Steel pipe welded joint

Abstract

A steel pipe welded joint (10) includes a base material part (1a, 1b) and a circumferential weld part (2). The circumferential weld part (2) is composed of a weld metal part (2a) and a weld heat-affected part (2b, 2c). The base material part (1a, 1b) has a predetermined chemical composition, with Pcm of 0.25 to 0.30. The weld metal part (2a) has a predetermined chemical composition, with B content of 0.0010% or less. The tensile strength of the base material part (1a, 1b) and the tensile strength of the joint in the circumferential weld part (2) in the tensile test are 980 MPa or more. The average hardness of the base material part (1a, 1b) is 300 HV10 or more. The average softening width of the weld heat-affected part (2b, 2c) is 4.0 mm or less, and the average softening degree is 80 HV10 or less.

Description

Technical Field

[0001] This invention relates to a welded joint for steel pipes. Background Technology

[0002] For cylindrical components in mechanical structures, in the past, after forging or stretching and rolling bar steel into the desired shape, or further machining it, heat treatment was carried out to give the mechanical structure the required mechanical properties.

[0003] However, in recent years, driven by the trend towards larger and higher yield strength mechanical structures, there has been a pursuit of weight reduction by replacing cylindrical mechanical structural components with hollow seamless steel tubes. In particular, for steel tubes used in crane booms, in addition to the increasing size of cranes used in high-rise buildings, there is also the necessity of operating in cold regions; therefore, high strength is required alongside high toughness. Specifically, recently, for crane boom applications, seamless steel tubes with tensile strengths exceeding 980 MPa and excellent toughness at temperatures as low as -40°C are required.

[0004] Various technologies are publicly available for high-strength and high-toughness seamless steel pipes and their manufacturing methods.

[0005] For example, Patent Document 1 discloses a method that allows for the manufacture of high-strength seamless steel pipes with excellent toughness through online processing and heat treatment without adding expensive alloy steel.

[0006] Patent document 2 discloses a seamless steel pipe and its manufacturing method with the following characteristics: tensile strength of 950 MPa or more, yield stress of 850 MPa or more, and Charpy impact absorption energy of 60 J or more at -40°C.

[0007] Patent document 3 discloses the following seamless steel pipe and its manufacturing method: tensile strength is above 950MPa, yield stress is above 850MPa, Charpy impact absorption energy at -40℃ is above 60J, and wall thickness is greater than 30mm.

[0008] Patent document 4 discloses a seamless steel pipe with the following characteristics: high strength with a tensile strength of 980 MPa or more, excellent low-temperature toughness, small Pcm (below 0.30), and excellent weldability.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Application Publication No. 2001-240913

[0012] Patent Document 2: International Publication No. 2010 / 061882

[0013] Patent Document 3: Japanese Patent Application Publication No. 2012-193404

[0014] Patent Document 4: International Publication No. 2018 / 025778 Summary of the Invention

[0015] The problem the invention aims to solve

[0016] However, when the aforementioned seamless steel pipes are typically used in large mechanical structures, multiple seamless steel pipes are joined together by circumferential welding to form a welded joint. Therefore, in order to achieve lightweight mechanical structures, both the strength of the seamless steel pipes and the strength of the welded joints are required.

[0017] In addition, welding cracks such as low-temperature cracks are prone to occur at the welded part of the weld joint. Therefore, from a safety point of view, the welded part is required to have excellent resistance to low-temperature cracks.

[0018] The present invention aims to provide a steel pipe welded joint with high joint strength and excellent resistance to low-temperature cracking.

[0019] Solution for solving the problem

[0020] This invention was made to solve the above-mentioned problems, and is based on the steel pipe welded joint shown below.

[0021] (1) A welded joint for steel pipes, comprising a base material portion and a circumferential weld portion,

[0022] The circumferential weld portion consists of a weld metal portion and a weld heat-affected portion.

[0023] The chemical composition of the base material is expressed in % by mass.

[0024] C: 0.10~0.20%

[0025] Si: 0.05~1.00%

[0026] Mn: 0.05~1.20%

[0027] P: below 0.025%

[0028] S: less than 0.005%

[0029] Cu: less than 0.20%

[0030] N: less than 0.007%

[0031] Ni: 0.20–0.50%

[0032] Cr: ≥0.30% and <0.50%

[0033] Mo: 0.30–0.50%

[0034] Nb: 0.01~0.05%

[0035] Al: 0.001~0.100%

[0036] B: 0.0005~0.0020%

[0037] Ti: 0.003~0.050%

[0038] V: 0.01~0.20%

[0039] The sum of any one or more of Ca, Mg, and REM: 0–0.0250%

[0040] Balance: Fe and impurities,

[0041] The value of Pcm, expressed by the following formula [A], is 0.25 to 0.30.

[0042] The chemical composition of the welded metal portion, expressed in mass percent.

[0043] C: 0.04~0.14%

[0044] Si: 0.05~1.00%

[0045] Mn: 1.00~2.00%

[0046] P: below 0.025%

[0047] S: less than 0.005%

[0048] Cu: less than 0.50%

[0049] N: less than 0.007%

[0050] Ni: 2.50–3.00%

[0051] Cr: ≥0.90% and <1.40%

[0052] Mo: 0.40–0.90%

[0053] Nb: below 0.010%

[0054] Al: below 0.010%

[0055] B: Below 0.0010%

[0056] Ti: 0.003~0.050%

[0057] V: 0.01~0.20%

[0058] The sum of any one or more of Ca, Mg, and REM: 0–0.0250%

[0059] Balance: Fe and impurities,

[0060] The tensile strength of the base material and the tensile strength of the joint in the circumferential weld are both above 980 MPa.

[0061] The average hardness of the base material is 300HV10 or higher, the average softening width of the weld heat-affected zone is 4.0 mm or less, and the average softening degree of the weld heat-affected zone is 80HV10 or less.

[0062] Pcm=C+(Si / 30)+(Mn / 20)+(Cu / 20)+(Ni / 60)+(Cr / 20)+(Mo / 15)+(V / 10)+5B…[A]

[0063] In formula [A], the element symbol represents the content (mass%) of each element in the steel, and is set to zero if the element is not present.

[0064] (2) According to the steel pipe welded joint described in (1) above, wherein,

[0065] The metallographic structure of the base material is tempered martensite, with an area percentage of over 90%.

[0066] (3) The steel pipe welded joint according to (1) or (2) above, wherein,

[0067] The welded metal part is a multi-layer weld overlay metal.

[0068] The effects of the invention

[0069] According to the present invention, steel pipe welded joints with high joint strength and excellent resistance to low-temperature cracking can be obtained. Attached Figure Description

[0070] Figure 1 This is a schematic diagram illustrating a steel pipe welded joint according to one embodiment of the present invention.

[0071] Figure 2 This is a diagram illustrating the measurement method for the average softening width and average softening degree of the heat-affected zone in welding.

[0072] Figure 3 This is a diagram illustrating the shape of the test plate used in the C-clamp constrained butt weld crack test method (FISCO).

[0073] Figure 4 This is a diagram illustrating the shape of the test plate used in the Y-shaped weld crack test method. Detailed Implementation

[0074] In the seamless steel pipe described in Patent Document 4, by limiting Pcm (weld crack sensitivity composition (%)) expressed in the following formula [A] to less than 0.30, and by making the seamless steel pipe contain an appropriate amount of B, hardenability is improved while taking into account both strength and toughness.

[0075] Pcm=C+(Si / 30)+(Mn / 20)+(Cu / 20)+(Ni / 60)+(Cr / 20)+(Mo / 15)+(V / 10)+5B…[A]

[0076] In formula [A], the element symbol represents the content (mass%) of each element in the steel, and is set to zero if the element is not present.

[0077] Therefore, based on the technology described in Patent Document 4, the inventors have repeatedly studied a method that balances high joint strength and excellent resistance to low-temperature cracking, and as a result, have obtained the following insights.

[0078] (a) By suppressing Pcm to a low level, low-temperature cracking during welding can be suppressed. However, on the other hand, the reduction of Pcm leads to a reduction in strength, so when the seamless steel pipe described in Patent Document 4 is used as the base material to manufacture the welded joint, there is a possibility that sufficient joint strength cannot be obtained.

[0079] (b) In addition, in the seamless steel pipe described in Patent Document 4, the strength is improved by optimizing the B content. However, if the base material contains B, B may also flow into the weld metal and cause solidification cracks, etc.

[0080] (c) In order to ensure joint strength and prevent welding cracks, it is effective to set a lower limit for Pcm, optimize welding conditions, and minimize the reduction in strength of the weld.

[0081] This invention is based on the above-mentioned insights. The technical features of this invention will be described in detail below.

[0082] (A) Overall Structure

[0083] Figure 1 This is a schematic diagram illustrating a steel pipe welded joint according to one embodiment of the present invention. Figure 1As shown, the steel pipe welded joint 10 includes a base material portion 1a, a base material portion 1b, and a circumferential weld portion 2. That is, the steel pipe welded joint 10 is formed by joining the base material portion 1a and the base material portion 1b using circumferential welding. Furthermore, the circumferential weld portion is composed of a weld metal portion 2a, a weld heat-affected portion 2b, and a weld heat-affected portion 2c. The base material portion 1a and the base material portion 1b are tubular, such as seamless steel pipes or welded steel pipes.

[0084] (B) Chemical composition of the base material

[0085] The reasons for limiting the chemical composition of the base material are as follows. In the following explanation, "%" for the content of each element means "mass %".

[0086] C: 0.10~0.20%

[0087] Carbon (C) is an essential element for improving strength. When the C content is less than 0.10%, it is difficult to achieve high tensile strengths such as 980 MPa or higher due to its interaction with other elements. On the other hand, if the C content is greater than 0.20%, weldability decreases significantly. Therefore, the C content is set to 0.10–0.20%. A C content of 0.12% or more is preferred, and 0.18% or less is even more preferred.

[0088] Si: 0.05~1.00%

[0089] Si has a deoxidizing effect, as well as improving strength and hardenability. To obtain these effects, the Si content needs to be set to 0.05% or more. However, if the Si content is greater than 1.00%, toughness and weldability decrease. Therefore, the Si content is set to 0.05 to 1.00%. The Si content is preferably 0.10% or more. In addition, the Si content is preferably 0.60% or less, and more preferably 0.40% or less.

[0090] Mn: 0.05~1.20%

[0091] Mn has a deoxidizing effect, as well as improving strength and hardenability. To obtain these effects, it is necessary to contain 0.05% or more Mn. However, if the Mn content is greater than 1.20%, the toughness decreases. Therefore, the Mn content is set to 0.05% to 1.20%. The Mn content is preferably 0.30% or more, more preferably 0.60% or more. In addition, the Mn content is preferably 1.10% or less.

[0092] P: below 0.025%

[0093] If the phosphorus (P) content exceeds 0.025%, the reduction in toughness becomes significant, making it difficult to ensure the predetermined Charpy impact value. Therefore, the P content as an impurity is set to 0.025% or less. Preferably, the P content is 0.020% or less.

[0094] S: less than 0.005%

[0095] If the sulfur content exceeds 0.005%, the reduction in toughness becomes significant, making it difficult to ensure the predetermined Charpy impact value. Therefore, the sulfur content as an impurity is set to 0.005% or less. Preferably, the sulfur content is 0.003% or less.

[0096] Cu: less than 0.20%

[0097] If the Cu content is greater than 0.20%, it can sometimes lead to a decrease in hot workability. Therefore, the Cu content as an impurity is set to 0.20% or less. The Cu content is preferably 0.15% or less, more preferably 0.10% or less, and even more preferably 0.05% or less.

[0098] N: below 0.007%

[0099] If the nitrogen (N) content exceeds 0.007%, coarse nitrides will form, or it will be difficult to ensure the solidification of boron (B). In particular, in thick-walled steel pipes, the hardenability-enhancing effect of B becomes insufficient, making it impossible to obtain a sufficiently hardened microstructure, or the reduction in toughness becomes significant. Therefore, it is difficult to ensure the predetermined Charpy impact value. Therefore, the N content as an impurity is set to 0.007% or less. The N content is preferably 0.006% or less.

[0100] Ni: 0.20–0.50%

[0101] Ni improves hardenability, strength, and toughness. To achieve these effects, a Ni content of 0.20% or more is required. On the other hand, if the Ni content exceeds 0.50%, the alloy cost increases. Therefore, the Ni content is set at 0.20% to 0.50%. Preferably, the Ni content is 0.30% or more, and more preferably 0.40% or less.

[0102] Cr: ≥0.30% and <0.50%

[0103] Cr has the effect of improving hardenability and strength. To obtain these effects, a Cr content of 0.30% or more is required. On the other hand, to ensure good hardenability, in the case of low-alloy steels containing Cr and Mo in combination with 0.0005% to 0.0020% B (discussed later), if the Cr content is 0.50% or more, coarse borocarbides may form during tempering, leading to a decrease in toughness. In addition, the Pcm (weld crack susceptibility component) becomes higher, making it easier for weld cracks to form. Therefore, the Cr content is set to 0.30% or more and less than 0.50%. The Cr content is preferably 0.35% or more, more preferably 0.40% or more. Furthermore, the Cr content is preferably 0.47% or less, more preferably 0.45% or less.

[0104] Mo: 0.30–0.50%

[0105] Mo has the effect of improving hardenability and strength. To obtain these effects, the Mo content needs to be 0.30% or more. On the other hand, to ensure good hardenability, in the case of low-alloy steels containing Mo and Cr in combination with 0.0005% to 0.0020% B (discussed later), if the Mo content is greater than 0.50%, coarse borocarbides may sometimes form during tempering, leading to a decrease in toughness. In addition, the Pcm (weld crack susceptibility component) becomes higher, making it easier for weld cracks to form. Therefore, the Mo content is set to 0.30% to 0.50%. The Mo content is preferably 0.35% or more, more preferably 0.40% or more. Furthermore, the Mo content is preferably 0.48% or less, more preferably 0.46% or less.

[0106] Nb: 0.01–0.05%

[0107] Nb combines with C and / or N to form fine precipitates, which help to suppress the coarsening of austenite grains and improve toughness. To reliably ensure this effect, a Nb content of 0.01% or more is required. However, if the Nb content exceeds 0.05%, the amount of precipitates increases, which can actually degrade toughness. Therefore, the Nb content is set to 0.01% to 0.05%. Preferably, the Nb content is 0.02% or more, and more preferably 0.04% or less.

[0108] Al: 0.001~0.100%

[0109] Al is an element with deoxidizing properties. To ensure this effect, it is necessary to contain more than 0.001% Al. On the other hand, even with more than 0.100% Al, the above-mentioned effect becomes saturated, and the formation of macroscopic streaks increases. Therefore, the Al content is set to 0.001 to 0.100%. The Al content is preferably 0.055% or less. Furthermore, the Al content in this invention refers to the content of acid-soluble Al (so-called "sol.Al").

[0110] B: 0.0005~0.0020%

[0111] Boron (B) is an extremely important element for ensuring that thick-walled steel pipes with a low Pcm value (below 0.30) for weldability purposes possess a sufficient hardened microstructure, and therefore requires a B content of 0.0005% or more. However, if the B content is greater than 0.0020%, even if the Cr content is less than 0.50% and the Mo content is less than 0.50%, the combined presence of these elements can lead to the formation of coarse borocarbides during tempering, resulting in a decrease in toughness. Therefore, the B content is set to 0.0005 to 0.0020%. A B content of 0.0008% or more is preferred, and 0.0016% or less is even more preferred.

[0112] Ti: 0.003~0.050%

[0113] During tempering, Ti precipitates as Ti carbides, which increases strength. It also ensures effective solid solution of B, allowing N to be fixed to Ti and thus improving the hardenability of B. These effects are obtained when the Ti content is 0.003% or higher. However, if the Ti content is greater than 0.050%, coarse Ti carbonitrides form in the high-temperature region during solidification, and the amount of Ti carbide precipitation during tempering becomes excessive, resulting in reduced toughness. Therefore, the Ti content is set to 0.003–0.050%. The Ti content is preferably 0.005% or higher, and more preferably 0.015% or lower.

[0114] In addition, as mentioned above, in order to keep N fixed, it is preferable to satisfy Ti / N≥48 / 14.

[0115] V: 0.01~0.20%

[0116] During tempering, v precipitates as v carbides, which increases strength. This effect is achieved when the v content is 0.01% or more. However, if the v content is greater than 0.20%, the amount of v carbide precipitated during tempering becomes excessive, thus reducing toughness. Furthermore, the Pcm (protrusion thickness) increases, making weld cracking more likely. Therefore, the v content is set to 0.01–0.20%. Moreover, the v content is preferably 0.04% or more. Additionally, the v content is preferably 0.15% or less, and more preferably 0.10% or less.

[0117] The sum of any one or more of Ca, Mg, and REM: 0–0.0250%

[0118] Ca, Mg, and REM all react with S to form sulfides, thereby improving the morphology of inclusions and increasing toughness. Therefore, one or more of Ca, Mg, and REM may be included as needed. To stably obtain this effect, the total content of these components is preferably 0.0005% or more. On the other hand, if the total content of these components exceeds 0.0250%, the amount of inclusions increases and the cleanliness of the steel decreases, thus reducing toughness. Therefore, the upper limit of the total content of these elements is set to 0.0250%. The total content is preferably 0.0100% or less, more preferably 0.0080% or less, and even more preferably 0.0050% or less.

[0119] In this invention, "REM" refers to a total of 17 elements, including Sc, Y, and the lanthanides. "REM content" refers to the content of a single REM element, and the total content of two or more REM elements. Furthermore, REM is generally supplied as an alloy of multiple REM elements, i.e., a mixed rare earth alloy. Therefore, it can also be contained by adding one or more individual elements, for example, it can be added in the form of a mixed rare earth alloy.

[0120] The base material of this invention contains the aforementioned elements and the balance: Fe and impurities. Here, "impurities" refers to components that are mixed into raw materials such as ores and waste during the industrial manufacturing of steel materials due to various major reasons in the manufacturing process, and are permissible to a extent that does not adversely affect this invention.

[0121] Pcm: 0.25~0.30

[0122] For the base material portion of the present invention, Pcm, expressed by the following formula [A], is 0.25 to 0.30. When Pcm is less than 0.25, it is difficult to ensure sufficient joint strength. On the other hand, by setting Pcm to 0.30 or less, low-temperature cracking of the circumferential weld portion can be prevented.

[0123] Pcm=C+(Si / 30)+(Mn / 20)+(Cu / 20)+(Ni / 60)+(Cr / 20)+(Mo / 15)+(V / 10)+5B…[A]

[0124] In formula [A], the element symbol represents the content (mass%) of each element in the steel, and is set to zero if the element is not present.

[0125] (C) Chemical composition of the welded metal

[0126] The reasons for defining the chemical composition of the weld metal are as follows. In the following explanation, "%" for the content of each element means "mass %". Furthermore, the chemical composition of the weld metal referred to here refers to the chemical composition of the initial weld layer.

[0127] C: 0.04~0.14%

[0128] Carbon (C) is an essential element for improving strength. However, if the C content exceeds 0.14%, weldability decreases significantly. Therefore, the C content is set between 0.04% and 0.14%. Preferably, the C content is 0.06% or more, and more preferably 0.12% or less.

[0129] Si: 0.05~1.00%

[0130] Si is an element that increases strength. To achieve this effect, the Si content needs to be 0.05% or more. However, if the Si content is greater than 1.00%, the toughness decreases. Therefore, the Si content is set between 0.05% and 1.00%. The Si content is preferably 0.10% or more, and more preferably 0.60% or less.

[0131] Mn: 1.00~2.00%

[0132] Mn is an element that improves strength. To achieve this effect, it needs to contain 1.00% or more Mn. However, if the Mn content is greater than 2.00%, the toughness decreases. Therefore, the Mn content is set to 1.00 to 2.00%. The Mn content is preferably 1.20% or more, and more preferably 1.80% or less.

[0133] P: below 0.025%

[0134] If the phosphorus (P) content exceeds 0.025%, the reduction in toughness becomes significant, making it difficult to ensure the predetermined Charpy impact value. Therefore, the P content as an impurity is set to 0.025% or less. Preferably, the P content is 0.020% or less.

[0135] S: less than 0.005%

[0136] If the sulfur content exceeds 0.005%, the reduction in toughness becomes significant, making it difficult to ensure the predetermined Charpy impact value. Therefore, the sulfur content as an impurity is set to 0.005% or less. Preferably, the sulfur content is 0.003% or less.

[0137] Cu: less than 0.50%

[0138] If the Cu content is greater than 0.50%, it can sometimes lead to a decrease in toughness. Therefore, the Cu content as an impurity is set to 0.50% or less. The Cu content is preferably 0.40% or less, and more preferably 0.30% or less.

[0139] N: below 0.007%

[0140] If the nitrogen (N) content exceeds 0.007%, coarse nitrides form, resulting in a significant decrease in toughness, making it difficult to ensure the desired Charpy impact value. Therefore, the N content as an impurity is set to 0.007% or less. Preferably, the N content is 0.006% or less.

[0141] Ni: 2.50–3.00%

[0142] Ni has the effect of improving strength and toughness. To achieve these effects, a Ni content of 2.50% or more is required. On the other hand, if the Ni content is greater than 3.00%, the alloy cost increases. Therefore, the Ni content is set at 2.50% to 3.00%. The Ni content is preferably 2.60% or more, and more preferably 2.80% or less.

[0143] Cr: ≥0.90% and <1.40%

[0144] Cr has the effect of increasing strength. To achieve this effect, it is necessary to contain 0.90% or more Cr. On the other hand, if the Cr content is 1.40% or more, it sometimes leads to a decrease in toughness. Therefore, the Cr content is set to 0.90% or more and less than 1.40%. The Cr content is preferably 1.00% or more. In addition, the Cr content is preferably 1.30% or less, and more preferably 1.20% or less.

[0145] Mo: 0.40–0.90%

[0146] Mo has the effect of increasing strength. To achieve this effect, a Mo content of 0.40% or more is required. On the other hand, if the Mo content is greater than 0.90%, it can sometimes lead to a decrease in toughness. Therefore, the Mo content is set to 0.40% to 0.90%. The Mo content is preferably 0.50% or more, preferably 0.80% or less, and preferably 0.70% or less.

[0147] Nb: below 0.010%

[0148] Nitrogen (Nb) is an element that can be incorporated into the base material. However, if the Nb content exceeds 0.010%, it can degrade toughness. Therefore, the Nb content is set to 0.010% or less. Preferably, the Nb content is 0.008% or less, and more preferably 0.005% or less.

[0149] Al: below 0.010%

[0150] Al is an element that inevitably mixes into the base material. However, if the Al content is greater than 0.010%, it leads to a decrease in toughness. Therefore, the Al content is set to 0.010% or less. The Al content is preferably 0.008% or less, and more preferably 0.005% or less. Furthermore, the Al content in this invention refers to the content of acid-soluble Al (so-called "sol.Al").

[0151] B: Below 0.0010%

[0152] Botanicals (B) are elements that inevitably mix into the base material. However, if the B content is greater than 0.0010%, solidification cracks may occur in the weld metal. Therefore, the B content is set to 0.0010% or less. The lower the B content, the better; preferably 0.0007% or less, more preferably 0.0005% or less, and even more preferably 0.0003% or less. On the other hand, when it is desirable to increase the strength of the weld metal, the presence of B can be actively pursued. To achieve this effect, the B content is preferably 0.0001% or more, more preferably 0.0003% or more.

[0153] Ti: 0.003~0.050%

[0154] Ti has the effect of increasing strength. This effect is obtained when the Ti content is 0.003% or more. However, if the Ti content is greater than 0.050%, the toughness decreases. Therefore, the Ti content is set to 0.003 to 0.050%. The Ti content is preferably 0.005% or more, and more preferably 0.015% or less.

[0155] V: 0.01~0.20%

[0156] V has the effect of increasing strength. This effect is obtained when the V content is 0.01% or more. However, if the V content is greater than 0.20%, the toughness decreases. Therefore, the V content is set to 0.01% to 0.20%. Furthermore, the V content is preferably 0.04% or more, and more preferably 0.15% or less.

[0157] The sum of any one or more of Ca, Mg, and REM: 0–0.0250%

[0158] Ca, Mg, and REM all react with S to form sulfides, thereby improving the morphology of inclusions and increasing toughness. Therefore, it is permissible to include any one or more of Ca, Mg, and REM as needed. To stably achieve this effect, the total content of these components is preferably 0.0005% or more. On the other hand, if the total content of these components exceeds 0.0250%, the amount of inclusions increases and the cleanliness of the steel decreases, thus reducing toughness. Therefore, the upper limit of the total content of these elements is set at 0.0250%. The total content is preferably 0.0100% or less, more preferably 0.0050% or less.

[0159] The welded metal portion of the present invention contains the aforementioned elements and balances: Fe and impurities. Here, "impurities" refers to components that are mixed into raw materials such as ores and waste during the industrial manufacturing of steel materials due to various major reasons in the manufacturing process, and are permissible within a range that does not adversely affect the present invention.

[0160] (D) Metallographic structure of the base material

[0161] Ideally, in order to achieve both high strength and high toughness, the base material of the present invention has a metallographic structure dominated by tempered martensite. Specifically, it is desirable that the area ratio of tempered martensite is 90% or more. There are no particular limitations on the microstructure of the remainder, and it may contain one or more of bainite, ferrite, and pearlite.

[0162] Furthermore, in this invention, the metallographic structure is measured using the following method. First, an observation test piece is collected from the base material with a cross-section including the central portion of the steel pipe's wall thickness and perpendicular to the rolling direction as the observation surface. In the case of a welded steel pipe, the observation test piece is collected at a position 180° away from the weld portion along the circumference of the steel pipe. In the following description, to distinguish it from the "circumferential weld portion" mentioned above, in welded steel pipes, the weld portion extending in the longitudinal direction of the steel pipe is referred to as the "longitudinal weld portion." After grinding the observation surface, it is etched using a nitric acid ethanol etching solution. Then, the area ratio of tempered martensite is determined from a microstructure photograph obtained using an optical microscope at 500x magnification.

[0163] (E) Mechanical properties

[0164] In the steel pipe welded joint of the present invention, the tensile strength of the base material and the tensile strength of the circumferential weld joint in the joint tensile test (hereinafter, in either case, also referred to as "TS") are both 980 MPa or higher. As long as the TS of both the base material and the circumferential weld joint are 980 MPa or higher, weight reduction can be stably achieved. Therefore, it can be used stably for applications such as crane booms that can handle large cranes.

[0165] The preferred lower limit for the yield stress (TS) of the base material and the circumferential weld joint is 1000 MPa. Furthermore, the preferred upper limit for the TS of the base material and the circumferential weld joint is 1100 MPa. In addition, in the steel pipe welded joint of the present invention, the yield stress of the base material and the yield stress in the tensile test of the circumferential weld joint (hereinafter, both are referred to as "YS") are preferably 890 MPa or more, and more preferably 900 MPa or more.

[0166] Furthermore, in this invention, the tensile strength and yield stress in the base material are measured by cutting a No. 12B test piece (a circular arc-shaped test piece with a width of 25 mm) as described in JIS Z2241:2011 from the base material and performing a tensile test at room temperature. Additionally, for the tensile strength and yield stress in the tensile test of the circumferential weld joint, the measurement is performed using a No. 3 test piece (width of the parallel section: 20 mm) according to JIS Z 3121:2013, with the longitudinal direction of the steel pipe weld joint aligned with its length direction and the circumferential weld joint located at the center of the parallel section. That is, the strength of the circumferential weld joint essentially becomes the joint strength. In the case where the base material is a welded steel pipe, the test piece for the tensile test is taken at a position 180° apart from the weld section along the circumference and longitudinal direction of the steel pipe.

[0167] In order to achieve the joint strength described above, it is necessary to minimize the softening of the weld heat-affected zone. Therefore, in this invention, the average hardness of the base material is set to 300 HV10 or more, the average softening width of the weld heat-affected zone is set to 4.0 mm or less, and the average softening degree is set to 80 HV10 or less.

[0168] Furthermore, there is no need to set a special limit on the maximum hardness of the weld heat-affected zone, but from the viewpoint of suppressing low-temperature cracking, it is preferable to set it to 415HV10 or below.

[0169] In this invention, the "average hardness of the base material", the "average softening width of the weld heat-affected zone", the "average softening degree of the weld heat-affected zone", and the "maximum hardness of the weld heat-affected zone" are calculated in the following order.

[0170] In a cross-section perpendicular to the longitudinal direction of the steel pipe, hardness measurements are taken at three points: 1.0 mm from the outer surface of the base material, at the center of the wall thickness, and 1.0 mm from the inner surface. The average value of each measurement is calculated to determine the average hardness of the base material.

[0171] Figure 2 This is a diagram illustrating the measurement method for the average softening width and average softening degree of the weld heat-affected zone. (Example) Figure 2 As shown in (a), a cross section parallel to the longitudinal direction of the steel pipe is cut through the axis of the steel pipe, including the base material parts 1a and 1b, the weld metal part 2a, and the weld heat-affected parts 2b and 2c.

[0172] Furthermore, hardness measurements were performed at 1.0 mm intervals along three lines parallel to the longitudinal direction of the steel pipe, located at a position 1.0 mm from the outer surface, the center of the wall thickness, and a position 1.0 mm from the inner surface. These lines included positions separated by 0.5 mm from the boundary between the welded metal portion 2a and the welded heat-affected portions 2b and 2c towards the base material portions 1a and 1b, respectively.

[0173] After that, as Figure 2 As shown in (b), the measurement point with the lowest hardness is determined and designated as the lowest hardness position. Furthermore, if multiple measurement points with the lowest hardness exist, the measurement point closest to the base material portion 1b is designated as the lowest hardness position. The difference between the average hardness of the base material portion and the hardness at the lowest hardness position is defined as the softening degree.

[0174] Next, in the region from the lowest hardness position toward the base material 1b side until the hardness changes to decrease, determine the measurement point farthest from the weld metal 2a among the measurement points on the base material 1b side where the hardness between two adjacent points is 10HV or higher, and set it as the outer softening limit position.

[0175] Next, within the region from the lowest hardness position to the boundary between the weld metal portion 2a and the weld heat-affected portion 2c, a hardness measurement point having the hardness closest to the aforementioned outer softening limit position is determined and designated as the inner softening limit position. Furthermore, the distance from the outer softening limit position to the inner softening limit position in a direction parallel to the longitudinal direction of the steel pipe is designated as the softening width.

[0176] The softening degree and softening width were measured at a total of six locations on both the weld heat-affected zone 2b and the weld heat-affected zone 2c side, at a position 1.0 mm from the outer surface, at the center of the wall thickness, and at a position 1.0 mm from the inner surface. The average value of these measurements was taken as the average softening width and average softening degree of the weld heat-affected zone.

[0177] Furthermore, the maximum value among all measured hardness values ​​at weld heat-affected zones 2b and 2c is designated as the highest hardness of the weld heat-affected zone. In addition, "HV10" refers to the "hardness symbol" when the Vickers hardness test is performed with a test force of 98 N (10 kgf) (refer to JIS Z 2244-1:2020).

[0178] Furthermore, in the steel pipe welded joint of the present invention, the Charpy impact value of the base material at -40°C is preferably 75 J / cm. 2 The above. If the Charpy impact value at -40℃ is 75 J / cm². 2Therefore, even when used as a crane boom operating in cold regions, it can be used with sufficient stability. A more preferred lower limit for the Charpy impact value of this seamless steel pipe at -40°C is 125 J / cm². 2 Ideally, the higher the value, the better.

[0179] (F) Wall thickness

[0180] No particular limitation is set on the wall thickness of the base material portion in the welded steel pipe joint of the present invention. However, if the wall thickness of the base material portion is greater than 45.0 mm, bainite is easily generated in the base material portion, making it difficult to form a structure dominated by tempered martensite. Therefore, the wall thickness of the base material portion is preferably 45.0 mm or less, more preferably 40.0 mm or less, 30.0 mm or less, or 20.0 mm or less. On the other hand, from the viewpoint of ensuring the strength of the welded steel pipe joint, thicker walls are more advantageous. Therefore, the wall thickness of the base material portion is preferably 5.0 mm or more, more preferably greater than 8.0 mm, and even more preferably greater than 12.0 mm. The reason for this is that the thicker the wall, the greater the tendency for the constraint force on HAZ softening inhibition and deformation caused by the increase in the cooling rate after welding to increase, thus increasing the strength of the welded steel pipe joint.

[0181] (G) Manufacturing method of the base material

[0182] The base material used in manufacturing the welded steel pipe joint of the present invention can be manufactured, for example, by the following method. Furthermore, in the following description, the case where the base material is a seamless steel pipe is used as an example, but it is not limited thereto.

[0183] After steel having the chemical composition described in section (B) is smelted using the same method as that used for general low-alloy steel, it is cast into ingots or billets. Alternatively, a so-called "round billet continuous casting" method can be used to produce billets with a round billet shape for pipe making.

[0184] As the next step, the cast ingot or billet undergoes initial rolling or hot forging. This step is to obtain the billet for the final hot-rolled tube (e.g., tube making by hot piercing, rolling and drawing processes, or tube making by hot extrusion). Alternatively, seamless steel tubes can be directly machined from billets formed into circular billet shapes using the aforementioned "round billet continuous casting" method; therefore, initial rolling or hot forging is not always necessary.

[0185] The seamless steel pipe of the present invention is manufactured by sequentially performing the following steps [i] to [iv] on the billet used for the final hot-rolled pipe produced by the above-mentioned initial rolling or hot forging and the round billet (hereinafter referred to as "bill").

[0186] [i]: The hot-rolled tube process involves heating a steel billet to 1200–1300°C and then processing it to achieve a reduction in cross-section of 40–99% to produce a tube blank.

[0187] After heating the steel billet to 1200–1300°C, it undergoes processing with a reduction in cross-section of 40–99% to produce a tube blank with a predetermined shape. If the billet heating temperature is below 1200°C, the deformation resistance increases during the next processing step with a reduction in cross-section of 40–99%, increasing the load on the tube-making equipment and sometimes resulting in defects or cracks. On the other hand, if the billet heating temperature is above 1300°C, it can sometimes lead to high-temperature grain boundary cracks or reduced ductility. Therefore, in the hot-rolled tube process, the billet heating temperature is initially set to 1200–1300°C.

[0188] Even if the billet heating temperature is within the aforementioned range, the following situation exists: if the reduction in cross-section of the hot-rolled tube after heating is less than 40%, then even after the cooling process described later in [ii], a fine quenched structure will not be formed in the quenching process in [iii], and the seamless steel tube will not possess the desired mechanical properties. On the other hand, in tube-making processes with a reduction in cross-section greater than 99%, there is a need to add tube-making equipment. Therefore, the hot-rolled tube process is designed to perform processing with a reduction in cross-section of 40% to 99%.

[0189] The heating temperature in process [i] refers to the surface temperature of the steel billet. While the holding time within the aforementioned temperature range also depends on the size and shape of the steel billet, it is preferably set to 60–300 minutes. Furthermore, the finishing temperature of the billet during hot rolling is preferably set to 850–950°C. The aforementioned finishing temperature of the billet refers to the temperature of the outer surface of the billet. In process [i], the preferred lower limit of the heating temperature is 1230°C, and the preferred upper limit is 1280°C. Moreover, the preferred lower limit of the section reduction rate is 50%, and the preferred upper limit is 90%.

[0190] [ii]: Cooling process, cooling the tube blank to a temperature lower than point Ac1.

[0191] The tube blank, finished into a predetermined shape, is cooled to a temperature below the Ac1 point in order to obtain a fine quenched structure in the quenching process of [iii]. There are no particular restrictions on the cooling rate at this time. Furthermore, for the tube blank after hot rolling, it may be temporarily cooled to room temperature and then reheated to perform the next process of [iii], or it may be directly heated from a suitable temperature below the Ac1 point after hot rolling to perform the next process of [iii]. The cooling temperature in process [ii] refers to the temperature of the outer surface of the tube blank.

[0192] [iii]: Quenching process, which involves heating the cooled tube blank to Ac3 point ~ 950℃ and then rapidly cooling it.

[0193] Next, the tube blank, which has cooled in step [ii], undergoes a quenching process involving heating to a temperature of Ac3 to 950°C followed by rapid cooling. If the heating temperature is lower than Ac3, austenitization is not complete, and therefore, the seamless steel pipe may not possess the desired mechanical properties. On the other hand, if the heating temperature is higher than 950°C, fine austenite grains cannot be obtained in a single quenching process, and the seamless steel pipe may not possess the desired mechanical properties. Therefore, the heating temperature during the quenching process is set to Ac3 to 950°C.

[0194] While the holding time at the aforementioned heating temperature also depends on the size of the tube blank, it is preferably set to 5 to 30 minutes. Even short-duration rapid heating treatments using induction heating are acceptable as long as the heating is generally uniform. The heating temperature in step [iii] refers to the temperature of the outer surface of the tube blank. Appropriate methods such as water cooling or oil cooling can be used for quenching, provided a sufficiently hardened structure is obtained. In step [iii], the preferred lower limit of the heating temperature is 880°C, and the preferred upper limit is 920°C.

[0195] [iv]: The tempering process involves heating the quenched tube blank to 500–600°C and then cooling the heated tube blank to room temperature.

[0196] To ensure that the tube blank quenched in step [iii] possesses the predetermined mechanical properties for a seamless steel tube, a tempering treatment is performed, involving heating to 500–600°C and then cooling to room temperature. In the case of the chemical composition described in section (B), if the tempering temperature is below 500°C, even if the predetermined strength (TS) can be ensured, the low-temperature toughness may decrease, and the Charpy impact value at -40°C may be below 75 J / cm. 2 On the other hand, if the tempering temperature exceeds 600°C, sometimes even if the desired low-temperature toughness (Charpy impact value at -40°C) can be obtained, the strength will be reduced, and it will be impossible to ensure a high strength of TS of 980 MPa or above. Therefore, the tempering temperature is set to 500–600°C.

[0197] While the holding time at the aforementioned heating temperature also depends on the size of the tube blank, it is preferably set to 30–60 minutes. The heating temperature in step [iv] refers to the temperature of the outer surface of the tube blank. There are no particular restrictions on the cooling rate during tempering. Therefore, natural cooling in the atmosphere, forced air cooling, spray cooling, oil cooling, water cooling, or other cooling methods appropriate to the equipment can be used. In step [iv], the preferred lower limit of the heating temperature is 525°C, and the preferred upper limit is 575°C.

[0198] (H) Manufacturing method of welded joints of steel pipes

[0199] With the pipe ends of the base material produced by the above method connected together, circumferential welding can be performed using welding materials such as solid welding wire or flux-cored welding wire to manufacture steel pipe welded joints.

[0200] To suppress softening of the heat-affected zone, welding requires a low heat input. Furthermore, even when the initial layer is welded with a low heat input to improve production efficiency, the heat input typically increases gradually for the second layer and subsequent layers. However, in this invention, from the viewpoint of ensuring joint strength, welding is performed with a low heat input of 0.5 kJ / mm or less from the initial layer to the final layer.

[0201] Furthermore, by performing welding with low heat input from the initial layer to the final layer, the inflow of alloying elements from the base material to the weld metal is minimized, and in particular, the boron content in the weld metal can be reduced. This, in turn, suppresses the formation of high-temperature cracks such as solidification cracks.

[0202] Furthermore, in conventional construction, preheating is performed before welding to prevent low-temperature cracking. However, in this invention, preheating is not performed to suppress softening of the heat-affected zone, and the interpass temperature is managed by lowering it. Specifically, the interpass temperature is set to 150°C or below.

[0203] Furthermore, other welding conditions can be performed under general conditions; for example, gas-shielded arc welding can be used. In this case, the welding current, voltage, welding speed, and shielding gas can be appropriately selected from known techniques. Additionally, there are no particular restrictions on the type of welding material, but it is necessary to select welding materials whose chemical composition of the weld metal meets the above-mentioned requirements.

[0204] Furthermore, when performing circumferential welding, multi-layer surfacing is preferred. When the wall thickness is 5.0 mm or more, it is difficult to perform welding in only one layer using conventional gas-shielded arc welding or similar methods. While laser welding or similar methods can achieve welding in only one layer, this requires either a high heat input or a narrower bevel interval and a smaller bevel angle. In the former case, as mentioned above, the inflow of weld metal (B) from the base material to the weld metal becomes significant, therefore, it is not preferred.

[0205] On the other hand, narrowing the bevel spacing and reducing the bevel angle can lead to welding defects, which may reduce the fatigue strength of the joint. Therefore, from the viewpoint of ensuring the fatigue strength of the joint, multi-layer welding is preferred, provided that the bevel spacing is sufficiently ensured. That is, the weld metal portion is preferably multi-layer weld metal.

[0206] For the same reason, Figure 2 The width W of the welded metal portion shown is preferably greater than 7.0 mm, and more preferably 9.0 mm or more. (Using...) Figure 2 The method for measuring the width W of the welded metal part is explained. For example... Figure 2 As shown, in a cross-section passing through the axis of the steel pipe and parallel to its longitudinal direction, the intersection point 2d between the boundary of the weld metal portion 2a and the weld heat-affected portion 2b and the outer surface of the welded joint of the steel pipe is determined. Similarly, the intersection point 2e between the boundary of the weld metal portion 2a and the weld heat-affected portion 2c and the outer surface of the welded joint of the steel pipe is determined. Furthermore, the distance in the longitudinal direction of the steel pipe between intersection point 2d and intersection point 2e is called the width W of the weld metal portion.

[0207] The present invention will be described in more detail below with reference to embodiments, but the present invention is not limited to these embodiments.

[0208] Example 1

[0209] Steels A through H, with the chemical compositions shown in Table 1, were smelted and then cast into rectangular billets using a converter-continuous casting process. The rectangular billets were further hot-forged into circular billets, which were then cooled to room temperature.

[0210] In Table 1, steels A to E are steels whose chemical compositions fall within the range specified in this invention. On the other hand, steels F to H are steels whose chemical compositions do not meet the conditions specified in this invention. Furthermore, Table 1 also shows the Ac1 and Ac3 points obtained from the following formulas (i) and (ii).

[0211] Ac1 point (℃)=723+29.1×Si-10.7×Mn-16.9×Ni+16.9×Cr…(i)

[0212] Ac3 point (°C) = 910 - 203 × C 0.5 +44.7×Si-15.2×Ni+31.5×Mo+104×V-(30×Mn+11×Cr+20×Cu-700×P-400×Al-400×Ti)…(ii)

[0213] [Table 1]

[0214]

[0215] The aforementioned circular steel billet was heated to 1240°C, and two seamless steel tubes with an outer diameter of 114.3 mm and a wall thickness of 8.6 mm were produced using the Mannesmann mandrel process with a finishing temperature ranging from 850 to 950°C. These tubes were then cooled to room temperature. The rationale for producing relatively thin-walled steel tubes is that, if the strength can be ensured by using thin-walled steel tubes, which are disadvantageous from the perspective of the strength of the welded joints, then the strength can also be sufficiently ensured by using thick-walled steel tubes, which are relatively thin-walled. The seamless steel tubes thus obtained were quenched and tempered under the conditions shown in Table 2 to produce the steel tube base material. Furthermore, all quenching was performed using water quenching. Cooling during tempering was entirely performed by natural atmospheric cooling.

[0216] Subsequently, test pieces were collected from the base material of each welded joint of the steel pipe, with the cross-section perpendicular to the rolling direction serving as the observation surface. After grinding the observation surface, nitric acid ethanol etching solution was used. Then, the area ratio of tempered martensite was determined from the microstructure photographs obtained by optical microscopy at 500x magnification.

[0217] [Table 2]

[0218] Table 2

[0219]

[0220] #1″≥90″ indicates that the tempered martensite area ratio is above 90%.

[0221] Next, after beveling the ends of the two obtained steel pipe base materials at a bevel angle of 60°, circumferential welding based on gas-shielded arc welding was performed under the conditions shown in Table 2 with the ends butted together, producing steel pipe welded joints (test numbers 1-12). During circumferential welding, high-tensile steel solid welding wire (YM-100A) manufactured by Nippon Steel Welding Industry Co., Ltd. was used as the welding material, and Ar-20% CO2 was used as the shielding gas. Additionally, a backing material was used, and the bevel spacing was set to 0 mm. The results of measuring the chemical composition of the initial weld layer in the weld metal of each obtained steel pipe welded joint are shown in Table 3.

[0222] [Table 3]

[0223]

[0224] Next, test pieces No. 12B (circular arc-shaped test pieces with a width of 25 mm) as described in Appendix E of JIS Z 2241:2011 were cut from the base material of each steel pipe weld joint. Tensile tests were performed at room temperature according to JIS Z 2241:2011 to determine YS and TS. Additionally, three 2 mm V-shaped test pieces, each 10 mm wide and 5 mm thick, were cut from the base material of each steel pipe weld joint, with the cut surface including both the pipe axis direction and the wall thickness (pipe diameter) direction, and the pipe axis direction aligned with the length direction. Furthermore, Charpy impact tests were performed at -40°C according to JIS Z 2242:2018. The impact value was determined from the average of the absorbed energy of each of the three pieces. Specifically, the measured absorbed energy (J) was divided by the cross-sectional area perpendicular to the length direction of the test piece at the cut location (e.g., in the case of a 5 mm thickness, width 8 mm × thickness 5 mm = 0.4 cm). 2 The impact value is then calculated.

[0225] Next, a tensile test was conducted on the joint of the circumferential weld using test piece No. 3 (width of parallel section: 20 mm) according to JIS Z 3121:2013, with the longitudinal direction and length direction of each steel pipe weld joint aligned and the circumferential weld section located in the center of the parallel section. YS and TS were then determined.

[0226] Furthermore, the following parameters were measured in the above order: "average hardness of the base material", "average softening width of the weld heat-affected zone", "average softening degree of the weld heat-affected zone", "maximum hardness of the weld heat-affected zone", and "width W of the weld metal".

[0227] Furthermore, the resistance to solidification cracking and low-temperature cracking were evaluated using the following methods. Specifically, the presence of solidification cracking was evaluated using a C-clamp-constrained butt weld cracking test, and the presence of low-temperature cracking was evaluated using a Y-shaped weld cracking test. The test methods are described in more detail below.

[0228] First, slabs were made from steels A to H as described above. After heating at 1250°C for 60 minutes, they were hot-rolled within a temperature range of 1000–1250°C to produce steel plates with a wall thickness of 8.6 mm. Next, quenching and tempering were performed under the conditions shown in Table 2 to obtain steel plates corresponding to each test number.

[0229] After cutting two 120mm × 200mm steel plates from the obtained steel plate, a bevel shape was formed, and the steel plate was manufactured. Figure 3A test plate of the shape shown was prepared. Then, a C-clamp-constrained butt weld crack test was performed according to JIS Z 3155:1993. Two weld beads were formed, and the welding conditions were set to be the same as those shown in Table 2. The presence or absence of cracks was then investigated using the method described in JIS Z 3155:1993. If no cracks were observed in either weld bead, it was evaluated as no solidification crack (A). If a crack was observed in one weld bead, it was evaluated as having a solidification crack (B), and if cracks were observed in both weld beads, it was evaluated as having a solidification crack (C). In this embodiment, excellent low-temperature crack resistance was determined only when no low-temperature cracks were observed (A).

[0230] Additionally, a 150mm x 200mm steel plate was cut from the aforementioned steel plate, and four 8mm diameter holes were formed at each point. Two 5mm wide grooves were then created by connecting the two holes at each location. Next, an electrical discharge machining (EDM) process was used to create a bevel between the two grooves, thus fabricating the product. Figure 4 The test plate was of the shape shown. Furthermore, for test plates other than those of this shape, a Y-shaped weld crack test was performed according to JIS Z 3158:2016. The welding conditions were set to be the same as those shown in Table 2. The presence or absence of cracks was then investigated using the method described in JIS Z 3158:2016. In addition, cracks were investigated in five cross-sections, each divided into four equal parts. If no cracks were observed in any of the cross-sections, it was evaluated as no low-temperature cracks (A). Conversely, if cracks were observed in two or fewer cross-sections, it was evaluated as having low-temperature cracks (B), and if cracks were observed in three or more cross-sections, it was evaluated as having low-temperature cracks (C). In this embodiment, the absence of solidification cracks (A) or the presence of solidification cracks (B) was determined to indicate excellent resistance to solidification cracks.

[0231] The results of the above surveys are summarized in Table 4.

[0232] [Table 4]

[0233]

[0234] As shown in Table 4, in test numbers 1 to 5, which all meet the requirements of this invention, the joints exhibit high joint strength, excellent resistance to solidification cracking, and resistance to low-temperature cracking. In contrast, in test numbers 6 to 12, which are comparative examples and do not meet the requirements of this invention, at least one of the joint strength, resistance to solidification cracking, and resistance to low-temperature cracking deteriorates.

[0235] Example 2

[0236] Similar to Example 1, steels I to P having the chemical compositions shown in Table 5 were melted and rectangular billets were cast using a converter-continuous casting process. The rectangular billets were then further hot-forged into circular billets, which were then cooled to room temperature.

[0237] [Table 5]

[0238]

[0239] The aforementioned circular steel billets were heated to 1240°C, and two seamless steel tubes with the outer diameters and wall thicknesses shown in Table 6 were produced using the Mannesmann mandrel process at a finishing temperature within the range of 850–950°C. These tubes were then cooled to room temperature. The resulting seamless steel tubes were then quenched and tempered under the conditions shown in Table 6 to produce the steel tube base material. Furthermore, all quenching was performed using water quenching. Cooling during tempering was entirely performed by natural atmospheric cooling.

[0240] Subsequently, test pieces were collected from the base material of each welded joint of the steel pipe, with the cross-section perpendicular to the rolling direction serving as the observation surface. After grinding the observation surface, nitric acid ethanol etching solution was used. Then, the area ratio of tempered martensite was determined from the microstructure photographs obtained by optical microscopy at 500x magnification.

[0241] [Table 6]

[0242] Table 6

[0243]

[0244] #1″≥90″ indicates that the tempered martensite area ratio is above 90%.

[0245] Next, after beveling the ends of the two obtained steel pipe base materials at a bevel angle of 60°, circumferential welding based on gas-shielded arc welding was performed under the conditions shown in Table 6 with the ends butted together, producing steel pipe welded joints (test numbers 13-20). During circumferential welding, welding materials with the chemical composition shown in Table 7 were used, with Ar-20% CO2 used as the shielding gas. Additionally, a backing material was used, and the bevel spacing was set to 0 mm. The results of measuring the chemical composition of the initial weld layer in the weld metal of each obtained steel pipe welded joint are shown in Table 8.

[0246] [Table 7]

[0247]

[0248] [Table 8]

[0249]

[0250] Next, the YS, TS, and Charpy impact value of the base material, the YS and TS of the joint tensile test of the circumferential weld, the "average hardness of the base material", the "average softening width of the weld heat-affected zone", the "average softening degree of the weld heat-affected zone", the "maximum hardness of the weld heat-affected zone", and the "width W of the weld metal" were measured using the same method as in Example 1. Furthermore, resistance to solidification cracking and resistance to low-temperature cracking were evaluated using the same method as in Example 1.

[0251] The results of the above surveys are summarized in Table 9.

[0252] [Table 9]

[0253]

[0254] As shown in Table 9, among all test numbers 13 to 20 that meet the requirements of this invention, the results show high joint strength, excellent resistance to solidification cracking, and resistance to low-temperature cracking.

[0255] Industrial availability

[0256] According to the present invention, a steel pipe welded joint with high joint strength and excellent resistance to low-temperature cracking can be obtained. Therefore, the steel pipe welded joint of the present invention is suitable for use in mechanical structural components, and is particularly suitable for use in crane booms.

[0257] Explanation of reference numerals in the attached figures

[0258] 1a, 1b, base material section; 2, circumferential weld section; 2a, weld metal section; 2b, 2c, heat-affected zone of weld; 2d, 2e, intersection point; 10, steel pipe welded joint.

Claims

1. A welded joint for steel pipes, comprising a base metal portion and a circumferential weld portion, wherein the circumferential weld portion is composed of a weld metal portion and a heat-affected zone, and the chemical composition of the base metal portion is, by mass%, C: 0.10–0.20%. Si: 0.05~1.00% Mn: 0.05~1.20% P: below 0.025% S: less than 0.005% Cu: less than 0.20% N: less than 0.007% Ni: 0.20–0.50% Cr: ≥0.30% and <0.50% Mo: 0.30–0.50% Nb: 0.01~0.05% Al：0.001～0.100％、 B：0.0005～0.0020％、 Ti: 0.003~0.050% V：0.01～0.20％、 The total of any one or more of Ca, Mg, and REM: 0–0.0250%, balance: Fe and impurities. The value of Pcm, expressed by the following formula [A], is 0.25 to 0.

30. The chemical composition of the welded metal part, in mass percent, is C: 0.04–0.14%. Si: 0.05~1.00% Mn: 1.00~2.00% P: below 0.025% S: less than 0.005% Cu: less than 0.50% N: less than 0.007% Ni: 2.50–3.00% Cr: ≥0.90% and <1.40% Mo: 0.40–0.90% Nb: below 0.010% Al: below 0.010% B: Below 0.0010% Ti: 0.003~0.050% V：0.01～0.20％、 The sum of any one or more of Ca, Mg, and REM: 0–0.0250% Balance: Fe and impurities, The tensile strength of the base material and the tensile strength of the joint in the circumferential weld are both above 980 MPa. The base material has an average hardness of 300 HV10 or higher, the average softening width of the weld heat-affected zone is 4.0 mm or less, and the average softening degree of the weld heat-affected zone is 80 HV10 or less. Pcm=C+(Si / 30)+(Mn / 20)+(Cu / 20)+(Ni / 60)+(Cr / 20)+(Mo / 15)+(V / 10)+5B···[A] wherein In formula [A], the element symbols represent the content of each element in steel, and are set to zero if the element is not present. The unit of the content of each element in steel is mass.

2. The steel pipe welded joint according to claim 1, wherein, The metallographic structure of the base material is tempered martensite, with an area percentage of over 90%.

3. The steel pipe welded joint according to claim 1 or claim 2, wherein, The welded metal part is a multi-layer weld overlay metal.

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