The Multiphase Micro- and Nanostructures of 0.2 and 0.4 C Direct-Quenched and Partitioned Steels

Abstract

Quenching and partitioning produces advanced high-strength steels that utilise transformation-induced plasticity for improved strength and deformability. Microstructures of these steels consist mainly of tempered martensite and carbon-enriched retained austenite. A novel processing route of direct-quenching and partitioning (DQP) facilitates carbon partitioning from supersaturated martensite to untransformed austenite directly from the quench-stop temperature in a decelerated cooling that simulates slow cooling of a coiled strip. A major advantage of DQP steels is that they keep both the costs and emissions down by inexpensive alloying and energy-efficient processing. In this study, we investigate the microstructures of 0.2C and 0.4C laboratory hot-rolled DQP steels with comparison to a direct-quenched variant with high-resolution transmission electron microscopy as the main research technique. We show that the structures of DQP steels have frequent nanotwinned regions and can contain three different crystal structures with characteristic length scales ranging from few nm to ~200 nm. This is in remarkable contrast to the traditional lath-martensitic microstructure of the as-quenched material. Density functional theory calculations provide further insight into these findings with the calculated results of energetics, and show that carbon helps in stabilising the newly found omega phase. These results give further insight to the aspects that must be considered when assessing their effect on essential mechanical properties like strain hardening and toughness.