Effect of Mn Partitioning during Intercritical Annealing on Following γ→α Transformation and Resultant Mechanical Properties of Cold-rolled Dual Phase Steels

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Sequential transformation phenomena of α→γ→α during intercritical annealing and subsequent cooling were investigated to achieve a more advanced control of mechanical properties in a low-carbon cold-rolled DP steel sheet with a chemical composition of 0.13mass%C-1.4mass%Si-2.0mass%Mn. The steel was intercritically annealed at 1073 K for 0–1000 s, then air-cooled to 873–1073 K (quenching start temperature: Tq), followed by water-quenching. The tensile strength increased with an extension of the annealing time, especially at the low Tq, corresponding to the increase in the volume fraction of martensite. This means the γ→α transformation during air-cooling was delayed by extending the annealing time. Microstructural observation and elemental analysis by EPMA indicated that the volume fraction of γ during annealing was almost saturated after annealing for 250 s, whereas the Mn content in γ was still increasing at that time. These results suggest that the retardation of the γ→α transformation during air-cooling by extending the annealing time results from the chemical stabilization of γ by the enrichment of Mn during intercritical annealing. In order to obtain the less scattering of mechanical properties in cold-rolled DP steel sheets, precise microstructural control considering the partitioning of substitutional alloying elements during intercritical annealing is quite important.