JPH0639444A - Warp correction method for welded steel pipe - Google Patents

Abstract

PURPOSE:To prevent drop of productivity by countervailing the warp of a pipe stock caused by a residual welding stress and reducing the load on a turks- head, etc., to stably manufacture a straight pipe in a manufacturing method for a welded steel pipe. CONSTITUTION:By setting the height of a roll throat of a final stand in a fin path provided immediately before a squeeze roll stand 7 higher than the height 11 of a mill path line by DELTAH, a bending strain is applied toward the bottom of pipe backing the seam line by three point bending with a fin path roll 5 on the fore stage of a roll 5a and three rolls of the squeeze roll stand 7 and the warp of pipe is reduced by countervailing the warp of pipe stock caused by the residual welding stress.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a welded steel pipe in which a seam wire extends parallel to the pipe axis, and more specifically, a method for straightening a warp in a welded steel pipe for suppressing the warp of a raw pipe after welding. Regarding

[0002]

2. Description of the Related Art FIG. 2 shows a general equipment outline of a welded steel pipe production line. The strip 1 wound into a coil is unwound by the uncoiler 2, flattened by the leveler 3, and then formed by the breakdown roll 4 and the fin pass roll 5 from a flat plate into a shape close to a circular cross section. The edge portion is melted, the molten steel is removed by the squeeze roll 7, and at the same time, the seam portion is welded. Next, the conventional method is to cool the welded part in the cooling zone 8 and then form it into a regular shape and a perfect circle with the sizing roll 9 and correct the warp with the turks head 10 to manufacture a straight steel pipe. In addition,
A cage roll may be used instead of the breakdown roll 4 for the rough forming of the strip 1.

[0003]

The outer diameter is relatively small,
When the plate thickness is large, the turks head 10 cannot sufficiently correct the warp of the pipe, and in some cases, the warp of the pipe remains and is sent downstream. In this case, the pipe does not roll smoothly on the transport table, and transport troubles often occur. Further, since the straightness of the pipe does not satisfy the product standard, it is often re-corrected by a rotary type straightening machine off-line. As described above, if the pipe is warped, the productivity is lowered due to transport troubles and extra steps. Therefore, when the pipe warps,
The operator adjusts the settings to deal with the turks head 10. However, it is not easy to take sufficient warpage.

As a countermeasure, there is a manufacturing facility in which a parallel roller type online warp straightening machine is arranged on the exit side of the sizing roll 9, but after the squeeze roll 7, the seam portion is twisted clockwise or counterclockwise. In some cases, it does not have a sufficient correction effect. FIG. 3 shows an example of seam twist generated after welding. The horizontal axis represents the distance from the center of the squeeze roll 7, and the vertical axis represents the twist angle. As shown in FIG. 3, the twist angle gradually increases toward the downstream side of the squeeze roll, and the twist angle becomes 90 ° at the location of the turkey head 10.
It may twist as much as 180 degrees.

As a countermeasure against such a warp of the pipe, for example, as shown in FIG. 4 of Japanese Patent Application Laid-Open No. 62-197222, a pair of twist correction rolls 21 inclined after the squeeze roll 7 in opposite directions are provided. , 22 are arranged, and the straightening rolls 21 and 22 are rotated as shown by an arrow 23, a rotational torque 24 is applied to the pipe 20 which advances in the direction of an arrow 25, and the seam position is held in a state parallel to the pipe axis and twisted. There has been proposed a technique for keeping the straightening effect of the turks head 10 or the online straightening machine constant by suppressing the above. However, this technique has the following drawbacks. (1) A straightening device including a straightening roll, a seam twist detection device, a feedforward control device, and the like becomes large-scaled, and the equipment cost increases. (2) The contact portion between the straightening roll and the tube surface slips and the speed difference is large, and roll flaws are likely to occur. (3) The straightening roll deforms the pipe and reduces the roundness,
The forming load of the sizing roll 9 in the subsequent process increases.

According to the present invention, the warp direction of the raw pipe after welding by the squeeze roll 7 is analyzed, and the warp in the direction opposite to the warp direction of the raw pipe is made into the raw pipe in the vicinity of the squeeze roll where the twist of the seam portion does not increase. The purpose is to reduce the amount of warpage of the pipe, reduce the load on the turks head 10 or the online straightening machine, stably manufacture a straight pipe, and prevent a decrease in productivity.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides a method for producing a welded steel pipe, in which a continuously fed strip is formed into a cylindrical raw pipe by a forming roll group and seam edges are seam welded. Pipe making by increasing the roll throat height of the fin pass final stand installed immediately before the squeeze roll stand for welding the edge part by a height within the range of 3 to 15% of the pipe outer diameter from the height of the mill's pass line. It is characterized by doing. In other words, a roll that changes the roll throat height, its pre-stage roll, and three rolls of a squeeze roll give bending strain in the pipe bottom direction with the seam line as the back, and warp of the raw pipe due to welding residual stress. It is characterized by suppressing and making pipes.

[0008]

The present inventor investigated and analyzed the factors causing the warpage of the pipe. As a result, at the stage of forming a cylindrical blank tube from the strip, if the roll rolling conditions of the break down roll and the fin pass roll are changed or the downhill conditions are changed, the warpage direction of the blank pipe after squeeze roll is somewhat I knew it would change. In addition, the direction of the warp may be slightly different when making a strip with a so-called camber in which the centerline of the strip is not a straight line, or when the mill equipment has poor accuracy and the center of the forming stand is misaligned. I knew it would change. However, it was found that the amount of warpage due to these factors was not large and was absorbed by the warpage due to the residual stress due to welding of the seam portion, which is the main cause. After welding with the squeeze roll, a compressive residual stress is generated in the seam portion, and the base pipe warps with the seam wire as an antinode. After the squeeze roll, the seam line is twisted in the circumferential direction, and for example, when the seam line is twisted by 90 ° at the location of the turks head, the warp direction of the pipe is also twisted by 90 °. This is because the seam line is always angry and warped. The reason why the correction effect is not stable in the Turks head or the online warp straightening machine is that the warp direction of the pipe is not constant due to the twisting of the pipe.

Therefore, as a method for correcting the warp of the pipe, a method of correcting the twist of the pipe to maintain the seam position in a state parallel to the pipe axis to keep the correction effect of the turks head or the like constant, and a method upstream from the turks head or the like. A method is conceivable in which strain in the direction opposite to the warpage of the raw pipe is added to reduce the warpage of the raw pipe and the molding load of the turks head or the like is reduced. The present invention is the latter method, in which the warp in the direction opposite to the warp direction of the raw pipe due to the residual welding stress is added in the vicinity of the squeeze roll where there is almost no twist in the seam portion. That is, as shown in FIG. 1, the fin throat final stand (roll 5a) installed immediately before the squeeze roll stand 7 has the roll throat height 1 in the direction indicated by arrow 15
Roll 5 by setting ΔH higher than 1
3 of fin pass roll 5 and squeeze roll 7 in front of a
By the three-point bending by the individual rolls, bending strain can be applied in the direction of the pipe bottom with the seam line as the back, and the warping of the raw pipe due to the welding residual stress can be offset and the warping can be reduced. As a result, the load on the turks head and the like is reduced, a straight pipe can be stably manufactured, and a decrease in productivity can be prevented.

The reason why the roll throat height increase ΔH of the fin pass final stand installed immediately before the squeeze roll 7 is limited to the range of 3 to 15% of the pipe outer diameter will be described. This is because if the lower limit value is set to less than 3%, the effect of three-point bending will be small and the warp of the raw pipe will not be sufficiently taken, and if the upper limit value exceeds 15%, bending strain will be too large and the seam portion will be totally This is because the warp remains on the back and the surface pressure of the flange portion of the roll increases, so that roll flaws are likely to occur.

[0011]

EXAMPLES Examples will be shown below. The method of the present invention was applied to the electric resistance welded pipe manufacturing mill having the line configuration shown in FIG. Outer diameter 3
Various pipe sizes in the range of 1.8 to 60.5 mm and plate thickness of 3 to 8 mm were produced. The rise ΔH of the roll throat height of the fin stand final stand installed immediately before the squeeze roll with respect to the mill pass line height is 0 to 20% of the pipe outer diameter.
By changing the range, it was examined whether the product tube was warped or the roll was flawed. The results are shown in Table 1. For each tube size, if the increase ΔH is in the range of 3 to 15% of the tube diameter, no roll defects occur, the warpage amount per 1 m of tube length is less than 1 mm, and a straight tube can be manufactured. It was confirmed.

[0012]

[Table 1]

[0013]

As described above, according to the present invention, by setting the roll throat height of the fin pass final stand installed immediately before the squeeze roll to be higher than the pass line height, the warp of the raw pipe due to the welding residual stress can be achieved. Can be offset, the load on the turks head and the like can be reduced, a straight pipe can be stably manufactured, and a decrease in productivity can be prevented.

[Brief description of drawings]

FIG. 1 is a side view of a warp straightening portion of a welded steel pipe for explaining the method of the present invention.

FIG. 2 is a system diagram showing a general equipment outline of a welded pipe manufacturing line.

FIG. 3 is a graph showing an example of the twist of the seam wire after welding.

FIG. 4 is an explanatory view of a conventional straightening roll for suppressing the warpage of a pipe.

[Explanation of symbols]

1 strip 2 uncoiler 3 leveler 4 break down roll 5,5a fin pass roll 6 welder 7 squeeze roll 8 cooling zone 9 sizing roll 10 turks head 11 pass line 15, 16 arrow 20 pipe 21, 22 straightening roll 24 rotational torque

Claims (1)

[Claims] 1. A method for producing a welded steel pipe in which a continuously fed strip is formed into a cylindrical raw tube by a forming roll group and the seam edge portion is welded, in a squeeze roll stand for welding the seam edge portion. A method for straightening a warp of a welded steel pipe, characterized in that the roll throat height of the immediately preceding fin pass final stand is raised from the height of the pass line of the mill by a height in the range of 3 to 15% of the pipe outer diameter.