Mill Shearing On Rolled Steel Products

July 24, 2012

Shearing occurs when a longitudinal strip of base metal is torn off a bar during rolling. This strip often reattaches as rolling continues, not necessarily to the same bar. Shearing can refer either to the the discontinuity resulting from the detachment  or to the subsequent reattachment. There are usually several occurrences of shearing with a single orientation along the bar.–AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects

Mill Shearing is commonly mistaken for Slivers, Scabs, Laps, and Seams.

Mill shearing is usually detected visually and appears longer than scabs. also, the surface below the defect is smoother and more uniform than found below scabs.

Excessive rubbing of the steel as it rolls through the mill causes overheating, shearing material off the bar, which is later picked up from mill components on the same or another billet.

Improved guiding, pass design, and better section control can reduce incidents of mill shearing.

Metallurgical comments:

  • Rolled in material may have come from some source other than the base material.
  • If the material which is removed by shearing is not reattached, the remaining gouge in the surface may form other defects upon further rolling.
  • Intergranular precipitates or segregation can contribute to mill shearing.
  • Adjust mill to reduce sources of friction(al) heating

The rolled in material may have come from some source other than the base metal.

In my experience, mill shearing presents as and is easily confused with laps and slivers. Confirming that it is a piece of foreign material that has been rolled into the product is easily confirmed with a pair of pliers to remove it. Mill shearing almost always is removable by such means, and will show as two completely separate pieces of material in a micro.


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.


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.