Welded joint and method for manufacturing the same

Abstract

The present invention provides a weld joint having high strength and excellent high temperature crack resistance and low temperature toughness. In the present invention, high Ni steel plates containing 6.5 to 10.0 mass % Ni are welded together to form a weld metal portion and joined to obtain a weld joint. The weld metal portion is made into the following weld metal portion composition: 13.0-25.0% Mn is contained in such a way that the austenite structure is still formed in the extremely low temperature region, Cr is limited to 3.8% or less in order to suppress the occurrence of high temperature cracks, and Ni: 1.0-12.0%, Mo: 0.1-5.0%, N: 0.080% or less, O: 0.100% or less is contained. On this basis, the weld metal portion strength is adjusted so that the yield strength BYS of the steel plate and the 0.2% proof stress WPS of the weld metal portion satisfy the following formula [WPS] ≤ [BYS] - 100MPa (1) and the tensile strength BTS of the steel plate and the tensile strength WTS of the weld metal portion satisfy the following formula [WTS] ≤ [BTS] + 100MPa (2). As a result, both the weld metal portion and the weld joint can maintain excellent low temperature toughness.

Description

Technical Field

[0001] The present invention relates to, for example, a welded structure such as a liquefied gas storage tank used in an extremely low temperature environment, and particularly to improving the strength and extremely low temperature toughness of a welded joint using a high-Ni steel plate containing 6.5 to 10.0 mass % Ni. Background Art

[0002] 9% Ni steel is mostly used in storage tanks for liquefied natural gas (LNG), liquid nitrogen, liquid oxygen, etc. In the welding of 9% Ni steel, a Ni-based alloy containing more than 50% Ni is generally used as a welding material. This is because when a welding material (eutectic welding material) composed of a composition similar to that of 9% Ni steel (eutectic welding material) is used as a welding material and welded, the low temperature toughness (extremely low temperature toughness) at an extremely low temperature of -196°C, which is equivalent to that of the 9% Ni steel base material, cannot be ensured in the welded joint portion in the welded state.

[0003] In contrast, for example, Patent Document 1 proposes a "flux-cored welding wire for welding 9% Ni steel". The welding wire described in Patent Document 1 is a flux-cored welding wire in which a flux is filled in a Ni-based alloy sheath, and the composition of the flux-cored welding wire is adjusted to contain, in mass % relative to the total mass of the welding wire, a total of Mn: 2.0 to 4.5%, Ni: 53 to 65%, Cr: 13 to 19%, Mo: 5 to 14%, Nb: 0.5 to 3.0%, Cu: 0.01 to 0.5%, Ti: 0.4 to 1.0%, C: 0.02% or less, Si: 0.2% or less, and in the flux, in mass % relative to the total mass of the welding wire, Ti oxide: T The total amount of iO2 conversion is 3.0-7.0%, the total amount of Si oxide: SiO2 conversion is 0.5-2.0%, the total amount of Zr oxide: ZrO2 conversion is 1.0-2.0%, the total amount of Al oxide: Al2O3 conversion is 0.01-0.1%, the total amount of Na oxide and K oxide: one or more of Na2O conversion and K2O conversion is 0.1-0.8%, CaO: 0.1-0.8%, the total amount of fluorine compound: F conversion is 0.1-1.0%, the total amount of slag forming agent composed of oxides and fluorine compounds: 6-12%. If this welding wire is used to make a 9% Ni steel weld joint, a weld metal with high strength and excellent toughness can be obtained, and a weld metal with excellent crack resistance and defect resistance such as water hole and excellent welding workability in all forms can be obtained.

[0004] In addition, Patent Document 2 describes a "welding material for low temperature steel". The welding material described in Patent Document 2 is a welding material having the following composition: a welding material in which the amount of Ni is reduced and austenite is stabilized by using Mn, and in the core wire or in the core wire and a part of its covering solution, C: 0.05-0.5%, Si: 0.15-0.75%, Mn: 20-50%, Cr: 4-17%, N: 0.005-0.5% are contained in the core wire weight ratio, and the remainder is composed of Fe and inevitable impurities. It should be noted that in addition to the above composition, W and Ta can be further contained up to 4%, or Ni and Mo can be contained up to 10%. As a result, the tensile properties of the weld metal and the impact toughness at a test temperature of -196°C are comparable to those of nickel alloys.

[0005] In addition, Patent Document 3 describes a "welding wire for flux-cored arc welding". The welding wire described in Patent Document 3 is characterized in that it contains, in terms of weight %, selected from C: 0.15-0.8%, Si: 0.2-1.2%, Mn: 15-34%, Cr: 6% or less, Mo: 1.5-4%, S: 0.02% or less, P: 0.02% or less, B: 0.01% or less, Ti: 0.09-0.5%, N: 0.001-0.3%, TiO2: 4-15%, a total of one or more selected from SiO2, ZrO2 and Al2O3: 0.01-9%, a total of one or more selected from K, Na and Li: 0.5-1.7%, one or more selected from F and Ca: 0.2-1.5%, and the remainder is Fe and other inevitable impurities. When welding a high-Mn steel material for ultra-low temperature as a welding base material using the welding wire described in Patent Document 3, a weld joint portion can be obtained which prevents high temperature cracking, has a high room temperature yield strength, and has excellent ultra-low temperature toughness.

[0006] In addition, "solid welding wire for gas metal arc welding" is described in Patent Document 4. The solid welding wire described in Patent Document 4 is a welding wire having a composition containing, by mass%, C: 0.2-0.8%, Si: 0.15-0.90%, Mn: 17.0-28.0%, P: 0.03% or less, S: 0.03% or less, Ni: 0.01-10.00%, Cr: 0.4-4.0%, Mo: 0.01-3.50%, B: less than 0.0010%, N: 0.12% or less, and the remainder being Fe and inevitable impurities. When steel plates having a composition of 0.5% C-0.4% Si-25% Mn-3% Cr-the remainder Fe are joined together by gas metal arc welding using the solid welding wire described in Patent Document 4, a high strength having a room temperature yield strength of 400 MPa or more and an absorbed energy vE at a test temperature of -196°C can be obtained.-196 The deposited metal has excellent extremely low temperature toughness of 28J or more.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2018-144045

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 49-052737

[0011] Patent Document 3: Japanese Patent Application No. 2017-502842

[0012] Patent Document 4: Japanese Patent No. 6621572 Summary of the invention

[0013] However, the welding material described in Patent Document 1 is a Ni-based alloy welding material containing more than 50% Ni, and has the problem of being expensive. In addition, according to the research of the present inventors, the high-Mn steel welding material described in Patent Document 2 has the problem of high-temperature cracking when made into a welded joint of 9% Ni steel. In addition, when the high-Mn steel welding wire described in Patent Documents 3 and 4 is applied to the welding of 9% Ni steel, it is found that the boundary between the base metal part and the weld metal part (hereinafter referred to as the weld joint (molten boundary)) is 1.5 Å, and the absorbed energy vE of the Charpy impact test at the test temperature: -196°C is 2.3 Å. -196 There is a problem that 27J cannot be ensured.

[0014] In order to solve the problems of the above-mentioned prior art, the present invention aims to provide a welded joint, which is a welded joint formed by welding high-Ni steel plates containing 6.5 to 10.0% Ni by mass, wherein the weld metal portion has excellent high-temperature crack resistance, high strength and excellent low-temperature toughness, and the low-temperature toughness of the welded joint is also excellent.

[0015] It should be noted that the "high strength" of the welded metal portion mentioned here refers to the case where the room temperature yield strength (0.2% yield strength) WYS is 400MPa or more and the room temperature tensile strength WTS is 660MPa or more. In addition, the "excellent low temperature toughness" of the welded metal portion and the welded joint here refers to the absorbed energy vE of the Charpy impact test at a test temperature of -196°C. -196 For cases above 27J.

[0016] In order to achieve the above-mentioned object, the inventors of the present invention firstly conducted in-depth studies on various important factors that cause high temperature cracking of a weld metal that contains 13% or more of Mn in the weld metal composition and becomes an austenite structure at extremely low temperatures. As a result, it was found that in order to suppress high temperature cracking of a weld metal that becomes an austenite structure by containing 13% or more of Mn, it is necessary to limit Cr to 4.0% or less of mass. It was also found that if the weld metal contains more than 4.0% of Cr, carbides (Cr 23 C6) precipitates at the austenite grain boundaries, which cause embrittlement, so that openings are caused by thermal deformation introduced during welding, resulting in high-temperature cracks.

[0017] In addition, we have repeatedly conducted in-depth research on various important factors that affect the low-temperature characteristics of welded joints when the weld metal becomes austenite at extremely low temperatures. In addition, we have observed the propagation path of damage for various Charpy impact test pieces tested at extremely low temperatures. As a result, we found that the absorbed energy vE -196 In a welded joint with excellent low-temperature toughness of 27 J or more, the absorbed energy vE is the energy required for the fracture to propagate in the weld metal. -196 In the welded joint with a reduced low-temperature toughness of less than 27 J, the damage propagates in the weld heat-affected zone. This is because: in the base metal portion of the high-Mn steel, the microstructure is constructed by heat treatment (quenching and tempering treatment), and the ultra-low temperature toughness is ensured by this heat treatment, but in the weld heat-affected zone, the microstructure is coarsened, so the ultra-low temperature toughness is reduced. On the other hand, in the weld metal portion containing 13% by mass or more of Mn and having an austenite structure, the excellent low-temperature toughness is maintained.

[0018] Based on the above, further studies were repeated, and as a result, it was found that in order to obtain a welded joint having excellent low-temperature toughness in which damage propagates not only in the weld heat affected zone of the weld joint but also in the weld metal portion, the 0.2% proof stress WPS of the weld metal portion and the yield strength BYS of the steel plate (base material portion) satisfy the following formula (1):

[0019] [WPS]≤[BYS]-100MPa……(1)

[0020] Here, WPS: 0.2% proof stress of weld metal (MPa), BYS: yield strength of steel plate (parent material) (MPa), and the tensile strength WTS of weld metal and the tensile strength BTS of steel plate (parent material) satisfy the following formula (2):

[0021] [WTS]≤[BTS]+100MPa……(2)

[0022] Here, it is effective to adjust the strength of the weld metal by setting the welding input heat to 0.5 to 6.0 kJ / mm so as to satisfy WTS: tensile strength of weld metal (MPa), BTS: tensile strength of steel plate (base material) (MPa).

[0023] The present invention has been completed after further investigation based on the above circumstances. The gist of the present invention is as follows.

[0024] [1] A welded joint characterized in that steel plates are welded to each other via a weld metal portion,

[0025] The above steel plate is made into a steel plate having the following steel plate composition:

[0026] That is, the steel plate composition is calculated by mass %, including:

[0027] C: 0.02~0.20%, Si: 0.05~0.50%,

[0028] Mn: 0.10-1.80%, P: 0.030% or less,

[0029] S: 0.030% or less, Ni: 6.5-10.0%,

[0030] N: 0.010% or less, O: 0.010% or less,

[0031] And, the remainder consists of Fe and inevitable impurities;

[0032] The weld metal portion has the following weld metal portion composition and tensile properties of 0.2% proof stress WPS: 400 MPa or more and tensile strength WTS: 660 MPa or more,

[0033] The weld metal portion comprises, in terms of mass %,

[0034] C: 0.10~0.80%, Si: 0.10~1.00%,

[0035] Mn: 13.0-25.0%, P: 0.030% or less,

[0036] S: 0.030% or less, Ni: 1.0-12.0%,

[0037] Cr: 0.4~3.8%, Mo: 0.1~5.0%,

[0038] N: 0.080% or less, O: 0.100% or less,

[0039] And, the remainder consists of Fe and inevitable impurities;

[0040] Furthermore, the yield strength BYS of the steel plate and the 0.2% proof stress WPS of the weld metal portion satisfy the following formula (1):

[0041] [WPS]≤[BYS]-100MPa……(1)

[0042] Among them, WPS: 0.2% proof stress of weld metal (MPa), BYS: yield strength of steel plate (MPa),

[0043] In addition, the tensile strength BTS of the steel plate and the tensile strength WTS of the weld metal portion satisfy the following formula (2):

[0044] [WTS]≤[BTS]+100MPa……(2)

[0045] Here, WTS is the tensile strength of the weld metal portion (MPa), and BTS is the tensile strength of the steel plate (MPa).

[0046] [2] The weld joint according to [1], characterized in that the weld metal portion composition is further composed of the following components in terms of mass % in addition to the above-mentioned weld metal portion composition:

[0047] One or more selected from the following (i) and (ii): (i) one or more of V: ​​1.0% or less, Ti: 1.0% or less, Nb: 1.0% or less, and W: 1.0% or less; and (ii) one or more of Cu: 2.0% or less, Al: 1.0% or less, Ca: 0.010% or less, and REM: 0.020% or less.

[0048] [3] The weld joint according to [1] or [2], characterized in that the steel plate composition is made to contain the following components in addition to the above steel plate composition in terms of mass %:

[0049] One or more selected from the following (iii) and (iv): (iii) one or more of Cu: less than 0.5%, Al: less than 0.1%, Cr: less than 1.0%, Mo: less than 1.0%, V: less than 0.2%, Nb: less than 0.2% and Ti: less than 0.2%; and (iv) one or more of B: less than 0.005%, Ca: less than 0.005% and REM: less than 0.020%.

[0050] [4] A method for producing a welded joint having excellent low temperature toughness, characterized in that the welded joint is obtained by forming a weld metal portion by gas metal arc welding steel plates using a solid welding wire.

[0051] The above steel plate is made into a steel plate having the following steel plate composition:

[0052] The steel plate composition contains, by mass%, the following:

[0053] C: 0.02~0.20%, Si: 0.05~0.50%,

[0054] Mn: 0.10-1.80%, P: 0.030% or less,

[0055] S: 0.030% or less, Ni: 6.5-10.0%,

[0056] N: 0.010% or less, O: 0.010% or less,

[0057] and the remainder consists of Fe and unavoidable impurities;

[0058] The solid welding wire is made into a welding wire having a welding wire composition containing the following components and the remainder consisting of Fe and inevitable impurities,

[0059] C: 0.15~1.00%, Si: 0.15~1.10%,

[0060] Mn: 17.0-30.0%, P: 0.030% or less,

[0061] S: 0.030% or less, Ni: 0.2-13.0%,

[0062] Cr: 0.4~3.8%, Mo: 0.1~5.0%,

[0063] N: 0.060% or less, O: 0.020% or less;

[0064] The gas metal arc welding conditions are adjusted so that the welding heat input is 0.5 to 6.0 kJ / mm, so that the weld metal portion has a weld metal portion composition containing the following components in mass % with the remainder being Fe and inevitable impurities, and tensile properties of 0.2% proof stress WPS: 400 MPa or more and tensile strength WTS: 660 MPa or more.

[0065] C: 0.10~0.80%, Si: 0.10~1.00%,

[0066] Mn: 13.0-25.0%, P: 0.030% or less,

[0067] S: 0.030% or less, Ni: 1.0-12.0%,

[0068] Cr: 0.4~3.8%, Mo: 0.1~5.0%,

[0069] N: 0.080% or less, O: 0.100% or less;

[0070] Furthermore, the relationship between the 0.2% proof stress WPS of the weld metal portion and the yield strength BYS of the steel plate satisfies the following formula (1):

[0071] [WPS]≤[BYS]-100MPa……(1)

[0072] Here, WPS: 0.2% proof stress of weld metal (MPa), BYS: yield strength of steel plate (MPa),

[0073] Furthermore, the relationship between the tensile strength WTS of the weld metal portion and the tensile strength BTS of the steel plate satisfies the following formula (2):

[0074] [WTS]≤[BTS]+100MPa……(2)

[0075] Here, the tensile strength of the weld metal portion (WTS) and the tensile strength of the steel plate (BTS) are satisfied.

[0076] [5] A method for manufacturing a welded joint according to [4], wherein the welding wire composition is prepared as follows: in addition to the above welding wire composition, it further contains, in terms of mass %, one or more selected from the following (v) and (vi): (v) one or more of V: ​​1.0% or less, Ti: 1.0% or less, Nb: 1.0% or less, and W: 1.0% or less; and (vi) one of Cu: 2.0% or less, Al: 1.0% or less, Ca: 0.010% or less, and REM: 0.020% or less. or two or more; and, the weld metal portion composition is made as follows: on the basis of the above weld metal portion composition, it further contains, in terms of mass %, one or more selected from the following (i) and (ii): (i) one or more of V: ​​1.0% or less, Ti: 1.0% or less, Nb: 1.0% or less, and W: 1.0% or less; and (ii) one or more of Cu: 2.0% or less, Al: 1.0% or less, Ca: 0.010% or less, and REM: 0.020% or less.

[0077] [6] The method for manufacturing a welded joint according to [4] or [5] is characterized in that the following steel plate composition is made: on the basis of the above steel plate composition, it further contains one or more selected from the following (iii) and (iv) in terms of mass %: (iii) Cu: less than 0.5%, Al: less than 0.1%, Cr: less than 1.0%, Mo: less than 1.0%, V: less than 0.2%, Nb: less than 0.2% and Ti: less than 0.2%; and (iv) B: less than 0.005%, Ca: less than 0.005% and REM: less than 0.020%.

[0078] Effects of the Invention

[0079] According to the present invention, it is possible to easily produce a welded joint having high strength and excellent low-temperature toughness without using expensive Ni-based welding materials and without causing high-temperature cracks, thereby achieving a special industrial effect.

[0080] In addition, according to the present invention, a welded joint having high strength and excellent low-temperature toughness can be provided. DETAILED DESCRIPTION

[0081] [Welding joints]

[0082] The welded joint of the present invention is a welded joint formed by welding high-Ni steel plates having a basic composition of C: 0.02-0.20%, Si: 0.05-0.50%, Mn: 0.10-1.80%, and Ni: 6.5-10.0% to each other via a weld metal portion. The high-Ni steel plates to be welded may be two or more. It should be noted that, hereinafter, the mass % in the composition is expressed only in %.

[0083] <High Ni Steel Plate>

[0084] The high Ni steel plate used as the base material (welded material) has a steel plate composition containing, by mass%, C: 0.02-0.20%, Si: 0.05-0.50%, Mn: 0.10-1.80%, P: 0.030%, S: 0.030% or less, Ni: 6.5-10.0%, N: 0.010% or less, O: 0.010% or less, and the remainder is Fe and inevitable impurities. It should be noted that, in addition to the above-mentioned components, alloy elements can be added as optional elements as needed. As the selected elements, one or more selected from Cu: 0.5% or less, Al: 0.1% or less, Cr: 1.0% or less, Mo: 1.0% or less, V: 0.2% or less, Nb: 0.2% or less and Ti: 0.2% or less, which can be expected to improve the strength and toughness, and / or one or more selected from B: 0.005% or less, Ca: 0.005% or less and REM: 0.020% or less, which can be expected to improve the toughness, can be exemplified.

[0085] <Welded metal part>

[0086] Furthermore, the weld metal portion of the weld joint of the present invention has a composition (weld metal portion composition) containing, by mass%, C: 0.10-0.80%, Si: 0.10-1.00%, Mn: 13.0-25.0%, P: 0.030% or less, S: 0.030% or less, Ni: 1.0-12.0%, Cr: 0.4-3.8%, Mo: 0.1-5.0%, N: 0.080% or less, O: 0.100% or less, and the remainder is Fe and inevitable impurities.

[0087] First, the reasons for limiting the composition of the weld metal portion will be described.

[0088] C: 0.10~0.80%

[0089] C is an element that stabilizes austenite and has the effect of increasing the strength of the weld metal by solid solution strengthening. In order to obtain such an effect, it is necessary to contain 0.10% or more. However, if it contains more than 0.80%, high temperature cracks during welding are likely to occur. Therefore, C is limited to the range of 0.10 to 0.80%. It should be noted that it is preferably 0.15% or more. And it is preferably 0.60% or less.

[0090] Si: 0.10~1.00%

[0091] Si stabilizes austenite by inhibiting the precipitation of carbides, thereby dissolving C in austenite. In order to obtain such an effect, it is necessary to contain more than 0.10%. However, if it contains more than 1.00%, Si segregates during solidification, generates a liquid phase at the interface of the solidification pool, and reduces the high temperature crack resistance. Therefore, Si is limited to the range of 0.10 to 1.00%. It should be noted that it is preferably more than 0.20%. And preferably less than 0.90%.

[0092] Mn: 13.0~25.0%

[0093] Mn is an element that stabilizes the austenite phase at low cost. In the present invention, it is necessary to contain 13.0% or more. When Mn is less than 13.0%, the stability of austenite is insufficient, so a hard martensite phase is generated in the weld metal, and the toughness is reduced. On the other hand, if Mn exceeds 25.0%, excessive Mn segregation occurs during solidification, resulting in high-temperature cracking. Therefore, Mn is limited to the range of 13.0 to 25.0%. It should be noted that it is preferably 15.0% or more. And it is preferably 22.0% or less.

[0094] P: 0.030% or less

[0095] P is an element that segregates at the crystal grain boundary and induces high temperature cracking. In the present invention, it is preferably reduced as much as possible. If it is 0.030% or less, it is acceptable. Therefore, P is limited to 0.030% or less. It should be noted that excessive reduction leads to an increase in refining costs. Therefore, P is preferably adjusted to 0.002% or more.

[0096] S: 0.030% or less

[0097] S is an element that segregates at the crystal grain boundary and induces high temperature cracking. In the present invention, it is preferably reduced as much as possible. If it is 0.030% or less, it is acceptable. Therefore, S is limited to 0.030% or less. It should be noted that excessive reduction leads to an increase in refining costs. Therefore, S is preferably adjusted to 0.001% or more.

[0098] Ni: 1.0~12.0%

[0099] Ni is an element that strengthens the austenite grain boundary, and suppresses the occurrence of high temperature cracks by controlling the embrittlement of the grain boundary. In order to obtain such an effect, it is necessary to contain more than 1.0%. In addition, Ni also has the effect of stabilizing the austenite phase. However, Ni is an expensive element, and containing more than 12.0% is economically disadvantageous. Therefore, Ni is limited to the range of 1.0 to 12.0%. It should be noted that it is preferably more than 2.0%. And preferably less than 11.0%.

[0100] Cr: 0.4~3.8%

[0101] Cr acts as an element that stabilizes the austenite phase at extremely low temperatures, and improves the low temperature toughness (extremely low temperature toughness) of the weld metal. In addition, Cr also has the effect of improving the strength of the weld metal. In addition, Cr effectively plays a role in reducing the solid-liquid coexistence temperature range and suppressing the occurrence of high temperature cracks. And Cr effectively plays a role in improving the corrosion resistance of the weld metal. In order to obtain such an effect, it is necessary to contain 0.4% or more. When Cr is less than 0.4%, the above effect cannot be ensured. On the other hand, if it contains more than 3.8%, Cr carbides precipitate at the grain boundaries, the grain boundaries become brittle, and the thermal deformation introduced during welding opens, causing high temperature cracks. Therefore, Cr is limited to the range of 0.4 to 3.8%. It should be noted that it is preferably 0.6% or more. And it is preferably 3.5% or less.

[0102] Mo: 0.1~5.0%

[0103] Mo is an element that strengthens the austenite grain boundary, and suppresses the occurrence of high temperature cracks by suppressing the embrittlement of the grain boundary. It should be noted that Mo also has the effect of improving the wear resistance by solidifying the weld metal. In order to obtain such an effect, it is necessary to contain 0.1% or more. On the other hand, if it contains more than 5.0%, the grains are too solidified, the grain boundaries become relatively weak, and high temperature cracks occur. Therefore, Mo is limited to the range of 0.1 to 5.0%. It should be noted that it is preferably 0.5% or more. And it is preferably 4.0% or less.

[0104] O: 0.100% or less

[0105] O (oxygen) is an element that is inevitably mixed in. In the weld metal, it forms Al-based oxides and Si-based oxides, which contribute to the suppression of the coarsening of the solidified structure. Such an effect becomes significant when it is contained at 0.003% or more, so it is preferably contained at 0.003% or more. When it is contained in a large amount exceeding 0.100%, the coarsening of the oxide is significant. Therefore, O (oxygen) is limited to 0.100% or less. It should be noted that it is preferably 0.003% or more. And it is preferably 0.060% or less.

[0106] N: 0.080% or less

[0107] N is an element that is inevitably mixed in. It effectively contributes to the improvement of the strength of the weld metal like C, stabilizes the austenite phase, and stably improves the extremely low temperature toughness. Such an effect becomes significant when it is contained at 0.003% or more, so it is preferably contained at 0.003% or more. However, if it is contained at more than 0.080%, nitrides are formed and the low temperature toughness decreases. Therefore, N is limited to 0.080% or less. It should be noted that it is more preferably 0.004% or more. And it is preferably 0.060% or less.

[0108] The above-mentioned components are basic components of the weld metal portion of the weld joint of the present invention. In the present invention, in addition to the above-mentioned basic components, as arbitrary optional components, one or more selected from V: 1.0% or less, Ti: 1.0% or less, Nb: 1.0% or less, and W: 1.0% or less and / or one or more selected from Cu: 2.0% or less, Al: 1.0% or less, Ca: 0.010% or less, and REM: 0.020% or less may be contained as needed.

[0109] One or more selected from V: 1.0% or less, Ti: 1.0% or less, Nb: 1.0% or less, and W: 1.0% or less

[0110] V, Ti, Nb, and W are all carbide-forming elements that precipitate fine carbides in austenite grains to contribute to increasing the strength of the weld metal. They are selected as necessary and one or more kinds may be contained.

[0111] V is a carbide-forming element, which precipitates fine carbides in austenite grains and helps improve the strength of the weld metal. In order to obtain such an effect, V is preferably contained in an amount of 0.001% or more. However, if it is contained in an amount exceeding 1.0%, excessive carbides become the starting point for the occurrence of fracture, and thus the low-temperature toughness decreases. Therefore, when V is contained, it is preferably limited to 1.0% or less. It should be noted that it is more preferably 0.002% or more. In addition, it is more preferably 0.600% or less.

[0112] In addition, Ti is a carbide-forming element like V, precipitates fine carbides, and contributes to improving the strength of the weld metal. In order to obtain such an effect, Ti is preferably contained in an amount of 0.001% or more. However, if contained in an amount exceeding 1.0%, excessive carbides become the starting point of fracture, thereby reducing low-temperature toughness. Therefore, when contained, Ti is preferably limited to 1.0% or less. It should be noted that it is more preferably 0.002% or more. In addition, it is more preferably 0.600% or less.

[0113] In addition, Nb is also a carbide-forming element like V and Ti, and precipitates fine carbides, thereby contributing to improving the strength of the weld metal. In order to obtain such an effect, Nb is preferably contained in an amount of 0.001% or more. However, if contained in an amount exceeding 1.0%, excessive carbides become the starting point for the occurrence of fracture, thereby reducing the low-temperature toughness. Therefore, when contained, Nb is preferably limited to 1.0% or less. It should be noted that it is more preferably 0.002% or more. In addition, it is more preferably 0.600% or less.

[0114] In addition, W is a carbide-forming element like V, Ti, and Nb, and precipitates fine carbides, which contributes to improving the strength of the weld metal. In order to obtain such an effect, W is preferably contained in an amount of 0.001% or more. However, if it is contained in an amount exceeding 1.0%, excessive carbides become the starting point for the occurrence of fracture, so the low-temperature toughness decreases. Therefore, when W is contained, it is preferably limited to 1.0% or less. It should be noted that it is more preferably 0.002% or more. In addition, it is more preferably 0.600% or less.

[0115] One or more selected from the group consisting of Cu: 2.0% or less, Al: 1.0% or less, Ca: 0.010% or less, and REM: 0.020% or less

[0116] Cu is an element that contributes to stabilization of austenite, Al is an element that functions as a deoxidizer, and Ca and REM are elements that contribute to suppression of high temperature cracking, and they may be selectively contained as necessary.

[0117] Cu: 2.0% or less

[0118] Cu is an element that stabilizes the austenite phase. In order to obtain such an effect, it is preferably contained at 0.01% or more. However, if it is contained in a large amount exceeding 2.0%, a low melting point liquid phase is generated at the austenite grain boundary, so high temperature cracking occurs. Therefore, when Cu is contained, it is preferably limited to 2.0% or less. It should be noted that it is more preferably 0.02% or more. In addition, it is more preferably 1.6% or less.

[0119] Al: 1.0% or less

[0120] Al acts as a deoxidizer, and has important functions of increasing the viscosity of the molten metal, stably maintaining the weld bead shape, and reducing the occurrence of spatter. In addition, Al reduces the solid-liquid coexistence temperature range, which helps to suppress the occurrence of high-temperature cracks in the weld metal. Such an effect is very significant when it is contained at 0.001% or more, so it is preferably contained at 0.001% or more. However, when it is contained at more than 1.0%, the viscosity of the molten metal becomes too high, and conversely, defects such as increased sputtering and poor fusion due to the non-diffusion of the weld bead will increase. Therefore, when contained, Al is preferably limited to a range of less than 1.0%. It should be noted that it is more preferably 0.002%. In addition, it is more preferably less than 0.8%.

[0121] One or two of Ca: 0.010% or less and REM: 0.020% or less

[0122] Ca and REM are both elements that contribute to suppressing high temperature cracks. Ca combines with S in the molten metal to form high melting point sulfide CaS, thereby suppressing high temperature cracks. This effect becomes significant when it is contained at 0.001% or more. On the other hand, if it is contained at more than 0.010%, the arc will interfere during welding, making it difficult to perform stable welding. Therefore, when Ca is contained, it is preferably limited to 0.010% or less. It should be noted that it is more preferably 0.002% or more. In addition, it is more preferably 0.008% or less.

[0123] REM is a strong deoxidizer and exists in the weld metal in the form of REM oxide. REM oxide becomes a nucleation site during solidification, thereby changing the solidification morphology of the weld metal and helping to suppress high-temperature cracks. This effect becomes significant when it is contained at 0.001% or more. However, if it is contained at more than 0.020%, the stability of the arc decreases. Therefore, when contained, REM is preferably limited to 0.020% or less. It should be noted that it is more preferably 0.002% or more. In addition, it is more preferably 0.016% or less.

[0124] The balance other than the above components is composed of Fe and inevitable impurities. Inevitable impurities include Bi, Sn, Sb, etc., and are permissible if the total amount is 0.2% or less.

[0125] The weld metal portion having the above-mentioned composition is a weld metal portion having high strength and excellent low-temperature toughness, with tensile properties of 0.2% proof stress WPS: 400 MPa or more and tensile strength WTS: 660 MPa or more.

[0126] The welding conditions are adjusted so that the weld metal composition is within the above range and the relationship between the 0.2% proof stress WPS of the weld metal and the yield strength BYS of the steel plate satisfies the following formula (1):

[0127] [WPS]≤[BYS]-100MPa……(1)

[0128] Furthermore, the relationship between the tensile strength WTS of the weld metal and the tensile strength BTS of the steel plate satisfies the following formula (2):

[0129] [WTS]≤[BTS]+100MPa……(2)

[0130] Specifically, the welding conditions were adjusted so that the welding heat input per weld pass was 0.5 to 6.0 kJ / mm.

[0131] It should be noted that if the weld metal portion satisfies both of the above (1) and (2), the fracture in the weld joint occurs on the weld metal portion side, showing high absorbed energy and becoming a weld joint portion excellent in low temperature toughness (ultra-low temperature toughness).

[0132] [Method for manufacturing welded joint]

[0133] Next, a preferred method for producing the welded joint of the present invention will be described.

[0134] First, two or more high-Ni steel plates having a desired plate thickness and the above-mentioned steel plate composition are prepared as the welded material. Then, the prepared steel plates are beveled so that a predetermined bevel shape is formed between them. The bevel shape to be formed does not need to be particularly limited, but as a welded structure, a common レ bevel, a V bevel, etc. can be exemplified.

[0135] Next, the steel plates with the beveled surfaces are joined together and welded using a welding material (solid wire) to form a multi-layer weld metal portion to produce a welded joint.

[0136] <Welding method>

[0137] The welding method used does not need to be particularly limited as long as it can form a weld metal portion (multi-layer) having desired characteristics. In order to form a weld metal portion and an adhesive portion having desired strength and excellent low-temperature toughness, multi-layer submerged gas metal arc welding with a heat input of 0.5 to 6.0 kJ / mm per weld pass is preferred. In addition, the welding material used does not need to be particularly limited as long as it can form the above-mentioned weld metal portion.

[0138] <Welding materials>

[0139] In addition, a preferred method for producing the welding material (solid wire) to be used will be described.

[0140] In the solid welding wire used in the present invention, any commonly used method for manufacturing solid welding wire can be applied.

[0141] The solid welding wire used is preferably a welding wire having a composition that includes C: 0.15 to 1.00%, Si: 0.15 to 1.10%, Mn: 17.0 to 30.0%, P: 0.030% or less, S: 0.030% or less, Ni: 0.2 to 13.0%, Cr: 0.4 to 3.8%, Mo: 0.1 to 5.0%, N: 0.060% or less, O: 0.02 0% or less as basic elements, or may further contain any alloying elements as one or more selected from V: 1.0% or less, Ti: 1.0% or less, Nb: 1.0% or less and W: 1.0% or less, and / or one or more selected from Cu: 2.0% or less, Al: 1.0% or less, Ca: 0.010% or less and REM: 0.020% or less, and the remainder is composed of Fe and inevitable impurities.

[0142] The molten steel having the composition of the welding wire is subjected to the following steps in sequence: a casting step of melting steel by a common melting method such as an electric furnace or a vacuum melting furnace, and casting into a mold of a specified shape to obtain a steel billet such as a steel block; a heating step of heating the obtained steel billet such as a steel block to a specified temperature; and a hot rolling step of hot rolling the heated steel billet to obtain a rod-shaped body of a specified shape. Preferably, the obtained rod-shaped body is then subjected to multiple cold rolling (cold wire drawing), and annealing is performed as needed to obtain a cold rolling step of a welding wire of a desired size. It should be noted that annealing is preferably performed at an annealing temperature of 1000 to 1200°C.

[0143] Hereinafter, the present invention will be further described based on examples.

[0144] Example

[0145] First, as a test plate, a high Ni steel plate having a plate thickness of 30 mm and having a steel plate composition shown in Table 1 was prepared. It should be noted that a No. 10 test piece specified in JIS Z 2241 taken from the center of the plate thickness of the steel plate was used to perform a tensile test at room temperature. The tensile properties (yield strength BYS, tensile strength BTS) of the obtained steel plate are collectively indicated in Table 1.

[0146] Next, a welding material (solid welding wire) is prepared.

[0147] Molten steel of the composition (wire composition) shown in Table 2 was melted in a vacuum melting furnace and cast to obtain a steel block (100 kgf). The obtained steel block was heated to 1200°C and then hot rolled to obtain a rod-shaped steel billet. Then, the obtained rod-shaped steel billet was further cold rolled (cold drawn) multiple times with annealing to obtain a 1.2 mmφ solid welding wire.

[0148] Next, the prepared test plate (high Ni steel plate: plate thickness 30 mm × width 150 mm × length 400 mm) was subjected to bevel processing in such a way that a bevel (bevel angle: 45°) could be formed. In addition, the obtained solid welding wire for welding was used as a welding material in the bevel, and gas metal arc welding was performed in a shielding gas to form a multi-layer weld metal portion to obtain a welded joint. It should be noted that the welding conditions are the conditions consisting of a current of 150 to 450 A (DCEP), a voltage of 20 to 40 V, a welding speed of 15 to 60 cm / min, a temperature between weld passes of 100 to 200°C, and a shielding gas of 80% Ar-20% CO2 in a downward form. It should be noted that in order to adjust the strength of the weld metal, the welding input heat of one weld pass was adjusted to a range of 0.5 to 6.0 kJ / mm in the present example (see Table 3). The air temperature during welding was 18°C ​​and the humidity was 40%.

[0149] A test piece for analysis was collected from the plate thickness and width of the weld metal portion of the obtained welded joint within the range of φ10 mm at the center position, and the chemical composition of the weld metal portion was analyzed by chemical analysis.

[0150] Next, the weld metal portion and weld heat affected zone of the obtained welded joint were observed using an optical microscope (magnification: 100 times) to check whether there was a weld crack (high temperature crack). If a crack was observed in the weld metal portion, the weld crack was evaluated as "presence", and if no crack was observed, the weld crack was evaluated as "absence".

[0151] In addition, from the plate thickness and the width center position of the weld metal portion of the obtained welded joint, a No. 14A tensile test piece (parallel portion diameter 12.5 mmφ) was taken based on the provisions of JIS Z 2241, and a tensile test was performed at room temperature to obtain the strength of the weld metal portion (0.2% proof stress WPS, tensile strength WTS). It should be noted that the tensile test was performed on 3 pieces each, and the average value was taken as the strength of the weld metal portion.

[0152] It should be noted that the right-hand side value and left-hand side value of equations (1) and (2) were calculated using the obtained 0.2% proof stress WPS and tensile strength WTS of the weld metal portion and the yield strength BYS and tensile strength BTS of the steel plate shown in Table 1, respectively, and the case where equations (1) and (2) were satisfied was set to "0", and the case where equations (1) and (2) were not satisfied was evaluated as "×".

[0153] In addition, a Charpy impact test piece (V-notch: 10 mm thick) was taken from the obtained welded joint, and a Charpy impact test was performed at a test temperature of -196°C according to the provisions of JIS Z 2242. The absorbed energy E at the test temperature of -196°C was calculated. -196(J) The test pieces for measuring the weld metal part and the weld joint part were set to three each, and the average values ​​were set as the absorbed energy E of the weld metal part and the weld joint part of the welded joint. -196 It should be noted that the two Charpy impact test pieces were selected in such a way that the center position of the thickness of the test piece was 7 mm from the steel plate surface in the plate thickness direction. In addition, the notch positions of the Charpy impact test pieces for measuring the weld metal part and the weld joint part were respectively taken at the center position in the width direction of the weld metal part and the weld joint part.

[0154] In addition, for the test piece in which the notch position in the Charpy impact test piece after the test is the welded joint, the center position of the thickness of the Charpy impact test piece was cut, and the cut surface was corroded with 2% sodium nitrate. The cut surface was observed using an optical microscope (magnification: 50 times) to observe whether the cracks generated from the notch propagated to any of the weld metal part and the heat affected zone (HAZ), and to investigate the damage propagation path from the notch.

[0155] The obtained results are shown in Table 4.

[0156]

[0157]

[0158]

[0159]

[0160]

[0161]

[0162] In the examples of the present invention, no weld cracks (high temperature cracks) were observed. In addition, in the examples of the present invention, the weld metal parts obtained all had a yield strength (0.2% yield strength) of 400 MPa or more at room temperature and a tensile strength of 660 MPa or more, and the absorbed energy vE of the Charpy impact test at a test temperature of -196°C was -196 The welded joint also has an absorption energy vE -196 The low-temperature toughness is excellent at 27 J or more. Therefore, the obtained welded joint can be said to be a welded joint having high strength and excellent low-temperature toughness.

[0163] On the other hand, in comparative examples that depart from the scope of the present invention, weld cracks occurred, or the strength of the weld metal portion was insufficient, or the low-temperature toughness of the weld metal portion was reduced, or the low-temperature toughness of the welded joint was reduced, and the desired weld joint could not be obtained.

[0164] In the comparative example where the 0.2% proof stress of the weld metal portion satisfies the formula (1) and / or the tensile strength of the weld metal portion does not satisfy the formula (2), the crack propagation path (crack propagation position) becomes the heat affected zone (HAZ portion), and the low temperature toughness decreases. When the 0.2% proof stress of the weld metal portion satisfies the formula (1) and the tensile strength of the weld metal portion satisfies the formula (2), the crack propagation path becomes the weld metal portion.

Claims

1. A welding joint, characterized in that: It is a welded joint where steel plates are welded together via a weld metal portion. The steel plate is made into a steel plate having the following steel plate composition: That is, the steel plate composition, in terms of mass %, contains: C: 0.02~0.20%, Si: 0.05~0.50%, Mn: 0.10-1.80%, P: 0.030% or less, S: 0.030% or less, Ni: 6.5-10.0%, N: 0.010% or less, O: 0.010% or less, and the remainder consists of Fe and unavoidable impurities; Furthermore, the weld metal portion has the following weld metal portion composition, and has tensile properties of 0.2% proof stress WPS: 400 MPa or more, and tensile strength WTS: 660 MPa or more, The weld metal portion contains, by mass%, C: 0.10~0.80%, Si: 0.10~1.00%, Mn: 13.0-25.0%, P: 0.030% or less, S: 0.030% or less, Ni: 1.0-12.0%, Cr: 0.4~3.8%, Mo: 0.1~5.0%, N: 0.080% or less, O: 0.100% or less, and the remainder consists of Fe and unavoidable impurities; Furthermore, the yield strength BYS of the steel plate and the 0.2% proof stress WPS of the weld metal portion satisfy the following formula (1), and the tensile strength BTS of the steel plate and the tensile strength WTS of the weld metal portion satisfy the following formula (2), [WPS]≤[BYS]-100MPa……(1) [WTS]≤[BTS]+100MPa……(2) Wherein, WPS: 0.2% proof stress of weld metal part, unit is MPa, WTS: tensile strength of weld metal part, unit is MPa, BYS: Yield strength of steel plate, in MPa, BTS: Tensile strength of steel plate, in MPa.

2. The welded joint according to claim 1, characterized in that: The weld metal composition is prepared as follows: in addition to the above weld metal composition, one or more selected from the following (i) and (ii) are further contained in terms of mass %: (i) V: 1.0% or less, Ti: 1.0% or less, Nb: 1.0% or less, and W: 1.0% or less; and (ii) one or more of Cu: 2.0% or less, Al: 1.0% or less, Ca: 0.010% or less, and REM: 0.020% or less.

3. The welded joint according to claim 1 or 2, characterized in that: The steel sheet composition is prepared as follows, namely, on the basis of the above steel sheet composition, further containing, in terms of mass%, one or more selected from the following (iii) and (iv): (iii) one or more of Cu: 0.5% or less, Al: 0.1% or less, Cr: 1.0% or less, Mo: 1.0% or less, V: 0.2% or less, Nb: 0.2% or less and Ti: 0.2% or less; and (iv) one or more of B: 0.005% or less, Ca: 0.005% or less and REM: 0.020% or less.

4. A method for manufacturing a welded joint having excellent low temperature toughness, characterized in that: Steel plates are welded together by gas metal arc welding using a solid wire to form a weld metal portion to obtain a weld joint. The steel plate is made into a steel plate having the following steel plate composition: That is, the steel plate composition, in terms of mass %, contains: C: 0.02~0.20%, Si: 0.05~0.50%, Mn: 0.10-1.80%, P: 0.030% or less, S: 0.030% or less, Ni: 6.5-10.0%, N: 0.010% or less, O: 0.010% or less, and the remainder consists of Fe and unavoidable impurities; Furthermore, the solid welding wire is made into a welding wire having the following welding wire composition: That is, the welding wire composition contains, by mass%, C: 0.15~1.00%, Si: 0.15~1.10%, Mn: 17.0-30.0%, P: 0.030% or less, S: 0.030% or less, Ni: 0.2-13.0%, Cr: 0.4~3.8%, Mo: 0.1~5.0%, N: 0.060% or less, O: 0.020% or less, and the remainder consists of Fe and unavoidable impurities; Furthermore, the welding conditions of the gas metal arc welding are adjusted to a welding heat input of 0.5 to 6.0 kJ / mm, so that the weld metal portion has the following weld metal portion composition and has tensile properties of 0.2% proof stress WPS: 400 MPa or more and tensile strength WTS: 660 MPa or more, The weld metal composition, in mass %, comprises: C: 0.10~0.80%, Si: 0.10~1.00%, Mn: 13.0-25.0%, P: 0.030% or less, S: 0.030% or less, Ni: 1.0-12.0%, Cr: 0.4~3.8%, Mo: 0.1~5.0%, N: 0.080% or less, O: 0.100% or less, and the remainder consists of Fe and unavoidable impurities; The relationship between the 0.2% proof stress WPS of the weld metal portion and the yield strength BYS of the steel plate satisfies the following formula (1), and the relationship between the tensile strength WTS of the weld metal portion and the tensile strength BTS of the steel plate satisfies the following formula (2), [WPS]≤[BYS]-100MPa……(1) [WTS]≤[BTS]+100MPa……(2) Wherein, WPS: 0.2% proof stress of weld metal part, unit is MPa, WTS: tensile strength of weld metal part, unit is MPa, BYS: Yield strength of steel plate, in MPa, BTS: Tensile strength of steel plate, in MPa.

5. The method for manufacturing a welded joint according to claim 4, characterized in that: The welding wire composition is made to contain, in addition to the above welding wire composition, one or more selected from the following (v) and (vi) in terms of mass %: (v) V: (vi) one or more of Cu: 2.0% or less, Al: 1.0% or less, Ca: 0.010% or less, and REM: 0.020% or less; and the weld metal composition is made to contain, in addition to the weld metal composition, one or more of the following (i) and (ii) in terms of mass %: (i) V: 1.0% or less, Ti: 1.0% or less, Nb: 1.0% or less, and W: 1.0% or less; and (ii) one or more of Cu: 2.0% or less, Al: 1.0% or less, Ca: 0.010% or less, and REM: 0.020% or less.

6. The method for manufacturing a welded joint according to claim 4 or 5, characterized in that: The steel plate composition is made to be further composed of one or more of the following (iii) and (iv) in terms of mass % on the basis of the above steel plate composition: (iii) one or more of Cu: less than 0.5%, Al: less than 0.1%, Cr: less than 1.0%, Mo: less than 1.0%, V: less than 0.2%, Nb: less than 0.2% and Ti: less than 0.2%; and (iv) one or more of B: less than 0.005%, Ca: less than 0.005% and REM: less than 0.020%.

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