FERRITIC STAINLESS STEEL

Abstract

Provided is a ferritic stainless steel including, as a ferritic stainless steel used in a separator for a fuel cell, a base material including, in weight %, C: 0.003% to 0.012%, N: 0.003% to 0.015%, Si: 0.05% to 0.15%, Mn: 0.3% to 0.8%, Cr: 20% to 24%, Mo; 0.1% to 0.4%, Nb: 0.1% to 0.7%, Ti: 0.03% to 0.1%, and the remainder being Fe and inevitable impurities. A first scale layer including chromium oxide is formed on a surface of the base material, and a second scale layer including chromium oxide and manganese oxide is formed on a surface of the first scale layer. A silicon content included in each of the first scale layer and the second scale layer is 0.2 weight % or less, and the following formula is satisfied: Nb+Mn≧8Si where Nb, Mn and Si are weight % amounts of corresponding components, respectively.

Description

TECHNICAL FIELD

The present disclosure relates to a ferritic stainless steel, and in particular, to a ferritic stainless steel capable of maintaining high conductivity under a high temperature oxidizing environment.

BACKGROUND ART

Stainless steel has excellent corrosion resistance and oxidation resistance, and thereby has been used in various fields from room temperature to high temperature. Among these, extensive studies for manufacturing components such as a separator of a fuel cell operated under a high temperature environment using stainless steel have been ongoing.

In order to use stainless steel in a high temperature fuel cell, a thickness of a scale formed on a surface of the stainless steel should not excessively increase or electrical conductivity should not be reduced under a high temperature oxidizing environment. When the scale thickness increases by a certain level or higher, the scale is peeled off damaging a material, and when electrical conductivity is low, fuel cell efficiency may decrease.

Accordingly, such properties need to be fulfilled to use stainless steel as a fuel cell component.

When stainless steel is oxidized, chromium oxide (Cr₂O) is formed on the surface, and corrosion resistance is obtained due to an oxidized scale formed with such chromium oxide. However, the scale formed at the time has low electrical conductivity while having excellent corrosion resistance. In addition, general stainless steel includes silicon in a certain amount, which causes a problem of exhibiting an insulating effect by forming silicon oxide at an interface between the stainless steel and the scale. The image of the scale famed with chromium oxide as above is shown in FIG. 3, the image of silicon oxide formed is shown in FIG. 4, and a result of analyzing components of a layer with clusters of silicon oxide is shown in FIG. 5.

Technologies of adding rare earths or controlling a silicon concentration to be very low in stainless steel have been developed in view of the above problems, however, such technologies are difficult to use in common mass production-type metal manufacturing processes, and manufacturing costs excessively increase.

Accordingly, development of stainless steel preventing silicon oxide formation and having high electrical conductivity even at a high temperature has been required.

DISCLOSURE

Technical Problem

The present disclosure has been made in view of the above, and the present disclosure is directed to providing a ferritic stainless steel capable of maintaining high electrical conductivity even under a high temperature oxidizing environment.

Technical Solution

In view of the above, a ferritic stainless steel according to one embodiment of the present disclosure includes, as a ferritic stainless steel used in a separator for a fuel cell, a base material including, in weight %, C: 0.003% to 0.012%, N: 0.003% to 0.015%, Si: 0.05% to 0.15%, Mn: 0.3% to 0.8%, Cr: 20% to 24%, Mo; 0.1% to 0.4%, Nb: 0.1% to 0.7%, Ti: 0.03% to 0.1%, and the remainder being Fe and inevitable impurities, wherein, when exposed to an oxidizing environment of 300° C. to 900° C., a first scale layer including chromium oxide is formed on a surface of the base material, a second scale layer including chromium oxide and manganese oxide is formed on a surface of the first scale layer, a silicon content included in each of the first scale layer and the second scale layer is 0.2 weight % or less, and the following formula is satisfied.

Formula: Nb+Mn≧8Si (Nb, Mn and Si are weight % amounts of corresponding components, respectively.)

A thickness of the second scale layer is ⅔ or greater of a thickness of the whole scale layer.

A third scale layer including niobium oxide is formed between the base material and the first scale layer.

Advantageous Effects

According to a ferritic stainless steel of the present disclosure, components capable of maintaining high electrical conductivity over a long period of time even when used in a separator of a fuel cell and the like under a high temperature oxidizing environment can be manufactured.

DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional TEM image of a ferritic stainless steel according to one embodiment of the present disclosure.

FIG. 2 shows EDS graphs of components of a first scale layer formed with chromium oxide and a second scale layer formed with chromium/manganese oxide.

FIG. 3 is a sectional TEM image of a comparative example forming only a chromium oxide layer.

FIG. 4 is a sectional TEM image of a comparative example forming a silicon oxide layer between a base material and a scale.

FIG. 5 is an EDS graph of components of the silicon oxide layer formed between the base material and the scale.

FIG. 6 is a graph showing and comparing silicon fractions in an example of the present disclosure and a comparative example depending on a depth.

FIG. 7 is a graph showing a niobium fraction in an example of the present disclosure depending on a depth.

MODE FOR DISCLOSURE

The terminology used herein is for describing particular embodiments only and is not intended to be limiting of the present inventive concept. Singular forms used herein include plural forms as well, unless the context clearly indicates otherwise. The meaning of “include” used in the specification specify the presence of stated specific features, areas, integers, steps, operations, elements, and/or components thereof, but do not preclude the presence or addition of one or more other specific features, areas, integers, steps, operations, elements, components and/or groups thereof.

Unless otherwise specified, all the terms including technical terms and scientific terms used herein have the same meanings commonly understandable to those skilled in the art relating to the present disclosure. In addition, the terms defined in generally used dictionaries are additionally interpreted to have meanings corresponding to relating scientific literatures and contents disclosed now, and are not interpreted either ideally or very formally unless defined otherwise.

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

The present disclosure relates to a ferritic stainless steel including, with iron as a matrix structure, C: 0.003% to 0.012%, N: 0.003% to 0.015%, Si: 0.05% to 0.15% or less, Mn: 0.3% to 0.8%, Cr: 20% to 24%, Mo; 0.1% to 0.4%, Nb: 0.1% to 0.7% Ti: 0.03% to 0.1% (hereinbefore, weight %), and satisfying the following formula.

Nb+Mn≧8Si   Formula:

Nb, Mn and Si are weight % amounts of corresponding components, respectively.

The above-mentioned formula limits the content of the manganese and the content of the niobium are greater than the content of the silicon by a certain level or higher, and exhibits a composition required for preventing silicon oxide formation. Manganese and niobium has high oxidization rate and diffusion rate, and are formed as oxides on an external surface layer of a scale or at an interface between a base material and the scale, and as a result, oxide production occurring from silicon oxidation may be prevented. Such an effect may not be expected when the manganese and the niobium are included at a certain level or lower compared to silicon, and therefore, satisfying the range of the above-described formula is important.

Hereinafter, reasons for limiting the range of each component will be described. Moreover, % described below all means weight %.

Carbon (C) is an element essentially included in a stainless manufacturing process. When a carbon content excessively increases, precipitates such as chromium carbide are formed, which may adversely affect base material composition and oxidation characteristics, and therefore, the upper limit is limited to 0.013%. However, limiting the carbon content to be extremely low causes an excessive increase in the costs and therefore, the lower limit is preferably limited to 0.003%.

When the nitrogen (N) content excessively increases, various nitrides are precipitated, or pores are produced adversely affecting product qualities, and therefore, the upper limit is limited to 0.015%. However, limiting the nitrogen content to be extremely low causes an excessive increase in the costs and therefore, the lower limit is preferably limited to 0.008% or greater.

Silicon (Si) forms an insulator film by forming film-type precipitates at an interface between a scale and a base material when the material is exposed to a high temperature, and is a component that needs to be strictly limited, and therefore, the upper limit is limited to 0.15%. However, in order to reduce the silicon content to 0.05% or lower, high-cost processes such as vacuum melting need to be carried out, and therefore, the lower limit is limited to 0.05% in the present disclosure.

Manganese (Mn) is quickly diffused when stainless steel is oxidized at a high temperature to form dense manganese/chromium oxide on an external layer of a scale, and therefore, needs to be added in 0.3% or greater. However, an excessive addition of manganese excessively facilitates scale growth causing a concern of scale peel-off, and therefore, the upper limit is limited to 0.8%.

Chromium (Cr) is an essential element for securing corrosion resistance of stainless steel. In order to prevent chromium exhaustion caused from an oxidation over a long period of time under a high temperature oxidizing environment, a minimum of 20% or greater thereof needs to be added. However, the upper limit is preferably limited to 24% in order for preventing an increase in the manufacturing costs, and precipitation of chromium carbide, intermetallic compounds and the like.

Molybdenum (Mo) is an element capable of increasing material strength under a high temperature environment. Accordingly, a minimum of 0.1% or greater thereof needs to be added, however, when considering that molybdenum is a high-priced element, the upper limit is preferably limited to 0.4% for suppressing an increase in the manufacturing costs.

Niobium (Nb) forms an oxide by being oxidized at a scale/base material interface due to its excellent oxidation characteristics, and suppresses formation of an insulating silicon oxide therethrough, and therefore, needs to be added in 0.1% or greater. Meanwhile, when added in excess, hot workability is inhibited and manufacturing costs increase, and therefore, the upper limit is preferably limited to 0.7%.

Titanium (Ti) increases material strength by forming an internal oxide right below an interface between a base material and a scale, that is, near a surface of the base material, at a high temperature, and therefore, the content of 0.03% or greater is required. However, an excessive addition causes an increase in the manufacturing costs and forms titanium oxide on the outside of the scale, and therefore, the upper limit is preferably limited to 0.1%.

When such a ferritic stainless steel is exposed to an oxidizing environment of 300° C. to 900° C., a first scale layer including chromium oxide is formed on a surface of the ferritic stainless steel, and a second scale layer including chromium oxide and manganese oxide is formed on a surface of the first scale layer, wherein a thickness of the second scale layer is ⅔ or greater of a thickness of the whole scale layer.

As shown in FIG. 1, there is a difference in the thickness between the first scale layer including chromium oxide and the second scale layer including chromium oxide and manganese oxide. Chromium oxide has low electrical conductivity and is not suitable to be used as a component of a fuel cell, however, manganese oxide has relatively high electrical conductivity and may be used as a component of a fuel cell. In order to have required electrical conductivity, the thickness of the second scale layer needs to be larger than the thickness of the first scale layer, and the thickness of the second scale layer needs to be larger by at least two times or greater than the thickness of the first scale layer. Accordingly, of the whole scale layer, the second scale layer preferably has a thickness of ⅔ or greater. In addition, as shown in FIG. 2, the second scale layer include manganese, chromium and the like, and the first scale layer includes chromium and the like.

Between the ferritic stainless steel and the first scale layer, a third scale layer including niobium oxide is preferably formed.

Between a base material, that is, stainless steel, and a scale layer formed on a surface thereof, readily oxidizable silicon normally forms an oxide layer. An image of such silicon oxide layer formation is shown in FIG. 4. Silicon oxide has extremely low electrical conductivity and may not be used as a component for a fuel cell. Accordingly, production of silicon oxide needs to be suppressed by forming, instead of silicon, an oxide having high electrical conductivity while being oxidized faster than silicon. For this, niobium is added to form niobium oxide between the base material and the scale layer in the present disclosure, and silicon oxide formation is capable of being suppressed. More preferably, silicon oxide production needs to be completely prevented, however, completely suppressing the production of silicon oxide is very difficult. When niobium oxide is produced, an opportunity of silicon oxidation is reduced as much, and accordingly, total silicon oxide production may be reduced, and a decrease in the electrical conductivity may be prevented therefrom.

FIG. 6 shows a graph comparing silicon fractions in an example of the present disclosure and a comparative example forming a silicon oxide layer depending on a depth. According to FIG. 6, it is seen that a silicon fraction appears to be high at depths of 1 μm to 5 μm in a comparative example, however, a silicon fraction does not increase in the same range in an example of the present disclosure.

Meanwhile, as shown in FIG. 7, a niobium content appears to be high at depths of 2 μm to 5 μm in an example of the present disclosure.

Hereinafter, compositions, whether the formula is satisfied or not, and whether silicon oxide is produced or not in examples of the present disclosure and comparative examples are compared in Table 1.

TABLE 1
										Silicon
Steel Type	C	N	Si	Mn	Cr	Mo	Nb	Ti	Formula	Oxide
Example 1	0.005	0.007	0.11	0.5	21.3	0.15	0.43	0.05	Satisfied	Not
										Produced
Example 2	0.009	0.004	0.14	0.6	22.6	0.22	0.72	0.08	Satisfied	Not
										Produced
Example 3	0.007	0.013	0.06	0.4	23.5	0.33	0.65	0.04	Satisfied	Not
										Produced
Example 4	0.011	0.006	0.08	0.7	23.3	0.11	0.25	0.04	Satisfied	Not
										Produced
Example 5	0.007	0.009	0.09	0.5	20.5	0.25	0.53	0.07	Satisfied	Not
										Produced
Comparative	0.001	0.008	0.12	0.4	22.3	0.2	0.15	0.08	Not	Produced
Example 1									Satisfied
Comparative	0.008	0.007	0.12	0.1	22.6	0.23	0.7	0.05	Not	Produced
Example 2									Satisfied

As shown in Table 1, it was seen that silicon oxide is formed greatly reducing electrical conductivity when the composition or the formula of the present disclosure is not satisfied.

Hereinbefore, embodiments of the present disclosure have been described with reference to the accompanying drawings, however, it is to be understood that those having common knowledge in the art to which the present disclosure belongs may implement the present disclosure in other specific forms without modifying technological ideas or essential characteristics of the present disclosure.

Therefore, embodiments described above need to be understood as illustrative rather than limitative in all aspects. The scope of the present disclosure is represented by the attached claims rather than the detailed descriptions provided above, and the meaning and the scope of the claims, and all modifications or modified formed deduced from equivalent concepts thereof need to be interpreted as being included in the scope of the present disclosure.

Claims

1. A ferritic stainless steel used for a separator for a fuel cell, comprising a base material including, in weight %, C: 0.003% to 0.012%, N: 0.003% to 0.015%, Si: 0.05% to 0.15%, Mn: 0.3% to 0.8%, Cr: 20% to 24%, Mo; 0.1% to 0.4%, Nb: 0.1% to 0.7%, Ti: 0.03% to 0.1%, and the remainder being Fe and inevitable impurities,

wherein a first scale layer including chromium oxide is formed on a surface of the base material, and a second scale layer including chromium oxide and manganese oxide is formed on a surface of the first scale layer,

a silicon content included in each of the first scale layer and the second scale layer is 0.2 weight % or less, and

the following formula is satisfied:

formula: Nb+Mn≧8Si (Nb, Mn and Si are weight % amounts of corresponding components, respectively).

2. The ferritic stainless steel of claim 1, wherein a thickness of the second scale layer is ⅔ or greater of a thickness of the whole scale layer.

3. The ferritic stainless steel of claim 2, comprising a third scale layer including niobium oxide formed between the base material and the first scale layer.