JP4081991B2 - Corrosion resistant steel for freight oil tank and method for producing the same - Google Patents

Corrosion resistant steel for freight oil tank and method for producing the same Download PDF

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JP4081991B2
JP4081991B2 JP2001123043A JP2001123043A JP4081991B2 JP 4081991 B2 JP4081991 B2 JP 4081991B2 JP 2001123043 A JP2001123043 A JP 2001123043A JP 2001123043 A JP2001123043 A JP 2001123043A JP 4081991 B2 JP4081991 B2 JP 4081991B2
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corrosion
steel
resistant steel
mass
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JP2002012940A (en
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秀途 木村
克身 正村
典巳 和田
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JFE Steel Corp
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JFE Steel Corp
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Description

【0001】
この発明は、タンカー用鋼に関し、特に油タンク用として耐食性に優れた、プライマー塗装併用型の貨油タンク用高耐食鋼およびその製造方法に関するものである。
【0002】
【従来の技術】
国際海事機構の規制により、新建造タンカーのダブルハル構造が義務づけられて以降、貨油タンク、バラストタンク、デッキプレート等の船舶構造物には新たな腐食問題が投じられることとなった。海水環境と湿潤高温環境が繰返されるバラストタンク材では、腐食要因を解析した結果、海水腐食と飛沫帯環境での腐食、又結露環境腐食に耐える鋼材が要求され、バラストタンク部に該当するデッキプレートにおいても、裏面における腐食環境として、バラストタンク上部と同様のものが想定されている。
【0003】
これらの用途に適した鋼材として、特開平7-267182号公報では、Cu-P,Cu-Cr,Cu-P-Cr系鋼材が優れた耐食性を示すとして提案されている。この技術による鋼材は、C:0.15%以下、Si:0.02〜1.5%、Mn:0.2〜5.0%、S:0.005%以下、Cu:0.1〜1.0%、P:0.01〜0.15%を含み、さらに、Ni:0〜1.5%、Nb:0〜0.03%、ならびにMo:0〜1.0%、V:0〜1.0%およびW:0〜1.0%のうちの1種または2種以上、Al:0〜1.0%、Ti:0〜0.5%のうちの1種または2種を含むバラストタンク用低合金鋼である。
【0004】
また、特開平7-310141号公報、特開平8-246048号公報では、いずれもl〜3%程度のCrを含有する耐食鋼が、バラストタンク環境で有効に使用可能な耐食性を示すとして提案されている。特開平7-310141号公報記載の耐食鋼は、Cr:0.5〜3.5%を主成分とし、Ni:1.5%以下、Mo:0.8%以下のうちの1種以上、あるいはさらにNb:0.005〜0.05%、Ti:0.005〜0.05%のうちの1種以上を含む鋼である。特開平8-246048号公報記載の耐食鋼は、C:0.1 %以下、Si:0.10〜0.80%、Mn:1.50%以下、Al:0.005〜0.050 %、Cr:1.0〜3.0 %、Ti:0.005〜0.03%、N:0.0020〜0.0120%を含む鋼である。
【0005】
【発明が解決しようとする課題】
一方、最近のタンカー、特にVLCCと呼称される大型の貨油船で、貨油タンク内面に著しい腐食が発見されるケースがある。この現象は、従来問題とされなかったことから、新たな腐食課題としてクローズアップされているというのが現状である。そのため、貨油タンク、もしくは貨油タンク該当部分のデッキプレートの腐食防止は、バラストタンクについて行われているような、腐食メカニズムの分類と推定がなされないまま、危急の課題とされている。
【0006】
貨油タンク用材料の上記のような腐食問題を解決する手段として、組立て前に適用される錆止めプライマー塗装および重塗装がある。しかし多くの場合、塗装は使用にともない傷みを生じ、表面損傷部位より錆が発生し、塗膜を破壊するいわゆる塗膜下腐食の進行により、通常の使用では長くても5〜10年の使用で、裸使用と変らないほどの腐食が認められる状況にあり、メンテナンス費用は膨大なものとなる。
【0007】
また、前述の特開平7-310141号公報または特開平8-246048号公報記載のバラストタンク用の鋼は、その実施例を見ると添加元素の量が多く、溶接性が著しく劣るばかりか合金コストの増分に見合う塗装併用下での耐食性向上効果が小さい。即ち、貨油タンク用の鋼材として必ずしも使用しやすいとは言えない。これは特開平7-267182号公報記載の技術でも同様で、その実施例ではCuを添加する場合は0.5〜0.14%、Pを添加する場合は0.045〜0.14%、Crを添加する場合は1〜5%となっている。
【0008】
この発明は、以上の問題点を解決し、貨油タンクにおける腐食メカニズムを解明してそれに対して十分な耐食性を有し、かつ溶接性の向上および合金コストの削減が可能な貨油タンク用耐食鋼を提供することを目的とする。
【0009】
【課題を解決するための手段】
この発明は、貨油タンクにおいて鋼材に作用する腐食環境について、詳細に調査する中でなされた。その結果、最近の大型タンカーにおける腐食要因としては、貨油タンク内に導入される原動機排ガスの影響および原油揮発成分中の硫化水素の影響が大きいことが明らかとなった。排ガスは原油からの揮発成分による爆発防止のために導入されるものであるが、排ガスには、酸素、窒素のほかに、相当量の炭酸ガス、SOx、場合によってはH2Sなど、腐食性ガスも含まれる。
【0010】
そのため、これら腐食性ガスの存在下で温度サイクルが存在して酸露点腐食が作用した場合、塗膜に微小な損傷部位が生じただけで該箇所より緻密性の低い錆が発達する。この錆は塗膜と鋼材の間を発達して進行し、ついには塗膜を連続的に剥離させる。さらに揮発成分に含まれる硫化水素は、鋼材の塗膜剥離部分に作用して腐食を進行させる。この硫化水素による腐食は油井管等の原油接触環境で広く経験されるものである。このような腐食が問題となるのは主にガスのたまる油タンク上部であり、上記のような油タンク内上部の腐食雰囲気を、以下タンク環境と記載する。
【0011】
上記のように、調査の結果、貨油タンクにおける腐食メカニズムは、解明された。酸露点腐食の場合には、たとえ合金を添加して耐食性を高めた鋼材を用いたとしても、塗装を施さない裸使用では実用的な耐食性は得られないので、塗装して使用することを前提とする。しかし上記のような塗膜損傷部分の錆が拡大する問題があるので、塗膜下の錆の進行を最小限とし、塗膜寿命を長期化せしめるという観点に立ち、耐食鋼の成分設計を繰り返し、当該雰囲気中で塗膜損傷部位も含めた十分な耐食性を示す鋼材を開発するに至ったものが本発明である。
【0012】
この発明の鋼材の成分設計においては、耐食性のみならず、50kJ/cmレベルの入熱溶接、特に100kJ/cmを超える大入熱溶接の適用を受ける際の機械的性質、溶接性等とのバランスについても、重要な要素として考慮されている。
【0013】
本発明は、上記に基づきなされたものであり、本件第1の発明は、プライマー塗装状態で使用する貨油タンク用耐食鋼において、化学成分として、mass%で、C:0.16%以下、Si:1.5%以下、Mn:3.0%以下、P:0.035%以下、S:0.01%以下 Al:0.23 %以下を含み、さらに、Cu:0.1%〜1.4%、Cr:0.2〜4%、Ni:0.05〜0.7%のうちの1種以上を含み、残部がFe および不可避不純物からなり、下記の式(1)で表されるPcmの値が0.22以下であることを特徴とする貨油タンク用耐食鋼である。
【0014】
Pcm=C+Si/30+Mn/20+Cr/20+Cu/20+Ni/60+Mo/15+V/10+5B≦0.22(1)
但し、元素記号はそれぞれの元素のmass%を示す。本件第2の発明は、プライマー塗装状態で使用する貨油タンク用耐食鋼において、化学成分として、mass%で、C:0.15%以下、Si:1.5%以下、Mn:0.2%以上3.0%以下、P:0.035%以下、S:0.005%以下、Al:0.23%以下を含み、さらに、Cr:0.2〜4%、Cu:0.2%〜1.0%、Ni:0.1〜0.7%のうち1種以上を含み、残部がFeおよび不可避不純物からなり、上記の式(1)で表されるPcmの値が0.22以下であることを特徴とする貨油タンク用耐食鋼である。本件第の発明は、本件第1または本件第の発明に記載の化学成分に加えて、さらにMo:0.5mass%以下を含むことを特徴する貨油タンク用耐食鋼である。本件第の発明は、本件第1ないし本件第の発明のいずれか1つに記載の化学成分に加えて、さらにmass%でNb:0.05%以下、V:0.12%以下、Ti:0.1%以下のうち、いずれか1種または2種以上を含むことを特徴する貨油タンク用耐食鋼である。本件第の発明は、本件第1ないし本件第の発明のいずれか1つに記載の化学成分に加えて、さらにB:0.01mass%以下を含むことを特徴する貨油タンク用耐食鋼である。本件第の発明は、仕上げ温度を800℃以上で圧延を行い、その後冷却速度2℃/sec以上で600℃以下まで冷却を行うことを特徴とする本件第1ないし本件第の発明のいずれか1つに記載の貨油タンク用耐食鋼の製造方法である。
【0015】
【発明の実施の形態】
以下、本発明の貨油タンク用耐食鋼およびその製造方法について詳しく説明する。
【0016】
まず化学成分の限定理由について述べる。単位はすべてmass%である。
【0017】
C:0.16%以下
Cは鋼の強化に役立つ元素であるが、過度の添加は溶接性と耐食性に悪影響を及ぼすため、添加量の上限を0.16%とする。より好ましくは、添加量の上限を0.15%とする。
【0018】
Si:1.5%以下
Siは鋼の脱酸に有用な元素であるが、過度の添加は溶接作業性に悪影響を及ぼすため、添加量の上限を1.5%とする。
【0019】
Mn:3.0%以下
Mnは鋼の強化と靭性向上に有効な元素であるが、過度の添加は溶接性を阻害するため、添加量の上限を3.0%とする。より好ましくは、添加量の上限を2.0%とする。鋼の強度を確保するためには0.2%以上の添加が好ましい。
【0020】
P:0.035%以下
Pは、溶接性を低下させる元素であり、含有量は低いほど望ましい。0.035%までは許容できる範囲であり、含有量を0.035%以下とする。
【0021】
S:0.01%以下
Sは鋼の熱間加工性、耐溶接割れ性等を低下させる元素であり、含有量は低いほど望ましい0.01%までは許容できる範囲であり、含有量の上限を0.01%とする。より好ましくは、添加量の上限を0.005%とする。
【0022】
Cr、Cu、Ni:1種以上
Cr、Cu、Niは、耐食性を確保する上で少なくともいずれか1種の添加が必要である。それぞれの添加量の範囲については次のようになる。
【0023】
Cu:0.1〜1.4%
Cuは鋼のタンク環境での耐塗膜下腐食性を著しく向上させる。0.1%以上添加しないとその効果は明瞭ではないが、1.4%を超える添加は溶接高温割れの傾向が顕著になるため、添加量を:0.1〜1.4%とする。好ましくは0.2〜1.0%が適当である。Cr、Cu、Niのうちいずれか1種を添加する場合はCuが最も効果的である。
【0024】
Ni:0.05〜0.7%
Niは高価な添加元素ではあるが、耐食性向上に有効で、かつCuによる溶接性への害を抑制する効果を持つ。0.05%以上添加しないとその効果は明瞭ではないが、添加量が0.7%を超えると、効果が飽和し、かえって鋼の経済性を損ない、溶接割れ性も低下するようになるため、添加量を0.7%以下とする。より好ましくは添加量の下限を0.1%とする。Cr、Cu、Niのうちいずれか2種を添加する場合はCuおよびNiの添加が効果的である。
【0025】
Cr:0.2〜4%
Crの添加は炭酸ガス腐食の抑制に効果があることが知られているが、タンク環境でも一定の防食効果および耐塗膜下腐食性が得られる元素である。0.2%以上添加しないとその効果は明瞭ではないが、添加量が4%を超えると、低温割れ抑止のために予熱や後熱が必要になり、また溶接作業性も低下するため、添加量を0.2%以上、4%以下とする。
【0026】
Al:0.8%以下
Alはタンク環境での耐塗膜下腐食性を向上させるので適宜添加できる。0.8%を超える添加を行うと溶接時にスラグを多発し、作業性を顕著に低下させるため、添加する場合は0.8%以下とする。
【0027】
Mo:0.5%以下
Moは該当する使用環境での耐食性に有害な元素であるが、鋼の強度特性を向上させるため、制限して使用することが出来る。0.5%を超える添加は耐塗膜下腐食性の低下の傾向を著しくするため、添加量を0.5%以下とする。
【0028】
Nb:0.05%以下、V:0.12%以下、Ti:0.1%以下
Nb、V、Tiは鋼中の炭素と結合して炭化物を形成し、溶接性に及ぼす炭素の影響を減じることが出来るため、一定量の添加を選択できる。ただし、Nbは0.05%、Vは0.12%、Tiは0.1%を超えて添加すると、炭化物が多量に析出し、溶接時にクラックを生じやすくなるため、添加量としてはNbは0.05%以下、Vは0.12%以下、Tiは0.1%以下とする。
【0029】
B:0.01%以下
Bは熱間加工性の向上に有効な添加元素であり、選択して添加が可能であるが、0.01%を超える添加は溶接高温割れの傾向を著しくするため、添加量を0.01%以下とする。
【0030】
Pcm値:0.22以下(Pcm=C+Si/30+Mn/20+Cr/20+Cu/20+Ni/60+Mo/15+V/10+5B (1))
以上の各元素に対する制限を設けたうえで、上記式(1)に相当するPcm値について規定する。これは溶接割れ感受性を示す特性値であり、この値が0.22を超えると溶接時の低温割れ発生率が著しく高くなるため、Pcm値を0.22以下に保持することが必要である。
【0031】
本発明の鋼の化学成分の内、上記の化学成分以外の残部は実質的にFeである。「残部が実質的にFeである」とは、本発明の作用効果を無くさない限り、不可避不純物をはじめ、他の微量元素を含有するものが本発明の範囲に含まれ得ることを意味する。
【0032】
本発明の実施にあたっては、一部の化学成分については、以下のようにすることもできる。
【0033】
C: Cは鋼の強化の観点からは、0.03%以上添加することが好ましい。
【0034】
S: Sについては安定して0.005%以下とするために、脱硫処理を行うことが好ましい。
【0035】
Cr: Crは添加に伴い防食効果が向上するが、添加量が0.6%を超えると防食効果の向上が鈍化する傾向が見られ、また鋼の溶接性は次第に劣化するので、添加する場合は0.6%以下とすることが好ましい。
【0036】
Cu: Cuは添加量が0.4%を超えると、やはり鋼の溶接性が次第に劣化する傾向が見られるので、添加する場合は0.4%以下とすることが好ましい。
【0037】
Ni: Niは、添加量が0.5%を超えると効果が飽和し始めるため、費用対効果の観点からは添加量を0.5%以下とすることが好ましい。
【0038】
Al: Alは添加量が0.4%を超えると、溶接時のスラグ発生により作業性に影響が出てくるので、添加する場合は0.4%以下とすることが好ましい。
【0039】
次に、製造方法について説明する。
【0040】
上記の化学成分の鋼は、通常の鋼と同様の方法で製造できる。例えば、鋼の溶製では、転炉等で主要5元素C、Si、Mn、P、Sを発明の範囲に調節するとともに、必要に応じてその他の合金元素を添加する。
【0041】
その後、連続鋳造等により得られた鋳片をそのままあるいは冷却後、圧延を行う。圧延条件については、耐食鋼としては特に条件を問わないが、機械的特性の観点からは適切な圧下比を確保する必要がある。
【0042】
圧延の際、熱間圧延後の冷却速度を制御すると、引張強度490N/mm2級以上の高強度鋼材とすることができる。その方法は、熱間圧延の仕上げ温度を800℃以上とし、その後2℃/s以上の冷却速度で600℃以下まで冷却するものである。仕上げ温度が800℃未満では靭性が劣り、冷却速度が2℃/s未満もしくは冷却停止温度が600℃超えの場合には、490MPa級以上の強度が得られない。冷却は一度冷却した後再加熱して冷却しても、直接冷却しても良い。
【0043】
本発明では、耐食性を発揮するため上記のように製造した鋼に、有機もしくは無機の塗装、あるいは錆止めプライマーによる塗膜を施して使用する必要がある。塗装およびプライマーの種類は問わないが、無機系ジンクプライマーによる塗膜を用いると最も効果的である。
【0044】
本発明による耐食鋼の適用形態としては、VLCCタンカーの貨油タンク上部の構造体ないしデッキプレートに、プライマー塗装ままで用いることが最も通常である。さらに、タンク内部もしくは天井部の梁、柱等の構造体として用いても好適な耐食性と機械的性質を発揮しうる。
【0045】
【実施例】
以下に本発明の実施例を示す。
【0046】
(実施例1) 本実施例では、溶解はすべて1470N(150kgw)真空誘導溶解炉により、鋳造も真空中で実施した。245N(25kgw)鋳塊となしたのち、1200℃に加熱して熱間圧延し、厚さ25mmの板材とした。この25mmの板材を溶接割れ性の評価に用いた。さらに、1180℃に再加熱して熱間圧延して厚さ6mmの板材とし、耐食性評価試験に供した。
【0047】
溶接高温割れ性の評価には、厚さ25mmの板材に深さ15mmのV溝を切り、溶接ビードを置いて、冷却後の溶着金属部の割れの有無を比較評価する方法を用いた。溶接方法はサブマージアーク溶接とし、溶接材料は市販の強度50キロ級(490N/mm2級)ワイヤとした。溶接条件は、電圧38〜45V、電流1000〜1250A、溶接速度40cm/分、入熱139kJ/cmとした。割れの検出にはX線透過法を用いた。また、同時にスラグの発生等によるビードの乱れ、作業性の悪化についても評価した。
【0048】
一方、溶接時の低温割れ感受性の評価には、日本工業規格JIS Z3158で規定されるy型溶接割れ試験を実施し、鋼板冷却後の割れの有無で評価した。溶接は市販の490N/mm2級被覆アーク溶接用ワイヤを使用し、溶接条件は、電圧24V、電流170A、溶接速度15cm/min、入熱16kJ/cmとした。割れの検出には断面切断法を用いた。
【0049】
さらに、耐食性については、厚さ6mmの板材から寸法6mm×55mm×45mmの腐食試験片を切り出し、全面にプライマー処理を施して腐食試験に供した。プライマー処理は、事前によくショット錆落としを実施した試験片に、亜鉛末入り顔料を一定の割合で含有する、アルキルシリケート樹脂ワニス溶剤使用のジンク系プライマーの吹付塗装を行い、室温で24時間乾燥させた。亜鉛末入り顔料の重量割合については、43%のものと48%のものを使用して比較した。
【0050】
耐食性の評価を加速するため、試験面には鋼材表面に達するX字型のカッティングを施し、これを模擬損傷箇所として腐食試験後の表面錆、塗膜下の錆の進行を表面積率で評価した。なお、試験前の損傷面積率は1.0%であった。
【0051】
腐食試験は、貨油タンク内の環境条件を模擬した雰囲気と温度サイクル中に、試験片を曝して、腐食箇所拡大率の評価を実施した。貨油タンク内模擬環境は、ガス組成10%CO2,8%O2,0.02%SOx,残部N2の混合ガスを過飽和水蒸気圧の下に充満させて、試験用の雰囲気とした。この雰囲気中に挿入した試験片には、ヒータと冷却装置によって30℃/60℃の繰返し温度サイクルを、1サイクル1日として90日間付与し、結露水による腐食を模擬できるようにした。
【0052】
表1、表2、表3は、それぞれ本発明鋼と比較鋼の成分分析結果、および(1)式によるPcmと、上記の評価方法を用いた溶接性と耐食性の評価結果、即ちV溝試験における高温割れの有無(割れなしを◎で示している)、作業性評価の結果(作業性の劣るものに*を付している)、低温割れ感受性(割れの無いものを◎で示している)、プライマー種類(43:亜鉛末顔料の重量割合43%、48:同48%)模擬環境における錆(腐食)面積率(単位%)をまとめて示している。
【0053】
【表1】
【0054】
【表2】
【0055】
【表3】
【0056】
表1と表2にまとめた本発明成分による鋼材(No.1〜5 7 15 18 29 31 35)は、すべて耐溶接割れ性と溶接時の作業性を兼ね備え、かつ耐塗膜下腐食性が良好で、好適に使用可能であることがわかる。一方、表3にまとめた比較鋼(No.36〜64)は、成分の限定理由の項で述べたような背景から、耐溶接割れ性、溶接作業性、模擬環境下での耐塗膜下腐食性の何れかが十分ではない。これより、これらを満足するためには本発明による成分設計が好適であることが理解できる。
【0057】
図1は、Pcm値とy型溶接割れ試験結果の関係を示す図である。この図より、Pcmの値が、0.22以下である場合は溶接割れが発生せず(図中no crack)、それより大きい場合は溶接割れが発生すること(図中 crack)が分かる。よって、Pcmの値が0.22を超えると溶接時の低温割れ発生率が著しく高くなるため、Pcm値を0.22以下に保持することが必要である。
【0058】
図2は、添加Cr、Cu量と塗膜付結露腐食試験結果の関係を示す図である。この図より、添加Cr、Cu量の増加により、結露腐食試験による錆面積率(図中Sで示す)が縮小されることが分かる。錆面積率の許容限度を15%以下とするには、Cr量を0.2%以上Cu量を0.1%以上とすればよいことが分かる。
【0059】
(実施例2) 本実施例では、溶解はすべて5t真空誘導溶解炉によって行い、鋳造も真空中で実施した。9800Nの鋳塊となしたのち、1200℃に加熱して各種の製造条件で熱間圧延し、厚さ25mmの板材とした。この25mmの板材を溶接割れ性の評価、引張り試験、シャルピー衝撃試験に供した。さらに、この25mmの板材を1180℃に再加熱して熱間圧延して厚さ6mmの板材とし、耐食性評価試験に供した。
【0060】
引張り試験片は圧延方向に全厚試験片を採取し、常温での引張り強度で評価した。引張り強度が490N/mm2以上の場合を良好とした。シャルピー衝撃試験は、板厚中央部よりVノッチシャルピー試験片を採取し、試験温度-40℃における3本平均の吸収エネルギーで評価した。試験温度-40℃以上で50J以上の吸収エネルギーを示す場合を良好と判定した。
【0061】
溶接高温割れ性の評価には、厚さ25mmの板材に深さ15mmのV溝を切り、溶接ビードを置いて、冷却後の溶着金属部の割れの有無を比較評価する方法を用いた。溶接方法はサブマージアーク溶接とし、溶接材料は市販の490N/mm2級ワイヤとした。溶接条件は、電圧38〜45V、電流1000〜1250A、溶接速度40cm/分、入熱139kJ/cmとした。割れの検出にはX線透過法を用いた。また、同時にスラグの発生等によるビードの乱れ、作業性の悪化についても評価した。
【0062】
一方、溶接時の低温割れ感受性の評価には、日本工業規格JIS Z3158で規定されるy型溶接割れ試験を実施し、鋼板冷却後の割れの有無で評価した。溶接は市販の490N/mm2級被覆アーク溶接用ワイヤを使用し、溶接条件は電圧24V、電流170A、溶接速度15cm/min、入熱16kJ/cmとした。割れの検出には断面切断法を用いた。また、同時にスラグの発生等によるビードの乱れなどの、溶接欠陥や作業性についても評価した。
【0063】
耐食性については、厚さ6mmの板材から寸法6mm×55mm×45mmの腐食試験片を切り出し、全面にプライマー処理を施して腐食試験に供した。プライマー処理は、事前によくショットによる錆落としを実施した試験片に、亜鉛末入り顔料を一定の割合で含有するアルキルシリケート樹脂ワニス溶剤使用のジンク系プライマーの吹付塗装を行い、室温で24時間乾燥させた。亜鉛末入り顔料の亜鉛の重量割合については、43%のものと48%のものを使用して比較した。
【0064】
耐食性の評価を加速するため、試験面には鋼材表面に達するX字型のカッティングを施し、これを模擬損傷箇所として腐食試験後の表面錆、塗膜下の錆の進行を損傷部の表面積率である錆面積率で評価した。なお、試験前の損傷部の面積率は1.0%であった。
【0065】
腐食試験は、貨油タンク内の環境条件を模擬した雰囲気と温度サイクル中に試験片を曝して、腐食箇所拡大率の評価を実施した。貨油タンク内模擬環境は、ガス組成10%CO2、8%O2、0.02%SOx、0.1%H2S、残部N2の混合ガスを過飽和水蒸気圧の下に充満させて、試験用の雰囲気とした。この雰囲気中に挿入した試験片には、ヒータと冷却装置によって30℃/60℃の繰返し温度サイクルを、1サイクル1日として90日間付与し、結露水による腐食を模擬できるようにした。
【0066】
表4、表5に本発明鋼(No.101〜111 113 115 118 129 132 135)および比較鋼(No.136〜151)の成分分析結果、(1)式によるPcm値を示す。本発明鋼であるNo.101、107、113、128については圧延条件を変化させて各種の試料を作製した。表6に各鋼材に対する圧延仕上げ温度、圧延後の冷却速度(冷速)、および冷却速度2℃/s以上に相当する水冷した場合の冷却停止温度を示す。さらに、引張試験より得られた強度、シャルピー衝撃試験より得られた-40℃における吸収エネルギーvE-40、上記の評価方法を用いた溶接性(溶接時の高温割れ性、低温割れ性については、各試験において割れが発生した場合には高温割れ・低温割れと記載し、割れなしの場合は◎を記載し、作業性不良のものや溶接欠陥が存在する場合にはその旨を記載して示す)、表面に適用したプライマー種類(43:亜鉛末顔料の重量割合43%、48:同48%、一部についてはプライマーの塗布を行わなかった)、模擬環境における錆(腐食)面積率(単位%)を併せて示す。
【0067】
【表4】
【0068】
【表5】
【0069】
【表6】
【0070】
化学成分が本発明の範囲内である鋼材(No.101〜111 113 115 118 129 132 135)は、すべて耐溶接割れ性と溶接時の作業性を兼ね備え、かつ耐塗膜下腐食性が良好で、油タンクに好適に使用可能であることがわかる。一方、化学成分が本発明の範囲外の比較鋼(No.136〜151)は、成分の限定理由の項で述べたような背景から、耐溶接割れ性、溶接作業性に問題がある、または錆面積率が8%以上で模擬環境下での耐塗膜下腐食性が十分でない。また鋼材No.101のプライマー塗布を行わなかったものについては、鋼材全面に渡って非常に大きな減肉が生じることが確認された。これより、耐溶接割れ性、溶接作業性、模擬環境下での耐塗膜下腐食性を満足するためには本発明による成分設計の鋼材を用いれば良いことが分かった。また本発明の範囲の製造条件を用いると、JIS規定の490MPa級鋼以上の強度が得られる事が分かった。
【0071】
図3は、Cu添加量と塗膜付結露腐食試験の錆面積率の関係を示す図である。この図より、Cu添加量が0.1%未満の場合は耐塗膜下腐食性に対するCu添加の効果が十分発揮されないことがわかった。一方、1.4%を超える添加は溶接高温割れが発生した。
【0072】
【発明の効果】
本発明により鋼の化学成分を適切に調製することにより、貨油タンクにおける腐食メカニズムに対して十分な耐食性を有し、溶接性の向上および合金コストの削減が可能な貨油タンク用耐食鋼を得ることができる。その結果、VLCCタンカーの貨油タンク上部の構造体乃至デッキプレート用に、重塗装して、または重塗装することなくプライマー適用ままで用い得るばかりでなく、タンク内部もしくは天井部の梁、柱等の構造体として長期間好適に用い得る鋼材が提供され、船舶の製造コスト、維持管理コストの低減などの経済効果を得ることができる。
【図面の簡単な説明】
【図1】 Pcm値とy型溶接割れ試験結果の関係を示す図である。
【図2】 Cr、Cu添加量と塗膜付結露腐食試験結果の関係を示す図である。
【図3】 Cu添加量と塗膜付結露腐食試験の錆面積率の関係を示す図である。
[0001]
This invention relates to a tanker for steel, especially excellent in corrosion resistance for the currency oil tank, to a high corrosion resistant steel and a manufacturing method for cargo oil tanks of the primer coating combination type.
[0002]
[Prior art]
Since the construction of a double hull structure for a newly built tanker has become mandatory under the regulations of the International Maritime Organization, new corrosion problems have been put on ship structures such as freight oil tanks, ballast tanks and deck plates. For ballast tank materials that are repeatedly subjected to seawater environments and humid high temperature environments, as a result of analysis of corrosion factors, steel plates that are resistant to seawater corrosion, splash zone environment corrosion, and condensation environment corrosion are required. In FIG. 2, the same corrosive environment on the back surface as that of the upper part of the ballast tank is assumed.
[0003]
As a steel material suitable for these applications, JP-A-7-267182 proposes that Cu-P, Cu-Cr, and Cu-P-Cr steel materials exhibit excellent corrosion resistance. Steel materials by this technology include C: 0.15% or less, Si: 0.02 to 1.5%, Mn: 0.2 to 5.0%, S: 0.005% or less, Cu: 0.1 to 1.0%, P: 0.01 to 0.15%, Ni: 0 to 1.5%, Nb: 0 to 0.03%, Mo: 0 to 1.0%, V: 0 to 1.0%, and W: 0 to 1.0%, Al: 0 to 1.0% %, Ti: Low alloy steel for ballast tank containing one or two of 0 to 0.5%.
[0004]
In addition, in JP-A-7-310141 and JP-A-8-246048, both corrosion-resistant steels containing about 1 to 3% of Cr are proposed as exhibiting corrosion resistance that can be effectively used in a ballast tank environment. ing. The corrosion resistant steel described in JP-A-7-310141 is mainly composed of Cr: 0.5 to 3.5%, Ni: 1.5% or less, Mo: 0.8% or less, or Nb: 0.005 to 0.05% , Ti: steel containing one or more of 0.005 to 0.05%. The corrosion-resistant steel described in JP-A-8-246048 has C: 0.1% or less, Si: 0.10 to 0.80%, Mn: 1.50% or less, Al: 0.005 to 0.050%, Cr: 1.0 to 3.0%, Ti: 0.005 to This steel contains 0.03% and N: 0.0020 to 0.0120%.
[0005]
[Problems to be solved by the invention]
On the other hand, in recent tankers, particularly large freighter ships called VLCC, there are cases where significant corrosion is found on the inner surface of the freight oil tank. Since this phenomenon has not been regarded as a problem in the past, the current situation is that it has been highlighted as a new corrosion problem. Therefore, the prevention of corrosion of the coin oil tank or the deck plate of the corresponding part of the coin oil tank is an urgent issue without the classification and estimation of the corrosion mechanism as is done for the ballast tank.
[0006]
As means for solving the above-mentioned corrosion problem of the material for the coin oil tank, there are rust-preventing primer coating and heavy coating applied before assembly. However, in many cases, the coating is damaged by use, rust is generated from the damaged part of the surface, and the progress of so-called under-coating corrosion that destroys the coating film, it is used for 5 to 10 years at most in normal use In this situation, corrosion that is not different from bare use is recognized, and the maintenance cost becomes enormous.
[0007]
Further, the steel for ballast tank described in the above-mentioned Japanese Patent Application Laid-Open No. 7-310141 or Japanese Patent Application Laid-Open No. 8-246048 has a large amount of additive elements in view of the examples, and not only the weldability is remarkably deteriorated but also the alloy cost. The effect of improving the corrosion resistance under the combined use of paint corresponding to the increment of is small. That is, it cannot always be said that it is easy to use as a steel material for a coin oil tank. This also applies to the technique described in JP-A-7-267182. In the examples, when Cu is added, 0.5 to 0.14%, when P is added, 0.045 to 0.14%, and when Cr is added, 1 to 5%.
[0008]
The present invention solves the above problems, elucidates the corrosion mechanism in the coin oil tank, has sufficient corrosion resistance to it, and can improve weldability and reduce alloy costs. The purpose is to provide steel.
[0009]
[Means for Solving the Problems]
The present invention has been made in a detailed investigation of the corrosive environment acting on steel materials in a coin oil tank. As a result, it became clear that the influence of the engine exhaust gas introduced into the freight oil tank and the influence of hydrogen sulfide in the crude oil volatile components are large as corrosion factors in recent large tankers. Exhaust gas is introduced to prevent explosion due to volatile components from crude oil, but in addition to oxygen and nitrogen, the exhaust gas is corrosive, such as a considerable amount of carbon dioxide, SOx, and sometimes H 2 S. Gas is also included.
[0010]
Therefore, when a temperature cycle exists in the presence of these corrosive gases and acid dew point corrosion acts, a rust having a lower density than that portion develops only when a minute damaged portion is generated in the coating film. This rust develops and progresses between the coating film and the steel material, and finally peels the coating film continuously. Furthermore, hydrogen sulfide contained in the volatile component acts on the coating film peeling portion of the steel material to cause corrosion. This corrosion by hydrogen sulfide is widely experienced in crude oil contact environments such as oil well pipes. Such corrosion from becoming a problem mainly a cargo oil tank upper accumulating the gas, the corrosion atmosphere cargo oil tank top as described above, hereinafter referred to tank environment.
[0011]
As described above, as a result of the investigation, the corrosion mechanism in the coin oil tank was elucidated. In the case of acid dew point corrosion, even if a steel material with increased corrosion resistance by adding an alloy is used, practical corrosion resistance cannot be obtained with bare use without coating, so it is assumed that it will be used after painting. And However, there is a problem that the rust of the damaged part of the paint film expands as described above. From the viewpoint of minimizing the progress of rust under the paint film and extending the life of the paint film, the component design of corrosion resistant steel is repeated. The present invention has led to the development of a steel material exhibiting sufficient corrosion resistance including the damaged part of the coating film in the atmosphere.
[0012]
In the composition design of the steel material of the present invention, not only corrosion resistance but also balance with mechanical properties, weldability, etc. when receiving heat input welding of 50 kJ / cm level, especially high heat input welding exceeding 100 kJ / cm Is also considered as an important factor.
[0013]
The present invention has been made based on the above, and the first invention of the present invention is a corrosion-resistant steel for freight oil tanks used in a primer coating state, as chemical components, in mass%, C: 0.16% or less, Si: 1.5% or less, Mn: 3.0% or less, P: 0.035% or less, S: 0.01% or less , Al: 0.23 % or less , Cu: 0.1% to 1.4%, Cr: 0.2 to 4%, Ni: 0.05 Corrosion-resistant steel for freight oil tanks, comprising one or more of ˜0.7%, the balance being Fe and inevitable impurities , and the Pcm value represented by the following formula (1) being 0.22 or less It is.
[0014]
Pcm = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ≦ 0.22 (1)
However, an element symbol shows mass% of each element. Present second invention, in cargo oil tank corrosion resistant steel for use in primers paint condition, as chemical components, in mass%, C: 0.15% or less, Si: 1.5% or less, Mn: 0.2% to 3.0% or less , P: 0.035% or less, S: 0.005% or less, Al: 0.23% or less, further Cr: 0.2-4%, Cu: 0.2% -1.0%, Ni: 0.1-0.7% In addition, the remaining portion is made of Fe and inevitable impurities, and the Pcm value represented by the above formula (1) is 0.22 or less. Present third invention, in addition to the chemical components according to the first or the present second inventions present, further Mo: a cargo oil tank corrosion resistant steel comprising the following 0.5 mass%. In addition to the chemical component described in any one of the first to third inventions, the fourth invention of the present invention further has mass% of Nb: 0.05% or less, V: 0.12% or less, Ti: 0.1% It is a corrosion-resistant steel for freight oil tanks including any one or more of the following. The fifth aspect of the present invention is a corrosion-resistant steel for freight oil tanks characterized by further containing B: 0.01 mass% or less in addition to the chemical component described in any one of the first to fourth aspects of the present invention. is there. The sixth invention of the present invention is any one of the first to fifth inventions characterized in that rolling is performed at a finishing temperature of 800 ° C. or higher, and then cooling is performed at a cooling rate of 2 ° C./sec or higher to 600 ° C. or lower. It is a manufacturing method of the corrosion-resistant steel for coin oil tanks described in any one.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the corrosion resistant steel for a coin oil tank and the method for producing the same according to the present invention will be described in detail.
[0016]
First, the reasons for limiting chemical components will be described. All units are mass%.
[0017]
C: 0.16% or less
C is an element useful for strengthening steel, but excessive addition adversely affects weldability and corrosion resistance, so the upper limit of addition is 0.16%. More preferably, the upper limit of the addition amount is 0.15%.
[0018]
Si: 1.5% or less
Si is a useful element for deoxidizing steel, but excessive addition has an adverse effect on welding workability, so the upper limit of the addition amount is 1.5%.
[0019]
Mn: 3.0% or less
Mn is an element effective for strengthening steel and improving toughness, but excessive addition inhibits weldability, so the upper limit of the addition amount is 3.0%. More preferably, the upper limit of the addition amount is 2.0%. In order to ensure the strength of the steel, addition of 0.2% or more is preferable.
[0020]
P: 0.035% or less
P is an element that decreases weldability, and the lower the content, the more desirable. Up to 0.035% is an acceptable range, and the content is 0.035% or less.
[0021]
S: 0.01% or less
S is an element that lowers the hot workability, weld crack resistance, etc. of steel, and the lower the content, the more acceptable range is 0.01%, and the upper limit of the content is 0.01%. More preferably, the upper limit of the addition amount is 0.005%.
[0022]
Cr, Cu, Ni: 1 or more types
At least one of Cr, Cu and Ni needs to be added to ensure corrosion resistance. The range of each addition amount is as follows.
[0023]
Cu: 0.1-1.4%
Cu significantly improves the corrosion resistance under the coating in the steel tank environment. If 0.1% or more is not added, the effect is not clear, but if it exceeds 1.4%, the tendency of hot welding cracking becomes prominent, so the addition amount is set to 0.1 to 1.4%. 0.2 to 1.0% is preferable. When adding any one of Cr, Cu, and Ni, Cu is the most effective.
[0024]
Ni: 0.05-0.7%
Although Ni is an expensive additive element, it is effective in improving corrosion resistance and has the effect of suppressing the damage to weldability caused by Cu. If 0.05% or more is not added, the effect is not clear. However, if the added amount exceeds 0.7%, the effect is saturated, and on the contrary, the economy of the steel is deteriorated and the weld cracking property is also lowered. 0.7% or less. More preferably, the lower limit of the addition amount is 0.1%. When adding any two of Cr, Cu, and Ni, the addition of Cu and Ni is effective.
[0025]
Cr: 0.2-4%
Although the addition of Cr is known to be effective in suppressing carbon dioxide gas corrosion, it is an element that can provide a certain anticorrosive effect and corrosion resistance under a coating film even in a tank environment. If 0.2% or more is not added, the effect is not clear. However, if the added amount exceeds 4%, preheating and post-heating are required to prevent low-temperature cracking, and welding workability is also reduced. 0.2% or more and 4% or less.
[0026]
Al: 0.8% or less
Al can be added as appropriate because it improves the corrosion resistance under a coating film in a tank environment. If over 0.8% is added, slag is frequently generated during welding, and the workability is remarkably reduced.
[0027]
Mo: 0.5% or less
Mo is an element that is harmful to the corrosion resistance in the corresponding usage environment, but can be used in a limited manner in order to improve the strength properties of the steel. Addition over 0.5% remarkably reduces the corrosion resistance under the coating film, so the addition amount should be 0.5% or less.
[0028]
Nb: 0.05% or less, V: 0.12% or less, Ti: 0.1% or less
Nb, V, and Ti combine with carbon in steel to form carbides and reduce the effect of carbon on weldability, so a certain amount of addition can be selected. However, if Nb is added to 0.05%, V is added to more than 0.12%, and Ti exceeds 0.1%, a large amount of carbide precipitates and cracks are likely to occur during welding. 0.12% or less, Ti 0.1% or less.
[0029]
B: 0.01% or less
B is an additive element effective for improving hot workability, and can be selected and added. However, addition exceeding 0.01% makes the tendency of hot cracking in welding remarkable, so the addition amount should be 0.01% or less. .
[0030]
Pcm value: 0.22 or less (Pcm = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B (1))
The Pcm value corresponding to the above formula (1) is defined after setting the restrictions on the above elements. This is a characteristic value indicating the sensitivity to weld cracking. If this value exceeds 0.22, the rate of occurrence of low-temperature cracking during welding becomes extremely high, so the Pcm value must be maintained at 0.22 or less.
[0031]
Of the chemical components of the steel of the present invention, the balance other than the above chemical components is substantially Fe. The phrase “the balance is substantially Fe” means that an element containing an inevitable impurity and other trace elements can be included in the scope of the present invention unless the effects of the present invention are lost.
[0032]
In carrying out the present invention, some chemical components may be as follows.
[0033]
C: C is preferably added in an amount of 0.03% or more from the viewpoint of strengthening the steel.
[0034]
S: It is preferable to perform a desulfurization treatment so that S is stably 0.005% or less.
[0035]
Cr: Cr improves the anticorrosion effect with addition, but when the addition amount exceeds 0.6%, the improvement of the anticorrosion effect tends to slow down, and the weldability of steel gradually deteriorates. % Or less is preferable.
[0036]
Cu: If the addition amount of Cu exceeds 0.4%, the weldability of the steel tends to deteriorate gradually. Therefore, when added, it is preferable to make it 0.4% or less.
[0037]
Ni: Since the effect of Ni begins to saturate when the addition amount exceeds 0.5%, the addition amount is preferably 0.5% or less from the viewpoint of cost effectiveness.
[0038]
Al: If the addition amount of Al exceeds 0.4%, workability is affected by the generation of slag during welding, so when added, the content is preferably 0.4% or less.
[0039]
Next, a manufacturing method will be described.
[0040]
The steel having the above chemical components can be produced in the same manner as ordinary steel. For example, in the melting of steel, the main five elements C, Si, Mn, P, and S are adjusted to the scope of the invention by a converter or the like, and other alloy elements are added as necessary.
[0041]
Thereafter, the slab obtained by continuous casting or the like is rolled as it is or after cooling. The rolling conditions are not particularly limited as the corrosion resistant steel, but it is necessary to ensure an appropriate reduction ratio from the viewpoint of mechanical properties.
[0042]
By controlling the cooling rate after hot rolling during rolling, a high-strength steel material having a tensile strength of 490 N / mm class 2 or higher can be obtained. In this method, the hot rolling finish temperature is set to 800 ° C. or higher, and then cooled to 600 ° C. or lower at a cooling rate of 2 ° C./s or higher. When the finishing temperature is less than 800 ° C, the toughness is inferior, and when the cooling rate is less than 2 ° C / s or the cooling stop temperature exceeds 600 ° C, the strength of 490 MPa or higher cannot be obtained. Cooling may be performed by cooling once and then reheating to cool or directly.
[0043]
In the present invention, it is necessary to use the steel produced as described above with a coating with an organic or inorganic coating or a rust-preventing primer in order to exhibit corrosion resistance. The type of coating and primer is not limited, but it is most effective to use a coating film made of an inorganic zinc primer.
[0044]
As a form of application of the corrosion-resistant steel according to the present invention, it is most usual to use it as it is with primer coating on the structure or deck plate above the coin oil tank of the VLCC tanker. Furthermore, suitable corrosion resistance and mechanical properties can be exhibited even when used as a structure such as a beam or column in the tank or on the ceiling.
[0045]
【Example】
Examples of the present invention are shown below.
[0046]
(Example 1) In this example, all the melting was performed by a 1470N (150 kgw) vacuum induction melting furnace, and casting was also performed in vacuum. After forming an ingot of 245N (25 kgw), it was heated to 1200 ° C. and hot-rolled to obtain a plate material having a thickness of 25 mm. This 25 mm plate was used for the evaluation of weld cracking. Further, it was reheated to 1180 ° C. and hot-rolled to obtain a plate material having a thickness of 6 mm, which was subjected to a corrosion resistance evaluation test.
[0047]
For the evaluation of weld hot cracking, a method of comparatively evaluating the presence or absence of cracks in the weld metal part after cooling by cutting a 15 mm deep V-groove on a 25 mm thick plate and placing a weld bead was used. The welding method was submerged arc welding, and the welding material was a commercially available 50 kg class (490 N / mm 2 class) wire. The welding conditions were a voltage of 38 to 45 V, a current of 1000 to 1250 A, a welding speed of 40 cm / min, and a heat input of 139 kJ / cm. An X-ray transmission method was used to detect cracks. At the same time, bead disturbance due to slag generation and deterioration of workability were also evaluated.
[0048]
On the other hand, for the evaluation of low temperature cracking susceptibility during welding, a y-type weld cracking test specified in Japanese Industrial Standard JIS Z3158 was conducted, and the presence or absence of cracking after cooling the steel sheet was evaluated. For welding, a commercially available wire for 490 N / mm class 2 coated arc welding was used. The welding conditions were a voltage of 24 V, a current of 170 A, a welding speed of 15 cm / min, and a heat input of 16 kJ / cm. A cross-section cutting method was used to detect cracks.
[0049]
Further, regarding corrosion resistance, a corrosion test piece having a size of 6 mm × 55 mm × 45 mm was cut out from a plate material having a thickness of 6 mm, subjected to a primer treatment on the entire surface, and subjected to a corrosion test. Primer treatment is performed by spraying a zinc-based primer containing an alkyl silicate resin varnish solvent containing a certain percentage of zinc dust pigment on a test piece that has been thoroughly shot rust removed beforehand, and dried at room temperature for 24 hours. I let you. The weight ratio of the pigment containing zinc dust was compared using 43% and 48%.
[0050]
In order to accelerate the evaluation of corrosion resistance, the test surface was cut in an X-shape that reached the steel surface. Using this as a simulated damage location, the surface rust after the corrosion test and the progress of rust under the coating were evaluated by the surface area ratio. . The damaged area ratio before the test was 1.0%.
[0051]
In the corrosion test, the test piece was exposed to an atmosphere simulating the environmental conditions in the coin oil tank and the temperature cycle, and the corrosion site expansion rate was evaluated. The freight oil tank simulated environment was filled with a gas mixture of 10% CO 2 , 8% O 2 , 0.02% SOx and the balance N 2 under supersaturated water vapor pressure to provide a test atmosphere. The test piece inserted into this atmosphere was subjected to a 30 ° C / 60 ° C repeated temperature cycle for 90 days, one cycle per day, using a heater and a cooling device to simulate the corrosion caused by condensed water.
[0052]
Tables 1, 2 and 3 show the results of component analysis of the steel of the present invention and the comparative steel, and the results of evaluation of weldability and corrosion resistance using the above evaluation method, Pcm according to the formula (1), that is, the V-groove test. Presence or absence of hot cracking (no cracks are indicated by ◎), workability evaluation results (* are marked for those with poor workability), low temperature cracking susceptibility (no cracks are indicated by ◎) ), Primer type (43: weight ratio of zinc dust pigment: 43%, 48: 48%) The rust (corrosion) area ratio (unit%) in a simulated environment is shown collectively.
[0053]
[Table 1]
[0054]
[Table 2]
[0055]
[Table 3]
[0056]
Table 1 and a steel according to the invention components are summarized in Table 2 (No.1~ 5, 7 ~ 15 , 18 ~ 29, 31 ~ 35) are all both workability in welding and resistance to weld cracking resistance, and resistance to It can be seen that the corrosiveness under the coating film is good and can be suitably used. On the other hand, the comparative steels (Nos. 36 to 64) summarized in Table 3 are weld crack resistance, welding workability, and under the coating resistance under simulated environment because of the background described in the section on the reasons for limiting the components. Any of the corrosiveness is not enough. From this, it can be understood that the component design according to the present invention is suitable for satisfying these.
[0057]
FIG. 1 is a diagram showing the relationship between the Pcm value and the y-type weld crack test result. From this figure, it can be seen that when the value of Pcm is 0.22 or less, no weld crack occurs (no crack in the figure), and when it is larger than that, a weld crack occurs (crack in the figure). Therefore, if the value of Pcm exceeds 0.22, the rate of occurrence of low temperature cracking during welding becomes extremely high, so it is necessary to keep the Pcm value below 0.22.
[0058]
FIG. 2 is a diagram showing the relationship between the amount of added Cr and Cu and the result of the dew corrosion test with coating film. From this figure, it can be seen that the rust area ratio (denoted by S in the figure) by the condensation corrosion test is reduced by increasing the amount of added Cr and Cu. It can be seen that the Cr content should be 0.2% or more and the Cu content 0.1% or more in order to set the allowable limit of the rust area ratio to 15% or less.
[0059]
(Example 2) In this example, all the melting was performed in a 5t vacuum induction melting furnace, and casting was also performed in vacuum. After forming an ingot of 9800N, it was heated to 1200 ° C and hot-rolled under various production conditions to obtain a plate with a thickness of 25 mm. This 25 mm plate was subjected to weld cracking evaluation, tensile test, and Charpy impact test. Further, this 25 mm plate was reheated to 1180 ° C. and hot-rolled to obtain a 6 mm thick plate, which was subjected to a corrosion resistance evaluation test.
[0060]
Tensile test specimens were collected in the rolling direction and were evaluated by tensile strength at room temperature. A case where the tensile strength was 490 N / mm 2 or more was considered good. In the Charpy impact test, V-notch Charpy test pieces were collected from the center of the plate thickness, and evaluated by the average absorbed energy of three at a test temperature of -40 ° C. A test temperature of -40 ° C. or higher and an absorption energy of 50 J or higher was judged as good.
[0061]
For the evaluation of weld hot cracking, a method of comparatively evaluating the presence or absence of cracks in the weld metal part after cooling by cutting a 15 mm deep V-groove on a 25 mm thick plate and placing a weld bead was used. The welding method was submerged arc welding, and the welding material was a commercially available 490 N / mm class 2 wire. The welding conditions were a voltage of 38 to 45 V, a current of 1000 to 1250 A, a welding speed of 40 cm / min, and a heat input of 139 kJ / cm. An X-ray transmission method was used to detect cracks. At the same time, bead disturbance due to slag generation and deterioration of workability were also evaluated.
[0062]
On the other hand, for the evaluation of low temperature cracking susceptibility during welding, a y-type weld cracking test specified in Japanese Industrial Standard JIS Z3158 was conducted, and the presence or absence of cracking after cooling the steel sheet was evaluated. For welding, a commercially available wire for 490 N / mm class 2 coated arc welding was used. The welding conditions were a voltage of 24 V, a current of 170 A, a welding speed of 15 cm / min, and a heat input of 16 kJ / cm. A cross-section cutting method was used to detect cracks. At the same time, weld defects such as bead disturbance due to slag generation and workability were also evaluated.
[0063]
Regarding the corrosion resistance, a corrosion test piece having a size of 6 mm × 55 mm × 45 mm was cut out from a 6 mm thick plate material, subjected to a primer treatment on the entire surface, and subjected to a corrosion test. Primer treatment is performed by spraying a zinc-based primer using an alkyl silicate resin varnish solvent containing a pigment containing zinc dust at a certain ratio on a test piece that has been thoroughly rust-removed by shot in advance, and drying at room temperature for 24 hours. I let you. The weight ratio of zinc in the pigment containing zinc powder was compared using 43% and 48%.
[0064]
In order to accelerate the evaluation of corrosion resistance, the test surface is cut in an X shape that reaches the surface of the steel material. Using this as a simulated damage location, the surface rust after the corrosion test and the progress of rust under the paint film are measured as the surface area ratio of the damaged portion. The rust area ratio was evaluated. The area ratio of the damaged part before the test was 1.0%.
[0065]
In the corrosion test, the test piece was exposed to an atmosphere simulating the environmental conditions in the coin oil tank and the temperature cycle, and an evaluation of the expansion rate of the corrosion portion was performed. The freight oil tank simulated environment is filled with a gas mixture of 10% CO 2 , 8% O 2 , 0.02% SOx, 0.1% H 2 S and the balance N 2 under supersaturated steam pressure for testing. The atmosphere. The test piece inserted into this atmosphere was subjected to a 30 ° C / 60 ° C repeated temperature cycle for 90 days, one cycle per day, using a heater and a cooling device to simulate the corrosion caused by condensed water.
[0066]
Tables 4 and 5 show the component analysis results of the steels of the present invention (No. 101 to 111 , 113 to 115 , 118 to 129 , 132 to 135) and the comparative steel (No. 136 to 151), and the Pcm value according to equation (1). Indicates. With respect to No. 101, 107, 113, and 128, which are steels of the present invention, various samples were produced by changing the rolling conditions. Table 6 shows the rolling finishing temperature for each steel material, the cooling rate after the rolling (cooling rate), and the cooling stop temperature in the case of water cooling corresponding to a cooling rate of 2 ° C./s or more. Furthermore, the strength obtained from the tensile test, the absorbed energy vE-40 obtained from the Charpy impact test at −40 ° C., and the weldability using the above evaluation method (for hot cracking and cold cracking during welding, If cracks occur in each test, describe them as hot cracks / cold cracks, if there are no cracks, indicate ◎, and if there are workability defects or welding defects, indicate that effect. ), The type of primer applied to the surface (43: 43% by weight of zinc dust pigment, 48: 48% of the same, some were not coated with primer), rust (corrosion) area ratio in simulated environment (units) %).
[0067]
[Table 4]
[0068]
[Table 5]
[0069]
[Table 6]
[0070]
Steel materials (No. 101 to 111 , 113 to 115 , 118 to 129 , 132 to 135) whose chemical components are within the scope of the present invention have both weld crack resistance and workability during welding, and have a coating film resistance. a lower corrosive good, it can be seen that it is suitably used in currency oil tank. On the other hand, the comparative steels (No. 136 to 151) whose chemical components are out of the scope of the present invention have problems in weld crack resistance and welding workability from the background described in the section on reasons for limiting the components, or The rust area ratio is 8% or more and the corrosion resistance under the coating film in the simulated environment is not sufficient. In addition, it was confirmed that the steel material No. 101 not subjected to the primer application had a very large thickness reduction over the entire surface of the steel material. From this, it was found that the steel material having the component design according to the present invention may be used in order to satisfy the welding crack resistance, the welding workability, and the corrosion resistance under the coating film under the simulated environment. Further, it was found that when the production conditions within the range of the present invention were used, a strength higher than 490 MPa class steel specified by JIS was obtained.
[0071]
FIG. 3 is a graph showing the relationship between the amount of Cu added and the rust area ratio in the dew corrosion test with coating film. From this figure, it was found that when the Cu addition amount is less than 0.1%, the effect of Cu addition on the corrosion resistance under the coating film is not sufficiently exhibited. On the other hand, when it exceeds 1.4%, weld hot cracking occurred.
[0072]
【The invention's effect】
By appropriately preparing the chemical components of the steel according to the present invention, a corrosion resistant steel for a coin oil tank that has sufficient corrosion resistance against the corrosion mechanism in the coin oil tank, and that can improve weldability and reduce alloy costs. Obtainable. As a result, the structure or deck plate above the VLCC tanker's oil tank can be used with or without heavy primer, as well as with the inside of the tank or on the ceiling beam, column, etc. A steel material that can be suitably used for a long period of time as a structural body is provided, and economic effects such as reduction in ship manufacturing costs and maintenance costs can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between a Pcm value and a y-type weld crack test result.
FIG. 2 is a graph showing the relationship between the amount of Cr and Cu added and the results of a dew corrosion test with a coating film.
FIG. 3 is a graph showing the relationship between the amount of Cu added and the rust area ratio in a dew corrosion test with a coating film.

Claims (6)

プライマー塗装状態で使用する貨油タンク用耐食鋼において、化学成分として、mass%で、C:0.16%以下、Si:1.5%以下、Mn:3.0%以下、P:0.035%以下、S:0.01%以下、Al:0.23%以下を含み、さらに、Cu:0.1%〜1.4%、Cr:0.2〜4%、Ni:0.05〜0.7%のうちの1種以上を含み、残部がFeおよび不可避不純物からなり、下記の式(1)で表されるPcmの値が0.22以下であることを特徴とする貨油タンク用耐食鋼。
Pcm=C+Si/30+Mn/20+Cr/20+Cu/20+Ni/60+Mo/15+V/10+5B≦0.22 (1)
但し、元素記号はそれぞれの元素のmass%を示す。
In the corrosion resistant steel for coin oil tanks used in the primer coating state, as chemical components, mass%, C: 0.16% or less, Si: 1.5% or less, Mn: 3.0% or less, P: 0.035% or less, S: 0.01% In the following, Al: 0.23% or less is included, and Cu: 0.1% to 1.4%, Cr: 0.2 to 4%, Ni: 0.05 to 0.7%, and the balance is made of Fe and inevitable impurities Corrosion-resistant steel for freight oil tanks, wherein the value of Pcm represented by the following formula (1) is 0.22 or less.
Pcm = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ≦ 0.22 (1)
However, an element symbol shows mass% of each element.
プライマー塗装状態で使用する貨油タンク用耐食鋼において、化学成分として、mass%で、C:0.15%以下、Si:1.5%以下、Mn:0.2%以上3.0%以下、P:0.035%以下、S:0.005%以下、Al:0.23%以下を含み、さらに、Cr:0.2〜4%、Cu:0.2%〜1.0%、Ni:0.1〜0.7%のうち1種以上を含み、残部がFeおよび不可避不純物からなり、下記の式(1)で表されるPcmの値が0.22以下であることを特徴とする貨油タンク用耐食鋼。
Pcm=C+Si/30+Mn/20+Cr/20+Cu/20+Ni/60+Mo/15+V/10+5B≦0.22 (1)
但し、元素記号はそれぞれの元素のmass%を示す。
In the corrosion resistant steel for coin oil tanks used in the primer coating state, as chemical components, mass%, C: 0.15% or less, Si: 1.5% or less, Mn: 0.2% or more and 3.0% or less, P: 0.035% or less, S : Contains 0.005% or less, Al: 0.23% or less, and Cr: 0.2-4%, Cu: 0.2% -1.0%, Ni: 0.1-0.7%, and the balance is Fe and inevitable impurities Corrosion resistant steel for freight oil tanks, characterized in that the value of Pcm represented by the following formula (1) is 0.22 or less.
Pcm = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ≦ 0.22 (1)
However, an element symbol shows mass% of each element.
請求項1または請求項に記載の化学成分に加えて、さらにMo:0.5mass%以下を含むことを特徴する貨油タンク用耐食鋼。In addition to the chemical component of Claim 1 or Claim 2 , Mo: 0.5 mass% or less is further included, Corrosion-resistant steel for freight oil tanks characterized by the above-mentioned. 請求項1ないし請求項のいずれか1つに記載の化学成分に加えて、さらにNb:0.05mass%以下、V:0.12mass%以下、Ti:0.1mass%以下のうち、いずれか1種または2種以上を含むことを特徴する貨油タンク用耐食鋼。In addition to the chemical component according to any one of claims 1 to 3 , any one of Nb: 0.05 mass% or less, V: 0.12 mass% or less, Ti: 0.1 mass% or less, or Corrosion resistant steel for freight oil tanks, characterized by containing two or more. 請求項1ないし請求項のいずれか1つに記載の化学成分に加えて、さらにB:0.01mass%以下を含むことを特徴する貨油タンク用耐食鋼。In addition to the chemical component according to any one of claims 1 to 4 , B: 0.01 mass% or less is further included. 仕上げ温度を800℃以上で圧延を行い、その後冷却速度2℃/sec以上で600℃以下まで冷却を行うことを特徴とする請求項1ないし請求項のいずれか1つに記載の貨油タンク用耐食鋼の製造方法。The coin oil tank according to any one of claims 1 to 5 , wherein rolling is performed at a finishing temperature of 800 ° C or higher, and then cooling is performed at a cooling rate of 2 ° C / sec or higher to 600 ° C or lower. Of manufacturing corrosion-resistant steel.
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