RU2648813C2 - Method for press-molding steel pipe and method for producing steel pipe - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: group of inventions relates to a method of press-forming a steel pipe having a heavy wall thickness, particularly to a method of manufacturing a steel pipe by press bending. Seam gap is formed during applying press bending of a final time to an open seam pipe which is a material to be formed and the predetermined seam gap, forming with the additional pressing-down amount based on the relationship is applied to the open seam pipe after the state that the predetermined seam gap is formed during applying press bending of the final time to the open seam pipe which is the material to be formed. Unconnected edges of the open seam pipe are then joined and sealed.

EFFECT: smallest gap between the open edges is provided by eliminating the spring.

3 cl, 6 dwg, 2 tbl

Description

Technical field

The objects of the present invention are a method of forming a steel pipe and a method of manufacturing a steel pipe by press-bending.

State of the art

As a method of forming a steel pipe with a large wall thickness, a stamping method was used in which the upper and lower parts of the stamp shown in FIG. 3 (hereinafter also referred to as "press brake").

This molding method is used to form pipes of plate steel of a certain width and length in order to create a steel pipe having a length in the direction of the axis of the pipe. With this method of molding, first, the edge sections of the thick steel sheet are bent (hereinafter, this operation is referred to as “edge flexible”), and then several times the steel sheet is pressed in the transverse direction, as a result of which a cylindrical blank is obtained.

In the main molding step performed after edge bending, as shown in FIG. 3, two matrix elements 1a, 1b are used, installed at a certain distance from each other, on which the steel sheet S is mounted, and the front punch element 22 together with the punch 2, which are part of the upper part of the stamp, are lowered into the space between the two matrix elements 1a, 1b, producing deformation of a sheet of steel S of large thickness. Then, the steel sheet S is moved to a certain distance in the transverse direction, and pressing is again performed by the upper part of the stamp. This pressing operation is repeated several times.

As a rule, the molding of a pipe from a steel sheet is carried out sequentially in the transverse direction from the edge of the sheet (from point A in the drawing) from one side of the sheet to the center of the steel sheet in the transverse direction (to point C in the drawing), as a result of which the steel sheet is formed S in the section from one edge of the sheet to the point immediately in front of the center in the transverse direction (pressing the first half of the sheet). After that, the pipe is formed from the steel sheet in the transverse direction from the edge of the sheet (from point B in the drawing) from the opposite side of the sheet to the center of the steel sheet in the transverse direction (to point C in the drawing), as a result, the steel sheet S is formed on section from one edge of the sheet to the point immediately in front of the center in the transverse direction (pressing the second half of the sheet). And finally, the central section of the steel sheet S is pressed in the transverse direction (point C in the drawing) (final pressing). Thus, an open pipe is obtained. An open pipe is a tubular-shaped body in which a cylindrical shape is given to the original steel sheet by molding, the unconnected edges of which are located opposite each other, not yet welded to each other. In FIG. 3, the dotted line shows the position of the steel sheet S in a state where the punch 2 has not yet come into contact with the steel sheet S. Not shown in FIG. 3 feed rollers are located to the left and right of the matrix elements 1a, 1b. These feed rollers can support the steel sheet S at points B or C, as shown by the dotted line in the upper left drawing in FIG. 3, and can also move the steel sheet S left or right according to the drawing.

In FIG. 4 is a schematic illustration of an open pipe. As shown in FIG. 4, the open pipe 3 is a cylindrical pipe made of an initial steel sheet by molding, the unconnected edges 31a, 31b of which, located opposite each other, are not yet welded to each other. The gap g between opposing unconnected edges is called the gap between the edges of the open pipe. The direction L along the longitudinal axis of the open pipe 3 coincides with the longitudinal direction of the punch.

So, after performing the above operation, the open pipe is moved in the longitudinal direction (in the direction perpendicular to the plane of the drawing in Fig. 3) to start the next operation. To carry out such movement of the open pipe after the final molding pass, which ends the pipe pressing, the gap g between the edges of the open pipe 3 should have a width greater than the thickness of the punch rod 21 on which the punch 22 of the upper part of the punch is fixed.

Then, using the forming device, the open pipe 3 is crimped so that the gap g between the edges of the open pipe 3 is closed. The unconnected edges of an open pipe that come into contact with each other are welded in this state by a welding machine, resulting in a straight welded steel pipe. If necessary, the cylindrical shape of the resulting welded steel pipe is adjusted by expansion or crimping in the radial direction.

The setting and parameters of the aforementioned pressing operations can be carried out by applying a stamp with a mechanism for adjusting the gap between the matrix elements of the lower part of the stamp disclosed in JP-A-11-129031.

Disclosure of invention

Technical problem

When crimping an open pipe with a crimping device in such a way as to close the gap between the edges of the open pipe and welding the connected edges with a welding machine, the crimping force of the crimping device has an upper limit. In this case, a situation may arise in which the unconnected edges of the open pipe with a gap between them cannot be joined to each other by crimping the open pipe, the size of the gap between the unconnected edges of which is greater than or equal to a certain value, depending on the size of the steel pipe. As a result, it is likely that the joined edges of the open pipe will be impossible to weld together.

Thus, the requirement for the final pressing of the pipe on the bending press is that the last bending pass provides the smallest possible gap between the unconnected edges.

The gap between the unconnected edges of the open pipe is controlled by gradually increasing the amount of compression during the final pressing of the open pipe. With increasing compression during final pressing, the gap between the edges decreases. Conversely, with a decrease in the amount of compression during the final pressing, the gap between the edges increases.

When removing the pressing force of the punch, since there is a spring effect, the gap between the unconnected edges of the open pipe becomes larger than it was when applying the pressing force of the punch. To reduce the influence of the spring effect after removing the pressing force of the punch, a technology was proposed in which the compression value at the last pressing was increased so that the punch continued to move further after the open edges of the pipe came into contact with the bearing element of the punch, providing further bending of the open pipe.

With this technology, the punch compression amount is excessively large, there is a possibility of an undesirable situation in which the unconnected edges of the open pipe strongly clamp the punch carrier, so in order to remove the pipe, it may be necessary to cut it along its entire length.

To eliminate this drawback, attention was paid to the fact that the amount of compression did not become excessively large. As a result, the probability was again returned that the gap between the edges to be welded would become too large.

It should also be noted that in addition to the above factors, the yield strength of plate steel is also affected by the gap between the unconnected edges of the open pipe. This is because during bending deformation of a sheet of steel of large thickness, the resulting spring effect depends on the yield strength of the steel. For example, if the final bale of the pressing is performed under the same conditions and then the pressing force of the punch is completely removed, then in the case when the yield strength of the steel sheet S is low, the gap between the unconnected edges of the open pipe becomes small, and in the case when the yield strength of the steel sheet S is high , the gap between the unconnected edges becomes large.

As mentioned above, since care was taken to ensure that the compression value does not become excessively large, there is a tendency for the gap between the unconnected edges of the open pipe to become excessively large. The influence of the yield strength of plate steel is combined with the aforementioned trend, which leads to an even greater increase in the change in the gap between the unconnected edges of the open pipe. With an excessively large gap between the unconnected edges of the open pipe, the crimping force required to crimp the open pipe and close the gap to perform welding also becomes large, as a result of which the required crimping device becomes large. In addition, to solve the problem of changing the gap between the unconnected edges, manual adjustment of the compression force of the compression device on the welding machine requires a lot of time.

In connection with the foregoing, to reduce the change in the gap between the unconnected edges of the open pipe after pressing, a technology was proposed according to which the setting of the pressing conditions is made for each type of steel sheet, as well as a technology that involves pre-compiling a table containing the yield strength and the corresponding conditions pressing, and determining the conditions of pressing according to this table.

The method disclosed in JP-A-11-129031 involves adjusting the shape of the profile of a pipe molded from steel plate by changing the distance between the matrix elements of the lower part of the stamp. As for changing the magnitude of the spring effect, this method involves measuring the shape of the profile of the steel pipe after molding from the steel sheet in the first stage, and then molding to adjust the shape in the second stage by changing the distance between the matrix elements of the lower part of the stamp. In other words, according to the method disclosed in JP-A-11-129031, the shape of the profile of the steel pipe is measured in the state after removing the pressing force in the first step, i.e. at the moment when the spring effect occurs, and then corrective molding is performed in the second stage by changing the pressing conditions, for example, the compression value, load, the distance between the matrix elements of the lower part of the stamp, etc., in accordance with the obtained measurement results. Accordingly, additional time is required to change the distance between the matrix elements of the lower part of the stamp. In particular, when performing corrective molding several times to create one steel pipe, it is necessary to measure the form as many times, and, therefore, there is a problem of a significant decrease in productivity. In addition, it is necessary to set the conditions for such corrective molding for each steel pipe from a large number of steel sheets having different values of the spring effect.

Accordingly, it is an object of the present invention to provide a method for forming a steel pipe, providing a small change in the size of the gap between the unconnected edges.

Solution

The objects of the present invention are:

[1] A method of forming a steel pipe by pressing a steel sheet by performing press-bending of the steel sheet several times, in which, in accordance with a previously obtained dependence of a predetermined amount of a gap between unconnected edges of an open pipe mold on an amount of additional compression required after reaching a predetermined gap between the edges at the final pass of press bending, after reaching the specified value of the gap between the edges, at the final pass of press bending, molding is performed Access the tube with additional to press.

[2] The method of forming a steel pipe according to [1], wherein the predetermined gap between the unconnected edges of the open pipe is the gap between the unconnected edges at a time when the unconnected edges of the open pipe come into contact with the punch rod of the upper part of the stamp.

[3] A method for manufacturing a steel pipe in which the unconnected edges of an open pipe formed using the method of [1] or [2] are joined together and welded to each other.

The beneficial effect of the invention

According to the present invention, the pressing is carried out under predetermined conditions, and therefore, it is possible to obtain a material that provides a small change in the gap between the unconnected edges, which eliminates the need for corrective molding or installation and adjustment of the distance between the matrix elements of the lower part of the stamp, resulting in significantly increased productivity.

Brief Description of the Drawings

In FIG. 1 is a graph showing the relationship between the unconnected edges of an open pipe and the change in yield strength according to the present invention.

In FIG. 2 shows a graph of the relationship between the unconnected edges of an open pipe and the change in yield strength for the prior art.

In FIG. 3 is a diagram illustrating a process for molding a steel pipe by bending.

In FIG. 4 shows a schematic illustration of an open pipe.

In FIG. 5 is a diagram showing an operation of edge bending, in which: FIG. 5A shows the installation of a workpiece; in FIG. 5B shows the state at the time the load is applied during edge bending, and FIG. 5C shows the shape of the sheet after unloading the edge bending.

In FIG. 6 is a diagram for explaining a pressing operation, in which: FIG. 6A shows a state upon application of a load; in FIG. 6B shows the state after unloading, and FIG. 6C shows a cross-sectional shape of a pipe after compression.

The implementation of the invention

The main standards that are guided by the creation of trunk pipelines (which mainly use direct welded steel pipes) are the standards of ANI (American Petroleum Institute). According to ANI standards, the range of variation of the yield strength of a welded steel pipe made of X80 steel from the upper to the lower limit is 138 MPa. The steel sheet used as a starting material in the production of welded steel pipes also has a yield strength, which essentially varies in the same range as the yield strength of a welded steel pipe. In particular, in the production of steel pipes with a high yield strength, steel plate is used as the starting material, manufactured using the TMSR technology (thermomechanical control process). Accordingly, the yield strength of plate steel may also vary depending on changes in the chemical composition of the components, rolling conditions and cooling conditions.

Further, the explanation of the present invention is carried out on the example of forming a steel pipe from steel grade X80 according to the classification of ANI, having an external diameter of 1219 mm and a wall thickness of 31.8 mm. However, the present invention is not limited to the embodiment described below.

The molding according to the proposed method begins with edge bending, as shown in FIG. 5. At 41 in FIG. 5, the lower part of the edge bending stamp is indicated, 42 indicates the upper part of the edge bending stamp. The edge bending is performed in such a way that the bending angle of the edge j (see Fig. 5 B) becomes 28 ° at the moment of applying the pressing force of the stamp on the section h (see Fig. 5A) with a width of 240 mm of the edge section of the steel sheet in the transverse direction. The bending angle k (see Fig. 5C) of the edge of the steel sheet in the transverse direction after removing the pressing force of the stamp is 23 °.

After this, the steel sheet is bent successively 11 times in the transverse direction, starting from its edge sections, with the help of a punch 2 (the upper part of the stamp) whose radius R is 415 mm (see Fig. 6A). The molding method is now illustrated with FIG. 6. As shown in FIG. 6A, the punch 2 (the upper part of the punch) consists of the rod 21 and the front part 22 of the punch, and the lower part of the punch consists of the matrix elements 1a, 1b. The full bending angle (angle ƒ in Fig. 6C) of a thick steel sheet (except for the gap between the unconnected edges of the open pipe) is obtained by bending the steel sheet 11 times and bending the edge sections of the sheet. More precisely, when performing a one-time operation of bending a steel sheet of steel with a yield strength of 640 MPa with the application of a one-time pressing force of the punch, calculated to obtain a bending angle (angle d in Fig. 6A) of 35 °, the bending angle after removing the pressing force of the punch (angle e on Fig. 6B) is 29 °. As for the bending portions of the steel sheet S, as shown in FIG. 6A, section “a” is a section that has been bent during the previous operation, section “b” is a section that is bent during this operation, and section “c” is a section that will be bent during the next operation and operations following her. With this bending method, the adjustment of the bending angle is usually carried out in such a way that the movement of the punch 2 is controlled directly by the stamping device. In this case, the molding is performed in such a way that the punch 2 is shifted down from the initial position, which is considered to be the position in which the front part 22 of the punch is in contact with the upper surface of the steel sheet (hereinafter, the displacement value of the punch 2 from the initial position will be designated as the amount of compression, and the upper surface steel sheet is the reference point, unless otherwise specified) at a given distance.

In FIG. Figure 2 shows a graph of the dependence of the gap between the unconnected edges of the open pipe on the change in the yield strength, which is the deviation of the yield strength from the reference value of the yield strength of the steel sheet, equal to 640 MPa, in the state of removal of the pressing load at the end of pressing.

Even in the case where the steel sheets have the same bending shape at the time of applying the bending load, i.e. at the time of pressing a steel sheet, with a high yield strength of the steel used, the spring effect becomes large. Accordingly, the bending angle of the steel sheet after molding becomes small, and the gap between the unconnected edges of the open pipe after removing the pressing load becomes large. As shown in FIG. 2, if the yield strengths of the original steel sheets of large thickness differ by 160 MPa from each other, the difference in the size of the gap between the unconnected edges of the open pipe after removing the pressing load becomes 170 mm. This value is 14% of the outer diameter of the open pipe and is extremely large.

Accordingly, in FIG. Figure 1 shows the dependence of the gap between the unconnected edges of the open pipe on the change in the yield strength after removing the pressing load at the end of the intermediate bending operation when molding is performed in the usual way and the way in which the compression value is changed at the final molding pass (during the 11th pressing operation) . For comparison, in FIG. 1 also shows a graph b for the prior art shown in FIG. 2.

The graph c corresponds to an implementation example of the present invention, in which the unconnected edges of the open pipe come into contact with the rod 21 of the punch of the upper part of the stamp, after which the steel sheet is further pressed to a distance of 9 mm to ensure the same amount of deformation during the final molding pass (11- st operation). The change in the gap between the unconnected edges of the open pipe after removing the pressing load is small and amounts to 20 mm Thus, it becomes clear that a substantially uniform gap between the unconnected edges of the open pipe can be obtained regardless of the yield strength of plate steel.

Schedule a corresponds to an example in which pressing is performed until the unconnected edges of the open pipe come into contact with the punch rod of the upper part of the stamp. Comparing this graph with the graph for the prior art (graph b, fixed stroke of the punch), we see that the change in the gap between the unconnected edges of the open pipe after removing the pressing load is small. However, this gap between the unconnected edges of the open pipe after removing the pressing load is large compared with the size of the gap between the unconnected edges of the open pipe for the above example according to the present invention.

The reason that the pressing is continued further by 9 mm after the unconnected edges of the open pipe come into contact with the rod 21 of the punch of the upper part of the stamp is based on the following technical principle.

In the manufacture of a plurality of steel pipes of the same size, a plurality of open pipes of the same size are first manufactured. During their production, the pressing conditions are essentially unchanged, and, therefore, the deformation mechanism when forming an open pipe from the original thick steel sheet is also practically the same. Thus, with a further increase in the amount of compression by a certain amount relative to the state in which a predetermined shape is achieved during the final pressing, the gap between the unconnected edges of the open pipe after releasing the pressing load also becomes constant.

As mentioned earlier, when forming an open pipe from a thick steel sheet using two matrix elements 1a, 1b and one punch 2, the use of the gap between the unconnected edges of the open pipe as an indicator of obtaining a given shape during the final pressing process during which the compression increases by a certain amount, is simple and effective.

In the above example, when the gap between the unconnected edges of the open pipe is used as an indicator of obtaining a given shape during the final pressing process, during which the compression value increases by a certain amount, the gap between the unconnected edges of the open pipe at the time when the unconnected edges of the open pipe is taken on both sides come in contact with the rod 21 of the punch. In this case, the amount of compression added further after the moment when both unconnected edges of the open pipe come into contact with the rod 21 of the punch of the upper part 2 of the stamp is determined by conducting preliminary preliminary molding operations or on the basis of data obtained during production before.

Regarding the use of the gap between the unconnected edges of the open pipe as an indicator of obtaining a mold, after which the compression value increases further by a constant value, an explanation was made for the case when both unconnected edges of the open pipe on both sides come into contact with the rod 21 of the punch 2, that is, for the case where the gap between the unconnected edges of the open pipe corresponds to the thickness of the punch rod 21. However, the present invention is not limited to such a case. For example, determining the moment when the gap between the unconnected edges of an open pipe reaches a predetermined value can be carried out using a detector that allows you to measure the position of the edges of the open pipe at any time, or using a simplified detector that determines the moment when the edges of the open pipe reach a certain position .

In particular, for example, a light projector and a light detector mounted on the punch rod 21 can be used for this. The intensity of the light entering the light detector changes when the edges of the open pipe block the optical path of light from the light projector to the light receiver. By the degree of change in the intensity of light entering the light detector, it is possible to determine the position of the unconnected edges of the open pipe. On the other hand, if you want to determine only the point in time when the unconnected edges of the open pipe come into contact with the rod 21 of the punch of the upper part 2 of the stamp, it is not necessary to constantly measure the position of the edges. For example, to determine the point in time at which the gap between the unconnected edges of an open pipe reaches a certain value can be done by measuring the change in electrical conductivity caused by the contact of the unconnected edges of an open pipe with a punch rod 21, or by confirming the presence or absence of contact by installing a piezoelectric element on the contact area.

As a method of regulating the molding with a further increase in the required amount of compression from the point in time at which the gap between the unconnected edges of the open pipe reaches a predetermined value, during the final pressing, for example, a signal sent to the stamping device at the moment when the gap between the unconnected edges open pipe becomes equal to a predetermined value, after which molding can be performed with an increase in compression by a predetermined value, i.e. the above signal can be used as a trigger. An additional compression value can be measured by measuring the displacement of the punch 2. In the case where the gap between the unconnected edges of the open pipe is used as the reference gap between the edges, when this gap becomes equal to the thickness of the punch rod 21, or the gap at the time when the unconnected the edges of the open pipe come into contact with the rod 21 of the punch, you can measure the distance by which the edges of the pipe that come into contact with the rod 21 slide up the rod 21, and up to The total compression value can be adjusted by the value of the given slip distance of the unconnected edges of the open pipe.

In the production of a steel pipe by manufacturing an open pipe by the pressing method described above, continuous welding of the unconnected edges of the open pipe with tack welds is performed using a continuous tack weld unit, after which main welding can be performed to weld the inner seam and weld the outer seam. It is preferable to increase the roundness of the steel pipe, the welding of which will be carried out by expanding it using an expander. During the expansion operation, the expansion coefficient (the ratio of the change in the external diameter after expansion to the external diameter of the pipe before expansion) is usually taken equal to from 0.3% to 1.5%. To obtain the optimal ratio between improving the roundness of the pipe and the required power of the press expander for pipe expansion, the preferred value of the coefficient of expansion of the pipe should be from 0.5 to 1.2%.

Example 1

For the production of steel pipes with an external diameter of 1219 mm, a wall thickness of 31.8 mm and a length of 12 m, 10 steel sheets 24 m long were prepared from AP1 X80 steel. Each steel sheet was cut into three parts in the longitudinal direction, and thus, three sets of samples were obtained, each of which included 10 sample sheets. Using machine processing, the width of each of these steel sheets was made equal to 3693 mm, after which the edge bending of the corresponding edge sections with a width of 180 mm of each steel sheet in the transverse direction was performed using a matrix with a radius of R 380 mm, and then these steel sheets were pressed.

When pressing as a punch 2 of the upper part of the stamp, a punch was used, the front part 22 of which had a radius of R 415 mm. As the lower part of the stamp, matrix elements 1 with a radius of R 100 mm installed at a distance of 540 mm from each other were used (the distance between the matrix elements means the distance between the upper points of the matrix elements 1a, 1b of the lower part of the stamp). Pressing was carried out by performing 11 separate operations. The thickness of the punch rod 21 was 100 mm.

The installation positions in the transverse direction from the first pass to the tenth pass (the distance between the center point between the two matrix elements of the lower part of the die and the center of the steel sheet in the transverse direction) and the corresponding compression values are presented in Table 1. The compression values were selected so that as a result of operations of edge bending and 11 operations of pressing a steel sheet with a yield strength of 615 MPa to obtain a bend of the sheet along approximately the entire circumference of an open pipe, except cheniem gap between the unbonded edges of the open pipe.

Regarding the order of passes during the molding operation in the example under consideration, as shown in FIG. 3, which shows a diagram of a pressing operation, pressing the first half of the steel sheet included making passes 1-5, pressing the second half of the sheet included making passes 6-10. When performing passes 1-5, i.e. when pressing the first half of the sheet, the sheet was sequentially formed from one edge portion on one side of the steel sheet to the center of the steel sheet in the transverse direction. Forming was carried out from the edge of the steel sheet from one side to a point directly in front of the center of the steel sheet in the transverse direction; at each pass, the steel sheet was bent by an amount corresponding to one pass. After this, passes 6-10 were performed, i.e. pressing of the second half of the sheet, in which the sheet was sequentially formed from the other edge portion on the opposite side of the steel sheet to the center of the steel sheet in the transverse direction. And finally, during the 11th pass, the pressing force of the punch was applied to the central section of the steel sheet in the transverse direction.

When pressing ten samples of steel sheets (steel sheets No.№A-J), ten passes in each direction were performed in the transverse direction (in the table, the values of the positions of the steel sheet are indicated with a “+” sign for positions from point A to center C, and with a sign “-” for positions from center C to point B). After this, the eleventh passage of the pressing was performed and the gap between the unconnected edges of the open pipe was measured in the state after removing the punch pressing load. The measurement results, showing the dependence of the gap between the unconnected edges of the open pipe after pressing on the yield strength of the steel sheet, are shown in Table 2.

In the final passage (eleventh passage), the steel sheet was installed so that its center in the transverse direction coincided with the center of the lower part of the stamp. In the example according to the present invention, to obtain a gap between the unconnected edges of the open pipe after pressing from a steel sheet with a yield strength of 615 MPa equal to 125 mm, after the unconnected edges of the open pipe came into contact with the punch rod 21, the pressing was continued, and additional the compression was 9 mm. In comparative example 1, to obtain a gap of 100 mm between the unconnected edges of a steel pipe having a yield strength of 560 MPa, after removing the pressing load of the punch, the compression value (the distance by which the punch 2 moves from the upper starting position, i.e., the point of contact with steel sheet surface) was set equal to 48.6 mm. In comparative example 2, the steel sheet was first pressed until the unconnected edges of the open pipe came into contact with the punch rod 21, the gap was checked in the state after removing the punch pressing load; then, the distance between the matrix elements 1a, 1b of the lower part of the stamp was adjusted, and then the pressing was repeated.

In the example according to the present invention, the change in the gap between the unconnected edges (the difference between the maximum and minimum values of the gap) after forming ten steel pipes is small; the time required for pressing is also small, so this method makes it possible to obtain both the correct shape of the steel pipe and high productivity.

On the contrary, despite the fact that comparative example 1 shows a slightly shorter required time in the case of the steel sheet J with the lowest yield strength, clamping of the punch rod 21 with unconnected edges of the open pipe occurs. Thus, there is a need to stop the production line to remove the molded pipe, and therefore, comparative example 1 is difficult to consider as an option for industrial use. As for comparative example 2, it demonstrates the required time, approximately 1.4 times longer than in the example according to the invention, which indicates an unconditionally lower productivity when using this method, despite the fact that the shape of the open pipe is stable.

Industrial applicability

The steel pipe pressing method and the steel pipe production method according to the present invention are not limited to the manufacture of steel pipes of only large diameter and large wall thickness, and are applicable to the production of all types of steel pipes by three-point bending.

List of Reference Items

1a, 1b: matrix elements

2: punch

21: punch bar

22: front of the punch

3: open pipe

31a, 31b: unconnected edges of an open pipe

41: lower part of the stamp for edge bending

42: upper part of the stamp for edge bending

Claims (5)

1. A method of obtaining an open steel pipe by extrusion molding a steel sheet, comprising multiple press bending of steel sheet by pressing force punch with barbell with the formation of an open pipe used for its formation in a steel pipe, characterized in that at the final press bending, a predetermined uniform gap between the edges of the open pipe is formed, after which an additional pressing force is applied to the central portion of the steel sheet, the value of which provides a predetermined uniform gap between the edges of the open pipe after removing the pressing force. 2. The method according to p. 1, characterized in that a uniform gap between the edges of the open pipe is formed upon contact with the punch bar. 3. A method of manufacturing a steel pipe from a steel sheet, comprising the steps of connecting the unconnected edges of an open steel pipe obtained by compression molding by the method of claim 1 or 2, and welding the unconnected edges to each other.

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