WO2015152108A1 - Steel-pipe shaping method and shaping device using three-point bending - Google Patents

Abstract

In this steel-pipe shaping method, in which a raw steel sheet is made into a substantially cylindrical shape via a first-half shaping process whereby three-point bending is performed a plurality of times from one widthwise edge of the raw steel sheet to the widthwise center thereof, a second-half shaping process whereby three-point bending is performed a plurality of times from the other widthwise edge of the raw steel sheet to the widthwise center thereof, and a final shaping process whereby three-point bending is performed on the widthwise center of the raw steel sheet, by dividing the first-half shaping process into a preceding shaping process performed before the second-half shaping process and a following shaping process performed after the second-half shaping process and setting the extent over which shaping is performed in the preceding shaping process to more than 0.17 but less than 0.46 times the width of the steel sheet, the maximum diameter with which steel pipes can be manufactured is increased without modifications to an existing press.

Description

Steel pipe forming method and forming apparatus by three-point bending press forming

The present invention relates to a method for forming a steel pipe used for a line pipe or the like, and more particularly to a method for forming a steel pipe using a plurality of three-point bending press forming on a raw steel plate and a forming apparatus therefor.

For steel pipes used for line pipes, etc., a raw steel plate having a predetermined width, length and thickness is press-formed into a U-shape, and then press-formed into an O-shape, and then the butt portion is welded. A steel pipe having a roundness increased by increasing the diameter of the material steel pipe (so-called pipe expansion), that is, a so-called UOE steel pipe is widely used. However, in recent years, steel pipes used for line pipes and the like have come to use high-strength, large-diameter, thick-walled steel pipes. For this reason, the UOE steel pipe manufacturing method requires a large pressing force to press the raw steel plate into a U shape or an O shape, so that the manufacturing range is limited or the manufacturability is limited by conventional manufacturing equipment. Is significantly reduced.

Therefore, as a method of reducing the pressing force when manufacturing a high-strength, large-diameter, thick-walled steel pipe, bending (so-called end bending) is applied to both width end portions of the raw steel plate, and then a plurality of times. A press-bend type steel pipe manufacturing method in which a steel plate is formed into a substantially cylindrical shape by welding bending and welding a butt portion and then expanded to straighten the shape to obtain a steel pipe has been put into practical use.

Several methods have been proposed for this press-bending type steel pipe manufacturing technology. For example, in Patent Document 1, after pressing one plate width end as a front stage, pressing the plate width center side part, and pressing the other plate width end as a back stage, the plate width center A method of pressing the side part is disclosed. In Patent Document 2, after press forming three times from one end of the material steel plate toward the center, the steel plate is greatly moved to the other side, and three times from the other end toward the center. Is formed into a round approximate shape with the central portion remaining, and press forming is performed on the remaining central portion. Further, in Patent Document 3, after performing bending molding by three points of the upper and lower molds, the plate material is fed in the width direction by a sizing feeder, and bending is performed at a location different from the previously molded location. A method of obtaining a semi-finished pipe product by repeating in sequence is disclosed.

JP 2005-324255 A JP 2007-090406 A JP 2011-056524 A

In any of the methods disclosed in Patent Documents 1 to 3, the half of the material steel plate is subjected to bending press from one plate width end portion toward the center of the material steel plate, leaving the portion to be finally bent and pressed. Is formed into a substantially semicircular shape and then subjected to bending press molding from the other plate width end. However, in this method, when bending press molding from the other plate width end portion is started, the half-shaped pre-formed portion formed into a substantially semicircular shape is lifted greatly before that, and the height is the product steel pipe dimension (outside May reach about 1.5 times the diameter.

In such a case, there is no room in the space that can be used for manufacturing the steel pipe due to the relatively small size of the press machine relative to the outer diameter of the steel pipe to be bent press formed, or due to incidental equipment installed in the press machine. In such a case, the steel plate of the above-mentioned preformed part and the press machine or its associated equipment may interfere with each other, and the equipment damage and the maximum outer diameter that can be manufactured are greatly restricted. . In order to deal with such problems, it is necessary to devise a design that increases the height of the press machine or changes the mounting position of the incidental equipment so that the steel plate and the press machine do not contact each other. In addition, when the equipment needs to be renewed, a large capital investment is required.

The present invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to modify the existing press machine when manufacturing a steel pipe by the three-point bending press method (press bend method). The present invention also proposes a method for forming a steel pipe capable of expanding the maximum diameter of the steel pipe that can be manufactured.

In order to solve the above-mentioned problems, the inventors have made extensive studies by paying attention to the change in the shape of the steel sheet during the forming according to the forming order of the steel pipe. As a result, bending press molding (first half molding), which is performed in the prior art from one plate width end to the center of the steel plate, is divided into two stages, the former molding and the latter molding, and the molding range in the former molding is appropriate. By setting the range, it was found that the lifted height of the pre-formed part can be reduced and the maximum diameter of the steel pipe that can be manufactured can be expanded, and the present invention has been developed.

That is, the present invention is a first half forming in which a three-point bending press is performed a plurality of times from one plate width end portion toward the plate width center of the material steel plate, and a plurality of times from the other plate width end portion toward the plate width center. In the method for forming a steel pipe in which the material steel plate is formed into a substantially cylindrical shape by the latter half forming in which the three-point bending press is performed and the final forming in which the central portion of the plate width is three-point bending pressed, the first half forming is performed before the second half forming. A method for forming a steel pipe, characterized in that it is divided into pre-stage forming and post-stage forming after the latter half forming, and the ratio of the forming range in the pre-stage forming to the steel sheet width is in the range of more than 0.17 and less than 0.46. is there.

The method for forming a steel pipe of the present invention is characterized in that the ratio of the forming range to the steel plate width in the former forming of the first half forming is in the range of 0.21 to 0.42.

Further, the method for forming a steel pipe according to the present invention is characterized in that the material steel plate is formed by applying end bending to both end portions of the plate width.

Further, the present invention is a steel pipe forming apparatus used for the steel pipe forming method according to any one of the above, wherein the uppermost portion of the punch support and the uppermost portion of the lower mold when the punch is lowered most during pressing. The distance between them is 1.4 times or less of the outer diameter of the steel pipe to be manufactured.

According to the present invention, since the highest position where the steel plate is lifted during press forming can be kept low, the maximum outer diameter of the steel pipe that can be manufactured can be increased without adding improvements to the existing press machine.
Further, according to the present invention, since the highest position where the steel plate is lifted during press forming can be kept low, the height of the press used for forming can be kept low, and the degree of design freedom can be increased. In addition, it is possible to reduce the capital investment by reducing the height of the building where the press machine is installed and reducing the depth of digging the floor.
In addition, according to the present invention, the highest reached position where the steel sheet is lifted is suppressed low, and after each bending press, the amount of steel sheet falling when releasing the pressing force is reduced, so the impact force when dropping the steel sheet is reduced, It can also be expected to have effects such as prevention of steel sheet defects, press machine breakage, and reduction of impact noise.

It is a figure which illustrates typically the forming method of the steel pipe by the press bend method. It is a schematic diagram explaining the change of the steel plate shape for every press in the conventional press bend method. It is a schematic diagram explaining the change of the steel plate shape when dividing the press of five times in the first half forming into the former 1 time and the latter 4 times. It is a mimetic diagram explaining change of the shape of a steel plate when five presses in the first half forming are performed by dividing into the former two times and the latter three times. It is a mimetic diagram explaining change of the shape of a steel plate when five presses in the first half forming are performed by dividing into the former three times and the latter two times. It is a schematic diagram explaining the change of the steel plate shape when performing the press of 5 times in the first half by dividing into the former stage 4 times and the latter stage 1 time. FIG. 7 is a schematic view collectively showing changes in the maximum height position of the steel sheet during the forming shown in FIGS. 2 to 6; It is a schematic diagram which shows the relationship between the space | interval of a raising / lowering guide, the position of an upper structure, and a steel plate shape. It is a figure which shows the relationship between the bending range and the front-end | tip maximum height of a steel plate in the first half first stage shaping | molding. It is a schematic diagram which shows typically the penetration | invasion presence or absence of the steel plate to the penetration prohibition area | region in the shaping | molding method of an Example.

FIG. 1 schematically shows a method of forming a steel pipe by the press bend method, and the direction perpendicular to the paper surface corresponds to the longitudinal direction of the material steel plate, that is, the longitudinal direction of the steel pipe. For forming the steel pipe, a steel plate as a raw material is set on a pair of lower molds arranged separately, a punch is lowered by a driving device (not shown), the steel plate is bent and pressed, and the punch is raised, This is performed by repeatedly feeding the steel plate in the plate width direction using a feeding device or the like (not shown) and then performing the next bending press. The punch has a punch tip that contacts the steel plate, and a punch support that connects the punch tip and the driving device. The punch tip and the punch support may be directly connected or may be connected via a spacer. The width of the punch support may be equal to the width of the punch tip, but is preferably narrower than the width of the punch tip.

First, as forming the first half, bending press and steel sheet feeding are repeated a plurality of times (a times) from the sheet width end of the material steel sheet toward the center of the sheet width (from A part to C part in FIG. 1), One half of the steel plate (excluding the central portion C of the steel plate width) is formed into a substantially semicircular shape. In the present invention, this molding step is hereinafter referred to as “first half molding”. In addition, in FIG. 1, the example using the steel plate which provided the end bending to the both width | variety edge part of a raw material steel plate is shown. The end bending is preferably applied from the viewpoint of suppressing the sharpening of the seam welded portion and improving the roundness, but may not be applied. When end bending is applied, the crimping press method disclosed in Japanese Patent Application Laid-Open No. 08-294727, Japanese Patent Application Laid-Open No. 51-76158, or the like can be suitably used.

Next, bending press and steel plate feeding are repeated a plurality of times (b times) from the other plate width end of the raw steel plate toward the center of the plate width (from B to C in FIG. 1), and one side of the raw steel plate Half (however, excluding the central part C of the plate width of the steel plate) is formed into a substantially semicircular shape. In the present invention, this molding step is hereinafter referred to as “second half molding”. At this time, in order to obtain the same shape as the substantially semicircular portion formed by the first half molding, the molding conditions such as the press times a and b, the bending angle, and the steel sheet feed amount are generally the same as those of the first half molding.

The material to be formed (steel plate) after the first half forming and the second half forming has a shape like the capital letter C of the alphabet in which the flat portion remains in the portion of the central portion C of the plate width and the butted portions at both ends of the steel plate are opened. It has become. Finally, the flat portion C at the center of the plate width is formed by one bending press, and the opening of the butt portion is closed to a degree slightly wider than the width of the punch support. In the present invention, this molding step is hereinafter referred to as “final molding”. Thereafter, the formed steel sheet is moved in the longitudinal direction and carried out of the press machine. The formed opening is pressed and closed, welded, and then expanded to obtain a product steel pipe.

FIG. 2 shows a steel pipe of each press accompanying the progress of pressing in the case of forming the steel pipe by the conventional press bend method shown in FIG. A change in shape is indicated by a solid line and a dotted line, and an arrow indicated by a broken line indicates a change in the maximum height position of the steel sheet lifted in each press.
From this figure, in the former half forming shown in FIG. 2 (a), the maximum height position where the steel plate is lifted is not so large, but in the latter half forming shown in FIG. 2 (b), the highest in the first press forming. Although the steel plate is lifted to a high position and the height decreases as the forming progresses, it can be seen that the steel plate position gradually approaches the press machine main body and may cause contact.

Generally, a press machine used for forming a steel pipe loads a forming force on a material to be formed (steel plate) from above via a punch by a cylinder mechanism installed at the top of the press machine. In the upper part, drive mechanisms such as a hydraulic cylinder, a hydraulic pump, a hydraulic tank, etc. are installed as ancillary equipment. However, the forming force necessary for forming the steel pipe increases as the strength of the steel plate increases and the plate thickness increases, and the incidental equipment increases in proportion thereto. For this reason, in a press machine that manufactures a high-strength and thick-walled steel pipe, incidental facilities such as a drive mechanism disposed on the upper part of the press machine are inevitably large and easily interfere with the steel plate. In each press, when the punch is lowered to the lowest position, the drive device connected at the upper end of the punch support is also lowered to the lowest position. It is important that the steel plate does not interfere with a drive mechanism or the like disposed on the upper part of the press even when it is lowered to a low position.

Therefore, the inventors divided the first half forming into two steps, the former forming and the latter forming, and the relationship between the press forming order, the shape of the steel sheet, and the lifting height, the number of presses a, The case where b was 5 times was examined.
FIG. 3 shows a case where one press is performed in the first half of the former molding, and five presses are performed in the second half of the molding, and then the remaining four presses are performed in the first half of the latter half of the molding. FIG. In the case where the second press is performed twice, the second half molding is performed five times, and the remaining three presses are performed in the first half latter stage molding, FIG. 5 illustrates that the first half first stage molding is performed three times, In the case where the second half molding is performed five times and the remaining two presses are performed in the first half latter stage molding, FIG. 6 shows that the first half first stage molding is performed four times, and the second half molding is performed five times. FIG. 5 shows the change in the steel plate shape and the change in the maximum height of the steel plate in each press of the second half forming and the first half latter forming when the remaining one press is performed in the first half latter forming. In each figure, (a) shows the latter half forming, and (b) shows the change in the steel plate shape and the maximum height position in which the steel plate is lifted in the former first half forming. For the first half first forming, FIG. ) And omitted. In each figure, the solid line and the dotted line represent the change in the shape of the steel sheet, and the broken line arrow represents the maximum height position where the steel sheet is lifted. For example, the solid lines in FIGS. 3 (a), 4 (a), 5 (a) and 6 (a) indicate the shape of the steel plate when the first press of the second half forming is performed. Moreover, the solid line of FIG.3 (b), FIG.4 (b), FIG.5 (b) and FIG.6 (b) shows the steel plate shape when the 1st press of back | latter stage shaping | molding is implemented among the first half forming. Show.

2 to 6, it can be seen that the maximum height position where the steel sheet is lifted is changed by changing the number of presses in the first half of the former stage forming. Therefore, FIG. 7 summarizes the change of the maximum height of the steel plate in FIGS. 2 to 6 as one figure, where (a) shows the latter half forming and (b) shows the first half latter stage forming. .
From FIG. 7 (a), in the press in the second half forming, the steel plate is lifted the highest when the first half former forming press is four times and the first half latter forming press is one time (first half forming: indicated as 4 + 1). When the first half former press is 3 times and the first half latter half press is 2 times (first half molding: indicated as 3 + 2), then the first half molding is not divided into two but is pressed 5 times at a time (first half molding: 5 + 0), then the first half former press is 2 times and the first half latter press is 3 times (first half molding: indicated as 2 + 3). The lowest is the first half former press. Thus, it can be seen that the press of the former half-stage molding is four times (first half molding: indicated as 1 + 4).

In the case of the first half forming: 4 + 1, the first half forming: 3 + 2 and the first half forming: 2 + 3, the maximum height of the steel sheet is almost the same as that in the first half forming: 5 + 0, but the first half forming: 4 + 1 Since the maximum height position of the material is closest to the press machine, there is a high possibility that the material to be formed (steel plate) will interfere with the press machine and incidental equipment. Therefore, in the latter half molding, the material (steel plate) is less likely to interfere with the press machine in the first half molding: 3 + 2, the first half molding: 2 + 3, and the first half molding: 1 + 4.

Also, from FIG. 7 (b), the steel plate is lifted highest by the press in the first half latter stage forming in the case of the first half forming: 1 + 4 where the lifting height of the steel plate is the lowest in FIG. 7 (a). The first half forming: 2 + 3, then the first half forming: 3 + 2, and the lowest lifting height of the steel sheet is the first half forming: 4 + 1. .

Therefore, when FIG. 7 (a) and FIG. 7 (b) are combined, in the case of forming a steel pipe by the press bend method, there is the least possibility that the workpiece will come into contact with the press machine. When the first half-stage press is 3 times and the first half-stage press is 2 times (first half-mold: 3 + 2), when the first half-first-stage press is twice and the first half-late stage press is 3 times (first half-mold: 2 + 3) This is considered to be the case where the number of presses is four times and the former half latter stage press is one time (first half molding: 4 + 1).

As described above, in the press molding of the present invention, it is preferable that the plurality of presses in the first half molding are performed separately in the former molding and the latter molding to optimize the number of presses in the former molding. However, the number of presses can be arbitrarily set. Therefore, in the present invention, preferable press conditions are defined by a range (hereinafter, referred to as “pre-formation range”) that is subjected to bending deformation by the press in the first half pre-form.

搬 The substantially cylindrical steel plate after the final press forming is carried out of the press by moving the substantially cylindrical steel plate in the longitudinal direction. In this case, the interval between the columns (also referred to as “elevating guides”) that support the upper structure of the press machine needs to be sufficiently wider than the width of the substantially cylindrical shaped body. Therefore, in order to form a steel pipe having an outer diameter of 1219.2 mm, the distance between the lifting guides is 1.4 times the product steel pipe outer diameter + 300 mm (maximum seam opening width) and the superstructure. The position of the body and the steel plate shape are shown in FIG. 8 (a) and FIG. 8 (b). The relationship between the maximum height of the steel sheet tip at this time and the pre-forming range is shown in FIG. In addition, when the former half former stage forming is 3 times or less, in the latter half forming (the former stage forming range / the steel sheet width is 0.3 or less range), as is apparent from FIG. The plot is omitted because there is no interference. Also, since the dimensions of the forming range and the absolute value of the maximum height of the steel plate tip depend on the outer diameter of the steel pipe to be manufactured, the former forming range is the ratio to the steel plate width, and the maximum height of the steel plate tip is the product steel pipe It is shown as a ratio to the outer diameter.

Here, the point marked with ● in FIG. 9 is the maximum height of the front end of the former molding portion when the latter half molding is performed, and corresponds to the front end position of FIG. The point where the ratio of the former forming range to the steel plate width is 0.46 is the case where the first half forming is formed without dividing it into the first and second stages (first half forming in FIG. 8A: 5 + 0), and the maximum height is The product has reached 1.4 times the outer diameter of the steel pipe. However, the maximum height decreases as the ratio of the former forming range to the steel plate width becomes smaller than 0.46. Accordingly, the ratio of the former forming range to the steel plate width needs to be less than 0.46, and is preferably 0.42 or less. If this ratio is 0.38 or less, the maximum height can be reduced by 10% or more compared to the case where the first half molding is not divided into the former stage and the latter stage, which is more preferable.

On the other hand, the point marked with ○ in FIG. 9 is the maximum height of the tip of the latter half molding portion when the former half latter stage molding is performed, and corresponds to the tip position in FIG. The maximum height decreases as the ratio of the former forming range to the steel plate width increases, and when the ratio of the former forming range to the steel plate width exceeds 0.17, the former half forming is formed without dividing the former stage into the latter stage, that is, The maximum height is smaller than the latter forming point when the ratio of the former forming range to the steel plate width is 0.46. Accordingly, the ratio of the former forming range to the steel plate width needs to be more than 0.17, and is preferably 0.21 or more. If this ratio is 0.29 or more, the maximum height can be reduced by 10% or more compared to the case where the first half molding is not divided into the first and second stages, which is more preferable.

As described above, in the above description, the case where the number of three-point bending presses is 5 times for the first half forming, 5 times for the second half forming, and 1 time for the final forming (11 times in total) is described as an example. The manufacturing method of a steel pipe is not limited to 11 times of press, and may be increased or decreased. However, if the number of times is reduced, the bending angle per one time increases, and accordingly, the lifting height of the steel plate increases, and the roundness after forming becomes inferior. On the other hand, when the number of times is increased, the lifting height of the steel sheet is lowered and the roundness after forming is also increased, but the productivity is lowered. Therefore, it is preferable to determine the number of presses in consideration of both advantages and disadvantages.
In the above description, as described above, from the viewpoint of making the steel plate shape after the first half forming the same as the steel plate shape after the second half forming, the number of presses in the first half forming and the second half forming and the feed amount of the steel plate are the same. It may be changed if the circularity can be secured.

From a thick steel plate (strength: X100) in which end bending of 17 degrees is applied to the range of 210 mm at both width ends of the plate width: 4428 mm by a bending press method using a press machine having a strength of 100 MN, outer diameter: 1422 An experiment was conducted to manufacture a steel pipe of 4 mm × length: 12.8 m × tube thickness: 12.7 mm.
In the bending press machine used in the experiment, the dimension in the width direction of the steel plate of the driving device to which the upper end of the punch support is connected is 2300 mm (1150 mm on one side from the center of the punch). In this press machine, the distance between the lowermost surface of the driving device to which the upper end portion of the punch support is connected and the upper surface of the lower mold when the punch is lowered most in press molding is 1890 mm.
In the pressing method using the bending press machine, the first half molding and the second half molding are pressed five times from the position of 1822 mm from the center of the plate width toward the center of the plate width, and finally the center of the width is finally formed (total 11). The steel plate feed amount in each press was set to 364 mm, and the bending angle was set to 29.5 degrees.

In addition, as shown in Table 1, the five presses in the first half molding are divided into two, the first stage molding and the second stage molding. After the first half first stage molding, a predetermined number of presses within 5 times are performed, and then the second half molding. 4 types of molding methods (A to D) in which the remaining press is performed in the first half latter-stage molding, and the conventional molding method E in which the first half molding is performed at once as a reference example. Carried out.
In addition, as shown in FIG. 10, the evaluation of each forming method is an area where the material to be formed (steel plate) enters the upper part of the punch installation portion of the press machine and causes contact with the press machine and its associated equipment, The degree of penetration of the steel sheet into this region was evaluated.

Table 1 also shows the results of forming the steel pipe by the five pressing methods A to E described above. The “bending range” shown in Table 1 is the distance from the plate end portion (including the end bending portion) of the press-formed portion in the former first half molding, and the numbers in parentheses are the above-mentioned portions. The ratio with respect to the steel plate width is shown. Moreover, the change of the maximum height position where the steel plate lifts up in each of the above conditions is shown in FIG. 10 together with the invasion prohibition region of the steel plate.

As shown in Table 1 and FIG. 10, in the case of the conventional method (E method) in which the ratio to the steel plate width is formed to 0.46 in the first half forming, the steel plate is prevented from entering by the second press of the second half forming. There is a large intrusion into the area, which may damage the press. Similarly, in the case of the A method (first half forming: 1 + 4) in which the ratio to the steel sheet width is formed to 0.13 in the first half first stage molding, the first press of the first half latter stage molding (the second press of the first half molding), There is a risk that the steel plate has greatly entered the intrusion prohibited area and the press machine may be damaged.
On the other hand, in the case of the B method (first half forming: 2 + 3) formed to the ratio of the steel sheet width in the first half first stage molding, the first press of the first half second stage molding (the third press of the first half molding), In the case of the D method (first half forming: 4 + 1) in which the steel sheet has caused a moderate intrusion into the intrusion-prohibited region, and the ratio to the steel sheet width is 0.41 in the first half former forming, the latter half forming 4 presses Visually, the steel plate has made a slight intrusion into the intrusion prohibited area, but it is in a range where plastic deformation does not occur in the workpiece (steel plate), and the product shape can be damaged, and the press machine can also be damaged. There was not.
Furthermore, in the case of the C method (first half forming: 3 + 2) method in which the ratio to the steel plate width is formed to 0.29 in the first half former stage forming, the steel plate does not enter the intrusion prohibited area in all the presses.

In this way, the first half molding is divided into the first and second stages, and the molding range in the first stage is set to an appropriate range, so that the material to be molded (steel plate) does not come into contact with the press or is damaged. I understand that I can do it. Therefore, by applying the present invention, it is possible to enlarge the maximum diameter that can be formed by the press machine. In addition, if it is a steel pipe with a small outer diameter, it is not necessary to apply this invention, and it cannot be overemphasized that it can fully shape | mold also by the conventional method.

Claims (4)

1. First half forming of a plurality of three-point bending presses from one plate width end of the material steel plate toward the center of the plate width, and a plurality of three-point bending presses from the other plate width end toward the center of the plate width. In the forming method of the steel pipe which forms the material steel plate into a substantially cylindrical shape by the latter half forming to be performed and the final forming to perform the three-point bending press on the center portion of the plate width
   The former half forming is divided into a former forming performed before the latter forming and a latter forming performed after the latter forming, and the ratio of the forming range in the former forming to the steel plate width is set to a range of more than 0.17 and less than 0.46. A method of forming a steel pipe characterized by the above.
2. The method for forming a steel pipe according to claim 1, wherein the ratio of the forming range to the steel plate width in the former forming of the first half forming is in the range of 0.21 to 0.42.
3. The method for forming a steel pipe according to claim 1 or 2, wherein the material steel plate is formed by applying end bending to both ends of the plate width.
4. A steel pipe forming apparatus for use in the method for forming a steel pipe according to any one of claims 1 to 3, wherein the punch is positioned between the uppermost part of the punch support and the uppermost part of the lower die when the punch is lowered most during pressing. A steel pipe forming apparatus characterized in that the distance is 1.4 times or less the outer diameter of the steel pipe to be manufactured.
   

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