Improvement of plasticity and toughness of ultra-high strength martensitic steels via tailoring trace ferrite

The susceptibility of ultra-high strength martensitic steels to cracking during processing and service has hindered their extensive application. This paper presents a novel strategy to improve plasticity and toughness of ultra-high strength martensitic steels via tailoring trace ferrite. Given this thought, we have successfully developed an ultra-high strength martensitic steel with modified ferrite. The modified ferrite is evenly distributed in the martensitic matrix, with a small size, low aspect ratio and high dislocation density. The strength class of this steel is comparable to conventional ultra-high strength martensitic steels at 1500 MPa, but the total elongation and impact toughness have been increased by 13.0% and 10.1%, respectively. The improved performance can be attributed to the strong synergistic relationship between modified ferrite and martensite, which effectively prevents the formation of microcracks at the interface between the two phases.