JP6068158B2 - Cast products having an alumina barrier layer - Google Patents

Description

  The present invention relates to an austenitic cast product having an alumina barrier layer, and more specifically to a cast product excellent in high-temperature tensile ductility.

  Heat-resistant cast steel products such as reaction tubes and cracking tubes for ethylene production, hearth rolls of carburizing heat treatment furnaces, radiant tubes, and metal dusting materials are exposed to high-temperature atmospheres, so austenitic heat-resistant alloys with excellent high-temperature strength are used. It is used.

In this type of austenitic heat-resistant alloy, a metal oxide layer is formed on the surface during use in a high temperature atmosphere, and this oxide layer serves as a barrier to protect the base material in a high temperature atmosphere.
On the other hand, if Cr oxide (mainly composed of Cr ₂ O ₃ ) is formed as these metal oxides, since the denseness is low, the function of preventing the entry of oxygen and carbon is not sufficient, and the inside of the inside in a high temperature atmosphere Oxidation occurs and the oxide film becomes enlarged. In addition, these Cr oxides are easy to peel off in repeated heating and cooling cycles, and even if they do not come off, they do not have sufficient functions to prevent oxygen and carbon from entering from the outside atmosphere, so they pass through the coating. This has the disadvantage of causing internal oxidation and carburization of the base metal.

On the other hand, by increasing the Al content over a general austenitic heat-resistant alloy, an oxide layer mainly composed of alumina (Al ₂ O ₃ ), which has high density and hardly permeates oxygen and carbon, is used as a base. It has been proposed to form on the surface of a material (for example, see Patent Document 1 and Patent Document 2).

However, since Al is a ferrite-forming element, if the content increases, the ductility of the material deteriorates and the high-temperature strength decreases. This tendency of decreasing ductility is observed especially when the Al content exceeds 4%.
Therefore, the austenitic heat resistant alloys of the patent documents described above, even can be expected to improve the barrier function of the Al ₂ O _3, there is a disadvantage that lead to decrease in ductility of the matrix.

Therefore, high temperature stability of Al ₂ O ₃ can be ensured without increasing the Al content to more than 4%, and an excellent barrier function is exhibited in a high temperature atmosphere without reducing the ductility of the material. In order to provide a cast product that can be processed, in Patent Document 3, after the inner surface is processed so that the surface roughness (Ra) of the cast body becomes 0.05 to 2.5 μm, heat treatment is performed in an oxidizing atmosphere. The cast product in which an alumina barrier layer containing Al ₂ O ₃ is formed on the inner surface of the cast body, and Cr-based particles having a higher Cr concentration than the base metal base are dispersed at the interface between the alumina barrier layer and the cast body. (See, for example, Patent Document 3).

  The casting product of Patent Document 3 can maintain excellent oxidation resistance, carburization resistance, nitridation resistance, corrosion resistance, etc. over a long period of time when used in a high temperature atmosphere due to the presence of a stable alumina barrier layer. it can.

JP-A-51-78612 JP-A-57-39159 International Publication No. WO2010 / 113830

  In the cast product of Patent Document 3, there is a product in which the tensile ductility at a high temperature is lowered.

  As a result of intensive studies, it has been found that the decrease in high-temperature tensile ductility is due in part to the constituents of rare earth elements contained in the casting for the purpose of promoting the formation and stabilization of an alumina barrier layer.

  An object of the present invention is to provide an austenitic cast product having an alumina barrier layer and excellent in high temperature tensile ductility.

Cast products having the alumina barrier layer of the present invention,
A cast product in which an alumina barrier layer containing Al ₂ O ₃ is formed on the surface of a cast body,
Cast bodies are in mass%, C: 0.3-0.7%, Si: 0.1% -1.5%, Mn: 0.1% -3%, Cr: 15-40%, Ni : 18-55%, Al: 2-4%, rare earth elements: 0.005-0.4%, W: 0.5-5% and / or Mo: 0.1-3%, balance 25 % Of Fe and unavoidable impurities,
80% or more of the rare earth element is La.

According to the cast product of the present invention, by increasing the content of La as a constituent of rare earth elements contained in the cast body, a cast body excellent in high-temperature tensile ductility can be obtained while stabilizing the oxide film. The formation efficiency of the alumina barrier layer containing Al ₂ O ₃ when the cast body is heat-treated is increased, and a firm alumina barrier layer can be formed in a thinner state.

  In addition, by setting the C concentration to 0.3 to 0.7%, it is easy to form carbides, and an environment in which the upper limit concentrations of W and Mo, which are elements that form carbides having a relatively large particle size, are reduced. By constructing it, making it an iron-based alloy having an Fe concentration of at least 25% or more, and making the La component of 80% or more among rare earth elements, Al easily moves to the surface during the formation of the alumina barrier layer. The alloy composition was used.

  The casting product of the present invention can increase the amount of Ni-La compound produced by setting La in the rare earth element to 80% or more, and has high-temperature tensile ductility, particularly high-temperature tensile ductility at 1100 ° C. or higher. It is.

  The cast product of the present invention can obtain an alloy structure in which a fine metal structure grows in the radial direction with orientation and Al easily moves by centrifugally casting the cast body. Therefore, it is possible to obtain a cast product in which a coating film having an excellent strength is obtained even in an environment of repeated heating, although the alumina barrier layer is thinner than the conventional one. As a cast product produced by centrifugal casting, a tube, particularly a reaction tube used under high temperature can be exemplified.

Hereinafter, embodiments of the present invention will be described in detail.
The present invention is a cast product in which an alumina barrier layer containing Al ₂ O ₃ is formed on the surface of a cast body. The cast body is C: 0.3 to 0.7%, Si: 0.1% to 1.5%, Mn: 0.1% to 3%, Cr: 15 to 40%, Ni: 18 to 55%, Al: 2 to 4%, Rare earth element: 0.005 to 0. 4%, W: 0.5 to 5% and / or Mo: 0.1 to 3%, with the balance being 25% or more of Fe and unavoidable impurities, the rare earth element being 80% or more of La To obtain a casting product. In the present specification, “%” is all mass% unless otherwise indicated.

<Description of reasons for limiting ingredients>

C: 0.3 to 0.7%
C has the effect of improving castability and increasing the high temperature creep rupture strength. For this reason, at least 0.3% is contained. However, if the content is too large, the primary carbide of Cr ₇ C ₃ is likely to be widely formed, and the movement of Al forming the alumina barrier layer is suppressed, so there is insufficient supply of Al to the surface portion of the cast body. As a result, local breakage of the alumina barrier layer occurs, and the continuity of the alumina barrier layer is impaired. Moreover, since secondary carbide precipitates excessively, it causes a reduction in ductility and toughness. For this reason, the upper limit is set to 0.7%. C is more preferably 0.4 to 0.6%.

Si: 0.1% to 1.5%
Si is contained at least 0.1% as a deoxidizer for the molten alloy and to increase the fluidity of the molten alloy. However, if the content is too large, the high temperature creep rupture strength is lowered, so the upper limit is 1.5. %. The upper limit of Si is more preferably 1.0%.

Mn: 0.1% to 3%
Mn is contained at least 0.1% as a deoxidizer for the molten metal alloy and for fixing S in the molten metal. However, if the content is too large, the high temperature creep rupture strength is reduced, so the upper limit is 3%. To do. The upper limit of Mn is more preferably 1.0%.

Cr: 15% to 40%
Cr is contained in an amount of 15% or more for the purpose of contributing to improvement in high temperature strength and repeated oxidation resistance. However, if the content is too high, the high temperature creep rupture strength is lowered, so the upper limit is made 40%. In addition, Cr is more preferably 20 to 30%.

Ni: 18% to 55%
Ni is an element necessary for ensuring repeated oxidation resistance and stability of the metal structure, so Ni is contained in an amount of 18% or more. However, even if the content exceeds 55%, the effect corresponding to the increase cannot be obtained, so the upper limit is made 55%. Ni is more preferably 20 to 45%.

Al: 2-4%
Al is an element effective for improving carburization resistance and coking resistance. Moreover, in this invention, it is an indispensable element in order to produce an alumina barrier layer on the surface of a casting. For this reason, it contains at least 2% or more. However, if the content exceeds 4%, the ductility deteriorates as described above, so the upper limit is defined as 4% in the present invention. The Al content is more preferably 2.5 to 3.8%.

Rare earth elements: 0.005 to 0.4%. However, more than 80% is La
The rare earth element means 17 kinds of elements obtained by adding Y and Sc to 15 kinds of lanthanum series from La to Lu in the periodic table, but the rare earth element contained in the heat-resistant alloy of the present invention is 80% The above is La. By setting La to 80% or more, it is possible to increase the amount of Ni-La compounds such as Ni ₂ La and Ni ₃ La that are excellent in high-temperature tensile ductility, particularly high-temperature tensile ductility at 1100 ° C. or higher.
The rare earth element has an S fixing ability and an ability to fix an oxide film with a rare earth oxide, and contributes to promoting the formation and stabilization of the alumina barrier layer, so it is contained in an amount of 0.005% or more. On the other hand, if the content is too large, ductility and toughness deteriorate, so the upper limit is made 0.4%.
Furthermore, the rare earth element preferably has a Ce content of 0.1% or less. By suppressing the Ce content, it is possible to reduce the amount of Ce compound that causes high temperature brittleness such as Ni ₂ Ce and Ni ₃ Ce, and to increase the high temperature tensile ductility. It is more preferable that the rare earth element is composed only of La without containing Ce.

W: 0.5-5% and / or Mo: 0.1-3%
W and Mo are dissolved in the matrix and strengthen the austenite phase of the matrix, thereby improving the creep rupture strength. In order to exert this effect, at least one of W and Mo is contained. In the case of W, 0.5% or more is contained, and in the case of Mo, 0.1% or more is contained.
However, when the content of W and Mo is too large, the ductility is lowered and the carburization resistance is deteriorated. Since W and Mo have a large atomic radius, the migration of Al is caused by solid solution in the base. It suppresses and prevents the formation of the alumina barrier layer.
For this reason, W is 5% or less, and Mo is 3% or less. In addition, it is more preferable that W is 0.5 to 3% and Mo is 2% or less.

At least one of Ti: 0.01 to 0.6%, Zr: 0.01 to 0.6% and Nb: 0.1 to 3.0% Ti, Zr and Nb are elements that easily form carbides. Since W and Mo do not dissolve in the base as much as possible, there is no particular effect on the formation of the alumina barrier layer, but it has the effect of improving the creep rupture strength. If necessary, at least one of Ti, Zr and Nb can be contained. The contents of Ti and Zr are 0.01% or more and Nb is 0.1% or more.
However, if added excessively, ductility is reduced. Nb further reduces the peel resistance of the alumina barrier layer. For this reason, the upper limits are 0.6% for Ti and Zr, and 3.0% for Nb. It is more preferable that Ti and Zr are 0.3% and Nb is 1.5%.

B: 0.1% or less B has an effect of strengthening the grain boundary of the cast body, and can be contained as necessary. In addition, since the fall of creep rupture strength will be caused when content increases, even when adding, it shall be 0.1% or less.

Fe: 25% or more The diffusion rate of Al in Fe, Ni, and Cr is considered to be faster as the atomic size is smaller. For this reason, by increasing Fe with small atoms and reducing the amount of Cr, the diffusion of Al in the alloy is enhanced, Al can be easily moved, and the formation of an Al ₂ O ₃ coating is promoted. Can do. Moreover, the production | generation of Cr oxide can also be suppressed by reducing Cr.
For the above reasons, Fe is contained at 25% or more. Note that Fe is more preferably 30% or more.

Inevitable Impurities P, S, and other impurities inevitably mixed when the alloy is melted may be present as long as it is normally acceptable for this type of alloy material.

<Cast body>
The cast body constituting the cast product of the present invention is prepared by melting a molten metal having the above component composition, and casting to the above composition by centrifugal casting, stationary casting, or the like.
The resulting casting can be shaped according to the intended application.
In addition, this invention is especially suitable for the cast body produced by centrifugal casting. This is because by applying centrifugal casting, a fine metal structure grows with orientation in the radial direction by the progress of cooling by the mold, and an alloy structure in which Al easily moves can be obtained. Thereby, in the heat treatment described later, it is possible to obtain a cast product having a coating film having excellent strength even in an environment of repeated heating while being an alumina barrier layer thinner than the conventional one.
As a cast product produced by centrifugal casting, a tube, particularly a reaction tube used in a high temperature environment can be exemplified.

  The cast body is subjected to a surface treatment on a target portion that comes into contact with a high-temperature atmosphere when the product is used, and after the surface roughness of the portion is adjusted, the heat treatment is performed in an oxidizing atmosphere.

<Surface treatment>
The surface treatment can be exemplified by a polishing treatment. This surface treatment is desirably performed on the entire target portion that comes into contact with a high-temperature atmosphere when the product is used.

  The surface treatment can be performed so that the surface roughness (Ra) of the target site is 0.05 to 2.5 μm. More desirably, the surface roughness (Ra) is 0.5 to 2.0 μm. When the surface roughness (Ra) is less than 0.05 μm, Cr oxidizes in preference to Al, but when it is 0.05 μm or more, the generation of Cr oxide scale can be suppressed, and the subsequent heat treatment makes it more An alumina barrier layer can be suitably formed. When the thickness is 2.5 μm or more, it is considered that a Cr oxide scale is easily generated due to residual processing strain. At this time, by adjusting the surface roughness by the surface treatment, the residual stress and distortion of the heat affected zone can be removed at the same time.

  When the surface treatment is performed by a polishing process, it is desirable to perform paper polishing with a count of 12 to 220 and then finish polishing with a count of 240 to 1200.

<Heat treatment>
After the surface treatment, heat treatment is performed under the following conditions.
The heat treatment is performed by performing a heat treatment in an oxidizing atmosphere.
An oxidizing atmosphere is an oxidizing gas containing 20% by volume or more of oxygen, or an oxidizing environment in which steam or CO ₂ is mixed. The heat treatment is performed at a temperature of 900 ° C. or higher, preferably 1000 ° C. or higher, more preferably 1050 ° C. or higher, and the heating time is 1 hour or longer.

<Casting products>
As described above, a cast product in which an alumina barrier layer containing Al ₂ O ₃ is stably formed on the surface of the cast body can be obtained by sequentially performing surface treatment and heat treatment on the cast body.

<Alumina barrier layer>
The alumina barrier layer containing Al ₂ O ₃ formed in the cast product of the present invention has a high density and functions as a barrier for preventing oxygen, carbon, and nitrogen from entering the base material from the outside. In the present invention, the surface treatment is performed on the portion that comes into contact with the high temperature atmosphere when the product is used, the surface roughness of the portion is adjusted, and then the portion is heat-treated in an oxidizing atmosphere, thereby casting. Al ₂ O ₃ can be continuously formed as an alumina barrier layer on the surface of the product.

The thickness of the alumina barrier layer formed on the cast body is preferably 0.05 μm or more and 3 μm or less in order to effectively exhibit the barrier function. If the thickness of the alumina barrier layer is less than 0.05 μm, the carburization resistance may be lowered. If the thickness exceeds 3 μm, the alumina barrier layer is affected by the difference in thermal expansion coefficient between the base material and the coating. There is a possibility that the peeling of the metal tends to proceed.
In order to avoid the above effects, the thickness of the alumina barrier layer is more preferably 0.1 μm or more and 2.5 μm or less. On the other hand, if there is a difference in the film thickness, there is a possibility that peeling of the coating may proceed when the temperature changes drastically. For this reason, the thickness of the alumina barrier layer is preferably 0.5 μm or more and 1.5 μm or less, and most preferably about 1 μm on average.

When the surface of the cast product of the present invention is examined by SEM / EDX, a part of the Cr oxide scale may be formed on the alumina barrier layer. This is because the Cr oxide scale formed inside the alumina barrier layer is pushed up to the product surface by Al ₂ O ₃ . However, it is preferable that the oxide scale is small, and it is preferable that Al ₂ O ₃ occupies 80 area% or more so that it is less than 20 area% on the product surface.

<About La>
The cast product of the present invention has a La content in the rare earth element of 80%, and as explained in the examples described later, the tensile ductility at high temperatures (specifically, 1100 ° C. or higher) is as much as possible. Can be increased.
The reason is that the melting temperature of the Ni—La compound is higher than the melting temperature of the Ni—Ce compound, and the high temperature embrittlement of the La additive is on the higher temperature side than 1200 ° C. More specifically, the melting points of Ni ₂ Ce and Ni ₃ Ce are 1000 ° C. and 1180 ° C., respectively, whereas the melting points of Ni ₂ La and Ni ₃ La are 1100 ° C. and 1240 ° C., respectively.

  Therefore, particularly when used in a reaction tube, it is effective to contain 80% or more of La as a rare earth element that does not become brittle in the temperature range of use (about 1100 ° C.).

In the present invention, in the rare earth element, the content of Ce is suppressed to contain 80% or more of La, but the Ce additive and La additive are maintained at 1050 ° C. for 10 hours, When a repeated oxidation test was performed in the atmosphere, almost no difference was observed in the peeling resistance of Al ₂ O ₃ between the Ce additive and the La additive.

  In addition, the bead placement test (crack sensitivity test: Japan Welding Association website http: / /www-it.jwes.or.jp/qa/details.jsp?pg_no=0100080100), but it was confirmed that there was almost no effect.

  The present invention is suitable as a cast product that has excellent high-temperature tensile ductility and can effectively prevent intrusion of oxygen, carbon, nitrogen, and the like from the outside atmosphere by the alumina barrier layer.

The molten metal was melted by melting in the air in a high-frequency induction melting furnace, and a test tube (outer diameter 59 mm, wall thickness 8 mm, length 3000 mm) having an alloy chemical composition listed in Table 1 below was cast by centrifugal force casting. Test No. 11 to 23 are invention examples, test Nos. 101 to 105 are comparative examples.
More specifically, the comparative example is a sample No. Comparative Examples Nos. 101 to 104, which contain more Ce than La for the alloy chemical composition of the present invention. 105 is a comparative example in which the content of La is less than 80% with respect to the total amount of La and Ce.

<Surface treatment>
These test tubes were subjected to a skiving process which is a rough process on the inner surface of the pipe and a surface treatment by paper polishing, and the surface roughness (Ra) was adjusted to 1.0 μm.

<Heat treatment>
After the surface treatment, all the test tubes were heated in air (approximately 21% oxygen) at 1000 ° C. for 10 hours, and then heated and cooled in the furnace.

<High temperature ductility test>
A tensile test piece was prepared from the test tube in accordance with JIS Z 2201, and a ductility test was performed. Specifically, the test piece was processed into a parallel part diameter of 10 mm and a parallel part length of 50 mm, and a ductility test was performed according to a metal material tensile test method of JIS G 0567. The test was conducted at 1100 ° C.

  Table 2 shows the results of the above tests.

<Consideration of test results>
Regarding the tensile strength, referring to Table 2, the test sample No. 11-No. No. 23 is a test No. which is a comparative example. 101-No. It turns out that it is almost equivalent to 105.

Regarding the elongation (high temperature tensile ductility), the inventive example is about 10 times that of the comparative example.
Sample No. which is an example of the invention. 11-No. 23 is excellent in elongation (high-temperature tensile ductility) because the content of La in the rare earth element is 80% or more, and Ni-La series such as Ni ₂ La and Ni ₃ La that are excellent in high-temperature tensile ductility. This is because the amount of the compound produced can be increased.
On the other hand, the poor in elongation test 101-105 are comparative examples (temperature tensile ductility) is high content of Ce in the rare earth element, i.e., the content of La is less than 80% results, _Ni 2 Ce This is because a large amount of Ni—Ce-based compounds such as Ni ₃ Ce and the like caused high temperature brittleness.

  Regarding the inventive examples, as the rare earth elements, the test No. 1 with La of 0.12% and Ce of 0.03%. No. 18 shows the same elongation (high temperature tensile ductility) as the other invention examples. This is because the production amount of the Ni—Ce compound could be suppressed by making Ce 0.1% or less.

  In both the inventive example and the comparative example, the film thickness and the area ratio of the alumina barrier layer are both good, and for the inventive example, the specimen is plated with Ni, covered with a stainless steel sheet, and further from above When resin coating was applied and cross-sectional SEM analysis was performed, it was found that a suitable alumina barrier layer having a thickness of 0.05 μm or more and 3 μm or less was formed.

  As shown in the above examples, the cast product of the present invention can form a uniform alumina barrier layer over the entire surface of the cast body, and effectively prevents the entry of oxygen, carbon, nitrogen, etc. from the external atmosphere. Of course, it has excellent high temperature tensile ductility.

  In addition, although the said Example produced the test tube by centrifugal casting, the same result can be obtained even if it is static casting.

  The present invention is useful as a cast product having an alumina barrier layer and excellent in high-temperature tensile ductility.

Claims (7)

A cast product in which an alumina barrier layer containing Al ₂ O ₃ is formed on the surface of a cast body,
Cast bodies are in mass%, C: 0.3-0.7%, Si: 0.1% -1.5%, Mn: 0.1% -3%, Cr: 15-40%, Ni : 18-55%, Al: 2-4%, rare earth elements: 0.005-0.4%, W: 0.5-5% and / or Mo: 0.1-3%, balance 25 % Of Fe and unavoidable impurities,
80% or more of the rare earth element is La , and the content of Ce is 0.05% or less of the mass of the cast body . A cast product on which an alumina barrier layer is formed.   The cast product according to claim 1, wherein the rare earth element does not contain Ce. Furthermore, it contains at least one selected from the group consisting of Ti: 0.01 to 0.6%, Zr: 0.01% to 0.6%, and Nb: 0.1 to 3.0%. Or the cast product of Claim 2 . The cast product according to any one of claims 1 to 3 , further comprising B: more than 0% and 0.1% or less. Cast product according to any of the alumina barrier layer has a thickness 0.05μm~3μm claims 1 to 4. The cast product according to any one of claims 1 to 5 , wherein a surface roughness (Ra) of the cast body covered with the alumina barrier layer is 0.05 µm to 2.5 µm. The cast product according to any one of claims 1 to 6 , wherein the cast body is manufactured by centrifugal casting.