Teach you how to inspect steel coils
Quality screening isn’t just a checkbox; it’s your first line of defense against costly production downtime. For steel coil buyers, a robust inspection protocol must bridge the gap between “what’s on the label” and “what’s on the warehouse floor.”
1. The “First Look” Inspection: Spotting Deal-Breakers
Visual inspection is about more than just a quick glance. We conduct these at two critical stages: initial arrival at the warehouse and right before uncoiling.
Surface Red Flags: While minor scratches can be marked, fatal defects like cracks, heavy mill scale, or delamination are immediate grounds for rejection. For coated products (PPGI/PPGL), pay close attention to the edges—if you see peeling or blistering of the paint, the batch is compromised.
Geometric Integrity: A “qualified” coil must be a perfect cylinder. We keep a strict eye on telescoping (max 20mm) and edge burrs (must be ≤0.1mm). If the burrs are too high, you’ll face slippage or tracking issues during unwinding.
The Packaging Trap: Don’t ignore the wrapper. Torn moisture-proof paper or snapped steel straps usually mean one thing: rust. Check that the labels (Batch #, Grade, Specs) perfectly match your MTC (Mill Test Certificate).
2. Precision Matters: Dimensional Tolerance Tracking
Processing suitability hinges on exact numbers. We don’t rely on “factory estimates”; we use multi-point measurements across the coil.
Thickness & Width: We measure at least 5 cross-sections per coil. For standard orders, we look for a deviation within ±3%. However, for precision-grade steel, we tighten that tolerance to ±0.02mm.
ID/OD Alignment: Most production lines require a standard Inner Diameter of Φ508mm or Φ610mm. An ID deviation of more than ±5mm can prevent the coil from fitting onto the decoiler mandrel.
Flatness & Wave: Check the edge waviness (ideally ≤0.5mm/m). If the edges are wavy, the steel won’t feed straight, leading to wasted material and potential equipment damage.
3. Lab Testing: Verifying the “Muscle” of the Steel
The Mill Test Certificate tells one story; a random lab test tells the truth. Our testing workflow focuses on three pillars:
Tensile Testing: We use microcomputer-controlled machines to pull the samples until they snap. For low-alloy high-strength (HSLA) coils, we expect a yield strength of ≥355MPa. If the elongation after fracture doesn’t hit the target, the material might be too brittle for your forming process.
Impact Resilience: Critical for structural steel (like bridge components). Using a Charpy pendulum, we verify if the material maintains its toughness—typically KV2 ≥ 27J—even in low-temperature environments.
Hardness Benchmarks: Depending on the grade, we toggle between Brinell and Rockwell scales. We take multiple readings to ensure the coil isn’t “hard in the middle and soft at the ends.”
4. Traceability: The Paper Trail
At the end of the day, an inspection is only as good as its record. We cross-reference every result against international standards like GB/T 228.1 or ASTM equivalents.
Every batch is assigned a unique identifier linked to its raw data. If a defect is found post-processing, we can trace it back to the exact hour it was rolled. This “closed-loop” system ensures that non-conforming material is isolated, returned, or scrapped—never shipped.