Phase transformation and mechanical properties of ultrahigh strength steels under continuous cooling conditions

The present work deals with phase transformation mechanical behavior and of newly developed ultrahigh strength steels. Bainitic and martensitic phase transformations have been studied in detail with respect to their transformation temperature range and microstructural evidences observed under various continuous cooling conditions. The various critical temperatures of Ac1, Ac3, Bs, Ms, and TNR of the investigated steels were initially calculated using numerical equations and validated with experimental results. The dilatometric tests consisted under different cooling rates from 0.1 to 100 °C/s and the respective continuous cooling transformation (CCT) diagrams have been determined. The microstructural features were revealed using optical and different electron microscopy techniques. A strong dependency of cooling rate on microstructure and mechanical properties has been found. The present steels have exhibited ultrahigh strength (1400–2000 MPa) with reasonable ductility (6–13% total elongation), which is attributed to combination of mixed microstructure comprising fine bainite and lath martensite along with thin film-like retained austenite. Fine precipitates of (Ti, Nb)C, Mo2C and/or Cu contribute to strengthening effect by increasing the hardness and strength values for a particular cooling rate. It has been witnessed that the hardness, yield strength and ultimate tensile strength have been increased with the rise of both cooling rate and carbon contents.