JP2003340518A - Manufacturing method of uoe steel pipe having good crush strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a steel pipe for an UOE line pipe capable of improving compression strength in the process of external press load. <P>SOLUTION: In a manufacturing method of an UOE steel pipe, a steel plate is treated by C press, U press, and O press in order and the both edges of the steel plate are seam welded, and after that, the pipe is expanded. The steel pipe having good crush strength is characterized by ≥0.35 of the ratio α/β of the upset ratio in O press shown as α/the expanding ratio shown as β in the method. The expanding ratio may be 2% or more. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for forming a steel pipe used for a line pipe or the like by a UOE manufacturing method,
A method for improving crush properties.

[0002]

2. Description of the Related Art In recent years, line pipes have become increasingly important as a long-distance transportation method for crude oil and natural gas, and in particular, submarine line pipes that cross the ocean have reached a depth of 3000 m. Generally, in the design of pipelines, the steel pipe inner diameter is first determined from the fluid transport amount, and then the wall thickness and material are determined by considering crack propagation characteristics and corrosion weight loss in order to keep the circumferential stress under internal pressure load to a constant value. Has been done. However, as the water pressure increases with the deepening of the sea, crushing strength, which has hitherto been less important, is becoming a problem. The crush strength has a correlation with the ratio of the outer diameter to the wall thickness, and by increasing the crush strength of the steel pipe, it is possible to increase the diameter and reduce the wall thickness. Therefore, the crush strength has begun to become the main design factor that determines the steel pipe size.

By the way, the crushing strength of steel pipes has been studied for a long time in oil country tubular goods, and many empirical formulas have been statistically proposed. Outer diameter / thickness ratio, yield strength, roundness, uneven thickness, and residual stress were the main controlling factors. Since these studies were mainly conducted on seamless steel tubes of homogeneous material, it was not necessary to discuss much about material anisotropy.

However, since a main line pipe used for long-distance transportation has a large diameter, a steel pipe manufactured by the UOE method is used. As shown in FIG. 1, the manufacturing process of a steel pipe by the UOE method includes C forming (pressing), U forming (pressing), O forming (pressing), seam welding, and pipe expanding. Further, in O-molding, the diameter is reduced by an O-mold as shown in FIG. 2, which is called an O-molding upset. Further, as shown in FIG. 3, the pipe expansion is a process of correcting the circularity by pushing and expanding the metal segments from the inside, and is subjected to a tensile stress in the circumferential direction to be plastically deformed. Since all of these bending, compression and tension forming are performed cold, the final product, that is, the steel pipe, has anisotropic mechanical properties due to the combination of work hardening and the Bauschinger effect. The Bausinger effect is a phenomenon in which the yield strength in the opposite direction to that after the material is subjected to plastic strain is reduced. Therefore, the UOE steel pipe to which the plastic strain in the tensile direction in the circumferential direction is applied has the compressive yield strength in the circumferential direction,
That is, the yield strength against an external pressure load decreases due to the Bausinger effect.

On the other hand, with respect to an axial load load, the main strain at the time of molding is orthogonal to the load direction. Therefore, stress behavior is unlikely to differ between axial tension and compression loads. Further, when the circumferential load is tensile stress, that is, when the internal pressure is applied, the strength design based on the value obtained from the full thickness tensile test does not cause any problem.

However, in recent years, the demand for UOE steel pipes applicable to deep-sea line pipes has increased, and the crushing strength of steel pipes due to external pressure has become a problem. Crushing is a phenomenon in which a steel pipe is crushed by external pressure and is one of buckling, so the yield strength of compression determines the crushing strength. Therefore, when the UOE steel pipe is applied to a line pipe that requires crushing strength, there is a problem that the compressive strength in the circumferential direction decreases due to the Bausinger effect.

In order to solve such a problem, a method of recovering the lowered compressive yield strength by heat treatment is disclosed in JP-A-9-3.
It is disclosed in Japanese Patent Laid-Open No. 545 and Japanese Patent Laid-Open No. 9-49025 and is reported in many research papers. According to these methods, the compressive yield strength of the UOE steel pipe decreased by forming is restored to the steel plate level before forming, and the crushing strength is improved to some extent.

[0008]

However, in the improvement of the crushing strength by the heat treatment, a heating step is added, which results in a significant increase in cost. An object of the present invention is to improve the crush strength of a steel pipe by optimizing the UOE manufacturing process without heat treatment.

[0009]

Means for Solving the Problems The inventors of the present invention have noticed that crushing strength is improved by applying a compressive plastic strain from the inner surface of a UOE steel pipe to the wall thickness center portion (hereinafter, inner surface side),
By optimizing the ratio between the upset ratio during O-forming and the pipe expansion ratio during pipe expansion, we succeeded in imparting compressive plastic strain to the inner surface side and minimizing the decrease in compressive yield stress. On the other hand, it has been found that when the pipe expansion ratio is 2% or more, the crush strength is improved together with the pipe expansion ratio regardless of the upset ratio of O molding.

That is, the gist of the present invention is that
It is as follows. (1) In a method for manufacturing a UOE steel pipe in which steel sheets are sequentially C-formed, U-formed, and O-formed, and the ends of the steel sheets are seam-welded and then expanded, the upset ratio α during O-forming and the pipe expansion ratio β during expansion are A method of manufacturing a UOE steel pipe having excellent crush strength, characterized in that the ratio is α / β ≧ 0.35. (2) In a method of manufacturing a UOE steel pipe in which steel sheets are sequentially C-formed, U-formed, and O-formed, and the ends of the steel sheets are seam-welded and then expanded, the pipe expansion ratio during pipe expansion is set to 2% or more. It is a method of manufacturing a UOE steel pipe having excellent crushing strength.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the cross section of a steel pipe manufactured by the UOE method, the inventors have shown that the compressive yield strength in the circumferential direction from the outer surface of a steel sheet to the center of wall thickness (hereinafter, outer surface side) is measured in the circumferential direction. Detailed sampling was performed by changing the sampling position. The test piece was a cylinder having a diameter of 6 mm and a length of 15 mm, and the longitudinal direction was the circumferential direction. The results are shown in FIG. 4, where the horizontal axis is the counterclockwise angle with the axially symmetrical portion of the seam welded portion of the steel pipe being 0 °, and the seam welded portion is 180 °. The compressive yield strength retention rate on the vertical axis is the percentage of the value obtained by dividing the compressive yield strength of the steel pipe by the compressive strength of the steel sheet before forming. As described above, the compressive yield strength of the outer surface side of the steel pipe is 80 to 9 of the compressive yield strength of the steel sheet.
It was found to have dropped to 0%. Therefore, as a result of analyzing the strain behavior in each process of C forming, U forming, O forming, and pipe expanding shown in FIG. 1, the largest tensile plastic strain occurs in U forming on the outer surface side, and compression occurs during upsetting of O forming. It was found that compressive plastic strain due to compressive stress slightly exceeding the yield strength occurs, and tensile plastic strain due to tensile stress exceeding tensile yield strength again occurs during pipe expansion.

Therefore, attention has been paid to the possibility of suppressing the decrease in the compressive yield stress on the outer surface side by controlling the balance between the compressive plastic strain at the time of upsetting in O-forming and the tensile plastic strain at the time of pipe expansion. The Bauschinger effect is a phenomenon in which the yield strength decreases in the direction opposite to the plastic strain, so by increasing the upset ratio to add more compressive plastic strain and decreasing the pipe expansion ratio to reduce the tensile prestrain. , The reduction of the compressive yield stress on the outer surface side was suppressed.

In the test, a steel pipe having an outer diameter / thickness of 18.7 was manufactured by changing the ratio of the upset ratio α and the pipe expansion ratio β of O forming, and a ¼ total area was formed from the outer surface of the welded portion and the axisymmetric portion. A compression test piece was collected centering on the thick part. The results are shown in FIG. 5 as changes in the compressive yield strength retention rate with respect to α / β. Here, the upset ratio and the pipe expansion ratio are defined as follows.

From this, it is clarified that the compression yield strength retention rate reaches 90% by setting α / β ≧ 0.35,
The lower limit of α / β was set to 0.35. The upper limit of α / β depends on the preferable range and the optimum range of the upper limit of α and the lower limit of β described later, and the preferable range is 1 or less, and the optimum range is 0.6 or less. The lower limit of α is preferably 0.3% or more from the viewpoint of minimizing the seam gap before welding and reducing the peaking of the welded portion as shown in FIGS. 6 and 7. Regarding the upper limit of α, the larger the value, the more the hindering the reduction of the compression yield strength, but the overlap or buckling of the groove portion as shown in FIG. However, it is preferably 0.5% or less. In order to prevent overlap and buckling of the groove portion, a protrusion shape having a taper that expands toward the axial center is formed at a portion corresponding to the seam weld portion on the inner surface of the O-molding die as shown in FIG. It is effective to provide them.

The inventors further examined the influence of the pipe expansion ratio on the compression yield strength retention ratio. The results are shown in FIG. 10, and it can be seen that when the expansion ratio exceeds 2%, the compression yield strength maintenance ratio stably exceeds 80%, and increases with the expansion ratio. Therefore, even if the upset ratio cannot be sufficiently obtained due to insufficient press capacity of the O-molding machine for thick-walled materials, the pipe expansion ratio should be set to 2% or more to maintain the compression yield strength maintenance ratio to 85% or more. I found that When the expansion ratio is increased, the compressive yield strength retention ratio is improved, but since more plastic strain is given, the yield ratio increases, the uniform elongation decreases, and the impact value decreases. The maximum tube expansion ratio is preferably 5% or less due to deterioration of these mechanical properties. Further, if the pipe expansion ratio is reduced, the circularity cannot be ensured, so 0.5% or more is preferable, and the optimum range is 0.6% or more. The rate of decrease in the compressive yield strength on the outer surface side due to the Bauschinger effect does not largely depend on the magnitude of tensile plastic strain during pipe expansion and the tensile plastic strain due to bending during pipe forming, which changes depending on the wall thickness. On the other hand, the increase in compressive yield strength on the inner surface side is
It depends on the degree of work hardening which varies depending on the magnitude and thickness of compressive strain during pressing. Therefore, as the wall thickness increases, the compressive yield strength on the outer surface side does not change and the compressive yield strength on the inner surface side increases. The effect of the present invention is at least the thickness,
The effect was confirmed in the range of 20 to 42 mm.

[0016]

[Example] The materials are X-65 and X80, and the outer diameter and the wall thickness are 660 to 711 mm and 25 to 38 m, respectively.
UOE steel pipes in the range of m were manufactured with the outer diameter / wall thickness ratio, upset ratio and pipe expansion ratio shown in Table 1. From this steel pipe, a compression test piece having a diameter of 6 mm and a length of 15 mm and having a longitudinal direction in the circumferential direction was cut out, and the compression yield strength was measured and shown as the compression yield strength retention rate. Further, for some of the steel pipes, a steel pipe having a length of 5 m was used as a crushing test body, and a uniaxial crushing test in which water pressure was applied so as to prevent axial force from being generated in the steel pipe was set in a pressure vessel to measure the crushing strength. The results are shown in Table 1.

Manufacturing No. Production Nos. 1 to 4 have the upset ratio and the pipe expansion ratio within the scope of the present invention, have the same material and outer diameter / wall thickness ratio, and have an α / β ratio of less than 0.35. 14
The retention rate of compressive yield strength is larger than -17. In addition, the manufacturing number. When a crushing test was performed on the steel pipes Nos. 1, 2 and 14, the production No. within the scope of the present invention. The crushing strengths of Nos. 1 and 2 are manufacturing numbers outside the range of the present invention. Higher than 14,
The effect of the present invention was verified. In addition, the manufacturing number. Production No. 5 has the same material and outer diameter / thickness ratio, and the α / β ratio is less than 0.35. The steel pipe compressive yield strength ratio to the plate yield strength is larger than that of No. 18.

Manufacturing No. Nos. 6 to 12 have a pipe expansion ratio of 2% or more, and regardless of the α / β ratio, the steel pipe compression yield strength ratio to the plate yield strength is the same as the material and the outer diameter / wall thickness ratio, and the pipe expansion ratio is 2%. Manufacturing No. less than Better than 14-17. Of these, manufacturing number. When a crushing test was performed on No. 6, the production No. It was found to have a crush strength higher than 14.

Manufacturing No. No. 13 was manufactured with a pipe expansion ratio of 4%. Manufacturing No. 13 of the same size and steel type. Compared with No. 18, the decrease in compressive yield strength was small.

[0020]

[Table 1]

[0021]

As described above, according to the present invention, the UO
It is possible to give a higher crush resistance by specifying the ratio of the upset ratio at the time of O molding of the steel pipe manufactured by the E method and the pipe expansion ratio at the time of pipe expansion, and by setting the pipe expansion ratio to 2% or more, It is possible to provide UOE steel pipes with excellent crushing strength at low cost. This can be used for line pipes for transportation of natural gas, crude oil, etc. even in an environment where high crushing strength is required such as deep sea. It has a very high contribution to the industry.

[Brief description of drawings]

FIG. 1 is a schematic view of a UOE method for manufacturing a steel pipe.

FIG. 2 is a schematic diagram of O molding.

FIG. 3 is a schematic view of tube expansion molding.

FIG. 4 is a distribution of the compressive yield strength retention rate of UOE steel pipes.

FIG. 5 shows changes in compressive yield strength retention rate depending on the upset ratio / tube expansion ratio.

FIG. 6 is a schematic diagram of a weld seam gap.

FIG. 7 is a schematic diagram of welded peaking.

FIG. 8 is a schematic diagram of overlap and buckling of the groove portion.

FIG. 9 shows an example of the O-molding die projection shape.

FIG. 10 shows changes in the compressive yield strength retention rate depending on the pipe expansion rate.

Claims (2)

[Claims] 1. A steel sheet is sequentially C-formed, U-formed, and O-formed,
A method of manufacturing a UOE steel pipe in which ends of a steel sheet are seam-welded and then expanded, wherein a ratio of an upset ratio α during O forming and a pipe expansion ratio β during expansion is α / β ≧ 0.35. A method for manufacturing a UOE steel pipe having excellent crush strength. 2. A steel sheet is sequentially C-formed, U-formed, and O-formed,
A method for producing a UOE steel pipe having excellent crushing strength, which is characterized in that a pipe expansion ratio during pipe expansion is 2% or more in a process for producing a UOE steel pipe in which pipe ends are seam-welded and then expanded.