JP3680796B2 - Cr-containing corrosion resistant steel for construction and civil engineering structures - Google Patents
Cr-containing corrosion resistant steel for construction and civil engineering structures Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は、溶接構造用部材としてのCr含有鋼、特に構造物の完成後に人目に触れず、しかも外壁材のような厳しい環境に曝されない用途での、建築・土木構造用部材として好適なCr含有耐腐食鋼に関するものである。
【0002】
【従来の技術】
建築・土木構造用鋼材としては、従来、主に SS400等の普通鋼、 SM490等の高張力鋼ならびにこれらの鋼材に塗装やめっきを施した材料が使用されてきた。
しかしながら、近年の建物の大型化や設計の多様化に伴い、各種の鋼材や材料の利用が検討され始めている。
特に環境問題への関心が高まる中、ライフサイクルコスト(LCC)を重視した材料の選定が検討されるようになってきており、例えば住宅に関しては 100年以上の寿命を前提とした設計が求められつつある。
【0003】
構造物の長寿命化を考えた場合、めっき鋼板のめっき厚を厚くする方法が考えられるが、溶接を必要とする建築構造物の場合には、溶接後の溶接部の処理に多大な負荷がかかるため、実用には適さないという問題がある。
このような中、耐食性に優れ、発銹に対する保守費用がほとんど必要なく、またリサイクルも容易であるFe−Cr系合金の、建築・土木構造用材料への適用が大いに期待されている。
【0004】
Cr含有鋼の代表であるステンレス鋼は、金属組織の違いから、SUS430に代表されるフェライト系ステンレス鋼、SUS304に代表されるオーステナイト系ステンレス鋼、SUS410に代表されるマルテンサイト系ステンレス鋼およびSUS329に代表される2相ステンレス鋼に大別される。
このような各種ステンレス鋼の中で、オーステナイト系ステンレス鋼は、材料強度、耐食性、溶接性、溶接部じん性および汎用性の点で優れるため、従来から建築・土木構造用材料としての適用が試みられてきた。
【0005】
しかしながら、オーステナイト系ステンレス鋼には、
(1) Ni,Cr等の合金元素を多量に含有しているため、普通鋼に比べると格段に高価である、
(2) 応力腐食割れ感受性が高い、
(3) 普通鋼に比べると熱膨張率が大きく、また熱伝導度が小さいため、溶接時の熱影響に起因した歪みが蓄積し易いことから、精度を要求される部材等への適用が難しい、
といった問題があり、従来、普通鋼や普通鋼に塗装あるいはめっきを施した材料が使用されていた汎用構造材への適用は難しく、適用範囲が制限されるという問題があった。
【0006】
このため、最近では、めっきや塗装を施した普通鋼の代替として、Cr含有量の少ない低Cr含有鋼の建築・土木用材料への適用が検討されており、特にマルテンサイト系ステンレス鋼の建築・土木用材料への適用が考えられている。
マルテンサイト系ステンレス鋼は、上述したような高価なNiを多量に含有するオーステナイト系ステンレス鋼に比べると格段に安価であり、また熱膨張率が小さくかつ熱伝導率が大きいことに加え、普通鋼に比べると著しく耐食性に優れ、しかも高い強度を有するという特徴がある。
また、マルテンサイト系ステンレス鋼では、高Cr含有鋼で問題となるσ脆化や475 ℃脆化等の心配がなく、さらにオーステナイト系ステンレス鋼で問題となる塩化物環境下での応力腐食割れのおそれもないという利点がある。
【0007】
しかしながら、SUS410鋼に代表されるマルテンサイト系ステンレス鋼は、C含有量が 0.1mass%程度と高いために、溶接部じん性や溶接部の加工性に劣り、しかも溶接に際しては予熱を必要とし溶接作業性にも劣ることから、溶接が必要な部材に対する適用には問題を残していた。
【0008】
上記の問題に対し、例えば特公昭51−13463 号公報では、Cr:10〜18mass%,Ni:0.1 〜3.4 mass%,Si:1.0 mass%以下およびMn:4.0 mass%以下を含み、さらにC:0.03mass%以下、N:0.02mass%以下に低減し、溶接熱影響部にマッシブマルテンサイト組織を生成させることによって、溶接部の性能を向上させた溶接構造用マルテンサイト系ステンレス鋼を提案している。
また、特公昭57−28738 号公報には、Cr:10〜13.5mass%,Si:0.5 mass%以下およびMn:1.0 〜3.5 mass%を含有すると共に、C:0.02mass%以下、N:0.02mass%以下に低減し、さらにNiを 0.1mass%未満に低減することによって、溶接前後における予熱、後熱を必要としない、溶接部じん性および加工性に優れた構造用マルテンサイト系ステンレス鋼が提案されている。
【0009】
ところで、構造用鋼を耐食性の観点から考えた場合には、Cr含有量は高い方が望ましい。しかしながら、実際に使用される構造用鋼の多くは、全く発銹がないような高耐食性を必ずしも必要とはしていない場合が多く、特に構造物の完成後に人目に触れず、しかも外壁材のような厳しい環境に曝されない部材では、完成後の長期間の使用に際して錆汁が垂れ出してこない程度の耐食性があれば充分であり、既存のステンレス鋼ほどの高い耐食性を必要としない。
また、建築・土木用構造材料として用いる場合、表面性状に対する要求が低いので、熱間圧延まま、あるいは熱間圧延後脱スケール処理を施したままでの使用が可能であることが経済的観点から望ましい。
このような要望に対し、Cr含有量を10mass%未満に低減し、しかも熱間圧延ままあるいは熱間圧延後脱スケールを施したままの状態での使用を前提として、コストを抑えた安価なCr含有鋼の開発が進められている。
【0010】
例えば特許第 3039630号公報には、Cr:6〜18mass%,Si:0.05〜1.5 mass%,Mn:0.05〜1.5 mass%を含有し、またC:0.005 〜0.1 mass%とし、さらに熱間圧延での仕上温度を 780℃以下とすることにより、酸化スケール直下に5μm以上のCr欠乏層を生成させることで局部腐食の発生を抑えた、建造構造部材用低腐食速度鋼が提案されている。
【0011】
また、特開平11−323505号公報には、Cr:5〜10mass%,Si:0.05〜1.0mass%,Mn:0.05〜2.0 mass%を含有し、かつC:0.005 〜0.03mass%、N:0.005〜0.03mass%に低減した鋼において、金属部最表層から 0.5〜10μm の深さのCr量を5mass%未満にすることにより、均一型の全面腐食を生じさせることで、強度低下や破壊を引き起こすような局部的かつ急激な肉厚の減少を抑え、腐食に伴う強度低下を抑制した鋼が提案されている。
【0012】
しかしながら、上記した特許第 3039630号公報および特開平11−323505号公報に開示の技術によっても、Cr含有量が10mass%未満の低Cr含有鋼の耐食性は十分とはいい難く、耐長期間腐食性の一層の改善が望まれていた。
しかも、特開平11−323505号公報に開示の技術は、クラッド法や溶射、めっきといった工程を前提としており、実用化に向けてコスト面で大きな問題が残されていた。
【0013】
また、発明者らは、先に、Ni, Cu, Cr, Moなどの元素を極端に増量することなく、またNb, Tiの添加、さらにはC, Nの過度の低減を必要とすることなしに、溶接性や耐初期発錆性に優れたFe−Cr合金を開発し、特願2001−148701号明細書において開示した。
具体的には、Crを8mass%超,15mass%未満の範囲で含有するFe−Cr合金について、特にCo,VおよびWを、それぞれCo:0.01mass%以上、0.5 mass%未満、V:0.01mass%以上、0.5 mass%未満およびW:0.001 mass%以上、0.05mass%未満の範囲で含有させると共に、次式(2) で示されるX値および好ましくは次式(3) で示されるZ値がそれぞれ、X値≦11.0、0.03≦Z値≦1.5 の範囲を満足するように成分調整するものである。
X値=Cr+Mo+1.5 Si+0.5 Nb+0.2 V+0.3 W+8Al
−Ni−0.6 Co−0.5 Mn−30C−30N−0.5 Cu --- (2)
Z値=(Co+1.5 V+4.8 W) --- (3)
また、さらに好ましくは、CとNの比(C/N)が0.60以下となるよう成分調整するものである。
【0014】
しかしながら、上記の技術は、Cr含有量が多く経済的に不利なだけでなく、特にCrを11mass%以上含むような場合には、軟質化を目的とした焼鈍が必要となるため、さらにコスト面で不利となる問題を残していた。
さらに、Crの含有量が多いと、長時間の使用に伴う腐食減量は少なくなるものの、局部腐食が起こり易くなり、強度低下の面ではむしろ全面腐食を起こす場合よりも不利となるところに問題を残していた。
【0015】
【発明が解決しようとする課題】
本発明は、上記の問題を有利に解決するもので、外観上の美麗さが問題とならないような溶接構造用材料に関し、100 年以上の寿命を満足するのに十分な耐食性を有し、熱間圧延ままあるいは熱間圧延後脱スケール処理を施したままでの使用を前提とし、特に構造物の完成後に人目に触れず、しかも外壁材のような厳しい環境にさらされない用途において、建築・土木構造用部材として好適な鋼材を、10mass%未満の低Cr含有量により実現し、同鋼材を安価に提供することを目的とする。
【0016】
本発明鋼は、熱間圧延ままで実質的にフェライト単相からなる組織を有し、引張強さ(TS)が 400〜550 MPa 級の強度を有し、特に建築・土木構造用部材としての使用に当たり、100 年以上の使用においても、腐食に伴う強度低下が使用前の10%以下、特に好適には5%以下という性能を有している。
また、本発明鋼では、溶接に際し、溶接部のじん性劣化の原因となる熱影響部における粗大粒の生成を、該熱影響部での組織を実質的にマルテンサイト組織とすることによって抑制し、良好な溶接部じん性を確保している。
さらに、本発明鋼は、溶接・加工によって鋼管あるいは形鋼に成形したのち、構造用部材として用いることもできる。
【0017】
【課題を解決するための手段】
さて、発明者らは、上記の目的を達成するために、各種元素の影響について綿密な検討を行った。特に、Co,V,Wに注目して、Cr含有量が10mass%未満の低Cr含有鋼において、耐発銹性に及ぼすこれらの元素の影響について調査した。
その結果、適量のCoを添加することによって溶接部じん性が格段に改善されること、またこれらの3元素を複合して適量添加することによって、Ni,Cu,Cr,Moなどの元素を極端に増量することや、Nb,Tiの添加あるいはC,Nの低減といったコストアップの要因を増やすことなしに、耐長期間耐食性を効果的に改善できるとの知見を得た。
本発明は、上記の知見に立脚して完成されたものである。
【0018】
すなわち、本発明の要旨構成は次のとおりである。
1.C:0.0015〜0.02mass%、 N:0.0015〜0.02mass%、
Si:0.1 〜1.0 mass%、 Mn:0.1 〜3.0 mass%、
Cr:5mass%超,10mass%未満、 Ni:0.01〜3.0 mass%、
Al:0.1 mass%以下、 P:0.05mass%以下および
S:0.03mass%以下
を含み、さらに
Co:0.010 〜1.0 mass%
を含有し、残部はFeおよび不可避的不純物の組成になり、耐長期間腐食性および溶接部じん性に優れることを特徴とする建築・土木構造用のCr含有耐腐食鋼。
【0019】
2.上記1において、Cr含有量が5mass%超,8mass%未満で、さらにVおよびWを、それぞれV:0.01〜0.5 mass%、W:0.001 〜0.05mass%の範囲で、かつ下記 (1)式で表されるZ値が0.03≦Z≦1.5 を満足する範囲において含有する組成になり、耐長期間腐食性および溶接部じん性に優れることを特徴とする建築・土木構造用のCr含有耐腐食鋼。
記
Z値=(〔%Co〕+ 1.5〔%V〕+ 4.8〔%W〕) ----(1)
ここで、〔%Co〕,〔%V〕,〔%W〕は各元素の含有量(mass%)
【0020】
3.上記2において、CrおよびW含有量がそれぞれ、Cr:5mass%超,7.5 mass%未満、W:0.005 〜0.03mass%である、耐長期間腐食性および溶接部じん性に優れることを特徴とする建築・土木構造用のCr含有耐腐食鋼。
【0021】
4.上記1〜3のいずれかにおいて、鋼が、さらに
Cu:3.0 mass%以下およびMo:3.0 mass%以下
のうちから選んだ1種または2種を含有する組成になり、耐長期間腐食性および溶接部じん性に優れることを特徴とする建築・土木構造用のCr含有耐腐食鋼。
【0022】
5.上記1〜4のいずれかにおいて、鋼が、さらに
B:0.0002〜0.0030mass%
を含有する組成になり、耐長期間腐食性および溶接部じん性に優れることを特徴とする建築・土木構造用のCr含有耐腐食鋼。
【0023】
【発明の実施の形態】
以下、本発明を由来するに至った実験結果について説明する。
まず、低Cr含有鋼へのCo添加効果について述べる。
図1に、7mass%Cr鋼に対して、Coを添加した場合の溶接部(熱影響部)じん性の変化について調べた結果を示す。
ここで、溶接部じん性は、板厚:5.5 mmの熱延板に溶接方向が鋼板の圧延方向に垂直な方向となるようにI開先を作製し、1.2 mmφの Y309Lタイプの溶接ワイヤを用い、半自動MAG溶接機により溶接継手を作製し、この溶接継手から、図2に示すように、Vノッチ先端位置が止端部から1mm溶接金属側の位置となるように、2mmVノッチサブサイズシャルピー試験片(JIS Z 2202)を採取し、−50℃における吸収エネルギー(vE-50)を測定することにより評価した。なお、Vノッチ先端位置における溶接金属部と母材部との比率a:bはおよそ1:4であった。
同図から明らかなように、Coを0.01mass%以上添加することによって、溶接部じん性が 150 J/cm2以上改善される。特にその効果は、0.03mass%以上の添加で200 J/cm2 以上と著しい。
【0024】
次に、Co,V,Wの複合添加の効果について説明する。
図3に、同じく7mass%Cr鋼板に対し、これら3元素を複合添加した場合における耐長期間腐食試験による熱延板の強度低下状況について調べた結果を、Z値(3元素の影響を示すパラメータ)との関係で示す。
ここで、Z値とは、次式(1)
Z値=(〔%Co〕+ 1.5〔%V〕+ 4.8〔%W〕) ----(1)
ここで、〔%Co〕,〔%V〕,〔%W〕は各元素の含有量(mass%)
で示される耐長期間腐食性の指標となる値である。
また、強度低下状況は、板厚:4mmの熱延板に対し、塩水噴霧(0.1mass%NaCl,35℃,3h)→乾燥(60℃,3h )→湿潤(50℃,2h)を1サイクルとする腐食試験を 300サイクル行った後に、腐食試験前後での強度(最大引張荷重)低下によって評価した。
なお、図3には、比較のため、Co,V,Wの単独添加または2種添加を行った場合の調査結果についても併せて示す。
同図に示したとおり、Z値が0.03以上になると長期間腐食に伴う強度低下が5%以下に激減し、耐長期間腐食性が著しく改善されることが分かる。しかも、3元素を複合添加していない場合に比べて強度低下が小さい。
【0025】
次に、本発明において、鋼材の成分組成を上記範囲に限定した理由について説明する。
C:0.0015〜0.02mass%,N:0.0015〜0.02mass%
CおよびNは、溶接熱影響部の加工性の改善ならびに溶接割れ防止の観点からは、可能な限り低減するのが好ましい。また、過度に添加すると熱間圧延ままでの強度が高くなりすぎ、目標とする強度が得られない。さらに、C,Nは、溶接熱影響部のマルテンサイト相の硬さに大きな影響を及ぼすばかりでなく、炭窒化物の析出に伴うCr欠乏層の形成を助長し、耐食性を劣化させる原因となる。このためC,Nの上限はそれぞれ0.02mass%とする必要がある。一方、C,N量の過度の低減は、精錬コストの増大を招くばかりでなく、熱間圧延ままでの強度が低下し、目標とする強度が得られなくなる。さらに、溶接熱影響部でのマルテンサイト生成能を低下させ、粗大フェライト粒の生成を助長し、溶接熱影響部のじん性を劣化させる。このためC,Nの下限はそれぞれ0.0015mass%とした。より好ましい組成範囲は、C,Nとも0.0020〜0.010 mass%である。
【0026】
Si:0.1 〜1.0 mass%
Siは、脱酸剤として有用な元素であるが、含有量が 0.1mass%未満では十分な脱酸効果が得られず、一方 1.0mass%を超える添加はじん性や加工性の低下を招くだけでなく、溶接熱影響部でのマルテンサイト生成能を低下させるので、Si量は 0.1〜1.0 mass%の範囲に限定した。より好ましい組成範囲は 0.1〜0.5 mass%である。
【0027】
Mn:0.1 〜3.0 mass%
Mnは、オーステナイト安定化元素であり、溶接熱影響部のマルテンサイト生成能を増加させ、じん性を改善する効果を有するだけでなく、Siと同様、脱酸剤としての働きをもつ。しかしながら、含有量が 0.1mass%未満ではその添加効果に乏しく、一方 3.0mass%を超えて添加すると加工性の低下やMnSの形成に伴う耐食性の低下を招くため、Mn量は 0.1〜3.0 mass%の範囲に限定した。より好ましい組成範囲は 0.1〜1.5 mass%である。
【0028】
Cr:5mass%超, 10mass%未満
Crは、耐食性を向上させる有用元素である。本発明では、外壁材のような厳しい環境下での使用は想定していないが、構造物の完成後に人目に触れず、よりマイルドな環境下での使用においても、長期間の使用に際して錆汁が垂れてこないようにする必要がある。
このための耐食性を確保するには、5mass%超の添加が必要である。一方、本発明に関わる安価なCr含有鋼においては、10mass%以上のCr添加はコスト増加を招く不利がある。従ってCr量は5mass%超, 10mass%未満の範囲に限定した。
なお、Co,V,Wを複合して添加した場合は、複合添加による局部腐食の発生抑制効果が十分に発揮されるように、Cr量を5mass%超, 8mass%未満とするのが好適である。より好適な成分範囲は、Cr量が5mass%超,7.5 mass%未満で、W量が 0.005〜0.03mass%である。このような成分範囲に調整することにより、局部腐食の発生が効果的に抑制され、長期間の使用に伴う強度低下をより低く抑えることが可能となる。
【0029】
Ni:0.01〜3.0 mass%
Niは、延性、じん性を向上させる元素であり、本発明では特に溶接部のじん性を向上させるために添加する。しかしながら、含有量が0.01mass%に満たないとその添加効果に乏しく、一方 3.0mass%を超えて添加しても効果は飽和に達し、むしろ素材が硬質化して加工性が劣化するため、Ni量は0.01〜3.0 mass%の範囲に限定した。
【0030】
Al:0.1 mass%以下
Alは、脱酸剤として作用する元素であるが、多量に含有すると酸化物系介在物が増加し、製鋼段階でのノズル詰まり等の原因となったり、へげ等の表面欠陥の原因となり耐食性の低下を招く。このためAl量は 0.1mass%以下に限定した。
【0031】
P:0.05mass%以下
Pは、熱間加工の際に割れを誘発し、また耐食性に対しても有害な元素であるが、含有量が0.05mass%までならその悪影響が顕著とならず許容できるので、P量は0.05mass%に制限した。より好ましくは0.03mass%以下である。
【0032】
S:0.03mass%以下
Sは、硫化物を形成し、鋼の清浄度を低下させるだけでなく、MnSを形成して発銹の起点となる。また、Sは、結晶粒界に偏析し粒界脆化を惹起する有害な元素でもあるので、できるだけ低減するのが好ましい。しかしながら、0.03mass%以下であれば、その悪影響が顕著とならず許容できる。
【0033】
Co:0.010 〜1.0 mass%
Coは、本発明の骨子となる元素であり、10mass%未満の低Cr含有鋼に対し、微量添加することで、溶接部じん性が著しく改善される。また、Coを添加しない場合に比べ、耐長期間腐食性も改善される。しかしながら、含有量が 0.010mass%未満ではその効果が得られず、一方 1.0mass%を超えて添加すると素材が硬質化し加工性が劣化するので、Co量は 0.010〜1.0 mass%の範囲に限定した。より好ましい添加範囲は 0.030〜1.0 mass%である。
【0034】
Co添加による溶接部じん性の改善効果は、Co添加に伴うオーステナイト生成能の増加によって溶接熱影響部にマルテンサイト相が形成し易くなり、しかもその硬化能がC,N等を添加した場合に比べて小さいことによるものと考えられる。また、Co添加によって耐長期間腐食性が改善される機構は明らかではないが、長期間腐食において、最も強度低下の原因となる局部的かつ急激な腐食に対し、鋼板表面あるいはスケール中に濃化したCoが有効に働き、被腐食面全体が均一に腐食されるようになるためと考えられる。
【0035】
以上、必須成分および抑制成分について説明したが、本発明では、その他にも以下の元素を適宜含有させることができる。
V:0.01〜0.5 mass%、W:0.001 〜0.05mass%で、かつ
Z値(〔%Co〕+ 1.5〔%V〕+ 4.8〔%W〕)=0.03〜1.5
Co,VおよびWは、本発明において特に重要な元素である。これまで、溶接熱影響部の溶接割れ感受性を改善するためには、PCM{=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B}あるいはNi当量、Cr当量といった値の適正化が検討されてきた。このため、溶接熱影響部の特性改善に加え、耐腐食性および延性・加工性の改善を図るために、これらのパラメータに大きく影響するCr,Mo,Niや、C,N,Nb,Tiといった元素に注目した検討が行われてきた。しかしながら、CoおよびWについては、耐食性やフェライト相、オーステナイト相の安定性に影響を与えるにもかかわらず、PCMあるいはNi当量、Cr当量といったパラメータに及ぼす影響や、熱延ままあるいは熱延後脱スケールままの鋼板の耐長期間腐食性に及ぼす影響について、詳細な検討が行われていなかった。
本発明では、熱延ままあるいは熱延後脱スケールままの鋼板の耐長期間腐食性に及ぼすCo,V,Wの影響、特にこれらを複合して添加した場合の効果を定量的に評価し、これら元素の適正な添加範囲および適正比率を明らかにした。
【0036】
これら3元素の効果の指標となるZ値は、耐長期間腐食性の指標となる値で、先に述べたように、このZ値が0.03以上となるようにCo,V,Wを複合添加することによって所望の効果が得られる。
ここに、これら3元素の複合添加によって耐長期間腐食性が改善される機構は明らかではないが、長期間腐食において、最も強度低下の原因となる局部的かつ急激な腐食に対して、鋼板表面あるいはスケール中に濃化したCo,V,Wが有効に働き、被腐食面全体が均一に腐食されるようになったためと考えられる。
一方、Z値が 1.5を超えるような添加を行った場合、耐長期間腐食性の効果は飽和する上、硬質化により加工性がかえって低下する。
このため、Z値は0.03〜1.5 の範囲に限定した。より好ましくは0.05〜1.0 の範囲である。
【0037】
また、V,Wの含有量はそれぞれ、V:0.01〜0.5 mass%,W:0.001 〜0.05mass%に限定する必要がある。というのは、上記したZ値が適正範囲 (0.03≦Z≦1.5)を満足していても、各々の含有量が下限値を下回ると複合添加による効果が得られず、一方Vの添加量が 0.5mass%、またWの添加量が0.05mass%を超えると、炭化物の析出が著しくなり、母材および溶接熱影響部のじん性が著しく低下するからである。より好ましい添加量は、V:0.05〜0.3 mass%、W:0.005〜0.03mass%である。
【0038】
このように、本発明による低Cr含有鋼へのCo添加による溶接熱影響部のじん性改善効果、およびCo,V,Wの複合添加による耐長期間腐食性向上効果を利用することにより、高価なNi,Cu,Cr,Moなどの元素を極端に増量することや、Nb,Tiの添加あるいはC,Nの低減といったコストアップの要因を増やすことなしに、溶接部じん性と、熱延ままあるいは熱延後脱スケールままの状態での耐長期間腐食性との両立が可能となったのである。
【0039】
Cu:3.0 mass%以下
Cuは、耐食性を向上させる元素であり、高い耐食性を必要とする場合に添加することが有効である。しかしながら、3.0 mass%を超えて添加すると、熱間圧延等における熱間割れのおそれが生じるため、Cuは 3.0mass%以下で含有させるものとした。なお、より好ましくは効果が顕著となる 0.1mass%を下限とし、1.0mass%以下で含有させることが望ましい。
【0040】
Mo:3.0 mass%以下
Moは、Cu同様、耐食性の改善に有効な元素である。しかしながら、3.0 mass%を超えて添加すると、加工性が低下するだけでなく、オーステナイト相の安定性が低下し、特に溶接熱影響部のじん性が低下する。このため、Moは 3.0mass%以下で含有させるものとした。なお、加工性と耐食性の両立という観点からは 0.1〜1.0 mass%の範囲が好適である。
【0041】
B:0.0002〜0.0030mass%
Bは、焼入れ性の向上を通じて特に溶接熱影響部のじん性改善に効果がある。しかしながら、含有量が0.0002mass%未満ではその効果に乏しく、一方0.0030mass%を超える添加では、硬化が大きくなり、母材、溶接熱影響部とも、じん性および加工性が損なわれる。
このため、Bは0.0002〜0.0030mass%の範囲で含有させるものとした。なお、より好ましい添加範囲は0.0005〜0.0010mass%である。
【0042】
次に、本発明鋼の好適製造方法について説明する。
まず、上記の好適成分組成に調整した溶鋼を、転炉または電気炉等の通常の溶製法にて溶製したのち、真空脱ガス(RH法)、VOD法、AOD法等の公知の精練方法で精錬し、ついで連続鋳造あるいは造塊−分塊法でスラブ等に鋳造して鋼素材とする。
ついで、鋼素材は、加熱され、熱間圧延工程により鋼板、形鋼、棒鋼等の所定の形状の鋼材とされる。熱間圧延工程における加熱温度は特に限定しないが、加熱温度が高すぎると結晶粒の粗大化を招き、じん性・加工性が劣化するばかりでなく、δフェライトが生成し熱間圧延時に割れが生じ易くなる場合がある。一方、加熱温度が低すぎると圧延が困難となる。このため加熱温度は1000〜1300℃程度とするのが好ましい。また、熱間圧延工程では所定の板厚・寸法の鋼材とすることができればよく、熱間圧延条件は特に限定されないが、熱間圧延の仕上温度は 800〜1100℃とするのが生産性の面から好ましい。
【0043】
熱間圧延後の鋼材は、そのまま、あるいはその後ショットブラスト、酸洗等による脱スケール処理を行ったのち、製品となる。必要に応じ、防錆剤等を熱延まま、あるいは脱スケール処理後の鋼材表面に塗布してもよい。また、より軟質な材料とする場合には、熱間圧延後に 600〜900 ℃に加熱・保持するバッチ式あるいは連続式の熱延板焼鈍を施してもよい。さらに、表面の硬質化あるいは表面粗さの低減や表面光沢を必要とする場合などは、脱スケール処理後に調質圧延により冷間での軽圧下を施すことは有利である。
製品となる鋼材は、そのまま構造用鋼材として用いることができ、また熱間圧延により得られる鋼板を必要に応じて角状あるいは円筒状のパイプ、各種形鋼等の素材として用いることもできる。
【0044】
【実施例】
実施例1
表1に示す成分組成の溶鋼を、転炉−2次精錬工程で溶製し、連続鋳造法でスラブとした。これらのスラブを、加熱後、熱間圧延により板厚:4mmおよび板厚:5.5 mmの熱延板とした。スラブ加熱温度は1100〜1200℃、熱間圧延の仕上温度は 800〜1050℃、巻き取り温度は 600〜900 ℃であった。また、得られた熱延板の一部は脱スケール処理を行った。
これらの鋼板から試験片を採取し、引張試験、腐食試験および溶接試験を行い、強度、伸び、耐長期間腐食性および溶接部じん性について評価した。
【0045】
測定方法は次のとおりである。
(1) 強度、伸び
板厚:4mmの熱延板(脱スケール材を含む)から、引張方向が圧延方向に平行になるようJIS 13号B試験片 (JIS Z 2201)を採取し、引張試験を実施して、伸び(El)および引張強さ(TS)を測定した。
【0046】
(2) 耐長期間腐食性
板厚:4mmの熱延板(脱スケール材を含む)に対し、塩水噴霧(0.1mass%NaCl,35℃,3h)→乾燥(60℃,3h )→湿潤(50℃,2h)を1サイクルとする腐食試験を 300サイクル行った。この試験方法により、100 年使用後相当における耐腐食性を評価することができる。腐食試験後の鋼板から引張方向が圧延方向に平行になるようJIS 13号B試験片を採取して引張試験を実施し、次式により腐食に伴う強度低下を求めた。
△TS=〔(Pmax0−Pmax)/Pmax0〕×100 (%)
ここで、Pmax0:腐食試験前の鋼板を用いた引張試験における最高荷重点での荷重
Pmax :腐食試験後の鋼板を用いた引張試験における最高荷重点での荷重
【0047】
(3) 溶接部じん性
板厚:5.5 mmの熱延板(脱スケール材を含む)より、溶接方向が鋼板の圧延方向に垂直な方向になるようにI開先を作製し、1.2 mmφの Y309Lタイプ溶接ワイヤを用い、半自動MAG溶接機により溶接継手を作製し、溶接熱影響部のじん性を評価した。溶接条件は、雰囲気ガス:Ar (流量:15リットル/min) +CO2(流量:4リットル/min)、電圧:20〜30V、電流:200 〜250 A、ギャップ:2〜3mm、溶接速度:30〜60 cm/min の1パス溶接とした。
得られた溶接継手から、図2に示したように、Vノッチ先端位置が止端部から1mm溶接金属側の位置となるように、かつ衝撃方向が溶接方向に一致するように、2mmVノッチ−サブサイズシャルピー試験片(JIS Z 2202) を採取し、−50℃における吸収エネルギーを測定した。なお、Vノッチ先端位置における溶接金属部と母材部との比率a:bはおよそ1:4であった。また、溶接金属が母材から盛り上がる部分は研削除去して試験片を作製した。
得られた結果を表2に示す。
【0048】
【表1】
【0049】
【表2】
【0050】
表2に示したとおり、本発明の成分組成範囲を満足する発明例は、 vE-50 が174 J/cm2 以上と良好な溶接部じん性を有し、同時に 100年使用後相当での強度低下が10%以内であり、良好な耐長期間腐食性を有することがわかる。
これに対し、比較例は、発明例に比べると溶接部じん性や耐長期間腐食性に劣っている。
【0051】
実施例2
表3に示す成分組成の溶鋼を、実施例1と同様に処理して得られた熱延鋼板から試験片を採取し、実施例1と同様の方法で引張試験、腐食試験および溶接試験を行い、強度、伸び、耐長期間腐食性および溶接部じん性について評価した。
得られた結果を表4に示す。
【0052】
【表3】
【0053】
【表4】
【0054】
表4に示したとおり、本発明の成分組成範囲を満足する発明例は、良好な溶接部じん性を有し、同時に 100年使用後相当での強度低下が5%以内であり、極めて良好な耐長期間腐食性を有することが分かる。
これに対し、比較例は、発明例に比べると溶接部じん性および耐長期間腐食性とも劣っていた。
【0055】
【発明の効果】
かくして、本発明によれば、溶接部じん性に優れるだけでなく、熱延ままあるいは熱延後脱スケールままの使用において耐長期間腐食性に優れたCr含有鋼を安定して得ることができる。
また、本発明のCr含有鋼は、建築・土木構造用材料としての用途をはじめとする、安価な材料の提供に対する要求に応えるものであり、またライフサイクルコストを大幅に低減することもでき、その工業的利用価値は極めて大きい。
【図面の簡単な説明】
【図1】 溶接部じん性に及ぼすCo添加量の影響を示したグラフである。
【図2】 シャルピー試験片のVノッチ先端位置と溶接部との位置関係を示した図である。
【図3】 長期間腐食に伴う強度低下とZ値との関係を示したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention is a Cr-containing steel as a member for welded structures, particularly Cr suitable as a member for building / civil engineering in applications that are not exposed to human eyes after completion of the structure and are not exposed to harsh environments such as outer wall materials. Containing corrosion-resistant steel.
[0002]
[Prior art]
Conventionally, steel for construction and civil engineering has been mainly used for ordinary steel such as SS400, high-strength steel such as SM490, and materials obtained by painting or plating these steels.
However, with the recent enlargement of buildings and diversification of designs, the use of various steel materials and materials has begun to be considered.
With a growing interest in environmental issues, the selection of materials with an emphasis on life cycle costs (LCC) is being considered. For example, housing is required to be designed with a life expectancy of 100 years or more. It's getting on.
[0003]
In order to extend the life of the structure, a method of increasing the plating thickness of the plated steel sheet can be considered, but in the case of a building structure that requires welding, a great load is imposed on the processing of the welded part after welding. Therefore, there is a problem that it is not suitable for practical use.
Under such circumstances, application of Fe-Cr alloys, which are excellent in corrosion resistance, require little maintenance cost for heat generation, and are easy to recycle, to construction and civil engineering materials is highly expected.
[0004]
Stainless steels, which are representative of Cr-containing steels, are ferritic stainless steels represented by SUS430, austenitic stainless steels represented by SUS304, martensitic stainless steels represented by SUS410, and SUS329 due to differences in metal structures. It is roughly divided into representative duplex stainless steels.
Among these various types of stainless steel, austenitic stainless steel is superior in terms of material strength, corrosion resistance, weldability, weld toughness, and versatility. Has been.
[0005]
However, for austenitic stainless steel,
(1) Because it contains a large amount of alloy elements such as Ni and Cr, it is much more expensive than ordinary steel.
(2) High stress corrosion cracking susceptibility,
(3) Compared to ordinary steel, the coefficient of thermal expansion is large and the thermal conductivity is small, so distortion due to the thermal effect during welding is likely to accumulate, making it difficult to apply to parts that require precision. ,
Conventionally, it is difficult to apply to general-purpose structural materials in which ordinary steel or a material obtained by coating or plating ordinary steel is used, and there is a problem that the application range is limited.
[0006]
For this reason, recently, the application of low Cr content steel with low Cr content to construction and civil engineering materials has been examined as an alternative to plated or painted ordinary steel, especially martensitic stainless steel architecture.・ Applicable to civil engineering materials.
Martensitic stainless steel is much cheaper than austenitic stainless steel containing a large amount of expensive Ni as described above, and has a low thermal expansion coefficient and high thermal conductivity. Compared to the above, it is remarkably excellent in corrosion resistance and has a high strength.
In martensitic stainless steels, there is no concern about σ embrittlement or 475 ° C embrittlement, which is a problem with high Cr steels, and stress corrosion cracking in a chloride environment, which is a problem with austenitic stainless steels. There is an advantage that there is no fear.
[0007]
However, martensitic stainless steel represented by SUS410 steel has a high C content of about 0.1 mass%, so it is inferior in weld toughness and weldability and requires preheating when welding. Since the workability is also inferior, there has been a problem in application to members that require welding.
[0008]
In response to the above problem, for example, Japanese Patent Publication No. 51-13463 includes Cr: 10 to 18 mass%, Ni: 0.1 to 3.4 mass%, Si: 1.0 mass% or less, and Mn: 4.0 mass% or less, and C: We propose martensitic stainless steels for welded structures that are reduced to 0.03 mass% or less and N: 0.02 mass% or less, and by generating a massive martensite structure in the weld heat-affected zone, improving the performance of the weld zone. Yes.
Japanese Patent Publication No. 57-28738 contains Cr: 10 to 13.5 mass%, Si: 0.5 mass% or less and Mn: 1.0 to 3.5 mass%, C: 0.02 mass% or less, N: 0.02 mass Proposed martensitic stainless steel for structural use with excellent weld toughness and workability that does not require preheating and postheating before and after welding by reducing Ni to less than 0.1 mass%. Has been.
[0009]
By the way, when structural steel is considered from the viewpoint of corrosion resistance, a higher Cr content is desirable. However, many of the structural steels that are actually used do not always require high corrosion resistance so that they do not cause any damage at all. In such a member that is not exposed to a harsh environment, it is sufficient if the corrosion resistance is such that rust juice does not spill out during long-term use after completion, and does not require as high corrosion resistance as existing stainless steel.
In addition, when used as a structural material for construction and civil engineering, since there is a low demand for surface properties, it is possible to use it as it is in hot rolling or after descaling after hot rolling. desirable.
In response to such demands, the Cr content is reduced to less than 10 mass%, and it is cheap Cr with reduced costs on the premise of use in a state of hot rolling or descaling after hot rolling. Development of contained steel is in progress.
[0010]
For example, Japanese Patent No. 3039630 includes Cr: 6 to 18 mass%, Si: 0.05 to 1.5 mass%, Mn: 0.05 to 1.5 mass%, and C: 0.005 to 0.1 mass%. A low corrosion rate steel for building structural members has been proposed in which the occurrence of local corrosion is suppressed by forming a Cr-deficient layer of 5 μm or more directly below the oxide scale by setting the finishing temperature of 780 ° C. or less.
[0011]
JP-A-11-323505 discloses Cr: 5 to 10 mass%, Si: 0.05 to 1.0 mass%, Mn: 0.05 to 2.0 mass%, and C: 0.005 to 0.03 mass%, N: 0.005. In steel reduced to ~ 0.03 mass%, by reducing the Cr content at a depth of 0.5-10 μm from the outermost layer of the metal part to less than 5 mass%, it causes uniform overall corrosion and causes strength reduction and breakage. Steels have been proposed in which such local and rapid wall thickness reduction is suppressed and strength reduction due to corrosion is suppressed.
[0012]
However, even with the techniques disclosed in the above-mentioned Patent No. 3039630 and JP-A-11-323505, it is difficult to say that the corrosion resistance of the low Cr-containing steel with a Cr content of less than 10 mass% is long-term corrosion resistance. Further improvement was desired.
Moreover, the technique disclosed in Japanese Patent Application Laid-Open No. 11-323505 is premised on processes such as a cladding method, thermal spraying, and plating, and has left significant problems in terms of cost for practical use.
[0013]
In addition, the inventors do not need to increase the amount of elements such as Ni, Cu, Cr, Mo and the like first, add Nb, Ti, and do not require excessive reduction of C, N. In addition, an Fe—Cr alloy excellent in weldability and initial rust resistance was developed and disclosed in Japanese Patent Application No. 2001-148701.
Specifically, for Fe-Cr alloys containing Cr in the range of more than 8 mass% and less than 15 mass%, especially Co, V and W are respectively Co: 0.01 mass% or more and less than 0.5 mass%, V: 0.01 mass % Or more, less than 0.5 mass% and W: 0.001 mass% or more and less than 0.05 mass%, and the X value represented by the following formula (2) and preferably the Z value represented by the following formula (3): The components are adjusted so as to satisfy the ranges of X value ≦ 11.0 and 0.03 ≦ Z value ≦ 1.5, respectively.
X value = Cr + Mo + 1.5 Si + 0.5 Nb + 0.2 V + 0.3 W + 8Al
-Ni-0.6 Co-0.5 Mn-30C-30N-0.5 Cu --- (2)
Z value = (Co + 1.5 V + 4.8 W) --- (3)
More preferably, the components are adjusted so that the ratio of C to N (C / N) is 0.60 or less.
[0014]
However, the above technique is not only economically disadvantageous with a large Cr content, but particularly when it contains 11 mass% or more of Cr, annealing for the purpose of softening is required. And left a disadvantageous problem.
Furthermore, if the Cr content is high, the corrosion weight loss associated with long-term use is reduced, but local corrosion is likely to occur, and there is a problem in that it is disadvantageous in terms of strength reduction rather than causing full corrosion. I left it.
[0015]
[Problems to be solved by the invention]
The present invention advantageously solves the above-mentioned problems, and has a corrosion resistance sufficient to satisfy a life of 100 years or more, and a thermal Building / civil engineering for applications that are not hot-rolled or unscaled after hot rolling, and that are not exposed to human eyes after completion of the structure and are not exposed to harsh environments such as exterior wall materials. An object of the present invention is to realize a steel material suitable as a structural member with a low Cr content of less than 10 mass% and to provide the steel material at a low cost.
[0016]
The steel of the present invention has a structure consisting essentially of a ferrite single phase as it is hot-rolled, and has a tensile strength (TS) of 400 to 550 MPa class, particularly as a member for construction and civil engineering structures. In use, even when used for more than 100 years, the strength is reduced by corrosion by 10% or less, particularly preferably 5% or less.
Further, in the steel of the present invention, during welding, the formation of coarse grains in the heat-affected zone that causes toughness deterioration of the weld zone is suppressed by making the structure in the heat-affected zone substantially a martensitic structure. , Ensuring good weld toughness.
Furthermore, the steel of the present invention can be used as a structural member after being formed into a steel pipe or a shaped steel by welding and processing.
[0017]
[Means for Solving the Problems]
Now, in order to achieve the above object, the inventors have conducted a thorough examination on the influence of various elements. In particular, focusing on Co, V, and W, the effects of these elements on the resistance to cracking were investigated in a low Cr-containing steel having a Cr content of less than 10 mass%.
As a result, weld toughness is remarkably improved by adding an appropriate amount of Co, and elements such as Ni, Cu, Cr, and Mo are drastically added by combining these three elements in an appropriate amount. It has been found that the corrosion resistance can be effectively improved for a long time without increasing the cost and increasing the cost increase factors such as the addition of Nb and Ti or the reduction of C and N.
The present invention has been completed based on the above findings.
[0018]
That is, the gist configuration of the present invention is as follows.
1. C: 0.0015 to 0.02 mass%, N: 0.0015 to 0.02 mass%,
Si: 0.1 to 1.0 mass%, Mn: 0.1 to 3.0 mass%,
Cr: more than 5 mass%, less than 10 mass%, Ni: 0.01-3.0 mass%,
Al: 0.1 mass% or less, P: 0.05 mass% or less and
S: 0.03 mass% or less
Including
Co: 0.010 to 1.0 mass%
Cr-containing corrosion-resistant steel for construction and civil engineering structures, with the balance being Fe and inevitable impurities, and excellent long-term corrosion resistance and weld toughness.
[0019]
2. In the above 1, the Cr content is more than 5 mass% and less than 8 mass%, and V and W are in the range of V: 0.01 to 0.5 mass%, W: 0.001 to 0.05 mass%, respectively, and the following formula (1) Cr-containing corrosion-resistant steel for construction and civil engineering structures, which has a composition containing Z value in a range satisfying 0.03 ≦ Z ≦ 1.5 and has excellent long-term corrosion resistance and weld toughness .
Record
Z value = ([% Co] + 1.5 [% V] + 4.8 [% W]) ---- (1)
Here, [% Co], [% V] and [% W] are the contents of each element (mass%).
[0020]
3. In the above 2, the Cr and W contents are Cr: more than 5 mass%, less than 7.5 mass%, and W: 0.005 to 0.03 mass%, respectively, characterized by excellent long-term corrosion resistance and weld toughness Cr-containing corrosion resistant steel for construction and civil engineering structures.
[0021]
4). In any one of the above 1-3, the steel is further
Cu: 3.0 mass% or less and Mo: 3.0 mass% or less
Cr-containing corrosion-resistant steel for construction and civil engineering structures, which has a composition containing one or two selected from the above, and is excellent in long-term corrosion resistance and weld toughness.
[0022]
5. In any one of the above 1 to 4, the steel further
B: 0.0002 to 0.0030 mass%
Cr-containing corrosion-resistant steel for construction and civil engineering structures, characterized by having a composition containing bismuth and excellent long-term corrosion resistance and weld toughness.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the experimental results that led to the present invention will be described.
First, the effect of Co addition to the low Cr steel will be described.
FIG. 1 shows the results of examining the change in toughness of the welded portion (heat affected zone) when Co is added to 7 mass% Cr steel.
Here, the toughness of the welded part is a hot rolled sheet with a thickness of 5.5 mm, and an I groove is prepared so that the welding direction is perpendicular to the rolling direction of the steel sheet, and a 1.2 mmφ Y309L type welding wire is used. 2 mm V notch sub-size Charpy so that the welded joint is produced by a semi-automatic MAG welder and the V-notch tip position is 1 mm from the toe to the weld metal side as shown in FIG. Take a test piece (JIS Z 2202) and absorb the absorbed energy at -50 ° C (vE -50 ) Was measured. The ratio a: b between the weld metal part and the base material part at the V notch tip position was approximately 1: 4.
As is apparent from the figure, the toughness of the weld is 150 J / cm by adding 0.01 mass% or more of Co. 2 This is improved. In particular, the effect is 200 J / cm with addition of 0.03 mass% or more. 2 This is remarkable.
[0024]
Next, the effect of combined addition of Co, V, and W will be described.
Fig. 3 shows the Z value (a parameter indicating the effect of the three elements). The results of the investigation of the strength reduction of the hot-rolled sheet by the long-term corrosion resistance test when these three elements were added to the 7 mass% Cr steel plate. ).
Here, the Z value is the following formula (1)
Z value = ([% Co] + 1.5 [% V] + 4.8 [% W]) ---- (1)
Here, [% Co], [% V] and [% W] are the contents of each element (mass%).
It is a value used as an index of long-term corrosion resistance.
In addition, the strength reduction state is one cycle of salt spray (0.1mass% NaCl, 35 ℃, 3h) → drying (60 ℃, 3h) → wetting (50 ℃, 2h) on a hot rolled sheet with a thickness of 4mm. After performing 300 corrosion tests, the strength (maximum tensile load) before and after the corrosion test was evaluated.
For comparison, FIG. 3 also shows the results of investigation when Co, V, and W are added alone or in combination of two.
As shown in the figure, it can be seen that when the Z value is 0.03 or more, the strength decrease due to long-term corrosion is drastically reduced to 5% or less, and the long-term corrosion resistance is remarkably improved. Moreover, the decrease in strength is small compared to the case where the three elements are not added in combination.
[0025]
Next, the reason why the component composition of the steel material is limited to the above range in the present invention will be described.
C: 0.0015 to 0.02 mass%, N: 0.0015 to 0.02 mass%
C and N are preferably reduced as much as possible from the viewpoint of improving the workability of the weld heat affected zone and preventing weld cracking. Moreover, when it adds excessively, the intensity | strength in hot rolling will become high too much, and the target intensity | strength will not be obtained. Furthermore, C and N not only have a great influence on the hardness of the martensitic phase in the weld heat affected zone, but also promote the formation of a Cr-deficient layer accompanying the precipitation of carbonitrides and cause the corrosion resistance to deteriorate. . For this reason, the upper limit of C and N needs to be 0.02 mass%, respectively. On the other hand, excessive reduction of the amount of C and N not only increases the refining cost, but also decreases the strength as it is in hot rolling, and the target strength cannot be obtained. Furthermore, the martensite formation ability in the weld heat affected zone is reduced, the formation of coarse ferrite grains is promoted, and the toughness of the weld heat affected zone is deteriorated. For this reason, the lower limits of C and N are each 0.0015 mass%. A more preferable composition range is 0.0020 to 0.010 mass% for both C and N.
[0026]
Si: 0.1 to 1.0 mass%
Si is an element useful as a deoxidizer, but if the content is less than 0.1 mass%, a sufficient deoxidation effect cannot be obtained, while addition exceeding 1.0 mass% only causes a decrease in toughness and workability. In addition, since the martensite formation ability in the weld heat affected zone is lowered, the Si content is limited to a range of 0.1 to 1.0 mass%. A more preferable composition range is 0.1 to 0.5 mass%.
[0027]
Mn: 0.1 to 3.0 mass%
Mn is an austenite stabilizing element that not only has the effect of increasing the martensite forming ability of the weld heat affected zone and improving toughness, but also acts as a deoxidizer, similar to Si. However, if the content is less than 0.1 mass%, the effect of addition is poor. On the other hand, if it exceeds 3.0 mass%, the workability and corrosion resistance associated with the formation of MnS are reduced, so the Mn content is 0.1 to 3.0 mass%. It was limited to the range. A more preferable composition range is 0.1 to 1.5 mass%.
[0028]
Cr: More than 5 mass%, less than 10 mass%
Cr is a useful element that improves corrosion resistance. In the present invention, it is not assumed to be used in a severe environment such as an outer wall material, but it is not visible to the public after the structure is completed, and even in a milder environment, rust juice is used for a long period of use. It is necessary to prevent dripping.
To ensure corrosion resistance for this purpose, it is necessary to add more than 5 mass%. On the other hand, in an inexpensive Cr-containing steel according to the present invention, the addition of 10 mass% or more of Cr has a disadvantage that causes an increase in cost. Therefore, the Cr content is limited to a range of more than 5 mass% and less than 10 mass%.
In addition, when adding Co, V, and W in combination, it is preferable that the Cr content is more than 5 mass% and less than 8 mass% so that the effect of suppressing the occurrence of local corrosion by the combined addition is sufficiently exhibited. is there. A more preferable component range is that the Cr amount is more than 5 mass% and less than 7.5 mass%, and the W amount is 0.005 to 0.03 mass%. By adjusting to such a component range, the occurrence of local corrosion is effectively suppressed, and it is possible to further suppress a decrease in strength associated with long-term use.
[0029]
Ni: 0.01-3.0 mass%
Ni is an element that improves ductility and toughness. In the present invention, Ni is added to improve the toughness of the welded portion. However, if the content is less than 0.01 mass%, the effect of addition is poor. On the other hand, even if added over 3.0 mass%, the effect reaches saturation, rather the material becomes harder and the workability deteriorates, so the amount of Ni Is limited to the range of 0.01 to 3.0 mass%.
[0030]
Al: 0.1 mass% or less
Al is an element that acts as a deoxidizer, but if it is contained in a large amount, the oxide inclusions increase, which may cause nozzle clogging at the steelmaking stage, and cause surface defects such as baldness. Cause a decline. For this reason, the amount of Al was limited to 0.1 mass% or less.
[0031]
P: 0.05 mass% or less
P is an element that induces cracking during hot working and is also harmful to corrosion resistance. However, if the content is up to 0.05 mass%, its adverse effect is not noticeable and can be tolerated. Restricted to mass%. More preferably, it is 0.03 mass% or less.
[0032]
S: 0.03 mass% or less
S not only forms sulfides and lowers the cleanliness of the steel, but also forms MnS and serves as a starting point for initiation. S is also a harmful element that segregates at the grain boundaries and causes embrittlement of the grain boundaries, so it is preferable to reduce it as much as possible. However, if it is 0.03 mass% or less, the adverse effect is not significant and is acceptable.
[0033]
Co: 0.010 to 1.0 mass%
Co is an element that forms the core of the present invention, and the toughness of the weld zone is remarkably improved by adding a small amount to a low Cr-containing steel of less than 10 mass%. In addition, the long-term corrosion resistance is improved as compared with the case where Co is not added. However, if the content is less than 0.010 mass%, the effect cannot be obtained. On the other hand, if the content exceeds 1.0 mass%, the material becomes hard and the workability deteriorates, so the Co content is limited to the range of 0.010 to 1.0 mass%. . A more preferable range of addition is 0.030 to 1.0 mass%.
[0034]
The effect of improving the toughness of the weld by adding Co is that when the austenite formation ability increases due to the addition of Co, a martensite phase is easily formed in the heat affected zone, and the hardening ability is added when C, N, etc. are added. This is probably due to the small size. In addition, the mechanism by which Co addition improves long-term corrosion resistance is not clear, but in long-term corrosion, it concentrates on the steel sheet surface or scale against local and rapid corrosion that causes the most strength reduction. This is thought to be due to the effective action of Co and the entire corroded surface being corroded uniformly.
[0035]
As described above, the essential component and the suppressing component have been described. However, in the present invention, the following elements can be appropriately contained.
V: 0.01 to 0.5 mass%, W: 0.001 to 0.05 mass%, and
Z value ([% Co] + 1.5 [% V] + 4.8 [% W]) = 0.03 to 1.5
Co, V and W are particularly important elements in the present invention. Until now, in order to improve the weld crack sensitivity of the heat affected zone, P cm {= C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B} Or optimization of values such as Ni equivalent and Cr equivalent has been studied. For this reason, in order to improve the corrosion resistance, ductility and workability in addition to improving the characteristics of the heat affected zone, such as Cr, Mo, Ni, C, N, Nb, Ti, etc. Studies that focus on elements have been conducted. However, although Co and W affect the corrosion resistance and the stability of the ferrite phase and austenite phase, P cm In addition, there has been no detailed study on the effects on parameters such as Ni equivalent and Cr equivalent and on the long-term corrosion resistance of steel sheets as-rolled or descaled after hot-rolling.
In the present invention, the influence of Co, V, W on the long-term corrosion resistance of a steel sheet as it is hot-rolled or descaled after hot-rolling, particularly the effect of adding these in combination, is quantitatively evaluated. The appropriate range and ratio of these elements were clarified.
[0036]
The Z value, which is an index of the effect of these three elements, is a value that is an index of long-term corrosion resistance. As described above, Co, V, and W are added together so that this Z value is 0.03 or more. By doing so, a desired effect can be obtained.
Here, the mechanism by which the long-term corrosion resistance is improved by the combined addition of these three elements is not clear, but in the long-term corrosion, the surface of the steel sheet is more sensitive to the local and rapid corrosion that causes the most strength reduction. Alternatively, it is considered that Co, V, and W concentrated in the scale worked effectively, and the entire corroded surface was corroded uniformly.
On the other hand, when the Z value exceeds 1.5, the effect of long-term corrosion resistance is saturated, and the workability is lowered due to hardening.
For this reason, Z value was limited to the range of 0.03-1.5. More preferably, it is in the range of 0.05 to 1.0.
[0037]
Moreover, it is necessary to limit content of V and W to V: 0.01-0.5 mass% and W: 0.001-0.05 mass%, respectively. This is because even if the above-described Z value satisfies the appropriate range (0.03 ≦ Z ≦ 1.5), if the respective contents are below the lower limit value, the effect of the composite addition cannot be obtained, while the amount of V added is This is because if 0.5 mass% and the added amount of W exceeds 0.05 mass%, precipitation of carbides becomes remarkable, and the toughness of the base metal and the weld heat affected zone is remarkably reduced. More preferable addition amounts are V: 0.05 to 0.3 mass% and W: 0.005 to 0.03 mass%.
[0038]
Thus, by utilizing the effect of improving the toughness of the weld heat-affected zone by adding Co to the low Cr-containing steel according to the present invention and the effect of improving long-term corrosion resistance by adding Co, V, and W together, it is expensive. Without increasing elements such as Ni, Cu, Cr, Mo, etc., adding Nb, Ti, or reducing C, N, increasing the cost toughness and maintaining hot toughness Alternatively, it is possible to achieve both long-term corrosion resistance in a state of descaling after hot rolling.
[0039]
Cu: 3.0 mass% or less
Cu is an element that improves corrosion resistance, and it is effective to add Cu when high corrosion resistance is required. However, if added over 3.0 mass%, there is a risk of hot cracking in hot rolling or the like, so Cu is contained at 3.0 mass% or less. In addition, it is more preferable that the lower limit is 0.1 mass% where the effect is remarkable, and the lower limit is 1.0 mass% or less.
[0040]
Mo: 3.0 mass% or less
Mo, like Cu, is an element effective for improving corrosion resistance. However, when it exceeds 3.0 mass%, not only the workability is lowered, but also the stability of the austenite phase is lowered, and particularly the toughness of the heat affected zone is lowered. For this reason, Mo is contained at 3.0 mass% or less. From the viewpoint of achieving both workability and corrosion resistance, the range of 0.1 to 1.0 mass% is preferable.
[0041]
B: 0.0002 to 0.0030 mass%
B is particularly effective for improving the toughness of the weld heat affected zone through the improvement of the hardenability. However, when the content is less than 0.0002 mass%, the effect is poor. On the other hand, when the content exceeds 0.0030 mass%, hardening increases, and the toughness and workability of both the base metal and the weld heat affected zone are impaired.
For this reason, B shall be contained in the range of 0.0002 to 0.0030 mass%. A more preferable addition range is 0.0005 to 0.0010 mass%.
[0042]
Next, the suitable manufacturing method of this invention steel is demonstrated.
First, the molten steel adjusted to the above preferred component composition is melted by an ordinary melting method such as a converter or an electric furnace, and then a known scouring method such as vacuum degassing (RH method), VOD method, AOD method or the like. Then, it is made into a steel material by continuous casting or casting into a slab by the ingot-bundling method.
Next, the steel material is heated and made into a steel material having a predetermined shape such as a steel plate, a shaped steel, a steel bar, etc. by a hot rolling process. The heating temperature in the hot rolling process is not particularly limited. However, if the heating temperature is too high, the crystal grains become coarse and not only the toughness and workability deteriorate, but also δ ferrite is generated and cracking occurs during hot rolling. It may be likely to occur. On the other hand, if the heating temperature is too low, rolling becomes difficult. For this reason, it is preferable that heating temperature shall be about 1000-1300 degreeC. In the hot rolling process, it is sufficient that the steel material has a predetermined thickness and size, and the hot rolling conditions are not particularly limited, but the hot rolling finishing temperature is set to 800 to 1100 ° C. From the aspect, it is preferable.
[0043]
The steel material after hot rolling becomes a product as it is or after being descaled by shot blasting, pickling or the like. If necessary, a rust preventive agent or the like may be applied hot-rolled or on the surface of the steel material after descaling. Moreover, when using a softer material, you may perform the batch type or continuous type hot-rolled sheet annealing which heats and hold | maintains at 600-900 degreeC after hot rolling. Furthermore, when the surface is hardened, the surface roughness is reduced, or the surface gloss is required, it is advantageous to perform a cold light reduction by temper rolling after descaling.
The steel material used as a product can be used as it is as a structural steel material, and a steel plate obtained by hot rolling can be used as a raw material for square or cylindrical pipes, various shaped steels and the like as required.
[0044]
【Example】
Example 1
Molten steel having the composition shown in Table 1 was melted in the converter-secondary refining process, and slab was formed by a continuous casting method. After heating, these slabs were hot rolled into hot rolled sheets having a plate thickness of 4 mm and a plate thickness of 5.5 mm. The slab heating temperature was 1100 to 1200 ° C, the hot rolling finishing temperature was 800 to 1050 ° C, and the winding temperature was 600 to 900 ° C. In addition, a part of the obtained hot rolled sheet was descaled.
Specimens were collected from these steel plates and subjected to a tensile test, a corrosion test, and a welding test to evaluate strength, elongation, long-term corrosion resistance, and weld toughness.
[0045]
The measurement method is as follows.
(1) Strength and elongation
Thickness: JIS 13B test piece (JIS Z 2201) was sampled from a hot-rolled sheet of 4 mm (including descaling material) so that the tensile direction was parallel to the rolling direction, and the tensile test was conducted to stretch. (El) and tensile strength (TS) were measured.
[0046]
(2) Long-term corrosion resistance
Thickness: One cycle of salt spray (0.1 mass% NaCl, 35 ° C, 3h) → drying (60 ° C, 3h) → wetting (50 ° C, 2h) on 4mm hot-rolled sheet (including descaling material) The corrosion test was conducted 300 cycles. This test method can evaluate corrosion resistance after 100 years of use. A JIS 13B test piece was taken from the steel plate after the corrosion test so that the tensile direction was parallel to the rolling direction, and the tensile test was performed.
△ TS = [(Pmax 0 -Pmax) / Pmax 0 ] × 100 (%)
Where Pmax 0 : Load at the highest load point in a tensile test using a steel plate before the corrosion test
Pmax: Load at the maximum load point in a tensile test using a steel plate after a corrosion test
[0047]
(3) Weld joint toughness
Thickness: I groove is made from a hot rolled sheet (including descaling material) of 5.5 mm so that the welding direction is perpendicular to the rolling direction of the steel sheet, and a Y309L type welding wire of 1.2 mmφ is used. Welded joints were produced with a semi-automatic MAG welder and the toughness of the weld heat affected zone was evaluated. Welding conditions are atmospheric gas: Ar (flow rate: 15 liters / min) + CO 2 (Flow rate: 4 liters / min), voltage: 20-30 V, current: 200-250 A, gap: 2-3 mm, welding speed: 30-60 cm / min.
From the obtained welded joint, as shown in FIG. 2, the 2 mm V notch − so that the V notch tip position is located on the 1 mm weld metal side from the toe and the impact direction coincides with the welding direction. Sub-size Charpy specimens (JIS Z 2202) were collected and the absorbed energy at −50 ° C. was measured. The ratio a: b between the weld metal part and the base material part at the V notch tip position was approximately 1: 4. Further, the test piece was prepared by grinding and removing the portion where the weld metal swells from the base material.
The obtained results are shown in Table 2.
[0048]
[Table 1]
[0049]
[Table 2]
[0050]
As shown in Table 2, the invention examples satisfying the component composition range of the present invention are: vE -50 174 J / cm 2 The above shows good weld toughness, and at the same time, the strength decrease after 100 years of use is within 10%, indicating that it has good long-term corrosion resistance.
In contrast, the comparative example is inferior in weld toughness and long-term corrosion resistance as compared with the inventive example.
[0051]
Example 2
Test specimens were taken from hot-rolled steel sheets obtained by treating molten steel having the composition shown in Table 3 in the same manner as in Example 1, and subjected to tensile tests, corrosion tests, and welding tests in the same manner as in Example 1. Strength, elongation, long-term corrosion resistance and weld toughness were evaluated.
Table 4 shows the obtained results.
[0052]
[Table 3]
[0053]
[Table 4]
[0054]
As shown in Table 4, the invention examples satisfying the component composition range of the present invention have good weld toughness, and at the same time the strength decrease after 100 years of use is within 5%, which is extremely good. It can be seen that it has long-term corrosion resistance.
In contrast, the comparative example was inferior in weld toughness and long-term corrosion resistance as compared with the inventive example.
[0055]
【The invention's effect】
Thus, according to the present invention, it is possible to stably obtain a Cr-containing steel that is not only excellent in weld joint toughness but also excellent in long-term corrosion resistance when used as hot rolled or as descaled after hot rolling. .
In addition, the Cr-containing steel of the present invention meets the demand for the provision of inexpensive materials, including applications as materials for construction and civil engineering structures, and can greatly reduce the life cycle cost, Its industrial utility value is extremely large.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of Co addition amount on weld toughness.
FIG. 2 is a view showing a positional relationship between a V-notch tip position of a Charpy test piece and a welded portion.
FIG. 3 is a graph showing the relationship between the decrease in strength accompanying long-term corrosion and the Z value.
Claims (5)
Si:0.1 〜1.0 mass%、 Mn:0.1 〜3.0 mass%、
Cr:5mass%超,10mass%未満、 Ni:0.01〜3.0 mass%、
Al:0.1 mass%以下、 P:0.05mass%以下および
S:0.03mass%以下
を含み、さらに
Co:0.010 〜1.0 mass%
を含有し、残部はFeおよび不可避的不純物の組成になり、耐長期間腐食性および溶接部じん性に優れることを特徴とする建築・土木構造用のCr含有耐腐食鋼。C: 0.0015 to 0.02 mass%, N: 0.0015 to 0.02 mass%,
Si: 0.1 to 1.0 mass%, Mn: 0.1 to 3.0 mass%,
Cr: more than 5 mass%, less than 10 mass%, Ni: 0.01-3.0 mass%,
Al: 0.1 mass% or less, P: 0.05 mass% or less and S: 0.03 mass% or less, and
Co: 0.010 to 1.0 mass%
Cr-containing corrosion-resistant steel for construction and civil engineering structures, with the balance being Fe and inevitable impurities, and excellent long-term corrosion resistance and weld toughness.
記
Z値=(〔%Co〕+ 1.5〔%V〕+ 4.8〔%W〕) --- (1)
ここで、〔%Co〕,〔%V〕,〔%W〕は各元素の含有量(mass%)In Claim 1, Cr content is more than 5 mass% and less than 8 mass%, Furthermore, V and W are the range of V: 0.01-0.5 mass%, W: 0.001-0.05 mass%, respectively, and following (1) Formula Cr-containing corrosion resistance for construction and civil engineering structures, which has a composition containing Z value represented by the formula 0.03 ≦ Z ≦ 1.5 and has excellent long-term corrosion resistance and weld toughness steel.
Z value = ([% Co] + 1.5 [% V] + 4.8 [% W]) --- (1)
Here, [% Co], [% V] and [% W] are the contents of each element (mass%).
Cu:3.0 mass%以下およびMo:3.0 mass%以下
のうちから選んだ1種または2種を含有する組成になり、耐長期間腐食性および溶接部じん性に優れることを特徴とする建築・土木構造用のCr含有耐腐食鋼。In any one of Claims 1-3, steel is further
Construction and civil engineering characterized by having a composition containing one or two of Cu: 3.0 mass% or less and Mo: 3.0 mass% or less and having excellent long-term corrosion resistance and weld toughness Cr-containing corrosion resistant steel for structural use.
B:0.0002〜0.0030mass%
を含有する組成になり、耐長期間腐食性および溶接部じん性に優れることを特徴とする建築・土木構造用のCr含有耐腐食鋼。In any one of Claims 1-4, steel is further B: 0.0002-0.0030mass%
Cr-containing corrosion-resistant steel for construction and civil engineering structures, characterized by having a composition containing bismuth and excellent long-term corrosion resistance and weld toughness.
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