JP3584742B2 - High strength thick steel plate excellent in weldability and toughness and method for producing the same - Google Patents
High strength thick steel plate excellent in weldability and toughness and method for producing the same Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は、橋梁、建築物などの大形鉄鋼構造物に用いられる、靱性の良好な溶接性にすぐれた高張力厚鋼板に関する。
【0002】
【従来の技術】
引張強さが570MPa以上の鋼板は、橋梁等の鉄鋼構造物に広く用いられている。近年の動向として、施工コストを削減する目的で、これら構造物の構成部品点数低減がはかられ、例えば橋梁などでは、主桁を少なくするような設計が採用される。このためフランジの板厚を増し、従来75mm厚程度までであった使用鋼板は、100mm厚前後のものが要求されるようになってきている。
【0003】
しかし、一般に強度をそのまま維持して板厚を厚くしようとすれば、靱性や溶接性のすぐれた鋼板の製造が困難になってくる。これは板厚が厚くなれば強制冷却時の冷却速度が遅くなって、低温変態生成物の形成が不十分となり、焼入れる場合では焼入れ硬さが不十分になるからである。この不十分な焼入れ状態で、強度を確保するために低温での焼戻しをおこなえば、靱性の大きく劣る鋼板になる。一方、遅い冷却速度でも、C量を増し、焼入れ性向上元素を多く添加すれば、低温変態生成物を形成させる、ないしは十分な焼き入れをおこなわせることは可能である。しかしながら、これらの元素の含有量増加は靱性を低下させる原因となる。
【0004】
橋梁等の大形鉄鋼構造物の建設においては、施工現場での溶接もおこなわれるため、溶接前の予熱不要の鋼板が求められ、これまでは約25℃の温度にて溶接割れが発生しない鋼板が適用されてきた。この溶接性に対して、C量増加やこれら焼入れ性向上元素の増加は、割れ感受性を増すため、板厚が厚く強度の高い鋼板の製造を一層困難にする。さらに、冬季の山間部での工事など、気温が0℃前後の環境で溶接をおこないたい場合があり、0℃ないしはそれ以下の温度においても、溶接割れが発生しない鋼板に対する要望が増しつつある。
【0005】
従来より、靱性および溶接性にすぐれた高強度の厚鋼板や、その製造方法の技術は数多く公表されている。しかしながら、強度が570MPaを超える場合は、厚さが50mm程度以下を対象とするものが主であり、強度が高く溶接性にすぐれ、板厚が75mmを超え、しかも割れ防止予熱温度が低い鋼板となるとほとんど報告されていない。例えば、特開平9−3596号公報には、板厚が100mmまでの、降伏比80%以下引張強さ590MPa以上の低降伏比型高張力鋼板、およびその製造方法の発明が提示されているが、溶接の割れ防止予熱温度は50℃以下としている。また、特開平9−13143号公報には、板厚40〜100mmを対象とした、溶接性にすぐれ600MPa級の鋼板およびその製造方法の発明が開示されている。しかし、実施例から判断される限り、溶接割れ防止予熱温度は25℃であり、また試験材の板厚は43〜75mmの範囲しか示されていない。
【0006】
【発明が解決しようとする課題】
本発明の目的は、引張強さ570MPa以上で、靱性にすぐれ、かつ溶接割れ防止のための鋼板温度を0℃以下とすることのできる、板厚が75mmを超える高張力厚鋼板およびその製造方法を提供することにある。
【0007】
【課題を解決するための手段】
本発明者らは、板厚中央部の引張強さが570MPa以上で、溶接時の鋼板温度を0℃以下にすることができ、靱性がすぐれた板厚75mm以上の鋼板を得るための組成範囲と、さらにその鋼板を圧延終了後その温度から焼入れる直接焼き入れ法にて製造する方法を調査した。
【0008】
まず焼入れ焼戻しによる調質鋼を前提とし、成分の検討をおこなった結果は、炭素量を選定し、その上で焼入れ性向上元素を添加することにより、板厚75mm以上にて所要強度は十分得ることができた。その強度を得た上で、さらに橋梁等の構造物に必要な鋼の靱性(vTs≦−5℃)と、溶接の割れ防止のための鋼板温度を低下できる組成の検討を進めた。割れ防止鋼板温度の目標は、JIS規格Z−3158の斜めY形溶接割れ試験法に基づき、試験片温度を0℃として試験溶接部に割れのないこととした。
【0009】
溶接の割れ感受性は、次式で示されるPcmの値を低くする必要がある。
【0010】
Pcm=C(%)+(Si(%)/30)+(Mn(%)/20)+(Cu(%)/20) +(Ni(%)/60)+(Cr(%)/20)+(Mo(%)/15) +(V(%)/10)+5B(%) ・・・・・ (b)
この鋼板組成によるPcmの値と、割れ防止鋼板温度との関係を調べた結果、鋼板温度を0℃以下にするには、Pcmを0.20以下にすればよいことがわかった。そこでPcmが0.20以下である範囲にて、厚さ100mm前後の鋼板で目標とする強度を得、その上で、十分な靱性を得るための合金成分の検討をさらに進めた結果、次のようなことが明らかになった。
【0011】
まず、靱性を損なうことなく焼入れ性を向上させるための、強度増加に有意な元素は、CrおよびCuであった。そこでこれらの元素は積極的に用いることとした。Moもよいが、添加量が増すとすると靱性をやや悪くする傾向があったため、多くは添加しないことにした。Bは少量の含有で焼き入れ性を大幅に向上させることができ、しかも炭素当量を増さないので、その利用を検討の結果、この板厚の厚い鋼板でも極めて有効であることがわかった。ただし、Bの添加は一定量以上含有させれば、その含有量に関わりなくほぼ同じ焼入れ性向上効果が得られるが、量が増すと靱性を劣化させる作用がある。そこでこの作用を抑止する対策を調べた結果、Cu、NiおよびMoの存在が有効であることがわかった。これらの元素が含有されていることにより、必要含有量よりも過剰にBが存在していても、靱性を悪くすることがないので、Bの含有許容範囲を拡大することができ、B添加を効果的に利用することができるのである。
【0012】
これらの検討の中で、焼入れ性を向上させ、焼戻し軟化を抑止し、さらに析出硬化も期待できるとして、一般に高張力鋼板に多用されるVの添加が、靱性を劣化させるため好ましくないことがわかった。Vは添加元素としてその効果を発揮させる場合、通常0.05%前後含有させるが、その利点よりも靱性劣化という難点の方が顕著であった。これは、一つには、板厚が厚くなると、焼入れ時の冷却ばかりでなく、焼戻し時の冷却も極めてゆっくりした冷却となり、微細であるべきその析出物が粗大化してしまうためではないかと思われる。そこで、Vは添加しないことにした。
【0013】
Vに代わる元素としては、Nbが有効であった。Nbを含有させることにより、強度増加と靱性の向上がもたらされ、とくに圧延終了後その温度から焼入れる直接焼き入れ法において効果的であった。Nb添加の効果は、焼入れ性向上よりは、結晶粒の微細化と、析出硬化にあると考えられる。すなわち結晶粒の微細化により、強度の向上と靱性の向上が得られ、その上Nbの炭化物や窒化物の微細析出物は、Vのそれに比較して凝集や粗大化が起こりにくい。
【0014】
以上のような検討結果から選定された組成の鋼は、通常の焼入れ焼戻しによる調質熱処理によって、目標とする特性の鋼板を得ることができる。しかしながら、製造工程の合理化の点からは、圧延加工の熱を利用して、圧延後直ちに焼き入れることが望ましい。そこで、この直接焼入れによる製造方法の条件について調査の結果、圧延素材の加熱温度は高めとし、Ar3変態温度直上で圧延を終了して直ちに急冷して焼入れ後、焼戻し温度は通常より低めとすることが、目標とする特性の鋼板を製造するために重要であることがわかった。圧延素材の加熱温度を高くするのは、Nbを圧延前に固溶状態にしておくことが、Nb添加の効果を十分発揮させるために必要であるからである。また、焼戻し温度は、通常、十分な靱性を得るために高めとすることが必要であるが、Vの代わりにNbを添加することによって、焼戻し温度を低くして十分な靱性を確保できることがわかったのである。これは、Nb添加による結晶粒の微細化が効果的に作用したと考えられる。Nbは、その微細析出による結晶粒成長抑止効果もあるが、熱間圧延のパス間での再結晶を抑止し、圧延直後における結晶粒を微細化する効果もあると考えられる。焼戻し温度を低くできることは、同じ強度を得るための鋼の成分の低減、ことにC量やMn量を少なくすることができ、これは鋼の靱性や溶接性の改善に有意である。
【0015】
以上のような知見に基づき、さらにそれぞれの元素の効果的な含有量の範囲限界、および製造条件の範囲を明確にし、本発明を完成させた。本発明の要旨は次のとおりである。
【0016】
(1) 重量%で、C:0.05〜0.14%、Si:0.01〜0.5%、Mn:0.5〜1.5%、P:0.015%以下、S:0.005%以下、Cu:0.30〜0.50%、Ni:0.15〜0.50%、Cr:0.30〜0.60%、Mo:0.05〜0.4%、Nb:0.015〜0.045%、Ti:0〜 0.030 %、Al:0.005〜0.10%、N:0.001〜0.007%、およびB:下記(a) 式で示される範囲、を含有し、残部はFeおよび不可避的不純物からなり、下記(b) 式で定義されるPcmの値が0.20以下である、引張強さ570MPa以上の靱性が良好で寒冷地における溶接性にすぐれた板厚75mmを超える高張力厚鋼板。
【0017】
0.0003≦B(%)≦0.0005+(Cu(%)/1000)+(Ni(%)/500) +(Mo(%)/500) ・・・・・ (a)
Pcm=C(%)+(Si(%)/30)+(Mn(%)/20)+(Cu(%)/20) +(Ni(%)/60)+(Cr(%)/20)+(Mo(%)/15) +(V(%)/10)+5B(%) ・・・・・ (b)
(2)上記の組成の素材を1050〜1250℃の温度に加熱後、圧延して800〜900℃にて所定の板厚に仕上げて直接焼き入れし、さらに350〜550℃の温度で焼戻し処理を施すことを特徴とする、上記(1)の高張力厚鋼板の製造方法。
【0018】
【発明の実施の形態】
本発明鋼板の化学組成、または製造条件の限定理由を以下に説明する。
Cの含有量を0.05〜0.14%とする。Cは鋼の強度を決定するための最も重要な元素である。含有量が0.05%未満の場合、必要とする570MPa以上の強度を得ることが困難になる。また0.14%を超える含有は、鋼板の靱性および溶接性が目標を下回る結果となる。したがって、その含有範囲を0.05〜0.14%とするが、強度が十分高く、靱性および溶接性のいずれもすぐれた、バランスした性能を得るためには、0.06〜0.10%の範囲にあることが望ましい。
【0019】
Siの含有は、靱性を悪くすることなく鋼の強度を向上させる効果があり、その含有量を0.01〜0.5%とする。0.01%を下回る含有量では効果がなく、0.5%を超える含有は鋼板の表面性状を悪くするからである。
【0020】
Mnは含有量を0.5〜1.4%とする。MnはCとともに鋼の強度を確保するために重要である。0.5%未満では鋼の強度が不足し、1.4%を超える場合は、靱性が悪くなる。強度が十分高く、靱性が良好な範囲として好ましいのは、1.0〜1.4%である。
【0021】
Pは鋼の不可避的不純物の一つで、鋼の靱性を劣化させ、溶接性を悪くするので、少なければ少ないほどよい。顕著な悪影響をおよぼさない範囲として、0.015%以下とする。
【0022】
Sも不可避的不純物の一つであり、鋼の靱性を著しく劣化させるので、その含有量は少なければ少ないほどよい。顕著な悪影響をおよぼさない範囲として、0.005%以下とする。
【0023】
Cuの含有量は0.30〜0.50%とする。Cuは溶接性や靱性を大きく損なうことなく、焼入れ性向上により、鋼の強度を増加させることができるので、積極的に利用する。このため、少なくとも0.30%以上含有させる。しかし、多くなりすぎると靱性に影響が現れるので、上限を0.50%までとする。
【0024】
Niは靱性や溶接性を損なわずに強度を上昇させることができる。ただし、高価であり、その効果を得るにはやや多量の添加が必要なので、使用を抑制したい元素である。しかし、Cuを鋼に添加する場合、Cuによる熱間脆性を抑止する効果があるので、含有させる必要がある。この目的には、Cu量の1/2から等量程度以上の含有が必要であるので、Niの含有量は0.15〜0.50%とする。
【0025】
Crの含有量は0.30〜0.60%とする。CrはCuと同様溶接性や靱性を損なうことなく、鋼の強度を増加させることができるので、積極的に利用する。その効果を十分発揮させるには、少なくとも0.30%以上の含有が必要であるが、含有量が増してくると靱性を悪くするので、多くても0.60%までである。望ましいのは、0.40〜0.60%である。
【0026】
Moの添加は鋼の強度を向上させ、またBの添加による靱性の低下を緩和する効果があり、その含有量を0.05〜0.4%とする。0.05%未満ではその効果が十分でなく、0.4%を超える含有は、鋼の靱性が低下してくるおそれがあるからである。好ましいのは、0.05〜0.15%の含有である。
【0027】
Bは、炭素当量を増すことなく、少量の含有で焼入れ性が向上するので、溶接性の劣化なしに強度を向上させ得る、重宝な元素である。その効果を十分発揮させるには、0.0003%以上の含有があればよい。しかし、より多く含有させてもその効果は飽和して変わらないが、靱性が劣化してくる傾向がある。この靱性劣化の傾向は、Cu、NiおよびMoの存在により緩和される。そこで、Bの含有量範囲を、
0.0003≦B(%)≦0.0005+(Cu(%)/1000)+(Ni(%)/500) +(Mo(%)/500) ・・・・・ (a)
とする。
【0028】
Nbの含有量は0.015〜0.045%とする。Nbの含有は鋼の結晶粒を細かくし、靱性を劣化させずに強度の向上を得ることができる。このような効果を十分得るためには、0.015%以上の含有が必要である。しかし多すぎる添加は、靱性の劣化や溶接性の低下を来たすので、多くても0.045%までである。望ましいのは、0.025〜0.035%のせまい範囲に管理することである。
【0029】
Tiは、添加しなくてもよい。しかし結晶粒を細かくし、Nを固定してB添加による効果を十分発揮させる効果があるので、必要により添加する。添加する場合その含有量は0.005〜0.030%とするのが望ましい。これは0.005%を下回る場合、その添加の効果は十分でなく、0.030%を超えると鋼の靱性が劣化してくるからである。
【0030】
Alは製鋼段階における鋼の脱酸のために添加し、その含有範囲は0.005〜0.10%とする。0.005%を下回る含有量の場合、鋳片に欠陥が残存するおそれがある。また多すぎてもその効果は飽和し、鋼の靱性を悪くするようになるので、0.10%までの含有とする。
【0031】
Nは鋼の不可避的不純物の一つであるが、少量の存在はAlやNbあるいはTiと結合して微細な析出物を形成し、特に溶接部の結晶の粗大化を抑止する効果がある。このためには、少なくとも0.001%以上の含有が望ましい。しかし、0.007%を超える含有は鋼の靱性を悪くするので、Nの含有量を0.001〜0.007%とする。
【0032】
前出の(b) 式で示される溶接の割れ感受性Pcmの値は、0.20以下となるよう各成分の量を管理する。Pcmの値が0.20を超えると、鋼板温度0℃で割れが発生するからである。
【0033】
鋼板の熱処理を直接焼入れ法にておこなう場合は、以下の条件にて処理する。熱間圧延する素材鋼の加熱温度は、1050〜1250℃とする。これは1050℃を下回るとNb添加による細粒化効果が十分得られなくなるからである。熱間圧延前にNbが十分固溶状態にならなくなるためと思われる。また、1250℃を超えて加熱しても得られた鋼板の特性改善には何の効果も認められず、酸化スケールや加熱燃料の損失を増すだけである。好ましい加熱温度は1050〜1160℃である。
【0034】
熱間圧延は、所定の板厚に仕上げる終了温度を800〜900℃とする。800℃未満では、変態温度を大きく下回ることになり、十分な焼入れ強度が得られなくなる。また900℃を超えると、鋼板の靱性が低下するおそれがある。好ましいのは830〜880℃である。なお、望ましいのは900℃以下で50%以上の累積圧下を付与することで、より一層の細粒化および靱性の向上が得られる。
【0035】
熱間圧延終了後、できる限り時間をおかずに強制冷却する。すなわち焼入れをおこなう。十分な焼入れ強度を得るためには、強制冷却は、水などの冷却媒体が鋼板面全体に均一にあたるようにして、板厚の中心部で1℃/s以上、望ましくは2℃/s以上の冷却速度でおこなうのがよい。なお強制冷却の開始は、多少遅れたとしても、鋼板表面にて750℃以上の温度から開始することが望ましい。
【0036】
焼戻しは350〜550℃の温度でおこなう。350℃未満の温度では靱性が不十分になる。550℃を超える温度で焼戻す場合、合金成分等を多く添加すれば強度と靱性を確保することは可能であるが、Pcmを十分低くすることが困難になってくる。また焼入れが不十分な場合、焼戻し温度を下げることによって強度を確保することは可能であるが、そうすると靱性の劣る鋼となる。したがって、上記成分範囲にて、Pcmが0.2以下で、所要板厚および強制冷却速度から、十分な焼入れがおこなわれ、かつ350〜550℃の焼戻し温度にて引張強さが570MPa以上となるように、個々の鋼成分とその量を選定すれば、靱性および溶接性のすぐれた高張力厚鋼板が得られるのである。
【0037】
【実施例】
表1に示す組成の鋼を鍛造して、厚さ300mmの圧延素材とし、表2に示す加熱温度、仕上げ温度、および板厚として熱間圧延をおこない、仕上げ圧延後直ちに焼入れを実施し、次いで表2中に示す温度で焼戻しをおこなった。得られた鋼板の板厚中央部より、圧延と直角の方向に試験片を採取して引張り試験をおこない、圧延と平行な方向に試験片を採取してシャルピー衝撃試験をおこなった。また、JIS規格Z−3158の斜めY形溶接割れ試験法に基づき、試験片温度を0℃として溶接試験をおこない、溶接部の割れの有無を調査した。
【0038】
【表1】
【0039】
【表2】
【0040】
鋼板の評価試験結果を表2に併記する。表1において、鋼記号A〜Fは本発明の組成範囲を満足するものであるが、鋼記号G〜Sは、いずれかの組成、またはPcm値が本発明で規制する範囲を逸脱している。表2において、性能評価の判断基準を、引張強さ570MPa以上、靱性はvTrsが−5℃以下、溶接性は試験片温度が0℃にて割れの発生がないこととすれば、本発明の有効性は明らかである。
【0041】
組成が本発明範囲を満足する鋼による試験結果を表2の試験番号1〜16に示すが、熱間圧延以降の製造条件が不十分な場合、目標性能が得られないことがある。例えば、試験番号3は仕上げ温度、ないしは焼入れ温度が低すぎ、試験番号5では、素材加熱温度が低すぎ、試験番号13は焼戻し温度が高すぎて、いずれの場合も、十分な強度が得られていない。また、試験番号4は仕上げ温度が高すぎ、試験番号8では加熱温度が高すぎ、試験番号9は焼戻し温度が低すぎて、いずれの場合も強度は十分であるが靱性が劣っている。しかしながら、製造条件が適正である場合、このように鋼組成を限定された鋼は、すぐれた強度、靱性および溶接性を得ることができる。
【0042】
これに対し、製造条件が十分満足すべき範囲に入っていても、鋼番号G〜Sの化学組成が本発明で規制する範囲を逸脱している場合、表2の試験番号17〜29の結果から明らかなように、強度、靱性、または溶接性のいずれもが目標特性を超える鋼板とはならないのである。
【0043】
【発明の効果】
本発明によれば、板厚中央部の引張強さが570MPa以上、靱性はvTrsが−5℃以下、溶接性は鋼板温度が0℃にて割れの発生がない、板厚75mmを超える高強度厚鋼板が得られ、また直接焼き入れ法により、その鋼板を容易に製造することができる。この鋼板は、橋梁など鋼構造物の施工条件を緩和し、その建設コストの低減に効果的に活用できる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength steel plate having excellent toughness and excellent weldability, which is used for large-sized steel structures such as bridges and buildings.
[0002]
[Prior art]
Steel plates having a tensile strength of 570 MPa or more are widely used for steel structures such as bridges. As a recent trend, in order to reduce construction costs, the number of component parts of these structures has been reduced. For example, in a bridge or the like, a design that reduces the main girder is adopted. For this reason, the thickness of the flange has been increased, and a steel sheet used up to about 75 mm in thickness has been required to be about 100 mm thick.
[0003]
However, in general, if it is attempted to increase the sheet thickness while maintaining the strength as it is, it becomes difficult to manufacture a steel sheet having excellent toughness and weldability. This is because the thicker the sheet thickness, the slower the cooling rate at the time of forced cooling, the insufficient formation of low-temperature transformation products, and in the case of quenching, the quenching hardness becomes insufficient. If tempering is performed at a low temperature in order to ensure strength in this insufficiently quenched state, a steel sheet having significantly poor toughness is obtained. On the other hand, even at a slow cooling rate, it is possible to form a low-temperature transformation product or perform sufficient quenching by increasing the amount of C and adding a large amount of a hardenability improving element. However, an increase in the content of these elements causes a decrease in toughness.
[0004]
In the construction of large steel structures such as bridges, since welding is also performed at the construction site, steel plates that do not require preheating before welding are required, and so far steel plates that do not crack at temperatures of about 25 ° C Has been applied. Contrary to this weldability, an increase in the amount of C and an increase in these hardenability improving elements increase cracking susceptibility, which makes it more difficult to produce a steel sheet having a large thickness and high strength. Further, there are cases where welding is desired to be performed in an environment in which the temperature is around 0 ° C., such as construction in a mountainous area in winter, and there is an increasing demand for a steel sheet that does not cause weld cracking even at a temperature of 0 ° C. or lower.
[0005]
2. Description of the Related Art Conventionally, many techniques of a high-strength steel plate excellent in toughness and weldability and a method of manufacturing the same have been disclosed. However, when the strength exceeds 570 MPa, the thickness is mainly about 50 mm or less, and the strength is high, the weldability is excellent, the plate thickness exceeds 75 mm, and the crack prevention preheating temperature is low. Very few have been reported. For example, Japanese Unexamined Patent Publication No. 9-3596 discloses a low-yield-ratio high-strength steel sheet having a thickness of up to 100 mm, a yield ratio of 80% or less, and a tensile strength of 590 MPa or more, and an invention of a method for producing the same. The preheating temperature for preventing cracks in welding is set to 50 ° C. or less. Also, Japanese Patent Application Laid-Open No. 9-13143 discloses an invention of a 600 MPa class steel sheet having excellent weldability and a method of manufacturing the same for a thickness of 40 to 100 mm. However, as determined from the examples, the welding crack prevention preheating temperature is 25 ° C., and the plate thickness of the test material is shown only in the range of 43 to 75 mm.
[0006]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION It is an object of the present invention to provide a high-strength steel plate having a plate thickness of more than 75 mm, which has a tensile strength of 570 MPa or more, has excellent toughness, and can maintain a steel plate temperature of 0 ° C. or less for preventing weld cracking, and a method of manufacturing the same. Is to provide.
[0007]
[Means for Solving the Problems]
The present inventors have found that the composition range for obtaining a steel plate having a plate thickness of 75 mm or more that has a tensile strength at the center of the plate thickness of 570 MPa or more, a steel sheet temperature at the time of welding of 0 ° C. or less, and excellent toughness can be obtained. Further, a method of manufacturing the steel sheet by a direct quenching method in which the steel sheet is quenched from the temperature after the completion of rolling was investigated.
[0008]
First, assuming a tempered steel by quenching and tempering, the result of examining the components shows that the required strength is sufficiently obtained at a plate thickness of 75 mm or more by selecting the amount of carbon and then adding a quenchability improving element. I was able to. After obtaining the strength, studies were further conducted on the toughness (vTs ≦ −5 ° C.) of steel required for structures such as bridges and the composition capable of lowering the steel sheet temperature for preventing cracking of welding. The target of the temperature of the crack-preventing steel sheet was determined to be 0 ° C. based on the oblique Y-shaped weld cracking test method of JIS standard Z-3158, so that there was no crack in the test weld.
[0009]
It is necessary to reduce the value of Pcm expressed by the following equation for the crack sensitivity of welding.
[0010]
Pcm = C (%) + (Si (%) / 30) + (Mn (%) / 20) + (Cu (%) / 20) + (Ni (%) / 60) + (Cr (%) / 20 ) + (Mo (%) / 15) + (V (%) / 10) + 5B (%) (b)
As a result of examining the relationship between the value of Pcm based on the composition of the steel sheet and the temperature of the crack-preventing steel sheet, it was found that the Pcm should be set to 0.20 or less in order to reduce the steel sheet temperature to 0 ° C or lower. Therefore, within the range where Pcm is 0.20 or less, a steel sheet with a thickness of about 100 mm was obtained to obtain the target strength, and further study of alloy components to obtain sufficient toughness was carried out. It became clear.
[0011]
First, Cr and Cu were significant elements for increasing the strength to improve hardenability without impairing toughness. Therefore, these elements were positively used. Mo is also good, but if the addition amount increases, the toughness tends to be slightly deteriorated, so that it was decided not to add much. A small amount of B can significantly improve the hardenability and does not increase the carbon equivalent. As a result of studying its use, it has been found that this thick steel plate is extremely effective. However, if B is added in a certain amount or more, almost the same hardenability improving effect can be obtained irrespective of the content, but if the amount is increased, it has the effect of deteriorating the toughness. Then, as a result of investigating a countermeasure for suppressing this effect, it was found that the presence of Cu, Ni and Mo was effective. By containing these elements, even if B is present in excess of the required content, the toughness is not deteriorated, so that the allowable range of B content can be expanded, and It can be used effectively.
[0012]
In these studies, it was found that the addition of V, which is commonly used in high-strength steel sheets, is not preferable because it improves hardenability, suppresses temper softening, and can also expect precipitation hardening, because it deteriorates toughness. Was. When V is used as an additive element to exert its effect, it is usually contained at about 0.05%, but the disadvantage of toughness deterioration is more remarkable than its advantage. One reason for this is that when the plate thickness is increased, not only cooling during quenching, but also cooling during tempering becomes extremely slow cooling, and the precipitates that should be fine become coarse. It is. Therefore, V was not added.
[0013]
Nb was effective as an element replacing V. The incorporation of Nb resulted in an increase in strength and an improvement in toughness, and was particularly effective in a direct quenching method in which quenching was performed at that temperature after the completion of rolling. It is considered that the effect of the addition of Nb lies in the refinement of crystal grains and the precipitation hardening, rather than the improvement in hardenability. In other words, the refinement of the crystal grains improves the strength and the toughness, and the fine precipitates of Nb carbides and nitrides are less likely to agglomerate or coarsen than those of V.
[0014]
The steel having the target properties can be obtained from the steel having the composition selected from the above examination results by the refining heat treatment by the usual quenching and tempering. However, from the viewpoint of rationalization of the manufacturing process, it is desirable to use the heat of the rolling process to quench immediately after rolling. Therefore, as a result of investigation on the conditions of the production method by the direct quenching, the heating temperature of the rolled material was set to be high, the rolling was finished immediately above the Ar 3 transformation temperature, immediately quenched, and then quenched, and the tempering temperature was set lower than usual. Has been found to be important for producing a steel sheet having desired properties. The reason why the heating temperature of the rolled material is increased is that it is necessary to bring Nb into a solid solution state before rolling in order to sufficiently exert the effect of Nb addition. Further, the tempering temperature usually needs to be set higher to obtain sufficient toughness, but it has been found that by adding Nb instead of V, the tempering temperature can be lowered to secure sufficient toughness. It was. It is considered that the refinement of the crystal grains by the addition of Nb effectively acted. Although Nb has an effect of suppressing crystal grain growth due to its fine precipitation, it is considered that Nb also has an effect of suppressing recrystallization between passes of hot rolling and making crystal grains fine immediately after rolling. The ability to lower the tempering temperature can reduce the composition of the steel to obtain the same strength, and in particular, reduce the amount of C and Mn, which is significant for improving the toughness and weldability of the steel.
[0015]
Based on the above findings, the present invention has been completed by clarifying the range limits of the effective content of each element and the range of the manufacturing conditions. The gist of the present invention is as follows.
[0016]
(1) By weight%, C: 0.05 to 0.14%, Si: 0.01 to 0.5%, Mn: 0.5 to 1.5%, P: 0.015% or less, S: 0.005% or less, Cu: 0.30 to 0.50%, Ni: 0.15 0.50%, Cr: 0.30 to 0.60%, Mo: 0.05 to 0.4%, Nb: 0.015 to 0.045%, Ti: 0 to 0.030 % , Al: 0.005 to 0.10%, N: 0.001 to 0.007%, and B: (a) The range shown by the formula is contained, the balance is composed of Fe and unavoidable impurities, the value of Pcm defined by the following formula (b) is 0.20 or less, and the toughness with a tensile strength of 570 MPa or more is good. High-strength steel sheet with a thickness of more than 75 mm that is excellent in weldability in cold regions.
[0017]
0.0003 ≦ B (%) ≦ 0.0005 + (Cu (%) / 1000) + (Ni (%) / 500) + (Mo (%) / 500 )
Pcm = C (%) + (Si (%) / 30) + (Mn (%) / 20) + (Cu (%) / 20) + (Ni (%) / 60) + (Cr (%) / 20 ) + (Mo (%) / 15) + (V (%) / 10) + 5B (%) (b)
(2) After heating the material of the above composition to a temperature of 1050 to 1250 ° C, rolling, finishing to a predetermined thickness at 800 to 900 ° C and directly quenching, and further tempering at a temperature of 350 to 550 ° C The method for producing a high-tensile steel plate according to (1) above, wherein
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
The reasons for limiting the chemical composition of the steel sheet of the present invention or the manufacturing conditions will be described below.
The content of C is set to 0.05 to 0.14%. C is the most important element for determining the strength of steel. If the content is less than 0.05%, it becomes difficult to obtain the required strength of 570 MPa or more. If the content exceeds 0.14%, the toughness and weldability of the steel sheet will be lower than the targets. Therefore, the content range is set to 0.05 to 0.14%. However, in order to obtain a balanced performance with sufficiently high strength and excellent toughness and weldability, 0.06 to 0.10% It is desirable to be within the range.
[0019]
The content of Si has an effect of improving the strength of the steel without deteriorating the toughness, and the content is set to 0.01 to 0.5%. This is because a content of less than 0.01% has no effect, and a content of more than 0.5% deteriorates the surface properties of the steel sheet.
[0020]
Mn has a content of 0.5 to 1.4%. Mn is important together with C in order to secure the strength of the steel. If it is less than 0.5%, the strength of the steel is insufficient, and if it exceeds 1.4%, the toughness deteriorates. The preferred range for sufficiently high strength and good toughness is 1.0 to 1.4%.
[0021]
P is one of the unavoidable impurities of steel, which deteriorates the toughness of the steel and deteriorates the weldability. The content is set to 0.015% or less as a range that does not have a significant adverse effect.
[0022]
S is also one of the unavoidable impurities and remarkably deteriorates the toughness of steel, so the smaller the content, the better. The content is set to 0.005% or less as a range that does not have a significant adverse effect.
[0023]
The content of Cu is set to 0.30 to 0.50%. Since Cu can increase the strength of steel by improving hardenability without significantly impairing weldability and toughness, Cu is actively used. Therefore, the content is at least 0.30% or more. However, if the content is too large, the toughness is affected. Therefore, the upper limit is set to 0.50%.
[0024]
Ni can increase the strength without impairing toughness and weldability. However, since it is expensive and requires a large amount of addition to obtain the effect, it is an element whose use is desired to be suppressed. However, when Cu is added to steel, it is necessary to contain Cu because it has an effect of suppressing hot brittleness due to Cu. For this purpose, the content of Ni is required to be か ら to about equal to or more than the amount of Cu. Therefore, the content of Ni is set to 0.15 to 0.50%.
[0025]
The content of Cr is set to 0.30 to 0.60%. Since Cr can increase the strength of steel without impairing weldability and toughness like Cu, it is actively used. In order to exert its effect sufficiently, the content is required to be at least 0.30% or more, but as the content increases, the toughness deteriorates. Therefore, the content is at most 0.60%. Desirable is 0.40 to 0.60%.
[0026]
The addition of Mo has the effect of improving the strength of the steel and mitigating the decrease in toughness due to the addition of B, and its content is made 0.05 to 0.4%. If the content is less than 0.05%, the effect is not sufficient, and if the content exceeds 0.4%, the toughness of the steel may be reduced. Preferred is a content of 0.05 to 0.15%.
[0027]
B is a useful element that can improve the strength without deteriorating the weldability because the hardenability is improved with a small content without increasing the carbon equivalent. In order to exert its effect sufficiently, the content should be 0.0003% or more. However, even if it is contained more, the effect is saturated and does not change, but the toughness tends to deteriorate. This tendency of toughness degradation is mitigated by the presence of Cu, Ni and Mo. Therefore, the content range of B is
0.0003 ≦ B (%) ≦ 0.0005 + (Cu (%) / 1000) + (Ni (%) / 500) + (Mo (%) / 500 )
And
[0028]
The content of Nb is 0.015 to 0.045%. The content of Nb makes the crystal grains of steel finer, and can improve strength without deteriorating toughness. In order to sufficiently obtain such an effect, the content needs to be 0.015% or more. However, too much addition results in deterioration of toughness and weldability, so that the addition is at most 0.045%. It is desirable to manage it within a narrow range of 0.025 to 0.035%.
[0029]
Ti may not be added. However, it has the effect of making the crystal grains fine, fixing N, and sufficiently exhibiting the effect of adding B, so that it is added as necessary. When it is added, its content is desirably 0.005 to 0.030%. This is because if it is less than 0.005%, the effect of the addition is not sufficient, and if it exceeds 0.030%, the toughness of the steel deteriorates.
[0030]
Al is added for deoxidation of steel in the steelmaking stage, and its content range is 0.005 to 0.10%. If the content is less than 0.005%, defects may remain in the slab. If the content is too large, the effect is saturated and the toughness of the steel deteriorates. Therefore, the content is limited to 0.10%.
[0031]
N is one of the unavoidable impurities of steel, but its presence in a small amount combines with Al, Nb or Ti to form fine precipitates, and has an effect of particularly suppressing the coarsening of the crystal at the weld. For this purpose, the content is preferably at least 0.001% or more. However, since the content exceeding 0.007% deteriorates the toughness of steel, the content of N is set to 0.001 to 0.007%.
[0032]
The amount of each component is controlled so that the value of the crack susceptibility Pcm of the welding represented by the above formula (b) becomes 0.20 or less. If the value of Pcm exceeds 0.20, cracks occur at a steel sheet temperature of 0 ° C.
[0033]
When the heat treatment of the steel sheet is performed by the direct quenching method, the treatment is performed under the following conditions. The heating temperature of the raw steel to be hot-rolled is 1050 to 1250 ° C. This is because if the temperature is lower than 1050 ° C., the effect of adding Nb to obtain a fine grain cannot be sufficiently obtained. This is presumably because Nb is not sufficiently dissolved before hot rolling. Further, even if the heating temperature exceeds 1250 ° C., no effect is recognized on the property improvement of the obtained steel sheet, and only the loss of the oxide scale and the heating fuel is increased. The preferred heating temperature is 1050 to 1160 ° C.
[0034]
In the hot rolling, the finishing temperature for finishing to a predetermined thickness is 800 to 900 ° C. If the temperature is lower than 800 ° C., the transformation temperature will be much lower, and sufficient quenching strength cannot be obtained. If the temperature exceeds 900 ° C., the toughness of the steel sheet may decrease. Preferred is 830-880C. Desirably, by applying a cumulative reduction of 50% or more at 900 ° C. or less, further refinement of grain size and improvement of toughness can be obtained.
[0035]
After the completion of hot rolling, forced cooling is performed as quickly as possible. That is, quenching is performed. In order to obtain a sufficient quenching strength, the forced cooling is performed such that a cooling medium such as water is uniformly applied to the entire surface of the steel sheet, and a temperature of 1 ° C./s or more, preferably 2 ° C./s or more is obtained at the center of the sheet thickness. It is better to do it at a cooling rate. The start of the forced cooling is desirably started at a temperature of 750 ° C. or more on the surface of the steel sheet even if it is slightly delayed.
[0036]
Tempering is performed at a temperature of 350 to 550 ° C. If the temperature is lower than 350 ° C., the toughness becomes insufficient. When tempering at a temperature exceeding 550 ° C., it is possible to secure strength and toughness by adding a large amount of alloy components and the like, but it becomes difficult to sufficiently reduce Pcm. If the quenching is insufficient, it is possible to secure the strength by lowering the tempering temperature, but this will result in steel with poor toughness. Therefore, in the above component range, when Pcm is 0.2 or less, sufficient quenching is performed from the required thickness and forced cooling rate, and the tensile strength becomes 570 MPa or more at a tempering temperature of 350 to 550 ° C. Thus, by selecting the individual steel components and their amounts, a high-tensile steel plate with excellent toughness and weldability can be obtained.
[0037]
【Example】
A steel having a composition shown in Table 1 was forged into a rolled material having a thickness of 300 mm, and hot rolling was performed as a heating temperature, a finishing temperature, and a sheet thickness shown in Table 2, and quenching was performed immediately after the finish rolling. Tempering was performed at the temperatures shown in Table 2. From the central part of the thickness of the obtained steel sheet, a test piece was sampled in a direction perpendicular to the rolling, and a tensile test was performed. A test piece was sampled in a direction parallel to the rolling, and a Charpy impact test was performed. Further, based on the oblique Y-shaped weld cracking test method according to JIS Z-3158, a welding test was performed at a test piece temperature of 0 ° C., and the presence or absence of cracks in the welded portion was investigated.
[0038]
[Table 1]
[0039]
[Table 2]
[0040]
Table 2 also shows the evaluation test results of the steel sheets. In Table 1, steel symbols A to F satisfy the composition range of the present invention, but steel symbols G to S have any composition or Pcm value outside the range regulated by the present invention. . In Table 2, if the criteria for the performance evaluation were as follows: tensile strength of 570 MPa or more, toughness of vTrs of -5 ° C. or less, and weldability of the test piece at 0 ° C. without cracking, The effectiveness is clear.
[0041]
Test results of steels whose composition satisfies the range of the present invention are shown in Test Nos. 1 to 16 in Table 2. However, if the production conditions after hot rolling are insufficient, the target performance may not be obtained. For example, Test No. 3 has too low finishing temperature or quenching temperature, Test No. 5 has too low material heating temperature, and Test No. 13 has too high tempering temperature, and in each case, sufficient strength is obtained. Not. Test No. 4 has too high finishing temperature, Test No. 8 has too high heating temperature, and Test No. 9 has too low tempering temperature. In each case, the strength is sufficient but the toughness is inferior. However, when the manufacturing conditions are appropriate, such a steel having a limited steel composition can obtain excellent strength, toughness, and weldability.
[0042]
On the other hand, even if the manufacturing conditions are within the range that should be sufficiently satisfied, when the chemical composition of the steel numbers G to S is out of the range regulated by the present invention, the results of the test numbers 17 to 29 in Table 2 are obtained. As is clear from the above, the steel sheet does not exceed the target properties in any of the strength, toughness, or weldability.
[0043]
【The invention's effect】
According to the present invention, the tensile strength at the central portion of the sheet thickness is 570 MPa or more, the toughness is vTrs of -5 ° C or less, the weldability is 0 ° C, the steel sheet temperature is 0 ° C, no crack occurs, and the high strength exceeds 75 mm in the sheet thickness. A thick steel plate is obtained, and the steel plate can be easily manufactured by a direct quenching method. This steel sheet can be used effectively to ease the construction conditions of steel structures such as bridges and to reduce the construction cost.
Claims (2)
0.0003≦B(%)≦0.0005+(Cu(%)/1000)+(Ni(%)/500) +(Mo(%)/500) ・・・・・ (a)
Pcm=C(%)+(Si(%)/30)+(Mn(%)/20)+(Cu(%)/20) +(Ni(%)/60)+(Cr(%)/20)+(Mo(%)/15) +(V(%)/10)+5B(%) ・・・・・ (b) By weight%, C: 0.05 to 0.14%, Si: 0.01 to 0.5%, Mn: 0.5 to 1.5%, P: 0.015% or less, S: 0.005% or less, Cu: 0.30 to 0.50%, Ni: 0.15 to 0.50% , Cr: 0.30 to 0.60%, Mo: 0.05 to 0.4%, Nb: 0.015 to 0.045%, Ti: 0 to 0.030 % , Al: 0.005 to 0.10%, N: 0.001 to 0.007%, and B: (a) The range shown by the formula is contained, the balance consists of Fe and unavoidable impurities, the value of Pcm defined by the following formula (b) is 0.20 or less, the toughness of tensile strength 570 MPa or more is good, High-strength steel plate exceeding 75mm in thickness with excellent weldability.
0.0003 ≦ B (%) ≦ 0.0005 + (Cu (%) / 1000) + (Ni (%) / 500) + (Mo (%) / 500 )
Pcm = C (%) + (Si (%) / 30) + (Mn (%) / 20) + (Cu (%) / 20) + (Ni (%) / 60) + (Cr (%) / 20 ) + (Mo (%) / 15) + (V (%) / 10) + 5B (%) (b)
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CN114182165A (en) * | 2021-10-29 | 2022-03-15 | 南京钢铁股份有限公司 | Low-yield-ratio high-toughness Q500qE bridge steel and production method thereof |
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CN115404400A (en) * | 2022-04-22 | 2022-11-29 | 安阳钢铁股份有限公司 | Preparation method of Q960 steel plate for welding coal mine hydraulic support with heat input of 17KJcm |
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CN110578089A (en) * | 2019-09-20 | 2019-12-17 | 舞阳钢铁有限责任公司 | high-strength quenched and tempered steel plate for ocean engineering and production method thereof |
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