Nano-Bainitic Low Density Steel

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Advanced grades of steel with reduced weight are essential for nowadays constructions. Aluminum as alloying element in steel reduces its density and has been used to produce low-density steels (LDS) offering an interesting combination of high mechanical properties at reduced densities. Moreover, increasing the strength of steels while maintaining their ductility enables down-gauging the parts, consequently lowering their weight. This strategy has been implemented via the development of advanced high strength steels (AHSS) in different levels.LDS have microstructures that could be ferrite based, austenite based, or a mixture of the two. Additionally, the high Al additions result in the presence of κ-carbides and – on occasion – a number of brittle ordered phases. It is essential to avoid larger precipitates of these phases at higher temperatures, as they negatively influence the ductility. On the other hand, generating fine well dispersed precipitates of these phases at low temperatures is reported to enhance the alloy’s strength. This heat treatment cycle is similar to that used to produce nano bainitic steels (nB) with microstructures of alternating nano-sized lamellae of bainitic ferrite and retained austenite, as well as blocky austenite grains, giving impressive strength and ductility combinations. However, the low transformation temperatures used necessitate long transformation times.The project aims to exploit the synergy between LDS and nB in terms of both alloying and heat treatment, introducing a novel concept of Nano-Bainitic Low Density Steel, as well as investigating the boundaries and limitations of this concept in order to provide an encompassing overview of the range of properties achievable via these newly introduced alloys.The synergy between nB and LDS is based on the following:1- High Al content will significantly accelerate the bainitic transformation and increase the fraction of bainitic ferrite formed leading to higher mechanical properties at faster transformation times.2- Quenching the alloys into low austempering temperatures will suppress the formation of large unwanted brittle precipitates at high temperatures, and if low temperature precipitates form, they will be well dispersed fine precipitates that further strengthen the microstructure.The foreseen final product is a high mechanical property steel with a microstructure of bainitic ferrite and retained austenite, having low density and produced by a low cost isothermal treatment at low temperature. The effect of varying C (between 0.5 – 1.0 wt.%), Al (5.0 – 8.0 wt.%) and Mn (1 – 8 wt.%) on the microstructure and bainitic transformation kinetics as well as the mechanical properties will be studied with eleven alloy combinations. These results, in combination with thermodynamic simulation will be used to design three final alloys along with suitable processing routes for optimal final properties.