Causes of eccentricity (uneven thickness) of seamless steel pipes

The finished products of seamless steel pipe and precision steel pipe will have the problem of uneven eccentric thickness, but how is it produced? Many people don't understand. Today, let's talk about how the eccentricity of seamless steel pipes such as cold-drawn steel pipes, cold-rolled steel pipes, and hot-rolled steel pipes is produced. Why is the concentricity of seamless steel pipes not so ideal? Eccentric steel pipes are most likely to occur in the production process of hot-rolled steel pipes, and most of the produced links are produced during hot piercing: According to the anatomical analysis of the steel pipes rolled by the automatic pipe rolling machine, we believe that the perforated capillary pipe is rolled by the automatic pipe rolling machine. After rolling, the uneven wall thickness in the vertical and horizontal directions of the steel pipe basically retains the distribution characteristics of the uneven wall thickness of the perforated capillary tube, that is, the steel pipe still has a spiral-shaped uneven wall thickness after rolling, and the horizontal wall thickness unevenness increases significantly.

The reasons for the uneven wall thickness of the automatic pipe rolling mill are:

① The existence form and severity of the uneven wall thickness of the perforated capillary directly affect the existing form and severity of the uneven wall thickness of the rolled steel pipe.

②When the pipe is rolled on the automatic pipe rolling machine, the position of the plug is deviated from the center of the pass due to the bending of the ejector rod, resulting in uneven wall thickness, and the wall thickness and minimum wall thickness position in the pipe and on each cross-section of the pipe head are almost fixed change; while the degree of wall thickness unevenness from the end of the pipe to the head of the pipe gradually increases. Therefore, reducing the residual curvature of the ejector rod and reducing the axial force of the ejector rod during pipe rolling have a significant effect on reducing the degree of wall thickness unevenness.

③The greater the amount of wall reduction, the more serious the uneven wall thickness of the waste pipe. When the amount of wall reduction is small, the automatic pipe rolling machine can reduce the uneven wall thickness of the perforated capillary tube.

④The pass adjustment is not correct. When the roll gap is not parallel, the uneven wall thickness of the waste pipe will be aggravated.

We carried out Fourier transform on the measured wall thickness data of blank tubes in the four rolling processes of Φ400mm automatic tube rolling unit, piercing, secondary piercing (extension), automatic tube rolling and leveling, and obtained the quantitative analysis and analysis of wall thickness unevenness. The reasons for its formation, and based on this, a way to improve the uneven wall thickness of steel pipes is proposed:

① After the secondary perforation (extension), the distribution characteristics of the spiral uneven wall thickness on the waste pipe are retained until the finished pipe. Therefore, improving the secondary perforation (extension) is the key link to improving the wall thickness accuracy of the finished pipe. The main measure is to improve The tool design improves the concentricity of the ejector pin and the plug with the rolling line during the rotation.

② Improving the uneven wall thickness of the perforated capillary is an important link. The main measures are to improve the heating uniformity of the tube blank, improve the accuracy of the centering hole, lengthen the length of the uniform belt of the plug and the length of the reverse cone, and improve the distance between the ejector pin and the plug. Concentricity with the roll line during rotation.

③ Although serious symmetrical wall thickness unevenness will occur during pipe rolling, it has a certain effect on reducing spiral wall thickness unevenness. Therefore, when rolling the tube, it should be rolled twice, and the blank tube should be turned 90° between the passes.

④The leveling process can eliminate the uneven wall thickness of symmetry, but it has little effect on eliminating the uneven wall thickness of the spiral shape. Therefore, the ability of the leveling machine should be improved.

⑤Fourier transform is an effective means to study the uneven wall thickness of the cross-rolling process, and this method can also be used for the research on the uneven wall thickness of other steel pipe production units.

Finish-rolled steel pipes and cold-drawn steel pipes also have eccentricity problems due to perforation:

Any factor that destroys the geometrical correctness of the deformation zone formed by the roll, the plug, and the guide plate will aggravate the uneven wall thickness of the capillary.

(1) Head.

① For the shape design of the plug, the rolling cone of the ideal plug should be parallel to the exit cone of the roll. If the plug is designed according to the traditional Matveyev formula, the rolling cone of the plug is not parallel to the exit cone of the roll. The deformation in such a gradually expanding gap will inevitably lead to insufficient rolling of the tube wall, resulting in uneven wall thickness of the capillary tube, and the uneven wall thickness of the capillary tube will become more serious with the increase of the feeding angle;

②Because the rigidity of the ejector rod is not enough, it bends during the piercing process so that the plug cannot maintain the centered position so that the wall thickness of the capillary pipe that passes through is uneven;

③ Uneven wear or damage to the plug.

(2) Guide plate.

①The distance between the guide plates is too large. During the perforation process, the center line of the perforation is maintained by the restriction of the guide plate. If the distance between the guide plates is large, the position of the plug changes greatly, making the plug unstable and causing uneven wall thickness of the capillary.

②The uneven wear of the upper and lower guide plates will also aggravate the uneven wall thickness.

(3) Roller.

①Roll center line deflection: During the production process, due to incorrect installation of the pressing screws on both sides of the piercing machine, or due to thread and bearing wear, the axial direction between the two rollers is horizontally deflected, and the feed angles of the two rollers are inconsistent. Distorts the deformed zone resulting in uneven wall thickness.

②The rolling cone of the plug and the roll is more non-parallel under the large feeding angle.

③ Improper roll speed will also affect the wall thickness accuracy.

(4) Centering and heating of the tube blank.

Both eccentric centering holes and uneven heating (male and female sides) will cause uneven wall thickness.

(5) Rigidity, structure, and adjustment of the punching machine.

The rigidity of the body of the piercing machine is not enough, and the locking mechanism on it is unreliable; the centering device of the ejector rod is not adjusted accurately, the operation is unreliable and the distance from the body is far; the adjustment of the rolling center line is generally lower than the center line of the rolling mill. , the purpose of which is to improve the stability of the rolled piece. If the adjustment is too large, the relative relationship between the tools in the deformation zone will change asymmetrically after the rolling line moves down, which will also affect the uneven wall thickness of the capillary.

Therefore, there is no way to completely avoid the uneven eccentric thickness of the seamless steel pipe, and it can only be strictly controlled step by step to minimize it.