Galvanized Coil: Characteristics, Classification and Industrial Batch Application Analysis

Galvanized coil refers to a functional coiled material made from cold-rolled or hot-rolled steel coils, coated with zinc through continuous processes such as hot-dip galvanizing and electroplating, while maintaining its coiled form. Compared to single-sheet galvanized sheet, its coiled form allows for direct integration into downstream continuous production lines, enabling integrated uncoiling, shearing, stamping, and bending operations, significantly improving processing efficiency. It is a core raw material for mass production in industries such as home appliances, automobiles, and construction. From the continuous stamping of refrigerator side panels to the batch profiling of factory roof panels, all rely on it as a base material.

I. Core Characteristics: Why is galvanized coil the preferred choice for industrial batch processing?

Strong adaptability to continuous processing: Galvanized coils can weigh 5-30 tons and range in length from several hundred meters to over a thousand meters. They can be directly connected to downstream uncoilers, continuous stamping lines, or profiling equipment, reducing restocking frequency (up to 60% efficiency improvement compared to single-sheet production). For example, when processing refrigerator side panels at a home appliance factory, 20 tons of galvanized coils can achieve 8 hours of continuous production without stopping to change materials, increasing daily output by 40% compared to using single sheets.

Highly uniform and stable zinc coating: Using a continuous galvanizing process, the zinc coating thickness tolerance can be controlled within ±5g/㎡ (far superior to the ±10g/㎡ of single sheets). The zinc coating's adhesion and surface consistency are extremely consistent across the same coil, eliminating "batch variations" in mass production. For example, in automobile body processing, the salt spray resistance of the zinc coating on doors stamped from the same roll of alloyed galvanized coil remains within 50 hours, ensuring uniform corrosion protection across the entire vehicle. Significant cost and logistics advantages: The coil format requires 60% less storage space than single sheets (30 tons of galvanized coil requires only 2 square meters of storage space, while a single sheet of the same weight requires 12 square meters), and transportation costs are reduced by 25% (the coil can be hoisted as a whole, eliminating the need for individual packaging). Furthermore, continuous processing reduces sheet cutting losses (from 8% to 3% for a single sheet), further reducing costs for downstream companies.

Wide process compatibility: The zinc layer thickness (10-200g/㎡), surface condition (spangle-free, passivated, oiled), and substrate strength (Q235-Q550) can be customized to meet downstream requirements. It also seamlessly integrates with subsequent processes (such as color coating and lamination). For example, in color-coated coil production, the galvanized coil can be directly used as the substrate for the roller coating process, eliminating the need for additional surface treatment.

II. Scientific Classification: Selecting the Right Raw Material Coil Based on Core Dimensions

(I) Classification by Galvanizing Process: Determining the Performance and Application of the Zinc Coating

Hot-Dip Galvanized Coil (GI Coil): The base steel coil is continuously immersed in molten zinc at 440-460°C, forming a 50-200g/㎡ zinc layer. This zinc layer bonds tightly to the base steel and offers strong corrosion resistance. GI coils account for over 75% of total galvanized coil production.

Advantages: Thick zinc layer, excellent outdoor corrosion resistance, and single coil lengths of up to 800-1500 meters, making them suitable for processing long products.

Specifications: Thickness 0.3-6.0mm, Width 600-1800mm.

Applications: Profiled raw material for building roof panels, base material for batch welding of municipal guardrails, and continuous bending of container side panels. Electro-Galvanized Coil (EG Coil): A continuous electrolytic process deposits a thin, uniform zinc layer of 10-50g/㎡ onto the substrate surface. The layer achieves a surface smoothness of Ra ≤ 0.4μm, making it suitable for fine stamping.

Advantages: Smooth surface and high precision (thickness tolerance ≤ ±0.02mm), suitable for direct use as raw material for home appliance panels.

Specifications: Thickness 0.2-2.0mm, Width 600-1250mm.

Applications: Refrigerator side panel stamping material, continuous cutting of washing machine back panels, and batch processing of precision electronic components.

Galvanized Alloy Coil (GA Coil): After hot-dip galvanizing, the zinc layer and substrate are continuously heated at 500-550°C to form a zinc-iron alloy (70%-90% zinc content). The surface hardness reaches HV200-250, offering excellent wear and high-temperature resistance. Advantages: Scratch resistance, excellent weldability, no discoloration after short-term use at 200°C, suitable for continuous stamping of automotive bodies.
Specifications: Thickness 0.4-1.2mm, Width 900-1600mm.
Applications: Material for stamping automotive doors and hoods, continuous forming of oven liners, and processing of high-temperature equipment housings.
(II) Classification by Substrate Type: Matching Processing Strength Requirements
Cold-Rolled Galvanized Coil: Based on cold-rolled steel coil, it offers a surface roughness Ra ≤ 0.8μm and a flatness tolerance ≤ 0.5mm/m, making it suitable for fine processing.
Compatible Processes: Electrogalvanized coil and alloyed hot-dip galvanized coil primarily utilize this substrate.
Applications: Home appliance panel stamping, automotive body processing, and precision electronic component production. For example, 0.3mm cold-rolled EG coil is used for continuous stamping of mobile phone charger housings. Hot-rolled galvanized coil: Based on hot-rolled steel coil, with a light surface scale (removed by pickling before galvanizing), thickness ranges from 1.5-6.0mm, tensile strength ≥345MPa, and cost 15%-20% lower than cold-rolled galvanized coil.

Suitable Process: Hot-dip galvanized coil primarily utilizes this substrate.

Applications: Raw materials for building steel structure purlins, batch welding of heavy equipment brackets, and continuous bending of container floors. For example, 2.0mm hot-rolled GI coil is used for continuous profiling of factory purlins.

(III) Classification by Surface Post-treatment: Adaptation to Downstream Functional Requirements

Ordinary zinc spangle coil: Hot-dip galvanizing naturally produces 1-5mm zinc spangles with a metallic luster. No additional treatment is required, resulting in the lowest cost and suitable for industrial substrate processing.

Applications: Profiling of building roof panels, welding of municipal guardrails, and cutting of machinery and equipment bases. Zinc-free coil: By adjusting the galvanizing process to suppress the formation of zinc spangles, the surface is smooth and free of grain, approaching the smoothness of electrogalvanized coil, making it suitable for subsequent painting or color coating.

Applications: Color-coated coil substrates (such as raw materials for color-coated panels for home appliances) and decorative parts requiring protective films (such as furniture panels).

Passivated coil: After continuous galvanizing, it is chromated to form a 1-3μm transparent or colored passivation film, which improves corrosion resistance by 30% and is fingerprint-resistant.

Transparent passivated coil: Used for stamping home appliance panels (such as refrigerator side panels) and electronic component processing, preserving the metallic color of the zinc layer.

Colored passivated coil (blue, yellow, and black): Used for mass production of decorative parts (such as shelf rails and display racks), combining aesthetics with corrosion resistance.

Oil-coated anti-rust coil: Coated with a thin layer of 5-10μm anti-rust oil, the surface improves short-term rust protection by 50%. Suitable for long-term storage or long-distance transportation. Cleaning and degreasing are required before use. Application Scenarios: Galvanized coils in stock that are not immediately processed, and raw material coils transported across regions (e.g., EG coils shipped from a steel mill to a home appliance factory).

III. Diverse Applications: Covering the entire industrial chain from raw materials to finished products

(I) Building Processing: Improving Efficiency in Mass Production

Roof Sheet Profiling: GI coils (0.3-0.8mm thick) (zinc coating 120-180g/㎡) are processed into interlocking roof panels with a corrugated height of 30-50mm using a continuous profiling machine. A single production line can produce up to 15,000㎡ of roof panels per day for logistics warehouses and steel structure factories. Wind-up resistance reaches 1.2kPa (withstands a Category 10 typhoon), and no painting is required during installation, shortening construction time. Guardrail Batch Welding: 1.0-2.0mm thick GI coils (zinc coating 150g/m2) are continuously uncoiled, sheared, and welded into municipal guardrails. A single production line can process 1,000 meters of guardrail per day. After welding, they are spray-coated with plastic and remain rust-free for 10 years, increasing efficiency by 30% compared to single sheet processing.

Steel Structure Purlin Processing: 1.5-3.0mm thick hot-rolled GI coils (zinc coating 100-120g/m2) are continuously roll-formed into C-shaped steel purlins. Daily production can reach 8,000 meters. These purlins are used for factory roof supports, spaced 1.5-2 meters apart, supporting the weight of the pre-painted steel sheets and preventing rust in humid environments. (II) Home Appliance Manufacturing: Precision Processing and Mass Production

White appliance mass production: Refrigerator factories use 0.3-0.5mm thick EG coils (20-30g/㎡ zinc coating, no spangles, and transparent passivation). Through a "uncoiling - continuous stamping - bending - assembly" production line, they produce 25,000 refrigerator side panels daily, with a surface flatness tolerance of ≤0.3mm, ensuring precise fit with refrigerator doors. Washing machine factories use 0.4-0.6mm thick GI coils (no spangles). These coils are continuously cut and welded into housings, eliminating the need for rustproofing and boasting a service life 4-6 years longer than ordinary steel housings. Small Appliance Processing: Microwave ovens use 0.2-0.4mm thick GA coils (zinc coating 40-50g/㎡), which are continuously stamped into outer covers. They are heat-resistant (200°C) and scratch-resistant, with strong paint film adhesion (no peeling after 100 friction tests). Electric oven inner shells use 0.3-0.5mm thick GA coils. After continuous bending and forming, they require no additional corrosion protection and are rust-resistant even after long-term exposure to high-temperature moisture. (III) Automotive and Transportation: Suitable for High-Strength Processing
Automotive body stamping: Passenger car factories use 0.6-1.0mm thick GA coils (zinc coating 40-60g/㎡, cold-rolled base), which are processed into doors and hoods through large-scale continuous stamping lines. A single production line can produce up to 3,000 sets of body parts per day. The zinc-iron alloy layer improves collision resistance and achieves a weld strength of 350MPa, meeting automotive safety standards. Truck bodies use 1.5-2.5mm thick GI coils (zinc coating 80-100g/㎡). These are continuously bent and welded into body side panels, resistant to cargo friction and outdoor rain, and have a service life 5-7 years longer than ordinary steel body panels. Transportation Equipment Processing: Bus shelter roofs are made from 1.0-1.2mm thick GI coils (no zinc spangles), continuously extruded and then spray-coated. Up to 100 roofs can be processed daily, offering easy installation and excellent weather resistance. Highway noise barriers are made from 1.2-1.5mm thick GI coils, continuously bent into a honeycomb structure, with a daily output of up to 5,000 square meters, ensuring both sound insulation and outdoor corrosion resistance.
(IV) Packaging and Other Applications: Meeting Diverse Batch Demands
Packaging Container Production: Food canning factories use 0.15-0.2mm thick EG coils (10-20g/㎡ zinc coating, oil-free), continuously stamped into cans. The surface is clean and free of impurities, meeting food hygiene standards, with a daily output of up to 100,000 pieces. Industrial turnover boxes are made from 0.3-0.5mm thick GI coils, continuously bent and welded into box bodies. They can be reused 3-4 times more often than ordinary steel boxes and are resistant to moisture and impact. Agricultural Machinery Parts Processing: 1.5-2.0mm thick hot-rolled GI coils (zinc coating 80-100g/㎡) are used for tractor fenders. Continuously cut and then stamped, we process 2,000 fenders daily. These coils resist corrosion from muddy fields and reduce maintenance frequency. 2.0-3.0mm thick GI coils are used for small agricultural machinery frames. These coils are continuously welded into frames, offering high strength and excellent weather resistance, making them suitable for outdoor operations.

IV. Purchasing Tips: Avoid Mistakes and Choose the Right Raw Material Coil

Determine the process based on the downstream processing scenario:

Outdoor bulk processing (roofing panels, guardrails): Choose GI coils with a zinc coating ≥ 120g/m2 (≥ 180g/m2 in coastal areas), preferably hot-rolled base (thickness 1.0mm or greater);

Indoor fine stamping (home appliances, electronics): Choose EG coils with a zinc coating 10-50g/m2, cold-rolled base (thickness 0.2-0.5mm), spangle-free and passivated;

High temperature/high wear (automotive bodies, ovens): Choose GA coils with a zinc coating 40-80g/m2, cold-rolled base (thickness 0.4-1.2mm). Select roll weight and length based on production efficiency:

Large-scale continuous production lines (such as automobile and appliance factories): Choose 20-30 ton rolls, 800-1500 meters in length, to reduce restocking (≤ 2 per day);

Small and medium-sized processing plants (such as small guardrail and hardware factories): Choose 5-10 ton rolls, 300-500 meters in length, to reduce storage pressure and capital investment;

Long products (such as 12-meter roof panels): Choose rolls ≥ 1200 meters in length to avoid splicing during processing (which can cause leaks). Check key parameters to assess quality:
Zinc coating quality: Tested using a zinc coating thickness gauge: GI coils ≥ 50g/m2, EG coils ≥ 10g/m2, and GA coils ≥ 40g/m2. Uniformity error ≤ 10% (to prevent rust from occurring due to thinning of the zinc coating).
Dimensional Accuracy: Cold-rolled substrate coils have a thickness error ≤ ±0.02mm, width error ≤ ±1mm; hot-rolled substrate coils have a thickness error ≤ ±0.05mm to ensure process compatibility.
Coil Shape and Surface: End surface flatness error ≤ 5mm (to prevent uncoiling deviation), with no pyramids or misaligned layers (to prevent processing stalls). The surface is free of missing plating, zinc bumps, or scratches, and passivated coils are free of yellowing. Post-treatment based on storage and transportation:

For long-term storage (over 3 months): Choose oil-coated rust-proof coils, and clean and degrease them before use.

For immediate processing or short-term storage: Choose oil-free passivated coils to avoid degreasing.

For transportation in humid areas: Choose color-passivated and oil-coated coils for dual rust protection, preventing white rust during transportation.

Select reputable manufacturers to ensure quality:

Preferably choose large steel mills (Baosteel, Ansteel, and Shougang) or reputable galvanizing companies. Request a material report (including zinc coating thickness, substrate material, and salt spray resistance test data). Avoid purchasing "non-standard" coils (inadequate zinc coating or adulterated substrate). These coils are prone to rust and cracking after processing, increasing rework costs.