1.What are the macroscopic morphological characteristics of double skin defects? How can they be initially identified with the naked eye?
Morphological Characteristics:
Tongue-shaped or scaly: The defect presents as an irregularly shaped thin layer of metal peeling up, one end connected to the substrate and the other end free, resembling a tongue or fish scales.
Strip-like distribution: Often appears as strips or blocks, discontinuously or continuously distributed along the rolling direction.
Edge curling: In severe cases, the edge of the peeling skin can be slightly lifted, even revealing the underlying voids or oxide scale.
Color and luster:
The inner side of the peeling skin (the surface in contact with the substrate) is usually dark gray or dark brown, due to oxidation caused by air entering the gaps during hot rolling or hot working.
A normal strip surface should have a uniform metallic luster (after cold rolling and annealing) or silver-gray.
Tactile inspection:
Lightly scratching the defect with a finger or a sharp object will reveal a noticeable edge step. Sometimes, a gentle scraping can peel off the peeling skin, leaving a pit on the substrate surface after peeling.
2.What are the typical locations of the heavy skin defect on cold-rolled coils?
Edge Area (Most Common):
Approximately 70% of heavy-strip defects are located on both sides of the strip's width (within 10-50 mm of the edge).
Cause: During the rolling process, the edges of hot-rolled slabs experience rapid temperature drop and complex stress. If corner cracks or edge bubbles exist, after multiple rolling passes, the metal is flattened and folded to the surface, forming strip-like peeling along the edge.
Any Location (Center or 1/4):
If the defect originates from subcutaneous bubbles or large inclusions within the slab, during rolling, the bubbles rupture and the inclusions are crushed and extended, causing heavy-strip defects to appear at any location along the strip's width, even at the center.
Head and Tail Sections:
The head and tail sections of hot-rolled coils often have a higher incidence of heavy-strip defects than the middle sections due to factors such as the impact of steel biting during rolling, uneven temperature, and poor slab grinding.
3.How can simple on-site testing (such as polishing and acid washing) be used to help identify re-peeled leather?
Grinding with an abrasive wheel:
Operation: Use a handheld abrasive wheel or angle grinder to grind the defect horizontally (perpendicular to the rolling direction) or longitudinally, with a grinding depth of approximately 0.2~0.5mm.
Identification:
If it is a crack or scratch, the grinding marks will usually disappear or become shallower after grinding.
If it is a layer of flaked skin: When grinding to the boundary between the flaked skin and the substrate, delamination or gaps will be clearly visible; sometimes, after the flaked skin is ground off, a pit will be exposed underneath, with a dark color at the bottom (oxidation color) and a clear boundary with the surrounding substrate.
Local acid pickling method:
Operation: Use a cotton swab to apply an appropriate amount of pickling solution (such as 10%~20% nitric acid alcohol or hydrochloric acid solution) to the defect area and observe the reaction.
Identification: The gaps in flaked skin usually contain iron oxide scale or dirt. After acid pickling, the metal around the defect becomes brighter, while the oxides in the gaps, after corrosion, may leave more obvious black lines or pits, making the defect outline clearer.
4.What surface defects (such as scratches and scabs) can be easily confused with double skin defects? How can they be distinguished?
Double skin: tongue-shaped or layered, the edges can be lifted. There are gaps/layers, and a pit appears below after lifting.
Scratches: Grooves, usually straight, with uniform width. No layering, only surface grooves.
5.If the lapis laminae are suspected to be caused by internal defects, what microscopic testing methods are needed for final confirmation?
Metallographic Microscopy Analysis:
Sample Preparation: Cut a sample perpendicular to the defect, mount it, and then grind and polish it.
Observation: Observe the cross-sectional morphology of the defect under a microscope. Typical characteristics of the flaked surface include folded metal layers and the presence of a distinct oxide or decarburized layer. Large inclusions underneath will also be clearly visible.
Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS):
Morphological Observation: Observe the microstructure of the inner side of the flaked surface and the pits in the matrix under high magnification, looking for molten state characteristics (slag entrapment) or a candy-like fracture surface (brittle fracture).
Compositional Analysis: Perform energy dispersive spectroscopy analysis on the residue inside the flaked surface.
If elements such as Na, K, Ca, and Si are detected, it indicates possible entrapment by mold flux.
If oxides of Al, Mg, and Ca are detected, it indicates possible deoxidation products or inclusion aggregation.
If FeO is predominant, it indicates that iron oxide scale generated during hot working was pressed in.