JP2005262253A - Seam welding method for high-strength uo steel pipe having excellent transverse crack resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a high-strength UO steel pipe capable of surely preventing the occurrence of the transverse crack in weld metal of a seam weld zone without the occurrence of an increase in a welding cost and the lowering in productivity in manufacturing the high-strength UO steel pipe having tensile strength of ≥800 MPa. <P>SOLUTION: In the seam welding method of the UO steel pipe of forming a steel sheet having the tensile strength of ≥800 MPa to a tubular form, then tack welding the butt section of the steel sheet and thereafter subjecting the steel sheet to inside surface seam welding followed by outside surface seam welding, the maximum ultimate temperature (T<SB>in</SB>) on the inside surface welded metal surface formed by inside surface seam welding at the time of outside surface seam welding satisfies the relational expression of T<SB>in</SB>≥0.6x [A<SB>c1</SB>transformation temperature of the inside surface welded metal]. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a large steel pipe used as a line pipe for transportation of natural gas or oil, and particularly relates to a method for producing a UO (also referred to as UOE) steel pipe having a tensile strength of 800 MPa or more.

A large-sized steel pipe used as a line pipe is generally called a UO steel pipe, and after a steel sheet is formed into a tubular shape by U-shaped press forming and further O-shaped press forming, the butt end of the steel sheet is temporarily welded It is manufactured by joining by inner surface seam welding and outer surface seam welding. At this time, for the tack welding, a gas shield arc welding method such as MAG welding or CO ₂ welding is mainly used, and recently, a laser welding method or the like is also used. Usually, tack welding is welded from the outer surface side of the steel plate butt portion formed into a tubular shape. In addition, seam welding mainly uses multi-electrode submerged arc welding (hereinafter also referred to as SAW welding method), which is superior in both welding efficiency and welding quality, as main welding after tack welding. Usually, in the seam welding, after the inner surface seam welding is performed from the inner surface side of the steel pipe seam portion that has been tack welded, the outer surface seam welding is performed from the outer surface side of the steel pipe seam portion.

  In recent years, line pipes tend to increase operating pressure from the viewpoint of improving the transportation efficiency of natural gas and oil and reducing the transportation cost as the usage environment. In addition, the thickness of the line pipe is required to be reduced from the viewpoint of improving the execution efficiency when assembling the line pipe. Based on these circumstances, the strength of UO steel pipes for line pipes has been increased.

  Currently, from the point of increasing the strength of UO steel pipes for line pipes, development of high-strength UO steel pipes with a tensile strength of 800 MPa or more, which exceeds the conventional X65 and X80 grades (American Petroleum Institute standard, API standard) is in progress. . However, on the other hand, when manufacturing a high-strength UO steel pipe having a tensile strength of 800 MPa or more, there has been a problem that a transverse crack, which is a kind of cold crack, occurs in the weld metal of the seam weld after seam welding. .

  In general, transverse cracks that occur in the weld metal at the weld after welding occur due to the content of diffusible hydrogen in the weld metal, tensile stress, structure hardness, etc., and the higher the content of diffusible hydrogen, the more It is said that the higher the tensile stress or the harder the structure, the easier the transverse cracks occur.

  In a high-strength UO steel pipe having a tensile strength of 800 MPa or more, it is necessary to increase the strength of the seam welded portion to be equal to or higher than that of the base steel material from the viewpoint of strength balance. With the increase in strength of seam welds, the hardenability alloy component amount increases, the cold cracking susceptibility increases, and the tensile stress and the structure hardness tend to increase, which is not seen in conventional low strength UO steel pipes. There has been a problem that the occurrence of lateral cracks becomes remarkable.

  As a conventional method for reducing the low temperature cracking of a steel pipe seam weld, as shown in Non-Patent Documents 1 and 2, etc., the reduction of the diffusible hydrogen content in the weld metal, which is the cause of the low temperature crack, or the seam weld Methods such as heat treatment such as preheating have been studied. However, these methods cause an increase in welding costs such as restrictions on the composition of the welding material used for welding and the need for new equipment such as a heating device, and workability and production accompanying the addition of heat treatment processes such as preheating. This will also cause problems such as deterioration of sex.

  In addition, for example, in Patent Document 1, as a method of reducing the tensile stress that is a cause of low temperature cracking, welding is performed by lowering the martensitic transformation point of the weld metal by defining the chemical composition of the weld metal of the seam weld. A method for preventing the occurrence of transverse cracks by reducing the residual stress has been proposed. In this method, after the weld metal melts and solidifies, in order to reduce the residual tensile stress resulting from thermal shrinkage during the cooling process to room temperature, the microstructure of the weld metal is caused on the low temperature side of the cooling process. This is a method of reducing the residual tensile stress by using the compressive stress due to transformation expansion.

  However, this method is a method of suppressing transverse cracking by lowering the structure transformation point of the weld metal, and in order to obtain this effect, the content of alloy elements necessary for lowering the structure transformation point is inevitably required. Will increase. Therefore, the toughness of the weld metal decreases with increasing alloying element content, and conversely, the sensitivity to cold cracking is increased to increase the risk of solidification cracking. This method sufficiently suppresses the occurrence of transverse cracks in the weld metal. There was a technical limit to do it.

JP 2001-71176 A “Journal of the Japan Welding Society” Vol. 46 (1977) No. 12, pages 875-880 "Journal of the Japan Welding Society" Volume 46 (1977) No. 8, pp. 561-566

  In view of these problems of the prior art, the present invention provides a weld metal for a seam welded portion without causing an increase in welding cost or a decrease in productivity when producing a high strength UO steel pipe having a tensile strength of 800 MPa or more. It aims at providing the manufacturing method of the high intensity | strength UO steel pipe which can prevent generation | occurrence | production of a lateral crack reliably.

  As a result of detailed examination of the occurrence of transverse cracks in the seam welds of high-strength UO steel pipes, the present inventors have found that the temperature of the inner surface seam weld metal surface during outer seam welding greatly affects the occurrence of transverse cracks. . The present invention has been made on the basis of this finding, and the gist thereof is as follows.

(1) In the seam welding method for a UO steel pipe, the steel sheet having a tensile strength of 800 MPa or more is formed into a tubular shape, the butt portion of the steel sheet is tack welded, and then the inner seam is welded and subsequently the outer seam is welded. A seam welding method for a high-strength UO steel pipe, characterized _in that a maximum ultimate temperature T _in on the inner surface weld metal surface formed by the inner surface seam welding satisfies the following expression (1) during welding.
T _in ≧ 0.6 × A _C1 transformation temperature of inner surface weld metal (1)
Where T _in is the highest temperature reached on the inner surface weld metal surface during outer seam welding, and A _{C1 is} the A _C1 transformation temperature of the inner surface weld metal.

  (2) The seam welding method according to (1), wherein the seam welding is performed using a submerged arc welding method.

  (3) The seam welding method for a high-strength UO steel pipe according to (1) or (2) above, wherein the outer surface seam welding keeps the inner surface weld metal surface warm.

  (4) The seam welding method for a high-strength UO steel pipe according to any one of (1) to (3), wherein the inner surface weld metal surface is heated simultaneously with the outer surface seam welding.

  (5) The high strength according to any one of (1) to (3) above, wherein the inner surface seam weld metal surface is heated immediately before the outer surface seam welding so that the heating temperature is 50 ° C. or higher. Seam welding method for UO steel pipe.

  According to the present invention, when producing a high-strength UO steel pipe having a tensile strength of 800 MPa or more, it is possible to reliably generate transverse cracks in the weld metal of the seam welded part without causing an increase in welding cost or a decrease in productivity. Can be prevented. Therefore, the manufacturing method of the high intensity | strength UO steel pipe used for the line pipe etc. which have a high quality weld part, maintaining high productivity can be provided, and the place which contributes industrially is large.

  Details of the present invention will be described below.

  As described above, a transverse crack, which is a kind of cold cracking that occurs in the weld metal of the seam weld when producing a UO steel pipe having a relatively high strength, is formed by forming a steel plate into a tubular shape and tack-welding the butt portion. It has been known in the art to occur after subsequent seam welding.

  However, the generation mechanism and conditions of the transverse crack are not necessarily clarified. Therefore, the present inventors first investigated in detail the occurrence and welding conditions of transverse cracks in the seam welded portion of a high-strength UO steel pipe having a tensile strength of 800 MPa or more, and then, based on the investigation results, We examined the welding method to suppress the occurrence.

  FIG. 1 shows an example of transverse cracks that occur in the weld metal of a seam welded portion of a high-strength UO steel pipe. In addition, a seam welded part is called seam welding (henceforth outer surface seam welding) from the outer surface side after seam welding (henceforth inner surface seam welding) the steel pipe seam part after tack welding from the inner surface side. ). Further, the inner surface weld metal indicates a weld metal portion formed by inner surface seam welding, and the outer surface weld metal indicates a weld metal portion formed by outer surface seam welding, respectively. The same shall apply).

  According to the investigation results of the present inventors, transverse cracks in the weld metal of the seam welded portion of the high-strength UO steel pipe are in the direction toward the inner surface from the fusion line 3 of the outer surface weld metal 1 and the inner surface weld metal 2 as shown in FIG. It has been confirmed that a large number of transverse cracks 4 occur in the inner surface weld metal separated by about 1 to 3 mm, and that the generation time is in a wide range from several hours to several days after the end of the outer surface seam welding.

  The inventors of the present invention heated the inner surface weld metal by outer surface seam welding to cause a large number of transverse cracks only in a specific region in the inner surface weld metal, and (a) diffusible hydrogen in the inner surface weld metal diffused and moved. It was considered that the starting point of the crack was generated by accumulating in the specific region, and (b) the residual tensile stress was generated in the specific region due to thermal contraction in the process of cooling the inner surface weld metal after heating.

FIG. 3 shows the relationship between the thickness direction distance from the outer surface weld metal surface and the temperature of the seam weld metal when the seam welded seam portion of the high-strength UO steel pipe is welded to the inner surface and then continuously welded to the outer surface. . FIG. 4 shows the relationship between the distance in the plate thickness direction from the outer surface weld metal surface during the same seam welding, the residual stress in the weld line direction, and the maximum temperature reached (T _in ) at the inner surface weld metal surface during the outer surface welding. Indicates.

The wall thickness of the UO steel pipe (steel plate thickness used for production) is 16 mm, and the heat input during outer seam welding is 2.2 kJ / mm. The seam weld metal contains C: 0.07 mass%, Si: 0.2 mass%, Mn: 1.8 mass%, Ni: 2 mass%, Cr: 1 mass%, Mo: 1 mass%, The _AC1 transformation point was about 640 ° C. The maximum temperature (T _in ) at the inner surface weld metal surface during outer seam welding shown in FIG. 4 is indicated by the relative value (T _in / A _C1 ) to the A _C1 transformation point of the seam weld metal. Residual stress was expressed as positive (+) residual tensile stress and negative (-) residual compressive stress.

As shown in FIG. 3, at the time of outer surface seam welding, a part of the inner surface weld metal is heated and melted to about 1600 ° C. by the welding heat input, and the inner surface weld metal surface 5 (from the outer surface weld metal surface 6 by the heat conduction). The thickness direction distance of 10 mm is equivalent to the fusion line with the outer surface weld metal) to about 1/3 of the thickness direction (the thickness direction distance from the outer surface weld metal surface: 10 to 12 mm) of the seam weld metal. It is heated to a temperature of 640 ° C. or higher, which is the A _C1 transformation point. At this time, diffusible hydrogen enters from the inner surface weld metal surface through the outer surface weld metal, but diffuses and moves, but the diffusion and movement speed becomes slower inside the plate thickness direction where the heating temperature is low. It is thought that it accumulates in the vicinity of the weld metal surface and becomes the starting point of cracking.

Also, A _C1 transformation point of the seam weld metal by the outer surface seam welding heat input from FIG. 4 (640 ° C.) over about from the inner surface weld metal surface 5 which is heated to a temperature in the thickness direction 1/3 of the area (the outer surface weld metal The plate thickness direction distance from the surface 6: 10 to 12 mm) is thermally contracted to room temperature after having undergone volume expansion by phase transformation at around 400 ° C. in the cooling process. At this time, if the maximum temperature (T _in ) at the inner surface weld metal surface during outer seam welding is high (T _in / A _C1 = 0.6, 0.7), if it is low (T _in / A Compared with _C1 = 0.55, 0.50), the area above the A _C1 transformation point of the weld metal on the inner surface increases, and the generation of compressive stress due to the volume expansion of the phase transformation during the cooling process increases. Even if tensile stress is introduced due to shrinkage, the residual stress state of the weld metal at room temperature can finally be made the residual compressive stress.

2, the ratio of the maximum temperature of the inner surface weld metal surface during the outer surface welding for A _C1 transformation point of the seam weld metal in the case of producing a high strength UO pipe using different steel sheet thickness of 14～30Mm ( The relationship between T _in / A _C1 ) and the occurrence of transverse cracks in the seam weld is shown. In addition, the evaluation of the transverse crack of the seam welded portion is carried out over the entire length of the seam welded portion of the UO steel pipe having a total length of 10 to 12 m by performing an ultrasonic flaw detection test and an X-ray nondestructive test to determine the presence or absence of the transverse crack. However, if it occurred, it was judged that transverse cracks occurred.

In 30mm below the thickness (steel pipe wall thickness) is from 2, regardless of the A _C1 transformation point of the steel sheet thickness and seam welding metal, transverse cracks occur in the seam weld zone of the high-strength UO steel pipes, the inner surface weld metal A _It can be reduced under the condition that the ratio (T _in / A _C1 ) of the maximum temperature (T _in ) at the inner surface weld metal surface during outer surface welding to the _C1 transformation point satisfies 0.6 or more. In other words, by performing the highest temperature (T _in) the outer surface seam welding so that 0.6 times the A _C1 transformation point determined by the chemical composition of the seam weld metal of the inner surface weld metal surface of high strength steel pipe seam The occurrence of transverse cracks in the weld can be reliably prevented.

From the above knowledge, in the present invention, after forming a steel plate having a tensile strength of 800 MPa or more into a tubular shape, the butt portion of the steel plate is tack welded, and then seam welding is performed on the inner surface, followed by seam welding on the outer surface. In the outer seam welding, the maximum attainable temperature T _in on the inner surface weld metal surface formed by the inner surface seam welding is made to satisfy the following expression (1).
T _in ≧ 0.6 × A _C1 transformation temperature of inner surface weld metal (1)
Where T _in is the highest temperature reached on the inner surface weld metal surface during outer seam welding, and A _{C1 is} the A _C1 transformation temperature of the inner surface weld metal.

With the configuration of the present invention, even when diffusible hydrogen that has entered through the outer surface weld metal diffuses, moves and accumulates near the inner surface weld metal surface during the seam welding of the outer surface of the high-strength steel pipe, By increasing the area above the A _C1 transformation point and increasing the generation of compressive stress due to volume expansion of the phase transformation during the cooling process, it becomes possible to make the residual stress state of the weld metal at room temperature the residual compressive stress. The occurrence of transverse cracks in the seam weld can be prevented.

  In the present invention, compared to the conventional method, there is no restriction on the welding method such as heat treatment of the seam welded part, the size of the UO steel pipe, the composition of the seam weld metal, etc., and the seam is maintained while maintaining good productivity. It is possible to manufacture a high-strength UO steel pipe with excellent welding quality that prevents transverse cracks in the welded portion.

  Further, in the present invention, in order to obtain the effect of suppressing the transverse crack of the seam welded portion, the seam welding method is not particularly limited, but multi-electrode submerged arc welding is used in terms of both welding efficiency and welding quality. Is preferred.

In the present invention, the method for adjusting the maximum temperature (T _in ) at the inner surface weld metal surface during the outer surface seam welding to a temperature that satisfies the above expression (1) is not particularly limited. For example, by adjusting either or both of the heat input and welding speed of outer surface seam welding, the maximum ultimate temperature (T _in ) on the inner surface weld metal surface during outer seam welding is adjusted to be higher. Can do. In addition, in the case of multi-electrode submerged arc welding, if the current of the first electrode or the first and second electrodes is increased, the penetration depth of the outer surface weld metal becomes deeper. As a result, the temperature of the inner surface weld metal surface also increases. Can be high. In addition to this, a method of increasing the current density by using a small-diameter welding wire to deepen the penetration is also conceivable. A method of adjusting the maximum temperature (T _in ) at the inner surface weld metal surface during outer surface seam welding by adjusting these welding conditions is preferable because it can be realized without lowering productivity and welding quality.

Further, in the present invention, in addition to the adjustment of the above welding conditions, a method of preventing heat dissipation from the inner surface weld metal surface by covering the inner surface weld metal surface with a heat insulating material or the like during outer surface seam welding. Is also preferable for adjusting (T _in ) the maximum temperature achieved on the inner surface weld metal surface during outer surface seam welding without lowering productivity and welding quality.

It is also effective to heat the inner surface weld metal surface simultaneously with the outer surface seam welding, for example, with a gas burner or a plasma torch. By heating the inner surface weld metal surface formed after inner surface seam welding with a gas burner or plasma torch at the same time as outer surface seam welding, the inner surface weld metal at the time of outer surface seam welding without reducing productivity or weld quality (T _in ) can be adjusted to the maximum temperature reached on the surface.

  It is also effective to heat the inner surface seam weld metal surface with, for example, a gas burner or a plasma torch immediately before the outer surface seam welding. In this case, in order to sufficiently obtain the heating effect on the inner surface seam weld metal surface, it is preferable to set the heating temperature of the inner surface seam weld metal surface to 50 ° C. or more, and immediately after this heating, the outer surface seam welding is performed.

The present invention is intended for high-strength UO steel pipes for line pipes having a tensile strength of 800 MPa or more and good toughness, and to suppress the occurrence of transverse cracks in seam welds of such high-strength UO steel pipes. Especially effective.
In the high-strength UO steel pipe for line pipes, the strength and toughness of the weld metal in the seam welded portion are required to be equal to or better than the base metal from the viewpoint of balance between strength and toughness between the base metal and the seam welded portion. For these reasons, the tensile strength of the weld metal in the present invention is preferably 800 MPa or more, which is equivalent to that of the base steel plate. The upper limit of the tensile strength is not particularly required, but the upper limit is preferably set to about 1200 MPa in order to ensure good toughness. In order to obtain such a weld metal having a tensile strength of 800 to 1200 MPa, the weld metal structure at room temperature is preferably mainly a bainite structure or a martensite structure.
In the present invention, the chemical component composition and each component content of the weld metal are not particularly limited, but the following component composition is desirable for a high-strength steel pipe having a tensile strength of 800 or more and excellent toughness.

  The content of C is preferably 0.02% by mass or more in order to ensure the strength of the weld metal. Further, when the content is 0.1% by mass or more, the weld metal becomes brittle and the toughness is lowered, so the upper limit of the content is preferably less than 0.1% by mass.

  Si has a deoxidizing action in the weld metal, and its content is preferably 0.05 mass% or more in order to obtain the effect. Further, if its content exceeds 0.35% by mass, the toughness of the weld metal is adversely affected, so the upper limit of its content is preferably 0.35% by mass.

  In order to improve the hardenability of the weld metal and ensure its strength, the content of Mn is preferably 0.3% by mass or more. Moreover, since toughness will fall when the content will be 2.0 mass% or more, it is preferable that the upper limit of the content shall be less than 2.0 mass%.

  Ni is preferably 0.5% by mass or more in order to improve the hardenability of the weld metal and ensure its strength. Moreover, since it becomes easy to generate | occur | produce a hot crack when the content will be 3.0 mass% or more, it is preferable to make the upper limit of the content less than 3.0 mass%.

In order to improve the hardenability and ensure the strength, Cr is preferably contained in an amount of 0.3% by mass or more. Moreover, since the toughness fall will be caused when the content will be 3.0 mass% or more, it is preferable to make the upper limit of the content into 3.0 mass% or less.
In order to improve the hardenability and to secure the strength, the Mo content is preferably 0.3% by weight or more. Moreover, since the toughness fall will be caused when the content will be 3.0 mass% or more, it is preferable to make the upper limit of the content into 3.0 mass% or less.

A high strength steel having a tensile strength of about 960 MPa is used as a base steel plate, and a UO steel pipe having a diameter of 750 to 900 mm is formed. After 72 hours from the pipe forming, an ultrasonic flaw test and an X-ray test are performed on the entire length of the seam weld. The presence or absence of transverse cracks was inspected by nondestructive inspection. Four types of plate thickness were prepared. Three-electrode submerged arc welding was used for inner seam welding and outer seam welding during pipe making. Two types of seam weld metal of about 1000 MPa class and 1100 MPa class were prepared. The A _C1 points of these weld metals are 640 ° C. and 620 ° C., respectively. The flux at the time of submerged arc welding was a melt type flux. The amount of diffusible hydrogen in the flux was measured using a No. 2 test piece in accordance with the method for measuring the amount of hydrogen in the high weld zone of JIS Z3118. As a result, the amount of diffusible hydrogen was about 10 ml / 100 g. The flux dries in normal welding, but in this example, the worst moisture absorption condition was intentionally assumed and used as it was without drying before use. Table 1 shows examples of the present invention and comparative examples.

Invention Examples 1 to 3 have a plate thickness of 14 mm, Invention Examples 4 and 5 have a plate thickness of 16 mm, Invention Examples 6 to 9 have a plate thickness of 20 mm, and Examples 10 and 11 have a plate thickness of 30 mm. Since the ratio (T _in / A _C1 ) between the maximum temperature T _in at the inner surface weld metal surface during seam welding and the A _C1 transformation point of the seam weld metal satisfies the condition of 0.6 or more, No transverse cracks occurred in the seam weld. In Invention Examples 6-9, but A _C1 transformation point is two using different seam weld metal, transverse cracks in the seam weld both cases did not occur at all.

Invention Examples 12 to 14 are examples in which a supplementary heat treatment is performed to increase the maximum temperature reached on the inner surface weld metal surface during outer surface seam welding. Invention Example 12 is a case where heat is maintained by covering the inner surface weld metal surface with ceramics, and Invention Example 13 is a case where the inner surface weld metal surface is heated by a gas burner during outer surface seam welding. In addition, Invention Example 14 is a case where the inner surface weld metal is heated by a gas burner so that the heating temperature of the inner surface weld metal becomes 50 ° C. immediately before the outer surface welding. In Invention Examples 12 to 14, the ratio (T _in / A _C1 ) between the maximum temperature T _in at the inner surface weld metal surface and the A _C1 transformation point of the seam weld metal during outer seam welding is 0.6 or more. Since the conditions were satisfied, no transverse cracks occurred in the seam weld.

On the other hand, Comparative Examples 1 to 5, welded to the seam welding conditions such as the heat input is incorrect, the A _C1 transformation point maximum temperature T _in the seam weld metal of the inner surface weld metal surface during the outer surface seam welding The ratios (T _in / A _C1 ) were both less than 0.6 and did not satisfy the conditions specified by the present invention, resulting in transverse cracks in the seam welds, resulting in poor weld quality.

It is a schematic diagram which shows the seam weld metal and transverse crack generation | occurrence | production area | region of a high intensity | strength UO steel pipe. It is a diagram showing the relationship between T _in / A _C1 and transverse cracks occurrence at the outer surface seam welding of high-strength UO pipe. It is a figure which shows the relationship between the seam weld metal plate thickness direction distance and temperature at the time of the outer surface seam welding of a high intensity | strength UO steel pipe. It is a figure which shows the relationship between the seam welding metal plate thickness direction distance at the time of the outer surface seam welding of a high intensity | strength UO steel pipe, and a welding line direction residual stress.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Outer surface weld metal 2 Inner surface weld metal 3 Welding line which is a boundary of outer surface weld metal and inner surface weld metal 4 Lateral crack 5 Inner surface weld metal surface 6 Outer surface weld metal surface

Claims (5)

In a seam welding method for a UO steel pipe, in which a steel plate having a tensile strength of 800 MPa or more is formed into a tubular shape, and then a butt portion of the steel plate is tack welded, then an inner seam weld is performed, and subsequently an outer seam weld is performed. A seam welding method for a high-strength UO steel pipe, characterized _in that the maximum temperature T _in the inner surface weld metal surface formed by the inner surface seam welding satisfies the following formula (1).
T _in ≧ 0.6 × A _C1 transformation temperature of inner surface weld metal (1)
Where T _in is the highest temperature reached on the inner surface weld metal surface during outer seam welding, and A _{C1 is} the A _C1 transformation temperature of the inner surface weld metal. The said seam welding is performed using the submerged arc welding method, The seam welding method of the high strength UO steel pipe of Claim 1 characterized by the above-mentioned. The method of seam welding of a high-strength UO steel pipe according to claim 1 or 2, wherein the outer surface seam welding retains the inner surface weld metal surface. The seam welding method for an advanced UO steel pipe according to any one of claims 1 to 3, wherein an inner surface welded metal surface is heated simultaneously with the outer surface seam welding. The method for seam welding a high-strength UO steel pipe according to any one of claims 1 to 3, wherein the inner surface seam weld metal surface is heated immediately before the outer surface seam welding so that the heating temperature is 50 ° C or higher. .

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