Investigations On The Microstructures-Properties Relationship And Deformation Mechanism In High Strength And High Ductility Low Density Steels

With rapid developments of automobile industry, automotive steels are also facing challenges such as safety, environment, resource, energy, cost and so on. In this circumstance, lightweight becomes an important development trend in the future development of automobile industry. A new class automotive steel has been developed by adding Al to the medium and high manganese steels, and through optimization of the alloy composition design and heat treatment processes, obtaining low density, optimal high strength and high ductility combination. With a great potential and a promising prospect, high strength and high ductility low density steels may be widely used in automotive structures. However, the research on high strength and high ductility low density steels for automobile aopplications has just started, and there are many theoretical problems to be solved. In the present paper, the mechanical properties and the microstructural evolution for high strength and high ductility low density steels during annealing and aging treatments were investigated and then microstructural evolution and deformation mechanisms during tensile deformation and high pressure torsion (HPT) were studied. To decode the deformation mechanisms for high strength and high ductility low density steels can provide us not only with an excellent opportunity to enrich the plastic deformation theory and to extend our understanding of the structure-propertie relationship of high strength and high ductility low density steels, but also provide some methods to optimize the microstructures and improve mechanical properties for high strength and high ductility low density steels. The main original works of this paper are present as follows:(1) Different annealing treatments were carried out for two typical high-manganese Fe-Mn-Al-C low density steels (Fe-26Mn-8Al-1C and Fe-26Mn-10Al-1C steels, which were designated 8A1 and 10A1 steels, respectively), microstructure and mechanical properties of these steels during annealing treatment processes were studied. Microstructural evolution for 8A1 and 10A1 steels during tensile deformation was investigated, and the relationship between the mechanical behavior and the deformation mechanisms of the steels was established. After the analysis and the comparison of the microstructural evolution and the mechanical behavior between these two high-manganese low density steels with different A1 contents, the effects of A1 content on the microstructural evolution and the mechanical properties were clarified. The result indicates that an increase in the critical stress is needed for dislocation slip with increasing Al content, which results in delaying the formation of dense microbands and plastic instability, and obtaining higher strain hardening ability and higher strength and higher toughness than 8A1 steel.(2) The microstructures and the mechanical properties of 8A1 and 10A1 steels under different aging treatments were investigated. The result indicates that the strength significantly increases and the elongation does not obviously decrease by the aging treatment at a short holding time. The morphology and the status of the precipitated phases for 8A1 and 10A1 steels were investigated by using XRD, TEM and HRTEM, and the roles of κ-carbide on the mechanical properties and the deformation mechanism of the high-manganese low density steels were clarified. The result indicates that spinodal decomposition and ordering coexistence appeared in the investigated high-manganese low density steels, with increasing A1 content, the average size and the volume fraction of the precipitates increased significantly, and 8A1 and 10A1 steels have excellent integrated mechanical properties through reasonable control of the precipitation carbides.(3) The relationship between the microstructures and the mechanical properties of medium-manganese Fe-Mn-Al low density steels (Fe-12Mn-8Al-1C steel, which was designated 8A112Mn steel) under different annealing and aging treatments was revealed. The effects of the phase distribution and the phase property on the deformation mechanism and the fracture behavior for the 8A112Mn steel were studied by using SEM, TEM and EBSD through the strain hardening rate-true strain analysis and microstructure observation. The result indicates that 8A112Mn steel has excellent integrated mechanical properties through reasonable control of the phase distribution and the phase property, and this steel exhibites a superior combination of strength and ductility (ultimate tensile strength of 920 MPa and elongation to failure of 46.4%) after an annealing treatment at 900℃. Lots of lamellar precipitation carbides appeare after an aging treatment at 600min, which results in a remarkable decrease in ductility as well as strain hardening ability, causing synchronous decrease in both strength and ductility.(4) The microstructural evolution and the mechanical properties of high-manganese Fe-Mn-Al low density steel (Fe-26Mn-3Al-1C steel, which was henceforth designated 3A1 steel) with different revolutions during HPT deformation were investigated. The deformation mechanism and grain refinement mechanism were revealed. The result indicates that the hardness value increased three times than the one before HPT deformation. The evolvement characteristics of the microstructures in 3A1 steel are present as follows:(a) lots of deformation twins were produced, twin bands intersected each other and twin bands were cut and block by S shape shear bands. (b) The dominated deformation characteristic was shear bands during HPT deformation at higher revolutions, and amorphous structures were formed in shear bands.