1.What defects are mainly checked during surface quality acceptance of cold-rolled coils? How is acceptance determined?
Surface quality is a key focus of visual inspection, and the following typical defects need to be checked:
Rust: Yellowish-brown, red rust, or black spots, mostly seen on edges or where packaging is damaged. Minor rust that can be wiped off is acceptable; large areas of severe rust on the entire roll are unacceptable (substandard).Scratches/Indentations: Longitudinal or transverse grooves on the surface, which can be felt by hand. Intermittent and slight scratches or no scratches are acceptable; periodic scratches that are noticeably tactile and obstruct the nail are unacceptable.Roller printing with periodic, uniformly sized and shaped unevenness is acceptable if it has no tactile feel; it is unacceptable if it has a tactile feel and micro-cracks along the roller print.
2.How to inspect the dimensional accuracy of cold-rolled coils?
Dimensional acceptance mainly involves measuring thickness and width, requiring the use of tools such as micrometers or calipers:
Thickness measurement: Take measurements at points at least 15mm from the edge on the inside. For each roll, take at least three sections: the beginning, middle, and end, and measure multiple points at each section.
Width measurement: Measured along the roll width direction. The allowable deviation for the cut edge steel strip is 0~+3mm, and for the uncut edge, it is 0~+5mm.
3.What are the acceptance criteria for plate shape (unevenness, camber)?
Sheet shape issues can affect subsequent processing such as leveling and stamping, and require careful inspection.
≤1000mm 3mm 3mm 2mm
1000-1250mm 3mm 3.5mm 3mm
Warping: The steel plate warps in both the longitudinal and transverse directions simultaneously. This is usually caused by uneven cooling or improper straightening machine settings. Severe warping should be rejected.
4.How to inspect coil quality (towering, overflowing edges, collapsed coils)?
Coil shape issues affect hoisting, uncoiling, and annealing processes. The following must be checked during acceptance:
Towering (misaligned edges): The top and bottom of the coil are uneven, with each coil higher or lower than the previous one. Allowable standard: Local overflowing edge height ≤ 3mm; towering overflow height within the core of the coil's inner diameter within 15 coils ≤ 10mm.
Overflowing edge: The edges of the coil are uneven, usually caused by poor alignment or poor plate shape during coiling. Coils exceeding the above standards should be required to undergo finishing.
Collapsed coil: The coil's shape is noticeably non-circular, existing in the inner, outer, or middle coils. Allowable standard: Inner coil ellipticity ≤ 15mm. Collapsed coils are usually caused by stacking compression or unstable coiling tension. Collapsed coils should be photographed and their impact on use assessed.
Inner diameter: The standard inner diameter of cold-rolled steel coils is 508mm (610mm for pickled coils). During acceptance, it must be confirmed that it meets the requirements of the uncoiling equipment.
5. What tools are needed for visual inspection? How to handle non-conformities?
Common inspection tools:
Micrometer/Ultrasonic thickness gauge: Measure thickness
Steel tape measure/calipers: Measure width and overflow height
Right ruler/feeler gauge: Measure unevenness and camber
Quality inspection ruler: Quickly determine if gaps and overflow exceed standards (commonly used self-made tools by companies such as Wuhan Iron and Steel Logistics)
Surface roughness tester: Measure surface roughness Ra value and peak count, referring to GB/T 2523-2022 standard
Profile detector: Measure the profile of steel coils, referring to YB/T Standard 6020-2022 (Applicable to cold-rolled steel coils with a thickness ≤3.0mm)
Non-conforming handling procedure:
Immediately stop unloading, take photos and videos as evidence, and record the location, extent, and measurement data of the defects.
Fill out the acceptance record form, retain samples or seal part of the goods.
Notify the supplier and the transporter within 24 hours and negotiate a solution.
Submit a return, exchange, downgrade acceptance, or claim according to the contract terms.
For minor defects, a concessionary acceptance can be negotiated with a deduction of the corresponding price difference.
Note: For high-requirement applications such as automotive steel sheets and appliance steel sheets, it is recommended to commission a third-party laboratory for re-inspection and confirmation before processing.