Interaction between deformation, recrystallization and precipitation in niobium steels

The present study was undertaken in order to establish the extent to which niobium (columbium) inhibits recovery and recrystallization of microalloyed austenite while present as solute on the one hand, and as carbonitride precipitates on the other. Three steels were used; the first was an HSLA steel containing 0.054 pct Nb, 0.05 pct C and 0.92 pct Mn. The second was prepared by treating the first in wet hydrogen at 1100°C so as to reduce the C and N content to about 10 ppm by wt. The third material was a plain carbon steel containing 0.055 pct C and 0.41 pct Mn. The isothermal recovery and re-crystallization of these materials, after an interval of hot working, was studied by means of interrupted compression tests. Samples were prestrained at 10-2 and 10-1s-1 at 900, 950 and 1000°C to natural strains of 0.10 and 0.25, and held isothermally prior to reloading. The results obtained in this way, indicate that 0.56 pct of substitutional solute can give rise to an order of magnitude decrease in the rate of recrystallization. When precipitation of Nb(CN) takes place either during deformation or in strained aus-tenite the mean precipitate size is ∼ 20 nm. The presence of such particles inhibits both static recovery and recrystallization; the magnitude of the effect being dependent on the volume fraction of precipitate. When the volume fraction reaches about 0.02 pct, static recrystallization is completely suppressed for the present prestraining conditions.