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The majority of defects encountered in steel bars in
our shops are found on the surface. Internal defects can also be
encountered, and we posted about central burst (chevron) defects here. This post describes Pipe Steel.
Definition: A central cavity formed by contraction of the metal during solidification is called pipe.
When this cavity is found in wrought or cast products, this is also called pipe.
 Pipe steel centerline defect in wrought steel bar. We had this specimen hard chrome plated and made it into a bookend.
In the days of ingot casting, the location of the shrinkage cavity
was controlled by ingot mold design and the addition of hot tops to
assure that after cropping the material containing the void off, there
would remain sufficient sound material to roll into product.
Today with modern computer controlled billet and bloom casting
processes, pipe steel and center porosity is very seldom encountered.
Recently a question was asked about centerline defects on cast billets in one of my LinkedIn Groups.
"hello can any one tell us why some times whe have holes along the center of the billets just casted thanks"
Despite a lack of specifics about grade, deoxidation, and many other
factors, we can make some comments based on the fact that this is
continuously cast billets according to question.
Here are my comments addressing the continuous billet casting
process and how it can be implicated in the creation of centerline voids
(pipe steel defects).
The three key parameters in the casting process that are most likely to result in centerline pipe are
- Casting speed
- Superheat
- Electromagnetic Stirring. EMS amperage and frequency (Together they drive intensity.)
1. Casting Speed- Incorrect casting speed
can result in pipe/ centerline looseness/ porosity. This can be
aggravated by issues with mold level control. Slow down your casting
speed to get sufficient solidification.
2. Superheat is critical to maintaining the
proper fluidity and solidification dynamics in the mold. Liquid metal
shrinks in three steps; 1) volume decreases the liquid cools goes from
the pouring temperature to the freezing temperature; 2) volume
decreases as the metal solidifies. This is reinforced by the driving out
of dissolved gases as the metal freezes; 3) the metal shrinks as it
cools from solidification temperature to ambient temperature.
3. Electromagnetic Stirring (EMS)- If you macroetch
transverse sections of the billets and still see columnar rather than
equiaxed grain structure in the cast billet, it is a sign that the EMS
is ineffective.
There are a host of other operating parameters as well as chemistry
and processes that can contribute to porous centers or central cavity
pipe steel defects. Here is a list of questions to help address these:
Do you have adequate cooling water through the molds? Are you
running EMS? What is the metallurgical distance on this caster? What is
the mold level control? Evidence of turbulence into the mold? Meters per
minute for casting speed? Shrouding status on nozzles? What was
superheat? What was water flow?
Do you have chemistry in control, steel deoxidized, so that the
large void is a result of solidification shrinkage, not divorce of gas
from the liquid steel? What is grade? What was deoxidizer?
Continuous casting of steel is a complex process with a
large number of operating parameters and processes that need to be in
close control. Understanding how these parameters can impact the final
product is critical to eliminating defects that result from lack of
control.
Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which you can finish reading here.
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Re: Pipe Steel: Internal Defect
