JP5609668B2 - Stainless clad steel with excellent seawater pitting resistance - Google Patents
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- 229910000831 Steel Inorganic materials 0.000 title claims description 77
- 239000010959 steel Substances 0.000 title claims description 77
- 239000013535 sea water Substances 0.000 title claims description 45
- 238000005260 corrosion Methods 0.000 claims description 61
- 230000007797 corrosion Effects 0.000 claims description 61
- 239000010935 stainless steel Substances 0.000 claims description 39
- 229910001220 stainless steel Inorganic materials 0.000 claims description 39
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- 239000000463 material Substances 0.000 claims description 23
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 claims description 18
- 238000005259 measurement Methods 0.000 claims description 12
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- 238000010438 heat treatment Methods 0.000 description 6
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- 238000009529 body temperature measurement Methods 0.000 description 5
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- KRHYYFGTRYWZRS-UHFFFAOYSA-N Fluorane Chemical compound F KRHYYFGTRYWZRS-UHFFFAOYSA-N 0.000 description 4
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 4
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- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 3
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- 206010070834 Sensitisation Diseases 0.000 description 1
- PMZURENOXWZQFD-UHFFFAOYSA-L Sodium Sulfate Chemical compound [Na+].[Na+].[O-]S([O-])(=O)=O PMZURENOXWZQFD-UHFFFAOYSA-L 0.000 description 1
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- 229910052938 sodium sulfate Inorganic materials 0.000 description 1
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Description
本発明は、海洋構造物や熱交換器に代表される各種用途で使用される耐海水孔食性に優れたステンレスクラッド鋼に関するものである。 The present invention relates to a stainless clad steel excellent in seawater pitting resistance used in various applications represented by offshore structures and heat exchangers.
近年、高効率化の観点からプラント操業が高温・高圧化する傾向にあり、化学プラントの設計においては強度を確保するため、より板厚が厚い鋼板を使用する割合が増加する傾向にある。さらに、産業設備と構造物のニーズとしては耐久性と長寿命化およびメンテナンスフリーが指向されており、ステンレス鋼はこれらのニーズに適合した材料として注目を集めている。一方で、ステンレス鋼の主原料であるNiやMo、Crに代表される合金元素は、価格の高騰や価格の上下動がある。そのため、ステンレス鋼に代わり、ステンレス鋼の優れた防錆性能をより経済的に利用でき、価格が安定しかつ安価な鋼材としてステンレスクラッド鋼が、最近、注目されている。 In recent years, plant operation tends to increase in temperature and pressure from the viewpoint of higher efficiency, and in the design of chemical plants, the ratio of using thicker steel plates tends to increase in order to ensure strength. Furthermore, the needs of industrial equipment and structures are oriented toward durability, long life and maintenance-free, and stainless steel is attracting attention as a material that meets these needs. On the other hand, alloy elements typified by Ni, Mo, and Cr, which are the main raw materials for stainless steel, have a rising price and a rising and falling price. Therefore, instead of stainless steel, stainless steel clad steel has recently been attracting attention as a steel material that can use the excellent antirust performance of stainless steel more economically, is stable in price, and inexpensive.
ステンレスクラッド鋼とは合わせ材にステンレス鋼、母材に普通鋼材と、二種類の性質の異なる金属を張り合わせた鋼材である。クラッド鋼は、異種金属を金属学的に接合させたもので、めっきとは異なり剥離する心配がなく単一金属および合金では達し得ない新たな特性を持たせることができる。 Stainless steel clad steel is a steel material in which two different types of metals are bonded together: stainless steel as a laminated material and ordinary steel as a base material. The clad steel is obtained by metallographically bonding dissimilar metals, and unlike the plating, there is no fear of peeling, and it can have new characteristics that cannot be achieved by a single metal and alloy.
ステンレスクラッド鋼は、使用環境毎の目的に合った防錆能を確保するため、使用環境毎に合わせ材であるステンレス鋼の種類を選択し、無垢材（全厚ステンレス鋼）と同等の防錆能を確保している。
このように、ステンレスクラッド鋼は、ステンレス鋼材の使用量が少なくてすみ、かつ、無垢材（全厚ステンレス鋼）と同等の防錆能を確保できるため、経済性と機能性が両立できる利点を有する。
Stainless steel clad steel has the same rust prevention as solid materials (full thickness stainless steel) by selecting the type of stainless steel that is suitable for each usage environment in order to ensure the anticorrosive ability suitable for the purpose of each usage environment. Performance is secured.
In this way, stainless steel clad steel requires only a small amount of stainless steel, and can secure the same rust prevention performance as solid wood (full thickness stainless steel). Have.
以上から、ステンレスクラッド鋼は非常に有益な機能性鋼材であると考えられており、近年そのニーズが各種産業分野で益々高まっている。 From the above, it is considered that stainless clad steel is a very useful functional steel material, and in recent years its needs have been increasing in various industrial fields.
このようなステンレスクラッド鋼を海洋構造物や海水と接する環境で使用される熱交換器、造船等に代表される各種用途で使用する場合には、上記防錆能に加え、厳しい海水腐食環境下で使用されるため、耐海水孔食性が要求される。ステンレス鋼の不動態皮膜は塩化物イオンにより破壊されやすくなり、その腐食形態は孔食腐食（Pitting Corrosion）の形態をとる。そこで硫酸やふっ酸などに代表される酸中での腐食形態が全面腐食を呈するのに対し、海水中では局部腐食の起点となる耐孔食性が重要な指標となる。 When such stainless clad steel is used in various applications typified by heat exchangers, shipbuilding, etc. that are in contact with offshore structures or seawater, in addition to the above rust prevention performance, Therefore, seawater pitting resistance is required. The passive film of stainless steel is easily destroyed by chloride ions, and its corrosion form takes the form of pitting corrosion. Therefore, while corrosion forms in acids such as sulfuric acid and hydrofluoric acid exhibit general corrosion, pitting corrosion resistance, which is the starting point of local corrosion, is an important index in seawater.
耐海水孔食性を改善する技術として、特許文献１には、固溶化熱処理条件と母材炭素鋼の成分を規定し、耐海水性に優れたステンレス鋼を合わせ材とし炭素鋼を母材としたステンレスクラッド鋼管を製造する方法が開示されている。しかしながら、特許文献１では、ステンレスクラッド鋼管の用途（例えば、海洋構造物や熱交換器等）毎に求められる耐食性および耐海水孔食性を確保するため、合わせ材として使用するステンレス鋼を用途毎に選択しなければならない。すなわちステンレス鋼の成分のみで調整する方法が示されているに過ぎず、ステンレスクラッド鋼の場合、接合界面の健全性と信頼性向上や母材と合わせ材の性能を同時に維持することを高級鋼材や多様な品種すべてに対応することは難しい。 As a technique for improving seawater pitting resistance, Patent Document 1 defines the conditions for solution heat treatment and the base material carbon steel, and uses stainless steel with excellent seawater resistance as a combination material and carbon steel as a base material. A method of manufacturing a stainless clad steel pipe is disclosed. However, in Patent Document 1, in order to ensure the corrosion resistance and seawater pitting resistance required for each use of a stainless clad steel pipe (for example, an offshore structure or a heat exchanger), stainless steel used as a laminated material is used for each application. Must be selected. That is, only the method of adjusting with the components of stainless steel is shown, and in the case of stainless clad steel, high-grade steel materials that maintain the soundness and reliability of the joint interface and maintain the performance of the base material and the laminated material at the same time. It is difficult to deal with all kinds of variety.
特許文献2には、優れた耐孔食性を有する耐海水腐食性オーステナイト系ステンレス鋳鋼及び海水用ポンプを提供する技術が開示されている。特許文献2では、課題を解決するために重量で、Ｃ：０.０８ｗｔ％ 以下、Ｓｉ：０.５〜１.５ｗｔ％、Ｍｎ：０.５〜２ｗｔ％ 、Ｐ：０.０４ｗｔ％以下、Ｓ：０.０１ｗｔ％以下、Ｎｉ：８.０〜９.５ｗｔ％、Ｃｒ：１８〜２１ｗｔ％を含むオーステナイト系ステンレス鋳鋼について、δフェライト相を６vol％ 以上にするか、あるいは、清浄度を０.１％ 以下にすることにより達成される成分が開示されている。しかしながら、特許文献2では鋼の成分および第二相の析出量を所定範囲に規定しているに過ぎず表面性状に関しては十分な技術が開示されていない。また、鋳鋼のため強度などの点で不利なものであり、さらにそのままではクラッド鋼の原料とはなりえない。 Patent Document 2 discloses a technology for providing a seawater corrosion-resistant austenitic stainless cast steel having excellent pitting corrosion resistance and a seawater pump. In Patent Document 2, in order to solve the problem, by weight, C: 0.08 wt% or less, Si: 0.5-1.5 wt%, Mn: 0.5-2 wt%, P: 0.04 wt% or less, About austenitic stainless cast steel containing S: 0.01 wt% or less, Ni: 8.0 to 9.5 wt%, Cr: 18 to 21 wt%, the δ ferrite phase is set to 6 vol% or more, or the cleanliness is 0 Ingredients achieved by less than 0.1% are disclosed. However, Patent Document 2 merely defines the steel components and the precipitation amount of the second phase within a predetermined range, and does not disclose a sufficient technique regarding the surface properties. Moreover, since it is a cast steel, it is disadvantageous in terms of strength and the like, and it cannot be a raw material for clad steel as it is.
本発明は、かかる事情に鑑み、耐海水孔食性に優れたステンレスクラッド鋼を提供することを目的とする。 In view of such circumstances, an object of the present invention is to provide a stainless clad steel excellent in seawater pitting resistance.
課題を解決するために、同一成分（鋼組成）および同一履歴で圧延から熱処理まで完了した複数のステンレスクラッド鋼に対して種々な鏡面仕上げ処理を施し、表面性状の詳細な検討を行った。
そして、表面粗さ、介在物、ステンレス鋼の耐食性（耐孔食性）を左右する不動態皮膜の強さCｒ/Fe比などに着目して検討を行ったところ、孔食指数PIが35.0以上であるステンレス鋼を合わせ材とするステンレスクラッド鋼であって、前記ステンレスクラッド鋼表面の、JIS B 0601−2001に準拠して測定される圧延（長手）方向（L）の平均粗さRzJIS（L）を1.0μｍ以下、圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）を1.0μｍ以下、各測定箇所での前記圧延（長手）方向（L）の平均粗さRzJIS（L）と前記圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）の比(L/C)の平均である平均L/Cを0.5〜2.0の範囲とすることで、耐海水孔食性が改善することを見出した。さらには、前記ステンレスクラッド鋼表面の硫化物系介在物の個数を2.5×105個/mm2以下、不動態皮膜部におけるCr濃度(at％)/Fe濃度(at％)と母相であるステンレス鋼のCr濃度(at％)/Fe濃度(at％)の比を1.2以上とすることで、より一層耐海水孔食性が向上することも見出した。
In order to solve the problem, various mirror finishing treatments were applied to a plurality of stainless clad steels having the same components (steel composition) and the same history and completed from rolling to heat treatment, and the surface properties were examined in detail.
And we examined the surface roughness, inclusions, and the strength Cr / Fe ratio of the passive film that affects the corrosion resistance (pitting corrosion resistance) of stainless steel. An average roughness Rz JIS (L) in the rolling (longitudinal) direction (L) of the stainless clad steel surface measured according to JIS B 0601-2001. ) the 1.0 .mu.m or less, 1.0 .mu.m or less vertical (average C) roughness Rz JIS (C) to the rolling direction, the average roughness Rz JIS of the rolling at each measurement point (lengthwise) direction (L) (L) and the average L / C, which is the average of the ratio (L / C) of the average roughness Rz JIS (C) in the direction perpendicular to the rolling direction (C), in the range of 0.5 to 2.0, It has been found that seawater pitting resistance is improved. Furthermore, the number of sulfide inclusions on the surface of the stainless clad steel is 2.5 × 10 5 pieces / mm 2 or less, and the Cr concentration (at%) / Fe concentration (at%) in the passive film part and the parent phase. It has also been found that seawater pitting resistance is further improved by setting the Cr concentration (at%) / Fe concentration (at%) ratio of stainless steel to 1.2 or more.
本発明は、以上の知見に基づいてなされたものであり、その要旨は以下のとおりである。
［１］孔食指数PIが35.0以上であるステンレス鋼を合わせ材とするステンレスクラッド鋼であって、前記ステンレスクラッド鋼表面の、JIS B 0601−2001に準拠して測定される圧延（長手）方向（L）の平均粗さRzJIS（L）が1.0μｍ以下、圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）が1.0μｍ以下、各測定箇所での前記圧延（長手）方向（L）の平均粗さRzJIS（L）と前記圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）の比(L/C)の平均である平均L/Cが、0.5〜2.0の範囲であることを特徴とする耐海水孔食性に優れたステンレスクラッド鋼。
なお、孔食指数PIとは、Cr（質量％）+3Mo（質量％）+16N（質量％）で示される指数である。
［２］前記［１］において、前記ステンレスクラッド鋼表面の硫化物系介在物の個数が2.5×105個/mm2以下であることを特徴とする耐海水孔食性に優れたステンレスクラッド鋼。
［３］前記［１］または［２］において、前記ステンレスクラッド鋼の表面に形成される不動態皮膜部におけるCr濃度(at％)/Fe濃度(at％)と母相であるステンレス鋼のCr濃度(at％)/Fe濃度(at％)の比が1.2以上であることを特徴とするの耐海水孔食性に優れたステンレスクラッド鋼。
This invention is made | formed based on the above knowledge, The summary is as follows.
[1] Rolling (longitudinal) direction measured according to JIS B 0601-2001 on a stainless steel clad steel having a pitting corrosion index PI of 35.0 or more as a combination material The average roughness Rz JIS (L) of (L) is 1.0 μm or less, the average roughness Rz JIS (C) in the direction perpendicular to the rolling direction (C) is 1.0 μm or less, and the rolling at each measurement point (longitudinal) ) Direction (L) average roughness Rz JIS (L) and average L / C which is the average of the ratio (L / C) of the average roughness Rz JIS (C) in the direction perpendicular to the rolling direction (C) Is a stainless clad steel excellent in seawater pitting corrosion resistance, characterized by being in the range of 0.5 to 2.0.
The pitting index PI is an index represented by Cr (mass%) + 3Mo (mass%) + 16N (mass%).
[2] The stainless clad steel excellent in seawater pitting corrosion resistance according to [1], wherein the number of sulfide inclusions on the surface of the stainless clad steel is 2.5 × 10 5 pieces / mm 2 or less.
[3] In the above [1] or [2], Cr concentration (at%) / Fe concentration (at%) in the passive film portion formed on the surface of the stainless clad steel and Cr of the stainless steel as the parent phase Stainless steel clad steel with excellent seawater pitting corrosion resistance, characterized in that the ratio of concentration (at%) / Fe concentration (at%) is 1.2 or more.
なお、耐海水孔食性に優れたとは、後述する耐海水孔食性の指標であるCPT（臨界孔食発生温度）が40℃以上、CCT（臨界隙間腐食発生温度）が20℃以上である。 “Excellent seawater pitting corrosion resistance” means that CPT (critical pitting corrosion occurrence temperature), which is an index of seawater pitting corrosion resistance described later, is 40 ° C. or higher, and CCT (critical crevice corrosion occurrence temperature) is 20 ° C. or higher.
本発明によれば、耐海水孔食性に優れたステンレスクラッド鋼が得られる。
海洋構造物や熱交換器に代表される、耐海水孔食性が要求される用途で、好適に用いることができる。
According to the present invention, a stainless clad steel excellent in seawater pitting resistance can be obtained.
It can be suitably used in applications that require seawater pitting resistance, such as marine structures and heat exchangers.
本発明のステンレスクラッド鋼は、Cr（質量％）+3Mo（質量％）+16N（質量％）で示される孔食指数PIが35.0以上であるステンレス鋼を合わせ材とするステンレスクラッド鋼であって、前記ステンレスクラッド鋼表面の、JIS B 0601−2001に準拠して測定される圧延（長手）方向（L）の平均粗さRzJIS（L）が1.0μｍ以下、圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）が1.0μｍ以下、各測定箇所での前記圧延（長手）方向（L）の平均粗さRzJIS（L）と前記圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）の比(L/C)の平均である平均L/Cが、0.5〜2.0の範囲である。さらに、前記ステンレスクラッド鋼表面の硫化物系介在物の個数が2.5×105個/mm2以下であることが好ましい。また、前記ステンレスクラッド鋼の表面に形成される不動態皮膜部におけるCr濃度(at％)/Fe濃度(at％)と母相であるステンレス鋼のCr濃度(at％)/Fe濃度(at％)の比が1.2以上であることが好ましい。 The stainless clad steel of the present invention is a stainless clad steel having a pitting corrosion index PI represented by Cr (mass%) + 3Mo (mass%) + 16N (mass%) of 35.0 or more as a combination material. The average roughness Rz JIS (L) in the rolling (longitudinal) direction (L) measured according to JIS B 0601-2001 on the surface of the stainless clad steel is 1.0 μm or less, and the direction perpendicular to the rolling direction ( The average roughness Rz JIS (C) of C) is 1.0 μm or less, the average roughness Rz JIS (L) in the rolling (longitudinal) direction (L) at each measurement point, and the direction perpendicular to the rolling direction (C ) Average roughness Rz JIS (C) ratio (L / C) average L / C is in the range of 0.5 to 2.0. Furthermore, it is preferable that the number of sulfide inclusions on the surface of the stainless clad steel is 2.5 × 10 5 pieces / mm 2 or less. In addition, Cr concentration (at%) / Fe concentration (at%) in the passive film formed on the surface of the stainless clad steel and Cr concentration (at%) / Fe concentration (at%) of the stainless steel as the parent phase ) Ratio is preferably 1.2 or more.
そして、上記のように表面性状を制御することにより、耐海水孔食性に優れたステンレスクラッド鋼が得られることになる。なお、本発明のステンレスクラッド鋼としては、熱延鋼板、熱延処理後に焼ならし熱処理を施した鋼板、いずれも含まれ、同様な効果が得られる。 And by controlling the surface properties as described above, a stainless clad steel having excellent seawater pitting resistance can be obtained. As the stainless clad steel of the present invention, hot-rolled steel sheets, steel sheets subjected to Annealing Rashi heat treatment after hot rolling treatment, both included, the same effect can be obtained.
また、機械的な処理に加え化学的な処理を組み合わせることで表面性状を制御し表面の特性を所定の範囲にすることができる。機械的な処理としては、通常のベルト研磨、砥石研磨などが挙げられ、これらの既存の各種表面研磨手法を組み合わせて行うことができる。これらを組み合わせることで表面の粗さを低く抑えることができる。また、表面の介在物除去と不動態皮膜強化の観点から、化学的な処理を行うことが好ましい。化学的な処理方法としては特に限定はしないが、硝酸やふっ硝酸、硫酸、塩酸中における酸洗処理またはこれら酸溶液中または中性塩溶液における電解処理（例えば、ルスナー法：20質量％硫酸ナトリウム溶液や硝酸ナトリウム）が好ましい。 Further, by combining chemical treatment with chemical treatment, the surface properties can be controlled and the surface characteristics can be kept within a predetermined range. Examples of the mechanical treatment include ordinary belt polishing, grinding wheel polishing, and the like, and these existing various surface polishing methods can be combined. By combining these, the surface roughness can be kept low. Moreover, it is preferable to perform a chemical treatment from the viewpoint of removing inclusions on the surface and strengthening the passive film. The chemical treatment method is not particularly limited, but pickling treatment in nitric acid, hydrofluoric acid, sulfuric acid, hydrochloric acid or electrolytic treatment in these acid solutions or neutral salt solutions (for example, Lusner method: 20% by mass sodium sulfate) Solutions and sodium nitrate) are preferred.
合わせ材：Cr（質量％）+3Mo（質量％）+16N（質量％）で示される孔食指数PIが35.0以上であるステンレス鋼
海洋構造物や熱交換器にステンレス鋼を無塗装で使用するためには、海水環境において十分な耐孔食性、すなわち、耐海水孔食性を有する必要がある。PIが35.0未満のステンレス鋼では海水中で孔食を生じ、本発明の特徴である表面の粗さを制御したとしてもJIS G 0578準拠のCPT（臨界孔食発生温度）：40℃以上、CCT（臨界隙間腐食発生温度）:20℃以上の特性を達成することができず、十分な耐海水孔食性が得られない。よって、本発明において、PIは35.0以上とする。好ましくは40.0以上である。
Laminated material: Stainless steel is used unpainted in stainless steel marine structures and heat exchangers with a pitting corrosion index PI of 35.0 or more, expressed as Cr (mass%) + 3Mo (mass%) + 16N (mass%) Therefore, it is necessary to have sufficient pitting corrosion resistance in a seawater environment, that is, seawater pitting corrosion resistance. Stainless steel with a PI of less than 35.0 causes pitting corrosion in seawater. Even if the surface roughness, which is a feature of the present invention, is controlled, CPT (critical pitting corrosion temperature) conforming to JIS G 0578: 40 ° C or higher, CCT (Critical crevice corrosion occurrence temperature): A characteristic of 20 ° C. or higher cannot be achieved, and sufficient seawater pitting resistance cannot be obtained. Therefore, in the present invention, PI is 35.0 or more. Preferably it is 40.0 or more.
JIS B 0601−2001に準拠して測定される圧延（長手）方向（L）の平均粗さRzJIS（L）が1.0μｍ以下、圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）が1.0μｍ以下、各測定箇所での前記圧延（長手）方向（L）の平均粗さRzJIS（L）と前記圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）の比(L/C)の平均である平均L/Cが、0.5〜2.0の範囲
表面の粗さはステンレスクラッド鋼の耐海水孔食性に大きな影響を及ぼす。表面の凹凸が大きい場合、海水浸漬中、海塩粒子やふじつぼなどをはじめとする付着物がステンレスクラッド鋼の表面につきやすくなり、隙間を形成しやすくなる。加えて、表面積が広がることで粒界や粒内に存在する可溶性析出部が多く表面に現れ、孔食の起点となる脆弱部が多くなる。また、粗さに異方性がある場合、方向によって表面の性状が異なることになる。さらに特定な方向に付着物が着きやすくなりクラッド鋼を使用する時に鋼板方向を気にして適用しなくてはいけないという実使用上の問題がある。
Average roughness Rz JIS (L) in the rolling (longitudinal) direction (L) measured in accordance with JIS B 0601-2001 is 1.0 μm or less, average roughness Rz JIS in the direction perpendicular to the rolling direction (C) (C) is 1.0μm or less, an average roughness Rz JIS of the rolling at each measurement point (lengthwise) direction average roughness Rz JIS (L) and the direction perpendicular to the rolling direction (L) (C) ( The average L / C, which is the average of the ratio (L / C), is in the range of 0.5 to 2.0. The surface roughness greatly affects the seawater pitting resistance of stainless clad steel. When the surface unevenness is large, during seawater immersion, deposits such as sea salt particles and Fujitsubo are likely to hit the surface of the stainless clad steel, and gaps are easily formed. In addition, since the surface area is widened, many soluble precipitates present in the grain boundaries and within the grains appear on the surface, and the fragile parts that are the starting points of pitting corrosion increase. In addition, when the roughness is anisotropic, the surface properties vary depending on the direction. Furthermore, there is a problem in practical use that deposits tend to adhere in a specific direction, and the clad steel must be applied with care when using the clad steel.
そこで、上記を考慮し、耐海水孔食性の向上と付着物排除の点で検討したところ、表面粗さとしてJIS B 0601−2001に準拠して測定される粗さRzJISを用い、圧延（長手）方向（L）の平均粗さRzJIS（L）を1.0μｍ以下、圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）を1.0μｍ以下、各測定箇所での前記圧延（長手）方向（L）の平均粗さRzJIS（L）と前記圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）の比(L/C)の平均である平均L/Cを、0.5〜2.0の範囲とすることで、海水に浸漬した時にステンレスクラッド鋼表面に付着物がつきにくくなり、耐海水孔食性が向上することがわかった。以上より、圧延（長手）方向（L）の平均粗さRzJIS（L）は1.0μｍ以下、圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）は1.0μｍ以下、各測定箇所での前記圧延（長手）方向（L）の平均粗さRzJIS（L）と前記圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）の比(L/C)の平均である平均L/Cは0.5〜2.0の範囲とする。好ましくは、圧延（長手）方向（L）の平均粗さRzJIS（L）が0.7μｍ以下、圧延方向に対して垂直方向（C）の平均粗さRzJIS（C）が0.7μｍ以下、平均L/Cが0.7〜1.5の範囲がよい。一方で、表面の粗さは小さいほうが好ましいが、ステンレスクラッド鋼の研磨には非常に負荷がかかるため、平均粗さRzJIS（L）および平均粗さRzJIS（C）の下限は0.1μｍが好ましい。また、圧延方向と垂直方向で完全に粗さを揃えることも工業的には非常に負荷がかかるため平均L/Cの範囲は0.9以下もしくは1.1以上がさらに好ましい。 Therefore, in consideration of the above, when considering the improvement of seawater pitting corrosion resistance and the removal of deposits, the surface roughness was measured using a roughness Rz JIS measured in accordance with JIS B 0601-2001, and rolled (longitudinal). ) The average roughness Rz JIS (L) in the direction (L) is 1.0 μm or less, the average roughness Rz JIS (C) in the direction perpendicular to the rolling direction (C) is 1.0 μm or less, and the rolling at each measurement point (longitudinal) direction average of a mean L ratio of the average roughness (L) Rz JIS (L) and the average roughness Rz JIS of the direction perpendicular to the rolling direction (C) (C) (L / C) It was found that by setting / C in the range of 0.5 to 2.0, the surface of the stainless clad steel is less likely to adhere when immersed in seawater, and the seawater pitting resistance is improved. From the above, the average roughness Rz JIS (L) in the rolling (longitudinal) direction (L) is 1.0 μm or less, and the average roughness Rz JIS (C) in the direction perpendicular to the rolling direction (C) is 1.0 μm or less. the rolling in the measurement points (lengthwise) direction average roughness Rz JIS of the (L) (L) and the ratio of the average roughness Rz JIS of the vertical (C) to the rolling direction (C) (L / C) The average L / C, which is the average of, is in the range of 0.5 to 2.0. Preferably, the average roughness Rz JIS (L) in the rolling (longitudinal) direction (L) is 0.7 μm or less, the average roughness Rz JIS (C) in the direction perpendicular to the rolling direction (C) is 0.7 μm or less, and the average L / C is preferably in the range of 0.7 to 1.5. On the other hand, it is preferable that the surface roughness is small, but the polishing of stainless steel clad steel is very burdensome, so the lower limit of average roughness Rz JIS (L) and average roughness Rz JIS (C) is 0.1 μm. preferable. Further, it is industrially very difficult to make the roughness completely uniform in the rolling direction and the vertical direction, so the average L / C range is more preferably 0.9 or less or 1.1 or more.
ステンレスクラッド鋼表面の硫化物系介在物の個数が2.5×105個/mm2以下（好適条件）
海洋構造物や熱交換器に代表される用途で使用されるステンレスクラッド鋼の場合は、主に耐海水孔食性が要求される。検討したところ、ステンレスクラッド鋼表面に析出する介在物の存在は、耐海水孔食性に関与していることがわかった。さらに、ステンレスクラッド鋼表面に析出する介在物としては、酸化物、炭化物、窒化物、硫化物等があげられるが、この中でも、特に粗大な硫化物（例えば、ＦｅＳ、Ｆｅ2Ｓ、ＭｎＳ、ＣｕＳ、Ｃｕ2Ｓ、ＣｄＳ、ＳｎＳ、ＮｉＳ、ＺｎＳなど）の存在は、耐海水孔食性の低下に大きく関与していることがわかった。以上の検討の結果から、本発明においては、硫化物系介在物を制御し、その個数を2.5×105 個/mm2以下とすることが好ましい。なお、硫化物系介在物とは、ＦｅＳ、Ｆｅ2Ｓ、ＭｎＳ、ＣｕＳ、Ｃｕ2Ｓ、ＣｄＳ、ＳｎＳ、ＮｉＳ、ＺｎＳなどである。硫化物系介在物の個数に関しては少ないほうが孔食の発生確率が低減するため、少ないほうが良い。ただし、実際にはすべての硫化物系介在物を除去することは実製造上困難になるため1.0×102 個/mm2以上が好ましい。
The number of sulfide inclusions on the surface of stainless clad steel is 2.5 × 10 5 / mm 2 or less (preferred conditions)
In the case of stainless clad steel used in applications typified by offshore structures and heat exchangers, seawater pitting corrosion resistance is mainly required. As a result of the investigation, it was found that the presence of inclusions precipitated on the surface of the stainless clad steel was involved in seawater pitting resistance. Further, the inclusions precipitated on the surface of the stainless clad steel include oxides, carbides, nitrides, sulfides, etc. Among them, particularly coarse sulfides (for example, FeS, Fe 2 S, MnS, CuS). , Cu 2 S, CdS, SnS, NiS, ZnS, etc.) was found to be greatly involved in the decrease in seawater pitting resistance. From the results of the above studies, in the present invention, it is preferable to control sulfide inclusions so that the number thereof is 2.5 × 10 5 pieces / mm 2 or less. The sulfide inclusions are FeS, Fe 2 S, MnS, CuS, Cu 2 S, CdS, SnS, NiS, ZnS, and the like. The smaller the number of sulfide inclusions, the lower the probability of occurrence of pitting corrosion. However, in practice, it is difficult to remove all sulfide inclusions in actual production, so 1.0 × 10 2 pieces / mm 2 or more is preferable.
なお、個々の介在物をエネルギー分散型Ｘ線分析装置を用いて元素分析を行い、組成を調べることにより、硫化物系析出物であるがどうか、同定できる。ただし、硫化物系介在物のサイズが1.0μｍΦ未満であれば硫化物系介在物が海水などに接触して溶解し孔食が発生しても、その進行が止まってしまう確率が高いので、同定時にカウントする介在物は1.0μmΦ以上のサイズに限定する。 In addition, it can be identified whether it is a sulfide type | system | group deposit by performing an elemental analysis for each inclusion using an energy dispersive X-ray analyzer, and investigating a composition. However, if the size of the sulfide inclusions is less than 1.0μmΦ, even if the sulfide inclusions dissolve in contact with seawater and the like, and the pitting corrosion occurs, there is a high probability that the progress will stop. Inclusions that are sometimes counted are limited to a size of 1.0 μmΦ or more.
ステンレスクラッド鋼の表面に形成される不動態皮膜部におけるCr濃度(at％)/Fe濃度(at％)と母相であるステンレス鋼のCr濃度(at％)/Fe濃度(at％)の比が1.2以上（好適条件）
不動態皮膜部におけるCr濃度(at％)/Fe濃度(at％)は、不動態皮膜の耐孔食性改善に非常に重要な要因となる。基本的には、不動態皮膜部におけるCr濃度(at％)/Fe濃度(at％)と母相であるステンレス鋼のCr濃度(at％)/Fe濃度(at％)の比（以下、Cr/Fe濃度比と略す）が高いほど表層に耐孔食性に優れた安定な不動態皮膜が形成されていることになり、耐食性の観点からCr/Fe濃度比は高いほうが良い。検討したところ、大気暴露試験や促進腐食試験によって耐食性（耐孔食性）の向上効果が明瞭に現れるにはCr/Fe濃度比が1.2以上であることがわかった。この知見を基に、Cr/Fe濃度比は好ましくは1.2以上とする。より好ましくは1.5以上である。
Ratio of Cr concentration (at%) / Fe concentration (at%) in the passive film formed on the surface of stainless clad steel and Cr concentration (at%) / Fe concentration (at%) of the stainless steel as the parent phase Is 1.2 or more (preferred conditions)
The Cr concentration (at%) / Fe concentration (at%) in the passive film part is a very important factor for improving the pitting corrosion resistance of the passive film. Basically, the ratio of Cr concentration (at%) / Fe concentration (at%) in the passive film part to Cr concentration (at%) / Fe concentration (at%) of the stainless steel as the parent phase (hereinafter referred to as Cr) The higher the (/ Fe concentration ratio), the more a stable passive film with excellent pitting corrosion resistance is formed on the surface layer. From the viewpoint of corrosion resistance, the higher Cr / Fe concentration ratio is better. As a result of the examination, it was found that the Cr / Fe concentration ratio is 1.2 or more so that the effect of improving the corrosion resistance (pitting corrosion resistance) appears clearly by the atmospheric exposure test and the accelerated corrosion test. Based on this knowledge, the Cr / Fe concentration ratio is preferably 1.2 or more. More preferably, it is 1.5 or more.
一方で、Cr/Fe濃度比を大きく上げようとすると酸浸漬や酸洗、電解処理が必要となる。ステンレスクラッド鋼板は普通鋼とステンレス鋼の合わせ鋼板であるため、所定の溶液中に浸漬し処理する場合に普通鋼が溶解しないように配慮する必要があり、無垢材（ステンレス鋼）以上に表層のCr濃縮割合を改善するには負荷がかかる。このように、過度なCr/Fe濃度比向上には設備的な負荷がかかるため、Cr/Fe濃度比は100以下が好ましい。 On the other hand, in order to increase the Cr / Fe concentration ratio, acid dipping, pickling, and electrolytic treatment are required. Since stainless clad steel plate is a combination of plain steel and stainless steel, it is necessary to take care not to dissolve ordinary steel when it is immersed in a prescribed solution. It takes a load to improve the Cr concentration ratio. As described above, an excessive load on the Cr / Fe concentration ratio imposes an equipment load, so the Cr / Fe concentration ratio is preferably 100 or less.
なお、メカニカルな研磨のみではCr/Fe濃度比向上が十分でない場合もある。そのため、何らかの化学的な表面制御手法と組み合わせることが重要となる。 In some cases, mechanical polishing alone may not sufficiently improve the Cr / Fe concentration ratio. Therefore, it is important to combine with some chemical surface control technique.
また、本発明において、Cr/Fe濃度比は、例えば、深さ方向にスパッタしながら元素の濃度プロファイル(at％)を測定し、各元素（Fe、Crなど）濃度プロファイルからFeとCrの原子比、Cr/Fe濃度比を求めることができる。この場合、図1に示すように、Cr、Feの濃度プロファイルの値(at％)がほぼ一定値になった領域を母相と仮定し、それよりスパッタ時間の短い領域を不動態皮膜部と定義する。不動態皮膜部では最もCr/Feが高い値を示した部位での値を不動態皮膜部におけるCr濃度(at％)/Fe濃度(at％)とする。 In the present invention, the Cr / Fe concentration ratio is measured by, for example, measuring the element concentration profile (at%) while sputtering in the depth direction, and determining the Fe and Cr atoms from each element (Fe, Cr, etc.) concentration profile. Ratio and Cr / Fe concentration ratio can be obtained. In this case, as shown in FIG. 1, it is assumed that the region in which the Cr and Fe concentration profile values (at%) are almost constant is the matrix phase, and the region with a shorter sputtering time is the passive film portion. Define. In the passive film portion, the value at the highest Cr / Fe value is defined as Cr concentration (at%) / Fe concentration (at%) in the passive film portion.
なお、本発明のステンレスクラッド鋼の母材としては、炭素鋼や低合金鋼を用いることができる。そして、本発明のステンレスクラッド鋼は、この母材の片面または両面に合わせ材としてステンレス鋼がクラッドされたものであり、母材と合わせ材とをクラッドにする製造方法については特に限定しない。熱間圧延法、爆着圧延法、拡散接合法、鋳包み法などを用いることができる。 Carbon steel or low alloy steel can be used as the base material of the stainless clad steel of the present invention. The stainless clad steel of the present invention is obtained by clad stainless steel as a mating material on one or both sides of the base material, and the manufacturing method for clad the base material and the mating material is not particularly limited. A hot rolling method, an explosive rolling method, a diffusion bonding method, a cast-in method, and the like can be used.
また、700℃〜1000℃の温度で、1分から2時間保持の焼なまし処理を行うこともできる。ステンレスクラッド鋼の合わせ材に使用するステンレス鋼に含有されるCrやMo含有量が多い場合、例えばCr含有量18質量％以上でMoを2質量％以上含有するような高合金鋼の場合、σ（シグマ）相やΧ（カイ）相、さらにM23C6、M6C（MはFe、Crが主成分）などが生成し、有効なCrが低下し鋭敏化により著しい耐食性低下を引き起こすことがある。このような場合に、本発明の表面が制御されたステンレスクラッド鋼は有効であり、脱Cr層の除去、鋭敏化部の健全化に寄与することができる。 In addition, at a temperature of 700 ℃ ~1000 ℃, it is also possible to carry out the burn of better processing of 1 minute to 2 hour hold. When the content of Cr and Mo contained in the stainless steel used in the stainless steel clad steel is large, for example, in the case of a high alloy steel containing at least 18% by mass of Cr and 2% by mass of Mo, σ (Sigma) phase, cocoon phase, and M 23 C 6 , M 6 C (M is mainly composed of Fe and Cr) are generated, and effective Cr is decreased and sensitization causes a significant decrease in corrosion resistance. There is. In such a case, the stainless clad steel having a controlled surface according to the present invention is effective, and can contribute to the removal of the Cr removal layer and the soundness of the sensitized portion.
以下に、本発明を詳細に説明する。
表1に示す成分組成からなるオーステナイト系ステンレス鋼とSS400成分系の普通鋼（以下、普通鋼と略す）溶鋼を、転炉、電気炉、真空溶解炉等の公知の方法で溶製し、連続鋳造法あるいは造塊−分塊法により鋼素材（スラブ）とした。次いで、得られた鋼素材を、通常用いられる条件で、熱間圧延、熱延板焼鈍（例えば箱焼鈍）、酸洗と順次処理して熱延板とし、さらに、冷延、仕上げ焼鈍（例えば連続焼鈍）し、冷延焼鈍板とした。得られた冷延焼鈍板をクラッドの合わせ材（オーステナイト系ステンレス鋼）および母材（普通鋼）として、表2に示す製造条件でステンレスクラッド鋼を製造した。
The present invention is described in detail below.
Austenitic stainless steel composed of the components shown in Table 1 and SS400 component ordinary steel (hereinafter abbreviated as ordinary steel) are melted by a known method such as a converter, electric furnace, vacuum melting furnace, etc. A steel material (slab) was obtained by a casting method or an ingot-bundling method. Next, the obtained steel material is subjected to hot rolling, hot-rolled sheet annealing (for example, box annealing) and pickling in order under the conditions normally used to form a hot-rolled sheet, and further cold-rolled and finish-annealed (for example, Continuous annealing) to obtain a cold-rolled annealed sheet. Stainless steel clad steel was produced under the production conditions shown in Table 2 using the obtained cold-rolled annealed sheet as a clad laminated material (austenitic stainless steel) and a base material (ordinary steel).
すなわち、表１に示す合わせ材（オーステナイト系ステンレス鋼、板厚20ｍｍ）と母材（普通鋼：SS400相当、板厚73ｍｍ）を、幅1890ｍｍ、長さ2060ｍｍに組み立てスラブ寸法とし、スラブ加熱温度（℃）：1250℃、圧延終了温度（℃）：1000℃、水冷開始温度（℃）：950℃、水冷終了温度（℃）:650℃、冷却速度（℃/ｓ）：3.0℃/ｓの条件で、ステンレスクラッド鋼（合わせ材：板厚3ｍｍ、母材：板厚11ｍｍ、幅2500ｍｍ、長さ8000ｍｍ）を製造した。さらに、ステンレスクラッド鋼の一部に対して、表３に示すように、950℃、10分または2時間の焼ならし熱処理を行った。 That is, the laminated material shown in Table 1 (austenitic stainless steel, plate thickness 20 mm) and the base material (regular steel: equivalent to SS400, plate thickness 73 mm) are assembled into slab dimensions with a width of 1890 mm and a length of 2060 mm, and the slab heating temperature ( ° C): 1250 ° C, rolling end temperature (° C): 1000 ° C, water cooling start temperature (° C): 950 ° C, water cooling end temperature (° C): 650 ° C, cooling rate (° C / s): 3.0 ° C / s Thus, stainless clad steel (laminated material: plate thickness 3 mm, base material: plate thickness 11 mm, width 2500 mm, length 8000 mm) was produced. Further, as shown in Table 3, normalizing heat treatment was performed on a part of the stainless clad steel at 950 ° C. for 10 minutes or 2 hours.
上記により得られたステンレスクラッド鋼に対して、ステンレス鋼の表面を表４に示す粗さ形状になるようにベルト研磨を行なった。具体的には、長手方向に対し多パスのベルト研磨を行なった後、長手垂直方向に対し多パスのベルト研磨を行なった。長手方向研摩の際には、JIS R6256：2006で規定するところの研磨ベルトP60〜P400であるものを用い、長手垂直方向研摩の際には、研磨ベルトP120〜P800であるものを用いた。
次に、硝酸水溶液を1L／分で所定時間ステンレスクラッド鋼のステンレス鋼の表面に噴霧し、その後水洗し、不動態化処理を行った。この際、表４に示すような不動態皮膜になるように、噴霧時間は5秒〜10分、硝酸水溶液濃度は5〜35質量％の範囲で調整しサンプルを作製した。
The stainless clad steel obtained as described above was subjected to belt polishing so that the surface of the stainless steel had the roughness shown in Table 4. Specifically, after performing multi-pass belt polishing in the longitudinal direction, multi-pass belt polishing was performed in the longitudinal and vertical direction. In the case of the longitudinal direction polishing, the polishing belts P60 to P400 as defined in JIS R6256: 2006 were used, and in the case of the longitudinal vertical direction polishing, the polishing belts P120 to P800 were used.
Next, an aqueous nitric acid solution was sprayed on the surface of the stainless steel clad steel at a rate of 1 L / min for a predetermined time, then washed with water and subjected to a passivation treatment. At this time, samples were prepared by adjusting the spraying time in the range of 5 seconds to 10 minutes and the nitric acid aqueous solution concentration in the range of 5 to 35% by mass so as to obtain a passive film as shown in Table 4.
以上により得られたステンレスクラッド鋼に対して、平均粗さRzJIS、硫化物系介在物の数、Cr/Fe濃度比、CPT（臨界孔食発生温度）、CCT（臨界隙間腐食発生温度）を測定し、耐海水孔食性を評価した。以上より得られた結果を表４に示す。平均粗さRzJIS、硫化物系介在物の数、Cr/Fe濃度比、CPT（臨界孔食発生温度）、CCT（臨界隙間腐食発生温度）の測定方法は下記の通りである。 The average roughness Rz JIS , the number of sulfide inclusions, Cr / Fe concentration ratio, CPT (critical pitting corrosion temperature), and CCT (critical crevice corrosion temperature) are measured for the stainless clad steel obtained above. The seawater pitting resistance was evaluated. The results obtained from the above are shown in Table 4. The average roughness Rz JIS , the number of sulfide inclusions, Cr / Fe concentration ratio, CPT (critical pitting corrosion temperature), and CCT (critical crevice corrosion temperature) are measured as follows.
平均粗さRzJIS
JIS B 601-2001に準拠し、同一位置で圧延方向（L）と圧延方向に対して垂直方向（C）の2方向に対して測定長さ10ｍｍで表面粗さRzJISを測定した。これを5箇所で行い、圧延方向（L）と垂直方向（C）の各々の平均を求め、圧延方向（L）の平均粗さRzJIS（L）と垂直方向（C）の平均粗さRzJIS（C）を求めた。また、各測定箇所での圧延方向（L）と垂直方向（C）の平均粗さRzJISの比(L/C)を算出し、5箇所のL/Cの算術平均(平均L/C)を求めた。
Average roughness Rz JIS
In accordance with JIS B 601-2001, the surface roughness Rz JIS was measured at a measurement length of 10 mm in two directions of the rolling direction (L) and the direction perpendicular to the rolling direction (C) at the same position. This is done at five locations, the average of each of the rolling direction (L) and the vertical direction (C) is obtained, the average roughness Rz in the rolling direction (L) JIS (L) and the average roughness Rz in the vertical direction (C) JIS (C) was determined. In addition, the ratio (L / C) of the average roughness Rz JIS between the rolling direction (L) and the vertical direction (C) at each measurement point was calculated, and the arithmetic average (average L / C) of L / C at 5 points Asked.
硫化物系介在物の数
EPMAでステンレスクラッド鋼の表面100μm×100μmを分析し、S量のマッピングを利用して得られた像を画像処理することで、円相当径を算出し、1.0μmΦ以上の硫化物の個数を計測した。なお、測定面積100μm×100μmの視野を各20視野測定しその平均値として求めた。上記したようにサイズが1.0μｍΦ未満の硫化物は介在物が溶解しても孔食の進行が止まってしまう傾向にある（いわゆる再不動態化性ピット）ので、1.0μｍΦ以上の硫化物をカウントした。
Number of sulfide inclusions
EPMA is used to analyze the surface of stainless clad steel 100μm × 100μm, and by processing the image obtained by mapping S amount, calculate the equivalent circle diameter and measure the number of sulfides of 1.0μmΦ or more. did. In addition, 20 fields of view each having a measurement area of 100 μm × 100 μm were measured and obtained as an average value. As described above, sulfides with a size of less than 1.0 μmΦ tend to stop the progress of pitting corrosion even if inclusions dissolve (so-called repassivation pits), so sulfides with a size of 1.0 μmΦ or more were counted. .
Cr/Fe濃度比
AES（装置名：PHISICAL ELECTONICS社製 PHI MODEL 660、加速電圧 : 5kV、試料電流量 ：0.2μA、測定領域 ：5μｍ×5μｍ）を用いて深さ方向にスパッタしながら測定した各元素（Fe、Cr）プロファイルからその原子比を求めた。なお、図１に示すように、Cr、Feの値がほぼ一定値になった領域を母相とし、それよりスパッタ時間の短い領域を不動態皮膜部と定義した。不動態皮膜部では最もCr/Feが高い値を示した部位での値をCr/Fe濃度とし、母相のCr/Fe濃度と比較した。
Cr / Fe concentration ratio
Each element (Fe, Cr) measured by sputtering in the depth direction using AES (PHI MODEL 660 manufactured by PHISICAL ELECTONICS, acceleration voltage: 5 kV, sample current: 0.2 μA, measurement area: 5 μm × 5 μm) ) The atomic ratio was obtained from the profile. In addition, as shown in FIG. 1, the area | region where the value of Cr and Fe became the substantially constant value was made into the parent phase, and the area | region where sputtering time was shorter than that was defined as the passive film part. In the passive film part, the value at the highest Cr / Fe value was taken as the Cr / Fe concentration and compared with the Cr / Fe concentration of the matrix.
CPT（臨界孔食発生温度）、CCT（臨界隙間腐食発生温度）
耐海水孔食性の指標として、ＡＳＴＭに規定されるCPT試験（臨界孔食発生温度測定試験）およびCCT試験（臨界隙間腐食発生温度測定試験）により、臨界孔食発生温度および臨界隙間腐食発生温度を求めた。
耐海水孔食性の指標であるCPT試験（臨界孔食発生温度測定試験）は：ＡＳＴＭＧ48(method C)に準拠し、6質量％FeCl3 ＋ 1質量％HCl溶液中、５℃間隔で浸漬試験を行った。試験サイズは幅20mm×長さ50mm×板厚2.0ｍｍである。浸漬試験を3回行い、発生した孔食のうちの最大孔食深さが0.025ｍｍに達した場合は不合格とした。3回とも孔食が発生しなかった場合は合格とし、その場合の最高温度をCPT（℃）とした。なお、CPTの目標値は４０℃以上、好ましくは５０℃以上である。
CCT試験（臨界隙間腐食発生温度測定試験）も、上記CPT試験（臨界孔食発生温度測定試験）と同様の方法で行った。発生した孔食のうちの最大孔食深さが0.025ｍｍに達した場合は不合格とした。3回とも孔食が発生しなかった場合は合格とし、その場合の最高温度をCCT（℃）とした。なお、CCTの目標値は2０℃以上、好ましくは3０℃以上である。
CPT (critical pitting corrosion temperature), CCT (critical crevice corrosion temperature)
As an index of seawater pitting corrosion resistance, the critical pitting corrosion temperature and critical crevice corrosion temperature are determined by the CPT test (critical pitting corrosion temperature measurement test) and CCT test (critical crevice corrosion temperature measurement test) specified by ASTM. Asked.
The CPT test (critical pitting corrosion temperature measurement test), which is an index of seawater pitting corrosion resistance, is based on ASTMG48 (method C) and is immersed in a 6% FeCl 3 + 1% HCl solution at 5 ° C intervals. went. The test size is 20 mm wide x 50 mm long x 2.0 mm thick. The immersion test was performed three times, and when the maximum pitting depth of the generated pitting corrosion reached 0.025 mm, it was determined to be rejected. When pitting corrosion did not occur in all three times, it was accepted, and the maximum temperature in that case was defined as CPT (° C.). The target value of CPT is 40 ° C. or higher, preferably 50 ° C. or higher.
The CCT test (critical crevice corrosion occurrence temperature measurement test) was also performed in the same manner as the CPT test (critical pitting corrosion occurrence temperature measurement test). When the maximum pitting depth of the generated pitting corrosion reached 0.025 mm, it was determined as rejected. When pitting corrosion did not occur in all three times, it was accepted, and the maximum temperature in that case was defined as CCT (° C.). The target value of CCT is 20 ° C. or higher, preferably 30 ° C. or higher.
表4より、本発明例では、CPT、CCT共に目標の４０℃以上、20℃以上となっており、耐海水孔食性に優れたステンレスクラッド鋼が得られていることがわかる。
No.7は、熱延後に９５０℃、１０分の焼なまし熱処理を行った本発明例である。表面粗さの制御をすることで、No.4と同等な特性が得られていることがわかる。
No.8、9は、No.7に対して、さらに、硫化物系介在物の個数を低減させた本発明例であり、CPTがNo.7に比べ高くなっており、No.9では60℃に達している。No.10は、No.7に対して、さらに硫化物系介在物の個数を低減し、Cr/Fe濃度比を向上させた本発明例である。CPTが65℃、CCTが50℃と非常に高くなっている。No.11は、No.10に対し、さらにCr/Fe濃度比を高め、2以上とした本発明例である。CPTが70℃に到達している。
No.13は硫化物系介在物の個数を低減し、Cr/Fe濃度比を1.2以上にすることでCPTが65℃、CCT:45℃が得られている。
これらの試験結果から、表面の粗さに加え、硫化物系介在物の個数、Cr/Fe濃度比を制御することでより一層耐海水孔食性向上することがわかる。
From Table 4, it can be seen that in the present invention example, both CPT and CCT are the target of 40 ° C. or higher and 20 ° C. or higher, and a stainless clad steel excellent in seawater pitting corrosion resistance is obtained.
No.7 is, 950 ℃ after the hot rolling, is an example of the present invention that was baked a better heat treatment for 10 minutes. It can be seen that by controlling the surface roughness, the same characteristics as No. 4 are obtained.
Nos. 8 and 9 are examples of the present invention in which the number of sulfide inclusions is further reduced as compared to No. 7, and CPT is higher than No. 7, with No. 9 being 60 It has reached ℃. No. 10 is an example of the present invention in which the number of sulfide inclusions is further reduced and the Cr / Fe concentration ratio is improved compared to No. 7. CPT is 65 ℃ and CCT is very high at 50 ℃. No. 11 is an example of the present invention in which the Cr / Fe concentration ratio is further increased to 2 or more with respect to No. 10. CPT has reached 70 ℃.
No. 13 has a CPT of 65 ° C and a CCT of 45 ° C by reducing the number of sulfide inclusions and setting the Cr / Fe concentration ratio to 1.2 or higher.
These test results show that seawater pitting resistance is further improved by controlling the number of sulfide inclusions and the Cr / Fe concentration ratio in addition to the surface roughness.
一方、比較例では、耐海水孔食性に劣っている。
No.1は平均粗さRzJIS（L）およびRzJIS（C）が1.0を超えている比較例である。CPT、CCT共に目標の40℃、20℃に未達であり、耐海水孔食性に劣っている。
No.2は平均粗さRzJIS（L）が1.0μｍを超えており、かつ、2方向の平均粗さRzJISの比の平均(平均L/C)が2.0を超えている比較例である。CPT、CCT共に目標の40℃、20℃に未達であり、耐海水孔食性に劣っている。
No.3は2方向の平均粗さRzJISの比を平均した値(平均L/C)が2.0を超えている比較例である。CPT、CCT共に目標の40℃、20℃に未達であり、耐海水孔食性に劣っている。
On the other hand, the comparative example is inferior in seawater pitting resistance.
No. 1 is a comparative example in which the average roughness Rz JIS (L) and Rz JIS (C) exceed 1.0. Both CPT and CCT have not reached the target of 40 ° C and 20 ° C, and they have poor seawater pitting resistance.
No. 2 is a comparative example in which the average roughness Rz JIS (L) exceeds 1.0 μm and the average ratio (average L / C) of the average roughness Rz JIS in two directions exceeds 2.0 . Both CPT and CCT have not reached the target of 40 ° C and 20 ° C, and they have poor seawater pitting resistance.
No. 3 is a comparative example in which the ratio of the average roughness Rz JIS in two directions (average L / C) exceeds 2.0. Both CPT and CCT have not reached the target of 40 ° C and 20 ° C, and they have poor seawater pitting resistance.
Claims (2)
なお、孔食指数ＰＩとは、Ｃｒ（質量％）＋３Ｍｏ（質量％）＋１６Ｎ（質量％）で示される指数である。 Rolled (longitudinal) direction (measured in accordance with JIS B 0601-2001) on a stainless clad steel surface comprising stainless steel having a pitting corrosion index PI of 35.0 or more as a combination material ( L) average roughness Rz JIS (L) is 1.0 μm or less, average roughness Rz JIS (C) in the direction perpendicular to the rolling direction (C) is 1.0 μm or less, and the rolling at each measurement point ( Longitudinal) direction (L) average roughness Rz JIS (L) and average ratio R / L of the average roughness Rz JIS (C) in the direction perpendicular to the rolling direction (C) (L / C) resistance C is a range der of 0.5 to 2.0 is, the number of sulfide inclusions of the stainless clad steel surface, characterized in der Rukoto 2.5 × 10 5 cells / mm 2 or less Stainless clad steel with excellent seawater pitting resistance.
The pitting index PI is an index represented by Cr (mass%) + 3Mo (mass%) + 16N (mass%).
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