Austenite processing and phase transformation of niobium and molybdenum alloyed high performance structural steel

Grain refinement and phase transformation control are significant issues for high performance structural steels. The use of a higher Nb content in low carbon Mn steel has an important effect on refining the prior austenite grains at intercritical temperatures by static recrystallization. The mechanism by which higher Nb levels produce ultra refinement of austenite is that recrystallization kinetics need longer incubation times and longer finish times, and higher solute Nb can slow down the mobility of recrystallized grain boundaries, and suppress grain coarsening effectively. Meanwhile, higher Nb can also suppress the softening between passes, and may increase the possibility of partial DRX (Dynamic Recrystallization) caused by high strain accumulation during hot strip finish rolling. Therefore, the start temperature of finish rolling should be lower to prevent DRX. Additionally, Nb and Mo are key elements for affecting phase transformation. Comparing results from an experimental Mn-Cr-Nb steel with Mn-Cr and Mn-Mo steels with higher Nb levels shows that a higher Nb content is beneficial in stabilizing austenite and widening the window for acicular ferrite and bainite formation during continuous accelerated cooling; whereas, Mo additions in low carbon higher Nb bearing steels widen the window for acicular ferrite and bainite transformation further. However, in order to obtain an optimum microstructure in low carbon-Mn-Nb-Mo steels, the ultra refined austenite should be cooled at a rate of more than 10ºC/s, and the finish cooling temperature should be below 450ºC. (AU) Copyright © 2014 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved.