Phase transformations during partitioning in a Q&P steel with blocky retained austenite

Abstract

Effect of partitioning on structure and mechanical properties of a Fe-0.44%C-1.8%Si-1.3%Mn-0.82%Cr-0.28%Mo steel processed by quenching-partitioning (Q&P) was considered. This Q&P steel exhibited a superior combination of the yield stress (YS) of ∼1150 MPa, ultimate tensile strength (UTS) of ∼1650 MPa and elongation-to-failure (δ) of ∼21 %. The product of strength and elongation (PSE – UTS × δ) was >30 GPa × % after partitioning, with a duration ranging from 60 to 500 s. During partitioning, three main processes occurred: the carbon partitioning between retained austenite (RA) and martensite/bainitic ferrite (BF), the transformation of blocky RA into bainite, and the precipitation of transition η–Fe2C carbides in the martensitic matrix. The enrichment of RA by carbon changed the mechanism of bainitic transformation and then suppressed the formation of BF and induced the reverse growth of RA upon 500 s partitioning. The rate of carbon partitioning depends on the diffusion path that produces a wide distribution of carbon concentration in areas of RA. The formation of secondary martensite and bainite takes place in areas of untransformed austenite with relatively low carbon content. The effect of RA volume fraction on ductility and the PSE is insignificant. Carbon enrichment of RA ceased its transformation into strain-induced martensite and induced twinning during tension.