A comprehensive description of 16 common defects in rolled steel billets
A comprehensive description of 16 common defects in rolled steel billets
The billets used by cold-rolling strip steel mills are hot-rolled strip steel. The poor quality of the billets themselves and incorrect process operations often result in various defects. These defects can have a significant impact on the quality of our cold-rolled products.
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FeO scale rolling in
The surface of hot-rolled steel strips is coated with a layer of iron oxide scale, which is mostly gray-black or reddish-brown in color. The distribution area varies from large to small, and it can be in the form of blocks or strips. The depth of the scale penetration also varies. The causes are that the high rolling temperature forms regenerated iron oxide scale which is pressed onto the surface of the steel strip, or the heating time is too long and the heating temperature is too high, resulting in an oxidative atmosphere and generating iron oxide scale which is pressed in during the rolling process. The thickness of the iron oxide scale produced usually depends on the heating conditions, the steel quality, the initial rolling and final rolling temperatures.
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(2) Curvature (commonly known as sickle-shaped curvature)
The length direction of the steel strip bends towards one side on the horizontal plane. The cause is that the roller profile is not processed correctly or the roller gap adjustment is uneven, resulting in inconsistent extension of the steel strip during rolling. Additionally, the uneven heating temperatures on both sides of the steel billet during heating will also cause different extensions during rolling. Moreover, improper adjustment of the reduction on both sides, uneven wear of the roller bearing shells, incorrect installation of the guide devices, etc., can all lead to the bending of the steel strip during rolling.
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(3) Cracked edges
When the edges of the steel strip are severely damaged, they are torn into jagged shapes and there are obvious metal fragments. The causes are quite complex and involve both the steel quality and the heating and rolling processes. In summary, the main reasons are the unreasonable design of the roller profile, incorrect adjustment of the rollers, low-temperature rolling, and secondly, the inappropriate chemical composition of the steel strip or the brittleness of the billet. Additionally, there may be cracks at the edge of the billet or the billet may be overburned, all of which are factors contributing to the edge cracking.
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(4) Scratches (Scratches)
The surface of the strip steel has longitudinal and transverse scratches that are of varying lengths and positions, and are generally distributed continuously or intermittently along the entire length or a certain section of the strip steel. The longitudinal scratches are mostly formed when the guide plates or rollers during rolling have sharp corners that come into contact with the strip steel, or during the winding process. The transverse scratches are caused by the strip steel moving laterally on the cold bed or during transportation and other processes when it is rubbed.
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(5) Indentation
There are various shapes and sizes of pits on the surface of the steel strip, and the pits are not in a periodic pattern. The cause is that the rolls and the pinch rolls cause sticking or adhesion of iron oxide scale. If foreign objects fall on the surface of the steel strip and fall off after rolling, they will also form pits on the surface of the steel strip. Additionally, during the stacking process of the steel strip, if it is pressed against hard objects or padded with substances with sharp corners, it will also cause indentation on the surface of the steel strip.
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(6) Roller marks
There are periodic roller impressions in the form of strips or sheets on the surface of the steel strip. The areas with the impressions are brighter, but there is no obvious protrusion or depression sensation. The cause is poor quality of the roller material, with low hardness that is prone to damage. The hardness of the steel strip is high or during the rolling process, the roller surface is not properly treated, resulting in imprints or the presence of iron filings.
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(7) Uneven thickness
The thickness of each part of the strip steel is not uniform. The non-uniformity of thickness can be divided into longitudinal (in the length direction) and transverse (in the width direction) types. That is, there is non-uniformity in the thickness of the head, middle and tail parts in the same longitudinal section, and in the thickness of the two sides and the middle or the two sides of the same transverse section. The causes are uneven heating temperature of the strip steel or unreasonable distribution of reduction in each pass, which leads to inconsistent plastic deformation of the strip steel. Improper configuration of the roller profile or bending deformation of the rollers can also cause periodic non-uniformity of thickness. In addition, the non-uniformity of the strip steel's thickness itself and severe wear of the rollers and the two bearing shells will also cause non-uniformity of thickness of the strip steel.
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(8) Scarring (also known as thickening of the skin)
The surface of the steel strip is composed of "tongue-shaped" or "fish-scale-like" raised thin sheets. Their area and thickness are not equal, and the outline is extremely irregular. There are both closed and open shapes, some are connected to the steel body and folded onto the surface of the strip without easy detachment, some are not connected to the steel body but adhered to the surface of the strip, and are prone to detachment. There are raised and non-raised ones. Generally, the scars on the rolled products do not tend to rise, leaving traces around, and there is iron oxide scale below. The scars caused by steelmaking tend to rise and expand easily, and there are inclusions below. The causes are that there are residual scars (heavy scales) on the surface of the steel ingot, which are pressed onto the surface of the strip after rolling, or the surface of the strip is uneven, the cleaning depth-to-width ratio is insufficient, the surface of the rolled piece is severely scratched before rolling, or in a certain previous rolling pass, the rolling roller falls off or has sand holes, and the surface of the strip generates protrusions, and after rolling, it presents periodic growth and scar formation, or due to the adhesion of metal substances on the strip surface, after passing through the rolling, pits are formed, and when rolled again, if the extension is inconsistent, the pits will also deform and scar. Additionally, during rolling, foreign metal substances fall on the surface of the strip and are brought into the rolling rollers, and after rolling, they are pressed onto the surface of the strip, forming non-rooting scars.
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(9) Surface inclusions
There are obvious point-like, block-like or strip-like non-metallic inclusions on the surface of the steel strip. Their colors are reddish-brown, light yellow and gray-white. The causes are poor slag-making during steelmaking, unclean ladles, or the cracking of refractory materials during the heating of the steel strip, with non-metallic inclusions adhering to the surface of the steel strip. These inclusions remain on the surface after rolling without being removed.
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(10) Pinholes
The surface of the steel strip shows local or continuous patches of roughness, with various shapes and different sizes of indentations. In severe cases, there are similar orange-peel-like marks that are larger and deeper than the rust spots. The cause is that during the heating process, the surface of the steel strip was severely oxidized. During the rolling process, the oxide scale was pressed onto the surface of the steel strip in patches or blocks. During the rolling process or after acid washing, these scales fell off, forming small pits, which are commonly known as oxidation rust spots. Poor quality of the rolling mill rolls and severe wear can also cause rust spots on the steel strip. Some are caused by corrosion by certain gases during the heating process, resulting in gas corrosion rust spots.
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(11) Fold
The steel strip forms a double-layer metal structure with local mutual folding. The cause is improper control of the roll shape and reduction amount, resulting in large waves that are pressed together after rolling, or improper operation causing local pressing together. A large temperature difference during heating leads to uneven local deformation and inconsistent extension on both sides, which also causes folding.
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(12) Stratification
There is a clear separation of the metal structure, which has disrupted the overall continuity of the strip steel. In severe cases, it splits into layers, 2, 3, etc., and there are sometimes visible inclusions between layers, and the location of the layering caused by the inclusions is not fixed. The cause is that during the rolling of the steel ingot into the strip steel billet, the shrinkage cavities were not completely removed, resulting in residual shrinkage cavities on the strip steel billet, or the steel ingot has concentrated inclusions. Additionally, severe chemical composition segregation may also cause layering.
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(13) Erythema
The surface of the steel strip appears reddish, but with certain depth of spots. The cause is that the furnace ash or the remaining red oxide scale has been pressed onto the surface of the steel strip.
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