Designing quadplex (four-phase) microstructures in an ultrahigh carbon steel

Here we present an approach to design a ferrite-based quadplex microstructure (ferrite/martensite/carbide/austenite) using a lean alloyed Mn–Si–Cr–Al ultrahigh carbon steel. The material has 1500 MPa tensile strength and 11% elongation. The thermomechanical processing includes two main steps, namely, first, the formation of a ferrite plus carbide duplex microstructure by warm rolling below Ae₁; and second, annealing just above Ae₁ for a short time (∼20 min). The quadplex microstructure consists of 57 vol% ultrafine ferrite (mean grain size ∼1.5 µm), 29 vol% martensite, 12 vol% spherical carbide and 2 vol% austenite. Fracture analysis after tensile deformation reveals a mixed ductile and brittle failure mode without necking. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and dilatometry tests were conducted to map the microstructure characteristics and the contribution of each phase to the overall deformation