JP2021075758A - Fe-Ni-Cr-Mo-Cu ALLOY HAVING EXCELLENT CORROSION RESISTANCE - Google Patents

Abstract

To provide an Fe-Ni-Cr-Mo-Cu alloy having excellent corrosion resistance even in an environment including sulfuric acid of about 45-65% concentration, which is the most severe corrosive environment for sulfuric acid, or a higher corrosive environment further including Cl-ions in addition to that.SOLUTION: An Fe-Ni-Cr-Mo-Cu alloy contains, in 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%, and Al: 0.001-0.1% with the balance being Fe and inevitable impurities, satisfying the following (1) formula: Ni+Cr+3Mo+5Cu+25N≥89.0.SELECTED DRAWING: Figure 1

Description

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 or a waste liquid treatment facility.

Due to its good corrosion resistance, Fe-Ni-Cr-Mo-Cu alloy is applied in various industrial fields such as chemical plants, water treatment facilities, pollution control equipment, oil well environment, food plants, thermal power / nuclear power plants, and seawater environment. ing. When carbon steel or general-purpose alloys such as SUS430 and SUS304 are applied in such an environment where high corrosion resistance is required, there are major restrictions on their use because the corrosion resistance of the alloy is insufficient.

In particular, in recent years, the standard values for exhaust gas from automobile and marine engines and exhaust gas from power plants have become stricter, and the emission of environmental pollutants such as _{SO x must be properly treated.} On the other hand, the use of inferior fuels containing a large amount of impurities, which are the source of environmental pollutants, is also expanding, and there is an urgent need to address environmental issues. In response to these trends, technological development of exhaust gas treatment equipment has progressed, and technology for detoxifying exhaust gas by spraying an alkaline solution, seawater, or a mixed solution thereof as an absorption liquid in the treatment equipment has been developed and put into practical use. There is.

Since the exhaust gas is continuously circulated in the exhaust gas treatment device, the corrosiveness of the waste liquid generated inside is an extremely corrosive solution in which ^{H +} ions and Cl ^{− ions are concentrated.} In addition, conventionally, in exhaust gas treatment equipment, _{sulfuric acid is generated due to SO x} contained in the exhaust gas, so that corrosion generally called dew point corrosion occurs, and alloys with improved sulfuric acid resistance have been developed so far. ing.

However, in recent years, such as by using the expanded use and seawater poor fuel absorption solution, Cl rather than just sulfuric acid environment ^- for also containing _ions, the corrosion environment have become more stringent. Conventionally, Cl ^- ions have the property of destroying the passivation film of corrosion-resistant alloys such as stainless steel and are well known to cause corrosion. However, when Cl ^- ions are mixed with sulfuric acid, the passivation film is formed. Since the property of destroying stainless steel and the dissolution by acid act as a synergistic effect and the total corrosion progresses at an accelerating rate, it is necessary to further improve the total corrosion resistance and develop a new alloy.

Under the above circumstances, for example, Patent Document 1 discloses an alloy in which Cu is added to an alloy based on SUS304 to improve sulfuric acid corrosion resistance, but the amount of Cu added to the alloy is small. It is up to 0.4%, and no studies have been conducted on alloys containing more Cu. The assumed environment has also been studied only in an environment with a low sulfuric acid concentration of 10%. Further, since the base alloy is SUS304, it is considered that the corrosion resistance is insufficient in the recent corrosive environment, which is becoming more severe.

On the other hand, in Patent Document 2, the amount of Cu added to the alloy is examined up to a relatively high concentration of 2.91%, but the concentration of sulfuric acid is examined only at an extremely high concentration of 98%. .. At a high concentration of 80% or more, the sulfuric acid solution changes its liquid property from non-oxidizing to oxidizing, and the corrosiveness decreases, so that the corrosion rate decreases. That is, it can be said that the corrosive environment becomes mild. Therefore, assuming a recent environmental pollution prevention device, it is considered that using 98% sulfuric acid does not simulate the most severe corrosive environment, so 45 to 65% sulfuric acid is used, which makes the corrosive environment the most severe. I think it should be.

In Patent Document 3, the amount of Cu added to the alloy is examined up to a very high concentration of 4.5%, but the sulfuric acid concentration is not described at all, the corrosive environment is unclear, and Cl. ^- it has not been examined in an environment with the addition of ions. Although it is presumed that assumes general corrosion Corrosion form, or assumes what corrosion because the description is not made is unclear.

In Patent Document 4, the amount of Cu added to the alloy is examined up to a relatively high concentration of 2.86%, and the test solution is also examined in a sulfuric acid environment containing ^{100,000 ppm Cl − using hydrochloric acid, ammonium chloride, sulfuric acid, and ammonium sulfate.} Has been made. However, the alloy under consideration is in a state of being heat-treated to simulate brazing heat treatment in consideration of harmful precipitates such as σ phase, assuming the applicable member of the alloy, and the state of the annealed metal structure. Has not been considered at all. Furthermore, the form of corrosion considers pitting corrosion rather than total corrosion.

By the way, since ^{Mo is effective in addition to Cu in an environment where Cl −} ions are mixed in an acid, a highly corrosion-resistant alloy containing 8.77% Mo has been developed as shown in Patent Document 5. However, since the form of corrosion assumed in this document is crevice corrosion according to ASTM G48 Method D, no consideration is given to total corrosion.

Japanese Unexamined Patent Publication No. 1-104748 Japanese Unexamined Patent Publication No. 56-93860 Japanese Unexamined Patent Publication No. 4-346638 Japanese Unexamined Patent Publication No. 2018-172709 Japanese Unexamined Patent Publication No. 2010-31313

Introduction to Stainless Steel 2015 (Published by Stainless Steel Association, "Introduction to Stainless Steel" by the Stainless Steel Association, Revised Committee, p. 64)

As described above, in the environmental pollution prevention device which has become more severe in recent years, it is considered that the corrosion resistance of the alloys is insufficient or the corrosion form considered is different, and the device has excellent total corrosion resistance. The development of materials is desired.

The present invention has been made in view of the above circumstances, and an object of the present invention is in a sulfuric acid environment having a concentration of about 45 to 65%, which is the most severe corrosive environment in sulfuric acid, or in a highly corrosive environment containing ^{Cl − ions.} The present invention is to propose a Fe-Ni-Cr-Mo-Cu alloy having excellent corrosion resistance.

The inventors have made extensive studies to solve the above problems. It has been found that adding Cu to a corrosion-resistant alloy such as stainless steel improves acid resistance, especially overall corrosion resistance, but it has not been sufficient to clarify the mechanism. The inventors considered that the reason for the alloy having excellent total corrosion resistance was the surface of the alloy in direct contact with the acid. Excellent corrosion resistance is guaranteed by the passivation film on the surface of the alloy of stainless steel, but it is considered that the composition is composed of hydroxide of Fe and Cr and oxide of Cr according to Non-Patent Document 1. ing. On the other hand, it was presumed that the reason why the acid resistance of the stainless steel was improved by adding Cu was that the composition, structure or morphology of the passivation film was changed.

Assuming the above-mentioned recent environmental pollution prevention device, sufficient corrosion resistance can be obtained because the corrosion resistance of the alloy body is low even if a large amount of Cu is added to general-purpose stainless steel such as SUS430 or SUS304 as the base material. I expected it not. Therefore, based on the so-called super austenitic stainless steel, which is an alloy component system with higher corrosion resistance than general-purpose stainless steel, 2.5% or more of Cu is added to the alloy, and a corrosion test is conducted in an environment where ^{acid and Cl − ions are mixed.} It was. As a result, it was found that a protective film mainly composed of Cu, which is different from the conventional passivation film, exists on the surface of the alloy, which brings about excellent corrosion resistance.

As a preliminary experiment, the present inventors have conducted 5, 10, 20 and 5, 10, 20 for an alloy containing Fe-25% Ni-23% Cr-6% Mo-2.7% Cu-0.20% N as a main component. Immersion tests were performed at 20, 40, 60, 80, 100 ° C. and boiling points of each concentration using 40, 60, 80 and 98% sulfuric acid, and an isocorrosion line indicating the corrosion rate in relation to the so-called temperature and sulfuric acid concentration. I made a figure. As a result, it was found that the corrosion rate was the highest in the range of the sulfuric acid concentration of 45 to 65%, that is, the concentration was the most severe in the corrosive environment. As for the temperature, the higher the temperature, the higher the corrosion rate. According to the conventional knowledge, it was confirmed that the corrosiveness of high-concentration sulfuric acid of 80% or more is lowered because the liquid property changes from non-oxidizing to oxidative.

Next, the raw material was melted in an electric furnace, and steel containing Fe-33% Ni-24% Cr-8% Mo-3% Cu-0.25% N as a basic component was melted using AOD or VOD. The melted steel was continuously cast into slabs and hot forged to obtain a forged plate with a thickness of 8 mm. Then, it was annealed and pickled, and then cold-rolled to a thickness of 2 to 3 mm, and annealed and pickled to prepare a cold-rolled plate. The final annealing temperature was 1150 ° C. for 1 minute. A 25 mm x 50 mm corrosion test piece is collected from this cold-rolled plate, the surface is wet-polished with No. 400 water-resistant abrasive paper, and after washing with water, ultrasonic cleaning is performed in a mixed solution of acetone and ethanol for a corrosion test. Served. In this melting, the alloy element concentrations of Ni, Cr, Mo, Cu and N were variously changed.

Using the above corrosion test piece, adjust the sulfuric acid concentration to 50% and hydrochloric acid to 0.5% in a solution of artificial seawater (Aquamarin manufactured by Yashima Pure Chemicals Co., Ltd.) concentrated 3.5 times as much as usual. , Was subjected to a corrosion test by immersion at a temperature of 80 ° C. for 96 hours. The immersion corrosion test is evaluated by the corrosion rate. If the corrosion rate is less than 0.17 mm / year, the corrosion resistance is excellent (◎), and if the corrosion rate is 0.17 to 0.30 mm / year, it is acceptable (○), 0.30 mm. When it exceeded / year, it was judged to be inferior (x). As a result, good corrosion resistance in alloys in which the characteristic formulas represented by Ni, Cr, Mo, Cu and N, which are effective alloy component elements for corrosive substances of sulfuric acid, hydrochloric acid and Cl ^{− ions, satisfy Ni + Cr + 3Mo + 5Cu + 25N ≧ 89.0.} Was found to be obtained.

As described above, it is considered that the alloy exhibiting excellent corrosion resistance has some action on the passivation film on the surface, and after the test piece after the corrosion test is taken out from the above corrosion test solution, it is immediately dissolved in distilled water. Was washed away, dried with cold air, and immediately after that, a detailed analysis of the alloy surface was performed using a Marcus-type high-frequency glow discharge emission surface analyzer (hereinafter, GDS). GDS is a device that performs element concentration profiling analysis in the depth direction by sputtering a sample with Ar plasma and causing the sputtered atoms to emit atoms, thereby measuring the element concentration profile from the surface after the corrosion test. As the measurement conditions, the excitation mode was pulse mode, the sputtering method was Ar sputtering, the sputtering pressure was 600 MPa, the sputtering output was 8.75 W, the pulse frequency was 100 Hz, and the analysis area was 4 mmφ. A GD-Profiler 2 manufactured by HORIBA, Ltd. was used as the GDS analyzer.

As mentioned earlier, the corrosion resistance of stainless steel is ensured by a passivation film composed of hydroxides of Fe and Cr and oxides of Cr. As a result of GDS analysis, a passivation film is applied on top of this passivation film. It was confirmed that there was another protective film mainly composed of Cu having a composition different from that of the above. That is, it was found that the film has a two-layer structure of a Cu-based protective film and a passivation film in this corrosive environment. It was considered that these films provided good corrosion resistance in a mixed solution of ^{acid and Cl − ions.} Further analysis was carried out by GDS to investigate the structure of the film in more detail. As a result, it was found that good corrosion resistance is exhibited when the two layers of the protective film mainly composed of Cu and the passivation film each have the following thickness and composition.

The Cu-based protective film has a thickness of 0.005 to 0.1 μm, and its composition is Cu: 45% or more, Ni: 10 to 30%, Cr: 5 to 20%, Fe: 10% or less. Including. The balance is composed of alloy components such as Mn and Si.

Next, the passivation film existing between the Cu-based protective film and the alloy base material has a thickness of 0.001 to 0.005 μm, and its composition is Cr: 40% or more, Ni: 10 to 40. %, Fe: 5 to 20%, O: 5 to 10%. The balance is composed of alloy components such as Cu, Mn, and Si.

The inventors inferred this mechanism as follows. First, it is considered that a general passivation film is present on the surface of the sample before the test as in stainless steel, but its structure is considered to be a hydroxide of Fe and Cr and an oxide of Cr, and it is thick. The size is said to be several nm. Next, since the pH of this test solution is 1 or less, it thermodynamically reaches the so-called depassivation pH range, but in reality, the passivation film is completely destroyed in this solution. It is considered that the passivation film is maintained while repeating dissolution and regeneration. This is because if the passivation film is not present at all, it is considered that the sample dissolves at a remarkable corrosion rate and does not retain the original shape of the test piece. In such a low pH solution, alloying elements such as Fe, Ni, Cr, Mo, and Cu are dissolved in the process of the so-called anodic reaction in which a metal is usually dissolved, and H ⁺ ions in the corrosive solution are reduced as a cathode reaction. Reactions occur mainly in pairs. In an extremely corrosive solution such as this test solution, the above-mentioned anodic reaction actively occurs, and when a certain amount of anodic reaction occurs, Cu ²⁺ ions accumulate in the solution, so that the cathode reaction is H ⁺ ions. From the reduction reaction of Cu ²⁺ ions, the reduction reaction of Cu 2+ ions is the main. Comparing the redox potentials of both, it can be inferred from the fact that the redox potential ^{of Cu 2+ exists at +0.337V with respect to the hydrogen electrode.} As a result, a protective film mainly composed of Cu was formed on the surface of the alloy, and it was presumed that it provided excellent corrosion resistance.

Further, as a result of subjecting pure Cu to the same corrosion test, it had better corrosion resistance than the steel of the present invention, but pure Cu is expensive and is completely unsuitable for structural materials in terms of strength. Although this film is mainly composed of Cu, Ni, Fe, and Cr were also present from the measurement results of GDS. The ^{redox potentials of Ni 2+} , Fe ²⁺ , and Cr ³⁺ are -0.257V, -0.447V, and -0.744V, respectively, and although they are all lower than the redox potential of ^{Cu 2+, they are contained in the film.} Since these elements are present in, it is considered that the reduction reaction of these ions actually occurs. Moreover, since the oxidation-reduction potential of ^{Fe 2+} is nobler than that of ^{Cr 3+, Fe 2+} is preferentially reduced than ^{Cr 3+,} although in the coating should Fe is concentrated from Cr, in the coating The Fe concentration was lower than the Cr concentration. As a mechanism for this, ^{even if Fe 2+} is reduced, the pH of the solution is 1 or less, so it is presumed that the reduced Fe is immediately redissolved.

That is, the Fe-Ni-Cr-Mo-Cu alloy of the present invention has C: 0.004 to 0.025%, Si: 0.02 to 1.0%, Mn: 0.03 in mass% below. ~ 0.35%, P: ≤0.040%, S: ≤0.003%, Ni: 30.0-37.0%, Cr: 22.0-25.0%, Mo: 7.0- It contains 9.0%, N: 0.15 to 0.30%, Cu: 2.5 to 4.0%, Al: 0.001 to 0.1%, and the balance is from Fe and unavoidable impurities. It is characterized by satisfying the following equation (1).
Ni + Cr + 3Mo + 5Cu + 25N ≧ 89.0… (1)

Further, in the Fe-Ni-Cr-Mo-Cu alloy of the present invention, in addition to the above components, Nb: 0.001 to 0.03%, V: 0.01 to 0.1%, B: 0. It may contain one or more selected from 0001 to 0.003% and Ti: 0.001 to 0.01%.

The Fe-Ni-Cr-Mo-Cu alloy of the present invention is characterized by having a film containing 45% or more of Cu on the surface of the alloy in addition to the composition of the above components.

The Fe-Ni-Cr-Mo-Cu alloy of the present invention is characterized in that the film containing 45% or more of Cu has a thickness of 0.005 to 0.1 μm.

Further, it is characterized by having a passivation film having a thickness of 0.001 to 0.005 μm made of Cr—Ni—Fe—O between this film and the alloy base material.

Further, the coating mainly containing Cu contains Cu ≧ 45%, Ni: 10 to 30%, Cr: 5 to 20%, Fe: 10% or less, and the balance is composed of alloy components such as Mn and Si. The passivation film contains Cr: 40% or more, Ni: 10 to 40%, Fe: 5 to 20%, O: 5 to 10%, and the balance is composed of alloy components such as Mn, Si, and Cu. It is a feature.

According to the present invention, since it is excellent in corrosion resistance, it can be suitably used as a material used in an environment where there is a concern that acid-induced total corrosion resistance may occur in an environmental pollution prevention device or various plants.

It is an element profile in the depth direction of an alloy having a protective film mainly composed of Cu and a passivation film.

Next, the composition components that the Fe-Ni-Cr-Mo-Cu alloy of the present invention should have will be described.
C: 0.004 to 0.025 mass%
C is an austenite phase stabilizing element. However, when added in a large amount, it combines with Cr, Mo and the like to form carbides, and the amount of solid solution Cr and solid solution Mo in the base material 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 a decrease in strength. Therefore, C is limited to 0.004 to 0.025 mass%. It is preferably 0.005 to 0.023 mass%, and more preferably 0.006 to 0.021 mass%.

Si: 0.02-1.0 mass%
Si is an element added as an antacid. Further, since Si is also an element that enhances the fluidity of molten steel and improves weldability, it is desirable to add 0.02 mass% or more. However, since Si is an element that promotes the precipitation of intermetallic compounds such as the σ phase and also increases the sensitivity to intergranular corrosion, it is set to 0.02 to 1.0 mass%. It is preferably 0.02 to 0.8 mass% or less, and more preferably 0.02 to 0.6 mass% or less.

Mn: 0.03 to 0.35 mass%
Since Mn is an element having a deoxidizing action, at least 0.03 mass% or more is required to obtain the effect. However, like Si, Mn also causes precipitation of intermetallic compounds such as the σ phase, so it is not preferable to add more than necessary. Therefore, it is necessary to set it to 0.03 to 0.35 mass%. It is 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 at the grain boundaries as a phosphide, which impairs hot workability and deteriorates overall corrosion resistance. Therefore, it is desirable to reduce it as much as possible. However, extremely reducing the P content leads to an increase in manufacturing cost. Therefore, in the present invention, P is limited to 0.040 mass% or less. It is 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 like P, and is easily segregated at grain boundaries, and is an element that significantly impairs hot workability and is harmful to corrosion resistance. is there. If it is contained in excess of 0.003 mass%, its harmfulness will be noticeable, so it is necessary to control it to 0.003 mass% or less. It is preferably 0.002 mass% or less, more preferably 0.001 mass% or less.

Ni: 30.0 to 37.0 mass%
Ni has the function of suppressing the precipitation of intermetallic compounds such as the σ phase, improving the overall corrosion resistance, and particularly reducing the dissolution rate of the alloy. If the content is less than 30.0 mass%, the precipitation of intermetallic compounds is promoted, while if it exceeds 37.0 mass%, the hot workability is deteriorated, the hot deformation resistance is increased, and the cost is increased. Therefore, the Ni content is 30.0 to 37.0 mass%. It is 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 that improves not only the overall corrosion resistance of the alloy but also the overall corrosion resistance such as pitting corrosion resistance, crevice corrosion resistance, and stress corrosion cracking resistance. In order to obtain the effect sufficiently, it is necessary to contain 22.0 mass% or more. However, if the content exceeds 25.0 mass%, precipitation of intermetallic compounds such as the σ phase is promoted and the corrosion resistance is deteriorated, so the content was set to 22.0 to 25.0 mass%. It is preferably 22.2 to 24.8 mass%, more preferably 22.4 to 24.6 mass%.

Mo: 7.0-9.0 mass%
Mo is also a useful element for improving overall corrosion resistance, pitting corrosion resistance and crevice corrosion resistance, and therefore requires a content of 7.0 mass% or more. However, excessive addition of Mo promotes the precipitation of intermetallic compounds such as the σ phase and lowers the corrosion resistance. Therefore, Mo is in the range of 7.0 to 9.0 mass%. It is preferably 7.2 to 8.5 mass%, more preferably 7.4 to 8.0 mass%.

Cu: 2.5-4.0 mass%
Cu is an element that is positively added because it is extremely effective in improving acid resistance, that is, improving overall corrosion resistance. In the present invention, a protective film mainly composed of Cu is formed on the surface of the alloy, and is an element that plays an extremely important role from the viewpoint of improving the total corrosion resistance. In order to obtain the effect, it is necessary to add 2.5 mass% or more. However, if it is contained in excess of 4.0 mass%, the denseness of the protective film mainly composed of Cu is impaired, and the corrosion resistance is rather lowered. Therefore, the amount of Cu added was set to 2.5 to 4.0 mass%. It is preferably 2.7 to 3.8 mass%, and more preferably 2.9 to 3.6 mass%.

N: 0.15 to 0.30 mass%
Like Cr, Mo, Ni, and Cu, N is a useful element for improving overall corrosion resistance, pitting corrosion resistance, and crevice corrosion resistance. In order to obtain the effect, it is necessary to add 0.15 mass% or more. However, if N is contained in excess of 0.30 mass%, nitride precipitation is caused, hot deformation resistance is extremely increased, and hot workability is impaired. Therefore, the N content is 0.15 to 0. It was set to 30 ass%. It is preferably 0.19 to 0.27 mass%, more preferably 0.20 to 0.26 mass%.

Al: 0.001 to 0.1 mass%
Al in the presence of _{CaO-SiO 2 -Al 2 O 3} -MgO-F slag to reduce S to promote desulfurization by deoxidation, positively added because it has an effect of improving the hot workability However, the addition of less than 0.001 mass% has no effect, and the addition of more than 0.1 mass% promotes the formation of huge inclusions that affect the aesthetics and corrosion resistance of the steel plate, and further N Since the precipitation of AlN, which is a compound of the above, becomes remarkable and the effect of N effective for corrosion resistance is reduced, the Al content is set to 0.001 to 0.1 mass%. It is preferably 0.001 to 0.07 mass%, more preferably 0.001 to 0.04 mass%.

Nb: 0.001 to 0.03 mass%
Nb precipitates at the grain boundaries as carbides of Nb, suppresses the precipitation of Cr carbides that occur when heat-affected by welding or the like, and prevents so-called sensitization. In order to obtain the effect, it is necessary to add 0.001 mass% or more. However, excessive addition promotes the precipitation of intermetallic compounds such as the σ phase and deteriorates the corrosion resistance, so the content was set to 0.001% to 0.03 mass%. It is preferably 0.003 to 0.028 mass%, more preferably 0.006 to 0.026 mass%.

V: 0.01 to 0.1 mass%
V is precipitated at the grain boundaries as a carbonitride of V, and prevents sensitization like Nb. In order to obtain the effect, it is necessary to add 0.01 mass% or more. However, excessive addition causes precipitation hardening and makes it difficult to process the material. Therefore, it was set to 0.01 to 0.1 mass%. It is preferably 0.02 to 0.09 mass%, more preferably 0.03 to 0.08 mass%.

B: 0.0001 to 0.003 mass%
B is effective in improving the hot workability, but if it is less than 0.0001 mass%, the effect is small, and if it exceeds 0.003 mass%, the hot workability is deteriorated. Therefore, B was set to 0.0001 to 0.003 mass%. It is preferably 0.0003 to 0.0027 mass%, more preferably 0.0006 to 0.0024 mass%.

Ti: 0.001-0.01 mass%
Ti precipitates at the grain boundaries as a carbide of Ti and prevents sensitization like Nb. However, excessive addition causes a large amount of carbides of Ti to precipitate on the welded portion when welding is performed, which causes corrosion called knife line attack. Therefore, Ti was set to 0.001 to 0.01 mass%. It is preferably 0.002 to 0.009 mass%, more preferably 0.003 to 0.008 mass%.

Ni + Cr + 3Mo + 5Cu + 25N ≧ 89.0 ... (Equation 1)
In order to improve the total corrosion resistance in an environment where acids and Cl ⁻ ions are mixed, it is effective to add Ni, Cr, Mo, Cu, and N to the alloy. As an action of each alloy element, Ni slows the dissolution rate due to corrosion of the alloy in a low pH environment, that is, in an acid solution. Cr forms a passivation film and is the source of the corrosion resistance of the alloy. Mo promotes the re-passivation of the passivation film by generating molybdate ions, and as an effect, slows the dissolution rate in the acid solution like Ni. N produces ammonia to neutralize the pH solution and slow down the rate of corrosion. That is, all the alloying elements contribute to the corrosion resistance as a synergistic effect, and if the sum of the equation (1) in which the contribution is weighted to each element satisfies 89.0 or more, sufficient corrosion resistance can be obtained. Therefore, it is necessary that Ni + Cr + 3Mo + 5Cu + 25N ≧ 89.0. It is preferably Ni + Cr + 3Mo + 5Cu + 25N ≧ 94.0, and more preferably Ni + Cr + 3Mo + 5Cu + 25N ≧ 98.0.

The Cu-based protective film has a thickness of 0.005 to 0.1 μm. As described above, in the Fe-Ni-Cr-Mo-Cu alloy of the present invention, Cu ²⁺ ions are mainly reduced in the acid solution. A protective film is formed on the surface of the alloy base material. If the thickness of the protective film is less than 0.005 μm, the thickness is insufficient and sufficient protection cannot be obtained. On the other hand, if the thickness exceeds 0.1 μm, the structure of the film becomes porous, the denseness is lost, and the protective property is impaired. Therefore, the Cu-based protective film needs to have a thickness of 0.005 to 0.1 μm. It is preferably 0.005 to 0.090 μm, more preferably 0.005 to 0.080 μm.

A passivation film with a thickness of 0.001 to 0.005 μm made of Cr-Ni-Fe-O is provided between the Cu-based film and the alloy base material. When the passivation film is exposed to a corrosive environment, the passivation film is formed. Corrosion gradually progresses while repeating dissolution and regeneration of itself, that is, repassivation. If the thickness is less than 0.001 μm, the dissolution rate is faster than the regeneration rate, so that the corrosion progress rate is high and sufficient corrosion resistance cannot be obtained. On the other hand, it is known that the thinner and denser the passivation film is, the better the corrosion resistance is given to the alloy. However, if it exceeds 0.005 μm, the denseness is impaired and good corrosion resistance cannot be obtained. Therefore, the passivation film needs to have a thickness of 0.001 to 0.005 μm, preferably 0.001 to 0.004 μm, and more preferably 0.001 to 0.003 μm.

Composition of Protective Film Mainly Cu The reason why the protective film mainly composed of Cu gives good corrosion resistance in an acid environment is that pure Cu has good corrosion resistance to acid. Therefore, it is desirable that the protective film is mainly composed of Cu, and the concentration thereof needs to be Cu ≧ 45%. Since Ni and Cr also ^{have corrosion resistance to acids and Cl −} ions, it is desirable to contain 10 to 30% and 5 to 20%, respectively. However, it is desirable that Fe that is easily dissolved in this environment is 10% or less, and the balance contains alloy components such as Mn and Si, but is preferably as small as possible.

Composition of passivation film When the alloy of the present invention is exposed to a corrosive environment, the passivation film has a function of ensuring corrosion resistance until the protective film mainly composed of Cu is formed, and the protective film mainly composed of Cu is destroyed. It has the function of ensuring corrosion resistance until it is regenerated. In order to sufficiently obtain the effect, it is desirable to contain 40% or more of Cr, and it is desirable that Ni, Fe, and O contain 10 to 40%, Fe: 5 to 20%, and O: 5 to 10%, respectively. .. The balance contains alloy components such as Cu, Mn, and Si, but it is desirable that the amount is as small as possible.

Definition of the thickness of the protective film mainly composed of Cu and the passivation film When the protective film mainly composed of Cu is elementally profiled from the surface layer in the depth direction by GDS, for example, as shown in FIG. 1, Cu is It is the profile that contains the most and gradually decreases in concentration along the depth direction. However, when it reaches a certain depth, a passivation film existing directly under the protective film of Cu is detected. Here, as a definition of the thickness of the protective film mainly composed of Cu, the depth from the surface layer to the depth at which Cu intersects the Cr of the passivation film is defined as the thickness of the protective film mainly composed of Cu.

Next, in the elemental profile of the passivation film, the Cr concentration is detected to be the highest, but the concentration immediately decreases and first intersects Ni, and then intersects Fe. After that, it becomes the profile of the base material component. As the definition of the thickness of the passivation film, Cr in the passivation film is Ni in the passivation film from the depth point where Cu in the protective film mainly composed of Cu intersects Cr in the passivation film. The thickness up to the intersection depth is defined as the thickness of the passivation film.

Definition of composition of protective film mainly composed of Cu and passivation film In the protective film mainly composed of Cu, the depth point where Cu is most concentrated on the outermost layer is defined as the composition of the protective film. Similarly, regarding the composition of the passivation film, the depth point at which the Cr concentration is highest is defined as the composition of the passivation 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, raw materials such as iron scrap, stainless scrap, ferronickel, and ferrochrome are melted in an electric furnace, and an AOD (Argon Oxygen Decarburization) furnace or a VOD (Rolling Oxygen Decarburization) furnace is used. After decarburizing and refining by blowing a mixed gas of oxygen and rare gas, rolling mill, Fe-Si alloy, Al, etc. are added to reduce Cr oxide in the slag, and then fluorite is added. _CaO-SiO 2 _-Al 2 O 3 to form a -MgO-F slag and deoxidation and desulfurization Te, continuous casting or ingot-making - the steel piece by slabbing method, then the steel strip, heat It is preferable to perform inter-rolling or cold-rolling to obtain various steel materials such as thin steel plates, thick steel plates, shaped steels, bar steels, and wire rods.

Raw materials prepared by adjusting scrap, ferrochrome, ferronick, stainless scrap, etc. to a predetermined ratio were melted in an electric furnace, and a mixed gas of oxygen and rare gas was blown in an AOD furnace or a VOD furnace for decarburization and refining. .. Thereafter, quicklime, Fe-Si alloy, after the addition of Al or the like reduction treatment of Cr oxides in the slag, a _{CaO-SiO 2 -Al 2 O 3} -MgO-F slag by addition of fluorite formation Then deoxidized and desulfurized. Then, it was made into a slab by a continuous casting method. After adjusting to various component compositions shown in Table 1, continuous casting was performed to obtain steel pieces (slabs). The composition of C and S shown in Table 1 is a carbon / sulfur simultaneous analyzer (combustion in oxygen stream-infrared absorption method), and the composition of N is an oxygen / nitrogen simultaneous analyzer (inert gas-impulse). The composition other than the above is the value analyzed by using the heat melting method) and by using fluorescent X-ray analysis.

Next, the steel piece (slab) was hot-rolled to 8 mm, and cold rolling, heat treatment, and pickling were repeated to produce a cold-rolled coil having a plate thickness of 1 to 3 mm. The final annealing temperature was 1150 ° C. for 1 minute. Corrosion test pieces having a width of 25 mm, a length of 50 mm, and a thickness of 2 to 3 mm were collected from the plate. The surface of the corrosion test piece was wet-polished with No. 400 water-resistant abrasive paper, washed with water, and then ultrasonically cleaned in a mixed solution of acetone and ethanol to be subjected to a corrosion test. In this dissolution, Fe-33% Ni-24% Cr-8% Mo-3% Cu-0.25% N was used as a basic component, and the amounts of Ni, Cr, Mo, Cu and N were variously changed.

Using the above corrosion test piece, adjust the sulfuric acid concentration to 50% and hydrochloric acid to 0.5% in a solution of artificial seawater (Aquamarin manufactured by Yashima Pure Chemicals Co., Ltd.) concentrated 3.5 times as much as usual. , Was subjected to a corrosion test by immersion at a temperature of 80 ° C. for 96 hours. The immersion corrosion test is evaluated by the corrosion rate. If the corrosion rate is less than 0.17 mm / year, the corrosion resistance is excellent (◎), and if the corrosion rate is 0.17 to 0.30 mm / year, it is acceptable (○), 0.3 mm. When it exceeded / year, it was judged to be inferior (x).

No. 1 shown in Table 1. The steel plates 1 to 13 are examples of inventions satisfying the conditions of the present invention, and have excellent corrosion resistance. In Table 1, numerical values that do not meet the scope of the present invention are shown in parentheses. In addition, No. For 3, 5 and 6, the numerical values relating to the protective film mainly containing Cu and the passivation film are shown in parentheses, but these examples are outside the scope of the dependent claims, which is a more preferable range. Although the corrosion rate is slightly higher, the scope of independent claim 1 is satisfied, and the corrosion rate of 0.30 mm / year or less, which is a practical level, is satisfied, and therefore, it is classified as an example of the present invention.

No. Since the steel sheet of No. 3 has a low Cu content of 2.50%, it is presumed that the thickness of the protective film mainly composed of Cu became thin and the concentration of Cu in the protective film mainly composed of Cu also decreased.

No. The steel sheet of No. 5 is characterized in that Cr is as low as 22.00%, so that it is difficult to form a passivation film, and Mn is as high as 0.35%. Since Mn has a property of easily dissolving the passivation film in the acid solution, it is presumed that the thickness of the passivation film is 0.0008 μm.

No. Since Mo of the steel sheet of No. 6 is as low as 7.00%, it is presumed that the effect of reimmobilization by molybdate ions could not be sufficiently obtained in this environment.

On the other hand, No. The steel plates 14 to 23 are comparative examples.
No. The steel sheet of No. 14 satisfies the equation (1), but has a low Ni content in the protective coating mainly composed of Cu and the passivation coating because the Ni content is low. Therefore, the dissolution rate in the acid solution is high and the corrosion resistance is inferior.
No. The steel sheet of No. 15 satisfies the equation (1), but has a low Cr concentration in the passivation film and is inferior in corrosion resistance because of its low Cr content.
No. The steel sheet of No. 16 satisfies the equation (1), but is inferior in corrosion resistance because the passivation film becomes thick due to the high Cr content and the denseness is impaired. Moreover, as a result of observing the metallographic structure, precipitation of the σ phase was observed due to the high Cr content. Precipitation of the σ phase is considered to be one of the causes of deterioration of corrosion resistance.
No. The steel sheet of No. 17 has a low Mo content and does not satisfy the equation (1), and the passivation power of the passivation film is low. Therefore, the Cr concentration in the passivation film is low and the corrosion resistance is inferior. Further, since the content of low Cu is low, the concentration of Cu in the protective film mainly composed of Cu is low, and the thickness is also insufficient.
No. The steel sheet of No. 18 satisfied the equation (1), but the precipitation of the σ phase was observed due to the high Mo content. As a result, it is inferior in corrosion resistance.
No. The steel sheet of 19 has a low Cu content and does not satisfy the equation (1). Further, since it contains low Cu, the Cu concentration in the protective film mainly composed of Cu is low, and the thickness is also insufficient. Therefore, it is inferior in corrosion resistance.
No. The steel of No. 20 satisfies the equation (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 precision is lost and the corrosion resistance is inferior.
No. The steel sheet of No. 21 has a low N content and does not satisfy the equation (1). Further, since the content is low N, the σ phase is precipitated and the corrosion resistance is inferior. It is considered that the precipitation of the σ phase caused insufficient thickness of the passivation film and insufficient Cr concentration, and increased the dissolution rate.
No. The steel sheet of No. 22 satisfied the equation (1), but precipitation of Cr-Mo-N-based nitride was observed due to the high N concentration. As a result, N, which greatly contributes to corrosion resistance, does not work effectively and is inferior in corrosion resistance.
No. The steel sheet of No. 23 satisfied the equation (1), but precipitation of Al—N was observed due to the high Al content. As a result, N, which greatly contributes to corrosion resistance, does not work effectively and is inferior in corrosion resistance.

Since the Fe-Ni-Cr-Mo-Cu alloy of the present invention has excellent corrosion resistance, it can be suitably used in an environment where total corrosion in which ^{acid and Cl-ion are mixed causes corrosion.}

Further, the coating mainly containing Cu contains Cu ≧ 45%, Ni: 10 to 30%, Cr: 5 to 20%, Fe: 10% or less, and the balance contains Mn and Si as unavoidable impurities . The passivation film is characterized by containing Cr: 40% or more, Ni: 10 to 40%, Fe: 5 to 20%, O: 5 to 10%, and the balance containing Mn, Si, and Cu as unavoidable impurities. And.

Claims (6)

Below by mass%
C: 0.004 to 0.025%,
Si: 0.02-1.0%,
Mn: 0.03 to 0.35%,
P: ≤0.040%,
S: ≤0.003%,
Ni: 30.0 to 37.0%,
Cr: 22.0 to 25.0%,
Mo: 7.0-9.0%,
N: 0.15 to 0.30%,
Cu: 2.5-4.0%,
An Fe-Ni-Cr-Mo-Cu alloy characterized by having Al: 0.001 to 0.1% and the balance consisting of Fe and unavoidable impurities, and satisfying the following equation (1).
Ni + Cr + 3Mo + 5Cu + 25N ≧ 89.0… (1) In addition to the above components, Nb: 0.001 to 0.03%, V: 0.01 to 0.1%, B: 0.0001 to 0.003%, Ti: 0.001 to 0.01%. The Fe-Ni-Cr-Mo-Cu alloy according to claim 1, which contains one or more selected from the above. The Fe-Ni-Cr-Mo-Cu alloy according to claim 1 or 2, wherein the alloy surface has a film containing 45% or more of Cu. The Fe-Ni-Cr-Mo-Cu alloy according to claim 3, wherein the film has a thickness of 0.005 to 0.1 μm. The Fe-Ni according to claim 3 or 4, wherein a passivation film having a thickness of 0.001 to 0.005 μm made of Cr—Ni—Fe—O is provided between the film and the alloy base material. -Cr-Mo-Cu alloy. The film containing 45% or more of Cu contains Ni: 10 to 30%, Cr: 5 to 20%, Fe: 10% or less in addition to Cu, and the balance contains a small amount of alloy components such as Mn and Si. And
The passivation film contains Cr: 40% or more, Ni: 10 to 40%, Fe: 5 to 20%, O: 5 to 10%, and the balance contains a small amount of alloy components such as Mn, Si, and Cu. The Fe-Ni-Cr-Mo-Cu alloy according to claim 5, wherein the Fe-Ni-Cr-Mo-Cu alloy is used.

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