Microstructural evolution in a new 770 MPa hot rolled Nb–Ti microalloyed steel

The microstructural evolution during hot rolling of a commercially developed hot rolled Nb–Ti steel with a yield strength of 770 MPa is described and analyzed in terms of strengthening mechanisms. The objective of the study is to examine the constituents of the microstructure (type of microstructure, nature of precipitates, dislocation density) that contributed to the attractive strength–toughness combination of a new high strength 770 MPa Nb–Ti microalloyed steel. From the transmission electron microscopy observations, the precipitates can be categorized into four classes depending on their size and shape. Type I were intergranular rod-like (Fe,Mn)3C precipitates, while type II were TiN precipitates of size range 120–500 nm containing small amounts of niobium. The type III precipitates identified as (Nb,Ti)C were ∼10–200 nm size and randomly distributed in the matrix, and type IV were spherical or needle-shaped (3–5 nm) (Nb,Ti)C precipitates that nucleated preferentially on sub-boundaries and dislocations in ferrite. The dislocation density was high in some grains and less in other grains. The high dislocation density and fine-scale precipitation are the dominant factors responsible for the high strength of 770 MPa microalloyed hot rolled steel.