Impact Toughness of Ultrafine-Grained Interstitial-Free Steel
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TRODUCTION
INTERSTITIAL-FREE (IF) steels are recently developed steel products including very low amounts of carbon/nitrogen interstitials. Therefore, they are extensively used in the automotive industry because of their excellent deep drawability and high planar anisotropy.[1] However, the IF steels have very low yield strength because of the absence of interstitials in the ferrite grains.[2,3] The low strength hinders their applications in conditions where high strength is demanded. Considering the single-phase microstructure of IF steels, the strengthening methods to enhance their mechanical properties are limited. Among them, grain refinement seems to be the most viable method.[4] In contrast, there is a limitation to obtaining grain refinement for steels by traditional methods such as rolling, forging, and extrusion. In recent years, new techniques imposing accumulated strains via severe plastic deformation (SPD) have been proposed to obtain ultrafine-grained (UFG) structures with grain sizes on the submicron level.[5–7] Among the different SPD techniques, equal-channel angular extrusion/pressing (ECAE/P) is the most attractive method for producing bulk UFG materials. Therefore, ECAE processing has been applied to IF steels in bulk and ONUR SARAY, Ph.D. Student, and GENCAGA PURCEK, Professor, are with the Department of Mechanical Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey. Contact e-mail: [email protected] IBRAHIM KARAMAN, Professor, is with the Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843. HANS J. MAIER, Professor, is with the Lehrstuhl fu¨r Werkstoffkunde, University of Paderborn, D-33095 Paderborn, Germany. Manuscript submitted February 15, 2012. Article published online June 8, 2012 4320—VOLUME 43A, NOVEMBER 2012
sheet forms, and excellent mechanical properties, such as high strength and enhanced fatigue behavior, were obtained.[2,8–15] In contrast to several reports on tensile and fatigue properties of UFG steels, data for dynamic fracture toughness (especially for IF steels) are hardly available.[4,11,16] Furthermore, no impact data were reported for UFG steels processed using ECAE. This is mainly because of the difficulty to obtain samples with large enough sizes from laboratory-scale ECAE setups. In contrast, strong and tough steels are always in demand to reduce weight and improve safety and performance in transportation and heavy machinery.[17] In addition, the demand for improving low-temperature toughness of these materials is increasing because steels with bodycentered cubic (bcc) crystal structure are very sensitive to a decrease in temperature in service under impact loading. In contrast to other strengthening methods, grain refinement is well known to be a strengthening mechanism that also increases toughness.[4] Some previous reports revealed that the ductile-to-brittle transition temperature (DBTT) can be reduced by grain refinement especially for two-phase steels.[4,18] However, there is no study on the impact properties of UFG steels f
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