High Strength and High Ductility of Ultrafine-Grained, Interstitial-Free Steel Produced by ECAE and Annealing GENCAGA PURCEK, ONUR SARAY, IBRAHIM KARAMAN, and HANS J. MAIER Interstitial-free steel (IF steel) underwent severe plastic deformation by equal-channel angular extrusion/pressing (ECAE/P) to improve its strength, and then it was annealed to achieve a good strength-ductility balance. The coarse-grained microstructure of IF steel was refined down to the submicron level after eight-pass ECAE. The ultrafine-grained (UFG) microstructure with high dislocation density brought about substantially improved strength but limited tensile ductility. The limited ductility was attributed to the small, uniform elongation caused by early plastic instability. The annealing at temperatures below 723 K (450 °C) for 1 hour did not lead to remarkable softening, whereas annealing at temperatures up to 923 K (650 °C) resulted in complete softening depending on the development of recrystallization. Therefore, the temper- ature of approximately 923 K (650 °C) can be considered as a critical recrystallization tem- perature for UFG IF steel. The annealing at 873 K (600 °C) for different time intervals resulted in different stress–strain response. Uniform tensile elongation increased at the expense of strength with annealing time intervals. After annealing at 873 K (600 °C) for 60 minutes, the yield strength, tensile strength, uniform elongation, and total elongation were found to be 320 MPa, 485 MPa, 15.1 pct, and 33.7 pct, respectively, showing the better combination of strength and ductility compared with cold-rolled samples. DOI: 10.1007/s11661-011-1063-7 Ó The Minerals, Metals & Materials Society and ASM International 2012 I. INTRODUCTION 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 con- sumption. Considering the monophase microstructure of IF steel, strengthening methods to enhance its mechanical properties are limited. Among them, equal-channel angular extrusion/pressing (ECAE/P), which induces grain refine- ment 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 character- istics, 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 fati- gue 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 intro- duced into the deformed microstructure with grains ranging from 200 nm to 100 lm. [15–19] In addition, the annealing process can be applied also to UFG materials processed by SPD techniques to obtain a microstructure with enhanced stability. [5] Although some studies have addressed the processing of IF steel by ECAE, [2,6–9] to the best of our knowledge, no systematic study has been published yet on the effect of annealing on the tensile behavior of UFG IF steel produced by ECAE. In view of the results cited, one can expect that high uniform elongation and high tensile strength will be 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: Purcek@hotmail.com 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 METALLURGICAL AND MATERIALS TRANSACTIONS A