High Strength and High Ductility of Ultrafine-Grained, Interstitial-Free Steel Produced by ECAE and Annealing
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INTERSTITIAL-free steel (IF steel) is a recently developed steel product with a low free carbon level. This steel is, therefore, used widely in the automotive industry because of its excellent deep drawability and high planar isotropy.[1] However, the IF steel in the coarse-grained (CG) condition possesses high ductility but low yield strength.[2] The amount of interstitial carbon and nitrogen atoms inside the ferrite grains decreases by stabilizing the microstructure with microalloying elements (Ti and/or Nb), which brings about the low strength of IF steel because of the decrease in the solid-solution-hardening effect of the interstitial atoms.[3] The low strength of this steel hinders its application in conditions where high strength is demanded. The low strength is a problem especially in automotive applications as thicker parts or sheets will be needed in a vehicle to obtain the desired strength and crash performance. This increases the global vehicle weight, leading to an increase in the fuel consumption. Considering the monophase microstructure of IF steel, strengthening methods to enhance its mechanical properties are limited. Among them, equal-channel angular GENCAGA PURCEK, Associate Professor, and ONUR SARAY, Ph.D. Student, are with the Department of Mechanical Engineering, Karadeniz Technical University, 61080-Trabzon, Turkey. Contact e-mail: [email protected] IBRAHIM KARAMAN, Associate Professor, is with the Department of Mechanical Engineering, Texas A&M University, College Station, TX. HANS J. MAIER, Professor, is with the Lehrstuhl fu¨r Werkstoffkunde, University of Paderborn, 33095 Paderborn, Germany. Manuscript submitted May 18, 2011. Article published online February 3, 2012 1884—VOLUME 43A, JUNE 2012
extrusion/pressing (ECAE/P), which induces grain refinement via severe plastic deformation (SPD), seems to be the most viable method because ultrafine-grained (UFG) materials tend to have improved mechanical and physical properties.[4,5] The grain size, grain boundary characteristics, and dislocation distributions are responsible mainly for improved mechanical behavior.[3] Therefore, this technique has been applied to IF steel in bulk and sheet forms,[2,3,5–11] and the obtained UFG IF steel showed excellent mechanical properties, such as high strength,[6–9] good low-temperature toughness,[12] and enhanced fatigue behavior.[2,6–8] However, the UFG microstructure brought about limited ductility with a few percent uniform elongation because of the reduced dislocation accumulation ability.[13] Considering the application, a reasonable combination of high strength and good ductility for IF steel is mandatory. Some previous reports have shown that the ductility of UFG materials can be enhanced without considerable strength loss by appropriate annealing treatments after severe plastic deformation.[12–15] This improvement was attributed to the bimodal grain size distribution introduced into the deformed microstructure with grains ranging from 200 nm to 100 lm.[15–19] In addition, the annealing process can b
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