During the production process of cold-rolled steel strips, the quality of the raw material, hot-rolled steel strips, is the cornerstone that determines the quality of the final product. Failure to properly control subsequent processes, combined with inherent deficiencies in hot-rolled coils, allows defects to enter the cold-rolling operation, ultimately impairing the surface smoothness, dimensional accuracy, and mechanical properties of the cold-rolled products.
This article will systematically summarize the 16 common defects of cold-rolled raw steel coils, analyze their macroscopic characteristics and the main causes of their occurrence, and help front-line technicians and quality managers quickly identify and trace the sources.
1. Surface defects
These defects mainly affect the appearance of the product and its subsequent coating performance, and are the most common quality issues.
Iron oxide scale pressing in
Characteristics: A layer of gray-black or reddish-brown oxide covers the steel strip surface, appearing in lumps, strips, or scattered dots, with indentations of varying depths.
Cause: During hot rolling, if operators apply excessive heating temperatures, prolong the heating duration, or select inappropriate rolling temperatures, they will cause regenerated iron oxide scale to form on the strip surface, which subsequent rolling passes, then press into the matrix. The oxidizing atmosphere in the heating furnace is also an important factor.
2. Scratch
Characteristics: There are longitudinal or transverse grooves on the surface of the strip steel, usually below the rolled surface, and they are distributed continuously or intermittently.
Causes: Longitudinal scratches are mostly caused by sharp corners on the guide plates, rollers, and other equipment that come into contact with the steel strip during rolling or coiling; transverse scratches are more common when the steel strip moves laterally on the cold bed or during transportation, where it rubs against the ground or other hard objects.
3. Indentation
Feature: There are irregular-shaped and varying-sized pits on the surface of the strip steel, and the distribution shows no periodic pattern.
Causes: Adhered oxide scale or other foreign substances on the rolling mill rolls or pinch rolls caused the surface defects.
4. Roll Printing
Characteristics: The surface of the strip steel shows regular and periodically distributed strip-like or sheet-like impressions. The marked areas are shiny, but there is usually no obvious unevenness.
Causes: Poor quality of the rolling mill rolls leads to surface damage; alternatively, rolling hard steel or improper operation causes the roll surface to develop indentations or even pick up metal shavings.
5. Scarring
Characteristics: There are “tongue-shaped” or “fish-scale-like” raised thin layers on the surface of the steel strip. The shape is irregular. Some are connected to the main body, while others are adhered on.
Causes: Rolling extends ingot scars, deforms surface scratches, and, from roll wear or pickup, creates periodic defects on the strip.
6. Surface inclusions
Characteristics: Point-like, block-like, or strip-like non-metallic inclusions cover the surface of the strip steel, typically appearing in reddish-brown, light yellow, or grayish-white.
Causes: A poor slag-making practice and an unclean pouring system introduced non-metallic inclusions into the steel billet. The peeling and falling of heating furnace refractory materials onto the billet surface caused them to be pressed in.
7. Nerve endings
Characteristics: The surface of the strip steel shows localized or extensive rough areas, covered with tiny pits. In severe cases, it may form bumpy patterns resembling the skin of an orange.
Causes: When you roll with oxidized scale on the strip, you press it into the surface, and acid washing later creates pits. Using worn or poor-quality rolls also damages the surface, and furnace gases can corrode the billet during heating.
8. Erythema
Characteristic: The steel strip surface displays red-brown spots of varying depth.
Cause: Hot rolling presses furnace ash or residual red iron oxide scale into the steel strip surface.
Plate Shape and Dimension Defects
These defects affect the geometric accuracy of the strip steel, making it difficult to perform strip feeding and plate shape control in the subsequent cold rolling process.
9. Scythe Bend
Feature: The strip steel bends towards one side in the horizontal direction, forming a “C” shape.
Causes: Processing errors in the roller profile or uneven adjustment of the roller gap result in inconsistent extension on both sides of the strip steel; uneven heating temperature of the slab; improper adjustment of the lowering amount on both sides of the rolling machine or uneven wear of the bearing; incorrect installation of the guide devices.
10. Cracking
Feature: The edges of the steel strip show cracks. In severe cases, they become jagged and even detach completely.
Causes: The steel itself is highly brittle; the heating or rolling temperature is too low; the roller profile is inappropriate; there are micro-cracks or overburning at the edges of the billet.
11. Uneven thickness
Feature: The thickness of the strip steel is inconsistent in the length direction (head, middle, tail) or the width direction (middle and edge).
Causes: Uneven heating temperature leads to inconsistent plastic deformation; Inappropriate distribution of reduction amounts in each pass; Bent or improperly configured rolls; Deviations in the thickness of the incoming material.
12. Fold
Feature: Localized areas on the steel strip surface create a folded structure where two metal layers overlap.
Causes: Operators who apply excessive local reduction cause metal stacking; those who allow large temperature differences during heating create uneven local elongation; and improper roll gap control produces large waves that later pass together.
Internal and Structural Defects
These types of defects are the most serious and often directly lead to the scrapping of the products.
13. Stratification
Characteristics: Delamination separates the internal metal structure of the strip, disrupting its overall continuity, with inclusions sometimes visible between the layers.
Causes: Failure to completely remove residual shrinkage cavities from the ingot leads to hollow or layered structures after rolling. Similarly, failing to control severe component segregation or concentrated large inclusions in the steel results in the same defects.
Conclusion
Understanding defect characteristics and causes enables targeted solutions. In production, optimizing processes, strengthening equipment management, and standardizing operations directly reduce these defects. This supplies the cold rolling process with clean, uniform raw materials.