Roll-Related Root Causes in Steel Strip Production
In modern hot and cold rolling lines, surface and shape defects in steel strips are rarely caused by a single factor. While steelmaking quality and heating conditions play important roles, the rolling mill roll system is the final and decisive control point that determines whether defects are eliminated or transferred to downstream products.
Below is a practical overview of common rolling defects, analyzed from a process path + rolling mill roll perspective, suitable for production engineers, quality managers, and procurement professionals.
Iron Oxide Scale Rolled In
Main process steps:
Heating Furnace → Roughing Mill → Finishing Mill Entry
Hot-rolled steel strips typically carry a layer of iron oxide scale, gray-black or reddish-brown in color. The scale may appear as patches or streaks with uneven distribution and varying penetration depths.
At high rolling temperatures, regenerated oxide scale forms rapidly. If reheating time is excessive or the furnace atmosphere is strongly oxidizing, thick oxide layers develop. During rolling, these oxides can be pressed into the steel surface, making them difficult to remove during pickling.
Although oxide scale thickness is influenced by heating regime, steel grade, and rolling temperature range, whether the scale is actually rolled into the strip is determined by roll surface condition.
Key roll-related factors include:
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Excessive surface roughness of roughing mill work rolls
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Insufficient anti-adhesion performance of roll material
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Poor coordination between roll cooling and descaling systems
Bending (Sickle-Shaped Strip)
Main processes involved:
Roughing Mill / Finishing Mill
Sickle-shaped bending occurs when the strip bends laterally along its length. The fundamental reason is uneven elongation across the strip width.
From a roll perspective, this defect is commonly linked to:
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Insufficient roll profile machining accuracy
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Non-parallel roll gap adjustment
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Uneven wear of roll bearings
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Inadequate compensation for roll thermal expansion
Uneven billet heating and improper reduction distribution further amplify the bending effect during rolling.
Torn Edge
Main stages:
Intermediate Rolling → Final Rolling
Torn edges appear as severe edge fractures with saw-tooth shapes and metal tearing. This defect is typically the result of combined mechanical and metallurgical factors.
Roll-related contributors include:
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Unreasonable roll edge profile design
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Concentrated roll wear at strip edges
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Stress concentration under low-temperature rolling conditions
When combined with brittle steel chemistry, billet edge cracks, or overheating during reheating, edge tearing becomes more likely.
Scratches
Main processes:
Finishing Rolling → Cooling → Coiling
Scratches can be longitudinal or transverse:
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Longitudinal scratches often originate in the finishing mill due to roll surface microcracks, adhered hard particles, or misaligned guides.
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Transverse scratches usually form during cooling bed transfer, transportation, or coiling.
Maintaining high surface integrity of finishing mill work rolls is critical for scratch prevention.
Indentation (Dents)
Main stage:
Finishing Rolling / Before Coiling
Indentations appear as random, non-periodic pits on the strip surface. Common causes include material buildup on work rolls or pinch rolls, oxide scale adhesion, or metal chips embedded in the roll surface.
This defect typically indicates:
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Insufficient roll anti-adhesion performance
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Inadequate cooling and lubrication management
Roller Printing (Roll Marks)
Main process:
Finishing Mill Stand
Roller printing appears as periodic bright strip-like or patch-like marks without obvious depth variation. Although visually subtle, they signal early roll surface damage.
Typical causes include:
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Improper roll material selection with insufficient hardness
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Early roll fatigue failure
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Iron filings or debris adhered to the roll surface
Uneven Thickness
Main process:
Entire Finishing Rolling Section
Thickness deviations may occur both longitudinally (head–middle–tail) and transversely (edge–center).
From the roll system perspective, key causes are:
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Unreasonable roll configuration
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Elastic bending deformation of rolls
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Severe wear of work rolls or backup rolls
High mill stiffness and effective roll shape control are essential for stable thickness accuracy.
Scarring (Callusing / Scabs)
Main processes:
Intermediate Finishing → Final Finishing Rolling
Scarring appears as irregular tongue-shaped or fish-scale-like raised flakes. Some are rooted and difficult to remove, while others are loosely attached.
Direct roll-related causes include:
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Roll surface defects such as chipping, pinholes, or material loss
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Metal buildup on roll surfaces
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Repeated magnification of roll surface pits during multi-pass rolling
Scarring is a classic symptom of roll surface integrity failure.
Surface Inclusions
Main sources:
Steelmaking → Heating → Rolling
Surface inclusions appear as point-like, block-like, or strip-like non-metallic particles. They are usually caused by poor slag control, ladle contamination, or refractory spalling during reheating, and are not fully removed during rolling.
Pockmarks (Oxidation Pits)
Main processes:
Heating → Rough Rolling / Finish Rolling
Pockmarks are rough surface patches with pits of varying depth. Severe cases resemble orange peel. They result from heavy oxidation during heating, where thick oxide layers are pressed into the surface and later detach.
Poor roll surface quality, severe roll wear, and low resistance to oxidation or thermal cracking further aggravate this defect.
Fold
Main processes:
Rough Rolling / Finishing Rolling
Folds occur when localized metal overlaps and forms a double layer. This is usually caused by mismatched roll profile design and reduction control, often worsened by uneven temperature distribution across the strip.
Delamination (Layered Structure)
Delamination refers to internal separation of the steel into multiple layers, sometimes with inclusions visible between layers. Root causes include residual shrinkage cavities, concentrated inclusions, and severe chemical segregation in the original billet.
Erythema (Red Spots)
Main processes:
Heating → Initial Rolling
Erythema appears as reddish spots with certain penetration depth. It is caused by furnace ash or unremoved red oxide scale being pressed into the strip surface during early rolling passes.
Final Thoughts
Across both hot-rolled and cold-rolled steel production, rolling mill rolls act as the final quality gate.
Even when defects originate upstream, roll material selection, surface integrity, profile accuracy, cooling efficiency, and wear control ultimately determine whether defects are suppressed or transferred to the finished strip.
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