The latest knowledge on steel pipe networks and recommended practices for addressing uneven wall thickness in steel pipes is essential for maintaining product quality and production efficiency. Uneven wall thickness is a common issue that can manifest in various forms, such as spiral wall thickness variations, localized thick or thin sections, and inconsistencies at the head and tail of the pipe. These irregularities are often influenced by the continuous rolling process, which plays a key role in determining the final wall thickness uniformity.
1. **Causes of Spiral Wall Thickness Variations**
- One primary cause is the non-uniform wall thickness from the piercing mill, where the angle of the two rolls may not be equal, or the pre-rolling amount at the beginning of the process is too small. This leads to uneven wall thickness distributed along the entire length of the pipe in a spiral pattern.
- Another factor is the premature opening of the centering roll, misalignment of the centering roll, or jerking of the ejector pin during rolling. These issues also result in helical patterns of uneven thickness across the pipe’s length.
**Solutions:**
- Adjust the alignment of the piercing mill so that both rolls have equal inclination angles, and follow the rolling table parameters for proper mill settings.
- For the second case, adjust the timing of the centering roll opening based on the capillary exit speed. Avoid opening the centering roll too early to prevent the top rod from shaking, which could cause thickness irregularities. The opening degree should be adjusted according to the diameter changes of the capillary tube and the size of the capillary bounce.
2. **Causes of Linear Wall Thickness Variations**
- Improper adjustment of the mandrel pre-worn saddle height can lead to uneven contact between the mantle and the capillary tube, causing rapid temperature drops and resulting in uneven wall thickness or even pulling and concave defects.
- Incorrect gap settings between tandem rolls, roller centerline deviation, and unbalanced rolling between single and double racks can also contribute to linear asymmetry in wall thickness.
- A broken safety cup or differences in inner and outer roll gaps may cause asymmetric wall thickness. Additionally, improper adjustment during continuous rolling, steel rolling, or pull steel rolling can lead to similar issues.
**Solutions:**
- Ensure the mandrel pre-worn saddle height is properly adjusted, and align the mandrel with the capillary tube.
- When changing hole types or rolling specifications, measure the roll gap to ensure it matches the rolling table values.
- Use an optical centering device to adjust the rolling center line and correct the pipe rolling mill center line during annual overhauls.
- Replace damaged safety cups and measure internal and external roll gaps regularly to identify and fix problems promptly.
- Avoid steel stacking during continuous rolling to maintain consistent wall thickness.
3. **Causes of Head and Tail Wall Thickness Irregularities**
- A steep slope at the front end of the tube blank or an improperly centered hole can cause uneven thickness at the pipe’s head.
- Excessive extension coefficient during perforation, high roller rotational speed, or unstable rolling conditions can lead to uneven thickness at the capillary tube’s end.
- Instability in the thrower’s steel feeding can also cause irregularities at the end of the pipe.
**Solutions:**
- Check the quality of the tube blank and avoid cutting off excessive taper at the front end. Correct the centering hole when changing hole shapes or during overhauls.
- Reduce the perforation speed to ensure stable rolling and uniform wall thickness. Adjust the guide plates accordingly when adjusting roller speeds.
- Monitor the condition of the guide plates and inspect the bolts regularly to minimize turbulence and ensure stable steel feeding.
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