Q:What are the core advantages of hot-dip galvanized coils?
A:Ultra-high strength and lightweight compatibility
Excellent corrosion resistance
Improved molding accuracy and surface quality
Environmental protection and process adaptability
Q:Where is the excellent corrosion resistance of hot-dip galvanized coil reflected?
A:Metallurgical bonding of coating:
During the hot forming process, the galvanized layer diffuses with the steel substrate to form a zinc-iron alloy layer (Fe-Zn alloy, containing about 10%-11% Fe), and the coating adhesion is significantly better than that of ordinary hot-dip galvanized steel sheets. Salt spray tests show that the red rust resistance of double-sided 10μm zinc-iron alloy coating can reach more than 800 hours, which is 1.6 times that of ordinary hot-dip galvanizing (500 hours).
Corrosion reinforcement of welding area:
During hot stamping, the oxide layer formed near the weld due to high temperature can be re-galvanized through online coating repair technology (such as zinc powder spraying + induction heating) to avoid the problem of coating damage caused by ordinary welding. Data from a factory showed that the corrosion rate of hot-formed welds was 78% lower than that of cold-formed welds.
Q:What are the main disadvantages of hot-dip galvanized coils?
A:High material and production costs
Narrow process window, high scrap rate
Difficulty in subsequent processing
Hydrogen embrittlement risk and recycling challenges
Q:Why is the scrap rate of hot-dip galvanized coils high?
A:Strict temperature control:
The heating temperature of the steel plate needs to be precisely controlled within the range of ±5℃. If it is lower than 880℃, it will lead to insufficient austenitization and insufficient strength; if it is higher than 920℃, it is easy to produce coarse grains and reduce toughness. During the initial commissioning of a factory, the scrap rate was as high as 15% due to the fluctuation of the temperature control system, and then it was reduced to less than 3% through infrared temperature measurement closed-loop control.
Rapid mold loss:
During the hot stamping process, the mold surface temperature frequently alternates between **200-500℃**, which makes the mold steel (such as H13 steel) prone to thermal fatigue cracks. On average, the mold needs to be repaired once every 5,000 pieces produced. The mold life is only 1/3 of that of the cold forming mold, and the maintenance cost increases by about 20%.
Q:Why is the subsequent processing of hot-dip galvanized coils difficult?
A:No secondary forming:
After hot forming, the hardness of the steel plate reaches more than 45HRC. If local correction (such as punching and flanging) is required, carbide tools (the cost is 5 times that of ordinary tools) must be used, and the processing efficiency is low (the feed speed is only 1/2 of cold-formed parts).
Many welding restrictions:
When welding with ordinary steel plates, laser welding or CMT cold metal transition welding is required to avoid softening of the martensitic structure near the weld due to heat input. After the welding process of a certain model was switched, the fluctuation coefficient of the weld strength dropped from ±15% to ±5%, but the equipment investment increased by 8 million yuan.