1.What is the core definition of galvanized medium and thick plate?
Substrate thickness: Defined as 4-60mm (thinner than 4mm is "thin galvanized sheet," thicker than 60mm is "thick galvanized sheet"). Sufficient rigidity and load-bearing capacity are required. Common substrate materials include Q235B, Q345B (carbon steel), and Q355ND (low-alloy weathering steel).
Galvanizing process: Hot-dip galvanizing is the primary method (electrogalvanizing is only used for small batches and high-precision applications due to its thin coating and high cost). The hot-dip galvanizing layer thickness is typically 50-150μm (corresponding to a zinc coating weight of Z100-Z275, or a total double-sided zinc coating weight of 100-275g per square meter). Salt spray resistance can reach 500-1000 hours (far exceeding that of ordinary carbon steel).
2.What are the key performance indicators of galvanized medium and thick plates?
Mechanical properties: Yield strength ≥235MPa (Q235B substrate), ≥345MPa (Q345B substrate), tensile strength 400-630MPa, can meet the requirements of load-bearing and impact resistance
Corrosion resistance: Hot-dip galvanizing must be uniform and free of leaks. In the neutral salt spray test (NSS), white rust should appear for ≥96 hours and red rust for ≥500 hours.
Surface quality: Slight zinc spangles and scratches are allowed on the zinc surface, but peeling, blistering and black spots are not allowed (which will affect the anti-rust effect).
Dimensional tolerance: thickness tolerance ±0.3mm (thick plate ±0.5mm), length/width tolerance ±2mm, flatness ≤5mm/m (to avoid processing deformation)
3.What are the common materials and brands?
Carbon structural steel: Q235B+Z, Q235C+Z. Low cost, good plasticity, used for building steel structures (such as support beams), general machinery bases, container frames
Low-alloy high-strength steel: Q345B+Z, Q355D+Z, Q460C+Z. High strength (yield strength 345-460MPa) and excellent weather resistance, used in construction machinery (excavator buckets), bridge components, and high-voltage equipment housings.
Weathering steel: Q355ND+Z, 09CuPCrNi-A+Z. Intrinsically weather-resistant, with a zinc layer for double rust protection, suitable for long-term outdoor service (e.g., docks, wind turbine towers)
4.What are the main processes for galvanized medium and thick plates?
Substrate Pretreatment:
Degreasing (using alkaline solution to remove oil from the substrate surface) → Pickling (using hydrochloric acid/sulfuric acid to remove oxide scale and prevent poor zinc coating adhesion) → Water washing + passivation (neutralizing the acid to prevent substrate corrosion).
Hot-Dip Galvanizing:
The substrate is immersed in molten zinc at 450-460°C (zinc purity ≥ 99.9%), forming a zinc-iron alloy layer (inner layer) + pure zinc layer (outer layer). The coating thickness is controlled by the zinc immersion time + substrate pull-out speed.
Post-Processing:
Cooling (air/water cooling to prevent zinc cracking) → Passivation (chromate treatment to improve zinc corrosion resistance) → Straightening (medium and thick plates are prone to deformation and require roller straightening to ensure flatness) → Inspection (measuring zinc layer thickness, adhesion, and surface quality).
5.What are the typical application scenarios of galvanized medium and thick plates?
Construction and Infrastructure: Load-bearing columns and crane beams for steel structure factories, guardrails and connectors for municipal bridges, and ventilation duct casings in subway stations (needing rust protection in humid environments).
Construction Machinery: Loader frames, road roller counterweights, and concrete pump booms (needing to withstand impact and resist corrosion from mud in construction environments).
Transportation: Heavy-duty truck floor panels (wear-resistant and rainwater-resistant), container corner posts and crossbeams (resistant to salt spray in shipping environments).
Machinery and Equipment: Large water pump casings, chemical storage tank support bases (preventing corrosion caused by liquid leakage), and wind turbine tower flanges (for long-term outdoor service).