Combinatorial design of transitory constitution steels: Coupling high strength with inherent formability and weldability through sequenced austenite stability

Abstract We introduce a novel alloying and processing scheme for high strength steels, which allows for precise and cost-effective cold forming due to high fractions of metastable austenite, and a subsequent low-distortion, coating-preserving strengthening through martensitic transformation induced by low temperature treatments. As the constitution is thus synchronised with the processing requirements, we refer to these materials as Transitory Constitution Steels. Suitable alloy compositions were identified by high throughput screenings through the exemplarily material systems Fe–5Ni–0.3C–(3–15)Mn and Fe–13.5Cr–6Mn–2Cu–0.2C–(0–2)Ni (wt.%) using combinatorial bulk metallurgical methods. The transformation behaviour, mechanical properties and underlying microstructural phenomena were studied in more detail after upscaling of selected compositions. The steel Fe–13.5Cr–6Mn–1.5Cu–0.2C (wt.%) exhibited an increase in yield strength from 300 to 1050 MPa after immersion into liquid nitrogen, as well as an ultimate tensile strength of more than 1700 MPa at a total elongation of more than 9%. Despite the ultra high strength, no embrittlement induced by Laser beam welding was observed, highlighting the inherent weldability of steels synthesised by the alloying and processing scheme presented here. Possibilities for flexible alloy design and processing variations are discussed.