Synergistic effects of holding time at intercritical annealing temperature and initial microstructure on the mechanical properties of dual phase steel

Abstract

The synergistic effects of the initial microstructure and the intercritical annealing time on the microstructure and tensile properties of low carbon steel were studied. It was found that by refining the pre-intercritical annealing microstructure, the resulting duplex DP microstructure becomes finer, which results in the enhancement of the tensile properties and work-hardening behavior. Based on the hardness measurements, three stages were identified for intercritical annealing: initial rise or fall, reaching a plateau, and final fall to a low value. The pre-intercritical annealing microstructure did not show any pronounced effects on these stages, with the exception of the short-lived initial stage. It was shown that the DP steel obtained right after reaching the abovementioned plateau can exhibit improved mechanical properties and work-hardening behavior. During the third stage, the enrichment of manganese in austenite and the concurrent grain growth were found to be the main factors taking part in the change of hardness and tensile strength and reappearance of the yield-point elongation. The impact of the microstructural features (austenite and ferrite) on the grain growth behavior during intercritical annealing and the effect of the grain size on the Lüders strain were also discussed.