EFFECTS OF COMPOSITION AND COILING TEMPERATURE ON PRECIPITATION AND TEXTURE FORMATION IN A FEW INTERSTITIAL FREE HIGH STRENGTH STEELS

Interstitial free high strength (IFHS) steels are widely used in the automobile sector for their excellent deep drawability coupled with adequate strength. However, very often deterioration of drawability has been reported in these steels, as compared to the normal interstitial free (IF) steels. A few batch annealed IFHS steels with varying chemical composition and coiled at different temperatures have been chosen for the present study with a view to determining the effects of composition and coiling temperature on precipitation and texture formation after cold rolling and annealing. It has been observed that the final texture developed after batch annealing depends on precipitation which in turn can be related to the composition and processing history of the steels. In these steels P and Mn were added as solid solution strengthening elements, whereas Ti and Nb additions were made to scavenge the C and I in solid solution. It appears that P is taken out of the solution due to the formation of FeTiP, thus leaving behind insufficient Ti to form TiC. As a result C remains in solid solution thereby degrading the beneficial {111} texture formation during recrystallization annealing. However, when the same steel is coiled at a higher coiling temperature, precipitation of carbosulphides takes place during the coiling itself which in turn takes the C out of solid solution leading to an enhancement of the favorable {111} texture formation during recrystallization anneal. Presence of FeTiP and absence of carbide type of precipitates have been identified as the key reasons for the deterioration of deep drawability in these steels when coiled at lower temperatures.