SE528027C2 - Use of a ferritic steel in catalysts for diesel engines - Google Patents

Abstract

Ferritic steel alloyed with a balanced quantity of chromium, aluminium and reactive elements and that has the following composition (in % by weight): Ni up to 1 Cr 15-25 Al 0.75-3.7 Si max 0.6 Mo and/or W 0-3 Ti and/or Nb 0-1 Cup to 0.2 N up to 0.2 one or more of the reactive elements Zr, Hf and REM>0 balance iron and normally occurring impurities, wherein the equation Cr+3AI>=26 is satisfied. The ferritic stainless steel is suitable for use as substrate material in catalytic converters for diesel engines in heavy vehicles at temperatures in the interval of 600-900° C. and a specific relation between the content of chromium and aluminium being satisfied.

Description

20 25 30 528 027 2 manufacturing costs. For example, a metal monolith can be made with thinner wall thicknesses, normally between 1/2 and 1/3 compared to the corresponding ceramic monolith, which gives lower pressure drop, so-called "back pressure", larger effective area and greater catalytic capacity. smaller and with a more flexible design.Ternary conductivity is better in metal, which entails less risk of overheating than in ceramic monoliths.

Diesel engines are often used in vehicles with long operating times and where the cost of downtime and catalyst replacement is high. The requirements for service life and reliability of the catalyst and thus also the support material are therefore also high. The most common metal foil used for catalytic exhaust purifiers for diesel engines is today the same as that used for petrol engines and consists mainly of an Fe-Cr-Al with 19-21% Cr, 5-7% Al, one or fl your reactive elements e.g. . Zr. Y. Hf. La and Ce and residual iron with naturally occurring impurities. This material has acceptable, but not optimal, oxidation properties for the temperature ranges used in diesel engines and has poor mechanical properties due to the high aluminum content - which results in poorer processing properties and a high manufacturing cost. There is thus a need for a material which has good properties in cyclic oxidation at maximum temperatures in the range 600-900 ° C and with good production economy.

Description of the Prior Art Today's exhaust gas cleaner material is optimized for the purification of exhaust gases from petrol engines which operate at temperatures above 900 ° C and preferably between 1000-1200 ° C. It is well known that Fe-Cr-Al alloys form an α-alumina at temperatures above 900-950 ° C, which gives lower oxide growth and better oxidation properties than γ-alumina. Experience has shown that Fe-Cr-Al alloys require an aluminum content of over 4.5% to form single-phase α-alumina applied in catalytic exhaust purifiers. Exhaust gas purifiers for diesel engines 'are assumed to have an operating temperature below 600 ° C which has temporary or regular' peaks of up to a maximum of 900 ° C, which means that the most critical temperature range for oxidation properties is 600-900 ° C and 528 027 3 these conditions therefore new requirements for the oxidation resistance of the material. At these temperatures, an Fe-Cr-Al alloy forms a mixed oxide consisting mainly of chromium oxide and γ-alumina where the chromium oxide is less stable and risks evaporating or peeling off from the surface while the alumina is considerably more stable. An oxide consisting for the most part of alumina is, from experience, also advantageous for the adhesion of the coating, a so-called wash-coat, which is applied to the metal surface of the finished monolith.

However, it is relatively unknown how this mixed oxide occurs in temperatures below 900 ° C and how the composition of the steel affects the growth and composition of the oxide. Japanese Patent Application No. JP2000297355 describes a steel for use in exhaust systems at temperatures 2800 ° C which is in principle an Fe-Cr alloy with additives, where e.g. copper is added for improved machinability. The low content of aluminum of max. 0.15%, however, gives a relatively high oxidation rate at temperatures 2800 ° C and the material is therefore not suitable for the production of thin film where the requirements for oxidation rate are higher. Japanese Patent Application No. JP2002004011 focuses on use in exhaust systems at temperatures between 650-800 ° C but is as in the previous example the material is not alloyed with aluminum and thus has similar oxidation resistance and is therefore not suitable for use as a foil material in a catalytic exhaust purifier in diesel vehicle.

The manufacturing costs of Fe-Cr-AI alloys are generally high due to low thermal ductility and crack sensitivity in hot working and low impact strength and machinability at room temperature. An improvement of these mechanical properties to reduce the manufacturing cost is therefore preferable and especially for catalytic exhaust purifiers for heavy diesel engines as these require a large amount of material and thus are more price sensitive than currently used materials as above. It is therefore an object of the invention to provide a ferritic stainless steel for use as a substrate material in catalysts for 528,027 4 diesel engines in heavy vehicles at temperatures in the range SOU-900 ° C and wherein a specific ratio of the content of chromium and aluminum is met.

Description of the invention These objects are achieved in accordance with the independent claims 1 and 3.

Additional embodiments are specified in the subclaims.

On the surface of the alloy of the present invention during operation at temperatures of up to 900 ° C, chromium oxide is formed which promotes the formation of a pure alumina where a relationship between high chromium contents and the aluminum content of the substrate according to the above composition can be ascertained.

The chromium content of the alloy should be in the range 15.0-25.0% by weight, preferably 17.0-23.0% by weight. In order to optimize the properties of the alloy in the application substrate material for catalysts in diesel engines for heavy vehicles, it has been shown that the best results for the life-determining properties such as strength and oxidation resistance are shown when the equation Cr + 3A | z 23 is satisfied, preferably Cr + 3A | 26, most preferably Cr + 3A | of 29.

Additives of reactive elements, such as Zr, Hf and / or rare earth metals (REM) such as Sc, Y, La and Ce, further improve the oxidation resistance by reducing the tendency for scaling and fl-aging, ie the tendency for the oxide to detach from the metal on cooling or mechanical deformation for both aluminas and chromium oxides.

The total content of rare earth metals (REM) shall be limited to a maximum of 0.2% by weight, whereby one or more of the elements Ce, La, Sc and / or Y may be added. The preferred content of REM should be in the range of 0.01-0.2% by weight. 10 15 20 25 30 528 027 5 In aluminum alloy ferritic steels have a nod! a embrittlement effect, so the content Ni should be limited to max. 1.0% by weight.

The molybdenum can be added to the alloy to achieve improved strength at temperatures above 600 ° C. In the alloy of the present invention, molybdenum can be completely or partially replaced by tungsten. The content Mo and / or W should be 0 up to 3% by weight, preferably> 0 up to 2.5% by weight and most preferably> 0 up to 1.0% by weight.

Addition of one or more of the substances Ti, Nb, Zr and Hf together with carbon and nitrogen gives precipitates of carbides and / or nitrides which provide the material with increased mechanical stability and resistance to grain growth and thereby improve the mechanical properties of the material.

Since hafnium is considered to have a similar effect on the properties of the alloy, this substance may replace zirconium in whole or in part. The preferred content of zirconium and / or hafnium should be a maximum of 0.2% by weight. Preferably this content should be in the range> 0 - 0.2% by weight.

The material according to the invention has, through a balanced balance of chromium and aluminum, obtained a considerable reduction of the oxidation rate in the temperature range 600-900 ° C, which is the critical temperature range for use as a carrier material in catalytic exhaust purifiers for diesel engines.

These properties are obtained when the alloy satisfies the form Cr + 3Al 2 23 within the given limits for chromium and aluminum, which gives a favorable oxide formation and oxide composition. Additives of rare earth metals, such as Sc, Y, La and / or Ce, also contribute to good adhesion of the oxide and therefore reduce the risk of scaling and fl-aggregation at these temperatures.

The alloy also has good mechanical properties in both hot and cold conditions, which results in low manufacturing costs and good economy in the finished product. 528 027 6 The final product of the material can be made in the form of strip or foil with a thickness less than 200μm or in the form of wire with a diameter of less than 200μm and is intended for use as a carrier material in catalytic exhaust gas purifiers for diesel engines at temperatures up to 600 -900 ° C.

Table 1 shows some examples of the composition of an alloy according to the present invention.

Table 1 Element Example 1 Example 2 Example 3 Example 4 (wt%) (wt%) (wt%) (wt%) Ni 0.18 0.24 0.28 0.08 Cr 20.95 20, 94 22.42 22.1 1 Al 3.2, 3.09 2.94 0.83 Mo <0.01 0.01 2 2.02 W <0.01 0.01 <0.01 <0.01 Ti 0.01 0.004 0.1 0.12 Nb <0.01 0.01 0.81 0.77 Zr <0.01 0.01 <0.01 0.002 C 0.1 0.01 0.105 0.097 Si 0, 89 0.28 0.45 0.11 P 0.011 0.013 0.016 <0.003 S 0.01 0.0001 0.01 0.001 N 0.013 0.018 0.022 0.032 Mn 0.1 0.11 0.18 0.11 Ce 0.036 0.044 0.016 0.009 La 0.018 0.022 0.008 0.0045 Co 0.02 0.03 _ 0.03 <0.01 V 0.05 0.05 0.09 0.028 Cu <0.01 0.01 0.03 <0.01 Mg 0.05 0.0065 < 0.05 <0.05 The alloy of the present invention can be made by conventional melt metallurgy and casting by casting or continuous casting followed by hot working and then cold working to the final dimension. The product can be further processed into foil, thin tape or wire. The alloy can also be directly cast into strip, sheet or foil with a width-to-thickness ratio> 50 with a thickness after casting below 5 mm, followed by cold working or a combination of hot and cold working. Possible alternative manufacturing difficulties are that a substrate material with a lower aluminum content is coated with pure aluminum or an aluminum alloy so that the correct composition is achieved. Coating of the substrate alloy with aluminum alloy can take place by previously known processes such as dipping in molten, electrolytic coating. rolling of strips of the substrate alloy and the aluminum alloy, deposition of solid Al alloy from a gas phase by so-called CVD or PVD technology. The coating with Al alloy can take place after the substrate alloy has been rolled down to the desired finished thickness for the product, or in a larger thickness. In the latter case, a diffusion annealing can be carried out to effect a homogenization of the material, after which rolling in one or more steps is carried out to obtain the finished product. Rolling can also be done directly on a coated product with a greater thickness than the desired finished thickness. In this case, the rolling may be followed by annealing. An example of film making of a Fe-Cr-Al alloy by PVD coating is described in U.S. Patent 6,197,132 B1.

Claims (3)

see; "i 528 027 Claims Use of a ferritic stainless steel alloy containing (in% by weight): No up to 1 Cr 015-25 Al 0.5-_3,7 Mo and / or W 0-3 Tl and / or Nb 0-1 Zr and / or Hf> 0-0.2 one or fl your rare earth metals (REM) such as Ce, lea, Sc; and Y 0-0.2 C 0-0.2 N 0-0.2 V 50.09 residual even and normally occurring pollutants, the equation Cr + 3Al 2 '29 being fulfilled as substrate material in catalysts for diesel engines in heavy vehicles. Use of a ferritic stainless steel alloy according to requirements at temperatures in the range 600-900 ° C. The use of a ferritic stainless steel alloy containing (in w / w%): Ni up to »1 Cr 15-25 Al 0,5-3,7 Mo and / or W> 0-2,5 Ti and / or Nb 0-1 Zr and / or Hf> 0-0.2 »one or fl era rare earth metals (REM) such as Ce, La, Se, and Y oo, 2 I c 090.2 528 027 N 0-0.2 v sa fi æ the rest equal and normally occurring impurities, whereby the equation cr + 3A: a zsar fulfill. as a substrate material in catalysts for diesel engines in heavy vehicles. . Use of a ferritic stainless steel alloy according to claim 3 at temperatures in the range 600-900 ° C. . Use according to claim 3 or 4, wherein the Cr:: content is 17.0-23.0% by weight. . ' Use according to any one of claims 3-5 wherein the content 1A1 is 1.5-3.5A weight - ', ° / .o; preferably 2.5-3.5 wt.% o. ". Use according to any one of claims 3-6 wherein the content of REM is 0.01.-0.2 wt.