JP4474731B2 - Structural electric resistance welded steel pipe with excellent hydroforming properties and strain aging - Google Patents

Structural electric resistance welded steel pipe with excellent hydroforming properties and strain aging Download PDF

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JP4474731B2
JP4474731B2 JP2000127077A JP2000127077A JP4474731B2 JP 4474731 B2 JP4474731 B2 JP 4474731B2 JP 2000127077 A JP2000127077 A JP 2000127077A JP 2000127077 A JP2000127077 A JP 2000127077A JP 4474731 B2 JP4474731 B2 JP 4474731B2
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steel pipe
steel
hydroforming
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JP2001303196A (en
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章男 登坂
裕二 橋本
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JFE Steel Corp
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JFE Steel Corp
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Description

【0001】
【発明の属する技術分野】
この発明は、自動車の構造部材や足回り部材などの使途に好適な鋼管であって、とくにハイドロフォーミングにおける加工性(ハイドロフォーミング性)に優れ、しかも歪み時効性を有する構造用電縫鋼管に関する。
【0002】
【従来の技術】
自動車用の構造部材として用いられる、種々の断面形状をもつ中空部材を製造するには、従来、鋼板のプレス加工によって成形した部品同士をその溶接代であるフランジ部でスポット溶接で接合する方法が採用されてきたが、品質の面でも、生産効率の面でも改善が求められていた。
一方、最近になり、構造用の中空部材に対しても、衝突時のより高い衝撃吸収能が求められるようになり、素材が一層高強度化してきた。このため、従来のプレス成形による方法では、成形欠陥のない、また成形品の形状・寸法精度が良好な部材を製造することが次第に困難になってきつつある。
【0003】
このような問題を解決するための新しい成形方法として、最近、ハイドロフォーミングによる成形法が注目されている。ハイドロフォーミングは、鋼管の内部に高圧液体を注入して塑性加工を行う方法であり、鋼管の断面寸法を拡管加工などにより変化させて、複雑形状部材の一体成形をはかるとともに、強度・剛性を高める機能をもつ優れた成形法である。
ところで、このハイドロフォーミングに供される鋼管としては、一般に、容易に強度が得られ、かつ安価であるC:0.20〜0.10%の中、低炭素鋼からなる電縫鋼管が用いられることが多かった。
【0004】
【発明が解決しようとする課題】
しかしながら、かかるC量の電縫鋼管にハイドロフォーミングを施しても、材料の加工性がよくないために、十分な拡管率が得られないという問題があった。
一方、電縫鋼管の素材そのものの加工性を高めるために、炭素量を著しく低減した極低炭素鋼を素材に用いることが考えられる。しかし、極低炭素電縫鋼管の場合には、ハイドロフォーミング性はよいものの、あらたに溶接に起因した別の問題を生じる。すなわち、極低炭素電縫鋼管では鋼管製造時における継目部の溶接熱により、熱影響部の結晶粒が粗大化して軟化し、拡管成形での変形が局部的に集中して、素材がもつ高延性を十分に発揮できないこと、また、ハイドロフォーミングした部材を他の部材と溶接した場合に、同様な軟化が生じて部材としての強度が十分に得られないことである。
このように、ハイドロフォーミングに十分耐えられ、かつ溶接熱影響部の軟化が生じない鋼管は未だ提案されていないのが現状である。
【0005】
そこで、本発明は、上述した従来技術が抱えていたこれらの問題に鑑み、ハイドロフォームに適した電縫鋼管についての新たな提案を行うものである。とくに、この発明は、ハイドロフォーミング性に優れるとともに、溶接軟化を生じないばかりか、ハイドロフォーミング後の塗装焼付処理で硬化する、いわゆる歪み時効性を具えた、構造用電縫鋼管を提案することを目的とする。また、本発明鋼管が目指す具体的な目標特性は、(鋼管のTS)×拡管率(軸方向圧縮の条件)で表したハイドロフォーミング性が13000MPa・%以上であり、焼付処理後の引張強度と鋼管の引張強度との差が40MPa以上の歪み時効硬化量を有するものとする。
【0006】
【課題を解決するための手段】
発明者らは、上記課題を達成するために、電縫鋼管の成分組成、製造方法などについて種々の検討を重ねた。その結果、C量を0.01〜0.05%未満の範囲としたセミ極低炭素材を用いること、固溶Nを適正量含有させること、継ぎ目部を電気抵抗溶接して造管した後に絞り率0.3 〜10%のサイジング(縮径)を行うことが極めて有効であることを見いだした。本発明は上記知見を基にして完成したものであり、その要旨構成は次のとおりである。
【0007】
(1)鋼組成が、質量%(以下単に、%)でC:0.01〜0.05%未満、Si:1.0%以下、Mn:3.0%以下、P:0.15%以下、S:0.015%以下、Al:0.04%以下、N:0.005〜0.02%を含有し、残部はFeおよび不可避的不純物の鋼組成からなる成分組成を有する鋼スラブを熱間圧延し、熱間圧延終了後0.5秒以内に40℃/s以上の冷却速度で650℃以下まで冷却し、冷却終了温度以下の温度で巻取るか、または、上記鋼スラブを熱間圧延した熱延鋼板を冷間圧延した後、焼鈍時の加熱温度を750℃以下とすることにより、固溶Nを0.003%以上含有させた熱延または冷延の帯状素材を円筒状に成形した後、継目部を電気抵抗溶接し、次いで、外周長の絞り率で0.3〜10%のサイジングを施した電縫鋼管であって、引張強度(MPa)×拡管率(%)が13000MPa・%以上で、歪み量10%のハイドロフォーミング後170℃×20分の熱処理を行う歪み時効処理による引張強度の上昇量が40MPa以上であることを特徴とするハイドロフォーミング性に優れ、歪み時効性を有する構造用電縫鋼管。
【0008】
(2) 上記 (1)において、鋼組成が、上記成分のほか、下記A群〜C群から選ばれる1種または2種以上を含有することを特徴とするハイドロフォーミング性に優れ、歪み時効性を有する構造用電縫鋼管。

A群:Nb:0.005 〜0.040 %、Ti:0.005 〜0.50%、B:0.0005〜0.020 %のうちの1種または2種以上
B群:Cu:0.02〜1.5 %、Ni:0.02〜1.0 %、Cr:0.02〜1.0 %、Mo:0.02〜1.0 %のうちの1種または2種以上
C群:Ca:0.0020〜0.02%、 REM:0.0020〜0.02%のうちの1種または2種
【0010】
【発明の実施の形態】
この発明における鋼成分の限定理由、電縫鋼管の製造方法などについて説明する。
C:0.01〜0.05%未満
Cは、鋼の強化に寄与する反面、成形性を低下させる元素であり、とくにC含有量が0.05%以上では成形性の低下が大きくなる。一方、0.01%に満たない含有量では、電縫鋼管製造時の抵抗溶接により溶接熱影響部の結晶粒が粗大化し、また、ハイドロフォーミングした部材をアーク溶接した際にも同様に結晶粒が粗大化し、強度低下や不均一な変形の原因となる。このため、C量は0.01〜0.05%未満の範囲とする。
【0011】
Si:1.0 %以下
Siは、鋼の強化に有用な元素であり、所望の強度に応じて添加する。しかし、1.0 %を超えて添加すると、鋼管用の素材となる熱延または冷延鋼板の表面性状が悪化し、結局、鋼管の表面性状の顕著な悪化につながり、結果的にハイドロフォーム時の耐バースト性を低下させる。このため、1.0 %以下の範囲で添加する。
【0012】
Mn:3.0 %以下
Mnは、表面性状および溶接性を低下させることなく、鋼板ひいてはハイドロフォーミング部材の強度を向上させるのに有効な元素であるが、3.0 %超えの添加はハイドロフォーム時の拡管率を低下させる。したがって、Mn含有量は3.0 %未満%とする。
【0013】
P:0.15%以下
Pは、強度の向上に有効な元素であるが、0.15%を超えて添加すると溶接性を顕著に劣化させる。とくに、Pによる強化作用がさほど必要ではないとき、またC量が高く溶接性の低下が懸念されるときには、0.02%以下に制限するのが望ましい。
【0014】
S:0.015 %以下
Sは、鋼中で非金属介在物として存在し、これが起点となってハイドロフォーミング時に鋼管を破断させる恐れがある。このため、S量は低いほど耐バースト性が改善され0.015 %以下とすればその効果があらわれる。なお、耐バースト性の一層の向上には、好ましくは0.010 %以下、さらに好ましくは0.005 %以下に制限するのがよい。
【0015】
Al:0.04%以下
Alは、鋼の脱酸に必要であるとともに、結晶粒の粗大化抑制のために有用な元素であるので、0.005 %以上の添加が望まれる。しかし、0.04%を超えて多量添加すると、固溶状態で残存するN量が減少し、歪み時効硬化量が低下する。このため、歪み時効硬化を発揮させるには、0.04%以下、好ましくは0.02%以下の範囲で添加する。
【0016】
N:0.005 〜0.02%、かつ固溶状態のNとして0.003 %以上
Nは、成形性(とくに延性)を低下することなく鋼を強化するのに有用な元素である。このような効果は、N量(全N量)で0.005 %以上、かつ固溶状態のN量で0.003 %以上存在させることが必要である。一方、0.02%を超えてNを含有すると、現状の製鋼技術では健全なスラブが製造しにくくなる。よって、N量は0.005 〜0.02%、かつ固溶状態Nは0.003 %以上の範囲とする。なお、固溶状態のN量は、地鉄を化学的に溶解し、抽出残査を分析する方法で求めることができる。
【0017】
Nb:0.005 〜0.040 %、Ti:0.005 〜0.50%、B:0.0005〜0.020 %
Nb、TiおよびBは、いずれも結晶粒の微細化に有用な元素である。このような効果は、Nb:0.005 %以上、Ti:0.005 %以上、B:0.0005%以上の添加であらわれる。しかし、その効果は、Nb:0.040 %、Ti:0.50%、B:0.020 %を超えて添加しても飽和するだけでなく、鋼の熱間変形抵抗を増大して製造性を阻害する。したがって、これら元素は上記範囲で添加する。
【0018】
Cu:0.02〜1.5 %、Ni:0.02〜1.0 %、Cr:0.02〜1.0 %、Mo:0.02〜1.0 %
Cu、Ni、Cr、Moは、鋼管の延性を損なうことなく、強度を向上させるのに有用な元素である。このような効果は、いずれも0.02%以上の添加で得られるが、その効果は、Cuで1.5 %、Ni、CrおよびMoで1.0 %を超えて添加しても飽和するほか、鋼の熱間加工性および冷間加工性を低下させる。したがって、これら元素は上記範囲で添加する。
【0019】
Ca:0.0020〜0.02%、 REM:0.0020〜0.02%
Ca、 REMは、鋼中のSを主体とした非金属介在物の形態を球状にして、その切欠作用を減少して、耐バースト性を向上させるのに有用な元素である。こうした効果は、Ca、 REMともに0.0020%以上の添加で得られるが、0.02%を超えて添加しても効果が飽和するかやや低下する傾向となる。したがって、これら元素は上記範囲で添加する。なお、Ca、 REMの両者を併用する場合には合計量で0.03%以下の範囲で添加するのが好ましい。
【0020】
また、本発明の鋼管は,引張強度(MPa)×拡管率(%)が 13000 MPa・%以上であるものとする。鋼管の引張強度が小さいと、高い衝撃吸収能が得られず、また、拡管率が小さいと、ハイドロフォーミングにより成形できる形状が限定されてしまう。本発明では、これらの2つの特性がバランスしていることが必要であるので、引張強度(MPa)×拡管率(%)を 13000 MPa・%以上に限定する。なお、引張強度は350MPa以上、拡管率は28%以上であることが好ましい。
ここで拡管率とは、外径do の鋼管を変形部長さlc =2do として、管端から管内面に液体を供給して液圧を負荷し、円形断面自由バルジ変形させ、バーストした時の最大外径dmax より、(dmax −do )/do ×100 で定義するものとする。この拡管率の測定は、自由バルジ試験により行なう。この自由バルジ試験は、例えば、図1および図2に示される金型2a,2bを、図3に示す構成のハイドロフオーミング加工装置を用いて、拡管を行なうことにより実施できる。
【0021】
図1は金型の斜視図であり、図2は金型の断面図である。金型2a,2bはそれぞれ、長さ方向両端側は、鋼管の外径do に略等しい径の半筒状面で構成される鋼管保持部3を有し、長さ方向中央部には、径dc の半円筒状変形部4および傾斜角θ=45°のテーパー状変形部5とよりなる変形部6を有し、変形部6の長さlc がdo の2倍となっている、上部金型2aおよび下部金型2bからなる。半円筒状変形部4の径dc は、鋼管の外径do の2倍程度あればよい。図3に示すように、この上部金型2aと下部金型2bとで、金型それぞれの鋼管保持部3に鋼管1が嵌まるように、鋼管1を挟み込む。この状態で、鋼管1の両端から該鋼管1の内面側に、軸押シリンダ7aを介して水等の液体を供給して、液圧Pを付与し、円形断面自由バルジ変形させてバーストした時の最大外径dmax を測定する。なお、図3中の8、9はそれぞれ金型2a、2bが鋼管を挟み込んだ状態に保持しておくための、金型ホルダ、アウターリングである。
【0022】
なお、ハイドロフォームでは、管の両端を固定する場合と、管の両端から圧縮力を加える場合(軸方向圧縮という)とがあるが、一般に、管端圧縮の方が高い拡管率を得ることが可能であり、本発明においても、高い拡管率が得られるよう、管の両端から圧縮力を適宜負荷するものとする。この圧縮力の負荷は、図3において、軸押シリンダ7a,7bに対して軸方向に圧縮力Fを負荷することにより実施できる。
【0023】
さらに、本発明の鋼管は、歪み量10%のハイドロフオーミング後、170 ℃×20分の熱処理を行なう歪み時効処理により、引張強度が40 MPa以上上昇する特性を有するものとする。ここで、歪み量10%のハイドロフオーミングは、図2に示した金型において、半円筒状変形部4の径dc が鋼管の外径do の1.1 倍のものを用い、鋼管を金型の変形部6に沿うまでハイドロフォームを行なうことにより実施する。また、 170℃×20分の熱処理は、成形部品の塗装焼付処理に相当するものである。
したがって、歪み時効処理により引張強度が40 MPa以上上昇するという上記の特性を有することにより、ハイドロフォームによる成形後の塗装焼付処理により、成形部品が高強度化して、高い耐衝突性を備えるようになるのである。
【0024】
次に、本発明鋼管の製造方法について説明する。
上述した成分組成にしたがう鋼を溶製した後、連続鋳造法あるいは造塊−分塊法によりスラブとする。スラブは、熱間圧延により熱延鋼板とするか、さらに冷間圧延−焼鈍により冷延鋼板とする。このようにして得られた熱延鋼板または冷延鋼板を素材として、ロール成形または曲げ加工により、ほぼ円筒状の形に成形し、両幅端部同士を突き合わせた継目部を電気抵抗溶接にて接合する。
ここで、造管用の素材となる熱延鋼板あるいは冷延鋼板の段階で固溶Nを0.003 %以上確保しておく必要がある。このような熱延鋼板は、上記の成分組成に従う鋼スラブの熱間圧延工程において、熱間圧延終了後0.5 秒以内に冷却を開始、おおむね40℃/s以上の冷却速度で650 ℃以下まで冷却し、冷却終了温度以下の温度で巻取ることにより製造できる。また、この熱延鋼板を冷間圧延した後、焼鈍時の加熱温度を750 ℃以下とすることにより、固溶Nを0.003 %以上含有する冷延鋼板を製造できる。
【0025】
電気抵抗溶接に次いで、外周長の絞り率で0.3 〜10%のサイジングを行う。サイジングを行う目的は、電縫鋼管をハイドロフォームに供するために、十分な形状精度を得ることと、材料変形の均一性を確保することにある。このような目的を達成するには、少なくとも0.3 %の絞り率が必要であるが、10%を超えて行うと加工硬化が顕著となり、延性の低下ひいては拡管率の低下を招いてしまう。したがって、電気抵抗溶接後は、外周長の絞り率で0.3 〜10%のサイジングを行うものとする。
本発明においては、歪み時効硬化性に寄与する固溶Nを所定量確保することが極めて重要であり、そのため、上記工程において、とくに鋼板の製造段階では、高温域(750 ℃以上)に滞留する時間を短くすることが有効である。また、素鋼管を製造する際にも同様に、不要な加熱を可能な限り抑止することが十分な量の固溶Nを確保するためには望ましい。
【0026】
【実施例】
表1に示す化学成分の鋼を溶製してスラブとした。このスラブを1220℃に加熱後、熱間圧延して板厚2.0 mmの熱延鋼板とするか、または、熱間圧延に引き続き、酸洗−冷間圧延−連続焼鈍の工程により板厚2.0 mmの冷延鋼板とした。ここで、熱間圧延終了後は、0.5 秒以内に冷却を開始し、おおむね40℃/s以上の冷却速度で650 ℃以下まで冷却し、冷却終了温度以下の温度で、かつ400 ℃以上の温度で巻き取った。また、冷延鋼板では焼鈍時の加熱温度を750 ℃以下とした。
これらの熱延鋼板または冷延鋼板を、円筒状に成形後、その継目部を電気抵抗溶接して、直径63.5mm、肉厚2.0 mmの鋼管とし、次いで外周長の絞り率で1.2 %のサイジングを行った。
【0027】
得られた電縫鋼管から、長手方向に引張試験片(JIS12号試験片)を採取し、鋼管素材の引張強度を求めた。また、電縫鋼管を500 mmの長さに切断しハイドロフォーム用の試験体とした。図1〜3で説明したように、この試験体の両端から水を供給して、円形断面自由バルジ変形させて、バーストしたときの拡管率を測定した。ここで、金型寸法は、図2におけるlc が127.0 mm、dc が127.0 mm、rd が5mm、lo が550 mm、θが45°のものとした。
各電縫鋼管の特性は、拡管率だけでなく、鋼管の強度TSとのバランスを考慮して、TS×拡管率でも表した。また、サイジングを施した電縫鋼管を用いて、歪み量10%でハイドロフォーム加工を行い、次いで170 ℃20分の塗装焼付処理相当の熱処理を施し、各工程終了後の引張強度(TS)を、鋼管の変形部位よりJIS12号引張試験片を切り出して、それぞれ測定した。
【0028】
【表1】

Figure 0004474731
【0029】
得られた結果を表2に示す。表1、2から、本発明にしたがう電縫鋼管は、TS×拡管率が高く、ハイドロフォーミング性が優れているとともに、歪み時効硬化量が大きいことがわかる。すなわち、発明例では、素材強度×拡管率の値で1300MPa・%以上が得られ、また焼付処理相当熱処理後のTS(D)と鋼管のTS(B)との差が114MPa以上、焼付相当熱処理後のTS(D)と10%ハイドロフォーミング後のTS(C)との差が58MPa以上、という大きな歪み時効硬化量が得られる。
一方、化学成分が適正でない比較例は、ハイドロフォーミング性が劣るか、歪み時効硬化量が少ないかのいずれかの難点を抱えており、ハイドロフォーミング部材の構造部材としての性能に欠けるものである。
【0030】
【表2】
Figure 0004474731
【0031】
また、表1中の管No. 1の鋼管について、サイジングの際の絞り率を、0.1 〜12%の間で変化させた場合の、電縫管のTS、拡管試験結果、歪み量10%のハイドロフォーム後のTS、塗装焼付熱処理後のTS、歪み時効硬化量を表3に示す。
表3より、サイジングの際の絞り率が0.3 〜10.0%の範囲内である場合に、TS×拡管率で 13000MPa・%以上が得られることがわかる。
【0032】
【表3】
Figure 0004474731
【0033】
【発明の効果】
以上説明したように、本発明によれば、ハイドロフォーミング性に優れ、しかも大きな歪み時効硬化量を有する構造用電縫鋼管を提供することが可能になる。したがって、本発明は、ハイドロフォーミング後、塗装焼付処理して製造される構造部材の高品質、安定生産に大きく寄与する。
【図面の簡単な説明】
【図1】自由バルジ試験に用いる金型を示す斜視図である。
【図2】自由バルジ試験に用いる金型を示す断面図である。
【図3】自由バルジ試験に用いるハイドロフォーミング加工装置の構成の例を示す断面図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel pipe suitable for use as a structural member or underbody member of an automobile, and particularly to a structural electric-welded steel pipe having excellent workability in hydroforming (hydroforming property) and strain aging.
[0002]
[Prior art]
In order to manufacture hollow members having various cross-sectional shapes used as structural members for automobiles, conventionally, there is a method of joining parts formed by press working of steel plates by spot welding at a flange portion which is a welding allowance thereof. Although it has been adopted, improvements have been demanded in terms of quality and production efficiency.
On the other hand, recently, higher impact absorbing ability at the time of collision has been demanded for structural hollow members, and the strength of materials has been further increased. For this reason, it is becoming increasingly difficult to produce a member having no molding defects and having a good shape and dimensional accuracy in the conventional press molding method.
[0003]
As a new molding method for solving such a problem, a molding method by hydroforming has recently been attracting attention. Hydroforming is a method of plastic processing by injecting high-pressure liquid into the inside of a steel pipe, and by changing the cross-sectional dimension of the steel pipe by pipe expansion processing, etc., it is possible to integrally form complex shaped members and increase strength and rigidity. It is an excellent molding method with functions.
By the way, as a steel pipe used for this hydroforming, generally, an electric resistance steel pipe made of low carbon steel is often used in C: 0.20 to 0.10%, which is easily obtained and inexpensive. .
[0004]
[Problems to be solved by the invention]
However, there has been a problem that even if hydroforming is applied to the C amount of ERW steel pipe, the workability of the material is not good, so that a sufficient pipe expansion rate cannot be obtained.
On the other hand, in order to improve the workability of the material itself of the electric resistance welded steel pipe, it is conceivable to use an ultra-low carbon steel with a significantly reduced carbon content as the material. However, in the case of an ultra-low carbon ERW steel pipe, the hydroforming property is good, but another problem due to welding is newly generated. In other words, in ultra-low carbon ERW steel pipes, the heat-affected zone crystal grains are coarsened and softened by the welding heat at the time of steel pipe production, and deformation during tube expansion is concentrated locally, resulting in high material properties. The ductility cannot be sufficiently exhibited, and when the hydroformed member is welded to another member, the same softening occurs and the strength as the member cannot be sufficiently obtained.
Thus, the present condition is that the steel pipe which can fully endure hydroforming, and does not produce the softening of a heat affected zone has not been proposed yet.
[0005]
Therefore, in view of these problems that the above-described prior art has, the present invention makes a new proposal for an electric resistance welded steel pipe suitable for hydroforming. In particular, the present invention is excellent in hydroforming properties, not only does not cause welding softens and cured in paint baking after hydroforming, equipped with so-called strain aging resistance, to propose a structure for an electric resistance welded steel pipe With the goal. In addition, the specific target characteristics of the steel pipe of the present invention are as follows. The hydroforming property expressed by (TS of steel pipe) x pipe expansion rate (axial compression condition) is 13000 MPa ·% or more, and the tensile strength after baking treatment and The difference from the tensile strength of the steel pipe shall have a strain age hardening amount of 40 MPa or more.
[0006]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the inventors have made various studies on the composition of the ERW steel pipe, the manufacturing method, and the like. As a result, a semi-very low carbon material having a C content in the range of 0.01 to less than 0.05% is used, a proper amount of solute N is contained, and the joint portion is subjected to electric resistance welding to form a pipe, and then a drawing ratio of 0.3 to It was found that 10% sizing (reducing diameter) is extremely effective. The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[0007]
(1) Steel composition in mass% (hereinafter simply referred to as%) C: 0.01 to less than 0.05%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15% Hereinafter, S: 0.015% or less, Al: 0.04% or less, N: 0.005 to 0.02%, the balance being a steel slab having a component composition consisting of a steel composition of Fe and inevitable impurities The steel slab is cooled to 650 ° C. or less at a cooling rate of 40 ° C./s or more within 0.5 seconds after the hot rolling is completed, After cold-rolling the hot-rolled hot-rolled steel sheet, the hot-rolled or cold-rolled strip material containing 0.003% or more of solid solution N is formed into a cylinder by setting the heating temperature during annealing to 750 ° C. or lower. After forming into a shape, the seam portion is subjected to electric resistance welding, and then the saijin having a squeezing ratio of the outer peripheral length of 0.3 to 10% A ERW steel pipe subjected to the tensile strength (MPa) × expansion ratio (%) is at 13000 mPa ·% or more, tensile due to strain aging treatment to conduct a heat treatment at 170 ° C. × 20 minutes after the distortion of 10% of hydroforming A structural electric-welded steel pipe having excellent hydroforming properties and strain aging, characterized in that the strength increase is 40 MPa or more.
[0008]
(2) In the above (1), the steel composition contains one or more selected from the following groups A to C in addition to the above components, and has excellent hydroforming properties and strain aging properties ERW steel pipe for structural use.
Group A: Nb: 0.005 to 0.040%, Ti: 0.005 to 0.50%, B: 0.0005 to 0.020%, or one or more types B Group: Cu: 0.02 to 1.5%, Ni: 0.02 to 1.0%, One or more of Cr: 0.02-1.0%, Mo: 0.02-1.0% Group C: Ca: 0.0020-0.02%, REM: One or two of 0.0020-0.02%
DETAILED DESCRIPTION OF THE INVENTION
The reason for limiting the steel components in the present invention, the method for producing the ERW steel pipe, etc. will be described.
C: 0.01 to less than 0.05% C contributes to the strengthening of steel, but is an element that lowers formability. In particular, when the C content is 0.05% or more, the formability is greatly reduced. On the other hand, if the content is less than 0.01%, the crystal grains in the weld heat-affected zone become coarse due to resistance welding during the manufacture of ERW steel pipes, and also when the hydroformed member is arc welded, the grains are also coarse. This causes strength reduction and non-uniform deformation. For this reason, C amount is taken as 0.01 to less than 0.05% of range.
[0011]
Si: 1.0% or less
Si is an element useful for strengthening steel, and is added depending on the desired strength. However, if added over 1.0%, the surface properties of the hot-rolled or cold-rolled steel sheet used as the material for the steel pipe deteriorate, eventually resulting in a marked deterioration in the surface texture of the steel pipe, resulting in the resistance to resistance during hydroforming. Reduce burstiness. For this reason, it adds in 1.0% or less of range.
[0012]
Mn: 3.0% or less
Mn is an element effective for improving the strength of the steel sheet and hence the hydroforming member without deteriorating the surface properties and weldability. However, the addition of more than 3.0% lowers the tube expansion rate during hydroforming. Therefore, the Mn content is less than 3.0%.
[0013]
P: 0.15% or less P is an element effective for improving the strength, but if added over 0.15%, the weldability is remarkably deteriorated. In particular, when the strengthening action by P is not so necessary, or when there is a concern about a decrease in weldability due to a high C content, it is desirable to limit it to 0.02% or less.
[0014]
S: 0.015% or less S is present as a non-metallic inclusion in steel, and this may be a starting point and break the steel pipe during hydroforming. For this reason, the lower the amount of S, the better the burst resistance. If the S content is 0.015% or less, the effect is exhibited. In order to further improve the burst resistance, it is preferably limited to 0.010% or less, more preferably 0.005% or less.
[0015]
Al: 0.04% or less
Al is an element that is necessary for deoxidation of steel and is useful for suppressing the coarsening of crystal grains, so addition of 0.005% or more is desired. However, if it is added in a large amount exceeding 0.04%, the amount of N remaining in a solid solution state is reduced, and the strain age hardening is reduced. Therefore, in order to exhibit strain age hardening, it is added in a range of 0.04% or less, preferably 0.02% or less.
[0016]
N: 0.005 to 0.02%, and N in a solid solution state is 0.003% or more. N is an element useful for strengthening steel without reducing formability (particularly ductility). Such an effect needs to be present at 0.005% or more in the N amount (total N amount) and 0.003% or more in the N amount in a solid solution state. On the other hand, when N is contained exceeding 0.02%, it becomes difficult to produce a sound slab with the current steelmaking technology. Therefore, the N amount is 0.005 to 0.02%, and the solid solution state N is 0.003% or more. In addition, the amount of N in a solid solution state can be obtained by a method in which ground iron is chemically dissolved and an extraction residue is analyzed.
[0017]
Nb: 0.005 to 0.040%, Ti: 0.005 to 0.50%, B: 0.0005 to 0.020%
Nb, Ti, and B are all useful elements for refining crystal grains. Such an effect appears when Nb is 0.005% or more, Ti is 0.005% or more, and B is 0.0005% or more. However, the effect is not only saturated when added in excess of Nb: 0.040%, Ti: 0.50%, B: 0.020%, but also increases the hot deformation resistance of the steel and impairs manufacturability. Therefore, these elements are added in the above range.
[0018]
Cu: 0.02-1.5%, Ni: 0.02-1.0%, Cr: 0.02-1.0%, Mo: 0.02-1.0%
Cu, Ni, Cr and Mo are useful elements for improving the strength without impairing the ductility of the steel pipe. All of these effects can be obtained with addition of 0.02% or more, but the effect is saturated even if added over 1.5% with Cu and over 1.0% with Ni, Cr and Mo, and the hot Reduces workability and cold workability. Therefore, these elements are added in the above range.
[0019]
Ca: 0.0020-0.02%, REM: 0.0020-0.02%
Ca and REM are useful elements for improving the burst resistance by reducing the notch action of the non-metallic inclusions mainly composed of S in the steel and making them spherical. Such an effect can be obtained by adding 0.0020% or more of both Ca and REM, but even if added over 0.02%, the effect tends to be saturated or slightly reduced. Therefore, these elements are added in the above range. When both Ca and REM are used in combination, it is preferable to add in a total amount of 0.03% or less.
[0020]
The steel pipe of the present invention shall have a tensile strength (MPa) x pipe expansion rate (%) of 13000 MPa ·% or more. If the tensile strength of the steel pipe is small, a high impact absorbing ability cannot be obtained, and if the pipe expansion rate is small, the shape that can be formed by hydroforming is limited. In the present invention, since these two characteristics need to be balanced, the tensile strength (MPa) × the tube expansion rate (%) is limited to 13000 MPa ·% or more. The tensile strength is preferably 350 MPa or more and the tube expansion rate is preferably 28% or more.
The pipe expansion rate is the maximum when a steel pipe with an outer diameter do is deformed part length l c = 2do, liquid is supplied from the pipe end to the inner surface of the pipe, the hydraulic pressure is applied, the circular section free bulge is deformed, and bursts. The outer diameter dmax is defined as (dmax−do) / do × 100. This expansion rate is measured by a free bulge test. This free bulge test can be carried out, for example, by expanding the molds 2a and 2b shown in FIGS. 1 and 2 using the hydroforming apparatus having the configuration shown in FIG.
[0021]
FIG. 1 is a perspective view of a mold, and FIG. 2 is a cross-sectional view of the mold. Each of the molds 2a and 2b has a steel pipe holding portion 3 composed of a semi-cylindrical surface having a diameter substantially equal to the outer diameter do of the steel pipe at both ends in the length direction. an upper metal having a deformed portion 6 comprising a semi-cylindrical deformed portion 4 of dc and a tapered deformable portion 5 having an inclination angle θ = 45 °, and the length lc of the deformed portion 6 is twice as long as do It consists of a mold 2a and a lower mold 2b. The diameter dc of the semi-cylindrical deformable portion 4 may be about twice the outer diameter do of the steel pipe. As shown in FIG. 3, the steel pipe 1 is sandwiched between the upper mold 2a and the lower mold 2b so that the steel pipe 1 fits in the steel pipe holding portion 3 of each mold. In this state, when a liquid such as water is supplied from both ends of the steel pipe 1 to the inner surface side of the steel pipe 1 via the axial cylinder 7a, and a hydraulic pressure P is applied, and a circular cross section free bulge is deformed and burst. The maximum outer diameter dmax is measured. In addition, 8 and 9 in FIG. 3 are a mold holder and an outer ring for holding the molds 2a and 2b in a state where the steel pipe is sandwiched, respectively.
[0022]
In hydroform, there are cases where both ends of the tube are fixed and cases where compressive force is applied from both ends of the tube (referred to as axial compression). Generally, tube end compression can obtain a higher tube expansion rate. In the present invention, a compressive force is appropriately applied from both ends of the tube so as to obtain a high tube expansion rate. The compression force can be applied by applying a compression force F in the axial direction to the axial cylinders 7a and 7b in FIG.
[0023]
Furthermore, the steel pipe of the present invention has a characteristic that the tensile strength increases by 40 MPa or more by strain aging treatment in which heat treatment is performed at 170 ° C. for 20 minutes after hydroforming with a strain amount of 10%. Here, for the hydroforming with a strain amount of 10%, in the mold shown in FIG. 2, a semi-cylindrical deformed portion 4 having a diameter dc of 1.1 times the outer diameter do of the steel pipe is used. It is carried out by carrying out hydrofoaming along the deformed portion 6. Moreover, the heat treatment at 170 ° C. × 20 minutes corresponds to a paint baking process for molded parts.
Therefore, by having the above characteristics that the tensile strength increases by 40 MPa or more by strain aging treatment, the molded parts are strengthened by the paint baking treatment after molding with hydrofoam, so that high impact resistance is provided. It becomes.
[0024]
Next, the manufacturing method of this invention steel pipe is demonstrated.
After melting the steel according to the above-described component composition, it is made into a slab by a continuous casting method or an ingot-bundling method. A slab is made into a hot-rolled steel sheet by hot rolling, or is further made into a cold-rolled steel sheet by cold rolling-annealing. Using the hot-rolled steel sheet or cold-rolled steel sheet obtained in this way as a raw material, it is formed into a substantially cylindrical shape by roll forming or bending, and the seam part where both width end parts are butted together is obtained by electric resistance welding. Join.
Here, it is necessary to secure a solid solution N of 0.003% or more at the stage of a hot-rolled steel sheet or a cold-rolled steel sheet as a material for pipe making. Such a hot-rolled steel sheet starts cooling within 0.5 seconds after the hot rolling is completed in the hot rolling process of the steel slab according to the above composition, and is cooled to approximately 650 ° C. or less at a cooling rate of approximately 40 ° C./s or more. And it can manufacture by winding at the temperature below the cooling end temperature. Moreover, after cold rolling this hot-rolled steel sheet, by setting the heating temperature during annealing to 750 ° C. or less, a cold-rolled steel sheet containing 0.003% or more of solute N can be produced.
[0025]
Following electrical resistance welding, sizing of 0.3 to 10% is performed with the drawing ratio of the outer peripheral length. The purpose of sizing is to obtain sufficient shape accuracy and to ensure uniformity of material deformation in order to use the ERW steel pipe for hydroforming. In order to achieve such an object, a drawing ratio of at least 0.3% is necessary. However, if it exceeds 10%, work hardening becomes remarkable, resulting in a decrease in ductility and a decrease in tube expansion ratio. Therefore, after electric resistance welding, sizing of 0.3 to 10% is performed at the drawing ratio of the outer peripheral length.
In the present invention, it is extremely important to secure a predetermined amount of solid solution N that contributes to strain age hardening, and therefore, in the above process, particularly in the manufacturing stage of a steel sheet, it stays in a high temperature region (750 ° C. or higher). It is effective to shorten the time. Similarly, when producing a raw steel pipe, it is desirable to suppress unnecessary heating as much as possible in order to secure a sufficient amount of solute N.
[0026]
【Example】
Steels having chemical components shown in Table 1 were melted to form slabs. This slab is heated to 1220 ° C. and then hot rolled to obtain a hot rolled steel sheet having a thickness of 2.0 mm, or, following hot rolling, a thickness of 2.0 mm is obtained by pickling, cold rolling, and continuous annealing processes. The cold-rolled steel sheet. Here, after the hot rolling is finished, cooling is started within 0.5 seconds, the cooling is generally performed at a cooling rate of 40 ° C./s or more to 650 ° C. or less, and the temperature is equal to or lower than the cooling end temperature and 400 ° C. or more. I wound up with. In the case of cold-rolled steel sheets, the heating temperature during annealing was set to 750 ° C. or lower.
These hot-rolled steel sheets or cold-rolled steel sheets are formed into a cylindrical shape, and the seams are electrically resistance welded to form steel pipes with a diameter of 63.5 mm and a wall thickness of 2.0 mm, and then a sizing of 1.2% with a drawing ratio of the outer peripheral length Went.
[0027]
A tensile test piece (JIS No. 12 test piece) was taken in the longitudinal direction from the obtained ERW steel pipe, and the tensile strength of the steel pipe material was determined. Moreover, the ERW steel pipe was cut into a length of 500 mm to obtain a test body for hydrofoam. As described with reference to FIGS. 1 to 3, water was supplied from both ends of the test body, and a circular cross section free bulge was deformed to measure the tube expansion rate when bursting. Here, in FIG. 2, lc was 127.0 mm, dc was 127.0 mm, rd was 5 mm, lo was 550 mm, and θ was 45 ° in FIG.
The characteristics of each electric resistance welded steel pipe are expressed not only by the pipe expansion ratio but also by TS × pipe expansion ratio in consideration of the balance with the strength TS of the steel pipe. In addition, using a sized sized ERW steel pipe, hydroforming is performed at a strain of 10%, followed by heat treatment equivalent to 170 ° C for 20 minutes of paint baking, and the tensile strength (TS) after each process is finished. The JIS No. 12 tensile test piece was cut out from the deformation part of the steel pipe and measured.
[0028]
[Table 1]
Figure 0004474731
[0029]
The obtained results are shown in Table 2. From Tables 1 and 2, it can be seen that the electric resistance welded steel pipe according to the present invention has a high TS × expansion ratio, excellent hydroforming properties, and a large strain age hardening amount. That is, in the invention example, a material strength × a tube expansion rate value of 1300 MPa ·% or more is obtained, and the difference between the TS (D) after the baking treatment equivalent heat treatment and the TS (B) of the steel pipe is 114 MPa or more. A large strain age hardening amount is obtained in which the difference between TS (D) after 10% and TS (C) after 10% hydroforming is 58 MPa or more.
On the other hand, the comparative example in which the chemical component is not appropriate has either a poor hydroforming property or a small strain age hardening amount, and lacks the performance of the hydroforming member as a structural member.
[0030]
[Table 2]
Figure 0004474731
[0031]
In addition, regarding the steel pipe No. 1 in Table 1, when the squeezing ratio during sizing was changed between 0.1 and 12%, the TS of the electric resistance welded tube, the result of the pipe expansion test, the distortion amount of 10% Table 3 shows the TS after hydroforming, the TS after paint baking heat treatment, and the strain age hardening amount.
From Table 3, it can be seen that when the drawing ratio during sizing is within a range of 0.3 to 10.0%, a TS × tube expansion ratio of 13000 MPa ·% or more can be obtained.
[0032]
[Table 3]
Figure 0004474731
[0033]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a structural electric-welded steel pipe having excellent hydroforming properties and a large strain age hardening amount. Therefore, the present invention greatly contributes to the high quality and stable production of the structural member manufactured by performing the coating baking process after hydroforming.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a mold used for a free bulge test.
FIG. 2 is a cross-sectional view showing a mold used for a free bulge test.
FIG. 3 is a cross-sectional view showing an example of the configuration of a hydroforming apparatus used for a free bulge test.

Claims (2)

鋼組成が、質量%で
C:0.01〜0.05%未満、
Si:1.0%以下、
Mn:3.0%以下、
P:0.15%以下、
S:0.015%以下、
Al:0.04%以下、
N:0.005〜0.02%を含有し、残部はFeおよび不可避的不純物の鋼組成からなる成分組成を有する鋼スラブを熱間圧延し、熱間圧延終了後0.5秒以内に40℃/s以上の冷却速度で650℃以下まで冷却し、冷却終了温度以下の温度で巻取るか、または、上記鋼スラブを熱間圧延した熱延鋼板を冷間圧延した後、焼鈍時の加熱温度を750℃以下とすることにより、固溶Nを0.003%以上含有させた熱延または冷延の帯状素材を円筒状に成形した後、継目部を電気抵抗溶接し、次いで、外周長の絞り率で0.3〜10%のサイジングを施した電縫鋼管であって、引張強度(MPa)×拡管率(%)が13000MPa・%以上で、歪み量10%のハイドロフォーミング後170℃×20分の熱処理を行う歪み時効処理による引張強度の上昇量が40MPa以上であることを特徴とするハイドロフォーミング性に優れ、歪み時効性を有する構造用電縫鋼管。
The steel composition is C: 0.01 to less than 0.05% by mass%.
Si: 1.0% or less,
Mn: 3.0% or less,
P: 0.15% or less,
S: 0.015% or less,
Al: 0.04% or less,
N: 0.005 to 0.02% is contained, and the balance is hot-rolled with a steel slab having a composition composed of a steel composition of Fe and inevitable impurities, and within 40 seconds within 0.5 seconds after the hot rolling is completed. After cooling to 650 ° C. or less at a cooling rate of at least ° C./s and winding at a temperature below the cooling end temperature, or cold rolling a hot-rolled steel plate obtained by hot rolling the steel slab, heating during annealing By setting the temperature to 750 ° C. or lower, a hot-rolled or cold-rolled strip material containing 0.003% or more of solute N is formed into a cylindrical shape, and then the seam portion is subjected to electric resistance welding, and then the outer peripheral length 170 ° C. after hydroforming with a tensile strength (MPa) × pipe expansion rate (%) of 13,000 MPa ·% or more and a strain amount of 10%. × Pull by strain aging treatment with 20 minutes heat treatment An electrical resistance welded steel pipe having excellent hydroforming properties and strain aging, characterized by an increase in tensile strength of 40 MPa or more.
請求項1において、鋼組成が、上記成分のほか、下記A群〜C群から選ばれる1種または2種以上を含有することを特徴とするハイドロフォーミング性に優れ、歪み時効性を有する構造用電縫鋼管。

A群:Nb:0.005〜0.040%、Ti:0.005〜0.50%、B:0.0005〜0.020%のうちの1種または2種以上
B群:Cu:0.02〜1.5%、Ni:0.02〜1.0%、Cr:0.02〜1.0%、Mo:0.02〜1.0%のうちの1種または2種以上
C群:Ca:0.0020〜0.02%、REM:0.0020〜0.02%のうちの1種または2種
In Claim 1, in addition to the said component, steel composition contains 1 type, or 2 or more types chosen from the following A group-C group, It is excellent in hydroforming property characterized by the above-mentioned, and has structural aging ERW steel pipe.
Group A: Nb: 0.005 to 0.040%, Ti: 0.005 to 0.50%, B: 0.0005 to 0.020%, or one or more B group: Cu: One or two or more of 0.02 to 1.5%, Ni: 0.02 to 1.0%, Cr: 0.02 to 1.0%, Mo: 0.02 to 1.0% Group C: Ca: 0.0020 to 0.02%, REM: One or two of 0.0020 to 0.02%
JP2000127077A 2000-04-27 2000-04-27 Structural electric resistance welded steel pipe with excellent hydroforming properties and strain aging Expired - Fee Related JP4474731B2 (en)

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