Faster Pickling of High Carbon and Alloy Steels
Harder steels – high carbon and alloy steels – have a different composition of mill scale that is more difficult to remove by acid pickling. This means the pickling line must be slowed considerably to achieve full scale removal – up to 75% slower than when running low carbon strip.
With EPS ‘mechanical pickling’ there is little line speed decrease when running high carbon or alloy steels. The EPS process is largely unaffected by the composition of the scale. Production experience shows that higher carbon steels (grade 1018 to 1074) and series 41XX, 51XX and 61XX alloy steels can be run on an EPS line at about the same speed as low carbon strip. The determining factor in line speed for EPS is the thickness of the strip – thicker material entails a thicker layer of scale – and not the composition of the scale.
|EPS vs. ACID PICKLING - COMPARE LINE SPEEDS|
|Low Carbon Steel||normal||normal|
|High Carbon Steel|
(1018 - 1074)
|50% - 60% slower**||15% - 20% slower|
(41XX, 51XX, 61XX)
|55% - 75% slower**||25% - 35% slower|
** Slowing line speed not only hurts economics, it increases risk of acid eroding the steel substrate, so inhibitors are added to the pickle liquor, making acid pickling of harder steels even more difficult and expensive.
“To fully descale the 1018 to 1074 carbon steels we deal with, the acid pickler slows the line down 30 to 40% of where they run low carbon grades. If it’s 41XX, 51XX or 61XX alloy steels, they slow it down even more: 25% to 30% of low carbon speeds. Our EPS cell is able to run these grades very close to low carbon speeds.”
Steel Technologies – Mishiwaka, IN
How EPS Can Improve Acid Pickling Economics
Acid picklers who run several grades can gain a significant advantage by installing an EPS line and moving the ‘difficult-to-descale’ products to that EPS line. Those products will run much faster and free up capacity on the acid pickling line to run more low carbon steel at higher speeds.
|VIEW EXAMPLE SCENARIO >|
For picklers already operating hydrofluoric acid pickling lines, an investment in EPS technology can have a ‘multiplier effect.’ It doesn’t just add the EPS capacity, but offers the opportunity to significantly boost the capacity of the acid pickling line by dedicating it to low carbon steel where it incurs no processing speed penalty.
EPS Pickling of Electrical Steel
Electrical Steel – also known as Silicon Steel due to its high Si content (up to 6%) which improves the electrical resistivity of the material – is notoriously difficult to pickle with hydrochloric acid. This difficulty stems from the presence of silica or silicate on the strip surface. The pickling line must be slowed significantly to achieve full scale removal, but ‘over-pickling’ can cause corrosion of the steel at grain boundaries and degrade its electrical properties.
EPS, being a mechanical process, is indifferent to the presence of silicates on the surface and can descale electrical steel with no danger of degrading the electrical properties of the strip.