JP3843059B2 - Manufacturing method of UOE steel pipe excellent in seam weld strength characteristics and internal pressure fatigue characteristics - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
In the manufacturing method of the UOE steel pipe shown in FIG. 1 in which the steel sheet is C-shaped, U-shaped, O-shaped in order, and the ends of the steel sheets are seam welded and then expanded, seam weld strength characteristics and internal pressure fatigue characteristics The present invention relates to a method for manufacturing a UOE steel pipe suitable for a line pipe or the like.
[0002]
[Prior art]
In recent years, line pipes have become increasingly important as a long-distance transportation method for crude oil and natural gas. In particular, to improve transportation efficiency by increasing pressure and to improve local construction efficiency by reducing the outer diameter and weight of line pipes. Now, there is a strong need for a high-strength line pipe that exceeds X100 (tensile strength of 760 MPa or more) in the API standard.
[0003]
With the increase in strength of line pipes, the softening of the weld heat affected zone (hereinafter HAZ), which was hardly considered a problem in conventional welding such as submerged arc welding of medium and low strength materials with a tensile strength of about 700 MPa, A new problem has arisen that a high strength material exceeding 760 MPa becomes large.
[0004]
On the other hand, when a general-purpose UOE steel pipe is used for a main line pipe, the pulsation of the internal pressure is small, and fatigue failure from the seam weld does not become a problem. However, in environments where pressure increase and decrease are repeated or in environments where internal pressure pulsations are frequently experienced, fatigue strength is required, and the fatigue strength of seam welds is much smaller than the fatigue strength of the tube, It has been desired to improve the fatigue strength of seam welds more than ever.
[0005]
For such a problem, Patent Document 1 discloses a method of increasing strength by applying compressive residual stress to HAZ during steel pipe welding. However, even if this method is applied to a seam welded portion in the UOE steel pipe manufacturing method, a remarkable effect was not obtained. In addition, when applying residual stress by shot blasting, offline work is a prerequisite for the processing of steel balls, making online equipment difficult.
[0006]
Further, Patent Document 2 discloses a method of applying stress by ultrasonic vibration along the seam welded portion immediately after welding to improve fatigue strength. However, in this method, the high temperature seam weld immediately after welding is blown by ultrasonic vibration, and since the seam weld and HAZ are high temperature, the yield strength is low and the plastic deformation is easily caused by the stress. Is relaxed, and compressive residual stress is hardly introduced. Even if compressive residual stress is introduced, it is canceled by thermal contraction in the subsequent cooling process to room temperature, so that the fatigue strength improvement effect by introducing compressive residual stress to the weld toe cannot be sufficiently obtained. Furthermore, since there is no disclosure of specific conditions for introducing compressive residual stress at the weld toe, such as the range of hitting with an ultrasonic vibrator, fatigue strength above a predetermined level can be stably achieved using this method. It is difficult to improve.
[0007]
[Patent Document 1]
JP 54-47839 A [Patent Document 2]
US Pat. No. 6,171,415
[Problems to be solved by the invention]
The present invention stabilizes the strength of seam welds and HAZ of high-strength UOE steel pipes without improving the molding conditions, shape of seam welds, and base metal components, and improves the internal pressure fatigue strength of general-purpose UOE steel pipes. Further, the present invention provides a method for manufacturing a UOE steel pipe excellent in seam weld strength characteristics and internal pressure fatigue characteristics.
[0009]
[Means for Solving the Problems]
In order to solve such problems, seam welds of high strength UOE steel pipes and HAZ tensile tests and Charpy impact tests are performed, fatigue tests of general-purpose UOE steel pipes are performed, and post-weld processing to weld toes And invented a manufacturing method for improving the tensile strength, toughness and fatigue properties of seam welds and HAZ. The gist of the present invention is as follows.
(1) In a method of manufacturing a UOE steel pipe in which steel plates are sequentially formed into C, U, and O, and the ends of the steel plates are seam welded and then expanded, at least the seam weld toe after the expansion is peened by ultrasonic vibration. A method for producing a UOE steel pipe excellent in seam weld strength characteristics and internal pressure fatigue characteristics, characterized by performing a treatment.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
First, as shown in FIG. 1, the inventor first forms a UOE steel pipe having a tensile strength of 600 to 1050 MPa by C-forming, U-forming, O-forming a steel plate, and seam welding the ends of the steel plate, and then expanding the pipe. Manufactured by. The outer diameter and wall thickness were 609 to 904 mm and 16 to 38 mm, respectively. After flattening these UOE steel pipes, a JIS Z 2201 No. 14B tensile test piece with a seam weld in the center was sampled and subjected to a tensile test in accordance with JIS Z 2241.
[0011]
The result is shown in FIG. 2, which shows the relationship between the fracture form and whether the fractured part is a joint or a tube and the tensile strength. Rupture) is the preferred form of rupture. When the tensile strength is 780 MPa or less, the tube breaks, but when the tensile strength is over 780 MPa and less than 950 MPa, both fracture forms of joint breakage and tube breakage are observed. It was found to break.
[0012]
Further, detailed analysis of the fracture surface after fracture of the joint fractured test piece was carried out visually and with a scanning electron microscope. As a result, it was found that the fracture origin was the weld toe on the inner surface. From this, it was concluded that the cause of breakage in the high-strength material was the softening of the seam weld toe, and it was possible to break the base metal by strengthening this part.
[0013]
As a method for improving the strength of the seam weld toe, as shown schematically in FIGS. 3 and 4, there is a method of introducing processing strain by shot blasting or reinforcing by TIG welding to the seam weld after expansion. Can be mentioned.
[0014]
That is, FIG. 3 and FIG. 4 schematically show the state of the cross-section after performing the above processing. In FIG. 3, the base material (steel plate) 1 is an outer surface weld metal 2 and an inner surface weld metal 3. And the shot blasting portion 6 is formed in a range covering both the weld metals 2 and 3 and the weld toe 5 and the HAZ 4 of the seam welded portion after the pipe expansion. Thereby, processing distortion is introduced into the seam welded portion.
[0015]
Further, in FIG. 4, as in FIG. 3, the base material (steel plate) 1 is seam welded by the outer surface weld metal 2 and the inner surface weld metal 3, and the weld toe 5 and HAZ 4 of the seam welded portion after pipe expansion. A TIG welded portion 7 is formed in a range that covers. This reinforces the seam weld.
[0016]
Shot blasting or TIG welding reinforcement was applied to seam welds of UOE steel pipes having a tensile strength, outer diameter and wall thickness of 600 to 1050 MPa, 609 to 904 mm, and 16 to 38 mm, respectively. Shot blasting or TIG welding reinforcement was applied to a range that sufficiently covered the HAZ including the toe. In shot blasting, the processing depth was 0.05 to 0.15 mm from the surface. The processing depth was measured by observing plastic deformation in the vicinity of the surface with a microphotograph and measuring Vickers hardness. TIG welding was performed at a heat input of 0.2 to 1.0 kJ / mm and a speed of 10 cm / min.
[0017]
After flattening these UOE steel pipes, a JIS Z 2201 No. 14B tensile test piece with a seam weld in the center was sampled and subjected to a tensile test in accordance with JIS Z 2241. FIG. 5 shows a test result of a steel pipe in which processing distortion due to shot blasting is introduced into the seam welded portion after pipe expansion, and FIG. 6 shows a test result of a steel pipe reinforced by TIG welding.
[0018]
From FIG. 5, when processing strain is introduced into the seam weld after pipe expansion by shot blasting, the boundary at which the joint breaks is 970 MPa, which is significantly improved compared to the result of FIG. The joint fracture still occurred for the steel pipe having the following strength. In addition, when TIG welding reinforcement was performed as shown in FIG. 6, the joint fracture did not occur even with a high-strength material of 1050 MPa, and the effect was more remarkable than shot blasting.
[0019]
As described above, by strengthening the seam welded portion by shot blasting and TIG welding reinforcement, an effect is seen in preventing joint breakage of the high-strength material. However, the effect of shot blasting is insufficient, and on-line equipment is difficult to process steel balls. Further, although TIG welding reinforcement is effective, it is necessary to newly add a welding facility, and production efficiency is poor, so that it cannot be manufactured at low cost.
[0020]
Therefore, we focused on peening by ultrasonic vibration, which is easier to implement on-line than shot blasting, has a deep machining depth, and can be expected to improve the structure. FIG. 7 shows a schematic diagram showing a state of a cross section after the peening treatment by ultrasonic vibration is performed so as to cover the heat affected zone including the toe portion of the steel pipe after the pipe expansion.
[0021]
That is, in FIG. 7, the base material (steel plate) 1 is seam welded by the outer surface weld metal 2 and the inner surface weld metal 3, and ultrasonic waves are applied so as to cover the HAZ 4 including the weld toe 5 of the seam welded portion after the pipe expansion. A peening processing unit 8 by vibration is formed.
[0022]
When plastic deformation is applied to the HAZ including the toe by peening by ultrasonic vibration, the processing depth reaches several millimeters. Therefore, the effect of hardening the softened HAZ is superior to that of shot blasting by the peening treatment by ultrasonic vibration, and as a result, a better tubular body fracture form is obtained.
[0023]
In addition, although the ultrasonic generator for ultrasonic vibration used in the present invention is not particularly limited, an ultrasonic wave is oscillated by an oscillator using a power source of 500 w to 1 kw, and the frequency is converted to 20 to 60 kHz by a transducer. Further, the amplitude is amplified by a wave guide, and an ultrasonic vibration terminal composed of a pin having a diameter of 2 mm to 6 mmφ is mechanically vibrated with an amplitude of 20 to 40 μm. Indentation with a depth of about several hundreds μm can be formed on the surface before hitting while maintaining the above.
[0024]
Here, the indentation is a shape change of a depth of about several hundred μm from the surface before the ultrasonic treatment of the portion subjected to the ultrasonic treatment, and can be measured by a macro photograph of a cross section or a stylus meter. The above-mentioned processing depth is a depth that has undergone plastic processing by ultrasonic vibration, and is a portion having a processed structure of several millimeters from the surface, and can be measured by a micrograph of a cross section. Moreover, it can measure by the Vickers hardness test based on JISZ2244 as a hardness change of the process structure | tissue with respect to a base material, for example.
[0025]
【Example】
Example 1
UOE steel pipes having joint tensile strengths of 620 to 1050 MPa, outer diameters and wall thicknesses of 609 to 904 mm and 16 to 38 mm, and tube strengths of 590 to 1030 MPa were produced. Seam welding was performed by submerged arc welding. After the pipe expansion, the HAZ including the toe portions of the inner and outer surfaces of the seam welded portion was subjected to a peening process by ultrasonic vibration with a tool amplitude of 40 μm and a vibration frequency of 30 kHz. The depth of the indentation was measured from a macro photograph of the sample cross section and was 0.05 to 0.1 mm.
[0026]
After flattening these UOE steel pipes, a JIS Z 2201 No. 14B tensile test piece with a seam weld in the center was sampled and subjected to a tensile test in accordance with JIS Z 2241. The results are shown in FIG. 8, and when the peening treatment by ultrasonic vibration was applied to the HAZ including the toe portion of the seam welded portion, the joint material was broken even at a joint strength of a maximum of 1050 MPa.
[0027]
Breaking from the base metal in the welded joint tensile test means breaking from the pipe body in a burst test using an actual pipe. Therefore, it is the most preferable fracture mode when used for a line pipe or the like.
[0028]
(Example 2)
UOE steel pipes having a tensile strength of 600 MPa and outer diameters and thicknesses of 660 mm and 25 mm, respectively, were produced. Seam welding was performed by submerged arc welding. After the pipe expansion, the HAZ including the toe portions of the inner and outer surfaces of the seam welded portion was subjected to a peening process by ultrasonic vibration with a tool amplitude of 40 μm and a vibration frequency of 30 kHz. The depth of the indentation was measured from a macro photograph of the sample cross section and was 0.05 to 0.1 mm. For comparison, some steel pipes were subjected to shot blasting on the HAZ including the toe ends of the inner and outer surfaces of the seam welded portion after pipe expansion. The shot blasting conditions were adjusted so that the surface roughness was 0.1 mm in Rmax and 0.15 mm in processing depth. Rmax was measured with a stylus meter, and the processing depth was measured as the depth of the processed structure by observing 3 to 5 views of the microstructure of the cross section.
[0029]
These steel pipes, that is, steel pipes obtained by peening treatment or shot blasting by ultrasonic vibration on the HAZ including the toe ends of the inner and outer surfaces of the seam welded part, and the steel pipes as expanded, were subjected to an actual pipe internal pressure fatigue test. The internal pressure fatigue test was performed by using oil as a pressure medium and increasing and decreasing the internal pressure so that a predetermined stress ratio was obtained at 10 seconds / cycle so that no axial force was generated. The internal pressure fatigue characteristics were evaluated as the number of cycles until a burst occurred when the stress ratio was changed. The results are shown in FIG.
[0030]
In FIG. 9, the result of the test material subjected to the peening treatment by ultrasonic vibration is shown as “ultrasonic peening”. The internal pressure fatigue characteristics of a steel pipe that had been subjected to peening treatment by ultrasonic vibration on the HAZ including the toes on the inner and outer surfaces of the seam welded portion were significantly improved compared to the steel pipe that had been expanded and shot blasted after the expansion.
[0031]
As described above, the steel pipe with shot blasting on the HAZ including the toes on the inner and outer surfaces of the seam welded part was subjected to the internal pressure fatigue test as a comparative material. However, the internal pressure fatigue characteristics of the steel pipe subjected to peening treatment by ultrasonic vibration And the effects of the present invention were demonstrated.
[0032]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a UOE steel pipe excellent in seam weld strength characteristics and internal pressure fatigue characteristics by performing peening treatment by ultrasonic vibration on the seam welded portion after pipe expansion by the UOE method. As a result, the industrial contribution is extremely high.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a steel pipe manufacturing process by a UOE method.
FIG. 2 is a view showing a comparison of the breaking mode of the joint as it is expanded, according to the breaking strength.
FIG. 3 is a schematic diagram showing a state of a cross section after performing a shot blast process on a seam welded portion.
FIG. 4 is a schematic view showing a state of a cross section after TIG welding reinforcement is applied to a seam welded portion.
FIG. 5 is a diagram showing a comparison of fracture forms of welded joints subjected to shot blasting treatment according to fracture strength.
FIG. 6 is a diagram showing a comparison of fracture forms of welded joints subjected to TIG welding reinforcement according to fracture strength.
FIG. 7 is a schematic diagram showing a state of a cross section after peening treatment by ultrasonic vibration of the present invention to a seam welded portion.
FIG. 8 is a diagram showing a comparison of fracture forms of UOE steel pipes subjected to peening treatment by ultrasonic vibration according to the present invention according to fracture strength.
FIG. 9 is a diagram showing a comparison of the effects of peening treatment by ultrasonic vibration of the present invention and other treatments on the strength of an internal pressure fatigue test of a UOE steel pipe.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Base material 2 ... Outer surface weld metal 3 ... Inner surface weld metal 4 ... HAZ
5 ... Weld toe 6 ... Shot blast 7 ... TIG weld 8 ... Peening treatment by ultrasonic vibration

Claims (1)

In the manufacturing method of a UOE steel pipe in which steel plates are sequentially formed into C, U, and O, and the ends of the steel plates are seam welded and then expanded, at least the seam weld toe after expansion is subjected to peening treatment by ultrasonic vibration. A method for manufacturing a UOE steel pipe excellent in seam weld strength characteristics and internal pressure fatigue characteristics.

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