Rust gets a closer look in ground­break­ing study

The Weekend Australian - Travel - - Resources - Sarah Belfield

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

Newspapers in English

Newspapers from Australia

© PressReader. All rights reserved.