Diffusional and displacive transformations in interstitial-free steel within the scope of a critical assessment of the mechanical property

Interstitial-free steel is fully ferritic at room temperature, extremely soft and ductile. The strengthening is a key focus of the present study through heat treatment. The samples are soaked in the austenitic temperature domain followed by different rates of cooling in furnace, air, and water quenching. The cooling curves obtained from the experiments are superimposed in the computed CCT diagram to predict and discuss various microstructure formations by diffusional and displacive mechanisms of the phase transformations which is lacking in the system. Essentially, the best combination of strength-ductility is obtained in the water quenched sample, compared to the reported techniques, for a detailed analysis. The electron microscopy hints at a lath type morphology with high dislocation density. The selected area diffraction pattern has indexed the lattice of the product phase as BCC. Further, the image quality (IQ) map, grain boundary misorientation, as well as Kurdjumov-Sachs (K-S) orientation relationships with parent austenite confirm the lath as martensite through variant selections. For the mechanical property assessment with EBSD, the role of the hierarchical structures of martensite (e.g., packet, block and sub-block boundaries) to the dislocation pinning is discussed against microstructural contributions to resolve ambiguity in the strengthening process.