Effect of Combined Addition of Nb and Ti on the Mechanical Properties of Extra Low Carbon Steel Sheets

Effect of combined addition of Nb and Ti on the mechanical properties of interstitial free (IF) steel sheets was studied comparing with Nb-bearing IF steel (Nb-IF) and Ti-bearing IF steel (Ti-IF).With the addition of Ti equivalent to nitrogen content together with Nb, mechanical properties are very much improved compared to Nb-IF, especially when steels are coiled at normal temperature or at the coilend when coiled at high temperature. This is attributed to the formation of large cubic TiN precipitates. On the other hand, fine AIN precipitates is formed in Nb-IF.If the content of Ti exceeds the amount equivalent to form TiN, excess Ti forms fine TiC precipitates, and the yield stress tends to increase, while r-value does not decrease so much compared to the above-mentioned case.If the Ti is added more than amount equivalent to nitrogen and carbon content, the behavior of mechanical properties tends to become nearly the same as that of Ti-IF. This is attributable to the increase of the size of TiC precipitates.