Texture development in high strength interstitial free steels

• Main Author: Cross, I. J.
• Published: Swansea University 2001
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636326

Interstitial free (IF) steels are used in applications in the automotive industry where their excellent formability can be taken advantage of. This results from the removal of interstitial elements from solution by the addition of elements with a high affinity for carbon and nitrogen. Whilst highly formable, these steels have an associated low proof stresses and low dent resistance. To allow the use of thinner gauges in cars, strengthening of IF steels has been carried out by the addition of solid solution strengthening elements such as manganese, phosphorus and silicon. Despite obtaining desirable strength increases, the formability is reduced in the high strength IF steels. Formability is known to be related to crystallographic texture. High r-values (a measure of formability) are seen when advantageous {111}<<I>uvw</I>> orientations have a strong intensity. Orientations deviated away from the favourable types, particularly {100} types, are detrimental to formability. The development of such textures in unalloyed, formable IF steels have been extensively studied using modern techniques such as Electron Back Scatter Diffraction (EBSD). The work presented in this thesis involves the study of texture development in a typical, commercially relevant, high strength IF steel, beginning with the hot rolled material through to the fully annealed condition. The literature review outlines the metallurgy of IF steels as well as discussing the factors affecting texture in IF steels. The mechanisms operating during rolling and annealing are also reviewed. The experimental work includes evaluation of textures throughout processing from hot rolling to annealing. Emphasis is placed on the study of the evolution and development of texture during recrystallisation, using the EBSD technique. An understanding of the mechanism controlling texture evolution during recrystallisation has been developed by studying grain orientations and grain boundaries at different stages of recrystallisation. Texture evolution in the steel studied is dependent on oriented nucleation. No evidence in support of favourable orientation relationships affecting growth was found. The lower r-values seen in high strength IF steels can be explained from the textures.