Weld joint

Abstract

To provide a weld joint in which hot cracking is suppressed.SOLUTION: A weld joint comprising a plurality of steel materials and a weld metal that joins the steel materials, wherein the weld metal has a bead index HI represented by the following formula (1) of 75 or more and an FCC phase fraction of 70% or more. In the formula (1), BW represents the width (mm) of the final bead, BT represents the thickness (mm) of the final bead, and S represents the cross-sectional area (mm2) of the final bead. Formula 1: HI=5×BW+0.5×BT+0.2×S.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to weld joints. [Background technology]

[0002] In recent years, austenitic welding materials, for example, have been used as welding materials that can obtain weld metals with excellent low-temperature toughness. For example, Ni-based low-temperature steels containing 6 to 9% Ni are used in structures such as liquid hydrogen tanks, liquid carbon dioxide tanks, and LNG tanks that store hydrogen fuel, natural gas, etc., because they are required to ensure toughness at extremely low temperatures (for example, -196°C). Furthermore, austenitic welding materials that can obtain weld metals with excellent low-temperature toughness are used to weld these Ni-based low-temperature steels.

[0003] For example, Patent Document 1 discloses a stainless steel wire for gas-shielded arc welding in which the surface of a stainless steel core wire is covered with a nickel plating having a surface hardness of 300 to 500 Hv and a thickness of 0.2 to 4 μm, and the free expansion diameter of the coated wire is set in the range of 350 to 800 mm. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-18587 Summary of the Invention [Problem to be solved by the invention]

[0005] As mentioned above, austenitic welding materials with an fcc fraction of 70% or more have been used to obtain weld metal with excellent low-temperature toughness. However, austenitic welding materials have the property of being prone to hot cracking. Therefore, welded joints in which hot cracking is suppressed are desired.

[0006] From the above viewpoints, an object of the present disclosure is to provide a welded joint in which the occurrence of hot cracking is suppressed. [Means for solving the problem]

[0007] The means for solving the problem include the following aspects. <1> A welding method for joining steel materials to each other, comprising: The weld metal is a welded joint having a bead index HI, represented by the following formula 1, of 75 or more and an fcc fraction of 70% or more. Formula 1: HI=5×BW+0.5×BT+0.2×S (In the above formula 1, BW is the width of the final bead (mm), BT is the thickness of the final bead (mm), and S is the cross-sectional area of ​​the final bead (mm 2 ) respectively. <2> The chemical composition of the weld metal is expressed in mass % relative to the total mass of the weld metal, C: 0.05~0.80%, Si: 0.03 to 0.50% Mn: 0 to 30.0% P: 0~0.050%, S: 0~0.050%, Cu: 0-5.0% Ni: 0 to 30.0% Cr: 0 to 20.0%, Mo: 0-10.0% Nb: 0 to 5.00% V: 0~5.00%, Ta: 0 to 5.00% Hf: 0 to 5.00% Ti: 0~5.00%, Zr: 0~5.00%, Co: 0 to 1.00%, Pb: 0~1.00%, Sn: 0 to 1.00% W: 0~20.00%, Mg: 0-0.1% Al: 0 to 0.400%, Ca: 0-5.0% B: 0~0.5000%, REM: 0~0.500%, N: 0 to 0.5000%, and O: Contains 0.0010 to 0.1500% the balance being Fe and impurities, The total of the Mn content and the Ni content (Mn+Ni) is 5.0% or more, the mass ratio (Ni / Mn) of the Mn content to the Ni content is 0.10 or more; <1> The welded joint described in [Effects of the Invention]

[0008] According to the present disclosure, a welded joint in which the occurrence of hot cracking is suppressed is provided. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a schematic cross-sectional view showing a base material having a groove used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0010] An embodiment that is an example of the present disclosure will be described. In this specification, when a numerical range expressed using "to" is not preceded or followed by "greater than" or "less than," it means a range that includes these numerical values ​​as the lower and upper limits. When "to" is preceded or followed by "greater than" or "less than," it means a range that does not include these numerical values ​​as the lower or upper limit. In the present specification, the upper limit of a numerical range may be replaced by the upper limit of another numerical range, or may be replaced by a value shown in an example. The lower limit of a numerical range may be replaced by the lower limit of another numerical range, or may be replaced by a value shown in an example. Furthermore, regarding the content, "%" means "% by mass." The content (%) of "0 or more" means that the component is an optional component and does not need to be contained.

[0011] <Welded joints> A welded joint according to an embodiment of the present disclosure has a plurality of steel materials serving as base materials and a weld metal that joins the steel materials together. The weld metal has a bead index HI of 75 or more, as expressed by the following formula 1. The weld metal also has an fcc fraction of 70% or more. Formula 1: HI=5×BW+0.5×BT+0.2×S (In Equation 1, BW is the width of the final bead (mm), BT is the thickness of the final bead (mm), and S is the cross-sectional area of ​​the final bead (mm 2 ) respectively.

[0012] In this disclosure, the term "final bead" refers to the bead formed last when the weld metal is formed by multi-layer welding, and refers to the bead formed in one pass when the weld metal is formed by one pass welding.

[0013] The weld metal according to the embodiment of the present disclosure has the above-described configuration, which suppresses the occurrence of hot cracking. The weld metal according to the embodiment of the present disclosure was discovered based on the following findings.

[0014] Conventionally, austenitic welding materials with an fcc fraction of 70% or more have been used as welding materials that can produce weld metal with excellent low-temperature toughness. However, austenitic welding materials have the property of being prone to hot cracking. This is thought to be because austenitic welding materials solidify in the γ phase (face-centered cubic lattice fcc), which causes severe solidification segregation of P, S, C, and Si, etc., resulting in a decrease in the melting point of the liquid phase in the (γ phase + liquid phase) state, and tensile stress is applied to the liquid phase due to solidification shrinkage, causing hot cracking. Therefore, one possible way to prevent hot cracking is to adjust the type and content of chemical components in the weld metal. For example, fixing the C and S in the weld metal is thought to be effective, but this can result in the formation of precipitates, which can reduce toughness.

[0015] In response to this, the inventors focused on controlling the bead shape as a means of suppressing the occurrence of hot cracking. Hot cracking occurs due to thermal contraction of the weld metal. If the amount of displacement due to this thermal contraction is constant, the larger the bead, the smaller the amount of strain. Therefore, they found that the occurrence of hot cracking can be suppressed by controlling the width, thickness, and cross-sectional area of ​​the bead, that is, by setting the bead index HI expressed by the above-mentioned Equation 1 to a certain value or higher. Note that controlling the bead index HI expressed by Equation 1 suppresses the occurrence of hot cracking not only in weld metal formed using austenitic welding materials, but also in other weld metals.

[0016] Hereinafter, welded joints according to embodiments of the present disclosure will be specifically described.

[0017] (Bead index HI) The weld metal of the welded joint according to the embodiment of the present disclosure has a bead index HI, represented by the following formula 1, of 75 or more. Formula 1: HI=5×BW+0.5×BT+0.2×S (In Equation 1, BW is the width of the final bead (mm), BT is the thickness of the final bead (mm), and S is the cross-sectional area of ​​the final bead (mm 2 ) respectively.

[0018] Hot cracking occurs due to thermal contraction of the weld metal, and hot cracking can be suppressed by controlling the width, thickness, and cross-sectional area of ​​the bead. Therefore, a bead index HI of 75 or more suppresses hot cracking.

[0019] In addition, the coefficients for "BW", "BT", and "S" in Equation 1 represent the degree to which the width BW, thickness BT, and cross-sectional area S contribute to the suppression of hot cracking, and were determined by experiment.

[0020] From the viewpoint of further suppressing the occurrence of hot cracking, the bead index HI is preferably 85 or more, and more preferably 95 or more. On the other hand, the upper limit of the bead index HI is not particularly limited, but from the viewpoint of ensuring the toughness of the weld heat affected zone, it is preferably 300 or less, and more preferably 200 or less.

[0021] The width (BW), thickness (BT), and cross-sectional area (S) of the final bead are controlled by the welding current, voltage, speed, waveform, groove shape, and the presence and degree of weaving (movement of the welding wire sideways relative to the welding line direction).

[0022] Here, a method for measuring the width BW, thickness BT, and cross-sectional area S of the final bead will be described. In this disclosure, the term "final bead" refers to the bead formed last when the weld metal is formed by multi-layer welding, or to the bead formed in one pass when the weld metal is formed by one pass welding.

[0023] The cross section of the final bead of the weld metal perpendicular to the weld line direction is observed at any five points, and the length of the longest part in the width direction, the length of the longest part in the thickness direction, and the cross-sectional area of ​​the cross section are determined. The arithmetic mean values ​​of the five cross sections are then defined as the width BW, thickness BT, and cross-sectional area S.

[0024] (FCC fraction) In a welded joint according to an embodiment of the present disclosure, the fcc fraction (volume %) in the weld metal is 70% or more. A high proportion of austenite in the structure of the weld metal enhances the low-temperature toughness of the weld metal. The fcc fraction is preferably 80% or more by volume, more preferably 90% or more by volume, and may even be 100% by volume. The remainder of the structure is bcc.

[0025] The fcc fraction in the structure of the weld metal can be determined by the following method. A sample is taken from the weld metal, and the bcc fraction (volume %) is measured on the sample surface using a FERITSCOPE (registered trademark) FMP30 (manufactured by Fischer Instruments Inc.) with a Fischer Instruments Inc. probe (FGAB 1.3-Fe) by magnetic induction method, and the arithmetic mean of the measured bcc fraction is calculated. The obtained mean bcc fraction is used to calculate the fcc fraction (volume %) in the weld metal structure using the following formula. fcc fraction=100-bcc fraction

[0026] (Chemical composition of weld metal) In the weld joint according to the embodiment of the present disclosure, the chemical composition of the weld metal is preferably as shown below. Note that in the description of the chemical composition of the weld metal, "%" means "mass % relative to the total mass of the weld metal" unless otherwise specified.

[0027] The chemical composition of the weld metal is C: 0.05~0.80%, Si: 0.03 to 0.50% Mn: 0 to 30.0% P: 0~0.050%, S: 0~0.050%, Cu: 0-5.0% Ni: 0 to 30.0% Cr: 0 to 20.0%, Mo: 0-10.0% Nb: 0 to 5.00% V: 0~5.00%, Ta: 0 to 5.00% Hf: 0 to 5.00% Ti: 0~5.00%, Zr: 0~5.00%, Co: 0 to 1.00%, Pb: 0~1.00%, Sn: 0 to 1.00% W: 0~20.00%, Mg: 0-0.1% Al: 0 to 0.400%, Ca: 0-5.0% B: 0~0.5000%, REM: 0~0.500%, N: 0 to 0.5000%, and O: Contains 0.0010 to 0.1500% the balance being Fe and impurities, The total content of Mn and Ni (Mn + Ni) is 5.0% or more, It is preferable that the mass ratio of the Mn content to the Ni content (Ni / Mn) is 0.10 or more.

[0028] (C: 0.05 to 0.80%) C is an element that improves the strength of the weld metal and ensures the strength of the weld metal. On the other hand, if the C content of the weld metal is excessive, the increase in strength of the weld metal has a large effect of deteriorating toughness, and the low-temperature toughness of the weld metal decreases. Therefore, the C content of the weld metal is set to 0.05 to 0.80%. The lower limit of the C content of the weld metal may preferably be 0.07%, 0.10%, 0.15%, or 0.20%. The upper limit of the C content of the weld metal is preferably 0.75%, 0.70%, 0.65%, 0.60%, 0.55%, or 0.50%.

[0029] (Si: 0.03 to 0.50%) Silicon is a deoxidizing element. If the silicon content of the weld metal is too low, the oxygen content of the weld metal increases. On the other hand, Si has a low solid solubility in the austenite phase, and the greater the Si content, the more likely solidification segregation occurs, resulting in hot cracking. Therefore, the Si content in the weld metal is set to 0.03 to 0.50%. The lower limit of the Si content in the weld metal is preferably 0.04%, 0.05%, or 0.08%. The upper limit of the Si content in the weld metal is preferably 0.48%, 0.45%, 0.40%, 0.35%, 0.30%, or 0.20%.

[0030] (Mn: 0 to 30.0%) Mn is an austenite stabilizing element and contributes to improving the low-temperature toughness of the weld metal. Mn also functions as a deoxidizer and improves the cleanliness of the weld metal. Furthermore, Mn is an element that neutralizes S in the weld metal by forming MnS, thereby improving the low-temperature toughness of the weld metal. In addition, Mn has the effect of preventing hot cracking. Therefore, Mn may be contained in the weld metal. On the other hand, if the Mn content in the weld metal is excessive, it is likely to segregate in the weld metal, causing significant embrittlement in the segregated areas. Therefore, the Mn content of the weld metal is set to 0 to 30.0%. The lower limit of the Mn content in the weld metal is preferably 0.1%, 0.3%, 0.5%, 0.8%, 1.0%, 1.5%, or 2.0%. The upper limit of the Mn content in the weld metal is preferably 28.0%, 25.0%, 23.0%, or 20.0%.

[0031] (P: 0~0.050%) P is an impurity element that promotes hot cracking or reduces toughness, so it is preferable to reduce the P content of the weld metal as much as possible. Therefore, the lower limit of the P content of the weld metal is set to 0%. However, from the viewpoint of reducing the dephosphorization cost, the P content of the weld metal should be 0.003% or more. On the other hand, if the P content in the weld metal is 0.050% or less, the adverse effects of P are within an acceptable range. Therefore, the P content of the weld metal is set to 0 to 0.050%. In order to effectively suppress hot cracking or a decrease in toughness, the P content of the weld metal is preferably 0.040% or less, 0.030% or less, 0.020% or less, 0.015% or less, or 0.010% or less.

[0032] (S: 0~0.050%) S is an impurity element that promotes hot cracking or reduces toughness, so it is preferable to reduce the S content of the weld metal as much as possible. Therefore, the lower limit of the S content of the weld metal is set to 0%. However, from the perspective of reducing the cost of desulfurization, the S content of the weld metal should be 0.003% or more. On the other hand, if the S content in the weld metal is 0.050% or less, the adverse effect of S on toughness falls within an acceptable range. Therefore, the S content of the weld metal is set to 0 to 0.050%. In order to effectively suppress hot cracking or a decrease in toughness, the S content of the weld metal is preferably 0.040% or less, 0.030% or less, 0.020% or less, 0.015% or less, or 0.010% or less.

[0033] (Cu: 0-5.0%) Cu is a precipitation strengthening element and may be contained in the weld metal to improve the strength of the weld metal. Cu is also an austenite stabilizing element and may be contained in the weld metal to improve the low-temperature toughness of the weld metal. On the other hand, if the Cu content in the weld metal is excessive, the above effect becomes saturated. Therefore, the Cu content in the weld metal is set to 0 to 5.0%. The lower limit of the Cu content in the weld metal is preferably 0.3%, 0.5%, or 0.7%. The upper limit of the Cu content in the weld metal is preferably 4.5%, 4.0%, or 3.5%.

[0034] (Ni: 0 to 30.0%) Ni is an austenite stabilizing element and may be contained in the weld metal to improve the low-temperature toughness of the weld metal. On the other hand, increasing the Ni content of the weld metal increases the cost of the weld metal. Therefore, the Ni content of the weld metal is set to 0 to 30.0%. The lower limit of the Ni content in the weld metal is preferably 0.2%, 0.5%, 1.0%, or 2.0%. The upper limit of the Ni content in the weld metal is preferably 28.0%, 25.0%, 23.0%, or 20.0%.

[0035] (Cr: 0 to 20.0%) Cr is a ferrite stabilizing element and may be contained in the weld metal to improve the low-temperature toughness of the weld metal. On the other hand, if the Cr content of the weld metal is excessive, the solid-liquid coexistence temperature range of the molten metal will be widened, making hot cracking more likely to occur. Therefore, the Cr content of the weld metal is set to 0 to 20.0%. The lower limit of the Cr content in the weld metal is preferably 1.0%, 2.0%, or 3.0%. The upper limit of the Cr content in the weld metal is preferably 18.0%, 15.0%, 13.0%, or 10.0%.

[0036] (Mo: 0-10.0%) Mo is a precipitation strengthening element and may be contained in the weld metal to improve the strength of the weld metal. On the other hand, if the Mo content in the weld metal is excessive, the strength of the weld metal becomes excessive and the low-temperature toughness decreases. Therefore, the Mo content in the weld metal is set to 0 to 10.0%. The lower limit of the Mo content in the weld metal is preferably 1.0%, 2.0%, or 3.0%. The upper limit of the Mo content in the weld metal is preferably 9.0%, 8.0%, or 7.0%.

[0037] (Nb: 0 to 5.00%) Nb is an element that forms carbides in the weld metal and increases the strength of the weld metal, and therefore may be contained in the weld metal. On the other hand, if the Nb content in the weld metal is excessive, there is a concern that hot cracking may occur in the weld metal. Therefore, the Nb content in the weld metal is set to 0 to 5.00%. The lower limit of the Nb content in the weld metal is preferably 0.02%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the Nb content in the weld metal is preferably 4.50%, 4.00%, 3.50%, 3.00%, or 2.50%.

[0038] (V: 0~5.00%) V is an element that forms carbonitrides in the weld metal and increases the strength of the weld metal, and therefore may be contained in the weld metal. On the other hand, if the V content in the weld metal is excessive, hot cracking of the weld metal may occur. Therefore, the V content of the weld metal is set to 0 to 5.00%. The lower limit of the V content in the weld metal is preferably 0.02%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the V content of the weld metal is preferably 4.50%, 4.00%, 3.50%, or 3.00%.

[0039] (Ta: 0 to 5.00%) Ta is an element that forms carbides in the weld metal and increases the strength of the weld metal, and therefore may be contained in the weld metal. On the other hand, if the Ta content in the weld metal is excessive, a large amount of coarse carbonitrides will precipitate, which may actually lead to a decrease in the toughness of the weld metal. Therefore, the Ta content in the weld metal is set to 0 to 5.00%. The lower limit of the Ta content in the weld metal is preferably 0.01%, 0.03%, 0.05%, 0.01%, 0.02%, 0.03%, or 0.05%. The upper limit of the Ta content in the weld metal is preferably 4.50%, 4.00%, 3.50%, 3.00%, 2.00%, 1.00%, 0.50%, 0.20%, or 0.10%.

[0040] (Hf: 0 to 5.00%) Hf is an element that forms carbides in the weld metal and increases the strength of the weld metal, and therefore may be contained in the weld metal. On the other hand, if the Hf content in the weld metal is excessive, the toughness of the weld metal may be reduced. Therefore, the Hf content in the weld metal is set to 0 to 5.00%. The lower limit of the Hf content in the weld metal is preferably 0.01%, 0.02%, 0.05%, 0.01%, 0.02%, 0.03%, or 0.04%. The upper limit of the Hf content in the weld metal is preferably 4.50%, 4.00%, 3.50%, 3.00%, 2.00%, 1.00%, 0.50%, 0.20%, or 0.10%.

[0041] (Ti: 0 to 5.00%) Ti is a deoxidizing element and may be contained in the weld metal to suppress welding defects and improve the cleanliness of the weld metal. On the other hand, if the Ti content in the weld metal is excessive, carbides may form in the weld metal, which may deteriorate the toughness of the weld metal. Therefore, the Ti content of the weld metal is set to 0 to 5.00%. The lower limit of the Ti content in the weld metal is preferably 0.003%, 0.01%, 0.02%, or 0.03%. The upper limit of the Ti content in the weld metal is preferably 4.50%, 4.00%, 3.50%, or 3.00%, 2.00%, or 1.50%.

[0042] (Zr: 0 to 5.00%) Zr can stabilize the bead shape during the welding operation to obtain the weld metal, so it may be contained in the weld metal. On the other hand, if the Zr content in the weld metal is excessive, the oxygen content in the weld metal increases, which may deteriorate the low-temperature toughness. Therefore, the Zr content of the weld metal is set to 0 to 5.00%. The lower limit of the Zr content in the weld metal is preferably 0.003%, 0.01%, 0.02%, or 0.03%. The upper limit of the Zr content in the weld metal is preferably 4.50%, 4.00%, 3.50%, or 3.00%, 2.00%, 1.50%, or 1.00%.

[0043] (Co: 0-1.00%) Co is an element that increases the strength of the weld metal through solid solution strengthening, and therefore may be contained in the weld metal. On the other hand, if the Co content in the weld metal is excessive, the ductility of the weld metal decreases and the toughness cannot be ensured. Therefore, the Co content of the weld metal is set to 0 to 1.00%. The lower limit of the Co content in the weld metal is preferably 0.01%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the Co content in the weld metal is preferably 0.95%, 0.90%, 0.85%, or 0.80%.

[0044] (Pb: 0 to 1.00%) Pb has the effect of improving the toe formability between the base steel material and the weld metal and improving the machinability of the weld metal, and therefore may be contained in the weld metal. On the other hand, if the Pb content in the weld metal is excessive, hot cracking occurs. Therefore, the Pb content in the weld metal is set to 0 to 1.00%. The lower limit of the Pb content in the weld metal is preferably 0.01%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the Pb content in the weld metal is preferably 0.95%, 0.90%, 0.85%, or 0.80%.

[0045] (Sn: 0 to 1.00%) Sn is an element that improves the corrosion resistance of the weld metal, and therefore may be contained in the weld metal. On the other hand, if the Sn content in the weld metal is excessive, there is a concern that cracks may occur in the weld metal. Therefore, the Sn content of the weld metal is set to 0 to 1.00%. The lower limit of the Sn content in the weld metal is preferably 0.01%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the Sn content in the weld metal is preferably 0.95%, 0.90%, 0.85%, or 0.80%.

[0046] (W: 0~20.00%) W is a solid solution strengthening element and may be contained in the weld metal to improve strength. On the other hand, if the W content in the weld metal is excessive, the strength of the weld metal becomes excessive, which may result in a decrease in toughness. Therefore, the W content of the weld metal is set to 0 to 20.00%. The lower limit of the W content in the weld metal is preferably 0.02%, 0.05%, 0.10%, 0.15%, or 0.20%. The upper limit of the W content of the weld metal is preferably 18.00%, 15.00%, 13.00%, or 10.00%.

[0047] (Mg: 0-0.1%) Mg is a deoxidizing element that is effective in reducing oxygen and improving toughness, and therefore may be contained in the weld metal. On the other hand, if the Mg content in the weld metal is excessive, the arc becomes unstable during the welding operation to obtain the weld metal, increasing spatter and blowholes, and deteriorating the welding workability. Therefore, the Mg content in the weld metal is set to 0 to 0.1%. The lower limit of the Mg content in the weld metal is preferably 0.01%, 0.02%, or 0.03%. The upper limit of the Mg content in the weld metal is preferably 0.09%, 0.08%, or 0.07%.

[0048] (Al: 0 to 0.400%) Al is a deoxidizing element and may be contained in the weld metal to suppress welding defects and improve the cleanliness of the weld metal. On the other hand, if the Al content in the weld metal is excessive, Al may form nitrides or oxides in the weld metal, which may reduce the low-temperature toughness of the weld metal. Therefore, the Al content of the weld metal is set to 0 to 0.400%. The lower limit of the Al content in the weld metal is preferably 0.001%, 0.005%, 0.010%, 0.020%, or 0.030%. The upper limit of the Al content of the weld metal is preferably 0.350%, 0.300%, or 0.200%.

[0049] (Ca: 0-5.0%) Ca changes the structure of sulfides in the weld metal and also has the effect of reducing the size of sulfides and oxides in the weld metal, and is therefore effective in improving the ductility and toughness of the weld metal, so Ca may be added to the weld metal. On the other hand, if the Ca content in the weld metal is excessive, coarsening of sulfides and oxides occurs, which may lead to deterioration of the low-temperature toughness of the weld metal. Therefore, the Ca content in the weld metal is set to 0 to 5.0%. The lower limit of the Ca content in the weld metal is preferably 0.1%, 0.2%, or 0.3%. The upper limit of the Ca content in the weld metal is preferably 4.8%, 4.5%, 4.3%, 4.0%, 3.0%, 2.0%, or 1.0%.

[0050] (B: 0~0.5000%) B is an austenite stabilizing element and an interstitial solid solution strengthening element, and may be contained in the weld metal to improve the low temperature toughness and strength of the weld metal. On the other hand, if the B content of the weld metal is excessive, M 23 (C,B)6 precipitates, causing a deterioration in toughness. Therefore, the B content of the weld metal is set to 0 to 0.5000%. The lower limit of the B content in the weld metal is preferably 0.0005%, 0.0010%, or 0.0020%. The upper limit of the B content of the weld metal is preferably 0.4800%, 0.4500%, 0.4300%, 0.4000%, 0.3000%, 0.2000%, or 0.1000%.

[0051] (REM: 0 to 0.500%) REM is an element that stabilizes the arc during welding work to obtain the weld metal, and therefore may be contained in the weld metal. On the other hand, if the REM content in the weld metal is excessive, spattering will become severe during the welding operation to obtain the weld metal, which may result in poor welding workability. Therefore, the REM content of the weld metal is set to 0 to 0.500%. The lower limit of the REM content of the weld metal is preferably 0.001%, 0.002%, or 0.005%. The upper limit of the REM content of the weld metal is preferably 0.480%, 0.450%, 0.430%, 0.400%, 0.300%, 0.200%, or 0.100%.

[0052] "REM" is a general term for 17 elements, including Sc, Y, and lanthanides, and the REM content refers to the total content of one or more REM elements. REM is generally contained in misch metal. For this reason, for example, it may be added in the form of misch metal so that the REM content falls within the above range. The same applies hereinafter.

[0053] (N: 0 to 0.5000%) N is an austenite stabilizing element and an interstitial solid solution strengthening element, and may be contained in the weld metal to improve the low temperature toughness and strength of the weld metal. On the other hand, if the N content in the weld metal is excessive, the occurrence of blowout increases, which causes welding defects. Therefore, the N content of the weld metal is set to 0 to 0.5000%. The lower limit of the N content in the weld metal is preferably 0.0002%, 0.0005%, 0.0010%, 0.0020%, or 0.0050%. The upper limit of the N content of the weld metal is preferably 0.4500%, 0.4000%, or 0.3500%, 0.3000%, 0.2000%, or 0.1000%.

[0054] (O: 0.0010~0.1500%) O is contained in the weld metal as an impurity. However, an excessive O content leads to deterioration of toughness and ductility, so the upper limit of the O content in the weld metal is set to 0.1500%. On the other hand, an extreme reduction in the O content leads to an increase in manufacturing costs, so the lower limit of the O content in the weld metal is set to 0.0010% or less. The lower limit of the O content in the weld metal is preferably 0.0020% or 0.0030%. The upper limit of the O content in the weld metal is preferably 0.1300% or 0.1000%.

[0055] (balance: Fe and impurities) The remaining components in the chemical composition of the weld metal are Fe and impurities. Impurities refer to components that are mixed in during industrial production of weld metal due to raw materials such as ores or scraps, or due to various factors in the production process, and are acceptable within a range that does not adversely affect the properties of the weld metal.

[0056] (Total of Mn and Ni contents (Mn + Ni)) Mn and Ni are austenite-stabilizing elements that improve the low-temperature toughness of the weld metal. Since Ni is an expensive metal, in order to improve the low-temperature toughness of the weld metal while suppressing the cost of the weld metal, it is preferable that the Mn and Ni contents in the weld metal each satisfy the above-mentioned ranges, and that the total of the Mn and Ni contents (Mn + Ni) be 5.0% or more. It is even more preferable that (Mn + Ni) be 8.0% or more, 10.0% or more, or 15.0% or more.

[0057] Moreover, if the Mn content is excessively increased, the stacking fault energy decreases and the toughness deteriorates. Therefore, from the viewpoint of suppressing the cost of the weld metal and improving the low-temperature toughness of the weld metal, it is preferable that the Mn content and Ni content in the weld metal each satisfy the above ranges, and that the total content of the Mn content and the Ni content (Mn + Ni) be 37.0% or less. The total content of Mn and Ni (Mn+Ni) in the weld metal is more preferably 35.0% or less, 32.0% or less, or 30.0% or less.

[0058] (Mass ratio of Mn content to Ni content (Ni / Mn)) Mn and Ni are austenite stabilizing elements that improve the low-temperature toughness of the weld metal. However, Ni is an expensive metal, and if Mn is added excessively, the stacking fault energy decreases, deteriorating the toughness. Therefore, from the viewpoint of improving the low-temperature toughness of the weld metal while suppressing the cost of the weld metal, it is preferable that the mass ratio of the Mn content to the Ni content (Ni / Mn) in the weld metal be 0.10 or more. The lower limit of the mass ratio (Ni / Mn) of the Mn content to the Ni content in the weld metal is more preferably 0.33, 0.50, 0.70, 1.00, 1.10, or 1.20. The upper limit of the mass ratio (Ni / Mn) of the Mn content to the Ni content in the weld metal is preferably 3.80, 3.50, 3.30, or 3.00.

[0059] (tensile strength) The tensile strength of the weld metal is preferably, for example, 590 to 1200 MPa. The tensile strength can be measured by conducting a tensile test on the weld metal in accordance with JIS Z3111:2005.

[0060] (Method for manufacturing welded joints (welding method)) A weld joint according to an embodiment of the present disclosure includes a plurality of base steel materials and a weld metal that joins the steel materials together. The region of the base steel adjacent to the weld metal is a heat-affected zone.

[0061] Here, a method for manufacturing a welded joint according to an embodiment of the present disclosure will be described. The manufacturing method described below is an example, and the method for manufacturing a welded joint according to an embodiment of the present disclosure is not limited to the following method.

[0062] A welded joint according to an embodiment of the present disclosure can be produced by welding a steel material, which serves as a base material, using a welding material.

[0063] For example, in a method for manufacturing a welded joint according to an embodiment of the present disclosure, a welded joint is obtained by gas-shielded arc welding of steel materials using a flux-cored wire. In this case, the chemical components of the weld metal include components derived from the flux-cored wire, which is the welding material, and the steel material, which is the base material.

[0064] Furthermore, the method for manufacturing a welded joint according to an embodiment of the present disclosure is achieved by submerged arc welding using a solid wire and flux. For example, in submerged arc welding, a typical submerged arc welding device can be used, in which granular flux is dispersed on the weld line in advance, the solid wire is fed into the flux, and welding is performed using arc heat generated between the solid wire and the steel material in the flux. In this case, the chemical components of the weld metal include components derived from the solid wire and flux, which are the welding materials, and the steel material, which is the base material.

[0065] Furthermore, the welded joint according to the embodiment of the present disclosure can be obtained by a welding method such as shielded metal arc welding, simple electrogas arc welding, electroslag welding, TIG welding, and gas-shielded welding using a solid wire, etc. In this case, the chemical components of the weld metal include components derived from the welding material and the steel base material.

[0066] The type of base material of the welded joint according to the embodiment of the present disclosure, i.e., the steel material (welded material) used in the manufacturing method of the above-mentioned welded joint, is not particularly limited, but for example, Ni-based low-temperature steel containing 6 to 9% Ni with a plate thickness of 20 mm or more can be suitably used. [Example]

[0067] Next, the feasibility and effects of the present disclosure will be explained in more detail using examples and comparative examples of the present disclosure. However, the following examples of the invention do not limit the present disclosure, and any design modifications that adhere to the above and below stated intent are all included in the technical scope of the present disclosure.

[0068] <1. Gas Metal Arc Welding (GMAW) using Flux-Cored Wire> Welded joints No. 1 and No. 2 shown in Table 1 were produced by gas shielded arc welding using FC-9NI (φ1.2 mm) manufactured by Nippon Steel Welding Industries Co., Ltd. as a flux-cored wire. A steel plate (size: 30 mm (t) × 120 mm (w) × 500 mm (l)) was used as the steel plate (base material) to be welded. As shown in Figure 1, a groove was formed in base materials 2A and 2B up to half the plate thickness so that the groove angle X met the conditions in Table 2. The base materials 2A and 2B were butted together with the gap adjusted to 1 mm. Single-bead welding was performed on these base materials 2A and 2B in one pass. The weld length was 500 mm. The welding conditions were the "GMAW1" and "GMAW2" conditions listed in Table 2. 100% CO2 was used as the shielding gas. In this way, a welded joint with weld metal was produced. The chemical composition of the weld metal in the produced welded joint was shown in "WM1" in Table 3.

[0069] 2. Submerged Arc Welding (SAW) using solid wire and flux Welded joints No. 3 to No. 11 shown in Table 1 were produced by submerged arc welding using NITTETSU FILLER 196 (φ2.4 mm) manufactured by Nippon Steel Welding Industries Co., Ltd. as the solid wire and NITTETSU FLUX 10H manufactured by Nippon Steel Welding Industries Co., Ltd. as the flux. A steel plate (size: 30 mm (t) × 120 mm (w) × 500 mm (l)) was used as the steel plate (base material) to be welded. As shown in Figure 1, a groove was formed in base materials 2A and 2B up to half the plate thickness so that the groove angle X met the conditions in Table 2. The base materials 2A and 2B were butted together with the gap adjusted to 1 mm. Single bead welding was performed on these base materials 2A and 2B in one pass. The weld length was 500 mm. The welding conditions were "SAW1" to "SAW4" as shown in Table 2. In this way, welded joints with weld metal were produced. The chemical compositions of the weld metal in the produced welded joints were those shown in Tables 3 and 4 as "WM2" to "WM7".

[0070] For the welded joints No. 1 to No. 11 obtained, the cross sections were observed at any five points perpendicular to the welding direction, and the width BW (mm), thickness BW (mm), and cross-sectional area S (mm 2 ) was obtained and the bead index HI was calculated.

[0071] <Evaluation test> (Hot cracking test) The welded joints were photographed using the radiographic testing method for steel welded joints (JIS Z3104-1995), and the weld metal parts, excluding the start and crater, were judged to have passed if they had no type 3 defects (cracks or similar defects). The results for the presence or absence of defects are shown in Table 1.

[0072] [Table 1]

[0073] [Table 2]

[0074] [Table 3]

[0075] [Table 4]

[0076] The results shown in Tables 1 to 4 show that the welded joints of the present disclosure, which have a bead index HI of 75 or more, have superior hot cracking resistance compared to the welded joints of the comparative examples, which have a bead index HI of less than 75. [Explanation of symbols]

[0077] 2A, 2B base material

Claims

1. A welding method for joining steel materials to each other, comprising: The weld metal has a bead index HI, represented by the following formula 1, of 75 or more, and an fcc fraction of 70% or more. Formula 1: HI=5×BW+0.5×BT+0.2×S (In the above formula 1, BW is the width (mm) of the final bead, BT is the thickness (mm) of the final bead, and S is the cross-sectional area (mm ２ ) and ) respectively.

2. The chemical composition of the weld metal is expressed in mass % relative to the total mass of the weld metal, C: 0.05-0.80%, Si: 0.03-0.50%, Mn: 0 to 30.0%, P: 0 to 0.050%, S: 0 to 0.050%, Cu: 0 to 5.0%, Ni: 0 to 30.0%, Cr: 0-20.0%, Mo: 0-10.0%, Nb: 0 to 5.00%, V: 0-5.00%, Ta: 0-5.00% Hf: 0-5.00% Ti: 0 to 5.00%, Zr: 0-5.00%, Co: 0-1.00%, Pb: 0 to 1.00%, Sn: 0-1.00%, W: 0-20.00%, Mg: 0-0.1%, Al: 0-0.400%, Ca: 0-5.0%, B: 0 to 0.5000%, REM: 0-0.500%, N: 0 to 0.5000%, and O: 0.0010 to 0.1500%; the balance being Fe and impurities; The total content of the Mn and the Ni (Mn + Ni) is 5.0% or more, The welded joint according to claim 1, wherein a mass ratio (Ni / Mn) of the Mn content to the Ni content is 0.10 or more.

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