Effect of thermo-mechanical treatments on the microstructure of micro-alloyed low-carbon steels

Micro-alloyed steels are known to exhibit superior mechanical properties through controlled rolling and specific thermo-mechanical treatments. Steel strength directly comes from a controlled precipitation process of carbides, nitrides and carbo-nitrides formed during the thermo-mechanical treatment, which are responsible for the fine-grained ferritic structure. In the present study, four different micro-alloyed, low-carbon steels have been studied: one baseline steel containing a small fraction of Al and N, and the other three with different fractions of Nb and V. Two thermo-mechanical treatments, which differed in the γ → α transformation temperature were evaluated. Micro-strengthening contributions of the carbo-nitrides were determined using the Ashby-Orowan approach on the basis of TEM characterization. It was found that for the all four steels the Ashby-Orowan approach was in quite good agreement with the bulk yield strength (σy), as determined by mechanical testing steel (accounting for 0.8σy). The pinning force of the particles was also calculated and compared to the recrystallization driving force. The steel containing Nb + V exhibited the highest pinning force, but the low fraction of the alloying elements, made this contribution only a fraction of ~1/3 with respect to the recrystallization driving force.