The causes of surface quality defects in mild steel cold-rolled coil are complex and diverse, involving multiple aspects, including raw materials, processes, equipment, environment, and operations. Systematic control is essential to minimize these defects. Raw material defects are one of the root causes of surface quality issues. If residual scale on the hot-rolled coil surface is not thoroughly removed, it can be pressed into the substrate during cold rolling, causing indentations and pitting after pickling. Surface defects such as cracks, scars, or heavy scale can easily develop into more severe surface damage during cold rolling, even leading to strip breakage. Prevention requires addressing these issues at the source. Optimize the hot-rolling heating process, control the oxidizing atmosphere within the furnace, and reduce scale formation. Furthermore, strengthen maintenance of the high-pressure water descaling system to ensure unobstructed nozzles and stable water pressure to enhance descaling effectiveness. Furthermore, conduct rigorous surface inspections of hot-rolled coils to eliminate material with severe defects.
Improper control of cold rolling process parameters can directly lead to surface defects. Excessive rolling force or improper distribution of reduction per pass can lead to internal stress concentration in the material, resulting in cracks or breakage. If the roll surface roughness is not properly matched, excessively coarseness can easily scratch the strip, while excessively fineness can reduce friction, causing slippage or abrasions. Emulsion concentration, temperature, and cleanliness significantly impact surface quality. If the emulsion is not thoroughly purged, residual liquid carbonizes at high temperatures, causing black spots or color variations. Prevention requires refined process control, adjusting rolling force and reduction according to material properties to avoid localized stress concentration. Regularly grinding the rolls to maintain uniform surface roughness and match the requirements of different rolling stages is essential. Emulsion filtration and circulation management should be strengthened to ensure unobstructed nozzles and adequate purging pressure to minimize residual material.
Equipment condition is a key factor affecting the surface quality of mild steel cold rolled coils. Roller sticking, peeling, or roll profile defects can cause periodic roll marks or indentations on the strip surface. Sharp edges or hard objects on auxiliary equipment such as guides and tension rollers can scratch the strip surface. Excessive coiling tension fluctuations can easily lead to interlayer misalignment, resulting in scratches or adhesion defects. Prevention requires strengthened equipment maintenance, establishing a regular roll inspection system, and promptly replacing defective rolls. High-wear components such as guides and tension rollers should be rounded to prevent scratches. The coiler tension control system should be optimized to ensure stable tension and reduce interlayer friction.
Environmental factors also have a significant impact on the surface quality of mild steel cold rolled coils. Excessively high humidity in the production environment can easily form a water film on the strip surface, which can electrochemically corrode with impurities in the emulsion, causing rust or yellowing. Inadequate purity or a high dew point in the annealing furnace can cause oxidation of the strip at high temperatures, resulting in oxidized discoloration or color shift. Prevention requires improving environmental conditions and controlling humidity in the production workshop to avoid condensation on the strip surface. Annealing furnace seal inspections should be strengthened, and the shielding gas purity and dew point should be regularly tested to ensure the oxidizing atmosphere does not exceed standards. Rapid cooling of the annealed strip should be implemented to reduce oxidation time.
Operational compliance directly impacts the defect rate of mild steel cold-rolled coils. Excessively fast unwinding and coiling speeds or improper tension control can easily cause the strip to vibrate, resulting in surface scratches or horizontal streaks. If the leveling fluid concentration is too low or purge is incomplete, residual fluid can corrode the strip surface, causing rust or discoloration. Prevention requires standardized operating procedures and the development of standardized work instructions to clearly define unwinding and coiling speed and tension parameters. Leveling fluid concentration monitoring and nozzle maintenance should be strengthened to ensure effective purge. Regular operator training should be provided to enhance quality awareness and operational skills.
Preventing surface defects in mild steel cold-rolled coils requires a comprehensive approach encompassing five key aspects: raw material control, process optimization, equipment maintenance, environmental improvements, and operational standards. Through comprehensive process control, defects can be reduced, improving product surface quality and market competitiveness.
