JP5181639B2 - Welded steel pipe for high-strength thick-walled line pipe excellent in low-temperature toughness and manufacturing method - Google Patents

Description

  The present invention relates to a welded steel pipe for a high-strength thick-walled line pipe excellent in low-temperature toughness suitable for a line pipe for transporting crude oil and natural gas.

  Currently, steel pipes for line pipes up to American Petroleum Institute (API) standard X80 (tensile strength of 620 MPa or more) have been put into practical use as materials for trunk pipelines for long-distance transportation of crude oil and natural gas. In recent years, increasing the internal pressure of pipelines has been studied in order to increase the efficiency of transporting crude oil and natural gas, and as a result, it is required to increase the thickness of steel pipes for high-strength line pipes of X80 or higher. Yes.

  On the other hand, a method of manufacturing a thick steel plate having good strength and toughness by using a metal structure as fine bainite by controlled rolling and controlled cooling has been proposed (for example, Patent Documents 1 to 3). The wall thickness of such a conventional high strength line pipe of X80 or higher is at most less than 25 mm, and a thick line pipe of 25 mm or more or 30 mm or more has not been required.

  In general, when manufacturing a thick steel plate, the rolling by control rolling tends to be insufficient at the center of the plate thickness, and it becomes difficult to secure a cooling rate by controlled cooling. Furthermore, when manufacturing a thick steel pipe, after forming a thick steel plate into a tube shape by a UO process, end parts are faced | matched and the seam part is welded by arc welding. In this seam welding, when the steel pipe is thickened, the heat input becomes large, and the particle size of the weld heat affected zone (referred to as Heat Affected Zone, HAZ) becomes coarse. Therefore, a decrease in low temperature toughness becomes an important problem.

  As a technique for improving the low-temperature toughness of the HAZ of the steel pipe for high-strength line pipe, a method for refining the HAZ structure using intragranular transformation has been proposed (for example, Patent Documents 4 to 6). The method proposed in Patent Document 4 is to generate acicular ferrite using an oxide as a nucleus, and the methods proposed in Patent Documents 5 and 6 are based on a complex inclusion with oxide and sulfide. It produces intragranular bainite.

  The use of intragranular bainite is extremely effective in improving the low temperature toughness of HAZ. However, if the cooling rate decreases due to the thickening of the steel pipe, transformation to bainite becomes insufficient, intragranular ferrite is generated, and the strength decreases. Therefore, it is difficult to increase the thickness of the steel pipe for high-strength line pipe excellent in low temperature toughness.

JP 2000-256777 A JP 2004-76101 A JP 2004-143509 A Japanese Patent Laid-Open No. 08-325635 JP 2001-355039 A JP 2003-138340 A

  The inventors made a prototype of a thick steel plate for a high-strength line pipe of X80 or more with a plate thickness of 25 mm or more. As a result, it was found that the problems caused by the increase in the thickness of the steel sheet were much more serious than expected. In particular, in the central portion of the plate thickness, the reduction by controlled rolling and the cooling rate by controlled cooling become insufficient, and the toughness is significantly reduced compared to the surface layer portion of the steel plate. Furthermore, as a result of investigating the metal structure in the central part of the plate thickness of the steel plate, it was found that it is extremely difficult to make the central part of the plate thickness into a fine bainite structure in the thick steel plate for high-strength line pipe.

  The present invention solves such a problem that could not be predicted from the prior art, and in particular, even if the wall thickness is 25 mm or more, and even 30 mm or more, excellent HAZ low temperature toughness can be ensured. The present invention provides a welded steel pipe for a high-strength thick-walled line pipe excellent in low-temperature toughness and a method for producing the same.

  In the present invention, C and Al are reduced, and appropriate amounts of Mo and B are added to improve hardenability, and the carbon equivalent Ceq, which is an index of hardenability, and the cracking sensitivity index Pcm, which is an index of weldability, are in an optimal range. The HAZ of the base steel plate and the welded steel pipe is made into a fine metal structure mainly composed of bainite, and further, the intragranular bainite generated with the oxide of Ti as the nucleus is utilized, and particularly the effective crystal grain size of the HAZ This is a welded steel pipe for high-strength, thick-walled line pipes that has been refined and has good low-temperature toughness, and the summary thereof is as follows.

(1) A steel pipe obtained by seam welding a base steel plate formed into a tubular shape, wherein the base steel plate is in mass%, C: 0.030 to 0.080%, Si: 0.01 to 0.50. %, Mn: 0.50 to 2.00%, S: 0.0001 to 0.0050%, Ti: 0.003 to 0.030%, Mo: 0.10 to 1.50%, O: 0.00. 0001 to 0.0080%, and further by mass, Cr: 0.02 to 1.50%, V: 0.010 to 0.100%, Nb: 0.001 to 0.200%, Zr: 0.0001-0.0500%, Ta: containing one or more of 0.0001-0.0500%, P: 0.050% or less, Al: limited to 0.020% or less, The balance has a component composition consisting of iron and inevitable impurities, and the Ceq calculated by the following (Equation 1) is 0.4. A ～0.56, Pcm found by the following equation (2) and is 0.15 to 0.21, and satisfies the following (Equation 3), 30% or less in the metal structure area ratio of the base material steel plate Of low-temperature toughness, characterized in that the effective crystal grain size is 20 μm or less and the effective crystal grain size of the weld heat affected zone is 150 μm or less. Welded steel pipe for high-strength thick-walled line pipe.

Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5
... (Formula 1)
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60
+ Mo / 15 + V / 10 + 5B (Formula 2)
10C + 100Al + 5Mo + 5Ni <3.3 (Formula 3)
Here, C, Si, Mn, Ni, Cu, Cr, Mo, V, B, and Al are contents [mass%] of each element.

(2) The welded steel pipe for high-strength thick-walled line pipe excellent in low-temperature toughness according to (1) above, wherein the thickness of the base steel plate is 25 to 40 mm.
(3) High strength excellent in low temperature toughness as described in (1) or (2) above, wherein the tensile strength of the base steel plate is 600 to 800 MPa, with the circumferential direction of the steel pipe as the tensile direction Welded steel pipe for thick line pipe.

(4) The above base metal steel sheet further comprising one or both of Cu: 0.05 to 1.50% and Ni: 0.05 to 5.00% in mass%. The welded steel pipe for high-strength thick-walled line pipe excellent in low-temperature toughness according to any one of 1) to (3).
( 5 ) The base material steel plate is further in mass%, Mg: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Y: 0.0001 to 0.0050%, Hf: 0.0001 to 0.0050%, Re: 0.0001 to 0.0050%, W: 0.01 to 0.50%, one or more The welded steel pipe for high-strength thick-walled line pipes according to any one of (1) to ( 4 ) above, which is contained.

( 6 ) The weld metal is in mass%, C: 0.010 to 0.100%, Si: 0.01 to 0.50%, Mn: 1.0 to 2.0%, Ni: 0.2 to Including 3.2%, Cr + Mo + V: 0.2 to 2.5%, Al: 0.001 to 0.100%, Ti: 0.003 to 0.050%, O: 0.0001 to 0.0500% , P: 0.020% or less, S: 0.010% or less, and the balance is made of iron and inevitable impurities, (1) to ( 5 ) above, A welded steel pipe for high-strength thick-walled line pipes with excellent low-temperature toughness.

( 7 ) When steel is melted, Si and Mn are added to perform weak deoxidation, and then Ti is added to any one of (1) and (4) to ( 5 ) above. The steel adjusted to the described components was cast, and the obtained steel slab was heated to 1000 ° C or higher, hot-rolled with a rolling ratio from 900 ° C or lower to the end of rolling being 2.5 or higher, and a stop temperature of 600 ° C. The steel sheet obtained by performing the water cooling described below was formed into a tubular shape in the UO process, and the butt portion was seam welded by submerged arc welding with a heat input of 4.0 to 10.0 kJ / mm from the inner and outer surfaces. After that, the pipe was expanded, and then the heat treatment of the seam welded portion was performed within a range of 300 to 500 ° C. Excellent in low temperature toughness described in any one of the above (1) to (6) Manufacturing method of welded steel pipe for high-strength thick-walled line pipe.

( 8 ) The method for producing a welded steel pipe for a high-strength thick-walled line pipe excellent in low-temperature toughness as described in ( 7) above, wherein the seam weld is heat-treated.
( 9 ) The method for producing a welded steel pipe for a high-strength thick-walled line pipe excellent in low-temperature toughness as described in ( 8 ) above, wherein the heat treatment of the seam weld is performed within a range of 300 to 500 ° C.

  According to the present invention, even if the wall thickness is 25 mm or more, further 30 mm or more, the low temperature toughness of the base steel plate of the welded steel pipe for line pipes, particularly the central portion of the thickness of the base steel plate and the HAZ is ensured. This makes it possible to provide a welded steel pipe for a high-strength thick-walled line pipe excellent in low-temperature toughness and a method for producing the same.

  The present invention is based on a steel material in which the C content is reduced and the toughness is improved as a low-temperature transformation structure mainly composed of bainite, and Mo is added to improve the hardenability and suppress the addition of Al. In particular, it is a welded steel pipe that utilizes intragranular bainite, and in particular, refines the effective crystal grain size of HAZ to improve low-temperature toughness. That is, the present invention reduces the amount of Al, controls the amount of oxygen, adds an appropriate amount of Ti, disperses fine inclusions that act extremely effectively as nuclei for intragranular transformation, The greatest feature is that the effective crystal grain size of the HAZ of the base steel plate and the welded steel pipe is refined by using it as a production nucleus. Hereinafter, the base steel plate is also simply referred to as a steel plate, and the welded steel pipe is also simply referred to as a steel pipe.

  The intragranular bainite of HAZ is obtained by transforming intragranular ferrite generated by intragranular transformation at a high temperature at the time of cooling using the above-described fine inclusions as production nuclei. Therefore, the addition amount of Mo and the hardenability index Ceq and the weldability index Pcm are within the optimum ranges, so that the steel pipe having a thick wall as in the present invention, that is, the intragranular bainite is reduced even when the cooling rate is slow. It is extremely effective for generating. Due to the formation of intragranular bainite, the low temperature toughness of the HAZ is remarkably improved without reducing the strength. Intragranular bainite may also contribute to the suppression of HAZ softening.

  The mechanism of formation of intragranular bainite is considered as follows. The cation vacancy-type oxide can take in a large amount of Mn ions, and MnS is likely to be complexly precipitated in the oxide. Therefore, a Mn-depleted layer is generated around the oxide and sulfide. This Mn-deficient layer acts as a transformation nucleus when the steel is heated to a high temperature such that the metal structure becomes an austenite phase, and usually petal-like intragranular ferrite is generated. This intragranular ferrite has a high degree of supercooling when the cooling rate is high or when the hardenability is high, so it transforms into bainite during cooling and becomes intragranular bainite.

  A typical example of the cation vacancy-type oxide is a fine oxide containing Ti as a main component, and petals-like intragranular bainite is generated using this as a nucleus. In addition, fine sulfides mainly composed of Mn may be combined and precipitated in the fine oxides mainly composed of Ti. Depending on the component composition of the steel, the oxide contains one or more of Al, Si, Mn, Cr, Mg, and Ca, and the sulfide contains one or more of Ca, Cu, and Mg. May be included. The size of the inclusions serving as the nuclei of intragranular bainite can be measured by a transmission electron microscope (referred to as TEM), and the diameter is preferably in the range of 0.01 to 5 μm.

  When a large amount of intragranular bainite is generated in the HAZ, a martensite / austenite hybrid (called Martensite-Austenite Constituent, MA) that becomes the starting point of fracture becomes finer, and the low-temperature toughness is greatly improved. When the amount of C is suppressed to 0.05% or less and fine inclusions are dispersed, intragranular bainite is generated, the structure within the grain is subdivided, and the Charpy fracture surface unit, that is, the effective crystal grain size is extremely small. Become. Furthermore, since intragranular bainite is harder than intragranular ferrite, the formation of intragranular bainite may suppress the softening of HAZ.

  As shown schematically in FIG. 1, in the HAZ of the central part of the welded steel pipe for high-strength line pipes (in the vicinity of the 1/2 part of the thickness and referred to as 1 / 2t part) Coarse MA present along the prior austenite grain boundaries of the thermal HAZ may become the starting point of fracture and impair toughness. The reheat HAZ is a portion where the weld metal and the HAZ in the vicinity of the melting line of the preceding welding are reheated by subsequent welding. Normally, HAZ is a part within 10 mm from the melting line, although it varies somewhat depending on the heat input during welding. For example, when a notch is provided at a position of 1 mm or 2 mm from the melting line, the Charpy absorbed energy at −40 ° C. is , Sometimes less than 50J. In particular, when the wall thickness is 25 mm or more, it is difficult to improve the HAZ toughness of the welded steel pipe for high-strength line pipe.

  The present inventors examined improvement of low temperature toughness by reducing the effective crystal grain size of HAZ and suppressing the formation of MA. First, samples are collected from steel materials having various component compositions, and heat treatment (reheating) that simulates the heat history of reheated HAZ, assuming submerged arc welding of a butt portion in the manufacture of a steel pipe having a wall thickness of 25 to 40 mm. HAZ reproduction test). This is because the steel material is heated to 1400 ° C. and immediately cooled to room temperature, further heated to 750 ° C. and immediately cooled to room temperature, and the cooling rate from 800 ° C. to 500 ° C. during cooling is 2 to 15 ° C./s. To do. In accordance with JIS Z 2242, a V-notch test piece was sampled from the steel material after the reheat HAZ reproduction test, and a Charpy impact test was performed at −40 ° C. The toughness results evaluated by the reheat HAZ reproduction test are shown in FIG.

  FIG. 2 shows the relationship between 10C + 100Al + 5Ni + 5Mo and the Charpy absorbed energy at −40 ° C. of reheated HAZ obtained by the simulation test. The present inventors examined the influence of C, Mo, and Ni that affect the formation of MA and Al that affects the intragranular transformation on the reproduced HAZ toughness. Further, based on the obtained results, the relationship between the addition amount of each element and the effect was linearly regressed, and an index of 10C + 100Al + 5Ni + 5Mo having a correlation with the reproduced HAZ toughness was obtained. From FIG. 2, it is clear that when 10C + 100Al + 5Ni + 5Mo is suppressed to less than 3.3, the Charpy absorbed energy of reheated HAZ at −40 ° C. is 50 J or more.

  As a result of measuring the effective crystal grain size of the sample having good low-temperature toughness of the reheated HAZ by EBSP (Electron Back Scattering Pattern), it was found to be 150 μm or less. In addition, as a result of investigating the metal structure and inclusions, fine oxides, composite oxides, and composite sulfides mainly composed of Ti are generated, and intragranular bainite is generated in HAZ using these as precipitation nuclei. It became clear that. That is, the effective crystal grain size of HAZ becomes 150 μm or less due to the formation of intragranular bainite, and the low temperature toughness is improved.

  Next, the present inventors conducted intensive research to satisfy the toughness of the base steel sheet. This is because when the wall thickness is 25 mm or more, the reduction ratio in the non-recrystallization temperature region cannot be secured, and the crystal grain size of 1/2 t part becomes coarse and the Charpy energy decreases. As a result of the study, the inventors determined that the area ratio of polygonal ferrite is 30% or less, the area ratio of bainite is 70% or more, and the effective crystal grain size of the base steel sheet is 20 μm or less. It has also been found that toughness is improved, and in particular, it is possible to suppress a reduction in toughness at the center of the plate thickness. Specifically, the Charpy absorbed energy at −40 ° C. of the test piece taken from the vicinity of the surface layer, that is, from about 2 to 12 mm from the surface of the steel material is 200 J or more, and the 1/2 t part, that is, the wall thickness The Charpy energy when collected from almost the center can be 100 J or more. The Charpy impact test was performed at −40 ° C. in accordance with JIS Z 2242 by collecting V-notch test pieces.

  The fine oxides, composite oxides, and composite sulfides mainly composed of Ti of the present invention are effective not only for the formation of HAZ intragranular bainite but also for the refinement of the effective crystal grain size of the base steel sheet. . In particular, the refinement of the effective crystal grain size in the 1 / 2t part of the base steel sheet, which has been difficult in the past, has been made possible by the fine oxides, composite oxides, and composite sulfides mainly composed of Ti. . The reason for this is considered as follows.

  First, when reduction in the non-recrystallization temperature range is secured, transformation from normal grain boundaries is promoted, so it is difficult to transform intragranularly from oxides, complex oxides, and complex sulfides. . This is considered to be because when the crystal grain size is reduced by securing the reduction, the growth rate of bainite nucleated from the grain boundary becomes too high as compared with the intragranular transformation. That is, it is considered that the transformation from the grain boundary is completed before the intragranular transformation is generated.

  On the other hand, when the reduction ratio in the non-recrystallization temperature region is insufficient, the crystal grain size becomes coarse, particularly at the center of the plate thickness, and the growth of bainite nucleated from the grain boundary is also slowed. Therefore, it is considered that the effective crystal grain size is refined in the grains due to intragranular transformation from oxides, complex oxides, and complex sulfides mainly composed of Ti. Further, it is considered that the fine oxide acts as pinning particles and suppresses the growth of crystal grains, which is also effective for refining the effective crystal grain size of the base steel plate.

Therefore, in the present invention, control of the oxygen amount in the steel making process is extremely important. In particular, when adjusting the component composition of steel, it is necessary to add Si and Mn so that the content is in the above-described range and perform weak deoxidation, and then add Ti. The oxygen concentration when adding Ti is preferably 0.001 to 0.003%. As a result, Ti oxide having a particle diameter of 0.01 to 10 μm and a number per area of 1 μm ^{2 of} 10 to 1000 / mm ² , specifically, Ti ₂ O ₃ can be dispersed. Thereby, the production | generation of an intragranular transformation is accelerated | stimulated and the effective crystal grain diameter of HAZ of a base material steel plate and a welded steel pipe refines | miniaturizes.

  When the steel composition obtained by adjusting the component composition and casting by such a steelmaking process is hot-rolled, the rolling ratio from 900 ° C. to the end of rolling is 2.5 or more, preferably 3.0 or more. By doing so, it is possible to make the effective crystal grain size of the base steel plate 20 μm or less.

  The effective particle size is a value obtained by converting the area of a portion surrounded by a boundary having a crystal orientation difference of 15 ° or more into an equivalent circle diameter using EBSP. Polygonal ferrite is observed in an optical microscope structure as a white massive structure that does not contain coarse precipitates such as coarse cementite and MA in the grains. In the optical microstructure of the base steel sheet, martensite, retained austenite, and MA may be included as the remainder of polygonal ferrite and bainite.

  In the present invention, bainite is defined as a structure in which carbides are precipitated between laths or massive ferrites or a structure in which carbides are precipitated in the laths. Further, martensite is a structure in which carbides are not precipitated between laths or within laths. Residual austenite is austenite in which austenite generated at a high temperature remains in a base steel plate or a welded steel pipe.

  Further, the heat treatment of the welded portion decomposes the coarse MA generated along the HAZ prior austenite grain boundaries into fine cementite, thereby improving the low temperature toughness. Thereby, the toughness at the meeting part of the 1/2 t part or the meeting part + 1 mm at a lower temperature is improved. For example, when the welded part is heated to a temperature of 300 to 500 ° C., the V-notch Charpy at a low temperature of −40 ° C. The absorbed energy can be 50 J or more. Therefore, when used at an extremely low temperature of −40 ° C. or lower, it is preferable to further heat-treat the structure in which the intragranular bainite is formed to obtain a mixed structure of intragranular bainite and cementite.

  Hereinafter, the reasons for limitation of the base steel sheet according to the present invention will be described. Since HAZ is a heat-affected zone that does not melt during welding, the component of HAZ is the same as that of the base material.

  C: C is an element that improves the strength of steel. However, in the present invention, the content of C is limited, a metal structure mainly composed of bainite is obtained, and both high strength and high toughness are achieved. . If the C content is less than 0.030%, the strength is insufficient, and if it exceeds 0.080%, the toughness decreases. Therefore, in the present invention, the optimum amount of C is set to a range of 0.030 to 0.080%.

  Si: Si is an important deoxidizing element in the present invention, and in order to obtain an effect, it is necessary to contain 0.01% or more of Si in the steel. On the other hand, if the Si content exceeds 0.50%, the toughness of the HAZ decreases, so the upper limit is made 0.50%.

  Mn: Mn is an element that is used as a deoxidizer, is necessary for ensuring the strength and toughness of the base steel sheet, and further produces sulfides such as MnS that are effective as nuclei for intragranular transformation. It is extremely important in the invention. In order to obtain these effects, it is necessary to contain 0.50% of Mn. However, if the Mn content exceeds 2.00%, the toughness of the HAZ is impaired. Therefore, the content range of Mn is set to 0.50 to 2.00%. In addition, since Mn is an inexpensive element, in order to ensure hardenability, it is preferable to contain 1.00% or more, and an optimal minimum is 1.50% or more.

  P: P is an impurity, and if it contains more than 0.050%, the toughness of the base steel sheet is significantly reduced. Therefore, the upper limit of the P content is 0.050%. In order to improve the toughness of the HAZ, the P content is preferably set to 0.010% or less.

  S: S is an important element in the present invention that produces sulfides such as MnS that are effective as nuclei for intragranular transformation. If the S content is less than 0.0001%, the amount of sulfide produced is reduced and no intragranular transformation occurs, so it is necessary to make it 0.0001% or more. On the other hand, if more than 0.0050% of S is contained in the base steel plate, coarse sulfides are generated and the toughness is lowered, so the upper limit of the amount of S is made 0.0050% or less. In order to improve the toughness of the HAZ, it is preferable that the upper limit of the S amount is 0.0030% or less.

  Al: Al is a deoxidizer, but in the present invention, in order to finely disperse the oxide of Ti, it is extremely important to limit the upper limit of the Al amount to 0.020% or less. In order to promote the formation of intragranular transformation, the Al content is preferably 0.010% or less. A more preferred upper limit is 0.008% or less.

  Ti: Ti is an extremely important element in the present invention because it finely disperses Ti oxides that effectively act as nuclei for intragranular transformation. However, if Ti is contained excessively, carbonitrides are produced and the toughness is impaired. Therefore, in the present invention, the Ti content needs to be 0.003 to 0.030%. Moreover, since Ti is a strong deoxidizing agent, if the amount of oxygen when adding Ti is large, a coarse oxide is generated. Therefore, at the time of steelmaking, it is necessary to deoxidize with Si and Mn in advance to reduce the amount of oxygen. When the Ti oxide is coarsened, intragranular transformation is less likely to occur, and the effect of pinning the grain boundary is reduced, so that the effective crystal grain size of the HAZ of the base steel plate and the welded steel pipe may become coarse.

  Mo: Mo is an element effective in improving hardenability, forming carbonitrides and improving strength, and in order to obtain the effect, addition of 0.10% or more is necessary. When Mo exceeds 1.50%, the toughness decreases, so the upper limit of the Mo amount is 1.50% or less.

  O: Oxygen is an element inevitably contained in the steel, but in the present invention, the amount of O needs to be limited in order to produce an oxide containing Ti. The amount of oxygen remaining in the steel at the time of casting, that is, the amount of O in the base steel plate needs to be 0.0001 to 0.0080%. This is because when the O content is less than 0.0001%, the number of oxides is not sufficient, and when it exceeds 0.0080%, coarse oxides increase, and the HAZ toughness of the base steel plate and the welded steel pipe is impaired. Because. Further, when the oxide mainly composed of Ti becomes coarse due to an increase in the amount of oxygen, the effective crystal grain size of the HAZ of the base steel plate and the welded steel pipe may become coarse.

  Furthermore, you may add 1 type (s) or 2 or more types among Cu, Ni, Cr, V, Nb, Zr, Ta as an element which improves an intensity | strength and toughness. In addition, these elements can be regarded as impurities because their content is less than the preferred lower limit because they do not have a particularly adverse effect.

  Cu, Ni: Cu and Ni are effective elements that increase the strength without deteriorating the toughness. In order to obtain the effect, the lower limit of the Cu content and the Ni content is preferably 0.05% or more. . On the other hand, the upper limit of the amount of Cu is preferably set to 1.50% in order to suppress the occurrence of cracks during heating of the steel slab and during welding. The upper limit of the amount of Ni is preferably set to 5.00% in order to impair the weldability if contained in excess. Note that Cu and Ni are preferably combined and contained in order to suppress generation of surface defects.

  Cr, V, Nb, Zr, Ta: Cr, V, Nb, Zr, Ta are elements that generate carbides and nitrides and improve the strength of steel by precipitation strengthening, and contain one or more. You may let them. In order to increase the strength effectively, the lower limit of Cr amount is 0.02%, the lower limit of V amount is 0.010%, the lower limit of Nb amount is 0.001%, the lower limit of Zr amount and Ta amount are Both are preferably 0.0001%. On the other hand, when Cr is added excessively, the strength increases due to the improvement of hardenability and the toughness may be impaired. Therefore, the upper limit of Cr content is preferably 1.50%. In addition, excessive addition of V, Nb, Zr, and Ta may coarsen carbides and nitrides and impair toughness. Therefore, the upper limit of V amount is 0.100% and the upper limit of Nb amount is 0.200%. It is preferable that the upper limit of Zr amount and Ta is 0.0500%.

  Furthermore, in order to control the form of inclusions and improve toughness, one or more of Mg, Ca, REM, Y, Hf, Re, and W may be added. In addition, these elements can be regarded as impurities because their content is less than the preferred lower limit, because they do not have a particularly adverse effect.

  Mg: Mg is an element that exerts an effect on oxide miniaturization and sulfide morphology control. In particular, fine Mg oxide acts as a nucleus for intragranular transformation, and 0.0001% or more is preferably added in order to obtain an effect of suppressing the coarsening of the particle size as pinning particles. On the other hand, if an amount of Mg exceeding 0.0100% is added, a coarse oxide is generated, which may reduce the toughness of the HAZ of the base steel plate and the welded steel pipe. % Is preferable.

  Ca, REM: Ca and REM are useful for controlling the form of sulfides, suppress the formation of MnS that forms granulated materials and extends in the rolling direction, and properties in the thickness direction of steel materials, especially lamellar resistance It is an element that improves. In order to obtain this effect, it is preferable that both the lower limits of the Ca content and the REM content be 0.0001% or more. On the other hand, when the upper limit of the Ca content and the REM content exceeds 0.0050%, the oxide increases, the fine Ti-containing oxide decreases, and the formation of intragranular transformation may be inhibited. It is preferable to set it to 0050% or less.

  Y, Hf, Re, W: Y, Hf, W, and Re are also elements that exhibit the same effect as Ca and REM, and if added excessively, the formation of intragranular transformation may be inhibited. Therefore, the preferable ranges of the Y amount, the Hf amount, and the Re amount are each 0.0001 to 0.0050%, and the preferable range of the W amount is 0.01 to 0.50%.

  Furthermore, in the present invention, in order to ensure the hardenability of the HAZ of the base steel plate and the weld metal, the area ratio of bainite of the base steel plate is 80% or more, and intragranular bainite is generated in the HAZ. , Ni, Cu, Cr, Mo, V The carbon equivalent Ceq of the following (Formula 1) calculated from the content [% by mass] is 0.40 to 0.56.

Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5
... (Formula 1)
Moreover, in order to ensure the low-temperature toughness of the HAZ of the base steel plate and the welded steel pipe, the following ( The cracking sensitivity index Pcm of the formula 2) is set to 0.15 to 0.21.

Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60
+ Mo / 15 + V / 10 + 5B (Formula 2)
Furthermore, in order to ensure the low temperature toughness of the HAZ, as described above, the content [mass%] of C, Al, Mo, and Ni needs to satisfy the following (formula 3).

10C + 100Al + 5Mo + 5Ni <3.3 (Formula 3)
In addition, since Ni, Cu, Cr, and V, which are elements that are selectively contained, are impurities when they are less than the preferred lower limit described above, they are 0 in the above (Formula 1) to (Formula 3). calculate.

  The metal structure of the base steel sheet to be a welded steel pipe has a good balance between strength and toughness when the area ratio of bainite is 70% or more and the area ratio of polygonal ferrite is 30% or less. In addition, if the effective crystal grain size is set to 20 μm or less due to the generation of an oxide mainly composed of Ti, the toughness of the base steel sheet becomes good. Polygonal ferrite is also effective in reducing the effective crystal grain size of the base steel sheet, and the area ratio is preferably 3% or more. The thickness of the base steel plate is preferably 25 mm or more, and the tensile strength in the direction corresponding to the circumferential direction of the steel pipe is preferably 600 MPa or more. This is to prevent breakage due to internal pressure when used as a line pipe. In addition, when it is necessary to raise an internal pressure, it is preferable that the thickness of a base material steel plate shall be 30 mm or more. On the other hand, the thickness of the base steel plate is preferably 40 mm or less, and the tensile strength in the direction corresponding to the circumferential direction of the steel pipe is preferably 800 MPa or less. This is because the load at the time of forming the base steel sheet in the UO process increases due to the increase in thickness and the increase in tensile strength. Normally, the direction corresponding to the circumferential direction of the steel pipe is the sheet width direction of the base steel plate.

  Next, a manufacturing method will be described.

  After melting the steel in the steel making process described above, it is cast into a steel slab. Casting may be performed by a conventional method, but continuous casting is preferable from the viewpoint of productivity. The billet is heated for hot rolling.

  The heating temperature of the hot rolling is 1000 ° C. or higher. This is because hot rolling is performed at a temperature at which the steel structure becomes an austenite single phase, that is, an austenite region, and the crystal grain size of the steel sheet is made fine. Although the upper limit is not specified, the reheating temperature is preferably 1150 ° C. or lower in order to suppress the coarsening of the effective crystal grain size.

  Since hot rolling may be started immediately after extraction from the heating furnace, the starting temperature of hot rolling is not particularly specified. In order to refine the effective crystal grain size of the base steel plate, it is preferable that the rolling ratio in the recrystallization region exceeding 900 ° C. is 2.0 or more. The reduction ratio in the recrystallization region is the ratio between the thickness of the steel slab and the thickness at 900 ° C.

  Next, if the reduction ratio in the non-recrystallized region at 900 ° C. or less is set to 2.5 or more, the effective crystal grain size of the base steel sheet becomes 20 μm or less after water cooling. In order to further reduce the effective crystal grain size of the base steel plate, it is preferable that the rolling ratio in the non-recrystallized region at 900 ° C. or lower is 3.0 or more. In the present invention, the reduction ratio of non-recrystallization zone rolling is a ratio obtained by dividing the plate thickness at 900 ° C. by the plate thickness after the end of rolling.

  Moreover, although the upper limit of the reduction ratio in the non-recrystallized region and the recrystallized region is not defined, it is usually 12.0 or less considering the plate thickness of the steel slab before rolling and the plate thickness of the steel plate after rolling.

The rolling end temperature is preferably higher than the temperature at which the steel structure becomes an austenite single phase. That is, the rolling end temperature is preferably Ar ₃ or higher, but a small amount of polygonal ferrite may be generated during rolling, and may be Ar ₃ -50 ° C. or higher.

Ac ₃ and Ar ₃ can be calculated from the content [% by mass] of C, Si, Mn, P, Cr, Mo, W, Ni, Cu, Al, V, and Ti.

Ac ₃ = 910−203√C−15.2Ni + 44.7Si + 104V + 31.5Mo
+ 13.1W-30Mn-11Cr-20Cu + 700P + 400Al
+ 400Ti
Ar ₃ = 910-310C-55Ni-80Mo-80Mn-15Cr-20Cu
Furthermore, although water cooling is implemented after completion | finish of rolling, if a water cooling stop temperature shall be 600 degrees C or less, the metal structure mentioned above will be obtained and the toughness of a base material steel plate will become favorable. The lower limit of the water cooling stop temperature is not specified, and the water cooling may be performed to room temperature. However, in consideration of productivity and hydrogen defects, the temperature is preferably set to 150 ° C. or higher.

  After forming the base steel plate into a tubular shape, when the butt portion is arc welded to form a welded steel pipe, the forming is preferably a UOE process in which the base steel plate is C-pressed, U-pressed, and O-pressed.

  For arc welding, it is preferable to adopt submerged arc welding from the viewpoint of the toughness and productivity of the weld metal. In particular, when manufacturing a welded steel pipe having a wall thickness of 25 to 40 mm, the heat input of submerged arc welding from the inner and outer surfaces is preferably set to 4.0 to 10.0 kJ / mm. If the heat input is within this range, in the welded steel pipe of the present invention having the above-described component composition, intragranular bainite is generated in the HAZ, the HAZ effective crystal grain size is 150 μm or less, and excellent low temperature toughness is obtained.

  In particular, when submerged arc welding is performed one pass at a time from the inner and outer surfaces, if the heat input is less than 4.0 kJ / mm, the weld metal of the tack welding performed prior to the main welding between the inner surface metal and the outer surface metal May remain. Moreover, if the heat input of submerged arc welding is set to 10.0 kJ / mm or less, the old austenite grain size of HAZ can be set to 500 μm or less even in a steel pipe having a thickness of 25 to 40 mm, and toughness It is effective for improvement. The heat input when welding from the inner surface and the heat input when welding from the outer surface do not have to be the same, and there may be a slight difference in heat input.

  When the heat input of the submerged arc welding from the inner and outer surfaces is set to 4.0 to 10.0 kJ / mm, the cooling rate from 800 ° C. to 500 ° C. during cooling of the HAZ when the thickness of the steel pipe is 25 to 40 mm Is 2 to 15 ° C./s. Even with such a slower cooling rate than usual, in the welded steel pipe of the present invention having the above-described component composition, intragranular bainite is generated in the HAZ, the effective crystal grain size of the HAZ is 150 μm or less, and excellent low temperature toughness is obtained. It is done.

  Moreover, it is preferable to use the following components for the wire used for welding in order to make the component composition of a weld metal into the range mentioned later in consideration of the dilution of the component by a base material steel plate. That is, in mass%, C: 0.010 to 0.120%, Si: 0.05 to 0.50%, Mn: 1.0 to 2.5%, Ni: 2.0 to 8.5% Containing Cr, Mo, or V in the range of Cr + Mo + V: 1.0-5.0%, and further Al: 0.100% or less, Ti: 0.050% or less It is a component composition which contains and the remainder consists of Fe and an unavoidable impurity. If necessary, B: 0.0001% to 0.0050% may be included.

  Furthermore, the component composition of the weld metal will be described.

  C is an element that is extremely effective for improving the strength, and preferably contains 0.010% or more. However, if the amount of C is too large, cold cracking is likely to occur, and in particular, the HAZ of the so-called T-cross portion where the on-site welded portion and the seam weld intersect may harden and impair toughness. Therefore, it is preferable that the upper limit of the C amount is 0.100%. In order to improve the toughness of the weld metal, the upper limit is more preferably 0.050% or less.

  Si is preferably contained in an amount of 0.01% or more in order to prevent the occurrence of blow holes that are welding defects. On the other hand, if it is excessively contained, the low temperature toughness is remarkably deteriorated, so the upper limit is preferably made 0.50% or less. In particular, when performing welding a plurality of times, the low temperature toughness of the reheat weld metal may deteriorate, so the upper limit is more preferably set to 0.40% or less.

  Mn is an element effective for ensuring a good balance between strength and toughness, and the lower limit is preferably 1.0% or more. However, when Mn is contained in a large amount, segregation is promoted and not only the low-temperature toughness is deteriorated, but also the production of a welding wire used for welding becomes difficult. Therefore, the upper limit is preferably made 2.0% or less.

  P and S are impurities, and it is preferable to set these upper limits to 0.020% and 0.010% in order to reduce the low temperature toughness of the weld metal and reduce low temperature cracking susceptibility. From the viewpoint of low temperature toughness, a more preferable upper limit of P is 0.010%.

  Ni is an element that enhances hardenability and ensures strength, and further improves low-temperature toughness, and it is preferable to contain 0.2% or more. On the other hand, if the Ni content is too high, hot cracking may occur, so the upper limit was made 3.2% or less.

  Cr, Mo, and V are all elements that enhance the hardenability, and for high strength of the weld metal, it is preferable to contain one or more of these in a total of 0.2% or more. On the other hand, if the total of one or more of Cr, Mo and V exceeds 2.5%, the low temperature toughness may deteriorate, so the upper limit is preferably made 2.5% or less.

  Al is an element that is added to improve the refining and solidification during the production of the welding wire, and suppresses the coarsening of the grain size of the weld metal by utilizing a fine Ti-based oxide. Therefore, it is preferable to contain 0.001% or more of Al. However, since Al is an element that promotes the formation of MA, the preferable upper limit of the content is 0.100% or less.

  Ti is an element that generates a fine oxide serving as a nucleus for intragranular transformation and contributes to the refinement of the grain size of the weld metal, and is preferably contained in an amount of 0.003% or more. On the other hand, if Ti is contained in a large amount, a large amount of Ti carbide is generated and the low-temperature toughness may be deteriorated, so the upper limit is preferably made 0.050% or less.

  O is an impurity, and the amount of oxygen finally remaining in the weld metal is often 0.0001% or more. However, if the amount of O remains over 0.0500%, coarse oxides increase, and the toughness of the weld metal may decrease, so the upper limit is preferably made 0.0500% or less. .

  The weld metal may further contain B.

  B is an element that increases the hardenability of the weld metal, and in order to increase the strength, it is preferable to contain 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, the toughness may be impaired, so the upper limit is preferably made 0.0050% or less.

  For the weld metal, elements other than the above, for example, Cu, Nb, Zr, Ta, Mg, Ca, REM, Y, Hf, Re, W, which are selectively added to the base metal by dilution from the base steel plate And may contain elements such as Zr, Nb, and Mg added as necessary in order to improve the refining and solidification of the welding wire. These are impurities inevitably contained.

  After seam welding, the pipe may be expanded in order to improve the roundness of the steel pipe. When the roundness of the steel pipe is increased by expanding the pipe, it is necessary to deform the plastic pipe to the plastic region. Therefore, the expansion ratio is preferably set to 0.7% or more. The expansion ratio is a percentage obtained by grading the difference between the outer peripheral length of the steel pipe after the expansion and the outer peripheral length of the steel pipe before the expansion by the outer peripheral length of the steel pipe before the expansion. If the expansion ratio exceeds 2%, the toughness may be lowered due to plastic deformation of both the base metal and the welded portion. Therefore, the tube expansion rate is preferably 0.7 to 2.0%.

  Moreover, it is preferable to heat-treat the welded portion and HAZ of the steel pipe, and particularly when heated to a temperature of 300 to 500 ° C., the coarse MA formed along the prior austenite grain boundaries decomposes into bainite and fine cementite. And toughness is improved. When the heating temperature is less than 300 ° C., the decomposition of coarse MA is insufficient and the effect of improving toughness may not be sufficient, so the lower limit is preferably set to 300 ° C. or higher. On the other hand, when the welded portion is heated to over 500 ° C., precipitates are generated and the toughness of the weld metal may be deteriorated. Therefore, the upper limit is preferably set to 500 ° C. or less. When MA produced in reheated HAZ decomposes into bainite and cementite, the shape is the same as that of MA as observed by SEM, but it contains fine white precipitates inside and can be distinguished from MA. it can.

  The heat treatment of the welded part and the HAZ may be performed by heating from the outer surface with a burner, or high-frequency heating may be performed. Although the outer surface may be cooled immediately after reaching the heat treatment temperature, it is preferably maintained for 1 to 600 seconds in order to promote the decomposition of MA. However, considering the cost of the equipment and productivity, the holding time is preferably 300 s or less.

  Next, examples of the present invention will be described.

The steel having the chemical components shown in Table 1 was melted by adjusting the oxygen concentration when adding Ti within a range of 0.001 to 0.003%, and a steel piece having a thickness of 240 mm was obtained. These steel slabs were heated to the heating temperature shown in Table 2, and hot-rolled in a recrystallization temperature range of 950 ° C. or higher to a thickness of 35 to 140 mm. Further, hot rolling was performed with the rolling ratio in the non-recrystallized region in the temperature range from 900 ° C. to the end of rolling as shown in Table 2. The end temperature of hot rolling was Ar ₃ −50 ° C. or higher, water cooling was started at 750 ° C., and water cooling was stopped at various temperatures.

  From the obtained steel plate, a V-notch test piece was prepared in which the plate width direction was the longitudinal direction and the notch was provided parallel to the plate thickness direction in accordance with JIS Z 2242. The sampling position of the Charpy test piece was a surface layer portion, that is, a position of about 2 to 12 mm from the surface, and a 1/2 t portion, that is, approximately the center of the wall thickness. The Charpy test was performed at −40 ° C. to determine the absorbed energy. The tensile property was evaluated using an API standard test piece. When a base steel plate having a thickness of 25 to 40 mm was formed into a welded steel pipe, it was confirmed by analysis using a finite element method that the influence of strain introduced by forming at the center of the plate thickness was small.

  The microstructure of the central portion of the base steel sheet was observed with an optical microscope, and the area ratios of polygonal ferrite and bainite were measured to confirm the remaining structure. The effective crystal grain size of the base steel plate was measured by EBSP.

  Next, in consideration of dilution with the base steel plate, by mass, C: 0.010 to 0.120%, Si: 0.05 to 0.50%, Mn: 1.0 to 2.5%, Ni : 2.0-8.5%, Al: 0.100% or less, Ti: 0.050% or less, and further, if necessary, one or more of Cr, Mo, V Cr + Mo + V: contained in the range of 1.0-5.0%, B: contained 0.0001-0.0050%, the balance is welded using a welding wire having a composition composed of Fe and inevitable impurities Submerged arc welding was performed for each pass from the inner and outer surfaces with a heat input of 4.0 to 10.0 kJ / mm to produce a welded joint. Some of the joints were heat treated at the temperatures shown in Table 2. In addition, the sample was extract | collected from the weld metal and the component analysis was performed. The tensile strength of the weld metal was measured according to JIS Z 3111. Table 3 shows the chemical composition and tensile strength of the weld metal.

  Small pieces were collected from the welded joint, and the effective crystal grain size of HAZ was measured by EBSP. Moreover, the bainite produced in the shape of a petal starting from inclusions was defined as intragranular bainite, and the area ratio was measured. Furthermore, the Charpy absorbed energy of HAZ was measured at −40 ° C. using a V-notch test piece in accordance with JIS Z 2242. The V notch was provided at a position of 1 mm on the base metal side from the melting line, and the measurement was performed at −40 ° C. In addition, taking the width direction perpendicular to the weld metal as the longitudinal direction of the test piece and making the weld metal approximately the center of the parallel part, an API standard test piece is taken and a tensile test is performed to determine the fracture position. Went. The results are shown in Table 4. The intragranular transformation structure of Table 4 is the area ratio of intragranular bainite.

  In addition, some base material steel plates are UO process, submerged arc welding, expanded to make steel pipes, the microstructure and mechanical properties are investigated, and the HAZ microstructure and mechanical properties of the base material steel plates and joints are equivalent. It was confirmed.

  Production No. 1 to 14 are examples of the present invention. The effective crystal grain size of the base steel sheet is 20 μm or less, and the effective crystal grain size of HAZ is 150 μm or less. Further, the Charpy absorbed energy of the base material and HAZ at −40 ° C. exceeds 50 J, and the low temperature toughness is good. In these examples of the present invention, the fracture position of the joint tensile test is the base material, and the softening of the HAZ is not a problem.

  On the other hand, production No. Nos. 15 to 19 and 22 show that the base steel plate component and the weld metal component are outside the scope of the present invention. Nos. 20, 21, 24, and 25 have base steel plate components outside the scope of the present invention. In Nos. 23, 26 and 27, the manufacturing conditions of the base steel plate are outside the scope of the present invention, and as shown in Table 4, these are comparative examples. Among these, production No. 15 is an example in which the amount of C is small, the area ratio of polygonal ferrite is increased, and the tensile strength is lowered. In addition, production No. 16 and 17 are examples in which the C amount and the Mn amount are large, the strength is increased, and the HAZ toughness of the base steel plate and the welded steel pipe is lowered. Production No. 18 and 19 are examples in which the amounts of impurities P and S are large and the toughness is lowered.

  Furthermore, production No. No. 20 has a large amount of Ti. No. 21 has a large amount of oxygen. No. 22 is an example in which the effective crystal grain size of HAZ is increased and the toughness is deteriorated due to the small amount of Ti. No. No. 21 has a large amount of oxygen, and the toughness of the base steel sheet is deteriorated. Production No. No. 24 has low strength because Ceq and Pcm are low. Since No. 25 has high Ceq and Pcm, the strength of the base steel plate is increased, the toughness is lowered, and furthermore, (Equation 3) is not satisfied. This is an example.

  In addition, production No. As shown in Table 2, Nos. 23 and 26 are examples in which the rolling reduction ratio of the rolling is small, the effective crystal grain size of the base steel plate is increased, and the toughness of the base steel plate is lowered. Production No. No. 27 is an example in which the strength is reduced because the water cooling stop temperature after hot rolling is high. In addition, production No. 16, 17 and 25, since the strength of the base steel plate is high, as a result of the joint tensile test, the HAZ fractures.

It is a schematic diagram of reheat HAZ. It is a figure which shows the influence of the component which acts on the toughness of reheat HAZ.

Explanation of symbols

1 Reheat HAZ
2 Hybrid of martensite and austenite 3 Old austenite grain boundary

Claims (9)

A steel pipe obtained by seam welding a base steel plate formed into a tubular shape, wherein the base steel plate is in mass%,
C: 0.030 to 0.080%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 2.00%,
S: 0.0001 to 0.0050%,
Ti: 0.003-0.030%,
Mo: 0.10 to 1.50%,
O: 0.0001 to 0.0080%
Including
Furthermore, in mass%,
Cr: 0.02 to 1.50%,
V: 0.010-0.100%
Nb: 0.001 to 0.200%,
Zr: 0.0001 to 0.0500%,
Ta: 0.0001 to 0.0500%
Containing one or more of them,
P: 0.050% or less,
Al: limited to 0.020% or less, the remainder having a component composition composed of iron and inevitable impurities, Ceq determined by the following (Formula 1) is 0.40 to 0.56, and the following (Formula 2 Pcm calculated by the above formula (1) is 0.15 to 0.21, satisfies the following (formula 3), and the metal structure of the base steel sheet is 30% or less of polygonal ferrite and 70% or more of the area ratio. A welded steel pipe for high-strength thick-walled pipes with excellent low-temperature toughness, characterized in that the effective crystal grain size is 20 μm or less and the effective crystal grain size of the weld heat affected zone is 150 μm or less.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5
... (Formula 1)
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60
+ Mo / 15 + V / 10 + 5B (Formula 2)
10C + 100Al + 5Mo + 5Ni <3.3 (Formula 3)
Here, C, Si, Mn, Ni, Cu, Cr, Mo, V, B, and Al are contents [mass%] of each element.   The welded steel pipe for a high-strength thick-walled line pipe excellent in low-temperature toughness according to claim 1, wherein the base steel sheet has a thickness of 25 to 40 mm.   The welding for high-strength thick-walled pipes with excellent low-temperature toughness according to claim 1 or 2, wherein the base steel plate has a tensile strength of 600 to 800 MPa, wherein the circumferential direction of the steel pipe is the tensile direction. Steel pipe. The base steel plate is further in mass%,
Cu: 0.05 to 1.50%,
Ni: 0.05-5.00%
One or both of these are contained, The welded steel pipe for high intensity | strength thick line pipe excellent in the low temperature toughness of any one of Claims 1-3 characterized by the above-mentioned. The base steel plate is further in mass%,
Mg: 0.0001 to 0.0100%,
Ca: 0.0001 to 0.0050%,
REM: 0.0001 to 0.0050%,
Y: 0.0001 to 0.0050%,
Hf: 0.0001 to 0.0050%,
Re: 0.0001 to 0.0050%,
W: 0.01 to 0.50%
The high-strength thick line pipe welded steel pipe according to any one of claims 1 to 4 , wherein one or more of them are contained. Weld metal is mass%,
C: 0.010 to 0.100%,
Si: 0.01 to 0.50%,
Mn: 1.0-2.0%,
Ni: 0.2-3.2%
Cr + Mo + V: 0.2 to 2.5%,
Al: 0.001 to 0.100%,
Ti: 0.003 to 0.050%,
O: 0.0001 to 0.0500%
Including
P: 0.020% or less,
S: Limiting to 0.010% or less, the balance being made of iron and inevitable impurities, for high-strength thick-walled pipe excellent in low-temperature toughness according to any one of claims 1 to 5 Welded steel pipe. When steel is melted, Si, Mn are added and weak deoxidation is performed, then Ti is added, and the steel adjusted to the component according to any one of claims 1, 4 to 5 is prepared. The steel piece obtained by casting is heated to 1000 ° C or higher, hot-rolled at a rolling ratio from 900 ° C or lower to the end of rolling at 2.5 or higher, and water-cooled to a stop temperature of 600 ° C or lower. the obtained steel sheet, according to claim 1, heat input butt portion was molded into a tubular in a UO process from inner and outer surfaces, characterized in that the seam welding by submerged arc welding is 4.0~10.0kJ / mm The manufacturing method of the high strength thick line pipe welded steel pipe excellent in the low temperature toughness described in any one of -6 . The method for producing a welded steel pipe for a high-strength thick-line pipe excellent in low-temperature toughness according to claim 7 , wherein the seam welded portion is heat-treated. The method for producing a welded steel pipe for a high strength thick line pipe excellent in low temperature toughness according to claim 8 , wherein the heat treatment of the seam welded portion is performed within a range of 300 to 500 ° C.

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