WO2000008223A1 - Oxidation-resistant metal foil - Google Patents

Abstract

The invention relates to a method for producing a metal foil consisting of an iron-chromium-aluminium alloy and presenting high oxidation-resistance at elevated temperatures, which foil is produced by hot-dip aluminizing an iron-chromium support strip using an aluminium-silicon alloy. The foil has the following composition, in % by weight: between 18 and 25 % Cr, between 4 and 10 % Al, between 0.03 and 0.08 %Y, a maximum of 0.01 % Ti, between 0.01 and 0.05 % Zr, between 0.01 and 0.05 % Hf and between 0.5 and 1.5 % Si, the remainder being made up of iron and process-related impurities. The total aluminium content of the coated metal foil is at least 7 % near the surface and decreases towards the centre without falling below 3 %. The compound is rolled and during rolling is subjected to process annealing at approximately 800 DEG C as a result of which the volume change caused by final annealing carried out after rolling is reduced to </= 0.5 %.

Description

 OXIDATION RESISTANT MET ALL FILM

The invention relates to a method for producing an iron-chromium-aluminum-metal foil with high resistance to high temperatures.

The US-A 4,414,023 describes a steel with 8.0-25.0% Cr, 3.0-8.0% Al, 0.002-0.06% Se, max. 4.0% Si, 0.06-1.0% Mn, 0.035-0.07% Ti, 0.035-0.07% Zr, including inevitable impurities.

EP-A 0 387 670 is an alloy with 20-25% Cr, 5-8% Al, max. 0.01% P, max. 0.01% Mg, max. 0.5% Mn, max. 0.005% S, remainder Fe, including unavoidable impurities, from which further alloying elements such as 0.03% Y, 0.004% N, 0.02 - 0.04% C, 0.035 - 0.07% Ti, 0.035-0.07% Zr and 0.035-0.14% Hf are added.

However, the documents mentioned are based on traditional manufacturing processes, namely the conventional casting of the alloy and the subsequent hot and cold forming. Here the disadvantage must be accepted that iron-chromium-aluminum alloys are difficult to produce by conventional rolling and annealing processes and that this disadvantage becomes more and more important with an increase in the aluminum content. With aluminum contents of more than 6%, the problems associated with these processes become so great that it is practically no longer possible to process this alloy on an industrial scale, so that such high-aluminum alloys have so far not been offered on the market. Higher proportions of aluminum are, however, indispensable in this production process in order to further improve the resistance to oxidation or to increase the electrical resistance, as is required for certain applications. To eliminate these disadvantages, US Pat. No. 5,336,139 specifies a method in which foils made of iron-chromium-aluminum alloys are produced by coating a suitable iron-chromium steel on both sides by roll-cladding with aluminum or aluminum alloys. This composite is exclusively cold rolled and finally diffusion annealed so that a homogeneous structure is created. The core material can be made of AISI 434 stainless steel, optionally with Ce and La additives.

EP-B 0 204423 describes another way of producing multilayer metal foils, namely that of fire aluminizing. However, this document is based on an iron-chromium alloy without reactive additives. Now, as described in Example 2 below, it has been shown that such materials are inadequate for use as catalysts because they are not sufficiently resistant to oxidation. Additions of reactive elements are absolutely necessary for use as a catalyst. Furthermore, the cited document describes that aluminum alloys containing silicon have not given satisfactory results in practice.

EP-B 0 516 097 has disclosed a scale-resistant Fe-Cr-Al alloy with additions of La, Y and Hf, which can be produced by coating, in particular roll cladding.

The invention is based on the object of providing an iron-chromium-aluminum material which has an oxidation resistance which is up to 1,100 ° C. better than that of conventional materials, in order in particular to take account of the requirements for environmental protection. If necessary, the material should be able to be varied in such a way that the electrical resistance increases, which is necessary for preheating, in particular of catalysts in the cold start phase, for example in the case of certain types of the pre-catalyst connected upstream of the actual main catalyst. The material should also be able to be produced inexpensively. For specific applications, the dimensions of a sheet made from this material should only change slightly in terms of final thickness even in the case of annealing up to approximately 1,150 ° C. This object is achieved by a method according to claim 1.

Advantageous further developments of the subject matter of the invention can be found in the subclaims.

As a result of the intermediate annealing, changes in the dimension of a sheet made of this material in terms of final thickness can also be realized in the case of final annealing up to approximately 1,150 ° C. below 0.5%.

If the composite is annealed, the temperature and holding time must be selected so that on the one hand rolling to the final thickness is possible without any problems. H. the formation of intermetallic phases should be suppressed as far as possible. On the other hand, such an intermediate annealing can be used to diffuse part of the aluminum into the carrier strip. Surprisingly, this has the advantage that the change in volume during the heat treatment to the final thickness can be significantly reduced.

The metal foil according to the invention can be obtained, for example, by block casting, but can be produced even more cost-effectively by continuous casting, as well as subsequent hot and cold forming. With a thickness between 0.5 and 2 mm, this tape is coated on each side with a layer made of aluminum with 8 - 13% silicon. The coating is applied by way of fire aluminizing. The composite produced in this way is preferably cold-rolled with at least one intermediate annealing to form a film and then also contains the mechanical requirements for the further processing steps, such as for example the corrugation which is required in the production of the catalyst.

A final heat treatment is advantageously carried out at temperatures between 700 and 1200 ° C., a further development which is meaningful in terms of economical production is that the heat treatment in the form of the diffusion annealing known per se after the final shaping of the end products produced from the metal composite foil and " in situ " after the completion of the catalyst devices, or only on the finished catalyst carrier body. For certain other applications, eg for use as a heating element, the diffusion annealing is carried out directly on the film.

Surprisingly, this leads to a significant improvement in the oxidation resistance, especially for high-temperature applications. It is particularly important to choose the type of reactive additives. As already described, it depends on the one hand on the type of additives and on the other hand on their upper limits. So under no circumstances should 0.08 mass% Y be exceeded. The silicon additives in the coating are also essential because they have an advantageous effect on the diffusion behavior in the desired manner.

A further advantageous embodiment of the invention consists in further alloying the iron-chromium-aluminum alloys described above with the aid of the coating with aluminum and silicon in order to increase their oxidation resistance and their electrical resistance. This is also possible if you start with a carrier tape that already contains up to 6% aluminum and then only requires a thinner coating.

The subject matter of the invention is explained in more detail by the examples below, examples 1 and 2 merely representing comparative examples of the subject matter of the invention.

example 1

The following alloy was melted and processed in the conventional way by rolling and annealing in mass%:

20.45% Cr 0.20% Si 0.05% Hf 0.02% Zr <0.01% Ti 5.55% Al 0.06% Y. Their oxidation behavior was examined after aging at 1,100 ° C. and compared with that of an alloy with rare earth additives, with a 20% smaller change in mass being found in the material with added Y and Hf.

If you now compare this with an alloy that contains Y, but also Ti and no additions of Hf, its mass also changes by more than 10% more than the alloy with additions of rare earths.

It follows from these investigations that the composition listed as Example 1 stands out from the other, more common alloys in terms of its oxidation resistance, as is necessary for applications in the catalyst and heating conductor area.

Example 2

A carrier material with the composition in mass%

15.91% Cr

<0.01% Y

<0.01% Zr

<0.01% Hf remainder iron and process-related impurities was by means of fire aluminizing

3.8% Al and 0.4% Si produced, formed into a film by rolling and then diffusion annealed. After aging for 400 hours at 1,100 ° C, the carrier material increased in mass 10 times as much as a comparative alloy with additives from SE; its length changed by a factor of 2 more.

Example 3

The alloy according to the invention was produced by fire aluminizing and has the following chemical composition (in% by mass):

18.35% Cr 0.59% Si 5.4% AI 0.03% Zr 0.04% Y 0.05% Hf balance iron with process-related impurities.

It was diffusion annealed at 1,100 ° C and then showed the following aluminum distribution over the strip thickness:

10% by mass of aluminum were determined on the surface, 5 μm below the surfaces about 5% and 3.5% inside the band.

This has a particularly advantageous effect on the resistance to oxidation. The change in mass at 1,100 ° C. is 25% less than in the case of a comparative alloy which was produced in the conventional way, for example as described in Example 1. Another advantage is that the cost of manufacturing by fire aluminizing is only about 75% of the cost of conventional alloys.

Example 4

A strip was cut out from the tape produced according to Example 3 at a thickness of 0.11 mm. Individual pieces were annealed at temperatures shown in the table below and then rolled to a final thickness of 50 μm. During the subsequent final annealing at 1,100 ° C, the length and width changed by less than 0.5%.

Temperature of change of

Intermediate annealing / ° C length or width /%

800 0.3

900 0.2

1,000 0.2

Claims

claims 1. Process for producing an iron-chromium-aluminum-metal foil with high high-temperature oxidation resistance, which is produced by fire-aluminizing an iron-chromium carrier tape with an aluminum-silicon alloy, the foil having the following composition in mass%: 18-25% Cr 4 - 10% AI 0.03 - 0.08% Y max. 0.01% Ti 0.01-0.05% Zr 0.01-0.05% Hf 0.5 - 1.5% Si remainder iron and process-related impurities, the total aluminum content of the coated metal foil near the surface being at least 7% and not falling below 3% towards the inside, the composite being rolled and during the rolling of an intermediate annealing at about Is subjected to 800 ° C, by which the volume change of a final annealing following the rolling process is reduced to <0.5%. 2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that an intermediate annealing is carried out at 400 ° C to 600 ° C during rolling, which reduces the volume change in the final annealing to less than 0.5%. 3. The method according to claim 1 or 2, wherein the total aluminum content near the surface is set to at least 10%, which does not drop below 5% towards the inside 4. The method according to any one of claims 1 to 3, characterized in that following the rolling process, a shaped body is first produced and this is then subjected to diffusion annealing. 5. Film, produced according to one of claims 1 to 4, which has an electrical resistance of more than 1.5 Ωmm / m. 6. Use of a metal foil produced according to one of claims 1 to 4 as a heating conductor resistor. 7. Use of a metal foil produced according to one of claims 1 to 4 as a carrier for exhaust gas catalysts.