Hot-rolled steel pipes are divided into general steel pipes, low- and medium-pressure boiler steel pipes, high-pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, geological steel pipes and other steel pipes.
Cold-rolled (pulled) seamless steel pipes include general steel pipes, low- and medium-pressure boiler steel pipes, high-pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes and other steel pipes, as well as carbon thin-walled steel pipes, alloy thin-walled steel pipes, stainless thin-walled steel pipes and special-shaped steel pipes. The outer diameter of hot-rolled seamless pipes is generally greater than 32mm, with a wall thickness of 2.5-75mm. The outer diameter of cold-rolled seamless steel pipes can reach 6mm, with a wall thickness of 0.25mm. The outer diameter of thin-walled pipes can reach 5mm, with a wall thickness of less than 0.25mm. Cold rolling has higher dimensional accuracy than hot rolling.
1. Appearance and physical characteristics identification
| Observation | Hot rolled steel pipe | Cold rolled steel pipe |
| Surface state | • Oxide scale or brown rust is present, and the touch is rough • Visible longitudinal rolling lines or slight bumps |
• Silvery white metallic luster, smooth and flat • Often covered with anti-rust oil film, with neat edges and no burrs |
| Section accuracy | • Ovality error＞1%, wall thickness tolerance±0.5mm • Ports may show irregular deformation |
• Ovality < 0.6%, wall thickness tolerance ≤ ± 0.05mm • Cutting surface is neat and sharp |
| Thickness range | Usually ≥5mm (mainly medium-thick wall tubes) | Most are less than 5mm (mostly thin-walled precision tubes) |
2. Analysis of material properties and processing traces
Differences in mechanical properties
Hot-rolled tubes : good plasticity (elongation > 20%), yield strength 355–460MPa, suitable for heavy-load structures but prone to cracking during stamping ; Cold-rolled tubes : high strength (yield strength ≥ 500MPa), hardness HRB ≥ 80, but low bending toughness, prone to brittle cracking due to excessive processing .
Microscopic features
Hot-rolled tubes have coarse grains (ISO 643 rating ≤ 5), with banded structures ; Cold-rolled tubes have grain size ≥ 7 (observed under a metallographic microscope), and the grains are uniformly refined .
Processing traces
Hot-rolled tubes often have scaly textures caused by high-temperature rolling; Cold-rolled tubes have uniform color spots formed by precision roller indentations or annealing.
3. Professional testing method verification
| Test Item | Hot rolled tube features | Cold rolled tube features |
|---|---|---|
| Chemical Composition | High C content (0.25%–0.35%) | Low carbon design (C≤0.23%) |
| Tolerance detection | Outer diameter deviation ±0.3mm (GB/T17395) | Outer diameter accuracy ±0.05mm (GB/T3639) |
| Non-destructive testing | Hidden cracks under oxidation skin (need to be detected after pickling) | Eddy current testing can detect μm-level inner wall defects. |
