Rust gets a closer look in groundbreaking study
UNTIL now, rust wasn’t given too much thought in the steel industry’s fight against an expensive problem — defects found in very thin steel strips originating from hot-strip mills. Now, following research at the University of Wollongong, rust is increasingly coming under scrutiny.
Hot-strip mills are where the strips are produced from slabs measuring about 12m long, 2.5m wide and 25cm thick. After the slab is heated to about 1200C, it is put through a roughing mill, which flattens it to about 3cm thick.
Then it is the job of finishing rolls to bring down the hot steel’s thickness to around 5mm, a process which sees the steel strip reach a length of around 1000m. The step requires high precision. The finishing rolls must produce uniform strips free of defects, because most of the strips will be used later to make tinplate for food and beverage cans.
Over time microscopic grooves become worn into the surface of the finishing rolls, but the grooves themselves are not what has been plaguing the global steel industry. The problem is what they are thought to do to the hot steel being flattened.
While the grooves are just 30 millionths of a metre deep — 30 microns — they are implicated in ridges that crop up sporadically and somewhat mysteriously in hot-rolled strips. In hot-rolled strips the ridges are imperceptible, but by the time the strips have been cold-rolled to a thickness of about half a millimetre and then annealed, the ridges have turned into waves called ridge-buckles.
They tend to be around 20 to 30cm apart and their peaks and troughs typically deviate from the strip’s flat parts by about 3mm. This buckling may not sound like a big deal, but it means manufacturers such as tin can makers can’t use tinplate made from those steel strips.
Rian Dippenaar, a engineering researcher with the University of Wollongong, says most defective strips are downgraded, while the few defective strips that cannot be sold into other, less stringent applications, are sold as scrap.
‘‘ That constitutes quite a cost,’’ he says. A defect-free strip cold-rolled is probably worth about $1400-$1500 a tonne, while downgraded product fetches between 20 and 50 per cent less. Scrap is even less valuable.
Understandably, the industry has been keen to discover how and why the buckles occur.
Dippenaar and fellow ridge-buckle researcher Wanda Melfo have come up with qualitative work indicating that the thin patina of rust found on strips during the hot-rolling process — known as scale — may be contributing to the problem.
They suspect that scale from strips rolled early in the working life of finishing rolls is making the rolls wear prematurely. This, in turn, leads to ridges in strips rolled later.
According to Melfo and Dippenaar, scale behaves as a lubricant in the roll-bite, the place where the top and bottom finishing rolls meet and squeeze the strip being flattened.
A lubricant lowers the friction between two surfaces. But the scale found on a hot-rolled strip isn’t necessarily the same everywhere. It can be made up of different types of iron oxides and certain parts of the strip may form more or less of the stuff. It means a set of finishing rolls will experience different amounts of friction across the width of the strip it is flattening.
Over time, differing amounts of friction across the roll-bite add up to differing wear rates, so that grooves form around the outer surface of each roll. The process is akin to a carpenter putting grooves in a table leg using a woodturning lathe.
A second way that scale may louse up finishing rolls relates to its tendency to fracture. Melfo and Dippenaar think that wherever a fracture occurs, there is an increase in the amount of heat reaching a finishing roll. The scale, which insulates the rolls from some of the strip’s heat, breaks into jigsaw-like pieces, exposing hot, bare metal in between adjacent pieces.
Just as scale plays havoc with the amount of friction across roll-bites, it also affects the heat levels across them. Wear rates across a given roll may increase wherever more heat reaches its surface. Again, where you find wear, you’ll find grooves.
Melfo and Dippenaar’s work was done with BlueScope Steel as a partner investigator, as part of an ARC-Linkage grant.
BlueScope metallurgical technologist Chris Killmore says the research has been very well received by the company’s operating department engineers because it has provided insights into one of the many factors that can influence roll wear. Melfo, a mechanical engineer by training but later a convert to the world of metallurgy, now works in the Netherlands for metals manufacturer Corus.
Melfo’s assessment is that steel industries around the world are putting greater emphasis on studying scale. ‘‘ The oxidation of steel at high temperature is a research topic that has been greatly explored in the last two or three years,’’ she says. ‘‘ It’s been found that the role of scale in production is more important than originally believed, and some of the most common mill problems might be caused by scale.’’
Scale experts: Rian Dippenaar and Wanda Melfo are focusing on defects in rolled steel