Influence of titanium and nitrogen on hot ductility of C-Mn-Nb-Al steels

The influence of small additions of titanium on the hot ductility of C-Mn-N b-Al steels has been examined. Titanium and nitrogen levels varied in the r anges 0.014-0.045 and 0.004-0.011 wt-%, respectively, so that a wide range of Ti/N ratios could be studied. The tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min(-1) to test temperature s in the range 1100-800 degrees C and strained to failure at a strain rate of 2 x 10(-3) s(-1). It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase i n the size of the titanium containing precipitates, and a decrease in the v olume fraction of precipitates. Coarser particles could be obtained by incr easing the Ti/N ratio above the stoichiometric ratio for TiN and by testing at higher temperatures. However; ductility was generally poor for these ti tanium containing steels and it was equally poor when niobium was either pr esent or absent. For steels with similar to 0.005 wt-%N ductility was very poor at the stoichiometric Ti/N ratio of 3.4 : 1. Ductility was better at t he higher Ti/N ratios but only two of the titanium containing niobium steel s gave better ductility than the titanium free niobium containing steels an d then only at temperatures below about 950-900 degrees C. One of these ste els had the lowest titanium addition (0.014 wt-%), thus limiting the volume fraction of fine Ti containing particles and the other had the highest Ti/ N ratio of 8 : 1. However, even for these two steels ductility was worse th an for the titanium free steels in the higher temperature range. The commer cial implications of these results are discussed. MST/4232.