1.How to choose the base material?
Low carbon steel is preferred: Use low carbon steel with a carbon content ≤0.25% (such as Q235, SPCC) to avoid stress concentration during bending due to the hard and brittle matrix of high carbon steel (C≥0.3%) (low carbon steel has an elongation of ≥25% and better bending performance).
Alloy element regulation:
Control the Si content to less than 0.1% to avoid the formation of brittle Fe-Si-Zn phase (e.g. the cracking rate of Si-containing steel during bending is 30% higher than that of Si-free steel);
Add 0.01-0.05% Al to refine the matrix grains and improve plasticity (when the grain size is reduced from 20μm to 10μm, the number of bending times can be increased by 50%).
2.How to optimize the coating composition?
Adding alloy elements to zinc liquid: Adding 0.1-0.2% Al to inhibit the excessive growth of the Fe-Zn alloy layer, so that the coating is mainly pure zinc layer (ζ phase) (the elongation of the pure zinc layer is ≥20%, and the bending resistance is strong);
Adding 0.05-0.1% Ni to refine the alloy layer grains (grain size <5μm) and reduce brittleness (when the Ni content is 0.08%, the bending cracking rate decreases by 40%).
Control coating thickness
The coating thickness of thin plates (≤2mm) should be controlled at 8-15μm, and that of thick plates (＞2mm) should be controlled at 20-30μm to avoid cracking due to high internal stress in thick coatings (e.g., the stress of a 30μm coating when bent is twice that of a 15μm coating).
3.How to regulate galvanizing process parameters?
Low temperature and short time immersion plating: The zinc liquid temperature is controlled at 430-440℃ (20-30℃ lower than the traditional 460℃), reducing the thickness of the Fe-Zn alloy layer (the alloy layer thickness is reduced from 10μm to 5μm);
The immersion plating time is ≤8s (thin plate ≤5s), avoiding excessive growth of the alloy layer (such as the immersion plating time is reduced from 15s to 8s, the δ phase thickness is reduced by 30%).
Cooling method optimization
Slow cooling: air cooling is used after galvanizing (cooling rate <5℃/s) to avoid quenching stress in the coating caused by water cooling (air cooling reduces the bending cracking rate by 60% compared with water cooling);
Annealing treatment: annealing at 200-250℃ for 1-2h after galvanizing to eliminate the internal stress of the coating (after annealing, the internal stress is reduced from 150MPa to below 50MPa).
4.How to perform coating softening treatment?
Tempering treatment: After galvanizing, tempering is performed at 200-300℃ for 1h to decompose the hard and brittle phases (such as Γ phase) in the alloy layer, reducing the hardness from 300HV to below 200HV, and improving the toughness (the number of bending times after tempering increases from 5 times to 15 times);
Mechanical finishing: Through rolling or shot peening, compressive stress is generated on the surface of the coating (residual compressive stress ≥ 100MPa) to offset the bending tensile stress (the cracking rate decreases by 25% after shot peening).
5.How to detect quality and predict defects?
Pre-test of bending test
Before production, a bending test is carried out according to GB/T 232 standard: using R=2t bending radius, observe the cracking of the coating after bending 180° (qualified standard: crack length <1mm and no shedding); use a metallographic microscope to observe the thickness of the alloy layer (target ≤5μm). If the δ phase thickness is >8μm, the galvanizing process needs to be adjusted.
Stress simulation and prediction
Finite element analysis is used to simulate the stress distribution of the coating during bending and optimize the mold design (such as predicting the stress concentration area and adjusting the fillet radius);
Test the coating bonding strength of key parts. When the bonding strength level is ≥1, the risk of bending cracking is low.