阅读说明：本技术 耐蚀性优异的Fe-Ni-Cr-Mo-Cu合金 (Fe-Ni-Cr-Mo-Cu alloy with excellent corrosion resistance ) 是由 矢部室恒 冈崎贤司 于 2020-03-02 设计创作，主要内容包括：本发明提出即使在硫酸中腐蚀环境变得最严酷的大约45-65%浓度的硫酸环境中或进一步在该硫酸环境中包含Cl～-离子的高腐蚀环境中,耐蚀性也优异的Fe-Ni-Cr-Mo-Cu合金。关于Fe-Ni-Cr-Mo-Cu合金,其特征在于,按以下质量%,C：0.004-0.025%、Si：0.02-1.0%、Mn：0.03-0.35%、P：≦0.040%、S：≦0.003%、Ni：30.0-37.0%、Cr：22.0-25.0%、Mo：7.0-9.0%、N：0.15-0.30%、Cu：2.5-4.0%、Al：0.001-0.1%,以及剩余部分由Fe和不可避免的杂质构成,满足下述的(1)式：Ni+Cr+3Mo+5Cu+25N≧89.0 (1)。(The present invention proposes to contain Cl in or further in a sulfuric acid environment of about 45-65% concentration even if the corrosive environment in sulfuric acid becomes severer ‑ An Fe-Ni-Cr-Mo-Cu alloy which is excellent in corrosion resistance even in an ionic highly corrosive environment. An Fe-Ni-Cr-Mo-Cu alloy characterized by comprising, in mass%, C: 0.004-0.025%, Si: 0.02 to 1.0%, Mn: 0.03-0.35%, P: 0.040%, S: ≦ 0.003%, Ni: 30.0-37.0%, Cr: 22.0-25.0%, Mo: 7.0-9.0%, N: 0.15-0.30%, Cu: 2.5-4.0%, Al: 0.001 to 0.1%, and the balance consisting of Fe and inevitable impurities, satisfying the following formula (1): ni + Cr +3Mo +5Cu +25N ≧ 89.0 (1).)

1. A Fe-Ni-Cr-Mo-Cu alloy is characterized in that,

in the following amount by mass%,

C：0.004-0.025%，

Si：0.02-1.0%，

Mn：0.03-0.35%，

P：≦0.040%，

S：≦0.003%，

Ni：30.0-37.0%，

Cr：22.0-25.0%，

Mo：7.0-9.0%，

N：0.15-0.30%，

Cu：2.5-4.0%，

al: 0.001-0.1%, and

the balance of Fe and inevitable impurities, and satisfies the following formula (1):

Ni+Cr+3Mo+5Cu+25N≧89.0 (1)。

2. the Fe-Ni-Cr-Mo-Cu alloy according to claim 1, characterized by containing, in addition to said constituents, a chemical element selected from the group consisting of Nb: 0.001-0.03%, V: 0.01-0.1%, B0.0001-0.003%, Ti: 0.001-0.01% of one or more selected from the group consisting of.

3. The Fe-Ni-Cr-Mo-Cu alloy according to claim 1 or 2, wherein a coating containing 45% or more of Cu is provided on the alloy surface.

4. The Fe-Ni-Cr-Mo-Cu alloy according to claim 3 wherein the coating has a thickness of 0.005 to 0.1 μm.

5. The Fe-Ni-Cr-Mo-Cu alloy according to claim 3, wherein a passive film having a thickness of 0.001 to 0.005 μm and made of Cr-Ni-Fe-O is provided between said film and the alloy base material.

6. The Fe-Ni-Cr-Mo-Cu alloy according to claim 4, wherein a passive film having a thickness of 0.001 to 0.005 μm and made of Cr-Ni-Fe-O is provided between said film and the alloy base material.

7. The Fe-Ni-Cr-Mo-Cu alloy according to claim 5,

the coating film containing 45% or more of Cu contains, in addition to Cu, Ni: 10-30%, Cr: 5-20%, Fe: less than 10%, the rest contains trace amount of alloy components such as Mn or Si,

the passive coating film contains Cr: more than 40%, Ni: 10-40%, Fe: 5-20%, O: 5-10%, and the rest contains trace amount of alloy components such as Mn, Si, Cu, etc.

8. The Fe-Ni-Cr-Mo-Cu alloy according to claim 6,

the coating film containing 45% or more of Cu contains, in addition to Cu, Ni: 10-30%, Cr: 5-20%, Fe: less than 10%, the rest contains trace amount of alloy components such as Mn or Si,

the passive coating film contains Cr: more than 40%, Ni: 10-40%, Fe: 5-20%, O: 5-10%, and the rest contains trace amount of alloy components such as Mn, Si, Cu, etc.

Technical Field

The present invention relates to an Fe-Ni-Cr-Mo-Cu alloy used in an environment where extremely excellent corrosion resistance is required, such as a chemical plant and a waste liquid treatment facility.

Background

Because of its excellent corrosion resistance, Fe-Ni-Cr-Mo-Cu alloys are suitable for use in various industrial fields such as chemical plants, water treatment facilities, pollution control equipment, oil well environments, food plants, thermal and nuclear power plants, and seawater environments. In an environment where high corrosion resistance is required as in these industrial fields, when carbon steel or SUS430 or SUS304, which are general-purpose alloys, is applied, the corrosion resistance of the alloy is insufficient, and thus, the use of the alloy is greatly restricted.

In particular, in recent years, exhaust gas from engines for automobiles and ships or exhaust gas from power stations has been required to be treated with SO appropriately with a strict reference value_xDischarge of environmental pollutants. On the other hand, the use of inferior fuels containing many impurities causing environmental pollutants is also expanding, and countermeasures against environmental problems are urgently needed. In response to such a trend, the development of a technology for an exhaust gas treatment apparatus has been advanced, and a technology for detoxifying exhaust gas has been developed and put into practical use by spraying an alkaline solution, seawater, or a mixed solution thereof as an absorbent in the treatment apparatus.

In the exhaust gas treatment device, since the exhaust gas is continuously circulated and treated, H is a corrosive property of the waste liquid generated in the device⁺Ions or Cl^-The ions are concentrated, extremely corrosive solutions. In addition, conventionally, in an exhaust gas treatment device, SO caused by exhaust gas is generated_xSo that corrosion, which is called sulfuric acid dew point corrosion, generally occurs, alloys having improved sulfuric acid resistance have been developed.

However, in recent years, since the use of inferior fuels has been expanded or seawater or the like is used as an absorption solution, not only the sulfuric acid environment but also Cl is contained^-Ions and thus the corrosive environment becomes more severe. As has been known, Cl^-The ions have a property of destroying the passive film of the corrosion resistant alloy such as stainless steel, and cause corrosion, but if Cl is contained, the corrosion is caused^-When ions are mixed in sulfuric acid, the property of the passive film is destroyed and dissolution by acid acts as a synergistic effect, and the dissolution is acceleratedThe general surface corrosion proceeds, and therefore, further improvement of the general surface corrosion resistance or development of a new alloy is required.

Under the circumstances as described above, for example, patent document 1 discloses an alloy in which the sulfuric acid corrosion resistance is improved by adding Cu to an alloy based on SUS304, but the amount of Cu added to the alloy is as small as 0.4%, and no study has been made on an alloy containing Cu in excess of the amount. Regarding the assumed environment, the study was also conducted only in an environment where the sulfuric acid concentration was 10% and low. Further, it is considered that since the alloy to be the base is SUS304, the corrosion resistance is inherently insufficient in the recent corrosive environment which has been increasing in severity.

On the other hand, in patent document 2, studies are conducted until the amount of Cu added to the alloy is 2.91% and a high concentration, but only at an extremely high concentration of 98% with respect to the concentration of sulfuric acid. At a high concentration of 80% or more of the sulfuric acid solution, the liquid properties change from non-oxidizing to oxidizing, and the corrosivity decreases, so that the corrosion rate decreases. That is, it can be said that the corrosive environment becomes mild. Therefore, when an environmental pollution prevention device in recent years is assumed, it is considered that 98% sulfuric acid is used without simulating the most severe corrosive environment, and it is considered that 45 to 65% sulfuric acid having the most severe corrosive environment should be used.

In patent document 3, although studies have been made to make the amount of Cu added to the alloy 4.5% and a very high concentration, the sulfuric acid concentration is not described in its entirety and is not clear about the corrosive environment, and Cl is not added^-The study was conducted in an ionic environment. It is assumed that the entire surface is etched with respect to the etching mode, but since it is not described, it is unclear how to consider etching.

Patent document 4 has studied until the Cu content of the alloy is 2.86% and a high concentration, and has included 100000ppm of Cl in the test solution by using hydrochloric acid, ammonium chloride, sulfuric acid, and ammonium sulfate^-The sulfuric acid environment of (a). However, the alloy to be studied is a member to which the alloy is applied and subjected to a heat treatment simulating a brazing heat treatment in consideration of harmful precipitates such as sigma phaseThe state of the annealed microstructure was not investigated at all. Further, the etching pattern is not full-surface etching, but pitting etching is considered.

However, since in Cl^-In the environment in which ions are mixed into acid, Mo is effective in addition to Cu, and therefore, as shown in patent document 5, a highly corrosion-resistant alloy containing 8.77% Mo has been developed, but since the corrosion mode assumed in this document is crevice corrosion according to ASTM G48 Method D, no consideration is given to the surface corrosion.

Patent document 1: japanese patent laid-open publication No. 1-104748;

patent document 2: japanese patent laid-open No. 56-93860;

patent document 3: japanese patent laid-open publication No. 4-346638;

patent document 4: japanese patent laid-open publication No. 2018-172709;

patent document 5: japanese patent laid-open publication No. 2010-31313;

non-patent document 1: stainless steel entry 2015 (issued by the stainless steel association, written by the stainless steel association "stainless steel entry" regulatory committee, page 64).

Disclosure of Invention

As described above, in recent years, in the environment pollution prevention device of increasing severity, development of a material having excellent corrosion resistance over the entire surface is desired in consideration of insufficient corrosion resistance of the above alloys and different corrosion forms.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an Fe — Ni — Cr — Mo — Cu alloy which: even if Cl is contained in or further to a sulfuric acid environment of about 45-65% concentration where the corrosive environment becomes severer in sulfuric acid^-The corrosion resistance is also excellent in an ionic highly corrosive environment.

The inventors have made extensive studies to solve the above problems. As a result of the findings, it has been understood that if Cu is added to a corrosion resistant alloy such as stainless steel, the acid resistance (particularly, the entire surface corrosion resistance) is improved, but the mechanism thereof is still insufficient. The inventors considered that this is due to the alloy surface in direct contact with an acid in the alloy having excellent corrosion resistance over the entire surface. Stainless steel ensures excellent corrosion resistance by passive coating on the alloy surface, but the composition of stainless steel is considered to be composed of a hydroxide of Fe and Cr and an oxide of Cr according to non-patent document 1. On the other hand, the reason why the acid resistance of stainless steel is improved by adding Cu is presumed to be due to a change in composition, structure, or form of the passive film.

When the above-described environmental pollution prevention device in recent years is assumed, even if a large amount of Cu is added to general-purpose stainless steel such as SUS430 or SUS304 with respect to the base material composition, the corrosion resistance of the alloy body is low, and thus it is expected that sufficient corrosion resistance is not obtained. Then, based on the alloy composition class higher in corrosion resistance than general-purpose stainless steel called super austenitic stainless steel, 2.5% or more of Cu is added to the alloy, and acid and Cl are added^-The corrosion test was performed in an environment where the ions were mixed together. As a result, it was found that a protective film mainly composed of Cu, which is separate from the conventional passive film, exists on the alloy surface, resulting in excellent corrosion resistance.

The present inventors conducted immersion tests at 20, 40, 60, 80, and 98% sulfuric acid at 20, 40, 60, 80, and 100 ℃ and boiling points of each concentration as a preliminary experiment on an alloy containing Fe-25% Ni-23% Cr-6% Mo-2.7% Cu-0.20% N as a main component, and generated an equi-corrosion graph showing the corrosion rate in terms of the relationship between the temperature and the sulfuric acid concentration. As a result, it was understood that the corrosion rate was the highest in the range of 45 to 65% sulfuric acid concentration, that is, the concentration at which the corrosion environment became the most severe. Further, with respect to temperature, the higher the temperature, the greater the etch rate. As in the conventional findings, it was confirmed that the liquid properties of the high-concentration sulfuric acid of 80% or more were changed from non-oxidizing to oxidizing, and the corrosivity was decreased.

Next, the raw materials were dissolved in an electric furnace, and a steel containing Fe-33% Ni-24% Cr-8% Mo-3% Cu-0.25% N as a basic component was dissolved by AOD or VOD. The molten steel was continuously cast into a cast slab (slab), and hot forged into a forged plate having a thickness of 8 mm. And then, annealing and pickling are carried out, further, cold rolling is carried out until the thickness is 2-3mm, and annealing and pickling are carried out to manufacture the cold-rolled sheet. Finally, annealing was carried out at 1150 ℃ for 1 minute. A corrosion test piece of 25mm × 50mm was taken out from the cold-rolled sheet, the surface was wet-polished with 400 # waterproof polishing paper, and after water washing, ultrasonic cleaning was performed in a solution in which acetone and ethanol were mixed, and a corrosion test was performed. When the dissolution is performed, the concentrations of the alloying elements of Ni, Cr, Mo, Cu, and N are variously changed.

The artificial seawater (sapphire (aquamarine) manufactured by octanday chemical corporation) was adjusted to a solution concentrated by a usual factor of 3.5 so that the sulfuric acid concentration was 50% and the hydrochloric acid was 0.5% using the corrosion test piece, and subjected to a corrosion test by immersing at a temperature of 80 ℃ for 96 hours. The immersion corrosion test is evaluated based on the corrosion rate, and if the corrosion rate is less than 0.17 mm/year, the corrosion resistance is judged as excellent (. circleincircle.), if the corrosion rate is 0.17 to 0.30 mm/year, the corrosion resistance is judged as acceptable (. largecircle.), and if the corrosion rate exceeds 0.30 mm/year, the corrosion resistance is judged as poor (. largecircle.). As a result, they have found that the compounds are derived from p-sulfuric acid, hydrochloric acid and Cl^-The alloy in which the characteristic expressions represented by Ni, Cr, Mo, Cu and N, which are alloy component elements effective in ionic corrosive substances, satisfy the condition that Ni + Cr +3Mo +5Cu +25N is not less than 89.0, and has good corrosion resistance.

As described above, considering that the alloy exhibiting excellent corrosion resistance does not exert any effect on the passive film on the surface, the alloy surface was analyzed in detail by a Marcus (マーカス) type high-frequency glow discharge emission surface analyzer (hereinafter, GDS) immediately after the test piece after the corrosion test was taken out from the corrosion test solution, after the solution was rinsed with distilled water and dried with cold air. The GDS is a device that performs analysis of element concentration distribution (profiling) in the depth direction by sputtering a sample with Ar plasma and causing atoms sputtered to emit light, thereby measuring the element concentration distribution from the surface after a corrosion test. As the measurement conditions, the excitation mode was the pulse mode, the sputtering method was Ar sputtering, the sputtering pressure was 600MPa, the sputtering output was 8.75W, the pulse frequency was 100Hz, and the analysis area was 4mm φ. The GDS analyzer was GD-Profiler 2 manufactured by horiba, Inc.

In the foregoing, it was described that the corrosion resistance of stainless steel is ensured by the passive film composed of the hydroxide of Fe and Cr and the oxide of Cr, but as a result of GDS analysis, it was confirmed that another protective film mainly composed of Cu having a composition different from that of the passive film exists on the passive film. That is, it was found that the film had a two-layer structure of a protective film and a passive film of Cu as a main body in the present corrosive environment. It is considered that these coatings are formed in acid and Cl^-The mixed solution of ions causes good corrosion resistance. In order to examine the structure of the coating film in more detail, the analysis was further advanced by GDS. As a result, it was found that when the two layers of the protective film mainly composed of Cu and the passivation film have the following thicknesses and compositions, respectively, good corrosion resistance is exhibited.

The protective coating film of the Cu host has a thickness of 0.005 to 0.1 μm, and the composition thereof contains Cu: more than 45%, Ni: 10-30%, Cr: 5-20%, Fe: less than 10%. The remainder is made of an alloy component such as Mn or Si.

Then, a passive film existing between the protective film of the Cu body and the alloy base material has a thickness of 0.001 to 0.005 μm, and a composition containing Cr: more than 40%, Ni: 10-40%, Fe: 5-20%, O: 5 to 10 percent. The remainder is composed of alloy components such as Cu, Mn, and Si.

The inventors speculate on this mechanism as follows. First, it is considered that a general passive film similar to stainless steel exists on the surface of the sample before the test, but it is structured as a hydroxide of Fe and Cr and an oxide of Cr, and has a thickness of several nm. Next, it is considered that the pH of the test solution is 1 or less, and thus thermodynamically reaches a range of so-called de-passivation pH, but actually, the dissolution and regeneration are repeated while maintaining the passive film without breaking the passive film at all in the test solution. This is because, if a passive coating film is not present at all, the sample dissolves at an abnormal corrosion rate, and the original shape of the test piece is not retained. So-called anodes in which the usual metal is dissolved in such a low pH solutionIn the reaction process, Fe, Ni, Cr, Mo, Cu and other alloy elements are eluted out to serve as a cathode reaction to corrode H in the solution⁺The reaction in which the ions are reduced mainly occurs in pairs. In a very corrosive solution such as this test solution, the above-mentioned anodic reaction actively occurs, and if a certain degree of anodic reaction occurs, Cu is present²⁺The ions accumulate in the solution and thus react with H in relation to the cathode⁺Reduction of ions by Cu²⁺The reduction reaction of the ions is dominant. If the oxidation-reduction potentials of the two are compared, the oxidation-reduction potential is based on Cu and also on the hydrogen electrode²⁺Is assumed to be + 0.337V. From this, it is presumed that a protective film mainly composed of Cu is formed on the alloy surface, resulting in excellent corrosion resistance.

Further, as a result of providing pure Cu for the corrosion test, it has more excellent corrosion resistance than the steel of the present invention, but pure Cu is high in cost and is not suitable at all for a structural material in view of strength. The coating was mainly composed of Cu, but Ni, Fe, and Cr were also present according to the measurement result of GDS. Considering, though Ni²⁺、Fe²⁺、Cr³⁺Has an oxidation-reduction potential of-0.257V, -0.447V and-0.744V, respectively, all being higher than that of Cu²⁺The oxidation-reduction potential of (2) is worse, but since these elements are present in the coating film, reduction reaction of these ions actually occurs. In addition, due to Fe²⁺Oxidation-reduction potential ratio of Cr³⁺Is more preferred, and thus Fe²⁺Specific Cr³⁺More preferentially, Fe should be present in the film at a higher concentration than Cr, but at a lower concentration than Cr. The mechanism is presumed to be due to the fact that even Fe²⁺Reduced, the pH of the solution is also below 1, so the reduced Fe redissolves immediately.

That is, the Fe — Ni — Cr — Mo — Cu alloy of the present invention contains C in the following mass%: 0.004-0.025%, Si: 0.02 to 1.0%, Mn: 0.03-0.35%, P: 0.040%, S: ≦ 0.003%, Ni: 30.0-37.0%, Cr: 22.0-25.0%, Mo: 7.0-9.0%, N: 0.15-0.30%, Cu: 2.5-4.0%, Al: 0.001 to 0.1%, and the balance of Fe and inevitable impurities, and satisfies the following formula (1):

Ni+Cr+3Mo+5Cu+25N≧89.0 (1)。

in addition, the Fe-Ni-Cr-Mo-Cu alloy of the present invention may contain, in addition to the above components, a component selected from the group consisting of Nb: 0.001-0.03%, V: 0.01-0.1%, B0.0001-0.003%, Ti: 0.001-0.01% of one or more selected from the group consisting of.

In the Fe-Ni-Cr-Mo-Cu alloy of the present invention, the following characteristics are provided: in addition to the above-described composition, the alloy surface has a coating containing 45% or more of Cu.

In the Fe-Ni-Cr-Mo-Cu alloy of the present invention, the following characteristics are provided: the film containing 45% or more of Cu has a thickness of 0.005 to 0.1. mu.m.

Further, the following features are provided: a passive film having a thickness of 0.001 to 0.005 μm and made of Cr-Ni-Fe-O is provided between the film and the alloy base material.

Further, a film mainly composed of Cu includes not less than 45% of Cu, Ni: 10-30%, Cr: 5-20%, Fe: 10% or less, and the balance of the alloy composition comprising Mn, Si, or the like. Characterized by the following: the passive coating film contains Cr: more than 40%, Ni: 10-40%, Fe: 5-20%, O: 5-10%, and the balance of alloy components such as Mn, Si, Cu, etc.

According to the present invention, since it is excellent in corrosion resistance, it can be suitably used as a material which is used in an environment where corrosion resistance of the entire surface is concerned to be caused by an acid in an environment pollution prevention device, various factories, or the like.

Drawings

Fig. 1 is an element distribution diagram in the depth direction of an alloy having a protective coating film of a Cu body and a passive coating film.

Detailed Description

Next, the composition that the Fe-Ni-Cr-Mo-Cu alloy of the present invention should have will be described.

C: 0.004-0.025 wt%

C is an austenite phase stabilizing element. However, if a large amount of Cr or Mo is added, Cr, Mo, etc. are bonded to form carbide, and the amount of Cr and Mo dissolved in the matrix decreases, resulting in a decrease in corrosion resistance. On the other hand, the lower limit of C is 0.004 mass% from the viewpoint of preventing the decrease in strength. Thus, C is limited to 0.004 to 0.025 mass%. Preferably 0.005 to 0.023 mass%, more preferably 0.006 to 0.021 mass%.

Si: 0.02-1.0% by mass

Si is an element added as a deoxidizer. Further, since Si is also an element which improves the fluidity of molten steel and improves weldability, it is preferable to add 0.02 mass% or more. However, Si is 0.02 to 1.0 mass% because it promotes the precipitation of intermetallic compounds such as sigma phase and is an element that increases the grain boundary corrosion susceptibility. Preferably 0.02 to 0.8 mass%, more preferably 0.02 to 0.6 mass%.

Mn: 0.03 to 0.35 mass%

Since Mn is an element having a deoxidizing effect, it is necessary to be at least 0.03 mass% or more in order to obtain the effect. However, Mn also causes precipitation of intermetallic compounds of σ or the like in the same manner as Si, and therefore, it is not preferable to add Mn more than necessary. Therefore, it is necessary to be 0.03 to 0.35 mass%. Preferably 0.03 to 0.30 mass%, more preferably 0.03 to 0.25 mass%.

P: 0.040 mass% or less

P is an element that is inevitably mixed as an impurity and segregates as a phosphide at crystal grain boundaries, thereby impairing hot workability and deteriorating corrosion resistance as a whole. Therefore, it is desirable to reduce the force as much as possible. However, extremely reducing the content of P incurs an increase in manufacturing cost. Therefore, in the present invention, P is limited to 0.040 mass% or less. Preferably 0.030 mass% or less, and more preferably 0.020 mass% or less.

S: 0.003 mass% or less

S is an element that is inevitably mixed as an impurity, as in P, and is easily segregated in crystal grain boundaries, and particularly, an element that significantly hinders hot workability and is detrimental to corrosion resistance. If the content exceeds 0.003 mass%, the harmful effect is remarkable, and therefore, it is necessary to control the content to 0.003 mass% or less. Preferably 0.002 mass% or less, more preferably 0.001 mass% or less.

Ni: 30.0 to 37.0 mass%

Ni suppresses precipitation of intermetallic compounds such as σ and improves the entire surface corrosion resistance, and in particular, has an effect of reducing the dissolution rate of the alloy. If the content is less than 30.0% by mass, precipitation of intermetallic compounds is promoted, while if it is more than 37.0% by mass, deterioration of hot workability, increase of heat distortion resistance and further increase of cost are caused. Therefore, the Ni content is 30.0 to 37.0 mass%. Preferably 31.0 to 36.0 mass%, more preferably 32.0 to 35.0 mass%.

Cr: 22.0 to 25.0 mass%

Cr is an extremely important element for improving not only the entire surface corrosion resistance of the alloy but also the corrosion resistance such as pitting corrosion resistance, crevice corrosion resistance, and stress corrosion cracking resistance as a whole. In order to sufficiently obtain this effect, it is necessary to contain 22.0 mass% or more. However, if the content exceeds 25.0 mass%, the content is 22.0 to 25.0 mass% because precipitation of intermetallic compounds such as σ phase is promoted and the corrosion resistance is rather deteriorated. Preferably 22.2 to 24.8% by mass, more preferably 22.4 to 24.6% by mass.

Mo: 7.0 to 9.0 mass%

Since Mo is also an element that is useful for improving the pitting corrosion resistance and the crevice corrosion resistance, the content of Mo needs to be 7.0 mass% or more. However, excessive addition of Mo promotes precipitation of intermetallic compounds such as σ phase, and lowers the corrosion resistance. Therefore, Mo is in the range of 7.0 to 9.0 mass%. Preferably 7.2 to 8.5% by mass, more preferably 7.4 to 8.0% by mass.

Cu: 2.5 to 4.0 mass%

Cu is an element that is positively added because it is extremely effective for improving acid resistance (i.e., improving the resistance to general corrosion). In the present invention, the element plays an extremely important role in terms of forming a protective film mainly composed of Cu on the alloy surface to improve the entire surface corrosion resistance. To obtain this effect, it is necessary to add 2.5 mass% or more. However, if the content exceeds 4.0 mass%, the density of the protective film mainly composed of Cu is impaired, and the corrosion resistance is rather lowered. Therefore, the amount of Cu added is 2.5 to 4.0 mass%. Preferably 2.7 to 3.8% by mass, more preferably 2.9 to 3.6% by mass.

N: 0.15 to 0.30 mass%

N is an element that is useful for improving the general corrosion resistance, pitting corrosion resistance, and crevice corrosion resistance, as with Cr, Mo, Ni, and Cu. In order to obtain this effect, it is necessary to add 0.15 mass% or more. However, if N is contained in an amount exceeding 0.30 mass%, precipitation of nitrides is caused, and the heat distortion resistance is greatly increased to inhibit the hot workability, so that the content of N is 0.15 to 0.30 mass%. Preferably 0.19 to 0.27 mass%, more preferably 0.20 to 0.26 mass%.

Al: 0.001-0.1% by mass

Because Al is in CaO-SiO₂-Al₂O₃In the presence of the-MgO-F slag, the desulfurization is promoted by deoxidation to reduce S, and the hot workability is improved, so that it is necessary to add it positively, but if the addition is less than 0.001 mass%, the effect is not exhibited, and if the content exceeds 0.1 mass%, a large intervening material affecting the appearance of the steel sheet and the corrosion resistance is formed, and furthermore, the precipitation of AlN as a compound with N becomes significant, and the effect of N effective for the corrosion resistance is reduced, so that the content of Al is 0.001 to 0.1 mass%. Preferably 0.001 to 0.07% by mass, more preferably 0.001 to 0.04% by mass.

Nb: 0.001-0.03% by mass

Nb precipitates as Nb carbides at crystal grain boundaries, suppresses precipitation of Cr carbides that occur when subjected to thermal influence by welding or the like, and prevents so-called quenching. In order to obtain this effect, it is necessary to add 0.001 mass% or more. However, the excessive addition of the intermetallic compound promotes the precipitation of the σ phase or the like, and deteriorates the corrosion resistance, and therefore, the amount is 0.001% to 0.03% by mass. Preferably 0.003 to 0.028% by mass, more preferably 0.006 to 0.026% by mass.

V: 0.01 to 0.1% by mass

V precipitates as a carbonitride of V at crystal grain boundaries, and prevents sharp sensitization similarly to Nb. In order to obtain this effect, it is necessary to add 0.01 mass% or more. However, excessive addition causes precipitation solidification, and makes it difficult to process the material. Therefore, it is 0.01 to 0.1 mass%. Preferably 0.02 to 0.09 mass%, more preferably 0.03 to 0.08 mass%.

B:0.0001-0.003 mass%

B is effective for improving hot workability, but if it is less than 0.0001 mass%, the effect is not so great, and if it is more than 0.003 mass%, hot workability is conversely deteriorated. Thus, B is 0.0001 to 0.003 mass%. Preferably 0.0003 to 0.0027% by mass, more preferably 0.0006 to 0.0024% by mass.

Ti: 0.001-0.01% by mass

Ti precipitates as carbide of Ti at crystal grain boundaries, and prevents sharp sensitization similarly to Nb. However, when excessive addition is performed, Ti carbide precipitates in a large amount in a welded portion, and causes corrosion called knife-edge corrosion (knifeline attack). Therefore, Ti is 0.001 to 0.01 mass%. Preferably 0.002 to 0.009 mass%, more preferably 0.003 to 0.008 mass%.

Ni + Cr +3Mo +5Cu +25N ≧ 89.0 (formula 1)

In the presence of an acid and Cl^-In an environment where ions are mixed together, in order to improve the entire surface corrosion resistance, it is effective to add Ni, Cr, Mo, Cu, and N to the alloy. As each alloying element functions, Ni slows down the rate of dissolution due to corrosion of the alloy in a low pH environment (i.e., an acid solution). Cr forms a passive film, which contributes to the corrosion resistance of the alloy. Mo generates molybdic acid ions, thereby promoting the occurrence of re-passivation of the passivation film, and as an effect thereof, the dissolution rate in the acid solution is slowed down in the same manner as Ni. N generates ammonia to neutralize the pH solution, which slows down the corrosion rate. That is, in any of the alloy elements, corrosion resistance is contributed as a synergistic effect, and if the sum of the expressions (1) obtained by weighting the contribution degrees of the elements satisfies 89.0 or more, sufficient corrosion resistance is obtained. Therefore, the alloy needs to be equal to or larger than 89.0 of Ni + Cr +3Mo +5Cu + 25N. Preferably, the ratio of Ni + Cr +3Mo +5Cu +25N is 94.0 or more, and more preferably, the ratio of Ni + Cr +3Mo +5Cu +25N is 98.0 or more.

The protective coating film of Cu body has a thickness of 0.005-0.1 μm

As described above, in the Fe-Ni-Cr-Mo-Cu alloy of the present invention, Cu is added in an acid solution²⁺Mainly of ionsFor Cu²⁺The ions are reduced to form a protective coating on the surface of the alloy base material. If the thickness of the protective coating is less than 0.005. mu.m, the thickness is insufficient and sufficient protection is not obtained. On the other hand, if it has a thickness exceeding 0.1 μm, the structure of the coating film becomes porous, the denseness is lost, and conversely, the protection is impaired. Therefore, the protective coating film of the Cu body needs to have a thickness of 0.005 to 0.1. mu.m. Preferably 0.005 to 0.090 μm, more preferably 0.005 to 0.080. mu.m.

A passive film having a thickness of 0.001-0.005 μm and made of Cr-Ni-Fe-O is provided between the Cu base film and the alloy base material

If the passive coating film is exposed to a corrosive environment, the dissolution and regeneration (i.e., re-passivation) of the passive coating film itself are repeated while the corrosion is slowly performed. If the thickness is less than 0.001. mu.m, the dissolution rate is higher than the regeneration rate, and therefore the corrosion rate is high, and sufficient corrosion resistance is not obtained. On the other hand, it is known that the thinner and denser the passive film is, the better corrosion resistance is given to the alloy, but if it exceeds 0.005 μm, the denseness is impaired and the better corrosion resistance is not obtained. Therefore, the passive film needs to have a thickness of 0.001 to 0.005. mu.m, preferably 0.001 to 0.004. mu.m, and more preferably 0.001 to 0.003. mu.m.

Composition of protective coating mainly containing Cu

The reason why the protective coating mainly containing Cu causes good corrosion resistance in an acid environment is because pure Cu has good corrosion resistance to acid. Therefore, the protective film is preferably mainly composed of Cu, and the Cu concentration is required to be Cu ≧ 45%. Since Ni and Cr also have p-acid and Cl^-Since the corrosion resistance of the ions is high, it is preferable that the ions are contained in an amount of 10 to 30% and 5 to 20%, respectively. However, in this environment, it is preferable that the content of Fe which is easily dissolved is 10% or less, and it is preferable that the remainder contains Mn, Si, or the like as an alloy component in a small amount as possible.

Composition of passive film

The passive film has a function of securing corrosion resistance before the above-mentioned protective film mainly composed of Cu is formed and a function of securing corrosion resistance before the protective film mainly composed of Cu is broken and regenerated when the alloy of the present invention is exposed to a corrosive environment. In order to obtain the function of the passive film sufficiently, it is preferable to contain 40% or more of Cr, each of which contains Ni: 10-40%, Fe: 5-20%, O: 5 to 10% is desirable. It is desirable that the remainder contains as small amounts as possible of Cu, Mn, Si, etc. as alloying components.

Definition of thickness of Cu-based protective coating and passivation coating

If the element distribution map is extracted from the surface layer in the depth direction by GDS with respect to the protective coating mainly containing Cu, for example, as shown in fig. 1, the distribution map contains the largest amount of Cu and the Cu concentration gradually decreases in the depth direction. However, if the depth reaches a certain level, a passive film existing directly below the Cu protective film is detected. Here, the thickness of the protective film mainly composed of Cu is defined as the thickness of the protective film mainly composed of Cu up to the depth at which Cu from the surface layer intersects with Cr of the passive film.

Next, with respect to the element profile of the passive film, the highest Cr concentration was detected, but the concentration immediately decreased, crossing first Ni and then Fe. Then, the distribution pattern of the base material components is obtained. As a definition of the thickness of the passive film, the thickness of the passive film is defined from a depth portion where Cu in the above-described protective film mainly containing Cu intersects Cr in the passive film to a depth portion where Cr in the passive film intersects Ni in the passive film.

Definition of composition of Cu-based protective coating film and passivation coating film

In a protective film mainly composed of Cu, a depth portion at which the Cu concentration in the outermost layer is the largest is defined as a composition of the protective film. Similarly, with respect to the composition of the passive film, the depth portion at which the Cr concentration becomes the highest is defined as the composition of the passive film.

Next, a method for producing the Fe-Ni-Cr-Mo-Cu alloy of the present invention will be described. In the Fe-Ni-Cr-Mo-Cu alloy of the present invention, it is preferable that raw materials such as scrap iron, stainless steel scrap, ferronickel, and ferrochrome are dissolved in an electric furnace and then subjected to Argon Oxygen Decarburization (AOD) in an Argon Oxygen Decarburization (AOD) furnaceOr a VOD (Vacuum Oxygen Decarburization) furnace, in which a mixed gas of Oxygen and a rare gas is blown to perform Decarburization refining, quicklime, an Fe-Si alloy, Al or the like is added to reduce Cr oxide in slag, and then fluorite is added to form CaO-SiO₂-Al₂O₃The MgO-F based slag is deoxidized and desulfurized, and is formed into a slab by a continuous casting method or an ingot-cogging rolling method, and then the slab is hot-rolled or further cold-rolled to form various steels such as a thin steel sheet, a thick steel sheet, a shaped steel, a bar steel, a wire rod, and the like.

Examples

Raw materials such as scrap steel, ferrochrome, ferronickel, and stainless steel scrap are dissolved in an electric furnace at a predetermined ratio, and a mixed gas of oxygen and a rare gas is blown in an AOD furnace or a VOD furnace to perform decarburization refining. Then, quicklime, Fe-Si alloy, Al or the like is added to reduce Cr oxide in the slag, and then fluorite is added to form CaO-SiO₂-Al₂O₃MgO-F slag and deoxidation and desulfurization. Subsequently, a cast slab is produced by a continuous casting method. After adjusting the compositions to various compositions shown in table 1, billets (slabs) were produced by continuous casting. The composition of C, S shown in table 1 was analyzed by an analyzer (combustion in oxygen gas flow-infrared absorption method) for the simultaneous analysis of carbon and sulfur values, the composition of N was analyzed by an analyzer (inert gas-pulse heating melting method) for the simultaneous analysis of oxygen and nitrogen values, and the compositions other than those described above were analyzed by fluorescent X-ray analysis.

Next, the above slab (slab) was hot-rolled to 8mm, and cold rolling, heat treatment and acid pickling were repeated to produce a cold-rolled coil having a thickness of 1 to 3 mm. Finally, annealing temperature 1150 ℃ for 1 minute. Selecting the width from the plate: 25mm, length: 50mm, thickness: 2-3mm corrosion test pieces. The corrosion test piece was subjected to a corrosion test by wet polishing the surface with a 400 # water repellent polishing paper, washing with water, and then ultrasonic cleaning in a solution of acetone and ethanol. When the dissolution was carried out, the amounts of Ni, Cr, Mo, Cu and N were varied variously using Fe-33% Ni-24% Cr-8% Mo-3% Cu-0.25% N as a basic component.

The artificial seawater (sapphire manufactured by yatsui chemicals) was adjusted to a solution concentrated by a common factor of 3.5 so that the sulfuric acid concentration was 50% and the hydrochloric acid was 0.5% using the above corrosion test piece, and subjected to a corrosion test by immersion at a temperature of 80 ℃ for 96 hours. The immersion corrosion test is evaluated based on the corrosion rate, and if the corrosion rate is less than 0.17 mm/year, the corrosion resistance is judged as excellent (. circleincircle.), if the corrosion rate is 0.17 to 0.30 mm/year, the corrosion resistance is judged as acceptable (. largecircle.), and if the corrosion rate exceeds 0.3 mm/year, the corrosion resistance is judged as poor (. largecircle.).

TABLE 1

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Steel sheets No. 1 to 13 shown in table 1 are invention examples satisfying the conditions of the present invention, and have excellent corrosion resistance. In table 1, the numerical values that do not satisfy the ranges of the present invention are shown by parentheses. Note that, in the case of nos. 3, 5, and 6, numerical values relating to a protective film mainly composed of Cu and a passive film are shown in parentheses, and although these examples deviate from the scope of the dependent claims which are more preferable ranges and the etching rate is slightly improved, the range satisfying the independent claim 1 and the etching rate of 0.30 mm/year or less which is a practical level are classified as the present invention examples.

In the steel sheet No. 3, since Cu is as low as 2.50%, it is estimated that the thickness of the protective film mainly composed of Cu becomes thin and the Cu concentration in the protective film mainly composed of Cu also becomes low.

Steel sheet No. 5 is characterized by the following: in addition to the difficulty in forming a passive film due to Cr as low as 22.00%, Mn is also as high as 0.35%. Since Mn has a property of easily dissolving the passive film in an acid solution, the thickness of the passive film is assumed to be 0.0008 μm.

In the steel sheet No. 6, Mo is as low as 7.00%, and therefore, it is assumed that the effect of re-passivation by molybdate ions is not sufficiently obtained in the present environment.

On the other hand, steel sheets No. 14 to 23 are comparative examples.

Although steel sheet No. 14 satisfies expression (1), the Ni content is low in the protective film mainly composed of Cu and the passivation film because the Ni content is low. Therefore, the dissolution rate in the acid solution is high, and the corrosion resistance is poor.

Although steel sheet No. 15 satisfies expression (1), since the Cr content is low, the passive film has a low Cr concentration and is poor in corrosion resistance.

Although steel sheet No. 16 satisfies expression (1), since the Cr content is high, the passive film becomes thick and the denseness is impaired, resulting in poor corrosion resistance. Further, as a result of observing the metal structure, since it contains high Cr, it is judged that the σ phase is precipitated. It is considered that the precipitation of the σ phase also causes the deterioration of the corrosion resistance.

In the steel sheet No. 17, the Mo content was low, and the formula (1) was not satisfied, and the re-passivation force of the passivation film was low. Therefore, the passive film has a low Cr concentration and poor corrosion resistance. In addition, the following is also one reason: since Cu is contained at a low content, the Cu concentration in the protective film mainly containing Cu is also low, and the thickness is also insufficient.

The steel sheet No. 18 satisfies the formula (1), but since the Mo content is high, it is judged that the σ phase precipitates. As a result, the corrosion resistance is poor.

In the 19 th steel sheet, the Cu content is low and does not satisfy the formula (1). Further, since Cu is contained at a low level, the Cu concentration in the protective film mainly containing Cu is low and the thickness is insufficient. Therefore, the corrosion resistance is poor.

Steel No. 20 satisfies expression (1), but since the Cu content is high, the protective film mainly composed of Cu is too thick, and the structure is porous. Therefore, the compactness is lost and the corrosion resistance is poor.

In the steel sheet No. 21, the N content is low and does not satisfy the formula (1). Further, since the content of N is low, a σ phase precipitates, and the corrosion resistance is poor. It is considered that the precipitation of the σ phase causes the thickness of the passive film to be insufficient and the Cr concentration to be insufficient, and the dissolution rate is increased.

The steel sheet No. 22 satisfied the expression (1), but the N concentration was high, and therefore, it was found that Cr-Mo-N based nitrides were precipitated. As a result, N greatly contributing to corrosion resistance does not effectively act, and corrosion resistance is poor.

Although steel sheet No. 23 satisfies expression (1), since Al is high, precipitation of Al — N is determined. As a result, N greatly contributing to corrosion resistance does not effectively act, and corrosion resistance is poor.

Since the Fe-Ni-Cr-Mo-Cu alloy of the present invention has excellent corrosion resistance, it can be suitably used for acids and Cl^-The entire surface of the ion-mixed layer is corroded by the ions, which causes corrosion.