Iron oxide scale rolled in
Main process steps: Heating furnace → Roughing mill → Finishing mill entry
Hot-rolled steel strips have a layer of iron oxide scale adhering to their surface, which is mostly gray-black or reddish-brown in color. The scale varies in size and distribution, appearing as patches or streaks, and the depth of penetration also varies. The cause of this is that high rolling temperatures lead to the formation of regenerated iron oxide scale which is pressed into the steel strip surface, or prolonged heating time and excessively high heating temperatures create an oxidizing atmosphere, resulting in the formation of iron oxide scale that is pressed in during rolling. The thickness of the resulting iron oxide scale usually depends on the heating conditions, the steel grade, and the initial and final rolling temperatures.
Direct relationship with the rolling mill rolls:
- Excessive surface roughness of the high-temperature roughing mill work rolls
- Insufficient anti-adhesion capability of the rolls, leading to easy adhesion of regenerated iron oxide scale
- Inadequate coordinated control of roll cooling and descaling
Key process points:
The thickness of iron oxide scale is closely related to the heating regime, steel grade, initial rolling and final rolling temperatures, but whether or not it is “rolled in” is ultimately determined by the surface condition of the rolling mill.
Bending (commonly known as sickle-shaped bending)
Main processes involved: Roughing mill / Finishing mill stands
The strip steel bends laterally in the horizontal plane along its length. This is caused by incorrect machining of the rolling mill rolls or uneven adjustment of the roll gap, resulting in inconsistent elongation of the strip steel. Uneven heating of the strip steel blank itself can also lead to differential elongation during rolling. Furthermore, improper adjustment of the reduction amount on both sides, uneven wear of the roll bearings, and incorrect installation of the guide devices can all contribute to the bending of the strip steel during rolling.
Reasons related to the rolling mill rolls include:
- Insufficient machining accuracy of the roll profile
- Non-parallel roll gap
- Uneven wear of roll bearings
- Insufficient thermal expansion compensation of the rolls
Furthermore, uneven heating temperatures on both sides of the billet and improper distribution of reduction during rolling will be further amplified under the action of the rolling mill.
Torn edge
Main process stage where this occurs: Intermediate and final rolling stages.
When the strip edge is severely fractured, it is torn into a sawtooth shape with noticeable metal fragments. The causes are complex, related to both the steel quality and the heating and rolling processes. The main reasons include unreasonable roll profile design and incorrect roll adjustment, as well as low-temperature rolling. Other contributing factors include unsuitable chemical composition of the steel strip or high brittleness of the billet, as well as cracks in the billet edge or overheating of the billet. All of these can lead to edge cracking.
Factors closely related to the rolling mill rolls:
- The roller edge design is unreasonable.
- Roller surface wear is concentrated at the edges.
- Under low-temperature rolling conditions, stress concentration occurs in the roller.
Edge cracking is often the result of a combination of factors, including the steel quality, heating regime, and improper matching of the rolling mill roll profile.
Scratch (scratches)
Main processes involved: finish rolling → coiling
The surface of the steel strip has longitudinal and transverse scratches that are shallower than the rolled surface, varying in length and location, and generally distributed continuously or intermittently along the entire length or a portion of the steel strip. Longitudinal scratches are mostly formed when sharp edges of guide plates or rollers come into contact with the steel strip during rolling, or during the coiling process. Transverse scratches are caused by abrasion during the lateral movement of the steel strip on the cooling bed or during transportation.
- Longitudinal scratches often occur during the finishing rolling process and are related to microcracks on the roll surface, hard particles, or improper guide positioning.
- Transverse scratches often occur during the cooling bed, transportation, or coiling process.
The surface quality of the finishing mill work rolls is key to controlling this type of defect.
indentation
Main process stage where this occurs: Finish rolling / Before coiling
The surface of the steel strip exhibits pits of varying shapes and sizes, which are not periodic. The causes include sticking of the rolling mill rolls or pinch rolls, or the adhesion of iron oxide scale. If foreign objects fall onto the steel strip surface and are subsequently removed after rolling, pits will also form. Additionally, dents can be formed on the steel strip during stacking if it comes into contact with hard objects or is placed on a surface with sharp edges.
Common causes of roll damage:
- Work rolls or pinch rolls are experiencing material buildup.
- Oxide scale or metal chips are adhering to the roll surface.
This indicates insufficient anti-adhesion properties of the rolling mill rolls and inadequate cooling and lubrication management.
Roller printing
Main process involved: finishing rolling mill stand
The surface of the steel strip has periodic strip-like or patch-like roller imprints. The imprinted areas are brighter, but there is no noticeable unevenness. The cause is poor roller material, low hardness making it easily damaged, or high hardness of the steel strip, or carelessness during rolling causing imprints or iron filings to adhere to the roller surface.
Typical causes:
- Improper selection of roll material, resulting in insufficient hardness.
- The roll surface develops fatigue marks or has iron filings adhering to it.
Roller marks are essentially a direct manifestation of early roller failure on the product.
Uneven thickness
Main process involved: the entire finishing rolling process.
The thickness of the steel strip is inconsistent across different parts. This thickness variation can be longitudinal (along the length) and transverse (across the width), meaning that the thickness differs at the head, middle, and tail sections of the same longitudinal cross-section, and also between the edges and the center of the same transverse cross-section. The causes include uneven heating temperature of the steel strip or unreasonable reduction distribution in each rolling pass, leading to inconsistent plastic deformation of the steel strip. Improper roll profile configuration or roll bending deformation can also cause periodic thickness variations. Additionally, inherent thickness variations in the steel strip itself, as well as severe wear of the rolls and bearings, can all contribute to uneven thickness in the steel strip.
Roll-related factors:
- Unreasonable roll configuration
- Elastic bending of the rolls
- Severe wear of the work rolls and backup rolls
In the finishing mill stand, the stiffness of the roll system and its ability to compensate for roll shape are crucial for thickness control.
Scarring (also known as callusing)
Main processes involved: Intermediate finishing rolling → Final finishing rolling
The surface of the steel strip exhibits “tongue-shaped” or “fish-scale-shaped” raised thin flakes. Their area and thickness vary, and their outlines are highly irregular. Some are closed, others are not; some are connected to the steel body and folded onto the surface of the steel strip, making them difficult to detach; others are not connected to the steel body but are adhered to the surface of the steel strip and are easily detached; some are raised, others are not. Generally, scabs on rolled products are not easily raised, leaving traces around them, with iron oxide scale underneath. Scabs caused by steelmaking are easily raised and opened, with inclusions underneath. The causes include residual scabs (heavy scale) on the surface of the steel ingot, which are pressed into the surface of the steel strip after rolling; or unevenness on the surface of the steel strip, insufficient cleaning depth and width, severe scratching of the workpiece surface before rolling, or a roll defect (such as a chipped roll or pinhole) in a certain pass before the finished product, resulting in bumps on the surface of the steel strip after passing through, which then form periodic rooted scabs after further rolling; or due to metal adhering to the rolls, forming depressions on the steel strip after passing through, and if the elongation is uneven during subsequent rolling, the depressions will also deform into scabs; in addition, foreign metal objects falling onto the surface of the steel strip during rolling are carried into the rolls, and after rolling, they are pressed into the surface of the steel strip, forming non-rooted scabs.
Situations directly related to the rolling mill rolls include:
- Roll surface defects such as material loss and sand holes
- Metal deposits adhering to the roll surface
- Surface pits on the roll are repeatedly magnified during multi-pass rolling
This is a typical consequence of roller surface integrity failure.
Surface inclusions
Main sources: Steelmaking → Heating → Rolling
The surface of the steel strip has noticeable point-like, block-like, or strip-like non-metallic inclusions, which are reddish-brown, light yellow, and grayish-white in color. The causes include poor slag formation during steelmaking, unclean ladles, or spalling of refractory materials during steel strip heating, resulting in non-metallic inclusions adhering to the steel strip surface that are not removed during rolling.
Pockmarks
Main process steps: Heating → Rough rolling / Finish rolling
The surface of the steel strip exhibits localized or continuous patches of roughness, with pits of varying shapes and sizes. In severe cases, there are larger and deeper blemishes resembling orange peel. The cause is severe oxidation of the steel strip surface during heating, where the iron oxide scale is pressed into the surface in sheets or chunks during rolling. These scales then detach during the rolling process or after pickling, forming small pits, commonly known as oxidation pits. Poor quality and severe wear of the rolling rolls can also cause pitting on the steel strip. In some cases, the pitting is caused by corrosion from certain gases during the heating process, resulting in gas corrosion pits.
Reasons related to the rolling mill rolls:
- Poor roller surface quality
- Severe roller surface wear
- Insufficient resistance to oxidation and thermal cracking
fold
Main processes involved: rough rolling / finish rolling
The steel strip forms a double layer of metal due to localized folding. This is caused by improper control of the roll profile and reduction ratio, resulting in large waves that are then pressed together during rolling, or by improper operation causing localized compression. Significant temperature differences during heating, leading to uneven local deformation and differing elongation on both sides, can also cause folding.
The root cause is a mismatch between the roll profile and the deformation control.
Layered
There is a noticeable separation of the metal structure, disrupting the overall continuity of the steel strip. In severe cases, it separates into 2 or 3 layers, with sometimes visible inclusions between the layers. The location of the delamination caused by inclusions is not fixed. The reasons for this are that the shrinkage cavities were not completely removed during the rolling of the steel ingot into a strip blank, leaving shrinkage cavity remnants on the steel strip blank, or that the steel ingot contained concentrated inclusions. Additionally, severe segregation of chemical composition can also lead to delamination.
erythema
Main processes involved: Heating → Initial rolling
The steel strip surface exhibits reddish spots of a certain depth. This is caused by furnace ash or unremoved red iron oxide scale being pressed into the steel strip surface.
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