Effect of Cooling Rate on Phase Transformation Kinetics and Microstructure of Nb-Ti Microalloyed Low Carbon HSLA Steel

Nb-Ti microalloyed low carbon high strength low alloy (HSLA) steels are used to fabricate components, particularly for the automobile and piping applications requiring optimum combination of mechanical properties along with good weldability. These properties depend on the transformed ferrite microstructure and grain size obtained after cooling from hot rolling temperatures which are always well above the austenitizing temperature. Hence, an attempt was made to study the austenite decomposition (phase transformation) kinetics and the accompanying microstructural evolution in Nb-Ti microalloyed steel by subjecting it to austenitization at 1100 degrees C for 3 min followed by cooling at different rates ranging from 1 to 100 degrees C/sec in a Bahr DIL 805 A/D dilatometer. The first derivative method was employed to identify critical transformation temperatures from the dilation curves. A modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis (used for non-isothermal conditions) was carried out to find the time exponent (n') values controlling the rate of transformation at different cooling rates. The nature of transformed ferrite was observed to change from polygonal to acicular type, and its grain size was found to decrease with an increase in cooling rate. EBSD analysis also revealed the cooling rate to have a profound effect on the microtexture evolution of the concerned alloy. The "gamma" fibre and rotated cube components are replaced by the transformed copper ("gamma" fibre) components owing to faster transformation and lack of recrystallization of transformed alpha with increase in cooling rate. Finally, a power law and logarithmic relationship of grain size and microhardness with the applied cooling rate were established.