Where can unevenness manifest in thick-walled square tubes?

Unevenness in a thick-walled square tube can manifest primarily as spiral wall thickness unevenness, linear wall thickness unevenness, and uneven thickness at the head or tail.

 

Spiral wall thickness unevenness in thick-walled square tube is caused by adjustments such as misalignment of the piercing mill's rolling centerline, unequal roll angles, or insufficient mandrel reduction. This unevenness typically occurs in a spiral pattern along the entire length of the tube.

 

The primary remedy is to adjust the piercing mill's rolling centerline to equalize the roll angles and adjust the tube mill according to the parameters specified in the rolling table.

 

 

Linear wall thickness unevenness is caused by improperly adjusted mandrel pre-piercing saddle height. During pre-piercing, the mandrel contacts the shell on one side, causing the shell to cool too quickly at the contact surface, resulting in unevenness and even concave defects in the thick-walled square tube. Alternatively, the gap between the continuous rolling rolls may be too small or too large.

 

Deviation in the tube mill's centerline. Uneven reduction between single and double stands can cause linear symmetry deviations, resulting in excessively thin (or thick) steel pipes in the single stand direction and excessively thick (or thin) steel pipes in the double stand direction. The primary measures are to properly adjust the height of the mandrel pre-piercing saddle and ensure alignment between the mandrel and the tube. The roll gap should be measured when changing the pass profile or rolling specifications to ensure that the actual roll gap is consistent with the rolling table. Use an optical alignment device to adjust the rolling centerline. The tube mill centerline must be calibrated during annual overhauls.

 

Uneven wall thickness at the head and tail can be caused by excessive bevel or curvature at the front end of the tube billet, as well as misaligned centering holes. This can lead to uneven wall thickness at the head of the tube. Excessive elongation during piercing, excessive roll speed, and unstable rolling can also cause uneven wall thickness at the tail end of the tube billet.

 

Measures include inspecting tube billet quality to prevent excessive bevel or reduction at the front end. The centering hole should be calibrated whenever the pass profile is changed or during maintenance. A lower piercing speed is used to ensure rolling stability and uniformity of thick-walled square tube wall thickness. When the roll speed is adjusted, the matching guide plate is also adjusted accordingly.