Effect of mn partitioning during intercritical annealing on Followingγ→α transformation and resultant mechanical properties of cold-rolled dual phase steels

Abstract

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 coldrolled DP steel sheet with a chemical composition of 0.13mass%C-1.4mass%Si- 2.0mass%Mn. The steel was intercritically annealed at 1 073 K for 0-1 000 s, then air-cooled to 873-1 073 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 → αtransformation during air-cooling by extending the annealing time results from the chemical stabilization of γby 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. © 2011 ISIJ.