Processing innovation opens up new pathway for specialist steel

Car manufacturers, trying to reduce the weight of vehicles, are interested in low-weight, high-strength grades of steel. But some of these metals have properties which make them difficult to manufacture and process in large quantities. Material scientists at the University of Warwick have now found a way to control the properties of these materials so they can be rolled and formed like conventional steel.

The problem up to now has been that strong, lightweight steel alloy grades tend to have brittle phases that make them hard, but stiff and unworkable. Two of these phases, according to the team led by Alireza Rahnama of Warwick Manufacturing group, are known as kappa-carbide and B-2 intermetallic, which occur in the steel grades Fe-15Mn-10Al-0.8C-5Ni and Fe-15Mn-10Al-0.8C, otherwise seen as having good strength and ductility properties.

In the journal Acta Materalia, Rahnama and colleagues describe how they found that raising the temperature at which metal is annealed (heated and then cooled slowly) changes the properties of these grades so that the brittle phases are less troublesome. At annealing temperatures between 900°C and 1200°C, the kappa-carbide phase can be removed altogether and the B-2 intermetallic forms as nano-scale discs that do not stiffen the bulk material.

This discovery, funded by the High Value Manufacturing Catapult, should make it possible to make malleable sheets of lightweight but strong steel that can be formed into streamlined components for vehicle bodies.

“Most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off,” Rahnama said. “This paper studies the kinetics and thermodynamics of microstructural evolution of lightweight steels through simulations and experiments and proposes a mechanism to achieve higher strength and larger ductility; a method that can be readily adopted by industry.”