Effect of Pre-Treatment on Copper Precipitation Characteristics in a Copper-Alloyed Interstitial Free Steel Studied by T
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INTRODUCTION
Interstitial free (IF) steels are the novel automotive grade steels having very high formability.[1–3] The strain hardening exponent (n), which is a measure of stretchability, is ‡0.22 and the Lankford parameter (rm), which is an index of deep drawability, is ‡1.8 in these steels.[1] IF steels are characterized by very low amounts of carbon and nitrogen achieved by the use of sophisticated vacuum degassing technologies that reduce the total C content to less than 30 ppm by weight and N content to less than 40 ppm by weight.[4,5] In addition, these steels also contain small but effective amount of stabilising elements such as Ti and/or Nb that react with the interstitial elements to form carbides, carbo-nitrides, carbo-sulphides of Ti and/or Nb during various processing steps.[6] Due to the precipitation of these compounds, the ferrite matrix is essentially interstitial free and therefore not sensitive to aging phenomenon.[7–9] Although IF steels meet the stringent formability requirement of the automotive industry,[1] their low strength (yield strength RADHAKANTA RANA, formerly DAAD Research Scholar, with Institut fu¨r Eisenhu¨ttenkunde, RWTH Aachen, Intzestraße 1, Aachen 52072, Germany and also Institute Research Scholar with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, Kharagpur, 721302, India, is now Principal Researcher, with the Tata Steel Research, Development and Technology, 1970CA, IJmuiden, The Netherlands. Contact email: [email protected]. VE´RONIQUE MASSARDIER, Assistant Professor, is with the Universite´ de Lyon, CNRS, INSALyon, MATEIS UMR5510, 69621 Villeurbanne, France. SHIV BRAT SINGH, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur and OMKAR NATH MOHANTY, formerly Tata Research Professor, with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, is now Director, with the Technology and Academic Initiative, RSB Metaltech., RSB Group, Bhubaneswar, India. Manuscript submitted March 31, 2011. Article published online September 22, 2012 186—VOLUME 44A, JANUARY 2013
£220 MPa; ultimate tensile strength £360 MPa)[10] has limited their use only to rear floor pan, spare wheel well, and the front and rear door inners[1] that constitute only about 4 pct of the total steel used in auto body.[11] Attempts have been made to address this drawback of IF steels by application of various metallurgical principles such as solid solution strengthening using P, Mn and Si, grain refinement by addition of excess Nb, strain aging by creating excess interstitials etc..[12–17] However, even with these modifications, the maximum tensile strength that has been achieved in the existing high strength IF (IF-HS) steels is around 440 MPa only and therefore, there is a scope for further improvement.[18] Copper has a great potential for strengthening IF steels through classical age hardening in iron[19–24] and this possibility has been
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