The production processes of seamless steel pipes are mainly divided into two categories: hot rolling (extrusion) and cold drawing (cold rolling). Among them, hot rolling is the most fundamental process with the largest output, while cold drawing is used to produce steel pipes with higher precision and better surface quality. The following is a detailed analysis of the production process of seamless steel pipes:
I. Process Flow of Hot-Rolled Seamless Steel Pipes (Mainstream)
The hot rolling process is mainly used to produce seamless pipes with large diameter, thick wall thickness or general precision.
1. Billet Preparation
- Raw material: Round steel (commonly referred to as billet), which can be made of carbon steel, alloy steel or stainless steel according to requirements.
- Cutting: Long round steel is sawn into fixed lengths (usually calculated based on the finished pipe length).
- Centering: A centering hole is drilled at one end of the billet (mandatory for two-roll piercing mills, optional for three-roll piercing mills), ensuring uniform wall thickness during piercing and guiding the plug.
2. Heating
The cut billets are fed into a rotary heating furnace for heating.
- Temperature control: Heating temperature is generally between 1200℃ and 1250℃ (austenitizing temperature).
- Requirements: Uniform temperature inside and outside the billet must be ensured to avoid overheating or burning.
3. Piercing
- Equipment: Commonly used are two-roll cross-roll piercing mills (Mannesmann piercing mills) or three-roll piercing mills.
- Principle: The heated billet passes between two relatively rotating and inclined rolls with a fixed plug in the center. Driven by the rolls, the billet rotates and advances forward. Under the action of the plug, the solid billet is pierced into a hollow shell (also called mother tube).
- Feature: This is the most critical step in seamless pipe manufacturing, which determines the wall thickness quality of the steel pipe.
4. Rolling
- Purpose: Wall reduction and elongation. The thick-walled hollow shell is rolled into a mother tube close to the finished wall thickness and length.
- Equipment:
- MPM (Multi-Stand Pipe Mill): Modern mainstream equipment with high efficiency and good quality.
- Assel Mill: Suitable for producing thick-walled pipes.
- PQF (Premium Quality Finishing) Mill: One of the most advanced technologies at present, capable of producing ultra-thin-walled steel pipes.
5. Sizing & Reducing
- Sizing: When the outer diameter of the steel pipe is close to the finished size, a sizing mill is used to precisely control the outer diameter within the standard range.
- Reducing: If a smaller outer diameter is required, the mother tube passes through a reducing mill (multi-stand continuous rolling) to shrink the outer diameter at high temperature, with a slight reduction in wall thickness.
6. Cooling
The steel pipe has a high temperature after leaving the sizing mill and needs to be cooled.
- Methods: Depending on the steel grade, either controlled water cooling (to control cooling rate and improve strength) or air cooling (stack cooling) is adopted.
7. Straightening
Hot-rolled steel pipes are usually bent and must be straightened by a straightening machine to meet the standard straightness requirements.
8. Finishing & Inspection
- End cutting: The defective parts at both ends (such as uneven wall thickness or cracks) are cut off.
- Testing: Ultrasonic Testing (UT) or Eddy Current Testing (ET) is performed to detect internal and surface defects.
- Hydrostatic test: Conducted on each pipe individually to verify sealability and strength.
- Surface treatment: Shot blasting to remove scale and rust.
- Packing & warehousing: Bundled, weighed, marked and stored in the warehouse.
II. Process Flow of Cold-Drawn (Cold-Rolled) Seamless Steel Pipes
Cold drawing is a further processing based on hot-rolled pipes, used to produce small-diameter, thin-walled, high-precision and high-finish steel pipes (such as hydraulic pillar tubes and bearing tubes).
1. Preparation
- Raw material: Hot-rolled steel pipes are usually used as blanks (also called mother tubes).
- Pickling: To remove scale and rust from the surface of hot-rolled pipes.
- Lubrication: Lubricants (such as lime milk or phosphating layers) are applied to the inner and outer surfaces of the steel pipe to reduce friction during drawing, preventing die wear and surface scratches.
2. Cold Drawing
- Equipment: Cold drawing machine.
- Principle: The lubricated steel pipe is pulled through a cemented carbide die with a smaller diameter.
- Result: The outer diameter and wall thickness of the steel pipe are reduced, while the length is increased.
- Multi-pass: The deformation per pass is limited; multiple drawing passes are generally required to achieve the final dimensions.
3. Heat Treatment
After cold drawing, the steel pipe undergoes work hardening, becoming hard and brittle, which makes further drawing or application impossible.Therefore, annealing or normalizing must be carried out during or after drawing to restore plasticity and toughness.
4. Finishing
- Straightening: Precision straightening.
- Pickling / Passivation: To remove scale generated during heat treatment.
- Testing & Inspection: Strict dimensional measurement and non-destructive testing.
- Finished product: Finally, precision seamless pipes with extremely high dimensional accuracy (tolerance up to ±0.05mm) and mirror-bright surface are obtained.
Summary
- Hot-rolled pipes: Short process, high efficiency, low cost, suitable for mass production, mainly used as structural pipes and fluid transportation pipes.
- Cold-drawn pipes: Long process, high cost, high precision, suitable for small-batch, high-demand pipes for mechanical processing.
