Cold rolling refers to the processing of steel plates or strips into various types of steel through cold working such as cold drawing, cold bending, and cold drawing at room temperature.
Cold-rolled precision seamless steel pipe is a kind of seamless pipe with high dimensional accuracy and good surface finish for precision mechanical structures, hydraulic equipment or steel sleeves.
The main advantage of cold-rolled steel pipe is that the reduction rate of the section is large, especially the wall reduction ability. For carbon steel, the reduction rate of section reduction in one rolling can reach 80% to 83%, and for alloy steel it can reach 72% to 75%. Its main disadvantage is the difficulty of tool replacement. In addition to being directly used to produce some high-precision cold-rolled tubes, the cold-rolling method is often used in conjunction with the cold-drawing method to form cold-drawn billets. In this way, the wall-reducing ability of cold rolling can be fully exerted, and the advantage of easy replacement of cold drawing tools can be cleverly utilized, which is conducive to improving productivity, expanding product production range and improving the surface quality of steel pipes.
Process Flow of Cold-rolled Seamless Steel Pipe:
Cold-rolled (drawn) seamless steel pipe: round tube billet → heating → perforation → heading → annealing → pickling → oiling (copper plating) → multi-pass cold drawing (cold rolling) → billet → heat treatment → straightening → water Pressure test (flaw detection) → marking → storage.
At present, the cold-rolled pipe production multi-cycle cold-rolled pipe mill. The working characteristic of the periodic cold rolling mill is that the steel pipe and the mandrel do not move, the rolling mill is driven by the reciprocating motion of the stand, and the rolling piece is compressed by the variable section profile to achieve the purpose of reducing diameter and wall.
Advantages: fast forming speed, high output, and no damage to the coating, can be made into a variety of cross-sectional forms to meet the needs of use conditions; cold rolling can cause great plastic deformation of the steel, thereby improving the yield of the steel point.
Disadvantages:
1. Although there is no hot plastic compression in the forming process, there is still residual stress in the section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. The style of cold-rolled steel is generally an open section, which makes the free torsional stiffness of the section low. It is prone to torsion when under bending, prone to bending-torsional buckling under compression, and has poor torsional performance;
3. The wall thickness of cold-rolled forming steel is small, and it is not thickened at the corners where the plates are connected, and the ability to withstand local concentrated loads is weak.
Difference between cold rolling and hot rolling in process:
1. Cold-rolled section steel allows local buckling of the section, so that the bearing capacity of the member after buckling can be fully utilized; while hot-rolled section steel does not allow local buckling of the section.
2. The reasons for the residual stress of
hot-rolled steel and cold-rolled steel are different, so the distribution on the cross-section is also very different. The residual stress distribution on the section of cold-formed thin-walled steel is curved, while the residual stress distribution on the cross-section of hot-rolled or welded steel is thin-film.
3. The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsion resistance of hot-rolled steel is better than that of cold-rolled steel.
Cold-rolled vs. Hot-rolled steel pipes: How to choose?
(1) Situations where cold-rolled steel pipes are preferred
High precision requirements: such as hydraulic cylinders and bearing sleeves, which require strict dimensional tolerances.
Thin-walled pipes (wall thickness <3mm): Cold rolling can produce ultra-thin-walled pipes (thinnest 0.2mm).
Surface quality sensitive: such as automobile exhaust pipes and home appliance housings, which require smoothness and no oxide scale.
Subsequent deep processing: such as bending and flaring (cold rolling has better ductility).
(2) Situations where hot-rolled steel pipes are preferred
Low-cost requirements: hot rolling has a simple process and is 20% to 40% cheaper than cold rolling.
Thick-walled pipes (wall thickness >5mm): hot rolling can produce large-diameter thick-walled pipes, while cold rolling is usually limited to thin walls.
Structural load-bearing parts: such as building steel frames and bridge pillars, which require high toughness and impact resistance.
Non-precision uses: such as transportation pipelines and pile foundation pipes, which do not require high surface quality.
Difference between cold-rolled and cold-drawn steel pipes:
Characteristics
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Cold-rolled steel pipe
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Cold-drawn steel pipe
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Processing method
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Room temperature rolling (continuous deformation through rollers)
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Room temperature drawing (stretching through molds in passes)
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Dimensional accuracy
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High (tolerance ±0.05~0.1mm)
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Very high (tolerance ±0.01~0.03mm)
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Surface roughness
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Ra 0.8~1.6μm (smooth, suitable for direct coating)
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Ra 0.4~0.8μm (smoother, mirror effect optional)
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Wall thickness range
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Wide (0.2~20mm)
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Thinner (0.1~10mm, ultra-thin wall has obvious advantages)
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Mechanical properties
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High strength (work hardening), good ductility retention
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Highest strength (significant cold hardening), slightly lower toughness
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Production efficiency
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High (continuous production)
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Low (single root drawing in passes)
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Cost
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Low (suitable for batches)
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High (large mold loss, mainly small batches)
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Cold rolled vs. Cold drawn steel pipe: How to choose?
(1) Situations where cold-rolled steel pipes are preferred
Mass production: such as automobile exhaust pipes and home appliance housings, the cost advantage is obvious when the demand is large.
Medium precision requirements: such as hydraulic supports and ordinary mechanical structural parts, which do not require extreme tolerances.
Thick wall thickness (>2mm): Cold rolling process is more economical for thick-walled tube forming.
Subsequent coating/spraying: High surface uniformity requirements (cold-rolled surface is more consistent).
(2) Situations where cold-drawn steel pipes are preferred
Ultra-high precision requirements: such as precision instrument shafts and aerospace components (tolerance ±0.01mm).
Ultra-thin wall or small diameter (<1mm wall thickness or <10mm diameter): Cold drawing can produce extremely thin wall tubes.
High strength requirements: such as gun barrels and high-pressure cylinders, which require the significant work hardening effect of cold drawing.
Special cross-sectional shapes: Special-shaped tubes (such as hexagonal and D-shaped) can only be achieved by cold drawing.
Read more: Hot Rolled Seamless Steel Pipe or Cold Drawn Seamless Tubing
