JP6767831B2 - Ferritic stainless steel and welded structures for welded structures with excellent high temperature fatigue characteristics - Google Patents

Description

The present invention is suitable for fuel cell high temperature members such as reformers and heat exchangers used when reforming hydrocarbon fuels such as city gas, methane, natural gas, propane, kerosene and gasoline into hydrogen. Regarding Al-containing ferrite-based stainless steel. In particular, it is suitable for a high temperature member in a welded structure of a solid oxide fuel cell (SOFC), which requires thermal fatigue characteristics in a high temperature environment including a reformed gas environment.

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. 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 700 ° C. in order to secure the amount of heat required for the hydrogen reforming reaction. Further, under such high temperature operation, it is exposed to an oxidizing atmosphere containing a large amount of water vapor, carbon dioxide, carbon monoxide, etc., and a heating / cooling cycle of starting / stopping is repeated according to the demand for hydrogen. So far, austenitic stainless steel represented by SUS310S (25Cr-20Ni) has been used as a practical material having sufficient durability in such a harsh environment. 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. Further, in the SOFC system, when applying a high Cr-containing stainless steel, it is necessary to select a steel type capable of preventing the ceramic electrode from being poisoned by the evaporation of Cr at the SOFC operating temperature. From the above background, the application of Al-containing ferritic stainless steel having high oxidation resistance of alumina to a fuel reformer is disclosed.

Patent Document 1 describes Cr: 8 to 35%, C: 0.03% or less, N: 0.03% or less, Mn: 1.5% or less, Si: 0.8 to 2.5% and / or Al: 0.6 to 6.0%, Nb: 0.05 to 0.80%, Ti: 0.03 to 0.50%, Mo: 0.1 to 4%, Cu: 0.1. Ferritic stainless steels for petroleum fuel reformers are disclosed, which contain 1 or 2 or more of ~ 4% and have a composition in which the total amount of Si and Al is adjusted to 1.5% or more. In a thermal fatigue test in which the material is repeatedly heated and cooled in a temperature range of 200 to 900 ° C. (restraint rate 50%), these stainless steels have 500 cycls (cycles) of repeated breakage in which the initial maximum tensile stress drops to 3/4. It is characterized by the above.

Patent Document 2 describes Cr: 8 to 25%, C: 0.03% or less, N: 0.03% or less, Si: 0.1 to 2.5%, Mn: 1.5% or less, Al: Contains 0.1 to 4%, and further contains Nb: 0.05 to 0.80%, Ti: 0.03 to 0.5%, Mo: 0.1 to 4%, Cu: 0.1 to 4%. Ferrite-based stainless steels for alcohol-based fuel reformers, including one or more, are disclosed. In a thermal fatigue test in which the material is repeatedly heated and cooled in a temperature range of 200 to 900 ° C. (restraint rate 100%), these stainless steels have a failure repetition rate of 1000 cyc or more in which the initial maximum tensile stress is reduced to 3/4. It is characterized by that.

Patent Document 3 describes Cr: 12 to 20%, C: 0.03% or less, N: 0.03% or less, Si: 0.1 to 1.5%, Mn: 0.95 to 1.5%. , Al: 1.5% or less, Nb: 0.1 to 0.8, Mo: 0.1 to 4%, Cu: 0.1 to 4.0, including one or more, A = A ferritic stainless steel for a hydrocarbon fuel reformer in which the A value defined by Cr + Mn + 5 (Si + Al) is adjusted in the range of 15 to 25 is disclosed. In a thermal fatigue test in which the material is repeatedly heated and cooled in a temperature range of 200 to 900 ° C. (restraint rate 100%), these stainless steels have 800 cyc or more of repeated breakage in which the initial maximum tensile stress is reduced to 3/4. It is characterized by that.

Patent Document 4 describes C: less than 0.02%, Si: 0.15 to 0.7%, Mn: 0.3% or less, P: 0.035% or less, S: 0.003% or less, Cr. : 13 to 20%, Al: 1.5 to 6%, N: 0.02% or less, Ti: 0.03 to 0.5%, Nb: 0.001 to 0.1% or less, solid in steel The amount of molten Ti is [Ti], the amount of solid-melt Nb in steel is [Nb], and when 13 ≦ Cr ≦ 16, 0 ≦ [Ti] ≦ [Nb] +0.05, 0 <[Nb] ≦ 0. When 10 is satisfied and 16 <Cr ≦ 20, 0 ≦ [Ti] ≦ 1/2 × [Nb] +0.15, [Ti] ≦ 0.12, and 0 <[Nb] ≦ 0.1 are satisfied. Al-containing ferritic stainless steel for fuel cells, characterized by the above, is disclosed. These stainless steels are characterized by having a creep rupture time of 4000 h or more at 750 ° C. and an initial stress of 10 MPa.

Patent Document 5 describes C: 0.001 to 0.03%, Si: 0.01 to 2%, Mn: 0.01 to 1.5%, P: 0.005 to 0.05%, S: Containing 0.0001 to 0.01%, Cr: 16 to 30%, N: 0.001 to 0.03%, Al: 0.8 to 3%, Sn: 0.01 to 1%, at 800 ° C. A high-purity ferrite-based stainless steel plate having excellent oxidation resistance and high-temperature strength, which is characterized by having a 0.2% strength of 40 MPa or more and a tensile strength of 60 MPa or more, is disclosed.

Patent Document 6 discloses a welding material for Al-containing ferritic stainless steel, in which a fuel reformer and a gas pipe may be connected by welding. That is, in Patent Document 6, C: 0.02% or less, Si: 0.5% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.005% or less, Ni: 0.5% or less, Cr: 15 to 20%, N: 0.03% or less, Nb and Ti: 1 or more in total 0.1 to 0.5%, Al: 1.5 to 3.5% FeCrAl alloy welded wires are disclosed that contain less than or equal to Fe and the balance consisting of Fe and unavoidable impurities.

Japanese Unexamined Patent Publication No. 2003-160840 Japanese Unexamined Patent Publication No. 2003-160844 Japanese Unexamined Patent Publication No. 2003-160842 Japanese Unexamined Patent Publication No. 2010-22638 Japanese Unexamined Patent Publication No. 2012-172160 JP-A-2015-199107

As described above, parts such as the fuel reformer and the heat exchanger are continuously operated in the temperature range of 200 to 800 ° C. However, when considering the safety of the SOFC system, it is preferable to select the material in consideration of the part having the lowest strength in the structure. Since the welded portion has coarser grains than the base metal portion, the strength is generally low. Further, in the case of ferritic stainless steel, it is known that a significant decrease in strength occurs at a high temperature exceeding 700 ° C. The present inventors have found that fatigue characteristics due to thermal stress in a structure are of the utmost importance in ensuring durability, assuming long-term operation. That is, in the fuel cell structure including the welded portion, the problem is that the welded portion has a lower high temperature strength than the base metal portion, and it is possible to improve the same characteristics of the welded portion, which is inferior in high temperature fatigue characteristics. It is mentioned as a new issue.

Patent Documents 1 to 3 are technical ideas for increasing the number of cycles in which a material is broken by a thermal fatigue test, Patent Document 4 is a technical idea for lengthening the time for a material to break in a creep test, and Patent Document 5 is a tensile test. It is based on the technical idea of increasing the high temperature strength. These focus on the characteristics of the base metal part. In the case of welding, since it undergoes a thermal history different from that of actual manufacturing of steel products, it is important to examine the components from the viewpoint of high temperature fatigue characteristics of the welded part, but in Patent Documents 1 to 5, from this viewpoint. No consideration has been given. The welding wire of Patent Document 6 is component-designed from the viewpoint of ensuring the toughness and Cr evaporation resistance of the welded portion, but the high-temperature fatigue characteristics are not considered.

The present invention has been made in view of such a situation, and it is an object of the present invention to improve the fatigue characteristics of the welded portion of ferritic stainless steel in order to secure durability even in a high temperature environment of 800 ° C. containing a reforming gas. And.

The present inventors diligently studied to solve the above problems and obtained the following findings.
・ In order to improve high temperature fatigue characteristics, in addition to solid solution strengthening by adding alloying elements, growth of subgrain boundaries can be suppressed and fine structure can be maintained, thereby suppressing both the formation and growth of fine cracks. Was found.
It was found that Sn is particularly liable to segregate at the subgrain boundaries formed in the steel when deformed at 800 ° C., as well as solid solution strengthening, and significantly suppresses the movement of the subcrystal grain boundaries.
-It was also found that B also segregates at the subgrain boundaries formed during the high temperature fatigue test in addition to the solid solution strengthening, and remarkably suppresses the movement of the subcrystal grain boundaries.
-Furthermore, in order to segregate B at the subgrain boundaries, it was found that it is effective to limit P, S and O, which are likely to segregate at the grain boundaries, so as to satisfy (Equation 1).
[P] + [S] + 5 × [O] ≤ 0.080 ... (Equation 1)
The present invention has been made based on these findings, and the gist thereof is as follows.

(1)
In terms of mass%, C: 0.03% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.050% or less, S: 0.015% or less, Cr: 11.0 ~ 25.0%, Al: 1.3 to 4.0%, N: 0.040% or less, Sn: 0.005 to 0.5%, B: 0.0005 to less than 0.0050%, O: It contains 0.010% or less, and further contains one or more types of Ti: 0.50% or less, Nb: 0.50% or less, V: 0.50% or less, and further satisfies the following (Equation 1). Ferritic stainless steel with excellent high temperature fatigue characteristics, characterized in that the balance is Fe and unavoidable impurities.
[P] + [S] + 5 × [O] ≤ 0.080% ... (Equation 1)
Here, [P], [S], and [O] indicate the content (mass%) of each element.
(2)
Further, the high temperature fatigue according to (1), wherein the ferritic stainless steel contains one or two types of Mg: 0.015% or less and Ca: 0.005% or less in mass%. Ferritic stainless steel with excellent characteristics.
(3)
Further, the ferritic stainless steel is, in terms of mass%, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, Co: 1.0. The ferritic stainless steel having excellent high temperature fatigue characteristics according to (1) or (2), which contains 1 type or 2 or more types of% or less.
(4)
Further, the ferritic stainless steel is Zr: 0.50% or less, Ga: 0.10% or less, Zn: 0.10% or less, Sb: 0.50% or less, La: 0.10 in mass%. % Or less, Y: 0.10% or less, Hf: 0.10% or less, Ta: 0.1% or less, REM: 0.10% or less, and one or more of them (. The ferritic stainless steel having excellent high temperature fatigue characteristics according to any one of 1) to (3).
(5)
The ferritic stainless steel having excellent high temperature fatigue characteristics according to any one of (1) to (4), wherein the ferritic stainless steel is used for a welded structure.
(6)
The ferritic stainless steel having excellent high temperature fatigue characteristics according to (1) to (5), wherein the ferritic stainless steel is applied to a fuel reformer, a heat exchanger, or a fuel cell high temperature member.
(7)
The ferritic stainless steel having excellent high temperature fatigue characteristics according to (1) to (6), wherein the ferritic stainless steel is applied to a welded structure for a fuel cell.

INDUSTRIAL APPLICABILITY According to the present invention, it is suitable for a fuel cell high temperature member such as a reformer and a heat exchanger used when reforming hydrocarbon fuels such as city gas, methane, natural gas, propane, kerosene and gasoline into hydrogen. In particular, it is possible to provide a ferrite-based stainless steel suitable for a high temperature member in a welded structure of a solid oxide fuel cell (SOFC), which is required to have thermal fatigue characteristics in a high temperature environment including a reformed gas environment.

In order to solve the above-mentioned problems, the present inventors have completed the present invention by repeating diligent experiments and studies on the relationship between the composition of ferritic stainless steel and the high temperature fatigue characteristics of welded joints produced using the same. It was. The findings obtained in the present invention will be described below.

(A) The crystal grain size of the welded portion is coarser than that of the base metal. As a result, the high temperature strength of the welded portion is lower than that of the base metal.

(B) As a result of thermal stress analysis assuming the operation of a stationary fuel cell system, a maximum stress of 40 MPa is repeatedly applied at 800 ° C.

(C) Under such stress, the applied strain undergoes deformation while forming subcrystal grain boundaries due to dynamic recovery. Subcrystal grains gradually coarsen and become nucleation sites for microcracks. These microcracks propagate in the subcrystal grains and eventually lead to breakage of the weld. Here, the subgrain boundaries inhibit the growth of microcracks. Therefore, in order to enhance the high temperature fatigue characteristics, in addition to strengthening the solid solution by adding alloying elements, the growth of subgrain boundaries is suppressed and the fine structure is maintained, thereby suppressing both the formation and growth of fine cracks. I found out that I can do it.

(D) It has been newly found that the high temperature fatigue characteristics are remarkably improved by adding a small amount of Sn regardless of the addition of excessive Al and Nb, Mo, Cu, etc. that contribute to solid solution / precipitation strengthening. It was found that Sn is particularly liable to segregate at the subcrystal grain boundaries formed in the steel when deformed at 800 ° C., as well as solid solution strengthening, and significantly suppresses the movement of the subcrystal grain boundaries. However, excessive Sn addition lowers the grain boundary strength and causes a decrease in high temperature fatigue characteristics.

Similar to Sn, (e) B also segregates into the subcrystal grain boundaries formed during the high temperature fatigue test in addition to solid solution strengthening, and remarkably suppresses the movement of the subcrystal grain boundaries. Further, in order to segregate B at the subgrain boundaries, it is effective to limit P, S and O, which are likely to segregate at the grain boundaries, so as to satisfy the formula (Equation 1). It is also effective to generate P, S and O, which are easily segregated at the grain boundaries, as non-metal inclusions and sulfides by adding appropriate amounts of Ca and Mg, and to secure segregation sites for B at the grain boundaries. is there.
[P] + [S] + 5 × [O] ≤ 0.080 ... (Equation 1)
On the other hand, when B is added in excess, B is deposited as BN at the grain boundaries of the welded portion, and promotes fracture starting from the grain boundaries during the high temperature fatigue test. Therefore, it is important to determine an appropriate amount of B added.

(F) In addition, Ti, Nb, V, Ni, Cu, Mo, W, Co, Zr, Ga, Sb, La, Y, Hf, Ta, and REM are effective in improving high temperature fatigue characteristics by strengthening solid solution. It is an additive element.

(G) The findings described in (b) to (f) above can be applied to the welded portion, and durability can be ensured even in a high temperature environment of 800 ° C., that is, fatigue of the welded portion of ferritic stainless steel. We have newly found that the characteristics can be improved.

Hereinafter, each requirement of the present invention will be described in detail. The "%" indication of the content of each element means "mass%" unless otherwise specified.

The reasons for limiting the chemical components will be described below.

<C: 0.03% or less>
C forms a solid solution or Cr23C6 in the ferrite phase and inhibits oxidation resistance. It also promotes the precipitation of Cr23C6 at the grain boundaries during welding. Therefore, the smaller the amount of C, the better, and the upper limit is 0.030%. However, since excessive reduction leads to an increase in refining cost, the lower limit is preferably 0.001%. More preferably, the lower limit is 0.002% and the upper limit is 0.020%.

<Si: 2.0% or less>
Si is an important element for ensuring oxidation resistance. It is also an element that enhances high-temperature strength by strengthening solid solution. In order to obtain these effects, the lower limit is preferably 0.01%. On the other hand, excessive addition may hinder the deterioration of steel toughness and workability and the formation of Al-based oxide film, so the upper limit is set to 2.0%. When the effect of Si is positively utilized, it is preferable that the lower limit of the Si content is 0.3% and the upper limit is 1.0%.

<Mn: 2.0% or less>
Mn dissolves in the oxide film together with Si in a reformed gas environment to enhance the protective property. In order to obtain these effects, the lower limit is preferably 0.1%. On the other hand, excessive addition inhibits the corrosion resistance of steel and the formation of an Al-based oxide film, so the upper limit is set to 2.0% or less. From the viewpoint of oxidation resistance and basic characteristics, the lower limit of the Mn content is preferably 0.2% and the upper limit is preferably 1.2%.

<P: 0.050% or less>
P inhibits manufacturability and weldability. Further, since it is an element that easily segregates at the grain boundaries during welding, it is an element that inhibits the grain boundary segregation of Sn and B. The lower the content, the better, so the upper limit is 0.050%. However, since excessive reduction leads to an increase in refining cost, the lower limit is preferably 0.003%. From the viewpoint of manufacturability and weldability, the preferable range is preferably 0.010% at the lower limit and 0.035% at the upper limit.

<S: 0.015% or less>
S is an unavoidable impurity element contained in the steel and lowers the protective property of the Al-based 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 during high-temperature, long-term use. Further, since it is an element that easily segregates at the grain boundaries during welding, it is an element that inhibits the grain boundary segregation of Sn and B. Therefore, the lower the amount of S, the better, so the upper limit is set to 0.015%. However, since excessive reduction leads to an increase in raw materials and refining costs, the lower limit is set to 0.0001%. From the viewpoint of manufacturability and oxidation resistance, the preferable range is 0.0001 to 0.0050%.

<Cr: 11.0 to 25.0%>
Cr is a constituent element that is basic for ensuring the protection of the surface oxide film in addition to corrosion resistance, and an amount of Cr of 11.0% or more is required to obtain these effects. On the other hand, excessive addition of Cr promotes precipitation of Cr23C6. It also promotes the formation of the σ phase, which is the embrittled phase. The upper limit is set to 25.0% because it may promote an increase in alloy cost and Cr evaporation. The preferable range is that the lower limit is 13.0% and the upper limit is 20.0%.

<Al: 1.3 to 4.0%>
In addition to the deoxidizing element, Al is an essential additive element for forming an Al-based oxide film and suppressing Cr evaporation. It is also an element that enhances high-temperature strength by strengthening solid solution. In order to obtain these effects, the lower limit is set to 1.3%. However, since excessive addition of Al promotes toughness of steel and brittle fracture in welds, the upper limit is set to 4.0%. The preferable range is that the lower limit is 1.5% and the upper limit is 3.0%.

<N: 0.040% or less>
N inhibits oxidation resistance like C. Therefore, the smaller the amount of N, the better, and the upper limit is 0.040%. However, since excessive reduction leads to an increase in refining cost, the lower limit is set to 0.001%. The preferable range is that the lower limit is 0.002% and the upper limit is 0.020%.

<O: 0.010% or less>
O is an unavoidable impurity, and when excess O is contained, Si, Mn, and Al are oxidized during welding to form slag that is easily peeled off, and the oxidation resistance is deteriorated. In addition, during the high temperature fatigue test, the slag and the weld metal interface are peeled off, resulting in a nucleation site for microcracks. Therefore, the upper limit needs to be 0.010% or less. Preferably, the upper limit is 0.008%.

<Sn: 0.005-0.5%>
Sn is an element that segregates at subgrain boundaries and remarkably suppresses grain boundary movement to enhance high temperature fatigue characteristics. In order to obtain this effect, the lower limit is set to 0.005%. On the other hand, the addition of excessive Sn weakens the grain boundary strength of the steel material, promotes grain boundary fracture and causes a decrease in manufacturability, so the upper limit needs to be 0.5%. The preferable Sn content is preferably 0.01% at the lower limit and 0.4% at the upper limit. Considering the manufacturability, the upper limit of Sn is preferably 0.12%, more preferably 0.05%.

<B: 0.0005 to less than 0.0050%>
B is an element that segregates at the subgrain boundaries and enhances high temperature fatigue characteristics by remarkably suppressing grain boundary movement. In order to obtain this effect, the lower limit is set to 0.0005%. On the other hand, the addition of excess B causes a decrease in the manufacturability of the steel material and promotes the precipitation of BN during welding, so that the upper limit must be less than 0.0050%. The preferable B content is preferably 0.001% at the lower limit and 0.0045% at the upper limit.

<[P] + [S] + 5 x [O] ≤ 0.080>
Similar to Sn, B also segregates at the subcrystal grain boundaries formed during the high temperature fatigue test in addition to solid solution strengthening, and remarkably suppresses the movement of the subcrystal grain boundaries. Further, in order to segregate B at the subgrain boundaries, it is effective to limit the amounts of P, S and O that are likely to segregate at the grain boundaries. Among these elements, O in particular forms Al and Al2O3 in steel, and these inclusions can serve as microcrack formation sites during fatigue tests. Therefore, it is necessary to control [P] + [S] + 5 × [O] to 0.080% or less. Preferably, [P] + [S] + 5 × [O] should be 0.070% or less.

<Ti, Nb, V: 0.5% or less>
Ti, Nb, and V are elements that contribute to suppressing the formation of Cr23C6 during welding by the action of stabilizing elements that fix C and N. Furthermore, it is an element that contributes to the improvement of high temperature strength and fatigue characteristics. In order to obtain these effects, the lower limit of each element is preferably 0.004%. On the other hand, excessive addition leads to an increase in alloy cost, a decrease in manufacturability and a decrease in oxidation resistance due to an increase in recrystallization temperature, so the upper limit is set to 0.5%. The preferable range is for each element, the lower limit is 0.03% and the upper limit is ~ 0.40%.

<Mg: 0.015% or less, Ca: 0.005% or less>
Mg and Ca are elements that indirectly contribute to the improvement of high temperature fatigue characteristics by forming P, S and O that are easily segregated at the grain boundaries as non-metal inclusions and sulfides. These elements shall contain one or more of these elements. Excessive addition of these elements lowers manufacturability and corrosion resistance of steel, so Mg: 0.015% or less and Ca: 0.005% or less are preferable. Further, in order to surely obtain this effect, it is preferable to contain Mg: 0.001% and Ca: 0.0005% or more.

<Ni, Cu, Mo, W, Co: 1.0% or less>
These elements increase high temperature fatigue properties by strengthening solid solution. It is preferable to contain one or more of these elements. On the other hand, excessive addition of these elements leads to a decrease in manufacturability and an increase in alloy cost. Therefore, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 1.0% or less, W: 1. It is preferable that the total amount of these elements is 2.0% or less, with 0% or less and Co: 1.0% or less. In order to surely obtain this effect, Ni: 0.020% or more, Cu: 0.010% or more, Mo: 0.010% or more, W: 0.001% or more, Co: 0.005% or more should be contained. Is preferable.

<Zr, Ga, Zn, Sb, La, Y, Hf, Ta, REM>
These elements increase high temperature fatigue properties by strengthening solid solution. In addition, Zr, La, Y, Hf, Ta, and REM are conventionally effective elements for improving hot workability, steel cleanliness, and oxidation resistance. Ga, Zn, and Sb are concentrated near the surface to suppress the oxidation of Cr. It is preferable to contain one or more of these elements. On the other hand, excessive addition of these elements leads to a decrease in crystal strength, a decrease in manufacturability, and an increase in alloy cost. Therefore, Zr: 0.50% or less, Ga: 0.10% or less, Zn: 0.1% or less. , Sb: 0.50% or less, La: 0.10% or less, Y: 0.10% or less, Hf: 0.10% or less, Ta: 0.50% or less, REM: 0.10% or less. It is necessary to contain one kind or two or more kinds of elements. The total amount of these elements is preferably 0.20% or less. In order to surely obtain these effects, Zr: 0.0001% or more, Ga: 0.001% or more, Zn: 0.01% or more, Sb: 0.003% or more, La: 0.0001% or more, Y : 0.0001% or more, Hf: 0.0001% or more, Ta: 0.002% or more, REM: 0.001% or more is preferable.

Various ferritic stainless steels whose components are shown in Table 1 were melted and hot-rolled, annealed pickled, and cold-rolled to produce a cold-rolled annealed steel sheet having a plate thickness of 1.0 mm. Here, the steels A1 to A16 are steels within the component range specified by the present invention, and the steels B1 to B14 are steels outside the component range specified by the present invention. A welding material having a width of 120 mm and a length of 250 mm was cut out from these cold-rolled annealed steel sheets. Next, using ferritic stainless steel sheets of the same steel type as the base material, TIG tanning welding was performed using Ar shield gas at a current of 80 to 100 A and a welding speed of 50 cm / min to obtain the ferritic stainless steel sheets in Table 1. Manufactured welded joints.

Further, Al-containing ferritic stainless steel welding materials having symbols C1 and C2 indicating the components in Table 2 were melted. Welded joints were produced using the steel types A1, B11, and B12 in Table 1 as base materials and the welding materials C1 and C2. The welding conditions were a current of 200 A, a welding speed of 50 cm / min, and a welding material supply amount of 8 g / min, and Ar gas shielding was performed. A sample for chemical analysis was collected from the weld metal part of the obtained welded joint, and component analysis was performed. Table 3 shows the chemical composition of the weld metal part of each welded joint. Here, the symbols in Table 3 mean that, for example, in the case of A1C1, the base metal is A1 and the welding material is a welded joint manufactured using C1.

The high temperature fatigue characteristics were evaluated by a plane bending fatigue test. For each welded joint, a plate-shaped test piece having a parallel portion width of 10 mm and a distance between gauge points of 35 mm was collected so that the weld bead was located at the center of the width of the test piece. The test conditions were 800 ° C., the stress range was 40 and 44 MPa, the frequency was 1700 Hz, and the stress ratio was -1, and the test cycle until fracture was measured. Here, the test is censored in 10 ^seven times, "○" the ones that did not break until 10 ^c times in the stress range 40MPa, the ones that did not break up to 10 ^seven times in the stress range 44MPa "◎", test The high temperature fatigue characteristics were evaluated by marking the one that broke in the middle as "x". In addition, the fatigue test was not carried out in the stress range of 44 MPa for those that broke in the middle of the test in the stress range of 40 MPa.

The results obtained are shown in Table 4.

A1 to A16 satisfy the steel components specified in the present invention. As a result, the grain boundary strengthening of Sn and B can be obtained enough, fatigue fracture does not occur until 10 ^seven times, was the evaluation of from "○", "◎".

In B1, the amount of Sn added exceeds the steel component specified in the present invention. As a result, during the fatigue test lead to grain boundary strength decreases with excessive grain boundary segregation of Sn, for intergranular fracture occurs, broke before reaching ¹⁰⁷ times stress range 40 MPa.

In B2, the amount of B added exceeds the steel component specified in the present invention. As a result, the grain boundary fracture due to the deposition of BN because resulting was broken before reaching 10 ⁷ times with stress range 40MPa during fatigue testing.

In B3, the amount of Al added is less than the steel component specified in the present invention. As a result, oxidation resistance is insufficient in fatigue test, since the thinning caused by the scale detachment occurs, fatigue fracture starting from the there occurred was broken before reaching 10 ⁷ times with stress range 40 MPa.

In B4, the amount of Al added exceeds the steel component specified in the present invention. As a result, starting from the excess _Al 2 _{O 3} generated in the weld fatigue failure occurs, broke before reaching ¹⁰⁷ times stress range 40 MPa.

In B5, the amount of P added exceeds the steel component specified in the present invention. As a result, it is impossible to obtain a grain boundary strengthening of Sn and B sufficiently, it broke before reaching ¹⁰⁷ times stress range 40 MPa.

In B6, the amount of O added exceeds the steel component specified in the present invention. As a result, excessive slag generated in the welded portion becomes a generation region of the micro crack, it broke before reaching ¹⁰⁷ times stress range 40 MPa.

In B7, P + S + 5O exceeds the steel composition specified in the present invention. As a result, it is impossible to obtain a grain boundary strengthening of Sn and B sufficiently, it broke before reaching ¹⁰⁷ times stress range 40 MPa.

In B8, the amount of Ti added exceeds the range specified in the present invention. As a result, excessive Ti-based precipitates produced in the welded portion becomes a generation region of the micro crack, it broke before reaching ¹⁰⁷ times stress range 40 MPa.

In B9, the amount of Nb added exceeds the steel component specified in the present invention. As a result, excessive Nb-based precipitates generated in the welded portion becomes a generation region of the micro crack, it broke before reaching ¹⁰⁷ times stress range 40 MPa.

In B10, the amount of Cr added exceeds the steel component specified in the present invention. As a result, excessive Cr-based precipitates generated in the welded portion becomes a generation region of the micro crack, it broke before reaching ¹⁰⁷ times stress range 40 MPa.

In B11 and B12, the amount of Sn and B added is less than the steel component specified in the present invention. As a result, it is impossible to suppress the movement of the sub-grain boundaries, it broke before reaching ¹⁰⁷ times stress range 40 MPa.

B13 exceeds the steel composition defined by Ga and Sb in the present invention. As a result, grain boundary strength of the welded portion is reduced, it broke before reaching ¹⁰⁷ times stress range 40 MPa.

In B14, Mg and Ca exceed the steel components specified in the present invention. As a result, the crack as a starting point the MgO and CaO in the weld formed and was broken before reaching 10 ⁷ times with stress range 40 MPa.

In B15, Cr is less than the steel component specified in the present invention. As a result, the welded joint by the scale peeling due to oxidation degradation of the steel material itself is broken before reaching 10 ⁷ times with thinning and stress range 40 MPa.

In B16, C, Mg and Ca exceed the steel components specified in the present invention. As a result, the crack as a starting point the MgO and CaO in the weld grain boundary Cr carbide and the weld grain precipitated on the formed and was broken before reaching 10 ⁷ times with stress range 40 MPa.

Both the base material and the welding material of A1C1 satisfy the steel components specified in the present invention. As a result, the grain boundary strengthening of Sn and B can be sufficiently obtained, fatigue fracture did not occur until ¹⁰⁷ times.

In B11C1 and B12C2, the base material does not satisfy the steel component specified in the present invention, but the component is adjusted so as to satisfy the steel component of the welded portion by the welding material. As a result, the grain boundary strengthening of Sn and B can be sufficiently obtained, fatigue fracture did not occur until ¹⁰⁷ times.

According to the present invention, it is possible to obtain a ferritic stainless steel having excellent high temperature fatigue characteristics of a welded portion in a high temperature environment including a modified gas environment without relying on the addition of Al, Nb, Mo and Cu. In particular, a ferritic stainless steel suitable for a welded structure for a fuel cell can be obtained.

Claims (7)

In terms of mass%, C: 0.03% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.050% or less, S: 0.015% or less, Cr: 11.0 ~ 25.0%, Al: 1.3 to 4.0%, N: 0.040% or less, Sn: 0.005 to 0.5%, B: 0.0005 to less than 0.0050%, O: It contains 0.010% or less, and further contains one or more types of Ti: 0.50% or less, Nb: 0.50% or less, V: 0.50% or less, and further satisfies the following (Equation 1). Ferritic stainless steel for welded structures with excellent high temperature fatigue properties, characterized in that the balance is Fe and unavoidable impurities.
[P] + [S] + 5 × [O] ≤ 0.080% ... (Equation 1)
Here, [P], [S], and [O] indicate the content (mass%) of each element. The high temperature fatigue according to claim 1, wherein the ferritic stainless steel contains one or two types of Mg: 0.015% or less and Ca: 0.005% or less in mass%. Ferritic stainless steel for welded structures with excellent characteristics. Further, the ferritic stainless steel is, in terms of mass%, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, Co: 1.0. The ferritic stainless steel for a welded structure having excellent high temperature fatigue characteristics according to claim 1 or 2, which contains 1 type or 2 or more types of% or less. Further, the ferritic stainless steel contains Zr: 0.50% or less, Ga: 0.10% or less, Zn: 0.10% or less, Sb: 0.50% or less, La: 0.10 in mass%. % Or less, Y: 0.10% or less, Hf: 0.10% or less, Ta: 0.1% or less, REM: 0.10% or less, one or more kinds of claims. Item 5. A ferritic stainless steel for a welded structure having excellent high temperature fatigue characteristics according to any one of Items 1 to 3. The welded structure, the fuel reformer, the welding structure for ferrite having excellent high-temperature fatigue properties according to any one of claims 1 to 4, characterized in that a heat exchanger or a fuel cell hot member Ferritic steel. A welded structure characterized in that a ferrite-based stainless steel having the component composition according to any one of claims 1 to 4 is used. The welded structure according to claim 6, which is a fuel reformer, a heat exchanger, or a fuel cell high temperature member.