JP6655962B2 - Al-containing ferritic stainless steel welded joint with excellent 475 ° C brittle resistance - Google Patents

Description

  INDUSTRIAL APPLICABILITY The present invention is suitable for a fuel cell high-temperature member such as a reformer and a heat exchanger used when reforming a hydrocarbon-based fuel such as city gas, methane, natural gas, propane, kerosene and gasoline into hydrogen. The present invention relates to an Al-containing ferritic stainless steel welded joint excellent in 475 ° C brittleness resistance and a method for producing the same.

Recently, due to problems such as depletion of fossil fuels represented by petroleum and global warming due to CO ₂ emission, the spread of new systems replacing conventional power generation systems is accelerating. As one of them, a "fuel cell", which has a high practical value as a distributed power source and a power source of an automobile, has attracted attention. There are several types of fuel cells. Among them, polymer electrolyte fuel cells (PEFCs) and solid oxide fuel cells (SOFCs) have high energy efficiency and are expected to be widely used in the future.

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

  The fuel reformer is usually operated at a high temperature of 200 to 900 ° C. in order to secure the heat required for the reforming reaction of hydrogen. Under such a high-temperature operation, a heating / cooling cycle by starting / stopping is repeated according to a demand for hydrogen by being exposed to an oxidizing atmosphere containing a large amount of water vapor, carbon dioxide, carbon monoxide and the like. Further, in the SOFC system, when a high Cr-containing stainless steel is applied, there is a problem of preventing poisoning of the ceramic electrode due to evaporation of Cr at the SOFC operating temperature.

  Further, in a ferritic stainless steel containing 15% or more of Cr, when exposed to a temperature range of about 400 to 500 ° C. for a long time, hardening due to spinodal decomposition of a high Cr concentration phase and a low Cr concentration phase occurs. There is a possibility. It is so-called 475 ° C brittleness. Therefore, in a high-temperature use environment of the SOFC system, there is a possibility that material deterioration due to 475 ° C. brittleness may occur.

For SOFC applications, the application of Al-containing ferritic stainless steel having good oxidation resistance and Cr evaporation resistance is recommended. However, Al-containing ferritic stainless steels have higher toughness than other ferritic stainless steels and therefore have lower toughness. For example, as disclosed in Non-Patent Document 1, the ductile brittle transition temperature of steel materials increases with the addition of Al. It is known to

  Patent Literature 1 discloses a ferritic stainless steel for a fuel cell having 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%. If the Cr content is 15% or less which does not exhibit 475 ° C brittleness, it is necessary to add 4% or more of Al to secure oxidation resistance. As described above, since the toughness of the steel sheet containing a large amount of Al is significantly reduced, actual production becomes difficult.

  Patent Literature 2 discloses a ferritic stainless steel excellent in oxidation resistance and secondary workability, and suitable for applications requiring oxidation resistance and high formability such as an element cover of a sensor installed in an automobile exhaust gas path. Is disclosed. In the case of the present stainless steel, since it contains about 18% of Cr substantially, it is considered that 475 ° C brittleness may be exhibited.

  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 steel foil. In the case of the present stainless steel, since it contains substantially 18% of Cr and 3% or more of Al, it is considered that brittleness at 475 ° C. is exhibited and toughness is reduced due to high Al content.

  Patent Document 4 discloses a heat-resistant ferritic stainless steel for supporting a catalyst excellent in weldability and workability. The present stainless steel is characterized by containing about 1 to 2.5% of Al and having excellent formability even after welding.

JP 2014-139342 A JP 2012-12677 A JP-A-2015-78415 JP 2001-316773 A

"The Incremental in a Brittle-to-ductile Transition Temperature in Fe-Al Single crystal", ISIJ International Vol. 15 No. 6 pp. 999-1004

In the present invention, it has been found that avoiding brittle fracture of a welded portion caused by 475 ° C. brittleness is a particularly important solution when a welded structure is employed in an SOFC system.
Patent Literatures 1 to 4 do not recognize a problem in a weld after hardening due to 475 ° C brittleness. In other words, there is no ferritic stainless steel developed from the viewpoint of ensuring 475 ° C brittleness resistance at the welded portion.
The present invention has been developed to solve the above-described problem, and provides an Al-containing ferritic stainless steel welded joint excellent in 475 ° C. brittleness resistance.

(1) The chemical composition of the weld metal portion is, in mass%, C: 0.03% or less, N: 0.03% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.040% or less, S: 0.01% or less, Cr: 15.0 to 25.0%, Al: 1.0 to 4.0%, Ti: 1.0% or less, Nb: 1 0.01% or less, V: 1.0% or less, a first group consisting of one or more kinds and satisfying the formula (1), and Mg: 0.010% or less and Ca: 0.005% or less. Al-containing ferritic stainless steel welded joint excellent in brittle resistance at 475 ° C., characterized by containing at least one of the second group consisting of one or more species and the balance being Fe and inevitable impurities. .
([Ti] + [Nb] + [V]) / (2 × [C] + [N]) ≧ 4.0 (1)
Here, [Ti], [Nb], [V], [C], and [N] indicate the content (% by mass) of each element.
(2) The chemical composition of the weld metal part is further
Ni: 1.0% or less, Cu: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, a third group composed of one or more kinds, and Co: 0. 10% or less, Sn 0.10% or less, As: 0.05% or less, B: 0.005% or less, Pb: 0.005% or less, Zr: 0.1% or less, Zn: 0.03% or less, Y: 0.10% or less, Ga: 0.10% or less, La: 0.10% or less, Hf: 0.10% or less, Sb: 0.10% or less, REM: 0.10% or less, Ta: An Al-containing ferrite system excellent in 475 ° C brittleness resistance according to (1), wherein at least one of the fourth group consisting of one or more kinds of 0.5% or less is contained. Stainless steel welded joint.
(3) The Al-containing ferritic stainless steel welded joint excellent in 475 ° C brittleness resistance according to claim 2, wherein the formula (2) is 0.20% or less by mass%.
[Co] + [Sn] + [As] + [B] + [Pb] + [Zr] + [Zn] + [Y] + [Ga] + [La] + [Hf] + [Sb] + [REM] ] + [Ta] Expression (2)
Here, [Co], [Sn], [As], [B], [Pb], [Zr], [Zn], [Y], [Ga], [La], [Hf], [Sb] , [REM], and [Ta] indicate the content (% by mass) of each element.
(4) The Al-containing ferritic stainless steel welded joint excellent in 475 ° C brittle resistance described in (1) to (3), wherein the crystal grain size in the weld metal portion is 350 µm or less.
(5) An Al-containing ferritic stainless steel welded joint excellent in brittle resistance at 475 ° C described in (1) to (4), which is applied to a fuel reformer, a heat exchanger, or a high-temperature member of a fuel cell. .
(6) An Al-containing ferrite system excellent in brittle resistance at 475 ° C according to any one of (1) to (4), which is applied to a high-temperature member used in at least one of a gas turbine and a power generation system. Stainless steel welded joint.
(7) The anti-durability described in any one of (1) to (4), which is applied to at least one of automobile members such as an exhaust manifold, a converter, a muffler, a turbocharger, an EGR cooler, a front pipe, and a center pipe. Al-containing ferritic stainless steel welded joint with excellent 475 ° C brittleness.
(8) Al content excellent in 475 ° C brittle resistance described in any one of (1) to (4), wherein the Al content is applied to a high-temperature member used in a combustion device of at least one of a stove and a fan heater. Ferritic stainless steel welded joint.
Hereinafter, the Al-containing ferritic stainless steel welded joints according to the above (1) to (8) are each referred to as the present invention.

  According to the present invention, an Al-containing ferritic stainless steel welded joint excellent in 475 ° C brittleness resistance can be provided.

  The present inventors have elucidated the brittle fracture mechanism caused by 475 ° C brittleness in ferritic stainless steel and conducted experiments and studies on the effects of various alloying elements on the brittle fracture behavior. Was completed. Hereinafter, the findings obtained by the present invention will be described.

(A) Since the crystal grain size in the welded portion is larger than that in the base material portion, fracture due to brittleness at 475 ° C. is likely to occur. In the case of this fracture, it exhibits a cleavage surface. The Al-containing ferritic stainless steel tends to have a larger crystal grain size during welding than a ferritic stainless steel not containing Al. The crystal grain size of the molten metal portion (welded metal portion, in the present invention, means a region that has been melted and solidified during welding) in the welded joint is preferably 350 μm or less. The curing behavior attributable to 475 ° C. brittleness can be evaluated by, for example, a Vickers hardness test. In order to ensure 475 ° C brittle resistance, the Vickers hardness after aging at 500 ° C is preferably 300 or less.

(B) As a result of detailed structure observation, twinning deformation is involved in the fracture caused by brittleness at 475 ° C., and shear strain is generated by the introduction of the deformation twinning. To release this shear strain, cleavage cracks are formed and propagate, leading to fracture. It was also found that a grain boundary crack may occur when the above-mentioned shear strain is released. In this case, grain boundary destruction due to a decrease in grain boundary strength due to grain boundary segregation of P, S, Sn or the like becomes a starting point of crack generation.

(C) In order to suppress fracture attributable to brittleness at 475 ° C. in the welded portion, (i) it is difficult to develop twinning deformation, (ii) the coarsening of crystal grains during welding is suppressed, (iii) ) It is important to suppress a 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 such as in a welded area and slip deformation is extremely difficult due to hardening caused by brittleness at 475 ° C., addition of an appropriate amount of Al increases twinning deformation stress and relatively facilitates slip deformation. I found it new.

(E) Normally, since the welded structure of an Al-containing ferritic stainless steel is constituted by the growth of columnar crystals, it tends to be larger than a ferritic stainless steel not containing Al. However, the formation of equiaxed crystals is promoted by controlling the chemical components in the weld metal to generate carbonitrides composed of (Ti, Nb, V) (C, N) or inclusions composed of MgO, CaO, and the like. It was found that the weld structure was refined.

(F) In ferritic stainless steel, the formation behavior of carbonitride Ti (C, N) is arranged by [Ti] / ([C] + [N]). However, in Al-containing ferritic stainless steels, since Al increases the activity of C, the precipitation behavior of Cr carbide is more strongly affected by the amount of C than ordinary ferritic stainless steels. Do you get it. In order to obtain the effect of promoting the formation of equiaxed crystals by the carbonitride (Ti, Nb, V) (C, N) described in (e), ([Ti] + [Nb] + [V]) / ( The value of (2 × [C] + [N]) is preferably set to 4.0 or more. Alternatively, it was found that it is preferable to add 0.0005% or more of Mg and 0.0001% or more of Ca in order to obtain the same effect.

(G) Elements that suppress grain boundary migration due to grain boundary segregation are Co, Sn, As, B, Pb, Zr, Zn, Y, Ga, La, Hf, Sb, REM, and Ta. It is preferable that one or two or more of these elements are contained, and the total amount is 0.010% or more. On the other hand, an excessive content of these elements promotes grain boundary destruction due to a decrease in grain boundary strength, so that the total amount is preferably set to 0.10% or less.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" of the content of each element means "% by mass".

  The reasons for limiting the chemical composition of the weld metal part will be described below.

<C: 0.03% or less>
C forms a solid solution or forms a Cr carbide in the ferrite phase and inhibits oxidation resistance. It also promotes the formation of Cr carbide at the grain boundaries during welding. Therefore, the smaller the C content, the better, and the upper limit is made 0.03%. However, since an excessive reduction leads to an increase in the refining cost, the lower limit is preferably set to 0.001%. A more preferred range is 0.002 to 0.020%.

<N: 0.030% or less>
N, like C, inhibits the oxidation resistance targeted by the present invention. Therefore, the smaller the amount of N, the better, and the upper limit is made 0.030%. However, an excessive reduction leads to an increase in refining costs, so the lower limit is set to 0.001%. The preferred range is 0.002 to 0.020%.

<Si: 2.0% or less>
Si is an important element for securing oxidation resistance. Si is slightly dissolved in the Al-based oxide film, and is also concentrated at the interface immediately below the oxide film / steel interface, thereby improving oxidation resistance in a reformed gas environment. In order to obtain these effects, the lower limit is preferably set to 0.2%. On the other hand, excessive addition promotes spinodal decomposition of Cr and lowers 475 ° C brittleness resistance. In addition, the upper limit is set to 2.0% because the toughness and workability of steel may be reduced and the formation of an Al-based oxide film may be hindered. A preferred upper limit is 1.2%.

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

<P: 0.040% or less>
P is an element that inhibits manufacturability and weldability, and lowers the grain boundary strength at the welded portion. Since the smaller the content, the better, the upper limit is made 0.040%. However, since an excessive reduction leads to an increase in the refining cost, the lower limit is preferably set to 0.003%. From the viewpoint of manufacturability and weldability, a preferable range is 0.004 to 0.028%.

<S: 0.010% or less>
S is an unavoidable impurity element contained in steel and lowers the protection of the Al-based film. In particular, the presence of Mn-based inclusions and solid solution S also acts as a starting point of the destruction of the Al-based oxide film when used at a high temperature for a long time. Further, it is an element that lowers the grain boundary strength in the welded portion. Therefore, the lower the S amount, the better, and the upper limit is set to 0.010%. However, an excessive reduction leads to an increase in raw materials and refining costs, so the lower limit is made 0.0001%. From the viewpoints of manufacturability, oxidation resistance, and 475 ° C brittleness, a preferable range is 0.0001 to 0.0010%.

<Cr: 15.0 to 25.0%>
Cr is a basic constituent element for ensuring the protection of the surface oxide film in addition to the corrosion resistance. To obtain these effects, a Cr content of 15.0% or more is required. On the other hand, excessive addition of Cr promotes formation of a σ phase, which is an embrittlement phase, when exposed to a high-temperature atmosphere, in addition to remarkable material hardening caused by 475 ° C. brittleness. The upper limit is set to 25.0% because the alloy cost may be increased and Cr evaporation may be promoted. The preferred range is 15.5 to 20.0%. A more preferred range is 16.0 to 20.0%.

<Al: 1.0 to 4.0%>
Al is an essential additive element in addition to the deoxidizing element to form an Al-based oxide film and suppress Cr evaporation. Further, it is an element that suppresses the appearance of twinning deformation in a welded portion. In the present invention, since these effects cannot be obtained at less than 1.0%, the lower limit is set to 1.0%. However, excessive addition of Al promotes the toughness of the steel and brittle fracture in the welded portion, so the upper limit is set to 4.0%. The preferred range is 1.2-3.5%.

<Ti, Nb, V: 1.0% or less>
Ti is an element that contributes to suppressing the formation of Cr carbide during welding by the action of a stabilizing element that fixes C and N. Further, it is an effective element for forming a Ti-based oxide on the outer layer side of the Al-based oxide film to suppress the evaporation of Cr. In order to obtain these effects, the lower limit is preferably set to 0.01%. On the other hand, excessive addition leads to an increase in alloy cost and a decrease in manufacturability and oxidation resistance due to an increase in recrystallization temperature, so the upper limit is made 1.0%. A preferred range is 0.10 to 0.50%.
Nb and V increase the material strength by solid solution strengthening, and excessive addition promotes brittle fracture. Therefore, the upper limits must each be 1.0%. A preferred component range is 0.008 to 0.7%.

<([Ti] + [Nb] + [V]) / (2 × [C] + [N])> (first group)
Further, in the welded joint according to the present invention, the content of Ti, Nb, V, C, and N in the weld metal portion (while the element group consisting of Ti, Nb, and V satisfies the chemical composition of the weld metal defined above). Mass%) preferably satisfies the following expression (1).
([Ti] + [Nb] + [V]) / (2 × [C] + [N]) ≧ 4.0 (1)
Here, [Ti], [Nb], [V], [C], and [N] indicate the content (% by mass) of each element in the weld metal part.

<Mg, Ca> (second group)
Mg and Ca are effective elements for improving the cleanliness and hot workability of steel. Further, the formation of inclusions made of MgO, CaO, or the like at the time of welding promotes the formation of equiaxed crystals, and is an element that contributes to the refinement of the weld structure. In order to obtain these effects, it is preferable that Mg contains 0.0005% or more and Ca contains 0.0005% or more. On the other hand, since excessive addition causes a decrease in productivity, Mg is preferably 0.010% or less and Ca is preferably 0.005% or less. In addition, a second group consisting of one or more of Mg: 0.0005% or more and 0.010% or less, and Ca: 0.0005% or more and 0.005% or less is contained in the weld metal portion to thereby perform welding. The same effect can be obtained as in the case where the second group that includes at least one of Ti, Nb and V of the metal part and satisfies the formula (1) is included. Further, the composition of the weld metal portion may include at least one of the first group and the second group, and the weld metal portion may contain Mg and / or Ca less than the above range (for example, impurity level concentration). Even if it is contained, the above formula (1) may be satisfied.

<Ni, Cu, Mo, W: 1.0% or less> (third group)
Ni, Cu, Mo, and W are effective elements for improving high-temperature strength and corrosion resistance, and at least one kind (hereinafter, also referred to as “third group”) may be added as necessary. However, since excessive addition leads to an increase in alloy cost and hinders manufacturability, the upper limits of Ni, Cu, Mo, and W are set to 1.0%. In order to exhibit the above effects, it is preferable that Ni, Co, Mo, and W be 0.02% or more. More preferably, it is 0.08% or more.

<Co, Sn, As, B, Pb, Zr, Zn, Y, Ga, La, Hf, Sb, REM, Ta> (fourth group)
These elements segregate at the grain boundaries and suppress the coarsening of crystal grains during welding. Zr, La, Y, Hf, REM, and Ta have been conventionally effective elements for improving hot workability, cleanliness of steel, and oxidation resistance. Ga, Zn, Sn and Sb are concentrated near the surface to suppress the oxidation of Cr.

  To obtain these effects, Co: 0.005% or more, Sn 0.003% or more, As: 0.001% or more, B: 0.0001% or more, Pb: 0.0001% or more, Zr: 0.0001 % Or more, Zn: 0.01% or more, Y: 0.0001% or more, Ga: 0.001% or more, La: 0.0001% or more, Hf: 0.0001% or more, Sb: 0.003% or more , REM: 0.001% or more, Ta: 0.002% or more, and preferably contains one or more of these elements, and the total amount is 0.010% or more. In the Al-containing ferritic stainless steel welded joint of the present invention, the weld metal portion is made of Co, Sn, As, B, Pb together with the third group or instead of the element group of Ni, Cu, Mo, W. , Zr, Zn, Y, Ga, La, Hf, Sb, REM, and a fourth group composed of at least one element of Ta may be contained within the above-mentioned content range.

On the other hand, excessive addition of these elements promotes grain boundary destruction due to a decrease in grain boundary strength. Therefore, the fourth group contains Co: 0.10% or less, Sn 0.10% or less, and As: 0.05% or less. , B: 0.005% or less, Pb: 0.005% or less, Zr: 0.1% or less, Zn: 0.03% or less, Y: 0.10% or less, Ga: 0.10% or less, La : 0.10% or less, Hf: 0.10% or less, Sb: 0.10% or less, REM: 0.10% or less, Ta: 0.5% or less Therefore, the total amount (the following formula (2)) needs to be 0.20% or less.
[Co] + [Sn] + [As] + [B] + [Pb] + [Zr] + [Zn] + [Y] + [Ga] + [La] + [Hf] + [Sb] + [REM] ] Equation (2)
Here, [Co], [Sn], [As], [B], [Pb], [Zr], [Zn], [Y], [Ga], [La], [Hf], [Sb] , [REM], and [Ta] indicate the content (% by mass) of each element.

The manufacturing method will be described below.
The Al-containing ferritic stainless steel welded joint excellent in 475 ° C brittleness resistance of the present invention is manufactured by tanning Al-containing ferritic stainless steel or welding using a welding rod. The welding joint of the present invention can be easily realized by manufacturing using the welding material of the present invention described later, but it goes without saying that the welding joint of the present invention is not limited to the manufacturing conditions. That is, by appropriately selecting the base steel material to be welded, the welding material to be used, the welding method, and the welding conditions, the chemical composition of the weld metal in the final welded joint can be controlled within the scope of the present invention. When a welding material is used, good oxidation resistance and Cr evaporation resistance are required for SOFC applications. Therefore, manufacturing a welded joint using an Al-containing ferritic stainless steel having these characteristics as a base material is the present invention. Is a preferred example. Further, from the viewpoint of facilitating the control of the components of the welded joint, it is more preferable that the base material or the welding material is a component system close to the welded joint of the present invention.

  The chemical composition of a suitable base material of the present invention is, in mass%, C: 0.02% or less, Si: 0.15 to 0.70%, Mn: 0.3% or less, Cr: 13 to 20%, Al: 1.5 to 6%, N: 0.02% or less, Ti: 0.03 to 0.5%, the balance being Fe and inevitable impurities. Further, the chemical composition of a preferable welding material of the present invention is, in mass%, C: 0.02% or less, Si: 0.1 to 0.6%, Mn: 0.05 to 0.4%, Cr: 13 to 20%, Ti: 0.05 to 0.4%, P: 0.020% or less, S: 0.015% or less, N: 0.040% or less, the balance being Fe and unavoidable impurities It is.

  In applications other than SOFC applications and applications requiring 475 ° C. brittleness resistance, the base material constituting the welded joint of the present invention is not limited to Al-containing ferritic stainless steel. Further, for example, in a high temperature environment such as a high temperature member used for a gas turbine, a power generation system, an exhaust member, an automobile member such as a converter, a muffler, a turbocharger, an EGR cooler, a front pipe, a center pipe, and a combustion device such as a stove fan heater. It is suitable for all members used.

Hereinafter, examples of the present invention will be described.
Ferritic stainless steels of steel types 1 to 35 whose components are shown in Tables 1-1 and 1-2 are melted, hot-rolled, annealed and pickled, cold-rolled, and then subjected to finish annealing and pickling to obtain a thickness of 1. A 0 mm cold-rolled annealed steel sheet was manufactured. The “composition of the invention example” in Table 1-1 means that the steel composition satisfies the composition of the weld metal portion of the Al-containing ferritic stainless steel welded joint of the present invention. The “composition of the comparative example” in Table 1-2 means that the steel composition does not satisfy the composition of the weld metal part of the Al-containing ferritic stainless steel welded joint of the present invention. Blanks in Tables 1-1, 1-2, 2 and 3 mean that the corresponding components were not added.

  Test pieces having a width of 120 mm and a length of 250 mm were cut out from ferritic stainless steel sheets of each steel type. Next, ferritic stainless steel sheets of steel types 1 to 31 are subjected to TIG tanning welding using ferrite stainless steel sheets of the same steel type as base materials at a current of 80 to 100 A and a welding speed of 50 cm / min using an Ar shielding gas. Each time a ferritic stainless steel welded joint was manufactured. The welding position was the center of the plate width, and the welding direction was the plate longitudinal direction.

  In addition, Al-containing ferritic stainless steel welding materials indicated by the symbols A to C indicating the components in Table 2 were melted. Welded joints were produced using the steel types 4, 5, 14, 22, and 29 in Tables 1-1 and 1-2 as base materials and using the welding materials of the symbols A to C. The welding conditions were a current of 200 A, a welding speed of 50 cm / min, a welding material supply of 8 g / min, and Ar gas shielding. A sample for chemical analysis was collected from the weld metal part of the obtained welded joint, and a component analysis was performed. Table 3 shows the chemical components of the weld metal part of each welded joint. Here, the symbol in Table 3 means that, for example, in the case of 4A, the base material is 4, and the welding material is a welded joint manufactured using C.

  A sample was cut out from the welded portion of each welded joint, filled with resin, and then etched with aqua regia. With respect to the sample after etching, the number of crystal grains in an area of 800 μm in thickness × 1600 μm in width at the center of the molten metal part was counted in the molten metal part photographed by an optical microscope. The number of crystal grains that crossed one side of the measurement region was 0.5, and the number of crystal grains that crossed 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 joint.

  Each welded joint was subjected to an aging treatment at 500 ° C. for 3000 hours in the atmosphere. The resistance to 475 ° C brittleness was evaluated by a V-bending test for these 500 ° C aged materials. The bending radius was set to 2.0 mm and the bending angle was set to 90 degrees, and V-bending was performed so that the press fitting was in contact with the back bead of the sample. When the V-bending molding was completed without cracking visually, the brittle resistance at 475 ° C was very good (（), and when the V-bending molding was completed but cracking was visually observed, good (○). When a crack penetrated during V-bending, the brittle resistance at 475 ° C was evaluated as poor (x).

  Table 4 shows the test results. The test pieces of symbols 1 to 32 are the weld metal parts of the welded joints obtained by tanning and welding ferritic stainless steel sheets of the steel type corresponding to each symbol, and thus are the same as the ferritic stainless steel sheets of the steel type corresponding to each symbol. Having the following composition: Therefore, the test pieces of symbols 1 to 22 satisfy the component range specified by the present invention. As a result, even after aging treatment at 500 ° C. for 3000 hours, generation of cracks due to V bending in the welded portion was not confirmed, and excellent 475 ° C. brittleness resistance was exhibited.

Symbol 8a is a test piece produced with a higher heat input than the welded joint of symbol H. The crystal grain size of the welded portion was 360 μm, which was larger than the preferred range of the present invention, but because it satisfied the component range specified by the present invention, it exhibited good 475 ° C. brittleness resistance.
The test piece of symbol 23 has a P content larger than the specified range of the present invention. As a result, the grain boundary strength of the weld metal decreased, and grain boundary fracture occurred.
The test piece denoted by reference numeral 24 has a Cr content larger than the range specified in the present invention. As a result, the material was significantly hardened due to the spinodal decomposition of Cr, and cleaved fracture occurred.
The test piece of symbol 25 has an Al content larger than the specified range of the present invention. As a result, the effect of suppressing the twinning deformation due to the addition of Al surpassed the remarkable hardening due to solid solution strengthening of Al, resulting in cleavage fracture.
The test pieces of symbols 26 and 30 have an Al content smaller than the specified range of the present invention. As a result, the effect of suppressing twin deformation due to the addition of Al could not be sufficiently obtained, and cleavage cleavage occurred.
The test piece denoted by reference numeral 27 has a Sn content exceeding 0.10%, and the total content of Sn, Sb and Ga is also larger than the specified range of the present invention. Further, in the test piece of symbol 28, the respective contents of Zn, B and Pb are larger than the specified range of the present invention, and the total content of Zn, B, As and Pb is also larger than the specified range of the present invention. As a result, the grain boundary strength of the welds of the test pieces of symbols 27 and 28 was significantly reduced, and grain boundary fracture occurred.
The test piece denoted by reference numeral 29 has a total amount represented by the formula (2) according to the present invention, although the content of the weld metal portions As, Zr, Y, Ga, and Ta is within the range specified by the present invention. More than the specified range. As a result, the crystal grain size of the weld became large, and cleavage cleavage occurred.
In the test piece denoted by the symbol 31, the value of the expression (1) is smaller than the specified range of the present invention. Further, the test piece indicated by the symbol 32 has a C content larger than the specified range of the present invention. As a result, in each of the test pieces, the crystal grain size of the welded portion was increased, and cleavage occurred.
The test piece of symbol 33 has an Sb content exceeding 0.10%. In addition, the test piece indicated by the symbol 34 has an As content exceeding 0.05% and a Pb content exceeding 0.005%. As a result, the grain boundary strength of the welds of the test pieces of symbols 33 and 34 was significantly reduced, and grain boundary fracture occurred.
The test piece of symbol 35 has a Ca content exceeding 0.005%. As a result, the inclusions excessively generated in the welded portion of the test piece of symbol 35 became the starting point of the fracture, and the cleavage fracture occurred.

  All of the test pieces with symbols 4A, 5C, 14A, 22C, and 29B satisfy the component range specified by the present invention. As a result, even after the aging treatment at 500 ° C. for 3000 hours, generation of cracks due to V bending in the welded portion was not confirmed, and excellent 475 ° C. brittleness resistance was exhibited. In particular, with respect to the test piece of 29B, since the brittle fracture occurs in the TIG tanned welded joint of the base metal X, the components of the base metal are not limited in the present invention, and the chemical composition of the weld metal is important. The result suggests that

  According to the present invention, city gas, methane, natural gas, propane, kerosene, reformer used when reforming hydrocarbon-based fuel such as gasoline to hydrogen, a fuel cell high-temperature member such as a heat exchanger, Excellent resistance to 475 ° C brittleness suitable for all parts used in high-temperature environments, such as exhaust manifolds, converters, mufflers, turbochargers, EGR coolers, front pipes, center pipes and other automotive parts, and stoves and fan heaters and other combustion equipment. In addition, it is possible to provide an Al-containing ferritic stainless steel welded joint.

Claims (7)

The chemical composition of the weld metal part is, in mass%, C: 0.03% or less, N: 0.03% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.040. %, S: 0.01% or less, Cr: 15 to 25%, Al: 1.0 to 4.0%,
A first group consisting of one or more of Ti: 1.0% or less, Nb: 1.0% or less, V: 1.0% or less, and satisfying the formula (1); and
Mg: 0.010% or less, Ca: 0.005% or less, containing at least one of the second group consisting of one or two or more,
Balance Fe and unavoidable impurities der is,
The weld metal grain size magnitude is Al-containing ferritic stainless steel welded joints having excellent 475 ° C. embrittlement, characterized in der Rukoto following 350μm in.
([Ti] + [Nb] + [V]) / (2 × [C] + [N]) ≧ 4.0 (1)
Here, [Ti], [Nb], [V], [C], and [N] indicate the content (% by mass) of each element. The chemical composition of the weld metal part is
A third group consisting of one or more of Ni: 1.0% or less, Cu: 1.0% or less, Mo: 1.0% or less, and W: 1.0% or less; and
Co: 0.10% or less, Sn 0.10% or less, As: 0.05% or less, B: 0.005% or less, Pb: 0.005% or less, Zr: 0.1% or less, Zn: 0. 03% or less, Y: 0.10% or less, Ga: 0.10% or less, La: 0.10% or less, Hf: 0.10% or less, Sb: 0.10% or less, REM: 0.10% Hereinafter, among the fourth group consisting of one or more of Ta: 0.5% or less,
The Al-containing ferritic stainless steel welded joint excellent in 475 ° C brittleness resistance according to claim 1, comprising at least one group. The Al-containing ferritic stainless steel welded joint excellent in brittle resistance at 475 ° C according to claim 2, wherein the formula (2) is not more than 0.20% by mass%.
[Co] + [Sn] + [As] + [B] + [Pb] + [Zr] + [Zn] + [Y] + [Ga] + [La] + [Hf] + [Sb] + [REM] ] + [Ta] Expression (2)
Here, [Co], [Sn], [As], [B], [Pb], [Zr], [Zn], [Y], [Ga], [La], [Hf], [Sb] , [REM], and [Ta] indicate the content (% by mass) of each element. The Al-containing ferritic stainless steel welded joint excellent in 475 ° C brittleness resistance according to any one of claims 1 to 3 , which is applied to a fuel reformer, a heat exchanger, or a high temperature member of a fuel cell. . The Al-containing ferritic stainless steel welding excellent in 475 ° C brittleness resistance according to any one of claims 1 to 3 , which is applied to a high-temperature member used in at least one of a gas turbine and a power generation system. Fittings. The 475 [deg.] C brittle resistance according to any one of claims 1 to 3 , which is applied to at least one member of an exhaust manifold, a converter, a muffler, a turbocharger, an EGR cooler, a front pipe, and a center pipe. Excellent Al-containing ferritic stainless steel welded joint. The Al-containing ferritic stainless steel excellent in 475 ° C brittle resistance according to any one of claims 1 to 3 , wherein the Al-containing ferrite stainless steel is applied to a high-temperature member used in a combustion device of at least one of a stove and a fan heater. Steel welded joint.