                Zrnik Jozef 1,3, a  Dobatkin V. Sergey 2,b Fujda Martin 3,c and Dzugan Jan 1,d 1 ComtesFHT, Inc., Dobrany, Czech Republic 2 Moscow Institute of Steel and Alloys, Moscow, Russia 3 Technical University of Kosice, Slovak Republic a jzrnik@comtesfht.cz, b dobatkin@ultra.imet.ac.ru, c martin.fujda@tuke.sk, d jdzugan@comtesfht.cz  carbon steel, thermomechanical treatment, ECAP, microstructure, mechanical properties. Abstract. By application of thermomechanical controlled rolling and accelerated cooling, the carbon steel grain refinement is limited to levels of ~ 5 µm in steels. The strain assisted or strain induced transformation could be considered for the refining process. The present work, likewise, deals with grain refinement of medium carbon steel containing 0.45 wt pct carbon having different initial microstructure modified by either thermal and/or thermomechanical treatment (TM) prior severe plastic deformation. In case of TM treated steel, structure refinement was conducted in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process which provided total strain of 3. Uniform and fine recrystallized ferrite structure with grain size of the order of 2-5 µm and with nest-like pearlite colonies was obtained. The further grain refinement of steel samples having different initial structure was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. The steel samples of different initial structure were then subjected to six ECAP pressing passes through die channel angle of 120°. The microstructure development was analyzed in dependence of effective strain introduced (ε ef ~ 2.5 - 4). Employment of this processing route resulted in extensive deformation of ferrite grains where mixture of subgrains and ultrafine grain was found regardless the preliminary treatment of steel. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrains and submicrocrystalline grains having high angle boundaries. The straining and moderate ECAP temperature caused the partial cementite lamellae fragmentation and spheroidization as straining increased. The lamellae cementite spheroidization was more extensive in TM treated steel samples. The tensile behavior was characterized by strength increase for both structural steel states; however the work hardening behavior was modified in steel where preliminary TM treatment was introduced to modified coarse ferrite-pearlite structure. Introduction In the last years, ultrafine grained materials have attracted considerable research interest because they tend to possess high strength without sacrificing toughness and ductility. Microstructural refinement of steel is usually achieved by alloying and/or thermomechanical treatments accompanies various types of phase transformation. Recently, advancement of severe plastic deformation (SPD) techniques provide another efficient access for grain refinement of metals and alloys. The fabrication of bulk materials with ultrafine grain sizes has attracted a great deal of attention over the past two decades because of the materials’ enhanced properties [1, 2]. In recent years a worldwide effort in manufacturing process to obtain ultrafine grain structures in steels is persiting. It has been already well known that severe plastic deformation (SPD) of metallic materials is capable of producing ultrafine grained (UFG) materials with submicrometer or nanometer grain size [3, 4]. Since ECAP was introduced in the literature as an innovative technology of manufacturing bulk UFG metallic materials, many research groups worldwide have devoted effort to discover not Materials Science Forum Vols. 638-642 (2010) pp 2013-2018 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.638-642.2013 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 147.232.31.55-23/11/09,12:16:22)