1.What are the symptoms and causes of cracking/wrinkling during deep drawing?
Symptoms: During deep stamping processes such as automotive panels and appliance door panels, cracking (the coating and the steel base break together) may occur at corners or arcs of the part; or surface wrinkling (due to insufficient material ductility and uneven stress distribution).
Causes: Uncontrolled annealing (the annealing temperature is too low or the annealing time is insufficient, resulting in insufficient recrystallization of the steel base, high hardness, and poor ductility. Typically, DX53D+Z requires a hardness of ≤90 HRB and an elongation of ≥38%); excessive alloying (an excessively thick zinc-iron alloy layer increases the brittleness of the coating, causing cracking during stamping and dragging the steel base with it); and fluctuations in the steel base composition (e.g., excessively high C and Mn content, resulting in excessive hardness).
2.What are the symptoms and causes of coating peeling/powdering during bending?
Symptoms: After bending processes (such as folding or curling), the coating at the bend may "skin" (separation from the steel base) or "powder" (fragmentation that can be rubbed off by hand).
Cause: Excessive brittle phases (such as Fe₃Zn₁₀ and FeZn₁₃) in the alloying layer (due to excessive alloying temperature and excessive zinc-iron reaction); or insufficient adhesion between the coating and the steel base (due to incomplete removal of the oxide film on the strip surface during pretreatment, resulting in poor zinc adhesion).
3.What are the manifestations and causes of insufficient corrosion resistance?
Symptoms: After short-term storage or outdoor exposure, uncoated steel sheets rapidly develop white rust (zinc corrosion products) and even localized red rust (corrosion of the steel base). During salt spray testing, the white rust appears shorter than the standard (typically, the salt spray white rust time for DX53D+Z is ≥72 hours without passivation and ≥200 hours after passivation).
Causes: The coating thickness is too thin (not meeting the design weight, e.g., 80g/m² required but only 50g/m² in practice); poor passivation (uneven thickness of the passivation film (e.g., chromate film), missing coating, or uncontrolled passivation solution concentration/temperature); pinholes/missing coating in the coating (direct exposure of the steel base leads to preferential corrosion).
4.What are the manifestations and causes of mechanical property deviations?
Symptoms: Abnormal steel plate hardness (too high or too low), such as a required hardness of ≤90 HRB but an actual test result of 100 HRB (leading to cracking during processing); or insufficient elongation in a tensile test (e.g., the standard requirement of ≥38% but an actual value of only 30%).
Causes: Incorrect annealing process parameters (excessively high heating temperature resulting in coarse grains, or excessively rapid cooling rate resulting in excessively high base steel hardness); or excessive hardness of the rolled hardened strip (the rolling pressure of the "hardened coil" prior to annealing was not controlled as required, resulting in an inability to eliminate internal stresses after annealing).
5.What are the symptoms and causes of scratches and indentations?
Symptoms: Linear "scratches" (of varying depths, possibly revealing the strip's underside) along the strip's running direction; or irregular "indentations" (such as roller marks, caused by foreign matter or deformation in the equipment's rollers).
Causes: Burrs or foreign matter on the guide and tension rollers in the production line (scratched during strip operation); uneven roller pressure or roller wear during the leveling/stretching process; or collision with hard objects (such as forklifts) during handling of the finished coil.