JP6515287B2 - Method of manufacturing welded joint - Google Patents

Description

  The present invention relates to a method of manufacturing a welded joint made of austenitic stainless steel used for high pressure hydrogen gas equipment, liquid hydrogen equipment and the like.

  In recent years, research on commercialization of transportation devices that use hydrogen as energy instead of fossil fuels has been actively promoted. At the time of practical use, it is also necessary to maintain a working environment where hydrogen can be stored and transported at high pressure, and development and application of high strength materials with tensile strength exceeding 800MPa used in parallel Is in progress.

  Under such background, as materials to be used, for example, in Patent Documents 1 to 4, the solubility of N is increased by increasing Mn, and the solid of N is added by adding V or Nb. An austenitic stainless steel has been proposed in which high strength is attempted by taking advantage of solution strengthening, precipitation strengthening of nitride, and further miniaturization due to its pinning effect.

  By the way, when using a high strength austenitic stainless steel containing such a large amount of N as a structure, it is required that assembly by welding is possible from the viewpoint of cost. Therefore, for example, Patent Documents 3, 5 and 6 propose welding materials (welding metals) in which tensile strength exceeding 800 MPa can be obtained by actively utilizing Al, Ti and Nb and performing heat treatment after welding. ing. Further, according to Patent Document 7, the post-welding heat treatment is performed by managing the N amount of the welding material (the filler metal), the shield gas at the time of welding, and the molten pool area and increasing the N amount of the weld metal. Instead, weld joints have been proposed to achieve high strength.

  On the other hand, in an actual structure, it is necessary to use butt welding by gas tungsten arc welding without using a filler metal because it is difficult to use the filler material depending on the use site. At that time, if the penetration depth is not sufficient, the unmelted butted surface remains as a defect, causing a problem that the required weld joint strength can not be obtained. As one of the measures, it is conceivable to increase the welding heat input, but the fusion zone becomes large, undercut or burnout occurs, and the soundness of the welded joint may be impaired.

In order to solve these problems, for example, Patent Documents 8 to 11 propose various fluxes containing metal oxides that increase the penetration depth during gas tungsten arc welding. Patent Document 8 is a mixture of a transition metal group IVa oxide or pure metal except Hf and a transition metal group VIa oxide other than W, and the ratio of oxygen atom to metal atom in this mixture is specified. Discloses a flux for TIG welding. Further, Patent Document 9 discloses a flux comprising SiO ₂ and Cr ₂ O ₃ and defining the component ratio in the range of 0.15 to 2.0, and Patent Document 10 discloses TiO ₂ There is disclosed a flux which contains 15 wt%, the balance is SiO ₂ and Cr ₂ O _3, and the component ratio is defined in the range of 0.5 to 1.2. Patent Documents 9 and 10 all propose a flux that increases penetration during TIG welding. Furthermore, Patent Document 11 proposes a flux containing essentially 50 mass% of W oxide and Ti oxide.

WO 2004/083476 WO 2004/083477 WO 2004/110695 International Publication No. 2012/132992 Unexamined-Japanese-Patent No. 5-192785 JP, 2010-227949, A International Publication No. 2013/005570 Japanese Patent Laid-Open No. 2000-71094 Unexamined-Japanese-Patent No. 2000-326090 Unexamined-Japanese-Patent No. 2000-326091 Japanese Patent Laid-Open No. 2002-205190

  The fluxes disclosed in Patent Documents 8 to 11 certainly have the effect of increasing the penetration depth in carbon steel and austenitic stainless steel. However, it has been revealed that, when N is used in a high strength stainless steel actively utilized, the required weld joint strength can not be obtained or the hydrogen embrittlement resistance is significantly reduced.

  The present invention has been made in view of the above-mentioned present situation, and when gas tungsten arc welding is performed without using a filler metal, sufficient penetration depth can be obtained, and excellent strength and resistance to hydrogen embrittlement can be obtained. It is an object of the present invention to provide a method of manufacturing a welded joint.

  As a result of repeated investigations in order to solve the above-mentioned problems, the present inventors obtained the following findings.

  That is, C: 0.005 to 0.07%, Si: 0.1 to 1.2%, Mn: 2.5 to 10%, Ni: 9 to 14%, Cr: 19 to 24% by mass , Mo: 1 to 4%, Nb: 0.1 to 0.4%, N: 0.20 to 0.45%, Al: 0.0005 to 0.05% Various types of austenitic stainless steels As a result of applying the flux containing metal oxide of the above and performing gas tungsten arc welding, it became clear that the penetration depth becomes deep regardless of the type of the flux, and the weldability improves.

However, it has also become clear that the tensile strength and hydrogen embrittlement resistance of the obtained weld joint do not satisfy the required performance depending on the type of flux. And, in order to obtain a sufficient penetration depth and to exhibit excellent strength and resistance to hydrogen embrittlement of the welded joint, the flux is one selected from TiO ₂ , Cr ₂ O ₃ and ZrO _2. It turned out that it is necessary to make the above contain 5 to 90% in total by mass%.

  The reason is considered as follows.

That is, when one or more elements selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ are contained in the flux, they dissociate in the weld pool during welding to increase the amount of O in the molten metal. Since O in the molten pool acts as a surface active element, it affects the molten metal flow and is considered to have increased the penetration depth. Further, dissociated Ti, Cr and Zr increase the solubility of N, reduce the scattering of nitrogen from the weld pool during welding, increase the yield of N in the weld metal, and increase the amount of solid solution N. In addition, some of these elements precipitate as nitrides in the weld metal. It is presumed that the strength of the weld metal is improved by the solid solution strengthening by N and the precipitation strengthening by nitride, and as a result, sufficient strength of the welded joint is obtained. On the other hand, when these are contained excessively, while the effect of the increase in the amount of solid solution N saturates, the amount of precipitation of nitride increases. It is judged that the nitride deposited in excess reduces the ductility in hydrogen, so that the hydrogen embrittlement resistance is not sufficient.

  This invention is completed based on said knowledge, The summary is in the manufacturing method of the weld joint shown to following (1)-(3).

(1) A method of manufacturing a welded joint obtained by applying a flux to a welded portion of austenitic stainless steel and performing gas tungsten arc welding without using a filler metal,
The above-described austenitic stainless steel is, by mass%, C: 0.005 to 0.07%, Si: 0.1 to 1.2%, Mn: 2.5 to 10%, Ni: 9 to 14%, Cr 19-24% Mo: 1-4% Nb: 0.1-0.4% N: 0.20-0.45% Al: 0.0005-0.05% V: 0- 0.5%, Ti: 0 to 0.5%, B: 0 to 0.01%, Cu: 0 to 3%, Ca: 0 to 0.05%, Mg: 0 to 0.05%, and REM: containing 0 to 0.5%, the balance being Fe and impurities, and O, P and S as impurities O: 0.008% or less, P: 0.03% or less, and S: Have a chemical composition that is less than 0.01%,
The flux contains, in mass%, a total of 5 to 90% of one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ , and
The manufacturing method of a weld joint whose oxygen content in a weld metal is 0.020% or less.

(2) The austenitic stainless steel is, by mass%, V: 0.001 to 0.5%, Ti: 0.001 to 0.5%, B: 0.0001 to 0.01%, Cu: 0 .005 to 3%, Ca: 0.0005 to 0.05%, Mg: 0.0005 to 0.05%, and REM: at least one selected from 0.001 to 0.5% The manufacturing method of the weld joint as described in said (1).

(3) The N content contained in the austenitic stainless steel and the total content of one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ contained in the flux are represented by the following formula (i) The manufacturing method of the weld joint as described in said (1) or said (2) which is satisfied.
14-20 N ≦ TiO ₂ + Cr ₂ O ₃ + ZrO ₂ ≦ 98-40 N (i)
However, in the formula (i), N, TiO ₂ , Cr ₂ O ₃ and ZrO ₂ represent the content (mass%) of the respective elements or oxides.

  According to the present invention, it is possible to provide a method for producing a welded joint in which sufficient penetration depth can be obtained and excellent strength and resistance to hydrogen embrittlement can be obtained when performing gas tungsten arc welding without using a filler metal. can do.

  It goes without saying that in the method of manufacturing a welded joint according to the present invention, if a welded joint satisfying required performance such as strength can be obtained, a filler metal can be used.

It is a figure which shows the shape and dimension of the bevel process used in the Example.

  The reasons for limiting the chemical compositions of the austenitic stainless steel, the flux and the weld metal in the method of manufacturing a weld joint according to the present invention are as follows. In the following description, “%” indication of the content of each component element means “mass%”.

<Austenitic stainless steel>
C: 0.005 to 0.07%
C is an element effective to stabilize the austenite structure. In order to acquire this effect, it is necessary to contain C 0.005% or more. A desirable range is 0.01% or more, and a further desirable range is 0.02% or more. However, if it is contained excessively, carbides are formed in grain boundaries by heating at the time of welding, and the corrosion resistance of the heat affected zone at the welding heat is deteriorated. Therefore, the C content is made 0.07% or less. A desirable range is 0.06% or less, and a further desirable range is 0.05% or less.

Si: 0.1 to 1.2%
Si is contained as a deoxidizer, but is an element effective for improving the corrosion resistance. In order to obtain the effect sufficiently, it is necessary to contain 0.1% or more. A desirable range is 0.15% or more, and a further desirable range is 0.2% or more. However, when it is contained in excess, the stability of the austenitic structure is reduced and the ductility is reduced. Furthermore, welds that do not use filler metal increase the susceptibility to solidification cracking of the weld metal. Therefore, the Si content is set to 1.2% or less. A desirable range is 1.1% or less, and a further desirable range is 1.0% or less.

Mn: 2.5 to 10%
Mn not only contributes to deoxidation during production, but also has the effect of stabilizing the austenitic structure and reducing the susceptibility to hydrogen embrittlement. Furthermore, it is effective to increase the solubility of N in the molten metal and to increase the strength during base metal production and welding. In order to fully utilize the effect of this Mn, it is necessary to contain Mn 2.5% or more. A desirable range is 2.8% or more, and a further desirable range is 3.0% or more. On the other hand, when it is contained excessively, the ductility is lowered, so the Mn content is made 10% or less. A desirable range is 8% or less, and a further desirable range is 6.5% or less.

Ni: 9 to 14%
Ni is an element essential for obtaining a stable austenite structure, which increases stacking fault energy and reduces the susceptibility to embrittlement in a hydrogen environment. In order to sufficiently obtain those effects, it is necessary to contain 9% or more of Ni. A desirable range is 9.5% or more, and a further desirable range is 10% or more. However, since it is an expensive element, addition of a large amount leads to an increase in cost, and reduces the solubility of N in the molten metal during base metal production and welding, leading to a reduction in strength. Therefore, the Ni content is 14% or less. A desirable range is 13.5% or less, and a further desirable range is 13% or less.

Cr: 19 to 24%
Cr is an essential element to ensure corrosion resistance in the use environment. Furthermore, in addition to increasing the solubility of N in the molten metal at the time of base material production and welding, it is also effective in generating carbides and enhancing strength. In order to obtain the effect sufficiently, Cr needs to be contained 19% or more. A desirable range is 19.5% or more, and a further desirable range is 20% or more. However, when it is contained in excess, the austenite structure becomes unstable and it causes embrittlement due to excessive formation of carbide. Therefore, the Cr content is 24% or less. A desirable range is 23.5% or less, and a further desirable range is 23% or less.

Mo: 1 to 4%
Mo is an element effective to improve the corrosion resistance in the use environment and to enhance the strength. Furthermore, it also has the effect of reducing the susceptibility to hydrogen embrittlement. In order to sufficiently obtain those effects, it is necessary to contain Mo at 1% or more. A desirable range is 1.2% or more, and a further desirable range is 1.5% or more. However, Mo is a very expensive element, and if it is contained excessively, the Mo content is 4% or less in order to destabilize the austenite structure. A desirable range is 3.8% or less, and a further desirable range is 3.5% or less.

Nb: 0.1 to 0.4%
Nb is an element effective for improving the strength by depositing as a solid solution or carbonitride on a substrate. In order to acquire the effect, it is necessary to contain Nb 0.1% or more. A desirable range is 0.12% or more, and a further desirable range is 0.15% or more. However, if it is contained in excess, in welding without using a filler metal, in addition to enhancing the solidification cracking susceptibility of the weld metal, it also causes a decrease in ductility. Therefore, the Nb content is 0.4% or less. A desirable range is 0.38% or less, and a further desirable range is 0.35% or less.

N: 0.20 to 0.45%
N is an essential element in order to form a fine nitride and obtain high strength while forming a solid solution in the matrix. In addition, it is an element that contributes to the stabilization of the austenite structure. In order to obtain these effects sufficiently, it is necessary to contain N 0.20% or more. A desirable range is 0.22% or more, and a further desirable range is 0.25% or more. However, if it is contained excessively, it causes a decrease in hot workability at the time of production. Therefore, the N content is set to 0.45% or less. A desirable range is 0.43% or less, and a further desirable range is 0.40% or less.

Al: 0.0005 to 0.05%
Al is contained as a deoxidizer like Si and Mn. In order to obtain the effect sufficiently, it is necessary to contain Al by 0.0005% or more. A desirable range is 0.0008% or more, and a further desirable range is 0.001% or more. However, if it is contained excessively, the solubility of N is reduced during welding to reduce the amount of solid solution N, leading to a decrease in strength, so the Al content is made 0.05% or less. A desirable range is 0.04% or less, and a further desirable range is 0.03% or less.

V: 0 to 0.5%
V, like Nb, precipitates as a solid solution or as a carbonitride in a substrate and is an element effective for improving the strength, and thus V may be contained. When it is contained, it is desirable to be 0.001% or more. A more desirable range is 0.005% or more, and a further desirable range is 0.01% or more. However, when it is contained excessively, a large amount of carbonitrides precipitates, which causes a decrease in ductility, so the V content is made 0.5% or less. A desirable range is 0.45% or less, and a further desirable range is 0.40% or less.

Ti: 0 to 0.5%
Like V and Nb, Ti may be contained as a solid solution or as a carbonitride precipitated on a substrate and is an element effective for improving the strength, and therefore, may be contained. When it is contained, it is desirable to be 0.001% or more. A more desirable range is 0.003% or more, and a further desirable range is 0.005% or more. However, when it is contained in excess, a large amount of carbonitride precipitates, which causes a decrease in ductility, so the content is made 0.5% or less. A desirable range is 0.45% or less, and a further desirable range is 0.40% or less.

B: 0 to 0.01%
B segregates at grain boundaries to enhance the grain boundary adhesion and contributes to the improvement of strength, and also has the effect of suppressing embrittlement in a hydrogen environment, and thus may be incorporated. When it contains it, it is desirable to set it as 0.0001% or more. A more desirable range is 0.0002% or more, and a further desirable range is 0.0005% or more. However, if it is contained in excess, in order to increase solidification cracking sensitivity in the weld metal when no filler metal is used, the content is made 0.01% or less. A desirable range is 0.008% or less, and a further desirable range is 0.005% or less.

Cu: 0 to 3%
Cu may be contained because it is an element effective to obtain a stable austenitic structure. When it is contained, it is desirable to make it 0.005% or more. A desirable range is 0.008% or more, and a further desirable range is 0.01% or more. However, when it is contained in excess, in the welding which does not use the filler, the solidification cracking sensitivity of the weld metal is enhanced. Therefore, the Cu content is 3% or less. A desirable range is 2.5% or less, and a further desirable range is 2.0% or less.

Ca: 0 to 0.05%
Ca may be contained because it has the effect of improving the hot workability. When it is contained, it is desirable to be 0.0005% or more. A more desirable range is 0.001% or more, and a further desirable range is 0.0015% or more. However, when the content of Ca is excessive, it combines with O to significantly reduce the cleanliness, and rather deteriorates the hot workability. Therefore, the Ca content is 0.05% or less. A desirable range is 0.03% or less, and a further desirable range is 0.01% or less.

Mg: 0 to 0.05%
Mg, like Ca, may be included because it has the effect of improving hot workability. When it is contained, it is desirable to make it 0.0005% or more. A more desirable range is 0.001% or more, and a further desirable range is 0.0015% or more. However, when the content of Ca is excessive, it combines with O to significantly reduce the cleanliness, and rather deteriorates the hot workability. Therefore, the Mg content is 0.05% or less. A desirable range is 0.03% or less, and a further desirable range is 0.01% or less.

REM: 0 to 0.5%
REM may be contained because it has a strong affinity with S and has an effect of improving hot workability. When it is contained, it is desirable to be 0.001% or more. A more desirable range is 0.002% or more, and a further desirable range is 0.005% or more. However, when the content of REM is excessive, it combines with O to significantly reduce the cleanliness, and rather deteriorates the hot workability. Therefore, the content of REM is 0.5% or less. A desirable range is 0.3% or less, and a further desirable range is 0.1% or less.
In addition, "REM" is a general term for 17 elements in total of Sc, Y and lanthanoid, and the content of REM indicates the total content of one or more elements of REM. In addition, REM is generally contained in misch metal. Therefore, for example, it may be added in the form of misch metal, and the amount of REM may be contained so as to fall within the above range.

  When two or more elements of V, Ti, B, Cu, Ca, Mg and REM are contained, the total content thereof is preferably 4.6% or less.

  In the welded joint and the method of manufacturing a welded joint according to the present invention, the austenitic stainless steel contains the above-described elements, and the balance is composed of Fe and impurities. The term "impurity" means a component mixed due to raw materials such as ore, scrap and the like and other factors when industrially manufacturing steel products. Of the impurities, O, P and S need to be strictly limited in their contents.

O: 0.008% or less O exists as an impurity, but when it exists as free oxygen in the molten pool, it has the effect of increasing the penetration depth. However, when it is contained in a large amount, it causes deterioration in hot workability and toughness and ductility during base material production. Furthermore, when the flux is applied and welded as in the present invention, the amount of oxygen in the weld metal increases, and the hydrogen embrittlement resistance decreases. Therefore, the O content needs to be 0.008% or less. A desirable range is 0.007% or less, and a further desirable range is 0.006% or less.

P: 0.03% or less P is contained as an impurity and inhibits the hot workability at the time of manufacture, and increases the solidification cracking susceptibility in the weld metal when the filler metal is not used. Therefore, although it is preferable to reduce as much as possible, extreme reduction leads to an increase in steelmaking costs. Therefore, the P content is made 0.03% or less. Desirably, it is 0.025% or less, more preferably 0.02% or less.

S: 0.01% or less S is contained as an impurity, and in the base material, it inhibits the hot workability at the time of production and increases the susceptibility to solidification cracking in the weld metal when the filler metal is not used. Therefore, although it is preferable to reduce as much as possible, extreme reduction leads to an increase in steelmaking costs. Therefore, the S content is 0.01% or less. Desirably, it is 0.008% or less, more preferably 0.005% or less.

<Flux>
One or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ : 5 to 90% in total
When the flux contains one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ , these metal oxides dissociate in the weld pool during welding, and increase the amount of O in the molten metal. Change the flow direction of the molten metal and increase the penetration depth. In addition, dissociated Cr, Ti and Zr increase the solubility of N, reduce the scattering of nitrogen from the weld pool during welding, and increase the amount of solid solution N in the weld metal after solidification. Some of the elements are precipitated as nitrides in the weld metal to increase the strength of the weld metal. In order to sufficiently obtain these effects, it is necessary to contain one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ in a total amount of 5% or more. Desirably, it is 8% or more, more preferably 10% or more. However, when it is contained in excess, the effect of increasing the amount of solid solution N saturates, and the amount of precipitation of nitride increases, which lowers the hydrogen embrittlement resistance. Therefore, the total content is 90% or less. Preferably it is 85% or less, more preferably 80% or less.

Furthermore, it is preferable that the total content of one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ and the N content contained in the austenitic stainless steel satisfy the following formula (i).
14-20 N ≦ TiO ₂ + Cr ₂ O ₃ + ZrO ₂ ≦ 98-40 N (i)
However, in the formula (i), N, TiO ₂ , Cr ₂ O ₃ and ZrO ₂ represent the content (mass%) of the respective elements or oxides.

  The reason is that, as described above, as the amount of N decreases, the amount of solid solution N and the amount of nitride in the weld metal increase, which is advantageous for securing the strength of the weld metal. On the other hand, when the amount of N is large, in addition to being able to secure strength without actively utilizing these effects, the amount of nitride becomes excessive and it becomes difficult to secure hydrogen embrittlement resistance. .

As the flux, in addition to TiO ₂ , Cr ₂ O ₃ and ZrO ₂ , metal oxides such as SiO ₂ , Fe ₂ O ₃ , Fe ₃ O ₄ , MoO ₃ , WO ₃ , MgO, CaO, K ₂ O etc. Alternatively, fluorides such as CaF ₂ and CaF may be contained. In addition, these fluxes are mixed with a solvent such as methanol, ethanol, propanol and the like and applied to a portion to be welded. In addition, dispersants such as polyoxyethylene alkyl ethers and amines may be mixed with the solvent to avoid flux aggregation.

<Weld metal>
Oxygen content: 0.020% or less When flux is applied and welded, the oxygen content in the weld metal increases. When the oxygen content in the weld metal exceeds 0.020%, a large amount of oxide is present, and the susceptibility to embrittlement in hydrogen is enhanced. Therefore, the oxygen content in the weld metal needs to be 0.020% or less. Desirably, it is 0.018% or less, more preferably 0.015% or less.

  EXAMPLES Hereinafter, the present invention will be more specifically described by way of examples, but the present invention is not limited to these examples.

  Materials having chemical compositions of steel types A to D shown in Table 1 are melted in a laboratory and cast from an ingot, hot forging, hot rolling, heat treatment and machining to a plate thickness of 3 mm, width 50 mm, length 100 mm Steel sheets were produced and used as sample materials.

  After I-shaped groove processing shown in FIG. 1 was performed in the longitudinal direction of the sample material, a flux having a chemical composition shown in Table 2 and methanol were mixed and applied to the butt portion. Thereafter, butt welding was performed by a gas tungsten arc welding method with a heat input of 5 kJ / cm without using a filler metal. From the above, welded joints of test numbers J1 to J19 shown in Table 3 were obtained.

(Weldability)
Among the obtained weld joints, those for which the back bead was not formed were judged as “Fail” for weldability, and those for which a back beat having a width of 1 mm or more was formed were “pass” for weldability. The results are shown in Table 3.

(Analysis of oxygen content in weld metal)
Samples for chemical analysis were taken from the weld metal of the weld joint for which the weldability was "pass", and the oxygen content was analyzed.

(Tensile test)
A plate-like tensile test specimen having the weld metal of the weld joint in which the weldability was "pass" at the center of the parallel portion was collected and subjected to a tensile test at normal temperature. And the thing of 800 Mpa or more which is the target intensity | strength of a base material was determined as "pass", and the thing of less than 800 Mpa was determined as "reject." Further, among the test pieces determined as "pass", the case where the base material is broken is referred to as "pass (good)", and the one where the weld metal is broken is referred to as "pass (acceptable)". The results are shown in Table 3.

(Low strain rate tensile test)
For welded joints that passed the tensile test, stepped plate-like low strain rate tensile test specimens having parallel parts of weld metal were collected and subjected to a low strain rate tensile test in air and under a high pressure hydrogen environment of 85 MPa. The strain rate is 3 × 10 ^-5 / s, and in the low strain rate tensile test, those that have a ratio of 90% or more in the high-pressure hydrogen environment to that in the high-pressure hydrogen environment with respect to the breaking in the atmosphere are "passed" Those with less than 90% are considered as "rejected". In addition, among the test pieces judged as “pass”, the ones for which the ratio of the throttling throttling in a high-pressure hydrogen environment to the throttling throttling in the atmosphere is 92% or more is “pass (good)”, the other is “pass” (OK) The results are shown in Table 3.

As apparent from Table 3, the welded joints J2 to J8 and J12 to J16 satisfying the requirements of the present invention have excellent weldability which can obtain a sufficient penetration depth, and at least the target tensile strength of the base material. It became clear that weld joint strength over 800MPa is obtained. In addition, it has been revealed that it also has a good resistance to hydrogen embrittlement in which the ratio of the throttling throttling in a high pressure hydrogen environment to the throttling in air is 90% or more. In particular, when the total content of one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ and the N content contained in the austenitic stainless steel sheet satisfy a predetermined relationship, the base material is broken and the atmosphere is broken. The ratio of the breaking restriction under high pressure hydrogen environment to the breaking restriction in the medium satisfied 92% or more.

On the other hand, weld joints J1 and J11 were not coated with flux, so the penetration depth was not sufficient and a sound weld joint could not be obtained. In addition, weld joints J9 and J17 have a total content of 2% of one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ in the flux, which is less than the necessary amount, and therefore the weld pool during welding It was not possible to sufficiently suppress the scattering of N from the weld joint, and the tensile strength of the welded joint was below 800 MPa. Furthermore, weld joint J10 had a total content of one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ in the flux in excess of 95%. Therefore, a large amount of nitrides precipitates in the weld metal after solidification, and the ductility in a high pressure hydrogen environment is not good, and the ratio of the breaking restriction in a high pressure hydrogen environment to the breaking restriction in the atmosphere is less than 90%. The In addition, in the welded joints J18 and J19, the oxygen content in the base metal was as high as 0.012%, so the oxygen content in the weld metal after welding exceeded 0.02%. Therefore, the ductility in a high pressure hydrogen environment is not good, and the ratio of the breaking restriction in a high pressure hydrogen environment to the breaking restriction in the atmosphere is less than 90%.
As described above, when only welded joints satisfying the requirements of the present invention are welded without using a filler metal, they have good weldability and have a tensile strength of 800 MPa or more and a reduction of area in the atmosphere. It can be seen that a welded joint excellent in resistance to hydrogen embrittlement can be obtained, in which the ratio of fracture reduction in a high pressure hydrogen environment exceeds 90%.

  According to the present invention, it is possible to provide a method for producing a welded joint in which sufficient penetration depth can be obtained and excellent strength and resistance to hydrogen embrittlement can be obtained when performing gas tungsten arc welding without using a filler metal. can do. Therefore, the manufacturing method of the weld joint of the present invention can be suitably used for various steel materials, such as equipment for high pressure hydrogen gas and equipment for liquid hydrogen.

Claims (3)

A method of manufacturing a welded joint obtained by applying a flux to a welded portion of austenitic stainless steel and performing gas tungsten arc welding without using a filler metal,
The austenitic stainless steel is, by mass%, C: 0.005 to 0.07%, Si: 0.1 to 1.2%, Mn: 2.5 to 10%, Ni: 9 to 14%, Cr 19-24% Mo: 1-4% Nb: 0.1-0.4% N: 0.20-0.45% Al: 0.0005-0.05% V: 0- 0.5%, Ti: 0 to 0.5%, B: 0 to 0.01%, Cu: 0 to 3%, Ca: 0 to 0.05%, Mg: 0 to 0.05%, and REM: containing 0 to 0.5%, the balance being Fe and impurities, and O, P and S as impurities O: 0.008% or less, P: 0.03% or less, and S: Have a chemical composition that is less than 0.01%,
The flux contains, in mass%, a total of 5 to 90% of one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ , and
The manufacturing method of a weld joint whose oxygen content in a weld metal is 0.020% or less.   The austenitic stainless steel is, by mass%, V: 0.001 to 0.5%, Ti: 0.001 to 0.5%, B: 0.0001 to 0.01%, Cu: 0.005 to The present invention contains one or more selected from 3%, Ca: 0.0005 to 0.05%, Mg: 0.0005 to 0.05%, and REM: 0.001 to 0.5%. The manufacturing method of the weld joint as described in 1. The N content contained in the austenitic stainless steel and the total content of one or more selected from TiO ₂ , Cr ₂ O ₃ and ZrO ₂ contained in the flux satisfy the following formula (i) The method of manufacturing a welded joint according to claim 1 or 2.
14-20 N ≦ TiO ₂ + Cr ₂ O ₃ + ZrO ₂ ≦ 98-40 N (i)
However, in the formula (i), N, TiO ₂ , Cr ₂ O ₃ and ZrO ₂ represent the content (mass%) of the respective elements or oxides.

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