RU2729804C1 - Matrix and method of making steel pipe - Google Patents

Abstract

FIELD: metal forming.SUBSTANCE: group of inventions relates to metal forming and can be used for production of steel welded pipes of flexible sheet. Preform of a U-shaped section is formed by bending sheet material, an open pipe is formed with a gap for welding and form a pipe by connection of a gap section for welding. Workpiece of preliminary shape during pressure processing takes shape, central angles in which within appropriate arcs, having diameter equal to or substantially equal to outer diameter of steel pipe, are 70 degrees or more and 150 degrees or less, and the sum of the central angles within the arcs having the diameter equal to or substantially equal to the outer diameter of the steel pipe is less than 360 degrees. At that, pressure treatment is performed by means of a pair of semi-female dies having arc-shaped sections with radii of 0.96–1.04 of external radius of steel pipe.EFFECT: upgraded quality of welded pipes.7 cl, 10 tbl, 17 dwg

Description

The technical field to which the invention relates

The present invention relates to a die for use in a steel pipe forming process and a method for manufacturing a steel pipe using the die.

State of the art

UOE forming methods are widely used for forming steel pipes. In UOE forming methods, the steel sheet is first pressurized to be U-shaped, and then the sheet is pressurized to be O-shaped by molding an open pipe, which is a tubular body having a weld gap portion between the edge portions of the sheet. located opposite each other in the circumferential direction. A portion of the weld gap of the open pipe is butted and welded to form a steel pipe, which is further expanded so that the diameter of the steel pipe is increased. However, the UOE forming method requires a large pressure force in the pressure bending process of the steel sheet to be U-shaped or O-shaped for forming an open pipe, and inevitably requires the use of large-sized pressing equipment.

In addition, in the manufacture of a steel pipe, a method of forming an open pipe by applying a reduced pressure force is used. For example, in practice, a pressure bending process is used in which portions of the edges in the width direction of the steel sheet are bent to produce curved portions of the edges, after which three-point pressure bending is performed repeatedly with a punch supported on a punch support and a die to make the steel sheet approximately circular. forms. The size of the open-ended weld gap of an open pipe obtained by forming by a pressure bending process is greater than the width of the punch support. If the size of the open weld gap is too large, the force required to join the opposite portions of the sheet edges and close the weld gap is increased in order to weld the weld gap. In this case, more powerful equipment is required to close the section of the weld gap. In addition, after welding the weld gap portion with an excessive amount of the open weld gap portion, the welded portion receives a spring force that opens the weld gap portion and tends to cause a weld defect. If the specified force is very large, the welded section is destroyed.

Methods for reducing the size of the open weld gap of an open pipe after pressure bending are described in Patent Literature 1-4. Patent Literature 1 describes a method for reducing the size of the open section of the weld gap of an open pipe by using a pivot joint section between the front end of the punch and the punch support to reduce the width of the punch support. Patent Literature 2 describes a method for reducing the size of the open section of the weld gap of an open pipe using a means for maintaining the gap to limit the movement of the sheet material in the direction perpendicular to the direction of movement of the punch, and applying a large pressure during the final bending without bringing the edge sections of the sheet into contact with the punch support. ... Patent Literature 3 describes a method for reducing the open pipe weld gap size by measuring the gap between the sheet edge portion and the punch support after the final pressure process and minimizing the gap. Patent Literature 4 describes a method for reducing the size of the open section of the weld gap of an open pipe regardless of the shape deviation during pressure bending, in which the value of the punch pressure at the final stage is determined based on the point in time when the distance between the sections of the edges of the sheet becomes equal to a predetermined value in the process of applying pressure during the final bending process.

Unfortunately, the methods described in Patent Literature 1-4 do not allow reducing the size of the open area of the open pipe welding gap to a width less than the width of the punch support. Methods for reducing the size of the open weld gap by post-bending the open pipe are described in Patent Literature 5-9. Patent Literature 5 describes a method for forming a pipe with less force by hot forming a steel pipe after pressure bending. Patent Literature 6 discloses a pressure bending method in which a tilt sensor is provided for detecting tilt or tilt of a push member attached to a slider, the push member being positioned to tilt or translate in response to tilt or tilt detection by the sensor skewing, and when the workpiece material is pressurized to form a pipe, the pressing member is tilted or translationally moved by an amount of inclination to reduce the amount of skewing. Patent Literature 7 describes a method in which a slotted pipe having a non-circular preliminary shape is molded by a slight change in shape compared to other bending steps, with at least one bending step affecting the inside of the sheet material from the right and the left sides with respect to the center defined by the longitudinal centerline of the tool located on the upper side of the gradually formed sheet material, after which the production of the slotted pipe is completed by applying in each case an appropriate pressure force acting on the sections previously subjected to a slight change in shape, with both sides of the center of the non-circular preform from the outside. Patent Literature 8 describes a method in which, in a preform having a flat portion between portions bent at least two pipe bends, plastic deformation is applied to at least one flat portion at a pre-formed pipe bend with a closed area with a slot. Patent Literature 9 describes a method for forming a closed slotted pipe. This method includes providing a small curved portion with slightly less curvature than other portions, or providing a non-curved non-curved portion for forming a preform, and applying a bending force without limiting the slightly curved portion or non-curved portion, which allows you to obtain an open pipe by means of pressure treatment from a preform. During the application of the bending force, it is recommended that the preform is held in the die in a U-shaped state with the open portion facing upward and supported at its lowest end.

List of opposed documents

Patent Literature

Patent Literature 1: Japanese Laid-open Patent Application No. 2004-82219

Patent Literature 2: Japanese Laid-open Patent Application No. 2011-56524

Patent Literature 3: WO2014 / 188468

Patent Literature 4: WO2014 / 192043

Patent Literature 5: Japanese Laid-open Patent Application No. 2005-324255

Patent Literature 6: Japanese Patent Laid-Open No. 2005-21907

Patent Literature 7: Japanese Laid-open Patent Application No. 2012-250285

Patent Literature 8: US Patent No. 4149399

Patent Literature 9: WO2016 / 084607

Disclosure of the essence of the invention

Technical problem

Unfortunately, the method described in Patent Literature 5 entails a significant increase in cost when considering the consumption of thermal energy for heating. In addition, in this method, if the sheet material made by the thermomechanical processing step is used to provide strength, toughness and weldability, the material performance may deteriorate. In the methods described in Patent Literature 6-8, a preform material with a non-circular preform is molded separately from the right side and the left side. If the amount of deformation on the right and left sides is different, there may be a level difference (misalignment) in the weld gap or in the notched portion serving as the weld portion. In these methods, deformation to obtain the desired shape in a separate step causes a local concentration of deformation that can degrade the roundness of the steel pipe. For this reason, deformation at multiple stages becomes unavoidable and causes limitation of effective molding. In the method described in Patent Literature 9, since the radius of the lower die is larger than the outer diameter of the pipe, the lowermost portion of the U-shaped preform bends backward causing deformation that opens the gap portion. This can inhibit the clearance in the slotted portion.

The present invention has been designed with the above problems in mind. An object of the present invention is to provide a die and a method for manufacturing a steel pipe for efficiently forming a steel pipe with proper roundness.

Solution to the problem

To solve the problem and solve the problem, a matrix is proposed for use in the process of forming a steel pipe according to the present invention, and the process of forming a steel pipe includes forming a preform having a U-shaped cross section by bending a sheet material, forming an open pipe which is a tubular body having a weld gap in the longitudinal direction of an open pipe by pressure forming a preform, and forming a steel pipe by joining a weld gap, the matrix being used in the step of pressure forming of a preform to obtain an open pipe. The die contains: a pair of half-matrices containing the first half-matrix and the second half-matrix, wherein the preform is placed on the second half-matrix, so that the first half-matrix is located opposite the U-shaped open side of the preform, and the preform is subjected to pressure processing, while the preform is the preliminary form is held between a pair of semi-matrices; and an arcuate portion formed in the surface of each half-matrix to be in contact with the preform so that the center of the arc is located at a position coincident with the bending center of the half-matrix, the arcuate portion having a diameter equal to or substantially equal to the outer diameter of the steel pipe, and the arcuate portion in each half-matrix has a central angle equal to or greater than 70 degrees, and in total, the central angles of the arcuate portions of both half-matrices are less than 360 degrees.

In addition, in the matrix of the present invention, each half-matrix comprises linear portions or arcuate portions of slight curvature having a curvature less than the curvature of the arcuate portion, wherein linear portions or arcuate portions of slight curvature are connected to both ends of the arcuate portion in the direction of the arc.

In addition, in the matrix of the present invention, the central angles of the arcuate portions of both half-matrices are equal to each other.

In addition, a method for manufacturing a steel pipe of the present invention is a method including: forming a preform having a U-shaped section by bending at least once a sheet material that has been folded at both ends in the width direction of the workpiece preliminary form; forming an open pipe, which is a tubular body having a weld gap in the longitudinal direction, by pressurizing a preform and forming a steel pipe by joining a weld gap, the preform being shaped during forming such that the center corners in the ranges corresponding to arcs having a diameter equal to or substantially equal to the outer diameter of the steel pipe are 70 degrees or more with the midpoints of the arcs located at the butting portion of both ends along the width of the sheet, and the lowermost portion of the U-shaped section, and the blank is preliminary the shape during forming takes on such a shape that in total the center angles in the ranges corresponding to arcs having a diameter equal to or substantially equal to the outer diameter of the steel pipe are less than 360 degrees.

In addition, in the method for manufacturing a steel pipe of the present invention, the preform is not in contact with the semi-matrices at portions outside the ranges corresponding to the arcs.

In addition, in the method for manufacturing a steel pipe of the present invention, the center angles in the ranges corresponding to the arcs are equal center angles in the range with the midpoints of the arcs located at the joint portion of both ends along the width of the sheet, and in the range with the midpoints of the arcs located on with the lower section of the U-shaped section.

In addition, the method for manufacturing a steel pipe of the present invention uses a die of the present invention.

Advantageous Effects of the Invention

The die and the method for manufacturing a steel pipe of the present invention provide an advantageous effect of being able to efficiently form a steel pipe with proper roundness.

Brief Description of Drawings

FIG. 1 is an external perspective view of a die and punch for use in forming a U-shaped preform using the pressure bending process of an embodiment;

fig. 2 is a flow diagram of a process for molding a preform having a U-shaped cross section using a pressure bending process;

fig. 3 is a cross-sectional view of a preform having a U-shaped section;

fig. 4 is a flow chart of a process for forming an open pipe by final forming a preform;

fig. 5 - arcuate sections, linear sections and central corners of the upper half-matrix and the lower half-matrix;

fig. 6 is a graph showing the relationship between the open weld gap size of an open pipe and the limiting angle in terms of pressure;

fig. 7 is a flow diagram of a deformation state during molding of an open pipe using an upper half-matrix and a lower half-matrix with a limiting angle of 0 degrees;

fig. 8 is a graph showing the relationship between the limiting angle and the roundness of a steel pipe before expanding the pipe when the weld gap portion of the open pipe is closed by welding;

fig. 9 is a graph showing the relationship between limiting angle and pressure force;

fig. 10 is a graph showing the magnitude of the open section of the weld gap for an open pipe versus changes in the individual limiting angles of the upper half-matrix and the lower half-matrix;

fig. 11 is a graph showing the roundness of a steel pipe before expanding a pipe formed by closing a weld gap portion of an open pipe by welding as a function of changes in the individual limiting angles of the upper half-matrix and the lower half-matrix;

fig. 12 is a graph showing the force of pressure versus changes in the individual limiting angles of the upper half-matrix and the lower half-matrix;

fig. 13 is a graph showing the magnitude of an open portion of the weld gap when the limiting angle of the upper half matrix and the limiting angle of the lower half matrix are the same, and the length of the slightly curved portion or non-curved portion of the preform is changed after pressure bending;

fig. 14 is a graph showing the roundness of a steel pipe before expanding the pipe when the limiting angle of the upper half-matrix and the limiting angle of the lower half-matrix are the same, and the length of a slightly curved portion or non-curved portion of a preform is changed after pressure bending;

fig. 15 is a graph showing pressure force when the limiting angle of the upper half-die and the limiting angle of the lower half-die are the same, and the length of a slightly curved portion or non-curved portion of a preform is changed after pressure bending;

fig. 16 is a graph showing the magnitude of the open section of the weld gap for an open pipe depending on the change in the radii of the arcuate sections of the upper half-matrix and the lower half-matrix;

fig. 17 is a graph showing the force of pressure versus changes in the radii of the arcuate portions of the upper half-matrix and the lower half-matrix.

Implementation of the invention

The following is a description of an embodiment of an injection die and a method for manufacturing a steel pipe using the injection die of the present invention. FIG. 1 shows a perspective external view of a die 1 and a punch 2 for use during forming a preform having a U-shaped section using the pressure bending process of the present embodiment. The die 1 is disposed on a sheet material conveying path containing a plurality of conveying rollers 3 for the sheet material S and comprises a pair of bar-shaped members 1a and 1b, left and right, for supporting the sheet material S at two points in the conveying direction of the sheet material. The distance e between the bar-shaped members 1a and 1b in the conveying direction of the sheet material can be varied depending on the size of the finished steel pipe.

The punch 2 is movable towards and away from the die 1 and has a downwardly projecting front end 2a of the punch for pressurizing the material S and a punch support 2b of the same width extending towards the rear surface (upper end surface) from the front end 2a of the punch to support the front end 2a of the punch. The punch support 2b has an upper end connected to a drive means, not shown. The drive means applies a pressing force to the front end 2a of the punch.

FIG. 2 shows the process of forming a preform S ₁ having a U-shaped cross section by pressure bending. This process is particularly illustrated by an example in which the pre-folded sheet material S is bent, the sheet material S being fed from top to bottom in the left column in FIG. 2, then from top to bottom in the middle column of FIG. 2, and finally bending is performed as shown in the right column in FIG. 2. The arrows near the punch 2 and the sheet material S in FIG. 2 indicate the direction in which the punch 2 or sheet material S moves at each step.

For shaping the sheet material S into a tubular shape using the sheet material S as a starting material, first of all, the edge bending of the sheet material S is preliminarily performed. This edge bending is performed at the end portion along the width of the sheet, the bending of which is relatively difficult compared to the flexible one performed on the sheet material S using a die 1 and a punch 2. Provided that curved edge portions are provided across the width at the end portions of the sheet material S by bending the edges, it is easy to ensure that the steel pipe is properly rounded compared to when the curved edge portions not provided. The roundness of a steel pipe is a measure of how close the cross-sectional shape of a steel pipe is to a circle and is a ratio obtained by dividing the difference between the maximum and minimum deviation from the approximate arc over the entire circumference of the steel pipe by the diameter of the steel pipe. For example, a steel pipe having an outer diameter D is divided into 8 equal parts, 12 equal parts, 16 equal parts, or 24 equal parts in the circumferential direction of the pipe at any given pipe length, and the outside diameters are measured at opposite positions. If the maximum diameter and minimum diameter of the measured outer diameters are equal to D _max and D _min , respectively, the roundness [%] is defined as {(D _max -D _min ) / D} × 100. The closer the roundness is to zero, the closer the cross-sectional shape of the steel pipe is to a perfect circle.

A sheet material S having curved edge portions is placed on a die 1 as shown in FIG. 1. While the sheet material S is periodically conveyed by a predetermined feed amount, bending (three-point bending) of the entire sheet material S is performed by the process shown in FIG. 2 to form a preform S ₁ having a generally U-shaped cross-section.

FIG. 3 is a cross-sectional view of a preform S ₁ having a U-shaped section. As shown in FIG. 3, a part of the preform S ₁ has a non-bent portion P that does not undergo bending and is located, in particular, approximately in the section W / 4 away from each of the end portions across the width of the sheet. This unbent portion P can be obtained by increasing the feed amount of the sheet material S and eliminating the pressure treatment with the punch 2. The part of the preform S ₁ instead of the unbent portion P may have a slightly bent portion with a curvature less than the curvature of other portions (with a small curvature compared with other sections), located, in particular, approximately in section W / 4 to the side of each of the end sections across the width of the sheet. In this case, if necessary, in the description below, the expression "non-curved portion P" can be read as "slightly curved portion". A slightly curved section can be obtained by applying a lower pressure to it with the punch 2 compared to other sections.

The punch 2 shown in FIG. 1 and FIG. 2 has an I-shape in which the width of the punch front end 2a in the sheet material conveying direction is equal to the width of the punch support 2b in the sheet material conveying direction. However, the shape of the punch 2 is not limited to this. For example, a punch 2 having an approximately inverted T shape may be used, in which the width of the front end 2a of the punch in the direction of conveying the sheet material is greater than the width of the punch support 2b in the direction of conveying the sheet material. If the width of the punch support 2b in the conveying direction of the sheet material is the same, the punch 2 having an approximately inverted shape T can apply pressure to a larger area of the sheet material S in one stroke compared to the punch 2 having an I-shape, thereby reducing number of clicks.

After performing the bending pressure of the sheet material S to form the preform S ₁ pre-form having a U-shaped cross section, operate final shaping to give O-shaped S ₁ preform having a preform, using the upper half-matrix 4 and a lower half-matrix 5, as shown in fig. 4, thereby obtaining an open pipe S ₂ which is a tubular body having a weld gap portion G between the widthwise end portions of the sheet opposite each other in the circumferential direction.

A process for performing the final shaping of the preform S ₁ to obtain an open tube S _{2 is} described below with reference to FIG. 4. First of all, as shown in FIG. 4 (a), the preform S _{1 is} positioned in the lower die 5 such that the upper die 4 and the U-shaped open side of the preform S ₁ are opposite each other (so that the U-shaped open side of the preform S ₁ facing up), and the preform S _{1 is} held between the upper half-matrix 4 and the lower half-matrix 5. As shown in FIG. 5, the surfaces of the upper half-matrix 4 and the lower half-matrix 5. which can be in contact with the preform S ₁ have arcuate portions 4a and 5a, respectively, with a diameter equal to or substantially equal to the outer diameter of the steel pipe to be formed, and with central angle θ. Hereinafter, the center angle θ of the arcuate portions 4a and 5a will be referred to as a limiting angle. The arcuate portion 4a has an arc center at a position coincident with the bending center O _{p4 of the} upper half-matrix 4. The arcuate portion 5a has an arc center at a position coinciding with the bending center O _{p5 of the} lower half-matrix 5. The upper half-matrix 4 has linear portions 4b ₁ and 4b ₂ , connected to both ends in the direction of the arc of the arcuate portion 4a. The lower half-matrix 5 has linear portions 5b ₁ and 5b ₂ connected to both ends in the arc direction of the arcuate portion 5a. Instead of linear portions 4b ₁ , 4b ₂ , 5b ₁ , and 5b ₂ , the upper half-matrix 4 and the lower half-matrix 5 may have arcuate portions of slight curvature having a curvature less than that of the arcuate portions 4a and 5a.

In the present invention, it is preferable that the linear portions 4b ₁ and 4b ₂ connected to the arcuate portion 4a are symmetrical about the bending center O _p4 , i.e. the center of the arcuate portion 4a from the point of view of improving the symmetry of the finished steel pipe. Similarly, it is preferred that the arcuate portions of slight curvature provided in place of the linear portions 4b ₁ and 4b ₂ connected to the arcuate portion 4a are symmetrical about the bending center O _p4 , i.e. the center of the arcuate portion 4a from the point of view of improving the symmetry of the finished steel pipe. Likewise, it is preferable that the linear portions 5b ₁ and 5b ₂ connected to the arcuate portion 5a are symmetrical about the bending center O _p5 , i.e. the center of the arcuate portion 5a in terms of improving the symmetry of the finished steel pipe. Similarly, it is preferable that the arcuate portions of slight curvature provided instead of the linear portions 5b ₁ and 5b ₂ connected to the arcuate portion 5a are symmetrical about the bending center O _p5 , i.e. the center of the arcuate portion 5a in terms of improving the symmetry of the finished steel pipe.

Subsequently, the preform S ₁ held between the upper half-matrix 4 and the lower half-matrix 5 is pressed by the upper half-matrix 4 and is subjected to final shaping, as shown in FIG. 4 (b). In this case, the portions of the preform S ₁ that are located opposite the arcuate portions 4a and 5a of the upper half-matrix 4 and the lower half-matrix 5 are limited by the upper half-matrix 4 and the lower half-matrix 5, while the unbent areas P of the preform S ₁ are not limited to the upper half-matrix 4 and lower half-matrix 5. Thus, the open pipe P ₂ , as shown in FIG. 4 (c) can be molded with a pressure force less than the pressure force required when the entire circumference of the preform S _{1 is} limited by the upper half-die 4 and the lower half-die 5.

In the case under consideration in the present invention, when the open pipe S _{2 is} formed by the final molding of the preform S ₁ using the upper half matrix 4 and the lower half matrix 5, a pressure force is applied to the W / 4 portion away from the center of the non-bent portion P towards the end portion along the width of the workpiece S _{1 of the} preliminary form. The reason for this is as follows. When the entire preform S ₁ takes the shape of a circle, the bending moment M = F⋅r⋅cosϕ (F is the pressure force, r is the radius of the circle) at the position where the central angle is away from the pressure application area, which corresponds to the angle ϕ, and is greatest at a position away from the pressure application area, which corresponds to an angle of 90 degrees, where the deformation is also greatest. The pressure force is applied at a position away from the center of the unbent portion P, which corresponds to an angle of 90 degrees, i.e. 1/4 of the whole circle, whereby the unbent portion P is effectively deformed. In this case, the bending moment is greatest in a position away from the position receiving pressure forces, which corresponds to an angle of 90 degrees, and decreases as the distance from this position increases. Based on this, it is preferable that a pressure force is applied to the section away from the center of the unbent portion P in the direction of the end portion in the width, which corresponds to W / 4 ± 0.07W, to ensure sufficient plastic deformation in the unbent portion P.

In the present embodiment, the center of the non-bent portion P is located in a section including a position away from the end portion in a width that corresponds to W / 4. The reason for this is as follows. Although it is preferable that a pressure force is applied to the section away from the center of the non-bent portion P in the direction of the end portion in the width, which corresponds to W / 4, as described above, the contact position between the upper half-die 4 and the preform S ₁ changes at the stage of forming the preform S ₁ in order to obtain an open pipe S ₂ . When the unbent portion P is located in a section including a position away from the end portion in width corresponding to W / 4 in the preform S ₁ , the pressure receiving portion is always the end portion of the width of the preform S ₁ so that the unbent portion is the most deformable. Thus, it is possible to deform the unbent portion P in one stroke without changing the position of applying the pressure. In addition, it is preferable that the conditions are ensured under which the unbent portion P is in the range W / 4 ± 0.07W from the position of receiving the pressure force, i.e. the end section along the width of the preform S ₁ .

Since the end portions across the width of the sheet are in contact with the upper half-die 4 in the initial forming state, as shown in FIG. 4 (a) and FIG. 4 (b), it is preferable that the unbent portion P is located in a section including a section away from the end portion across the width of the preform S ₁ , which corresponds to W / 4.

FIG. 6 is a graph showing the relationship between the size of the open section G of the weld gap of the open pipe S ₂ and the limiting angle in combination with the pressure force. The relationship between the open area size and the bounding angle shown in FIG. 6, and the pressure value is the relationship obtained for a steel pipe with a tensile strength of 630 MPa, an outer diameter of 660.4 mm and a pipe thickness of 40.0 mm, formed by welding both edges of the open pipe S _2, and then performing shape correction by expansion pipes with a pipe expansion coefficient of 1%.

The preform S ₁ after pressure bending has an unbent portion P of length W / 12 in the W / 4 portion from each of the ends along the sheet width on both sides, and this preform S _{1 is} held between the upper half-matrix 4 and the lower half-matrix 5 with one and the same bounding angle. The pressure value is set such that the distance between the portions W / 2 of the open pipe S ₂ is equal to the diameter before the pipe expansion (the pressure value during the final molding is set so that the diameter in the longitudinal direction matches the diameter before the pipe expansion). As can be seen from FIG. 6, the larger the limiting angle, the smaller the open-ended portion G of the weld gap of the open pipe S ₂ .

FIG. 7 (a) to 7 (c) schematically show a deformation state when an open pipe S ₂ is formed using an upper half-matrix 4 and a lower half-matrix 5 with a limiting angle of 0 degrees. When the limiting angle of the upper half-matrix 4 and the lower half-matrix 5 is 0 degrees, the arcuate portions 4a and 5a are arcs having a diameter equal to 1.16 of the outer diameter of the steel pipe, so that the upper half-matrix 4 is in contact only with both edges of the preform S _1. forms, and the lower half-matrix 5 is in contact only with the central region along the width of the sheet of the blank S _{1 of the} preliminary form. As shown in FIG. 7 (a), the diameter of the arcuate portion 5a of the lower half-matrix 5 is greater than the diameter of the steel pipe, so that when comparing the cross-section of the preform S ₁ with the clock, only the 6 o'clock portion is in contact with the lower half-matrix 5. Thus, as shown in FIG. ... 7 (b), the 6 o'clock portion of the preform S ₁ and the adjacent portions are bent back so that they correspond to the arcuate portion 5a of the lower half matrix 5 during final forming, and the radius of curvature becomes larger than the diameter of the steel pipe. As a result, after the final molding, as shown in FIG. 7 (c), the size of the open portion G of the weld gap of the open pipe S ₂ is large in combination with the springback at the "3 o'clock" portion and the "9 o'clock" portion of the preform S ₁ .

FIG. 8 is a graph showing the relationship between the limiting angle and the roundness of the steel pipe before expanding the pipe when the weld gap portion G of the open pipe S ₂ is closed by welding. As can be understood from FIG. 8, when the bounding angle is 60 degrees, the roundness will be worse than when the bounding angle is 0 degrees. However, as the limiting angle increases, the roundness improves. When the bounding angle is 70 degrees or more, the roundness will be better than when the bounding angle is 0 degrees. You can also understand that roundness improves most when the bounding angle is between 100 degrees and 110 degrees.

FIG. 9 is a graph showing the relationship between the limiting angle and the pressure force. As can be understood from FIG. 9, when the limiting angle increases, the pressure force also increases. An increase in the limiting angle reduces the size of the open section G of the weld gap for the open pipe S ₂ , but the increased pressure force requires the use of more powerful pressing equipment. Therefore, it is preferable to reduce the limiting angle in the range in which the required size of the open area of the weld gap is achieved. For example, the limiting angle is set to 150 degrees or less to set the pressure force to 90% or less of the pressure force required when the separate limiting angles of the upper half-matrix 4 and lower half-matrix 5 to constrain the entire circumference of the preform S ₁ with the upper half-matrix 4 and the lower half-matrix 5 are equal to 180 degrees.

FIG. 10 is a graph showing the magnitude of the open area G of the weld gap of the open pipe S ₂ as a function of the variation of the individual limiting angles of the upper half-matrix 4 and the lower half-matrix 5. FIG. 11 is a graph showing the roundness of a steel pipe before expanding a pipe formed by closing the weld gap G of an open pipe S ₂ by welding as a function of changes in the individual limiting angles of the upper half matrix 4 and the lower half matrix 5. FIG. 12 is a graph showing the force of pressure as a function of changes in the individual limiting angles of the upper half matrix 4 and the lower half matrix 5. The data in FIG. 10-12 were obtained for a steel pipe with a tensile strength of 630 MPa, an outer diameter of 660.4 mm and a pipe thickness of 40.0 mm, i.e. these characteristics are the same as those in relation to FIG. 6, figs. 8 and FIG. 9. The horizontal axis represents the mean of the limiting angles of the upper half-matrix 4 and the lower half-matrix 5, and the different limiting angles in the lower half-matrix 5 are indicated by different symbols. In these figures, for example, the expression "bottom 60 degrees" means that the limiting angle in the bottom half-matrix 5 is 60 degrees.

As can be understood from FIG. 10, regardless of the individual limiting angles of the upper half-matrix 4 and the lower half-matrix 5, when the average value of the limiting angles of the upper half-matrix 4 and the lower half-matrix 5 increases, the size of the open area G of the weld gap S ₂ decreases. As can be understood from FIG. 11, when the limiting angle of one of the half-matrix, the upper half-matrix 4 or the lower half-matrix 5 becomes less than 60 degrees, the roundness of the steel pipe deteriorates. Accordingly, although the individual limiting angles of the upper half-matrix 4 and the lower half-matrix 5 need not be equal, it is desirable that the limiting angles of the upper half-matrix 4 and the lower half-matrix 5 exceed 60 degrees to obtain a satisfactory roundness of the steel pipe. From FIG. 12 it can also be understood that the larger the average value of the limiting angles of the upper half-matrix 4 and the lower half-matrix 5, the greater the pressure force. Therefore, when the upper limit value of the allowable pressure force is set, the range of the average value of the used limiting angles of the upper half matrix 4 and the lower half matrix 5 can be determined from the upper limit value of the pressure force.

FIG. 13 is a graph showing the magnitude of the open portion G of the weld gap when the limiting angle of the upper half matrix 4 and the limiting angle of the lower half matrix 5 are the same, and the length L of the unbent portion P of the preform S ₁ changes after pressure bending. FIG. 14 is a graph showing the roundness of a steel pipe before expanding the pipe when the limiting angle of the upper half-matrix 4 and the limiting angle of the lower half-matrix 5 are the same, and the length L of the unbent portion P of the preform S ₁ changes after pressure bending. FIG. 15 is a graph showing the pressure force when the limiting angle of the upper half-matrix 4 and the limiting angle of the lower half-matrix 5 are the same, and the length L of the unbent portion P of the preform S ₁ changes after pressure bending. FIG. 13-15 the horizontal axis shows the average value of the limiting angle of the upper half-matrix 4 and the limiting angle of the lower half-matrix 5.

As can be understood from FIG. 13, regardless of the length L of the unbent portion P of the preform S ₁ , when the average value of the limiting angle of the upper half-matrix 4 and the average value of the limiting angle of the lower half-matrix 5 increase, the amount of the open portion G of the weld gap increases. It is also clear that when the average value of the limiting angle of the upper half-matrix 4 and the average value of the limiting angle of the lower half-matrix 5 are the same, the larger length L corresponds to the smaller value of the open area G of the weld gap. As can be understood from FIG. 14 and FIG. 15, when the mean value of the limiting angle of the upper half-matrix 4 and the limiting angle of the lower half-matrix 5 are the same, there is no significant difference in the roundness of the steel pipe and the pressure on it due to the length L of the unbent portion P of the preform S ₁ . Thus, when the mean value of the limiting angle of the upper half-matrix 4 and the limiting angle of the lower half-matrix 5 are the same, the size of the open portion G of the weld gap of the open pipe S ₂ can be reduced by increasing the length L of the unbent portion P of the preform S ₁ without causing a difference in roundness steel pipe and the force of pressure on it, which is due to the length L.

FIG. 16 is a graph showing the magnitude of the open section G of the weld gap of the open pipe S ₂ as a function of the change in the radii of the arcuate sections of the upper half-matrix 4 and the lower half-matrix 5. FIG. 17 is a graph showing the force of pressure as a function of changes in the radii of the arcuate portions of the upper half-matrix 4 and the lower half-matrix 5. FIG. 16 and FIG. 17, the central angles of the arcuate portions 4a and 5a of the upper half-matrix 4 and the lower half-matrix 5 are 45 degrees, and as the radii of the arcuate portions, which are the radii of the arcuate portions 4a and 5a, change, a steel pipe with a tensile strength of 630 MPa, an outer diameter of 660, 4 mm and a pipe thickness of 40.0 mm is pressurized by final shaping so that the diameter in the longitudinal direction matches the diameter before expanding the pipe. FIG. 16 and FIG. 17, the horizontal axis represents the relationship between the radius of the arcuate portion and the outer radius of the steel pipe (the radius corresponding to the outer diameter of the steel pipe). When the radius of the arcuate portion is greater than the outer radius of the steel pipe, this ratio is greater than 1.0, and when the radius of the arcuate portion is less than the outer radius of the steel pipe, the ratio is less than 1.0.

As shown in FIG. 16, when the radius of the arcuate portion is equal to the outer radius of the steel pipe (1.0 on the horizontal axis in FIG. 16), the amount of the open-ended portion G of the weld gap is the smallest. On the other hand, when the radius of the arcuate portion is larger than the outer radius of the steel pipe, spring deformation occurs in the 6 o'clock portion of the preform S ₁ and nearby portions as shown in FIG. 7 so that the size of the open-ended portion G of the weld gap increases as the radius of the arcuate portion increases. When the radius of the arcuate portion is less than the outer radius of the steel pipe, spring deformation occurs in portions where the arcuate portions 4a and 5a of the upper half-matrix 4 and the lower half-matrix 5 end, so that the size of the open-ended portion G of the weld gap increases as the radius of the arcuate portion decreases. Thus, although it is most preferable that the radius of the arcuate portion is equal to the outer radius of the steel pipe, the value of the open-ended portion G of the weld gap remains 40 [mm] or less when the radius of the arcuate portion is a radius equivalent to the outer radius of the steel pipe ± 3.5%.

However, as can be understood from FIG. 17, the pressure force increases as the radius of the arcuate portion decreases. In particular, when the radius of the arcuate section is small, it is necessary to determine the radius, taking into account the load on the pressing equipment.

Example 1

A weld-edged steel sheet having a width of 1928 mm, a length of 1000 mm, a thickness of 40 mm and a tensile strength of 635 MPa was flanged followed by flexible pressure to obtain a preform S ₁ . Subsequently, the final shaping of the preform S ₁ was carried out using a 30 MN press equipment using the upper half matrix 4 and the lower half matrix 5 with different limiting angles to form the preforms A, B and C. Tables 1-3 show the preform profiles A, B and C. In Tables 1-3, the initial alphabetic characters A, B, C in the "#" column denote the profiles of the preforms (preforms A, B and C), and the numbers following the alphabetic characters A, B, and C denote a combination of limiting corners of the upper half-matrix 4 and the lower half-matrix 5.

Table 1 shows the data for preform A with a 161 mm wide unbent portion (W / 12) at approximately W / 4 from the edge of the sheet as condition A. Table 2 shows data for preform B with 321 unbent portion mm (W / 6) (double the width in relation to the width of condition A) at approximately W / 4 from the edge of the sheet as condition B. Table 3 shows data for a preform C with an unbent area of 321 mm wide approximately at the area W / 6 from the edge of the sheet as condition C. Preforms A, B and C are symmetrical about a straight line connecting the center of the edge of the sheet and 1/2 of the sheet width, and tables 1-3 indicate 1/2 of the sheet width ... The final molding pressure was set such that the distance between the outer surface side of the W / 2 portion and the outer surface side of the sheet edge was 654 mm.

After measuring the size of the open weld gap of the open pipe S ₂ after the final shaping of the preforms A, B, and C, the weld gap G of the open pipe S ₂ was welded to form a steel pipe having an outer diameter of 654 mm. Thereafter, the diameter of the steel pipe was measured at eight points in 22.5 degree increments in the circumferential direction, and the difference between the maximum diameter and the minimum diameter was obtained. Tables 1-3 also show the profile of the semi-matrix (limiting angle), the pressure force, the values of the open area of the gap for welding and roundness. In this case, the roundness is the value obtained by dividing the difference between the maximum and the minimum by the outer diameter of the steel pipe (the average of all measured diameters).

The welder used in this example was unable to close a hole in a pipe having an open weld gap greater than 40 mm after final shaping. In this case, both ends and the center in the axial direction of the pipe were temporarily welded to close the hole using another pressing equipment, after which the section G of the weld gap was finally welded along the entire length. A roundness of 2.5% before pipe expansion was accepted as acceptable. This is because if the roundness before pipe expansion is 2.5% or less, the roundness after pipe expansion is considered satisfactory if it is 1% or less.

In samples No. A1-A7, A9 and A10 in Table 1, No. B1-B7, B9 and B10 in Table 2 and No. C1-C7, C9 and C10 in Table 3, which are included in the group of samples of the present invention, the value the open area of the weld gap is small, and the roundness is also satisfactory. In particular, specimens with a restraining angle between 90 degrees and 110 degrees have a roundness of 1.0% or less even without pipe expansion. The lower the average value of the limiting angle, the lower the pressure force.

In contrast, in samples # A8 and A11 in Table 1, # B8 and B11 in Table 2, and # C8 and C11 in Table 3, in which the limiting angles of the upper half-matrix 4 and the lower half-matrix 5 are a combination of 60 degrees and 90 degrees, the size of the open area of the weld gap is small, but the roundness is unsatisfactory. In specimens No. A12 to A16 in Table 1, No. B12 to B16 in Table 2, and No. C12 to C16 in Table 3, in which the average limiting angle is 60 degrees or less, the open weld clearance is large. Particularly, in the sample No. A15 and A16 in Table 1, Sample No. B16 in Table 2, and Sample No. C16 in Table 3, the roundness cannot be measured because the welded portion collapsed after welding the G-gap section.

In a specimen formed using a preform B having a non-bent portion wider than a non-bent portion of a preform A compared to a specimen formed using a preform A, the pressure force and roundness are almost the same, but the size of the open weld gap small.

In the sample formed using the preform C, in which the position of the non-bent portion is closer to the edge of the sheet than in the preform B, as compared to the sample formed using the preform B, the pressure force, the size of the open area of the weld gap and the roundness is almost the same. In sample No. C17 in Table 3, in which the limiting angles of the upper half matrix 4 and the lower half matrix 5 are 180 degrees, despite the application of a maximum force of 30 MN / m press equipment, the distance between the outer surface of the section W / 2 and the outer surface of the sheet edge was 658 mm and the pressure was less than for other samples. Thus, the size of the open area of the weld gap is satisfactory, but the roundness is unacceptable. To achieve a 2.5% roundness before tube expansion, it may be necessary to perform a final press equivalent to the number of pressings for other products using a larger press.

Notwithstanding the foregoing description of the embodiments in which the present invention is employed, it is not intended to limit the present invention to the description and drawings that form part of the disclosure of the present invention by the embodiments. In other words, all other embodiments, examples, operating procedures and the like performed by those skilled in the art based on the embodiments are within the scope of the present invention.

Example 2

A weldable steel sheet having a width of 1639 mm, a length of 1000 mm, a thickness of 31.8 mm and a tensile strength of 779 MPa was folded and followed by flexible pressure to obtain a preform S ₁ . Subsequently, the final shaping of the preform S ₁ was carried out using a 30 MN press equipment using the upper half matrix 4 and the lower half matrix 5 with different limiting angles to form the preforms A, B and C. Tables 4-6 show the preform profiles A, B and C. In Tables 4-6, the initial alphabetic characters A, B, C in the "No." column denote the profiles of the preforms (preforms A, B and C), and the numbers following the alphabetic characters A, B, and C indicate a combination of limiting corners of the upper half-matrix 4 and the lower half-matrix 5.

Table 4 shows the data for a preform A with a 137mm wide (W / 12) unbent portion at approximately W / 4 from the edge of the sheet as condition A. Table 5 shows data for a preform B with a 273 unbent portion mm (W / 6) (double the width in relation to the width of condition A) approximately at area W / 4 from the edge of the sheet as condition B. Table 6 shows data for a preform C with an unbent area of 273 mm wide approximately at area W / 6 from the edge of the sheet as condition C. The preforms A, B, and C are symmetrical about a straight line from the center of the edge portion of the sheet to 1/2 the width of the sheet. Tables 4-6 indicate 1/2 sheet width. The final molding pressure was set such that the distance between the outer surface side of the W / 2 portion and the outer surface side of the sheet edge was 553 mm.

After measuring the size of the open weld gap of the open pipe S ₂ after the final shaping of the preforms A, B and C, the weld gap G of the open pipe S ₂ was welded to form a steel pipe having an outer diameter of 553 mm. Thereafter, the diameter of the steel pipe was measured at eight points in 22.5 degree increments in the circumferential direction, and the difference between the maximum diameter and the minimum diameter was obtained. Tables 4-6 also show the die profile (bounding angle), pressure force, open-ended weld gap and roundness. In this case, the roundness is a value obtained by dividing the difference between the maximum and minimum by the outer diameter of the steel pipe.

The welder used in this example was unable to close a hole in a pipe having an open weld gap greater than 40 mm after final shaping. In this case, both ends and the center in the axial direction of the pipe were temporarily welded to close the hole using another pressing equipment, after which the section G of the weld gap was finally welded along the entire length. A roundness of 2.5% before pipe expansion, which reaches 1.0% or less after pipe expansion, was accepted as acceptable.

In samples No. A1-A7, A9 and A10 in Table 4, No. B1-B7, B9 and B10 in Table 5 and No. C1-C7, C9 and C10 in Table 6, which are included in the group of samples of the present invention, the value the open area of the weld gap is small, and the roundness is also satisfactory. In particular, specimens with a restraining angle between 90 degrees and 110 degrees have a roundness of 1.0% or less even without pipe expansion. The lower the average value of the limiting angle, the lower the pressure force.

In contrast, in samples # A8 and A11 in Table 4, # B8 and B11 in Table 5, and # C8 and C11 in Table 6, where the limiting angles of the upper half matrix 4 and the lower matrix 5 are a combination of 60 degrees and 90 degrees, the size of the open area of the weld gap is small, but the roundness is unsatisfactory. In the samples No. A12 to A16 in Table 4, No. B12 to B16 in Table 5, and No. C12 to C16 in Table 6, in which the average value of the limiting angles is 60 degrees or less, the open weld clearance is large. Particularly, in the samples # A15 and A16 in table 4, sample # B16 in table 5 and sample # C16 in table 6, roundness cannot be measured because the welded portion collapsed after welding the G-gap section.

In a specimen formed using a preform B having a non-bent portion wider than a non-bent portion of a preform A compared to a specimen formed using a preform A, the pressure force and roundness are almost the same, but the size of the open weld gap small.

In the sample formed using the preform C, in which the position of the non-bent portion is closer to the edge of the sheet than in the preform B, as compared to the sample formed using the preform B, the pressure force, the size of the open area of the weld gap and the roundness is almost the same. In the sample # C17 in Table 6, in which the limiting angles of the upper half-matrix 4 and the lower half-matrix 5 are 180 degrees, despite the application of a maximum force of 30 MN / m press equipment, the distance between the outer surface side of the section W / 2 and the outer surface side of the sheet edge was 556 mm, and the pressure value was less than for other samples. Thus, the size of the open area of the weld gap is satisfactory, but the roundness is unacceptable. To obtain a roundness of 2.5% prior to pipe expansion, it may be necessary to perform a final shaping to the equivalent pressure in other specimens using high-pressure pressing equipment.

Example 3

A weldable steel sheet having a width of 2687 mm, a length of 1000 mm, a thickness of 50.8 mm and a tensile strength of 779 MPa was folded and followed by flexible pressure to obtain a preform S ₁ . Subsequently, the final shaping of the preform S ₁ was carried out using a 30 MN press equipment using the upper half matrix 4 and the lower half matrix 5 with different limiting angles to form the preforms A, B and C. Tables 7-9 show the preform profiles A, B and C. In Tables 7-9, the initial alphabetic characters A, B, C in the "No." column denote the profiles of the preforms (preforms A, B and C), and the numbers following the alphabetic characters A, B, and C indicate a combination of limiting corners of the upper half-matrix 4 and the lower half-matrix 5.

Table 7 shows the data for a preform A with a 224 mm wide (W / 12) unbent portion at approximately W / 4 from the edge of the sheet as condition A. Table 8 shows data for a preform B with a 448 unbent portion mm (W / 6) (double the width in relation to the width of condition A) at approximately the area W / 4 from the edge of the sheet as condition B. Table 9 shows the data for a preform C with a 448 mm wide unbent area approximately at the area W / 6 from the edge of the sheet as condition C. The preforms A, B, and C are symmetrical about a straight line from the center of the edge portion of the sheet to 1/2 the width of the sheet. Tables 7-9 indicate 1/2 sheet width. The final molding pressure was set such that the distance between the outer surface side of the W / 2 portion and the outer surface side of the sheet edge was 905 mm.

After measuring the size of the open weld gap of the open pipe S ₂ after the final shaping of the preforms A, B, and C, the weld gap G of the open pipe S ₂ was welded to form a steel pipe having an outer diameter of 905 mm. Thereafter, the diameter of the steel pipe was measured at eight points in 22.5 degree increments in the circumferential direction, and the difference between the maximum diameter and the minimum diameter was obtained. Tables 7-9 also show the die profile (limiting angle), pressure force, open-to-weld clearance and roundness. In this case, the roundness is a value obtained by dividing the difference between the maximum and minimum by the outer diameter of the steel pipe.

The welder used in this example was unable to close a hole in a pipe having an open weld gap greater than 40 mm after final shaping. In this case, both ends and the center in the axial direction of the pipe were temporarily welded to close the hole using another pressing equipment, after which the section G of the weld gap was finally welded along the entire length. A roundness of 2.5% before pipe expansion, which reaches 1.0% or less after pipe expansion, was accepted as acceptable.

In samples No. A1-A7, A9 and A10 in Table 7, No. B1-B7, B9 and B10 in Table 8 and No. C1-C7, C9 and C10 in Table 9, which are included in the group of samples of the present invention, the value the open area of the weld gap is small, and the roundness is also satisfactory. In particular, specimens with a restraining angle between 90 degrees and 110 degrees have a roundness of 1.0% or less even without pipe expansion. The lower the average value of the limiting angle, the lower the pressure force.

In contrast, in the samples # A8 and A11 in Table 7, # B8 and B11 in Table 8 and # C8 and C11 in Table 9, in which the limiting angles of the upper half-matrix 4 and the lower half-matrix 5 are a combination of 60 degrees and 90 degrees, the size of the open area of the weld gap is small, but the roundness is unsatisfactory. In the samples # A12 to A16 in Table 7, # B12 to B16 in Table 8, and ## C12 to C16 in Table 9, where the average limiting angle is 60 degrees or less, the weld gap is large. Particularly, in the samples # A15 and A16 in table 7, sample # B16 in table 8 and sample # C16 in table 9, roundness cannot be measured because the welded portion collapsed after welding the G-gap section.

In a specimen formed using a preform B having a non-bent portion wider than a non-bent portion of a preform A compared to a specimen formed using a preform A, the pressure force and roundness are almost the same, but the size of the open weld gap small.

In the sample formed using the preform C, in which the position of the non-bent portion is closer to the edge of the sheet than in the preform B, as compared to the sample formed using the preform B, the pressure force, the size of the open area of the weld gap and the roundness is almost the same. In the sample no. C17 in Table 9, in which the limiting angles of the upper half matrix 4 and the lower half matrix 5 are 180 degrees, despite the application of a maximum force of 30 MN / m press equipment, the distance between the outer surface side of the W / 2 area and the outer surface side of the sheet edge was 915 mm, and the pressure value was less than for other samples. Thus, the size of the open area of the weld gap is satisfactory, but the roundness is unacceptable. To obtain a roundness of 2.5% prior to pipe expansion, it may be necessary to perform a final shaping to the equivalent pressure in other specimens using high-pressure pressing equipment.

Example 4

To manufacture a steel pipe with a given outer diameter from 621 mm to 687 mm, a steel sheet with edges for welding, having a width of 1826-2032 mm, a length of 1000 mm, a thickness of 40 mm and a tensile strength of 635 MPa, was subjected to edge bending followed by bending pressure to obtain a preform S ₁ . Subsequently, the final shaping of the preform S ₁ was carried out using a 30 MN pressing equipment using a series of upper half-matrices 4 and lower half-matrices 5 with a radius of the arcuate portion of 327 mm and a limiting angle of 45 degrees for forming preforms D1 - D11. Table 10 shows the bending conditions for preforms D1 - D11. Each of the preforms D1 to D11 has an unbent portion of a width W / 12 approximately at a portion W / 4 from the edge of the sheet according to the original width W of the sheet. During the final forming, the pressure treatment was performed such that the distance between the outer surface side of the portion W / 2 and the outer surface side of the sheet edge reaches a value corresponding to the original width L of the sheet, as shown in Table 10. Table 10 also indicates the outer diameter of the steel pipes after pressure treatment during final forming.

The size of the open area of the weld gap of the open pipe S ₂ after the final forming of the preforms D1 to D11 was measured. Table 10 also shows the pressure force and the size of the open area of the weld gap.

In the sample No. D6 in Table 10, in which the ratio of the radius of the arcuate portion to the outer radius of the steel pipe is 1.00, the weld gap is the smallest, and when the outer radius of the steel pipe decreases or increases, the open weld gap increases. An open weld gap of 40 mm or less that can be closed with the welding machine used in Example 1 was achieved in Sample Nos. D2 to D10 in Table 10, and the ratio of the radius of the arcuate portion to the outer radius of the steel pipe is 0.96-1.04. An open weld gap of 50 mm, which cannot cause fracture of the welded portion in example 1, was also achieved in samples Nos. D2 to D10 in Table 10, and the ratio of the radius of the arcuate portion to the outer radius of the steel pipe was 0.96 - 1.04.

Although the open weld gap that can be closed by welding the G gap and the open weld gap that does not damage the weld will vary depending on the welding equipment and welding method, the recommended arc radius values sections of the upper half-matrix 4 and the lower half-matrix 5 are 0.96-1.04 of the outer radius of the steel pipe.

Industrial applicability

The die and the method for manufacturing a steel pipe of the present invention effectively form a steel pipe having a proper roundness.

List of item numbers

1 - matrix

1a - bar-shaped element

1b - bar-shaped element

2 - punch

2a - punch front end

2b - punch support

3 - transport roller

4 - upper half-matrix

4a - arcuate section

4b ₁ - linear section and arcuate section of slight curvature

4b ₂ - a linear section and an arcuate section of small curvature

5 - lower half-matrix

5a - arched section

5b ₁ - linear section and arcuate section of small curvature

5b ₂ is a linear section and an arcuate section of slight curvature.

Claims (20)

1. A die for use in a steel pipe forming process, including forming a preform having a U-section by bending a sheet material, forming an open pipe that is a tubular body having a longitudinal weld gap portion of the open pipe, by pressure forming the preform and forming a steel pipe by joining the weld nip portion, the die being used in the step of forming the preform to form an open pipe; matrix contains: a pair of half-matrices, including the first half-matrix and the second half-matrix, the second half-matrix is made with the possibility of installing the blank of the preliminary U-shaped section into it, and the first one is capable of being positioned opposite the U-shaped open side of the blank of the preliminary shape, and the pair of said half-matrices are made with the ability to hold the preform blank when processing it by pressure, a first arcuate portion formed on the surface of the first half-matrix in contact with the preform such that the center of the first arc is located at a position coincident with the bending center of the first half-matrix, the first arcuate portion having a diameter equal to or substantially equal to the outer diameter of the steel pipe , and has a center angle of 70 degrees or more and 150 degrees or less; and a second arcuate portion formed on the surface of the second half-matrix in contact with the preform such that the center of the second arc is located at a position coincident with the bending center of the second half-matrix, the second arcuate portion having a diameter equal to or substantially equal to the outer diameter of the steel pipe , and has a center angle of 70 degrees or more and 150 degrees or less; the sum of the central angles of the first arcuate section and the second arcuate section is less than 360 degrees, wherein the radius of the first arcuate portion and the radius of the second arcuate portion are 0.96 to 1.04 of the outer radius of the steel pipe. 2. The matrix according to claim 1, in which the first half-matrix contains one or more linear portions or arcuate portions of small curvature having curvature less than the curvature of the first arcuate portion, and one or more linear portions or arcuate portions of small curvature are connected to at least one end of the first arcuate section in the direction of the arc, and the second half-matrix contains one or more linear sections or arcuate sections of small curvature having a curvature less than the curvature of the second arcuate section, and one or more linear sections or arc-shaped sections of small curvature are connected to at least one end of the second arcuate section in the direction of the arc. 3. The matrix of claim 1 or 2, wherein the center corners of the first arcuate portion and the second arcuate portion are equal to each other. 4. A method of manufacturing a steel pipe, comprising: forming a preform having a U-shaped cross section by bending at least once a sheet material that has been folded at both ends in a width direction of the preform; forming an open pipe, which is a tubular body having a longitudinal weld nip portion, by pressurizing the preform; and the formation of a steel pipe by connecting a section of the weld gap, moreover, the preform during forming takes a shape such that the center angles within the respective arcs having a diameter equal to or substantially equal to the outer diameter of the steel pipe are 70 degrees or more and 150 degrees or less; and with the midpoints of the arcs located at the joining section of both ends along the width of the sheet, and the lowest section of the U-shaped section, and the preform is shaped during forming such that the sum of the center angles within arcs having a diameter equal to or substantially equal to the outer diameter of the steel pipe is less than 360 degrees, the pressure treatment is carried out by a first half-matrix having a first arcuate portion formed on the surface of the first half-matrix in contact with the preform, so that the center of the first arc is located at a position coinciding with the bending center of the first half-matrix, the radius of the first arcuate portion being 0.96 - 1, 04 outside radius of steel pipe, and a second half-matrix having a second arcuate portion formed on the surface of the second half-matrix in contact with the preform so that the center of the second arc is located at a position coincident with the bending center of the second half-matrix, the radius of the second arcuate portion being 0.96 - 1 .04 of the outer radius of the steel pipe. 5. The method according to claim 4, wherein the preform is not in contact with the semi-matrices in areas outside the said arcs. 6. The method according to claim 4 or 5, in which the central angles within the said arcs are equal between the central angle, which is in the same range with the midpoints of the arcs, which are located at the joining section of both ends along the width of the sheet, and the central angle located within the same limits with the midpoints of the arcs, which are located at the lowest part of the U-shaped section. 7. A method according to any one of claims. 4 - 6, which uses a matrix according to any one of claims. 13.

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