The Role of Retained Austenite on Tensile Properties of Steels with Bainitic Microstructures

Abstract

In high-carbon, silicon-rich steels it is possible to obtain a very fine bainitic microstructure by transformation at low temperatures (200-300 C). This microstructure consists of slender ferrite plates, with thicknesses of several tens of nm, in a matrix of retained austenite. Whereas strength is mainly provided by to the fine scale of the ferrite plates (stronger phase), ductility is mostly controlled by the retained austenite (softer phase). Further improvement in ductility is achieved by strain induced transformation of austenite to martensite, the so called TRIP effect. In order to take full advantage of this effect, the mechanical stability of the austenite, i.e., its capability to transform to martensite under strain, must not be too low nor excessively high.Two main aspects of the mechanical stability of the retained austenite, morphology and chemical composition, have been studied to determine the role that these play on the ductility behaviour of the bainitic steels studied. It is suggested that the chemical composition has the strongest effect on the ductility of these new high strength alloys.

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

“…These findings explain the observed low ductility in steel 1 and support the earlier suggestion on the strong effect of chemical composition of the RA in nanostructured bainitic steels on their ductility. [58] B. TRIP Steel 3…”

Properties of fine-grained steels generated by displacive transformation

“…A prominent feature is that they all fail when the retained austenite content is reduced to about 10%. An Table 1 T I , V γ , σ Y and σ U T S stand for isothermal transformation temperature, the volume fraction of retained austenite, the 0.2% proof and ultimate tensile strengths respectively [24].…”

Tensile Ductility of Nanostructured Bainitic Steels: Influence of Retained Austenite Stability

“…More recent work, based on measurements of retained austenite content before tensile tests and calculated evolutions, proposed that the stability of retained austenite would influence the material ductility, and indicated that there could be an optimum stability [20]. Data from this same publication were later re-interpreted to propose the existence of a percolation mechanism, whereby ductility was imparted by a percolating network of retained austenite in the matrix of bainitic ferrite, and fracture occurred at an approximately constant volume fraction of 10% retained austenite [17].…”