What are the Differences Between Cold Drawing and Cold Rolling of Seamless Steel pipes?

Comparison Dimension

Cold Drawing

Cold Rolling

Processing Principle

External "tensile force" is used to pull the steel pipe through a fixed conical die (the die hole diameter is smaller than the original outer diameter of the steel pipe), forcing the cross-section of the steel pipe to shrink and the length to extend, thereby achieving dimensional adjustment.
(Analogous to "pulling noodles": dough is stretched through a small-hole die, resulting in a reduced cross-section.)

External "compressive force" is used to pass the steel pipe through two or more sets of relatively rotating rolls (the roll gap is smaller than the original outer diameter of the steel pipe). The rolling action of the rolls uniformly reduces the wall thickness/outer diameter of the steel pipe, accompanied by slight elongation.
(Analogous to "rolling dough": dough is thinned by rotating rolling pins, resulting in uniform thickness.)

Core Equipment

Cold drawing machine (horizontal/vertical), dies (tungsten steel/alloy material, requiring regular replacement to ensure accuracy), traction device, annealing furnace (to eliminate work hardening after cold drawing)

Cold rolling mill train (multi-roll type, e.g., 20-roll, 12-roll cold rolling mills), rolls (chrome-plated/alloy material, requiring precision grinding), tension control system, straightening machine

Deformation Characteristics

Large deformation per pass (reduction rate of outer diameter/wall thickness can reach 30%-50%), but uneven deformation (especially for wall thickness, prone to "eccentricity"); significant length elongation (elongation rate can reach 100%-200%).

Small deformation per pass (reduction rate of outer diameter/wall thickness is only 5%-15%), requiring continuous multi-pass rolling; extremely high deformation uniformity (wall thickness tolerance can be controlled within ±0.05mm); slight length elongation (elongation rate of 10%-30%).

Product Characteristic

Cold-Drawn Seamless Steel Pipes

Cold-Rolled Seamless Steel Pipes

Dimensional Accuracy

Moderate accuracy:
- Outer diameter tolerance: ±0.1-0.3mm (depending on die accuracy);
- Wall thickness tolerance: ±5%-10% (prone to deviation due to die wear or pipe eccentricity);
- Length accuracy: Custom lengths available (up to 10-15m), but general straightness (subsequent straightening required).

High accuracy:
- Outer diameter tolerance: ±0.02-0.1mm (roll gap can be precisely adjusted);
- Wall thickness tolerance: ±1%-3% (multi-pass rolling + tension control, wall thickness uniformity far superior to cold drawing);
- Length accuracy: Extremely high straightness (synchronous straightening during rolling), no additional processing required.

Surface Quality

Average:
- Inner wall prone to residual "die scratches" or "scale indentations" (if surface treatment of the pipe is incomplete before cold drawing, defects will be stretched and amplified);
- Outer wall may have "drawing marks" (caused by slight friction during traction), with a surface roughness Ra value of approximately 1.6-6.3μm.

Excellent:
- Smooth inner and outer walls without scratches (precision grinding of rolls + no die friction during rolling);
- Minimal surface scale (most pipes undergo pickling and passivation before cold rolling, and no high-temperature oxidation occurs in room-temperature processing);
- Surface roughness Ra value can reach 0.4-1.6μm, close to a "mirror finish".

Mechanical Properties

More significant work hardening:
- Large deformation per pass leads to high internal dislocation density of the pipe, resulting in a significant increase in hardness and tensile strength (e.g., the hardness of Grade 20 steel after cold drawing is HB180-210, 30%-50% higher than that in the annealed state);
- Obvious decrease in plasticity (elongation δ decreases from 25% to 10%-15%), requiring "recrystallization annealing" to restore plasticity.

Mild work hardening:
- Multi-pass rolling with small deformation results in uniform and low internal dislocation density, leading to a moderate increase in hardness and tensile strength (e.g., the hardness of Grade 20 steel after cold rolling is HB150-180, 10%-30% higher than that in the annealed state);
- Good plasticity retention (elongation δ is approximately 18%-22%), usable directly in some scenarios without additional annealing.

Microstructure Uniformity

Poor:
- Deformation concentrates in the cross-sectional shrinkage area, prone to "banded structure" (especially for pipes with thick walls);
- Poor microstructure consistency along the length (different deformation amounts between the head and tail of the pipe).

Excellent:
- Multi-pass rolling + uniform roll extrusion refine and homogenize the microstructure (finer ferrite grains);
- High microstructure consistency along the length (stable deformation amount due to full-process tension control).

Process

Typical Application Scenarios

Examples of Representative Products

Cold Drawing

1. General fluid transportation (low accuracy requirements, e.g., tap water, natural gas);
2. Structural components (e.g., mechanical frames, supports, requiring custom lengths but not high accuracy);
3. Preprocessing of low-cost precision parts (e.g., steel pipes requiring subsequent machining, with machining allowance reserved after cold drawing).

Grade 20 cold-drawn fluid pipes, Q235 cold-drawn structural pipes, Grade 10 cold-drawn mechanical pipes

Cold Rolling

1. High-precision fluid transportation (e.g., hydraulic systems, lubrication systems, requiring strict control of wall thickness tolerance to prevent leakage);
2. Precision mechanical parts (e.g., bearing sleeves, cylinder liners, requiring high surface quality to reduce wear);
3. Pipes for medical equipment and food industry (requiring smooth inner walls to prevent contamination).

Grade 20 cold-rolled hydraulic pipes, 1Cr18Ni9Ti cold-rolled food-grade pipes, cold-rolled pipes for precision bearings