Strain Hardening Behavior of Dual-Phase Steels
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CAR-REDUCED weight and elevated safety parameters are the modern design criteria for car manufactures, being essential to fulfill customer’s expectations, legal requirements, and standards. Innovative steel grades with increased formability, high strength level, and high strain hardening index have been developed not only for weight reduction purposes, but also to improve vehicles’ crash safety.[1] Advanced high strength steels, especially dual-phase (DP), transformation induced plasticity (TRIP), and twinning induced plasticity (TWIP) steels, exhibit promising results in terms of superior mechanical properties, also considering that their extraordinary mechanical properties can be tailored and adjusted by a correct choice of alloying elements and processing routes. Dual-phase steels constitute a family of high strength strip grades, with a characteristic multiphase structure, consisting of hard second-phase islands (usually martensite at around 20 pct) dispersed in a ferrite matrix. Main characteristics of DP steels are continuous yielding behavior, lower yield/tensile strength ratios, higher work hardening rates at low strain, and higher levels of uniform and total elongation[2] than HSLA steels with similar yield strength. Also, TRIP steels consist of a ferrite matrix with a uniform dispersion of hard second phases (martensite or bainite). These steels also contain retained austenite in volume fractions greater than 5 pct, which progressively V. COLLA, Technical Research Manager, and A. DIMATTEO, Young Researcher, are with the Scuola Superiore di Studi Universitari e di Perfezionamento Sant’Anna – Pisa, Italy. Contact e-mail: [email protected] M. DE SANCTIS and A. SOLINA, Associate Professors, G. LOVICU, Research Assistant, and R. VALENTINI, Assistant Professor, are with the Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei materiali, Universita` di Pisa – Pisa, Italy. Manuscript submitted March 18, 2008. Article published online September 3, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
transforms to martensite during plastic deformation, thus increasing the work hardening rate at higher strain levels. Finally, TWIP steels are austenitic alloys in which mechanical twinning is the prominent deformation mode. These steels exhibit exceptional combinations of ductility, work hardening, and ultimate strength. The strain-hardening rate of TWIP steel is lower than that of DP and TRIP steels, but their local hardening capacity is superior, resulting in increased uniform elongation. In particular, the excellent capacity of local hardening of TWIP steel is mainly related to its single-phase structure, compared with DP and TRIP steel.[3] The present work is focused on better characterizing strain hardening behavior of conventional DP steels. In order to produce the DP microstructure, cold rolling of ferrite/pearlite microstructure, followed by intercritical annealing in the (c + a) region between A1 and A3 and final quenching at sufficiently high cooling rate, is usually imposed. The cool
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