JPH04128346A - Stainless steel foil for metal support - Google Patents

Abstract

PURPOSE:To improve abnormal oxidation resistance and adhesive strength of film without adding Y and other rare earth elements by adding specific amounts of Mg in the heat resisting foil used for combustion exhaust gas cleaning device and consisting of an Fe-Cr-Al alloy having a specific composition. CONSTITUTION:This stainless steel foil has a composition consisting of, by weight, 4.5-6.5% Al, 13-25% Cr, <=0.025% C, <=0.02% N, 0.01-0.1% Mg, and the balance iron and satisfying C+N<=0.03%. By the addition of Mg, the adhesive strength of a film as well as the resistance to the occurrence of abnormal oxidation in the exhaust gas can also be improved, thus sufficient oxidation resistance can be provided. Further, the effect of remarkably reducing the elongation of foil attendant upon the growth of oxide film (greater than the effect produced by Y and other rare earth elements) can be provided. Accordingly, the occurrence of structural damage can be prevented and the service life of the catalyst support can be improved while obviating the necessity of expensive Y and other rare earth elements.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a heat-resistant stainless steel foil used as a catalyst carrier for a combustion exhaust gas purification device. For more details, please refer to 50μ
Heat-resistant stainless steel foil has a high resistance to abnormal oxidation in combustion exhaust gas when used in the form of a foil of about 100%, and has a great effect on improving the structural durability when used in the honeycomb body of the catalyst carrier. Involved.

[Conventional technology]

Ceramic honeycombs have traditionally been used in combustion exhaust gas purification devices for automobiles, etc., but by replacing them with heat-resistant stainless steel foil, it is possible to reduce the thickness of the honeycomb walls, resulting in a reduction in ventilation resistance and heat capacity. Since it is possible to improve engine performance and save expensive catalyst precious metals, this honeycomb body is used, for example, as disclosed in Japanese Patent Application Laid-open Nos. 50-92286, 51-48473, and 57-71898. A technique has been proposed in which a heat-resistant metal foil of Fe-Cr-Al system is used.

In this case, oxidation resistance and film adhesion are attracting attention as properties required of the alloy, and therefore, the material has generally been used as a heating wire or Based on the Fe-Cr-Al alloy, which has been widely used as high-temperature parts in heating appliances, activated alumina CT is used for its oxidation resistance and as a direct support for catalysts.
A1103) Foil with improved adhesion to the coating layer is used. All of the techniques disclosed in the above publications utilize Y as a means to improve the oxidation resistance of the material.
Since Y is an extremely expensive element, its range of use is limited.

On the other hand, in JP-A-58-177437, Fe-Cr
-Al! , 0.002 to 0.05% by weight of La, Ce, Pr, mainly to prevent peeling of the oxide film of the alloy.
, Nd, containing up to 0.06% by weight of rare earth elements, and Zr to stabilize the alloy, and Nb to ensure high temperature creep strength. An alloy has been proposed in which C and N are added within specific relationship ranges. According to this patent, alloys with a total rare earth element content exceeding 0.06% by weight not only have little improvement in oxidation resistance compared to alloys containing less than 0.06% by weight, but also cannot be processed at normal hot working temperatures. It states that

In JP-A No. 63-45351, the same <Fe-Cr
It has been proposed to add only Ln or La, excluding Ce, in a range of 0.05 to 0.2% by weight, since adding Y to alloys based on -Al systems would be expensive. This is because the presence of Ce is the cause of the decrease in hot workability due to the addition of Ln, and also because Ce has the effect of decreasing oxidation resistance. The company states that this is based on the knowledge that it enables intermediate processing and improves oxidation resistance. Furthermore, JP-A No. 63-42356 describes a Fe-Cr-Al2 alloy with excellent oxidation resistance and peeling resistance of oxide scale containing a total of 0.01% of La, Ce, Pr and Nd.
So far, alloys containing 0.30% or less have been disclosed.

All of these conventional technologies mainly propose the addition of Y or lanthanoids as a means to improve these properties based on the results of studies on the adhesion and oxidation resistance of the oxide film of the foil that constitutes the catalyst carrier. However, the influence of the foil material on the structural durability of the honeycomb body, which is a practically important characteristic required for the foil constituting the honeycomb body of the catalyst, has not been sufficiently investigated.

[Problem to be solved by the invention]

However, in the case of catalyst carriers in automobiles, for example, it is rare for the catalyst carriers to reach the end of their service life due to the foil's lack of oxidation resistance under normal use environments, but rather due to structural damage. It is more common to reach the end of life.

In this case, the causes of structural damage are represented by the following two. The first one is that the foil that makes up the honeycomb body is thin, so when it is used in high-temperature exhaust gas, the oxide film grows on the foil surface and the metal/oxide film boundary forms. The foil creep deforms due to the stress generated in the process (hereinafter referred to as oxide film growth stress) and gradually stretches, which promotes the deformation of the honeycomb body.

Furthermore, the second major cause is that damage to the honeycomb body progresses due to thermal fatigue caused by repeated heating and cooling in conjunction with the driving conditions of the vehicle.

In other words, the honeycomb body of the catalyst carrier is susceptible to elongation of the foil due to the growth stress of the surface oxide film due to oxidation in such a high temperature environment,
In most cases, cells reach the end of their structural life due to cell collapse or extreme deformation due to the accumulation of thermal strain that occurs inside them due to repeated heating and cooling, or due to a combination of these.

In addition, since the foil is exposed to high-temperature exhaust gas with a significantly large surface area relative to its volume, it must also have excellent oxidation resistance, and it must also have good adhesion to the so-called washcoat layer of the catalyst directly supported. Of course, sexuality must be ensured.

When adding Y or other rare earth elements as a means of imparting these properties, special consideration must be given to the addition method and equipment used during the steel manufacturing stage, since these elements are highly active. For example, when molten steel after adding rare earths comes into contact with the atmosphere, a severe oxidation phenomenon occurs, which not only significantly reduces the yield of rare earths, but also causes problems such as frequent surface defects and deep cracks in the steel ingot during casting. be. Furthermore, the slag used to shield the molten steel from such an oxidizing atmosphere, or the so-called continuous powder added during continuous casting, reacts with the rare earth elements in the molten steel and alters its quality, resulting in the above-mentioned flaws in the steel ingot. There is a problem with doing so.

In addition, for widespread general use such as in passenger cars, low cost and stable supply are also important characteristics required.

In view of these current issues, the present inventors have developed a method that is effective in improving the structural durability of a catalyst carrier and also ensures oxidation resistance and adhesion of the oxide film, as described above. The present invention was developed as a result of various studies in order to provide a foil that can avoid manufacturing problems caused by the addition of Y or lanthanoids.

[Means to solve the problem]

First, we investigated the oxidation resistance of the foil, and found that M
It has been revealed that the addition of g makes it possible to improve the resistance to the occurrence of abnormal oxidation in exhaust gas and the adhesion of the oxide film, and that the catalyst carrier has sufficient oxidation resistance under the actual usage environment of the catalyst carrier.

In addition, it was also found that Mg has the effect of significantly reducing the elongation of the foil due to the growth of the oxide film, and that this effect is greater than that of Y and other rare earth elements. This effect of Mg is extremely important because, as mentioned above, the actual life of the catalyst carrier is determined in most cases by its structural durability.

On the other hand, in order to reduce damage caused by thermal fatigue of the catalyst carrier due to heating and cooling, it is necessary to
It is important to improve the yield strength at temperatures up to 850°C, and it has become clear that the addition of Nb + Ta + Mo and W, either alone or in combination, is effective for this purpose.

That is, based on the above study results, the present invention has been developed to ensure sufficient oxidation resistance as a foil and adhesion of the film even in high-temperature exhaust gas, while also improving the structural durability of the catalyst carrier. This was achieved with the aim of providing a stainless steel for catalyst carriers, which has the property of having low elongation due to foil oxidation, which is important for improvement, and which is easy to manufacture and can be supplied at low cost.

Furthermore, the purpose is to improve the high-temperature proof strength of such foils, thereby improving the structural durability of the catalyst carrier under harsh repeated heating and cooling environments.

Therefore, the first concrete means is weight %: 1: 4.5% or more and 6.5% or less Cr: 13% or more and 25% or less C: 0.025% or less N: 0.02% The following C+N: 0.03% or less Mg: 0.01% or more and 0.1% or less, the balance F
This is a heat-resistant stainless steel foil for use as a catalyst carrier for combustion exhaust gas purification, which is characterized by consisting of e.g. This ensures good adhesion.

Furthermore, the second one is in weight percent: Al: 4.5% or more and 6.5% or less Cr: 13% or more and 25% or less C: 0.025% or less N: 0.02% or less C+N: 0.03% or less Mg: Contains 0.01% or more and 0.1% or less, and further contains at least one selected from 3% or less Nb, 3% or less Ta, 4% or less Mo, and 4% or less W. , Ta+Nb: 3% or less, Mo+W: 4% or less, and is a heat-resistant stainless steel foil for use in combustion exhaust gas purification catalysts, which significantly improves high-temperature yield strength and is particularly suitable for repeated heating and cooling environments. It can improve the structural durability of the catalyst support underneath.

[For production]

Next, the reasons for limiting the components in the present invention and their effects will be explained in detail. It should be noted that all chemical compositions in this specification are in percent by weight.

(1) Al: Al is a basic element that ensures oxidation resistance in the present invention, and if it is less than 4.5%, the oxidation resistance of the foil in exhaust gas cannot be ensured, and it easily causes abnormal oxidation. Because of the order, it cannot withstand its use as a catalyst carrier. On the other hand, even if the content exceeds 6.5%, the oxidation resistance improvement effect is saturated, but the toughness of the hot rolled sheet is extremely reduced, manufacturability is impaired, and the coefficient of thermal expansion of the foil is significantly reduced. This results in increased thermal fatigue due to repeated heating and cooling when used as a catalyst carrier. Therefore, in the present invention, the range of l is 4.5% or more and 6.5% or less.

(2) Cr: Cr is a basic element that ensures corrosion resistance of stainless steel. In the present invention, the main component of oxidation resistance is A f 203
Although it is present in the film, if Cr is insufficient, its adhesion and protective properties will decrease. On the other hand, if Cr is excessive, the toughness of the hot rolled sheet decreases, so the range is 13% or more and 25% or less.

(3) C, N: In the present invention, both C and N need to be kept low because they significantly reduce the toughness of the hot rolled sheet.

In addition, Nb, which is added as a high-temperature strengthening element, has this negative effect.
Although it can be further reduced by the action of Ta,
If C exceeds 0.025% or N is 0.02%
or when the total amount of C+N exceeds 0.03%, it becomes difficult to improve toughness. Therefore, from this point of view, the range is C: 0.025% or less, N: 0.02% or less, and C+N: 0.03% or less.

(4) Mg: In the present invention, Mg secures the oxidation resistance of the foil and the adhesion of the film, and at the same time suppresses the elongation deformation caused by the growth of the oxide film and improves the structure of the catalyst carrier during high-temperature use. It is the most important element added for the purpose of improving durability.

In the present invention, in order to avoid the production problems caused by the addition of Y and other rare earth elements mentioned above, these elements are not added. Ensuring sufficient oxidation resistance and film adhesion is M.
This is achieved by adding Mg, and from this point of view, Mg needs to be at least 0.01% or more. On the other hand, if Mg is added in excess, the effect on oxidation resistance will be saturated, but the steel ingot after casting will be prone to cracking (and hot workability will be reduced), and since Mg is also an active element, The same manufacturing problems as Y and other rare earth elements arise.Therefore, from this point of view, the amount of Mg added is limited to 0.1% or less.

Furthermore, Mg has the important effect of significantly reducing the elongation of the foil due to oxidation, which is an important function in improving the structural durability of the catalyst carrier during high-temperature use. It is an added element. M from this point
According to the inventors' study, the amount of g added is as follows:
This is the same range where the effect on oxidation resistance appears.

Therefore, the addition range of Mg is 0.01% or more and 0.1% or less. Furthermore, by adding Y and other rare earth elements, M
Similarly to g, not only the oxidation resistance but also the elongation of the foil due to oxidation is reduced, but this latter effect is greater when Mg is added.

By the way, although Mg and Y are different from other rare earth elements, they are so-called active elements and have high reactivity in molten steel, but at high temperatures, such as over 1600°C, their reactivity decreases. In high Al-containing steels like the one of the present invention, the Al content is almost the same as that of An, or even more so on the high temperature side! , the reactivity is higher, so that manufacturing problems that occur when Y or other rare earth elements are added are less likely to occur.

(5) Nb, Ta, Mo, W: Nb, Ta,
In the present invention, Mo and W are added to particularly improve the high-temperature yield strength of the foil, thereby improving the thermal fatigue resistance during repeated heating and cooling of the carrier.

That is, when the honeycomb body of the catalyst carrier is subjected to severe repeated heating and cooling, damage may occur due to thermal fatigue.
A high yield strength at 0.degree. C. improves resistance to thermal fatigue, and from this point of view, the addition of the four elements mentioned above is effective.

The action of Nb and Ta combines with C and N in the steel to form carbonitrides, and in addition to this exerting a so-called precipitation strengthening effect, the excess is dissolved in the metal base, resulting in solid solution strengthening. This effect improves high-temperature yield strength. From this point of view, the amount of Nb and Ta added is at least Nb: (9
3・C%/12+93・N%/14) X 1.5 or more, or Ta: (181・C%/12+ 181・N
%/14) X It is desirable to add 1.5 or more, but excessive addition will not only cause cracks to occur during cooling after casting of this type of steel, but also affect hot workability, toughness, or oxidation resistance. In order to reduce this, the upper limit values are set to 3% for both Nb, Ta, and Nb + Ta. In addition, the addition of Nb and Ta has the desirable effect of also improving the toughness of the hot-rolled sheet in the Fe-Cr-A1 alloy like the present invention, and this effect is also sufficient within the above addition range. Demonstrated.

Like Nb and Ta, Mo and W are added for the purpose of improving high-temperature strength, but because they dissolve in solid solution without forming a precipitate phase to a considerable extent, they have a relatively large strengthening effect especially at high temperatures. is obtained stably, and almost no change in metallographic structure occurs even when heated at high temperatures for long periods of time, so this strengthening effect hardly deteriorates over time.

The amount of Mo and/or W to be added is determined from this point of view, and the inventors have found that in order to obtain a sufficient solid solution strengthening effect, it is desirable to add at least 2% or more of the total amount of Mo + W; As the amount of Mo and W added increases, the base material becomes stronger, but the hot workability and toughness of the hot rolled sheet tend to decrease accordingly.From this point of view, the amount of Mo and W added is limited to 4% or less in Mo+W. be done.

(6) Other impurities: Si: In the present invention, Si, like Mg, has the effect of suppressing the elongation of the foil due to oxidation, and from this point of view
% or more, the effect begins to appear, but on the other hand, it also has the effect of reducing the toughness of this type of alloy. In the present invention, since elongation due to oxidation of the foil can be suppressed by the action of Mg, it is desirable that Si be included within a range that is unavoidable in steel manufacturing.

Mn: In the present invention, Mn is particularly concentrated in the very early stage of the oxide film, harming the subsequent formation of the Al2O3 film, and causing structural defects to remain in the film.
It is desirable to limit it to 3% or less.

Since SO3 reduces oxidation resistance, it is desirable to suppress it to 0.003% or less in the present invention.

In the present invention, if FDP is included in a large amount, it will reduce the toughness of the material, so it is desirable that it is included in mass sales, which is unavoidable in the normal steelmaking process, and the amount is about 0.03% or less. .

The Fe-Cr-A1 alloy of the present invention having such a structure can be produced by combining melting, hot rolling, and cold rolling processes similar to those of the mass production process of ordinary ferritic stainless steel, and an appropriate annealing process as necessary. By doing this, it is possible to manufacture foils up to about 50.1111 mm. In addition, the foil produced in this manner, the exhaust gas purification catalyst carrier and the catalyst device constructed using this foil not only have high resistance to abnormal oxidation even in a high-temperature combustion exhaust gas atmosphere, but also have high resistance to abnormal oxidation caused by oxidation of the foil. Since the elongation deformation is very small, the amount of deformation of the honeycomb body is small even when it is used as a catalyst carrier at high temperature for a long time, and therefore it has excellent structural durability.

Furthermore, if the high-temperature yield strength of the foil is improved by adding at least one of Nb, Ta, Mo, and W, the resistance to thermal fatigue as a honeycomb body will be significantly increased. This results in even better structural durability even under repeated usage conditions.

〔Example〕

Next, the effects of the present invention will be explained in more detail with reference to Examples.

(Example 1) Table 1 shows the chemical composition of the steel in which the oxidation resistance of the foil related to the present invention and the elongation due to oxidation at high temperatures when used as a catalyst carrier were investigated. In addition, at this time, Mn is 0.3% for both steels.
Hereinafter, S was 0.003% or less.

All of these steels are melted in vacuum, hot rolled and cold rolled, combined with annealing and descaling processes as necessary.
The thickness of the foil was adjusted to approximately 50 nm. At this time, for B5, cracks occurred during hot rolling and the steel ingot was divided, so no further tests were conducted.

The foil thus obtained was heated at 1100°C for 25 hours in a heating furnace into which engine exhaust gas was introduced, repeated 16 times (400 hours), and the oxidation status was observed.
Abnormal oxidation occurred in the foil of B1 at 100 hours, and similar abnormal oxidation was observed in B2 at 150 hours, whereas all other foils remained silvery white even after 400 hours. The patient appeared to be in good health.

The results are also listed in Table 1.

Next, a sinusoidal corrugation with a period of 3.5 m and an amplitude of 3.2 m is applied to the foil of each component, and the processed (corrugated sheet) and the unprocessed foil (flat sheet) are overlapped and wound. Apparent external shape 35■
A honeycomb-shaped cylindrical body with a length of 65 mm was prepared, and the contact portion between the corrugated plate and the flat plate was brazed and fixed with a commercially available Ni-based brazing material, and then a so-called wash was applied to these honeycomb bodies in a normal supporting process. It was coated and turned into a catalyst.

The catalyst thus obtained was heated at 1000° C. for a total of 300 hours in a heating furnace into which exhaust gas from a gasoline engine was introduced, and the deformation of the honeycomb body at this time was observed. The results are shown in Table 2.

Honeycomb bodies A1 to A5 within the scope of the present invention all have a small elongation amount in the longitudinal direction of 2% or less, whereas B1 of the comparative example. B2 had a large growth rate of 4% or more (B3 and B4 also showed slightly large growth of 2% or more).

From the above, the steel within the scope of the present invention has excellent oxidation resistance, has small elongation deformation even during high-temperature use as a catalyst carrier, and has excellent structural durability during high-temperature long-term use. It is clear that there are

Margin below (Example 2) Next, the results of investigating the effect of the foil according to claim 2 of the present invention will be described.

Table 2 shows the chemical composition of the removed steel. After vacuum melting, each of these steels was hot-rolled and cold-rolled into a foil with a thickness of about 50p, and the same process as in Example 1 was carried out to obtain an outer diameter of 65mm and a length of 10mm.
After making a honeycomb body of 0 mm, it was inserted into a 5US304 cylinder with an inner diameter of 59.5 mm, a length of 102 mm, and a thickness of 1.0 llIn, and each contact part was brazed with a commercially available Ni-based hard solder to form a catalyst carrier. This was further washed coated to prepare a catalyst. After heating these catalysts with a gas oil burner at 1030°C for 12 minutes, cooling air was immediately introduced and forced air cooling was performed 750 times until the inside of the carrier was below 60"C. The damage to the exterior was visually observed, mainly on the outside.The results are shown in Table 3.

In addition, during this test, plate-shaped tensile test pieces taken from cold rolled sheets of these steels were used to test 600, 700 and 800
The high temperature yield strength at °C was measured. These results are summarized in Table 3. The criteria for high temperature resistance is 60
20kg/cii or more at 0'C 13kg/cii at 700"C
cd or more, 4.5 kg/cij or more at 800℃,
Those that satisfy cholera are marked with an ○, and those that exceed this are marked with an X.

For products with high high-temperature strength ranging from A6 to All within the scope of the present invention, some deformation of the carrier, slight foil breakage, and cell deformation are observed even after 750 repeated tests of heating and cooling. There was no damage, and it can be judged that the structural durability as a carrier is ensured. On the other hand, in Comparative Examples 86 and B7, the high-temperature strength is comparable to that of A6 to All, but the elongation due to oxidation of the foil is large, so blisters and steps occur on the end face, and there is a large difference on the outer periphery of the honeycomb body. Cell blockage and foil breakage occurred due to deformation. Also, 8
In all honeycomb bodies made with foils with low high temperature strength of 8 to Bll, after 600 cycles or less, severe cell deformation, cell collapse, and foil breakage occurred frequently on the outer periphery, and in some cases, several layers from the outer periphery Foil breaks on the corrugated or flat plate occurred almost all around the honeycomb body, and the inner cylindrical part protruded toward the entry side, and a portion of the end face on the entry side was chipped or fell off. .

Based on these results, items judged to have excellent structural durability are marked with an O mark, and those with poor structural durability are marked with an × mark in the column for determining the damage status after the thermal test in Table 4. did.

From the above, the catalyst carrier made of the foil of the present invention, which has improved high-temperature yield strength by adding Nb, Ta, Mo, and W, can maintain its structural integrity even under harsh repeated heating and cooling environments. It is clear that it has excellent durability.

Below is a blank table [Effects of the invention] As is clear from the examples, FeCr-A according to the present invention
jl! Stainless steel foil not only has high resistance to abnormal oxidation in engine exhaust gas, but also has low elongation due to oxidation, so when used as a honeycomb body for a catalyst carrier, it has excellent structural durability during high-temperature use. Furthermore, this foil material with improved high-temperature strength is resistant to heating and
Since the heat fatigue resistance of the honeycomb body during repeated cooling and use is improved, structural durability can be further improved.

Therefore, the foil according to the present invention is highly effective in improving the structural durability of the catalyst carrier, and due to these effects, the stainless steel foil according to the present invention is suitable for forming catalyst carriers for automobiles and the like.

Claims (1)

[Claims] 1. In weight percent, Al: 4.5% or more and 6.5% or less Cr: 13% or more and 25% or less C: 0.025% or less N: 0.02% or less C+N: 0. A heat-resistant stainless steel foil for a combustion exhaust gas purification catalyst carrier, characterized in that it contains Mg: 0.01% or more and 0.1% or less, and the balance consists of Fe and inevitable impurities. 2. In weight percent, Al: 4.5% or more and 6.5% or less Cr: 13% or more and 25% or less C: 0.025% or less N: 0.02% or less C+N: 0.03% or less Mg: 0. 01% or more and 0.1% or less, further including 3% or less Nb, 3% or less Ta, and 4%
It is characterized by containing at least one selected from the following Mo and 4% or less W in the range of Ta + Nb: 3% or less, Mo + W: 4% or less, and the balance consists of Fe and inevitable impurities. Heat-resistant stainless steel foil for combustion exhaust gas purification catalyst.