Facile route to bulk ultrafine-grain steels for high strength and ductility

Steels with sub-micron grain sizes usually possess high toughness and strength, which makes them promising for light-weighting technologies and energy saving strategies. To date, industrial fabrication of UFG (ultrafine-grained) alloys, which generally relies on the manipulation of diffusional phase transformation, is limited to steels with austenite to ferrite transformation(1–3). Moreover, the limited work hardening and uniform elongation of these UFG steels(1,4,5) hinder their widespread application. Herein, we report the easy mass production of UFG structures in a typical Fe-22Mn-0.6C TWIP (twinning-induced plasticity) steel via minor Cu alloying and manipulating the recrystallization process by intragranular nanoprecipitation (within 0.5 min) of a coherent disordered Cu-rich phase. The timely rapid and copious nanoprecipitation not only prevents the growth of the freshly recrystallized sub-micron grains, but also substantially enhances thermal stability of the obtained UFG structure due to their strong and sustainable Zener pinning effect. Importantly, the precipitates exhibit weak interactions with dislocations under loading due to their full coherency and disordered nature. Consequently, a fully-recrystallized UFG structure with 800 ± 400 nm grain size was developed without the introduction of any detrimental lattice defects such as brittle particles and segregated boundaries. The resultant mechanical performance is strikingly enhanced, i.e., the yield strength of the UFG steel was doubled (to ~ 710 MPa), with simultaneously large uniform ductility of 45 % and high tensile strength (~ 2000 MPa). The current grain refinement concept can be extended to other alloy systems, and the manufacturing processes can also be readily applied to existing industrial production lines.