Effect of holding time on the microstructure and mechanical properties of dual-phase steel during intercritical annealing

Abstract

Continuous annealing simulation tests were conducted by using a continuous annealing thermomechanical simulator. Holding times of 5, 60, 180, and 480 seconds for an intercritical annealing temperature of 820 °C were adopted to investigate the evolution of the microstructure and mechanical properties of ferrite-bainite dual-phase steel. The ferrite-bainite dual-phase steel was characterized by high strength and low yield ratio due to the presence of the constituents (polygonal ferrite, bainite, martensite and retained austenite) of the steel microstructure. Specimen 3 exhibits the highest value of A50 (7.67%) and a product of R _m × A ₅₀ (10453MPa%) after a 180s holding. This is likely attributed to the presence of a C-enriched retained austenite in the microstructure. And the effect of martensite islands and carbide precipitate is thought to be able to contribute in strengthening the present steel. It is expected that equilibrium of austenite fraction would be reached for reasonable intercritical holding period, regardless of the heating temperature. The results suggest that long increasing holding times may not be needed because the major phase of the microstructure does not change very significantly. It is favorable for industrial production of DP steels to shorten holding times.

Keywords

ferrite-bainite dual-phase steel / holding time / martensite islands / mechanical properties

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