rolling defects

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Slivers On Rolled Steel ProductsMay 17, 2012

Slivers are elongated pieces of metal attached to the base metal at one end

only. They normally have been hot worked into the surface and are common to

low strength grades which are easily torn, especially grades with high sulfur,

lead and copper.-AISI Technical Committee on Rod and Bar Mills, Detection,

Classification, and Elimination of Rod and Bar Surface Defects

Slivers are loose or torn segments of steel that have been rolled into the surface of the

bar.

Slivers may be caused by bar shearing against a guide or collar, incorrect entry into a

closed pass, or a tear due to other mechanical causes. Slivers may also be the result of a

billet defect that carries through the hot rolling process.

This is my lab notebook sketch for slivers back in the day

Slivers often originate from short rolled out point defects or defects which were not

removed by conditioning.

Billet conditioning that results in fins or deep ridges have also been found to cause

slivers and should be avoided. Feathering of of deep conditioning edges can help to

alleviate their occurrence.
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Slivers often appeared on mills operating at higher rolling speeds.

When the frequency and severity of sliver occurrence varies between heats, grades, or

orders, that is a clue that the slivers probably did not originate in the mill.

This is how Slivers present under the microscope. Note decarburization (white

appearance.)

Slivers are often mistaken for shearing, scabs, and laps. We will post about these other

defects in the future.

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Laps On Rolled Steel Products

Laps are longitudinal crevices at least 30 degrees off radial, created by folding

over, but not welding material during hot working (rolling). A longitudinal

discontinuity in the bar may exist prior to folding over but the defect generally

is developed atthe mill.-AISI Technical Committee on Rod and Bar Mills, Detection,

Classification, and Elimination of Rod and Bar Surface Defects

Here is my lab notebook entry for a lap back in 1985

In plain language, a lap is a rolled over condition in a bar where a sharp over fill or fin

has been formed and subsequently rolled back into the bars surface.

Photo of a lap from AISI Surface Defects Manual.
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An etch of the full section shows what is going on in the mill. Laps were often related to

poor section quality on incoming billets, although overfill scratches, conditioning gouges

from chipping have also been shown to cause laps.

Cross section of steel bar exhibiting laps (white angular linear indications). When two

laps are present 180 degrees apart, the depth to which they are folded over can indicate

where in the rolling the initial over fill ocurred. White indicates decarburization, which

confirms my interpretation that this lapping occurred early in the rolling.

Laps are often confused with slivers, and mill shearing which we shall describe and

post soon.

The term lap seam is sometimes used, but it is careless usage; it implies the lap is

caused by a seam it is not; a seam is a longitudinally oriented imperfection, and so is

used in this mongrel term as a shorthand way of saying longitudinal.

Modern speakers sometimes try to use the word laminationto describe laps but as we

will see, not all lamination type imperfections are laps

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