Coil Decoiling & Straightening

Why Rebar Coils Are Supplied in Coil Form

Coil decoiling and straightening is the first processing step whenever rebar coils are used in a fabrication facility or on a large site with its own stirrup-bending or mesh-welding equipment. Coils — typically B500A or B500B in diameters ∅6–16 mm — are supplied in ring form because coil is far more economical to transport than straight bar. A standard coil weighs 1–3 tonnes and occupies a fraction of the volume of equivalent straight-bar bundles, reducing shipping costs significantly for international buyers.

However, coiled steel has a residual curvature (the “coil set”) that must be removed mechanically before the bar can be used in a cutting-and-bending line or fed into an automated stirrup machine. This process is called decoiling and straightening.

Types of Decoiling Equipment

Passive (Free-Rotating) Decoilers

The simplest decoiling system: the coil sits on a rotating mandrel or cradle. As the feed machine pulls wire/bar, the coil spins freely. Passive decoilers are suitable for smaller diameters (∅6–10 mm) and lower-speed lines. The bar still carries significant curvature at this stage and requires a separate straightener unit downstream.

Motorised (Powered) Decoilers

Powered decoilers actively unwind the coil at a controlled speed, synchronised with the downstream processing line. This reduces tensile stress on the bar during feeding and is preferred for ∅12–16 mm coils or high-speed stirrup machines processing thousands of pieces per shift. Motorised systems also allow coil-end detection and automatic stopping to prevent the tail end of the coil from whipping through the line.

Turntable vs. Vertical-Axis Decoilers

Turntable (horizontal-axis) decoilers are common for heavy coils. Vertical-axis (pay-off reel) decoilers are used where floor space is constrained — the coil stands upright and bar exits from the top. The latter requires careful guide alignment to avoid kinking.

Straightening Mechanisms and Their Effect on Steel Properties

There are two principal straightening technologies:

For B500B material (Agt ≥ 5.0%), preserving ductility through the straightening process is critical. DIN 488 requires that the straightened bar retains its grade properties. Excessive rotary straightening can reduce the actual Agt of B500B bar to below the B500A threshold (2.5%), rendering it non-conforming. Machine settings should be validated with periodic tensile tests on samples cut from straightened bar.

Straightness Tolerance After Processing

DIN 488-1 specifies a maximum bow (deviation from straight) of 6 mm per 1,000 mm for processed straight bars. Bars failing this criterion must be re-straightened or rejected before entering a bending line. A simple check: lay the straightened bar on a flat surface and measure the maximum gap between bar and surface at any point along a 1 m gauge length.

Integration with Automated Stirrup and Mesh Lines

Modern automated stirrup benders (e.g. Schnell, MEP, EVG) incorporate an integrated decoiler and rotary straightener in a single machine head. The coil feeds directly into the straightener, then into a flying-shear cutter and bender — producing stirrups or links to DIN 488 shape codes at rates exceeding 50 pieces per minute. For cut-and-bend operations, this integrated approach eliminates the separate straightening step and reduces handling.

When ordering coil from us for automated lines, specify your machine make and model: coil inner diameter (typically 800–1,000 mm), outer diameter, and maximum coil weight (typically 1–3 t) must match your decoiler’s rated capacity. We can advise on appropriate coil specifications for your line.

Coil vs. Straight Bar — When to Choose Coil

• Choose coil when: you operate an in-house stirrup or mesh line; your site has a rebar processing facility; you are importing by container and want to maximise tonnes per container; diameter is ∅6–16 mm.
• Choose straight bar when: bars will be used without further cutting (e.g. standard length columns); you lack straightening equipment on site; diameter exceeds ∅16 mm (not available in coil); a project specification explicitly requires straight mill bar.