1. Spiral Uneven Wall Thickness: Causes include uneven wall thickness due to misalignment of the piercing mill rolling centerline, unequal inclination angles of the two rolls, or insufficient pre-pressurization of the mandrel. This unevenness typically presents a spiral distribution along the entire length of the steel pipe. Other causes include premature opening of the centering roll during rolling, improper adjustment of the centering roll, and mandrel vibration.
- Measures: Adjust the piercing mill rolling centerline to ensure equal inclination angles of the two rolls. Adjust the rolling mill according to the parameters given in the rolling schedule. For this situation, adjust the centering roll opening time based on the tube exit speed. Avoid premature opening of the centering roll during rolling to prevent mandrel vibration and uneven wall thickness. The centering roll opening degree needs to be adjusted appropriately according to changes in the tube diameter, taking into account the amount of tube runout.
2. Causes of Linear Uneven Wall Thickness: Improper adjustment of the mandrel pre-penetration saddle height, causing the mandrel to contact the tube on one side during pre-penetration, resulting in excessively rapid temperature drop on the contact surface, leading to uneven wall thickness or even denting defects. Other causes include: Insufficient or excessive gap between rolling mill rolls; deviation of the rolling mill centerline; uneven reduction in single and double stands, causing linear symmetrical deviations in the single-stand (overly thick) and double-stand (overly thick) directions of the steel pipe; a broken mandrel and a large difference in the inner and outer roll gaps, causing linear asymmetrical deviations in the steel pipe; and improper continuous rolling adjustments, such as steel stacking or pulling, which can cause linear uneven wall thickness.
- Measures: Adjust the height of the mandrel pre-penetration saddle to ensure alignment between the mandrel and the tube. Measure the roll gap when changing the roll pass and rolling specifications to ensure the actual roll gap matches the rolling schedule. Adjust the rolling centerline using an optical alignment device and correct the rolling mill centerline during the annual overhaul. 1. Replace broken stands promptly. Measure the inner and outer roll gaps of the continuous rolling mill rolls and replace any damaged sections immediately. Avoid pulling or piling steel during continuous rolling.
3. Causes of uneven wall thickness at the head and tail: Excessive skewness or curvature of the billet tip, and misalignment of the billet centering hole can easily cause uneven wall thickness at the head of the steel pipe. Excessive elongation coefficient during piercing, excessively high roll speed, and unstable rolling can also cause uneven wall thickness at the tail of the tube.
- Measures: Inspect the billet to prevent excessive skewness and reduction at the tip. Correct the centering hole when changing the die type or during maintenance. Use a lower piercing speed to ensure rolling stability and uniform tube wall thickness. When adjusting the roll speed, adjust the matching guide disc accordingly. Monitor the condition of the guide discs and increase the inspection intensity of the guide disc bolts to reduce the axial movement of the guide discs during rolling and ensure stable steel ejection.
Post time: Dec-03-2025