JP2004143556A - Thick, large-sized straight uoe steel pipe satisfying request for strict toughness, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick, large-sized straight UOE steel pipe which satisfies request for strict inside-outside toughness, and to provide a production method therefor, with respect to a both side one layer submerged arc welding method for a thick, high toughness UOE steel pipe having a plate thickness of ≥30 mm, by establishing an effective submerged arc welding technique for preventing the reheat embrittlement of an inside weld metal caused by outside welding. <P>SOLUTION: The composition of a base metal 1 is limited to a specified range. Further, the compositions (particularly, the contents of Mo and B) in an inside weld metal 2 subjected to reheat by outside welding and in an outside weld metal 3 subjected to no reheat, and Pcm are limited to respective specified ranges. <P>COPYRIGHT: (C)2004,JPO

Description

【００３２】
【表２】

【００３３】
【表３】

【００３４】
【表４】

【００３５】
【表５】

【００３６】
溶接終了後、図２に示すように溶接継手の外面位置（ａ：母材１外面から２ｍｍ深さの位置）、内面位置（ｂ：母材１内面から２ｍｍ深さの位置）、ルート位置（ｃ：試験片中心線６が会合部７を通る位置）よりそれぞれ、１０ｍｍ×１０ｍｍサイズのシャルピー衝撃試験片４を採取し、ＪＩＳ　Ｚ　２２４２に従いシャルピー衝撃試験を行った。表６に溶接金属の化学組成を示す。外面、内面、ルートの各位置から採取した試料に基づく衝撃試験結果は表７に示したとおりである。
【００３７】
【表６】

【００３８】
【表７】

【００３９】
表７より明らかなように、本発明例相当鋼管の溶接金属では、外面により再熱を受ける内面とどこからも再熱されない外面の両方とも高い靭性を示す。なお、内面と外面の溶接金属の組成及びＰｃｍを変えるにあたり、本実施例においては、それぞれ組成の異なる溶接ワイヤを様々な組合せで１、２、３、４電極に使用することで対処した（表５）。
【００４０】
【発明の効果】
本発明によれば、低温用高靭性ＵＯＥ鋼管の母材の化学成分ならびに両面１層サブマージドアーク溶接により形成される内面および外面溶接金属の化学成分をそれぞれ区別して限定したので、内外面ともに優れた低温靭性を有する厳格靭性要求を満たす厚肉大径ストレートＵＯＥ鋼管を得ることができる。
【図面の簡単な説明】
【図１】実施例における開先形状（ａ：板厚３１．４ｍｍ、ｂ：板厚３１．８ｍｍ）を示す断面図である。
【図２】実施例におけるシャルピー衝撃試験片採取位置を示す断面図である。
【符号の説明】
１　母材
２　内面溶接金属
３　外面溶接金属
４　シャルピー衝撃試験片
５　ノッチ
６　試験片中心線
７　会合部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is intended to obtain a strict toughness in a weld metal of a large-diameter UOE steel pipe, particularly a thick-wall UOE steel pipe manufactured by a large heat input submerged arc welding on the inner and outer surfaces, and a thick large-diameter straight pipe satisfying a strict toughness requirement. The present invention relates to a UOE steel pipe and a method for manufacturing the same.
[0002]
[Prior art]
In recent years, undersea pipelines have been required to have a thick wall and strict toughness, reflecting the deepening of oil and gas development, particularly in laying in sea areas exceeding a depth of 2000 m. As the most effective method for securing the toughness of a submerged arc weld metal, welding is performed using a welding material containing Mo, Ti, B, and the like, and is called an acicular ferrite (hereinafter, abbreviated as AF). A method of obtaining a fine weld metal structure is known (for example, see Patent Documents 1 and 2). Ti acts as a nucleation site for AF transformation in the weld metal as TiO 2, and B segregates at the former austenite grain boundary, thereby suppressing the coarsening of the proeutectoid ferrite at the grain boundary.
[0003]
However, in large heat input welding, the addition of Ti-B alone cannot completely prevent the formation of coarse grain boundary ferrite, so it is necessary to add Mo, an element that enhances hardenability, to achieve a uniform and fine structure. There is. Therefore, Mo-Ti-B-based submerged arc welding wires are widely used in seam welds of steel pipes requiring high toughness even at low temperatures.
[0004]
However, when performing single-sided welding on both sides with a Mo-Ti-B welding wire, high toughness is obtained by the fine AF structure in the inner and outer surface weld metal original portions, but the portion reheated by the outer surface welding of the inner surface weld metal. However, there is a problem that toughness deteriorates (reheat embrittlement). As measures against such reheat embrittlement, the weld metal is reduced in oxygen (Patent Document 3), the addition of Mn and Mo is suppressed, and Ni is added instead (Patent Document 4), and the Ceq is limited. Then, a technique of optimizing the rolling conditions (Patent Document 5) is disclosed.
[0005]
Reheat embrittlement is based on the fact that Mo, Nb, V, Ti, etc., which are in solid solution in the as-welded state, are precipitated by reheating, and the formula: PHIW = Mo + 5Ti + 10Nb + 20V + 200 B A method for suppressing reheat embrittlement by setting a parameter PHIW represented by (mass%) to 1.5% by mass or less and 0.165% by mass or less for Pcm (Patent Document 6), Is to generate upper bainite (hereinafter referred to as UB), which is inferior in toughness in the cooling process at the time of reheating, and to reduce the generation of UB, the hardenability of the inner surface weld metal, that is, Pcm is set to the outer surface weld metal. (Patent Document 7) is disclosed.
[0006]
[Patent Document 1]
JP-A-53-63238 [Patent Document 2]
JP-A-53-22137 [Patent Document 3]
JP-A-59-156599 [Patent Document 4]
JP-A-62-34694 [Patent Document 5]
JP-A-61-266126 [Patent Document 6]
JP-A-5-375 [Patent Document 7]
JP-A-9-1344
[Problems to be solved by the invention]
The present inventors have diligently pursued the cause of the reheat embrittlement of the Mo-Ti-B-based weld metal. In an actual joint (welded portion actually constructed), reheating of the inner surface welding metal by outer surface welding is local, and it is difficult to measure the toughness of the reheated portion itself. Therefore, single-sided welding is performed under the heat input conditions and welding wire conditions equivalent to inner surface welding, a test piece of sufficient size to extract a Charpy test piece from the weld metal is cut out, and heat that simulates reheating by outer surface welding is simulated. Cycle was given. As a result of examining the effect of C, Mo, Ti, and B on reheat embrittlement, a conventional alloy composition of Mo-Ti-B-based weld metal can obtain good toughness in the original part, but has a temperature of 900 to 1200 ° C. It was observed that the toughness was remarkably deteriorated in the reheated portion with the temperature range as the highest point.
[0008]
From this, the present inventors interpret the reheat embrittlement mechanism of the conventional weld metal having the Mo-Ti-B composition as follows. The original weld metal has a fine structure mainly composed of AF and exhibits high toughness. However, when reheated to a temperature range of 900 to 1200 ° C., AF transforms to austenite, and during the cooling process, the austenite grain boundary Polygonal ferrite (Polygonal Ferrite: hereinafter referred to as PF) which forms and grows becomes a dominant structure, and as a result, UB and island-like martensite (Martensite-Austenite: hereinafter referred to as MA) are formed between PF grains. It becomes a mixed tissue containing. UB and MA are structures that promote embrittlement, that is, the present inventors consider that their formation is the main cause of reheat embrittlement. Furthermore, the present inventors believe that the amounts of C, Si, Mn, Mo, Ti, and B in the weld metal, particularly Mo and B, have a great influence on the formation of the MA structure in the reheat portion.
[0009]
The present invention is directed to a thick UOE steel pipe having a thickness of 30 mm or more. The seam portion of such a thick steel pipe is constructed in a double-sided, one-layer submerged welding process, but the welding metal has a large heat input, so that the cooling rate of the weld metal is reduced. Therefore, in order to obtain a fine AF structure in the weld metal original portion, it is necessary to limit the amounts of Mo and B to be added, as compared with a weld metal having a plate thickness of less than 30 mm. In the double-sided, single-layer submerged welding process, the amount of Mo and B can be increased relatively easily because the weld metal on the outer surface applied after the inner surface is not reheated.
[0010]
On the other hand, in the inner surface weld metal which is reheated by outer surface welding, Mo and B have a great effect on reheat embrittlement. An amount that does not become excessive must be selected in order to suppress embrittlement of the hot part. In particular, the reheat embrittlement region in the inner surface weld metal is enlarged by the outer surface welding with a large heat input. Therefore, the route that must include the reheat embrittlement portion of the inner surface weld metal, the definition of not only the absorbed energy (vE) but also the ductile fracture surface area (SA) in the Charpy impact test at the inner surface position is being generalized. Some consumers have demanded a minimum of 40% SA at -20 ° C. In order to achieve this, an absorption energy of 120 to 130 J or more is required.
[0011]
In view of such a trend, the present invention can effectively prevent the hottest embrittlement of the inner surface weld metal by outer surface welding in a double-sided, single-layer submerged arc welding method for a thick-walled high toughness UOE steel pipe having a thickness of 30 mm or more. It is an object of the present invention to provide a thick-walled large-diameter straight UOE steel pipe that satisfies strict toughness requirements on both the inner surface and the outer surface together with a manufacturing method thereof by establishing a suitable submerged arc welding technique.
[0012]
[Means for Solving the Problems]
The present invention that has achieved the above object is a thick-walled large-diameter straight UOE steel pipe having a base material having a thickness of 30 mm or more and having a welded portion on each of an inner surface and an outer surface having an inner surface formed first at one location in the circumferential direction of the tube. And
The composition of the base material is% by mass, including C: 0.10% or less, Si: 0.5% or less, Mn: 2.0% or less, P: 0.020% or less, and S: 0.0020% or less. Further, Cu: 0.5% or less, Cr: 0.5% or less, Ni: 0.5% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.05% or less, Ti: 0.1% or less, B: One or more kinds selected from 0.0030% or less, the balance being Fe and unavoidable impurities,
The composition of the weld metal on the inner surface side is% by mass, C: 0.08% or less, Si: 0.5% or less, Mn: 0.8 to 1.8%, Cu: 0.50% or less, Cr: 0.5% or less. 50% or less, Ni: 0.50% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.045% or less, Ti: 0.005 to 0.030%, B: 0 0.0005 to 0.0015%, N: 0.0080% or less, O: 0.035% or less, the balance being Fe and unavoidable impurities, and Pcm represented by the following formula being 0.120 to 0. 180%,
The composition of the weld metal on the outer surface side is% by mass, C: 0.10% or less, Si: 0.5% or less, Mn: 0.8 to 1.8%, Cu: 0.50% or less, Cr: 0. 50% or less, Ni: 0.50% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.1 to 0.4%, Ti: 0.005 to 0.050%, B: 0.0005% to 0.0060%, N: 0.0080% or less, O: 0.035% or less, the balance being Fe and inevitable impurities, and Pcm represented by the following formula is 0.140. It is a thick, large-diameter straight UOE steel pipe satisfying strict toughness requirements, characterized by being 0.200% or less.
[0013]
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B
In the formula, C, Si, Mn, Cu, Cr, Ni, Mo, V, and B represent mass% of each element.
Further, the present invention provides a thick-walled large-diameter UOE steel pipe comprising a step of joining a peripheral end butted portion of a UO-processed steel material having a plate thickness of 30 mm or more by submerged arc welding of inner and outer inner layers one by one by submerged arc welding. In the manufacturing method of
As the UO processed steel material (corresponding to the base material of the UOE steel pipe of the present invention), in mass%, C: 0.10% or less, Si: 0.5% or less, Mn: 2.0% or less, P : Not more than 0.020%, S: not more than 0.0020%, Cu: not more than 0.5%, Cr: not more than 0.5%, Ni: not more than 0.5%, Nb: not more than 0.07%, V: 0.07% or less, Mo: 0.05% or less, Ti: 0.1% or less, B: One or more kinds selected from 0.0030% or less, with the balance Fe and inevitable Using a composition consisting of impurities,
On the inner surface side, in mass%, C: 0.08% or less, Si: 0.5% or less, Mn: 0.8 to 1.8%, Cu: 0.50% or less, Cr: 0.50% or less , Ni: 0.50% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.045% or less, Ti: 0.005 to 0.030%, B: 0.0005 to 0.0005% 0.0015%, N: 0.0080% or less, O: 0.035% or less, the balance being Fe and unavoidable impurities, and the Pcm represented by the following formula is reduced to 0.120 to 0.180%. A weld metal of the composition
On the outer surface side, in mass%, C: 0.10% or less, Si: 0.5% or less, Mn: 0.8 to 1.8%, Cu: 0.50% or less, Cr: 0.50% or less , Ni: 0.50% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.1 to 0.4%, Ti: 0.005 to 0.050%, B: 0 0.0005% to 0.0060%, N: 0.0080% or less, O: 0.035% or less, the balance is composed of Fe and inevitable impurities, and Pcm represented by the following formula is 0.140 to 0. The above-mentioned submerged arc welding is performed using a welding material selected so as to form a weld metal having a composition of 200%. A method for manufacturing a thick-wall large-diameter straight UOE steel pipe satisfying strict toughness requirements. is there.
[0014]
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B
In the formula, C, Si, Mn, Cu, Cr, Ni, Mo, V, and B represent mass% of each element.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the reasons for limiting the chemical components of the base material in the UOE steel pipe according to the present invention will be described.
C: An element that has a very large effect on the strength and toughness of the base material. If it exceeds 0.10%, the toughness and ductility are adversely affected. In addition, Preferably, it is 0.03-0.08%.
[0016]
Si: Si is an element that is inevitably included in the process of deoxidizing steel, but if present in excessive steel, the toughness of the HAZ (heat affected zone) is deteriorated. It is. Preferably, the content is 0.1 to 0.4%.
Mn: a very important element that simultaneously improves the strength and toughness of the base material, but if it exceeds 2.0%, the segregation or the like adversely affects the steel material, so the upper limit was made 2.0%. Incidentally, the content is preferably 1.0 to 2.0%.
[0017]
P, S: P and S are elements that promote center segregation and are desirably low. The upper limits are 0.020% and 0.0020%, respectively.
Cu: an element necessary for securing the strength of the base material, but if the content exceeds 0.5%, the base material and the HAZ portion are hardened, so the upper limit is 0.5%.
Cr: an element necessary for securing the strength of the base material, but if contained in excess of 0.5%, the toughness of the HAZ portion is deteriorated, so 0.5% was made the upper limit.
[0018]
Ni: an element for improving the strength and toughness of the base material. However, if the content exceeds 0.5%, the HAZ portion is hardened, so the upper limit is 0.5%.
Nb, V: Nb and V are added to secure the strength and toughness of the base material and the HAZ portion. However, if both Nb and V exceed 0.07%, the toughness is adversely affected. V was set to 0.07% or less. Preferably, Nb: 0.02 to 0.06% and V: 0.06% or less.
[0019]
Mo: is an element necessary for securing the strength of the base material, but if contained in excess of 0.05%, the HAZ portion is hardened, so the upper limit was 0.05%.
Ti: Ti is an element necessary for ensuring the toughness of the base material. However, if contained in excess of 0.1%, the toughness of the base material is adversely deteriorated, so the upper limit is set to 0.1%. Incidentally, the content is preferably 0.005 to 0.03%.
[0020]
B: B has the effect of segregating at austenite grain boundaries during rolling to increase hardenability, but if it exceeds 0.0030%, the toughness of the HAZ portion is deteriorated, so the upper limit was made 0.0030%. . Preferably, it is 0.0005% or less.
The reasons for limiting the component composition of the weld metal are as follows.
C: C is a high quench hardening component, and an increase in the content causes carbides and martensite to be generated, resulting in a decrease in toughness. Therefore, the content needs to be 0.08% or less on the inner surface and 0.10% or less on the outer surface. Preferably, it is 0.03-0.07% on the inner surface and 0.03-0.08% on the outer surface.
[0021]
Si: Si is added as a deoxidizing agent, but at the same time, since it is a hardenable component, if it is added excessively, UB is formed and the toughness is reduced. Preferably, both the inner and outer surfaces are 0.1 to 0.4%.
Mn: Mn is necessary as a deoxidizing agent and a hardenable component, but the effect is poor when the content is less than 0.8% on both the inner surface and the outer surface. On the other hand, when the content exceeds 1.8%, UB is formed and the toughness is reduced. Therefore, the content was set to 0.8 to 1.8%.
[0022]
Cu: Cu is a hardenable component and is a component mixed from the base metal dilution and the plating of the wire. However, if the content exceeds 0.50% on both the inner surface and the outer surface, the hardenability becomes excessive and the toughness is impaired. The upper limit was 0.50%.
Cr: Cr is a hardenable component and is contained by dilution of the base material. However, if the content exceeds 0.50% on both the inner and outer surfaces, the hardenability becomes excessive and the toughness is impaired, so the upper limit is 0.50%. And
[0023]
Ni: Ni is a hardenable component and is contained by dilution of the base material. However, if the content exceeds 0.50% on both the inner and outer surfaces, the hardenability becomes excessive and the toughness is impaired, so the upper limit is 0.50%. And
Nb, V: Nb and V are hardenable components and are contained by dilution of the base material. However, if the content exceeds 0.07% on both the inner surface and the outer surface, the hardenability becomes excessive and the toughness is impaired. The upper limit of V was 0.07%. Preferably, both the inner surface and the outer surface are Nb: 0.02 to 0.06% and V: 0.06% or less.
[0024]
Mo: Mo is a hardenable component, which refines the structure and improves the toughness in the original weld metal part, but promotes the formation of UB and MA in the reheated part and lowers the toughness. Therefore, it must be suppressed to 0.045% or less on the inner surface. On the other hand, at least 0.1% is required on the outer surface to improve toughness by refining the structure, but if added over 0.4%, hardenability becomes excessive and martensitic structure occurs, impairing toughness. 0.1 to 0.4%.
[0025]
Ti: Ti forms fine ferrite to improve toughness. However, if the content is less than 0.005% on both the inner surface and the outer surface, this effect is poor, so at least 0.005% is required. On the other hand, if the content exceeds 0.030% on the inner surface and more than 0.050% on the outer surface, solid solution Ti increases and the toughness decreases due to precipitation of carbides and nitrides. Therefore, the inner surface is set to 0.005 to 0.030%, and the outer surface is set to 0.005 to 0.050%.
[0026]
B: B is a high quench hardening component and forms fine AF in the original weld metal due to a synergistic effect with Ti to improve toughness. However, if both the inner and outer surfaces are less than 0.0005%, this effect is poor. . On the other hand, the reheat portion promotes the formation of UB and MA, and lowers the toughness. Therefore, on the inner surface, the content needs to be suppressed to 0.0015% or less. On the other hand, if the content exceeds 0.0060% on the outer surface, martensite is formed in the raw material part and the toughness is reduced, so the upper limit was made 0.0060%.
[0027]
N: N is a component inevitably contained in the weld metal. However, since B is nitrided to promote the formation of grain boundary ferrite and reduce the toughness, it is necessary to make the inner and outer surfaces 0.0080% or less. There is.
O: O is a component inevitably contained in the weld metal, but if both the inner surface and the outer surface exceed 0.035%, the toughness decreases due to the increase in inclusions in the weld metal, so the upper limit is 0.035%. And
[0028]
Pcm: Pcm indicates the hardenability of the entire composition of the weld metal. If the hardenability is insufficient, pro-eutectoid ferrite precipitates and the toughness deteriorates. In order to obtain the toughness of the raw material portion without considering reheat embrittlement, 0.140% or more is required. Therefore, the lower limit on the outer surface is set to 0.140%. On the other hand, it is necessary to consider the balance between the inner surface and the portion reheated by the outer surface welding. On the other hand, if it exceeds 0.200%, the hardenability becomes excessive, so that the toughness is deteriorated even in the original part. Therefore, the upper limit of the outer surface is set to 0.200%. Further, if it exceeds 0.180%, UB and MA precipitate in the reheated portion, and the toughness is deteriorated. Therefore, the upper limit is set to 0.180% on the inner surface.
[0029]
In order to manufacture the UOE steel pipe of the present invention, a UO-processed steel material having a thickness of 30 mm or more, which is a base material joined by inner and outer surface single-layer submerged arc welding, having a base material composition of the present invention, The inner and outer welding materials (wire and / or flux) used for the submerged arc welding may be selected so that the inner and outer weld metal compositions fall within the scope of the present invention, respectively.
[0030]
【Example】
When the pipes PA and PB of the API standard X65 class UO pipes PA and PB having the chemical composition and plate thickness shown in Table 1 are joined at the peripheral ends by submerged arc welding of the inner and outer surfaces one by one on the inner surface side to form a welded pipe. The welding material used for the inner surface and the outer surface was variously changed, and welding was performed under the same welding conditions to produce a steel pipe equivalent to the present invention and a steel pipe equivalent to the comparative example deviating from the present invention, and their toughness was compared. In this comparative test, a submerged arc welding was performed using a combination of a molten flux having a chemical composition shown in Table 2 and a wire having a chemical composition shown in Table 3 on one side of each of the four electrodes. The welding conditions for each plate thickness in this case are as shown in Table 4, and the groove shape is as shown in FIG. Table 5 shows the combinations of the base materials and the wires of the respective electrodes in the examples and comparative examples.
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
[Table 3]

[0034]
[Table 4]

[0035]
[Table 5]

[0036]
After the welding is completed, as shown in FIG. 2, the outer surface position of the welded joint (a: a position 2 mm deep from the outer surface of the base material 1), the inner surface position (b: a position 2 mm deep from the inner surface of the base material 1), the root position ( c: A 10 mm × 10 mm size Charpy impact test piece 4 was sampled from each of the test piece center lines 6 passing through the associated portion 7) and subjected to a Charpy impact test according to JIS Z2242. Table 6 shows the chemical composition of the weld metal. The results of the impact test based on the samples taken from the outer surface, the inner surface, and the route are shown in Table 7.
[0037]
[Table 6]

[0038]
[Table 7]

[0039]
As is clear from Table 7, in the weld metal of the steel pipe equivalent to the present invention, both the inner surface that is reheated by the outer surface and the outer surface that is not reheated from anywhere show high toughness. In addition, in changing the composition and Pcm of the weld metal on the inner surface and the outer surface, in the present embodiment, various combinations of welding wires having different compositions were used for the 1, 2, 3, and 4 electrodes (see Table 1). 5).
[0040]
【The invention's effect】
According to the present invention, the chemical composition of the base material of the high-toughness UOE steel pipe for low temperature and the chemical composition of the inner surface and the outer surface weld metal formed by single-sided, single-layer submerged arc welding are separately limited, so that both the inner and outer surfaces are excellent. A thick large-diameter straight UOE steel pipe satisfying strict toughness requirements having low-temperature toughness can be obtained.
[Brief description of the drawings]
FIG. 1 is a sectional view showing a groove shape (a: plate thickness: 31.4 mm, b: plate thickness: 31.8 mm) in an example.
FIG. 2 is a cross-sectional view showing a Charpy impact test piece sampling position in an example.
[Explanation of symbols]
Reference Signs List 1 base metal 2 inner surface weld metal 3 outer surface weld metal 4 Charpy impact test specimen 5 notch 6 test specimen center line 7

Claims (2)

A thick large-diameter straight UOE steel pipe having a base material having a thickness of 30 mm or more and having a welded portion on each of the inner and outer surfaces each having an inner surface formed first at one location in the circumferential direction of the tube,
The composition of the base material is% by mass, including C: 0.10% or less, Si: 0.5% or less, Mn: 2.0% or less, P: 0.020% or less, and S: 0.0020% or less. Further, Cu: 0.5% or less, Cr: 0.5% or less, Ni: 0.5% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.05% or less, Ti: 0.1% or less, B: One or more kinds selected from 0.0030% or less, the balance being Fe and unavoidable impurities,
The composition of the weld metal on the inner surface side is% by mass, C: 0.08% or less, Si: 0.5% or less, Mn: 0.8 to 1.8%, Cu: 0.50% or less, Cr: 0.5% or less. 50% or less, Ni: 0.50% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.045% or less, Ti: 0.005 to 0.030%, B: 0 0.0005 to 0.0015%, N: 0.0080% or less, O: 0.035% or less, the balance being Fe and unavoidable impurities, and Pcm represented by the following formula being 0.120 to 0. 180%,
The composition of the weld metal on the outer surface side is% by mass, C: 0.10% or less, Si: 0.5% or less, Mn: 0.8 to 1.8%, Cu: 0.50% or less, Cr: 0. 50% or less, Ni: 0.50% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.1 to 0.4%, Ti: 0.005 to 0.050%, B: 0.0005% to 0.0060%, N: 0.0080% or less, O: 0.035% or less, the balance being Fe and inevitable impurities, and Pcm represented by the following formula is 0.140. Thick large-diameter straight UOE steel pipe satisfying strict toughness requirements characterized by being 0.200% or less.
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B
In the formula, C, Si, Mn, Cu, Cr, Ni, Mo, V, and B represent mass% of each element. A method for manufacturing a thick-wall large-diameter UOE steel pipe, comprising a step of joining a peripheral end butt portion of a UO-processed steel material having a thickness of 30 mm or more by submerged arc welding of inner and outer inner layers one by one by submerged arc welding to form a pipe.
As the UO processed steel material, in mass%, C: 0.10% or less, Si: 0.5% or less, Mn: 2.0% or less, P: 0.020% or less, S: 0.0020% or less. Further, Cu: 0.5% or less, Cr: 0.5% or less, Ni: 0.5% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.05% or less , Ti: 0.1% or less, B: 0.0030% or less, the composition containing one or more kinds and the balance being Fe and unavoidable impurities,
On the inner surface side, in mass%, C: 0.08% or less, Si: 0.5% or less, Mn: 0.8 to 1.8%, Cu: 0.50% or less, Cr: 0.50% or less , Ni: 0.50% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.045% or less, Ti: 0.005 to 0.030%, B: 0.0005 to 0.0005% 0.0015%, N: 0.0080% or less, O: 0.035% or less, the balance being Fe and unavoidable impurities, and the Pcm represented by the following formula is reduced to 0.120 to 0.180%. A weld metal of the composition
On the outer surface side, in mass%, C: 0.10% or less, Si: 0.5% or less, Mn: 0.8 to 1.8%, Cu: 0.50% or less, Cr: 0.50% or less , Ni: 0.50% or less, Nb: 0.07% or less, V: 0.07% or less, Mo: 0.1 to 0.4%, Ti: 0.005 to 0.050%, B: 0 0.0005% to 0.0060%, N: 0.0080% or less, O: 0.035% or less, the balance is composed of Fe and inevitable impurities, and Pcm represented by the following formula is 0.140 to 0. A method for producing a thick-wall, large-diameter straight UOE steel pipe satisfying strict toughness requirements, wherein said submerged arc welding is performed using a welding material selected to form a weld metal having a composition of 200%.
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B
In the formula, C, Si, Mn, Cu, Cr, Ni, Mo, V, and B represent mass% of each element.

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