to claim 1, wherein after the pipe making and welding, at least a portion including a weld metal portion and a weld heat affected zone is subjected to post heat treatment.