1.What types of problems can cause coating damage and peeling?
Causes of edge coating peeling: Wear of the cutting tool or improper clearance during shearing/slitting causes separation of the coating from the substrate at the cutting edge.
Impact: Exposure of the steel substrate and accelerated edge rusting.
Surface scratching/scratching
Cause: The processing equipment (guide rollers, molds) has hard objects or rough surfaces, which cause friction during transportation.
Impact: Destruction of coating continuity and reduced corrosion resistance.
Powdering or peeling of zinc layer
Cause: Excessive deformation during bending/stamping (*r/t* value is too small, r=bending radius, t=thickness).
Insufficient coating adhesion (defective annealing process or abnormal zinc liquid composition).
Manifestation: The zinc layer in the deformed area falls off in powder form.
2.What are the types and causes of machining defects?
Causes of bending cracking: Poor material ductility (insufficient annealing or high carbon content of the substrate).
The bending direction is parallel to the rolling direction (it should be perpendicular to avoid anisotropy).
The bending radius is too small
Wrinkling/cracking during stamping: Causes: Insufficient lubricity of the coating increases die friction. The substrate n value (hardening index) and r value (plastic strain ratio) do not match the deep drawing requirements.
3.What are the types and causes of welding problems?
Zinc vapor pores: Cause: Zinc evaporates (low boiling point) at high welding temperature (＞907℃), and the vapor is trapped in the molten pool to form pores.
High-incidence processes: arc welding, laser welding.
Welding spatter intensifies: Reason: Zinc vapor disturbs the stability of the molten pool, causing disordered spattering of molten droplets.
Weld embrittlement: Cause: Zinc penetrates into the weld to form a brittle Fe-Zn alloy phase, reducing the toughness of the joint.
4.What are the types and causes of surface quality abnormalities?
Black spots/oxidation: Cause: Local high temperature during processing (such as welding heat affected zone) causes the zinc layer to oxidize and generate ZnO.
Passivation film damage: Cause: Processing destroys the surface chromate passivation layer and reduces corrosion resistance.
5.What are the types and causes of size and shape out-of-control problems?
Uneven plate shape
Reason: Uneven stress release during processing (residual stress of the original roll has not been eliminated).
Improper tension setting during roll forming.
Large springback deviation in thickness direction
Reason: The hardness of the coating is higher than that of the substrate (the hardness of the zinc layer is about 60-70 HV), which changes the overall elastic modulus of the material.