Microstructure evaluation of an advanced high strength steel with superior results in terms of strength-ductility trade-off

Major improvements in new advanced high strength steels, especially related to microstructural features, have been made by the steel sector to access the trade-off between strength and ductility. In this context, the present study provides a detailed analysis of the microstructure of a cold rolled steel with a minimum tensile strength of 980 MPa, which possesses superior elongation when compared to other conceptions of steels from the same strength level. The annealing process was simulated in a Gleeble machine, and the microstructural characterization was done using optical and scanning electron microscopy, EBSD and XRD analysis. Austenite decomposition, using dilatometric test, and mechanical properties were also evaluated. The steel characterization revealed a microstructure consisting of ferrite matrix with martensite islands and retained austenite particles, in a fraction equivalent to that of conventional TRIP steels, dispersed throughout. The carbon content in the austenite, however, was less than 1.0% w/w, which results in a relatively low stability. Therefore, the increase in strain hardening capacity enabled by the deformation-induced transformation of austenite to martensite produces increased ductility during straining, distinguishing the analyzed material from other steels of the same strength level.