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Defects of Reducing Diameters of Steel Pipes and Their Prevention

Posted: 06/15/2022 10:52:03 Hits: 6

The purpose of reducing the diameter of the steel pipe is to size the pierced billet with a larger diameter to a finished steel pipe with a smaller diameter, and to ensure that the outer diameter and wall thickness of the steel pipe and its deviation meet the relevant technical requirements.

The defects caused by the reduced diameter of the steel pipe mainly include the geometrical dimension of the steel pipe being out of tolerance, the lines of the reduced diameter, nail prints, scarring, scratches, pockmarks, inner convexity and inner square.

Out-of-tolerance geometric dimensions of steel pipes: the outer diameter, wall thickness or ovality of the steel pipe after reducing diameters do not meet the requirements of sizes and deviation specified in the relevant standards.

Out-of-tolerance diameters and ovality of the steel pipe: it is caused by improper assembly of the roller and improper adjustment of the hole of the reduced diameter machine, unreasonable distribution of deformation, poor machining accuracy or serious wear of the reduced diameter roll, and too high or low temperature of the pierced billet and nonuniform axial temperatures. It is mainly reflected in the shape of the pass and assembly of the roll, the diameter reduction of the pierced billet and the heating temperature of the pierced billet.

Out-of-tolerance wall thickness of steel pipes: The out-of-tolerance wall thickness of steel pipes after the diameter of the pierced billet that is determined is mainly manifested in the uneven wall thickness and non-circular inner hole of the steel pipe, which is mainly affected by accuracy of the wall thickness and hole shape of the pierced billet, adjustment of the hole type, tension of the reducing diameter, the diameter reduction of the pierced billet, and the heating temperature of the pierced billet.

Lines of steel pipes: the line of the steel pipe is caused by the dislocation of the rollers in a certain frame or several frames of the sizing machine, resulting in the hole shape not being round. The edge of a roller cut the surface of the steel pipe to a certain depth, forming a green line or several green lines on the surface of the whole steel pipe.

The nail print is caused by the difference in the linear speed between the roller edge of the roller and other parts of the 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 pipe body, and its appearance is in a short arc shape, similar to a "nail" shape, so it is called "nail print." If the "green line" and "nail print" are serious, the steel pipe may be rejected.

In order to eliminate the "green line" and "nail print" on the surface of the steel pipe, it is necessary to ensure the hardness of the reduced diameter roll and keep it well cooled. When designing the roll pass or adjusting the roll pass, it is necessary to ensure the appropriate opening angle of the pass's side wall and the roll's gap value to prevent the pass from being dislocated.

In addition, the rolling reduction of the single-stand pass should also be properly controlled to avoid the excessive expansion of the pierced billet in the pass when rolling the low-temperature pierced billet, so that the bearings won't be damaged because the metal is squeezed into the roll gap of the roll and the rolling pressure is too great. The practice has shown that the use of tension reduction is beneficial to limiting the lateral expansion of the metal, and has a very positive effect on reducing the defects of green lines and nail prints in steel pipes.

Scarring of steel tubes: scarring is distributed on the surface of the tube body in an irregular form. The scarring is mainly caused by the sticking of steel on the surface of the size roll. It is related to factors such as the hardness and cooling condition of the roll, the depth of the pass, and the reduced diameter of the pierced billet. Measures such as improving the material of the roll, increasing the hardness of the roll surface, ensuring good cooling conditions for the roll, reducing the reduced diameter of the pierced billet and relative sliding speed between the roll surface and the metal surface are beneficial to reducing the chance of the roll sticking to steel. Once scars are found for the steel pipe, find the frame where the scars are generated according to the shape and distribution of the defects. Check and repair the parts of the rolls that stick to the steel, and the rolls that cannot be removed or repaired should be replaced in time.

Scratches of steel pipes: they are mainly caused by the "ears" between the reduced diameter frames and the surfaces of the inlet guide or outlet guide sticking to the steel, scratching and injuring the surface of the steel pipe in motion. Once the surface of the steel pipe is found to be scratched, the guide cylinder should be checked for sticking steel or other attachments in time, or the iron "ears" between the reduced diameter frames should be removed.

Pitted surface of steel pipes: The generation of pitted surfaces of steel pipes is caused by the wear of the surface of the roller, or the excessively thick oxide scale on the surface of the pierced billet due to high temperatures, but it has not been well removed. Before the sizing of the pierced billet, high-pressure water should be used to remove the iron oxide scale on the outer surface of the pierced billet in a timely and effective manner to reduce the occurrence of a pitted surface on the outer surface of the steel pipe.

The convex steel pipe: when the diameter of the pierced billet is reduced, the wall of the steel pipe is bent inward due to the excessively reduced diameter of the single frame of the fixed diameter machine, forming a raised linear defect on the inner wall of the steel pipe. This defect does not happen very often. It is mainly caused by errors in the combination of the roll stands of the sizing machine, or serious errors in the alignment of the pass, or mechanical failure of the stand when sizing the thin-walled steel pipe. Increasing the tension coefficient can increase the critical diameter reduction. Under the condition of the same diameter reduction, the internal resistance of the steel pipe can be effectively avoided. Reducing the diameter reduction can improve the stability of the pierced billet during deformation, and can also effectively prevent inward convexity of the steel pipe. In production, the roll configuration should be carried out in strict accordance with the rolling table, and the roll pass should be carefully adjusted to prevent the occurrence of internal convexity in the steel pipe.

"Inner square" of steel pipes: it means that after the diameter of the pierced billet is reduced by a sizing machine; the inner hole of the cross section is "square" (two-roll type sizing machines or "hexagonal" (three-roll sizing machine). The "inner square" of the steel pipe will affect its accuracy of wall thickness and inner diameter. The inner square of the steel pipe is related to the D/S value of the pierced billet, the amount of diameter reduction, the size of the tension when the diameter is reduced, the shape of the pass, and the rolling speed and rolling temperature. When the D/S value of the pierced billet is smaller, the tension is smaller and the diameter reduction is greater. The rolling speed and rolling temperature are higher, the steel pipe is more likely to have an uneven lateral wall thickness, and the inner square is also more obvious.

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