EP0236823B1 - Metallic semi-finished product, process for its manufacture and uses of the semi-finished product - Google Patents

Abstract

The invention relates to a metallic semi-finished product based on iron and/or nickel and/or cobalt and containing 2 to 16% of aluminum, 12 to 30% of chromium and at least one highly reactive element X, in particular from the group consisting of the rare earth metals and/or their dispersed oxides. The characteristic feature of the invention is that metallic columnar crystals are formed at least in the surface region of the semi-finished product and columnar crystals consisting predominantly of aluminum oxide and/or chromium oxide grow out of the said metallic columnar crystals, the axes of the two types of columnar crystals being predominantly at right angles to the geometric surface of the semi-finished product.

Description

The invention relates to a method for producing semi-finished metal products in the form of wire, rods, billets, tubes, in particular sheet metal or strips, for applications in which a high thermal shock resistance and large surface area and / or chemical resistance and / or low thermal conductivity of the surface layer is required , in particular for catalyst supports, soot filters, heating conductors, aerosol filters, linings of chemical and energy converting systems. The semi-finished product is based on iron and / or nickel and / or cobalt with 2 to 16% aluminum, 12 to 30% chromium and up to 4% of at least one highly reactive element from the group Y, Zr, Ti, Ce, Sm, Hf , La, Th, U, V, W, Nb, Mo, Gd, Si, Mg, Ca and / or their dispersed oxides. An alloy from this area was made from J. Electrochem. Soc., Solid-State Science Technology, Volume 131, No. 4, pages 923-931. By annealing strips of this alloy at temperatures above 1000 ° C in oxygen, oxide-containing surfaces can be created.

It is known (Strafford KN, High temperature Corrosion of alloys Containing rare earth of refractory elements: a review ..., High Temperature Technology Vol. 1 No. 6, Nov. 1983) that metal alloys of the MCrAlX and MCrAlZX types with M as iron and / or cobalt and / or nickel as the base and X small additions by weight of highly reactive elements such as Y, Zr, Ti, Ce, Sm, Hf, La, Th, U, V, W, Ta, Nb, Mo, Gd, Si, Mg, Ca and Z as one element or its oxide from the series of X, but a different element than that chosen for X, provide an improvement in the oxide layer properties. The adhesion of the oxide layer, which consists of individual oxide grains, is improved and the oxidation behavior is also favorably influenced.

It is also known (Ramanarayan TA, Raghavan, M. and Petkovic-Luton, R., "The Characteristics of Alumina Scales Formed on Fe-Based Yttria-Dispersed Alloys", J. Electrochem. Society, April 1984, Vol. 131 No 4., 923-931) that in a base alloy particularly finely dispersed oxides of rare earth metals, such as Y₂O₃, exert a similarly improving influence.

The alloys mentioned form, depending on the composition, either layers composed predominantly of chromium oxide or aluminum oxide with high selectivity or layers composed of Al₂O₃-chromium oxide mixed crystals. At use temperatures of about 900 ° C and more, the alloys are selected that form Al₂O₃ layers.

The disadvantage of the layers and components made of the described alloys produced by the described methods, in the case of use in the case of particularly frequent temperature changes with high temperature differences, is that individual areas of the oxide layer flake off. The resulting defects are healed again by the specified alloys under suitable conditions, but the occurrence of flaking under high temperature cycling limits the service life and leaves the materials known today for some applications, e.g. do not appear to be suitable as a carrier for catalytically active substances, in particular precious metals for the detoxification of combustion gases.

Another disadvantage is that alloys with more than about 6% aluminum, for example US Pat. No. 4,414,023, cannot be produced as a foil by rolling, or their production involves considerable costs. For temperature stability over a long time, however, it is necessary to heal defects in the oxide layers by means of aluminum to be supplied from the alloy, and consequently the highest possible Al content makes sense.

From US-A-4 414 023 a ferritic stainless steel is known which can be processed hot and is resistant to temperature changes, oxidation and scaling at elevated temperatures. The iron-chromium-aluminum alloy contains cerium, lanthanum and other rare earth metals and is suitable for forming a textured aluminum oxide film on the surface. The alloy is produced in the usual way by melting. The typical steel impurities such as oxygen, nitrogen and sulfur are reduced before the rare earth metals are added to the melt. Any conventional process, including arc furnaces, AOD and vacuum induction melting processes, is applicable. The melt is poured into blocks, bars, strips or sheets. The steel is then hot and / or cold rolled and subjected to the usual procedures such as descaling and annealing. The ferritic stainless steel is then heat treated to form an alumina surface.

From EP 0232793 A1 the formation of an oxide film with primarily aluminum oxide grains on an aluminum-containing stainless steel is known by pouring the molten metal onto cooled rollers and allowing it to solidify at a rate of more than 1000 K / s and then oxidizing the strip.

The object of the invention is to provide a semi-finished metal product which, preferably without an intermediate layer (wash coat), represents a temperature change-resistant adhesive base for a catalytically active coating. The task also includes the creation of a method which leads in the simplest possible way to a metallic semi-finished product with the property mentioned.

To achieve the object, a semi-finished metal product based on iron and / or nickel and / or cobalt with 2 to 16% aluminum, 12 to 30% chromium and up to 4% of at least one highly reactive element from the group Y, Zr, Ti , Ce, Sm, Hf, La, Th, U, V, W, Ta, Nb, Mo, Gd, Si, Mg, Ca and / or their dispersed oxides, preferably at least 0.01% of these elements or oxides, and Inevitable impurities proposed according to the invention that the semifinished product is solidified from the molten state in a front, which is predominantly parallel to the geometric surface, with a cooling rate of 10³ to 10⁶ K / s, then briefly in a bonded oxygen at a temperature of 800 to 1000 ° C Form contained gas, in particular CO₂, is set under reducing conditions and then in the temperature range from 800 to 1000 ° C for up to 25 h in air, at least in the surface area of Semi-finished metal stem crystals are formed, from which stem crystals consisting predominantly of aluminum oxide and / or chromium oxide have grown, the axes of both types of stem crystal being predominantly perpendicular to the geometric surface of the semi-finished product.

Normal steel contaminants are carbon, nitrogen, oxygen, phosphorus, sulfur, manganese, copper and nickel.

The surface structure can be clearly seen from the attached micrographs.

Figure 1 shows a semi-finished product surface produced using conventional methods with irregularly lying whisker-like oxidic stem crystals.

Figure 2 shows the semi-finished product surface as it is formed when the method according to the invention is carried out. The large areas are the metal stem crystals, from the surface of which the oxidic stem crystals have grown like hair.

Figure 3 shows the oxide stem crystals in 10 times magnification compared to Figure 2. According to this, the axes of the oxide stem crystals are predominantly perpendicular to the geometric surface of the semi-finished product, which is formed by the surfaces of the metal stem crystals.

Contents of 14 to 25% chromium and 5 to 9% aluminum in the matrix made of iron and / or nickel and / or cobalt have proven to be particularly suitable for the formation mainly of aluminum and chromium oxide or, depending on the annealing conditions, practically exclusively of aluminum oxide existing oxidic stem crystals. In addition to the elements mentioned, other elements which do not impair or even favor the formation of stem crystals can be present.

The average grain diameter D of the metal stem crystals can be 5 to 50 µm. Depending on the alloy and wall thickness, the cooling rate is preferably chosen so that the average grain diameter is D = 5 to 30 μm. The length of the grains should be L = 15 µm to the thickness of the tape, preferably L = 20 to 100 µm, with L: D ≧ 3.

It has been shown that, due to the crystallographically predominantly uniform alignment of the grains of the structure, there are closely spaced but separate oxide grains with a diameter of d = 0.05 to 3 μm, depending on the growing conditions, and lengths of l = 0.5 form up to 15 µm with l: d ≧ 3.

The invention provides a semi-finished metal product in which metal stem crystallites are formed at least in the surface area by the directional solidification of the metal alloy. Due to the high cooling rate and thermal treatment, the additional elements from the aforementioned group X, which are highly reactive, i.e. are highly oxygen-affine, or their oxides, which are present in finely divided form in the surface area, in the subsequent heat treatment as germs. At the same time, as a kind of dowel, they ensure a high level of adhesive strength for the thermally grown oxidic stem crystallites. A further improved nucleation is achieved by briefly heating as the first thermal treatment stage under reducing conditions or those that occur during this treatment. This is followed by the long-term annealing treatment under oxidizing conditions, preferably in air, in which the oxidic stem crystallites grow, which mainly consist of aluminum oxide and / or chromium oxide, the axes of which are also predominantly perpendicular to the geometric surface of the semi-finished product. Due to this stem crystal formation, an extraordinarily large surface is created, in which a metallic and / or ceramic coating can be very well anchored.

• das Metallband braucht im wesentlichen nicht mehr plastisch verformt zu werden; Bereiche plastischer Verformung führen bekanntlich zu unkontrolliertem Wachstum der Körner des Metallgefüges bei den anschließenden Behandlungsschritten bei erhöhten Temperaturen und damit zur unregelmäßigen Ausbildung der Körner der Oxidschicht;
• die Rolle bzw. Rollen können aus einem Material wie Stahl oder Keramik mit einer geringeren Wärmeleitfähigkeit hergestellt werden, als dies mit Kupferrollen für die Herstellung amorpher Bänder notwendig ist;
• die Rolle bzw. Rollen werden wenn nötig mit Hilfe von erwärmtem, umlaufendem Öl auf eine vorgegebene Temperatur gehalten, wodurch auf diese Weise die gewünschte Kristallgröße des Metallgefüges eingestellt wird und eine besondere Wärmenachbehandlung zur Korngrößeneinstellung entfallen kann.

The size of the grains of the metal structure is determined by the cooling rate and the heat content of the molten metal. For the production of tapes with a thickness of approx. 50 µm, As they are needed, for example, as a carrier for catalytic converters for motor vehicles and power plants, it has proven to be advantageous to work with a device in which, under a crucible, which has a long, thin gap as a nozzle, the gap possibly using a chamfered ceramic plug rod can be opened, one or two rollers are arranged. The roller or rollers can have a predetermined waviness on their peripheral surface, so that the metal beam solidifies on one roller or in the predetermined gap between the two rollers in the geometric shape required for the intended use. The arrangement has the following advantages:

• the metal strip essentially no longer needs to be plastically deformed; Areas of plastic deformation are known to lead to uncontrolled growth of the grains of the metal structure in the subsequent treatment steps at elevated temperatures and thus to the irregular formation of the grains of the oxide layer;
• the roll or rolls can be made of a material such as steel or ceramic with a lower thermal conductivity than is necessary with copper rolls for the production of amorphous strips;
• if necessary, the roller or rollers are kept at a predetermined temperature with the aid of heated, circulating oil, as a result of which the desired crystal size of the metal structure is set and a special heat treatment for grain size adjustment can be omitted.

It has now been found that profiled, finely crystalline strips of approximately 20 to 200 μm can be produced in this way, with the aid of cooled copper rolls, possibly up to approximately 500 μm and more. It is preferable to produce tapes approximately 40 to 70 µm thick.

In this way it is possible to produce aluminum oxide layers which consist of individual oxide grains which have grown separately but largely uniformly and have a diameter of preferably d = 0.1 μm to 0.3 μm and a length l = 4 to 15 µm exist, so that the deposition of catalytically active substances can be carried out without an additional intermediate layer (wash coat), such as in the case of purely oxidic catalyst supports.

The described stem crystal formation at least in the surface area of the semi-finished product can alternatively be achieved by pouring onto cooled rolls by briefly melting a surface layer of the cast or rolled semi-finished product with subsequent self-cooling.

The invention is explained below using three exemplary embodiments.

example 1

With the help of a roller made of steel, which was kept at a constant temperature by an oil circuit, a metal strip 50 µm thick was produced at a cooling rate of 10rate to 10⁵ K / s. The material composition is 20% chromium, 5% aluminum, 0.15% cerium, 0.01% lanthanum, the rest iron with small amounts of Si, Mn, C, S, P, Ni as trace elements.

The tape was then preheated to 900 ° C dry carbon dioxide gas for one minute and then held in air at 925 ° C for 16 hours. A cold rolled sheet of the same material composition was also treated after recrystallization annealing.

It can be seen that the stalk-shaped oxide grains of the cast strip have diameters of approximately 0.2 μm and lengths of on average approximately 4 μm and that they are arranged almost uniformly perpendicular to the surface of the metal grains, while the sample produced by rolling is scaly , partially in contact with each other grains of different orientation and length of up to about 3 microns.

Example 2

A 10 mm thick plate made of the material described in Example 1 was bombarded with an electron beam in such a way that a spot approximately 0.5 mm in diameter was melted down to a depth of 100 μm. The plate was then treated in CO₂ gas at 900 ° C for 1 minute. The procedure was as in Example 1.

It was found that the oxide grains or whiskers produced in the area of the melting spot have a similar quality to that of the cast sample according to Example 1.

The sample was subjected to several temperature changes, being heated to approximately 1000 ° C. and quenched in an oil bath. The stem oxide grains in the area of the melting spot were not attacked by the treatment, the oxide layer on the rest of the sample surface shows individual flaking.

Example 3

A plate according to Example 2 with a similar melting spot was produced, then an etching was carried out so that the grain boundaries were exposed to a depth of 20 μm and the procedure according to Example 1 was then continued. It was found that a radiation-like oxide-grain layer of the same quality as in Example 1 had grown on the metal grains which had been partially exposed.

In this way, it is possible to produce a particularly large surface area, so that preferably no or only a very thin (20 μm thick) intermediate layer (wash coat) appears to be necessary for use as a catalyst support.

It is possible to additionally coat the oxide layer produced in this way with sanding, slip, flame or plasma spraying or also by other known methods with ceramic or metals. The sequence of the steps of slip-drying-plasma spraying even allows almost gas-tight oxide layers to be applied, which are only firmly connected to the metal body via the stem-shaped oxide grains and therefore have a high resistance to temperature changes.

Claims (9)

1. A process for the production of semi-finished metal products on the basis of iron and/or nickel and/or cobalt with 2 to 16% aluminium, 12 to 30% chromium and up to 4% of at least one highly reactive element from the group Y, Zr, Ti, Ce, Sm, Hf, La, Th, U, V, W, Nb, Mo, Gd, Si, Mg, Ca and/or their dispersed oxides,
characterized in that the semi-finished product is allowed to solidify from the molten condition predominantly parallel with the geometrical surface at a cooling speed of 10³ to 10⁶ K/sec, whereafter annealing is performed first briefly at a temperature of 800 to 1000°C in a gas, more particularly CO₂, containing oxygen in the bonded form, in reducing conditions produced thereby, and annealing then being performed in air over the same temperature range for up to 25 hours, metal fringe crystals being formed at least in the surface zone of the semi-finished product, from which fringe crystals consisting mainly of aluminium oxide and/or chromium oxide are grown, the axes of the two kinds of fringe crystal lying predominantly perpendicularly to the geometrical surface of the semi-finished product.

2. A process according to claim 1, characterized in that the semi-finished product is produced by strip casting using at least one cooled roller or by continuous casting.

3. A process according to claim 1, characterized in that the surface of the semi-finished product produced is briefly melted and cooled in air, in vacuo or under inert gas.

4. A process according to one of claims 1 to 3, characterized in that the annealing in air at a temperature in the range of 850 to 1000°C is performed for 4 to 20 hours.

5. A process according to claims 1 to 4, characterized in that the annealing is performed in reducing conditions at a temperature in the range of 880 to 980°C for 0.5 to 4 minutes.

6. A process according to one of claims 1 to 5, characterized in that 14 to 25% chromium is added to the iron and/or nickel and/or cobalt alloy.

7. A process according to one of claims 1 to 6, characterized in that 5 to 9% aluminium is added to the iron and/or nickel and/or cobalt alloy.

8. Use of a semi-finished product produced according to claims 1 to 7 as an adhesive base, resistant to changes in temperature, for a metal or ceramic coating.

9. Use of a semi-finished product produced according to claims 1 to 7 for catalyst supports.

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