Nb(C,N) Precipitation and Austenite Recrystallization

The influence of boron addition, amount of deformation, and solution heat-treatment temperature on the precipitation and recrystallization behaviors of a family of high-strength low-alloy (HSLA) steels was studied. A stress relaxation technique was employed to detect the occurrence of austenite recrystallization and to determine the precipitation start(P _s) and finish(P _f) times. After preheating to 1100 °C or 1200 °C for 30 minutes, the specimens were cooled to test temperatures between 800 °C and 1000 °C. They were subsequently deformed to true strains of 5 or 25 pct and subjected to stress relaxation. The advent of recrystallization produced a sharp increase in relaxation rate, while the occurrence of carbonitride precipitation led to the appear- ance of a stress plateau. The results indicate that the presence of boron (1) accelerates carbo- nitride precipitation and (2) retards austenite recrystallization when present in combination with Nb. The precipitation-time-temperature (PTT) diagrams determined in this investigation are C-shaped for both the B-free as well as the B-modified Nb steels. These data were analyzed in terms of the classical theory of nucleation, on the basis of which it is demonstrated that the acceleration of the nucleation kinetics of precipitation can be attributed to the segregation of boron and of boron-vacancy complexes to dislocations and grain boundaries, as well as to the faster diffusion of Nb in the presence of boron.