WO2011111646A1 - Ferritic stainless steel having excellent corrosion resistance in condensed water environment produced by exhaust gas from hydrocarbon combustion - Google Patents

Abstract

Disclosed is ferritic stainless steel which has excellent corrosion resistance in a condensed water environment produced by an exhaust gas from hydrocarbon combustion, said condensed water environment being typified by the use environment of a member for a secondary heat exchanger. Specifically disclosed is ferritic stainless steel which contains, in mass%, 0.030% or less of C, 0.030% or less of N, 0.4% or less of Si, 0.01-0.5% of Mn, 0.05% or less of P, 0.01% or less of S, 16-24% of Cr, 0.3-3% of Mo, 0.05-0.25% of Ti, 0.05-0.50% of Nb, 0.01-0.2% of Al and 0.4% or less of Cu, with the balance made up of Fe and unavoidable impurities. The ferritic stainless steel satisfies formula (A): Cr + Mo + 10Ti ≥ 18 and formula (B): Si + Cu ≤ 0.5.

Description

Ferritic stainless steel with excellent corrosion resistance in condensed water environment generated from hydrocarbon combustion exhaust gas

The present invention relates to a heat exchanger, a hot water heater using LPG or petroleum as a fuel, and a ferritic stainless steel having excellent corrosion resistance used for its members in a secondary heat exchanger that generates low pH condensed water. It is about steel.

BACKGROUND ART A heat exchanger is a device that gives heat generated by burning various fuels to a medium centered on water, and is used in various fields from a steam generator for nuclear power generation to a hot water heater in general households. Of these, heat exchangers are incorporated in ordinary household gas or oil water heaters in order to convert the combustion heat into hot water. Conventionally, this heat exchanger has been made of copper, which can be easily processed into a fin structure or the like and has excellent thermal conductivity in order to increase thermal efficiency. However, CO ₂ reduction is also required for hot water heaters due to environmental problems in recent years, and a latent heat recovery type hot water heater that further utilizes the heat of conventional exhaust gas has been developed for the purpose of further improving thermal efficiency. This water heater uses another heat exchanger (secondary heat exchanger) to further use the heat of the exhaust gas that burned gas and oil after passing through the conventional heat exchanger (primary heat exchanger). Have. Since the exhaust gas after passing through the primary heat exchanger is about 150 to 200 ° C. and contains a large amount of water vapor, in the secondary heat exchanger, not only direct heat but also heat of condensation in which water vapor becomes water droplets. In other words, the total heat efficiency is improved to 95% or more by collecting latent heat. An example of the structure of this latent heat recovery type water heater is disclosed in Patent Document 1, for example.
Here, the condensed water generated in the secondary heat exchanger is generated from exhaust gas in which hydrocarbon-based raw materials such as city gas, LPG, and petroleum are burned. It is known that a weakly acidic aqueous solution containing sulfuric acid ions and pH of about 3 or less is obtained. In this low pH solution, conventionally used copper (corrosion at pH 6.5 or less) cannot be used. Other ordinary steels (corrosion at pH below 7) and aluminum (corrosion at pH about 3) may corrode in this environment. Therefore, stainless steel is currently selected as a material for secondary heat exchangers, which is excellent in corrosion resistance in a weakly acidic region. Among the general-purpose stainless steels, austenitic stainless steel with superior corrosion resistance is important to emphasize corrosion resistance. SUS316L (18Cr-10Ni-2Mo) is mainly adopted. However, although SUS316L satisfies the corrosion resistance necessary for the secondary heat exchanger member applied to the latent heat recovery type water heater, the raw material contains a large amount of Ni and Mo, which are very unstable in price stability. Contains. The latent heat recovery type water heater is expected to be widely spread to the general public as a trump card for CO ₂ reduction, and further cost reduction is strongly demanded in order to realize this. Naturally, proposals for alternative materials with lower costs are also expected in SUS316L, which is a secondary heat exchanger material. Also, although there is no problem of corrosion resistance in a general usage environment, SUS316L can also corrode in areas such as the coast where sea salt particles are likely to fly, which is one of the factors that hinder the corrosion resistance of stainless steel. Sex cannot be denied. In this case, in SUS316L, stress corrosion cracking, which is one of the weak points of austenitic stainless steel, may occur.
In order to solve such problems that occur when applying austenitic stainless steel, in recent years, attempts have been made to apply ferritic stainless steel to secondary heat exchanger members (Patent Documents 1 to 3). .

JP 2002-106970 A JP 2003-328088 A JP 2009-299182 A

Patent Document 1 uses ferritic stainless steels SUS436J1L, SUS436L, and SUS444 in a heat exchanger for recovering latent heat, thereby having excellent heat conductivity, corrosion resistance, brazing property, and relatively inexpensive pipes and fins. It is said that a heat exchanger for latent heat recovery can be obtained.
Patent Document 2 states that as a ferritic stainless steel exhibiting durability in a high-temperature steam environment in a heat exchanger environment, the Cr, Mo, Si, and Al contents are added in relation to the intended use temperature. It is a proposal. Moreover, patent document 3 has prescribed | regulated Nb, C, and N as a ferritic stainless steel suitable as a heat exchanger member used for brazing.
However, in Patent Document 1, the average corrosion depth was used as an index of corrosion resistance. However, in stainless steel originally excellent in corrosion resistance, pitting corrosion is mainly generated, and even in some cases, pitting corrosion is caused. If it penetrates, it cannot be used as a material. In this respect, the conditions disclosed in Patent Document 1 still require improvement, and according to the study by the present inventors, among the ferritic stainless steels described in Patent Document 1, Some were inferior in corrosion resistance when used in a heat exchanger for latent heat recovery. Patent Document 2 has a problem that the material becomes very hard and brittle because the amount of Al added is large, and the temperature assumed in Patent Document 2 is 300 to 1000 ° C., which is much higher than the latent heat recovery water heater of this time The materials used in a high temperature environment are specified. Patent Document 3 uses Nb as an essential element for the purpose of preventing crystal grain coarsening during heat treatment during brazing, but there is no indication of improvement in corrosion resistance.
Thus, conventionally, it could not be said that ferritic stainless steel suitable as a secondary heat exchanger member has been sufficiently disclosed. In view of such circumstances, an object of the present invention is to provide a ferritic stainless steel that is inexpensive and excellent in corrosion resistance and can be suitably used as a member for a secondary heat exchanger.

In order to solve the above problems, the present inventors have evaluated the corrosion resistance of various stainless steels in such an environment. As a result, the Cr and Ti contents are large, particularly when they are concentrated on the surface of the passive film. In particular, it was clarified that it has excellent corrosion resistance. Moreover, from the evaluation of the generated corrosion starting point, it was found that the corrosion resistance in this environment can be improved by reducing Cu and Si. The present invention has developed a ferritic stainless steel having excellent corrosion resistance in the secondary heat exchanger environment as a result of diligent investigation of the corrosive environment in the secondary heat exchanger.
That is, the present invention is a ferritic stainless steel excellent in corrosion resistance in a condensed water environment generated from hydrocarbon combustion exhaust gas having the following characteristics.
(1) By mass%, C: 0.030% or less, N: 0.030% or less, Si: 0.4% or less, Mn: 0.01 to 0.5%, P: 0.05% or less, S: 0.01% or less, Cr: 16-24%, Mo: 0.30-3%, Ti: 0.05-0.25%, Nb: 0.05-0.50%, Al: 0. 01 to 0.20%, Cu: 0.4% or less, the balance is composed of Fe and inevitable impurities, and (A) formula: Cr + Mo + 10Ti ≧ 18, and (B) formula: Si + Cu ≦ 0.5 High corrosion resistance ferritic stainless steel with excellent corrosion resistance in condensed water environment generated from hydrocarbon combustion exhaust gas, characterized by satisfying.
However, Cr, Mo, Ti, Si, and Cu in the formula mean the content (% by mass) of each element.
(2) Further, by mass, Ni: 0.3 to 3%, B: 0.0001 to 0.003%, V: 0.03 to 1.0%, Sn: 0.005 to 1.0% Sb: One or more of 0.005 to 1.0% is contained, and high corrosion resistance excellent in corrosion resistance in a condensed water environment generated from hydrocarbon combustion exhaust gas according to (1) Ferritic stainless steel.
(3) The test piece was semi-immersed in an aqueous solution containing the steel described in (1) or (2) having a pH of 2.5 and containing 100 ppm nitrate ions, 10 ppm sulfate ions, and 10 ppm chloride ions, and 80 ° C. The maximum corrosion depth after 14 cycles of the wet and dry repeated test held for 24 hours is 50 μm or less, and the condensed water environment generated from the hydrocarbon combustion exhaust gas according to (1) or (2) High corrosion resistance ferritic stainless steel with excellent corrosion resistance at low temperatures.

According to the present invention, it is possible to provide a ferritic stainless steel excellent in corrosion resistance in a secondary heat exchanger environment, not an austenitic stainless steel to which a large amount of expensive Ni or Mo is added. Moreover, it becomes possible to exhibit excellent corrosion resistance not only as a water heater but also as a device material used in a condensed water environment of combustion gas using LNG, petroleum or other hydrocarbon as fuel.

Fig.1 (a) is the figure which showed the shape of the sample used for the test.
FIG.1 (b) is the figure which showed the shape of the sample used for the test.
FIG. 2 is a diagram showing the relationship between the maximum corrosion depth after the test and the constituent elements.
FIG. 3 is a diagram showing the relationship between the maximum corrosion depth after the test and the constituent elements.
FIG. 4 is a diagram showing the results of Examples and Comparative Examples in relation to the maximum corrosion depth after the test and the component elements.

　そこで、Ｃｒ＋Ｍｏ＋１０Ｔｉ及びＳｉ＋Ｃｕという２つのパラメータによって、腐食深さがどのような影響を受けるのかについて評価を行った。その結果を表１、図２、図３に示す。表１、図２および図３に示すように、Ｃｒ＋Ｍｏ＋１０Ｔｉが１８以上でかつＳｉ＋Ｃｕ≦０．５を満たすフェライト系ステンレス鋼では最大腐食深さは５０μｍ以下を示した。なお、Ｃｒ＋Ｍｏ＋１０Ｔｉが１８以上の値を示しても、Ｓｉ＋Ｃｕ≦０．５を満たさない場合には、最大腐食深さは５０μｍを超える結果となった。一方、オーステナイト系ステンレス鋼ではＣｒ＋Ｍｏ＋１０Ｔｉが１８以上の値を示しても、汎用鋼ではＳｉ＋Ｃｕ≦０．５を満たさないため最大腐食深さが５０μｍを超える結果となった。このように、低ｐＨで硝酸イオンと硫酸イオンが所定の比率以上で存在する溶液中で乾湿繰り返し環境となる場合には、Ｃｒ＋Ｍｏ＋１０Ｔｉ≧１８で、かつＳｉ＋Ｃｕ≦０．５を満たすフェライト系ステンレス鋼が優れた耐食性を有することを明らかとなった。ここで、式中のＣｒ、Ｍｏ、Ｔｉ、Ｓｉ、Ｃｕは、それぞれの元素の含有量（質量％）を意味する。
　このＣｒ＋Ｍｏ＋１０Ｔｉが１８以上で本試験条件における最大腐食深さが小さくなる理由は以下のように考えている。
　本試験で腐食深さが５０μｍ以下のサンプルの試験後の不働態皮膜をＡＥＳで分析すると表面皮膜にはＣｒの他にＴｉが濃縮していることが確認された。また本乾湿繰り返し試験は、硝酸イオンと硫酸イオンを含む低ｐＨ試験用溶液の濃縮・乾燥過程による孔食発生・再不働態化の繰り返しの腐食環境であることから、本試験での腐食機構は孔食発生が支配的な腐食機構であり、ＣｒとＴｉの不働態皮膜への濃縮が有効であったと考えられる。
　Ｍｏも一般的な耐孔食性指標として知られる耐孔食指数ＰＩ値＝Ｃｒ＋３．３Ｍｏ＋１６Ｎでも示されるように、孔食が発生する初期段階で再不働態化に寄与することが知られ、本試験環境においてもその効果が示されたが、その寄与率はＰＩ値のそれよりも小さいことが明らかとなった。この二つの機構により、本試験環境における最大腐食深さが結果的にＣｒ＋Ｍｏ＋１０Ｔｉで示される指標で表されたと考えられる。
　一方、Ｓｉ＋Ｃｕが０．５以下で最大腐食深さが小さくなる理由は、以下のように考えている。Ｃｕは、通常、活性溶解速度を低下させて耐食性を高める元素であるが、いったん腐食が生じた場合には鋼中のＣｕが溶出し、特に本試験環境のような酸化剤となる硝酸イオンが多い環境においては、溶出したＣｕイオンがＣｕ^２＋の酸化剤となってカソード反応を促進することで腐食速度を増大せしめ、腐食深さが大きくなる等が原因と推定される。
　Ｓｉの効果は、上記の試験液中において乾湿繰り返し試験を実施した場合、Ｓｉを含有した供試材は、気液界面を中心にＳｉ酸化物の析出が確認され、その近傍で腐食が生じていることが確認された。これは、析出物と供試材間で生じたすき間ですき間腐食が生じ、腐食を促進させたと考えられ、さらにこのとき環境にＣｕ^２＋が存在したために、より腐食が加速されると推定している。なお、オーステナイト系ステンレス鋼でも、Ｃｒ＋Ｍｏ＋１０Ｔｉが１８以上でも腐食深さは５０μｍを超える結果となったが、これは汎用オーステナイト系ステンレス鋼では、その製鋼条件のため、ＳｉやＣｕが必然的に高くなっており、Ｓｉ＋Ｃｕが０．５以下とならないものが殆どであるためと考えられる。
　さらにオーステナイト系ステンレス鋼はＭｎＳ等の水溶性介在物がフェライト系ステンレス鋼よりも多く、これが本試験用溶液中での溶解速度が大きいことも一因と推定される。
　なお、Ｃｒ＋Ｍｏ＋１０Ｔｉは、より望ましくは２０以上であり、更に望ましくは２２以上である。またＳｉ＋Ｃｕは、より望ましくは０．３未満であり、更に望ましくは０．２未満である。
　本発明のステンレス鋼の組成成分の詳細な規定について、以下に説明する。
　Ｃｒは、ステンレス鋼の耐食性を確保する上で最も重要な元素であり、フェライト組織を安定化させるために少なくとも１６％は必要である。Ｃｒを増加させると耐食性も向上するが、加工性、製造性を低下させるため、上限を２４％とする。望ましくは１８．５~２３％であり、より望ましくは１９．０~２２．０％である。
　Ｔｉは、一般にはフェライト系ステンレス鋼の溶接部においてＣ，Ｎを固定することで粒界腐食を抑制し、加工性を向上させる非常に重要な元素である。更に本腐食環境においては、耐食性上、重要な元素である。Ｔｉは酸素との親和力が非常に強いが、これが硝酸イオンを含む本腐食環境でＣｒとともにステンレス鋼の表面被膜を形成し、孔食発生を抑制するのに非常に有効であることを知見したものである。皮膜形成や、安定化元素としてのＣ、Ｎの固定には（Ｃ＋Ｎ）の４倍以上が必要である。しかしながら過剰な添加は製造時の表面疵の原因となるため、その範囲を０．０５~０．２５％とする。より望ましくは、０．０８~０．２％とする。
　Ｍｏは、不働態皮膜の補修に効果があり，耐食性を向上させるのに非常に有効な元素で、特にＣｒとの組み合わせで耐孔食性を向上させる効果がある。そのためＭｏは少なくとも０．３０％含有させることが必要である。Ｍｏを増加させると耐食性は向上するが、加工性を低下させ、またコストが高くなるため上限を３％とする。より望ましくは、０．５０~２．００％である。
　Ｃｕは、スクラップを原料として用いた場合に不可避不純物として０．０１％以上含まれ得る。ただし、本環境においては、Ｃｕは腐食を促進させるため望ましくない。その理由は前述のように、一度腐食が開始した場合、溶出したＣｕイオンがカソード反応を促進させると推定されるためである。そのためＣｕは少ないほど望ましく、その範囲を０．４％以下とする。より望ましくは、０．１０％以下である。
　Ｓｉは、原料から不可避的に混入する元素であり、一般的に耐食性、耐酸化性にも有効であるが、本環境においては腐食の進行を促進する作用があるだけでなく、また、過度な添加は加工性，製造性を低下させる。そのため上限を０．４％とする。より望ましくは０．２％未満である。また、極度に低減させることはコストの増加を招くため、通常は不可避的に０．０５％程度以上含有している。
　さらに本発明のステンレス鋼で規定される他の化学組成成分について、以下に詳しく説明する。
　Ｃは、強度向上や安定化元素との組合せによる結晶粒粗大化抑制等の効果があるが、溶接部の耐粒界腐食性、加工性を低下させる。高純度系フェライト系ステンレス鋼ではその含有量を低減させる必要があるため、上限を０．０３０％とした。過度に低減させることは精錬コストを悪化させるため、より望ましくは、０．００２~０．０２０％である。
　Ｎは、Ｃと同様に耐粒界腐食性、加工性を低下させるため、その含有量を低減させる必要があることから、その上限を０．０３０％とする。ただし過度に低減させることは精錬コストを悪化させるため、より望ましくは、０．００２~０．０２０％である。
　Ｍｎは、脱酸元素として重要な元素であるが、過剰に添加すると腐食の起点となるＭｎＳを生成しやすくなり、またフェライト組織を不安定化させるため、その含有量を０．０１~０．５％とする。より望ましくは、０．０５~０．３％である。
　Ｐは、溶接性、加工性を低下させるだけでなく、粒界腐食を生じやすくもするため、低く抑える必要がある。そのため含有量を０．０５％以下とする。より望ましくは０．００１~０．０４％である。
　Ｓは、先述のＣａＳやＭｎＳ等の腐食の起点となる水溶性介在物を生成させるため、低減させる必要がある。そのため含有率は０．０１％以下とする。ただし過度の低減はコストの悪化を招くため、より望ましくは０．０００１~０．００６％である。
　Ａｌは、脱酸元素として重要であり、また、非金属介在物の組成を制御し組織を微細化する効果もある。しかし過剰に添加すると非金属介在物の粗大化を招き、製品の疵発生の起点になる恐れもある。そのため、下限値を０．０１％、上限値を０．２０％とする。より望ましくは０．０３％~０．１０％である。
　Ｎｂは、Ｔｉと同様にＣ、Ｎを固定し、溶接部の粒界腐食を抑制し加工性を向上させる上で非常に重要な元素である。そのためにはＮｂをＣとＮの和（Ｃ＋Ｎ）の８倍以上添加することが必要である。ただし、過剰な添加は、加工性を低下させるため、その範囲を０．０５~０．５０％とする。より望ましくは０．１~０．３％である。
　本発明のステンレス鋼は、上記の組成成分のほかにＮｉ、Ｂ、Ｖ、ＳｎおよびＳｂの１種または２種以上を必要に応じて添加することができる。
　Ｎｉは、活性溶解速度を抑制し、かつ不働態化に非常に効果があるため、必要に応じて０．３％以上添加する。ただし、過剰な添加は、加工性を低下させ、フェライト組織を不安定にするだけでなくコストも悪化するため、上限を３％とする。望ましくは０．８~１．５０％である。
　Ｂは、二次加工脆性改善に有効な粒界強化元素であるため、必要に応じて添加することができる。しかし、過度の添加はフェライトを固溶強化して延性低下の原因になる。このため下限を０．０００１％、上限を０．００３％とする。より望ましくは０．０００２~０．００２０％である。
　Ｖは、耐銹性や耐すき間腐食性を改善し、Ｃｒ、Ｍｏの使用を抑えてＶを添加すれば優れた加工性も担保することができるため、必要に応じて添加することができる。ただしＶの過度の添加は加工性を低下させる上、耐食性向上効果も飽和するため、Ｖの下限を０．０３％、上限を１．０％とする。より望ましくは０．０５~０．５０％である。
　Ｓｎ、Ｓｂも、耐流れさび性を確保するために必要に応じて添加することができる。これらは腐食速度を抑制するのに重要な元素であるが、過剰な添加は製造性及びコストを悪化させるため、その範囲をいずれも０．００５~１．０％とする。より望ましくは０．０５~０．５％である。
　なお、本発明の鋼材は、熱交換器として使用しうる鋼であり、鋼板、型鋼、棒、線材、管などの形態とすることができるが、主として鋼板として製造される。上記（１）または（２）に記載の組成を有する鋼を、通常の溶製方法、例えば転炉、電気炉などを用い、必要に応じて真空精錬などの二次精錬を行って、溶製し、連続鋳造により鋼片とするか又はインゴットに鋳造した後、圧延して鋼片とする。溶製、鋳造については、通常のフェライト系ステンレス鋼の溶製、鋳造に準じて行うことができる。この鋼片を加熱後、熱間圧延して所要の形状の鋼材とする。熱間圧延に関する条件も特に限定されるものではなく、通常のフェライト系ステンレス鋼の熱間圧延の加熱、圧延条件に準じて行えばよい。なお、鋼板の場合、熱間圧延した鋼板を、さらに必要に応じて、酸洗、焼鈍を施した後、冷間圧延して冷間圧延鋼板とし、さらに焼鈍、酸洗等を施して所要の冷間圧延鋼板とすることができる。 As a result of earnest development, the inventors have provided ferritic stainless steel that exhibits excellent corrosion resistance as a secondary heat exchanger material of a latent heat recovery type water heater using hydrocarbon fuels such as LNG and petroleum. The following was found.
(I) Corrosion depth in repeated wet and dry tests in condensed water generated from combustion exhaust gas is stainless steel satisfying the formula (A): Cr + Mo + 10Ti ≧ 18 and (B) formula: Si + Cu ≦ 0.5 among ferritic stainless steels. Steel showed 50 μm or less.
(Ii) In the said environment, the maximum corrosion depth of the ferritic stainless steel was smaller than that of the austenitic stainless steel showing the same value in the formula (A).
First, a test method that simulates the environment will be described.
As described above, condensed water generated from general LNG or petroleum combustion exhaust gas exhibits acidity of pH = 3 or less containing nitrate ions and sulfate ions. In addition, the secondary heat exchanger becomes an environment in which exhaust gas of 150 to 200 ° C. is sent from the primary heat exchanger during use and returns to room temperature when stopped. For this reason, as a simulation test, a test solution was prepared from the reagents at pH = 2.5, nitrate ions 100 ppm, sulfate ions 20 ppm, and Cl ⁻ ions = 10 ppm. These compositions, which is obtained by simulating a condensed water generated from the LNG in the combustion exhaust gas, Cl ^- for the ions, a place where there is a fact several ppm, the operation at high beach environment and corrosive environments Assuming the situation, the concentration was set high at an accelerated rate. 1 ml of this test solution 3 is filled in a test tube 1 as shown in FIG. 1A, cut into 1 × 15 × 100 mm, and wet-polished with # 600 emery paper on the entire surface. Was soaked in half so that almost half of the longitudinal direction was soaked in the solution (see FIG. 1B). The test tube containing this sample is placed in a warm bath at 80 ° C. and held for 24 hours, and then a completely dried stainless steel sample is taken out, washed lightly with distilled water, and the test solution is again added to the newly washed test tube as described above. The sample was semi-immersed again and the sample was held at 80 ° C. for 24 hours and repeated 14 times (14 cycles). The reason why the temperature to be maintained is set to 80 ° C. is that the temperature of the exhaust gas is 150 to 200 ° C., but the temperature is lowered by the generation of condensed water, and the actual member temperature is in contact with the generated condensed water. This is because the temperature is lower than 100 ° C. and a relatively high temperature is aimed at accelerating corrosion.
The specimen after 14 cycles was rusted and its corrosion depth was measured by a focusing method using a 200 × microscope. About the pitting corrosion hole produced here, the depth of five points from the thing with a large hole diameter was measured, and the maximum value was made into the maximum corrosion depth. This has the same meaning as the maximum pitting depth. In addition, 12 steel types shown in Table 1 were used for the test material. As a result of this test, when the maximum corrosion depth exceeded 50 μm, it was determined that it reached the perforation in the long term, and in this case, it was determined that there was no corrosion resistance, and the case where it was 50 μm or less was corrosion resistance.
When ferritic stainless steel containing Cr was used as a base and the amount of Cr was increased or Mo or Ti was added, it was found that the corrosion depth was improved in any case. When the effect of increasing the Cr unit content is 1, the effect of increasing the Mo unit content is the same as that of Cr, and the effect of increasing the Ti unit content is about 10 times that of Cr. I found out that It was also found that both Si and Cu worsen the corrosion depth in ferritic stainless steel, and the contribution ratios of Si and Cu were almost equal.

Thus, an evaluation was made as to how the corrosion depth is affected by the two parameters Cr + Mo + 10Ti and Si + Cu. The results are shown in Table 1, FIG. 2 and FIG. As shown in Table 1, FIG. 2 and FIG. 3, the maximum corrosion depth is 50 μm or less in the ferritic stainless steel in which Cr + Mo + 10Ti is 18 or more and Si + Cu ≦ 0.5. Even when Cr + Mo + 10Ti shows a value of 18 or more, the maximum corrosion depth exceeded 50 μm when Si + Cu ≦ 0.5 was not satisfied. On the other hand, even if Cr + Mo + 10Ti shows a value of 18 or more in the austenitic stainless steel, the general corrosion steel does not satisfy Si + Cu ≦ 0.5, and thus the maximum corrosion depth exceeds 50 μm. Thus, when it becomes a wet and dry repeated environment in a solution in which nitrate ions and sulfate ions exist at a predetermined ratio or more at low pH, a ferritic stainless steel satisfying Cr + Mo + 10Ti ≧ 18 and Si + Cu ≦ 0.5 is obtained. It was revealed that it has excellent corrosion resistance. Here, Cr, Mo, Ti, Si, and Cu in the formula mean the content (% by mass) of each element.
The reason why the maximum corrosion depth in this test condition is small when Cr + Mo + 10Ti is 18 or more is considered as follows.
In this test, the passive film after the test of the sample having a corrosion depth of 50 μm or less was analyzed by AES, and it was confirmed that Ti in addition to Cr was concentrated on the surface film. In addition, this dry / wet repeated test is a corrosive environment in which pitting corrosion generation and repassivation occur repeatedly during the concentration / drying process of a low pH test solution containing nitrate ions and sulfate ions. It is considered that the generation of corrosion is the dominant corrosion mechanism, and the concentration of Cr and Ti to the passive film was effective.
It is known that Mo also contributes to repassivation at the initial stage where pitting corrosion occurs, as shown by pitting corrosion index PI value = Cr + 3.3Mo + 16N, which is also known as a general pitting corrosion resistance index. The effect was also shown in Fig. 1, but the contribution rate was found to be smaller than that of the PI value. With these two mechanisms, it is considered that the maximum corrosion depth in the test environment was expressed as an index represented by Cr + Mo + 10Ti as a result.
On the other hand, the reason why the maximum corrosion depth is small when Si + Cu is 0.5 or less is considered as follows. Cu is an element that usually increases the corrosion resistance by decreasing the active dissolution rate. However, once corrosion occurs, Cu in the steel elutes, and nitrate ions that become an oxidizing agent particularly in the test environment are present. In many environments, it is presumed that the eluted Cu ions become an oxidant of Cu ²⁺ to accelerate the cathode reaction, thereby increasing the corrosion rate and increasing the corrosion depth.
As for the effect of Si, when the wet and dry repeated test was carried out in the above test solution, the Si-containing test material was confirmed to precipitate Si oxide mainly at the gas-liquid interface, and corrosion occurred in the vicinity thereof. It was confirmed that This is presumed that crevice corrosion occurred between the precipitate and the specimen, which promoted the corrosion, and further, since Cu ²⁺ was present in the environment at this time, the corrosion was estimated to be accelerated. Yes. Even in the austenitic stainless steel, even when Cr + Mo + 10Ti is 18 or more, the corrosion depth exceeded 50 μm. However, in general austenitic stainless steel, Si and Cu are inevitably high due to the steelmaking conditions. This is probably because Si + Cu is almost 0.5 or less.
Furthermore, austenitic stainless steel has more water-soluble inclusions such as MnS than ferritic stainless steel, which is presumed to be due to the high dissolution rate in the test solution.
Note that Cr + Mo + 10Ti is more preferably 20 or more, and even more preferably 22 or more. Si + Cu is more desirably less than 0.3, and even more desirably less than 0.2.
The detailed definition of the composition components of the stainless steel of the present invention will be described below.
Cr is the most important element for ensuring the corrosion resistance of stainless steel, and at least 16% is necessary to stabilize the ferrite structure. Increasing Cr improves corrosion resistance, but lowers workability and manufacturability, so the upper limit is made 24%. It is desirably 18.5 to 23%, and more desirably 19.0 to 22.0%.
Generally, Ti is a very important element that suppresses intergranular corrosion and improves workability by fixing C and N in a welded portion of ferritic stainless steel. Furthermore, in this corrosive environment, it is an important element for corrosion resistance. Ti has a very strong affinity for oxygen, but it has been found that this forms a surface coating of stainless steel with Cr in this corrosive environment containing nitrate ions, and is extremely effective in suppressing pitting corrosion. It is. For film formation and fixation of C and N as stabilizing elements, 4 times or more of (C + N) is required. However, excessive addition causes surface flaws during production, so the range is made 0.05 to 0.25%. More preferably, the content is 0.08 to 0.2%.
Mo is effective in repairing the passive film, and is an extremely effective element for improving the corrosion resistance. In particular, Mo is effective in improving the pitting corrosion resistance in combination with Cr. Therefore, it is necessary to contain Mo at least 0.30%. Increasing Mo improves corrosion resistance, but lowers workability and increases cost, so the upper limit is made 3%. More desirably, it is 0.50 to 2.00%.
Cu can be contained in an amount of 0.01% or more as an inevitable impurity when scrap is used as a raw material. However, in this environment, Cu is undesirable because it promotes corrosion. This is because, as described above, once corrosion starts, it is estimated that the eluted Cu ions promote the cathode reaction. Therefore, the smaller the amount of Cu, the better, and the range is made 0.4% or less. More desirably, it is 0.10% or less.
Si is an element inevitably mixed from the raw material, and is generally effective in corrosion resistance and oxidation resistance, but not only has an effect of promoting the progress of corrosion in this environment, but also is excessive. Addition reduces processability and manufacturability. Therefore, the upper limit is made 0.4%. More desirably, it is less than 0.2%. Moreover, since extremely reducing causes an increase in cost, it is usually inevitable that the content is about 0.05% or more.
Further, other chemical composition components defined in the stainless steel of the present invention will be described in detail below.
C has effects such as improvement in strength and suppression of grain coarsening by a combination with a stabilizing element, but reduces intergranular corrosion resistance and workability of the weld. In high-purity ferritic stainless steel, the content must be reduced, so the upper limit was made 0.030%. Since excessive reduction deteriorates the refining cost, it is more preferably 0.002 to 0.020%.
N, like C, lowers the intergranular corrosion resistance and workability, so its content needs to be reduced, so its upper limit is made 0.030%. However, excessive reduction deteriorates the refining cost, so 0.002 to 0.020% is more desirable.
Mn is an important element as a deoxidizing element. However, if added excessively, MnS, which becomes a starting point of corrosion, is easily generated and the ferrite structure is destabilized. 5%. More desirably, it is 0.05 to 0.3%.
P not only deteriorates weldability and workability, but also easily causes intergranular corrosion, so P needs to be kept low. Therefore, the content is made 0.05% or less. More desirably, it is 0.001 to 0.04%.
Since S produces water-soluble inclusions that serve as starting points for corrosion such as CaS and MnS, it is necessary to reduce S. Therefore, the content is made 0.01% or less. However, excessive reduction leads to cost deterioration, so 0.0001 to 0.006% is more desirable.
Al is important as a deoxidizing element, and also has an effect of controlling the composition of non-metallic inclusions and refining the structure. However, if it is added excessively, the non-metallic inclusions become coarse, which may be the starting point for product wrinkles. Therefore, the lower limit is set to 0.01% and the upper limit is set to 0.20%. More desirably, it is 0.03% to 0.10%.
Nb is an extremely important element for fixing C and N as well as Ti, suppressing intergranular corrosion of the welded portion, and improving workability. For that purpose, it is necessary to add Nb at least 8 times the sum of C and N (C + N). However, excessive addition reduces workability, so the range is made 0.05 to 0.50%. More desirably, it is 0.1 to 0.3%.
The stainless steel of this invention can add 1 type, or 2 or more types of Ni, B, V, Sn, and Sb other than said composition component as needed.
Ni suppresses the active dissolution rate and is very effective in passivating, so 0.3% or more is added as necessary. However, excessive addition reduces workability and not only makes the ferrite structure unstable but also worsens the cost, so the upper limit is made 3%. Desirably, it is 0.8 to 1.50%.
B is a grain boundary strengthening element effective for improving the secondary work brittleness, and therefore can be added as necessary. However, excessive addition causes the solid solution strengthening of ferrite and causes a decrease in ductility. For this reason, the lower limit is made 0.0001% and the upper limit is made 0.003%. More desirably, it is 0.0002 to 0.0020%.
V improves the weather resistance and crevice corrosion resistance, and can suppress the use of Cr and Mo, and if V is added to ensure excellent workability, it can be added as necessary. However, excessive addition of V reduces workability and also saturates the effect of improving corrosion resistance, so the lower limit of V is 0.03% and the upper limit is 1.0%. More desirably, it is 0.05 to 0.50%.
Sn and Sb can also be added as needed to ensure flow rust resistance. These are important elements for suppressing the corrosion rate, but excessive addition deteriorates manufacturability and cost, so the range is 0.005 to 1.0% in all cases. More desirably, it is 0.05 to 0.5%.
The steel material of the present invention is steel that can be used as a heat exchanger, and can be in the form of a steel plate, die steel, bar, wire, tube, etc., but is mainly manufactured as a steel plate. Steel having the composition described in (1) or (2) above is subjected to secondary refining such as vacuum refining using a normal melting method, for example, a converter, an electric furnace, etc. Then, it is made into a steel slab by continuous casting or cast into an ingot and then rolled into a steel slab. Melting and casting can be performed in accordance with normal melting and casting of ferritic stainless steel. After this steel slab is heated, it is hot-rolled to obtain a steel material having a required shape. The conditions relating to hot rolling are not particularly limited, and may be performed according to the heating and rolling conditions of normal ferritic stainless steel. In the case of a steel plate, the hot-rolled steel plate is further pickled and annealed as necessary, and then cold-rolled into a cold-rolled steel plate, and further subjected to annealing, pickling, etc. It can be a cold rolled steel sheet.

　表２のＮｏ．１~２０が本発明例、Ｎｏ．２１~２９が比較例である。本発明範囲から外れる数値にアンダーラインを付している。また、表２の結果を図４に示す。
　この結果、表２、図４に示すように、本発明範囲の成分を含有するとともに、Ａ）式：Ｃｒ＋Ｍｏ＋１０Ｔｉ≧１８を満たし、かつＢ）式：Ｓｉ＋Ｃｕ≦０．５の本発明範囲を満たすＮｏ１~２０については、いずれも最大腐食深さは５０μｍ以下となった。なお，Ａ）式で２０以上を満たし、かつＢ）式で０．３未満を満たした本発明例は、より最大腐食深さが小さくなり、さらにＡ）式で２２以上を満たし、かつＢ）式で０．２未満を満たした本発明例は、最大腐食深さが２０μｍ以下と、耐食性にきわめて優れる結果を示した。
　一方、（Ａ）と（Ｂ）式の一方または両方を満たさない条件では、何れも最大腐食深さは５０μｍを超える結果となった。以上の結果から、本発明により、二次熱交換器相当の炭化水素燃焼排ガスから発生する凝縮水環境における耐食性に優れるフェライト系ステンレス鋼を提供することが可能であることが明らかとなった。 Steel having the chemical composition shown in Table 2 was produced by a conventional method for producing high purity ferritic stainless steel. That is, first, an ingot having a thickness of 40 mm was manufactured after vacuum melting, and this was hot rolled to a thickness of 4 mm. Thereafter, after heat treatment at 900 to 1000 ° C. for 1 minute based on each recrystallization behavior, the scale was ground and removed, and a steel plate having a thickness of 1.0 mm was manufactured by cold rolling. This was heat-treated at 900 to 1000 ° C. for 1 minute based on the respective recrystallization behavior as final annealing, and subjected to the following tests. In the case of austenitic stainless steel, the heat treatment temperature was 1100 ° C.
The wet and dry repeated test was the same test as described above. The test solution was nitrate ion NO ³⁻ : 100 ppm, sulfate ion SO ₄ ²⁻ : 10 ppm, chloride ion Cl ⁻ : 10 ppm, pH = 2.5. A test tube as shown in FIG. 1 is filled with 10 ml of a test solution, cut into 1 × 15 × 100 mm, and various types of stainless steel samples wet-polished with # 600 emery paper are approximately 1 in the longitudinal direction. / 2 was soaked in half so that it was immersed in the solution. The test tube containing the sample is placed in a warm bath at 80 ° C. After 24 hours, the completely dried sample is washed with light distilled water, and the newly washed test tube is refilled with the test solution, and the sample is filled again. In the same manner as described above, the operation of half-immersion and holding at 80 ° C. for 24 hours was performed 14 cycles.

No. in Table 2 1 to 20 are examples of the present invention, No.1. 21 to 29 are comparative examples. Numerical values that fall outside the scope of the present invention are underlined. Moreover, the result of Table 2 is shown in FIG.
As a result, as shown in Table 2 and FIG. 4, while containing the components of the present invention range, A1 satisfying the present invention range of A) formula: Cr + Mo + 10Ti ≧ 18 and B) formula: Si + Cu ≦ 0.5 For ~ 20, the maximum corrosion depth of all was 50 μm or less. In addition, the present invention example satisfying 20 or more in the formula A) and less than 0.3 in the formula B) has a smaller maximum corrosion depth, further satisfying 22 or more in the formula A), and B). The example of the present invention satisfying the formula of less than 0.2 showed extremely excellent corrosion resistance with a maximum corrosion depth of 20 μm or less.
On the other hand, under the conditions that did not satisfy one or both of the formulas (A) and (B), the maximum corrosion depth exceeded 50 μm. From the above results, it has been clarified that the present invention can provide a ferritic stainless steel having excellent corrosion resistance in a condensed water environment generated from hydrocarbon combustion exhaust gas equivalent to a secondary heat exchanger.

The present invention can be applied as a material for a heat exchanger, particularly as a material for a secondary heat exchanger of a latent heat recovery type water heater. Specifically, the present invention can be applied to any material such as a heat exchanger pipe as well as a case and a partition plate. Furthermore, this material can be similarly provided not only in the combustion exhaust gas of hydrocarbon fuel, but also in an environment where repeated drying and wetting are exposed to a low pH solution containing nitrate ions and sulfate ions. Specifically, heat exchangers, outdoor exterior materials for acid rain environments, building materials, roofing materials, outdoor equipment, water storage / hot water storage tanks, home appliances, bathtubs, kitchen equipment, and other general outdoor / indoor applications .

1 Test tube 2 Sample 3 Test solution

Claims (3)

In mass%, C: 0.030% or less, N: 0.030% or less, Si: 0.4% or less, Mn: 0.01 to 0.5%, P: 0.05% or less, S: 0 0.01% or less, Cr: 16 to 24%, Mo: 0.30 to 3%, Ti: 0.05 to 0.25%, Nb: 0.05 to 0.50%, Al: 0.01 to 0 20%, Cu: 0.4% or less, the balance being made of Fe and inevitable impurities, and satisfying the formula (A): Cr + Mo + 10Ti ≧ 18 and (B) formula: Si + Cu ≦ 0.5 High corrosion resistance ferritic stainless steel with excellent corrosion resistance in the condensed water environment generated from hydrocarbon combustion exhaust gas.
However, Cr, Mo, Ti, Si, and Cu in the formula mean the content (% by mass) of each element. Further, in terms of mass%, Ni: 0.3 to 3%, B: 0.0001 to 0.003%, V: 0.03 to 1.0%, Sn: 0.005 to 1.0%, Sb: The highly corrosion-resistant ferritic stainless steel having excellent corrosion resistance in a condensed water environment generated from hydrocarbon combustion exhaust gas according to claim 1, characterized by containing one or more of 0.005 to 1.0%. steel. The steel according to claim 1 or 2 is semi-immersed in an aqueous solution having a pH of 2.5 and containing 100 ppm nitrate ion, 10 ppm sulfate ion, and 10 ppm chloride ion, and held at 80 ° C. for 24 hours. The maximum corrosion depth after 14 cycles of the wet and dry repeated test is 50 μm or less, high corrosion resistance excellent in corrosion resistance in a condensed water environment generated from hydrocarbon combustion exhaust gas according to claim 1 or 2 Ferritic stainless steel.

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