JP5217773B2 - High-strength welded steel pipe for low temperature having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat-affected zone toughness and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength welded steel pipe excellent in the low-temperature toughness of a weld heat-affected zone having a pipe thickness of 50 mm or less and a tensile strength of 570 MPa to 760 MPa, which is suitable for transportation of natural gas and crude oil.

  In recent years, welded steel pipes used for 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, an X120 grade steel pipe having a tensile strength exceeding 900 MPa is also in the stage of specific examination.

  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 island martensite (MA) in the upper bainite structure in the coarse region of the weld heat affected zone is reduced to increase the toughness of the HAZ.

In Patent Document 2, the microstructure of the weld heat affected zone is the lower bainite, and in order to improve toughness, welding is performed on the outer surface side after removing the temporary attachment of the seam portion, thereby reducing the heat input to welding. A method for manufacturing a welded steel pipe is described.
JP 2004-99930 A Japanese Patent No. 3702216

However, welded steel pipe Patent Document 1 is intended, the tensile strength relates to high strength welded steel pipe above 800 MPa, tensile strength about the steel pipe to make a base material 760MPa or less and _{P CM} is lower or more components 570MPa Production guidelines are not available.

Further, the method of Patent Document 2, in order to completely lower bainite weld heat-affected zone of the microstructure, is necessary to limit to a very narrow range of range of P _CM, production stability is concerned The

  Accordingly, the present invention provides a low-temperature high-strength welded steel pipe having a tensile strength of 570 MPa or more and 760 MPa or less excellent in low-temperature toughness of a welded heat-affected zone of a longitudinal seam weld in order to solve the above-described problems. .

In order to develop a low-temperature high-strength welded steel pipe excellent in toughness of the weld heat-affected zone with a pipe thickness of 50 mm or less and a tensile strength of 570 MPa or more and 760 MPa or less, the present inventors have conducted intensive research and found the following knowledge Got.
(1) The site where the toughness is most reduced in the heat affected zone (LBZ: Local Brittle Zone) is a CGHAZ structure near the bond on the outer surface side, and the CGHAZ structure on the inner surface is a two-phase region (Ac1- In the ICCGHAZ structure reheated to Ac3 point), the HAZ coarse grain region (region where the prior austenite particle size near the melting line is 50 μm or more: Coarse-grain HAZ, hereinafter CGHAZ) is the pre-structure. The root portion refers to the vicinity of the meeting portion where the inner surface weld metal and the outer surface weld metal cross.
(2) CGHAZ microstructure does not depend on the outer surface side and inner surface side, a P _CM value of the base material, in cooling after welding, the cooling rate in the temperature range of 500 ° C. from 800 ° C. to transformation gamma-alpha phase In combination, the upper bainite structure containing a large amount of hard island martensite (MA) and cementite between the laths and the high-strength martensite structure are suppressed to a certain fraction or less, and a baini having an equivalent circle diameter of 5 μm or less. Toughness is most improved when bainite (MA-free bainite) in which carbides mainly composed of cementite are precipitated is mainly contained in the lath and / or between laths of the tick ferrite.
(3) In order to suppress MA formation at the lath interface of bainitic ferrite, it is effective to reduce Si and add an appropriate amount of B.

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 0.2%,
Mn: 1.2-2.2%,
P: 0.015% or less,
S: 0.003% or less,
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: 0.005% or less,
B: 0.0003 to 0.0020%
Further,
Cu: 0.01 to 1%,
Ni: 0.01 to 1%,
Cr: 0.01-1%,
Mo: 0.01 to 0.2 %,
V: 0.01 to 0.1%
Containing one or more of
_{P CM} value calculated by the following formula (1) (mass%) satisfies the 0.12 ≦ _{P CM} ≦ 0.20,
A base material portion comprising the balance Fe and inevitable impurities;
In the lath and / or lath of bainitic ferrite with an average grain size of 5 μm or less in the microstructure of the weld heat affected zone where the prior austenite grain size is 50 μm or more in the seam weld zone of the steel pipe welded layer by layer from the inner and outer surfaces There is at least 50% area ratio of bainite with cementite mainly composed of cementite in between, and the balance is upper bainite, martensite, pearlite or mixed structure containing cementite between island martensite and / or lath. A high-strength welded steel pipe for low temperatures having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat-affected zone toughness, characterized by having a certain longitudinal seam welded joint.
P _CM (mass%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1)
However, each element shows content (mass%).
2. The base material part is further 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%
The high-strength welded steel pipe for low temperature having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat-affected zone toughness according to 1, characterized by containing one or more of the above .
3. The low-temperature high-strength welded steel pipe having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat affected zone toughness according to 1 or 2, wherein the hardness of the welding heat affected zone satisfies the following formula (2) :
200 ≦ Hv (98N) ≦ 300 (2)
4.1. A method for producing a high-temperature steel pipe for low temperature having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat-affected zone toughness according to any one of 1 to 3, comprising forming a raw steel plate into a cylindrical shape, The welding heat input of each of the inner and outer surfaces when the butt portion is welded one layer at a time from the inner and outer surfaces is 80 kJ / cm or less, and the heat input balance on the outer surface side and the inner surface side satisfies the following formula (3). A method for producing a high-strength welded steel pipe for low temperature having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat-affected zone toughness .
Inner surface heat input ≦ Outer surface heat input (3)
5. Tensile strength with excellent weld heat affected zone toughness according to 4, characterized in that after welding one layer at a time from the inner and outer surfaces in the longitudinal direction of the steel pipe, the pipe is expanded at a tube expansion ratio of 0.4% or more and 2.0% or less. Is a manufacturing method of high-strength welded steel pipe for low temperature of 570 MPa or more and 760 MPa or less .

  ADVANTAGE OF THE INVENTION According to this invention, the high strength welded steel pipe for low temperature which has the tensile strength of 570 MPa or more and 760 MPa or less excellent in the toughness of the welding heat affected zone of a longitudinal seam welded part is obtained, and it is very useful industrially.

In the present invention, the composition of the base material, the microstructure of the weld heat affected zone where the prior austenite grain size near the fusion line in the seam weld zone of the steel pipe welded layer by layer from the inner and outer surfaces in the longitudinal direction of the steel pipe is 50 μm or more. Stipulate.
[Component composition of base material]
In the following description, “%” means “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 0.2%
Si is an element that acts as a deoxidizing material and increases the strength of the steel material by solid solution strengthening, but if it is less than 0.01%, the effect is hardly obtained. On the other hand, when Si is added in excess of 0.2%, when the heat input is high, the formation of upper bainite containing a large amount of MA harmful to the toughness of the weld heat affected zone becomes significant, so the upper limit is 0.2%. And A preferable range is 0.01% or more and less than 0.12%.

Mn: 1.2-2.2%
Mn acts as a hardenability improving element. The effect can be obtained by addition of 1.2% or more. However, in the continuous casting process, the concentration rises at the center segregation part remarkably. If the addition exceeds 2.2%, the cause of delayed fracture at the center segregation part. Therefore, the upper limit is made 2.2%.

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. Precipitated TiN suppresses the austenite grain coarsening by the pinning effect and contributes to the improvement of the toughness of the base metal and the heat affected zone of the weld. . 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. 0.001% to suppress austenite grain coarsening due to pinning effect by TiN
B: 0.0003 to 0.0020%
B segregates at the austenite grain boundary in the heat affected zone and has the effect of improving hardenability, and fine cementite is precipitated inside or between the laths constituting the bainitic ferrite with a small alloy composition. Facilitates the production of bainite free of MA.

  This effect is prominent when added in an amount of 0.0003% or more and 0.0020% or less, and if added over 0.0020%, a large amount of B-based carbides and nitrides are generated and the toughness is lowered. The upper limit is made 0.0020%. A preferable range is 0.0005% or more and 0.0015% or less.

Cu, Ni, Cr, Mo, V, one or more of Cu, Ni, Cr, Mo, V all act as a hardenability improving element, so for the purpose of increasing the strength, one of these elements, 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, since addition of 1% or more causes deterioration of toughness, the upper limit is set to 1% and 0.01 to 1% when added.

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

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 when added, the upper limit is made 1%, and 0.01 to 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 when added, the upper limit is made 1% and 0.01 to 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 remarkably reduces toughness, so when added, the upper limit is made 0.1%.

O: 0.005% or less, P: 0.015% or less, S: 0.003% or less In the present invention, O, P, and S are inevitable impurities and define the upper limit of the content. O is 0.005% or less in order to suppress the formation of inclusions that are coarse and adversely affect toughness. If the P content is large, the central segregation is remarkable and the base material toughness is deteriorated. If the content of S is large, the amount of MnS produced increases remarkably and the toughness of the base material deteriorates.

P _CM (mass%): 0.12 to 0.20
P _CM (mass%) is a weld cracking susceptibility index represented by C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B. Each element is a content (mass%), and an element not contained is 0. .

In the present invention, in order to achieve the tensile strength of the steel pipe: 570 MPa or more and 760 MPa or less, P _CM (mass%) is set to 0.12 or more and 0.20 or less.

  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 to improve the toughness by suppressing the austenite grain coarsening in the heat affected zone by the pinning effect. Good.

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 Ca addition amount exceeds 0.01%, coarse CaO is likely to be generated, the toughness including the base material is lowered, the nozzle of the ladle is blocked, and the productivity is hindered. 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%.

  The balance other than the above components is composed of Fe and inevitable impurities.

[Base material manufacturing method]
In the present invention, the steel having the above-described component composition is hot-rolled by a conventional method and then subjected to accelerated cooling to obtain a steel plate having a predetermined plate thickness and strength. Depending on the plate thickness, high-frequency heat tempering is performed on the same line (inline) so that mechanical properties such as strength and toughness have desired values.

  Specifically, for example, steel having the above-described component composition is heated to a temperature of 1000 to 1200 ° C., hot-rolled at a rolling end temperature of 600 ° C. or higher, and then cooled at a cooling rate of 20 ° C./s or higher. Accelerated cooling to a temperature of from ℃ to 600 ℃.

In the present invention, reheating treatment is performed to 500 to 750 ° C. at a temperature rising rate of 0.5 ° C./s or more according to the thickness of the steel plate so that the tensile strength of the steel pipe is 570 MPa or more and 760 MPa or less.
[Production conditions for steel pipes]
The steel pipe which concerns on this invention manufactures the steel material (henceforth a base material) mentioned above with a normal UOE steel pipe manufacturing process.

  Seam welding is performed by multi-electrode submerged arc welding with a heat input of 80 kJ / cm or less for each of the inner surface (back side) and outer surface (final side) of the steel pipe after temporary welding. By controlling the inner and outer surface heat input balance satisfying the expression (3) with a heat input of 80 kJ / cm or less, stable joint HAZ toughness can be obtained even at a low temperature of −30 ° C. or less.

  The joint HAZ has the lowest toughness in the CGHAZ structure in the vicinity of the bond on the outer surface side, and the ICCGHAZ structure in which the CGHAZ structure in the root part is reheated to a two-phase region (Ac1 to Ac3 points) by seam welding on the outer surface side. Lowest.

  And since LBZ (Local Brittle Zone) whose toughness is most deteriorated in the joint HAZ becomes an ICCGHAZ structure on the inner surface side, in the present invention, the heat input of the seam welding on the outer surface side and the inner surface side is satisfied to satisfy the equation (3). Set to suppress the coarsening of the γ grains in the CGHAZ part, which is the front structure of the ICCGHAZ structure on the inner surface side.

Inner surface heat input ≦ Outer surface heat input (3)
Further, according to the present invention, the seam on the outer surface side and the inner surface side is stable so that an impact value of 100 J or more can be obtained in a stable Charpy test (notch position: FL, test temperature-30 ° C.) on the outer surface side and the inner surface side. The heat input of welding is 80 kJ / cm or less. The lower the heat input, the better from the viewpoint of HAZ toughness, so no lower limit is set. However, the heat input is such that one inner and outer surface welding is possible. “Securing stably” means that the cumulative failure probability is 1% or less in a Charpy test of 100 or more tests at a test temperature of −30 ° C. or less.

  The flux used in seam welding is not particularly limited, and may be a melt type or a fired type. Moreover, preheating before welding or heat treatment after welding is performed as necessary. After seam welding, pipe expansion is performed at a pipe expansion rate of 0.4% or more and 2.0% or less according to the required roundness.

[Microstructure]
Bainitic ferrite with an average grain size of 5 μm or less in the microstructure of the weld heat-affected zone where the prior austenite grain size in the vicinity of the melt line in the seam weld is 50 μm or more by the combination of the base material having the above composition and the welding conditions. The area ratio of bainite (MA-free bainite) in which carbides mainly composed of cementite are precipitated in the lath and / or between the laths is present at least 50%, and the balance is island martensite (MA) and / or lath. Upper bainite, martensite, pearlite, or a mixed structure containing cementite in between is obtained.

  The microstructure is identified by randomly observing at least 10 fields of view near the melting line at a position 6 mm from the outer surface of the steel pipe in the thickness direction with a scanning electron microscope (5000 magnifications).

  Moreover, the average particle diameter of bainitic ferrite takes the average value of 100 or more grains by observation with an optical microscope. In addition, CGHAZ of the welding heat affected zone where the prior austenite grain size is 50 μm or more in the vicinity of the melting line is observed near the FL on the outer surface side of the steel pipe and on the inner surface side before outer surface welding.

  In the weld heat affected zone where the prior austenite grain size near the melting line is 50 μm or more, in the case of the above microstructure, the Vickers hardness (load: 98 N) is 200 ≦ Hv (98 N) ≦ 300, and the desired joint strength, Toughness is obtained. In addition, the hardness of a welding heat affected zone measures Vickers hardness 10 points or more with a load of 98N, and makes it the average value.

  Steel having a chemical composition shown in Table 1 is melted in a converter and made into a slab of 220 mm thickness by continuous casting, and then a steel plate having a thickness of 25 to 32 mm under hot rolling, accelerated cooling, and reheating conditions shown in Table 2. A to J were produced. As the tempering treatment, reheating treatment was performed with an induction heating type heating device installed on the same line as the accelerated cooling equipment.

  The obtained steel sheet was formed by U press and O press in a normal UOE pipe manufacturing equipment, and then the inner and outer surfaces of the steel pipe were seam welded by submerged arc welding, and the pipe was expanded at a expansion ratio of 0.6 to 1.2%. Thus, a UOE steel pipe having an outer diameter of 400 to 1626 mm was obtained.

  In order to evaluate the tensile strength of the obtained steel pipe, a full-thickness tensile specimen according to API-5L was taken in the circumferential direction, and a tensile test was performed.

  Furthermore, a V-notch Charpy impact test piece of JIS Z2202 (1980) was taken 2 mm below the outer surface and from the root portion on the inner surface from the seam welded joint of the steel pipe. The Charpy impact test was carried out at the notch position of the Charpy impact test piece at FL (ratio of HAZ and weld metal is 1: 1) at a test temperature of −30 ° C., and the average value of 100 test pieces was obtained.

  FIG. 1 (a) shows the sampling position and notch position of the Rurpee test piece at the root portion on the outer surface side and FIG. 1 (b) at the root portion on the inner surface side.

  On the outer surface side CGHAZ, the hardness of the weld heat affected zone where the prior austenite grain size in the vicinity of the melting line is 50 μm or more was measured by a Vickers hardness test (load: 98 N).

  Table 3 summarizes the test results of the tensile strength of the steel pipe, the microstructure and hardness of CGHAZ, and the toughness of CGHAZ (hereinafter referred to as HAZ toughness). In the description of this example, bainite of bainitic ferrite having an equivalent circle diameter of 5 μm or less in which carbides mainly composed of cementite are precipitated in the lath and / or between the laths is referred to as an MA free bainite structure.

  The steel pipe has a tensile strength of 570 MPa or more and 760 MPa or less, and a Charpy absorbed energy (vE-30) of 100 J or more at a test temperature of the welded bond portion of −30 ° C. is within the scope of the present invention.

  Invention Example No. 1 to 12 have a microstructure defined in the present invention, and were confirmed to have high HAZ toughness (100 J or more and cumulative failure probability of 1% or less). Invention Example No. In Nos. 1 to 12, the heat input of the outer surface and the inner surface satisfies heat input of 80 kJ / cm or less and the inner surface heat input ≦ the outer surface heat input.

  On the other hand, Comparative Example No. In Nos. 13 to 16, the heat input of the outer surface and the inner surface was 80 kJ / cm or more, and the MA-free bainite structure fraction could not be ensured by 50% or more, so the HAZ toughness decreased.

  Comparative Example No. Nos. 17 to 20 are examples in which the composition of the base material is outside the range of the present invention, and Comparative Example No. In No. 17, since the fraction of MA-free bainite structure could not be ensured by 50% or more in the microstructure, HAZ toughness was lowered.

Comparative Example P _CM is below the lower limit of the present invention No. In No. 18, the CGHAZ structure became a pearlite-based structure, the CGHAZ hardness decreased, and the HAZ toughness decreased.

Comparative Example P _CM value exceeds the upper limit of the present invention No. No. 19 is a comparative example No. 19 in which the strength of the base material is outside the range of the present invention and the amount of Si exceeds the upper limit of the present invention. In No. 20, since the fraction of MA-free bainite structure could not be secured by 50% or more, the HAZ toughness was lowered.

  Comparative Example No. in which inner surface heat input exceeded outer surface heat input 21 or No. In No. 22, since the fraction of MA-free bainite structure could not be ensured by 50% or more, the Charpy impact value was 100 J or less, and the inner surface heat input was lower than the outer surface heat input.

It is a figure explaining the collection position and notch position of a Charpy test piece, (a) is a collection position: outer surface side, notch position: FL, (b) Collection position: root part on the inner surface side, notch position: FL.

Claims (5)

% By mass
C: 0.03-0.12%,
Si: 0.01 to 0.2%,
Mn: 1.2-2.2%,
P: 0.015% or less,
S: 0.003% or less,
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: 0.005% or less,
B: 0.0003 to 0.0020%
Further,
Cu: 0.01 to 1%,
Ni: 0.01 to 1%,
Cr: 0.01-1%,
Mo: 0.01 to 0.2 %,
V: 0.01 to 0.1%
Containing one or more of
_{P CM} value calculated by the following formula (1) (mass%) satisfies the 0.12 ≦ _{P CM} ≦ 0.20,
A base material portion comprising the balance Fe and inevitable impurities;
In the lath and / or lath of bainitic ferrite with an average grain size of 5 μm or less in the microstructure of the weld heat affected zone where the prior austenite grain size is 50 μm or more in the seam weld zone of the steel pipe welded layer by layer from the inner and outer surfaces There is at least 50% area ratio of bainite with cementite mainly composed of cementite in between, and the balance is upper bainite, martensite, pearlite or mixed structure containing cementite between island martensite and / or lath. A high-strength welded steel pipe for low temperatures having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat-affected zone toughness, characterized by having a certain longitudinal seam welded joint.
P _CM (mass%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (1)
However, each element shows content (mass%). The base material part is further 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%
The high-strength welded steel pipe for low temperatures having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat-affected zone toughness according to claim 1, comprising one or more of the following . The hardness of the weld heat affected zone satisfies the following formula (2) : The high strength welded steel pipe for low temperature having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat affected zone toughness according to claim 1 or 2 .
200 ≦ Hv (98N) ≦ 300 (2) It is a manufacturing method of the high-strength steel pipe for low temperature whose tensile strength excellent in the welding heat affected zone toughness as described in any one of Claims 1 thru | or 570MPa or less is 760MPa or less, Comprising: A raw steel plate is shape | molded cylindrically, The welding heat input of each of the inner and outer surfaces when the butt portion is welded one layer at a time from the inner and outer surfaces is 80 kJ / cm or less, and the heat input balance on the outer surface side and the inner surface side satisfies the following formula (3). A method for producing a high-strength welded steel pipe for low temperature having a tensile strength of 570 MPa or more and 760 MPa or less excellent in weld heat-affected zone toughness .
Inner surface heat input ≦ Outer surface heat input (3) The welded heat-affected zone toughness according to claim 4, wherein after welding one layer at a time from the inner and outer surfaces in the longitudinal direction of the steel pipe, the pipe is expanded at a tube expansion ratio of 0.4% or more and 2.0% or less . A method for producing a low-temperature high-strength welded steel pipe having a tensile strength of 570 MPa or more and 760 MPa or less .

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