An Overview on Precipitation and Texture Formation in IF and IFHS Steels During Processing

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Interstitial free steels, commonly known as interstitial free (IF) steels, are widely used by auto manufacturers on account of their excellent formability as compared to other steels. As the name suggests, IF steels are almost free of interstitial elements, C and N, and this IF matrix results in excellent formability. In IF steels, C and N are kept very low (generally <30 ppm), and Ti and/or Nb are added to tie up with the carbon and nitrogen left behind. However, the problem with IF steels is their poor strength and thus the higher strength version of IF steels, popularly known as interstitial free high strength (IFHS) steels have been developed. These steels contain Mn and P as solid solution strengthening elements. Though IF and IFHS steels are considered as “clean” steel, precipitation plays a very important role as it directly affects the textural evolution, and thereby the formability. Precipitation is closely controlled by composition as well as by processing history of the steels. The precipitation behavior is rather different and much more complex in IFHS steels as compared to IF steels. Textural evolution in both these grades is very much dependent on the morphology and distribution of the precipitates, their sequence of occurrence, and their volume fractions. In this article, an overview of the complex precipitation behavior and its correlation with textural development in IF and IFHS steels during the different stages of processing have been critically discussed. An attempt has also been made to correlate these aspects with the mechanical behavior of such steels.