JP6750082B1 - Fe-Ni-Cr-Mo-Cu alloy with excellent corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Fe-Ni-Cr-Mo-Cu alloy having excellent corrosion resistance even 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. I will provide a. SOLUTION: In the following mass%, C: 0.004 to 0.025%, Si: 0.02 to 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 to 9.0%, N: 0.15 to 0. 30%, Cu: 2.5 to 4.0%, Al: 0.001 to 0.1%, and the balance consists of Fe and unavoidable impurities, and Fe-Ni-Cr- satisfying the following equation (1). Mo-Cu alloy. Ni + Cr + 3Mo + 5Cu + 25N ≧ 89.0 ... (1) [Selection diagram] Fig. 1

Description

TECHNICAL FIELD The present invention relates to an Fe—Ni—Cr—Mo—Cu alloy used in environments such as chemical plants and waste liquid treatment facilities where extremely excellent corrosion resistance is required.

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

Particularly in recent years, the standard values have become stricter with respect to exhaust gas from automobiles, engines for ships, and exhaust gas from power plants, and the emission of environmental pollutants such as SO _x must be properly treated. On the other hand, the use of inferior fuel containing a large amount of impurities, which are the source of environmental pollutants, is also expanding, and addressing environmental issues is an urgent task. In response to such trends, technological development of exhaust gas treatment equipment has progressed, and technology for detoxifying exhaust gas by spraying alkaline solution, seawater, or a mixed solution thereof as an absorbing 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 a highly corrosive solution in which H ⁺ ions and Cl ⁻ ions are concentrated. Further, conventionally, in an exhaust gas treatment apparatus, sulfuric acid is generated due to SO _x contained in the exhaust gas, so that corrosion generally called sulfuric acid dew point corrosion occurs, and an alloy having increased sulfuric acid resistance has been developed so far. ing.

However, in recent years, due to the expansion of use of poor fuel and the use of seawater as an absorbing solution, not only a simple sulfuric acid environment but also Cl ⁻ ions are contained, so _{that the} corrosive environment becomes more severe. Cl Conventionally ^- ions have the property of destroying the passive film corrosion resistant alloy such as stainless steel, it has been well known for generating corrosion, Cl sulfate ^- the ion is mixed, the passivation film Since the property of breaking iron and the dissolution by acid act as a synergistic effect, and general corrosion progresses at an accelerated rate, further improvement of general corrosion resistance and development of new alloys are required.

In the above situation, 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 made on alloys containing more Cu. As for the assumed environment, only the environment with a low sulfuric acid concentration of 10% has been studied. Moreover, since the alloy serving as the base is SUS304, it is considered that the corrosion resistance is insufficient in the first place in the recent severe corrosive environment.

On the other hand, in Patent Document 2, the addition amount of Cu 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%. .. When the sulfuric acid solution has a high concentration of 80% or more, the liquid property changes from non-oxidizing property to oxidizing property and corrosive property is lowered, so that the corrosion rate is lowered. That is, it can be said that the corrosive environment becomes mild. Therefore, assuming a recent environment pollution control device, it is considered that the use of 98% sulfuric acid does not simulate the most severe corrosive environment. Therefore, 45 to 65% sulfuric acid having the most severe corrosive environment is used. It should be considered.

In Patent Document 3 as well, the amount of Cu added to the alloy has been studied up to a very high concentration of 4.5%, but the sulfuric acid concentration is not described at all, and 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 Cu addition amount of the alloy is studied 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 done. However, the alloy under consideration is a state in which a heat treatment simulating brazing heat treatment in which harmful precipitates such as σ phase are taken into consideration is assumed on the assumption of a member to which the alloy is applied, and a state of an annealed metal structure. Has not been examined at all. Furthermore, the corrosion form takes into consideration pitting corrosion, not general corrosion.

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

JP-A-1-104748 JP-A-56-93860 JP-A-4-346638 JP, 2018-172709, A JP, 2010-31313, A

Introduction to Stainless Steel 2015 (published by the Stainless Association, published by the Stainless Association “Introduction to Stainless Steel” Revised Committee, p. 64)

As described above, in environmental pollution control devices that are becoming increasingly severe in recent years, it is considered that the corrosion resistance of the above alloys is insufficient, or the considered corrosion mode is different, and has excellent general corrosion resistance. Development of materials is desired.

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

The inventors have conducted extensive studies to solve the above problems. It has been known so far that the addition of Cu to a corrosion-resistant alloy such as stainless steel improves the acid resistance, particularly the general corrosion resistance, but it was not sufficient to clarify the mechanism. The inventors considered that the reason for the alloy having excellent general corrosion resistance is the alloy surface that is in direct contact with the acid. Although stainless steel has excellent corrosion resistance secured by the passivation film on the surface of the alloy, according to Non-Patent Document 1, the composition is considered to be composed of Fe and Cr hydroxides and Cr oxides. ing. On the other hand, it is speculated that the reason why the acid resistance of stainless steel is improved by adding Cu is that the composition, structure or morphology of the passive film is changed.

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

As a preliminary experiment, the inventors have found that Fe-25%Ni-23%Cr-6%Mo-2.7%Cu-0.20%N based alloys are 5, 10, 20, An immersion test was carried out using 40, 60, 80 and 98% sulfuric acid at 20, 40, 60, 80, 100°C and boiling points of each concentration, and an isocorrosion line showing the corrosion rate in the relationship between so-called temperature and sulfuric acid concentration. I made a diagram. As a result, it was found that the corrosion rate was the highest in the sulfuric acid concentration range of 45 to 65%, that is, the corrosion environment was the severest. Regarding the temperature, the higher the temperature, the higher the corrosion rate. As has been conventionally known, it has been confirmed that 80% or more high-concentration sulfuric acid changes its liquidity from non-oxidizing to oxidizing, so that its corrosiveness decreases.

Next, the raw material was melted in an electric furnace, and the 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 molten steel was continuously cast into a slab (slab) and hot forged into a forged plate having a thickness of 8 mm. After that, annealing and pickling were performed, cold rolling was further performed to a thickness of 2 to 3 mm, annealing and pickling were performed, and a cold rolled sheet was produced. The final annealing temperature was 1150° C. for 1 minute. A 25 mm x 50 mm corrosion test piece was sampled from this cold rolled plate, the surface was wet-polished with No. 400 water-resistant abrasive paper, and ultrasonic cleaning was performed in a solution of acetone and ethanol mixed after water washing for corrosion test. I served. In this melting, the alloy element concentrations of Ni, Cr, Mo, Cu and N were variously changed.

Using the above corrosion test piece, a solution prepared by concentrating artificial seawater (Aquamarine manufactured by Yasu Chemical Co., Ltd.) to 3.5 times the normal concentration was adjusted so that the sulfuric acid concentration was 50% and the hydrochloric acid was 0.5%. It 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 0.17 to 0.30 mm/year, acceptable (∘), 0.30 mm When it exceeded /year, it was judged as poor (x). As a result, good corrosion resistance is achieved in alloys whose characteristic formulas represented by Ni, Cr, Mo, Cu and N, which are alloy component elements effective for corrosive substances of sulfuric acid, hydrochloric acid and Cl ⁻ ions, satisfy Ni+Cr+3Mo+5Cu+25N≧89.0. It was found that

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 taking out the test piece after the corrosion test from the corrosion test solution, immediately dissolve the solution in distilled water. Was washed off, dried with cold air, and immediately after that, a detailed analysis of the alloy surface was performed by a Marcus type high frequency glow discharge emission surface analyzer (hereinafter referred to as GDS). The GDS is an apparatus for performing element concentration profiling analysis in the depth direction by sputtering a sample with Ar plasma and causing the sputtered atoms to emit atomic light. With this, the element concentration profile from the surface after the corrosion test was measured. As the measurement conditions, the excitation mode was a 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. As the GDS analyzer, GD-Profiler2 manufactured by Horiba Ltd. was used.

As mentioned above, the corrosion resistance of stainless steel is ensured by the passivation film composed of Fe and Cr hydroxides and Cr oxides. As a result of the GDS analysis, the passivation film is formed on this passivation film. The existence of another protective film mainly composed of Cu having a different composition was confirmed. That is, it was found that in this corrosive environment, the film has a two-layer structure of a Cu-based protective film and a passive film. It was considered that these films give good corrosion resistance in a mixed solution of acid and Cl ⁻ ions. In order to investigate the structure of the film in more detail, further analysis was conducted by GDS. As a result, it has been found that good corrosion resistance is exhibited when the two-layer coating consisting of a protective coating mainly composed of Cu and a passive coating has 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 consists 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: 10-40. %, Fe: 5 to 20%, O: 5 to 10%. The balance consists of alloy components such as Cu, Mn and Si.

The inventors have speculated that this mechanism is as follows. First, it is considered that a general passivation film is present on the surface of the sample before the test as in the case of stainless steel, but its structure is assumed to be a hydroxide of Fe and Cr and an oxide of Cr. The length is set to several nm. Next, since the pH of this test solution is 1 or less, it has reached a so-called depassivation pH region thermodynamically, but actually, the passivation film is completely destroyed in this solution. However, it is considered that the passive film is maintained even after repeated 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 usually dissolved during the so-called anodic reaction in which the metal is dissolved, and H ⁺ ions in the corrosion solution are reduced as a cathode reaction. Reactions mainly occur in pairs. Is extremely corrosive strong solution as in the present test solution occurs actively anode reaction described above, to come to accumulate Cu ²⁺ ions in the solution when the place is some degree of anode reaction and the cathode reaction is H ⁺ ions The main reaction is the reduction reaction of Cu ²⁺ ions. It can be inferred from the fact that the redox potentials of Cu ²⁺ exist at +0.337 V with reference to the hydrogen electrode when the redox potentials of the both are compared. As a result, it was speculated that a protective film mainly composed of Cu was formed on the surface of the alloy, giving 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 high in cost and completely unsuitable for a structural material from the viewpoint of strength. Although this film was mainly composed of Cu, Ni, Fe and Cr were also present from the GDS measurement results. The redox potentials of Ni ²⁺ , Fe ²⁺ , and Cr ³⁺ are −0.257 V, −0.447 V, and −0.744 V, respectively, and although they are all lower than the redox potential of Cu ²⁺ , they are in the film. It is considered that the reduction reaction of these ions actually occurs due to the presence of these elements in. 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, it was assumed that even if Fe ²⁺ was reduced, the pH of the solution was 1 or less, so that the reduced Fe was immediately redissolved.

That is, in the Fe-Ni-Cr-Mo-Cu alloy of the present invention, 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- 9.0%, N: 0.15 to 0.30%, Cu: 2.5 to 4.0%, Al: 0.001 to 0.1%, with the balance being Fe and unavoidable impurities. And satisfies the following formula (1).
Ni+Cr+3Mo+5Cu+25N≧89.0 (1)

Further, the Fe-Ni-Cr-Mo-Cu alloy of the present invention has Nb: 0.001 to 0.03%, V: 0.01 to 0.1%, B: 0. One or two or more selected from 0001 to 0.003% and Ti: 0.001 to 0.01% may be contained.

The Fe-Ni-Cr-Mo-Cu alloy of the present invention is characterized by having a coating 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 in that a passivation film made of Cr-Ni-Fe-O having a thickness of 0.001 to 0.005 μm is provided between the film and the alloy base material.

Further, the film mainly composed of Cu contains Cu≧45%, Ni:10 to 30%, Cr:5 to 20% and Fe:10% or less, and the balance contains Mn and Si as unavoidable impurities . The passive film contains Cr: 40% or more, Ni: 10-40%, Fe: 5-20%, O: 5-10%, and the balance contains Mn, Si, Cu as unavoidable impurities. And

INDUSTRIAL APPLICABILITY According to the present invention, since it has excellent corrosion resistance, it can be suitably used as a material used in an environment pollution control device, various plants, etc. in an environment where there is a concern that general acid corrosion resistance will occur.

3 is an elemental profile in the depth direction of an alloy having a Cu-based protective film and a passive 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 it is added in a large amount, it combines with Cr and Mo to form a carbide, 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 the strength from decreasing. 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 a deoxidizer. Further, Si is an element that enhances the fluidity of molten steel and improves weldability, so addition of 0.02 mass% or more is desirable. However, Si is an element that promotes precipitation of intermetallic compounds such as σ phase and also increases intergranular corrosion susceptibility, so the content is made 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 necessary to obtain the effect. However, Mn causes precipitation of an intermetallic compound such as a σ phase like Si, so addition of Mn more than necessary is not preferable. Therefore, it is necessary to set the content 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 as a phosphide at the grain boundaries to impair hot workability and deteriorate overall corrosion resistance. Therefore, it is desirable to reduce it as much as possible. However, extremely reducing the P content causes 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, more preferably 0.020 mass% or less.

S: 0.003 mass% or less S is an element that is unavoidably mixed as an impurity like P, is easily segregated at the grain boundaries, and particularly impairs hot workability, and is an element harmful to corrosion resistance. is there. If it is contained in excess of 0.003 mass%, the harmfulness thereof becomes remarkable, so it is necessary to control the content 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 a function of suppressing precipitation of intermetallic compounds such as σ phase, improving general corrosion resistance, and particularly decreasing the dissolution rate of the alloy. When the content is less than 30.0 mass%, precipitation of intermetallic compounds is promoted, while when it exceeds 37.0 mass%, deterioration of hot workability, increase of hot deformation resistance, and further increase of cost are caused. 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 general corrosion resistance of the alloy but also overall corrosion resistance such as pitting corrosion resistance, crevice corrosion resistance and stress corrosion cracking resistance. In order to sufficiently obtain the effect, it is necessary to contain 22.0 mass% or more. However, if the content exceeds 25.0 mass %, precipitation of intermetallic compounds such as σ phase is promoted and the corrosion resistance is rather deteriorated. Therefore, it is 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 an element that is useful for improving general corrosion resistance and pitting corrosion resistance and crevice corrosion resistance, so a content of 7.0 mass% or more is required. However, excessive addition of Mo promotes the precipitation of intermetallic compounds such as σ phase and reduces the corrosion resistance. Therefore, Mo is set 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 general 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 general corrosion resistance. In order to obtain the effect, it is necessary to add 2.5 mass% or more. However, if the content exceeds 4.0 mass%, the denseness of the protective film mainly composed of Cu is impaired and the corrosion resistance is rather deteriorated. Therefore, the addition amount of Cu is set to 2.5 to 4.0 mass %. It is preferably 2.7 to 3.8 mass%, more preferably 2.9 to 3.6 mass%.

N: 0.15 to 0.30 mass%
N, like Cr, Mo, Ni and Cu, is an element useful for improving general 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, when N is contained in an amount of more than 0.30 mass %, precipitation of nitrides is caused, hot deformation resistance is extremely increased, and hot workability is impaired. Therefore, the content of N 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%
In the presence of CaO—SiO ₂ —Al ₂ O ₃ —MgO—F slag, Al has the effect of promoting desulfurization by deoxidation to reduce S and improving hot workability, and therefore is actively added. However, addition of less than 0.001 mass% has no effect, and addition of more than 0.1 mass% promotes formation of large inclusions that affect the aesthetics and corrosion resistance of the steel sheet, and further N The precipitation of AlN, which is a compound of, becomes remarkable, and the effect of N, which is effective in corrosion resistance, is reduced, so the Al content was made 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 is precipitated as a carbide of Nb at the crystal grain boundaries and suppresses the precipitation of Cr carbide that occurs when it is thermally affected by welding or the like, and prevents so-called sensitization. In order to obtain the effect, addition of 0.001 mass% or more is necessary. However, excessive addition promotes precipitation of intermetallic compounds such as σ phase and deteriorates corrosion resistance, so the content was made 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 precipitates as a carbonitride of V at the crystal grain boundary 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 is 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 deteriorates. Therefore, B is 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 to 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 Ti carbide to precipitate in the welded portion during welding, which causes corrosion called knife line attack. Therefore, Ti is 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 general corrosion resistance in an environment in which acid and Cl ⁻ ions are mixed, it is effective to add Ni, Cr, Mo, Cu and N to the alloy. As a function of each alloying element, Ni delays the dissolution rate due to corrosion of the alloy in a low pH environment, that is, in an acid solution. Cr forms a passive film and is the source of the corrosion resistance of the alloy. Mo promotes repassivation of the passivation film by generating molybdate ions, and its effect is to delay the dissolution rate in an acid solution like Ni. N produces ammonia to neutralize the pH solution and slow the rate of corrosion. That is, any alloy element contributes to the corrosion resistance as a synergistic effect, and if the sum of the equation (1) in which the degree of 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. 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 and the denseness is lost and conversely the protective property is impaired. Therefore, the Cu-based protective film is required to have a thickness of 0.005 to 0.1 μm. The thickness is preferably 0.005 to 0.090 μm, more preferably 0.005 to 0.080 μm.

Between the Cu-based coating and the alloy base material, there should be a passivation coating made of Cr-Ni-Fe-O with a thickness of 0.001 to 0.005 µm. Passivation coating when the passivation coating is exposed to a corrosive environment. Corrosion gradually progresses by repeating dissolution and regeneration of itself, that is, repassivation. When the thickness is less than 0.001 μm, the rate of dissolution is faster than the rate of regeneration, so that the rate of progress of corrosion is fast 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 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 Cu-Based Protective Film The reason why the Cu-based protective film gives good corrosion resistance in an acid environment is that pure Cu has good corrosion resistance to acids. Therefore, the protective film is preferably composed mainly 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, Fe, which is easily dissolved in this environment, is preferably 10% or less, and the balance contains alloy components such as Mn and Si, but is preferably as small as possible.

Composition of Passive Film When the alloy of the present invention is exposed to a corrosive environment, the passive film has a function of ensuring corrosion resistance until the protective film mainly composed of Cu is produced 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 action, it is desirable to contain Cr in an amount of 40% or more, 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 thickness of protective film mainly composed of Cu and passivation film When an element profile of a protective film mainly composed of Cu is taken in the depth direction from the surface layer by GDS, for example, as shown in FIG. The profile is the most contained, and the concentration gradually decreases along the depth direction. However, when the depth reaches a certain level, the passivation film existing immediately below the protective film of Cu is detected. Here, as the definition of the thickness of the protective film mainly composed of Cu, the thickness of the protective film mainly composed of Cu is defined up to the depth where Cu from the surface layer intersects Cr of the passive film.

Next, the elemental profile of the passivation film is detected with the highest Cr concentration, but the concentration immediately decreases and first intersects with Ni and then with Fe. After that, the profile of the base material component is obtained. As a definition of the thickness of the passivation film, from the depth point where Cu in the above-mentioned Cu-based protective film intersects with Cr in the passivation film, Cr in the passivation film becomes Ni in the passivation film. The thickness of the passive film is defined up to the intersecting depth point.

Definition of composition of protective film mainly composed of Cu and passivation film In a protective film mainly composed of Cu, a depth point where Cu is most concentrated in the outermost layer is defined as a composition of the protective film. Similarly, regarding the composition of the passive film, the depth point where the Cr concentration is 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.
The Fe-Ni-Cr-Mo-Cu alloy of the present invention melts raw materials such as iron scrap, stainless scrap, ferro-nickel, and ferrochrome in an electric furnace, and then uses an AOD (Argon Oxygen Decarburization) furnace or a VOD (Vacuum Oxygen Decarburization) furnace. At that, a mixed gas of oxygen and a noble gas is blown to decarburize and refine, and quick lime, Fe-Si alloy, Al, etc. are added to reduce the Cr oxide in the slag, and then fluorite is added. CaO—SiO ₂ —Al ₂ O ₃ —MgO—F slag is formed, deoxidized and desulfurized, and made into a steel piece by a continuous casting method or an ingot-slump rolling method, and then the steel piece is heat-treated. It is preferable to carry out hot rolling or cold rolling to obtain various steel materials such as thin steel plate, thick steel plate, shaped steel, bar steel and wire rod.

A raw material prepared by adjusting scrap, ferrochrome, ferronickel, stainless scrap, etc. to a predetermined ratio is melted in an electric furnace and decarburized by blowing a mixed gas of oxygen and a rare gas in an AOD furnace or a VOD furnace. .. 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 And deoxidized and desulfurized. Then, it was made into a slab by the continuous casting method. After adjusting to the various component compositions shown in Table 1, it was continuously cast into a steel slab (slab). The composition of C and S shown in Table 1 is obtained by using a carbon/sulfur simultaneous analysis apparatus (combustion in an oxygen stream-infrared absorption method), and the composition of N is determined by an oxygen/nitrogen simultaneous analysis apparatus (inert gas-impulse). The composition other than the above is the value analyzed by fluorescent X-ray analysis.

Then, the steel slab (slab) was hot-rolled to 8 mm, and cold rolling, heat treatment and pickling were repeated to manufacture a cold-rolled coil having a plate thickness of 1 to 3 mm. The final annealing temperature was 1150° C. for 1 minute. A corrosion test piece having a width of 25 mm, a length of 50 mm and a thickness of 2 to 3 mm was sampled from the plate. The surface of the corrosion test piece was wet-polished with water-resistant abrasive paper No. 400, and then ultrasonic cleaning was performed in a solution of acetone and ethanol mixed after water washing and subjected to the 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, a solution prepared by concentrating artificial seawater (Aquamarine manufactured by Yasu Chemical Co., Ltd.) to 3.5 times the normal concentration was adjusted so that the sulfuric acid concentration was 50% and the hydrochloric acid was 0.5%. It 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 0.17 to 0.30 mm/year, acceptable (∘), 0.3 mm. When it exceeded /year, it was judged as poor (x).

No. shown in Table 1 The steel sheets 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 satisfy the scope of the present invention are shown in parentheses. In addition, No. Regarding 3, 5, and 6, the numerical values for the protective film and the passivation film mainly containing Cu are written in parentheses, but these examples are out of the scope of the dependent claims which are more preferable ranges, Although the corrosion rate is slightly high, the range of independent claim 1 is satisfied, and since the corrosion rate of 0.30 mm/year or less which is a practical level is satisfied, it is classified as an example of the present invention.

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

No. The steel sheet of No. 5 is characterized by having a low Cr content of 22.00%, which makes it difficult to form a passivation film, and has a high Mn content of 0.35%. It is presumed that the thickness of the passivation film was 0.0008 μm because Mn has a property of easily dissolving the passivation film in the acid solution.

No. It is speculated that since the steel sheet of No. 6 had a low Mo content of 7.00%, the effect of repassivation by molybdate ions was not sufficiently obtained in this environment.

On the other hand, No. Steel plates 14 to 23 are comparative examples.
No. The steel plate of No. 14 satisfies the formula (1), but since the Ni content is low, the Ni content in the protective film mainly containing Cu and the passivation film is low. Therefore, the dissolution rate in the acid solution is high and the corrosion resistance is poor.
No. The steel plate of No. 15 satisfies the formula (1), but since the Cr content is low, the Cr concentration in the passivation film is low and the corrosion resistance is poor.
No. The steel plate of No. 16 satisfies the formula (1), but the Cr content is high, so that the passivation film becomes thick and the denseness is impaired, resulting in poor corrosion resistance. Further, as a result of observing the metal structure, the precipitation of σ phase was recognized because of the high Cr content. It is considered that the precipitation of the σ phase is one of the factors that deteriorate the corrosion resistance.
No. The steel sheet of No. 17 has a low Mo content and does not satisfy the formula (1), and the repassivation force of the passivation film is low. Therefore, the Cr concentration in the passivation film is low and the corrosion resistance is poor. Further, because of the low Cu content, the Cu concentration in the protective film mainly composed of Cu is low, and the thickness is also insufficient.
No. The steel plates of No. 18 satisfied the formula (1), but the precipitation of σ phase was recognized because of the high Mo content. As a result, the corrosion resistance is poor.
No. The steel plate of No. 19 has a low Cu content and does not satisfy the formula (1). Further, since the Cu content is low, the Cu concentration in the protective film mainly composed of Cu is low and the thickness is insufficient. Therefore, it is inferior in corrosion resistance.
No. The steel No. 20 satisfies the formula (1), but since the Cu content is high, the protective film mainly containing Cu was too thick and the structure was porous. Therefore, the denseness is lost and the corrosion resistance is poor.
No. The steel plate of No. 21 has a low N content and does not satisfy the formula (1). Further, since the content of N is low, the σ phase is precipitated and the corrosion resistance is poor. It is considered that the precipitation of the σ phase caused the thickness of the passivation film to be insufficient and the Cr concentration to be insufficient, thereby increasing the dissolution rate.
No. The steel plate of No. 22 satisfied the formula (1), but the precipitation of Cr-Mo-N-based nitride was observed because the N concentration was high. As a result, N, which greatly contributes to the corrosion resistance, does not act effectively and the corrosion resistance is poor.
No. The steel plate of No. 23 satisfied the formula (1), but the precipitation of Al-N was recognized because of the high Al content. As a result, N, which greatly contributes to the corrosion resistance, does not act effectively and the 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 in an environment in which general corrosion in which acid and Cl ^- ions are mixed causes corrosion.

Claims (6)

In mass% below,
C: 0.004 to 0.025%,
Si: 0.02-1.0%,
Mn: 0.03 to 0.35%,
P: ≤0.040%,
S: ≤ 0.003%,
Ni: 30.0-37.0%,
Cr: 22.0 to 25.0%,
Mo: 7.0-9.0%,
N: 0.15 to 0.30%,
Cu: 2.5-4.0%,
Al: 0.001-0.1%, the balance consisting of Fe and unavoidable impurities, and satisfying the following formula (1): Fe-Ni-Cr-Mo-Cu alloy.
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, containing one or more selected from the above. The Fe-Ni-Cr-Mo-Cu alloy according to claim 1 or 2, wherein a film containing 45% or more of Cu is provided on the surface of the alloy. The Fe-Ni-Cr-Mo-Cu alloy according to claim 3, wherein the coating has a thickness of 0.005 to 0.1 µm. The Fe-Ni according to claim 3 or 4, wherein a passivation film made of Cr-Ni-Fe-O having a thickness of 0.001 to 0.005 µm is provided between the film and the alloy base material. -Cr-Mo-Cu alloy. The coating 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 Mn and Si as unavoidable impurities ,
The passivation film contains Cr: 40% or more, Ni: 10-40%, Fe: 5-20%, O: 5-10%, and the balance contains Mn, Si, Cu as unavoidable impurities. The Fe-Ni-Cr-Mo-Cu alloy according to claim 5.

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