The purpose of steel pipe sizing (reduction) is to size (reduce) the rough pipe with a larger diameter to the finished steel pipe with a smaller diameter and to ensure that the outer diameter and wall thickness of the steel pipe and their deviations meet the relevant technical requirements.
The quality defects caused by sizing (reduction) of steel pipes mainly include: geometric dimension deviation of steel pipes, “blue line” of sizing (reduction), “nail mark”, scar, abrasion, pockmark, inner convexity, inner square, etc.
① Geometric dimension deviation of steel pipes: The geometric dimension deviation of steel pipes mainly refers to the outer diameter, wall thickness, or ovality of steel pipes after sizing (reduction) that does not meet the size and deviation requirements specified in the relevant standards.
② Out-of-tolerance of outer diameter and ovality of steel pipes: The main reasons are: improper roller assembly and hole adjustment of the sizing (reduction) mill, unreasonable deformation distribution, poor processing accuracy, or severe wear of the sizing (reduction) rollers, too high or too low temperature of the rough pipe, and uneven axial temperature. It is mainly reflected in the hole shape and roller assembly, the diameter reduction of the rough pipe, and the heating temperature of the rough pipe.
③ The wall thickness of the steel pipe exceeds the tolerance: the wall thickness of the rough pipe after the rough pipe is sized (reduced), which is mainly manifested as uneven wall thickness and non-circular inner hole of the steel pipe. It is mainly affected by factors such as the wall thickness accuracy of the rough pipe, the hole shape and hole adjustment, the tension during sizing (reducing) the size of the rough pipe diameter reduction, and the heating temperature of the rough pipe.
④ “Blue line” and “fingernail mark” of steel pipe: The “blue line” of steel pipe is caused by the misalignment of the rollers in one or several frames of the sizing (reducing) machine, resulting in the hole shape not being “round”, causing the edge of a certain roller to cut into the surface of the steel pipe to a certain depth. The “blue line” runs through the outer surface of the entire steel pipe in the form of one or more lines.
“Fingernail mark” is caused by the difference in linear speed between the roller edge and other parts of the rolling groove, which causes the roller edge of the roller to stick to the steel and then scratch the surface of the steel pipe. This defect is distributed along the longitudinal direction of the tube body, and its morphology is a short arc, similar to the shape of a “fingernail”, so it is called a “fingernail mark”. When the “blue line” and “fingernail mark” are serious, the steel pipe may be scrapped.
To eliminate the “blue line” and “fingernail mark” defects on the surface of the steel pipe, the hardness of the sizing (reducing) roller must be guaranteed and its cooling must be kept good. When designing the roll hole or adjusting the roll hole, it is necessary to ensure the appropriate hole side wall opening angle and roll gap value to prevent the hole from being misaligned.
In addition, the reduction amount of the single-frame hole should be properly controlled to avoid excessive expansion of the rough pipe in the hole when rolling the low-temperature rough pipe, causing the metal to squeeze into the roll gap of the roll and damage the bearing due to excessive rolling pressure. The practice has shown that the use of tension reduction technology is conducive to limiting the lateral expansion of the metal, which has a very positive effect on reducing the “blue line” and “fingernail mark” defects of steel pipes.
① Steel pipe scarring: Steel pipe scarring is distributed in an irregular form on the surface of the pipe body. Scarring is mainly caused by steel sticking to the surface of the sizing (reducing) roller. It is related to factors such as the hardness and cooling condition of the roller, the depth of the hole type, and the sizing (reducing) amount of the rough pipe. Improving the material of the roller, increasing the roller surface hardness, ensuring good roller cooling conditions, reducing the rough pipe sizing (reducing) amount, and reducing the relative sliding speed between the roller surface and the metal surface is conducive to reducing the chance of roller sticking to steel. Once the steel pipe is found to have scarring, the frame where the scarring is produced should be found according to the shape and distribution of the defect, and the roller part that sticks to the steel should be inspected, removed, or repaired. Rollers that cannot be removed or repaired should be replaced in time.
② Steel pipe scratches: Steel pipe scratches are mainly caused by the “ears” between the sizing (reducing) machine frames and the surfaces of the inlet guide tube or outlet guide tube sticking to the steel, rubbing and damaging the surface of the moving steel pipe. Once the surface of the steel pipe is scratched, the guide tube should be checked in time to see if there is any sticking steel or other attachments, or the iron “ears” between the sizing (reducing) machine frames should be removed.
③ Outer hemp surface of steel pipe: The outer hemp surface of steel pipe is caused by the roughness of the roller surface due to wear, or the excessive temperature of the rough pipe makes the surface iron oxide too thick, but it is not well removed. Before the rough pipe is sized (reduced), the iron oxide on the outer surface of the rough pipe should be removed promptly and effectively with high-pressure water to reduce the occurrence of defects on the outer hemp surface of the steel pipe.
④ Convexity in steel pipe: Convexity in steel pipe refers to the situation that when the rough pipe is sizing (reducing) the diameter, the wall of the steel pipe is bent inward (sometimes in a closed state) due to the excessive sizing (reducing) amount of the single frame of the sizing (reducing) mill, forming a raised linear defect on the inner wall of the steel pipe. This defect does not occur often. It is mainly caused by errors in the combination of the roller racks of the sizing (reducing) mill, serious errors in the hole adjustment, or mechanical failure of the rack when sizing (reducing) thin-walled steel pipes. Increasing the tension coefficient can increase the critical diameter reduction. Under the same diameter reduction conditions, can effectively avoid the internal resistance of the steel pipe. Reducing the diameter reduction can improve the stability of the rough pipe during deformation, and can also effectively prevent the steel pipe from convex. In production, the roll matching should be carried out strictly according to the rolling table, and the roll hole type should be carefully adjusted to prevent the occurrence of convex defects in the steel pipe.
“Inner square” of steel pipe: “Inner square” of steel pipe means that after the rough pipe is sized (reduced) by the sizing (reducing) mill, the inner hole of its cross-section is “square” (two-roller sizing and reducing mill) or “hexagonal” (three-roller sizing and reducing mill). The “inner square” of the steel pipe will affect its wall thickness accuracy and inner diameter accuracy. The “inner square” defect of the steel pipe is related to factors such as the D/S value of the rough pipe, the diameter reduction, the tension during sizing (reducing), the hole shape, the rolling speed, and the rolling temperature. When the D/S value of the rough pipe is smaller, the tension is smaller, the diameter reduction is larger, and the rolling speed and rolling temperature are higher, the steel pipe is more likely to have uneven transverse wall thickness, and the “inner square” defect is more obvious.
Post time: Jan-17-2025