METHOD FOR PRODUCING STEEL PIPE

Abstract

 A method for manufacturing a steel pipe includes forming a steel plate into a cylindrical shape by a bending press method that presses down the steel plate with a punch having, at a leading edge thereof, an outer circumferential surface with a circular arcuate section so that edges in a width direction of the steel plate face each other; butting and welding together the edges in the width direction of the steel plate; and expanding the steel pipe from the inside using a pipe expanding machine in which a plurality of pipe expanding dies having an outer circumferential surface with a circular arcuate section are circumferentially arranged, wherein a pipe expanding R ratio that is a ratio of a curvature radius of the outer circumferential surface of each of the pipe expanding dies to a target radius of an inner surface of the steel pipe and a punch R ratio that is a ratio of a curvature radius of an outer circumferential surface of a leading end of the punch to the target radius of the inner surface of the steel pipe are both less than 1, and the pipe expanding R ratio and the punch R ratio satisfy a predetermined conditional expression. As a result, a steel pipe with high roundness can be manufactured without cracking a pipe expanding die.

Description

Field

[0001] The present invention relates to a method for manufacturing a steel pipe that includes a pipe expanding step of expanding a steel pipe from the inside using a pipe expanding machine in which a plurality of pipe expanding dies having an outer circumferential surface with a circular arcuate section are circumferentially arranged.

Background

[0002] In general, a manufacturing process for UOE steel pipes for, for example, oil well pipes and line pipes includes a step of applying beveling to edges in a width direction of a thick steel plate served as a material; a forming step of bending the width-direction edges of the thick steel plate using a pressing machine, and thereafter forming the thick steel plate into a cylindrical shape by forming the thick steel plate sequentially into a U-shape and an O-shape so that the width-direction edges of the thick steel plate face each other; a tack welding step of restraining the cylindrical thick steel plate, abutting the facing width-direction edges of the thick steel plate against each other, and tack-welding the edges together; a permanent welding step of seam-welding the inner and outer surfaces of the butted portion of the thick steel plate using a submerged arc welding method; and a pipe expanding step of forming a steel pipe to have predetermined levels of roundness, straightness, and outside diameter by expanding the steel pipe from the inside using a pipe expanding machine in which a plurality of pipe expanding dies having an outer circumferential surface with a circular arcuate section are circumferentially arranged.

[0003] In a method for manufacturing welded steel pipes for, for example, oil well pipes and line pipes, the forming step of forming the thick steel plate into a cylindrical shape so that the edges in the width direction of the thick steel plate face each other often uses a bending press method that presses down, with a punch, a thick steel plate placed on two linear dies arranged in parallel with a certain space therebetween, instead of the method that performs the U-forming and the O-forming using the pressing machine. Using the bending press method needs only a smaller pressing force than that in the case of performing the U-forming and the O-forming, although the number of times of pressing increases. For this reason, the bending press method is particularly suitable for manufacturing steel pipes of thick materials or high-strength materials.

[0004] The UOE steel pipes for oil well pipes or line pipes are used by joining the pipe ends of the UOE steel pipes together using circumferential welding at a laying site. Hence, if the roundness of a UOE steel pipe is low, the pipe ends of the UOE steel pipes cannot be abutted and joined together during the circumferential welding. If the roundness of the UOE steel pipe is low, the steel pipe is likely to be deformed by pressure when the UOE steel pipe is laid in high-pressure ambience such as a deep sea. From such a background, technologies have been proposed that improve the roundness of the UOE steel pipe in the pipe expanding step (refer to Patent Literature 1 to 4).

Citation List

Patent Literature

[0005] 

Patent Literature 1: Japanese Patent Application Laid-open No. 2010-167440

Patent Literature 2: Japanese Patent Application Laid-open No. H03-094936

Patent Literature 3: Japanese Patent Application Laid-open No. S59-183943

Patent Literature 4: Japanese Patent Application Laid-open No. S61-147930

Summary

Technical Problem

[0006] To improve the roundness of the UOE steel pipe, the ratio of a curvature radius R1 of the outer circumferential surface of each of the pipe expanding dies to a target radius R0 of the inner surface of the UOE steel pipe (hereinafter expressed as a pipe expanding R ratio) is desirably close to 1.0. However, the contact pressure of the pipe expanding die to the inner surface of the UOE steel pipe increases as the pipe expanding R ratio is closer to 1.0, so that the pipe expanding die cracks. For this reason, in the conventional pipe expanding step, the pipe expanding R ratio has been limited up to approximately 0.9, and it has been difficult to further improve the roundness of the UOE steel pipe. From such a background, it has been desired to provide a technology that can manufacture steel pipes with high roundness without cracking the pipe expanding die.

[0007] In view of such a problem, the present invention is made and aims to provide a method for manufacturing a steel pipe that can manufacture the steel pipe with high roundness without cracking a pipe expanding die.

Solution to Problem

[0008] A method for manufacturing a steel pipe according to the present invention includes: forming a steel plate into a cylindrical shape by a bending press method that presses down the steel plate with a punch having, at a leading edge thereof, an outer circumferential surface with a circular arcuate section so that edges in a width direction of the steel plate face each other; butting and welding together the edges in the width direction of the steel plate; and expanding the steel pipe from the inside using a pipe expanding machine in which a plurality of pipe expanding dies having an outer circumferential surface with a circular arcuate section are circumferentially arranged, wherein a pipe expanding R ratio that is a ratio of a curvature radius of the outer circumferential surface of each of the pipe expanding dies to a target radius of an inner surface of the steel pipe and a punch R ratio that is a ratio of a curvature radius of an outer circumferential surface of a leading end of the punch to the target radius of the inner surface of the steel pipe are both less than 1, and the pipe expanding R ratio and the punch R ratio satisfy Expression (1) below:

[0009] In the above-described method for manufacturing the steel pipe according to the present invention, the pipe expanding R ratio further falls in a range from 0.94 to less than 1.0.

Advantageous Effects of Invention

[0010] According to the present invention, a steel pipe with high roundness can be manufactured without cracking a pipe expanding die.

Brief Description of Drawings

[0011] 

FIG. 1 is a conceptual diagram illustrating a procedure of a forming step by a bending press method in a method for manufacturing a steel pipe as according to an embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a configuration of a pipe expanding machine used in the method for manufacturing the steel pipe as according to the embodiment of the present invention.

FIG. 3 is a schematic diagram illustrating a section of the steel pipe.

FIG. 4 is a diagram illustrating the outside diameter and the inside diameter of the steel pipe after the steel pipe is formed by the forming step by the bending press method and expanded at a pipe expanding R ratio of 0.903.

FIG. 5 is a diagram illustrating the outside diameter and the inside diameter of the steel pipe after the steel pipe is formed by the forming step by the bending press method and expanded at a pipe expanding R ratio of 0.986.

FIG. 6 is a diagram illustrating a relation among the pipe expanding R ratio, a punch R ratio, and roundness of the steel pipe.

FIG. 7 is a diagram illustrating another relation among the pipe expanding R ratio, the punch R ratio, and the roundness of the steel pipe.

Description of Embodiments

[0012] As a result of intensive studies by the inventors of the present invention, a steel pipe has a better shape after being seam-welded if a forming step by a bending press method (also called a press bending method) is employed instead of press forming into a U-shape and an O-shape (hereinafter called a UO process) than if the UO process is performed. It has been found that, as a result, increasing a pipe expanding R ratio does not cause a pipe expanding die to crack when compared with the case of performing the UO process. This is considered to be because of the following mechanism. In the forming step by the bending press method, a steel plate as a raw material is press-formed in a manner divided into a plurality of portions in the plate width direction. As a result, undeformed potions do not continue long in the circumferential direction when the steel plate is formed into a tubular shape, even though small undulation exists as a whole. In contrast, in the case of the UO process, each of the forming operations into a U-shape and an O-shape is performed in one step. As a result, depending on the pressing conditions, undeformed potions may continue long in the circumferential direction when the steel plate is formed into a tubular shape. When an undeformed potion exists in the circumferential direction, a very large load is applied to a pipe expanding die that presses the undeformed potion when the pipe is expanded, so the pipe expanding die is likely to crack. This phenomenon is more significant as the pipe expanding R ratio is closer to 1 at which the outer circumferential surface of the pipe expanding die contacts the steel pipe inner surface in a larger area when the pipe is expanded. However, in the case of the steel pipe formed by the bending press method, the undeformed potion does not continue long in the circumferential direction. Hence, it is considered that, compared with the UO process, the pipe expanding operation can be performed without cracking the pipe expanding die even when the pipe expanding R ratio is larger. The following describes a method for manufacturing a steel pipe serving as an embodiment of the present invention that has been conceived based on the above-described findings, with reference to the drawings.

[Forming step by a bending press method]

[0013] First, with reference to FIG. 1, a description will be made of a procedure of the forming step by the bending press method in the method for manufacturing the steel pipe as according to the embodiment of the present invention. FIG. 1 is a conceptual diagram illustrating the procedure of the forming step by the bending press method in the method for manufacturing the steel pipe according to the embodiment of the present invention. The arrows in FIG. 1 indicate movements of a steel plate S and a punch 2

[0014] As illustrated in FIG. 1, to form the steel plate S into a cylindrical shape by the bending press method, the outer surface side of the steel plate S is supported on two linear dies 1a and 1b arranged in parallel with a certain space therebetween, and, at the same time, an operation of pressing down, with the punch 2, a position on the inner surface side of the steel plate S corresponding to the space between the dies 1a and 1b is repeatedly performed from both edges in the width direction toward the center in the width direction of the steel plate S. This can form the steel plate S into the cylindrical shape.

[Configuration of a pipe expanding machine]

[0015] Next, with reference to FIGS. 2 and 3, a description will be made of a configuration of a pipe expanding machine used in the method for manufacturing the steel pipe as according to the embodiment of the present invention. FIG. 2 is a schematic diagram illustrating the configuration of the pipe expanding machine used in the method for manufacturing the steel pipe as according to the embodiment of the present invention. FIG. 3 is a schematic diagram illustrating a section of the steel pipe. The configuration of the pipe expanding machine to be used in the present invention is not limited to the configuration illustrated in FIGS. 2 and 3.

[0016] As illustrated in FIG. 2, this pipe expanding machine 11 used in the method for manufacturing the steel pipe as according to the embodiment of the present invention includes a shaft-like boom 12 with the leading end thereof inserted from one end of a steel pipe P and a pipe expanding head 14 that is provided at the leading end of the boom 12 and expands the steel pipe P when the diameter is expanded in the direction orthogonal to the axis. The pipe expanding head 14 includes a tapered outer circumferential surface 15 and a pipe expanding die 16.

[0017] The tapered outer circumferential surface 15 is provided at the leading end of the boom 12, and decreases in diameter from the leading end side toward the base end side of the boom 12. The pipe expanding die 16 slidingly contacts the tapered outer circumferential surface 15, and includes a tapered inner circumferential surface 16a that decreases in diameter from the leading end side toward the base end side of the boom 12 and a die outer circumferential surface 16b that faces the inner circumferential surface of the steel pipe P. As illustrated in FIG. 3, a plurality of such pipe expanding dies 16 are provided along the circumferential direction of the tapered outer circumferential surface 15.

[0018] To expand the steel pipe P using the pipe expanding machine 11 having such a configuration, first, the pipe expanding dies 16 are positioned in a pipe expansion start position, and the boom 12 is moved back from the pipe expansion start position. This displaces each of the pipe expanding dies 16 slidingly contacting the tapered outer circumferential surface 15 in the radial direction by way of a wedge action, and expands the steel pipe P. The undulation in the cross-sectional shape of the steel pipe P becomes small, and the cross-sectional shape of the steel pipe P becomes close to a perfect circular shape.

[0019] Next, the boom 12 is moved forward to the pipe expansion start position, and after the pipe expanding dies 16 are returned inward in the direction orthogonal to the axis by a release mechanism, which is not illustrated, the boom 12 is further moved forward by an amount corresponding to a pitch (length in the axial direction) of the pipe expanding dies 16. The pipe expanding dies 16 are positioned in a new expansion position, and then, the operation described above is repeated. This can expand the steel pipe P over the entire length thereof in increments of the pitch of the pipe expanding dies 16.

[Embodiment]

[0020] FIGS. 4 and 5 are diagrams illustrating the outside diameter (solid line) and the inside diameter (dashed line) of the steel pipe P after the steel pipe P is formed by the forming step by the bending press method and expanded at a pipe expanding R ratio of 0.903 and 0.986, respectively. Each of FIGS. 4 and 5 illustrates a deviation from an average radius of radii at respective points of the steel pipe P at a scale enlarged by a common factor.

[0021] As illustrated in FIGS. 4 and 5, in the steel pipe P formed by the forming step by the bending press method, increasing the pipe expanding R ratio from 0.903 to 0.986 reduced the difference between the outside diameter and the inside diameter, and improved the roundness. No cracking of the pipe expanding dies was found after the pipe expanding R ratio was increased.

[0022] FIGS. 6 and 7 are diagrams each illustrating a relation among the pipe expanding R ratio, a punch R ratio, and the roundness of the steel pipe. The punch R ratio refers to a ratio of a curvature radius RP1 of the outer circumferential surface at the leading end of the punch to a target radius R0 of the inner surface of steel pipe P. As illustrated in FIG. 6, it could be demonstrated that the steel pipe P having high roundness is obtained when the pipe expanding R ratio and the punch R ratio are both less than 1, and satisfy Expression (1) given below. A straight line L1 in FIG. 6 is a straight line represented by a relational expression, Pipe expanding R ratio = -0.3 × punch R ratio + 1.18. As illustrated in FIG. 7, the value of the pipe expanding R ratio may be in a range from 0.94 to less than 1.

[0023] From what has been described above, it has been verified that the roundness of the steel pipe can be improved without cracking the pipe expanding die by forming the steel pipe using the forming step by the bending press method, and performing the forming step by the bending press method and the pipe expanding step so that the pipe expanding R ratio and the punch R ratio are both less than 1, and satisfy Expression (1) given above.

[0024] While the description has been made above of the embodiment to which the invention made by the inventors is applied, the present invention is not limited by the description and the drawings constituting parts of the disclosure of the present invention by way of the present embodiment. In other words, for example, other embodiments, examples, and operational techniques made, for example, by those skilled in the art based on the present embodiment are all included in the category of the present invention.

Industrial Applicability

[0025] According to the embodiment of the present invention, a steel pipe with high roundness can be manufactured without cracking the pipe expanding die.

Reference Signs List

[0026] 

1a, 1b

die

2

punch

11

pipe expanding machine

12

boom

14

pipe expanding head

15

tapered outer circumferential surface

16

pipe expanding die

16a

tapered inner circumferential surface

16b

die outer circumferential surface

P

steel pipe

S

steel plate

Claims

1. A method for manufacturing a steel pipe, the method comprising:

forming a steel plate into a cylindrical shape by a bending press method that presses down the steel plate with a punch having, at a leading edge thereof, an outer circumferential surface with a circular arcuate section so that edges in a width direction of the steel plate face each other;

butting and welding together the edges in the width direction of the steel plate; and

expanding the steel pipe from the inside using a pipe expanding machine in which a plurality of pipe expanding dies having an outer circumferential surface with a circular arcuate section are circumferentially arranged, wherein

a pipe expanding R ratio that is a ratio of a curvature radius of the outer circumferential surface of each of the pipe expanding dies to a target radius of an inner surface of the steel pipe and a punch R ratio that is a ratio of a curvature radius of an outer circumferential surface of a leading end of the punch to the target radius of the inner surface of the steel pipe are both less than 1, and the pipe expanding R ratio and the punch R ratio satisfy Expression (1) below:

2. The method for manufacturing the steel pipe according to claim 1, wherein the pipe expanding R ratio further falls in a range from 0.94 to less than 1.0.

Drawing