Finished seamless steel pipes and precision steel pipes often exhibit eccentricity and uneven thickness, but how does this occur? Many people don't understand. Today, we'll discuss how eccentricity arises in seamless steel pipes such as cold-drawn, cold-rolled, and hot-rolled steel pipes, and why the concentricity of seamless steel pipes is never ideal.

Eccentricity is most likely to occur during the hot-rolling process, primarily during hot piercing. Based on the dissection and analysis of steel pipes rolled by automatic pipe rolling mills, we believe that after the pierced tube is rolled by an automatic pipe rolling mill, the longitudinal and transverse wall thickness unevenness of the steel pipe essentially retains the distribution characteristics of the pierced tube's wall thickness unevenness. That is, the rolled steel pipe still has a spiral-shaped wall thickness unevenness, and the transverse wall thickness unevenness is significantly increased.

The reasons for uneven wall thickness in seamless steel pipes produced by automatic rolling mills are as follows:
① The form and severity of uneven wall thickness in the pierced tube directly affect the form and severity of uneven wall thickness in the rolled seamless steel pipe.
② During the rolling of seamless steel pipes on an automatic rolling mill, the bending of the mandrel causes the mandrel position to deviate from the center of the die, resulting in uneven wall thickness. The wall thickness and minimum wall thickness positions on each cross-section of the seamless steel pipe at the beginning and middle sections remain almost constant; however, the unevenness gradually increases from the end to the beginning of the seamless steel pipe. Therefore, reducing the residual bending of the mandrel and decreasing the axial force of the mandrel during rolling significantly reduces the degree of uneven wall thickness.
③ The greater the wall reduction, the more severe the uneven wall thickness of the rough pipe. Smaller wall reductions can reduce the uneven wall thickness of the pierced tube.
④ Incorrect die adjustment, such as non-parallel roll gaps, will exacerbate the uneven wall thickness of the rough pipe.

We performed a Fourier transform on the measured wall thickness data of rough tubes from four rolling processes—piercing, secondary piercing (extension), automatic rolling, and leveling—on a Φ400mm automatic tube rolling mill. This yielded a quantitative analysis of wall thickness unevenness and its causes. Based on this analysis, we proposed ways to improve wall thickness unevenness in steel tubes:
① The spiral-shaped wall thickness unevenness distribution on the rough tube after secondary piercing (extension) is retained until the finished tube. Therefore, improving secondary piercing (extension) is a key step in improving the wall thickness accuracy of the finished tube. The main measures are to improve tool design and increase the concentricity of the mandrel and the mandrel with the rolling line during rotation.
② Improving the wall thickness unevenness of the rough tube after piercing is also important. The main measures are to improve the heating uniformity of the billet, improve the accuracy of the centering hole, lengthen the length of the mandrel leveling zone and the reverse cone, and improve the concentricity of the mandrel and the mandrel with the rolling line during rotation.
③ Although rolling seamless steel tubes produces severe symmetrical wall thickness unevenness, it has a certain effect on mitigating the spiral-shaped wall thickness unevenness. Therefore, seamless steel pipes should be rolled in two passes, with the rough pipe rotated 90° between passes.
④ The leveling process can largely eliminate symmetrical wall thickness unevenness, but its effect on eliminating spiral wall thickness unevenness is minimal. Therefore, the capacity of the leveling machine should be improved.
⑤ The Fourier transform is an effective means of studying wall thickness unevenness during skew rolling. This method can also be used to study wall thickness unevenness in other steel pipe production units.

Regarding precision-rolled and cold-drawn steel pipes, eccentricity problems can also occur due to piercing: any factor that disrupts the correct geometry of the deformation zone formed by the rolls, mandrel, and guide plate will exacerbate the wall thickness unevenness of the rough pipe.
(1) Mandrel:
① Mandrel shape design: Ideally, the mandrel's rolling cone should be parallel to the roll exit cone. If the mandrel is designed according to the traditional Matveyev formula, its rolling cone is not parallel to the roll exit cone. The metal deforms within this gradually widening gap, inevitably causing insufficient rolling of the tube wall, resulting in uneven tube wall thickness. Moreover, the unevenness in tube wall thickness worsens with increasing feed angle.
② Due to insufficient rigidity of the mandrel rod, bending occurs during piercing, preventing the mandrel from maintaining its center position, thus resulting in uneven tube wall thickness.
③ Uneven wear or damage to the mandrel.
(2) Guide Plates:
① Excessive guide plate distance: During piercing, the guide plates are used to maintain the piercing centerline. A large guide plate distance causes significant changes in the mandrel's vertical position, making the mandrel unstable and leading to uneven tube wall thickness.
② Uneven wear of the upper and lower guide plates also exacerbates the unevenness in wall thickness. 
(3) Rolls:
① Roll centerline misalignment: During production, incorrect installation of the screws on both sides of the piercing mill, or wear of the threads and bearings, can cause axial horizontal misalignment between the two rolls. Inconsistent feed angles between the two rolls lead to distortion in the deformation zone, resulting in uneven wall thickness.
② Large feed angles cause the mandrel and the rolling cone of the rolls to become less parallel.
③ Improper roll speed can also affect wall thickness accuracy.
(4) Billet centering and heating: Eccentricity of the centering hole and uneven heating (yin-yang sides) will both cause uneven wall thickness. (5) Rigidity, Structure, and Adjustment of the Piercing Mill: Insufficient rigidity of the piercing mill body leads to unreliable locking mechanisms; inaccurate adjustment of the centering device on the push rod results in unreliable operation and a distance too far from the mill body; the adjustment of the rolling centerline, generally lower than the mill centerline, aims to improve the stability of the rolled piece. However, excessive adjustment causes asymmetrical changes in the relative relationship between tools within the deformation zone after the rolling line shifts downward, affecting the uneven wall thickness of the tube.

Therefore, eccentric thickness unevenness in seamless steel pipes cannot be completely avoided; it can only be strictly controlled step by step to minimize it.