JP6098393B2 - Welded joint manufacturing method and welded joint - Google Patents

Description

  The present invention relates to a method for manufacturing a welded joint and a welded joint. More specifically, the present invention relates to a method for manufacturing a welded joint for obtaining a joint excellent in high strength and hydrogen embrittlement resistance, which are characteristics required for high pressure gas piping, particularly high pressure hydrogen gas piping, and an austenitic steel welded joint obtained thereby. .

  In recent years, research on practical application of transportation equipment using hydrogen, natural gas, or the like as energy has been actively promoted. For practical use, it is necessary to prepare a use environment in which these gases can be stored and transported at high pressure, and the strength of the materials used there is also increased. And the utilization of the material which raised the tensile strength 550MPa of the austenitic steel currently widely used to 724MPa, and also the development and application examination of a material exceeding 800MPa are advanced in parallel.

  Under such a background, as materials to be used, for example, in Patent Documents 1 to 3, the solubility of N is increased by increasing Mn and V is contained, or V and Nb are combined. Thus, an austenitic stainless steel has been proposed in which the solid solution strengthening of N and the precipitation strengthening of nitride are utilized to try to increase the strength.

  When these high-strength austenitic steels containing a large amount of N are used as structures, assembly by welding is necessary, but the welded parts are required to have the same strength as the base metal from the viewpoint of use performance. Therefore, for example, Patent Documents 3 to 5 propose a filler material (welding material) having a tensile strength exceeding 800 MPa by actively utilizing Al, Ti, and Nb.

  However, both of these filler metals and weld metals obtained using the filler materials require post-weld heat treatment in order to achieve high strength. On the other hand, when considered as an actual large structure, such a long-time post-weld heat treatment is a major limitation and may cause an extreme increase in manufacturing cost.

  Therefore, in Patent Document 6, the N content of the filler metal, the shield gas at the time of welding, the weld pool area, etc. are managed to increase the N content of the weld metal without performing post-weld heat treatment. A welded joint that achieves high strength has been proposed.

  In an actual structure, it may be difficult to use a filler material depending on the use site, such as a thin member. And when high-strength austenitic steel containing a large amount of N is welded without using a filler metal, N is scattered from the molten pool during welding, so the N content of the resulting weld metal is the base metal. Since the content of other alloy elements is the same as that of the base metal, the resulting weld metal has a lower strength than that of the base material, and there is a problem that the required weld joint strength is not satisfied. Sometimes.

International Publication No. 2004/083476 International Publication No. 2004/083477 International Publication No. 2004/110695 JP-A-5-192785 JP 2010-227949 A International Publication No. 2013/005570

  The present invention has been made in view of the above situation, and has high strength and excellent hydrogen embrittlement resistance, which are characteristics required for high-pressure gas piping, particularly high-pressure hydrogen gas piping, without using a filler material. It is an object of the present invention to provide a method for producing a welded joint capable of obtaining a joint having the same, and a welded joint obtained thereby.

  The present inventors have repeated investigations to solve the above-described problems. As a result, the following items were first confirmed.

  When a welded joint is manufactured by welding a thin member by a tungsten gas arc method without using a filler metal, a portion that forms a hypothetical line called a “weld line” from the viewpoint of welding efficiency ( Hereinafter, it is generally called “welded line”) and is completed by welding once.

  However, in mass%, C: 0.005-0.1%, Si: 1.2% or less, Mn: 2.5-6.5%, Ni: 8-15%, Cr: 19-25%, When the above welded wire is welded once using an austenitic steel containing Al: less than 0.05% and N: 0.15-0.45%, a weld pool is formed during welding. Since N is scattered and the N content of the weld metal is reduced, the weld joint strength exceeding the required strength of the base material cannot be obtained.

  Then, the present inventors performed detailed examination about optimization of the welding method in order to solve the above-mentioned problem.

  As a result, the strength of the weld metal can be improved by carrying out the following (a) when manufacturing a welded joint by welding by the tungsten gas arc welding method, which is essential to back shield and shield. It was clarified that a welded joint satisfying the required strength was obtained.

  (A) Improving the tensile strength of the weld metal by performing subsequent welding on the weld metal obtained by first welding the welded wire with a heat input lower than the weld heat input at the time of first welding. be able to. Then, it is effective that the number of subsequent weldings is at least one and not more than eight.

  Furthermore, when producing a welded joint by welding by the above-described back shield and tungsten gas arc welding method, which is essential to shield, one or more of the contents selected from the following (b) to (e) are also implemented. As a result, it has become clear that the effect of improving the strength of the weld metal becomes greater.

(B) A gas composed of Ar: 0 to 100% and N ₂ : 0 to 100% in volume% is used as the back shield gas.

(C) A gas composed of Ar: 50 to 100% and N ₂ : 0 to 50% in volume% is used as a shielding gas.

(D) The heat input λ _i of the i-th subsequent welding with respect to the heat input λ ₀ of welding at the time of first welding,
0.18 ≦ λ _i / λ ₀ ≦ 0.98
The following equation is satisfied.

  (E) The welding heat input in the subsequent welding is made lower than the welding heat input in the immediately preceding subsequent welding.

In the above (b) and (c), N ₂ is vol%, and “0%” indicates that it is an Ar single gas. Further, in the above (b), Ar is a volume%, and "0%" refers to a N ₂ alone gas.

  The reason why the strength of the weld metal is improved by the implementation of the contents described in the above (a) to (e) is as follows.

  First, as described above, when a welded wire is welded once to form a welded joint, since N scatters from the molten pool during welding, the N content of the weld metal is reduced. Strength decreases.

  However, if subsequent welding is performed on the weld metal obtained by first welding with a weld heat input lower than the welding heat input at the time of first welding, a part of the weld metal obtained by the first welding is obtained. Strain is introduced due to thermal deformation caused by subsequent welding. In addition, the thermal cycle produces carbonitrides in the weld metal. As a result, the tensile strength of the weld metal is higher than when welding once.

  And although the above effect becomes remarkable by repeating the subsequent welding a plurality of times, considering the application to actual construction, it is preferable that the number of the subsequent welding is small from the viewpoint of construction efficiency and cost. Furthermore, when the number of times exceeds 8, an excessive carbonitride is generated, and the ductility of the weld metal is lowered. For this reason, 1-8 times is appropriate for the frequency | count of subsequent welding.

Further, when performing the i-th subsequent welding, the welding heat input (λ _i ) is related to the welding heat input (λ ₀ ) at the time of the first welding, and “λ _i / λ ₀ ” is 0.18. If it is less than 1, the heat input will be too small, sufficient carbonitride may not be generated, and sufficient strength improvement effect may not be stably obtained, while “λ _i / λ ₀ ” is If it exceeds 0.98, the penetration shape becomes almost the same as that of the first welding, and a sufficient strength improvement effect may not be obtained stably. Therefore, in order to stably secure the strength improvement effect due to the introduction of processing strain and the precipitation of carbonitride, the above-mentioned “λ _i / λ ₀ ” is controlled within the range of 0.18 to 0.98. preferable.

  Furthermore, in the subsequent welding, if the welding heat input is made smaller than the immediately preceding subsequent welding heat input, the penetration shape gradually decreases, so that further processing distortion is introduced by thermal deformation caused by the subsequent welding. For this reason, it is more desirable that the welding heat input in the subsequent welding is lower than the welding heat input in the immediately preceding subsequent welding.

In addition, when performing back shield as an indispensable requirement at the time of welding, a gas composed of Ar: 0 to 100% and N ₂ : 0 to 100% in volume% is used as the back shield gas, so that the first layer welding is performed. Since the scattering of N from the backside molten pool surface of the steel can be stably prevented, the yield of N of the weld metal can be increased, the strength can be reduced, and the above gas can be used as a back shield gas during subsequent welding. As a result, the vicinity of the surface is nitrided, which contributes to improving the strength, albeit slightly.

Furthermore, when shielding as an indispensable requirement at the time of welding, by using a gas composed of Ar: 50 to 100% and N ₂ : 0 to 50% in volume% as a shielding gas, the surface of the molten pool during welding As a result, it is possible to stably suppress the scattering of N from the steel, so that a decrease in strength can be reduced and a slight improvement in strength can be expected by nitriding near the surface.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in a welded joint manufacturing method and a welded joint described below.

(1) By mass%, C: 0.005 to 0.1%, Si: 1.2% or less, Mn: 2.5 to 6.5%, Ni: 8 to 15%, Cr: 19 to 25% Al: less than 0.05% and N: 0.15-0.45%,
The balance consists of Fe and impurities,
A base material having a chemical composition in which O, P, and S as impurities are respectively O: 0.02% or less, P: 0.05% or less, and S: 0.04% or less,
A method for manufacturing a welded joint in which welding is performed by a tungsten gas arc welding method without using a filler material, and subsequent welding is performed 1 to 8 times on the weld metal obtained by the first welding. ,
A method for manufacturing a welded joint, wherein the welding heat input in the subsequent welding is lower than the welding heat input in the first welding.

(2) Manufacture of a welded joint according to (1) above, wherein a gas composed of Ar: 0 to 100% and N ₂ : 0 to 100% in volume% is used as a back shield gas. Method.

(3) by volume% Ar: 50 to 100% and N _2: a gas composed of 0-50%, welding described above, characterized by using as a shielding gas (1) or (2) A method for manufacturing a joint.

(4) The welding heat input λ _i in the i-th subsequent welding with respect to the welding heat input λ ₀ in the first welding satisfies the following formula (1): The manufacturing method of the welded joint in any one of to 3).
0.18 ≦ λ _i / λ ₀ ≦ 0.98 (1).

  (5) The method for manufacturing a welded joint according to any one of (1) to (4), wherein the welding heat input in the subsequent welding is lower than the welding heat input in the immediately following subsequent welding.

  (6) The chemical composition of the base material is selected from V: 0.5% or less, Nb: 0.5% or less, and Mo: 4.5% or less in mass% instead of part of Fe. The method for producing a welded joint according to any one of (1) to (5) above, which contains a seed or more.

  (7) A welded joint obtained by the method for manufacturing a welded joint according to any one of (1) to (6).

  The “impurities” in the “Fe and impurities” as the remainder refers to those mixed from ore, scrap, or production environment as raw materials when the steel material is industrially produced.

  According to the present invention, it is possible to obtain an austenitic steel welded joint having high strength and excellent hydrogen embrittlement resistance, which are characteristics required for high-pressure gas piping, particularly high-pressure hydrogen gas piping, without using a filler material. Can do.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element in the chemical composition of the base material means "mass%".

(A) Base material chemical composition:
C: 0.005-0.1%
C is an element effective for stabilizing austenite. In the weld metal, C is mainly bonded to Cr and precipitated as carbide by the thermal cycle of the subsequent welding, and contributes to the strength improvement. In order to acquire these effects, it is necessary to contain C 0.005% or more. However, if C is contained excessively, in the base metal, carbides are formed at the grain boundaries by heating during welding to deteriorate the corrosion resistance, and in the weld metal, a large amount of carbides precipitate and the ductility is lowered. Invite. Therefore, the upper limit of C content is 0.1%. The lower limit of the C content is preferably 0.01%, and more preferably 0.02%. Further, the upper limit of the C content is preferably 0.08%, and more preferably 0.06%.

Si: 1.2% or less Si is an element that is added as a deoxidizer and is effective in improving corrosion resistance. However, excessive Si content lowers the stability of austenite and lowers ductility, and in weld metals, segregates at columnar crystal boundaries during solidification to lower the melting point of the liquid phase and increase the susceptibility to solidification cracking. . Therefore, the Si content is 1.2% or less. The Si content is desirably 1.1% or less, and more desirably 1.0% or less. In addition, it is not necessary to set a lower limit in particular for the content of Si, but an extreme decrease does not provide a sufficient deoxidation effect, and the cleanliness of the steel increases and the cleanliness deteriorates. Invite rise. Therefore, the lower limit of the Si content is desirably 0.01%, and more desirably 0.05%.

Mn: 2.5 to 6.5%
Mn is an element that is added as a deoxidizer and is also effective in stabilizing austenite. Further, Mn indirectly contributes to increasing the solubility by increasing the solubility of N in the molten metal during the production of the base material and during welding. In order to fully utilize this strength improvement effect, it is necessary to contain 2.5% or more of Mn. However, excessive inclusion of Mn causes a decrease in ductility. Therefore, the upper limit of the Mn content is set to 6.5%. The lower limit of the Mn content is preferably 2.8%, and more preferably 3.0%. Further, the upper limit of the Mn content is desirably 6.2%, and more desirably 6.0%.

Ni: 8-15%
Ni is an essential element for obtaining stable austenite, and a content of 8% or more is necessary to sufficiently obtain the effect. However, since Ni is an expensive element, its inclusion in a large amount causes an increase in cost and reduces the solubility of N in the molten metal during the production of the base material and during welding. Therefore, the upper limit of the Ni content is 15%. The lower limit of the Ni content is desirably 8.5%, and more desirably 9%. Further, the upper limit of the Ni content is desirably 14.5%, and more desirably 14%.

Cr: 19-25%
Cr is an essential element for ensuring corrosion resistance in the environment of use. Further, Cr is effective in increasing the solubility of N in the molten metal during the production of the base material and during welding, and a Cr content of 19% or more is necessary in order to sufficiently obtain the effect. . However, excessive content of Cr makes austenite unstable and causes embrittlement depending on the type of gas contact environment. Therefore, the upper limit of Cr content needs to be 25%. The lower limit of the Cr content is preferably 19.5%, and more preferably 20%. Further, the upper limit of the Cr content is desirably 24.5%, and more desirably 24%.

Al: less than 0.05% Al is added as a deoxidizing agent, as is Si and Mn. However, when Al is contained excessively, a large amount of nitride is formed and ductility is reduced. Therefore, the Al content is less than 0.05%. The Al content is desirably 0.04% or less, and more desirably 0.03% or less. In addition, although it is not necessary to provide a lower limit in particular for the content of Al, an extreme decrease does not provide a sufficient deoxidation effect, and the cleanliness of the steel increases and the cleanliness deteriorates. Invite rise. Therefore, the lower limit of the Al content is desirably 0.001%, and more desirably 0.002%.

N: 0.15-0.45%
N is an essential element for obtaining a high strength by forming a fine nitride while forming a solid solution in the matrix. In particular, in weld metal, it is precipitated by the thermal cycle of subsequent welding, which greatly contributes to strength improvement. In order to sufficiently obtain this effect, an N content of 0.15% or more is necessary. However, when N is contained excessively, the hot workability at the time of manufacture is lowered in the base metal, and in the weld metal, it causes blowholes and / or pits during welding and is caused by excessive precipitation. A reduction in ductility occurs. Therefore, the upper limit of N content is set to 0.45%. The lower limit of the N content is preferably 0.18%, and more preferably 0.20%. Further, the upper limit of the N content is preferably 0.42%, and more preferably 0.40%.

  In the present invention, the base material has a chemical composition in which the elements from C to N described above, the balance is Fe and impurities, and the contents of O, P and S as impurities are limited to the ranges described below. It is what you have.

O: 0.02% or less O is present as an impurity, but when contained in a large amount, the base metal causes a decrease in hot workability during production and also causes deterioration in toughness and ductility. Therefore, the content of O needs to be 0.02% or less. The upper limit of the O content is desirably 0.015%, more desirably 0.01%.

P: 0.05% or less P is contained as an impurity. In the base metal, hot workability during production is inhibited, and in the case of weld metal, the melting point of the liquid phase is lowered during solidification and the solidification cracking sensitivity is increased. Let Therefore, it is preferable to reduce the P content as much as possible. However, since extreme reduction leads to an increase in steelmaking cost, it is set to 0.05% or less. The upper limit of the P content is desirably 0.045%, and more desirably 0.04%.

S: 0.04% or less S is contained as an impurity, as in P. In the base metal, hot workability during production is inhibited, and in the case of a weld metal, the melting point of the liquid phase is lowered during solidification, and solidification occurs. Increase crack susceptibility. For this reason, it is preferable to reduce the S content as much as possible as in the case of P, but extreme reduction leads to an increase in steelmaking costs. Therefore, the upper limit of the S content is 0.04%. The upper limit of the S content is desirably 0.035%, and more desirably 0.03%.

  In the present invention, the base material may contain one or more elements selected from V, Nb, and Mo, if necessary, instead of a part of the Fe.

  That is, since V, Nb, and Mo all have an effect of increasing the strength, the above elements may be contained in order to obtain this effect. This will be described in detail below.

V: 0.5% or less V may be contained because it is an element effective for improving the strength by precipitating as a solid solution or carbonitride in the matrix. However, when the V content is excessive, a large amount of carbonitride precipitates, resulting in a decrease in ductility. For this reason, the amount of V in the case of containing is 0.5% or less. The upper limit of the V content is desirably 0.4%, and more desirably 0.3%.

  On the other hand, in order to stably obtain the effect of V described above, the V content is preferably 0.05% or more, and more preferably 0.1% or more.

Nb: 0.5% or less Nb, like V, precipitates as a solid solution or carbonitride in the matrix, and is an element effective for improving the strength, so may be contained. However, when the Nb content is excessive, a large amount of carbonitride precipitates, resulting in a decrease in ductility. For this reason, the amount of Nb when contained is 0.5% or less. The upper limit of the Nb content is desirably 0.4%, and more desirably 0.3%.

  On the other hand, in order to stably obtain the effect of Nb described above, the Nb content is desirably 0.05% or more, and more desirably 0.1% or more.

Mo: 4.5% or less Mo is an element effective for increasing the strength. Mo is an element that is also effective in improving the corrosion resistance under the use environment. For this reason, in order to acquire these effects, you may contain Mo. However, Mo is a very expensive element, and excessive content makes austenite unstable. For this reason, the upper limit of the amount of Mo in the case of making it contain shall be 4.5%. The upper limit of the Mo content is preferably 4.0%, and more preferably 3.5%.

  On the other hand, in order to stably obtain the effect of Mo described above, the Mo content is preferably 0.5% or more, and more preferably 1.0% or more.

  Said V, Nb, and Mo can be contained only in any one of them, or 2 or more types of composites. In addition, although the total amount of these elements in the case of making it contain may be 5.5%, it is preferable to set it as 5.0% or less.

(B) Tungsten gas arc welding method:
The method for manufacturing a welded joint according to the present invention is the first time when a base metal having the chemical composition described in the above (A) is welded by a tungsten gas arc welding method without using a filler material to manufacture a welded joint. A method for manufacturing a welded joint in which subsequent welding is performed 1 to 8 times on a weld metal obtained by welding, characterized in that the welding heat input in the subsequent welding is lower than the welding heat input in the first welding. To do.

  Hereinafter, the tungsten gas arc welding method according to the present invention will be described in detail.

(B-1) Subsequent welding:
When producing a welded joint by welding a base material having the chemical composition described in the above item (A), especially a thin base material by a tungsten gas arc welding method without using a filler material, When a weld is simply welded once to form a welded joint, N is scattered from the weld pool during welding, and the N content of the weld metal is reduced, resulting in a welded joint strength that exceeds the required strength of the base metal. I can't. However, on the weld metal obtained by the first welding (in other words, on the weld metal obtained by first welding the welding line), the welding heat input is lower than the welding heat input in the first welding. If it welds, the fall of the weld metal intensity | strength by N scattering from said molten pool can be suppressed.

  This is because a part of the weld metal obtained by the first welding is processed by thermal deformation due to subsequent welding, strain is introduced, and carbonitride is generated in the weld metal by thermal cycling. In order to obtain the above effect, it is necessary to perform at least one subsequent welding under the above-described conditions.

  Although the above effect becomes prominent by repeating the subsequent welding multiple times, considering the application to the actual construction, from the viewpoint of construction efficiency and cost, it is preferable that the number of subsequent weldings is small, while the number of times is When it exceeds eight times, an excessive carbonitride is generated and the ductility of the weld metal is lowered. Therefore, the number of subsequent weldings performed under the above conditions is 1 to 8 times. As for the number of subsequent welding, it is desirable that the lower limit is 2 times and the upper limit is 7 times, and it is extremely desirable that the lower limit is 3 times and the upper limit is 6 times.

In order to suppress a decrease in weld metal strength, it is necessary to perform subsequent welding on the weld metal obtained by the first welding with a welding heat input lower than the welding heat input in the first welding. to enhance the, compared welding heat input lambda ₀ in the welding for the first time, a welding heat input lambda _i of i-th subsequent welding, the aforementioned _{0.18 ≦ λ i / λ 0 ≦} 0.98 ··· ( 1)
It is better to satisfy the equation.

When “λ _i / λ ₀ ” is less than 0.18, the heat input is too small, and sufficient carbonitride is not generated, and sufficient strength improvement effect may not be stably obtained. On the other hand, when “λ _i / λ ₀ ” exceeds 0.98, the penetration shape becomes almost the same as that of the first welding, and the strength improvement effect by the introduction of processing strain and the precipitation of carbonitride is stably obtained. It may not be possible. For this reason, in order to stably secure the strength improvement effect due to the introduction of processing strain and the precipitation of carbonitride, the above-mentioned “λ _i / λ ₀ ” is controlled within the range of 0.18 to 0.98. It is preferable. More preferably, “λ _i / λ ₀ ” has a lower limit of 0.20 and an upper limit of 0.95.

  Moreover, if the welding heat input in the subsequent welding is made lower than the welding heat input in the immediately preceding subsequent welding, the penetration shape gradually decreases, so that further processing distortion is introduced by thermal deformation due to the subsequent welding. Therefore, it is more effective for strength improvement. Therefore, it is desirable that the welding heat input in the subsequent welding is lower than the welding heat input in the immediately following subsequent welding.

(B-2) About back shield gas:
When producing a welded joint by welding a base material having the chemical composition described in the above item (A), particularly a thin base material by a tungsten gas arc welding method without using a filler material, a weld metal In order to stably suppress the decrease in strength, it is preferable to use a gas composed of Ar: 0 to 100% and N ₂ : 0 to 100% in volume% as the back shield gas.

  Back-shielding with the above gas during welding can stably prevent the scattering of N from the backside molten pool surface during the first layer welding, thus increasing the N yield of the weld metal and reducing the strength reduction. This is because the vicinity of the surface is nitrided by back-shielding with the above gas even during subsequent welding, which contributes to a slight improvement in strength.

In addition, since the effect of further improving the weld metal strength can be obtained by increasing the partial pressure of N ₂ in the back shield gas, the above-described back shield gas may include Ar: 0 to 98% and N in volume%. _2: it is more preferable to use the composed gas 2-100%, by volume% Ar: 0 to 95% and N _2: still more preferably, to use a formed gas in 5-100% .

(B-3) About shielding gas:
When producing a welded joint by welding a base material having the chemical composition described in the above item (A), particularly a thin base material by a tungsten gas arc welding method without using a filler material, a weld metal In order to stably suppress a decrease in strength, it is preferable to use a gas composed of Ar: 50 to 100% and N ₂ : 0 to 50% by volume as the shielding gas.

  If shielding with the above gas at the time of welding, the scattering of N from the surface of the molten pool during welding can be stably suppressed, so that the yield of N of the weld metal is increased and the above gas is also shielded at the time of subsequent welding. This is because the vicinity of the surface is nitrided and contributes to the strength improvement.

By increasing the partial pressure of N ₂ in the shielding gas, an effect of further improving the weld metal strength can be obtained. Therefore, the shielding gas is, as a volume%, Ar: 50 to 99.5% and N ₂ : it is more preferable to use the composed gas in 0.5~50%, Ar: 50~99% and N _2: it is more preferable to use a formed gas 1 to 50%.

Note that the N ₂ constituting the above-described shielding gas, by volume%, in the case of more than 50%, N dissolved in the weld metal at high temperatures, no longer completely dissolved in the weld metal together with coagulation, N ₂ may form blowholes and / or pits, or the welder may be nitrided and deteriorated. Therefore, N ₂ is in volume%, and the upper limit is 50%, but the upper limit is 30%. This is more preferable.

  Since the weld metal has a rapidly solidified structure, unlike a base material manufactured by performing a solution heat treatment, ferrite generated at a high temperature in the solidification process may remain up to room temperature.

  Ferrite becomes brittle in the hydrogen environment and becomes the starting point of fracture. Especially when ferrite with an area ratio exceeding 20% continuously exists, it can be connected and propagated to reduce the hydrogen embrittlement resistance of the weld metal. However, when the base metal having the chemical composition described in the item (A) is welded by the tungsten gas arc welding method described in the item (B) to produce a welded joint, the ferrite amount in the weld metal is stabilized. The area ratio can be 20% or less. For this reason, the welded joint obtained by the method for manufacturing a welded joint according to the present invention can be used in a hydrogen environment without any problem.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Thickness 2mm, width 50mm, length 100mm by hot forging, hot rolling, heat treatment and machining from ingots prepared by melting and casting materials A to E having chemical compositions shown in Table 1 A steel plate was prepared for use as a welding base material.

  Butt welding was carried out in the longitudinal direction of the above-mentioned welded base steel plate under the conditions shown in Table 2 without using a filler metal by the tungsten gas arc welding method.

  A plate-like tensile test piece having a weld metal at the center of the parallel part was taken from the obtained welded joint and subjected to a tensile test at room temperature.

  In addition, as what has the high intensity | strength which is a characteristic requested | required of high-pressure hydrogen gas piping, what exceeds 724 MPa which is the minimum required strength of a base material is "possible", 800 MPa which is the target strength of a base material Those exceeding 720 MPa were judged as “excellent”, and those exceeding 820 MPa exceeding 800 MPa by 20 MPa or more were judged as “excellent”.

For welded joints that have been judged as “excellent”, “good” or “possible” as a result of the tensile test, a stepped plate-shaped low strain rate tensile test piece having a weld metal parallel part is collected and It was subjected to a low strain rate tensile test in a medium and 85 MPa high pressure hydrogen environment. The strain rate is 3 × 10 ⁻⁵ / s, and a low strain rate tensile test in which the ratio of the fracture drawing in a high-pressure hydrogen environment to the fracture drawing in the atmosphere is 90% or more is “pass”. On the other hand, a case where the above-mentioned ratio was less than 90% was regarded as “fail”.

  Table 3 shows the tensile test results and the low strain rate tensile test results.

  From Table 3, it is clear that the test codes J2 to J18 of the examples of the present invention that satisfy the requirements of the present invention have a tensile strength exceeding at least the minimum required strength of the base material and also have hydrogen embrittlement resistance.

In addition, among the test codes of the above-described example of the present invention, the effect of improving the strength can be easily obtained by including N ₂ in the shield gas or the back shield gas from the comparison of the test codes J3 and J10 to J14. Is clear.

  Further, from the comparison between the test code J3 and the test code J15, and the comparison between the test code J5 and the test code J16, in the subsequent welding, the effect of improving the strength can be obtained by making the welding heat input smaller than the immediately preceding pass. It is clear that it is easy.

  On the other hand, the test code J1 of the comparative example in which the welding joint is formed by one welding without performing the subsequent welding, the weld metal strength decreases due to the scattering of N from the weld metal, and the tensile strength is the minimum required strength of the base metal. It did not exceed 724 MPa which is.

  Further, the test code J19 of the comparative example using the base material D having a low N content of 0.13% and deviating from the conditions specified in the present invention did not improve the strength even after the subsequent welding. The strength did not exceed 724 MPa, which is the minimum required strength of the base material.

  Furthermore, in the test code J20 of the comparative example using the base material E in which the contents of Cr and Ni deviate from the conditions specified in the present invention, the tensile strength exceeds the minimum required strength of the base material, 724 MPa, but the austenite Since the stability is poor and there is a large amount of ferrite in the weld metal, the ratio of the fracture drawing in a high-pressure hydrogen environment to the fracture drawing in the atmosphere is less than 90% in a low strain rate tensile test and is susceptible to hydrogen embrittlement. Was expensive.

  According to the present invention, it is possible to obtain an austenitic steel welded joint having high strength and excellent hydrogen embrittlement resistance, which are characteristics required for high-pressure gas piping, particularly high-pressure hydrogen gas piping, without using a filler material. Can do.

Claims (7)

In mass%, C: 0.005 to 0.1%, Si: 1.2% or less, Mn: 2.5 to 6.5%, Ni: 8 to 15%, Cr: 19 to 25%, Al: Less than 0.05% and N: 0.15 to 0.45%,
The balance consists of Fe and impurities,
A base material having a chemical composition in which O, P, and S as impurities are respectively O: 0.02% or less, P: 0.05% or less, and S: 0.04% or less,
A method for manufacturing a welded joint in which welding is performed by a tungsten gas arc welding method without using a filler material, and subsequent welding is performed 1 to 8 times on the weld metal obtained by the first welding. ,
A method for manufacturing a welded joint, wherein the welding heat input in the subsequent welding is lower than the welding heat input in the first welding. _{2. The} method for manufacturing a welded joint according to claim 1, wherein a gas composed of Ar: 0 to 100% and N ₂ : 0 to 100% in volume% is used as a back shield gas. 3. The method for manufacturing a welded joint according to claim 1, wherein a gas composed of Ar: 50 to 100% and N ₂ : 0 to 50% by volume is used as a shielding gas. The welding heat input λ _i in the i-th subsequent welding with respect to the welding heat input λ ₀ in the first welding satisfies the following expression (1): 4. The manufacturing method of the welded joint as described in 1 ..
0.18 ≦ λ _i / λ ₀ ≦ 0.98 (1).   The method for manufacturing a welded joint according to any one of claims 1 to 4, wherein the welding heat input in the subsequent welding is lower than the welding heat input in the immediately preceding subsequent welding.   Instead of a part of Fe, the chemical composition of the base material is at least one selected from mass%, V: 0.5% or less, Nb: 0.5% or less, and Mo: 4.5% or less. It contains, The manufacturing method of the welded joint in any one of Claim 1 to 5 characterized by the above-mentioned.   The welded joint obtained by the manufacturing method of the welded joint in any one of Claim 1-6.