Influence of Partial Replacement of Si by Al on Microstructure and Properties of Nanostructured Martensitic Steel

Nanostructured martensitic steels comprising nanoscale-martensite and retained austenite were obtained by quenching-partitioning-tempering (Q-P-T) treatment. The influence of partial replacement of Si by Al on the microstructure and properties were studied. Results showed that with partial replacement of Si by Al, except nanoscale-martensite and retained austenite, a little ferrite was also clearly observed. By contrast, with partial replacement of Si by Al, although the ultimate tensile strength (1392 MPa against 1215 MPa) was slightly lower, the elongation after fracture (16.7% against 19.9%) and the toughness (equivalent to the area under the stress-strain curve) (43.1 × 10⁶ against 37.1 × 10⁶ J/m³) were relatively higher. This was because the retained austenite in the steel with partial replacement of Si by Al had higher carbon content, similar volume fraction of film-like morphology, lower volume fraction of blocky morphology and was surrounded by ferrite, which played significant role in the transformation-induced plasticity (TRIP) effect.