The role of nitrogen in the hot working of niobium microalloyed steels.

A detailed and critical survey of the Chemical and Physical metallurgy principles of the hot working of niobium microalloyed steels has been carried out emphasizing the role of nitrogen. A series of steels of different nitrogen and niobium contents were initially given a single pass rolling to study the recrystallization kinetics. Subsequently two pass rolling schedules with a roughing pass to recrystallize the austenite followed by a finishing pass at a series of low temperatures were applied and slabs were then held for different times before quenching. The recrystallization kinetics after the second pass were studied by quantitative metallography and precipitation kinetics were followed by chemical analysis of electrolytically extracted precipitates. Accurate lattice parameters of the extracted carbonitride were determined by X-ray Diffraction to obtain their composition and size. Those observations were complemented by limited Electron Microscopy and free N analysis. A thermodynamic model was developed to predict carbonitride composition and solution temperatures. This is based on published solub ility data and involves the assumption of ideal solution in the carbonitride, as information about the non-ideal characteristies of the solution of niobium carbides and nitrides is not available. Results obtained were compared with published observations on Reheating and with the present results on precipitation. Good agreement was obtained with solution temperatures but it is clear that the thermodynamic data is not comprehensive enough for the model to predict closely the observed carbonitride composition which becomes N rich at high temperatures. The effect of Mn and Si were also considered and it was found that Mn had a greater effect than Si on the solution temperature. On reheating all the experimental steels, grain coarsening occurred below the solution temperatures as a result of abnormal grain growth. The roughing pass at 1100°C caused the recrystallization to produce austenite grain sizes of 50 - 70ym in all the steels and resulted in a significant fraction of carbonitride precipitation. After finishing, recrystallization was increasingly retarded with increasing N or Nb contents. In the former case this was attributed to the refinement of the precipitate size and in the latter case by the increase in volume fraction of precipitates. Room temperature properties of air cooled slabs were found to be relatively poor as a result of the coarse (15um) ferrite grain size after the two pass schedules.