Effect of copper on annealing characteristics of interstitial free steels Radhakanta Rana Æ Shiv Brat Singh Æ Omkar Nath Mohanty Received: 7 June 2006 / Accepted: 22 January 2007 / Published online: 18 May 2007 Ó Springer Science+Business Media, LLC 2007 Abstract The effect of copper on the annealing behav- iour of interstitial free (IF) steel has been investigated using thermoelectric power (TEP) and resistivity measurements. Kinetics of annealing of cold rolled copper-containing IF steel was found to be sluggish when compared with the base steels. TEP measurement revealed that copper has a negative coefficient of TEP in a-iron. Introduction Interstitial free (IF) steels are superformable steels (Lankford parameter, r m ‡ 1.8; strain hardening exponent, n ‡ 0.22) with exceptionally low levels of carbon and nitrogen (typically, total C < 0.003 wt.% and total N < 0.004 wt.%) [1, 2]. Small amounts of ‘stabilising’ elements such as Ti and/or Nb are added to these steels to precipitate out the interstitials in the form of carbide, nitride, sulphide or carbo- sulphide thus leaving the matrix essentially interstitial free. These steels are widely used for making auto-body compo- nents. A major drawback of IF steels is their low strength ( £ 360 MPa). This has been addressed to some extent through the application of various metallurgical concepts such as solid solution strengthening (using P, Mn and Si), work hardening and strain ageing to achieve strength of about 450 MPa [3]. It is possible to achieve even higher strength (>550 MPa) in IF steels by making use of classical age hardening effect of copper in a-iron [4–6]. Thus, addition of copper to highly formable IF steels can make them stronger and dent resistant, which are desirable for better fuel-economy and improved performance of the vehicle. The high formability of IF steels would be utilised by press forming the material in the as-annealed condition when it is soft and formable, and then it can be aged to give rise to high strength. The stabilisation of interstitials by addition of Ti and/or Nb would still be required in the presence of copper, since interstitials must be removed from the solid solution to ensure the high formability [1]. The annealing behaviour of IF steels is important because it governs the development of texture in the material which in turn deter- mines its formability [7, 8]. Accordingly, present work was undertaken to study the annealing behaviour of a new cop- per-alloyed interstitial free (IF-Cu) steel and compare it with that of IF steel and rephosphorised high strength IF (IF-HS) steel. The kinetics of recrystallisation during annealing of the three steels was studied using resistivity and hardness measurements and by microstructure characterisation. In addition to these conventional techniques, a relatively no- vel method based on thermo electric power (TEP) mea- surement was also used for studying the annealing behaviour. The principle of TEP is based on the Seebeck effect and it is emerging as a powerful technique for studying various metallurgical phenomena like cold working, annealing, precipitation, measurement of solute content in solid solution etc. [9–13]. Experimental The copper-containing IF (IF-Cu) steel was melted and cast in the form of 20 g button using IF steel and electrolytically R. Rana (&) Institut fu ¨r Eisenhu ¨ttenkunde, RWTH Aachen, Aachen 52072, Germany e-mail: rana@iehk.rwth-aachen.de S. B. Singh  O. N. Mohanty Department of Metallurgical & Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India 123 J Mater Sci (2007) 42:7508–7513 DOI 10.1007/s10853-007-1544-7