Interstitial Free Steels...

Interstitial Free Steels  The term ‘Interstitial Free steel or IF steel’ refers to the fact, that there are no interstitial solute atoms to strain the solid iron lattice, resulting in very soft steel. IF steels have interstitial free body centered cubic (bcc) ferrite matrix. These steels normally have low yield strength, high plastic strain ratio (r-value), high strain rate sensitivity and good formability. Conventional IF steels which were developed commercially in Japan during 1970s following the introduction of vacuum degassing technology contained carbon (C) in the range of 40 – 70 ppm and nitrogen (N) in the range of 30 -50 ppm. Later, niobium (Nb) and/or titanium (Ti) were added to these steels to stabilize the interstitial C and N atoms. IF steel is termed as ‘clean steel’ as the total volume fraction of precipitates is very less. In spite of this, the precipitates appear to have a very significant effect on the properties of IF steels. Liquid steel is processed to reduce C and N to levels low enough that the remainder can be ‘stabilized’ by small additions of Ti and Nb. Ti and Nb are strong carbide/nitride formers, taking the remaining C and N out of solution in liquid iron, after which these latter two elements are no longer available to reside in the interstices between solidified iron atoms. IF steel has ultra low carbon content, achieved by removing carbon monoxide, hydrogen, nitrogen, and other gasses during steelmaking through a vacuum degassing process. Interstitial elements like nitrogen or carbon are also in the form of nitrides and carbides due to the alloying elements such as Nb and/or Ti used for the stabilization of the residual interstitials. Therefore, IF steels posses typically non aging properties. Because of their non ageing properties, IF steels...