Analysis of the reasons for the eccentricity (uneven thickness) of seamless steel pipes

The finished products of seamless steel pipes and precision steel pipes 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, and why the concentricity of seamless steel pipes is not so ideal.

Eccentric steel pipes are most likely to be produced in the production process of hot-rolled steel pipes, and most of the links are produced during hot perforation:

Based on the anatomical analysis of steel pipes after rolling by automatic tube rolling mills, we believe that after the perforated rough pipes are rolled by the automatic tube rolling mill, the uneven longitudinal and transverse wall thickness of the steel pipes retains the distribution characteristics of the uneven wall thickness of the perforated rough pipes, that is, the rolled steel pipes still have spiral wall thickness unevenness, and the transverse wall thickness unevenness is significantly increased.

The reasons for the uneven wall thickness of the automatic tube rolling machine are:

① The existence form and severity of the uneven wall thickness of the perforated rough tube directly affect the existence form and severity of the uneven wall thickness of the steel tube after rolling.

② When rolling the tube on the automatic tube rolling machine, the bending of the push rod causes the position of the push head to deviate from the center of the hole type, resulting in uneven wall thickness. The maximum wall thickness and minimum wall thickness positions on each cross-section of the tube and the tube head are almost fixed; while the uneven wall thickness from the tail to the tube head gradually increases. Therefore, reducing the residual bending of the push rod and reducing the axial force of the push rod during tube rolling have a significant effect on reducing the uneven wall thickness.

③ The greater the wall reduction, the more serious the uneven wall thickness of the rough tube. When the wall reduction is small, the automatic tube rolling machine has the effect of reducing the uneven wall thickness of the perforated rough tube.

④ If the hole type is not adjusted correctly when the roll gap is not parallel, the uneven wall thickness of the rough tube will be aggravated. ;

Our seamless steel pipe mill has carried out Fourier transformation on the measured wall thickness data of rough pipes in the four rolling processes of piercing, secondary piercing (extension), automatic pipe rolling and leveling of the Φ400mm automatic pipe rolling unit, and obtained a quantitative analysis of the uneven wall thickness and its causes, and proposed ways to improve the uneven wall thickness of steel pipes on this basis:

① The distribution characteristics of the spiral wall thickness unevenness on the rough pipe after secondary piercing (extension) have been retained until the finished pipe, so improving the secondary piercing (extension) is the key link to improving the wall thickness accuracy of the finished pipe. The main measures are to improve the tool design and improve the concentricity of the ejector and the plug with the rolling line during the rotation process.

② Improving the uneven wall thickness of the rough pipe after piercing is an important link. The main measures are to improve the heating uniformity of the pipe blank, improve the accuracy of the centering hole, lengthen the length of the plug leveling belt and the length of the reverse cone, and improve the concentricity of the ejector and the plug with the rolling line during the rotation process.

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

④ The leveling process can eliminate symmetrical wall thickness unevenness, but it has little effect on eliminating spiral wall thickness unevenness. Therefore, the capacity of the leveling machine should be improved.

⑤ Fourier transform is an effective means to study the uneven wall thickness during the oblique rolling process. This method can also be used to study the uneven wall thickness of other steel pipe production units.

Finished rolled steel pipes; cold drawn steel pipes; due to perforation; eccentricity problems will also occur:

Any factor that destroys the correctness of the geometric shape of the deformation zone formed by the roller, the head, and the guide plate will aggravate the uneven wall thickness of the rough tube. ;

(1) Head.

① The shape design of the plug. The ideal plug-rolling cone should be parallel to the roller outlet cone. If the plug is designed according to the traditional Matveyev formula, the plug rolling cone is not parallel to the roller outlet cone. The metal deforms in such a gradually expanding gap, which will inevitably cause insufficient tube wall rolling and uneven rough tube wall thickness. Moreover, the uneven rough tube wall thickness becomes more serious with the increase of the feed angle;

② Due to the insufficient rigidity of the ejector rod, bending occurs during the piercing process, so that the plug cannot maintain the centering position, resulting in uneven rough tube wall thickness;

③ Uneven wear or damage to the plug. ;

(2) Guide plate.

① The guide plate distance is too large. During the piercing process, the center line of the piercing is maintained by the limiting effect of the guide plate. The large guide plate distance causes a large change in the upper and lower positions of the plug, making the plug unstable and resulting in uneven rough tube wall thickness.

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

(3) Roller. ① Deflection of the center line of the roll: During the production process, due to incorrect installation of the press screws on both sides of the punching machine, or due to wear of the threads and bearings, the axial direction between the two rolls is horizontally deflected, and the feed angles of the two rolls are inconsistent, causing the deformation zone to be distorted and resulting in uneven wall thickness.

② Under large feed angles, the head and the rolling cone of the roll are more non-parallel.

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

(4) Centering and heating of the tube blank.

The eccentricity of the centering hole and uneven heating (positive and negative sides) will cause uneven wall thickness. ;

(5) Rigidity, structure, and adjustment of the punching machine. The body rigidity of the punching machine is not enough, and the locking mechanism on it is unreliable; the centering device of the top 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 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 rough pipe.

Summary:

In summary, there is no way to completely avoid the uneven eccentric thickness of seamless steel pipes, and it can only be strictly controlled step by step to minimize it. If you have any ideas about our analysis, please leave us a message for discussion.