WO2016105095A1 - Ferritic stainless steel - Google Patents

Abstract

A ferritic stainless steel according to the present invention comprises 0.003-0.012 wt% of C, 0.003-0.015 wt% of N, 0.05-0.15 wt% or less of Si, 0.3-0.8 wt% of Mn, 20-24 wt% of Cr, 0.1-0.4 wt% of Mo, 0.1-0.7 wt% of Nb, 0.03-0.1 wt% of Ti, and the remainder being Fe and inevitable impurities, and satisfies the following formula. Formula: Nb+Mn≥8Si (Nb, Mn and Si are the wt% amounts of corresponding components, respectively)

Description

Ferritic Stainless Steel

The present invention relates to a ferritic stainless steel, and more particularly to a ferritic stainless steel capable of maintaining high conductivity in a high temperature oxidizing environment.

Stainless steel has been applied to various fields from room temperature to high temperature because of its excellent corrosion resistance and oxidation resistance. Among them, a lot of research is being conducted to fabricate parts such as a separator of a fuel cell operating in a high temperature environment with stainless steel.

In order to apply stainless steel to a high temperature fuel cell, the thickness of the scale formed on the surface of the stainless steel in a high temperature oxidizing environment should not be excessively thick or the electrical conductivity is lowered. If the thickness of the scale is thicker than a certain amount, the scale may peel off and damage the material. If the electrical conductivity is low, the efficiency of the fuel cell may be reduced.

Therefore, in order to apply stainless steel as a fuel cell component, these characteristics must be provided.

When the stainless steel is oxidized, chromium oxide (Cr ₂ O) is formed on the surface and has corrosion resistance due to the oxidation scale composed of such chromium oxide. However, the scale formed at this time has excellent corrosion resistance but low electrical conductivity. In addition, the general stainless steel contains a certain amount of silicon, due to which silicon oxide is formed at the interface between the stainless steel and the scale, there is a problem showing the insulation effect. The scale of the chromium oxide is shown in FIG. 3, the silicon oxide is formed in FIG. 4, and the results of component analysis of the layer in which the silicon oxide is collected are shown in FIG. 5.

To solve this problem, a technique of adding rare earth or controlling the silicon concentration in stainless steel to very low is being developed. However, this technique is difficult to apply to a general mass production metal manufacturing process, causing excessive manufacturing cost. do.

Therefore, it is necessary to prevent the formation of silicon oxide and to develop stainless steel having high electrical conductivity even at high temperatures.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a ferritic stainless steel that can maintain high electrical conductivity even in a high temperature oxidation environment.

In order to achieve the above object, the ferritic stainless steel according to an embodiment of the present invention, in weight%, C: 0.003-0.012%, N: 0.003-0.015%, Si: 0.05-0.15% or less, Mn: 0.3- 0.8%, Cr: 20-24%, Mo; 0.1-0.4% Nb: 0.1-0.7% Ti: 0.03-0.1%, remainder Fe and an unavoidable impurity are included, and it is characterized by the following formula.

Formula: Nb + Mn ≥ 8Si (Nb, Mn, Si are each weight percent content of the component)

When the ferritic stainless steel is exposed to an oxidizing environment at 300 to 900 ° C., a first scale layer containing chromium oxide is formed on the surface of the ferritic stainless steel, and chromium oxide and manganese oxide are formed on the surface of the first scale layer. A second scale layer is formed, wherein the thickness of the second scale layer is characterized in that more than 2/3 of the thickness of the entire scale layer.

A third scale layer containing niobium oxide is formed between the ferritic stainless steel and the first scale layer.

According to the ferritic stainless steel according to the present invention, even when applied to a separator plate of a fuel cell in a high temperature oxidizing environment, it is possible to manufacture a component capable of maintaining high electrical conductivity for a long time.

1 is a cross-sectional TEM photograph of a ferritic stainless steel according to an embodiment of the present invention,

2 is an EDS graph showing components of a first scale layer composed of chromium oxide and a second scale layer composed of chromium / manganese oxide;

3 is a cross-sectional TEM photograph of a comparative example in which only a chromium oxide layer is formed;

4 is a cross-sectional TEM photograph of a comparative example in which a silicon oxide layer is formed between a base material and a scale;

5 is an EDS graph showing components of a silicon oxide layer formed between a base material and a scale;

6 is a graph showing the fraction of silicon according to the depth of the Examples and Comparative Examples of the present invention,

7 is a graph showing the fraction of niobium according to the depth of the embodiment of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” include plural forms as well, unless the phrases clearly indicate the opposite. As used herein, the term "comprising" embodies a particular characteristic, region, integer, step, operation, element, and / or component, and other specific characteristics, region, integer, step, operation, element, component, and / or group. It does not exclude the presence or addition of.

Unless defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Commonly defined terms used are additionally interpreted to have a meaning consistent with the related technical literature and the presently disclosed contents, and are not interpreted in an ideal or very formal sense unless defined.

Hereinafter, a ferritic stainless steel according to a preferred embodiment of the present invention with reference to the accompanying drawings will be described.

The present invention is based on iron as a known structure, C: 0.003-0.012%, N: 0.003-0.015%, Si: 0.05-0.15% or less, Mn: 0.3-0.8%, Cr: 20-24%, Mo; 0.1-0.4% Nb: 0.1-0.7% Ti: Ferritic stainless steel containing 0.03-0.1% (above weight%), and is steel of the composition which satisfy | fills following formula.

Nb + Mn ≥ 8Si

Nb, Mn, and Si each mean a weight% content of the corresponding component.

The above formula is to limit the content of manganese and niobium to a certain amount or more than silicon, and represents a composition necessary to prevent the formation of silicon oxide. Manganese and niobium are formed as oxides at the outer surface layer of the scale or at the interface between the base material and the scale because of their fast oxidation rate and diffusion rate, thereby preventing the silicon from oxidizing to form oxides. When manganese and niobium have a predetermined content or less than silicon, it is important to satisfy the above-described formula because such effects cannot be expected.

The reason for limiting the range of each component is described below. In addition, all% described below means the weight%.

Carbon (C) is an essential element contained in the stainless steel manufacturing process. If the carbon content is excessively increased, precipitates such as chromium carbide are formed, which may adversely affect the composition and oxidation characteristics of the base metal, so the upper limit is limited to 0.013%. However, controlling the carbon content extremely low causes excessive cost increase, so it is desirable to limit the lower limit to 0.003%.

Nitrogen (N) is limited to an upper limit of 0.015% because excessively increased content because various nitrides precipitate or adversely affect the quality due to the generation of pores. However, controlling the nitrogen content extremely low causes excessive cost increase, so it is desirable to limit the lower limit to 0.008% or more.

Silicon (Si) is a component that must be strictly limited because the film forms a precipitate in the form of a film at the interface between the scale and the base material when exposed to high temperatures, so the upper limit is limited to 0.15%. However, in order to reduce the silicon content to 0.05% or less, a high cost process such as vacuum melting is required, so the lower limit is limited to 0.05% in the present invention.

Manganese (Mn) diffuses rapidly when the stainless steel is oxidized at high temperature to form dense manganese / chromium oxide in the scale outer layer, so 0.3% or more should be added. However, excessive addition of manganese excessively accelerates the growth of the scale, so that peeling of the scale may occur, the upper limit is limited to 0.8%.

Chromium (Cr) is an essential element to secure corrosion resistance of stainless steel. At least 20% should be added to prevent chromium depletion from prolonged oxidation in high temperature oxidation environments. However, in order to prevent an increase in manufacturing cost and precipitation of chromium carbide, intermetallic compounds, etc., the upper limit is preferably limited to 24%.

Molybdenum (Mo) is an element that can increase the strength of the material in high temperature environment. Therefore, it is necessary to add at least 0.1% or more, but since it is an expensive element, it is preferable to limit the upper limit to 0.4% in order to suppress the increase in manufacturing cost.

Niobium (Nb) is oxidized at the scale / base material interface due to its excellent oxidation characteristics to form an oxide, and thus suppresses the formation of insulating silicon oxide, thereby adding 0.1% or more. On the other hand, excessive addition may inhibit hot workability and increase the manufacturing cost, and therefore, it is preferable to limit the upper limit to 0.7%.

Titanium (Ti) forms an internal oxide just below the interface between the base material and the scale at a high temperature, that is, near the surface of the base material, thereby increasing the strength of the material, so a content of 0.03% or more is required. However, it is preferable to limit the upper limit to 0.1% because excessive addition causes an increase in manufacturing cost and forms titanium oxide outside the scale.

When the ferritic stainless steel is exposed to an oxidizing environment at 300 to 900 ° C, a first scale layer containing chromium oxide is formed on the surface of the ferritic stainless steel, and the chromium oxide and manganese oxide are included on the surface of the first scale layer. A second scale layer is formed, and the thickness of the second scale layer is 2/3 or more of the thickness of the entire scale layer.

As shown in FIG. 1, there is a difference in thickness between the first scale layer including chromium oxide and the second scale layer including chromium oxide and manganese oxide. Chromium oxide is unsuitable for use as a fuel cell component because of its low electrical conductivity, but manganese oxide can be used as a fuel cell component because of its relatively high electrical conductivity. In order to have the required electrical conductivity, the thickness of the second scale layer must be thicker than the thickness of the first scale layer, and at least twice as thick as the thickness of the first scale layer. Therefore, it is preferable that the second scale layer has a thickness of 2/3 or more in the entire scale layer. Also, as shown in FIG. 2, it can be seen that the second scale layer includes manganese and chromium, and the first scale layer includes chromium and the like.

It is preferable that a third scale layer containing niobium oxide is formed between the ferritic stainless steel and the first scale layer.

Usually, silicon which is easy to oxidize becomes an oxide layer between a base material, ie, stainless steel, and the scale layer formed in the surface. Thus formed silicon oxide layer is shown in FIG. Since silicon oxide has extremely low electrical conductivity, it cannot be used as a fuel cell component. Therefore, it is necessary to suppress the production of silicon oxide by forming an oxide having high electrical conductivity while oxidizing faster than silicon instead of silicon. To this end, in the present invention, niobium may be added to form niobium oxide between the base material and the scale layer to suppress the formation of silicon oxide. More preferably, the production of silicon oxide should be completely prevented, but it is very difficult to completely suppress the production of silicon oxide. However, the production of niobium oxide reduces the chance of oxidizing silicon by that amount, which can reduce the total amount of silicon oxide produced, thereby preventing the degradation of electrical conductivity.

6 is a graph comparing silicon fractions according to depths of the examples of the present invention and the comparative examples in which the silicon oxide layer is formed. According to FIG. 6, in the comparative example, the fraction of silicon appears high at a depth of 1 to 5 μm, but the embodiment of the present invention does not increase the fraction of silicon in the same range.

On the other hand, as shown in Figure 7, it can be seen in the embodiment of the present invention that the content of niobium is high at a depth of 2 ~ 5㎛.

Hereinafter, the composition of the present invention and the comparative examples, the satisfaction of the formula and the production of silicon oxide was compared in Table 1.

Steel grade	C	N	Si	Mn	Cr	Mo	Nb	Ti	expression	Silicon oxide
Example 1	0.005	0.007	0.11	0.5	21.3	0.15	0.43	0.05	satisfied	Unproduced
Example 2	0.009	0.004	0.14	0.6	22.6	0.22	0.72	0.08	satisfied	Unproduced
Example 3	0.007	0.013	0.06	0.4	23.5	0.33	0.65	0.04	satisfied	Unproduced
Example 4	0.011	0.006	0.08	0.7	23.3	0.11	0.25	0.04	satisfied	Unproduced
Example 5	0.007	0.009	0.09	0.5	20.5	0.25	0.53	0.07	satisfied	Unproduced
Comparative Example 1	0.001	0.008	0.12	0.4	22.3	0.2	0.15	0.08	dissatisfaction	produce
Comparative Example 2	0.008	0.007	0.12	0.1	22.6	0.23	0.7	0.05	satisfied	produce

As shown in Table 1, it can be seen that if the composition or formula of the present invention is not satisfied, silicon oxide is formed, which greatly reduces the electrical conductivity.

Although embodiments of the present invention have been described above with reference to the accompanying drawings, those skilled in the art to which the present invention pertains may implement the present invention in other specific forms without changing the technical spirit or essential features thereof. I can understand that.

Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive. The scope of the present invention is shown by the following claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present invention. .

Claims (3)

1. By weight%, C: 0.003-0.012%, N: 0.003-0.015%, Si: 0.05-0.15%, Mn: 0.3-0.8%, Cr: 20-24%, Mo; 0.1-0.4%, Nb: 0.1-0.7%, Ti: 0.03-0.1%, balance Fe and inevitable impurities,

   A ferritic stainless steel, characterized by satisfying the following formula.

   Formula: Nb + Mn ≥ 8Si (Nb, Mn, Si are each weight percent content of the component)
2. The method according to claim 1,

   A first scale layer containing chromium oxide is formed on the surface of the ferritic stainless steel, a second scale layer containing chromium oxide and manganese oxide is formed on the surface of the first scale layer,

   The thickness of the said 2nd scale layer is 2/3 or more of the thickness of a full scale layer, Ferritic stainless steel.
3. The method according to claim 2,

   A ferritic stainless steel, characterized in that a third scale layer containing niobium oxide is formed between the ferritic stainless steel and the first scale layer.

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