Analysis and prevention of the causes of external surface defects that are easy to appear in hot-rolled seamless steel pipes

1. Concave correction
Features: The outer surface of the steel pipe is spirally concave.
Causes:
1) Improper adjustment of the straightening machine roller angle and excessive pressure reduction.
2) Severe wear of the straightening roller, etc.
Inspection: The outer surface of the steel pipe is allowed to have concave corrections without obvious edges and corners the inner surface is not protruding, and the outer diameter size meets the tolerance requirements. The excessive concave correction should be removed.

2. Rolling fold
Features: The steel pipe wall presents wrinkles with concave and convex outside and inside along the longitudinal direction locally or throughout the length, and the outer surface is strip-shaped.
Causes:
1) The hole width expansion coefficient is too small.
2) Improper adjustment of the rolling mill causes the hole to be misaligned or the rolling center line to be inconsistent.
3) Improper distribution of the pressure reduction of each frame of the continuous rolling mill, etc.
Due to the above reasons, the metal enters the gap between the rollers during the rolling process of the steel pipe or the steel pipe loses stability, causing the pipe wall to wrinkle.
Inspection: It is not allowed to exist. It should be cut off or scrapped.

3. Pull-out cracking
Features: There is a phenomenon of pulling and cracking on the surface of the steel pipe, which mostly occurs on thin-walled pipes.
Causes:
1) Due to the uneven heating temperature of the tube billet, the deformed part is tensile rolled in the low-temperature part. When the tensile force is large, the steel pipe will be pulled and cracked.
2) Improper adjustment of the speed and roll gap of each frame of the continuous rolling mill causes the steel to tear.
3) The influence of the rough pipe wall thickness. When the rough pipe wall thickness supplied by the piercing machine to the continuous rolling mill is small, the metal deformation of the continuous rolling mill is smaller than the designed deformation, causing the continuous rolling mill to roll tensilely, and tearing when the tensile force is large.
4) There are serious inclusions in the tube billet itself.