JPH10258312A - Manufacture of welded tube excellent in roundness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the welded tube of product having high roundness in all of the tip, rear end and the middle. SOLUTION: After bending a metallic strip 2 in the width direction and executing butt welding 7 of both end parts in the width direction in a state where the cross section is held in an approximately round shape, the welded tube stock 8 is flatly worked with over-bend rolls 9 so that the distance from the weld zone to the tube-bottom part becomes smaller than the targeted outside diameter of product and, next, the welded tube stock is formed into an approximately round shape with straightening rolls 11. In flattening work, it is preferable to flatten the welded tube stock so that the distance from the weld zone to the tube-bottom part is 3-10% smaller than the targeted outside diameter of product. When the out-of-roundness in the middle part of the tube is degraded by the flattening work, the rolling reduction with the straightening rolls 11 is adjusted in accordance with the amount of flattening with the over-bend rolls 9. Then, because residual stress is removed by the flattening work and straightening, the roundness of the welded tube of product which is obtained by cutting the welded tube stock 8 into standard length is not degraded even at the tube ends.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a welded pipe having excellent roundness over its entire length including the pipe end.

[0002]

2. Description of the Related Art When a welded pipe is manufactured by a roll forming method,
A metal strip slit to a predetermined width is used as a tube material. The metal strip is sequentially bent in the width direction at each of the forming stands arranged in multiple stages, and is finally formed into a cylindrical shape in which both ends in the width direction are abutted. Next, the both ends in the width direction are butt-welded to obtain a raw tube. This raw tube is passed through a sizing process, and the diameter of the tube is reduced by a plurality of forming rolls to improve dimensional accuracy and roundness. Then, it is cut to a fixed size to form a product tube. In the roll forming method, a multi-stage arrangement of forming stands is required. Moreover, if the diameter of the tube to be manufactured is different, it is required to change the forming rolls of each stand accordingly. Changing the forming rolls and adjusting between the stands is a very troublesome operation, and is a bottleneck in improving productivity. Therefore, the present inventors have developed a rollless forming method that does not require a large number of forming rolls by utilizing the property that the metal band curls in the plate width direction when bending and bending-back distortion is applied to the metal band ( Japanese Patent Publication No. 6-77773. In this method, a metal strip is continuously bent and bent back with the inner surface of the pipe inside using a small-diameter bending roll parallel to the sheet width direction, and the bending in the sheet width direction due to bending and bending back is used. To form a metal strip into a cylindrical shape.

[0003]

In the pipe forming by the roll forming method, the curvature of the roll surface varies depending on the circumferential position of the pipe in contact with the roll surface. Therefore, depending on the circumferential position of the tube, the form of processing and the amount of distortion received by the forming rolls are different. The welded pipe that has undergone such a forming process has an inherent stress distribution that varies along the pipe circumferential direction, and when the pipe is cut, the cross-sectional shape of the cut end surface changes due to residual stress. Lowers the roundness of the part. In particular, when the welded pipe to be manufactured is a thin-walled pipe having a small thickness ratio (wall thickness / outer diameter of the pipe) or a pipe made of a high-strength material, the roundness tends to be reduced. Even when the pipe is formed by the rollless forming method, the roundness at the pipe end is still reduced, though not as much as the roll forming method. The present invention has been devised to solve such a problem, and releases residual stress by flattening after welding, suppresses deformation caused by residual stress even at the pipe end, and has a high roundness. The purpose is to manufacture welded tubes.

[0004]

In order to achieve the object, a method for manufacturing a welded pipe according to the present invention is such that a metal strip is bent in the sheet width direction and the cross section is maintained in a substantially circular shape in the sheet width direction. After butt-welding the ends, the welded tube is flattened with an overbend roll so that the distance from the welded portion to the tube bottom is smaller than the target product outer diameter, and then the welded tube is straightened into a substantially circular shape It is characterized by being formed by a roll. In the flattening, it is preferable to flatten the welded tube so that the distance from the welded portion to the tube bottom is smaller than the target product outer diameter by 3 to 10%. The roundness of the pipe end is ensured by the flattening, but the roundness of the central part of the pipe may be reduced. In such a case, it is preferable to adjust the diameter reduction ratio by the straightening roll according to the flatness by the overbend roll so that the numerical value given by Expression (1) is in the range of 5 to 11. Flatness rate (%) + 10 × diameter drawing rate (%) ... (1)

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a tube forming by a roll forming method, a metal band is sequentially bent in a sheet width direction at each forming stand and formed into a cylindrical shape. In the rollless forming method shown in FIG. 1, the metal band 2 unwound from the uncoiler 1 is bent / unbent by a small-diameter roll 3. Metal strip 2
Is curled in the plate width direction in the process of reaching the side rolls 4 due to the bending in the plate width direction generated by the bending / unbending process, and is deformed into a cylindrical shape. Metal strip 2 formed into a cylindrical shape
Is fed to the squeeze roll 6 while being guided at both ends in the sheet width direction by the seam guide rolls 5, and both ends in the sheet width direction are butt-welded by the welding device 7. After the welding pipe 8 is flattened by the overbend roll 9, the drawing device 10 is used.
After that, it is processed into a perfect circular shape by the straightening roll 11. Next, it is cut to a fixed size by the cutting device 12 to form a product welded pipe.

[0006] In both the roll forming method and the rollless forming method, the shape and magnitude of the distortion generated in the process of forming the metal band 1 into a cylindrical shape vary with respect to the width direction of the metal band 1. different. Also, when adjusting the cross-sectional shape of the welding pipe 8 with the straightening roll 11, the straightening roll 1
The distortion from 1 is not uniform in the circumferential direction of the welding pipe 8. As described above, the non-uniform residual stress is distributed along the circumferential direction in the weld pipe 8 having passed through the complicated deformation path, and the distribution state of the residual stress cannot be grasped in advance. The residual stress is in a state of being balanced at a position away from the pipe end. Therefore, when the straightening condition by the straightening roll 11 is properly selected, the roundness of the welding pipe 8 can be ensured. However, when the weld pipe 8 is cut, the balance of the residual stress in the vicinity of the cut end face is broken, and the residual stress is balanced again with the pipe end deformed. Therefore, even if the straightening condition is selected so that the roundness of the welding pipe 8 becomes good,
In a product weld pipe obtained by cutting the weld pipe 8, roundness near the pipe end deteriorates.

In the present invention, in order to prevent deformation as a result of such a state in which the residual stress is out of balance,
After the metal strip 1 is welded while being held in a substantially perfect shape by the squeeze roll 6, the overbend roll 9 flattens the distance from the welded portion to the pipe bottom so as to be smaller than the target product outer diameter. As the overbend roll 9, a target product radius × 1.1 to 1.4 is added to a portion including the roll minimum diameter portion.
Rolls with calibers of a moderate radius are preferred.
By such an overbend roll 9, the welding blank 8 is flattened with a flattening amount of 3 to 10% based on the target product outer diameter. Regarding the caliber shape, it is difficult to flatten the caliber with a radius equal to the target product radius at all, so it is necessary to make the caliber a radius larger than the target product radius. Specifically, it is possible to perform a desired flat deformation with a caliber having a radius of about 1 to 1.4 times the target product radius. As shown in Table 1, in order to secure good roundness of the pipe in combination with diameter drawing by a straightening roll performed following flattening, the flattened pipe is flattened within a range of 3 to 10% of the diameter of the welded pipe. It is necessary to adjust the rate and further restrict the numerical value given by the equation (1) to a range of 5 to 11. Flatness rate (%) + 10 x diameter drawing rate ... (1)

[0008]

If the flatness is out of the range of 3 to 10% of the diameter of the welded pipe, or if the numerical value given by the equation (1) is out of the range of 5 to 11, the roundness deteriorates as shown in FIG. . In addition, the cross section of the pipe immediately after welding has a so-called roofing in which the weld is deformed into a roof shape due to residual stress because the weld is hot and has lower strength than the other parts. In this case, a vertically long cross-sectional shape is obtained. Therefore, it is necessary to improve the shape of the entire cross section of the pipe and the vicinity of the welded portion by pressing down the pipe after welding from above and below to make the pipe flat in the flat direction. When the welding pipe 8 is flattened, new residual stress is generated. This residual stress is
The effect of the residual stress generated by the processing history up to that point is almost eliminated. The residual stress generated in the flattening is offset by the residual stress in the opposite direction generated when the welding pipe 8 is formed into a perfect circular shape by the straightening roll 11. Therefore, the residual stress of the welding pipe 8 that has passed through the straightening roll 11 is significantly reduced as compared to the state where the pipe 8 remains welded after forming. Therefore, even when the welding tube 8 is cut to a fixed size, deformation such as opening and closing of the mouth does not occur on the cut end surface. The effect of flattening on the improvement of roundness shows a different tendency at the center, tip and rear end of the product welded pipe. That is,
The required roundness is maintained at the pipe end as the flattening amount increases, but the pipe may be excessively flattened at the center.
In such a case, the roundness of the center of the pipe is secured by performing a process of reducing the diameter of the welding pipe 8 with the straightening roll 11 and adjusting the amount of reduction.

[0010]

EXAMPLE As a pipe material, C: 0.065% by weight, S
i: 0.50% by weight, Mn: 0.30% by weight, P: 0.
025% by weight, S: 0.001% by weight, Ni: 2.00
%, Cr: 16.30% by weight, N: 0.010% by weight, thickness 0.35 mm, 0.2% proof stress 93
A high-strength stainless steel strip of 5 MPa, tensile strength of 1170 MPa, elongation of 9.0%, and Vickers hardness HV380 was used. This stainless steel strip is slit into a width of 79.3 mm, and is continuously cut in the longitudinal direction of the steel strip 2 by a small-diameter bending roll 3 having a diameter of 8 mm arranged in parallel with the width direction of the steel strip 2 as shown in FIG. Was subjected to bending and bending back processing. Steel strip 2 utilizing the warpage in the sheet width direction generated by bending and unbending
Was formed into a cylindrical shape, and both ends of the steel strip 2 in the sheet width direction were butt-welded at a welding speed of 10 m / min with the squeeze roll 6 holding the cross section in a substantially circular shape. As shown in FIG. 3, the squeeze roll 6 has a diameter d _s = 25.4 mm.
Roll pairs forming a gap of.

Subsequently, the welding blank 8 was flattened by the overbend roll 9 such that the distance from the welded portion to the bottom of the tube was smaller than the target product outer diameter of 25.4 mm.
The overbend roll 9 has a caliber radius R shown in FIG.
A roll pair having _B = 15.5 mm, roll bottom diameter D ₁ = 60 mm, and roll body diameter D ₂ = 82 mm was used. Also, an overbend roll 9 having a caliber corresponding to an appropriate flattening amount.
As shown in FIG. 5, the major radius of the caliber R _B1 = 1
6.04Mm, short radius R _B2 = 11.03mm, roll bottom diameter D ₁ = 60 mm, it was used roll pairs of the roll barrel diameter D ₂ = 82.4mm. The roll of FIG. 4 has a region where the roll does not come into contact with the pipe at the roll joining portion. In the roll of FIG.
It contacts the pipe surface almost entirely in the circumferential direction.
In this respect, the roll method shown in FIG. 5 is preferable because the roll method shown in FIG. 5 does not cause local bending deformation at the rolled portion. After the flattened welding tube 8 was pulled out by the drawing device 10, the welding tube 8 was formed into a substantially perfect shape by the straightening roll 11 without drawing in the diameter direction.
As shown in FIG. 6, the straightening roll 11 has a diameter d _c =
Bottom diameter D ₃ = 60 mm forming a gap of 25.4 mm
Roll pairs were used. The formed weld pipe 8 was cut to a fixed size to obtain a product weld pipe having a diameter of 25.4 mm.

In flattening, the flattening amount is from 0 mm to 3 m
m, and the roundness (longitudinal diameter-horizontal diameter) at the front end, the center, and the rear end of the product welded pipe cut to the fixed size was measured.
As can be seen from the measurement results of FIG. 7, the roundness of the product welded pipe not subjected to flattening was a value exceeding 4% of the pipe outer diameter. The condition of flatness 0 is different from No. 1 in Table 1,
Since the reduction of the flat roll was set to 0, the roofing shape of the welded portion remained as it was, and the center of the pipe became extremely elongated vertically. Normally, when a certain degree of roundness at the center is secured, the shape of the pipe end often collapses. However, in the case of such extreme conditions, although the pipe end is still vertically elongated due to the balance of the residual stress, it is presumed that the extremely vertically elongated center is deformed in a direction in which the degree of the elongated shape is reduced. . On the other hand, in the flat product welded pipe, the pipe section tended to change from vertical to horizontal with increasing flatness at any position. In particular, when the flattening is performed with the flattening amount in the range of about 1.5 to 2 mm, the product welded pipe has a good shape with roundness of about 1% or less of the outer diameter of the pipe at any of the front, center, and rear ends. was gotten. The flatness of 1.5 to 2 mm corresponds to 5.9 to 7.9% of the target product outer diameter.

As shown in FIG. 8, the ratio of change from the vertical to the horizontal as the flattening amount increases is larger at the center of the tube than at the front and rear ends. Therefore, if the flatness is set so as to secure the roundness at the front end and the rear end, there is a possibility that the center of the pipe may become excessively flattened horizontally. Therefore, the flattening amount was fixed to 2 mm, and the straightening roll 11 was pressed down to a maximum of 0.15 mm, thereby performing a process of reducing the diameter of the welding pipe 8. In this case, as shown in FIG. 8, there was a tendency that the center of the product welded pipe was changed from the horizontal to the vertical, and the front and rear ends were changed from the vertical to the horizontal as the reduction amount was increased. From this result, even when the cross-sectional shapes of the tube central portion and the tube end do not fall within the desired ranges at the same time when flattening is performed, the adjustment of the amount of reduction by the straightening roll 11 is combined with the flattening to obtain substantially the same cross section in the longitudinal direction. It can be seen that a product welded tube having a shape can be obtained. The appropriate relationship between the flattening amount and the amount of reduction of the straightening roll 11 is that the magnitude of the residual stress itself varies depending on factors such as the diameter of each roll, the arrangement of the roll stand, the dimensions of the pipe such as the diameter and plate thickness, and the material characteristics. Can not be determined in a straightforward manner. Therefore, it is necessary to experimentally determine the relationship between the two,
Under the conditions used, the flatness was 3 to 10 times the outer diameter of the tube.
%, And the amount of reduction of the pipe diameter due to the reduction of the straightening roll 11 was about 0 to 0.5% of the outer circumference of the pipe.

[0014]

As described above, in the present invention, by flattening a welded pipe and then forming it into a perfect circular shape by using a straightening roll, residual stress at the time of pipe making is removed, and When the pipe is cut into a product weld pipe, the roundness does not decrease even at the pipe end. Therefore, a welded pipe having a good cross-sectional shape over the entire length is manufactured at low cost without the need for annealing for removing residual stress.

[Brief description of the drawings]

FIG. 1 is a pipe production line by a rollless forming method employed in an embodiment of the present invention.

Fig. 2 Effect of molding conditions on roundness

FIG. 3 is a profile of a squeeze roll in the pipe making line of FIG.

Fig. 4 Profile of overbend roll in the same pipe production line

FIG. 5 Profiles of other overbend rolls in the pipe production line

Fig. 6 Profile of straightening roll in the same tube making line

Fig. 7 Effect of flatness on roundness of product welded pipe

FIG. 8: Effect of reduction amount of straightening roll on roundness of product welded pipe

[Explanation of symbols]

1: Uncoiler 2: Metal strip (steel strip) 3: Small-diameter roll 4: Side roll 5: Seam guide roll 6: Squeeze roll 7: Welding device 8: Welding tube 9: Overbend roll 10: Pulling device 11: Straightening roll 12: cutting device d _s: crown diameter R _B of squeeze rolls, R _B1, R _B2: over caliber of the bend roll radius D _1: over bend roll of the bottom diameter D _2: over-bend the roll body diameter d _c: straightening rolls Diameter D ₃ : Bottom diameter of straightening roll

Claims (3)

[Claims] 1. A metal strip is bent in a sheet width direction, and both ends in a sheet width direction are butt-welded in a state where a cross section is maintained in a substantially circular shape, and then a distance from a welded portion to a pipe bottom is out of a target product. A method of manufacturing a welded pipe having excellent roundness, wherein the welded pipe is flattened with an overbend roll so as to have a diameter smaller than the diameter, and then the welded pipe is formed into a substantially circular shape with a straightening roll. . 2. The roundness of the welded pipe according to claim 1, wherein the welded pipe is flattened by an overbend roll so that the distance from the welded portion to the pipe bottom is smaller than the target product outer diameter by 3 to 10%. Manufacturing method of welded pipe. 3. The perfect circle according to claim 1, wherein the reduction ratio of the diameter by the straightening roll is adjusted so that the numerical value given by the expression (1) is in the range of 5 to 11 according to the flatness by the overbend roll. Manufacturing method of welded pipe with excellent degree. Flatness rate (%) + 10 × diameter drawing rate (%) ... (1)