Unraveling the Initial Microstructure Effects on Mechanical Properties and Work-Hardening Capacity of Dual-Phase Steel
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DUAL-phase (DP) steels fall within the category of advanced high-strength steels (AHSS). These are low-carbon steels having a microstructure consisting of 10 to 40 vol pct of a hard martensite phase in a matrix of soft ferrite. They possess high ultimate tensile strength while maintaining a good percent elongation to failure value (normally 400 to 1000 MPa and 10 to 30 pct), high initial work hardening rate, continuous yielding behavior, and good weldability with all conventional welding techniques. These properties make them suitable for automotive and other industries.[1–3] At a given martensite fraction, the mechanical properties and work-hardening behavior of the DP steels are mainly determined by the morphology, size, and distribution of the martensite and by the ferrite grain size, which themselves are dependent on the initial microstructure. The initial microstructure is dependent on the heat treatment[4–9] and thermomechanical processing
HAMED MIRZADEH, MOHAMMAD ALIBEYKI, and MOSTAFA NAJAFI are with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted February 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
routes.[10–20] Although severe plastic deformation techniques have been used for extreme microstructural refinement before intercritical annealing to produce ultrafine grained (UFG) DP steels,[21] production of the initial UFG microstructure based on the tempering of cold-rolled martensite as an advanced thermomechanical processing route[22–24] has been practiced in recent years. Several studies have been focused on heat treatment routes. Cai et al.,[4] Das et al.,[5] Ahmad et al.,[6] Kalhor and Mirzadeh,[7] Seyedrezai et al.,[8] and Schemmann et al.[9] showed that changing the initial microstructure can significantly affect the attributes of the DP microstructure and the resulting mechanical properties. The martensite initial microstructure was experimentally found to be one of the appropriate ones for obtaining a good combination of mechanical properties.[4–7] Other studies have used the thermomechanical processing routes for DP steels. Calcagnotto et al.[10] and Papa Rao et al.[11] focused on warm deformation and intercritical annealing, where the importance of the bimodal ferrite grain structure was also discussed.[11] Azizi-Alizamini et al.[12] studied the effects of cold rolling of tempered martensite, subsequent tempering step, and heating rate to intercritical annealing temperature. Nakada and coworkers[13,14] investigated the effect of heating rate to the intercritical region on the cold-rolled martensite, where the importance of chain-networked martensite grains formed on grain boundaries in the recrystallized ferrite matrix was
discussed. Alternatively, Karmakar et al.,[15] Mazaheri et al.,[16] and Etesami and Enayati[17] intercritically annealed cold-rolled DP microstructures and Ashrafi et al.[18] considered double intercritical annealing of cold-
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