Transmission Electron Microscopy Characterization of the Bake-Hardening Behavior of Transformation-Induced Plasticity an
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ONE of the main aims of steel research for automotive body applications is to develop materials with the optimum combination of relevant properties, cost, and productivity. Transformation-induced plasticity (TRIP) and dual-phase (DP) steels have demonstrated a high ultimate tensile strength (~800 to 1000 MPa), while maintaining a high level of ductility (30 to 40 pct).[1,2] The microstructure of DP steels consists of a soft, ductile, polygonal, ferrite matrix with hard martensite islands, and, in some cases, a small amount of retained austenite.[3] The strength-ductility balance in this steel is controlled by the volume fraction of martensite.[3] The TRIP steel microstructure contains polygonal ferrite, bainite, retained austenite, and a minor quantity of martensite.[4] Both steels exhibit continuous yielding behavior, low yield point, and a high strain-hardening coefficient.[5,6] These characteristics have been attributed to an increase in the work-hardening parameters, through the continuous transformation of retained austenite to martensite in the TRIP steels and the formation of mobile dislocations and martensite twinning in the DP steels during forming.[7] This results in a localized increase of the strain-hardening coefficient that I.B. TIMOKHINA, Research Fellow, is with the Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia. Contact e-mail: [email protected] P.D. HODGSON, Professor, ARC Federation Fellow, is with the Center for Material and Fiber Innovation, Deakin University, Geelong, VIC 3217, Australia. E.V. PERELOMA, Professor, Director of BlueScope Steel Metallurgy Centre, is with the School of Mechanical, Material and Mechatronics Engineering, University of Wollongong, Wollongong, NSW 2500, Australia. Manuscript submitted September 17, 2006. Article published online August 29, 2007. 2442—VOLUME 38A, OCTOBER 2007
delays the onset of necking and ultimately leads to the high elongation without compromising the strength.[8] In industrial processing, these steels undergo a finishing treatment, where the paint coating of the automotive body is baked at the temperatures of 150 C to 200 C for 20 to 30 minutes after forming.[9] During this bakehardening (BH) treatment, interstitial solute atoms, such as carbon, move to the dislocations produced during pressing and pin dislocations.[10] The driving force for this solute redistribution is to reduce lattice energy. This reduction leads to solute diffusion toward dislocations and the formation of a Cottrell atmosphere.[11] The use of the BH treatment has been proposed for both TRIP and DP steels, in addition to the work hardening, to increase the yield strength of these steels in the final body structure.[9] This proposal should potentially lead to an increase in strength for the same thickness of steel sheet. The maximum increase (100 MPa) has been observed in the TRIP and DP steels after 2 to 10 pct of prestraining (PS) in an interrupted tensile test to simulate the process of forming and paint baking of the ca
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