Cold-rolled coils offer significant application advantages over other steel products: superior surface quality, enabling direct use in visible components without additional treatment; high dimensional accuracy, reducing subsequent machining allowances and enhancing production efficiency; stable mechanical properties with minimal batch-to-batch variation, facilitating quality control; excellent workability, permitting complex forming processes such as stamping, bending, and welding; Comprehensive product specifications cater to diverse industry requirements; Superior corrosion resistance achievable through surface treatments (e.g., galvanising, colour coating); Certain high-strength grades enable lightweight design, reducing product weight and energy consumption. These advantages secure cold-rolled coils' pivotal role in modern manufacturing.

Various surface defects may occur during cold-rolled coil production, primarily including: Scratches, caused by damage from rollers or guide devices; Pitting, resulting from rough surfaces of pickling rolls; oxidation discolouration, surface colour changes arising from inadequate protective atmosphere during annealing; roller marks, periodic imprints on the steel sheet caused by surface defects on the rolling mill rollers; pitting, localised surface depressions potentially caused by foreign object indentation or excessive pickling; edge cracks, fissures appearing along the sheet edges, often related to raw material quality or rolling process; inclusions, defects formed by the exposure of non-metallic inclusions within or on the surface of the steel sheet. These defects compromise product appearance and functional performance, necessitating control through process optimisation and enhanced inspection during production.

Finishing constitutes the final stage of cold-rolled coil production, primarily comprising the following operations: trimming, which removes irregular edges to ensure width precision; re-coiling, where large coils are cut into smaller coils according to customer specifications; straightening, which corrects wave and bend deformations in the steel plate using a straightening machine; surface inspection, where surface defects are detected manually or via online systems to reject non-conforming products; oiling, applying rust-preventive oil as required; packaging, following standard procedures to safeguard product quality during storage and transport; labelling, marking packaging with product information (grade, specification, weight, production date, etc.). Finishing processes directly impact final product quality and user experience, necessitating strict control of process parameters and operational standards at every stage.

There is no standardised weight for cold-rolled coils. It is generally determined based on customer requirements, transport conditions, and production equipment capacity, with common ranges between 5 and 20 tonnes. Excessively light coils increase changeover frequency, reducing production efficiency; conversely, excessively heavy coils complicate transport and handling while demanding greater equipment load-bearing capacity. In actual production, coil weight can be controlled by adjusting rolling length and thickness. For instance, thin-gauge cold-rolled coils (e.g., 0.3mm) may reach several thousand metres in length with weights exceeding 15 tonnes, whereas thick-gauge coils (e.g., 2.0mm) have shorter lengths and comparatively lower weights. Coil weight information is typically indicated on product packaging or quality certificates.

Cold rolling mills are categorised by structure and operational mode: single-stand reversible mills feature simple construction and low investment, suited for diverse, small-batch production; multi-stand continuous mills (e.g., five-stand, six-stand) offer high efficiency for large-volume, single-product runs; dual-stand reversible mills balance efficiency between these extremes; Twenty-roll cold rolling mills (Senjimir mills) are suitable for producing ultra-thin, high-precision cold-rolled strip steel, particularly stainless steel and high-alloy steels; planetary cold rolling mills achieve large reduction ratios through multiple sets of small-diameter work rolls, making them suitable for producing difficult-to-deform materials. The selection of a cold rolling mill requires comprehensive consideration of product specifications, material properties, and production scale.

The annealing temperature for cold-rolled coils must be determined based on material composition, cold-rolling deformation, and performance requirements, generally ranging between 600°C and 700°C. For ordinary carbon steels (e.g., SPCC, DC01), a recrystallisation annealing temperature of approximately 650°C is typically employed. For high-strength low-alloy steels, achieving an optimal balance of strength and ductility may necessitate slightly elevated temperatures or extended holding times. Stainless steels require higher annealing temperatures, generally between 1000°C and 1100°C, to ensure complete microstructural transformation and carbide dissolution. During annealing, the heating rate, holding time, and cooling rate must be controlled to prevent excessive grain growth from overheating or incomplete work hardening from insufficient temperatures. Temperature control methods differ slightly between continuous annealing furnaces and bell annealing furnaces.

The mechanical properties of cold-rolled coils serve as a crucial basis for evaluating their service performance. Key indicators include: Tensile strength (σb), denoting the material's resistance to failure under tensile loading; Yield strength (σs or Rp0.2), representing the stress at which plastic deformation commences; Elongation (δ), indicating the relative elongation at fracture and reflecting plasticity; Hardness (e.g., HV, HRC), indicating the material's resistance to localised deformation; Cold bending performance, denoting the material's ability to withstand bending deformation at ambient temperatures, assessed via cold bending tests; Impact toughness, signifying the material's resistance to impact loads (particularly crucial for materials used in low-temperature environments). Mechanical property requirements vary across different grades and specifications of cold-rolled coils; selection should be based on specific application scenarios.

To enhance corrosion resistance and extend service life, cold-rolled coils typically undergo the following rust prevention treatments: Oil coating: Applying a thin layer of rust-inhibiting oil to form a protective film; Passivation: Chemically forming a dense oxide layer on the surface (e.g., chromate passivation); Galvanising (hot-dip or electro-galvanising): Depositing a zinc layer to protect the substrate through sacrificial anodic action; Colour coating: Applying a decorative and protective coating to the surface; Film lamination: Covering with a plastic film to prevent scratches and moisture exposure during transport and storage. The cost and effectiveness vary across these rust prevention methods, necessitating selection based on the usage environment and storage duration. For instance, oil coating may suffice for short-term storage, while galvanising or colour coating is recommended for long-term outdoor applications.

Various sheet form defects may occur during cold-rolled coil production, primarily including: - Waviness (centre wave, edge wave, double-rib wave, etc.), manifesting as periodic undulations on the steel surface; - Sickle bend, referring to lateral curvature along the coil's length; - Buckling, denoting irregular convex-concave deformation within the plane of the steel sheet; Thickness non-uniformity, where thickness variations within a single coil or sheet exceed tolerance limits. These flatness issues primarily stem from uneven pressure distribution during rolling, roller wear, improper tension control, and inconsistent annealing temperatures. Poor plate form compromises subsequent processing precision and reduces product yield rates. Therefore, production must control these issues through measures such as adjusting rolling parameters, replacing rollers, and optimising annealing processes.

Pickling is a critical process in cold-rolled coil production, primarily serving to remove scale (FeO, Fe₂O₃, Fe₃O₄) from the surface of hot-rolled coils. This scale impairs contact between rollers and steel sheets during cold rolling, increasing rolling forces, causing surface scratches, and potentially damaging equipment. Acid pickling yields a clean, uniform surface, creating optimal conditions for subsequent cold rolling. Common pickling solutions include hydrochloric acid and sulphuric acid. During pickling, acid concentration, temperature, and duration must be strictly controlled to ensure complete removal of scale without excessive corrosion of the base metal. Post-pickling rinsing and drying are essential to prevent residual acid from corroding the steel sheet. Pickling quality directly impacts the surface finish and formability of cold-rolled coils.

The dimensional tolerance standards for cold-rolled coils vary according to different national and regional specifications, with common examples including the Chinese national standard GB/T 708, the Japanese standard JIS G3141, and the American standard ASTM A653. Taking the Chinese national standard as an example, thickness tolerances typically range from ±0.02mm to ±0.05mm (varying according to thickness), while width tolerances generally fall between ±1mm and ±3mm. Flatness requirements typically stipulate that wave height per metre must not exceed a specified value (e.g., 2mm/m). Permissible defect levels for different surface quality grades are also clearly defined. In actual production, enterprises often establish internal control standards stricter than national standards to meet customer requirements. When purchasing, the applicable product standard and tolerance requirements should be clearly specified.

High-strength cold-rolled coils (such as HSLA steel, dual-phase steel DP, and transformation-induced plasticity steel TRIP) feature high strength and lightweight properties. They are primarily used in automotive manufacturing for producing body structural components, door impact beams, chassis parts, etc., enabling reduced vehicle weight while enhancing safety and fuel economy. Additionally, they are employed in products demanding high strength, such as construction machinery, shipping containers, and high-pressure vessels. Through the addition of microalloying elements (e.g., Nb, V, Ti) and controlled rolling processes, these coils maintain sufficient ductility and formability to meet complex forming requirements while ensuring high strength. Demand for high-strength cold-rolled coils is growing steadily with the development of new energy vehicles.

Stainless steel cold-rolled coils are manufactured from standard hot-rolled stainless steel coils through processes including pickling, cold rolling, annealing, and levelling. They exhibit the following properties: exceptional corrosion resistance, capable of withstanding various acidic and alkaline media; a smooth, aesthetically pleasing surface finish, achievable in mirror or satin effects; High strength, good plasticity, and excellent workability; Good high-temperature resistance, suitable for diverse thermal environments; Strong chemical stability, resistant to rusting; Superior hygienic properties, compliant with food contact requirements. Stainless steel cold-rolled coils are widely used in food machinery, chemical equipment, decorative materials, home appliances, automotive applications, and other sectors, representing a high-value-added cold-rolled product.

The packaging method for cold-rolled coils must be determined based on material composition, storage conditions, and transportation mode. Common approaches include: wrapping the inner layer with moisture-proof paper or plastic film to prevent water ingress; protecting the outer layer with kraft paper, corrugated cardboard, or galvanised iron sheets to enhance impact resistance; Installing protective rings (typically wooden or plastic) at both ends of the core to prevent deformation; securing the coil with steel or plastic strapping to ensure compactness; for export products, employing waterproof packaging with an outer waterproof cover, and labelling the packaging with product information and moisture-proof markings. Stainless steel cold-rolled coils usually utilise neutral packaging to avoid contact with other metals and prevent corrosion. Appropriate packaging safeguards product quality during storage and transportation.

Work hardening (also termed cold working) refers to the phenomenon where a metal material undergoes plastic deformation at ambient temperatures, resulting in increased strength and hardness while decreasing plasticity, toughness, and elastic modulus. During cold rolling, steel plates endure immense rolling forces that flatten and elongate grains, increase grain boundary area, and elevate dislocation density. This enhances interatomic bonding, making further deformation difficult. While work hardening improves material strength, it complicates subsequent processing. To restore plasticity, annealing is typically required. This involves heating to recrystallise the internal structure and accumulate dislocations, thereby reinstating the material's workability.

Levelling is a crucial subsequent process in cold-rolled coil production, primarily serving to: correct sheet form by eliminating defects such as waves and warping arising from cold rolling and annealing; adjust surface finish by controlling the roughness of levelling rolls to achieve the desired surface texture; partially relieving internal stresses to enhance dimensional stability; slightly increasing material strength and hardness (via minor cold-rolling deformation); and providing optimal surface conditions for subsequent processing (e.g., coating, stamping). The elongation rate during levelling is typically controlled between 0.5% and 3%, with specific parameters adjusted according to product requirements and material properties.

Annealing treatment can be categorised into several types based on purpose and process parameters: - Full annealing (heating to the austenitising temperature followed by slow cooling to achieve complete recrystallisation of the microstructure) - Partial annealing (heating below the austenitising temperature, resulting in partial recrystallisation of the microstructure) - Isothermal annealing (holding at a constant temperature after heating to ensure more uniform microstructural transformation) spheroidising annealing (promoting carbide spheroidisation to reduce hardness and improve machinability), and recrystallisation annealing (primarily used for work-hardened materials to restore ductility). In cold-rolled coil production, recrystallisation annealing is commonly employed. This process involves heating to induce grain nucleation and growth in cold-deformed grains, forming new equiaxed grains to mitigate work-hardening effects.

Despite their superior properties, cold-rolled coils present certain drawbacks: complex production processes and substantial equipment investment result in higher costs; limited thickness range precludes manufacturing thicker specifications; work hardening during cold rolling necessitates annealing to restore ductility, extending production cycles and increasing expenses; thin-gauge coils are prone to deformation during transport and processing, demanding higher operational precision; Certain carbon steel cold-rolled coils exhibit poor corrosion resistance, necessitating additional surface treatments; high rolling forces demand stringent equipment precision and maintenance, with production prone to flatness issues (e.g., waviness, sickle bending).