The uneven wall thickness of the seamless tube(SMLS) is mainly manifested in the phenomenon of uneven wall thickness of the spiral shape, uneven wall thickness of the straight line, and thicker and thinner walls at the head and tail. The influence of continuous rolling process adjustment of seamless tubes is an important factor leading to uneven wall thickness of finished pipes. Specifically:
1. The spiral wall thickness of the seamless tube is uneven
The causes are:
1) The wall thickness of the seamless steel pipe is uneven due to adjustment reasons such as the incorrect rolling center line of the piercing machine, the inclination angle of the two rolls, or the small amount of reduction before the plug, and is generally distributed in a spiral shape along the entire length of the steel pipe.
2) During the rolling process, the centering rollers are opened too early, the centering rollers are not adjusted properly, and the wall thickness is uneven due to the vibration of the ejector rod, which is generally distributed in a spiral shape along the entire length of the steel pipe.
Measure:
1) Adjust the rolling center line of the piercing machine so that the inclination angles of the two rolls are equal, and adjust the rolling mill according to the parameters given in the rolling table.
2) For the second case, adjust the opening time of the centering roller according to the exit speed of the capillary tube, and do not open the centering roller too early during the rolling process to prevent the ejector rod from shaking, resulting in uneven wall thickness of the seamless steel pipe. The opening degree of the centering roller needs to be properly adjusted according to the change of the diameter of the capillary, and the beating amount of the capillary should be taken into consideration.
2. The linear wall thickness of the seamless tube is uneven
Cause:
1) The height adjustment of the mandrel pre-piercing saddle is not appropriate. When the mandrel is pre-piercing, it contacts the capillary on one side, causing the temperature of the capillary to drop too quickly on the contact surface, resulting in uneven wall thickness of the seamless steel pipe or even a concave defect.
2) The gap between the continuous rolling rolls is too small or too large.
3) Deviation of the center line of the rolling mill.
4) The uneven reduction of the single and double racks will cause the linear symmetrical deviation of the steel pipe to be ultra-thin (ultra-thick) in the direction of the single rack and ultra-thick (ultra-thin) in the direction of the double racks.
5) The safety abutment is broken, and the difference between the inner and outer roll gaps is large, which will cause asymmetric deviation of the straight line of the steel pipe.
6) Improper adjustment of continuous rolling, stacking steel and drawing rolling will cause uneven wall thickness in a straight line.
Measure:
1) Adjust the height of the mandrel pre-piercing saddle to ensure the centering of the mandrel and capillary.
2) When changing the pass type and rolling specification, the roll gap should be measured to keep the actual roll gap consistent with the rolling table.
3) Adjust the rolling center line with optical centering device, and the center line of the rolling mill must be corrected during the annual overhaul.
4) Timely replace the frame with broken safety mortar, measure the inner and outer roll gaps of continuous rolls, and replace them in time if there is a problem.
5) During continuous rolling, steel drawing and stacking should be avoided.
3. The wall thickness of the seamless tube head and tail is uneven
Cause:
1) The cutting slope and curvature of the front end of the tube blank are too large, and the centering hole of the tube blank is not correct, which will easily cause the wall thickness of the steel pipe head to be uneven.
2) When piercing, the elongation coefficient is too large, the roll speed is too high, and the rolling is unstable.
3) Unstable steel throwing by the piercer may easily cause uneven wall thickness at the end of the capillary tube.
Measure:
1) Check the quality of the tube blank to prevent the front end of the tube blank from cutting inclination and large reduction, and the centering hole should be corrected when changing the pass type or overhauling.
2) Use a lower piercing speed to ensure the stability of rolling and the uniformity of capillary wall thickness. When the roll speed is adjusted, the matching guide plate is also adjusted accordingly.
3) Pay attention to the use status of the guide plate and increase the inspection of the guide plate bolts, reduce the range of movement of the guide plate during steel rolling, and ensure the stability of steel throwing.