Effects of martensite morphology and tempering on dynamic deformation behavior of dual-phase steels
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9/27/04
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Effects of Martensite Morphology and Tempering on Dynamic Deformation Behavior of Dual-Phase Steels HAN SANG LEE, BYOUNGCHUL HWANG, SUNGHAK LEE, CHANG GIL LEE, and SUNG-JOON KIM The effects of martensite morphology and tempering on the quasistatic and dynamic deformation behavior of dual-phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens, which had different martensite morphologies and tempering conditions, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and fracture mode. Bulky martensites were mixed with ferrites in the step-quenched (SQ) specimens, but small martensites were well distributed in the ferrite matrix in the intermediate-annealed (IA) specimens. Under a dynamic loading condition, the fracture mode of the SQ specimens was changed from cleavage to ductile fracture as the tempering temperature increased, whereas the IA specimens showed a ductile fracture mode, irrespective of tempering. These phenomena were analyzed in terms of a rule of mixtures applied to composites, microstructural variation, martensite softening and carbon diffusion due to tempering, and adiabatic shear-band formation.
I. INTRODUCTION
IN strategic industrial fields such as the defense, automotive, precision machinery, and space aircraft industries, demands for structural materials that can be used under severe conditions like dynamic loading have been increasing.[1–10] The resistance to deformation and fracture under dynamic loading is generally lower than under quasistatic loading, which can fatally affect the overall fracture toughness of structural materials. Thus, detailed studies employing effective and quantitative evaluation should be made to determine how to maintain the deformation and fracture resistance of the materials so that they can be widely applied to various industrial fields. Dual-phase steels composed of ferrite and martensite have a better deformability than other high-strength low-alloy (HSLA) steels with similar strength.[11–15] Under quasistatic loading, they are also characterized as having a continuous yielding during plastic deformation, whose strength increases with increasing martensite volume fraction, in accordance with a rule of mixtures like in composite materials.[12] These studies on dual-phase steels have been primarily concerned with phenomena occurring only under either static or quasistatic loading, but very few studies have been done on the their dynamic deformation and fracture behavior. Particularly in dual-phase steels containing brittle martensites, resistance to deformation and fracture can abruptly drop under dynamic loading rather than under quasistatic loading, and, thus, the dynamic deformation and fracture behavior should be closely HAN SANG LEE, formerly Research Assistant, with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is Junior Researcher with Device and BYOUNGCHUL HWANG, Research Assista
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