Cold-rolled coils are strip steel products manufactured from hot-rolled coils as raw material through continuous rolling at ambient temperatures in cold rolling mills. They derive their name from being wound into coils for storage and transportation. As the core foundational product within the cold-rolled steel system, they share the same fundamental origin as the previously mentioned cold-rolled sheets, differing only in form.

I. Core Production Processes for Cold-Rolled Coils

The production of cold-rolled coils represents a ‘refining upgrade’ from hot-rolled coils, characterised fundamentally by rolling without reheating. The specific process can be divided into three major stages, effectively mitigating surface defects inherent in hot-rolled products while enhancing performance and precision:

Raw Material Pre-treatment: Hot-rolled coils (typically 3-12mm thick) undergo ‘pickling’ to remove surface scale (rust layer) formed during hot rolling, preventing its embedding into the steel during subsequent rolling and compromising surface quality. For premium applications, a ‘degreasing’ step may be added to eliminate surface oils.

Cold rolling: The pickled hot-rolled coils enter cold rolling mills (typically multi-roll mills such as four-roll or six-roll mills). Under ambient temperature, continuous rolling pressure from the rolls reduces the thickness to the target specification (usually 0.15–4 mm). During rolling, plastic deformation induces ‘work hardening’ in the steel (increased hardness and reduced ductility), necessitating subsequent treatment based on performance requirements.

Finishing and Product: Post-rolling strip steel undergoes ‘levelling’ (to eliminate surface waviness and adjust surface finish) and ‘trimming’ (to correct width and ensure edge integrity). It is ultimately coiled (typically with an inner diameter of 508mm or 610mm, outer diameter varying by weight), though some is directly uncoiled into cold-rolled sheets (flat plates).

II. Key Characteristics of Cold-Rolled Coils

Compared to hot-rolled coils, cold-rolled coils offer core advantages in ‘precision’ and ‘surface quality’, summarised in four points:

High dimensional accuracy: Thickness tolerances are controllable within ±0.01–0.05mm (significantly superior to hot-rolled coils' ±0.1–0.3mm), with minimal width deviation and excellent flatness (free from noticeable warping or corrugation), making them suitable for applications demanding stringent dimensional precision.

Superior surface quality: Acid pickling removes oxide scale, and the absence of high-temperature oxidation during cold rolling yields a highly polished finish (classified as matt, semi-matt, or high-gloss). Free from pitting or scaling defects, it requires no additional polishing for direct use in visible components.

Controllable mechanical properties: By adjusting rolling deformation and subsequent annealing (heating to soften and eliminate work hardening), the hardness, strength, and ductility of cold-rolled coils can be controlled. For instance: - “Hard-rolled coils” (unannealed) exhibit high hardness, suitable for structural components; ‘Soft-state cold-rolled coils’ (post-annealing) exhibit excellent ductility, making them suitable for deep drawing applications (e.g., automotive body panels).

Excellent subsequent workability: With a smooth surface and dimensional stability, they facilitate subsequent processes such as stamping, bending, welding, and coating (painting, galvanising) without requiring complex pre-treatment.

III. Common Specifications and Grades of Cold-Rolled Coils

1. Common Specifications

Thickness: Mainstream range 0.15–4 mm (most commonly 0.3–2 mm), with special requirements extending to 0.05 mm (ultra-thin cold-rolled coil).

Width: Typically 600–2000 mm (customised per downstream requirements, e.g., 1250 mm or 1500 mm for household appliances).

Weight: Single coil weight generally 5–30 tonnes (calculated based on coil diameter and thickness for convenient hoisting and transportation).