Tensile Properties and Deformation Behavior in Duplex Lightweight Steel

Abstract

Recently, the reduction in vehicle’s weight has been devoted to reduce vehicle’s exhaust emission and to improve fuel efficiency. The most efficient method is the use of materials lighter than conventional ones. Many efforts have been directed towards applying lightweight materials such as Al alloys or Mg alloys, despite their high costs. However, these alloys show poor formability, which restricts the application. The development of new advanced automotive steels, namely lightweight steels, is recognized as a more realistic measure. As a part of this study, a considerable amount of Al has been added to automotive steels to obtain the lightweight effect. It has been shown that the addition of 1 wt.% of Al leads to 1.5% of weight reduction in comparison with conventional steels. However, the addition of Al drops the ductility, and thus the method utilizing microstructures such as full austenitic phase strengthened with κ-carbides or duplex phase of ferrite+austenite has been suggested. The full austenitic microstructure displays the low work hardening capacity, in spite of the excellent combination of strength and ductility, which results in a problem of poor formability. When the austenite is utilized as a secondary phase in ferrite matrix, high strength-high ductility lightweight steels having high work hardening capacity can be developed by the austenite/martensite transformation during the deformation, namely TRansformation Induced Plasticity (TRIP) mechanism. In addition to lightweight needs, automotive steels require a combination of specific strength and ductility for forming complex shapes as well as improving crashworthiness qualities. In structural reinforcement components, like A- and B-pillar, side sill, and front cross member, require ultra-high strength above 1 GPa and good ductility (total elongation of 15-20%). In particular a high yield strength is required due to importance of the stiffness. Firstly, in the present study, the ferrite + austenite duplex lightweight steel showing the operation of both TRIP and TWIP mechanisms was developed by varying annealing conditions of an Fe-0.3C-8.5Mn-5.6Al steel, and tensile properties were evaluated. Detailed deformation mechanisms were investigated in relation with microstructural evolution by electron back-scatter diffraction (EBSD) and transmission electron microscopy (TEM) analyses, and the correlation between microstructural evolution process and tensile ductility was verified. Secondly, although the utilization of austenite is considered as a proper approach in the lightweight steels, so far relatively long time aging treatment and high Mn content are required for high strength application affecting cost and productivity. In the present study, therefore, the ferrite + austenite duplex lightweight steel showing strengths and ductility above 1 GPa and 25% was developed by varying simply annealing temperature of an Fe-12Mn-5.5Al-0.7C steel. Detailed deformation mechanisms were investigated in relation to microstructural evolution by electron back-scatter diffraction (EBSD) and transmission electron microscopy (TEM) analyses, and the correlation between microstructural evolution process and tensile properties was verified.