Scabs On Rolled Steel Products

May 29, 2012

Scabs are irregularly shaped, flattened protrusions caused by splash, boiling or other problems from teeming, casting, or conditioning.-AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects

Scabs are always present prior to rolling.

(Teeming refers to the process of filling an ingot mold with molten steel from the ladle. We’ll point out some continuous casting analogs  later in this post.)

Scabs have scale and irregular surfaces beneath them; they tend to be round or oval shaped and concentrated to only certain blooms or  billets. Scabs are always the same chemistry as the steel bloom or billet.

(If the  gross irregular surface protrusion characteristic is appearing on all product, it is not likely to be a scab. If the protrusion is a different analysis, it is likely to be mill shearing.)

To differentiate between scabs and rolled in scale,  scabs are ductile when bent while scale is brittle and crumbles.

If the protrusion is brittle, it may be rolled in scale.

Scabs are primarily an ingot process issue related to teeming, but we have seen them on continuous cast  products as a result of mold and tundish anomalies.

Scabs present with scale beneath; Cracks may (but are not always)  be present associated with the scab due to stress concentration causing the material underneath to crack. (Not the crack causing the scab…)

Ingots or blooms showing scabs should be conditioned to remove the scabs. Thermal conditioning of billets (hot scarfing or torch conditioning) can sometimes leave artifacts which present as scabs upon rolling.

While scabs can be confused with slivers, shearing, rolled in scale, or tearing, their ductility eliminates them as rolled in scale. Scabs are distinct from shearing as scabs are isolated by occurrence and have an irregular surface beneath them, while shearing usually presents as multiple instances in a single orientation along the bar. Tearing is characterized by chevron shaped breaks rather than oval shaped protrusions.

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Slivers On Rolled Steel Products

May 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.

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

May 15, 2012

“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 at the 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 bar’s surface.’

Photo of a lap from AISI Surface Defects Manual.

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 ‘lamination’ to describe laps but as we will see, not all lamination type imperfections are laps…

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