JP6971185B2 - Ferritic stainless steel welded joints and fuel cell components - Google Patents

Description

The present invention relates to a ferrite stainless steel welded joint and a member for a fuel cell.

Recently, due to problems such as the depletion of fossil fuels such as petroleum and the _{phenomenon of global warming due to CO 2} emissions, the spread of new systems that replace conventional power generation systems is accelerating. As one of them, "fuel cells", which have high practical value as distributed power sources and power sources for automobiles, are attracting attention. There are several types of fuel cells, but among them, polymer electrolyte fuel cells (PEFC) and solid oxide fuel cells (SOFC) are highly energy efficient, and their widespread use is expected in the future.

A fuel cell is a device that generates electric power through a reaction process opposite to the electrolysis of water, and requires hydrogen (fuel hydrogen) as a fuel. Fuel hydrogen is produced by reforming a hydrocarbon fuel such as city gas (LNG), methane, natural gas, propane, kerosene, and gasoline in the presence of a catalyst. Among them, a fuel cell using city gas as a raw material has an advantage that hydrogen can be produced in an area where city gas piping is installed.

The fuel reformer is usually operated at a high temperature of 200 to 900 ° C. in order to secure the amount of heat required for the hydrogen reforming reaction. In addition to the fuel reformer, the operating temperature of the combustor that heats the reformer, the heat exchanger, the battery body, and the like becomes extremely high.
Further, in such a fuel cell under high temperature operation, an atmosphere containing a large amount of water vapor, carbon dioxide, carbon monoxide, a large amount of hydrogen, and a trace amount of hydrogen sulfide derived from a hydrocarbon fuel (hereinafter, carburizing property / It will be exposed to a reducing / sulfurizing environment). When, for example, a steel material is exposed to such an atmosphere, the surface of the material is carburized and corroded by sulfurization, which makes the operating environment harsh.

Further, in the case of high Cr-containing stainless steel, if it is exposed to a temperature range of about 400 to 500 ° C. for a long time, it is considered that hardening due to spinodal decomposition of the high Cr concentration phase and the low Cr concentration phase may occur. It is so-called 475 ° C brittle. Therefore, in a high temperature use environment in an SOFC system or PEFC system, it is conceivable that material deterioration due to brittleness at 475 ° C. may occur.

Here, it is recommended to use Al-containing ferritic stainless steel having good oxidation resistance for high temperature member applications in fuel cells. However, the Al-containing ferritic stainless steel has higher strength than other ferritic stainless steels and therefore has low toughness. For example, as disclosed in Non-Patent Document 1, the ductile brittle transition temperature of the steel material is increased by the addition of Al. It is known to do. Further, the Al-containing stainless steel has a problem that deterioration due to brittleness (σ brittleness) due to precipitation of the intermetallic compound σ phase in a high temperature range occurs.
Furthermore, cost reduction is indispensable for the widespread use of fuel cell systems in the future, and reduction of alloy costs by optimizing the materials used is an important issue.

Patent Document 1 discloses a ferrite-based stainless steel for a fuel cell, which has both oxidation resistance in a high-temperature steam-containing atmosphere and creep rupture life. The Cr content of this stainless steel is 13 to 20%, but if the Cr content is 15% or less that does not show brittleness at 475 ° C, it is necessary to add 4% or more of Al to ensure oxidation resistance. As described above, the toughness of the steel sheet containing a large amount of Al is significantly reduced, which makes actual production difficult.

Patent Document 2 describes ferrite stainless steel, which has excellent oxidation resistance and secondary workability, and is suitable for applications that require oxidation resistance and high moldability, such as element covers for sensors installed in automobile exhaust gas paths. Is disclosed. In the case of this stainless steel, since it contains about 18% Cr, it is considered that brittleness at 475 ° C. may develop.

Patent Document 3 discloses a high Al-containing ferritic stainless steel sheet having excellent surface properties, a method for producing the same, and a stainless foil. In the case of this stainless steel, since it contains about 18% Cr and 3% or more Al, it is considered that brittleness at 475 ° C. and a decrease in toughness due to a high Al content occur.

Patent Document 4 discloses a heat-resistant ferrite-based stainless steel for supporting a catalyst, which has excellent weldability and workability. This stainless steel is characterized in that it contains about 1 to 2.5% of Al and has excellent formability even after welding.

Japanese Unexamined Patent Publication No. 2014-139342 Japanese Unexamined Patent Publication No. 2012-12674 Japanese Unexamined Patent Publication No. 2015-78415 Japanese Unexamined Patent Publication No. 2001-316673

"The Increase in a Brittle-to-ductile Transition Temperature in Fe-Al Single crystal", ISIJ International, Vol. 51 No. 6, pp. 999-1004

The reformed gas of a fuel cell using the city gas or the like as a raw material may contain a large amount of hydrogen and a sulfide component added as an impurity or an odorant in addition to water vapor, carbon dioxide and carbon monoxide. be. However, conventionally, the oxidation resistance of ferritic stainless steel has been evaluated and examined only in an atmosphere containing water vapor and carbon dioxide as main components, an atmosphere containing water vapor and oxygen as main components, or an environment such as the atmosphere. .. That is, the oxidation characteristics of ferritic stainless steel under a harsh environment (carburizing / reducing / sulphurizing environment) containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfide component are unknown.
Further, in the case of an SOFC system or a PEFC system, the operating temperature of the fuel cell becomes high, so that further improvement in high temperature strength is required.
Further, when a welded structure is adopted as a member for a fuel cell, it is also required to have a welded structure in which brittle fracture of a welded portion due to brittleness at 475 ° C. or σ brittleness can be avoided.

In the stainless steels of Patent Documents 1 to 4, no mention is made of oxidation characteristics and high temperature strength under more severe environment such as embrittlement / reducing / sulfurizing environment containing a large amount of hydrogen and hydrogen sulfide, and 475 ° C. The embrittlement characteristics of welds due to brittleness and σ brittleness have not been recognized. That is, a high-performance ferrite stainless steel weld joint that improves the embrittlement characteristics of the welded portion and has excellent oxidation characteristics and high-temperature strength has not yet been realized.

The present invention has been devised to solve the above-mentioned problems, and is in an environment containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfide component (carburizing / reducing / sulfide environment). However, it provides a ferritic stainless steel welded joint and a member for a fuel cell, which has high oxidation resistance, excellent high temperature strength, and excellent brittleness.

The gist of the present invention is as follows.
[1] A welded joint composed of a ferritic stainless steel base material and a weld metal part.
The chemical composition of the weld metal part is mass%.
Cr: 12.0 to 16.0%,
C: 0.030% or less,
Si: 2.50% or less,
Mn: 1.00% or less,
P: 0.050% or less,
S: 0.0030% or less,
Al: 1.00 to 2.50%,
N: 0.030% or less,
Nb: 1.00% or less,
V: 0-1.00%,
Ni: 0-1.0%,
Cu: 0-1.0%,
Mo: 0-1.0%,
W: 0-1.0%,
Co: 0-0.50%,
As: 0-0.05%,
Pb: 0 to 0.005%,
Zr: 0-0.10%,
Zn: 0 to 0.03%,
Y: 0 to 0.10%,
La: 0 to 0.10%,
Hf: 0 to 0.10%,
Sb: 0 to 0.10%,
Ta: 0-0.5%
REM: 0 to 0.10%
Including, and further
B: 0.0200% or less,
Sn: 0.20% or less,
Ga: 0.0200% or less,
Mg: 0.0200% or less,
Ca: A ferrite-based stainless steel welded joint containing one or more of 0.0100% or less, satisfying the following formula (1), and having the balance composed of Fe and impurities.
10 (B + Ga) + Sn + Mg + Ca> 0.020 ... (1)
The element symbol in the formula (1) indicates the content (mass%) of each element in the weld metal portion.
[2] The chemical composition of the weld metal portion contains Nb: 0.001 to 1.0% and V: 0.001 to 1.0%, and is characterized by satisfying the following formula (2). The ferritic stainless steel welded joint according to the above [1].
(Nb + V) / {2 × (C + N)} ≧ 5.0 ・ ・ ・ (2)
The element symbol in the formula (2) indicates the content (mass%) of each element in the weld metal part.
[3] The chemical composition of the weld metal portion is further increased by mass%.
One or more of Ni: 0.02 to 1.0%, Cu: 0.02 to 1.0%, Mo: 0.02 to 1.0%, W: 0.02 to 1.0% The first group consisting of and
Co: 0.10 to 0.50%, As: 0.001 to 0.05%, Pb: 0.0001 to 0.005%, Zr: 0.0001 to 0.10%, Zn: 0.01 to 0.03%, Y: 0.0001 to 0.10%, La: 0.0001 to 0.10%, Hf: 0.0001 to 0.10%, Sb: 0.003 to 0.10%, Ta It is characterized by containing at least one of the second group consisting of one or more of 0.002 to 0.5% and REM: 0.001 to 0.10%. The ferrite-based stainless steel welded joint according to 1] or [2].
[4] The ferrite-based stainless steel welded joint according to any one of the above [1] to [3], wherein the size of the crystal grain size in the weld metal portion is 600 μm or less.
[5] The ferritic stainless steel welded joint according to any one of the above [1] to [4], which is applied to a fuel reformer, a heat exchanger or a fuel cell member.
[6] The ferrite-based stainless steel welded joint according to any one of the above [1] to [5], which is applied to a combustor or a burner member.

[7] A fuel cell member using the ferrite-based stainless steel welded joint according to any one of the above [1] to [6].

According to the present invention, high oxidation resistance and excellent high temperature strength even in an environment containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfide component (carburizing / reducing / sulphurizing environment). Further, it is possible to provide a member for a ferrite-based stainless steel welded joint and a fuel cell having excellent brittleness characteristics.

In order to solve the above-mentioned problems, the present inventors have completed the present invention by repeating diligent experiments and studies on welded joints of Al-containing ferritic stainless steel having both oxidation resistance, high temperature strength and embrittlement characteristics. I let you. The "high temperature strength" in the present embodiment is a characteristic that can exhibit an excellent 0.2% proof stress even in a high temperature range of around 750 to 800 ° C., and the "oxidation resistance" is carbon dioxide and monoxide. It means the oxidation characteristics in a reformed gas environment (carburizing / reducing / sulfurizing environment) containing carbon, a large amount of hydrogen, and a sulfurizing component. Further, the "embrittlement property" indicates the stability of the structure in which σ brittleness and 475 ° C. brittleness can be suppressed.
The findings obtained in the present invention will be described below.

"About embrittlement characteristics"
(A) Since the crystal grain size of the welded portion is larger than that of the base metal portion, fracture due to brittleness at 475 ° C. or σ brittleness is likely to occur. In the case of this fracture, it presents a cleavage plane. In the Al-containing ferritic stainless steel, the crystal grain size at the time of welding tends to be coarser than that in the ferritic stainless steel containing no Al. The crystal grain size of the weld metal portion (meaning a region melted and solidified during welding in the present embodiment) in the welded joint is preferably 600 μm or less.

(B) As a result of detailed microstructure observation, twinning deformation is involved in fracture caused by 475 ° C brittleness and σ brittleness, and shear strain occurs due to the introduction of deformed twinning. In order to release this shear strain, a cleft crack is generated and propagates, leading to fracture. It was also found that grain boundary cracks may occur when the above-mentioned shear strain is released. In this case, the grain boundary fracture due to the decrease in grain boundary strength due to the segregation of grain boundaries such as P and S becomes the starting point of crack formation.

(C) In order to suppress the fracture caused by 475 ° C brittleness and σ brittleness in the welded part, (i) it is difficult to develop twinning deformation, and (ii) it is necessary to suppress the coarsening of crystal grains during welding. , (Iii) It is important to suppress the decrease in grain boundary strength due to excessive grain boundary segregation of impurity elements.

(D) Al has conventionally been known to be an element that increases the strength of ferritic steel and increases the ductile brittle transition temperature, that is, promotes brittle fracture. However, in a situation where the crystal grains are coarse like a welded part and slip deformation is very difficult due to hardening due to 475 ° C brittleness or σ brittleness, adding an appropriate amount of Al increases the twin crystal deformation stress and causes relative slip. It was newly found that it facilitates deformation.

(E) Since the welded structure of Al-containing ferritic stainless steel is usually composed of the growth of columnar crystals, it tends to be coarser than that of ferritic stainless steel that does not contain Al. However, the formation of equiaxed crystals is promoted by forming carbonitrides composed of (Nb, V) (C, N) or inclusions composed of MgO, CaO, etc. by controlling the chemical components in the weld metal. It was found that the weld structure became finer.

(F) In order to obtain the effect of promoting the formation of equiaxed crystals by the carbonitride (Nb, V) (C, N) described in the above (e), the chemical composition of the weld metal portion is 10 (B + Ga) + Sn + Mg + Ca. Was found to be important to be greater than 0.020. In the case of weld metal, Mg, Ca and Ga can generate oxides and sulfides and improve the cleanliness of grain boundaries. In addition, since Mg, Ca, and Ga also effectively act as nucleation sites for Nb (C, N), it is possible to promote the formation of equiaxed crystals of the weld metal. It was also found that it is preferable to set the value of (Nb + V) / {2 × (C + N)} to 5.0 or more in order to efficiently obtain such an action and effect.

(G) Further, adjusting the content of Cr, Si, Nb, and Al in the composition of the weld metal portion is effective in suppressing the precipitation (σ brittleness) of the intermetallic compound σ phase and the brittleness itself at 475 ° C. It turned out to be. The σ brittleness and the 475 ° C. brittleness are derived from the formation of intermetallic compounds containing Cr as the main component and Si and Al, and the formation sites are often grain boundaries. That is, in order to suppress σ brittleness and 475 ° C. brittleness, it can be said that it is effective to suppress the formation of the intermetallic compound itself and reduce the formation sites thereof. As a result of further studies by the present inventors on these, the formation of the intermetallic compound itself is suppressed by limiting the amount of Cr, and the structure is stabilized by suppressing the formation site by segregation of Nb into the grain boundaries. As a result, it was found that σ brittleness and 475 ° C. brittleness can be suppressed. Furthermore, by limiting the amount of Cr and adding Nb, the formation of intermetallic compounds containing Si and Al can be suppressed, and the amount of Si and Al that contributes to the oxidation resistance described later can be secured, so that oxidation resistance and tissue stability can be secured. It is also possible to achieve both sex.

"About high temperature strength"
(H) In order to suppress deformation, which is usually a problem in structures operating in the high temperature range of around 750 to 800 ° C, the high temperature strength of the material ferrite stainless steel, especially 0.2% at around 750 ° C. It is effective to increase the proof stress and suppress the decrease of 0.2% proof stress at around 800 ° C.

(I) It was found that the improvement and suppression of the decrease in 0.2% proof stress in the high temperature region described above are remarkably improved by adding a small amount of B, Nb, Sn, Mg, Ca and Ga and adjusting the amount thereof. .. That is, in ferrite stainless steel, the property of increasing the 0.2% proof stress at around 750 ° C and suppressing the decrease in 0.2% proof stress at around 800 ° C can be achieved by adding these trace elements. Findings were obtained. There are still many unclear points about the action of improving the high temperature strength, but the mechanism of action described below is inferred based on the experimental facts.

(J) The addition of a small amount of B contributes not a little to the increase in proof stress and tensile strength at 750 to 800 ° C., and in particular has the effect of significantly improving the proof stress by 0.2%. The addition of a small amount of B suppresses the formation of cavities (nano-sized gaps) generated from the grain boundaries due to the segregation of B at the grain boundaries, delays the grain boundary slip, and causes the dislocation density in the crystal grains. It has the effect of increasing the internal stress associated with the rise of. In addition, we have found new findings that the action and effect of B are remarkable in Nb-added steel.

(K) The action and effect of B, which is remarkable in the above-mentioned Nb-added steel, is superimposed by the combined addition of Mg, Ca, and Ga. Mg and Ca generate non-metal inclusions and sulfides, increase the cleanliness of the crystal grain boundaries, promote the segregation of B grain boundaries, and more efficiently express the above-mentioned action and effect of B. Further, since Ga also improves the cleanliness of the steel, the above-mentioned action and effect of B can be efficiently exhibited by the combined addition with B.

(L) Further, in order to further exert the effect of increasing the internal stress accompanying the increase in the dislocation density in the grain described in (j) above, the combined addition with Sn is effective. Although Sn is a grain boundary segregation element, when it is added in combination with B, it also has a large action as a solid solution strengthening element in the crystal grains, and is effective in increasing the high temperature strength with an increase in internal stress.

"About oxidation resistance"
(M) Further, in order to enhance the oxidation resistance in the reformed gas environment containing the above-mentioned hydrogen and sulfide components, Al-based oxidation is performed by adjusting the contents of Si, Al, Nb and Mn within a predetermined range. It is effective to promote the formation of a film and enhance the protective property of the film. Furthermore, the addition of B, Nb, Sn, Mg, Ca, and Ga in ferritic stainless steel does not impair the oxidation resistance in a modified gas environment. Rather, the addition of a small amount of Mg and Sn does not impair the oxidation resistance of the Al-based oxide film. It also enhances the protection of stainless steel and has the effect of oxidation resistance. Since Si is also an element that promotes columnar crystallization of the weld structure like Al, if the amount of Si is increased from the viewpoint of promoting the formation of an Al-based oxide film, there is a concern that the weld metal portion may become coarse. NS. However, since columnar crystallization of the welded structure can be sufficiently suppressed by adding a small amount of Nb, Sn, Mg, Ca, and Ga, even when the amount of Si is relatively high as in the present embodiment, the Al-based oxide film can be used. It is possible to achieve both promotion of formation and suppression of coarsening of the weld metal portion. Here, in the present embodiment, the surface film before being exposed to the high temperature reformed gas environment is "passive film", and the passive film exposed to the high temperature reformed gas environment is composed by various reactions. The changed one will be described separately from the "Al-based oxide film".

(N) The reformed gas environment (carburizing / reducing / sulphurizing environment) described above produces defects in the Al-based oxide film in the ferritic stainless steel as compared with the steam oxidation environment containing no atmosphere or hydrogen. easy. The reason why the reformed gas environment facilitates the formation of defects in the oxide film is not clear, but it is presumed that the sulfide produced under the reformed gas containing a sulfide component has some adverse effect on the oxide film. .. If a defect occurs in the Al-based oxide film in the reformed gas environment, the oxidation of Cr and Fe may proceed in the exposed base material. In response to the promotion of oxidation in such a reforming gas, Mg dissolves in an Al-based oxide film, and Sn delays the outward diffusion of Cr and Fe by segregation on the surface of the base metal, thereby causing an Al-based solution. The protective property of the oxide film can be further enhanced. As a result, the oxidation resistance of the ferritic stainless steel can be improved.

Hereinafter, an embodiment of the ferritic stainless steel welded joint of the present invention will be described.

<Ingredient composition>
First, the reasons for limiting the chemical composition of the weld metal part will be described below. In addition, "%" display of the content of each element means "mass%".

<Cr: 12.0 to 16.0%>
In addition to corrosion resistance, Cr is a basic constituent element for improving high-temperature strength and ensuring the protection of the surface oxide film, and in order to obtain these effects, an amount of Cr of 12.0% or more is required. be. It is preferably 13.0% or more. On the other hand, excessive Cr content promotes the formation of the σ phase, which is an embrittled phase, when exposed to a high temperature atmosphere, in addition to the remarkable hardening of the material due to brittleness at 475 ° C. In addition, the upper limit is set to 16.0% or less because it may promote an increase in alloy cost and Cr evaporation. It is preferably 15.0% or less.

<C: 0.030% or less>
C forms a solid solution or Cr carbide in the ferrite phase and inhibits oxidation resistance. It also promotes the formation of Cr carbides at the grain boundaries during welding. Therefore, the smaller the amount of C, the better, and the upper limit is 0.030% or less. It is preferably 0.020% or less. However, since excessive reduction leads to an increase in refining cost, the lower limit of the amount of C is preferably 0.001% or more. More preferably, it is 0.002% or more.

<Si: 2.50% or less>
Si is an important element for ensuring oxidation resistance. Si slightly dissolves in the Al-based oxide film and also concentrates directly under the oxide film / at the steel interface, improving the oxidation resistance in a modified gas environment. In order to obtain these effects, it is preferably 0.50% or more. On the other hand, excessive inclusion of Si promotes spinodal decomposition of Cr and lowers brittleness resistance at 475 ° C. Further, since Si is also an element that promotes columnar crystallization of the welded structure, if it is contained in a large amount, the weld metal portion may be coarsened. Further, since the toughness and workability of steel may be deteriorated and the formation of an Al-based oxide film may be hindered, the upper limit is set to 2.50% or less. The preferred upper limit is 1.70% or less.

<Mn: 1.00% or less>
Mn can be dissolved together with Si in the Al-based oxide film or directly under the reformed gas environment to enhance the protection property and contribute to the improvement of the oxidation resistance. In order to obtain these effects, the lower limit is preferably 0.10%. On the other hand, if it is contained excessively, the corrosion resistance of the steel and the formation of the Al-based oxide film are hindered, so the upper limit is set to 1.00% or less. From the viewpoint of oxidation resistance and basic characteristics, 0.90% or less is preferable.

<P: 0.050% or less>
P is an element that hinders manufacturability and weldability and lowers the grain boundary strength in the welded portion. The lower the content, the better, so the upper limit is 0.050% or less. However, since excessive reduction leads to an increase in refining cost, the lower limit is preferably 0.003% or more. From the viewpoint of manufacturability and weldability, the preferable range is 0.005 to 0.040%.

<S: 0.0030% or less>
S is an impurity element inevitably contained in the steel and lowers the protective property of the Al-based oxide film. In particular, the presence of Mn-based inclusions and solid solution S also acts as a starting point for destruction of the Al-based oxide film at high temperature and long-term use. It is also an element that lowers the grain boundary strength in the weld. Therefore, the lower the amount of S, the better, so the upper limit is 0.0030% or less. However, since excessive reduction leads to an increase in raw materials and refining costs, the lower limit is preferably 0.0001% or more. From the viewpoint of manufacturability, oxidation resistance, and brittleness at 475 ° C., a preferable range is 0.0001 to 0.0020%.

<Al: 1.00 to 2.50%>
In addition to being a deoxidizing element, Al is an essential element for forming an Al-based oxide film and contributing to the improvement of oxidation resistance. It is also an element effective in suppressing the occurrence of twinning deformation in welds and improving the embrittlement characteristics. In the present embodiment, since these effects cannot be obtained when the Al amount is less than 1.00%, the lower limit is set to 1.00% or more. It is preferably 1.50% or more. However, since Al is also an element that promotes columnar crystallization of the weld structure like Si, if it is contained in a large amount, the weld metal portion may be coarsened. Further, if Al is contained excessively, the toughness of the steel is lowered and the brittle fracture in the welded portion is promoted. Therefore, the upper limit is set to 2.50% or less. It is preferably 2.30% or less.

<N: 0.030% or less>
N is an element that inhibits oxidation resistance like C. Therefore, the smaller the amount of N, the better, and the upper limit is 0.030% or less. However, since excessive reduction leads to an increase in refining cost, the lower limit is preferably 0.002% or more. From the viewpoint of oxidation resistance and manufacturability, the preferable range is 0.005 to 0.020%.

<Nb: 1.00% or less>
Nb is a stabilizing element that fixes C and N, and contributes to suppressing the formation of Cr carbides during welding. Furthermore, the intermetallic compound that causes σ brittleness and 475 ° C. brittleness proceeds to precipitate mainly at the grain boundaries, but this formation site is reduced by segregation of Nb into the grain boundaries. , The stability of the structure is increased, and as a result, σ brittleness and 475 ° C. brittleness can be suppressed. In order to obtain these effects, the lower limit of Nb is preferably 0.001% or more, more preferably 0.15% or more. On the other hand, since excessive inclusion of Nb promotes brittle fracture in addition to an increase in alloy cost, the upper limit of Nb is set to 1.00% or less. Both are preferably 0.60% or less.

<B, Sn, Ga, Mg, Ca>
As described above, B, Sn, Ga, Mg, and Ca are elements that can further exert the effect of increasing the high temperature strength. In addition, these elements are also elements that promote the formation of an Al-based oxide film and contribute to the improvement of oxidation resistance. Further, Sn, Ga, Mg and Ca have an action of concentrating in the vicinity of the surface and promoting selective oxidation of Al. Therefore, in addition to the above-mentioned component composition, it contains one or more of B, Sn, Ga, Mg, and Ca.
B can delay grain boundary slip by segregating grain boundaries, increase internal stress due to an increase in dislocation density in the crystal grains, and improve 0.2% proof stress. Mg and Ca are effective elements for improving the cleanliness and hot workability of steel. Further, it is an element that promotes the formation of equiaxed crystals by generating inclusions composed of MgO, CaO, etc. at the time of welding, and contributes to the miniaturization of the welded structure. In order to obtain these effects, it is preferable that Sn is 0.005% or more, and B, Ga, Mg, and Ca are 0.0002% or more, respectively. On the other hand, excessive inclusion of these elements causes an increase in steel refining cost and a decrease in manufacturability. Therefore, Sn is 0.20% or less, B, Ga and Mg are 0.0200% or less. Ca is 0.0100% or less.

<10 (B + Ga) + Sn + Mg + Ca>
Further, in the welded joint according to the present embodiment, the content (% by mass) of B, Sn, Ga, Mg and Ca in the weld metal portion is satisfied while B, Sn, Ga, Mg and Ca satisfy the chemical composition limited above. ) Satisfies the following equation (1).

10 (B + Ga) + Sn + Mg + Ca> 0.020 ... (1)
If the left side of the formula (1) is 0.020 or less, the crystal grains may be coarsened. Therefore, the content of B, Sn, Ga, Mg, and Ca in the weld metal portion shall satisfy the formula (1), and preferably the left side of the formula (1) is 0.035 or more.
The upper limit of the formula (1) is not particularly specified by the upper limit values of B, Sn, Ga, Mg, and Ca, but is preferably 0.2 or less from the viewpoint of high temperature strength and manufacturability.
Here, B, Sn, Ga, Mg, and Ca in the formula (1) indicate the content (mass%) of each element in the weld metal portion.

The weld metal portion of the ferritic stainless steel welded joint according to the present embodiment is composed of Fe and impurities other than the above-mentioned elements (remaining portion), but can also contain arbitrary elements described later. Therefore, the lower limit of the content of V, Ni, Cu, Mo, W, Co, As, Pb, Zr, Zn, Y, La, Hf, Sb, Ta, and REM is 0% or more.
The "impurity" in the present embodiment is a component that is mixed by various factors in the manufacturing process including raw materials such as ore and scrap when industrially manufacturing steel, and is inevitably mixed. Also includes ingredients.

<(Nb + V) / {2 × (C + N)}>
The welded joint according to the present embodiment contains Nb: 0.001 to 1.0% and V: 0.001 to 1.0%, and contains Nb, V, C, and N in the weld metal portion. It is preferable that (% by mass) satisfies the following formula (2).

Like Nb, V is a stabilizing element that fixes C and N, and contributes to suppressing the formation of Cr carbides during welding. Therefore, V is preferably 0.001% or more, more preferably 0.15% or more. On the other hand, since excessive inclusion of V promotes brittle fracture in addition to an increase in alloy cost, the upper limit of V is set to 1.00% or less. Both are preferably 0.60% or less.

(Nb + V) / {2 × (C + N)} ≧ 5.0 ... Equation (2)
When the Nb, V, C, and N amounts of the weld metal portion satisfy the equation (2), even if a carbonitride is formed, the Nb amount required for ensuring the high temperature strength can be sufficiently secured. However, if the left side of the above formula (2) is less than 5.0%, the high temperature strength may decrease. Therefore, the content of Nb, V, C, N in the weld metal portion preferably satisfies the formula (2), and more preferably the left side of the formula (2) is 10.0 or more.
Here, Nb, V, C, and N in the formula (2) indicate the content (mass%) of each element in the weld metal portion, but when they are not contained (when the content is 0%), they are 0. Is substituted and calculated.

Further, the chemical composition of the weld metal portion of the joint according to the present embodiment is, if necessary, the first group consisting of one or more of Ni, Cu, Mo and W, and Co, As, Pb and Zr. , Zn, Y, La, Hf, Sb, Ta, REM may contain at least one of the second group consisting of one or more.

<Ni, Cu, Mo, W: 1.0% or less> (Group 1)
Ni, Cu, Mo, and W are elements effective for increasing high temperature strength and corrosion resistance, and may contain at least one of them if necessary. However, if it is excessively contained, the alloy cost will increase and the manufacturability will be impaired. Therefore, the upper limit of each content of Ni, Cu, Mo and W is set to 1.0% or less. In order to exhibit the above effect, Ni, Co, Mo and W are preferably 0.02% or more, respectively. More preferably, it is 0.08% or more.

<Co, As, Pb, Zr, Zn, Y, La, Hf, Sb, Ta, REM> (Group 2)
These elements segregate at the grain boundaries and suppress the coarsening of crystal grains during welding. Further, Zr, La, Y, Hf, Ta, and REM are also effective elements for improving hot workability, steel cleanliness, and oxidation resistance. Zn and Sb are concentrated near the steel surface to promote selective oxidation of Al and suppress oxidation of Cr.
In order to obtain these effects, Co: 0.005% or more, As: 0.001% or more, Pb: 0.0001% or more, Zr: 0.0001% or more, Zn: 0.01% or more, Y: 0 One of .0001% or more, La: 0.0001% or more, Hf: 0.0001% or more, Sb: 0.003% or more, Ta: 0.002% or more, REM: 0.001% or more or It is preferable that two or more kinds are contained and the total amount thereof is 0.010% or more. In the Al-containing ferritic stainless steel welded joint of the present embodiment, the weld metal portion contains the second group together with the first group or instead of the first group within the above-mentioned content range. May be good.
On the other hand, excessive addition of these elements promotes grain boundary destruction due to a decrease in grain boundary strength. Therefore, in the second group, Co: 0.50% or less, As: 0.05% or less, Pb: 0.005. % Or less, Zr: 0.10% or less, Zn: 0.03% or less, Y: 0.10% or less, La: 0.10% or less, Hf: 0.10% or less, Sb: 0.10% or less , Ta: 0.5% or less, REM: 0.10% or less, and it is necessary to make an element group consisting of one kind or two or more kinds.
The REM is an element belonging to atomic numbers 58 to 71 excluding La and Y and Sc (scandium), and is, for example, Ce, Pr, Nd and the like. Further, the REM referred to in the present embodiment is composed of one or more selected from the elements belonging to atomic numbers 58 to 71 and Sc, and the REM amount is the total amount of these.

The ferritic stainless steel welded joint according to the present embodiment is composed of Fe and impurities (including unavoidable impurities) other than the elements described above, but the effects of the present invention are impaired in addition to the elements described above. It can be contained in a range that does not exist. It is preferable to reduce Bi, Se and the like as much as possible, including the above-mentioned P and S, which are general impurity elements. On the other hand, the content ratio of these elements is controlled to the extent that the problem of the present invention is solved, and if necessary, one or more of Bi ≦ 100 ppm and Se ≦ 100 ppm may be contained.

Here, the ferritic stainless steel welded joint of the present embodiment is a welded joint composed of a base material (base material portion) made of ferritic stainless steel and a welded metal portion, and both the base metal portion and the welded metal portion are made of metal. The structure consists of a ferritic single phase structure. This means that it does not contain the austenite phase or the martensite structure. When an austenite phase or a martensite structure is contained, the raw material cost is high and the yield such as ear cracks is likely to decrease during manufacturing. Therefore, the metal structure of the welded joint is a ferrite single-phase structure. It should be noted that although precipitates such as carbonitrides are present in the steel, they are not considered because they do not greatly affect the effect of the present invention, and the above description describes the structure of the main phase.

Further, as described above, since the crystal grain size of the welded portion is larger than that of the base metal portion, fracture due to brittleness at 475 ° C. or σ brittleness is likely to occur. Therefore, in the present embodiment, the crystal grain size of the weld metal portion is preferably 600 μm or less.

The particle size of the weld metal portion can be obtained as follows.
A sample is cut out from the welded portion of the welded joint, etched, and then the number of crystal grains in the region of 800 μm in thickness × 1600 μm in width at the center of the weld metal portion is counted in the weld metal portion photographed by an optical microscope. The number of crystal grains straddling one side of the measurement region is 0.5, and the number of crystal grains straddling two sides is 0.25. Then, the square root of (area of measurement area) / (number of crystal grains in measurement area) is defined as the crystal grain size of the weld metal portion.

The welded joint of the present embodiment is composed of the weld metal portion described above and the base metal portion made of ferritic stainless steel. In the present embodiment, the component composition of the base material portion is not particularly limited, but it is preferable that the base material portion is an Al-containing ferritic stainless steel capable of efficiently exerting the effect in the present invention. Further, from the viewpoint of facilitating the component control of the welded joint, it is more preferable that the base metal has a component system similar to that of the welded joint of the present invention.
That is, the suitable chemical composition of the base material portion of the present embodiment is, in mass%, C: 0.020% or less, Si: 2.50% or less, Mn: 1.00% or less, Cr: 12.0 to 16.0%, Al: 2.50% or less, N: 0.030% or less, Nb: 1.00% or less, V: 1.00% or less, and B: 0.0200% or less, Sn: 0.20% or less, Ga: 0.0200% or less, Mg: 0.0200% or less, Ca: 0.0100% or less containing one or more types, and satisfying the following formula (1'). The balance is Fe and impurities.

10 (B + Ga) + Sn + Mg + Ca> 0.020 ... (1')
In addition, each element symbol in the formula (1') indicates the content (mass%) of each element in the base metal part.

The shape of the ferrite-based stainless steel as the base material is not particularly limited to plate materials, pipe materials, wire materials, and the like.

<Manufacturing method>
Next, a method for manufacturing the ferritic stainless steel welded joint of the present embodiment will be described.
The ferritic stainless steel welded joint of the present embodiment is manufactured by tanning ferritic stainless steel or welding using a welding rod. The welded joint of the present embodiment can be easily realized by manufacturing using a welding material described later, but it goes without saying that the welded joint is not limited to the manufacturing conditions. That is, the chemical composition of the weld metal in the final welded joint can be controlled within the scope of the present embodiment by appropriately selecting the base steel material to be welded, the welding material to be used, the welding method, and the welding conditions.
When welding materials are used, good oxidation resistance, high temperature strength, and embrittlement resistance are required for fuel cell system applications. Therefore, welded joints are manufactured using Al-containing ferritic stainless steel that has these characteristics as a base material. It is preferable, and specifically, it is more preferable to use stainless steel having the above-mentioned chemical composition as a base material. Further, in the sense of facilitating the component control of the welded joint, it is more preferable that the base material and the welded material have a component system similar to that of the welded joint of the present embodiment.

The chemical composition of the suitable welding material of this embodiment is Cr: 12.0 to 16.0%, C: 0.020% or less, Si: 0.60 to 2.50%, Mn: 0 in mass%. .30% or less, Al: 1.00 to 2.50%, Nb: 0.001 to 1.00%, P: 0.020% or less, S: 0.0030% or less, O: 0.010% or less , N: Contains 0.030% or less, and the balance is Fe and impurities.

According to this embodiment, high oxidation resistance and excellent high temperature strength even in an environment containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfide component (carburizing / reducing / sulphurizing environment). Further, it is possible to provide a ferritic stainless steel welded joint having excellent structure stability capable of suppressing σ phase precipitation and brittleness at 475 ° C.
Therefore, the welded joint of the present embodiment includes a fuel reformer, a heat exchanger, and the like used for reforming hydrocarbon fuels such as city gas, methane, natural gas, propane, kerosene, and gasoline into hydrogen. It is suitable for a fuel cell member, and is particularly suitable for a high temperature member of a solid oxide fuel cell (SOFC) or a solid polymer fuel cell (PEFC) having a high operating temperature. Further, it can be suitably used for all peripheral members of a fuel cell, such as a burner and a combustor for storing the burner, which are in contact with a reforming gas and are used in a high temperature environment.

Next, an example of the present invention will be shown. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is used in the following examples. It is not limited to the conditions. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
The underlined lines in the table below indicate those outside the scope of the present invention.

Ferritic stainless steels of steel grades A to O whose components are shown in Table 1 are melted, and after hot rolling, annealing pickling, and cold rolling, finish annealing and pickling are performed to obtain a 0.8 mm thick cold-rolled annealed plate. Manufactured.

A test piece having a width of 120 mm and a length of 250 mm was cut out from a ferritic stainless steel plate of each steel type. Next, using test pieces of the same steel type as a base material, TIG tanning welding was performed to manufacture ferritic stainless steel welded joints A to O for each of the ferritic stainless steel sheets of steel types A to O. The welding conditions were a current of 85 A, a voltage of 8.5 V, a welding speed of 100 cm / min, and an Ar gas adjusted to a flow rate of 30 l / min was used as the shield gas. The welding position was at the center of the plate width, and the welding direction was the plate longitudinal direction. Table 1 shows the compositions of the weld metal portions of the obtained welded joints A to O. Since the TIG tanning welding does not use a welding material, the obtained weld metal of the welded joint has the same composition as the ferritic stainless steel plate used as the base material.

The "composition of the example of the invention" in Table 1 means that the composition of the steel satisfies the composition of the weld metal portion of the Al-containing ferritic stainless steel welded joint of the present invention. Further, the "composition of the comparative example" in Table 1 means that the composition of the steel does not satisfy the composition of the weld metal portion of the Al-containing ferritic stainless steel welded joint of the present invention.

Next, the Al-containing ferritic stainless steel welding material (welding material) of the symbol W1 shown in Table 2 is melted, the steel types B and C in Table 1 are used as the base material, and the welding material of the symbol W1 is used for TIG. Welding was performed to produce welded joints BW and CW. Welding conditions were current 60A, voltage 7.5V, welding speed 60cm / min, welding material supply amount 8g / min, and Ar gas adjusted to a flow rate of 30l / min was used as the shield gas. The welding position was at the center of the plate width, and the welding direction was the plate longitudinal direction.
Next, samples for chemical analysis were collected from the weld metal parts of the obtained welded joints BW and CW, and component analysis was performed. Table 3 shows the chemical composition of the weld metal part of each welded joint. Here, regarding the symbols in Table 3, "BW" means a welded joint manufactured by using steel type B as a base material, and "CW" means a welded joint manufactured by using steel type C as a base material. Means that

[Measurement of crystal grain size]
A sample was cut out from the weld metal part of each welded joint, filled with resin, and then etched with aqua regia. For the sample after etching, the number of crystal grains in the region of 800 μm in thickness × 800 μm in width at the center of the weld metal portion was counted in the weld metal portion photographed by an optical microscope. The number of crystal grains straddling one side of the measurement area was 0.5, and the number of crystal grains straddling two sides was 0.25. The square root of (area of measurement area) / (number of crystal grains in measurement area) was defined as the crystal grain size of the weld metal part.

[Oxidation resistance]
Oxidation test pieces having a width of 20 mm and a length of 25 mm were cut out from the welded portion of each welded joint. At this time, it was cut out so that the welding line was arranged at the center of the width of the oxidation test piece, that is, the longitudinal direction of the test piece and the bead direction were parallel to each other. Atmosphere oxidation test, assuming the reformed gas in which the city gas as fuel, 28 _vol% H 2 _O-10% by volume% CO-8 _{vol% CO 2 -0.01% H 2 S} -bal. Was an atmosphere of H _2. In the atmosphere, the oxidation test piece was heated to 650 ° C., held for 1000 hours, cooled to room temperature, and the oxidation increase ΔW (mg / cm ² ) was measured.
The evaluation of oxidation resistance was as follows.
⊚: Weight increase ΔW is less than ^{0.2 mg / cm 2.}
〇: Weight increase ΔW is 0.2 to 0.3 mg / cm ² .
X: Weight increase ΔW is more than ^{0.3 mg / cm 2.}
As for the oxidation resistance, the cases of "◎" and "○" were regarded as acceptable.

[High temperature strength]
A plate-shaped high-temperature tensile test piece (plate thickness: 0.8 mm, parallel portion width: 10.5 mm, parallel portion length: 35 mm) was cut out from each welded joint. At this time, the weld metal portion was cut out so as to be arranged at the center of the tensile test piece in the longitudinal direction (center of the parallel portion). At 750 ° C and 800 ° C, high-temperature tensile tests were conducted with a strain rate of 0.3% / min up to 0.2% proof stress and then 3 mm / min, and 0.2% proof stress (750 ° C proof stress) at each temperature. , 800 ° C proof stress) was measured (according to JIS G 0567).
In the evaluation of high temperature strength, when the proof stress of 750 ° C is more than 120 MPa and the proof stress of 800 ° C is more than 40 MPa, it is evaluated as a pass (“〇”), and when any one of them is not satisfied, it is rejected (“×”). evaluated. When the proof stress of 750 ° C. was more than 150 MPa and the proof stress of 800 ° C. was more than 60 MPa, it was evaluated as having particularly excellent high-temperature strength (indicated by "⊚" in the table).

[Structural stability (σ brittleness / 475 ° C brittleness)]
Two samples were taken from the weld metal part of the welded joint so that the center on the cross section perpendicular to the plate surface (center of plate thickness: near t / 2) could be observed, and one was heat-treated at 500 ° C for 1000 hours. (500 ° C. heat treatment), and the other was heat-treated at 650 ° C. × 1000 hours (600 ° C. heat treatment). The atmosphere of these heat treatments was set to the atmosphere. Next, after each sample after the heat treatment is embedded in a resin and polished, the Vickers hardness Hv 500 ° C after the heat treatment at _{500 ° C. and the Vickers hardness Hv 650 ° C.} after the heat treatment at 650 ° C. are loaded according to JIS Z 2244, respectively. _{The hardness increase amount ΔHv 500 ° C.} and ΔHv _{650 °} C. from the Vickers hardness before heat treatment, which were measured at 8.N and measured in advance before the heat treatment, were calculated.
For the evaluation of tissue stability (σ brittleness / 475 ° C brittleness), _{those with ΔHv 500 ° C} and ΔHv _{650 ° C} both less than 20 are evaluated as acceptable (“〇”), and if either one is 20 or more, it is evaluated. It was rejected (“x”) because the hardness increased significantly after the heat treatment and the structure was unstable.

Table 4 shows the test results. In the welded joints A to I, BW, and CW, the weld metal component satisfies the component specified in the present invention, and the evaluation of all the characteristics is "◯" or "◎".

In the weld metals J to O, the weld metal component deviates from the component specified in the present invention, and the crystal grain size and each characteristic targeted by the present invention cannot be compatible with each other. It became.

Claims (7)

A welded joint consisting of a ferritic stainless steel base material and a weld metal part.
The chemical composition of the weld metal part is mass%.
Cr: 12.0 to 16.0%,
C: 0.030% or less,
Si: 2.50% or less,
Mn: 1.00% or less,
P: 0.050% or less,
S: 0.0030% or less,
Al: 1.00 to 2.50%,
N: 0.030% or less,
Nb: 1.00% or less,
V: 0-1.00%,
Ni: 0-1.0%,
Cu: 0-1.0%,
Mo: 0-1.0%,
W: 0-1.0%,
Co: 0-0.50%,
As: 0-0.05%,
Pb: 0 to 0.005%,
Zr: 0-0.10%,
Zn: 0 to 0.03%,
Y: 0 to 0.10%,
La: 0 to 0.10%,
Hf: 0 to 0.10%,
Sb: 0 to 0.10%,
Ta: 0-0.5%
REM: 0 to 0.10%
Including, and further
B: 0.0200% or less,
Sn: 0.20% or less,
Ga: 0.0200% or less,
Mg: 0.0200% or less,
Ca: A ferrite-based stainless steel welded joint containing one or more of 0.0100% or less, satisfying the following formula (1), and having the balance composed of Fe and impurities.
10 (B + Ga) + Sn + Mg + Ca> 0.020 ... (1)
The element symbol in the formula (1) indicates the content (mass%) of each element in the weld metal portion. Claim 1 is characterized in that the chemical composition of the weld metal portion contains Nb: 0.001 to 1.0% and V: 0.001 to 1.0% and satisfies the following formula (2). Ferritic stainless steel welded joints described in.
(Nb + V) / {2 × (C + N)} ≧ 5.0 ・ ・ ・ (2)
The element symbol in the formula (2) indicates the content (mass%) of each element in the weld metal portion. The chemical composition of the weld metal portion is further increased by mass%.
One or more of Ni: 0.02 to 1.0%, Cu: 0.02 to 1.0%, Mo: 0.02 to 1.0%, W: 0.02 to 1.0% The first group consisting of and
Co: 0.10 to 0.50%, As: 0.001 to 0.05%, Pb: 0.0001 to 0.005%, Zr: 0.0001 to 0.10%, Zn: 0.01 to 0.03%, Y: 0.0001 to 0.10%, La: 0.0001 to 0.10%, Hf: 0.0001 to 0.10%, Sb: 0.003 to 0.10%, Ta The claim is characterized by containing at least one of a second group consisting of one or more of 0.002 to 0.5% and REM: 0.001 to 0.10%. The ferrite-based stainless steel welded joint according to 1 or 2. The ferrite-based stainless steel welded joint according to any one of claims 1 to 3, wherein the size of the crystal grain size in the weld metal portion is 600 μm or less. The ferrite-based stainless steel welded joint according to any one of claims 1 to 4, wherein the joint is applied to a fuel reformer, a heat exchanger, or a fuel cell member. The ferrite-based stainless steel welded joint according to any one of claims 1 to 5, which is applied to a member of a combustor or a burner. A fuel cell member using the ferrite-based stainless steel welded joint according to any one of claims 1 to 6.