Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Abstract

Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.

Understanding the Behavior of Advanced High-Strength Steels Using Atom Probe Tomography

“…It is well known that the RA is subject to both chemical and mechanical stabilization. [27,56,57] In order for transformation to martensite to take place under the load, the size of the RA should be sufficient for the martensite nuclei to form and its carbon content should be an intermediate one. If the carbon content is too low, then the RA will transform very quickly on initial loading and will not contribute significantly to the work hardening and ability of the steel to withstand the load.…”

“…It is well documented that the realization of the TRIP effect depends on a combination of various microstructural characteristics, such as (1) the volume fraction of the RA; (2) the morphology and size/thickness of the RA crystals; (3) the carbon content of the RA; (4) the location of the RA, if dealing with multiphase steel; and (5) the dislocation density of the BF. [27,28] The lowering of carbon content in the steel composition could be partially compensated by the formation of low-carbon polygonal ferrite and careful control of morphology and distribution of phases in the microstructure. [29] The effectiveness of this approach, in particular, will also be evaluated in this work.…”

Cyclic Deformation of Advanced High-Strength Steels: Mechanical Behavior and Microstructural Analysis

“…For TRIP steels, the main parameters that can affect the monotonic deformation in this respect are retained austenite and its carbon content, morphology, and distribution. [19] The cyclic deformation of TRIP steels will be controlled by the accommodation of the small repeated plastic strains by the different phases within the microstructure. It is of interest to understand whether the role of the retained austenite in a TRIP steel in the control of the cyclic properties is as important as for the monotonic properties.…”

The Impact of Strain Heterogeneity and Transformation of Metastable Austenite on Springback Behavior in Quenching and Partitioning Steel

“…Compared to the volume fraction of RA in specimens prior to bending (15.9%), only 1.9% retained austenite transformed when specimens strained at a 74 • angle. It is a known fact that the RA transformation is affected by the surrounding phase [31,32]. In this research, the surrounding phase of retained austenite was only martensite due to the quenching and partitioning process.…”