A 150-year-old idea finally looks like working
ALTHOUGH he is best known for developing a way to mass-produce steel, Henry Bessemer was a prolific British inventor. In the 1850s in Sheffield his converters blasted air through molten iron to burn away impurities, making steel the material of the industrial revolution. But Bessemer knew he could do better, and in 1865 he filed a patent to cast strips of steel directly, rather than as large ingots which then had to be expensively reheated and shaped by giant rolling machines.
Bessemer’s idea was to pour molten steel in between two counter-rotating water-cooled rollers which, like a mangle, would squeeze the metal into a sheet. It was an elegant idea that, by dint of having fewer steps, would save time and money. Yet it was tricky to pull off. Efforts to commercialise the process were abandoned.
Until now. Advances in production technology and materials science, particularly for new types of high-tech steel, mean that Bessemer’s “twin-roll” idea is being taken up successfully. An alternative system that casts liquid steel directly onto a single horizontally moving belt is also being tried. Both techniques could cut energy consumption—one of the biggest costs in steelmaking—by around 80%. Other savings in operating and capital costs are also possible. If these new processes prove themselves, steelmaking could once again be transformed.
On a roll
Steelmakers are cautious about new technologies. It was not until the 1960s that the industry ventured from casting ingots to building giant integrated plants for the continuous casting of steel. This involves pouring molten steel through a bottomless mould which, being cooled by water, partially solidifies it. The steel is then drawn down through a series of rolls to form sheet steel or other shapes required by factories and construction companies. Most of the 1.6bn tonnes of steel produced annually worldwide is now made this way.
Continuous casting, however, still takes a lot of rolling to reduce slabs cast 80-120mm thick to the 1-2mm required by many producers, such as carmakers. Casting any thinner causes quality problems and flaws in the steel’s microstructure. One reason for that is the bottomless mould has to be oscillated to ensure molten steel does not stick to its sides. The new techniques of twin-roll and single belt-casting are, in effect, “moving moulds”—the rollers and the belt move with the steel as it cools and solidifies. This allows direct casting to a thickness of just a few millimetres, requiring only minimal rolling thereafter.
The new techniques are particularly good for making higher-value, specialist steels, says Claire Davis, a steel expert with the Warwick Manufacturing Group at the University of Warwick in Britain. Ms Davis and her team are developing new high-tech steels especially for belt casting, including advanced low-density steels that are stronger, lighter and more flexible than conventional steel.
A twin-roll process, much as Bessemer conceived, is already employed by Nucor, a giant American steelmaker. Called Castrip, it is producing steel in two of its plants. A big advantage of twin-roll and belt-casting is compactness. Nucor reckons a Castrip plant needs only 20 hectares (50 acres) and provides a good investment return from the production of only 500,000 tonnes of steel a year. A conventional steel plant, by comparison, may sprawl over 2,000 hectares and need to produce some 4m tonnes a year to turn a profit.
Other firms are licensing Castrip as well. Shagang, a large Chinese steelmaker, is replacing a less energy-efficient plant with the new technology. The numbers look compelling enough to encourage a startup, too: Albion Steel is talking to investors about building a £300m ($370m) Castrip plant in Britain. The plant would be “fed” by a low-cost mini-mill that melts scrap and produces steel for galvanising, mostly for the construction industry, says Tony Pedder, one of Albion’s founders. Mr Pedder is the chairman of Sheffield Forgemasters, an engineering company, and a former boss of British Steel (which later became Corus). Britain has a surplus of scrap but imports galvanised steel. The plant would employ only about 250 people; traditional integrated operations need a thousand or so. “We believe in the technology,” says Mr Pedder. “In our view it is past the point of being experimental.”
Salzgitter, a German steelmaker, opened the first commercial single belt-caster at Peine, near Hanover, in 2012. It began by making construction steel but has progressed to more specialist steels. The trick is to keep the water-cooled belt perfectly flat, says Roderick Guthrie of McGill University in Canada, one of the pioneers of the technology. Salzgitter uses a vacuum under the belt to do that, whereas Mr Guthrie employs powerful magnets to the same effect on a pilot plant at the university. His research group is working with a number of companies, including a big carmaker. Whereas twin-roll casting is constrained by practical limitations, such as the size of the rollers, horizontal single belt-casting is less so, argues Mr Guthrie.
The techniques may end up being complementary. Their spatial efficiency and low cost would also allow production to be located closer to customers. Mr Guthrie thinks it is not inconceivable for such a plant to be integrated within a car factory. “If we can make the quality as good as the big slab-casting plants, it would change the face of the steel industry,” he says. New technologies might just blast a dose of fresh air through an old industry, much as Bessemer’s converter did 150 years ago.