1.How to observe powder characteristics?
Color: Grayish-black usually indicates iron powder or the combustion/oxidation products of rolling oil; grayish-white may contain elements such as zinc and aluminum (if it's a plated sheet) or lubricant residue.
Wipeable: If easily wiped off, it's likely surface contaminants (such as dust, oil, or rust-preventive oil oxide layer). If firmly attached or peeled off from the substrate, it indicates damage to the material's surface layer itself.
Location: Is it precisely at the point of maximum tensile stress on the outer edge of the bend? If so, it's almost certainly a crack in the brittle surface layer.
2.What are the initial response measures?
Clean the sheet metal: Thoroughly clean the sheet surface with a specialized metal cleaner before bending to remove any potential contaminants.
Adjust the bending process:
Increase the bending radius: This is the most direct and effective method to reduce tensile strain on the outer surface.
Bend along the rolling direction: Try to make the bending line perpendicular to the rolling direction of the sheet metal (i.e., bend laterally), which usually improves the material's ductility at the bend.
Reduce the bending speed: Reduce the impact effect and allow the material to deform more smoothly.
Inspect the die: Ensure that the radius (R) of the bending die is smooth, unworn, and clean.
3.What impact will issues with the pickling and annealing processes have?
Over-pickling/Hydrogen Embrittlement:
Cause: Excessive pickling concentration/temperature or time leads to over-corrosion of the base metal, even hydrogen atom penetration.
Symptoms: Rough, dark surface; brittle flakes starting from the surface during bending; fine powder.
Inspection: Check pickling process records, especially free acid and iron ion concentrations.
Annealing Surface Oxidation/Carburization:
Cause: Improper dew point control (too high) of the protective atmosphere (e.g., nitrogen-hydrogen mixture) in the annealing furnace causes oxidation of the steel plate surface at high temperatures, forming an extremely thin iron oxide layer; or excessively high carbon potential in the atmosphere leads to slight surface carburization, forming a brittle carbide layer.
Symptoms: The steel plate surface may exhibit a slight bluish-gray or iridescent color; powder is flaky during bending.
Inspection: This is the most crucial area to investigate. Check the annealing furnace's sealing, the purity of the protective gas, the dew point (usually required to be below -40°C), and residual oxygen levels.
4.What impact will problems in the rolling process have?
Poor Roll Surface Condition:
Causes: Inappropriate roll roughness (possibly too rough), wear, or micro-peeling, which "plows" or "tears" the material surface during rolling, forming micro-cracks.
Symptoms: Powdering may be accompanied by fine, directional scratches.
Poor Rolling Lubrication:
Causes: Unclean lubricant, uneven spraying, or insufficient amount, leading to excessive frictional heat during rolling, causing localized "burns" or "adhesion," altering the surface structure.
Symptoms: Irregular dotted or streaky dark spots may appear on the surface.
5.What impact do issues with raw materials or coatings have?
Abnormalities in the hot-rolled substrate: The hot-rolled coil itself contains a thick layer of abnormal iron oxide scale (such as "red rust"), which was not completely removed by pickling before cold rolling, and the residue is pressed into the surface.
Poor adhesion of the coating: For products such as galvanized (GI) and aluminized zinc (GL), if the adhesion between the coating and the substrate is poor, the coating will peel off in pieces when bent, appearing as "powdering".