JP2007210023A - High strength welded steel pipe having excellent weld zone embrittlement crack property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method with which horizontal cracks caused by hydrogen produced in the seam weld zone of a high strength welded steel pipe having a tensile strength of ≥850 MPa and subjected to seam welding from the inside and outside faces are prevented. <P>SOLUTION: In the method for producing a welded steel pipe having excellent embrittlement crack properties in a weld metal, and in a method for producing a welded steel pipe where a steel plate having a tensile strength of ≥850 MPa is cylindrically formed, and the butted parts are subjected to submerged-arc welding from the inside and outside faces, the height in the thickness direction of the weld metal is reduced by 0.2 to 10% over the whole length in a welding direction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength welded steel pipe in which the tensile strength in the circumferential direction of a base metal and a weld metal formed by arc welding used for a natural gas or crude oil transportation line pipe is 850 MPa or more, and a method for producing the same.

  In recent years, laying of high-strength large-diameter line pipes having a tensile strength of 850 MPa or more has been studied from the viewpoints of efficient transportation and cost reduction of incidental facilities in long-distance pipelines that transport natural gas or crude oil. Such a line pipe is usually produced by UOE method, UOC method, JOE method or bending roll method by making a steel plate into a cylindrical shape and seam welding the butt portion. In this case, the seam welded portion serving as a joint is usually formed in the order of inner surface welding and outer surface welding by submerged arc welding. However, in the nondestructive inspection after outer surface welding, cracks in the direction perpendicular to the steel pipe axis direction, so-called transverse cracks, are sometimes found in the seam welds.

  When steel pipes with such transverse cracks are used in frozen land zones, there is a risk of breaking due to tensile stress exceeding the yield strength of the pipe in the axial direction due to seasonal fluctuations in temperature, and repeated stress loading There is a risk that the crack will develop and the transport fluid will leak, leading to a major accident. For this reason, it is necessary to prevent the occurrence of cracks during production or to reliably detect and remove the cracks that have occurred by nondestructive inspection.

  A transverse crack in a seam weld is a kind of embrittlement crack in a high-strength material. This transverse crack is generally caused by hydrogen and is also called hydrogen embrittlement crack, and it is difficult to occur when the strength of the weld metal is lowered. However, if the strength of the seam welded portion is reduced, embrittlement cracks are less likely to occur, but it is also assumed that deformation from the seam welded portion is selectively promoted during internal pressure loading, leading to fracture from the welded portion. Therefore, there is a need for a method for preventing hydrogen embrittlement cracking while maintaining the strength of the weld metal at or above the base metal strength.

  Since hydrogen embrittlement cracks depend on the hydrogen concentration, load stress, material properties, and particularly strength, it is necessary to control these to below the limit value so that hydrogen embrittlement cracks do not occur due to complex effects. As a method for reducing the hydrogen concentration, there is a method in which after seam welding from the inner and outer surfaces, the temperature is raised to 100 ° C. or higher, preferably 200 ° C. or higher, and held for an appropriate time. This is so-called post-heating, in which the weld metal is heated to diffuse hydrogen to obtain a hydrogen concentration below the limit at which transverse cracking occurs.

  From this point of view, Patent Document 1 discloses a technique for preventing hydrogen embrittlement cracking in a seam welded portion of a high-strength material by compositely suppressing the strength, base metal strength, and welding conditions of the weld metal of the UOE steel pipe. ing. Although Patent Document 1 describes that transverse cracks in welds occur frequently in the preceding seam welds, specific conditions are disclosed regarding prevention of transverse cracks by suppressing hydrogen concentration and welding residual stress. Absent.

  Moreover, although the method of preventing the toughness fall and solidification cracking by quenching and tempering the whole steel pipe after welding is proposed in Patent Document 2, the hydrogen concentration and welding residual stress are not mentioned. Similarly, as a technique for preventing cracking of a high-strength UOE steel pipe, Patent Document 3 proposes a method of appropriately controlling the height ratio of the inner and outer surfaces of the weld and relieving residual stress that causes hydrogen embrittlement cracking. ing.

  Japanese Patent Application Laid-Open No. 2003-228561 controls the residual stress so that hydrogen cracking does not occur in a seam welding in which submerged arc welding is performed from the inner and outer surfaces, with the ratio between the inner metal height and the outer metal height being in an appropriate range. However, according to this method, it is necessary to control the welding conditions within a very narrow range in order to eliminate other problems in seam welding, for example, removal of groove remaining after welding, undercut, etc., and blunting of the toe angle. , May impair productivity.

  In addition, as a method of relieving residual stress, which is a factor that induces hydrogen embrittlement cracking, plastic welding is applied to the welded part by so-called stress-relieving tempering, which is heated to about 700 ° C. after welding, or hammering. A method for reducing the residual stress is proposed. However, these methods do not take into account the influence of the relationship between the hydrogen concentration and the residual stress on the transverse cracking, and thus it cannot be said that the hydrogen embrittlement cracking resistance has been reliably improved. In addition, stress relief annealing requires an excessive amount of time for heating and cooling, and hammer peening needs to be performed immediately after welding, requiring a special processing device. Therefore, these methods are not necessarily appropriate methods for application to seam welds in consideration of the manufacturing process and manufacturing cost.

  Also, as a refinement of the welding material, a fine precipitate that acts as a hydrogen trap site, such as VN, is generated in the weld metal to reduce diffusible hydrogen harmful to transverse cracks, There is a method of reducing the residual stress at room temperature. However, utilization of hydrogen trap sites is not always a useful method for high-strength materials, and the use of low-temperature transformation melts causes a significant cost increase.

JP 2003-313121 A JP 57-35636 A JP 2005-246403 A

  The present invention provides a method for manufacturing a welded steel pipe that prevents hydrogen-induced lateral cracking that occurs in a seam welded portion of a high-strength welded steel pipe having a tensile strength of 850 MPa or more when performing seam welding from the inner and outer surfaces.

The present invention has been made to solve the above-described problems. After seam welding, the shape of the weld metal is changed. Specifically, the height in the thickness direction of the weld metal is reduced by compressive deformation or cutting. This is a method for reducing the residual stress generated in the weld metal previously welded inside the weld metal and preventing hydrogen-induced cracking, and the gist thereof is as follows.
(1) In a method for manufacturing a welded steel pipe in which a steel sheet having a tensile strength of 850 MPa or more is formed into a cylindrical shape and the butt portion is submerged arc welded from the inner and outer surfaces, the height in the thickness direction of the weld metal is increased over the entire length in the welding direction. The manufacturing method of the welded steel pipe excellent in the brittle cracking property of the weld metal characterized by reducing by 0.2 to 10%.
(2) The weld metal is excellent in embrittlement cracking as described in (1) above, wherein the thickness in the thickness direction of the weld metal is reduced by 0.2% or more and 3% or less by compressive deformation. Manufacturing method of welded steel pipe.
(3) Manufacture of a welded steel pipe excellent in the brittle cracking property of the weld metal according to (1), wherein a portion corresponding to 2 to 10% of the height in the thickness direction of the weld metal is cut off Method.
(4) Any of the above (1) to (3), wherein the maximum value of the residual stress of the preceding weld metal among the inner surface weld metal and the outer surface weld metal is less than 80% of the tensile strength of the weld metal. The manufacturing method of the welded steel pipe excellent in the brittle cracking property of the weld metal of Claim 1.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to prevent generation | occurrence | production of the hydrogen embrittlement crack in the weld part of the high intensity | strength welded steel pipe used for natural gas and a crude oil transportation line pipe etc. whose tensile strength is 850 MPa or more.

  When manufacturing high-strength welded steel pipes with a tensile strength of 850 MPa or more by the UOE pipe making process, the end of the steel plate is bent by the C press, bent into a U shape by the U press, and then formed into a cylindrical shape by the O press. Thereafter, usually, after provisional attachment from the outer surface, inner surface welding by submerged arc welding is performed, then outer surface welding is performed, and roundness is adjusted by pipe expansion or contraction correction.

  When a defect in the seam welded portion of this UOE steel pipe was detected by ultrasonic flaw detection in accordance with JIS G 058, transverse cracks were found infrequently. From the result of ultrasonic flaw detection, it was found that when the position where the defect was detected was specified, the transverse crack occurred in the weld metal on the inner surface welded in advance. Moreover, as a result of observing the fracture surface of the transverse crack with a scanning electron microscope, it was found that the fracture surface peculiar to hydrogen embrittlement crack was exhibited.

  As a result, transverse cracks that occur in seam welds of high-strength welded steel pipes are hydrogen embrittlement cracks caused by fluxes, groove condensation, moisture taken into the weld metal from the atmosphere, and hydrogen residual stress. It was concluded. However, after performing inner surface welding, an attempt was made to detect defects by ultrasonic flaw detection without performing outer surface welding, and it was found that transverse cracks did not occur with inner surface welding.

  The steel plate (base material) used as the material of the UOE steel pipe described here has a steel composition of mass%, C: 0.02 to 0.10%, Si: 0.01 to 0.6%, Mn: 1.5 to 2.5%, P: 0.015% or less, S: 0.003% or less, Ni: 0.1 to 2.0%, Mo: 0.15 to 0.60%, Nb: 0 0.001 to 0.10%, Ti: 0.005 to 0.030%, Al: 0.06% or less, and B: 0.0001 to 0.005%, N: 0 as necessary. 0.0001-0.006%, V: 0.001-0.10%, Cu: 0.01-1.0%, Cr: 0.01-1.0%, Zr: 0.0001-0.005 %, Ta: 0.0001-0.005%, Ca: 0.0001-0.01%, REM: 0.0001-0.01%, Mg: 0.0001-0.00 % Of one or contain two or more types, in which the steel balance consisting of Fe and unavoidable impurities is obtained by hot controlled rolling.

  In the production of the UOE steel pipe, the thick steel plate (base material) having the steel composition described above is, in mass%, C: 0.01 to 0.12%, Si: 0.3% or less, Mn: 1.2 -2.4%, Ni: 4.0-8.5%, Cr + Mo + V: 3.0-5.0%, Ti: 0.005-0.15%, Al: 0.02% or less, Welding is performed at a heat input of 1.5 kJ / mm to 6.3 kJ / mm using a welding wire composed of the remaining Fe and inevitable impurities.

  About the weld metal obtained in this way, a component is the mass%, C: 0.04-0.14%, Si: 0.05-0.4%, Mn: 1.2-2.2 %, P: 0.01% or less, S: 0.010% or less, Ni: 1.3 to 3.2%, Cr + Mo + V: 1.0 to 2.5%, Ti: 0.003 to 0.050% Al: 0.02% or less, B: 0.005% or less, O: 0.01 to 0.03%, and the balance is Fe and inevitable impurities.

  The present inventors obtained the residual stress of the weld metal part before pipe expansion in the UOE process by a numerical analysis simulation by a finite element method (hereinafter also referred to as FEA). This is because it is difficult to measure the residual stress inside the weld metal seam-welded from the inner and outer surfaces in a nondestructive manner.

  The inventors of the present invention formed a weld metal having a tensile strength of 950 MPa by seam welding in the order of the inner surface and the outer surface, and assumed a state before pipe expansion. By FEA, the center line of the weld metal in the circumferential section of the steel pipe The residual stress in the steel pipe axial direction on (welding center line, one-dot chain line in FIG. 1) was determined, and the distribution in the thickness direction is shown in FIG. Thermal conductivity, specific heat, and density, which are physical properties necessary for analysis, are measured, and heat transfer coefficient, which is a state value, is a temperature history obtained by actually measuring changes in internal temperature during external welding, and changes in internal temperature due to FEA. The analysis value was set to the same value.

  The horizontal axis in FIG. 2 is the distance from the inner surface of the steel pipe on the welding center line. Furthermore, the residual stress of the weld metal on the inner surface and outer surface of the steel pipe measured by X-ray diffraction is also shown in FIG. The predicted value by FEA of the residual stress on the inner and outer surfaces of the weld metal is in good agreement with the measured value by X-ray diffraction. From this, it is considered that the predicted value by FEA of the residual stress inside the weld metal shown in FIG. 2 accurately estimates the residual stress of the actual weld metal.

  As shown in FIG. 2, the residual stress has a maximum value on the inner side weld metal side welded in advance, and the value exceeds the yield strength of the weld metal, 800 MPa. In addition, it was found that the position where the residual stress was maximized coincided with the occurrence of lateral cracking detected by ultrasonic flaw detection. Therefore, it is considered that hydrogen embrittlement cracking can be prevented if the residual stress is reduced by any method.

  In order to ascertain the relationship between the hydrogen concentration and the residual stress at which the lateral cracking is next generated, the present inventors generate hydrogen embrittlement cracking in a weld metal having a tensile strength of 850 MPa or more as follows. The relationship between tensile stress and hydrogen concentration was investigated. Therefore, the relationship between the stress at which hydrogen embrittlement cracking occurs in the weld metal of a high strength welded steel pipe having a tensile strength of 850 MPa or more and the amount of hydrogen was investigated as follows. A sample having a size of 200 mm × 200 mm in the circumferential direction and the axial direction was taken from the welded steel pipe so as to include the inner and outer surface weld metals, and immediately cooled with dry ice and stored. A round bar tensile test piece having a longitudinal direction parallel to the welding direction and a parallel part diameter of 6 mm was collected from the weld metal of this sample. These round bar tensile specimens were cadmium plated so that hydrogen would not escape. Next, a constant load was applied to the tensile test piece for 240 hours, and the presence or absence of breakage, that is, the occurrence of hydrogen embrittlement cracking was examined. Furthermore, the amount of hydrogen was measured in accordance with the gas chromatographic method employed in the hydrogen measurement method for steel welds of JIS Z 3118, using a round bar tensile test piece having a parallel part diameter of 6 mm that was collected in the same manner. .

  The results are shown in FIG. 3. The amount of hydrogen was measured by the above measurement method, that is, the amount of diffusible hydrogen collected by holding at 45 ° C. for 72 hours, and the volume of hydrogen contained per 100 g of test piece [ cc]. The vertical axis in FIG. 1 represents the constant load applied to the test piece divided by the cross-sectional area of the parallel part of the test piece and expressed as stress σ [MPa].

  Since the residual stress corresponding to the maximum amount of hydrogen of 0.42 cc / 100 g in the weld metal measured with an actual machine is 680 MPa from FIG. 3, if the residual stress is less than 80% of the tensile strength of 850 MPa, It will not occur.

  That is, from FIG. 3, an index was obtained that hydrogen cracking does not occur when the residual stress generated in the weld metal is less than 80% of the tensile strength of the weld metal.

When the hydrogen concentration exceeds 0.2 cc per 100 g of weld metal, the hydrogen amount H [cc] and the tensile stress σ [MPa]
(H−0.1) × (σ−550) ≦ 45
If it is satisfied, it can be estimated that hydrogen embrittlement cracking does not occur. Therefore, if the amount of hydrogen contained in the weld metal formed by the preceding seam welding is H [cc] and the tensile residual stress applied to the weld metal is [MPa] satisfies the relationship of the above formula, the strength is high. Hydrogen embrittlement cracking of the welded steel pipe can be prevented.

  The present inventors examined a method of reducing the residual stress of the weld metal by redistributing it by changing the shape of the weld metal from the as-welded state. By changing the shape of the weld metal, the performance inherent in the weld metal, particularly toughness, must not be critically degraded. The present inventors first studied a method for reducing the residual stress by compressing and plastically deforming the inner and outer surfaces of the weld metal.

  Specimens with a welded metal strength of 950 MPa, an outer diameter of 914 mm, a wall thickness of 16 mm, and a weld metal height of 19.5 mm from a UOE steel pipe with inner and outer surfaces of 300 mm in the circumferential direction and 100 mm in the axial direction centered on the seam weld. Was cut out. The seam welded portion of this test piece was pressed in the thickness direction so as to be sandwiched from the inner and outer surfaces (FIG. 4), and the processing ratio was calculated by the following equation (1) from the shape change at that time.

  After the processing, electrolytic polishing was performed, and the axial residual stress on the surface of the weld metal on the inner surface of the steel pipe was measured by an X-ray diffraction method. The results are shown in FIG.

  In addition, assuming the case where the actual weld metal was compressed at various processing ratios, the estimated values of residual stress obtained by FEA were also plotted in the figure. The FEA analysis was performed by setting the pipe making method, physical property values, and state values to the same conditions as those in the analysis when FIG. 2 was obtained, and further adding compressive deformation in the thickness direction of the weld metal. By slightly compressing and deforming the weld metal, the residual stress in the surface layer portion is greatly reduced. Moreover, since the FEA value and the measured value were in agreement, it was confirmed that the stress state inside the weld metal after deformation could be predicted by FEA.

  Next, the change in the residual stress in the axial direction of the weld metal depending on the degree of processing was analyzed by FEA and shown in FIG. This is a result obtained by assuming a position where the residual stress of the weld metal shown in FIG. 2 is maximized. The FEA analysis was performed in the same manner as the analysis when obtaining FIG. The residual stress, which reaches a maximum of 800 MPa as welded, is drastically reduced by compressive deformation like the inner surface. It was found that the maximum stress can be suppressed to 750 MPa or less which does not cause cracking even by applying a compression plastic strain of only 0.2%.

  Next, attention was focused on toughness among the mechanical properties that deteriorate when plastic deformation is applied to the weld metal. It has long been known that when submerged arc welding is performed from the inside and outside surfaces, the toughness of the meeting surface is minimized and the toughness is deteriorated by cold forming. Therefore, the relationship between the processing ratio and the plastic strain amount at the meeting surface of the inner surface weld metal and the outer surface weld metal was evaluated by the same FEA analysis as that in FIG. 6 and shown in FIG. From this, it became clear that when the processing ratio exceeds 3%, distortion starts to concentrate on the meeting surface.

  Next, after applying compression processing within a range of 0.1 to 5%, three V-notch Charpy specimens were collected from the welded joints on the inner and outer surfaces in accordance with JIS Z2202, FIG. 8 shows the result of an impact test conducted at room temperature in accordance with Z2242. Although the dispersion of Charpy absorbed energy is large at each processing ratio, it can be seen from an average value that when the processing ratio exceeds 3%, the deterioration of the absorbed energy gradually becomes remarkable. Therefore, in the present invention, deterioration in toughness can be minimized by setting the processing rate of compression processing to 3% or less.

  From the above, it is possible to reduce the residual stress without significantly degrading the performance of the weld metal by applying compression processing of 0.2% or more and 3% or less from the inner and outer surfaces to the weld metal, and hydrogen embrittlement cracking It was found that can be prevented. In the above examination, the cut sample is subjected to plastic deformation by press working. However, in an actual pipe, the weld metal may be pressed from the inner and outer surfaces, or may be compressed by a rolling roll.

  The compression processing of the weld metal is the most efficient method because it gives plastic strain simultaneously with the shape change and can release the residual stress equivalent to the yield strength with the minimum shape change.

  Next, the present inventor examined a method of cutting off the surface layer portion of the outer surface weld metal as a method of redistributing residual stress only by a change in shape of the weld metal and preventing hydrogen cracking without giving plastic deformation. .

First, as shown in FIG. 9, the present inventors removed the height of the weld metal by H ₂ from the outer surface of the steel pipe by cutting, and measured the residual stress changing at that time. FIG. 10 shows the residual stress change of the inner surface weld metal measured by X-ray diffraction on the inner surface of the steel pipe and the residual stress change obtained by FEA according to the resection ratio shown by Formula 2. The FEA analysis was performed assuming that the pipe making method, physical property values, and state values were the same as those in the analysis when FIG. 2 was obtained, and that the surface layer of the outer surface weld metal was cut.

  As the cutting rate increases, the residual stress on the surface of the weld metal on the inner surface of the steel pipe decreases, and the tendency agrees well with the value calculated by FEA. Although it was cut on the outer surface weld metal side, it was found that the residual stress on the inner surface also decreased as a result of redistribution of the residual stress.

  FIG. 11 shows the transition of the maximum axial direction residual stress inside the weld metal obtained by FEA. This is a result obtained by FEA analysis in the same manner as when FIG. 10 is obtained, assuming the position where the residual stress of the weld metal shown in FIG. 2 is maximized. If the cutting rate exceeds 2% of the weld metal height in the thickness direction, the reduction rate of the maximum stress becomes remarkable, and it can be estimated that it falls below 750 MPa, the residual residual stress that does not cause hydrogen cracking with respect to the strength of 950 MPa. It was.

  As for the upper limit of the amount of cut of weld metal, at least the notch to the base metal impairs the strength of the welded joint. Therefore, it is necessary to leave the outer surface surplus. The cutting effect of 10% or more slows down the maximum residual stress reduction effect. Therefore, in the actual manufacturing range, the excision ratio is preferably 2% to 10%.

  As for the cutting method, the same effect can be obtained by grinding by a grinder, gouging or electric discharge machining, in addition to the cutting by milling used in the above examination.

  In the present invention, the redistribution of residual stress when the weld metal on the outer surface is cut to 2% or more is shown. However, the same or higher effect can be obtained by cutting the weld metal on the inner surface. However, in general, the weld metal on the inner surface is lower than the weld metal on the outer surface, and the cutting of the weld metal on the inner surface of the steel pipe is not easy compared to the cutting of the outer surface weld metal. For this, it is preferable to cut off the outer surface weld metal.

  Residual stress was measured only for a weld metal with a tensile strength of 950 MPa, but whether the residual stress below the critical stress can be suppressed with the same processing rate or cutting rate even if the strength of the weld metal is different. The result of the examination is shown in FIG. The critical stress for hydrogen cracking is about 80% of the tensile strength. When 0.2% compression processing and 2% outer surface weld metal are cut, the weld metal is below the critical stress in any strength range. It was confirmed that the method of the present invention can be applied if the tensile strength of 850 MPa is 850 MPa or more.

  An ultra-high-strength line pipe having a strength of 850 MPa or more is generally formed by a UOE forming method, but may be formed by a bending roll.

  The effects of the implementation of the present invention will be described below with reference to examples of the present invention and comparative examples.

  Steel composition is mass%, C: 0.02-0.10%, Si: 0.01-0.6%, Mn: 1.5-2.5%, P: 0.015% or less, S : 0.003% or less, Ni: 0.1 to 2.0%, Mo: 0.15 to 0.60%, Nb: 0.001 to 0.10%, Ti: 0.005 to 0.030% , Al: steel containing 0.06% or less and the balance Fe and unavoidable impurities were hot-rolled to obtain a thick steel plate as a material for a UOE steel pipe. Some thick steel plates are: B: 0.0001 to 0.005%, N: 0.0001 to 0.006%, V: 0.001 to 0.10%, Cu: 0.01 to 1.0% Cr: 0.01-1.0%, Zr: 0.0001-0.005%, Ta: 0.0001-0.005%, Ca: 0.0001-0.01%, REM: 0.0001 -0.01%, Mg: 0.0001-0.006% of 1 type or 2 types or more are contained. The tensile strength of these thick steel plates is 850 to 1000 MPa.

  After forming these thick steel plates in the order of C press, U press, and O press, submerged arc welding was performed in the order of inner surface and outer surface. Welding was performed with water droplets from the spray spray attached to the groove before the inner surface welding. This is an accelerated test of hydrogen that is inevitably introduced due to condensation and flux that can occur in actual operation. Submerged arc welding is mass%, C: 0.01-0.12%, Si: 0.3% or less, Mn: 1.2-2.4%, Ni: 4.0-8.5%, Cr + Mo + V: 3.0 to 5.0%, Ti: 0.005 to 0.15%, Al: 0.02% or less, and using a welding wire composed of the remainder Fe and inevitable impurities, heat input The measurement was performed in the range of 1.5 to 6.3 kJ / mm.

  The composition of the weld metal is mass%, C: 0.04 to 0.14%, Si: 0.05 to 0.4%, Mn: 1.2 to 2.2%, P: 0.01% Hereinafter, S: 0.010% or less, Ni: 1.3 to 3.2%, Cr + Mo + V: 1.0 to 2.5%, Ti: 0.003 to 0.050%, Al: 0.02% or less , B: 0.005% or less, O: 0.01-0.03%, consisting of the balance Fe and inevitable impurities, and confirming that the tensile strength is 850-1000 MPa.

  A sample of 200 mm × 200 mm including the seam weld was collected within 20 minutes after the outer surface welding was completed, and stored in dry ice. Further, the time of exposure to normal temperature was set within 1 hour, and the seam welded portion was sandwiched from the inner and outer surfaces, and compression was performed by a press. From the change in the height of the weld metal before and after pressing, the processing ratio was calculated according to Equation 1.

  Both the as-welded test piece and the pressed test piece were left at room temperature for 72 hours or more, and inspected for the presence or absence of cracks generated in the seam welded portion by ultrasonic flaw detection in accordance with JIS G 058. Here, the number of cracks generated is the number of cracks detected per 1 m in the welding direction of the seam weld.

  On the other hand, in order to evaluate toughness, three V-notch Charpy specimens were collected from the inner and outer surface welding meeting surfaces in accordance with JIS Z 2202, and subjected to an impact test at −30 ° C. in accordance with JIS Z 2242. The average value of the three absorbed energy was determined.

  Moreover, the residual stress was calculated | required by analysis according to the manufacturing method, physical-property value, and state value of each steel pipe by FEA. This residual stress is the maximum axial stress generated inside the weld metal.

  Table 1 shows the results. At a processing rate of 0.2% or more, the residual stress was less than 80% of the tensile strength, and there were no cracks. On the other hand, cracks were detected as welded or when the processing ratio did not reach 0.2%. Further, the absorbed energy at a processing rate of 3% or less did not fall below 85J.

  Next, after forming and submerged arc welding from the inner and outer surfaces by the same method as described above, the flux was removed, and the weld metal was cut off from the outer surface of the steel pipe by milling within at least 20 minutes. The excision ratio was calculated by Equation 2 from the change in the height of the weld metal before and after excision.

  Both the as-welded steel pipe and the steel pipe subjected to the cutting process were inspected for cracks generated in the seam welded part by ultrasonic flaw detection after lapse of 72 hours or more at room temperature in accordance with JIS G 058.

  Moreover, the residual stress was calculated | required by analysis according to the manufacturing method, physical-property value, and state value of each steel pipe by FEA. This residual stress is the maximum axial stress generated inside the weld metal.

  Table 2 shows the results. At a processing rate of 2% or more, the residual stress was less than 80% of the tensile strength, and there were no cracks. On the other hand, cracks were detected as-welded or when the processing ratio did not reach 2%.

The figure which shows the position which performed the FEA analysis of the residual stress. The figure which showed the axial direction residual stress distribution in the seam welding center of the UOE steel pipe before pipe expansion of size (phi) 1016x19mm and tensile strength 950MPa with respect to the position from an inner surface. The figure which shows the relationship between the hydrogen concentration and stress in a weld metal. The figure which shows a process ratio. The figure which shows the relationship between a process ratio and the axial direction residual stress of an inner surface weld metal surface. The figure which shows the relationship between a process ratio and the welding part maximum axial direction stress. The figure which shows the relationship between a processing ratio and the amount of plastic strain in an inner-outer surface welding meeting surface. The figure which shows the relationship between a processing ratio and the absorbed energy in an meeting surface. The figure which shows the resection ratio. The figure which shows the relationship between an excision ratio and the axial direction residual stress of an inner surface weld metal surface. The figure which shows the relationship between an excision ratio and a welding part maximum axial direction stress. The figure which shows the relationship between the intensity | strength of a weld metal, limit stress, the maximum stress at the time of 0.2% processing, and 2% excision.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Tube 2 Outer surface metal 3 Inner surface metal 4 Processing tool

Claims (4)

  In a method for manufacturing a welded steel pipe in which a steel sheet having a tensile strength of 850 MPa or more is formed into a cylindrical shape and the butt portion is submerged arc welded from the inner and outer surfaces, the height in the thickness direction of the weld metal is 0.2 over the entire length in the welding direction. The manufacturing method of the welded steel pipe excellent in the brittle cracking property of the weld metal characterized by reducing by 10%.   2. The welded steel pipe with excellent embrittlement cracking property according to claim 1, wherein the thickness in the thickness direction of the weld metal is reduced by 0.2% or more and 3% or less by compressive deformation. Method.   The method for producing a welded steel pipe excellent in embrittlement cracking property of weld metal according to claim 1, wherein a portion corresponding to 2 to 10% of the height in the thickness direction of the weld metal is excised.   The maximum value of the residual stress of the preceding weld metal among the inner surface weld metal and the outer surface weld metal is set to be less than 80% of the tensile strength of the weld metal. A method for producing a welded steel pipe excellent in embrittlement cracking of a weld metal.

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