Development of a Lean Alloyed Trip Assisted Bainitic Steel by Austempering Treatment

A lean alloyed low carbon Si and Mn steel was austenitized at three different temperatures in the inter-critical zone (790, 810 and 830 °C) followed by austempering in salt bath in the temperature range of 425 and 525 °C to develop an attractive combination of strength and ductility with a TRIP assisted bainitic ferrite microstructure. The microstructure revealed a combination of ferrite, bainite along with martensite/retained austenite. Superior tensile strength and ductility were found in the steel austempered at about 450 °C due to the presence of higher bainitic, lower ferrite and moderate martensite/retained austenite. The product of tensile strength and percentage of elongation obtained were greater than 20 GPa% at some of the heat-treated conditions qualifying for third generation AHSS. The microstructural evolutions were correlated with the theory of bainite formation.