JP5055899B2 - Method for producing high-strength welded steel pipe excellent in weld heat-affected zone toughness and having tensile strength of 760 MPa or more, and high-strength welded steel pipe - Google Patents

Description

  The present invention is suitable for transportation of natural gas and crude oil, has a pipe thickness of 6 mm or more, a tensile strength of 760 MPa or more, a high strength welded steel pipe excellent in low temperature toughness of a weld heat affected zone (hereinafter referred to as HAZ), and It relates to the manufacturing method.

  In recent years, welded steel pipes used for the transportation of natural gas and crude oil have been strengthened year by year in order to improve transport efficiency by increasing pressure and to improve local welding work efficiency by reducing wall thickness, and X100 grade steel pipes are already in practical use. It has become. Further, the demand for the X120 grade exceeding the tensile strength of 900 MPa is being realized.

  Regarding the heat-affected zone toughness of such a high-strength welded steel pipe, for example, in Patent Document 1, after the final welding, the cooling rate of the welded portion is cooled from 600 ° C to 400 ° C at least 1 ° C / s or more, It is described that the amount of MA in the upper bainite structure of the HAZ coarse-grained region is reduced to increase the toughness of the HAZ.

  In addition, the reheated HAZ part of the SCCGHAZ or ICCGHAZ part, which is a problem in Patent Document 1, is a very small area, and is near the thickness center where the existence of defects is unlikely. Therefore, it is considered that the low toughness of this part is not particularly problematic in the use of the welded steel pipe.

Patent Document 2 describes a welded steel pipe with a small heat input by performing welding on the outer surface side after removing the temporary attachment of the seam portion in order to improve the toughness using the HAZ microstructure as the lower bainite. The manufacturing method is described.
JP 2004-99930 A Japanese Patent No. 3702216

However, welded steel pipe Patent Document 1 is intended is for the P _CM lower component as the base material, manufacturing guidelines for steel pipe tensile strength exceeds 760 MPa, the base material P _CM is high component I can't get it.

Further, the method of Patent Document 2, in order to completely lower bainite structure the HAZ microstructure, is necessary to limit to a very narrow range of range of P _CM, production stability is concerned.

  Therefore, the present invention provides a high-strength welded steel pipe having a tensile strength of 760 MPa or more and a method for producing the same, which is excellent in HAZ toughness of a longitudinal seam welded part, in order to solve the above-described problems.

但し、数１において、ｈ：板厚（ｍｍ）、θ _０ ：初期温度（℃）、Ｑは溶接入熱（Ｊ／ｃｍ）を示す。溶接入熱（ｋＪ／ｍｍ）はΣ（Ｉ _ｉ ×Ｖ _ｉ ）×６０／ｖ／１０００とする。ここで、
Ｉ _ｉ ：ｉ番目の電極の電流（Ａ）、Ｖ _ｉ ：ｉ番目の電極の電圧（Ｖ）、ｖ：溶接速度（ｍｍ／ｍｉｎ）とする。
２．更に、質量％で、
Ｃａ：０．０００５〜０．０１％
ＲＥＭ：０．０００５〜０．０２％
Ｚｒ：０．０００５〜０．０３％
Ｍｇ：０．０００５〜０．０１％
Ｂ：０．０００１〜０．００１０％
の一種または二種以上を含有することを特徴とする１記載の溶接熱影響部靭性に優れた引張り強さが７６０ＭＰａ以上の高強度溶接鋼管の製造方法。
３．縦シーム溶接後、拡管することを特徴とする１または２に記載の溶接熱影響部靭性に優れた引張り強さが７６０ＭＰａ以上の高強度溶接鋼管の製造方法。
４．溶接ボンド部から溶接ボンド＋１ｍｍの領域の溶接熱影響部硬さが、２５０≦ＨＶ（９８Ｎ）≦３５０を満たすことを特徴とする１乃至３のいずれかに記載の製造方法で製造された溶接熱影響部靭性に優れた引張り強さが７６０ＭＰａ以上の高強度溶接鋼管。 In order to develop a high-strength welded steel pipe having a tensile strength of 760 MPa or more and excellent HAZ toughness, the present inventors have conducted intensive research and obtained the following knowledge.
1. The site where the toughness is most reduced in the HAZ is a HAZ coarse grain region (Coarse-grain HAZ, hereinafter referred to as CGHAZ), which is a region of at least a weld bond + 1 mm from the weld bond portion.
2. Microstructure of CGHAZ includes a P _CM value of the base material, in cooling after welding, by a combination of cooling rate in temperature range of 500 ° C. from 800 ° C. to transformation gamma-alpha phase, the upper containing MA of the rigid large amount When the bainite structure or the lower bainite structure in which the high-strength martensite structure is suppressed to a certain fraction or less is mainly used, the toughness is most improved.
The present invention has been made by further study based on the above knowledge, that is, the present invention,
1. % By mass
C: 0.03-0.12%
Si: 0.01 to 0.5%
Mn: 1.5 to 3.0%
Al: 0.01 to 0.08%
Nb: 0.01 to 0.08%
Ti: 0.005 to 0.025%
N: 0.001 to 0.010%
O: 0.003% or less S: 0.003% or less
Cu: 0.01 to 1%
Ni: 0.01 to 1%
Cr: 0.01 to 1%
Mo: 0.01 to 1%
V: 0.01 to 0.1%
Containing one or more of
P _CM value satisfies 0.19 ≦ P _CM ≦ 0.30, welding base material and the balance Fe and unavoidable impurities was formed in the tubular by cold working, the longitudinal seam submerged arc welding from the welding-bonding unit As a welding condition in which the microstructure of the bond + 1 mm region is a lower bainite with an area ratio of at least 50% or more, and the balance is upper bainite, martensite, or a mixed structure thereof, the following formula (1) is satisfied, and 800 A method for producing a high-strength welded steel pipe having a tensile strength of 760 MPa or more excellent in weld heat-affected zone toughness, characterized in that the cooling rate is from 5 to 40 ° C./s within a range of 5 to 40 ° C./s.
0.228 ≦ P _CM + 0.0174 × ln (v _{(between 800-500 ° C.)} ) ≦ 0.350 (1)
_{However, P CM} than those obtained for composition of the material to be _{welded, P CM = C + Si /} 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × each element in B indicates the content. “v” is a cooling rate (° C./s) between 800 to 500 ° C. of the welded portion and is obtained by the equation (2).
v _{(between 800-500 ℃) (℃ / s)} = 300 / t ( _{between 800-500 ℃) (s) ··· (} 2)
t _{(between 800-500 ° C) (s)} is the welded part (welded bond + 1mm from welded bond)
It calculates | requires by Formula 1 in the cooling time to 800-500 degreeC.
[Equation 1]

In Equation 1, h: plate thickness (mm), θ ₀ : initial temperature (° C.), and Q indicate welding heat input (J / cm). The welding heat input (kJ / mm) is Σ (I _i × V _i ) × 60 / v / 1000. here,
I _i : current (A) of i-th electrode, V _i : voltage (V) of i-th electrode, v: welding speed (mm / min).
2. Furthermore, in mass%,
Ca: 0.0005 to 0.01%
REM: 0.0005 to 0.02%
Zr: 0.0005 to 0.03%
Mg: 0.0005 to 0.01%
B: 0.0001 to 0.0010%
The method for producing a high-strength welded steel pipe having a tensile strength of 760 MPa or more and excellent in weld heat-affected zone toughness according to 1, characterized by containing one or more of the above.
3. 3. The method for producing a high-strength welded steel pipe having a tensile strength of 760 MPa or more excellent in weld heat-affected zone toughness according to 1 or 2 , wherein the pipe is expanded after longitudinal seam welding.
4). The welding heat produced by the production method according to any one of 1 to 3, wherein the hardness of the weld heat affected zone in the region from the weld bond portion to the weld bond + 1 mm satisfies 250 ≦ HV (98N) ≦ 350. High-strength welded steel pipe with a tensile strength of 760 MPa or more with excellent affected zone toughness.

  According to the present invention, a high-strength welded steel pipe having a tensile strength of 760 MPa or more and a manufacturing method thereof excellent in HAZ toughness of a longitudinal seam welded portion and a method for producing the same can be obtained, which is extremely useful industrially.

  In the present invention, the composition of the base material, the microstructure in the region from the weld bond of the longitudinal seam weld to the weld bond + 1 mm, and the welding conditions necessary for obtaining the structure are defined.

[Component composition]% is mass%.
C: 0.03-0.12%
C contributes to an increase in strength by being supersaturated in a low temperature transformation structure. In order to obtain this effect, 0.03% or more of addition is necessary, but if added over 0.12%, the hardness of the circumferential welded portion of the steel pipe is remarkably increased and cold cracking is likely to occur. Therefore, the upper limit is made 0.12%.

Si: 0.01 to 0.5%
Si is an element that acts as a deoxidizer and increases the strength of the steel by solid solution strengthening. However, if less than 0.01%, there is no effect, and if added over 0.5%, the toughness decreases significantly. Therefore, the upper limit is made 0.5%.

Mn: 1.5 to 3.0%
Mn acts as a hardenability improving element. The effect can be obtained by addition of 1.5% or more, but in the continuous casting process, the concentration rises at the center segregation part, and if it exceeds 3.0%, it causes delayed fracture at the center segregation part. Therefore, the upper limit is made 3.0%.

Al: 0.01 to 0.08%
Al acts as a deoxidizing element. A sufficient deoxidation effect can be obtained with addition of 0.01% or more, but if added over 0.08%, the cleanliness in the steel is lowered and the toughness is deteriorated, so the upper limit is 0.08%. And

Nb: 0.01 to 0.08%
Nb has an effect of expanding the austenite non-recrystallized region at the time of hot rolling, and 0.01% or more is added to make the non-recrystallized region at 950 ° C. or less. On the other hand, if added over 0.08%, the toughness of HAZ is significantly impaired, so the upper limit is made 0.08%.

Ti: 0.005-0.025%
Ti forms nitrides and is effective in reducing the amount of solute N in the steel. The precipitated TiN suppresses the coarsening of austenite grains by the pinning effect and contributes to the improvement of the toughness of the base material and HAZ. Addition of 0.005% or more is necessary to obtain the pinning effect, but if added over 0.025%, carbides are formed, and the toughness is significantly deteriorated by precipitation hardening. Is 0.025%.

N: 0.001 to 0.01%
N usually exists as an inevitable impurity in steel, but TiN is formed by addition of Ti. In order to suppress the coarsening of the austenite grains due to the pinning effect by TiN, it is necessary to be present in the steel in an amount of 0.001% or more. Since TiN decomposes in a region heated to a temperature higher than or equal to 0 ° C. and the adverse effect of solute N is significant, the upper limit is made 0.01%.

One or more of Cu, Ni, Cr, Mo, and V Cu, Ni, Cr, Mo, and V all act as a hardenability improving element. Therefore, for the purpose of increasing the strength, Or two or more of them are added.

Cu: 0.01 to 1%
Cu contributes to the hardenability improvement of steel by adding 0.01% or more. However, if 1% or more is added, toughness deterioration occurs, so the upper limit is made 1%.

Ni: 0.01 to 1%
Ni contributes to improving the hardenability of steel by adding 0.01% or more. In particular, even if added in a large amount, it does not cause toughness deterioration, so it is effective for toughening. However, it is an expensive element, and even if added over 1%, the increase in strength is saturated, so the upper limit is 1%. And

Cr: 0.01 to 1%
Cr also contributes to improving the hardenability of steel by adding 0.01% or more. On the other hand, if added over 1%, the toughness deteriorates, so the upper limit is made 1%.
Mo: 0.01 to 1%
Mo also contributes to improving the hardenability of steel by adding 0.01% or more. On the other hand, if added over 1%, the toughness deteriorates, so the upper limit is made 1%.
V: 0.01 to 0.1%
V forms precipitation strengthening by forming carbonitride, and contributes especially to the softening prevention of a weld heat affected zone. This effect can be obtained by addition of 0.01% or more, but if added over 0.1%, precipitation strengthening is remarkably reduced in toughness, so the upper limit is made 0.1%.

O: 0.003% or less, S: 0.003% or less In the present invention, O and S are inevitable impurities and define the upper limit of the content. O is 0.003% or less in order to suppress the formation of inclusions that are coarse and adversely affect toughness. If the content of S is large, the amount of MnS produced increases remarkably and the toughness of the base material deteriorates.

P _CM: 0.19~0.30
P _CM in weld crack sensitivity index expressed by C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B, each element and content elements that do not contain is 0.

In the present invention, in order to achieve joint strength ≧ 760 MPa, P _CM : 0.19 or more. FIG. 1 shows the hardness distribution at a position 1 mm below the outer surface of a welded steel pipe subjected to inner / outer surface single-layer submerged arc welding, and shows the temperature just above the austenitizing temperature of steel (Ac ₃ points) under the heat effect of one cycle. The vicinity of the heated region becomes the HAZ most softened portion.

  From the analysis result by FEM calculation, assuming that the strength of the HAZ part is constant at the softened part strength, in order to achieve the joint strength ≧ 760 MPa, the hardness of the HAZ softened part is 210 (Hv (98N) ) I understood that it should be enough.

Therefore, reproduction in simulated heat cycle test pieces taken from the experimental steel ingot of various P _CM, resulting in HAZ highest softening unit in more welding submerged arc welding, imitating a heat cycle of the immediately above Ac ₃ point and maximum heating temperature A hardness test was conducted by applying a thermal cycle. The heating temperature was 900 ° C., and the cooling rate between 800 ° C. and 500 ° C. was 5 ° C./s.

FIG. 2 shows the test results. When P _{CM is} 0.19 or more, 210 (Hv (98 N)), which is the hardness of the HAZ most softened portion necessary to achieve joint strength ≧ 760 MPa, is obtained.

On the other _hand, the upper limit of the _{P CM} is 0.30 in terms of the circumferential weldability ensured. Simulated thermal cycle test the collected experimental steel ingot y-groove weld cracking test using: perform (test atmosphere 30 ° C. over 80%), 0.30 as the upper limit of P _CM that cold cracking is prevented at 100 ° C. preheating Obtained.

The above is the basic component composition of the steel according to the present invention. When the toughness of the weld is further improved, one or more of Ca, REM, Zr, and Mg are added.
Ca, REM, Zr, Mg
Ca, REM, Zr, and Mg form oxysulfides or carbonitrides in steel, and are mainly added for the purpose of suppressing the austenite grain coarsening in the weld heat affected zone by the pinning effect and improving toughness.

Ca: 0.0005 to 0.01%
In the steelmaking process, when the Ca addition amount is less than 0.0005%, it is difficult to secure CaS due to the deoxidation reaction control, and a toughness improving effect cannot be obtained, so the lower limit of Ca is set to 0.0005%.

  On the other hand, when the amount of Ca added exceeds 0.01%, coarse CaO is likely to be generated, and the toughness including the base material is lowered, and the nozzle of the ladle is blocked and the productivity is hindered. The upper limit is 0.01%, and when added, 0.0005 to 0.01%.

REM: 0.0005 to 0.02%
REM forms an oxysulfide in steel and provides a pinning effect to prevent the weld heat affected zone from becoming coarse by adding 0.0005% or more. However, since it is an expensive element and the effect is saturated even if added over 0.02%, the upper limit is made 0.02%, and when added, it is made 0.0005 to 0.02%.

Zr: 0.0005 to 0.03%
Zr forms carbonitrides in steel and brings about 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. However, when the addition exceeds 0.03%, the cleanliness in the steel is remarkably lowered and the toughness is lowered. Therefore, the upper limit is made 0.03%, and when added, the content is made 0.0005 to 0.03%.

Mg: 0.0005 to 0.01%
Mg is produced as fine oxides in the steel during the steelmaking process, and in particular, has a pinning effect that suppresses the coarsening of austenite grains 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 0.01%, the cleanliness in the steel is lowered and the toughness is lowered. The upper limit is 0.01%, and when added, 0.0005 to 0.01%.

B: 0.0001 to 0.0010%
B segregates at the austenite grain boundaries in the weld heat affected zone, and has the effect of increasing the hardenability, and facilitates the formation of lower bainite or martensite with a lower component. This effect is prominent when added in an amount of 0.0010% or less, and if added over 0.0010%, the toughness decreases, so the upper limit is made 0.0010%. -0.0010%.

  In the present invention, the steel having the above-described component composition is hot-rolled and accelerated and cooled by a conventional method, and then tempered to obtain a steel plate having a predetermined plate thickness.

[Microstructure]
In the present invention, the microstructure in the region from the weld bond of the longitudinal seam weld to the weld bond + 1 mm is the lower bainite and the upper bainite, martensite, or a mixed structure thereof having an area ratio of at least 50%. The microstructure is defined at 2 mm below the surface.

  In the case of the above microstructure, the region from the weld bond portion to the weld bond + 1 mm has a hardness of 250 ≦ HV (98N) ≦ 350, and excellent weld toughness is obtained.

  FIG. 3 shows the influence of hardness on the reproducible thermal cycle Charpy test result of the weld bond, and excellent low temperature toughness is obtained when 250 ≦ HV (98N) ≦ 350. The hardness test was performed on a test piece provided with the same reproducible thermal cycle as the reproducible thermal cycle Charpy test. It should be noted that the rules relating to the microstructure are intended for CGHAZ, and are applied when the area exceeding the weld bond + 1 mm is CGHAZ.

[Vertical seam welding conditions]
In the longitudinal seam welding of the steel pipe according to the present invention, the microstructure in the region from the weld bond to the weld bond + 1 mm becomes the lower bainite with an area ratio of at least 50% and the upper bainite, martensite, or a mixed structure thereof. Next, select the welding conditions. Specific welding conditions may be selected in advance using test materials, and are not particularly defined in the present invention.

In the steel according to the present invention, when the welding method is submerged arc welding and the welding condition is represented by the cooling rate between 800-500 ° C. of the welded part, the welding condition is selected so as to satisfy the expression (1). A microstructure is obtained.
0.228 ≦ P _CM + 0.0174 × ln (v _{(between 800-500 ° C.)} ) ≦ 0.350 (1)
_{However, P CM} those obtained for composition of the material to be welded, v is at a cooling rate of between 800 to 500 ° C. of the weld (℃ / s) (2) determined by the equation.
v _{(between 800-500 ° C.) (° C./s)=300/t (between 800-500 ° C.) (s)} (2)
t _(between 800 and 500 ° _{C.) (s)} can be _{obtained by Equation} 1 in terms of the cooling time from 800 to 500 ° C. in the welded portion (weld bond portion to weld bond + 1 mm).

In Equation 1 , h : plate thickness (mm), θ ₀ : initial temperature (° C.), Q indicates welding heat input ( J / cm 2 ).

The welding heat input (kJ / mm) is Σ (I _i × V _i ) × 60 / v / 1000. here,
I _i : current of the i-th electrode (A), V _i : voltage of the i-th electrode (V), v: welding speed (m
m / min).

In the formula (1), when the parameter: P _CM + 0.0174 × ln (v _{(between 800-500 ° C.)} ) is less than 0.228, the fraction of the upper bainite structure containing a large amount of MA exceeds 50%, HAZ toughness deteriorates.

On the other hand, when it exceeds 0.350, the fraction of martensite structure exceeds 50%, and the HAZ toughness deteriorates. FIG. 4 illustrates a range satisfying the expression (1).
The steel pipe according to the present invention is manufactured by expanding the pipe after vertical seam welding according to the required roundness.

  Steel having the chemical composition shown in Table 1 was melted in a converter and made into a slab of 220 mm thickness by continuous casting, and then steel plates A to H were produced under the hot rolling, accelerated cooling, and reheating conditions shown in Table 2. . Reheating was performed for the purpose of tempering, and was performed using an induction heating type heating device installed on the same line as the accelerated cooling equipment.

  Furthermore, after forming these steel plates by U press and O press, they were submerged arc welded from the inner surface and the outer surface. Thereafter, the pipe was expanded to obtain a steel pipe having an outer diameter of 400 to 1626 mm.

  In order to evaluate the joint strength of the obtained steel pipe, a full-thickness tensile test piece based on API-5L was sampled and a tensile test was performed.

  Furthermore, a V-notch Charpy impact test piece of JIS Z2202 (1980) was sampled from the center position of the thickness of the welded joint of the steel pipe, and a Charpy impact test was performed. The notch was located at a position where the base metal and the weld metal exist at a ratio of 1: 1 in the center of the plate thickness in CGHAZ (FIG. 5). Table 3 summarizes the test results of the hardness of CGHAZ and the toughness of CGHAZ (hereinafter referred to as HAZ toughness).

  The joint strength of the steel pipe is 760 MPa or more, and the Charpy absorbed energy at the test temperature of the welded bond portion at −30 ° C. −100 J or more is within the scope of the present invention.

The combination of P _CM and the HAZ cooling rate calculated from the chemical composition is within the scope of the present invention, invention example No. 1-18 had the desired microstructure and exhibited high HAZ toughness.

On the other hand, low HAZ portion cooling _rate, comparing the combination of _{P CM} and HAZ portion cooling rate falls below the lower limit of the present invention example No. In 19-23, since the fraction of the upper bainite structure was increased, the CGHAZ hardness decreased and the HAZ toughness decreased.

Also, high the HAZ cooling _rate, Comparative Example a combination of _{P CM} and HAZ portion cooling rate exceeds the upper limit of the present invention No. In No. 24, the fraction of martensite structure increased, so the CGHAZ hardness increased and the HAZ toughness decreased.

In _Comparative Example _{P CM} is below the lower limit of the present invention No. 25, the base metal strength is below 760 MPa, in _addition, since the combination of _{P CM} and HAZ portion cooling rate falls below the lower limit of the present invention, together with CGHAZ tissue becomes upper bainite, CGHAZ hardness decreases, HAZ Toughness decreased.

In _Comparative Example _{P CM} value exceeds the upper limit of the present invention No. 26, the base material toughness is degraded, in _addition, since the combination of _{P CM} value and a HAZ cooling rate exceeds the upper limit of the present invention, together with CGHAZ structure becomes martensite, CGHAZ hardness increases, HAZ Toughness decreased.

The figure which shows an example of the hardness distribution (1 mm under an outer surface) of a welded joint part. It shows the effect of P _CM on top softening the hardness of the weld heat affected zone. The figure which shows the relationship between the low temperature toughness and hardness of the weld bond part by a reproduction thermal cycle. The lower bainite structure fraction to 50% or more in _CGHAZ, shows a range of _{P CM} and HAZ cooling rate. The figure explaining the notch position in a welded joint Charpy test.

Claims (4)

% By mass
C: 0.03-0.12%
Si: 0.01 to 0.5%
Mn: 1.5 to 3.0%
Al: 0.01 to 0.08%
Nb: 0.01 to 0.08%
Ti: 0.005 to 0.025%
N: 0.001 to 0.010%
O: 0.003% or less S: 0.003% or less
Cu: 0.01 to 1%
Ni: 0.01 to 1%
Cr: 0.01 to 1%
Mo: 0.01 to 1%
V: 0.01 to 0.1%
Containing one or more of
P _CM value satisfies 0.19 ≦ P _CM ≦ 0.30, after the base material and the balance Fe and unavoidable impurities was formed into a tubular by cold working, the longitudinal seam submerged arc welding, from the weld bond portion As welding conditions in which the microstructure of the weld bond + 1 mm region is a lower bainite with an area ratio of at least 50% or more, and the balance is an upper bainite, martensite or a mixed structure thereof, the following formula (1) is satisfied, and Production of a high-strength welded steel pipe having a tensile strength of 760 MPa or more excellent in weld heat-affected zone toughness, characterized in that the cooling rate is from 800 ° C to 500 ° C within a range of 5 to 40 ° C / s. Method.
0.228 ≦ P _CM + 0.0174 × ln (v _{(between 800-500 ° C.)} ) ≦ 0.350 (1)
_{However, P CM} than those obtained for composition of the material to be _{welded, P CM = C + Si /} 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × each element in B indicates the content. “v” is a cooling rate (° C./s) between 800 to 500 ° C. of the welded portion and is obtained by the equation (2).
v _{(between 800-500 ° C.) (° C./s)=300/t (between 800-500 ° C.) (s)} (2)
t _{(between 800-500 ° C.) (s)} is _{determined by Equation} 1 in terms of the cooling time from 800 to 500 ° C. in the welded portion (weld bond portion to weld bond + 1 mm).

In Equation 1, h: plate thickness (mm), θ ₀ : initial temperature (° C.), and Q indicate welding heat input (J / cm). The welding heat input (kJ / mm) is Σ (I _i × V _i ) × 60 / v / 1000. here,
I _i : current of the i-th electrode (A), V _i : voltage of the i-th electrode (V), v: welding speed (m
m / min). Furthermore, in mass%,
Ca: 0.0005 to 0.01%
REM: 0.0005 to 0.02%
Zr: 0.0005 to 0.03%
Mg: 0.0005 to 0.01%
B: 0.0001 to 0.0010%
The method for producing a high-strength welded steel pipe having a tensile strength of 760 MPa or more excellent in weld heat-affected zone toughness according to claim 1, comprising one or more of the following.   3. The method for producing a high-strength welded steel pipe having a tensile strength of 760 MPa or more excellent in weld heat-affected zone toughness according to claim 1 or 2, wherein the pipe is expanded after longitudinal seam welding.   4. The heat-affected zone hardness in a region from the weld bond portion to the weld bond + 1 mm satisfies 250 ≦ HV (98N) ≦ 350, and is manufactured by the manufacturing method according to claim 1. A high-strength welded steel pipe with a tensile strength of 760 MPa or more with excellent weld heat-affected zone toughness.

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