1.What types of online full-inspection equipment are available?
Cold rolling continuous production lines (continuous annealing/leveling/rewinding) typically operate at speeds of 300-1200 m/min, which cannot be monitored manually in real time and must rely on surface defect detection systems.
Equipment: Imported: Cognex (USA), ISRA (Germany), Parsytec (Germany) Domestic: Manufacturers affiliated with Beijing University of Science and Technology and Huazhong University of Science and Technology
2.What are some examples of bright field + dark field combinations?
Bright field (vertical illumination): The pit appears as a dark spot (light reflected off the lens).
Dark field (low-angle ring illumination): The edge of the pit appears as a bright ring (diffuse reflection at the edge).
Conclusion: Bright field illumination alone has an extremely high false negative rate (especially for shiny steel strips). A dark field channel is necessary; otherwise, the pits will be almost invisible after smoothing.
3.What are some algorithmic pitfalls?
Ordinary vision systems may misreport emulsion spots, watermarks, and dust as pits.
3D laser profiling must be enabled:
2D only shows grayscale changes;
3D measures depth data; real pits show negative depth, while dust shows positive height.
Recommendation: If budget is limited, at least use dome lighting + multi-angle 2D fusion.
4.How is offline sampling inspection conducted?
Visual inspection + tactile examination (most basic, still effective)
Method: Observe at a 45° angle against the light, and scratch with your fingernail while observing.
Tactile examination: The edges of the pits feel slightly rough; the raised areas will snag on your fingernail.
Limitations: Only applicable to depths >5μm, and relies on the user's experience.
Oilstone Polishing (Destructive, but Extremely Precise)
Tools: Square oilstone (400-600 grit).
Procedure: Apply kerosene or light oil and polish the suspected area 10-20 times in one direction.
Judgment Criteria:
Genuine Pits: After polishing away the surface roughness, the dark spots remain with clear boundaries.
Fake Pits (Dirty/Watermarked): Disappear after a few polishes.
Film-coating method (microscopic inspection) is used for dispute review and evidence collection for claims.
Method: Soften the film with cellulose acetate membrane and acetone, apply it tightly to the dent, and peel it off after drying.
The three-dimensional morphology can be measured under a microscope to distinguish whether the dent is caused by a roller pressing in or a foreign object.
5.How to detect in special scenarios?
Scenario 1: Tin-plated/Chromium-plated substrate (extremely thin)
Characteristics: Thickness 0.15-0.30mm, pits often lead to punching through.
Method: Pinhole detector.
Principle: High-voltage electric field causes breakdown discharge at the thinned area of the pit.
Can only detect fully penetrated pits, not shallow pits.
Scenario 2: Automotive Outer Panel (High Surface Requirements)
Requirement: No dents larger than 0.1mm are allowed.
Method: Oil Film Method (Oiled Surface Check).
Apply a thin layer of anti-rust oil and expose it to strong, oblique light.
Differences in oil retention at the dents will create variations in the oil sheen.
Scenario 3: Pickled/Cold-Rolled Coil (Unannealed) The surface is rough, covered by iron oxide scale, making it extremely difficult to detect visually.
Method: Eddy Current Array.
It can measure differences in surface conductivity; the eddy current signal changes abruptly at pits.
Disadvantage: Sensitivity to shallow pits is lower than visual inspection.