First, the uniform surface that results from the EPS process mitigates a class of surface irregularities that are collectively termed “mill defects.” These include pitted surface, rust, roll marks and “tail mark”. For pitted and rusted surface defects, both acid pickling and EPS are effective in reducing the severity of the defect. Both processes removed all of the oxide which, by itself, makes any pitting cavity less deep. However, the EPS process also served to smooth out the contours of any cavities that remained on the steel substrate.
As the following photos show, more widespread pitting remained after acid pickling and Slurry Blasting the rusted coupons, due to the severity of the rusting. Here the EPS ability to “smooth out” the pitting seemed to offer this distinct advantage: When removing oil from the samples, small pieces of the cleaning media became trapped in the jagged cavities of the acid pickled sheet. The blue circles show only a few of the many locations where these small reddish particles became lodged in the cavities. No such particles were trapped in the cavities of the EPS coupon due to the contour smoothing from Slurry Blasting.
The roll mark samples showed little difference
between the untreated hot roll and acid pickled coupons. Both retained the brighter “scrape” mark across
the steel surface. The roll mark was
barely visible on the EPS coupon since it blended into the very uniform
reflective surface, as shown in the photo below.
Silicon
(chemical symbol Si) is an extremely
common element found, to some extent, in all steels. It is deliberately
added to levels of approximately 4% for “electric steel”, extensively used in
alternating current magnetic circuits, because it increases electrical
resistance and lowers hysteresis loss.
In all other hot rolled steels, it is desirable to minimize the silicon
content and it is typically held to less than 0.50% by weight.
A primary reason is excess silicon often deposits on the
surface of the hot rolled steel, producing silicon “streaks” (see photo,
following page) which detract from the surface appearance and are not removed
by acid pickling of the steel.
Samples of hot rolled sheet with very apparent silicon
streaks were run through the EPS process, then analyzed for surface chemistry
to determine if EPS can mitigate the silicon streak surface defect.
Specifically, half of the sample with severe silicon streaks was masked off
from exposure to the Slurry Blast stream, then the entire sample run through
the EPS small test unit. Afterwards, the
masking was removed, revealing a sample where the masked, untreated half (left side)
still shows the silicon streaks, but the EPS-treated right half shows no
remaining evidence of streaks as the photo below shows:
An EDS spectrum analysis of the chemical composition of the
surface of each side showed that the Si content had been reduced by 57% on the
EPS-treated right side. More
importantly, the uniform surface of the EPS-treated side makes the material
eminently useable, thereby avoiding a possible rejection or mill claim that
might result should such material undergo acid pickling and the streaking
remain visible.
