Microstructure Optimization of Dual-Phase Steels Using a Representative Volume Element and a Response Surface Method: Pa
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A. Dual-Phase Steels (DP)
ADVANCED high-strength steels (AHSS) offer the ability to produce stronger, safer, and lighter cars. To meet the demands of automobile manufacturers for these materials, steelmakers have developed new, lowcost, high-strength steels (HSS) and AHSS with superior strength and ductility.[1] In addition, they have developed a descriptive terminology for AHSS along with a number of different grades. These materials include the
TAREK M. BELGASAM is with the School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99163, and also with the Mechanical Engineering Department, Faculty of Engineering, University of Benghazi, Benghazi, Libya. Contact email: [email protected] HUSSEIN M. ZBIB is with the School of Mechanical and Materials Engineering, Washington State University. Manuscript submitted March 23, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
first generation AHSS with mainly ferrite-based microstructures and second generation AHSS with high manganese, austenitic-based microstructures. The first generation AHSS consist of transformation-induced plasticity (TRIP), DP, complex phase (CP), and martensitic (MART) steels.[2] The most important second generation austenitic AHSS grade is composed of twinning induced plasticity (TWIP) steels.[3] There is a growing need for third generation AHSS characterized by greater strength and formability than in first generation AHSS, and lower cost than that of the second generation AHSS. Efforts have been made to expand the range of first and second generation AHSS.[2] Demand is growing for stronger steel with lower mass, better stretchability (to improve formability), better crash energy management, reduced alloy requirements, and simpler manufacturing processes to reduce costs.[4] Dual-phase steels have a range of strength and formability combinations depending on their microstructure. The microstructure of DP steels includes martensite phase particles dispersed in the soft ferritic matrix. Generally, DP steels contain a purely ferrite phase as a matrix with about a 3.3 to 47 pct fraction of martensite islands spread as a hard phase over a matrix.[4]
B. Plastic Deformation and Work Hardening Behavior of DP Steels Flow stress of DP steels relies not solely on the characteristics of the ferrite and martensite phases but also on the volume fraction and morphology of the martensite phase.[5–9] During plastic deformation of DP steels, ferrite phase properties govern the yield behavior of DP steels, which is determined by the ferrite’s composition and grain size. The plastic deformation starts in the ferrite grain. Even though the martensite phase has a significant effect on strain hardening of DP steels, the martensite phase behavior generally shows elasticity unless deformation reaches high-stress levels.[10–12] In DP steels, the strength of the ferrite phase depends on the initial dislocation density resulting from compatible strains when the austenite phase alters into the martensite phase during cooling.[13,14] The martens
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