Why Cold Rolling Control Decides Tube Quality
Cold rolling is the step that converts a semi-finished hollow into a precision round tube with the final outer diameter, wall thickness, roundness and surface finish. Because the material is deformed below its recrystallization temperature, the process simultaneously sets dimensional accuracy, surface integrity and the level of work hardening left in the finished tube. A small drift in roll gap, tension or lubrication therefore does not simply produce an out-of-tolerance tube: it can generate cracks, laminations or a hardness profile that has to be corrected by additional annealing. Cold-drawn and cold-rolled precision tubes are commonly supplied to GB/T 3639, and the process windows below are the ones that most often decide whether a batch meets that specification.
Dimensional Accuracy: Roll Gap, Tension and Pass Schedule
The core target of rolling is to hit the specified outer diameter and wall thickness with a consistent roundness along the whole length. Three process variables dominate the result.
| Variable | Typical setting | Failure if ignored |
|---|---|---|
| Roll gap calibration | Gap error ≤0.01 mm, verified with a standard sample tube | Oversize or undersize diameter, wall thickness eccentricity |
| Online diameter monitoring | Check every 100 tubes; stop and adjust if deviation exceeds ±0.05 mm | Slow dimensional drift discovered only at final inspection |
| Rolling tension | 10–30 MPa for mild steel, 30–50 MPa for high-strength steel, fluctuation ≤±5% | Too low: bending and ovality above 0.8% of outer diameter. Too high: wall thinning and necking cracks |
| Pass allocation | Carbon steel such as Q235: 3–5 passes at 15–20% per pass. Higher strength steel such as Q275: 5–8 passes at 10–15% | Single-pass reduction above 20% gives rolling cracks and wrinkles; too few passes concentrate work hardening |
Pass scheduling deserves particular attention because it trades deformation efficiency against residual ductility. Concentrating the reduction into a few heavy passes raises hardness quickly and leaves little reserve for subsequent sizing or bending. Spreading the same total reduction over more light passes keeps the tube ductile, improves roundness and makes the following annealing step more predictable. The chosen schedule should always be recorded together with the material grade so that a repeat order can reproduce the same structure.
Surface Quality: Rolls, Pickling and Lubrication
Round tubes for hydraulic lines and automotive use carry strict surface requirements, often a maximum roughness of Ra 3.2 µm. Three surface defects account for most rejections.
Contaminated or worn rolls. Oil and scale on the roll surface are pressed into the tube and appear as indentations or pitting, while a worn roll produces longitudinal scratches and pushes roughness above Ra 6.3 µm. Rolls should be wiped clean before rolling, inspected every 500 tubes, and replaced or re-polished once their own surface roughness exceeds Ra 0.8 µm.
Incomplete raw material preparation. Residual oxide scale that was not fully removed by pickling breaks away during rolling and forms pits, and residual oil carbonises at the roll bite to form dark spots. Incoming strip or hollow should be sampled before rolling; a white cloth wipe that shows black stains indicates that degreasing or pickling must be repeated.
Wrong lubrication. Insufficient lubricant raises friction and causes scoring, while excessive residue is difficult to remove and spoils the finish. A lubricant matched to the material should be applied by spray at a controlled coverage of about 5–10 g/m², with rolling speed kept in the 5–15 m/min band so that the film does not degrade thermally.
Stress and Deformation Control: Annealing and Cooling
Cold rolling is a cold working operation, so work hardening and internal stress accumulate in proportion to the total reduction. If they are not relieved in time, the tube becomes hard and prone to longitudinal cracking during later passes.
| Parameter | Recommended practice | Problem if exceeded |
|---|---|---|
| Intermediate annealing frequency | Every 2–3 passes for carbon steel, every 1–2 passes for high-strength steel | Hardness above HB180 makes further rolling crack-sensitive |
| Annealing temperature and soak | 650–700 °C for 1–2 hours, then controlled cooling | Below 600 °C leaves residual stress; hardness stays above HB120 |
| Rolling speed | 5–10 m/min for thin wall, 10–15 m/min for heavy wall | Above 15 m/min the roll bite overheats and properties become uneven |
| Cooling | Water cooling of rolls and tube, tube temperature ≤50 °C | Heat build-up causes thermal wear of rolls and thermal deformation of the tube |
Incoming material quality closes the loop on internal soundness. Loose structure and non-metallic inclusions in the hollow can open up during rolling, producing laminations that only become visible as leakage during a later hydrostatic test. Eddy current or ultrasonic inspection of the incoming material, and 100% inspection for critical products such as high-pressure oil tubes, is far cheaper than scrapping a finished batch.
Equipment Condition and Operating Discipline
Process parameters can only be held if the mill itself is stable. Roll bearing clearance should be checked daily, typically requiring a value of 0.02 mm or less, together with the meshing of the transmission gears; loose bearings show up as unstable roll rotation and spiral marks on the tube, while a loose drive produces speed fluctuation and variable wall thickness. Weekly lubrication and immediate shutdown on abnormal noise or vibration prevent small faults from becoming dimensional scrap.
Operator skill is equally important. Personnel should understand the quantitative link between a deviation and the correction: for example, if the outer diameter measures 0.1 mm oversize, roll pressure is reduced by a defined increment rather than by an arbitrary amount, which avoids over-rolling and excessive wall reduction. A written response procedure for scratches, dimensional drift and surface defects ensures that the machine is stopped before a whole batch is affected.
Taken together, dimensional control, surface protection, timely stress relief and stable equipment form a single system. Auditing roll gap and tension records, annealing charts, lubricant consumption and inspection results against the same batch number gives a reliable early warning when the process begins to drift.
FAQ
Q: What roll gap accuracy should be maintained before rolling?
Calibrate with a standard sample tube so that the gap error is 0.01 mm or less, then monitor the outer diameter online and stop the mill if the deviation exceeds ±0.05 mm.
Q: How many passes are needed for cold-rolled round tubes?
Carbon steel such as Q235 is usually rolled in 3–5 passes at 15–20% reduction each, while higher-strength steel such as Q275 needs 5–8 passes at 10–15%.
Q: When is intermediate annealing required?
After every 2–3 passes for carbon steel and every 1–2 passes for high-strength steel, holding at 650–700 °C for 1–2 hours to bring hardness back below HB120.
Q: How can scratches and pitting on the tube surface be prevented?
Keep rolls clean and free of wear, confirm that pickling and degreasing are complete, and apply a matched lubricant evenly at about 5–10 g/m².
Q: Why does rolling tension have to be controlled so tightly?
Too little tension lets the tube bend and lose roundness, while too much tension thins the wall and can cause necking cracks, so tension should be held within ±5% of the set value.
Q: How is internal defect risk managed before rolling?
Eddy current or ultrasonic testing screens the incoming material, and critical products such as high-pressure oil tubes are inspected 100% before entering the mill.