steel pipe welded joint

Abstract

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

Description

Technology Field

[0001] The present invention relates to a welded joint of a steel pipe. Background Technology

[0002] In the case of cylindrical machine structural members, conventionally, steel bars were often shaped into a desired form by forging or elongating them, or by additionally machining, followed by heat treatment to impart the necessary mechanical properties.

[0003] However, recently, due to the trend toward larger and higher strength of mechanical structures, lightweighting is being sought by replacing cylindrical mechanical structural members with hollow, seamless steel tubes. In particular, for steel tubes used in crane booms, high toughness along with high strength is required because of the need to work in cold regions in addition to the increasing size of cranes for high-rise construction. Specifically, recently, for use as crane booms, seamless steel tubes with a tensile strength of 980 MPa or more and excellent toughness at low temperatures such as -40°C have also become required.

[0004] Various technologies have been disclosed regarding seamless steel pipes of high strength and high toughness and methods for manufacturing the same.

[0005] For example, Patent Document 1 discloses a method that makes it possible to manufacture a high-strength seamless steel pipe with excellent toughness by online processing heat treatment without adding expensive alloy steel.

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

[0007] Patent document 3 discloses a seamless steel pipe having a tensile strength of 950 MPa or more, a yield stress of 850 MPa or more, and a Charpy absorption energy of 60 J or more at -40°C, and a thickness of more than 30 mm, and a method for manufacturing the same.

[0008] Patent document 4 discloses a seamless steel pipe having high strength with a tensile strength of 980 MPa or more, excellent low-temperature toughness, and also having a Pcm of 0.30 or less and excellent weldability. Prior art literature

[0009] Japanese Patent Publication No. 2001-240913, International Publication No. 2010 / 061882; Japanese Patent Publication No. 2012-193404, International Publication No. 2018 / 025778 The problem to be solved

[0010] However, when the above-mentioned seamless steel pipes are used for large machine structures, it is common practice to join multiple seamless steel pipes by circumferential welding to form welded joints. Therefore, to achieve lightweighting of the machine structure, strength of the welded joints is required in addition to the strength of the seamless steel pipes.

[0011] In addition, since welding cracks such as low-temperature cracking are prone to occur in the welded joint, excellent low-temperature crack resistance is required in the welded joint from the perspective of safety.

[0012] The present invention aims to provide a steel pipe welded joint having high joint strength and excellent low-temperature crack resistance. means of solving the problem

[0013] The present invention was made to solve the above problem and is characterized by the steel pipe welded joint shown below.

[0014] (1) A steel pipe welded joint including a base material and a circumferential weld,

[0015] The above circumferential weld is composed of a weld metal part and a weld heat-affected zone, and

[0016] The chemical composition of the above base material is, in mass%,

[0017] C: 0.10~0.20%,

[0018] Si: 0.05~1.00%,

[0019] Mn: 0.05~1.20%,

[0020] P: 0.025% or less,

[0021] S: 0.005% or less,

[0022] Cu: 0.20% or less,

[0023] N: 0.007% or less,

[0024] Ni: 0.20~0.50%,

[0025] Cr: 0.30% or more and less than 0.50%,

[0026] Mo: 0.30~0.50%,

[0027] Nb: 0.01~0.05%,

[0028] Al: 0.001~0.100%,

[0029] B: 0.0005~0.0020%,

[0030] Ti: 0.003~0.050%,

[0031] V: 0.01~0.20%,

[0032] Total of one or more of Ca, Mg, and REM: 0~0.0250%,

[0033] Remainder: Fe and impurities,

[0034] The value of Pcm expressed by the following [A] formula is 0.25~0.30, and

[0035] The chemical composition of the above weld metal part, in mass%,

[0036] C: 0.04~0.14%,

[0037] Si: 0.05~1.00%,

[0038] Mn: 1.00~2.00%,

[0039] P: 0.025% or less,

[0040] S: 0.005% or less,

[0041] Cu: 0.50% or less,

[0042] N: 0.007% or less,

[0043] Ni: 2.50~3.00%,

[0044] Cr: 0.90% or more and less than 1.40%,

[0045] Mo: 0.40~0.90%,

[0046] Nb: 0.010% or less,

[0047] Al: 0.010% or less,

[0048] B: 0.0010% or less,

[0049] Ti: 0.003~0.050%,

[0050] V: 0.01~0.20%,

[0051] Total of one or more of Ca, Mg, and REM: 0~0.0250%,

[0052] Remainder: Fe and impurities,

[0053] The tensile strength of the base material and the tensile strength in the joint tensile test of the circumferential weld are both 980 MPa or higher, and

[0054] The average hardness of the base material is 300 HV10 or higher, the average softening width of the weld heat-affected zone is 4.0 mm or lower, and the average softening degree of the weld heat-affected zone is 80 HV10 or lower,

[0055] Steel pipe welded joint.

[0056] Pcm=C+(Si / 30)+(Mn / 20)+(Cu / 20)+(Ni / 60)+(Cr / 20)+(Mo / 15)+(V / 10)+5B··· [A]

[0057] [A] The element symbols in the formula represent the content (mass%) of each element in the steel, and if not contained, they are set to zero.

[0058] (2) The metal structure of the above base material is, in area %,

[0059] Tempered martensite: 90% or more,

[0060] The steel pipe welded joint described in (1) above.

[0061] (3) The above weld metal part is a multipass weld metal,

[0062] The steel pipe welded joint described in (1) or (2) above. Effects of the invention

[0063] According to the present invention, it is possible to obtain a steel pipe welded joint having high joint strength and excellent low-temperature crack resistance. Brief explanation of the drawing

[0064] FIG. 1 is a schematic diagram showing a steel pipe welded joint according to one embodiment of the present invention. Figure 2 is a figure illustrating a method for measuring the average softening width and average softening degree in the weld heat-affected zone. Figure 3 is a diagram illustrating the shape of a test plate used in the C-type jig restrained butt weld crack test method. Figure 4 is a diagram illustrating the shape of a test plate used in the Y-type weld crack test method. Specific details for implementing the invention

[0065] In the seamless steel pipe described in Patent Document 4, the Pcm (composition of weld crack susceptibility (%)) expressed by the following formula [A] is limited to 0.30 or less, and by including an appropriate amount of B, the hardenability is increased, thereby achieving both strength and toughness.

[0066] Pcm=C+(Si / 30)+(Mn / 20)+(Cu / 20)+(Ni / 60)+(Cr / 20)+(Mo / 15)+(V / 10)+5B··· [A]

[0067] [A] The element symbols in the formula represent the content (mass%) of each element in the steel, and if not contained, they are set to zero.

[0068] Accordingly, the inventors of the present invention, based on the technology described in Patent Document 4, repeatedly examined a method to achieve both high joint strength and excellent low-temperature crack resistance, and as a result, arrived at the following findings.

[0069] (a) By keeping Pcm low, it is possible to suppress low-temperature cracking during welding. However, on the other hand, since reducing Pcm leads to a decrease in strength, when a welded joint is manufactured using a seamless steel pipe described in Patent Document 4 as the base material, it may not be possible to obtain sufficient joint strength.

[0070] (b) Also, in the seamless steel pipe described in Patent Document 4, strength is increased by optimizing the B content, but if B is included in the base material, B may be introduced into the weld metal and cause solidification cracks, etc.

[0071] (c) In order to ensure joint strength while preventing weld cracking, it is effective to set a lower limit on Pcm and optimize welding conditions to minimize the reduction in strength in the welded area.

[0072] The present invention was made based on the above findings. Below, each requirement of the present invention will be explained in detail.

[0073] (A) Overall composition

[0074] FIG. 1 is a schematic diagram showing a steel pipe welded joint according to an embodiment of the present invention. As shown in FIG. 1, the steel pipe welded joint (10) includes a base material portion (1a, 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) by circumferential welding. The circumferential weld portion is composed of a weld metal portion (2a) and a weld heat-affected portion (2b, 2c). The base material portion (1a, 1b) has a pipe shape and is, for example, a seamless steel pipe, a welded steel pipe, etc.

[0075] (B) Chemical composition of the base material

[0076] The reasons for limiting the chemical composition of the base material are as follows. In the following explanation, "%" for the content of each element refers to "mass%".

[0077] C: 0.10~0.20%

[0078] Carbon is an indispensable element for increasing strength. If the carbon content is less than 0.10%, it may be difficult to obtain high strength, such as a tensile strength of 980 MPa or higher, due to its interaction with other elements. On the other hand, if the carbon content exceeds 0.20%, weldability is significantly reduced. Therefore, the carbon content is set to 0.10 to 0.20%. It is preferable that the carbon content be 0.12% or higher, and 0.18% or lower.

[0079] Si: 0.05~1.00%

[0080] Si has a deoxidizing effect and also improves strength and hardenability. To obtain these effects, the Si content needs to be 0.05% or higher. However, if the Si content exceeds 1.00%, toughness and weldability decrease. Therefore, the Si content should be 0.05% to 1.00%. It is preferable that the Si content be 0.10% or higher. Furthermore, it is preferable that the Si content be 0.60% or lower, and more preferable that it be 0.40% or lower.

[0081] Mn: 0.05~1.20%

[0082] Manganese has a deoxidizing effect and also improves strength and hardenability. To obtain these effects, it is necessary to include at least 0.05% of Manganese. However, if the Manganese content exceeds 1.20%, toughness decreases. Therefore, the Manganese content is set to 0.05% to 1.20%. It is preferable that the Manganese content be 0.30% or higher, and more preferable that it be 0.60% or higher. Additionally, it is preferable that the Manganese content be 1.10% or lower.

[0083] P: 0.025% or less

[0084] If the P content exceeds 0.025%, the decrease in toughness becomes significant, making it difficult to secure the required Charpy impact value. For this reason, the P content as an impurity is set to 0.025% or less. It is preferable that the P content be 0.020% or less.

[0085] S: 0.005% or less

[0086] If the S content exceeds 0.005%, the decrease in toughness becomes significant, making it difficult to secure the required Charpy impact value. For this reason, the S content as an impurity is set to 0.005% or less. It is preferable that the S content be 0.003% or less.

[0087] Cu: 0.20% or less

[0088] If the Cu content exceeds 0.20%, it may result in a decrease in hot workability. For this reason, 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.

[0089] N: 0.007% or less

[0090] If the N content exceeds 0.007%, coarse nitrides are formed, or it becomes difficult to secure solid solution B. In particular, in thick steel pipes, the effect of B on improving hardenability becomes insufficient, making it impossible to obtain a sufficiently hardened structure. Consequently, the decrease in toughness becomes significant, making it difficult to secure the required Charpy impact value. For this reason, the N content as an impurity is set to 0.007% or less. It is preferable that the N content be 0.006% or less.

[0091] Ni: 0.20~0.50%

[0092] Ni has the effect of improving hardenability, strength, and toughness. To obtain these effects, it is necessary to include at least 0.20% Ni. On the other hand, if Ni is included in excess of 0.50%, the alloy cost increases. Therefore, the Ni content is set to 0.20 to 0.50%. It is preferable that the Ni content be 0.30% or more, and 0.40% or less.

[0093] Cr: 0.30% or more and less than 0.50%

[0094] Cr has the effect of improving hardenability and strength. To obtain these effects, it is necessary to include at least 0.30% Cr. Meanwhile, in the case of a low-alloy mesh containing a combination of Cr and Mo along with 0.0005% to 0.0020% B (described later) to ensure good hardenability, if the Cr content exceeds 0.50%, coarse boron carbides may form during tempering, leading to a decrease in toughness. Additionally, the Pcm (composition susceptible to weld cracking) increases, making weld cracks more likely to occur. Therefore, the Cr content should be 0.30% or more and less than 0.50%. It is preferable that the Cr content be 0.35% or more, and more preferable that it be 0.40% or more. Furthermore, it is preferable that the Cr content be 0.47% or less, and more preferable that it be 0.45% or less.

[0095] Mo: 0.30~0.50%

[0096] Mo has the effect of improving hardenability and strength. To obtain these effects, it is necessary to include at least 0.30% of Mo. Meanwhile, in the case of a low-alloy mesh containing Mo and Cr in combination with 0.0005% to 0.0020% of B (described later) to ensure good hardenability, if the Mo content exceeds 0.50%, coarse boron carbides may form during tempering, leading to a decrease in toughness. Additionally, the Pcm (composition susceptible to weld cracking) increases, making weld cracks more likely to occur. Therefore, the Mo content is set to 0.30% to 0.50%. It is preferable for the Mo content to be 0.35% or higher, and more preferable for it to be 0.40% or higher. Furthermore, it is preferable for the Mo content to be 0.48% or lower, and more preferable for it to be 0.46% or lower.

[0097] Nb: 0.01~0.05%

[0098] Nb combines with C and / or N to form fine precipitates, which suppress the coarsening of austenite grains and improve toughness. To reliably secure the above effect, it is necessary to include at least 0.01% of Nb. However, if an amount of Nb exceeding 0.05% is included, the amount of precipitates increases, which may actually degrade toughness. Therefore, the Nb content is set to 0.01% to 0.05%. It is preferable that the Nb content be 0.02% or more and 0.04% or less.

[0099] Al: 0.001~0.100%

[0100] Al is an element that has a deoxidizing effect. To secure this effect, it is necessary to include at least 0.001% of Al. On the other hand, if Al is included in excess of 0.100%, the above effect becomes saturated, and the occurrence of defects in the substrate increases. Therefore, the Al content is set to 0.001 to 0.100%. It is preferable that the Al content be 0.055% or less. Furthermore, the Al content of the present invention refers to the content in acid-soluble Al (so-called "sol. Al").

[0101] B: 0.0005~0.0020%

[0102] B is a very important element for providing a sufficient quenched structure in thick steel pipes with a Pcm value of 0.30 or lower for weldability, and it is necessary to include at least 0.0005%. However, if the B content exceeds 0.0020%, the Cr content is less than 0.50%, and even if the Mo content is 0.50% or less, if they are combined, coarse boron carbides may form during tempering, which may lead to a decrease in toughness. Therefore, the B content is set to 0.0005~0.0020%. It is preferable that the B content be 0.0008% or higher and 0.0016% or lower.

[0103] Ti: 0.003~0.050%

[0104] Ti precipitates as Ti carbides during tempering, thereby improving strength. Ti also has the function of fixing N and securing solid solution B, which is effective in improving the hardenability of B. These effects are obtained when the Ti content is 0.003% or higher. However, if the Ti content exceeds 0.050%, coarse Ti carbonitrides are formed in high-temperature regions such as during solidification, and the amount of Ti carbide precipitation during tempering becomes excessive, resulting in a decrease in toughness. Therefore, the Ti content is set to 0.003% to 0.050%. It is preferable that the Ti content be 0.005% or higher, and 0.015% or lower.

[0105] In addition, as mentioned above, in order to fix N, it is desirable to satisfy Ti / N ≥ 48 / 14.

[0106] V: 0.01~0.20%

[0107] V precipitates as V carbides during tempering, thereby improving strength. This effect is obtained when the V content is 0.01% or higher. However, if the V content exceeds 0.20%, the amount of V carbides precipitated during tempering becomes excessive, which lowers toughness. Additionally, the Pcm increases, making it easier for weld cracks to occur. Therefore, the V content is set to 0.01% to 0.20%. Furthermore, it is preferable that the V content be 0.04% or higher. It is also preferable that the V content be 0.15% or lower, and more preferable that it be 0.10% or lower.

[0108] Total of one or more of Ca, Mg, and REM: 0~0.0250%

[0109] Ca, Mg, and REM all have the function of improving toughness by improving the form of inclusions through reaction with S to form sulfides. For this reason, one or more of Ca, Mg, and REM may be included as needed. To stably obtain this effect, it is desirable that the total content of these components be 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, which conversely leads to a decrease in 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.

[0110] In the present invention, "REM" refers to a total of 17 elements including Sc, Y, and lanthanides, and "REM content" refers to the content of REM when there is only one type, and the total content of REM when there are two or more types. In addition, REM is generally supplied as mischmetal, which is an alloy of multiple types of REM. Therefore, individual elements may be added and contained in one or two or more types, and, for example, may be added in the form of mischmetal.

[0111] The base material according to the present invention consists of each of the elements described above, and the remainder consists of Fe and impurities. Here, "impurities" refers to components that are incorporated due to raw materials such as ore and scrap, or various factors of the manufacturing process when steel materials are manufactured industrially, and are permitted within a range that does not adversely affect the present invention.

[0112] Pcm: 0.25~0.30

[0113] The base material according to the present invention has a Pcm of 0.25 to 0.30, as indicated by the following [A] formula. If the Pcm is less than 0.25, it becomes difficult to secure sufficient joint strength. On the other hand, by making the Pcm 0.30 or less, it becomes possible to prevent low-temperature cracking in the circumferential weld.

[0114] Pcm=C+(Si / 30)+(Mn / 20)+(Cu / 20)+(Ni / 60)+(Cr / 20)+(Mo / 15)+(V / 10)+5B··· [A]

[0115] [A] The element symbols in the formula represent the content (mass%) of each element in the steel, and if not contained, they are set to zero.

[0116] (C) Chemical composition of the weld metal

[0117] The reasons for limiting the chemical composition of the weld metal are as follows. In the following description, "%" for the content of each element refers to "mass%". Furthermore, the chemical composition of the weld metal referred to herein means the chemical composition of the first layer weld.

[0118] C: 0.04~0.14%

[0119] C is an indispensable element for increasing strength. On the other hand, if the C content exceeds 0.14%, weldability is significantly reduced. Therefore, the C content is set to 0.04 to 0.14%. It is preferable that the C content be 0.06% or higher, and 0.12% or lower.

[0120] Si: 0.05~1.00%

[0121] Si is an element that has a strength-enhancing effect. To obtain this effect, the Si content needs to be 0.05% or more. However, if the Si content exceeds 1.00%, toughness decreases. Therefore, the Si content is set to 0.05~1.00%. It is preferable that the Si content be 0.10% or more and 0.60% or less.

[0122] Mn: 1.00~2.00%

[0123] Mn is an element that improves strength. To obtain this effect, it is necessary to include at least 1.00% Mn. However, if the Mn content exceeds 2.00%, toughness decreases. Therefore, the Mn content is set to 1.00 to 2.00%. It is preferable that the Mn content be 1.20% or more and 1.80% or less.

[0124] P: 0.025% or less

[0125] If the P content exceeds 0.025%, the decrease in toughness becomes significant, making it difficult to secure the required Charpy impact value. For this reason, the P content as an impurity is set to 0.025% or less. It is preferable that the P content be 0.020% or less.

[0126] S: 0.005% or less

[0127] If the S content exceeds 0.005%, the decrease in toughness becomes significant, making it difficult to secure the required Charpy impact value. For this reason, the S content as an impurity is set to 0.005% or less. It is preferable that the S content be 0.003% or less.

[0128] Cu: 0.50% or less

[0129] If the Cu content exceeds 0.50%, it may result in a decrease in toughness. For this reason, the Cu content as an impurity is kept below 0.50%. It is preferable that the Cu content be below 0.40%, and more preferable that it be below 0.30%.

[0130] N: 0.007% or less

[0131] If the N content exceeds 0.007%, coarse nitrides are formed, and the decrease in toughness becomes significant, making it difficult to secure the required Charpy impact value. For this reason, the N content as an impurity is set to 0.007% or less. It is preferable that the N content be 0.006% or less.

[0132] Ni: 2.50~3.00%

[0133] Ni has the effect of improving strength and toughness. To obtain these effects, it is necessary to include at least 2.50% Ni. On the other hand, if Ni is included in excess of 3.00%, the alloy cost increases. Therefore, the Ni content is set to 2.50 to 3.00%. It is preferable that the Ni content be 2.60% or more, and 2.80% or less.

[0134] Cr: 0.90% or more and less than 1.40%

[0135] Cr has the effect of improving strength. To obtain this effect, it is necessary to include at least 0.90% Cr. On the other hand, if the Cr content exceeds 1.40%, it may lead to a decrease in toughness. Therefore, the Cr content should be 0.90% or more and less than 1.40%. It is preferable that the Cr content be 1.00% or more. Furthermore, it is preferable that the Cr content be 1.30% or less, and more preferable that it be 1.20% or less.

[0136] Mo: 0.40~0.90%

[0137] Mo has the effect of improving strength. To obtain this effect, it is necessary to include at least 0.40% of Mo. On the other hand, if the Mo content exceeds 0.90%, it may result in a decrease in toughness. Therefore, the Mo content should be 0.40% to 0.90%. It is preferable that the Mo content be 0.50% or more, 0.80% or less, and 0.70% or less.

[0138] Nb: 0.010% or less

[0139] Nb is an element that can be incorporated from the base material. However, if the Nb content exceeds 0.010%, it may degrade toughness. Therefore, the Nb content is kept below 0.010%. It is preferable that the Nb content be below 0.008%, and more preferable that it be below 0.005%.

[0140] Al: 0.010% or less

[0141] Al is an element that is inevitably incorporated from the base material. However, if the Al content exceeds 0.010%, it leads to a decrease in toughness. Therefore, the Al content is set to 0.010% or less. It is preferable that the Al content be 0.008% or less, and more preferable that it be 0.005% or less. Furthermore, the Al content of the present invention refers to the content in acid-soluble Al (so-called "sol. Al").

[0142] B: 0.0010% or less

[0143] B is an element that is inevitably incorporated from the base material. However, if the B content exceeds 0.0010%, there is a risk of solidification cracking occurring in the weld metal. Therefore, the B content should be 0.0010% or less. A lower B content is preferable, preferably 0.0007% or less, more preferably 0.0005% or less, and even more preferably 0.0003% or less. On the other hand, if it is desired to improve the strength of the weld metal, it may be actively included. To achieve this effect, the B content is preferably 0.0001% or more, and more preferably 0.0003% or more.

[0144] Ti: 0.003~0.050%

[0145] Ti has the effect of improving strength. This effect is obtained when the Ti content is 0.003% or higher. However, if the Ti content exceeds 0.050%, toughness decreases. Therefore, the Ti content is set to 0.003~0.050%. It is preferable that the Ti content be 0.005% or higher, and 0.015% or lower.

[0146] V: 0.01~0.20%

[0147] V has the effect of improving strength. This effect is obtained when the V content is 0.01% or higher. However, if the V content exceeds 0.20%, toughness decreases. Therefore, the V content is set to 0.01% to 0.20%. In addition, it is preferable that the V content be 0.04% or higher and 0.15% or lower.

[0148] Total of one or more of Ca, Mg, and REM: 0~0.0250%

[0149] Ca, Mg, and REM all react with S to form sulfides, thereby improving the form of inclusions and enhancing toughness. For this reason, one or more of Ca, Mg, and REM may be included as needed. To stably obtain this effect, it is desirable that the total content of these components be 0.0005% or more. On the other hand, if the total content of these components exceeds 0.0250%, the amount of inclusions increases, which lowers the cleanliness of the steel and consequently reduces toughness. Therefore, the upper limit of the total content of these elements is set to 0.0250%. It is desirable that the total content be 0.0100% or less, and more desirable that it be 0.0050% or less.

[0150] The weld metal portion according to the present invention consists of each of the elements described above, and the remainder is Fe and impurities. Here, "impurities" refers to components that are incorporated due to raw materials such as ore and scrap, or various factors of the manufacturing process when steel materials are manufactured industrially, and are permitted within a range that does not adversely affect the present invention.

[0151] (D) Metal structure of the base material

[0152] In order to achieve both high strength and high toughness, the base material according to the present invention preferably has a metal structure mainly composed of tempered martensite. Specifically, it is preferable that the area ratio of tempered martensite be 90% or more. There are no particular restrictions on the structure of the remainder, but it may include one or more selected from bainite, ferrite, and pearlite.

[0153] In addition, in the present invention, the metal structure is measured by the following method. First, a test specimen for observation is taken from the base material such that the cross-section perpendicular to the rolling direction, including the central part of the thickness of the steel pipe, becomes the observation surface. Here, if the steel pipe is a welded steel pipe, the test specimen for observation is taken at a position 180° away from the weld in the circumferential direction of the steel pipe. In the following description, to distinguish it from the "circumferential weld" mentioned above, the weld extending in the longitudinal direction of the steel pipe in a welded steel pipe is referred to as the "longitudinal weld." Then, after polishing the observation surface, Nital etching is performed. After that, the area percentage of tempered martensite is determined from a microstructure image taken with an optical microscope at a magnification of 500x.

[0154] (E) Machine characteristics

[0155] In the steel pipe welded joint according to the present invention, the tensile strength of the base material and the tensile strength in the joint tensile test of the circumferential weld (hereinafter referred to as "TS" in both cases) are both 980 MPa or higher. Since the TS of both the base material and the circumferential weld is 980 MPa or higher, it is possible to reliably achieve weight reduction, and thus it can be reliably used as a crane boom capable of responding to the increase in the size of cranes.

[0156] The preferred lower limit of TS for the base material and the circumferential weld is 1000 MPa. Also, the preferred upper limit of TS for the base material and the circumferential weld is 1100 MPa. Furthermore, in the steel pipe weld joint according to the present invention, the yield stress of the base material and the yield stress in the joint tensile test of the circumferential weld (hereinafter referred to as "YS" in both cases) are preferably 890 MPa or higher, and more preferably 900 MPa or higher.

[0157] In addition, in the present invention, the tensile strength and yield stress in the base material are measured by cutting out a No. 12B test specimen (an arc-shaped test specimen with a width of 25 mm) specified in JIS Z 2241:2011 from the base material and performing a tensile test in a room temperature atmosphere. In addition, the tensile strength and yield stress in the joint tensile test of the circumferential weld are measured using a No. 3 test specimen (width of the parallel section: 20 mm) conforming to JIS Z 3121:2013, which is taken such that the longitudinal direction of the steel pipe weld joint coincides with the longitudinal direction and the circumferential weld is located at the center of the parallel section. That is, the strength of the circumferential weld substantially becomes the joint strength. Here, when the base material is a welded steel pipe, the test specimen for the tensile test is taken at a position 180° away from the longitudinal weld in the circumferential direction of the steel pipe.

[0158] Here, in order to achieve the joint strength described above, it is necessary to suppress softening in the weld heat-affected zone as much as possible. Therefore, in the present invention, the average hardness of the base material is set to 300 HV10 or higher, and the average softening width in the weld heat-affected zone is set to 4.0 mm or less, and the average degree of softening is set to 80 HV10 or less.

[0159] In addition, there is no need to impose a special limit on the maximum hardness in the weld heat-affected zone, but from the perspective of suppressing low-temperature cracking, it is desirable to keep it 415HV10 or lower.

[0160] In the present invention, the "average hardness in the base material," the "average softening width in the weld heat-affected zone," the "average degree of softening in the weld heat-affected zone," and the "maximum hardness in the weld heat-affected zone" are determined in the following order.

[0161] 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 thickness, and 1.0 mm from the inner surface. By calculating the average of each measurement, the average hardness of the base material is obtained.

[0162] FIG. 2 is a diagram illustrating a method for measuring the average softening width and average softening degree in the weld heat-affected zone. As shown in FIG. 2a, a cross-section parallel to the longitudinal direction of the steel pipe is cut, passing through the axis of the steel pipe, and including a base material part (1a, 1b), a weld metal part (2a), and a weld heat-affected zone (2b, 2c).

[0163] Then, on three lines parallel to the length direction of the steel pipe passing through a position 1.0 mm from the outer surface, a position at the center of the thickness, and a position 1.0 mm from the inner surface, hardness measurements are performed at intervals of 1.0 mm, including a position 0.5 mm away from the boundary between the weld metal part (2a) and the weld heat-affected zone (2b, 2c) toward the base material part (1a, 1b).

[0164] After that, as shown in FIG. 2b, a measurement point where the hardness is lowest is identified and designated as the lowest hardness position. In addition, if there are multiple measurement points where the hardness is lowest, the measurement point closest to the base material (1b) among them is designated as the lowest hardness position. Then, the difference between the average hardness in the above base material and the hardness at the lowest hardness position is defined as the degree of softening.

[0165] Next, in the area from the lowest hardness position toward the base material part (1b) side until the hardness changes to a decrease, among the measurement points on the base material part (1b) side where the hardness between two adjacent points is 10 HV or more, the measurement point furthest from the weld metal part (2a) is identified and designated as the outer softening limit position.

[0166] Next, within the region from the lowest hardness position to the boundary between the weld metal part (2a) and the weld heat-affected zone (2c), a measurement point having the hardness closest to the hardness at the outer softening limit position is identified and designated as the inner softening limit position. Then, the distance in the direction parallel to the length direction of the steel pipe from the outer softening limit position to the inner softening limit position is defined as the softening width.

[0167] The above degree of softening and softening width are measured at a total of six locations, each on the side of the weld heat-affected zone (2b) and the side of the weld heat-affected zone (2c), at a position 1.0 mm from the outer surface, at the center of the thickness, and at a position 1.0 mm from the inner surface, and the average of these values ​​is taken as the average softening width and average degree of softening in the weld heat-affected zone.

[0168] In addition, the maximum value among the total hardness measurements in the weld heat-affected zone (2b, 2c) is defined as the maximum hardness in the weld heat-affected zone. Also, “HV10” refers to the “hardness symbol” when a Vickers hardness test is performed with a test force of 98 N (10 kgf) (refer to JIS Z 2244-1:2020).

[0169] In addition, in the steel pipe welded joint according to the present invention, the Charpy impact value of the base material at -40℃ is 75J / cm 2 It is desirable that the Charpy impact value at -40℃ be 75 J / cm² 2 If this is the case, it can be used sufficiently reliably even as a crane boom for work performed in cold regions. A more desirable lower limit for the Charpy impact value of the seamless steel pipe at -40°C is 125 J / cm² 2 And the higher it is, the more desirable it is.

[0170] (F) Thickness

[0171] There is no particular limitation on the thickness of the base material in the steel pipe welded joint according to the present invention. However, if the thickness of the base material exceeds 45.0 mm, bainite is prone to form in the base material, making it difficult to form a structure mainly of tempered martensite. Therefore, it is preferable that the thickness of the base material be 45.0 mm or less, and more preferable that it be 40.0 mm or less, 30.0 mm or less, or 20.0 mm or less. On the other hand, from the perspective of securing the strength of the steel pipe welded joint, it is advantageous for the thickness to be thicker. For this reason, it is preferable that the thickness of the base material be 5.0 mm or more, more preferable that it exceed 8.0 mm, and more preferable that it exceed 12.0 mm. This is because as the thickness increases, the strength of the steel pipe welded joint tends to increase due to the suppression of HAZ softening caused by the increased cooling rate after welding and the increased restraining force against deformation.

[0172] (G) Method for manufacturing the base material

[0173] The base material used for manufacturing the steel pipe welded joint according to 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 is not limited thereto.

[0174] A steel having the chemical composition described in item (B) above is melted in the same way as a general low-alloy steel and then made into an ingot or a billet by casting. In addition, it may be made into a billet with a round billet shape for can making by the so-called "Round CC" method.

[0175] As a next step, the cast ingot or billet is subjected to bloom rolling or hot forging. This process is a process for obtaining a material to be used for final hot pipe making (e.g., pipe making by hot drilling, rolling, and drawing processes, or pipe making by hot extrusion press). In addition, since the billet formed into a round billet shape by the above "Round CC" method can be directly used to finish it into a seamless steel pipe, it is not necessary to perform bloom rolling or hot forging.

[0176] A seamless steel pipe of the present invention is manufactured by sequentially performing the processes [i] through [iv] shown below on a material used for final hot pipe making and a cast billet shaped like a round billet (hereinafter referred to as "steel billet") manufactured by the above-mentioned breakdown rolling or hot forging.

[0177] [i]: A hot tube manufacturing process in which a steel billet is heated to 1200~1300℃ and then processed by a cross-sectional reduction rate of 40~99% to produce a small tube.

[0178] After heating the aforementioned steel billet to 1200–1300°C, processing is performed with a cross-sectional reduction rate of 40–99% to manufacture a small tube having a predetermined shape. If the heating temperature of the steel billet is below 1200°C, the deformation resistance increases when processing with a cross-sectional reduction rate of 40–99%, increasing the load on the tube-making equipment and potentially causing processing defects such as flaws or cracks. On the other hand, if the heating temperature of the steel billet exceeds 1300°C, it may result in high-temperature intergranular cracking or a decrease in ductility. Therefore, in the hot tube-making process, the heating temperature of the steel billet is first set to 1200–1300°C.

[0179] Even if the heating temperature of the steel billet is within the above range, if the cross-sectional reduction rate in the hot pipe forming process after heating is less than 40%, even if the cooling process [ii] described later is performed, a fine quenched structure is not formed in the quenching process [iii], and the desired mechanical properties cannot be provided to the seamless steel pipe. On the other hand, in a pipe forming process where the cross-sectional reduction rate exceeds 99%, it may be necessary to expand the pipe forming equipment. Therefore, the hot pipe forming process is to perform processing with a cross-sectional reduction rate of 40 to 99%.

[0180] The heating temperature in the process of [i] refers to the temperature on the surface of the steel billet. The holding time in the above temperature range varies depending on the size and shape of the steel billet, but it is preferable to set it to 60 to 300 minutes. In addition, the small tube finishing temperature in hot tube making is preferably 850 to 950°C. The above-mentioned small tube finishing temperature refers to the temperature on the outer surface of the small tube. In the process of [i], the preferred lower limit of the heating temperature is 1230°C, and the preferred upper limit is 1280°C. In addition, the preferred lower limit of the cross-sectional reduction rate is 50%, and the preferred upper limit is 90%.

[0181] [ii]: A cooling process for cooling the above-mentioned tube to a temperature below the Ac1 point.

[0182] The small tube, finished in a predetermined shape, is cooled to a temperature below the Ac1 point in the quenching process of [iii] to obtain a fine quenched structure. There is no particular limit on the cooling rate at this time. Additionally, for the small tube after hot forming, it may be cooled to room temperature and then reheated to perform the next process of [iii], or after hot forming, it may be cooled to a suitable temperature below the Ac1 point and then heated directly from that temperature to perform the next process of [iii]. The cooling temperature in this process of [ii] refers to the temperature on the outer surface of the small tube.

[0183] [iii]: Quenching process of heating the cooled tube to Ac3 point to 950℃ and then rapidly cooling it.

[0184] Next, a quenching treatment is performed on the pipe cooled in the process of [ii] above, by heating to a temperature of Ac3 to 950°C and then rapidly cooling. If the heating temperature is below the Ac3 point, austenitization is not completed, so it may not be possible to provide the desired mechanical properties to the seamless steel pipe. On the other hand, if the heating temperature exceeds 950°C, fine austenite grains cannot be obtained in a single quenching treatment, so it may not be possible to provide the desired mechanical properties to the seamless steel pipe. Therefore, the heating temperature during the quenching treatment is set to Ac3 to 950°C.

[0185] The holding time at the above heating temperature varies depending on the size of the tube, but it is preferable to keep it between 5 and 30 minutes. As long as nearly uniform heating is possible, a short-duration rapid heating treatment using induction heating is acceptable. The heating temperature in this [iii] process refers to the temperature on the outer surface of the tube. For rapid cooling, a suitable method such as water cooling or oil cooling may be used as long as a sufficient quenched structure can be obtained. In the process of [iii], the preferred lower limit of the heating temperature is 880°C, and the preferred upper limit is 920°C.

[0186] [iv]: A tempering process in which the quenched tube is heated to 500–600°C and then cooled to room temperature.

[0187] In order to provide the small tube quenched in the process of [iii] above with the desired mechanical properties as a seamless steel tube, a tempering treatment is performed by heating to 500 to 600°C and then cooling to room temperature. In the case of the chemical composition described in item (B) above, if the heating temperature for tempering is below 500°C, the desired strength (TS) can be secured, but the low-temperature toughness is reduced, and the Charpy impact value at -40°C is 75 J / cm 2There are cases where it falls below that. On the other hand, if the heating temperature for tempering exceeds 600°C, even if the required low-temperature toughness (Charpy impact value at -40°C) can be obtained, the strength decreases, and there are cases where high strength such as a TS of 980 MPa or higher cannot be secured. Therefore, the heating temperature during tempering treatment is set to 500~600°C.

[0188] The holding time at the above heating temperature varies depending on the size of the tube, but it is preferable to keep it between 30 and 60 minutes. The heating temperature in this [iv] process refers to the temperature on the outer surface of the tube. There is no particular limit on the cooling rate during tempering. Therefore, cooling according to the equipment, such as cooling in the atmosphere, forced air cooling, mist cooling, oil cooling, or water cooling, may be performed. In the process of [iv], the preferred lower limit of the heating temperature is 525°C, and the preferred upper limit is 575°C.

[0189] (H) Method for manufacturing welded steel pipe joints

[0190] A steel pipe welded joint can be manufactured by butting the pipe ends of the base material parts manufactured by the above method and performing circumferential welding using a welding material such as solid wire or flux-containing wire.

[0191] In order to suppress softening in the heat-affected zone of a weld, it is necessary to perform welding with low heat input. Also, to increase production efficiency, even if the first layer is welded with low heat input, it is common practice to gradually increase the heat input from the second layer onwards. However, in the present invention, from the perspective of securing joint strength, welding is performed with low heat input of 0.5 kJ / mm or less from the first layer to the final layer.

[0192] In addition, by performing welding with low heat input from the first layer to the final layer, the inflow of alloying elements from the base metal to the weld metal can be minimized, and in particular, the B content in the weld metal can be reduced. Accordingly, the occurrence of high-temperature cracks, such as solidification cracks, can be suppressed.

[0193] In addition, in normal construction, preheating is performed before welding to prevent low-temperature cracking. However, in the present invention, preheating is not performed to suppress softening in the weld heat-affected zone, and the inter-pass temperature is also managed to be low. Specifically, the inter-pass temperature is set to 150°C or lower.

[0194] In addition, other welding conditions may be performed under general conditions, for example, gas shielded arc welding is used. In this case, the welding current, voltage, welding speed, and shielding gas can be appropriately selected from known techniques. Furthermore, although there are no specific restrictions on the type of welding material, it is necessary to select a welding material whose chemical composition of the weld metal satisfies the above specifications.

[0195] In addition, when performing circumferential welding, it is desirable to perform multipass welding. When the thickness is 5.0 mm or more, it is difficult to perform welding in a single layer using conventional gas shielded arc welding, etc. Although it is possible to perform welding in a single layer using laser welding, etc., in that case, it is necessary to use a high heat input, narrow the gap between the bevel tips, and reduce the bevel angle. In the former case, as mentioned above, it is not desirable because the inflow of B from the base material to the weld metal becomes significant.

[0196] On the other hand, if the gap between grooves is narrow and the groove angle is small, welding defects may occur, which may consequently reduce the fatigue strength of the joint. Therefore, from the perspective of ensuring the fatigue strength of the joint, it is desirable to perform multipass welding after ensuring a sufficient gap between grooves. In other words, it is desirable for the weld metal to be multipass weld metal.

[0197] For the same reason, the width (W) of the weld metal part shown in FIG. 2 is preferably greater than 7.0 mm, and more preferably greater than 9.0 mm. Using FIG. 2, a method for measuring the width (W) of the weld metal part is explained. As shown in FIG. 2, in a cross-section parallel to the longitudinal direction of the steel pipe passing through the axis of the steel pipe, an intersection point (2d) is identified where the boundary between the weld metal part (2a) and the weld heat-affected zone (2b) intersects the outer surface of the steel pipe weld joint. Likewise, an intersection point (2e) is identified where the boundary between the weld metal part (2a) and the weld heat-affected zone (2c) intersects the outer surface of the steel pipe weld joint. Then, the distance between the intersection point (2d) and the intersection point (2e) in the longitudinal direction of the steel pipe becomes the width (W) of the weld metal part.

[0198] The present invention will be explained more specifically below by way of examples, but the present invention is not limited to these examples.

[0199] Example 1

[0200] Steels A to H having the chemical compositions shown in Table 1 were melted and rectangular billets were cast by a converter-continuous casting process. The rectangular billets were further formed into round billets by hot forging and cooled to room temperature.

[0201] Steels A to E in Table 1 are steels whose chemical composition is within the range specified in the present invention, and steels F to H are steels whose chemical composition deviates from the conditions specified in the present invention. In addition, Table 1 also shows the Ac1 and Ac3 points obtained from the following equations (i) and (ii).

[0202] Ac1 point (°C) = 723 + 29.1 × Si - 10.7 × Mn - 16.9 × Ni + 16.9 × Cr···(i)

[0203] 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)

[0204]

[0205] The above-mentioned circular billet was heated to 1240°C, and two seamless steel pipes with an outer diameter of 114.3 mm and a thickness of 8.6 mm were produced using the Mannesmann-Mandrel method, with the finishing temperature ranging from 850 to 950°C, and then cooled to room temperature. The reason for producing relatively thin steel pipes here is that it was assumed that if the corresponding strength could be secured in thin steel pipes, which are disadvantageous in terms of the strength of welded steel pipe joints, then the corresponding strength could also be sufficiently secured in thicker steel pipes. Each seamless steel pipe obtained in this manner was subjected to quenching and tempering under the conditions shown in Table 2 to produce steel pipe base materials. Furthermore, all quenching was performed by water quenching. Cooling during tempering was performed by air cooling in the atmosphere.

[0206] After that, observation specimens were taken from the base material of each steel pipe weld joint so that the cross-section perpendicular to the rolling direction was the observation surface, and after polishing the observation surface, Nital etching was performed. After that, the area percentage of tempered martensite was determined from the microstructure image taken with an optical microscope at a magnification of 500x.

[0207]

[0208] Next, grooves were machined at the ends of the two obtained steel pipe base materials to have a groove angle of 60°. Then, with the ends butted together, circumferential welding was performed by gas shielded arc welding under the conditions shown in Table 2 to produce steel pipe welded joints (test numbers 1 to 12). When performing circumferential welding, a solid wire for high-strength steel (YM-100A) manufactured by Nittetsu Welding Industry Co., Ltd. was used as the welding material, and Ar-20% CO2 was used as the shielding gas. In addition, a backing material was used, and the spacing of the grooves was set to 0 mm. For each obtained steel pipe welded joint, the results of measuring the chemical composition of the first layer weld of the weld metal are shown in Table 3.

[0209]

[0210] Next, from the base material of each steel pipe welded joint, a No. 12B test specimen (an arc-shaped test specimen with a width of 25 mm) as specified in Annex E of JIS Z 2241:2011 was cut. In accordance with JIS Z 2241:2011, a tensile test was performed in an atmosphere at room temperature to determine YS and TS. In addition, from the base material of each steel pipe welded joint, three 2 mm V-notch test specimens with a width of 10 mm and a thickness of 5 mm were cut, with the notch surface being a plane that includes the pipe axis direction and the thickness (pipe diameter) direction, and the pipe axis direction and the length direction being aligned. Then, in accordance with JIS Z 2242:2018, a Charpy impact test was performed at -40°C. The impact value was determined from the average of the absorbed energies of each of the three specimens. Specifically, the measured absorbed energy (J) is perpendicular to the longitudinal direction of the test specimen and also at the cross-sectional area at the notch position (for example, in the case of a thickness of 5 mm, width 8 mm × thickness 5 mm = 0.4 cm 2 It was calculated by dividing by ).

[0211] Next, using a No. 3 test specimen (width of parallel section: 20 mm) in accordance with JIS Z 3121:2013, which was taken such that the longitudinal direction of each steel pipe weld joint coincided with the longitudinal direction and the circumferential weld was located at the center of the parallel section, a tensile test of the circumferential weld joint was performed to obtain YS and TS.

[0212] In addition, the "average hardness in the base material," "average softening width in the weld heat-affected zone," "average softening degree in the weld heat-affected zone," "maximum hardness in the weld heat-affected zone," and "width (W) of the weld metal part" were measured in the order described above.

[0213] In addition, resistance to solidification cracking and resistance to low-temperature cracking were evaluated by the following methods. Specifically, the presence or absence of solidification cracking was evaluated by the C-type jig restrained butt weld cracking test method, and the presence or absence of low-temperature cracking was evaluated by the Y-type weld cracking test method. Each test method is described in detail below.

[0214] First, slabs were produced from the above steels A to H, heated at 1250°C for 60 minutes, and then hot-rolled in a temperature range of 1000 to 1250°C to produce steel plates with a thickness of 8.6 mm. Subsequently, quenching and tempering were performed under the conditions shown in Table 2 to obtain steel plates corresponding to each test number.

[0215] Two steel plates measuring 120 mm × 200 mm were cut from the obtained steel plate, and an improved shape was formed to produce a test plate with the shape shown in Fig. 3. Then, a C-type jig restrained butt weld crack test was performed in accordance with JIS Z 3155:1993. At this time, two weld beads were formed, and the welding conditions were the same as those shown in Table 2. Afterward, the presence or absence of cracks was investigated using the method described in JIS Z 3155:1993. When no cracks were observed in either of the two weld beads, it was evaluated as "no solidification cracks" (A). In addition, if a crack was observed in one, it was evaluated as "solidification cracks present" (B), and if cracks were observed in both, it was evaluated as "solidification cracks present" (C). In this embodiment, it was determined that it possesses excellent low-temperature crack resistance only when "no low-temperature cracks" (A) was observed.

[0216] In addition, a steel plate measuring 150 mm × 200 mm was cut from the aforementioned steel plate, and four holes with a diameter of 8 mm were formed. Then, two grooves with a width of 5 mm were formed to connect two of the holes. After that, a groove was formed between the two grooves by electrical discharge machining to produce a test plate with the shape shown in Fig. 4. Furthermore, regarding the shape of the test plate, a Y-shaped weld crack test was performed in accordance with JIS Z 3158:2016. At this time, the welding conditions were the same as those shown in Table 2. Subsequently, the presence or absence of cracks was investigated using the method described in JIS Z 3158:2016. In addition, the crack investigation was performed on five cross-sections of the formed weld bead divided into four equal parts. When no cracks were observed in any of the cross-sections, it was evaluated as "no low-temperature cracks" (A). In addition, if cracks are observed in two or fewer cross-sections, it is evaluated as having low-temperature cracks (B), and if cracks are observed in three or more cross-sections, it is evaluated as having low-temperature cracks (C). In this embodiment, the case of having no solidification cracks (A) or solidification cracks (B) was determined to have excellent resistance to solidification cracks.

[0217] Table 4 summarizes the results of each of the above investigations.

[0218]

[0219] As shown in Table 4, test numbers 1 to 5, which satisfy all the specifications of the present invention, resulted in high joint strength and excellent resistance to solidification cracking and low-temperature cracking. In contrast, test numbers 6 to 12, which are comparative examples that do not satisfy the specifications of the present invention, resulted in at least one of joint strength, resistance to solidification cracking, and resistance to low-temperature cracking deteriorating.

[0220] Example 2

[0221] As in Example 1, steels I to P having the chemical composition shown in Table 5 were melted and a rectangular billet was cast by a converter-continuous casting process. The rectangular billet was further formed into a round billet by hot forging and cooled to room temperature.

[0222]

[0223] The above-mentioned circular billet was heated to 1240°C, and two seamless steel pipes having the outer diameter and thickness shown in Table 6 were produced using the Mannesmann-Mandrel method so that the finishing temperature was in the range of 850 to 950°C, and then cooled to room temperature. Each seamless steel pipe obtained in this way was subjected to quenching and tempering under the conditions shown in Table 6 to produce steel pipe base materials. In addition, all quenching was performed by water quenching. Cooling during tempering was all performed by air cooling in the atmosphere.

[0224] After that, observation specimens were taken from the base material of each steel pipe weld joint so that the cross-section perpendicular to the rolling direction was the observation surface, and after polishing the observation surface, Nital etching was performed. After that, the area percentage of tempered martensite was determined from the microstructure image taken with an optical microscope at a magnification of 500x.

[0225]

[0226] Next, grooves were machined at the ends of the two obtained steel pipe base materials to have a groove angle of 60°. Then, with the ends butted together, circumferential welding was performed by gas shielded arc welding under the conditions shown in Table 6 to manufacture steel pipe welded joints (test numbers 13–20). When performing circumferential welding, welding materials having the chemical composition shown in Table 7 were used, and Ar-20% CO2 was used as the shielding gas. Additionally, backing material was used, and the spacing of the grooves was set to 0 mm. For each obtained steel pipe welded joint, the results of measuring the chemical composition of the first layer weld of the weld metal are shown in Table 8.

[0227]

[0228]

[0229] Next, in the same manner as in Example 1, the YS, TS, and Charpy impact values ​​of the base material, the YS and TS in the joint tensile test of the circumferential weld, the "average hardness of the base material," the "average softening width in the weld heat-affected zone," the "average degree of softening in the weld heat-affected zone," the "maximum hardness in the weld heat-affected zone," and the "width (W) of the weld metal" were measured. In addition, resistance to solidification cracking and resistance to low-temperature cracking were evaluated by the same method as in Example 1.

[0230] Table 9 summarizes the results of each of the above investigations.

[0231]

[0232] As shown in Table 9, test numbers 13 to 20, which satisfy all the specifications of the present invention, resulted in high joint strength and excellent resistance to solidification cracking and low-temperature cracking.

[0233] [Industrial Applicability]

[0234] According to the present invention, it is possible to obtain a steel pipe welded joint having high joint strength and excellent low-temperature crack resistance. Therefore, the steel pipe welded joint according to the present invention is suitable for machine structural members, and in particular for crane booms. Explanation of the symbols

[0235] 1a, 1b base material 2 Circular weld 2a Welded metal part 2b, 2c Weld Heat-Affected Zone Intersection point 2d, 2e 10 Steel pipe welded joints

Claims

Claim 1 A steel pipe welded joint comprising a pair of base material portions, which are steel pipes, and a circumferential weld portion joining the pair of base material portions, wherein the circumferential weld portion is composed of a weld metal portion and a pair of weld heat-affected zones, and the chemical composition of the base material portion is, in mass%, C: 0.10~0.20%, Si: 0.05~1.00%, Mn: 0.05~1.20%, P: 0.025% or less, S: 0.005% or less, Cu: 0.20% or less, N: 0.007% or less, Ni: 0.20~0.50%, Cr: 0.30% or more and less than 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%, total of one or more of Ca, Mg and REM: 0~0.0250%, remainder: Fe and impurities, and the value of Pcm represented by the following formula [A] is 0.25~0.30, and the chemical composition of the weld metal part is, in mass%, C: 0.04~0.14%, Si: 0.05~1.00%, Mn: 1.00~2.00%, P: 0.025% or less, S: 0.005% or less, Cu: 0.50% or less, N: 0.007% or less, Ni: 2.50~3.00%, Cr: 0.90% or more and less than 1.40%, Mo: 0.40~0.90%, Nb: 0.010% or less, Al: 0.010% or less, B: 0.0010% The following is the composition: Ti: 0.003~0.050%, V: 0.01~0.20%, total of at least one of Ca, Mg and REM: 0~0.0250%, remainder: Fe and impurities; the tensile strength of the base material and the tensile strength in the joint tensile test of the circumferential weld are both 980 MPa or higher; the yield stress of the base material and the yield stress in the joint tensile test of the circumferential weld are both 890 MPa or higher; the metal structure of the base material is tempered martensite: 90% or higher in area %; the average hardness of the base material is 300 HV10 or higher; and the average softening width in the weld heat-affected zone is 4.The thickness is 0 mm or less, and the average softening degree in the weld heat-affected zone is 80 HV10 or less; the steel pipe comprises the pair of base material parts, the weld metal part, and the pair of weld heat-affected zones, and a cross-section parallel to the longitudinal direction of the steel pipe passing through the axis of the steel pipe is cut, and hardness measurements are performed at 1.0 mm intervals along three lines parallel to the longitudinal direction passing through a position 1.0 mm from the outer surface of the base material part, a position at the center of the thickness, and a position 1.0 mm from the inner surface, so as to include a position 0.5 mm away from the boundary of the weld metal part and the weld heat-affected zone toward the base material part, and a measurement point where the hardness is minimum is identified and designated as the minimum hardness position, the difference between the average hardness in the base material part and the hardness at the minimum hardness position is defined as the softening degree, and within the region extending from the minimum hardness position toward the base material part until the hardness changes to a decrease, the hardness between two adjacent points A steel pipe welded joint, wherein among the measurement points on the base material side having a hardness of 10 HV or more, the measurement point furthest from the weld metal part is designated as the outer softening limit position, and within the area from the lowest hardness position to the boundary between the weld metal part and the weld heat-affected zone, a measurement point having a hardness closest to the hardness at the outer softening limit position is designated as the inner softening limit position, and the distance in a direction parallel to the length direction from the outer softening limit position to the inner softening limit position is designated as the softening width, and the degree of softening and the softening width are measured at a total of six locations on each side of the pair of weld heat-affected zones, including a position 1.0 mm from the outer surface, a position at the center of the thickness, and a position 1.0 mm from the inner surface, and the average of these values ​​is used as the average softening width and the average degree of softening in the weld heat-affected zone.Pcm=C+(Si / 30)+(Mn / 20)+(Cu / 20)+(Ni / 60)+(Cr / 20)+(Mo / 15)+(V / 10)+5B ··· [A] where the element symbols in [A] represent the content (mass%) of each element in the steel, and zero is used when the element is not present. Claim 2 A steel pipe welded joint according to claim 1, wherein the weld metal portion is a multipass weld metal. Claim 3 delete

Images