JP4975354B2 - Manufacturing method of high strength welded steel pipe - Google Patents

Description

  The present invention relates to a method for producing a high-strength welded steel pipe having a tensile strength of 850 MPa or more, which is used for a natural gas / crude oil transportation line pipe or the like.

  In the long-distance pipeline for transporting natural gas, the laying of an ultra-high-strength large-diameter line pipe having a tensile strength exceeding 850 MPa has been studied from the viewpoint of improving transportation efficiency and reducing the cost of incidental facilities. Such a line pipe is usually formed by the UOE method, and the seam welded portion that becomes a joint is welded from the inner and outer surfaces by submerged arc welding. Thereafter, in order to adjust the roundness of the steel pipe, correction by pipe expansion is performed.

  Important issues in the manufacture of high-strength welded steel pipes by the UOE pipe making process are cracks from the seam welds during pipe expansion (expansion cracks) and the difference between the maximum and minimum thicknesses (uneven thickness). . However, there have been few inventions that have devised the tube expansion process for the purpose of preventing the tube expansion crack and uneven thickness. The proposal concerning the pipe expansion process is a technique that can improve the roundness and straightness of the steel pipe with respect to the general-purpose material (for example, Patent Documents 1 to 3), and can prevent the pipe expansion crack and uneven thickness for the high-strength material. It wasn't.

Patent Documents 1 and 2 is a method for tube expansion by a segment having a high coefficient of friction, roundness due to high friction, it is described for improving the true Jikado. However, there is no description about the interrelationship between the pipe expansion crack, the thickness deviation, and the pipe expansion ratio, and there is no description about preventing the pipe expansion crack while securing the thickness deviation with a high-strength steel pipe.

  Patent Document 3 discloses a method of changing the friction coefficient at the circumferential position at the time of pipe expansion, and describes that a plurality of segments having relatively different friction coefficients are used for the purpose of ensuring the straightness of the steel pipe. Has been. However, it is possible to prevent expansion cracking by increasing the friction coefficient between the inner surface of the steel pipe and the segment, and when considering the uneven thickness, there is an optimum range in the part where the friction coefficient and the friction coefficient should be increased. Not listed.

JP 57-94434 A JP-A-57-94433 JP-A-9-182924

  The present invention provides a method for manufacturing a high-strength welded steel pipe that can prevent pipe expansion cracks and uneven thickness that occur in a seam weld when a high-strength steel pipe after seam welding is mechanically expanded and straightened.

The present invention has been made in order to solve the above-described problems. By setting the coefficient of friction between the inner surface of the steel pipe near the welded portion and the inner surface of the steel pipe and the expanded portion of the other portion to a specific value, the distortion generated in the welded portion Is a method of manufacturing a high-strength steel pipe that prevents the occurrence of expansion cracks and uneven wall thickness, the gist of which is to form a steel sheet having a tensile strength of 850 MPa or more into a cylindrical shape, and seam welds by abutting the ends of the steel sheets After the steel pipe is expanded, when expanding the steel pipe, the high friction coefficient application area where the friction coefficient between the inner surface of the steel pipe and the expanded segment is 0.15 or more is used. in the range below 25 ° more than 180 °, the coefficient of friction between the steel pipe inner surface and the segments in the portion other than the high friction coefficient application areas tube expansion as less than 0.15, the tube expansion cracking及occurring seam weld A process for producing a high strength welded steel pipe, characterized in that to prevent uneven thickness.

  According to the present invention, a high-strength steel pipe having a tensile strength of 850 MPa or more used for a natural gas / crude oil transportation line pipe or the like can be produced without causing expansion cracking and uneven thickness, which contributes to the industry. Extremely prominent.

  In the UOE pipe making process, the edge of a steel plate is bent with a C press, bent into a U shape with a U press, then formed into a steel tube with an O press, and then usually by tack welding from the outer surface and then by submerged welding. Inner surface welding is performed, followed by outer surface welding, and the roundness is further adjusted by pipe expansion to form a UOE steel pipe. The pipe expansion is usually performed by giving a plastic deformation of about 1% in the circumferential direction. However, in steel pipes having a tensile strength exceeding 850 MPa, so-called expanded cracks that break from the seam weld toe before giving 1% plastic strain have resulted in a significant loss of productivity. Tube expansion cracks can be prevented by reducing the tube expansion rate, but the roundness of the product is significantly deteriorated.

  The inventors of the present invention provide a method for preventing the occurrence of expansion cracking by increasing the friction coefficient between the inner surface of the steel pipe and the segment in the vicinity of the seam weld, and lowering the friction coefficient between the inner surface of the steel pipe and the segment in other portions. Was examined.

  FIG. 1 shows strain concentration at the toe when the friction coefficient between the inner surface of the steel pipe and the segment is increased to 0.15 to 0.40 (referred to as a high friction coefficient application region) at the circumferential position. Is analyzed by FEA. The friction coefficient of the part other than the high friction coefficient application area was less than 0.15. Here, the horizontal axis indicates the high friction coefficient application area by the angle of the central angle of the steel pipe, and the center of the seam weld is 0 °. It was found that the strain began to rise when the high friction coefficient application area was less than 40 °, and increased rapidly when it fell below 25 °, resulting in a pipe expansion crack. That is, if the seam welded portion is a central portion in the circumferential direction (center angle 0 °) and a portion having a central angle of 25 ° or more with the seam welded portion as a high friction coefficient application region, a remarkable distortion reduction effect can be obtained. FIG. 1 shows that pipe expansion cracking can be prevented.

  Next, in order to examine the upper limit of the high friction coefficient application range, the relationship between the range of the high friction coefficient application range and the thickness deviation was examined. The thickness deviation of a steel pipe is expressed as a percentage of (maximum wall thickness-minimum wall thickness) / average wall thickness. The result is shown in FIG. 2. As a result, when the friction coefficient is 0.4, the high friction coefficient application area makes the seam welded portion the central portion (center angle 0 °) in the circumferential direction, and the center angle is sandwiched between them. It can be seen that if the angle exceeds 180 ° (half the circumference of the steel pipe), uneven thickness becomes significant.

  In order to verify this analysis result, a pipe expansion test was performed on the UOE steel pipe, and the occurrence of cracks in the seam portion was evaluated. The range of the high friction coefficient application region was a steel pipe center angle of 25 ° with the seam weld in the center in the circumferential direction, and the friction coefficient other than the high friction coefficient application region was 0.1. FIG. 3 shows the relationship between the expansion cracking caused by the expansion test and the friction coefficient. Here, 10 tests are performed for each friction condition, and the frequency means the number of cracks when the circumferential strain is added to 1.5%. When the friction coefficient was 0.12 or less, cracking occurred, and the generation site started from the seam portion and started from the weld toe. On the other hand, when the friction coefficient was 0.15 or more, no crack was generated.

  Based on the above results, in the present invention, at least the friction coefficient between the inner surface of the steel pipe and the segment within the range where the center angle is at least 25 ° and at most 180 ° with the seam welded portion as the center in the circumferential direction is set to 0. The coefficient of friction between the inner surface of the steel pipe and the segment in other parts was set to less than 0.15, and the pipe expansion was mechanically corrected. As a result, it has been found that pipe expansion can be performed without causing pipe expansion cracks and without significantly deteriorating the uneven thickness.

  Although the present invention does not specify the number of segments at the time of pipe expansion, it is desirable to divide the entire circumference by 8 to 14 in order to efficiently reduce the polygonalization of the steel pipe.

  In the present invention, it is important that the seam welded portion is the center in the circumferential direction, and the inner surface of the steel pipe where the central angle of the steel pipe is at least 25 ° or more and 180 ° or less is the high friction coefficient application region. It does not specify the number of segments corresponding to. When using 12 equally divided segments, the segment corresponding to the seam part, that is, if the friction coefficient for 30 ° is 0.15 or more and the other friction coefficient is less than 0.15, the seam welding position is included for 3 segments. Even if the friction coefficient for 90 ° is 0.15 or more and other friction coefficients are less than 0.15, it is included in the present invention.

  In the present invention, when the tensile strength of the steel sheet is less than 850 MPa, pipe expansion cracking does not occur, so the steel sheet is limited to a steel pipe having a tensile strength of 850 MPa or more as a base material. The upper limit of the tensile strength of the steel sheet is not limited, but at present, it is difficult to manufacture a steel pipe from a steel sheet having a tensile strength exceeding 1200 MPa.

  The thick steel plate (base material) that is the material of the UOE steel pipe has a steel composition of 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: 0.1-2.0%, Mo: 0.15-0.60%, Nb: 0.001- 0.10%, Ti: 0.005 to 0.030%, Al: 0.06% or less, and further, 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 to 1.0%, Zr: 0.0001 to 0.005%, Ta : 0.0001 to 0.005%, Ca: 0.0001 to 0.01%, REM: 0.0001 to 0.01%, Mg: 0.0001 to 0.006% It contains two or more kinds, and Steels balance consisting of Fe and unavoidable impurities, the cast steel strip may be produced by hot controlled rolling.

  Moreover, although the method of manufacturing a steel pipe from a steel plate is mainly a UOE pipe making process, it is applicable also when the steel pipe shape | molded with the bending roll is mechanically corrected from an inner surface. Seam welding of steel pipes is mass%, C: 0.01 to 0.12%, Si: 0.3% or less, Mn: 1.2 to 2.4%, Ni: 4.0 to 8.5% , Cr + Mo + V: 3.0 to 5.0%, Ti: 0.005 to 0.15%, Al: 0.02% or less, heat input is 1.5 kJ / mm to 6.3 kJ / Mm and may be performed by submerged arc welding.

  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 high-strength steel pipe manufactured by the method of the present invention has a relatively low strain concentration at the seam weld toe, compared to the steel pipe manufactured by the prior art, so the burst strength when the internal pressure is applied is also seam. It is possible to avoid breakage from the part, which is advantageous.

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

  Table 1 shows the expansion of a UOE steel pipe having a steel pipe size of φ914 × 12t, φ914 × 16t, φ1016 × 19t, φ1219 × 20t, steel pipe strength of 850 to 1040 MPa, in the circumferential region including the seam weld shown in region A of FIG. This is a result obtained by changing the angle and changing the friction coefficient between the inside of the steel pipe other than the region A and the region A and the segment, and shows the occurrence of the expansion crack and the thickness deviation. As shown in Table 2, as a method of changing the coefficient of friction for each region, a method in which segments having different surface properties are partially arranged, and the type of lubricant or the presence / absence of application are changed in the circumferential position. That was realized.

  As shown in FIG. 5, the friction coefficient is obtained by performing lubrication equivalent to pipe expansion on a sample cut out from a steel plate before being formed into a tubular shape using a die that has been processed in the same manner as the surface of the segment. The sample was sandwiched from both sides at 100 MPa and pulled out from one side by a tensile tester, and the half of the ratio between the pulling load, P and the sandwiching load, and W at that time was determined as the friction coefficient μ (the following formula (1)).

  Table 2 shows the test conditions of this test method and the obtained friction coefficient. In the column of the surface property of the segment in Table 2, a method for processing the surface of the mold is shown. In shot blasting, the surface roughness was Rmax = 20 μm, and in sharpening, grooves were provided at a depth of 0.1 mm in the direction of the drawing load and 45 °. In addition, lubricant compound grease and soluble oil were applied to the sample, but the solid lubricant was coated with a phosphate film on the mold surface and sprayed with polyamideimide resin containing molybdenum disulfide and baked. Is.

  In Examples 1 to 13, the region A is 25 ° or more and 180 ° or less, the friction coefficient is 0.15 or more, and the friction coefficient is less than 0.15 except in the region A. The uneven thickness was 3% or less.

  The thickness deviation of the steel pipe is expressed as a percentage of (maximum wall thickness−minimum wall thickness) / average wall thickness, and is the average of the results of expanding 10 pipes under each friction condition. The numerical value of the pipe expansion crack is a percentage obtained by calculating the rate of occurrence of cracks from the number of pipe expansion cracks when ten pipe expansions were performed under each friction condition. The tube expansion was performed with the circumferential strain set at 1.0%.

  In Comparative Examples 1, 3, 5, and 6, since the region A was less than 25 ° and the high friction coefficient application region having a friction coefficient of 0.15 or more was smaller than the range of the present invention, pipe expansion cracking occurred. In Comparative Example 3, since the friction coefficient other than the region A is more than 0.15, the thickness deviation is more than 3%. In Comparative Example 2, the high friction coefficient application region where the friction coefficient was 0.15 or more exceeded 180 °, and the uneven thickness degree exceeded 3%. In Comparative Examples 4 and 8, since the range of the region A is wide and the friction coefficient is less than 0.15, that is, the friction coefficient of the portion to be a high friction coefficient application region is less than 0.15, the pipe expansion crack Produced. In Comparative Example 7, the range of the region A was 25 ° or more, but the coefficient of friction was less than 0.15, so that a pipe expansion crack occurred.

The figure which shows the relationship of the plastic strain to a high-friction coefficient application area | region and a seam weld toe part. The figure which shows the relationship between a high friction coefficient application area | region and thickness deviation. Histogram of pipe cracking against coefficient of friction between steel pipe inner surface and segment. The figure which shows the area | region which specified the friction coefficient. The figure which shows the measuring method of a friction coefficient.

Explanation of symbols

1 Steel pipe up to pipe expansion 2 Seam weld 3 Pipe expansion segment

Claims (1)

A steel plate having a tensile strength of 850 MPa or more is formed into a cylindrical shape, end portions of the steel plates are butted together and seam welded to form a steel pipe. Then , when the steel pipe is expanded, the coefficient of friction between the inner surface of the steel pipe and the expanded segment is 0.15. The above-described high friction coefficient application area is a steel pipe center angle in the range of 25 ° to 180 ° with the seam weld in the center in the circumferential direction, and the friction between the inner surface of the steel pipe and the segment in the area other than the high friction coefficient application area. A method for producing a high-strength welded steel pipe, characterized in that the pipe is expanded with a coefficient of less than 0.15 to prevent pipe cracking and uneven thickness occurring in a seam weld .

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