Cold rolling and annealing textures in low carbon and extra low carbon steels

Abstract

The cold rolling texture of low and extra low carbon steels is primarily made up of a nearly perfect normal direction (ND) fibre and of two other components, centred at 001 and 112, which lie along the rolling direction (RD) fibre. This texture is influenced significantly by the hot band texture and not particularly by such metallurgical parameters as steel chemistry or the presence of precipitates. The annealing of heavily cold rolled materials strengthens the ND fibre and reduces the intensity of the RD fibre. The annealing texture, particularly the strength of the 111 components, depends significantly on the hot band texture and grain size, as well as on coiling temperature, cold reduction, and alloy chemistry. The 111 fibre is particularly beneficial for imparting good deep drawability (high r_m value) to sheet steel, whereas the 001 has a detrimental effect. In conventional batch annealed Al-killed steels, cold reductions of around 70%, low coiling temperatures and slow heating rates induce the development of the most desired annealing textures and correspondingly the highest r_m values. The optimum cold reduction increases to about 90% in Nb or Ti stabilised interstitial free (IF) steels. Grain growth after recrystallisation increases r_m; thus longer annealing cycles and higher annealing temperatures are beneficial, if grain growth occurs. In the case of box annealing, the practical upper limit of annealing temperature is ∼720°C. In the continuous annealing process, which involves higher heating and cooling rates, cold rolled steels can be annealed to advantage in the intercritical γ + α range. Carbon in solution and/or in the form of carbides is the single most deleterious element that impedes the development of sharp 111 annealing textures. Dissolved N, P, and Mn have effects similar to that of carbon. Niobium, Ti, or Si in solid solution enhances the intensity of the 111 or near 111 components.