JP5862464B2 - Corrosion resistant steel for coal ships or coal / ore combined ships - Google Patents
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Description
本発明は、石炭船または石炭・鉱石兼用船の船倉に用いられる耐食性に優れた鋼材に関する。 The present invention relates to a steel material having excellent corrosion resistance used for a cargo ship or a coal / ore combined ship.
ばら積み貨物船において、１９９０年代初頭に海難事故が相次ぎ国際問題となった。特に、石炭船や石炭・鉄鉱石兼用船で事故が多く報告されおり、その原因の大部分は船倉内の損傷であった。ばら積み貨物船では、積荷を直接船倉（以下「ホールド」とも言う）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・鉄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。 In bulk cargo ships, marine accidents became an international issue one after another in the early 1990s. In particular, many accidents were reported on coal ships and coal / iron ore combined ships, most of which were damage in the hold. Bulk cargo ships are loaded directly into the hold (hereinafter also referred to as “hold”) and are therefore susceptible to corrosive loads, especially in the hold of coal ships and coal / iron ore combined ships. It is considered a problem that the strength locally decreases due to pitting corrosion at the side wall portion.
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おり、事故防止のために非特許文献１において、ホールド側壁部鋼材の切替基準を図面板厚の７０％以下の場合、ホールド肋骨部鋼材の切替基準を図面板厚の７５％以下の場合（但し、図面板厚−腐食代−腐食余裕厚より大きな値とする必要はない）と定めている。 Cases in which this pitting corrosion has progressed remarkably and cases in which the thickness of the rib portion ensuring the strength of the ship has been extremely reduced have been reported. When the switching standard is 70% or less of the drawing plate thickness, the switching standard of the hold rib steel material is 75% or less of the drawing plate thickness (however, it is necessary to make the value larger than the drawing plate thickness-corrosion allowance-corrosion margin thickness No).
前記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隔てているだけである。そして、船倉内の温度は、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。 The side wall of the bulk carrier where pitting occurs is a single hull, and the load and seawater are separated from each other only by one steel material. And since the temperature in a hold rises by the self-heating property which coal has, dew condensation water tends to arise in a hold side wall part by the temperature difference between seawater and a hold.
こうした、船倉側壁部に結露水が生じた場所に、石炭のＳＯ４ ２−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。そこで、低ｐＨ環境に対しては「水素発生反応を抑制」すること、鉄の溶解のカウンターアニオンとなるＳＯ４ ２−の地鉄−錆界面への透過に対しては、「ＳＯ４ ２−の錆透過抑制」することの２つの防食メカニズムが必要となる。 Since coal SO 4 2- dissolves in the place where dew condensation water is generated on the side wall of the cargo hold and reacts with the dew condensation water to produce sulfuric acid, the inside of the hold has a low pH environment in which sulfuric acid corrosion is likely to occur. Yes. Therefore, for the low pH environment that "hydrogen generation reaction inhibition", the SO 4 2-which is a counter anion of dissolution of iron base steel - for the penetration into rust interface, "SO 4 2- Two anticorrosion mechanisms are required to “suppress rust permeation”.
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施されている。しかし、石炭や鉄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 to 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 iron ore, and scratches and wear 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や一部補修する方法が取られているが、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、ライフサイクルコストを低減させることが課題となっている。 Therefore, as a countermeasure against corrosion, methods of repainting and partial repairs are taken regularly, but such methods are very expensive and reduce life cycle costs including ship maintenance costs. It is a problem to make it.
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。 By the way, steel developed for cargo oil tanks and ballast tanks is known as a corrosion resistant steel for ships.
カーゴオイルタンクの上甲板裏面は、防爆対策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、フィルムが剥離した箇所でお椀型の局部腐食が生じる環境に曝される。例えば、特許文献１では、「ｐＨ低下抑制による耐食性向上」および「硫化物微細分散による耐局部腐食性向上」の防食メカニズムを利用することによる耐食鋼が提案されている。 The upper back of the cargo oil tank is exposed to a corrosive gas environment such as O 2 , CO 2 , SO 2 contained in the inert gas blown into the tank for explosion-proof measures, and H 2 S volatilized from crude oil. . Although there is a protective film derived from crude oil, the bottom plate is exposed to an environment in which bowl-shaped local corrosion occurs at the point where the film is peeled off. For example, Patent Document 1 proposes a corrosion-resistant steel using the anti-corrosion mechanism of “improvement of corrosion resistance by suppressing pH decrease” and “improvement of local corrosion resistance by fine dispersion of sulfide”.
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。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ないため、電気防食が機能せず、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部分で、腐食環境ではあるが、電気防食作用が機能する。 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. The back side of the upper deck of the ballast tank is not immersed in seawater and is not in a state of being splashed with seawater, so the anticorrosion function does not function, and the temperature of the steel material rises due to sunlight. It becomes a severe corrosive environment and receives severe corrosion. Moreover, although the side wall surface and the bottom surface of the ballast tank are completely immersed in seawater and are in a corrosive environment, the anticorrosive action functions.
しかし、積荷が無く運行する場合には、バラストタンクに海水が注入されておらず、バラストタンク全体で、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。例えば、特許文献２では、錆を緻密化することにより、Ｃｌ−の透過を抑制することが、特許文献３では、ＷＯ４ ２−により、電気化学的にＣｌ−の透過を抑制する防食メカニズムを利用した耐食鋼が提案されている。 However, when operating without cargo, seawater is not injected into the ballast tank, and the entire ballast tank does not have anti-corrosion protection. Therefore, it is subject to severe corrosion due to repeated dry and wet environments and residual adhered salt. . For example, in Patent Document 2, it is possible to suppress the passage of Cl − by densifying rust, and in Patent Document 3, a corrosion prevention mechanism that electrochemically suppresses the transmission of Cl − by WO 4 2−. Corrosion-resistant steel has been proposed.
前述したように、石炭船および石炭・鉱石兼用船においては、乾湿繰り返しで硫酸の濃縮が起こる低ｐＨ環境の場合、水素発生反応の抑制およびＳＯ４ ２−の錆−地鉄界面への透過を抑制しなければならない。このように、石炭船および石炭・鉱石兼用船の船倉とバラストタンクおよびオイルタンクにおいては、腐食環境や防食メカニズムが異なるためバラストタンク用およびオイルタンク用の耐食鋼をそのまま転用することは出来ない。このため、石炭船または石炭・鉱石兼用船の船倉用の鋼としては、独自の材料設計や特性評価が必要とされる。 As described above, in a coal ship and a coal / ore combined ship, in a low pH environment where sulfuric acid is concentrated by repeated drying and wetting, suppression of hydrogen generation reaction and permeation of SO 4 2− to the rust-steel interface are prevented. Must be suppressed. As described above, the corrosive environment and the anticorrosion mechanism are different in the hold of the coal ship and the coal / ore combined ship, and the corrosion resistant steel for the ballast tank and the oil tank cannot be diverted as they are. For this reason, original material design and characteristic evaluation are required for steel for coal ships or coal / ore combined ships.
また、石炭船または石炭・鉱石兼用船の船倉用途に言及した従来技術としては、特許文献１、４および５がある。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化学成分組成として、特許文献１にはＣｕおよびＭｇを必須成分組成とした鋼材が、特許文献４にはＣｕ、ＮｉおよびＳｎを必須成分組成とした鋼材が、そして、特許文献５にはさらにコスト面の改善を目的とした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。 Further, Patent Documents 1, 4 and 5 are known as conventional techniques referring to the use of a coal ship or a coal / ore combined ship. As chemical composition compositions of corrosion-resistant steel for shipbuilding under the use environment of coal ships and coal / ore combined ships, Patent Document 1 discloses steel materials containing Cu and Mg as essential component compositions, and Patent Document 4 discloses Cu and Ni. Steel materials having Sn and Sn as essential components, and Patent Document 5 disclose steel materials having Cu and Sn as essential components for the purpose of further improving cost.
しかしながら、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、鉱石船カーゴホールド等の共通的な使用環境での優れた鋼材を対象としているため、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の結果が良好であることを挙げているが、石炭船および石炭・鉱石兼用船のホールド使用環境下を考慮した試験結果は示されていない。 However, since the steel materials disclosed in Patent Document 1 are intended for excellent steel materials in common use environments such as ship outer plates, ballast tanks, cargo oil tanks, ore ship cargo holds, etc., the corrosion resistance of steel materials is evaluated. As a method, the results of corrosion tests of cargo oil tanks and ballast tanks are cited as good, but no test results have been shown that take into account the hold use environment of coal ships and coal / ore combined ships.
また、特許文献４と５では、石炭船や石炭・鉱石兼用船の環境を模擬した塗膜下における耐食性を評価しているものの、ホールド使用環境下では不可避といえる石炭や鉄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および鋼板の切替基準となる最大孔食深さの評価を行っていない。 In Patent Documents 4 and 5, although corrosion resistance under a paint film simulating the environment of a coal ship or coal / ore combined ship is evaluated, mechanical damage caused by coal or iron ore can be said to be unavoidable in a hold use environment. The evaluation test assuming the situation where peeling easily occurs and the evaluation of the maximum pitting corrosion depth which is the standard for switching the steel sheet are not performed.
以上、石炭船または石炭・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、石炭船または石炭・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であるにもかかわらず、従来技術においては、この観点は考慮されていなかった。 As described above, in the development of steel materials with excellent corrosion resistance for use in coal ships or coal / ore combined ships, the corrosion environment unique to coal ships or coal / ore combined ships is taken into account, and at the same time, the coated film is peeled off. Despite the importance of evaluating the corrosion of steel in the absence of this, this viewpoint has not been considered in the prior art.
そこで、本発明の目的は、乾湿を繰返しかつ低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または石炭・鉱石兼用船の船倉用耐食鋼を提供することにある。 Accordingly, an object of the present invention is to provide a corrosion-resistant steel for a ship of a coal ship or a coal / ore combined ship capable of suppressing corrosion after peeling of a coating film in a low pH environment with repeated drying and wetting.
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、その鋼材の表面には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り、鋼材が乾湿繰返しかつ低ｐＨ環境下に曝される。ここでは、鋼材の表面の防食塗膜の剥離後も耐食性の発揮できる鋼材の開発を行った。 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 the steel materials is used with a corrosion-resistant coating film applied. 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, and the steel material is repeatedly exposed to dry and wet conditions and exposed to a low pH environment. Here, the steel material which developed corrosion resistance even after peeling of the anticorrosion coating film on the surface of the steel material was developed.
そこで、本発明者らは、石炭船または石炭・鉱石兼用船の船倉内の環境を模擬した試験法を開発し、その試験法を用いて各合金元素の影響を検討した結果、Ｓｂの添加により、石炭船または石炭・鉱石兼用船の船倉内の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ある。そこで、錆層を緻密にすることで腐食因子の地鉄への到達を抑制するＳｎ、Ｔａの添加により、耐食性が向上することを知見した。
本発明は、以上の知見に基づきなされたもので、その要旨は以下のとおりである。
Therefore, the present inventors have developed a test method simulating the environment in the hold of a coal ship or a coal / ore combined ship, and examined the influence of each alloy element using the test method. It was found that the corrosion resistance of the steel material after peeling the coating film in the hold of a coal ship or a coal / ore combined ship is improved. However, Sb is an environmentally hazardous substance, and there is a possibility that the Sb content will be regulated in the future. Therefore, it has been found that the corrosion resistance is improved by adding Sn and Ta that suppress the arrival of the corrosion factor to the ground iron by making the rust layer dense.
The present invention has been made based on the above findings, and the gist thereof is as follows.
[１] 成分組成が、質量％で、Ｃ：０．０１〜０．２０％、Ｓｉ：０．０１〜０．５０％、Ｍｎ：０．１〜２．０％、Ｐ：０．０２５％以下、Ｓ：０．０３５％以下、Ａｌ：０．００５〜０．１０％、Ｓｎ：０．０１〜０．５％、Ｔａ：０．００１〜０．１％を含有し、残部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または石炭・鉱石兼用船の船倉用耐食鋼。 [1] Component composition is mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.1 to 2.0%, P: 0.025% Hereinafter, S: 0.035% or less, Al: 0.005-0.10%, Sn: 0.01-0.5%, Ta: 0.001-0.1%, the remainder Fe and unavoidable Corrosion-resistant steel for cargo ships of coal ships or coal / ore combined ships, characterized by comprising impurities.
[２] さらに、質量％で、Ｃｕ：０．０５〜１．０％、Ｎｉ：０．０５〜１．０％の中から選ばれる１種以上を含有することを特徴とする上記[１]に記載の石炭船または石炭・鉱石兼用船の船倉用耐食鋼。 [2] The above-mentioned [1], further comprising at least one selected from Cu: 0.05 to 1.0% and Ni: 0.05 to 1.0% by mass% Corrosion-resistant steel for hold of coal ships or coal / ore combined ships described in 1.
[３] さらに、質量％で、Ｍｏ：０．０１〜０．５％、Ｗ：０．０１〜０．５％の中から選ばれる１種以上を含有することを特徴とする上記[１]または[２]に記載の石炭船または石炭・鉱石兼用船の船倉用耐食鋼。 [3] The above-mentioned [1], further comprising at least one selected from Mo: 0.01 to 0.5% and W: 0.01 to 0.5% by mass% Or the corrosion-resistant steel for a cargo ship of [2] or a coal / ore combined-use ship.
[４] さらに、質量％で、Ｃｒ：０．１％以下を含有することを特徴とする上記[１]乃至[３]の何れかに記載の石炭船または石炭・鉱石兼用船の船倉用耐食鋼。 [4] Corrosion resistance for cargo ships of coal ships or coal / ore combined ships according to any one of the above [1] to [3], further comprising Cr: 0.1% or less by mass% steel.
[５] さらに、質量％で、Ｓｂ：０．０１〜０．１０％を含有することを特徴とする上記[１]乃至上記[４]の何れかに記載の石炭船または石炭・鉱石兼用船の船倉用耐食鋼。 [5] The coal ship according to any one of [1] to [4] above, further comprising Sb: 0.01 to 0.10% by mass%. Corrosion-resistant steel for ship holds.
[６] さらに、質量％で、Ｃａ：０．０００５〜０．０１０％を含有することを特徴とする上記[１]乃至[５]の何れかに記載の石炭船または石炭・鉱石兼用船の船倉用耐食鋼。 [6] The coal ship according to any one of the above [1] to [5], further containing Ca: 0.0005 to 0.010% by mass%. Corrosion resistant steel for ship holds.
[７] さらに、質量％で、Ｎｂ：０．００１〜０．１０％、Ｖ：０．００２〜０．２％、Ｔｉ：０．００１〜０．０３０％およびＺｒ：０．００１〜０．０５０％の中から選ばれる１種以上を含有することを特徴とする上記[１]乃至[６]の何れかに記載の石炭船または石炭・鉱石兼用船の船倉用耐食鋼。 [7] Further, by mass%, Nb: 0.001 to 0.10%, V: 0.002 to 0.2%, Ti: 0.001 to 0.030%, and Zr: 0.001 to 0.00. Corrosion-resistant steel for hold of coal ships or coal / ore combined ships according to any one of the above [1] to [6], comprising at least one selected from 050%.
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または石炭・鉱石兼用船の船倉用耐食鋼を得ることができるので、船舶の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ができる。 According to the present invention, the corrosion resistance of a coal ship or a coal / ore combined-use ship that can suppress corrosion after peeling of the coating film in a dry / wet repetitive and low pH environment in the coal ship / coal / ore combined-use hold. Since steel can be obtained, the maintenance cost of the ship can be suppressed and the life cycle cost of the ship can be reduced.
以下に本発明の各構成要件の限定理由について説明する。 The reasons for limiting the respective constituent requirements of the present invention will be described below.
１．成分組成について
はじめに、本発明の鋼の成分組成を規定した理由を説明する。なお、成分％は、すべて質量％を意味する。
1. About component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.
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を得るために０．０１％以上の含有とする。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量は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。
C: 0.01 to 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. On the other hand, the content exceeding 0.20% lowers the weldability and the toughness of the heat affected zone. Therefore, the C amount 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、また鋼の強度を高める元素であるので、０．０１％以上を含有させる。しかしながら、０．５０％を超える含有は、鋼の靱性を劣化させるので、Ｓｉ量は０．０１〜０．５０％の範囲とする。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０．２０〜０．４０％の範囲である。
Si: 0.01 to 0.50%
Si is added as a deoxidizer and is an element that increases the strength of steel, so 0.01% or more is contained. However, if the content exceeds 0.50%, the toughness of the steel deteriorates, so the Si content is in the range of 0.01 to 0.50%. In addition, Si improves the corrosion resistance by forming an anticorrosion film in an acidic environment. In order to acquire this effect, Preferably it is 0.20 to 0.40% of range.
Ｍｎ：０．１〜２．０％
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できる元素であるので、０．１％以上含有させる。しかしながら、２．０％を超える含有は、鋼の靱性および溶接性を低下させるため、Ｍｎ量は０．１〜２．０％の範囲とする。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: 0.1 to 2.0%
Mn is an element that can increase the strength of the steel at low cost and can prevent hot brittleness, so it is contained in an amount of 0.1% or more. However, if the content exceeds 2.0%, the toughness and weldability of the steel are lowered, so the Mn content is in the range of 0.1 to 2.0%. In addition, from a viewpoint of ensuring intensity | strength and inclusion suppression, Preferably it is 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の含有量が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。好ましくは、０．０１５％以下である。
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 the welded portion toughness increases. Therefore, the P content is 0.025% or less. Preferably, it is 0.015% 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る。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、極力低減することが望ましく、本発明では０．０３５％以下とする。好ましくは０．０２５％以下であり、さらに好ましくは０．０１５％以下である。
S: 0.035% or less S generates Cu and an intermetallic compound Cu 2 S, and improves sulfuric acid resistance. However, Mn and MnS that are the starting point of local corrosion are formed, and the local corrosion resistance is lowered. Furthermore, since it is a harmful element that deteriorates the toughness and weldability of steel, it is desirable to reduce it as much as possible. In the present invention, it is 0.035% or less. Preferably it is 0.025% or less, More preferably, it is 0.015% or less.
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％の範囲とする。好ましくは、０．０１０〜０．０５０％の範囲である。
Al: 0.005-0.10%
Al is added as a deoxidizer. For this purpose, a content of 0.005% or more is required. However, a content exceeding 0.10% lowers the toughness of the weld metal part when welding. Therefore, the Al content is in the range of 0.005 to 0.10%. Preferably, it is 0.010 to 0.050% of range.
Ｓｎ：０．０１〜０．５％
Ｓｎは大きな水素過電圧を持つため、Ｓｎ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、腐食生成物を緻密にし、地鉄へのＨ２Ｏ、Ｏ２、ＳＯ４ ２−、Ｃｌの拡散を抑制する。この効果を得るには０．０１％以上の含有が必要であるが、０．５％を超える含有は、鋼材の靭性を著しく低下させ、さらにＳｎ添加効果も飽和するため、Ｓｎ量は０．０１〜０．５％の範囲とする。好ましくは、０．０２〜０．２％の範囲である。
Sn: 0.01-0.5%
Since Sn has a large hydrogen overvoltage, the hydrogen generation reaction is suppressed at the portion where Sn is deposited, and the corrosion resistance is improved. Further, the corrosion product is made dense, and the diffusion of H 2 O, O 2 , SO 4 2− , and Cl into the ground iron is suppressed. In order to obtain this effect, the content of 0.01% or more is necessary. However, if the content exceeds 0.5%, the toughness of the steel material is remarkably lowered and the effect of adding Sn is saturated. The range is 01 to 0.5%. Preferably, it is 0.02 to 0.2% of range.
Ｔａ：０．００１〜０．１％
Ｔａは無定形およびガラス状の酸化皮膜を生成することで、アノード活性点を減少させる。また、Ｔａは水素過電圧が大きいため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り、この効果を得るには０．００１％以上の含有が必要であるが、０．１％を超えて含有すると靭性が低下するため、Ｔａ量は０．００１〜０．１％の範囲とする。
Ta: 0.001 to 0.1%
Ta generates an amorphous and glassy oxide film, thereby reducing the anode active point. Further, Ta is an element that not only improves acid resistance because of a large hydrogen overvoltage, but also increases the strength of steel. To obtain this effect, it is necessary to contain 0.001% or more, but 0.1% If the content exceeds V, the toughness decreases, so the Ta content is in the range of 0.001 to 0.1%.
以上が本発明の基本化学成分であり、残部はＦｅ及び不可避的不純物からなるが、更に耐食性の観点からＣｕ、Ｎｉの一種以上、Ｍｏ、Ｗの一種以上、Ｓｂを選択元素として含有してもよい。また、Ｃｒの含有量を制限してもよい。更に、強度、靭性の観点からＣａ、Ｎｂ、Ｔｉ、Ｚｒ、Ｖの一種以上を選択元素として含有してもよい。 The above is the basic chemical component of the present invention, and the balance is composed of Fe and inevitable impurities. From the viewpoint of corrosion resistance, one or more of Cu and Ni, one or more of Mo and W, and Sb as a selective element may be contained. Good. Further, the Cr content may be limited. Furthermore, you may contain 1 or more types of Ca, Nb, Ti, Zr, and V as a selection element from a viewpoint of intensity | strength and toughness.
Ｃｕ：０．０５〜１．０％、Ｎｉ：０．０５〜１．０％の中から選ばれる1種以上
Ｃｕは腐食生成物を緻密にし、地鉄へのＨ２Ｏ、Ｏ２、ＳＯ４ ２−、Ｃｌ−の拡散を抑制する。これにより、鋼の耐食性が向上する。この効果を得るには、０．０５％以上の含有が必要であるが、１．０％超えでは、溶接性や母材の靭性が低下する。そのため、Ｃｕを含有する場合は、Ｃｕ量は０．０５〜１．０％の範囲とすることが好ましい。さらに好ましくは０．０５〜０．５０％の範囲である。いっそう好ましくは０．０５〜０．３５％の範囲である。また、ＣｕとＳｂの金属間化合物であるＣｕ２Ｓｂを形成することで、耐食性が向上する効果もある。
One or more kinds selected from Cu: 0.05 to 1.0% and Ni: 0.05 to 1.0% Cu densifies the corrosion products and adds H 2 O, O 2 , SO to the steel 4 2− , suppresses diffusion of Cl − . Thereby, the corrosion resistance of steel improves. In order to acquire this effect, 0.05% or more of content is required, but if it exceeds 1.0%, the weldability and the toughness of the base material are deteriorated. Therefore, when it contains Cu, it is preferable to make Cu amount into the range of 0.05-1.0%. More preferably, it is 0.05 to 0.50% of range. More preferably, it is 0.05 to 0.35% of range. Further, forming Cu 2 Sb, which is an intermetallic compound of Cu and Sb, has an effect of improving the corrosion resistance.
ＮｉはＣｕと同様に腐食生成物を緻密にし、地鉄へのＨ２Ｏ、Ｏ２、ＳＯ４ ２−、Ｃｌ−の拡散を抑制する。これにより、鋼の耐食性が向上する。この効果は、０．０５％以上で得られるが、１．０％を超えると効果が飽和すると共にコストも上昇するため、Ｎｉを含有する場合は、Ｎｉ量は０．０５〜１．０％の範囲とすることが好ましい。さらに好ましくは０．０１０〜０．５０％の範囲である。 Ni, like Cu, densifies the corrosion products and suppresses diffusion of H 2 O, O 2 , SO 4 2− , and Cl − into the ground iron. Thereby, the corrosion resistance of steel improves. This effect is obtained at 0.05% or more. However, if the content exceeds 1.0%, the effect is saturated and the cost is increased. When Ni is contained, the amount of Ni is 0.05 to 1.0%. It is preferable to set it as the range. More preferably, it is 0.010 to 0.50% of range.
Ｍｏ：０．０１〜０．５％、Ｗ：０．０１〜０．５％の中から選ばれる1種以上
Ｍｏ、Ｗはいずれも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ぎ、耐食性を向上させる．さらにはＭｏ、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．これらの効果を得るためには、いずれも０．０１％以上を含有させることが好ましい。しかし、０．５％を超えて含有しても効果が飽和するだけでなく、コストが嵩むため、Ｍｏ、Ｗを含有する場合は、Ｍｏ、Ｗとも０．０１〜０．５％の範囲とすることが好ましい。さらに好ましくは、０．０１〜０．３％の範囲である。
One or more selected from Mo: 0.01 to 0.5%, W: 0.01 to 0.5% Mo and W both form oxygen acids when they are eluted from the base material. The anion is repelled electrically, preventing the anion from penetrating to the surface of the steel, and improving the corrosion resistance. Furthermore, Mo and W improve corrosion resistance by forming hardly soluble corrosive substances such as FeMoO 4 and FeWO 4 . In order to acquire these effects, it is preferable to contain 0.01% or more of all. However, if the content exceeds 0.5%, the effect is not only saturated, but also the cost increases. Therefore, when Mo and W are contained, both Mo and W are in the range of 0.01 to 0.5%. It is preferable to do. More preferably, it is 0.01 to 0.3% of range.
Ｃｒ：０．１％以下
Ｃｒは、低ｐＨ環境で加水分解を起こすため、耐食性を低下させる元素であるので無添加でよい。強度調整のため添加することができるが、特にその含有量が０．１％を超えると耐食性の低下が著しくなるため、Ｃｒを含有する場合は、０．１％以下とすることが好ましい。さらに好ましくは、０．０３％以下である。
Cr: 0.1% or less Since Cr causes hydrolysis in a low pH environment, it is an element that lowers corrosion resistance, so it may not be added. Although it can be added for strength adjustment, particularly when its content exceeds 0.1%, the corrosion resistance is remarkably lowered. Therefore, when Cr is contained, it is preferably 0.1% or less. More preferably, it is 0.03% or less.
Ｓｂ：０．０１〜０．１０％
Ｓｂは鋼材に合金元素として０．０１％以上含有すると、低ｐＨ環境において地鉄近傍に濃縮する。Ｓｂは大きな水素過電圧を持つため、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。さらに、腐食生成物を緻密にし、地鉄へのＨ２Ｏ、Ｏ２、ＳＯ４ ２−、Ｃｌ−の拡散を抑制する。一方、０．１０％を超えて含有すると靭性が低下する。よって、Ｓｂを含有する場合は０．０１〜０．１０％の範囲とすることが好ましい。より好ましくは、０．０３〜０．１０％の範囲である。
Sb: 0.01 to 0.10%
When Sb is contained in an amount of 0.01% or more as an alloy element in a steel material, it is concentrated in the vicinity of the ground iron in a low pH environment. Since Sb has a large hydrogen overvoltage, the hydrogen generation reaction is suppressed at the portion where Sb is deposited, and the corrosion resistance is improved. Furthermore, the corrosion product is made dense, and diffusion of H 2 O, O 2 , SO 4 2− , and Cl − into the ground iron is suppressed. On the other hand, when it contains exceeding 0.10%, toughness will fall. Therefore, when it contains Sb, it is preferable to set it as 0.01 to 0.10% of range. More preferably, it is 0.03 to 0.10% of range.
Ｃａ：０．０００５〜０．０１０％
Ｃａは数ｐｐｍから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、少なくとも０．０００５％含有することが好ましい。しかし、０．０１０％を超えて含有させると、粗大な介在物を形成し、母材の靱性を劣化させるので、Ｃａを含有する場合は０．０００５〜０．０１０％の範囲とすることが好ましい。さらに好ましくは、０．００１０〜０．００３０％の範囲である。
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. Ca is an element that controls the form of inclusions to increase the ductility and toughness of steel. In order to exhibit such an effect, it is preferable to contain at least 0.0005%. However, if the content exceeds 0.010%, coarse inclusions are formed and the toughness of the base material is deteriorated. Therefore, when Ca is contained, the content may be in the range of 0.0005 to 0.010%. preferable. More preferably, it is 0.0010 to 0.0030% of range.
Ｎｂ：０．００１〜０．１０％、Ｖ：０．００２〜０．２％、Ｔｉ：０．００１〜０．０３０％およびＺｒ：０．００１〜０．０５０％の中から選ばれる１種以上
Ｎｂ、Ｖ、Ｔｉおよび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、Ｎｂ、ＴｉおよびＺｒは、いずれも０．００１％以上、Ｖは０．００２％以上含有させることが好ましい。しかしながら、Ｎｂは０．１０％を超えて、Ｖは０．２％を超えて、Ｔｉは０．０３０％を超えて、およびＺｒは０．０５０％を超えて含有すると靱性が低下するため、それぞれ、Ｎｂを含有する場合は０．００１〜０．１０％の範囲と、Ｖを含有する場合は０．００２〜０．２％の範囲と、Ｔｉを含有する場合は０．００１〜０．０３０％の範囲と、およびＺｒを含有する場合は０．００１〜０．０５０％の範囲とすることが好ましい。さらに好ましくは、Ｎｂは０．００５０〜０．０２０％の範囲であり、Ｖは０．００５〜０．１０％の範囲であり、Ｔｉは０．００５〜０．０２０％の範囲であり、およびＺｒは０．００５〜０．０２０％の範囲である。
One selected from Nb: 0.001 to 0.10%, V: 0.002 to 0.2%, Ti: 0.001 to 0.030%, and Zr: 0.001 to 0.050% Nb, V, Ti and Zr are all elements that increase the strength of steel, and can be selected and contained according to the required strength. In order to obtain such an effect, it is preferable that Nb, Ti and Zr are contained in an amount of 0.001% or more and V is contained in an amount of 0.002% or more. However, if Nb exceeds 0.10%, V exceeds 0.2%, Ti exceeds 0.030%, and Zr exceeds 0.050%, the toughness decreases. When Nb is contained, a range of 0.001 to 0.10%, when V is contained, a range of 0.002 to 0.2%, and when Ti is contained, 0.001 to 0.1%. In the case of containing 030% and Zr, the content is preferably 0.001 to 0.050%. More preferably, Nb is in the range of 0.0050 to 0.020%, V is in the range of 0.005 to 0.10%, Ti is in the range of 0.005 to 0.020%, and Zr is in the range of 0.005 to 0.020%.
２．製造方法について
上述した成分組成を有する鋼を転炉、電気炉等の溶製手段で常法により溶製し、連続鋳造法または造塊〜分塊法等で常法によりスラブ等の鋼素材とし、そのまま、あるいは冷却後再加熱して熱間圧延を行う。耐食性を発揮させる為の熱処理条件は問わないが、機械的特性の観点からは適切な圧下比を確保することが好ましい。熱間圧延の仕上温度が７５０℃未満となると変形抵抗が大きくなり、形状不良が起きるため、仕上温度は、７５０℃以上とすることが好ましい。
2. About the manufacturing method Steel having the above-described composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and is made into a steel material such as a slab by a conventional method using a continuous casting method or an ingot-bundling method. The hot rolling is performed as it is or after reheating after cooling. The heat treatment conditions for exhibiting corrosion resistance are not limited, but it is preferable to ensure an appropriate reduction ratio from the viewpoint of mechanical properties. When the finishing temperature of hot rolling is less than 750 ° C., deformation resistance increases and shape failure occurs. Therefore, the finishing temperature is preferably 750 ° C. or higher.
例えば、仕上温度を７５０℃以上、その後１５０℃／ｍｉｎ以上の冷却速度で６００℃以下まで冷却することで、引張強さ４９０ＭＰａ級以上の鋼材を製造することができる。
冷却速度１５０℃／ｍｉｎ未満では引張強さ４９０ＭＰａ級以上の鋼材を得られない。
For example, a steel material having a tensile strength of 490 MPa or higher can be produced by cooling the finishing temperature to 750 ° C. or higher and then cooling to 600 ° C. or lower at a cooling rate of 150 ° C./min or higher.
If the cooling rate is less than 150 ° C./min, a steel material having a tensile strength of 490 MPa or higher cannot be obtained.
本発明者らは、石炭船および石炭・鉱石兼用船の船倉（以下ホールドと呼ぶ）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結果、以下のようであった。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試験をおこなった。 As a result of investigating the mechanism of pitting corrosion that most affects the destruction of ships due to corrosion in the hold (hereinafter referred to as hold) of coal ships and coal / ore combined ships, the present inventors have found the following. . The side wall of the bulk carrier is a single hull, and the cargo and seawater are separated from each other only by one piece of steel. 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 wet, 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 for corrosion of the steel material in the meniscus portion, 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 considered that H + is supplied from the coal surface to the meniscus portion. Unreacted H + adheres to the coal surface again during the drying process, and is used for the corrosion reaction in the next dew condensation process. This process takes place in a long-term cycle, causing more corrosion in the meniscus area and pitting corrosion. Will be formed. Based on this mechanism, the following tests were conducted to simulate pitting corrosion in the hold of coal ships and coal / ore combined ships.
表１に示す成分の鋼を、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℃に加熱し、仕上圧延終了温度８００℃とする熱間圧延により２５ｍｍ厚の鋼板とした。 Steels having the components shown in Table 1 were made into slabs by continuous casting after melting or converter melting in a vacuum melting furnace. Subsequently, the slab was charged into a heating furnace and heated to 1200 ° C., and a steel plate having a thickness of 25 mm was formed by hot rolling at a finish rolling finishing temperature of 800 ° C.
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５０ｇを敷き詰め、低温恒温恒湿器により、図１に示す雰囲気Ａ（温度６０℃、湿度９５％、２０時間） ⇔ 雰囲気Ｂ（温度３０℃、湿度９５％、３時間） 、遷移時間０．５時間の温湿度サイクルを８４日間与えた。ここで、記号「 ⇔ 」は繰り返しという意味で使用している。 A test piece of 5 mm t × 50 mm W × 75 mm L was collected from the steel plate, and the surface of the test piece was shot blasted to remove surface scale and oil. 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 a silicon seal, they are fitted into an acrylic jig, 50 g of coal is laid on top, and the atmosphere A shown in FIG. 1 (temperature 60 ° C., humidity 95%, 20 hours) 雰 囲 気 Atmosphere B (temperature 30 ° C., humidity 95%, 3 hours), a temperature and humidity cycle with a transition time of 0.5 hours was given for 84 days. Here, the symbol “⇔” is used to mean repetition.
なお、石炭は、５ｇを秤量し、常温で１００ｍｌの蒸留水に２時間浸漬したのち、ろ過を行ない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。 In addition, 5 g of coal was weighed, immersed in 100 ml of distilled water at room temperature for 2 hours, filtered, and the coal leachate diluted to 200 ml having a pH of 3.0 was used.
本実施例は、上記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。 In this example, the test is performed under the above-described conditions, thereby simulating the temperature / humidity environment and the dew condensation state that greatly affect the corrosion in the hold of the coal ship and the coal / ore combined ship. After the test, using a rust remover, the rust of each test piece was peeled off, 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.
ここで、最大孔食深さの値は対象とする面積が大きいほど、増加し、それに伴い最大孔食深さも深くなる。そこで、実船での各期間の最大孔食深さを予測するために、極値統計を用い本試験片面積での測定値から実船ホールド相当面積の最大孔食深さを算出した。 Here, the value of the maximum pitting depth increases as the target area increases, and the maximum pitting depth increases accordingly. Therefore, in order to predict the maximum pitting depth for each period on the actual ship, the maximum pitting depth in the area equivalent to the actual ship hold was calculated from the measured values in the area of this specimen using extreme value statistics.
ここで、本開発鋼の適用部位であるホールド肋骨部は両面からの腐食のため、各期間の最大孔食深さを２倍し、それらの値をy=axbで外挿することで船舶寿命である２５年後の最大板厚減を推定した。その結果を表２に示す。 Here, because the hold rib, which is the application site of the developed steel, is corroded from both sides, the maximum pitting corrosion depth for each period is doubled and the value is extrapolated at y = ax b. The maximum thickness reduction after 25 years, which is the lifetime, was estimated. The results are shown in Table 2.
適用部位の板厚は１５〜２０ｍｍであり、腐食代は３．５〜４．０ｍｍ、腐食余裕厚は０．５ｍｍであることを前提条件とし、鋼船規則ＣＳＲ−Ｂ編の鋼板切替基準から、２５年後の最大板厚減のクライテリアを４．０ｍｍとした。なお、ここで、最大板厚減とは船舶における図面板厚から局所的な腐食によりもっとも減肉した鋼板の厚さを言う。 Based on the assumption that the plate thickness of the application site is 15 to 20 mm, the corrosion allowance is 3.5 to 4.0 mm, and the allowance for corrosion is 0.5 mm. The criterion for the maximum thickness reduction after 25 years was 4.0 mm. Here, the maximum thickness reduction refers to the thickness of the steel plate that has been most reduced by local corrosion from the drawing thickness in the ship.
表２から、本発明例の２５年後の最大板厚減推定値は、鋼船規則ＣＳＲ−Ｂ編の切替基準から算出された２５年後の最大板厚減のクライテリアである４．０ｍｍを下回っていることが分かる。Ｔａを除く全ての耐食元素を添加した比較例Ｎｏ．３１の最大板厚減推定値がクライテリアを上回る４．４７ｍｍであることからＴａの効果が大きいことが分かる。 From Table 2, the maximum thickness reduction estimated value after 25 years of the example of the present invention is 4.0 mm, which is the criterion of the maximum thickness reduction after 25 years calculated from the switching standard of Steel Ship Rules CSR-B. You can see that it is below. Comparative Example No. added with all corrosion resistant elements except Ta It can be seen that the effect of Ta is great because the estimated maximum plate thickness reduction value of 31 is 4.47 mm, which exceeds the criteria.
以上、本発明の効果が確認された。本実施例では、石炭船または石炭・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。また、雰囲気Ａ、Ｂの条件、遷移時間、サイクル、石炭の調整方法、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、鋼材のホールド内での使用環境に応じて、適宜変更することができる。 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 simulating the environment in a coal ship or 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, coal adjustment method, pH value of coal leachate are not limited to the above examples, and depending on the use environment in the steel hold. Can be changed as appropriate.
本発明に係る鋼材は、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、さらに乾湿繰返しかつ低ｐＨ環境下に曝される、石炭船または石炭・鉱石兼用船ホールドの構成部材として使用することができる。 The steel material according to the present invention is used as a constituent member of a coal ship or a coal / ore combined ship hold that is easily peeled off due to mechanical damage of coal or ore and is exposed to repeated dry and wet conditions and a low pH environment. Can do.
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