Enhancing yield stress and uniform elongation in an ultrathin packaging steel via controlling dislocation density

Ultrathin sheet steels used in the global packaging industry for lightweight packaging applications have been achieved by an extra cold processing (double reduction) to increase yield stress, but usually at a severe loss of uniform elongation and formability. From an industrial point of view, achieving the synergy of strength and uniform tensile ductility in ultrathin sheets is difficult, owing to the stringent industry demands for low alloying and simple processing routes that are often neglected on a laboratory scale. In this work, we demonstrate that high strength and large uniform elongation of ultrathin sheet steels can be simultaneously achieved at low costs by simply using the standard industrial procedures. An optimized double reduction process, which exploits the beneficiary effects of dislocation density control and bake hardening, was designed. This optimized process leads to an ultrathin sheet steel that shows a significantly improved uniform elongation without sacrificing yield stress, thus offering an efficient way to improve the formability of ultrathin sheets with high yield stress. This enhanced uniform elongation is due to the initially unsaturated dislocation density that allows for the further multiplication of dislocations during plastic straining. The results highlight the important role of precisely controlling dislocations in producing the low-cost ultrathin sheet steels with high strength and high uniform tensile ductility, which are highly demanded for large-scale industrial production.