WO2014192091A1

WO2014192091A1 - Method for producing welded steel pipe

Info

Publication number: WO2014192091A1
Application number: PCT/JP2013/064852
Authority: WO
Inventors: 正之堀江; 征哉田村; 俊博三輪; 宏司堀際
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2013-05-29
Filing date: 2013-05-29
Publication date: 2014-12-04
Also published as: JP6070967B2; CN105246608A; EP3006128A4; EP3006128B1; RU2621747C1; CN105246608B; EP3006128A1; JPWO2014192091A1

Abstract

In this method that is for producing a welded steel pipe by means of a press bending method and that produces a steel pipe by causing a starting-material steel plate to be three-point bend press-molded into an open pipe by means of a pair of dies, which are disposed at a predetermined distance in the direction of steel plate transfer, and a punch, which presses the steel plate between the pair of dies, and then welding the open pipe, after performing a plurality of repetitions of first-half press molding from one end in the widthwise direction of the steel plate towards the center in the widthwise direction (but excepting the center in the widthwise direction), performing a plurality of repetitions of second-half press molding from the other end in the widthwise direction towards the center in the widthwise direction (but excepting the center in the widthwise direction), and when finally performing final press molding in the center in the widthwise direction to result in an open pipe, causing the portion of the steel plate supported at the dies at the center side in the widthwise direction of the steel plate at the final pass of the second-half press molding to be an unmolded portion, thus producing an open pipe having a small amount of irregularity in the welded portion.

Description

Manufacturing method of welded steel pipe

The present invention relates to a method for manufacturing a large-diameter and thick-walled welded steel pipe used for a line pipe or the like, and more specifically, an open with high roundness by a press bend method in which a plurality of three-point bending press forming is performed. It relates to a method of manufacturing a tube. In the present invention, the above-mentioned open pipe (open seam pipe) is a state in which the plate ends (open seam edges) facing each other are not welded after the plate material is formed into a cylindrical shape. Refers to the molded product.

For large diameter and thick steel pipes used for line pipes, etc., a steel plate with a predetermined width, length and thickness is press-formed into a U shape and then pressed into an O shape to form an open pipe. Thereafter, the so-called “UOE steel pipe”, in which the open pipe is butt welded to form a steel pipe and the diameter thereof is further expanded (expanded) to increase the roundness, is widely used. However, in order to manufacture this UOE steel pipe, when a steel plate is press-formed into a U shape or an O shape, a great deal of pressure is required, and thus it is necessary to use a large-scale press.

Therefore, as a technique for reducing the pressing pressure when manufacturing a large-diameter and thick-walled steel pipe, for example, bending (so-called edge-crimping) is applied to the end of the steel plate in the width direction, and then the steel plate is processed in the width direction. Press bend to make a steel pipe after correcting the shape after forming a steel plate into a substantially circular open tube by performing a three-point bending press several times while feeding a predetermined amount to A method of manufacturing a steel pipe by the method has been put into practical use.

However, in the press bend method described above, since the press forming in the width direction of the steel sheet is separate, the bending shape is likely to vary due to slight fluctuations in the thickness and strength of the steel sheet. As a result, when the open pipe is butted, a step is generated at the butt portion, which causes a mistake in the welded portion. This misunderstanding of the welded portion causes local concentration of the tensile stress in the circumferential direction caused by the internal pressure, which greatly impairs the reliability of the product.

In order to prevent this weld from being mistaken, it is necessary to finely adjust the press conditions (for example, the amount of reduction) in the width direction of the steel sheet, which has been an obstacle to automation and mass production. Also, if there is a mistake in the welded part, the right and left butt parts are restrained and welded. However, if the steel plate is a high-strength or thick-walled material, a large restraining force is required, so it can be manufactured. There is also the problem that the range is limited.

As a technique corresponding to such a problem, for example, Patent Document 1 discloses a punch that constitutes an upper mold, a cradle that is fixed at the position opposite to the punch, and that serves as a bottom dead center of the punch. First and second dies that are arranged opposite to the right and left sides of the cradle and are capable of reciprocating in opposite directions are provided, and the lower die is configured by the cradle, the first die, and the second die. A press mold is disclosed. In Patent Document 2, an outer mold in which a concave molding surface having a radius corresponding to the outer diameter is formed to a predetermined length and an inner mold in which a convex molding surface having a radius corresponding to the inner diameter is formed to a predetermined length are brought close to each other. In addition to bending the press equivalent part between both molds, the inner mold with the roller body installed on both sides of the outer mold and receiving the steel plate at a position protruding inward from the extended surface of the outer mold A technique has been disclosed in which the bending is made accurate by bringing the shape close to the outer mold and bending the portion corresponding to the press. Further, in Patent Document 3, a steel plate is press-formed, bent, and welded and joined in a state where the groove portions are substantially in contact with each other to form a semi-formed round steel tube, and the entire semi-formed round steel tube is heated. After that, a method for manufacturing a round steel pipe is disclosed in which the forming surface is hot-formed by passing between a plurality of semi-circular forming rolls corresponding to the final radius, and the shape is adjusted.

Japanese Patent Laid-Open No. 11-129031 JP 2007-090406 A JP 2005-324255 A

However, in the method described in Patent Document 1, the plate thickness is reduced at the bottom dead center because the steel plate is clamped between the punch and the cradle. Therefore, when the narrowed range is local, the tube thickness becomes non-uniform, and there is a possibility that a predetermined dimension cannot be satisfied. Further, in the method described in Patent Document 2, although the problem of Patent Document 1 is solved by narrowing the entire range of the bending formation with the outer mold and the inner mold, the proper inner mold and outer mold dimensions are solved. However, since it differs depending on the diameter and thickness of the steel pipe, it is necessary to prepare molds with various dimensions, and the frequency of mold replacement increases, resulting in poor productivity. In addition, the method described in Patent Document 3 requires a heating process in which the shape correction is performed hot, which causes a significant increase in manufacturing cost. Moreover, when using the steel plate manufactured by the thermomechanical process for a raw material, there exists a possibility that intensity | strength, toughness, and weldability may be impaired by heating.

The present invention has been made in view of the above-described problems of the prior art, and its purpose is to produce a welded steel pipe that can easily produce an open pipe with a small amount of misalignment of the welded portion by the press bend method. To propose a method.

The inventors conducted a detailed investigation focusing on changes in the shape of the steel sheet in three-point bending press forming in order to reduce the occurrence of misunderstandings in the welded portion as much as possible. As a result, after multiple press forming (first half press forming) from one width end of the steel sheet toward the width center, multiple press forming (second half press forming) from the opposite end to the width center. However, in the manufacturing method of the open tube in which the center part of the width is finally formed, in the final pass of the first half press forming, when the steel plate is set on the die, one die comes into contact with the unformed steel plate, and the other While the die is in contact with the already formed steel plate, in the final pass of the second half press forming, depending on the feed amount of the set steel plate, both dies may contact the already formed steel plate, In such a case, there is a difference in the processing shape between the first half press molding and the second half press molding, and a large mistake occurs in the butt portion of the open pipe. Therefore, in order to prevent the above mistake In the final pass of the second half of the press molding, it found that the steel sheet of the unshaped the plate width center side of the die needs to'll so as to be positioned, has led to the development of the present invention.

[Correction 09.07.2014 based on Rule 91]
That is, according to the present invention, a material steel plate is three-point bending press-formed with a pair of dies arranged at a predetermined interval in the steel plate feeding direction and a punch for pressing the steel plate between the pair of dies. In the method of manufacturing a steel pipe by welding the open pipe after the open pipe, from the width end of one of the steel plates toward the width center (however, leaving the width center) after the first half press molding, When the second half press-molding is performed from the opposite width end toward the width center (however, leaving the width center) and finally the center of the width is finally press-molded to form an open tube, A method for producing a welded steel pipe, characterized in that a steel plate supported by a die on the center side of the steel plate width is an unformed part.

In the method for producing a welded steel pipe according to the present invention, the final pass of the latter half press forming is the following formula (1):
Β _b · W <α _b · L _b + L _n (1)
Here, L _b : Molding range in the final pass of the second half press molding (mm)
L _n : Molding range in final press molding (mm)
W: Die spacing (mm)
α _b : Steel sheet position shift rate in the final pass of the second half press forming (−)
β _b : Die position shift rate in the final pass of the second half press forming (−)
It is characterized by satisfying.

According to the present invention, there is no adverse effect on quality such as a reduction in the thickness of the steel sheet due to the narrow pressure between the lower mold and the upper mold, and the work efficiency is not reduced by replacing the lower mold. There is no need to change the molding conditions in the press molding, and an open tube having no level difference (mismatch) at the butt portion can be obtained. In addition, according to the present invention, since hot shape correction is not required, it is possible to provide a steel pipe that maintains the characteristics built in the manufacturing stage of the material steel plate.

It is a schematic diagram explaining the manufacturing method of the open pipe | tube of this invention. It is a schematic diagram explaining the last pass of the first half press molding. It is a schematic diagram explaining the final pass of the latter half press forming when the feed amount of the steel sheet is relatively large with respect to the die interval. It is a schematic diagram explaining the final pass of the latter half press forming when the feed amount of a steel plate is relatively small with respect to the die interval. It is a schematic diagram explaining the steel plate feed in the final pass of the latter half press forming when the feed amount of the steel plate is relatively small with respect to the die interval. It is a figure which shows the positional relationship of a steel plate, die | dye, and a punch when the steel plate is arrange | positioned in the state before the last pass of the second half press forming. It is a figure explaining the influence which die space | interval W exerts on the force required for three-point bending press molding. It is a figure explaining the amount of mistakes of the butting part of an open pipe.

Hereinafter, embodiments of the present invention will be specifically described.
FIG. 1 shows a press using a three-point bending press forming machine having a pair of dies arranged at predetermined intervals in the steel sheet feeding direction and supporting the steel sheet at two locations and a punch for pressing the steel sheet between the dies. The process which shape | molds the open pipe before welding of a welded steel pipe by the bend method is shown typically. In addition, in FIG. 1, although the steel plate which provided the end bending is used, it is the same also when the end bending is not provided.

First, as the first half press forming, from the A part to the C part in FIG. 1, the three-point bending press forming and the feeding of the steel sheet are repeated a plurality of times (a times) to form the half of the steel sheet into a substantially circular shape. At this time, the first half press forming is completed without forming the steel plate center portion C. In this specification, this process is referred to as “first half press molding”.

Subsequently, the three-point bending press forming and feeding are repeated a plurality of times (b times) from the other end of the steel plate, from the B part to the C part, and the remaining half is formed into a substantially circular shape. In this latter half press forming, it is preferable that the forming conditions such as the feed amount of the steel sheet and the number of presses (pass times) are the same as the first half press forming so that the shape of the forming part is the same as the first half press forming. . Further, even in the latter half press forming, the steel plate center portion C is not formed. This process is called “second half press molding”. The steel plate after the latter half press forming has a C-shaped shape in which a flat portion remains at the center of the width and the butt portion is greatly opened.

Finally, the flat part at the center of the width of the steel plate is subjected to three-point bending press forming, and the butt opening is closed. This process is referred to as “final press molding”.

The amount of reduction (positional relationship between the die and punch) in the first half press molding and the second half press molding can be arbitrarily selected for each press molding pass to control the molding shape, but the first half press molding and the second half press molding. In order to make the molding shapes of the same the same, it is preferable to make them constant. However, if it is known that the end bending shape, plate thickness, strength, etc. are different between the first half press forming side and the second half press forming side, or if an asymmetric shape is desired in consideration of the subsequent processes, The feed amount, the number of presses, the reduction amount, etc. may be changed between the first half press molding and the second half press molding. In that case, it is preferable to adjust the amount of reduction that can be easily changed on the spot.

In addition, it is preferable that the feed amount of the steel plate for each pass of the steel plate is not more than the die interval. This is because, when the feed amount exceeds the die interval, an undeformed portion remains in the formed steel sheet, resulting in a result that the roundness of the open pipe and thus the product steel pipe is extremely inferior.

FIG. 2 is a schematic diagram for explaining the final pass (a-th) of the first half press molding. When the feeding of the steel plate is completed, the left die in the figure is in contact with the steel plate that has not yet been formed, but the other right die has a steel plate portion with a curvature that has already been formed. Will float from the die. Therefore, when the steel plate is pressed with a punch, the pre-formed side having a curvature is lowered, and press forming starts from a state where the steel plate is inclined. Furthermore, in addition to the difference between the left and right at the start of press forming, a large amount of the pre-formed side is drawn during the press, so that the forming region of the steel plate at the punch bottom dead center is asymmetric with respect to the center of the upper die.

On the other hand, FIG. 3 and FIG. 4 are schematic diagrams for explaining the final pass (b-th) of the latter half press forming. In the latter half press forming, the forming shape changes greatly depending on the relative relationship between the die interval and the feed amount of the steel plate. For example, as shown in FIG. 3, when the feed amount of the steel plate is relatively large with respect to the die interval, the right die in the drawing is in contact with the unformed portion on the center side in the width direction of the steel plate. The other left-side die has a steel plate portion having a curvature that has already been formed, so that the steel plate floats from the die. That is, it is the same state as FIG.

On the other hand, as shown in FIG. 4, when the feed amount of the steel sheet is relatively small with respect to the die interval, the steel sheet portion processed by the first half press forming also comes to the right die in the figure. Therefore, both the left and right dies are in contact with the already formed part. For comparison, the position of the steel plate in FIG. 2 is reversed left and right in FIG. At this time, the amount of inclination of the steel plate at the start of press forming (FIG. 4B) is smaller than that in FIG. 2, and the deformation region at the punch bottom dead center (FIG. 4C) is as shown in FIG. And different. For this reason, a difference arises in a right-and-left shape.

In addition, in the case of FIG. 4, in addition to the difference in the shape at the bottom dead center of the punch, the punch position at the start of press molding is not the same as in the case of FIG. 2 or FIG. A difference occurs in the shape after molding in quantity.

Further, in the case of FIG. 4, the shape of the right steel plate is different from the shape of the left steel plate in FIG. 3, and thus, for example, a device (guide or the like) for feeding the steel plate disposed on the machine side of the press machine Thus, when setting the steel plate position, the set steel plate is displaced at the same guide position as in the first half press forming shown in FIG. 3, leading to a difference in forming shape.

FIG. 5 schematically shows the situation when the steel plate is fed for the final pass of the second half press forming when the feed amount of the same steel plate as in FIG. 4 is relatively small with respect to the die interval. In order to send the steel plate to the final pass position, the steel plate is sent toward the steel plate width end side (left side of the figure), but before reaching the final pass position, the center of gravity of the steel plate is the center of the width of the steel plate. Since it exceeds the right side die in the figure on the side, the left side of the steel plate width side is lowered and comes into contact with the left side die. At this time, the position at which the left end of the steel plate starts to fall and the position in contact with the die also vary depending on the inertial force when feeding the steel plate and the frictional resistance with the die due to the difference in the surface state of the steel plate, so press forming It also causes variation in range.

As described above, as shown in FIG. 4, when the feed amount of the steel sheet is relatively small with respect to the die interval, the range subject to deformation in the final pass differs between the first half press forming and the second half press forming. As shown in FIG. 5, since the posture of the material to be formed at the time of feeding the steel plate becomes unstable, a difference occurs between the left and right in the shape of the steel plate after press forming.

Therefore, the inventors examined the press conditions for preventing the state as shown in FIG. 4 in the latter half press forming.
FIG. 6 shows the positional relationship between the steel plate, the die and the punch when the steel plate is placed in a state before the final pass (b-th) of the second half press forming. Here, L _b , L _n , W, α _b and β _b shown in the figure are defined as follows.
L _b : Molding range (mm) in the final pass of the second half press molding
L _n : Molding range in final press molding (mm)
W: Die spacing (mm)
α _b : Steel sheet position shift rate in the final pass of the second half press forming (−)
β _b : Die position shift rate in the final pass of the second half press forming (−)
However, 0 ≦ α ≦ 1, 0 ≦ β ≦ 1, and when α is 0.5, the center of L _b coincides with the center of the punch, and when α is smaller than 0.5, L _b The center is shifted to the left side from the center of the punch, and when β is 0.5, the center between the dies and the center of the punch coincide, and when β is smaller than 0.5, the center of the die is It will shift to the left from the center of the punch. In this case, the steel sheet feed amount to the final forming (n-th) after the final pass (b-th) of the second half press forming is α _b × L _b + L _n / 2.

As can be seen from FIG. 6, the steel plate portion formed by the first half press forming comes into contact with the right die in the drawing, that is, the same state as FIG.
β _b · W <α _b · L _b + L _n (1)
This is when the condition is satisfied.
Here, when the center of the punch is coincident with the center between the dies and the center of the steel sheet forming range in the final pass is coincident, that is, when α = 0.5 and β = 0.5,
W <L _b + 2L _n (2)
Further, when L _b = L _n , that is, when α = 0.5, β = 0.5 and the steel sheet feed amount is constant,
W <3L _b (3)
Thus, it is understood that the steel sheet feed amount needs to be set to 1/3 or more of the die interval W.

Next, the influence of the die interval W on the force required for the three-point bending press forming will be examined with reference to FIG.
In three-point bending, the material to be formed (steel plate) is deformed by yielding at the end of the forming range. Therefore, a bending moment necessary for plastic deformation of the material to be formed is present at the end of the forming range. Must act. Here, the bending moment required for plastic deformation is a value _Mf determined by the thickness of the molding material and the deformation resistance, while the force acting on the molding material from the die is the reaction force P ₁ and P received from the die. ₄ , and a moment obtained by multiplying this by the distance (L ₁ and L ₄ ) to the deformation point (end of the forming range) acts. Then, when any one or more of P ₁ × L ₁ and P ₄ × L ₄ exceeds M _f , the deformation is started.
However, when the die interval is reduced, the distances L ₁ and L ₄ are also reduced, so that the reaction forces P ₁ and P ₄ required for the deformation are increased and exceed the force of the press machine, and in this case, molding is impossible. It becomes. Accordingly, there is a lower limit value determined by the amount of force of the press, the size of the material to be molded, and the strength of the die interval in the three-point bending press molding.

In order to manufacture a steel pipe using an open pipe press-molded under the above press conditions, for example, after continuous tack welding of an open seam edge of an open pipe using a continuous tack welding apparatus, internal welding and external welding are performed. What is necessary is just to implement this welding in order. Next, in order to increase the roundness of the steel pipe, the main welded steel pipe is preferably expanded using a pipe expanding device. The above expansion ratio (= (outer diameter of pipe after expansion−outer diameter of pipe before expansion) / outer diameter of pipe before expansion × 100 (%)) is usually in the range of 0.3 to 1.5%. However, from the viewpoint of balancing the effect of improving the roundness and the ability required of the pipe expansion device, the pipe expansion ratio is preferably in the range of 0.5 to 1.2%.

After a plate width of 2755 mm, a length of 12192 mm, a plate thickness of 31.8 mm and a strength of API X80 Grade (actual tensile strength of 759 to 778 MPa) is subjected to end face processing with an edge mirror to a plate width of 2745.3 mm, End bending of a bending angle of 18 degrees was performed in the range of 210 mm from the plate ends of both widths using a R280 mm mold.
Next, the end-bending thick steel plate was subjected to three-point bending press forming with a three-point bending press machine having a force of 100MN and variously changing the steel plate feed amount and the die interval, and the outer diameter was 914.4 mm, the length was 12192 mm, The tube was formed into an open tube having a thickness of 31.8 mm, and the amount of misalignment at the butt portion defined in FIG. 8 was measured. The punch outer peripheral surface of the three-point bending press molding was R315 mm, and the die outer peripheral surface was R100 mm.

Table 1 shows the measurement result of the misalignment amount of the butt portion together with the pressing conditions. As the pressing conditions, the number of passes in the second half of the press molding, bending angle theta _b per one _pass, forming range L _b at the final pass of the second half of the press _molding, the range L _n in the final _{press-molding,} the final late press molding The punch shift rate α _b , die shift rate β _b, and die interval W in the pass are shown. For the other passes of the latter half of press forming, the bending angle, forming range, punch shift rate and die shift rate per pass are the same as the final pass of the second half press forming, and the first half press forming is the same number of presses as the second half press forming. , And conditions.
In addition, “insufficient strength” described in the remarks column of Table 1 indicates that the die interval is narrow, and press molding could not be performed due to insufficient strength of the press. On the other hand, regarding the conditions where the pressing force amount was within the capability range, the minimum value and the maximum value of the misalignment amount when five open pipes were formed were shown.
In addition, the evaluation of the amount of misalignment of the butt portion shown in Table 1 is based on the ability to correct the restraint machine at the time of tack welding installed in the welded steel pipe production line used in this example. The following was determined to be acceptable (O) and exceeding 5 mm as unacceptable (x).

From Table 1, the final pass of the second half press forming was performed under the condition that α _b = 0.5, β _b = 0.5 and the condition of the formula (1) of the present invention was satisfied. In any case, the misalignment amount of the butt portion is 5 mm or less, which is the correctable range, in any condition, but exceeds the correctable range if the condition of the expression (1) is not satisfied. It can be seen that there is a mistaking amount.

[Correction 09.07.2014 based on Rule 91]

In the same manner as in Example 1, a thick steel plate having an API X80 Grade strength, an outer diameter of 914.4 mm, a length of 12192 mm, and a plate thickness of 31.8 mm was subjected to three-point bending press forming into an open pipe for a welded steel pipe. At this time, the number of passes of the second half press molding was changed to 9 and 5 times, and the die interval was changed to 360 mm, 380 mm, 620 mm and 640 mm, and the shift amounts α _b and β _b of the final pass in the second half press molding were further changed under the above conditions. Various changes were made. For the other passes of the latter half press forming, the bending angle and forming range per pass were the same as those of the final pass, and the punch shifting rate and die shifting rate were both 0.5. In the first half press molding, the same number of press operations and conditions as in the second half press molding were used.
The difference in the welded portion of each oven tube thus obtained was measured and evaluated in the same manner as in Example 1, and the results are shown in Table 2.

From Table 2, when the final pass of the second half press forming is performed under the condition that satisfies the condition of the expression (1) of the present invention, the amount of misalignment of the welded portion of the open pipe regardless of the values of α _b and β _b Is a small value within the correctable range, and even if the misalignment amount is large, by changing α _b and β _b to appropriate values satisfying the condition of equation (1), It turns out that it can reduce to the range which can be corrected.

In the same manner as in Examples 1 and 2, an open pipe for a welded steel pipe having an API X80 Grade strength, an outer diameter of 914.4 mm, a length of 12192 mm, and a pipe thickness of 31.8 mm was manufactured. At this time, the number of passes of the second half press molding was set to nine, the die interval was set to 360 mm and 380 mm, and the molding range L _b in the final pass of the second half press molding and the molding range L _n in the final press molding are shown in Table 3. Various changes were made as indicated. Further, the bending angle θ _b in the final pass of the second half press molding and the bending angle θ _n in the final press molding were also changed as shown in Table 3. For the other passes of the latter half of press forming, the bending angle, forming range, punch shift rate and die shift rate per pass are the same as the final pass of the second half press forming, and the first half press forming is the same number of presses as the second half press forming. , And conditions.
The difference in the welded portion of each oven tube thus obtained was measured and evaluated in the same manner as in Example 1, and the results are also shown in Table 3.

From Table 3, when the 3-point bending press forming was performed by equation (1) satisfies the conditions of the present invention, regardless of the size of the forming range L _b and L _n, eye welds open pipe It can be seen that the difference amount can be reduced to a value smaller than the correctable range. In particular, the second half even when the number of passes of the press molding 5 times and less, by increasing the molding range L _n to the last press, it can be seen that the suppressed small correctable range tongue and groove volume.

Open pipes for welded steel pipes having various strengths and dimensions were produced in the same manner as in Examples 1 to 3. Table 4 shows the strength grade and dimensions of the product, the radius of the tool used as the end bending condition, the working width (end bending range), the bending angle, and the pressing condition. The punch shifting rate and die shifting rate were set to 0.5. In the other passes of the second half press molding, the bending angle per one pass, the molding range, the punch shift rate and the die shift rate are the same as the final pass of the second half press molding. The same press times and conditions were used.
The difference in the welded portion of each oven tube thus obtained was measured and evaluated in the same manner as in Example 1, and the results are also shown in Table 4.

From Table 4, when three-point bending press molding is performed under the conditions satisfying the condition of the expression (1) of the present invention, regardless of differences in the molding range and bending angle caused by the strength and size of the steel pipe, It can be seen that the amount of misalignment of the welded portion is kept small within a correctable range.

Claims

After the raw steel plate is made into an open pipe by three-point bending press forming with a pair of dies arranged at a predetermined interval in the steel plate feeding direction and a punch for pressing the steel plate between the pair of dies. In the method of manufacturing a steel pipe by welding the open pipe,
From one width end of the steel sheet toward the center of the width (however, leaving the width center) multiple times of the first half press forming, from the width end of the opposite side toward the center of the width (however, the width center (Leave) When the second half press molding is performed multiple times, and finally the center of the width is final press molded to form an open pipe.
A method of manufacturing a welded steel pipe, characterized in that a steel plate supported by a die on the center side of the steel plate width is an unformed portion in the final pass of the latter half press forming.
The method for manufacturing a welded steel pipe according to claim 1, wherein a final pass of the latter half press forming satisfies the following expression (1).
Β b · W <α b · L b + L n (1)
Here, L b : Molding range in the final pass of the second half press molding (mm)
L n : Molding range in final press molding (mm)
W: Die spacing (mm)
α b : Steel sheet position shift rate in the final pass of the second half press forming (−)
β b : Die position shift rate in the final pass of the second half press forming (−)