JP5949526B2 - Corrosion resistant steel for holding coal ships and coal / ore combined ships - Google Patents
Corrosion resistant steel for holding coal ships and coal / ore combined ships Download PDFInfo
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Description
本発明は、石炭船、石炭・鉱石兼用船ホールドに用いられる耐食性に優れた鋼材に関するものである。
また、本発明の鋼材は、強度や延性、靱性に優れ、さらには溶接熱影響部（HAZ）靱性にも優れるので、溶接構造用鋼材としても適合するものである。
The present invention relates to a steel material having excellent corrosion resistance used for a coal ship and a coal / ore combined ship hold.
In addition, the steel material of the present invention is excellent in strength, ductility, and toughness, and is also excellent in weld heat affected zone (HAZ) toughness, and is therefore suitable as a steel material for welded structures.
ばら積み貨物船において、1990年代初頭に海難事故が相次いで発生し、国際問題となった。特に、石炭船や石炭・鉱石兼用船での事故が数多く報告されており、その原因の大部分は船倉（以下、「ホールド」とも言う）内での損傷であった。ばら積み貨物船では、積荷を直接ホールドに積載するため、腐食性の積荷の影響を受け易く、船倉内の腐食、特に石炭船、石炭・鉱石兼用船の船倉内の側壁部での孔食により、局所的に強度が減少することが問題と考えられている。この孔食が著しく進行した事例や、船の強度を確保する肋骨部分の板厚が極端に減少している事例が報告されている。 In bulk cargo ships, marine accidents occurred one after another in the early 1990s, which became an international issue. In particular, many accidents were reported on coal ships and coal / ore combined ships, most of which were caused by damage in the hold (hereinafter also referred to as “hold”). Bulk carriers are loaded directly on the hold, so they are easily affected by corrosive loads, and due to corrosion in the cargo hold, especially pitting corrosion on the side walls of the coal ship and coal / ore combined ships, It is considered that the strength is locally reduced. Cases in which this pitting corrosion has progressed remarkably and cases in which the thickness of the rib portion that ensures the strength of the ship has been extremely reduced have been reported.
上記したような孔食の発生するばら積み貨物船の側壁部は、シングルハルとなっていて、積荷と海水とは鋼材一枚で隔てられているだけである。
ホールド内の温度は、石炭が有する自己発熱性により上昇するため、海水と船倉内の温度差により、船倉側壁部には結露水が生じやすい。船倉側壁部に結露水が生じた場所では、石炭の硫黄成分が溶け出し、結露水と反応して硫酸を生成するので、船倉内は硫酸腐食が生じやすい低pH環境となっている。
The side wall portion of the bulk carrier that generates pitting corrosion as described above is a single hull, and the load and the seawater are only separated by a single steel material.
Since the temperature in the hold rises due to the self-heating property of the coal, condensed water tends to be generated on the side wall of the hold due to the temperature difference between the seawater and the hold. In the place where dew condensation water is generated in the side wall of the hold, the sulfur component of the coal dissolves and reacts with the dew formation water to produce sulfuric acid, so the inside of the hold is in a low pH environment where sulfuric acid corrosion is likely to occur.
このような船倉内の腐食対策として、船倉内には変性エポキシ系塗装が被覆厚さ約150〜200μmで施されている。しかし、石炭や鉱石によるメカニカルダメージや積荷搬出の際の重機による傷・磨耗により、塗装が剥がれる場合が多いため、十分な防食効果が得られていない。 As a countermeasure against such corrosion in the hold, a modified epoxy coating is applied in the hold with a coating thickness of about 150 to 200 μm. However, since the coating is often peeled off due to mechanical damage caused by coal or ore and damage and abrasion caused by heavy machinery during loading and unloading, sufficient anticorrosion effects are not obtained.
そこで、さらに腐食対策として、定期的に再塗装を行ったり、一部補修する方法が採られているが、このような方法は、非常に大きなコストがかかるため、船舶のメンテナンス費用を含めて、ライフサイクルコストの低減が課題となっている。 Therefore, as a countermeasure against corrosion, re-painting or partial repair methods are taken regularly, but since such a method is very expensive, including ship maintenance costs, Reduction of life cycle cost is an issue.
ところで、船舶用の耐食鋼としては、カーゴオイルタンク用やバラストタンク用に開発された鋼が知られている。しかし、石炭船および石炭・鉱石兼用船のホールド使用環境は、腐食環境（温度、湿度、腐食性物質など）の違い、および内容物によるメカニカルダメージの有無などの点で、カーゴオイルタンクやバラストタンク使用環境と全く異なっている。このため、石炭船および石炭・鉱石兼用船ホールド用の鋼としては、独自の材料設計や特性評価が必要とされる。 By the way, steel developed for cargo oil tanks and ballast tanks is known as a corrosion resistant steel for ships. However, the holding environment of coal ships and coal / ore combined ships is cargo oil tanks and ballast tanks in terms of differences in corrosive environments (temperature, humidity, corrosive substances, etc.) and the presence or absence of mechanical damage due to contents. It is completely different from the usage environment. For this reason, original material design and characteristic evaluation are required for steel for holding coal ships and coal / ore combined ships.
石炭船および石炭・鉱石兼用船ホールド用途に言及した従来技術としては、特許文献１、２および３が知られている。すなわち、石炭船および石炭・鉱石兼用船のホールド使用環境下での造船用耐食鋼の成分組成として、特許文献１にはCuおよびMgを必須成分組成とする鋼材が、また特許文献２にはCu，NiおよびSnを必須成分組成とする鋼材が、さらに特許文献３には、コストの改善を加味して、CuおよびSnを必須成分組成とした鋼材が、それぞれ開示されている。 Patent Documents 1, 2, and 3 are known as conventional techniques referring to a coal ship and a coal / ore combined ship holding application. That is, as a component composition of corrosion resistant steel for shipbuilding under the holding use environment of coal ships and coal / ore combined ships, Patent Document 1 discloses a steel material containing Cu and Mg as essential component compositions, and Patent Document 2 discloses Cu. , Ni and Sn are disclosed as steel components, and Patent Document 3 discloses steel materials containing Cu and Sn as essential components in consideration of cost improvement.
上記特許文献１に示された鋼材は、船舶外板、バラストタンク、カーゴオイルタンク、鉱炭船カーゴホールド等の使用環境で優れた鋼材と記載されている。しかし、この特許文献１では、鋼材の耐食性の評価方法として、カーゴオイルタンクとバラストタンクの腐食試験結果が示されているだけで、その試験結果については良好であることが掲げられているが、石炭船および石炭・鉱石兼用船のホールド使用環境を考慮した試験結果については示されていない。 The steel material disclosed in Patent Document 1 is described as an excellent steel material in a use environment such as a ship outer plate, a ballast tank, a cargo oil tank, and a coal carrier cargo hold. However, in this patent document 1, as a method for evaluating the corrosion resistance of steel materials, only the corrosion test results of the cargo oil tank and the ballast tank are shown, and the test results are stated to be good. Test results considering the use environment of coal ships and coal / ore combined ships are not shown.
前述したとおり、石炭船および石炭・鉱石兼用船のホールド内環境は、カーゴオイルタンクやバラストタンク内の環境と全く異なっている。
すなわち、ホールド内には塗装が施されているものの、石炭や鉱石は直接ホールドに積載されるため、ホール内の鋼材は石炭や鉱石によるメカニカルダメージを受ける。そのため、ホールド内の塗膜は剥がれやすい状況にあり、鋼材が直接腐食環境に曝される。
また、石炭の自己発熱性によりホールド内の温度は上昇する。一方、船側外板は海水と接しているため、海水と船倉内の温度差により、船倉側壁部は結露水に起因する乾湿繰返し環境にある。そして、この結露水と石炭に含まれる硫黄成分とが反応し、希硫酸が生じることが日本海事協会により報告されている。
As described above, the environment in the hold of the coal ship and the coal / ore combined ship is completely different from the environment in the cargo oil tank or the ballast tank.
That is, although the hold is painted, the coal and ore are loaded directly on the hold, so that the steel in the hole is mechanically damaged by the coal and ore. Therefore, the coating film in the hold is easily peeled off, and the steel material is directly exposed to the corrosive environment.
Moreover, the temperature in the hold rises due to the self-heating property of coal. On the other hand, since the ship side skin is in contact with seawater, the side wall of the hold is in a dry and wet environment due to dew condensation due to the temperature difference between the sea and the hold. It has been reported by the Japan Maritime Association that this condensed water reacts with the sulfur component contained in the coal to produce dilute sulfuric acid.
これに対して、カーゴオイルタンクの上甲板裏面は、防爆のためにタンク内に吹き込まれるイナートガス中に含まれるO2，CO2，SO2や原油から揮発するH2S等の腐食性ガス環境に曝される。底板は、原油由来の保護性フィルムがあるものの、お椀型の局部腐食が生じる環境に曝される。
また、バラストタンクは積荷がない時には、海水を注入して船舶の安定航行を可能にする役目を担うものであり、極めて厳しい腐食環境下におかれている。すなわち、バラストタンクの上甲板の裏側は、海水に浸からず、海水の飛沫を浴びる状態におかれているため、このような部位では電気防食が機能しない。さらに、この部位は、太陽光によって鋼材の温度が上昇するため、厳しい腐食環境となり、激しい腐食を受ける。また、バラストタンクの側壁面や底面は、海水に完全に浸漬されている場合には電気防食が働くものの、バラストタンクに海水が注入されていない場合には電気防食が全く働かないため、乾湿繰り返し環境と残留付着塩分の作用によって、激しい腐食を受ける。
In contrast, the back of the upper deck of the cargo oil tank is a corrosive gas environment such as O 2 , CO 2 , SO 2 contained in the inert gas blown into the tank for explosion prevention, and H 2 S volatilized from crude oil. Exposed to. Although the bottom plate has a protective film derived from crude oil, it is exposed to an environment in which bowl-shaped local corrosion occurs.
Further, the ballast tank plays a role of enabling stable navigation of the ship by injecting seawater when there is no cargo, and is placed in an extremely severe corrosive environment. That is, since the back side of the upper deck of the ballast tank is not immersed in seawater and is in a state of being splashed with seawater, the anticorrosion does not function in such a part. Furthermore, since the temperature of the steel material is increased by sunlight, this part becomes a severe corrosive environment and is severely corroded. In addition, the side and bottom surfaces of the ballast tank are electrically protected when they are completely immersed in seawater. However, when seawater is not injected into the ballast tank, they are not protected at all. It is subject to severe corrosion due to the environment and residual adhered salt.
上述したとおり、カーゴオイルタンクやバラストタンクと石炭船、石炭・鉱石兼用船のホールド内の腐食環境は全く異なっている。しかるに、特許文献１では、カーゴオイルタンクとバラストタンクに対する評価のみで、石炭船や石炭・鉱石兼用船にも特許文献１で示される鋼材が適応可能と述べているが、石炭船、石炭・鉱石兼用船のホールド内の腐食環境でも十分な耐食性が得られるというには問題がある。 As described above, the corrosive environment in the hold of the cargo oil tank or ballast tank and the coal ship or coal / ore combined ship is completely different. However, Patent Document 1 states that the steel materials shown in Patent Document 1 can be applied to coal ships and coal / ore combined ships only by evaluating the cargo oil tank and the ballast tank. There is a problem that sufficient corrosion resistance can be obtained even in a corrosive environment in the hold of the combined ship.
また、特許文献２，３では、石炭船や石炭・鉱石兼用船の環境を模擬した塗膜下腐食を評価し、耐食性に優れた鋼材を示している。しかしながら、ホールド内の塗装は石炭や鉱石によるメカニカルダメージで剥離しやすい状況にあるため、塗膜がない状態での鋼材の腐食の評価が重要となるが、この点については記載されていない。
すなわち、特許文献２，３では、ホールド使用環境下では不可避といえる石炭や鉱石によるメカニカルダメージで剥離しやすい状況を想定した評価試験を行っていない。
Further, Patent Documents 2 and 3 show a steel material excellent in corrosion resistance by evaluating under-coating corrosion simulating the environment of a coal ship or a coal / ore combined ship. However, since the coating in the hold is easily peeled due to mechanical damage caused by coal or ore, it is important to evaluate the corrosion of the steel material without the coating film, but this point is not described.
That is, Patent Documents 2 and 3 do not perform an evaluation test assuming a situation in which peeling is likely due to mechanical damage caused by coal or ore, which is unavoidable in a hold use environment.
上述したとおり、石炭船および石炭・鉱石兼用船ホールドに用いられる耐食性に優れた鋼材の開発には、石炭船および石炭・鉱石兼用船ホールド特有の腐食環境を考慮すると同時に、塗膜が剥離して塗膜がない状態での鋼材の腐食の評価が重要なのであるが、従来技術ではこの点について考慮が払われていなかった。
本発明は、上記の現状に鑑み開発されたもので、乾湿繰返し環境かつ低pH環境下において、塗膜剥離後の腐食を抑制することができる石炭船および石炭・鉱石兼用船ホールド用の耐食鋼を提供することを目的とする。
また、本発明は、上記した耐食性だけでなく、強度や延性、靱性、さらにはHAZ靱性にも優れ、溶接構造用鋼材としても適合する耐食鋼を提供することを目的とする。
As mentioned above, the development of steel materials with excellent corrosion resistance for use in coal ships and coal / ore combined ships holds, taking into account the corrosive environment peculiar to coal ships and coal / ore combined ships, and at the same time, the coating film peels off. Although it is important to evaluate the corrosion of steel in the absence of a coating film, the prior art has not taken this point into consideration.
The present invention was developed in view of the above-mentioned present situation, and is a corrosion resistant steel for holding a coal ship and a coal / ore combined ship that can suppress corrosion after peeling of the coating film in a dry and wet repeated environment and a low pH environment. The purpose is to provide.
Another object of the present invention is to provide a corrosion-resistant steel that is excellent not only in the above-described corrosion resistance but also in strength, ductility, toughness, and HAZ toughness, and is suitable as a steel material for welded structures.
一般に、船舶は、厚鋼板や薄鋼板、形鋼、棒鋼等の鋼材を溶接して建造されており、かかる鋼材の表面に防食塗膜が施されて使用される。しかし、石炭船、石炭・鉱石兼用船ホールド環境では、石炭・鉱石のメカニカルダメージで塗装は剥がれやすい状況にあるため、鋼材は乾湿繰返し環境かつ低pH環境下に曝される。したがって、塗膜剥離後も耐食性の発揮できる鋼材の開発が望まれている。 Generally, a ship is constructed by welding steel materials such as thick steel plates, thin steel plates, shaped steels, and steel bars, and the surface of such steel materials is used with an anticorrosion coating. However, in a coal ship or coal / ore combined ship hold environment, the paint is easily peeled off due to mechanical damage of the coal / ore, so that the steel is exposed to a dry and wet repeated environment and a low pH environment. Therefore, development of a steel material that can exhibit corrosion resistance even after the coating film is peeled is desired.
そこで、本発明者らは、石炭船、石炭・鉱石兼用船ホールド内の環境を模擬したラボ試験を開発し、その試験法を用いて各合金元素の影響を検討した。
その結果、Cu，Sbの添加で石炭船および石炭・鉱石兼用船ホールド内塗膜剥離後の鋼材の耐食性が向上することを見出した。しかしながら、Sbは環境負荷物質であり、今後Sb含有量が規制されていく可能性が高い。また、Cu添加の際には、割れを防ぐために高価なNiを必要とする。
Therefore, the present inventors have developed a laboratory test that simulates the environment in a coal ship and a coal / ore combined-use ship hold, and examined the influence of each alloy element using the test method.
As a result, the addition of Cu and Sb was found to improve the corrosion resistance of the steel material after peeling of the paint film in the coal ship and coal / ore combined ship hold. However, Sb is an environmentally hazardous substance, and there is a high possibility that the Sb content will be regulated in the future. Further, when adding Cu, expensive Ni is required to prevent cracking.
そこで、次に、Cu，Sbに代わる成分について種々検討を加えた。
その結果、
(1) Mo, Ｗを添加すると、それらの酸素酸により錆中のアニオン透過が抑制されると共に、FeMoO4やFeWO4といった難溶性の腐食生成物が形成されて耐食性が向上し、
(2) また、Nb，Taは、価格的にはNiと同程度であるが、これらの微量添加で耐酸性のある酸化物Nb2O5やTa2O5を形成させることにより、耐食性はさらに向上し、
(3) その結果、Cu量を減少でき、かつNiフリーとでき、
(4) さらに、Nbは、耐酸性の酸化物を形成するだけでなく、鋼中で微細な炭化物や窒化物あるいは炭窒化物を形成して、構造用用途においても十分な強度や延性、靱性を呈し、さらにはHAZ靱性の劣化防止に関しても極めて有効である
ことの知見を得た。
本発明は、上記の知見に立脚するものである。
Then, next, various examination was added about the component replaced with Cu and Sb.
as a result,
(1) When Mo and W are added, anion permeation in rust is suppressed by these oxygen acids, and insoluble corrosion products such as FeMoO 4 and FeWO 4 are formed to improve corrosion resistance.
(2) Also, Nb and Ta are about the same price as Ni, but the corrosion resistance is reduced by forming acid-resistant oxides Nb 2 O 5 and Ta 2 O 5 with the addition of these trace amounts. Further improve,
(3) As a result, the amount of Cu can be reduced and Ni-free,
(4) In addition, Nb not only forms acid-resistant oxides, but also forms fine carbides, nitrides, or carbonitrides in steel, which is sufficient for structural applications. In addition, we have obtained knowledge that it is extremely effective in preventing the deterioration of HAZ toughness.
The present invention is based on the above findings.
すなわち、本発明の要旨構成は次のとおりである。
１．質量％で、
Ｃ：0.01〜0.20％、
Si：0.01〜0.50％、
Mn：0.10〜2.0％、
Ｐ：0.025％以下、
Ｓ：0.035％以下、
Nb：0.001〜0.050％、
Al：0.005〜0.10％および
Cu：0.05％未満
を含有し、かつ
Mo：0.01〜0.50％および
Ｗ：0.01〜0.50％
のうちから選んだ１種または２種を含有し、残部はFeおよび不可避的不純物からなることを特徴とする石炭船および石炭・鉱石兼用船ホールド用の耐食鋼。
That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.01 to 0.20%
Si: 0.01 to 0.50%
Mn: 0.10 to 2.0%,
P: 0.025% or less,
S: 0.035% or less,
Nb: 0.001 to 0.050%,
Al: 0.005-0.10% and
Cu: contains less than 0.05%, and
Mo: 0.01-0.50% and W: 0.01-0.50%
Corrosion-resistant steel for holding coal ships and coal / ore combined ships, characterized in that it contains one or two selected from among them, and the balance consists of Fe and inevitable impurities.
２．さらに、質量％で、Ta：0.001〜0.10％を含有することを特徴とする前記１に記載の石炭船および石炭・鉱石兼用船ホールド用の耐食鋼。 2. Further, the corrosion-resistant steel for holding a coal ship and a coal / ore combined ship according to the above 1, characterized by containing Ta: 0.001 to 0.10% by mass%.
３．さらに、質量％で、Sb：0.01〜0.048％を含有することを特徴とする前記１または２に記載の石炭船および石炭・鉱石兼用船ホールド用の耐食鋼。 3. Further, the corrosion-resistant steel for holding a coal ship and a coal / ore combined ship according to the above 1 or 2, characterized by containing Sb: 0.01 to 0.048 % in mass%.
４．さらに、質量％で、Ca：0.0005〜0.010％を含有することを特徴とする前記１〜３のいずれかに記載の石炭船および石炭・鉱石兼用船ホールド用の耐食鋼。 4). Further, the corrosion-resistant steel for holding a coal ship and a coal / ore combined ship according to any one of the above 1 to 3, characterized by containing Ca: 0.0005 to 0.010% by mass%.
５．さらに、質量％で、Ｖ：0.002〜0.20％およびZr：0.001〜0.10％のうちから選んだ１種または２種を含有することを特徴とする前記１〜４のいずれかに記載の石炭船および石炭・鉱石兼用船ホールド用の耐食鋼。 5. Furthermore, by mass%, V: 0.002~0.20% Contact and Zr: Collier according to any one of 1 to 4, characterized by containing one or two chosen from among from 0.001 to 0.10% Corrosion resistant steel for holding coal and ore ships.
本発明によれば、石炭船、石炭・鉱石兼用船ホールド内の乾湿繰返し環境かつ低pH環境下において、優れた耐食性を発揮させることができ、その結果、メンテナンス費用を抑え、船舶のライフサイクルコストを低減させることのできる耐食性に優れた鋼材を得ることができる。
また、本発明の鋼材は、上記した耐食性だけでなく、十分な強度や延性、靱性を有し、さらにはHAZ靱性にも優れているので、溶接構造用鋼材としても有用である。
According to the present invention, it is possible to exhibit excellent corrosion resistance in a dry and wet repetitive environment and a low pH environment in a coal ship, a coal / ore combined ship hold, and as a result, the maintenance cost is reduced, and the life cycle cost of the ship is reduced. It is possible to obtain a steel material with excellent corrosion resistance that can reduce the above.
Further, the steel material of the present invention is useful not only as the above-mentioned corrosion resistance but also as a steel material for welded structure because it has sufficient strength, ductility and toughness, and also has excellent HAZ toughness.
以下、本発明を具体的に説明する。
まず、本発明において、鋼材の成分組成を前記の範囲に限定した理由について説明する。なお、以下の成分組成を表す％は、特に断らない限り質量％を意味するものとする。
Ｃ：0.01〜0.20％
Ｃは、鋼の強度を上昇させるのに有効な元素であり、本発明では所望の強度（490〜620MPa）を得るために0.01％以上含有させる。一方、0.20％を超える含有は、溶接性および溶接熱影響部の靭性を低下させる。よって、Ｃは0.01〜0.20％の範囲とする。好ましくは0.05〜0.15％の範囲である。
Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
C: 0.01-0.20%
C is an element effective for increasing the strength of steel. In the present invention, C is contained in an amount of 0.01% or more in order to obtain a desired strength (490 to 620 MPa). On the other hand, the content exceeding 0.20% lowers the weldability and the toughness of the heat affected zone. Therefore, C is in the range of 0.01 to 0.20%. Preferably it is 0.05 to 0.15% of range.
Si：0.01〜0.50％
Siは、脱酸剤として添加される、また鋼の強度を高める元素であり、本発明では0.01％以上を含有させる。しかしながら、0.50％を超える添加は、鋼の靱性を劣化させるので、Siの上限は0.50％とする。加えてSiは酸性環境下で、防食皮膜を形成して耐食性を向上させる。この効果を得るには、好ましくは0.02〜0.15％の範囲である。
Si: 0.01-0.50%
Si is an element that is added as a deoxidizer and increases the strength of steel, and in the present invention, 0.01% or more is contained. However, addition exceeding 0.50% deteriorates the toughness of the steel, so the upper limit of Si is 0.50%. In addition, Si improves corrosion resistance by forming an anticorrosion film in an acidic environment. In order to obtain this effect, it is preferably in the range of 0.02 to 0.15%.
Mn：0.10〜2.0％
Mnは、低コストで鋼の強度を上げることができ、さらに熱間脆性を防止する効果があるので、0.10％以上含有させる。しかしながら、2.0％を超える添加は、鋼の靱性および溶接性を低下させるため、Mnは2.0％以下とする。なお、強度の確保と介在物抑制の観点から、好ましくは0.8〜1.4％の範囲である 。
Mn: 0.10 to 2.0%
Mn can increase the strength of the steel at a low cost, and further has an effect of preventing hot brittleness, so 0.10% or more is contained. However, addition exceeding 2.0% lowers the toughness and weldability of steel, so Mn is made 2.0% or less. From the viewpoint of securing strength and suppressing inclusions, the content is preferably in the range of 0.8 to 1.4%.
Ｐ：0.025％以下
Ｐは、粒界に偏析することで、鋼の母材靱性のみならず、溶接性および溶接部靱性を劣化させる有害な元素であるので、できるだけ低減することが望ましい。特に、Ｐ含有量が0.025％を超えると、母材靱性および溶接部靱性の低下が大きくなる。よってＰ量は0.025％以下とする。
P: 0.025% or less P is a harmful element that deteriorates not only the base metal toughness of steel but also the weldability and weld toughness by segregating at the grain boundaries. In particular, when the P content exceeds 0.025%, the deterioration of the base metal toughness and weld zone toughness becomes large. Therefore, the P content is 0.025% or less.
Ｓ：0.035％以下
Ｓは、Cuと金属間化合物Cu2Sを生成し、耐硫酸性を向上させる。しかしながら、Mnと局部腐食の起点となるMnＳを形成し、耐局部腐食性を低下させ、さらに鋼の靱性および溶接性を劣化させる有害な元素であるので、本発明では0.035％以下とした。
S: 0.035% or less S generates Cu and an intermetallic compound Cu 2 S and improves sulfuric acid resistance. However, since it is a harmful element that forms Mn and MnS, which is a starting point of local corrosion, lowers the local corrosion resistance, and further deteriorates the toughness and weldability of the steel, it is set to 0.035% or less in the present invention.
Nb：0.001〜0.050％
Nbは、酸化皮膜Nb2O5を生成して耐酸性を向上させるだけでなく、鋼の強度を高める元素である。すなわち、Nbは、鋼中で微細な炭化物や窒化物あるいは炭窒化物を形成して、構造用鋼材として必要な強度や延性、靱性を得るのに有用である。さらに、Nbは、HAZ靱性の劣化防止に関しても極めて有効である。このような効果を得るためには、0.001％以上含有させる必要があるが、0.050％を超えて添加すると靱性が低下するため、Nbは0.001〜0.050％の範囲で含有させるものとした。好ましくは0.001〜0.030％の範囲である。
Nb: 0.001 to 0.050%
Nb is an element that not only generates an oxide film Nb 2 O 5 to improve acid resistance but also increases the strength of steel. That is, Nb is useful for forming fine carbides, nitrides, or carbonitrides in steel to obtain the strength, ductility, and toughness necessary for structural steel materials. Furthermore, Nb is extremely effective in preventing deterioration of HAZ toughness. In order to obtain such an effect, it is necessary to contain 0.001% or more. However, if added over 0.050%, the toughness decreases, so Nb is contained in the range of 0.001 to 0.050%. Preferably it is 0.001 to 0.030% of range.
Al：0.005〜0.10％
Alは、脱酸剤として添加される。このためには0.005％以上の含有を必要とするが、0.10％を超える含有は溶接した場合に、溶接金属部の靱性を低下させる。よってAl量は0.005〜0.10％の範囲とした。
Al: 0.005-0.10%
Al is added as a deoxidizer. For this purpose, a content of 0.005% or more is required, but a content exceeding 0.10% lowers the toughness of the weld metal part when welding. Therefore, the Al content is set in the range of 0.005 to 0.10%.
Cu：0.05％未満
Cuは、腐食生成物を緻密にし、地鉄へのH2O，O2，Cl-の拡散を抑制する。これにより、鋼の耐食性が向上するが、圧延により表面割れを発生させる。これを防止するにはCu添加量の半分程度のNi添加が必要であるが、Niは高価であるため、コストの面からCuは0.05％未満に抑制するものとした。
Cu: less than 0.05%
Cu densifies the corrosion products and suppresses the diffusion of H 2 O, O 2 , and Cl − into the steel. This improves the corrosion resistance of the steel, but causes surface cracks by rolling. In order to prevent this, it is necessary to add Ni that is about half of the Cu addition amount. However, since Ni is expensive, Cu is suppressed to less than 0.05% from the viewpoint of cost.
Mo：0.01〜0.50％およびＷ：0.01〜0.50％のうちから選んだ１種または２種
Mo，Ｗはそれぞれ、耐酸性に優れた元素で、裸材さらには塗膜下腐食を抑制する効果も有している。これらの元素は、母材から溶出した際に酸素酸を形成し、これらがアニオンを電気的に反発させ、アニオンが地鉄表面まで侵入することを防止して、耐食性を向上させる。さらに、MoおよびＷは、FeMoO4やFeWO4といった難溶性の腐食性物質を形成することで耐食性を向上させる。しかし、Mo，Ｗ含有量がそれぞれ0.01％に満たないとその添加効果に乏しく、一方0.50％を超えて添加しても効果が飽和するだけでなく、むしろコスト高となるため、Mo，Ｗ量はそれぞれ0.01〜0.50％の範囲とした。
One or two selected from Mo: 0.01-0.50% and W: 0.01-0.50%
Each of Mo and W is an element having excellent acid resistance, and has an effect of suppressing corrosion under the bare material and further under the coating film. When these elements are eluted from the base material, they form oxygen acids, which electrically repel anions and prevent the anions from penetrating to the surface of the ground iron, thereby improving corrosion resistance. Furthermore, Mo and W improve corrosion resistance by forming a hardly soluble corrosive substance such as FeMoO 4 or FeWO 4 . However, if the contents of Mo and W are less than 0.01%, the effect of addition is poor. On the other hand, adding more than 0.50% not only saturates the effect but also increases the cost. Respectively in the range of 0.01 to 0.50%.
以上、基本成分について説明したが、本発明では、必要に応じて、以下に述べる元素を適宜含有させることができる。
Ta：0.001〜0.10％
Taも、Nb同様、酸化皮膜Ta2O5を生成して耐酸性を向上させるだけでなく、鋼の強度を高める元素であるので、必要とする強度に応じて選択して含有させることができる。このような効果を得るためには、Taは0.001％以上含有させる必要があるが、0.10％を超えて添加すると靱性が低下するため、上記の範囲で含有させるものとした。
The basic components have been described above. In the present invention, the following elements can be appropriately contained as necessary.
Ta: 0.001 to 0.10%
Ta, as well as Nb, is an element that not only improves the acid resistance by generating an oxide film Ta 2 O 5 but also increases the strength of the steel, so it can be selected and contained according to the required strength. . In order to obtain such an effect, Ta needs to be contained in an amount of 0.001% or more, but if added over 0.10%, the toughness is lowered, so that it is contained in the above range.
Sb：0.01〜0.10％
Sbは、鋼材に合金元素として含有させると、低pH環境においてSbとして錆層の地鉄近傍に析出する。Sbは大きな水素化電圧を持つため、Sbが析出した部分では水素発生反応が抑制され、耐食性が向上する。また、Cuと金属間化合物であるCu2Sbを形成することで、さらに耐食性が向上する。一方、Sbは環境負荷物質である。よって、Sbは0.01〜0.10％の範囲に制限した。
Sb: 0.01-0.10%
When Sb is contained as an alloy element in steel, it precipitates in the vicinity of the ground iron in the rust layer as Sb in a low pH environment. Since Sb has a large hydrogenation voltage, the hydrogen generation reaction is suppressed at the portion where Sb is deposited, and the corrosion resistance is improved. Further, by forming the Cu 2 Sb is Cu intermetallic compound, to further improve the corrosion resistance. On the other hand, Sb is an environmentally hazardous substance. Therefore, Sb was limited to a range of 0.01 to 0.10%.
Ca：0.0005〜0.010％
Caは、数ppm〜100ppm程度の添加で腐食界面のpHを上昇させる効果があるため、石炭腐食環境のような硫酸環境では腐食抑制効果が認められる。また、Caは、介在物の形態を制御して鋼の延性および靱性を高める元素である。このような効果を発揮させるためには、Caは少なくとも0.0005％含有させる必要がある。しかし、過度に添加すると、粗大な介在物を形成し母材の靱性を劣化させるので、Ca量の上限は0.010％とした。
Ca: 0.0005 to 0.010%
Since Ca has the effect of increasing the pH of the corrosion interface when added in the range of several ppm to 100 ppm, a corrosion inhibiting effect is recognized in a sulfuric acid environment such as a coal corrosive environment. Further, Ca is an element that increases the ductility and toughness of steel by controlling the form of inclusions. In order to exert such an effect, it is necessary to contain Ca at least 0.0005%. However, if added excessively, coarse inclusions are formed and the toughness of the base material is deteriorated, so the upper limit of the Ca content was made 0.010%.
Ｖ：0.002〜0.20％、Ti：0.001〜0.030％およびZr：0.001〜0.10％のうちから選んだ１種または２種以上
Ｖ，Ti，Zrはいずれも、鋼の強度を高める元素であり、必要とする強度に応じて選択して含有させることができる。このような効果を得るためには、Ｖは0.002％以上、Tiは0.001％以上、Zrは0.001％以上含有させることが好ましい。しかしながら、Ｖは0.20％、Tiは0.030％、Zrは0.10％を超えて添加すると靱性が低下するため、Ｖ，Ti，Zrはそれぞれ、上記の範囲で含有させることが好ましい。
One or more selected from V: 0.002 to 0.20%, Ti: 0.001 to 0.030%, and Zr: 0.001 to 0.10% V, Ti, and Zr are elements that increase the strength of steel and are necessary It can be selected and contained depending on the strength. In order to obtain such an effect, it is preferable that V is 0.002% or more, Ti is 0.001% or more, and Zr is 0.001% or more. However, when V is added to 0.20%, Ti is added to 0.030%, and Zr exceeds 0.10%, the toughness is lowered. Therefore, V, Ti, and Zr are preferably contained in the above ranges.
本発明における成分組成のうち、上記以外の成分はFeおよび不可避的不純物である。ただし、本発明の効果を害さない範囲内であれば、上記以外の成分の含有を拒むものではない。 Among the component compositions in the present invention, components other than those described above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.
次に、本発明に係る耐食鋼材の好適製造方法について説明する。
連続鋳造などにより得られた鋼材を、そのまま、あるいは一旦冷却後、加熱したのち、熱間圧延を行う。耐食性を発揮させるための熱処理条件は問わないが、機械的特性からは適切な圧下比を確保する必要がある。熱間圧延の際、圧延後の冷却速度を制御することで、引張強さ：490MPa級以上の鋼材を製造することができる。例えば、仕上温度を750℃以上とし、その後150℃/min以上の冷却速度で600℃以下まで冷却することが好ましい。仕上げ温度が750℃未満では変形抵抗が大きくなって形状不良が生じやすく、また冷却速度が150℃/min未満では490MPa級以上の強度が得られ難い。
Next, the suitable manufacturing method of the corrosion-resistant steel material which concerns on this invention is demonstrated.
A steel material obtained by continuous casting or the like is heated as it is or after being cooled and then hot-rolled. The heat treatment conditions for exhibiting corrosion resistance are not limited, but it is necessary to ensure an appropriate reduction ratio from the mechanical characteristics. By controlling the cooling rate after rolling during hot rolling, a steel material having a tensile strength of 490 MPa or higher can be produced. For example, it is preferable to set the finishing temperature to 750 ° C. or higher and then cool to 600 ° C. or lower at a cooling rate of 150 ° C./min or higher. When the finishing temperature is less than 750 ° C, deformation resistance increases and shape defects tend to occur, and when the cooling rate is less than 150 ° C / min, it is difficult to obtain a strength of 490 MPa or higher.
表１に示す成分になる鋼を、真空溶解炉で溶製または転炉溶製後、連続鋳造によりスラブとした。ついで、スラブを加熱炉に装入して1200℃に加熱後、仕上圧延終了温度：800℃の熱間圧延により25mm厚の鋼板とした。 The steel which becomes a component shown in Table 1 was made into a slab by continuous casting after melting or converter melting in a vacuum melting furnace. Next, the slab was charged into a heating furnace and heated to 1200 ° C., and then finished to a finish rolling temperature of 800 ° C. to obtain a 25 mm thick steel plate.
本発明者らは、石炭船および石炭・鉱石兼用船のホールド内の腐食でもっとも船舶の破壊に影響を与える孔食発生のメカニズムを調査したところ、次のとおりであった。
すなわち、ばら積み貨物船の側壁部は、シングルハルとなっていて、積荷と海水とは鋼材１枚を隔てているだけである。そのため、海水と船倉内の温度差により、船倉側壁部には結露水が生じ、鋼材及び石炭表面が濡れ、石炭表面に吸着しているH2SO4由来の物質が水膜に滲出する。メニスカスを形成する石炭下で孔食が進展し、メニスカス部分では、鋼材の腐食にＨ+が消費されていくため、Ｈ+濃度が減少していく。一方、石炭表面にはＨ+が多く存在するため、石炭表面とメニスカス部分でＨ+濃度の差が生まれる。その化学ポテンシャルの差を駆動力とし、メニスカス部分に石炭表面 からＨ+が供給されると考えられる。そして、乾燥過程で未反応のＨ+は再び石炭表面に固着し、次の結露過程で腐食反応に使用され、この過程が長期的なサイクルで起こり、メニスカス部分で腐食がより進行し、孔食が形成されていく。
本メカニズムを基に、石炭船および石炭・鉱石兼用船のホールド内の孔食を実験室的に模擬すべく以下の条件とした。
The inventors of the present invention have investigated the mechanism of pitting corrosion that most affects the destruction of ships due to corrosion in the hold of coal ships and coal / ore combined ships.
That is, the side wall portion of the bulk carrier is a single hull, and the load and seawater only separate one steel material. Therefore, due to the temperature difference between the seawater and the hold, dew condensation water is generated on the side wall of the hold, the steel material and the surface of the coal are wetted, and the H 2 SO 4 -derived substance adsorbed on the surface of the coal oozes into the water film. Pitting corrosion progresses under the coal that forms the meniscus, and H + is consumed in the meniscus portion due to the corrosion of the steel material, so the H + concentration decreases. On the other hand, since a large amount of H + exists on the coal surface, a difference in H + concentration is produced between the coal surface and the meniscus portion. The difference in chemical potential is used as the driving force, and it is thought that H + is supplied to the meniscus from the coal surface. Unreacted H + adheres to the coal surface again during the drying process, and is used for the corrosion reaction in the next condensation process. This process occurs in a long-term cycle, and the corrosion progresses more in the meniscus area, causing pitting corrosion. Will be formed.
Based on this mechanism, the following conditions were used to simulate pitting corrosion in the hold of coal ships and coal / ore combined ships.
前記鋼板から、５mmt×50mmW×75mmLの試験片を採取し、その試験片の表面をショットブラストして、表面のスケールや油分を除去した。この面を試験面とすることにより、塗膜剥離後の鋼材の耐食性を評価した。裏面と端面をシリコン系シールでコーティングした後、アクリル製の治具に嵌め込み、その上に石炭５ｇを敷き詰め、低温恒温恒湿器により、図１に示すように、雰囲気Ａ（温度：60℃、湿度：95％、20時間）と雰囲気Ｂ（温度：30℃、湿度：95％、３時間）を遷移時間：0.5時間で繰り返す温湿度サイクルを28日間与えた。なお、石炭は50ｇを秤量し、常温で100mlの蒸留水に２時間浸漬したのち、ろ過を行い、200mlに希釈した石炭浸出液のpHが3.0になるものを用いた。本実施例は、こうした条件で試験を行うことにより、石炭船および石炭・鉱石兼用船のホールド内の腐食に大きな影響を及ぼす温湿度環境、結露状況を模擬している。試験後、錆剥離液を用い、各試験片の錆を剥離し、鋼材の重量減少量を測定して腐食量とした。また、生じた最大孔食深さをデプスメーターを用いて測定した。 A test piece of 5 mmt × 50 mmW × 75 mmL was collected from the steel plate, and the surface of the test piece was shot blasted to remove the scale and oil on the surface. By using this surface as a test surface, the corrosion resistance of the steel material after coating film peeling was evaluated. After coating the back and end surfaces with silicon seals, they are fitted into acrylic jigs, and 5 g of coal is laid on top of them, and the atmosphere A (temperature: 60 ° C, Humidity: 95%, 20 hours) and atmosphere B (temperature: 30 ° C., humidity: 95%, 3 hours) were repeated for 28 days at a transition time: 0.5 hour. In addition, 50 g of coal was weighed and immersed in 100 ml of distilled water at room temperature for 2 hours, followed by filtration. The coal leachate diluted to 200 ml had a pH of 3.0. In this example, the test is conducted under such conditions, thereby simulating a temperature / humidity environment and a dew condensation state that greatly affect the corrosion in the hold of the coal ship and the coal / ore combined ship. After the test, a rust remover was used to remove the rust of each test piece, and the weight loss of the steel material was measured to obtain the amount of corrosion. Further, the maximum pitting corrosion depth was measured using a depth meter.
さらに、前記鋼板を用い、溶接入熱：10kJ/mmのCO2溶接により、溶接継手を作製し、得られた溶接継手の溶接金属と母材の境界（ボンド）から３mm母材側の溶接熱影響部（HAZ）にVノッチの底が対応するようにシャルピー試験片を採取して、シャルピー衝撃試験を行った。この試験は、０℃で３本の試験片について行い、その平均値を衝撃吸収エネルギーとした。この衝撃吸収エネルギーが70Ｊ以上であれば、溶接部靱性に優れていると言える。
得られた結果を表２に示す。
Furthermore, a welded joint was prepared by CO 2 welding with a welding heat input of 10 kJ / mm using the steel sheet, and the welding heat on the side of the 3 mm base metal from the boundary (bond) between the weld metal and base metal of the welded joint obtained. Charpy specimens were collected so that the bottom of the V notch corresponded to the affected area (HAZ), and a Charpy impact test was performed. This test was performed on three test pieces at 0 ° C., and the average value was taken as the impact absorption energy. If this shock absorption energy is 70 J or more, it can be said that the weld toughness is excellent.
The obtained results are shown in Table 2.
No.１，３，５，７〜９，13〜17，20〜22に示す発明例は、No.26に示す比較例に比べて、腐食量で12〜26％、最大孔食深さで16〜43％低減しており、耐食性に優れていることが分かる。また、これらの発明例は、HAZ靱性も75Ｊ以上と極めて良好であり、溶接構造用鋼材として十分に適合することが分かる。これに対し、Nbを含有しない比較例２，４，６，10〜12，18〜19，23〜25は、耐食性は良好であったが、HAZ靱性に劣っていた。
一方、No.27，28から、Ni単独では耐食性向上は確認できず、またNo.32のSn単独添加でも耐食性の向上は見られなかった。さらに、No.29〜31のCr添加では、腐食量、最大孔食深さが共に大きくなり、Crが耐食性を劣化させたことが分かる。よって、本発明では、Crは不可避的不純物の範囲までとし、0.050％以下であることが好ましい。なお、No.32の比較例で示すように、Sbの代わりにSnを含有させても腐食減量および最大孔食深さを抑制する効果はない。さらに、Snは、Cuと共存するとCuの融点を下げ、さらに鉄への固溶度も下げるため、Cuが鋼材表面の粒界に析出し、熱間割れを引き起こす。そのため、Snの添加は行わず、不可避的不純物レベルとし、0.005％未満とすることが好ましい。
Inventive examples shown in No. 1, 3, 5, 7-9, 13-17, 20-22 are 12-26% in corrosion amount and maximum pitting depth compared with comparative examples shown in No. 26. It is reduced by 16 to 43%, indicating that the corrosion resistance is excellent. In addition, it can be seen that these inventive examples have extremely good HAZ toughness of 75 J or more, and are sufficiently suitable as steel materials for welded structures. In contrast, Comparative Examples 2, 4, 6, 10 to 12, 18 to 19, and 23 to 25 containing no Nb had good corrosion resistance but were inferior in HAZ toughness.
On the other hand, from Nos. 27 and 28, no improvement in corrosion resistance could be confirmed with Ni alone, and no improvement in corrosion resistance was observed with the addition of No. 32 Sn alone. Furthermore, it can be seen that with the addition of Cr of Nos. 29 to 31, both the corrosion amount and the maximum pitting corrosion depth increased, and Cr deteriorated the corrosion resistance. Therefore, in the present invention, Cr is in the range of inevitable impurities, and is preferably 0.050% or less. As shown in the comparative example No. 32, even if Sn is contained instead of Sb, there is no effect of suppressing the corrosion weight loss and the maximum pitting corrosion depth. Furthermore, Sn coexists with Cu, lowering the melting point of Cu and lowering the solid solubility in iron, so Cu precipitates at the grain boundaries on the steel surface, causing hot cracking. For this reason, Sn is not added, and the inevitable impurity level is preferably set to less than 0.005%.
以上、本発明の効果が確認された。本実施例では、石炭船および石炭・鉱石兼用船ホールド内の環境を模擬した試験法として図１に示した方法に拠ったが、実際にホールド内に設置して評価した場合と極めて整合性がある結果が得られている。なお、雰囲気Ａ，Ｂの条件、遷移時間、サイクル数、石炭の調整方法および石炭浸出液のpHの値等の条件は、上述の例に限られるものではなく、鋼材のホールド使用環境に応じて、適宜変更することができるのは言うまでもない。 As mentioned above, the effect of this invention was confirmed. In this example, the method shown in FIG. 1 was used as a test method for simulating the environment in a coal ship and a coal / ore combined-use ship hold, but it was extremely consistent with the case where it was actually installed in the hold and evaluated. Some results have been obtained. In addition, conditions such as atmosphere A and B conditions, transition time, cycle number, coal adjustment method and coal leachate pH value are not limited to the above-described examples, Needless to say, it can be changed as appropriate.
本発明鋼材を用いれば、石炭や鉱石のメカニカルダメージにより塗膜が剥離し易く、さらに乾湿繰返し環境かつ低pH環境下にある石炭および石炭・鉱石兼用船ホールドにおいて、腐食を効果的に抑制することができ、ひいては船舶のライフサイクルコストの低減を図ることができる。 If the steel of the present invention is used, the coating film is easily peeled off due to mechanical damage of coal and ore, and the corrosion is effectively suppressed in coal and coal / ore combined ship hold in dry and wet repeated environment and low pH environment. As a result, the life cycle cost of the ship can be reduced.
Claims (5)
Ｃ：0.01〜0.20％、
Si：0.01〜0.50％、
Mn：0.10〜2.0％、
Ｐ：0.025％以下、
Ｓ：0.035％以下、
Nb：0.001〜0.050％、
Al：0.005〜0.10％および
Cu：0.05％未満
を含有し、かつ
Mo：0.01〜0.50％および
Ｗ：0.01〜0.50％
のうちから選んだ１種または２種を含有し、残部はFeおよび不可避的不純物からなることを特徴とする石炭船および石炭・鉱石兼用船ホールド用の耐食鋼。 % By mass
C: 0.01 to 0.20%
Si: 0.01 to 0.50%
Mn: 0.10 to 2.0%,
P: 0.025% or less,
S: 0.035% or less,
Nb: 0.001 to 0.050%,
Al: 0.005-0.10% and
Cu: contains less than 0.05%, and
Mo: 0.01-0.50% and W: 0.01-0.50%
Corrosion-resistant steel for holding coal ships and coal / ore combined ships, characterized in that it contains one or two selected from among them, and the balance consists of Fe and inevitable impurities.
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