Evolution of microstructure and texture during annealing of two high strength interstitial free steels containing Nb and or Ti A. Saha Podder*, D. Bhattacharjee, Sudin Chatterjee and R. K. Ray Interstitial free steel containing Nb, recrystallised at a slower rate compared to Ti stabilised steel. The coincidence site lattice boundaries of the type S3, S13b and S39a appear to play a major role during the first stage of grain growth in both the Nb z Ti and Ti containing interstitial free steels. They do not seem to have any major role in the development of final texture in these materials. The distribution of grain boundary misorientation angles also changes with annealing time and show two maxima close to the misorientation angles 60u and 20–40u. Keywords: IF steel, Misorientation, Texture, CSL boundary, EBSD Introduction Interstitial free (IF) steels are commonly used in auto- motive applications because of their high formability. Good formability of IF steels is achieved by the reduc- tion of interstitial elements (C and N) to ultra low level and through recrystallisation annealing of the cold rolled material. Recrystallisation depends on composi- tion as well as temperature and time of holding, and occurs by the movement of grain boundaries. Therefore, the nature of grain boundaries play very important role during the progress of recrystallisation. Conventional IF steels are stabilised with Nb and or Ti in order to remove all C and N from solid solution. 1,2 In practice the low strength of IF steels poses problems due to their poor dent resistance. Therefore, new grades of IF steel were developed 3 which were strengthened by the addition of substitutional solutes such as Mn and P (repho- sphorised). Thus good formability and strength (dent resistance) are combined in these high strength IF steels. As mentioned above, grain boundaries play a vital role in controlling recrystallisation and thereby the final properties of IF steels. It is established that grain boundary mobility is linked to the misorientation between the neighbouring grains. 4 The role of special grain boundaries such as the coincidence site lattice (CSL) boundaries is quite important in this connection. Considerable work has been carried out on the role of CSL boundaries during recrystallisation of fcc materi- als. 5,6 In steels, the majority of work in this area has been mainly focused on secondary recrystallisation of Fe–Si steels. 7–11 In fact, two different hypotheses have been proposed to explain the formation of Goss oriented grains during secondary recrystallisation in Fe–3Si steel. One of these hypotheses 7–9 suggests that S3–S9 boundaries are responsible for development of the Goss texture due to the enhanced mobility of these CSL boundaries. This hypothesis was based on a micro- structure where only 13% of the grain boundaries were of the type S3–S9. According to the second group, 10,11 30% of boundaries surrounding a grain must be highly mobile in order that it may grow continuously in the matrix during annealing. In the IF steels it is predomi- nantly the {111} grains which form during recrystallisa- tion annealing and impart high formability to the material. 12–15 The role of the grain boundaries, specially the CSL boundaries in the evolution of microstructure and texture of IF steels has not, unfortunately, been looked into in details. In the present work, an attempt has been made to establish the relationship, if any, between the CSL boundaries, grain size and texture during the progress of recrystallisation in two high strength IF steels. Experimental The composition of the steels studied are shown in Table 1. These steels were industrially hot rolled to 3?2 mm thickness. The slab reheating temperature was 1200uC. The hot rolling finishing temperature was 910uC (above the Ar 3 temperature), while the coiling tempera- ture was 670uC. This product was subsequently cold rolled 62% in five passes to a final thickness of 1?2 mm. The samples were heated at 650uC in a muffle furnace for the durations of 30 min, 1, 3, 6, 8, 10, 20, 30 and 40 h in order to study the progress of recrystallisation. This temperature was selected because the recrystallisation is sluggish at this temperature making it easy to observe the course of the recrystallisation and grain growth reactions. Heat treated specimens were cut and mounted, polished and etched in 2% nital solution for microstructural analysis. A digital image analysis system was used to measure the grain size distribution. For each specimen, y500 grains were chosen randomly and grain size was measured using equivalent area method. R&D and Scientific Services Division, Tata Steel, Jamshedpur 831001, India *Corresponding author, email arijit.sahapodder@tatasteel.com ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 13 April 2007; accepted 14 August 2007 DOI 10.1179/174328407X245102 Materials Science and Technology 2009 VOL 25 NO 12 1417