The properties of SECC galvanized steel sheets (such as corrosion resistance, workability, and surface quality) are influenced by a variety of factors throughout the entire process, from raw materials to production and processing. These factors can be categorized as follows:
1. Properties of the Substrate (Cold-Rolled Steel Sheet)
The substrate is the foundation of SECC, and its properties directly impact the overall performance after galvanizing. These factors primarily include:
Chemical Composition:
The carbon (C), manganese (Mn), phosphorus (P), and sulfur (S) content of the substrate (such as SPCC) are crucial. For example, excessively high carbon content increases substrate hardness and reduces stamping formability; excessively high sulfur content can lead to hot brittleness, impairing weldability.
Mechanical Properties:
The yield strength, tensile strength, and elongation of the substrate determine the upper limit of SECC processing. Insufficient elongation (e.g., less than 30%) can lead to cracking during deep drawing after galvanizing; excessively high yield strength increases bending difficulty and may cause the coating to flake off. Surface Quality:
Surface roughness, scale, oil stains, or scratches on the substrate will be directly transferred to the galvanized layer. For example, if scale is not removed from the substrate, the galvanized layer will not adhere well, resulting in localized peeling or bulging.
II. Galvanized Layer Parameters
SECC's core performance (such as corrosion resistance) depends primarily on the galvanized layer, and key influencing factors include:
Coating Thickness:
Coating thickness is a key indicator of corrosion resistance (usually expressed as the weight of the zinc coating on a single surface, e.g., E8-E32, corresponding to 1.4-7.0μm). Insufficient thickness will result in reduced corrosion resistance and premature red rusting during salt spray testing; excessive thickness, on the other hand, may increase costs and may cause "zinc accumulation" during bending, leading to cracking.
Coating Uniformity:
Uneven current distribution during the galvanizing process can result in significant variations in coating thickness across different areas of the steel plate (e.g., too thick at the edges, too thin in the center), resulting in localized corrosion resistance and a reduced overall service life. Zinc Coating Purity and Structure:
Imponents such as lead (Pb) and iron (Fe) in the coating may reduce its ductility and cause cracking during processing. The zinc coating's crystal structure (e.g., columnar or equiaxed crystals) can affect its adhesion strength, and a loosely structured coating is prone to detachment.
III. Surface Treatment Process
SECC surface treatment (e.g., passivation, oiling, and anti-fingerprint treatment) is key to enhancing its functionality. Specific factors influencing this include:
Passivation Film Type and Thickness:
Chromate passivation (traditional process) forms a dense protective film, improving corrosion resistance, but is less environmentally friendly. Chromium-free passivations (e.g., silane and titanate) are more environmentally friendly, but may offer slightly lower corrosion resistance. A passivation film that is too thin provides insufficient protection, while a film that is too thick may impair subsequent painting or welding.
Anti-Fingerprint Treatment Quality:
The uniformity and adhesion of anti-fingerprint coatings (e.g., SECC-N5) are crucial. Flawed coatings or poor adhesion can lead to fingerprint residue and wear, resulting in a loss of anti-fingerprint functionality. Oiling Amount and Type:
Oil application prevents rust during storage and improves lubricity during processing. However, excessive oiling can affect subsequent coating (e.g., poor paint adhesion); insufficient oiling can cause cracking of the steel sheet during stamping due to excessive friction.
IV. Production Process Parameters
Process control during the galvanizing process directly impacts the performance stability of SECC, primarily including:
Electrolysis Parameters:
The current density, temperature, and electrolyte concentration during galvanizing affect the rate and uniformity of zinc deposition. For example, excessively high current density can lead to a rough zinc layer, increased porosity, and reduced corrosion resistance; excessive temperature fluctuations can result in uneven coating thickness.
Substrate Pretreatment:
The quality of degreasing (degreasing) and pickling (removing oxide scale) before galvanizing is crucial. Incomplete degreasing results in poor adhesion between the zinc layer and the substrate; excessive pickling can cause excessive corrosion on the substrate surface, affecting mechanical properties. Post-Processing:
Improper temperature and time control during passivation and drying can lead to passivation film failure (e.g., decomposition due to excessively high drying temperatures) or residual moisture, causing corrosion during storage.
V. Storage and Usage Environment
Even if qualified SECC is produced, subsequent storage and usage conditions can affect its performance, including the following:
Storage Environment:
Humid, high-temperature environments, or those with corrosive gases (such as those in industrial areas or at the seaside) can accelerate SECC corrosion. If stacked without isolation (e.g., direct contact with moisture), "contact corrosion" may occur, leading to localized rust.
Processing Methods:
During processes such as stamping and bending, insufficient mold lubrication or excessive pressure can cause the zinc layer to wear and peel, losing its protective effect. High temperatures during welding can damage the passivation film, requiring additional treatment to prevent rust at the weld points.
Coating and Post-Processing:
If oil or impurities are not removed from the SECC surface, subsequent coating (e.g., spray painting) can cause blistering and flaking, losing its secondary protective effect. Summary: SECC performance is a function of the combined effects of substrate characteristics, zinc coating parameters, surface treatment, production process, and storage and operating environment. Substrate quality is fundamental, while the zinc coating and passivation film provide the core guarantees of corrosion resistance. Production process control determines performance stability, while storage and processing conditions influence actual service life. These factors must be comprehensively considered during product selection and use to ensure compatibility with specific applications (e.g., household appliances, electronic equipment, and other applications requiring surface quality and corrosion resistance).