JP2009202167A - Welded steel pipe having excellent weld heat-affected zone toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-cost welded steel pipe for a line pipe with a pipe thickness of ≥25 mm, which has excellent weld heat-affected zone toughness and whose productivity is high. <P>SOLUTION: Disclosed is the welded steel pipe with the pipe thickness of ≥25 mm provided with the following seam-welded zones each by one layer at the internal and external faces, in which the seam welding is performed in such a manner that either the internal face or the external face is made first: 1. in the coarse grain region in the vicinity of the melting line in the Back side seam welding, the average grain size of old austenite in the region reheated to Ac1 to Ac3 transformation points by the Final side welding is ≤100 μm; 2. the angle formed by the tangent of the melting line in the plate surface side of 6 mm and the center line of Final side weld bead, in the Final side welding is ≥20°; and 3. the grain size of old austenite in the coarse grain region in the vicinity of the melting line in the Final side welding is ≤300 μm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a welded steel pipe having a thickness of 25 mm or more manufactured by forming a thick steel plate into a cylindrical shape by cold bending and welding the butt portion, and is suitable as a welded steel pipe for natural gas and crude oil line pipes. Further, the present invention relates to a material having excellent weld heat affected zone toughness having a tensile strength of 565 Mpa or more.

  Line pipes used for the transportation of natural gas and crude oil are increasing in strength and thickness year by year in order to achieve improved transport efficiency through high-pressure operation.

  Also, in the submarine pipeline system that transports natural gas, the deeper the laying depth, the better the buckling strength when laying, and the thicker line pipe from the viewpoint of water pressure resistance during operation and safety against tidal currents. Is required.

  In addition, the development of gas and oil fields has a tendency to expand to extremely cold regions such as Russia and Alaska, and cold sea regions such as the North Sea, and line pipe materials require toughness at lower temperatures of the base material and weld heat affected zone. Is done.

  Welded steel pipes for line pipes are generally manufactured by forming a thick steel plate into a cylindrical shape and seam-welding the butt part one layer at a time from the inner and outer surfaces. The heat input of seam welding increases and it becomes difficult to ensure the toughness of the heat affected zone.

  Patent Document 1 relates to a method of manufacturing a steel plate having excellent weld heat affected zone toughness, and Ti and Mg are added to steel that does not substantially contain Al to produce a stable oxide at high temperature and dispersed in the steel. It is described that the coarsening of the weld heat affected zone structure is suppressed.

Patent Document 2 relates to a manufacturing method by welding thick-walled large-diameter steel pipes, and seam welding, which is usually performed in one layer for each inner and outer surfaces, has an inner surface of one layer, an outer surface of two layers, an inner surface of 30 mm or more in accordance with the increase in thickness. A method for reducing the welding heat input per pass is described, in which two outer layers and two layers are multilayered.
Japanese Patent No. 3378433 Japanese Patent No. 2650601

  However, when Al is not substantially contained in the steel as described in Patent Document 1, deoxidation using Al, which is an inexpensive and strong deoxidizing element, cannot be performed, and not only the steelmaking cost increases. There is a concern about an increase in oxide impurities.

  In addition, Mg-based oxides are stable at high temperatures and thus exhibit the pinning effect of the weld heat-affected zone structure, but the oxides are uniformly and finely dispersed in steel to provide a sufficient pinning force. Is difficult.

The multi-layer seam welding as described in Patent Document 2 not only significantly reduces the productivity of steel pipes, but is also formed by ICCGHAZ (Inter Critical Coarse Grain HAZ), which is considered to have the most remarkable deterioration in toughness. The ratio of the welded heat-affected zone made of coarse grains near the melted line to the Ac _{1 to} Ac ₃ transformation point by the subsequent welding is increased in the seam weld section.

  Then, this invention aims at providing the welded steel pipe excellent in the weld heat affected zone toughness of the pipe thickness of 25 mm or more which is excellent in productivity, in order to solve the problem mentioned above.

  In order to ensure high productivity, the present inventors decided to perform seam welding with one layer on each of the inner and outer surfaces, and reproduced thermal cycle tests on microstructures and penetration shapes that provide excellent weld toughness in the seam welds. Various tests were conducted using test weld joints.

  As a result, 1. The portion with the lowest toughness in the weld zone is a region where the coarse grain region near the melting line is reheated to a specific temperature range. 2. Toughness is improved by making the region finer; It was found that the toughness value evaluated by the Charpy impact test is affected by the penetration shape of the weld.

  In the following description, the welding performed in advance in the welding of the inner and outer surfaces one layer at a time is assumed to be the back side welding, and the welding performed subsequently is referred to as the final side welding.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
(1) A welded steel pipe having a pipe thickness of 25 mm or more provided with the following seam welded portion of each inner and outer surface layer, which was performed in advance on either the inner surface or the outer surface.
1. The average grain size of the prior austenite grains in the region reheated to the Ac1 to Ac3 transformation points by the final side welding in the coarse grain region near the melting line of the back side seam welding is 100 μm or less.
2. The angle formed by the tangent line of the melt line at a position 6 mm below the surface of the final side welding and the center line of the final side weld bead is 20 ° or more.
3. The prior austenite grain size in the coarse grain region near the melting line of final side welding is 300 μm or less.
(2) The chemical composition of steel is
% By mass
C: 0.03 to 0.08%
Si: 0.05% or less Mn: 1.0 to 2.0%
P: 0.006% or less S: 0.005% or less Al: 0.02-0.05%
Nb: 0.005 to 0.025%
Ti: 0.005 to 0.030%
N: 0.001 to 0.010%
In addition,
Cu: 0.10 to 0.60%
Ni: 0.10 to 1.20%
Cr: 0.05-0.40%
Mo: 0.05 to 0.40%
Containing one or more of
The carbon equivalent (Ceq) represented by the following formula satisfies 0.30 ≦ Ceq ≦ 0.45, and consists of the balance Fe and unavoidable impurities, and has excellent weld heat affected zone toughness according to (1) Welded steel pipe.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5
(3) Furthermore, in mass%,
Zr: 0.0005 to 0.0300%
Ca: 0.0005 to 0.0100%
Mg: 0.0005 to 0.0100%
REM: 0.0005 to 0.0200%
The welded steel pipe having excellent weld heat affected zone toughness according to 2, characterized by containing one or more of the above.

  According to the present invention, it is possible to manufacture a welded steel pipe for a line pipe excellent in weld heat-affected zone toughness having a pipe thickness of 25 mm or more at a low cost and with high productivity, which is extremely effective industrially.

The present invention defines the shape of the weld and the microstructure of the weld heat affected zone. The reason for limitation will be described below.
1. Lamination method The seam welded part is welded with one layer each on the inner and outer surfaces, which was performed on either the inner surface or the outer surface in advance. A multi-electrode submerged arc welding method is used in order to make seam welding of a welded steel pipe having a thickness of 25 mm or more, which is an object of the present invention, one-layer welding on each of the inner and outer surfaces. Either the inner surface or the outer surface may be preceded, but the normal factory layout often precedes the inner surface welding. Therefore, in the following explanation, the preceding is performed as inner surface welding, and the inner surface side welding is back. Side welding and outer surface welding are referred to as final side welding.
2. Back-side seam welding The region with the least toughness in each seam welded portion of the inner and outer surfaces is ICCGHAZ of Back-side seam welding, so that the average grain size of the prior austenite grains is 100 μm or less in the microstructure of the region.

  Table 1 shows the results of a reproducible thermal cycle Charpy impact test (test temperature of -30 ° C., test number of 3) using line pipe steel sheets having various component compositions. The reproducible heat cycle was as follows, which simulated the thermal history obtained by seam welding of a welded steel pipe having a pipe thickness of 25 mm or more at four locations of the weld.

1. final-side melt line vicinity coarse grain region (hereinafter, final-side CGHAZ (CGHAZ is an abbreviation for Coarse Grain HAZ)),
2. back side melt line vicinity coarse grain region (hereinafter, back side CGHAZ),
3. A region where the coarse grain region near the melting line of the back side welding is reheated to the Ac _{1 to} Ac ₃ transformation point by the final side welding (hereinafter referred to as ICCGHAZ (abbreviation of Inter Critical Coarse Grain HAZ)),
4). The thermal cycle in the region where the rough flow region near the melting line formed by the back side welding was reheated from 600 ° C. to the Ac ₁ transformation point by the final side welding (hereinafter referred to as SCCGHAZ (abbreviation of Sub Critical Coarse Grain HAZ)) was used. .

  From the obtained test results, it is recognized that the back side ICCGHAZ has the worst toughness. In FIG. The result of having investigated the influence of the prior austenite particle size on the toughness of back side ICCGHAZ about 1-3 is shown. The prior austenite particle size was changed by changing the holding time at the maximum temperature (1400 ° C.) and the cooling time from 1400 ° C. to 800 ° C. in the reproduction thermal cycle of the back side CGHAZ.

  FIG. 1 shows that the smaller the prior austenite grain size, the higher the Charpy impact value. The toughness value of ICCGHAZ is extremely small, but when evaluating the Charpy impact value of an actual welded joint, only about 0.5 to 2.0 mm is included in the notch bottom.

Therefore, when the correlation between the Charpy impact value of the simulated thermal cycle material simulating the welded joint and ICCGHAZ was taken, if the average value of the three replicated thermal cycle materials was 25 J or more, 40 J was stably secured by the welded joint. I knew it was possible. By setting the prior austenite grain size of ICCGHAZ to 100 μm or less, it is possible to achieve the target value.
3. Final side seam welded portion Since the Charpy impact test of the seam welded portion of the welded steel pipe is usually performed by collecting a Charpy impact test piece from the final side, in the present invention, the penetration shape of the welded portion in the final side seam welded portion, Defines the prior austenite grain size in CGHAZ.

  Table 2 shows the steel No. 1 in Table 1 having a plate thickness of 26.8 mm and a V groove. 1 shows a Charpy impact test of a welded joint prepared by performing single-sided welding simulating final-side welding by 4-electrode submerged arc welding with a welding heat input of 6 to 12 kJ / cm.

  Four-electrode submerged arc welding was performed with four types of groove angles in order to change the penetration shape, and with a welding heat input of 6 to 12 kJ / cm for each groove angle in order to change the prior austenite grain size.

  For the Charpy impact test, Charpy impact test pieces were collected from each position in the plate thickness direction on the final side of the joint, and the test was performed at a notch position: CGHAZ, a test temperature: −30 ° C., and the number of tests: 9.

  From Table 2, when the angle formed by the tangent of the melt line at the position of 6 mm below the surface of the final side welding and the center line of the final side weld bead is 20 ° or more, and the prior austenite grain size in CGHAZ is 300 μm or less, It can be seen that excellent Charpy impact test results are obtained.

  Note that the position 6 mm below the surface of the final-side welded portion is the sampling position of the outer surface FL Charpy test of DNV-OS F101 applied to the material evaluation of the seabed line pipe.

  When considering the fracture performance against internal pressure, the closer to the outer surface, the higher the stress state, so the closer to the outer surface and generally the inclination angle of the melt line with the tube thickness direction becomes acute, increasing the risk of fracture Therefore, it is a suitable position for evaluating steel pipe fracture performance.

  The Charpy impact test at this position is suitable for evaluating the fracture resistance of cracks and defects that have entered the pipe thickness direction against the stress acting in the pipe circumferential direction due to the internal pressure received when the line pipe is in service.

The component composition of the steel sheet suitable for the case where the welded steel pipe according to the present invention has a tensile strength of 565 MPa or more will be described below. In addition, all% of component amount is the mass%.
C: 0.03-0.08%
C contributes to an increase in strength by dissolving in supersaturation in a low temperature transformation structure. In order to obtain this effect, addition of 0.03% or more is necessary. However, if the addition exceeds 0.08%, the hardness of the heat-affected zone with high heat input is increased and island martensite is generated in the structure. Since the toughness deteriorates, the upper limit is made 0.08%.

Si: 0.05% or less Si is an element that acts as a deoxidizer and further increases the strength of the steel by solid solution strengthening. When the structure of the weld heat affected zone is upper bainite, island martensite The generation of heat and significantly deteriorates the weld heat-affected zone toughness. In the present invention, it has been found that the toughness of the weld heat affected zone when the weld heat affected zone is upper bainite is remarkably improved by substantially not containing Si. Therefore, it is desirable to reduce Si as much as possible, but 0.05% is allowed. Preferably it is less than 0.04%.

Mn: 1.0-2.0%
Mn acts as a hardenability improving element, and its effect can be obtained by addition of 1.0% or more. However, when a continuous casting process is applied, the concentration of the central segregation part is remarkably increased, and the addition exceeds 2.0%. If done, the toughness of the segregation part deteriorates, so the upper limit is made 2.0%.

P: 0.006% or less P is an element that increases the strength by solid solution strengthening. However, in order to deteriorate the toughness and weldability of the base metal and the weld heat affected zone, the content can be generally reduced. desired. In the present invention, by reducing P, the hardness of the weld heat affected zone is reduced and the weld heat affected zone toughness is improved. In particular, P is made 0.006% or less in order to significantly improve the weld heat affected zone toughness by making it 0.006% or less.

S: 0.005% or less S is present as an inevitable impurity in steel. In particular, the segregation at the center segregation part is an element that promotes toughness deterioration due to the segregation part of the base material. Therefore, it is desirable to reduce S as much as possible, but up to 0.005% is allowed due to restrictions in the steelmaking process.

Al: 0.02 to 0.05%
Al acts as a deoxidizing element. Sufficient deoxidation effect can be obtained with addition of 0.02% or more, but if added over 0.05%, the cleanliness in the steel is lowered and toughness deteriorates, so the upper limit is 0.05%. To do.

Nb: 0.005 to 0.025%
Nb has the effect of expanding the austenite non-recrystallized region at the time of hot rolling, and in order to make the non-recrystallized region up to 900 ° C., addition of 0.005% or more is necessary. On the other hand, when the amount of Nb added is increased, island martensite is generated in the weld heat affected zone, particularly in the heat affected zone of high heat input welding, and further, precipitation embrittlement occurs in the reheat weld heat affected zone during multi-layer welding. And the toughness deteriorates significantly, so the upper limit is made 0.025%. The amount of Nb added is preferably as low as possible from the viewpoint of weld heat affected zone toughness.

Ti: 0.005-0.030%
Ti forms nitrides and is effective in reducing the amount of solute N in steel. The precipitated TiN suppresses the coarsening of the austenite grains in the base metal and the weld heat affected zone during slab heating before hot rolling, especially the weld heat affected zone in high heat input welding due to the pinning effect, and the base metal and welding heat Contributes to improved toughness of the affected area. In order to obtain this effect, addition of 0.005% or more is necessary, but if added over 0.030%, the base material and weld heat affected zone toughness deteriorate due to precipitation of coarse TiN and carbides. Therefore, the upper limit is made 0.030%.

N: 0.001 to 0.010%
N is usually present as an inevitable impurity in steel, but is defined to form TiN that suppresses austenite coarsening by adding Ti as described above. In order to obtain the required pinning effect, it is necessary to be present in the steel in an amount of 0.001% or more. However, if it exceeds 0.010%, the base material and the weld heat affected zone due to an increase in solute N Therefore, the upper limit is made 0.010%.

  In the present invention, one or more of Cu, Ni, Cr, and Mo are further added. Cu, Ni, Cr, and Mo all act as a hardenability improving element, and by adding one or more of these elements, it is possible to increase the strength of a thick steel plate having a thickness of 6 mm or more.

Cu: 0.10 to 0.60%
Cu contributes to the hardenability improvement of steel by adding 0.10% or more. On the other hand, if added in excess, the toughness of the base metal and the weld heat affected zone is deteriorated, so when added, the upper limit is made 0.60%.

Ni: 0.10 to 1.20%
Ni contributes to improving the hardenability of steel by adding 0.10% or more. In particular, even when added in a large amount, deterioration in toughness is small compared to other elements, and it is an effective element for toughening. However, since it is an expensive element and is added in excess of 1.20%, the hardenability is excessively increased and the weld heat affected zone toughness deteriorates. Therefore, when added, the upper limit is made 1.20%.

Cr: 0.05-0.40%
Cr contributes to the improvement of hardenability of steel by adding 0.05% or more. On the other hand, if added in excess, the toughness of the base metal and the weld heat affected zone is deteriorated. Therefore, when added, the upper limit is made 0.40%.

Mo: 0.05-0.40%
Mo contributes to improving the hardenability of steel by adding 0.05% or more. On the other hand, when the amount of Mo added is increased, the toughness of the high heat input welded portion is deteriorated. Moreover, since precipitation embrittlement is caused in the reheat welding heat-affected zone during multi-layer welding, and the toughness deteriorates, the upper limit is made 0.40% when added. The amount of Mo added is preferably as low as possible from the viewpoint of weld heat affected zone toughness.

Ceq: 0.30 to 0.45
Ceq (= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14. Each element is a content (mass%), and an element not included is 0) is an estimate of the effect of a quenching element such as C and Mn. It can be used as an index and is preferably controlled to 0.30 or more from the viewpoint of securing strength. On the other hand, if it exceeds 0.45, toughness and weldability will be impaired, so the upper limit is made 0.45%.

  The basic component composition of the present invention is as described above, but when further improving toughness, one or more of Zr, Ca, Mg, and REM can be added.

  Zr, Ca, Mg, and REM all control the morphology of MnS, which is a non-metallic inclusion in steel, or form oxides or nitrides, mainly pinning the austenite grain coarsening in the weld heat affected zone Suppress with.

Zr: 0.0005 to 0.0300%
Zr forms carbonitrides in steel and brings about a pinning effect that suppresses coarsening of austenite grains, particularly in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary, but when adding over 0.0300%, the cleanliness in the steel is remarkably lowered, and the toughness is lowered. When adding, it is 0.0005 to 0.0300%.

Ca: 0.0005 to 0.0100%
Ca is an element effective for controlling the form of sulfide in steel, and the addition of 0.0005% or more suppresses the generation of MnS harmful to toughness. However, if added over 0.0100%, a CaO-CaS cluster is formed and the toughness is deteriorated. Therefore, when added, the content is made 0.0005 to 0.0100%.

Mg: 0.0005 to 0.0100%
Mg is produced as fine oxides in the steel during the steel making process, and has a pinning effect that suppresses the coarsening of austenite grains, particularly in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary, but if added over 0.0100%, the cleanliness in the steel decreases, and the toughness decreases, When adding, it is made into 0.0005 to 0.0100%.

REM: 0.0005 to 0.0200%
REM is an element effective for controlling the form of sulfide in steel, and by adding 0.0005% or more, it suppresses the generation of MnS harmful to toughness. However, since it is an expensive element and the effect is saturated even if added over 0.0200%, when added, the content is made 0.0005 to 0.0200%.

  The balance is Fe and inevitable impurities. In the present invention, positive addition of V which is an element harmful to HAZ toughness and B which is an element harmful to weldability is not performed. These elements are handled as one of inevitable impurities. Desirably, V is 0.005% or less and B is 0.0005% or less.

  In the welded steel pipe according to the present invention, the steel having the above-described composition is made into a steel sheet having a thickness of 25 mm or more, then formed into a tubular shape, and the butt portion is seam welded one layer at a time from the inner and outer surfaces, and thereafter, the pipe is expanded or contracted Manufactured with tube straightening. The production conditions of the steel sheet are not particularly defined, but when the tensile strength is 565 MPa or more, it is preferably produced by accelerated cooling or direct quenching-tempering.

  Molten steel having chemical components shown in Table 3 was melted in a vacuum melting furnace, and a 150 mm thick slab was formed by a continuous casting method. Subsequently, these slabs were hot-rolled and subsequently subjected to accelerated cooling to obtain thick steel plates (25.4 to 30.9 mm).

  Using the obtained steel plate, a steel pipe material is formed by the UO forming method. After tack welding, the first layer is welded from the inner surface to the second layer to the seam portion by 4-electrode submerged arc welding, and then the pipe is expanded. To obtain a welded steel pipe. FIG. 2 shows the groove shape, and Table 4 shows the dimensions of each part of the groove shape. The groove shape was an X groove provided with grooves on the outer surface 11 side and the inner surface side 12 of the steel plate 1.

  At this time, the austenite grain size of the inner and outer surfaces was changed by changing the heat input balance of the inner and outer surfaces, and the angle of the outer surface melt line was changed by changing the groove shape of the outer surface.

  Cut out a weld cross-section sample from the obtained welded steel pipe, perform mirror polishing and nital etching, observe the joint shape with a digital camera, and position 1 / 5t from the outer surface of the outer weld bead on the steel pipe (6mm below the plate thickness surface) ) And the angle (θ) formed between the melt line and the bead center line of the outer surface welding.

  Further, the sample was observed with an optical microscope, and the average grain size (D1) of prior austenite grains in the region where the heat affected zone near the fusion line formed by inner surface welding was reheated to the Ac1 to Ac3 transformation point by outer surface welding. And the average particle diameter (D2) of the prior austenite grains in the weld heat affected zone near the fusion line formed by outer surface welding was measured.

  For the average particle diameter of the prior austenite, 20 or more circle equivalent diameters of the prior austenite of the weld heat affected zone in contact with the melt line of the relevant part were obtained by image processing, and the average value was adopted. In addition, the particle | grains considered to be the reverse transformation austenite produced by being reheated to the Ac1-Ac3 transformation point were excluded from the measurement.

  Moreover, as shown in FIG. 3, a V-notch Charpy test piece conforming to the JIS Z 2202 standard was collected from the welded portion of the obtained welded steel pipe, and a Charpy test conforming to the JIS Z 2242 standard was performed. The absorbed energy at 30 ° C. was measured three times for each condition, and the average value and the minimum value were obtained.

Table 5 shows the measurement results of the weld microstructure of the welded steel pipe and the Charpy test results.
In each of the inventive examples, the minimum value of 50 J or more was achieved in the inner and outer surface FL Charpy test and the meeting part FL Charpy test. On the other hand, in the comparative example that is outside the scope of the present invention, the minimum value is less than 50J.

  Steel pipe No. In Nos. 2 and 6, since the outer surface groove is one step, the angle formed by the melt line of the outer surface welding and the bead center line of the outer surface welding is less than 20 °, and therefore, the outer surface FL Charpy test exhibits low absorbed energy. .

  Steel pipe No. No. 4 has a large internal welding heat input, so 10 is not added with Ti and austenite grains are not pinned by TiN in the steel, so the average grain size of the prior austenite grains in the region reheated to the Ac1 to Ac3 transformation point exceeds 100 μm. Part FL Charpy test shows low energy.

  Steel pipe No. 10 and 11 are not added with Ti and austenite grains are not pinned by TiN in the steel, and the heat input to the outer surface is large. The austenite grains have an average grain size exceeding 400 μm and exhibit low energy in the outer surface FL Charpy test.

The figure which shows the result of having investigated the influence of the prior austenite particle size on the toughness of the back side ICCGHAZ. The figure explaining groove shape. The figure which shows the Charpy test piece collection position.

Claims (3)

A welded steel pipe having a pipe thickness of 25 mm or more and having the following seam welded portions, each of which has one inner and outer surface layers, which is performed in advance on either the inner surface or the outer surface.
1. The average grain size of the prior austenite grains in the region reheated to the Ac1 to Ac3 transformation points by the final side welding in the coarse grain region near the melting line of the back side seam welding is 100 μm or less.
2. The angle formed by the tangent line of the melt line at a position 6 mm below the surface of the final side welding and the center line of the final side weld bead is 20 ° or more.
3. The prior austenite grain size in the coarse grain region near the melting line of final side welding is 300 μm or less. The chemical composition of steel
% By mass
C: 0.03 to 0.08%
Si: 0.05% or less Mn: 1.0 to 2.0%
P: 0.006% or less S: 0.005% or less Al: 0.02-0.05%
Nb: 0.005 to 0.025%
Ti: 0.005 to 0.030%
N: 0.001 to 0.010%
In addition,
Cu: 0.10 to 0.60%
Ni: 0.10 to 1.20%
Cr: 0.05-0.40%
Mo: 0.05 to 0.40%
Containing one or more of
The carbon equivalent (Ceq) represented by the following formula satisfies 0.30 ≦ Ceq ≦ 0.45, and consists of the balance Fe and unavoidable impurities, and has excellent weld heat affected zone toughness according to claim 1 Welded steel pipe.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 Furthermore, in mass%,
Zr: 0.0005 to 0.0300%
Ca: 0.0005 to 0.0100%
Mg: 0.0005 to 0.0100%
REM: 0.0005 to 0.0200%
The welded steel pipe excellent in weld heat-affected zone toughness according to claim 2, comprising one or more of the following.

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