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

摘要

Interstitial free high strength (IFHS) steels are widely used in the automobile sector fortheir excellent deep drawability coupled with adequate strength. However, very oftendeterioration of drawability has been reported in these steels, as compared to the normalinterstitial free (IF) steels. A few batch annealed IFHS steels with varying chemicalcomposition and coiled at different temperatures have been chosen for the present study with aview to determining the effects of composition and coiling temperature on precipitation andtexture formation after cold rolling and annealing. It has been observed that the final texturedeveloped after batch annealing depends on precipitation which in turn can be related to thecomposition and processing history of the steels. In these steels P and Mn were added as solidsolution strengthening elements, whereas Ti and Nb additions were made to scavenge the C andN 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 therebydegrading the beneficial { 1 11 } texture formation during recrystallization annealing. However,when the same steel is coiled at a higher coiling temperature, precipitation of carbosulphidestakes place during the coiling itself which in turn takes the C out of solid solution leading to anenhancement of the favorable { 111 }texture formation during recrystallization anneal. Presenceof FeTiP and absence of carbide type of precipitates have been identified as the key reasons forthe deterioration of deep drawability in these steels when coiled at lower temperatures.