1.What are roll marks? How are they identified?
📖 Definition and Appearance: Roll marks are irregularly shaped protrusions or depressions that are periodically distributed (i.e., the distance between defects is fixed) on the surface of cold-rolled coils. They may appear as dots, blocks, or stripes.
🤔 How to Identify:
Observe the Pattern: This is the most crucial characteristic for identifying roll marks. Measure the distance between two adjacent defects with a ruler. If the interval is fixed, it can be confirmed as a roll mark.
Classification: Depending on the production process, roll marks are divided into mill roll marks and flat roll marks. Flat roll marks usually have a smooth, shiny appearance, while mill roll marks are darker and have a certain degree of roughness.
2.How to distinguish between surface inclusions, scale, and delamination?
These three defects are all related to the internal purity of the steel coil, but they differ significantly in appearance and processing methods.
Characteristics: Surface inclusions, scale, delamination
Appearance: Dot-like, block-like, linear, or elongated shapes, with colors ranging from brownish-red, yellowish-brown, grayish-white, or grayish-black. Irregularly shaped thin metal flakes, appearing leaf-like, feather-like, or fish-scale-like, clearly raised. Linear separation appears on the cross-section of the steel strip, often with grayish-white or dark black inclusions between layers.
Feel: Rough to the touch or feels like foreign matter. Sharp, raised feel to the touch. Delamination is usually only discovered after observing the end face or applying external force.
Root Causes: Non-metallic inclusions in the molten steel are exposed on the surface during rolling. Original defects in the billet (such as scale, peeling) were not removed, or surface-adhered foreign matter was pressed in. Non-metallic inclusions or slag concentrated in the substrate are extended after rolling but cannot be pressed together.
3.What are some typical edge defects that may occur on cold-rolled coils? How are they identified?
Edge defects mainly occur on the two sides of the steel coil, and the three most common are:
Edge cracks: Serrated cracks appear on the edge of the steel strip, sometimes forming open fissures, usually without a periodicity.
Edge streaks: During the edge trimming process, broken edges do not completely detach and remain attached to the edge of the steel strip, easily identifiable with the naked eye.
Overflowing edges: Uneven winding occurs in certain areas of the steel coil, resulting in jagged edges that appear "off-center."
4.How to determine rust defects? What is the difference between rust and oxidation?
Both rust and oxidation are related to the oxidation of steel, but their formation stages and treatment methods are completely different.
Characteristics: Rust vs. Oxidation
Appearance: Usually appears as reddish-brown or yellow spots. Oxidation: Appears as yellowish-brown, blue, or grayish-black, with lighter edges.
Forming Stage: Oxidation occurs after finished products leave the factory, during storage, transportation, etc., when exposed to water or humid air. Oxidation occurs during annealing due to impure protective gas or excessively high furnace temperature; these are considered production defects.
Determination: Mild rust can be negotiated and treated; severe rust (large area, deep corrosion) is considered unacceptable. Severe oxidation will also be considered unacceptable.
5.How to determine scratch defects? How to distinguish between pickling scratches and coiling scratches?
Scratches are the most frequent type of defect, accounting for 18% of all cold-rolled defects according to 2022 statistics. They typically appear as continuous or intermittent grooves. For ease of differentiation, the table below compares the two main types of scratches:
Characteristics: Coiling scratches vs. Pickling scratches
Typical Distribution: Distributed transversely along the strip, mostly located at the edges. Often occurs at loopers, tension rolls, etc., in the pickling line.
Microscopic Features: Composed of multiple dot-like scratches; metal accumulation is often present at the scratch tail. Scratches are usually deeper, and the metal accumulation characteristics may differ.
Causes: Improper coiling tension, relative slippage between coil layers, etc.; Abnormality of a roller in the pickling line, strip misalignment, etc.