Casting back to 1856
Castrip’s Frank Fisher and Brunel University’s Professor Zhongyun Fan talk to Khai Trung Le about the unmet potential of Bessemer’s twin-roll casting process.
In 1856, Sir Henry Bessemer changed the way we mass-produce steel forever. Recognised as the first inexpensive industrial method for steel, the Bessemer process blasts compressed air through molten iron to burn out impurities. However, nine years later, Bessemer patented another process, twin-roll casting, which has largely eluded the steelmaking industry – that is, perhaps, until now.
Rather than produce steel into large ingots that require reheating and shaping, the twin-roll process involves pouring molten steel between two rotating, water-cooled rollers that would squeeze metal into a sheet. Beyond timesaving benefits, there are advantages in energy consumption – one of the dominant issues surrounding the UK steel trade – and manpower and operating costs. Twin-roll casting has been used in production of specialist metals, but can the process be adopted as a solution by the steel trade?
‘From the 1980s to early 2000s, there were around 20–30 various twin-roll casting processes ongoing throughout the world,’ said Frank Fisher, President of Castrip LLC, part of USA-based steel producer Nucor. ‘Steelmakers recognised it as a potential game changer in producing exotic steels and metals. But most of these fell during the financial and steel crises throughout the world.’
Castrip’s focus has been on developing new steels ‘that cannot be produced with conventional compact strip production and high rolling processes. We’re eliminating cold stream downrolling and leveraging rapid solidification and transformation,’ Fisher said, declining to offer more information.
Despite the relative simplicity of the process, Fisher noted that the process has only been possible to capitalise on fully due to technological advances. ‘You’re talking about going from liquid to solid steel in less than two-tenths of a second. The parameters around the steel chemistries, gases and roll texture are tightly controlled. Without high-speed automation and computerisation, I can’t imagine how you could come close to making it work back in 1865.’
Similarly, Brunel University London, UK, launched the Advanced Metal Casting Centre (AMCC) in April 2016, intended to bring research to an industrial scale and includes a twin-roll magnesium strip caster paired with melt conditioning. Zhongyun Fan, Professor of Metallurgy at Brunel, described the AMCC process as focused on treating the liquid metal before casting to disperse oxide and other nanoscale particles, eliminating their harmful properties.
Despite the clear advantages offered, twin-roll casting has yet to reach critical mass. Fan said, ‘The two main reasons are low productivity and low quality, with the existence of central line segregation,’ which causes cracking in subsequent rolling and is linked with other defects like surface bleeding. ‘The most successful application so far is in aluminium and low-quality applications. If twin-roll casting can produce high-quality strips that can be rolled into automotive sheets, that would be significant.’ Fisher contested the claim of low quality, stating that the quality is apparent from the amount of coils distributed by Castrip, but concurs with Fan on the clear appeal to the automotive sector.
Although Fisher noted that Castrip is not pursuing twin-roll casting as a means of producing conventional steel – ‘Nucor has 27 million tonnes of steel production, they don’t want another “me-too” process’ – a recent article from the Economist notes that Castrip is in discussions with Albion Steel to develop a plant in the UK focused on scrap and producing primarily for the construction industry. Tony Pedder, Albion Steel founder and chairman of Sheffield Forgemasters, commented to the Economist, ‘We believe in the technology. In our view, it is past the point of experimental.’