JP2017213588A - Manufacturing method of austenite stainless steel weld joint - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of an austenite stainless steel weld joint capable of obtaining required strength and hydrogen embrittlement resistance without using welding material.SOLUTION: A manufacturing method of an austenite stainless steel weld joint includes: a process of preparing a first member of a chemical composition which contains C of 0.005 to 0.1%, Si of 0.2 to 1.2%, Mn of 2.5 to 6.5%, Ni of 8 to 15%, Cr of 19 to 25%, Mo of 0.01 to 4.5%, V of 0.01 to 0.5%, Nb of 0.01 to 0.5%, Al less than 0.05%, N of 0.15 to 0.45%, etc. in terms of mass%; a process of preparing a second member of the austenite stainless steel in which Nieq stipulated by formula (1) is 27% or more; and a process of welding the first member to the second member according to gas tungsten arc welding without using welding material. Therein, heat input Q is 7 kJ/cm or less and satisfies formula (2): Nieq=Ni+Mo+Mn+0.6Cr+0.3Si+12(C+N)...(1), Q≥-0.18[H]+4.8...(2).SELECTED DRAWING: None

Description

  The present invention relates to a method for manufacturing an austenitic stainless steel welded joint.

  In recent years, research on practical application of transport equipment using hydrogen as energy as an alternative to fossil fuels has been underway. In order to put it into practical use, it is necessary to prepare an environment in which hydrogen can be stored and transported at high pressure (hereinafter also referred to as a hydrogen facility). The hydrogen facility is, for example, a high-pressure hydrogen gas device or a liquid hydrogen device. Materials used for hydrogen facilities are required to have hydrogen embrittlement resistance.

  International Publication Nos. 2004/083476, 2004/083477, and 2004/110695 disclose high-strength austenitic stainless steel. In these documents, the solubility of N is increased by increasing Mn and V is contained, or by combining V and Nb, the solid solution strengthening by N and the precipitation strengthening by nitride. It has been proposed to utilize

  When austenitic stainless steel is used as a structure, assembly by welding is required, and the welded portion is required to have the same strength as the base material. In JP-A-5-192785, JP-A 2010-227949, and the above-mentioned International Publication No. 2004/110695, the tensile strength exceeding 800 MPa is obtained by actively utilizing Al, Ti and Nb. A welded joint is disclosed.

  These welded joints require post-weld heat treatment to increase the strength. However, in a large structure, long-time post-weld heat treatment becomes a major limitation and may increase the manufacturing cost. In International Publication No. 2013/005570, the N content of the welding material is controlled to increase the N content of the weld metal so that high strength can be obtained without performing post-weld heat treatment. A welded joint is disclosed.

  Japanese Unexamined Patent Publication No. 9-137255 discloses austenitic stainless steel with improved weldability.

International Publication No. 2004/083476 International Publication No. 2004/083477 International Publication No. 2004/110695 International Publication No. 2012/132992 JP-A-5-192785 JP 2010-227949 A International Publication No. 2013/005570 JP-A-9-137255

  In an actual structure, it is not necessary that all the parts are made of the high-strength austenitic stainless steel as described above, and different austenitic stainless steels may be welded depending on the use parts. In this case, high strength is not required for the welded joint. That is, in the case of a dissimilar welded joint, it is only necessary to have a strength equivalent to that of the lower strength base material. However, it is required to have excellent embrittlement characteristics in a hydrogen environment.

  The use of a welding material may be difficult, for example, when a thin member (for example, a member having a thickness of 3 mm or less) is welded. In this case, without using the welding material, the steel materials are brought into contact with each other and welded by gas tungsten arc welding. If welding is performed without using a welding material, an unmelted portion may remain as a defect on the butt surface, and the required strength may not be obtained.

  International Publication No. 2004/083476, International Publication No. 2004/083477, International Publication No. 2004/110695, International Publication No. 2012/132992, Japanese Unexamined Patent Publication No. 5-192785, Japanese Unexamined Patent Publication No. 2010-227949, and International Publication No. In the publication 2013/005570, examination regarding welding workability is not made.

  JP-A-9-137255 describes that welding workability can be improved by adjusting the O content in accordance with the Al content. However, since Al has a strong affinity for N, even if this technique is applied to austenitic stainless steel in which N is positively contained, a sufficient effect may not be obtained.

  An object of the present invention is to provide a method for producing an austenitic stainless steel welded joint that can obtain necessary strength and hydrogen embrittlement resistance without using a welding material.

In the method for manufacturing a welded joint according to an embodiment of the present invention, the chemical composition is mass%, C: 0.005 to 0.1%, Si: 0.2 to 1.2%, Mn: 2.5 to 6.5%, Ni: 8-15%, Cr: 19-25%, Mo: 0.01-4.5%, V: 0.01-0.5%, Nb: 0.01-0.5 %, Al: less than 0.05%, N: 0.15 to 0.45%, Ti: 0 to 0.5%, Cu: 0 to 3.0%, B: 0 to 0.01%, balance: A first member is prepared which is Fe and impurities, and O, P, and S as the impurities are O: 0.02% or less, P: 0.05% or less, and S: 0.04% or less, respectively. A process and a second member which is an austenitic stainless steel containing C, Si, Mn, Ni, Cr, Mo and N, and whose Nieq defined by the formula (1) is 27% or more. A step of, Ar gas, as any shielding gas in the mixed gas of the mixed gas, and Ar and _{N 2} and _{H 2} in _{N 2} gas, a mixed gas of Ar and _{N 2,} and Ar and _{H 2,} welding Welding the first member and the second member by gas tungsten arc welding without using a material,
The heat input Q of the gas tungsten arc welding is 7 kJ / cm or less and satisfies the following formula (2).
Nieq = Ni + Mo + Mn + 0.6Cr + 0.3Si + 12 (C + N) (1)
Q ≧ −0.18 [H ₂ ] +4.8 (2)
However, the content of each element of the second member is substituted by mass% for each element symbol in the formula (1). In the formula (2), the unit of Q is kJ / cm, and the mixing ratio of H _{2 in} the shielding gas is substituted by volume% for [H ₂ ].

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of an austenitic stainless steel welded joint from which required intensity | strength and hydrogen embrittlement resistance are acquired is obtained, without using a welding material.

FIG. 1 is a schematic diagram showing groove processing of a steel sheet for a weld base material.

  In order to solve the above-mentioned problems, the present inventors have conducted a detailed study. As a result, the following items were confirmed first.

  When welding a thin portion by gas tungsten arc welding without using a welding material, from the viewpoint of welding efficiency, a portion that forms a hypothetical line called a “weld line” (hereinafter, covered) The welding line is generally completed by welding once.

  In mass%, C: 0.005-0.1%, Si: 0.2-1.2%, Mn: 2.5-6.5%, Ni: 8-15%, Cr: 19-25% , Mo: 0.01-4.5%, V: 0.01-0.5%, Nb: 0.01-0.5%, Al: less than 0.05%, N: 0.15-0. When the austenitic stainless steel (first member) containing 45% or the like and another austenitic stainless steel (second member) are welded by gas tungsten arc welding without using a welding material, the strength of the weld metal is increased. It may be less than the second member. That is, it may not be possible to make an overmatch joint. This is presumably because N is scattered from the molten pool during welding, and the N content of the weld metal is reduced.

  In this specification, “welding material” means a filler material, that is, a material (welding rod, welding wire, etc.) added during welding, and “welding metal” means a part of the welded portion. Means a metal that has melted and solidified during welding.

  In order to solve this problem, the present inventors have conducted a detailed study on optimization of a method for manufacturing a welded joint. As a result, the following findings (a) to (c) were obtained.

(A) As described above, the decrease in the strength of the weld metal is considered to be caused by N scattering from the molten pool during welding. In order to suppress this, the heat input during welding may be reduced. If the heat input is reduced, the area of the molten pool is reduced, and the scattering of N can be suppressed.

  On the other hand, if the heat input is reduced in welding without using a welding material, there may be a lack of penetration. If insufficient penetration occurs, an unmelted portion is formed on the welded line that is the butt surface, and the strength of the welded joint is reduced.

  Therefore, in order to weld the austenitic steel having the above chemical composition without using a welding material, it is necessary to appropriately manage heat input. Specifically, if the heat input at the time of welding is managed in the range of 4.8 to 7 kJ / cm, insufficient melting and N scattering can be suppressed.

(B) When H ₂ is mixed with the shielding gas, the penetration depth increases, and welding is possible with a smaller heat input. Specifically, if the heat input Q is 7 kJ / cm or less and the following conditions are satisfied, insufficient melting can be suppressed.
Q ≧ −0.18 [H ₂ ] +4.8 (1)
However, the unit of Q is kJ / cm, and the mixing ratio of H _{2 in} the shielding gas is substituted into [H ₂ ] by volume%.

(C) When N ₂ is mixed in the shield gas, the scattering of N from the molten pool can be suppressed, and N can be supplemented into the weld metal. Therefore, the N content of the weld metal can be further increased, and a weld metal with higher strength can be obtained.

  Based on the above findings, the present invention has been completed. Hereinafter, the manufacturing method of the austenitic stainless steel welded joint by one Embodiment of this invention is explained in full detail.

  The manufacturing method of an austenitic stainless steel welded joint according to an embodiment of the present invention includes a step of preparing a first member and a second member, and a step of welding the first member and the second member by gas tungsten arc welding. Prepare.

[Step of preparing first member and second member]
In the following description, “%” of the element content means mass%. In the following description, the first member and the second member may be referred to as “base material” without being distinguished from each other.

[First member]
The first member has a chemical composition described below.

C: 0.005-0.1%
Carbon (C) is an element effective for stabilizing austenite. In order to sufficiently obtain this effect, the C content needs to be 0.005% or more. However, if the C content is too high, carbides are formed at the grain boundaries due to heating during welding, and the corrosion resistance and toughness of the steel are reduced. Therefore, the C content is 0.005 to 0.1%. The lower limit of the C content is preferably 0.008%. The upper limit of the C content is preferably 0.08%.

Si: 0.2-1.2%
Silicon (Si) is an element effective as a deoxidizer and an element effective in improving corrosion resistance. Si further contributes indirectly to increasing the solubility of N during the production of the base material and increasing the strength of the steel. If the Si content is less than 0.2%, this effect cannot be obtained sufficiently. However, if the Si content is too high, an intermetallic compound may be formed with Ni, Cr, etc., and hot workability may be significantly reduced. In addition, weld metal may segregate at the columnar crystal boundary during solidification to lower the melting point of the liquid layer and increase the susceptibility to solidification cracking. Therefore, the Si content is 0.2 to 1.2%. The lower limit of the Si content is preferably 0.25%. The upper limit of the Si content is preferably 1.0%.

Mn: 2.5 to 6.5%
Manganese (Mn) is an element effective as a deoxidizer and also an element effective for stabilizing austenite. Further, Mn increases the solubility of N during the production of the base material and contributes indirectly to increasing the strength. In order to sufficiently obtain this effect, the Mn content needs to be 2.5% or more. On the other hand, if the Mn content is too high, the toughness decreases. Therefore, the Mn content is 2.5 to 6.5%. The lower limit of the Mn content is preferably 2.7%. The upper limit of the Mn content is preferably 6%.

Ni: 8-15%
Nickel (Ni) is an essential element for obtaining stable austenite. In order to obtain this effect sufficiently, the Ni content needs to be 8% or more. However, since Ni is an expensive element, a large content causes an increase in cost. Moreover, when Ni content is too high, the solubility of N at the time of base material manufacture will become small. Therefore, the Ni content is 8 to 15%. The lower limit of the Ni content is preferably 9%. The upper limit of the N content is preferably 14.5%.

Cr: 19-25%
Chromium (Cr) is an essential element for ensuring corrosion resistance under the usage environment. Further, Cr indirectly contributes to increasing the solubility of N during the production of the base material and increasing the strength. In order to sufficiently obtain this effect, the Cr content needs to be 19% or more. On the other hand, if the Cr content is too high, austenite becomes unstable, and the steel becomes brittle depending on the type of gas contact environment. Therefore, the Cr content is 19 to 25%. The lower limit of the Cr content is preferably 19.2%. The upper limit of the Cr content is preferably 24.5%.

Mo: 0.01 to 4.5%
Molybdenum (Mo) dissolves in the matrix or precipitates as carbonitride to increase the strength of the steel. Mo is also an element effective for improving the corrosion resistance under the use environment. In order to sufficiently obtain this effect, the Mo content needs to be 0.01% or more. However, since Mo is an expensive element, a large content causes an increase in cost. Moreover, when the Mo content is too high, austenite becomes unstable. Therefore, the Mo content is 0.01 to 4.5%. The lower limit of the Mo content is preferably 1%. The upper limit of the Mo content is preferably 4%.

V: 0.01 to 0.5%
Vanadium (V) dissolves in the matrix or precipitates as carbonitride to increase the strength of the steel. In order to acquire this effect, it is necessary to make V content 0.01% or more. However, if the V content is too high, a large amount of carbonitride precipitates and ductility decreases. Therefore, the V content is 0.01 to 0.5%. The lower limit of the V content is preferably 0.05%. The upper limit of the V content is preferably 0.4%.

Nb: 0.01 to 0.5%
Niobium (Nb) dissolves in the matrix or precipitates as carbonitride to increase the strength of the steel. In order to obtain this effect, the Nb content needs to be 0.01% or more. However, if the Nb content is too high, a large amount of carbonitride precipitates and ductility is lowered. Therefore, the Nb content is 0.01 to 0.5%. The lower limit of the Nb content is preferably 0.05%. The upper limit of the Nb content is preferably 0.4%.

Al: Less than 0.05% Aluminum (Al) is contained as a deoxidizer, as is Si and Mn. However, if the Al content is too high, a large amount of nitride is formed and the ductility of the steel is reduced. Therefore, the Al content is less than 0.05%. The lower limit of the Al content is preferably 0.0003%. The upper limit of the Al content is preferably 0.04%.

N: 0.15-0.45%
Nitrogen (N) dissolves in the matrix and forms fine nitrides to increase the strength of the steel. In order to sufficiently obtain this effect, the N content needs to be 0.15% or more. However, if the N content is too high, the hot workability at the time of production is lowered. In addition, welding defects such as blow holes may occur during welding. Therefore, the N content is 0.15 to 0.45%. The lower limit of the N content is preferably 0.16%. The upper limit of the N content is preferably 0.42%.

  The balance of the chemical composition of the first member is Fe and impurities. The impurity here refers to an element mixed from ore and scrap used as a raw material of steel, or an element mixed from the environment of the manufacturing process.

  Among impurities, the contents of O, P, and S are limited to the ranges described below.

O: 0.02% or less Oxygen (O) is contained as an impurity in the steel. When the O content is too high, the hot workability during production is lowered, and the toughness and ductility of the weld metal are lowered. Therefore, the O content is 0.02% or less. The O content is preferably 0.01% or less.

P: 0.05% or less Phosphorus (P) is contained as an impurity in the steel. When P content is too high, the hot workability at the time of manufacture will fall. The lower the P content, the better. However, the extreme reduction leads to an increase in manufacturing cost. Therefore, the P content is 0.05% or less. The P content is preferably 0.03% or less.

S: 0.04% or less Sulfur (S) is contained as an impurity in the steel. When S content is too high, the hot workability at the time of manufacture will fall. The lower the S content, the better. However, the extreme reduction leads to an increase in manufacturing cost. Therefore, the S content is 0.04% or less. The S content is preferably 0.03% or less.

  The chemical composition of the first member may further contain an element described below instead of a part of Fe. The elements (Ti, Cu, B) described below are all selective elements. That is, the chemical composition of the first member may not contain some or all of these elements.

Ti: 0 to 0.5%
Titanium (Ti) dissolves in the matrix or precipitates as carbonitride to increase the strength of the steel. However, if the Ti content is too high, a large amount of carbonitride precipitates and ductility decreases. Therefore, the Ti content is 0 to 0.5%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%. The upper limit of the Ti content is preferably 0.45%.

Cu: 0 to 3.0%
Copper (Cu) is an element effective for obtaining a stable austenite structure. However, if the Cu content is too high, the effect is saturated and the toughness is reduced. Therefore, the Cu content is 0 to 3.0%. The lower limit of the Cu content is preferably 0.005%, more preferably 0.008%, and still more preferably 0.01%. The upper limit of the Cu content is preferably 2.5%, more preferably 2%.

B: 0 to 0.01%
Boron (B) segregates at the grain boundary to increase the grain boundary fixing force and contributes to strength improvement. B also has an effect of suppressing embrittlement in a hydrogen environment. However, if the B content is too high, the cracking sensitivity during welding is increased when no welding material is used. Therefore, the B content is 0 to 0.01%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, and further preferably 0.0005%. The upper limit of the B content is preferably 0.008%, more preferably 0.005%.

The first member preferably has Nieq defined by the following formula of 30% or more.
Nieq = Ni + Mo + Mn + 0.6Cr + 0.3Si + 12 (C + N)
For each element symbol in the above formula, the content of each element of the first member is substituted by mass%.

  The higher the value of Nieq, the higher the stability of the austenite structure and the hydrogen embrittlement resistance. The Nieq of the first member is more preferably 32% or more.

[Second member]
The second member is an austenitic stainless steel containing C, Si, Mn, Ni, Cr, Mo and N, and Nieq defined by the following formula is 27% or more.
Nieq = Ni + Mo + Mn + 0.6Cr + 0.3Si + 12 (C + N)
For each element symbol in the above formula, the content of each element of the second member is substituted by mass%.

  In order to increase the hydrogen embrittlement resistance of the base material, it is necessary to increase the stability of the austenite structure and to suppress the formation of ferrite structure and martensite structure. In addition, it is necessary to increase the stacking fault energy and suppress the localization of dislocations. In order to obtain sufficient hydrogen embrittlement resistance, the Nieq of the second member needs to be 27% or more. Nieq of the second member is preferably 27.5% or more, and more preferably 28% or more.

  The second member preferably has a chemical composition described below.

C: 0.001 to 0.06%
Carbon (C) is an element effective for stabilizing austenite. In order to sufficiently obtain this effect, the C content needs to be 0.001% or more. However, if the C content is too high, carbides are formed at the grain boundaries due to heating during welding, and the corrosion resistance and toughness of the steel are reduced. Therefore, the C content is 0.001 to 0.06%. The lower limit of the C content is preferably 0.005%. The upper limit of the C content is preferably 0.05%.

Si: 0.01 to 1.0%
Silicon (Si) is an element effective as a deoxidizer and an element effective in improving corrosion resistance. If the Si content is less than 0.01%, this effect cannot be obtained sufficiently. However, if the Si content is too high, an intermetallic compound may be formed with Ni, Cr, etc., and hot workability may be significantly reduced. In addition, weld metal may segregate at the columnar crystal boundary during solidification to lower the melting point of the liquid layer and increase the susceptibility to solidification cracking. Therefore, the Si content is 0.01 to 1.0%. The lower limit of the Si content is preferably 0.03%. The upper limit of the Si content is preferably 0.9%.

Mn: 0.01 to 6.0%
Manganese (Mn) is an element effective as a deoxidizer and also an element effective for stabilizing austenite. In order to sufficiently obtain this effect, the Mn content needs to be 0.01% or more. On the other hand, if the Mn content is too high, the toughness decreases. Therefore, the Mn content is 0.01 to 6.0%. The upper limit of the Mn content is preferably 5.5%. Mn also has the effect of increasing the solubility of N. When N is actively used, the Mn content is preferably set to 2.0% or more. On the other hand, if N is not actively used, the Mn content may be 2.0% or less.

Ni: 8-15%
Nickel (Ni) is an essential element for obtaining stable austenite. In order to obtain this effect sufficiently, the Ni content needs to be 8% or more. However, since Ni is an expensive element, a large content causes an increase in cost. Moreover, when Ni content is too high, the solubility of N at the time of base material manufacture will become small. Therefore, the Ni content is 8 to 15%. The lower limit of the Ni content is preferably 9%. The upper limit of the N content is preferably 14.5%.

Cr: 15-25%
Chromium (Cr) is an essential element for ensuring corrosion resistance under the usage environment. In order to sufficiently obtain this effect, the Cr content needs to be 15% or more. On the other hand, if the Cr content is too high, austenite becomes unstable, and the steel becomes brittle depending on the type of gas contact environment. Therefore, the Cr content is 15 to 25%. The lower limit of the Cr content is preferably 16%. The upper limit of the Cr content is preferably 24.5%.

Mo: 0.01-4.0%
Molybdenum (Mo) dissolves in the matrix or precipitates as carbonitride to increase the strength of the steel. Mo is also an element effective for improving the corrosion resistance under the use environment. In order to sufficiently obtain this effect, the Mo content needs to be 0.01% or more. However, since Mo is an expensive element, a large content causes an increase in cost. Moreover, when the Mo content is too high, austenite becomes unstable. Therefore, the Mo content is 0.01 to 4.0%. The lower limit of the Mo content is preferably 1%. The upper limit of the Mo content is preferably 3.5%.

Al: Less than 0.05% Aluminum (Al) is contained as a deoxidizer, as is Si and Mn. However, if the Al content is too high, a large amount of nitride is formed and the ductility of the steel is reduced. Therefore, the Al content is less than 0.05%. The lower limit of the Al content is preferably 0.0003%. The upper limit of the Al content is preferably 0.04%.

N: 0.001% or more and less than 0.2% Nitrogen (N) dissolves in the matrix and forms fine nitrides to increase the strength of the steel. On the other hand, if the N content is too high, the hot workability at the time of production decreases. In addition, welding defects such as blow holes may occur during welding. Therefore, the N content is 0.001% or more and less than 0.2%. The lower limit of the N content is preferably 0.003%. The upper limit of the N content is preferably 0.18%.

  The balance of the chemical composition of the second member is Fe and impurities. The impurity here refers to an element mixed from ore and scrap used as a raw material of steel, or an element mixed from the environment of the manufacturing process.

  Among impurities, the contents of O, P, and S are limited to the ranges described below.

O: 0.02% or less Oxygen (O) is contained as an impurity in the steel. When the O content is too high, the hot workability during production is lowered, and the toughness and ductility of the weld metal are lowered. Therefore, the O content is 0.02% or less. The O content is preferably 0.01% or less.

P: 0.05% or less Phosphorus (P) is contained as an impurity in the steel. When P content is too high, the hot workability at the time of manufacture will fall. The lower the P content, the better. However, the extreme reduction leads to an increase in manufacturing cost. Therefore, the P content is 0.05% or less. The P content is preferably 0.03% or less.

S: 0.04% or less Sulfur (S) is contained as an impurity in the steel. When S content is too high, the hot workability at the time of manufacture will fall. The lower the S content, the better. However, the extreme reduction leads to an increase in manufacturing cost. Therefore, the S content is 0.04% or less. The S content is preferably 0.03% or less.

  The chemical composition of the second member may further contain an element described below instead of a part of Fe. The elements described below (V, Nb, Ti, Cu, B, Ca, Mg, and REM) are all selective elements. That is, the chemical composition of the second member may not contain some or all of these elements. In addition, when 2 or more types of elements are contained among Ti, Cu, B, Ca, Mg, and REM, it is preferable to make the total content 4.6% or less.

V: 0 to 0.5%
Vanadium (V) dissolves in the matrix or precipitates as carbonitride to increase the strength of the steel. However, if the V content is too high, a large amount of carbonitride precipitates and ductility decreases. Therefore, the V content is 0 to 0.5%. The lower limit of the V content is preferably 0.01%. The upper limit of the V content is preferably 0.4%. As described above, since the second member does not necessarily have high strength, the V content may be 0.01% or less.

Nb: 0 to 0.5%
Niobium (Nb) dissolves in the matrix or precipitates as carbonitride to increase the strength of the steel. However, if the Nb content is too high, a large amount of carbonitride precipitates and ductility is lowered. Therefore, the Nb content is 0 to 0.5%. The lower limit of the Nb content is preferably 0.01%. The upper limit of the Nb content is preferably 0.4%. As described above, since the second member does not necessarily have high strength, the Nb content may be 0.01% or less.

Ti: 0 to 0.5%
Titanium (Ti) dissolves in the matrix or precipitates as carbonitride to increase the strength of the steel. However, if the Ti content is too high, a large amount of carbonitride precipitates and ductility decreases. Therefore, the Ti content is 0 to 0.5%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%. The upper limit of the Ti content is preferably 0.45%.

Cu: 0 to 3.0%
Copper (Cu) is an element effective for obtaining a stable austenite structure. However, if the Cu content is too high, the effect is saturated and the toughness is reduced. Therefore, the Cu content is 0 to 3.0%. The lower limit of the Cu content is preferably 0.005%, more preferably 0.008%, and still more preferably 0.01%. The upper limit of the Cu content is preferably 2.5%, more preferably 2%.

B: 0 to 0.01%
Boron (B) segregates at the grain boundary to increase the grain boundary fixing force and contributes to strength improvement. B also has an effect of suppressing embrittlement in a hydrogen environment. However, if the B content is too high, the cracking sensitivity during welding is increased when no welding material is used. Therefore, the B content is 0 to 0.01%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, and further preferably 0.0005%. The upper limit of the B content is preferably 0.008%, more preferably 0.005%.

Ca: 0 to 0.05%
Calcium (Ca) improves the hot workability of steel. However, if the Ca content is too high, it combines with O to deteriorate the cleanliness, and the hot workability is deteriorated. Therefore, the Ca content is 0 to 0.05%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.001%, and still more preferably 0.0015%. The upper limit of the Ca content is preferably 0.03%, more preferably 0.01%.

Mg: 0 to 0.05%
Magnesium (Mg) improves the hot workability of steel. However, if the Mg content is too high, it combines with O to deteriorate the cleanliness, and the hot workability is deteriorated. Therefore, the Mg content is 0 to 0.05%. The lower limit of the Mg content is preferably 0.0005%, more preferably 0.001%, and further preferably 0.0015%. The upper limit of the Mg content is preferably 0.03%, and more preferably 0.01%.

REM: 0 to 0.5%
Rare earth elements (REM) have a strong affinity with S and improve the hot workability of steel. However, if the REM content is too high, it combines with O to deteriorate the cleanliness, and the hot workability is deteriorated. Therefore, the REM content is 0 to 0.05%. The lower limit of the REM content is preferably 0.001%, more preferably 0.0015%, and still more preferably 0.002%. The upper limit of the REM content is preferably 0.45%, more preferably 0.4%.

  “REM” is a general term for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM indicates the total content of one or more elements in REM. Further, REM is generally contained in Misch metal. For this reason, for example, it may be contained in the form of misch metal so that the REM content falls within the above range.

[Gas tungsten arc welding]
The first member and the second member are welded by gas tongue ten arc welding without using a welding material. Gas tungsten arc welding is performed under the following conditions.

[Shielding gas]
In the gas tungsten arc welding in the present embodiment, any one of Ar gas, N ₂ gas, a mixed gas of Ar and N ₂ , a mixed gas of Ar and H _2, and a mixed gas of Ar, N ₂ and H ₂ Is used as a shielding gas.

When H ₂ is mixed with the shielding gas, the penetration depth increases, and welding is possible with a smaller heat input. By welding with a small heat input, scattering of N from the molten pool can be suppressed, and the strength of the welded joint can be improved. The higher the H ₂ mixing ratio, the greater the penetration depth. Therefore, the shielding gas preferably contains H _2. The mixing ratio of H ₂ is preferably 0.1 to 20% by volume. The lower limit of the mixing ratio of H ₂ is more preferably 1% by volume.

When N ₂ is mixed with the shielding gas, the scattering of N from the molten pool can be suppressed, and N can be supplemented into the weld metal. Therefore, the N content of the weld metal can be further increased, and a weld metal with higher strength can be obtained. For this reason, the shield gas preferably contains N ₂ . The mixing ratio of N ₂ is preferably 0.1 to 50% by volume. The lower limit of the mixing ratio of N ₂ is more preferably 0.5% by volume. The upper limit of the mixing ratio of N ₂ is preferably 40% by volume, and more preferably 30% by volume.

In the gas tungsten arc welding in the present embodiment, a back shield gas may be used. The component of the back shield gas is not particularly limited, and examples thereof include Ar, N ₂ , and a mixed gas thereof. In particular, it is preferable to use _{N 2.} By using N ₂ for the back shield gas, the strength of the weld metal can be stably improved.

[Heat input]
In the gas tungsten arc welding in the present embodiment, the heat input Q is 7 kJ / cm or less and satisfies the following formula.
Q ≧ −0.18 [H ₂ ] +4.8
However, in the above formula, the unit of Q is kJ / cm, and the mixing ratio of H _{2 in} the shielding gas is substituted for [H ₂ ] by volume%. When the shielding gas is Ar gas, N ₂ gas, or a mixed gas of Ar and N ₂ , 0 is substituted for [H ₂ ].

As described above, when H ₂ is mixed with the shielding gas, the penetration depth increases, and welding is possible with a smaller heat input. If the heat input Q is smaller than −0.18 [H ₂ ] +4.8, the penetration depth may be insufficient, and an unmelted part may occur on the weld line. On the other hand, when the heat input Q exceeds 7 kJ / cm, the melted portion becomes too large, and the soundness of the welded joint may be deteriorated, such as undercut or burnout.

The smaller the heat input Q is, the smaller the area of the molten pool becomes, and it is possible to suppress N scattering. Thereby, the N content of the weld metal can be increased, and the strength of the welded joint can be improved. Accordingly, the heat input Q is preferably smaller in the range larger than −0.18 [H ₂ ] +4.8. The heat input Q is preferably 6.5 kJ / cm or less, and more preferably 6.2 kJ / cm or less.

  A welded joint is manufactured by the above manufacturing method. The welded joint manufactured by this embodiment includes a base material and a weld metal formed by melting and solidifying the base material during welding.

  According to the manufacturing method of the present embodiment, it is possible to suppress insufficient penetration during welding. Therefore, the welded joint manufactured by the manufacturing method of the present embodiment has the necessary strength even when manufactured by one-time welding.

  In general, since the weld metal has a rapidly solidified structure, unlike a base material manufactured by performing a solution heat treatment or the like, ferrite generated at a high temperature may remain at room temperature in the solidification process. In a hydrogen environment, ferrite becomes brittle and becomes a starting point of fracture. In particular, when ferrite having an area ratio exceeding 20% is continuously present, it is connected and propagated to reduce the hydrogen embrittlement resistance of the weld metal.

  According to the manufacturing method of the present embodiment, the amount of ferrite of the weld metal can be stably reduced to 20% or less. Therefore, the welded joint manufactured by the manufacturing method of this embodiment has excellent hydrogen embrittlement resistance.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these examples.

  From an ingot in which the materials 1A to 1E having the chemical composition shown in Table 1 are melted and cast in the laboratory, the thickness is 2.5 mm, the width is 60 mm, and the length is 100 mm by hot forging, hot rolling, and heat treatment. A steel plate for welding base material (first member) was prepared. Note that “-” in Table 1 indicates that the content of the element is at the impurity level.

  Furthermore, from an ingot obtained by melting and casting materials 2A to 2D having chemical compositions shown in Table 2 by hot forging, hot rolling, and heat treatment, a plate thickness of 2.5 mm, a width of 60 mm, and a length A steel plate (second member) having a thickness of 100 mm was prepared. Note that “-” in Table 2 indicates that the content of the element is at the impurity level.

  The groove processing shown in FIG. 1 was performed on the end face parallel to the longitudinal direction of each steel sheet for welding base material. The numerical values in FIG. 1 are dimensions. Then, the end surface which gave the groove processing of the steel plate for two welding base materials was faced | matched on the conditions shown in Table 3, and it welded once by gas tungsten arc welding, without using a welding material.

[Tensile test]
A plate-like tensile test piece having a weld metal at the center of the parallel portion was produced from each weld joint. A tensile test was performed on the test piece at room temperature. In the tensile test, what was fractured by the weld metal was rejected, and what was fractured by the base material (what was fractured by the second member) was evaluated as acceptable.

[Low strain rate tensile test]
In order to evaluate the hydrogen embrittlement resistance under a high-pressure hydrogen environment, a low strain rate tensile test was performed on the welded joint evaluated as acceptable in the tensile test. A plate-shaped low strain rate tensile test piece having a weld metal at the center of the parallel part was produced from the welded joint evaluated as acceptable in the tensile test. Using this test piece, a low strain rate tensile test was performed in the atmosphere and in a high-pressure hydrogen environment of 85 MPa. The strain rate was 3 × 10 ⁻⁵ / sec. When the value of the squeeze squeeze in a high-pressure hydrogen environment is 80% or more of the value of the squeeze squeeze in the atmosphere, it is evaluated as acceptable. Then I evaluated. On the other hand, those that were less than 80% were evaluated as rejected.

  Table 4 shows the results of the tensile test and the low strain rate tensile test. In the column of “tensile test”, “◯” indicates pass and “x” indicates failure. In the “Low Strain Rate Tensile Test” column, “◎” indicates that the value of the fracture drawing in the high-pressure hydrogen environment is 90% or more in the atmosphere, and “◯” indicates 80%. It is less than 90%, and “x” indicates less than 80%. In the same column, “-” indicates that a low strain rate tensile test is not performed.

  The welded joints with test codes J2-4, 6-8, 10-19, 22-27, 29-31, 39, and 40 all passed both the tensile test and the low strain rate tensile test. Among these, the welded joints of test codes J2, 4, 6, 7, 10-19, 24-27, and 29-31 have the value of the squeeze squeeze in the high-pressure hydrogen environment of the squeeze squeeze value in the atmosphere. It was 90% or more, and particularly excellent hydrogen embrittlement resistance was exhibited. The welded joints with test numbers J3, 8, 22, 23, 39 and 40 had less than 90% of the fracture drawing value in the high-pressure hydrogen environment, although they were 80% or more of the fracture drawing value in the atmosphere. It was. This is presumably because the welding heat input was high, the N content in the weld metal was lowered, and the Nieq of the weld metal was partially lowered.

  The welded joints of test codes J1, 5, and 9 did not pass the tensile test because an unmelted portion remained on the butt surface. This is presumably because the welding heat input was too low and the penetration depth was insufficient.

  The weld joints with test codes J20, 21, 28, and 32 did not pass the low strain rate tensile test. This is presumably because Nieq of the second member was too low.

  The welded joints with test codes J33 and 34 did not pass the tensile test. This is probably because the N content of the first member was too low.

  The welded joints with test codes J35 and 36 did not pass the low strain rate tensile test. This is presumably because the Cr content of the first member was too high or the Ni content was too low.

  The welded joints with test codes J37 and 38 did not pass the tensile test. This is presumably because the weld heat melted away because the welding heat input was high, and a sound weld was not formed.

  As mentioned above, although embodiment of this invention was described, embodiment mentioned above is only the illustration for implementing this invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  ADVANTAGE OF THE INVENTION According to this invention, the welded joint suitable for hydrogen facilities, such as a high pressure hydrogen gas apparatus and a liquid hydrogen apparatus, can be provided.

Claims (5)

Chemical composition is mass%, C: 0.005-0.1%, Si: 0.2-1.2%, Mn: 2.5-6.5%, Ni: 8-15%, Cr: 19-25%, Mo: 0.01-4.5%, V: 0.01-0.5%, Nb: 0.01-0.5%, Al: less than 0.05%, N: 0.00. 15 to 0.45%, Ti: 0 to 0.5%, Cu: 0 to 3.0%, B: 0 to 0.01%, balance: Fe and impurities, O, P as the impurities, Preparing a first member in which S is O: 0.02% or less, P: 0.05% or less, and S: 0.04% or less,
A step of preparing a second member which is an austenitic stainless steel containing C, Si, Mn, Ni, Cr, Mo and N, and whose Nieq defined by the formula (1) is 27% or more;
Any of Ar gas, N ₂ gas, mixed gas of Ar and N ₂ , mixed gas of Ar and H _2, and mixed gas of Ar, N ₂ and H ₂ is used as a shielding gas, and no welding material is used. And a step of welding the first member and the second member by gas tungsten arc welding,
The manufacturing method of the austenitic stainless steel welded joint whose heat input Q of the said gas tungsten arc welding is 7 kJ / cm or less, and satisfy | fills following formula (2).
Nieq = Ni + Mo + Mn + 0.6Cr + 0.3Si + 12 (C + N) (1)
Q ≧ −0.18 [H ₂ ] +4.8 (2)
However, the content of each element of the second member is substituted by mass% for each element symbol in the formula (1). In the formula (2), the unit of Q is kJ / cm, and the mixing ratio of H _{2 in} the shielding gas is substituted by volume% for [H ₂ ]. It is a manufacturing method of the austenitic stainless steel welded joint according to claim 1,
The chemical composition of the second member is% by mass, C: 0.001 to 0.06%, Si: 0.01 to 1.0%, Mn: 0.01 to 6.0%, Ni: 8 to 15%, Cr: 15-25%, Mo: 0.01-4.0%, Al: less than 0.05%, N: 0.001% or more and less than 0.2%, V: 0 to 0.5% Nb: 0 to 0.5%, Ti: 0 to 0.5%, Cu: 0 to 3.0%, B: 0 to 0.01%, Ca: 0 to 0.05%, Mg: 0 to 0% 0.05%, REM: 0 to 0.5%, balance: Fe and impurities, O, P and S as the impurities are respectively O: 0.02% or less, P: 0.05% or less, And S: The manufacturing method of the austenitic stainless steel welded joint which is 0.04% or less. A method for producing an austenitic stainless steel welded joint according to claim 1 or 2,
The shielding gas, by _volume% H 2: containing 0.1% to 20%, the production method of the austenitic stainless steel welded joints. It is a manufacturing method of the austenitic stainless steel welded joint according to any one of claims 1 to 3,
The shielding gas, by _volume% N 2: containing from 0.1 to 50% manufacturing method of austenitic stainless steel welded joints. It is a manufacturing method of the austenitic stainless steel welded joint according to any one of claims 1 to 4,
The chemical composition of the first member is a method for manufacturing an austenitic stainless steel welded joint, wherein Nieq defined by the formula (3) is 30% or more.
Nieq = Ni + Mo + Mn + 0.6Cr + 0.3Si + 12 (C + N) (3)
However, the content of each element of the first member is substituted by mass% for each element symbol in the formula (3).

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