improvement of tensile properties of low-carbon steels via short-time intercritical annealing

The effect of low volume fraction formation of martensite on the tensile properties of low carbon steel was evaluated. First, steel samples with ferrite-cementite microstructure were produced. The thermomechanical treatment used included austenitizing at 1000 °C and then quenching in ice brine solution, tempering the obtained martensitic structure for 1 h at 650 °C, 80% cold rolling, and re-tempering for 2 h at 650 °C. In order to form a low volume fraction of martensite, steel samples with ferrite-cementite microstructure were intercritically annealed for 30 seconds at 740 °C. As a result of intercritical annealing treatment, 6.2% martensite was formed. The results of tensile test showed that the formation of 6.2% martensite led to the elimination of yield point phenomenon and Lüders banding, decrease of yield stress and increase of true stress at maximum load, while true uniform strain did not change significantly. The work hardening rate also increased significantly. Based on the results of modeling of the flow behavior with the Holloman equation, the work hardening capability of the steel sample including ferrite-cementite decreased after a certain plastic strain, while the work hardening capacity remained constant with the formation of a low volume fraction of martensite in the microstructure.