JP3618270B2 - High-tensile steel plate with excellent weldability and base metal toughness - Google Patents
High-tensile steel plate with excellent weldability and base metal toughness Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は、溶接性(耐低温割れ性及びHAZ靭性)に優れると共に、良好な母材靭性(特にvE−80≧47Jレベル、好ましくはvE−100≧47Jレベルの高靭性)も兼ね備えた高張力鋼板に関する。本発明の高張力鋼板は、特に高度の母材靭性が要求される海洋構造物等に好適に用いられる。
【0002】
【従来の技術】
780MPa級以上の高張力鋼板では、母材強度の確保という観点から合金成分を多量に添加する為、小入熱溶接条件では冷却速度が速い為、HAZ(溶接熱影響部)が硬化して溶接割れ(低温割れ)を生じ易く、かかる溶接割れの防止を目的として、溶接施工時に100℃程度の予熱が行う必要がある。従って、この予熱工程を省略できれば施工効率が大きく上昇し、且つコストダウンにもつながる為、耐低温割れ性に優れた780MPa級以上の高張力鋼板の提供が切望されている。
【0003】
ところで、耐低温割れ性の指標としては下式で定義されるPcm(%)というパラメーターが用いられている。
Pcm=[C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5×[B]
(式中、[ ]は各元素の含有量を示す)
【0004】
そして、従来は上記Pcmを制御することにより耐低温割れ性を改善すると共に、合金成分の含有量制限に伴う母材強度低下を、製造方法を改良する等して補っていた。これにより、780MPa級以上の高張力鋼板において、母材製造時の焼入れにおける冷却速度が比較的速い薄物(≦34mm)では予熱フリーを達成できたが、冷却速度が遅い厚物(≧40mm)では予熱フリーと母材強度の両立を達成することができなかった。また、Cuの析出を利用して母材強度を確保する方法も開示されているが、冷却速度が遅い厚物では充分な母材強度が得られなかった。
【0005】
この様に小入熱溶接においてHAZ部は高温に加熱され、且つ冷却速度が速い為、硬化して低温割れを起こし易い。一方、母材は板厚が厚くなる程冷却速度が遅くなる為、圧延後の焼入れで強度が確保し難くなる。従って、780MPa級以上の高張力鋼板での厚物では、小入熱溶接時の低温割れを防止する為、冷却速度が速い場合に硬くならない様にした上で、鋼板製造時の焼入れ過程において冷却速度が遅い場合に如何に強度を確保するかが最重要課題となる。
【0006】
更に近年、特に海洋構造物等の分野においては、母材靭性の更なる向上が切望されており、母材靭性vE−80≧47J、vE−100≧47Jといった極めて高度の靭性を確保すべく検討が進められている。
【0007】
一般に母材靭性の改善に当たっては、焼入れ性を高めることが提案されているが、焼入れ性の増大は耐割れ性を劣化することが知られている為、好ましい手段ではない。また、特開平6−240353には引張強さが780MPa級で、溶接性にも優れた低温靭性の良好な高張力鋼板が提案されているが、これは、一段目の加工熱処理で微細な炭化物及び窒化物の分散を有するベイナイト組織とすることにより、再加熱の際に微細なオーステナイト粒を得るもので、これにより母材靭性の向上を図るものである。ところが上記方法は、圧延後に2回の加工熱処理からなる操作を1回以上繰返し行った後、焼戻し処理するものであり、繁雑な製造工程を経なければならず、しかも、得られる母材靭性にしても、例えばvE−80≧47J、vE−100≧47Jレベルといった極めて高度の靭性を確保するまでには至っていない。
【0008】
【発明が解決しようとする課題】
本発明は、上記事情に着目してなされたものであり、その目的は、溶接性(耐低温割れ性及びHAZ靭性)に優れると共に、良好な母材靭性(特にvE−80≧47J、好ましくはvE−100≧47J)も兼ね備えた高張力鋼板を提供することにある。
【0009】
【課題を解決するための手段】
上記課題を解決し得た本発明に係る溶接性及び母材靭性に優れた高張力鋼板とは、
C :0.010〜0.06%(質量%の意味、以下同じ),
Mn:1.0〜3.0%,
Cr:0.1〜2.0%,
Mo:0.1〜1.5%,
B :0.0006〜0.0050%,
Nb:0.010〜0.05%
を含有し、更に
Si:1.0%以下 (0%を含まない),
Cu:2.0%以下 (0%を含む),
Ni:6%以下 (0%を含む),
V :0.10%以下 (0%を含む),
Al:2.0%以下 (0%を含む),
N :0.020%以下(0%を含まない)
を含有し、更に
Ti:0.03%以下,
Zr:0.05%以下,及び
Hf:0.10%以下
よりなる群から選択される少なくとも一種を含有し、
残部:鉄及び不可避不純物であり、且つ、
下式(1)で表されるKPがKP≧3.4を満足すると共に、
下式(2)で表されるKNが0.0≦KN≦3.0を満足する(但し、上記鋼板の組成のうち、Si:0.05〜0.50%,Mn:1.25〜2.5%,Cu:0.25%未満,Nb:0.010〜0.030%を満足する鋼板は除く)ものであるところに要旨を有するものである。
KP=[Mn]+1.5×[Cr]+2×[Mo] … (1)
KN=([N]/14−[Ti]/48−[Zr]/91−[Hf]/178)×104 …(2)
(式中、[ ]は各元素の含有量(%)を意味する)
【0010】
本発明において、更にCa:0.0005〜0.005%を含有する高張力鋼板や;上記KPがKP≧4.0を満足する高張力鋼板や;母材靭性vE−100が47J以上である高張力鋼板は、溶接性及び靭性が一層高められるので好ましい態様である。
【0011】
【発明の実施の形態】
前述した通り、490〜590MPa級の高張力鋼板では、Pcmの制御により耐低温割れ性の改善と母材強度の確保を両立させることができたが、780MPa級以上の高張力鋼板ではPcmによる成分制御を行ったとしても、特に厚物において両特性の両立を図ることは困難であった。そこで本発明では成分設計に当たり、これまで耐低温割れ性の指標とされてきたPcmではなく、全く別のパラメータにより耐低温割れ性を制御することができないか鋭意検討した。その結果、鋼組織を考慮した上式(1)で表されるKPを用い、更にC量を極低減化し、Bを添加することにより耐低温割れ性と母材強度とを両立できることが明らかになると共に、一方、極めて高度の母材靭性を確保する為には、Nbを所定量添加し、旧γ粒径を微細化することが有効であることを見出し、本発明を完成したのである。
【0012】
まず、本発明における耐低温割れ性の改善法について説明する。上述した通り、本発明では、Cを極低Cにすると共に、焼入れ性向上元素であるMn,Cr及びMoを積極的に添加し、当該焼入れ性向上元素よって定められるKP値を適切に制御すると共に、更にBを添加することにより耐低温割れ性の向上を図るものである。これらの成分を適切に添加することにより、ベイナイトのCCT線(図4のCCT線図を参照)が短時間側且つ低温度側に移動すると共に、フェライトのCCT線が長時間側に移動する(実線→破線へと移動)。
【0013】
従って、従来によれば、高冷却速度ではマルテンサイト、低冷却速度ではフェライトまたは高温ベイナイトを生成するために、硬さの冷却速度感受性が大きく、小入熱溶接時のHAZ部の硬さ低減(耐低温割れ性の改善)と母材強度の確保が両立できず、予熱フリーの達成が困難であったが、本発明によれば、高冷却速度、低冷却速度のいずれにおいても低温ベイナイトを生成し、硬さの冷却速度感受性が低下し、溶接時のHAZ部の硬さ低減(耐低温割れ性の改善)と母材強度確保を両立ならしめたのである。
【0014】
尚、上述した耐低温割れ性向上に対するアプローチは、本発明の出願前に明らかになったものであり、これについては既に出願を済ませている(特願平10−336268)。この先願発明は、特に780MPa級以上の高張力鋼板において、大入熱溶接時にHAZ靭性が劣化し、実際の溶接施工時では入熱制限(5kJ/mm以下)を行う必要があるという実状に鑑み検討されたものであり、本発明の如く、溶接時におけるHAZ部の硬さ低減(耐低温割れ性の改善)と母材強度確保の両立は勿論のこと、大入熱溶接時におけるHAZ靭性を改善する為には、前述の方法を採用することが有効であることを見出し、出願されたものである。従って、上記先願発明も本願発明も、共に小入熱溶接時における耐低温割れ性及びHAZ靭性の向上を目指すものである点では一致するが、上記先願発明では、通常入熱溶接時に加え、更に大入熱溶接時におけるHAZ靭性向上を目的として鋭意検討されたものであるのに対し、本発明では、通常入熱溶接時における優れたHAZ靭性を確保しつつ、更にvE−80≧47J、vE−100≧47Jといった極めて高度の母材靭性を確保する目的で検討されたものである点で、両者は相違する。実際のところ、先願発明ではvE−40の母材靭性を評価しているに過ぎず、本発明の如くvE−100レベルの苛酷な条件下における母材靭性の改善については全く考慮していなかった。そして、本発明によれば、極めて高度の母材靭性を確保する為には、更にNbを適切に制御することが有効であることを見出したところに特徴の一つを有するものであり、先願発明では開示されていなかった新しい技術的思想が付加されている点で、本願発明は、先願発明とは異なる発明であると言える。即ち、本発明は、「小入熱溶接時における耐低温割れ性及びHAZ靭性の向上」という課題に対しては、先願発明のアプローチをそのまま踏襲していくと共に、本発明独自に提起された「極めて高度の母材靭性確保」という課題に対しては、新たに見出したNb添加による旧γ粒径の微細化により達成した次第であり、両者は解決すべき課題及び達成手段が異なるものである。
【0015】
以下、耐低温割れ性向上に寄与する成分及びKP値について説明する。
【0016】
C:0.010〜0.06%
Cは、溶接時におけるHAZ部の耐低温割れ性と母材強度確保を両立させる為に必要な元素である。Cが0.06%を超えると高冷却速度側で低温ベイナイトではなくマルテンサイトが生成する様になり、耐低温割れ性が改善されない。好ましくは0.055%以下である。尚、0.010%未満では必要最小限の母材強度が得られない。好ましくは0.030%以上である。
【0017】
Mn:1.0〜3.0%
Cr:0.1〜2.0%
Mo:0.1〜1.5%
これらの元素は焼入れ性を改善する作用を有し、高冷却速度〜低冷却速度で低温ベイナイトを生成し易くすると共に、前述の通り、極低Cとし、同時に所定のB量を添加することにより溶接時におけるHAZ部の耐低温割れ性と母材強度の確保を両立させることができる点で有用である。
【0018】
まず、Mn,Cr及びMoの含有量は、夫々1.0%以上,0.1%以上,0.1%以上であることが必要である。これらの含有量に満たないと所望の焼入れ性改善作用が発揮されず、母材強度が不足する。好ましくはMn:1.25%以上,Cr:0.3%以上、Mo:0.3%以上である。但し、Mn,Cr及びMoの含有量が、夫々3.0%,2.0%,1.5%を超えると母材の靭性が低下する。好ましくはMn:2.5%以下,Cr:1.5%以下、Mo:1.3%以下である。
【0019】
更に、これらの元素で定められるKP値は3.4以上であることが必要である。KP値が3.4未満では、上記作用を有効に発揮させることができず、高温ベイナイトまたはフェライトが生成する様になり、780MPa以上の母材強度が得られなくなる(後記する図1を参照)。KP値は大きい程良く、好ましくは4.0以上である。尚、その上限は、Mn,Cr,Moの各添加量の上限に基づいて定められる範囲であれば特に制限されないが、母材靭性等を考慮すれば7以下、より好ましくは6以下に制御することが推奨される。
【0020】
B:0.0006〜0.0050%
Bは焼入れ性改善元素で、低冷却速度で低温ベイナイトを生成させ易くすると共に、前述の通り、極低Cとし、同時に適量のMn,Cr,Moを添加することにより熱溶接時におけるHAZ部の耐低温割れ性と母材強度確保を両立させることができる点で有用である。Bが0.0006%未満では、焼入れ性改善効果が期待できず、母材強度が不足してしまう。好ましくは0.0007%以上である。但し、Bが0.0050%を超えると、かえって焼入れ性が低下し、母材強度が不足する。好ましくは0.0030%以下である。
【0021】
以上が、主に耐低温割れ性等の溶接性向上に寄与する成分及び要件である。そして、本発明のもう一つの課題である極めて高度の母材靭性を得る為には、Nbを0.010〜0.05%の範囲で添加することが必要であり、これによりvE−80≧47J、好ましくはvE−100≧47Jという極めて高レベルの母材靭性が達成されるのである。
【0022】
Nb添加により極めて優れた母材靭性が得られるのは、Nb添加によりγ粒径が微細化され、変態後のベイナイトブロックサイズが微細化されることに起因するものと考えられる。尚、490〜590MPa級の高張力鋼板では、γ粒の微細化による母材靭性の改善が一般に行われているが、これは、フェライト組織が主体である鋼板において有用な方法であり、780MPa級の高張力鋼板では、ベイナイト組織及びマルテンサイト組織が主体となる為、γ粒径を微細化した場合は焼入れ性が劣化し、むしろ母材靭性が劣化すると考えられていた。これに対し、本発明では前述の如く低C及びKP値の制御による高焼入れ性を確保し、これがNb添加によるγ粒微細化作用と相俟って、結果的に高度の母材靭性を達成できたものと思料される。この様なNb添加による靭性向上作用を有効に発揮させる為には0.010%以上の添加が必要である。好ましくは0.020%以上、より好ましくは0.030%超である。但し、Nbの添加量が0.05%を超えるとHAZ靭性等が低下する。好ましくは0.040%以下である。
【0023】
以上が本発明の課題を解決する為に不可欠な要素・成分であり、残部:鉄及び本発明の作用を損なわない許容成分である。
【0024】
尚、本発明では、更に一層優れた特性の付与を目指して、Ti:0.03%以下,Zr:0.05%以下,及びHf:0.10%以下よりなる群から選択される少なくとも一種を含有し、上式(2)で表されるKNが−1≦KN≦4.0を満足する様制御することが推奨される。
【0025】
上記Ti,Zr,Hfの元素は、不純物として含まれるNを固定する作用を有し、溶接時におけるHAZ部でNが固溶Bと結合し、Bが消費されてB添加による作用が損なわれるのを防止する作用もある。更に、Ti等の窒化物は溶接時におけるHAZ部のγ粒を微細化し、HAZ靭性改善にも寄与する。かかる観点から、これらの元素は鋼中のN含有量に応じ、必要があれば積極的に添加することが推奨される。その場合、上記元素のうちTiは必ず含まれる様に添加し、他の元素(Zr,Hf)は必要に応じてTiと共に添加することが好ましい。具体的には、Ti:0.03%,Zr:0.05%,Hf:0.10%を超えると母材の靭性が劣化するので、これ以下に制御することが推奨される。
【0026】
更に上記元素を添加する場合には、上式(2)で定義されるKN値が−1〜4.0であることが好ましい。例えばN量が多いにもかかわらず上記元素の添加量が少ない為、KN値が4.0を超えるときには、B添加による作用が有効に発揮されず、HAZ靭性が低下する(後記する図2を参照)。一方、上記元素の添加量が多すぎてKNが−1未満になると、母材の靭性が劣化する。より好ましくは0.0以上、3.0以下である。
【0027】
更に本発明では、一層優れた溶接性・母材靭性の向上を目指して、下記元素を積極的に添加することが推奨される。
【0028】
Si:1.0%以下
Siは脱酸剤として有用な元素であり、この様な作用を有効に発揮させる為には、0.05%以上添加することが好ましい。但し、1.0%を超えて添加すると溶接性及び母材靭性が低下するので、その上限を1.0%とすることが好ましい。より好ましくは0.50%以下である。
【0029】
Cu:2.0%以下
Cuは固溶強化及び析出強化により母材強度を向上させると共に、焼入れ性向上作用も有する元素である。但し、2.0%を超えて添加するとHAZ靭性が低下する為、その上限を2.0%とすることが好ましい。より好ましくは1.5%以下である。
【0030】
Ni:6%以下
Niは母材靭性向上に有用な元素であるが、6%を超えて添加するとスケール疵が発生し易くなる為、その上限を6%とすることが好ましい。より好ましくは4%以下である。
【0031】
V:0.10%以下
Vは少量添加により焼入れ性及び焼戻し軟化抵抗を高める作用がある。但し、0.10%を超えて添加するとHAZ靭性が低下する為、その上限を0.10%とすることが好ましい。より好ましくは0.07%以下である。
【0032】
Al:0.20%以下
Alは脱酸元素であると共に、Nを固定し、固溶Bを増加させることによりBの焼入れ性を高める元素である。この様な作用を有効に発揮させる為には0.01%以上添加することが好ましい。但し、0.20%を超えて添加すると靭性が劣化するので、その上限を0.20%とすることが好ましい。より好ましくは0.10%以下である。
【0033】
N:0.020%以下
NはBと結合して固溶Bを減少させ、Bの焼入れ性向上作用を阻害し、母材の靭性及びHAZ靭性を低下させる。Nの含有量が0.020%を超えるとその作用が顕著になる為、Ti等の添加によるKN値制御によるHAZ靭性・母材靭性の向上、Al添加による焼入れ性向上効果を有効に発揮させることができない。より好ましくは0.010%以下である。
【0034】
Ca:0.0005〜0.005%
CaはMnSを球状化し、介在物の形態制御による異方性を低減する効果を有する元素である。この様な作用を有効に発揮させる為には0.0005%以上添加することが好ましい。但し、0.005%を超えて過剰に添加すると母材靭性が低下するのでその上限を0.005%とすることが好ましい。
【0035】
次に、本発明の鋼板を製造する方法について説明する。
【0036】
本発明の鋼板は、上記成分組成を満足する鋼を用い、加熱、熱間圧延、及び焼入れした後、焼戻しすることにより所望の高張力鋼板を得ることができる。各工程の条件(温度、時間等)は特に限定されず、通常用いられる高張力鋼板の製造条件を適宜採用することができる。具体的には、例えば約1000〜1200℃で2時間以上加熱した後、熱間圧延を行い、900〜950℃で圧延を完了し、その後冷却する。次いで約880〜950℃で10分以上保持した後、水冷することが推奨される。また、焼戻し工程では、約550〜650℃で約5〜15分保持して行うことが推奨される。この様に本発明によれば、高張力鋼板の製造に当たり、通常実施される製造条件を適用することにより、溶接性及び母材靭性に優れた高張力鋼板が得られ、前述の特開平6−240353の如く、圧延後に2回の加工熱処理からなる操作を1回以上繰返し行った後、焼戻し処理するといった繁雑な製造工程を経る必要がない点で、極めて有用である。
【0037】
以下、実施例に基づいて本発明を詳細に述べる。ただし、下記実施例は本発明を制限するものではなく、前・後記の趣旨を逸脱しない範囲で変更実施することは全て本発明の技術的範囲に包含される。
【0038】
【実施例】
表1〜2に示す成分組成の鋼を通常の溶製法により溶製し、スラブとした後、1100〜1150℃で2時間保持した後、熱間圧延し、900〜950℃で圧延を完了して徐冷した。その後、再加熱し、900℃で15分保持した後、水冷し、表3〜4に記載の焼戻し条件で所定の板厚からなる高張力鋼板を製造した。
【0039】
この様にして得られた各鋼板について、下記要領で母材特性[強度及び靭性(vE−100)]を評価し、本発明で基準とする母材特性レベル(強度≧780MPa、vE−100≧471)をクリアしたものについては、更に溶接性(耐低温割れ性及びHAZ靭性)を評価した。
【0040】
[母材特性試験]
▲1▼引張試験:各鋼板の板厚1/4部位からJIS4号試験片を採取し、引張試験を行うことにより0.2%耐力及び引張強さを測定した。本発明では、引張強さ≧780MPaを合格とした、
▲2▼衝撃試験:各鋼板の板厚1/4部位からJIS4号試験片を採取し、シャルピー衝撃試験を行うことにより吸収エネルギー(vE−100)を得た。本発明では、vE−100≧47Jを合格とした。
【0041】
[溶接性試験]
▲1▼HAZ靭性:入熱5kJ/mm(サブマージ溶接法)で溶接を行い、図4に示す部位からJIS4号試験片を採取してシャルピー試験を行い、ボンド部の吸収エネルギー(vE−10)を求めた。本発明では、vE−10≧47Jを合格とした、
▲2▼耐低温割れ性:JIS Z 3158に記載のy形溶接割れ試験法に基づいて、入熱1.7kJ/mmで被覆アーク溶接を行い、ルート割れ防止予熱温度を測定した。本発明では25℃以下を合格とした。
【0042】
これらの結果を表3〜4に併記する。
【0043】
【表1】
【0044】
【表2】
【0045】
【表3】
【0046】
【表4】
【0047】
表3及び表4より以下の様に考察することができる。
【0048】
まず、表1の鋼板は本発明の要件を満足する実施例であり、表3に示す通り、いずれの鋼板も母材特性及び溶接性に優れていた。
【0049】
これに対し、表2の鋼板は本発明の要件を満足しない比較例であるが、これらは表4に示す不具合を有している。
【0050】
まず、No.23はC量が本発明の下限値を下回る例であり、所望の母材強度が得られなかった。また、No.24はC量が本発明の上限値を超える例であり、耐低温割れ性が低下した。
【0051】
No.25はSi量が本発明の上限値を超える例であり、所望の母材靭性が得られなかった。
【0052】
No.26はKP値が本発明の下限値を下回る例であり、所望の母材強度が得られなかった。
【0053】
No.27はMn量が本発明の下限値を下回る例であり、所望の母材強度が得られなかった。また、No.28はMn量が本発明の上限値を超える例であり、母材靭性が低下した。
【0054】
No.29はNi量が本発明の上限値を超える例であり、耐低温割れ性が低下した。
【0055】
No.30はCr量が本発明の上限値を超える例であり、所望の母材靭性が得られなかった。
【0056】
No.31はMo量が本発明の上限値を超える例であり、所望の母材靭性が得られなかった。
【0057】
No.32はNb量が本発明の上限値を超える例であり、HAZ靭性が低下した。また、No.33はNb量が本発明の下限値を下回る例であり、母材靭性が低下した。
【0058】
No.34はCu量が本発明の上限値を超える例であり、母材靭性が低下した。
【0059】
No.35はV量が本発明の上限値を超える例であり、HAZ靭性が低下した。
【0060】
No.36/No.37はB量が本発明の下限値/上限値を下回る/超える例であり、いずれも所望の母材強度が得られなかった。
【0061】
No.38はTi量が本発明の上限値を超える例であり、母材靭性が低下した。
【0062】
No.39はKN値が本発明の下限値を下回る例であり、母材靭性が低下した。また、No.43はKN値が本発明の上限値を超える例であり、HAZ靭性が低下した。
【0063】
No.40はZr量が本発明の上限値を超える例であり、母材靭性が低下した。
【0064】
No.41はHf量が本発明の上限値を超える例であり、母材靭性が低下した。
【0065】
No.42はCa量が本発明の上限値を超える例であり、母材靭性が低下した。
【0066】
No.44はN量が本発明の上限値を超える例であり、HAZ靭性が低下した。
【0067】
図1は、上記結果に基づき、母材強度(引張強さ)とKP値の関係をグラフ化したものであるが、KP値を3.4以上に制御することにより780MPa以上の引張強度が得られることが分かる。
【0068】
図2は、上記結果に基づき、入熱5kJ/mmの溶接時のHAZ靭性(vE−1 00)とKN値の関係をグラフ化したものえあるが、KN値を−1.0〜4.0の範囲に制御することにより47kJ以上のHAZ靭性が得られることが分かる。
【0069】
【発明の効果】
本発明法は以上の様に構成されており、溶接性(耐低温割れ性及びHAZ靭性)に優れると共に、良好な母材靭性(特にvE−80≧47J、好ましくはvE−100≧47J)も兼ね備えた高張力鋼板を提供することができた。
【図面の簡単な説明】
【図1】母材強度とKP値の関係を示すグラフである。
【図2】HAZ靭性とKN値の関係を示すグラフである。
【図3】サブマージアーク溶接時のボンド靭性の試験片採取位置を示す概略説明図である。
【図4】本発明の成分設計の考え方を説明するための模式的なCCT線図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is excellent in weldability (cold crack resistance and HAZ toughness), and also has high base metal toughness (particularly high toughness of vE- 80 ≧ 47J level, preferably vE- 100 ≧ 47J level). It relates to steel plates. The high-tensile steel plate of the present invention is suitably used for offshore structures and the like that require a particularly high base metal toughness.
[0002]
[Prior art]
In high-tensile steel sheets of 780 MPa class or higher, a large amount of alloy components are added from the viewpoint of securing the strength of the base metal, and because the cooling rate is fast under the low heat input welding conditions, the HAZ (welding heat affected zone) hardens and welds. Cracking (cold cracking) is likely to occur, and it is necessary to preheat at about 100 ° C. during welding for the purpose of preventing such welding cracking. Therefore, if this preheating step can be omitted, the construction efficiency will be greatly increased and the cost will be reduced. Therefore, it is desired to provide a high-tensile steel plate of 780 MPa class or higher that has excellent cold cracking resistance.
[0003]
By the way, a parameter called Pcm (%) defined by the following formula is used as an index of cold cracking resistance.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B]
(In the formula, [] indicates the content of each element)
[0004]
Conventionally, by controlling the above Pcm, the cold cracking resistance was improved, and a decrease in the strength of the base material due to the limitation of the content of the alloy component was compensated by improving the manufacturing method. As a result, in a high-tensile steel plate of 780 MPa class or higher, preheating-free was achieved with a thin material (≦ 34 mm) with a relatively fast cooling rate in quenching during manufacturing of the base material, but with a thick material (≧ 40 mm) with a slow cooling rate. It was not possible to achieve both preheating free and base material strength. Moreover, although the method of ensuring the base material strength using precipitation of Cu is also disclosed, sufficient base material strength was not obtained with a thick material having a slow cooling rate.
[0005]
As described above, in the small heat input welding, the HAZ portion is heated to a high temperature and has a high cooling rate, and thus is hardened and easily causes a low temperature crack. On the other hand, since the cooling rate of the base material increases as the plate thickness increases, it is difficult to ensure the strength by quenching after rolling. Therefore, in order to prevent low-temperature cracking at the time of small heat input welding, thick materials made of high-strength steel sheets of 780 MPa class or higher should not be hardened when the cooling rate is high, and then cooled in the quenching process during steel sheet production. The most important issue is how to secure the strength when the speed is low.
[0006]
Furthermore, in recent years, especially in the field of offshore structures and the like, further improvement of the base metal toughness is eagerly desired, and studies are made to ensure extremely high toughness such as base metal toughness vE- 80 ≧ 47J, vE- 100 ≧ 47J. Is underway.
[0007]
In general, to improve the toughness of the base metal, it has been proposed to increase the hardenability. However, since it is known that the increase in the hardenability deteriorates the crack resistance, it is not a preferable means. Japanese Patent Laid-Open No. 6-240353 proposes a high-tensile steel sheet having a tensile strength of 780 MPa and excellent weldability and low-temperature toughness. In addition, by forming a bainite structure having nitride dispersion, fine austenite grains are obtained during reheating, thereby improving the base material toughness. However, the above method involves tempering after repeatedly performing an operation consisting of two heat treatments after rolling once, and has to go through a complicated manufacturing process, and to obtain the toughness of the base material to be obtained. However, it has not yet reached a very high toughness such as vE- 80 ≧ 47J, vE- 100 ≧ 47J level.
[0008]
[Problems to be solved by the invention]
The present invention has been made by paying attention to the above circumstances, and its purpose is excellent in weldability (cold crack resistance and HAZ toughness) and good base material toughness (particularly vE- 80 ≧ 47 J, preferably The object is to provide a high-tensile steel sheet that also has vE- 100 ≧ 47J).
[0009]
[Means for Solving the Problems]
The high-tensile steel plate excellent in weldability and base material toughness according to the present invention that has solved the above-mentioned problems,
C: 0.010 to 0.06% (meaning mass%, the same shall apply hereinafter),
Mn: 1.0 to 3.0%,
Cr: 0.1 to 2.0%,
Mo: 0.1 to 1.5%,
B: 0.0006 to 0.0050%,
Nb: 0.010 to 0.05%
Contains further Si: 1.0% or less (not including 0%),
Cu: 2.0% or less (including 0%),
Ni: 6% or less (including 0%),
V: 0.10% or less (including 0%),
Al: 2.0% or less (including 0%),
N: 0.020% or less (excluding 0%)
In addition, Ti: 0.03% or less,
Containing at least one selected from the group consisting of Zr: 0.05% or less and Hf: 0.10% or less,
The balance: iron and inevitable impurities, and
KP represented by the following formula (1) satisfies KP ≧ 3.4, and
KN represented by the following formula (2) satisfies 0.0 ≦ KN ≦ 3.0 (in the composition of the steel sheet, Si: 0.05 to 0.50%, Mn: 1.25 (Excluding steel sheets satisfying 2.5%, Cu: less than 0.25%, Nb: 0.010 to 0.030%).
KP = [Mn] + 1.5 × [Cr] + 2 × [Mo] (1)
KN = ([N] / 14- [Ti] / 48- [Zr] / 91- [Hf] / 178) × 10 4 (2)
(In the formula, [] means the content (%) of each element)
[0010]
In the present invention, a high-tensile steel plate further containing Ca: 0.0005 to 0.005%; a high-tensile steel plate in which the KP satisfies KP ≧ 4.0; and a base metal toughness vE- 100 is 47 J or more. A high-strength steel plate is a preferred embodiment because the weldability and toughness are further improved.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
As described above, in the high-strength steel sheet of 490 to 590 MPa class, it was possible to achieve both the improvement of the low temperature cracking resistance and the securing of the base material strength by the control of Pcm. Even if the control is performed, it is difficult to achieve both characteristics particularly in a thick product. Therefore, in the present invention, in designing the components, it has been intensively investigated whether the cold crack resistance can be controlled by completely different parameters instead of Pcm, which has been used as an index of cold crack resistance until now. As a result, using KP represented by the above formula (1) considering the steel structure, further reducing the amount of C and adding B, it is clear that both cold cracking resistance and base metal strength can be achieved. On the other hand, in order to ensure a very high base metal toughness, it has been found that it is effective to add a predetermined amount of Nb and refine the old γ grain size, thereby completing the present invention.
[0012]
First, a method for improving cold cracking resistance in the present invention will be described. As described above, in the present invention, C is made extremely low C, and hardenability improving elements Mn, Cr and Mo are positively added to appropriately control the KP value determined by the hardenability improving element. At the same time, the addition of B further improves the cold cracking resistance. By appropriately adding these components, the CCT line of bainite (see the CCT diagram of FIG. 4) moves to the short time side and the low temperature side, and the CCT line of ferrite moves to the long time side ( Move from solid line to broken line).
[0013]
Therefore, according to the prior art, in order to produce martensite at a high cooling rate and ferrite or high-temperature bainite at a low cooling rate, the hardness is highly sensitive to the cooling rate, and the hardness of the HAZ part during small heat input welding is reduced ( Improvement in cold cracking resistance) and securing of base material strength were incompatible, and it was difficult to achieve preheating free. However, according to the present invention, low-temperature bainite was produced at both high and low cooling rates. However, the sensitivity to the cooling rate of the hardness is reduced, and the reduction of the hardness of the HAZ part during welding (improvement of cold cracking resistance) and the securing of the base material strength are both achieved.
[0014]
Note that the above-mentioned approach to improving cold cracking resistance has been clarified prior to the filing of the present invention, and an application has already been filed for this (Japanese Patent Application No. 10-336268). In view of the fact that the invention of the prior application particularly has a high strength steel sheet of 780 MPa class or higher, the HAZ toughness deteriorates at the time of high heat input welding, and it is necessary to perform heat input restriction (5 kJ / mm or less) at the time of actual welding construction. As studied in the present invention, the HAZ toughness at the time of high heat input welding as well as the reduction of the hardness of the HAZ part during welding (improvement of cold cracking resistance) and the securing of the strength of the base metal are achieved. In order to improve, it has been found out that it is effective to adopt the above-mentioned method, and has been filed. Therefore, although both the above-mentioned prior application invention and the present invention coincide with each other in that they aim to improve low temperature cracking resistance and HAZ toughness during small heat input welding, In addition, the present invention has been intensively studied for the purpose of improving the HAZ toughness at the time of high heat input welding. In the present invention, while ensuring excellent HAZ toughness at the time of normal heat input welding, it is further possible to achieve vE- 80 ≧ 47J. , VE- 100 ≧ 47J, and the two are different in that they have been studied for the purpose of ensuring a very high base metal toughness. Actually, the invention of the prior application merely evaluates the base material toughness of vE- 40 , and does not consider the improvement of the base material toughness under severe conditions of vE- 100 level as in the present invention. It was. And according to this invention, in order to ensure extremely high base material toughness, it has one of the characteristics in the place where it discovered that it was effective to control Nb more appropriately. It can be said that the present invention is an invention different from the prior invention in that a new technical idea that has not been disclosed in the claimed invention is added. That is, the present invention follows the approach of the invention of the prior application as it is to the problem of “improvement of low temperature cracking resistance and HAZ toughness at the time of small heat input welding”, and has been proposed by the present invention. The issue of "ensuring extremely high base metal toughness" depends on the newly found refinement of the old γ grain size by the addition of Nb, both of which have different issues and means of achievement. is there.
[0015]
Hereinafter, components and KP values that contribute to the improvement of cold cracking resistance will be described.
[0016]
C: 0.010 to 0.06%
C is an element necessary for achieving both low temperature cracking resistance of the HAZ part during welding and ensuring the strength of the base material. When C exceeds 0.06%, martensite is generated instead of low-temperature bainite on the high cooling rate side, and the low-temperature cracking resistance is not improved. Preferably it is 0.055% or less. In addition, if it is less than 0.010%, the necessary minimum base material strength cannot be obtained. Preferably it is 0.030% or more.
[0017]
Mn: 1.0-3.0%
Cr: 0.1 to 2.0%
Mo: 0.1 to 1.5%
These elements have the effect of improving the hardenability, make it easy to produce low-temperature bainite at a high cooling rate to a low cooling rate, and, as described above, extremely low C and simultaneously adding a predetermined amount of B. This is useful in that both low temperature cracking resistance of the HAZ part during welding and securing of the base material strength can be achieved.
[0018]
First, the contents of Mn, Cr, and Mo are required to be 1.0% or more, 0.1% or more, and 0.1% or more, respectively. If these contents are not satisfied, the desired hardenability improving effect is not exhibited and the base material strength is insufficient. Preferably, Mn: 1.25% or more, Cr: 0.3% or more, Mo: 0.3% or more. However, if the contents of Mn, Cr, and Mo exceed 3.0%, 2.0%, and 1.5%, respectively, the toughness of the base material decreases. Preferably, Mn: 2.5% or less, Cr: 1.5% or less, Mo: 1.3% or less.
[0019]
Furthermore, the KP value determined by these elements needs to be 3.4 or more. If the KP value is less than 3.4, the above-described effect cannot be exhibited effectively, high-temperature bainite or ferrite is generated, and a base material strength of 780 MPa or more cannot be obtained (see FIG. 1 described later). . A larger KP value is better, and is preferably 4.0 or more. The upper limit is not particularly limited as long as it is a range determined based on the upper limit of each addition amount of Mn, Cr, and Mo, but is controlled to 7 or less, more preferably 6 or less in consideration of the base material toughness and the like. It is recommended.
[0020]
B: 0.0006 to 0.0050%
B is an element for improving hardenability, and facilitates the formation of low-temperature bainite at a low cooling rate. As described above, it is extremely low C, and at the same time, by adding an appropriate amount of Mn, Cr, Mo, the HAZ part at the time of heat welding is added. This is useful in that both low-temperature cracking resistance and securing of the base material strength can be achieved. If B is less than 0.0006%, the effect of improving the hardenability cannot be expected, and the base material strength is insufficient. Preferably it is 0.0007% or more. However, if B exceeds 0.0050%, the hardenability is lowered and the base material strength is insufficient. Preferably it is 0.0030% or less.
[0021]
The above are the components and requirements mainly contributing to the improvement of weldability such as cold cracking resistance. And, in order to obtain a very high base metal toughness which is another problem of the present invention, it is necessary to add Nb in a range of 0.010 to 0.05%, whereby vE- 80 ≧ A very high base metal toughness of 47 J, preferably vE- 100 ≧ 47 J, is achieved.
[0022]
It is considered that the extremely excellent base material toughness is obtained by adding Nb because the γ grain size is refined by adding Nb and the bainite block size after transformation is refined. Note that, in a high-tensile steel plate of 490 to 590 MPa class, improvement of the base metal toughness by refinement of γ grains is generally performed. This is a useful method in a steel sheet mainly composed of a ferrite structure, and is a 780 MPa class. In the high-strength steel sheet, since a bainite structure and a martensite structure are mainly used, it has been considered that when the γ grain size is refined, the hardenability deteriorates, and rather the base material toughness deteriorates. On the other hand, in the present invention, as described above, high hardenability is ensured by controlling the low C and KP values, and this, combined with the γ grain refinement effect by adding Nb, results in achieving high base metal toughness. It is thought that it was made. Addition of 0.010% or more is necessary in order to effectively exhibit such a toughness improving effect by adding Nb. Preferably it is 0.020% or more, More preferably, it exceeds 0.030%. However, if the amount of Nb added exceeds 0.05%, the HAZ toughness and the like deteriorate. Preferably it is 0.040% or less.
[0023]
The above are elements / components indispensable for solving the problems of the present invention, and the balance: iron and acceptable components that do not impair the action of the present invention.
[0024]
In the present invention, at least one selected from the group consisting of Ti: 0.03% or less, Zr: 0.05% or less, and Hf: 0.10% or less, with the aim of imparting even better properties. It is recommended that control be performed so that KN represented by the above formula (2) satisfies −1 ≦ KN ≦ 4.0.
[0025]
The elements of Ti, Zr, and Hf have an action of fixing N contained as an impurity, and N is bonded to solute B in the HAZ part at the time of welding, and B is consumed and the action of B addition is impaired. There is also an action to prevent this. Furthermore, nitrides such as Ti refine γ grains in the HAZ part during welding and contribute to improving HAZ toughness. From this point of view, it is recommended that these elements be positively added if necessary according to the N content in the steel. In that case, it is preferable to add Ti so as to be included in the above elements, and add other elements (Zr, Hf) together with Ti as necessary. Specifically, when the Ti content exceeds 0.03%, Zr: 0.05%, and Hf: 0.10%, the toughness of the base material deteriorates.
[0026]
Furthermore, when adding the said element, it is preferable that the KN value defined by the said Formula (2) is -1-4.0. For example, even though the amount of N is large, the amount of the above element added is small, so when the KN value exceeds 4.0, the effect of B addition is not effectively exhibited, and the HAZ toughness decreases (see FIG. 2 described later). reference). On the other hand, if the amount of the element added is too large and KN is less than -1, the toughness of the base material deteriorates. More preferably, it is 0.0 or more and 3.0 or less.
[0027]
Furthermore, in the present invention, it is recommended to positively add the following elements with the aim of further improving weldability and base material toughness.
[0028]
Si: 1.0% or less Si is an element useful as a deoxidizer, and in order to effectively exhibit such action, it is preferable to add 0.05% or more. However, if added over 1.0%, the weldability and the base metal toughness are lowered, so the upper limit is preferably made 1.0%. More preferably, it is 0.50% or less.
[0029]
Cu: 2.0% or less Cu is an element that improves the strength of the base metal by solid solution strengthening and precipitation strengthening and also has an effect of improving hardenability. However, if added over 2.0%, the HAZ toughness decreases, so the upper limit is preferably made 2.0%. More preferably, it is 1.5% or less.
[0030]
Ni: 6% or less Ni is an element useful for improving the toughness of the base metal. However, if added over 6%, scale flaws are likely to occur, so the upper limit is preferably made 6%. More preferably, it is 4% or less.
[0031]
V: 0.10% or less V has the effect of increasing hardenability and temper softening resistance by adding a small amount. However, if added over 0.10%, the HAZ toughness decreases, so the upper limit is preferably made 0.10%. More preferably, it is 0.07% or less.
[0032]
Al: 0.20% or less Al is a deoxidizing element, and is an element that fixes the N and increases the solid solution B to increase the hardenability of B. In order to effectively exhibit such an action, it is preferable to add 0.01% or more. However, since the toughness deteriorates if added over 0.20%, the upper limit is preferably made 0.20%. More preferably, it is 0.10% or less.
[0033]
N: 0.020% or less N combines with B to reduce solid solution B, inhibit the hardenability improving effect of B, and lower the toughness and HAZ toughness of the base material. When the N content exceeds 0.020%, the effect becomes remarkable. Therefore, the HAN toughness and base material toughness are improved by controlling the KN value by adding Ti and the like, and the effect of improving the hardenability by adding Al is effectively exhibited. I can't. More preferably, it is 0.010% or less.
[0034]
Ca: 0.0005 to 0.005%
Ca is an element having an effect of reducing the anisotropy by spheroidizing MnS and controlling the shape of inclusions. In order to effectively exhibit such an action, it is preferable to add 0.0005% or more. However, if the addition exceeds 0.005% excessively, the base material toughness decreases, so the upper limit is preferably made 0.005%.
[0035]
Next, a method for producing the steel plate of the present invention will be described.
[0036]
The steel plate of the present invention uses steel that satisfies the above component composition, and after heating, hot rolling, and quenching, it can be tempered to obtain a desired high-tensile steel plate. The conditions (temperature, time, etc.) for each step are not particularly limited, and the commonly used production conditions for high-tensile steel sheets can be adopted as appropriate. Specifically, for example, after heating at about 1000 to 1200 ° C. for 2 hours or more, hot rolling is performed, rolling is completed at 900 to 950 ° C., and then cooled. Next, it is recommended to hold at about 880 to 950 ° C. for 10 minutes or more and then cool with water. In addition, it is recommended that the tempering step be performed at about 550 to 650 ° C. for about 5 to 15 minutes. Thus, according to the present invention, a high-tensile steel plate excellent in weldability and base metal toughness can be obtained by applying production conditions that are usually performed in the production of a high-strength steel plate. Like 240353, it is extremely useful in that it is not necessary to go through a complicated manufacturing process such as tempering after repeatedly performing an operation consisting of two processing heat treatments after rolling.
[0037]
Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are encompassed in the technical scope of the present invention.
[0038]
【Example】
Steels having the composition shown in Tables 1 and 2 were melted by a normal melting method to form a slab, held at 1100 to 1150 ° C. for 2 hours, hot rolled, and rolled at 900 to 950 ° C. And slowly cooled. Then, after reheating and hold | maintaining for 15 minutes at 900 degreeC, it water-cooled and manufactured the high-tensile steel plate which consists of predetermined | prescribed board thickness on the tempering conditions of Tables 3-4.
[0039]
For each steel plate thus obtained, the base material properties [strength and toughness (vE- 100 )] are evaluated in the following manner, and the base material property level (strength ≧ 780 MPa, vE− 100 ≧) based on the present invention. Those that cleared 471) were further evaluated for weldability (cold crack resistance and HAZ toughness).
[0040]
[Base material characteristics test]
(1) Tensile test: A JIS No. 4 test piece was taken from a 1/4 thickness portion of each steel plate, and 0.2% proof stress and tensile strength were measured by conducting a tensile test. In the present invention, the tensile strength ≧ 780 MPa was accepted,
{Circle around (2)} Impact test: JIS No. 4 test piece was taken from a 1/4 thickness portion of each steel plate, and absorbed energy (vE- 100 ) was obtained by conducting a Charpy impact test. In the present invention, vE- 100 ≧ 47J was accepted.
[0041]
[Weldability test]
(1) HAZ toughness: Welding was performed at a heat input of 5 kJ / mm (submerged welding method), and a JIS No. 4 specimen was taken from the site shown in FIG. 4 to perform a Charpy test, and the absorbed energy of the bond part (vE- 10 ) Asked. In the present invention, vE −10 ≧ 47 J was accepted,
(2) Cold cracking resistance: Based on the y-type weld cracking test method described in JIS Z 3158, covered arc welding was performed at a heat input of 1.7 kJ / mm, and the root cracking prevention preheating temperature was measured. In this invention, 25 degrees C or less was set as the pass.
[0042]
These results are also shown in Tables 3-4.
[0043]
[Table 1]
[0044]
[Table 2]
[0045]
[Table 3]
[0046]
[Table 4]
[0047]
From Tables 3 and 4, it can be considered as follows.
[0048]
First, the steel plates in Table 1 are examples that satisfy the requirements of the present invention, and as shown in Table 3, all the steel plates were excellent in base material characteristics and weldability.
[0049]
On the other hand, although the steel plate of Table 2 is a comparative example which does not satisfy the requirements of this invention, these have the faults shown in Table 4.
[0050]
First, no. No. 23 is an example in which the amount of C is lower than the lower limit of the present invention, and a desired base material strength was not obtained. No. No. 24 is an example in which the amount of C exceeds the upper limit of the present invention, and the low temperature cracking resistance was lowered.
[0051]
No. No. 25 is an example in which the Si amount exceeds the upper limit of the present invention, and the desired base material toughness was not obtained.
[0052]
No. No. 26 is an example in which the KP value is lower than the lower limit of the present invention, and a desired base material strength was not obtained.
[0053]
No. No. 27 is an example in which the amount of Mn falls below the lower limit of the present invention, and a desired base material strength was not obtained. No. No. 28 is an example in which the amount of Mn exceeds the upper limit of the present invention, and the base material toughness was lowered.
[0054]
No. No. 29 is an example in which the Ni content exceeds the upper limit of the present invention, and the low temperature cracking resistance was lowered.
[0055]
No. No. 30 is an example in which the Cr amount exceeds the upper limit of the present invention, and the desired base material toughness was not obtained.
[0056]
No. No. 31 is an example in which the Mo amount exceeds the upper limit of the present invention, and the desired base material toughness was not obtained.
[0057]
No. No. 32 is an example in which the Nb amount exceeds the upper limit of the present invention, and the HAZ toughness was lowered. No. No. 33 is an example in which the Nb amount is lower than the lower limit of the present invention, and the base material toughness was lowered.
[0058]
No. No. 34 is an example in which the amount of Cu exceeds the upper limit of the present invention, and the base material toughness was lowered.
[0059]
No. No. 35 is an example in which the V amount exceeds the upper limit of the present invention, and the HAZ toughness was lowered.
[0060]
No. 36 / No. No. 37 is an example in which the amount of B is below / exceeds the lower limit value / upper limit value of the present invention, and none of the desired base material strength was obtained.
[0061]
No. No. 38 is an example in which the Ti amount exceeds the upper limit of the present invention, and the base material toughness was lowered.
[0062]
No. No. 39 is an example in which the KN value is lower than the lower limit of the present invention, and the base material toughness was lowered. No. No. 43 is an example in which the KN value exceeds the upper limit of the present invention, and the HAZ toughness was lowered.
[0063]
No. No. 40 is an example in which the amount of Zr exceeds the upper limit of the present invention, and the base material toughness was lowered.
[0064]
No. No. 41 is an example in which the amount of Hf exceeds the upper limit of the present invention, and the base material toughness was lowered.
[0065]
No. No. 42 is an example in which the Ca content exceeds the upper limit of the present invention, and the base material toughness was lowered.
[0066]
No. No. 44 is an example in which the N amount exceeds the upper limit of the present invention, and the HAZ toughness was lowered.
[0067]
FIG. 1 is a graph showing the relationship between the base material strength (tensile strength) and the KP value based on the above results. By controlling the KP value to 3.4 or more, a tensile strength of 780 MPa or more is obtained. You can see that
[0068]
FIG. 2 is a graph showing the relationship between the HAZ toughness (vE- 1 00 ) and the KN value during welding with a heat input of 5 kJ / mm based on the above results. It can be seen that a HAZ toughness of 47 kJ or more can be obtained by controlling to 0 range.
[0069]
【The invention's effect】
The method of the present invention is configured as described above, and is excellent in weldability (cold crack resistance and HAZ toughness) and also has a good base metal toughness (particularly vE- 80 ≧ 47J, preferably vE- 100 ≧ 47J). We were able to provide a high-strength steel sheet that had both.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between base material strength and KP value.
FIG. 2 is a graph showing the relationship between HAZ toughness and KN value.
FIG. 3 is a schematic explanatory view showing a specimen collection position for bond toughness during submerged arc welding.
FIG. 4 is a schematic CCT diagram for explaining the concept of component design of the present invention.
Claims (4)
Mn:1.0〜3.0%,
Cr:0.1〜2.0%,
Mo:0.1〜1.5%,
B :0.0006〜0.0050%,
Nb:0.010〜0.05%
を含有し、更に
Si:1.0%以下 (0%を含まない),
Cu:2.0%以下 (0%を含む),
Ni:6%以下 (0%を含む),
V :0.10%以下 (0%を含む),
Al:2.0%以下 (0%を含む),
N :0.020%以下(0%を含まない)
を含有し、更に
Ti:0.03%以下,
Zr:0.05%以下,及び
Hf:0.10%以下
よりなる群から選択される少なくとも一種を含有し、
残部:鉄及び不可避不純物であり、且つ、
下式(1)で表されるKPがKP≧3.4を満足すると共に、
下式(2)で表されるKNが0.0≦KN≦3.0を満足するものであることを特徴とする溶接性及び母材靭性に優れた高張力鋼板(但し、上記鋼板の組成のうち、Mn:1.25〜2.5%,Nb:0.010〜0.030%,Si:0.05〜0.50%,Cu:0.25%未満を満足する鋼板は除く)。
KP=[Mn]+1.5×[Cr]+2×[Mo] … (1)
KN=([N]/14−[Ti]/48− [ Zr ] /91−[Hf]/178)×10 4 …(2)
(式中、[ ]は各元素の含有量(%)を意味する)C: 0.010 to 0.06% (meaning mass%, the same shall apply hereinafter),
Mn: 1.0 to 3.0%,
Cr: 0.1 to 2.0%,
Mo: 0.1 to 1.5%,
B: 0.0006 to 0.0050%,
Nb: 0.010 to 0.05%
Further,
Si: 1.0% or less (excluding 0%),
Cu: 2.0% or less (including 0%),
Ni: 6% or less (including 0%),
V: 0.10% or less (including 0%),
Al: 2.0% or less (including 0%),
N: 0.020% or less (excluding 0%)
Further,
Ti: 0.03% or less,
Zr: 0.05% or less, and
Hf: 0.10% or less
Containing at least one selected from the group consisting of:
The balance: iron and inevitable impurities, and
KP represented by the following formula (1) satisfies KP ≧ 3.4 , and
KN represented by the following formula (2) satisfies 0.0 ≦ KN ≦ 3.0, a high-tensile steel plate excellent in weldability and base metal toughness (however, the composition of the steel plate) Among them, steel sheets satisfying Mn: 1.25 to 2.5%, Nb: 0.010 to 0.030%, Si: 0.05 to 0.50%, Cu: less than 0.25% are excluded) .
KP = [Mn] + 1.5 × [Cr] + 2 × [Mo] (1)
KN = ([N] / 14- [Ti] / 48- [Zr] / 91- [Hf] / 178) × 10 4 ... (2)
(In the formula, [] means the content (%) of each element)
Ca:0.0005〜0.005%を含有するものである請求項1に記載の高張力鋼板。The high-tensile steel plate according to claim 1 , further comprising Ca: 0.0005 to 0.005%.
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