Achieving ultrafine dual-phase structure with superior mechanical property in friction stir processed plain low carbon steel

Ultrafine dual-phase structure was successfully prepared in a plain low carbon steel via friction stir processing (FSP) at a tool rotation rate of 400 rpm and a travel speed of 50 mm/min under rapid water cooling. Ultrafine polygonal ferrite grains (~1 μm) were distributed around the martensite phase consisting of nanostructured laths with a width of ~200 nm. FSP steel exhibited a good combination of ultrahigh yield strength of ~1100 MPa and large uniform elongation of ~7%. This work provides an effective strategy to prepare high strength low carbon steels.