Design of a new Ni-free austenitic stainless steel with unique ultrahigh strength-high ductility synergy

• A conceptual approach was used to design a new Ni-free austenitic stainless steel.
• Unique combination of ultrahigh strength-high ductility was achieved.
• Dislocation hardening mechanism contributed to the ultrahigh yield strength of the steel.
• Solid solution hardening mechanism contributed to the ultrahigh yield strength of the steel.
• During deformation both martensitic transformation and twinning occurred.

A conceptual approach was used to design a new Ni-free austenitic stainless steel with a unique combination of ultrahigh strength and ductility. The concept was based on the alloying of the 0.05C–18Cr–12Mn (wt.%) steel by 0.39%N and heavy warm rolling (84% reduction) at 1173 K (900 °C) to achieve the yield strength of minimum 1 GPa and high tensile strength and elongation due to a proper stability of the austenite as a result of the optimized stacking fault energy (SFE). The yield strength of 1010 MPa, tensile strength of 1150 MPa and high fracture strain of 70% were measured for the steel designed. Dislocation and solid solution hardening mechanisms are introduced as the main contributors for the ultrahigh yield strength of the steel. The strain hardening is gradual and the hardening rate reaches a high level of ∼2400 MPa at a high true strain of 40% due to slow α′-martensitic transformation and mechanical twinning. Consequently, the ductility of the designed steel is excellent.