Precipitation behavior in Nb-Ti microalloyed HSLA steels

The precipitation behavior of NbC carbides has been investigated in both Nb steel and Nb-Ti steel. Precipitation-Time-Temperature (PTT) diagrams were obtained by measuring softening ratios at high temperatures from two-stage interrupted compression tests. It was found that both precipitation start time (P-s) and finish time (P-f) were delayed in the Nb-Ti steel compared to those of the Nb steel. After a reheating treatment, all the precipitates were dissolved in the Nb steel, but, in the Nb-Ti steel, undissolved (Ti,Nb)(C,N) carbonitrides were observed. In addition, after the first prestrain of 0.3 at test temperatures, (Nb,Ti)C carbides strarted to precipitate preferentially at the undissolved (Ti,Nb)(C,N) carbonitrides. The delayed matrix precipitation of NbC carbides observed in the Nb-Ti steel seemed to be the cause of insufficient amounts of soluble Nb in austenite. That is, in the Nb-Ti steel, a significant fraction of Nb was lost by both the undissolved (Ti,Nb)(C,N) carbonitrides and the consequent heterogeneous nucleation of (Nb,Ti)C carbides. Therefore, it is suggested that the lowered supersaturation of Nb can result in the delay of precipitation of strain-induced NbC carbides formed in the matrix of Nb-Ti steel.