JP2021080523A - Fe-based alloy - Google Patents

Abstract

To provide an Fe-based alloy having excellent corrosion resistance in a high-temperature steam environment containing sulfur oxide.SOLUTION: An Fe-based alloy comprises, in mass%, C: 0.04-0.15%, Si: 2.5-4.5%, Mn: 0.5-1.0%, P: 0.10% or less, S: 0.010% or less, Ni: 30.0-35.0%, Cr: 18.0-23.0%, Ti: 0.05-0.30%, Al: 0.02-0.10%, B: 0.02-0.08%, N: 0.01-0.03%, REM: 0.002-0.01%, Cu: 0-0.5%, Mo: 0-0.35%, Mg: 0-0.005%, Ca: 0-0.003%, with the balance being Fe and impurities, and satisfies [Si/14-(Al/18+TiSO+MnSO)≥0.165].SELECTED DRAWING: None

Description

The present invention relates to Fe-based alloys.

For the realization of a hydrogen society in the future, it is necessary to develop a process for producing hydrogen using clean energy that does not depend on fossil fuels, that is, does not generate carbon dioxide. Examples of the energy source include renewable energy such as solar power or wind power, and nuclear energy. Among them, nuclear energy is considered to have the best energy quantity, energy density, and supply stability. In a light water reactor, hydrogen is produced by a method such as electrolysis using the electric energy once taken out. Further, in recent years, a high-temperature gas reactor in which a core is cooled by helium gas and heat exchange is performed to extract high-temperature heat has attracted attention.

The high-temperature heat generated in the high-temperature gas reactor can be used not only for power generation but also for direct hydrogen production. As a method for producing hydrogen, an IS process incorporating thermal decomposition of sulfuric acid and hydrogen iodide, high temperature water electrolysis (steam electrolysis), and the like are considered. The IS process is also called chemical hydrogen production and is suitable for mass production of hydrogen. In principle, it is as follows, and includes decomposition of high-temperature sulfuric acid and hydrogen iodide, and these reactions proceed with a catalyst such as platinum.

H ₂ SO ₄ → SO ₃ + H ₂ O (pyrolysis of sulfuric acid: 300 degrees or more)
SO ₃ → SO ₂ + 1 / 2O ₂ ( _{decomposition of SO 3} : ≧ 850 ° C)
2HI → H ₂ + I ₂ (decomposition of hydrogen iodide: 400 ° C)
SO ₂ + I ₂ + 2H ₂ O → 2HI + H ₂ SO ₄ (Bunsen reaction: 200 ° C)
H ₂ O → H ₂ + 1 / 2O ₂ (overall reaction)

The main issues of this process are improvement of corrosion resistance of materials used for equipment / plant and improvement of catalyst performance. Since a high-temperature strong acid is used for the reaction, the corrosion or deterioration of equipment and pipes becomes serious. In particular, the sulfate-decomposed part is exposed to an extremely strong corrosive environment, so corrosion is unavoidable with ordinary metal materials. Regarding the process using a catalyst, the activity and durability of the catalyst itself are insufficient, and along with the development of a catalyst material, chemical engineering efforts to improve the contact efficiency of the gas phase or the liquid phase are being promoted.

Patent Document 1 discloses a stainless steel pipe having caulking resistance and carburizing resistance, which is provided with a Cr-deficient layer on the surface and an oxide scale layer mainly composed of Cr on the outside (surface side) thereof.

Japanese Unexamined Patent Publication No. 2005-48284

An alloy material provided with an oxide film centered on chromium is expected to have corrosion resistance based on the barrier property of the film, but the corrosion resistance of the base material itself is not sufficient in a high-temperature sulfuric acid environment such as the IS process.

An object of the present invention is to provide an Fe-based alloy having excellent corrosion resistance in a high-temperature steam environment containing sulfur oxides such as _{SO 3 and sulfuric acid.}

In order to achieve the above object, the present inventors conducted an experiment in which various Fe-based alloys were exposed to a high-temperature steam environment containing sulfur oxides such as _{SO 3 and sulfuric acid, and evaluated the corrosion resistance.} Of the main components of Fe-based alloys, Fe, Cr and Ni, Cr is most easily oxidized, and the Cr oxide film is known as a corrosion resistant film. However, in the above-mentioned high-temperature steam environment, there was an oxide film having a lower corrosion rate than the Cr oxide film. It is a corrosion-forming film mainly composed of Si (hereinafter, also simply referred to as "Si oxide film"). The present inventors investigated the conditions for stably forming a Si oxide film, and obtained the following findings.

(A) In the above-mentioned high-temperature steam environment, first, among the elements contained in the base material, an element having a high affinity for oxygen (hereinafter, referred to as "element A") is selectively oxidized. In the initial stage of corrosion, an oxide of element A is formed on the surface of the base metal in an island shape. After that, when the content of the element A of the base material is sufficiently high, the scattered island-shaped A oxides gradually expand and grow into an oxide film covering the entire surface of the base material. However, when the content of the element A is not sufficiently high, the oxide of the element that is easily oxidized next to the element A (hereinafter referred to as "element B") is not covered with the oxide of the element A. It forms an island on the surface of the base metal and grows. Then, when the formation of the oxide of the element B is completed, the next easily oxidized element is formed in an island shape and grows, and the reaction proceeds in sequence. However, when the high-temperature steam environment to which the base material is exposed has an oxidation potential in which both element A and element B are oxidized, the oxidation of element A precedes, but the oxidation of not only element A but also element B proceeds at the same time. ..

(B) In order to stably form a Si oxide film, it is necessary to reduce Al and Ti, which are elements that are more easily oxidized than Si, and Mn, which is easily oxidized next to Si. However, Al, Ti and Mn have a role as deoxidizers during steelmaking, and there is a limit to the reduction of the amount of these elements.

(C) Therefore, the conditions for stably forming the Si oxide film even when a certain amount of Al, Ti and Mn are contained are further investigated. Oxides of each element formed in the above high-temperature steam environment are respectively _{SiO 2, Al 2 O 3,} TiO 2 and MnO _2, 2 oxygen atoms whereas Si atom one oxide, Al atom 1 For each oxidation, 1.5 oxygen atoms are required, for the oxidation of one Ti atom, two oxygen atoms are required, and for the oxidation of one Mn atom, two oxygen atoms are required. That is, it is necessary to sufficiently increase the calculated value of the following equation (a). Theoretically, if this calculated value is set to 0 or more, a Si oxide film will be formed. Actually, it is affected by the diffusion rate of each element.
Si / 14- (Al / 18 + Ti / 24 + Mn / 55) ... (a)

(D) Ti has a higher affinity for nitrogen than oxygen. Therefore, it is necessary to correct the Ti term in the above equation (a) based on the amount of solid-dissolved Ti remaining after the formation of TiN (Ti _SO = [Ti] / 48- [N] / 14). .. However, in calculation, the value of [Ti] / 48- [N] / 14 may be a negative value, but in that case, it means that there is no solid solution Ti, so the above formula (a) Substitute 0 for the Ti term inside.

Since (E) Mn has a high affinity with S, the Mn term in the above formula (a) is corrected based on the _{amount of solid solution Mn remaining without forming MnS (Mn SO).} There is a need. Here, since S is an element that easily forms sulfide not only with Mn but also with Ca and Mg, it is necessary to consider CaS and MgS in the calculation of S that contributes to the formation of MnS. Therefore, the amount Mn _SO of the solid solution Mn is set to a value obtained by [Mn] / 55-([S] / 32- [Ca] / 40- [Mg] / 24). However, in calculation, the value of [Mn] / 55- ([S] / 32- [Ca] / 40- [Mg] / 24) may be a negative value, but in that case, the solid solution Mn Since it means that does not exist, 0 is substituted for the Mn term in the above equation (a).

(F) From the above, in order to form a uniform Si oxide film in a high-temperature vapor environment containing sulfur oxides, it is important to adjust the chemical composition to satisfy the following formulas (1) to (3).
Si / 14- (Al / 18 + Ti _SO + Mn _SO ) ≧ 0.165 ... (1)
Ti _SO = [Ti] / 48- [N] / 14 ... (2)
Mn _SO = [Mn] / 55- ([S] / 32- [Ca] / 40- [Mg] / 24) ... (3)
However, each element symbol in the above formula means the content (mass%) of each element, and when the calculated value on the right side of the above formula (2) is a negative value, the Ti _{SO in} the above formula (1) Substitute 0 for the term, and if the calculated value on the right side of the above equation (3) is a negative value, substitute 0 for _{the Mn SO term of the above equation (1).}

The present invention is based on the above findings, and the following Fe-based alloys are the gist of the present invention.
By mass%
C: 0.04 to 0.15%,
Si: 2.5-4.5%,
Mn: 0.50 to 1.00%,
P: 0.10% or less,
S: 0.010% or less,
Ni: 30.0 to 35.0%,
Cr: 18.0 to 23.0%,
Ti: 0.05 to 0.30%,
Al: 0.02 to 0.10%,
B: 0.02 to 0.08%,
N: 0.01-0.03%,
REM: 0.002-0.01%,
Cu: 0-0.5%,
Mo: 0-0.35%,
Mg: 0-0.005%,
Ca: contains 0 to 0.003%,
Remaining: Fe and impurities,
An Fe-based alloy that satisfies the following equations (1) to (3).
Si / 14- (Al / 18 + Ti _SO + Mn _SO ) ≧ 0.165 ... (1)
Ti _SO = [Ti] / 48- [N] / 14 ... (2)
Mn _SO = [Mn] / 55- ([S] / 32- [Ca] / 40- [Mg] / 24) ... (3)
However, each element symbol in the above formula means the content (mass%) of each element, and when the calculated value on the right side of the above formula (2) is a negative value, the Ti _{SO in} the above formula (1) Substitute 0 for the term, and if the calculated value on the right side of the above equation (3) is a negative value, substitute 0 for _{the Mn SO term of the above equation (1).}

According to the present invention, it is possible to provide an Fe-based alloy having excellent corrosion resistance in a high-temperature steam environment containing sulfur oxides such as _{SO 3 and sulfuric acid.}

The reasons for defining the chemical composition of the steel of the present invention will be described below. In addition, "%" regarding the content of each component means "mass%".

C: 0.04 to 0.15%
C stabilizes the austenite structure and improves the strength of the steel. Therefore, the content is set to 0.04% or more. However, if it is contained in an excessive amount, carbides are precipitated and embrittled during use at a high temperature, so the upper limit is 0.15%. The preferred lower limit is 0.06% and the preferred upper limit is 0.12%.

Si: 2.5-4.5%
Si forms a Si oxide film in high-temperature steam containing sulfur oxides and suppresses corrosion. Therefore, the content is set to 2.5% or more. On the other hand, if Si is excessively contained, the workability is lowered, so the content is 4.5% or less. The preferred lower limit is 2.8%. The preferred upper limit is 4.0% and the more preferred upper limit is 3.5%.

Mn: 0.50 to 1.00%
Mn is an element effective for deoxidizing steel, and S is fixed as MnS to improve heat-sensitive workability. Therefore, the content is set to 0.50% or more. However, Mn oxide does not have excellent corrosion resistance in high-temperature steam containing sulfur oxides, and is incorporated into the Si oxide film to inhibit the formation of Si oxide and deteriorate the corrosion resistance. Therefore, the upper limit is 1. It is set to 00%. It is preferably 0.80% or less, and more preferably 0.70% or less.

P: 0.10% or less P is an element contained in steel as an impurity. Since P deteriorates the weldability of the steel material, the content should be small. In particular, if the P content exceeds 0.10%, the weldability is significantly reduced, so the upper limit is set to 0.10%. More preferably, it is 0.05% or less. Since it is better that the P content is small, a lower limit is not set in particular, but from the viewpoint of manufacturing cost, 0.010% is a practical lower limit.

S: 0.010% or less S is an element contained in steel as an impurity. Since S deteriorates the hot workability of the steel material, it is preferable that the content is small. The S content shall be 0.010% or less. The upper limit of S is more preferably 0.0005%. Since it is better that the S content is small, a lower limit is not particularly set, but from the viewpoint of manufacturing cost, 0.0001% is a practical lower limit.

Ni: 30.0 to 35.0%
Ni is an indispensable element for maintaining the corrosion resistance of the steel itself and ensuring the high temperature strength. Therefore, Ni is contained in an amount of 30.0% or more. The preferred lower limit is 30.5% and the more preferred lower limit is 31.0%. On the other hand, if it is excessively contained, the diffusion of Si is suppressed, it becomes difficult to form a good Si corrosion resistant film, and as a result, the corrosion resistance is lowered, so the upper limit is set to 35.0%. It is preferably 34.0% or less, more preferably 33.0% or less.

Cr: 18.0 to 23.0%
Cr is an essential element for maintaining the corrosion resistance of the steel itself. In particular, in order to maintain oxidation resistance in a high-temperature oxidation environment such as heat exchange, the content must be 18.0% or more. The preferred lower limit is 19.0%, and the more preferable lower limit is 20.0% or more. However, since excessive content reduces hot workability, the upper limit is set to 23.0%. The preferred upper limit is 22.5% and the more preferred upper limit is 22.0%.

Ti: 0.05 to 0.30%
Ti is contained in 0.05% or more because it dissolves in the steel to improve the strength of the steel. However, Ti is easily oxidized and hinders the formation of a Si oxide film, so the upper limit is set to 0.30%. It is preferably 0.20% or less, and more preferably 0.15% or less.

Al: 0.02 to 0.10%
Al has a strong affinity for oxygen and is an element effective for deoxidizing steel, so it is contained in an amount of 0.02% or more. However, if the Al content is high, the formation of a Si oxide film is hindered, so the upper limit is set to 0.10%. It is preferably 0.08% or less, and more preferably 0.05% or less. The preferred lower limit is 0.03%.

B: 0.02 to 0.08%
B is an element effective for maintaining the stability of the metal structure at a high temperature, and contains 0.02% or more. If it is contained in an excessive amount, chromium boride precipitates during use at a high temperature and promotes embrittlement, so the upper limit is 0.08%. The preferred lower limit is 0.03%.

N: 0.01-0.03%
N is an element effective for ensuring the strength of steel by solid solution strengthening, and is contained in an amount of 0.01% or more. However, N combines with Ti in the steel to form a nitride to stably form a Si oxide film. On the other hand, since the formation of nitrides lowers the hot workability, the upper limit of the N content is set to 0.03%. The preferred lower limit is 0.015% and the preferred upper limit is 0.025%.

REM: 0.002-0.01%
REM is an element effective for improving the hot workability of P, and contains 0.002% or more. However, if it is contained in an excessive amount, the cleanliness of the alloy is lowered as a non-metal inclusion, which causes defects. Further, the content of excess REM inhibits the formation of the Si oxide film, so the upper limit is set to 0.01%. The preferred lower limit is 0.003% and the more preferred lower limit is 0.005%. The preferred upper limit is 0.008% and the more preferred upper limit is 0.007%. REM is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total amount of the above elements.

Cu: 0-0.5%
Cu may be contained because it improves corrosion resistance in high-temperature steam containing sulfur oxides. Since excessive content reduces hot workability, the upper limit is 0.5%. In order to obtain the above effect, it is preferable to contain 0.08% or more.

Mo: 0 to 0.35%
Mo may be contained because it improves corrosion resistance at room temperature such as during storage of steel. Since excessive content reduces hot workability, the upper limit is set to 0.35%. In order to obtain the above effect, it is preferable to contain 0.04% or more.

Mg: 0 to 0.005%
Since Mg is an element effective for fixing S and improving hot workability and weldability, it may be contained in the range of 0.005% or less. In order to obtain the above effect, it is preferable to contain 0.0003% or more.

Ca: 0 to 0.003%
Since Ca is an element effective for fixing S and improving hot working and weldability, it may be contained in the range of 0.003% or less. In order to obtain the above effect, it is preferable to contain 0.0015% or more.

The balance in the chemical composition of the Fe-based alloy according to the present invention is Fe and impurities. Impurities mean components that are allowed in the industrial production of steel materials as long as they are mixed by raw materials such as ores and scraps and other factors and do not impair the effects of the present invention.

The Fe-based alloy according to the present invention stably forms a Si oxide film in a high-temperature vapor environment containing sulfur oxides to improve corrosion resistance. For this purpose, as described above, the contents of Al and Ti, which are more easily oxidized than Si, are reduced, but the desired corrosion resistance cannot be obtained only by reducing the contents of these elements. Then, in order to stably form the Si oxide film, it is important to adjust the chemical composition to satisfy the following formulas (1) to (3).
Si / 14- (Al / 18 + Ti _SO + Mn _SO ) ≧ 0.165 ... (1)
Ti _SO = [Ti] / 48- [N] / 14 ... (2)
Mn _SO = [Mn] / 55- ([S] / 32- [Ca] / 40- [Mg] / 24) ... (3)
However, each element symbol in the above formula means the content (mass%) of each element, and when the calculated value on the right side of the above formula (2) is a negative value, the Ti _{SO in} the above formula (1) 0 is substituted for the term, and when the calculated value on the right side of the above equation (3) is a negative value, 0 is substituted for _{the Mn SO term of the above equation (1).}
The rvalue in the equation (1) is preferably 0.180, more preferably 0.225.

There are no particular restrictions on the method for producing the Fe-based alloy according to the present invention, and various production methods can be adopted depending on the purpose of use. As an example of the manufacturing method, a manufacturing method of the Fe-based alloy tube will be described.
First, a molten steel having the above chemical composition is prepared by an electric furnace, an AOD (Argon Oxygen Decarburization) furnace, a VOD (Vacum Oxygen Decarburization) furnace, a VIM (Vacum Induction Melting) furnace, or the like. A material is obtained from molten steel through hot working and heat treatment processes. Hollow billets are produced from materials (ingots or slabs), for example, by machining or vertical drilling.
Hot extrusion is performed on the hollow billet. Hot extrusion is, for example, the Eugene-Sejurne method. The Fe-based alloy tube is manufactured by the above steps. Steel pipes may be manufactured by hot working other than hot extrusion. Cold working such as cold rolling and / or cold drawing may be further carried out on the hot-worked steel pipe. A final heat treatment, such as a solution treatment, may be performed to obtain the desired mechanical properties. In the above-mentioned example of the manufacturing method, the manufacturing method of the Fe-based alloy tube has been described. However, the Fe-based alloy may be a plate material, a welded pipe, a bar material, or the like. In short, the product shape of the Fe-based alloy is not particularly limited.

50 kg of ingots having the chemical compositions shown in Table 1 were prepared by vacuum melting. The obtained ingot (outer diameter 250 mm) is heated at 1200 ° C., and then hot forged to obtain a hot forged material having a thickness of 40 mm, a width of 100 mm, and a length of 150 mm. Rolling was performed to obtain a cold-rolled steel sheet having a thickness of 5 mm, a width of 100 mm, and a length of 1000 mm. The obtained cold-rolled steel sheet was heat-treated at 1100 ° C. and then water-quenched to prepare a test steel sheet.

(Evaluation of corrosion resistance)
A test piece for a corrosion test having a thickness of 1 mm, a width of 10 mm, and a length of 55 mm was produced by machining from the central portion of the wall thickness of the test steel plate. The entire surface of the test piece was finished with wet emery abrasive paper No. 600, washed and degreased with acetone, and then the mass of the test piece before the test was measured. A test piece set on a jig was placed in a quartz tube for a corrosion test, and heated to 850 ° C. while flowing argon gas. Then, supplying a 98% sulfuric acid by using a micro-pump from the upstream side, to generate SO ₃ gas vaporized and decomposed, ventilation as 20% SO ₃ gas while flowing argon gas containing pure argon gas or SO ₂ The test was carried out for 50 hours. After the test, the test piece was descaled with a potassium permanganate solution and a diammonium citrate solution until the test piece had a metallic luster, and then the mass was measured after the test. ^{The corrosion rate (g / m 2} ) was measured by dividing the mass difference between the test piece before and after the test piece by the test time. Comparative material No. When the corrosion rate of 9 (steel conforming to ALLOY 800H) is 1, the corrosion rate of 0.7 or less is evaluated as "◎" (excellent), and the corrosion rate of more than 0.7 and less than 0.8 is "○". (Good) and those over 0.8 are listed in Table 1 as "x" (defective).

(Evaluation of workability)
When a defect such as cracking occurred in the process of performing hot rolling and cold rolling on the above hot forged material and a homogeneous cold-rolled steel sheet could not be obtained, the workability was set to "x" (defective). ..

As shown in Table 1, No. In No. 9, the Si content was too low and the equation (1) was not satisfied. Therefore, a sufficient Si oxide film was not formed on the surface of the steel material, and the corrosion resistance was deteriorated.
No. In No. 10, as a result of an excessive amount of Mn, excess Mn oxide was incorporated into the Si oxide film to form a non-uniform Si film, and the Mn oxide was corroded, resulting in deterioration of corrosion resistance.
No. In No. 11, since the amount of Al was too large and the equation (1) was not satisfied, a sufficient Si oxide film was not formed on the surface of the steel material, and the corrosion resistance was deteriorated.
No. In No. 12, the amount of Ti was too large and the equation (1) was not satisfied. Therefore, a sufficient Si oxide film was not formed on the surface of the steel material, and the corrosion resistance was deteriorated.
No. In No. 13, the corrosion resistance deteriorated because the excess REM inhibited the formation of a uniform Si oxide film.
No. In No. 14, the content of each element was within the range specified in the present invention, but since the equation (1) was not satisfied, a sufficient Si oxide film was not formed on the surface of the steel material, and the corrosion resistance was deteriorated.
No. In No. 15, the amount of Si was too small and the equation (1) was not satisfied. Therefore, a sufficient Si oxide film was not formed on the surface of the steel material, and the corrosion resistance was deteriorated.
No. In No. 16, the amount of Si was too large, so that the workability was deteriorated.
On the other hand, No. Since 1 to 8 satisfied all the configurations of the present invention, they were excellent in corrosion resistance and processability.

According to the present invention, it is possible to provide an Fe-based alloy having excellent corrosion resistance in a high-temperature steam environment containing sulfur oxides such as _{SO 3 and sulfuric acid.}

Claims (1)

By mass%
C: 0.04 to 0.15%,
Si: 2.5-4.5%,
Mn: 0.50 to 1.00%,
P: 0.10% or less,
S: 0.010% or less,
Ni: 30.0 to 35.0%,
Cr: 18.0 to 23.0%,
Ti: 0.05 to 0.30%,
Al: 0.02 to 0.10%,
B: 0.02 to 0.08%,
N: 0.01-0.03%,
REM: 0.002-0.01%,
Cu: 0-0.5%,
Mo: 0-0.35%,
Mg: 0-0.005%,
Ca: contains 0 to 0.003%,
Remaining: Fe and impurities,
An Fe-based alloy that satisfies the following equations (1) to (3).
Si / 14- (Al / 18 + Ti _SO + Mn _SO ) ≧ 0.165 ... (1)
Ti _SO = [Ti] / 48- [N] / 14 ... (2)
Mn _SO = [Mn] / 55- ([S] / 32- [Ca] / 40- [Mg] / 24) ... (3)
However, each element symbol in the above formula means the content (mass%) of each element, and when the calculated value on the right side of the above formula (2) is a negative value, the Ti _{SO in} the above formula (1) 0 is substituted for the term, and when the calculated value on the right side of the above equation (3) is a negative value, 0 is substituted for _{the Mn SO term of the above equation (1).}