The main challenge in the development of the next generation of ultra-high-strength stainless steels is the concurrent optimisation of strength and ductility and minimisation of alloying costs. The research carried out within the LIGHT programme focused on fundamental studies aimed at optimisation of alloying, and strengthening and deformation mechanisms of austenitic microstructures. Another important topic was the delayed cracking phenomenon, one of the main barriers to the development of the next generation of cost-efficient, ultra-high-strength stainless steels.
A world-class knowledge platform on high strength austenitic stainless steels was created. The programme enhanced understanding of how alloying affects the stacking fault energy and deformation mechanisms, i.e. transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects. Based on this knowledge, new alloying concepts with optimised properties were developed and tested in pilot scale. This knowledge is also the key for optimisation of the mechanical properties of austenitic stainless steels. Another important theme was development of thermo mechanical treatments providing superior properties compared to conventional austenitic steels strengthened by temper rolling.
A substantial improvement was achieved in the understanding of the mechanisms and kinetics of delayed cracking, including clarification of the roles of hydrogen, residual stresses, martensite content and chemical composition of the steel. Design guidelines for avoiding the cracking phenomenon were developed and coupled with finite-element-forming simulations.
The simultaneous optimisation of steel strength, ductility and toughness is a classic dilemma for the metallurgist. The cost of raw materials should also be kept at a minimum with, in the case of stainless steels, no compromise in corrosion resistance. Optimisation of material properties requires a thorough understanding of the microstructural features affecting the material behaviour and deformation mechanisms. Increased understanding of these features and application of the generated knowledge in the development of next generations of ultra-high-strength steels were the targets of the project.
Markets for the ultra-high-strength steels are expected to grow rapidly in the future. There is growing demand for new ultra-high-strength stainless steels for developing lightweight structures in many application areas. Weight reduction in moving vehicles is particularly attractive, as lower weight means higher payload and lower fuel consumption, with resulting cost savings over the vehicle life cycle. Development of lightweight structures using ultra-high-strength steels is also important from the environmental viewpoint, as reduced use of material and lower fuel consumption means lower emissions. These steels will not only provide competitive advantage to Outokumpu in existing markets, but will open totally new markets for stainless steels.
Juho Talonen, Outokumpu Oyj
Outokumpu Oyj, Ruukki Metals Oy, University Of Oulu, Aalto University