Abstract
The retained austenite (RA) in advanced high-strength steel (AHSS) grades, such as dual-phase (DP) steels, plays an important role on their formability. Thanks to the transformation-induced plasticity (TRIP) effect that occurs during the mechanically induced transformation of RA into martensite, additional ductility is obtained. Martensite has a higher flow stress than austenite; hence, the transformation results in an apparent hardening, which is beneficial for the stability of deformation. The stability of RA at a given temperature strongly depends on its carbon content, which, in AHSS, is not uniform but distributed. The aim of this study is to build a model that predicts the transformation as well as TRIP in a DP steel grade with RA. A physics-based kinetic model is presented that captures the transformation of retained austenite based on the thermodynamic driving force of the applied stress. A direct analytical estimate of transformation plasticity is provided, which is consistent with the kinetic model. Transformation kinetics is incorporated in a self-consistent, mean-field homogenization-based constitutive model. Finally, an indication of the effect of transformation of retained austenite on formability is given.
Highlights
Recent generations of advanced high-strength steel (AHSS) grades exploit complex microstructural effects such as stress and strain partitioning into various constituent phases in order to gain increased strength and formability
The behavior of DP steels can be further enhanced by allowing a certain amount of retained austenite (RA) phase to remain in the microstructure, resulting in those which are sometimes referred to as enhanced ductility dual-phase steels (DH)
The behavior of multi-phase steels and especially those that comprise the retained austenite phase was modeled using a combination of the mean-field modeling approach and a physics-based, mechanically induced transformation model
Summary
Recent generations of advanced high-strength steel (AHSS) grades exploit complex microstructural effects such as stress and strain partitioning into various constituent phases in order to gain increased strength and formability. The behavior of DP steels can be further enhanced by allowing a certain amount of retained austenite (RA) phase to remain in the microstructure, resulting in those which are sometimes referred to as enhanced ductility dual-phase steels (DH). It is observed that RA undergoes mechanically induced transformation into martensite, which gives rise to two beneficial effects. During transformation, an additional inelastic strain is obtained, which is referred to as transformation-induced plasticity (TRIP), which increases the ductility of the material. Second, the transformation into the harder martensite phase stabilizes the deformation, thereby increasing formability. DH steels retain the promising weldability of DP due to lower proportions of retained austenite and less alloying elements which influence weldability adversely
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