1.What mechanical properties of galvanized sheet need to meet?
Yield strength: This determines the sheet's ability to resist permanent deformation. For example, galvanized sheet used for building exteriors requires sufficient yield strength to prevent deformation under pressure, while appliance casings require a balance between strength and ease of processing (yield strength is typically between 235-550 MPa, depending on the base material grade, such as Q235 or SPCC).
Tensile strength: This measures the maximum tensile force a sheet can withstand before breaking, directly impacting its load-bearing capacity or impact resistance (for example, galvanized sheet used in automotive chassis requires a higher tensile strength, typically ≥300 MPa).
Elongation: This reflects the sheet's ability to deform plastically and is a key indicator for processing (such as bending and stamping). Galvanized sheet with too low an elongation is prone to cracking during stamping. For applications requiring processing, such as appliance panels and pipe fittings, an elongation of 15% or higher is typically required (depending on the complexity of the process). Hardness: It needs to match the application scenario. For example, the hardness of galvanized sheet used for packaging (carton lining) should not be too high (to avoid scratches), while galvanized sheet used for mechanical parts needs a certain hardness to resist wear (usually marked with Brinell hardness HB or Vickers hardness HV).
2.What is the core function of the galvanized layer?
Galvanized coating thickness: This is the foundation of corrosion resistance-the thicker the coating, the longer the corrosion lifespan (in a dry environment, each additional 1μm of zinc coating extends the lifespan by approximately 1-2 years). Requirements vary depending on the application:
Indoor applications (such as appliance housings): The zinc coating thickness is generally required to be ≥80g/㎡ (double-sided, approximately 11μm/side);
Outdoor humid/industrial environments (such as architectural color-coated steel sheets and outdoor piping): ≥120g/㎡ (approximately 17μm/side);
Coastal environments with high salt spray (such as ship accessories and port facilities): ≥275g/㎡ (approximately 39μm/side), and a combination of a thick zinc coating and passivation may be used.
Galvanized coating adhesion: The zinc coating must adhere firmly to the substrate; otherwise, it will peel off (for example, the zinc coating will peel when bent, thus losing its corrosion protection). This is usually verified through a "bend test": the galvanized sheet is bent 180° (or at a standard angle). Acceptance is determined by the absence of flaking or cracking in the zinc coating. Passivation Performance (Required in Some Applications): After galvanizing, the surface is typically subjected to a passivation treatment (such as chromate passivation or chromium-free passivation) to form a passive film, further enhancing corrosion resistance (especially resistance to salt spray and moisture). This must be verified by a salt spray test: For example, in a neutral salt spray test, the white rust-free time must be ≥48 hours (indoors) or ≥96 hours (outdoors).
Corrosion Resistance Uniformity: The zinc layer must evenly cover the sheet surface to avoid localized "missing plating" (where the substrate is directly exposed and susceptible to rust). This can be verified by a "drip test": a specific solution (such as copper sulfate solution) is added to the surface to observe whether red spots quickly appear (red spots indicate exposed iron, indicating missing plating).
3.How does the surface quality of galvanized sheet affect its appearance and suitability for subsequent processing?
No Obvious Defects: The surface must be free of cracks, peeling, blistering, inclusions, or scratches. These defects not only affect the appearance but also damage the integrity of the zinc coating (e.g., scratches can easily corrode). This is particularly true for "exposed" applications such as home appliances and architectural decoration, where surface defects are extremely stringent (typically requiring "first-class" quality, meaning no visible defects).
Zinc Coating Uniformity: In addition to uniform thickness, the surface spangle (the crystalline pattern formed during the galvanizing process) must be consistent (e.g., "small spangle" or "no spangle," depending on customer needs). Avoid excessively large or dense spangles, which can lead to uneven appearance and affect subsequent painting (uneven spangles can cause uneven paint adhesion).
Surface Roughness: The surface must be tailored to the application. For example, galvanized sheet used for printing/laminated films (such as billboard panels) must have a smooth surface (Ra ≤ 1.6μm) to avoid streaks that affect print clarity. For structural parts that do not require secondary processing, the roughness can be relaxed.
4.How is the process adaptability of galvanized sheet reflected?
Machinability:
Bending/Stamping: Must meet the requirements of "no cracking and no zinc layer detachment" (as mentioned above, this depends on elongation and zinc layer adhesion).
Cutting/Cutting: Cut edges must be smooth and burr-free (to avoid scratches or assembly problems), and damaged zinc layers must be easily recoated with corrosion protection after cutting (for example, cold cutting is easier to maintain clean edges than hot cutting).
Weldability: If welding is required (such as for building steel structures and mechanical parts), the galvanized sheet must meet welding requirements. Zinc vapor may be generated during welding, so weld porosity and cracking must be avoided. An appropriate welding process (such as argon arc welding) is generally required, and the weld seam must be re-coated with corrosion protection after welding.
Paintability: If subsequent coating (such as spraying or painting) is required, the surface must be clean (free of oil stains and oxide layers), and the passivation film must be compatible with the coating to prevent coating detachment. This can be verified through an "adhesion test," such as a cross-cut test to ensure no coating detachment.
5.What are the key "core essential performance" priorities for different scenarios?
Interior Decoration/Home Appliances: Emphasizes surface quality (defect-free, smooth), elongation (suitable for stamping), and basic corrosion protection (standard zinc coating thickness).
Exterior Architecture/Industrial: Emphasizes corrosion resistance (thick zinc coating, passivation, salt spray resistance), and yield strength (anti-deformation).
Automotive/Mechanical Parts: Emphasizes mechanical properties (tensile strength, hardness), zinc coating adhesion (no peeling during processing), and weldability.
Highly Corrosive Environments (Coastal/Chemical Industries): The core requirement is "ultra-thick zinc coating + efficient passivation," and long-term corrosion resistance must be verified (e.g., salt spray test ≥ 500 hours).