KR100774097B1 - Ferritic type stainless steel containing yttrium - Google Patents

Abstract

The present invention relates to a yttrium (Y) -containing ferritic stainless steel, containing yttrium and reducing the film resistance of the surface of the material at a temperature of 800 ° C. or lower, thereby providing excellent electrical conductivity and excellent oxidation resistance and low coefficient of thermal expansion. In addition, it can be very useful as a high temperature material used in a connection material of a solid oxide fuel cell or an automobile muffler, a high temperature boiler, an energy installation, and the like.

Description

Yttrium-containing ferritic stainless steel {FERRITIC TYPE STAINLESS STEEL CONTAINING YTTRIUM}

1 is a graph showing a change in weight per unit area with time in a circulating oxidation atmosphere in 800 ℃ air in the oxidation resistance test of the ferritic stainless steel sheet according to the embodiment and the prior art according to the present invention.

The present invention relates to a ferritic stainless steel containing yttrium, specifically, by adding yttrium to Fe-Cr ferritic stainless steel to reduce the film resistance of the material surface at high temperature, it is excellent in electrical conductivity, A ferritic stainless steel with improved oxidative and thermal expansion coefficients.

Recently, many studies have been made on solid oxide fuel cells (SOFCs), and the core, which is a core component of unit cell stacking, has high electrical conductivity at high temperature while maintaining airtightness to supply gas. At the same time, it should satisfy stability against oxidizing and reducing atmospheres, mechanical stability, and thermal shock resistance. Until now, the LaCrO ₃ series of ceramics has been the mainstay of such connecting materials, but recently, with the development of research to lower the operating environment of SOFC, the thickness of electrolyte, the characteristics of components, and the design of batteries have been improved. As a result, the battery performance has been improved to have a high power density at low temperatures, thereby allowing the application of metallic materials instead of conventional ceramic materials.

Metal connectors generally have the advantages of high density required for the connectors, mechanical strengths such as high deformation resistance in SOFC operating environments, thermal conductivity, and economics, but they form oxides on the surface in high temperature oxidizing atmospheres. This increases electrical conductivity, and also has disadvantages such as a difference in coefficient of thermal expansion with other battery components.

Recently, in order to solve these problems, research on the surface coating with the development of a new alloy is in progress. Among these, conductive ceramic coatings are used to increase oxidation resistance and electrical conductivity of existing products, and to suppress evaporation of chromium (Cr), LSM (La _1-x SrxMnO ₃ ), LSC (LaxSr _1-x CoO ₃₎ , LSCF An oxide of a perovskite structure is coated, such as (La _1-x SrxCo _1-y Fe _y O ₃ ). However, the perovskite coating has problems such as interfacial reaction between the substrate and the coating layer and internal oxidation through the coating layer when exposed to a high temperature for a long time, and increases the electrical resistance of the metal interconnect when the oxides thus formed are insulating oxides. This reduces the performance of the SOFC. Therefore, the development of a new alloy capable of forming a conductive oxide is needed.

In general, in order to improve oxidation resistance at high temperatures, alloying elements such as Al, Si, Cr, etc. may be added to protect metals with low growth rates such as Al ₂ O ₃ , SiO ₂ , Cr ₂ O _3, etc. at high temperature oxidation. By forming an oxide film, an alloy is developed in the direction which suppresses oxidation of a metal in high temperature oxidizing atmosphere. However, SiO ₂ and Al ₂ O ₃ are insulative oxides and therefore are not suitable as metal connectors because they degrade the electrical properties of SOFCs.

Therefore, in order to improve the electrical conductivity, alloys forming a Cr ₂ O ₃ protective film are mainly applied. Such alloys include Cr-based alloys, Ni-based alloys, Fe-based alloys, and Fe-Cr ferrites, which are Fe-based alloys. Stainless steels dominate. However, Fe-Cr-based alloys have a disadvantage in that Cr oxides are volatilized at high temperatures and electrical conductivity is reduced.

The representative Fe-Cr alloys developed so far are ZMG232, an alloy of 22% Cr, 0.04% La, and 0.22% Zr, and 0.08 to lower the volatilization and thermal expansion coefficient of Cr at high temperatures. There is a Crofer22 APU from ThyssenKrupp with% La added. A common problem with ZMG232 and Crofer22 APU is that it contains a small amount of La, but La ₂ O ₃ is concentrated on the surface of the material, resulting in high film resistance.

Therefore, the present inventors have studied to develop ferritic stainless steel having excellent oxidation resistance and electrical conductivity at low temperature and low film resistance on the surface of the material. As a result, yttrium, which is a rare earth element, is added to the Fe-Cr alloy. By controlling the amount, it is possible to provide a ferritic stainless steel material having excellent electrical conductivity and improved oxidation resistance and coefficient of thermal expansion while having low film resistance on the surface of the material in a high temperature oxidizing atmosphere.

Accordingly, it is an object of the present invention to provide a ferritic stainless steel having low film resistance at the surface of a material in a high temperature oxidizing atmosphere and excellent electrical conductivity while improving oxidation resistance and thermal expansion coefficient.

In the present invention to achieve the above object,

It provides a ferritic stainless steel comprising 0.01 to 0.1% by weight of yttrium (Y).

Hereinafter, the present invention will be described in more detail.

The ferritic stainless steel of the present invention is characterized by reducing the film resistance on the surface of the material and improving the oxidation resistance and thermal expansion coefficient by adding yttrium, which is a rare earth element, to the existing Fe-Cr alloy in an amount of 0.01 to 0.1% by weight. It is done. At this time, if the content of yttrium is less than 0.01% by weight, both conductivity and oxidation resistance deteriorate, and if it exceeds 0.1% by weight, oxidation resistance is excellent but conductivity is not good.

Preferably, the ferritic stainless steel of the present invention is 0.01 to 0.1 wt% of yttrium (Y), more preferably 0.04 to 0.08 wt%, 0.02 wt% or less of carbon (C), and 0.3 to 1.5 wt. Of silicon (Si). %, Manganese (Mn) 0.4 to 1.2 wt%, nickel (Ni) 0.22 to 0.26 wt%, chromium (Cr) 14 to 25 wt%, aluminum (Al) 0.2 to 1.5 wt%, zirconium (Zr) 0.1 to 0.5 wt% % And the remaining amount of iron (Fe).

According to the present invention, yttrium, which is a rare earth element, is added to the Fe—Cr alloy to reduce the resistance of the ferritic stainless steel film at high temperature to improve electrical conductivity. When yttrium is added to the alloy, a spinel structure of Fe-Mn-Cr-O oxide, which improves electrical conductivity on the outermost surface of the alloy, is formed more than that of La and at the same time Cr In addition to preventing volatilization, the yttrium oxide produced is concentrated at the interface between the grain boundaries of the oxide and the oxide / base metal, rather than being concentrated on the surface of the material, thereby suppressing the diffusion of cations. It becomes thinner and the film resistance becomes relatively low.

In the present invention, the ingot (ingot) of the composition may be prepared in the form of a steel sheet by homogenizing at 1,200 ° C. for 24 to 48 hours and heat-treating at 800 ° C. for 1 to 2 hours, and the ferritic stainless steel produced according to the present invention. The area specific resistance of the film is At 800 ° C., it is 41 mΩ · cm ² or less, preferably 20 mΩ · cm ² or less, and the average coefficient of thermal expansion is 12.6 × 10 ⁻⁶ / K or less, preferably 12.5 × 10 ⁻⁶ / at room temperature to 1,000 ° C. It is good that it is K or less.

For example, the area resistance of the coating of the alloy containing 0.04 and 0.08% by weight of yttrium, respectively, produced in an 800 ° C. atmospheric oxidizing atmosphere is 20 mΩ · cm ² and 12 mΩ · cm ² , respectively, at room temperature to 1,000 ° C. The average coefficient of thermal expansion of is 12.5 × 10 ^-6 / K and 12.0 × 10 ^-6 / K. It is ZMG232 (area resistance: 54 mΩcm ²⁾ manufactured by Hitachi Metals Co., Ltd., which contains lanthanum in an amount of 0.04% by weight instead of yttrium. And average coefficient of thermal expansion: 12.8 × 10 ⁻⁶ / ° C. [JP Albellan et al., 5 ^th European SOFC Forum, Switzerland, Vol. 1 (2002) pp 248-256].

In addition, as a result of the oxidation resistance test, it can be seen that the addition of yttrium in an amount of 0.08 wt% significantly increased the oxidation resistance than the addition of lanthanum in an amount of 0.04 wt%.

Accordingly, the yttrium-containing ferritic stainless steel of the present invention has a low film resistance at the surface of the material at a high temperature compared to the conventional ferritic stainless steel, and thus has excellent electrical conductivity and excellent oxidation resistance and thermal expansion coefficient. It can be usefully used as a high temperature material used in the metal connection material of the battery, or automobile muffler, high temperature boiler, energy equipment.

Hereinafter, the present invention will be described in more detail based on the following examples. However, the following examples are not intended to limit the invention only.

<Production of Yttrium-Containing Ferritic Stainless Steel Sheet>

Example 1

Arc melting of 0.08% yttrium, 0.02% carbon, 0.4% silicon, 0.5% manganese, 0.22% nickel, 22% chromium, 0.21% aluminum, 0.22% zirconium and the rest of Fe Melted in) to 0.5 kg ingot was homogenized for 24 hours at 1,200 ℃ and heat-treated for 1 hour at 800 ℃ to prepare a ferritic stainless steel sheet.

Examples 2-4

A ferritic stainless steel sheet containing yttrium was manufactured in the same manner as in Example 1 except that the yttrium content was 0.04 wt%, 0.03 and 0.09 wt%, respectively.

Comparative example

ZMG232 from Hitachi Metals Co., Ltd., which uses 0.04% by weight of lanthanum instead of yttrium, was purchased and used.

Test Example 1 : Measurement of area resistance and average thermal expansion coefficient

The ferritic stainless steel sheets prepared in Examples 1 to 4 and the ZMG232 of the comparative example were oxidized for 100 hours in an atmospheric oxidizing atmosphere at 800 ° C., and then, two-probe ac impedance in an argon (Ar) gas atmosphere. The area resistance of the alloy coating and the average coefficient of thermal expansion at room temperature to 1,000 ° C were measured (see K. Huang et al., Solid State Ionics 129 (2000) pp. 237-250). Table 1 shows.

As shown in Table 1, the ferritic stainless steel sheet of the embodiment of the present invention has a lower film resistance than ZMG232 containing 0.04% by weight of lanthanum instead of yttrium, and thus has excellent electrical conductivity and low average thermal expansion coefficient. .

Test Example 2 : Oxidation Resistance Experiment

The oxidation resistance of the ferritic stainless steel plates prepared in Examples 1 and 2 and the ZMG232 of the comparative example was evaluated by a cyclic mass gain test for 200 hours at 800 ° C., and the results are shown in FIG. 1.

As shown in FIG. 1, the ferritic stainless steel sheets of Examples 1 and 2 containing yttrium at 0.08 wt% and 0.04 wt%, respectively, have a mass increase amount due to oxide generation than ZMG232 containing 0.04 wt% lanthanum in the comparative example. Since there is little, it turns out that oxidation resistance is excellent.

As described above, the ferritic stainless steel according to the present invention contains a small amount of yttrium and has a low film resistance at the surface of the material at high temperatures, thereby providing excellent electrical conductivity, excellent oxidation resistance, and low coefficient of thermal expansion. It may be usefully used as a metal connecting material of an oxidizing fuel cell or a high temperature material used in automobile mufflers, high temperature boilers, energy installations and the like.

Claims (6)

0.02 wt% or less carbon (C), 0.3-1.5 wt% silicon (Si), 0.4-1.2 wt% manganese (Mn), 0.22-0.26 wt% nickel (Ni), 14-25 wt% chromium (Cr), aluminum Ferritic stainless steel comprising 0.2 to 1.5 wt% (Al), 0.1 to 0.5 wt% zirconium (Zr), 0.04 to 0.08 wt% yttrium (Y) and the balance of iron (Fe). delete delete delete delete delete

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