DEVELOPMENT OF ULTRA HIGH STRENGTH BAINITIC STEEL WITH EXCELLENT DUCTILITY

Abstract

In this work entitled ‘Development of ultra high strength carbide free bainitic steel (hot & cold rooling route) with excellent ductility was studied in detail. The primary objective of this work is to develop carbide-free bainitic steel to deliver an excellent combination of strength and ductility. By refining the microstructure through different heat treatment processes, carbide-free bainite steel exhibits excellent strength, toughness and wear resistance. The higher strength is achieved by the thinning of bainitic laths (100 to 150 nm) and the higher dislocation density due to bainitic transformation. Higher ductility originates from sufficient retained austenite in the microstructure, which was thermally stable at room temperature. The study starts with theoretical and experimental validation of the development of carbide-free bainitic steel with lean composition, which can deliver excellent strength and elongation. This set of experiments was carried out with hot-rolled (HR) samples. The steel containing C, Mn, Cr, and Si are primary alloying elements. We have conducted an extensive analysis and quantification of microstructural components, and their relationship with the tensile properties has been established using semi-empirical equations. Application of such HR materials are limited to some specific areas of an automobile like door sill whereas a cold rolled (CR) steel can find a much wider applications like door impact beams or bumpers etc. However, reducing the thickness of an HR material of ultra high strength by cold rolling is very difficult, if not impossible, because of several reasons including increase of rolling load to a great extent as well as the springback effect as well as and complications in achieving lower thickness due to cracking. The softer microstructure after hot rolling will be needed for performing the cold rolling operation. It was decided to have a fully pearlitic microstructure after casting and hot rolling to facilitate the subsequent cold rolling operation and finally the cold rolled strip would be transformed to carbide-free bainitic structure by suitable heat treatment. The composition was tailored through performing several virtual experiments in order to develop the desired microstructure.