Researchers in Warwick have made a breakthrough in controlling the undesired brittle stages during lightweight steel production, meaning the metal can be produced on an industrial scale.
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The new processing route – developed at WMG at the University of Warwick – allows low density steel-based alloys to be produced with maximum strength, while still remaining durable and flexible, something which has been largely impossible until now.
Current processes for strengthening lightweight steels make them less flexible – and therefore less marketable. The new method, developed by Dr Alireza Rahnama, is a game changer, and could lead to a revolution in safer, greener, more fuel-efficient cars.
The new processing route is a simple heat treatment at a temperature between 900 °C to 1150 °C for a duration of 30 minutes, following by a fast cooling to ambient temperature. This annealing leads to the transformation of coarse stringer bands of B2 to nano-sized disk-like B2.
Dr Alireza Rahnama
B2 refers to one of two brittle phases can occur in these steels: kappa-carbide (k-carbide) and B2 intermetallic – which make the steels hard but limits their ductility, so they are difficult to roll.
“Depending on the respective Al concentration, lightweight steels can have different complex strengthening precipitates,” says Rahnama. Two of these precipitates are k (kappa)-carbide and B2 intermetallic. They are not easily shearable so they significantly enhance the strength of the lightweight steels. But their drawback is that they are very brittle and thus make our steel susceptible to crumbling especially when they are coarse.
Researchers found through simulation and experimentation that at certain high annealing temperatures, these brittle phases become more controllable, permitting the steels to retain their ductility. Between 900 °C to 1200 °C, the k-carbide phase can be removed from production, and the B2 intermetallic brittle phase becomes manageable – forming the disk-like, nano-sized morphology, as opposed to a coarser product.
The key finding of this research is that we could form nano-sized precipitates in a ductile matrix so we could mix the strength of precipitates with the desirable ductility (large deformation) of the matrix. In such way we could successfully enhance the strength and increase the ductility.
Dr Alireza Rahnama
The team tested two lightweight steels - and Fe-15Mn-10Al-0.8C - for their potential to achieve maximum strength and ductility.
“Fe-Mn-Al-C steels show a wide range of mechanical properties,” explains Rahnama. “They have both corrosion resistance and low density of aluminium and low cost of manufacturability of steel. Ni on the other hand, is a strong B2 intermetallic former which can in turn significantly increase the strength of this specific type of steel. So we chose a 5 wt.% Ni Fe-Mn-Al-C steel and a Ni- free steel to enable us to compare their microstructure and properties.”
It is hoped that this work could lead to more environmentally-friendly cars: those made of stronger and lighter materials are safer for drivers, emit less CO2 and consume less fuel. More malleable steels will also allow Manufacturers to form car parts into desirable, streamlined shapes.
Dr Rahnama comments: “Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. Lightweight steels are one of the candidates to address these concerns.”
“This steel can be easily produced through the technological route explained in the published paper [Acta Materialia]. The industry just need to change their annealing profile to one suggested in the paper,” he continues. “These steels have reduced weight, increased strength, large ductility (they can deform largely for example in the case of a car crash) and their usage for future transportation can significantly reduce the CO2 emissions.”
We as materials scientists always look for new alloys with enhanced mechanical properties (high strength large ductility) while our aim is to decrease the density because we want reduced CO2 emissions into our environment. Also the low cost manufacturability is very important factor for us and industry. So any next step is to design and develop new alloys with an increasingly high strength and low density which can deform largely and can be easily produced with low cost.
Dr Alireza Rahnama
Reference: University of Warwick
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