JP4430222B2 - Manufacturing method of welded steel pipe with excellent formability - Google Patents

Manufacturing method of welded steel pipe with excellent formability Download PDF

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JP4430222B2
JP4430222B2 JP2000306580A JP2000306580A JP4430222B2 JP 4430222 B2 JP4430222 B2 JP 4430222B2 JP 2000306580 A JP2000306580 A JP 2000306580A JP 2000306580 A JP2000306580 A JP 2000306580A JP 4430222 B2 JP4430222 B2 JP 4430222B2
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welded
steel pipe
value
point
thickness
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JP2002115780A (en
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康浩 篠原
均 朝日
展弘 藤田
直樹 吉永
学 高橋
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新日本製鐵株式会社
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【0001】
【発明の属する技術分野】
本発明は、成形あるいは引き抜きなどの加工性の優れた機械構造用鋼管およびその製造方法に関する。本発明は特に自動車部品などに用いられる材料に適する。
【0002】
【従来の技術】
機械構造用鋼管は、その形状に起因する軽量化が可能な構造部材として、自動車分野などに多く使用されており、各種部品になるうるまでの工程で冷間あるいは熱間で加工される。そのため、これら鋼管には加工性が良好であることは言うまでもなく、局所的な塑性変形することなく均一的変形が高いことが要求される。従来の構造用鋼管はその製造方法が溶接鋼管であるため、造管時に既に多くの塑性変形が加わっており、製品時の降伏比は極めて高くまた均一変形能は殆どなく、極めて加工性が悪い鋼管が多かった。特に高強度鋼管ではその傾向が強い。
【0003】
近年、例えば自動車分野では各種部品軽量化し、同時に製造コストを抑制する動きが活発化しており、特開平10−175026号公報に開示されている静水圧塑性変形による新たな鋼管の成型工程として、ハイドロフォームが注目されている。これら複雑かつ局部的な大変形が伴う加工あるいは複数回の加工必要な鋼管成型に対して、上記理由より十分な塑性変形能を有する鋼管は、現在、ほとんど供給できない状況である。
【0004】
上述した事情より溶接鋼管の加工性向上には、鋼管そのものの塑性変形能を高めることが重要である。特に、最近注目されているハイドロフォームなどの厳しい成型加工に応える技術の開発が望まれている。ここで鋼管の塑性変形能とは、具体的に冷間加工性に対する指標であり、さらに詳しくは例えば鋼管肉厚の局部減少のしにくさの指標であるn値あるいは鋼管長手方向の材料伸縮性を表すr値、特に管軸方向のr値を意味する。これらn値(以降加工硬化指数を意味する)と管軸方向r値(通常冷延鋼板などで定義される平均r値のうち、圧延方向のr値と同義)を同時に高めることが重要であることが、詳細な研究結果より本発明者らによって明らかになった。さらに、n値は造管成型時の歪みにより大きく低下することから、ハイドロフォーム成形のような厳しい加工に耐えうるn値を確保するためには、造管後にn値を回復するための再加熱処理が必要であることが分かった。
【0005】
ところで、従来、溶接鋼管で造管ままでは溶接部組織は変態点以上に加熱されその後ただちに急冷されるため焼き入れ組織となっており、一般に母材と比べて溶接部の強度は高い。また、溶接部の肉厚は、一般に溶接後のビードカットにより母材のそれと比較し減肉しており、従来の溶接鋼管では溶接部の肉厚制御はされていない。従って、溶接ままである従来の溶接鋼管では、溶接部の肉厚分布が多少あっても、溶接部の強度が母材と比べて高いため拡管などの成形で溶接部が破断することはなかった。
【0006】
しかしながら、ハイドロフォームのような厳しい成形に耐えうるためには、溶接ままの鋼管ではn値が不十分であり、造管後のn値回復を目的とした再加熱処理が必要である。この熱処理によって溶接部の組織は回復し、母材と溶接部の強度差はほぼなくなる。従って、従来の溶接鋼管では溶接部の肉厚分布の制御をしていないため、局所的に薄くなっている場合はハイドローフォームといった厳しい成形では溶接部で破断し良好な成形加工が出来ない問題があった。
【0007】
【発明が解決しようとする課題】
本発明は、良好な成形性を有する機械構造用溶接鋼管を供給する目的で、上述した従来技術が有する課題である溶接部における材質不均質あるいは肉厚変化を制御した鋼管の提供とその製造方法である。
【0008】
【課題を解決するための手段】
本発明は、溶接部における加工性の見地から材質あるいは形状の不均質を制御することで、加工性に優れた鋼管を提供する。すなわち、本発明の要旨は次のとおりである。
)Ac1点以上に加熱された溶接熱影響部を含む溶接部の最小板厚が母材の平均板厚より0.8以上、1.2以下になるよう溶接部を研削した溶接鋼管を素管とし、管全体をAc1点−200℃以上、Ac1点+50℃以下の温度に再加熱し、前記温度範囲に少なくとも10秒以上保持する再加熱処理を施すことを特徴とする成形性の優れた溶接鋼管の製造方法。
(2)前記再加熱処理前に前記素管のAc1点以上に加熱された熱影響部を含む溶接部をAc3点−50℃以上、Ac3点+150℃以下の温度でシーム部を熱処理することを特徴とする()記載の成形性に優れた溶接鋼管の製造方法。
(3)平均r値が1.2以上の高r値鋼板を前記素管の素材として管軸方向に溶接して製造した溶接鋼管を用いることを特徴とする()または()記載の加工性の優れた溶接鋼管の製造方法。
(4)溶接鋼管用母材鋼板の幅方向の板厚形状で端部から板厚の10倍の長さまでの領域の板厚と鋼板中央部の板厚の比が、1.0以上、1.3以下であることを特徴とする()〜()のいずれかの項に記載の加工性の優れた溶接鋼管の製造方法。
【0009】
【発明の実施の形態】
以下に、本発明を詳細に説明する。
まず、加工性に優れた溶接鋼管について説明する。
ハイドロフォーム成形のような厳しいかつ複雑な加工に耐えうる鋼管の性能として、n値あるいはr値が重要である。n値は、等方的に加工が加わる場合に特に重要であり、下限を0.15とした。n値は高いほど成形性は向上するので、上限は特に定めない。n値は、JISの引張試験法における歪量が5〜10%あるいは3〜8%で求められる値とする。
【0010】
一方、r値は軸押しすることで材料を流入する加工で重要であり、そのような部位の加工性を確保するため、管軸方向r値を1.5以上とした。管軸方向r値は高いほど成形性は向上するので、上限は特に定めない。管軸方向r値は、鋼管の管軸方向からJIS12号引張試験片を採取し、JISにある引張試験で歪み量10または5%で求められる値とする。
【0011】
このように、鋼管性能としてn値およびr値が高いと加工性は向上するが、溶接鋼管では熱影響部を含んだ溶接部において形状あるいは材質の不均質が存在し、厳しい加工では溶接部で破断し十分な加工性が得られない場合がある。ただし、溶接熱影響部とは、Ac1点以上に加熱された領域である。本発明は、溶接鋼管の溶接部における形状あるいは材質を制御することで、ハイドロフォーム成型のような厳しい加工で溶接部が破断することなく成形性に優れた溶接鋼管が得られることである。溶接鋼管とは、電縫溶接、TIG,MIGなどのアーク溶接、鍛接、あるいはレーザー、電子ビーム溶接によって、あるいはこれらを複合して造管した鋼管である。従来の溶接鋼管では、溶接部ビードはアーク溶接、レーザーあるいは電子ビーム溶接では余盛まま、電縫溶接あるいは鍛接では造管後余盛りを内外面母材の板厚より薄くなるよう研削しているが、特に溶接部の形状制御はしていない。また、溶接部組織は焼き入れ組織となっていることから溶接部強度は母材と比較して高いから、拡管等の成形では溶接部で破断する問題は生じなかった。しかしながら、ハイドロフォーム成型のような厳しい加工では、造管時に低下するn値を再加熱処理によって回復することが必要である。このような溶接部の形状制御していない従来の溶接鋼管では、再加熱処理により溶接部と母材との強度差がほぼなくなり、従来の溶接鋼管では溶接部の形状不均質により局所的に肉厚が薄い部では応力集中が生じ、成型初期で溶接部が破断し良好な成形が出来ないことが分かった。そこで、本発明者らは、溶接部の肉厚と母材の肉厚の比の値が及ぼす成形性の影響について実験的に検討した結果、Ac1点以上に加熱された溶接熱影響部を含む溶接部の最小肉厚と母材の平均肉厚の比が、0.8〜1.2の範囲である場合、溶接部で破断することなく良好な加工性を示した。加工性の評価は、軸押し量1mmで、内圧を100bar昇圧する単純拡管試験のバースト時の最大拡管率である。従って、Ac1点以上に加熱された溶接熱影響部を含む溶接部の最小肉厚と母材の平均肉厚比を0.8以上、1.2以下にした。
【0012】
さらに、溶接部が破断することなく溶接部と母材が均一に加工されるためには、溶接部の肉厚制御に加えて溶接部の強度分布、すなわち硬さ分布制御が必要である。溶接部の硬さ分布は溶接条件によって変化する。本発明者らは、電縫溶接において、溶接条件と造管後のビード研削を変化させて造管した鋼管の加工性を調査し、加工性に及ぼす溶接部の肉厚と溶接部硬さの関係の影響を実験的に検討した。図1にその結果を示すが、Ac1点以上に加熱された溶接熱影響部を含む溶接部と母材の肉厚比Δt、およびAc1点以上に加熱された溶接熱影響部を含む溶接部と母材の硬さ比ΔHの関係をΔH/100+20Δtで整理し、成形性の影響をみると、25≧ΔH/100+20Δt≧18である場合、溶接部で応力集中が発生せず溶接破断することなく優れた成形性を示した。従って、Ac1点以上に加熱された溶接熱影響部を含む溶接部最小肉厚と母材の平均肉厚の比ΔtおよびAc1点以上に加熱された溶接熱影響部を含む溶接部と母材の平均硬さ比ΔHの関係を25≧ΔH/100+20Δt≧18に定めた。
【0013】
次に、本発明で規定した鋼管製造法について説明する。ハイドロフォーム成型ような厳しく加工に耐えうる鋼管を得るためには、n値およびr値の確保とともに、溶接部の板厚制御が重要となる。n値は、造管歪みにより大きく低下する。厳しい加工に耐えうるために必要なn値を確保するためには、造管後の後熱処理は必須である。造管歪みを除去しn値を確保するためには、Ac1点−200℃以上の加熱が必要である。Ac1点+50℃以上の加熱では、α→γ変態によりr値が低下するため、再加熱温度はAc1点−200℃以上、Ac1点+50℃以下に定めた。また、この温度域で管全体が均一に加熱保持されるためには、保持時間を10秒以上必要とする。上限は特に定めない。
【0014】
また、溶接部の内外表面では特に高い造管歪み導入されているため、再加熱処理によって異常粒成長による局所的な軟化部が生じる。これを回避するため、造管後、予め溶接熱影響部を含む溶接部だけをシーム熱処理することで溶接部の歪みを開放できる。従って、溶接部の表層における高歪領域の解消および組織の均質化を図るため、シーム熱処理は熱影響部を含む溶接部をAc3点−50℃以上に加熱する必要がある。Ac3点+150℃以上では、結晶粒粗大化により強度が低下するため、シーム熱処理の加熱温度は、Ac3点−50℃以上、Ac3点+150℃以下に定めた。
【0015】
また、厳しい加工に耐えるために必要な管軸方向のr値を確保するためには、造管前の鋼板のr値が高い必要であるが、平均的にr値が高いと複雑な加工に耐えうることができる。従って、ハイドロフォーム鋼板の平均r値を1.2以上に定めた。r値は、JISにある引張試験で求めた値であり、平均r値とは圧延方向、圧延方向から45°方向、板幅方向のr値の平均である。
【0016】
さらに、鋼管の溶接部の板厚制御するためには、鋼管の素材となる鋼板の端部の板厚制御が重要ある。鋼管の溶接部の板厚制御を可能とするためには、鋼板の端部から板厚の10倍の長さまでの領域の板厚と鋼板中央部の板厚の比が、1.0以上、1.3以下に定めた。
【0017】
【実施例】
熱間あるいは冷間圧延にて製造した、引張強度300〜800MPaの鋼板を、複数の圧延スタンドを有する成型圧延機あるいはベンディングロール成型機によって塑性加工して溶接鋼管用母管とし、これを電縫溶接、TIG,MIG,レーザー溶接、電子ビーム溶接、固相圧接などを用いて溶接し溶接鋼管と成した。溶接後、ただち、鋼管内外面をビードカッターにより溶接部を所定の肉厚になるよう研削した。一部は、溶接部のシーム熱処理を施した。これら鋼管は、鋼管性能を向上させるため再加熱処理を施した。再加熱処理は、ガス炉あるいは電気抵抗炉を用い所定の温度まで管全体を再加熱し、その温度に保持した。加工性は、軸押し量1mm,100bar/mmの条件の単純拡管試験を実施し、バースト時の最大拡管率で評価した。表1に、使用した鋼板の圧延方向r値および平均r値、端部の板厚と中央部の板厚比、および本発明で規定した造管、再加熱処理条件を示す。
【0018】
表2に、本発明で得られた鋼管の溶接部の形状および硬さ、およびn値、r値、さらに最大拡管率を示す。溶接部の形状および硬さを制御かつn値およびr値の高い鋼管は、最大拡管率1.25を越える極めて加工性に優れた鋼管である。
表3に比較に本発明以外の鋼管の特性を示す。
【0019】
【表1】
【0020】
【表2】
【0021】
【表3】
【0022】
【発明の効果】
本発明は、ハイドロフォームをはじめとした極めて厳しい成型加工に耐えうる加工性の優れた溶接鋼管を提供するものであり、例えば自動車用部品あるいは他の塑性加工を必要とする機械部品の供給が可能になる。
【図面の簡単な説明】
【図1】溶接部と母材の平均肉厚比と平均硬さ比と最大拡管率との関係を示す図。
[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel pipe for machine structure having excellent workability such as forming or drawing and a method for producing the same. The present invention is particularly suitable for materials used for automobile parts and the like.
[0002]
[Prior art]
Machine structural steel pipes are frequently used in the automobile field and the like as structural members that can be reduced in weight due to their shapes, and are processed cold or hot in processes until they become various parts. Therefore, it goes without saying that these steel pipes have good workability, and are required to have high uniform deformation without local plastic deformation. Since conventional steel pipes are manufactured using welded steel pipes, many plastic deformations have already been applied during pipe making, the yield ratio during production is extremely high, there is almost no uniform deformability, and workability is extremely poor. There were many steel pipes. This tendency is particularly strong in high-strength steel pipes.
[0003]
In recent years, for example, in the automobile field, various parts have been reduced in weight, and at the same time, there has been an active movement to reduce manufacturing costs. As a new steel pipe forming process by hydrostatic pressure plastic deformation disclosed in JP-A-10-175026, Forms are attracting attention. For steel pipe forming that requires complicated and local large deformation or steel pipe forming that requires a plurality of processes, a steel pipe having sufficient plastic deformability cannot be supplied at present.
[0004]
From the circumstances described above, it is important to improve the plastic deformability of the steel pipe itself in order to improve the workability of the welded steel pipe. In particular, it is desired to develop a technology that responds to severe molding processing such as hydrofoam, which has recently been attracting attention. Here, the plastic deformability of the steel pipe is specifically an index for cold workability, and more specifically, for example, an n value that is an index of the difficulty of local reduction in the thickness of the steel pipe or the material stretchability in the longitudinal direction of the steel pipe. R value representing the value, particularly the r value in the tube axis direction. It is important to simultaneously increase these n value (hereinafter referred to as work hardening index) and the tube axis direction r value (of which the r value in the rolling direction is the same as the average r value normally defined in cold-rolled steel sheets). This has been clarified by the present inventors from detailed research results. Furthermore, since the n value greatly decreases due to distortion during pipe forming, in order to secure an n value that can withstand severe processing such as hydroforming, reheating to recover the n value after pipe forming is performed. It turns out that processing is necessary.
[0005]
By the way, conventionally, if a welded steel pipe is made as it is, the welded portion structure is heated beyond the transformation point and then rapidly cooled, so that it is a quenched structure, and the strength of the welded portion is generally higher than that of the base material. In addition, the thickness of the welded portion is generally reduced compared to that of the base metal due to the bead cut after welding, and the thickness of the welded portion is not controlled in conventional welded steel pipes. Therefore, in the conventional welded steel pipe that is still welded, even if there is some thickness distribution of the welded part, the welded part is not broken by molding such as pipe expansion because the strength of the welded part is higher than the base metal. .
[0006]
However, in order to be able to withstand severe forming such as hydroforming, the steel pipe as welded has insufficient n value, and reheating treatment for the purpose of recovering the n value after pipe forming is necessary. By this heat treatment, the structure of the welded portion is recovered, and the difference in strength between the base metal and the welded portion is almost eliminated. Therefore, the conventional welded steel pipe does not control the thickness distribution of the welded part, so if it is locally thin, severe molding such as hydrofoam breaks at the welded part and a good molding process cannot be performed was there.
[0007]
[Problems to be solved by the invention]
The present invention is to provide a welded steel pipe for machine structure having good formability, and to provide a steel pipe with controlled material inhomogeneity or thickness change in a welded portion, which is a problem of the above-described prior art, and a method for manufacturing the same It is.
[0008]
[Means for Solving the Problems]
The present invention provides a steel pipe excellent in workability by controlling material or shape heterogeneity from the viewpoint of workability in a welded portion. That is, the gist of the present invention is as follows.
( 1 ) Ac Welded steel pipe whose welded portion is ground so that the minimum thickness of the welded portion including the weld heat affected zone heated to one or more points is 0.8 or more and 1.2 or less than the average plate thickness of the base metal. , And the whole tube is reheated to a temperature of Ac 1 point −200 ° C. or higher and Ac 1 point + 50 ° C. or lower, and subjected to a reheating treatment for holding at least 10 seconds in the temperature range. For producing welded steel pipes with excellent properties.
(2) The welded portion including the heat-affected zone heated to Ac 1 point or higher of the raw pipe before the reheating treatment is heat-treated at a temperature of Ac 3 point −50 ° C. or higher and Ac 3 point + 150 ° C. or lower. ( 1 ) The manufacturing method of the welded steel pipe excellent in the formability as described in the above.
(3) A welded steel pipe manufactured by welding a high r-value steel plate having an average r value of 1.2 or more in the pipe axis direction as a raw material of the raw pipe is used. ( 1 ) or ( 2 ) Manufacturing method of welded steel pipe with excellent workability.
(4) The ratio of the plate thickness in the width direction of the base steel plate for welded steel pipes in the width direction to a length 10 times the plate thickness and the plate thickness in the central portion of the steel plate is 1.0 or more, 1 The method for producing a welded steel pipe having excellent workability according to any one of ( 1 ) to ( 3 ), wherein the method is 3 or less.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
First, a welded steel pipe excellent in workability will be described.
The n value or the r value is important as the performance of a steel pipe that can withstand severe and complicated processing such as hydroforming. The n value is particularly important when processing isotropically, and the lower limit is set to 0.15. Since the moldability improves as the n value is higher, the upper limit is not particularly defined. The n value is a value obtained when the strain amount in the JIS tensile test method is 5 to 10% or 3 to 8%.
[0010]
On the other hand, the r value is important in the process of flowing the material by pushing the shaft, and the r value in the tube axis direction is set to 1.5 or more in order to ensure the workability of such a part. Since the moldability improves as the pipe axis direction r value is higher, the upper limit is not particularly defined. The pipe axis direction r value is a value obtained by taking a JIS No. 12 tensile test piece from the pipe axis direction of a steel pipe and obtaining a strain amount of 10 or 5% in a tensile test in JIS.
[0011]
As described above, when the n value and the r value are high as the steel pipe performance, the workability is improved. However, in the welded steel pipe, there is a heterogeneity of the shape or material in the welded portion including the heat affected zone. It may break and sufficient workability may not be obtained. However, the welding heat affected zone is a region heated to Ac 1 point or higher. It is an object of the present invention to obtain a welded steel pipe excellent in formability by controlling the shape or material in the welded part of the welded steel pipe without rupturing the welded part by severe processing such as hydroforming. The welded steel pipe is a steel pipe made by electric welding, arc welding such as TIG, MIG, forging, laser, electron beam welding, or a combination of these. In conventional welded steel pipes, the weld bead is ground by arc welding, laser or electron beam welding, and is ground so that the surplus after pipe forming is thinner than the thickness of the inner and outer surface base metal in electric welding or forging. However, the shape of the weld is not particularly controlled. Further, since the welded portion structure is a hardened structure, the welded portion strength is higher than that of the base material, so that there is no problem of breakage at the welded portion in the molding such as pipe expansion. However, in severe processing such as hydroforming, it is necessary to recover the n value, which decreases during pipe making, by reheating treatment. In such a conventional welded steel pipe that does not control the shape of the welded part, the difference in strength between the welded part and the base metal is almost eliminated by reheating treatment. It was found that stress concentration occurred in the thin part, and the welded part was broken at the initial stage of molding, and good molding was not possible. Therefore, the present inventors experimentally studied the influence of the formability exerted by the ratio of the thickness of the welded portion to the thickness of the base metal, and as a result, the welding heat affected zone heated to Ac 1 point or higher was determined. When the ratio of the minimum thickness of the welded part to be included and the average thickness of the base metal was in the range of 0.8 to 1.2, good workability was exhibited without breaking at the welded part. The evaluation of workability is the maximum tube expansion rate during burst in a simple tube expansion test in which the axial pressure is 1 mm and the internal pressure is increased by 100 bar. Therefore, the minimum thickness of the welded portion including the weld heat affected zone heated to Ac 1 point or higher and the average thickness ratio of the base material were set to 0.8 or more and 1.2 or less.
[0012]
Further, in order to uniformly process the welded portion and the base material without breaking the welded portion, it is necessary to control the strength distribution of the welded portion, that is, the hardness distribution control in addition to the thickness control of the welded portion. The hardness distribution of the weld varies depending on the welding conditions. The present inventors investigated the workability of steel pipes made by changing welding conditions and bead grinding after pipe making in ERW welding, and determined the thickness of welds and the hardness of welds on workability. The influence of the relationship was examined experimentally. The results are shown in Figure 1, including the thickness ratio Δt of the weld and the base material including the weld heat affected zone which is heated above Ac 1 point, and the weld heat affected zone which is heated above Ac 1 point welding When the relationship between the hardness ratio ΔH of the part and the base material is arranged as ΔH / 100 + 20Δt and the influence of formability is observed, when 25 ≧ ΔH / 100 + 20Δt ≧ 18, stress concentration does not occur in the welded portion and welding breaks. Excellent moldability was exhibited. Therefore, the ratio Δt of the minimum thickness of the welded portion including the weld heat affected zone heated to the Ac 1 point or higher and the average thickness of the base metal, and the welded portion and the mother including the weld heat affected zone heated to the Ac 1 point or higher. The relationship of the average hardness ratio ΔH of the material was set to 25 ≧ ΔH / 100 + 20Δt ≧ 18.
[0013]
Next, the steel pipe manufacturing method prescribed | regulated by this invention is demonstrated. In order to obtain a steel pipe that can withstand severe processing such as hydroform molding, it is important to control the thickness of the welded part as well as to secure the n value and the r value. The n value is greatly reduced due to tube-forming distortion. Post-heat treatment after pipe forming is indispensable to ensure the n value necessary to withstand severe processing. In order to remove the tube-forming distortion and secure the n value, heating at Ac 1 point-200 ° C. or higher is necessary. In heating at Ac 1 point + 50 ° C. or higher, the r value decreases due to the α → γ transformation, so the reheating temperature was set to Ac 1 point −200 ° C. or higher and Ac 1 point + 50 ° C. or lower. In order to uniformly heat and hold the entire tube in this temperature range, a holding time of 10 seconds or more is required. There is no particular upper limit.
[0014]
In addition, since a particularly high pipe-forming strain is introduced on the inner and outer surfaces of the welded portion, local softened portions due to abnormal grain growth are generated by the reheating treatment. In order to avoid this, the distortion of the welded portion can be released by performing a seam heat treatment only on the welded portion including the weld heat affected zone in advance after pipe making. Therefore, in order to eliminate the high strain region in the surface layer of the weld zone and to homogenize the structure, the seam heat treatment needs to heat the weld zone including the heat affected zone to Ac 3 point −50 ° C. or higher. At Ac 3 point + 150 ° C. or higher, the strength decreases due to the coarsening of the crystal grains. Therefore, the heating temperature of the seam heat treatment was set to Ac 3 point −50 ° C. or higher and Ac 3 point + 150 ° C. or lower.
[0015]
In addition, in order to secure the r value in the tube axis direction necessary to withstand severe processing, the steel plate before pipe forming needs to have a high r value. However, if the r value is high on average, complex processing is required. Can withstand. Therefore, the average r value of the hydroformed steel sheet was set to 1.2 or more. The r value is a value obtained by a tensile test in JIS, and the average r value is an average of r values in the rolling direction, the 45 ° direction from the rolling direction, and the sheet width direction.
[0016]
Furthermore, in order to control the thickness of the welded portion of the steel pipe, it is important to control the thickness of the end of the steel plate that is the material of the steel pipe. In order to enable plate thickness control of the welded portion of the steel pipe, the ratio of the plate thickness in the region from the end of the steel plate to 10 times the plate thickness and the plate thickness in the central portion of the steel plate is 1.0 or more, It was set to 1.3 or less.
[0017]
【Example】
A steel plate with a tensile strength of 300 to 800 MPa manufactured by hot or cold rolling is plastically processed by a forming rolling mill or bending roll forming machine having a plurality of rolling stands to form a welded steel pipe main pipe, which is electro-sewn. Welding, TIG, MIG, laser welding, electron beam welding, solid phase pressure welding, etc. were used to form a welded steel pipe. Immediately after welding, the inner and outer surfaces of the steel pipe were ground with a bead cutter so that the welded portion had a predetermined thickness. Some were subjected to seam heat treatment of the weld. These steel pipes were reheated to improve the steel pipe performance. In the reheating treatment, the entire tube was reheated to a predetermined temperature using a gas furnace or an electric resistance furnace, and kept at that temperature. The workability was evaluated by performing a simple tube expansion test under conditions of a shaft push amount of 1 mm and 100 bar / mm, and the maximum tube expansion rate during burst. Table 1 shows the rolling direction r value and average r value of the steel plates used, the plate thickness ratio of the end portion to the center portion, and the pipe making and reheating treatment conditions defined in the present invention.
[0018]
Table 2 shows the shape and hardness of the welded portion of the steel pipe obtained in the present invention, the n value, the r value, and the maximum tube expansion rate. A steel pipe that controls the shape and hardness of the weld and has a high n-value and r-value is a steel pipe that has an extremely high workability exceeding the maximum expansion ratio of 1.25.
Table 3 shows the characteristics of steel pipes other than the present invention for comparison.
[0019]
[Table 1]
[0020]
[Table 2]
[0021]
[Table 3]
[0022]
【The invention's effect】
The present invention provides a welded steel pipe excellent in workability that can withstand extremely severe forming processes such as hydroform, and can supply, for example, automotive parts or other mechanical parts that require plastic working. become.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between an average thickness ratio, an average hardness ratio, and a maximum tube expansion ratio between a welded portion and a base material.

Claims (4)

Ac1点以上に加熱された溶接熱影響部を含む溶接部の最小板厚が母材の平均板厚より0.8以上、1.2以下になるよう溶接部を研削した溶接鋼管を素管とし、管全体をAc1点−200℃以上、Ac1点+50℃以下の温度に再加熱し、前記温度範囲に少くとも10秒以上保持する再加熱処理を施すことを特徴とする成形性の優れた溶接鋼管の製造方法。Ac A welded steel pipe whose ground is welded so that the minimum thickness of the weld including the weld heat-affected zone heated to 1 or more points is 0.8 or more and 1.2 or less than the average thickness of the base metal. The entire tube is reheated to a temperature of Ac 1 point −200 ° C. or higher and Ac 1 point + 50 ° C. or lower, and subjected to a reheating treatment for holding at least 10 seconds in the temperature range. An excellent welded steel pipe manufacturing method. 前記再加熱処理前に前記素管のAc1点以上に加熱された熱影響部を含む溶接部をAc3点−50℃以上、Ac3点+150℃以下の温度でシーム部を熱処理することを特徴とする請求項記載の成形性に優れた溶接鋼管の製造方法。Heat treating the seam portion of the welded portion including the heat-affected zone heated to the Ac 1 point or higher of the base tube at a temperature of Ac 3 point −50 ° C. or higher and Ac 3 point + 150 ° C. or lower before the reheating treatment. The method for producing a welded steel pipe having excellent formability according to claim 1 . 平均r値が1.2以上の高r値鋼板を前記素管の素材として管軸方向に溶接して製造した溶接鋼管を用いることを特徴とする請求項または記載の成形性の優れた溶接鋼管の製造方法。The excellent formability according to claim 1 or 2 , wherein a welded steel pipe manufactured by welding a high r-value steel plate having an average r value of 1.2 or more in the axial direction as a raw material of the raw pipe is used. Manufacturing method of welded steel pipe. 溶接鋼管用母材鋼板の幅方向の板厚形状で端部から板厚の10倍の長さまでの領域の板厚と鋼板中央部の板厚の比が、1.0以上、1.3以下であることを特徴とする請求項のいずれかの項に記載の成形性の優れた溶接鋼管の製造方法。The ratio of the plate thickness in the width direction of the base steel plate for welded steel pipes in the width direction to the length 10 times the plate thickness and the plate thickness in the central portion of the steel plate is 1.0 or more and 1.3 or less The method for producing a welded steel pipe having excellent formability according to any one of claims 1 to 3 .
JP2000306580A 2000-10-05 2000-10-05 Manufacturing method of welded steel pipe with excellent formability Expired - Fee Related JP4430222B2 (en)

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