KR20060127063A - Cr-al-steel for high-temperature applications - Google Patents

Abstract

The present invention relates to a product of ferritic stainless steel manufactured according to the process of this invention, which product has increased resistance to cyclic and continuous thermal load and oxidation at elevated temperatures and which has improved mechanical properties at said temperatures as well as use thereof in the form of wire, strip, foil and/or tube in high-temperature applications such as in catalytic converter applications, in heating and furnace applications and which has the following composition (in % by weight): less than 1 % of Ni, 15-25 % of Cr, 4,5-12 % of Al, 0,5-4 % of Mo, 0,01-1,2 % of Nb, 0-0,5 % of Ti, 0-0,5 % of Y, Sc, Zr and/or Hf, 0-0,2 % of one or more rare earth metals (REM) such as, for instance, Ce or La, 0-0,2 % of C, 0-0,2 % of N, with the balance iron and normally occurring impurities.

Description

Cr-Aｌ steel for high temperature applications {Cr-Aｌ-STEEL FOR HIGH-TEMPERATURE APPLICATIONS}

The present invention relates to a ferritic stainless steel product manufactured according to the process of the present invention, which has improved resistance to cyclic and continuous thermal load and oxidation at elevated temperatures, and also to improved mechanical properties at this temperature. In addition, the present invention also relates to the use of such products in the form of wires, strips, foils and / or tubes in high temperature applications such as catalytic converter applications, heating and furnace applications.

Fe-Cr-Al alloys have been widely used at temperatures above 900 ° C. Thanks to the protective oxide on the surface, the alloy can withstand periodic and repetitive thermal loads and oxidations until, for example, oxide formers such as Al are depleted from the material. The total content of Al and the mechanical strength are the limiting factors for the manufacture and life of the whole device.

For example, metal high temperature materials used in catalytic converters or heat resistant applications are typically wires or thin strips of ferritic Fe-Cr-Al alloys with at least 4.5 wt.% Al and small amounts of reactive elements added. Based on Due to the high ductility of the metal it has good mechanical and thermal fatigue resistance. An aluminum content of about 4.5% by weight gives the material the possibility of forming a thin and protective aluminum oxide upon heating with the reactive element. In addition, the reactive element causes oxygen to be significantly reduced by peeling or flaking, ie loosening from the metal upon cooling or mechanical deformation. However, conventional Fe—Cr—Al alloys have a significant disadvantage: they are very weak mechanically at high temperatures, so they tend to deform significantly under small stresses due to acceleration, pressure changes, mechanical shock or temperature changes. have. The alloy disclosed in EP-B-290 719 is intended for use not only in the construction of catalytic converters but also in the production of heating elements, such as for resistive heating furnaces, and in connection with the protective oxide layer as a result of the addition of Ti and Zr to the alloy. Thereby solving the problem of reducing elongation of the substrate.

Ferritic steel materials with a low carbon content are also brittle by grain growth when used at temperatures above 800 ° C. In order to obtain the optimum oxidation resistance of the alloy and to enable plastic cold working, a low content of carbon is required because the carbon content of 0.02% by weight or more is brittle by increasing the brittle transition temperature of the material. The elements used in solid solution hardening of high temperature materials such as Mo and / or W are thought to have a significant negative effect on the oxidative properties, so the content of these elements may be up to 1% by weight as disclosed in US 4859649 or at most as disclosed in EP 0667400. 0.10% by weight is preferred.

It is an object of the present invention to provide a ferritic stainless steel alloy having improved resistance to cyclic and continuous thermal loads and oxidation at elevated temperatures.

Another object of the present invention is to provide a ferritic stainless steel with improved mechanical properties for use in applications with periodic and continuous thermal loading and oxidation, such as auxiliary materials in converter applications such as catalysts at elevated temperatures.

Another object of the present invention is to provide a ferritic stainless steel for use in heating applications and furnace applications.

Another object of the present invention is to provide ferritic stainless steel in the form of wires, strips, foils and / or tubes.

Another object of the present invention is to provide a process for producing the alloy product.

The above objects are met by using a ferritic stainless steel having the following composition (% by weight):

Less than 1% Ni,

15-25% Cr,

4.5-12% Al,

0.5-4% Mo,

0.01-1.2% Nb,

0-0.5% Ti,

0-0.5% of Y, Sc, Zr and / or Hf,

0-0.2% of one or more rare earth metals (REM), such as Ce or La,

0-0.2% C,

0-0.2% of N,

The rest is iron and impurities that normally occur.

The final product is manufactured in the form of wires, strips, foils and / or tubes.

The final product according to the invention is made of a homogeneous material or laminate or a material having a concentration gradient of Al, with the content of Al increasing towards the surface of the product. Thus, by coating the base material and the base alloy with Al or Al alloy, respectively, in particular, the strip of the base alloy having a thickness of 1 mm or less is coated with Al alloy to affect the manufactured product.

With this two-step process, the oxidation and mechanical properties of the alloy can be improved and each can be optimized independently. This process can also simplify the production process when conventional dry metallurgical preparations of materials having an average Al content of 4.5% by weight or more are associated with a significant reduction in production due to brittleness. Another advantage of this process is that the final material is made to have an Al concentration gradient, thus increasing the content of Al towards the surface, which is accompanied by improved oxidation resistance, because it prevents the formation of fast-growing oxides such as chromium and iron oxides. This is because the mechanical properties of the final material are improved.

 For example, the base alloy is prepared by powder metallurgy or conventional dry metallurgy of the planned composition, and the alloy is hot-cold rolled to the final desired dimensions.

In preparation by the coating process, the base material prior to coating has the following composition (% by weight):

Less than 1% Ni,

15-27% Cr,

0-5% Al,

0.5-5% Mo,

0.01-2% Nb,

0-0.5% Ti,

0-0.5% of Y, Sc, Zr and / or Hf,

0-0.2% of one or more rare earth metals (REM), such as Ce or La,

0-0.2% C,

0-0.2% of N,

The rest is iron and impurities that normally occur.

The most preferred composition (% by weight) of the base material is as follows:

Less than 1% Ni,

16-25% Cr,

0.5-4% Al,

0.7-4% Mo,

0.25-1.0% Nb,

0-0.5% of Y, Sc, Zr and / or Hf,

0-0.5% Ti,

One or more rare earth metals (REM), such as Ce or La, from 0 to 0.1%,

0.02 ~ 0.2% C,

0-0.05% N,

The rest is iron and impurities that normally occur.

The material can be used in a coated state or after diffusion annealing. If the substrate contains 2-4% by weight of Al, the most desirable composition is obtained before coating. This aluminum content imparts improved oxidation resistance to the final product, which simplifies the manufacturing process, i.e. the risk of product failure is significantly reduced compared to the production of materials having an aluminum content of at least 4% by weight. After coating with Al alloy, the total Al content of this material is greater than 4.5% by weight.

Carbide and / or nitride precipitation of one or more Ti, Nb, Zr, Hf elements provides resistance to particle growth and mechanical stability. The presence of Mo and / or W in the solid solution provides improved strength at high temperatures, ie temperatures above about 800 ° C. In the alloy according to the invention, W replaces Mo in whole or in part as long as the effect on the alloy is maintained.

 The addition of Zr and / or Hf and REM and / or Y and / or Sc provides improved resistance to peeling and flaking of the oxide layer formed. These elements may be added to the base alloy and / or to the Al alloy used in the coating to supply the final product content of a homogeneous material. The alloy according to the invention as a whole contains at least 0.1% by weight of Ti + Nb + Zr + Hf.

 Conventional metallurgy can produce most of the alloy composition according to the invention. However, the material can be obtained in a two-step process according to the invention, in which the microstructure of the material is controlled, the oxidation properties of the material are improved, the mechanical properties of the material are optimized and improved, and the maximum aluminum content of the material is It is usually not limited by the brittle effect given to Al content of at least about 5% by weight in both cold / hot working. In addition, the process of coating the substrate with an Al alloy provides a final product, the content of Mo, Nb and C in the final product being higher than that of conventionally prepared materials that do not contain these elements, resulting in a significant reduction in oxidative properties. .

    Conventional processes such as immersion in water, electrolytic coating, rolling strips of base alloys and aluminum alloys, and deposition of Al solid solution alloys from the gas phase using so-called CVD or PVD techniques, may affect base alloy coatings using Al alloys. Can be. After rolling the base alloy to the desired final or larger thickness of the article, it may affect the Al alloy coating. In the latter case, diffusion annealing is performed to homogenize the material, followed by one or more rolling steps to provide the final product. Rolling directly affects the coated article according to the invention with a thickness larger than the desired final thickness. In this case, annealing is performed after rolling.

The thickness of the coated Al layer can vary depending on the thickness of the substrate, the desired aluminum content of the final product and the aluminum content of the substrate. However, the total Al content of the final product should be at least 4.5 wt% as above. The product can be used in the form of a material or laminate having an Al concentration gradient or an annealed homogeneous material, the Al content being higher at the surface than at the center of the material. In materials with a concentration gradient, if the aluminum content is greater than 6.0% by weight at least 5 μm from the surface, lower total content and average content lower than 4.0% by weight are possible, respectively.

Examples of useful aluminum alloys are pure Al, 0.5-25 wt% Si and Al alloys, 0-2 wt% Al alloys with one or more elements of Ce, La, Y, Zr, Hf. Therefore, when coating from the melt, it is desirable that a homogeneous material or eutectic mixture be deposited with a low melting point. Depending on the coating by rolling, it is required that the material is ductile and have similar mechanical properties as the substrate so that the coating and the substrate are formed in a similar manner.

1 shows the results of an oxidation test at 1000 ° C. as a function of mass change versus time as well as D and E as well as Comparative Examples 1 and 3. FIG.

FIG. 2 shows the results of the oxidation test of 1100 ° C. as a function of mass change versus time as well as Comparative Example 1 as well as 1100 ° C. as well as C, E and G. FIG.

Example  One

Table 1 shows the composition of the alloys tested. Example C and Comparative Example 1 were prepared by conventional methods of dry metallurgy and hot working. In Comparative Example 1, 50 μm thick strips were also prepared by hot rolling and cold rolling. Comparative Example 1 is an alloy used as an auxiliary material in today's catalytic converters. This material is sufficient for oxidation resistance as an auxiliary material. However, the mechanical strength of this material is low, which is a factor in limiting the lifetime of the entire device.

Very low ductility (2% elongation at break) of the alloy according to example C means that this alloy is hardly made in the form of a thin strip. However, as shown in Table 1, the same alloy has excellent strength at high temperatures, for example at 700 ° C. and 900 ° C., ultimate strength is about 100% greater than Comparative Example 1. The oxidation resistance of Comparative Example 1 and Example C at 1100 ° C. is shown in FIG. 2. The oxidation rate of Example C is 5% larger than that of Comparative Example 1, which means that the material is considered equivalent to oxidation resistance.

Example  2

Table 1 shows the composition of the alloys tested. Examples A and B and Comparative Examples 1 and 2 were prepared by conventional methods by dry metallurgy and hot working. Thereafter, a 50 탆 thick strip was also prepared by hot rolling and cold rolling. The alloys according to Examples A and B both have sufficient ductility at room temperature to allow cold rolling into very thin strips with good productivity.

Examples D and E and Comparative Example 3 correspond to cold-rolled strips of the alloys according to Examples B and C and Comparative Example 2, respectively, and the total amount of Al by coating with a considerable amount of Al deposition or sputtering on both sides is 5.5 to 6 weight. Corresponds to% (Table 3).

TABLE 3

The thickness of Al obtained by the glow discharge optical spectroscopy (GDOES) method was measured, and the thickness and composition of the thin surface layer can be accurately measured by this method. The results of the analysis show that a total content of Al of 5-6% by weight is obtained. These samples were oxidized at 1000 ° C. for 620 hours and are shown in FIG. 1. The alloys according to Examples D and E are superior to the alloys according to Comparative Example 3, whereas the Fe-Cr-Al alloys prepared by the conventional method of Comparative Example 1 are significantly superior to the Examples D and E alloys according to the invention. It is oxidizable.

Example  3

Examples F and G and Comparative Example 4 had the same composition as the alloys according to Examples D and E and Comparative Example 3 and were annealed at 1050 ° C. for 10 minutes to give the same Al content to the material. The bending test determined the ductility of the material and determined the smallest bending radius at which the material can bend without breaking, as shown in Table 4.

Table 4

The smallest radius tested material was 0.38 mm. The alloy according to the present invention has superior ductility than Comparative Example 4. The alloy according to Comparative Example 4 was brittle and proved that this alloy was less suitable for use as a catalytic converter. The alloy according to example G has an ultimate strength at 900 ° C. and is equally superior to the material prepared by the conventional method according to the invention of example C, compared to the Fe-Cr-Al alloy prepared by the conventional method of 1 in comparison. 2 times better. This means that, on the assumption that the oxidation resistance is sufficient, an alloy having a thickness of half the thickness of the conventional material can be used, thus reducing the material cost and increasing the efficiency of the production of the catalytic converter.

The alloy according to Example G was oxidized at 1100 ° C. with the alloys according to Examples C and E as well as Comparative Example 1, as shown in FIG. 2. Improved oxidation resistance was obtained with the alloy according to example G, which was obtained by comparing an alloy prepared by the conventional method with the same material without diffusion annealing (example E). The comparison between Examples G and C is particularly interesting because the compositions are similar but the manufacturing methods correspond to other alloys, the alloy according to Example G is prepared by cold rolling to the desired thickness followed by Al coating and annealing, On the other hand, the alloy according to example C is prepared from the beginning with the desired Al content. Apart from the improved manufacturing properties of the material prepared by the method of Example G, this alloy further has better oxidation resistance than Example C. The negative effect on oxidation resistance due to the presence of Mo and Nb in the alloys according to Example C can explain the relatively lower-looking oxidation resistance of Example C compared to 1 in comparison. These elements are known to reduce the oxidation resistance of the alloy. In Example G, there are no such negative effects, which can be explained by the positive result of Example G prepared with Al coating. Thus, this production method is preferable for the oxidation resistance of the alloy.

In summary, the effects of the high content of Mo and Nb combine to provide a significantly improved strength compared to the materials used today, as well as the dimensions and products mentioned above which are weak for this material using the process described above. Oxidation resistance is imparted when used at high temperatures.

Ferritic stainless steel products made in accordance with the process of the present invention have improved resistance to cyclic and continuous thermal loads and oxidation at elevated temperatures and have improved mechanical properties at these temperatures, which include wires, strips, foils and / or the like. Or in the form of tubes, making them suitable for use in high temperature applications such as catalytic converter applications, heating and furnace applications.

Table 1

TABLE 2

Claims (8)

A ferritic steel alloy having the following composition (% by weight). Less than 1% Ni, 15-25% Cr, 4.5-12% Al, 0.5-4% Mo, 0.01-1.2% Nb, 0-0.5% Ti, 0-0.5% of Y, Sc, Zr and / or Hf, 0-0.2% of one or more rare earth metals (REM), such as Ce or La, 0-0.2% C, 0-0.2% of N, The rest is iron and impurities that normally occur. 2. The ferritic steel alloy according to Claim 1, wherein W replaces Mo in whole or in part. The ferritic steel alloy according to claim 1 or 2, comprising at least one rare earth metal (REM). 2. The ferritic steel alloy according to claim 1, which contains at least 0.1% by weight of Ti, Nb, Zr and / or Hf in total. The method for producing a ferritic steel alloy according to any one of claims 1 to 4, wherein the base alloy is coated with Al alloy or Al to have a base alloy having the following composition (% by weight). Less than 1% Ni, 15-27% Cr, 0-5% Al, 0.5-5% Mo, 0.01-2% Nb, 0-0.5% Ti, 0-0.5% of Y, Sc, Zr and / or Hf, 0-0.2% of one or more rare earth metals (REM), such as Ce or La, 0-0.2% C, 0-0.2% of N, The rest is iron and impurities that normally occur. Products in the form of wires, strips, foils and / or tubes for use in high temperature applications characterized in that they are made of a ferritic steel alloy according to any one of claims 1 to 4. Use of the ferritic steel alloy according to any one of claims 1 to 4 as an auxiliary material in catalytic converter applications. Use of the ferritic steel alloy according to any one of claims 1 to 4 in heating and furnace applications.

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