Materials Technology - Carbon Steels Laboratory Investigations on Copper-alloyed Interstitial Free Steel - Part I: Effect of Annealing Radhakanta Rana 1 ), 2), Wolfgang Bleck 1 ), Shiv Brat Singh 2 ) and Omkar Nath Mohanty2) 1) Department of Ferrous Metallurgy, RWTH Aachen University, Aachen, Germany 2) Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, India Effect of 1.18 wI.% copper on the annealing behaviour and the resulting mechanical properties in conventional mild interstitial free steel has been investigated. It is revealed that copper forms complex precipitates during batch annealing and modifies the batch annealing charac- teristics as compared with the base steel. Accordingly, strength-formability parameters are influenced as well. In case of continuous an- nealing, solute copper controls the mechanical properties. Keywords: interstitial free steel; copper alloying; annealing; mechanical properties. Introduction The increasing demand for lighter and fuel economic au- tobodies has led the steel industry towards the development of higher strength steels. One major group of automotive steels is the interstitial free (IF) steels having superforma- bility [1-3]. Improvements in steelmaking technology in the last decades materialised the production of IF steels with exceptionally low levels of interstitial contents (typically, total C < 0.003 wt.%, N < 0.004 wt.%). Further, addition of stabilising elements, such as Ti and/or Nb, which combine the remaining interstitials as various types of carbides, ni- trides, sulphides, carbo-nitrides and carbo-sulphides [4-6], makes these steels non-ageing. However, though IF steels meet the stringent formability requirements for the automo- tives (Lankford parameter, rm O!: 1.8 and strain hardening exponent, nO!: 0.22), the major drawback of these steels are their lower strength levels (yield strength s 220 MPa, ten- sile strength s 360 MPa) as compared to other automotive steels such as dual phase (DP), multiphase (MP) or trans- formation induced plasticity (TRIP) steels [7]. Metallurgi- cal concepts, such as solid solution strengthening (through P, Mn and Si), strain ageing, work hardening and grain re- finement by the addition of Nb have been applied to the de- velopment of high strength IF (IF-HS) steels, dent resistant bake hardenable (BH) steels etc. However, even then the tensile strength of IF steels has been limited to the maxi- mum of -450 MPa. On the other hand, there is a possibility of strengthening mild IF steels through classical age hardening by copper in Fe-based systems [8-10] which is hitherto unemployed met- allurgical phenomenon in these steels. Copper has been suc- cessfully alloyed to high strength low alloy (HSLA) and pressure vessel steel grades with benefits for strength, toughness, weldability and corrosion resistance. Fe-Cu is a model alloy for precipitation hardening causing significant strengthening and the prospect of copper-alloyed IF (IF-Cu) steels are being discussed in recent times to develop a cate- gory of post heat treatment (PHT) steels [11-13]. The con- cept lies in the fact that IF-Cu steels would be strengthened through precipitation hardening by copper upon an ageing treatment of the stamp-formed steel. The greatest advantage of high formability of IF steels would be utilised during 612 press forming of the annealed steel when it is in soft condi- tion. Thus ageing of a pre-deformed steel would yield peak strength in a shorter time as an added advantage. Precipita- tion behaviour of copper has been studied extensively in the past with the use of novel characterisation techniques like high resolution transmission electron microscopy (HRTEM), atom probe field ion microscopy (APFIM), small angle neutron and X-ray scattering (SANS and SAXS), extended X-ray absorption fluorescence spec- troscopy (EXAFS), X-ray absorption near edge spec- troscopy (XANES) etc. It has been established that useful copper precipitation occurs in the temperature range of 450- 600°C and the precipitation sequence of copper in a-iron is, bee Cu -+ 9R Cu -+ 3R Cu -+ £ Cu [8, 14-16]. In the peak aged state copper precipitates are in the form of bee clusters having a size of 2-3 nm. However there are still ambiguities regarding composition of bee clusters, existence of 3R tran- sition etc. From application point of view, studies on copper precipitation have been limited to steels containing higher carbon levels than in IF steels. Though preliminary propos- als for IF-Cu steels have been pointed out based on only good ageing response, no systematic scientific work has been carried out so far on the processing of such steels. Therefore, to reach a conclusion on the possibility of devel- opment of these steels, it is necessary to understand their behaviour during various processing steps. In the current in- vestigation annealing chacteristics, effect of coiling condi- tions on annealing behaviour and the ageing behaviour of an IF-Cu steel have been studied and the basic indicator properties for automotive use have been documented in three parts. While the major beneficial effect of copper in IF steels would be precipitation strengthening, it is expected that copper should modify the annealing characteristics of IF steels due to the presence of copper in precipitate and/or solute form. Further, mechanical properties are considered to be the basic parameters determining the performance of these kind of steels during press forming and ultimate use. In the present part the difference in annealing behaviour and the resulting mechanical properties of IF-Cu steel with ref- erence to conventional IF steels have been cited and the possibility of future use of this steel in competition with ex- isting IF steels has been discussed. steel research int. 78 (2007) No. 8