1.What is the effect of coating weight on the bending properties of galvanized steel?
Coating weight is too low (e.g., below Z40): The zinc layer is too thin, and localized stress concentration (e.g., at rounded corners) during bending may cause "localized wear or exposure" of the zinc layer. If the bending angle is large (e.g., a 180° flat bend), the thin zinc layer may be "torn" by the deformation of the substrate, losing its protective effect and easily causing surface scratches.
Coating weight is too high (e.g., above Z180): The zinc layer is too thick, and the deformation of the zinc layer during bending cannot match the tensile/compressive deformation of the substrate (e.g., the zinc layer is in tension on the outside of the bend and in compression on the inside). This can easily lead to cracking, wrinkling, or flaking of the coating due to "insufficient zinc plasticity," especially at small bend radii. For example, the probability of zinc cracking in Z275 thick-coated steel when bent at a small 90° radius is significantly higher than that in Z100-coated steel. Suitable range: Most bending processes (such as building brackets and home appliance frames) are more suitable for Z60-Z120 coatings. The zinc layer has a moderate thickness, which can meet the deformation coordination requirements while not being too thin to expose the bottom.
2.What is the impact on stamping performance?
Coating weight and friction coefficient: The "spangle morphology" of the zinc coating (e.g., ordinary spangle, smooth-finished spangle-free) affects the friction coefficient, but coating weight indirectly correlates to surface roughness. Thick coatings (e.g., Z150 and above) without smooth finishing may have coarser spangles and a rougher surface. This increases frictional resistance with the die during stamping, making it more susceptible to scratches and material jamming, or uneven friction leading to excessive localized deformation (e.g., bulging and wrinkling) of the workpiece.
Conversely, thin coatings (e.g., Z60-Z80) or smooth-finished thick coatings (e.g., Z120 smooth-finished steel) offer a smoother surface, more stable friction coefficient, and are better suited to the "uniform deformation" requirements of deep drawing. (For deep-drawn galvanized steels like DX53D+Z, Z80-Z100 coatings are generally recommended, balancing formability and protection.) Coating adhesion and delamination risk: During stamping, the workpiece surface is subjected to extrusion and tensile stress from the mold. If the adhesion of a thick coating is insufficient (for example, an oxide layer exists at the interface between the coating and the substrate), delamination of the zinc layer and the substrate may easily occur at stress concentration points (such as stamping fillets and areas with greater drawing depth). This can manifest as surface "bubbling" or zinc layer flakes falling off, which not only affects the appearance but may also lead to a decrease in coating adhesion during subsequent coating (such as spray painting).
3.What is the impact on welding process performance?
Excessive coating weight: Thick zinc layer → Excessive zinc vapor evaporates during welding. If this vapor cannot escape quickly, it can easily form pores and slag inclusions in the weld (especially in arc welding, where zinc vapor can interfere with arc stability). Excessive zinc vapor can also lead to poor weld fusion or the formation of a "zinc brittle layer" near the weld (zinc reacts with iron to form brittle compounds), reducing weld strength.
For example, when welding Z180-coated steel, the incidence of pores is over 30% higher than that of Z80-coated steel. This requires additional control of welding current and speed (e.g., increasing current to accelerate zinc vapor escape), increasing process complexity.
Excessive coating weight: Thin zinc layer → Minimal zinc vapor evaporates during welding, reducing the risk of weld defects. However, attention should be paid to protecting the heat-affected zone (HAZ). A thin zinc layer can oxidize at high welding temperatures, leading to zinc failure in the HAZ and subsequent rust formation (re-applying anti-rust paint is necessary).
4.What is the impact on shearing/cutting performance?
Excessive coating weight: The zinc layer is thick and relatively brittle. During shearing, the zinc layer at the edges is easily broken and detached due to the "shear impact," resulting in a discontinuous zinc layer at the cut, requiring additional treatment (such as passivation) to prevent rust. If laser cutting is used, the evaporation of the thick zinc layer may produce a large amount of zinc slag, which adheres to the cut surface and increases cleaning costs.
Excessive coating weight: The zinc layer is thin and more tightly bonded to the substrate. The zinc layer is less likely to detach at the edges during shearing, resulting in a cleaner cut surface and easier subsequent processing.
5.What is the "core influencing logic" of coating weight on processing performance?
Low coating weight (Z40-Z80): Suitable for processes such as welding, shearing, and simple bending (low deformation and low coating integrity requirements), but requires attention to local protection of the thin zinc layer.
Medium coating weight (Z80-Z120): Offers the best overall performance and is suitable for most processing techniques (bending, stamping, welding, etc.), making it a universal choice for applications such as automotive, home appliances, and construction.
High coating weight (Z150 and above): Recommended only for applications requiring low deformation (such as outdoor billboards and static brackets). Complex stamping and small-radius bending processes should be avoided, and welding requires special control parameters.