The core of the cold-rolled round tube process is to transform hot-rolled raw materials into high-precision, high-surface-quality tubes through multiple rolling passes and precision machining at room temperature. The entire process focuses on the three goals of "diameter reduction, wall thickness control, and quality improvement," requiring strict control of temperature, rolling force, and dimensional accuracy.
I. Complete Process Flow Diagram
Cold-rolled round tube production involves four major stages: "Raw Material Pretreatment → Cold Rolling → Subsequent Finishing → Quality Inspection," encompassing a total of eight core steps. The detailed process is as follows:
Raw Material Preparation → 2. Pickling and Degreasing → 3. Cold Rolling → 4. Intermediate Heat Treatment
Finish Rolling/Cold Drawing (Optional) → 6. Finished Product Heat Treatment → 7. Finishing → 8. Quality Inspection and Packaging
II. Detailed Explanation of the Core Processes in Each Stage
Stage 1: Raw Material Pretreatment - Laying the Foundation for Processing
The goal of this stage is to remove surface defects in the raw material and ensure "no impurity contact" during subsequent rolling, thereby preventing any degradation of the tube surface quality. Raw Material Selection: Hot-rolled seamless steel tube billets (outer diameter 20-150mm, wall thickness 3-20mm) or hot-rolled coils (thickness 1-5mm) are primarily used. The material must meet the requirements of the finished product (e.g., low-carbon steel Q235, medium-carbon steel 20#), and the raw material must be free of surface defects such as cracks and folds.
Cutting/Piercing (for coils): If using coils, they must first be cut into fixed-width strips using a slitting shear. These strips are then rolled into round tube billets using a "piercing mill" (forming an "open tube" with a gap). The gaps are then welded to fill the gaps (this is only used for some welded tubes; seamless tubes can be made directly from the billet).
Stage 2: Pickling and Degreasing – Cleaning the surface to prevent sticking during rolling.
Before cold rolling, the raw material surface must be free of scale and oil. Otherwise, the scale will be pressed into the tube surface by the rollers, causing "pitting" defects. Pickling: Immerse the raw material in a 15%-20% hydrochloric acid solution (40-60°C) for 10-30 minutes to remove surface oxide scale (Fe₂O₃ + 6HCl = 2FeCl₃ + 3H₂O) through a chemical reaction.
Degreasing: After pickling, rinse with clean water and then immerse in an alkaline degreasing agent (such as sodium hydroxide solution) to remove surface oil and prevent it from adhering to the rolls during rolling and affecting the pipe's precision.
Drying: Finally, dry the raw material with hot air (80-120°C) to ensure the surface is dry and prevent subsequent rusting.
Stage 3: Cold Rolling - Core Forming and Dimensional Accuracy Control
This is a critical step in determining the size and surface quality of cold-rolled round tubes. Through "multiple continuous rolling passes," the raw material is gradually rolled to the target specifications. The entire process is carried out at room temperature (as opposed to the high-temperature rolling process of hot rolling). Equipment: A two-roll cold rolling mill or a multi-roll cold rolling mill (such as a four-roll or six-roll cold rolling mill) is used. The roll surfaces are polished (roughness Ra ≤ 0.4μm) to ensure a smooth outer surface of the pipe.
Process Features:
Multi-pass rolling: The deformation of each rolling pass is controlled at 10%-20% to avoid excessive deformation that may cause cracking in the pipe. Three to eight passes are typically required to gradually reduce the outer diameter from the raw material's 20-150mm to the target 5-220mm, and the wall thickness from 3-20mm to 0.5-25mm.
Dimensional Control: The outer diameter is monitored in real time after each rolling pass using an online diameter gauge. Roll pressure is automatically adjusted if the deviation exceeds ±0.05mm, ensuring the final outer diameter tolerance of the finished product is within ±0.05-±0.2mm. Tension Control: During the rolling process, a "tension roller" applies a steady tension (10-50 MPa) to prevent bending or excessive ovality in the tube and ensure straightness (≤1 mm per meter).
Stage 4: Intermediate Heat Treatment - Stress Relief and Cracking Prevention
During the cold rolling process, the metal undergoes internal "work hardening" (increased hardness and decreased toughness). Continued rolling can easily lead to cracking in the tube, so "intermediate annealing" is required between multiple rolling passes.
Process: The rolled tube is placed in a continuous annealing furnace and heated to 600-700°C under a protective atmosphere (such as nitrogen to prevent oxidation). The temperature is maintained for 1-2 hours, and then the tube is cooled to room temperature.
Purpose: To eliminate internal stresses generated by cold rolling, reduce the tube hardness (e.g., reducing the hardness of 20# steel from HB180 to HB120), and restore toughness to prepare for subsequent rolling. Stage 5: Finish Rolling/Cold Drawing (Optional) – Improving Precision
For the production of ultra-high-precision tubing (such as barrels for hydraulic cylinders), a "finish rolling" or "cold drawing" process is required after cold rolling.
Finish rolling: Using a precision cold rolling mill, a smaller deformation (5%-10%) is employed to further reduce the outer diameter tolerance to ±0.03mm and the inner wall roughness to Ra ≤ 0.8μm.
Cold drawing: The cold-rolled tube is passed through a "die hole" (with a diameter slightly smaller than the tube's outer diameter) and forcibly pulled under tension. This further improves dimensional accuracy and surface finish, but is less efficient than finish rolling and is primarily used for small-batch, high-precision tube production.
Stage 6: Finished Product Heat Treatment – Customizing Mechanical Properties
The tube undergoes a final heat treatment to adjust its mechanical properties based on the intended use (e.g., high strength for load-bearing components, high toughness for curved parts). Annealing: Heating to 650-750°C, holding, and then slowly cooling to reduce hardness and increase toughness. Suitable for pipes requiring bending (such as brake lines).
Normalizing: Heating to 850-950°C, holding, and then air cooling to refine grain size and achieve uniform microstructure. Suitable for components requiring both strength and toughness (such as chassis connecting rods).
Quenching and Tempering (Quenching + Tempering): Only used for medium- and high-strength steels (such as 45# steel). First, heating to 820-860°C and quenching (water cooling), followed by heating to 500-600°C and tempering, significantly increases strength (yield strength increases from 355MPa to over 500MPa). Suitable for load-bearing components such as drive shafts. Stage 7: Finishing - Correcting the shape for ease of use
Straightening: The pipe is straightened using a multi-roll straightening machine (usually 11 or 13 rollers) to ensure a straightness of ≤0.5mm/m, preventing deviations during subsequent cutting or assembly.
Cutting: Based on customer requirements, the pipe is cut into fixed lengths (usually 2-12m) using a saw or laser cutter, with a length tolerance of ±2mm.
Surface Treatment: Depending on corrosion protection requirements, galvanizing (hot-dip galvanizing or electroplating, with a zinc layer thickness ≥80μm), plastic coating (such as epoxy coating), or polishing (for decorative pipes, with a surface roughness Ra ≤0.4μm) is performed.
Stage 8: Quality Inspection and Packaging - Ensuring qualified delivery
Dimensional Inspection: Calipers and micrometers are used to check the outer diameter and wall thickness, and an internal diameter gauge is used to check the internal diameter to ensure compliance with specifications. A straightness gauge is used to check straightness. Mechanical Property Testing: Sampling is subject to a "tensile test" (for tensile strength and yield strength), a "hardness test" (for HB or HRB), and a "flattening test" (for weld quality, only for welded pipes).
Surface Inspection: Visual inspection or eddy current testing is performed to detect surface defects such as cracks, pitting, and scratches. High-pressure pipes (such as oil and gas pipelines) are subject to a "hydrostatic test" (applying 1.5 times the working pressure and maintaining the pressure for 30 minutes without leakage).
Packaging: Qualified pipes are wrapped in waterproof paper or plastic film and then tied into bundles with steel strapping. The bundles are labeled with the specification, material, and batch number for easy transportation and storage.