Microstructural evolution and mechanical properties of high strength microalloyed steels: Ultra Fast Cooling (UFC) versus Accelerated Cooling (ACC)

We describe here the microstructural evolution and mechanical properties of high strength microalloyed steels processed using different cooling trajectory. Pilot-scale studies demonstrated that high strength of ∼700 MPa can be obtained in a microalloyed steel using ultra fast cooling (UFC) positioned at the exit of hot rolling mill, while the yield strength obtained via the conventional thermo-mechanical controlled processing (TMCP) with accelerated cooling (ACC) is ∼100 MPa less. The underlying reason is that ultra fast cooling positioned immediately after hot rolling enhances strengthening associated with precipitation and grain refinement. Theoretical calculations and experiments indicated that grain refinement and precipitation in TMCP with in-front UFC led to strength increment of ∼49 and 54 MPa, respectively over the conventional TMCP with ACC process. Furthermore, the microstructural characterization indicated that the density of high angle grain boundaries was increased and the average size of precipitates was reduced from ∼34 nm to ∼10 nm, when the cooling pattern is changed from ACC to UFC. The theoretical estimate also indicated that when the cooling profile is changed from the conventional ACC to UFC+ACC, and to UFC, a higher degree of precipitation is responsible for increase in strength in UFC processed hot rolled microalloyed steels.