Ultrafine-grained ferrite steel with dispersed cementite particles

While preserving the easiness of recyclability and weldability in low C–Si–Mn steels, the grain refinement without microalloying was performed for the low C–Si–Mn steel. The ultrafine grain size was carried out for 18 mm thick and 20 m long square bars. The yield strength was raised from 300 MPa in 20 μm grain size to 740 MPa in 0.5 μm grain size. The ductile-to-brittle transition temperature, DBTT, was lower than 120 K. However, the ductility deteriorated as a result of the grain refinement. Thus, finely dispersed cementite particles were designed to obtain a better ductility, especially for the uniform elongation. A uniform elongation of 7% and a total elongation of 20% were obtained for the highest strength level.