THERMOMECHANICAL PROCESSING OF HIGH AL FERRITIC LOW DENSITY STEELS

Abstract

In the past, rapid industrialization and urbanisation increased greenhouse gas emissions, which became an environmental problem in the current situation. Similar amounts of greenhouse gas emissions are produced by the power, agriculture, and industrial sectors; a sizeable quantity is also produced by the transportation sector. With this goal in mind, the automobile industry is currently concentrating on reducing the weight of an automotive to improve fuel efficiency, which lowers greenhouse gas emissions from the transportation sector. From the standpoint of a material, there are many ways to reduce the weight of steel, including steel-based laminates, steel foams, and steel-matrix composites. The most popular method of lowering the density of the steel is to add low density components (such as Al and Si). In terms of density, solubility, alloying, and workability, aluminium (density = 2.7 gm cm3) is the most desired elemental addition to the steel for density reduction. However, Al addition also leads to some thermodynamic phenomenon like stabilization of brittle intermetallic phases above 11 wt.% addition, formation of kappa carbide (ordered FCC structure) and stabilization of ferrite upto melting point of the Al added steel. So, Mn and C is can be a desired addition in these Al added steels to stabilize austenite at elevated temperatures. This makes the steel eligible for heat treatments in the intercritical (ferrite + austenite) region. A slight increase in Al and C content in the steel leads to the formation of significant amount of kappa carbides which will deteriorate ductility in these steels, compromising the formability needed in an automotive steel. Austenitic Fe-Al-Mn-C low density steels are explored extensively due to various strengthen and ductility enhancing mechanism associated with the austenite phase. Ferritic Fe-Al-Mn-C low density steels have been studied relatively less. In the present study, effect of thermomechanical processing on high Al Fe-11Al-(5,10)Mn-1C ferritc low density steels are studied. Phase transformation studies suggest that ThermoCalc underestimates the phase transformation temperature for these alloys when confirmed through DTA, dilatometry and quenching experiments. The hot rolling of these steels exhibit a partially recrystallized ferrite matrix exhibiting the difficulty of recrystallization of the ferrite phase during hot rolling. The microstructure consists of ferrite matrix and coarse kappa carbides. The interface of ferrite/ kappa carbide acts like a nucleation site for microscrack. The propagation through interlinking of these microcracks along ferrite/kappa carbide interfaces leads to the tensile failure in the hot rolled alloys. The hot deformation study was conducted for Fe-11Al-10Mn-1C alloy to understand the effect of different hot deformation parameter on Dynamic recrystallization (DRX) in ferritic as well as ferrite-austenite (intercritical) region. A constitutive model for the hot deformation of the alloy showing high accuracy was in predicting the flow stress was also generated. The hot deformation study suggested that hot working of these alloys in intercritical region leads to an appreciable DRX and grain refinement of microstructure. Effect of Nb addition on the microstructure of as-cast as well as processed condition of these alloys have also been studied. The hot rolled alloys exhibit a banded microstructure consisting of a partially recrystallized ferrite band and secondary bands. Secondary bands comprise of an exterior region of ferrite + kappa carbide. The interior of the band consists of retained austenite with nano-size kappa carbide precipitation in it. The annealing of hot rolled alloys leads to an appreciable refinement and DRX in the microstructure which consist of ferrite grains and coarse kappa carbides at ferrite grain boundary. The cold rolling and annealing of hot rolled and annealed alloys leads to further grain refinement and DRX in the ferrite matrix. The hot rolled alloys show higher strength while annealed alloys show higher ductility. These alloys show solid solution strengthening, precipitation strengthening by fine NbC precipitates, dislocation strengthening as primary strengthening mechanisms. Annealing of the hot rolled alloys lead to the decomposition of retained austenite to ferrite + kappa carbides increasing the solid solution strengthening in these alloys as ferrite is more responsive for solid solution strengthening. In hot rolled alloys, microvoid formation near kappa carbide in ferrite+ kappa carbide bands lead to the formation of microcracks which propagates by interlink together in this band leading to the tensile failure. In annealed alloys, the crack initiates at ferrite/kappa carbide interface and propagates through ferrite grain boundaries leading to the tensile failure. The hot deformation studies of Nb containing alloys were also conducted to optimize the hot deformation parameters for these alloys. Constitutive modelling for hot deformation was also generated. The models show similar levels of accuracy in predicting the flow stress. A faster DRX kinetics and higher degree of grain refinement during the hot deformation was observed in ferrite-austenite region in comparison to ferrite region. The coarse NbC precipitates and kappa carbides induce Particle Stimulated Nucleation (PSN) in these alloys during the hot deformation. Also grain refinement was observed by pinning of grain boundaries by NbC precipitates in ferrite as well as austenite phase.