JP2009242933A - Ferritic stainless steel having excellent corrosion resistance in urea water - Google Patents
Ferritic stainless steel having excellent corrosion resistance in urea water Download PDFInfo
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- 230000007797 corrosion Effects 0.000 title claims abstract description 62
- 238000005260 corrosion Methods 0.000 title claims abstract description 62
- WTHDKMILWLGDKL-UHFFFAOYSA-N urea;hydrate Chemical compound O.NC(N)=O WTHDKMILWLGDKL-UHFFFAOYSA-N 0.000 title claims abstract description 48
- 229910001220 stainless steel Inorganic materials 0.000 title claims abstract description 26
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 11
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 6
- 239000012535 impurity Substances 0.000 claims abstract description 5
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 5
- 229910052710 silicon Inorganic materials 0.000 claims abstract description 5
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 4
- 229910052758 niobium Inorganic materials 0.000 claims description 12
- 229910052719 titanium Inorganic materials 0.000 claims description 12
- 229910052759 nickel Inorganic materials 0.000 claims description 6
- 229910052802 copper Inorganic materials 0.000 claims description 5
- 229910052721 tungsten Inorganic materials 0.000 claims description 3
- 229910052720 vanadium Inorganic materials 0.000 claims description 2
- 238000010828 elution Methods 0.000 abstract description 29
- 239000000203 mixture Substances 0.000 abstract description 4
- 229910000831 Steel Inorganic materials 0.000 description 33
- 239000010959 steel Substances 0.000 description 33
- XSQUKJJJFZCRTK-UHFFFAOYSA-N Urea Chemical compound NC(N)=O XSQUKJJJFZCRTK-UHFFFAOYSA-N 0.000 description 25
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- 229910052742 iron Inorganic materials 0.000 description 7
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- 239000000126 substance Substances 0.000 description 4
- 229910021529 ammonia Inorganic materials 0.000 description 3
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- 229910002092 carbon dioxide Inorganic materials 0.000 description 2
- 239000001569 carbon dioxide Substances 0.000 description 2
- 239000007806 chemical reaction intermediate Substances 0.000 description 2
- 239000003638 chemical reducing agent Substances 0.000 description 2
- 239000010960 cold rolled steel Substances 0.000 description 2
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- VHUUQVKOLVNVRT-UHFFFAOYSA-N Ammonium hydroxide Chemical compound [NH4+].[OH-] VHUUQVKOLVNVRT-UHFFFAOYSA-N 0.000 description 1
- 229910002089 NOx Inorganic materials 0.000 description 1
- BVCZEBOGSOYJJT-UHFFFAOYSA-N ammonium carbamate Chemical compound [NH4+].NC([O-])=O BVCZEBOGSOYJJT-UHFFFAOYSA-N 0.000 description 1
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- KXDHJXZQYSOELW-UHFFFAOYSA-N carbonic acid monoamide Natural products NC(O)=O KXDHJXZQYSOELW-UHFFFAOYSA-N 0.000 description 1
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- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 238000002161 passivation Methods 0.000 description 1
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Abstract
Description
本発明は、主にディーゼルエンジンを主体とする内燃機関において尿素水を用いて排ガス中のＮＯｘを低減する装置、特に自動車尿素ＳＣＲ（Ｓｅｌｅｃｔｉｖｅ Ｃａｔａｌｙｔｉｃ Ｒｅｄｕｃｔｉｏｎ）システムなどに使用される機器類、具体的には尿素水を貯蔵、製造あるいは輸送する際に使用される機器類の材料として好適に用いられる尿素水中での耐食性に優れたフェライト系ステンレス鋼に関する。 The present invention mainly relates to a device for reducing NOx in exhaust gas using urea water in an internal combustion engine mainly composed of a diesel engine, particularly equipment used in an automotive urea SCR (Selective Catalytic Reduction) system, specifically, Relates to a ferritic stainless steel excellent in corrosion resistance in urea water, which is preferably used as a material for equipment used when storing, manufacturing or transporting urea water.
近年、環境問題に対する意識の高まりから、排ガス規制がより強化されると共に、炭酸ガス排出抑制に向けた取り組みが進められている。自動車分野においては、バイオエタノールやバイオディーゼル燃料といった燃料面からの取り組みに加え、軽量化や排気熱を熱回収する熱交換器をとりつけて燃費向上を図ったり、ＥＧＲ（Ｅｘｈａｕｓｔ Ｇａｓ Ｒｅｃｉｒｃｕｌａｔｉｏｎ）、ＤＰＦ（Ｄｉｅｓｅｌ Ｐａｒｔｉｃｕｌａｔｅ Ｆｉｌｔｅｒ）、尿素ＳＣＲシステムといった排ガス処理装置を設置するといった取り組みを実施している。 In recent years, due to increasing awareness of environmental issues, exhaust gas regulations have been further strengthened, and efforts are being made to reduce carbon dioxide emissions. In the automotive field, in addition to efforts from the fuel side, such as bioethanol and biodiesel fuel, weight reduction and heat exchangers that recover exhaust heat are installed to improve fuel efficiency, EGR (Exhaust Gas Recirculation), DPF (DPF) Efforts are being made to install exhaust gas treatment equipment such as Diesel Particulate Filter) and urea SCR system.
このうち、尿素ＳＣＲシステムは、ＮＯｘ低減システムの一つで、尿素水をＮＯｘの還元剤として使用する。尿素水は、液体アンモニアやアンモニア水を還元剤として使用する場合に比べ、安全で取り扱いが比較的容易との利点があり、自動車のみならず都市部に設置される分散型電源設備など定置型のＮＯｘ低減システムへの適用も検討されている。 Among these, the urea SCR system is one of NOx reduction systems and uses urea water as a reducing agent for NOx. Compared to using liquid ammonia or aqueous ammonia as a reducing agent, urea water has the advantage of being safe and relatively easy to handle. Application to NOx reduction systems is also being studied.
尿素ＳＣＲシステムでは、排出ガス中に噴射された尿素水が、熱及び水分により分解してアンモニアを生成し、アンモニアとＮＯｘが触媒上で選択還元されて無害な窒素に分解される。このとき使用される尿素水は２５〜４５％といった高濃度の尿素水溶液である。自動車尿素ＳＣＲシステムにおいては、一般に凝固点の最も低い約３２．５％の尿素水溶液が使用されており、ＪＩＳ Ｋ２２４７−１によって、「ディーゼル機関ＮＯｘ還元添加材−ＡＵＳ３２−第一部：性状」（自動車規格ＪＡＳＯ Ｅ５０２も同様の規定）に規定されている。この規格中には、不純物元素の濃度も厳しく規定されており、ステンレス鋼に関連する元素では、Ｆｅ：＜０．５、Ｃｒ：＜０．２、Ｎｉ：＜０．２、Ｃｕ：＜０．２（以上、ｍｇ/ｋｇ）と規定されている。
尿素ＳＣＲシステムにおいて使用される尿素水を貯蔵、製造あるいは輸送する機器類からの溶出によって、前記規定をオーバーすることは許容されないため、尿素ＳＣＲシステムの機器類に使用される材料には非常に優れた耐食性が必要である。
In the urea SCR system, urea water injected into exhaust gas is decomposed by heat and moisture to generate ammonia, and ammonia and NOx are selectively reduced on the catalyst and decomposed into harmless nitrogen. The urea water used at this time is a high concentration urea aqueous solution of 25 to 45%. In an automobile urea SCR system, an aqueous urea solution having a lowest freezing point of about 32.5% is generally used. According to JIS K2247-1, “diesel engine NOx reducing additive-AUS32—part 1: properties” (automobile) The standard JASO E502 is also defined in the same rule). In this standard, the concentration of the impurity element is also strictly defined. For elements related to stainless steel, Fe: <0.5, Cr: <0.2, Ni: <0.2, Cu: <0. .2 (above, mg / kg).
The elution from the equipment that stores, manufactures, or transports the urea water used in the urea SCR system is not allowed to exceed the above regulations, so it is very good for materials used in the equipment of the urea SCR system. Corrosion resistance is required.
特許文献１には、高品位尿素水の供給装置とその装置を用いた高品位尿素水の供給方法が開示されており、エアー抜き機構付き高品位尿素水供給口及びガンノズル付き排出ホースを備えた電動ポンプを備えている、１２００〜１５００Ｌの実容量を有する高密度ポリエチレン製ＩＢＣ（Ｉｎｔｅｒｍｅｄｉａｔｅ Ｂｕｌｋ Ｃｏｎｔａｉｎｅｒ）タンクからなる供給装置が示されている。このうちの電動ポンプには、強化プラスチック製が好ましいと記載されており、ポンプのシャフトにはステンレス合金（ＳＵＳ３０４）、ハステロイ、インコネル合金が好ましいと記載されている。ＳＵＳ３０４はオーステナイト系ステンレス鋼であり、フェライト系ステンレス鋼に関して直接的に記載されていない。 Patent Document 1 discloses a high-grade urea water supply device and a high-grade urea water supply method using the device, and includes a high-grade urea water supply port with an air vent mechanism and a discharge hose with a gun nozzle. A supply device comprising an IBC (Intermediate Bulk Container) tank made of high-density polyethylene having an actual capacity of 1200 to 1500 L, equipped with an electric pump, is shown. Among these, it is described that the reinforced plastic is preferable for the electric pump, and that stainless steel alloy (SUS304), Hastelloy, or Inconel alloy is preferable for the pump shaft. SUS304 is an austenitic stainless steel and is not described directly with respect to ferritic stainless steel.
特許文献２には、尿素製造プラント用二相ステンレス鋼、溶接材料、尿素製造プラントおよびその機器が開示されており、Ｃｒ：２６％以上２８％未満、Ｎｉ：6〜１０％、Ｍｏ：０．２〜１．７％、Ｗ：２％を超え３％までを含む二相ステンレス鋼が示されている。尿素は、アンモニアと炭酸ガスを原料として、高温高圧下で合成されるが、アンモニウムカーボメイトなどの合成反応中間生成物の存在によって激しい腐食性を有する。このため、尿素製造プラントの構成材料として、腐食による減肉に耐え、内部物質を漏洩させないような材料が必要となる。
尿素ＳＣＲシステムなど常温付近で使用される高濃度の尿素水環境は、尿素合成プラントにおける高温高圧の腐食環境よりも、低温である点や合成反応中間生成物が存在しないといった点などからマイルドである。しかしながら、前記ＪＩＳ規格の規定を遵守するためにはＦｅ、Ｃｒ、Ｎｉ、Ｃｕといったステンレス鋼の構成元素の溶出を抑制する必要があり、溶出抑制といった観点で優れた耐食性が必要となる。 The high-concentration urea water environment used near room temperature such as the urea SCR system is milder than the high-temperature and high-pressure corrosive environment in the urea synthesis plant because of its low temperature and the absence of synthesis reaction intermediate products. . However, in order to comply with the provisions of the JIS standard, it is necessary to suppress elution of constituent elements of stainless steel such as Fe, Cr, Ni, and Cu, and excellent corrosion resistance is required from the viewpoint of suppressing elution.
本発明は、主にディーゼルエンジンを主体とする内燃機関において尿素水を用いて排ガス中のＮＯｘを低減する装置、特に自動車尿素ＳＣＲシステムなどに使用される機器類、より具体的には尿素水を貯蔵、製造あるいは輸送する際に使用されるタンクや配管といった機器類に好適に用いられ、高濃度尿素水への溶出が小さく、尿素水中での耐食性に優れたフェライト系ステンレス鋼を提供することを目的とする。 The present invention mainly relates to a device for reducing NOx in exhaust gas using urea water in an internal combustion engine mainly composed of a diesel engine, particularly equipment used in an automobile urea SCR system, more specifically urea water. To provide a ferritic stainless steel that is suitably used for equipment such as tanks and pipes used for storage, manufacture or transportation, has low elution into high-concentration urea water, and has excellent corrosion resistance in urea water. Objective.
本発明者等は、前述の課題を解決すべく鋭意検討した結果、２５〜４５％といった高濃度尿素水への鋼からの溶出を小さくし、優れた耐食性を発現させるには、鋼の表面にＣｒを含む不動態皮膜を形成させることが重要であり、適正量のＣｒの添加された鋼とする必要があることを知見した。
鋼の表面にＣｒを含む不動態皮膜を形成することにより、鋼の耐食性が向上することはよく知られている。しかし、例えば、自動車尿素ＳＣＲシステムにおいて使用される機器類では、大気中に曝された使用前の通常の状態から高濃度尿素水に曝された瞬間に、機器類を構成する鋼の不動態皮膜自身が溶出したり、不動態皮膜の下地となっている鋼が溶出したりする恐れがあった。
本発明者等は、フェライト系ステンレス鋼に１０％以上のＣｒを含有させることで、自動車尿素ＳＣＲシステムにおいて使用される２５〜４５％といった高濃度尿素水中においても、鋼の表面に形成した不動態皮膜自身の溶出を抑制できるとともに、不動態皮膜の下地となっている鋼からの不動態皮膜を介した溶出を抑制できる一様な不動態皮膜を形成できることを見出した。
As a result of intensive investigations to solve the above-mentioned problems, the present inventors reduced elution from steel into high-concentration urea water such as 25 to 45% and developed excellent corrosion resistance on the surface of steel. It has been found that it is important to form a passive film containing Cr, and it is necessary to use a steel to which an appropriate amount of Cr is added.
It is well known that the corrosion resistance of steel is improved by forming a passive film containing Cr on the surface of the steel. However, for example, in the equipment used in the automobile urea SCR system, the passive film of steel constituting the equipment at the moment when it is exposed to high-concentration urea water from the normal state before use exposed to the atmosphere. There was a risk that the steel itself may elute or the steel that is the base of the passive film may be eluted.
The present inventors have added 10% or more of Cr to the ferritic stainless steel, so that the passivation formed on the surface of the steel even in the high concentration urea water such as 25 to 45% used in the automobile urea SCR system. It has been found that a uniform passive film can be formed that can suppress the elution of the film itself and suppress the elution from the steel that is the base of the passive film through the passive film.
また、自動車尿素ＳＣＲシステムにおいて尿素水を貯蔵、製造あるいは輸送する際に使用されるタンクや配管といった機器類は、溶接あるいはろう付けされることにより接合されて組み立てられる場合が多い。溶接（あるいはろう付け）された溶接部（あるいはろう付け部）を構成する鋼の表面には酸化皮膜が形成される場合があるが、酸化皮膜が形成されている状態であっても、高濃度尿素水中での鋼からの溶出が抑制されなければならない。フェライト系ステンレス鋼は、オーステナイトステンレス鋼に比べて、Ｃｒの拡散が早いため、酸化皮膜直下におけるＣｒ欠乏が抑制される。酸化皮膜が形成された鋼からの高濃度尿素水への溶出を抑制するためには、酸化皮膜直下におけるＣｒ量の多いことが重要であるので、フェライト系ステンレス鋼は、オーステナイト系ステンレ鋼に比べて、より少ないＣｒ量で溶接部（あるいはろう付け部）からの鋼の溶出を抑制することが可能である。 In addition, equipment such as tanks and piping used when storing, manufacturing, or transporting urea water in an automobile urea SCR system is often joined and assembled by welding or brazing. An oxide film may be formed on the surface of the steel constituting the welded (or brazed) welded part (or brazed part), but even if the oxide film is formed, the concentration is high. Elution from steel in urea water must be suppressed. Since ferritic stainless steel diffuses Cr faster than austenitic stainless steel, Cr deficiency directly under the oxide film is suppressed. In order to suppress elution of steel with an oxide film into high-concentration urea water, it is important to have a large amount of Cr immediately below the oxide film. Therefore, ferritic stainless steel is more difficult than austenitic stainless steel. Thus, it is possible to suppress elution of steel from the welded portion (or brazed portion) with a smaller amount of Cr.
さらに、本発明者等は、鋭意研究を重ね、酸化皮膜直下のＣｒ量を確保し、酸化皮膜の形成された状態において上記の高濃度尿素水中への鋼の溶出規定（Ｆｅ：＜０．５、Ｃｒ：＜０．２、Ｎｉ：＜０．２、Ｃｕ：＜０．２（以上、ｍｇ/ｋｇ）ＪＩＳ Ｋ２２４７−１）を満足するためには、下記（Ｉ）式、下記（ＩＩ）式もしくは下記（ＩＩＩ）式（但し、（Ｉ）〜（ＩＩＩ）式中における元素記号は、その元素の含有量（質量％）を示す。また、元素記号の前の数値は定数である。）で示される有効Ｃｒ量を１０％以上とする必要があることを見出した。
Ｎｂ、Ｔｉを含有しない場合またはＮｂのみを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ ・・・ （Ｉ）
Ｔｉのみを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ−１０Ｔｉ ・・・ （ＩＩ）
ＮｂおよびＴｉを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ−（１０Ｔｉ−３Ｎｂ） ・・・ （ＩＩＩ）
Furthermore, the present inventors have conducted extensive research to secure the amount of Cr immediately below the oxide film, and in the state where the oxide film is formed, the elution regulation of steel in the high-concentration urea water (Fe: <0.5 In order to satisfy JIS K2247-1), Cr: <0.2, Ni: <0.2, Cu: <0.2 (more than mg / kg), the following formula (I) and the following (II) Formula (III) (wherein the element symbol in the formulas (I) to (III) represents the content (% by mass) of the element. The numerical value before the element symbol is a constant). It was found that the effective Cr amount indicated by the above should be 10% or more.
When Nb and Ti are not contained or when only Nb is contained Cr + 4Si-2Mn (I)
When containing only Ti Cr + 4Si-2Mn-10Ti (II)
When Nb and Ti are contained Cr + 4Si-2Mn- (10Ti-3Nb) (III)
なお、本発明においては、上記（Ｉ）〜（ＩＩＩ）式より算出される数値を「有効Ｃｒ量」と呼ぶ。上記（Ｉ）〜（ＩＩＩ）式は、鋼に含まれるＳｉ、Ｍｎ、Ｔｉ、ＮｂによるＣｒの耐食性向上作用への影響が考慮された合金元素指標であり、鋼の耐食性向上に寄与する有効なＣｒ量の指標となる数値としての「有効Ｃｒ量」を算出するためのものである。
本発明において、有効Ｃｒ量は、十分に高い耐食性を得るために、１０％以上とする必要があり、好ましくは１３％以上、より好ましくは１６％以上とされる。
In the present invention, the numerical value calculated from the above formulas (I) to (III) is referred to as “effective Cr amount”. The above formulas (I) to (III) are alloy element indexes in consideration of the effect of Si, Mn, Ti, and Nb contained in steel on the effect of improving the corrosion resistance of Cr, and are effective in contributing to the improvement of the corrosion resistance of steel. This is for calculating the “effective Cr amount” as a numerical value serving as an index of the Cr amount.
In the present invention, in order to obtain a sufficiently high corrosion resistance, the effective Cr amount needs to be 10% or more, preferably 13% or more, more preferably 16% or more.
上記（Ｉ）〜（ＩＩＩ）式に挙げたＳｉ、Ｍｎ、Ｔｉ、Ｎｂの作用について、十分理解されているわけではないが、各元素の作用については次のように推察される。
Ｓｉは、鋼の表面に形成されたＣｒ酸化物の直下で酸化物を形成して、Ｃｒの酸化を抑制する有用な元素である。Ｍｎは、ＣｒとＭｎとを含むスピネル型酸化物の生成を促進して、鋼の溶出の抑制に寄与する有効Ｃｒ量を減少させる。Ｔｉは、Ｃｒ酸化物の成長を顕著に促進して有効Ｃｒ量を減少させる効果が大きい。Ｎｂは、ＴｉのＣｒ酸化物成長促進効果を減じる効果があるため、Ｔｉによる有効Ｃｒ量の減少を抑制する。
Although the actions of Si, Mn, Ti, and Nb listed in the above formulas (I) to (III) are not fully understood, the action of each element is presumed as follows.
Si is a useful element that suppresses the oxidation of Cr by forming an oxide immediately below the Cr oxide formed on the steel surface. Mn promotes the formation of a spinel oxide containing Cr and Mn, and reduces the effective Cr amount contributing to the suppression of steel elution. Ti has a great effect of significantly promoting the growth of Cr oxide and reducing the amount of effective Cr. Since Nb has the effect of reducing the Cr oxide growth promoting effect of Ti, it suppresses the decrease in the effective Cr amount due to Ti.
本発明は、このような知見に基づいてなされたものであり、高濃度尿素水への溶出が小さく、尿素水中での耐食性に優れたフェライト系ステンレス鋼を提供するものである。本発明の要旨とするところは、下記の通りである。 The present invention has been made on the basis of such knowledge, and provides a ferritic stainless steel that is small in elution into high-concentration urea water and excellent in corrosion resistance in urea water. The gist of the present invention is as follows.
（１）質量％で、Ｃ：０．０５％以下、Ｎ：０．０５％以下、Ｓｉ：０．０２〜１．５％、Ｍｎ：０．０２〜２％、Ｃｒ：１０〜２２％を含有し、かつ下記（Ｉ）式（但し、（Ｉ）式中における元素記号は、その元素の含有量（質量％）を示す。また、元素記号の前の数値は定数である。）で示される有効Ｃｒ量が１０％以上であり、残部がＦｅおよび不可避不純物からなることを特徴とする尿素水中での耐食性に優れたフェライト系ステンレス鋼。
Ｃｒ＋４Ｓｉ−２Ｍｎ ・・・ （Ｉ）
（２）Ｎｂ、Ｔｉのいずれか１種または２種をそれぞれ０．０３〜１％の範囲で含有し、かつ下記（Ｉ）式、下記（ＩＩ）式もしくは下記（ＩＩＩ）式（但し、（Ｉ）〜（ＩＩＩ）式中における元素記号は、その元素の含有量（質量％）を示す。また、元素記号の前の数値は定数である。）で示される有効Ｃｒ量が１０％以上であることを特徴とする請求項１に記載の尿素水中での耐食性に優れたフェライト系ステンレス鋼。
Ｎｂのみを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ ・・・ （Ｉ）
Ｔｉのみを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ−１０Ｔｉ ・・・ （ＩＩ）
ＮｂおよびＴｉを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ−（１０Ｔｉ−３Ｎｂ） ・・・ （ＩＩＩ）
（３）Ｍｏ：３％以下、Ｎｉ：３%以下、Ｃｕ：３％以下、Ｖ：３％以下、Ｗ：５％以下の範囲で、1種または２種以上を含むことを特徴とする（１）または（２）に記載の尿素水中での耐食性に優れたフェライト系ステンレス鋼。
（４）Ａｌ：１％以下、Ｃａ：０．００２％以下、Ｍｇ：０．００２％以下、Ｂ：０．００５％以下の範囲で1種または２種以上を含むことを特徴とする（１）から（３）のいずれかに記載の尿素水中での耐食性に優れたフェライト系ステンレス鋼。
(1) By mass%, C: 0.05% or less, N: 0.05% or less, Si: 0.02-1.5%, Mn: 0.02-2%, Cr: 10-22% Contained and expressed by the following formula (I) (where the element symbol in the formula (I) indicates the content (% by mass) of the element, and the numerical value before the element symbol is a constant). Ferritic stainless steel excellent in corrosion resistance in urea water, characterized in that the amount of effective Cr is 10% or more, and the balance consists of Fe and inevitable impurities.
Cr + 4Si-2Mn (I)
(2) One or two of Nb and Ti are contained in the range of 0.03 to 1%, respectively, and the following formula (I), the following formula (II), or the following formula (III) (where ( In the formulas (I) to (III), the element symbol indicates the content (% by mass) of the element, and the numerical value before the element symbol is a constant. The ferritic stainless steel having excellent corrosion resistance in urea water according to claim 1.
When containing only Nb Cr + 4Si-2Mn (I)
When containing only Ti Cr + 4Si-2Mn-10Ti (II)
When Nb and Ti are contained Cr + 4Si-2Mn- (10Ti-3Nb) (III)
(3) Mo: 3% or less, Ni: 3% or less, Cu: 3% or less, V: 3% or less, W: 5% or less, including 1 type or 2 types or more ( A ferritic stainless steel having excellent corrosion resistance in urea water as described in 1) or (2).
(4) Al: 1% or less, Ca: 0.002% or less, Mg: 0.002% or less, and B: 0.005% or less, including 1 type or 2 types or more (1 ) To (3), a ferritic stainless steel excellent in corrosion resistance in urea water.
本発明によれば、高濃度尿素水への溶出が小さく、尿素水中での耐食性に優れたフェライト系ステンレス鋼を提供できるので、主にディーゼルエンジンを主体とする内燃機関において尿素水を用いて排ガス中のＮＯｘを低減する装置、特に自動車尿素ＳＣＲシステムなどに使用される機器類、より具体的には尿素水を貯蔵、製造あるいは輸送する際に使用されるタンクや配管といった機器類に好適に用いられる材料を提供できる。 According to the present invention, it is possible to provide a ferritic stainless steel that is small in elution into high-concentration urea water and excellent in corrosion resistance in urea water. Suitable for use in equipment that reduces NOx in the inside, especially equipment used in automobile urea SCR systems, and more specifically, equipment such as tanks and piping used when storing, manufacturing, or transporting urea water Material can be provided.
以下、本発明で規定される尿素水中での耐食性に優れたフェライト系ステンレス鋼の化学組成についてさらに詳しく説明する。 Hereinafter, the chemical composition of ferritic stainless steel excellent in corrosion resistance in urea water defined in the present invention will be described in more detail.
Ｃ：Ｃは、鋼の耐粒界腐食性、加工性を低下させるため、その含有量を低く抑える必要がある。そのため、Ｃの含有量を０．０５％以下とした。しかしながら、Ｃの含有量を過度に低めると、鋼の精練コストが上昇するため、Ｃの含有量を０．００２％以上とすることが望ましい。 C: Since C reduces the intergranular corrosion resistance and workability of steel, it is necessary to keep the content low. Therefore, the content of C is set to 0.05% or less. However, if the C content is excessively reduced, the steel refining cost increases, so the C content is preferably 0.002% or more.
Ｎ：Ｎは、鋼の耐孔食性に有用な元素であるが、耐粒界腐食性、加工性を低下させるため、その含有量を低く抑える必要がある。そのため、Ｎの含有量を０．０５％以下とした。しかしながら、Ｎの含有量を過度に低めると、鋼の精練コストが上昇するため、０．００２％以上とすることが望ましい。 N: N is an element useful for pitting corrosion resistance of steel, but its content needs to be kept low in order to reduce intergranular corrosion resistance and workability. Therefore, the N content is set to 0.05% or less. However, if the N content is excessively reduced, the steel refining cost increases, so it is desirable that the N content be 0.002% or more.
Ｓｉ：Ｓｉは、脱酸元素として有用であると共に、耐食性を向上させるために有効な元素であるが、加工性を低下させるため、その含有量を０．０２〜１．５％とした。 Si: Si is useful as a deoxidizing element and is an effective element for improving corrosion resistance, but its content is set to 0.02 to 1.5% in order to reduce workability.
Ｍｎ：Ｍｎは、脱酸元素として有用であるが、過剰に含有させると耐食性を劣化させるので、０．０２〜２％とした。 Mn: Mn is useful as a deoxidizing element. However, if it is excessively contained, the corrosion resistance deteriorates, so the content was made 0.02 to 2%.
Ｃｒ：Ｃｒは、本発明において最も重要な元素であり、高濃度尿素水中への溶出が極めて小さく、優れた耐食性を発現する鋼とするには、少なくとも１０％以上含有させる必要がある。Ｃｒの含有量を増加させるほど、鋼の溶出特性が安定化するが、加工性、製造性が低下するため、上限を２２％以下とした。また、本発明のフェライト系ステンレス鋼の用途では外面耐食性が要求される場合も多く、外面耐食性と高濃度尿素水中における耐食性とを両立させるためには、Ｃｒの含有量を１３％以上とすることが望ましく、より望ましくは１６〜２２％である。 Cr: Cr is the most important element in the present invention, and it is necessary to contain at least 10% or more in order to make steel exhibiting extremely low elution into high-concentration urea water and exhibiting excellent corrosion resistance. As the Cr content increases, the elution characteristics of the steel become more stable, but the workability and manufacturability decrease, so the upper limit was made 22% or less. In addition, the application of the ferritic stainless steel of the present invention often requires outer surface corrosion resistance, and in order to achieve both outer surface corrosion resistance and corrosion resistance in high-concentration urea water, the Cr content should be 13% or more. Is desirable, more desirably 16 to 22%.
Ｎｂ、Ｔｉ：ＮｂおよびＴｉは、Ｃ、Ｎを固定し、溶接部の耐粒界腐食性を向上させる上で有用な元素であり、必要に応じて含有させることができる。溶接部の耐粒界腐食性を確保しつつ、良好な加工性、製造性を得るには一方または両方をそれぞれ０．０３〜１％とすることが望ましく、耐粒界腐食性の観点から（Ｎｂ＋Ｔｉ）／（Ｃ＋Ｎ）≧８（式中における元素記号は、その元素の含有量（質量％）を示す。）であることが望ましい。 Nb, Ti: Nb and Ti are elements useful for fixing C and N and improving the intergranular corrosion resistance of the welded portion, and can be contained as necessary. In order to obtain good workability and manufacturability while ensuring the intergranular corrosion resistance of the weld, it is desirable that one or both be 0.03 to 1%, respectively, from the viewpoint of intergranular corrosion resistance ( It is desirable that Nb + Ti) / (C + N) ≧ 8 (the element symbol in the formula indicates the content (mass%) of the element).
有効Ｃｒ量：本発明においては、下記（Ｉ）式、下記（ＩＩ）式もしくは下記（ＩＩＩ）式（但し、（Ｉ）〜（ＩＩＩ）式中における元素記号は、その元素の含有量（質量％）を示す。また、元素記号の前の数値は定数である。）で示される有効Ｃｒ量を１０％以上とする。
Ｎｂ、Ｔｉを含有しない場合またはＮｂのみを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ ・・・ （Ｉ）
Ｔｉのみを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ−１０Ｔｉ ・・・ （ＩＩ）
ＮｂおよびＴｉを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ−（１０Ｔｉ−３Ｎｂ） ・・・ （ＩＩＩ）
Effective Cr amount: In the present invention, the following formula (I), the following formula (II) or the following formula (III) (wherein the element symbol in the formulas (I) to (III) is the content of the element (mass) The numerical value before the element symbol is a constant.) The effective Cr content indicated by 10) is 10% or more.
When Nb and Ti are not contained or when only Nb is contained Cr + 4Si-2Mn (I)
When containing only Ti Cr + 4Si-2Mn-10Ti (II)
When Nb and Ti are contained Cr + 4Si-2Mn- (10Ti-3Nb) (III)
上記（Ｉ）〜（ＩＩＩ）式により算出される有効Ｃｒ量は、鋼が溶接あるいはろう付けにより接合された場合のように鋼の表面に酸化皮膜が形成された状態において、酸化皮膜直下のＣｒ量を確保して、高濃度尿素水中への溶出が極めて小さく、ＪＩＳ Ｋ２２４７−１を満足する優れた耐食性を発現するフェライト系ステンレス鋼とするには、１０％以上とする必要がある。
また、本発明のフェライト系ステンレス鋼の用途では外面耐食性が要求される場合も多く、外面耐食性と高濃度尿素水中における耐食性とを両立させるためには、有効Ｃｒ量を１３％以上とすることが望ましく、より望ましくは１６％以上である。
The effective Cr amount calculated by the above formulas (I) to (III) is the Cr immediately below the oxide film when the oxide film is formed on the surface of the steel as in the case where the steel is joined by welding or brazing. In order to secure the amount and make ferritic stainless steel exhibiting excellent corrosion resistance satisfying JIS K2247-1 with extremely small elution into high-concentration urea water, it is necessary to be 10% or more.
Further, in the application of the ferritic stainless steel of the present invention, the outer surface corrosion resistance is often required, and in order to achieve both the outer surface corrosion resistance and the corrosion resistance in high-concentration urea water, the effective Cr amount may be 13% or more. Desirably, more desirably 16% or more.
Ｍｏ：Ｍｏは、耐食性を向上させるために、必要に応じて３％以下の範囲で含有させることができる。Ｍｏの含有量が０．３％以上であると、耐食性を向上させる効果が安定して得られる。また、Ｍｏの過剰な添加は、加工性を劣化させると共に、Ｍｏが高価であるためコストアップにつながる。したがって、Ｍｏは、０．３〜３％含有させるのが望ましい。 Mo: Mo can be contained in a range of 3% or less as necessary in order to improve the corrosion resistance. If the Mo content is 0.3% or more, the effect of improving the corrosion resistance is stably obtained. Further, excessive addition of Mo deteriorates workability and leads to an increase in cost because Mo is expensive. Therefore, it is desirable to contain Mo by 0.3 to 3%.
Ｎｉ：Ｎｉは、耐食性を向上させるために、必要に応じて３％以下の範囲で含有させることができる。Ｎｉの含有量が０．２％以上であると、耐食性を向上させる効果が安定して得られる。また、Ｎｉの過剰な添加は、加工性を劣化させると共に、Ｎｉが高価であるためコストアップにつながる。したがって、Ｎｉは、０．２〜３％含有させるのが望ましい。 Ni: Ni can be contained in a range of 3% or less as necessary in order to improve corrosion resistance. If the Ni content is 0.2% or more, the effect of improving the corrosion resistance is stably obtained. Further, excessive addition of Ni deteriorates workability and leads to an increase in cost because Ni is expensive. Therefore, Ni is desirably contained in an amount of 0.2 to 3%.
Ｃｕ：Ｃｕは、耐食性を向上させるために、必要に応じて３％以下の範囲で含有させることができる。Ｃｕの含有量が０．２％以上であると、耐食性を向上させる効果が安定して得られる。また、Ｃｕの過剰な添加は、加工性を劣化させると共に、Ｃｕが高価であるためコストアップにつながる。したがって、Ｃｕは、０．２〜３％含有させるのが望ましい。 Cu: Cu can be contained in a range of 3% or less as required in order to improve the corrosion resistance. If the Cu content is 0.2% or more, the effect of improving the corrosion resistance is stably obtained. Further, excessive addition of Cu deteriorates workability and leads to an increase in cost because Cu is expensive. Therefore, it is desirable to contain Cu 0.2 to 3%.
Ｖ：Ｖは、耐食性を向上させるために、必要に応じて３％以下の範囲で含有させることができる。Ｖの含有量が０．２％以上であると、耐食性を向上させる効果が安定して得られる。また、Ｖの過剰な添加は、加工性を劣化させると共に、Ｖが高価であるためコストアップにつながる。したがって、Ｖは、０．２〜３％含有させるのが望ましい。 V: V can be contained in a range of 3% or less as required in order to improve corrosion resistance. When the V content is 0.2% or more, the effect of improving the corrosion resistance is stably obtained. Further, excessive addition of V deteriorates processability and leads to an increase in cost because V is expensive. Therefore, V is desirably contained in an amount of 0.2 to 3%.
W：Wは、耐食性を向上させるために、必要に応じて５％以下の範囲で含有させることができる。Wの含有量が０．５％以上であると、耐食性を向上させる効果が安定して得られる。また、Wの過剰な添加は、加工性を劣化させると共に、Wが高価であるためコストアップにつながる。したがって、Wは、０．５〜５％含有させるのが望ましい。 W: W can be contained in a range of 5% or less as required in order to improve the corrosion resistance. If the W content is 0.5% or more, the effect of improving the corrosion resistance is stably obtained. Further, excessive addition of W deteriorates workability and leads to cost increase because W is expensive. Therefore, it is desirable to contain 0.5 to 5% of W.
Ａｌ：Ａｌは、脱酸効果等精練上有用な元素であり、成形性を向上させる効果があることから、必要に応じて１％以下の範囲で含有させることができる。Ａｌの含有量が０．０１％以上であると、Ａｌを添加したことによる安定した効果が得られる。しかし、Ａｌの過剰な添加は靭性を劣化させる。したがって、Ａｌは、０．０１〜１％含有させるのが望ましい。 Al: Al is an element useful for scouring, such as a deoxidizing effect, and has an effect of improving moldability. Therefore, it can be contained in a range of 1% or less as necessary. When the Al content is 0.01% or more, a stable effect due to the addition of Al is obtained. However, excessive addition of Al deteriorates toughness. Therefore, Al is desirably contained in an amount of 0.01 to 1%.
Ｃａ：Ｃａは、脱酸効果等精練上有用な元素であり、必要に応じて０．００２％以下の範囲で含有させる。Ｃａを含有させる場合には、Ｃａを添加したことによる安定した効果が得られるように０．０００２％以上含有させることが望ましい。 Ca: Ca is an element useful for scouring such as a deoxidizing effect, and is contained in a range of 0.002% or less as necessary. When Ca is contained, it is desirable to contain 0.0002% or more so as to obtain a stable effect due to the addition of Ca.
Ｍｇ：Ｍｇは、脱酸効果等精練上有用な元素であり、また、組織を微細化し、加工性、靭性を向上させるためにも有用であることから、必要に応じて０．００２％以下の範囲で含有させる。Ｍｇを含有させる場合には、Ｍｇを添加したことによる安定した効果が得られるように０．０００２％以上含有させることが望ましい。 Mg: Mg is an element useful for scouring such as a deoxidation effect, and is also useful for refining the structure and improving workability and toughness. Include in the range. When Mg is contained, it is desirable to contain 0.0002% or more so that a stable effect can be obtained by adding Mg.
Ｂ：Ｂは、２次加工性を向上させるのに有用な元素であり、必要に応じて０．００５％以下の範囲で含有させる。Ｂを含有させる場合には、Ｂを添加したことによる安定した効果が得られるように０．０００２％以上含有させることが望ましい。 B: B is an element useful for improving secondary workability, and is contained in a range of 0.005% or less as necessary. When it contains B, it is desirable to make it contain 0.0002% or more so that the stable effect by adding B may be acquired.
また、不可避不純物であるＰについては、溶接性の観点から０．０４％以下とすることが望ましい。また、Ｓについては、耐食性の観点から０．０１％以下とすることが望ましい。 Further, P, which is an inevitable impurity, is preferably 0.04% or less from the viewpoint of weldability. Moreover, about S, it is desirable to set it as 0.01% or less from a corrosion-resistant viewpoint.
表１に示す化学組成および有効Ｃｒ量を有する実験例１〜１３の鋼を溶製し、通常の熱延、冷延、焼鈍工程を経て、板厚１ｍｍの鋼板を製造した。このようにして得られた実験例１〜１３の冷延鋼板を用いて以下に示すように尿素水溶液中で浸漬試験を行った。 Steels of Experimental Examples 1 to 13 having chemical compositions and effective Cr amounts shown in Table 1 were melted, and steel plates having a thickness of 1 mm were manufactured through normal hot rolling, cold rolling, and annealing processes. An immersion test was performed in an aqueous urea solution as described below using the cold-rolled steel sheets of Experimental Examples 1 to 13 obtained as described above.
実験例１〜１３の冷延鋼板より、幅２０mm、長さ４０mmの試験片を切り出し、エメリー紙にて粗さ＃６００まで湿式研磨を施した。その後、溶接熱影響部の表面状態を模擬するために、大気中にて７００℃で１秒保持する熱処理を実施した。続いて、熱処理を施した実験例１〜１３の試験片を、６０℃、３０％の尿素水溶液中に１４４時間浸漬する腐食試験を行った。なお、比液量は、ＪＩＳ Ｋ２２３４「不凍液」中の金属腐食性試験に準じて３．６ｍｌ・ｃｍ−２とし、尿素水溶液の調製に用いた尿素には特級試薬を用いた。
腐食試験終了後、実験例１〜１３の試験片を秤量して腐食速度を求めると共に、ＩＣＰＳによる溶液分析を行った。溶液分析の元素はＦｅ、Ｃｒ、Ｎｉ、Ｃｕとした。腐食速度および溶液分析の結果を表２に示す。
A test piece having a width of 20 mm and a length of 40 mm was cut out from the cold-rolled steel sheets of Experimental Examples 1 to 13, and wet-polished to a roughness # 600 with emery paper. Thereafter, in order to simulate the surface state of the weld heat affected zone, a heat treatment was performed in the atmosphere at 700 ° C. for 1 second. Then, the corrosion test which immerses the test piece of Experimental Examples 1-13 which heat-processed in 60 degreeC and 30% urea aqueous solution for 144 hours was done. The specific liquid amount was 3.6 ml · cm −2 in accordance with the metal corrosion test in JIS K2234 “Antifreeze”, and a special grade reagent was used for urea used for preparing the urea aqueous solution.
After completion of the corrosion test, the test pieces of Experimental Examples 1 to 13 were weighed to determine the corrosion rate, and solution analysis was performed by ICPS. Elements for solution analysis were Fe, Cr, Ni, and Cu. The results of corrosion rate and solution analysis are shown in Table 2.
表２より、化学組成および有効Ｃｒ量が本発明範囲内である実験例１〜１０の鋼では、腐食速度が０．００１ｇ・ｍ−２・ｈ−1以下と極めて小さいと共に、試験後溶液中のＦｅ、Ｃｒ、Ｃｕ、Ｎｉ量（溶出量）がＪＩＳ Ｋ２２４７−１の規定（Ｆｅの溶出量：＜０．５、Ｃｒの溶出量：＜０．２、Ｎｉの溶出量：＜０．２、Ｃｕの溶出量：＜０．２（以上、ｍｇ/ｋｇ））を満足している。 From Table 2, in the steels of Experimental Examples 1 to 10 in which the chemical composition and the effective Cr amount are within the scope of the present invention, the corrosion rate is extremely small as 0.001 g · m −2 · h −1 or less and in the solution after the test. Fe, Cr, Cu, Ni amount (elution amount) of JIS K2247-1 (Fe elution amount: <0.5, Cr elution amount: <0.2, Ni elution amount: <0.2) Cu elution amount: <0.2 (more than mg / kg)).
これに対し、Ｃｒの含有量が本発明範囲からはずれ、かつ微量にしかＣｒを含まない実験例１１の鋼は、腐食速度が０．３５ｇ・ｍ−２・ｈ−1と大きく、試験後溶液中のＦｅ量がＪＩＳ Ｋ２２４７−１の規定をはるかに超える値となっている。
また、Ｃｒの含有量ならびに有効Ｃｒ量が本発明範囲外である実験例１２では、腐食速度は０．００５ｇ・ｍ−２・ｈ−1と小さいものの、試験後溶液中のＦｅ、Ｃｒ量がＪＩＳ Ｋ２２４７−１の規定を満足していない。
また、有効Ｃｒ量が本発明範囲外である実験例１３においても、腐食速度は０．００３ｇ・ｍ−２・ｈ−1以下と小さいものの、試験後溶液中のＦｅ、Ｃｒ量がＪＩＳ Ｋ２２４７−１の規定を満足していない。
In contrast, the steel of Experimental Example 11 in which the Cr content is out of the scope of the present invention and contains only a small amount of Cr has a large corrosion rate of 0.35 g · m −2 · h −1, and the solution after the test The amount of Fe in the sample is a value far exceeding the standard of JIS K2247-1.
Further, in Experimental Example 12 in which the Cr content and the effective Cr amount are outside the scope of the present invention, although the corrosion rate is as small as 0.005 g · m −2 · h −1 , the Fe and Cr amounts in the solution after the test are small. It does not satisfy the requirements of JIS K2247-1.
Also in Experimental Example 13 where the effective Cr amount is outside the range of the present invention, although the corrosion rate is as small as 0.003 g · m −2 · h −1 or less, the amounts of Fe and Cr in the solution after the test are JIS K2247−. Not satisfying the provision of 1.
また、実験例１〜１３における試験後溶液中のＦｅ量（溶出量）およびＣｒ量（溶出量）の結果と、有効Ｃｒ量との関係を図１に示す。
図１に示すグラフより、溶接熱影響部の表面状態とされた鋼において、有効Ｃｒ量が１０％以上である場合には、ＦｅおよびＣｒの溶出量がＪＩＳ Ｋ２２４７−１の規定を満足することが分かる。さらに、図１より、有効Ｃｒ量が１３％以上である鋼では、ＦｅおよびＣｒの溶出量が非常に少なく、耐食性が非常に優れていることが分かる。
Moreover, the relationship of the result of the amount of Fe (elution amount) and the amount of Cr (elution amount) in the solution after a test in Experimental Examples 1-13, and the amount of effective Cr is shown in FIG.
From the graph shown in FIG. 1, when the effective Cr amount is 10% or more in the steel in the surface state of the weld heat affected zone, the elution amount of Fe and Cr satisfies the JIS K2247-1 standard. I understand. Furthermore, it can be seen from FIG. 1 that the steel with an effective Cr content of 13% or more has a very low elution amount of Fe and Cr and very excellent corrosion resistance.
本発明の尿素水中での耐食性に優れたフェライト系ステンレス鋼は、主にディーゼルエンジンを主体とする内燃機関において尿素水を用いて排ガス中のＮＯｘを低減する装置、特に自動車尿素ＳＣＲシステムにおいて用いられる尿素水を貯蔵、製造あるいは輸送する際に使用されるタンクや配管といった機器類の材料として好適である。 The ferritic stainless steel excellent in corrosion resistance in urea water of the present invention is mainly used in an apparatus for reducing NOx in exhaust gas using urea water in an internal combustion engine mainly including a diesel engine, particularly in an automobile urea SCR system. It is suitable as a material for equipment such as tanks and pipes used for storing, manufacturing or transporting urea water.
Claims (4)
Ｃｒ＋４Ｓｉ−２Ｍｎ ・・・ （Ｉ） In mass%, C: 0.05% or less, N: 0.05% or less, Si: 0.02-1.5%, Mn: 0.02-2%, Cr: 10-22%, And the effective Cr shown by the following formula (I) (however, the element symbol in the formula (I) indicates the content (mass%) of the element. The numerical value before the element symbol is a constant). A ferritic stainless steel excellent in corrosion resistance in urea water, characterized in that the amount is 10% or more and the balance consists of Fe and inevitable impurities.
Cr + 4Si-2Mn (I)
Ｎｂのみを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ ・・・ （Ｉ）
Ｔｉのみを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ−１０Ｔｉ ・・・ （ＩＩ）
ＮｂおよびＴｉを含有する場合
Ｃｒ＋４Ｓｉ−２Ｍｎ−（１０Ｔｉ−３Ｎｂ） ・・・ （ＩＩＩ） One or two of Nb and Ti are contained in the range of 0.03 to 1%, respectively, and the following formula (I), the following formula (II), or the following formula (III) (provided that (I) to The element symbol in the formula (III) indicates the content (mass%) of the element, and the numerical value before the element symbol is a constant.) The ferritic stainless steel having excellent corrosion resistance in urea water according to claim 1.
When containing only Nb Cr + 4Si-2Mn (I)
When containing only Ti Cr + 4Si-2Mn-10Ti (II)
When Nb and Ti are contained Cr + 4Si-2Mn- (10Ti-3Nb) (III)
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| JPWO2010010916A1 (en) * | 2008-07-23 | 2012-01-05 | 新日鐵住金ステンレス株式会社 | Ferritic stainless steel for urea water tank |
| JP2012112025A (en) * | 2010-11-26 | 2012-06-14 | Nippon Steel & Sumikin Stainless Steel Corp | Ferritic stainless steel sheet for part of urea scr system and method for production thereof |
| JP2012211379A (en) * | 2011-03-31 | 2012-11-01 | Nisshin Steel Co Ltd | FERRITIC STAINLESS STEEL EXCELLENT IN SECONDARY WORKABILITY AND Cr EVAPORATION RESISTANCE |
| JP2013010981A (en) * | 2011-06-28 | 2013-01-17 | Jfe Steel Corp | Ferritic stainless steel for egr cooler |
| WO2016035241A1 (en) * | 2014-09-02 | 2016-03-10 | Ｊｆｅスチール株式会社 | Ferritic stainless steel sheet for casing for urea-scr |
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